Scientific Bases for the Preparation of Heterogeneous Catalysts

Studies in Surface Science and Catalysis 143

SCIENTIFIC BASES FOR THE PREPARATION OF HETEROGENEOUS CATALYSTS

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S t u d i e s in S u r f a c e S c i e n c e a n d C a t a l y s i s Advisory Editors:

B. Delmonand J.T. Yates

Vol. 143

SCIENTIFIC BASES FOR THE PREPARATION OF

HETEROGENEOUS CATALYSTS

Proceedings of the 8t" International Symposium, Louvain-la-Neuve, Belgium,September 9 -12, 2002

Edited by E. G a i g n e a u x * , D.E. D e V o s * * , P. G r a n g e * , P.A. J a c o b s * * , J . A . M a r t e n s * * , P. R u i z * a n d G. P o n c e l e t *

* Universit# Catholique de Louvain, Louvain-la-Neuve, Belgium ** Katholieke Universiteit Leuven, Heverlee (Leuven), Belgium

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FOREWORD This volume of the Studies in Surface Science and Catalysis series contains the Proceedings of the Eighth International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, held on the campus of the "Universit6 catholique de Louvain" (UCL) in Louvain-la-Neuve, Belgium, on September 9-12, 2002. This symposium is jointly organized by the "Unit6 de catalyse et chimie des mat6riaux divis6s" of the "Universit6 catholique de Louvain", and the "Centrum voor oppervlaktechemie en katalyse" of the "Katholieke universiteit Leuven". The topic of this series of symposia, which was initiated in 1975 and organized at four-year intervals, has invariably been the discussion of the fundamentals behind the unit operations in the preparation of industrially relevant solid catalysts. Heterogeneous catalysis has always been a lively research field, although in the time period covered by these symposia, the emphasis of catalysis research has significantly shifted and spread from traditional applications in petroleum and bulk chemical production to, among others, the synthesis of fine chemicals, agro- and oleo-chemicals, pharmaceuticals and the environmental protection systems. This area, via the automotive applications, is presently dominating the catalyst market. The future for catalysis research seems bright, as it is now generally recognized as an essential element of sustainable development. In parallel with the broadening of the applications, the diversity of catalyst materials is booming, not only with respect to active elements and the sophistication of molecularly engineered active sites, but also with respect to the never ending discovery of new approaches for structuring porous matrices at the different relevant length scales, including the nanometer, the micrometer as well as the millimeter scale. Catalyst preparation techniques and physico-chemical catalyst characterisation tools likewise are subject to continuous developments. The organisers of these symposia, assisted by a Scientific Committee composed of eminent researchers holding an industrial appointment, try to keep trace of the relevant developments in the field and solicit contributions dealing with preparation aspects of relevant new generations of catalyst materials. In the organisation of this Eighth Symposium, the Scientific Committee was faced with the difficult task to select 139 papers (37 oral communications and 102 posters) out of the more than 200 submitted abstracts. When hesitating among excellent contributions, the decisive criterion was always the catalyst preparation aspect of the work. The organisers hope to be

able to offer a valuable platform for discussion of the science behind catalyst preparation. The attendance of over 250 suggests that the effort is appreciated by the scientific community. The organisers are indebted to public and private sponsors without whom the organisation of this symposium would have been financially very difficult. For obvious reasons, the sponsoring Companies and Agencies cannot be acknowledged properly by citing them in the Proceedings. The same holds true for all those who have contributed to the preparation of the meeting, secretaries, technicians, students and postdocs. Fortunately, the organisers are in position to express their appreciation towards the Rector of the UCL, Professor M. Crochet, and the "Service des Auditoires", for allowing the event to be patronized again by the university. Our grateful acknowledgements also go specifically to Ms Marianne Saenen, who has decisively contributed to the success of the symposium. Thankyou very much Marianne I

The Editors

Georges Poncelet has been a driving force of this Symposia series. Given his retirement next year, the moment has come to express our appreciation and gratefulness for his devotion and dedication to these meetings. Georges, it is with a warm heart that we dedicate this volume to you.

Eric

Dirk

Johan

Patricio

Paul

P~rre

vii ORGANIZING COMMITTEE Dr. D. DE VOS, Katholieke Universiteit Leuven Dr. E.M. GAIGNEAUX, Universit6 catholique de Louvain Prof. P. GRANGE, Universit6 catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Prof. J. MARTENS, Katholieke Universiteit Leuven Dr. G. PONCELET, Universit6 catholique de Louvain Dr. P. RUIZ, Universit6 catholique de Louvain SCIENTIFIC COMMITTEE Dr. A. ANUNDSKAS, Norsk Hydro, Norway Dr. M.P. ATKINS, BP Amoco, United Kingdom Dr. G. BELLUSSI, EniTecnologie, Italy Dr. J.-L. BOUSQUET, TotalFinaElf, France Dr. J.A. DELGADO, Repsol, Spain Dr. D. DE VOS, KUL, Belgium Dr. E.M. GAIGNEAUX, UCL, Belgium Prof. P. GRANGE, UCL, Belgium Dr. J. GROOTJANS, Atofina Research, Belgium Dr. K. HARTH, BASF, Germany Dr. G. HECQUET, Atofina, France Dr. S.D. JACKSON, Synetix, United Kingdom Prof. P.A. JACOBS, KUL, Belgium Dr. K. JOHANSEN, Haldor Topsoe, Denmark Dr. S. KASZTELAN, Institut Franqais du P6trole, France Dr. J.-P. LANGE, Shell International, The Netherlands Prof. J.A. MARTENS, KUL, Belgium Dr. L.R. MARTENS, Exxon Mobil, Belgium Dr. R. PARTON, DSM Research, The Netherlands Dr. M.A. PEREZ, CEPSA, Spain Dr. G. PONCELET, UCL, Belgium Dr. P. RUIZ, UCL, Belgium Dr. F. SCHMIDT, Sfid Chemie, Germany Dr. J.-P. SCHOEBRECHTS, Solvay, Belgium Dr. M. SCHOONOVER, UOP, USA Dr. C. STOCKER, Sumitomo Deutschland, Germany Dr. M. TWIGG, Johnson Matthey, United Kingdom

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ix

Contents Foreword Aspects of scale-up of catalyst production Keld Johansen

Quantitative structure-activity relationships in zeolite-based catalysts: influence of framework structure J.L. Casci and M.D. Shannon

17

Cogelation: an effective sol-gel method to produce sinter-proof finely dispersed metal catalysts supported on highly porous oxides B. Heinrichs, S. Lambert, C. Alig, J.P. Pirard, G. Beketov, V. Nehasil and N. Kruse

25

Steam reforming of CH4 over Ni/Mg-A1 catalyst prepared by spc-method from hydrotalcite T. Shishido and K. Takehira

35

Toward a molecular understanding of noble metal catalyst impregnation J.R. Regalbuto, M. Schrier, X. Hao, W.A. Spieker, J.G. Kim, J.T. Miller and A.J. Kropf

45

Support modification of cobalt based slurry phase Fischer-Tropsch catalysts S. Barradas, E.A. Caricato, P.J. van Berge and J. van de Loosdrecht

55

The effects of nature and pretreatment of surface alumina support on the catalytic nickelsilicate membrane formation C. Constantin, V. Parvulescu, A. Bujor, G. Popescu and B.L. Su

67

Supports and catalysts preparation by using metal alkoxides grafting technique E. Santacesaria, A. Sorrentino, M. Di Serio and R. Tesser

77

Combinatorial approaches for speeding up heterogeneous catalyst discovery, optimisation and scaling-up C. Mirodatos

89

High surface area metal oxides from matrix assisted preparation in activated carbons M. Schwickardi, T. Johann, W. Schmidt, O. Busch and E Schiith

93

Effects of the impregnating and drying process factors on mechanical properties of a PCoMo/A1203 hydrotreating catalyst Dongfang Wu and Yongdan Li

101

Influence of CeO2 content on R h / Y i O 2 monolithic catalysts for N20 decomposition S. Suarez, M. Yates, F.J. Gil Llambfas, J.A. Martfn, P. Avila and J. Blanco

111

Pt combustion catalysts prepared from W/O microemulsions J. Rymes, G. Ehret, L. Hilaire and K. Jir6tov6

121

Preparation of stable catalysts for N20 decomposition under industrial conditions S. Alini, E Basile, A. Bologna, T. Montanari and A. Vaccari

131

The Anderson-type heteropolyanions in the synthesis of alumina- and zeolitesupported HDS oxidic precursors E. Payen, G. Plazenet, C. Martin, C. Lamonier, J. Lynch and V. Harl~

141

Sol-gel preparation of pure and silica-dispersed vanadium and niobium catalysts active in oxidative dehydrogenation of propane P. Moggi, G. Predieri, D. Cauzzi, M. Devillers, P. Ruiz, S. Morselli and O. Ligabue

149

Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas D.J. Draelants, Y. Zhang, H. Zhao and G. V. Baron

159

Preparation of gold-titanosilicate catalysts for vapor-phase propylene epoxidation using H2 and 02 A.K. Sinha, S. Seelan, S. Tsubota and M. Haruta

167

Sol-gel synthesis of colloids and triflates containing hybrid type catalysts A.N. Parvulescu, B.C. Gagea, M. Alifanti, V. Parvulescu and V.L Parvulescu

177

Preparation of zeogrids through interposed stapling and fusion of MFI zeolite type nanoslabs S.P.B. Kremer, C.E.A. Kirschhock, M. Tielen, E Collignon, P.J. Grobet, P.A. Jacobs and J.A. Martens

185

Large scale synthesis of carbon nanofibers by catalytic decomposition of hydrocarbon L. Pesant, G. Wine, R. Vieira, P. Leroi, N. Keller, C. Pham-Huu and M.J. Ledoux

193

Synthesis and characterization of carbon nanofiber supported ruthenium catalysts M.L. Toebes, E E Prinsloo, J.H. Bitter, A.J. van Dillen and K.P. de Jong

201

Synthesis of high pore volume and specific surface area mesoporous alumina L. Sicard, B. Lebeau, J. Patarin and E Kolenda

209

xi Investigation on acidity of zeolites bound with silica and alumina X. Wu, A. Alkhawaldeh and R.G. Anthony

217

Preparation of BN catalyst supports from molecular precursors. Influence of the precursor on the properties of the BN ceramic J.A. Perdigon-Melon, A. Auroux, J.M. Guil and B. Bonnetot

227

Monitoring of the particle size of MoSx nanoparticles by a new microemulsionbased synthesis K. Marchand, M. Tarret, L. Normand, S. Kasztelan and T. Cseri

239

Transition metal phosphides. Novel hydrodenitrogenation catalysts V. Zuzaniuk, R. Prins, C. Stinner and T. Weber

247

The application of non-hydrothermally prepared stevensites as support for hydrodesulfurization catalysts M. Sychev, R. Prihod'ko, A. Koryabkina, E.J.M. Hensen, J.A.R. van Veen and R.A. van Santen

257

NiMo/HNazY(s)-A1203 catalysts for the hydrodesulfurization of hindered dibenzothiophenes: effect of the preparation method T. Klimova, D. Solis, J. Ramirez and A. L6pez-Agudo

267

Chiral dirhodium catalysts confined in porous hosts H.M. Hultman, M. de Lang, M. Nowotny, I. W. C.E. Arends, U. Hanefeld, R.A. Sheldon and T. Maschmeyer

277

Synthesis and characterization of zeolite encaged enzyme-mimetic copper histidine complexes J.G. Mesu, H.J. Tromp, D. Baute, E.E. van Faassen and B.M. Weckhuysen

287

Strategies for the heterogenization of rhodium complexes on activated carbon J.A. Diaz-Au~ton, L.C. Romdn-Mart[nez, C. Salinas-Mart[nez de Lecea and H. Alper

295

Heterogeneous metathesis initiators M. Mayr, B. Mayr and M.R. Buchmeiser

305

Preparation of physically heterogeneous and chemically homogeneous catalysts on the base of metal complexes immobilized in polymer gels A.A. Efendiev, T.N. Shakhtakhtinsky and N.A. Zeinalov

313

Hydrocracking catalyst to produce high quality diesel fraction R. Galiasso Tailleur

321

xii Thermostable yttria-doped inorganic oxide catalyst supports for high temperature reactions E. Elaloui, R. Begag, B. Pommier and G.M. Pajonk

331

Preparation and characterization of WOx-CeO2 catalysts M. Alifanti, C.M. Visinescu, V.1. Parvulescu, P. Grange and G. Poncelet

337

Preparation of iridium catalysts by deposition precipitation: room temperature oxidation of CO M. Okumura, E. Konishi, S. Ichikawa and T. Akita

345

New approach to preparation and investigation of active sites in sulfated zirconia catalysts for skeletal isomerization of alcanes N.A. Pakhomov, A.S. Ivanova, A.E Bedilo, E.M. Moroz and A.M. Volodin

353

Supported ruthenium carbido-cluster catalysts for the catalytic removal of nitrogen monoxide and sulfur dioxide: the preparation process monitored by sulfur K-edge X-ray absorption near-edge structure Y. Izumi, T. Minato, K.-L Aika, A. Ishiguro, T. Nakajima and Y. Wakatsuki

361

Catalytic transformation of dichloromethane over Y and X zeolites L. Pinard, J. Mijoin, R. Lapeyrolerie, P. Magnoux and M. Guisnet

369

Preparation of new solid super-acid catalyst, titanium sulfate supported on zirconia and its acid catalytic properties J.R. Sohn, E.H. Park and J.G. Kim

377

Superacid WOx/ZrO2 catalysts for isomerization of n-hexane and for nitration of benzene V.V. Brei, O.V. Melezhyk, S.V. Prudius, M.M. Levchuk and K.I. Patryliak

387

Preparation of copper-oxide catalyst systems for hydrogenation Y. Sakata, N. Kouda, Y. Sakata and H. Imamura

397

Application of experimental design for NOx reduction by Pd-Cu catalysts M. Rebollar, M. Yates and M.A. Valenzuela

407

Marked difference of catalytic behavior by preparation methods in CH4 reforming with CO2 over Mo2C and WC catalysts S. Naito, M. Tsuji, Y. Sakamoto and T. Miyao

415

Synthesis and properties of new catalytic systems based on zirconium dioxide and pentasils for process of NOx selective catalytic reduction by hydrocarbons V.L. Struzhko, S.N. Orlyk, T.V.Myroniuk and V.G. Ilyin

425

xiii Preparation of chitosan based catalysts for several reactions of liquid phase hydrogenation V. Isaeva, A. Ivanov, L. Kozlova and V. Sharf

435

Preparation of Mo/A1203 sulfide catalysts modified by Ir nanoparticles J. Cinibulk and Z. Vit

443

Peptization mechanisms of boehmite used as precursors for catalysts D. Fauchadour, E Kolenda, L. Rouleau, L. Barr~ and L. Normand

453

Influence of the treatment of Y zeolite by ammonium hexafluorosilicate on physicochemical and catalytic properties: application for chlororganics destruction R. Lopez-Fonseca, J.L Guti~rrez-Ortiz, B. de Rivas, S. Cibrian and J.R. Gonz61ez-Velasco

463

Preparation of SiO2 modified SnO2 and ZrO2 with novel thermal stability Y.-Z Zhu, J.-Y. Wei, L. Zeng, X.-D. Zhao, W. Lin and Y.-C. Xie

471

Control of the textural properties of cesium 12-molybdophosphate-based supports S. Paul V. Dubromez, L. Zair, M. Fournier and D. Vanhove

481

MnOx/CeO2-ZrO2 and MnOx/WO3-TiO2 catalysts for the total oxidation of methane and chlorinated hydrocarbons E. Kantzer, D. D6bber, D. Kiessling and G. Wendt

489

Catalytic behaviour of Rh-supported catalysts on lamellar and zeolitic structures by anchoring of organometallic compound C. Blanco, R. Ruiz, C. Pesquera and E Gonzalez

499

The use of sol-gel technique to prepare the TiO2-A1203 binary system over a wide range of Ti-A1 ratios A.Yu. Stakheev, G.N. Baeva, N.S. Telegina, L V. Mishin, T.R. Brueva, G.L Kapustin and L.M. Kustov

509

Catalytic performance in the complete acetone oxidation of manganese and cobalt oxides supported on alumina and silica A. Gil, S.A. Korili, M.A. Vicente and L.M. Gandia

517

Unsupported and supported manganese oxides used in the catalytic combustion of methyl-ethyl-ketone L.M. Gandia, S.A. Korili and A. Gil

527

Ni/Hfl zeolite catalysts prepared by the deposition-precipitation method R. Nares, J. Ramirez, A. Guttierrez-Alejandre, R. Cuevas, C. Louis and T. Klimova

537

xiv Sol-gel A1203 structure modification by Ti and Zr addition. A NMR study J. Escobar, J.A. de Los Reyes and T. Viveros

547

Promotion of Ru/ZrO2 catalysts by platinum A.M. Serrano-S6nchez, F. Blas-Sudrez, P. Steltenpohl, M.P. Gonzdlez-Marcos, J.A. Gonzdlez-Marcos, and J.R. Gonzdlez-Velasco

555

Catalysts based on RhMo6 heteropolymetalates. Bulk and supported preparation and characterisation C.I. Cabello, I.L. Botto, M. Mu~oz and H.J. Thomas

565

Metallosilicate mesoporous catalysts prepared by incorporation of transition metals in the MCM-41 molecular sieves and their catalytic activity in selective oxidation of aromatics (styrene and benzene) V. Parvulescu and B.L. Su

575

Controlled surface modification of alumina-supported Mo or Co-Mo sulfides by surface organometallic chemistry J.-S. Choi, C. Petit-Clair and D. Uzio

585

Novel one step synthesis of cobalt (II) phtalocyanine-hydrotalcite catalysts for mercaptan oxidation in light oil sweetening I. Chatti, A. Ghorbel and J.M. Colin

595

Structural and catalytic properties of Zr-Ce-Pr-O xerogels S. Rossignol, C. Descorme, C. Kappenstein and D. Duprez

601

Influence of the precursor (nature and amount) on the morphology of MoO3 crystallites supported on silica D. Navez, C. Weinberg, G. Mestl, P. Ruiz and E.M. Gaigneaux

609

Single step synthesis of metal catalysts supported on porous carbon with controlled texture N. Job, E Ferauche, R. Pirard and J.P. Pirard

619

A g - S i Q and Cu/SiO2 cogelled xerogel catalysts for benzene combustion and 2-butanol dehydrogenation S. Lambert, N. Tcherkassova, C. Cellier, E Ferauche, B. Heinrichs, P. Grange and J.P. Pirard

627

Preparation of zeolite catalysts for dehydrogenation and isomerization of n-butane M. Inaba, K. Murata, M. Saito, I. Takahara, N. Mimura, H. Hamada and Y. Kurata

637

XV

The application of well-dispersed nickel nanoparticles inside the mesopores of MCM-41 by use of a nickel citrate chelate as precursor D.J. Lensveld, J.G. Mesu, A.J. van Dillen and K.P. de Jong

647

Preparation of Ce-Zr-O composites by a polymerized complex method T.G. Kuznetsova, V.A. Sadykov, E.M. Moroz, S.N. Trukhan, E.A. Paukshtis, V.N. Kolomiichuk, E.B. Burgina, V.L Zaikovskii, M.A. Fedotov, V.V. Lunin and E. Kemnitz

659

Sol-gel routes for the preparation of heterogeneous catalyst based on Ru, Rh, Pd supported metals P. Moggi, S. Morselli and G. Predieri

669

Development of novel heterogeneous catalysts for oxidative reactions: preparation and performance of Co-Nx catalysts in partial oxidation of toluene and n-butane M.L. Kaliya, S.B. Kogan and M. Herskowitz

679

Synthesis and modification of basic mesoporous materials for the selective etherification of glycerol J.-M. Clacens, Y. Pouilloux and J. Barrault

687

Carbon nanotubes: a highly selective support for the C=C bond hydrogenation reaction J.-P. Tessonier, L. Pesant, C. Pham-Huu, G. Ehret and M.J. Ledoux

697

Raman studies of the templated synthesis of zeolites P.P.H.J.M. Knops-Gerrits and M. Cuypers

705

Templateless synthesis of catalysts with narrow mesoporous distribution N. Yao, G. Xiong, S. Sheng, M. He and K.L. Yeung

715

Control of pore structures of titanias and titania/aluminas using complexing agents M. Toba, S. Niwa, N. Kijima and Y. Yoshimura

723

Tungstophosphoric acid immobilized in polyvinyl alcohol hydrogel beads as heterogeneous catalyst L.R. Pizzio, C. C6ceres and M.N. Blanco

731

Functionalized SiMCM-41 as support for heteropolyacid based catalysts L.R. Pizzio, A. Kikot, E. Basaldella, P. Vazquez, C. Cdceres and M.N. Blanco

739

Influence of the preparation method on the surface properties and activity of alumina-supported gallium oxide catalysts A. Petre, B. Bonnetot, A. Gervasini and A. Auroux

747

xvi Preparation and properties of bimetallic Ru-Sn sol-gel catalysts: the influence of catalyst reduction J. Hajek, N. Kumar, H. Karhu, L. Cerveny, J. Vayrynen, T. Salmi and D. Yu. Murzin

757

A new insight into molybdate/boehmite interaction D. Minoux, F. Diehl, P Euzen, J.-P Jolivet and E. Payen

767

Controlled coating of high surface area silica with titania overlayers by atomic layer deposition J. Keriinen, E. Iiskola, C. Guimon, A. Auroux and L. Niinist6

777

Concept of the synthesis of novel platinum catalysts for selective hydrogenation of unsaturated carbonyl compounds J. Kijenski and P. Winiarek

787

Storage and supply of hydrogen mediated by iron oxide: modification of iron oxides S. Takenaka, C. Yamada, T. Kaburagi and K. Otsuka

795

Catalytic activity of bulk and supported sulfated zirconia I.J. Dijs, L.W. Jenneskens and J.W. Geus

803

New one-step synthesis of superacid sulfated zirconia L. Zanibelli, A. Carati, C. Flego and R. Millini

813

Elaboration and characterization of a realistic Phillips model catalyst for ethylene polymerisation P.G. Di Croce, E Aubriet, P. Bertrand, P. Rouxhet and P. Grange

823

Preparation of new basic mesoporous silica catalysts by ammonia grafting H. Yoshida, Y. Inaki, Y. Kajita, K. Ito and T. Hattori

837

Titania-silica catalysts prepared by sol-gel method for photoepoxidation of propene with molecular oxygen C. Murata, H. Yoshida and T. Hattori

845

Preparation of large surface area MnOx-ZrO2 for sorptive NOx removal M. Machida, M. Uto and T. Kijima

855

Preparation of CuOx-TiO2 nano-composite photocatalysts from intercalated layer structure M. Machida, S. Nagasaki and T. Kijima

863

Vanadia-doped titanium pillared clay: preparation, characterization and SCR activity of NO by ammonia L. Khalfallah Boudali, A. Ghorbel, P. Grange and S.M. Jung

873

xvii Advanced preparation by sol-gel method of the encapsulated Pd/A1203 catalysts for methane combustion S. Fessi, A. Ghorbel, A. Rives and R. Hubaut

881

Non-ionic surfactant templated synthesis of mesoporous silica in the presence of platinum salts M.A. Aramendfa, V. Borau, C. Jimdnez, J.M. Marinas, EJ. Romero and EJ. Urbano

891

Synthesis and acid-base properties of catalysts based on magnesium and sodium-magnesium mixed phosphates M.A. Aramendfa, V. Borau, C. Jimdnez, J.M. Marinas, R. Rolddn, EJ. Romero and EJ. Urbano

899

Preparation of Pd-Ce/ZrO2 catalysts for methane oxidation L.S. Escand6n, S. Ord6ftez, E V. Dfez and H. Sastre

907

The effect of cerium introduction on vanadium-USY catalysts C. Ramos Moreira, M. Schmal and M.M. Pereira

915

Rh-Co mordenite catalysts for the selective reduction of NO by methane C.E. Quincoces, M. Incolla, A. De Ambrosio and M.G. Gonzdlez

925

Surface characterization of WO3-TiO2/A1203 catalysts and reactivity on selective catalytic reaction of NO by NH3 S. Egues, N.S. de Resende and M. Schmal

933

Catalytic materials for the synthesis of hydrofluorocarbons P. Cuzzato, V. Giammetta, R. Trabace and E Trifiro

941

Preparation, characterization and reactivity in m-cresol methylation of new heterogeneous materials having basic properties E Cavani, C. Felloni, D. Scagliarini, A. Tubertini, C. Flego and C. Perego

953

The effect of glycols in the organic preparation of V/P mixed oxide catalyst for the oxidation of n-butane to maleic anhydride S. Albonetti, E Cavani, S. Ligi, E Pierelli, E Trifiro, E Ghelfi and G. Mazzoni

963

Synthesis and characterization of nanostructured M02C on carbon material by carbothermal hydrogen reduction C. Liang, Z Wei, Q. Xin and C. Li

975

Carbon composite-based catalysts: new perspectives for the low-temperature H2S removal J.-M. Nhut, R. ~eira, N. Keller, C. Pham-Huu, W. Boll and M.J. Ledoux

983

xviii Active carbon surface oxidation to optimize the support functionality and metallic dispersion of a Pd/C catalyst V. Dubois, Y. Dal and G. Jannes

993

n-Butane isomerization over Al-promoted sulfated zirconias. Influence of the sulfate content J.A. Moreno and G. Poncelet

1003

Influence of preparation procedure on physical and catalytic properties of carbon-supported Pd-Au catalysts P. Canton, E Menegazzo, M. Signoretto, E Pinna, P. Riello, A. Bedetti and N. Pernicone

1011

Preparation of mesoporous highly dispersed Pd-Pt catalysts for deep hydrodesulfurization X. Xu, P. Waller, E. Crezee, Z. Shan, E Kapteijn and J.A. Moulijn

1019

Preparation of highly ordered CMI-1 and wormhole-like DWM mesoporous silica catalyst supports using C16(EO)10 as surfactant A. L~onard, J.L. Blin and B.L. Su

1027

Synthesis and characteriation of nanostructured mesoporous zirconia catalyst supports using non-ionic surfactants as templating agents J.L. Blin, L. Gigot, A. L~onard and B.L. Su

1035

Effect of preparation parameters on the catalytic activities of sulfated ZrO2SiO2 catalysts obtained by sol-gel process R. Akkari and A. Ghorbel

1045

Non-aggressive way for preparation of zirconium sulfate pillared clay using zirconium acetate developing high sulfur thermal stability over 830~ S. Ben Chaabene, L. Bergaoui, A. Ghorbel and J.E Lambert

1053

Influence of the preparation conditions on the structure of the active phase and catalytic properties of Ni-Co-molybdate propane oxydehydrogenation catalysts M.M. Barsan, A. Maione and E C. Thyrion

1063

Oxidized diamond as a new catalyst support T. Suzuki, K. Nakagawa, N.-O. Ikenaga and T. Ando

1073

Preparation of catalytic membranes, micro-capsules and fabrics active in immobilized Fenton chemistry J. Fernandez, V. Nadtochenko, A. Bozzi, T. Yuranova and J. Kiwi

1081

Preparation of vanadium-based catalysts for selective catalytic reduction of nitrogen oxides using titania supports chemically modified with organosilanes H. Kominami, M. Itonaga, A. Shinonaga, K. Kagawa, S. Konishi and Y. Kera

1089

xix Design, preparation and testing of effective FeOx/SiO2 catalysts in methane to formaldehyde selective oxidation E Arena, F. Frusteri, L. Spadaro, A. Venuto and A. Parmaliana

1097

New Fe-Mo-Ti mixed oxides prepared via the sol-gel method: comparison of the textural properties with solids obtained by impregnation S.R.G. Carrazan, C. Martin, C. M. Pedrero and J. Saunders

1107

Index

1115

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Aspects of scale-up of catalyst production Keld Johansen Research & Development Division, Haldor Topsoe A/S Nymollevej 55, DK-2800 Lyngby, Denmark, Tel: +45-45272491, E-mail: [email protected]

1. S C O P E OF C A T A L Y S T P R O D U C T I O N Catalyst production has a significant influence on the economy, since 80-90% of the chemicals used in a modern society are exposed to a catalyst. The value of the US catalyst market was 2.2 billion $ in 2000 [1]. A number of specialised catalyst companies in the world, many global, produce and supply a large number of products to the industry. It is said that products corresponding to 10% of the GNP of the industrialised countries are dependent on the availability of catalyst. Catalyst plants produce quantities of 0.1100 t/day of each product dependant on the type. Thus, transfer of new products and implementation in catalyst plants are important technological disciplines for each catalyst company. 2. N A T U R E OF T H E SCALE-UP P R O B L E M Before the decision is taken on transferring a new recipe from research and development departments to an existing catalyst plant or a new investment, considerable work has already been carried out - many test samples have been prepared and activitytested. All candidates for a new product have one thing in common: in the starting phase they have been prepared and selected from the laboratory processes.

2.1. Description of laboratory prepared samples Laboratory or bench scale prepared catalyst samples for screening are typically made in gram scale (10-50 g). The catalysts can be prepared in many ways depending on the type, but steps for three commonly used preparation routes are shown below in Fig. 1 and, for each step, examples of the typical laboratory equipment are given.

Io Typical co-precipitated catalyst manufacture Preparation step Typical equipment 1. Dissolution of agents Beaker with stirrer 2. Precipitation Pump + beaker with stirrer 3. Ageing Electric heating, thermostatic bath 4. Filtration Buchner funnel 5. Washing Demineralised water on Buchner funnel 6. Drying Drying cabinet 7. Calcination Muffle furnace 8. Lubricant aid addition Powder mixer Single station excenter press 9. Tabletting Electrically heated muffle furnace 10. Calcination Small reactor with H/N2 once through 11. Activation

II. Typical impregnated catalyst carrier process Typical equipment Preparation step 1. Forming a support (from another route) Heated muffle furnace 2. Activation of support Beaker with stirrer (pure chemicals) 3. Dissolution of impregnation liquid(s) Net in a beaker 4. Impregnation Drying cabinet 5. Drying Muffle furnace 6. Decomposition 7. Re-impregnation back to 4. Reactor sulphidation with H2/N2 once 8. Activation through III. Typical process for mixed/compounded catalyst Typical equipment Preparation step From flasks 1. Powders Beaker with stirrer 2. Dissolution of active metals Beaker with stirrer 3. Dissolution of extrusion aids Laboratory kneader 4. Mixing Laboratory piston extruder 5. Extrusion Drying cabinet 6. Drying Muffle furnace 7. Calcination/decomposing Laboratory sieve 8. Sieving Small reactor with H2/N2 9. Activation through Fig. 1. Three examples of commonly used preparation methods

etc. once

The laboratory equipment used is normally characterised by: Small dimensions with short mass and heat transport distances - High energy intensity per volume for stirrer mixers and kneaders - Pure chemicals Small layers in muffle furnaces with low temperature gradients, but long hea cycle H2 activation with low pH20 Drying with low but undefined pH20 Precipitation within small volume dimension Filtration without respect of agglomerate size Washing without respect of time or leakage of particles or ions - Tabletting/forming of non-representative granules - Generated heat during processes easily dissipated to cooling surface -

-

-

-

-

-

-

A modern catalyst laboratory will analyse and describe in detail all intermediates final catalysts from the above three manufacturing routes by means of the metho&, follows: main chemical elements and trace elements, phases (if not amorphous), 1: distribution and BET or selective surface area. A more complete list of commonly u methods is given in Table 1 below: Table 1. Physical and chemical characterisation methods C h e m i c a l

Main chemical elements Trace elements Oxidation state Element distribution

ICP, AAS, XRF and electron micro probe analysis AAS, ICP Electron micro probe analysis, EDS in SEM, ED5 in TEM

P h y s i c a l

Surface area Pore volume- total Pore size distribution Phases, crystallite size Surface composition Surface properties Particle size distribution Structure and texture Thermogravimetry Specially for finished product form Abrasion resistance for granules Attrition resistance for fluid cat and powders Crushing strength

BET (N2, Ar, K), or specific area, chemisorption of H2,CO, N20 Water absorption-Hg intrusion He, or Hg intrusion, N2 adsorption XRD, TEM, Raman Spectroscopy XPS, SIMS IR, microcalorimetry, chemisorption/desorption (TPD,TPR) LLD, SAXS, sieves, TEM, SEM TEM, SEM, optical microscopy TGA, DTA, dilatometer

Attrition loss Texture analyser

However, all the above methods in Table 1 cannot give a scientifically exhaustive description of the intermediates nor of the final catalyst. Amorphous phases often obtained from precipitation cannot be characterised sufficiently (how many kinds of amorphous phases exist?) Furthermore, the final catalyst granule is formed from agglomerates of crystallites. Both crystallites (forming primary agglomerates) and the agglomerates have their own particle size distribution and binding properties. Particle size distribution of primary and secondary agglomerates controls the final pore size distribution. The pore size distribution and particle strength will have significant influence on the final performance of the product in the reactor. Even considering the methods listed in Table 1, there is no method or combined methods today that can give a full description of the crystalliteagglomerate multi-parameter system. To further illustrate the problem, it should be mentioned that even if the overall chemical composition is the same for two different manufacturing routes, the pore size distribution is most probably different. Thus, it is not possible to characterise an intermediate or final catalyst so you can be sure to have the same catalyst without preparing it in the same reproducible way.

2.2 Catalyst manufacturing- unit operations The catalyst plant is operating in ton scale (typically 1-100 t/day) with processes and equipment completely different from bench scale as sketched in Fig. 1, even if the preparation steps are the same. In the open literature, description of catalyst manufacturing processes and equipment is sparse. The reference list contains important monographs and papers [2-24]. The patent literature gives some information, but catalyst manufacturing technologies are often not patented but kept secret. The single step in manufacturing is called a unit operation and can be performed by several types of equipment. Table 2 shows most of the unit operations used and examples of equipment for each. Most of the typical equipment will have more different time constants, heat transfer, flow patterns, temperature profiles etc. than bench scale equipment and, therefore, the final catalyst will achieve other properties.

2.3 Optimal combinations For every commercial catalyst an optimal combination of unit operation sequence exists for the manufacture of that specific catalyst and there will for each unit operation exist preferential process equipment, i.e. fluid bed calciner for calcination. The sequence of unit operations with the special selection of process equipment and all process parameters forms the know-how for manufacturing a catalyst product of large commercial value. But know-how does not mean that you always know why the desired properties are obtained due to the insufficient scientific characterisation of the catalyst material as described above under 2.1. Even small adjustments of the process can change strength, pore size distribution, bulk density, crystallite size etc. of the product and, thus, harm the performance in the industrial reactor. It has normally been costly and time-consuming to reach the final recipe and, therefore, all catalyst companies want to keep it secret. I f a single unit operation is changed it will often influence the optimisation of most of the other unit operations, and much of the development will have to be redone.

Table 2. List of unit operations with typical equipment Unit operation Typical equipment Tanks with stirrer 1. Dissolution Pumps, specially designed reactors and stirrers 2. Precipitation Temperature-regulated tanks, autoclaves 3. Ageing and maturation, gel formation Belt filter 4. Filtration Drum filter Centrifuge Filter press Belt filter 5. Washing Drum filter Centrifuge Belt conveyor furnace 6. Drying Spray drying Fluid bed drying Rotary kiln Vacuum dryer Z-mixer 7. Wet mixing (kneading) Double screw mixer Nauta mixer 8. Dry mixing Double cone mixer Ribbon blender Jet mill 9. Grinding Roller mill Universal mill Pearl mill Screen 10. Sieving Tabletting 11. Forming Extrusion Granulation Spray drying Corrugation Belt conveyor furnace 12. Calcination Rotary kiln Shaft furnaces Chamber and muffle furnace Tunnel furnace Fluid bed Pore filling- incipient wetness 13. Impregnation Immersion in liquid Controlled chemisorption See under calcination 14. Decomposition Electrical hearth 15. Fusion Prereduction reactor 16. Activation Fluid bed, chamber and muffle furnace 17. Cooling and annealing Washcoater, dragee pan 18. Coating Tanks with stirrer 19. Leaching Tanks with stirrer, kneader 20. Reslurrying . .

2.3.1 Examples of combinations A process scheme example of a precipitated catalyst is given below in Fig. 2.

Metal salts

Bases

Dissolution of raw materials I

[

Precipitation

I

I

Ageing

I

I

Filtration

I

Washing

Recycle_= -=

Re-slurrying

]

l

Spray drying

I

Rotary kiln calcination

I

Powder mixing

I

Tabletting

I

I Lubricants i -i [

Salt Salt

}=

H20 ~.

I Conveyor belt calcination I

Recycle I I

Sieving

.]

Finished product

I

Fig. 2. The scheme for production of an extruded and impregnated catalyst can be as shown in Fig. 3. Raw materials Recycled

Additives

I~ Mixing of raw materials

-I

Extrusion

I r

Impregnation ~ liquid I

Recycle

[i E

Drying Calcination Impregnation Drying Calcination Sieving Finished product

Fig. 3.

H20

HaO HaO }= H20 + gases

3. P E R F O R M I N G SCALE-UP E X P E R I M E N T S When the unit operation processes and equipment have such a major influence on the final catalyst properties and performance, it would be logical and desirable to perform the catalyst test preparations in the catalyst plant process equipment or in a pilot production plant in order to avoid using development time on "wrongly prepared" samples. However, there must be at least some selection of recipes. The scale-up factor from laboratory to plant is approximately 1,000,000 corresponding to going from g/day to tons/day. A pilot manufacturing plant will typically produce 1-50 kg/day. Then the scale-up factors will be: - From laboratory to pilot: 1000 - From pilot to plant: 1000

3.1. Plant experiments Catalyst plants are large investments in a number of production lines. Some of the lines are dedicated to one product, others are multipurpose lines where various products can be made. The advantages of performing experiments in the plant with the realistic processes and equipment in tons/day scale must be compared to the drawbacks as follows: a. Expensive raw material, energy and labour. b. Expensive disposal/recycling of unusable product. c. Lack of flexibility of production lines. d. Production lines are booked and lost contribution will also be costly. The large economic consequences and lack of flexibility will dictate to perform most of the manufacture development work in a pilot scale, where the cost of experiments is 1001000 times lower, and only to use the plant lines for the last process adjustments.

3.2. Pilot production plants Catalyst pilot production plants have process and equipment facilities that are similar to equipment used in the large scale plant as listed in Table 2, but at the kg capacity scale typically 10-50 kg/day. Equipment in a catalyst pilot production is to a great extent industrial equipment downscaled by a factor 50-1000. It is necessary that equipment can easily be moved, so flexible production lines of new combinations of unit operations can be arranged. A modern pilot plant will contain most of the equipment listed in Table 2. 4. P H Y S I O C H E M I C A L D E S C R I P T I O N OF UNIT O P E R A T I O N S

4.1. Methods of study of unit operations The methods of characterisation of intermediates and final product coming from each unit operation are of course the same as given in Table 1. However, if the unit operation process itself is to be analysed and understood more profoundly in situ or in line, analytical equipment must be installed and mathematical models for heat and mass transport must be set up.

4.2. Examples of unit operation studies 4.2 91. Precipitation High surface activated alumina carrier is important for a large number of catalysts. The precipitation can take place from a number of aluminium salts: AI(NO3)3, A12(SO4)3, A1C13 and NaAIO2 with bases or acids as NaOH, NH3, KOH, HNO3, HC1 and H2SO4. Precipitation is carried out by a controlled mixing of the reactants in order to obtain a supersaturated solution from which nucleation takes place. Amorphous primary particles are formed that later crystallise into desired phases and in parallel agglomerate to larger secondary particles 9 Precipitation processes need in line pH meters and possibilities for automatic particle size distribution analysis coupled to the ageing vessel. The potential formed types of aluminium compounds present in the solution are numerous (25). The first phases formed are far from equilibrium and, as a consequence of Ostwald's rule of stages, transform within minutes and hours. This illustrates the importance of control of history 9The precipitation methods and parameters in combination with carefully controlled ageing determine agglomerate size distribution, agglomerate packing and agglomerate strength. The precipitation may be controlled by pH and mass flows, but the study of the particle size distribution of amorphous particles and agglomerates within milliseconds is difficult, since only few relevant methods as SAXS and SANS exist. As an example of SAXS measurements, crystallite size distribution of the long dimension of precipitated pseudoboehmite is given for two sets of precipitation conditions (Fig. 4). The samples are taken out during the precipitation.

9 "~

0.05

.4,,.a

'~ 9

0.04

N r~

0.03

C,J

0.02

03 0.01

i/ ,/

0.00 i

20

'

410

'

Particle diameter (nm)

Fig. 4.

!

60

4.2.2. Calcination Controlled calcination of alumina carrier extrudates or tablets is important for obtaining the desired pore volume, BET area and pore size distribution. All these parameters are influenced by the partial pressure of water vapour at a given time and temperature. As an example a dried alumina carrier is used, which should be calcined to the highest possible surface area, but the temperature must reach at least 500~ and the use of a rotary kiln with countercurrent flow of air is considered. The kiln is heated indirectly. From test calcinations of small samples in controlled atmosphere we know the influence of the water vapour pressure. It reduces the surface area and it is quite clear that the water vapour pressure in the kiln must be kept at the lowest possible values. This means drying the air fed to the kiln, if possible, and maximising the ratio between the air flow/carrier flow. The generation of water from calcining the alumina can be found by a simple thermo-gravimetric analysis: The profile of the water vapour pressure in a countercurrent rotary kiln follows the thermogravimetric profile - with adjusted scales and minimum values equal to the water vapour pressure of the feed gas - therefore, it can be expected to reach a high surface area, if the water vapour generated in the first part of the kiln is diluted to a sufficiently low partial pressure. If this is tried disappointing results will probably be obtained due to the diffusion resistance: a. The carrier in the kiln forms a bed of a certain thickness. The carrier is heated by a number of processes: heat transfer from the gas sweeping over the bed surface and from the kiln wall. Therefore, all the carrier at the bottom of the bed, as well, will generate water vapour at a given time. The water vapour can only escape the bed by diffusion or when the bed "folds over". To get an idea of the influence hereof, a one-dimensional equation for diffusion from a bed where all particles generate gas can be solved [26]:

(<-Po) I= H In

dp* ( 1 - r

r

Plotting this for the high water generation rate, r, of 0.0001 g/g*sec for two bed thicknesses of 2 and 5 cm gives the following bed profiles of realistic values for the bed density (dp*(1 - e)) - for p0 (the water vapour pressure in the kiln gas) = 0 atm (Fig. 5), where it can be seen that most of the bed - potentially- will have a water vapour pressure of close to 1 atm as long as water is generated (at high rates) from the carrier. This problem can either be solved by running the kiln with a low loading, bed height = 1-2 pellet diameters, or by another design where (dry) air is passed through the bed, and this has the potential for changing the situation much as seen in: b. where we will solve the diffusion of an internally generated gas (i.e. with a net flow out of the pellet) in a cylindrical pellet:

10

DeUPtRgT[ln ~ ) ( PPs) t - ] = R2-4 x2 dp*r

Calculating for a porosity of the carrier O to 0,67 and a tortuosity (1:) of 1.6, the effective diffusion coefficient becomes 0.19 and 0.38 cm2/sec, respectively. The calculations are still made for a high water generation rate of 0.0001 g/g*sec and using a dp of 1.3 g/ml. The calculation is made for two diameters of the cylindrical carriers 1 and 2 mm and shown in Fig. 6. Bed profiles at 6wt%/10min po = 0 atm

N: d.

"

\\

0,6

i"

0,0

0

,

'

'

0,5

1

1,5

2

2,5

3

3,5

4

4,5

"

400~

cm

--"

400~

cm

-'-"

600~

cm

-- 600~

cm

5

X, r

Fig. 5 Fig. 6 shows that the diffusion out of small pores in the carriers is sufficiently fast to allow for obtaining low water vapour pressures internally at even high rates of water vapour generation. In conclusion, the above analysis shows that in order to obtain a dry calcination, even with industrially high heating rates and high loading, the crucial point is to design the calcination process so that the evolved gas will only have to diffuse short distances. 4.2.3. Forming of carrier Extrusion of a carrier from a paste is very dependent on the particle size distribution and strength of agglomerates. The small pores (below 1000 ,~) of the finished process are not influenced by the mixing and extrusion processes. The physical mixing does not alter the crystallites or primary particles (they are only attacked by chemicals), but larger pores are definitely influenced by the mixing and physical processes. Different types of extruders have a varying degree of energy deposition in the paste and the large agglomerates will be degraded to some extent. Therefore, the choice of extruder type and mixing time is

11 important for every catalyst development, since the result can vary a lot concerning pore volume and pore distribution. An example is given in Fig. 7. Pellet profiles at 6wt%ll0min Ps = 0 atm 0,0035

0,0030

0,0025

"

0,0020

"

400~

"'400~ "~"

0,0015

"--

0,0010

0,0005

" =" =='= =' "= ='== ,,, m=,= =',

"~ :,,,,~ ,,, 0,0000

. 0,01

.

. 0,02

. 0,03

0,04

r

.

0,05

0,06

.

. 0,07

. 0,08

0,09

x, cm

Fig. 6. Pore volume depending of choice of extruder and mixing time for a soft paste with same formulation/dry matter

Fig. 7

0,1

mm mm

600=K,1 mm --600~

mm

12 5. STABLE P R O D U C T I O N

5.1. Identification of critical catalyst process parameters A stable production with constant quality is important since the product must have reproducible performance and lifetime in the customer's reactor as previous deliveries. Thus, it is important to identify the critical parameters of the manufacturing process parameters that have large influence on the product quality and that should be fixed within a narrow interval. Examples could be: a. pH and mixing mass flows during precipitation for obtaining washable slurry and right density of agglomerates b. time and temperature during ageing and re-crystallisation for obtaining right phases and surface area c. temperature and pH20 during calcination of a carrier for obtaining right surface area of pore distribution. 5.1.1. Critical process parameters from successful home work A successful implementation of a manufacturing recipe in the plant is dependent on these critical parameters, and corresponding control intervals are found in the pilot production experiments. Identification of critical parameters and their allowed intervals is often time-consuming. 5.1.2. Critical process parameters from troubleshooting Even with a thorough test programme in pilot production facilities, it still happens that the recipe does not give the expected product quality in large-scale plant. Then a costly investigation must take place in the plant itself and the development team will be under heavy pressure in solving the problem. Maybe a critical parameter is overseen. Sometimes is it only a minor adjustment of a parameter or small physical changes of the equipment that make the difference.

5.2 Stable production and quality assurance Today, customers expect catalyst production to be certified by a quality system, i.e. ISO 9000, ISO 14000 or QS 9000. Thus, identification of the critical parameters is necessary for obtaining a reliable QA system. 6. C O S T - E F F E C T I V E AND W A S T E - F R E E M A N U F A C T U R I N G

6.1. Economic pressure on the process development To this point, only the aspects concerning the technical success of the scale-up process have been treated, but it is evident that the final production process will only be successful if the catalyst can be produced in a cost-effective way. Therefore, it is important to select from the first development work the manufacturing routes that can give the lowest cost and concentrate the R&D effort accordingly in the future development and test period. The raw material cost normally constitutes 60-80% of the direct cost and, therefore, the selection of raw material is an important task. The initial experiments are perhaps made on analytical

13

to select raw materials after the first enrichments of the ore. The price of raw material is tightly connected to the impurity level and a balance must be found. Some impurities such as sulphur, chlorine and alkaline metals can be harmful to the final catalyst, thus, a smart process to clean out the impurities must be found or a more expensive quality must be purchased. Furthermore, in part 2.3 we have seen that there can be several optimal technical unit operation combinations, but one will have lower direct cost than the other. A general rule is to keep the manufacturing process as simple as possible. 6.2. The waste problem A technical and cost-effective manufacturing process has now been developed and proved to give a very competitive product but the process work is not yet finished. An optimal catalyst manufacturing process must be able to handle any waste. For precipitation the cheapest and mostly used salts of the metals are nitrates, sulphates and chlorides. The cheap acids HNO3, H2SO4 and HCl are used as acid precipitation agents, and NaOH, KOH, Na2CO3 and NH3 are used as basic agents for the cheap bases. When a precipitated catalyst is made the corresponding counter-ions form salts as NaNO3, Na2SO4, NaCl, (NH4)2SO4 and NHaC1, which must end somewhere! Some of these salts have zero value or may be of a negative value, since the plant may not be allowed to discharge them to the sewage system, river or fjord. Anyway, any heavy metal particles slipping through filtration and washing must be removed before the discharge. A challenge is to develop processes where the counter-ions form a product (salt) that has a positive value and is sellable, i.e. NH4NO3. Nitrates, carbonates, hydroxides, and NH4 have the advantage that they do not leave impurities in the product after decomposition in calcinations, but they can form harmful gaseous emissions in chimneys as NH3, NO,,, NH4NO3, NH4NO2 and SO,,. Absorption and recycling of these compounds are normally necessary today. Particles from dusty processes must be collected in bag filters recycled by introducing the material in a proper place in the process. Recycle of used catalyst, material out of specifications and sieving scrap may be re-dissolved and recycled in the process. Today, much development work is concerned with closed loops in the manufacturing process layout. 6.3. Investment

A newly developed manufacturing process may be implemented in an existing production line if it is not fully booked and an investment is saved. However, if there is no free capacity or if the new product requires new unit operations and processes a new plant needs to be built. The simpler manufacturing process the cheaper investment, but the investment part of today's closed loops due to environmental concerns, often constitute 30% of the total investment. The capacity of a new plant must be decided. A larger capacity is more expensive, although it is not a linear relationship. The investment cost is seen as a capital cost in the cost calculus of a new product, and it is dependent on the capacity utilisation. Therefore, the decision of a planned capacity will be taken in close cooperation with the company's marketing people.

14 7. FINAL C O N F I R M A T I O N 7.1. Long time end user confirmation problem A typical lifetime of commercial catalysts is 1-10 years. Thus, for a new catalyst product, the final confirmation of the product performance in industry cannot take place until many years after the product has been produced even if accelerated ageing tests have been carried out. 7.2. Success rate

Development and implementation of a production method of a new product as described above is a total optimisation with many constraints, and only a few development programs come so far. Thus, the success rate from idea to product is rather low, and each success is quite an achievement for the development teams and project managers. 8. CONCLUSIONS Scale-up of catalyst manufacturing processes is a multidisciplinary teamwork where science, empirical experiments and knowledge of equipment technology are integrated. Production cost and environmental issues set tight limits on possible manufacturing solutions. A successful work is not frequent, and it can take some years before you really know you have a technological as well as an economical success. REFERENCES

1. 2. 3. 4.

Chemical week September 12 (2001): "New markets Drive Growth for Catalysts". A.B. Stiles and T.A. Koch, "Catalyst Manufacture", (1995), Dekker. J.F. Le Page et al., "Catalyse de contact" (1998), Technip. N. Pernicone and F. Traina, "Commercial Catalyst Preparation" (1984) in Applied Industrial Catalysts, Vol. 3, Academic Press. 5. C.N. Satterfield, "Heterogeneous Catalysis in Practice", p. 68 (1980) Mc Graw-Hill. 6. M.S. Spencer, in "Catalyst Handbook" 2 nd edition, p. 34, (1989), Wolfe Publishers. 7. M.V. Twigg, "The Catalyst: Preparation, Properties and Behaviour in Use" in Catalysis and Chemical Processes, Editors Pearce and Patterson, Leonard Hill. 8. G.W. Higginson, "Making Catalysts - An Overview" Chem. Eng. September 30, (1974), 98. 9. E.F. Sanders and E.J. Schlossmacher, "Catalyst Scale-up-Pitfall or Payoff?" (1983) in Applied Industrial Catalysis, Vol. 1, Academic Press. 10. W. Pletch, "Granulate Dry Particulate Solids by Compaction and Retain Key Powder Particle Properties", Chemical Engineering Progress, April (1997), 24. l l . W . Pletch, "Successfully Use Agglomeration for Size Enlargement", Chemical Engineering Progress, April (1996), 29. 12. J.W. Fulton, "Making the Catalyst", Chem. Eng., Jul 7 (1986). 13. F. Traina and N. Pernicone, "Preparation Techniques and their influence on the Properties of the Solid Catalysts", Chim. Ind., 52 (1970) 1. 14. A.H. Thomas and C.P. Brundrett, "Catalyst Development: Lab to Commercial Scale" Chem. Eng. Progr., June (1980) 41.

15 15. J. Petro, "Preparation of Catalysts" in Contact Catalysis(Szabo and Kallo Eds.), Elsevier, Vol. 2 (1976) 13. 16. I.P. Muchlenov et al., "Tehchnologie der Katalysatoren" (1976), VEB Leipzig. 17. S.P.S. Andrew, "The black art of designing and making catalysts", Chemtech March (1979). 18. G. Berrebi and P. Bernusset, "Making Industrial Catalysts", (1976) in ref. 20, vol. 1. 19. S.P.S. Andrew, "Heterogeneous Catalyst Preparation, The Fabrication of Microstructure", (1976) in below ref. 20, Vol. 1. 20. B. Delmon et al. (Eds.), "Preparation of Catalysts", Vol. 1-Vol. 7, 1976-1998 Elsevier. 21. X. Xu and J.A. Moulijn, "Catalyst Preparation and Characterisation", in Structured Catalysts and Reactors, A. Cybulsky and J.A. Moulijn (Eds.), Marcel Dekker (1998) p. 599. 22. A. Cybulsky and J.A. Moulijn, "Monoliths in Heterogeneous Catalysis", Catal. Rev.Sci. Eng. 36 (2) (1994) 179. 23. T. Beecroft and A.W. Miller, "Preparation and Characterisation of Solid Catalysts", Repts. Progr. Appl. Chem. 55 (1970) 385. 24. T.A. Nihuis et al., "Preparation of Monolithic Catalysts", Catal. Rev.-Sci. Eng., 43(4) (2001) 345. 25. J. Dohrup, PhD study: "Krystallisation af aluminiumhydroxyd til katalysatorb~erere", Copenhagen, 1997. 26. B. Donnis, Haldor Topsoe A/S: Non-published results.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

17

Quantitative structure-activity relationships in zeolite-based catalysts: influence of framework structure J. L. Casci a and M.D. Shannon b a

Synetix, RT&E Department, PO Box 1, Belasis Avenue, Billingham TS23 1LB, UK

b ICI Measurement Science Group, PO Box 90, Wilton Centre, Middlesbrough, TS90 8JE, U.K. This paper will explore the links between framework structure and catalyst activity in toluene disproportionation (TDP) catalysis. The catalyst system that will be described is an intergrowth of framework types EUO and NES[1]: designated NU-85 [2]. The paper will describe the crystallisation of a series of intergrowth "members", their characterisation by powder x-ray diffraction and (Transmission) Electron Microscopy and their use in TDP. It will be shown that activity in TDP, as measured by the temperature required to achieve a particular conversion, is directly proportional to specific features in the x-ray powder patterns that are related to the presence of increasing levels of the intergrowth structure. 1. I N T R O D U C T I O N Quantitative structure-activity or structure-performance (property) relationships (QSAR and QSPR respectively) are of increasing interest in a wide diversity of technology areas. With some notable exceptions, most QSA(P)R studies associated with heterogeneous catalysis are restricted to zeolite-based catalysts. The reasoning is simplethe well defined 3-D structures, of molecular-sieve zeolites, are a reasonable representation of the catalyst "surface", hence bulk characterisation provides information on the catalyst. It is for a similar reason that the majority of modelling studies involves zeolites and zeotypes. In zeolite-based catalysts a number of factors can influence performance (activity, selectivity, life). These have been reviewed recently by Venuto [3]. In a previous study we have explored the impact of surface composition [4] (SiOa/AI203 ratio) on catalytic performance, in TDP, and this paper aims to extend this study by describing the effect of framework structure in the same reaction. TDP is an industrially important reaction [4,5], it provides a means of "upgrading" toluene to more valuable benzene and (ortho-, meta-, para-) xylene: key intermediates in the formation of nylon and polyethyleneterephthalate (PET) respectively. While the study is specific in that it deals with a particular reaction, and structure type, there are general lessons to be learned about the linkage between preparation,

18 characterisation and catalyst performance. In addition this paper follows a natural progression from our previous paper on TDP catalysis using EUO type zeolites [6]. 2. EXPERIMENTAL Samples of zeolite NU-85 were prepared by crystallisation from reaction mixtures of molar composition: 60 SiO2 - x A1203 - 10 Na20 - 10 Hex Br2 - 3000 H20 where Hex Br2 is hexamethonium bromide, the organic template: hexane-l,6bis(trimethylammonium) bromide. Raw materials, gel make-up and autoclave type were as described previously [2,5]. The amount of alumina, x in the above molar composition, crystallisation temperature and time will be described later. Active catalysts were prepared by calcining the as-made materials, in air, at 450~ for 24 hours followed by 550~ for 24 hours. The resulting, template-flee zeolites were ion-exchanged with 1M HC1 (single exchange for either 2 or 4 hours) using 10 cm -3 acid per g zeolite. The resulting materials were filtered, washed and dried (110~ 16 hours) then compacted into pellets that were broken down and sieved to give a 425-1000 microns fraction. l g of the sieved fraction was placed in a 4mm internal diameter stainless steel reactor and calcined at 500~ in air for 16 hours, at atmospheric pressure. The air was replaced by nitrogen and the reactor cooled to 350~ Hydrogen was then passed through the reactor and the pressure raised to 2069 kPa. The hydrogen flow rate was set at 1728 cm -3 per hour (measured at atmospheric pressure). After 1 hour, toluene was introduced into the hydrogen stream at a rate of 1.9 cm -3 of liquid per hour. The mole ratio of hydrogen to toluene was 4:1 and the weight of toluene per unit weight of solid was 1.64. The reaction was continued for 7 days during which the temperature was increased stepwise in order to maintain 47% conversion of toluene: the final temperature was taken as a measure of catalyst activity. Materials were characterised by powder xrd (Philips APD 1700; Cu K(x radiation employing an automatic theta-compensating divergence slit; peak/line intensities were based on height) and EM (lattice image and electron diffraction data were obtained using either a Philips EM400T operating at 120KeV or a Philips CM30ST operating at 300KeV; the former has a "point resolution" of 0.37nm and the latter 0.20nm; electron dose was controlled to minimise beam damage to the zeolite crystals under observation: damage is not responsible for the structures shown/described here). 3.

RESULTS AND DISCUSSION

The starting point for this work was the preparation of zeolite EU-1 [7], that is, crystallisation from a reaction mixture of (molar) composition: 60 SiO2 - x A1203 - 10 Na20 - 10 Hex Br2 - 3000 H20 This material had been shown to have good activity and selectivity in TDP and consequently a more detailed study of its preparation was initiated. Part of this, extensive, study examined materials with different SiO2/A1203 ratios (different values of x in the above composition). For preparations carded out under similar conditions

19 (temperature, reagent sources, method, agitation etc) it was observed that subtle changes occurred in the powder xrd patterns as the A1 content was increased: deviations from the standard pattern for EU-1. These changes are indicated in Fig. 1 which shows diffractograms for a typical sample of EU-1 and a diffractogram for a sample containing these "subtle changes": this material was termed NU-85.

1 4 0 0 0 -_

13000 12000 11000--

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.~ 7000 6000

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4000 3000

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Degrees Two Theta Fig. 1 XRD patterns for EU-1 (upper) and NU-85 (lower) with affected lines indicated by arrows Highlighted, by arrows, in Figure l b are three lines that are affected by this increasing A1 content (decreasing SiO2/A1203 ratio). Overall, it can be seen that there is good correspondence between the patterns with the most apparent differences being alterations (reductions) in intensity of certain reflections - marked by arrows in Fig. 1. Of particular note are the lines at (ca.) 1.03nm and 0.372nm. More detailed examination shows that the 1.03 and 0.386nm lines are also selectively broadened and shifted to higher d-spacings (lower angles.) Table 1 shows details of a series of preparations with SiO2/A1203 ratios from 60/1 to 32.5/1. All five samples were crystallised at the same temperature (160~ using similar reagents etc, although it may be observed that there are some significant differences in crystallisation time. Also included in Table 1 are ratios of two of the xrd lines highlighted by arrows in Fig.1 above - the lines at 1.03 and 0.372nm. In each case the line intensity has been taken as a ratio to an adjacent line: such that the line at 1.03nm is taken as a ratio to the line at 1.11 and the line at 0.384nm is taken as a ratio to the line at 0.372nm. This method has been employed rather than the more

20 conventional intensity relative to the strongest line (ca. 0.434nm in the patterns) to allow for different diffractometer geometries: specifically the use of fixed slit Vs divergent slit geometries which can significantly alter the (relative) intensities of the low angle reflections. Table 1 Products obtained from reaction mixtures of composition: 60 SiO2 - x A1203 - 10 Na20 - Hex Br2 - 3000 H20 carried out at 160~ 1 or 2 litre autoclaves Sample

I III II IV V

x

SiO2/A1203 ratio

1.000 1.500 1.714 1.791 1.846

60/1 40/1 35/1 33.5/1 32.5/1

Crystallisation Ratio of xrd lines t/h 1.03/1.11 0.384/0.372 142 240 264 263 312

0.6 0.6 0.5 0.4 0.4

1.3 1.3 1.1 1.0 0.9

Impurities

None None None < 1% ANA* < 1% ANA

* ANA was identified from the xrd pattern and quantified by a calibration study in which ANA as doped, at different levels, into a pure EU-1 sample. Also included in Table 1 (final column) is an indication of product purity. The key points to note are: (a) In such a study where relative intensities are being used it is essential to ensure that these changes in relative intensity are not caused by the presence of impurities. (b) Detailed analysis of the NU-85 samples was undertaken and it was observed that the many samples co-crystallised with analcime (ANA) and a detailed calibration study was undertaken to assess the level of the impurities. (c) The presence of ANA (particularly at these low levels) has no effect on the relative intensities described above. From Table 1 it can be observed that there is a downward trend in the relative intensities with decreasing SiO2/A1203 ratios - this is highlighted in Fig. 2 which contains a plot of xrd relative intensity against SiO2/A1203 ratio. The sharp decline in relative intensity at SIO2/A1203 ratios (reaction mixture) below about 35/1 is evident. The detailed explanation of the significance of this ratio encompasses a description of the yield on SiO2 (and A1203 ) [7] and some knowledge of the maximum A1 content of the EUO unit cell [8] (based on structure and template content) - such a description is beyond the scope of this paper and will be described elsewhere. A more extensive study showed that this reduction of intensity in the EU-1 xrd pattern (or formation of NU-85) was a complex interplay between SIO2/A1203 ratio, crystallisation temperature and silica source, such that, a family of curves (xrd line intensity against SIO2/A1203 ratio) could be obtained for each temperature using

21 particular raw materials. However, each member of this series, as defined by their xrd line intensities has specific properties and it is these properties which will now be discussed. 1.4 -~

0.3~/0.372

O ~ ,...~ .,,.a

r/3 (D o...~

t

0.6-

1.03/1.11

0.2

. . . .

20

I

. . . .

I

. . . .

I

. . . .

3~iO2/A12034o0f Reaction ~r

I

70

60

Fig. 2. Plot of relative xrd line intensities as a function of SiO2/A1203 ratio. 0.384nm line relative to 0.372nm; 1.03nm line relative to 1.1 lnm Table 2 contains details of some NU-85 samples together with their activities in TDP (as measured by the temperature required for 47% conversion, after 7 days on line - see experimental section above). Table 2 Nu-85 samples: preparative details, xrd line intensities and activity in TDP catalysis SiO2/A1203" ratio

SiO2 source

60/1.714"** 60/1.714"** 60/2.18'* 60/2.18 60/1.714"**

Cabosil Cabosil Syton X30 Syton X30 Cabosil

Crystallisation Temp./~ 160 180 160 160 150

xrd line ratio 0.384/0.372nm 0.9 1.1 0.6 0.7 1.0

TDP Activity Temp/~ 382 409 359 366 390

* Reaction mixtures of composition: 60 SiO2-x A1203-10Na20-10 Hex Br2-3000 H20 ** 2 litre autoclave *** 19 litre autoclave

22 These materials, as described in Table 2, were prepared at different compositions, crystallisation temperatures and autoclave types: conditions which give rise to unique NU-85 materials in terms of the xrd line intensity 9 When the xrd line intensity (a measure of their NU-85 "character") is plotted against TDP activity - see Fig. 3 - a straight-line results with a correlation coefficient (R2) of 0.96.

1.2-

r r

1.1-

oO r

1-

9. 0.9r,r

= 0.80.70.60.5 350

i

i

i

i

i

i

360

370

380

390

400

410

TDP activity: temperature (~

420

required for 47%

Fig.3. Plot of xrd line intensity (ratio of 0.384/0.372nm) against TDP activity (temperature required for 47% conversion) To understand the underlying reason behind the "reduced" xrd line intensity in NU-85 (compared to EU-1) a detailed structural study was carried out with the most critical information being obtained from a combination of lattice imaging and electron diffraction. It was from this data that the structure of NU-85 was discerned: it was an intergrowth of EU-1 and NU-87 (structure types EUO and NES). In general, lattice images were preferred to electron diffraction patterns as a method of determining the structure of NU-85 because this gave direct visual evidence for the intergrowths since there is a 1:1 correspondence between the structure of a crystallite and the lattice image, when properly recorded. Such a lattice image is shown in Fig. 4. This figure shows a crystal that contains discreet bands of fringes with spacings of ca. 2.02 and ca.l.25nm parallel to one another. The 2.02nm bands are EU-1 and the 1.25nm bands belong to NU-87. In this crystal there are three bands of zeolite NU-87 separated by two bands of zeolite EU-1 - as marked in the Figure.

23

Fig. 4. Lattice Image of NU-85: with fringes associated with EU-1 (2.02nm) and NU-87 (1.25nm) marked. The use of this technique to estimate the proportion of NU-87 in the (different) samples of NU-85 is fraught with difficulties, in particular the need to examine a sufficient number of crystals by lattice imaging to ensure that the results obtained are representative of the whole sample. This is particularly important where a sample is believed to contain a significant amount of either (or both) of the two parent zeolites, EU-1 and NU-87, together with the intergrowth. It is also important for NU85 samples that are close to the end-member materials, such as those containing large amounts e.g. 95% by volume, of EU-1. A further complication is that as the proportion of intergrown NU-87 in NU-85 falls there will be an increasing proportion of pure EU-1 crystallites in any sample. This is for two related reasons. Firstly the crystallites are of finite size, typically 20-100 nm, which means that most crystallites will contain only a few ten's of 2.02nm layers of EU-1. This coupled with the observation that the average band width of NU-87 in intergrown crystals appears to be relatively insensitive to total NU-87 content means that a small number of crystallites will contain a significant fraction of intergrown NU-87 in EU-1. The corollary is that a sample that contains 1% by volume NU-87 as an intergrowth in EU-1 may contain only 1 in 20 crystallites in an orientation that shows any intergrowth. It can then be necessary to examine more than 100 crystallites in the appropriate orientation to characterise the sample by lattice imaging. Even within these limitations sufficient work has been carried out on selected samples to support the view that the reduction in (relative) intensity in the xrd patterns is linearly related to an increase in the NU-87 content of the intergrowth. Thus the linear increase in activity (in TDP) shown in Figure 3 is consequence of increasing amounts of NU-87 in the samples. As may be evident, the work presented here was part of a much larger study and Fig. 3 was shown to apply to over 30 samples. The

24 excellent correlation observed was used, within our laboratories, to guide further development and scale-up, such that xrd was used routinely to predict catalytic activity thereby saving considerable time and effort. To this point the temperature required for 47% conversion has been described as a measure of catalyst "activity". However, since this was taken after 7 days on-line it is evident that it also contains information on catalyst lifetime (or deactivation). NU87 is a 2-D channel system while EU-1 contains uni-dimenional channels. Hence it may be expected that there would be differences in deactivation rates. A separate, but related, topic concerns selectivity. The TDP reaction also generates light gas and C9+ aromatics ("heavy ends"). As with "activity" it may be expected that the difference in pore structure would be reflected in differences in the selectivity. Indeed there is evidence for differences in the xylenes and benzene balance in NU-85 samples (compared to EU-1). Both topics (deactivation and selectivity) are complex and beyond the scope of this contribution and will be addressed in a separate paper. 4.

CONCLUSIONS

An example of a Quantitative-Structure-Activity-Relationship has been found for a material designated NU-85. Under certain combinations of temperature and SIO2/A1203 ratio a range of materials with subtle, but significantly different, xrd patterns were obtained. These materials (designated NU-85) had different activities in TDP catalysis and it was possible to relate the xrd pattern intensities, firstly, with their activity in TDP and then, secondl~y, with the presence of increasing amounts of the NU-87 structure intergrowing with EU-1. ACKNOWLEDGEMENTS The authors gratefully acknowledge the contributions of Dr Ivan Lake (TDP catalysis) and Mr. S Huntley (zeolite synthesis/activation) and thank Synetix for permission to publish this paper. REFERENCES

1. W.M. Meier, D. H. Olson and L. B. McCusker, Atlas of Zeolite Structure Types, Fourth Revised edition, 1996. 2. J.L. Casci, I.J.S. Lake and M.D. Shannon, Eur. Pat. 462745 (1991). 3. P.B. Venuto, Stud. Surf. Sci. Catal., 105 (1997) 811. 4. J.L. Casci and A. Stewart, Proc. 12 th Int. Catalysis Congress, Stud. Surf. Sci. and Cat., 130 (2000) 851. 5. A. Azzouz, V. Hulea, B. Zaoui, M. Attou and E. Dumitriu, J. Soc. Alger. Chim., 3(1993)38. 6. J.L. Casci, B.M. Lowe and T.V. Whittam, Proc. Vlth Int. Zeolite Conf., Butterworths (1984) 894. 7. B.M. Lowe, Zeolites, 3 (1983) 123. 8. J.L. Casci, Stud. Surf. Sci. Catal., 84 (1994) 133-140.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

25

Cogelation: an effective sol-gel method to produce sinter-proof finely dispersed metal catalysts supported on highly porous oxides B. Helnnchs a' , S. Lambert a, C. Ali6, .,a J.-P. Pirard a, G. Beketov b, V. Nehasil b, and N. Kruse b aLaboratoire de G6nie Chimique, B6a, Universit6 de Li6ge, B-4000 Li6ge, Belgium bChimie Physique des Mat6riaux (Catalyse- Tribologie), Universit6 Libre de Bruxelles, C.P. 243, Campus Plaine, B-1050 Bruxelles, Belgium Pd/SiO2, Ag/SiO2, Cu/SiO2 and Pd-Ag/SiO2 catalysts have been prepared by cogelation, that is concomitant hydrolysis and condensation of a complex containing both the active metal cation and hydrolysable methoxy groups -OCH3 with Si(OC2H5)4 (TEOS). The resulting materials contain metal nanocrystallites trapped inside microporous primary silica particles, as a result of the role that the complex plays as nucleation agent of SiO2 particle formation. The primary SiO2 particles are arranged in aggregates such that a highly porous catalyst texture results. This makes the catalysts sinter-proof at high temperature and allows a very easy diffusion of reactants. High activity and selectivity have been found for various reactions. X-ray electron spectroscopy has been applied along with ionic sputtering techniques to demonstrate that metal nanocrystallites are encapsulated in the primary SiO2 particles. This is particularly true for Pd/SiO~. 1. INTRODUCTION As compared to traditional ways of solid catalyst preparation frequently involving twostep procedures [1, 2], sol-gel chemistry allows to do so in one single step. For example, rather than dispersing the active metal component on a previously synthesized support, both metal dispersion and support synthesis can be realized simultaneously. Several authors prepared metal supported catalysts by the sol-gel route and demonstrated high dispersion on gels of finely controlled texture. Most frequently, the metal of interest is introduced in the initial solution (whose main components are for example aluminum tri-sec-butoxide (ATB) or tetraethoxysilane (TEOS) and water in alcohol) in the form of a salt (e.g. H2PtC16, PdC12, Pd(CH3CO2)2, RuC13, etc.) [3-5]. During the past ten years, a particularly interesting method to homogeneously disperse nanometer-sized metal particles in a silica gel has been developed that consists in using modified alkoxides of the type (RO)3Si-X-A [6-10]. In such molecules, a functional organic group A, able to form a chelate with a cation of a metal like palladium, platinum, nickel, silver, copper, etc., is connected to the hydrolysable alkoxide E-mail: [email protected]

26 moiety (RO)3Si- via an inert and hydrolytically stable spacer X. The concomitant hydrolysis and condensation of such molecules with a network-forming reagent such as Si(OC2H5)4 (TEOS), that is their cogelation, result in materials in which the catalytic metal is anchored to the SiO2 matrix. This is illustrated in Fig. 1 where X - propyl and A = diamine. The aim of the present paper is to study the mechanisms of this cogelation and to show that it leads to catalysts with remarkable structure and properties. ! ! OR [ RO--S~--OR

I

OR' [ +

/CH2--- C~2

R'O -- Si -- CH2-- CH2-- CH2-- HN"

OR

1

OR'

"-,M~/

.NH 2

+ H20

-Ro~ - R'OH

O

I

-- o - - s i / N / ~ / N / N

I

M 'v

0

I

--Si--

Fig. 1. Anchoring of the catalytic species M to the SiO2 matrix.

I

2. E X P E R I M E N T A L 2.1. Catalyst preparation Five series of catalysts have been prepared: aerogels Pd/SiO2, xerogels Pd/SiO2, xerogels Ag/SiO2, xerogels Cu/SiO2 and xerogels Pd-Ag/SiO2 (bimetallic catalysts). For each given sample, A or X denote whether the initial wet gel was aerogel-dried under supercritical conditions or xerogel-dried under vacuum, respectively. Metal weight percentages follow the elemental symbol of the metal and are sometimes higher than those expected from synthesis operating variables because of a loss, during gel drying, of unreacted tetraethoxysilane (TEOS), which is the precursor of the silica support. The general synthesis procedure was as follows. For monometallic catalysts, metal precursor (Pd(II) acetylacetonate, Ag(I) acetate or Cu(II) acetate) and 3-(2-aminoethyl)aminopropyltrimethoxysilane, H2NCH2CH2NH(CH2)3-Si(OCH3)3 (EDAS), as ligand are mixed together in half the total volume of ethanol. The slurry is stirred at room temperature until a clear solution (the color of which is characteristic of the metal complex formed) is obtained (about half an hour). After addition of TEOS, a solution of aqueous 0.18N NH3 in the remaining half volume of ethanol is added to the mixture under vigorous stirring. The vessel is then tightly closed and heated to 70~ for 3 days (gelation and aging). For bimetallic catalysts Pd-Ag/SiO2, Pd(II) acetylacetonate and EDAS are mixed in a quart of the volume of ethanol, Ag(I) acetate and 3-(aminopropyl)triethoxysilane, HEN(CHE)aSi(OC2Hs)3(AES), are mixed in another quart of the total volume of ethanol and those two solutions are mixed together after formation of Pd and Ag complexes. Note that, for one particular Ag/SiO2 catalyst (XAgl.7), Ag(I) acetate was mixed with AES instead of EDAS. Nevertheless, in this preparation the addition of EDAS (same amount than AES) after the introduction of all reactants was still necessary to obtain gelation of the solution. After aging, a first series of Pd/SiO2 catalysts were dried under supercritical conditions at 327~ and 12 MPa to produce aerogels (series A). The other gels (Pd/SiO2, Ag/SiO2, Cu/SiO2 and Pd-Ag/SiO2) were dried under vacuum at 150~ for 72 h to produce xerogels (series X). All xerogels

27 were subsequently calcined under air at 400~ during 12 h and reduced under H2 at 350~ (400~ for Cu) during 3 h.

2.2. Catalyst characterization and catalytic experiments The size of metallic particles was examined by transmission electron microscopy (TEM). For Pd/SiO2 samples, the Pd dispersion was additionally calculated from CO chemisorption. The bulk composition of bimetallic particles in Pd-Ag/SiO2 samples was examined by X-ray diffraction (XRD). Texture was examined by N2 adsorption at 77 K, Hg porosimetry, and TEM. Elemental distributions of Ag and Pd in the silica framework were additionally studied by X-ray photoelectron spectroscopy (XPS) along with ionic sputtering. Pd/SiO2 aerogels were tested in H2 combustion, Pd/SiO2 xerogels in C2H4 hydrogenation, Ag/SiO2 and Cu/SiO~ in C6H6 combustion and Pd-Ag/SiO2 in selective hydrodechlorination of 1,2dict'doroethane into ethylene. More details about preparation, characterization and catalytic test can be found in [8, 9, 11-14].

3. RESULTS Fig. 2 shows a structural model of the cogelled catalysts suggested on account of TEM results. Except for XAgl.7, mono- or bimetallic nanocrystallites (Pd, Ag, Cu or Pd-Ag) with diameters between 2 and 4 nm are located inside the primary silica particles. The narrow size distribution of these latter silica particles reveals mean values generally ranging between 10 and 50 nm depending on the sample. Most frequently, larger metal crystallites (10 to 30 nm) are also observed to be deposited on the outer surface of the primary SiO2 particles. The number density of such larger metallic particles is higher for Ag/SiO2 and Cu/SiO2 than for Pd/SiO2. Primary SiO2 particles are arranged in aggregates constituting Fig. 2. Structural model of cogelled the catalyst pellet. In the case of XAgl.7 prepared catalysts suggested from TEM (M using both AES and EDAS, the structure is = metal, A= alloy), different" silver crystallites exhibit a broad range of sizes (1 to 20 nm) and no individual and welldefined primary SiO2 particles are observed. The texture of the cogelled catalysts was examined by N2 adsorption and Hg porosimetry. Except for XAgl.7, all samples exhibit a similar and very broad pore size distribution. An example is given in Fig. 3 for the case of XPdI.9Ag3.7. Cumulative distributions over the complete pore size range were obtained by applying a combination of various methods within their respective validity domains and by summing up the porous volumes corresponding to these domains [9, 16].

28 In the micropore domain, the catalysts exhibit a very narrow pore size distribution centered around a mean o value of about 0.8 nm that corresponds micro E to the steep volume increase followed by a plateau. In the range of meso- and 0.| " macropores, all samples exhibit a broad distribution starting at about 2 nm and 0.01 extending up to several hundred nanometers. Catalyst XAgl.7 has a continuous pore size distribution from 0.001 the smallest micropores (0.7 nm) to the 0.1 1 10 100 1000 largest macropores (2000 nm). It is particularly interesting to note that the Pore width (nm) pore volurnes, measured by Hg Fig. 3. Pore size distribution. porosimetry, of xerogel catalysts, which are between 1 and 7 cm3/g, are almost in the same order of magnitude as the pore volumes of Pd/SiO2 aerogels (9 to 25 cm3/g) or other aerogels described in the literature [15]. For this reason, those materials are called 'low-density xerogels'. The reason for the high porosity of cogelled xerogels has been examined by Ali6 et al. [ 16]. One Pd/SiO2 sample (XPd3.3: only EDAS as ligand) and two Ag/SiO2 samples (XAgl.5: only EDAS as ligand; XAgl.7: AES + EDAS) were subjected to a combined XPS and SSIMS analysis. We limit the presentation of results largely on the XPS evidence (before and after Ar ion sputtering), which was obtained by inspecting the Pd and Ag 3d doublets along with the background on the high-energy side (low kinetic energy side). Figs. 4 and 5 demonstrate that the spectral features remain essentially unchanged when reducing the calcined catalysts. No noticeable change is seen in either line positions or background, both qualitatively and quantitatively. This may be taken as an indication that both Pd and Ag could be in the reduced (metallic) form after calcination of the catalysts. Comparison of the 3d spectra of Pd/SiO2 with those of Pd metal (foil) yet shows significant differences. Apart from a shift in the binding energies of the characteristic lines (that is, at least partially, due to electrostatic charging) the background tail structure measured for Pd/SiO2 is different from Pd foil. This difference suggests energy losses of Pd 3d electrons to take place while travelling through the SiO~ matrix. The correctness of this suggestion is demonstrated by comparing the Si 2p background features of SiO2 ('model line' in Fig. 4, appropriately scaled to the 3d spectral region) with those of Pd 3d in Pd/SiO2. In fact, it turns out that while the electron energy loss spectrum (EELS) in SiO~ is qualitatively similar to the one in Pd/SiO2, intensities are remarkably different. Note that the tails in the EELS of the SiO2 matrix are mainly associated with the occurrence of interband transitions and plasmons, the latter peaking by about 22 eV above the main lines (zl, z2). e~0

10-

m

F

29

.....

Model 3d line

Model 3d line

3400 1400 4 3200 1200

3

3000

1000

I

2

2800

2600

800

2400

1 600

~o

~

~o

Eb [eV]

;o

~o

Fig. 4. Pd 3d line of XPd3.3" (1) calcined, (2) reduced, (3) sputtered lh, (4) sputtered 2h, (5) Pd foil.

,

440

.

420

,

.

400

,

380

.

360

,

340

Eb[eV]

Fig. 5. Ag 3d line" (1) XAgl.7 calcined, (2) XAgl.5 calcined, (3) XAgl.5 reduced, (4) XAgl.5 sputtered lh.

Turning now to the Ag 3d spectral features (Fig. 5) it is evident that the background tail structure coincides fairly well with the 2p Si model line. A considerable difference in the morphological structure of Pd/SiO2 and Ag/SiO2 can be seen from the above XPS results. While the strong background tail on the high-energy side of the 3d lines pleads for a model in which Pd particles are encapsulated in the SiO2 matrix, the close similarity of the background intensities in Ag 3d and Si 2p is in line with a homogeneous in-depth distribution of Ag metal in SiO2. To provide further support for the 'encapsulation model' of Pd/SiO2 catalysts prepared via EDAS complexation, SSIMS measurements were performed. Although not shown explicitly in this paper, we mention that in the early stages of the 'static' sputtering, Pd ion species were hardly discernible. Only after prolonged sputtering did these species gain in intensity. Clearly, sputtering must have an influence on the XPS background features as well. This is demonstrated in Figs. 4 and 5 for both Pd/SiO: and Ag/SiO2 catalysts. Clearly, while prolonged sputtering causes considerable changes in the background intensity on the high-energy side of 3d Pd no such changes are seen in the respective spectral region of 3d Ag. Silica particle sizes of Pd/SiO2 catalysts can be estimated on the basis of the simple model of photoelectron attenuation. Forming the intensity ratios of the 2d Pd lines before and after sputtering allows us to estimate values of 2.5 to 3.5 nm for the SiO2 particle radii. As to the catalytic performance of the catalysts, we mention that all of them have been proved to be active for the various catalytic reactions mentioned in paragraph 2.2. In

.

30 particular, Pd-Ag/SiO2 cogelled catalysts are highly selective in C2H4 during C1CH2-CH2C1 hydrodechlorination which makes them attractive from an industrial point of view [9,12,17]. 4. DISCUSSION

In cogelled Pd/SiO2, Ag/SiO2 (except for XAgl.7), Cu/SiO2 and Pd-Ag/SiO2 catalysts, TEM micro graphs show many small metal particles to be located inside primary silica particles (see Fig. 2). It has been suggested that such encapsulation could be the result of a nucleation process initiated by the ligand, in our case EDAS (H2NCH2CHzNH(CH2)3Si(OCH3)3) [8,9], used for metal complexation in the sol-gel process. Ali6 et al. [ 16, 18, 19] have examined this hypothesis of nucleation by EDAS in xerogels without metal. Due to the hydrolysable functions of EDAS, Si-O-Si bonds can form all around this additive. Furthermore, the fact that gelation occurs in half an hour in the presence of EDAS, whereas it would take days without EDAS under the same conditions, indicates that EDAS reacts faster than TEOS. This is not surprising on account of the presence of methoxy groups in EDAS as compared to ethoxy groups in TEOS. Thus, in cogelation, silica particles are formed with a hydrolyzed EDAS core (containing the active metal) and a shell mainly made of hydrolyzed TEOS. In order to assess the validity of the nucleation hypothesis, the relation between the SiO2 particle volume and the TEOS and EDAS concentrations has been examined. Expressing that the volume occupied by the silica skeleton is proportional to the sum of TEOS and EDAS amounts which have reacted to form the gel, one obtains: Np Vp ( 1 - ) - C~ ([TEOS] + [EDAS]) (1) where Np is the number of SiO2 particles in the gel, Vp is the volume of one SiO2 particle (nm3), and is its void fraction, C~ is a constant, [TEOS] and [EDAS] are the concentrations of TEOS and EDAS (mol/1). The hypothesis of nucleation by EDAS leads to: Np = C2 [EDAS] (2) where C2 is a constant (1/C2 is proportional to the number of EDAS molecules in one nucleus). Combining (1) and (2) and expressing Vp as a function of dp (diameter of SiO2 particles, nm), one obtains: d 3 = C3 ([TEOS] + [EDAS]) / ( ( 1 - ) [EDAS]) (3) where C3 = 6C1 / ( C2). d o was measured by TEM for all samples including xerogels without metal studied in [ 16, 18, 19] and the void fraction of silica particles, , was derived from texture analysis. Results are shown in Figure 6. One observes that relation (3) represents experimental data in a satisfactory manner. This result, combined with TEM observations, reinforces the hypothesis of nucleation by EDAS and of metal crystallites to be encapsulated in the primary SiO2 particles. Both XPS and SSIMS results obtained with Pd/SiO2 (XPd3.3), are in complete agreement with this.

31 70 ~

On the other hand, different completely

|

60 t 50

9Pd/SiO2, aerogels 9 Pd/SiO2, xerogels

= 40 30 '1 "@ 20 I

9 Ag/SiO2, xerogels 9 Cu/SiO2, xerogels o SiO2 without metal, xerogels __--.

10 0 0

~ ~ ~ ,200 400 600 800 1000 ([TEOS] + [EDAS]) / ((1-) [EDAS])

Fig. 6. SiO2 particle nucleation by EDAS.

results have been obtained with sample XAgl.5 in which EDAS is used as ligand. In that sample, a homogeneous in-depth distribution of Ag metal in SiO2 is found. This could be in ~ agreement with the fact 1200 that, according to TEM analysis, more active metal is located outside silica particles in

Ag/SiO2 (and Cu/SiO2) catalysts than in Pd/SiO2 catalysts. That difference could be explained by the fact that EDAS forms more stable complexes with Pd than with Ag. Note that in xerogel Y~A.gl.7, no nucleation is indicated by TEM. In this sample, AES (containing ethoxy groups) rather than EDAS (containing methoxy groups) was used. EDAS was only added after the introduction of all other reactants so as to obtain the gelation of the solution. For this catalyst, XPS and SSIMS also detected a homogeneous in-depth distribution of Ag metal in SiO2. A very important concern about cogelled catalysts is the accessibility of the active metal which is located inside the silica particles. However, in the case of Pd/SiO2 aerogels and xerogels, the sizes of Pd crystallites obtained by TEM analysis are in agreement with those determined by CO chemisorption. This result demonstrates metal crystallites to be accessible for CO (and possibly leading to structural rearrangements of Pd particles) or other molecules of similar size. This conclusion is reinforced by the activity found for all cogelled catalysts in various catalytic reactions. The complete pore size distribution (Fig. 3) combined with TEM images (Fig. 2) suggests that the narrow micropore size distribution centered around 0.8 nm is located inside the primary SiO2 particles (10 dp 50 nm) and that the continuous meso- and macropore size distribution is caused by voids between these particles and between aggregates of these particles. It has been shown that such a structure allows an easy diffusion of reactants from the external surface of the catalyst pellet to the active metal crystallites located inside SiO2 particles without any di~sional limitations [13]. Moreover, because they are larger than the micropores of the silica particles in which they are located, the highly dispersed metal crystallites in cogelled catalysts are trapped and probably more stable in against physical and chemical processes. In fact, these crystallites are sinter-proof during high temperature activation and reaction.

32 5. CONCLUSIONS Cogelation, comprising concomitant hydrolysis and condensation of a complex containing both the active metal cation and hydrolysable methoxy groups-OCH3 with a network-forming reagent such as Si(OC2H5)4 (TEOS), allows to produce catalysts in which metal nanocrystaUites are trapped inside microporous primary SiO2 particles. The latter are arranged in larger aggregates thus constituting the macroscopic catalyst pellet. The localization of metal crystallites inside the SiO2 particles is a result of the role played by the ligand of the metal cation during nucleation and formation of silica particles. This encapsulation makes metal crystallites sinter-proof during operation at high temperatures. Moreover, even simply dried under vacuum, the resulting xerogels catalysts have a porosity comparable to the one of aerogels dried in supercritical conditions. This high porosity combined with a hierarchical structure - SiO2 particle, aggregate, macroscopic pellet allows a very easy diffusion of reactants from the external surface of the catalyst pellet to the active metal crystallites located inside SiO~ particles without any diffusional limitations. Monometallic as well as bimetallic cogelled catalysts have been shown to be active and selective in various catalytic reactions. In some cases, their performance is much higher as compared to more conventional catalysts. ACKNOWLEDGMENTS

The Rrgion Wallonne, the F.R.I.A. (S.L.), the F.N.R.S. and the Communaut6 Fran~aise de Belgique are gratefully acknowledged for financial support. REFERENCES

1. K. Foger, in 'Catalysis: Science and Technology', J.R. Anderson and M. Boudart (eds.), Vol. 6, p. 227, Springer-Verlag, Berlin, 1984. 2. M. Che, O. Clause and Ch. Marcilly, in 'Handbook of Heterogeneous Catalysis', G. Ertl, H. KnSzinger and J. Weitkamp (eds.), Vol. 1, p. 191, Wiley-VCH, Weinheim, 1997. 3. K. Balakrishnan and R.D. Gonzalez, J. Catal., 144 (1993) 395. 4. J.N. Armor, E.J. Carlson and P.M. Zambri, Appl. Catal., 19 (1985) 339. 5. T. Lopez, P. Bosch, M. Asomoza and R. Gomez, J. Catal., 133 (1992) 247. 6. B. Breitscheidel, J. Zieder and U. Schubert, Chem. Mater., 3 (1991) 559. 7. A. Kaiser, C. Gtirsm_ann and U. Schubert, J. Sol-Gel Sci. Technol., 8 (1997) 795. 8. B. Heinrichs, F. Noville and J.-P. Pirard, J. Catal., 170 (1997) 366. 9. B. Heinrichs, P. Delhez, J.-P. Schoebrechts and J.-P. Pirard, J. Catal., 172 (1997) 322. 10. A.I. Serykh, O.P. Tchachenko, V.Y. Borovkov, V.B. Kazansky, K.M. Minachev, C. Hippe, N.I. Jaeger and G. Schulz-Ekloff, Phys. Chem. Chem. Phys., 2 (2000) 2667. 11. B. Heinrichs, F. Noville, J.-P. Schoebrechts and J.-P. Pirard, J. Catal., 192 (2000) 108. 12. B. Heinrichs, J.-P. Schoebrechts and J.-P. Pirard, J. Catal., 200 (2001) 309. 13. B. Heinrichs, J.-P. Pirard and J.-P. Schoebrechts, AIChE J., 47 (2001) 1866. 14. S. Lambert, N. Tcherkassova, C. Cellier, F. Ferauche, B. Heinrichs, P. Grange and J.-P. Pirard, published in the present volume of Stud. Surf. Sci. Catal. (2002). 15. G.M. Pajonk, Appl. fatal., 72 (1991) 217.

33 16. C. Ali6, R. Pirard, A.J. Lecloux and J.-P. Pirard, J. Non-Cryst. Solids, 246 (1999) 216. 17. P. Delhez, B. Heinrichs, J.-P. Pirard and J.-P. Schoebrechts, Process for the Preparation of a Catalyst and its Use for the Conversion of Chloroalkanes into Alkenes Containing Less Chlorine, US Patent No. 6 072 096 (2000). 18. C. Ali6, F. Ferauche, R. Pirard, A.J. Lecloux and J.-P. Pirard, J. Non-Cryst. Solids, 289 (2001) 88. 19. C. Ali6, R. Pirard, A.J. Lecloux and J.-P. Pirard, J. Non-Cryst. Solids, 285 (2001) 135.

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35

Steam reforming of CH4 o v e r N i / M g - A I c a t a l y s t p r e p a r e d b y spc-method from hydrotalcite T. Shishido and K. Takehira* Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-hiroshima, Hiroshima 739-8527, Japan Ni-supported catalyst was prepared by solid phase crystallization

(spc-) method starting

from Mg-AI hydrotalcite (HT) anionic clay as the precursor and tested for the steam reforming of CH4. The HT precursor was prepared by co-precipitation method from metal nitrates, thermally decomposed and finally reduced to form Ni-supported catalyst,

i.e., spc-Ni/Mg-Al. Ni z+ can well replace the Mg 2+ sites in the HT precursor, followed by the formation of Ni-Mg-O solid solution, resulting in the formation of stable and highly dispersed Ni metal particles on the catalyst, spc-Ni/Mg-A1 catalyst showed higher activity

(imp-) method. Moreover, this catalyst showed higher activity as well as higher stability than a commercial Ni/a-Alz03 catalyst during the than those prepared by the impregnation

reaction for 600h. 1. INTRODUCTION The conversion of hydrocarbon to hydrogen will play an important role in 21st century, especially, for providing hydrogen for polymer electrolyte fuel cell (PEFC).

Currently,

steam reforming of hydrocarbons, especially of CHa (1), is the largest and generally the most economical way to make H2. Alternative industrial chemical approach includes

2CH4

CH4

+

H20

=

CO

+

H20

+

1/202

=

+

3H2 2CO

+

(1) 5H2

(2)

oxidative reforming of CH4 (2). However, the Hz production for PEFC requires enormously high efficiency taking account that the reformer should be compact in the FC system not only on-board of vehicle but also in the stationary FC system. A sufficient

36 amount of H2 should be continuously produced in a small reformer and fed to the PEFC and, therefore, the reforming catalyst must work under far higher space velocity compared to those for methanol or Fisher-Tropsch synthesis. A new concept of the catalyst preparation is required. We have proposed the highly dispersed

metal-supported

catalyst,

spc-method for starting

preparing the stable and

from the crystalline

precursor

containing active metal species in the structure [1]. In this paper, we report the preparation of

spc-Ni/Mg-Al catalyst

from Mg-A1 hydrotalcite like anionic clays as the precursor and

its successful use in the steam reforming of CH4.

2. EXPERIMENTAL 2.1. Catalysts preparation The catalyst,

spc-Ni/Mg-Al, was

clay as the precursor. replaced by Ni 2+.

prepared starting from the hydrotalcite (HT) anionic

Mg3AI-HT was used as the precursor, in which a part of Mg 2+ was

The HT precursor was prepared by co-precipitation from nitrates of Ni,

Mg and Al, thermally decomposed to mixed oxides at 1123K for 5h, and reduced to form supported Ni catalysts and

(spc-Ni/Mg-A1) at

imp-Ni/Al203 catalysts

imp-Ni/Mg-Al,imp-Ni/MgO impregnation (imp-) method, cz- and

1073K for 0.5h.

were prepared by the

7-A1203 (Kanto and JRC-ALO-4) and MgO "smoke" powder (JRC-MgO-4) were used as the support and impregnated in an aqueous solution of nickel nitrate. In the case of

imp-Ni/Mg-Al, Mg-Al

mixed oxide (MgaAl) was prepared by calcining Mg3AI-HT at

1123K for 5h and used as the support.

2.2. Steam reforming of CH4 The catalytic reactions were carried out at atmospheric pressure in a conventional flow reactor using a U-shaped quartz tube (O 10 mm) with a fixed bed catalyst. was diluted with quartz beads.

The catalyst

CH4 and H20 were mixed with N2 in the ratio of 1/2/2.

The flow rate of CH4 and N2 was controlled with a mass flow controller (STEC SEC-400 Mark3).

Distilled water was fed into the reactor with a liquid pump (Shimadzu

LC-10ADvp) through a vaporizer. The space velocity changed from 6.0 x 104 to 3.0 x 105 ml h-lg-cat -1. The products were analyzed by three on-line TCD gas chromatographs with Porapak-Q and Molecular Sieve 5A columns.

37

2.3 Characterization of the catalysts The metal compositions in the HT precursors were determined by ICP analyses

using

OPTIMA3000.

Perkin Surface

Elmer area

of

ICP the

:

3.0 kcps

catalyst was measured by the BET method using N2 at 77 K with a BEL ]

Japan BELSORP18. X-ray diffraction (XRD) was performed on an X-ray

j

~

_

tO

(a)

o

diffractometer (Rigaku RINT2550VHF) using Cu Ka radiation (L=1.5405,~). Transmission

electron

tO

microscopy o

(TEM) observation was carried out on JEOL JEM3000E The dispersion of Ni

o

~

9

o

(c)

o

on the catalysts was measured by the H2 pulse method at room temperature. One hundred mg of catalyst was first reduced

I

20

I

I

I

40

60

80

2 0 / degree

at 1073 K for 0.5h in 20 vol% H2 in N2 stream and then used for the measurement.

Fig. 1. XRD patterns of

spc-Nio.25/Mg2.ys-Al,

(a) before calcination, (b) calcined at 1123 K and (c) reduced at 1073 K; (O) MgO, NiO,

3. RESULTS AND DISCUSSION

(O) Ni, ([]) hydrotalcite

3.1. Structure of the Ni-supported catalysts The atomic ratio of Ni/Mg/AI in HT precursors determined by ICP analysis were very close to that calculated from the amount of reagents, regardless of the content of Ni. sequential result of XRD analyses of

A

spc-Ni/Mg-Al during the preparation is shown in

Fig. 1. The Ni-containing precursor after drying at 393 K (Fig. la) showed the diffraction lines of well-crystallized HT phase (JCPDS 22-700) alone, suggesting that Ni2§ is incorporated in the Mg 2+ site and dispersed uniformly in brucite- (magnesium hydroxide-) like layer of the HT structure. After calcination at 1123K, HT like layered structure collapsed to MgO (JCPDS 45-948) and NiO (JCPDS 47-1049) as seen in the strong lines overlapping each other (Fig. lb), suggesting a formation of solid solution between NiO and MgO. Very weak lines due to spinel phase (MgAl204: JCPDS 33-853) were also observed, indicating that the spinel phase was formed during the calcination. After the

38 reduction at 1073K, weak lines of Ni metal appeared, indicating that a part of NiO was reduced to Ni metal and segregated from Ni-Mg-O cubic lattice during the reduction. Fig. 2a shows the XRD pattern of Mg-AI mixed oxide obtained by calcining Mg-AI HT at 1123K.

~ 3.0kcps

Strong lines of MgO

i

o

were observed in the Mg-AI mixed oxide, but the value of the lattice parameter a (0.420 nm) was smaller than that of pure

o

9

MgO (0.421 nm), suggesting the formation

0

1

0

of a Ni-Mg-O solid solution as well as the presence of AI3+ ion in the Ni-Mg-O cubic

o

.'1=I (D

lattice as reported by several researchers [2].

o

Sato et al. [3] also reported that the oxide residue obtained on thermal decomposition

9

of the Mg-A1 HT below 1373K comprises a defect rock salt phase,

-

Mgl-xAlzx/3~bx/30.

Interestingly, upon impregnating the Mg-A1 mixed oxide in an aqueous solution of nickel nitrate, the lines of the HT layered structure appeared instead of decreasing lines of MgO, indicating that a significant reconstruction of the HT structure took place in the sample during the impregnation (Fig. 2b). During

o o

I

20

I

I

40 60 2 0 /degree

i

80

Fig.2 XRD patterns of imp-6.3wt% Ni/Mg-AI. (a) before calcination, (b) calcined at 1123K and (c) reduced at 1073K; (O) MgO, NiO, (O) MgAlzO4 (0) Ni, (m) hydrotalcite

the impregnation procedure, the HT like layered structure might be regenerated by the rehydration of MgO to brucite layer as frequently observed [3-8]. Rajamathi et al. [4] proposed two possible mechanisms for the regeneration of the HT like structure: (i) the insertion of protons/hydroxyl ions at the defect sites of the oxide or (ii) a dissolution-reprecipitation mechanism. They concluded that the reconstruction of HT phase occurs via a dissolution-reprecipitation mechanism based on the following facts: (i) the magnesium hydroxide has relatively high solubility in aqueous solutions, (ii) the defect rock salt phase is not essential to carry out the reconstruction and (iii) the defect rock salt phase causes a rapid reconstruction of the parent hydroxide, followed by the formation of layered HT like structure. This reconstruction via a dissolution-reprecipitation mechanism may induce a replacement of a substantial part of Mg z+ sites with Ni z+, and Ni ions may be dispersed in octahedral sites in

39 brucite layers of the hydrotalcite phase. It is likely that, during the preparation, the

imp-Ni/Mg-Al are similar to that of spc-Ni/Mg-A1. Actually, after the calcination at 1073 K, the XRD pattern of imp-Ni/Mg-Al (Fig. 2c)

circumstances around Ni species in

showed the presence of Ni-Mg-O solid solution and MgAI204. Even after the calcination at 1073K, the BET surface areas of

spc-Ni/Mg-Al catalysts were >100 m 2 g-l, regardless the

Ni content. After the reduction at 1073 K, the lines due to Ni metal appeared in addition to those of MgO phase (Fig. 2d), indicating that a part of NiO was reduced and segregated from the MgO cubic lattice during the reduction.

Nonetheless such circumstances, the

imp-Ni/Mg-Al was still stronger than that in spc-Ni/Mg-Al, indicating that the particle size of Ni metal on imp-Ni/Mg-A1 is larger than that on spc-Ni/Mg-A1. intensity of the lines of Ni metal in

3.2. Catalytic activity of the Ni-supported catalysts We

have

reported

spc-Ni/Mg-Al showed

that

the

100

higher

90

activity for both partial oxidation [9] and

COz

reforming

[10]

of

CH4

80 o

70

compared to the Ni catalysts prepared

imp-method, such as imp-Ni/Alz03, imp-Ni/MgO and imp-Ni/Mg-A1. Even at high space velocity, spc-Ni/Mg-A1 by

showed

no

decline

in

the

CH4

conversion. This may be due to the formation of highly dispersed stable Ni metals on

and

spc-Ni/Mg-Al.

o

60504030 0.0

I

I

1.0

2.0

3.0

GHSV / ml h -1 g-cat -1

Fig. 3 shows the CH4 conversion

Fig. 3. Effect of space velocity on steam reforming over both spc- and of CH4 imp-catalysts in the steam reforming of Reaction temp.: 1073K, CH4/HzO/Nz=I/2/2, Ni: CH4 at the ratio of steam to carbon of 6.3 wt%; (0) spc-Nio.z5/Mgz.75-A1, (A) obtained

2.0 at 1073K when the space velocity

imp-Ni//T-Al203 (e) imp-Ni/Mg-Al imp-Ni/MgO, (.) imp-Ni/a-Al203

(11)

was increased. The CH4 conversion calculated from thermodynamic equilibrium was above 99%. The Ni content in

spc-Nio.zs/Mgz.75-Al was equal to that in imp-6.3wt% Ni-supported catalysts. Evidently, spc-Ni/Mg-Al showed the highest activity among the catalyst tested including

40

imp-Ni-supported catalysts

loaded on various supports at the space velocity of 6.0 x 10 4 ml

h -1 g-cat -l. As the space velocity increased, CH4 conversion significantly decreased on the

imp-Ni/MgO and imp-Ni/c~-Al203. In the

imp-Ni/Mg-Al and imp-Ni/7-Al203, CH4 the space velocity, spc-Ni/Mg-Al showed

cases of

conversion gradually decreased with increasing

the highest activity even at high space velocity of 3.0 x 105 ml h -1 g-cat -a. It is clearly demonstrated that the activity of spc-Ni/Mg-Al was higher than

imp-Ni/Mg-Al at the

same

Ni loading. This strongly suggests that the dispersion of Ni metal is important for the catalytic activity. When ~/-A1203 (JRC-ALO-4) was used as the support, spinel structure NiAlzO4 formed on the catalyst surface, probably resulting in the high dispersion of Ni. Nonetheless such preferable effect expected on

imp-6.3wt%Nihl-Alz03, its

activity was

still lower than spc-Nio.zs/Mgz.75-Al. When

spc-Nio.s/Mga.5-A1was

tested for the steam reforming of CH4, a high and stable

activity was observed for 600 h of the reaction even at the steam to carbon ratio of 1.5. Life test of the catalysts was carried out at 1013K and under SV=2,500h -1. During the life test, the activity was checked under the more severe conditions,

i.e.,

at the lower

temperature of 933K at the higher SV of 10,000 h -1. A commercial Ni/a-AlzO3 catalyst showed a gradual decrease in the activity during the

100

reaction for 600 h. Thus, it

80

was clearly demonstrated that

the

activity

spc-Ni/Mg-A1 was stable

and

of

showed

4

20

for 600h. Stability

supported

40

no

decline during the reaction

3.3

60

high and

of

the

0

I 10

Ni catalysts

spc-Nio.s/Mga.s-Al

was

tested for the steam reforming of CH4 at the steam to carbon ratio of 1.0 and the space velocity of 2.4x105 ml h -1 g-cat -1. The low

I I 20 30 Time on stream / h

I 40

50

Fig. 4. Stability of Ni-supported catalysts at 1073K 5 1 1 CH4/HzO/Nz=I/1/2, GHSV=2.4xl0 ml h- g-catNi: 12.6 wt%; (0) spc-Nio.5/Mg2.s-Al, (A) imp -Ni//~,-Al203 (o) imp-Ni/Mg-Al (~) imp-Ni/MgO, (.)

imp-Ni/oc-Al203

ratio of steam to carbon caused a lowering of equilibrium CH4 conversion to around 92% at 1073K. The other catalysts prepared by

imp-method were

also tested and the results are

41 shown in Fig. 4. imp-12.6wt%Ni/a-A1203 showed a low activity at the beginning of the reaction and moreover a drastic decline in the activity during 50 h of the reaction. The activities of both imp-12.6wt%Ni/MgO and imp-12.6wt%Ni/Mg3-A1 were also lower than that of spc-Nio.5/Mgz.5-Al and decreased gradually during the time course. The best one among the imp-catalysts was imp-12.6wt%Ni/~,-AIzO3, but its activity still decreased along the time for 50 h even though the activity was enough high at the beginning of the reaction. This may be partly due to the severe conditions of the low steam carbon ratio of 1.0.

It is

thus demonstrated that spc-Ni/Mg-A1 was enough sustainable in the steam reforming of CH4 compared to the imp-catalysts.

3.4. Dispersion of Ni metal on the catalysts The adsorption of Hz on the supported Ni catalyst was measured by the H2 pulse method and the dispersion of Ni metal on the catalysts was calculated (Table 1). The amount of H2 adsorbed increased in the order of imp-Ni/a-Al203 < imp-Ni/MgO < imp-Ni/Mg-Al < imp-Ni/'y-Al203 < spc-Ni/Mg-Al, and inevitably the dispersion of Ni metal on the catalyst was calculated in the same order. Interestingly, the dispersion of Ni metal obtained from H2 adsorption was higher on spc-Ni/Mg-Al than on the corresponding

imp-Ni/Mg-Al. TEM observation also showed that spc-Ni/Mg-Al afforded rather sharp distribution of small size of Ni metal, while imp-Ni/Mg-A1 showed a broad distribution of relatively large size of Ni metal. In the spc-preparation starting from HT as the precursors, Ni species might be distributed homogeneously, i.e., not only on the surface but also in the bulk of the catalyst. Such a distribution may cause a decreasing value in the calculation of Ni dispersion. It is noteworthy that spc-Ni/Mg-Al showed higher dispersion than

imp-Ni/Mg-Al nonetheless such an unfavorable circumstance. During the reduction of Ni species in the catalysts, a substantial part of Ni may migrate from the oxide structure to the surface and may form the ultra-highly dispersed and stable Ni metal particles. The high activity observed on imp-Ni/Mg-Al may be due to the fact that a part of Ni ions was incorporated in the HT phase during the preparation, resulting in the formation of stable and highly dispersed Ni metals. This phenomenon is exclusively due to the reconstruction of the Mg-Al HT structure and seems to be important in the catalyst preparation.

42 Table 1 Dispersion of Ni metal on the catalyst a) Catalyst Hz uptake

spc-Nio.5/Mgz.5-A1 spc-Nio.zs/Mgz.75-A1 imp- 12.6wt%Ni/Mg3.0-Al imp-6.3wt%Ni/Mg3.o-A1 imp-6.3wt%Ni/MgO imp-6.3wt%Ni/a-Alz03 imp-6.3wt%Ni/~l-AlzO3

Ni dispersion

/ pmol g-cat -1

/%

76.4

7.1

38.4

7.2

72.5

6.8

17.5

3.3

7.1

1.3

15.7

2.9

36.2

6.8

a) reduced at 1073K for 0.Sh

4. CONCLUSION

spc-Ni/Mg-A1 showed higher activity than imp-catalysts and, moreover, higher stability than the commercial Ni/a-Alz03 catalyst during the reaction for 600h. During the preparation of spc-Ni/Mg-Al, Ni 2+ replaces well the Mg 2+ sites in the HT precursor, followed by the formation of Ni-Mg-O solid solution after the calcination, resulting in the formation of stable and highly dispersed Ni metal particles on the catalyst after the reduction. The high and stable activity observed on spc-Ni/Mg-A1 is due to the stable and highly dispersed Ni metal particles. ACKNOWLEDGEMENT A part of this work was supported by the Hiroshima Prefectural Industrial Technology Promotion Organization. We thank Dr. P. Wang in Japan Science and Technology Corporation and Dr. D. Shouro in Hiroshima Prefectural Institute of Industrial Science and Technology for TEM observation and their helpful discussions. REFERENCES

1. a) T. Hayakawa, H. Harihara, A. G. Andersen, A. E E. York, K. Suzuki, H. Yasuda and K.Takehira, Angew. Chem., Int. Ed. Engl., 35 (1996) 192.

b) K. Takehira, T. Shishido, M.

Kondo, R. Furukawa, E. Tanabe, K. Ito, S. Hamakawa and T. Hayakawa, Stud. Surf. Sci. Catal., 130 (2000) 3525.

43 2. a) D. C. Puxley, I. J. Kitchener, C. Komodroms and N. D. Parkins, Stud. Surf. Sci. Catal., 16 (1983) 237. b) J. H. Ross, In: Special. Periodical Reports, Vol 7, eds. G. C. Bond and G. Webb (Royal Society of Chemistry, London,1985) pp.1-45 and references therein. 3. T. Sato, K. Kato, T. Endo and M. Shimada, React. Solids, 2 (1986) 253. 4. M. Rajamathi, G. D. Nataraja, S. Ananthamurthy and E V. Kamath, J. Mater. Chem., 10 (2000) 2754. 5. s. Miyata, Clays Clay Miner., 28 (1980) 50. 6. N. S. Puttaswamy and P. V. Kamath, J. Mater. Chem., 7 (1997) 1941. 7. W. T. Richle, Chemtech, January (1986) 58. 8. F. Cavani, F. Torifiro and A. Vaccari, Catal. Today, 11 (1991) 173. 9. T. Shishido, M. Sukenobu, H. Morioka, M. Kondo, Y. Wang, K. Takaki and K. Takehira, Appl. Catal. A: General, 223 (2002) 35. 10. T. Shishido, M. Sukenobu, H. Morioka, R. Furukawa, H. Shirahase and K. Takehira, Catal. Lett., 73(2001) 21.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

45

Toward a molecular understanding of noble m e t a l c a t a l y s t impregnation J.R. Regalbuto 1, M. Schrier l, X. Hao 1, W.A. Spieker ~, J.G. Kim 2, J.T. Miller 3, and A.J. Kropf 4 1Dept. of Chem. Eng., U. of Illinois at Chicago, 810 S. Clinton Street, Chicago, IL 60607 2Soonchunhyang University, Chunan, Korea 3Bp Research Center,E-IF, 1510 W. Warrenville Rd., Naperville, IL 60563 4Argonne National Laboratory, CMT, 9700 Cass Ave., Argonne, IL 60439 1. INTRODUCTION There have been many recent advances in the understanding of noble metal catalyst impregnation, dating from the landmark postulation of Brunelle that the adsorption of noble metal complexes onto common oxides supports was essentially coulombic in nature [1]. The hydroxyl groups which populate oxide surfaces become protonated and so positively charged or deprotonated and negatively charged below a characteristic pH value. This pH, at which the surface is neutral, is termed the point of zero charge (PZC). This surface chemistry is depicted in the left side of Fig. 1. Brunelle cited many instances in which oxides placed in solutions at pHs below their PZC would adsorb anions such as hexachloroplatinate [PtCI6] -2, while at[ pH values above their PZC would adsorb cations such as tetraamonium platinate (TAP), (NH3)4Pt] 2+ [1]. This electrostatic mechanism was semi-quantitatively developed by another landmark work, that of Contescu and Vass [2], who studied the adsorption of [PdC14]-2 and [(NH3)4Pd] +2 over alumina at low and high pH, respectively. Much more work on the experimental verification and quantification of this "physical" adsorption process has been conducted in our own laboratory for the chloroplatinic acid (HzPtCI6, or CPA)/alumina system [3,4]. The Revised Physical Adsorption (RPA) model [4], with which all known sets of Pt/alumina adsorption data can be satisfactorily simulated with no adjustable parameters, is a result of these efforts. The basis of the RPA model is the a priori calculation of adsorption equilibrium constants, seen as the center regime of Fig. 1. Most recently, the mechanism of adsorption has been probed at the molecular scale using techniques such as 195Pt NMR and EXAFS to study the coordination chemistry of dissolved and adsorbed complexes arising from CPA [5,6, and references within]. This is the chemistry depicted in the right-hand portion of Fig. 1.

46 This paper will present some of our recent results in the latter two areas, that is, the application of the RPA model to several systems beyond CPA/alumina, and EXAFS characterization of dissolved and adsorbed Pt and Au complexes. 2. EXTENSION OF THE RPA M O D E L TO TAP/SIO2 The RPA model can predict, to a reasonable degree, the adsorption of the aqueous phase metal complexes arising from chloroplatinic acid (CPA) onto alumina as occurs during Oxide surface !

Oxide-water interface I

I

I

Bulk liquid solution I

I

K~I PtC16-2 + H20 OH2 +

PZC

A

I

K.,~2

OH

I

surface charging and pH shift

PtC15(H20)- + H20

Kay3

O"

~ K~5 I

~

PtC15(H20)- + C1-

PtC15(H20)-

PtC15(H20)-2 + H+

PtCI4(HzO)2

PtC14(OH)(H20) + H+

PtCI4(OHXH20)I

adsorption equilibrium constants

PtC14(H20)2+ C1-

PtC14(OH)2-2 + H+

I

i

liquid phase speciation

Fig. 1. Three regimes of fundamental studies of catalyst impregnation. catalyst impregnation [3,4]. The model consists of a non-Nernstian description of the charging of surface hydroxyl groups, a proton balance to account for the shifts in pH caused by the oxide buffeting effect [7], and an adsorption equilibrium constant based solely on the coulombic free energy of adsorption [4]. The model has been mainly applied to CPA adsorption over alumina at low pH, in which a negatively charged Pt complex adsorbs over a positively charged oxide surface. In this section we present the results for the complementary case; cationic Pt tetraamine complexes adsorbing at high pH over a negatively charged silica surface. 2.1 TAP/SiO2 Experimental Adsorption surveys were conducted in which the uptake of TAP was recorded as a function ofpH at constant initial metal concentration. Metal concentrations were measured by ICP in solutions before and after contact with silica. One hour contact time was found to be

47 sufficient for attainment of adsorption equilibrium. Three fumed and two precipitated silicas were employed; their properties are given in Table 1. The precipitated silicas contained substantial amounts of Na impurities, which typically rendered their PZCs above 6.0. After copious washing with deionized water, the PZCs came down to within the range exhibited by the essentially pure fumed silicas. Table 1. Properties of silica samples supplier name Cabosil L90 Cabosil M-7d Cabosil EH-5 Degussa Vn-3s Degussa Fk-300

type fumed fumed fumed precipitated precipitated

surface area 90 200 380 175 300

PZC 4.0 3.7 3.6 3.9 3.6

2.2 TAP/SiO2 results and discussion The adsorption results for tetraamonium platinate are shown in Fig. 2. The PZC of all pure samples was 3.8 _ 0.2, and so one model curve can represent all samples. Specific surface areas varied between 90 and 380 m2/gm; to account for this, different masses of silicas were added to the solutions such that the same surface loading, or (m 2 oxide)/(liter of solution), was attained. The value used, 30,000 mZ/liter, is close to incipient wetness (pore filling) in some of the samples. Two concentrations were employed; the higher, 2400 ppm TAP, corresponds to about one monolayer of TAP assuming a maximum adsorption density of 0.8 ~mol/m 2 [8]. The lower concentration, 600 ppm, corresponds to about 1A of a monolayer of TAP. The model simulates the data to a reasonable degree, without any adjustment of parameters. It would then appear that TAP adsorption over many types of silica is very similar and can be adequately described by the RPA model. The practicality of these "wet impregnation" (WI) catalysts was demonstrated by another set of experiments in which the final Pt dispersion of catalysts prepared by this method were compared to catalysts prepared by incipient wetness or "dry impregnation" (DI). The results are shown in Table 2. Utilizing wet impregnation with a pH only slightly above the PZC of silica, only a small amount of Pt is adsorbed, but is well dispersed after calcination (Table 2, row 1). Higher metal loadings are obtained by adsorbing at higher pH, and the dispersion is excellent after calcination. The poor dispersion of the DI preparations can be explained by the oxide buffering effect [7]. Even when starting with a moderately basic solution in dry impregnation, the silica surface is insignificantly charged and the solution pH will approach the PZC of silica. Platinum complexes are not strongly adsorbed and sinter during calcination.

48 3000

~

2500

o

2000

9 Eh-5

2400 ppm

9 Vn3s 9

0

0

0

L90 9 M7d

x

1500

/ x

1000

~

500

4

6

x

.

=

1

600 p p m

8

Fk30t

"---'--Mode

10

12

14

&

Eh-5

o

Vn-3,,

o

L90

u

M7d

+

Fk30

--" "-Mode

pH final

Fig. 2. Uptake - pH curves for adsorption of TAP over various types of silicas. The amount of silica was adjusted to 30,000 m2/liter by adding appropriate masses of silicas with different specific surfaces areas. Table 2. Effect of method of preparation on Pt dispersion on silica* %Pt Method pH base 0.25 WI 5.0 none 1.06 WI 9.0 NHnOH 1.63 WI 9.5 KOH 1.50 DI 5.0 (initial) none 2.0 DI 9.0 (initial) NI-hOH * Dispersion determinedby 1-12chemisorption. Sampleswere calcinedat 250~ 3. " I M P R E G N A T I O N CARBON

ENGINEERING"

H/Pt 0.87 1.28 1.26 0.29 0.33

OF CPA AND TAP ON ACTIVATED

An exciting application suggested by our accrued understanding of impregnation, at least for adsorption systems that follow an electrostatic mechanism, is the potential to alter the adsorptive properties of the support by changing the support PZC. We attempted this type of control with ion-doped silica and alumina and failed [9]. The RPA model predicts that a K § or Na + silica with a PZC of 8 should behave like alumina in the low pH range and adsorb high amounts of CPA, and conversely CI doped alumina with a PZC 4 should behave like silica, and adsorb high amounts of TAP in the basic regime. In actual practice the doped oxides behaved no differently than the pure oxides. We have most recently found that the dopant redissolves in the pH range of interest [10]; that is, it does not appear that the PZCs of silica and alumina can be irreversibly changed with ion doping.

49

On the other hand, we have achieved success with activated carbon. When treated with successively more rigorous oxidative pretreatments, the PZC of carbon can drop from about 9 to below 3. In this section we will show that a carbon with a PZC of 9 behaves like alumina and that with a PZC of 3 behaves like silica for the uptake of CPA and TAP as a function ofpH. 3.1 "Impregnation Engineering" of CPA and TAP on activated carbon Three types of activated carbon with different oxidative pretreatments and different PZCs are shown in Table 3. Adsorption of cationic and anionic Pt is measured as a function of pH, using constant metal concentration. As before, the extent of uptake is determined by ICP analysis of the liquid phase before and after contact with the support.

Table 3. Activated Carbon Samples Activated Carbon

SurfaceArea (mZ/g)

Pretreatment

PZC

Norit CA1

1400

3.0

Darco KB-B

1500

Norit SX ULTRA

1200

Chemically activated by the phosphoric acid process By chemical activation of hardwood Acid washed steam activated carbon

5.6 9.0

3.2 CPA and TAP/carbon results and discussion 200 180 .. 160

a~

b~

I

"

...'.. § ~

~=140

< 120 o-o 100

9SX,PZC=9.4,1Hr

I , O,,Hr I--,-KB B.PZC=S.8.1H

; ./&~

~ 8o ~9

0

1

2

3

4

5

"'i

6 pH

7

8

.....

9

i. 4

10 11 12

9~,pzc3.o

vl=" 120

I /

A SX,PZC9.4

= 100

w

~ 60 40 20 0

9KB-B,PZC 5.6

14o

\

I /

,.

model for CAl, faetot0.51 t /

80 60

\

,~',$

9

mode,.rKB-B.fa,or0.55/ ,//~..:i~ - - modelforSX.factor0.94

~, 40 20

1 0

2

4

I

6 pH 8

I~

/ i 10

;'. ~, "~."

12

14

Fig. 3. Uptake - pH curves for adsorption of a) CPA and b) TAP over carbons of various PZCs. The amount of carbon was adjusted in all cases to 2000 m2/liter. The uptake of CPA and TAP over these three materials is shown in Fig. 3; the carbon with the PZC of 3 behaves similarly to silica, in that it takes up high amounts of TAP (Fig. 3b)

50

as does silica (Fig. 2), but little CPA (Fig. 3a). The carbon with the PZC of 9 behaves similarly to alumina, adsorbing little TAP at high pH (Fig. 3b) but much CPA at low pH (Fig. 3a). The PZC 5.6 carbon exhibits adsorption behavior in between the other two materials. The uptake over silica can be modeled with the RPA model with an additional constraint that only a certain fraction of the highly porous (and very narrow pore size) material is accessible to the Pt complex. The uptake of CPA onto the PZC 3.0 carbon, while low, is not negligible and is higher than predicted by the RPA model. We are in the process of a minor revision of the model to reflect the more complex hydroxyl group chemistry on the carbon surface. 4. EXAFS C H A R A C T E R I Z A T I O N OF D I S S O L V E D AND A D S O R B E D Pt AND Au COMPLEXES The high sensitivity of the Advanced Photon Source (APS) at Argonne National Laboratory has permitted the study of dilute solutions (200 ppm and above) of the noble metal complexes HzPtCI6 and NaAuCI4. The changes in coordination chemistry of these materials upon adsorption have been investigated. This molecular characterization permits a more detailed study of the adsorption mechanism 4.1 EXAFS experimental EXAFS measurements were performed at the MRCAT undulator beam-line equipped with a double-crystal Si (111) monochromator with resolution of better than 4 eV at 11.5 keV (Pt L3 edge). Spectra of the metal solutions contained in plastic cuvettes were taken in fluorescence mode and those of solids as pressed powders in transmission mode. Phase-shift and backscattering amplitudes were obtained from various solid reference compounds. Details of the experimental and fitting procedures can be found in [6]. 4.2 EXAFS results and discussion 9

I

""'

I

"

I

"

0.03

I 0.02

0

1

2

3

4

radial coordinate R [A]

Fig. 4. Magnitude of the Fourier Transform for Pt coordination sphere: solid-CPA at pH= 1.5 HCI (kZ: Ak = 3.0- 10.7, Ar = 1.0-2.7; fit: 6.0 Pt-CI); dashed-CPA at pH=2.7 (kZ: Ak = 3.0-10.0, Ar = 1.0-2.4; fit 2.8 Pt-CI and 3.2 Pt-O); dotted-CPA at pH=12.5 (kZ: Ak = 3.0-8.6, Ar = 1.02.4; 1.8 Pt-C1 and 4.2 Pt-O)

51 In dilute CPA solutions the degree of hydrolysis is much greater than has been reported in the literature for higher concentrations. For instance, the species immediately formed by dissolving 200 ppm CPA in water, which results in a pH of 2.70, is [PtCI3(H20)3] +1, and upon 24 hrs aging becomes [PtCI2(OH)2(H20)z] ~ at pH 2.40 [6]. Fourier transforms of EXAFS spectra of 200 ppm Pt solutions at various pH values are shown in figure 4. The spectrum of CPA in HCI at a pH of 1.5 is characteristic of [PtCI6]-2 while at a pH of 12 in NaOH, the average Pt coordination is 1.8 Pt-CI and 4.2 Pt-O bonds. The spectrum for fresh CPA at its natural pH (about 2.7) indicates the chlorine coordination between the two. A surprising result was obtained when the adsorption of the chloroaquohydroxo complexes was compared to hexachloroplatinate [PtCl6] -z, which at the same dilution can be obtained with excess chloride [6]. These samples are shown in Table 4. Surface loading (amount of oxide) was varied for three target Pt loadings of 1.0, 4.8 and 7.2 wt %. One sample of each pair was prepared without NaCI, the other with. Table 4 summarizes the important experimental parameters of these samples and the measured chlorine coordination numbers (the balance of the six-fold coordination is oxygen and is not listed in the Table). Upon adsorption, both types of complexes lose cfiloride (much more for the hexachloroplatinate complexes) and in fact converge to an identical, low chlorine content of about 2.0 ligands. The loss of CI and gain of O during adsorption has been taken as an indication of adsorption via surface ligand exchange [5]. An alternate explanation, and one that is consistent with a completely physical mechanism of adsorption, is that the adsorbing complexes respond to the local environment of the surface, where the CI- concentration would be lower and the pH higher than the bulk. Low chloride is consistent with our prior measurements, which indicated CI- is not adsorbed over alumina in this pH range [3]. Secondly, the rise in pH is consistent with electric double layer theory and is due to the equality of electrochemical potentials between bulk and adsorbed protons. Table 4. Chlorine coordination number as a function of added NaCI and surface loading. Sample

Target wt% Pt

A 1 A2 A3 A4 A5 A6

1 1 4.8 4.8 7.2 7.2

Surface Loading NaCI [m2/i] [mol/I] 5000 5000 1000 1000 650 650

--0.1 --0.1 --0.01

Initial pH

CN CI liquid

Final pH

CN CI solid

Actual wt% Pt

2.63 2.60 2.59 2.54 2.50 2.55

2.7 6 2.7 6 2.7 (6)*

4.34 5.59 2.84 3.43 2.81 2.87

1.6 1.5 2.1 1.9 2.1 2.2

1.0 0.7 3.7 1.9 4.1 3.2

*estimated from [6], not measured The effect of increased surface pH is seen most clearly in the EXAFS analysis of dissolved and adsorbed complexes arising from tetrachloroaurate, [AuCI4]-. In Fig. 5, the AuCl coordination number is plotted versus pH (the coordination is always 4 and the balance of ligands are O-containing and are not shown). Unlike Pt(1V) complexes [6], the Au(II)

52

solutions show little dependence on Au or excess C1-concentration. The Au-CI coordination numbers for the adsorbed samples, if plotted versus the equilibrium bulk pH at which the sample was prepared, fall below the species in solution. If they are plotted versus the pH at the adsorbed layer [11], however, the chloride coordination numbers of the adsorbed complexes overlap with the coordination of the liquid phase species. Once again, this is to say that the adsorbing complexes speciate as if in the liquid solution, but at the local conditions of the adsorption plane. These results provide a molecular-scale refinement of RPA model; ligand exchange with the oxide surface, a feature of chemical adsorption mechanisms [5], needs not be invoked.

[] 200 ppm AuCI4-

3.5

l=

A

A

A,

," 2.5 .9 Itl

._

@500 ppm AuCI4-

3

Z

"O

i~ I Am

A

2

[] 200 ppm AuCI4-, 50x CIAadsorbed Au at bulk pH

A

A adsorbed Au at surface pH

O

o 1.5 o m o

:~

1

AA

0.5 0

,

0

2

9

4

,

6

JhA

A

~

8

10

12

14

pH (bulk or surface)

Fig. 5. Au-CI coordination numbers of dissolved and adsorbed complexes stemming from [AuCI4]-

REFERENCES 1. J.P. Brunell, Pure and Appl. Chem., 50 (1978) 1211. 2. C. Contescu and M.I. Vass, Appl. Catal. 33 (1987) 259. 3. J.R. Regalbuto, A. Navada, S. Shadid, M.L. Bricker and Q. Chen, J. Catal., 184 (1999) 335. 4. W.A. Spieker and J.R. Regalbuto, Chem. Engg Sci. 56 (2001) 1. 5. B. Shelimov, J.-F. Lambert, M. Che and B. Didillon, J. Mol. Catal. A: Chem., 158 (2000) 91. 6. W.A. Spieker, J. Liu, J.T. Miller, A.J. Kropf and J.R. Regalbuto, App. Catal. A: General, in press. 7. J. Park and J.R. Regalbuto, J. Colloid Interf. Sc., 175 (1995) 239. 8. N. Santhanam, TA.. Conforti, W.A. Spieker and J.R. Regalbuto, Catal. Today 21 (1994) 141.

53 9. W.A. Spieker and J.R. Regalbuto, Stud. Surf. Sci. Catal. 130 (2000) 203. 10. J. Komh, W.S. Spieker and J.R. Regalbuto, manuscript submitted 11. W.A. Spieker, X. Hao, J. Liu, J.T. Miller, A.J. Kropf and J.R. Regalbuto, manuscript submitted.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

55

Support modification of cobalt based slurry phase Fischer-Tropsch catalysts S. Barradas, E.A. Caricato, P.J. van Berge, J. van de Loosdrecht Heterogeneous Catalysis Research, Sasol Technology (Pty) Ltd, PO Box 1, Sasolburg 9570, South Africa

1. I N T R O D U C T I O N In the Fischer-Tropsch slurry phase conversion of synthesis gas into distillates, the catalytic metal of choice is cobalt [1,2]. As cobalt is relatively more expensive than iron as an alternative, maximum and optimum metal utilisation is a prerequisite, an objective that can be effected through the application of pre-shaped support materials. The commercial supports of choice appear to be alumina, titania and silica. The Fischer-Tropsch synthesis part of the Gas-To-Liquids (GTL) process is preferably performed in a slurry phase bubble column reactor [1], a system favouring cobalt based catalysts due to its gradientless nature [1,3]. The churning nature of the three-phase bubble column reactor, however, exerts significant mechanical stress on the suspended catalyst, placing a high premium on its mechanical integrity. In addition to this demand of sufficient mechanical integrity, it is also important to ensure proper anchoring of the Fischer-Tropsch synthesis active phase (i.e. metallic cobalt). Insufficient anchoring of these metallic crystallites could result in the washing out of cobalt rich ultra-fine particulates from the porous pre-shaped support material during commercial slurry phase Fischer-Tropsch synthesis [4]. These cobalt rich ultra-fine particulates (less than 1 ~tm in diameter) are extremely difficult to remove from the produced waxy product. Not only may the cobalt rich ultra fine particulates poison some of the downstream processes (e.g. the catalytic hydroprocessing of waxy product to gas-oil), but may also contribute to a loss in the specific catalytic productivity, thus compromising the requirement of extended Fischer-Tropsch synthesis activities. Support modification has been reported earlier in the open literature [5,6,7,8,9]. Zirconia modification of silica supports was used to prevent the formation of unreducible cobalt-silicates [5]. Zr, Ce, Hf, or U modification of titania supports was reported to prevent the formation of cobalt-titanates during regeneration [6]. To increase the porosity of titania supports, they were modified with small amounts of binders, e.g. silica, alumina or zirconia [7]. Lanthanum oxide promotion of alumina was reported to be beneficial for improved production of products with higher boiling points [8], and zirconia modification of alumina supports was carried out to decrease the interaction of cobalt with alumina [9]. All these modified supports were either used for fixed bed cobalt based Fischer-Tropsch synthesis catalysts or they were used for slurry phase cobalt catalysts, but not tested under realistic Fischer-Tropsch synthesis conditions in large scale slurry bed reactors.

56 It is well known that alumina, titania [10,11,12] and magnesium oxide [13,14] dissolve in acidic aqueous solutions and even at pH values close to the isoelectric point [15,16]. In this study, it will be shown that these support surfaces were modified with promoters to increase the inertness thereof to acidic/aqueous environments, and not to stabilise the support against sintering and loss in surface area at high temperatures [17,18]. This paper will deal with the modification of alumina and titania supports for cobalt based slurry phase Fischer-Tropsch catalysts to ensure the successful operation of slurry phase bubble column reactors on commercial scale.

2. EXPERIMENTAL 2.1.

Catalyst preparation

Two alumina-supported catalysts were prepared by means of a two-step slurry phase impregnation method, using an aqueous cobalt nitrate solution [4,19,20,21]. A high-purity spray-dried Puralox SCCa 2/150 (trademark) alumina support, in the form of spherical particles, obtainable t~om Sasol Chemic GmbH of Uberseering 40, 22297, Hamburg, Germany, was used in all alumina-supported catalysts. The alumina had a surface area of 150 mE/g and a pore volume of 0.5 ml/g as determined by the B.E.T. method. After impregnation and vacuum drying, the dried intermediate was calcined in air at 250~ for 6 hours. The calcined catalyst was reduced at 425~ in pure hydrogen using a heating rate of l~ Platinum was incorporated as a reduction promoter. The composition of these catalysts were for catalyst A: 30gCo/0.075gPt/100gA1203 and catalyst B: 30gCo/0.075gPt/1.5gSi/100gAl203.

2.2.

Support modification

Silica support modification in the case of catalyst B was performed as reported earlier by Beguin et al. [22], prior to the catalyst preparation, by means of a non-aqueous slurry phase impregnation step using Tetra Ethoxy Ortho Silicate (TEOS) as precursor. TEOS dissolved in dry ethanol was used as impregnation solution. The alumina powder was added and after impregnation, the slurry was dried at 60~ under vacuum for 6 hours. The dried modified support was calcined in air at 500~ for 2 hours [4]. The silica level was found to be 2.5 Si atoms/nm2 fresh support. Zirconium support modification of alumina was performed by adding zirconium, in the form of zirconium isopropoxide, under an inert atmosphere to isopropanol. Alumina was added to this solution, and the mixture stirred at 60 ~ for 1 hour. The solvent was removed under a vacuum of 3kPa (a) with a jacket temperature of 95 ~ The resultant modified support was subsequently calcined at 600 ~ for 2 hours to obtain a protected modified catalyst support. The amount of precursor was found to be 0.1 Zr atoms/nm2 fresh support. Particulate titanium dioxide (Degussa P25 (trademark)) support was calcined at 650 ~ for 16 hours, spray dried and classified to 75-150 micron. The support had a rutile content of 80% and a surface area of 27mE/g.

57

2.3.

Dissolution behaviour The dissolution behaviour of the supports were characterised by monitoring the conductivity of a support in a model acidic aqueous environment. The conductivity was monitored by a Metrohm 712 conductometer with a platinum electrode. The support was added to an acidic aqueous medium with a pH of 2 at 25~ The conductivity was measured continuously for 30 hours with stirring at 25~ and a constant pH 2. The pH was maintained at 2 with a 10% HNO3 solution by continuous and automatic titration with a Metrohm 718 STAT Titrino fitted With a Ag, AgC1 glass and reference electrode. 2.4.

Fischer-Tropsch synthesis tests Laboratory Fischer-Tropsch synthesis tests were performed in a slurry-phase Constant S_tirred Tank Reactor. The pre-reduced catalyst (20-30 g) was suspended in ca 300 ml molten Fischer-Tropsch wax. Realistic Fischer-Tropsch conditions were employed, i.e." 220 ~ 20 bar; commercial synthesis gas feed: 50 vol% H2, 25 vol% CO and 25 vol% inerts; synthesis gas conversion levels in excess of 50%. Use was made of the ampoule sampling technique as the selected on-line synthesis performance monitoring method [23]. Larger scale Fischer-Tropsch synthesis runs were performed in a pilot plant slug-flow slurry reactor using 3-Skg catalyst as well as in a slurry phase bubble column demonstration unit using 500-1500kg catalyst. The reaction conditions were similar to those in the laboratory CSTR runs. The reactor wax production varied between 5 and 30kg per day for the pilot plant runs and up to 60 bbl/day for the demonstration unit. On-line catalyst samples were taken for particle size distribution measurements and _Scanning Electron Microscope analyses. 2.5.

Characterisation techniques Particle size distribution measurements were performed by means of a Leeds & Northrup Microtrac Particle Size Analyser, equipped with a laser beam to determine the particle sizes. A water suspension was prepared by adding ca 2g of the sample and a few drops of a suitable detergent. The sample was ultrasonically treated for 1 minute before the analysis was performed. An average of three rtms was used to calculate the final particle size distribution. SEM analyses were performed on a Cambridge 360 Scanning Electron Microscope at an accelerating voltage of 3.0 kV in the SE mode. T_ransmission Electron Microscopic analyses were performed on a Phih'ps CM200 with a LaB6 electron source at the University of the Witwatersrand, South Africa. High Resolution SEM analyses were performed on a JEOL 6000F ultra high resolution Field Emission Gun Scanning Electron Microscope at the Unit for Electron Microscopy at the University of Pretoria, South Africa. 3. RESULTS AND DISCUSSION

3.1.

Pilot Plant synthesis tests The unmodified almnina-supported cobalt catalyst (catalyst A) was tested in the pilot plant slug-flow reactor under realistic Fischer-Tropsch conditions. The produced wax was

58 secondary filtered over a 2 micron Whatman no 42 filter and analysed for cobalt content by ICP. Initially, clean waxy products were produced (i.e. cobalt free) as depicted in Fig. 1.

400 -

9

300 E O. r

=

200

0

o

100 0 o y

0

10

9

1

9

20

s 30

t 40

Time (days)

Fig. 1. Cobalt content in secondary filtered wax during pilot plant scale Fischer-Tropsch synthesis runs, using catalyst A: 30gCo/0.075gPt/100gA1203 (run F102).

After about 10 days on stream, however, the secondary filtered wax started discolouring and high levels of cobalt contamination were observed (refer: Fig. 1). This is not desired, as cobalt loss from the reactor decreases catalytic activity, and contamination of the produced waxy products may poison the downstream processes, e.g. the catalytic hydroprocessing of the waxy products to gas-oil. It also increases the cost of the commercial GTL plant due to the increased difficulty in removing the ultra fine contaminants from the produced waxy products. To ascertain the composition of these ultra fine cobalt contaminants, the produced waxy products were then secondary filtered through a 0.1 [am alumina membrane filter and the retained residue was analysed by XRD. The XRD analysis indicated that the residue contained well-crystallised Co304, and hardly any alumina. The ultra fine particles were found to be cobalt rich. During these extensive Fischer-Tropsch synthesis runs in both Pilot Plant and Demonstration Unit scale reactors, the mechanical integrity of the catalyst was monitored by means of Particle Size Distribution (PSD) measurements on extracted catalyst samples. The average particle size did not change even after 8 months of operation, and was qualitatively demonstrated by SEM analyses (refer: Fig. 2). Catalyst break-up is thus not an explanation for the observed contamination of secondary filtered wax with cobalt rich ultra fine particulates.

59

Fig. 2. (a) A SEM image of the alumina-supported cobalt catalyst A after a FischerTropsch synthesis test run in the demonstration unit. (b) Relative mean PSD values of the supported catalyst (catalyst B) after an 8 months test tun on the Pilot Plant slug-flow reactor.

3.2. Characterisation of freshly prepared cobalt supported alumina catalyst and the pure alumina support. The freshly prepared catalyst A was studied by high resolution FESEM (refer: Fig. 3).

Fig. 3. High resolution FESEM image of a fresh alumina-supported cobalt catalyst A 30gCo/100gA1203 Small white crystals were observed on the surface of the particles. A linescan elemental analysis was performed on a small crystal and was found to be pure boehmite (A10(OH)). The alumina used in this study has been calcined at 750 ~ and XRD analysis has shown that there was no boehmite present in the fresh support. It was reported earlier that alumina supports could (partially) dissolve in aqueous solutions [24], even at pH values close to the isoelectric point [15]. Partial dissolution of the alumina used during the preparation of catalysts A was experimentally confirmed during the aforesaid model dissolution test (refer: Fig. 4).

60 0.60 Unmodified alumina

o o.

" 0.40

%

~

-

_J

~, 0.20

1.5 g Si/AI203

o~

E

0.00

I

0

10

I

I

20

30

40

Time(h)

Fig. 4. The dissolution behaviour of an alumina support (used to prepare catalyst A) and a silica modified support (used to prepare catalyst B) It can be seen that rapid dissolution occurs within the first hour, followed by a more gradual dissolution that proceeds continuously (i.e. about 30 hours). A mechanism of metal oxide dissolution is reported in literature [24] (equations 1 and 2).

~Al/

H 0

~Al/

-OH2

+ 3 H+(aq)

~AI /

__~

~O /

~OH

H2 O~ ~A1 ~ /

~O / H

.OH2 Al/ (1)

/ /

3+

I-I2 o ,~ NO/ H

NOH 2

3+ .OH2 AI/ ~OH2

.OH2 in water

,~ slow

~AI "/ /

+ 21-120+ A13+(aq)

(2)

~OH

The abstraction of an aluminium ion from the bulk proceeds via 3 stepwise protonations of the surface hydroxyl groups. Alumina, therefore, dissolved in an acidic aqueous solution, similar to the environment during the aqueous phase cobalt nitrate impregnation step. Five hydroxyl groups have been identified [25] that could be responsible for the dissolution of alumina. These hydroxyl groups vary in acidity, therefore protonation of the most negatively charged would be protonated first, and were the most likely candidates to initialise dissolution. The abstraction of aluminium ions into solution via the mechanism shown above would be more rapid than for the more acidic hydroxyl groups. Therefore, the dissolution rate in the start of the dissolution test should be higher.

61 3.2.

The identification of an amorphous layer on the catalyst surface

An amorphous layer was observed on the surface of the particles (refer: Fig. 5) as analysed by TEM. The possible origin of this amorphous layer can be described by the following hypothesis: During the aqueous phase cobalt nitrate catalyst preparation step, the alumina support dissolves and the dissolved aluminium ions can precipitate either as boehmite or may, in combination with cobalt ions, precipitate as a cobalt aluminium hydrotalcite-like layer [16], producing a physically amorphous layer, uniformly covering the surface of the bulk support material [15]. It was, however, not possible to characterize the observed layer. It is hypothesised that during the aqueous phase impregnation and drying of the slurry, cobalt nitrate was precipitated on the amorphous cobalt-containing layer. The presence of this amorphous layer, prior to the actual impregnation process, resulted in an inferior anchoring of the eventually catalytically active cobalt metal. During the extended testing of the cobalt catalyst in a slurry phase bubble column reactor at Fischer-Tropsch synthesis conditions, the cobalt crystallites dislodged, thus causing the contamination of the produced waxy products, as observed in Fig. 1.

Fig. 5: TEM image of a fresh alumina-supported cobalt catalyst A: 30gCo/100gm1203 3.4.

Prevention of alumina dissolution

The dissolution behaviour of the alumina support during the slurry phase impregnation steps of the catalyst preparation procedure was considered to be the cause of the high levels of cobalt-rich ultra fine particulates in secondary filtered wax. To prevent dissolution of the alumina support during the slurry phase impregnation step of the catalyst preparation procedure, a support modification procedure was developed to prevent acidic attack of the hydroxyl groups on the surface of the support. A modifier was chosen which had to conform to the following requirements: i) Lower the dissolution rate of the modified support in an aqueous/acidic solution ii) The catalyst activity should not be compromised. iii) The porosity should not be decreased to such a point that the maximum cobalt loading on the alumina support, i.e. 30g Co/100g A1203, would not be achieved [4]. iv) No shift in catalyst selectivity, i.e. an increase in methane selectivity.

62 It was therefore necessary to prevem proton attack on the surface hydroxyl groups by either modifying the hydroxyl groups with a alkoxide silica modification agent (Terta exthoxy silane TEOS) or modifying the surface to form a stable spinel structure. The silica modification agent, TEOS, was added in such a manner that monolayer coverage was achieved. A monolayer will have the smallest impact and have the highest probability to pass the four requirements listed above. The support modification step was performed as a slurry impregnation using TEOS dissolved in dry ethanol as impregnation solution [4]. During support modification, TEOS reacted with the hydroxyl groups of the alumina surface. The three remaining ethoxy groups could be decomposed during a calcination step at 500~ in air, but was not compulsory. The support can also be modified by other methods with TEOS, e.g. chemical vapour deposition [26,27,28]. A maximum of 2.5 Si atoms/nm/ support was added during one impregnation step due to the bulkiness of the organic groups of the silicon alkoxide. The ethoxy groups reacted with surface hydroxyl groups and during decomposition all the remaining ethoxy groups on the TEOS molecule react with the surface hydroxyl groups, virtually coveting the total surface with a monolayer. The dissolution behaviour of the silica modified alumina was measured (refer: Fig 4), and was found to be significantly inhibited. It is believed that the silicon increases the inertness of the alumina support in an acidic/aqueous solution. The support geometry is important with respect to Fischer-Tropsch synthesis performance (i.e. activity and selectivity) of the resultant catalyst based thereon. The modified alumina support after silica modification did indeed produce a catalyst with the desired Fischer-Tropsch synthesis characteristics [4]. To verify the influence this monolayer silica had on catalyst activity and selectivity, the intrinsic Fischer-Tropsch synthesis activity as well as the selectivity of the supported cobalt catalysts studied in this paper (i.e. catalyst A and B), were determined at realistic Fischer-Tropsch synthesis conditions: (refer: Table 1). Table 1. Fischer-Tropsch synthesis data for catalysts A and B, as tested under realistic reactor conditions (refer: Experimental). Catalyst B Catalyst A 30Co/0.075Pt/1.5Si/100AI203 30Co/0.075 Pt/100A1203 Run number 1755 98F 15 20 Time on line (h) 75 76 Syngas conversion (%) 6.8 x 10-6 7.1 x 10-6 FT reaction rate (mol/s) 0.34 0.36 Productivity (gHC/gcat/la) 4 7 CH4 selectivity (%C-atom) From Table 1, it can be concluded that silicon modification of alumina did not have any substantial effect on the Fischer-Tropsch synthesis activity nor did it substantially influence the selectivity of the silica modified catalyst B. The silica modified supported cobalt catalyst, Catalyst B, were tested in a Pilot Plant slurry bubble column reactor under realistic Fischer-Tropsch synthesis conditions and it

63 was demonstrated that silica modification of the alumina support successfully eliminated the undesired phenomenon of ultra-fine cobalt rich contamination of filtered reactor wax. 400 3,50

9 Catalyst A

3OO

9Catalyst B

i i

i ta.

~200

8 ~50

J

lOO 50 liakr1#~ 0

"

".3/.: .

~.

m

r

o

10

20

~. i-

30

. -

mt -i

9

9

40

i

50

9

El J

_ --

Fig. 6. Cobalt content in secondary filtered wax during Pilot Plant scale Fischer-Tropsch synthesis rtms, using catalyst A: 30gCo/O.O75gPt/ 100g A1203 (run F102) and catalyst B: 30 Co/0.075 Pt/1.5g Si/100g A1203 (l'un F 117).

_, --I

60

70

8O

Time (days)

added to the pure support to form spinels with A1, i.e. aIIal2IIIo4, which are known [4,29,30] to exhibit decreased solubilities in comparison to alumina. After impregnation of the support with the modifier in an organic solvent, the modified support is calcined at high temperatures to create a protective surface layer of the desired spinel, i.e. aIIal2IIIo4 . In Fig. 7, an example of zirconia modification is shown. 0.6

0.5 1:: O 0. 0.4 0.

Unmodified alumina

o~

E 0.3 0.1 Zr atoms/nm 2 support

0.2 E 0.1

0.0

~'

I

I

I

0

10

20

30

40

Time(h)

Fig. 7. The dissolution behaviour of an unmodified alumina support (used to prepare catalyst A) and a zirconia modified support. The dissolution of titania supports was also investigated. Titania also (partially) dissolves in aqueous solutions and silica modification, using TEOS, effectively inhibited support dissolution [4].

64 4.

CONCLUSIONS

It was demonstrated that the production of clean waxy products (i.e. free of any cobalt contamination) during large scale slurry phase Fischer-Tropsch synthesis runs, was successfully effected with cobalt catalysts that were prepared on modified supports (i.e. supports displaying inhibited dissolution behaviour in aqueous environments). As an example, the silicon modification of alumina supports was discussed in detail. REFERENCES

1. P.J. van Berge, S. Barradas, J. van de Loosdrecht and J.L. Visagie, Erd61Erdgas Kohle, 117:3 (2001), 138. 2. Eisberg and R.A. Fiato, Studies in Surface Science and Catalysis, 199 (1998), 961. 3. P.J. van Berge and R.C. Everson, Studies in Surface Science and Catalysis, Vol. 107 (1997), 207. 4. P.J. van Berge, J. van de Loosdrecht, E.A. Caricato and S. Barradas, EP 1058580,2000. 5. Hoek, J.K. Minderhout, and P.W. Lednor, EP Patent No.110449 (1983). 6. C.H. Mauldin, S.M. Davis, and K.B. Arcuri, US Patent No. 4,663,305 (1987). 7. C.H. Mauldin, and K.L. Riley, US Patent No. 4,992,406 (1991). 8. S. Eri, J.G. Goodwin Jr., G. Marcelin and T. Riis, US Patent No. 4,880,763 (1989). 9. F. Rohr, A. Holmen, K.K. Barbo, P. Warloe and E.A. Blekkan, Studies in Surface Science and Catalysis, Vol 119 (1998) 107. 10. E. Bright and D.W.Readey, J. Am. Ceram. Soc., 70112] (1987) 900. 11. W.H. Casey, M.J. Cart and R.G Graham, Geochim. Cosmochim. Acta, 52 (1988) 1545. 12. M.L. Machesky, D.A. Palmer and D.J. Wesolowski, Letter to Geochim. Cosmochim. Acta, 58 (1994) 5627. 13. A.K. Datye, A.D. Logan and K.J. Blankenburg, Ultramicroscopy, 34 (1990) 47. 14. J.A. Mejias, A.J. Berry, K. Refson and D.G. Fraser, Chem. Phys. Lett., 314 (1999) 558. 15. J.-B. d'Espinose de la Caillerie and O. Clause, Studies in Surface Science and Catalysis, Vol. 101 (part B) (1996) 1321. 16. J.-B. d'Espinose de la Caillerie, C. Bobin, B. Rebours and O. Clause, Prep. of Cat. VI (eds G. Poncelet et al., Elsevier Science B.V.) (1995) 169. 17. A.I. ~[~li,A.E. Aksoylu and Z.I. Onsan, Turk.J.Chem., 22 (1998) 253. 18. T. Horiuchi, Y. Teshima, T. Osaki, T. Sugiyama, K. Suzuki and T. Mori, Catal. Lett., 62 (1999) 107. 19. R.L. Espinoza, J.L. Visagie, P.J. van Berge and F.H. Bolder, US Patent No. 5,733,839 (1998). 20. P.J. van Berge, J. van de Loosdrecht, E.A. Caricato, S. Barradas and B.H. Sigwebela, WO 00/20116 (2000). 21. P.J. van Berge, J. van de Loosdrecht and J.L. Visagie, WO 01/39882. 22. B. Benguin, E. Garbowski and M. Primet, J. Catal, 127 (1991) 595. 23. E. Bright and D.W.Readey, J. Am. Ceram. Soc., 70112] (1987) 900. 24. G. Furrer and W. Stumm, Geochim. Cosmochim Acta, 50 (1986) 1847. 25. H. Kn6zinger and P. Ratnasamy, Catal. R e v . - Sci. Eng. 1711] (1978) 31. 26. T. Jin and J.M. White, Surf. Interf. Anal., 11 (1988) 517.

65 27. 28. 29. 30.

N. Katada and M. Niwa, Chemical Vapour Deposition, 2[4] (1996) 125. S. Sato, M. Toita, Y.-Q. Yu, T. Sodesawa and F. Nozaki, Chem. Lett., (1987) 1535. P.J. van Berge, J. van de Loosdrecht andS. Barradas, ZA patent no. ZA 2001/6213. P.J. van Berge, J. van de Loosdrecht and S. Barradas, WO 02/07883, 2002.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

67

The effects of nature and pretreatment of surface alumina support on the catalytic nickelsilicate membrane formation C. Constantin l'z, V. Pfirvulescu 1'3#, A. Bujor z, G. Popescu z and B.L. Su 1. 1-Laboratoire de Chimie des Mat~riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61 rue de Bruxelles, B-50 Namur, Belgium Z-Research Center for Molecular Materials and Membranes, Spl. Independentei 206, Bucharest, Romania 3-Institute of Physical Chemistry, Spl. Independentei 202, Bucharest, Romania. Catalytic nickelsilicate membranes were synthesized by in-situ techniques on pretreated a- and ~,- alumina supports. The resulting films and bulk materials have been characterized by XRD, Nz adsorption-desorption, SEM, TEM and FTIR spectroscopy. Their permeation performances of gases and liquids and catalytic activity in the selective oxidation of styrene with HzOz have been evaluated. The formation, morphology, permeation and catalytic properties of the membranes have been correlated with nature of alumina supports, their surface pretreatment and conditions of the hydrothermal synthesis. The results show a strong effect of ~,-alumina surface on the morphology and properties of the membranes. 1. I N T R O D U C T I O N The new application fields of membranes in fuel cells and in catalytic membrane reactors expect new membrane materials. The recent researches are focused on three types of inorganic membranes: zeolite, microporous membranes obtained by sol-gel, and dense metallic and perovskite membranes [1-4]. Zeolitic membranes attract a particular attention due to their resistance to high temperatures, shape selectivity and sorption affinities [4-9]. However, the synthesis of zeolite films on various substrates is very sensitive to the chemical and structural properties of the support surface, the conditions of preparation and the substrate position in the reactor [10]. The films prepared by ex-situ coating or in-situ crystal growth methods contain often the intercrystal porosity, the molecular sieve effect of zeolite is thus greatly reduced. Furthermore, the adhesion between the crystals and the supports remains still a great problem [11]. The successful applications of zeolite membranes in the transformation and permeation of the small molecules and the properties of mesoporous silicates, designated as M41S, explain the actual interest in the synthesis of mesoporous molecular sieve membranes. Incorporation of the transition metals into the #: SSTC and DGRE-DRI-RW (Belgium) research fellow Corresponding author ([email protected])

68

MCM-41 network conducts to the active catalytic materials in the oxidation reactions with hydrogen peroxide [12-16]. In this paper we present our recent results on the formation of nickelsilicate membranes on alumina substrates and the effects of alumina surface treatment on the film morphology, permeation and catalytic performances. 2. E X P E R I M E N T A L

2.1. Synthesis Nickelsilicate membranes were hydrothermally synthesized on the porous a or ~, alumina supports. The chemical modification of the supports was achieved by acid and base treatments, deposition of silica and adsorption of sodium silicate or surfactant molecules. Mineral acid as hydrochloric and hydrogen peroxide were used for cleaning support materials. ~,-Alumina support disk (0 =28.5 mm and 3 mm in thickness, 0.5 vm in diameter of the pores and 186 m2/g in surface area) and a-alumina tube (12.3 mm o.d. and 8 mm i.d., 0.2 t.tm in diameter of the pores and 14.9 mZ/g in surface area) were cleaned in an ultrasonic bath containing a HCI acidified solution mixture of 2-propanol, ethanol and water (volume ratio of 1:2:2). To investigate the effects of surface functional groups on the membrane formation, the supports were wetted during 0.Sh or 12h, respectively, with an aqueous solution of a single component such as NaOH, NH4OH, TMAOH and sodium silicate or a mixture of NH4OH+TMAOH, NH4OH+CTMABr and TMAOH+H202. After this pretreatment, the supports were dipped or hydrothermally treated in the gel. The gel with the molar composition of 1.00 SiO2:0.04 Ni2+: 0.48 CTMABr: 0.28 N a 2 0 : 2 . 7 0 TMAOH: 196.00 H20 was obtained from sodium silicate (25.5-28.5% silica), cethyltrimethylammonium bromide (CTMABr), tetramethylammonium hydroxide (25 wt % TMAOH in water), Ni(CH3COO)z-4H20, and H2SO4 and aged in air for 2 days under ambient conditions. The films, deposited only on the top face of the support, or in the interior of the tube, were washed with distilled water. Crystals that were not, or weakly, bonded to the supports were removed by ultrasonic treatment in water for 1 h. The samples were dried in air at 100 ~ overnight and calcined at 550 ~ for 8 h. 2.2. Characterization The supports and obtained membranes were characterized by XRD (Philips PW 170 diffractometer), N2 adsorption-desorption (Tristar, Micromeritics), SEM (Philips XL-20 microscope), TEM (Philips Tecnai microscope), FTIR (Spectrum 2000, Perkin Elmer) and porometry (Colter porometer) techniques. Permeation of 02 through nickelsilicate membranes was carried out in a WickeKallenbach cell at 298K to evaluate the quality of the synthesized membrane. The membranes were tested in a catalytic reactor for the oxidation of styrene with hydrogen peroxide (30% wt) at 323K and molar ratio of styrene/ acetonitrile was 1/7.2. The oxidation products were analyzed using a Carlo Erba gas chromatograph with a stainless steel column containing OV-101 connected to a FID detector. The amount of the H202 was quantitatively analyzed by conventional iodometry. The used catalysts were dried at 373K for 8 h and then used for recycling experiments. Permeation measurements of the H202 and styrene were carried out under the reactor conditions.

69 3. RESULTS AND DISCUSSION The synthesis of the MCM-41-type metallosilicate membranes, using in-situ techniques, has shown a significant effect of the chemistry of the surface support. Nickelsilicate membranes were obtained by two different methods of the hydrothermal treatment. First, the support was treated into the gel and in the second case, the support was coated by dipping and treated in vapor atmosphere. Formation of the heterophase at the gel-substrate interface depends on the substrate surface properties and the vapor atmosphere. Those explain the effects of the support nature, pretreatment of the substrate and conditions of the hydrothermal treatment (time, vapor atmosphere) on the structure, morphology, uniformity and thickness of the films. 3.1 Effects of the support nature on the structure and morphology of the membranes Pretreatment of the alumina supports can clean the surface and change its reactivity. y-Alumina surface after pretreatment under the basic conditions contains a high concentration of the O- and OH functional groups. A high concentration of the cations (NH4 +, TMA § Na + and CTMA +) can be adsorbed by these very active surfaces. Interaction of the synthesis gel with these adsorbed cations leads to the nucleation, the growth and the aggregation of the micro- or sub-microsized globular species and the formation of the multilayer film on support surface when ~,-alumina was used as support. The morphology can be quite different if the pretreatment was different (Fig. 1). Both morphologies were very often encountered in metallic ions modified mesoporous MCM-41 silicas [15, 16]. Therefore, from the morphology point of view, it is very possible that the films obtained contain a hexagonal MCM-41 structure. The TEM and XRD techniques whose results will be discussed in the following section will supply us more information. It is observed that the membrane obtained on the y-alumina disk pretreated with a mixture of TMAOH and H202 was quite homogeneous (Fig. l a). While the treatment with NaOH or TMAOH conducts to a non-uniform composition with a different percent of the zeolite crystals in mesoporous membranes. Contrary of these, no significant effect of pretreatment on the the formation of membranes was noted when a-alumina support was used since membranes with similar morphology, thickness and structure were obtained both on pretreated or untreated a-alumina support (Figs. 2 and 3).

Fig. 1. SEM images of the membranes synthesized on 3,-alumina disk support pretreated in an aqueous solution of TMAOH+H2Oz+H20 (a) and NH4OH (b)

70

Fig. 2. SEM images of the membranes synthesized on a- alumina tubular support untreated (a) and pretreated in an aqueous solution of NH4OH (b) and NaOH+H202 (c) The same morphology was observed on all the membranes obtained on the a-alumina (Figs. 2 and 3). Nickelsilicate films obtained on a-Al203 have a typical morphology and structure for MCM-41 materials (Figs. 2 and 3), but the thickness is higher compared to the membranes synthesized on ~,-alumina disk. A difference in the morphology between outer and inner sides of the film was observed for all the membranes (Fig. 3a). It has to noted that the particle size of outer layers is much smaller than that of inner layers. Typical features in these images (Figs. 2 and 3a) are the small size of the grains on the substrate surface and their relatively homogeneous distribution on the surface.

Fig. 3. SEM images of the sectional view of nickelsilicate film synthesized on a- alumina pretrated in an aqueous solution of TMAOH+H202 (a) and of nickelsilicate powder obtained from the gel in autoclave (b)

71

Fig. 4. SEM images of the nickelsilicate membranes synthesized on 7-alumina pretreated during 2 h with an aqueous solution of TMAOH (a) and 12 h with TMAOH+H202 (b) and 12 h with NH4OH (c)

properties of the 3.2. Effect of the pretreatment of alumina support on the membranes Membranes synthesized on ?- A1203 have various morphology (Figs. 1, 3a and 4). The typical results obtained are shown in Table 1 together with synthesis conditions. The materials obtained are amorphous, MFI type zeolite or MSU mesoporous materials with a disordered wormhole like structure, depending on the support pretreatment and synthesis conditions. The membranes obtained after adsorption of sodium silicate have a non-uniform composition with a high percent of the amorphous phase. Pretreatment with NHnOH solution conducts to membranes with low uniformity. The zeolite or amorphous nickelsilicate with very small particles form these membranes. The ordered mesoporous nickelsilicate membranes were obtained on ~,-alumina treated with an aqueous solution of TMAOH+H202. The formation mechanism of mesoporous nickelsilicate membrane on the alumina substrate is somehow different compared with that of bulk materials. TEM images (Fig.5) and X-ray diffraction diagrams of both the membrane synthesized on ~,-alumina support pretreated with the mixture of TMAOH+H202 and the nickelsilicate powder obtained in the same autoclave (Fig. 6) exhibit a mesoporous structure with a hexagonal channel array of pore system. Anyway, comparing with the bulk nickelsilicate powder, the membrane material has a less resolved XRD patterns. As we discussed in the previous section that the morphology of the particles forming the membranes and that of powders obtained from the gel in autoclaves are identical, we can conclude on the basis of TEM, SEM and XRD results that the membranes obtained contain really an ordered mesoporous MCM-41 structure.

72

Fig. 5. T E M image of the nickelsilicate film

Fig. 6. XRD patterns of the nickelsilicate materials

Table 1. Preparation of the nickelsilicate membranes on 7-A1203 under various synthesis conditions and characteristics of their porous structure Run N ~ Support treated by a Membrane Atmosphere Product SBZT, aqueous solution of growth in autoclave Film P o w d e r (a) m2/g method 14 NH4OH immersion Gel + + 884 29 NaOH immersion Gel no + 13 NaOH,+HzOz immersion Gel + + 542 9 T M A O H + HzOz immersion Gel + + 920 A3 TMAOH dipping NH3 no 22 T M A O H + H202 dipping NH3 * 284 23 T M A O H + H z O z dipping DDA +,* 33 Na silicate immersion Gel no + A2 NHnOH dipping NH3 *,+ 480 29 NaOH dipping Vapor of the * gel 25 TMAOH+H202 dipping H20 no 31 TMAOH dipping NH3 * 28 T M A O H + H202 immersion Gel + + 920 +: Ni-MCM-41, *: MFI zeolite (a) : powder obtained from the gel in autoclaves The N2 adsorption-desorption profiles and the porosimetry measurements confirmed the presence of mesopores in the nickelsilicate membranes obtained by immersion, into the gel, of the alumina support treated with a solution of T M A O H + H z O z + H 2 0 . The average pore diameter was varied between 2.4 and 4.2 nm. Table 2 lists some permeation

73 experimental results of the gases and liquids through the nickelsilicate membrane. The permeation values are influenced by the preparation conditions (pretreatment of alumina support, ex. membranes 9 and 14 and time of the same treatment, ex. membranes 9 and 28). The evolution of the permeation evaluation with pressure on the surface membrane did not show the presence of any cracks on the surface of these membranes, indicating the high quality of these membranes. Table 2. Single gas and liquid permeability at 298 K for some nickelsilicate membranes Membrane Po2 x 102, PN2 x 102, PH2o2 x 102, cm3/cm2.min.atm cm3/cm2.min.atm cm3/cmZ.min 9 1.9 4.2 3.21

5

20

Ps_ty. x 10 2, cm3/cm2.min 0.94

14

4.8

9.2

8.24

2.82

28

3.5

6.7

5.47

1.46

...........

+9 +

-

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14

.~15 Fig. 7. Variation of styrene conversion with time in membrane reactor

~ 5 o

The mesoporous nickelsilicate membranes obtained are active and very selective to benzaldehyde and condensation products in the oxidation of styrene with hydrogen peroxide. The variation of the synthesis parameters and the pretreatment of the supports affects the permeation (Table 2) and the conversion of styrene (Fig. 7). In comparison with the conventional static reactor with a control of the H202 feed, the conversion of hydrocarbon on membranes after 12 h reaction was lower, but the efficiency of the H202 and selectivity to benzaldehyde and condensation products are higher. A variation of the pressure in the feed room favors the control of the rate of the oxidation. 3.3. Variation of the membranes properties with the synthesis parameters The parameters of interest are the film thickness, surface relief and their temporal evolution. These parameters depend substantially on the substrate surface properties and

74 pre-treatment. The dependences of the film thickness and morphology on the experimental conditions are discussed for two types of nickel-silicate films obtained by hydrothermal treatment in gel or vapour atmosphere. Ni-MCM-41 materials were obtained by immersion of the support into nickelsilicate gel or dip coating of a layer of synthesis gel. Hydrothermal treatment of the coated gel layer in atmosphere of dodecylamine (DDA) or ammonia leads to the zeolite or amorphous materials. From Table 1 and Fig. 8 it can be seen the influence of the atmosphere of the hydrothermal treatment on the resulted materials. The large difference in the surface morphology was observed. Thickness and uniformity of the membranes can be controlled by the time of the pretreatment (Fig. 4 a and b and Fig. 9) and the hydrothermal treatment. Results of our study show a significant effect of time of the pretreatment on the morphology, thickness and uniformity of the membrane surface.

Fig. 8. SEM images of nickelsilicate membranes synthesized on 7-A1203 pretreated for lh with an aqueous solution of TMAOH. The membranes were obtained by dip coating of a layer of gel which was subjected to hydrothermal treatment in the vapor (a) and by immersion of treated support in the gel (b)

SEM images of the membranes obtained by immersion of the support into nickelsilicate gel and hydrothermal treatment of the gel during 1, 2, 3 or 5 days show the formation of membrane only after 2 days. Except the membrane synthesized on 7-A1203, pretreated with TMAOH+H202+H20 mixture. However, the uniformity of the membrane obtained after 2 days of the hydrothermal treatment is not very good. Thickness of the membrane synthesized during 5 days is very high with a bad uniformity of the surface. Xray diffraction diagrams of the aggregates formed on the surface of these membranes show a typical MCM-41 materials.

75

Fig. 9. SEM images of nickelsilicate membranes synthesized on 7-AIzO3 pretreated for 30 min (a) and 2 h with a solution of TMAOH+H20 by immersion of the support into the gel 4. CONCLUSIONS The membranes obtained are insensitive to the pretreatment of c~-alumina support. While the structure and morphology of the nickelsilicate membranes synthesized on yalumina support are significantly modified by the synthesis conditions and the pretreatment of the substrate. Nickelsilicate membranes are active in oxidation reaction of styrene but their permeability and the catalytic activity are significantly modified by the synthesis conditions and the pretreatment of the support. The higher permeability for the gases and liquids and the higher conversion for the membranes synthesized on y-alumina supports pretreated during 12 h with TMAOH+HzOz+HzO mixture were obtained. ACKNOWLEDGEMENTS This work was performed within the frame of PAI-IUAP and a bilateral scientific cooperation between the R~gion Wallonne of Belgium and Romania. The VP and CC thanks the SSTC (Federal scientific, technological and cultural office of Prime Minister, Belgium) and DGRE-DRI of R6gion Wallonne, Belgium, for the research scholarships. REFERENCES

1. C. Lange, S. Storck, B. Tesche and W.F. Maier, J. Catal, 175 (1998) 280. 2. V. P~rvulescu, V.I. P~rvulescu, G. Popescu, A. Julbe, C. Guizard and L. Cot, Catal. Today, 25 (1995) 385. 3. S.J. Xu and W.J. Thomson, AIChE Journal, 473 (1997) 2731. 4. U. Illgen, R. Sch~ifer, M. Noack, P. K61sch, A. K/ihnle and J. Caro, Catal. Commun., 2 (2001) 339. 5. J. Caro, M. Noak, P. K61sch and R. Sch~ifer, Microporous Mesoporous Mater., 38 (2000) 3.

76 6. N. van der Puil, F.M. Dautzenberg, H. van Bekkun and J.C. Jansen, Microporous Mesoporous Mater., 27 (1999) 95. 7. Y. Uang, S. R. Chaudhuri and A. Sarkar, Chem. Mater., 8 (1996) 473. 8. M. Matsukata, N. Nishiyama and K. Ueyama, J. Chem. Soc., Chem. Commun., (1994) 339. 9. M.C. Lovallo and M. Tsapatsis, AIChE Journal, 42 (1996) 3020. 10. V. Valtchev, S. Mintova and L. Konstantinov, Zeolites, 15 (0995) 679. 11. Y. Yan, S.R. Chaudhurin and A. Sarkar, Chem. Mater., 8 (1996) 473. 12. M. Stockenhuber, R. W. Joyner, J.M. Dixon, M.J. Hudson and G. Grubert, Microporous Mesoporous Mat., 44-45 (2001) 367. 13. S. Biz and M.L. Occelli, Catal. Rev.Sci. Eng., 40 (1998) 329. 14. T. Blasco, A. Corma, M.T. Navarro and J.P. Pariente, J. Catal. 156 (1995) 65. 15. V. P~rvulescu, C. Dascalescu and B.L. Su, Stud. Surf. Sci. Catal. 135 (2001) 4772 16. V. Parvulescu and B.L. Su, Catal. Today, 69 (2001) 315.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

77

Supports and catalysts preparation by using metal alkoxides grafting technique E. Santacesaria (*), A. Sorrentino, M. Di Serio, R.Tesser Dipartimento di Chimica, Universit~ degli Studi di Napoli Federico II, Complesso di Monte S.Angelo via CINTHIA 80126 Napoli (Italy) E-mail [email protected]

Preparation methods for some acid and redox catalysts and some supports, obtained by grafting alkoxides on the surface of oxides rich of hydroxyls, are described. Acid and redox properties of the prepared catalysts have been determined and discussed. Catalysts have been evaluated in suitable test reactions and the most relevant performances will be reported. 1. I N T R O D U C T I O N The commercial availability of the metal alkoxides has strongly encouraged the use of these interesting compounds for producing new ceramic materials and thin coating films by the innovative techniques of sol-gel and chemical vapour deposition (CVD) [1]. Alkoxides are also largely used for the preparation of catalysts and/or catalytic supports, normally obtained by precipitation or co-precipitation as a consequence of hydrolysis [2]. Some papers have recently dealt with the preparation of catalysts obtained by grafting metal alkoxides on the surface hydroxyls of different carriers. A group of papers, for example, describes grafting of vanadyl alkoxide on oxides such as ZrO2, TiO2, SiO2 and TiO2-SiO2 [3-6]. Others describe the grafting of titanium alkoxides on silica [7-9], leading to a TiOzcoated material with a very high dispersion, which is useful as a catalytic support for vanadia. Few papers, at last, deal with grafting alkoxides with the aim of modifying the acidity of a surface [ 10-15]. Metal alkoxides grafting technique can usefully be used for changing the acid-base and/or the redox properties of the surface of an oxide rich in hydroxyls. To this purpose, oxides surfaces can be modified by grafting metal alkoxides, in different ways as, for example: (i) with low surface coverage to obtain, after steaming and calcination, a high dispersion of the corresponding oxide or isolated acid or redox sites of particular strength, (ii) with a monolayer, by completely altering the nature of the original surface in terms of its hydroxyls density, acidity or redox properties, (iii) with a multilayer, by repeating the grafting operation several times together with the treatments of steaming and calcination after each grafting step, to obtain a solid with a chemically different surface, but retaining To whom correspondence must be send. Work supported by MIUR (PRIN 2000 Funds)

78 the original mechanical and structural properties. It is then possible to modify the available alkoxide: (a) by changing, for example, the alkoxide group with another one having a longer alkyl chain for obtaining a compound more soluble in apolar solvents; (b) by reacting with an electron withdrawing compound such as, for example, a mineral acid in order to increase the Lewis acidity of the metal; (c) by reacting with a stoichiometric amount of water to obtain a partial condensation of the alkoxide molecules before grafting; (d) by preparing, always before grafting, a two metals heterometallic alkoxide. Alkoxide reactions with the hydroxyls of an oxide surface can be performed by contacting the surface with a solution of the alkoxide in the parent alcohol or in another solvent, more or less polar. The role of the solvent can be very important for the anchorage efficiency. The alkoxide concentration to be used can be choisen by knowing the surface hydroxyl density, the prevailing stoichiometry and the alkoxide-surface reactivity. In the present work, the methods of preparation of catalysts and supports, by grafting alkoxides on the most common commercial supports, will be described. Silica, alumina and titania, used as original supports were commercially available, while silica coated with TiOz, used as a support, has been obtained by grafting in three successive steps titanium alkoxide on silica. All the prepared catalysts and supports have been characterized by using different techniques and test reactions. In particular, we observed, in many cases, completely different properties and behaviour with respect to the corresponding catalysts prepared with the traditional impregnation technique [16]. The performances of the catalysts prepared by grafting were evaluated by measuring their activities and selectivities in adequate test reactions such as: methanol dehydration, skeletal isomerization of 1-butene to isobutene and alkane isomerizazion and cracking. The redox properties of vanadium based catalysts have been tested using three different reactions: the SCR (Selective Catalytic Reduction) of NO with NH3, the oxidative dehydrogenation (ODH) of ethanol to acetaldehyde, and the ODH of propane to propene. Very promising results have been obtained in some of the mentioned reactions as a consequence of the peculiarity of the catalysts obtained with the alkoxide grafting technique. In this paper the best results obtained will be described and interpreted on the basis of some of the results of the catalysts characterization. 2. E X P E R I M E N T A L S E C T I O N

2.1. Catalysts and supports preparation methods Catalysts have normally been prepared by contacting a given support with an alkoxide solution, generally kept under inert gaseous atmosphere, at temperature that can be room or solvent boiling temperature, according to the observed reactivity. Apolar solvents are largely preferred giving place to higher grafting yields, provided that the alkoxides are soluble in those solvents. Often, the parent alcohol is used as a solvent, but in this case it must be considered that the following equilibrium occurs: Surface .... OH + Me (OR), ~

Surface--- O--- Me (OR)n-1 + ROH

(1)

rendering the reaction much less favoured in the presence of an excess of the alcohol. However, this could be an advantage when a high dispersion of the catalyst is required. The concentration of alkoxide in the contacting solutions must be decided on the basis of

79 the amount of the element that we want to anchor on the support, but taking into account the yields obtainable, depending on the equilibrium (1) and/or on the reactivities of the involved reacting species. By assuming a conventional stoichiometry of 1 alkoxide molecule for 1 hydroxyl on the surface, we can roughly estimate the amount of alkoxide that is enough for obtaining a monolayer coating. After a grafting experiment, we can determine the yield of the reaction by measuring the amount of residual alkoxide in the solution. After grafting, the solid obtained is normally washed with the used solvent and then submitted to a steam treatment, at 150-190~ for eliminating, by hydrolysis, the residual alkoxide groups from the surface. The solid is then calcined at about 500~ In order to obtain a multilayer coating, the described procedure of grafting, washing, steaming and calcination is repeated several more times. In Tables 1 and 2, the most representative catalysts and supports prepared by using the grafting technique, together with the conditions adopted for preparation and the obtained yields, are reported. Table 1 List of the acid catalysts prepared with the grafting yields. Grafting have been made at room temperature for Si(OEt)4 at the solvent boiling point for Zr(OR)4 Metal initial Anchored Supported Precursor/solvent/support Acronym amount metal oxide (mmol/g) (mmol/g) (% wt.) Z1B 0.08 0.01 0.1 Zr(n-but)4/butanol/Al203 Z2B 0.61 0.02 0.2 Zr(n-but)4/butanol/AlzO3 Z3B 1.26 0.08 1.0 Zr(n-but)4/butanoffAlzO3 Z4B 2.17 0.14 1.7 Zr(n-but)4/butanol/Al203 Z1T 0.61 0.10 1.2 Zr(OCsH17)4/toluene/A1203 Z2T 1.26 0.14 1.7 Zr (OC8H17)4/toluene/AlzO3 ZTSS 2.73 0.52 Zr (OCsH17)2SO4/toluene/SiO2(Aldrich) ATSS 4.33 1.83 AI (i-OPr)SO4/toluene/SiOz(Aldrich) AS1 1.05 0.07 0.4 Si(OEt)4/ethanol/Al203 Si (OEt)4/ethanol/Al203 AS2 2.13 0.14 0.9 Si (OEt)Jeth an ol/A1203 AS3 4.10 0.28 1.7 Si(OEt)4/no-solvent/Al203 ASM 11.0 0.50 3.0 Si(OEt)4/no-solvent/ASM (Two times) ASM3 11.0 0.71 4.3 Since catalysts prepared by grafting are often very dispersed, the stoichiometry of the reaction between an alkoxide and the hydroxyls of the surface can be responsible for the change of the hydroxyls density giving place to respectively: an increase of the density for 1 to 1 stoichiometry, a contraction for a 3 hydroxyls for 1 alkoxide, while 2 to 1 stoichiometry gives invariance. Moreover, the overall stoichiometry can change with the concentration of the alkoxide solutions and this brings, in practice, to an average behaviour falling between 1:1 and 2: 1. The use of an apolar solvent largely favours grafting reactions. Therefore, alkoxides can be modified for obtaining more soluble compounds. It is possible to exchange, for example, one or more alkoxide groups by reacting the alkoxide with fatty alcohols: Me(OR)n + x R ' O H ~

Me(OR)n_x(OR')• + x ROH

80 Table 2 List of the redox catalysts prepared and related support with the grafting yields. Grafting have been made at room temperature for VO(i-OPr)3 and by refluxing at the boiling point of the solvent for Ti(OR)4. Grafting of VO(i-OPr)3 is quantitative in n-hexane. Acronym Metal initial Anchored Supported Precursor/solvent/support amount metal oxide (mmol/g) (mmol/g) (% wt.) Ti (i-OPr)4/toluene/SiO2 (Aldrich) ATS1 2.27 1.5 12.6 Ti (i-OPr)4/toluene/SiO2(Aldrich) ATS2 2.27 2.2 18.4 Ti (i-OPr)4/toluene/SiO2(Aldrich) ATSM 2.27 3.3 27.7 Ti (i-OPr)n/toluene/SiO2(Grace) GTS1 0.95 0.7 5.9 Ti (i-OPr)n/toluene/SiO2(Grace) GTS2 0.95 1.15 9.7 GTSM 0.95 1.41 11.3 Ti (i-OPr)n/toluene/SiOz (Grace) VO(i-OPr)3/n-hexane/ATSM AVTS1 0.32 0.32 2.9 AVTS2 0.71 0.71 6.4 VO(i-OPr)3/n-hexane/ATSM VO(i-OPr)3/n-hexane/ATSM AVTS3 1.17 1.17 10.6 VO(i-OPr)3/n-hexane/GTSM GVTS1 0.09 0.09 0.8 VO(i-OPr)3/isopropanol/GTSM GVpTS 0.09 0.06 0.5 VO(i-OPr)3Hydrol/isopropanol/GTSM GVhTS 0.10 0.9 VO(i-OPr)3nydroR+4Ti(i-OPr)4/ G(VhT4)S 0.09 0.8 Isopropanol/SiO2 Shifted alcohol, having low boiling point, is continuously removed by distillation thus favouring the reaction (see catalysts Z1T, Z2T of Table 1). Another possibility of modifying the grafting precursor is the reaction in a suitable solvent with an acid, for example sulphuric acid, having a strong electron-withdrawing effect on the metal. We obtain sulphated complexes of the type Me(OR)n_2SO4 that can be grafted giving place to very strong acid catalysts (see catalysts ATSS, ZTSS of Table 1). It is possible, at last, to favour the condensation of the alkoxides molecules, before grafting, by reacting them with stoichiometric amounts of water, in the presence of traces of HCI for promoting the condensation reaction by partial hydrolysis. The condensation reaction can be made by using only one type of alkoxide or, alternatively, by mixing two different alkoxides that can react with each other, giving place to a bimetallic alkoxide. However, other procedures can be followed for obtaining hetero-metallic alkoxides [17] to be grafted. Partial hydrolysis has been used by us for obtaining different dispersions of V205 catalysts. The preparation of hetero-metallic alkoxides opens a new perspective in the preparation of catalysts with the grafting technique, and we used this technique for preparing a vanadiumtitanium alkoxide that has been directly grafted on silica. 2.2 Catalysts characterization Three main aspects of catalysts characterizations, related to the changes observed as a consequence of grafting, that will be considered here and discussed are: (1) the change of the specific surface area and/or of the hydroxyls density; (2) the change of the acid-base properties; (3) the change of the redox properties. The changes obtained in the specific surface area and/or of the hydroxyls density, as a consequence of grafting alkoxides and of the following other described treatments, are not dramatic. Grafting is normally accompanied by a relatively small decrease of the specific surface area, as it can be

81

appreciated from the data reported in Tables 3 and 4. This behaviour can be explained by assuming that alkoxides, which are normally aggregates of 2-4 molecules, can occlude by grafting narrower pores. There are some exceptions to the decrease of the specific surface area or to the hydroxyls densities probably due to the preeminence of the 1 to 1 stoichiometry. However, in any case, it is very important to observe that the specific surface area remains very high when compared with that of the pure supported oxides (see for example the difference between pure TiOz anatase and TiOz anchored on silica after 1, 2 or 3 grafting steps reported in Table 4. The acidity has been characterized by using essentially two different techniques: (1) potentiometric titration of the acid sites with the determination of the ZPC (Zero Point Charge) or IEP (Isoelectric Point) and pKal, pKaz (Surface intrinsic ionization constants in water); (2) TPD (Thermal Programmed Desorption) of adsorbed organic bases such as pyridine and dimethyl pyridine to discriminate between Bronsted and Lewis acidic sites. Table 3 Properties of acid catalysts obtained by grafting Acronym ZPC OH pkal pkaz or (mmol/g) support weak

SiOz(Aldrich) ~'-AlzO3 ATSM Z1B Z2B Z3B Z4B Z1T Z2T AS1 AS2 AS3 ASM ASM3

2.2 7.5 2.7 7.5 7.6 7.7 7.9 7.8 7.9 7.4 7.1 6.6 5.6 5.2

2.2 1.05 2.0 1.20 1.32 1.44 1.34 1.42 1.38 1.01 0.95 0.82 0.97 0.91

0.6 5.7 1.1 . . 6.0 . 6.2 5.3 4.9 4.1 3.4 3.2

3.5 9.3 4.6 . . 10.1 . 10.3 9.1 8.8 8.1 7.6 7.5

Acids sites distributions by TPD Pyridine 2-6 Dimethylpyridine (~tmol/g) (~tmol/g) medium

strong

weak

74 84 155

3 35 46

10 2 7 .

10 12 16 . .

.

.

. 9 14

.

16 .

. 43 37 . . . 19

71 59 .

. . 12

.

. . . .

. . 80 . 80

.

. .

.

strong

59 18 114

3 9 39

16 13

7 5

24

7

30

6

. 3 3

.

. . .

. . 4 . 3

. 9 .

medium

.

. .

The main acid-base properties of some of the prepared catalysts are summarized in Table 3. However, as it will be seen also, the catalytic activities in different test reactions give information about the surface acidity characteristics. Redox properties of the catalysts can be determined, on the contrary, by submitting them to TPR (Thermal Programmed Reduction) with hydrogen (see for example Fig.l). Reduced catalysts can be reoxidized with pulses of oxygen so determining also the catalyst dispersion. These determinations have been made for vanadium based catalysts. It is very interesting to observe, for example, that clusters of vanadia of different sizes, corresponding to different dispersion indexes, can show very different redox properties, as it can be seen in Table 4 and Fig. 1.

82

Table 4 Properties of redox catalysts and related supports. Acronym V205 % Specific Pore Pores distributions I Surface volume Area (cm3/g) d< 20 @ 20 100 @ (mZ/g) SiO2 Aldrich SiO2 Grace TiO2 ATS1 ATSM GTS1 GTS2 GTSM AVTS1 AVTS2 AVTS3 GVTS1 GVpTS GVhTS G(VhT4)S

2.9 6.4 10.6 0.8 0.5 0.9 0.8

450 282 125 372 355 237 267 299 283 261 284 249 214 314 335

0,0020

0.72 1.02 0.38 0.50 0.51 0.23 0.26 0.27 0.46 0.41 0.48 0.29 0.24 0.26 -

17 6 15 27 7 5 4 37 22 43 -

82 97 84 83 32 92 94 95 37 28 38 -

1 3 10 _

2

41 1 1 1 26 50 19

0.13 0.19 0.11 0.73 1.00 0.62 0.45

600

GVTS1 (a) GV.TS (b)

5OO

> 0,0015 E

o O

4o0

(,~ 0,0010N--

300

"~ 0,0005-

200 E 100

0,00000

20

40

60

80

100

120

time (minutes)

Fig. 1. A n e x a m p l e o f T P R GVhTS

obtained for the catalysts:

Dispersion Oads/Vsupp

(a) C a t a l y s t G V T S 1 ,

(b)

83 2.3 Most significant catalyst performances obtained in test reactions 2.3.1 Acid catalyzed reactions All the catalysts prepared with acid properties listed in Tables 1 and 3 have been submitted to the most suitable test reaction, chosen from: (i) dehydration of methanol, that is a reaction promoted by acid sites of medium strength; (ii) skeletal isomerization of 1butene to isobutene, promoted by sites of relatively weak acidity; (iii) isomerization and cracking of n-hexane, a reaction promoted only by very strong acid sites. Relevant results of both activities and selectivities have been achieved in all the mentioned reactions by using acidic catalysts prepared by grafting. In particular, methanol dehydration is strongly promoted over zirconium oxide supported on ~,-alumina. The presence of only 0.08 mmoles of Zr/(g of alumina) increased by a factor of 2 the reaction rate of 4 mmol/g h obtained on pure alumina, at 180~ The synergistic effect reaches a maximum value for the mentioned content of ZrO2 on the surface, but the activities always remain greater than on pure alumina. Silicated alumina prepared by grafting silicon tetraethoxyde (TEOS) on ~,-alumina has recently been proposed [10] as a catalyst useful for the isomerization of 1-butene to isobutene, a reaction of great industrial relevance for the production of MTBE (methyl tertbutyl ether), i.e., the most important oxygenated compound used as additive in lead-free gasoline. This reaction is very useful as a reaction test, because the first step of the reaction, that is the double bond shifting giving 2-butene (both cis and trans), is promoted by waek acid sites, while the subsequent skeletal isomerization to isobutene requires more acidic sites. However, if the acidity is too high, other undesired reactions occur such as dimerization to octene, cracking, and coke formation, the latter being followed by catalyst deactivation. The activities and selectivities shown by silicated alumina of the ASM and ASM3 types are always higher and less affected by deactivation than the ones shown by respectively alumina, silica alumina and silicated alumina of the type AS1-3, as it can be 100

%

80.

co

i

60,

o O 20

0

S 0

28-

U

20"

b

~ 4o ----- NO AVTS2 ~ NO3ArnVTSg2natedcat.

y 0-| 150

9i 200

- - o - - NO impregnated cat.

2.4o 3~0 3.4o 4~0 4.4o s~o Temperature ~

Fig. 2. Conversion from butene to isobutene, at 430~ 5 ml/min, N2 50 ml/min, Cat=2g) over catalysts AS2, ASM and ASM3 compared with alumina [ 14].

36

t

l~

12-

e

o

9

i

i

-"

i

1

AS4 .

.

.

.

.

.

ASM AS2

9

e

n

i

~, Alumina

50

100

150

200

250

300

time (minutes)

350

4.00

Fig. 3. Comparison of the performances of a grafted and of an impregnated catalyst containing the same amount of V2Os in the SCR of NOn with NH3 [16]. (700 ppm of NH3 and of NO, 60 N1/h He, 0.3 g of cat.)

84 appreciated in the examples reported in Fig. 2. In particular, we observed a great improvement of both the activities and selectivities when the preparation of the catalyst occurs by contacting directly the support with the silicon ethoxide TEOS in the absence of any solvent. The use of ethanol, as a solvent, has a detrimental effect, as it can be seen in Fig. 2. From experimental evidence, sylicated alumina catalysts show many islands of silica on the surface of alumina [14]. When the grafting procedure is repeated seeral times, TEOS only reacts with the hydroxyls of the already supported silica, increasing the height of the silica islands. Consequently, catalyst ASM3 prepared repeating the grafting 3 times shows activities and selectivities that are comparable with those of ASM obtained by grafting in one single step (see Fig. 2). This also means that the active sites are probably located at the borderline between silica islands and not covered alumina surface [14]. The n-hexane isomerization reaction, always accompanied by cracking, is promoted only by very strong acid sites as the ones obtained by reacting some alkoxides directly with 100% sulfuric acid. Catalysts prepared and tested in the mentioned reaction are reported in Table 1 together with the conditions adopted in the preparation. Contrary to expectation, sulfated alumina alkoxides grafted on silica gave better results than sulfated zirconia alkoxides [13] especially after the regeneration in air at 400-500~ This is probably due to the instability of the sulfated zirconia prepared in this way, at the relatively high regeneration temperature. However, the activities and selectivities obtained in the reaction by using the two mentioned catalysts are compared in Table 5. Table 5. Product distibution in n-hexane isomerization and cracking, at 250~ (Space velocity- 90 cm3/(h'gcat) ) on sulphated catalysts ATSS and ZTSS of Table 1. Catalyst Cony. % C1 C2 C3 C4 C5 2-MP 3-MP MCP I

ATSS

1.6

-

4.0

16.3

15.1

7.3

29.1

14.3

ZTSS

1.0

-

-

30.1

26.3

11.1

22.5

10.0

l

l

13.9

2.3.2 Redox catalyzed reactions All the redox catalysts prepared and tested are vanadium based catalysts. On the basis of the information largely reported in the literature, the performances of these catalysts are greatly increased when V205 is supported on TiOz. But TiO2 supports, generally in the form of anatase, have a relatively low surface area susceptible to decrease by sintering. For this reason, different attempts have been made for preparing supports of silica impregnated with TiO2, or alternatively of silica coated with TiOz by grafting Ti alkoxide [7-9]. We devoted some efforts [15] in preparing and characterizing supports of silica coated with a multilayer of titania by repeating several times the grafting procedure. The obtained results are reported in Tables 2 and 4. It is interesting to observe that the amount of titania supported after three grafting steps is different according to the hydroxyl density of silica, changing according to the type of silica and the conditions of calcination. Supports prepared in this way have optimal chemical and physical properties with high and stable specific surface area and interact very well with vanadyl tri-isopropoxide in the preparation of vanadia catalysts by grafting. Many different vanadia catalysts have been prepared with

85 the aim to evaluate the effect of the vanadium load and dispersion on the redox properties. For this purpose, we attempted to increase the molecular size of the alkoxide species to be grafted on TiO2/SiO2, by submitting to partial hydrolysis vanadyl tri-isopropoxide dissolved in isopropanol. Other experiments have been made by submitting a mixture of vanadyl tri-isopropoxide and titanium isopropoxide to partial hydrolysis, then grafting the obtained product directly on SiO2. As vanadyl tri-isopropoxide readily reacts with water, it gives place first to autocondensation and then reacts with titanium isopropoxide giving place to a bimetallic alkoxide. The different catalysts prepared are summarized in Tables 2 and 4. These catalysts have been tested in different reactions, such as: (1) selective reduction of NOx (SCR) with NH3 [17]; (2) oxidative dehydrogenation (ODH) of ethanol to acetaldehyde [18]; (3) ODH of propane to propene [19]. In these reactions, relevant results have been obtained. In particular, in the SCR of NOx with NH3, vanadium pentoxide catalysts prepared by grafting on TiO2/SiOz supports resulted in much more active and selective catalysts than the corresponding ones obtained by impregnation of metavanadate, both containing 6 % by weight of VzO5 on the same support. This can be appreciated in Fig. 3. It is important to point out that for these catalysts, the redox as well as the acid properties of the catalysts are important, because of the strong basic character of one of the reagents. Therefore, for this reaction, morphological aspects related to the size of vanadium pentoxide clusters seem to be less important, even if no experiments have been made for clarifying this point. On the contrary, many experiments have been made on the other ODH reactions by changing, for example, vanadium loading from 3 % to 10 % by weight, or by changing the vanadium dispersion for the same loading (about 1% by weight). These last catalysts have been prepared by following different already described techniques based on the partial hydrolysis of vanadyl tri-isopropoxide alone grafted on TiOz/SiOz support, or alternatively treated with titanium isopropoxide and grafted, in this case, directly on SIO2. In Fig 4 the catalysts activities in ODH of ethanol to acetaldehyde for two different vanadium loadings of 0.8 and 6 % by weight, are compared together with the ones of catalysts prepared by controlling the extent of vanadium alkoxide hydrolysis [18]. As it can be seen, the selectivities for this reaction are insensitive to the type of catalyst and are correlated only with the extent of ethanol conversion. The activities, on the contrary, are affected both by the amount of titanium loading and the preparation method but, from the characteristics reported in Tables 2 and 4, we can conclude qualitatively that the activity is related only to the available surface of VzO5 for all the catalysts, except for G(VhT4)S which is much less active. Probably, the active sites necessary for this reaction, occurring at very low temperature, are not abundant on this catalyst while, as will be seen later, the same catalyst has better performances in ODH of propane. This means that by changing the preparation method, it is possible to change the type of sites available on the surface and, then, that those sites involved in ODH of ethanol, are reasonably different than those promoting ODH of propane. Concerning ODH of propane, Fig. 5 shows the propane conversions and selectivities to propene obtained at 500~ for, respectively, (i) catalysts having the same vanadium loading (0.8-0.9 % by weight) but different dispersions due to the different methods adopted for the preparation and (ii) catalysts with different vanadium loadings (from 3 to 10 % by weight) [20]. The three catalysts with the same vanadium loading gave comparable conversions but different selectivities, and catalyst G(VhT4)S, the worst for ODH of ethanol, was the best for ODH of propane.

86 The three catalysts with different vanadium loadings gave different activities and selectivities and the catalyst with 6.4% by weight of VzO5 was the most active and selective. A similar trend has been also obtained in the runs performed at 400~ These results demonstrate the sensitivity of ODH of propane to the structure of the vanadium clusters, but it seems that other factors must also be considered, such as the acidity of these clusters and the effect of titanium that, as we have demonstrated, can be amorphous, in this case, and not necessarily present as a homogeneous surface but simply surrounding vanadium oxide sites. 100

Conversion 1,~176 1 U . . ~ 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 ,

0,9 m' # ' ' 0,8

r

.... t.

,

,

i

,

i

.

,

,

i

,

.

I

,

9 t . . . . . -*'--L~_... ..........

0,7

o,1

80

.%,~

.

,

.~

,

.

,

--. ,--T-:--.,

/"~' ,

.

,

.

[-----] propane conversion selectivity to propene

70 60 0,7

.-~ ---9 0,5 "6 (~ 0,4 "~ 0,6

/

,

80

0,8

9

. ~ . m

9O

'q ,0 0,9

"Z"

._O 0,6 m 0,5: (1)> 0,4- - - ' - - G ( V " T 4 ) S I ('I - - , - - GV,TS ] o O0,3 l 0,2 ---"~-0,0

,

.

,

.

,

90 100 110 120 130 140 150 160 170

.

o~

50 40

30

0,3 O')

20

0,2

10

0,1

0

0,0 8O

GVTS1 GVhTS G(VhT4)SAVTS1 AVTS2 AV-;'S3 Catalyst

Temperature (~

Fig. 4. Conversions and selectivities for ODH of ethanol on different catalysts [18].(1.1 cm3/h of liquid ethanol, 7.7 cm3/min of 02, 22 cm3/min of He, 0.5 g of catalyst.

Fig. 5. Activities and selectivities at 500~ for propane to propene (6%02/29% propane plus N2,flow rate 10-80 cm 3 ,0.03g cat. For the first 3 cat.; 5% O2/10% propane and flow rate 50cm3/min for the other 3)

REFERENCES

1. L.L. Hench and J.K. West, Chem. Rev., 90 (1990) 33. 2. S. Klein, S. Thorimbert and W.F. Mayer, J. Catal. 163 (1996) 476. 3. J. Kijenski, A. Baiker, M. Glinski, P. Dollenmeier and A. Wokaun, J. Catal., 101 (1986) 1. 4. M. Schraml-Marth, A. Wokaun and A. Baiker, J. Catal., 124 (1990) 86. 5. U. Scharf, M. Schraml-Marth, A. Wokaun and A. Baiker, J. Chem. Soc. Faraday Trans., 87 (1991) 3299. 6. M.A. Reiche, E. Ortelli and A. Baiker, Appl. Catal. B: Environ., 23 (1999) 187. 7. A. Fernandez, J. Leyer, A.R. Gonzalez-Elipe, G. Munuera and H. Kn6zinger, J. Catal., 112 (1988) 489. 8. S. Srinivasan, A.K. Dayte, M.H. Smith and C.H.F. Peden, J. Catal., 145 (1994) 565. 9. R. Castillo, B. Koch, P. Ruiz and B. Delmon, J. Catal., 161 (1996) 524.

87 10. F. Buonomo, V. Fattore and B. Notari, US Patents, 4,013,589 (1977), 4,013,590 (1977) assigned to Snamprogetti SpA. 11. B.P. Nielsen, J.H. Onuferko and B.C. Gates, Ind. Eng. Chem Fund., 25 (1986) 337. 12. M. Stocker, T.Riis and H. Hagen, Acta Chem. Scandinavia B, 40 (1986) 200. 13. P. Iengo, M. Di Serio, A. Sorrentino, V. Solinas and E. Santacesaria, Appl. Catal. A: Gen., 167 (1998) 85. 14. P. Iengo, M. Di Serio, V. Solinas, D. Gazzoli, G. Salvio and E. Santacesaria, Appl. Catal. A: Gen., 170 (1998) 225. 15. P.Iengo, G. Aprile, M. Di Serio, D. Gazzoli, E. Santacesaria, Appl. Catal. A: General 170 (1999) 97-109 16. A. Sorrentino, S. Rega, D. Sannino, A. Magliano, P. Ciambelli, E. Santacesaria; Appl. Catal. A: General 209 (2001) 45-57 17. K.C. Caulton, L.G. Hubert-Pfalzgraf; Chem. Rev. 90 (1990) 969-995 18. E. Santacesaria, A Sorrentino, M. Di Serio, R. Tesser; Submitted for publication 19. R. Monaci, E. Rombi, V. Solinas, A. Sorrentino, E. Santacesaria, G. Colon; Appl. Catal. A: General 214 (2001) 203-212 20. A. Comite, A. Sorrentino, G. Capannelli, M. Di Serio, E. Santacesaria; Submitted for publication

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89

Combinatorial approaches for speeding up heterogeneous catalyst discovery, optimisation and scaling-up Claude Mirodatos Institut de Recherches sur mirodatos @catalyse.univ-lyon 1 .fr

la

Catalyse

-

CNRS,

Villeurbanne-

France,

Over the past five years, combinatorial chemistry applied to heterogeneous catalysis has been dealt with in more and more articles, reviews and patents (Fig. 1). This methodology remains very controversial, however. Today, within universities as well as within public and private research centres, attitudes toward combinatorial methods run the gamut from fascination to scepticism (or even outright rejection). The debate usually originates from a misunderstanding of the strategies at hand. As such, "combinatorial catalysis" is too often mistaken for a random, undisciplined mixing of various chemicals. On the contrary, the combinatorial approach embodies conventional catalysis, micro mechanics, robotics, analytical methodology and information technology.

Fig 1. Number of publications devoted to combinatorial heterogeneous catalysis. Publication numbers for 2001 are estimated over the first 6 months. The SciFinder (CA) search was performed by entering "combinatorial" and "heterogeneous catalysis" Industry essentially seeks to use the combinatorial approach in order to accelerate the discovery of new materials and reduce time-to-market, and this is generally well accepted. The role of academia, however, remains a matter of debate. Some of the most frequently asked questions are:

90

-

Is combinatorial catalysis an accelerated conventional process f o r catalyst preparation or a new methodology? Does academic combinatorial research aim only at discovering entirely new materials ? Are the formulas discovered by combinatorial approach easily scalable to industrial catalysts? Are creativity and fundamental knowledge still required ofscientists?

This presentation aims to clarify the debate. In a first part, the application of combinatorial chemistry to heterogeneous catalysis is analysed in terms of current strategies and perspectives on the industrial and academic levels. Potential methodologies for academic research laboratories are proposed with emphasis on both theoretical and practical considerations. As a case study, the European consortium "COMBICAT .... Catalyst Design and Optimisation by Fast Combinatorial Procedures" is presented focusing on the chosen strategy [1]. That consortium gathers research institutions with widespread basic knowledge on catalyst development, experienced SME's as specialists for development of chemical research software and high-tech robotics hardware and large catalyst production companies as well as catalyst end users (engineering entities) of the European chemical industry. In a second and main part, various aspects of the running research will be presented: - analysis of the combinatorial approach to heterogeneous catalysis, - strategies and technologies for secondary screening, preparation and testing of catalyst libraries : development of hard and software tools adapted to case studies, - scaling up of "hits" or "leads" discovered via the combinatorial loop to catalysts which have to be validated by conventional methods for further industrial developments. These aspects will be illustrated through the case study of catalyst development for the water gas shift reaction : - initial steps of manual, then robotic preparation of egg-shell formulae, optimisation via a sequence of combinatorial cycles : synthesis of a generation / high throughput testing / data analysis for preparing the further generation, and so -

-

-

-

on,

-

- scaling up of the egg-shell powdered materials to catalyst deposition over a monolith structure to fulfil the operating conditions of the targeted industrial application (GHSV, pressure drop, thermal stability, etc).

As a general conclusion, the importance of robotics with respect to scientific creativity is likely overestimated in the HT approach and the discovery of new materials is likely to come essentially from ideas and strategies combining synthesis, screening, scaling up and further optimisation via data mining. Combinatorial catalysis is not a new field in science, but an interdisciplinary topic involving many different research communities. Its

91 success should rely on combining scientist creativity and advanced technology, leading both to discoveries and to a better understanding of catalysis [2-4]. ACKNOWLEDGEMENTS I. Vauthey, L. Baumes, L. Villegas, A.S. Quiney, S. P. Teh, P. Denton and D. Farrusseng are thanked for having produced most of the presented results and the EU programme "COMBICAT" for granting that study. REFERENCES

1. website of COMBICAT programme: www.ec-combicat.org 2. A. Holzwarth, P. Denton, H. Zanthoff and C. Mirodatos, Catal. Today, 2441 (2001) 1. 3. D. Farrusseng, L. Baumes, I. Vauthey, C. Hayaud, P.Denton, C. Mirodatos, Proceedings of the NATO-ASI Conference, July 16-27/2001, Vilamoura, Portugal, Kluwer, 2002 (in press). 4. D. Farruseng, L. Baumes and C. Mirodatos, Data Management for Combinatorial Heterogeneous Catalysis: Methodology and Development of Advanced Tools, Kluwer, 2002 (in press).

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93

High surface area metal oxides from matrix assisted preparation in activated carbons M. Schwickardi, T. Johann, W. Schmidt, O. Busch, F. Schfith Max-Planck-Institut f/ir Kohlenforschung, Mfilheim a.d. Ruhr, Germany High surface area oxides are attractive materials for numerous applications in catalysis and sorption [1]. There are many techniques to manually prepare these materials, such as precipitation, sol-gel pathways, templating routes and so on [2,3,4,5]. We have developed a novel versatile route which offers a simple and straightforward manner to prepare a great variety of different oxides with even higher surface areas. This method avoids filtering steps and handling of suspensions which enables simple pipette robotic systems to prepare these materials. The method is suitable for the preparation of defined phases, such as spinels or perowskites, but also for the synthesis of amorphous or multiphase mixed metal oxides and can easily be parallelized. 1. I N T R O D U C T I O N High surface area binary and multinary metal oxides are interesting materials which show a high potential for several catalytic applications. The different oxides can either be used as catalyst or as a catalyst support due to their high surface areas. This could lead to new metal oxide supported catalysts with interesting catalyst/metal oxide interactions and therefore potentially new interesting properties. The conventional synthesis of high surface area metal oxide materials is normally performed either by precipitation, sol-gel reactions, or templating routes. Most of these methods require either the handling of large amounts of liquids or suspensions, complex reaction sequences, and/or expensive precursors. For the synthesis of fine chemicals, expensive precursors are not important. For large scale industrial applications, however, where several tons of catalyst may be used, it is of great advantage to avoid expensive precursors or noble metals, and any substance that is potentially dangerous. Because of the complicated preparation, a broad investigation of these metal oxides by high throughput techniques was not possible due to lack of automation. Further, the surface areas of materials resulting from the conventional preparation were sometimes not sufficiently high to be considered for demanding catalytic applications. Recently, we developed a novel preparation method for high surface area metal oxides, which is easy to perform, which does not contain any flammable or explosive compound, which requires no large amounts of liquids, which is applicable for the preparation of binary as well as multinary oxides with high specific surface areas, and which is therefore easily applicable for high throughput screening techniques.

94

2.

PREPARATION

The basic idea of the method is the formation of the oxides by a route in which a supporting material provides confined spaces to restrict the particle size of the oxides formed. This leads to the desired oxide and the supporting material as an exo-template which has to be removed after the internal formation of the metal oxide. Therefore suitable materials had to be found which provided sufficient pore volume for high oxide yield and suitable pore sizes to be impregnated by different metal containing precursor solutions. The support should ideally be removable by gentle procedures which only removes the support without losing the structural information of the templating material by sintering of the oxide or other processes which could harm the small particles and reduce the surface area.

We examined several different materials with respect to the mentioned abilities. For MgO which was used as an oxide model, different supporting materials are listed in Table 1. As can be seen, even corn starch and other well known materials provide good abilities in producing high surface area metal oxides after thermal treatment. However best results were achieved by using certain activated carbons (Table 2). Table 1 Different supporting materials for the formation of MgO and the surface areas of the resulting MgO. MgO-surface area (BET) Impregnated material _ 9m2/g Corn starch 146 m2/g Rice starch 61 m2/g Cellulose-Powder 98 m2/g Agarose 126 m2/g Gelatine-Powder 83 m2/g Polyvinylpyrrolidone 76 m2/g Table 2 Different activated carbon supports for the formation of MgO. Activated carbon BET-surface area ' Yield (g MgO / g Carbon) _ 9m2/g Aldrich: Darco| KB-B 193 m2/g 0.33 Fluka 05120 171 m2/g 0.30 Fluka 03866 57 m2/g 0.045 , ,

....

Further experiments were performed by using Fluka 05120 or Darco| activated carbon. As the TEM pictures show, small particles are formed which can be detected before and after calcinations (Fig. 1). The concentration of the precursor solution and even the thermal treatment before the calcination have a significant influence on the material due to the pre-formation of the precursor particles within the support. We indeed found differences in the surface area

95 which were related to the drying process of the wet support and the concentration of the precursor solution as can be seen in Table 3.

Fig. 1. Activated carbon impregnated with precursor solution of NiAl204.

Fig. 2. NiA1204 aiter calcination at 800 ~

Table 3 Influence of different concentrations of Mg(NO3)2 precursor solution on the BET surface area. ' c0neentration . . . . . . . . BET-surface a r e a Yieid (g Mg() / g'cari~oni.......... saturated saturated solution diluted by factor of 2 saturated solution diluted by factor of 5 .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

198 m2/g 168 m2/g 102 m2/g

0.36 0.19 0.07 ,

For some compounds like titanium and zirconium, alkoholates are the best compounds, but for most of the metals we found that very good results can be achieved by using highly concentrated nitrate solutions. This is very important because most of the metal species are available as nitrate compounds, which are normally quite stable and can easily be handled in air. Experiments with halogenides and acetates showed a decreased surface area compared to the nitrates. As can be seen in Table 3, the surface area is higher for higher concentrated metal salt solutions. In some cases, like iron oxides, even molten nitrate compounds can be used and successfully turned into high surface area oxides. The thermal treatment should be as gentle as possible to avoid sintering processes which occur at high temperatures. In air the activated carbon normally requires temperatures above 600 ~ for full combustion. However, we found a strong catalytic effect of most of the metal precursors which decreases the maximum temperature of the full combustion down to 500 ~ for selected compounds. Some examples are shown in Fig. 3. We further found differences in the maximum temperature which were related to

96 the treatment of the impregnated material before calcination. These results are outlined in Fig. 3 as well.

TG [%] 100-

~"a,..,. ............ ., ~ Activated carbon ' ~ . " k , . . . Mg(NO3) " ,, 80__ \ ~'., ........ 9............. ", Cr(NO3)3 dried 60. ":,_., " L-;..--- _.... ""'..,.... ~ _ Water 40Cr(NO3)3 " ' ~ ' " ~ : . ~ . ""~-.:~. N 'x 20-Cu(N~O3)2 TM .'7",,~~~~.2.2.1.L..Z.~...~.:._..~...2.._..._..~..._..:

i

100

230

3~)0

' 400

l

' 500 600 Temperature [~

' 700

' 800

' 900

1000'

Fig. 3. Thermogravimetric measurement of activated carbon impregnated with different precursor solutions. A further advantage of nitrates as precursors is their decomposition at relatively low temperatures. We found that this decomposition always took place before the combustion of the activated carbon (Fig. 3). We further found an increase in surface area if the combustion is performed slowly under reduced oxygen partial pressure. This could either be due to uncontrolled combustion which leads to local overheating and therefore local sintering processes or due to combustion of the templating carbon matrix before the oxide is finally formed. For metals which show a strong effect on carbon combustion and therefore a violent reaction which reduces the surface area when p2erforming the calcination under air, such as chromimn, we achieved surface areas of 155 m / g when performing part of the synthesis under argon. The impregnated carbon was first heated up to certain temperatures where the nitrates are decomposed and afterwards the partial pressure of oxygen in the feed was slowly raised up to atmospheric conditions. 3. PARALELLIZATION To use the activated carbon procedure in high throughput catalyst screening we designed a 77 well plate with quartz vials and set up a pipette robot with concentrated metal nitrate solutions (Fig. 4). A computer based algorithm which automatically produces random metal composition has been developed which directly controls the robot. Subsequently, the plate can be transferred to an oven for calcinations. This enables the production of 77 new

97 materials in less than one day which is basically limited by the cooling period of the calcination oven.

Fig. 4. Set up of the pipette robot for automated preparation of activated carbon based metal oxides. 4. MATERIALS

After calcinations at 450-500 ~ the materials basically consist of small crystalline particles. These sometimes show very broad reflections in XRD due to the small particle size which is below 5 nm at low temperatures. However, XRD patterns show increasing particle size with increasing temperatures. This is consistent with decreasing values in sorption measurements due to sintering processes. The synthesis of pure spinel or perovskite phases (Table 4) could be performed by mixing the desired molar ratios of the materials and applying temperatures above 700 ~ Table 4 Selected metal oxides prepared by the activated carbon route. For further information see experimental section. ........... Composition Phase ....... 13ET'surfa'ce ar'ea [m~g'''I]. . . . . MgO MgO 200 Fe203 maghemite/hematite 120 Cr203 eskolaite 160 TiO2 anatase/rutile 200 ZnCr204 spinel 70 CoA1204 spinel 130 NiAl204 spinel 240 LaFeO3 (perovskite) ~ , 50

98 To find optimal conditions for the preparation of the spinel phases, various calcination temperatures and times have been tested. Some examples are listed in Table 5. Table 5 Surface areas and calcinations temperatures of CoCr204 and NiA1204. CoCr204 Calcination procedure 500 ~ 600 ~ 45 min 600 ~ 2 h 700 ~ 800 ~ min 800 ~ 1 h

Niml204 surface area (BET) 126 m2/g 111 m2/g-107 m2/g . . . . . 89 m2/g 77 m2/g 68 m2/g ,,,

Calcination procedure 700 ~ 800 ~ 1 min 800 ~ 15 min 800 ~ h

,......

" .

800 ~ .

.

.

.

.

.

.

Surface area (BET) 236 m2/g 234 m2/g 220 m2/g 192 m2/g

5h .

.

.

.

165 m2/g .

.

.

.

.

.

.

.

.

XRD analyses of mixed oxide samples showed that spinel structures or mixtures of spinel phases and binary oxides occurred at a calcination temperature of 500 ~ However, at such low temperatures an amorphous character of part of the sample is very likely. The crystalline part of the samples consists of small particles of about 5 nm in size according to TEM studies. These oxides showed interesting catalytic activities in CO-oxidation and are currently a topic of intense investigation.

5. EXPERIMENTAL 5.1. General procedure The activated carbon was treated in two different ways. Method A: The activated carbon was dried at 100~ / 0.1 mbar and stored under Argon. 1.7 - 1.8 g were impregnated with 7 ml of a saturated aqueous metal nitrate solution. The mixture was magnetically stirred for 20 min. The mushy dispersion was filtered by applying vacuum and using a commercially available filter paper. The residual solid was mechanically squeezed and dried for 30 min at 80 ~ in an oven. Some metal nitrates like iron and chromium show nitrous oxide evolution. Method B: The activated carbon was directly used for the synthesis. 1.87 g of the carbon were drop by drop impregnated with 4 ml of a saturated metal nitrate solution. The mixture was manually kneaded. The crumbly mixture was then pyrolyzed in an oven.

5.2. Preparation of selected metal oxides MgO: Activated carbon (Aldrich Darco| KB-B) was impregnated by an aqueous saturated magnesium nitrate solution (method B). The impregnated carbon was calcined without being dried (1 h at 500 ~ rate 2 ~ / min).

99 Product: white, 0.36 g / g carbon XRD: broad MgO P2attem N2-sorption: 201 m / g (BET) Fe203: Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated Fe(NO3)3 solution (method A) and dried at 80 ~ for 1 h. During drying, nitrous oxide is produced. The impregnated carbon was then calcined (1 h at 450 ~ rate 2.4 ~ / min) and taken out of the oven while hot. Product: reddish brown, 0.29 g / g carbon XRD: Fe203 maghemite + hematite pattem N2-sorption: 123 m2/g (BET) Cr203: Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated Cr(NO3)3 solution (method A) and dried at 80 ~ for 1 h. During drying, nitrous oxide is produced. To maintain the structural properties of the activated carbon the sample was heated under protective atmosphere up to 450 ~ at a rate of 3 ~ / min and kept there for 30 min. The sample was then slowly calcined within 1 h with slowly increasing air ratio. Product: green, 0.31 g / g carbon XRD: Cr203 escolaite, syn pattern N/-sorption: 156 m~/g (BET) TiO2:1.76 g of vacuum dried activated carbon (Fluka 05120) was impregnated under Argon by 3.0 ml of titania(IV)-butylate (Aldrich) and exposed to air. It was calcined for 1 h at 450 ~ (rate 3 ~ / min) and taken out ofthe oven while hot. Product: 0.45 g / g carbon XRD: TiO2 anatase, syn pattern N2-sorption: 200 m2/g (BET) ZnCr204". Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated Zn(NO3)E/Cr(NO3)3 (Zn 9Cr, 1 : 2) solution (method A) and dried at 80 ~ for 1 h. The impregnated carbon was then calcined (15 min at 800 ~ rate 3 ~ / min) and taken out of the oven while hot. Product: grey green, 0.33 g / g carbon XRD: spinel pattern N2-sorption: 72 m2/g (BET)

CoCr204: Dried activated carbon (Aldrich Darco| KB-B) was impregnated by an aqueous saturated Co(NO3)ffCr(NO3)3 (Co:Cr; 1:2) solution (method A) and calcined without drying. The impregnated carbon was then calcined (1 h at 500 ~ rate 3 ~ / rain) and taken out of the oven while hot. Product: 0.35 g / g carbon XRD: spinel pattern N2-sorption: 126 m2/g (BET)

100

NiA1204: Dried activated carbon (Aldrich Darco| KB-B) was impregnated by an aqueous saturated Ni(NO3)2/AI(NO3)3 (Ni:Al; 1:2) solution (method A) and calcined without drying. The impregnated carbon was then calcined (1 h at 700 ~ taken out of the oven while hot. XRD: spinel pattern Nz-sorption: 236 m2/g (BET)

rate 3 ~ / min) and

LaFeO3: Dried activated carbon (Fluka 05120) was impregnated by an aqueous saturated

La(NO3)3/Fe(NO3)3 (La:Fe; 1:1) solution (method A) and dried at 80 ~ for 1 h. The impregnated carbon was then calcined (20 min at 700 ~ the oven while hot. Product: brown XRD: perovskite pattern Nz-sorption: 49 m2/g (BET)

rate 3 ~ / min) and taken out of

5.3. Parallel preparation The activated carbon used for impregnation was Fluka 05120 and Darco| KB-B, 100 supplied by Aldrich. Metal compounds were hydrates of nitrate salts supplied by Fluka. The precursor solutions were all saturated. Catalyst preparation was automatically performed using a commercial pipette system (Gilson XL 232) which distributed the metal salt solution to quartz vials filled with certain amounts of activated carbon. The carbon was transferred into the vials by using fixed volumes which were equal to 230 mg (+ 10 mg) in weight. We used a total amount of 450 ml of mixed precursor solution. The composition of the precursor solution was determined by a random algorithm which first determines whether a precursor is chosen as an ingredient or not, and afterwards which quantity of the chosen precursor contributes to the total composition. The last step is a calculation for the pipette robot which directly gives volumetric values for the robot. The impregnated samples were then calcined on a 77 well plate covered with gauze to avoid oxide particles from being expelled during carbon combustion. Samples were dried at 90 ~ overnignt and slowly heated to 500 ~ within 3 h. After remaining at 500 ~ for 2 h, which led to fully combustion of the carbon, they were allowed to cool down. REFERENCES 1. A. Beretta and P. Forzatti, J. Catal., 200 (2001) 45. 2. F. Schiith, in: Handbook of Porous Solids, F. Schfith, K. Sing, J. Weitkamp (Eds.), Wiley-VCH, Weinheim, 2002, in print. 3. T.J. Barton et al., Chem. Mater., 11 (1999) 2633. 4. M.A. Valenzuela et al., Appl. Catal. A:General, 148 (1997) 315. 5. C. Otero Ardan et al., Mater. Lett., 39 (1999) 22.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

101

Effects of the impregnating and drying process factors on the mechanical properties of a PCoMo/AI203 hydrotreating catalyst D. Wu, and Y. Li* Department of Catalysis Science and Technology and State Key Lab on C1 Chemical Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China Mathematical models for the mechanical properties of a PCoMo/AlaO3 hydrotreating catalyst in the impregnating and drying processes are developed with a response surface methodology. A central composite design is performed to study collectively the effects of process factors on the mean strength, Weibull modulus, and pellet density of the catalyst. Analysis of variance reveals that the models developed are adequate. The validity of the models is verified by experimental data. Results show that the impregnating and drying process factors have great effects on the mechanical properties and pellet density, and that there exists a great potential for increasing the mechanical reliability at low density. 1. I N T R O D U C T I O N The mechanical strength of solid catalysts is one of the key parameters for the efficient performance of a fixed bed converter [1]. Solid catalysts are typical brittle materials, and their mechanical failure is due to brittle fracture arising from a sudden catastrophic growth of a critical flaw under tensile stress induced in the catalyst bulk phase [2, 3]. Variations of size, shape and orientation of the flaws, e.g. pores, defects, dislocations and discontinuations, result in a large scattering range of the strength data. It has been proposed that the mechanical strength data of solid catalysts can be described well by Weibull

Pf (L) = l - e x p ( - / 3 L m )

(1)

distribution [2-4], where Pf is the probability of failure, L the load at failure, m Weibull modulus, [3 a size parameter. The higher the Weibull modulus, the smaller is the scattering range of the strength data. During catalyst production, various process factors have effects on the material properties and microstructures of catalyst particles, and hence have effects on the Corresponding author. E-mail: [email protected] (Y. D. Li).

102

mechanical properties. In the literature, several authors have studied the effects of some factors for several catalysts [5-7]. In previous publications from this group, the mechanical strength of a high-temperature water-gas shift catalyst was optimized in the calcination and reduction processes [8, 9]. In this work, the factors in the impregnating and drying of a PCoMo/AI203 hydrotreating catalyst were examined. A response surface methodology (RSM), an empirical statistical modeling technique [10, 11], was used to develop the mathematical models relating the process factors to the mechanical properties. 2. E X P E R I M E N T A L

2.1. Impregnating and drying The PCoMo/A1203 catalyst was prepared by a co-impregnation technique using pore filling method. The support was commercial ~-A1203 (3.82 mm cylindrical extrudates). After drying at 200 ~ for 3 h, 30 g of support was co-impregnated with 60 ml of a mixed aqueous solution of ammonium heptamolybdate, cobalt nitrate and phosphoric acid. After impregnation, the solids were left at room temperature for 2 h. The drying in air was carried out in a drying cupboard, with putting in the samples after the temperature reached the preset value. Table 1 Experimental range and levels of the factors Factors

Units

Notations Ti

Range and levels -2 -1 0 20 35 50

Impregnating temperature Impregnating time Drying temperature Drying time

~

+1 65

+2 80

h ~ h

ti To td

4 100 4

10 175 10

12 200 12

6 125 6

8 150 8

2.2. Experimental design Four experimental variables, i.e. impregnating temperature, impregnating time, drying temperature and drying time were selected as controlling factors. Table 1 gives their experimental range and levels, where each factor was coded linearly so that the upper and lower limits were +2 and -2. A central composite rotatable design [10], showed in Table 2 was used, which consists of 31 experiments. The xl, x2, x3 and x4 are coded notations of the above four factors respectively. The results were modeled with a second-order polynomial equation, i.e. each responsey was fitted by a quadratic model, given as follows for four factors. 4 j=l

4

i<j

j=l

103

w h e r e y is the p r e d i c t e d response, b0 the offset term, effect, and

bjj

bj

the linear effect,

b O.the

interaction

the s q u a r e d effect. In this work, the m e a n strength, W e i b u l l m o d u l u s , and

pellet density w e r e o b t a i n e d as the responses of the d es i g n e d e x p e r i m en t . Table 2 Central c o m p o s i t e rotatable design and e x p e r i m e n t al results No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Design matrix

Experimental results

X1

X2

X3

X4

-1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 -2 2 0 0 0 0 0 0 0 0 0 0 0 0 0

-1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1 1 1 1 1 0 0 -2 2 0 0 0 0 0 0 0 0 0 0 0

-1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 0 0 0 0 -2 2 0 0 0 0 0 0 0 0 0

-1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 0 0 0 0 0 0 -2 2 0 0 0 0 0 0 0

~9, g/cm 3

1.72 1.78 1.70 1.72 1.82 1.83 1.74 1.79 1.78 1.81 1.71 1.74 1.78 1.86 1.74 1.72 1.70 1.78 1.76 1.87 1.82 1.74 1.77 1.76 1.70 1.72 1.79 1.76 1.77 1.75 1.77

L-, N 24.9 26.5 39.2 40.6 37.3 41.0 33.6 35.3 30.0 35.0 35.2 34.5 36.1 36.6 34.0 35.2 33.7 28.3 38.6 40.1 36.6 43.6 33.5 31.8 32.3 30.9 31.4 29.1 28.2 28.4 33.5

m

[3

Pf(5)

Pe(10)

4.63 3.74 4.53 4.14 4.32 3.28 4.89 3.90 2.77 2.79 3.33 3.55 3.61 2.86 3.53 3.32 5.01 3.76 3.53 4.17 4.04 3.59 3.70 3.28 3.66 3.37 3.60 4.07 3.63 3.98 3.63

2.25x10 -7 3.26x10 -6 3.96x10 -8 1.45x10 -7 1.06x10 -7 3.54x10 -6 2.22x10 -a 6.37x10 -7 5.82x10 -s 3.57x10 -s 5.02x10 -6 2.42x10 -6 1.61x10 -6 2.46x10 -s 2.75x10 -6 5.15x10 -6 1.43x10 -8 2.39x10 -6 1.71x10 -6 1.39x10 -7 3.20x10 -7 8.86• -7 1.57x10 -6 8.29x10 -6 2.05x10 -6 6.52x10 -6 2.77x10 -6 7.52x10 -7 3.75x10 -6 1.13x10 -6 2.02x10 -6

3.89x10 -4 1.34x10 -3 5.84x10 -s 1.14x10 -4 1.11xl0 -4 6.99x10 -4 5.83x10 -s 3.36x10 -4 5.02x10 -3 3.19x10 -3 1.06x10 -3 7.30x10 -4 5.37x10 -4 2.44x10 -3 8.03x10 -4 1.08• -3 4.55x10 -s 1 . 0 1 x l 0 -3 5.03x10 -4 1.14x10 -4 2.14x10 -4 2.87x10 -4 6.02x10 -4 1.63x10 -3 7.39x10 -4 1.49x10 -3 9.14x10 -4 5.23x10 -4 1.29x10 -3 6.78x10 -4 6.94x10 -4

9.59x10 -3 1.77x10 -2 1.35x10 -3 2.02x10 -3 2.23x10 -3 6.79x10 -3 1.73x10 -3 4.99x10 -3 3.37x10 -2 2.19x10 -2 1.06x10 -2 8.51• .3 6.54x10 -3 1.75x10 -2 9.22x10 3 1.07x10 -2 1.47x10 -3 1.36x10 -2 5.80x10 -3 2.05x10 -3 3.51x10 -3 3.46x10 -3 7.78x10 -3 1.57x10 -z 9.28x10 -3 1.53x10 -z 1.11xl0 2 8.72x10 -3 1.59x10 -2 1.06x10 -z 8.54x10 -3

2.3. Determination of the responses The

mechanical

strength

was

measured

with

a ZQJ-II

strength

tester

described

d e t aile dly e l s e w h e r e [3]. A k n i f e - e d g e cutting strength was adopted, since it is m o r e suitable for the e x t r u d a t e s than the c r u s h i n g s t r e n g t h [3]. For each s a m p l e 30 pellets w e r e

104

tested to obtain representative mechanical properties [12]. Mean strength was determined by arithmetical average of the strength data. Weibull parameters were estimated with linear regression [3]. A scale with an accuracy of 0.01 mm was used to measure the pellet size, and a balance with a precision of 0.1 mg was used for determining the pellet weight. The pellet density was determined with 5 pellets using the weight-volume method. Table 3 Multiple regression results and significance of regression coefficients Term

Xl

b0 bl

xz

b2

X3 X4

b3 b4 X1X2 hi2 XlX3 hi3 XIX4 b14 XzX3 b23 XzX4

X3X4 Xl2 z xz 2 X3 z X4

Mean strength Coef. t 30.8 45.8 -0.529 -1.07 1.09 2.21 1.42 2.87

P 0.000 0.297* 0.037 0.009

-1.12

-1.85

0.077

-2.88

-4.75

0.000

b24 b34 bll bz2

b33 b44

1.99 2.18

4.44 4.85

Weibull modulus Coef. t P 3.71 45.1 0.000 -0.425 -7.03 0.000 0.062 1.03 0.313" 0.095 1.57 0.131 -0.203 -3.36 0.003

0.163

2.20

0.039

-0.122

-1.65

0.113

0.139

2.53

0.019

Pellet density Coef. t 1.75 277.6 0.0095 1.69 0.023 4.11 -0.028 -5.04 0.0098 1.75 -0.011 -1.62

0.014

0.000 0.000 -0.085

-1.56

2.69

P 0.000 0.104 0.000 0.000 0.093 0.119

0.013

0.134

R 0.881 0.887 Hierarchical terms added after backward elimination regression

0.841

Table 4 Analysis of variance (ANOVA) Source Mean strength SS a DFb MSc F P REG a 470.0 7 67.1 11.40.000 RESe 135.2 23 5.88 LF f 110.4 17 PE g 24.88 6 a

6.49 4.15

1.560.302

Weibull modulus SS DF MS F P 7.14 8 0.89 10.2 0.00 1.93 22 0.09

Pellet density SS DF MS F P 0.044 6 0.0073 9.67 0.000 0.018 24 0.0008

1.59 16 0.34 6

0.013 18 0.006 6

0.10r. 77 0.247 0.06

0.0007 0.0009

0.75

0.705

Total 605.2 30 9.07 30 0.062 30 Sum of squares; b Degrees of freedom; c Mean square; a Regression; e Residual; f L a c k o f

f i t ; g P u r e error.

The significance of the coefficients was tested with Student's t-tests [11]. The P-value is used as a tool to check the significance of each coefficient or corresponding model term. Firstly, x2x3, x22 and x32 have the most significant effects on the mean strength (P=0.000),

105 followed by x3 (P<0.01) and then by xz (P<0.05). Secondly, Xl has the largest effect on the Weibull modulus (P=0.000), followed by x4 (P<0.005). The terms of Xl2 and XlX4 are also considerably pronounced (P<0.05). Finally, x2 and x3 make the greater contributions towards the pellet density (P=0.000), followed by x22 (P<0.05). The model adequacy was checked with analysis of variance (ANOVA) [11], as shown in Table 4. Due to replicates at the center point of the design domain, the significance of the lack of fit can also determined. P-values for the lack of fit imply that three models exhibit no significant lack of fit relative to the pure error, even at 80% confidence level (P>0.20). P-values for the regression indicate that all the models are greatly significant (P=0.000). Model reduction, i.e. exclusion of insignificant terms from full model improves model adequacy. Consequently, the models developed can be used to navigate the design space. 3.2. Prediction and confirmation An additional experiment was chosen to verify the validity of the models and to check their predictive ability, with the condition of Xl = xz = x3 - x4 = 0.5, i.e. Ti = 57.5 ~ ti = 9 h, To = 162.5 ~ and td = 9 h. The model-predicted and the experimental values of the responses are shown in Table 5. The observed values are within the 95% prediction interval, and within the 95% confidence interval of the predicted ones. The observed values are in good agreement with the predicted ones; therefore the three models are valid and have satisfactory predictive ability. It is also confirmed that RSM is an effective tool for the mathematical modeling of the mechanical properties of solid catalysts. Table 5 Comparison between experimental and model-predicted values Responses

Experimental Predicted

Mean strength Weibull modulus Pellet density

33.00 3.388 1.760

31.84 3.495 1.761

95% Confidence interval 30.32-33.36 3.30-3.69 1.74-1.78

95% Predictioninterval 26.60-37.08 2.85-4.14 1.70-1.82

4. DISCUSSION 4.1. Statistical properties of the mechanical strength Statistics shows that the knife-edge cutting strength data of all the samples follow the Weibull distribution well. It can be deduced that there is still a single flaw distribution existed in the samples after impregnation and drying. Weibull statistics provides a method for calculating the failure probability of a catalyst pellet under a specific load. Table 2 gives the probabilities at 5 and 10 N/pellet. These probabilities in low loading range can be used as an index to the mechanical reliability of solid catalysts. The values of Pf(5) and Pf(10) in Table 2 show remarkable differences between the samples obtained at different operating conditions. The Pf(5) and Pf(10) of sample 17 with

106 the largest Weibull modulus are 2 and 1 orders of magnitude lower than those of sample 9

r~

l~176t

8.0X10-4

/ I

9

~

~ 6"0X10-41

/11~

4~176

//5;/

.....................

~85

I "

.80

1+,7+

>,

~- 2.0x10 .4

1.72

00

1.74

1.78 ~

40

0 2 4 6 8 10 The knife-edge cutting strength N/pellet '

1.78

10 O,"~"

/8

irn;;ea20 ~ 1 1 : : ~ j 6 ^~,~,~eo~ o"~t~ng ternn 70 n,, 4 ~',e'e perature, oC ou \~-,, Fig. 2. Response surface and contour plot of the pellet density.

Fig. 1. Weibull distribution curves of the typical samples in low loading range

with the lowest Weibull modulus respectively. However, the two probabilities of sample 22 with the largest mean strength are very close to those of sample 1 with the lowest mean strength. Fig. 1 gives the probability of strength failure of the typical samples in low loading range. These results reveal that the probability of strength failure in low loading range is mainly related to the Weibull modulus. The mean strength shows less effect on the probability of failure than the Weibull modulus. Therefore, the Weibull modulus is a decisive parameter for the mechanical reliability of solid catalysts. The density of the samples shows no obvious relationship with the mean strength and

60

.................................. . . . . . . . . . . ~. . . . . . . . . . . / - .

Z

g

50 "+ . .........

". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45

z

. . . . . . . . . . .

_E 40

........

m 31

c-

2u

i

.

". . . . . . . . . . .

............

................ ,,4,..,~---~ ~_.~.~._..-----~ ~

4 5 67 ~ ~ ~ ~ - ~ 4 0

35

~

j//

~i

200 o0 2 80 e 160 x~' 2n ~;~:_~_____

140

~r~

ir~ref .._8 ~ 4 8 / 1 2 0 .,,o~~ -~,latit :1t i ~ l ~ lO0 :.r Urne, h ]z "~ a

9 30

u 30 ~

-- 31~ _ ~ . . . . . . /

~

3 3 ~ ~

200 eO 160 x~" ~

140

~

Irnpre.._,9OatZ zlU 50 " ~ t ) U 37.____.7120 ~.e~"~ ng temp e 70 ^_ 100 .~o~ b

Fig. 3. Response surfaces and contour plots of the mean strength.

oc

+

107 with the Weibull modulus. Sample 17 has the lowest density, while its Weibull modulus is the highest in these samples. This indicates that it is possible to increase the mechanical reliability at low density. 4.2. Effects of the factors As can be seen from Table 2, after impregnation and drying, the Weibull moduli of all m the samples decrease, as compared with the original A1203 support (L = 40.17 N/pellet, m - 7.937 and / 3 - 1 . 3 5 • -13). Except sample 4, 6 and 22, the mean strength of the others decreases too. Thereby the impregnation and drying are unfavorable to the mechanical properties of solid catalysts. The results in Table 2 also show that the process factors have great effects on the mechanical properties and pellet density of the catalyst. To facilitate a straightforward examination of the effects of the factors, three-dimensional response surfaces and their corresponding contour plots were constructed using the models developed, with two factors kept at zero level and changing the other two in the design domain, as illustrated in Figures 2-4. These figures can be used to predict the responses for different values of the factors and to identify the interactions between the factors. The response surfaces in Figs. 2 and 3b are a part of distorted parabolic cylinder, which show a minimum ridge in the experimental domain. The response surface in Figure 3a is an inverted paraboloid (dome), and the corresponding contour plot is elliptical. The stationary point, which is the point at which the slope of the response surface is zero when taken in all directions, on this surface is within the design domain; however, it is a minimum point. The response surfaces in Figs. 4a and 4b have a saddle behavior or minimax nature. The stationary point is not a maximum or a minimum point, but a saddle point. According to these figures along with the models, several remarks can be made. Firstly, as the impregnating time or drying temperature increases, the mean strength decreases at first, and then increases. The mean strength takes a minimum at about t i - 7 h and To -

6 5

....

4

...........

~. . . . . . . . . . . . . . .

ffl

,g~ E

=1 :::3 "1:3 0

....

4.0

,,

_

.-.....,.

,,., ,-,. . . . .

__

s 3.5.........

12

N 3.o

12 1 ......" ~ ^ . /

2 " 5 ~ ~ \

rature oC

80

/ . 3 . 7 : _ . q , q ~ (

/~'~

4.0______..._______5/'8

45 , r n ~ 6

b Fig. 4. Response surfaces and contour plots of the Weibull modulus.

10

~

108 140 ~ In the experimental domain examined, the impregnating temperature has no significant effect on the mean strength. There is no the term of the drying time in the model, thus the factor has no effect on the mean strength. Secondly, the Weibull modulus increases with the increase of the impregnating time and with the decrease of the impregnating temperature. The effect of the drying time on the Weibull modulus is complex, but statistically less significant than the other factors. For the Weibull modulus, the drying temperature has no squared effect and interaction effect with others. The linear effect shows that as the drying temperature increases the Weibull modulus increases. Finally, the pellet density increases with the increase of the impregnating temperature. As the impregnating time increases, the pellet density decreases at first, and then increases. The model for pellet density reveals that the pellet density increases with increasing drying time and with decreasing drying temperature. 5. CONCLUSIONS RSM was used to develop mathematical models for the mechanical properties of a PCoMo/AI203 hydrotreating catalyst in the impregnating and drying processes. A central composite rotatable design was conducted to study simultaneously the effects of process factors on the mean strength, Weibull modulus and pellet density of the catalyst. A model is obtained for each response with backward regression method. The adequacy of the models is checked with the analysis of variance. The validity of the models is verified also by experimental data. Statistics of the mechanical strength reveals that the Weibull modulus is a decisive parameter for the mechanical reliability of the catalyst. Results show that the impregnation and drying are unfavorable to the mechanical properties. The impregnating and drying process factors have great effects on the mechanical properties and pellet density, and there exists a great possibility in increasing the mechanical reliability at low density. It is also confirmed that RSM is a good and useful method in mathematical modeling and factors analysis of the mechanical properties of solid catalysts. REFERENCES 1. S.ES. Andrew, Chem. Eng. Sci., 36 (1981) 1431. 2. Y.D. Li, X.M. Li, et al., Catal. Today, 51 (1999) 73. 3. Y.D. Li, D.E Wu, et al., Powder Technol., 113 (2000) 176. 4. W. Weibull, J. Appl. Mech., 18 (1951) 293. 5. EK. Gupta, D.S. Chabbra and A.C. Sengupta, Fertilizer Technol., 18 (1981) 193. 6. E.L. Furen, D.V. Gernet and T.A. Semenova, Kinet. Katal., 13 (1975) 107. 7. B.M. Fedorov, V.I. Nekhoroshev, et al., Kinet. Katal., 33 (1992) 157. 8. Y.D. Li and L. Chang, Ind. Eng. Chem. Res., 35 (1996) 4050. 9. Y.D. Li, R.J. Wang, et al., Catal. Today, 30 (1996) 49. 10. G.E.E Box, W.G. Hunter and J.S. Hunter, Statistics for experimenters: an introduction

109 to design, data analysis and model building, Wiley, New York, 1978. 11. R.H. Myers and D.C. Montgomery, Response surface methodology: process and product optimization using designed experiments, John Wiley & Sons, New York, 1995. 12. D.E Wu, Y.D. Li, et al., Chem. Eng. Sci., 56 (2001) 7035. 13. J. Piexoto, The American Statistician, 44 (1990) 120.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Influence of C e O 2 decomposition

content

on R h / T i O 2

111

monolithic

catalysts for

N20

S. Su~rez a*, M. Yates a, F.J. Gil Llambias b, J.A. Martin a, P. Avila a, J. Blanco a. a* Instituto de Catfilisis y Petroleoquimica, CSIC, Camino de Valdelatas s/n, 28049 Madrid, Spain, Tel.: 34 91 585 48 80, Fax: 34 91 585 47 60, e-marl: [email protected] b Facultad de Quimica y Biologia USACH, Alameda del Libertador Bernardo O'Higgins, 3363, Chile, Tel.: 2-681 25 75, Fax: 2-681 21 08, e-mail: [email protected] In this study the properties of TiOz-CeO2 samples with a cerium oxide content between 0-15 wt.%, prepared by wet impregnation, were analysed by electrophoretic migration technique, XRD, thermogravimetry, N2 adsorption and mercury intrusion porosimetry. The electrophoretic migration results, showed that the sample with a cerium oxide content around 2 wt.%, present a zero point charge higher than that of the single oxides. Also, a maximum in the BET area, pore volume and XRD intensity of TiOzanatase peaks was observed for this sample. Taking into account these results, Rh/TiOzCeO2 monolithic catalysts with different CeO2 content and silicate as binders, were prepared and tested in N20 decomposition reaction. A maximum in NzO conversion was achieved for catalysts with a CeOz content around 2 wt.% with respect to TiOz. The monolithic catalysts behaviour was related with the properties of TiOz-CeO2 powder samples and it is suggested that the formation of an new Ti-O-Ce phase could be responsible for the different properties of this system. 1. I N T R O D U C T I O N In recent years, NzO has been recognised as a contributor to the destruction of stratospheric ozone and as a greenhouse gas [1]. Combustion processes and chemical production such as adipic and nitric acid are among the major anthropogenic sources of NzO emissions [2]. For the year 2010, this emission should be reduced by around 60-90 % [3]. The catalytic decomposition of N20 to nitrogen and oxygen is the preferable technology for nitrous oxide elimination from stationary sources. For this purpose, although catalysts with different metals (Fe, Co, Ni) have been tested [4-7], those based on Rh have shown excellent results especially in the presence of oxygen and water in the gas mixture [8-10]. Although displaying good catalytic properties even at low temperature, at present there are no catalysts used industrially. Honeycomb catalysts have been extensively employed in industrial clean up processes because they offer attractive cost and operational advantages [11].

112 Thus, the main aim of this work has been to study the effect of CeO2 concentration on TiO2 properties, using powdered samples, to determine the optimum composition of Rh/TiO2-CeO2 monolithic catalysts. The studies carried out with powdered samples were critical to analyse the catalytic behaviour in the N20 decomposition reaction. 2. EXPERIMENTAL

2.1 Sample preparation In order to determine the effect of CeO2 content on the titania properties, TiO2CeO2 powder samples (0< [CeO2]< 15 wt%) were prepared by incipient impregnation of TiO2 with Ce(NO3)3 and subsequently heat treated at 500~ in air. TiO2-CeO2 monolithic supports were manufactured by extrusion of doughs prepared by kneading TiO2-Ce(NO3)3 mixtures with natural silicates as permanent binders and water, varying the CeO2 content between 0-4 wt%. After heat treatment at 500~ in air, catalysts were prepared by impregnation of the formed monoliths with an aqueous solution of RhC13"H20 and heat treated at 450~ for 4 hours in NH3/N2 gas stream. The final Rh concentration was 0.15 wt%. All monolithic catalysts had the following geometric dimensions: square cell size 6.76 mm 2, wall thickness 1.0 mm, geometric surface area 8.02 cm 2 cm "3, and cell density 7.72 cells cm -2.

2.2 Characterisation techniques The cerium and titanium oxide contents were determined by inductively coupled plasma (ICP) optical emission spectroscopy (Perkin-Elmer Optima 3300DV) of dissolution of the ground catalysts in acid solutions. The composition of the powder samples and monolithic supports are shown in Tables l a) and 2 respectively. The isoelectric points (IEP) of the individual materials and the zero point charge (ZPC) of the mixtures as defined by Parks [12], were determined by electrophoretic migration, measuring the zeta potentials as a function of the solution pH, as in previous studies [ 13] using a Zeta-Meter Inc. Instrument model 3.0+. Experiments were determined with 30 mg of approximately 2 ~tm diameter particles, suspended in 300 ml of 10.3 M KC1, adjusting the pH value with 0.2 M KOH and HC1 solutions. Each curve obtained was recorded at least twice to ensure reproducibility. Surface areas were measured by nitrogen ad/desorption with a Carlo Erba Sorptomatic 1800 on samples previously outgassed at 250~ overnight. The pore volume of monolithic catalysts were determined by mercury intrusion porosimetry (MIP) using CE Instruments Pascal 140/240 porosimeters, after drying the samples in an oven at 110~ overnight. The TG-DSC curves were measured on a Netzsch 409 EP Simultaneous Thermal Analysis device, using 20 mg of powdered sample. To analyse the effect of heat treatment of the samples, the experiments were carried out heating in air at a rate of 5 ~ rain -1, from ambient to 1000~ using c~-alumina as a reference. X-ray diffraction (XRD) patterns of ground samples were recorded on a Phillips PW 1710 powder diffractometer, using CuK~ radiation: (~, = 0.154 nm), between 20-80 ~ sampling data every 20 = 0.02 ~ with an accumulation time of 3 s. Wall sections of the monolithic support whith the best catalytic performance was analysed by Electron Probe Microanalysis by Wavelength Dispersion Spectroscopy

113 (EPMA-WDS) obtaining the line profile for Ti, Ce, Mg and Si. The axial crushing strength was measured on monolithic samples with a Chantillon LTCM dynamometer. The composition and properties of powdered samples and monolithic supports are collected in Tables 1 and 2. Table 1 a) Properties ofTiO2, CeO2 and TiCex powder samples Sample [CeO2] [TiO2] BET area ZPC I(XRD)TiO2 I(XRD)Ce02 wt.% wt.% m2 g~ pH 2 =25.0 2 =28.5 TiO2 0.0 100.0 67 6.5 97 TiCel 1.1 98.9 71 6.5 99 TiCe2 1.9 98.1 80 7.0 143 TiCe4 3.7 96.3 81 6.6 75 7 TiCel0 10.1 89.9 81 6.5 53 10 TiCel4 14.5 85.5 76 6.3 60 28 CeO2 4000(2 100.0 0.0 5.8 35 CeO2 500~ 100.0 0.0 2.0 40 CeO2 600~ 100.0 0.0 2.2 60 b) Pore volume of TiCex samples, obtained by N2 ad/desorption Sample Total pore volume* Mesopore volume, cm3 g-1 Average pore diameters cm3 g-1 narrow wide nm TiO2 0.28 0.16 0.12 13.7/17.3 TiCel 0.29 0.17 0.12 11.9/17.2 TiCe2 0.30 0.18 0.12 11.0/16.5 TiCe4 0.29 0.17 0.12 9.6/16.5 TiCel0 0.27 0.16 0.11 8.9/16.3 TiCe14 0.24 0.14 0.09 8.9/14.5 * calculated from volume of gas adsorbed at p/p0 = 0.96 (0-50 nm pore diameter) Table 2 Properties of monolithic supports treated at 500~ hours in air .atmosphere Sample [CeO2]* [TiO2] Pore volume** BET area Axial crushing strength wt.% wt.% cm3 g-1 m 2 g-1 MPa A 0.0 50 0.73 108 9.8 B 2.0 48 0.80 108 12.3 C 4.0 44 0.75 106 15.0 * respect to the TiO2 content. measured by mercury intrusion porosimetry (3 m-7.5 nm pore diameter)

2.3. Activity tests Catalytic activity measurement of Rh/TiO2-CeO2 monolithic catalysts for nitrous oxide decomposition was studied, using a fixed bed continuous tubular reactor, working in an integral regime close to isothermal axial profile. The inlet and outlet N20 concentrations were determined using an ADC Double Beam Lufi Type Infra-red Gas Analyser. The experiments were carried out at P = 120 kPa, T = 400-430~ GHSV(NTP) = 8000 h -l, VL =

114 0.91 m s~ using N20 (500 ppm) and N2 as gas balance. For these experiment monoliths of 5 cells and 17 cm length were used. 3. RESULTS AND DISCUSSION

3.1 TiO2-CeO2 powdered samples Fig. 1 shows the zeta potential v s . pH curves of titania (TiCe0), mixtures of TiO2CeO2 with a cerium oxide content around 2 wt% (TiCe2) and 14 wt% (TiCe14) as well as CeO2 treated at 400~ and 500~ in air. The IEP of titania and CeO2 as well as the ZPC of all TiCex samples are listed in Table 1a). These values were calculated by interpolation, if possible, and extrapolation, when the IEP was beyond the accessible pH range.

30-

2O

-10-20-

-30-

o

pH Fig. 1. Zeta potential as a function of the suspension pH at 25~ for samples of TiCex and CeO2 treated at different temperatures, kneaded in water. (11) TiCe0, (O) TiCez, (0) TiCel4, (-)~) CeO2 at 400~ (I-l) CeO2 at 500~ The IEP of TiO2 was 6.5, in accordance with previous results obtained in our group [ 14]. The CeO2 sample treated at 400~ had an IEP value of 5.8 whereas samples treated at 500~ and 600~ had similar values around 2.0 (Table 1a). These results indicate that IEP of CeO2 samples, markedly depends on the temperature treatment. This effect can be explained taking into account the TG-DSC results obtained for the TiCea4 sample before temperature treatment (Fig. 2). The weight loss observed in the temperature range 30400~ corresponds to the elimination of water and nitrates present in the sample after impregnation. According to the literature [15] also Ce203 was formed. At higher temperature, the TG curve shows a band centered at 700~ associated to an increase in the sample weight. This mass gain has been related to an oxidation process from Ce203 to Ce204. The same effect was observed after heat treatment at 500~ when maintained for 4 hours. Thus, the significant change in the ICP from 5.8 to 2.0 after heat treatment at 400~ hours and 500~ hours, respectively, can be related to the change in oxidation state of cerium (Ce +3 to Ce+4).

115 The zero point charge (ZPC) of mixtures TiO2-CeO2 should follow the equations: ZPC -- IEPce*Xce+ IEPTi* XTi Xce+ XTi "- 1

(1)

(2)

where IEPTi and IEPce are the isoelectric point of titania and ceria respectively, whereas Xce Xvi are the surface mole fraction of each component in the mixture. Thus, if the TiO2 surface was poorly covered by ceria, the ZPC of TiO2-CeO2 sample will be close to the IEP of TiO2 but if well covered, close to that of CeO2. 0

i t-

-15 0

200

400

600

800

101)0

Temperature, ~ Fig. 2. TG-DSC curves of TiCel4 sample previous calcination. Cerium impregnated over the titania surface, treated at 500~ in air atmosphere, could show two different behaviours. The cerium could form a superficial oxide similar to CeO2 (Ce +4) with an isoelectric point near 2.0 or the Ce may have a strong interaction with the TiO2 surface giving rise a new Ti-O-Ce species. The curve for TiO2-CeO2 sample with a cerium oxide content of 1.9 wt. % (Fig. 1), showed a ZPC = 7.0. This value was beyond the IEP of the pure oxides and thus suggested the formation of a new Ti-O-Ce species. The ZPC of TiCex mixtures are collected in Table 1. The data pass through a maximum for TiCe2 sample, and at higher CeO2 content the ZPC decreased towards the IEP of CeO2. The creation of a new species with different properties than the single oxides, have been proposed in the literature for Ni/A1203 and Co/A1203 at low metal content [ 16]. In both of these systems, the presence of NiO and COO3, have been only detected for contents above 2-4 wt%, respectively. The CeO2 crystal phase was first detected for TiCe4 and the peak intensities increased with CeO2 concentration. The data of the peak intensity at 20 = 25.0 (TiO2) and 20 = 28.5 (CeO2) are summarised in Table 1. The data reflect a maximum in titania crystallinity for sample TiCe2. Above this value, the crystallinity related to CeO2 start to increase due to the formation of a cerium oxide phase. On careful examination of TiCe2 XRD (Fig. 3), a shift of the anatase peaks may be detected. This effect could be related

116 with the inclusion of the cerium in the titania structure modifying its characteristic diffraction pattern. The textural nature of the titania and TiCex were characterised by nitrogen ad/desorption isotherms. The specific surface areas are presented in Table l a. None of the materials were found to be microporous from t-plot analyses of the adsorption isotherms. From the desorption curves, the mesopore size distribution was calculated using the BJH method. All of the samples had bimodal mesopore size distributions. The volumes of the narrow and wide mesopores, presented in Table lb), were calculated from the minima between the two distributions. These results indicated, that as the amount of ceria incorporation rose, the bimodal mesopore size distribution became narrower. For the titania sample the mesopores were centred in diameters of approximately 14 and 17 nm. With ceria incorporation both of these diameters were reduced until at the highest ceria content they were approximately 9 and 15 nm. Fig. 3. X-ray diffraction patterns for TiCex and pure CeO2, treated at 500~

i

Ceq

.[

TiCe-14 TiCqo TiC& TiCe~ TiO2

The XRD of TiCex samples and CeO2 are presented in Fig. 3. The TiO2 had the characteristic peaks of TiO2-anatase (ASTM 21-1272). The cerium oxide presented typical peaks at 20 = 28.5 and 33.2 assigned to cerinaite (ASTM 43-1002). The XRD patterns of TiCex samples showed these two phases, depending on the CeO2 content.

2o a'o ;i6 g6 6o 29:6

Analysis of these results showed that the incorporation of 1.0 wt% ceria into the titania support led to a 5% increase in the specific surface area. With incorporation of 1.9-10.1 wt% ceria the surface area was 20% higher than the original titania. But, at 14.5 wt% ceria the surface area began to fall, although still 14% higher than the non-doped support. The mesopore volume rose to a maximum at a ceria content of 1.9 wt% and then steadily fell as the ceria content was further increased. This behaviour suggested, that after the new species had been produced reaching a limit at 1.9% ceria, a pore filling mechanism was more pronounced, causing the reduction in the mesopore volume. Division of the mesopore range into the two fractions (wide and narrow, Table 1b) showed that the volume of the wider mesopore remained practically constant up to 3.7% ceria content, and then fell. However, the narrow mesopore volume increased with up to 1.9% ceria incorporation and then steadily decreased. These results suggested that with ceria incorporation a new species was formed that increased both the surface area and the pore volume. As more ceria was incorporated the surface area was maintained but pore narrowing due to deposition in the mesoporous network led to a reduction in the overall pore volume. Finally, at the highest levels of ceria incorporation the surface area also began to fall due to the formation of CeO2.

117

3.2

Monolithic systems

Taking into account the results with the powdered samples, monolithic supports were manufactured, with CeO2 contents between 0-4 wt%. The silicate binders were necessary for the conformation of these structures [17]. The axial profile and mapping obtained by EPMAWDS microscopy revealed that Ti and Ce concentration deviation over the wall cross-section were parallel and contrary to Si and Mg, principal components of the binders. This result indicated that cerium was selectively deposited on titania particles and not on the silicates. The CeO2 content added to titania and confirmed with ICP spectroscopy, total pore volume and BET area for the three monolithic supports, calcined at 500~ for 4 hours are collated in Table 2. For the monolithic samples the presence of the inorganic binder tended to mask the differences between samples due to its high surface area. However, analysis of the pore size distribution from the nitrogen isotherm of the material with no ceria, showed the characteristic diameters for the titania at 14 and 17 nm and a further diameter at about 48 nm associated with the binders. With ceria incorporation the pores at about 48 nm were unchanged but the narrower pores due to the titania were reduced to 9 and 17 nm then to 9 and 15 nm at 2 and 4% respectively. These changes in the pore size distribution of the monolithic samples suggested that the ceria was associated with the titania and not with the binders, in agreement with the results from EMPA-WDS line profiles. 80

Fig. 4. N20 conversion as a function of the cerium oxide content for Rh-monolithic catalysts at (A) 400~ and (O) 430~ Operation conditions: P = 120 kPa, GHSV (NTP)= 8000 h~, VL= 0.91 m s-1. Feed composition: [N20] = 500 ppm and [N2] = balance.

o~ =- 60 0

>

c

8

0

Z"

40 A j

20

'

0

I

1

'

I

2

'

I

3

'

I

4

[Ce02], wt.%

Finally, the effect of the cerium oxide content on the catalytic activity of rhodium catalysts was analysed for N20 decomposition reaction. The rhodium content was kept constant for all samples. The activity curves are represented in Fig. 4 at 400~ and 430~ Data were acquired after 2 hours of reaction to ensure steady state conditions. The systems present catalytic activity at temperature above 300~ Catalysts "A" prepared without ceria, displayed a good catalytic performance for N20 elimination even at 400~ reaching at 430~ a N20 conversion of 55 %. Nevertheless, a maximum was achieved for catalysts with a CeO2 content of 2 wt% with a conversion of 70%. These behaviour can be related with the result obtained with TiCex powder samples. The proposed new specie (Ti-O-Ce) formed when the cerium oxide content was around 2 wt%, could be responsible for the better performance of

118 the monolithic catalyst "B", compared with the others. This phase could increase the rhodium dispersion on the catalysts, and favour the reduced metal state as has been proposed previously in the literature for other Rh catalysts [18]. The reduction in the catalytic activity observed at CeO2 content above 2.0 wt.% (catalyst "C") can be connected with the detection of cerium oxide crystal phase observed for TiCe4 powder sample (Fig. 3) with approximately the same ceria content. The results indicated that the presence of CeO2 crystals reduce the catalytic activity for the N20 decomposition reaction. 4. CONCLUSIONS The influence of CeO2 content on titania samples prepared by wet impregnation have been studied by different techniques, in order to prepare active monolithic catalysts for N20 decomposition reaction for industrial application. The CeO2 formation depends closely on the temperature treatment of sample and affect strongly the zero point charge. From the results obtained we can conclude that the addition of a ceria concentration around 2 wt.% lead to an new Ti-O-Ce phase, that contribute to the better performance of the Rh/TiO2 monolithic catalysts for the N20 decomposition reaction. The presence of CeO2 crystal phase is responsible for a decrease in the catalytic activity. The catalysts tested in this study are the first monolithic systems developed for N20 abatement. These systems can be a way to reduce the N20 emission for industrial processes with relatively low installation cost and lower perturbation in the production process. ACKNOWLEDGEMENTS

We are grateful for the financial support from CICYT (Spain) Project MAT2000-0080-P4-02, the CYTED program (R.T.V.C.) and CSIC-USACH agreement (Project: 99CL0022). REFERENCES

1. F. Kapteinj, J. Rodriguez-Mirasol, and J.A. Moulijn, Appl. Catal. B., 9 (1996) 25. 2. S. Imamura, J. Tadani, Y. Saito, Y. Okamoto, H. Jindai and C. Kaito. Appl. Catal. A. 201 (2000) 121. 3. G. Centi, A. Galli, B. Montanari, S. Perathoner and A. Vaccari, Catal. Today, 35 (1997) 113. 4. H.C. Zeng and X.Y Pang. Appl. Catal. B., 13 (1997) 113. 5. G. Centi and F. Vanazza. Catal. Today, 53 (1999) 683. 6. R. S. Drago, K. Jurczyk and N. Kob. Appl. Catal. B., 13 (1997) 69. 7. Z.H. Zhu, S. Wang, G.Q. Lu and D.K. Zhang. Catal. Today, 53 (1999) 669. 8. G. Centi, L. DallOlio and S. Perathoner, Appl. Catal. A., 194 (2000) 79. 9. K. Yuzaki, T. Yarimizu, K. Aoyagi, Shin-Ichi Ito and K. Kunimori. Catal. Today, 45 (1998) 129. 10. G.Centi, L. Dall'Olio, S. Perathoner, Catal. Lett., 67 (2000) 107. 11. S. Irandoust and B. Andersson, Catal. Rev. Sci. Eng. 30 (1988) 341. 12. G.A. Parks, Chem. Rev., 65 (1965) 177. 13. F. J. Gil-Llambias and A. M. Escudey Castro, J. Chem. Comm., (1982) 478.

119 14. C. Knapp, F.J. Gil-Llambias, M. Gulppi-cabra, P. Avila and J. Blanco, J. Mater. Chem., 7 (8) (1997) 1641. 15. T. Arii, A. Kishi, M. Ogawa and Y. Sawada, Anal. Sci., 17 (2001) 875. 16. F.J. Gil-Llambias, A.M. Escudey-castro and J. Santos-Blanco, J. Catal. 83 (1983) 225. 17. J. Blanco, P. Avila, M. Yates and A. Bahamonde, Surf. Sci., (1995) 755. 18. F.L. Normand, L. Hilaire, K. Kili and G. Maire, J. Phys. Chem., 92 (1988) 2561.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Pt

121

combustion catalysts prepared from w/o mlcroemulslons

J. Ryme~ a, G. Ehret b, L. Hilaire c and K. Jir~tovfi a aInstitute of Chemical Process Fundamentals, 165 02 Praha 6, Rozvojovfi 135, Czech Republic, Tel. +420 2 20390295, E-mail: [email protected] blPCMS, UMR 7504, CNRS-Universite Louis Pasteur, Strasbourg, France CLMSPC-UMR 7515, CNRS-Universite Louis Pasteur, Strasbourg, France Platinum catalysts were prepared by impregnation of formed supports with reverse microemulsions (water-in-oil) or water containing chloroplatinic acid. Microemulsion catalysts were more active in combustion of toluene in toluene-air mixtures than those prepared classically from water solutions. The size of platinum in classically impregnated catalysts was three times higher than that of the catalysts prepared from microemulsions. In the case of microemulsion preparation method, platinum is located near the pellet surface or its position in the pellet can be optimised. The viscosity of the microemulsion affects the concentration profile of platinum in the catalysts. 1. I N T R O D U C T I O N Microemulsions are thermodynamically stable isotropic dispersions of oil in water (o/w) or of water in oil (w/o) containing domains of nanometer dimensions stabilized by an interracial film of surfactant(s). The most typical oil phases are alkanes, the choice of cyclic or aromatic hydrocarbon being dependent on further application. Extensive study of microemulsions has been stimulated by their great potential for practical applications in different fields [1,2] such as pharmaceuticals, cosmetics, enhanced oil recovery and material science (catalysts, semiconductors, etc.). A variant of the microemulsion method was first applied in the preparation of hydrogenation catalysts [3]. The principle of this and following studies consists in the preparation of solid particles in water droplets of w/o microemulsions. The resulting mixture is used a) after separation as a catalyst [3,4] or b) as an initial suspension for further catalyst preparation [5].

The authors (J. R. and K. J.) thank the Grant Agency of the Academy of Sciences of the Czech Republic for the financial support (Grant No. A4072904/1999).

122

Our approach to the preparation of supported combustion catalysts is different: We applied w/o microemulsion containing water-soluble Pt compound directly on a formed catalytic support of defined shape. As the complete characterisation of catalysts with low concentration of platinum is often difficult, we prepared a set of catalysts with 0.3 wt. % Pt. But in addition, we prepared another set of catalysts with 0.1 wt. % Pt. Such catalysts are especially interesting for combustion of VOC, because the price of the catalysts is not so high. This contribution describes some physical properties of reverse w/o microemulsions, physical-chemical properties of platinum catalysts prepared by classical impregnation from water solutions of HzPtCI6 and from reverse w/o microemulsions comprising chloroplatinic acid and their activity in combustion of volatile organic compounds (VOC). In our study we concentrated on Tween80 (polyoxiethylen(20)sorbitan monoleate), as the surfactant does not comprise any potentially harmful ions (like sodium in AOT or bromide in CTAB) that could affect the activity of platinum in catalytic combustion. 2. E X P E R I M E N T A L

Preparation of microemulsions. Microemulsions were prepared of oil (cyclohexanol, cyclohexane or heptane), of surfactant Tween 80 - polyoxiethylen(20)sorbitan monoleate with M=1309.68 and, for comparison, of AOT (sodium bis(2-ethylhexyl)sulfosuccinate) or CTAB (cetyltrimethyl ammonium bromide) and water. The molar ratio R of water and surfactant moved in the range 2 7 0 - 1110. Catalyst preparation. Chloroplatinic acid, in such amounts as to obtain the desired concentration of Pt in the catalysts, was added to previously prepared w/o microemulsions. Two types of catalytic supports, ~,-alumina in the form of full pellets (diameter 5 mm) and 0-alumina in the form of hollow pellets (diameter 5 mm, hole 2 mm), both manufactured in Chemopetrol Litvinov, Czech Republic, were chosen for catalysts preparation. After calcination at 500 ~ q,-alumina had SBET=166 mZ/g, pore volume 0.459 ml/g, mean pore radius 7.0 nm and water absorption capacity 40 %. After calcination at 900 ~ 0-alumina showed SBET=145 mZ/g, pore volume 0.445 ml/g, mean pore radius 8.1 nm and water absorption capacity 50 %. The catalysts with 0.3 and 0.1 wt. % Pt were prepared by impregnation with HzPtCI6 water solutions (denoted as I) or Pt microemulsions (denoted as M). The catalysts were dried 2 h at 120-160~ and calcined 2 h in air at 550 ~

Characterization of microemulsions The viscosity of microemulsions was measured with a Rheotest 2 (Medingen, Germany) instrument in the range of shear rates 5 - 1800 s -1.

Characterization of the catalysts Surface areas of the supports were determined from desorption isotherms of nitrogen adsorbed at -198 ~ after evacuation at 350 ~ for 5 h on ASAP 2010, Micromeritics, USA. Pore size distributions were obtained from mercury porosimeter working in the range 0.1400 MPa (AutoPore III, Micromeritics, USA). Size of platinum particles. High-resolution transmission electron microscopy (HRTEM) was performed to determine the size of Pt particles supported on alumina and to check the

123 dispersion of the platinum particles. HRTEM studies were performed on a TOPCON EM002B apparatus with a top entry device and operating at 160 kV (resolution 0.5 nm). Distribution of Pt throughout the catalyst pellet. Electron microprobe Jeol JXA-50A equipped with an energy dispersive X-ray spectrometer EDAX TV 9400 was used for the determination of platinum concentration as a function of position in a pellet. Catalytic experiments were done with 75 ml of a catalyst and F/W=10 000 m3/h/kgcat. Combustion of toluene was chosen as a model volatile organic compound and non-steadystate reaction condition (linear increase 3.5 ~ in reaction temperature) was applied in the catalyst activity evaluation. Concentration of toluene in air was 1 g / m 3. Temperatures of gaseous reaction mixture entering and leaving the catalyst layer were measured by thermocouples. Catalytic activities expressed as the inlet temperatures Ts0 or T90, at which 50 or 90 % conversions of toluene were achieved, were taken as a measure of the catalytic activity. 3. RESULTS AND D I S C U S S I O N

3.1. Viscosity of microemulsions W/o microemulsions can be either Newton or non-Newton liquids depending on many factors, such as the type of surfactant, oil, the amount of water etc. In our case of catalyst preparation the viscosity plays an important role and so we decided to characterize the flow properties of microemulsions we have prepared from non-polar oils (cyclohexane, heptane) and polar oil (cyclohexanol). The viscosity of microemulsions prepared from nonpolar oils exhibited the typical behavior of non-Newton liquids: With increasing shear rate the viscosity was sharply decreasing (Fig. 1, left). In contrast with that, the viscosity of microemulsions prepared from polar oil, cyclohexanol, showed the properties of nearly Newton liquids: With increasing shear rate the viscosity practically was not changing (Fig. 1, right). When comparing the viscosity of both microemulsion systems at the same conditions (3,=10 s -1 and 1000 s-l), we obtained data shown in Table 1. The viscosity of microemulsions prepared from cyclohexanol, especially at low shear rates, is substantially lower than those prepared from heptane. In the case of microemulsions prepared from cyclohexanol a variation in water amount did not show great effect on the viscosity value. On the contrary, decreasing the amount of water in microemulsions prepared from heptane lead to microemulsions of very viscous, gel consistence. The stability of such a type of preparation was limited in time. Table 1 Properties of microemulsions obtained at 25 ~ (oil: COL - cyclohexanol, H-heptane) Oil 78.5 COL 85.0 COL 94.5 H 89.3 H 83.8 H 77.8 H

Wt. % Tween80 1,4 0,7 0,4 0,7 1,1 1,5

R Water 20 14,3 5,1 10,0 15,1 20,7

1007 1516 927 1039 998 1003

Viscosity, mPa.s At 1000 s -1 At 3,=10 s -1 16 17,8 13 14 12 660 61 330 36 170 25

124

1:~:)

1~ C~

~ ~

5~ H~O IO%HzO

9 14.3% H20 9 20.0%H20

23

10

O0

'

600

'

1~}0

'

1800

Fig. 1. Dependence of the microemulsions viscosity on shear rate T, S-1 (left-CyclohexaneTween80-water microemulsions with R = 923-1039, r i g h t - Cyclohexanol-Tween80-water microemulsions with R -- 1007-1516 and various amount of water) 3.2. Concentration profile of Pt in the catalysts Adding chloroplatinic acid to the w/o microemulsions prepared resulted in an impregnation solution useful for the preparation of catalysts with 0.3 wt. % Pt. 0-Alumina in the form of hollow pellets is a good example for studying the effect of viscosity on Pt concentration profiles (Fig. 2). In the upper part of the figure concentration profiles of the catalysts prepared from the solution of low-viscous microemulsions (cyclohexanol-Tween80-water) and H2PtCI6, measured in the half of the pellets, are shown. With all catalysts the Pt concentration determined on the outer and inner part of the hollow pellets was almost the same. The lower part of the figure documents much higher concentration of Pt on the outer part of the pellets, when microemulsions of high viscosity (heptane or cyclohexane-Tween80-water) were applied in the catalyst preparation (3-4 times higher than in the case of impregnation by water solution of H2PtC16). Obviously, high viscosity of the impregnation solution does not allow a sufficiently fast penetration into the central part of the hollow pellets. Pt catalysts prepared over full pellets of T-alumina showed very non-uniform Pt concentration not depending on the quality of microemulsion, i.e. whether w/o microemulsions were of low or high viscosity (Fig. 3). Highest Pt concentration on the outer part of the pellet was observed with the catalyst (M33) that was prepared from very viscous w/o microemulsion. A lower viscosity of the impregnation solution did not lead to such steep change in the Pt concentration profile of the pellet (M02): Compared to previously mentioned catalyst, the concentration of platinum on the surface of this pellet was lower, but was detected more deeply inside the pellet. A lower viscosity of the impregnation solution did not hinder entering of the solution into the center of the pellet so much as in the case of high-viscous solutions. An extreme case is the catalyst (I01) prepared from water solution of HzPtCI6. The catalyst showed an almost uniform Pt concentration profile throughout the pellet.

125

4

M05

4.

1-

M02

1.

4.

1-

-o9_-e,~ ~,s o,o..Q3 o& o,e

g

Radial c~rdinate r/R Fig. 2 Concentration profiles from microemulsions (M24, M21, HzPtCI6 (103) over 0-alumina. The viscosity of impregnation solutions, 2,5

of platinum in hollow pellets of the catalysts prepared M21, M19, M17, M25) and from water solution of total concentration of Pt is 0.3 wt. %. Upper part: low lower part: high viscosity

Z5

i0~ ~/~/~~

.

2 !

1,5

L

1,0

t

E-..9. o,5 o,c

M25

1,5. 1,o 0,5 o,o

.•,•/ 0,9

-0,6 -0,3

. . . .

0,0

0,3

0,6

i

0,9

Radial c c ~ m a t e i R Fig. 3 Concentration profiles of platinum in full pellets of catalysts prepared from microemulsions (M05, M02, M33) and from water solution of HzPtC16 over 7-alumina (I01). Total concentration of Pt is 0.3 wt. %.

9

126

3.3. Size of platinum particles Transmission electron microscopy measurements revealed that the typical size of platinum particles in the calcined catalysts differ very much depending on the way the catalyst was prepared. The microemulsion catalysts exhibited similar and relatively small platinum particles with diameter (arithmetic mean) from 5 to 55 nm. In contrast with that, the I02 catalyst impregnated by classical method showed much larger Pt particles. An example of histograms of Pt particles observed in various catalysts is given in Fig. 4. The microemulsion catalysts have narrower particle size distribution located between 4 - 90 nm, while the catalysts impregnated by water solutions of chloroplatinic acid have much broader particle size distribution reaching diameters up to around 300 nm. The platinum particle sizes found in our calcined oxidation catalysts are much larger than those observed [6] in the reduced microemulsion catalysts moving in the range 0.5 3.5 nm. Calcination in air of platinum catalysts including microemulsion catalysts apparently evolves a growth of primary platinum particles by a sintering process. if)

M08

M32

4O 33 29

~O

1'o " ~ / / // // //

O

" 3o " ~

" ~

0

1'o " ~o " 3o " do "

M06

" J0" dO" ~0" ~0" f0" ~0" Platinum particle size, n m

I02

.

.

.

.

.

~XDo

Platinum particle size, nm

Fig. 4 Histograms of platinum particles determined by TEM in the catalysts prepared from microemulsions (0.1% Pt)

3.4. Catalyst activity in toluene combustion Table 2 comprises important details concerning microemulsions preparation, characterisation of calcined catalysts and their activity in combustion of toluene expressed

127 as temperatures T50 and T90. The Pt catalysts (0.1 wt. %) prepared over 0-alumina using w/o microemulsions of lower viscosity (e.g. M22) combusted toluene at much lower temperatures (T50_--150~ than the catalyst (I03) with even higher Pt concentration (0.3 wt.%) prepared by conventional impregnation method from water solution of H2PtCI6 (T50_--183~ Microemulsions of higher viscosity lead to the catalyst (M20) with high concentration of platinum on the outer surface of the pellets, but its catalytic activity was somewhat lower than that of comparable microemulsions catalysts. Substantially lower catalytic activity showed the M32 catalyst prepared from CTAB instead of Tween80. As documented in Table 2, the size of platinum particles in the calcined catalyst is five times smaller, and this parameter could be decisive. Table 2 Survey of microemulsion composition (COL-cyclohexanol, CAN-cyclohexane, Hheptane), concentration of surfactant in microemulsion Csu~f, molar ratio R of water and surfactant, Pt concentration in water droplets, nominal concentration of Pt in the catalysts, size of platinum particles dpt (arithmetic mean), and catalytic activity expressed as Ts0 and T90 No.

Oil

Tween80 wt. %

Water wt. %

1.5 1.5 a 1.5 0

20.7 20.7 20.7 100

Csurf

R

~tmol/1

Pt wt. %

Pt mmol/1

dpt nm

T50 ~

T90 ~

37 40 37 27.4

55 9.4 53.3 46.7

153 208 160 183

167 235 172 202

45.9 45.9 9.8 128.5 138.1

33.3 5.7 107.4 33.8 8.3

156 205 209 141 153

172 228 237 155 167

O-Alumina, hollow pellets M22 M32 M20 I03

COL CAN H

10.8 9.5 8.3 0

1011 279 1011 ~

0.1 0.1 0.1 0.3

y-Alumina, full pellets M06 CAN M08 b CAN I02 M05 CAN M33 H aCTAB, bdried

1.4 18.6 1.4 20.6 0 100 1.4 18.6 1.5 20.6 at 200 ~ only

8.6 8.6 8.8 8.6 0

982 1064 ~ 982 982

0.1 0.1 0.1 0.3 0.3

A set of the catalysts with 0.1 and 0.3 wt. % Pt prepared from microemulsions of similar viscosity over 7-alumina (full pellets) showed a great variety of Ts0 (140 to 205 ~ whereas the catalyst prepared by classical method of impregnation showed the lowest activity of all (T50=209~ Comparing the catalysts of the same Pt concentration, we can discuss the effect of Pt concentration in water droplets: The lowest calculated concentration of chloroplatinic acid was found for the solution used during classical impregnation of the support. Low concentration of chloroplatinic acid in water, i.e. low extent of saturation [7], can lead to the growth of Pt particles, which is documented by TEM measurements. The I02 catalyst with the lowest Pt concentration in the impregnation solution showed the highest size of platinum particles (107 nm), whereas the Pt size of the catalyst from microemulsions moved within limit of 5 - 27 nm.

128 More precise effect of platinum particle size on the activity of catalysts having 0.1 wt% Pt can be seen from Fig. 5. The figure demonstrates that the highest activity of the catalysts was observed with those who had size of platinum particles (arithmetic mean) around 4 0 - 50 nm. This statement is supported by our following observation: The M08

Z20

s 0

~ 180

-

1%

MO6A

.... ~ '

Mgg A~:)" 60' I~0 ' 1~:)' lED C~, I'I1TI

Fig. 5 Dependence of the activity of the catalysts prepared from water solution of chloroplatinic acid and from Pt microemulsions in catalytic combustion of toluene on the size of Pt particles catalyst, only dried, showed very low catalytic activity and had the smallest Pt particles (4.4 nm). After repeating the catalytic test with the same catalyst (i.e. the catalyst was in contact with air at 400 ~ its catalytic activity remarkably increased (T50=160 ~ Evidently, the platinum particles during the catalytic test (calcination in air) increased their size. So we can state that the optimal size of platinum in the supported catalysts intended for combustion of toluene should move within the limits 40 - 50 nm. 4. C O N C L U S I O N Microemulsion impregnation of a catalytic support can be a very good method for preparation of heterogeneous catalysts used in reactions with mass transfer limitation, for which high concentration of active metal in the outer shell of catalyst pellets and narrow metal size distribution is important. REFERENCES

1. M.P. Pileni, Langmuir 13 (1997) 3266. 2. K. L. Mittal and P. Kumar, Handbook of Microemulsion, Science and Technology, Marcel Dekker, New York, 1999.

129 3. M. Boutonnet Kizling, C. Bigey and R. Touroude, Appl. Catal. A: Gen., 135 (1996) L13-L17. 4. A. Claerbout and J. B.Nagy in Preparation of Catalysts V, p. 705. (G. Poncelet, P. A. Jacobs, B. Delmon, Eds.), Elsevier Science Publishers, Amsterdam, 1991. 5. H. H. Ingelsten, K. Berquist, J.-Ch. Beziat, A. Palmquist, M. Skoglundh and K. Holmberg, Europacat-V, 2-7 September 2001, Limerick, Ireland. Abstract 21-O-16. 6. M. Boutonnet, J. Kizling, R. Touroude, G. Maire and P. Stenius, Catal. Lett. 9 (1991) 347. 7. B.H. Robinson, A.N. Khan-Lodhi and T. Towey in Structure and Reactivity in Reverse Mice|les, Elsevier, Amsterdam, 1989, Vol. 65, p. 198.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

131

Preparation of stable catalysts for N20 decomposition under industrial conditions S. Alini a, F. Basile b*, A. Bologna a, T. Montanari b and A. Vaccari b a

Radici Chimica S.p.A. via Fauser, 50, 28100 Novara (Italy)

b Dipartimento di Chimica Industriale e dei Materiali, Universitfi degli Studi di Bologna, Viale Risorgimento, 4, 40136 Bologna (Italy) - E-mail: [email protected] The key parameters in the preparation of active and stable catalysts for N20 decomposition under conditions of industrial interest were examined using mixed oxides obtained form hydrotalcite-type (HT) precursors. The relationship between activity and composition of the catalysts was studied, with attention focused on the nature and amount of metal ions present in the HT precursor (Rh/Mg/A1 or Rh/Co/A1). An activation procedure was developed which makes it possible to prepare catalysts, active and stable even under the severe reaction conditions of industrial interest. The relationship between active phase and composition of the bulk matrix was also investigated. Moreover, the flexible structure of the HT precursor made it possible to investigate the use of other active elements (Pd or La) to develop synergetic effects, without any interactions due to side phase segregation, or lack of homogeneity. 1. I N T R O D U C T I O N Nitrous oxide, N20, exercises significant negative effects on the environment, being a relatively strong greenhouse gas and one of the species contributing to the depletion of ozone in the stratosphere [1-4]. The estimated human contribution to N20 emissions to the atmosphere amounts to 4.7-7.0"106 t/a, about 30-40% of the total emissions including natural sources [2,5]. However, the only emissions that can be reduced in the short term are those from energy-related and chemical processes [6-8], with the reasonable target of achieving about 60-90% reduction by 2010 [5]. Primary chemical sources of N20 are the production of nitric and adipic acids. For example, in the latter case for each molecule of adipic acid one molecule of N20 is produced, yielding high concentration levels of nitrous oxide in the outlet stream. Much effort is currently being made to reduce N20 emissions from industrial adipic acid plants (outlet gas stream: 20-25% N20, 5%02, 1-2% H20, 3% CO2 and N2 the remainder) [6-9], for which the high concentration of pollutant and the presence of 02 and H20 have dramatic effects on catalyst activity and stability. Hydrotalcite-type (HT) compounds, having the general formula [MIIl _xMIIIx(OH) 2]x+ (An-)x/n " mHzO, have been reported as useful precursors of catalysts to be employed under severe reaction conditions [10-12]. Previous works [8,11-17] reported the use of several mixed oxides obtained by calcination of HT precursors in N20 decomposition, although

132 they were mainly tested in diluted gas streams and in the absence of O2 and/or H20. The aim of this study on HT precursors was to shed light on the main parameters involved in the development of active and stable catalysts, able to decompose N20 under conditions of industrial interest. 2. E X P E R I M E N T A L SECTION The catalysts (compositions as in Table 1) were obtained by calcination at 650 ~ for 12 h of HT precursors prepared by coprecipitation at 60~ and constant pH (10.0+0.1). A solution ([Mn]+[M III] = 1M)containing the nitrates of the elements in the right atomic ratio was added to an excess of 1 M solution of Na2CO3 (CO32-/MnI = 3); the pH was maintained constant by dropwise NaOH addition. In the case of La-containing precursors to avoid the precipitation of La2(CO3)3, the synthesis was carried out under N2 flow and using NaNO3 instead of Na2CO3. The precipitates were kept in suspension under stirring at 60~ for 40 min., then filtered and washed until a Na20 content lower that 0.02 wt% was obtained. The precipitates were dried overnight at 90~ and calcined for 12 h at 650~ Table 1. Composition, B.E.T. surface area and phases identified by XRD for the samples dried overnight at 90~ or calcined at 650~ for 12 h. Composition

Atomic ratio

(%)

B.E.T. surface area (m2/g)

XRD phase composition

Dried Calcined Dried Calcined Co/A1 71.0:29.0 108 46 HT SP Rh/Co/AI 0.5:74.5:25.0 116 56 HT SP Rh/Mg/A1 1.0:71.0:28.0 110 156 HT M.O. Rh/Mg/A1 0.5:71.0:28.5 130 136 HT M.O. Rh/Mg/AI 1.0:80.0:19.0 8 122 HT M.O. Rh/Pd/Mg/AI 0.5:0.5:70.0:28.0 94 132 HT M.O. + PdO Rh/Pd/Mg/AI 0.5:1.0:70.0:28.5 98 140 HT M.O. + PdO Rh/La/Mg/AI 1.0:5.0:71.0:23.0 74 109 HT + La(OH)3 M.O + LazC05 HT= hydrotalcite-type phase. M.O. = MgO-type mixed oxide phase. SP = spinel-type phase The samples were characterised by XRD powder analyses using a Philips PW1050/81 diffractometer, equipped with a graphite monochromator in the diffracted beam and controlled by a PWl710 unit 0~ = 0.15418 nm, 40kV, 40mA). A 20 range from 10 ~ to 80 ~ was investigated at a scanning speed of 70~ The lattice constants were determined by least-square refinements, from the well-defined positions of the most intense peaks. The surface area was determined by N2 adsorption using a Carlo Erba Sorpty 1700. The temperature-programmed-reduction (TPR) of the catalysts was analysed in the range 80930~ with a heating rate of 15~ using a ThermoQuest TPDRO 1100, operating with a 50 ml/min flow of a Ha/Ar (4:96 v/v) gas mixture. The structure of the calcined samples was analysed by TEM using a TOPCON EM002K HRTEM and operating at 200

133 kV, after deposition on a carbon grid of a few drops of a suspension of a small amount of powder in ethanol. The catalytic tests were carried out using the samples calcined at 650~ as such or after reduction in a 160ml/min flow of Hz/Nz (5:95 v/v) at 750~ The catalytic tests were carried out in a lab-scale fixed bed reactor, operating at atmospheric pressure, in the range Tove, = 300-500~ and feeding a gas mixture = 20wt.% NzO, 5wt.% Oz, 2.6wt.% HzO, and He the remainder. A GHSV value = 50,000 h -1 was employed, significantly higher than the values reported in the literature (5,000-15,000 h -1) [11-17] to improve the economy of the process.

I :I ~

HT o* La(OH)3 + Mixed oxide ^ La2C05 A

t

+ /'~ A

~.

d

+

u.a.

.

o

o

.

b

.

a

10

20

30

40

50

60

70

~

80

Fig. 1. XRD powder patterns of the samples: (a and c) Rh/Mg/A1 (1.0/71.0/28.0, as atomic ratio %) and (b and d) Rh/La/Mg/A1 (1.0/5.0/71.0/23.0, as atomic ratio %) dried overnight at 90~ (a and b) or calcined for 12h at 650~ (c and d) 3. R E S U L T S AND DISCUSSION The composition of the dried samples determined by quantitative analysis (Table 1) is the same as that calculated on the basis of the composition of the solutions used for the preparation. With the exception of the La-containing sample, the XRD patterns of the dried samples show the presence only of a well crystallised HT phase. The absence of side phases for the Pd-containing sample is in agreement with that previously reported in the literature [18,19], even though the ionic radius of Pd z+ ions in octahedral coordination (0.100 nm) [20] is at the limit of the values suitable to prepare HT compounds [10]. For the Rh/Mg/AI HT phases, the c lattice parameter increases as a function of the Mg/(AI+Rh) atomic ratio, being respectively 23.23(6)* for the 71.0/29.0 ratio and 23,90(4)A for the 80.0/20.0 ratio, due to the decrease in the charge density in the cationic layers and the consequent decrease in the strength of interaction between anionic and cationic layers. Finally, the Rh/La/Mg/A1 sample shows the partial segregation of La(OH)3 (Fig. 1), while

134 the residual fraction of La 3+ ions inside the HT framework is responsible for the shift in the d003 diffraction peak towards lower angles (20= 11~ corresponding to a higher value of the c parameter. The samples calcined at 650~ show different XRD patterns as a function of the composition (Table 1). The M/Mg/AI samples (M= Rh or Pd) exhibit the presence of a cubic MgO-type mixed oxide phase with, furthermore, segregation of PdO for the two Pdcontaining samples. On the other hand, the two Co-containing samples show the presence of a spinel-type phase due to the oxidation of Co 2+ to Co 3+ ions during the calcination step [10,17,21]. Finally, the Rh/La/Mg/AI sample shows the formation of La2COs, together with a cubic MgO-type mixed oxide phase, confirming the high tendency of La 3+ ions to react with CO2 to form carbonates [22,23]. The reducibility of Rh in the cubic MgO-type mixed oxides was studied by TPR analysis. The Rh/Mg/AI (1.0/71.0/28.0 as atomic ratio %) sample shows an unresolved peak starting at 400~ attributable to the reduction of Rh 3+ ions to Rh ~ evidence of the presence of these ions in a crystalline-oxide matrix. Furthermore, the TEM analysis of this latter sample (Fig. 2) shows a homogeneous dispersion of the metallic Rh ~ particles on the Mg/Al-matrix surface, with the particle size distribution in the range between 1-3 nm. The catalysts derived from HT precursors provide information on the role of the type and amount of active metal, activation procedure and composition of the inert matrix, without any interaction due to side phase segregation or lack of homogeneity. Unlike that claimed previously by Fig. 2. TEM print of the reduced Rh/Mg/A1 Swamy and coworkers [13, 14], the catalyst (1.0/71.0/28.0, as atomic ratio %). Co/AI (75.0/25.0, as atomic ratio %) Zoom factor 1000Kx, 200kV. mixed oxide shows a very low activity, due to the severe reaction conditions and, mainly, the presence of water, in agreement with previous data for transition metals supported on cubic mixed oxides derived from HT precursors [15]. The addition of a small amount of Rh [i.e., the Rh/Co/AI (0.5/75.0/24.5, as atomic ratio %) catalyst] improves the catalytic performances significantly, giving higher conversions than those of the analogous Rh/Mg/A1 catalysts. Even so the level of conversion is still not satisfactory. Activation by reduction of the Rh-containing cubic mixed oxides changes the order of activity, since the performances of all the Rh/Mg/AI catalysts improved considerably in terms of both activity and stability (Fig. 3). The complete decomposition of N20 (residual amount lower than 50ppm) is reached using an oven temperature of 350-400~ as a

135 function of the Rh-content. On the contrary the Rh/Co/AI sample after an initial flash of activity, exhibits poorer performances, with only 77% of conversion at 500 ~ and quite poor stability. These data evidence the key role of the solubility of Rh 3§ ions in the cubic mixed oxides derived from HT precursors and the enhancement of activity due to the activation step with formation by reduction of well dispersed Rh particles, stabilised inside the Mg/A1 matrix. On the other hand, the activation of the Rh/Co/Al mixed oxide under H2 gives rise to reduction of Rh 3§ and Co3§ 2§ ions, with consequent destruction of the mixed oxide matrix. The reduced metal particles obtained are very active and produce an initial peak of activity, although the fast reoxidation of Co o and Rh ~ leads to rapid deactivation. Rh/Co/AI samples have been reported to have better performances in the presence of residual Na remaining in trace amounts after the washing step [14], thus the role of the basicity was also investigated for the Rh/Mg/A1 catalysts by varying the Mg/A1 atomic ratio in the HT precursors (1.0/71.0/28.0 and 1.0/80.0/19.0 as atomic ratio %, respectively), but maintaining the same structure. The two samples give comparable results in the catalytic decomposition of N20, evidencing that the basicity is not a key parameter in severe reaction conditions. More important to achieve high performances and stability is the unique interaction between Rh ~ particles and the Mg/Aloxide matrix. The activity and stability are strongly affected by the Fig. 3. Catalytic activity of reduced Rh/Co/Al and presence of significant Rh/Mg/Al catalysts with and without the presence of amounts of H20, since it 2.6wt.% H20 in the feed (oven temperature 450~ adsorbs preferentially on the active sites. Reduced Rh/Co/A1 and Rh/Mg/Al catalysts were tested also feeding 2.6 wt.% H20 in the gas stream [24]. When water is present (Fig. 3) in the feed, the activity of the Rh/Co/Al catalyst decreases 32% ca., while the Rh/Mg/A1 catalyst (1.0/71.0/28.0, as atomic ratio %) shows almost complete conversion (> 99.9%) at 450 ~ and the Rh/Mg/A1 catalyst (0.5/71.0/28.5, as atomic ratio %) reaches the same result only at 500~

136 The Rh/Mg/A1 catalyst (1.0/71.0/28.0, as atomic ratio %) was also tested on a larger scale and using an industrial flue stream. In the absence of H20 no decrease in activity was observed after more than 1,000h of time-on-stream, while in the presence of H20, the activity started to decrease after 400h ca. A careful characterisation carried out before and after reaction, evidences that this loss of activity is attributable to the partial surface reoxidation of the Rh ~ particles, without any structural modification. In agreement with these findings, further reduction of the catalyst recovered the initial activity [24], evidencing the high hydrothermal stability of the catalysts obtained from HT precursors and the reversibility of the deactivation process. Considering the strong exothermicity of the process, the hydrothermal stability is a key factor since H20 is always present. Water not only reduces the catalytic activity, but provides information on the role of the basicity of the support, since the catalyst with a higher Mg/AI ratio deactivates faster (Fig. 4). This behavioUr may be attributed to the Fig. 4. Deactivation with time-on-stream of reduced competition of H20 for the Rh/Mg/A1 catalysts with different Mg/A1 atomic ratios in sites responsible for O2 N20 decomposition in the presence of 2.6wt.% H20 removal from the metal (oven temperature 450~

Fig. 5. Catalytic activity as a function of the oven temperature of reduced Rh/Mg/A1 and Rh/Pd/Mg/A1 catalysts in the presence of 2.6wt.% H20

surface [25], that may be affected by the basicity, i.e., by the Mg/A1 ratio. The presence of a high amount of 02 on the Rh ~ surface induces its faster reoxidation, responsible for the deactivation with timeon- stream. Since the HT structure is flexible and allows the insertion of many different elements during the catalysts preparation, the possible development of

137 synergetic interactions between Rh and other elements was investigated. In particular, Pd has been previously claimed to enhance release of oxygen from the catalyst surface [16,22]. The catalytic tests carried out on the two reduced Rh/Pd/Mg/AI samples show catalytic results worse than those obtained using the reduced Rh/Mg/AI sample (0.5/71.0/28.5, as atomic ratio %) (Fig. 5). It may be hypothesised that the significant segregation of PdO occurring during the calcination step does not allow significant interdispersion of the elements, worsening the catalytic activity. Increasing the amount of Pd produces a further decrease in the activity at low temperature, confirming that the segregation of PdO inhibits the synergetic interaction between Rh ~ and Pd ~ A further attempt to generate synergetic effects between two i00 active elements inside the HT precursors, was carried out by the t - 80 insertion of La 3§ ions inside the HT O framework. The interest in the 60 preparation of La-containing tO u40 materials is .due to the fact that 0 La203 have been claimed as active - - + - Rh/Mg/AI 1171128 f,q Z 20 in N20 decomposition [26]. - , , - Rh/La/Mg/AI i/5/71123 Furthermore, La-containing catalysts obtained from HT 300 3so 4o0 450 500 precursors have already been Temperature (~ proposed as active materials [16,22,23], although the presence of Fig. 6. Catalytic activity as a function of the La 3§ ions inside the HT structure has oven temperature of the reduced Rh/Mg/AI and not been clearly demonstrated. The Rh/La/Mg/AI catalysts in the presence of insertion of La 3§ ions in the HT 2.6wt.% H20 framework is not favoured, due to the large atomic radius of La 3§ ions [20]. Furthermore, since in the preparation of La-containing HT phases it is necessary to avoid the formation of LaaCOs, the preparation was carried out under an N2 atmosphere using nitrates as the anionic species for the interlayer. The catalytic tests show a decrease in activity in comparison to the analogous sample without La (Fig. 6). The formation of La2CO5, which it is not active, partially hinders the active surface of the Rh ~ particles and their interaction with the support. It would be possible to avoid the formation of the carbonate by heating the HT precursors under N2, but this step is useless since consistent amounts of CO2 are always present in the industrial outlet streams and, therefore, the segregation and/or formation of La2CO5 would only be delayed. In conclusion, the preparation of HT precursors did not allow the interdispersion of the different active elements investigated to date and therefore it was not possible to obtain any indications of synergetic effects between Rh- and La- or Pd-containing phases. A

.

I

138 5. C O N C L U S I O N S The catalysts obtained from HT precursors have been claimed as active in the decomposition of N20, but the conditions and feeds used so far are almost ideal and thus do not provide indications for their application in conditions of industrial interest. Therefore, in the present work the decomposition of N20 using catalysts obtained from HT precursors was studied using conditions near to those of industrial interest: a concentrated stream, with high percentages of NzO, O2 and H20. In these conditions, the cubic mixed oxide Rh/Co/A1 is more active than the analogous Rh/Mg/A1, although neither have sufficient activity. The activation of the calcined samples by reduction, gives rise to a significant enhancement of the activity for the Rh/Mg/A1, with a total conversion at 400-450~ while the reduced Rh/Co/A1 suffers faster deactivation. The higher activity may be attributed to the formation of well dispersed Rh ~ particles (< 3 nm), stabilised inside an inert Mg/Al-oxide matrix. The presence of large amounts of H20 (3wt.% ca.) gives rise to deactivation due to the reoxidation of metallic Rh ~ particles, although this process is completely reversible by further heating the catalyst under H2. Catalytic tests carried out on a larger scale, using the outlet stream of an industrial adipic acid plant confirm the high activity of Rh-containing catalysts obtained from HT precursors and their stability with time-on-stream. The study of the Mg/AI ratio (i.e. of the basicity of the oxide matrix) did not evidence significant differences in activity in the absence of H20, while when water is present in the feed, increasing the Mg/A1 ratio increases the deactivation rate probably due to slower desorption of the 02 from the metal surface. The flexible structure of HT was used to promote synergetic effects between Rh and Pd or La. Pure HT precursors were obtained with Pd, although the insertion of Pd in the HT structure was not maintained after calcination, due to segregation of increasing amounts of PdO and corresponding decreases in the catalytic activity. The synthesis of a La-containing HT was carried out under N2 to avoid the formation of carbonates, although the La partially segregates as La(OH)3 due its large ionic radius. Calcination of the La-containing HT gave rise to the formation of La2COs, which decreases the catalytic activity by hindering the Rh ~ particles or their interaction with the Mg/Al-oxide matrix. In conclusion, Rh-containing catalysts obtained from HT precursors by calcination and further reduction are very promising for N20 decomposition under conditions of industrial interest (i.e., concentrated N20 stream, also containing 02 and H20). These catalysts exhibit high conversion and stability with time-on-stream. On the contrary, any attempt to improve their performances by addition of Pd or La was fruitless, due to their segregation during calcination, which does not allow the required interdispersion of the elements to be achieved and inhibits any synergetic interaction.

6. ACKNOWLEDGEMENTS The financial support from Radici Chimica acknowledged.

SpA

(Novara,

Italy)

is gratefully

139 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

H. Rodhe, Science 248 (1990)1217. M.A. Wojtowicz, J.R. Pels and A.J. Moulijn, Fuel Proc. Technol. 34 (1993) 1. J.C. Kramlich and W.P. Linak, Prog. Energy Combust. Sci. 20 (1994) 149. A.R. van Amstel and R.J. Steward, Fert. Res. 37 (1994) 213. C. Kroeze, Sci. Total Environ. 152 (1994) 189. E. De Soete, Rev. Inst. Fr. Petr. 48 (1993) 4123. G. Centi, S. Perathoner and F. Vazzana, CHEMTECH 12 (199) 48. F. Kapteijn, J. Rodriguez-Mirasol and J.A. Moulijn, Appl. Catal. B9 (1996) 25. M. Schmidt, H. Glatzel-Mattheier and H. Sartorius, J. Geophysical Res. 106 (2001) 5507. 10. F. Cavani, F. Trifir5 and A. Vaccari, Catal. Today 11 (1991) 173 and references therein. 11. A. Vaccari, Appl. Clay Sci. 14 (2000) 161 and refereces therein. 12. F. Basile and A. Vaccari, in "Layered Double Hydroxides: Present and Future" (V. Rives, Ed.), Nova Science, New York, 2001, p. 285 and references therein. 13. S. Kannan and C.S. Swamy, Appl. Catal. B3 (1994) 109. 14. C. S. Swamy, S. Kannan, Y. Li, J. N. Armor and T. A. Brymer, Eur. Pat. 640,379A1 (1994). 15. G. Centi, A. Galli, B. Montanari, S. Perathoner and A. Vaccari, Catal. Today 35 (1997) 113. 16. J. P6rez-Ramirez, J. Overeijnder, F. Kapteijn and J. A. Moulijn, Appl. Catal. B23 (1999) 59. 17. M.C. Romfin-Martinez, F. Kapteijn, D. Cazorla-Amor6s, J. P6rez-Ramirez and J.A. Moulijn, App. Catal. A225 (2002) 87. 18. F. Basile, G. Fornasari, M. Gazzano and A. Vaccari, Appl. Clay Sci. 16 (2000) 185. 19. F. Basile, G. Fornasari, M. Gazzano and A. Vaccari Appl. Clay Sci. 18 (2001) 51. 20. R.D. Shannon, Acta Crystallogr. A32 (1976) 751. 21. S. Kannan, S. Velu, V. Ramkumar and C.S. Swamy, J. Mater. Sci. 30 (1995) 1462. 22. J. P6rez-Ramirez, F. Kapteijn and J.A. Moulijn, Catal. Letters 60 (1999) 133. 23. J.N. Armor, T.A. Braymer, T.S. Farris, Y. Li, F.P. Petrocelli, E.L. Weist, S. Kannan and C.S. Swamy, Appl. Catal. B7 (1996) 397. 24. S. Alini, A. Bologna, F. Basile, T. Montanari and A. Vaccari, Eur. Pat. 1,830,354A1 (2001). 25. A.L. Yakolev, G.M. Zhidomirov and R.A. van Santen, Catal. Letters, 75 (2001) 45. 26. X. Zhang, A.B. Wakers and M.A. Vannice, Appl. Catal. B4 (1994) 237.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

141

The Anderson-type heteropolyanions in the s y n t h e s i s of aluminaand zeolite-supported H D S o x i d i c p r e c u r s o r s E. Payen a, G. Plazenet a, C. Martin a, C. Lamonier a, J. Lynch b, V. Harl6 b aLaboratoire de Catalyse de Lille, USTL, Brit. C3, 59655 Villeneuve d'Ascq Cedex, France bInstitut Franqais du P6trole, l&4 Avenue de Bois-Pr6au, 92852 Rueil-Malmaison Cedex, France This study is aimed at using and identifying A1- and Co-containing Anderson-type heteropolyanions in the synthesis of (Co)Mo/alumina and (Co)Mo/zeolite HDS catalysts oxidic precursors. Various techniques such as Raman spectroscopy, NMR and EXAFS are used to show the apparition of the AlMo6024H63- entity upon impregnation of an alumina or a zeolite with an ammonium heptamolybdate solution. The question of the Mo-Co interaction in the promoted precursors is also tackled through the use of the CoMo6024H63entity. 1. I N T R O D U C T I O N With increasing social awareness of environmental risks, the will to reduce toxic emissions has increased and the quality of the fuels becomes a key factor. In particular, the legislation concerning the quantity of sulfur emission is increasingly severe, and limits are expected to go down to 50 ppm in Europe by 2005 and to 10 ppm in the following years. This imposes improvements in catalytic hydrotreatment (HDT) of the petroleum feedstocks in general, and in hydrodesulfurization (HDS) in particular. Catalyst improvement requires a better understanding of the structure and genesis of the active phases consisting of well dispersed molybdenum disulfide on a high specific surface area support. These nanocrystallites are generally promoted by cobalt or nickel atoms. The active phases are obtained through sulfidation of an oxidic precursor generally prepared by impregnation of the support with an aqueous solution of the elements to be deposited. The aim of the present study is to determine firstly the exact nature of the oxidic precursor in order to understand more clearly the formation of the active phases. Concerning the most common support, i. e. alumina, it has long been thought that the entities present in the impregnating solution remained intact upon adsorption on the alumina surface. However, recently, Carrier et al. [1-5] showed that some aluminium atoms of the support are extracted during the equilibrium impregnation to give the Anderson-type aluminomolybdate anion AlMo6Oa4H63- (AIMo6), whereas a Raman spectroscopic study [6,7] of the oxomolybdate phase of a Mo/AlzO3 prepared by incipient wetness impregnation revealed the presence of well dispersed aluminomolybdate entities interacting with the support, these entities being preserved during the drying step. In this paper, we will give further evidence of this phenomenon, and see whether it is possible to

142 impregnate directly the A1M06 entity. As the alternative support -zeolites- contain, in addition to framework atoms, variable amounts of amorphous alumina and AI 3+ ions, we will examine whether the same dissolution-precipitation phenomenon can be extended to them. We will also study the case of the Co-promoted compounds and work on the Mo-Co interaction thanks to the use of the Anderson-type cobaltomolybdate anion CoMo6024H63(CoM06). 2. E X P E R I M E N T A L 2.1. Precursors preparation and nomenclature The samples were prepared by incipient wetness impregnation. An aqueous solution of ammonium heptamolybdate (AHM), A1Mo6 or CoMo6 was prepared, its concentration depending on the desired Mo loading. The support was then impregnated with the appropriate volume of solution, depending on its porous volume. After the maturation step (2 h in ambient air), the sample was dried at 100~ overnight. The samples were analysed in the dried state, since the species formed upon impregnation are at stake here. The samples will be designated accordingly to the starting material used (AHM, A1Mo6, CoMo6...) and the support (A for alumina, Z for HY zeolite), separated with a slash. For instance, AHM/Z will identify the sample prepared by impregnation of a HY zeolite with an ammonium heptamolybdate solution. The porous volumes of both supports were 1 mL/g. The alumina had a specific surface area of 200 mZ/g, and the HY zeolite had bulk and framework Si/AI ratios of respectively 13.6 and 19, as determined respectively by Xray fluorescence and 29Si NMR.

2.2. Physical measurements The Raman spectra of the samples, maintained at room temperature, were recorded using a Raman microprobe (Infinity from Dilor), equipped with a photodiode array detector. The exciting light source was the 532 nm line of a Nd. YAG laser, and the wavenumber accuracy was 2 cm -1. 27A1 MAS NMR (magic angle spinning nuclear magnetic resonance) measurements of the samples were carried out on a Bruker AC400 spectrometer. Mo and Co K-edge EXAFS (Extended X-ray Absorption Fine Structure) measurements were carried out in the Laboratoire pour l'Utilisation du Rayonnement Electromagn6tique (Orsay) at the EXAFS D42 and D44 beamlines using synchrotron radiation from the DCI storage ring running at 1.85 GeV with an average current of 250 mA. At the Mo K-edge, the data were taken in the transmission mode through a double crystal monochromator Ge (400) using two ion chambers as detectors. The time measuring an X-ray absorption spectrum (19900-20900 eV) was about 15 rain, and three spectra for each sample were recorded. Due to their low loadings, some samples had to be analysed at the Co K-edge in the fluorescence mode through a channel cut monochromator Si (111) using a multielement detector. Six spectra for each sample were recorded. The Mo and Co K-edges EXAFS regions of the spectra were extracted and analysed using A. Michalowicz's software packages EXAFS 98 ppc and Round Midnight [8]. The EXAFS spectrum was first transformed from k-space (k 3, Kaiser window 3.6-15 ,~-1 for the Mo K-edge and 2.6-14 ~-1 for the Co K-edge) to R-space to obtain the radial distribution

143 function (RDF). The EXAFS spectrum for one or several co-ordination shells was isolated by inverse Fourier transform of the RDF over the appropriate region and fitted using the single scattering EXAFS equation with amplitude and phase functions calculated by FEFF [9-12] or extracted from experimental reference spectra. The UV (Ultra-Violet)-visible spectroscopy experiments were carried out on a Perkin Elmer spectrometer UV-VIS-PIR Lambda 19. 3. RESULTS AND DISCUSSION 3.1. Synthesis and characterisation of ammonium and exchanged salts of andersontype heteropolyanions 3.1.1. Starting materials (NH4)3AIMo6Oz4H6 was prepared according to Hall [13]: a solution of aluminium nitrate (20 mL, 3.1.10 .3 tool) is added to a boiling ammonium heptamolybdate solution (80 mL, 4.2.10 .3 mol). A white precipitate appears and the still warm solution is filtrated. The precipitate is recrystallised twice in water. (NH4)3CoMo6024H6 was prepared according to Nomiya et al. [14]: 2 g of 30% hydrogen peroxide are added to a cobalt sulfate solution (30 mL, 0.015 mol). This solution is slowly added to a boiling ammonium heptamolybdate solution (260 mL, 0.025 mol). A green precipitate appears, which is recrystallised twice in water.

225

947 898

546 566

CoM~

2 -coMo61

952 225

390

90~

6

........ S ~ k. 315

8

10

12

k (~-1)

14

(b)

C0M0/~

897, 919

i

i

i

i

190

490

790

1090

iii

i

1390 0

W o v e n u m b e r (cr~ 1) Fig. 1. Raman spectra of AIM06, CoM06, AIMo6/A, CoMo6/Z and CoMo/Z. *: lines of the zeolite.

1

2

3

R(A) o

4

5

6

Fig. 2. (a) Mo K-edge EXAFS of AIMo6 and CoMo6. (b) Corresponding Fourier transforms.

144 3.1.2. Characterisations The Raman spectra of the bulk AIMo6 and CoMo6 salts are presented in Fig. 1. They are characteristic of the Anderson heteropolyanions according to literature data [2]. A1Mo6 and CoMo6 differ by a shift of the two most intense lines, corresponding to the symmetric and anti-symmetric stretching modes of M o Q ( t ) groups (where t stands for terminal oxygen) observed respectively at 952 and 902 cm -1 for AIMo6, 947 and 898 cm -1 for CoMo6. Another difference concerns the vibrational mode of the bonds with the central heteroatom: a line at 575 cm -1 is to be seen for A1Mo6, whereas a doublet at 566 and 546 cm -1 is present for CoMo6. Finally, a shift of the deformation mode of the MoO2(t) group is also observed: 390 cm -1 for AIMo6, 380 cm -1 for CoMo6. The Infra-Red (IR) spectra (not reported here) exhibit the 1400 cm -1 band characteristic of the N-H bond, which shows the presence of the ammonium counterions. The Mo K-edge EXAFS spectra and their Fourier transforms are shown in Fig. 2. According to previous FEFF simulations [15-17], the first co-ordination shell (1-2.5 A, not phase corrected) corresponds to three types of oxygen neighbours, and the second one (2.53.5 A, not phase corrected) to the neighbouring molybdenum atoms and the central heteroatom. In Table 1 are presented the fitting parameters of these spectra, giving the numbers of neighbours and their distances. Table 1 Fitting parameters of the Mo K-edge EXAFS spectrum of the ammonium or exchanged salts of AIMo6 and CoMo6 Mo-O (1) Mo-O (2) Mo-O (3) Mo-Mo Mo-AI/Co N 2.0 2.0 2.0 2.2 1.0 AIMo6 Ao (A) 0.02 0.06 0.06 0.06 0.02 R (A) 1.72 1.92 2.34 3.33 3.46 AE (eV) 5.4 5.4 5.4 5.4 5.4 N 2.4 2.0 2.2 2.5 1.3 CoMo6 Ao (A) 0.06 0.08 0.08 0.09 0.05 R (A) 1.71 1.95 2.30 3.32 3.31 AE (eV) 1.1 1.1 1.1 1.1 1.1 3.2. Synthesis of non-promoted oxidic precursors 3.2.1. Zeolite-supported samples The appearance of A1Mo6 is observed upon impregnation of a HY zeolite with an AHM solution. The 27A1 MAS NMR spectra of three AHM/Z samples are presented in Fig. 3 and show the presence of AIMo6 whatever the Mo loading. The Mo K-edge EXAFS spectra show this species to be majority at 3 Mo wt.%. The Raman spectra (Fig. 4) indicate that at the intermediate and high loadings, the parallel apparition of AHM is observed, with the main line at 937 cm -1. The study of other HY zeolites containing more or less extraframework aluminium atoms showed that the precipitation of AHM is dependent on this quantity of non-framework A1 atoms. The first reaction to take place upon impregnation of a zeolite is the dissolution-precipitation phenomenon leading to A1Mo6 from solubilisation of amorphous alumina located outside the zeolitic cavities. When amorphous alumina lacks for the formation of AIMo6, AHM precipitates at the surface of the support [15,17]. Upon

145 calcination, AIMo6 evolves into a surface "Ala(MoO4)3" phase (main line at 1003 cm -1) and a not determined polymolybdate phase (line at 980 cm-1), whereas AHM precipitate leads to crystalline MoO3 (Fig. 5). When the sample is put back to ambient atmosphere, MoO3 remains whereas A1Mo6 is recovered (Fig. 6) and represents the majority component on the 3 Mo wt.% sample, as confirmed by Mo K-edge EXAFS (spectrum not reported here).

60

-~ ~ , , .

937 ......902~j \__AH__;M__/ (8_%) Z 93,~ /952

~,]

15

~n

902 .7/3

~" 11 868,\A

L.._..... ~y' \ AHM/Z(5%) 952

130

ppm Fig. 3. 27A1MAS NMR of AHM/Z at 3, 5 and 8 Mo wt.%.

430

730

1030

1330

Wavenumber (cm1)

Fig. 4. Raman spectra of A H M / Z at and 8 Mo wt.%.

818 995

3, 5

818 995 AHM/Z(8~) 980 \j

995

~ 1003

~M_./z_(35! 1006

382 270

470

980 952.1

827 ~029 x~8 ~ LAI2(MoO4) 3 670

870

1070

Wavenumber (cm1)

AHM/Z(5%)

o

1270

Fig. 5" Raman spectra of calcined AHM/Z at 3, 5 and 8 Mo wt.%. *: lines of the glass of the in situ cell. +: lines of the zeolite.

1;o

~;o

go

~;o

~3o

o 11;o

Wavenumber (cmI )

1~;o

Fig. 6: Raman spectra of rehydrated A H M / Z at 3, 5 and 8 Mo wt.%. +: lines of the zeolite.

146 3.2.2. Alumina-supported samples Comparison with 10 Mo wt.% Mo/A has therefore been performed as the formation of this AiMo6 has been shown to occur upon impregnation of an alumina with AHM. Further evidence on this can be gained by 27A1 MAS NMR and Mo K-edge EXAFS experiments. In 27A1 MAS NMR, a shoulder is evidenced on the high chemical shifts side of the line characteristic of the octahedral atoms of the support at 0 ppm (Fig. 7). The corresponding line is clearly evidenced by subtraction of the spectrum of the alumina from the AHM/Ai203 one (Fig. 7). The characteristic peak of A1M06 at 15 ppm(2)appears, confirming the presence of the aluminomolybdate anion as already seen by Raman spectroscopy (7). However, liquid state 27A1NMR experiments carried out on the matured samples (not reported here) showed a difference between zeolite- and alumina-supported samples: the characteristic peak of AIMo6 is sharp for AHM/Z and broad for AHM/A. This tends to indicate a confinement of the entity on the alumina. Raman features also gave evidence of the dispersion of A1M06 in AHM/A and of its precipitated form in AHM/Z. The Mo K-edge EXAFS spectrum of AHM/A (not reported here) presents differences with those of A1M06. The fit realised with FEFF-calculated contributions shows that these are adapted to the spectrum, but leads to parameters that do not exactly match those of reference A1M06. The other techniques showing no evidence of heterogeneity, the distortion of the entity resulting from its dispersion might explain the results A!203 of the fit. Confirmation was sought through impregnation of an aluminomolybdate-containing solution. But the low solubility of the starting material (which limits the loading of the sample at 1.4 Mo wt.%) and the buffer effect of alumina impose the decomposition of this species (stable in the pH range 2-6 [18]). It consequently leads to free and adsorbed tetrahedral entities, as shown by the Raman lines at respectively 897 and 919 cm 1 (Fig. 1). Thus the presence of A1Mo6 at high Mo loadings on AHM/A should be ascribed to the high dispersion and to the interaction with the alumina support. Upon calcination, the presence of a surface aluminium molybdate "A12(MoO4)3" is shown by Raman spectroscopy, but A1Mo6 is recovered upon rehydration of the sample [7]. Fig. 7:27A1 MAS NMR spectra of A1M06, A1203 .........

and AHM/A.

147 3.3. Synthesis of promoted oxidic precursors 3.3.1. Dried CoMo/A and CoMo/Z Like for AHM/A, whatever the starting material used for the preparation (CoMo6 or coimpregnation with AHM and cobalt nitrate), in Z7Al NMR, the characteristic peak of AIMo6 at 15 ppm appears (not reported here). Thus, we conclude that CoMo6 is decomposed upon impregnation and leads to the formation of the aluminomolybdate as for solids prepared by co-impregnation with AHM and cobalt nitrate. This decomposition is also evidenced by UV-visible spectroscopy, with the disappearance of the band at 600 nm characteristic of the Co 3+ ions. The molybdenum atoms react to give A1Mo6, without interacting with the cobalt atoms which go into the surface sites of the alumina. Fig. 1 also shows the Raman spectra of two zeolite-supported Co-promoted oxidic precursors. The one prepared by co-impregnation of ammonium heptamolybdate and cobalt nitrate (CoMo/Z) presents the Raman features of A1Mo6, whereas the one prepared by impregnation with ammonium cobaltomolybdate (CoMo6/Z) presents those of CoMo6. Co K-edge EXAFS spectra (Fig. 8) indicate a Mo-Co contribution for CoMo6/Z, whereas it is absent in CoMo/Z. It appears that upon co-impregnation, the molybdenum atoms react to give A1Mo6, without interacting with the cobalt atoms which go into the cationic sites of the zeolite (19-23). This may explain the absence of promoting effect observed before (24). In opposition to that, the use of a CoMo6 solution leads to the preservation of an entity inside of which molybdenum and cobalt are in an interacting position. 3.3.2. Calcined CoMo/A and CoMo/Z In the case of CoMo/A, which presents the same molybdate entities as in the nonpromoted sample, their behaviour upon calcination is identical, and AIM06 evolves into an "aluminium molybdate" phase, and re-appears upon transfer to air. In the case of CoMo/Z, the calcination generally performed at 500~ to eliminate the ammonium counterions induces a decomposition of the cobaltomolybdate entity. Upon rehydration, the so freed molybdenum atoms associate with aluminium as in the impregnation of AHM to give AIM06. Thus, the calcination-rehydration steps have a levelling effect on the structure of the promoted catalyst: the same entities are finally identified whatever the support or the synthesis route.

(a)

(b) -- CoMo6

CoMo6

-- CoMo6/Z

CoMo6/Z

CoMo/Z

CoMo/Z ,

4.7

&7

8.7

k (A -1)

10,7

12,7

14,7

o

1

2

,

,

3

4

5

R (,&,)

Fig. 8. (a) Co K-edge EXAFS of CoM06, CoMo6/Z and CoMo/Z. (b) Corresponding Fourier transforms.

148 4. C O N C L U S I O N This study enables us to draw a parallel between the behaviours of alumina and HY zeolite upon impregnation with oxomolybdate entities. In some conditions, when the buffer effect of the alumina is avoided, both supports see extraction of aluminium atoms and their inclusion in an Anderson-type heteropolyanion, AIMo6024H63-. This entity is well dispersed on the alumina support whereas it appears as a bulk compound located in the macropores of the HY zeolite. However, it is still unknown whether the formation of this species is to be favoured. For promoted samples, a significant difference exists between both supports, since CoMo6024H63- can be preserved upon impregnation on a zeolite. Yet, the preparation of the catalysts has to be controlled: the final calcination-rehydration steps tend to level the various synthesis and lead to the presence of AIMo6024H63-. REFERENCES 1. X. Carrier, Thesis, Paris, 1998. 2. X. Carrier, J.-F. Lambert and M. Che, J. Am. Chem. Soc., 119 (1997) 10137. 3. X. Carrier, J.-F. Lambert and M. Che, Stud. Surf. Sci. Catal., 121 (1998) 311. 4. X. Carrier, J.-F. Lambert and M. Che, Stud. Surf. Sci. Catal., 118 (1998) 469. 5. X. Carrier, J.-F. Lambert and M. Che, Stud. Surf. Sci. Catal., 130 (2000) 1049. 6. L. Le Bihan, Thesis, Lille, 1997. 7. L. Le Bihan, P. Blanchard, M. Fournier, J. Grimblot and E. Payen, J. Chem. Soc., Faraday Trans., 94 (1998) 937. 8. A. Michalowicz, J. Phys. IV, 7 (1997) 235. 9. A. L. Ankoudinov, Thesis, Washington, 1996. 10. J. J. Rehr, J. Mustre de Leon, S. I. Zabinsky and R. C. Albers, J. Am. Chem. Soc., 113 (1991) 5135. 11. J. J. Rehr, S. I. Zabinsky and R. C. Albers, Phys. Rev. Lett., 69 (1992) 3397. 12. S. I. Zabinsky, J. J. Rehr, A. Ankudinov, R. C. Albers and M. J. Eller, Phys. Rev. B, 52 (1995) 2995. 13. R. D. Hall, J. Am. Chem. Soc., 29 (1907) 692. 14. K. Nomiya, T. Takahashi, T. Shirai and M. Miwa, Polyhedron, 6 (1987) 213. 15. G. Plazenet, Thesis, Lille, 2001. 16. G. Plazenet, E. Payen and J. Lynch, Phys. Chem. Chem. Phys., Submitted. 17. G. Plazenet, E. Payen, J. Lynch and B. Rebours, J. Phys. Chem. B., Submitted. 18. G. A. Tsigdinos, Top. Curr. Chem., 76 (1978) 1. 19. P. W. de Bont, Thesis, Delft, 1998. 20. P. W. de Bont, M. J. Vissenberg, E. Boellaard, V. H. J. de Beer, J. A. R. van Veen, R. A. van Santen and A. M. van der Kraan, J. Phys. Chem. B, 101 (1997) 3072. 21. M. J. Vissenberg, Thesis, Eindhoven, 1999. 22. M. J. Vissenberg, P. W. de Bont, J. W. C. Arnouts, L. J. M. van de Ven, J. W. de Haan, A. M. van der Kraan, V. H. J. de Beer, J. A. R. van Veen and R. A. van Santen, Catal. Lett., 47 (1997) 155. 23. M. J. Vissenberg, P. W. de Bont, E. M. van Oers, R. A. de Haan, E. Boellaard, A. M. van der Kraan, V. H. J. de Beer and R. A. van Santen, Catal. Lett., 40 (1996) 25. 24. P. Leyrit, Thesis, Paris, 1999.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

149

Sol-gel preparation of pure and silica-dispersed vanadium and niobium catalysts active in oxidative dehydrogenation of propane P. Moggia, G. PredierP, D. CauzzP, M. Devillers~, P. Ruiza, S. Morselli~' and O. Ligabuea aDepartment of Organic and Industrial Chemistry, University of Parma, Parco Area delle Scienze 17/A, 1-43100 Parma, Italy t'Department of General and Inorganic Chemistry, Analytical Chemistry, Physical Chemistry, University of Parma, Parco Area delle Scienze 17/A, 1-43100 Parma, Italy cunit6 de Chimie des Mat6riaux Inorganiques et Organiques, Universit6 catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-La-Neuve, Belgium dunit6 de Catalyse et de Chimie des Mat6riaux Divis6s, Universit6 catholique de Louvain, Croix du Sud 2/17, B-1348 Louvain-La-Neuve, Belgium Hydrolytic and non-hydrolytic sol-gel routes are implemented to prepare various pure and silica-dispersed vanadium- or niobium-based oxide catalysts corresponding to the compositions Nb-V, Sb-V and Nb-V-M (M = Sb, Mo, Si). Starting reagents in the hydrolytic procedure are isopropanol solutions of the metal alkoxides. The non-hydrolytic route is based on reactions between metal and Si alkoxides and hexane suspensions of niobium(V) chloride. The catalysts are tested in propane oxidative dehydrogenation. NbV05, SbVO4 and Nb2Mo3Oll are the major crystalline phases detected in the fresh catalysts, but structural modifications are in some cases observed after the use in the catalytic tests. At 500~ propane conversions of 30 % and selectivities to propene between 20 and 40 % are attained. When the space velocity is decreased, acrolein is in some cases found as by-product. 1. INTRODUCTION The key point to reach high catalytic performances with multimetallic oxides appears to be the ability to tune the catalyst architecture, by controlling the nature and homogeneity of the active phase and, in the case of multiphasic catalysts, the interdispersion of the various oxide phases. This can be accomplished by implementing adequate preparative procedures such as sol-gel methods. The sol-gel preparation of catalytic materials [1] has intrinsic advantages compared with other methods, because it allows a precise control of the synthetic conditions and hence of factors such as purity, stoichiometry, homogeneity (or Corresponding author. Tel. +39-0521-905464 E-mail: [email protected]

150 controlled heterogeneity) that are all relevant to the catalytic activity. Most sol-gel preparation procedures are carried out in the presence of water, and are based on inorganic polymerisation reactions, for which metal alkoxides are used as starting materials. A macromolecular network is then obtained via hydrolysis and subsequent hydroxyl condensation. Alternatively, the so-called non-hydrolytic sol-gel process is available to overcome disadvantages due to the presence of water. In this process, M-O-M' bonds are obtained by direct reaction between suitable precursors, such as metal chlorides or alkoxides; water is not required for hydrolysis, nor produced by condensation [2]. Sol-gel methods have been found to be particularly powerful to control the intimacy of molecular scale mixing in mixed oxides [3]. Niobia and silica-niobia systems were previously prepared by us by the hydrolytic sol-gel method from Nb(OEt)5 and Si(OMe)4 as precursors, obtaining high surface area mixed-oxide materials in a wide composition range, displaying variable acidities [4]. Since transition metal alkoxides with a d o configuration, such as Ti(IV) or Nb(V), are very reactive towards hydrolysis, pure niobia gels were obtained by introducing a mineral acid and/or a complexing agent such as oxalic acid in the solution of Nb precursor, in order to stabilise the sol and prevent the precipitation of niobic acid. Moreover, to obtain uniform high surface area mixed Nb-Si oxides, suitable combinations of metal precursors were used: the rate of hydrolysis and condensation of the individual precursors must be comparable in order to avoid segregation of single oxides. For this reason, the alternative non-hydrolytic sol-gel method was applied by us for the first time to produce mixed Nb-V and Nb-V-Si oxide systems. These materials were preliminary tested in the propane oxidative dehydrogenation (ODH) reaction, also in the presence of a potential promoter such as Sb204 [5]. The results obtained demonstrated the feasibility of the non-hydrolytic sol-gel route to prepare V-Nb mixed-oxide catalysts. Moreover, it seemed that interesting catalysts for the ODH of propane could be obtained by refining the preparation procedure and using suitable promoters. In this work, pure or silica-dispersed Nb-V systems were prepared by either the hydrolytic or the non-hydrolytic procedures. Moreover, the hydrolytic sol-gel route was extended to the preparation of pure or silica-dispersed Sb-V and Nb-V-M (M = Sb, Mo) oxide systems. The catalytic performances of these mixed-oxide systems in propane oxidative dehydrogenation were investigated.

2. EXPERIMENTAL 2.1. Catalysts preparation 2.1.1. Preparation of Nb-V and Nb-V-Si oxides by the non-hydrolytic sol-gel route NbC15, VO(OiPr)3, SiCh and Si(OEt)4 were the precursors adopted in the nonhydrolytic sol-gel preparations. The syntheses were conducted under inert atmosphere. VO(OiPr)3 and eventually SiCh and/or Si(OEt)4 were added dropwise to stirred suspensions of NbCI5 in hexane. By raising the temperature to 100~ limpid solutions were obtained, giving aiter few hours green gels. The metal precursors were mixed in order to obtain the following atomic ratios: 1-1 for the binary systems Nb-V and l" 1:5 for the ternary systems Nb-V-Si. The obtained xerogels were activated in air at 550~ for 5 h.

151

2.1.2. Preparation of Nb-V, Sb-V and Nb-V-M (M = Si, Mo, Sb) oxides by the hydrolytic sol-gel route Nb(OiPr)5, VO(OiPr)3, Sb(OiPr)3, Mo(OiPr)5 and Si(OEt)4 were the precursors adopted in the hydrolytic preparations. The syntheses were conducted under inert atmosphere by adding mixtures of water and 2-propanol to stirred mixtures of the alkoxides in 2-propanol. Each synthesis led to a stable sol, which turned into gel within few days. The metal precursors were mixed in order to obtain the following atomic ratios: 1:1 for the binary systems Nb-V and Sb-V; 1:1: l, 1:2:1 and 1:1:5, for the ternary systems Nb-V-Mo, Nb-VSb and Nb-V-Si, respectively. All the obtained xerogels were activated in air at 550~ for 5h. 2.2. Catalysts characterization The precursor gels were characterized by TGA and the calcined catalysts by FTIR and Raman spectroscopy, XRD, SEM and surface area determination. FTIR spectra in the range 4000-400 cml were recorded by using a Nicolet 5PC spectrophotometer. BET surface areas of the activated catalysts were measured with a Micromeritics ASAP 2000 analyzer using nitrogen at 77K. TGA analysis was carried out with a Mettler Toledo TGA/SDTA 851 e analyzer. Raman spectra were recorded on a DILOR-JOBIN-YVON-SPEX Olympus DX 40 spectrometer using a He-Ne laser ( = 632.8 nm). Powder XRD patterns were measured on a Siemens D-5000 diffractometer using the Cu-K radiation. SEM images were obtained with a Philips XL30 instrument, at the LAMEL Institute (CNR, Bologna, Italy). 2.3. Catalytic tests Propane oxidation experiments were generally carded out with 0.5g catalyst at atmospheric pressure, in the temperature range 400-500~ at a space velocity of 60 ml.min-1 geat-1. The gas feed composition was 10% C3H8, 10% 02 and 80% He (total flow rate 30 ml/min). Some experiments were also performed at a lower space velocity of 36 ml.min~gca(l, with a reactant mixture corresponding to 17% C3H8, 17% 02 and 66% He (total flow rate 18 ml/min). 3. RESULTS AND DISCUSSION All the obtained xerogels were amorphous solids, characterized by very broad IR bands in the region 1000-600 cm1. TGA analysis of all the prepared xerogels showed two successive weight losses associated with endothermal peaks (evidenced by the differential calorimetric scan at 10~ min~) at about 100~ due to the elimination of residual water and 2-propanol, and 200~ related to the elimination of residual alkoxide ligands for all the prepared xerogels. BET surface area measurements are reported for the calcined catalysts in Table 1.

152 Table 1. BET specific surface areas of the catalysts calcined at 550~ Surface Area (m2/g) Catalysts 1:1 Nb-Vnon-hydrolytic

6.4

1:1 Nb-V hydrolytic

1.3

1:1 Sb-V

31

1:2:1 Nb-V-Sb

7.9

1:1:1 Nb-V-Mo

3.0

1:1:5 Nb-V-Si non-hydrolytic

146

1:1:5 Nb-V-Si hydrolytic

132

The observed BET surface areas are rather low for all the samples, with the only exception of the ternary 1:1:5 Nb-V-Si, both hydrolytic and non-hydrolytic, and the binary 1:1 Sb-V systems. The calcination at 550~ of the 1:1 Nb-V binary system prepared either via the nonhydrolytic method or the hydrolytic one led to the formation of crystalline NbVO5, as evidenced by the XRD pattern reported in Fig. 1, which matches exactly the JCPDS file of NbVO5 (46-0046). The SEM image of the Nb:V 1:1 catalyst after the thermal treatment at 550~ (Fig. 2, for the non-hydrolytic preparation) evidenced a rather uniform distribution of micrometer sized crystallites of the NbVO5 phase.

CO t'4-,

400 350 300 250" 20O150

100-

500

0

I

~ I1,.

,

20

fl.I '

40

2-Thel

I

!

60

80

Fig. 1. XRD pattern of 11 Nb:V after the calcination treatment at 550~ practically pure NbVO5 phase.

1 00 containing the

153

Fig. 2. SEM image of 1"1 Nb:V (non-hydrolytic preparation) after the calcination treatment at 550~ evidencing the uniform distribution of the NbVO5 phase. After the catalytic runs, the NblsV4Os5 phase was recognized in the XRD pattern illustrated in Figure 3. Decomposition of the NbVO5 phase to the Nb-enriched NblsV4055 phase and V205 was also hypothesized from the Raman spectra, in which the characteristic pattern of V205 (146, 290, 404, 480, 527, 702 and 994 cm1) was recognized (Figure 4). However, there was no evidence of crystalline V205 provided by XRD.

o,..~ r~

90 8O 70 60 50 40 30 20 10 0

x

x

i

I

|

. !

!

i

25

50 2-Theta

75

Fig. 3. XRD pattern of 1"1 Nb-V after the catalytic reactions, containing Nb~sV4055.

100

154 5000

4000 3000 2000 I 000

0

0

200

400 600 800 wavenumbers (cm-1)

1000

1 200

Fig. 4. Raman spectrum of l'l Nb:V (non- hydrolytic preparation) after calcination at 550~ In the case of the binary system 1:1 Sb-V, calcination at 550~ led to the formation of

crystalline SbVO4, as evidenced by XRD analysis (Fig. 5). Unlike the 1:1 Nb-V system, crystalline SbVO4 was maintained even after the catalytic runs. AS far as the ternary system 1:1:1 Nb-V-Mo is concerned, the Nb2M03011 and (V0.07Mo0.93)5O14 phases were detected by XRD after the thermal treatment (Fig. 6). Similarly to the 1:1 Sb-V material, the XRD pattern of the 1:1:1 Nb-V-Mo system was unchanged after the catalytic runs. In the case of the ternary system 1:2:1 Nb:V:Sb, besides SbVO4, the NblsV4055 phase was again evidenced. After the catalytic runs, only traces of NblsV4055 were detected besides SbVO4, indicating some structure modifications. Again, although no evidence of crystalline V205 was given by XRD, the characteristic pattern of V205 was recognized in the Raman spectrum. In the case of the ternary system 1:1:5 Nb-V-Si, prepared either via the non-hydrolytic method or the hydrolytic one, the XRD spectra evidenced a predominantly amorphous structure, but with the presence of the NbVO5 phase. However, the characteristic pattern of V205 was recognized in the Raman spectra. Quite good results were achieved in the propane ODH experiments. In Table 2, propane conversion (Xc3HS), selectivity to propene (Sc3H6), selectivity to carbon dioxide (Sco2) and yield in propene (Yc3H6) are reported. Catalytic data were collected at the reaction temperature of 500~ feed composition: 10% C3H8, 10% 02, 80% He and space velocity of 60 ml minl g-1. The 1:1 Sb-V and 1:1:5 Nb-V-Si systems only were tested at the different space velocity of 100 ml min l g~. Almost all the prepared systems exhibited propane conversions of about 30% and propene selectivities higher than 20%. The most selective catalysts with respect to propene formation were the 1:1 Nb-V prepared via the hydrolytic method and 1:1 Sb-V systems (Sc3H6 ca 40o//0). The 1:1 Sb-V and 1:1:5 Nb-V-Si prepared via the non-hydrolytic method gave the best results in terms of propane conversion and yield in propene. In these two cases, while the higher conversion is in contradiction with

155 the lower amount of catalyst loading, the high yield in propene can be explained by the lower contact time adopted during the catalytic test, which preserves propene from further oxidation to carbon dioxide. Moreover, a minor amount of catalyst favours a better control of the reaction thermal efficiency inside the catalytic bed, reducing phenomena of complete oxidation due to hot spots. 400 350 300 250 r~

= 200 150 100

x

50 ........

,_

X ~

L

,r

X

._/t_ .

.

.

.

.

~

0

20

_

i

i

40

60

80

100

2-Theta Fig. 5. XRD pattem of 1"1 Sb:V after the calcination treatment at 550~ practically pure SbVO4 phase. 400 350 300 .~ 250 = o 200 150 100 50 0

containing the

A,A !

i

i

i

20

40

60

80

100

2-Th eta

Fig. 6. XRD pattern of 1"1"1 V:Nb:Mo after the calcination treatment at 550~ containing the mixed NbEM03Oll (x) and (Vo.07Moo.93)5014 (0) phases.

156 Table 2 Catalytic results in propane ODH. Temperature 500~ 02 and 80% He. Catalyst loading: 0.5 g. Total flow: 30 Catalysts XC3H8 . . . . . . . . (%) 1:1 Nb-V non-hydrolytic 33

a

Feed composition: 10% C3H8, 10% ml/min

8C3H6

8CO2

YC3H6

(%) 23

(%) 30

(%) 7

1:1 Nb-V hydrolytic

8

37

24

3

1:1 Sb-V a

34

38

39

13

1:2:1 Nb-V-Sb

33

23

30

7

1:1:1 Nb-V-Mo

27

25

20

7

1:1:5 Nb-V-Si non-hydrolytic a

39

30

22

11

1"1"5 Nb-V-Si hydrolytic

31

20

20

6

catalyst loading: 0.3g

The 1:1:5 Nb-V-Si catalyst prepared via the non-hydrolytic method was tested also at the higher temperature of 550~ Propane conversion of 37%, slightly lower than at 500~ and propene selectivity of 51% were obtained. The lower conversion was probably related to some catalyst deactivation; however, propene selectivity and consequently the yield in propene were significantly higher than at 500~ Under these conditions, propene productivity was 0.23 kilograms of propene per kilogram of catalyst per hour, in very good agreement with the best propene productivity data reported in literature for the V/Nb/O catalysts [6]. A set of ODH experiments was carried out at the lower space velocity of 36 ml min"1 g-1 in order to study the effect of contact time on the selectivity behaviour. Acrolein was detected among the reaction products of some catalysts, the highest selectivity value of 8% being achieved with the 1"1"5 Nb-V-Si catalyst prepared via the non-hydrolytic method at 500~ 4. CONCLUSIONS In this work, 1:1 Nb-V, 1:1 Sb-V, 1:2:1 Nb-V-Sb, 1:1:1 Nb-V-Mo and 1:1:5 Nb-V-Si mixed-oxide systems were prepared by applying sol-gel techniques and tested as catalysts in the oxidative dehydrogenation of propane. High metal oxides interdispersions were obtained, as confirmed by the practically pure phases NbVO5 and SbVO4 identified by XRD in the silica-free 1:1 Nb-V and Sb-V catalysts. Several mixed-oxide phases were detected in the 1:2"1 Nb-V-Sb and 1:1-1 Nb-V-Mo catalysts, while the systems containing silica were amorphous and characterized by quite high surface areas. All the prepared systems were active in propane ODH and displayed promising selectivities to propene, particularly the 1:1:5 Nb-V-Si prepared via the non-hydrolytic sol-gel method and the 1:1 Sb-V prepared via the hydrolytic route.

157 ACKNOWLEDGEMENTS

This work was supported by MIUR (Ministero dell'Istruzione, Universit/t e RicercaRome) and the Belgian National Fund for Scientific Research (Brussels). Omar Ligabue was recipient of a Socrates-Erasmus grant for a four-month stay in Louvain-la-Neuve. LAMEL Institute of CNR (Bologna, Italy) is acknowledged for the SEM analyses. REFERENCES

1. C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, San Diego, 1990. 2. J. B. Miller and E. I. Ko, Catal. Today, 35 (1997) 269. 3. R.H. Smiths, K. Seshan and J. H. R. Ross, Stud. Surf. Sci. Catal., 72 (1992) 213. 4. S. Morselli, P. Moggi, D. Cauzzi and G. Predieri, Stud. Surf. Sci. Catal., 118 (1998) 65. 5. F. Barbieri, D. Cauzzi, F. De Smet, M. Devillers, P. Moggi, G. Predieri and P. Ruiz, Catal. Today, 61 (2000) 353. 6. S. Albonetti, F. Cavani and F. Trifir6, Catal. Rev. Sci. Eng., 38 (1996) 413.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

159

Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas D.J. Draelants, Y. Zhang, H. Zhao, G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium To deposit the nickel catalyst in the filter, two preparation methods, depositionprecipitation of nickel nitrate with urea and conventional impregnation with nickel nitrate, were used and compared. In the deposition-precipitation with urea, experimental parameters such as reaction time and urea/nickel molar ratio were investigated to obtain a high controlled fixation of precursor during the slow decomposition of urea. SEM-EDX characterization showed that the urea method gave a fairly uniform spatial distribution of nickel throughout the filter substrate. On the contrary, with the conventional impregnation method, most of the precursor was deposited on the outer surface of the substrate. Tar cracking over nickel-modified catalytic filter prepared by the urea method was studied with a synthetic biomass gasification gas (free of H2S and dust). Benzene and Naphthalene were used as the model compounds, respectively. It was found that both benzene and naphthalene could be completely converted at 800-900~ over the catalytic filter with 1 wt% nickel. 1. INTRODUCTION Catalytic filters and membranes as multifunctional reactors, coupling a catalytically promoted reaction and a separation allowed by the filter or membrane itself, have gained an increasing attention in recent years due to demand for process efficiency improvement as well as environmental concerns [1-6]. The applications of catalytic filters in hot gas cleaning by the simultaneous removal of solid particles and some pollutants such as tar and ammonia from biomass gasification gas are also attracting public interest [1]. Apparently, the novel idea to combine reaction and separation in a single process unit may lead to a higher energy efficiency but also simplify the entire gas cleaning process with a potential reduction in investment costs and space saving, which is an important factor to make biomass gasification a feasible alternative energy source. Tars are always present in gasification gas as a side product and can easily plug downstream process equipment. Catalytic high-temperature gas cleaning is one of the potential solutions to the operational problems caused by tars. It has been demonstrated that nickel-based catalysts are very efficient in decomposing tars in biomass gasification To whom correspondenceshould be addressed. Email: [email protected]; Tel: +32-2-6293250; Fax: +322-6293248.

160 gas at 900 ~ [7]. Regarding the particulate removal, the ceramic candle filters can be applied to remove particles down to micrometer size at high temperature. So, a catalytic filter for simultaneous removal of solid particles and tar from biomass gasification can be achieved if a nickel catalyst for tar decomposition is placed on the pore walls of the inorganic filters through optimised deposition techniques. The most straightforward method is conventional impregnation with a nickel salt solution into the support body, followed by drying to bring about the deposition of salt inside the pores of the support. However, the drying is critical since the solution migrates and the precursor is deposited mainly where the solvent evaporates. Even when performed carefully, some nonuniformity must always be expected [8]. An improvement can be achieved by depositionprecipitation to immobilize the precursor in the pores of the substrate with a precipitant before the drying step [9]. On the other hand, it is required that the precursor and the precipitant are distributed uniformly throughout the pores before the onset of precipitation. Therefore, urea can serve well as a precipitant because it decomposes slowly at 90~ in an aqueous solution with generation of hydroxyl ions to make the precursor precipitate homogeneously throughout the pores [8, 10]. In this work, catalytic ceramic filters with uniform nickel-distribution were attained by the urea precipitation method, which was compared with the conventional impregnation method. To gain more insight on the urea method, the influence of the reaction time of urea decomposition and urea/nickel molar ratio on the fixation of the precursor on the support prior to drying was investigated. Finally, the catalytic performance of the nickel-modified filter in hot gas cleaning was tested on lab-scale using a simulated biomass gasification gas with benzene and naphthalene as mr model compounds. 2. EXPERIMENTAL 2.1. Preparation and characterisation of catalytic filter Some a-A1203-based filter discs (Schumacher, Germany) were vacuum impregnated with a solution containing appropriate amounts of nickel nitrate and urea. After the excess solution was drained off, the discs were placed in a closed vessel and kept at 90 ~ for a certain period, resulting in precipitation of nickel precursor by the slow hydrolysis of urea in the pores of the discs. After reaction, the filter discs were dried at 110 ~ for a few hours and calcined at 450 ~ for 4 h. Then the nickel-modified ceramic filter discs were obtained. For the conventional impregnation method, other steps and experimental conditions were identical to the preparation procedure with the urea method except for the absence of urea in the impregnation solution. The two-dimensional distribution of nickel throughout the modified filter disc was examined with SEM/DEX (energy dispersive X-ray) on a polished radial cross section of the disc. The analysis was performed on a JSM 6400 (JEOL) equipped with a NORAN Xray analysis system. The K line of nickel was used as the X-ray analysis line. The electron probe worked at an acceleration voltage of 15-25 kV and a current intensity of 3 x 109 A. 2.2. Test of reaction performance For reaction tests, the catalytic filter disc was fixed in the middle of a reactor tube (internal diameter 3 cm and length 50 cm), which was made of dense c~-A1203. The

161 catalytic performance was tested in a laboratory reaction setup, which has been described in detail in a previous publication [ 11 ]. In studies of tar cracking using a separate catalyst bed, two types of tar sources are applied, one directly drawn from a biomass gasifier and the other from model compounds. According to VTTs work [ 12], the tar consists mainly of highly stable compounds such as benzene (60-70 wt %), naphthalene (10-20 wt %), and other polyaromatic hydrocarbons (10-20 wt %), which can amount to 15-20 g of tar/Nm 3 in biomass gasification. So, benzene and naphthalene were used in this work as tar model compounds with a fixed concentration of 15 g/Nm 3 (4300 ppm) for benzene and 5 g/Nm 3 (875 ppm) for naphthalene, respectively. The gas composition used was 50 vol % N2, 12 vol % CO, 10 vol % HE, 11 vol % CO2, 12 vol % H20, 5 vol % CH4, 4300 ppm benzene (or 875 ppm naphthalene), which is a typical composition of the product gas from a biomass fluidised bed gasifier operated with air. The reaction tests were performed under three filtration gas velocities 2.5, 4 and 6 cm/s. All experimental points were monitored for at least 60 min after the reaction reached an apparent steady state at the selected operation condition. 3. RESULTS AND DISCUSSION 3.I. Influence of reaction time and urea/nickel molar ratios Urea decomposes slowly at 90~ in aqueous solution [ 10] according to reaction (1). Consequently, hydroxyl groups are slowly generated, uniformly throughout the pores and the precipitation takes place homogeneously. CO(NH2)2 + H2O ~ 2OH- + 2NH4 + + CO2

(1)

In the deposition-precipitation method with urea, experimental parameters such as the reaction temperature, the reaction time and the concentrations of urea and Ni 2+ in the impregnation solution determine the fixation degree of nickel, which is the amount of nickel precursor fixed by precipitation on the support before drying relative to the maximum amount that can be deposited. At present, there is no general agreement in the literature [9, 13-14] about the choice of these experimental parameters. So, it is necessary to investigate the urea method first, to determine adequate values of reaction time and urea/nickel molar ratio for the impregnation of the ceramic substrates. The study of the influence of the reaction time for urea decomposition and the urea concentration in the urea method was performed on ct-A12O 3 powder rather than a ceramic filter disc since the supply of the ceramic discs was limited. Experimental details have been reported elsewhere [ 11]. To investigate the role of the reaction time, the urea/nickel molar ratio was fixed at a value of 1.3. The results are shown in Fig. 1. The precipitation amount of nickel precursor and the suspension pH both increased with reaction time. The amount of nickel precipitate sharply increased during the first 6 h reaction time and increased slowly after 6 h. This suggested that at least 6 h reaction time was needed to fix a reasonable amount of precursor on the support during the wet stage of the urea method. The nickel precipitate was identified By XRD analysis as Ni3(NOa)2(OH)4 in all cases. The effect of the urea/nickel molar ratio on the precipitation was investigated in a range of ratios from 1 up to 3 for a reaction time of 24 h (see Fig. 2). It was found that the

162 amount of the nickel precipitate hardly increased when the molar urea/nickel ration was varied between 1 and 1.7, while for higher urea/nickel molar ratios, the amount of nickel precipitate surprisingly decreased although the pH continued to increase. This decrease of nickel precipitate is due to the increase of complex-formation of the Ni 2+ ion with NH3. The detail explanation for the behaviour in Fig. 2 can be found in our previous publication [15]. In addition, a maximum fixation degree of nickel precursor precipitated on the support (about 75 %) was obtained when the urea/nickel molar ratio is 1.7 after 24 h reaction. Therefore, such optimum experimental parameters as reaction time of 24 h and urea/nickel moral ratio of 1.7 were used to prepare the nickel-activated catalytic filter. 8

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Fig. 1. Precipitation amount of the nickel precursor and the suspension pH as a function of reaction time (urea/nickel molar ratio: 1.3)

Fig. 2. Precipitation amount of the nickel precursor and the suspension pH as a function of urea/nickel molar ratio (reaction time/24 h)

3.2. Nickel distribution throughout the catalytic filter disc To compare the nickel distribution obtained with the urea method, some catalytic filter discs were prepared by conventional impregnation with a solution containing only nickel nitrate. The distribution of NiO throughout the filter disc was investigated by SEM/DEX. Their Ni element mappings are shown in Fig. 3 (urea method) and Fig. 4 (conventional impregnation), respectively. The X-ray area scanning was made of part of a polished radial cross section of the discs till a depth of about 3.5 mm (vertical direction) in the disc. The white dots represent the EDX-mapping for the element Ni and the black background is from both the pores and the ~-A1203 particles of the filter disc. It appears that nickel displays a fairly uniform spatial distribution throughout the filter disc prepared by the urea method. However, throughout the disc prepared with the conventional impregnation method, most of the NiO is situated in the pores close to the outer surface (top area of Fig. 4), while less NiO is detected near the middle of disc (bottom area of Fig. 4). To increase the NiO loading further, the deposition cycle was performed several successive times on the filter disc under the same conditions previously used. SEM/EDX analyses were performed on a cross-section of the twofold impregnated filter discs. The Ni

163 element mapping showed that the urea method still gave a fairly homogeneous distribution of nickel throughout the disc after two cycles. However, the nickel distribution throughout the disc seemed to be less uniform with the conventional impregnation, since more precipitation occurred in the outer surface of the substrate. Therefore, the urea method is reliable and reproducible to deposit nickel catalyst on the filter with a uniform distribution of nickel.

Fig. 3. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the urea method.

Fig. 4. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the conventional impregnation method.

3.3. Catalytic performance of the nickel-activated filter discs Fig. 5 shows the benzene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. As a reference, the benzene conversions using a blank disc (no catalyst inside) in the same conditions are displayed in the figure. It was found that full conversion of benzene was obtained at typical filtration gas velocities (2.5 and 4 crn/s) and in a temperature range from 750~ to 900~ Even at a higher filtration velocity such as 6 cm/s, a complete conversion is still reached above 800 ~ Fig. 6 shows the naphthalene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. The blank disc was also used as a reference. As shown in Fig. 6, almost complete naphthalene cracking was achieved above 800 ~ with any gas velocity lower than 4 crn/s. Even with a gas velocity of 6 cm/s, the conversion still remains about 97 %. However, below 800 ~ the conversion of naphthalene significantly decreased as the reaction temperature decreased and the gas velocity increased. In addition, benzene was identified as one reaction product of naphthalene cracking at 750 and 800 ~ Table 1 lists the amount of benzene in the outlet gas after reaction over the nickel-modified filter disc. It is evident that for all gas velocities at 750 ~ benzene is present in the outlet gas with a significant amount. Above 800 ~

164 nearly no benzene was found in the outlet gas. Since no benzene formation was detected during the tests with the blank disc, it can be concluded that benzene was derived from the catalytic naphthalene cracking. Apparently, the nickel-activated filter disc displayed an excellent catalytic performance for the decomposition of either benzene or naphthalene as tar model compound in the simulated biomass gasification gas at a typical filtration gas velocity when the reaction temperature was in the range of 800-900 ~ Table 1 Yield of benzene as a reaction product of catalytic naphthalene cracking over a 1 wt % nickel-modified filter disc at different temperatures and gas velocities. Gas velocity Temperature (~ 2.5 cm/s 4 cm/s 6 cm/s 750 800 850 900 a Not detectable.

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Fig. 6. Naphthalene conversion as a function of reaction temperature and gas velocity over a 1 wt% nickel-modified and blank filter disc.

4. CONCLUSIONS The urea method can be applied to obtain nickel-modified catalytic filter with a uniform spatial distribution of nickel. The study of urea method showed that a reaction time of at least 6 h for urea decomposition is necessary and that a higher urea/nickel molar ration than 1.7 that led to less fixation of nickel precipitation. A maximum fixation of 75 % of the

165 precursor was found for a reaction time of 24 h and an urea/nickel molar ratio of 1.7. Moreover, the complete conversion of tar model compounds, such as benzene or naphthalene in a simulated sulfur-free and dust-free biomass gasification gas over a nickelmodified filter disc was obtained at 800-900 ~ using typical filtration gas velocities. This demonstrated that it is potential to develop a nickel-activated catalytic filter for simultaneous tar and particle removal from biomass gasification gas. This is encouraging to further develop more improved catalysts by the urea method for the removal of more complex tar mixtures and for the tar removal from real biomass gasification gas. ACKNOWLEDGEMENT

This work was financed by Research in Brussels (grant No. RIB-96/32) and the EU 5th Framework Programme (Contract No. ENK5-2000-00305). REFERENCES

1. A. Cybulski and J.A. Moulijn, Structured Catalysts and Reactors, Marcel Dekker Inc.: New York, 1998. 2. S.R. Ness, G.E. Dunham, G.F. Weber and D.K. Ludlow, Environ. Prog., 14 (1995) 69. 3. G. Saracco and V. Specchia, Catal. Rev. Sci. Eng., 36 (1994) 305. 4. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 5. G. Saracco and V. Specchia, Chem. Eng. Sci., 55 (2000) 897. 6. J.N. Armor, Appl. Catal. A: General, 49 (1989) 1. 7. A.V. Bridgwater, Appl. Catal. A, 116 (1994) 5. 8. J.T. Richardson, Principles of Catalyst Development, Plenum Press, New York, 1989. 9. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, in B. Delmon, P. Grange, P. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts V, Elsevier, Amsterdam, 1991, p. 166. 10. W.H.R. Shaw and J.J. Bordeaux, J. Am. Chem. Soc., 77 (1955) 4729. 11. H. Zhao, D.J. Draelants and G.V. Baron, Ind. Eng. Chem. Res., 39 (2000) 3195. 12. P. Simell, Catalytic hot gas cleaning of gasification gas. Espoo 1997, VTT Publications 330; Technical Research Center of Finland: Finland, 1997. 13. K.B. Mok, J.R.H. Ross and R.M. Sambrook, in: G. Poncelet, P. Grange, P. Jacobs (Eds.), Preparation of Catalysts III, Elsevier, Amsterdmn, 1983, p. 291. 14. X. Xu, H. Vonk, A. Cybulski and J.A. Moulijn, in G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs, P. Grange (Eds.), Preparation of Catalysts VI, Elsevier, Amsterdam, 1995, p. 1069. 15. H. Zhao, D.J. Draelants and G.V. Baron, Catal. Today, 56 (2000) 229.

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Studies in Surface Science andCatalysis143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

167

Preparation of gold-titanosilicate catalysts for vapor-phase propylene epoxidation using H2 and 02 A.K. Sinhaa, S. Seelan a, S. Tsubota a and M. Haruta b aEnvironmental Catalysis Research Group, Special Division of Green Life Technology, AIST, 1-8-31, Midorigaoka, Ikeda 563-8577, Japan bResearch Institute for Green Technology, AIST, Onogawa 16-1, Tsukuba 305-8569, Japan Vapor-phase epoxidation of propylene using H2 and O2 was carried out over gold catalysts supported on mesoporous ordered (MCM-41) and disordered titanosilicates prepared hydrothermally or by modified sol-gel method. Gold nanoparticles were homogeneously dispersed on the titanosilicate supports by deposition-precipitation (DP) method. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. NaOH was found to be the best precipitant to prepare Au catalysts with optimum propylene oxide yields and H2 efficiency. The extent of catalysts washing during preparation was found to affect the activity of the catalyst. The activity and hydrogen efficiency was found to depend on the type of mesoporous support used. 1. INTRODUCTION Propylene oxide (PO) is an industrially important chemical for the manufacture of polyurethane, unsaturated resins, surfactants and other products. Industrially PO is produced using two processes: Chlorohydrin process and Halcon (hydroperoxide) process [1]. The former process produces environmentally unfriendly chlorinated organic byproducts as well as calcium chloride, while the latter process produces equimolar amounts of co-products and requires heavy capital investment. Extensive efforts are being made since long to develop alternative process for direct gas-phase propylene epoxidation using oxygen [2, 3] because it is a promising method of tremendous industrial significance which can replace currently used environmentally disadvantageous chlorohydrin process and hydroperoxide processes. Recently Enichem utilized TS-1 (MFI) as a catalyst for the epoxidation of propylene in the liquid phase using hydrogen peroxide [4-6]. However, due to the very high production cost of H202 and its handling problems it would be highly desirable to produce H202 in-situ. Toso Co. Ltd. [7] has developed Pd/TS-1 catalyst for the in-situ generation of H202 from H2 and O2. Recently H61derich and coworkers [8] modified the catalyst developed by Toso with Pt and reported that the improved yield of PO could be ascribed to the maintainance of Pd in its +2 oxidation state.

168 Our research work on the catalysis by gold [9-11] has opened a new stage for the direct epoxidation of propylene using Ha and Oz in the vapor phase. In a series of papers we have reported the vapor phase epoxidation of propylene over highly dispersed nanosize Au particles supported on TiO2, TiO2/SiO2 [12, 13] and titanosilicates such as TS-1, TS-2, Ti-[-3, Ti-MCM-41 and Ti-MCM-48 [15-17]. These findings are now being followed by few other researchers [18-20] and companies [21,22]. But the problems for industrial viability still exist due to low PO yields and low H2 efficiency. Efficiency of supported metal catalysts often depends on metal dispersion and metal-support interaction. The presence of well dispersed tetrahedrally coordinated Ti sites and the Au nanoparticles on the support surface is thought to be necessary for epoxidation activity [15-17]. The synthesis of dispersed gold nanoparticles is highly sensitive towards the preparation methods and synthesis conditions. It has been shown that Au particles of critical size < 3.0 nm deposited on to titania or titanosilicate (TS-1, Ti-MCM-41, Ti-MCM-48) supports are essential for propylene epoxidation activity and high selectivity (> 90 %), larger Au particle size resulting in CO2 formation [12, 13]. Modified Deposition-Precipitation (DP) method has been found to be most suitable method for preparing supported gold catalysts with desirable gold particle size. The present work intends to study the role of precipitant, catalyst washing, calcinations and pretreatment in different gas streams, during the preparation of Au catalysts by DP method, onto mesoporous titanosilicates of different porosities and pore structures and its effect on the propylene epoxidation activity and selectivity in order to fine tune the synthesis of gold-titanosilicate catalysts for propylene epoxidation reaction. 2. EXPERIMENTAL Mesoporous Ti-MCM-41 support was prepared by hydrothermal crystallization according to literature procedure [23]. Disordered mesoporous titanosilicate Ti-Meso was synthesized by modified sol-gel method and hydrothermal crystallization (at 100~ 5 days) and using cetyltrimethyl ammonium bromide as template following the procedure similar to that for the synthesis of disordered mesoporous silica [24]. A modified sol-gel method without hydrothermal crystallization was used to prepare mesoporous titanosilicates, TiO-SiO(1) and TiO-SiO(2) [25]. Ti grafting on Ti-MCM-41 support was carried out according to literature procedure [26]. The materials were characterized by XRD (Rigaku R i n t - 2400, Cu-Kc~ radiation, 40 kV, 40 mA), UV-Vis (Photal Otsuka Electronics, MC-2530 UV/VIS light source), and nitrogen adsorption/desorption, BET surface area measurements (Micromeritics ASAP 2010 apparatus). Gold nanoparticles were deposited on the supports by deposition precipitation method [13-15] using aqueous HAuCl4 solution (corresponding to 2-4 wt % Au) and NaOH, NH3, urea, NaHCO3 and Na2CO3 as precipitants (at pH 7-9) followed by calcination in air at 300~ The Au particle size and its distribution was observed by TEM (Hitachi H-9000). The catalytic tests were carried out in a vertical fixed-bed U-shaped quartz reactor (i.d. 10 mm) using a feed containing 10 vol% each C3H6, H2 and Oa diluted with Ar passed over the catalyst (0.15 g) bed at a space velocity of 4000 h-lcm3/g.cat. The temperature was

169 controlled and measured using a glass tube covered Cr-A1 thermocouple located in the center of the catalyst bed. Prior to testing, the catalysts were first pretreated at 250~ for 30 min. in a stream of 10 vol% H2 in Ar, followed by 10 vol% Oz in Ar streams. The feeds and products are analyzed using on-line GCs equipped with TCD (Porapak Q column) and FID (HR-20M column) detectors and auto injector. 3. RESULTS AND DISCUSSION XRD spectra for the various mesoporous titanosilicates are shown in Fig. 1. The spectra of Ti-MCM-41 samples correspond to a regular, well-ordered, hexagonal mesoporous structure. But the titanosilicate Ti-Meso prepared hydrothermally shows a single broad peak in the 20 range 2 . 0 - 3.0~ the titanosilicate TiO-SiO(1) prepared by modified sol-gel method shows a broad peak 20 range 1.0 - 2.0 ~ Presence of a single broad peak implies that these titanosilicates are mesostructured amorphous materials [2425]. Absence of any higher order peaks implies that these mesostructured materials do not have any long-range order. UV-Vis spectra of the titanium containing MCM-41 and disordered mesoporous titanosilicate samples are shown in Fig. 2. The UV-Vis analysis of these samples show a band near 220 nm range due to tetrahedrally coordinated Ti. Generally a shoulder at --330 nm is expected in the spectrum if the sample contains some bulk titania, but such a shoulder could not be observed. Absorption band at 260-270 nm has been generally attributed to the presence of Ti atoms in 5- and 6- fold coordinations, which are most likely generated through hydration of the tetrahedrally coordinated sites [24]. Ti/Ti-MCM-41 and TiO-SiO(2) samples with higher Ti content than that for the other titanosilicate samples studied here show broader UV-Vis bands with some red-shift. Table 1 summarizes the surface properties of the various mesoporous titanosilicate samples. The BET surface areas of these titanosilicates (850-1250 m2g-1) was typical for that shown by mesoporous materials. The BET surface area and BJH average pore diameter is found to decrease after Ti grafting onto Ti-MCM-41 sample. The postsynthesis grafted Ti is expected to react with the surface silanol groups of the walls in a random fashion in the most accessible sites near the pore mouth and wider pores. As a consequence there is clear decrease in pore size after titanium grafting. The pore size distribution is also found to become narrower around the average pore diameter. The BET surface areas of disordered mesoporous materials is found to be lower than that of the ordered MCM-41 type materials. BJH average pore diameter of mesoporous titanosilicates TiO-SiO(1) and TiO-SiO(2) prepared by modified sol-gel method is much lower than that of disorderd mesoporous Ti-Meso and ordered Ti-MCM-41 samples crystallized hydrothermally. But all these samples show a narrow pore size distribution. All the samples exhibit isotherms of type IV, typical of mesoporous materials, with a H2 hysteresis loop (Fig. not shown).

170

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Table 1 Surface properties of mesoporous titanosilicate Catalyst Ti/Si Surface area (mag-1) Ti-MCM-41 0.015 1270.4 Ti/Ti-MCM-41 0.03 1016.6 Ti-Meso 0.015 882.9 TiO-SiO(1) 0.01 891.0 TiO-SiO(2) 0.02 834.4

.

.

.

.

.

.

200 250 300 350 400 450 500 550 Wavelength (nm) Fig. 2. UV-Vis spectra of titanium containing mesoporous silica materials.

support samples. Pore size Pore volume (cm3g-1) (~) 38.4 1.80 29.2 0.87 29.6 0.72 14.1 0.55 18.9 0.86

Table 2 shows the results of influence of the nature of the precipitant used for depositing Au on the Ti-MCM-41 support on the propylene epoxidation reactions over supported Au catalysts. The Au/Ti-MCM-41 catalyst prepared by using NaOH as the precipitant showed the best activity and hydrogen efficiency. Catalyst prepared by using urea as the precipitant shows lower activity and hydrogen efficiencies. Catalysts prepared

171 by using NaHCO3 and Na2CO3 as precipitants show the lowest activities. While catalyst prepared by using NH3 as precipitant shows poor PO selectivity due to more CO2 formation. Among different precipitants for Au deposition, NaOH has found to be the most suitable for getting uniform, well dispersed Au particles and maximum Au loading (from TEM observation and elemental analysis) which could be responsible for the best activity and hydrogen efficiency shown by this catalyst. Table 2. Influence of precipitant on propylene epoxidation activity and selectivity for 2 wt% Au loaded Ti-MCM-41 (TOS = 3 h) Precipitant Conversion (%) of PO selectivity H2 efficiency C3H6 H2 (%)1 (%) Urea 1.3 32.3 90.3 4.0 NaHCO3 0.5 28.2 96.8 1.8 NH3 1.4 32.2 75.1 4.4 Na2CO3 0.8 24.5 95.8 3.3 NaOH 2.5 16 92 15.6 Space velocity, 4000 h-lcmg/gcat; catalyst, 0.15 g; feed, Ar/CaH6/H2/Oz = 70/10/10/10; Reaction temperature, 423 K; TOS = 3 h. 1Propylene Oxide selectivity (mol %). Table 3 shows the results of the influence of catalyst washing and pretreatment on the propylene epoxidation activity and PO selectivity. Using NaOH as precipitant it is found that strong washing of the supported Au precipitate (Ti-MCM-41a, Table 3) results in diminished catalytic activity. The catalytic activity is found to decrease with increasing severity of washing. This can be attributed to the washing out of Au and Na + from the support surface due to comparatively weaker Au-support interaction owing to scant and highly dispersed Ti sites on the support. Partially charged Au nanoparticles are held on the support surface mostly due to electrostatic interaction with charged Ti sites while neutral silica surface cannot interact with the Au nanoparticles. The catalyst prepared from unwashed supported precipitate is found to be the best with uniform sized and well dispersed Au nanoparticles on the support (from TEM observation) and shows the best activity (Ti-MCM-41c). Au and Na analysis for different catalysts shows decreasing Au and Na + concentration for catalysts Ti-MCM-41a-c, with increasing severity of the washing. The role of Na ions in the epoxidation activity is still doubtful and it needs further investigation. The promoting effect of alkali ions for propylene epoxidation has been shown earlier [2, 16]. Catalyst calcinations at around 300~ followed by pretreatment in dilute H2 stream and then dilute O2 streams at 250~ is necessary for getting improved activity. Dried catalyst without any pretreatment shows better Ha efficiency but lower propylene oxide yield (Ti-MCM-41d). Pretreatment in NO stream increase propylene conversion and decreases PO selectivity due to more CO2 formation (TiMCM-41e). Typical TEM images of the Au nanoparticles supported on Ti-MCM-41 and TiOSiO(1) samples are shown in Figs. 3a and 3b, respectively. The TEM pictures for the Au deposited catalysts did not show the presence of any bulk titania phase in the samples. Au nanoparticles were found to be uniformly dispersed on the surface of titanosilicate samples. But the surface of mesoporous TiO-SiO(1) and Ti-Meso supports showed more

172 homogeneous Au dispersion than the Ti-MCM-41 support. The mean size of the gold nanoparticles on the various titanosilicate supports varied between 2.8 nm to 3.5 nm. Table 3. Influence of catalyst washing and pretreatment on propylene epoxidation activity and selectivity for 2 wt% Au loaded mesoporous titanosilicates (TOS = 3 h) Catalyst Catalyst Pretreatment Conversion (%) of PO selectivity C3H6 H2 (%) a support washing method TiMCM-41a strong washing Calcination --+ H2 0.8 14 93 -+ 02 TiMCM-41b medium Calcination --+ Ha 1.9 15 92 washing -~ O2 TiMCM-41c no washing Calcinations --) 2.5 16 92 H2 --) 02 TiMCM-41d no washing No calcinations or 1.8 6 92 pretreatment TiMCM-41e no washing Calcination --~ 3.5 26 77 2%NO Space velocity, 4000 h-lcm3/gcat; catalyst, 0.15 g; feed, Ar/C3H6/H2/O2 - 70/10/10/10; Reaction temperature, 423 K; TOS = 3 h. 1Propylene Oxide selectivity (mol %). -+ represents the next pretreatment step.

Figure 3. TEM images of Au supported on Ti-MCM-41 sample (a) sample (b).

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Fig. 4. Influence of the nature of support on propylene conversion, PO selectivity and hydrogen conversion. Au/Ti-MCM-41(II), Au/Ti/Ti-MCM-41(@), Au/Ti-Meso(O), Au/TiO-SiO(1) (A) and Au/TiO-SiO(2) (V)

174

The results of propylene epoxidation at 150~ over Au/Ti-MCM-41, Au/Ti/TiMCM-41, Au/Ti-Meso, Au/TiO-SiO(1) and Au/TiO-SiO(2) catalysts are compared in Fig. 4. The increasing order for propylene conversion after TOS > 150 rain. over Au catalysts supported on various mesoporous titanosilicates is: Au/TiO-SiO(1)< Au/TiMCM-41
175 shows higher propylene conversion, hydrogen efficiency and PO selectivity than Au catalysts supported on ordered Ti-MCM-41 support and disordered mesoporous (Ti-Meso) supports prepared by hydrothermal crystallization. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

S.L. Ainsworth, Chem. Eng. News 9 (1992); M. McCoy, Chem. Eng. News, 19 (2001). K. Murata and Y. Koyozumi, Chem. Commun., (2001) 1356. H. Orzesek, R. P. Schulz, U. Dingerdisses and W. E. Maier, Chem. Eng. Technol., 22 (1999) 8. M.G. Clerici, G. Bellusi and U. Romano, J. Catal., 129 (1991) 159. B. Notari, Catal. Today, 18 (1993) 163. E. Occhiello, Chem. Ind., 761 (1997). A. Sato, T. Miyake and T. Saito, Shokubai (Catalysts), 34 (1992) 132. R. Meiers, U. Dingerdissen and W. F. H61derich, J. Catal., 176 (1998) 376 M. Haruta, Catalysis Surveys of Japan, 1 (1991) 61 and references therein. M. Haruta, Catal. Today, 36 (1997) 123 and references therein. M. Haruta, Stud. Surf. Sci. Catai., 110 (1997) 123 and references therein. T. Hayashi, K. Tanaka and M. Haruta, Shokubai, 37 (1995) 72. T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 178 (1998) 566. Y. A. Kalvachev, T. Hayashi, K. Tanaka and M. Haruta, Stud. Surf. Sci. Catal., 110 (1997) 965. M. Haruta, B. S. Uphade, S. Tsubota and A. Miyamoto, Res. Chem. Intermed., 24 (1998) 329. B.S. Uphade, M. Okumura, S. Tsubota and M. Haruta, Appl. Catal. A: Gen., 190 (2000) 43. B. S. Uphade, Y. Yamada, T. Nakamura and M. Haruta, Appl. Catal. A: Gen., 215 (2000) 137. T. A. Nijhuis, H. Huizinga, M. Makkee and J. A. Moulijn, Ind. Eng. Chem. Res., 38 (1999) 884. E.E. Stangland, K. B. Stavens, R. P. Andres and W. N. Delgass, J. Catal., 191 (2000) 332. G. Mul, A. Zwijnenburg, B. van der Linden, M. Makkee and J. A. Moulijn, J. Catal., 201 (1) (2001) 128. R.G. Bowman, H. W. Clark, J. J. Maj and G. E. Hartwell, PCT/US97/l1414, PCT Pub. No. WO 98/00413 (1998). T. Hayashi, M. Wada, M. Haruta and S. Tsubota, Jpn. Pat. Pub. No. H10-244156, PCT Pub. No. WO97/00869, U.S. Patent 5,932, 750 (1999). P.T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 368 (1994) 321. R. Ryoo, J. M. Kim, C. H. Ko and C. H. Shin, J. Phys. Chem., 100 (1996) 17718. Z. Shan, E. Gianotti, J. C. Jansen, J. A. Peters, L. Marchese and T. Maschmeyer, Chem. Eur. J., 7 (2001) 1437. T. Maschmeyer, F. Ray, G. Sankar and J. M. Thomas, Nature, 378 (1995) 159. M.G. Clerici and P. Inagallina, J. Catai., 140 (1993) 71.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

177

Sol-gel synthesis of colloid and triflates containing hybrid type catalysts A.N. P~.rvulescu l, B. C. Gagea 1, M. Alifantil, V. Pfirvulescu 2 and V. I. P~rvulescu 1 1-University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, fax" 401-3320588, Email: v_ [email protected] 2_ Institute of Physical Chemistry of Romanian Academy of Sciences, Splaiul Independentei 172, Bucharest, Romania. Various colloid and triflate derivatives embedded catalysts were prepared using sol-gel procedure. Ru, Pt and Pd-Au stabilized colloids were embedded in various inorganic matrices like silica, zirconia or silica-tantalum mixed oxide. Mono- or bifunctional catalysts resulted in function of the properties of the matrix support. Lanthanum, silver and tert-butyldimethylsilyl-trifluoromethanesulfonate in supramolecular ensembles with an ammonium-quternary salt were embedded using the same technique. The preparative conditions leading to the integrity of the embedded structures and the homogeneity of the resulted materials were investigated. To provide such information the catalysts were characterized using adsorption-desorption isotherms of nitrogen at 77 K, H2chemisorption, 1H, 13C and 29Si solid state MAS-NMR, XRD, TEM, SEM, XPS, TG-DTA, NH3-FTIR. 1. INTRODUCTION Literature is abundant in data concerning homogeneous catalytic performances of colloids and triflates [1-2]. Although these catalysts exhibit very high activity and selectivity in a large number of reactions of practical interest, they are rather non-stable and their recovery from the solution is difficult. Therefore the preparation of hybrid catalysts preserving these species unaltered is of current interest. In the case of colloids, the current methodology involves incorporation of these materials in polymers [3-4]. Covalent attachment of nanometer-scale colloidal particles to organosilane-coated substrates is a flexible approach to formation of macroscopic surfaces with well-defined nanostructure [5]. Variations in substrate (nature of the support), geometry (planar, cylindrical), functional group (-C,-NH2, -PR2, -SH) and particle diameter may lead to a large diversity in properties of these materials. Several studies have also been reported in the last years on the heterogeneization of triflic acid [6] and various triflates by impregnation [7, 8] or polymer incorporation [9, 10], and on selective acidcatalyzed reactions using these materials. The use of the sol-gel technique has been proved very effective in immobilization of various species leading to effective heterogeneous catalysts [ 11 ]. In addition, the sol-gel

178 procedure allows the control of texture (very high surface areas and the possibility to tailor the pore size) and may combine the properties of the immobilized species and the support leading to multifunctional catalysts [12-13]. The incorporation of molecular organic additives [14] or co-hydrolysis of alcoxides with alcoxide-derivatives [15] may provide introduction of active surface functional groups and changes in the catalyst morphology. The aim of this contribution is to present data on the preparation of catalysts containing as embedding species a large family of colloids such as colloids of ruthenium, platinum, or palladium-gold alloys and triflate derivatives such as lanthanum and silver triflate or tert-butyldimethylsilyltrifluoromethanesulfonate (BMSTM). Silica, zirconia and tantalum oxides were used as carrier. All these preparations considered the polymeric solgel route using as starting materials silicon, zirconium or tantalum alcoxides. The main problem raised by these preparations is to adapt the procedure to the solvent compatible with the embedding materials. Both the colloids and the triflate derivatives are soluble in a limited number of solvents, mostly not the alcohols generally used in such preparations. Therefore, the protocol procedure described in this contribution followed a specific sol-gel route for each hybrid catalyst. During these preparations we investigated: i) the formation of the sol by hydrolysis of the alcoxide in the selected solvent; ii) the addition of the stabilized colloid or triflate derivative in the formed sol; iii) the addition of a surfactant in the case of triflates; iv) the gelation, and finally v) the drying and calcining of the materials. 2. E X P E R I M E N T A L

The colloids were prepared using a reported procedure [1], while the triflate derivatives were commercially available. The present study considered the following stabilized colloids: Pt, Ru, and Pd-Au. Ru and Pd-Au were stabilized with N+(CgHlv)4Br (TOAB) and N+(CH3)3C16H33Br(TMCB) type ligands, while for Pt several other commercial surfactants were used (see Table 1). The first condition to provide a homogeneous catalyst is to carry out the sol-gel process in the same solvents as for embedding the species. As a function of the stabilizer ligand nature, the colloids were soluble either in tetrahydrofurane, ethanol, 2-propanol or toluene. Lanthanum and silver triflates were soluble in ethanol, and BMSTM in CC14. Accordingly, the alcoxides were dissolved in the above solvents and the preparation of the sols was adapted to these conditions. The hydrolysis was initiated in the presence of hydrochloric acid. All the hybrid materials precursors are either air or moisture sensitive and therefore the syntheses were carried out under inert atmosphere, namely in argon. The materials were dried under vacuum. The characterisation of the colloids both in the free and in the embedded state was first performed using transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and atomic absorption spectroscopy (AAS). In addition, nitrogen 29, adsorption-desorption curves at 77 K, Hz-chemisorption measurements, solid state SiNMR, XRD, SAXS, XPS, MAS-NMR, NH3-FTIR, and 197AuM6ssbauer spectroscopy were applied. For the embedded triflates, the catalysts were characterized by nitrogen adsorption-desorption isotherms at 77K, TG-DTA, 1H, 13C, and 29Si solid state MASNMR, XRD, TEM, SEM, XPS and, FTIR after adsorption of NH3.

179

3. RESULTS AND DISCUSSIONS 3.1. Sol-gel encapsulation of colloids Ru embedded colloids The colloids as prepared were embedded in a zirconia (samples Ru-Zr-TOAB and Ru-Zr-TMCB) or silica sol-gel matrix (samples Ru-Si-TOAB and Ru-TMCB) using the procedure reported elsewhere [19]. According to this method, the Ru colloids were added in the oxide gelation step. Tetraethoxisilicate (TEOS) was used as the precursor for the silica support. However, since the NR4-stabilized Ru colloids are completely insoluble and even decompose at elevated temperatures in alcoholic solutions, the normal sol-gel procedure must be modified using THF as solvent. The molar composition of the sol was TEOS:THF:H20:HC1 = 1:3.5:4:0.05. The colloid was added as a 4.5 wt.% THF-solution at room temperature, after refluxing. The sol was stirred vigorously at 70 ~ (under reflux) until the gelification was complete (after 2 days). To avoid any decomposition in the presence of air, all steps were carried out under Ar. The resulting gel was dried under vacuum at 110 ~ using a ramp of 0.12 ~ rain ~. For zirconia, the gel was obtained using zirconium propoxide (ZP) as precursor. Hydrolysis of ZP was carried out in the presence of acetic acid and the molar composition of the sol was ZP:THF:HzO:CH3COOH = 1:3.5:4:0.05. Samples with 2 wt. % Ru were prepared. Pt embedded colloids Platinum colloids (3% Pt) were incorporated in SiO2 and SiO2-Ta205 (15% Ta2Os) polymeric networks. TEOS and Ta(OC2Hs)5 (PET), respectively, were used as precursors. Since Pt colloids were soluble in ethanol, the sol composition (molar ratio) was: TEOS:EtOH:H20:HC1 of 1:4.5:4.0.02 for silica, and TEOS:PET:EtOH :H20:HC1 of 1:0.05:10:4.5:0.0.3 for SiO2-Ta205 mixed oxide. For mixed oxide, because of the differences in the hydrolysis rate of TEOS and PET, the silica sol was prepared in the first step. The pre-hydrolysis and condensation step was carried out at 80 ~ for 2h, then the mixture was cooled to room temperature. PET was separately solved in ethanol and added at room temperature to the preformed silica sol. Because of the high instability of PET and colloid against water, the key point in these syntheses is the step when they are added. In the present experiments PET and colloid were introduced at room temperature, under vigorous stirring, in the silica gel resulted by refluxing the silica sol at 80 ~ for 2h. After this the hydrolysis was finished, the rate of the condensation processes became stationary and the cross-linking chains process was dominant. The addition of the tantalum oxide sol to the silica preformed gel led to an important enhancement of the gelification rate. PET can be considered to be a cross-linking agent [17]. The water molecules are then dispersed among the silica species resulting from condensation and have no more influence upon the stability of PET and colloid. The silica gel formed during refluxing in acid conditions had a small condensation degree and this favors a better dispersion of tantalum and colloid and a homogeneous incorporation during polymerization and polycondensation. The control of the gelification rate was achieved by the parameters of the sol-gel process (pH and quantity of the solvent). The gelification was conducted at room temperature. Platinum colloids (see Table 1) were solved in ethanol.

180 Table 1. Pt containing catalysts Catalyst Composition SPt0 SIO2-3% Pt SPtl SIO2-3% Pt SPt2 SIO2-3% Pt SPt3 SIO2-3% Pt SPt4 SPt5 SPt6

SIO2-3% Pt SIO2-3% Pt SIO2-3% Pt

SPt7 TSPt0 TSPtl TSPt2 TSPt3

SIO2-3% Pt SiO2-15%Ta205-3% SiO2-15%Ta205-3% SiO2-15%Ta205-3% SiO2-15%Ta205-3%

TSPt4 TSPt5 TSPt6

SiO2-15%Ta205-3% Pt SiO2-15%Ta205-3% Pt SiO2-15%Ta205-3% Pt

TSPt7

SiO2-15%Ta205-3% Pt

Pt Pt Pt Pt

Stabilizer ligand Polyethyleneglycol-dodecylether 2-Hydroxy-propionic acid ARQUAD 2HT-75 QUAB 342 (3-chloro-2-hydroxypropyldimethyldodecylammonium chloride) REWO AGPVY 3 TWENN 40 (polyoxyethylene (20) sorbitan monopalmitate) N+(CsHly)4Br Polyethyleneglycol-dodecylether 2-Hydroxy-propionic acid ARQUAD 2HT-75 QUAB 342, 3-chloro-2-hydroxypropyldimethyldodecylammonium chloride REWO AGPVY 3 TWENN 40 (polyoxyethylene (20) sorbitan monopalmitate) N+(CsHly)aBr

Pd-Au alloys embedded colloids Embedding of the Pd/Au colloid was carried out using TEOS as precursor for the silica support. However, since the NR4-stabilized Pd/Au colloids are completely insoluble and even decompose at elevated temperatures in alcoholic solutions, the normal sol-gel procedure must be modified using THF as solvent. The molar composition of the sol was TEOS:THF:H20:HC1 = 1:3.5:4:0.05. The colloid was added as a 4.5 wt.% THF-solution at room temperature, after refluxing. The sol was stirred vigorously at 70 ~ (under reflux) until gelification was complete (after 2 days). To avoid any decomposition in the presence of air, all steps were carried out under Ar. The resulting gel was dried at 110 ~ using a ramp of 0.12 ~ min 1 then calcined under air at 450 ~ with a ramp of 0.3 ~ min -1. A few samples were dried by lyophilization. 3.1.1. Properties of the encapsulated colloids The stabilization of the colloids used various surfactants, which generate a protector layer. Incorporation of these entities in the sol-gel matrix resulted in large porous materials with pores developed around the stabilized-colloids. The size of the resulting pores was in concordance with the size of the embedded colloid. Some of these properties are summarized in Table 2. XRD, TEM, and M6ssbauer spectroscopy indicated no change in the colloid structure after entrapment in the sol-gel matrix (Figs.1 and 3). 29Si solid state MAS-NMR spectra gave no indication of a direct colloid-silica bond. Table 2 indicates that the resulting hybrid materials exhibit a rather high surface area corresponding to a large

181 pore texture. H2-chemisorption measurements also indicated large availability of metals to hydrogen. These data led to a model in which after drying the oxide matrix embedded a colloid which preserves its initial structure and remains surrounded by the stabilizer ligand. The catalytic data of these materials in hydrogenation or hydrogenolysis reactions confirm such a model [13,18]. The hybrid catalysts containing Ru and Pd-Au colloids were investigated in selective C=C (styrene), CrlC (hexynol) and allylic (cinnamaldehyde) hydrogenation. Drying the samples by lyophilization resulted in smaller surface areas and hydrogen up-take characteristics. Following this route, catalysts with a large spectrum of properties resulted. Only silica embedded colloid catalysts exhibited excellent hydrogenation properties. Mixing the silica with a second oxide like tantalum led to support acid properties as resulted from NH3-FTIR measurements, which conferred to these catalysts bi-functional properties. Fig. 2 shows in-situ NH3-DRIFT spectra. The adsorption of NH3 is consistent with the formation of a new band at 1460 cm ~ for SPT2, and 1440 cm l for TSPT2 as a contribution of v(H-N-H) in adsorbed NH4 +. The bands at 3240, 3045, and 2840 cm -1 result from the same contribution. The adsorption of NH3 also led to the disappearance of the band due to adsorbed water. These data indicated a Br6nsted acidity only for tantalum containing catalysts. No Lewis acid sites were detected. Table 2. General properties of the investigated hybrid catalysts Support Embedded species/loading Surface area m 2 -1 SiO2(SPt7) Pt colloid/3 wt% metal 76 SiO2 Pd-Au alloy/1 wt% metal 193 SiO2 (TOAB) Ru colloid/2 wt% metal 217 ZrO2 (TOAB) Ru colloid/2 wt% metal 51 Ta205 Pt colloid/3 wt% metal 23 SiO2- Ta205 (TSPt7) Pt colloid/3 wt% metal 198 SiO2 (A1) BMSTM 423 SiO2 (LaT1) La triflate 438 SiO2 (AgT1) Ag triflate 415 a-resulted from H2-chemisorption

Pore size

nm 3.5 3.7 4.7 4.7 3.6 3.5 2.5 2.5 2.5

Metal size a nm 2.8 2.9 4.3 4.3 2.8 2.8

3.1.2. Effect of calcining and reduction of colloids The dried samples were either reduced in flowing hydrogen (30 ml min l ) at 450 ~ using a ramp of 0.12 ~ rain l or previously calcined in air at the same temperature, and then reduced in similar conditions. As 13C solid state MAS-NMR showed during the thermal treatment, the stabiliser ligand was decomposed leading to carbonaceous deposits. The Hz-chemisorption was much suppressed, indicating that these carbonaceous deposits cover the colloid surface (Table 3). The same conclusion results from the textural measurements and catalytic activity data. Both reduced and calcined and reduced catalysts exhibited very small surface areas at least three times smaller than only dried catalysts. These data may allow the conclusion that such hybrid materials are effective catalysts only for reactions resulting at lower temperatures, namely, in conditions in which the organic part remained unaffected.

182

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Table 3. Influence of the calcination on Hz-up-take and surface area of colloid embedded catalysts Catalyst Surface area, m 2 g-1 Hz-uptake, cm 3 g-1 dried calcined dried calcined Ru-SiO2-TMCB 242 10 0.11 0.05 Ru-ZrO2-TMCB 38 1.5 0.20 0.07 Pd-Au-SiO2 193 15 0.20 0.06 SPtt 54 1.5 0.20 0.07 TSPtl 198 2.0 0.29 0.06 SPt2 48 3.8 0.20 0.08 TSPt2 138 3.4 0.31 0.09

3.2. Sol-gel encapsulated triflates The immobilization of triflate derivatives followed a different strategy. Two kinds of materials were prepared, one in which the triflate derivative is in a direct interaction with the surfactant leading to supramolecular entities and another in which only the triflate derivative existed. However, these are rather small molecules as compared to colloids, and thus in both cases the resulting pore size corresponded to mesoporous materials. The catalysts embedding BMSTM in a silica sol-gel matrix were prepared as following. Since the silyl triflate derivative is only soluble in carbon tetrachloride and triethylamine, the sol-gel synthesis was carried out in inert atmosphere (Ar), using an adapted route in which the silica sol was obtained by acid hydrolysis of a solution of tetraethoxyorthosilicate (TEOS) (10.4 g) in acidified CC14 (TEOS:CC14 molar ratio of 4, pH 1.5 with HC1 37%). Water was then added to the acidic solution in TEOS:H20 molar ratio of l:10 (samples A1 and A2) or 1:4 (sample A3), and the mixture was refluxed at 70~ for 2h. After cooling the solution at room temperature, the silyl-triflate derivative (0.38 ml) was added under vigorous stirring as a 0.3 M solution in CC14 (samples A1 and A3) or as such (sample A2). Hexadecyltrimethyl-ammonium bromide, as a surfactant, was then introduced (surfactant:TEOS ratio of 0.1) and the gelation was carried out at 90~ for

,

183 two days in a teflon cylinder in an autoclave (samples A2 and A3) or at room temperature for 6 days (sample A1). The resulting gel was dried under vacuum at room temperature for 24 h and then at 100~ for 6 h. Samples without surfactant were prepared as well.

d

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1600

1800

2000

2200

Wavenumber, cl~ Fig. 4. NH3-FTIR spectra of embedded lanthanum triflate

The immobilization of lanthanum (LaT) and silver (AgT) triflate was made in the same way, but in ethanol. The samples for which the gelation was carried out at 90 ~ in a teflon cylinder within an autoclave where denoted as LaT2, AgT2, LaT3 and AgT3, or at room temperature for 6 days as LaT1 and AgT1. The immobilization and storage of AgT were carried out in a black chamber to avoid decomposition. The resulting gel was dried under vacuum, first at room temperature for 24 h then at 100 ~ for 6 h. Samples with 15 wt% triflate were thus obtained. The immobilization of BMSTM, LaT and AgT in a silica matrix using the sol-gel technique led to mesoporous materials with a monomodal pore size distribution. The surface area of these materials was rather high (Table 2). Characterization of these materials indicated that the triflate remained essentially unaffected during this process. XPS analysis showed a good dispersion of triflate as inferred from the comparison of the chemical and XPS S:Si ratios and a good integrity as shown from the analysis of the XPS S: F ratios. However, FTIR analysis suggested that in BMSTM, a small part of triflate was hydrolyzed, probably leading to a very small fraction of impregnated triflates. No similar evidence was found for LaT or AgT. FTIR analysis also indicated the existence of both Lewis and Br6nsted acid sites (Fig.4). The surfactant also remained embedded in the silica matrix, more probably incorporated as a supramolecular structure with triflate in the solid mesopores than in the silica network. The catalytic performances of the resulting materials were checked in specific reactions. All these catalysts showed excellent activity and selectivity at near room

184 temperature conditions. The hybrid catalysts containing triflates were tested in reaction of in n-hexane and cyclohexane isomerization [14, 19]. 4. CONCLUSIONS New hybrid catalysts might be obtained by embedding colloids or triflates derivatives and surfactants using the sol-gel technique. The nature of the solvent and pH at which are conducted these syntheses are the most important factors controlling the homogeneity and integrity of the embedded species. The morphology of the catalysts and the accessibility to the embedded species is preserved only in dried catalysts. Calcining led to the stabilizer decomposition, and to either the blocking of the active sites or partial decomposition of the active species. In conclusion, these hybrid catalysts are effective in low temperature reactions. ACKNOWLEDGMENT

The authors gratefully acknowledge the valuable support of Prof. H. B6nnemann from MPI Germany, for kindly supplying stabilized metal colloids and Prof. P. Grange, Belgium, for XPS and DRIFT characterizations. REFERENCES

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

185

Preparation of Zeogrids through interposed stapling and fusion of MFI zeolite type nanoslabs S.P.B. Kremer, C.E.A. Kirschhock, M. Tielen, F. Collignon, P.J. Grobet, P.A. Jacobs and J.A. Martens Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium Ordered porous materials, called Zeogrids, are obtained by aggregation of Silicalite-1 nanoslabs upon addition of a secondary template (cetyltrimethylammonium bromide), followed by calcination. The synthesis leads to a lamellar arrangement and fusion of the slabs and a high pore volume, distributed over ultra-micropores (0.14 mL/g) and supermicropores (0.55 mL/g). The use of a secondary template with shorter alkyl chain (dodecyltrimethylammonium bromide) leads to a similar structure, but results in a lower super-microporosity (0.33 mL/g). The interaction of the secondary template with the nanoslabs and their subsequent aggregation were monitored by DLS. The materials were characterized using SEM, XRD, TGA, N~ adsorption and tracer gas chromatography. I. INTRODUCTION Zeolites are currently manufactured as micron size crystals and compacted into millimeter size pellets for applications as packed beds. In many catalytic and adsorptive applications, mass and heat transfer properties could potentially be improved by structuring the zeolite in a different way. Research in this area led already to significant achievements. Alternatively structured zeolite matter are e.g. delaminated zeolites [1], supported zeolite films and membranes [2], hybrid structures with microporosity in walls of ordered mesoporous materials [3-6] and nanosized zeolites such as those synthesized in confined space [7]. The common property of these alternative zeolites is that at least in one direction, the zeolite framework has a dimension of around a nanometer. Recently, we developed an alternative approach to synthesize nanoscopically arranged zeolite material. The preparation departs from Zeosil nanoslabs having the Silicalite-1 framework type and measuring 1.3 x 4.0 x 4.0 nm3 [8, 9]. The formation of these nanoslabs occurs upon the room temperature hydrolysis of tetraethylorthosilicate (TEOS) in concentrated aqueous solution of tetrapropylammonium hydroxide (TPAOH). We discovered a method to obtain an interposed fusion of these nanoslabs into a new material refered to as Zeogrid [10]. The formation process of this microporous solid involves nanoslab stapling in layers with intercalated surfactant molecules. Removal of the surfactant through calcination causes facial and lateral fusion of the nanoslabs (Fig.l). Empty spaces lett laterally are responsible for the super-microporosity. The ultramicropores are the zeolitic channels inside the fused nanoslabs.

186

Fig.1. Genesis of Silicalite-1 Zeogrid. The rectangles represent the Silicalite-1 nanoslabs. In this work we investigated the formation process of two different types of Zeogrids (ZG) in presence of cetyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB), respectively. 2. E X P E R I M E N T A L

Preparation of ZGCTABand ZGDTAB.Silicalite-1 nanoslab suspension was obtained by hydrolyzing 37.32 g TEOS (Acros, 98%) in 32.13 g aqueous TPAOH solution (Alfa, 40 wt.- %) under continuous stirring. After the hydrolysis recognized by the homogenization of the two liquids, 30.55 g de-ionized water was added and stirring continued for 24 h. To an amount of 20 mL of this suspension, a same volume of ethanol (technical grade) was added. Subsequently, 60 mL of a 12 wt.-% solution of CTAB (Acros, 99%+) in ethanol was added dropwise under stirring (30 mL/h). A white precipitate formed during CTAB addition. The product was left undisturbed for 24 h, whereafter it was recovered by filtration over 8 ~tm pore diameter paper (Whatman 1440 090, grade 40), washed with ethanol on the filter and dried at 60~ Part of the precipitate was then compressed into flakes and the flakes crushed and sieved in order to obtain particles with diameters of 0.250.5 mm. An amount of 1 g of sample was loaded as a packed bed in a quartz tube and subjected to a first calcination step at 400~ under nitrogen gas and a second step at 400~ under oxygen gas. The heating rate was 0.5~ and the gas flow 30 mL/min. A second preparation (ZGDTAB) was done using DTAB as template (Acros, 99%). The template solution was added at once under stirring. No precipitation was observed. A white precipitate formed after lh under quiescent conditions. Characterization of ZGCTABand ZGDTAB. Dynamic light scattering (DLS) was done using an ALV-NIBS High Performance Particle Sizer, calibrated with polystyrene spheres of a diameter of 20 + 1.5 nm (Duke Scientific Co.). The samples were filtered (PTFE filters with 200 nm pore diameter, Alltech). The measurements were done in new polystyrene cells. The particle sizes were corrected for variation of the viscosity. The relative viscosities of the samples were determined using a U-tube Ostwald viscometer (Brand). Scanning electron microscopy (SEM) was performed with a JXA-733 JEOL instrument on gold plated samples. X-ray diffraction (XRD) was performed using a Siemens 5000 diffractometer with a Cu K a source (L = 0.154 nm). Thermogravimetric analysis (TGA) was done in a Setaram TGA 92 balance, at a heating rate of 1~ under 20% oxygen in helium atmosphere. Nitrogen adsorption experiments (-196~ were carried out in an Omnisorp instrument (Coulter), operated in the continuous flow mode. The samples were pretreated during 15 hours at 200~ under vacuum (1 Pa).

187 3. RESULTS AND DISCUSSION

3.1. Formation of the Zeogrids The formation of ZGCTABand ZGDTABis followed by in situ DLS (Fig.2). The formation of a precipitate from clear nanoslab suspension following the usual recipe is immediate upon template addition. Therefore, the amount of CTAB solution is reduced from 60 mL to 20 mL to slow the process and to allow for DLS investigation. The maximum in the particle size distribution in the original suspension, before CTAB addition (0'), is at 2.8 nrn, in agreement with the presence of nanoslabs measuring 1.3 x 4.0 x 4.0 nm3 [9]. Addition of CTAB leads first to an increase in particle size in the nanometer range (15' and 25'), followed by a sudden emergence of larger particles (35'). The nanoslabs first aggregate to particles of ca. 100 nm, whereafier a second population at ca. 1,000 nm appears (40'). The size increase in the nanometer range is ascribed to template adsorption on the surface of the nanoslabs and moderate nanoslab aggregation. The observed evolution of particle sizes suggests the existence of a critical CTAB surface coverage coupled with limited nanoslab aggregation, from which the elementary units rapidly organize into large particles. In the ZGDTABpreparation, the original particle size of 2.8 nm increases to ca. 10 nm within the first 55'. After 65', particles beyond a size of 100 nm are formed. The critical elementary unit is significantly larger in the DTAB system (Fig.2). The differences must originate from the size differences of the cetyl versus dodecyl chain. 0' A

15'

25' 40' 35'

5

CTAB

F_

f

0' A

40'

10 10' 20' 30' 45'

55'

i

100 1

90'

:5

t

I

I

10

100

1000

,,~5'

65'

90'

DTAB

.E I

1

10 size (nm)

i

100 1

i

10

100

1000

size (nm)

Fig.2. Normalized particle size distribution during ZGCTAB(top) and ZGDTAB (bottom) formation as a function of time (indicated near the curves) determined with DLS. Time zero corresponds to the starting nanoslab suspension before template addition.

188 3.2. Characterization of the Zeogrids

SEM reveals the as synthesized ZGCTAB and ZGDTAB to be composed of spherical particles with an average diameter between 2 and 10 micrometer (Fig.3a and b). Many spheres are twinned. Sometimes there is complete fusion of differem spheres. Given the layered nature of Zeogrid (Fig.l), it is assumed that in these spheres, the layers are packed in a concemric manner.

Fig.3. SEM micrographs of as synthesized ZGCTAB(a) and ZGDTAB(b). X-ray diffractograms of as synthesized ZGCTAB (b) and ZGDTAB(a) are shown in Fig.4. Both patterns reveal the presence of a layered compound showing first and second order diffraction. The repeat distance in ZGcT~ and ZGDT~ are 3.1 nm and 3.0 ran, respectively. Bragg type diffraction of crystalline (001) material at wider angles is not observed. It is our experience that with CTAB, the repeat distance is sensitive to slight =l variation in the preparation procedure, and especially to W the stirring conditions. The dC values can vary from 4.0 [10] C to 3.0 nm. After removal of the secondary template by (OOl) 9 calcination and annealing of (002) a the Silicalite-1 nanoslabs, the repeat distance invariably is 5 10 15 20 25 30 35 40 3.0 nm (Fig.l). A

t

b

dllll =m

2theta

(~

Fig.4. X-ray diffraction patterns of as synthesized ZGDTAB(a) and ZGcTAB(b).

189 Thermogravimetric analysis is performed on as synthesized ZGCTAB (Fig.5) and ZGDTAB. The ZGcTA~ sample undergoes 53% weight loss upon heating from ambient temperature to 600~ (Fig.5a). DTG curve reveals three consecutive weight losses (Fig.5b). Weight loss I (4%) is situated below 100~ and is ascribed to water desorption. The further weight losses occur around 175~ (II, 37%) and 275~ (III, 7%). Weight loss IH coincides with a significant exothermic effect according to DTA (Fig.5c). From a -lO previous TG analysis of nanoslabs [9], it is known that the occluded TPA gives rise to ,-. -20 exothermic weight losses at temperatures ~ ' ~ -30 exceeding 240~ Weight loss III thus is (3 I-- -40 attributed to occluded TPA. Weight loss II is attributed to the evacuation of surfactant. -50 The relatively low temperature at which -60 CTMA is evacuated and the absence of appreciable thermal effects can be explained by the co incidence of exothermic condensation of silanol groups r -0,1 and endothermic desorption of the E secondary template. The TG/DTG/DTA ~ -o,2 curves of ZGDTAB (not shown) are similar i-. -0,3 to those of ZGcTAB. The total weight loss is 45%, distributed over weight losses of 5%, 29% and 6%, centered at 40~ 180~ and -0,4 285~ respectively. The main difference between ZGcTAB and ZGDTAB is found in 3 C weight loss II, ascribed to surfactant evacuation (37% for ZGcTAB and 29% for :i 2 ZGDTAB). The similar d-values for ZGcTAB and ZGDT~ observed with XRD (Fig.4) suggest that the higher surfactant content of ZGcT~ compared to ZGDT~ is located 0 between individual nanoslabs composing the layers, resulting in a wider lateral of the nanoslabs in as 0 100 200 300 .400 500 600 spreading synthesized ZGCTAB. t e m p e r a t u r e (*C) i

i

i

i

A

, m

A

i

i

i

i

i

Fig.5. Thermogravimetric analysis (a), differential thermogravimetric analysis (b), and differential thermal analysis (c) of as synthesized ZGcTAB sample. Nitrogen adsorption is performed on calcined ZGcTAB and ZGDTAB, as well as on reference Silicalite-1 with a particle size of about 100 nm. The three adsorption isotherms are shown in Fig.6 (black traces). Both Zeogrids exhibit a type I isotherm with a long, almost horizontal plateau, characteristic of a microporous material with a small specific external surface area (Fig.6a and b). Silicalite-1 also exhibits a type I isotherm with a

190 shorter plateau at a lower nitrogen uptake, followed by an upward deviation owing to capillary condensation in the void spaces in between the crystallites (Fig.6c). The Silicalite-1 isotherm is fitted using both a Dubinin-Radushkevich equation for the ultra-micropore contribution and a BET equation for the meso- and macropore contribution [11]. The Zeogrid curves are fitted using similar procedure, except that the supermicropores are characterized using a modified BET type expression, accounting for a finite number of adsorbed nitrogen layers [ 10]. The fittings are represented in Fig.6 (gray traces) and the results are given in Table 1. The presence of a same ultra-micropore volume in ZGCTAB and ZGDTAB samples reflects the presence of a same quantity of Silicalite-1 nanoslabs per weight of calcined material. It is striking that the super-micropore volume is significantly smaller in ZGDT~ (0.33 mL/g) compared to ZGcT~ (0.55 mL/g). 500

a 400

/

/

"-" 300 _J

/

/

f

f--'-

J

b

E

./

200

100

f

0

i

i

i

i

0,2

0,4

0,6

0,8

1

pipo Fig.6. N2 physisorption isotherm at -196~ of calcined ZGcTAB (a), ZGDTAB (b) and Silicalite-1 (c) (black traces). The respective fitted curves are shown (gray traces).

Table 1. Fitting of experimental N2 adsorption isotherms with a model accounting for ultramicropore filling and multilayer adsorption in super-micropores i

Vultra-mieropore (mL/g)

Vsuper-micropore (mL/g)

Vmeso-,maeropore (mL/g)

Ssuper-micro-,meso-, CBET macropore (m2/g) ........

Silicalite- 1

0.138

-~ 0

ZGCTAB

0.138

0.55

ZGDTAB

0.138

0.33

0.40

147

83

- 0

1224

48

-~0

785

84

191 The super-micropores arise from the interposed fusion of the nanoslabs during the calcination [10]. In the as synthesized ZGCTAB, the nanoslabs are laterally more separated than in the as synthesized ZGDTAB. This wider spreading seems to be maintained after the calcination. The smaller super-micropore volume of ZGDTAB is most likely the result of a smaller diameter of the individual super-micropores. The Zeogrids are loaded in 3 cm long chromatographic columns and evaluated as packing for molecular separations using a pulse chromatographic setup. Chromatograms of a octane/iso-octane mixture injected as pulses at 130~ on ZGCTAB and ZGDTAB columns are shown in Fig.7. On both columns, iso-octane elutes faster than octane. This sequence reflects the a exclusion of iso-octane from the Silicalite-1 type ultra-micropores, present in the nanoslabs building the Zeogrids (Fig.l). A moment analysis of the chromatographic peaks is provided in Table 2. ZGCTAB and ZGDTAB show similar separation b factors a between iso- and linear octane, pointing to the presence and to the integrity of identical ultra-microporous building blocks, being the nanoslabs, used for the preparation of both materials. The mass transfer resistance of ZGcTAB is lower than the one of ZGDTAB, as reflected by ,,,,, ""~, 3 , 4 , 5 the lower 02/2~ 2 value. This is in agreement with narrower super-micropores present in the ZGDTAB 0 1 2 3 4 5 sample compared to ZGCTAB. time (rain) !

Fig.7. Chromatograms of binary mixtures of octane and iso-octane separated on ZGcTAB column (a) and ZGDTAB column (b) at 130~ Table 2. Moment analysis of chromatographic peaks : retention times at peak maximum (tMAX), mean retention times (~t), separation factors (a), standard deviations of chromatographic response peaks (o) and mass transfer resistances (o2/2p z) in pulse chromatographic experiments with a mixture of octane (n-C8) and iso-octane (2,2,4-TMC5) separated on ZGcTAB and ZGDTABcolumn at 130~ i

ZGCTAB tMAX(S) (s)

ZGDTAB

n-C8

2,2,4-TMC5

n-C8

2,2,4-TMC5

1.13

0.67

2.72

1.67

1.04

0.61

2.47

1.53

a (s)

1.70

1.61

o (s)

0.174

0.093

0.529

0.307

o 2 / 2~ 2 (xlO 3)

14

12

23

20

192 4. CONCLUSIONS The formation of Zeogrid starting from a Silicalite-1 nanoslab suspension and using CTAB or DTAB as surfactant proceeds according to a similar mechanism. DLS reveals that the surfactant molecules first adsorb onto the nanoslabs, which undergo limited aggregation. After a critical size is reached, the slabs staple into micrometer size spherical particles according to SEM, having a layered structure according to XRD and a large content of surfactant according to TGA. These three observations can be reconciled by the presence of concentric layers of nanoslabs, intercalated with surfactant. In the nanoslab annealing process upon removal of the surfactant, empty spaces are created. These spaces represent super-micropores. The smaller volume of super-micropores in ZGDTAa, determined with N2 adsorption, and the larger mass transfer resistance in the column packed with ZGDTAa, determined by tracer chromatographic experiments, indicate that the super-micropores are narrower in ZGoTAB compared to ZGcTAB. The more "open" stapling of the nanoslabs in ZGCTAB compared to ZGDTAa originates from a wider lateral spreading of the slabs in the aggregates, resulting from a larger surfactant volume between nanoslabs in the individual layers, as derived from TGA and XRD analyses. ACKNOWLEDGEMENTS

The authors acknowledge the IAP-PAI and the PRODEX office for financial support. P.J.G. acknowledges the Flemish F.W.O. for a position as Research Director. REFERENCES

1. A. Corma, V. Fomes, S.B. Pergher, T.L. Maesen and J.G. Buglass, Nature, 396 (1998) 353. 2. J. Caro, M. Noack, P. Kolsch and R. Schafer, Microporous Mesoporous Mater., 38

(2000) 3. 3. Z. Zhang, Y. Han, F.S. Xiao, S. Qiu, L. Zhu, R. Wang, Y. Yu, Z. Zhang, B. Zou, Y. Wang, H. Sun, D. Zhao and Y. Wei, J. Am. Chem. Soc., 123 (2001) 5014. 4. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao and F.S. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 5. Y. Liu, W. Zhang and T.J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 6. Y. Liu and T.J. Pinnavaia, Chem. Mater., 14 (2002) 3. 7. I. Schmidt, C. Madsen and C.J.H. Jacobsen, Inorg. Chem., 39 (2000) 2279. 8. C.E.A. Kirschhock, V. Buschmann, S. Kremer, R. Ravishankar, C.J.Y. Houssin, B.L. Mojet, R.A. van Santen, P.J. Grobet, P.A. Jacobs and J.A. Martens, Angew. Chem. Int. Ed., 40 (2001) 2637. 9. R. Ravishankar, C.E.A. Kirschhock, P.P. Knops-Gerrits, E.J.P. Feijen, P.J. Grobet, P. Vanoppen, F.C.E.A. De Schryver, G. Miehe, H. Fuess, B.J. Schoenmn, P.A. Jacobs and J.A. Martens, J. Phys. Chem. B, 103 (1999) 4960. 10.S.P.B. Kremer, C.E.A. Kirschhock, M. Tielen, F. Collignon, P.J. Grobet, P.A. Jacobs and J.A. Martens, Adv. Funct. Mat., 12 (2002) 286. 11 .M.J. Remy and G. Poncelet, J. Phys. Chem., 99 (1995) 773.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

193

Large scale synthesis of carbon nanofibers by catalytic decomposition of hydrocarbon L. Pesant, G. Win~, R. Vieira, P. Leroi, N. Keller, C. Pham-Huu*, M.J. Ledoux Laboratoire des Mat6riaux, Surfaces et Proc~d~s pour la Catalyse (LMSPC), UMR 7515 du CNRS, ECPM, Universit~ Louis Pasteur, 25 rue Becquerel, BP 08, 67087 Strasbourg Cedex 02, France. Tel: +33 3 90 24 26 75, Fax: +33 3 90 24 26 74, * e-mail: [email protected]

Large scale carbon nanofibers with homogeneous diameter, i.e. 50 nm, have been synthesised by catalytic hydrocarbon decomposition over a nickel supported catalyst. The carbon nanofiber yield of several dozen of grams per day has been obtained by optimizing the different synthesis conditions. At medium synthesis temperature the carbon nanofibers only expose prismatic planes on the outer surface of the material providing a high interaction with any deposited metal particle. As an example palladium particles were well dispersed on this surface with an average particle size of around 5 nm. The strong interaction between the metal and the carbon nanofiber surface led to faceted metal particles. This catalyst exhibited a high activity in the hydrogenation of nitrobenzene into aniline under moderate pressure. 1. I N T R O D U C T I O N At the beginning of the 21 st century, it is thought that the improvement of the existing chemical industry will pass through the development of new kinds of catalysts to meet the latest environmental requirements. Since their discovery in 1991 [1], carbon nanotubes and nanofibers have received increasing interest in academic research and for several potential applications [2-4]. It has been reported by different authors that carbon nanofibers could be efficiently used as catalyst support for several catalytic reactions either in gas or in liquid phase [5-7]. In both cases, the carbon nanofiber based catalysts always exhibited higher catalytic performances compared to those observed on equivalent conventional catalysts. Carbon nanofiber materials exhibit several advantages with respect to the conventional support materials: (i) a high external surface area in a nanometric-scale size which significantly improves the mass transfer, especially in a liquid medium, (ii) a strong interaction generated by the exposed prismatic planes towards the active phase, which can induce peculiar catalytic activities and selectivities. However, to be used in an efficient way the carbon nanofibers should be able to be synthesized on a large scale with a uniform diameter and a low price. From the numerous reviews published on the subject its seems

194 that Chemical Vapor Deposition (CVD) is the most efficient method for the production of a large amount of carbon nanostructured materials with high selectivity when compared to the physical methods such as laser ablation or arc-discharge [8-12]. The aim of the present article is to report the large scale (several hundred grams per gram of active phase) synthesis of uniform carbon nanofibers (average diameter ranging between 40 and 60 nm) by the catalytic decomposition of a mixture of ethane and hydrogen over a nickel catalyst supported on carbon nanotubes. To illustrate their catalytic potential, the as-synthesized carbon nanofibers are subsequently used as catalyst support for palladium in the hydrogenation of nitrobenzene in a liquid phase reaction. 2. E X P E R I M E N T A L S E C T I O N

2.1 Catalyst preparation The nickel based catalyst was prepared by immersion of the carbon nanotube support (under stiring) in a small excess of an aqueous solution of nickel nitrate (Ni(NO3)2.6H20, Merck) relatively to the pore volume of the material. The tubes were then impregnated with a volume of benzene corresponding to the tubule internal volume. Due to the high affinity of the benzene with the carbon nanotube the aqueous solution containing nickel salt was immediately pushed outside the tubules. After calcination (air at 300 ~ for 2 h) and reduction (H2 at 400 ~ for 2 h) the carbon nanotubes decorated with nickel metal were obtained. The concentration of nickel measured by ICP-MS was 20 wt.%. The TEM micrograph of the sample reduced under flowing hydrogen at 400~ unambiguously proved that the nickel phase was exclusively located on the outer walls of the carbon nanotubes with a particle mean diameter of about 40-50 nm [12]. It should be noted that despite the low reactivity of the exposed basal planes of the carbon nanotubes the nickel particles were well dispersed without formation of aggregates.

2.2 Carbon nanofiber synthesis The carbon nanofiber (CNF) synthesis was performed at atmospheric pressure at temperatures ranging from 550 to 650 ~ and under various mixtures of ethane and hydrogen. After synthesis the material was cooled to room temperature under the synthesis mixture and then discharged. Due to the high carbon nanofiber yield no post-treament was needed as usually encountered with other preparation methods such as acidic treatments in order to remove the metal catalyst from the final product.

2.3 Characterisation techniques The as-prepared sample was characterised by SEM (Philips) and TEM (Topcon EM002-B with a point-to-point resolution of 0.17 nm). The specific surface area was measured by the BET method using N2 as adsorbant at LN2 (SA-3100 sorptometer). The sample was outgassed under dynamic vacuum at 200 ~ for 2 h before measurement.

195 3. RESULTS AND DISCUSSION 3.1 Yield of carbon nanofibers

The total weight of the resulting sample was significantly increased compared to the starting weight of the metal catalyst, i.e. 17 g of carbon nanofibers were obtained from 0.04 g of the metal catalyst. The cumulated yield of the carbon nanofibers at 650 ~ and after 12 hours of synthesis was > 8000 wt. % (425 g of CNFs per gram of catalyst). The high carbon nanofiber yield observed was attributed to the high external surface area of the carbon nanotube support towards the gaseous reactants which significantly increases the number of nuclei for carbon nanofibers formation. The presence of hydrogen in the feed keeps the metal catalyst surface free and provides adsorption-decomposition sites for the hydrocarbon which, in turn, leads to the production of carbon nanofibers. Hydrogen also rapidly saturates the dangling bonds of the carbonized product during the synthesis. However, the hydrogen concentration should be keep below a certain level in order to avoid carbon gaseification which in turn, leads to a carbon loss [13]. Fig. 1. SEM micrograph of the assynthesized carbon nanofibers after CVD at 600~

3.2 Microstructure and surface area characteristics

The carbon nanofibers formed were extremely homogeneous with a mean diameter of about 50 nm and lengths up to several hundred micrometers (Fig. 1). From statistical SEM observation, it seemed that their diameter did not depend on the synthesis temperature but only on the initial diameters of the nickel catalyst particles [12]. This observation was attributed to the absence of significant sintering of the supported nickel phase when the reaction temperature increased from 550 ~ up to 650 ~ The TEM image presented in Fig. 2a clearly showed that the carbon filaments formed were nanofibers in nature with graphene planes stacked in a fishbone structure along the fiber axis with an angle of 75 o between them. The TEM image (Fig. 2b) indicates that the carbon nanofiber growth occurred in several directions from single nickel catalyst particles (octopus-like) which indicates that during the course of the synthesis the nickel surface underwent faceting with a concomitant formation of several active faces for carbon dissolution and precipitation. Fig. 2c shows the high resolution TEM image of the interface between the nickel particle and the carbon nanofiber growing from it.

196

Fig. 2. (a) TEM image of the carbon nanofibers showing graphene planes stacked in a fishbone structure along the fiber axis. (b) TEM image showing the formation of carbon nanofibers in several directions from a single nickel particle. (c) High resolution TEM image showing the nickel face which was active for carbon nanofibers growth. The carbon nanofibers obtained had a specific surface area measured by N2 adsorption ranging from 100 to more than 250 ma/g depending on the synthesis temperatures. Such a surface area was of the same order as those reported in the literature [5,14,15]. The presence of graphite edges which could act as adsorption sites for nitrogen was proposed to explain this high surface area. Fig. 3 shows the TEM micrographs of the carbon nanofibers as a function of the synthesis temperature at 600 and 700 ~ At low synthesis temperature the carbon nanofiber structure was more disordered, with a non-regular diameter (Fig. 3a). The structure became more ordered when the synthesis temperature was increased to 700 ~ (Fig. 3b). High-resolution TEM micrographs (not shown) also revealed the presence of a nickel particle at some fiber tips which indicated that during the synthesis both tip and base growth mechanisms occurred. 3.3 Nitrobenzene hydrogenation The hydrogenation of nitrobenzene was carried out on palladium supported on the carbon nanofibers prepared according to the preceding method without further purification and compared to a commercial palladium catalyst supported on a high surface area activated charcoal (Aldrich, 970 m2/g).

197

Fig. 3. TEM images of the carbon nanofibers structure as a function of the synthesis temperatures: (a) 600 ~ (b) 700 ~

Fig. 4. TEM micrograph of the palladium (5 wt.%) supported on carbon nanofibres showing the presence of faceted metal particles.

The reaction was performed in a batch reactor in an autoclave under 20 atmospheres of hydrogen and at room temperature. A liquid sampler allowed the progress of the reaction to be followed by product analyses. The products were analysed by gas chromatography in a Varian GC-3400 equipped with a capillary column (PONA) allowing the separation of the different reactant and products. The TEM image of the CNFbased catalyst is presented in Fig. 4. The palladium particles were significantly dispersed on the entire surface of the support with an average particle size of ca. 5 nm. The metal particle was highly faceted which indicates the existence of a strong metal-support interaction between the prismatic graphite planes and the metalparticles.

198

The catalytic activity is reported in Fig. 5 as a function of time. The hydrogenation activity, expressed in terms of conversion, was slightly higher over the palladium supported on carbon nanofibers when compared to that of the commercial catalyst supported on a high surface area activated charcoal (AC). The high hydrogenation activity of the CNF-based catalyst, despite its lower specific surface area when compared to that of the activated charcoal, was attributed to the existence of a strong interaction between the metal and the prismatic planes exposed by the support which in turn, modified the electronic properties and the turnover frequency of the metallic palladium active site. Similar results have also been reported by Baker's group [16] for nickel catalyst supported on a high surface area filamentous graphite during the hydrogenation of linear olefins in gas phase reactions. The authors have attributed this catalytic behaviour to the modification of the nickel morphology, i.e. flat metal particle instead of hemispheric particle, due to the strong interaction with the support surface.

100

o~ v

80-

I

I

I

I

F7 Pd/C m Pd/Aq

C .O m t.b > C O O C N C O

"='9z

60-

40-

20-

0

180

270

360

450

Time on stream (h) Fig. 5. Hydrogenation activity, expressed in terms of conversion, over the Pd/CNFs and Pd/AC catalysts at room temperature and under 20 bar total pressure. 4. C O N C L U S I O N Low surface area carbon nanotubes can be efficiently used as a catalyst support for the large scale synthesis of carbon nanofibers under relatively mild synthetic conditions, i.e. < 650 ~ The material obtained has a relatively high surface area (100-250 mZ/g depending on the synthesis temperature) with a mesoporous distribution. Both high carbon

199 yield and selectivity to the fibers can allow the direct use of these nanofibers as catalyst or catalyst support, without any traditional acidic and sonication purifications. The high external surface area and the high density of carbon edges (low-temperature synthesis) of such materials make them very attractive for use as catalyst supports for liquid phase reactions. The Pd/CNF catalyst displays a catalytic activity as high as that obtained on a commercial catalyst supported on activated charcoal despite the large difference between the two supports surface area, i.e. 100 m2/g for the CNFs instead of 1000 m2/g for the activated charcoal. The high hydrogenation activity observed on the CNF-based catalyst was attributed to the high external surface area of the support and to the peculiar interaction existing between the prismatic planes and the metallic particles. Recently, a significant improvement was introduced via a new synthesis route which allows the possibility for these nanoscopic materials, to be supported on a macroscopic host structure [17]. ACKNOWLEDGEMENTS The authors would like to thank members of the Inorganic Chemistry Department of the Fritz-Haber-Institut in Berlin for performing part of the experimental work and Prof. R. Schl6gl for helpful discussions.

REFERENCES .

2. ,

4. 5. 6. 7.

10. 11.

12. 13. 14. 15.

S. Iijima, Nature, 354 (1991) 56. M.S. Dresselhaus, G. Dresselhaus and P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1995. C. Laurent, E. Flahaut, A. Peigney and A. Rousset, New J. Chem., (1998) 1229. P.M. Ajayan, Chem. Rev., 99 (1999) 1787. C. Park and R.T.K. Baker, J. Phys. Chem. B, 102 (1998) 5168. F. Salman, C. Park and R.T.K. Baker, Catal. Today, 53 (1999) 385. C. Pham-Huu, N. Keller, G. Ehret, L. Charbonni6re, R. Ziessel and M.J. Ledoux, J. Mol. Catal. A : Chemical, 170 (2001) 155. C. Pham-Huu, N. Keller, G. Ehret and M.J. Ledoux, J. Catal., 200 (2001) 400. In Carbon Nanotubes : preparation and properties (Ed. T.W. Ebbesen) , CRC, Boca Raton, 1997. P.M. Ajayan, Chem. Mater., 11 (1999) 3862. P.M. Ajayan and O.Z. Zhou, in Carbon Nanotubes. Synthesis, Structure, Properties and Applications, Topics in Applied Physics, Vol. 80 (Eds. M.S. Dresselhaus, G. Dresselhaus and Ph. Avouris), Springer, Heidelberg, 2001, pp.391-425. C. Pham-Huu, N. Keller, V.V. Roddatis, G. Mestl, R. Schl6gl and M.J. Ledoux, Phys. Chem.-Chem. Phys., 4(3) (2002) 514. L. Ci, Y. Li, B. Wie, J. Liang, C. Xu and D. Wu, Carbon, 38 (2000) 1933. K. Hernadi, A. Fonseca, J.B. Nagy, D. Bernaerts and A.A. Lucas, J. Mol. Cat. A: Chemical, 107 (1996) 159. M.-S. Kim, N.M. Rodriguez and R.T.K. Baker, Mater. Res. Symp. Proc., 368 (1995) 99.

200 16. 17.

A. Chambers, T. Nemes, N.M. Rodriguez and R.T.K. Baker, J. Phys. Chem. B, 102 (1998) 2251. C. Pham-Huu, R. Vieira, M.J. Ledoux, L. Charbonni~re and R. Ziessel, French Pat. Appl. No. 01-15178, assigned to Sicat, 2001.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

201

Synthesis and characterization of carbon nanofiber supported ruthenium catalysts M.L.Toebes a, F.F. Prinsloob, J.H. Bitter a, A.J. van Dillena and K.P. de Jong a* aUtrecht University, Debye Institute, Department of Inorganic Chemistry and Catalysis, P.O. Box 80083, 3508 TB Utrecht, The Netherlands

bSaso1Technology R&D, P.O. Box

1, Sasolburg 9570, Republic of South Africa

Homogeneous deposition precipitation (HDP) is explored for the preparation of carbon nanofiber supported ruthenium catalysts. First, carbon nanofibers (CNF, 177 m~/g) are oxidized using nitric acid thus activating the graphitic carbon surface. Second, ruthenium (hydr)oxide is deposited homogeneously onto the CNF by hydrolysis of urea at 363K. Electron microscopy and hydrogen chemisorption studies showed that, after reduction in hydrogen, the CNF were readily homogeneously covered with 1-2 nm metal particles. This high dispersion remained almost unchanged upon heating in inert gas up to 973 K. These results clearly demonstrate the applicability of the HDP technique for the preparation ofthermostable CNF-supported metal catalysts. 1. INTRODUCTION For a number of reasons the properties of carbon nanofibers (CNF) as a catalyst support are advantageous. Carbon nanofibers have a relatively large external surface area (100 - 200 m2/g) combined with good accessibility and the absence of micropores. During the growth process the fibers interweave and form mechanically strong agglomerates while the dimensions of the skeins allow simple filtration. However, the chemical inertness of the graphitic carbon fibers brings along a problem: the application and anchoring of the catalytically active phase. However, the surface can be modified by treatment of the CNF in concentrated nitric acid. In this way oxygen-containing surface groups are introduced for the anchoring of the active phase or its precursor and to obtain a more hydrophilic surface necessary for the aqueous solution of the metal precursor [ 1]. The great potential of CNF as catalyst support material is demonstrated by several researchers. Metals like Pt [2], Pd [3,4] and Ni [58] are applied on CNF and tested in various reactions, a.o., selective hydrogenations. Selective hydrogenation reactions are important for the fine chemical industry. Ruthenium is one of the active metals for this reaction. Its performance is sensitive to subtle changes in dispersion and nature of the support. In this study we have applied ruthenium on a CNF support. For CNF supported ruthenium catalysts much higher selectivities (up to 92%) to cinnamylalcohol were found

202 [9] than with ruthenium supported on alumina [10](20-30%) and active carbon [11](3040%). Well-known techniques for the application of active phases on a support are, a.o., impregnation, ion-exchange and homogeneous deposition precipitation (HDP). The limits of impregnation often relate to poor reproducibility and very broad particle size distributions, while with ion-exchange only low loadings of the active phase can be achieved [12]. Hoogenraad et al. [13-15] have used ion-exchange to apply palladium on CNF and they found a metal loading of 3 wt% at most. With HDP using oxidic support materials this method leads to catalysts with both a high loading and a high dispersion. Geus and co-workers [16,17] have extensively studied this method for powdered supports and described the background of HDP. In HDP the active phase or its precursor is deposited onto an existing support by slowly and homogeneously introducing the precipitating agent in such a way that nucleation in the solution itself is avoided. To achieve this, locally high degrees of super saturation must be prevented. Notorious is the use of the hydrolysis of urea to precipitate compounds by a homogeneous increase of the hydroxyl concentration. To obtain a uniform layer of small particles the nucleation homogeneously over the surface of the support must proceed at a lower concentration than in the bulk of the solution [16]. This procedure has been used with success to produce a number of different catalysts. For example, Burattin et al. [18,19] have given a detailed description of the molecular mechanism of the HDP of Ni(II) on silica. They concluded that during HDP surface compounds like 1:1 Ni-phyllosilicates are formed. Our goal was to investigate whether HDP is suitable also to achieve high metal dispersions in case no surface compounds can be formed and the interaction is confined to that of the precursor ions with a relatively small number of surface sites.

2. EXPERIMENTAL 2.1. Catalyst preparation For the growth of CNF a 20 wt% Ni/SiO2 was prepared by homogeneous deposition precipitation (HDP) [ 16]. Prior to the fiber growth the Ni-catalyst was reduced in situ for 2 hours in a flow of 20% H2/Ar (1 bar) at 973 K. Next, the CNFs were grown at 823 K in CO/HE/Ar (= 20/7/73) for 24 hours. The CNFs were refluxed for 1 hour in a 1 M KOH solution in order to remove the silica support. For the activation of the CNFs and the removal of nickel, the CNFs were refluxed in concentrated nitric acid for 2 hours and washed with demi-water. Ruthenium (5 wt%) was deposited on the fibers according to the HDP-method. In literature in the majority of the studies on the preparation of ruthenium-supported catalysts ruthenium chloride was utilized as the precursor. Commercial ruthenium chloride can contain as much as 80% Ru(IV) which is only soluble in aqueous solution at very low pH. We attempted the preparation of CNF supported ruthenium catalysts via HDP using this precursor salt. Catalysts with good dispersions (0.25) and relatively small average particle sizes (3 nm) were obtained. TEM and SEM images of these dried catalysts, however, revealed the presence of large lumps of ruthenium oxide between the CNF, see Fig. 1. For this reason RuNO(NO3)3.nH20 could be a better choice as precursor salt for the HDP.

203 An acidified suspension (pH=0.5) of 5 gram CNF in 250 ml demi-water was heated up to 363 K and urea (1.56 gram) and RuNO(NO3)3.nH20 (0.823 gram, 5 wt%) were added. After 6 hours the loaded CNF was filtered and washed thoroughly with demi-water, dried at 393 K and reduced with Hz at 473 K for 1 hour (heating rate = 5 K/min).

Fig. la. TEM image of Ru/CNF prepared from ruthenium chloride

Fig. lb. SEM back scatter image of Ru/ CNF prepared from ruthenium chloride

2.2. Catalyst characterization The carbon fiber support and the catalysts before and after reduction were characterized with various techniques, viz. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), temperature programmed reduction (TPR), N2-physisorption, inductively coupled plasma emission spectrometry (Vista AZ CCD simultaneous ICP-AES) and hydrogen chemisorption. XRD patterns were recorded at room temperature with a Nonius PDS 120 powder diffractometer system equipped with a position-sensitive detector with a 20 range of 120 o using Co K a l (L=1.78897 ,~) radiation. The CNF and the CNF-supported ruthenium catalysts were examined in a Philips CM-200 FEG TEM operated at 200 kV. Samples were prepared by suspending the fibers in ethanol under ultrasonic vibration. Some drops of the thus produced suspension were brought onto a holey carbon film on a copper grid. SEM analysis was performed on a JEOL JSM 6000F scanning electron microscope. TPR measurements were performed with a TPDRO 1100 instrument from Thermo Quest CE Instruments. Prior to temperature programmed reduction the reduced catalysts were oxidized in a mixture of oxygen (10 ml/min) and nitrogen (10 ml/min) at 453 K. A sufficient amount of sample (4 0.02 g) was placed in a fixed bed reactor for analysis. A flow containing 5 % hydrogen and 95 % argon (Hock Loos) was passed downward through the catalyst bed at a rate of 20 ml/min (STP). From the dried flow (molsieves) the hydrogen consumption was measured as a function of temperature (heating rate = 10 K/min) using a tungsten thermal conductivity detector. Specific surface areas (BET) of the carbon nanofibers were calculated from nitrogen physisorption data measured at 77K with a Micromeritics ASAP 2400 apparatus. Prior to the physisorption experiments the

204 samples were evacuated at 473 K for at least 16 hours. Hydrogen chemisorption measurements were performed using an ASAP 2010C, Micromeritics instrument. Before chemisorption measurement, a sample was dried in He at 393 K for 1 hour, and reduced in flowing H2 (flow rate = 50 ml/min STP) at 473 K for 2 hours, heating rate 5K/min. After reduction the catalyst was degassed for two hours at 10-7 Pa at the reduction temperature in order to eliminate the chemisorbed hydrogen and water. The isotherms were measured at 308 K. The H/Ru ratios are based on the adsorbed amounts of hydrogen determined with this isotherm by extrapolation of the linear part to zero pressure. Estimated particle sizes are based on spherical geometry and an adsorption stoichiometry of H/Rus = 1. 3. RESULTS 3.1 Characterization of the support The CNFs are of the fishbone type, meaning that the graphite planes are oriented at an angle to the central axis. After treatment with nitric acid the fibers show a surface area of 177 m2/g and are non-microporous. In Fig. 2, a SEM image of an untreated CNF sample, is shown that the fibers form a highly porous structure. X-ray diffraction and electron microscopy showed that the graphitic structure of the CNF was not affected during activation. A TEM image of CNF after activation is presented in Fig. 3. The average diameter of the CNF is 25 nm and both Figs. 2 and 3 display the narrow diameter distribution of the CNF. Besides the introduction of the oxygen functional groups, the activation procedure in nitric acid also removes the nickel of the growth catalyst from the CNF. Only the nickel particles, which are encapsulated by a few graphite layers are still present.

Fig. 2. SEM image of untreated CNF

Fig. 3. TEM image of CNF after oxidation

Treatment of the carbon nanofibers with nitric acid introduces various oxygen-containing surface groups. The amount of acidic groups determined with titration could give rise to a ruthenium loading of at most 2-3 wt% using ion-exchange. This is confirmed by the results of Hoogenraad et al. [13,14,15]. They have used ion-exchange to apply palladium on CNF

205 and they also found a metal loading of 3 wt% at most. This implies that an alternative method is required to obtain a 5 wt% CNF supported metal catalyst. 3.2 Characterization of the catalysts The loading of the CNF supported ruthenium catalysts are studied with ICP-AES and tumed out to be the aimed 5 wt%. This indicates that not all ruthenium is applied via ionexchange, because using this method a loading of about 3 wt% can be obtained with CNF as support. TPR is used to obtain information on the reduction behavior of the catalysts. The results showed that the CNF supported ruthenium catalysts can be reduced at 473 K. The reduction temperature is higher than for bulk RuO2. If it is assumed that all ruthenium was present in the original catalyst as RuO2, a theoretical maximum H2/Ru ratio of 2 should be observed. The ratios calculated from the TPR profiles of the catalysts are given in Table 1.

Table 1 H2/Ru ratios calculated from the TPR data for the 5 wt% CNF supported ruthenium catalysts Sample Treatment H2/Ru Ru/CNFnp Dried 4.7 Ru/CNFred Reduced 473 K 2.1 Ru/CNF773 Reduced 473 K, 573 K in N2 2.3 2.2 Ru/CNF973 Reduced 473 K, 973 K in N2 Except for the Ru/CNFnp catalyst the calculated ratios are in close correspondence with the theoretical value of two. In the case of the Ru/CNFnp catalyst the higher ratio indicates that there must be a contribution to the H2 consumption due to the reduction of the surface oxygen functional groups.

Fig. 4a. TEM image of CNF-supported ruthenium catalyst, 5 wt%

Fig. 4b. TEM image of CNF-supported ruthenium catalyst, 5 wt%

206 In Fig. 4 two TEM images are shown, demonstrating the high metal dispersion of the reduced CNF supported ruthenium catalysts. The particle sizes estimated from the TEM images are small and they fall within the narrow range between 1.1-2.2 nm. The resulting average crystallite diameter is listed in Table 2. With TEM and SEM no large ruthenium particles were found, only in case the catalysts were prepared using ruthenium chloride as the metal precursor. In Fig. 4a also the fishbone structure of the CNF is visible. The metal dispersion and the average particle sizes calculated from the hydrogen chemisorption data are listed in Table 2 and are in close correspondence with the TEM estimated values. The reproducibility of the HDP procedure was checked several times and always high dispersions and ruthenium particles were found with TEM in the 1-2 nm range. Table 2 Dispersions and average particles sizes of reduced Ru-catalysts treated at different temperatures for 2 hours in N2 atter reduction at 473 K. T (K) in N2 wt% Ru H/Ru dm (nm) dTEM(rim) Ru/CNF 5.0 0.41 2.0 1.5 Ru/CNF (duplo) 5.0 0.46 1.8 1.2 Ru/CNF 773 5.0 0.38 2.2 2.2 Ru/CNF 973 5.0 0.26 3.2 1.8 To examine the stability of the ruthenium catalysts, the catalysts were treated after reduction at elevated temperatures in nitrogen. The influence of this treatment on the average metal particle sizes determined with TEM can be seen in Table 2. In this table also the results obtained with hydrogen chemisorption are listed. A slight but significant decrease of the metal dispersion was observable with the results of the hydrogen chemisorption measurements and the TEM images after heating beyond 773 K, which is quite a remarkably high temperature. In Figs. 5a and b the TEM images of the catalysts treated at 773 and 973 K are displayed.

Fig. 5a. TEM image of CNF-supported 5wt % ruthenium catalyst, reduced and treated at 773 K in N2

Fig. 5b. TEM image of CNF-supported 5wt % ruthenium catalyst, reduced treated at 973 K in N2

207 4. CONCLUSIONS We have shown that small uniform ruthenium particles can be applied on activated CNFs in a reproducible manner when the HDP method is used with RuNO(NO3)3 as catalyst precursor. A very uniform distribution of 1-2 nm sized ruthenium particles at an appreciable loading has been obtained. This high dispersion remained almost unchanged upon heating in inert to 973 K. These results clearly demonstrate the applicability of the HDP technique for the preparation of CNF supported metal catalysts, though no surface compound between precursor and support material can be formed.

ACKNOWLEDGEMENTS The authors are grateful to Cor van der Spek for performing the TEM measurements. These investigations are supported by Sasol Technology R&D and the Council for Chemical Sciences of the Netherlands Organization for Scientific Research with financial aid from the Netherlands Technology Foundation (CW/STW 349-5357).

REFERENCES

1. K.P. de Jong and J.W. Geus, Catal. Rev. Sci. Eng., 42 (2000) 481. 2. R. T. K. Baker, K. Laubernds, A. Wootsch and Z. PM1, J. Catal., 193 (2000) 165. 3. C. Pham-Huu, N. Keller, L.J. Charbonniere, R. Ziessel and M.J. Ledoux, Chem. Commun., (2000) 1871. 4. M.S. Hoogenraad, R.A.G.M.M. van Leeuwarden, G.J.B. van Breda Vriesman, A. Broersma, A.J. van Dillen and J.W. Geus, Stud. Surf. Sci. Catal., 91 (1995) 263. 5. S. K. Shaikhutdinov, L.B. Avdeeva, B.N. Novgorodov, V.I. Zaikovskii and D.I. Kochubey, Catal. Letters, 47 (1997) 35. 6. F. Salman, C. Park and R.T.K. Baker, Catal. Today, 53 (1999) 385. 7. A. Chambers, T. Nemes, N.M. Rodriguez and R.T.K. Baker, J. Phys. Chem. B, 102 (1998) 2251. 8. C. Park and R.T.K. Baker, J. Phys. Chem. B, 102 (1998) 5168. 9. J.M. Planeix, N. Coustel, B. Coq, V. Protons, P.S. Kumbhar, R. Dutartre, P. Geneste, P. Bernier and P.M. Ajayan, J. Am. Chem. Soc., 116 (1994) 7935. 10. B. Coq, P.S. Kumbhar, C. Moreau, P. Moreau and M.G. Warawdekar, J. Mol. Catal. 85 (1993) 215. 11. S. Galvagno, G. Capanelli, G. Neri, A. Donato and R. Pietropaolo, J. Mol. Catal., 64 (1991)237. 12 .K.P. de Jong, Current Opinion in Solid State & Materials Science, 4 (1999) 55. 13. M.S. Hoogenraad, Ph.D. thesis, Utrecht University, 1995 14. B.L. Mojet, M.S. Hoogeraad, A.J. van Dillen, J.W. Geus and D.C. Koningsberger, J. Chem. Soc., Faraday Trans. 93 (1997) 4371.

208 15. M.S. Hoogenraad, M.F. Onwezen, A.J. van Dillen and J.W. Geus, Stud. Surf. Sci. Catal., 101 (1996) 1331. 16. A.J. van Dillen, J.W. Geus, L.A.M. Hermans and J. van der Meijden, J. Proc. Int. Congr. Catal. 6th, 2 (1977) 677. 17. L.A.M. Hermans and J.W. Geus, Stud. Surf. Sci. Catal., 3 (1979) 113. 18. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B, 101 (1997) 7060. 19. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B, 102 (1998) 2722.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

209

Synthesis of high pore volume and specific surface area mesoporous alumina. L. Sicard a, B. Lebeau a, J. Patarin a, F. Kolenda b. a LMM (UMR CNRS-7016, CNRS), ENSCMu, Universit6 de Haute Alsace, 3, rue A. Werner, 68093 Mulhouse, France. b IFP, BP3, 69390 Vernaison, France.

The present study focuses on the synthesis of mesostructured aluminas. In the first part of the article, the nature of the interactions between the surfactant and the inorganic framework within a sodium dodecylsulfate templated alumina is studied. The mechanism of formation of this material is also described. In the second part, the synthesis of high pore volume mesoporous alumina prepared under basic conditions is reported.

1. I N T R O D U C T I O N Mesoporous materials synthesized in the presence of surfactants present a high surface area and pore volume. These properties should be interesting in adsorption and catalysis fields for example. The first studies have been focusing on the preparation of silicabased materials before extending the synthesis procedure to other oxides. However, few studies reporting the synthesis of mesostructured alumina have been published. Moreover, the majority of the mesostructured alumina obtained have a lamellar [1-3] or a disordered [2-14] structure (presence of a single diffraction peak between 20 = 1 and 10 ~ (Cu-Kal radiation) on the X-ray diffraction (XRD) pattern). To our knowledge, the only mesostructured alumina which presents a hexagonal structure has been synthesized in the presence of sodium dodecylsulfate (SDS) by Yada et al. [15-18]. The organization of the material is obtained by a slow increase of the pH, provoked by the decomposition of urea at 80~ However, the mesostructure collapses upon calcination. The use of a pH modifier (urea) seems to be the key point to obtain hexagonal structures. Indeed, very recently, thin layers of mesoporous alumina with a hexagonal structure have been prepared in the presence of a triblock copolymer and urea [19]. In this paper, two studies are presented. The first one concerns the system described by Yada et al. [15, 16]. The aim was to understand the unstability of the material upon calcination by thermal analysis and to elucidate the mechanism of formation of such a material by fluorescence techniques. The second part of the article describes an original method to prepare mesoporous alumina with high pore volume and surface area in the

210 presence of a mixed micelle composed of cetyltrimethylammonium bromide (CTMABr) and sodium palmitate. 2. E X P E R I M E N T A L SECTION

2.1. Reactants Aluminum nitrate nonahydrate (98%), sodium aluminate (92%), SDS (98%), CTMABr (98%) and sodium palmitate (98%) were purchased from Fluka, urea (99.5%) from Prolabo and hydrochloric acid Titrisol | from Merck. 2.2. Synthesis The procedure used to prepare the hexagonal alumina is similar to that described by Yada et al. [15, 16]. 7.6 g of aluminum nitrate nonahydrate, 11.9 g of SDS and 36.2 g of urea are solubilized in 21.6 g of deionized water under stirring at 40~ Consequently, the composition of the initial mixture is 1 Al(NOa)3"9H20 : 2 SDS : 30 urea : 60 Ha0. When a clear solution is obtained, the temperature is raised up to 80~ to decompose urea. A precipitate appears at pH - 5.5 but the solution is cooled down only when the pH reaches 7. The solid is then recovered by filtration and extensively washed with hot water. For the second system, all the solutions employed are heated at 50~ Typically, 0.58 g of sodium palmitate in 25 ml water is added to 2.35g of CTMABr solubilized in 15 ml water under vigorous stirring. 1.96 g of sodium aluminate in 10 ml water is then added to the mixture. The acidification is realized by adding 22 ml of a 1 tool.1-1 HC1 solution. The molar composition of the reactive mixture is 1 sodium aluminate : 0.36 CTMABr : 0.09 sodium palmitate : 1 HCI: 182 H20. The suspension is left under vigorous stirring at 50~ during 15 minutes. The product is then separated from the solution by centrifugation and carefully washed with a solvent (water, ethanol or acetone). 2.3. Characterization The powder X-ray diffraction (XRD) patterns were obtained with Cu-Kctl radiation on a STOE STADI-P diffractometer equipped with a curved germanium (111) primary monochromator and a linear position-sensitive detector. Typically, the diffractograms were collected at angles 20 between 1 and 10 ~ Scanning and transmission electron microscopies (SEM and TEM, respectively) were realized at IFP. SEM micrographs were obtained on a JEOL 6340 F and TEM micrographs on a JEOL 100 CX or a JEOL 120 CX. Sample Controlled Thermal Analysis (SCTA) was carried out under a constant residual pressure of 5.10 .3 mbar on a home-made apparatus [20]. The totality of the evolved gases was analyzed in situ via a mass spectrometer (VG Quadrupoles) with a maximum detection of m/z = 100. Fluorescence techniques were used to study micelles in clear solutions. Timeresolved fluorescence quenching (recording of the fluorescence decay curves on a single photon counting apparatus) was used to determine the pyrene (used as the probe molecule) fluorescence lifetime (~). Micelle aggregation numbers (N), which correspond to the number

211 of surfactant molecules per micelle, were determined in the presence of dodecylpyridinium chloride as fluorescence quencher [21]. The nitrogen adsorption-desorption isotherms of the calcined samples were determined at -196~ on a Micromeritics ASAP 2010 apparatus. Prior to the measurements, the samples were outgassed at 90~ during 1 hours and then at 350~ for 16 hours. The BET equivalent surface area was calculated using the BET equation [22]. 3. RESULTS AND DISCUSSION The materials synthesized in the presence of SDS, whatever the final pH (between 6 and 7), present a hexagonal structure as confirmed by the presence of the (100), (110) and (200) XRD peaks. SEM and TEM micrographs showing respectively a worm-like morphology and a honeycomb network are similar to that observed for MCM-41 materials. Finally, elemental analysis revealed the incorporation of dodecylsulfate (DS) with a molar ratio DS/A1 of 0.25-0.3. It is noteworthy that no sodium nor nitrogen are detected. After calcination at 450~ the structure is disorganized (presence of a single XRD peak at 20 < 10 ~ and the material presents a I-b <> nitrogen isotherm characteristic of materials having large micropores (diameters ranging from 1.5 to 2.0 nm). In order to explain the degradation of the material upon heating and to characterize the interactions between the surfactant and the inorganic framework, a sample controlled thermal analysis (SCTA) was conducted on the mesostructured material and on pure SDS [23]. On the one hand, it was found that the sulfate head group of the surfactant was removed at a higher temperature when occluded within the alumina (450~ than in pure SDS (250~ This indicates a strong interaction between the sulfate group and the inorganic framework in the mesostructured material. On the other hand, the alkyl chain of the DS was removed at a lower temperature when occluded (100~ compared to pure SDS (200~ The strong interaction between the sulfate group and the alumina would result in a weaker bound between the sulfate group and the alkyl chain for the occluded surfactant. Mesostructured aluminas were calcined at 150, 250, 400 and 580~ by SCTA. They all present a single XRD peak below 20 = 10 ~ indicating a decrease in the organization order, and a nitrogen isotherm typical for microporous solids (pore diameters below 2.0 nm). The specific surface area (SBET) and the pore volume (Vp) increase with the temperature up to 250~ (SBET = 750 m2/g and Vp = 0.3 cm3/g) and decrease at higher temperature. This is in good agreement with the strong interaction between the sulfate group of the surfactant and the aluminum framework: the porosity increases when the alkyl chain is removed and the pore network collapses when trying to remove the embedded sulfate group. The formation mechanism of the material was also studied by fluorescence techniques, and more particularly by time-resolved fluorescence [24]. In these experiments, the synthesis temperature was lowered down to 60 ~ in order to slow down the kinetics of precipitation. The value of the aggregation number (N = 104) at the beginning of the experiment indicates that the micelles in the precursor solution have a quasi-spherical shape (axial ratio equal to 1.8) and interact with urea molecules. The pyrene fluorescence lifetime decreases when aluminum nitrate is added to a solution containing SDS and urea. Consequently, nitrate anions, which are fluorescence quenchers, are at the micelle surface.

212 As the micelles are negatively charged, the interaction with nitrate anions has to be indirect, via aluminum cations. At the beginning, the micelles are thus surrounded by urea molecules and by aluminum and nitrate anions in such a proportion that the micelles are quasi-neutral. The decomposition of urea results in the liberation of hydroxyl ions and in the polymerization of aluminum species. The increase of the pyrene fluorescence lifetime with time indicates that the nitrate anions would progressively leave the micelle surface. This release would be caused by the polymerization of the bound aluminum species and, consequently, by their electrical charge decrease. During the whole experiment, i.e. until a precipitate is clearly visible, the micelle aggregation number remains constant, indicating that the micelles keep the same shape and size. The polymer-surfactant complexes would grow and reorganize shortly before the precipitation. The previous system is very interesting for fundamental studies. However, it is impossible to liberate the porosity without collapsing the structure. Consequently, an original approach to make mesoporous alumina was developed: the precipitation of the material was obtained by decreasing the pH of a sodium aluminate solution in the presence of surfactants. The precursor solution had a pH slightly superior than 12. As the aluminum species are negatively charged at such a pH, a globally positively charged micelle composed of 80 molar % of CTMABr and 20% of sodium palmitate was used. The material was precipitated at pH - 10.5 by adding HCI. The suspension, similar to a gel, was washed by centrifugation with water instead of filtration because the filter pores clog up. The XRD pattern of the dried material indicated a pseudo-boehmite structure but no mesoscopic organization (absence of a XRD peak at 20 ranging from 1 to 10~ Only 0.02 mol of CTMA § and 0.01 of palmitate per aluminum were incorporated and no counterion (Br- or Na § were detected. SEM micrographs (Fig. 1) showed small aggregates of platelets, about 10-20 nanometers wide whose arrangement creates textural (intergranular) mesoporosity.

Fig. 1. Scanning electron micrograph of the alumina synthesized in the presence of CTMABr and sodium palmitate washed with water.

213 After calcination at 450~ hard blocks were obtained. The solid presents a ~/alumina structure and a type IV nitrogen isotherm (see Fig. 2-a), typical for mesoporous materials. The surface area was found equal to 400 + 50 mZ/g and the pore volume to 0.5 + 0.1 cm3/g. The value of the pore diameters (centered on 5 nm) is in good agreement with SEM observations which indicate a textural mesoporosity. Many parameters of the reaction were studied. First, the composition of the initial mixture (dilution, global surfactants concentration and CTMABr/sodium palmitate ratio), secondly the procedure parameters (temperature, aging time, final pH and addition order of the chemicals) and third the nature of the chemicals (nature of the counterion of CTMA + and nature of the acid) were varied. However, all the materials washed with water presented the same characteristics as described previously.

Fig. 2. Nitrogen adsorption isotherms of the aluminas synthesized in the presence of CTMABr and sodium palmitate washed with a- water, b- ethanol, c- acetone. Textural porosity shall be affected during the washing and drying treatments. Consequently, attempts have been made with different washing solvents. They can be divided into two groups: in the first one, solutions made with water, in the other one, organic solvents. After washing with solvents of the first group (aqueous solution of CTMABr, Triton X-100 or equivolumic mixture of ethanol and water) or in absence of washing, the solids present the same characteristics obtained after washing with water only. On the contrary, the materials washed with absolute ethanol or acetone (see Fig. 2-b and 2c respectively) have very high mesopore volumes (1.1 cm3/g) with surface areas of 450 _+50 mZ/g. However, the pore diameter distribution is very large (diameters between 2 and 30 nm). Contrary to the aqueous suspensions, the ethanol or acetone suspensions can be filtered. In the case of ethanol, hard blocks are obtained after drying but in the case of acetone, the sample is a white dusty powder. This latter material is the only one which presents macropores as can be visualized on the nitrogen adsorption isotherm presented on

214 Figure 2-c. This macroporosity is due to the presence of particles, 100 to 700 nm diameter, composed by the aggregation of smaller platelets (see Fig. 3). It is noteworthy that the aggregation is reversible before drying. Indeed, solids washed with water first and then with ethanol or acetone present pore volumes close to 1.0 cm3/g whereas solids washed with ethanol or acetone first and then with water present low pore volumes (0.5 cm3/g). The syntheses were reproduced in the absence of surfactants and the resulting materials are similar to those obtained in the presence of surfactants. Therefore, the role of the latter seems to be negligible.

Fig. 3. Scanning electron micrograph of the alumina synthesized in the presence of CTMABr and sodium palmitate washed with acetone. The influence of alcohols and acetone as washing solvents has already been shown by White et aL [25]. However, the pore volumes Vp and surface areas SB~v obtained were less important in this case (Vp = 0.72 cm3/g and SBET = 302 m2/g for acetone, Vp = 0.36 cm3/g and SBET = 353 m2/g for ethanol in comparison with 0.34 cm3/g and 283 m2/g for water. The values of pore volumes were obtained by mercury intrusion for pore diameters between 3.6 and 100 nm). The authors explain the differences in porosity by the displacement of water by ethanol or acetone which would lead to the reduction of the surface tension and to less pore collapse during calcination. In fact, another parameter must be taken into account during the drying step: the higher vapor pressure of acetone and ethanol compared to water can create a network expansion and can make move particles away from one to another. During the washing step, other forces can play a role. First, the aspect of the suspension in water is similar to a gel, suggesting the formation of bounds between the alumina particles and the water molecules. After drying, the resulting solid would be a xerogel having little porosity. These bounds would not form in ethanol or acetone. Secondly, it was observed that the pH of the suspension during the last washing step is in the range 8 to 9, value close to the isoelectric point of alumina (pH = 8.7). Consequently, the particles are quasi-neutral and can aggregate. At the contrary, ethanol and acetone would allow a repulsion between the particles. A solid was washed with water

215 at a constant pH of 10.5. In this case, the pore volume was doubled (Vp = 1.1 cm3/g), showing the consistency of the hypothesis. 4. CONCLUSION Two aspects of the synthesis of mesoporous alumina were investigated: the mechanism of formation of a mesostructured hexagonal alumina and the preparation of an interesting solid for catalysis. The hexagonal alumina was precipitated in the presence of SDS by decomposition of urea. However, the interaction between the sulfate head group of the surfactant and the inorganic framework is very strong and do not allow to liberate the mesoporosity: the calcination of the sample leads to a microporous material. A mechanism of formation of this alumina could be established. In the starting solution, an ionic exchange is observed at the micelle surface between sodium and aluminum ions. Nitrate anions also present at the surface assure the neutrality of the miceUe. During the whole experiment, until the appearance of a precipitate, the micelles remain quasi-spherical. The aluminum species would polymerize at the micelle surface as the pH increases to form hybrid complexes. The second aim of the study, the preparation of mesoporous alumina, was reached by acidification of a solution containing sodium aluminate and by washing with ethanol or acetone. Materials having pore volumes as high as 1.1 cm3/g and specific surface area of about 450 m2/g were obtained. To our knowledge, these characteristics have never been reached before. As a matter of comparison, the solids prepared with the same procedure but washed with water present pore volume of 0.5 cm3/g only, certainly because of an aggregation of the particles during washing and drying. The mesoporous aluminas obtained could be good candidates as catalysis supports. ACKNOWLEDGEMENTS

We thank Drs. P.L. Llewellyn and R. Zana for fruitful collaboration within the GDR 690 in the SCTA and fluorescence studies, respectively. IFP and CNRS are also gratefully acknowledged for their financial support. REFERENCES

1. Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F.Schtith and G.D. Stucky, Chem. Mater., 6 (1994) 1176. 2. S. Acosta, A. Ayral, C. Guizard and L. Cot, J. Sol-Gel Sci. Technol., 8 (1996) 195. 3. S. Valange, J.L. Guth, F. Kolenda, S. Lacombe and Z. Gabelica, Microporous Mesoporous Mater., 35-36 (2000) 597. 4. F. Vaudry, S. Khodabandeh and M.E. Davis, Chem. Mater., 8 (1996) 1451. 5. X. Liu, Y. Wei, D. Jin and W.H. Shih, Mater. Lett., 42 (2000) 143. 6. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltr~n, D. Beltrhn, S. Mendioroz, M.D. Marcos and P. Amor6s, Adv. Mater., 5 (1999) 379.

216 7. S. Cabrera, J. E1 Haskouri, C. Guillem, J. Latorre, A. Beltr~.n-Porter, D. Beltr~Jn-Porter, M.D. Marcos and P. Amor6s, Solid State Sci., 2 (2000) 405. 8. S.A. Bagshaw and T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl., 10 (1996) 1102. 9. W. Zhang and T.J. Pinnavaia, Chem. Comm., (1998) 1185. 10. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Chem. Mater., 10 (1999) 2813. 11. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. 12. N.M. Eswaramoorthy, Proe. Indian Acad. Sci., 2 (1998) 143. 13. V. Gonzalez-Pena, I. Diaz, C. Marquez-Alvarez, E. Sastre and J. Perez-Pariente, Microporous Mesoporous Mater., 44-45 (2001) 203. 14. V. Gonzalez-Pena, C. Marquez-Alvarez, E. Sastre and J. Perez-Pariente, Stud. Surf. Sci. Catal., 135 (2001). 15. M. Yada, M. Machida and T. Kijima, Chem. Commun., (1996) 769. 16. M. Yada, H. Hiyoshi, K. Ohe, M. Machida and T. Kijima, Inorg. Chem., 36 (1997) 5565. 17. M. Yada, H. Hiyoshi, M. Machida and T. Kijima, J Porous Mater., 5 (1998) 133. 18. M. Yada, M. Ohya, M. Machida and T. Kijima, Chem. Commun., (1998) 1941. 19. N. Idrissi-Kandri, A. Ayral, M. Klotz, P.A. Albouy, A. E1 Mansouri, A. Van der Lee and C. Guizard, Mater. Lett., 50 (2001) 57. 20. J. Rouquerol, S. Bord&e and F. Rouquerol, Thermochim. Acta, 203 (1992) 193. 21. P. Somasundaran, L. Huang and A. Fan, in B.P. Binks (Ed.), Modem Characterization of Surfactant Systems, Surfactant Science Series, Vol. 83, M. Dekker Inc., New York, 1999, p. 213. 22. S. Brunauer, P.H. Emmet and E. Teller, J. Am. Chem. Sot., 60 (1938) 309. 23. L. Sicard, P.L. LLewellyn, J. Patarin and F. Kolenda, Microporous Mesoporous Mater., 44-45 (2000) 195. 24. L. Sicard, J. Fr/isch, M. Soulard, B. Lebeau, J. Patarin, T. Davey, R. Zana and F. Kolenda, Microporous Mesoporous Mater., 44-45 (2000) 25. 25. A. White, A. Walpole, Y. Huang and D.L. Trimm, Appl. Catal., 56 (1989) 187.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

217

Investigation on acidity of zeolites bound with silica and alumina X. Wu, A. Alkhawaldeh, and R.G. Anthony a Kinetics, Catalysis and Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, USA a Phone: 1-979-845-3370, Fax: 1-979-845-6446, E-mail: [email protected]

Understanding the effect of binder on acidity of zeolites would be helpful to select a binder and make a better zeolite-based catalyst. NH3-TPD and FTIR were used to investigate the acidic properties of silica- and alumina-bound Y and ZSM-5 zeolites with SIO2/A1203 ratios of 5 to 280. The BET surface areas and pore volumes were also determined for the bound and unbound zeolite samples. The results show that both total acid and Lewis acid densities are increased for alumina-bound zeolites but decreased for silica-bound zeolites. Silica-bound Y zeolite decreases significantly in Brtinsted acid density, but silica-bound ZSM-5 does not change as much. Br/3nsted acid density of silica-bound ZSM-5 increases slightly in the region of lower SIO2/A1203 ratios with a small decrease in Lewis acid density. Alumina binder gives an increase in Br6nsted acid density for Y zeolite and has no significant effect on Br6nsted acid for ZSM-5. In terms of the strength of acid, silica-bound Y and ZSM-5 zeolites reduce significantly the strength of strong acid and have no effect on the strength of weak acid. However, alumina-bound Y and ZSM-5 zeolites have little effect on the strength of both strong and weak acids. Pore volume and surface area of zeolites Y and ZSM-5 are reduced with both silica and alumina binders. 1. I N T R O D U C T I O N Zeolites as catalysts have been employed in many applications in the chemical industry. Zeolites are of poor self-binding property and have to be bound with a binder (matrix) such as silica, alumina or clay to give a desired physical shape and mechanical strength for industrial application. For catalytic applications of zeolites, silica and alumina as a binder have been the most widely utilized commercially. In most catalysis cases, zeolites are applied as an acidic catalyst. For the acidity of a solid used as a catalyst, three important acidic properties of the solid acid have to be addressed, that is, acid density, nature of acid (i.e. Bronsted acid or Lewis acid), and strength of the acid. Therefore, a better understanding of the acidity of bound-zeolites would be a great help for catalyst research and for preparing commercial zeolite catalysts. However, the binder-zeolite effect on acidity of the final catalysts have beenpaid less attention than it should have among

218 public available communication medium. Only a few papers have been published that address this issue. For alumina binder, Choudhary and his co-workers[I,2] investigated the binder effects on the acidity of H-gallosilicate and found that alumina binder had no significant effect on the intra-crystalline acidity but causes a decrease in inter-crystalline acidity, resulting in a small decrease in total acidity. Cao et al. [3] studied ~-A1203 made by different manufactures as a binder for mordenite and showed that the alumina reacted during pelletizing with the zeolite leading to a significant increase in Lewis acidity and a smaller change in Bronsted acidity. Zholobenko et al.[4] studied the acidity of pentasilzeolite (Si/AI=21) with different A1203 contents with a diffuse scattering IR spectroscopy and found a new Lewis acid center associated with the extra-lattice A1 3+ ions in the bound-zeolite. Chang et al.[5] and Shihabi et al.[6] have found direct insertion of aluminum into the high silica (SiO 2/A1203 o1600) porous pentasil frameworks and thus resulting in an increases in acidity. For silica binder, Choudhary and his co-workers[I,2] showed in the case of Hgallosilicate that inter-crystalline and intra-crystalline acidity decreased appreciably, resulting in a decrease in total acidity. As in the case of alumina binder, Sousa-Aguiar et al.[7] found a similar interaction of silica binder with ultra-stable Y zeolite, generating an acidic silica-alumina compound. Until now, to the authors' best of knowledge, there has been no published work that systematically discusses the effect of the alumina and silica binders on the acid properties i.e. the nature, density, and strength of the acid sites in ZSM-5 and Y zeolites with different silica to alumina ratios. This paper will address this issue, and at the same time, report the changes of surface area and pore volume of zeolite Y and ZSM-5 with a range of SiO 2/A1203 ratio between 5 and 280 after bound with silica and alumina.

2. EXPERIMENT AND MATERIALS 2.1.

Acidity determination Acid density and nature of acids of either a particular zeolite or its binder-bound form were determined by NH3-TPD and pyridine adsorption FTIR, respectively. NH3TPD was conducted on a Micromeritics PulseChemiSorb 2705. Helium was used as a carrier gas with a flowrate of 40 ml/min. When the catalyst sample was thermally equilibrated at 105~ NH3 was injected with a syringe until the sample was saturated with ammonia. Then the same flow of helium through the sample was maintained to eliminate free ammonia and physically adsorbed ammonia for about 1 hour at this temperature until no further change in detector signal could be found. Temperature programmed desorption of the adsorbed ammonia was then conducted out from 105~ to 685~ with a ramping rate of 15~ The catalyst sample acidities, i.e. total acid density, strong acid density and weak acid density, were obtained from the desorption profile of ammonia[8,9]. FTIR analysis was performed on a Nicolet Magna-IR 560 spectrometer with a MCT detector at a resolution of 4 cm -~ and an accumulation of 96 scans by applying diffuse reflectance FTIR spectrometry. Each catalyst sample was treated at elevated temperature in-situ to remove adsorbed water. The sample was then cooled under a flow of helium to 25~ Pyridine was brought into contact with the sample by means of bubbling helium at the same flow rate

219 through a liquid of pyridine. Desorption of pyridine was conducted out under the same flow of helium at 200~ IR spectra were taken with KBr as background. The IR absorption bands at around 1540cm -1 and 1450cm -1 were used to determine Br6nsted and Lewis acids, respectively[9,10].

2.2. Surface Area and Pore Volume The surface area and pore volume were measured on Micromeritics ASAP2000 with N2 as sorbate at liquid nitrogen temperature. 2.3.

Silica-bound Catalysts ZSM-5 zeolite samples with SiO:/AI203 ratios of 30, 80 and 280 and Y zeolite samples with SiO:/AlzO3 ratios of 5, 30 and 80 were provided by Zeolyst Co. Ludox HS40 (from Aldrich Chemical Co., contained sodium ion for stabilizing conterion, SiOe/Na:O=95) was used to bind these zeolite samples. The procedure to make the silicabound zeolite samples was as follows. The powder of a particular zeolite sample was mixed thoroughly with a mixture of de-ionized water and Ludox HS-40 to make a homogenous mud-like viscous gel. The gel was then dried at 110 ~ for 12 hours and calcined at 550 ~ for 4 hours. Every final catalyst sample contained 28% by weight of silica binder. 2.4.

Alumina-bound Catalysts The alumina-bound zeolite samples were provided by Zeolyst Co. in extrudated form. The parent ZSM-5 samples had SiO 2/A1203 ratios of 30, 80 and 280, respectively. The parent Y zeolite samples had SiO 2/A1203 of 5, 30 and 80. The alumina binder was 20% by weight for all of the samples. Before testing, all of the samples were ground into powder and calcined at 560~ for 8 hours. 3. RESULT AND DISCUSSION

3.1 Alumina-bound zeolites The total acidities of both alumina-bound Y and ZSM-5 are increased compared with the parent zeolites. However, the total acidity for alumina-bound Y zeolite has a little bit more enhancement than alumina-bound ZSM-5 (Fig.l). For alumina-bound zeolite Y, the total acidity does not seem to change with SIO2/A1203 ratio of the parent zeolite Y, while it seems to change slightly for alumina-bound ZSM-5. Br6nsted acidity is increased only for alumina-bound Y and does not change for alumina-bound ZSM-5, as shown in Fig. 2. The reaction of A1203 from the binder with SiO2 in the zeolite at high temperature such as at calcination, which results in new Br6nsted acid sites[5,6], is responsible for the increase in Br6nsted acid density. The reason that the alumina-bound Y samples were with increased Br6nsted acid sites may be due to free SiO2 (non-frame silica) present in the zeolite or easier insertion of alumina into Y zeolite than into ZSM-5. Lewis acidity is increased for alumina-bound Y and ZSM-5 zeolites, as shown in Fig. 3.

220 From Fig.1 to Fig.3, one can attribute the total acidity increase of alumina-bound ZSM-5 mainly to the increase in Lewis acidity and that of alumina-bound Y to the increase in both Lewis and Br6nsted acidity. Figs.4 and 5 present the strong and weak acidity changes, respectively, for alumina-bound Y and ZSM-5 zeolites. Compared with the parent zeolites, strong acids are increased for both of the alumina-bound Y and ZSM-5 zeolites, while weak acids are increased for alumina-bound Y and decreased for alumina-bound ZSM-5 with lower SiOffA1203 ratio of 30, but increased for higher SIO2/A1203 ratios of 80 and 280. Ammonia temperature programmed desorption (NH3-TPD) technique has been used to demonstrate the acidity strength of solid acidic sites. In NH3-TPD profiles, the temperatures corresponding to desorption peaks present the strength of acid sites. The higher the temperature, the stronger the acidic sites. The strength of strong acids is reduced for alumina-bound ZSM-5, but keeps almost unchanged for alumina-bound Y. Alumina binder has little effect on the strength of weak acids (ref. Fig. 6). Surface area and pore volume are reduced for Y and ZSM-5 after alumina is bound with the zeolite samples. Table 1 shows the results. 3.2 Silica-bound zeolites The total acidity of silica-bound zeolite samples is reduced compared with the theirunbound counterparts, especially for Y zeolite. The total acidity of silica-bound ZSM5 zeolite samples is not reduced much. Fig. 7 shows the changes. Br/Jnsted acidity is reduced for silica-bound Y, but slightly increased for ZSM-5 with low SIO2/A1203 ratio, as shown in Fig. 8. Lewis acidity is greatly reduced both for the silica-bound Y and ZSM-5 at SIO2/A1203 ratio, as shown in Fig.9. As SIO2/A1203 ratio increases, the difference in Lewis acidities between bound zeolites and unbound zeolites is greatly reduced. At high temperatures, the reaction of silica from the binder with extra-framework alumina, which has the characteristics of Lewis acid, results in a new Br6nsted acid sites [7]. Thus the decrease in Lewis acid and the increase in Br6nsted acid would be expected to be equal in quantity and the quantity depends at least on the amount of extra-framework alumina of the zeolite. The more the extra-framework alumina, the more increase in Br6nsted acid sites and decrease in Lewis acid sites would result. By XRD diffraction, the zeolite Y sample showed more crystalline than the ZSM-5 samples. Therefore, silicabound Y samples would have less reduce in Lewis acid density and less increase in Bronsted acid density than silica-bound ZSM-5 samples. That also means the total acid density of silica-bound Y sample should have less change than silica-bound ZSM-5 samples. However, the Ludox HS-40 contained some sodium ions, which can neutralize Br6nsted acid sites[l 1l, resulting a decrease in Br6nsted acid density and total acid density. The sodium in Ludox HS-40 would reduce Br6nsted acid sites by about 0. l mmol/g-zeolite if all of the sodium could neutralize the acid sites. Therefore, the total acid density and Br6nsted acid density of zeolite Y with few extra-framework alumina are reduced. Fig.10 and Fig.11 present the strong and weak acidity changes, respectively, for silica-bound Y and ZSM-5 zeolites. Compared with the unbound zeolites, strong acid density is greatly reduced but weak acids are slightly increased for both silica-bound Y and

221 ZSM-5 zeolites. The strong acids almost disappear at high SIO2/A1203 ratio for both Y and ZSM-5 zeolites. Fig.12 shows the NH3-TPD profiles for silica-bound Y and ZSM-5. The strength of strong acids is greatly reduced for silica-bound ZSM-5 and Y, while that of weak acids keeps almost unchanged. Strong acids sites are reduced as SIO2/A1203 ratio increases, and almost vanish at high SIO2/A1203 ratio. Based on the above results, one can attribute the total acidity decrease of silicabound Y and ZSM-5 mainly to the decrease in strong acids. Surface area and pore volume are reduced for both Y and ZSM-5 after silica is bound with the zeolites. Table 1 lists the results.

Fig.2 Br6nsted acid change of zeolites bound alumina.

Fig.3 Lewis acid change of zeolites bound alumina.

222

Fig. 6. NH3-TPD profiles for alumina-bound ZSM-5(a) and Y (b). Numbers following the letter Z or Y denote SIO2/A1203 molar ratio of the zeolite

223

Fig.8 Br6nsted acid change of zeolites bound silica.

Fig.9 Lewis acid change of zeolites bound silica.

Table 1. BET surface area and pore volume of zeolites and their binder-bound counterparts. BET surface area, m2/g Pore volume, ml/g SiO2/ Y-zeolite ZSM-5 zeolite Y zeolite ZSM-5 zeolite A1203 UnA1203 SiO2 UnA1203 SiO2 UnA1203 SiO2 UnA1203 ratio bound bound bound bound bound bound bound bound bound bound bound 5 592 484 501 / / / 0.213 0.204 0.179 / / 30 817 601 617 -427 423 322 0.286 2.245 0.212 0.152 0.147 80 806 642 541 475 396 312 0.276 0.310 0.180 0.168 0.139 280 / / / 417 305 366 / / / 0.156 0.109

SiO2 bound / 0.108 0.110 0.123

224

Fig 12. NH3-TPD profiles for silica-bound ZSM-5(a) and Y(b). Numbers following the letter Z or Y denote SIO2/A1203 molar ratio of the zeolite. 4. CONCLUSION With alumina as a binder, total acid density is increased and this increase is mainly due tothe increase in Lewis acid density. With silica as a binder, both strong acid density and the strength of the strong acids are reduced. The reaction of silica from the binder with extra-framework alumina in the zeolite and the insertion of alumina from binder into the framework of the zeolite take a major role in the change of the nature of acids.

225 Pore volume and surface area of a zeolite are both decreased with silica or alumina binder. REFERENCES

1. V.R. Choudhary, P. Devadas, A.K. Kinage and M. Guisnet, Appl. Catal. A: General, 162 (1997) 223. 2. P. Devadas, A.K. Kinage and V.R. Choudhary, in "Recent Advances in Applied Aspects of Industrial Catalysis" (T.S.R. Prasada Rao and G. Murali Dhar eds), Studies in Surface Science and Catalysis, Vol. 113 (1998), 425, Elsevier Science B.V. 3. Y. Cao, L. Lu, W. Cheng and D. Yang, Acta Petrolei SINICA (Petroleum Processing Section), special issue, 111 (1997). 4. V.L. Zholobenko, L.M. Kustov, S.A. Isaev and V.B. Kazanskii, Kinet. Katal., 33(1) (1992), 242. 5. C.D. Chang, C.T-W. Chu, R.F. Bridger, J.N. Miale and R.B. Calvert, J. Am. Chem. Soc., 106 (1984),8143. 6. D. Shihabi, W.E. Garwood, P. Chu, J.N. Miale, R.M. Lago, C. T-W. Chu and C.D. Chang, J. Catal, 93 (1985), 471. 7. F.E. Sousa-Aguiar, M.B. De Almeida Bezerra and V. Murta, Lat. Am. Appl. Res., 26(2) (1996) 99. Chemical Abstracts No., 126:91740. 8. L. Forni, Catalysis Review, 8(1) (1973) 65. 9. P.A. Jacobs, Carboniogenic Activity of Zeolites, Elsevier Scientific Publishing Co., New York, 1977. 10. K.G. Proctor and D.E. Leyden, in "Chemically Modified Oxide Surfaces: Proceedings of the Chemically Modified Surfaces Symposium"(D.E. Leyden and W.T. Collins, Eds), Midland, Michigan, June 28-30, 1989, 137-149, OPA(Amsterdam) B. V. 1990. 11. S. Kotrel, M.P. Rosynek and J.H. Lunsford, J. Catal.,182 (1999) 278.

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227

Preparation of BN catalyst supports from molecular precursors. Influence of the precursor on the properties of the BN ceramic. Jos6 Antonio Perdig6n-Mel6n 1(*), Aline Auroux 1 , Jos6 Maria Guil 2 and Bernard Bonnetot 3 llnstitut de Recherches sur la Catalyse, CNRS, 2, Av A Einstein, 69626 Villeurbanne cedex, France. 2Instituto de Quimica Fisica "Rocasolano", CSIC, Serrano, 119, 28006 Madrid, Spain 3Laboratoire des Multimat4riaux et Interfaces, UMR CNRS 5615, UCB Lyon I, 69622 Villeurbanne Cedex, France; [email protected] Thin powders and foams of boron nitride have been prepared from various molecular precursors in order to be used as noble metal supports in the catalytic conversion of methane. Different kinds of precursors have been studied issued from borazines bearing different substituents. To get boron nitride presenting the required porosity and specific areas, thermal or chemical treatments have been performed to favour these properties. The best results were obtained using a molecular precursor derived from trichloroborazine (TCB) which, after reaction with ammonia at room temperature and then thermolysis up to 1800~ led to BN powders with a specific area of more than 300 mZ.g-1. Comparable results were obtained using polyborazylene thermolysed under ammonia. Surprisingly, the aminoborazine-derived precursors did not yield such high surface areas when thermolysed, but the BN texture was more expanded and looked like a foam. Attempts have been made to incorporate directly a molecular precursor of a noble metal into the BN precursor before the thermal treatment. Good results have been obtained for the incorporation of the metal up to 1100~ but the high temperature required to stabilise the BN powder has limited the choice of metal to platinum, whose presence has been characterised on the BN support. This new catalyst is currently being tested. 1. I N T R O D U C T I O N The reduction of methane emissions using its catalytic combustion has drawn great interest because methane is a well-known greenhouse gas and the most stable and abundant alkane. Most of the proposed solutions are based on the oxidation of methane using supported noble metals like palladium or platinum. The reaction involved is highly energetic and the catalyst has to bear very hard working conditions. A large part of the literature is devoted to noble

228

metals supported on alumina [1-6], other oxides like zirconia [7-9] or more generally oxide supports. All these compounds possess high insulation properties [10]. Recently, taking into account the poor properties of the oxide supports [11], research has focused on using the same active catalysts, noble metals, but with new supports such as carbides [12] or nitrides[11]. However the choice of a good support is governed by several constraints: it has to be a good thermal conductor, very inert with respect to oxidation and to the supported metal, its surface area should be as high as possible, and it must be as inexpensive as possible. Taking these constraints into account, boron nitride could be a good candidate. Boron nitride is one of the most studied non-oxide ceramics and its specific properties have focused attention to its potential applications in many industrial fields [13,14]. BN, with its hexagonal structure, looks like graphite, and its electrical properties and chemical inertia are very promising for applications involving its use under hard conditions. Usually, bulk BN is prepared by high temperature reactions of inexpensive materials [15] but the powders obtained in this way are not suitable for the preparation of BN fibres, coatings, or high surface area powders, which are the required forms for high-technology applications. High surface area BN substrates have been prepared from aerogels following a method described in the literature [16]. The surface area was shown to be very dependent on the ceramisation temperature, decreasing from more than 800 mZ.g-1 to 400 mZ.g1 when BN was stabilised up to 1600~ as the smallest pores (about 0.1 nm) were closed by annealing of the aerogel [17,18]. As a catalyst support, high surface area BN would be interesting taking into account its main qualities: the chemical inertia of BN powders allows their use under very corrosive conditions up to 800~ without morphological changes of the support, while their thermal conductivity makes possible the spreading of the high energy produced by the combustion of hydrocarbon residues promoted by noble metals. Recently, finely ground BN powder has been used as support for noble metal catalysts and has given better results than traditional metal oxide supports [19]. In the present work, porous boron nitride samples have been prepared from different molecular precursors. The nature and the pretreatment of the precursors have been investigated in order to determine accurately the optimal conditions leading to high surface area porous boron nitride materials suitable to be used as catalysts supports. The best results led to BN powders presenting surface areas of about 250 to 300 mZ.g-1. The porosity resulted from the presence of two kind of pores: mesopores down to 2 nm, and a microporosity corresponding to pores from 0.5 to 1 nm. Attempts were made to incorporate directly a noble metal precursor into the BN precursor. Up to 1100~ Pd or Pt were kept in the porous structure, but the very high temperature required to stabilise BN powders lowered drastically the metal concentration on the catalyst As a consequence, only the platinum sample could be obtained and characterized; this catalyst is currently being tested.

2. EXPERIMENTAL 2.1. General procedure and products 2.1.1. Starting materials

All experiments were performed under an atmosphere of pure argon using a standard

229

vacuum-line, Schlenk techniques and an efficient dry box with solvents purified using standard methods [20]. The starting materials for the synthesis of polyborazylene, NH4CO3 and NaBH4 (Aldrich ACS), were used as received. The trichloroborazine (TCB) was prepared from BCI3 (Alphagaz) and NH4CI (Aldrich, ACS reagent). Ammonium chloride was dried under vacuum at 100~ for 12 hours before the synthesis. Methylamine (Fluka, 97%), required for the synthesis of trimethylaminoborazine, was used as supplied without further purification. 2.1.2. Characterisation. liB, 1H and 13C NMR spectra were recorded on a Bruker DRX 300 respectively at 96.26 MHz (external reference EtzOBF3 non decoupled, with positive value down field), 300 MHz (reference TMS) and at 75 MHz (reference TMS, total decoupling). The chemical shifts were expressed in ppm. The IR spectra were recorded on a FTIR Nicolet Magna 550 spectrophotometer in KBr pellets. The ceramic yield was determined using a TGA B70 apparatus. DCS analyses were run on a TA 8000 Mettler-Toledo under argon atmosphere. SEM images were obtained from a Jeol 55 CF (CMEABG Lyon). X-ray powder diffraction (XRD) spectra were obtained with Cu Ka radiation using a Philips PW 3710/3020 diffractometer equipped with a monochromator. The BET surface areas were measured using nitrogen at 77 K. All the samples were outgassed at 673 K for 4 hours prior to the absorption measurements.

2.2. Elaboration of high surface area boron nitride.

I

H

H

PI P II P III Fig. 1. Starting molecules for the synthesis of the precursors, respectively P I: trichloroborazine, P II: borazine and P III: trimethylaminoborazine. Boron nitride preparations start usually from borazinic (BN)3 precursors [13,14]. Among these compounds, three types of precursors can be studied according to the starting borazine group: the most simple, trichloroborazine (TCB), containing an amount of TCB polymers (precursor P I); a polyborazylenic polymer obtained from the thermolysis of borazine [HBNH]3 (precursor P II); and polytrimethylaminoborazines (- (NMe) 3- [BNH] 3) obtained through the polymerisation of amino borazines (MeNH[BNH])3 (precursor P I I I ) . Boron nitride can be prepared from all these precursors using appropriate thermolysis conditions. The results are different taking in account the nature of the precursor and the physical and chemical treatment applied during the pyrolysis. The precursor I was known to yield BN through high temperature reactions [21]. Its conversion into high surface area BN has been

230

studied and the optimal ceramisation conditions determined. Trimethylaminoborazine or MAB is a well-known compound which has been widely studied because of its ability to lead to thin and long BN fibres [13,14]. In a preliminary work, a study of the influence of the bulky amine part fitted to the BN framework has led to the conclusion that, unlike the case of zeolites, no residual porosity was induced by the large amine during the pyrolysis [22]. The starting molecules used for the synthesis of the precursors are given in Fig. 1. 2.2.1. Synthesis and characterisation of the trihaloborazine precursor (P I). Trichloroborazine (TCB) was prepared using the standard method by reaction of BC13 with NH4CI in toluene [23]. TCB was recovered with a 80% yield; however, polymers of TCB were obtained during the synthesis, as confirmed by the elemental analysis, but no further purification was performed because TCB polymers can also be transformed into BN. Elemental analysis : Theoretical for C13B3N3H3 (Mw 183.83) : B, 17.63; H, 1.63; N, 22.83; C1, 57.91. Experimental: B, 19.6; H, 1.6; N, 27.4; C1, 51.2 2.2.2. Synthesis and characterisation of polyborazylene (P II) Borazine was prepared using a two step classical method from ammonia borane, NH3BH3, which was pyrolysed in glyme as described in literature [24]. However, the borazine was not distilled but rather polymerised in situ, yielding directly a polymer. The solvent was eliminated by filtration under an argon atmosphere, and the remaining borazine and the volatile species were removed as recommended in the preparation of poly(aminoborane) [25,26]. A white waxy polymer was isolated, consisting of a mixture of polyborazylene and polyaminoborane. The presence of organic moieties in the polymer was confirmed by the elemental analysis (in weight%) : B = 23.0 ; N = 45.8 ; H = 7.1 ; C = 18.0 ; the oxygen content could not measured in presence of boron and was calculated to be about 6 wt%. 2.2.3. Synthesis and characterisation of the triaminoborazine precursor (PIII). Trimethylaminoborazine (MAB) was prepared by reaction of trichloroborazine with methylamine at low temperature. The trimethylaminoborazine was recovered after evaporation of the solvent under low pressure at room temperature [27]. The MAB was characterised in situ. To prepare the precursor (P III), MAB underwent a thermal polymerisation process conducted under an argon stream. The polymerisation of MAB evolved methylamine; each methylamine molecule evolved corresponds to an intercyclic bonding, making it possible to monitor the reaction by determination of the quantity of amine evolved. The polymerisation was carried out following the standard process described in ref. [28]. For the precursor (P III), the ratio of the quantity of amine evolved versus the amount of monomer was 0.5, which led to a polymer with glass transition Tg = 85.5 ~ and a ceramic yield of 55 % at 1000~ The elemental analysis was (in weight %) : B = 23.5; N = 47.8; H = 7.2; C = 18.8; oxygen could not measured in presence of boron but amounted to less than 3%.

231

2.2. 4. Conversion of the precursors into boron nitride. The conversion of the molecular precursors into boron nitride requires a ceramisation under an ammonia flow. Ammonia is used as a reductive atmosphere, and its ability to replace halo or amino groups on the borazine framework is well known [12,13]. Ammonia is also a curing reagent in the borazine polymerisation [25]. So, for the preparation of BN from molecular precursors, one usually needs to perform a low temperature ceramisation, up to 600~ at least, under an ammonia flow. The conversion of each precursor into BN has been studied using TGA up to 1000~ in order to optimise the ceramisation conditions and the properties of the obtained ceramic. The measurements were realised in a pure ammonia flow up to 650~ and then under nitrogen. The three tested compounds presented original behaviours related to their formulation and their reactivity towards ammonia. 22. 4.1. Conversion of precursor P I into BN. The mixture of TCB and its oligomers, P I, was converted into BN by reaction with ammonia. This reaction is well-known, but the best way to get high surface area BN remained to be investigated. When TCB was heated under an ammonia flow, the melting of this compound at 88~ lowered the reaction of the gas phase. The conversion of solid P I into boron nitride under an ammonia flow is given on Fig. 2. To avoid the melting of TCB, the reaction with ammonia was performed at room temperature on finely ground precursor. As shown on Fig. 2, the first step of the reaction was the addition of ammonia to the precursor. This reaction was rapid at its beginning, but like all gas / solid reactions its was slowed down by kinetic considerations. The reaction was run for more than 15 hours in order to reach the limit of the addition, which occurred when 6 molecules of ammonia were added to one of TCB. This corresponds to 2 NH3 for each BC1 group of the precursor. AM/Mo

Temperature

60 50 40 30 20 lO

.

0

"'

(~ 1200 1000

-10 -20 -30 -40 -50 -60

800

................ .: ............. . a . . M / M . . o . . %

-

.......................

600

Temperature ~

i

400

.~"

-70

200

~'

-8o

0

:"

5

10

Time (Hours) 1

5

20

25

0

Fig. 2. TGA evolution during the reaction of P I with ammonia. When the addition step was completed, the temperature was raised. Very little of the fixed

232

ammonia survived heating to 350~ The reaction of ammonia with TCB led to a very stable complex. At 400~ a white compound, which was characterised as NH4CI, was deposited on the silica tube of the TGA apparatus. Then, the weight loss slowed down to reach a value very close to the theoretical ceramic yield of TCB of 40 %. This reaction can be written as CI3B3N3H3 solid q- 6 NH3 gas'-) [(NH3)zC1BNH]3 solid

"-) 6 NH4CI gas -1- B N solid

The fixation of ammonia on TCB was accompanied by a severe dilatation of the compound, and very often the silica crucible was broken by this change in the solid volume. 22. 4.2. Conversion o f precursors P II and P III into BN. The behaviour of the precursors P II and P I I I was tested under ammonia under the same experimental conditions used for precursor P I. No significant reaction took place at room temperature, and Fig. 3 shows the results of a ceramisation. For both precursors, the reaction was very slow up to 100~ For the precursor P II, the reaction with ammonia became very important in the range of temperature from 200 to 300~ The total weight loss for precursor P II was more important than expected (the boron content of the polymer was consistent with a ceramic yield of 52 %, while the weight loss reached 55 %). This could be explained by a stripping off of volatile borazine from the precursor [25]. AM/Mo % 0

.-,:.c -

-10 -15

"

zA M / M o %MAB

-20

~z

-25 -30 -35 -40 -45 -50 -55 "60

I

0

I

I

|

100

I

I

"

I

200

"

"

"

I

300

'

,

9 I

400

.

.

9 I

500

9

, .

I

600

9

,

I

700

9

9

9 I

800

.

.

.

I .

900

.

.

1000

Temperature ~

Fig. 3. Ceramisation of the precursors P II (curve with circles) and P III. 2.2.4.3. Preparation of mix precursors containing a BN precursor and a noble metal precursor. One of the goals of this work was to prepare directly noble metals supported on BN powder in a single operation. The molecular precursor of the noble metal must react with the support

233

precursor in order to obtain a highly dispersed metal on the support after ceramisation. The study of the BN powder synthesis showed that precursor P I seemed to be the most promising, as reported in the results section. Consequently, a synthesis has been performed by reaction between P I and an organic platinum precursor in an appropriate solvent. The required quantity of platinum acetylacetonate was dissolved in ethylacetate and the required quantity of P I was added. The solution was refluxed for 12 hours and the solvent removed. Acetylacetone was displaced by the precursor, and a solid precursor containing platinum was recovered. This mixture was treated following the ceramisation conditions. 2.2.5. Ceramisation and stabilisation of the boron nitride powders. Taking into account the TGA analysis, a standard procedure was established to treat the precursors. The precursor P I reacted for 48 hours at room temperature with ammonia in a graphite crucible. Then the temperature was raised up to l l00~ with a heating rate of 100~ -1. The ceramisation was performed up to 650~ under pure ammonia and then under nitrogen. For the precursors P II and PIII, the room temperature treatment was omitted. High surface BN powders are known to be very reactive, like thin films [29], so a high templerature stabilisation of the powders was carried out up to 1800~ (heating rate 600~ - under nitrogen atmosphere). This treatment decreases the porosity [17], but the utilisation of BN under the experimental conditions for catalytic applications requires a very stable support. 3. RESULTS

3.1. BN powders. The three precursors all led to the formation of porous powders or foams. The results are given in Table 1. The surface areas of the samples obtained from P III were always smaller than for the samples obtained from P I or P II. The crystaUisation of the BN was very low although a very high temperature had been used. This was related to the important specific area which hindered the growth of the crystallites. The crystallinity of the samples was characterised by the determination of Lc, length along the c axis, and La, length along the a axis, of the graphite-like BN crystallites [30]. Table 1 Characteristics of the obtained powders. Crystallite size Precursor Lc / n m La/nm PI 1.6 5.4 P II 1.8 7.2 P III 1.6 4.5

Specific area / mZ.g-1. 310.2 + 7.3 482.7 + 16.9 58.7 + 2.8

Several experiments have been performed using each precursor, and the results given here are the most representative. The error on the surface area is given by the measurement apparatus. The specific areas obtained are very different depending on the precursor

234

formulation. It turned out to be impossible to obtain surface areas of more than 100 m z.g-a using precursor P III. This has been explained using SEM images, which are given in Figs. 4 and 5, respectively, for powders obtained from precursors P I and P III

Fig.6. Pore repartition in the samples

Fig. 7 . Surface area of the samples as a function of pore diameter.

The samples prepared from precursor P III always present the same microstructure, with a smooth external skin covering a more porous structure (Fig. 5). This porous structure is not open and the measured surface area is lower.The number of pores as a function of their size and the total surface area of the samples as a function of the diameter of the pores are reported respectively in Figs. 6 and 7. Although the surfaces were very different, the pore repartitions seemed to be homogeneous. Two types of pores are present: large mesopores from 4 to 20 nm and a small microporosity with pores smaller than 2 nm enhancing the surface area. The

235 microporosity of the sample obtained from precursor P II is more important than for the other samples, and the pores seemed to be a little smaller down to 1 nm in a large number. In fact the curves obtained from these three precursors show similar behaviours. The main difference lies in the global surface area, but the pore diameter distributions are not very different.

3.2. Characterisation of a Pt BN supported catalyst. As reported in the experimental part, a direct synthesis of platinum supported on BN powder has been carried out.

t~00

eoo I

i Reductsampie ;

1 I I t

i ~

Fig. 8. XRD spectra evolution during the synthesis of the BN-supported platinum sample. The XRD spectra recorded after each step of the synthesis are given in Fig. 8. The three steps correspond to the ceramisation up to 1100~ (bottom of the figure), then after annealing at 1800~ (middle), and finally after a thermal treatment of the sample under a reductive atmosphere up to 600~ (upper spectrum). The diffraction rays of platinum remain present after the annealing. That was not the case for palladium which was volatilised. This sample is currently the focus of further investigation to characterise the surface and the metal repartition on the BN support. 4. C O N C L U S I O N The preparation of high surface area boron nitride powders and foams from various molecular precursors has been studied. The highest specific areas were obtained using the poly-borazylenic precursor. However the difference with the measured surface area of the samples prepared from the polyhaloborazinic precursor was specially related to an enhanced micro-porosity of the samples prepared from the borazinic precursor. Curiously, the samples obtained from aminoborazinic precursors always exhibited a smaller specific surface area. This has been related to the ability of this precursor to melt and stay under a waxy form at

236

high temperature. The chemical treatment performed led to a non porous skin of the foam embedding a porous structure. This porous structure was shown to be captive and non efficient. Using polyhaloborazinic precursors, specific surface areas of more than 250 mZ.g-1 are commonly obtained, and the preparation of samples of 10 grams of BN powder is classical. Testing of these powders for impregnation by noble metal precursors is currently underway in order to evaluate the abilities of the BN support in comparison with the classical oxide supports. Following the initial idea directing this work, a platinum catalyst supported on a BN powder has been prepared directly from the mixture of the two precursors. The first analyses showed clearly that platinum was present, but further investigation is necessary in order to characterise the dispersion of platinum on the surface of the nitride and the efficiency of this new catalyst. REFERENCES

1. R. Burch, P.K. Loader and F.J. Urbano, Catal. Today 27 (1996) 243.. 2. P. Briot and M. Primet, Appl. Catal. 68 (1991) 301. 3. E. Garbowski, C. Feumi-Jantou, N. Mouaddib and M. Primet, Appl. Catal. A Gen. 109 (1994) 277. 4. R.J. Ferrauto, J.K. Lampert, M.C. Hobson and E.M. Waterman, Appl. Catal. B : Environ. 6 (1995) 263. 5. M. Lyubovsky, L. Pfefferle, Catal. Today, 47 (1999) 29. 6. S. Yang, A. Maroto-Valiente, M. Benito-Gonzalez, I. Rodriguez-Ramos and A. GuerreroRuiz, Appl. Catal. 28 (2000) 223. 7. F.H. Ribeiro, M. Chow and R.A. Dalla Betta, J. Catal. 146 (1994) 537. 8. K. Fujimoto, F.H. Ribeiro, M. Avalos-Borja and E. Iglesia, J. Calal. 179 (1998) 431. 9. W.S. Epling and G.B. Hoflund, J. Catal. 182 (1999) 5. 10. A.B. Stiles, Calalyst Supports and Supported Catalysts. Theoretical and Applied Concepts. Butterworths Publishers, Stoneham, MA, USA, p57-85. 11. C. M6thivier, J. Massardier and J.C. Bertholini, Appl. Catal. A: Gen. 182 (1999) 337. 12. C. M6thivier, B. B~guin, M. Brun, J. Massardier and J.C. Bertholini, J. Catal. 173 (1998) 374. 13. R.T Paine and C.K. Narula, Chem. Rev., 90 (1990) 73. 14. R.T Paine and L.G. Sneddon, Chemtech, 7 (1994) 29. 15. J. Economy and R. Anderson, Inorg. Cherrt 5,6 (1966) 968. 16. C.K. Narula, R. Schaeffer and R.T. Paine, J. Am. Chem. Soc. 109 (1987) 5556. 17. D.A. Lindquist, T.T. Borek, S.J. Kramer, C.K. Narula, G.Johnston, R. Schaeffer, D.M. Smith and R.T. Paine, J. Am; Ceram. Soc. 73 (1990) 757. 18. D.A. Lindquist, D.M. Smith, A.K. Datye, G.P. Johnston, T.T. Borek, R. Schaeffert and R.T Paine, Mat. Res. Soc. Symp. Proc. 180 (1990) 73. 19. J.C. Wu, Z. Lin, J.W. Pan and M.H. Rei, Proceedings of Am. Catal. Soc., 2001, 294. 20. D.D. Perrin, W.L. Armarengo and D.R. Perrin, Purification of Laboratory Chemicals, Pergamon Press, London 1996. 21. M. Houdayer, J. Spitz and D. Tran Van, French Pat. N~ 22163, 1981; U.S. Pat. N~ 472

237 454, 1984. 22. J.A. Perdigon-Melon, A. Auroux, D. Cornu, P. Miele, B. Toury and B. Bonnetot, Proceedings of Euroboron 2, Dinard 2-6/09/2001 J. Organometallic Chem. In press. 23. K. Niedenzu and J.W. Dawson, J. Am. Chem. Soc., 81 (1959) 3561. 24. P.J. Fazen, J.S. Beck, A.T. Lynch, E.E. Remsey, and L.G. Sneddon, Chem. Mater. 2 (1990) 1942. 25. E. Framery, Thesis N ~ 1616, University of Rennes (1996) 93-99. 26. D.P. Kim, K.T. Moon, J.G. Kho, J. Economy, C. Gervais and F. Babonneau, Polym. Adv. Technol., 10 (1999) 702 27. R.H. Toeniskoetter and F.R. Hall, Inorg. Chem., 2 (1963) 29. 28. K.J.L. Paciorek, W. Krone-Schmidt, D.H. Harris, R.H. Kratzer and K.J. Wynne, ACS Symposium Series 360, Chap 32, M. Zeldin, K.J. Wynne and H.R. Allcock Eds.(1988) 392. 29. T. Matsuda, J. Mater. Sci., 24 (1989) 2353. 30. J. Thomas, N.E. Weston and T.E. O'Connor, J. Am. Chem. Soc., 84 (1962) 4619.

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Studies in SurfaceScienceand Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

M o n i t o r i n g of the p a r t i c l e size of MoSx n a n o p a r t i c l e s microemulsion-based synthesis

239

by a n e w

Karin Marchand a*, Maud Tarret a, Laurent Normand b, Slavik Kasztelan a, Tivadar Cseri a

a Kinetics & Catalysis Division, IFP 1-4, avenue de Bois-Pr6au, F-92852 Rueil-Malmaison, France b Physics & Analysis Division, IFP 1-4, avenue de Bois-Pr~au, F-92852 Rueil-Malmaison, France

Molybdenum sulfide nanoparticles in the size-range 3-10 nm have been synthesized in mild conditions using a microemulsion-based route. The reverse microemulsion phase, AOT/ n-heptane/ water, was first characterized by Transmission Electron Microscopy (TEM) of Freeze Fractures (FF) obtained via High Pressure Freezing (HPF) as well as Dynamic Light Scattering (DLS). The impacts of various parameters such as water-tosurfactant molar ratio w and the addition of a nonionic cosurfactant were then studied. The reverse microemulsion phase was further used to tailor the size of MoSx nanoparticles. The mean particle size obtained by this method makes those particles particularly interesting for further catalytic applications. 1.

INTRODUCTION

Molybdenum sulfide-based catalysts are widely used in heterogeneous catalysis, especially in hydrotreating of petroleum cuts. The control of the morphology and size of MoSx nanoparticles is thus of great interest to obtain highly active and selective catalysts [1]. Various methods have been used to synthesize nanoparticles with controlled morphology and structure, and the use of microemulsions seems especially suited for tailoring particle size at the nanolevel [2]. Reverse microemulsions are thermodynamically stable transparent isotropic media with a continuous oil phase and discrete water droplets. Those are compartmentalized by a surfactant monolayer into nanometer-sized liquid entities. The advantage of the microemulsion-based route is that it is a soft technique, i.e. it does not require extreme conditions of pressure and temperature. But it is the dynamic of micellar dispersions that makes themso relevant for this kind of purpose : the droplets are indeed subject to Brownian motion and collide continuously, leading to the formation of short-lived dimers and to the exchange of the aqueous contents of the micelles. This dynamic process ensures a homogeneous repartition of the reactants among the aqueous droplets or 'water pools' and thus the formation of very monodispersed particles [3].

240

Schematically, water pools act as microreactors monitoring the nucleation and growth of nanoparticles and thus theirparticle size. The microemulsion-based route has already provided encouraging results, for instance in the synthesis of ZnS or CdS nanoparticles [4]. Their success is due to the easiness of finding simple cationic forms for Zn and Cd, which is of course much more difficult in the case of molybdenum. The main idea developed by Pileni and coworkers [3], which consists in forcing the supersaturation of the aqueous medium by using functionalized surfactants like (AOT)2Zn instead of (AOT)Na, cannot be applied to molybdenum either. An attempt to apply the microemulsion-based route to molybdenum sulfide nanoparticles has been carried out by Boakye et al. [2], but sizes obtained were in the size range 10-30 nm, which is still too large for catalytic applications to hydrotreating reactions. The objective of this study is thus to prepare MoSx nanoparticles by a microemulsion-based route in the size range 3-10 nm for catalytic applications, the particle size being directly monitored by the reverse microemulsion phase.

2.

EXPERIMENTAL

2.1. Materials The surfactants : Bis(2-ethylhexyl)sulfosuccinate sodium salt [dioctylsulfosuccinate sodium salt or AOT] (99 % purity), polyoxyethylene(5)nonylphenylether [Igepal CO 520 or NP-5] (99% purity), as well as ammonium tetrathiomolybdate [ATTM] (99.97 % purity) were purchased from Aldrich. The other chemicals used were obtained as follows: nheptane (purity > 99 %) was from Prolabo and sulfuricaAcid (purity > 97 %) from Merck. All were used as received.

2.2.Preparation of the reverse microemulsion phase Reverse microemulsions were prepared by dissolving 8.9 g of AOT in n-heptane (200 mL) and adding precisely defined volumes of water or of a 0.5 M sulfuric acid aqueous solution to this mixture in order to have the desired water-to-surfactant molar ratio. AOT concentration was kept constant and equal to 0.1 M with respect to the total microemulsion volume. When required, NP-5 was used as a cosurfactant at a concentration of 0.002 M with respect to the total volume. Samples were sealed and vigorously shaken to reach the thermodynamic equilibrium faster. Microemulsions thus obtained were stable over weeks when stored at room temperature. 2.1

Preparation of the MoSx nanoparticles Molybdenum sulfide nanoparticles were prepared by adding via a syringe 6.9 mL of a 0.005 M ammonium tetrathiomolybdate (ATTM) solution to a 0.1 M AOT/n-heptane/ 0.5 M sulfuric acid microemulsion system with a water-to-surfactant molar ratio w of 20. It should be noted that the water-to-surfactant molar ratio accounts for both ATTM and sulfuric acid solution. The reaction can be schematically represented as follows :

241

MoSaZ + (2-x) H + --o MoSx + (4-x) H2S

(1)

Stirring gives a brownish-yellow optically homogeneous solution. This color disappears progressively as the molybdenum sulfide nanoparticles form. 2.2

Characterization

Microemulsions were characterized by Dynamic Light Scattering (DLS) to determine the size distribution of the water pools. Measurements were performed on a Malvern Zetasizer. Interactions existing between aqueous droplets were neglected which is a prerequisite to use the hard sphere model. The linear mode of the apparatus was chosen ; this corresponds to a cumulative analysis. Morphology was observed by Transmission Electron Microscopy (TEM) of Freeze Fractures (FF) obtained through High Pressure Freezing (HPF). HPF has the advantage of slowing down - if not stopping - all dynamic processes in microemulsions due to the increase of viscosity linked with high pressure. A more in depth description of FF and HPF methods is given by Moor [6]. Nanoparticles were observed by TEM and analyzed by EDX. TEM measurements were performed on a Jeol 2010 High Resolution microscope working at 200 kV. In order to characterize the population of droplets, their diameters were measured with the aid of a SAISAM image analyzer system. Statistics were based on at least 300 water pools. Samples for electron microscopy containing MoSx nanoparticles were prepared by direct dropping of a very small amount of MoS• dispersion on carbon-coated copper grids and drying at room temperature. 3. 3.1

RESULTS AND DISCUSSION Control of the reverse m i c r o e m u l s i o n phase

The reverse microemulsion phase chosen was a well documented one : it included AOT as the main surfactant. AOT or Bis(2-ethylhexyl)sulfosuccinate is an edge-shape ion, as represented in Fig. 1. It favors the formation of spherical water-in-oil droplets at low water content but, on increasing the water-to-surfactant molar ratio w, interconnected cylinders have been reported [7, 3]. The choice of AOT was made in order to concentrate the reaction medium, i.e. increase the volume of dispersed phase and so the yield of the formation of MoS,, with respect to the total Fig. 1. Surfactant AOT microemulsion volume. Replicas of the reverse microemulsion phase obtained after HPF and observed by TEM were of excellent quality and presented the same homogeneous features as can be seen in Fig. 2.. The arrangement of droplets appears to be random with maybe a slight tendency to agglomeration, which might be a consequence of HPF. Nevertheless, for w < 20, features all appeared homogeneous and in every way identical to the one presented in Fig. 2..

242

Fig. 3. TEM image of a Freeze Fracture replica of the system 0.1M AOT/n-heptane / water (w=30) The water-to-surfactant molar ratio w was varied from 10 to 30 while the surfactant concentration was kept constant and equal to 0.1 .M. An increase in w led to cylindrical structures (as can be seen in Fig. 3.) which is in good agreement with Petit et al. [7]. These structures are also consistent with less good fits reported by Fletcher et al. [8], which they think might be consistent with non-spherical structures. Because of the too slow cooling rate of the High Pressure Freezing technique (10,000 K/s), water pools mean sizes were overestimated by TEM. Indeed, HPF essentially guarantees to produce a "snapshot" of the reverse microemulsion phase though this

243

representation may apply to temperatures inferior to the ambient temperature. This is why DLS measurements were carded out on the same samples. DLS results have been reported in Fig. 4; they are in good agreement with DLS data already published as well SANS measurements by Fletcher et al. [8].

Fig. 4. DLS Hydrodynamic diameter of the water pools of the system 0.1 M AOT/ water/ n-heptane Vs the water-to-surfactant molar ratio w

3.2

Monitoring of the particle size through the reverse microemulsion phase MoSx nanoparticles were synthesized as described above with a water-to-surfactant molar ratio w=20, which corresponds to water pools of a diameter of about 8 nm according to the relation found between the hydrodynamic diameter D and w: D (nm) = 0.3 w +1.6 (2) The MoSx nanoparticles synthesized were amorphous and the EDX analysis of portions of TEM grid containing the particles showed that the high contrasted particles contained molybdenum and sulfur. They were of a size of about 8 nm which matches exactly the size of the water pools determined previously.

HO~ 0

~0

~0

~0

~0

Fig. 5. NP-5 cosurfactant

In order to obtain a smaller particle size with no change in the amount of aqueous phase dispersed, NP-5 (see Fig. 5.) was used as a cosurfactant at a concentration well below that of AOT (0.002 M versus 0.1 M).

244 As a result, the interfacial tension between the aqueous and the organic phase was modified (as is represented in Fig. 6), which led to a new thermodynamic equilibrium of the microemulsion phase. The system 0.002 M NP-5/0.1 M AOT/water / n-heptane thus contains smaller but also more numerous water pools according to sizes estimated by TEM observation of FF obtained through HPF.

N/N/N/

Fig. 6. Schematic representation of an interface with a surfactant (AOT), a cosurfactant (NP-5) and organic molecules

Fig. 7.

MoSx mean Particle size distribution on using a cosurfactant

On using NP-5 (poloxyethylene(5)nonylphenylether as represented in Fig. 5) as a cosurfactant at a concentration of 0.002 M the nanoparticles of MoSx had an average size of 4 nm. Fig. 7 shows the evolution of the distribution of particle sizes. The population obtained is rather narrow compared to what has been previously obtained. This can be explained by a higher fluidity of the modified interface. So, exchanges of the aqueous

245 contents are more frequent, which means that more ATTM are involved in the nucleation stage and fewer left for the growth stage of the MoSx nanoparticles. This is in line with results obtained by FF and TEM pointing out a relative decrease of the microemulsion droplet mean size as well as results already published in the literature [9]. 4.

CONCLUSION

In this study we showed that the control of the nucleation-growth of MoS• nanoparticles could be directly monitored via the microemulsion phase. Furthermore, the mean particle size obtained by the reverse microemulsion-based route is in the range 4-8 nm, which makes those particles particularly interesting for further catalytic applications. ACKNOWLEDGEMENTS We thank F. Gaill for the use of the facilities of her laboratory. We are indebted to J.P. Lechaire and G. Frebourg for their excellent work with the preparation of the replicas. We are also grateful for the provision of the High Pressure Equipment by the R6gion Ilede-France. We finally wish to give special thanks to our colleagues : R. Revel, J. Rousseau, and D. Frot for their contribution to this work. REFERENCES

1. P. Da Silva, N. Marchal, V. Harl6, T. Cseri and S. Kasztelan, Symposium on Recent Advances in Heteroatom Removal Presented Before the Division of Petroleum Chemistry, Inc. 215 th National Meeting, American Chemical Society, Dallas (TX), March 29-April 3to, 1998. 2. E. Boakye, L.R. Radovic and K. Osseo-Asare, J. Coll. Inter. Sci. 163 (1994) 120. 3. M.P. Pileni (ed.), Reactivity in Reverse Micelles, Elsevier, Amsterdam, New York, Oxford, Shannon, Tokyo, 1989. 4. J. Eastoe and B. Warne, Curr. Opin. Colloid Interface. Sci., 1 (1986) 800. 5. J. Cizeron, M.P. Pileni, J. Phys. Chem., 99 (1995) 17410. 6. H. Moor, Cryotechniques in Biological Electron Microscopy, R.A. Steinbrecht and K. Zierhold (eds.), Springer Verlag, Berlin, 1987. 7. C. Petit, P. Lixon and M.P. Pileni, Langmuir, 7 (1991) 2620. 8. P.D.I. Fletcher, B.H. Robinson, F. Bermejo-Barrera, D.G. Oakenfull, J.C. Dore and D.C. Steytler, Microemulsions, I.D. Robb (ed.), Plenum Press, New York and London, 1982. 9. R.P. Bagwe and K.C. Khilar, Langmuir, 16 (2000) 905.

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247

Transition metal phosphides: novel hydrodenitrogenation catalysts V. Zuzaniuk, C. Stinner, R. Prins and Th. Weber Laboratory for Technical Chemistry, Swiss Federal Institute of Technology (ETH), ETH H6nggerberg, 8093 Z/irich, Switzerland

Bulk transition-metal phosphides (Co2P, NizP, MoP, WP, CoMoP, NiMoP) were prepared by reduction of a metal oxide/phosphate mixture in a flow of Ha and characterized by powder X-ray diffractometry and 3ap MAS NMR spectroscopy. The XRD powder patterns and the NMR shifts of all phosphides were in accordance with literature data. SiO2supported Ni2P, NilzP5 and Ni3P phosphides were also prepared by pore volume impregnation of nickel nitrate and di-ammonium hydrogen phosphate. The dried and calcined samples were reduced at elevated temperatures in a mixture of 5% H2 and N2. 1. INTRODUCTION Transition-metal sulfides, typically Co- or Ni-promoted MoSz-type phases supported on 7A1203, are widely used catalysts for the removal of sulfur and nitrogen from oil fractions [1-3]. However, future more stringent environmental legislation requires significant improvements in the activity of such catalysts. The presence of phosphate in sulfided (Ni-)Mo/7-Al203 catalysts enhances the hydrodenitrogenation (HDN) activity [4,5]. However, it is difficult to understand if phosphate is directly involved in the reactions or if it acts as a primary promoter. One explanation could be that part of the phosphate is reduced by H2 and reacts with metal centers to form metal phosphides. Besides the sulfides, transition metal nitrides, carbides and phosphides have been proved to be active catalysts in hydrotreating reactions. However, in contrast to the extent of research spent on carbides and nitrides only few examples of the use of phosphides in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysis have been reported in the literature [6,7]. We prepared several unsupported phosphides by reduction of metal oxide/phosphate precursors with H2 [8,9]. In order to obtain a high dispersion, industrial catalysts are usually dispersed on a support such as ]t-Al203 or SiO2. This results in a larger active surface area and a higher thermal stability. Thus, we adapted the method of preparing bulk metal phosphides to the preparation of SiOa-supported phosphides. The synthesis of supported transition-metal phosphides is, however, more complicated than that of unsupported samples due to the possible interaction of either the metal or the phosphorus with the support. This could lead to segregation of the metal and phosphorus. The choice of SiOa rather than ~,-A1203 as a support derives from the fact that phosphate interacts strongly with ~,-AlaO3 resulting in the formation of surface AIPO4. The reaction conditions

248 for the reduction of the calcined precursors were changed and the resulting phosphide phases were identified by XRD and 31p MAS NMR.

2. EXPERIMENTAL 2.1. Preparation of the catalysts For the preparation of the binary phosphides (Co2P, Ni2P, MoP, WP), the following procedure was applied: di-ammonium hydrogen phosphate (NH4)2HPO4 was used as phosphorus source and was dissolved in deionized water. Subsequently a solution of the metal salt (listed in Table 1) was added to the phosphate solution. In the case of the two ternary phosphides (CoMoP, NiMoP), a solution of ammonium heptamolybdate (NH4)6Mo7024" 4H20 followed by a solution of cobalt nitrate Co(NO3)2" 6H20 or nickel nitrate Ni(NO3)2" 6H20 was added to the phosphate solution. For all the compounds, the amounts of metal salt(s) and phosphate were chosen according to the stoichiometry of the corresponding phosphide. After evaporation of the water, the obtained solid was calcined in air at 773 K for 5 h and then reduced under H2 (99.999%, 300 ml.min -1) at 823 K (Co2P, Ni2P), 923 K (MOP, WP, NiMoP) and 1023 K (CoMoP) at a heating rate of I K-rain -1. The reduction conditions are summarized in Table 1. Finally, the samples were passivated in a flow of 0.5% O2/He (30 ml.min -1) for 2 h at room temperature. The passivated phosphides can be handled in air. Table 1 Synthesis conditions for unsupp0r[ed and supported metal phosph!des Catalyst Metal Salt Molar Gas Flow / Reduction Precursor ratio ml.min -1 Temp. / K MI:Mo:P a CozP Co(NO3)2" 6H20 2 : 0:1 H2 300 823 NizP Ni(NO3)2" 6H20 2:0:1 H2 300 823 MoP (NH4)6Mo7024 " 4H20 0:1:1 H2 300 923 WP (NHn)6H2W12040" 18H20 1:0:1 H2 300 923 NiMoP (NH4)6Mo7024" 4H20 1:1:1 H2 300 923 Ni(NO3)2" 6H20 CoMoP (NH4)6Mo7024 94H20 1:1:1 H2 300 1023 Co(NO3)2" 6H20 Ni2P/SiO2 Ni(NO3)2" 6H20 2" 0" 5%H2/N2 200 1023 b 1.3 Ni12Ps/SiO2 Ni(NO3)2" 6H20 2" 0" 1 5%H2/N2 50 1023 b Ni3P/SiO2 Ni(NO3)2 " 6H20 2 "0 " 1 5%H2/N2 .... 10 1023b ~ a M1 stands for the metal used in the corresponding phosphides, M1 - Co, Ni or W b The temperature was first increased to 523 K at a heating rate of 2 K'min -1 and then to the final temperature at 1 K'min -1 SiO2-supported phosphides (Ni2P/SiO2, Ni12Ps/SiO2, Ni3P/SiO2) were prepared by pore volume impregnation of a silica support. The silica support (chromatography gel, C-560, CU Chemie Uetikon, surface area 500 m2"g-1, pore volume 1 ml-g 1) was first ground,

249 sieved (63-90 ~tm) and then dried for 12 h at 393 K. A similar procedure to that applied to the synthesis of bulk phosphides was used for the supported samples. However, in order to avoid the formation of an insoluble precipitate of nickel phosphate upon mixing aqueous solutions of Ni(NO3)2 96H20 and (NH4)2HPO4, a two-step impregnation procedure had to be followed for the preparation of supported nickel phosphides. The metal solution was first introduced onto the SiO2 surface, followed by a drying step at 393 K. The (NH4)2HPO4 solution was then added to the support, which was subsequently dried at 393 K and calcined at 623 K. The oxidic precursor obtained was then reduced in a flow of 5% HflN2 at a flow rate ranging from 10 to 200 ml.min-1. Reduction conditions are summarized in Table 1.

2.2. Characterization of the catalysts XRD measurements were carried out using a Siemens D-5000 powder X-ray diffractometer (Cu I ~ radiation) with Bragg-Brentano geometry. The sample was pressed into a flat sample holder, which was rotated during the measurement. All the patterns were compared with calculated patterns (obtained from the Inorganic Crystal Structure Database) using the software PowderCell 2.3 [10]. Temperature-programmed reduction experiments were obtained using a Micromeritics AutoChem 2910 apparatus. The sample was placed in a quartz U-tube and reduced in a flow of a 4.8% Hz/Ar mixture. 31p NMR spectra were measured with an Advance 400 WB Bruker spectrometer equipped with a magic-angle-spinning probe. The probe was tuned to 162 MHz, and an external sample of 85% phosphoric acid was used as the reference standard. Samples were ground; in the case of the unsupported samples they were mixed with 50 wt% of pure silica powder to provide electrical insulation between the metallic particles and, thus, to avoid the appearance of eddy currents within the sample. Samples were packed in a 4 mm diameter rotor and spun at 10 to 15 kHz. The spectra were obtained by Fourier transforming the free induction decay signals. Spectra were acquired using a single 1.8 ~ts pulse causing a flip angle of Jr/4 and a recycle time of 1 s. The measurements were performed at room temperature. 1800 scans were recorded. The isotropic shifts of the signals were obtained by comparing spectra measured at different spinning rates. Using a modified version of the Bruker Winfit software, we fitted the centerband and the sideband pattern [11]. 3. RESULTS AND DISCUSSION

3.1. Bulk transition-metal phosphides The bulk transition-metal phosphides were synthesized according to the procedure described in the experimental section of this paper (see Table 1 for reduction conditions).

250 The XRD patterns of all the synthesized bulk phosphides are in agreement with literature data [12]. Small amounts of an unidentified crystalline phase accompanied CoMoP, while NiMoP contained small amounts of MoP. In all the other cases, only one crystalline phase was detected. Fig. l a shows the XRD pattern observed for WP and the calculated pattern, while Fig. 1b represents the corresponding crystallographic structure.

Fig. 1. (a) Powder XRD patterns of WP. Upper pattern: experimental, lower pattern: calculated. (b) Crystallographic structure of WP Catalysis usually requires highly dispersed active phases, which cannot be observed by XRD due to the broadness of the reflections. 31p MAS NMR could be an alternative technique to the detection and characterization of small transition-metal phosphide particles. Fig. 2 shows the 31p MAS NMR spectra obtained for NizP at a spinning rate of 10 kHz. Two signals were observed at 1487 and 4076 ppm. These two NMR signals correspond to the two P sites, which can be distinguished in the structure of Ni2P [13]. For all the other phosphides only one signal was found. Co2P has the same M/P ratio as Ni2P but crystallizes in the orthorhombic CozSi structure with only one P site surrounded by 9 Co atoms that form a distorted trigonal tricapped prism [13]. Co2P has a susceptibility about 10 times larger than that of Ni3P [14] but a Knight shift similar to that of Ni3P [15], which shows the clear influence of the structure on the Knight shitt. If compounds have the same structure type, then they may have a similar s electron density, and the Knight shiR might be proportional to the susceptibility. CoMoP crystallizes in the same structure type as CozP, but half of the Co atoms are replaced by Mo atoms in an ordered fashion [16]. The reported Pauli susceptibilities are 6.25-10 -4 emu/mol (Co/P) and 1.67-10-4 emu/mol (CoMoP) [14], and the Knight shifts are 1839 and 318 ppm, respectively (Table 2).

251

I' T

(b) I ,

~ ~ I

I

~ ~ I

I

I

l

4300 4200 4100 4000 3900 [ppm]

I

!

I

I

I

I,

I

1800 1700 1600 1500 1400 1300 1200 [ppm]

Fig. 2. 31p MAS NMR spectra of Ni2P at a spinning rate of 10 kHz (the centerbands are marked by triangles). (a) experimental data, (b) fitted spectra Table 2. 31p NMR results c'atalYst' ' Structure type C02P C02Si Ni2P Fe2P

rot / k H z

15 10

MoP WP NiMoP CoMoP Ni2P/SiO2

WC MnP Fe2P C02Si Fe2P

10 10 10 10 10

Nil2Ps/SiO2

NilEP5

10

NiaP/SiO2 Fe3P 10 a F MH: Full width at half maximum

iso i ppm 1839 1487 4076 214 257 97 318 1487 4076 1941 2259 1796

FWMH'"/kHz 10.0 1.2 4.1 5.4 2.0 6.0 4.0 1.0 2.5 1.8 1.3 1.0 ,

J

,

,,

_

_

A lower Knight shift is also observed in other compounds that contain 4d and 5d metals such as MoP (214 ppm) and WP (257 ppm) (see Fig. 3). MoP crystallizes in the WC structure with one P site that is trigonal prismatically surrounded by 6 Mo atoms [13]. WP crystallizes in the MnP structure that can be understood as a distorted NiAs structure, in which the P atoms are surrounded by 6 W atoms that form a distorted trigonal prism [ 13] (Fig. l b). The rather small shifts lead to the question as to whether MoP and WP are metallic at all and whether the observed shift is not a "normal" chemical shift. Ripley, however, reported that MoP and WP have metallic conductivity [17] and Jones reported that WP shows a Knight shift that is not dependent on temperature [18]. NiMoP crystallizes in the Fe2P structure like NiEP, from which it can be derived by replacing half of the Ni atoms by Mo atoms [13]. The structure has two P sites and hence two NMR signals should be observable. Wada et al. found only one signal at 70 ppm, which is

252

probably the result of their static measurement conditions, which resulted in broad lines [19]. A signal was detected at 97 ppm for our bulk NiMoP while another signal might exist around 50 ppm. However, it is difficult to exactly define where the second signal is located as the larger signal at 97 ppm is not well resolved. Moreover, problems arise from the fact that the strong sidebands of the MoP impurity overlap at different spinning rates with the signals of NiMoP. Under MAS conditions the linewidths of all the NMR signals were narrowed to values between 1 and 4 kHz; only the NMR signal of CozP and NiMoP had a linewidth of 10 kHz and 6 kHz, respectively. Most transition-metal monophosphides and metal-rich phosphides (M/P > 1) show metallic behaviour [18,20]. The short M - M distance implies strong metal bonding, which results in metallic conductivity. The magnetic susceptibility of NiaP and other phosphides hardly depends on temperature, which is further proof of their metallic character and shows that they can be classified as Pauli paramagnets. The metallic character of the phosphides explains why the observed NMR signals are shifted to such high values (Table 2). In a first approximation, the observed Knight shift (Fermi contact interaction term) is proportional to the product of Pauli susceptibility and average s electron density probability at the Fermi level [15]. In heavy and transition metals, electrons in the p and d orbitals can make large contributions by the orbital (van Vleck) paramagnetism and by coupling of the d orbitals with the core s orbitals (core polarization) [15].

1 (a)

(b) I

700

I

600

I

500

I

400

I

I

300

[ppm]

200

I

100

I

0

,I

-100

Fig. 3: 31p NMR spectra of WP at a spinning rate of 10 kHz (the centerband is marked by a triangle): (a) observed and (b) fitted spectra

3.2. SiO2-supported transition-metal phosphides Our research on supported transition-metal phosphides started with the preparation of nickel phosphides supported on an amorphous SiO2 carrier. SiO2 was preferred to A1203 as it interacts to a lesser extent with phosphates, which are known to form aluminum phosphates in the presence of A1203 [21]. The latter was already used as a support for MoP [22], but the authors had to introduce high loadings of molydenum and phosphorus

253 onto the low surface area A1203 support (91 m2.g1) in order to be able to detect MoP phases by X-ray diffraction. At such high loadings, only part of the Mo and P will interact with the support and the danger of segregation is lower. There was a clear influence of the support on the reduction behaviour of the phosphide species: while pure bulk Ni2P could be obtained from the reduction of the corresponding oxidic precursor containing a stoichiometric Ni to P ratio of 2 to 1, an excess of phosphate had to be introduced onto the SiO2 support in order to synthesize SiO2-supported Ni2P. A Ni to P ratio equal to two lead to the formation of compounds such as NiI2P5 or Ni3P depending on the reduction conditions but Ni2P could not be obtained. Other parameters, such as the reduction temperature and the flow rate were also found to have an influence on the type of phase of the Ni phosphides synthesized. Three different Ni phosphides (Ni2P, NinPs, Ni3P) were then prepared on SiO2 and characterized by powder X-ray diffraction and 31p MAS NMR spectroscopy. The influence of the flow rate on the reduction behaviour is illustrated in Fig. 4, which shows the TPR profiles obtained from the reduction of the oxidic precursor Ni-P(2:I) (where the numbers in brackets refer to the stoichiometric ratio between Ni and P) under a flow of 4.8% H2 in Ar using two different flow rates (10 and 50 ml.min'l). Although the shape of the two profiles is similar, the temperature maxima are shifted towards higher values when a lower flow rate is used. While the reduction starts at around 520 K when 50 ml'min~ of 4.8% H2/Ar are used, it only starts at 600 K and finishes around 1100 K under a flow rate of 10 ml.min"l. This is explained by the formation of water during the reduction process, the removal of which being favoured by a higher flow rate [23]. This contrasts with the results obtained for the unsupported Ni2P, in which the flow rate was shown to have hardly any influence on the reduction behaviour.

5 t-

._o

./"

(b) / .............

/

\

,,,-----..._._...-

,,

'\

J

Q.

E

g i-

8

400

6~o

8~o

Temperature [K]

10'0o

12'00

Fig. 4. TPR profiles of the oxidic precursor of Ni-P (2:1) (a) 1023 K, 2 K.min1, 4.8%H2/Ar, 50 ml-min1 (b) 1173 K, 5 K.min"1, 4.8%HJAr, 10 ml.min"l Reduction experiments carried out in a separate micro-reactor under a flow of 5% H2/N2 confirmed the importance of the flow rate: different types of Ni phosphides were formed

254

when the flow rate was varied from 10 to 200 ml.min -1. With a flow rate as low as 10 ml'min -1, Ni3P was formed on SiO2, while NilzP5 could be observed when a flow rate of 50 ml'min -1 was used. NizP was not detected at any time under those conditions. An excess of phosphate (Ni:P=2:1.3) had to be used during the impregnation of the SiOz carrier, together with a high flow rate of 200 ml.min -1 in order to achieve the synthesis of NizP/SiO2. It is suggested that the reduction of the oxidic precursor supported on SiO2 to NizP/SiO2 occurs through different reaction steps, involving the formation of NilzP5 and Ni3P as intermediates. The different Ni phosphide samples supported on SiO2 were also studied by 31p MAS NMR and the results are summarized in Table 2. The high chemical shifts observed for the NMR signals are explained by the metallic character of the Ni phosphides. As for the unsupported NizP sample, NizP/SiO2 revealed two NMR signals with isotropic chemical shifts of 1487 and 4076 ppm. The NMR spectrum of the NilzPs/SiOz sample also displayed two signals centered at 1941 and 2259 ppm. On the contrary, one signal only was detected for Ni3P/SiOz with an isotropic shift of 1796 ppm. In addition to those signals, peaks were detected around and below 0 ppm (i.e. +2, -8 and-21 ppm) for all three supported samples. Such shifts are typical of phosphate species such as HnPO4(3-n)-, P2074- and (PO3-)n [24] These results differ from those obtained for the unsupported materials, in which no other signals than the ones corresponding to the phosphides were detected. This illustrates one of the possible effects of the support, which could segregate the Ni and phosphate and prevent the complete reduction of the latter. It has to be noticed, however, that no signals corresponding to silicon phosphate were detected below 30 ppm [25]. 4. CONCLUSIONS A variety of bulk transition-metal phosphides (CozP, NizP, MoP, WP, CoMoP, NiMoP) were successfully prepared by reduction of a metal oxide/phosphate precursor with H2 at different reduction temperatures. These materials were characterized by powder X-ray diffractometry and 31p MAS NMR spectroscopy. The latter technique enabled the determination of Knight shifts characteristic of metallic-type of phases. NizP, Nil2P5 and Ni3P supported on SiO2 were also prepared by reducing an oxidic precursor and by changing reaction parameters such as the P content, the reduction temperature or the reductant flow rate. The influence of the support on the reduction process was clearly demonstrated: pure NizP could be obtained when a Ni to P ratio of two was used during the preparation of the unsupported material, while an excess of phosphate had to be introduced onto the SiOz support to yield NizP. 31p MAS NMR proved to be a useful technique to characterize transition-metal phosphides and could be an alternative technique to the detection and characterization of small metal phosphide crystallites supported on a carrier. REFERENCES 1. R. Prins, V.H.J. de Beer and G.A. Somorjai, Catal. Rev.-Sci. Eng., 31 (1989) 1. 2. H. Topsoe, B.S. Clausen and F.E. Massoth, "Hydrotreating Catalysis", Springer, NewYork, 1996.

255 3. Th. Weber, R. Prins and R.A. van Santen, "Transition Metal Sulphides - Chemistry and Catalysis", Kluwer (Eds.), Dordrecht, 1998. 4. S. Eijsbouts, J.N.M. van Gestel, J.A.R. van Veen, V.H.J. de Beer and R. Prins, J. Catal., 131 (1991) 412. 5. M. Jian and R. Prins, J. Catal., 179 (1998) 18. 6. W.R.A.M. Robinson, J.N.M. van Gestel, T.I. Koranyi, S. Eijsbouts, A.M. van der Kraan, J.A.R. van Veen and V.H.J. de Beer, J. Catal., 161 (1996) 539. 7. W. Li, B. Dhandapani and S.T. Oyanm, Chem. Lett., 3 (1998) 207. 8. C. Stinner, R. Prins and Th. Weber, J. Catal., 191 (2000) 438. 9. C. Stinner, R. Prins and Th. Weber, J. Catal., 202 (2001) 438. 10. G. Nolze and W. Kraus, Powder Diffr., 13 (1998) 256. 11. J. Herzfeld and A.E. Berger, J. Chem. Phys., 73 (1980) 6021. 12. PDF 3-953 (Ni2P); PDF 32-306 (Co2P); PDF 24-771 (MOP); PDF 29-1364 (WP); PDF 31-873 (NiMoP); PDF 32-299 (CoMoP). 13. P. Villars and L.D. Calvert, Pearson's Handbook of Crystallographic Data for Intermetallic Phases, (2nd ed.) ASM International, Materials Park, 1991. 14. S. Ohta and H. Onmayashiki, Physica B, 253 (1998) 193. 15. W.D. Knight and S. Kobayashi, "Encyclopedia of Nuclear Magnetic Resonance", Eds. D.M. Grant and R.K. Harris, Wiley, Chiehester, 1996 (p. 2672). 16. R. Guerin and M. Sergent, Aeta Crystallogr. B, 34 (1978) 3312. 17. R.L. Ripley, J. Less Comm. Met., 4 (1962) 496. 18. E.D. Jones, Phys. Rev., 158 (1967) 295. 19. S. Wada, T. Matsuo, S. Takata, I. Shirotani and C. Sekine, J. Phys. Soc. Jpn., 69 (2000) 3182. 20. B.F. Stein and R.H. Walmsley, Phys. Rev., 148 (1966) 933. 21. P.J. Mangnus, J.A.R. van Veen, S. Eijsbouts, V.H.J. de Beer and J.A. Moulijn, Appl. Catal., 61 (1990) 99. 22. S.T. Oyama, P. Clark, V.L.S. Teixeira da Silva, E.J. Lede and F.G. Requejo, J. Phys. Chem. B, 105 (2001) 4961. 23. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B 104 (2000) 10482. 24. K. Eichele and R.E. Wasylishen, J. Phys. Chem., 98 (1994) 3108. 25. T.R. Krawietz, P. Lin, K.E. Lotterhos, P.D. Torres, D.H. Barieh, A. Clearfield and J.F. Haw, J. Am. Chem. Soe., 120 (1998) 8502.

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257

The application of non-hydrothermally prepared stevensites as support for hydrodesulfurization catalysts M. Sychev a, R. Prihod'ko a, A. Koryabkina b, E.J.M. Hensen b, J.A.R. van Veen ~ and R.A. van Santen b aFaculty of Chemical Technology, National Technical University of Ukraine, 03056, Kiev, pr. Peremogy 37, Ukraine

bSchuit Institute of Catalysis, Eindhoven University of Technology, Den Dolech 2, PO Box 513 5600 MB Eindhoven, The Netherlands CShell International Chemicals B.V., Badhuisweg 3, 1031 CM Amsterdam, The Netherlands The stevensite-like materials containing Mg, Mg-Ni, and Mg-Co in the octahedral sheets were synthesized under non-hydrothermal conditions. The nature of divalent octahedral cations influences the surface area and pore volume of these materials. The thiophene HDS activities of stevensite-supported catalysts prepared by addition of Mo or W to the Nicontaining supports, especially to the NiZ+-exchanged stevensite, were superior in relation to those of their counterparts made by conventional co-impregnation of the Mg-stevensite with NiMo and NiW. The use of chelating agents, NTA and EN, affects beneficially the HDS activity of the catalyst which, however, is less obvious in the case of the CoMo catalysts. The positive role of the chelating agents is explained by weakening of the Ni(Co)-support interaction and by a change in the sulfidation sequence of Ni(Co) and Mo(W). The Ni z+- and Co Z+-exchanged Mg-stevensite or that isomorphously substituted with these cations are promising candidates for application as supports for hydro-processing catalysts. The thiophene HDS activity of the NiMo and NiW stevensite-supported catalysts is comparable with that of the corresponding ~,-AIzO3 commercial ones. 1. INTRODUCTION The necessity to develop hydrotreating catalysts with enhanced activity stimulates the search for alternative catalyst supports. It was shown that clay-supported transition metal sulfides can efficiently catalyze hydrodesulfurization (HDS) of thiophene [1-3]. However, the large scale application of the catalysts based on natural clays is still hampered, mainly due to the difficulties in controlling the chemical composition and textural properties. Synthetic clays do not suffer from these drawbacks. Recently, a novel non-hydrothermal approach was proposed for the synthesis of some trioctahedral smectites, namely saponite

258 and stevensite [2, 4-6]. Here, we report on the preparation and characterization of Mo(W)S~ promoted by the Co(Ni)-catalysts obtained using synthetic stevensites differing in the chemical composition as a support material. An attempt has been made to investigate the influence of chelating agents on the catalyst behavior in the thiophene HDS. 2. EXPERIMENTAL 2.1. Sample preparation Stevensite-like materials were synthesized on the basis of the theoretical composition of stevensite {N,jzZ+(M2+6_x.x)[Sis]O20(OH)4n H20}, where Nxz§ represents an interlayer cation and 9 a vacant site, respectively. The starting silica-aqueous suspension was obtained by heating (363 K, 1 h) the water-dispersed aerosil with the solid/liquid ratio of 1:50. Next, a solution containing 70 mmol of Mg(NO3)2 or a mixture of 63.7 mmol of Mg(NO3)2 with 6.3 mmol Ni(NO3)2 or Co(NO3)2 and 0.3 mol of urea was slowly added to this suspension and the reacting mixture was kept under stirring at 363 K for 36 h. The solid products were separated, washed, and dried at 383 K for 12 h. The samples with Mg2§ MgZ+-Niz§ and Mg2+-Co2+ (the cation ratio = 10:1) in the octahedral sheets are denoted as MgST, NiST, and COST, respectively. Ni2§ and Co2+-exchanged MgSTs were obtained conventionally and are referred to as NiexST and CoexST, respectively. The NiMo and CoMo catalysts were prepared by impregnation of the support with an aqueous solution containing ammonium heptamolybdate (AHM) or ammonium metatungstate (AMT) and Ni(Co)(NO3)2 with the Ni(Co) to Mo(W) molar ratio of 0.33 in the absence or in the presence of ammonia and a chelating agem, nitrilotriacetic acid (NTA) or ethylendiamine (EN). These chelating agents were chosen because of their different complexing properties. The NiW catalysts were obtained in the presence of the chelating agents only. For NTA, the ligand: Ni(Co) molar ratio was 1:1, while in the case of EN, this ratio was 4. 2.2.

Apparatus used and conditions of measurements

XRD: oriented powdered specimens, a DRON-3 diffractometer, CuIQ and CoK~ radiation. XRF: a high-vacuum device, Si(Li) detector (180 eV at 5.9 keV), sample weight of 200 mg. UV-visible diffuse reflectance spectroscopy (DRS): Shimadzu UV 2401, reference BaSO4. FTIR: Perkin-Elmer 2000, KBr technique. N2 adsorption: ASAP 2010 (Micromeritics), degassing at 403 K, 10.4 mbar, 5 h, pore volume calculated by the smethod. Temperature-programmed reduction (TPR): 66% H2 in Ar, flow-rate of 20 ml/min, temperature range of 298-1073 K, heating rate of 5 K/min, thermal conductivity detector. The thiophene HDS activity was measured at 673 K in a microflow reactor (1 bar pressure, 4 vol.-% thiophene in H2, 50 std cm3 minl). The samples were sulfided in situ using a gas mixture of 10 % H2S in H2 (60 std crn1, 6 K min1 from 293 to 673 K, 2 h at 673 K).

259

3. RESULTS AND DISCUSSION 3.1. Characterization of supports and catalyst precursors

The XRD patterns of the samples obtained (Fig. 1) are consistent with those of synthetic and natural stevensites [7,8], evidencing formation of the trioctahedral smectite structure, as indicated by the (060) reflection at about 1.52 A. However, intensities of the X-ray reflections vary for the particular solids, which is caused probably by differences in the crystallinity and/or in size and stacking of particles. All the samples show a typical smectite ability to swell in ethylene glycol and exhibit ion-exchange capacity comparable with that of the natural stevensite (of about 50 mequiv/g). The FTIR spectra of all the samples contain lattice vibration I ~" bands at about 1010, 660, 440, ]l o and 450 crn-1 and a shoulder at ,", 1 around 420 cm-1 assigned, respectively, to Si-O stretching, d S" OH libration, Si-O bending and ~ ' ~ ~" ~ 9 ~ g vibration of a Si-O-M 2+ bond ~ I ~ _ ~ ~ ~ _ _j~ characteristic of stevensite [8]. ~ -__.-._ _ 1" No silica hydrogel vSiO2 at 1100 t 2 cm-1 [7] was detected. The diffuse reflectance 3 spectra of NiST display bands at approximately 380, 670, and 0 ' 2'0 ' 4'0 ' 6'0 . . . . 80 740 nm, which originate from 2 Theta octahedrally coordinated Ni 2+ cations due to the transitions Fig. 1. X-ray diffraction patterns of stevensites air-dried from 3A2g(F) to 3Tlg(P) 3Tlg (F) at 383 K: (1) MgST, (2) NiST, (3) COST. and 3T2g(F) [9]. The DR spectra * CuK~ radiation. of CoST show a band at 528 nm and shoulders at 500 and 646 nm, assigned to the v3 (4m2g-')4Tlg) and v2 (4Tlg(P)'~4Tlg) transitions, respectively, typical of Co 2+ in the octahedral coordination [10]. Since Ni 2+ and Co 2+ admittedly accommodated in the non-framework positions were removed before the DRS measurements by the ion-exchange with NH4+, the observed DR bands can be attributed to the cations located in the octahedral sheets of the stevensite structure. The temperature programmed reduction (TPR) of NiST proceeds in two steps with maxima at around 700 and 815 K (Fig. 2). The observed TPR peaks are indicative for the presence of Ni 2+ with different reactivity for hydrogen toward the zero-valent state. Since the non-crystalline silica-alumina phase gave a much poorly resolved TPR profile (Fig. 2), the peaks should be related to the crystal structure of NiST. The nickel cations placed inside the octahedral sheets being coordinated with six framework oxygen atoms should be more stable than Ni 2+ exposed at the edges of the clay platelets. Consequently, the first reduction step can most likely be attributed to the reduction of the Ni 2+ cations located at those positions. Since the reduction of NiO takes place at approximately 570 K, these data show

260 absence of the Ni(OH)2 impurity. While in the case of CoSP virtually no Co(OH)2 phase was detected (623 K), the reduction takes place at considerably higher temperatures as compared to that for NiST. The TPR pattern of CoST contains also two reduction steps centered at 816 and 1074 K (Fig. 2). By analogy with NiST, they can be ascribed to the reduction of Co 2§ exposed at the edges of the clay platelets and inside the octahedral sheets, respectively. The sharper main reduction step, compared to that for NiST, shows that the Co 2§ cations are better distributed in the stevensite structure. 1074

682~06 ,

J 400

600

800

I000 400

4

600

800

3

I000

1200

Temperature [ K] Fig. 2. TPR patterns of (1) NiST and (3) COST. (2) Ni- and (4) Co-containing noncrystalline" silica-alumina phases are included as references. The N2 adsorption isotherms of all stevensites prepared are close to type II, being typical of mesoporous solids [11 ], with the H2 hysteresis representative of ink-bottle shaped pores (Fig. 3). The closure of the loop at P/Po = 0.45-0.5 indicates predominance of the mesopores with the size just below 4 nm. ,__,400

f

o

"~ 113 300

40

/

200

1

/,,

t

.,.o r

O

e.r 0

-,am-,-v o o~ ,=,7 ~.%....~" gX"" ":

#-_/....'+

/

..,.o- +" -"

.,o.,b,,/o'

1oo

o

0'.2 " 0.4

o16

oi~

1'.o

o~

0'.4

oi,5

oi~

~io

Relativepressure [PIP]

Fig. 3. N2 adsorption-desorption isotherms of (1) NiST, (3) COST, and (2,4) their sulfided counterparts.

261 The sulfidation of NiST and CoST at 673 K affects the shape of the N2 adsorption isotherms (Fig. 3), causing also some decrease in the surface area and pore volume (Table 1). The samples suffered from some loss of crystallinity, as indicated by broadening of the X-ray reflections. These results show that significant fractions of the Ni2§ and Co 2~ cations are sulfided and extracted from the octahedral sheets, which leads to partial amorphization of the stevensite structure. However, its main part persists with virtually no collapse. All the presented data confirm the formation of stevensite-like materials, the textural properties of which depend on the composition of octahedral sheets (Table 1). Table 1 Abundance of octahedral cations (XRF, in molar ratios), surface areas (SBET), and total pore volumes (Vto~0 of the studied air-dried stevensites and their sulfided counterparts Sample MgST NiST CoST NiSTsulf. Co STsulf.

Mg 5.62 5.11 5.12 5.11 5.12

Ni or Co -0.52 0.51 0.52 0.51

SBET(m2/g) 455 383 430 322 372

Vtotal(cc/g) 0.385 0.587 0.482 0.415 0.418

Treatment of the supports with impregnating solutions decreased their surface area and total pore volume by 10-15%, indicating incorporation of active phases in the stevensite interlayer space.

3.2. Thiophene HDS activity The sulfided Mg-stevensite itself does not exhibit any thiophene HDS activity. When Mo or W are introduced, the clays become active, exhibiting the conversion of 15 and 10% at 5 h run-time, respectively. The NiST and CoST samples (Ni2§ and Co 2§ are located together with Mg 2§ in the lattice) show comparable activities. The addition of Mo and W via impregnation substantially increases the HDS activity of these materials. A slight difference in activity is observed for the catalysts prepared from NiST or CoST and those obtained by the impregnation of MgST with the Ni-Mo or Co-Mo solutions (Fig. 4). This indicates that the Mo(W) cations can interact with Ni2§ or Co 2§ located in the clay lattice. The possibility that these cations can migrate from the framework upon sulfidation thus forming "NiMoS", "CoMoS", and "NiWS" phases cannot be excluded. This suggestion is consistent with the data obtained by means of the N2 adsorption. The thiophene HDS activity of the stevensite-supported catalysts was found to be sensitive to the mode of preparation. The catalysts prepared by addition of Mo or W to the Ni-containing supports (NiexST and NiST) were superior to their counterparts made by convemional co-impregnation of the Mg-stevensite with Ni and Mo(W) (Fig. 4). In the case of the CoMo catalysts, this effect was less pronounced. The beneficial effect of the presence of a complexing agent on the thiophene HDS activity was restricted to the catalysts containing both Ni(Co) and Mo(W). These findings are in accordance with those reported by Prins and co-workers [12]. The extent of the activity increase depends on the

262 ~

::::::::::::::::::::::::::::::::::::::::::::::::::::: :~:~'~'~:~-~'~ ":~-~:'

==================================================== ..........':q .......... :'::::"..... ~ , . M , ' , " . ' " ...........................

ata.ysts preparo wit

~

the

i!] i:' ": 3 0

............................ ~. .,,,,,,,,,,,,:~.-:-.~:--~:-.c::-.~:;--:~.-~ ,~.~...:~:~.::~:- "_l xL I, i l t." t,f.',' .~, . , ,

composition of the active phase and properties of the chelating agent. For the

i!~ ":

activity sequence was: NiMo NiW > "~:J!i!i ::'!i!..... ~'-ti[il::i......... ". :b1:~i~i Z0 CoMo, while the presence of EN resulted in !!~!i] i ~ ,l'!,!,, the following activity order: NiMo CoMo > NiW (Fig. 4). ~iii EN The obtained results can be explained ~?.:.-.---'-..-.......--.".'~~2 ~i:!l .....~ ! 1 ~~~" J NTA EI~O while taking into account several functions NiexST NiST MgST* of the chelating agents [12-14]. Firstly, they can inhibit formation of nickel silicates by Irarl 2o 7,e prohibiting hydrolytic interaction of the Ni2+ cations with silanol groups present on the support surface [12]. Most likely, this phenomenon can be propagated onto the 00 ..~-- 9 catalysts containing CoMo. NTA behaves differently than EN, forming much more ~!.".....,.. ......~.'_.:~:..'.:t.............~.!:..v..:.>.!....,,...~:.>..~ NTA CeexST CoST M~ST* stable complexes with Ni than EN does [13], and therefore better protecting nickel and hindering interaction with the support. This can explain higher activities of the NiMo and ~::|::: ::.';i~:." :::::::::::::::::::::::: ::::.'::::" NiW catalysts obtained from NiexST in the presence of NTA, as compared to that ~iWi 9 ~ililili!i ii~g :!!i!: ~'o prepared with use of EN. However, in the case of Co 2+, such a preventing functionality of the complexing ligands seems to be less pronounced (Fig. 4). N:c~:~T N~ST MgST* Secondly, adding NTA and EN to the Fig. 4. Initial thiophene HDS activity (5 impregnating solutions prevents the min run) of stevensite-supported catalysts sulfidation of Ni at low temperatures, thereby increasing the formation of the prepared in the presence or in the absence "NiMoS" phase, as was deduced from the of chelating agents in the impregnating increased activity in the thiophene HDS [12- solutions. 14]. Courier et al. [15] also showed that the * Samples obtained via co-impregnation. same effect, namely stabilization of cobalt against the sulfidation, explains the role of NTA in enabling the formation of "CoMoS" in the silica-supported CoMo catalysts. Some effect of the chelating agents on the dispersion of the MoS2 (WS2) particles also cannot be excluded [15]. This leads to the improved catalyst behavior. These conceptions, explaining the functions of the chelating agents, were mainly developed for the silica-supported HDS catalysts [12-15]. However, the data presented here indicate their vafidity for the stevensite-supported HDS catalysts. Nevertheless, taking into account differences in the crystal structure, surface chemistry of the smectites, and the corresponding characteristics of silica, additional study is necessary to elucidate the case.

d

!liiil

263 The catalysts under study deactivate relatively strongly during the initial 5 to 35 min of the test experiment (Fig. 5). This is probably due to the coke formation, although a contribution to the activity decrease arising from the establishment of a new equilibrium under reaction conditions cannot be excluded [16]. The extent of the catalyst deactivation depends on the active phase composition; the CoMo catalysts showed the strongest deactivation (Fig. 5). The use of the chelating agents, especially NTA, improves the catalyst stability. This effect can be attributed to a better dispersion of the sulfide active phase that hampers its sintering during the sulfidation and HDS test. As concerns selectivity, it can be stated that generally the thiophene HDS over the stevensite-supported catalysts resulted mainly in the formation of butane and butenes. Only during the initial stage of the catalytic test, when the conversion ofthiophene was very high, a small amount of C1-C3 hydrocarbons could be detected. Distribution of the C4 products depends on the composition of the metal-sulfide phase (Fig. 5B). The NiW catalysts produce more n-butane, likely because of their higher hydrogenation activity. Hence, catalytic tests in the hydrodesulfiafzation of thiophene showed that the behavior of the catalysts under study depends on the metal-sulfide phase composition and the mode of its formation. The presence of the chelating agents in the impregnating solutions influences both activity and stability of the catalysts. A.

35 =I

NiMoST

5o

u

NiWST

~30

,._.,

10

~--~

CoMoS T

I0

5

0

0

2

4 Run time [h]

6

g

-

-

-

n-butane l-butene 2-t-butene 2-c-butene

Figure 5. (A) Thiophene HDS over stevensite-supported catalysts: (1) NiexSTW(NTA), (2) NiexSTMo(NTA), (3) CoexSTMo(NTA), (4) NiexSTMo(H20), and (5) CoexSTMo(H20). (B) C4 products selectivity as a function of the metal-sulfide composition. 4. CONCLUSIONS The results obtained when using a variety of methods indicate that Mg-, Mg-Ni-, and Mg-Co-containing stevensite-like materials were synthesized under non-hydrothermal conditions. The composition of the octahedral layers affects the textural properties of these materials that are predominantly mesoporous.

264 The thiophene HDS activity of the stevensite-supported catalysts depends on the mode of their preparation. The materials prepared by addition of Mo or W to the Ni-containing supports, especially to the Ni2+-exchanged stevensite, were superior to their counterparts made by conventional co-impregnation of the Mg-stevensite with Ni and Mo(W). In the case of the CoMo active phase, introduction of the Co cations before those of Mo influences the catalyst activity less favorably. The activities of the NiMoST and NiWST catalysts prepared with NTA and EN exhibited an improvement, suggesting that Ni is responsible for the increase in the catalytic activity. This suggestion is based on the fact that these chelating ligands prefer coordination with Ni rather than with Mo and W. In the case of the CoMo catalysts, the beneficial effect of the NTA and EN is less obvious. The positive role of the chelating agents is explained by weakening of the Ni(Co)-support interaction and by a change in the sulfidation sequence of Ni(Co) and Mo(W). The Ni 2+- and C02+-exchanged Mg-stevensite or that isomorphously substituted with these cations are promising candidates for application as supports for hydro-processing catalysts. The NiMo and NiW stevensite-supported catalysts showed the thiophene HDS activity comparable with that of the corresponding %,-A1203commercial ones. ACKNOWLEDGMENTS

The authors thank Ms. M.C. Mittelmeijer-Hazeleger (University of Amsterdam, The Netherlands) for the adsorption measurements and Dr. K. Erdmann (Nicholas Copernicus University, Torun, Poland) for help. These investigations were supported in part by the Ukrainian Ministry of Education and Science and by a Spinoza grant (to R.A.v.S.) from the Dutch Science Foundation. REFERENCES

1. M. Sychev, V.H.J. de Beer, A. Kodentsov, E.M. van Oers and R.A. van Santen, J Catal. 168 (1997) 245. 2. R.G. Leliveld, W.C.A. Huyben, A.J. van Dillen, J.W. Geus and D.C. Koningsberger, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 106 (1997) p. 137. 3. E. Hayashi, E. Iwamatsu, M.E. Biswas, Y. Sanada, S. Ahmed, H. Hamid and T.Yoneda, Appl. Catal. A: Gen. 179 (1999) 203. 4. R..J.M.J. Vogels, M.J.H.V. Kerkhoffs and J.W. Geus, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 91 (1995) 1153. 5. M. Sychev and R. Prihod'ko, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 118 (1998) 967. 6. M. Sychev, R. Prihod'ko, I. Astrelin, P.J. Stobbelaar and R.A. van Santen, Book Abstract, EUROCLAY'99 Conf., Krakow, Poland (1999) 135. 7. N. Takahashi, M. Tanaka, T. Satoh, T. Endo and M. Shimada, Micropor. Mater., 9 (1997) 35. 8. G.T. Faust, J.C. Hathaway and G. Millot, Amer. Miner., 44 (1959) 342. 9. A.P. Hagan, M.G. Lofthouse, F.S. Stone and M.A. Trevethan, Stud. Surf. Sci. Catal.,

265 Elsevier, Amsterdam, 3 (1979) p.417. 10. J. Dedecek and B, Wichterlova, J. Phys. Chem. B., 103 (1999) 1462. 11. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by Powder and Porous Solids. Principles, Methodology and Applications, Acad. Press, San Diego, 1999, pp.439-441. 12. L. Medici and R. Prins, J. Catal., 163 (1996) 28. 13. R. Cattaneo, T. Shido and R. Prins, J. Catal., 185 (1999)199. 14. L. Coulier, V.H.J. de Beer, J.A.R. van Veen and J.W. Niemantsverdriet, J. Catal., 197 (2001) 26. 15. L. Courier, V.H.J. de Beer, J.A.R. van Veen and J.W. Niemantsverdriet, Topics Catal., 13 (2000) 99. 16. V.H.J. de Beer, C. Bevelander, T.H.M. van Sient Fiet, P.G.A.J. Werter and C.H. Amberg, J. Catal., 43 (1976) 68.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

NiMo/HNaY(x)-AI203

dibenzothiophenes:

267

catalysts for hydrodesulfurization of hindered the preparation method

effect of

T. Klimova a'*, D. Soils a, J. Ramirez a and A. L6pez-Agudo b aUNICAT, Departamento de Ingenieria Quimica, Facultad de Quimica, Universidad Nacional Aut6noma de M6xico, Cd. Universitaria, Coyoacfin, 04510 M6xico D.F., M6xico Fax: +52+55+56225366, e-mail: [email protected] bInstituto de Catfilisis y Petroleoquimica, CSIC, Cantoblanco, 28049 Madrid, Spain Three NiMo-HNaY-alumina catalysts with similar composition were prepared by different methods and tested in the hydrodesulfurization of dibenzothiophene (DBT) and 4,6-dimethyl-DBT. It was found that the catalyst preparation method induces some changes of the characteristics of the deposited metallic species as well as of the acidic properties of the zeolite component. These changes affect the catalytic behavior in the hydrodesulfurization of DBT and 4,6-DMDBT. Acidic properties of the catalyst seem to be more important for the conversion of alkyl-substituted DBT. 1. I N T R O D U C T I O N In recent years, the interest in new efficient HDS catalysts is growing due to more severe environmental legislation with respect to sulfur level in fuels and the need to process increasing amounts of high sulfur containing crude oil. To achieve the necessary efficiency, the work should be directed towards the development of catalysts with high desulfurization activity with respect to strongly hindered sulfur containing molecules, such as dibenzothiophene (DBT) and its alkyl-substituted derivatives: 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) [1]. The latter compounds particularly resilient with respect to HDS cannot be completely removed using current catalyst formulations, like NiMo or CoMo on ~,-AlzO3 [2, 3]. The extremely low reactivity of 4-MDBT and 4,6-DMDBT was ascribed to steric hindrance that occurs between the methyl groups of the dibenzothiophene molecule and the active sites of the catalyst. It was shown that hydrogenation of methyl-substituted DBT's prior to sulfur removal is able to alleviate this steric hindrance and, therefore, facilitate the HDS reaction [4]. Another possibility to increase desulfurization of 4,6-DMDBT is to use zeolite containing HDS catalysts [5, 6]. In this case desulfurization can take place more easily after cracking or inter and intramolecular migration of the methyl groups on the dibenzothiophene structure. However, there are no reports about the best method that can be used to prepare NiMo/~,alumina catalysts modified with zeolite and about the effect that the method of preparation

268

has on their catalytic behavior. It can be expected that, if the zeolite component is incorporated into the alumina matrix before the active phase deposition, some changes in Ni and Mo location and dispersion, as well as in zeolite acidity, can take place and they may affect the HDS performance of the catalyst. In order to obtain more information about zeolite containing systems, we characterized a series of catalysts with the same HNaY content (20 wt%) prepared by different methods. It is the object of the present work to study the effect that the preparation method has on the metallic and acid functionalities of the catalyst and its performance in the hydrodesulfurization of DBT and 4,6-DMDBT. 2. E X P E R I M E N T A L Three NiMo/HNaY-AI203 catalysts with the same zeolite loading (20 wt%) were prepared by different methods: i) mechanical mixing of the zeolite with a conventional NiMo/AI203 catalyst ((NiMo/AI203)+HNaY(MM)), ii) using y-alumina binder to join the zeolite and NiMo/AI203 ((NiMo/AI203)+HNaY(B)), and iii) by impregnation of Mo and Ni on the composed zeolite-alumina support prepared by the peptization method (NiMo/HNaY-AI203(P)) [7]. A commercial NaY zeolite (Si/A1 ratio = 2.42), 58% exchanged with ammonium acetate solution to obtain HNaY, was used as the zeolite source and pseudo-boehmite Catapal B as the 7-A1203 source. The impregnation of Ni and Mo (Mo first) on the supports was made by the pore volume method, using aqueous solutions of ammonium heptamolybdate and nickel nitrate to reach nominal composition of 12 wt% of MoO3 and 3.45 wt% of NiO. The catalysts were dried at 100~ (24 h) and calcined at 500~ (2h). The catalysts were characterized by N2 physisorption, XRD, thermodesorption of Py (FT-IR), NH3 TPD, TPR and NO chemisorption. The DBT and 4,6-DMDBT hydrodesulfurization activity tests were conducted in a 300 ml batch reactor at 300~ and 7.3 MPa total pressure. Before the activity tests, the catalysts were sulfided ex-situ in a tubular reactor at 400~ 4 h, in a stream of H2S (15 vol%)-H2. The course of the reaction was followed by taking liquid samples and analyzing them by GC or GC-MS. 3. RESULTS AND DISCUSSION

3.1. Catalyst characterization The results from N2 physisorption (Table 1) indicate that the use of binder during catalyst preparation produces some loss of zeolite microporosity, probably due to the micropore blockage. This effect was not observed in the (NiMo/AI203)+HNaY(MM) sample prepared by mechanical mixing.

269 Table 1 Textural properties of NiMo catalysts and HNaY support Sample

Surface area (mZ/g)

SBET NiMo/AlzO3 NiMo/HNaY-AIzO3(P) (NiMo/AIzO3)+HNaY(B) (NiMo/AIzO3)+HNaY(MM) NiMo/HNaY HNaY

Pore volume (cm3/g)

Smicropores

200 186 199 238 239 582

Vtotal

0 48 45 102 189 478

Vmicropores

0.33 0.30 0.36 0.34 0.14 0.31

Average pore diameter (A)

0 0.022 0.020 0.046 0.088 0.222

46 60 68 70 52 -

The XRD of NiMo catalysts (Fig. 1) reveal the presence of faujasite crystalline phase in all the cases. However, the intensity of the characteristic faujasite reflections is significantly higher in the diffractogram of the (NiMo/AIzO3)+HNaY(MM) sample than in those catalysts prepared with binder. A comparison of the diffractograms of the NiMo/HNaYAIzO3(P) catalyst with that of a mechanical mixture of the HNaY-AlzO3(P) support with 12

600

Q

o

=

400

=

200

,

i

I

20

,

I

40

,

a

I

60

,

80

De~rees (20} Fig. 1. X-ray powder diffraction patterns of NiMo catalysts: (a) (NiMo/AlzO3)+HNaY(MM), (b) (NiMo/AIzO3)+HNaY(B), (c) NiMo/HNaY-AIzO3(P), and (d) mechanical mixture of 12 wt% of MoO3 with HNaY-AIzO3(P) support (*faujasite, 9 molybdite). wt% MoO3 (curves c and d, Fig. 1), show that the impregnation of Mo and Ni species, and subsequent calcinations are responsible for the crystallinity loss of the faujasite phase. Previously, similar destruction of the NaY and HY zeolite structures was observed as a

270 result of Mo impregnation [8]. It was also found that the extent of crystallinity loss of NaY increased as the Mo content increased. Therefore, it can be supposed that in both catalysts prepared here with binder some destruction of the zeolite crystals takes place. It is possible that this deterioration of the zeolite structure is due to the penetration of the Mo species into the zeolite cages during the preparation of the catalysts, probably during the calcination step. The possibility of such migration of Mo species into zeolite was reported previously for MoO3/NaY [9]. The results from surface acidity measurements by NH3 TPD in the catalysts show that the total number of acid sites was proportional to the accessible zeolite area (Tables 1 and 2). The same trend was found also for the Br6nsted acid sites quantified by FT-IR of Py. Table 2 Acidity of NiMo catalysts Total acidity*

Br6nsted acidity** (gmol Py/g)

Sample

NiMo/AI203 NiMo/HNaY-AI203 (P) (NiMo/AIzO3)+HNaY(B) (NiMo/AI203) +HNaY (MM) NiMo/HNaY

pmol NH3/m 2

~tmol NH3/g

150~

250~

350~

25.71 38.45 34.92 46.31 64.74

5142 7268 6932 11031 15409

0 4.6 3.4 7.1 8.3

0 2.9 2.7 6.0 7.1

0 2.3 1.8 5.1 5.4

* Total number of acid sites determined by NH3 TPD ** The amount of pyridine adsorbed on Br6nsted acid sites per gram catalyst determined by FT-IR at different temperatures of Py desorption [10] In order to envision the effect that the catalyst preparation method has on the type and reducibility of the Mo and Ni species present in the catalyst precursor, TPR experiments were performed with all the catalyst samples in their oxide form. Fig. 2 shows the TPR patterns. For the pure alumina-supported catalyst, a TPR trace typical of a NiMo/AI203 catalyst is obtained. It presents two main reduction peaks, assigned to the reduction of octahedral Mo species (peak at about 400~ and tetrahedral Mo species in strong interaction with the alumina support (peak at 770~ [11]. It is also observed the presence of shoulders at 517~ and 600~ due to the reduction of octahedral Mo species with different degrees of polymerization. As HNaY is incorporated into the catalyst, by different methods, some changes in the proportion of tetrahedral and octahedral Mo species and in their agglomeration take place. Thus, in the thermogram of NiMo/HNaY-AIzO3(P) sample, a new reduction peak at 480~ is clearly observed. The position of this peak corresponds well with the reduction of octahedral Mo species supported on HNaY, where the surface agglomeration of anionic Mo species takes place during the impregnation of ammonium heptamolybdate [9]. This catalyst presents the highest proportion of octahedral Mo species. In contrast, for the (NiMo/AIzO3)+HNaY(B) sample, an increase in the intensity of hightemperature reduction peak, corresponding to tetrahedral Mo species, is observed (curve c,

271 Fig. 2). This increased proportion of tetrahedral Mo species is characteristic for low Mo loading MoO3/AIaO3 catalysts. It seems that in the (NiMo/AIaO3)+HNaY(B) catalyst the migration of Mo species towards the surface of alumina used as a binder takes place. In the case of the (NiMo/AIzO3)+HNaY(MM) catalyst the proportion of octahedral and tetrahedral Mo species is almost the same as in the initial NiMo/AlzO3 catalyst. Therefore, 'no significant migration of the Ni and Mo species is suggested.

// 300

200

d

J .~ = 1 o0

0 m

0

I

200

~

I

,

I

,

I

400 600 800 Temperature (~

t

//

,

1000

Fig. 2. TPR patterns of NiMo catalysts: (a) NiMo/Al203, (b) NiMo/HNaY-AlzO3(P), (c) (NiMo/AIzO3)+HNaY(B) and (d) (NiMo/AlzOa)+HNaY(MM). Isothermal period (1000~ after the axis break. Clearly, the changes in the characteristics of metal oxidic species in the catalysts, induced by the differences in the catalyst preparation procedure, may alter the morphology and dispersion of the final active MoSz phase and, therefore, the amount of catalytic sites. Therefore, chemisorption of NO was used to quantify the CUS (coordinatively unsaturated Mo sites) supposed to be the active sites of hydrotreatment catalysts [12]. The NO chemisorption results (Table 3) indicate that the amount of NO adsorbed on the three zeolite-alumina-containing catalysts is intermediate between NiMo/AlzOa and NiMo/HNaY samples. The results show that for the (NiMo/AlzO3)+HNaY (MM) and (NiMo/AlzOa)+HNaY(B) catalysts, the number of the accessible MoS2 active sites is lower when the binder is used in the preparation of the catalyst. The lowest number of active sites observed for the (NiMo/AIzO3)+HNaY(B) catalyst can be due to: i) the highest proportion of difficult to sulfide tetrahedral Mo species in strong interaction with the alumina support, observed by TPR, and ii) the partial coverage of surface Ni and Mo species by the alumina binder. The NiMo/HNaY-AIzOa(P) catalyst, prepared by the impregnation of Ni and Mo species on the HNaY-AIzO3(P) support, shows the highest amount of chemisorbed NO

272 among zeolite-alumina-containing samples, in line with the observed highest proportion of easy to sulfide octahedral Mo species. 3.2. Hydrodesulfurization activity The catalytic activity tests indicate that the activity in DBT hydrodesulfurization decreases in the following order: NiMo/AlzO3 > NiMo/HNaY-AIzO3(P) > (NiMo/AIzO3)+HNaY(MM) > (NiMo/AlzOa)+HNaY(B) > NiMo/HNaY (Table 3). This activity order correlates well with the amount of the MoSz active sites determined by NO chemisorption. For the transformation of DBT, no clear effect of the presence of zeolite on the total conversion is observed. In the 4,6-DMDBT HDS reaction, the highest conversion was found for (NiMo/AIzO3)+HNaY(B) catalyst, followed by the (NiMo/AIzO3)+HNaY(MM) formulation. In this case, there is no clear correlation between the observed catalytic activity and the amount of the MoSz active sites determined by NO chemisorption, or the number of the accessible acid sites of the zeolite. It is known that in this reaction, both types of active sites, metal sulfide and acidic, participate in the transformations of the methyl-substituted DBT molecules [5, 6]. Products formed on one type of sites can be subsequently transformed on the other. Therefore, it could be supposed that for the best catalytic activity in the hydrodesulfurization of alkyl-substituted DBT derivatives, an optimum ratio between acid and metal sulfide functions must be attained in the catalyst. Additional information about the catalytic performance of the catalysts can be obtained from the analysis of the product distribution, which is affected by the metallic and acid functionalities. Tables 4 and 5 compare the product distributions obtained in the DBT and 4,6-DMDBT reactions with the NiMo/AI203, NiMo/HNaY and NiMo catalysts with 20% of HNaY in their formulation. In the case of DBT, zeolite incorporation into the catalyst changes the contributions of the direct desulfurization (DDS) pathway, which yields biphenyl-type compounds, and of the desulfurization through hydrogenation (HYD) pathway, which gives cyclohexylbenzene-type compounds. Also, the proportion of CHB in the reaction products and the liquid yield decrease with the number of accessible zeolite acid sites in the catalyst. This effect is due to the cracking of CHB on the zeolite acid sites. On the other hand, the formation of DCH is enhanced on the catalysts where Mo precursor phase is more polymerized (NiMo/HNaY-AIzO3(P) and NiMo/HNaY formulations). In the case of the 4,6-DMDBT the DDS pathway was much more inhibited giving rise to the formation of hydrogenated products in larger proportion. The participation of the acid function of the zeolite is evidenced by the formation of significant amounts of cracking products like toluene, benzene, cyclohexane and light products and in the appearance of different isomers of MCHT and DMDCH.

273

Table 3 NO chemisorption results and catalytic activity of NiMo catalysts in the DBT and 4,6DMDBT hydrodesulfurization reactions ........ DBT Conversi6n (%) Catalyst

pmol NO/g

NiMo/AI203 NiMo/HNaY- A1203 (P) (NiMo/AIzO3)+HNaY(B) (NiMo/AI203) +HNaY (MM) NiMo/HNaY

130.5 51.1 45.8 49.6 25.5

DMDBT Conversi6n (%)

4h*

8h

4h

8h

57.5 57.0 45.6 56.6 25.0

90.9 90.7 82.0 88.5 42.0

27.7 26.4 43.7 38.7 18.3

61.1 58.5 65.3 61.3 36.3

* Reaction time Table 4 Product distribution (wt. %)* and liquid yield in the DBT hydrodesulfurization reaction Catalyst Compound**

NiMo/HNaY (NiMo/AI203) NiMo/HNaY +HNaY(MM) -A1203(P)

NiMo/AI203

(NiMo/AI203) +HNaY(B)

BP CHB DCH CH BZ LP

71.32 27.72 0.96 -

81.50 16.11 1.25 0.22 0.74 0.19

74.51 12.63 2.78 1.85 4.32 3.93

82.88 8.99 1.84 0.83 2.59 2.87

34.95 35.61 6.00 23.44

Liquid yield (%)

95.8

87.3

84.8

82.5

80.0

* At 60% DBT conversion for all the catalysts with the exception of NiMo/HNaY where DBT conversion was 42%. ** BP, biphenyl; CHB, cyclohexylbenzene; DCH, dicyclohexyl; CH, cyclohexane; BZ; benzene; LP, light products (C3-C6) 4. C O N C L U S I O N S The results obtained in the present work indicate that the method used for the catalyst preparation leads to some changes in the characteristics and performance of the obtained catalytic formulations. These changes are due to differences in the characteristics of both the deposited metallic species (their coordination state, location and dispersion) and the contribution of the acidic component of the catalyst (number of accessible acid sites and partial destruction of zeolite structure). Therefore, the importance of the method used for the preparation of zeolite-containing catalysts is clearly observed.

274 Table 5 Product distribution (wt. %)* and liquid yield in the 4,6-DMDBT HDS reaction Catalyst Compound** NilVIo/AI203 NiMo/AI203 NiMo/HNaY NiMo/AI203+H NiMo/HNaY +HNaY(B) -AI203(P) NaY(MM) THDMDBT HHDMDBT DMBP MCHT DMDCH BP CHB DCH TL CH BZ LP

4.64 1.05 27.70 54.37 7.94 1.45 2.64 0.21 -

6.04 1.41 24.28 16.38 5.99 3.04 0.95 5.55 1.23 35.13

6.30 1.28 39.42 26.53 4.15 0.56 0.59 1.16 13.42 0.64 0.99 4.95

36.69 22.72 3.35 3.02 2.53 3.14 1.79 26.75

6.45 0.35 51.43 0.17 8.22 6.52 4.49 22.4

Liquid yield (%)

92.5

87.5

90.8

82.5

80.0

* At 60% of 4,6-DMDBT conversion for all the catalysts with the exception of NiMo/HNaY where 4,6-DMDBT conversion was 36%. ** THDMDBT, tetrahydrodimethyldibenzothiophene; HHDMDBT, hexahydrodimethyldibenzothiophene; DMBP, dimethylbiphenyl; MCHT, methylcyclohexyltoluene; DMDCH, dimethyldicyclohexyl; BP, biphenyl; CHB, cyclohexylbenzene; DCH, dicyclohexyl; TL, toluene; CH, cyclohexane; BZ, benzene; LP, light products (C3-C6) ACKNOWLEDGEMENTS Financial support by DGAPA-UNAM (grant IN-103599), DGEP-UNAM, CONACyTCSIC program and IMP-FIES program are gratefully acknowledged. We would like to thank M. Cecilia Salcedo for obtaining XRD patterns.

REFERENCES 1. K.G. Knudsen, B.H. Cooper and H. Topsoe, Appl. Catal. A: General, 189 (1999) 205. 2. R. Shaft and G.J. Hutchings, Catal. Today, 59 (2000) 423. 3. F. Bataille, J.-L. Lemberton, P. Michaud, G. P~rot, M. Vrinat, M. Lemaire, E. Schulz, M. Breysse and S. Kasztelan, J. Catal., 191 (2000) 409. 4. T. Isoda, S. Nagao, X.L. Ma, Y. Korai and Y. Mochida, Energy Fuels, 10 (1996) 482. 5. M.V. Landau, D. Berger and M. Herskowitz, J. Catal., 159 (1996) 236. 6. P. Michaud, J.L. Lemberton and G. P6rot, Appl. Catal. A: General, 169 (1998) 343.

275 7. T. Klimova, D. Soils, J. Ramirez and A. L6pez Agudo, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 127 (1999) 373. 8. A. Lopez Agudo, R. Cid, F. Orellana, J.L.G. Fierro, Polyhedron, 5 (1986) 187. 9. Y. Okamoto, Catal. Today, 39 (1997) 45. 10. C.A. Emeis, J. Catal., 141 (1993) 347. 11. R. L6pez Cordero and A. L6pez Agudo, Appl. Catal. A: General, 202 (2000) 23. 12. L. Portela, P. Grange and B. Delmon, Catal. Rev.-Sci. Eng., 37 (1995) 699.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

277

Chiral dirhodium catalysts confined in porous hosts H.M. Hultman a'b, M. de Lang a, M. Nowotny a, I.W.C.E. Arends b, U. Hanefeld a, R.A. Sheldon b, T. Maschmeyer a* Delft University of Technology, Applied Organic Chemistry and Catalysis a and Biocatalysis and Organic Chemistry b, Julianalaan 136, 2628 BL Delft, The Netherlands. By immobilising homogeneous chiral catalysts on the inner surface of porous solids, considerations like separability, re-use and selectivity may be addressed simultaneously [1]. Dirhodium carboxamide complexes, Rhz(MEPY)4 and Rhz(BNOX)4, were attached to the surfaces of MCM-41 and silica through ligand exchange with carboxylic acid functionalised tethers. Their activity was probed in the cyclopropanation of styrene with ethyl and tert-butyl diazoacetate, as well as in the Si-H insertion of dimethylphenylsilane with methyl phenyldiazoacetate. Improvements found in regio- and enantioselectivity are discussed in terms of steric constraints imposed by the surface. 1. I N T R O D U C T I O N Metal complexes have been immobilised in many different ways, e.g.: covalent anchoring (by grafting or tethering) to inorganic supports, immobilisation by occlusion in zeolitic micro- or mesopores (ship-in-a-bottle concept), or as supported liquid-phase catalysts [2]. In recent reviews the potential of (chiral) metal complexes immobilised by these different methods has been evaluated [3,4,5]. Although silica has flexible Si-OH groups that are readily functionalised, its large pore size distribution prevents any confinement effects from occurring to a significant degree. An alternative carrier structure is MCM-41 [6,7], which is a mesoporous silica or aluminosilicate material. It has large channels ranging from 15 to 100 *, ordered in an hexagonal array [8], which can be prepared with an almost uniform pore size. Because of its mesopores, MCM-41 offers new opportunities for the encapsulation of large catalyst species, and for the catalytic conversion of substrates too large to fit into zeolites [9]. In this study chiral dirhodium catalysts (Rh2(MEPY)4) and Rh2(BNOX)4), developed by Doyle [10] (see Scheme 1) were immobilised. It can be anticipated that the spatial constraints induced by the carrier (MCM-41 or silica), and especially by the pores of MCM-41, are able to increase the influence of the chiral ligands. Earlier research [11,12] showed that enantioselective reduction catalysed by a palladium complex immobilised inside the pores of MCM-41 resulted in a threefold increase in enantioselectivity compared to the homogeneous palladium complex. In order to immobilise the homogeneous catalysts on the surface, an organic linker group was * The NRSCC (H.H.) and the KNAW (U.H.) are acknowledged for financial support.

278 introduced. The (inner) surface silanol groups were readily functionalised with tethers bearing a carboxylic acid group. Immobilisation proceeded via exchange of a carboxamide ligand.

x N

Y ....

Y....

oi,, Y y _l./O1[..-a ' ' J ' / + \. /R,h~Hh E X ~ . . ~ O O NI Y "~"~X

--O~ MCM-41 _ v/ or SiO2

I O I..N ~,. fRh~ .Rh toluene y---N:,,....IIO I RT, 5 days L'xr ON / N

/O si_R_c ,,

--O

.....

y \

OH

Y

C I R I Si /IX OOO I I I

R = (0H2)2 R = (0H2)3 R = P'C6H4

L = MEPY; X=CH 2, Y=COOMe

MCM-41 or SiO2

L = BNOX; X=O, Y=CH2Ph

Scheme 1. Immobilisation of dirhodium complexes on different carriers By introducing three different types of tethers (-(CH2)2-, -(CH2)3-, and -p-C6H4-, scheme 1) their influence on the catalyst could be evaluated. All silanol groups were silylated after the introduction of the tethers and before the immobilisation of the catalysts. Chiral dirhodium carboxamide complexes have attracted considerable interest as enantioselective catalysts [13]. As model reactions the Si-H insertion of methyl phenyldiazoacetate (1) with dimethylphenylsilane (2) (reaction (1)) as well as the cyclopropanation of styrene (4) with diazoacetates (5a/b) (reaction (2)) were investigated (scheme 2).

[•/COOMe +

Me,, , , P h Me/Sill

N2 1

@

C--C + 4

catalyst

CH2Ci2 ~. N2, reflux

2

f?

(1)

3

cata,,st

N2CH--C--OR 5a/b a: ethyl, b: tert-butyl

Scheme 2. Catalytic test reactions

COOMe H

Me2PhSr

~ RT/reflux N2

,~, cis 6a/b

+

* COOR trans 6a/b

(2)

279 2. E X P E R I M E N T A L SECTION

2.1. Materials and methods All reactions and manipulations were performed under an atmosphere of dry nitrogen using standard Schlenk-type techniques. Silica sources for the MCM-41 synthesis were CabO-Sil M5 (fumed silica, Fluka) and a solution of sodium silicate (14% NaOH, 27% SiO2, Aldrich). All other reagents were purchased from Aldrich, Acros or Baker and were used without further purification. Trans/cis ratios for cyclopropanes were determined by GC analysis using a CP-wax 52 CB column (50 m*0.53 mm, 2.0 om (film thickness)). Chiral GC analysis for cyclopropanes was performed using a B-DA or B-PH column (40m*0.25mm) at 110~ HPLC analysis for Si-H insertions was performed using a chiral OD-column (25 cm), using 98/2 hexane/isopropanol at 1 ml/min, with 254 nm UV detection. Rhodium contents were analysed by ICP-OES after dissolving the solid samples in l%v/v HF and 1.3%v/v H2804 in water. Loading of the COOH-tether was determined by CHN-elemental analysis of the corresponding CN-tether. Full conversion from CN to COOH was determined by IR analysis on a Perkin Elmer Spectrum One FT-IR spectrometer. BET surface analysis was performed by N2 adsorption at 77 K on a Quantachrome Autosorb-6B after drying the samples at 200~ in vacuum. XRD patterns were recorded using CuKz radiation on a Philips PW 1840 diffractometer equipped with a graphite monochromator. The samples were scanned in the range of 0.105 to 50.005~

2.2. Catalyst Preparation General synthesis of carrier with acid tether: 4.08 g MCM-41 (prepared according to Beck et al. [7] was activated at 200~ in vacuo for two hours. The outer surface silanol groups (10 % of the total amount of silanol groups) were protected by reaction with dimethoxydimethylsilane (0.197 g, 1.63 mmol) in refluxing toluene (40 ml). After 3 hours 1.04 mmol 4-(trichlorosilyl)butyronitrile or 3-(triethoxysilyl)propionitrile (equivalent to 10 % of the theoretical number of surface silanol groups) was added and the mixture was refluxed overnight. The remaining (inner surface) silanol groups were then protected by addition of dimethoxydimethylsilane (2.30 g, 0.0192 mol), and the mixture was refluxed for three hours. The solid was filtered off, washed with water and ethanol and dried at 100~ in vacuo. Hydrolysis of the nitrile was achieved by addition to 50% aqueous sulphuric acid and heating for two hours at 150~ After filtration, the solid was washed with water until the filtrate was neutral. It was then dried overnight at 80~ in vacuo. For silica samples (Aerosil 200) the same procedure was applied, only the second step (protection of the outer surface) was omitted. General immobilisation of Rh-complex: MCM-41-(CHz)3COOH (0,0397 g, 3,14"10 .5 mol carboxylic acid groups) and Rhz(5R-MEPY)4 (0,0235 g, 3,03"10 .5 mol) were stirred at room temperature for two days in toluene (6 ml). The resulting solid product was Soxhlet extracted with dichloromethane until the washings were colourless and then dried in vacuo.

2.3. Catalytic Procedures General Cyclopropanation Procedure: 0.2 g chlorobenzene, 3 ml dichloromethane and 4 (0.490 g, 4.70 mmol) were added to the catalyst (1 mol% maximum Rh-loading) and the mixture was stirred. Over a period of 3-5 hours a solution of 5a (0.0517 g, 0.453 mmol) or 5b (0.0599 g, 0.461 mmol) in 3 ml dichloromethane was added. After stirring overnight at room temperature (5a) or under reflux (5b), the solvent was evaporated in vacuo and the residue was chromatographed (silicagel, hexane/ethyl acetate 9/1). Before chiral GC analysis,

280 6a was converted into the corresponding methyl esters by treatment with a 0.1 molar solution of NaOH in MeOH. General Si-H insertion procedure: 0.2 g 1,2-dichlorobenzene and 1 ml dichloromethane were added to the catalyst (2 mol% maximum Rh-loading) and the mixture was stirred. Subsequently 1 (0.0973 g, 0.56 mmol) in 0,5 ml dichloromethane and 2 (0.0832 g, 0.62 mmol) in 1 ml dichloromethane were added. The resulting mixture was refluxed overnight. After evaporation of the solvent in vacuo, the reaction mixture was chromatographed (silicagel, petrolether/ethyl acetate 19/1). Leaching test: The cyclopropanation reaction was performed according to the general procedure. After stirring overnight at room temperature, the solid catalyst was allowed to settle. The supernatant solution was transferred to another vial through a syringe filter to remove traces of immobilised catalyst. 5a (0,2195 g, 1,92 mmol) was added to the filtrate and two minutes later a GC sample was taken. The mixture was left to stir overnight. Samples were taken after 15,5 h and 87,5 h. Recycling test: The cyclopropanation reaction was performed following the general procedure. After stirring overnight at room temperature, the solid catalyst was allowed to settle. The supernatant solution was transferred to another vial through a syringe filter to remove traces of immobilised catalyst. The rhodium content of this solution was determined by AAS. The trans/cis ratio of the products and the conversion were determined by GC. The remaining solid was washed with dichloromethane and dried. It was then used again following the same procedure. After three cycles, the rhodium content of the catalyst was determined by ICP OES. 3. RESULTS AND DISCUSSION 3.1. Catalyst preparation The preparation procedures of the carriers are outlined in Scheme 3. The alkyl tether groups were attached to the surface by the reaction of 3-(triethoxysilyl)propionitrile or 4(trichlorosilyl)butyronitrile with MCM-41 or silica. The nitrile function was hydrolysed with 50% aqueous sulphuric acid. The -p-C6H4COOH tether group was prepared by treating MCM-41 or silica with [4-(dimethoxymethyl)phenyl]trimethoxysilane. This acetal was then hydrolysed with 5% aqueous trifluoroacetic acid. This aldehyde, thus released, was finally oxidised with peracetic acid. Subsequently, the residual surface silanol groups were protected with dimethoxydimethylsilane. From C,H,N analysis of the CN-tether, the loading could be determined. IR analysis of the COOH-tether showed that no CN-tether was present after hydrolysis. CP MAS solid state NMR confirmed the presence of the tethers on the surface of the carriers. The analysis of the immobilised complexes by CP MAS NMR is hampered by the low loading of the complex on the carrier. Only very broad peaks with low intensity were observed. In order to validate the exchange of ligand as a way of immobilisation on the carriers, the model reaction of Rh2(5S-MEPY)4 with one equivalent acetic acid was followed by liquid 1H NMR [14]. Initially, the carboxylic acid exchanged with the axial ligands. This reaction was followed by exchange of chiral ligand with the acid. FAB-MS analysis indicated that more than one chiral ligand might be exchanged with an acid ligand. The theoretical maximum loading of dirhodium catalyst on the carrier (Table 1) was calculated from the loading of the CN-tether (determined by C,H,N analysis). Actual rhodium loadings are usually around 70% of the CN-loading, but drop to 15% in some cases.

281

-OH ~ ,O1~ MCM-41 --OH + (MeO)3Si---k~,~)/~---C,` -OH ~ OMe Silica

-OH surface --OH + --OH

___/~/CN

(EtO)3Si

,_ peracelJ&acid

,.~

"-CF3CO~

H20

H2SO ~ ~

"

"MCM-41

Silica

7si

_/~COOH

surface

Scheme 3. Synthesis of the different functionalised carriers Table 1. Immobilised catalysts used in this study with the maximum loading of dirhodium complex entry catalyst maximum loading (mmol catalyst/g) 1 MCM-41- (CH2) 2COO-Rh2 (5S-MEPY) 3 0,81 2 Si02-p-C6H4COO-Rhz(5S-MEPY)3 0,12 3 SiO2- (CH2) 2COO-Rh2 (5S-MEPY) 3 0,064 4 5

MCM-41 -p-C6H4COO-Rh2 (4S-BNOX) 3 SiOz- (CH2) 2COO-Rh2 (4S-BNOX) 3

0,80 0,085

6 7 8 9

MCM-41- (CHz)zCOO-Rh2 (4R-BNOX) 3 SiO2-p-C6H4COO-Rh2 (4R-BNOX) 3 SIC2- (CHz)2COO-Rha (4R-BNOX)3 SIC2- (CH2)3COO-Rh2 (4R-BNOX) 3

0,81 0,12 0,064 0,057

The nitrogen desorption measurements showed that the surface area, the total pore volume and the pore size decreased after attachment of the tether (Table 2). The XRD plots (Scheme 4) indicate, together with the N2 desorption experiments, that the structure of MCM41 remained intact when the CN-tether was introduced. The decrease in pore volume can be attributed to the presence of the tether inside the pore. However, after heating in aqueous sulphuric acid, the pore structure was partially damaged. However, 60% of the channel structure remained intact (determined by the decrease of the total pore volume, see Table 2, entry 2 and 3). Table 2. Results N2 desorption measurements entry sample ................... SBET(m2/g) 1 MCM-41 960 4- 13 2 MCM-41-(CH2)3CN 825 + 21 3 MCM-41-(CH2)3COOH 566 + 10

t0ial' pore vo]um'e""(cm3/g) 1.01 0.78 0.41

pore size (nm) 2.4 2.1 1.9

282

I

- - - - MCM-41 MCM-41-(CH2)3CN

8

--

10

20

30

40

MCM-41-(CH2)3COOH

50

2 theta

Scheme 4. Comparison XRD plots of MCM-41, (CHz)3COOH

MCM-41-(CHz)3CN and MCM-41-

3.2. Si-H insertion test reactions

In the Si-H insertion reaction (reaction 1) significant differences can be observed between the homogeneous and immobilised catalysts (Table 3). The homogeneous catalysts display no (entry 1) or low (entry 6) enantioselectivity. In the case of the catalysts immobilised on silica, however, the selectivities increased more than 10-fold (entries 2,3). This is clear evidence that despite the loss of one chiral ligand due to the method of immobilisation, the spatial confinement leads to a significant improvement of enantioselectivity. In contrast to the catalysts immobilised on silica, none of the catalysts immobilised inside MCM-41 showed significant activity. Even after refluxing overnight, large amounts of unmodified 1 remained. Possibly there is not enough space inside the pores of MCM-41 for the reaction to take place. Indeed the average pore diameter is 19 * (determined by nitrogen desorption analysis) and the catalyst size is similar (approximately 15 A). A transition state requiring a space demanding conformation might therefore be too constrained under these circumstances. Table 3. Results Si-H insertion reaction (1) entry catalyst Rhz(4R-BNOX)4 SiO2- (CH2) 2COO-Rh2 (4R-BNOX) 3 SiOz-p-C6H4COO-Rhz(4R-BNOX)3 MCM-41-(CHz)zCOO-Rhz(4R-BNOX)3 MCM-41-p-C6H4COO-Rhz(4S-BNOX)3 6 7 8

Rhz(5S-MEPY)4 SiO2-(CHz)zCOO-Rhz(5S-MEPY)3 Si02-p-C6H4COO-Rhz(5S-MEPY)3

yield of 3 (%) 73 88 67 only traces of product detected only traces of product detected

ee (%) 2 20 28 -

70 78 65

37 2 1

.................

Recycling experiments were performed for the Si-H insertion (reaction 1). Since all catalysts were immobilised in the same manner it can be assumed that these results are

283 representative for all the different immobilised catalysts described here. The yield of 3 decreased from 79% in the first cycle, to 31% in the second and 19% in the third cycle. After the third cycle, the liquid phase was removed from the solid catalyst and its activity was investigated. This solution did not catalyse the Si-H insertion. A rhodium analysis of the catalyst (SiO2-(CH2)2COO-Rh2(4S-BNOX)3) showed that no rhodium was left on the carrier. Further studies into the stability of this catalytic system are underway.

3.3. Cyclopropanation test reactions In the case of the cyclopropanation reaction (reaction 2) the trans/cis ratio is determined by the steric repulsion between the phenyl group of the styrene and the ester group of the intermediate carbene. In the homogeneous reaction, the small ligand does not significantly restrict the incoming styrene, and thus the difference in the formation of trans and cis product is not very large (Table 4, entries 1,4,7,10). In the heterogeneous reaction the steric hindrance by the bulky carrier surface forces the carbene slightly out of plane, away from the carrier surface. This, and the bulk of the ester group, directs the incoming styrene in such a way that more trans compound is formed than in the homogeneous reaction. If the more bulky tert-butyl diazoacetate (TBDA, 5b) is used instead of ethyl diazoacetate (EDA, 5a), both effects are even more pronounced. If the catalysts are immobilised on silica the effect is smaller than if the catalysts are immobilised inside MCM-41. This is due to the confinement in the pores that restricts the incoming styrene even more than the silica surface. A loss in enantioselectivity can be attributed to the loss of one chiral ligand during the immobilisation. The influence of the tether group was not significant. Only minor differences in selectivity were obtained. Table 4. Comparison EDA and TBDA inthe Cyc!0pr0panation reaction (2) entry catalyst diazo compound

yield 6a/b

trans/cis

(%) 1 2 3

Rhz(5S-MEPY)4 SiO2- (CH2)2COO-Rh2 (5S-MEPY) 3 MCM-41-(CHz)2COO-Rh2(5S-MEPY)3

5a 5a 5a

59 73 65

56/44 59/41 60/40

4 5 6

Rh2(5S-MEPY)4 SiO2- (CH2)2COO-Rh2(5S-MEPY) 3 MCM-41- (CH2)2COO-Rh2(5S-MEPY) 3

5b 5b 5b

50 62 50

60/40 71/29 74/26

7 8 9

Rh2(4R-BNOX)4 SiO2- (CH2) 2COO-Rh2(4R-BNOX) 3 MCM-41-(CH2)2COO-Rh2(4R-BNOX)3

5a 5a 5a

79 84 51

46/54 60/40 70/30

10 11 12

Rh2(4R-BNOX)4 SiO2- (CH2) 2COO-Rh2 (4R-BNOX) 3 MCM-41- (CH2) 2COO-Rh2 (4R-BNOX) 3

5b 5b 5b

64 53 51

59/41 66/34 72/28

3.4. Cyclopropanation leaching and recycling In order to evaluate whether any active rhodium complex leaches from SiO2(CH2)aCOO-Rh2(4R-BNOX)3 during the cyclopropanation reaction (reaction 2), the filtrate of a reaction mixture was tested for its catalytic activity. This filtrate displayed modest activity:

284 34% of the diazo compound was still present after 16 hours, decreasing to 11% after three more days. In comparison, in a typical cyclopropanation experiment, the conversion was already 100% after one minute. The activity of the filtrate is therefore approximately only 0.5% of the activity of the immobilised catalyst. Only a very small amount of catalytically active material leaches during reaction and the actual catalysis is performed by the heterogenised complex. In addition, we performed recycling experiments for the same reaction (SiO2C6H4COO-Rhz(5S-MEPY)3 as catalyst). The results (Scheme 5) show that the second and third cycle proceed more slowly. However, after stirring overnight, all diazo compound is consumed. From the results of the leaching and recycling experiments combined with rhodium analysis of the catalyst before and after recycling, it can be concluded that the catalyst remains active even after recycling for three times. However, 50% of the rhodium leached.

] 9

I.u,.. 40 60 . ~ "~

m -,,,_ cycle1 - -~ - cycle

o 200

0

i

200

i

400

i

600

l

800

i

1000 1200

reaction time (minutes)

Scheme 5: Recycling of SiO2-C6H4COO-Rh2(5S-MEPY)3 in the cyclopropanation of styrene with EDA 4. C O N C L U S I O N The immobilisation of the homogeneous dirhodium catalysts on silica surfaces and inside the pores of MCM-41 affords a significant improvement in regioselectivity (cyclopropanation reaction) and enantioselectivity (Si-H insertion) of these catalysts. The improvement is attributed to the confinement resulting from immobilisation. From leaching tests it was established that the reactions performed were indeed heterogeneous. The immobilised catalysts have been successfully re-used for three times in the cyclopropanation reaction. In the Si-H insertion the catalyst however, is less stable. The increased temperature utilised for this reaction is probably responsible for this difference. REFERENCES

1. J.M. Thomas, T. Maschmeyer, B.F.G. Johnson and D.S. Shephard, J. Mol. Catal. A, Chem., 141 (1999)139. 2. H.H. Wagner, H. Hausmann and W.F. H61derich, J. Catal., 203 (2001) 150 and references cited therein.

285 3. A. Baiker, Curr. Opin. Solid State Mater. Sci., 3 (1998) 86. 4. T. Bein, Curt. Opin. Solid State Mater. Sci., 4 (1999) 85. 5. K. Fodor, S.G.A. Kolmschot and R.A. Sheldon, Enantiomer, 4 (1999) 497. 6. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli and J.S. Beck, Nature, 359 (1992) 710. 7. J.S. Beck, J.C. Vartulli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 8. A. Corma, Chem. Rev., 97 (1997) 2373. 9. C. Huber, K. Moiler and T. Bein, J. Chem. Soc., Chem. Commun., (1994) 2619. 10. M.P. Doyle, W.R. Winchester, J.A.A. Hoorn, V. Lynch, S.H. Simonsen and R. Ghosh, J. Am. Chem. Soc., 115 (1993) 9968. 11. B.F.G. Johnson, S.A. Raynor, D.S. Shephard, T. Maschmeyer, J.M. Thomas, G. Sankar, S. Bromley, R. Oldroyd, L. Gladden and M.D. Mantle, Chem. Commun., (1999) 1167. 12. S.Raynor, J. Thomas, R. Raja, B. Johnson, R. Bell and M. Mantle, Chem. Commun., (2000) 1925. 13. M.P. Doyle, in: Comprehensive Organometallic Chemistry II. A review of the literature 1982-1994, E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds), Volume 12, Transition Metal Organometallics in Organic Synthesis, Elsevier Science Ltd., Pergamon Press, 1995. 14. Manuscript in preparation.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

287

Synthesis and characterization of zeolite encaged enzyme-mimetic copper histidine complexes J. Gerbrand Mesu*, Debbie Baute*, Henk J. Tromp*, Ernst E. van Faassen* and Bert M. Weckhuysen *'w Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB, Utrecht, The Netherlands * Centrum voor Oppervlaktechemie en Katalyse, Departement Interfasechemie, K.U. Leuven, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium * Interface Physics, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA, Utrecht, The Netherlands Histidine was complexed with copper cations and immobilized in zeolite Y by an ion exchange procedure. The influence of the initial copper concentration in the ion exchange solution and the ion exchange time on the structure of the final zeolite encaged copper complexes was studied. Two different copper complexes were found on the zeolite: a mono-histidine complex (A) and a bis-histidine complex (B). The complex ratio A/B decreases with increasing copper loading in the ion exchange solution. The dynamics of the exchange was also studied. The A/B ratio does not change during this process. The exchange process itself is very fast, as it takes place within an hour. After that the it is slowed down by orders of magnitude. 1. I N T R O D U C T I O N Histidine, a naturally occurring amino acid, was complexed with copper cations and incorporated inside the supercages of zeolite Y [1,2]. The obtained complexes mimic the active center of natural enzymatic counterparts, such as galactose oxidase. The pore geometry of the zeolite induces shape selectivity in reactions and allow for intra-particle transport of reactants and products. The zeolite host material also induces additional stability of the incorporated active center, thereby expanding the range(s) of operating temperatures and pressures as well as solvents. The zeolite encaged copper histidine (Curtis) complex has already been shown to exhibit promising catalytic activity for the epoxidation of alkenes with peroxides [3]. ESEEM experiments on the zeolite occluded Curtis systems showed the presence of (at least) two different Curtis complexes (A and B) on the zeolite [4,5]. Complex A exhibits 27A1 modulations, which indicates that the Cu(II) coordinates to zeolite framework wCorresponding author, e-maih [email protected] Tel: +31-30-2534328, Fax: +31-30-2511027

288 oxygen. Complex B, however, shows no 27A1 modulation. Complex A is proposed to be a mono-histidine complex where both the amino and imino nitrogens of the histidine are coordinated to Cu 2+, whereas the other equatorial ligands are provided by a zeolite oxygen and a water molecule. The complex is stabilized by the presence of protons in the zeolite framework. Complex B is proposed to be a bis-histidine complex, situated in the center of the supercage. The two histidine molecules coordinate to Cu 2+ in a square planar geometry: the amino and imino nitrogens of one histidine molecule and the imino nitrogen and carboxylate oxygen of the second histidine molecule. A number of parameters might affect the A/B ratio. These parameters are: 1. The pH of the exchange solution; 2. The copper concentration in the ion exchange solution; 3. Duration of the ion exchange. The effect of the first parameter has been discussed in a previous paper [4]. In this paper, we investigate the effect of the other two parameters. Once it is possible to set the A/B complex ratio by tuning the synthesis conditions during the ion exchange procedure, the next step is to relate this ratio to catalytic yield and/or selectivity of the resulting heterogeneous Cu(II)-catalyst. 2. E X P E R I M E N T A L

2.1. Preparation Aqueous solutions of Curtis complexes, prepared in bi distilled water with a His:Cu(II) ratio of 5:1 at pH 7.3 were used for ion exchange with NaY (ZEOCAT, Si:A1 = 2.71). A series of zeolite samples, differing in their amount of Curtis complexes, were prepared using solutions with different copper concentrations (0.1, 0.25, 0.50, 1.0, 1.5 and 4.5 copper/unit cell (Cu/UC)), while keeping the His:Cu(II) ratio in the solution at 5:1 and the pH at 7.3. The pH was adjusted with 0.1 M NaOH and/or 0.1 M HC1 solutions. All samples were stirred for 24 hours at room temperature. The pH of the exchange solution was measured regularly and adjusted if needed. All samples were dried at 333 K after washing and filtration. The duration of ion exchange affects the copper concentration of the zeolite: longer exchange times lead to higher copper concentration. The exchange dynamics was studied by taking a series of zeolite samples from the ion exchange solution at different points of time. 2.2. Characterization CW-EPR X-band measurements were performed on a Bruker ESP 300E Spectrometer at a temperature of 120 K. The Curtis complexes are paramagnetic due to the S=1/2 spin of the Cu 2§ ion. Nitrogen physisorption was performed with a Micromeritics ASAP 2400 apparatus. Measurements were done at 77 K. Prior to the measurements the zeolite samples were degassed for 24 hours at 373 K in vacuum. Micropore volumes and pore size distributions were determined with standard BET and BJH theory. Diffuse Reflectance Spectroscopy of the Curtis complex encapsulated zeolite samples were taken on a Varian Cary 5 UV-Vis-NIR spectrophotometer at room temperature. The DRS spectra were recorded against a halon white reflectance standard in

289 the range 2500-200 nm. Atomic Absorption Spectrometry (AAS) measurements for quantitative analysis of Cu 2+ in the zeolite samples were performed using an Instrumentation Laboratory Inc. apparatus with a nitrous oxide-acetylene flame. Measurements were done at a wavelength of 324.7 nm using a hollow cathode lamp. The amount of Cu 2+ was determined after dissolution of known quantities of ion-exchanged zeolite materials in HF/HzSO4. 3. RESULTS AND DISCUSSION The X-band CW-EPR spectra, recorded at 120 K, are shown in Fig. 1. The EPR intensity is proportional to the number of copper ions taken up by the zeolite during the ion exchange. Each of the spectra consists of two distinct EPR subspectra, which can be attributed to two different complexes, viz. complex A and complex B.

.A 4.5 Cu."lJC 1.5 Cu/UC 1.0 Cu/UC 0.5 CLdt.JC 0.25 Cu/UC O. 1 Cu!UC

2.~

2.75

3.0

3.25

3.S

3.75

B (kG)

Fig. 1. CW-EPR spectra as a function of the external copper concentration in the ion exchange solution. A change in the relative amounts of these two subspectra becomes visible upon going to higher copper concentrations in the ion exchange solution. At low copper concentrations only the subspectrum of complex A is visible, but at higher copper concentrations also the subspectrum of complex B appears. The shape of the spectrum is particularly sensitive to the values of g//and A//. The larger g//and smaller A//of complex A indicate lower density of the unpaired electron at the site of the copper nucleus. It suggests that the electronic orbital is affected by fewer nitrogen atoms in the first coordination sphere of the Cu(II) ion compared to complex B. The EPR spectra can be simulated as a superposition of two different subspectra: A respectively B. The shape of each sub spectrum is chosen as a molecule with an axial Zeeman interaction plus an axial hyperfine interaction to the I = 3/2 copper nucleus.

290 Table 1.

Calculated EPRparame!ersof the zeolite-Y enc._a,~.s.u,!atedCuH~ c_0.mp!exes_ Complex A ComplexA ComplexB ComplexB gll

All

gll

All

2.32

154

2.27

173

100

o~

8O

o~ 6O

-'~'-Complex A ""D-Complex B

t~ t-

0 L

o.

40 20

O~ 0.0

2.0

4.0

Cu/UC in exchange solution

Fig. 2. Relative amounts of complex A and complex B on the zeolite as a function of the copper concentration in the ion exchange solution (estimated from EPR spectra). Hyperfine couplings to nitrogen are omitted because they are experimentally not resolved. The calculated values for g//and AH for both complexes are presented in Table 1. The experimental accuracy did not require simulation as a mixture of 63Cu/65Cuisotopes. The relative amounts of complex A and complex B on the zeolite can be estimated from the EPR spectra in Figure 1. These amounts are depicted in Fig. 2 as a function of the copper concentration in the ion exchange solution. At the lowest copper concentration only complex A is found on the zeolite. With increasing copper concentration in the ion exchange solution an increasing amount of complex B is found, with a maximum of approximately 40 % for the highest copper concentrations. The amount of copper in the zeolite was measured by quantitative analysis. The results are presented in Fig. 3 as a function of the copper concentration in the ion exchange solution. The EPR intensity also gives an indication of the amount of copper exchanged onto the zeolite. These results are also depicted in Fig. 3. At low copper concentrations (up to 1 Cu/UC) the amount of Cu(II) exchanged onto the zeolite increases linearly with the copper concentration. In this region, all the copper in the exchange solution is deposited on the zeolite. At higher copper concentrations in the exchange solution (above 1 Cu/UC), almost no extra copper can be deposited on the zeolite. An explanation for the observed behavior might be the congestion of the zeolite crystals by immobilized histidine or Curtis complexes in the outer pore system and supercages of the zeolite crystals.

291

07

2.0 j~) r

0

o6 x5

1.5 ~ m=

~4 c3

1.o

d~

~."

,-2 re1

0.5:3

LUO

o.o

.m=

0.0

O~

~"

1.0 2.0 3.0 4.0 Cu/UC in exchange solution

Fig. 3. EPR intensity and quantitative analysis (in Cu/UC) of the zeolite occluded copper complexes as a function of the copper concentration in the exchange solution.

0.35 0.34 0.33 "~ 0.32 o~, 0.31 .o 0.30 0 0.29 0.28 ~: =

0

1

2

3

4

Cu/UC in exchange solution Fig. 4. Micropore volume as a function of the copper concentration in the ion exchange solution. The process of ion exchange will affect the available pore volume inside the zeolite. The micropore volume has been measured by N2 physisorption. The N2 physisorption isotherms of the Curtis loaded zeolite samples are of Langmuir type I. The evolution of the micropore volume as a function of the initial copper concentration in the ion exchange solution is presented in Figure 4. The micropore volume decreases from 0.34 ml/g for a pure Y zeolite to 0.29 ml/g for the highest copper loading. The curve displays a bend in going from an initial copper concentration of 0.25 Cu/UC to an initial copper concentration of 0.50 Cu/UC. This phenomenon may be attributed to an increase in the relative amount of the more bulky complex B in the pore system of the zeolite.

292 E = E

760

"~

740

E ~,

720

o c

700

~0

680

" <1:

660

Q.

u)

0

1

2

3

4

Cu/UC in exchange solution

Fig. 5. Maxima of the absorption band in the DRS spectra of the Curtis complexes in zeolite Y as a function of the copper concentration in the ion exchange solution. 1.2

i

+'-9 1 . 0 -

o N

0.8

c0 0.6 = o

0.4

o

0.2 0.0

I

I

I

l

l

0

5

10

15

20

~

=~

25

Exchange time (hours) Fig. 6. Copper loading on the zeolite as a function of the duration of exchange reaction. External copper concentration is 1.5 Cu/UC. At higher copper loadings the micropore volume is decreasing with the higher copper concentrations. This indicates an increasing fill up of the pore system and/or an increasing congestion of the pore system. Diffuse Reflectance Spectroscopy (DRS) in the UV-Vis-NIR region was used to measure the ligand field strength of the different samples. The maximum of the d-d absorption band shifts from 758 nm to 675 nm with higher copper concentrations in the ion exchange solution (see Figure 5). This shows that the ligand field strength of the Cu(II) in complex B is higher than that in complex A. This is an indication for an increase in the number of nitrogens in the first coordination sphere of the Cu(II) ion and is in agreement with the trend observed in the EPR parameters. The influence of the ion exchange time on the A/B ratio in the resulting zeolite samples was studied by CW-EPR. It was found that this ratio was independent of the ion exchange time (data not shown). The results of the quantitative analysis of the samples are presented in Figure 6. In this figure the amount of copper exchanged onto the zeolite is represented

293 as a function of time. The figure shows that the ion exchange is a fast process. Most copper is exchanged onto the zeolite within the first hour of the ion exchange process. The exchange process slows down considerably in the later stages, presumably due to partial congestion in the pore system of the zeolite. 4. C O N C L U S I O N S Two different copper complexes were found on the zeolite, a mono histidine complex (A) and a bis-histidine complex (B). The influence of the initial copper concentration in the ion exchange solution on the A to B ratio was studied. A high A/B ratio was found at low copper concentrations and vice v e r s a. For high copper concentrations the ratio leveled off at a value of A/B - 1.5. Full exchange was only achieved at low concentrations up to 1.0 Cu/UC. At higher copper concentrations the copper was no longer totally exchanged on the zeolite and the A/B ratio became independent of the external copper concentration. We also studied the dynamics of exchange. The A/B ratio does not change during this process. The exchange process itself is very fast, as it takes place within an hour. After that the exchange process is slowed down by orders of magnitude. ACKNOWLEDGEMENTS We would like to thank D. Goldfarb (Weizmann Institute of Science, Israel) and R.A. Schoonheydt (K.U. Leuven) for valuable discussions. REFERENCES

1. D.E. De Vos, P.P. Knops-Gerrits, R.F. Parton, B.M. Weckhuysen, P.A. Jacobs and R.A. Schoonheydt, J. Inclus. Phen. Molec. Recogn. Chem., 21 (1995) 185. 2. B.M. Weckhuysen, A.A. Verberckmoes, I.P. Vannijvel, J.A. Pelgrims, P.L. Buskens, P.A. Jacobs and R.A. Schoonheydt, Angew. Chem. Int. Ed. Engl., 34 (1995) 2652. 3. B.M. Weckhuysen, A.A. Verberckmoes, L. Fu and R.A. Schoonheydt, J. Phys. Chem., 100 (1996) 9456. 4. R.A. Grommen, P. Manikandan, Y. Gao, T. Shane, J.J. Shane, R.A. Schoonheydt, B.M. Weckhuysen and D. Goldfarb, J. Am. Chem. Soc. 122 (2000) 11488. 5. P. Manikandan, B. Epel and D. Goldfarb, Inorg. Chem., 40 (2001) 781.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

295

S t r a t e g i e s for the heterogenization of rhodium complexes on a c t i v a t e d

carbon J.A. Diaz-Aufi6n 1' 2, M.C. Romhn-Martinez 1, C. Salinas-Martinez de Lecea I and H. Alper z. 1 Departamento de Quimica Inorg~nica, Facultad de Ciencias. Universidad de Alicante. Apdo. de Correos 99, Alicante 03080. Spain. z Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K 1N 6N5. The present communication reports our recent results on the preparation of heterogenized rhodium complexes as hydroformylation catalysts. The [Rh(p-CI)(CsHlz)]z complex (shortened here as Rh(COD)) has been used as Rh precursor. Several strategies have been covered: i) ion exchange of Rh(COD) on activated carbons; ii) addition of diphosphine ligands to increase stability; iii) functionalization of the supports to allow covalent bonds with phosphine ligands. The ion exchange method showed low stability and only in the first catalytic run gave a catalytic activity similar to that of the homogeneous Rh(COD) complex. With the added diphosphine ligands, the complexes were even less stable and less active. The functionalization of the support to bind the diphosphine ligands produces highly active catalysts stable in four consecutive catalytic runs. The best results are obtained when the Rh diphosphine complex is synthesised before the heterogenization step (ligand-support bonding). 1. INTRODUCTION An important research area of catalysis is the heterogenization of metal complexes showing excellent activity and selectivity in the homogeneous phase (hybrid catalysts) [1]. The main advantage of supported metal complexes over unsupported ones is the ease of separation from the reaction media, which makes them economically convenient [2]. On the other hand, the main problems related with the heterogenization on solid supports are metal complex leaching and decrease in catalytic activity. The hydroformylation of olefins is the most important industrial process using transition metal complexes as homogeneous catalysts. Rhodium complexes are the most efficient catalysts for this reaction in terms of both activity and selectivity [3]. Under hydroformylation conditions, leaching usually takes place by substitution of the :t-bonding ligands (like PR3, dienes, etc.) by CO, forming carbonyls that wash out easily from the support [2,4]. To enhance the anchorage, support functionalization (using phosphine, amine, silanol or sulphur groups) [5-7], and ion exchange processes (with ionexchange resins, clays, zeolites, etc. as supports) have been used [2, 8-10].

296 The present paper is a review of our study on the heterogenization of Rh complexes on activated carbon. The [Rh(~t-C1)(C8Hlz)]2 complex (named here Rh(COD)) is a wellknown, air stable, active catalyst for the hydroformylation of olefins [11]. It has been used in homogeneous phase and also supported by ion exchange on clay materials [12] and resins [7]. Rh(COD) is also a suitable precursor to get different diphosphine complexes only by addition of ligands [13]. Activated carbon was selected as the support because it has a large surface area and the surface chemistry can be easily modified [14]. This work is focused on the use of two different ways of supporting complexes on activated carbon: by ion exchange and by a grafting process that involves a covalent-type bond between support and ligand and/or complex. Three different strategies were investigated for the hydroformylation of 1-octene.

2. EXPERIMENTAL The activated carbons used as supports were ROX-0.8 and GF-45 (from Norit), and ROX-N and GF-N (obtained from ROX-0.8 and GF-45 by oxidation with HNO3 35%). Characterization of these carbons is reported in a previous publication [15]. The heterogeneous catalysts were prepared using different methods described in the following sections. The catalysts have around l wt.% Rh. The metal content was determined according to a procedure described in the literature [7]. Catalytic activity experiments were carried out in a stainless steel stirred tank reactor operated at 353 K and with a stirring velocity of 600 rpm. The hydroformylation of 1octene was carried out at a total pressure of 4-5 MPa (with H2:CO ratio of 1:1) in a solution of 5 vol% of the olefin in hexane. The reaction was carried out for seven hours. Reactants and products were analysed by gas chromatography and 1H NMR spectroscopy. Heterogeneous samples were characterized by 31p solid state NMR and XPS. 3. R E S U L T S AND DISCUSSION

3.1. Rh(COD) heterogenized on activated carbon by ion exchange In this case, attention is focussed on the effect of the support surface chemistry on the anchoring of the complex by ion-exchange. To favour the ion-exchange, the carbon surface has been oxidized to create surface oxygen complexes [16, 17]. The expected ion-exchange process is presented in Scheme 1. The complex was anchored by the adsorption from solution method. A solution of Rh(COD) in MeOH (in a concentration to get lwt.% of Rh in heterogeneous samples) was put in contact with the activated carbon. The HCI released (see Scheme 1) was followed by conductiometry. Table 1 shows some data corresponding to the heterogenized catalysts prepared with carbons ROX-0.8 and ROX-N. The conductivity measurements during the heterogenization process indicate that the complex was almost not ion-exchanged in the ROX-0.8 support, while it was largely exchanged in the functionalized carbon ROX-N. The displacement of the binding energy of Rh 3ds/z (determined by XPS) to higher values in sample Rh(COD)/ROX-N suggests that Rh is bonded to oxygen [18].

297 After reaction, the catalyst was recovered by filtration after releasing reactor pressure at the reaction temperature. Leaching is high in both catalysts, but it is lower in the oxidized sample. A test was carried out by exposing the samples to the solvent for 24h under He; in this case the complex is not leached from support ROX-N. This indicates that the anchoring is effective, and that leaching is due to the nature of the reaction. To decrease leaching, the method to recover the catalyst was modified: after reaction, the CO:H2 mixture was substituted by He (keeping the pressure) then, the system was cooled down and the catalyst was washed several times avoiding contact with air. A significant decrease of Rh loss was found (only 13%), making possible the reutilization of the catalyst.

z

MeOH 40 *C He

~CI j 2 HCi

I,.

o 0 Rh

,, Rh '

O

Scheme 1 Table 1. Catal~.sts characterization "Entry I Catalyst 1 ,, 2

I Rh(COD)/ROX-0.8 [ Rh(COD)/ROX-N

%Rh (wt.) 1.1 1.0

%Rh exchanged 6.7 63.6

b.e. (eV) (Rh 3d5/2) 308.6 309.5

%Rh Leached 58 48(13)

Fig. 1 shows 1-octene conversion versus time for the homogeneous and heterogenized complex on supports ROX-0.8 and ROX-N, for the last case, results in three consecutive reaction runs are included. Products are a mixture of linear (L) and branched (B) isomers of aldehydes. Selectivity at 2 and 7 h (expresed as L/B) is also included in the figure. In general, the curves present a shape with two steps with different rates. These data show that: i) in the first step (faster), the linear aldehyde is mainly obtained. Increase of the branched product is related to the easy isomerization of 1-octene to 2-octene in these kinds of complexes mainly when the diene ligands are replaced by CO; ii) the selectivity of these

298 systems at high conversion level is low [19]; iii) catalyst Rh(COD)/ROX-0.8 is even faster than the homogeneous complex, probably due its apolar nature [15] that can enhance a high substrate concentration on its surface. In relation with this, the sample Rh(COD)/ROX-N, with a more polar surface, needs an induction time; iv) in successive rims, probably due to leaching of the complex, the reaction is slower, and in the third run, the activity aider 7 h is similar to the activity at 2 h in the first rtm.

.-.

g e-

100 90

0.9

8o

._o 70 a~ >

60

(o

40

t-

30 20 10 0

to

a)

"6 O

/--

1.4

, , , ~ 1.6

50 2.5

,, ,, ~ 2 . 7 S

.

.

.

.

.

.

,j

. "7--

2 "

m,

, . i . .

-

m ' l l ' "

u"

I

I

I

I

I

3

4

5

6

7

t(h)

Rh(COD) h o m o g e n e o u s 9 Rh(COD)/ROX-N Cycle 1 - 4 - Rh(COD)/ROX-N Cycle 3

- - I - - Rh(COD)/ROX-0.8 ---z~--Rh(COD)/ROX-N Cycle 2

Fig. 1.1-Octene conversion of different catalysts and in consecutive runs. It can be concluded that by oxidation of the carbon surface the complex anchoring takes place by ion-exchange and the catalyst is more stable to lixiviation by solvent. However, under reaction conditions the catalyst is not stable and a slow deactivation in several reaction runs is observed. 3.2. RH-diphosphine complexes ion-exchanged on activated carbon With the objective of increasing the stability of the catalysts, we considered the possibility of preparing diphosphine rhodium complexes supported on ROX-N (mainly by ion-exchange). Apart from the stabilization effect of the diphosphines, this kind of ligands does not favour the isomerization reaction and the complexes are more selective to linear products [19]. There are several studies in which Rh-diphosphine complexes are generated in situ using Rh(COD) as Rh precursor and adding diphosphines. Results are similar to those obtained by synthesizing the Rh-diphosphine complex prior to the reaction [20]. In this study the diphosphines (DPPB) are added to the hexane suspension of the Rh(COD)/ROX-N catalyst in a P:Rh ratio of 4:1. The synthesis of the cationic complex [Rh(DPPB)2]BF4 and its anchorage on the activated carbon ROX-N has also been

299 investigated. The [Rh(DPPB)2]BF4 complex (named here Rh(DPPB)) was synthesized following the method described in the literature [21 ] and was supported following the same method used in section 3 to obtain a catalyst with 1 wt.% Rh. Table 2 shows the catalytic results obtained with the phosphine based catalysts. Data corresponding to the homogeneous complex and the previously commented Rh(COD)/ROX-N catalyst have been also included. The addition of diphosphines to the reaction media produces a slight increase of the selectivity to linear aldehydes, but the catalyst is noticeably less active in the second run than the Rh(COD)/ROX-N. This could be due to difficulties for the readsorption of the more voluminous diphosphine complex. The 31p{1H} spectra of the concentrated solution (with a pale orange color) after the first run indicated a noticeable leaching of the complex. Table 2. Catal~ic properties and Rh leach ofhetero~enized samples Conversions (%) Rh Selec. Catalyst Leach Add. Entry Run Total 2-octene Aldehyde Alcohol (L/B) (%) Rh(COD) hom.

None

3

None

4

Rh(COD)/ROX-N DPPB Rh(DPPB)/ROX-N None

5 6

1 1 2 3 4 1 2 3 1 2

>99 >99 77 24 <10 93 27 <10 67 <10

0 10 23 6 . 19 9 . 16 .

>99 89 54 17 . . 74 18 . . 40 . .

0 0 0 0

0.9 0.7 1.9 1.8

0 0

1.4 2.0

11

4.2

>90

. 67

. .

>90

The bis-diphosphine rhodium complex [Rh(DPPB)z]BF4 supported on activated carbon ROX-N (entry 6) shows lower activity (but better selectivity) in the first run and faster deactivation. As a conclusion, the use of diphosphines as ligands instead of the COD ligand does not produces stable rhodium catalysts under hydroformylation conditions.

3.3. RH-diphosphine complex heterogenized by covalent bond Another way to support complexes involves the use of bifimctional ligands that contain, apart of the coordination group, a functional group able to react with the support surface, forming a chemical bond. On the basis of works dealing with the anchorage of metal complexes and dendrimers on SiO2 [22,23], we have developed a system to bind diphosphines to the activated carbon surface. The procedure requires the functionalization of the carbon surface and the synthesis of a ligand with a suitable functional group. In order to generate good electrophilic groups on the activated carbon surface, the supports were functionalized by treatment with SOC12 in CH2C12 [24]. In this way the carboxylic groups present on the activated carbon surface (previously oxidized by

300 treatment with HNO3 [16, 17]) can be transformed into acid chlorides as represented in scheme 2. For this study the commercial activated carbon GF-45 (NORIT) and GF-N have been used.

O OH -I-

N2, ,2h, CH2CI 2 SOCI2 = - SO2

.

O~ CI

- HCI

Activated Carbon Oxidized

Activated Carbon Chlorinated

Scheme 2 A diphosphine ligand with a hydroxyl group was synthesized following the sequence shown in scheme 3 [25]. The mixture of formaldehyde (0.6 ml 37 wt.% in water, 8.0 mmol) and diphenylphosphine (1.2 ml, 6.8 mmol) in methanol (5 ml) was heated under inert atmosphere at 70~ for 30 min. The reaction mixture was cooled down to room temperature and treated with 0.25 ml of the 3-amine-propanol (HO(CH2)3NH2, 3.3 mmol). After 30 min, the solution was heated to 70~ overnight, and then it was cooled to room temperature. The product was concentrated and dried in vacuum. The ligand HO(CH2)3N(CH2P(C6Hs)2)2 (named here HONP) was characterized by 1H, 13C and 31p{1H}, getting the expected results [23, 25]. HOJ~~NH2

-I- 2 HO~PPh2

H O ~ N ~ P PlX./pph h2 2 (HONP)

Scheme 3 The heterogenization of the complex was carried out via an esterification reaction between the acid chloride groups on the activated carbon surface and the hydroxyl group of the ligand. This reaction was carried out by the addition of a solution of HONP (0.1179 g) in CH2C12 to 1 g of support. The slurry was stirred and heated to reflux (70~ under N2 for l h. The solution was cooled to RT and dried in a vacuum oven at 70~ for 4 h. To anchor the metal complex two ways were followed: i) anchoring of ligand HONP on the support before complexation with Rh (sample named GF-ONP). To create a supported metal complex a solution of Rh(COD) in methanol was added (sample Rh(COD)/GF-ONP); ii) complexation of the ligand with Rh, and then, heterogenization of the homogeneous complex (sample Rh(HONP)/GF-C1). The complex [Rh(HONP)(COD)]C1 (scheme 4) was synthesized by the addition of 11.8 mg of Rh(COD) (0.023 mmol) to a solution of HONP in 20 ml MeOH (with molar ratio of 1:1) and stirring at RT under N2 for 2 h getting a darkred solution, in contrast to the pale-yellow solution of Rh(COD). The complex was characterized by 31p{IH} and 1H NMR with the expected results [25]. [Rh(HONP)(COD)]C1 complex was supported, but adding the required amount of complex in a methanol solution to the chlorine-functionalized support to obtain 1 wt.% Rh.

301

2

HO/~/'~..../~PPh2 PPh2

+

th.~cl~RI,

2

H O ~ N ~ ' P P h 2

<~,,,.. ~

Scheme 4 Samples GF-ONP, Rh(COD)/GF-ONP and Rh(HONP)/GF-CI were characterized by XPS (Table 3 and 4) and solid state 31p. Table 3 shows the binding energies found for Rh, P, N and C1. Support GF-45 contains P, N and CI, the oxidizing treatment (with HNO3) removes CI and the organic N (peak (1)), and inorganic N like NOx (peak 2) appears. With the chlorination and phosphine anchorage (sample GF-ONP) organic N and organic C1 (peak 2 in both cases) appear. The coordinated Rh, in both catalysts, shows an electronic state like that in the Wilkinson complex (Rh bonded with phosphines) [18]. Table 4 shows the atomic ratios obtained from XPS measurements. The nitric acid treatment produces an important removal of of P and C1, and an important increase of N and O. The esterification produces an increase of P, N and C1, and there are no significant changes when the complex is supported (sample Rh(COD)/GF-ONP). Sample Rh(HONP)/GF-C1 showed a lower amount of P, C1 and N than Rh(COD)/GF-ONP, but higher than its precursor GF-N. These results suggest that the esterification has been made. An investigation by NMR of these catalysts shows that when the ligand HONP is bonded to the support previous to the complexation with rhodium, the phosphines become oxidized (31p solid state NMR peak: +29 ppm). Table 3. XPS results (I): Binding ene,,rgies "" GF'45 GF-N

GF-ONP

Rh P N .... CI

(1) (2) (1) (2) (1) (2)

133.1 134.7 398.6/400.6 197.8 -

132.9 134.0 405.8

132.4 133.6 399.7 405.8

I

199.9

Rh(COD)/ GF-ONP 339.0 132.4 133.6 399.6/401.3 405.8 198.0 199-9

Rh(HONF)/ GF-CI 339.0 132.1 133.6 400.0 405.8 197.7 197.8

Table 5 shows the results obtained with these catalysts. They are quite interesting. In both cases the conversion is high in four consecutive catalytic runs. The catalysts prepared by heterogenization of the previously synthesized Rh(HONP)(COD) complex must be pointed out because conversion is above 99% and it is fully stable in the four tests, in spite of leaching of the complex.

302 Table 4. ,XPS results ( [I): quantitative results GF-45 GF-N Rh/C1 Rh/N Rh/C P/C CI/C N/C O/C

0.0118 0.0025 0.0050 0.1604

GF-ONP

0.0031

0.0101 0.0103 0.0263 0.2361

0.0204 0.2432

Rh(COD)/ GF-ONP 0.2261 0.1861 0.0048 0.0116 0.0210 0.0254 0.2427 ,,

Rh(HONP)/ GF-C1 0.2642 0.1135 0.0026 0.0043 0.0010 0.0232 0.2456 ,,

Although there is some conversion to the isomerization product, 1-octene, the selectivity to linear aldehyde is high, because of that the production of 1-nonanal is similar to the one obtained with the homogeneous complex. The lower activity of the catalyst prepared by Rh complexation on the ONP-GF support can be explained, because the phosphine oxide ligands are more labile than phosphine ligands. Finally, Fig. 2 shows the total nonanal conversion for all the catalysts used in several runs. As it can be observed, only the catalysts heterogeneized by a covalent-bond with the support did not show deactivation in further runs (entries 7 and 8) and only the sample Rh(HONP)/GF-CI showed a nonanal conversion in further runs similar to that of the Rh(COD) complex in homogeneous phase (47%). These results indicate that this new method to support complexes on activated carbon gives interesting catalytic results and it must be studied more extensively. Table 5: Catalytic Activity

i

........ Conversions (%) Catalyst

Rh(COD)/GF-N

Rh(HONP)/GF-C1

Entry Run Total 2-octene Aldehyde Alcohol 80 76 60 55 >99 >99 >99 >99

31 26 17 12 34 32 19 21

48 48 43 43 66 68 81 79

0 0 0 0 0 0 0 0

Select. (L/B) 2.4 2.4 2.5 2.5 2.0 2.1 1.2 1.9

Rh Leach

(%) 59

43

303

Fig. 2. Nonanal conversion over the heterogeneous catalysts

4. CONCLUSIONS Activated carbons can be effective supports for the heterogenization of Rh complexes to produce active heterogenized catalysts for hydroformylation. The carbon functionalization to create a ligand-support bond has shown to be a very promising method to give active and stable catalysts. With this kind of catalysts it is possible to obtain a nonanal conversion similar to that of Rh(COD) in homogeneous phase, even after 4 consecutive rims. ACKNOWLEDGMENTS

This study was made possible by the financial support from CICYT PB98-0983 and NSERC of Canada. REFERENCES

1. E. Lindner, F. Auer, A. Baumann, P. Wegner, H.A. Mayer, H. Bertagnoli, U. Rein6hl, T.S. Ertel and A. Weber, J. Mol. Catal. A: Chem. 157 (2000) 97. 2. J.M. Basset, J.P. Candy and C.C. Santini in: Transition Metals for Organic Synthesis, Vol. 2, p. 387, Eds. M. Belier, C. Bolm, Wiley-VCH. (1998), Weinheim (Germany). 3. V.A. Likholovov and B.L. Moroz, in: G. Ertl, H. Kn6zinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 5, p. 2231, Wiley-VCH Verlag Weinheim (Germany), 1997. 4. M. Lenarda, L. Storaro and R. Ganzerla, J. Mol. Catal. A: Chem., 111 (1996). 5. J.P. Arhancet, M.E. Davis, J.S. Merola and B. Hanson, Nature, 339 (1989) 454.

304 6. M. Iglesias-Hemhndez and F. S~nchez-Alonso in: Studies in Surface Science and Catalysis 1340, A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Eds.), Vol. 130D, p. 3395, Elsevier Science B.V., Amsterdam (The Netherlands), 2000. 7. J. Bahie and J.C. Bay6n, J. Mol. Catal. A: Chem., 137 (1999) 193. 8. K. Nozaki, F. Shibahara, Y. Itoi, E. Shirakawa, T. Ohta, H. Takaya and T. Hiymna, Bull. Chem. Soc. Jpn., 71 (1999) 1911. 9. A.M. Tzreciak and J.J. Zi61kowski, J. Mol. Catal., 88 (1994) 13. 10. A.J. Seen, A.T. Townsend, J.C. Bellis and K.J. Cavell, J. Mol. Catal. A: Chem., 149 (1999) 233. 11. R. M. Desphande, Pm-vvanto and H. Delmas, Ind. Chem. Res., 35 (1996) 3927. 12. V.L.K. Valli and H. Alper, Chem. Mat., 7 (1995) 359. 13. Z. Zhou, G. Facey, B.R. James and H. Alper, Organometallics ,15 (1996) 2496. 14. L.R. Radovik and F.R. Reinoso in: Peter A. Thrower (Ed.), Chemistry and Physics of Carbon, Vol. 25, p. 243, Marcel Dekker, New York (USA), 1997. 15. J.A. Diaz-Aufi6n, M.C. Rom~-Martinez and C. Salinas-Martinez de Lecea, J. Mol. Catal. A: Chem., 170 (2001) 81. 16. H. P. Boehm, High Temperatures-High Pressures, 22 (1990) 275 17. H.E. Van Dam and H. Van Bekkmn, J. Mol. Catal., 131 (1991) 335. 18. D. Briggs and M.P. Seah in: Practical Surface Analysis, Vol. 1, John Wiley and Sons, Chiechester (UK), 1993. 19. J. Hagen, in: Industrial Catalysis. A Practical Approach, p. 17, Wiley-VCH Verlag, Weinheim (Germany), 1999. 20. H. Alper and J-Q. Zhou, J. Chem. Soc., Chem. Commtm., (1993) 316. 21. M.P. Anderson and L.H. Pignolet, Inorg. Chem., 20 (1981) 4101. 22. S.C. Bourque, H. Alper, L.E. Manzer and P. Arya, J. Am. Chem. Sot., 122, (2000) 956. 23. C.M. Crudden, D. Allen, M.D. Mikoluk and J. Sun, Chem. Commun., (2001) 1154. 24. H.P. Boehm, Carbon, 32 (1994) 759. 25. M.T. Reetz and S.R. Waldvogel, Angew. Chem. Int. Ed. Engl., 36 (1997) 8.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

305

Heterogeneous metathesis initiators I M. Mayr, B. Mayr, M. R. Buchmeiser* Institute of Analytical Chemistry and Radiochemistry, University of Innsbruck, Innrain 52 a, A-6020 Innsbruck, AUSTRIA The synthesis of heterogeneous N-heterocyclic carbene- (NHC-) based metathesis initiators is described. Two entirely different approaches have been developed. The first consists of a "grafting from" approach, were polymerizable NHC-precursors have been grafted onto a norborn-2-ene (NBE) based monolithic support prepared via ring-opening metathesis polymerization (ROMP), taking advantage of the living character of ROMP. The second synthetic route is based on a "grafting to" approach and entails the synthesis of oligomeric NHC-precursors and their selective chain-end functionalization with tri(ethoxy)silane groups. These telechelic polymers were grafted on silica using standard silane chemistry. All heterogenized NHC precursors were successfully converted into the corresponding NHC-based second generation Grubbs catalysts and used for various metathesis reactions including ROMP, RCM and cross-metathesis. 1. I N T R O D U C T I O N Metathesis-based reactions represent valuable tools in synthetic organic chemistry, polymer chemistry and technology. So far, a broad range of well-defined homogenous systems including those for asymmetric synthesis is available.[1-3] In contrast to these well-defined homogeneous systems, only few reports exist on analogous, stable, permanently immobilized heterogeneous systems prepared by a molecular approach.[4-6] In this lecture, the synthesis of new, well-defined heterogeneous metathesis systems will be reported. We already reported on the synthesis of heterogeneous C-C coupling and ATRP systems[7-9]. In contrast to the grafting and precipitation polymerization techniques developed for the synthesis of these materials, two entirely different approaches were applied for the fabrication of new heterogeneous NHC-based metathesis catalysts. While other groups focus on the alkylidene moiety of metathesis initiators for immobilization purposes[10-13], we solely use NHCs for this goal since they are the most strongly bound ligands in these systems. In order to minimize polymer-analogue transformation to a minimum, we generally pursue a concept where entire ligands or at least their immediate precursors, which can be converted into the desired systems by a few simple synthetic steps, are attached to a carrier. This ensures a maximum analogy to the parent homogeneous systems and allows the direct comparison of catalytic data. The concepts for immobilization as well as selected results shall be outlined briefly in the following.

1 Grant number Y-158 provided by the FWF (Austrian Science Fund), Vienna, AUSTRIA.

306

2. RESULTS AND DISCUSSION 2.1 Heterogeneous metathesis catalysts based on monolithic supports Monolithic supports used for the present application consist of one piece and possess a permanent and interconnected porous structure. In principle they can be either inorganic (e.g. silica) or organic (e.g. PS-DVB). Nevertheless, in order to provide both maximum chemical stability and an easy access to functionalization, we already developed a completely new class of monolithic supports based on norborn-2-ene (NBE) and a NBEderived cross-linker[14-16]. These monolithic media are generally characterized by a high mass transfer within the interphase, which allows to run catalytic reactions in a continuous flow set-up at significantly elevated flow rates of up to 10 - 20 mm/s. They are synthesized within the confines of the reactor in a one-pot reaction procedure. Since the Ru-based initiator C12Ru(CHPh)(PCy3)2, which allows a "living" setup, is used during synthesis, the active catalytic sites can be used for derivatization purposes after synthesis of the support.

inner surface

1. n / ~

O

o CY3C

~' ~ .9

~~cy~

~2.~,,

~--x~

R-N~N- R I

o-~

BF4O_ /

i M,,c:z

H

FFN~N-R monolith

~v

1. base 2. CI2Ru(PCY3)2(CHPh)

~,

~

H

BF4-

O R..N~L--~N_R PCY3

Scheme 1. Surface-derivatization precursors.

of monolithic supports with polymerizable NHC-

In order to generate sufficient porosity, monoliths with a suitable microporosity (40 %) and microglobule diameter (1.5 + 0.5 lam) were synthesized. Consecutive ,,in-situ" derivatization was successfully accomplished using a mixture of norborn-2-ene and the corresponding NHC-precursor in methylene chloride (Scheme 1). The use of norborn-2ene significantly enhances grafting yields for the functional monomer. Using this setup,

307 tentacles of copolymer with a degree of oligomerization of 2 - 5 of the functional monomer may be generated. The free NHC necessary for recomplexation may simply be generated using 4-dimethylaminopyridine (DMAP). In a last step, excess base is removed by extensive washing and finally the catalyst is immobilized/formed by passing a solution of ClaRu(CHPh)(PCy3)2 over the rigid rod. Loadings of up to 1.4 % of Grubbs-catalyst on NHC base may be achieved. Monolith-immobilized metathesis catalysts prepared by this approach show high activity in various metathesis-based reactions such as ROMP and RCM. The cis/trans ratio of polymers (90 %) exactly corresponds to the one found with homogeneous systems. The use of chain-transfer agents (CTAs, e. g. cis-l,4-diacetoxybut2-ene, diethyldiallylmalonate, 2-hexene) allows the regulation of molecular mass, in particular in the case of cyclooctene. Typical values for the molecular weight and polydispersity (PDI) of poly(cyclooctene) were in the range of 1500 - 2500 and 1 . 2 - 1.9, respectively. The corresponding values for poly(norbornene) are 12000 and 1.2. The presence of CTAs additionally enhances the lifetime of the catalytic centers by reducing the average lifetime of the ruthenium methylidenes (Scheme 2).

monolith , ~

inner surface

0 ---/

CTA

regeneration

O R-N N R CI."~ CI~:Ru=-R PCy 3

[P-Ru=CH 2]

catalytic cycles

,,,,,

EtO2C CO2Et

+

EtOeC CO2Et

Scheme 2. Structure and reactivity of monolith-based, heterogeneous metathesis initiators. This is of enormous importance, since these methylidenes decompose in a unimolecular process and can only be suppressed by the use of a highly reactive CTA. In particular cis1,4-diacetoxybut-2-ene turned out to be well suited for these purposes. It allows the repetitive use of these systems, particularly important in RCM. Fig. 1 illustrates the enhanced long-term stability if CTAs are used in RCM. In terms of reaction kinetics, both

308 the tentacle-type structure and the designed micro structure of the support reduce diffusion to a minimum.

O O

O A A / % 6(]

5~ A

A A

0

. . . .

0

I

10

. . . .

I

20

. . . .

I

30 t/rain

. . . .

I

40

. . . .

I

50

. . . .

I

60

. . . .

I

70

Fig. 1. Difference in activity (A, expresses in % of the original value Ao) with (e) and without (A) the use of cis-l,4-diacetoxybut-2-ene. Thus, these systems behave as predicted by theory and must therefore be considered as successful alternatives to standard PS-DVB supports. The fast kinetics as well as an enhanced stability quantitatively translate into a high average turnover frequency (TOF) in RCM of up to 25 min-1, thus exceeding even the homogeneous analogue (TOF = 4 mini; 45 ~ Maximum tumover numbers are around 60 (homogeneous < 20). The catalytic systems presented here may be used as pressure stable catalytic reactors as well as one-way systems for use in combinatorial chemistry. The use of NHC-ligands successfully suppresses any bleeding leading even in RCM to virtually Ru-free products with a ruthenium-content of less than 0.07 %.

2.2 Heterogeneous metathesis catalysts based on silica 2.2.1 Surface-grafted silica Metathesis-based grafting techniques have already been successfully applied to the synthesis of other silica-based catalytic supports, e.g. those for heterogeneous ATRP[7, 18] as well as for heterogeneous Heck-type reactions[ 19]. This tempted us to investigate as to which extent these grafting techniques might be applied to the synthesis of silicaimmobilized NHC-precursors. The x-ray structure of such a polymerizable NHCprecursor is shown in Fig. 2.

309

F(I}

g(31

Fig. 2. X-ray structure of the polymerizable NHC-precursor 1,3-Di(1-mesityl)-4{[ (b icyc 1o[2.2.1 ] hept- 5-en- 2- ylcarbo nyl) o xy] methyl }-4,5- dihydro- 1H- imidazo 1-3- ium tetrafluoroborate. Scheme 3 summarizes the synthesis of a triethoxysilyl-telechelic oligomeric NHC precursor. This oligomer was grafted onto non-porous silica using standard silica chemistry[20]. Reaction of the grafted support with KO-tBu in THF at -30~ yielded the free carbene which was subsequently reacted with CI2Ru(CHPh)(PCy3)2 to yield the immobilized second generation Grubbs catalyst. After leaching of the support with aqua regia under microwave conditions, the ruthenium content of the solution was measured by inductively plasma-optical emission spectroscopy (ICP-OES). In terms of catalyst loading it is worth mentioning that only 13 % of the NHC ligand were converted into the corresponding catalyst, leading to a catalyst loading of 0.5 weight-%. This value is much lower than the one found in systems based on monolithic supports, were roughly 40 % of the NHC precursor could be used for immobilization, resulting in 1.4 weight-% catalyst loading[16]. Though a non-porous support should facilitate the accessibility of any surface-bound groups, this particular silica shows a reduced accessibility of the corresponding NHC-sites. We attribute this fact to the strong tendency of this support to agglomerate. Preliminary RCM experiments were carried out with diethyldiallylmalonate. The catalyst was added to a solution of this monomer in 1,2-dichlorobenzene and the mixture was heated to 50 ~ for 2 hours.

310

~,._~ BF4"

1. Mo(N-Ar')(CHCMe2Ph)(OR')2 2. (EtO)3Si-(CH2)3-N=C=O

(E(O)3Si~

[=

A

1~" - [ ' ~

.,~CM~Ph Jn

R Ar' = 2,6-/-Pr2-CeH3 R ' = CMe(CF3)2

n=7

O oA N.j~_~)

BF4-

Scheme 3. Synthesis of a triethoxysilyl-telechelic, oligomeric NHC precursor. Irrespective of the reaction conditions used (i. e. ultrasound, microwave, changing reaction times, temperature and solvents), the maximum turnover number (TON) that was achieved was 75. In principle, second generation Grubbs-type initiators immobilized on non-porous silica should behave similar to those immobilized on monolithic supports[16]. In fact, catalysts immobilized onto monolithic supports give similar maximum TONs (< 65) in the absence of any chain transfer agent (CTA). Ruthenium measurements by means of ICPOES revealed quantitative retention of the original amount of ruthenium at the support within experimental error ( 5 % ) , thus offering access to metal free products. 2.2.2 Surface-coated silica

A very simple approach to surface-functionalized supports lies in the use of copolymers that are used for simple coating techniques. While this method is certainly among the most straightforward ones, some general impediments need to mentioned. Generally, coating techniques result in a significant loss of specific surface and pore volume of the support. In due consequence, significant amounts of any catalytic site incorporated into such polymers are no longer accessible. In order to evaluate the general applicability of coating techniques for the synthesis of heterogeneous metathesis catalysts, copolymers of a NHC-precursor with norborn-2-ene were prepared and used for coating purposes as outlined in Scheme 4.

311

1. n

~ O

1 .COPOLYMER SYNTHESIS

O

R-N~..~-R [ BF4H r

CI2Ru(PCy3)2(CHPh)

3.

==~

~ m COPOLYMER

o--~

",,~.O Ph O ~ o BF4R-N~N-R H

2. COATING

COPOLYMER thermalcoating

r~

COATED SILICA

Scheme 4. Synthesis of coated silica supports. Typical amounts of NHC ligand that were immobilized by this approach were within a range of 90 - 130 mmol (ca. 5 - 7 %). Conversion of the polycationic precursor polymer into the polymeric NHC was accomplished using either KO-t-Bu or dimethylaminopyridine (DMAP). The former allows the synthesis of "protected" NHC precursors [17] that are thermally converted into the free NHC, while DMAP results in the instantaneous formation of the free NHC. In due consequence, ruthenium loadings are significantly higher in the case of the t-butoxide protected precursors (16 % v s 5 % for the DMAP route). Despite the high ruthenium loadings, the catalytic activity of such compounds is comparably low, in particular when compared with monolithic systems. Thus, typical values for the TON in the RCM of diethyldiallymalonate were < 10 (monolithic systems < 60!). Since the identical chemical approach in terms of monomers and conversion is used, these findings must obviously be attributed to diffusion-based processes within the pore structure of silica. This again underlines the high synthetic value of the monolith-based catalytic supports described in section 2.1.

312 REFERENCES

1. K.J. Ivin, J.C. Mol, Olefin Metathesis and Metathesis Polymerization, Academic Press, San Diego (1997). 2. M.R. Buchmeiser, Chem. Rev., 100 (2000) 1565. 3. A.H. Hoveyda and R.R. Schrock, Chem. Eur. J., 7 (2001) 945. 4. J.M. Basset and A. Choplin, J. Mol. Catal: A Chemical, 21 (1983) 95. 5. M. Chabanas, A. Baudouin, C. Cop6ret and J.-M. Basset, J. Am. Chem. Soc., 123 (2001) 2062. 6. R. Buffon, A. Choplin, M. Leconte, J.-M. Basset, R. Touroude and W.A. Herrmann, J. Mol. Catal: A Chemical, 72 (1992) L7. 7. R. Kr611, C. Eschbaumer, U.S. Schubert, M.R. Buchmeiser and K. Wurst, Macromol. Chem. Phys., 202 (2001) 645. 8. M.R. Buchmeiser and K. Wurst, J. Am. Chem. Soc., 121 (1999) 11101. 9. J. Silberg, T. Schareina, R. Kempe, K. Wurst and M.R. Buchmeiser, J. Organomet. Chem., 622 (2000) 6. 10. S.B. Garber, J.S. Kingsbury, B.L. Gray and A.H. Hoveyda, J. Am. Chem. Soc., 122 (2000) 8168. ll. J. Dowden and J. Savovic, Chem. Commun., (2001) 37. 12. A.G.M. Barrett, S.M. Cramp and R.S. Roberts, Org. Lett., 1 (1999) 1083. 13. Q. Yao, Angew. Chem., 112 (2000) 4060. 14. F. Sinner and M.R. Buchmeiser, Angew. Chem., 112 (2000) 1491. 15. F. Sinner and M.R. Buchmeiser, Macromolecules, 33 (2000) 5777. 16. M. Mayr, B. Mayr and M.R. Buchmeiser, Angew. Chem., 113 (2001) 3957. 17. S.C. Schfirer, S. Gessler, N. Buschmann and S. Blechert, Angew. Chem., 112 (2000) 4062. 18. U.S. Schubert, C.H. Weidl, C. Eschbaumer, R. Kr611 and M.R. Buchmeiser, Polym. Mater. Sci. Eng., 84 (2001) 514. 19. M.R. Buchmeiser, S. Lubbad and K. Wurst, Inorg. Chim. Acta, submitted (2002). 20. M.R. Buchmeiser, J. Chromatogr. A, 918/2 (2001) 233.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

313

In memory

of Professor

V!adimir Smetanyuk Preparation of physically heterogeneous and chemically homogeneous catalysts on the base of metal complexes immobilized in polymer gels A.A. Efendiev a, T.N. Shakhtakhtinskib and N.A. Zeinalovb alnstitute of Polymer Materials of the Azerbaijan National Academy of Sciences, 124 Samed Vurgun str., Sumgait 373204, Azerbaijan Republic bM.F.Nagiev Institute of Theoretical Problems of Chemical Technology of the Azerbaijan National Academy of Sciences, 29 H.Javid Avenue, 370143, Baku, Azerbaijan Republic A number of polymer gels have been prepared using tertiary ethylene-propyleneethylidenenorbornene copolymer as a rubber base with grafted poly-4-vinylpyridine, polymethacrylic acid and polymethacrylamide ligand chains. The grafted copolymers were crosslinked and complexes of nickel, zirconium and titanium were immobilized in the formed crosslinked copolymers. After treatment with organoaluminium compounds the obtained catalysts demonstrate high catalytic activity in the reactions of dimerization of lower olefins. 1. INTRODUCTION Supported complexes of transition metals combine the advantages of heterogeneous catalysts such as simplicity of separation from the reaction media and high stability with the advantages of homogeneous catalysts such as high activity and selectivity and the possibility of obtaining more accurate information about the structure of their active centers and thus, the mechanism of catalytic processes [1]. The use of polymer ligands as supports opens new possibilities to vary ligand surrounding and control the catalytic properties of complexes

[2,3]. We developed a new principle of preparation of metal complexes immobilized in polymer gels able to swell in hydrocarbon substrate thus, providing an easy access of the reagents to the active centers. These are two phase systems, wherein nonpolar rubber base is the dispersion medium containing fairly regularly distributed domains of graft chains of macromolecular ligands. The dispersion is crosslinked to a certain degree followed by treatment with transition metal compounds. As a result, metal complexes are formed in sites of the macromolecular ligands. Due to the rubber base, the catalysts can swell in hydrocarbon media up to several hundred vol. % forming a gel accessible for the reagents. The immobilized complex catalysts are actually physically heterogeneous but chemically homogeneous catalysts because the rate of diffusion in highly swollen polymers is comparable with that in liquids. On the other hand, the gel immobilized complexes can be easily separated from the reaction medium, as heterogeneous catalysts and used repeatedly.

314

The term "gel immobilized metal complex catalysts" was introduced by Kabanov and Smetanyuk [4,5], and then research continued by the authors of this paper in collaboration with laboratory of late Prof. Dr. V.I. Smetanyuk. This paper summarises the results of the preparation and investigation of nickel, zirconium and titanium complexes immobilized in rubber base with grafted macromolecular ligands [610]. 2. RESULTS AND DISCUSSION We have synthesized a number of polymer gels using tertiary ethylene-propylene-ethylidene norbornene copolymer (CEP) as a rubber base with grafted poly-4-vinylpyridine (PVP), polymethacrylic acid (PMA) and polymethacrylamide (PMAA) ligand chains [7,8]. The above mentioned monomers were added to the solution of tertiary copolymer in n-heptane together with 1-2% of azobis-isobutyronitrile and heated at 75-80 ~ for 6-10 hours. The grafted copolymers were crosslinked by adding 2-4 mass % of benzoperoxide to the solution. The crosslinked graft--copolymers were precipitated from the reaction medium in a form of swollen gels which were dried and granulated. The crosslinked graft-copolymers were contacted with hydrocarbon solvent (n-heptane, toluene) and in the swollen form they were treated with salts of nickel (nickel chloride, nickel acetylacetonate), titanium (dibutoxytitanium dichloride) or zirconium (dibutoxyzirconium dichloride). The resulting gel complexes were repeatedly washed with toluene-methanol mixture and nheptane to remove the excess of the metal salt until the washing gave a negative test for metal, and then treated with an organoaluminium compound (OAC). Diisobutylaluminium chloride (DIBAC), ethylaluminium dichloride (EADC), diethylaluminium chloride (DEAC), ethylaluminium sesquichloride (SCEA) and triisobutylaluminium (TIBA) have been chosen for such treatment. The swelling capacity of the obtained gel immobilized catalysts in nheptane was in the range of 600-800 vol.%. Comparison of IR-spectra of CEP and CEP-PVP shows that bands at 1620, 950 and 930 cm -1 characterizing non-saturation of CEP disappear after grafting of PVP and band at 1600 cm -~ associated with pyridine ring appears. After treatment of CEP-PVP with dibutoxyzirconium dichloride bands at 1640 cm -~, 1600 cm -1, and 1500 cm -1 with shoulder at 1529 cm -1 appear. Similar picture can be also observed in the case of treatment of CEP-PVP with dibutoxytitanium dichloride. One could assume that in these cases, coordination with zirconium and titanium takes place not only with nitrogen atom but also with ~-electron system of the pyridine ring, i.e. an arene complex is formed. Taking into account these data, as well as data of elemental analysis, the structures of zirconium and titanium complexes with CEP-PVP may be illustrated by the following schemes:

315

/ / / / _

/

CI

C4H9 Zr-Cl

N

.

""ke.~-ff" I C4H9

N

\ \ \ \

"-.

N

\"

In IR-spectrum of CEP-PMAA, there is a band in the 3320-3400 cm -1 region characteri the N-H bond of the amide group. After treatment with nickel chloride, this band is shifte the long-wave region. On the other hand, a band associated with the C=O bond of the aJ group does not change. One might assume that coordination of nickel takes place only N-H groups. Thus, the structure of nickel complexes of CEP-PMAA could be represente follows:

H\I~. C H

H

%o

NTiLz \ ' 'b

',

C

%O

H H

', \

H

o,

'N/

~C/

\N /

H

N/

~C/

H

In the IR-spectrum of the nickel complexes of CEP-PVP, we observe a band at 1640, which characterizes complexation of nitrogen atom and a decrease of intensity of the ba~ 1600cm -1 which characterizes free pyridine ring. From these data and elemental analysis can assume that the structure of the nickel complexes with CEP-PVP might be illustrate follows:

316

N

N

N

I

N

I

sNiL2,,

NiL2 I

% s

In the IR-spectrum of CEP-PMA there is a band at 1720 cm -a characterizing C=O group. After treatment of CEP-PMA with dibutoxyzirconium dichloride, this band is shifted to the long-wave region. From these data, it might be assumed that both CO and OH-groups take part in coordination with the metal. In the spectrum of zirconium complex with CEP-PMA bands at 1100 cm -1 and 570 cm -1 characterizing C=O and Zr-O bonds are observed, and bands characterizing Zr-CI bonds are absent. Based on these data, the structure of zirconium complexes with CEP-PMA might be illustrated as follows:

C4H9 I O

.O.. . - ' T Z r _'-'.

O .-~Zr "-

"y H

C4H9

%

~ C -

"'"o H

Similarly, the structure of complexes of CEP-PMA with titanium can be represented. After treatment of the complexes with OAC active gel-immobilized catalysts are formed. The catalytic activity of the obtained catalysts was studied in the reactions of dimerization of ethylene and propylene. It is known that in the dimerization of ethylene in the presence of homogeneous nickel complexes, 1-butene is initially formed, its major part being isomerized into cis-2-butene. The

317 latter, in turn, is isomerized into trans-2-butene which is a more stable compound in terms of thermodynamics. The quilibrium mixture resulting from the dimerization of ethylene in the presence of homogeneous nickel complexes has the following composition [11]: 1-butene 3 % mass cis-2-butene 27 % mass trans-2-butene 70 % mass Meanwhile, it is well known that 1-butene has more practical applications. Dimerization of ethylene in the presence of homogeneous titanium and zirconium complexes proceeds with mass selectivity up to 98-99 % with respect to 1-butene, but one always observes at least 0.5 - 1% mass of polymer formation which creates problems when scaling up. We carried out the dimerization of ethylene in the presence of the obtained gel immobilized complexes of nickel, titanium and zirconium. The reaction was carried out in 0,5-1itre thermostatted stainless steel reactor fitted with stirrer and manometer, n-Heptane was used as a solvent. Temperature range was 293-353 K; pressure range 0,2-4 MPa; molar ratio A1/Me varied in the range of 3 - 10. The catalytic activity of the catalysts was evaluated according to decrease of pressure in the reactor. Gas-liquid chromatography method was used for the analysis of the reaction products. Results of dimerization of ethylene at different temperatures in the presence of CEP-PVP-Ni (ac.ac.)z -DEAC are given in Fig.1.

g CzH 4 g Cat., h 120 100 80 60 40

I

I

I

I

293

313

333

353

p,,

T,K Fig.1. Dimerization of ethylene at various temperatures: catalysts CEP-PVPNi (ac.ac.)z, [] = EADC; 9 = DEAC; pressure- 0,2 MPa; molar ratio A1/Ni- 10. It is seen that optimum temperature range is 313-333 K. It is known that homogeneous nickel complexes are not stable at temperatures higher than 293 K [11] The process of dimerization is exothermic one and to prevent overheating at large scale complicated system of heat tapping is required. Gel immobilized nickel complexes remain active for a long time at higher temperatures, up to 353 K.

318

Fig.2 shows the dependence of dimerization of ethylene in the presence of CEP- PVP-Ni (ac.ac.)2- DEAC on pressure.

g C2H4 500 ~ g Cat., h 400 300 200 -

IOOF~i

l I 0,2

0,4

l

I

I

0,6 P, MPa

l

.-_

0,8

Fig.2. Dimerization of ethylene at different pressures: catalyst CEP- PVP- Ni (ac.ac)2 DEAC; temperature- 313 K; molar ratio A1/Ni = 10. As seen from Fig.2, the rate of dimerization increases linearly in the pressure range 0,1 1,0 MPa. Results of dimerization of ethylene using the same catalysts with various molar ratios AI/Ni are presented in Fig.3.

gC2H 4 g Cat., h 100 80 60 40 20 I

I

I

I

2

4

6

8

I

I

10 12 A1/Ni

I

I

I

14

16 18

i

,,..--

20

Fig.3. Dimerization of ethylene with different molar ratios of A1/Ni: catalyst CEP-PVP-Ni (ac.ac.)2-DEAC" pressure- 0,2 MPa; temperature- 313 K.

319 As it can be seen in Fig.3, the maximum catalytic activity is achieved with molar ratio 10. Further increase of the molar ratio does not lead to an increase of the catalytic activity. It is known that homogeneous nickel complexes are usually used when molar ratio A1/Ni is 50 100. Thus, it can be seen from the above mentioned data that gel-immobilized nickel complexes have significant advantages compared to homogeneous nickel complexes, as they can be used at elevated temperatures and with much lower A1/Ni molar ratio. The results of dimerization of ethylene in the presence of the obtained nickel, titanium and zirconium complexes with different macroligands and OAC are given in Table 1. The temperature in all the experiments was 313 K; pressure 0,2 MPa; OAC/Ni molar ratio of 10 and OAC/Ti or Zr of 4. Table 1 Dimerization of ethylene in the presence of gel immobilized complex catalysts Composition of the reaction Catalyst products, % mass 1-butenetrans-2-butene cis-2-butene 1 CEP-PMAA-NiClz-SCEA 86,0 11,0 3,0 99,9 traces traces CEP-PVP-Ti (OC4H9)zCIz-TIBA 67,0 33,0 traces CEP-PMA-Ti (OC4H9)zCIz-TIBA 99,9 traces traces CEP-PMAA-Ti (OC4H9)zClz-TIBA 99,9 traces traces CEP-PVP-Zr (OC4H9)zCIz-TIBA 99,9 traces traces CEP-PMA-Zr (OCzH9)zClz-TIBA 99,9 traces traces CEP-PMAA-Zr (OC4H9)zCIz-TIBA It can be seen from Table 1 that when using gel immobilized complexes of titanium and zirconium, very high selectivity with respect to 1-butene, up to 99.9 % mass practically, can be achieved without any formation of polymer. Besides, homogeneous complexes are not stable enough and loose their activity after a few hours, whereas gel immobilized complex catalysts remain active for hundred hours and more. It can be also seen from Table 1 that nickel complexes with PMAA macroligands demonstrate 86% selectivity with respect to 1-butene, whereas in case of homogeneous nickel complexes, as it was already mentioned, the selectivity does not exceed 3 %. We also studied the dimerization of propylene in the presence of gel-immobilized nickel complexes. It is known that in the dimerization of propylene with homogeneous nickel complexes a mixture of dimers containing 4-methyl-l-pentene, 4-methyl-2-pentene, 2-methy2-1pentene, 2,3-dimethyl-2-butene, hexene and other compounds is formed, and the content of 4-methyl-l-pentene does not exceed 8% mass [11]. It is also known that 4-methyl-l-pentene has more practical application as its polymer is widely used in electric power engineering, electronics, medicine, etc. The reaction was carried out using the same unit as with dimerization of ethylene. The conditions of the reaction were: pressure - 0,2 MPa; temperature - 313 K; molar ratio AI/Ni 10. CEP-PMAA-NClzo6HaO-SCEA was used as a catalyst. Analysis of the reaction products has shown that there was 46.0% mass of 4-methyl-l-pentene; 41,0%mass of 4-methyl-2pentene and 13,0% mass of other isomers in the mixture. Thus, using gel-immobilized nickel

320 complexes, one carl significantly increase the yield of 4-methyl-l-pentene in the dimerization of propylene. 3. CONCLUSION The results obtained show that immobilization of metal complexes in polymer gels allows to prepare physically heterogeneous and chemically homogeneous catalysts and leads to an important increase in their activity, selectivity and stability in the reactions of dimerizatiorl of lower olefirls. The immobilization of the complexes opens new possibilities of macromolecular design of the catalysts with desired structural organization and will contribute to the development of general principles of synthesis of highly efficient and environmentally friendly catalytic systems for liquid phase processes. ACKNOWLEDGEMENT The results discussed have been obtained in collaboration with V.A. Kabanov of Moscow State University. The authors would like to appreciate the contribution of late Professor Vladimir Smetanyuk. REFERENCES

1. F.R. Hartley, Supported Metal Complexes, D.Reidel Publ. Co., Dordrecht, 1985. 2. P. Hodge and D.C. Sheringtorl (eds.), Polymer-Supported Reactions in Organic Synthesis, Wiley & Sons, Chichester, 1983. 3. A.D. Pomogailo, Immobilized Polymeric Metal-Complex Catalysts, Nauka, Moscow 1996. 4. V.A. Kabarlov, V.I. Smetanyuk and V.G. Popov, Dokladi AN SSSR 225, (1975) 1377. 5. V.A. Kabarlov and V.I. Smetarlyuk, Macromol.Chem., 5 (1981) 121. 6. N.A. Zeinalov, A.V. Ivanyuk, A.I. Prudnikov, V.I. Smetarlyuk, M.V. Ulyanova and A.A. Eferldiev, Dokladi Akademii Nauk, 348 (1996) 207. 7. O.I. Adrov, G.N. Borldarenko, N.A. Zeinalov, A.V. Ivarlyuk, V.I. Smetanyuk, V.S. Stroganov, M.V. Ulyanova and A.A. Eferldiev, Vysokomolekulyarnye Soedineniya, Ser.B, 38 (1996) 1608. 8. N.A. Zeirlalov, A.V. Ivarlyuk, O.I. Adrov, G.N. Bondarerlko, M.A. Martynova, V.I. Smetanyuk, M.V. Ulyanova, A.A. Efendiev and A.I. Prudnikov, Vysokomolekulyarnye Soyedirlerliya, Ser.A., 39 (1997) 888. 9. N.A. Zeirlalov, O.I. Adrov, A.V. Ivanyuk, G.N. Borldarenko, V.A. Kabanov, M.V. Ulyarlova, A.I. Prudrlikov, V.I. Smetanyuk and A.A. Eferldiev, in: Book of Abstracts, International Symposium on Ionic Polymerization, Istarlbul, 1995, p.121. 10. A.A. Efendiev and N.A. Zeirlalov, in: Proceedings of XVI Mendeleev Congress of General and Applied Chemistry, Moscow, 1998, v.2, p.251. 11. V.Sh. Feldblyum, Dimerization and Disproportionation of Olefirls, Moscow, Chimiya, 1978.

Studies in SurfaceScienceand Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

321

Hydrocracking catalyst to produce high quality Diesel fraction Roberto Galiasso Tailleur PDVSA Intevep POBox 76343 Caracas 101 Venezuela, [email protected]

A hydrocracking catalyst oriented to the production of a high quality Diesel fraction was optimized by treating the AIzO3-SiOz support with steam-ammonia. The catalyst characterization shows that aluminum migrates from a tetrahedral coordination to pentahedral-octahedral coordination. This fact seems to increase the total conversion and selectivity of the VGO hydrocracking reaction. The results are associated to a higher metal dispersion and higher Lewis acid strength. 1. I N T R O D U C T I O N Catalytic hydrocracking is a modern refinery tool to produce clean fuels (Gasoline and Diesel). The versatility of this process is due to the catalyst formulation. When Diesel fraction quality is targeted, catalyst must produce a highly isomerized product to increase the cetane number, but still having a good cloud point. Hydrocracking of vacuum gas oil has been tested in a broad range of conversion levels, catalysts, and feedstocks [1,2]. A new generation of catalysts capable of transforming aromatics into iso-Paraffins has recently been introduced [3]. The 2000"s bring forward many challenges for the refining industry with more stringent environmental specifications for fuels, especially in diesel production and a general trend toward converting more difficult feedstocks. These changes are making hydrogen availability and catalyst stability a limiting factor in many refineries. As part of PDVSA-Intevep broad development, a new hydrocracking catalyst with improved diesel selectivity was targeted by modifying a "conventional" A1203-SiO2 support. 2. E X P E R I M E N T A L Three WNiPt/AlzO3-SiOz catalysts were prepared by steam ammonia treatment of the same AIzO3-SiOz support, characterized, and their hydrocracking activity tested using conventional pilot plant test. Previous studies [4-5] had shown the effect of the acidity and metal active center on activity and selectivity. Two hydrocracking severities have been used in this study to understand the catalyst impact in product quality. The catalysts were characterized using infrared spectroscopy (IR), nuclear magnetic resonance (NMR), X-Ray Photoelectron Spectroscopy (XPS), and programmed Thermal-Gravimetric Ammonia Desorption (TGAD).

322

2.1 Catalyst preparation Three hydrocracking catalysts (MHCK) were prepared by impregnation of an A1203-SIO2 support treated under steam ammonia atmosphere. The support was prepared by coprecipitation of silica and alumina gel. The product was dried at 120 ~ extruded in a lx5 mm cylinder shape and treated in air at 450 ~ for 4 hours to near constant weight. Then, it was dealuminated using steam-ammonia at 200 ~ 0.3 bar of ammonia partial pressure, 200 l/h of gas flow, during one, two and three hours, respectively, to generate support SI, SII and SIII. After that, the three supports were impregnated using the same amount of active species in two stages. First, they were impregnated by an ammoniumtungstate and nickel nitrate-water soluble solution, followed by two hours drying in air at 120 ~ Second, by impregnation with Pt-diammine water-soluble salt, drying at 120 ~ and calcined at 550~ in air during six hours. The three catalysts (CI, CII, and CIII) were then presulfided.

2.2 Catalyst characterization To understand the difference between the three supports and catalyst, SI-CI, SII-CII and SIII-CIII were submitted to a serie of analyses and tests. Chemical characterization: total metal analysis has been performed using atomic adsorption spectroscopy (Varian Techtron analyzer). Metals were reported in % by weight (bulk) of total metal oxides in the support (W, Ni, Pt / A1-Si support). See Table 1. Physical method: Surface, pore volume, and average pore diameter were measured using standard nitrogen adsorption and mercury porosimetry methods. See Table 1. XPS: Spectra were obtained in a Leybolh-Hereaus LHS-10 apparatus (Mg cathode) using Alfalfa with 50 eV of power. XPS method was used to assess the metal dispersion on supports using the peak area intensity (corrected) to measure atomic concentration. The Defoss6 et al [6] method was applied. Binding energies in sulfided catalyst were between Ni: 853.4 and 856.3 eV (2p3/2), W: 34,4 and 32,5 eV (4f5/2-7/2), Al: 74.2 eV (2p). In this way W, Ni, Pt and Al were measured using the peak deconvolution and integration to obtain the area and reported here as a ratio of metal/total metal in surface. Platinum at this low concentration was poorly detected as a large shoulder. See Table 2 for dispersion and Fig. 1 for typical XPS spectra of W and Ni species. Ratios of W+4/W+5 and Ni+2/Ni + were measured by deconvolution of the corresponding spectra. NMR: Solid 27A1 MAS NMR was used to determine the structure of the support (Si-A1) based on the method described by Nagy et al. [7]. The spectra provide information on the different type of aluminum structure in the support (tetrahedral-octahedral coordination). See Fig. 2 NHaTPD: Thermal desorption of ammonia was used to characterize the acid strength of the support and catalysts. A McBain microbalance was employed using 1 mg of sample. Ammonia was adsorbed at room temperature and the total mol of NH3*102/m 2 adsorbed were measured. Then the sample was heated using a ramp of 6 ~ and the remaining amount of NH3/m 2 calculated at three temperatures (200/300/400 ~ See Table 3. FT-IR: Infrared spectroscopy was used to determine the acidity of the MHCK catalysts. The apparatus was a Perkin Elmer 2865 with Fourier transform capabilities. The typical plots of absorbance as a function of increasing wavelength were obtained for each catalyst and reported in Fig. 3. To improve the plot, 4% and 8% of transmittance were added to the

323

Table 1- Physicochemical Properties Catalysts NiO wt% WO3 wt% PtO wt % A1203 wt% SiO2 wt% Surface m2/g Volume cm3/g particle diameter m aver. pore diameter A

CI CII CIII 4.0 4.2 4.3 13.4 13.2 13.5 0.1 0.1 0.1 15 14.7 14.5 Complement 222 234 228 0.52 0.54 0.51 0.001 0.001 0.001 120 105 110

Table_2- Surface Catalysts Ni/Total Me W/Total Me A1 /Total Me

Metal dispersion XPS CI CII CIII 2.2 2.7 2.1 6.4 5.8 6.0 4.4 3.8 3.5

Table 3- Ammonia TPD mmol*10-2/sm Temperature ~ SI SII SIII/CIII 200 5.3 4.6 4.3/2.8 300 1.2 1.3 1.1/0.4 400 0.4 0.6 0.7/0.2 _

signal for supports SII and SIII and catalyst CIII. Acidic absorption bands in the range 3500-3750 cm -1 were recorded because this region is associated to the stretching bands of the OH groups. Strong acidic bands appear in the region of 3600-3650 cm -1, and weak ones in the region of 3550 cm -1 [8]. See Fig. 3. Table 4. Reaction feed and products (wt%) Temperature ~ 380 380 380 400 400 LHSV h-1 0.75 0.75 0.75 0.75 0.75 Catalysts Feed CI-1 CII-1 CIII-1 CI-2 CIII-2 Conversion 100 60 64 69 70 72 Diesel 0.0 45.0 49.5 54.7 52.0 55.0 Nafta 0.0 10.8 10.2 9.9 13.0 12.0 Gas 0.0 4.5 4.8 5.0 5.6 5.8 Note: there is a dramatic change in percent conversion (10%) between CI-1 and CI-2 when going from 380 to 400~ while there is only a 3 % change f o r CIII-1 and CIII-2 in the same conditions and almost all went to the gasoline fraction (it seems selectivity shifts towards naphtha formation). Also note that there is no match between % conversion and the sum of individual fractions

Pilot plant The effect has been studied in a small-scale pilot plant (see detail of the plant in reference [1]). This unit has a 60 cm 3 down-flow fixed bed reactor that operates isothermally. The hydrogen and the hydrocarbon feed were preheated before entering the reactor. After reaction, the liquid product (C5+) was fractionated and analyzed using conventional ASTM method. In addition, a Mass Spectrometry coupled with gas chromatograph (GC MS) was used to measure aromatics, paraffins and naphthenics compounds distribution in the feed and in the products. In addition, a special NMR analysis was performed to determine the PNA. The VGO was desulfurized using commercial catalyst (not described here) and the product characteristics are shown in Table 4 as well as the feed. The MHCK catalysts were tested at 380 and 400 ~ LHSV=0.75 and 100 bar of total pressure, using 800 m3/m3 of HJHC ratio at the inlet of the reactor. The

324 HCK products quality are shown in Table 4. As example, CI-1 and CI-2 mean: catalyst I severity 1, and Catalyst I severity 2, respectively. To compare the catalysts, the same severity was used. Hydrogen purity was 100%; the catalytic system was diluted in the reactor with 50% inert material, and used a particle size of 0,1cm x 0,1 cm (cylinder). This special precaution was taken to ensure proper fluid dynamics according to De Bruijn results [9]. Catalysts were sulfided with light virgin gasoil at 300 ~ during six hours. Sulfur and carbon contents in all the fresh catalysts were nearly the same (6 wt% and 0.1 wt%) 3. RESULTS

3.1 Catalysts The three catalysts show nearly similar bulk composition in tungsten and Platinum, and a small difference in nickel, attributed to the impregnation method. Aluminum seemed to slightly decrease when the steam-ammonia treatment period increased. The small difference in surface, total pore volume, and average pore diameter (calculated by integrating the pore volume distribution curve in the range of 10 to 300 A) could not be correlated with the ammonia treatment and were in the range of the analysis errors.

3.1.1 Metals in surface (XPS) on sulfided catalyst The XPS spectra in the Nizp and W4f regions after sulfidation are shown in Fig. 1. The binding energy (BE, eV) of the support is in the range of 102.6 to 102.9 eV for the Sizp Table 5. XPS dispersion (IMe/ITotal) Sample CI W +6 0.121 WSz 0.334 IW/IT 0.455 NiO 0.09 NiSx 0.144 INi/IT 0.234 IAI/IT 0.452 Is/IT 0.314

CII 0.115 0.352 0.467 0.086 0.157 0.243 0.434 0.343

CIII 0.129 0.383 0.512 0.081 0.17 0.251 0.423 0.388

86618621858]8541850[848 W

C

...... .."..... ...-" .,..." :...., "....

I

~

....

and in the range of 74.5 eV for the Al2p. The sulfur 401 38 i 36 ! 341 321 30 was detected from 162.0-162.2 eV [10]. The shape Fig. 1. XPS spectra of the Ni2p envelope with a satellite peak at 860.6 eV shows almost the same presence of non sulfided Ni 2+ species in all catalysts (CI-CII-CIII) with no shift at all. The Nip3/2 signal (856.4 eV) is due to non sulfided Ni L+, probably in the Si-O-AI framework. It slightly increases from catalyst CI to CIII, with a maximum shift of 0.2 eV (856.4-856.2 eV). The second Ni2p3/2 peak corresponds to NiS [11]. The amount increases as a function of ammonia treatment with a shift of the signal by 0.4 eV (853.4853.7 eV). The NiS/NiO surface ratio is presented in Table 5. It increases from CI to CIII, indicating that the modification of the A1+2+O+3+-SIO2 framework changes to some

325

extent in the nickel structure at the active surface, and is probably also modified by Pt species during sulfating. The W4f7/2 doublets appear at 32.2. and 34.5 eV, and at 35.5 and 37.9 eV, which are ascribed to WS2 and non sulfided W § species. The position of the two doublets did not change for the three catalysts. The parameters for the "sulfided" species were obtained by curve fitting of sample CI and allowing the peak position and FWHM to relax into their local minimum. Table 5 shows that the proportion between WSz/W +6 species increases from 72 for CI to 78% for CIII, indicating larger sulfide species in catalyst with longer period of steam ammonia treatment and AIzO3-SiOz framework modification. Again the Pt species may have played some role in the active surface modification. Sulfur signal at 162.1 eV increases (with no relative shift) from CI to CIII in agreement with previous statement that surface sulfided species have increased. Previous sulfiding studies [12] speculated about the role of Pt on Ni and W migration from the framework, which could explain in part the present results. Table 5 shows that aluminum dispersion is reduced from CI to CIII, in agreement with the reduction of the bulk composition.

3.1.2 Acidity of support and oxide catalysts NH3 is one of the probing molecules for measuring the Lewis and Br6nsted acidity of the surface. The acidity of SI, SII, and Sill was measured as the amount of NH3 retained at each temperature. The results are presented in Table 3. The mass spectrometry analysis of the gases desorbed did not indicate any NH3 decomposition into amide, imido, hydrazine, and dimers species below 400 ~ Here the AIzO3-SiO2 sites are the main agent for the NH3 adsorption, but metals as W +6 and NiO contributed to the total acidity. Comparing the adsorption for the support Sill with catalyst CIII in Table 3, it can be concluded that impregnation reduced the total acidity by 40-60% and changed the acidity profile, as expected. Most of the metal oxides during the impregnation are deposited on top of the AIzO3-SiO2 framework, reducing the number of acid centers. The modification of the framework by the steam ammonia treatment changes these adsorption and metal dispersions, as shown above, and the exposed acid sites are reduced. Comparing support SI with SII and Sill, it was observed that the longer the steam-ammonia treatment period, the lower is the total acidity at 200 ~ but the higher is the acid strength (higher amount of NH3 retained at 400 ~ The infrared analysis of adsorbed pyridine (not shown here) confirmed that the strength of the Lewis MASNMR wppm fom [AI (H20)613+ acid sites was higher in tetrahedral pentacoordinates steam-ammonia sample CIII ~' octahedral (bands at 1350 cm -1). Figure 2 presents the Z7A1NMR sll analysis of CI CII and CIII samples without sulfiding. The CI Sl sample presents larger bands attributed to o Fig. 2. A1MASNMR spectra for SI, SII,SIII and CIII

326

aluminum in tetrahedral coordination and small bands attributed to aluminum occupying octahedral positions, with barely any penta-coordinated aluminum sites [14]. When the support is treated during a longer period with steam ammonia, the spectra changes. The AI in tetrahedral coordination decreases and turns to penta and octahedral coordinated sites, and no tetrahedral ones (see in the AI-MASNMR spectra above the peaks at 5, 30 and 60 ppm, respectively, for catalyst II and III in comparison with I). It seems that steam ammonia reorganizes the framework structure by dissolving aluminum and formation of A1203 over the -Si surface. Figure 3 presents the FTIR spectra in the region assigned to the acid centers. Bands centered at 3555 cm -1 are associated to Lewis acid centers and those of 3650 cm -1 to Br6nsted sites [10]. It can be seen that support SI has the lowest Lewis and the highest Br6nsted acidity while support Sill has the opposite surface composition with the higher Lewis to Br6nsted acid sites ratio. Support II has both type of sites in a similar amount/proportion. It is well known [11] that interactions between WO3-SiOz-AI203 are critical for the formation of Lewis acid centers. The new signal at 3570 cm -1 is attributed to non-framework OH stretching in silanol sites near a vacancy [12]. That signal decreases from catalyst CI to CIII. After metals impregnation, most of the bands are reduced (see dashed line in Fig. 3 for catalysts CIII).

~

CIII

"'" "LS-LS

'•

I

c mq 3680

_

"saIt - - - _- ~ _ ~ ' ~

I

3630

-

I

3580

14 12 10 8 !6 "4 2 3530

Fig. 3. FTIR of SI SII SIII and CIII 3.2 Catalytic test

The pilot plant test was done on desulfurized VGO. The sulfur, nitrogen and carbon Conradson properties are typical of those used in commercial hydrocracking for the conversion stage. Table 4 shows the results at two severities. Let us compare the catalyst at 380 ~ Activity is defined as the VGO conversion in weight, and selectivity as the diesel produced related to the total conversion. It can be seen that treatment produces an increase in VGO conversion and diesel yields at the expense of nafta production. Gas also slightly increases. This result is also confirmed at high severity (400 ~ The effect of temperature is almost the same in catalyst CI as in CII (same activation energy and similar selectivity). Catalyst CIII has almost the same pore structure, particle diameter and was tested under identical operating conditions. Neither fluid dynamics or diffusion differences can explain the higher activity. The active phase modification may be responsible of the activity and

327 selectivity improvement. The increase of acidity and the higher metal dispersion seem to be responsible for higher conversion and selectivity. The hydrogen consumption due to the hydrocracking reaction decreases from catalyst I to III due to the higher selectivity to diesel production (the higher the gasoline and C1-C4 production, the higher the hydrogen consumption). 3.3 Product quality The cracking and hydrogenation balance in the catalyst depends on acid center/metal ratio. In our case, the main target is the diesel quality. The results of diesel PNA analysis are shown in Table 6 for catalysts CI, CII and CIII operating at 380 ~ The analysis shows that the steam-ammonia treatment increases the paraffins content and decreases naphthenes and aromatics content. This indicates a higher hydrogenationhydrogenolisis activity in CIII than in CII and CI. In addition, the NMR analysis indicates Table 6 Selectivity to hydrogenation Catalyst n+isopar wt% Monocyclop. wt% Dicyclop. wt% Tricyclop. wt% Mono arom. wt% Di arom. wt% Triarom. wt% Cetane Number

CI-1 26.00 18.40 12.18 3.20 32.19 8.14 0.00 54.00

CII-1 26.90 19.15 12.60 3.00 30.80 7.60 0.00 55.00

CIII-1 28.20 19.14 13.40 2.20 30.00 7.20 0.00 57.00

a slightly higher iso/n-paraffins ratio in CII and CIII than in CI. As a consequence of the physicochemical modification of the catalysts, the cetane number increases from 51 to 54 (54 to 57 according to Table 6) and the cloud point is reduced f r o m - 2 2 ~ to -24 ~ in the hydrocracked products. 4. DISCUSSION Upon steam-ammonia treatment and then calcination at 500 ~ of the support samples, the -AI- coordination at the surface changed. Aluminum migrated from a well organized surrounding with tetrahedral coordination into a distorted penta and hexa coordination. Aurox et al. [14] discussed the heat of adsorption of ammonia on different types of alumina and concluded that the highest heat of adsorption of ammonia is associated to the abundance of pentacoordinated aluminum. They measured, for some superacid alumina with high content of pentacoordinated aluminum, a high LI+L2/NH3 ratio and the largest distribution of the acid centers (in agreement with their CO adsorption study [15]). Our FTIR results of Fig. 3 show a new band at 3570 cm -1, that may be attributed to the aluminum migration out of framework position. In addition, the change in

328

the AI-NMR signal (Fig. 2) at around 20ppm confirms the migration into a pentacoordinated-Al-framework position. When the support was impregnated using tungsten nickel and platinum watersoluble salts, the metals were preferentially adsorbed on the aluminum surface. The adsorbed metal salts were then decomposed in the air treatment, generating a more complex Me-AI-Si interaction (clusters) and most of the A1 NMR signal disappeared as well as the FTIR acid bands. Using ammonia adsorption, it was shown by XPS that metal dispersion changes from catalyst CI to CIII, which indicated the effect of steam-ammonia surface modification on the metals adsorption and migratiort The steam-ammonia treatment dissolved some aluminum, increasing the vacant sites by dealumination and migration to a different silica oriented environment. These vacant sites may be occupied by a tungsten and nickel species during impregnation. As a consequence, the electronic coordination of these metals is also modified in different ways after suffering an "equivalent" sulfiding procedure. The XPS spectra show the increase of the NiS/NiO and W+4/W+6 ratios, induced by support modification. Thus, not only are the acid sites modified, but the metals sites are also affected. At the present stage, it is not fully clear if the active phase is composed by nickel enclosed in a nickel-tungstate-sulfide layer where Pt could play an important role. The catalytic test proved that the steam ammonia treatment increased the initial conversion of VGO into diesel fraction, but did not proceed further to gasoline and gas formation by secondary cracking. This behavior is associated to a larger number of very strong (accessible) Lewis type acid centers in a non-shape selective support. Moreover, the change in aromatic hydrogenation shown in Table 4 confirms that the active metal phase was promoted, and the support modification (higher dispersion and higher Ni and W sulfur species). Both the acid and metal sites increased the isomerization (higher n/iso-paraffins ratio) in the complex bifunctional reaction path. 5. C O N C L U S I O N S It has been shown that steam ammonia treatment of an A1203-SIO2 hydrocracking support promotes the formation of pentacoordinated aluminum species. The mechanism is associated to a dealumination and aluminum migration from tetrahedral coordination into more distorted AI-Si environment. This generates a larger proportion of strong Lewis acid centers and a broad distribution of acid strength. The support modification also promotes a higher metal dispersion and a higher sulfur content at the surface. Both gave a higher activity and selectivity to diesel production. Diesel quality is also improved by a higher hydrogenation and a higher isomerization activity in steam- ammonia dealuminated catalyst. ACKNOWLEDGMENT I wish to acknowledge the efforts of the process engineers, and pilot plant and catalyst characterization people at PDVSA-Intevep, especially Jose Arroyo who did the bench scale catalytic tests. In particular, I thank Prof. Hercules who performed the XPS study and Z. Gabellica who carried out the AI-NMR studies. I would also like to thank Intevep for permission to publish this information.

329 REFERENCES

1. R. Galiasso, M. Di Marco and A. Salazar, 13th World Petroleum Congress, Proceedings (1991) 233. 2. J. Scherzer and A.J. Gruia, Hydrocracking Science and Technology, Marcel Decker, Inc. (1996) 96. 3. R. Prada, R. Galiasso, G. Romero and E. Reyes, US Pat. 4465792 (1989). 4. R. Galiasso and R Prada, Preprint 4 th International Conference on Refining Processing, Aiche Meeting, Houston A 22 (2001) 327. 5. R. Galiasso, Appl. Catal. (2002) (submitted). 6. C. Defoss6, P. Canesson, P.G. Rouxhet, and B.J. Delmon, J. Catal., 25 (1972) 407. 7 J. B.Nagy, Z. Gabelica, G. Debras, E.G. Derouanne, J.P. Gilson and P.A. Jacobs, Zeolites 2 (1969) 59. 8. J.B. Uytterhoeven, R. Schoonheydt, V.Liengme and W.K. Hall, J. Catal., 13 (1969) 425. 9. A. De Bruijn, 6th. Int. Congress on Catalysis, London, paper B34, 1976. 10. R.B. Shalvoy and R.J. Reucroft, J. Vac. Sci. Technol., 16 (1979) 567. 11. B. Pawelec, L. Daza, L.L.G. Fierro and J.A. Anderson, Appl. Catal. A: Gen., 145 (1906) 307. 12. R. Galiasso, WNiPt SIO2A1203 presulfiding, Appl. Catal. (in preparation). 13. D. Coster, A.L. Blumenfeld and J.J Fripiat, J. Phys Chem., 99 (1995) 321. 14. M. Aurox and M. Muscas, Catal. Lett., 28 (1994) 179. 15. V Gruver and J.J. Fripiat, J. Phys. Chem. 98 (1994) 8549.

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331

Thermostable yttria-doped inorganic oxide catalyst supports for high temperature reactions E. Elaloui
332

drying mode [2] with respect to alcohol solvents in order to prepare the doped oxides. Their behaviours versus heat treatments were observed with the help of the BET technique and XRD patterns. All heat treatments were performed in air unless otherwise mentioned. 2. SYNTHESIS AND PHYSICAL P R O P E R T I E S OF THE DOPED A E R O G E L S In each case, a pure and its corresponding doped oxide were prepared with a weight content of yttria of 8 %, the volumetric ratio of precursor to solvent was 1/1, and the supercritical drying temperature was 265 ~ while a stoichiometric quantity of water was reacted for the hydrolysis reactions. All reactants were vigorously stirred at ambient temperature to carry out the sol-gel achievement. Table 1 gives the precursors and corresponding solvent of the synthesis. No catalyst was added to the reactant mixtures. Table 1 Reactants used to prepare the gels Oxide Precursor SiO2 TMOS (Si(OCH3)4 A1203 Al-sec-butylate AI(OC4H9) 3 ZrO2 Zr-isopropylate Zr(OC3H7) 4 TiO2 Ti-tetrabutoxylate Ti(OC4H9)4 Y203 Y-acac Y (CH3COCHCOCH3) 3 2.1.

Solvent methanol sec-butanol isopropanol n-butanol methanol

Pure yttria

Table 2 gives the BET areas and XRD patterns of a yttria aerogel as a function of the temperature of heat treatments. Table 2 BET areas and XRD patterns of the yttria aerogel versus heat treatments Temperature in ~ BET areas in m2/g XRD patterns 300 107 amorphous with traces of cubic form 600 92 cubic 900 17 nd 1200 2.2 nd To the best of our knowledge, this is the first time that such data are reported for an yttria aerogel.

2.2.

Pure and yttria-doped aerogels

2.2.1. Pure and doped silica Table 3 shows the properties of these aerogels. It can be seen that the yttria-doped silica exhibits a very good resistance towards sintering by comparison with pure silica even at relatively low temperature (300 ~ Pure silica transformed into cristobalite at 900 ~

333 while the doped sample remained amorphous up to 900 ~ shifted to the quartz form.

and, at higher temperature,

Table 3 BET areas and XRD patterns for pure silica and yttria-doped silica versus heat treatments. BET areas in mZ/g XRD patterns Temperatures in ~ pure silica doped silica pure silica doped silica 300 517 794 amorphous amorphous 600 431 706 amorphous amorphous plus traces of YzSiO5 900 220 531 cristobalite amorphous 1200 4 8 cristobalite quartz 2.2.2. Pure and doped alumina In Table 4 the crystalline and textural stabilisation effects of yttria towards alumina appeared to be in line with previous results of Ponthieu et al. [2]. Table 4 BET areas and XRD patterns for pure alumina and yttria-doped alumina versus heat treatments. BET area in XRD pattern Temperatures in ~ pure alumina doped alumina pure alumina doped alumina 300 320 656 amorphous amorphous 600 300 423 amorphous amorphous 900 148 288 ~,-alumina amorphous 1200 4 16 (32 under N2) a-alumina a-alumina plus Y2A112(A104)3

mZ/g

It should be noted that for the present doped sample, no transition form of alumina was observed contrary to previous results where ~,-alumina appeared from 400 and up to 800 ~ for yttria-alumina [2]. 2.2.3. Pure and doped zirconia Doping zirconia resulted in a much smaller effect than the one observed for silica and alumina. In particular, only the monoclinic phase was detected for all the temperatures with pure and doped zirconia samples, as it can be seen from the results presented in Table 5.

334 Table 5 BET areas and XRD patterns for pure zirconia and yttria-doped zirconia versus heat treatments. BET areas in XRD patterns Temperature in ~ pure zirconia doped zirconia pure zirconia doped zirconia 300 203 277 monoclinic monoclinic plus Y0.15Zr0.8501.93 600 71 38 monoclinic monoclinic plus Y0.15Zr0.8501.93 900 15 20 monoclinic monoclinic 1200 1 2 monoclinic monoclinic plus Y0.15Zr0.8501.93

mZ/g

Comparing these results with those of St6cker and Baiker [3] who prepared highand low-temperature non doped zirconia aerogels in the presence of mono- and dicarboxylic acids as catalysts, one can notice that there was a strong effect of the nature of the acid catalysts, associated with the high temperature dried aerogel, in terms of surface areas measured after a treatment of calcination at 500 ~ whereas our work did not involve any addition of a catalyst at the sol-gel step as mentioned above. Two other papers dealing with yttria (cubic)-stabilised zirconia (YSZ) showed that in the case of a precipitation method using this time inorganic precursors, the results were of the same order of magnitude as ours, after a temperature treatment at 600 ~ [4,5]. 2.2.4. Pure and doped titania In the case of titania, the stabilising effect of yttria was not observed below a heat treatment of 600 ~ as shown in Table 6. Pure titania shifted from anatase to rutile at 900 ~ and the phase transformation was accompanied by a very strong decrease of the BET surface area. The doped sample, although it kept the anatase structure until reaching a treatment temperature of 1200 ~ was also characterised by a very strong diminution of its surface area at 900 ~ which, however, could not be explained by a sole crystalline change as shown in Table 6. Table 6 BET areas and XRD patterns for pure titania and yttria-doped titania versus heat treatments BET area in XRD pattern Temperatures in ~ pure titania doped titania pure titania doped titania 300 172 147 anatase anatase 600 64 110 anatase anatase 900 3 9 rutile anatase 1200 2 3 rutile rutile

mZ/g

Schneider and Baiker in their review on the many methods of preparation of titania gels in the form of xero- and aero-gels-[6], mentioned either amorphous or crystalline non

335 heat treated aerogels, which did not seemingly depend upon the supercritical mode selected, i.e., high or low temperature but rather to the sol to gel chemistry. Our results are close to those depicted by Schneider [7], but with much larger BET surface areas. 3. DISCUSSION As already published in the literature, sol-gel associated to supercritical drying always gives highly developed and divided materials which can have many applications in heterogeneous catalysis. Addition of a small part of YaO3 to silica, alumina, zirconia, and titania revealed a beneficial effect on the thermal resistance of these materials. However, it is difficult to compare our results to those already published because in our work, the volumetric ratio of precursor to solvent (1/1) was much higher than the one generally used by other authors on the one hand, and on the other hand, we never added any catalyst, either acidic or basic, contrary to most of the works recently described in the literature. The effect of yttria was found particularly strong with silica and alumina and moderate with the other oxides, zirconia and titania. This behaviour may be related to the findings that for the two former doped oxides, only amorphous materials were synthesised before any temperature treatments while in the case of the two latter ones, the solids as obtained exhibited already crystalline structures in the absence of any thermal treatment. 4. CONCLUSION The extension of our previous work on yttria-doped alumina to three other inorganic oxide supports currently employed in catalysis resulted in similar advantages. The next step will be to deposit a precious metal on the doped oxides and to compare the results obtained in a reaction test with the non-doped supported catalysts. REFERENCES

1. 2. 3. 4. 5.

G.M. Pajonk, Catal. Today, 52 (1999) 3. E. Ponthieu, J. Grimblot, E. Elaloui and G. M. Pajonk, J. Mater. Chem., 3 (1993) 287. C. St6cker and A. Baiker, J. Sol-Gel Sci. Techn., 10 (1997) 269. Y. Z Chen, B. J Liaw, C. F. Kao and J. C. Kuo, Appl. Catal. A: General, 217 (2001) 23. Y. B. Khollam, A. S. Deshpande, A. J. Patil, H. , S. B. Deshpande and S.K. Date, Mater. Chem. Phys., 71 (2001) 235. 6. M. Schneider and A. Baiker, Catal. Today, 35 (1997) 339. 7. M. Schneider, Dissertation ETH n ~ 10685. Swiss Federal Institute of Technology. Z/irich, 1984.

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337

Preparation and characterization of WOx-CeO2 catalysts M. Alifanti 1'2, C. M. Visinescu 1'2, V. I. Pfirvulescu 1, P. Grange 2 and G. Poncelet z 1_ University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, fax: 401-3320588, email: v_ [email protected] 2- Universit6 catholique de Louvain, Unit6 de catalyse et chimie des mat6riaux divis6s, Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium, fax: 32 10 473649, e-mail: grange @ cata. uc I.ac.be Tungsten-ceria catalysts were prepared by co-precipitation and impregnation techniques looking for the effect of the tungsten loading and calcination temperature. The catalysts were characterized by adsorption-desorption isotherms of Nz at 77 K, in-situ XRD, NH3-DRIFT, Raman spectroscopy, and XPS. It was found that the preparation method influences the texture of the catalyst, the tungsten loading, the dispersion of the WOx domains which correspond to a different interaction with ceria, and the temperature of formation of the Ce2(WO4)3 species which seem to be responsible for the strong Lewis/Br6nsted acid sites. 1. I N T R O D U C T I O N Isomerization and hydroisomerization of normal paraffins to branched isomers are important processes for obtaining high-octane gasoline. These reactions must be carried out with high selectivities, thus with minimum of (hydro-)cracking and aromatization products. The classical processes of alkane isomerization use corrosive liquid catalysts (H2SO4, HF, AICl3) or halogen-promoted metal oxides (CI-AI203) [1]. In order to avoid the environmental problems caused by these catalysts, several new acid materials have been investigated. Among these, sulfated zirconia has been thoroughly studied mainly in isomerization of butane, pentane [2-5] and hexane [6]. In spite of its high selectivity in the isomerization of light paraffins at low temperatures [7], this catalyst is unstable [6]. For this reason, the interest has been shifted to other catalysts, and tungsten oxide-based materials appeared to be more stable than sulfated oxides [8]. WOx/ZrO2 was shown to be a strong acid solid, active in butane isomerization [9]. It is worth to note that cracking is much reduced compared to sulfated zirconia and, consequently, the stability of this catalyst is higher. The structure and catalytic behavior of WOx/ZrOz are dependent on WOx concentration, preparation conditions, and thermal treatment. In pentane isomerization, the maximum rates were observed for catalysts with 19-24 wt% WO3 after calcination at temperatures in the range 923 - 1123 K [2, 9, 10, 11]. These conditions correspond to the formation of WOx domains of intermediate size on zirconia surface, which seem to be the active species in such reactions [12-14]. These "islands" appear to provide a compromise

338 between reducibility and accessibility of WOx clusters. They are necessary to delocalize a temporary charge imbalance, which generates Br6nsted acid sites in the presence of hydrogen, and stabilizes carbocation intermediates. The addition of Pt to WOx-ZrO2 affects the reducibility of the WOx domains. 0.05% Pt is sufficient to increase the activity and stability of Pt/WOx-ZrO2 for hexane isomerization [ 15]. Hydrogen present in the reaction stream dissociates on Pt centers and spills over onto WOx centers, generating Br6nsted acid sites [16]. This positive effect of Pt was also detected in the case of Pt/SO42-ZrO2 [8]. In spite of the tremendous effort, it is still not clear which are the active centers and why such a high amount of tungsten is necessary. It is also not clear if the reaction is initiated by the acid sites produced by tungsten or via an alkane oxidation caused by the same species, and why temperatures as high as 923 - 1123 K are required to generate the active species. In order to give answer to these questions, this study presents data on the effect of WOx loading, calcination temperature, and preparation methodology on the characteristics of the WOx-CeO2 catalysts. If, indeed, the oxidative behavior is the one that promotes the isomerization, then the replacement of ZrO2 with CeO2 should improve the performances of this catalyst in isomerization reactions.

2. EXPERIMENTAL 2.1. Catalyst preparation Two series of WOx-CeO2 catalysts containing 5, 10, 12 and 15 wt% WOx were prepared by co-precipitation and impregnation, respectively. The first series (denoted as PP) was obtained via the reaction between an aqueous solution of cerium nitrate (Ce(NOa)3.6H20, Fluka) with ammonium metatungstate ((NH4)6H2W12041 H20, Riedel-de Haen). The solution of ammonium tungstate (10 -2M) was added under vigorous stirring to the solution of cerium nitrate (0.85 M). Co-precipitation was achieved by adding aqueous ammonia (25 wt%) up to a pH=7.5. The resulting precipitate was thoroughly washed with hot deionized water until pH=7, then dried in a vacuum oven under a pressure of 30 mbar, at room temperature for 10 h and then at 373 K for another 10 h. The obtained materials were crushed and calcined for 3 h in static air at different temperatures in the range 973 1173 K. The second series of catalysts (denoted as CIT) was prepared by incipient wetness impregnation of CeO2 with ammonium metatungstate. CeO2 was prepared by the citrate method, as follows: cerium nitrate (Fluka) was dissolved in deionized water in such an amount as to give a 0.85 M solution. Citric acid (Merck) was added with an excess of 10 wt% over the stoichiometric quantity for the complete complexation of the Ce cations. The solution was partially dehydrated in a rotary evaporator at 310 K until the appearance of a colorless gel. The viscous material was then dried for 16 h in a vacuum oven set at 343 K under a pressure of 30 mbar. During this treatment~ an intense production of nitrogen oxide vapors occurred. The spongy yellowish amorphous cerium citrate obtained was decomposed in static air at 1173 K for 3 h, giving CeO2 with a specific surface area of 12.5 m2/g. The support was impregnated with 2.5 ml g-1 solutions containing different amounts of ammonium metatungstate. Drying and calcination of the catalysts were carried out after 24 h using the same protocol as for the PP catalysts.

339

2.2. Catalyst characterization Nitrogen sorption isotherms were recorded at 77 K with a Micromeritics ASAP 2000 instrument after outgassing the samples at 423 K for 12 h under a pressure of 0.1 Pa. BET formalism was applied for determination of the specific surface area. Table 1 presents the surface area of the prepared catalysts. XRD patterns were recorded on powders (40-751am) mounted on silicon monocrystal sample holders, by means of a Kristalloflex Siemens D5000 diffractometer using the Cu-Ka radiation at )~= 1.5418 J~. Data acquisition was realized in the 20 range 265 ~ with scan steps of 0.03 ~ A standard furnace disposed in the analysis chamber was used to follow the change of crystallinity with temperature. The freshly dried precursor powders (40-75pm) were mounted on a Pt sample holder and heated from room temperature to 1173 K at a rate of 1 K/s, the spectra being taken after stabilization for 1 h at each temperature. The 20 range was 20-55 ~ with scan steps of 0.03 ~ Table 1 Specific surface area of catalysts calcined at 1173 K Catalyst

WOx content, wt%

.....

CW5 5 CW10 10 CW12 12 CW15 15 CeO2 CeO2 - H20 a aCeO2 calcined at 1173 K for 3 h, impregnated another 3 h

Specific surface area, m 2 g-1 pp CIT 4.6 8.8 3.3 8.5 2.0 8.8 2.0 6.8 12.5 12.8 with water and recalcined at 1173 K for

XPS spectra were recorded on a SSX-100 spectrometer Model 206 (from Surface Science Instrument), at room temperature and under a residual pressure of 1.33 mPa. Monochromatized A1-Ka radiation (hv=1486.6eV), obtained by bombarding the A1 anode with an electron gun operating at a beam current of 12 mA and an accelerating voltage of 10kV was used. The spectrometer energy scale was calibrated using the Au 4f7/2 peak energy centered at 84 eV. The charge correction was made considering the C ls signal of adventitious carbon (C-C or C-H bonds) centered at 284.8 eV. The composite peaks were decomposed by a fitting routine included in the ESCA 8,3 D software. Atomic surface composition was calculated using the sensitivity factors (Scofield) provided by the same software, applied to the surface below the corresponding fitted XPS signals. NH3-DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectra were collected with a Brucker IF $88 spectrometer performing 200 scans with a resolution of 4 cm l . Pure samples were placed inside a commercial controlled environmental chamber (Spectra-Tech 0030-103) attached to a diffuse reflectance accessory (Spectra-Tech collector). To investigate the stability of the adsorbed ammonia species during temperature elevation, the spectra were recorded under helium (30ml/min) at room temperature, 373, 473, and 573 K, after exposure to an ammonia flow for 30 min at room temperature.

340 Raman spectra were obtained with a Dilor-Jobin Yvon-SPEX spectrometer equipped with an optical multi-channel analyzer. The Raman spectra were excited with the 488-nm line of an Ar-ion laser. 3. R E S U L T S

3.1. Textural changes induced by tungsten loading and preparation procedure The BET surface areas of the investigated catalysts as well as of CeO2 used as support for the CIT-samples are listed in Table 1. PP-catalysts have significantly lower surface area (2 to 4 times) than the corresponding CIT-materials. For the PP catalysts~ the increase of the WOx loading is consistent with a small decrease of the surface area. The CIT catalysts exhibit lower surface areas than the parent CeO2 support and, except for the CW 15 catalyst, the increase of the tungsten loading caused almost no change of the surface area. Water-"impregnated" ceria exhibited no change of the surface area compared to the initial support, indicating that a re-hydration of the support has no morphologic effect.

3.2. Effect of calcination temperature on the structure Figure 1 shows the in-situ XRD patterns of PP-samples recorded at different temperatures. The formation of cerianite form of CeO2 (JCPD-ICDD 43-1002) was observed at around 573 K, and well defined at 973 K. The formation of Ce2(WO4)3 phase (JCPD-ICDD 31-0340) was irreversible only at 973 K. The reflections corresponding to this phase were well defined at 1123 K, and the crystallization process was completed at 1173 K (Fig. 1B). The same behavior was also observed for the CIT samples (Fig. 2). The formation of this phase seems to occur as a solid-solid reaction between CeO2 and WOx. It is worth to mention that in none of the investigated catalyst, reflections attributable to tungsten oxide species have been detected.

(A)

t

a J!

,+

I

573 K

I

773 K,. 973 K ' 1173 K,

l

/

973 K' 773 K,' 20

25

30

35

40 20

45

+

i

*

+

l+

i )L,:

/ .3 , 9 ,)~ I)lj 1173K "' / . . . . . . . . . . . . . . . . . . . . -i- ~-l-IU ~'-------7,' I It , l ~ J ~ 1123K./

/ ......._J, .................!a_g

1o7_

=,.

573 K

^ ~ / . . . . . . . . . ". . . . . . / .......__..,.

I~ 973 K, ~ .........~ ~'----'----"-'7" 573 K /

50

20

40

55

25

30

35

45

50

55

20

Fig. 1. XRD patterns at increasing and decreasing temperatures for PP-CW 15 (A) and PP-CW10 (B); (I):Ce2(WO4)3; (+)" CeO2; (*) Pt sample-holder reflections.

3.3. Segregation of tungsten oxide species and covering of ceria surface The binding energy of W4fT/2 core level was in a very narrow range~ 35.5-35.7 eV, regardless of the preparation route and tungsten loading (Fig. 3). This interval corresponds to tungsten in the +6 oxidation state [17]. A very good uniformity in the binding energy

I

341 (882.5 eV) was also obtained for the Ce3ds/2 level in all tungsten-containing catalysts. This value was about 0.5 eV higher than the energy measured for Ce3ds/2 in ceria used as support [18], indicating a partial maintenance of Ce 3+ in the presence of tungsten. At the same time, the O1~ level was shifted by 0.6-0.8 eV to higher binding energies with respect to pure CeO2, again pointing to a direct contribution of tungsten-containing species. It should be mentioned that the O1~ signal was symmetric, which may indicate a homogeneous superficial composition.

~-..

5

10

15

20

25

30

-'l

-

s

. . . .

35

,

..

4O 45

50

55

60

65

20 Fig. 2. XRD pattern for CW15-CIT catalyst calcined at 1173 K; (*): Ce2(WO4)3.

Fig. 3. XPS signal in W4f region for CW10-CIT (a) and CW10-PP (b) catalysts.

Fig. 4. Variation of cation ratio as a function of W loading and preparation method.

Fig. 4 shows the variation of the XPS W/Ce atomic ratios for the two series of catalysts compared with the bulk ratios. For the PP-catalysts, except for CW15, this ratio increased with the tungsten loading. The higher surface than bulk ratios indicated a partial enrichment of tungsten at the catalysts surface, which occurred even for the co-precipitated catalysts. The smaller value of CW15 might be explained by the increase of the tungsten

342 domains, leading to XPS-silent tungsten atoms. The CIT catalysts also contained segregated tungsten species. The apparent decrease of the W/Ce ratio with the tungsten content is an artifact because impregnation resulted in several layers of tungsten which became hidden and XPS silent. 3.4. Structural modification caused by the tungsten content and p r e p a r a t i o n procedure Figures 5 and 6 present Raman spectra of selected samples. Figure 5 shows the structural changes occurring with the temperature. The spectrum of the sample calcined at 973 K contained the characteristic band of ceria at 450 cm -1 [19], and weak and broad bands corresponding to the W-O-W (700-850 cm -1) and W=O (350, 800 and 900 cm -1) modes [20, 21]. Calcination at 1173 K improved the structural organization of the samples, resulting in well-defined bands (Fig. 5, spectrum b). The intensity of these bands correlates well with the tungsten loading and their intensity ratio does not change with the loading (Fig. 6). At the same time, no shift of the bands position has been found, neither with the tungsten loading nor with the preparation route, indicating that no specific interaction occurred when the W content increased. This observation is in good concordance with the XRD patterns which presented only reflections of CeO2 and Ce2(WO4)3 in the whole range of compositions. Contrarily to the co-precipitated samples, the Raman spectra of the impregnated samples indicated different features. The increase of the tungsten loading led to an almost total disappearance of the bands assigned to ceria, while the bands corresponding to tungsten oxide became more prominent. W O

CW5

C

W

I

~

~

L__

.,... o,l (D

=

W-O-W

CW12 ~

_A.J

CW15 1(~0"2;0"3;0"4;0 5;0"6;0" 7;0" 8'00" 9;0"1000 Raman shift, cm-1 Fig. 5. Raman shift for CW5-CIT calcined at 973 K (a) and 1173 K (b).

600 650 700 750 800 850 900 950 1000 Raman shift, cm-1 Fig. 6. Raman shift for different WOx loadings" all catalysts calcined at 1173 K.

3.5. Acidity of the catalysts NH3-DRIFT spectra recorded at different temperatures showed bands due to ammonia adsorbed both on Br6nsted (1438-1390 cm -1) and Lewis (1597-1530 cm -1) acid sites. Fig. 7 shows the spectra obtained for CW10 calcined at 973 K (spectra a-d) and 1173 K (spectra e-h). The large band in the range 2500-3500 cm -1 at room temperature results from the contribution of Vas(N-H), vs(N-H), 26as(H-N-H), 26s(H-N-H) and 6as(H-N-H)

343 species adsorbed on Lewis acid sites of the surface. The increase of the desorption temperature led to a decrease of the intensity of the band located at 1430 cm1, which also shifted and splitted in two bands. The increase of the calcination temperature of the samples, from 973 to 1173 K, made the splitting clearer. This behavior might be associated to two different BrSnsted acid species. A decrease of the intensity also occurred for the band due to ammonia adsorbed on Lewis acid sites. Both Lewis and Br6nsted acid sites remained present only for the samples calcined at 1173 K. Fig. 8 shows that the acidity increased with the W loading.

Oa)

I(o)

(g) <

"~"~"'f l : ~

473

(c)_._ 573 K_

~

L ~

I (a)_.~-298K"

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber, cm 1

Fig. 7. NH3-DRIFT spectra of CW10-PP catalyst calcined at 973 K (spectra a-d) and 1173 K (spectra e-h).

4000

i

i

i

I

3500

3000

2500

2000

i

1500

!

1000

'

500

Wavenumber, cm "I

Fig. 8. NH3-DRIFT spectra for CWS-PP (spectra a, b, c) and CWl 0-PP (spectra d, e,f) calcined at 1173 K

4. DISCUSSION

Impregnation brings about a reduction of the surface area, depending on the Wcontent (Table 1). Since the water used for the impregnation and the thermal treatment applied after calcination do not affect the texture and the structure of CeO2, it appears that for the CIT-materials, the decrease of the surface area with respect to the support is linked to textural and structural modifications induced by the presence of W-containing species at the surface. For the co-precipitated catalysts, the decrease of the surface area with W content is less pronounced. The X-Ray patterns as well as the Raman spectroscopy results point to the exclusive formation of Ce2(WO4)3 with no detectable WOx crystallites on the PP-samples, as observed in the case of TiO2 or ZrO2 [20, 22]. On these catalysts, the WOx domains are well dispersed,_leading to an increased interaction between Ce and W. The case of the CITsamples is different: the XPS and Raman results indicate the formation of successive layers of W-containing species that causes a dramatic decrease of the acidity compared to the PPcatalysts.

344 5. CONCLUSIONS In conclusion, in the acid catalysts, superficial tungsten species exist as Ce2(WO4)3 structures. These structures might be formed in co-precipitated catalysts where, even for high WOx loadings~ no crystalline WO3 has been found. The appearance and the strength of the acidic sites strongly depend on the calcination temperature. NH3-DRIFT acidity measurements show that in the temperature range where alkanes isomerization occurs, the surface acidy is given both by Lewis and Br6nsted centers. The abundance of these species is related to the amount of W species. REFERENCES

1. 2. 3. 4. 5. 6. 7.

J.M. Thomas, Sci. Am., 266 (1992) 112. M. Hino and K. Arata, J. Chem. Soc. Chem. Commun., (1980) 851. K. Arata, Adv. Catal., 37 (1990) 165. B.H. Davis, R.A. Keogh and R. Srinivan, Catal. Today, 20 (1994) 219. X. Song and A. Sayari, Catal. Rev. Sci. Eng., 38 (1996) 329. S.R. Vandagna, R. A. Comelli and N. S. Figoli, Catal. Lett., 47 (1997) 259. K. Arata and M. Hino in "Proceedings, 9th International Congress on Catalysis, Calgary 1988. (M. J. Philips and M. Ternan, Eds.), p. 1727, Chem. Inst. of Canada, Ottawa, 1988. 8. D.G. Barton, S. L. Soled and E. Iglesia, Topics Catal., 6 (1998) 87. 9. M. Hino and K. Arata, J. Chem. Soc. Chem. Commun., (1987) 1259. 10. J. G. Santiestbean, J. C. Vartuli, S. Han, R.D. Bastian and C. D. Chang, J. Catal., 168 (1997) 431. 1 1.M. Scheithauer, T.-K. Cheung, R. E. Jentoft, R. K. Grasselli, B. C. Gates and H. Kn6zinger, J. Catal., 180 (1998) 1. 12. D. G. Barton, S. L. Soled, G. D. Meitzner, G. A. Fuentes and E. Iglesia, J. Catal., 181 (1999) 57. 13. R. D. Wilson, D. G. Barton, C. D. Beartsch and E. Iglesia, J. Catal., 194 (2000) 175. 14. C.D. Beartsch, S. L. Soled and E. Iglesia, J. Phys. Chem. B, 105 (2001) 1320. 15. M. G. Falco, S. A. Canavase, R. A. Comelli and N. S. Figoli, Appl. Catal. A: Gen., 201 (2000) 37. 16. K. Ebitani, J. Tsuji, H. Hattori and K. Kita, J. Catal., 135 (1992) 609. 17. J. F. Moulder, W. F. Sticle, P. E. Sobol and K. D. Bomben, "Handbook of XPS" (J. Chastain Ed.), Publ. Perkin Elmer Corp., 1992. 18. E. Paparazzo, G. M. Ingo and N. Zacchetti, J. Vac. Sci. Technol. A, 9 (1991) 1416. 19. J. R. McBride, K. C. Hass, B. D. Pointdexter and W. H. Weber, J. Appl. Phys., 76 (1994) 2435. 20. S. Eibl, B. C. Gates and H. Kn6zinger, Langmuir, 17 (2001) 107. 21. S. Kuba, P. C. Heydorn, R. K. Grasselli, B. C. Gates, M. Che and H. Kn6zinger, Phys. Chem. Chem. Phys., 3 (2001) 146. 22. W. Ji, J. Hu and Y. Chen, Catal. Lett., 53 (1998) 15.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation temperature

of

iridium

catalysts

345

by

deposition

precipitation:

room

oxidation of CO

M. Okumura, E. Konishi, S. Ichikawa and T. Akita National Institute of Advanced Industrial Science and Technology, Midorigaoka 1-8-31, Ikeda 563, Japan Iridium was supported on different types of metal oxides by deposition precipitation (DP) and liquid phase grafting (LG) methods and was examined for the oxidation of CO and H2. From these experiments, it was found that iridium supported on TiO2 prepared by DP was much more active for CO oxidation than Ir/Al203 and Ir/Fe203, and furthermore was active below room temperature. TEM observations showed that Ir was spread over the TiO2 surface as a thin layer of 2 nm thickness, the structure of which was completely different from those of other noble metal catalysts. 1. I N T R O D U C T I O N Catalytic CO oxidation has lately drawn considerable attention due to the growing applications for air purification, pollution control in automobiles, and incinerator exhaust gases. In addition to many different metal oxide catalysts, a wide variety of precious metal catalysts have been studied for low-temperature CO oxidation. Among them, it is noteworthy that Au nanoparticles deposited on oxide supports, such as A1203, SiO2, TiO2, MnOx, Fe203, and NiO, are very active for CO oxidation at room temperature [1-4]. Although Pd/SnO2 and Pt/SnO2 were known to be active for the low-temperature oxidation of CO, they often required complicated pretreatments and relatively long induction periods [5-7]. A Pd/CeO2-TiO2 catalyst was also recently reported to exhibit high catalytic activity for CO oxidation at low temperature [8]. Although Ir belongs to the 5f orbital metals as do Pt and Au, it has been investigated as a catalyst only for limited reactions, such as deNOx for automobile exhaust gases [9], hydrazine decomposition for rocket thrusters [10], and the hydrogenation of unsaturated hydrocarbons [11]. This is because Ir resources are rare and expensive. It is also worth noting that the conventional Ir catalysts were all prepared using the impregnation method. As the selection of preparation method was crucial in the production of suitable active Au catalysts, it should be considered important for us to examine the various preparation methods for Ir catalysts. In the research reported in this paper, supported Ir catalysts were prepared using the deposition precipitation (DP) and liquid phase grafting (LG) methods, and were studied for the oxidation of CO and H2 at low temperatures below 473 K.

346 2. E X P E R I M E N T A L

2.1. Catalyst preparation The metal oxides used as supports were A1203 (a reference sample from the Catalysis Society of Japan, JRC-ALO-7, specific surface area 180 m2/g), TiO2 (Degussa., P-25, 60 m2/g), and a-Fe203, prepared by calcinations in air at 627 K of the precipitates obtained from an aqueous solution of iron(III) nitrate by neutralizing with sodium carbonate. As an Ir precursor, reagent grade IrCl4 (Kishida Chemicals) and Ir(CH3COCHCOCH3)3 (Tri Chemical Laboratory) were used. (Ir(CH3COCHCOCH3)3 will hereafter be abbreviated Ir(acac)3.) Iridium catalysts were prepared by DP using the following procedure. The metal oxide support (2 g) was dispersed in an appropriate amount of an aqueous solution of IrCl4, the pH of which was adjusted to 7 (except where stated otherwise). The content of Ir in the starting solution was 1.8wt% with respect to the weight of the support. The dispersion was aged at room temperature for 1 h and was washed with distilled water several times. The solid material was vacuum-dried at 0.4 Pa for 12 h and calcined in air at 673 K for 4 h. Iridium catalysts were also prepared by LG using the following procedure. Ir was deposited on several metal oxide supports by using Ir(acac)3 as an iridium precursor. It was used without further purification of the reagent available from Tri Chemical Laboratory Inc. Instead of the aqueous solution of IrCl4, Ir(acac)3 was dissolved in acetone. A weighed quantity of metal oxide support was then introduced into this solvent and the resultant solution kept in a refrigerator overnight. The solvent and metal oxide were separated by filtration and the metal oxide with the gold absorbed precursor was calcined in air at 673 K. 2.2. Characterization TEM observations were performed with a JEOL JEM-3000F electron microscope at an accelerating voltage of 300 keV. All TEM photographs were taken at a magnification of 200,000. The TEM images were processed digitally from the negative films using a film scanner. Size distribution measurements for Ir particles were performed on digital images using the image analyzing software Image-Pro. One pixel corresponded to 0.06 nm in the images. The electron energy loss spectroscopy (EELS) spectra were measured by CCD camera at an exposure time of 2-3 s using a Gatan Imaging Filter under an energy dispersion of 0.2 eV per pixel. Energy resolution estimated from the full width at half-maximum (FWHM) of the zero-loss peak was approximately 1.2 eV. The amount of gold loading for each catalyst sample was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES).

2.3. Catalytic activity measurements and Characterization The powder catalyst sample (100 rag) was placed on a ceramic wool plug in a quartz tube with an inner diameter of 6 ram. Before measurement of catalytic activity, the catalyst sample was pretreated in a stream of 20 vol% H2 in Ar at space velocity (SV) = 20 000 h-lml/g-cat, and at 523 K for 1 h in a fixed-bed flow reactor. The activity

347 measurements for the oxidation of CO and of Ha were performed by passing the reactant gas (lvol% CO or Hz in air) at a flow rate of 33 ml/min (SV = 20 000 h-lml/g-cat). The reactant and effluent gases were analyzed using a gas chromatograph (Shimadzu GC-8A) with a thermal conductivity detector (TCD) and a column of molecular sieves 13X (5 m) at 333 K. 3. R E S U L T S AND DISCUSSION 3.1. the oxidation of CO and H2 o v e r Ir catalysts Fig. 1 shows the conversion vs. temperature curves for CO oxidation over Ir catalysts prepared by DP and LG methods. The pH of the Ir precursor solutions used in the DP method was adjusted to be 7. The conversions in CO oxidation at each temperature shown in Fig. 1 were after 30 minutes duration. Among the Ir catalysts, only the Ir/TiOz catalyst prepared by DP showed extremely high catalytic activity at room temperature. Fig. 2 shows the conversion vs. temperature curves for Hz oxidation over Iridium catalysts pretreated by hydrogen reduction. Comparing the obtained conversions in Ha oxidation with those in CO oxidation, it was found that the temperatures for 50% conversion of Hz oxidation over Ir catalysts, except for Ir/TiOz-DP, was similar to those in CO oxidation. Over Ir/AIzO3-DP and Ir/FezO3-DP catalysts, Hz oxidation proceeds at lower temperatures than CO oxidation. This feature is similar to those of other typical noble metal catalysts. It should be noted that CO oxidation over the Ir/TiOz catalyst takes place at temperatures even below room temperature and at much lower temperature than Hz oxidation. This feature is the same as that of highly dispersed Au catalysts [1-4]. These results indicated that the support effect for the CO oxidation was much larger over Ir catalysts than that over Au catalysts.

100

t--A

9

~

9

loo

_/J /U

/.J

/r/ o

IL /

/i2/o ,i

60

40

40

o/? /Af

r~ ./ / A / / /

20 / .-" t , ~ 7" 275 300 325 350 375 400 425 450 475 500 525 550 575 Reaction temperature / K

Fig. 1. Conversion of CO over Ir catalysts prepared by deposition precipitation (DP) and liquid phase grafting (LG).

0 ' 300 250

2

/ @ - 9 ~//'---

350

400

/J /t

/

// / I

/ /

450

.

,

500

I - o - - Ir/Fe'O'-DP --A-- Ir/Ti~) -~DP --n--Ir/al(~-LG I --A-- Ir/Ti~) -3LG

.l

550

600

650

Reaction temperature / K

Fig. 2. Conversion of Hz over Ir catalysts prepared by deposition precipitation (DP) and liquid phase grafting (LG).

348 From ICP analyses of the Ir/TiO z, Ir/Al20 3, and Ir/Fe20 3 catalysts prepared by DP, the actual Ir loadings were 1.14, 1.57 and 0.81 wt%, respectively. Comparatively, the actual Ir loadings of Ir/TiO 2, Ir/AlzO 3, and Ir/FezO 3 prepared by LG were 0.31, 0.22 and 0.03 wt%, respectively. These results indicate that the actual Ir loadings of Ir catalysts prepared by LG are much lower than those prepared by DP. This might be due to the bulky ligand structure of Ir(acac) 3, causing the interaction between the support surface and the precursor to become weak. 3.2. Effect of the pH control of the deposition precipitation In order to investigate the preparation conditions of DP for Ir/TiOz, the pH control of the aqueous solution of IrCl4 was examined. This is because the pH of the aqueous solution of HAuC14 is an important factor in controlling the size of Au particles deposited on supports during Au catalyst preparation. The pH of aqueous IrC14 solution was adjusted (using NaOH) to a fixed point, ranging from 3 to 8. Fig. 3 shows the time-on-stream changes for the conversion of CO over the Ir/TiOz catalysts prepared by DP under different pH conditions. Although gradual deactivation was observed, the complete oxidation of CO over Ir/TiOz prepared by DP at 7 pH took place for at least 1 h at 300 K. In comparison, those prepared at 3, and 5 pH showed much less activity and that prepared at 8 pH showed higher activity. The Ir/TiOz catalyst prepared at 8 pH was pretreated in an air stream instead of a hydrogen-containing stream at 523 K for 30 rain; it was not active for CO oxidation at room temperature, which was clearly different from the high catalytic activity of Ir/TiOz pretreated in a hydrogen stream. This difference indicates that the Ir metal phase produced by hydrogen reduction pretreatment is active for CO oxidation at low temperatures, whereas the oxidized phase of iridium metal is much less active. ITITN--IIIIIIIIIII ff I 100 ~ I - I T q ,

80 o

i

O

60

0 0 .,.~

0 r..)

40

20 ~

0

,

I

~

I

a

I

n

I

n

I

n

I

,

I

,

I

,

I

,

50 100 150 200 250 300 350 400 450 500 550

Time on stream / min Fig. 3. Conversion of CO as a function of the reaction time in CO oxidation over Ir/TiOz catalyst prepared by the DP method under different pH conditions and the in the stream of humidified reactant gas. 9 :pH=3 (6ppm HzO), 9 :pH=5 (6ppm HzO), 9 :pH=7 (6ppm HzO) 9 :H=8 (6ppm HzO), 9 : pH=8(6000ppm HzO).

349 From ICP analyses of the Ir/TiOz catalysts prepared at 3, 5, 7, and 8 pH, the actual Ir loadings were 0.57, 0.64, 1.14, and 1.48 wt%, respectively, showing that the Ir loading was maximum at 8 pH. The interaction between Ir 4+ and the TiOz surface increases above the point of zero charge (PZC) of TiOa because its surface is negatively charged. The solubility of iridium hydroxide decreases with an increase in pH of the starting solution. These two reasons can account for the maximum Ir loading at 8 pH. 3.3. Moisture effect for CO oxidation over Ir catalysts In order to investigate the moisture effect on CO oxidation, the reactant gas containing moisture at approximately 6 ppm was humidified to a level of 6000 ppm. As shown in Fig. 3, the catalytic activity of Ir/TiOa prepared by DP at 8 pH increased with the increase of the moisture in the reactant gas. The Ir/TiOa catalyst could maintain 100 % conversion of CO oxidation for 7 h in the presence of 6000 ppm moisture. This characteristic is advantageous for practical applications as a low-temperature environmental catalyst. The water-promoted oxidation of CO was also reported over Pd/SnO2 and Au/TiOa [12, 13]. 3.4. TEM observations of Ir catalysts TEM images of Ir/AlzO3 and Ir/FezO3 are shown in Fig. the deposited Ir was found to spread over the AlaO3 surface as a 5 nm thickness. Conversely, small clusters were deposited on results indicated that the selection of support greatly influences deposited.

4. In the case of Ir/AlzO3, thin layer of approximately the FeaO3 surface. These the surface structure of Ir

Fig. 4. (a) TEM image of Ir/FeaO3 prepared by DP and (b) TEM image of Ir/AlaO3 prepared by DP. The TEM images of the Ir/TiOa prepared by DP at 8 pH are shown in Fig. 5 (a-l), (a-2) and (b). The deposited Ir was found to spread over the TiOa surface as a thin layer of approximately 2 nm thickness, i.e., much thinner than that of Ir/AlaO3. These structures were completely different from those of other noble metal catalysts, where the noble metals

350

Fig. 5. TEM images of the Ir/TiO2 catalysts prepared by the DP method followed by the pretreatment at 523 K (a-l) prepared at pH 8 and calcined in air, (a-2) prepared at pH 8 and calcined in air, (b) prepared at pH 8 and calcined in the hydrogen stream, (c) prepared at pH 3 and calcined in air, (d) prepared at pH 5 and calcined in air.

351 were deposited on the supports as fine particles. Figure 6 shows the O K-edge EELS spectra obtained from the TEM images. EELS spectra were obtained for both Ir-rich and Ir-poor parts of Ir/TiOz. From this analysis, it was found that Ir particles were selectively deposited onto the rutile TiOz surface. This suggested that the interaction between Ir precursor and the rutile and anatase TiOz surface could be different. Comparing the Ir phase lattice constant calculated from the TEM images of Ir/TiOz, the metal phase of Ir was found to be produced by the hydrogen reduction pretreatment, while the iridium oxide was formed by calcination in air. Also, the deactivation during the time-on-stream in the room temperature reaction seemed to occur while the active metal phase of the catalyst was re-oxidized during the catalytic reaction. TEM images of Ir/TiOz prepared by DP at 3 and 5 pH are also shown in Fig.5 (c) and (d). In the case of these catalysts, aggregates of clusters and small clusters were respectively deposited as the major species on the TiOz surfaces. This result and that of ICP analysis produce good evidence that O-Kedge the amount of loading and the structure of the Ir can be controlled by changing the pH ,~. of the precursor solution while using the "~ DP method. From these analyses, it was found that the 2 nm thick Ir layer had a hetero-junction between the rutile TiOz .~' surface and the large surface area of the =~ exposed Ir metal surface, due to the thin 2 layered structure of Ir. Our work therefore suggests that the activity of CO oxidation was directly associated with the structure of the Ir metal phase having nano-level I I I 520 540 560 580 600 thickness on the TiOz support, and the E n e r g y L o s s [ e V ] selection of the support. -

9 I

"

"!"

Fig. 6. EELS spectra of (1) Ir rich part of Ir/TiOz and (2) Ir poor part of Ir/TiOz 4. C O N C L U S I O N S As the actual loadings of Ir catalysts prepared by DP were higher than those obtained by LG, the DP method was found to be more efficient for the preparation of Ir catalysts. In the view point of the catalytic activity, the reaction temperature for the catalytic oxidation of CO over the Ir/TiOz catalyst prepared by DP was lower by more than 100 K than the Ir/TiO2 prepared by DP, Ir/Fe203 and Ir/A1203 prepared by DP and LG, with those showing full conversion only at temperatures above 400 K. From TEM observation, it was found that the deposited Ir was spread over the rutile TiO2 surface as a thin layer of approximately 2 nm thickness.

352 It can therefore be assumed that this structure is a key to promoting high activity for CO oxidation at low temperatures. ACKNOWLEDGMENTS This research work was supported by a NEDO Technology Research Grant Program in ~)0 (Project ID: 00A38006b). REFERENCES

1. M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet and B. Delmon, J. Catal., 144 (1993) 175. 2. M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal., 115 (1989) 301. 3. M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma and M. Haruta, Cat. Lett., 51 (1998) 53. 4. M. Okumura and M. Haruta, Chem. Lett., (2000)396. 5. T., Engel and G. Ertl, Adv. Catal., 28 (1979) 1. 6. D.S. Stark and M. R. Harris, J. Phys., E21,715 (1988) 715. 7. D. R. Schryer, B. T. Upchurch, J. O. van Norman, K. G. Brown and J. Schryer, J. Catal., 122 (1990) 193. 8. G. Dong, J. Wang, Y. Gao and S. Chen, Cat. Lett., 58 (1999) 37. 9. R. Butch, Catal. Today, 35 (1997) 27. 10. K. Komatsu, SHOKUBAI, 39 (1997) 216. 11. E. N. Bakhanova, A. S. Astakhova, K. A. Brikenshtein, V. G. Dorokhov, V. I. Savchenko and M. L. Khidekel, Izv. Akad. Nauk SSSr, Ser. Khim., 9 (1993) 1972. 12. G. Croft and M. J. Fuller, Nature, 269 (1977) 585. 13. M. Date and M. Haruta, J. Catal., 201 (2001) 221.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 ElsevierScienceB.V. All rights reserved.

353

New approach to preparation and investigation of active sites in sulfated zirconia catalysts for skeletal isomerization of alkanes N.A. Pakhomov 1, A.S. Ivanova 1, A.F. Bedilo 2, E.M. Moroz 1 and A.M. Volodin 1 1Boreskov Institute of Catalysis, Prospect Akademica Lavrentieva, 5, Novosibirsk 630090, Russia 2 Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, USA

Catalysts active in the isomerization of n-butane have been synthesized by depositing sulfate ions on well-crystallized defective cubic structures based on ZrOz. This technique for introduction of sulfates does not result in any significant changes in the bulk properties of zirconium dioxide matrix. Active sulfated catalysts were prepared on the basis of cubic solid solutions of ZrO2 with calcium oxide and on the basis of cubic anion-doped ZrO2. The dependence of the catalytic activity on the amount of calcium appeared to have a maximum corresponding to 10 mol.% Ca. Radical cations formed after adsorption of chlorobenzene on activated catalysts have been used as spin probes for detection of strong acceptor sites on the surface of the catalysts and estimation of their concentration. A good correlation has been observed between the presence of such sites on a catalyst surface and its activity in isomerization of n-butane. 1. I N T R O D U C T I O N Great interest in the development and investigation of novel catalysts for isomerization of normal butane to isobutane is stimulated by an increasing demand in isobutane and isobutene used as precursors for preparation of high-octane and ecologically benign gasoline components - methyl-tert-butyl ether and alkylates. Sulfated zirconia catalysts are some of the most promising new materials suggested for this purpose. The interest to these materials was characterized by an exponential growth in the number of publications devoted to them in the late 90s [1-4]. However, our analysis of the published papers indicates that despite unique activity of sulfated zirconia shown at laboratory scale, the overall performance and stability of these catalysts are not sufficient for practical industrial application. Their main drawback consists not only in fast decrease in the catalytic activity due to coking but also in irreversible deactivation of the catalysts after their oxidative regeneration. Significant breakthrough in the development of an effective catalyst based on sulfated zirconia can be obtained only though fundamental studies on the nature of the

354

catalytic action and structure of the active sites. Unfortunately, after a decade of intensive investigation there is still no unambiguous understanding of exact structure of active sites accounting for the acidic and catalytic properties of sulfated zirconia and processes leading to their formation. This confusion has a lot to do with the traditional method used for preparation of active catalysts when hydrated zirconia used as a precursor is impregnated with a solution containing sulfate ions, with the following calcination at desired temperature [4]. In this case sulfates can be redistributed between the surface and bulk of the new ZrO2 phase formed during the dehydration. Obviously, this makes it difficult to follow the genesis of the active surface sites. We have recently suggested a new approach to the preparation of active sites in sulfated zirconia catalysts [5, 6]. In this case, the catalysts are prepared by deposition of sulfate ions on crystalline zirconium dioxide samples with highly defective structure. According to numerous reports, the monoclinic phase typical for ZrO2 is not suitable for this purpose. We have shown that active materials could be obtained by impregnation of zirconia-based oxides with cubic crystalline structure. It should be noted that the cubic structure is not thermodynamically stable for pure zirconia at low temperatures. It can be stabilized by introducing different additives, in particular, alkaline-earth metal cations [7]. Recently, similar results have been obtained for ZrO2 stabilized by Y203 [8]. In the present communication we report the regularities in the preparation of high surface area zirconia-based solid composites stabilized by calcium and barium, as well as the structural and catalytic properties of materials obtained by their sulfation. 2. E X P E R I M E N T A L

2.1. Samples Zirconium dioxide and zirconium-calcium (or barium) oxide composites were prepared by precipitation from solutions of corresponding nitrates with aqueous bases at definite pH and temperature followed by washing, drying and calcination according to [8]. The final thermal treatment was calcination at 700~ in dry air flow for 4 h. Then, the samples were subjected to incipient wetness impregnation with ammonium sulfate (Fisher, Certified A.C.S.) to yield nominal SO3 !oading of 8 wt.%, dried at 120~ overnight and calcined in a muffle furnace at 600~ for 2 h. The sulfating procedure had practically no effect on the specific surface area of the samples. Sulfated samples will be hereafter designated as NZSCa(Ba) -A, where N is the number of sample, A is the molar concentration of CaO or BaO in the initial composites. A sample of sulfated zirconia prepared according to a traditional method (denoted as SZ) was used for comparison. Hydrous zirconia was precipitated from an aqueous solution of ZrOCI2*8H20 (Alfa Aesar, 99.9% metals basis) with aqueous ammonium hydroxide (Fisher, 28-30 wt%) added dropwise under continuous stirring up to pH ~10. The precipitate was filtered, thoroughly washed with hot distilled water, dried at 120~ overnight, and subjected to incipient wetness impregnation with ammonium sulfate (Fisher, Certified A.C.S.) to yield nominal sulfur loading of 17 mol.%. The resulting material was dried at 120~ overnight and calcined at 600~ in a furnace for 2 h.

355 2.2. Testing The catalytic activity in n-butane isomerization was tested in an integral flow reactor in the kinetic region at 200~ and n-butane space velocity 600 h -1. One cm 3 of a catalyst with the grain size in the range of 0.25-0.5 mm was used. Prior to the catalytic tests, the samples were subjected to a standard pretreatment. They were calcined in situ in a dry air flow at 500~ for 1 h, and cooled down to the reaction temperature. Then, the reactor was purged with nitrogen for 15 min prior to the introduction of n-butane. The reaction products were analyzed in a gas chromatograph with the thermal conductivity detector. Besides isobutane, which was the major product, methane, propane, and pentane were detected. After 1 h on stream the catalysts were subjected to oxidative regeneration in flowing air for 30 minutes at 500~ XRD spectra were recorded on a URD-6 diffractometer with Cu K~ irradiation. The lattice parameters were determined with the precision of + 0.002 ,~. X-band ESR spectra were recorded at room temperature on an ERS-221 spectrometer. Prior to adsorption of chlorobenzene (Reakhim, For chromatography), the catalysts (0.1 g) were activated in quartz ampoules in air at 500~ for 2 h. 3. RESULTS AND DISCUSSION Table 1 presents the results of the XRD analysis of zirconia-based materials before and after the sulfation stage. The materials were prepared using different types and amounts of alkaline earth metal oxides and different precipitation pHs. In good agreement with earlier reported data [7], introduction of a small amount of Ca (as low as 5 mol.%) results in the formation of a cubic solid solution Zrl_• An increase of the CaO concentration to 10 mol.% leads to complete disappearance of the monoclinic phase in the material. This is accompanied by a small growth of both the lattice parameter (a) of the cubic phase and the surface area. The increase of the CaO concentration to 50 mol.% results in the formation of CaZrO3 perovskite and significant decrease in the amount of the solid solution based on cubic ZrOz. The average size of the cubic phase crystallites (coherent scattering area) determined from the XRD data does not vary much from sample to sample and is about 12-13 rim. As shown earlier [7], the effect of BaO addition on the phase composition is different from that of CaO. The monoclinic phase is observed up to the BaO content of 25 mol.%. At lower concentrations tetragonal ZrOz is present beside the monoclinic phase, while the cubic phase is not formed (Table 1, sample 9ZrBa-10). Thus, the effect of the barium oxide addition is similar to that of sulfate ions when they are added to hydrated zirconia before calcination. The classic method for the preparation of sulfated zirconia catalysts involving impregnation of hydrated zirconia with a source of sulfate ions followed by calcination at a temperature not exceeding 600-700~ is known to yield tetragonal ZrO2 [3]. The data on the phase composition of ZrOz samples having no alkaline earth metals is worth discussion as well. Their phase composition is largely determined by the precipitation pH. Sample 1ZS-0 precipitated under basic conditions (pH = 9) shows only monoclinic phase after the thermal treatment. Meanwhile, the sample prepared by precipitation under neutral condition (pH = 7) has both monoclinic and cubic crystalline phases. The latter can be attributed to the anionic modification of ZrOa, in this case, with

356 NO3- anions. The precipitation of hydrated zirconia is known to involve the formation of polyhydroxocomplexes (PHC), which have different composition depending on pH [10]. At low pH, the PHCs are formed from positively charged complexes of low molecular weight. In this case, the PHCs may include some anions present in the solution. Therefore, the precipitate formed under neutral conditions may have some inclusion anions, which may be not removed during the washing stage. Some of these anions may be retained even after calcination and stabilize the cubic crystalline phase. As hydrated zirconia precipitated under basic conditions does not contain anions in any significant concentration, its subsequent calcination results in the formation of a standard monoclinic crystalline phase of ZrO2. Table 1 Phase compositions and surface areas of zirconium-calcium (barium) oxides after calcination and sulfation MO

Sample

Precipitation pH

Type

Calcination at 700~

Concentration, mol.%

BET S.A.,

mZ/g

Sulfation and calcination at 600~

Phase composition*

a (cubic), nm

Phase composition

a

(cubic), nm

1ZS-0

7

-

0

75

C > M

0.5120

C > M

2ZS-0

9

-

0

50

M

_

M

_

3ZSCa-5

9

CaO

5

60

C>> M

0.5124

C>> M

0.5124

4ZSCa-10

9

CaO

10

70

C

0.5129

C

0.5129

5ZSCa-25

9

CaO

25

90

C

0.5130

C+ CaSO4

6ZSCa-50

9

CaO

50

70

C (traces) + CaZrO3

0.5150

C (traces) + CaZrO3 + CaSO4

7ZSBa- 10

9

BaO

10

85

T+M

T +M

*C - cubic 9T- tetragonal; M- monoclinic phases, respectively. As one can see from Table 1, subsequent sulfation of calcined samples does not lead to any significant changes of their bulk properties. For instance, neither undoped zirconia nor calcium-zirconium oxide samples experience any changes in the phase composition or the lattice parameter. Minor differences are observed only for samples with CaO concentration of 25 mol.% or higher. Here XRD detects the formation of the CaSO4 phase. Most likely, it is formed by sulfation of free CaO that is not bound into the solid solution. Such free calcium oxide may be present in the initial samples in a highly dispersed state. Sulfation also does not result in any significant changes in the surface area of the samples.

357 Figure 1 presents the dependence of the catalytic activity and isobutane selectivity of samples precipitated at pH - 9 on the CaO concentration. This study made it possible to evaluate the role of the cubic solid solution in their catalytic properties. One can see an obvious maximum on the dependence of the activity on the alkaline earth metal concentration in the structure of Zrl_,,Ca• solid solution. Without calcium the catalytic activity is low. This agrees with the literature data that sulfation of a calcined monoclinic phase without any special treatment does not yield an active catalyst [9]. The maximum activity is observed for the samples with the CaO concentration of 5-10 mol.%. The material with the calcium concentration of 50 mol.% does not show any catalytic activity at all. This sample is mostly composed of perovskite crystalline phase CaZrO3. A comparison of the catalytic activity of the samples with their surface areas clearly indicates that the activity growth is mostly caused by the formation of the cubic phase rather than just by an increased surface area.

i00

E

A v

80 30-

9

1

25-

A

1

2 3

201

151050-

0

10

20 30 CaO, mol. %

40

50

Fig. 1. Dependence of n-butane conversion (X) and isobutane selectivity (S) of ZrOz-CaO-SO4 z- catalysts on the CaO concentration. The samples were precipitated at pH = 9 (Table 1). Time on stream: 5 min (1), 30 rain (2), and 60 rain (3).

The conclusion that cubic zirconia yields active catalysts after sulfation finds further proof in the comparison of the properties of ZrO2 samples without any alkaline earth metal dope. Table 2 shows that sample 1ZS-0 prepared by precipitation under neutral conditions and having some cubic phase has much higher activity than purely monoclinic sample 2ZS-0. However, the anion stabilization of the cubic phase does not seem to present any practical interest since catalyst 1ZS-0 is subjected to a relatively fast deactivation. Its activity after the first isomerization-regeneration cycle is several times lower than its initial

358

activity. This deactivation also results in a decrease of the surface area, apparently, due to the further loss of the modifying anions. The difference between the effects of barium oxide and calcium oxide dopes on the phase composition of the binary systems is reflected in the catalytic properties of sulfated samples as well. As shown in Table 3, at the same concentration of the additive the catalytic activity the calcium-doped sample 4ZSCa-10 is higher than that of the bariumdoped sample 7ZSBa-10. We attribute this difference to different compositions of the samples. According to the XRD data (Table 1), prior to sulfation sample 4ZSCa-10 was composed primarily from the solid solution with cubic structure, while sample 7ZSBa-10 contained a mixture of the monoclinic and tetragonal phases. Unlike the monoclinic phase, the tetragonal phase can also yield sites active in butane isomerization upon sulfation. Table 2 Structure of ZrO2-SO4 2- samples prepared by various methods and their catalytic properties in n-butane isomerization Sample

Precipitation pH

Calcination T, oC

Sulfation a) method

structure

2ZS-0

9

700

1

M

1ZS-0

7

700

1

C>M

SZ

10

600

2

ZrOz

Cycle

T

a) 1 -- sulfation of calcined crystalline zirconia, 2 zirconia; b) x - n-butane conversion, S - isobutane selectivity.

BET,

X b)

mZ/g

%

1

50

6.7

93.0

1

75

25.5

86.8

2

43

9.2

90.8

1

110

22.1

92.2

'

S b)

' mol.%

2 110 16.3 90.2 sulfation of amorphous hydrated

Table 3 Catalytic properties of ZrOz-MO-SO4 2- samples in n-butane isomerization MO Sample Type

Concentration, mol.%

n-Butane conversion, %

Selectivity to isobutane, mol.%

4ZSCa-10

CaO

10

23.8

91.4

7ZSBa-10

BaO

10

15.0

92.8

The textural and catalytic properties of the most active ZrOz-CaO-SO4 2- sample prepared by our technique are similar to those of a traditional sulfated zirconia catalyst

359 with tetragonal crystalline structure prepared by sulfur deposition on amorphous hydrated zirconia (Table 2). These results seem to be a conclusive evidence of the fact that stabilization of zirconium dioxide in the tetragonal phase is not a mandatory requirement for synthesis of active catalysts. An important feature of catalysts prepared by sulfation of cubic solid solutions is their improved resistance to deactivation after the oxidative regeneration from carbonaceous residues formed during the reaction. A traditional sulfated zirconia catalyst was completely deactivated after 2-4 isomerization-regeneration cycles under our conditions. Meanwhile, ZrOz-CaO-SO4 2- catalysts could sustain more than a dozen of such cycles (Fig. 2). Subsequent deposition of additional sulfates on a partially deactivated sample resulted in complete regaining of its initial activity. As the catalyst deactivation from cycle to cycle is, most likely, due to a partial loss of sulfate groups from the surface, it appears that the sulfate groups are bound stronger to the calcium-containing cubic solid solutions than to tetragonal zirconia. Since the structure of the solid solutions is not affected by the sulfur loss, the active sites could be generated again by a second sulfation in almost the same number and strength. Meanwhile, the removal of sulfur from tetragonal sulfated zirconia catalysts results in their transformation to the monoclinic phase, which does not yield active catalysts upon sulfation.

100 t3

E

90

2

80 25 20 15 10 5

9

0

4

8

12

16

Number of cycle

20

2

24

Fig. 2. Dependence of the initial n-butane conversion (X) and isobutane selectivity (S) of 4ZSCa-10 (1) and SZ (2) on the number of isomerization - regeneration cycles. The arrow shows the growth of the catalytic activity after a second sulfation of 4ZSCa- 10. Finally, we would like to say a few words on the nature of the active sites in sulfated zirconium-calcium catalysts. For testing of strong acceptor sites on the surface of sulfated zirconia catalysts we have developed an original ESR technique that uses chlorobenzene and other organic molecules as spin probes. This technique had been described in detail

360 earlier [6, 11]. A good correlation has been found between the concentration of strong surface acceptor sites registered by ESR and the catalytic activity of various sulfated materials of different composition and crystalline structure in butane isomerization reaction. This approach makes it possible to use express ESR analysis to predict the activity of different catalysts in this reaction without performing the catalytic experiments. Thus, we have suggested a new approach not only to the generation of active sites in sulfated zirconia-based catalysts for skeletal isomerization of alkanes, but also to investigation of their formation mechanism. The possibility of synthesis of active surface sites by deposition of sulfate ions on crystalline doped zirconia materials with defective cubic structure without changing the bulk properties of the samples opens many new opportunities for investigation of their nature. ACKNOWLEDGEMENTS This work has been supported by the Russian Foundation for Basic Research (Grants 00-03-32441 and 00-15-97440). REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

K. Arata, Adv. Catal., 37 (1990) 165 T. Yamaguchi, Appl. Catal., 61 (1990) 1 X. Song and A. Sayari, Catal. Rev. - Sci. Eng., 38 (1996) 329 D. Farcasiu, A. Ghenciu and J.Q. Li, J. Catal., 158 (1996) 116 A.F. Bedilo, A.S. Ivanova, N.A. Pakhomov and A.M. Volodin, 4 th European Congress on Catalysis. EUROPCAT-IV. Book of Abstracts. (1999) 118. A.F. Bedilo, A.S. Ivanova, N.A. Pakhomov and A.M. Volodin, J. Mol. Catalysis, A 158 (2000) 409. A.S. Ivanova, E.M. Moroz, and G.S. Litvak, React. Kinet. Catal. Lett., 65 (1998) 169. C. Morterra., G. Cerrato., G.Meligrana, M.Signoretto, F.Pinna, and G. Strukul, Catal. Lett., 73 (2001) 113. C.R. Vera, J.C. Yori, and J.M. Parera, Appl. Catal. A: General, 167 (1998) 75. A.S. Ivanova, Kin. Catal., 42 (2001) 354. G.V. Timoshok, A.F. Bedilo and A.M. Volodin, React. Kinet. Catal. Lett., 59 (1996) 165.

Studiesin Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

361

Supported ruthenium carbido-cluster catalysts for the catalytic removal of nitrogen monoxide and sulfur dioxide: the preparation process monitored by sulfur K-edge X-ray absorption near-edge structure Yasuo Izumi," Taketoshi Minato," Ken-ichi Aika," Atsushi Ishiguro, b Takayuki Nakajima, b and Yasuo Wakatsuki b

aDepartment of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan

blnstitute of Physical and Chemical Research, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan

The preparation process of ruthenium carbido-cluster catalysts for the reduction of sulfur dioxide was traced by means of sulfur K-edge X-ray absorption near-edge structure (XANES). During the activation process, a pair of peaks at 2472.5 - 2472.8 and 2 4 8 2 . 7 2482.8 eV appeared. The pair was ascribed to the RuS x phase. Once the catalysts were activated at 503 - 573 K, another pair of peaks at 2474.2 and 2479.2 eV appeared. The pair was assigned to the n* and o* transition peaks, respectively, of adsorbed SO 2 molecules on the catalyst surface.

1.

INTRODUCTION

Catalytic removal of nitrogen oxides [1] and sulfur oxides [2] is one of the most important issues

in

environmental

problems.

carbido-cluster catalysts derived

We

have

investigated

from [Ru6C(CO)16]2- crystal.

supported

ruthenium

The supported cluster was

362

found to be good catalyst for sulfur dioxide reduction to elemental sulfur by hydrogen gas [3, 4] and nitric oxide reduction by carbon monoxide [5]. The difference in catalytic activity between supported cluster catalysts and conventional impregnated catalysts is interpreted on the basis of the metal site structure (coordination and particle size), the structural/electronic effects of support materials to the metal site, and the kinds of adsorbed/intermediate species on the surface [6-8]. In the cases of SO 2 and NO reduction, the former two factors were recently investigated by ruthenium K-edge extended X-ray absorption fine structure (EXAFS) [4,5,9]. In this manuscript, the third factor was studied by sulfur K-edge X-ray absorption near-edge structure (XANES). Infrared absorption spectroscopy is often used to monitor the adsorbed/intermediate species on the surface. However, by infrared absorption, the atoms dissociated from reactant molecules that are buried into the catalysts are often inaccessible, e.g. sulfur atom of SO 2 dissociated and reacted to form the RuS x phase. The major objective of this paper is to monitor both adsorbed and buried sulfur atoms by S K-edge XANES in the preparation process of supported ruthenium catalysts.

2.

EXPERIMENTAL SECTION

The crystal of [(Ph3P)zN]z[Ru6C(CO)16] (1) was reacted in purified THF in argon atmosphere with titania. The support material was fixed to TiO 2 because the cluster on TiO 2 exhibited higher activity for the SO 2 reduction than the cases supported on other inorganic oxides. As a reference, a conventional Ru catalyst was impregnated with aqueous RuCI3~

solution. Ru content was 1.5 wt% in both cases. These catalysts are denoted as

[RuaC]/TiO 2 and conv-Ru/TiO 2, respectively. The incipient catalysts were heated in vacuum for 1.5h and then reduced for 3h with hydrogen both at 573 K. The catalysts were reacted in a gas flow system (flow rate 60 cm 3 min 1) of SO 2 (33.8 kPa) + H 2 (67.5 kPa) at 4 4 3 - 573 K. Produced elemental sulfur was flushed in the argon flow at each reaction temperature for 2h to be separated from the catalyst, and the catalyst sample was transferred to in-situ glass cell and sealed by fire. The cell is made of Pyrex glass with the 12.5 pm-Kapton (Toray-Dupont) films on both sides.

363 S K-edge XANES data were measured in fluorescence excitation mode at KEK-PF (Tsukuba, Japan) using beamline 9A with a Ni/Rh coated harmonic rejection mirror and a fully-tuned S i ( l l l ) double crystal monochromator under ring conditions of 2.5 GeV and 390 - 270 mA. The entire path of the X-rays beam was in a He atmosphere. The samples were at room temperature. The detection gas was nitrogen for ion chamber to collect the X-ray fluorescence originated from the sample [10]. The photon energy was calibrated by assigning the maximum of the first pre-edge feature in the XANES spectrum of NazS203o5H20 to 2472.02 eV [11].

3.

RESULTS

The sulfur K-edge XANES spectra of ruthenium catalysts are shown in Fig. 1A. The [Ru6C]/TiO 2 samples were reacted in SO 2 + H 2 at 443 (a) and 503 K (b). The conv-Ru/TiO 2 samples were reacted in SO 2 + H 2 at 473 (e) and 573 K (f). Both catalysts were inactive at 443 K, and became active when elevated to 468 and 503 K, respectively, for the reaction to catalytically produce elemental sulfur. These spectra in the process of active catalyst preparation are compared to spectra of amorphous RuS x (x = 2, 3, 6) species prepared from the mixture of H2S (10.1 kPa) and H 2 (91.2 kPa) gas at 573 K [9] (c), SO 2 (101.3 kPa) adsorption at 290 K (d) both over [Ru6C]/TiO 2 catalysts, and standard inorganic and organometallic compounds (Fig. 1B) for elemental sulfur S8 (g), bulk R u S (x - 2, 3, 6; the mixture of these, but 2 may be major) (h), the crystal of Ru3(CO)9(~tz-H)z(~t3-S) (i), Ru6C(CO)I~(p3-SO) (j), and [(Ph3P)zN]2[Ru6C(CO)15(~t2-SO2)] (k)[12]. Two typical strong peaks were observed at 2474.2 and 2479.2 eV for activated [Ru6C]/TiO 2 catalyst (Fig. lb). The two peaks were also observed at the same energy positions for activated conv-Ru/TiO 2 catalyst (f). These two peaks are denoted as Pair A. Compared to these activated catalysts, two strong peaks were observed at 2472.8 and 2482.7 eV for inactive [Ru6C]/TiO 2 catalyst (a) and at essentially the same energy positions for inactive conv-Ru/TiO 2 (e). These two peaks are denoted as Pair B. The trend of peak intensity increase/decrease was followed by progressive augmentation of the reaction temperature: 443 (Figure la), 473 (not shown), and 503 K (b) for the [Ru6C]/TiO 2 catalyst and at 473 (e), 523 (not shown), and 573 K (f) for the conv-Ru/TiO~ catalyst. The ratio of peak intensity

364

did not change in each Pair A and B, strongly suggesting that two peaks in a pair were derived from the common kind of sulfur site. Two strong peaks appeared at similar energies (2475.6 and 2479.3 eV) to Pair A for [Ru6f(CO)]5(~tz-SOz)] z- (Figure lk). The differences from the corresponding peak in Pair A

(Figure lb, f) were 0.1 - 1.4 eV. When SO z gas was adsorbed on the [Ru6C]/TiO z at 290 K (d), two strong peaks appeared exactly at the same energies as (b). Thus, Pair A was ascribed to adsorbed S O z molecules on the [Ru6C]/TiO z catalyst.

Two peaks at 2474.2 and

2479.2 eV are ascribed to the transitions to :t* and o*, respectively [13].

I

I

I

I

.f__ 24 e c c

16

-0

C

N

E 0

Z

8-

2460 Figure 1.

b-

F 0 l I

_

a I

I

I

I

2480 2500 Energy (eV)

(A) S K-edge spectra for the [Ru6C]/TiO z catalysts in SO 2 + H 2 at 443 (a),

503 K (b), in HzS + H 2 at 573 K (c), and in SO z at 290 K (d), and conv-Ru/TiO 2 catalysts in SO 2 + H 2 at 473 (e) and 573 K (f).

365 I

,

.I

I

................. i

k 20 , , ~ m m

C

0 C

m m , , ~

70 0 14

10

E

h

0 Z

g - -

I

_ _ m l

_ _

2460

I

!

I ......

I

2480 2500 Energy (eV)

Figure 1. (B) S K-edge spectra for elemental sulfur S 8 (g), R u S (x = 2, 3, 6) (h), and the

crystal

of

Ru3(CO)9(~q-H)z(~t3-S)

[(ph3p)zN]z[Ru6c(cO),s(~tz-sOz)](k).

(i),

RuaC(CO),5(Pq-SO)

(j),

and

366 Table 1. The Peak energy positions of sulfur K-edge XANES for [Ru6C]/TiO z and conv-Ru/TiO z catalysts and reference inorganic/organometallic compounds

Catalyst/ Sample [Ru6C]/Ti 02

ConvRu/TiO z

Conditions

Peak energy positions (eV)

SO z + H z 443 K SO z + H z 503 K H2S + H z 573 K SO z 290 K SO z + H a 473 K SO 2 + H 2 573 K

2472.8(s, br)

S8

2474.2(m ) 2474.2(s)

2479.0(m ) 2479.2(s)

2472.9(s, br)

2497(w, br) 2496.7(w, br)

2480.7(w, br) 2474.2(s)

2472.5(s, br) 2472.6(w)

2482.7(s,br)

2474.2(s)

2479.2(s)

2479.2(s)

2483.0(w)

2497(w, br)

2482.8(s,br)

2498(w, sh)

2483.0(w)

2496(w, sh)

2472.6(m

2481.1 (w)

2472.1(s, br) 2471.6(s, br)

2482.5(s,br)

2498 (w, sh)

2480.7(w, br)

2505(w, sh)

)

RuS x

Ru3(CO)9(~z-H)z(~3-S) Ru6C(CO),5([-t3-50) [ (Ph3P)zN] z[Ru6C (CO)15( ~t,-SO~)]

2474.4(s) 2475.6(s)

2477.6(s) 2479.3(s)

2484.4(w, br) 2482.1 (m)

2496.4 (w, sh)

The two peaks of Pair B (Figure l a, e) were broader than those of Pair A. Relatively broad two peaks were observed at 2472.1 and 2482.5 eV for RuS x (h). The differences from the corresponding peaks in Pair B were 0.2 - 0.7 eV. Two peaks were also observed at 2472.6 and 2481.1 eV for S 8 (g). The differences from the corresponding peaks in Pair B were 0.1 -

1.7 eV. Note that the peak intensity and the broadness of Figure lg varied as the elemental sulfur sample was progressively diluted by boron nitride. Thus, Pair B is ascribed basically to sulfide, but the possibility that a part of produced elemental sulfur remained after the argon flush at the reaction temperature cannot be excluded.

The first

peak (2472.9 eV) in the spectrum for RuS x species prepared from HzS + H z on the [Ru6C]/TiO 2 catalyst (Figure lc) appeared at similar energy to that for RuS x (2472.1 eV, h). However, the second peak at 2480.7 eV was very weak in (c) compared to that in (h). The

367 reason is unclear at the moment.

4.

DISCUSSION

Pairs A and B were assigned to adsorbed SO 2 and sulfide bonded to ruthenium atoms, respectively. catalysts.

The activation of Ru catalysts was dependent on the heating temperature of

Pair B appeared when the catalysts were still inactive. Hence, ruthenium metal

microparticles were first sulfided and then exhibited the catalytic reactivity to form elemental sulfur from SO 2. The difference of sulfidation temperature for the [Ru6C]/TiO 2 (503 K) and conv-Ru/TiO 2 catalysts (573 K) may be originated from each Ru particle size. In general, smaller [Ru 6] cluster is more reactive than larger Ru particles ( 1 0 - 50.A) of conventional catalysts [14]. Pair A was exclusively observed in addition to the weak shoulder peaks of Pair B (Fig. 1A). Therefore, the surface during the catalysis of the SO 2 + H 2 reactions should be predominantly occupied by SO 2. The elementary step of SO 2 dissociation to SO(ads) may be the rate-determining step of overall reaction. The database to identify and evaluate the adsorbed, intermediate, and incorporated sulfur species was listed in Table 1 to investigate the SO 2 reduction and possibly also the desulfurization reaction. Further discussion of reaction mechanism of catalysis will need the combination of S K-edge XANES and pulse reaction measurements at beamline.

ACKNOWLEDGEMENT

The experiments were performed under the approval of the KEK-PF Program Review Committee (2000P018).

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1.

M. Shelef, Chem. Rev., 95 (1995) 209.

2.

S.C. Paik and J. S. Chung, Appl. Catal. B Environmental, 8 (1996) 267.

3.

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4.

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6.

Y. Izumi and Y. Iwasawa, CHEMTECH, 24(7) (1994) 20.

7.

Y. Izumi and K. Aika, J. Phys. Chem., 99 (1995) 10336.

8.

Y. Izumi and K. Aika, J. Phys. Chem., 99 (1995) 10346.

9.

Ishiguro, T. Nakhjima, T. Iwata, M. Fujita, T. Minato, F. Kiyotaki, Y. Izumi, K. Aika, M. Uchida, K. Kimoto, Y. Matsui and Y. Wakatsuki, Chem. Eur. J., submitted.

10. E W. Lytle, R. B. Greegor, D. R. Sandstrom, E. C. Marques, J. Wong, C. L. Spiro, G. E Huffman and E E. Huggins, Nucl. Instr. Meth. Phys. Res. 226 (1984) 542. 11. Y. Izumi, T. Glaser, K. Rose, J. McMaster, E Basu, J. H. Enemark, B. Hedman, K. O. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 121 (1999) 10035. 12. Y. Wakatsuki and T. Chihara, Bull. Chem. Soc. Jpn., 72 (1999) 2357. 13. H. Sekiyama, N. Kosugi, H. Kuroda and T. Ohta, Bull. Chem. Soc. Jpn., 59 (1986) 575. 14. Y. Izumi, Y. Iwata and K. Aika, J. Phys. Chem., 100 (1996) 9421.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

369

Catalytic transformation of dichloromethane over NaFAU(X,Y) and HFAU(Y) L. Pinard, J. Mijoin, R. Lapeyrolerie, P. Magnoux* and M. Guisnet Laboratoire de Catalyse en Chimie Organique, UMR 6503, 40 av. du Recteur Pineau, 86022 Poitiers Cedex, France The transformation of dichloromethane was carried out over FAU zeolites under the following conditions: flow reactor, temperatures between 220 and 450~ feed constituted of air with 4% steam and 1000 ppm CHzCI2. With all zeolites (NaX, NaY, NaHFAU and HFAU(Y)), dichloromethane can be selectively transformed into formaldehyde plus chlorhydric acid. The zeolite activity was shown to be related to the heat of dichloromethane adsorption, NaX being more active than NaY and much more active than HFAU(Y) and than NaHFAU(Y) zeolites without accessible sodium cations. 1. I N T R O D U C T I O N Catalytic oxidation is one of the emerging technologies to reduce the emissions of Volatile Organic Compounds (VOCs) from stationary sources [1-4]. For the destruction of chlorinated VOCs (e.g. dichloromethane, trichloroethylene which are frequently used as solvents), catalysts should be selective to chlorhydric acid (no production of chlorine), COz and H20 and also resistant to chlorocompounds. Oxidation catalysts (noble metals, etc) supported on zeolites were shown to be active for destruction of chlorocarbons [5]; the adsorption of these compounds on zeolites was also investigated [6-8], chloroform being proposed as a probe molecule for basicity characterisation [9,10]. In this paper, the activity and selectivity of pure FAU zeolite samples: NaX, NaY, HFAU and NaHFAU(Y) for transformation of traces (1000 ppm) of dichloromethane in presence of air with 4% steam were determined. With all zeolites, dichloromethane can be transformed into formaldehyde plus chlorhydric acid then into CO and COz. The catalytic properties of the zeolites will be discussed in the light of their physicochemical properties. 2. E X P E R I M E N T A L The techniques used for characterising the catalyst samples are described in part 3.1. Dichloromethane transformation was carried out in presence of steam (4%) under reconstituted air in a fixed bed reactor with a constant catalyst weight (140 mg). CHzCIz

* Corresponding author Tel: +33 5 49 45 34 98, Fax: +33 5 49 45 37 79, E-mail: [email protected]

370 was introduced with a concentration of 1000 ppm from a gas bottle (mixture CH2C12 and air), the space velocity being maintained at 20,000 h-1. The effluent gases were analysed on-line using GC equipped with TCD and FID. The conversion of dichloromethane was determined by comparing the surface area of the corresponding GC peaks in the effluent gases and in the feed. Organic compounds were analysed with a CP-Sil 5 capillary column, C02 and CO with respectively a Porapak Q column and a 13X molecular sieve column. Chlorhydric acid was recovered in water and its amount estimated by pH measurement. 3.

RESULTS AND DISCUSSION

3.1. Preparation and characterisation of the zeolite samples NaFAU(X) and NaFAU(Y) also called here NaX and NaY were supplied by Union Carbide. HFAU resulted from calcination of a NH4Y zeolite (CBV 500 from PQ) under air flow at 500~ for 12 hours. NaHFAU samples were prepared by ion exchange of Na (Y) with NH4OH solutions then calcination under dry air flow at 500~ for 6 hours. These samples will be called NaHFAU followed by the percentage of Na exchange by protons determined by elemental analysis carried out by the "Centre d'Analyse du CNRS (Vernaison) ". Table 1 Physicochemical characteristics of the zeolite samples Catalyst

Unit cell formula

Si/AI

(%) Acidity Exchange (1018H+.g-1)

Pore Volume (cm3.g-1) Micro Meso

NaFAU Na56A1565i1360384 2.45 (NAY) HNaFAU(7) H4NaszAI56Si1360384 HNaFAU(13) HT.zNan8.8AI56Si1360384 HNaFAU(33) H17.7Na38A1565i1360384 HNaFAU(39) H33Naz3AI56Si1360384 HNaFAU(62) H35NazlA156Si1360384 HFAU H3zAI32Si1600384,22.4EFAL 2.9(5)* NaFAU(NaX) Na88A188Si1040384 1.3 * Framework Si/AI ratio estimated from the position

0

0

0.313

0.021

7 13 33 35 0.320 0.038 59 62 133 100 403 0.255 0.060 0 0 0.281 0.002 of the IR structure bands [ 11].

The acidity of all samples was characterized by pyridine adsorption followed by IR spectroscopy. No hydroxyl band can be observed in the spectra of NaX and NaY; the spectra of NaHFAU samples present two main bands corresponding to bridging hydroxyls in the supercages (3640-3645 cm -1) and to silanol groups (3745 cm-1); in agreement with other studies [12,13], the band corresponding to bridging OH located in the hexagonal prisms appear only above 50% exchange. The spectrum of HFAU is more complex with 9 six bands, at 3745 cm-1 (silanols), 3665 cm-1 (OH of extraframework aluminium species), 3630 and 3555 cm -1 (bridging OH in the supercages and in the hexagonal prisms, respectively), 3600 and 3525 cm -1 corresponding to the bridging OH groups interacting with Lewis acid extraframework aluminium species [14,15].

371 The concentration of Br6nsted acid sites was calculated from the integrated intensity of the pyridinium ion band at 1545 cm -1 obtained after a desorption treatment at 150~ The value of the extinction coefficient (1.8 lamol-lcm) is the one previously determined by Lavalley and coll. [16]. The concentration of protonic sites is reported in Table 1: no protonic sites are found over NaX and NaY; as could be expected, the concentration of protonic sites able to retain pyridine adsorbed at 150~ as pyridinium ion increases with the degree of exchange; moreover, the ratio between this concentration and the theoretical one estimated from the unit cell formula increases significantly with the degree of protonic exchange confirming an increase in the acid strength. The pore volume was determined by nitrogen sorption-desorption a t - 1 9 6 ~ with the gas adsorption system ASAP 2010 (Micromeritics). All the zeolite samples present a large micropore volume (0.25 to 0.32 cm3g-1). NaY, NaHFAU and especially HFAU (Table 1) have also mesopores. With this latter zeolite, it is due to dealumination during thermal treatment of NH4FAU, this treatment causing the formation of extraframework aluminium species (Table 1). [.

140 120

~" 100

I

O

[i

80 60

t--i

i-'1

I-'1

40 20

0

i

i

!

i

i

i

1

2

3

4

5

6

Nads

Fig. 1. Heat of CH2C12 adsorption on NaX ([[]), NaY (A) and HY (O)as a function of the amount of CH2C12 adsorbed at 25~ (Nads: molecule per supercage ). A calorimetry study of dichloromethane adsorption was carried out at 25~ using a Setaram DSC 111 instrument linked to a volumetric line allowing the introduction of small sorbate amounts. With NaX, the heat of adsorption of the first molecules of dichloromethane is very high : 140 kJmo1-1 (Fig. 1). This high value was confirmed in three different experiments. After adsorption of an average of 0.75 molecule per supercage, the heat of adsorption becomes constant and equal to 64 kJ mo1-1 up to a value of 4-4.5 molecules by supercage then decreases. This latter decrease corresponds most likely to physical adsorption of dichloromethane. For approximately 6 molecules per supercage, which according to molecular simulation corresponds to a complete filling of supercages, the heat of adsorption becomes very low (Fig. 1). With NaY and HFAU, the initial heats of adsorption are much lower: 72 and 60 kJ mo1-1 and a plateau is obtained after adsorption of less than 0.2 molecule per supercage, the adsorption heat being then equal to 54 and 40 kJ mol -~ respectively. These experiments demonstrate that the sites for dichloromethane adsorption are in average stronger on NaX than on NaY and especially on HFAU.

372

Moreover, NaX presents very strong adsorption sites which do not exist with the other zeolites. The much higher initial adsorption energy found with NaX is most likely due to the location of Na cations in sites III and III'. These sites are of higher potential energy than sites I, I' and II [8] which are the only sites occupied by Na cations in the case of NaY. However, the heat values at the plateau which could correspond to adsorption of dichloromethane on Na cations in sites II (sites I and I' located in the hexagonal prisms and in sodalite cages are inaccessible by organic molecules) are also higher with NaX than with NaY. Similar results were obtained for chloroform and trichloroethylene adsorption on these zeolites and a good agreement was found between experimental values and values estimated through Monte Carlo simulations considering that the interaction between chlorocarbons and zeolites involves: i) strong van der Walls-type attractions between chlorine atoms and framework oxygens, ii) electrostatic interactions between chlorine atoms and Na cations and iii) hydrogen bonding with the framework oxygens [6-8]. The large difference between NaY and HFAU (Y) could be expected from large changes in parameters ii and iii. 3.2.

Dichloromethane

transformation

o v e r N a X , N a Y a n d HFAU(Y)

3.2.1. Activity and stability of the catalysts Over NaY and HFAU and whatever the temperature, there is practically no change in conversion (no deactivation) during the first 120 minutes reaction. That is not the case with NaX for which there is a rapid initial decrease in conversion during the first hour of reaction followed by a plateau (Fig. 2). This deactivation is not due to coking: no carbonaceous residues can be found after reaction on NaX(as well as on NaY and HFAU).

100

-------~,oo

8 0 ~

OO

NaX -,i

o

~

40 - t

o

20

~

0

NaY <

IL_I._I._I--~IIIII

1 0

!

i

1

i

-!

20

40

60

80

100

120

Time on stream (min.) Fig. 2. CH2C12 conversion as a function of time on stream over NaX (0) NaY (D) at 260~

160 140 120 100 80 60 40 20 0

~ ~'

0

1

2

3

4

5

6

Sads

Fig. 3. Adsorption heats of CHzCI2 on fresh NaX (D) and deactivated NaX (0) at 25~ as a function of the number of molecules adsorbed per supercage (Nads).

The effect of NaX deactivation on the heat of dichloromethane adsorption was determined. Fig. 3 shows that the very strong adsorption sites which were found over the

373

fresh NaX sample (adsorption heat of 140 kJ mo1-1) no longer exist over the deactivated sample. Therefore, it can be suggested that deactivation is due to the elimination of these very strong adsorption sites. As proposed in paragraph 3.1. these sites would be Na cations located in sites III (and neighbouring framework oxygens). As demonstrated elsewhere [12,13], the elimination of these sites is most likely due to the substitution of Na cations by protons with formation of sodium chloride. Fig. 3 shows also that on the remaining sites, the heat of dichloromethane adsorption is identical to the heat of adsorption with the fresh NaX sample on which more than 0.75 molecule of dichloromethane are adsorbed per supercage. 100

.= 8 0 e,l L,,

~.

60-

= .2 ~1,)

40-

=

0

20

-

200

I

I

I

I

250

300

350

400

450

Temperature (~

Fig.4. CH2C12 conversion over NaX (r temperature.

NaY (t'l) and HFAU (A) as a function of reaction

Fig. 4 compares the dichloromethane conversion after 120 minutes reaction over NaX, NaY and HFAU in a large range of temperature; although the activity of NaX is measured after deactivation, this catalyst is more active than NaY and especially than HFAU(Y). Thus, at 300~ the conversion which is complete on NaX, is equal to 60% on NaY and to 12% only on HFAU; with NaY, a complete conversion is obtained at 360~ and with HFAU only for temperatures higher than 460~ It should be remarked that, according to Ramachandran et al. [17], HFAU catalysts would be more active than NaY in dichloromethane transformation. This has led us to choose for this study the most active sample of a series of HFAU zeolites with framework Si/Al ratios between 5 and 100. The difference observed between the results of Ramachandran et al and ours could have different origins: presence of an alumina binder that is able to catalyse dichloromethane oxidation in their case and experiments carried out in absence of water with as a consequence a very fast deactivation of NaY [17]. Another strong argument in favour of the higher activity of NaY is the decrease which is observed when Na cations are exchanged by protons (NaHFAU series). Fig. 5 shows that this decrease is particularly pronounced at low exchange rates, the activity becoming very weak when 60% of Na cations are exchanged: 10% conversion at 300~ instead of 60% on NaY. This strong initial decrease is in agreement with the preferential formation of the 3640cm -1 band which corresponds to hydroxyls located in the supercages; hydroxyls located in hexagonal prisms (band at 3540 cm -1) appear only in significant amount above 60% exchange (Fig. 5 and [12-13]). Therefore, it can be considered that above this percentage of exchange, the remaining Na cations are not accessible by dichloromethane molecules.

374 ~"

60

60

v

"

A

40

0

5

i 40

/ i

i

2O "~

~ 2o ~

,ti U

,4,

0

~

v - 1

----IS

20

r

i

40

60

80

100

% exchang e

Fig.5. CH2C12 conversion as a function of the percentage of sodium exchange over NaHFAU and absorbance of the band at 3555 cm -1 (bridging OH of the hexagonal prisms). The higher activity of NaX compared to NaY could be related to the greater number of Na cations per unit cell (88 and 56) and to differences in their location: with NaX, Na cations are located in all sites (I,I',II,II',III,III~) whereas with NaY, there are no cations in sites III/III'; with as a consequence a greater percentage of cations accessible by organic molecules in NaX [8]. Experiments at 300~ carried out on mixtures of inert silica and NaX and NaY samples (Fig. 6) show that for the same concentration of Na atoms, the NaX-SiO2 mixtures (despite the initial deactivation) are much more active (at least 6 times) than the NaY-SiO2 mixtures. This difference in activity seems too large to be explained by differences in the number of sites accessible by dichloromethane molecules only. lOO

._.o NaX

80~

60-

o

40=

e,I

20-

~

o 0

!

|

|

|

10

20

30

40

nNa (1 0 20

50

atoms.g "1)

Fig.6. CH2C12 conversion as a function of Na content (nNa) over NaY-SiO2 (El) and NaXSiO2 (0) mixtures at 300~ This difference is rather due to the large difference in the polarity of zeolites shown by dichloromethane adsorption. Indeed, the order in activity is the same as the order in heat of dichloromethane adsorption of the plateau in Figure 1. NaX (AH - -64 kJ.mo1-1) is more active than NaY (AH = -54 kJ.mo1-1) and much more active than HFAU (AH = -40 kJ.mol-1).

375

3.2.2. Selectivity of the catalysts Over NaY and NaX, the only reaction products are CO, COz, HCI and formaldehyde; over HFAU, methyl chloride is also observed but only above 380~ No formation of C12 was detected over the three catalysts. Furthermore, no organic products other than formaldehyde and methyl chloride can be observed. The yields in formaldehyde, methyl chloride, CO and CO2 obtained on HFAU and NaY are reported in Figs. 7 and 8 as a function of reaction temperature. With both catalysts, no CO and COz can be observed at 300~ i.e. only formaldehyde and HCI are formed. With NaY, the yield in formaldehyde passes through a maximum (80%) at 340~ CO appears at this temperature and CO2 at 420~ the yields in these latter products increasing with the temperature at the expense of the yield in formaldehyde. A scheme with successive formation of formaldehyde, CO then COz can therefore be proposed. A similar scheme can be advanced for dichloromethane transformation over HFAU, with however an additional formation of methyl chloride at high temperatures. The transformation of dichloromethane into formaldehyde is much faster over NaY than over HFAU: at 300~ the yield in formaldehyde is 5 times greater over NaY (50%) than over HFAU (10%). The higher activity of NaY is confirmed by the lower temperature of CO and CO2 appearance in the reaction products: 350~ and 420~ for CO and 420~ and > 500~ for CO2 over NaY and HFAU respectively.

lOO

lOO

80

80&

~. 6 o -

&

40

60 N 4O

20 o

~_.r!

250

t~l

,T

300 350 400 Temperature (~

-

-

|

-

450

Fig. 7:CH2C12 conversion (0), conversion of CH2C12 into CO (El), CH20 (A), CH3CI (*) and CO2 (o)as a function of reaction temperature over HFAU

0 250

~

_

_

,

300 350 400 Temperature (~

9

2 450

Fig. 8" CH2C12 conversion (0), conversion of CH2C12 into CO ([~), CH20 (A)and CO2 (o) as a function of reaction temperature over NaY.

The positive effect of Na in FAU zeolites in the formation of oxidation products has already been reported in the literature. Thus, pyrene trapped in the supercages of HFAU zeolites was shown to be totally oxidised above 400~ over NaY and only above 550~ over HFAU [18]. Moreover, during coke oxidation over a series of NaHFAU zeolites, the CO/CO2 ratio was found to decrease with increasing Na amount in the zeolite, the authors concluding that the effect was probably due to a catalytic role of Na cations in CO oxidation [18]. The easier formation of oxidation products which is observed here with NaY seems to confirm the positive role of Na cations in oxidation.

376 4.

CONCLUSION

Over FAU zeolites in presence of air and water, dichloromethane is selectively transformed into formaldehyde and chlorhydric acid. NaX is more active than NaY, the protonic exchange of this latter zeolite causing a large decrease in activity. The activity was shown to be related to the heat of dichloromethane adsorption. ACKNOWLEDGMENTS L. Pinard gratefully acknowledges the "Agence de rEnvironnement et de la Maitrise de l'Energie" (ADEME) and the "R~gion Poitou-Charentes" for a scholarship. REFERENCES

1. J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. 2. S. Chatterjee and H. L. Greene, J. Catal. 130 (1991) 76. 3. R . W . van den Brink, P. Mulder, R. Louw, G. Sinquin, C. Petit and J. P. Hidermann, J. Catal., 180 (1998) 153. 4. Ph. D~g~, L. Pinard, P. Magnoux and M. Guisnet, Appl. Catal. B., 27 (2000) 17. . 5. L. Pinard, J. Tsou, P. Magnoux and M. Guisnet, Stud. Surf. Sci. Catal., 135 (2001) 173. 6. C.F. Mellot, A. K. Cheetham, S. Hams, S. Savitz, R. J. Gorte and A. L. Myers, J. Am. Chem. Soc., 120 (1998) 5788. 7. C.F. Mellot, A. K. Cheetham, S. Hams, S. Savitz, R. J Gorte and A. L. Myers., Langnuir, 14 (1998) 6728. 8. S. Butterfey, A. Boutin, C. Mellot-Drazniecks and A. H. Fuchs, J. Am. Chem. B, 105 (2001) 9569. 9. E.A. Pauksthis, N. S. Kotsararenko and L. G. Karakchiev, React. Kinet. Catal. Lett., 12 (1979) 315. 10. J. Xie, M. Huang and S. Kaliaguine, React. Kinet. Catal. Lett., 58 (1996) 217. 11. A. P. de Carvalho, Q. L. Wang, G. Giannetto, D. Cardoso, M. Brotas de Carvalho, F. Ramoa Riberio, J. B. Nagy, J. el Hage-Ai Aswad, E.G. Derouane and M. Guisnet, J Chim. Phys., 87 (1990) 271. 12. J. W. Ward and R. C. Hansford, J. Catal., 13 (1969) 364. 13. J. W. Ward, Zeolite Chemistry and Catalysis, ACS Monograph 171, (1979) chapter 3, 118. 14. A. Corma, V. Fornes and F. Rey, Appl. Catal., 59 (1990) 267. 15. F. Lonyi and J. H. Lunsford, J. Catal., 136 (1992) 566. 16. S. Khabtou, T. Chevreau and J. C. Lavalley, Microporous Mater., 3 (1994) 133. 17. B. Ramachandran, H. L. Greene and S. Chatterjee, Appl. Catal. B., 8 (1996) 157. 18. K. Moljord, P. Magnoux and M. Guisnet, Appl. Catal. A: General, 121 (1995) 245.

Studies in SurfaceScienceandCatalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

377

Preparation of new solid super-acid catalyst, titanium sulfate supported on zirconia and its acid catalytic properties J.R. Sohn, E-H. Park, and J-G. Kim Department of Industrial Chemistry, Engineering College, Kyungpook National University, Taegu 702-701, Korea Titanium sulfate supported on zirconia catalysts were prepared by drying of powdered Zr(OH)4 with titanium sulfate aqueous solution followed by calcining in air at high temperature. The characterization of prepared catalysts was performed using Fourier transform infrared (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by the measurement of surface area. The addition of titanium sulfate to zirconia shifted the phase transition of ZrOz from amorphous to tetragonal to higher temperature because of the interaction between titanium sulfate and zirconia. The catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. 1. INTRODUCTION Solid acid catalysts play an important role in hydrocarbon conversion reactions in the chemical and petroleum industries [1,2]. Many kinds of solid acids have been found; their acidic properties on catalyst surfaces, their catalytic action, and the structure of acid sites have been elucidated for a long time, and those results have been reviewed by Arata [3]. The strong acidity of zirconia-supported sulfate has attracted much attention because of its ability to catalyze many reactions such as cracking, alkylation, and isomerization. Sulfated zirconia incorporating Fe and Mn has been shown to be highly active for butane isomerization, catalyzing the reaction even at room temperature [4]. It has been reported by several workers that the addition of platinum to zirconia modified by sulfate ions enhances catalytic activity in the skeletal isomerization of alkanes without deactivation when the reaction is carried out in the presence of hydrogen [5,6]. The high catalytic activity and small deactivation can be explained by both the elimination of the coke by hydrogenation and hydrogenolysis [5], and the formation of Br6nsted acid sites from H2 on the catalysts [6]. On the other hand, many metal sulfates generate fairly large amounts of acid sites of moderate or strong strength on their surfaces when they are calcined at 400-700)~ [1]. The acidic property of metal sulfate often gives high selectivity for diversified reactions such as hydration, polymerization, alkylation, cracking, and isomerization [1]. However, structural and physicochemical properties of supported metal sulfates are considered to be in different states compared with bulk metal sulfates because of their interaction with

378

supports [7]. This paper describes the preparation of new solid super-acid catalyst, titanium sulfate supported on zirconia and its acid catalytic properties. The characterization of the samples was performed by means of Fourier transform infrared (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and surface area measurements. For the acid catalysis, the 2-propanol dehydration and cumene dealkylation were used as test reactions. 2. EXPERIMENTAL

2.1. Catalyst Preparation The precipitate of Zr(OH)4 was obtained by adding aqueous ammonia slowly into an aqueous solution of zirconium oxychloride at room temperature with stirring until the pH of mother liquor reached about 8. The precipitate thus obtained was washed thoroughly with distilled water until chloride ion was not detected. The catalysts containing various titanium sulfate contents were prepared by adding an aqueous solution of titanium sulfate [Ti(SO4)2"4H20] to the Zr(OH)4 powder followed by drying and calcining at high temperatures for 2 h in air. This series of catalysts are denoted by their weight percentage of Ti(SOn)z. For example, 5-Ti(SO4)a/Zr02 indicates the catalyst containing 5 wt% Ti(SO4)z.

2.2. Characterization FTIR spectra were obtained in a heatable gas cell at room temperature using Mattson Model GL6030E FTIR spectrophotometer. The self-supporting catalyst wafers contained about 9 mg/cm 2. Prior to obtaining the spectra the samples were heated under vacuum at 400-500 ~ for 1.5 h. Catalysts were checked in order to determine the structure of the catalysts by means of a Philips X'Pert-APD X-ray diffractometer, employing Cu K a (Nifiltered) radiation. DSC measurements were performed in air by a PL-STA model 1500H apparatus, and the heating rate was 5 ~ per minute. The specific surface area was determined by applying the BET method to the adsorption of N2 at -196 ~ Chemisorption of ammonia was also employed as a measure of the acidity of catalysts. The amount of chemisorption was determined based on the irreversible adsorption of ammonia [8,9]. 2-propanol dehydration was carried out at 160-180 ~ in a pulse micro-reactor connected to a gas chromatograph. Catalytic activity for 2-propanol dehydration was represented as mole of propylene converted from 2-propanol per gram of catalyst. Cumene dealkylation was carried out at 400-450 ~ in the same reactor as above. Catalytic activity for cumene dealkylation was represented as mole of benzene converted from cumene per gram of catalyst. 3. RESULTS AND DISCUSSION

3.1. Infrared spectra The IR spectra of 5-Ti(SO4)z/ZrO2 (KBr disc) calcined at different temperatures (400800 ~ are given in Fig. 1.5-Ti(SO4)z/ZrO2 calcined up to 700 ~ showed IR absorption bands at 1339, 1265, 1122 and 1030 cm -1 which are assigned to bidentate sulfate ion coordinated to the metal such as Zr4+ or Ti4+ [9]. However, for the calcination at 800 ~ IR

379 bands by the sulfate ion disappeared because of the complete decomposition of sulfate ion, as shown in Fig. 1. These results are in good agreement with those of thermal analysis described later. In general, for the metal oxides modified with sulfate ion followed by evacuating above 400 ~ a strong band assigned to S=O stretching frequency is observed at 1380-1370 cm -1 [10,11]. In this work, the corresponding band for samples exposed to air was not found because water molecules in air were adsorbed on the surfaces of catalysts. These results are very similar to those reported by other authors [10,11]. However, in a separate experiment IR spectrum of self-supported 5-Ti(SO4)flZrO2 after evacuation at 400 ~ for 2 h was examined, so that there was an intense band at 1376 cm -1 accompanied by broad and intense bands below 1250 cm -1 because of the overlapping of the ZrO2 skeletal vibration, indicating the presence of different adsorbed species depending on the treatment conditions of the sulfated sample [10].

c

"~ e [-

1376

~ !

|

|

Fig 1. Infrared spectra of 5-Ti(SO4)2/ZrO2 calcined at different temperatures for 2 h ; (a) 800~ (b) 700~ (c) 600~ (d) 500~ (e) 400~ and (f) after evacuation at 400~ for 2 h.

Wavenumber, cm -1

3.2. Crystalline structures of catalysts The crystalline structures of catalysts calcined in air at different temperatures for 2 h were examined. In the case of zirconia support, ZrO2 was amorphous to X-ray diffraction up to 300 ~ with a tetragonal phase at 350 ~ a two-phase mixture of the tetragonal and monoclinic forms at 400-800 ~ and a monoclinic phase at 900 ~ However, in the supported titanium sulfate catalysts, the crystalline structures of the samples were different from structure of the ZrO2 support. For the 5-Ti(SO4)2/ZrO2, as shown in Figure 2, ZrO2 is amorphous up to 500 ~ X-ray diffraction data indicated a tetragonal phase of ZrO2 at 600-700 ~ a two-phase mixture of the tetragonal and monoclinic ZrO2 forms at 800-900 ~ It is assumed that the interaction between titanium sulfate and ZrO2 hinders the transition of ZrO2 from amorphous to tetragonal phase [11,12]. The presence of titanium sulfate strongly influences the development of textural properties with temperature. For 5-Ti(SO4)z/ZrO2, the crystalline phase of titanium sulfate was not

380 observed at any calcination temperature, indicating that most of titanium sulfate is present as amorphous form and the titanium sulfate is well dispersed on the surface of zirconia. Moreover, for the sample of 15-Ti(SO4)z/ZrOz the transition temperature of ZrOz from amorphous to tetragonal phase was higher by 350 ~ than that of pure ZrOz. That is, the higher the content of titanium sulfate, the higher the phase transition temperature. However, as shown in Fig. 3, X-ray diffraction patterns of 15-Ti(SO4)z/ZrOz were quite different from those of 5-Ti(SO4)z/ZrOz. X-ray diffraction data indicated that an orthorhombic phase of Zr(SO4)2 was predominantly observed together with a tiny amount of orthorhombic phase of Ti(SO4)z at 400-700 ~ indicating that zirconium sulfate between titanium sulfate and zirconia was formed during the catalyst preparation. For the calcinations of 700 ~ a tiny amount of tetragonal and monoclinic phases of zirconia was observed because of the transition of ZrOz from amorphous to tetragonal phase. However, as shown in Fig. 3, from 800 ~ a large amount of monoclinic phase of zirconia was observed because of the decomposition of zirconium or titanium sulfate.

c_j"

o 9

o

9

900oc 800 ~

o

900 ~

9 9

t3

9

9

|

o 9

|

800 ~

@

~

|

|

@

9

|174

700 ~ o

700o, 2__.

6

6 0 0 5oo~

500 ~

~

.......,~L.2

400~ ld

...........

2~ 3d

40' 20

50'

60'

70'

Fig. 2. X-ray diffraction patterns of 5Ti(SO4)z/ZrO2 calcined at different temperatures for 2 h : O, tetragonal phase of ZrO2 ; e, monoclinic phase of ZrO2.

4 1;

0

0

~

0

0

~

2;

3;

46 5; 6; 7(~ 20 Fig. 3. X-ray diffraction patterns of 15Ti(SO4)2/ZrO2 calcined at different temperatures for 2 h : O, tetragonal phase of ZrO2 ; O, monoclinic phase of ZrO2; | orthorhombic phase of Zr(SO4)2; A, orthorhombic phase of Ti(SO4)2.

The XRD patterns of Ti(SO4)z/ZrOz containing different titanium sulfate contents and calcined at 500 ~ for 2 h are shown in Fig. 4. XRD data indicated a two-phase mixture of tetragonal and monoclinic forms of ZrO2 at the region of zero wt% of titanium sulfate (pure ZrOz). However, ZrO2 is amorphous at the region of 5 wt% because the interaction

381 between titanium sulfate and ZrO2 hinders the phase transition of ZrO2 from amorphous to tetragonal in proportion to the titanium sulfate content [12,13]. At the region of 10-20 wt%, an orthorhombic phase of zirconium sulfate through the reaction between titanium sulfate and zirconia during the catalyst preparation was predominantly observed together with a tiny amount of orthorhombic phase of titanium sulfate 9 Moreover, for 25-Ti(SO4)z/ZrOz sample, only triclinic phase of zirconium sulfate penta-hydrate [Zr(SO4)z'5H20] was observed, indicating that the surface structure is different depending on the content of impregnant, as shown in Fig. 4. Titanium sulfate [Ti(SO4)2] sample calcined at 500 ~ exhibited only orthorhombic form of titanium sulfate. 100%

[TI(SO~

...................

a

...l.'j

3

25% e

|

9

20%

9

|

"

" " ""

9

9

|

f

15% ~Z

10%

g 0%

(Zr02) ~ 10

20

~~,.~~~~ 30

40' 20

50'

60'

70'

Fig. 4. X-ray diffraction patterns of Ti(SO4)2/ZrO2 having various Ti(SO4)2 contents and calcined at 500 ~ for 2 h : O, tetragonal phase of ZrO2; O, monoclinic phase of ZrOz; | orthorhombic phase of Zr(SO4)2; A, orthorhombic phase of Ti(SO4)2; O, triclinic phase of Zr(SO4)'5HzO.

I

I

I

0

200 400 600 800 Temperature, ~ Fig. 5. DSC curves of precursor for Ti(SO4)z/ZrO2 having different Ti(SO4)2 contents ; (a) ZrOz, (b) 3-Ti(SO4)z/ZrO2, (c) 5-Ti(SOn)z/ZrO2, (d) 10-Ti(SO4)z/ZrO2, (e) 20-Ti(SO4)z/ZrO2, (f) 25-Ti(SO4)z/ZrO2, and (g) Ti(SO4)2"4H20.

3.3. Thermal Analysis The X-ray diffraction patterns in Figs. 2-4 clearly showed that the structure of Ti(SO4)2/ZrOz was different depending on the calcined temperature 9 To examine the thermal properties of precursors of Ti(SO4)2/ZrOz samples more clearly, their thermal analysis has been carried out and are illustrated in Fig. 5. For pure ZrO2, the DSC curve shows a broad endothermic peak below 180 ~ because of water elimination, and a sharp and exothermic peak at 438 ~ because of the ZrO2 crystallization [13]. However, it is of interest to see the influence of titanium sulfate on the crystallization of ZrOz from

382

amorphous to tetragonal phase. As Figure 5 shows, the exothermic peak due to the crystallization appears at 438 ~ for pure ZrO2, while for Ti(SO4)2/ZrO2 samples it is shifted to higher temperatures because of the interaction between Ti(SO4)2 and ZrO2. The shift increases with increasing titanium sulfate content. Consequently, the exothermic peaks appear at 473 ~ for 3-Ti(SO4)2/ZrO2 and 517 ~ for 5-Ti(SO4)a/ZrO2. The endothermic peaks around 730 ~ for Ti(SO4)2/ZrO2 samples are due to the evolution of SO3 decomposed from sulfate ion bonded to the surface of zirconia [7]. For 10Ti(SO4)a/ZrO2 and 20-Ti(SO4)2/ZrO2, three endothermic peaks below 300 ~ are due to the dehydration of zirconium sulfate which was confirmed by X-ray diffraction [Fig. 5(d) and (e)], indicating that the dehydration occurs in three steps. However, for 25-Ti(SO4)z/ZrOa, four endothermic peaks below 300 ~ was observed, indicating that the dehydration of zirconium sulfate penta-hydrate occurs in four steps. For 25-Ti(SO4)z/ZrO2, zirconium sulfate penta-hydrate was also confirmed by X-ray diffraction [Fig. 5(0]. As shown in Fig. 5(g), for Ti(SO4)a'4H20, two endothermic peaks due to dehydration appeared below 200 ~ and an endothermic peak due to the decomposition of sulfate ion was observed around 610 ~ lower than 730 ~ of decomposition temperature of sulfate ion bonded to zirconia, indicating that the bonding force of sulfate ion bonded to Ti4+ is weaker than that of sulfate ion bonded to zirconia.

3.4. Surface Properties It is necessary to examine the effect of titanium sulfate on the surface properties of catalysts, that is, specific surface area, acidity, and acid strength. The specific surface areas of samples calcined at 600 ~ for 2 h are listed in Table 1. The presence of titanium sulfate strongly influences the surface area in comparison with the pure ZrO2. Specific surface areas of Ti(SO4)2/ZrO2 samples are much larger than that of pure ZrO2 calcined at the same temperature, showing that surface area increases gradually with increasing titanium sulfate content up to 5 wt% of Ti(SO4)2. It seems likely that the interaction between titanium sulfate and ZrO2 protects catalysts from sintering. The acidity of catalysts calcined at 600 ~ as determined by the amount of NH3 irreversibly adsorbed at 230 ~ [7,8], is listed in Table 1. As listed in Table 1, the acidity increases abruptly upon the addition of titanium sulfate [0.5 wt% of Ti(SO4)2] to ZrO2, and then the acidity increases very gently with increasing titanium sulfate content up to 5 wt% of Ti(SO4)2. In view of Table 1, it is clear that the acidity runs parallel with the surface area. Infrared spectroscopic studies of ammonia adsorbed on solid surfaces have made it possible to distinguish between Br6nsted and Lewis acid sites [14]. The IR spectra of ammonia adsorbed on 5-Ti(SO4)2/ZrO2 samples evacuated at 400 ~ for 1 h were examined. For 5-Ti(SO4)2/ZrO2 the band at 1443 cm -1 is the characteristic peak of ammonium ion, which is formed on the Br6nsted acid sites and the absorption peak at 1622 cm -1 is contributed by ammonia coordinately bonded to Lewis acid sites [14], indicating the presence of both Br6nsted and Lewis acid sites on the surface of 5-Ti(SO4)a/ZrO2 sample. Acid stronger than Ho <- -11.93, which corresponds to the acid strength of 100 % H2SO4, are superacids [1,3,15].

383 Table 1 Specific Surface Area and Acidity of Ti(SO4)z/ZrO2 Calcined at 600 ~ for 2 h Catalysts Surface area (mZ/g) Acidity (Mmol/g) ZrO2

56.1

41.3

0.5-Ti(SO4)z/ZrO2

107.4

96.2

1-Ti(SO4) 2/ZrOz

108.2

185.4

3-Ti(SO4) 2/ZrO2

113.3

187.3

5-Ti(SO4) 2/ZrO2

137.2

189.0

10-Ti(SO4) 2/ZrO2

132.3

183.0

15-Ti(SO4) 2/ZrOz

126.5

137.3

20-Ti(SO4) 2/ZrOz

108.3

123.0

25-Ti(SO4) z/ZrO2

91.7

120.5

The strong ability of the sulfur complex to accommodate electrons from a basic molecule such as ammonia is a driving force to generate superacidic properties [11]. The acid strength of Ti(SO4)2/ZrO2 catalysts was examined by a color change method, using Hammett indicator in sulfuryl chloride [12,13]. The acid strength of Ti(SO4)z/ZrO2 catalysts was found to be Ho < -14.5. Consequently, Ti(SO4)z/ZrO2 catalysts would be solid superacids, in analogy with the case of ZrOz modified with sulfate group [7,8].

3.5. Catalytic Activities for Acid Catalysis It is interesting to examine how the catalytic activity of acid catalyst depends on the acid property. The catalytic activities for the 2-propanol dehydration are measured and the results are illustrated as a function of Ti(SO4)z content in Fig. 6, where reaction temperatures are 160-180 ~ In view of Table 1 and Fig. 6, the variations in catalytic activity for 2-propanol dehydration are well correlated with the changes of their acidity, showing the highest activity and acidity for 5-Ti(SO4)2/ZrO2. It has been known that 2propanol dehydration takes place very readily on weak acid sites [16]. Good correlations have been found in many cases between the acidity and the catalytic activities of solid acids. For example the rates of both the catalytic decomposition of cumene and the polymerization of propylene over SIO2-A1203 catalysts were found to increase with increasing acid amounts at strength Ho -< + 3.3 [17]. Cumene dealkylation takes place on relatively strong acid sites of the catalysts [16]. Catalytic activities for cumene dealkylation against Ti(SO4)z content are presented in Fig. 7, where reaction temperature is 400-450 ~ Comparing Table 1 and Fig. 7, the catalytic activities are also correlated with the acidity. The correlation between catalytic activity and acidity holds for both reactions, cumene dealkylation and 2-propanol dehydration, although the acid strength required to catalyze acid reaction is different depending on the type of reactions. As seen in Figs. 6 and 7, the catalytic activity for cumene dealkylation, despite higher reaction temperature, is lower than that for 2-propanol dehydration. Catalytic activities of 5-Ti(SO4)z/ZrO2 for 2-propanol dehydration and cumene dealkylation were examined as a function of calcination temperature. The activities for both reactions increased with the calcination temperature, giving a maximum at 600 ~

384

12[

20 ~18 o16 E ,,O 14 12

> ,,..~

<

~1o

• .~

8

6 _--O-

-I-

450~

j

4

180~ <

4

0 !

i

!

!

!

0 5 10 Ti(SO4)2 content, wt%

!

15

Fig. 6. Catalytic activities of Ti(SO4)z/ZrO2 for 2-propanol dehydration as a function of Ti(SO4)2 content. 4.

0

5 10 15 Ti(SO4)2 content, wt%

20

Fig. 7. Catalytic activities of Ti(SO4)z/ZrOz for cumene dealkylation as a function of Ti(SO4)2 content.

CONCLUSIONS

The interaction between titanium sulfate and zirconia influenced the physicochemical properties of prepared catalysts with calcination temperature. The presence of titanium sulfate delays the phase transitions of ZrO2 from amorphous to tetragonal and from tetragonal to monoclinic. The specific surface area and acidity of catalysts increase in proportion to the titanium sulfate content up to 5 wt% of Ti(SO4)2. The correlation between catalytic activity and acidity holds for both reactions, cumene dealkylation and 2-propanol dehydration, although the acid strength required to catalyze acid reaction is different depending on the type of reactions. ACKNOWLEDGEMENTS This work was supported by grant No.(2001-1-30700-006-2) from the Basic Research Program of the Korea Science and Engineering Foundation and was partially supported by the grant of Post-Doc. Program of Kyungpook National Universty, in Korea.

REFERENCES 1. K. Tanabe, M. Misono, Y. Ono and H. Hattori, "New Solid Acids and Bases", Elsevier Science, Amsterdam (1989). 2. J. R. Sohn and E. H. Park, J. Ind. Eng. Chem., 6 (2000) 297. 3. K. Arata, Adv. Catal., 37 (1990) 165. 4. T. K. Cheung and B. C. Gates, J. Catal., 168 (1997) 522. 5. T. Hosoi, T. Shimadzu, S. Ito, S. Baba, H. Takaoka, T. Imai and N. Yokoyama, Prepr.

385 Symp. Div. Petr. Chem. American Chemical Society, Los Angeles, 562 (1988). 6. K. Ebitani, J. Konishi and H. Hattori, J. Catal., 130 (1991) 257. 7. J. R. Sohn and E. H. Park, J. Ind. Eng. Chem., 4 (1998) 197. 8. J. R. Sohn, S. G. Cho, Y. I. Pae and S. Hayashi, J. Catal., 159 (1996) 170. 9. J. R. Sohn, H. W. Kim, M. Y. Park, E. H. Park, J. T. Kim and S. E. Park, Appl. Catal. A: General, 128 (1995) 127. 10. B. A. Morrow, R. A. McFarlane, M. Lion and J. C. Lavalley, J. Catal., 107 (1987) 232. 11. T. Yamaguchi, Appl. Catal., 61 (1990) 1. 12. J. R. Sohn and S. G. Ryu, Langmuir, 9 (1993) 126. 13. J. R. Sohn and S. Y. Lee, Appl. Catal. A: General, 164 (1997) 127. 14. A. Satsuma, A. Hattori, K. Mizutani, A. Furuta, A. Miyamoto, T. Hattori and Y. Murakami, J. Phys. Chem., 92 (1988) 6052. 15. E G. A. Olah, G. K. S. Prakash and J. Sommer, Science, 206 (1979) 13. 16. S. J. Decanio, J. R. Sohn, E O. Paul and J. H. Lunsford, J. Catal., 101 (1986) 132. 17. K. Tanabe, "Solid Acids and Bases", Kodansha, Tokyo(1970).

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

387

Superacid WOx/ZrO2 catalysts for isomerization of n-hexane and for nitration of benzene V.V. Brei a, O.V. Melezhyk a, S.V. Prudius a, M.M. Levchuk b, K.I. Patrylak b

a Institute of Surface Chemistry of the National Academy of Sciences of Ukraine e-mail: [email protected] b Institute of Bioorganic and Petroleum Chemistry of the National Academy of Sciences of Ukraine Methods of synthesis of superacid catalysts based on WO3/ZrOz system (surface area 4 0 - 250 mZ/g) are developed. This catalysts exhibit high activity in n-hexane isomerization reaction: yield of branched isomers at 230 - 250~ amounts to 65 - 70% with selectivity for i-C6 of 70 - 94% and 80% conversion of n-hexane. Promoting of the catalysts with Fe and Mn compounds does not effect their activity in n-hexane isomerization. WO3/ZrO2 catalysts synthesized are highly active in reaction of benzene nitration with 70% nitric acid. The yield of nitrobenzene is 65-80% with 98 - 99% selectivity at 170~ 1.

INTRODUCTION

Today WO3/ZrOz - based systems are intensively investigated as promising 'environmentally benign' catalysts for n-C5 - C6 alkane isomerization [1-4]. WO3/ZrOz and WOx/ZrOz systems (Ho~-14.5) are less acidic compared to SOnZ-/ZrOz (Ho ~ -16), but they are more stable. It is accepted that superacidic sites are clusters of H-tungstate bronzes, (WO3)m (W6-n 03) (n H+), which are formed when reducing WO3 clusters on surface of tetragonal ZrOz by dihydrogen [1]. Only clusters of some optimal size are catalytically active [2]. The role of tetragonal ZrOz consists in electron accepting from WOx clusters that weakens coulomb attraction of protons to the tungstate clusters. Usually WOx/ZrOz catalysts are synthesized by co-precipitation of zirconium and tungsten hydroxides or by impregnation of dried zirconium hydroxide with ammonium meta-tungstate solution with subsequent calcining of samples at 700-800~ [1-6]. However the samples so obtained possess relatively low surface area ( 40 - 60 mZ/g). In this article the methods of synthesis of WO3/ZrOz systems with surface area from 40 to 250 mZ/g are described. These samples exhibit high activity in n-hexane isomerization and gas-phase benzene nitration reactions.

388 2. E X P E R I M E N T A L Synthesis of WO3/ZrO2 samples was performed by co-precipitation method. To solution of ZrOC12"8H20 (30 g in 300 ml of water) 2.60 ml of aqueous meta-tungstate solution (containing 800 mg W in 1 ml) was added. This proportion of reagents corresponds to an atom ratio Zr:W = 8.25:1 and mass content of WO3 in oxide material (ZrO2+WO3) of 18,6%. The mixed solution was heated to boiling and then solution of 5.6 g of urea in 10 ml of water was added. The reaction solution was boiled under reflux 6 h. Then 11 ml of 12 M ammonia was added to the solution at room temperature (pH 9.2 - 9.6), precipitate was filtered off and washed with water up to disappearance of chloride in water. The product was dried first in stream of hot air and then for 24 h in drying box at 100~ The dried product was then calcined for 2 h at 800~ Surface area of WO3/ZrO2 sample after 800~ was 53 m2/g. Other samples were obtained by the same method but with different ageing times and amount of urea. Samples of WO3/ZrO2 dopped with Fe and Mn ions were synthesized by addition of FeCI3 or MnCI2 solutions to the starting reaction mixture with all the further operations being the same. Amount of Fe and Mn added was 2 at. % to zirconium. Surface area of FeWO3/ZrO2 sample calcined at 800~ was 59 M2/g, and 40 M2/g for Mn-WO3/ZrO2 Samples of mesoporous WO3/ZrO2 were obtained by the method described in [7] including formation of zirconia-tungstate sol in the presence of aqueous polyvinylalcohol (PVA) solution with subsequent sol-gel transformation. Xerogel containing polymeric template was carbonized in nitrogen stream and carbon formed was then burned out in air. Surface area of samples after calcining at 700~ was 150 - 200 m2g. For testing in n-hexane isomerization reaction the catalysts were promoted with 0.5wt % of Pt by impregnation with H2PtCI6 solution. Prior to testing the platinated catalysts were reduced for 4 h at 400~ in dihydrogen. Surface area of samples was measured by the standard low-temperature argon desorption method. Diffraction patterns were recorded using a DRON-UM1 diffractometer (CoI~ radiation). UV-Vis reflectance spectra of powdered samples were recorded with use of Specord M-40 spectrophotometer using MgO as a reference. Concentration of acid centers was determined by desorption of ammonia at different temperatures in vacuum with use of gravimetric installation, as well as by measuring of 4-nitrotoluene adsorption from solution. WOx/ZrO2(Pt) catalysts were tested in n-hexane isomerization reaction. Samples of catalysts (particle size 0.8 - 1.0 mm, catalysts layer volume 4 cm 3) were placed into an isothermal zone of the reactor and activated for 4 h at 400~ in flowing hydrogen. Then the reactor was brought to stable operating conditions (pressure 3 MPa, temperature 230270~ and fed with a mixture of hydrogen and n-hexane (H2:n-C6H14 = 1:1, LHVS= 1 h-l). Samples of products were taken for analysis at a time interval of 1 h. The catalysts retained their stable activity throughout the time period (6 h) taken by the experiments. Analysis of products was made in a gas chromatograph Khrom-4 using a flame-ionization detector and capillary column (50 m, 0.25 mm in diameter) with dinonylphthalate as a stationary phase. Catalytic experiments on benzene nitration were performed in a flow glass reactor with diameter of 0.7 cm with a fixed catalyst bed at atmospheric pressure. The volume of the catalyst

389 was 1 cm3. Carrier gas (nitrogen) was passed through the reactor at a rate of 6 ml/min. Benzene and 70% nitric acid were introduced into the reactor through capillaries, with their rate of feed being preliminarily calibrated. Experiments were conducted at 170 and 200~ as a rule, the experiment time was equal to 5 h. Benzene was removed from the reaction products mixture by evaporation. For analysis of samples Hewlett-Packard G1530A gas chromatograph was used with polyethylene glycol as a stationary phase. 3. R E S U L T S AND D I S C U S S I O N

Presence of tetragonal phase of ZrOz is a necessary condition for obtaining of superacid tungstate-containing catalyst [1-6]. The tetragonal and cubic modifications of ZrO2, characterized by the presence of anion lattice vacancies [8], which cause electronaccepting properties of the oxide matrix. In the XRD patterns of WO3/ZrOz samples synthesized as described above after calcining at 800~ predominantly the tetragonal phase is observed [6, 7]. The XRD patterns of WO3/ZrOz-PVA xerogels after consecutive treatment at gradually increasing temperatures are displayed in Fig. 1. After 200~ a maximum in the low-angle region is observed indicating the formation of ordered mesostructure. The formation of tetragonal ZrO2 phase takes place at temperatures above 500~

Fig. 1. XRD (CoK) of WO3/ZrOz-PVA samples prepared by sol-gel p olyvinylalcohol-terr~late method after consecutive treatment at: 1- 100~ 20 h inair 2- 200~ 20 h in air 1000.

3- 400~ 2 h in nitrogen h

A2t

,.~

4- 500~ 2 h in nitrogen 5- 600~ 2 h in nitrogen

10

20

30

4o

50

6o

7o

6- 500~ 5 h in air

200

Another necessary condition for superacidity in these systems is presence of WO3 clusters of definite optimal size on the surface of tetragonal ZrO2 [2]. Such optimal clusters are characterized by the width of forbidden zone E0 = 3.0-3.2 eV. The size of WO3 clusters

390 increases with increasing of calcination temperature and mass content of tungsten in the samples [2]. Fig. 2 displays UV-Vis diffuse reflectance spectra of some synthesized WO3/ZrO2 mesoporous powdered samples after different calcination temperatures. The inflexions of curves in the low-energy region of the plots indicate the presence of WO3 crystallites (E0=2.64 eV). Sample 5 exhibited high activity in n-hexane isomerization reaction. The use of Hammet indicators for the evaluation of the acidity of tungstatecontaining zirconia is complicated by the yellow color of the calcined WO3/ZrO2 samples and the dark-blue color of WOx/ZrO2 samples after reduction in dihydrogen. We evaluated the concentration of the strong acid surface centers by measuring the amount of indicator 4-nitrotoluene (4 - NT, pKa = -11.38) adsorbed on the superacidic catalysts samples from toluene solution. Concentration of the indicator adsorbed was determined by photometric method after its elution with dimethylformamide. It was found that calcined (800~ WO3/ZrO2 adsorbed 0.028 mmole of 4-NT/g of catalyst, while reduced WOx/ZrO2(Pt) adsorbed only 0.004 mmole of 4-NT/g. The last value is close to the content of acid sites active in n-pentane isomerization determined from the data of catalysts poisoning with 2,6dimethylpiridine [3]. One can suppose that the increased adsorption of 4-NT on WOs/ZrO2 is caused by formation of surface charge-transfer complexes between aromatic indicator molecule and Lewis acid sites. 6

lO

~ E

~

-

5"

/

/./f"

,-"~ -1z E

5-"

4-

W O J Z r O 2 non-reduced

+

W O /x Z r O 2 (Pt) reduced

3-

"1:3 t~

O

2-

"1:3

2-

,2,3

._~ E

0

Eo--Z ~ a

3.40eV

0

,

i

50

,

i

100

,

i

150

,

i

200

,

i

250

,

i

300

,

350

400

Temperature, ~

3.17

Fig. 2. UV-Vis diffuse reflectance spectra of mesoporous WOa/ZrO z samples. 1,2,3,4 - samples after calcining at 500, 600, 700, and 800~ (W:Zr=-1:8.25 at).

Fig.3. Gravimetric curves of thermodesorption of ammonia from reduced and non-reduced zirconiatungstate catalysts

5 - sample with W:Zr--1:4.12, 700~ Another method for evaluating the strength and concentration of surface acid sites is based on gravimetric measurements. Ammonia was preliminary adsorbed on the

450

391 samples dried at 400~ and weight of ammonia retained by the sample after desorption in vacuum at different temperatures was recorded (Fig. 3). Reduction of WO3/ZrQ (Pt) in dihydrogen (4 h, 400~ resulted in some increase of the amount of strongly adsorbed ammonia which desorbed at temperatures above 150~ This concentration amounted to 0.16 mmol/g compared to 0.12 mmol/g for non-reduced WO3/ZrO2. For the WOx/ZrOz(Pt) sample, the concentration of acid centers stronger than in H-mordenite (ammonia desorption above 330~ was found to be 0.035 mmol/g (Fig. 3). One more method used was the titrimetric determination of n-butylamine adsorbed on the samples from toluene solution. It was found that both reduced and non-reduced samples adsorbed near 0.13 mmole of n-butylamine/g which is close to the data on ammonia adsorption. WOx/ZrOz(Pt) catalysts prepared exhibit high activity in n-hexane isomerization reaction (Table 1). The yield of branched hexane isomers at 230 - 250~ amounts 65 70%, selectivity for i-C6 - 70 - 94%, conversion of n-hexane about 80%. These results are close to the patent data [5]. Similar characteristics were observed for H-mordenite (Pd) at 280 - 300~ [9] and for SO42-/ZrOz(Pt) at 220~ [10]. However, for WOx/ZrO2(Pt) catalysts, higher yield of the most valuable product - 2,2-dimethylbutane (2,2-DMB/Y~iC6=17-21%) was observed. It should be noted that WOx/ZrO2(Pt) samples obtained by impregnation of zirconium hydroxide xerogel with ammonia meta-tungstate solution exhibited considerably lower catalytic activity (about 15% n-hexane conversion). Tablel. Activity and selectivity of catalysts on the basis of tungstate-containing zirconia in the nhexane isomerization reaction (LHVS = I h-l; P = 3.0 MPa; Hz:C6H14= 1:1; Pt content = 0.5 wt.%) Sample t,~ n-C6H14 Selec- Contentof Reaction products composition, wt. % conver- tivity 2,2sion, For dimethylb 23C6 n-& 2,2-di % Ei-C6, utane in iso- methyl- methyl- methyl% wt ~i-C6, pen- butane pentane pentane %wt tanes WOx/ZrO2 250 81,1 93,8 21,5 0,6 0,6 3,7 16,3 39,0 20,8 (Pt)-I 270 78,8 87,9 16,0 1,4 0,6 7,4 10,9 36,1 21,3 WOx/ZrO2 230 79,8 96,3 18,7 0,3 0,7 1,7 14,4 40,6 21,9 (Pt)-2 250 80,8 88,4 16,8 3,4 0,8 5,1 12,0 39,0 20,5 270 86,8 69,0 20,3 9,7 0,4 16,8 12,1 29,7 18,1 WOx/ZrOz 230 77,0 84,3 12,9 3,6 0 8,5 8,4 37,7 18,8 (Pt)-Fe203 270 80,8 69,6 17,1 8,9 0 15,7 9,6 28,9 17,8 WOx/ZrOz 230 80,0 87,1 14,5 3,0 0 7,3 10,1 38,8 20,7 (Pt)-MnOx 250 79,9 83,3 13,5 4,7 0 8,7 9,0 37,3 20,3 270 Very low yield of liquid products Mesopor. 210 72,5 99,4 10,3 0,1 0 0,3 7,4 42,1 22,6 WO• 230 77,4 91,5 15,7 1,4 0 5,2 11,1 38,6 21,0 (Pt) 250 82,3 87,0 17,9 3,9 0 6,7 12,8 38,3 20,5

392 Catalytic activity of mesoporous samples was close to that of WOx/ZrOz(Pt) samples obtained without use of template (PVA). However due to higher surface area and pore volume the space time yield (STY) of i-C6 over the mesoporous catalysts was about 30% higher. So, STY = 5.3 mmol/gcat h at 250~ and LHVS -1 h -1. As seen in Table 1, doping with Fe and Mn compounds did not result in substantial change of the catalytic activity and selectivity in n-hexane isomerization. On the contrary, it is known that in case of sulfated zirconia, promotion with Fe and Mn greatly increases the catalytic activity [11]. It was supposed that the promoting effect of the transition metal in S04Z-/ZrOz(Pt) system was due to the promotion of the hydrogenation-dehydrogenation of the alkane by a redox mechanism. In case of WOx/ZrOz(Pt) system, this process probably proceeds without other promoters. But the not platinized WOx/ZrO2 demonstrated low activity [6]. A convenient method for testing the strength of catalysts acid sites is temperatureprogrammed reaction of cumene dealkylation using mass-spectrometer for the analysis of the reaction products. We used this method for testing of samples before isomerization catalytic experiments. Both WO3/ZrO2 and WO,,/ZrOz(Pt) exhibited high activity in cumene cracking. In the temperature-programmed reaction (TPR) spectra, the peak of formation benzene from cumene (preliminary adsorbed on superacid WOx/ZrO2) was observed at 100-130~ while for less acidic H-mordenite this peak appeared at 170~ [6]. However it should be noticed that there was no direct correlation between catalytic activity of different WOx/ZrOz samples in cumene cracking and n-hexane isomerization. The use of superacid WO3/ZrO2 materials may be promising for various catalytic processes. Particularly, it is technologically advantageous to use solid superacid catalysts for the nitration of benzene. Nitrobenzene (NB) is a starting material for obtaining valuable products such as aniline and benzidine. The main disadvantage of the existing industrial process of benzene nitration by liquid acids is the considerable waste of sulfuric acid and the necessity of utilization of the acid mixture after the reaction. The processes of gasphase nitration of several aromatic compounds [12], toluene [13], benzene [14] were investigated with different catalysts including superacid oxide systems based on WO3/ZrO2 [14]. Prepared WO3/ZrOz systems exhibit high activity in the vapour-phase (170 ~ benzene nitration reaction with use of 70% HNO3 as nitrating agent (Table 2). Chromatographic analysis of the reaction products revealed that the content of mononitrobenzene amounted to 98.2-99.8% provided that the starting molar ratio C6H6/HNO3>I. The main impurity was benzene. Other admixtures were negligible (less than 0.01 wt%). Thus, the nitration under the above-mentioned conditions proceeds selectively with formation of mono-nitrobenzene. However, when nitric acid was present in excess, one could observe the formation of products which were solid at room temperature, apparently consisting of a mixture of polynitrobenzenes. The yield of NB amounts to 65-80% of theoretical calculated on the spent nitric acid. Repeating of the process with previously used catalyst samples at similar conditions leads to only minor loss of catalytic activity. WO3/ZrO2 based catalysts worked stably for at least 15 h at 170~ but an increase of temperature to 200~ resulted in a decrease of NB yield to 49-56%. This can be caused by destruction of the catalysts or surface active sites in these conditions.

393 The samples of mesoporous and co-precipitated WO3/ZrO2 have similar catalytic activity. Preliminary reduction of WO3/ZrO2 to WOx/ZrOz does not effect substantially the catalytic activity. Obviously oxidation of WOx clusters to WO3 proceeds in presence of nitric acid. This is indicated by change of dark-blue color of reduced samples to yellow after catalytic reaction. The dependence of the NB yield on the nitric acid feed rate is plotted in Fig. 4. One can see that an increase of 70%-HNO3 feed rate up to LHVS=0.9 results in a nearly proportional increase of NB space time yield (STY) while relatively little change of NB yield (65-80%). 0,7

0,6 -

0,5 -

0,4

>_Ico

Fig. 4. Space time yields of nitrobenzene in benzene nitration reaction catalysed

0,3

0,0

o,o

o12

oi,,

o16

o18

11o

112

1,4

by WO3/ZrO z (170~ Squares - mesoporous, Circles - co-precipitated WO3/ZrO z

LHSV of 70% H N O 3, h -1

The optimal combination of STY and NB yield (0.4 gNB/gcatalysth at 66% NB yield) was observed at LHVS(70% HNO3) = 0.9 (Table 2, sample # 9). Mesoporous WO3/ZrOz catalysts are characterized by higher STY than catalysts obtained without use of template (Fig. 4). This is caused by the higher surface area and pore volume of the mesoporous samples. Compared to superacid WO3/ZrOz catalysts, the activity of the other catalysts was low. NB yield was 14% for HZSM-5 zeolite (Si/AI=26), and 25% for WO3/TiO2. While testing this samples, the formation of considerable amount of nitrogen dioxide was observed, in contrast to WO3/ZrOz catalyzed. The decrease of the catalytic activity in the order WO3/ZrO2 > WO3/TiO2 > HZSM5 corresponds to decrease of their acid strength. This agrees with the well-known mechanism of aromatic compounds nitration via formation of intermediate nitronium cation NO2 §

394 Table 2. Data on vapour-phase nitration of benzene (170~ Catalystand weight (grams) of 1 cm 3 of LHSV, h-1 its bulk volume # 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Mesoporous WO3/ZrO2 (1.16) Sample after the previous experiment (Repeatedly) Repeatedly Co-precipitated WO3/ZrO2 (1.80) Repeatedly WO• (1.85) Co-precipitated WO3/ZrO2 (1.99) Repeatedly Co-precipitated WO3/ZrO2 (1.95) Co-precipitated WO3/ZrO2 (2.04) Repeatedly Mesoporous WO3/ZrOz (1.16) WOflTiO2 (1.30) HZSM-5 (0.597)

C4M6/ Yield HNO3 of NB, Mole %

Space time yield of NB

C6H6

70% HNO3

0.37 0.59

0.13 0.16

2.08 2.47

75 68

0.13 0.10

0.49 0.43 0.49 0.41 0.46 0.42 1.56 1.23 2.46 2.30 0.56 0.41

0.15 0.15 0.16 0.19 0.30 0.11 0.90 0.75 1.33 1.07 0.27 0.23

2.37 2.05 2.22 1.57 1.12 2.73 1.26 1.19 1.35 1.57 1.48 127

68 77 70 75 80 65 66 69 54 40 25 14

0.14 0.09 0.08 0.10 0.15 0.05 0.40 0.32 0.51 0.64 0.07 0.08

4.

CONCLUSIONS Investigations carried out have shown that controlled hydrolysis co-precipitation method and sol-gel method with use of polyvinylalcohol as template allow to prepare superacid WO3/ZrO2 materials which are highly active in n-hexane isomerization and vapour-phase benzene nitration with nitric acid. Doping of WO3/ZrO2 with Fe and Mn ions, as well as with silica and alumina [6] does not improve the catalytic properties in n-hexane isomerization. Mesoporous catalysts based on WO3/ZrO2 system have higher productivity due to the higher surface area and pore volume compared to similar materials obtained by the coprecipitation method. REFERENCES

1. E. Iglesia, S.L. Soled and G.M. Kramer, J. Catal., 144 (1993) 238. 2. D.G. Barton, M. Shtein, R.D. Wilson, S.L. Soled and E. Iglesia, J. Phys. Chem. B, 103 (1999) 630. 3. J.G. Santiesteban, J.C. Vartuli, S. Han, R.D. Bastian and C.D. Chang, J. Catal., 168 (1997) 431. 4. S. Kuba, P. Lukinskas, P.K. Grassell, B.C. Gates, M. Che and H. Kn6zinger, Abstr., EuropaCat-V, Limerick (2001) 10-O-07. 5. C.D. Chang et al., Catalyst, US Pat. No. 5,382,731 (1995).

395 6. V.V.Brei, N.N. Levchuk, J.V.Melezhyk and K.I.Patrylak, Kataliz and Petroleumchemistry, 5-6 (2000) 59. 7. O.V.Melezhyk, S.V.Prudius and V.V.Brei, Microporous Mesoporous Mater., 49 (2001) 39. 8. V. N. Strekalovskiy, Yu.M.Polezhaev and S.V.Pal'guev. Oxides with admixture disordering, Moscow, 1987. 9. K.I. Patryliak, F.M. Bobonich, Yu.G. Voloshina, M.M. Levchuk, V.G. II'in, O.M. Yakovenko, L.A. Manza and I.M. Tsupryk, Appl. Catal. A: Gen., 174 (1998) 187. 10.i A.V. Ivanov, T.V. Vasina, O.V. Masloboishchikova, E.G. Khelkovskaya-Sergeeva, L.M. Kustov and P. Zoiten, Kinetika i Kataliz, 39 (1998) 396. 11. T.Wan Kam, C.B.Knouw and M.E.Davis, J. Catal., 158 (1996) 311. 12. O.V. Baklashov, N.F. Salakhutdinov and K.G. Ione, Abstr. Intern. Conference PostCongress Symposium EuropaCat-3 "Zeolite Catalysis and Industrial Progress", Krakow, (1997) 57. 13. D. Vassena, A. Kogelbrauer and R. Prins, Catal. Today, 60 (2000) 275. 14. A.A. Greish, S.S. Demygin and L.M. Kustov, Recent Reports of the 12th ICC, Granada, (2000) R-071.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

397

Preparation of copper-oxide catalyst systems for hydrogenation Yoshihisa Sakata, Naotake Kouda, Yasuyuki Sakata and Hayao Imamura Department of Advanced Materials Science and Engineering, Faculty of Engineering Yamaguchi University, 2-16-1 Tokiwadai Ube, 755-8611 Yamaguchi, JAPAN The preparations of copper-oxide catalyst systems for hydrogenation were investigated by the production of methanol by CO hydrogenation. The catalyst systems were prepared by the reduction of various mixed oxide precursors. The copper-lanthanide oxide system prepared from copper-lanthanide mixed oxide, Cu608Ln(NO3) (Ln=Ho to Yb), exhibited high activity for methanol production at relatively low temperatures compared with conventional methanol synthesis catalysts, where the catalytic property was examined by copper-ytterbium oxide system. The catalytic activity varied by changing the dispersion of ytterbium oxide in the catalyst, where the dispersion was controlled by the reduction condition of the precursor oxide. The optimum condition of the catalyst for the reaction is a homogeneous mixture of copper and ytterbium oxide, where fine ytteribium oxide particles are homogenously dispersed in copper metal. On the basis of the information from the investigation of the catalyst system prepared from Cu608Ln(NO3), the catalyst was prepared from the reduction of the mixed oxide obtained from the calcination of the mixtures of copper and ytterbium hydroxide coprecipitate. The catalytic activities were dependent on the condition of the precursor and the catalytic performance was improved by changing the content of ytterbium oxide. The catalytic properties of the catalyst system of copper and the other oxides were also investigated. 1. INTRODUCTION Copper is one of the suitable metals for hydrogenation catalyst and shows characteristic performances in various hydrogenations [1,2]. However, the catalytic performance of the metal itself is not enough, so that various modifications, such as the combination of suitable elements as promoter, have been carried out in order to improve the performance [3-6]. For the promoted copper catalysts, lanthanide elements have been reported as the effective promoter for hydrogenation e.g. the activity of ethene hydrogenation over copper metal catalyst was remarkably improved by adding Yb and Eu, where the catalyst was prepared by Cu metal with Yb or Eu metal dissolved in liquid NH3

[31. On the other hand, it is known that copper based catalyst, such as Cu/ZnO/AIzO3 and Cu/ZnO/CrzO3, exhibited characteristic performance to methanol production by CO or CO2

398 hydrogenation [1], while, when copper-lanthanide alloys were employed as catalysts to CO hydrogenation, methanol produced selectively with high activity from relatively low temperatures compared with Cu based methanol synthesis catalyst [4-6]. The alloy was oxidized under the reaction condition and the condition of the catalyst under the steady state is the mixture of copper metal and lanthanide oxide. These are clearly suggesting that oxides, particularly lanthanide oxides, are effective promoters for copper metal catalyst for hydrogenation. We investigated the catalytic properties of copper-lanthanide mixed oxide, Cu608Ln(NO3) [7] and found that this oxide easily reduced to copper metal and lanthanide oxide. The catalytic property of copper-ytterbium oxide system prepared from Cu6OsYb(NO3) to hydrogenation of ethene and acetone was also investigated [8]. The catalytic activity increased markedly compared with the corresponding precursor mixed oxide and changed by changing the dispersion of Yb203 in Cu. The key point of the optimum condition of the catalyst was the dispersion of Yb203 particles in Cu. Furthermore, the high catalytic performance of this catalyst system was also confirmed to methanol production by CO hydrogenation [9, 10]. In this work, we investigate the preparation of copper-oxide catalyst system for hydrogenation, particularly CO hydrogenation for methanol production. We report the relations between the condition and morphology of the catalyst originated from the preparation condition of the catalyst and the catalytic activity for CO hydrogenation as well as the catalytic activity of various oxide contained copper. 2. E X P E R I M E N T A L

Two kinds of catalyst precursors were mainly employed in this work. One was the mixed oxide of Cu608Ln(NO3), prepared by pyrolysis of the mixture of corresponding nitrates under O2 flow at 673 K, referred to in previous reports [7,11,12]. The other was the homogeneous mixture of the corresponding oxides, which was prepared by calcination of the hydroxide coprecipitate, obtained by the hydrolysis of the mixed nitrate solution added with NazCO3 solution (0.05 mol/l) and washing the obtained coprecipitates with distilled water several times until the pH of the supernatant solution was dropped to 7, at prescribed temperature. The copper-oxide catalyst systems were obtained by the reduction of the prepared precursors under Hz (300 Torr; 1 Torr=-133.32 Pa) at prescribed temperatures before the reaction. The pretreatment and reaction were carried out in a closed gas circulation system equipped with vacuum line and gas chromatograph sample inlet. The reaction was carried out under the mixed gas of H2 and CO (Hz/CO=3; 400 Torr) at prescribed temperatures. The products were condensed in a liquid-nitrogen cooled sampling tube in the system and then analyzed by on-line gas chromatograph equipped with TCD and FID detectors. The morphology and condition of the catalysts and precursors were examined by XRD, TEM-EDS and XPS.

399 3. RESULTS AND DISCUSSION The fundamental catalytic properties of copper-lanthanide oxide system were examined by employing copper-ytterbium oxide catalyst prepared from Cu608Yb(NO3). When the CO hydrogenation was carried out over the catalyst, CO2 and CH4 produced at the beginning of the reaction and the activity decreased with passing time. Particularly, the production of CH4 was only observed at the beginning of the reaction. On the other hand, methanol produced constantly after the induction period for 2 hours. The product of hydrogenation under steady state was methanol and the selectivity was more than 99%. Beside the products of hydrogenation, the production of CO2 was observed. Here, the activity of the catalyst was defined the amount of the produced methanol from 1 gram of catalyst per hour. Fig. 1 shows (a) the dependence of the activity upon the reduction temperature of Cu6OsYb(NO3) and (b) XRD pattern of the catalysts prepared from the mixed oxide reduced at various temperatures. As shown in Fig. 1 (a), the catalytic activity depends on the reduction temperature of the precursor mixed oxide and it is noticed that the precursor reduced at 523 K is preferable for CO hydrogenation. In order to know the difference of the catalyst originated from reduction temperature, various measurements were carried out. From the results of XRD measurement as shown in Fig. l(b), when the precursor was pretreated at preferable temperature, only the pattern attributable to copper metal is observed and the diffraction peaks are sharper by raising the reduction temperatures. It is also observed that the diffraction pattern attributable to Yb203 accompanied that of copper metal can be observed, as also shown in Fig. l(b), to the catalyst reduced over 673 K. Furthermore, the surface areas of the catalyst prepared by reduction of the precursor mixed Cu

100 Cu

8O Yb203

60

~

(o)

40

%

o~

"~

20

(b)

0

i

400

i

i

500 600 700 Reduction Temp. / K

800

(~) i

(a)

30

i

40

20

i

I

50

60

(b)

Fig.1. (a) Dependence of the activity upon the reduction temperature of the mixed oxide precursor of Cu608Yb(NO3) and (b) XRD patterns of the prepared catalyst obtained by the reduction of the precursor at (a) 523 K, (b) 623 K and (c) 723 K.

400

oxide were examined and the values are listed in Table 1. Table 1 gives the particle size of the components in the catalysts, calculated from the results of XRD by the Scherrer formula, as well as the catalytic activities. From Table 1, the surface area in the preferable catalyst for CO hydrogenation is 72 ma/g, while that of the precursor oxide was 10 mZ/g. The surface area of the catalyst was noticeably increased by the reduction of the precursor. It is also observed that the surface area of the catalyst decreased with raising the reduction temperature, while the particle size of copper metal increased and the catalytic activity decreased. This is probably caused by the crystal growth of copper metal and the copper and Yb203 crystallized individually with raising the reduction temperatures. In order to know the morphology and condition of the catalysts, TEM and EDX were applied to the catalyst prepared at various reduction temperatures. Table 1 The catalyst parameter of Cu-YbzO3 prepared by the reduction of Cu6OsYb(NO3) at various reduction temperatures. Reduction temp. Surface area Particle size Activity /K /m 2g-1 Cu Yb203/nm 10-6mol/h-g 473 64 17 42 523 72 14 93 573 41 17 71 623 40 18 40 673 37 20 8 33 723 23 20 9 20 From TEM measurement, the particles between 15 and 20 nm in diameter were observed in the catalyst obtained by the reduction at 523 K (catalyst 523R), while the particles between 15 and 20 nm as well as a few nm in diameter were observed in the catalyst obtained by reduction at 623 K (catalyst 623R). From the simultaneous EDX examination, EDX spectra clearly showed that the particles in catalyst 523 R consisted of stoichiometrical mixture of Cu and Yb, while the particles in catalyst 623R consisted of mainly Cu for those between 15 and 20 nm and mainly Yb for those of a few nm in the diameter. This suggests that the morphology of the preferable catalyst is a homogeneous mixture of copper metal and ytterbium oxide, where fine ytterbium oxide particles (probably the particle size was sub-nm in the diameter) homogeneously dispersed in copper metal. By raising the reduction temperature, copper metal and ytterbium oxide crystallized individually and separated from each other. This suggests that the interaction between the oxide and copper metal decreased with raising the reduction temperatures. These are some of the reasons why the catalytic activity decreased with increase of the reduction temperatures. The catalytic CO hydrogenation activity for methanol production over Cu based catalysts are listed in Table 2. In Table 2, the activity of conventional copper based methanol synthesis catalysts, Cu/ZnO/CraO3 (Cu:Zn:Cr = 6:3:1) and Cu/ZnO/AIzO3 (Cu:Zn:AI = 4:5:1), as well as Cu-YbaO3 prepared in the present work at various reaction

401 temperatures are listed. From the results in Table 2, the Cu-Yb203 catalyst prepared in this work exhibits noticeable catalytic performance to methanol production by CO hydrogenation compared to the conventional methanol synthesis catalysts under the present experimental condition. Particularly, the special feature of Cu-Yb203 catalyst shows the high catalytic activity of CO hydrogenation for methanol production at relatively low reaction temperatures. The precursor mixed oxide of Cu608Ln(NO3) can be prepared with copper and metal ions where the oxide of the metal ion express In203 type crystal structure [11,12]. Therefore, the metal ion of Ln can be changed to In and some lanthanide ions (Ho to Lu). The catalytic activity of Ho, Er and Yb contained copper prepared from the corresponding mixed oxides are listed in Table 3. From the results in Table 3, the catalytic activity is changed by changing the oxide and ytterbium oxide contained copper show the highest activity. There are various factors, e.g. the nature and condition of the precursor oxide, to explain the difference. However, the difference cannot be explained further from the results in this work. Table 2. Activities of methanol production over various Cu based catalysts. Catalysts Red. Temp./K React. Tem./K Activity/10-6mol/h-g 523 473 93 Cu-Yb203 573 473 71 573 523 51 573 573 13 Cu/ZnO/Cr203

573 573 573

473 523 573

3 20 7

Cu/ZnO/AI203

523 623

473 523

10 22

Table 3. Catalytic activity of Cu-Ln203 prepared from various Cu608Ln(NO3) precursors to methanol production. Precursor Red. Temp./K React. Temp./K Activityl06mol/h-g Cu6OsHo(NO3) 523 473 37 Cu608Er(NO3) 523 473 34 CuaO8Yb(NO3) 523 473 93 On the basis of the preferable condition of Cu-Yb203 catalyst system prepared from mixed oxide precursor, the preparation of the catalyst from the homogeneous mixture of the corresponding oxides were investigated. Here, the precursors obtained from the calcination of copper and ytterbium hydroxide coprecipitate were examined. When the catalyst prepared from the copper and ytterbium oxide mixture was

402

applied to CO hydrogenation, the same catalytic behavior, products and selectivity as that prepared from the mixed oxide was observed. The relation between the conditions of the catalyst precursor and the catalytic activities were investigated. 300

I

Yb203~u~

CuO

CuO CuO

200

~

623K

100

523K

0 400

I

450

,

I

500 Calcination

I

I

550 Temp.

600 /K

650

20

i

20

'

20

a

423K |

so

20 /deg

Fig.2 (a) The dependence of the activity for methanol production upon the calcination temperature of the precursor of the copper and ytterbium oxide mixture and (b) XRD pattern of the precursor oxide mixture calcined at various temperatures. Fig. 2 shows (a) the dependence of the activity at 473 K upon the calcination temperatures of the precursor of copper and ytterbium oxide mixture, where the content of Yb is 14 atomic % to Cu and the precursors were reduced at 523 K, and (b) the XRD pattern of the precursor at various calcination temperatures. As shown in Fig. l(a), the catalytic activity improved by the calcination of the obtained coprecipitate and the catalyst prepared from the precursor calcined at 473 K exhibit the optimum activity. The activities of the catalysts prepared from the precursor calcined above 473 K decreased with raising the calcination temperatures. From the XRD patterns of the precursors calcined at various temperatures as shown in Fig.2 (b), only the pattern attributed to CuO is observed to the precursors calcined up to 623 K and the peaks are sharper with raising the calcination temperatures, while the pattern attributed to Yb203 is also observed to the precursors calcined over 723 K. These results suggest that the condition of the precursor of the oxide mixture is seriously affected by the catalytic behavior. This suggests that the condition of the oxide mixture is one of the important factors for the preparation of active catalysts from the mixture. The condition of the precursors was also observed by XPS. Table 4 shows the XPS peak intensity ratio of Cu (2p)/Yb (3d) for the catalyst precursors, the content of Yb is 14 atomic %, calcined at various temperatures. From the results in Table 4, the values of the ratio decrease with raising the calcination temperatures. This suggests that ytterbium oxide gradually deposits over the surface when raising the calcination temperatures. Therefore, the optimum condition of the catalyst precursor,

403 calcined at 473 K, is under crystallizing CuO where fineYb203 particles dispersed homogeneously over the surface and in the bulk. Table 4. XPS peak ratio of Cu2p/Yb4d of the catalyst precursor (Yb = 14 atomic %) calcined at various temperatures. Calc. Temp /K Cu2p/Yb4d 373 162 423 132 523 76 623 52 723 48 Table 5. Relation between the catalytic activity of methanol production at 473 K and the content of Yb ion in the Cu-Yb203 catalyst. Yb content/atomic % Activity 10-6mol/h-g 14 a 93 14 170 3 81 5 142 8 275 25 63 80 34 a: Prepared from mixed oxide precursor of Cu608Yb(NO3). The catalytic activities of Cu-Yb203 catalysts prepared from the precursor of oxide mixtures are listed in Table 5, as well as those of the catalyst prepared from the mixed oxide precursor. It can be seen that the catalyst prepared from the oxide mixture containing the same amount of Yb203 shows higher activity than that from the mixed oxide of Cu608Yb(NO3). This indicates that Cu-oxide catalyst systems which have high catalytic ability for CO hydrogenation can be prepared the precursor of mixed hydroxide coprecipitate. Therefore, this result expresses the possibility for the preparation of the various kinds of copper-oxide catalyst systems, such as the catalyst containing different amounts of the oxides and combined with various kinds of oxides. In Table 5, the activities of the Cu-Yb203 catalysts with different contents of Yb203 are also listed. From the results, the amount of Yb203 is significantly affected to the catalytic activity and, particularly, the catalyst which contains 8 atomic % of Yb203, exhibited maximum activity. Therefore, it is clear that the preparation of the copper-oxide catalyst system from the precursor of oxide mixture is the effective way.

404

350 300

250 ~200 ~150

0

I

La

i

Ce

I

Pr

i

Sm

i

Ho

1 Yb

Fig. 3. Catalytic activity of various lanthanide oxide contained Cu-oxide systems to CO hydrogenation: [7 = methanol production and II = CO2 production. Fig. 3 shows the catalytic activity of copper-oxide catalyst, where the catalysts contain various lanthanide oxide (8 atomic %). Fig. 3 shows both of the activities of methanol and CO2 production to lanthanum, cerium, praseodymium, samarium, holmium and ytterbium oxides containing catalysts. Samarium holmium and ytterbium oxide containing catalysts exhibited relatively high catalytic activities for methanol production compared with lanthanum, cerium and praseodymium oxides containing catalyst, while the activities for CO2 production are nearly the same for all catalysts. Particularly, lanthanum oxide containing catalysts show noticeably low activity for methanol production. These differences are probably caused by the degree of activation of the copper metal by interacting with the added oxides. However, in the present investigation, few evidence were obtained to make clear the reasons of the differences. Further investigations are necessary for concluding how the differences in the catalytic activity are produced by changing the combined oxides. 4. CONCLUSION It is concluded that copper-oxide catalyst system, particularly copper-lanthanide oxide catalyst system, show a high activity for CO hydrogenation. The effective condition of the catalyst is the homogeneous mixture of copper and oxide where a fine particle of the oxide dispersed homogeneously in the copper metal and the active catalyst can be prepared from the mixed oxide of Cu608Ln(NO3) and the homogeneous oxide mixture obtained from the calcination of the hydroxide coprecipitate. If the combined oxides were changed,

405 the activities also changed and these are probably caused by the difference of the degree of activation of copper metal by the combined oxides. REFERENCES

1. K. Klier, Adv. Catal., 31 (1982) 243. 2. T. Turek, D. L. Trimm and N. W. Cant, Catal. Rev., 36 (1994) 645. 3. H. Imamura, M. Yoshinobu, T. Mihara, Y. Sakata and S. Tsuchiya, J. Mol. Catal., 69 (1991) 271. 4. R. M. Nix, T. Rayment, R. M. Lambert, J. R. Jennings and G. Owen, J. Catal., 106 (1987) 216. 5. G. Owen, C.M. Hawkes, D. Lloyd, J.R. Jennings, R.M. Lambert and R.M. Nix, Appl. Catal., 33 (1987) 405. 6. A.E Walker, R. M. Lambert, R.M. Nix and J.R. Jennings, J. Catal., 138 (1992) 694. 7. Y. Sakata, S. Sakai, H. Imamura, S. Tsuchiya, R. Sugise and K. Ohdan, J. Mol. Catal., 88 (1994) 103. 8. Y. Sakata, S. Nobukuni, E. Kikumoto, K. Tanaka, H. Imamura and S. Tsuchiya, J. Mol. Catal. A, 141 (1999) 269. 9. Y. Sakata, S. Nobukuni, T. Hashimoto, E Takahashi, H. Imamura and S. Tsuchiya, Chem. Lett., (1998) 1211. 10. Y. Sakata, S. Tsuchiya, N. Kouda, E Takahashi and H. Imamura, Stud. Surf. Sci. Catal., 130 (2000) 3705. 11. I. Yazawa, R. Sugise, N. Terada, M. Jo, K. Oda and H. Ihara, Jpn. J. Appl. Phys., 29 (1990) L 1693. 12. R. Sugise, K. Ohdan, T. Hashimoto, K. Kashiwagi, M. Shirai, I. Yazawa and H. Ihara, Jpn. J. Appl. Phys. 32 (1993) L940.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

407

Application of experimental design for NOx reduction by Pd-Cu catalysts M. Rebollar a, M. Yates a and M.A. Valenzuela b. alnstituto de Catfilisis y Petroleoquimica, Cantoblanco 28049 Madrid, Spain. bLaboratorio de Catfilisis y Materiales, ESIQIE-Instituto Polit~cnico Nacional, UPALM 07738, D.F., M6xico. The possibilities of working with experimental design and the corresponding model to determine the formulation of new catalysts and calculate a priori their expected conversion values was studied. Eight Pd-Cu catalysts supported on mixed A1-Mg oxides prepared via a sol-gel route were produced and evaluated in the reduction of NO employing propane as the reducing agent. This technique greatly reduces the number of experiments necessary in order to define the factors of greatest importance in the preparation and operational variables, thus reducing the time needed to fine tune a catalyst composition towards the particular reaction system under study. Key words: Experimental design, NO reduction, environmental catalysis 1. I N T R O D U C T I O N Experimental design has been employed for more than forty years in industry and chemical research [1]. However, it is not normally used to produce new catalysts due to the different behaviour that may be generated depending on the materials and compounds used, their characteristics, content and order or manner of preparation that can make its usefulness doubtful. Thus, some sectors of the chemical industry have opted for methods that insure that all possibilities are covered, as in the case of the pharmaceutical industry with systems of combinational chemistry that are now beginning to be applied in homogeneous catalysis [2,3]. However, the use of automatic methods or libraries with information of over 1000 different combinations are techniques that are beyond the capabilities of most conventional heterogeneous catalysis research laboratories. The use of molecular simulation although presenting a large scope is not a method that can easily be applied to the preparation of heterogeneous catalysts at present since it is still at an early stage of development [4]. Thus, the need to reduce research time and costs permits the consideration of experimental design as an alternative to define the optimum composition for a catalyst formulation more rapidly. In this investigation the experimental design was employed, using a factorial model to define the optimum formulation of a Pd-Cu/AI-Mg-O catalyst that presents the desired DeNOx conversion with a propane-CO mixture as the reducing agent. The procedure was

408

based on obtaining a mathematical model by experimental design with which both the catalyst preparation and reaction variables were defined. The results obtained from these experiments led to the mathematical model. The degree of confidence in this model was then subsequently analysed by testing the catalytic activity of some of the catalysts in conditions that were different from those used to obtain the model but within the designed working limits of catalyst formulation and reaction conditions. 2. E X P E R I M E N T A L

2.1 Experimental design The initial step in any experimental design is to define the independent variables that are to be considered. In this case Pd-Cu/A1-Mg-O catalysts for the reduction of NO in gas flows with different contents of water vapour, propane, NO and CO was chosen as the reaction to be modelled. The minimum and maximum values of the independent variables of support preparation, active phase content and reaction conditions are given in Table 1. Table 1 Independent variables considered in the experimental design Key A B C D E F G H

Variable Pd content Cu content AI/AI+Mg ratio Reaction Temperature NO Inlet concentration Amount of reducing agent Water Inlet concentration CO Inlet concentration

Minimum Maximum 0.05% 0.50% 0% 1% 0.18 0.5 200 550 200 ppm 900 ppm 300 mol/mol NO 900 mol/mol NO 0% 10% 0% 10%

Due to the number of variables conventional testing of all the different combinations would require a minimum of 256 experiments: eight variables tested at their minimum and maximum values only giving 28 experimental combinations. If apart from the extreme conditions any of the variables were further analysed at intermediate values the total number of experiments would be greatly increased. Thus, in order to reduce the number of experiments required to analyse this number of variables a factorial experimental design of base 2 broken down to 1/16 reduces this to only sixteen experiments. The experimental design determines the combination of the values of the independent variables for the 16 experiments. Employing the established limits for the composition led to the preparation of eight catalysts. Within these eight catalysts four were bimetallic, four had high Pd content, four had high A1/AI+Mg ratio. The composition of the catalysts prepared in order to cover all of the preparation variables is presented in Table 2. The combination of these eight catalysts with the reaction conditions produced by this experimental design, together with the measured NO conversions are shown in Table 3. To

409 obtain the mathematical model the catalytic conversion after 20 minutes in reaction was considered. Table 2 Catalyst composition With Without Cu Cu CAT 1 CAT 5 CAT 2 CAT 6 CAT 3 CAT 7 CAT 4 CAT 8

Pd AI/AI+Mg Content (%) Ratio High (0.50) 0.2 High (0.50) 0.5 Low (0.05) 0.5 Low (0.05) 0.2

2.2 Catalyst preparation The supports were synthesised by sol-gel hydrolysis of aluminium trisecbutoxide (Aldrich) and magnesium nitrate (Mallinckrodt) [4]. The resulting gels were dried at 120~ for 8 h to eliminate the excess alcohol then calcined in flowing air at 600~ for 8 h. The catalysts were prepared by impregnation with excess solution of the supports and then subsequently dried in a rotary evaporator. The palladium employed was the granular metal dissolved in nitric acid (Productos Quimicos Monterrey) while the copper was its nitrate (Aldrich). All of the catalysts were dried at 120~ for 8 h and subsequently calcined at 500~ in flowing air for a further 8 h. For the bimetallic Pd-Cu catalysts the supports were initially impregnated with copper, dried and calcined then subsequently impregnated with palladium then dried and calcined. The drying and calcination routines between each impregnation were the same as that employed with the monometallic catalysts. The catalysts were characterised by atomic adsorption spectroscopy, TPR, TPD-NH3, TPD-CO, these values have been published elsewhere [5].

2.3 Catalytic activity measurements The catalytic activities were evaluated in a computer controlled tubular stainless steel XYTEL micro-reactor employing 0.1 g of catalyst in the packed bed. The sample was previously treated in a 10% Ha in He gas flow of 100 cm3min -1 at 500~ for 1 h. The analyses were carried out by in line automatic sampling of the inlet and outlet gasses with a Perkin Elmer Auto System XL gas chromatograph with a PE-Molosiev capillary column. The working conditions of catalyst weight and gas flows were previously established by an analysis of conversion against W/F in order to ensure that the reactor was working in a differential regime. A total gas flow of 7 cm 3 min -1 was employed in all of the experiments. The rest of the experimental variables determined by the experimental design are shown in Table 3.

2.4 Generation and testing of the model From the reactivity results of the 16 experiments a mathematical model was generated that permitted the prediction of the catalytic behaviour of any other PdCu/AlMg-O catalyst for NO reduction produced and tested within the set limits of preparation and reaction variables. This model was subsequently tested by prediction of the expected

410

NO conversion of one of the catalysts under conditions that were totally different from those used in the catalytic tests where it was employed as part of the model development. By comparison of the predicted NO conversion values with those determined experimentally the confidence level of the model was determined.

3. RESULTS 3.1 Experimental distribution and NO conversions The experimental distribution: catalyst to be tested, reaction temperature, inlet gas composition: NO concentration, propane concentration, presence of water vapour and presence of CO and the measured NO conversion values are collated in Table 3. A simple direct correlation from the NO conversion results presented in Table 3 with the experimental variables was not obvious. However, by application of the factorial design the effects and interactions between different variables may be defined by a mathematical model. Table 3 Experimental conditions and NO conversions Experiment Catalyst Temp 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

CAT CAT CAT CAT CAT CAT CAT CAT CAT CAT CAT CAT CAT CAT CAT CAT

1 4 3 2 1 6 2 8 5 7 8 3 7 6 4 5

~ 550 200 550 550 200 200 200 200 550 200 550 200 550 550 550 200

NO

C3H8

ppm 200 200 200 900 900 200 200 200 200 900 900 900 200 900 900 900

mol/molNO 900 300 300 300 300 300 900 900 300 300 300 900 900 900 900 900

H20 CO 10% 10% No Yes Yes No No Yes Yes Yes No No No Yes Yes No No Yes Yes No Yes Yes No No No No Yes No No No Yes Yes Yes Yes

NO Conversion

(%) 6.69 2.64 16.68 5.26 3.72 11.13 3.96 12.85 0.90 10.21 16.70 13.03 16.75 16.67 3.19 1.02

To simplify the comparative analysis the results were codified using equation 1, where all of the values lie between values of +1 or -1, and each variable is designated by the letter used in Table 1. Xe - X ;~=

Xma• X

(1)

411 Where Xe is the value to be codified, X is the average value of the defined limits for each variable and Xmaxthe maximum limit of the variable. From the results of these evaluations by use of the algorithm developed by Yates [6] employing the State-ease "Design Expert (5.0.9)" programme, the effects of each variable were calculated and plotted against a half normal percent probability, shown in Fig. 1 where those variables with little or no effect on the NO conversion value fall on the line. Variables that cause the greatest effect over the NO conversion are those that are located away from the line.

Fig. 1. Half normal plot of effect of design variables. The most significant independent variables are collated in Table 4. From this table it may be appreciated that the factor with the highest influence was the presence of water vapour (G). The negative effect of the water vapour over the NO conversion was probably due to it acting as a poison to the catalytic surface as reported by Kobylinsky and Taylor [7]. The increase in the AI/(AI + Mg) ratio (C) had a positive effect. Increased acidity, confirmed by ammonia TPD [5], coincided with the results of Hamada [8] and Kikuchi [9],, among others. The negative effect of Pd indicated that with increased content the NO conversion was reduced. This was in accordance with the results of Resasco [10], in that case attributed to the increase in the crystal size with a subsequent decrease in the active surface area, confirmed by the CO chemisorption results [9]. Although the addition of Cu tended to reduce the NO conversion it led to a higher Pd dispersion. The major effect of the Cu was the increased stability of the Pd particles. An increase in the reaction temperature

412

favoured the NO conversion. The other factors and interactions between them had only minor effects. Table 4 Effect of the variables over the NO conversion Key

Variable

Effect

Coefficients

A B C D

Pd Cu AI Temperature

-5.93 -3.88 6.64 3.04

-2.97 -1.94 3.32 1.52

E F G H AC BE CE EF

NO Propane Water Vapour CO Pd-AI Cu-NO AI-NO NO-Propane

-0.23 0.87 -6.69 -0.92 0.47 -0.97 -0.61 -0.68

-0.11 0.43 -3.35 -0.46 0.24 -0.48 -0.31 -1.36

3.2 Mathematical model The mathematical model as a function of the variables involved was obtained by fitting the data to a 1 st order lineal equation of the following type" Y = Prediction

z

o f conversion

= Average standard deviation

o f the coefficients

+

hEffect i

Obtaining the following equation: Y

= 3.20-2.97A-1.94B+3.32C+1.52D-0.11E+0.43F-3.35G-0.46H + 0.24 AC - 0.48 BE - 0.31 CE - 0.68 EF

(2)

Decoding these coefficients permits substitution for the variables used" Y

=

4.43 - 13.87 Pd - 2.36 Cu + 19.35 (A1 + Mg)/AI + 0.00867 Temp - 0.00681 NO + 0.00536 C3H8 - 0.67 Hz0 - 0.092 CO + 5.903 Pd-AI- 0.00276 Cu-NO - 0.005469 AI-NO - 6x10 .6 NO-C3H8 (3)

This equation had a R z = 0.97, and a standard deviation of 1.94.

3.3 Verification of the model The mathematical model thus obtained was verified experimentally both qualitatively and quantitatively. From the qualitative method it was possible to predict the

413 results from the evaluation of three catalysts, in this case CAT 3, CAT 4 and CAT 7. Catalysts CAT 4 and CAT 3 had the same active phase contents: 0.05% Pd - 1% Cu, but different supports. CAT 3 had a higher AI concentration while CAT 4 had a higher Mg concentration. From the model and the effect of AI, CAT 3 should have a higher conversion. In the case of CAT 3 and CAT 7 produced with the same support but CAT 7 did not contain Cu and thus from the model a higher conversion would be expected. In the evaluation of these catalysts presented in Fig. 2 both estimations were completed but only during the first minutes of reaction. Afterwards there was a steady reduction in the conversions due to deactivation or low stability of the catalysts. In the quantitative method a catalyst was chosen, in this case CAT 4, and then the reaction conditions defined in order to predict the conversion with the model using equation [2]. The defined reaction conditions for the quantitative testing of CAT 4 were the following: gas flow 7cm3min-1, [NO] = 714 ppm, [C3H8] 400 mol/mol NO. The results after 20 minutes in reaction are presented in Table 5.

50

9

30

20 8

10 0

, 0

40

80

,

,

120

160

200

Tree Fig. 2. NO conversion against time for samples: CAT 3 A, CAT 4 n and CAT 7 O. Table 5 Catalytic conversion results of CAT 4 Temperature Estimated ~ NO conversion 200 8.9 300 9.8 550 12.0

Experimental NO conversion 4.4 8.8 13.9

Repeat Experimental NO conversion 3.4 4.2 11.1

414 4. CONCLUSIONS The use of experimental design permits from a limited number of experiments to obtain a mathematical model with which the calculated conversion values were in agreement with the experimentally determined ones. A simple experimental analysis of the eight catalysts and all the variables xvould have required a minimum of 256 experiments. However, in this case the relationships obtained were only valid during the first 40 minutes of reaction since they were determined from the initial conversion values, although at 550~ the difference between the calculated and experimentally determined values was only 2.5%. A further model could be derived where the effects of catalyst stability and deactivation were also considered in order to define the best selection of variables in the catalyst preparation. REFERENCES

1. D. Cox, "Planning of Experiments", I st Edition, John Wiley & Sons Inc., New York, (1958). 2. F. Gennari, Catal Rev Sci Eng., 42 (2000) 385. 3. R.H. Crabtree, Chemtec, 10 (1996) 21. 4. C.M. Fremman, G. Fitzgerald, D. King and J.M. Newsam, Chemtec, 10 (1999) 27. 5. L.L. Hench, Chem Rev., 44, (1990) 639. 6. M. Rebollar Barcel6, Reducci6n Selectiva de Oxidos de Nitr6geno con Catalizadores de Paladio Soportados en Oxidos Mixtos AI-Mg, Tesis de Maestria ESIQIE-IPN, M6xico, (2000). 7. G. Box, W.G. Hunter and J.S. Hunter, "Statistics for experimenters. An introduction to design, data analysis and model building", John Wiley & Sons Inc., New York, (1986). 8. T.P. Kobylinsky and B.W. Taylor, J. Catal., 25 (1973) 149. 9. Y. Kintaichi, H. Hamada, M. Tabata, M. Sasaki and T. Ito, Catal. Lett., 6 (1990) 239. 10. E. Kikuchi, K. Yogo, M. Umeno and H. Watanabe, Catal. Lett., 19, (1993) 131. 11. Y.H. Chin, A. Pisanu, L. Serventi, W.E. Alvarez and D.E. Resasco, Catal. Today, 54 (1999) 419.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

415

Marked difference of catalytic behavior by preparation methods in CH4 reforming with COa over MoaC and WC catalysts Shuichi Naito*, Miyuki Tsuji, Yousuke Sakamoto, and Toshihiro Miyao Department of Applied Chemistry, Faculty of Engineering, University, 3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama, Japan.

Kanagawa 221-8686,

Dependence of the activity and durability upon the preparation methods was investigated in the reforming of CH4 with COa over molybdenum and tungsten carbide catalysts. It was found that deactivation of MozC was suppressed even at ambient pressure over catalysts prepared through nitridation of molybdenum oxides before carburization. On the contrary, in the case of WC the catalysts prepared through nitridation exhibited much lower durability against the oxidation of carbide during CH4-COz reaction than those prepared by direct carburization. 1. I N T R O D U C T I O N Early transition metal carbides with high surface area are active catalysts for various reactions such as hydrodenitrogenation(HDN) [1,2], hydrogenation [3,4], Fischer-Tropsch synthesis [5,6], hydrocarbon isomerization [7]. Synthesis of these materials has attracted great attention over the years and a number of procedures have been reported for the preparation of carbides with high surface areas suitable for catalysts. The most commonly employed method is temperature programmed reaction of metal oxide with a mixture of hydrogen and methane, which was developed by Boudart and co-workers [8-10]. It is well known that supported group VIII precious metals like Pt, Rh and Ru exhibit high activities for reforming of CH4 to synthesis gas but, because of their cost, cheaper catalysts are desired. Recently, Green et al. have reported that carbides of molybdenum and tungsten prepared by direct carburization method are extremely active for the oxidation of CH4 to synthesis gas with COz, Oz or steam [11-13]. At ambient pressure the carbides deactivated in all the processes due to the oxidation of the catalysts to MoOz, while operation at an elevated pressure resulted in stabilization of the carbide and no catalyst deactivation. We also investigated the activity of MozC dispersed on various supports for the CH4-COz reaction. The performance depended strongly on the properties of the supports, with the ZrOz-supported

416

MozC catalysts exhibiting the highest activity and durability for this reaction. Moreover, deactivation of MozC/ZrOz at ambient pressure was suppressed by decreasing the loading amount of MozC [14]. In the present study we have investigated the dependence of the activity and durability in the reforming of CH4 with CO2 upon the preparation methods of molybdenum and tungsten carbide catalysts. 2. E X P E R I M E N T A L

The carbides were prepared by the following two methods. The first one was direct carburization of molybdenum and tungsten oxides in a 20 % CH4/H2 mixture at 973 K for 3 h (heating rate of 1 K/min). The second one was the nitridation of these oxides in NH3 at 973K for 3h before carburization. Both preparation procedures were carried out in a flow type microreactor operated at atmospheric pressure. In the case of m o l y b d e n u m carbides, the first catalyst is designated as ~-MozC, while the latter is a-MoCl_• in the literature, but here they will be denoted simply as MozC(I) and MozC(II) [9,15]. The same designation was used for WC(I) and WC(II) [15]. The reaction was carried out in the same flow reactor mentioned above, using 0.5g of catalyst pretreated in hydrogen at 773k for 3h. The composition of the feed gases was CH4/CO2=1:1 and the products were analyzed by FID and TCD (both porapak Q columns) gas chromatography. The characterization of the catalysts was carried out by various techniques such as X-ray power diffraction (XRD), BET surface area measurement and CO chemisorption. Catalysts were passivated in flowing 3% O2/He for 12 h at room temperature before removal from the reactor for XRD and BET measurements. CO chemisorption was performed at room temperature of MozC assuming a stoichiometry of one CO molecule per exposed Mo atom. X-ray photoelectron spectroscopy (XPS) was also employed to investigate the electronic state of the MozC surface. 3. R E S U L T S AND D I S C U S S I O N Fig. 1 shows the changes in conversions of CH4 and CO2 with time on stream in CH4-CO2 reaction at 1123K over M02C(I) and M02C(II) catalysts. The characteristic feature of the reaction was that the conversion of CO2 was larger than that of CH4. Fig. 2 shows the corresponding formation rates of Ha and CO over these catalysts. The rate of CO formation was considerably larger than that of H2 formation. The mass balance analysis suggested the occurrence of the reverse water gas shift reaction (CO2+H2---~CO+H20) during CH4 reforming with CO2. The activity of MozC(II) was more than three times higher than that of Mo2C(I). In addition, its high activity was maintained for a moderate reaction period, and the conversions of CH4 and COz after 16 h on stream were about 90 % of the initial conversions. On the other hand, the

417 activity of Mo2C(I) decreased abruptly to two thirds within 1 h. In Figs. 1 and 2, the results of CH4-CO2 reaction at 1123K over MoO2 catalysts (without carburization) are also shown, with conversions similar to the cases of Mo2C(I). This result may suggest that surface carbide structure is necessary to maintain high activity and durability. Table 1 summarizes the results of the characterization of these two kinds of carbide catalysts prepared by different methods, as well as their turnover frequencies (TOF) for H2 and CO formation and their ratios in CH4-CO2 reaction at 1123 K. TOF was estimated from the initial formation rate of H2 and CO divided by the dispersion, assuming that the catalytic active sites are the exposed Mo atoms measured by CO chemisorption. It is worth noticing that the dispersion of Mo2C(II) was much larger than Mo2C(I). It has been well recognized that carbides of the fcc structure formed through nitridation possess higher surface areas than those with hcp structure made through direct carburization from the oxides [9,15]. This is also the case in our study, as seen from the results of BET measurements, suggesting the formation of smaller particle size crystallites in Mo2C(II) catalyst. Eventually, the TOF of Mo2C(II) was estimated to be 10~20 times lower than that of Mo2C(I), although its overall conversion was much higher than Mo2C(I). Table 1 Characterization and TOF(1123K) of molybdenum carbide catalysts TOF [xl O-2s-1] Catalysts BET surf.area CO uptake Dispersion [ m2/g ] [ ~t mol/g ] [ xl0 -2 ] CO H2/CO H2 Mo2C(I) 15.4(0.6") 14.4 0.1 10.6 74.3 0.1 MozC(II) 121.1 (21.3") 823.8 8.4 1.4 3.3 0.4 ( *" after the reaction) 80 - - ~ - ~ - " ~

70 6O

--O-" --A--

50 0 ,p,,~

Mo2C I Mo2C I Mo2C II Mo2C II Mo02 MoO2

CH 4 C02 CH4 C02 CH 4 C02

40 30

Fig.1

2O

I

I

I

I

l

I

0

I

2

3

4

5

R e a c t i o n T i m e (h)

6

Changes of the conversion of CH4-CO2 reaction over various Mo catalysts at 1123K.

418 30 ~

.....

^

,

MozC I H2

25 rgl

m ~" 20 @

--I"-l---O--

0

15-

MozC

I CO

M o2C

II H 2

MozCII CO MoO2 Hz MoOz CO

Fig.2 v

v

v

v

v

v

v

5 E

- ~ - -f~- z : r - - t ~ - - ~ . - ~ . - ~ . - ~ . - c J

00

1

2 3 4 Reaction Time [h]

5

Changes of the formation rates of CH4-CO2 reaction over various Mo catalysts at 1123K

The situation was rather different in the cases of t u n g s t e n carbide catalysts with different preparation methods. Fig. 3 shows the changes in CH4 and CO2 conversion with time on stream over WC(I), which was prepared by direct carburization. The c o n v e r s i o n levels were 61% and 42% for CO2 and CH4, respectively, and stayed almost constant for more than 10 hours. Fig. 4 shows the c o r r e s p o n d i n g formation rates of H2 and CO. The rate of CO formation was considerably larger than that of H2 formation, which again suggests the occurrence of the reverse water gas shift reaction during CH4 reforming with CO2, similar to the cases of m o l y b d e n u m carbides. After the first run, the catalyst was reduced by h y d r o g e n at 773K for three hours to r e m o v e the accumulated inactive carbons, and then the second run was carried out. As shown in the broken lines, the initial c o n v e r s i o n was very high (more than 90%), and dropped abruptly as the reaction p r o c e e d e d to lower values than that of the first run. This result suggests that H2 reduction after CHa-COz reaction may produce very active species (probably finely dispersed W metals ) for this reaction, which is more easily oxidized during the second reaction. On the contrary, when WC(II) was prepared through the nitridation of WO3, initial activity was similar to that of direct carburization catalysts, which decreased linearly as the reaction proceeded as shown in Fig. 4.

419

- -o- CH4 2 n d r u n - - ~ - - CO z 2 n d r u n

Zx

l

(a)

1I 6O

1st

run

C02

1st

run

~

~

_'~ 50

i

o .,-4 4 0 m >

CH4

at

--i I

r--n

.~.

~

~

"/x-

-A-- - A - - ~

- --A- - A -

I Fig.3 Changes of the conversion in CH4-CO2 reaction over WC(I) catalyst at 1123K

- A - - -A

3O

o

c~

" ~-

-o-

-o-

- Q-

-o

- -o-

- o-

20

0 0

-(3

t

•

~

_1

L

1

2

3

4

5

Time

6

[h]

80F

I=cH

60

I~H,

70

COz

i_+_ c0

so

-

35

a0g~ ~ 25 ~ ~

=o 4 0

-

~ o

30

-15~

o o

20

-

10

-

0

'-n

20

10

5

~

Fig.4 Changes of the conversion in CH4-CO2 reaction over WC(II) at 1123K

o~

r

0 0

2

4

6 Time

8

10

12

[h]

Figs. 5 and 6 represent XRD patterns of these carbide catalysts before and after the reaction. Crystal structures of MozC(I) and MozC(II) as prepared were different from each other, the former being hcp and the latter fcc, as mentioned already. In the case of MozC(I), several new peaks assignable to MoOz e m e r g e d after the reaction as shown in (b). This was not observed in the case of MozC(II), which r e m a i n e d stable (shown in (d)). These results suggest that the oxidation of MozC to MoOz at the initial stage of the reaction might cause an abrupt deactivation in the case of MozC(I). The XRD pattern of MozC(II) after the reaction indicated that the crystal structure was t r a n s f o r m e d from fcc to hcp during the reaction, a c c o m p a n i e d by an increase in crystallite size. In the case of tungsten carbide catalysts (Fig. 6), no oxide patterns were observed after the first run, but small oxide peaks at 0=26 ~ and 53 ~ e m e r g e d after the second run. These results also suggest that the oxidation of WC to WOz at the second run is the main cause of the abrupt deactivation.

420

(b)

20

(a)

20

1 30

I 40

1 60

20[

I 60

~ ]

I 70

I 80

I

30

I

40

50

60

70

80

20[ ~ ]

9~ XRD patterns of WC(I) (a)" after 1st run, (b)'after 2 nd run

Fig. 5 XRD patterns of Mo2C(I) and (II) (a)(b)" Mo2C(I), (C)(d)" Mo2C(II) (a)(c)" before reaction, (b)(d)" after reaction,

(e)" MoO2 after reaction

Fig. 7 illustrates the Mo3d and C l s XPS spectra of Mo2C(I) and (II) before and after the CH4-CO2 reaction. As summarized in Table 2, both catalysts exhibited almost the same binding energies of Mo3d and C l s before reaction, which can be assigned Mo2C (Mo3d=227.4 and 230.6 and C1S=282.8 eV). After the CH4-CO2 reaction at 1123K, the binding energies for Mo3d transition shifted to the higher eV side in both catalysts, whose extent was larger in the case of MozC(I) catalysts (0.7-1.0 eV) than Mo2C(I) catalysts (0.3-0.4 eV). Moreover, characteristic Cls peak of 282.8-230.0 eV, which can be assigned to carbide carbon almost disappeared in the case of Mo2C(I) after the reaction, indicating that the carbide structure of the Mo2C(I) surface may be destroyed by CH4-CO2 reaction. These results are consistent with the XRD bulk information, which is the main cause for deactivation of the catalysts.

90

421

_••

Mo3d

I

2oo

. ~ 1

S

c)

295

290

285 B.E.

Fig. 7

B.E. leVI XPS spectra of (a),(b)" Mo2C(I) (a),(c)" after the reaction

280

275

[eV]

and (c),(d); MoZC(II) (b),(d)" before the reaction

Table 2. Binding energies of XPS data Catalysts Mo2C(I) Mo2C(II)

Treatments before reaction after reaction before reaction After reaction

Mo 3d3/2 230.8 231.5 230.6 230.9

Binding energy [ eV ] Mo 3d5/2 C 1s 227.5 284.8 282.8 228.5 284.8 227.4 285.0 283.0 227.8 284.9 228.9

According to Green's results for the carbides prepared by direct carburization (corresponding to MozC(I) in this study), elevated pressures were needed to maintain a constant high activity for 72 hours, while their activity dropped abruptly after 7 hours in the reaction at ambient pressure [11]. They proposed a redox type reaction mechanism for the formation of syngas. In this mechanism, after the dissociation of CO2 the formed O(a) reacts with carbon in the carbide surface to leave vacancies. These are then filled with either carbon from methane, reforming the carbide, or oxygen to form MOO2. For the former step to remain predominant, an elevated pressure is required. To elucidate the different catalytic behavior of MozC(I) and Mo2C(II) in this study, CH4-CD4 isotopic exchange reaction was carried out over both catalysts. The rate of CH3D and CHD3 formation was several times faster over Mo2C(II) compared to that over Mo2C(I) at 373K, suggesting that dissociation of methane was much easier over Mo2C(II). Accordingly, oxidation of vacancies with oxygen in the redox mechanism may be more

422 effectively prevented over Mo2C(II), resulting in the durability of the catalytic activity compared to MozC(I). These situations are schematically summarized in Fig. 8.

H2

CO

CO

j

j

/i .........

.......i MoO2

Inactive carbon

Fig. 8 Schematic view of the mechanism of CH4-CO2 reaction 4. CONCLUSION The dependence of the activity in reforming CH4 with CO2 was investigated in depth upon the preparation methods of molybdenum and tungsten carbides. In the case of

MozC, catalytic performance of the

catalyst, prepared through nitridation of the oxide before carburization, was considerably different from that prepared by direct carburization of the oxide. The deactivation was significantly suppressed in the former catalyst, although its TOF for syngas formation was smaller than the latter. The situation was rather different in the case of tungsten carbides, and both direct carburization and nitridation-carburization catalysts exhibited similar initial activity, but the durability was much better in the former catalyst. ACKNOWLEDGMENTS This study was supported by High Tech Research Project of Ministry of Education, Science, Sport and Culture of Japan.

423 REFERENCES

1. C.C. Yu, S. Ramanathan, F. Sherif and S.T. Oyama, J. Phys. Chem., 98 (1994) 13038. 2. F. Garin, V. Keller, R. Ducros, A. Muller and G. Maire, J. Catal., 160 (1997) 136. 3. H. Abe and A.T. Bell, J. Catal., 142 (1993) 430. 4. G. Djega-Mariadassou, M. Boudart, G. Bugli and C. Sagay,

Catal. Lett.,

31 (1995)411. 5. G.S. Ranhotra, A.T. Bell and J.A. Reimer, J. Catal., 108 (1987) 40. 6. J.-L. Dubois, K. Sayana and H. Arakawa, Chem. Lett., (1992) 5. 7. V. Keller, P. Weher, F. Garin, R. Ducros, and G. Maire, J. Catal., 153 (1995) 9. 8. V. Volpe and M. Boudart, J. Solid State Chem., 59 (1985) 332. 9. J.S. Lee, L. Volpe, F.H. Ribeiro and M. Boudart, J. Catal., 112 (1988) 44. 10. J.T. Wroloski and M. Boudart, Catal. Today, 15 (1992) 349. 11. J.B. Claridge, A.P.E. York, A.J. Brungs, C. Marquez-Alvarez, J. Sloan, S.C. Tsang and M.L.H. Green, J. Catal., 180 (1998) 85. 12. A.J. Brungs, A.P.E. York and M.L.H. Green, Catal. Lett, 57 (1999) 65. 13. A.J. Brugs, A.P.E. York, J.B. Claridge, C. Maroquez-Alvarez and M.L.H. Green, Catal. Lett., 70 (2000) 117. 14. M. Tsuji, T. Miyao and S.Naito, Catal. Lett., 69 (2000) 195. 15. J.S. Lee, L. Volpe, F.H. Ribeiro and M. Boudart, J. Catal., 112 (2000) 195. 16. T. Xiao, A.P.E. York, V.C. Williams, H. AI-Megren, A. Hanif, X. Zhou and M.L.H. Green, Chem. Mater., 12 (2000) 3896.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

425

Synthesis and properties of new catalytic systems based on zirconium dioxide and pentasiis for process of NOx selective catalytic reduction by hydrocarbons V. L. Struzhko, S. N. Orlyk, T. V. Myroniuk, V. G. Ilyin L. V. Pisarzhevsky Institute of Physical Chemistry of NAS of Ukraine Pr. Nauki, 31, 03039, Kiev, Ukraine This work is devoted to the synthesis of ZrO2 by various methods, the synthesis of zirconium-containing pentasils and ZrO2- W-zeolite based binary carriers. These materials were used as carriers of transition metal oxides (chromimn, cobalt) and their catalytic properties were characterized in the selective reduction of NO by methane and propanebutane mixture, the acidic properties of the samples were investigated by thermoprogrammed desorption and IR-spectroscopy methods. 1. INTRODUCTION Zirconium dioxide and zeolites of pentasil structure are widely used as catalysts and efficient carriers in many heterogeneous reactions, and particularly in the process of selective catalytic reduction of nitrogen oxides by hydrocarbons (SCR-process) [1,2]. Synthesis of new catalytic systems for NOx SCR-process by CnHm is therefore related with searching for their optimum composition and preparation methods to attain maximum activity in this reaction. It is well known that the composition, texture and crystalline structure of zirconium dioxide are rather sensitive to the conditions of its preparation, resulting in changes of their acidic and catalytic properties [3]. The transition metal oxides as dispersed onto the oxide or zeolite carriers surface are active catalysts in the process of nitrogen oxides selective reduction by hydrocarbons (NOx/HC/O2) [4]. For example, the systems Pt-ZrO2- A1203 and MnOy- ZrO2 have been studied in the reduction of NO with propane [5]. The reduction of nitrogen oxides (NOx) with propane on platinum deposited on La203, ZrO2, and their mixture has been studied [6]. Recently, much interest has been focused on zeolite systems, in which inorganic oxides are introduced into the matrix as catalysts, including the selective catalytic reduction process. Cobalt containing zeolites are known to have high activity when methane is used as a reductant for nitrogen monoxide [7,8]. We have previously reported the cation exchanging ZSM-5-type zeolites (Co- and Cacontaining samples), and MexOy/ZrO2 oxide systems were the active catalysts in the process of nitrogen monoxide SCR by methane and propane-butane mixture [9,10].

426 This work is devoted to the synthesis of ZrO2 by various methods, the synthesis of zirconium-containing pentasils and ZrO2- H+-Zeolite based binary carriers as well as the study of their acidic and catalytic properties in the SCR-process. The catalytic properties of samples doped with cobalt and chromium oxide, based on zirconium dioxide, zirconiumcontaining pentasils and ZrO2- H+-Zeolite based binary carriers have been characterized in the reduction of NO with methane and propane-butane mixture in an oxidizing atmosphere, and also the acidic properties of the various catalyst samples by thermoprogrammed desorption of ammonia (TPDA) and IR-spectroscopy. 2. EXPERIMENTAL 2.1. Preparation of the samples Pure zirconia was obtained by precipitation and the sol-gel method. Analysis of the literature data showed that precipitation is the most commonly accepted method. Thus, we prepared a sample of starting zirconia by precipitation of the hydroxide from 0.5 M aqueous solution of zirconium oxychloride by adding 2.5 M aqueous ammonia under vigorous stirring at 20 ~ and constant pH 9. After precipitation, Zr(OH)4 was maintained in contact with the mother liquor for five days and the precipitate was then washed with aqueous ammonia with pH 8 until there was a negative test for chloride ion. The sample was then dried at 170 ~ for 3-4 h. The subsequent zirconia sample was obtained by hydrolysis of zirconium isopropoxide in aqueous ethanol in the presence of NH4OH as the catalyst. Water and the organic solvent were removed from the zirconium hydrogel by drying in air at 150 ~ XRD (CuK) data show that upon thermal processing of zirconium hydroxide xerogels, at 460 ~ the tetragonal modification of zirconium dioxide synthesized by sol-gel method, and monoclinic modification of ZrO/synthesized by co-deposition were formed. Exothermic effect of amorphous Zr(OH)4 conversion to T-ZrO/ or M-ZrO2 occurs at 460 ~ [10]. The reaction mixture of initial zirconium silicagel was prepared by mixing aqueous solutions of NaOH and ZrOC12 8H20 in the presence of a complexing agent, [(C4H9)4N]J solid salt with aerosil. Zirconium-containing pentasils were synthesized via the crystallization of zirconium silicagels under hydrothermal conditions in an autoclave. After the crystallization terminated, the obtained deposits were separated, rinsed up to 7-8 pH, and dried in air at 120 ~ followed calcinating at 550 ~ for 6 h. When heating the initial o reaction mixture in the autoclave at 175 ~ 100~A-content crystalline phase zeolite was formed in 48 hours. X-ray photographic study of zirconium-containing zeolite revealed the identity of its structure with that of pentasil (the analog of zeolite ZSM-11). 100%-phase purity zirconium-containing zeolite has the following chemical composition: 0.057 Na20.1.00 SiO2.0.01 ZrO2.0.043 R20.18 H20. The presence of ZrO2 in formed pentasil was confirmed by X-ray fluorescence spectroscopy. NH4+ exchanged forms were prepared by multiple treatment of the original sodium formed by 0.5 N aqueous solution of NH4C1 at about 90 ~ accompanied by washing to remove the chloride ions. Samples were dried at 120 ~ and calcined at 540 ~ for 4 h in air. The sodium amount and degree of exchange in different H +- forms were determined by flame photometric analysis.

427

ZrOz-H+-Zeolite catalytic systems were synthesized in the following way: H+-Zeolite suspension was added to aqueous suspension of zirconium hydrogel washed off from salts, and the resulting mixture was then vigorously stirred for 45 rain to achieve more homogeneous sample. The deposit was then squeezed and dried at 100 ~ and calcinated at 500 ~ for 3 hours. To vary the amorphous and crystalline phases ratio, carriers with diverse compositions were obtained. The active phase of transition metals (Co, Cr) oxides was deposited by the precipitation method, ionic exchange from nitrate salts solutions and ionic exchange in solid phase as well [8,10]. The catalysts were obtained by impregnation of zirconia obtained by both methods with aqueous solutions of the corresponding salts (cobalt or chromium nitrate), subsequent drying at 100 ~ and roasting at 320 ~ for 6 h. The MexOy/ZrOz samples containing 5-10 wt% metal oxides on the support (relative to the metal) were prepared by this method.

2.2. Catalytic tests The catalytic activity of the catalyst samples obtained were characterized in the selective reduction of NO with methane and propane-butane mixture by conversion of NO to Nz (NzO) which was determined in a gradientless reactor with chromatograph analysis of the products. The NO concentration was determined with a gas analyzer with a chemiluminescence detector [8].

2.3. Investigation of acidic properties Studies of the acidic properties of the surfaces of samples by the TPDA method were carried out as follows. Samples (0.2 g) with 1-2 mm grain size were placed in a flow reactor (d=0.6 cm) and were conditioned in a stream of helium (V= 60 ml/min) for 1 h at 550 ~ After decreasing the temperature to 100 ~ the sample was saturated with ammonia. Completion of saturation was monitored by titration of the ammonia at the exit of the reactor. The saturated sample was treated with helium at 100 ~ to remove the physically adsorbed ammonia (30 min). The sample was then subjected to programmed heating in a stream of helium at a rate of 26~ The thermodesorption process was monitored with a catharometer and the amount of ammonia desorbed was determined by titration with HCI. The acidic properties were also studied by adsorption of carefully dried pyridine which was carried out at 150 ~ for 20 min, after which the sample was evacuated for 1 h at the same temperature to remove the physically adsorbed pyridine. Infrared spectra were recorded at room temperature on a Zeiss Specord 751R spectrophotometer. 3. RESULTS AND DISCUSSION The data on the catalytic activity of the samples MexOy/ZrOz in the process of NOx SCR by hydrocarbons are given in Table 1. It is seen that the activity of zirconium dioxidebased oxide catalysts dependent on a method to prepare ZrO2; and 10% CrzO3/T-ZrOz sample prepared by sol-gel method was found to be a more active catalyst in the reaction with propane-butane.

428 The Cr203/ZrO2 catalysts showed activity in the SCR of NO by a propane-butane mixture, which depended on the means of preparation of the zirconium dioxide. Thus, the conversion of NO to N~ was 13-17% at 350 ~ on 5-10 wt.% Cr203/ZrO~ catalysts obtained by precipitation, while the conversion of NO to N2 was 54% at 300 ~ on catalysts with analogous composition obtained through an alcogel step. This more active sample was also tested in the presence of SO2 (0.02%) in the reaction mixture. The conversion of NO in this case was also enhanced and reached 60% at 300-350 ~ This increase in activity by the action of sulfur dioxide may be attributed to the formation of sulfate since sulfated zirconium dioxide is a solid superacid and catalyzes the SCR of NO by hydrocarbons [ 11]. Table 1 Activity of synthesized MexOy/ZrO2 samples in the selective reduction of NO by hydrocarbons (HC)/0.05% NO + 0.09% CnHm+ 5% O2 + Ar; V = 6000 h-I/ No. Catalyst (preparation method) NO Conversion,% / T, ~ (HC) 1 10% Cr203/M-ZrO2 (deposition method) 13/300 (C3Hs-C4Hlo) 2 10% Cr203/T-ZrO2 (sol-gel method) 54/300 (C3Hs-C4HIo) 3 10% CoO/ZrOz (deposition method) 75/310 (CH4) 4 10% CoO/ZrO2 (sol-gel method) 72/300 (CH4) With 10% CoO/ZrO2 catalyst, conversion of NO (in reaction with methane) reached 75% at 310 ~ while the selectivity with respect to nitrogen decreased from 100% at 415 ~ to 63% at 310 ~ (the remainder was N20). There was no dependence of the catalytic activity of samples of CoO/ZrO2 on the method used to prepare zirconium dioxide. Both samples (No. 3 and No. 4, Table 1), in which zirconium dioxide was made by precipitation and the sol-gel method, respectively, had similar activity. This difference of behavior between samples of the CoO/ZrO2 and Cr203/ZrO2 catalysts may be explained by differences in the interactions of CoO and Cr203 with zirconium dioxide and, consequently, different influence of these catalysts on activation of methane and propanebutane. In order to elucidate the reasons for the dependence of the catalytic properties of these samples on their preparation method, we studied the acid surface properties of cobalt- and chromium-modified ZrO2 catalysts by ammonia thermoprogrammed desorption and IRspectroscopy. Our results again indicated that the activity of these catalysts in the SCR of NOx by hydrocarbons is a function both of the surface acidity and content of the active metal. The acid site concentration of the starting ZrO2 samples prepared by various methods is significant (0.13 and 0.23 mmol/g) but these samples are inactive, while 10% Cr203/ZrO/prepared by the sol-gel method displays considerable activity in the reaction studied with lower surface acidity. The acid site concentration of the sample with the same composition prepared by the precipitation method is reduced by a factor of 2.5 and, thus, this catalyst has much lower activity in the selective catalytic reduction. IR-spectra for zirconium dioxide samples obtained by the sol-gel method with and without 10% Cr203 shows that modification of zirconium dioxide by Cr203 leads to local IR vibrations, which cannot be attributed to characteristic modes of the ZrO2 and Cr203 frameworks. The finding of bands at 800, 1025, and 1170 cm1 in the spectrmn of sample

429 10%Cr203/ZrO2 may indicate the formation of new metal-oxygen bonds of the Zr-O-Cr type in the zirconium dioxide surface layer. Fig? 1 gives IR spectra for pyridine adsorbed on previously dehydrated samples. The spectrum of starting zirconium dioxide obtained through an alcogel step lacks the band characteristic for Br6nsted acid sites. The addition of Cr203 into zirconium dioxide leads to acidic B-sites characteristic for pyridinium ions with a band at 1540 cm-1. This may be related to formation of structure such as [3]:

H

/

H

L

I

O

O

mZrm

\ mCrm

] /\ It is known that for zeolite catalysts, BrOnsted acid sites are necessary for the selective reduction of NO by methane on Ga-H-ZSM-5, while the activity of Cu-, Ce-, and Cocontaining pentasils in the SCR of nitrogen oxide by hydrocarbons correlates with the strength of the Br6nsted acid sites of these catalysts. This correlation suggests that activation of the hydrocarbons reducing agent may occur specifically on the Bronsted acid sites [9]. Our results confirm the important role of BrOnsted acid sites in the reaction studied on ZrO2 systems. Thus, our results showed that zirconium dioxide modified by transition metal oxides (Co and Cr) displays significant activity in the selective reduction of NO by methane and propane-butane, which depends on the method of preparation of the ZrO2 sample.

I

r 9

|

J

9

9

~

|

I

Fig.1. IR spectra of zirconium samples obtained by the sol-gel method before and after pyridine adsorption: 1) ZrO2 aider vacuum heating at 550 ~ in vacuum, 2) ZrO2, and 3) 10% CrzO3/ZrO2.

,,

1600 1550 1500 1450 cm 1

The data on the catalytic activity of the synthesized samples of CoO/H+-pentasils in the NO+O2 NO2 reaction and acidic properties of these samples characterized by total surface acidity determined by TPDA method are given in Table 2. It is seen that 10% CoO

430 deposited by the precipitation on these carriers showed considerable oxidative activity in relation to NO, while only samples Nos. 1 and 3 showed the SCR-activity (conversion NO to N2 in reaction NO-CH4/O2 is 30-50%). It is also seen that these samples have crystalline structure and definite value of surface acidity which is necessary for activation of hydrocarbon-reductant. Table 2 Catalytic and acidic properties of CoO/I-F-pentasils /0. 05% NO+5% o2+mI'; V -- 6000 hl/ No. Catalyst SIO2/A1203 ~(~NH3, NO Conversion,%/ (ZrO2, A12Oaq-ZrO2) mmol/g T, ~ 1 i0%CoO/H +- pentasil 100 0.23 70/310

(SIO2,A1203) 2 3

10%CoO/H+-pentasil (SiO2,ZrO2) 10%CoO/i-F-pentasil

100

0.06

59/300

100

0.16

60/300

100

0.29

50/300

(SIO2,A1203, ZrO2) 4

10%CoO/H +-

(8iO2,A1203, ZrO2), amorphous 5 , !0%CoO/Si02(silicalite)

60/300

90-

o-%0. .070. r '" *-.

2

~o. 3020

0

J

Zr02

I

20

u

I

40

~

I

60

)

Content, %

I

80

~

I

1O0

H-TsVN --b-

Fig. 2. Dependence of the conversion of NO on the chemical composition of the binary carrier ZrO2 - H-TsVN (310 ~ 1) experimental curve; 2) calculated curve for direct additivity. The data on the catalytic activity of the samples on binary carriers 10% CoO/(H § pentasil - ZrO2) in the process ofNOx SCR by methane are given in Table 3.

431 During investigation of cobalt-zeolite catalysts the dependence of activity on the manner by which the active phase was introduced was established. Sample of 10% CoO/HTsVN (SiO2/A1203=37) (in which cobalt was introduced by soaking) had low activity in the selective catalytic reduction of NO with CH4. Conversion of 25% of NO was achieved at 320 ~ which is considerably lower than for cobalt containing cation-decationated form of zeolite with the pentasil structure, obtained by ion exchange in the solid phase (e.g., on Co-H-TsVN an 80% conversion of NO was obtained at 310 ~ [8]. Table 3 Activity of synthesized samples 10% CoO/(H-pentasils - ZrO2) in selective reduction of NO by methane/0.05% NO + 0.09% CH4 + 5% O2 + Ar; V - 6000 hl/ No. Catalyst NO Conversion, % / T, ~ 1 10% CoO/H-TsVN 25/320 2 10% COO/(35% H-TsVN-65% ZrO2) 67/300 3 10% COO/(50% H-TsVN-50% ZrO2) 72/300 4 10% COO/(65% H-TsVN-35% ZrO2 ) 81/310 5 10% COO/(35% H-TsVN-65% ZrO2 )* 69/300 */CoO was introduced by ion exchange in solid phase. Fig. 2 shows the dependence of the activity of cobalt containing catalysts CoO/ (zeolite-ZrO2) on the composition of the carrier. The observed activity of these systems is greater than that calculated on the assumption of addition of the catalytic properties of the components of the catalysts. Such differences indicate interaction between the components of the ZrO2-zeolite carrier, possibility forming new active centers. It is seen from Table 3 and Fig. 2 that increase in the zeolite content in the zeolite-ZrO2 system from 35 to 65% leads to increase in the catalytic activity which achieves 81% conversion at 310 ~ i.e., an activity was achieved equivalent to that of the cobalt containing cationdecationized zeolitic catalyst Co-H-TsVN [8]. To elucidate the reasons for differences in activity of cobalt-containing catalysts with the same amount of active phas, we studied the acidic properties of the surfaces of these samples by the previously described temperature programmed desorption of ammonia. Analysis of the results (Fig. 3, Table 4) indicated a complex correlation of the catalytic activity in the selective catalytic reduction of NO with CI-I4 with the chemical composition, the concentration and strength of the acid centers at the surface of the cobalt-zirconium, cobalt-zeolite, and binary systems based on them. The most important factor for the selective catalytic reduction activity is the localization of the metal (cobalt) active centers, which is determined by the method used to introduce cobalt into the catalyst. It may be claimed that, of two catalysts prepared in the same way, but with differing carrier composition (ratio of zeolite to ZrO2) the more active sample has the greater concentration of acid centers (samples No.2 and No.3 in Table 4). The low activity of the sample 10% CoO/H-TsVN (No.l) with considerable total acidity of the surface (0.66 mmol/g) and the presence of strong acid centers may be explained by different localization of the cobalt in comparison with the ion exchange sample Co-H-TsVN, obtained by ion exchange in the solid state [8].

432

,,m

r.f) r r

100

I

200

~

I

300

~

T,~

I

400

*

I

500

~

I

600

Fig. 3. Spectra of temperature programmed desorption of ammonia from the surface of cobalt catalysts on ZrO2 -zeolite binary carriers" 1) 10% CoO/H-TsVN, 2) 10% COO/(65% H-TsVN, 35% ZrO2), 3) 10%COO/(35% H-TsVN, 65% ZrO2). Table 4 Concentration of acid centers on CoO/(ZrO2 - Zt) catalysts determined by desorption of NH3 and their activity in SCR NO with CH4 No Catalyst Concentration of acid centers, )(No % / T, ~ mmol/g 150-260~ 400-500~ ~rfNH3 1 10% CoO/H-TsVN 0.38 0.28 0.66 25/320 2 10% COO/(65% H-TsVN0.19 0.15 0.34 81/310 35% ZrO2) 3 10% COO/(35% H-TsVN0.13 0.08 0.21 67/300 65% ZrO2) It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Br6nsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction NO+O2~NO2, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% COO/(65% H-Zeolite - 35% ZrO2)

433 catalyst exhibits maximum activity, and the catalytic properties of such samples are not an additive function of the carrier composition. REFERENCES

1. M.P. Fokema and J.Y. Ying, Catal. Rev., 41 (2001) 1. 2. Y. Traa, B. Burger and J. Weitkamp, Microporous Mesoporous Mater., 30 (1999) 3. 3. K. Tanabe, Catalysts and Catalytic Properties [Russian translation], Mir, Moscow, 1993. 4. R. Burch and T. C. Watling, Appl. Catal. B, 14 (1997) 207. 5. K. Eguchi and T. Hayashi, Catal. Today, 45 (1998) 109. 6. V. Pitchon and A. Fritz, J. Catal., 186 (1999) 64. 7. Yu. Li and J. N. Armor, J. Catal., 145 (1994) 1. 8. S.N. Orlik, V. L. Struzhko and V. P. Stasevich, Teor. Eksp. Khim., 32 (1996) 47. 9. S.N.Orlik and V. L. Struzhko, Teor. Eksp. Khim., 35 (1999) 373. 10. T.V. Mironyuk, V.L. Struzhko and S.N. Orlik, Ibid., 36 (2000) 307. 11. H. Hamada, Y. Kintaichi and M. Tabata, Chem. Lett., 1 (1991) 2179.

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Studiesin Surface Science andCatalysis143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

435

Preparation of the chitosan based catalysts for several hydrogenation reaction in the liquid phase V. Isaeva, A. Ivanov, L. Kozlova, V Sharf N.D. Zelinsky Institute for Organic Chemistry, Russian Academy of Sciences Leninsky pr. 47, Moscow 119991, Russia Novel chitosan based catalytic systems for hydrogenation of unsaturated compounds in the liquid phase were prepared. The catalytic performance of the obtained systems depended significantly on the chitosan forms (as the micro beads or chitosan deposited on the mineral supports), their preparation method and chemical modification of chitosan as well. The obtained chitosan based carriers and catalysts were examined by transmission and diffuse-reflectance FTIR spectroscopy. 1. I N T R O D U C T I O N It is only recently that chitosan (natural biopolymer, deacetylated chitin derivative) is being used as a carrier for metal catalyst preparation. Several papers and patents regarding chitosan application in reactions of selective and enantioselective reduction of organic compounds have been published in the last years [1-2]. Natural biopolymer chitosan (deacetylated chitin derivative) is attractive as carrier for heterogeneous catalysts due to its environmentally friendly nature, in particular, biodegradability. In addition, free amino groups in chitosan fragments facilitate metal deposition on chitosan from metal salts and metal complexes. Previously, we demonstrated the possibilities of chitosan as a macroligand for Rh and Ru complexes immobilization. The obtained metal complex systems showed activity and selectivity in transfer hydrogenation of several carbonyl compounds. Free amino groups in chitosan allow its immediate use as a macroligand without or with preliminary functionalization. The most attractive functionalization from our point of view is to modify the chitosan via reaction of chitosan aminogroups with carbonyl compounds leading to Shift's base formation. On the one hand, it allows to accomplish chitosan crosslinking with dicarbonyl compounds, commonly, with diglutar aldehyde [3,4]. On the other hand, it allows to introduce several functional groups in chitosan according to reaction with substituted carbonyl compounds. We chose 2pyridinealdehyde for such modification. The main obstacle for chitosan application in catalysis is to obtain stable regular micro beads separable after reaction for reusing. That is why this problem is of great interest [3]. The goal of this work was the investigation of alternative methods of Pd/chitosan based catalyst preparation for several reactions of hydrogenation in the liquid phase. The work focused on the following directions:

436 1. Preparation of the stable regular micro beads of desired size. 2. Development of cross-linking of chitosan and functionalization with 2pyridinealdehyde. 3. Deposition of cross-linked chitosan on the mineral support surface. The developed procedures were used for the synthesis of Pd catalysts for cyclopentadiene and 1,4-butynediol hydrogenation in the liquid phase. 2. E X P E R I M E N T A L

2.1.Catalyst preparation Several methods for the preparation of chitosan micro beads of regular size were tested. The main procedure was precipitating of hydrochloride chitosan solution by adding it dropwise in the bath containing the precipitating agent (alkali solution). Adding dropwise was performed with the equipment for micro drop forming through quartz die (d=0.1 - 0.2 mm) at 5 atm. Chitosan micro beads of diameter 0.5-1 mm were formed. The chitosan micro beads obtained by alcali precipitation method were cross-linked by reaction of chitosan amino group with diglutar aldehyde. Cross-linking extent was 7%. In addition to the chitosan native form, chitosan succinate form (70% of succine groups) was used. In several experiments, chitosan succinate form was used without crosslinking, as fibers or gels in aqueous and alcohol- aqueous systems. It should be noted, that cross-linking of chitosan succinate was performed via residual amino group. Chemical modification of the obtained chitosan micro beads was carried out by treatment with 2-pyridinealdehyde boiling solution in benzene for 24 h. The resulting chitosan forms were rinsed with benzene, THF and MeOH. Deposition of chitosan on SiOa (0.06-0.02 mm) and ZrOa was performed by multiple steeping of a portion of carrier in a solution of chitosan in 1% acetic acid and filtered. Wet or semi-dry chitosan-coated carrier was added directly to methanol or propanol-2 and stirred for 0.5-2h after addition of the calculated amount of glutaraldehyde. Calculated cross-linking extent was ~ 10-15%. Chemical modification of chitosan was performed by carrier treatment with pyridinealdehyde-2 boiling solution in benzene. Two procedures for metal introduction in chitosan base were used: impregnation and coprecipitation. According to the first procedure the metal deposition on chitosan micro beads was carried out from aqueous and alcohol solutions of NazPdCl4, H2PdC14, RhC13, Rh2(CH3COO)4, ZnSO4 and Pb(CH3COO)2. Pd and Pb/Zn in bimetallic catalysts was deposited by subsequent precipitation. Pd-Pb (Zn) atomic ratios were 1/1. Metal contents in the resulting samples were 0.5 - 4 % . According the second procedure, the metal complex with chitosan acidic (hydrochloride) form was synthesized and after that precipitated in the bath containing the precipitating agent. Metal contents in the resulting samples were 0.5-2%. The catalytic behavior of the catalysts was examined in reactions of hydrogenation of cyclopentadiene and 1,4-butynediol. Hydrogenation reactions were carried out at atmospheric pressure and 20~ (cyclopentadiene) and 45~ (1,4-butynediol). Hydrogenation rate was determined as the ratio of consumed H2 volume per unit time, ml/min.

437

2.2. Catalyst study by IR-spectroscopy The obtained chitosan carriers and catalytic systems on their base were studied by transmission and diffuse-reflectance FTIR spectroscopy. IR-spectra were obtained in "Nicolet Impact 410" equipment. To record the diffuse-reflectance spectra the samples were evacuated at 100~ for 2 h. The quantitatively spectrum analyses were performed using Kubelka-Munk equation according to the program OMNIC [5]. 3. RESULTS AND DISCUSSION 3.1. Catalyst characterization AB 3413 cm -1, characteristic for valence vibrations of NH pyridine group (Figs. 1, 2) and AB at 1591, 1567, 1475 and 1439 cm -~ corresponding to valence vibrations o f C=C a n d - C = N - bonds of pyridine ring are presented in the spectra of chitosan modified with pyridine fragments.

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IR-transmission examination of chitosan/SiOz deposited systems gave little information, due to intensive absorption of chitosan and silica gel in the same region. In contrary, characteristic chitosan AB in the regions 1 6 6 0 - 1300 cm -1 and 1100 cm -1 are presented in the spectrum of chitosan/ZrOz system (Fig. 3). Diffuse-reflectance IR-spectra of CO adsorbed on Pd/chitosan/ZrO2 catalyst are presented in Fig 4. Two characteristic adsorption bands (AB) at 2070 cm -1 and 1900 cm -1, corresponded to the vibrations of CO, adsorbed on Pd ~ in the linear and in the bridge (three-fold coordinated) form, respectively, arc presented in the IR spcctrum of the system. The presence of bridged or three-fold coordinated CO points to the formation of Pd ~ metal clusters. Noteworthy is that frequencies of linear and bridged forms of CO are shifted toward lower wave numbers compared to conventional Pd/support systems (e. g. 2070 v s 2100 cm -1 for linear form) [6, 7]. This is indicative of negative charging of Pd ~ clusters. Negative charging of Pd ~ clusters is also confirmed by lower stability of the linear form of CO compared to the bridged form. After evacuation at 200~ the linear form disappears completely, while intensity of bridged bonded one remains almost the same.

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Fig.4 Diffuse-reflectance IR-spectrum of CO adsorbed on Pd/chitosan/ZrO2 Presumably, small Pd ~ clusters are stabilized by NH2 -groups, which donate edensity to Pd ~ clusters. 3.2. Examination of catalytic performance of synthesized Pd-based systems. The hydrogenation activities of catalytic systems prepared by the coprecipitation method were low. It could be explained taking into account that the resulting samples with

439 low metal contents were formed because Pd was partially leached from chitosan during the precipitation in alcali solution. In general, activities of chitosan based catalysts prepared by impregnation method in hydrogenation of unsaturated organic compounds were comparable with those of traditional heterogeneous catalyst (as calculated per 1 mole of metal). It should be noted that the chitosan pretreatment influenced very much the catalyst activity. For instance, immediate Pd deposition from alcohol solution on dry chitosan fibers or micro beads led to almost completely inactive catalytic systems, regardless of the metal content. On the other hand, metal deposition on chitosan micro beads or fibers preliminary swollen in water dramatically improved the catalytic activity. 3.2.a. Hydrogenation of cyclopentadiene The obtained catalysts showed similar activity in cyclopentadiene hydrogenation. The selectivity data in cyclopentadiene hydrogenation are given in Table 1. The selectivity of consecutive reaction was determined as a rate ratio of cyclopentene/cyclopentane formation. On the contrary, the chitosan modification influenced essentially the selectivity of the catalyst on it basis. Table 1 Selectivity of Pd/chitosan based systetms in hydrogenation of cyclopentadiene Catalyst 0.1-0.5 g, Substrate 0.5 - 2 ml, 20~ 20 ml EtOH N ~ Catalyst composition Selectivity to Reaction rate ratio, cyclopentene, % Wolefine/Wdiene 1 Pd/chitosan 99.9 0.90 2 Pd/chitosan succinate 91.2 1.00 3 Pd/chitosan/Si02 94.9 0.02 4 Pd/chitosan-Pyr/SiO2 96.0 0.01 As can be seen from Table 1, the selectivity to cyclopentene obtained using the chitosan form containing carboxylic groups (succinate chitosan form, catalyst N ~ 2) was rather low. The highest selectivity to cyclopentene was achieved using catalyst N~ (Pd/chitosan). However, the cyclopentane formation rate (stage 2) decreased very slightly over this catalyst. Introducing pyridine fragments in chitosan led to a decrease of cyclopentene into cyclopentane hydrogenation rate up to ~ 3 orders. Deposition of chitosan on silica gel as well as modifying it with pyridine groups also resulted in a decrease of cyclopentene hydrogenation rate (catalysts N ~ 3,4). Taking into account the selectivity data and the hydrogenation ratio Wolefine/Wdiene, the best catalyst for this reaction was catalyst N ~ 4 (Pd/chitosan deposited on silica gel and modified with pyridine groups).

3.2.b. 1,4-butynediol hydrogenation The main product of 1,4-butynediol hydrogenation for this reaction is cisbutenediol. Simultaneously, the parallel reaction of 1,4-butenediol hydrogenation as well as cis-trans transformation takes place at the second stage. For this process, cis-l,4butenediol formation is most interesting from a practical point of view. Processes

440

selectivity was determined as ratio of product contents in the reaction mixture (cis/an+cis+trans, %) at 50% conversion (1 mole). As shown in Table 2, catalyst N~ exhibited a rather low selectivity. Introducing the second metal in chitosan (Pb or Zn) as well as pyridine fragments (catalysts N~ improved the selectivity regarding cis-l,4-butenediol. However, the second stage rate remained almost the same. Changing the support influenced remarkably the reaction performance, e.g. substitution SiOz (catalyst N~ for ZrOz (catalyst N~ enhanced the selectivity. In this case, Pb introduction (catalyst N~ almost completely suppressed the cis-trans isomerisation. Using succinate chitosan form very much improved the reaction selectivity: the maximum of selectivity to cis-l,4-butenediol was achieved over catalyst N~ But in this case, the reaction was not terminated at the stage of 1,4-butenediol formation. The rate of further hydrogenation of 1,4-butenediol (second stage) was rather high. Introducing Pb in chitosan completely suppressed the further 1,4-butenediol hydrogenation. The reaction was spontaneously finished over catalyst N~ after consumption of 1 mole of Ha. Simultaneously, the selectivity with respect to cis-l,4-butenediol was improved by that chitosan modification. Table 2 Selectivity of Pd/chitosan based systems in 1,4-butynediol hydrogenation. Catalyst 0.1-0.5 g, Substrate 0.25 - 2 ml, 20~ EtOH 20 ml Selectivity on 1,4Selectivity on Reaction N ~ Catalyst composition butenediol (cis/an + 1,4-butenediol rate ratio cis + trans) (cis) (Wz/W1)* 1 Pd/chitosan 0.88 0.84 0.6 2 Pd/chitosan succinate 0.97 0.91 1.3 3 Pd-Pb/chitosan 0.97 0.93 ~ succinate 5 Pd-Pb/chitosan 0.90 0.93 1 6 Pd-Zn/chitosan-Pyr 0.94 0.89 1 7 Pd-Pb/chitosan/SiO2 0.88 0.84 1,3 8 Pd/chitosan/ZrO2 0.88 0.87 1 9 Pd-Pb/chitosan/ZrO2 0.92 0.96 0.8 * W l - 1,4-butynediol hydrogenation rate (first stage), W a - 1,4-butenediol hydrogenation (second stage) 4.CONCLUSIONS Thus, our results demonstrated that the trends of selectivity for hydrogenation of 1,4-butenediol and cyclopentadiene are in tight connection with preliminary chitosan chemical modification as well as mineral support nature. Pd/chitosan modified with 2pyridinealdehyde deposited on SiOz demonstrated high selectivity in hydrogenation of cyclopentadiene into cyclopentene. 1,4-butynediol into cis-l,4-butenediol hydrogenation proceeded very selectively over Pd-Pb catalytic systems based on chitosan succinate form.

441 The catalytic systems based on chitosan/ZrOa exhibited improved selectivity in 1,4butynediol hydrogenation compared with those based on chitosan/SiO2.

REFERENCES

1. M.-Y. Yin et al., J. Mol. Catal. A: Chem., 147 (1-2) (1999) 93. 2. V Isaeva., V. Shaft, N. Nifant'ev, V. Chernetskii and Zh. Dykh., Stud. Surf. Sci Catal., 118 (Preparation of Catalysts VII), (1998) 237. 3. T. Ando and S. Kataoka., JP Patent No 61278354 A2 (1986). 4. W. Wang, F. Wood and G.A.F. Roberts, Advanc. Chit. Sci., II (1997) 920. 5. L.C.A van den Oetelaar et al, J. Phys. Chem. B, 102 (1998) 3445. 6. T. Rades, C. Pac, R. Ryoo, M. Polisset-Thfoin and J. Fraissard, Catal. Lett., 29 (1994), 91. 7. A.V. Ivanov, A. Yu. Stakheev, L.M. Kustov and Izv. An, Set. Khim. (rus.), No 7 (1999), 1265.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

443

Preparation of Mo/AlzO3 sulfide catalysts modified by lr nanoparticles J. Cinibulk and Z. Vit Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojovfi 135, 165 02 Prague 6 - Suchdol, Czech Republic. Preparation and catalytic properties of Ir/alumina and Ir-Mo/alumina sulfide catalysts were studied in hydrodenitrogenation (HDN) of pyridine and hydrodesulfurization (HDS) of thiophene. The Ir added to a Mo/alumina catalyst in amount 0.1-0.8 wt % increased activities in both reactions by a factor of about 2-3. The main factors, leading to improvement of activity of modified Ir-Mo catalysts, were Ir dispersion, and amount and state of the Mo phase before Ir deposition, i.e. whether oxidic or sulfided and the way of catalyst activation. The effects of starting Ir compounds and deposition order of Ir and Mo seem to be of a smaller importance. 1. I N T R O D U C T I O N Sulfides of some noble metals (Ru, Rh, Pd and Pt) are highly active in reactions such as HDS, HDN or hydrogenation (HY). Modification of conventional Mo catalysts by noble metals represents a possible way of improvement of their efficiency [1-3]. The majority of such modifications has been done up to now with Ru [1,2,4], whereas other noble metals have been studied less frequently. In contrast to a relatively large knowledge collected up to now on the addition of conventional promoters Co (Ni) to a Mo system, information about addition of noble metals or sulfides is rather limited. It was often reported that modification of a Mo catalyst by noble metals led to improvement of activity [1,2,4]. However, it some cases such modification did not bring any improvement or even led to decrease of activity. In contrast to conventional Co (Ni) promoters, it is desirable to keep the amount of noble metal in the mixed catalyst as low as possible. This is not only because of a high price, but also due to decrease of dispersion at higher loadings. Another problem is the way of deposition of noble metal and final catalyst pretreatment, affecting also dispersion and contact with the Mo phase. Preparation of conventional Mo/alumina system usually includes a calcination step in air after deposition of Mo and Co (Ni) salts, which are mostly ammonium heptamolybdate (AHM) and corresponding nitrates, before final sulfiding. However, for some noble metals such a step is not suitable because of possible decrease of the dispersion via oxidation or undesirable interaction with the support [1,5-7]. It was reported that the activity of mixed Ru-Mo catalysts was higher when these catalysts were directly sulfided and not calcined after Ru deposition [1,7]. It seems therefore that the preparation of systems based on a combination of noble metal and Mo needs a specific procedure taking into account the properties of noble metals.

444

One of the most active noble metal sulfides in hydrotreating reactions is Ir sulfide. Its exceptional activity was reported in HDN of different compounds [8] and in the parallel HDN of pyridine and HDS of thiophene [9,10]. Recently, we found a positive effect of the addition of a small amount of Ir to an Mo/alumina catalyst during HDS and HDN [11]. The aim of this contribution is to focus on some factors during preparation of mixed Ir-Mo sulfide catalysts and to evaluate their effect on catalyst activity. 2. E X P E R I M E N T A L The support was ~,-AlaO3 with BET surface area of 255 m2/g and pore volume of 0.76 ml/g (S/id-Chemie A.G., Germany), ground to particles 0.16-0.32 mm. The MoO3/alumina catalyst was prepared by the pore filling method using an aqueous solution of AHM. The product was calcined at 500~ for 2 h in air. The BET surface area was 213 m2/g. The Ir/alumina catalysts were prepared by impregnation of the support with cyclohexane or water solutions of Ir compounds (Ir4(CO)lz, acetylacetonate Ir(AcAc)3, HaIrCI6 and (NH4)zIrC16) by procedures described earlier [10]. Because of a low solubility of Ir carbonyl, which is 0.38 g per 1 of cyclohexane, a modified procedure facilitating the impregnation was developed. Ir carbonyl was placed in a cartridge of Soxhlet extractor and dissolved by hot circulating cyclohexane. The impregnation proceeded in the bottom flask in suspension of carrier and boiling solution of Ir4(CO)lz. A similar procedure was adopted for dissolution of Ir(AcAc)3, while a simple impregnation of the alumina by water solutions of HzIrCI6 and (NH4)aIrCI6 was used. The solvents were removed in a vacuum rotary evaporator. The surface area of all Ir catalysts was close to 230 mZ/g. Different mixed Ir-Mo/alumina catalysts were prepared. The details of procedures and catalyst characterization were published elsewhere [10,11]. In the first series, the Mo was deposited first. The samples were prepared from MoOJalumina or MoSz/alumina catalyst by adsorption of Ir4(CO)la from a cyclohexane solution. This procedure was the same as for the Ir catalysts. In another series, an inverse order of impregnation was used. The Ir was deposited first and then Mo was deposited from an aqueous solution of AHM. One catalyst was prepared by coimpregnation of alumina by an aqueous solution of AHM and (NH4)2IrCI6. The samples containing Ir were sulfided or reduced without calcination in air in order to avoid Ir sintering. The surface areas of mixed catalysts varied between 200208 m2/g. Reduction was performed by H2 using a temperature gradient 6~ up to 400~ and by keeping this temperature for 2 h. The dispersion of Ir in the reduced catalysts was determined by pulse H2 adsorption at 22~ and expressed by H/Ir ratios. The size of the Ir particles in reduced Ir catalysts was calculated under assumption of H/Ir=l stoichiometry [12] according to Anderson and Pratt [13] or estimated on some Ir-Mo samples by transmission electron microscopy (TEM) on a JEM-2000EX Jeol instrument. Sulfidation of catalysts was performed by H2S/Hz mixture (10 % H2S) using a temperature gradient 6~ up to 400~ and by keeping this temperature for 2 h. The TPR of sulfided catalysts was performed in a conventional apparatus by monitoring of H2 consumption. The catalysts were heated at a rate of 5~ in mixture 5 % of Ha in Ar (35 ml/min). Relative reducibilities of sulfided catalysts were defined as areas under the TPR curves in the range 100-600~ and related to the weight of catalyst. The content of metals and sulfur was

445 determined by the inductively coupled plasma (ICP). The contents of chlorine and carbon were determined by argentometric titration and by combustion, respectively. The BET surface area was measured by N2 adsorption on a Digisorb 2600 instrument. The activity of catalysts was tested in the parallel HDN/HDS of pyridine (PY) and thiophene (TH) in an flow reactor at 320~ and 20 bar. In case of reduced Ir samples, single HDN of pyridine was performed. The feed contained 220 ppm of PY and 240 ppm of TH (or only PY) in H2 at a flow rate of 0.4 mol/h. The HDS of thiophene was described by pseudo-first-order rate constant kTH. The HDN was described for simplicity by two rate constants for pyridine HY, kpy, and piperidine HDN, kc5. Further details concerning the evaluation of the catalyst activity can be found elsewhere [10,11]. 3. RESULTS AND DISCUSSION

3.1. Monometallic Ir/alumina catalysts In attempts to obtain information about the influence of the starting Ir compounds and Ir dispersion on the catalyst activity, the monometallic Ir/alumina catalysts were studied at first. The metal loading, content in carbon and chlorine and H/Ir values, evaluated from H2 adsorption, are summarized in Table 1. The values H/Ir of all catalysts exceeded slightly 1, which confirmed the presence of a well dispersed Ir phase. Differences between H/Ir values were rather small, despite different Ir precursors and preparation procedures. Table 1 Preparation, composition and dispersion of Ir/alumina catalysts Cat. Precursor Solvent Composition (wt%) Ir C C1 1 Ir4(CO) 12/alumina C6H12 0.85 0.23 2 Ir(AcAc)3/alumina C6H12 0.90 0.34 3 HzlrC16/alumina H20 1.10 0.12 0.86 4 (NH4)2IrC16/alumina H20 0.91 0.09 1.00 a Determined for reduced catalysts. Data taken from Ref. [10].

H/Ir a 1.06 1.19 1.26 1.18

The mean diameter of the Ir particles, calculated from H2 uptake, was near 0.91 nm. However, despite similar Ir amount and dispersion, the sample prepared from Ir carbonyl was more active than other samples (Fig. 1). We explain this difference by the lower amount of impurities (C and Cl) originating from the decomposition of the starting Ir compounds, rather than by the influence of the Ir dispersion. Differences in the H/ir values of the catalysts were rather small and did not allow to evaluate the effect of dispersion on activity. Thus, two additional samples with lower Ir dispersion were prepared by sintering of sample 3. This was achieved by calcination in air at 450~ As was shown by Foger and Jaeger [6], such treatment leads to the formation of IrO2 crystallites and to their segregation. After reduction, the H2 uptake on these catalysts was indeed much lower than on the original sample (Table 2). The mean diameter of the Ir particles, calculated from H2 uptake, increased roughly two times. Results of catalytic tests

446

of sintered catalysts and of the original sample 3 in HDN are shown in Table 2. Sintering of the catalysts suppressed significantly the rate constants of piperidine HDN, while it influenced only negligibly the rate constants of pyridine HY [10].

Fig. 1. Effect of precursor on activity of Ir/alumina sulfide catalysts in parallel HDN of pyridine and HDS of thiophene. Table 2 H2 adsorption, Ir dispersion and activity of reduced Ir/alumina catalysts in HDN Catalyst

Ha adsorption a (ml/gcat)

H/Ir a

dmean (nm)

Reduced It/alumina (No.3) 0.81 1.26 Sintered 450~ h 0.58 0.90 Sintered 450~ h 0.41 0.64 a Determined for reduced catalysts. Data taken from Ref.

0.9 1.2 1.7 [10].

kpy kc5 (mol/h.kgcat) 1.7 1.7 1.4

9.0 5.7 5.2

On the basis of these experiments, the carbonyl was chosen as the most suitable precursor for the preparation of the mixed Ir-Mo catalysts. At the same time, these experiments showed that it is desirable to achieve a good Ir dispersion, because it affects the rate of piperidine HDN and controls in this way the overall rate of transformation of pyridine.

3.2. Mixed Ir-Mo/alumina catalysts The prepared Ir-Mo catalysts contain almost the same amount of Mo and differ in the Ir loading and in the way of deposition of both components. The majority of these

447 catalysts was prepared by using the Ir carbonyl and the Ir amount was kept below 0.8 %. Precursors of the catalysts were prepared by three different routes, which are shown by the following scheme: Ir4(CO)12 MoO3/A1203

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The precursors were transformed into active sulfide (or metallic Ir) state by activation, which was either sulfidation or reduction. The sulfided catalysts were examined by the TPR and by evaluation of activity in the parallel HDN/HDS reaction. Some reduced samples were studied by TEM in order to compare the size of the metallic Ir particles. The BET surface areas of all mixed catalysts was similar and close to 200-208 ma/g. The amount of sulfur found by ICP in the sulfided Mo/alumina catalyst corresponded to a S/Mo molar ratio of 1.9, similar to the values reported for the MoS2 phase. The ratios S/(Ir+Mo) of the mixed catalysts were between 1.6-2.3, which suggests the presence of elemental sulfur [11]. The list of precursors of the Ir-Mo catalysts is given in Table 3. Table 3 Preparation and composition of the mixed Ir-Mo/alumina catalysts Cat. 5 6 7 8 9 10 11 12 13 14 15

Precursor Ir4 (CO) 12-MoO3/alumina Ir4(CO) 12-MoO3/alumina Ir4 (CO) 12-MoSz/alumina Ir4 (CO) 12-MoO3/alumina (NH4)2IrCl6-AHM/alumina AHM-Ir4 (CO) 12/alumina Ir4(CO)12-MoO3/alumina AHM-Ir/alumina AHM-Ir4 (CO) 1z/alumina Ir4(CO) 12-MoO3/alumina Ir4(CO) 12-MoSz/alumina

Loading Ir 0.11 0.24 0.34 0.53 0.69 0.73 0.79 0.73 0.73 0.16 0.13

(wt%) Mo 8.4 8.4 8.4 8.4 8.7 9.1 8.4 9.1 9.1 8.8 8.8

Preparation Mo first Mo first Mo first Mo first Coimpregnation Ir first Mo first Ir first and reduced Ir first, simultaneous reduction Mo first, new series Mo first, new series

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Ir loading, % Fig. 2. Activity of Ir-Mo sulfide catalysts in the parallel HDN/HDS reaction as a function of Ir loading, a- Thiophene HDS, b- Pyridine HY, c- HDN of piperidine. Mo (<>), Samples 5,6,8,11 (l-l), Sample 7 (11), Sample 9 (O), Sample 10 (~x).

Fig. 2 a-c shows the activities of the Ir-Mo/alumina sulfide catalysts in HDS of thiophene, HY of pyridine and HDN of piperidine during the parallel HDN/HDS, plotted against Ir amount in the catalysts. It is seen that addition of Ir to the Mo catalyst led to a substantial increase of activity. This increase was about 2 in HDS and about 3 in both steps of pyridine HDN. The data show that an optimum Ir amount in modified catalysts was found between 0.3-0.5 %. Above this Ir content, the activities in HDS and pyridine HY remained almost unaffected while activity in piperidine HDN clearly diminished. This decrease was explained by a diminution of the Ir dispersion, as evaluated from TEM measurements. The mean diameter of the majority of the Ir particles in reduced Ir-Mo sample with 0.53 % Ir was below 0.8 nm and some particles approached 1 nm. On the other hand, when the Ir amount increased to 0.79 %, the mean size of the majority of the particles approached 0.8-1.5 nm and the mean size of some smaller fractions (-10 %) increased up to 1.5-2.5 nm [11]. The effect of the way of deposition of Mo and Ir can be seen from the comparison of activities of samples 9-11. These samples had similar Ir loading but were prepared by different procedures, including deposition of Ir4(CO)12 from cyclohexane on an Mo/alumina catalyst, impregnation of an Ir4(CO)la/alumina precursor by an aqueous solution of AHM or coimpregnation of alumina by AHM and (NH4)2IrCI6. Fig. 2 a-c shows that the activities of these samples were in all reactions very similar. This suggests that the deposition order of both components, for Ir amount between 0.7 0.8 %, was of minor importance.

449 Correlation between the catalytic o activity in pyridine HY and relative ~176176 o~176176176176 reducibilities of the Ir-Mo sulfide cataZX ....... b ~0 lysts is demonstrated in Fig. 3. Similar m .~176176176176176176176176176176176176176176 [] ~176176176 dependence was also obtained for the ,~176176176176176176176176176176176176 ~1~ rate constants of thiophene HDS, as ~176176176176 recently shown elsewhere [11]. These ~176176176176176 .,.~.~176176 relations suggest that the activity of the mixed catalysts is closely related to the amount of hydrogen consumed during the TPR and, obviously, to the number I I 0 of reducible sulfur surface species. This 0.5 1.0 0.0 is in accordance with a generally accepted idea of the formation of anionic Relative reducibility, a.u. sulfur vacancies on the MoS2 phase, which are assumed to be the catalytic Fig. 3. Correlation between reducibility of sites in the HDS and hydrogenation the Ir-Mo sulfide catalysts and activity in the reactions. pyridine HY during parallel HDN/HDS. Denotations as in Fig. 1. Sample 12 (~), Sample 13 (v). A standard activation procedure for almost all mixed Ir-Mo catalysts was direct sulfidation of the precursors listed in Table 3. The only exceptions were samples prepared by deposition of AHM on reduced Ir/alumina catalyst (sample 12) or on an Ir4(CO)lz/alumina precursor and then reduced before sulfiding (sample 13). This aimed at keeping the Ir dispersion as high as it was in the starting It/alumina catalyst or, in the second case, at trying if such treatment in H2 could not lead to higher catalyst activity. The activity of samples 12 and 13 were compared with that of sample 10 with the same composition, but directly sulfided. Results of different activation procedures are shown in Table 4. The best activity was achieved in both HDS and HDN reactions after direct sulfidation of catalyst precursor as in sample 10. Table 4 Effect of activation on activity of Ir-Mo sulfide catalysts in parallel HDN of pyridine and HDS of thiophene kTH kpy kc5 Cat. Activation (mol/h.kgcat) 10 Sulfidation of AHM-Ir4(CO)12/alumina 1.9 2.5 3.0 12 Sulfidation of AHM-Ir/AI203 1.5 1.7 2.4 13 Reduction and sulfidation of AHM-Ira(CO)I2/AI203 0.9 1.5 1.1 Sample 12 was a little less active, which could probably be explained by a partial covering of an already developed Ir surface by Mo phase, decreasing in this way its accessibility. A third procedure, in which AHM and Ir4(CO)12 were reduced simultaneously before sulfiding, led to the lowest activity. In this case, it can be assumed that Ir4(CO)12 and AHM decompose easily to metallic Ir and MoO3 in H2 between 300-400~ [14,15]. On the

450

basis of our recent TPR study of the oxide Ir-Mo/alumina system, we speculate that the contact of well dispersed Ir particles with Mo oxide species could lead to the formation of some kind of non-reducible species [16]. Such an interaction can lead to a partial loss of Ir, keeping it in an Ir-O-Mo form. This assumption is consistent with earlier findings reported in the literature for the Ru-Mo system [1] and suggests to avoid thermal treatment of Ir-Mo catalyst precursor in the absence of HzS. The mixed Ir-Mo catalysts, in which Mo was deposited first, were prepared by impregnation of an oxidic or sulfided Mo/alumina catalyst. It was observed already that the sample prepared from presulfided Mo catalyst (sample 7), possessed a slightly higher activity than the samples prepared from oxidic Mo catalyst with comparable Ir loading. This is obvious from the comparison of the values k~i~ and kc5 plotted against Ir loading in Fig. 2. In order to confirm such an effect, an additional pair of mixed Ir-Mo samples was prepared by deposition of Ir4(CO)lz on a new charge of Mo/alumina catalyst (samples 14,15). The catalytic activity of both samples is compared in Fig. 4. The catalyst prepared from the presulfided Mo/alumina was clearly more active, approximately twice, than the catalyst prepared from oxidic Mo catalyst, in both reactions of thiophene and pyridine. A similar phenomenon was observed recently by Pinz6n et al. [17] after modification of sulfided and oxidic Mo/alumina catalyst by Ru, Rh and Pd, in HDS of dibenzothiophene and HY of naphthalene. The reason for the higher activity of the catalysts prepared by modification of a sulfided Mo/alumina matrix is not yet clear. We speculate that it could be connected with a closer contact between IrS• and MoSz phases in the catalyst, either through a higher Ir dispersion or due to a different distribution of Ir between the Mo phase and alumina.

Fig. 4. Effect of sulfidation of the Mo catalyst before Ir deposition on activity of Ir-Mo catalyst in parallel HDN/HDS. 4. CONCLUSIONS Deposition of Ir carbonyl, acetylacetonate, HzlrC16 and (NH4)zlrCI6 on alumina and subsequent reduction led to formation of Ir nanoparticles of about 1 nm. Direct sulfidation of deposited Ir precursors led to highly active Ir/alumina sulfide catalysts. A most active catalyst was obtained from Ir carbonyl. Addition of small amounts of Ir to Mo/alumina catalyst increased the reducibility of the MoSz phase, which possibly reflected in an enhanced activity

451 of the mixed catalysts in thiophene HDS and pyridine HY. However, activity in piperidine HDN decreased at Ir loadings above 0.5 %, in accordance with a decreased Ir dispersion. Other factors playing a major role were the state of the Mo phase before deposition of Ir, and the way of catalyst activation. Impregnation of the sulfided Mo catalyst by Ir4(CO)lz led to significantly higher HDS and HDN activities than impregnation of the oxidic Mo catalyst. The direct sulfidation of Ir-Mo catalyst precursors gave the most active catalysts. On the other hand, the starting Ir compound and the deposition order of Ir and Mo probably play a less important role. ACKNOWLEDGEMENT The authors thank the Grant Agency of the Academy of Sciences for financial support (grant A 4072103), Stid-Chemie A.G. (Germany) for providing the alumina carrier, and AIST (Tsukuba, Japan) for TEM measurements. REFERENCES

1. P.C.H. Mitchell, C.E. Scott, J.P. Bonnelle and J.G. Grimblot, J. Catal. 107 (1987) 482. 2. C.E. Scott, T. Romero, E. Lepore, M. Arruebarrena, P. Betancourt, C. Bolivar, M.J. P6rez-Zurita, P. Marcano and J. Goldwasser, Appl. Catal. 125 (1995) 71. 3. S.A. Giraldo de Le6n, P. Grange and B. Delmon, Catal. Lett. 47 (1997) 51. 4. A.S. Hirschon, R.B. Wilson Jr. and R.M. Laine, Appl. Catal. 34 (1987) 311. 5. A.G. Graham and S.E. Wanke, J. Catal. 68 (1981) 1. 6. K. Foger and H. Jaeger, J. Catal. 70 (1981) 53. 7. C.E. Scott, J. Guevara, A. Scaffidi, E. Escalona, C. Bolivar, M.J. P6rez-Zurita and J. Goldwasser, in: Stud. Surf. Sci. Catal., 130 (A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro, Eds.), Elsevier Science, 2000, p. 2813. 8. S. Eijsbouts, C. Sudhakar, V.H.J. de Beer and R. Prins, J. Catal. 127 (1991) 605. 9. Z. Vit and M. Zdrazil, J. Catal. 119 (1989) 1. 10. J. Cinibulk and Z. Vit, Appl. Catal. 180 (1999) 15. 11. J. Cinibulk and Z. Vit, Appl. Catal. 204 (2000) 107. 12. P. Marecot, J.R. Mahoungou and J. Barbier, Appl. Catal. 101 (1993) 143. 13. J.R. Anderson and K.C. Pratt, in "Introduction to Characterization and Testing of Catalysts", Academic Press (Harcourt Brace Jovanovish, Publishers), New York, p. 55, 1985. 14. K. Tanaka, K.L. Watters and R.F. Howe, J. Catal. 75 (1982) 23. 15. C. Thomazeau, V. Martin and P. Afanasiev, Appl. Catal. 199 (2000) 61. 16. Z. Vit and J. Cinibulk, React. Kinet. Catal. Lett. 72(2) (2001) 189. 17. M.H. Pinz6n, L.I. Merino, A. Centeno and S.A. Giraldo, in "Hydrotreatment and Hydrocracking of Oil Fractions", (B. Delmon, G.F. Froment and P. Grange, Eds.), Elsevier Science B.V., Amsterdam, 1999, p. 97.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

453

Peptization mechanisms of boehmite used as precursors for catalysts D. Fauchadour l, F. Kolenda 2, L. Rouleau 2, L. Barr~ 1, L. Normand 1 1 Institut Fran~ais du P6trole, 1-4 Avenue de Bois Pr6au, 92852 Rueil Malmaison Cedex, France 2 Institut Fran9ais du P6trole, CEDI "Rend Navarre", BP3, 69390 Vemaison, France In this work, using boehmite powders of different properties and under mild agitation, we determine key parameters, such as solid morphology, pH and ionic strength, and we identify the mechanisms which govern the peptization and account for observed behaviours (colloidal stability, sedimentation, gelification). We particularly reveal the role of dissolution on the dispersion and the aggregation of dense and open morphologies. This dissolution mechanism allows us to explain the behaviour of different boehmites during shaping processes and the textural properties of the elaborated supports. It is thus possible to explore better control of material forming and in particular of catalyst support elaboration. I. INTRODUCTION Boehmite is an important material in many fields such as petroleum, chemical or medicine industries. In the petroleum industry, it is mainly used as a precursor of gamma alumina (TAI203) which is a carder in the heterogeneous refining catalysts. Properties such as surface texture chemistry, porosity and thermal stability determine the performances of these supported catalysts [ 1]. In order to optimize the specific surface area, pore size and pore distribution of the alumina carrier, the best preparative schemes are sought. Making supported catalysts involves basically 4 steps : first is precipitation of boehmite (meaning synthesis of an aluminium oxyhydroxide) followed by washing and spray drying to produce a powder suitable for handling. The second step, called peptization, consists of the dispersion of the boehmite in an acidic solution in order to carry out a specific shaping technique such as oildrop or extrusion. The third step corresponds to a drying followed by a calcination at temperatures ranging from 500 to 700~ in order to transform the boehmite into the gamma phase structure of alumina. Metal deposition such as Pt, Mo or Co is finally performed by wet or dry incipient impregnation techniques [2]. Design of a tailored texture requires a good knowledge of the effect of each unit operation, and especially of the peptization, on the properties and characteristics of the support. For example, if one wants to obtain specific texture on the final alumina support, the particle size and the powder dispersion must be controlled during the peptization step. Moreover, use and performance of specific manufacturing process (such as oil-drop or extrusion) depends on the dispersion of boehmite and may even require the use of stable suspensions. If stability of boehmite has been studied as a function of pH, salinity and

454 concentration [3, 4, 5], very few studies take into account the nature of boehmite powders of different dispersability. This paper deals with 4 different kinds of boehmite used to identify key parameters controlling the behavior of boehmite during the peptization process. We begin to identify peptization mechanisms on a "model" boehmite that has been accurately characterized and which presents good peptization properties. Then we study mechanisms involved in the peptization process of 3 other kinds of boehmite with different properties in terms of peptization rate. Importance of dissolution is particularly emphasized. 2. MATERIALS AND PEPTISATION BEHAVIORS 2.1. Materials To succeed in this study, it is important to get a reference sample that can be dispersed to a rate of 100% in a specific test. This 100% peptizable boehmite powder (B 1) was provided by CONDEA Petrochemie Gesellshait. As indicated by the manufacturer, this boehmite is generated as a side product in the manufacture of alcohol straight-chain. We have verified this is a microcristalline boehmite of high purity and surface area (307 m2.g-1). The formula of the sample (A1OOHnHEO;n-0.45), as determined from thermogravimetric measurements, indicated a water excess from the well crystallized boehmite form (A1OOH), principally due to water physically adsorbed at the crystallite surface. X-ray diffraction and small angle X-ray scattering measurements showed the plate-like shape (length to width-~3) of the boehmite particles which have lengths of 100A and 30A along the a- and b-axes (planes (200), (020)) respectively. More detailed characterization on the powder initial state is given further in this paper. The three other boehmites were generated by aqueous precipitation. Two of them were provided by LA ROCHE CHEMICALS (B2 and B4) and one by PROCATALYSE (B3). They were chosen because they cover a large set of peptization properties in the reproducible peptization test described below.

Fig. 1. : Schematic representatlon ot Ume evolution ot -1 B1, B2, B3 and B4 suspensions in 0.1mol.l initial nitric acid solution. Note that B1 does not present any sedimentation

2.2. Boehmite peptization In order to be able to extrapolate further results to industrial conditions, boehmite sols were prepared by peptizing 6.67% weight of alumina in an aqueous solution containing various concentrations of nitric acid.

According to the weight concentration of boehmite and aggregation ionic strength limit determined elsewhere (0.1M to 0.15M for a monovalent salt [6]), initial concentration of acidic solution was 0.1 mol.1-1 (called classic peptization). Slow magnetic agitation was used

455 in order to obtain a homogeneous mixing of the powder with liquid without providing too much mechanical energy to the system. Following the peptization, one can identify 2 states: transitory and static. During the transitive state, 2 minutes after the end of the introduction of the boehmite powder, for 0.1mol.1l nitric acid initial concentration B1 is slightly opalescent and whitish. Under gentle agitation, B2, B3 and B4 are white and opaque homogeneous suspensions. After a few hours, an equilibrium pH is reached. If agitation is stopped, B 1 is still a stable suspension evolving slowly (over a few days) toward a low cohesive homogeneous gel. For the 3 other suspensions, stopping the agitation produces a fast sedimentation of a large part of the powder (sediment 1). If this sediment 1 is separated from this suspension, one can observe a slow sedimentation during a few weeks (sediment 2). These behaviors as well as the weight percentages of boehmite staying in the suspension are reported in Fig. 1 and Table 1, respectively. Table 1. Weight percentage of solid in suspension % weight of solid in susi~ensions (initial [HNOal = Suspension 1 Suspension 2 B1 100 100 B2 82 / B3 53 49 B4 33 29

In order to explain these differences we first characterized accurately properties of the powders of all the boehmite. Then we studied properties and behaviors of boehmite in acidic solution. We have then separated overfloatings from sediments in order to compare solids of these 2 phases to initial powder. Finally we discuss the results all together and conclude about the different mechanisms involved in the peptization process.

3. POWDER CHARACTERISTICS 3.1. Chemical and structural characteristics Characterizations by fluorescence X have shown various kinds of impurities, generally in small quantity, in the boehmites. Apart from giving us some hints on the synthesis process, these amounts of impurities can not be related directly to the peptization properties of the solids. XRD results have shown that there is no other crystalline phase in the initial powders and that the coherent domains are all around 3nm in thickness and 10nm in length. 3.2. Morphology characteristic SEM and TEM characterizations have shown that B1 boehmite is a "dense" agglomerate of platelet like crystallites developing a small dimension porosity (Fig. 2). B2,

456 B3 and B4 are made of two kinds of morphologies: a dense morphology similar to the one seen in B 1 and an open morphology with bigger porosity (Fig. 3). Although it is easy to identify tittle crystallites in the dense morphology (about 3nm thick and 10nm long like the coherent domains measured with XRD), it is impossible to identify clearly complete objects making up the open area. The large platelike curved crystals seen on TEM and SEM images are not independent from each others and this open morphology looks like a sponge. Thanks to XRD and HREM results, large plate-like curved crystals seem to be oriented agglomeration of small crystals. Because of the intimate mixing of the 2 phases, attempts to quantify these two morphologies from cross section images were not successful. N2 adsorption-desorption measurements show that BET specific areas are all about the same for the 4 powders (300m2g'l). However, total porosity is increasing going from B 1 to B4 and the isotherms show more and more macroporosity from B2 to B4. This suggests a higher quantity of open morphology.

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CRTA loss mass curve as a function of temperature for boehmites (a) and localisation model of water on crystallites for B 1 (b).

457 3.3. Boehmite-Air interface Because it has been shown that water at the surface could modify the dispersion properties [7], excess water of our boehmite has been studied by Controlled Transformation Rate Thermal Analysis (CRTA) [8]. This technique which respects the thermodynamic equilibrium allows to distinguish every type of adsorbed water and to estimate the apparent desorption activating energy [9]. 3 domains can be seen on the mass loss curve (Fig. 4) (apart from t h e 30/30~ domain corresponding to desorption of water due to relative humidity). The third domain corresponds to OH diffusion and OH desorption from the surface and from the structure due to phase transformation toward ~,-alumina. Domains 1 and 2 correspond to 2 different types of water. Considering the activating energies of these two domains (from 2 to 10 times lower than for phase transformation), it is clear that "excess" water (AIOOHnH20 ;n-0.45) compared to the alumina monohydrate A1OOH is not located between the octahedral double layers of the structure, but on the surface of plate-like boehmite crystals: water chimisorbed to A1 on [100] and [001] faces for the strongest links represented by domain 2 ; and water physisorbed to surface hydroxyls on [010] faces and to water molecules of the [100] and [001 ] faces. For the first time, this clarifies previous ATDATG experiments which showed an intermediate signal (between 100~ and 400~ attributed to possible strongly linked water on the surface [10,11 ]. This could validate as well the qualitative model proposed by Baker (Fig.4) [12]. Quantities of water on the surface of the 3 boehmites are not very different from each other and they correspond to 1 to 3 layers of water on surfaces of a model crystallite boehmite. However, the activation energy is a little bit higher for B 1, which could suggest a better crystallization or less amorphous microdomains in B 1 boehmite than in the other ones. We can finally conclude that B1 is of colloidal size and has a lot of water on the surface. B 1 could then behave as model particles weakly linked to each others. Although the other boehmite powders have about the same quantity of water on the surfaces, the open morphology identified on B2, B3, and B4 suggests particular contacts between elementary crystals. 4. BEHAVIOR IN LIQUID

Zero Charge Points (ZCP) measured by KNO3 addition technique showed that all boehmites have the same ZCP within the technique precision (8.9 0.1). Surface charge as a function of pH was measured on all the boehmites by comparing reference solutions (made from ultrafiltration of the corresponding boehmite suspensions) to boehmite suspemions. The results show that the variation of the surface charges for pH greater than 4 are all similar for the 4 boehmite powders. For pH smaller than 4, there is a slight divergence of the curves, probably due to a difference of dissolution below pH 4. Indeed, the study of pH change with time (which represents part of the dissolution) shows that there are important differences between powders, both in terms of pH variation rate and equilibrium pH (Fig. 5). These changes can be easily related to the variation of AI in solution as shown in Fig. 5 as well (measured by ICP on ultrafiltered solutions), pH is slowly increasing with time due to the dissolution kinetics of boehmite. Differences between the 4 boehmites are then due to differences of solubility (Fig. 6). In Fig. 6, one can clearly see that B1 has the lowest solubility since B1 solution always contains the smallest quantity of dissolved aluminum for a given equilibrium pH below 4.2.

458

Fig. 5.

: pH (a)and A1 in solution (b) variation as a function of time for an initial nitric acidic concentration of 0.1 mol.1-1 and 6.67% weight of boehmite.

However, it is surprising that at equilibrium and for a given initial quantity of acid, B 1 has the highest quantity of A1 dissolved (namely the lowest quantity of HNO3§ participating to the pH of the solution) and the lowest pH at equilibrium at the same time. In order to explain such a "paradox", calculations of surface charges were done by taking into account 1) the initial nitric acid introduced in the solution, 2) the A1 dissolution (by ICP), 3) the speciation of A1 (by NMR in order to know what kind of ion is present in the solution and what is the associated consumption of proton) and 4) the final pH at equilibrium. It was found that small differences of surface charge (estimated around 4 to 5 lxC/cm2) between B 1 and B4 can easily explain a greater dissolution of A1 and a lower pH at the same time.

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3,2 3,4 3,6 3,8 4 4,2 4,4 4,6 4,8 5 pn We can finally stmmmrize that above pH 4.2 the 4 boehmite powders have the same behavior in terms of charge density. At pH 4.2 and below, there is dissolution of part of the alumina. Differences between powders can be interpreted in terms of difference of solubility probably due to the presence of a low crystallinity nano-phase on B2, B3 and B4 (that is not detectable in XRD). Dissolution of this phase is not zero at the beginning of the peptization but it is very low, especially for B 1. At the equilibritm~ charge density may be slightly different but keeps in the same order of magnitude. Charge density, dissolution and morphology seem then to be the main parameters to tmderstand the differences in peptization.

459 Characterizations by means of cryo TEM replica show that there are elementary particles dispersed in all the suspensions. This TEM work with light scattering studies of the overfloating suspensions 1 for the 4 powders show that they all contain aggregates with hydrodynamic mean Trace o~lementary p.a~_icles and aggregate~sofelementary particles radius in the order of 100nm (Fig. 7). For B 1, these aggregates are made of elementary particles as seen in the dense morphology of the dry powder. In B2, B3 and B4, it is interesting to see that some of these aggregates are made of elementary particles, whereas some others are clearly made of bigger objects identified in the open morphology. Thus these 2 morphologies have been separated by peptization and they can both constitute the suspension. Of course, the biggest aggregates will slowly segregate to form sediment 2, and suspension 2 has a smaller hydrodynamic mean radius (Table 2). Table 2 Hydrodynamic mean radius Rh (in nm) of suspensions and % weight of solid dispersed in suspensions 1 for different preparations. Classic Prep. No dissolution Increased dissolution (initial [HNO3] = 0,1mol.1l) Rh Rh % wt. Solid Rh % wt. solid Rh % wt. solid Suspens. Suspens. suspension Suspens. suspension Suspens. suspension B1 B2

1 110 110

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showing only open morphology

By separating sediments from suspensions, we could see by SEM that sediments are all made of the open morphology with almost no trace of the dense morphology (Fig. 8). SEM on sediments 2 shows an agglomeration of

460 micro grains (of about 1~tm diameter) made of open morphology.

5. DISCUSSION 5.1. Peptization of BI B1 has a unique morphology made of a random arrangement of nanometric crystals. Furthermore, contacts between crystals, which are covered by 1 to 3 layers of water, are probably indirect and weak. This must involve an easy peptization under sott conditions as shown on alumina [7]. Actually, we could see that B1 is completely dispersed at the very beginning of the mixture, even when the dissolution is still very limited. This suggests that the increase of the charge density on surfaces of crystals (when [pH-ZPC [ increases) must be sufficient to "break" aggregates of the powder into colloidal particles. Peptization with no dissolution (with pH higher than 4.2), and with a comparable charge density adjusted by ionic strength shows a total peptization of B 1 with no sedimentation over a few weeks (table 2). This bears out that charging of surfaces is the main mechanism involved in the dispersion of B 1. Dissolution is not necessary for peptization of B 1 but it can modify the behavior of the suspension. Indeed, dissolution slowly increases the ionic strength of the solution. Taking into account the initial nitric acid quantity, pH, dissolved A1 in the solution, and speciation of aluminl'um ions (monomer [Al(H20)6]3§ by NMR), we can calculate the evolution

Fig. 9. Evolution of ionic strength calculated for B 1 classic peptisation from measured pH, AI concentration and speciation and initial nitric acid quantity introduced.

of the ionic strength I(t) with time (i.e. as a function of dissolution) with the formula: +

I(t) = 1/2 ([NO3"]initial + [H30 free](t) + 9[Alsolution](t)). Ionic strength reaches about 0.12moF1 after 3 days of peptization of 6.67% weight of alumina in an aqueous solution containing 0.1 mol.1-1 of acid nitric. Considering the aggregation ionic strength limit (around 0.1M to 0.15M for a monovalent salt), one can easily understand that this rise produces the formation of low cohesive gel due to aggregation kinetic of boehmite crystals over a few days.

5.2. Peptization of B2, B3 and B4 The dense morphology of the 3 other boehmite is completely dispersed during the classic peptization test used in this study. Similarities of properties with B1 suggest that this dispersion originates from charging of the elementary crystal surfaces. Sediments are made of open morphology only, indicating that contacts between objects of this morphology are more resistant than indirect solvated contacts between elementary crystals of the dense morphology. These contacts may be made of a lower crystallinity phase or an amorphous phase as it has been identified by CRTA and solubility measurements. However, part of the open

461 morphology can be dispersed in our test condition because of heterogeneity and dissolution. In a no-dissolution peptization, the quantity of solid in suspension is lower whereas it is largely increased when dissolution is increased (pH and ionic strength being constant, Table 2). This clearly indicates that open morphology is difficult to disperse with charging effect only. On the other hand, it is not an absolute obstacle to peptization since dissolution can break micro grains apart or separate micro grains from each others to make colloidal objects. Similarly to B1, if dissolution is too high, ionic strength may reach the aggregation limit and gel or flocculation may occur. 6. CONCLUSION This work allows us to propose a general representation of boehmite peptization, taking into account solid morphology, surface charge and dissolution. Powder made of a random agglomeration of nanometric crystals covered of water can only be dispersed by an increase of surface charge density (B 1). If the surface charge density is not high enough, the desagglomeration is not complete. In this case, aggregates up to a few hundreds of nm may subsist in suspension. They can create a bimodal pore size distribution like it has been seen in shaped alumina carrier. Other morphologies, like the ~ open morphology >>identified in this work, need more than only surface charging to be dispersed because they are made of bigger objects strongly linked together. Dissolution is then necessary to peptize such a morphology. Thus, one can understand why elaborated supports present mesopores around 9nm and macropores around 800nm when acidic concentration is not sufficient (2% for B4 with a few minutes of peptization, for example). When dissolution takes place (for 5 to 10% of acid for B4), macropores disappear because a progressive desegregation of open morphology. Finally, it is possible to play with the solid and the acidic concentrations and the ionic strength (that can be settled by different means) in order to control the rheology of the boehmite prior to the shaping and to obtain the targeted textural properties of catalyst supports. REFERENCES

1. R.K. Oberlander, in Applied Industrial Catalysis, Leach B.E. (Ed.), Vol. 3, 63, 1984. 2. J.F. Le Page, Preparation of Catalysts, Chapter 5, in Applied Heterogeneous Catalysts, Ed. Technip, p. 75-123, 1987. 3. J. Ramsay and S. Daish, J. Chem. Soc., Faraday Disc., (1978) 65. 4. C. Evanko, R. Delisio, D. Dzombak and J. Novak, Coll. Surf. A, 125 (1997) 95. 5. M. Van Bruggen, M. Donker, H. Lekkerkerker and T. Hughes, Coll. Surf. A, 150 (1999) 115. 6. F. Mange, Internal report IFP, 1998. 7. S. Desset, O. Spalla and B. Cabane, Langmuir 16 (2000) 10495. 8. D. Fauchadour, "Etude de la peptization de l'alumine boehmite", PhD Thesis, France, 2000. 9. J. Rouquerol, Thermochim. Acta, 144, (1989) 209. 10. P.A. Buining, C. Pathmamanohararg M. Bosboom, J.B.H. Jansen and H.N.W. Lekkerkerker, J. Am. Ceram. Soc., 73 (8) (1990) 2385. 11. P.A. Buining, C. Pathmamanoharan, M. Bosboom, J.B.H. Jansen and H.N.W. Lekkerkerker, J. Am. Ceram. Soc., 74 (6) (1991) 1303. 12. B.R. Baker and R.M. Pearson, J. Catal., 33 (1974) 265.

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Studiesin Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

463

Influence of the treatment of Y zeolite by ammonium hexafluorosilicate on the physicochemical and catalytic properties: application for chlororganics destruction R. L6pez-Fonseca, J.I. Guti6rrez-Ortiz, B. de Rivas, S. Cibri~in, and J.R. Gonz~tlezVelasco* Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad del Pais Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +34-94-6012681; Fax: +34-944648500; E-mail address: [email protected] The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite for oxidative destruction of chlorinated VOC. A series of Y zeolites with various Si/AI ratios was prepared from a commercial sample, then characterised and tested for the catalytic decomposition of chlorinated VOC (1,2dichloroethane and trichloroethylene). In general, these modified Y zeolites exhibited a higher activity with respect to that of the parent material, the zeolite subjected to 50% dealumination resulting in the most active catalyst. This increase in activity was associated with the development of strong Br6nsted acidity due to dealumination. 1. I N T R O D U C T I O N The increasing amounts of chlorinated volatile organic compounds (VOC), such as 1,2-dichloroethane (DCE) and trichloroethylene (TCE), released in the environment, together with their suspected toxicity and carcinogenic properties, have prompted researchers world-wide to find clean effective methods of destruction [1]. The abatement of chlorinated volatile organic compounds by catalytic combustion has been widely utilised in several technical processes. The lower temperatures required for catalytic combustion result in a lower fuel demand and can therefore be more cost effective than a thermal oxidation process [2]. In addition, the catalytic process also exerts more control over the reaction products and is less likely to produce toxic by-products, like dioxins, which may be generated by thermal combustion [3]. Most of the previous work related to catalysts for chlorinated VOC abatement is focused on the development of two type of catalysts, namely those based on noble metals and on transition metal oxides. By contrast, the utility of zeolites as effective catalysts for the decomposition of chlorinated organics has not been explored in detail, when it is reported that metal loaded catalysts employed in commercial applications are susceptible to deactivation by the HC1 and C12 produced during reaction [4]. In our previous works [5,6] it was found that H-zeolites showed a high activity for chlorinated VOC destruction under dry and humid conditions, and that their activity was controlled by the presence of

464 Br6nsted acidity. In the present study, an H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [7,8] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The scope of this work is to analyse the catalytic behaviour of a series of H-Y zeolites with different Si/AI in the oxidative decomposition of chlorinated hydrocarbons (DCE and TCE) in air, at lean concentration conditions (around 1000 ppm) between 200 and 550~ 2. E X P E R I M E N T A L AND M E T H O D S

2.1.

Materials and zeolite preparation

The Y zeolite (CBV400) in its H-form (H-Y) was supplied from Zeolyst Corp. and used as received. The series of dealuminated samples H-Y(d) was prepared as follows: prior to dealumination the starting material was obtained by two successive ion exchanges with a 3 M ammonium nitrate solution of the commercial H-Y sample to reduce the sodium content. Then, the NH4H-Y zeolite was preheated in a 0.5 M ammonium acetate solution at 80~ An aqueous solution of ammonium hexafluorosilicate was added dropwise at a rate of 50 cm 3 h 1 under vigorous stirring. The (NH4)2SiF6-to-zeolite ratio was adjusted to remove 15, 30, 50 and 75% of the aluminium in the zeolite, respectively. Afterwards, the temperature was raised to 95~ and the slurry was kept at this temperature for 3 hours to ensure that silicon could be inserted into vacancies created by the extraction of aluminium. Finally, the zeolite was recovered by filtration and repeatedly washed with hot deionised water to remove the unreacted (NH4)2SiF6 completely. The zeolites were pelletised using methylcellulose as a temporary binder which was removed by calcination in air. Then the pellets were crushed and sieved to grains of 0.3-0.5 mm in diameter and used for catalytic runs without further activation.

2.2.

Catalyst characterisation

The BET surface areas of the zeolite samples were determined by N2 adsorptiondesorption at -196~ in a Micromeritics ASAP 2010 equipment. The adsorption data were treated with the full BET equation. The <>method was applied in order to obtain an estimation of the micropore volume. The determination of the compositions was carried out using a Philips PW 1480 wavelength dispersive X-ray fluorescence (XRF) spectrometer. The crystallinity and the unit cell size were established by a Philips PW 1710 X-ray diffractometer (XRD) with CuK~t radiation (~,=1.5406,&) and Ni filter. The number of aluminium atoms per unit cell, NAI, was calculated from a0 using the correlation given by Fichtner-Schmittler et al. [9]. The atomic framework Si/A1 ratio was derived from the calculated N AI. The number of extra-framework aluminium atoms per unit cell was calculated by the difference between the total aluminium, as determined by XRF analysis, and the framework aluminium N AI. Diffuse reflectance infrared (DRIFT) spectra of pyridine adsorbed on the zeolite samples were obtained with a Nicolet Proteg6 460 ESP spectrometer, equipped with a controlled-temperature and environment diffuse reflectance chamber (Spectra-Tech) with KBr windows and a liquid nitrogen-cooled HgCdTe detector. All spectra were collected in the range of 4000-1000 cm -1 averaging 400 scans at an instrumental resolution of 1 cm -1,

465

and analysed using OMNIC software. Temperature-programmed desorption (TPD) of ammonia was performed on a Micromeritics AutoChem 2910 instrument. Prior to adsorption experiments, the samples were first pre-treated in a quartz U-tube in a nitrogen stream at 550~ Subsequently, the desorption was carried out from 100 to 550~ at a heating rate of 10~ min -1 in an Ar stream (50 cm 3 minl). This temperature was maintained for 15 min until the adsorbate was completely desorbed.

2.3. Experimental device and product analysis Catalytic oxidation reactions were carried out in a conventional fixed bed reactor under atmospheric pressure [10]. The flow rate through the reactor was set at 500 cm 3 mini and the gas hourly space velocity (GHSV) was set at 15000 h -1. The residence time based on the packing volume of the catalyst was 0.24 s. Following the reactor, a portion of the effluent stream was delivered and analysed on-line using a Hewlett Packard 5890 Series II gas chromatograph (GC) equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD), and controlled with HP ChemStation software. The concentration of the chlorinated feeds was determined by the ECD after being separated in a HP-VOC column. 3. RESULTS AND DISCUSSION

3.1. Catalyst characterisation Expectedly, increasing amounts of AHFS added led to increased degrees of dealumination of the samples. For moderate dealumination levels (<50%) in which 15, 30 and 50% of the original aluminium was calculated to be removed, respectively, the reaction was stoichiometric within the experimental error. Hence the actual dealumination degrees obtained were 16, 32 and 50%, respectively. However, for higher degrees (75%) the reaction was not complete any more since only 64% dealumination was achieved. Table 1. Textural and structural properties of dealuminated Y zeolites. . . . . Zeolite H-Y H-Y (dl 6%) H-Y(d32%) H-Y(ds0%) H-Y(d64%)

Crystal., %

a0, ,~

SBEV,m z g-1

Wpore,cm 3 g-1

Vmesopore, cm 3 g-1

100 99 97 95 40

24.52 24.51 24.49 24.46

900 955 865 820 155

0.382 0.406 0.355 0.334 0.085

0.070 0.072 0.050 0.049 0.105

466 Table 2. Textural and structural properties of dealuminated Y zeolites. Zeolite H-Y H-Y(d16%) H-Y(d32%) H-Y(ds0%)

H-Y(d64o/o)

(Si/A1)bum (Si/A1)fr~. 2.6 4.9 3.3 5.2 4.3 5.6 6.2 6.5 8.9 11.5

Alframework Alextraframework

Alto~ 53.3 44.6 36.2 26.7 19.4

32.2 31.1 28.9 25.5 15.4

21.1 13.5 7.3 1.2 4.0

Table 3. Total acidity and acid strength distribution of Y zeolites. Zeolite Total acidity, mmol NH3 g-1 Weak sites, % H-Y 0.65 74.8 H-Y(d16%) 0.61 65.2 H-Y(d32o/o) 0.56 54.4 H-Y(dso%) 0.48 40.3 H-Y(d64%) 0.35 57.2

Strong sites, % 25.2 34.8 45.8 59.7 42.8

The crystallinity of the dealuminated samples is listed in Table 1. It was observed that H-Y(d16o/o), H-Y(d32o/o) and H-Y(ds0%) retained high degrees of crystallinity [11]. However, a noticeable crystallographic degradation was noted for H-Y(d64%) sample since the crystallinity decreased to 40% as a result of the massive aluminium extraction [12]. For the parent material XRF analysis gave aluminium contents that were noticeably larger than the framework aluminium concentration indicating the presence of large amounts of extralattice or non-framework aluminium. A comparison of the results from Table 2 clearly demonstrate that at relatively low dealumination levels, i.e. 16 and 32%, the AHFS treatment preferentially removed EFAL species while both EFAL and FAL were removed when A o, I the AHFS concentration was ::i H-Y ai increased [13]. Hence, for H-Y(ds0%) sample, almost 100% EFAL and 30% et FAL were extracted from the parent .Q material. Indeed, it was found for this < sample that the framework Si/A1 ratio obtained from the unit cell size tt-Yld~%) was very close to the value obtained by XRF measurements. The results 1700 1650 1600 1550 1500 1450 1400 from Tables 1 and 2 were consistent Wavenumbers, cm -1 with the results found in the literature Fig. 1. IR spectra of adsorbed pyridine on Y zeolites. where it has been reported that dealumination with AHFS up to 50% dealumination did not damage the I

,

t

,

i

i

1

1

,

,

I

i

,

9

1

1

1

i

i

9

i

,

1

1

i

I

1

,

,

i

I

467 zeolite structure and EFAL was practically absent from such dealuminated samples [14]. According to N2 adsorption, the microporous character of the modified zeolites was largely retained and these samples did not possess any appreciable mesopore volume (Table 1). This finding was in agreement with their unaffected crystallinity. By contrast, the values / ~H'Y(de4%) of surface area and pore volume for the sample with a dealumination level of 64% were severely affected, ~ - ~o,d probably due to the collapse of the crystalline structure and to the blockage of the zeolite pore system to a very large extent [15]. As mentioned above, the structure ,i collapse was also revealed by XRD .~ m analysis. On the other hand, it is seen ,~ O from Table 1 that as the framework ~" Si/AI ratio increased the unit cell size H-Yff~,.) of AHF S - t r e a t e d samples significantly contracted due to the smaller size of silicon atoms [ 16]. Temperature-programmed desorption (TPD) of ammonia and \ infrared spectroscopy (IR) of t ~ . H-Y adsorbed pyridine are probably the most extensively used methods for ::::::::::::::::::::::::::::::::::::::::::::::::::::::::: characterising acidity (number, 50 leo 250 350 450 550 strength and nature of acid sites) in zeolites [17,18]. Fig. 1 shows the Temperature'~ diffuse-reflectance infrared spectra of Fig.2. Profiles of NH3-TPD from Y zeolites. pyridine adsorbed on H-Y, HY(ds0%) and H-Y(d64%) samples at 200~ in the region 1700-1400 cm 1. Pyridine bound to Br6nsted acid sites is associated with an IR band at 1545 cm ~ and that bound to Lewis acid sites with a band at 1450 cm 1. A band at 1490 cm ~ arises due to pyridine adsorbed on both Bronsted and Lewis sites [19]. H-Y possesses a large number of both types of acid sites with a ratio of Bronsted to Lewis acid sites, measured as the ratio of the integrated areas of the respective pyridine bands, of 0.8. Nevertheless, in the case of H-Y(ds0%) the band at 1450 cm ~ considerably decreased indicating that the dealumination treatment led to a sample with very few Lewis sites [20]. On the other hand, the band at 1545 cm I remained almost unchangeable. The results from NH3-TPD experiments are plotted in Figure 2. The NH3 desorbed above 100~ was considered as chemisorbed NH3 and subsequently used for acidity determination. The data from Table 3 indicates an overall loss of the total number of acid sites with dealumination [21]. As can been observed, the TPD profile of the parent zeolite was characterised by the display of a major desorption peak around 150~ [22]. This peak

468 was indicative of the presence of a notable number of weak acid sites (around 75% of the total number of acid sites). However, it showed almost no inflection in the high temperature range. Interestingly, as Si/A1 ratio was increased, a second desorption peak with a maximum at 350~ became more and more prominent indicating a noticeable increase in the number of strong acid sites with progressive dealumination [23 ]. This result paralleled the observation that the number of weak acid sites decreased from 75% of the total acidity for H-Y sample to 45% for H-Y(ds0%) sample. Table 3 summarises the ratio of strong acid sites obtained by integration of the high temperature signal. Hence, it could be concluded that a portion of the weak acidity turned into sites, mostly Bronsted-type sites, with high acid strength with increasing Si/A1 ratio [23]. 3.2. Catalytic activity results in chlorinated VOC conversion The series of dealuminated samples prepared by AHFS treatment were evaluated for the catalytic decomposition of DCE, which was considered as model reactions of chlorinated VOC destruction. The results of DCE and TCE conversion as a function of of reaction temperature over Y zeolites are shown in Fig. 3. It was noted that all dealuminated samples except H-Y(d64%) zeolite exhibited an enhanced performance in comparison with that of the parent material. The 50% dealuminated sample H-Y(ds0%) was the most active catalyst achieving complete conversion at 350~ for DCE and at 550~ for TCE. The following order of activity for chlorinated VOC conversion was observed: H-Y(ds0%)>HY(d32%)>H-Y(d16%)>H-Y>H-Y(d64%). Hence, H-Y(ds0%) zeolite showed a light-off temperature or Ts0 (temperature at which 50% conversion was attained) of 265~ 100 H-Y lower than that of H-Y(d32%), H-Y(d16%) H-Y(d1 90 1 ~ H-Y H-Y(d3"/' and H-Y, 280, 300 and 325~ d5'2% respectively. H-Y(d64%), however, 80 t H.YId.o'/, f showed a less active behaviour with a Tso 70 i value of 350~ Unlike DCE, TCE combustion required significantly higher / temperatures [24,25]. Ts0 values were 475, 475, 500, 510 and 520~ over Ho 40 1 Y(dso%), H-Y(d32%), H-Y(d16%), H-Y and / H-Y(d64%), respectively. 30 The combined characterisation 211 ~ and catalytic evaluation of the Y zeolites 111 obtained by progressive dealumination via the (NH4)2SiF6 method revealed that 200 250 300 350 400 450 500 550 the strength of the acid sites had a Temperature, *C dominant effect on the catalytic behaviour [26,27]. The zeolite activity increased for Fig. 3. Light-off curves of DCE and TCE Si/A1 ratios from 2.6 to 6.2 since the combustion over Y zeolites. decline in the acid site density was more than compensated for by the concomitant increase in the population of acid sites with high strength. Upon further removal of aluminium (c.a. Si/AI=8.4) the catalytic activity destruction dramatically dropped due to

469 the decrease in the number of acid sites and a partial loss of crystallinity, as evidenced by the low conversion of H-Y(d64o/o) sample. Similarly, Greene et al. [28] and Prakash et al. [29] obtained a substantial improvement in C.C14 conversion when using a Y zeolite subjected to SIC14 dealumination. 4. CONCLUSIONS The scope of this work was to evaluate the catalytic performance of a series of (NH4)2SiF6-dealuminated Y zeolites for the oxidative decomposition of chlorinated VOC in dry air, at lean concentration conditions (around 1000 ppm) between 200 and 550~ The highly active performance of chemically AHFS-dealuminated zeolites for chlorinated VOC destruction could be accounted for by the generation of new strong acid sites, which were preferentially BrOnsted sites, due to dealumination treatment. It could be concluded that a zeolite with a modest concentration of BrOnsted sites, which were primarily of high acid strength, demonstrated to be effective for catalytic purposes. Likewise, it was established that chlorinated VOC oxidative decomposition was a type of reaction that required strong BrOnsted acidity. ACKNOWLEDGEMENTS

The authors wish to thank Universidad del Pais Vasco/EHU (9/UPV 0069.31013517/2001) and Ministerio de Ciencia y Tecnologia (PPQ2001-1364) for the financial support. R. L-F. acknowledges Ministerio de Educaci6n y Cultura for the FPI grant (QUI96-0471). REFERENCES

1. E.C. Moretti, Practical Solutions for Reducing Volatile Organic Compounds and Hazardous Air Pollutants, Center for Waste Reduction Technologies of the American Institute of Chemical Engineers, New York, 2001. 2. G.J. Hutchings and S.H. Taylor, Catal. Today, 49 (1999) 105. 3. J.C. Lou and Y.S. Chang, Combust. Flame, 109 (1997) 188. 4. J.J. Spivey and J.B. Butt, Catal. Today, 11 (1992)465. 5. J.R. Gonz~lez-Velasco, R. L6pez-Fonseca, A. Aranzabal, J.I. Guti6rrez-Ortiz and P. Steltenpohl, Appl. Catal. B, 24 (2000) 233. 6. R. L6pez-Fonseca, P. Steltenpohl, J.R. Gonz/tlez-Velasco, A. Aranzabal and J.I. Guti6rrez-Ortiz, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 130 (2000) 893. 7. D.W. Breck and G.W. Skeels, US Patent 4 503 023, 1985. 8. G.W. Skeels and D.W. Breck, in: D. Olson, A. Bisio, (Ed.), Proceedings of the 6th International Zeolite Conference, Butterworths, Guilford, 1984, p. 87. 9. H. Fichtner-Schmittler, U. Lohse, G. Engelhardt and V. Patzelova, Cryst. Res. Technol., 19 (1984) K 1. 10. J.R. Gonz/flez-Velasco, A. Aranzabal, J.I. Guti6rrez-Ortiz, R. L6pez-Fonseca and M.A. Guti6rrez-Ortiz, Appl. Catal. B, 19 (1998) 189. 11. Q.L. Wang, G. Giannetto and M. Guisnet, Zeolites, 10 (1990) 301.

470 12. A.P. Matharau, L.F. Gladden and S.W. Carr, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 94 (1995) 147. 13. A. Gola, B. Rebouis, E. Milazzo, J. Lynch, E. Benazzi, S. Lacombe, L. Delevoye and C. Fernandez, Microporous Mesoporous Mater., 40 (2000) 73. 14. H. Ajot, J.F. Joly, J. Lynch, F. Raatz and P. Caullet, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 62 (1991) 583. 15. A.V. Abramova, E.V. Slivinskii and E.A. Skryleva, Kinet. Katal., 39 (1998) 411. 16. J.A. Lercher, C. Cmindling and G. Eder-Mirth, Catal. Today, 27 (1996) 353. 17. T. Barzetti, E. Selli, D. Moscotti and L. Forni, J. Chem. Soc., Faraday Trans., 92 (1996) 1401. 18. G. Zi and T. Yi, Zeolites, 8 (1988) 232. 19. T. Masuda, Y. Fujiyata, H. Ikeda, S-I. Matsushita and K. Hashimoto, Appl. Catal. A, 162 (1997) 29. 20. A. Macedo, F. Raatz, A. Boulet, A. Janin and J.C. Lavalley, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 37 (1987) 375. 21. H.G. Karge and V. Dondur, J. Phys. Chem., 94 (1990) 765. 22. C.S. Triantafillidis, A.G. Vlessidis and N.P. Evmiridis, Ind. Eng. Chem. Res., 39 (2000) 307. 23. B. Chauvin, M. Boulet, P. Massiani, F. Fajula, F. Figueras and T. Des Couri6res, J. Catal., 126 (1990) 532. 24. R. L6pez-Fonseca, J.I. Guti6rrez-Ortiz, A. Aranzabal and J.R. Gonzalez-Velasco, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 135 (2001) 4995. 25. A. Aranzabal, J.A. Gonzhlez-Marcos, R. L6pez-Fonseca, M. A. Guti6rrez-Ortiz and J.R. Gonzhlez-Velasco, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 130 (2000) 1229. 26. R. L6pez-Fonseca, A. Aranzabal, J.I. Guti6rrez-Ortiz, J.I. Alvarez-Uriarte and J.R. Gonzhlez-Velasco, Appl. Catal. B, 30 (2001) 303. 27. A. Aranzabal, R. L6pez-Fonseca, J.R. Gonzhlez-Velasco, J.I. Guti6rrez-Ortiz, M.A. Guti6rrez-Ortiz and J.A. Gonz~.lez-Marcos, Abstr. Pap. - 221st Am. Chem. Soc. (2001) CATL-027. 28. H. Greene, D. Prakash, K. Athota, G. Atwood and C. Vogel, Catal. Today, 27 (1996) 289. 29. D.S. Prakash, K.V. Athota, H.L. Greene and C.A. Vogel, AIChE Symp. Ser., 91 (1995) 1.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

471

Preparation of SiO2 m o d i f i e d SnO2 a n d Z r O 2 w i t h novel t h e r m a l stability Y-X. Zhu, J-Y. Wei, L. Zeng, X-D. Zhao, W. Lin and Y.-C. Xie Institute of Physical Chemistry, Peking University, 100871 Beijing, China SnO2 and ZrO2 samples prepared by digesting precipitation in a glass flask or polytetrafluoroethylene beaker were investigated. It is found that silica could be dissolved from the glass flask during digestion in basic condition and existed in the samples obtained as a surface modifier, which significantly enhanced the surface area and thermal stability of the samples. Therefore, some silica-doped samples were prepared by adding silica sol into the precipitating system before digestion and similar results were observed. High surface area SiO2-ZrO2 and SiO2-SnOz samples with novel thermal stability were obtained. 1. I N T R O D U C T I O N Both SnOz and ZrOz are important catalysts and catalyst supports. SnOz is widely used in the selective catalytic reduction of NO because of its good hydrothermal stability as well as its fine oxidative selectivity [1,2] and SnOz-based composite oxides are very active catalysts for CH4 deep oxidation [3,4]. ZrOz as a catalyst or catalyst support is used in many catalytic processes [5]. Besides their wide applications in catalysis, SnO2 and ZrOz are useful materials as sensors, ceramics and solid electrolytes. Although the applications of SnO2 and ZrO2 are quite different, high surface area and good thermal stability are always indispensable for their properties. Much work has been done to enhance the surface area of SnOz and ZrO2. Suzuki et al. [6] reported a "solvent replacement" method to prepare high surface area SnO2 and got SnOz with a surface area of 108mZ/g after calcination at 500 ~ for 3 h. Xu et al. [7] investigated the promoting effect of additives on thermal stability of SnOz and obtained modified SnOz sample with a surface area of more than 40mZ.g -1 after calcination at 900 ~ for lh. For the preparation of high surface area ZrOz, the most interesting results were reported by Chuah and his coworkers [8-11]. They have obtained zirconia with surface area of > 90 mZ/g after calcination at 900~ for 12 h by adding 10wt% aqueous solution of zirconium(IV) chloride dropwise to 5M aqueous ammonia and then digesting the precipitate at 100~ for 96 h at a pH of about 9.4. They found that digestion of the hydrous zirconia in mother liquid with a pH value of 9-13 is the key to obtain high surface area zirconia without the necessity of adding other oxides or doping agents. We prepared SnO2 with a similar method [12] and

472

got SnO2 with a surface area of 53 m2/g after calcination at 1000~ for 2 h. This is really unusual for a pure SnO2 powder considering that the melting point of SnO2 is only about 1600 ~ We suggest that the digested sample might have some amount of silica on its surface to hinder its sintering because the hot basic solution can dissolve some silica from the glass bottle. Chuah [11] has reported that digestion at acidic condition (pH=3) resulted in a much lower surface area. This also gave a hint of the influence of silica, because silica could not come out of the glass flask at acidic condition. Satoshi Sato et al. [13] also reported that silica could be dissolved from the pieces of quartz glass tube immersed in the digesting solution and deposited on zirconia, and that ZrO(OH)2 precipitate could facilitate the dissolution of the glass chip. We prepared some SnO2 and ZrO2 samples with high surface areas using the digesting method and examined them with X-ray fluorescence spectroscopy to determine the content of silica. As we expected, some amount of silica existed in these samples and improved the thermal stability and surface areas of SnO2 and ZrO2. Therefore, some silica-doped samples prepared by adding silica sol into the precipitating system before digestion were also investigated. High surface area SiO2-ZrO2 and SiO2-SnO2 samples with novel thermal stability were obtained. 2. E X P E R I M E N T A L

2.1 Sample preparation The SnO2 and ZrO2 samples were usually prepared in a glass flask by adding NH3.H20 or NaOH solution and SnCI4 (or ZrOCI2) solution simultaneously to a certain amount of NH4HCO3 solution, keeping a pH 7-8 (this method is referred as co-current co-precipitation). The precipitate was digested at certain temperature for several hours in the mother liquid in a glass round-bottom-bottle, then filtered, washed with NH4HCO3 solution and distilled water until the filtrate was free of C1- ions as tested with 0.5M AgNO3. Then the product was washed twice with ethanol and dried at ll0~ followed by calcination at required temperature for 2 to 4 hours. The silica-flee sample P-ZrO2 was Table 1 prepared with the same method in a Physical properties of some SnO2 samples polytetrafluoroethylene beaker instead of a glass flask. Sample t (h) SiO2 (wt %) S (mZ/g) The silica-doped samples SnO2-D SnO2-1 0 0 34 and ZrO2-D were prepared in the SNO2-2 12 0.15 35 polytetrafluoroethylene beaker and a SNO2-3 24 0.26 38 certain amount of silica sol was added SNO2-4 36 3.6 74 Sn02-5 48 4.5 93 into the beaker after precipitation. The Calcined at 500~ for 4h. following digestion and after-treatment was always described.

the

same

as

above

473 2.2 Sample characterization The silica content of the samples was determined with a Rigaku 3271E X-ray spectrometer. BET surface areas were measured on a Micromeritics ASAP 2010 Analyzer. X-ray powder diffraction patterns were recorded on a Rigaku D/MAX-2000 with Cu Kc~ radiation (The common SnO2 samples were analysed with a BD-86 X-ray Diffractometer). DTA-TG measurements were carried out on a Thermal Analysis SDT 2960 with a heating rate of 10~ 3. RESULTS AND DISCUSSION 3.1 SnO2 samples prepared in glass flask Table 1 shows the compositions and surface areas of some SnO2 samples prepared by co-current co-precipitation method with NaOH solution as the precipitating agent and digested at 100~ for different times (t). Samples digested for less than 24 h contain no or very small amount of silica and possess lower surface areas. Sample number 4 digested for 36 h contains 3.6wt% silica and AI AI shows a surface area more than twice that of samples number 1 to 3. The sample digested for 48 h contains the largest amount of silica and consequently has the highest surface area. Fig.1 shows the XRD patterns of the samples calcined at 500~ ~ Sn02-1 (The A1 diffraction peak comes

' ~ ~ ~ a ~ Sn02.5 ~

Sn02-4

~

Sn02-2

'6'0'7'0

from the sample frame made of 2'0 3'0 4'0'5'0 aluminium). It can be seen that all samples show a diffraction pattern of SnOz without any peak Fig. 1 XRD patterns of some SnOz samples characteristic for silica, and that the longer the digestion time, the Table 2 weaker and broader the SnO2 peaks, Physical properties of SnOz-D samples indicating a smaller particle size and lower crystallinity. Sample SiO2 S (mZ/g) After calcination at higher (wt %) 500~ 800~ 1000~ temperature, namely 800 ~ and SnOz-D1 5.2 206 166 132 SnO2-D2 9.5 216 169 49 1000 ~ the surface area of sample SnOz-5 remained at 75mZ/g and 53mZ/g, respectively, while the

20/~

474 samples with no or less silica sintered severely and had very low surface areas. Evidently, silica has a significant effect on the surface area and the thermal stability of the sample.

_

SnO~-D2

,ooo~

20 i

~"

30 'i

',

40 i

'"

,

5'0

',

60 f"

20/~

,

~

1000~

800~

800oc

oooc

ooooc

7'0

2'0

,

3'0

,

40 i

,

""

50 !

,

60 i

'

'

'

70 i

20/o

Fig. 2 XRD patterns of silica-doped SnO2 samples calcined at different temperatures

3.2 Silica-doped SnO2 samples Two silica-doped samples were also investigated. Table 2 lists the compositions and surface areas of the samples. The XRD patterns of these two silica-doped samples calcined at different temperatures are shown in Fig. 2. It can be seen that the silica-doped samples show quite high thermal stability, especially SnOz-D1, with a surface area as high as 132 mZ/g after calcination at 1000~ Both samples with a silica content of 5.2wt% and 9.5wt% show no diffraction peaks of silica, only the peaks of SnOz. Samples calcined at 500~ and 800~ have much broader diffraction peaks than those calcined at 1000~ and the corresponding surface areas are also quite high, more than 160mZ/g. At low calcination temperature, sample SnOz-D2 with higher silica content has a higher surface area than sample SnOz-D 1 with lower silica content, but its thermal stability is not so good as sample SnOz-D1, its surface area decreases to 49mZ/g after calcination at 1000~ This is probably because of the aggregation of amorphous silica. According to the "close-packed" monolayer model [14], the utmost monolayer dispersion capacity of SiOz on the surface of the support is about 0.048g/100m 2 or 0.081g/g SnOz (169mZ/g), so 9.5wt% silica is higher than this value and therefore amorphous silica species besides monolayer-dispersed silica might be formed. Details still need further investigation with solid state NMR and other techniques.

475

3.3 ZrO2 samples prepared in glass flask and silica-free P-ZrO2 sample Some zirconia samples were Table 3 also prepared using NaOH or NH3 Physical properties of some ZrO2 samples solution as the precipitating agent digested for 48 h at 100~ (A) and 30~ (B) and digested at 100~ or 30~ Sample SiO2 (wt %) S(m2/g) respectively for 48 h followed by ZrO2-Na-A 3.2 176 calcination at 500~ for 4 h. The ZrO2-Na-B 1.8 125 silica contents and surface areas of ZrO2-NH3-A 2.3 163 these samples are listed in Table 3. ZrO2-NH3-B 1.9 122 Samples digested at 100~ have more silica than the samples Table 4 digested at 30~ This is because Composition and surface areas of some ZrOz samples more silica can be dissolved from SiO2 Surface area (m2/g) the glass flask at higher temperature. Sample (wt %) 600~ 800~ 1000~ Similar to the results of SnO2 P-ZrO2-13 a 0 35 24 19 samples, the higher the silica ZrO2-6 a 0.89 111 73 33 content, the higher the surface area ZrO2-9 a 2.0 157 85 34 of the sample. XRD analysis ZrO2-13 a 5.2 218 140 64 a pH after digestion 3,0

4.4.

,--- 2 . 5 =.

" .O'I

I--

c

4.2

2.0

1.5

.,_.,

4.0

"~

3.8

c

b 1.0

0.5

3.6

0

.

, 200

.

, 400

. T/~

, 600

.

, 800

.

., 1000

a

3.4 0

'

200

'

460

660

860 '1oo6

T/~

Fig. 3 DTA-TG results of some hydrous zirconium oxide samples The corresponding oxides are: a. ZrO2-6; b. ZrO2-9; c. ZrO2-13 (figures not shown) finds no silica species, only tetragonal ZrO2, indicating that silica is probably in a highly dispersed state on the surface of the sample as a surface modifier. Further experiments confirmed the above prediction. Several silica-containing samples were prepared by digesting hydrous zirconium oxide at 100~ for 24 h at different pH in a glass flask and a silica-free sample was prepared in similar conditions in a polytetrafluoroethylene beaker with a cover. The resulting hydrous oxides dried at l l0~

476

were analyzed with DTA-TG technique and the surface areas of the samples calcined at different temperatures were also measured. Table 4 lists the composition and surface areas of the zirconium oxide samples. Fig. 3 shows the DTA-TG results. The silica-free zirconium hydrous oxide gives no crystallization peak in DTA-TG measurement. XRD analysis (Fig. 4) shows that the pure zirconium hydrous oxide dried at l l0~ is in a well-crystallized monoclinic state with small amount of tetragonal phase. Obviously, the existence of silica

j, 2'0'3'0'4'0'5'0'6'0'7'0 20/~

Fig. 4 XRD pattern of pure zirconium hydrous oxide dried at 110~

P-Zr02-13 1O00~ 1ooo~

~ ~ 2~3

800~

8oooc

600~

600~

3'0

.

4'0'5'0 20/~

.

.

.

6'0

,

. . . .

7'0

7'0

2'0'3'0'4'0'5'0'6'0 20/~

ZrO2-9

Zr02-13

__j

~

..... _A.,,_ ....... 1O00~

1O00~

__.J 2'0

800~

800~ 600~

3'0

4'o

20/~

~'o

6'o

7'0

600~ 2'o

'

3'o

'

4'o

s'o

'

6'o

'

20/~

Fig. 5 XRD patterns of some ZrO2 samples calcined at different temperatures

7'O

477 significantly elevates the crystallization temperature (Fig. 3) of the hydrous oxide as well as the surface areas of the corresponding oxides. Higher digesting pH results in higher silica content and consequently higher crystallization temperature and larger surface area of the corresponding oxide. The oxide samples calcined at different temperatures Table 5 were also characterized by XRD (Fig. 5). Surface areas of ZrOz-D samples Similar to the above-mentioned results, Surface area (m2/g) there is no diffraction peak of silica in all Sample 600oc 800~ 1000~ the samples. The pure zirconium oxide ZrOz-D1 241 136 95 P-ZrO2-13 displays mainly the peaks of ZrO2-D2 233 196 139 monoclinic ZrO2. However, tetragonal ZrO2 is the dominant phase in the samples containing a certain amount of silica. Not surprisingly, the higher the silica content, the higher the percentage of the tetragonal phase, especially the sample ZrOa-13 with the largest amount of silica. It consists of little monoclinic phase even after calcination at 1000~ for 4 h. All the results reveal that silica in the sample exhibits a typical effect of surface modification.

3.4 Silica-doped ZrO2 samples Two silica-doped samples with silica content of 2.7wt% (ZrOz-D1) and 5.2wt% (ZrO2-D2) respectively were also prepared. Their surface areas are listed in Table 5. The XRD patterns are shown in Fig. 6.

I

ZrO2-D1

ZrO2.D2

1000~ 800~ 2'0'3'0'4'0'5'0 20/o

6'0'7'0

600~

2'0'3'0'4'0'5'0'6'0' 20/~

1000~ 800~ 600~ 7'0

Fig.6. XRD patterns of silica-doped ZrO2 samples

The surface areas of silica-doped samples are higher than those of the samples listed in Table 4 though the silica content is comparable. ZrOa-D2 with 5.2wt% silica has a

478

specific surface area of 139m2/g after calcination at 1000~ for 4 h. From Fig.5 and Fig.6, it can be seen that the phase compositions of the two kinds of samples are also different. Both ZrO2-D1 and ZrO2-D2 contain monoclinic phase after calcination at 600~ 800~ and 1000~ However, ZrO2-9 and ZrO2-13, when calcined at 600~ and 800~ show only tetragonal phase. When calcined at 1000~ ZrO2-9 and ZrO2-13 exhibit only very small amounts of monoclinic phase. This can be probably attributed to the difference in the precipitation process. ZrO2-D1 and ZrO2-D2 were prepared in a polytetrafluoroethylene beaker, and silica sol was added before the digestion, so there is no silica source during the precipitation. Since ZrO2-9 and ZrO2-13 were prepared in a glass flask, in the course of precipitation, traces of silica dissolved from the glass vessel could deposit on the freshly formed ZrO(OH)x precipitate and help to hinder the formation of monoclinic ZrO2. The inhibition of monoclinic phase by silica can also be observed in the silica-doped samples. As can be seen in Fig. 6, after calcination at 600~ 800~ and 1000~ ZrO2-D2 with higher silica content always contains less monoclinic phase than ZrO2-D1. 4. CONCLUSION SnO2 and ZrO2 prepared by co-current co-precipitation and digestion in basic conditions in a glass flask contain certain amount of silica, and the silica as a surface modifier can improve the thermal stability and surface areas of SnO2 and ZrO2 by hindering their sintering. This is the main reason for the high surface areas and good thermal stability of thus prepared SnO2 and ZrO2. Silica-doped SnO2 and ZrO2 were also prepared and investigated. These samples exhibit novel thermal stability. 5.2wt% SiO2/ZrO2 maintains a specific surface area of 139 m2/g after calcination at 1000~ for 4 h, while 5.2wt% SiO2/SnO2 exhibits a specific surface area of 132 m2/g after calcination at the same temperature for 2 h. Silica can also help hinder the formation of monoclinic ZrO2.The effects of digestion on silica-flee sample and precipitation process on ZrO2 structure need further investigation. ACKNOWLEDGEMENT We gratefully acknowledge the financial support from National Science Foundation of China (29803001) and The Major State Basic Research Development Program (Grant No. G2000077503) REFERENCES

1. M.C. Kung, E W. Park and D. W. Kim, J. CataL, 18 (1999) 1. 2. J. Ma, Y. X. Zhu and J. Y. Wei, Stud. Surf. Sci. Cata[, Elsevier, Amsterdam, 130 (2000) 617.

479 3. 4. 5. 6. 7. 8. 9. 10. 11.

X. Wang and Y. C. Xie, Chem. Lett. (2001) 216. X Wang and Y C Xie, Appl. Catal. B, 35 (2001) 85. T. Yamaguchi, Catalysis Today, 20 (1994) 199. K. Suzuki, A. Sutsuma and H. Yoshida, Chem. Lett., 1997, 279. C. Xu, J. Tamaki and N. Miara, J. Mater. Sci. Lett., 8 (1989) 1092. P. Fornasiero, R. Di Monte and J. Kaspar, J.Catak, 151 (1995) 168. G.K. Chuah and S. Janenicke, Appl. Catal. A: General, 163 (1997) 261. G. K. Chuah, S. Janenicke and B. K. Pong, J. Catal,. 175 (1998) 80. G. K. Chuah, S. H. Liu, S. Janenicke and J. Li, Microporous Mesoporous Materials, 39 (Z000) 381. 12. J. Y. Wei, Y. X. Zhu and Y. C. Xie, Acta. Phys. -Chim. Sift, 17 (2001) 577. 13. S. Sato, R. Takahashi and T. Sodesawa, J. Catal., 196 (2000) 190. 14. Y. Xie and Y. Tang, Adv. CataL, 37 (1990) 1.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

481

Control of the textural properties of cesium 12-molybdophosphatebased supports S. Paul w V. Dubromez w L. Zatr, " + M. Fourmer, " + D. Vanhove w Laboratoire de Catalyse de Lille (ESA8010 CNRS) Equipe G6nie Chimique, Ecole Centrale de Lille et E.N.S.C.L., BP 48 59651 Villeneuve d'Ascq Cedex France + Equipe Catalyse H6t6rog6ne, Universit6 des Sciences et Technologies de Lille, Batiment C4, 59655 Villeneuve d'Ascq Cedex France

* Corresponding author- dominique, [email protected] An experimental design is applied in order to point out the influence of the more significant preparative parameters on the textural properties of Cs3PMol2040-based supports. With this in mind, a Hadamard matrix design including seven factors (i.e. stoichiometry, reactants concentrations, reaction temperature, addition rate, addition order, maturation time and thermal treatment procedure) has been used. Syntheses were carried out in an especially designed reactor in order to be able to control carefully mixing, temperature, concentrations and addition rate of reactants. Analysis of the results leads to the determination of optimal operating conditions to prepare supports with reproducible and controlled textural properties. 1. INTRODUCTION 12-Molybdophosphoric and 1-vanado, l l-molybdophosphoric acids are Keggintype heteropolyacids (HPA) well known as catalysts for mild oxidation reactions [1-15]. Nevertheless, these solids suffer from poor thermal stability and thus deactivate progressively under reaction conditions [16-19]. For this reason, coupled with their low surface area (1-5 m2/g), the influence of their dispersion on a support aiming at enhancing and stabilising the activity of the catalyst was tested. Various types of solids have been studied to play this role (silica, alumina, carbon, titania,...) but it is generally found that at low HPA contents strong interactions between the support and the active phase occur leading to the degradation of the latter [20, 21]. Promising results have been obtained by doping a silica support with alkaline ions [22]. In this case it is proposed that a heteropolysalt interface is formed between the silica network and the heteropolyacid crystal orientating and stabilising the structure of the active phase. Other interesting performances are also achieved when heteropolyacids are directly supported on a

482 heteropolyacid alkaline salt [23, 24]. Recently, we reported [25] that H4PMollVO40 deposited onto CsaHPMollVO40 is 15 times more active for the selective oxidation of isobutane to methacrylic acid and is also more stable than the pure acid alone. It was also observed in that work that apparently identical synthesis procedures lead to different solids as far as their textural properties are concerned. Moreover, Karmakar et al. [26-28] have shown a direct relation between the initial selectivity for acrylic acid and the proportion of porous volume related to large mesopores and macropores (pore radii Rp> 10 nm) in HPA supported-catalysts used for the selective oxidation of propane. This result shows that the textural properties of heteropolyanionic supports are the key parameters for the achievement of good selectivity. Lapham and Moffat [29] studied the influence of preparative parameters over several heteropolyoxometalates but the emphasis was put on 12-tungstophosphates. The role of thermal treatment was underlined and it was shown that the higher the temperature, the lower the surface area. In this context, cesium salts were much more stable than other heteropolysalts. Among the HPA alkaline salts, Cs3PMoI2040 (further noted Cs3) seems to be a judicious choice to play the role of support. Indeed, this solid is easy to prepare by a simple cationic exchange between H3PMo12040 (further noted H3) and a cesium salt. Moreover, Cs3 presents a good thermal stability and a high surface area is often reported [15,22,30,31]. Nevertheless, as far as this textural property is concerned, a great variation is found in the literature depending on the operating conditions used for the synthesis and the calcination [8,32,33]. In this study, we tried to clarify this important point by implementing a careful control of the Cs3 preparation procedure. A more particular attention was paid to the chemical engineering aspect of the reactor (agitation, temperature and concentrations homogeneity and addition rate of the reactants). Classically, the research of the optimal preparative conditions of a solid consists in a first step of listing the more significant parameters and then in varying intuitively a single one at a time while keeping the others constant. This work leads to a large number of tedious experiments and is uncertain to reach the objectives because of the potential interactions between the parameters. Experimental design avoids these drawbacks and enables the determination of the influence of the parameters with a minimum of experiments. Seven factors (i.e. stoichiometry, reactants concentration, reaction temperature, addition rate, addition order, maturation time and thermal treatment procedures) were studied using a Hadamard matrix design [34] with the objective to better understand their influences on the surface area, porous volume and porous volume distribution of the supports.

2. EXPERIMENTAL 2.1 [13 and Cs3 syntheses According to previous results [35], the synthesis of H3PMoI2040 was achieved in two steps: the formation of the acidic salt Na2HPMol2040,xH20 and its dissolution by acidification and purification by ether extraction. i) 218.8g of NaEMoO4,2 H20 (0.9 mole) were dissolved in 317 ml of deionised water. 5.15 ml of HaPO4 (85%) and then 250 ml of HC104 (60%) were added dropwise to

483 the solution. Pale yellow crystals of Na2HPMoI2040 precipitated and were collected by filtration and dried overnight at ambient temperature. ii) The disodic salt was dissolved in 4 ml/g of a 10% HC1 solution. A red orange solution was obtained. HaPMo12040 was extracted as heavy layer by diethyl ether, and then a quantity of water equivalent to half of the volume of the organic phase was added to it. After evaporation of the ether, the remaining aqueous solution was placed at 4~ to crystallise. The hydrated crystals (29 H20) were dried under air flow leading to the room temperature stable hydrated form (13 H20). Cs3 syntheses were performed by a simple cationic exchange between Ha (issued from the same batch for all preparations) and Cs2CO3 in a thermostated vessel; cesium carbonate being chosen to avoid the presence of residual counter-anions in the final solid. The reactor was especially designed to permit a constant and controlled addition rate of the reactants and an efficient and reproducible mixing. To this purpose, the vessel was equipped with baffles avoiding a vortex formation in the liquid and achieving therefore a quick mixing which was checked using a colorimetric tracer. Ha and Cs2CO3 solutions were both thermostated at reaction temperature (one in the reactor and the other (the so-called added reactant) in a separate vessel) before starting the addition, the rate of which was controlled by a peristaltic pump. In the reactive media, the pH was constantly monitored during the reaction. During the maturation, the reactor was kept under constant stirring and at constant temperature (the same as during the reaction). The mixture was then evaporated under vacuum at 70~ and ground in a mortar. In order to try to stabilize the properties of the support, a calcination was carded out. Two different procedures were followed. In the first one, the cold solid was placed in a furnace at 100~ and immediately heated up to 200~ (50~ The temperature was kept constant at this level for 2 hours and was then risen at 350~ (100~ and stabilised for 3 hours. The furnace was then switched off and allowed to cool down to ambient temperature overnight. The second procedure consisted in putting the cold solid in the furnace directly at 200~ and then in following the same thermal treatment as above. All the supports were then analysed by N2 adsorption-desorption over an ASAP 2010 Micromeritics apparatus after outgassing for 4h at 200~ BET [36] and BJH [37] methods were used to determine surface areas, porous volumes and porous volume distributions. The reproducibility of the synthesis and calcination procedure as far as textural properties are concerned was checked and validated.

2.2 Experimental design Hadamard matrices of experiments are generally used to point out the more influent qualitative and/or quantitative factors within a given experimental domain. In this method, two levels are attributed to the factors (noted -1 and +1) as presented in Table 1. To study the seven factors mentioned above, eight experiments are needed. The matrix of experiments is presented in Table 2 where each line corresponds to a synthesis while the columns correspond to the factors. Estimations of the effect of each factor were calculated by adding the responses modified by the sign of the level for the considered factor and by dividing this sum by eight.

484

The responses studied were the surface area, the porous volume and the porous volume distribution split in 3 classes of pore radii (Rp<5nm, 5nm10nm). Table 1 Factors levels Reactants concentrations (mol/1)

reactant

Reaction maturation temp. (~

quantities

Level +1

0.05

Stoich.

Cs2CO3

Level -1

0.1

20% mol. Cs excess

H3

Relative

Added

Initial thermal

Maturation

treatment (~

Addition rate (ml/min)

65

100

10

17

45

200

2

2

time (h)

Table 2 Matrix of experiments Initial R eact ion Add it io n maturation thermal Maturation rate treatment (ml/min) time (h) temp. (~ (oc)

Synthesis number

React ants conc. (mol/1)

Relative quantities

Added reactant

1

0.05

Stoich.

Cs2CO3

45

100

2

2

2

0.1

Stoich.

Cs2CO3

65

200

10

2

3

0.1

Stoich.

H3

45

100

10

17

4

0.1

Cs excess

Cs2CO3

65

100

2

17

5

0.05

Cs excess

Cs2CO3

45

200

10

17

6

0.1

Cs excess

H3

45

200

2

2

7

0.05

Stoich.

H3

65

200

2

17

8

0.05

Cs excess

H3

65

100

10

2

3. R E S U L T S Table 3 presents the main textural properties of the supports. It can be seen that the influence of operating conditions is important. Surface areas (SBET) actually vary from 15 to 82 and even if porous volumes are less scattered, the pore distributions are really diversified. The use of an excess of cesium carbonate leads systematically to a drastic fall of surface area and to an increase of the mean pore diameter (MPD) (syntheses 4, 5, 6 and 8). The porous volume, Vp, is always greater when 0.1M reactants concentrations are used

mZ/g

485 (syntheses 2, 3, 4 and 6). Syntheses 4 and 5 leading to the maximum proportions of large pores give also the lower surface areas. Conversely, high surface areas and low proportions of large pores are obtained for syntheses 1, 2 and 3. The preparation of a support presenting both a high surface area and an important proportion of large pores is therefore not possible. Table 3 Textural properties of the supports

Synthesis SBZT number (m2/g)

Total porous Mean pore volume diameter Vp (gL/g)

MPD (nm)

Pores distribution (in % of the total porous volume) Rp<5nm

Rp=5-10nm

Rp> 10nm

1

61

76

7

42%

49%

9%

2

82

101

8

42%

30%

28%

3

67

90

5

63%

20%

17%

4

15

80

22

6%

23%

71%

5

16

76

18

15%

21%

64%

6

28

92

10

43%

19%

38%

7

45

67

7

47%

7%

46%

8

23

71

10

38%

29%

33%

Table 4 Effect of the operating conditions over the textural properties Reaction Initial Reactants Relative Added Addition Maturation maturation thermal conc. quantities reactant rate time temp. treatment

SBET(mZ/g)

-6.0

+21.6

+1.4

-0.9

-0.6

+5.0

-6.3

Vp (laL/g)

-9

+2

+2

-2

-2

+3

-3

% Vp (Rp<5nm)

-2

+12

-11

-4

0

+2

-4

% Vp (Rp=5-10nm)

+2

+2

+6

-2

+6

0

-7

% Vp (Rp>10nm)

0

-13

+5

+6

-6

-3

+11

486

Estimations of the effects of the 7 factors on the different responses studied are listed in Table 4. These values represent half of the change of the response when the considered factor is moved from its lower (-1) to its upper level (+1). For instance, using 0.05M reactants concentrations instead of 0.1M leads to a mean decrease of 12 ma/g of the surface area. 4. D I S C U S S I O N The factorial analysis permits to draw some conclusions on the influent factors for surface area optimisation or porous volume distributions adjustment. Thus, the influence of the stoichiometry on the surface area is substantial: using a 20% stoichiometric excess of Cs2CO3 leads actually to a mean surface area decrease of 43 It is supposed that the cesium carbonate in excess blocks the pores reducing the available surface. The concentration of reactants, the maturation time and the rate of addition can also be considered as influent but to a lesser degree. The influence of other operating conditions are negligible. Conversely to the results reported by Lapham and Moffat [29] for 12-tungstophosphates, we have not found in this work any significant effect of the thermal treatment on the surface area. The great thermal stability of Cs3PMo12040 could be the explanation of this result. In order to obtain a support presenting a high surface area, it is necessary to use stoichiometric conditions, high reactants concentrations and addition rate and a short maturation time. The total porous volume of the support is mainly enhanced by the concentrations of the reactants that must be high. The distribution of the porous volume is also presented in Table 4. It is clear that the stoichiometry and the addition of HPA into carbonate favour the formation of micropores. Conversely, meso and macropores (Rp>10nm) are favoured by the use of an excess of cesium carbonate and a long maturation time. A reaction temperature of 65~ a slow addition rate of the carbonate to the HPA solution, and a calcination starting at 200~ are also preferred.

mZ/g.

Table 5 Optimised operating conditions

Synthesis number

Reactants conc. (mol/1)

9

0.1

Initial React ion Relative Added maturation thermal Addition rate Maturation treatment (ml/min) time (h) quantities reactant temp. (~ (~ 25% mol. Cs2C03 Cs excess

65

200

1

48

In order to maximize the proportion of the porous volume attributed to large pores (Rp>10nm), a synthesis has been done with the operating conditions presented in Table 5. It can be noticed that the values have been changed compared to the Hadamard matrix levels in a view to amplify the effects. The textural properties of the solid obtained are in

487 good accordance with the expected ones. The proportion of porosity attributed to large pores is actually 77% which is higher than the best result obtained in the Hadamard experimental design. The reproducibility of this particular synthesis and calcination as far as textural properties are concerned have been checked and validated as shown in Fig.1.

E

0,0020

f\

i, !

C

E

0,0015

i,/ I J"

E

/ 'L /

0

> 0,0010

ill'

0

>

,\ \

i

0,0005 r~f

\

\

'k,

0,0000 1

10

100

Pore Radius (nm)

Fig. 1 9BJH desorption derivative pore volume 5. C O N C L U S I O N This study has shown the strong influence of preparative operating conditions on the textural properties of the cesium 12-molybdophosphate-based supports. The use of stoichiometric quantities of reactants leads to a high surface area and microporous solid probably close to pure Cs3PMo12040 whereas high reactants concentrations favour the porous volume. Pores distributions can also be adjusted by modifying the preparative operating conditions. The formation of large pores is actually favoured by the use of an excess of Cs2CO3 and a long maturation time. In a way, the selection of the adapted operating conditions allows to design a support with "tailor made" textural properties. Experiments are now in progress in order to disperse an active phase on these supports and compare the reactivity of the catalysts thus obtained. REFERENCES 1. T. Okuhara, N. Mizuno and M. Misono, Adv. Catal., 41 (1996) 113. 2. N. Mizuno and M. Misono, Curr. Op. Sol. Sta. & Mat. Sci., 2 (1) (1997) 84. 3. I.V. Kozhevnikov, Chem. Rev., 98 (1) (1998) 171. 4. N. Mizuno and M. Misono, Chem. Rev., 98(1) (1998) 199. 5. M. Ai, J. Catal., 71 (1981) 88. 6. H. Mori, N. Mizuno and M. Misono, J. Catal., 131 (1991) 133. 7. N. Mizuno, T. Watanabe and M. Misono, Bull. Chem. Soc. Jpn., 64 (1991) 243. 8. K. Eguchi, I. Aso, N. Yamazoe and T. Seiyama, Chem. Lett. (1979) 1345.

488 9. L. M. Deusser, J. C. Petzoldt, J. W. Gaube and H. Hibst, Ind. Eng. Chem. Res., 37 (1998) 3230. 10. Y. Konishi, K. Sakata, M. Misono and Y. Yoneda, J. Catal., 77 (1982) 169. 11. J. Hu and R. C. Burns, J. Catal., 195 (2000) 360. 12. T. Ilkenhans, B. Herzog, T. Braun and R. Schl6gl, J. Catal.,153 (1995) 275. 13. M. Akimoto, H. Ikeda, A. Okabe and E. Echogoya, J. Catal., 89 (1984) 196. 14. T. Haeberle and G. Emig, Chem. Eng. Technol., 11 (1988) 392. 15. G. B. Mc Garvey and J. B. Moffat, J. Catal., 132 (1991) 100. 16. C. Rocchiccioli-Deltcheff, A. Aouissi, M.M. Bettahar, S. Launay and M. Fournier, J. Catal. 164 (1996) 16. 17. O. Watzenberger, T. Haeberle, D.T. Lynch and G. Emig, New Devel. in Select. Oxid., Stud. Surf. Sci. Catal., Vol. 55, Elsevier Science Publishers, Amsterdam, 1990, 843 18. G. Lischke, R. Eckelt and G. Ohlmann, React. Kinet. Catal. Lett., 31(2) (1986) 267. 19. G. Mestl, T. Ilkenhans, D. Spielbauer, M. Dieterle, O. Timpe, J. Kr6hnert, F. Jentoft, H. Kn6zinger and R. Schl6gl, Appl. Catal. A: Gen., 210 (2001) 13. 20. P. G. Vazquez, M. N. Blanco and C. V. Caceres, Catal. Lett., 60 (1999) 205. 21. M. Prevost, Y. Barbaux, L. Gengembre and B. Grzybowska, J. Chem. Soc., Faraday Trans., 92(24) (1996) 5103. 22. C. Desquilles, M. J. Bartoli, E. Bordes, G. Hecquet and P. Courtine, Erdol Erdgas Kohle, 109(3) (1993) 130. 23. K. Briickman, J. Haber, E. Lalik and E. M. Serwicka, Catal. Lett., 1 (1988) 35. 24. K. Brfickman, J. M. Tatiboui~t, M. Che, E. Serwicka and J. Haber, J. Catal., 139 (1993) 455. 25. M. Sultan, PhD thesis report, Universit6 de Technologie de Compi6gne, n~ D1215, 19/07/1999. 26. S. Karmakar, A. F. Volpe Jr., P. E. Ellis Jr. and J. E. Lyons, US Patent 6043184, 03/28/2000, assigned to Sunoco Inc. and Rohm and Haas. 27. J. E. Lyons, A. F. Volpe Jr., P. E. Ellis Jr. and S. Karmakar, US Patent 5990348, 11/23/1999, assigned to Sunoco Inc. and Rohm and Haas. 28. A. F. Volpe Jr., J. E. Lyons, P. E. Ellis and S. Karmakar, Prep. Am. Chem. Soc., Div. Pet. Chem., 44(2) (1999) 156. 29. D. Lapham and J. B. Moffat, Langmuir, 7 (1991) 2273. 30. B. Che|ighem, S. Launay, N. Essayem, G. Coudurier and M. Fournier, J. Chim. Phys., 94 (1997) 1831. 31. M. Akimoto, Y. Tsuchida, K. Sato and E. Echigoya, J. Catal., 72 (1981) 83. 32. C. Marchal-Roch, N. Laronze, R. Villaneau, N. Guillou, A. T6z6 and G. Herv6, J. Catal., 190 (2000) 173. 33. N. Mizuno, M. Tateishi and M. Iwamoto, J. Catal., 163 (1996) 87. 34. R. Perrin and J.P. Scharff, ~ Chimie industrielle ~, Vol.1, Paris, Masson, 1993. 35. C. Rocchiccio|i-Deltcheff, M. Fournier, R. Franck and R. Thouvenot, Inorg. Chem., 22 (1983) 207. 36. S. Brunauer, P. H. Emmet and E. Teller, J. Am. Chem. Soc., 60 (1938) 47, 309. 37. E. P. Barret, L. G. Joyner and P. O. Halenda, J. Am. Chem. Soc., 73 (1951) 373, 104, 114, 116, 127.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

489

MnOx/CeO2-ZrO2 and MnOx/WO3-TiO2 catalysts for the total oxidation of methane and chlorinated hydrocarbons E. Kantzer, D. D6bber, D. KieBling and G. Wendt Institut Rir Technische Chemic, Universit~it Leipzig, Linn6straBe 3, 04103 Leipzig, Germany Zirconia and titania-supported manganese oxide catalysts for the combustion of methane and chloromethane were prepared by precipitation and impregnation and characterized by various techniques. The characterization studies showed that, in contrast to titania-supported catalysts the manganese oxide phases on zirconia-supported manganese oxide catalyst is highly dispersed. The different states of the manganese oxide phase are reflected in the reducibility. Addition of ceria to zirconia and tungsten oxide to titania enhances the reducibility of the manganese oxide species. Catalytic activity in the methane oxidation is related to the dispersity of the catalytically active manganese oxide phase. In contrast to the methane oxidation the zirconia and titania supports are catalytically active themselves for chloromethane oxidation. Zirconia-supported manganese oxide catalysts showed a lower catalytic activity than ceria- zirconia catalysts with low ceria content. The best results for the titania-supported catalysts were obtained on catalysts with low manganese oxide and tungsten oxide loadings. 1. INTRODUCTION Catalytic total oxidation of volatile organic compounds (VOC) is widely used to reduce emissions of air pollutants. Besides supported noble metals supported transition metal oxides (V, W, Cr, Mn, Cu, Fe) and oxidic compounds (perovskites) have been reported as suitable catalysts [1,2]. However, chlorinated hydrocarbons (CHC) in industrial exhaust gases lead to poisoning and deactivation of the catalysts [3]. Otherwise, catalysts for the catalytic combustion of VOCs and methane in natural gas burning turbines to avoid NOx emissions should be stable at higher reaction temperatures and resists to thermal shocks [3]. Therefore, the development of chemically and thermally stable, low cost materials is of potential interest for the application as total oxidation catalysts. Manganese oxides have long been known to be catalysts for a variety of gas clean-up reactions. Manganese/copper mixed oxide (Hopcalite) is the catalytically active component in gas mask filters for CO; CO is converted to CO2 at room temperature [4]. Further applications of manganese oxide catalysts are the NH3 oxidation to N2 [5], the combustion of VOC [6,7] and methane [8], the oxidation of methanol [7], the 03 decomposition [9] and the NOx reduction[14]. Perovskite-type oxide catalysts (e.g. LaMnO3) have been proven to be effective catalysts for the total oxidation of chlorinated hydrocarbons [10]. Several studies have shown that besides preparation method and calcination temperature the kind

490 of support materials determines the structural and catalytic properties of the resulting manganese oxide catalysts [9,11-13]. The aim of the present work was to examine supported MnOx catalysts and to clarify the influence of the support (TiO2, WO3-TiO2,ZrO2, CeO2-ZrO2) on the textural, structural and catalytic properties for the total oxidation of methane and chloromethane, considering catalyst deactivation and formation of by-products. 2. EXPERIMENTAL

ZrO2 and CeO2-ZrO2 support materials were prepared by precipitation of the hydroxides from the nitrates with NHa-solution followed by drying and calcination at 600 ~ TiOE-anatase and TiOE-rutile were supplied by the Sachtleben Chemic GmbH (Germany). WOa-TiO2 samples were prepared by impregnation of TiO2 with aqueous (NH4)10W12041-solution and, after drying, calcined in air at 400 ~ for 3 h. The MnOxAVOa-TiO2 catalysts were prepared by impregnation of the calcined supports with an aqueous solution of Mn(NO3)2. MnOx/CeOE-ZrO2 catalysts were obtained by coprecipitation of the hydroxides from the metal nitrates with NHa-solution in presence of H~O2. The dried precursors were calcined in air at 600 and 800 ~ resp. for 6 h. InFfigures and Tables, the following abbreviations were used: xMn/yCe(W)Zr(Ti) for x mol% MnEO3/y mol% CeO2 (WO3) - (100 - y) mol% ZrO2 (TiO2 anatase). The catalysts were characterized by thermoanalytical (DTG/MS), powder X-ray diffraction (XRD), nitrogen adsorption (BET) and temperature programmed reduction (TPR) measurements. A continuous fixed bed reactor coupled on-line with a GC (FID/TCD) was used for the catalytic experiments. The amount of catalyst (particle size diameter: 0.1 - 0.3 mm) loaded was 0.6 g. The catalytic behaviour of the catalysts was investigated in the total oxidation of methane and chloromethane (1 vol.% in air, feed stream 5 l/h) considering by-product formation. 3. RESULTS AND DISCUSSION 3.1. MnOdCeO2-ZrO2 catalysts XRD analysis of the calcined ZrO2 supports revealed a mixture of monoclinic and tetragonal ZrO2. With increasing MR203 content the cubic ZrO2 modification was favoured and stabilized at higher MnOx content (Table 1). ~-Mn203 was detected at Mn203 contents of 40 mol%. Depending on the Mn203 content, a maximum of the specific surface areas was observed between 20 and 40 mol% for the precipitated catalysts; the specific surface areas for the impregnated catalysts are lower [15]. Using TPR, it was shown that reduction of the MnOx/ZrO2 samples is a multistep process and obviously depends on the MnO• loading. With increasing temperature peaks for amorphous MnOx species, ~-Mn203, MR304 and MR3+ ions incorporated in the ZrO2 lattice were determined (Fig. 1). In comparison to pure -Mn203 supported MnOx species are reduced at lower temperatures. Incorporation of CeO2 in the ZrO2 framework leads to a higher mobility of surface and bulk oxygen and can thus influence the catalytic properties [ 16,17]. At low CeO2 contents, beside monoclinic and tetragonal ZrO2, the Zr0.84Ce0.1602phase was identified. In addition,

491 crystalline CeO2 was found at higher CeO2 contents. The specific surface areas of the CeOz-ZrO2 samples did not differentiate significantly; a decrease was observed at higher CeO2 contents (Table 1). The reduction of CeO2 and CeOa-ZrO2 samples with high CeO2 contents occurred in two steps. The low temperature peak is assigned to the reduction of amorphous CeO2 and that at higher temperature, to crystalline CeO2 (Fig. 1). Table 1 Specific surface areas (aBET) and phase analysis by XRD of selected catalysts calcined at different temperatures (Tcalc.) Catalyst Tcalc.: 600 ~ Zcalc.: 800 ~ SBET(mZ/g) phases SBET(m2/g) phases 10Mn/Zr 84 c(t)-ZrO2 13 m-ZrO2, t-ZrO2 20Mn/Zr 147 c-ZrO2 39 c(t)-ZrO2, m-ZrO2 40Mn/Zr 144 Mn304, c-ZrO2 25 Gt-Mn203,t-ZrO2 20Ce/Zr 78 t-ZrO2(CeZr), m-ZrO2 45 t-ZrOz(CeZr), m-ZrO2 80Ce/Zr 59 CeO2 32 CeO2 20 Mn/20CeZr 108 t-ZrO2 9 t-ZrO2, Mn304 20Mn/80CeZr 72 CeO2 13 CeO2 3Mn/Ti(A) 42 10Mn/Ti(A) 29 3Mn/3WTi(A) 95 10Mn/3WTi(A) 72 3Mn/10WTi(A) 87 10Mn/10WTi(A) 68 m, t, c: monoclinic, tetragonal, R: rutile

Gt-Mn203,A 4 Gt-Mn203,A 4 A 7 Gt-Mn203,A 6 Gt-Mn203,A 10 0t-Mn203,A 6 cubic; CeZr: Zr0.84 Ce0.1602; MnTi:

~t-Mn203,A, R a-Mn203, A, R WO3, A, R a-Mn203, R WO3, A, R Gt-Mn203,R, MnTi MnTiO3; A: anatase;

The loading of MnOx on the CeOz-ZrO2 supports increases the specific surface areas (Table 1). It is important to point out that there are CeO2-MnOx interactions on unsupported as well as on supported catalysts. Using cerimetric redox titration [16] it was found that in comparison to the unsupported MnOx an increase of the Mn oxidation number is observed for CeO2-MnOx samples calcined at low temperatures, whereas a decrease was observed at higher calcination temperatures [15]. The obtained results are in accordance with those of Imamura et al. [16]. TPR measurements on MnOx/CeO2-ZrO2 samples revealed that CeO2 incorporation in the ZrO2 structure leads to a small shift of the low temperature reduction peaks of MnOx phases to lower temperature; thus, the presence of Ce ions in the ZrO2 structure enhances the reducibility of the MnOx species. The catalytic activity was investigated for the total oxidation of methane and chloromethane as testing reactions. Selected results are presented in Fig. 2 and Fig. 3 Compared with the impregnated MnOx/ZrO2 catalysts the catalytic activity of the precipitated catalysts for the methane conversion is higher [15]. The best results were obtained for MnOx loadings between 20 and 40 mol% (Fig. 2). With respect to the structural investigations it is suggested that the amount of X-ray amorphous (dispersed) MnO• species on the ZrO2 surface is responsible for the catalytic activity. Moreover, with

492 increasing calcination temperature of the samples the catalytic activity decreases. This is explained mainly by the decrease of the specific surface areas and by the migration of Mn 3+ ions in the ZrO2 framework under formation of solid solutions which lead to a depletion of the catalytically active sites. The catalytic activity of the ZrO2 and CeO2-ZrO2 supports is low. For the CeO2-ZrO2 samples, an activity maximum was found on the catalyst with a CeO2 content of 80 mol% [15]. 1~0 3

~ho~ - - ~ o ~ - - ~ o

,", / ', ~

10MIV3WTi 10Mn/10WH 10~Zr

Tert~alure (~ Fig. 1. TPR profiles of selected catalysts (3 l/h 8 vol.% H2 in Ar; heating rate: 10 l/h) Fig. 2 shows that at a constant MR203 content of 20mo1%, MnOx/CeO2-ZrO2 catalysts exhibit nearly the same catalytic activity as the MnOx/ZrO2 catalysts. However, comparing the catalyst behaviour of both catalyst systems, the activity of MnOx/CeOa-ZrO2 catalysts are lower than that of the MnOx/ZrO2 catalysts. On the basis of textural investigations, this effect is explained by the lower specific areas of the CeO2 containing catalysts. Furthermore, the decrease of the Mn oxidation number determined by cerimetric titration [15] is due to MnOx-CeO2 interactions at calcination temperatures > 400 ~ which affect the catalytic behaviour of the three-component system MnOx/CeO2-ZrO2 for the total oxidation of methane.

493

'~176,o~, 80

lON,kl/3WIi lOMn/lOWTi / /

-

p~'->~

/?

2o~

r

/

/f

~Zr

=o

:z

=>

'0t o. c ~ - _ - ~ _ ~ ~ ~ - o - - , 300 400

, 500

ge~on~

|

6~

700

(~

Fig. 2. Conversion of methane on selected catalysts vs. reaction temperature

y.

100

- ~ ;

-

,

0-

~6o. lOMCfi 3Mu3WIi IOMa/IOWIi --9 20MC/r v 40MqZr --

=> 40.

20-

~ y ~Y

2 0 ~

-v

300

'

~0

'

~o

'

&

Fig. 3. Conversion of chloromethane on selected catalysts vs. reaction temperature

494 The catalytic activity for chloromethane conversion over mixed oxide catalysts is characterized by a reversible deactivation of the catalysts and depends on the kind of CHC and reaction conditions [ 10]. After an initial period of up to 60 min, a nearly constant CHC conversion is observed. The conversion of chloromethane as a function of reaction temperature for selected catalysts is shown in Fig. 3. The results were obtained at steady state regime. With increasing MnOx content up to a loading of about 40 mol%, the catalytic activity increases. As in the case of methane conversion, the catalytic activity in chloromethane conversion is attributed to the dispersed MnOx species at the ZrO2 surface. Moreover, in comrast to methane oxidation, the ZrO2-support itself is catalytically active [ 15]. The investigations of CeO2-ZrO2 samples over a wide composition range showed that the best results were obtained with the sample 20 mol% CeO2-80 mol% ZrO2, which is active at low temperatures. It is suggested that the activity of these catalysts is determined by the incorporation of Ce 3§ ions in the ZrO2 matrix, which leads to a higher mobility of bulk and surface oxygen species. Loading of the CeO2-ZrO2 samples with MnOx causes a decrease of the catalytic activity (Fig. 3) which is explained by the CeO2-MnOx interactions (see above). Besides the main reaction products, HC1, CO2 and H20, several by-products were determined in the exit gases (Table 2). Only higher chlorinated chloromethanes were formed up to a reaction temperature of 500 ~ Remarkably low amounts of by-products were found over the catalyst 20 mol% CeO2-80 mol% ZrO2. With the MnOx comaining catalysts, more by-products were obtained. Furthermore, chlorine was formed by Deacon reaction. Table 2 Concentrations of organic by-products in exit gas (vpm) in the oxidation of chloromethane on selected catalysts Reaction temperature (~ Catalyst By-products 300 350 400 450 500 550 20Mn/Zr CH2C12 30 160 450 270 CHC13 30 CCh . . . . 10 40Mn/Zr CH2C12 50 340 730 110 CHC13 10 20 40 CCh

20Ce/Zr 20Mn/20CeZr

10Mn/Ti 3Mn/3WTi 10Mn/10WTi

CH2C12 CHC13 CH2C12 CHC13 CCh CH2C12 CHC13 CH2C12 CH2C12 CHCI3 fEb

-

60 . . .

-

-

10 220 .

. <10 -

.

.

.

.

50 480 . 80 <10 20 . .

lo

50 10 360 20 500 <10 <10 150

-

10 10 40 100 20 <10 20 30

495

3.2. MnOxfWOa-TiO2 catalysts DTG measurements and XRD phase analysis showed that loading of TiO2 support with MnOx lowered the TiO2 (anatase) to TiO2 (rutile) transformation temperature with increasing MnOx content and calcination temperature. (x-Mn203 was identified at MnOx loadings as low as 3 mol%. By thermoanalytical investigations it was shown that an evolution of oxygen takes place at peak maximum temperatures of about 500 and 800 ~ [18]. The former peak is assigned to the transformation of MnOx species (x > 1.5) to aMn203, the latter to the formation of MnTiO3. The formation of MnTiO3 is confirmed by XRD investigations on samples containing higher amounts of MnOx and calcined at higher temperatures. TPR measurements of the MnOx samples revealed that in contrast to the MnOx/ZrO2 samples, there are only weak interaction between the support and MnOx; thus well crystallized particles of 0t-Mn203 were formed on the TiO2 surface during calcination of the precursor at 600 ~ in accordance with the XRD investigations. Specific surface areas are lower than those of the MnOx/ZrO2 samples and decrease with increasing loading of MnOx and increasing calcination temperature (Table 1). Addition of WO3 to the TiO2 support changes the properties of the supported MnOx catalysts. WO3 was detected by XRD with WO3 loadings > 3 mol% at higher calcination temperatures. Compared to the MxlOx/TiO2 samples MnOx loading of the WOa-TiO2 supports does not change the phase composition significantly up to WO3 content of 10 mol%. However, TPR measurements have shown that WO3 addition increases the dispersion of the MnOx species especially at lower WO3 contents since the reduction of the MnOx species is observed at lower reduction temperatures. For MnOx/WO3-TiO2 samples with higher amounts of WO3 a significant decrease of the HE consumption was found (Fig. 1). This effect can be interpreted by solid state reactions among the catalyst components forming MnTiO3 and MnWO4. A remarkable enhancement of the specific surface areas were observed for the supported MnOx catalysts after loading of the TiO2 support with WO3 (Table 1). Similar results were reported for the influence of WO3 on the textural properties OfVEO5fWO3-TiO2 for the Denox process [19]. In Fig. 2 the catalytic properties of selected MnOxfWOa-TiO2 catalysts for the conversion of methane are compared with those of the MnOx/CeO2-ZrO2 catalysts. In general, the ZrO2 based catalysts exhibited a clearly higher catalytic activity than supported TiO2 catalysts. The support TiO2 (anatase) as well as the WO3-TiO2-support are nearly catalytically inactive at reaction temperatures < 600 ~ In the case of the MnOx/TiO2 catalysts the catalytic activity does not depend significantly on the MR203 content in the range up to l0 mol %; at higher Mn203 contents the catalytic activity decreases. With respect to the results obtained on ZrOE-supported MnOx catalysts, the low catalytic activity of the TiO2-supported MnOx catalysts is explained by their lower specific surface areas and the lower dispersion of the catalytically active MnOx phases. WO3 addition does not influence the catalytic activity at low WO3 contents. Higher amounts of WO3 lead to a considerable decrease of the catalytic activity caused by solid state reactions between the catalyst components forming MnTiO3 and MnWO4 phases (Fig. 2). As shown in Fig. 3 the conversion of chloromethane takes place at significant lower reaction temperatures than methane on both investigated catalyst systems. TiO2- and WOaTiO2-supports are catalytically active themselfes in the temperature range between 400 and

496 500 ~ For the catalysts with 3 and 10 mol% Mn203 nearly the same catalytic activity was determined; Mn203 content does not influence the catalytic activity significantly [18]. In contrast to the conversion of methane WO3 addition to the MnOx/TiO2 catalysts increases the catalytic activity for the chloromethane conversion. This effect is explained mainly by the enhancement of the specific surface areas by addition of WO3 to MnOx/TiO2 samples. However, higher contents of Mn203 and WO3 leads to a decrease of the catalytic activity caused by solid state reactions between the catalyst components (see above). For the best catalyst (3 mol% Mn203/3 mol% WO3-97 mol% TiO2) remarkable low amounts of byproducts were determined in the exit gas (Table 2). Furthermore, chlorine is formed mainly on catalysts containing MnOx. 4. CONCLUSIONS The kind and composition of the ZrO2 and TiO2 based supports influence the dispersity of the supported MnOx phases. Using ZrO2 as support a highly dispersed supported MnOx phase was found whereas on the TiO2-support well crystallized a-Mn203 was formed even at low loadings. The reduction behaviour of the supported MnOx catalysts is related to the dispersity; with TPR it is possible to differentiate between amorphous (dispersed) and crystallized MnOx species. The dispersity (reducibility) of the supported MnOx species was enhanced by the addition of CeO2 to ZrO2 and of WO3 to TiO2. The catalytic activity for methane oxidation depends on the Mn203 content and dispersity of the supported MnOx phases. In the case of ZrO2 supported MnOx catalysts a maximum of the catalytic activity was observed at a Mn203 content of 40 mol%. Addition of CeO2 to ZrO2 leads to a decrease of the catalytic activity caused by decrease of the specific surface areas and MnOx-CeO2 interactions. Compared with ZrO2 based catalysts, the catalytic activity of TiO2-supported MnOx catalysts is significantly lower because of the lower dispersity of the catalytically active MnOx phase. The catalytic activity for chloromethane oxidation is characterized by a reversible deactivation and formation of byproducts (mainly chloromethanes) at low reaction temperatures. In contrast to methane conversion the supports are catalytically active themselves. ZrO2-supported MnOx catalysts exhibit a lower catalytic activity than CeO~-ZrO~ catalysts with low CeO2 contents. The most efficient TiO2-supported catalysts are catalysts with low Mn203 and WO3 contents. At higher MR203 and WO3 contents a decrease of the catalytic activity was observed which can be explained by solid state reactions among the catalyst components. REFERENCES

1. J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. 2. F.J. Jansen, in Handbook of Heterogeneous Catalysis, (G. Ertl, H. Kn6zinger, J. Weitkamp, Eds.), VCH Verlagsgesellschatt mbH, Vol. 4 (1997) 1664. 3. B.K. Hodnett, Heterogeneous catalytic oxidation, J. Wiley & Sons Ltd. (2000) 189 and 285. 4. S. Veprek, D.L. Cocke, S. Kehl and H.R. Oswald, J. Catal., 100 (1986) 250. 5. A. W611ner, F. Lange, H. Schmelz and H. Kn6zinger, Appl. Catal. A: General, 94 (1993) 181.

497 6. A. Nishino, Catal. Today, 10 (1991) 107. 7. M.Baldi, V. Sanchez Escribano, J.M. Gallardo Amores, F. Milella and G. Busca, Appl. Catal. B: Environ., 17 (1998) 175. 8. H.M. Zhang, Y. Teraoka and N. Yamazoe, Catal. Today, 6 (1989) 155. 9. R. Radhakrishan and S.T. Oyama, J. Catal., 199 (2001) 282. 10. D. Kiel31ing, R. Schneider, P. Kra~, M. Hattendom and G. Wendt, Appl. Catal. B Environ., 19 (1998) 143. 11. J. Ma, G.K. Chuah, S. Jaenicke, R. Gopalakfishnan and K.L. Tan, Ber. Bunsenges. Phys. Chem. 99 (1995) 184 and 100 (1996) 585. 12. F. Kapteijn, A.D. van Langefeld, J.A. Moulijn, A. Andreini, M.A. Vuurman, A.M. Turek, J.-M. Jehn and I.E. Wachs, J. Catal., 150 (1994) 94. 13. J.M. Gallardo-Amores, T. Armaroli, G. Ramis, E. Finocchio and G. Busca, Appl. Catal. B: Environ., 22 (1999) 249. 14. M. Kantcheva, J. Catal., 204 (2001) 479. 15. D. D0bber, Ph. D. Thesis, Universitiit Leipzig, 2002. 16. S. Imamura, H. Shono, N. Okamoto, A. Hamada and S. Ishida, Appl. Catal. A: Genera, 142 (1996) 279. 17. C. de Leitenburg, A. Trovarelli, J. Llorca, F. Cavani and G. Bini, Appl. Catal. A: General, 139 (1999) 161. 18. E. Kantzer, unpublished results. 19. L. Alemany, L. Lietti, N. Ferlazzo, P. Forzatti, G. Busca, E. Gianello and F. Bregani, J. Catal., 155 (1995) 117.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

499

Catalytic behaviour of Rh-supported catalysts on lamellar and zeolitic structures by anchoring of organometallic compound C. Blanco, R. Ruiz, C. Pesquera and F. Gonz~ilez Departamento Ingenieria Quimica y Quimica Inorg/mica. Universidad de Cantabria. 39005-Santander, Spain. e-mail: [email protected] Rhodium catalysts were prepared by hydrogen reduction at atmospheric pressure of a cationic organometallic rhodium complex and anchored onto lamellar and zeolitic products. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts prepared. The new rhodium catalysts were applied in the hydrogenation of acetone. The reaction was carried out under milder conditions.

The lamellar and zeolitic materials synthesized are suitable forpreparing heterogeneous catalysts by anchoring an organometallic complex by ion exchange. A higher metal content was achieved in the supports with lamellar structures, while better dispersion was shown by the catalysts supported on zeolitic structures. Activity, selectivity and durability depend on the nature of the support used in the preparation of the catalyst. The acidic centres of the pillared montmorillonite favoured the formation of methyl isobutyl ketone, whereas the other heterogeneous catalysts favoured greater selectivity towards isopropanol. 1. I N T R O D U C T I O N Metal complexes from the platinum group have been extensively used as catalysts on hydrogenation reactions in homogeneous phase as well as heterogeneous. The advantages of using supported complex against unsupported complex are mainly due to the easy separation of the catalyst from the reaction medium and also in the better recovery of the catalyst. The use of supported complex has the advantage over supported metal that it presents a high catalytic activity under milder conditions. The synthesis of heterogeneous catalysts from surface organometallic complexes is a method used to stabilize homogeneous catalysts and/or to obtain small metallic particles, which are nearby uniform in size and structure and in some cases have resistance to sintering. The zeolites and modified clays (PIL-clays) are suitable products over which metallic phases can be dispersed to obtain a bifunctional catalysts providing the acid and basic centres on the mentioned structures. In addition, clays and zeolites have the advantage as supports that they are chemically and physically robust and inexpensive [1]. In our study we chose the hydrogenation of acetone, in which acetone is competitively transformed through bifunctional catalysts into 2-propanol (IPA) and methyl isobutyl ketone (MIBK). The aim of the study was to analyze the suitability of the use of new supports (different aluminosilicate structures based on modified clays in zeolitic products and PIL-clays) in

500 heterogeneous catalysis by inmobilizing cationic organometallic complex. The study analyzed the influence of the support characteristics on the activity/selectivity of these catalysts in the vapor phase hydrogenation of acetone under milder conditions. 2. E X P E R I M E N T A L SECTION

Supports were prepared from a montmorillonite (BENT) supplied by GADOR (Spain). The mineralogical composition (weight %) deduced from XR diffractogram was: montmorillonite (82 %), feldspar (11%), mica (5%), quartz (2%) with traces of calcite, dolomite and kaolin. The montmorillonite was purified and homogenized with sodium and designated BENa. This material was used to synthesize a pillared clay and zeolitic products. The pillared clay was prepared by the method described elsewhere [2] using aluminium oligomers as polyoxycations and is designated BENPIL. The zeolitic materials were obtained through alkaline treatment of the montmorillonite [3]. The conditions of treatment and the media used as solvent varied. When the medium was seawater (obtained from a part of the bay of Santander (Spain)), samples are denominated ZESE, and when distilled water was used, ZEDI. To designate the two conditions tested, a final -P is added when the reacting mixtures were kept in an autoclave at 160 ~ at autogeneous pressure for 24 h without stirring; and a final -X, when the treatment took place at boiling point, using a reflux system with continuous stirring for 24 h. The samples were washed in dialysis membranes, oven dried at 105 ~ powdered and kept in a desiccator. The final products were, therefore, denoted: ZESEP, ZEDIP, ZESEX and ZEDIX. Rhodium catalysts supported on six different aluminosilicate structures were prepared by reduction with hydrogen of cationic organometallic rhodium complex anchorage to the support. The precursor active phase was incorporated, in acetone medium, through ion exchange using [Rh(Me2CO)x(NBD)]C104 as metal precursor, -(NBD) = 2,5norbornadiene and (Me2CO)x = acetone-. Support and solutions were used in the appropriate quantities to obtain a metal loading of 1% wt assuming adsorption to be complete on the supports [4,5]. The catalysts were denoted Rh/support (where support is: BENa, BENPIL, ZESEP, ZEDIP, ZESEX and ZEDIX). The amount of rhodium loaded was determined by UV-VIS spectroscopy ( ~ , = 385 nm), as the difference between the metal concentration in the filtered liquid and that in the initial suspension. All the catalysts were reduced in situ at 200~ under a flow of hydrogen (5).The reduction temperature of Rh was determined by thermal analysis. The %Rh-dispersion (the number of exposed metal atoms divided by the number of total metal atoms of the same kind in the catalyst sample times 100) was determined by H2/O2 chemisorption by procedure of Benson and Boudart (6). Supports and catalysts were characterized (3,5) by different physicochemical techniques: Specific surface area, (SBET), was determined from N2 adsorption isotherms, Xray diffractometry, Scanning Electron Microscopy (SEM) and IR spectroscopy. The pyridine adsorption method was used to identify by IR the nature and character of the surface acidic groups of supports, precursors and catalysts. The catalytic activity analysis was done in gaseous phase (atmospheric pressure and temperature of reaction between 50 and 180~ in a fixed-bed reactor. The products of the

501 hydrogenation of acetone reaction were analyzed in a gas chromatograph equipped with an FID. The working conditions were selected so that the conversion should in no case exceed 20% and, therefore, the effects that inverse reactions might have on the activity can be ignored. 3. RESULTS AND DISCUSSION 3.1.Characterization of the supports The specific surface area (Table 1) shows an increase in BENa (homogenized montmorillonite) and in the pillared sample. The increase up to 247 mE/g is related to the creation of micropores in the pillaring process. Of the zeolitic samples, those treated more severely (ZE--P) show values similar to that of the starting material, whereas the ZE--X samples present a slight increase. These results can be attributed to the formation in ZE--P of a structure with a pore size smaller than the N2 used in the isotherm analysis, whereas the resulting ZE--X products have a more open structure.

Table 1 Specific surface area, SaEV (mE/g), of the supports Support BENT BENa BENPIL ZESEP SBET(m2/g) 64 87 247 70

ZEDIP 63

ZESEX 97

ZEDIX 90

The XR diffractograms for the starting montmorillonite and BENa showed the high purity of the sample. The pillaring process was confirmed by the values of the basal spacing, d(001), of the montmorillonite intercalated with the corresponding aluminium oligomeric cations. The resulting basal spacing in BENPIL reached 18 A. The XR diffractograms of the zeolitic products obtained by the alkaline treatment showed, Fig. 1, the attack on the montmorillonite structure. The characteristic peak at 19.7A ( 2 ) [3,7] was greatly reduced or disappeared. The diffractograms of the resulting products indicated a crystallized structure with crystalline minerals in a few proportion. A milder transformation was obtained in the ZE - -X samples. The XR dii~actograms show the characteristic peak of montmorillonite [7], and a significantly increased intensity in the peaks associated with what are considered impure products: feldspar and quartz. Thus a higher transformation is achieved in more severe conditions and in sea water than in their counterpart treatments. In addition, less heterogeneity is observed in the samples undergoing greater transformation. The IR spectra of the supports confirm what was observed in the XR diffractograms. The SEM photomicrographs (Fig. 2) showed the change from a lamellar structure in the original sample to a spherical granular structure in the zeolitic materials obtained with alkaline treatment. The spherical units formed are attributed to the Na + cation [8], which controls the crystalline phase, while the unit particle size depends on the characteristics of the medium used (distilled or sea water) because of the presence of ions in the latter. In most studies on the use of clays as catalysts or catalyst supports, the surface acidity of the clays has been considered a determinant of their catalytic activity [9,10]. All the supports (of lamellar and zeolitic structures) have acid centers, whose strength and number varied depending on the peculiar structure of the synthesized constituent materials [11].

502 The highest acidity was shown by the pillared clay, BENPIL. The analysis of the zeolitic materials showed ZESEX and ZEDIP to have an apparently higher number of centers than their counterparts [5].

I ZESEX

Q

S

S

.

s F

9~

s

s

s

ZEDIP

5

15

25

35

45

55

2

Fig. ].- XRD of the zeolitic products synthesized with different treatment conditions and in different media. S= sodalite, M = montmorillonite, F = Feldspar, Q = quartz.

Starting montmorillonite, BENT Montmorillonite treated in seawater, ZESEP Fig. 2.- SEM photomicrographs. 3.2. Catalyst characterization

The amount of metal retained in the different supports, when anchoring is by ion exchange, depends on the extent of the interaction between metal precursor and the support, i.e., on the number and strength of anchoring centers, the exchange capacity and the dimension of the channels in zeolites. The values of rhodium loaded (Table 2) indicate that incorporation of the organometallic cation is almost complete. The zeolitic products have a different degree of transformation depending on the treatment conditions and media employed in their synthesis, as seen in the characterization of the supports. These differences are reflected in the different metal loads. In these cases, it is necessary to take into account the sieve effect of the channels of these materials and therefore the dimensions of a cationic complex which is exchanged will be determined by its accessibility in these channels. The structure and the composition of the zeolitic

503 compounds are more important than the specific surface area, and there is no evident correlation between SBETand metal load. Table 2 also shows that the resulting Rh dispersion is not a function of the metal loading but of the structure of the support [12,13]. The nature of the support influences dispersion and therefore the size of the metallic particles. There was no correlation between metallic load and dispersion. Lamellar structures (BENa and BENPIL) incorporated larger amounts of rhodium complex but had lower dispersion than the catalysts synthesized on zeolitic products. The influence of the support structure is also reflected in the results for the zeolitic product synthesized with different treatment conditions or in different media, which determine the channel dimensions and the number of anchoring centres in the resulting samples. Higher dispersion was achieved in the more transformed samples, ZE--P, than in ZE--X, and in those synthesized in distilled water (ZEDI-) than in sea water medium (ZESE-). Table 2. Effect of the support on the catalytic activity of the heterogeneous catalysts expressed as conversion (%) and TOF (mol isopropanol, mo1-1 Rh min-1), Rh loading and estimated dispersion (D%)

T(~

Rh/BENa Conv TOF

Rh/BENPII Rh/ZEDIP Rh/ZESEP Conv. TOF Conv TOF Conv TOF

Rh/ZEDIX Conv TOF

100 6.9 10.2 4.6 6.3 14.4 9.7 10.6 9.6 5.7 5.5 90 7.5 11.0 1.5 2.6 8 5.4 6.6 5.9 3.7 3.6 80 4.7 6.9 0.7 1.3 5.3 3.5 4.1 3.7 2.2 2.2 70 2.9 4.3 0.4 0.7 3.1 2.1 2.4 2.2 1.4 1.4 60 1.7 2.6 0.2 0.3 1.7 1.1 1.4 1.3 0.8 0.7 50 1.0 1.5 0.08 0.1 1 0.7 0.7 0.7 0.4 0.4 Rh (%) 0.95 0.97 0.93 0.96 0.94 D (%) 48 37 >100" 78 74 "This data indicates that the stoichoimetryused in the titration is not correct [5].

Rh/ZESEX Conv TOF 7.6 4.4 2.6 1.5 0.9 0.5

8.9 5.2 3.1 1.7 1.0 0.6 0.92 63

3.3. Activity of catalysts 3.3.1. Influence of the support Works about catalysts with a similar content in active metals have shown the different behaviour of the catalysts depending on the characteristics of the support over which the metal is anchored, and also on the accessibility of the active centres [14]. The values of conversion decreases along with the fall in the temperature of the reaction (Table 2). The slight increase in conversion in Rh/BENa, when the temperature decreases, indicates diffusional effects, that can be attributed to the location of the active centres in the interlamellar space and not only in the external surface as in zeolites. The differences of the conversion between the catalysts prepared on zeolitic supports (higher in ZE- -P than in ZE- -X) are caused by the zeolitic structure rather than the metal loading. The difference in the results obtained in quite similar conditions (rhodium loading and metal dispersion)

504

confirms the effect of the support on activity; the interaction that exists between rhodium and the support can change the catalytic properties of the metal [15]. The decrease in the activity of the rhodium catalyst supported on the pillared clay (Rh/BENPIL) after the reaction was carried out at 100 ~ is significant. The catalytic activity was also calculated on the basis of surface sites, calculated by titration method, to obtain turnover frequencies, TOF (expressed as mol isopropanol formed mo1-1 Rh.min-1). TOF values for Rh/ZEDIP, Rh/ZESEP and Rh/ZESEX are nearly the same, although the dispersion values are different. This fact confirms that the acetone hydrogenation is insensitive to the structure of the metal, its reaction rate is proportional to the amount of exposed rhodium atoms, and is not affected by the metallic particle size or the crystallographic plane exposed. Rh supported on BENa and zeo|ites studied produced IPA with 100 % selectivity but Rh/BENPIL yielded IPA, propane and MIBK as products of the reaction. The formation of MIBK indicates the bifunctionality of the catalyst over whose active centres the formation of macromolecules that remain trapped within the interlamellar space may occur. 3.3.2. Effect of the t e m p e r a t u r e of the reaction In order to study the effect the reaction temperature has, the reaction was carried out with the same method but in the 180-80~ temperature range (the difference in temperature between consecutive steps was 20 ~ The resultant conversion and TOF show an increase in the values when the reaction took place at higher temperatures. Table 3 Selectivity (%) of the catalysts in the 180-120 ~ temperature range Rh/BENPIL Rh/ZEDIP Rh/ZEDIX Rh/ZESEX Temp. (-~ Prop. IPA MIBK Prop. IPA MIBK Prop. IPA Prop. IPA MIBK 180 73.6 24.5 1.9 0.7 98.5 0.5 0.3 99.8 0.3 99.6 0.3 160 23.0 74.6 2.3 0.2 99.8 . . . . 100 -100 -140 3.1 95.1 1.8 -100 . . . . 100 -100 -120 -100 . . . . 100 . . . . 100 -100 -Prop. = Propane The increase in temperature favoured the simultaneous paths towards IPA and MIBK formation (Table 3), except in those catalysts supported on BENa and ZESEP that yielded only IPA. The bifunctional character of the catalysts (Rh/BENPIL, Rh/ZEDIP and Rh/ZESEX) is corroborated by the MIBK formation, although in traces, in those catalysts supported in the materials which exhibited higher acidity. The overall conversion of acetone increased when the reaction temperature increased up to 180-~ but the selectivity towards IPA decreased. The results confirm that the activation energy of the condensation reaction is much higher than that of the hydrogenation reaction [16]. 3.3.3. A p p a r e n t activation energy The values of apparent activation energy, are in the order of or less than others cited in literature [14,17-20] for catalysts with other supports (oxides, activated carbon..) and more

505 traditional synthesis methods (non-organometallic via). The results indicate that the reaction is controlled chemically rather than dit~sionally [21 ]. 3.3.4. Deactivation of the catalysts The activity of rhodium catalyst samples was monitored at 100~ for 7 hours, during which period the activity declined. All the catalysts reach a constant activity after 200 minutes from the start of the reaction approximately (Fig. 3). The percentage of residual activity (percentage of the final activity versus the maximum value reached) for the catalysts supported on zeolites (Rh/ZEDIP, 71%; Rh/ZESEP, 83 %; Rh/ZEDIX, 76% and Rh/ZESEX, 57%) indicates the resistance to poisoning. The deactivation of Rh/BENPIL (36%) is relatively rapid particularly during the first hour of reaction as mentioned, and this can be related to the formation of heavy secondary products formed in the MIBK formation route [20]. These molecules remain within the pores of the pillared clay sample, and therefore they block the access of the reactant to the active centres, hence causing a decrease in the activity of the catalyst.

18 15-

.

12-

xx xx

Q o

"~

9-

> A A

0

U

6-

~

~---4...0.._0...O.0... ~

9

A

_

0 0

I

I

I

100

200

300

I

400 Time (minutes)

Fig. 3. Conversion of the catalysts at 100~ versus time: r Rh/BENa, [] Rh/BENPIL, Rh/ZEDIX, ~ Rh/ZEDIP, • Rh/ZESEP,. Rh/ZESEX. 4. CONCLUSIONS The zeolitic materials obtained by the alkaline treatment and pillaring process, produced new materials with a specific surface area higher than the raw material and also with a higher acidity.

506 The montmorillonite structure of the starting material was converted by the alkaline treatment into a mixture of zeolitic products and crystalline minerals. There was a remarkable influence of the nature of the medium (sea or distilled water) on the crystallization process. The zeolitic products synthesized in sea water showed higher crystallinity and less heterogeneity than their counterparts synthesized in distilled water. The PIL-clay and zeolitic materials synthesized are suitable as support for preparing heterogeneous catalysts by anchoring an organometallic complex by ion exchange. The structure of the materials used as supports had a great influence on the catalyst prepared. A higher metal content was achieved in the supports with lamellar structures, while better dispersion was shown by the catalysts supported on zeolitic structures. The behaviour of the catalysts in the hydrogenation of acetone, in gaseous phase and under milder conditions, shows that all the catalysts have a high activity at all the temperatures tested and comparable to more traditional metal/support systems used under stronger conditions. Activity, selectivity and durability of the new heterogeneous rhodium catalysts depend on the nature of the support used in the preparation of the catalyst. The lamellar structures mainly yield methyl isobutyl ketone (MIBK), whereas the zeolitic structures, in general, yield isopropanol (IPA). The activity remains steady throughout the period tested (7h), except when the support is the BENPIL: indeed, the deactivation of Rh/BENPIL is relatively rapid particularly during the first hour of reaction. The turnover numbers frequencies (TOF) obtained with these catalysts might be attributed to the structure of the metal no sensitive reaction the hydrogenation acetone. ACKNOWLEDGMENTS

To the DGESIC (Project: PB98-1107) and CICYT (Project: MAT1999 1093-C02-02) for financial support of this work. REFERENCES

1. J. Weitkamp and L. Puppe (eds.), Catalysis and Zeolites. Fundamentals and Applications, Springers Verlag, Berlin, 1999. 2. F.Gonzfilez, C. Pesquera, C. Blanco, I.Benito and S. Mendioroz, Inorg. Chem., 31 (1992) 727. 3. R. Ruiz, C. Pesquera, F. Gonzfilez and C. Blanco, In Studies in Surface Science and Catalysis. Zeolites and mesoporous materials at the dawn of the 21 st century, A. Galarneau, F. Di Renzo, F. Fajula, J. Vedrine (eds.), Elsevier, Amsterdam, 2001. 4. J.T. Richardson, Principles of Catalysis Development, Plenum Press, New York, 1989. 5. C. Blanco, R. Ruiz, C. Pesquera and F. Gonzfilez, Appl. Organomet. Chem., 16 (2002) 1. 6. J.E. Benson and M. Boudart, J. Catal., 4 (1965) 704. 7. I.Th. Rosenquist, Nor.Geol.Tidsskr., 39 (1959) 350. 8. D. Kall6 and Kh. M. Minachev (eds.), Catalysis on zeolites, Akademia Kiad6, Budapest, 1988. 9. C. Blanco, J. Herrero, S. Mendioroz and J.A. Pajares, Clays Clay Miner., 36 (1988) 364.

507 10. J.L.G. Fierro, Chemisorption of Probe Molecules. In Spectroscopy Characterization of Heterogeneous Catalysts. Part B, J.L.G. Fierro (ed), Elsevier, Amsterdam, 1990. 11. A. Corma, J. P6rez- Pariente, V. Fomes and A. Mifsud, Clay Miner., 19 (1984) 673. 12. S. Fuentes and F. Figueras, J. Catal., 61 (1980) 443. 13. B.Coq and F. Figueras, J. Mol. Catal., 40 (1987) 93. 14. B. Sen and M.A.Vannice, J. Catal., 113 (1988) 52. 15. G. Del Angel, B.R. Coq, R. Dutartre and F. Figueras, J. Catal., 87 (1894) 27. 16.Y.Z. Chen, C. M. Hwang and C.W. Liaw, Appl. Catal., 169 (1998) 207. 17. F. Rositani, S. Glavagno, Z. Poltarzewski, P. Staiti and P.L. Antonucci, J. Chem. Tech. Biotechnol., 35 (1985) 234. 18. L. Melo, A. Llanos, L. Garcia, P. Magnoux, F. Alvarez, M. Guisnet and G. Giannetto, Catal. Letters, 51 (1998) 51. 19. A.M. Fuente, G. Pulgar, F. GortzAlez, C. Pesquera and C. Blanco, Appl. Catal. A: Gen., 208 (2001) 35. 20. L.M. Gandia, A. Diaz and M. Montes, J. Catal., 157 (1995) 461. 21. O. Levenspiel, Ingenieria de las reacciones quimicas, Ed. Revert6, Barcelona, 1979.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

509

The use of the sol-gel technique to prepare TiO2-AI203 binary system over a w i d e r a n g e of T i - A i ratios A. Yu. Stakheev*, G. N. Baeva, N. S. Telegina, I. V. Mishin, T. R. Brueva, G. I. Kapustin, and L. M. Kustov Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991, Leninsky Prosp. 47, Moscow, Russia Series of TiO2-A1203 mixed oxides with different TiO2 contents (2.5-97 mol %) was prepared by the sol-gel method and studied by XRD, XPS, and N2 physisorption. At low Ti contents, Ti ions tend to incorporate into the A1203 lattice thus increasing the specific surface area. With an increase in the Ti content ( 50 mol %), the TiO2 crystalline phase starts to form. A1203 segregates at the surface of TiO2 clusters, thereby preventing their agglomeration. 1. INTRODUCTION Different types of alumina are the most widely used materials in heterogeneous catalysis. A1203 is used as a carrier for metal and oxide supported catalysts since it is able to stabilize a number of active phases in a dispersed state. Moreover, alumina bears relatively strong Lewis acid centers and demonstrates intrinsic activity in some reactions. However, in many cases, the use of TiO2 as a carrier makes it possible to achieve better catalytic activity, for example, in the case of a vanadium catalyst for selective reduction of NOx with NH3 [1] or hydrotreating catalysts [2]. The main disadvantage of titania is a lower surface area and a higher cost compared to alumina. Titanias with a high surface area are also less stable at higher temperatures. A possible approach to combine the advantages of both solids is the preparation of mixed oxide systems. Therefore, several studies were devoted recently to the synthesis of TiO2-AI203 mixed oxides with different TiO2/A1203 ratios [3, 4]. Particular attention was paid to the sol-gel method [5, 6], since this method allows a good control of the composition and characteristics of the resulting material [7, 8]. It was found that modification of TiO2 with A1203 by the sol-gel method appears to be an effective method for enhancement of the surface area and of the thermal stability [6, 9]. Therefore, the main objective of this work was to reveal the nature of the interaction between TiO2 and A1203 in the mixed oxide system prepared by the sol-gel method. We prepared a series of TiO2-A1203 mixed oxide systems by the sol-gel method, while varying TiO2/Al203 in a wide range, and studied the structure of these materials as a function of the TiO2 content by combination of X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and N2 physisorption (BET method). corresponding author, e-mail: [email protected]

510 2. EXPERIMENTAL 2.1. TiO2-A1203 p r e p a r a t i o n Preparation of TiO2-A1203 mixed oxide compositions with different molar percentages of TiO2 (mol % TiO2 = TiO2/(TiO2+A1203) x 100 ) was carried out by the "sol-gel" method by mixing of A1- and Ti-hydroxide sols prepared beforehand. The mixture of A1- and Ti-hydroxide sols was dried on a rotary evaporator at 60~ and the product was dried overnight at 120~ and calcined at 500~ for 2 h in flowing 02. The Al-hydroxide sol was prepared by a standard procedure proposed by Yoldas [10]. The Ti-hydroxide sol was prepared by dissolving Ti isopropoxide in isopropanol followed by hydrolysis carried out by dropwise addition of the required amount of the isopropanol-H20-HNO3 mixture. The ratio H+i-rNo3/Tiwas 0.05 and the ratio H20/Ti was 4. 2.2. C h a r a c t e r i z a t i o n

techniques

Specific surface areas of the synthesized TiO2-A1203 samples was estimated by nitrogen physisorption at 77 K using the BET method. XRD measurements were performed with a DRON-2 diffractometer using the CuKct radiation with a Ni filter and scanning over a range of 20=6-60 ~ with a rate of 2 ~ per minute. The catalysts were characterized by X-ray photoelectron spectroscopy using an XSAM-800 spectrometer (Kratos) with AI Kctl,2 radiation for spectra excitation. Spectra in the O ls, Ti 2p, and A1 2p regions were collected. The C Is binding energy (BE) at 285.0 eV was taken as a reference. The spectra of the Ti 2p3/2 line was analyzed by a conventional peak synthesis procedure using Gaussian functions. 3. RESULTS AND DISCUSSION 3.I. Dependence

of the specific surface area on the Ti02 content

Variation of the specific surface area of TiO2-A1203 mixed oxides as a function of the 400 TiO2 content is presented in Fig. 1. For the samples containing a low content of titania (mol % 3OO TiO2 < 20%) a clear tendency of increasing the SBETvalue with E 200 respect to that of pure alumina I ~ i ".i obtained by the sol-gel method O9 was observed. S~ET for the 100 samples containing 9-15 mol % . I . i j i v,, ................ 1................. r ................. ................. '.......... '- ..... I TiO2 is ca. 330-350 m2/g, while SBET of plain A1203 does not 0 20 40 60 80 100 exceed 300 m2/g. However, mol % Ti02 when the TiO2 content exceeds 50 mol %, the specific surface Fig. 1. Dependence of the specific surface area (BET) area decreases steadily. of TiO2-A1203 mixed oxides on the TiO2 content.

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

r

m

511 The observed tendency of increasing SBETfor the samples containing a relatively low amount of TiO2 is in a good agreement with the results reported by Klimova [ 11] and Linacero [6]. Noteworthy that the similar trend toward an increase in SBETwas observed upon modification of A1203 with a minor amount of ZrO2 followed by a decline in the surface area at higher ZrO2 contents [12]. The phenomenon of the enhancement of the surface area by doping SiO2 with a minor amount of other cations was studied by Bondar at al. [13, 14]. It was attributed to a distortion of the SiO2 structure by incorporation of modifying cations. The overall tendency of a decrease in the specific surface area for the samples containing a high concentration of TiO2 (mol % TiO2 > 50) is also in agreement with the data reported by Klimova, Linacero, and Toba [4, 6, 11]. 3.2. X-Ray Powder Diffraction

The X-ray powder diffraction patterns are shown in Fig. 2. The X-ray diffraction pattern of alumina precipitated from gel shows two broad maxima at 20=46 and 67 ~ characteristic of ~'-A1203. Introduction of small amounts of titania markedly reduces the intensity of these peaks. Accordingly, the sample with 15% appears to be virtually amorphous to X-rays. This result is in agreement with the data of Ramirez [15]. Amorphisation of A1203by TiO2 also explains the increase in SBET(see above).

I

9- anatase o- rutile

o

~1 ~ ~..,__ ]l il

0,

,

Mech. mixture: 50 mol % TiO 2

o

"

A

~-"~" ~___

._^

+ AI203

,~

85 mol % T i O 2

~~'f

~'~~..,,,.,.~.,....,_~~

50 mol O/oTiO2

~,~

' " ~ ~ ~ _ , ~

15 mol % TiO 2

I

20

30

40

A 50

60

70

80

20, ~

Fig. 2. X-ray diffraction pattems of the samples with different TiO2 contents. The data for mechanical mixture of bulk TiO2 and sol-gel A1203 are given for comparison

512 Appearance of crystalline TiO2 phase was observed only when TiO2 content reaches 50 mol %. Diffraction pattern of this sample exhibits broad maxima indicating the presence of minor amount anatase phase. The crystallite size of titania in the 50%TiO2-A1203 sample estimated from the Scherrer equation shows values around 100,~. No rutile phase was detected in the sample. This picture is essentially different from the pattern yielded by an equimolar mixture of TiO2-AI203 prepared by mechanically mixing parent titania and alumina. Diffraction pattern of an equimolar mixture TiOa-A1203 precipitated from gel The mechanical mixture reveals sharp diffraction peaks due to anatase and rutile. The diffractogram of the sample with 85% of TiO2 shows sharper peaks then the 50% TiO2 sample. An increase of the titania content from 50 to 85% considerably increases the crystallite size of titania. However, anatase is nearly the only phase in this sample. The estimation of rutile fraction from the intensities of the [101] and [110] reflection planes for anatase (Ig) and rutile (IR) respectively by applying the equation XR=1/[1+1.26 (IA/ IR )] gives a proportion of 55 and 5% respectively, since the fraction of rutile does not exceed 3-5% in the high-titania samples. The results obtained by XRD are in accordance with the results reported by Linacero [6] and Ramirez and Gutierrez-Alejandre [15] for A1203-TiO2 mixed oxides with different TiO2 content. These authors also reported a formation of TiO2 crystalline phases when TiO2 content exceeds ~ 70 mol %. The samples with lower TiO2 contents were found to be x-ray amorphous. 3.3. Study the surface composition by XPS Surface concentrations o f Ti a n d Al. In order to evaluate deviations in the surface concentrations of Al and Ti from the overall content, the coefficients of surface segregation (/5) were calculated by the method similar to that proposed by Seach [16]:

[JTi =

XPS

bulk

where ~Ti and ~Al are the coefficients of the surface segregation of Ti and AI, respectively (Ti/A1)xps and (Al/Ti)xps

are the element atomic ratios on the surface calculated from XPS data;

(Ti/Al)xps and (Al/Ti),,ps

are the overall (bulk) element atomic ratios determined by chemical analysis

513 Variations of 15~iand 15A1with the TiO2 content are displayed in Fig. 3. Evidently, titania does not show a pronounced tendency to segregation. In the samples with a low Ti content, the surface composition remains essentially the same as the bulk composition. Therefore, the coefficients of surface segregation [~Ti and I~A~are close to 0. We can conclude that at low TiO2 contents, Ti ions are homogeneously distributed in TiO2-AI203 like in a solid 4 solution. _

3 ................. !~ ......' ~........."..................."1"

"t2 a7 ~

'~2

.....................................

0"1 I . . . . '1~'I .... ~/ ............... . . . . . . . . ~J.............. [ ". .D. ." ~ ~, ............... . ; ; . . . ~ ":i~. .............. ,. t -

~

0

l

!

i

l

!

20

40

60

80

100

-1

tool % TiO2 Fig. 3. Dependence of the coefficients of surface segregation of Ti and A1 on the TiO2 content.

Unlike Ti, A1 demonstrates very pronounced tendency to segregate on the surface of mixed oxide. This tendency becomes particularly distinct with decreasing A1 content. Thus, for the sample containing 15 and 3 mol % A1203 (the TiO2 content is 85 and 97 mol %, respectively), the A1/Ti surface atomic ratio exceeds the bulk value by a factor of 5-6 ([~Al "" 4-5).

We can conclude that TiO2 tends to interact with A1203 at low TiO2 contents, which results in homogeneous distribution of Ti. A 1 2 0 3 shows a tendency to segregate on the surface of T i O 2 species. This tendency is particularly pronounced at low A1203 contents.

Chemical states of O, Ti, and A1. XPS spectra in the O ls, Ti 2p, and A1 2p regions for the samples with different molar contents of TiO2 are shown in Fig. 4. Analysis of variations and XPS line positions and shapes allow us to reveal several evident relationships. 0 ls region. For the samples containing 2.5-15 mol % TiO2, the binding energy of the 0 Is peak is - 531.5 eV, the peak is symmetrical, which is typical for plain A 1 2 0 3 . The position and lineshape of the O ls peak remains essentially unaltered even for the sample containing 50 mol % TiO2. However, the furher increase in the TiO2 content up to 85 mol % leads to broadening of the O ls peak and the peak shifts toward higher binding energies by -~ 1.5 eV. Curve fitting analysis reveals two peaks: one at-~ 531.7 eV (characteristic of A1203) and another one at -~ 530.3 eV with pronounced asymmetry, which is typical of TiO2. Note that the appearance of the second peak is in agreement with XRD data showing the formation of the TiO2 phase in this sample. Ti 2p region. Analysis of the Ti 2p peak allows us to reveal two components at-~ 458.5 and 460.5 eV in the samples containing 2.5-5 mol % TiO2. The component at 458.5 eV can be assigned to the Ti ions in the octahedral coordination typical for TiO2. The component at 460.5 eV implies the presence of lower coordinated Ti ions, presumably in

514 the tetrahedral coordination [17]. With increasing Ti content, the intensity of the signal of octahedral Ti increases. This peak predominates in the spectra and the signal of tetrahedral Ti becomes invisible. Presumably, the signal of tetrahedral Ti points to certain incorporation of Ti ions into the A1203 lattice and occupation of some tetrahedral positions. Probably, this process is not pronounced due to the difference in the A1 and Ti radii, and at higher TiO2 contents, the octahedral coordination predominates, which points to the preferable formation ofTiO~ species.

AI 2p region. The position and shape of the A12p line remain the same in the whole range of TiO2 contents. We can tentatively conclude that even for the samples with a low A1 content, A1 tends to form separate A1203 species without marked interaction with the TiO~ phase. Pronounced segregation of A1 on the surface (see above, Fig. 3) indicates that A1203 species are located on the surface of TiO2 particles. 4. CONCLUSIONS The data obtained allow us to propose the following mechanistic scheme describing oxide-oxide interaction in the TiO2-A1203 system. At low TiO2 contents (0-15 mol % TiO2), there is a pronounced interaction between two oxides resulting in the formation of AI-Ti mixed oxide. In this oxide, a part of Ti ions appears to occupy tetrahedral positions of the A1203 lattice. Incorporation of Ti ions lowers the crystallinity of A1203, which, in turn, results in an increase in the specific surface area of the mixed oxide. As a result of Ti-A1 interaction, formation of the TiO2 phase is negligible and Ti is uniformly distributed in the oxide particles. At higher TiO2 contents up to 50 mol %, the TiO2 phase begins to form. However, A1203 appears to segregate on the surface of TiO2 and encapsulate TiO2 clusters thus preventing their agglomeration. Therefore, the size of TiO2 clusters remains relatively small (- 100 A) and SBET does not decrease significantly. Upon the following increase in the TiO2 content up to 85 mol %, formation of a bulk TiO2 phase takes place, which is accompanied by an abrupt decline in SBET.

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516 REFERENCES

I 2. 3. 4. 5.

o

10. 11. 12 13. 14. 15. 16. 17.

H. Bosh and F. Janssen, Catal. Today 2 (1988) 369. J. Ramirez, S. Fuentes and G. Diaz, Appl. Catal. 52 (1989) 211. J. Ramirez, L. Ruiz-Ramirez and L. Cedeno, Appl. Catal. A 93 (1993) 163. M. Toba, F. Mizukami and S. Niwa, J. Mater. Chem., 4 (1994) 585. A. Gutierrez-Alejandre, M. Gonzalez-Cruz and M. Trombetta, Microporous Mesoporous Mater., 23 (1998) 265. R. Linacero, M. L. Rojas-Cervantes and J. de D. Lopez-Gonzalez, J. Mater. Sci., 35 (2000) 3279. T. Lopez, A. Romero and R. Gomez, J. Non-Cryst. Solids 127 (1991) 105. A. Z. Khan and R. Ruckenstein, Appl. Catal. 90 (1992) 199. R. Linacero, M. L. Rojas-Cervantes and J. de D. Lopez-Gonzalez, J. Mater. Sci., 35 (2000) 3269. B.E. Yoldas, Am. Ceram. Soc. Bull., 54 (1975) 286 T. Klimova, Y. Huerta and M. L. Rojas-Cervantes, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 91 (1995) 411. T. Klimova, M. L. Rojas-Cervantes and P. Castillo, Microporous Mesoporous Mater., 20 (1998) 293. L.M. Kustov, L.A. Bondar and V.Yu. Borovkov, Kinet. Katal., 27(6) (1986) 1392. I. E. Neimark, Synthetic Mineral Adsorbents and Catalyst Carriers (Russ.), Kiev, Naukova Dumka, 1982. J. Ramirez and A. Gutierrea-Alejandre, J. Catal., 170 (1997) 108. M. P. Seaeh, J. Catal., 57 (1979) 450. A. Yu. Stakheev, E. S. Shpiro and J. Apiok, J. Phys. Chem., 97 (1993) 5668.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

517

Catalytic performance in the complete acetone oxidation of manganese and cobalt oxides supported on alumina and silica A. Gila, S.A. Korili a, M.A. Vicente b and L.M. Gandia a aDepartamento de Quimica Aplicada. Edificio Los Acebos, Universidad Pfiblica de Navarra. Campus de Arrosadia, s/n. E-31006 Pamplona. Spain bDepartamento de Quimica Inorgfinica. Facultad de Ciencias Quimicas, Universidad de Salamanca. Plaza de la Merced, s/n. E-37008 Salamanca. Spain A series of 7-alumina- and silica-supported oxides as Mn203, C0304 and Sm-Mn, Sm-Co bimetallic combinations was prepared by incipient wetness impregnation of the supports with aqueous solutions containing the nitrates of the metals and citric acid. The results obtained with these catalysts in the acetone complete oxidation reaction have shown a predominant effect of the support and the transition metal nature. The silica-supported oxides exhibited a bulk-like behaviour with relatively little difference between the Mn and Co catalysts. In contrast, when supported on alumina the performance of the Mn catalysts was remarkably better than that of the Co-based ones. In this case, whereas the Sm-Mn catalysts maintained a rather good performance relative to that of Mn/AI203, the temperatures required for acetone combustion over the Sm-Co catalysts were significantly lower than the ones over C0/A1203. 1. I N T R O D U C T I O N Catalytic combustion processes are gaining growing attention due to their potentiality for both waste gas emissions prevention and end-of-pipe control. Indeed, low-temperature catalytic combustion of lean fuel-air mixtures in gas turbines represents an efficient method for energy production with minimum formation of unburned hydrocarbons, CO and NO~ [1, 2]. On the other hand, catalytic combustion for controlling Volatile Organic Compounds (VOCs) and odour emissions is a well-established technology that is being used in a variety of industrial applications [3, 4]. In this process, very high destructive efficiencies can be achieved at relatively low operating temperatures resulting in considerable environmental and economic benefits in comparison to the more conventional thermal combustion.

Financial support by the Ministry of Science and Technology (MAT2000-0985) and the Department of Education and Culture of the Navarre Government (Orden Foral 143/1998) is gratefullyacknowledged.

518 A suitable combustion catalyst has to fulfil several strict requirements for a successful commercial application [1]. Very high catalytic activity is a prerequisite in order to initiate and ensure the combustion of lean fuel-air mixtures or air streams containing very low VOCs concentrations (below 10,000 ppmv) at temperatures as low as possible and high space velocities. Moreover, as the operating conditions in catalytic burners are usually very severe, it is crucial to attain an adequate balance between the catalyst activity and its long-term durability based on properties such as good thermal and mechanical stability and resistance to poisoning, material vaporisation, phase transformation and specially sintering [5]. It is well known that supported noble metals (typically Pd and Pt) and transition metal oxides (mainly oxides of Co, Cr, Cu, Mn and Ni) are active and selective catalysts for the complete oxidation reactions involved in catalytic combustion [1, 3]. Both categories have advantages and drawbacks and, as far as the complete oxidation of VOCs is concerned, the choice of the good catalyst to be used may be somewhat problematic since it depends in great part on the detailed composition of the waste gas stream [6]. In this connection, there seems to be wide agreement that noble metal-based combustion catalysts are among the most active ones at low reaction temperatures and, as a matter of fact, they are widely used for the abatement of VOCs emissions. However, it has been claimed that metal oxides are not necessarily less active than noble metal catalysts [7], which can be particularly true for the combustion of saturated hydrocarbons and stable oxygenate compounds, like acetates. Nevertheless, single transition metal oxides lack mechanical strength, and in some cases they are considerably volatile and have very low sintering resistance at moderate operating temperatures [5]. In recent years much attention has been paid to various mixed oxides like rare earth transition metal oxide perovskites [8-12] and transition metal-substituted hexaaluminates [13] as promising combustion catalysts mainly due to their excellent thermal stability properties. Properly prepared perovskite-type oxides have been found suitable catalysts for the complete oxidation of VOCs showing catalytic performances comparable to that of the noble metals [9-11]. Further, these mixed oxides are stable in a wide range of temperatures under wet atmosphere and show good resistance to poisoning by sulphur compounds [2]. The perovskite oxides have the general formula ABO3 where A and B are usually rare earth and transition metal cations, respectively. The high stability of their structure allows the accommodation of structural defects as anionic or cationic vacancies and a widely variable oxygen nonstoichiometry, but still preserving the perovskite structure. This fact is of particular interest because it has been found that the catalytic behaviour of the perovskites in oxidation reactions is controlled by the nonstoichiometric character of the surface [12]. In a similar way as it happens when considering the single oxides of the fourth period transition metals [1], the complete oxidation of VOCs over perovskites exhibits a characteristic activity pattern the oxides containing Co, Mn and Ni at B position being the most active ones [8-10]. In this work, we present a comparative study of the catalytic performance of a series of 7alumina- and silica-supported oxides as Mn203, Co304 and Sm-Mn, Sm-Co bimetallic

519 combinations in the complete oxidation of acetone, which is a representative oxygenate VOC. We extend here our previous study [14] concerning unsupported single (Mn203, Co304) and perovskite-type (SmlV[nO3, SmCoO3) transition metal oxides to the use of these active phases deposited on supports commonly used for the preparation of VOCs combustion catalysts. 2. E X P E R I M E N T A L

2.1. Catalysts preparation The catalysts were prepared by incipient wetness impregnation of the ?-AlaO3 (Spheralite 505, Procatalyse) and SiOa (AF125, Kali Chemic) supports (200-300 l.tm size fraction) previously calcined for 16 h at 1073 K. With the aim of obtaining a good dispersion of the perovskites on the supports, the bimetallic Sm-Mn and Sm-Co catalysts were prepared according to an adaptation of the procedure proposed by Zhang et al. [15]. This method applies the well-known amorphous citrate process for synthesising bulk metal oxides to the preparation of supported perovskites. To this end, aqueous impregnation solutions were prepared at a samarium / transition metal molar ratio of 1 by first dissolving suitable amounts of the transition metal nitrates (Mn(NOa)z'4HzO, Merck, PA and Co(NOa)a'6HaO, Panreac, PA) and Sm203 (Sigma 99.9%) previously treated with the stoichiometric quantity of HNO3 (Panreac 60%, PA). Next, citric acid (Panreac, PA) was added in order to obtain a total metal / citric acid equivalent ratio of 1 and the resulting solutions were finally left to boil for 30 min. The required volume of the impregnating solutions was slowly added to the supports to give catalysts with a final transition metal content of about 6 wt.%. Then, the solids were dried for 12 h at 343 K under reduced pressure (5-103 Pa) and calcined in a muffle for 4 h at 823 K. A second series of supported bimetallic catalysts was obtained by further calcining a separate portion of these solids for 4 h at 1073 K. The catalysts based on the single Mn and Co oxides were prepared in a similar way to that above-mentioned for the preparation of the supported bimetallic combinations. In this case, the aqueous impregnation solutions were prepared at a transition metal / citric acid equivalent ratio of 1 and left to boil for 30 rain. After impregnation of the supports in order to obtain catalysts with a final transition metal content of about 6 wt.% and drying as described above for the bimetallic catalysts, the solids were calcined in a muffle for 4 h at 823 K. The catalysts are designated hereafter as SmM/S-T (bimetallic combinations) or M/S-T (single oxides), where M, S and T refer to the used transition metal (Mn or Co), support (A1 for ?-AlzO3 or Si for SiOz) and calcination temperature (823 or 1073 K), respectively.

2.2. Characterisation techniques Physicochemical characterisation of the catalysts included nitrogen adsorption, X-ray diffraction (XRD) and chemical analyses. Textural analyses were carried out from the corresponding Nz adsorption-desorption isotherms at 77 K using a static automatic volumetric

520

apparatus (Micromeritics ASAP 2010 adsorption analyser). Prior to analysis the samples were degassed for 6 h at 473 K. X-ray powder diffraction patterns were obtained by using a Siemens D-500 diffractometer and employing nickel-filtered Cu K a radiation. In the case of the single oxide catalysts, the mean crystallite diameters of the supported oxides were estimated from application of the Scherrer equation. XRD line broadening (XRDLB) measurements were carried out from the width at half-maximum of the a-MnzO3 (222) (33.0 ~ in 20) and Co304 (311) (36.9 ~ in 20) peaks, both corrected for instrumental broadening. The manganese and cobalt contents were determined by atomic absorption spectroscopy using a Perkin-Elmer 2100 spectrophotometer. The samples subjected to analysis were first treated by hot aqua regia and concentrated

HzSO4 digestion.

2.3. Catalytic performance evaluation The acetone combustion reaction was carried out in a tubular (8 mm i.d.) fixed-bed Pyrex glass reactor at atmospheric pressure. The catalyst was diluted with inert solids (100-200 ~tm Pyrex glass beads) in a volume ratio of 1:4 approximately, forming a catalytic bed of about 1 cm of depth. A thermocouple placed inside the reactor, in the centre of the catalyst bed monitored the reaction temperature. Mass flow controllers (Bronkhorst) monitored and controlled the flow of gases used to obtain the feed mixture and to pretreat the catalyst. An air stream saturated with acetone (Panreac, PA) was created using a saturator equipped with temperature and pressure control, and then diluted with pure air (Air Liquide, 99.999%), resulting in a 600 ppmv acetone concentration in the reactor feed. Prior to each experiment the catalyst was treated under 100 cm 3 min -~ (STP) of pure air (Air Liquide, 99.999%) for 1 h at 573 K. Subsequently, the catalyst was cooled to 413 K, and prior to obtaining the light-off curves for acetone combustion, the catalytic bed was stabilised for 3 h at 413 K under the feed stream. The temperature was then raised at a controlled heating rate of 2.5 K min -~ until complete acetone combustion was obtained. The lightoff tests were performed at a W/Fi, ratio of 1.9 g min mmol-lacetonebased on the amount of transition metal (Mn or Co) loaded in the reactor, and gas hourly space velocity (GHSV) of about 34,000 h 1 (STP) based on the total catalytic bed volume. On-line analysis of the product stream was performed on a Hewlett Packard 6890 gas chromatograph equipped with a 6 ft HayeSep Q column connected to a TCD for COz determination, and an HP-INNOWax 30 m x 0.32 mm i.d. column connected to an FID for acetone analysis. 3. RESULTS AND DISCUSSION The results of the physicochemical characterisation of the catalysts, consisting of their specific BET surface area (SBET), total pore volume (Vp), transition metal content and, in the case of the single oxide catalysts the mean a-MnzO3 or C0304 crystallite diameter (dxRo), are summarised in Table 1. As expected from the relatively high metal oxide content, the prepared catalysts have a

521 specific surface area and total pore volume significantly lower than those of the supports. This effect is more pronounced for the Sm-Mn and Sm-Co combinations than for the single oxide catalysts, which is also in accordance with the higher total metal oxide content of the bimetallic samples. After calcination at 823 K, the presence in the single metal oxide catalysts of a-MnzO3 and Co304 has been detected by XRD, which is consistent with what has been found for other supported Mn [16, 17] and Co [18] catalysts of comparable metal loading and calcination temperature. The mean crystallite diameters estimated from XRDLB measurements for both aMn203 and Co304 are significantly lower than the values found when the catalysts are prepared by pore volume impregnation of the supports with aqueous solutions containing only the nitrates of the transition metals [19, 20]. This suggests that the presence of citric acid in the impregnating solution can help to prepare catalysts with a relatively well dispersed metal oxide phase on the support surface. With regard to the bimetallic catalysts, the XRD patterns of the samples calcined at 823 K do not show any diffraction peak corresponding to crystalline Mn or Co oxides. Only in the case of the SiOz-supported catalysts, the presence of an amorphous phase can be detected for both Sm-Mn and Sm-Co combinations. Table 1 Physicochemical properties and T50 values for acetone combustion of the prepared catalysts Sample ),-AlzO3 Mn/AI-823 SmMn/AI-823 Co/AI-823 SmCo/A1-823 SiOz Mn/Si-823 SmMn/Si- 823 Co/Si-823 SmCo/Si-823

SBET (m z g-l)

Vp (cm 3 g-l)

185 88 66 79 63 304 269 231 275 204

0.426 0.349 0.238 0.126 0.198 0.804 0.674 0.518 0.698 0.499

Mn or Co content (wt. %)

dXRD (nm)

Ts0 (K)

5.69 5.10 7.31 6.62

15

483 488 605 535

6.50 5.03 7.53 6.92

12

8

6

520 520 513 543

The light-off curves for acetone combustion over the catalysts based on the single Mn and Co oxides are presented in Fig. 1, and the corresponding values of the temperature Ts0 at which the acetone conversion reaches 50% are included in Table 1. It is clear from these results that the behaviour of the alumina-supported single metal oxides contrasts strongly with that of the silicasupported ones. The performance of Mn/Si-823 is very similar to the one of Co/Si-823 in accordance with the tendency of the respective unsupported Mn and Co oxides for this reaction

522

[14], which suggests a bulk-like behaviour of the ct-Mn203 and silica.

Co304

particles supported on

The results obtained with Mn/A1-823 and Co/A1-823 catalysts are worth mentioning since these samples present, respectively, the lowest and the highest Ts0 values for acetone combustion among all the catalysts considered in this work. It is well-known that a variety of surface and subsurface Co-AI phases with spinel structure may be formed from C O 3 0 4 crystallites in aluminasupported cobalt catalysts [18]. This is because of the solid-state diffusion of Co z§ and AI3§ ions readily taking place at temperatures above ca. 800 K, resulting in a very low C o 3 0 4 content even for high cobalt loaded catalysts. The Co-A1 mixed oxides with spinel structure are considerably less reducible than Co304 [19] and, as a result, almost inactive in combustion reactions [12, 21]. Therefore, it may be suggested that the behaviour showed by the Co/AI-823 catalyst be determined in part by the possible presence of poorly active cobalt aluminates. In contrast, the interaction between manganese and ]t-A1203 is weak compared to the case of cobalt [16], the result being the formation of bulk manganese oxides even at low metal loading [16, 17]. This feature of the Mn/AI203 systems can contribute to the very good performance of the Mn/AI-823 catalyst, which attains complete acetone conversion at the relatively low temperature of 515 K under the reaction conditions used. In that connection, and taking into consideration previous results on acetone combustion over supported Mn203 catalysts [20], and the reactivity of ~/-A1203 towards acetone [22], a possible cooperative effect between Mn203 and specific sites on the support may be also suggested. Finally, the Ts0 value for acetone combustion over Mn/AI-823 is about 50 K lower than the one required under the same reaction conditions over a catalysts prepared from Mn nitrate precursor [20].

1.00 o *" o

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Temperature (K) Fig. 1. Light-off curves for acetone combustion over the single oxide catalysts. The light-off curves for acetone combustion over the series of catalysts based on the Sm-Mn and Sm-Co bimetallic combinations are presented in Figs. 2 and 3, respectively. The results obtained with the Sm-Mn samples calcined at 823 K are very similar to the ones attained with the

523

Mn203 catalysts. Again, the use of ~{-A1203 as support improves the performance of the manganese oxide based catalysts relative to the use of silica, resulting in a reduction of the Ts0 value for acetone combustion by about 30 K. Despite the fact that from the XRD results the SmMn catalysts calcined at 823 K do not contain a-Mn203, at least as a well-crystallised phase, it seems that manganese is in a similar chemical environment in both, single and mixed, series of oxide catalysts. When calcined at higher temperature (1073 K), the Ts0 value of the silicasupported Sm-Mn catalyst increases dramatically, something that points to an excessive sintering of the supported oxides. In contrast, the SmMn/AI-1073 catalyst maintains a rather good catalytic performance. A good resistance to sintering due to an effective anchorage of the Mn oxides to the alumina surface, the stabilisation of the A1203 lattice by Sm ions inhibiting the formation of Mn-AI spinels and SmMnO3 formation would be among the several factors that can account for this result.

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700

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550

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600

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Fig. 2. Light-off curves for acetone

Fig. 3. Light-off curves for acetone

combustion over the Sm-Mn catalysts.

combustion over the Sm-Co catalysts.

When compared to the case of the Co/SiO2 catalysts, it can be seen that whereas over SmCo/Si-823 the temperatures required for total acetone combustion increase slightly, over the SmCo/Si-1073 catalyst these temperatures increase by more than 100 K. These results are somewhat similar to the ones obtained with the series of silica-supported manganese oxide catalysts. Despite the possible presence of SmCoO3 in the catalyst calcined at 1073 K and the good catalytic performance of this perovskite in the acetone combustion [14], it seems likely that the behaviour of the SmCo/Si-1073 catalyst be mainly due to a poor dispersion of the supported metal oxides. This point is of particular importance for the perovskite oxides because the severe sintering taking place at temperatures above 973 K not only reduces the exposed surface area but also the density of surface defects active in oxidation reactions [23]. An interesting result from this study is the remarkable improvement of the Co304/A1203 catalyst performance achieved with the SmCo/Al-823 sample. Indeed, the alumina-supported Sin-

524 Co bimetallic combination shows a Ts0 value for acetone combustion that is about 70 K lower than the one of the Co/AI-823 catalyst. It is well-known that the reaction of perovskite components with the support to form stable and fairly inactive aluminates is a serious problem when preparing perovskite catalysts (specially those containing Co) supported on alumina-based oxides. The precoating of alumina with oxides of the lanthanide elements in order to stabilise the support is an effective procedure in preventing this problem [15, 24]. In our case, such a stabilisation of the alumina support prior to the preparation of the catalysts has not been carried out. However, this could have been done in part on calcining by the samarium ions deposited by impregnation, but with the purpose of perovskite formation. If so, Co would be in excess relative to Sm and be able to form accessible cobalt oxides on the surface of the Sm-stabilised alumina, contrary to the case of the Co304/AIzO3 catalyst for which the formation of poorly active cobalt aluminate would be predominant, as noted above. REFERENCES

1. M.F.M. Zwinkels, S.G. J~ir~s, P.G. Menon and T.A. Griffin, Catal. Rev.-Sci. Eng., 35 (1993) 319. 2. D. Klvana, J. Chaouki, C. Guy and J. KirchnerovL Combust. Sci. Tech., 121 (1996) 51. 3. J.J. Spivey in G.C. Bond and G. Webb (Senior Reporters), Complete Oxidation of Volatile Organics, Catalysis, Vol. 8, The Royal Society of Chemistry, Cambridge, 1989, p. 157. 4. S. Vigneron, J. Hermia and J. Chaouki (eds.), Characterization and Control of Odours and VOC in the Process Industries, Elsevier, Amsterdam, 1994, p. 263. 5. P.O. Thevenin, A.G. Ersson, H.M.J. Kusar, P.G. Menon and S.G. J~ir~s. Appl. Catal. A, 212 (2001) 189. 6. J. Hermia and S. Vigneron, Catal. Today, 17 (1993) 349. 7. C. Lahousse, A. Bernier, P. Grange, B. Delmon, P. Papaefthimiou, T. Ioannides and X. Verykios, J. Catal., 178 (1998) 214. 8. L.G. Tejuca, J.L.G. Fierro and J.M.D. Tasc6n, Adv. Catal, 36 (1989) 237. 9. N. Yamazoe and Y. Teraoka, Catal. Today, 8 (1990) 175. 10. T. Seiyama, Catal. Rev.-Sci. Eng., 34 (1992) 281. 11. A. Musiliak-Piotrowska and K. Syczewska, Catal. Today, 59 (2000) 269. 12. M.A. Pefia and J.L.G. Fierro, Chem. Rev., 101 (2001) 1981. 13. G. Groppi, C. Cristiani and P. Forzatti in J.J. Spivey (Senior Reporter), Preparation and Characterization of Hexaaluminate Materials for High-temperature Catalytic Combustion, Catalysis, Vol. 13, The Royal Society of Chemistry, Cambridge, 1997, p. 85. 14. A. Gil, N. Burgos, M. Paulis, M. Montes and L.M. Gandia, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 130 (2000) 2153. 15. H.M. Zhang, Y. Teraoka and N. Yamazoe, Appl. Catal., 41 (1988) 137.

525 16. B.R. Strohmeier and D.M. Hercules, J. Phys. Chem., 88 (1984) 4922. 17. F. Kapteijn, A.D. van Langeveld, J.A. Moulijn, A. Andre'l'ni, M.A. Vuurman, A.M. Turek, J.M. Kehng and I.E. Wachs, J. Catal., 150 (1994) 94. 18. R.L. Chin and D.M. Hercules, J. Phys. Chem., 86 (1982) 360. 19. P. Arnoldy and J.A. Moulijn, J. Catal., 93 (1985) 38. 20. M. Paulis, L.M. Gandia, A. Gil, J. Sambeth, J.A. Odriozola and M. Montes, Appl. Catal. B, 26 (2000) 37. 21. P. Thorm~ihlen, E. Fridell, N. Cruise, M. Skoglundh and A. Palmqvist, Appl. Catal. B, 31 (2001) 1. 22. B.E. Hanson, L.F. Wieserman, G.W. Wagner and R.A. Kaufman, Langmuir, 3 (1987) 549. 23. S. Kaliaguine, A. Van Neste, V. Szabo, J.E. Gallot, M. Bassir and R. Muzychuk, Appl. Catal. A, 209 (2001) 345. 24. N. Mizuno, Catal. Today, 8 (1990) 221.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Unsupported and supported manganese oxides used in t h e combustion o f m e t h y l - e t h y l - k e t o n e

527

catalytic

L.M. Gandia, S.A. Korili and A. Gil Departamento de Quimica Aplicada. Edificio Los Acebos, Universidad Pfiblica de Navarra. Campus de Arrosadia, s/n. E-31006 Pamplona. Spain. In the present work, the catalytic combustion of methyl-ethyl-ketone (MEK) has been studied over unsupported (MnzO3, Mn304) and a-alumina-, ?-alumina-, magnesia-, silica-alumina- and silica-supported manganese oxides. The catalytic performance, as judged by the light-off curves for MEK combustion, is quite similar for Mn203 and Mn304. As regards the support nature the following order of improving performance can be established among the supported manganese oxide catalysts: Mn/(MgO) ~ Mn/(c~-A1203) < Mn/(SiO2) < Mn/(?-AlzO3) ~ Mn/(SiO2-A1203). The performance of the catalysts in the combustion reaction when alkali and acid ions are added to the solids has also been investigated. The addition of sodium and cesium results in a considerable improvement of the performance, while sulfate has a negative effect. 1. I N T R O D U C T I O N Catalytic combustion is a well-established technology for controlling Volatile Organic Compounds (VOCs) and odour emissions and has been successfully applied in a great variety of commercial installations [1,2]. This technology is a challenging alternative route to the typical thermal process, mainly due to the environmental and economic benefits that can be achieved [13]. Because of the large gas volumes that must be treated and the low VOC concentrations [2], catalysts that are very active and selective towards complete oxidation reactions are required. Noble metals, usually supported on 7-A1203, and single or appropriate combinations of transition metal oxides are active VOC combustion catalysts [1,3-5]. Both categories have advantages and drawbacks and the final choice depends greatly on the detailed composition and the physical properties of the waste gas stream. Manganese oxide-based systems constitute a very interesting type of heterogeneous catalysts. The chemical properties and redox behaviour of manganese, allowing the stabilization of several intermediate oxidation states and crystalline or pseudocrystalline oxide forms [6], produce a potential suitability for various kinds of catalytic processes as the combustion of VOCs. Despite the considerable interest of the manganese oxide-based systems, and due in part to their inherent

528

complexity and difficult characterization, several issues as the nature and surface structures of the active phases as well as the effects of the manganese precursors, supports and additives are still insufficiently established [7-11 ]. In this study, we present the preparation, characterization and catalytic performance in the complete oxidation of MEK of various series of manganese oxide-based catalysts. We extend here our previous studies concerning the effect of the manganese precursors, the nature of the supports and the additives on the physicochemical properties and catalytic performance of both unsupported and supported MnOx catalysts [12-15]. 2. E X P E R I M E N T A L

2.1. Unsupported manganese oxides The unsupported manganese oxides were prepared by the citrate method [16]. The starting materials, Mn(NO3)2-4H20 (Merck, PA) and citric acid (Panreac, PA), were dissolved in deionised water at an equivalent ratio of 1:1. The resulting solution was slowly evaporated in a rotavapor at 333 K to form a dense gel, which was further dehydrated in a vacuum oven at 343 K. The final precursors were heated in air (5 K/min) up to 823 or 1373 K in a muffle, where they were calcined for 4 h. The acid modified samples were prepared by treating 3 g of the manganese oxides with 50 cm 3 of 0.01 N H2SOa (Merck, 98%) or citric acid (Panreac, PA). In the case of the alkali modified catalysts, the quantity of Na or Cs necessary to form half a monolayer on the surface of the manganese oxide was estimated on the basis of the BET specific surface area of the initial oxides (12.5 and 0.9 mZ/g, respectively). Taking as a basis for the calculations the bigger ionic radius, 50 cm 3 of NaNO3 (Panreac, PA) and CsNO3 (Aldrich, 99%) solutions were prepared and mixed with the manganese oxide in suitable ion proportions. The slurries resulting from the mixing of the oxide with the acid or salt solutions were agitated at room temperature for 3 h, then evaporated slowly at 333 K under reduced pressure in a rotavapor. The resulting solids were dried for 12 h at 383 K and finally calcined for 4 h at 773 K. For a better comparison of the catalysts, the manganese oxides were also treated with 50 cm 3 of deionised water, then evaporated, dried and calcined in the same way as the modified samples. The catalysts are designated hereafter as MnT-X, where T refers to the calcination temperature of the precursors (823 or 1373 K) and X refers to the additive used, that is S for sulfuric acid, Cit for citric acid, Na for sodium nitrate and Cs for cesium nitrate. No suffix was used in the case of the pure unmodified manganese oxides (Mn823 and Mn1373).

2.2. Supported manganese oxides The supported manganese oxide catalysts were prepared by incipient wetness impregnation of the 200 - 300 btm particle size fraction of various commercial supports (c~-A1203, sds; 3,-A1203, Spherelite 505, Procatalyse; MgO, sds; SiO2, Aerolyst 350, Degussa-Hfils; SIO2-A1203, sds) previously calcined at 973 K for 4 h. The required amount of an aqueous solution of

529 Mn(NO3)2"4H20 (Merck, P.A.) was slowly added to the supports to give solids with a content of about 6 wt.% Mn. The solids were subsequently dried at 393 K for 16 h and calcined at 773 K for 4 h in a muffle. A series of acid and alkali-modified silica-supported manganese oxide catalysts was also prepared. The silica support was first treated with acid (H2SO4 or citric acid) or alkali (Na or Cs) in a way similar to the one for the bulk manganese oxides, and then impregnated by incipient wetness with an aqueous solution of manganese, dried at 393 K for 16 h and calcined at 773 K for 4 h. For a more reliable study of this series of supported catalysts, the blank supports were also treated with 50 cm 3 of deionised water, then evaporated, dried and calcined in the same way as for the acid and alkali-modified silica-supported catalysts. These catalysts are designated hereafter as Mn/(Sup)X, where Sup refers to the commercial support used, and X refers to the additive used in the case of the silica-supported catalysts.

2.3. Characterization techniques The catalysts were characterized by X-ray diffraction (XRD) analysis using a Siemens D-500 powder diffractometer, using nickel-filtered Cu Kct (X - 1.5404 ,~) radiation. The textural analyses were carried out from the corresponding krypton (Air Liquide, 99.995 %) and nitrogen (Air Liquide, 99.999 %) adsorption at 77 K using a static automatic volumetric apparatus (Micromeritics ASAP 2010 adsorption analyser) for the unsupported and supported manganese oxides, respectively. The samples were previously degassed (less than 0.1 Pa) for 6 h at 473 K. Chemical analyses of the catalysts, in order to determine the manganese and additives contents, were carried out by Activation Laboratories, Ancaster, Ont., Canada, using instrumental neutron activation analysis (INAA) and infrared analysis (IR). 2.4. Catalytic combustion of methyl-ethyl-ketone (MEK) The MEK combustion reaction was carried out in a tubular (8 mm i.d.) fixed-bed Pyrex glass reactor at atmospheric pressure. The catalyst was diluted with inert solids ( 1 0 0 - 200 ~tm Pyrex glass beads) in a volume ratio of 1:4 approximately, forming a catalytic bed of about 1 cm of depth. A thermocouple placed inside the reactor, in the center of the catalyst bed, monitored the reaction temperature. Mass flow controllers (Bronkhorst) monitored and controlled the flow gases. An air stream saturated with MEK (Panreac, PA) was created using a saturator, equipped with temperature and pressure control, and then diluted with pure air (SEO, 99.999 %), resulting in a 600 ppmv MEK concentration in the reactor feed. Prior to the reaction, the catalysts were treated under 100 cm3/min of air for 1 h at 573 K. The light-off curves for MEK combustion were obtained at a controlled heating rate of 2.5 K/rain and GHSV of about 34,000 h -1 based on the total volume of the catalytic bed. The catalytic runs were performed keeping constant the S~ET/Qan ratio in the case of the unsupported catalysts (1.575 mZ'min/cm3MEK for the Mn823 series and 0.536 m2.min/cm3MEK for the Mn1373 series), or the WMn/Qinratio in the case of the supported ones (0.030 g'min/cm3MEK). On-line analysis of the product stream was performed using a

530

Hewlett-Packard 6890 gas chromatograph equipped with two column systems, a 6 ft HayeSep Q connected to a TCD for CO2 determination, and an HP-INNOWax 30 m x 0.32 mm i.d. column connected to an FID for MEK analyses. 3. R E S U L T S AND D I S C U S S I O N

3.1. Unsupported manganese oxides The XRD patterns of the unsupported catalysts revealed the presence of only one phase, aMn203 for the material calcined at 823 K and ct-Mn304 for the one calcined at 1373 K. The BET specific surface areas and the additive content of the manganese oxides are summarized in Table 1. When Mn823 is treated with citric acid, a slight increase of the specific surface area takes place, that can be explained by a partial dissolution of the manganese oxide during the period of contact with the citric acid solution, since this procedure was very similar to the citrate method used for the synthesis of the parent oxide. In contrast, the incorporation of sodium and cesium causes a loss of specific surface area that can be related to a cement effect [17], which sticks the oxide and the alkali metal salt together in a sort of conglomerate. The effects of the additives on the specific surface area of Mn1373 series are not clear, because of the very low surface area of these oxides. Table 1 Physicochemical properties and Ts0 values of the unsupported manganese oxides Sample

SBET a

(mZ/g)

Additive (wt.%)

Sample

558

Mn1373

0.9

601

Mn1373-S

0.8

SBET a

(m2/g)

Additive (wt.%)

12.5

Mn823-S

11.6

Mn823-Cit

14.6

560

Mn1373-Cit

1.1

Mn823-Na

8.7

0.41 (Na)

526

Mn1373-Na

0.9

0.09 (Na)

583

Mn823-Cs

6.3

1.03 (Cs)

509

Mn1373-Cs

1.0

0.18 (Cs)

600

0.25 (S) --

--

T5o (K)

Mn823

a

--

Tso (K)

0.29 (S) --

600 609 600

Data from krypton adsorption at 77 K. A comparison of the ignition curves for MEK combustion over Mn823 and Mn1373 at a

SBET/Qin ratio of 1.575 m2"min/cm3MEK, is presented in Fig. 1. The results show only a slight shift to higher combustion temperatures over Mn1373 compared to the case of the Mn823 catalyst. It should be also noted that the shape of the light-off curves is essentially the same over both oxides. Taking into account that the shape of a light-off curve depends to a great extent on the reaction kinetics [18], the results in Fig. 1 indicate that the catalytic performance of both Mn823 and Mn1373 for MEK combustion is very similar. As a first approach, this fact might be related to the nucleophilicity of the oxides surface among other factors. Busca et al. [19-21] have proposed that ketones are oxidised to CO2 over transition metal oxides according to a Mars-van Krevelen type

531

mechanism, with the combustion being produced at the expense of nucleophilic lattice oxygen species. For a given transition metal, the nucleophilic character of the O z. anions in the corresponding oxides considerably increases as the oxidation state for the metal decreases [19]. According to this approach, a similar nucleophilicity can be expected for both Mn823 and Mn1373, and therefore also a similar catalytic behaviour, in accordance with the results in Fig. 1.

1.00 o 9

Mn823 Mn1373

oe

0.75 o 9

0.50

o OID

0.25

r,)

Oe C~

~laP ,..

0.00

,..-.o ; " ~ t ~

400

450

500

,

,

550

i

600

,

,

i

650

700

T e m p e r a t u r e (K)

Fig. 1. Light-off curves for MEK combustion over Mn823 and Mn1373 oxides. The ignition curves for MEK combustion over the Mn823 and Mn1373 series of modified manganese oxides are shown in Figs. 2 and 3, respectively. Interesting catalytic performance differences for the ketone combustion can be observed among the oxides of each series. As judged from the light-off curves and the values (included in Table 1) of the respective temperature (Ts0) at which the conversion of MEK reaches 50 %, the addition of sodium and cesium results in an improvement of the catalytic performance, while sulfate has a negative 1.00

1.00

9 9 o [] 9

0.75

g

Mn823 Mn823-S Mn823-Cit Mn823-Na Mn823-Cs

9

9

[]

[]

o

[]

9

,

0.00 400

[]

0

0.50

o

-~~_~_g==,=== 500

9 9

Mn1373 Mn1373-S

~ ~_

o [] ~I,

Mn1373-Cit Do 9 Mn1373-Na O~ Mn1373-Cs ~1, t~4D O 17 9 9

~ 9e

0.25

[]

9 ,.1• ~ 9 uO0

450

0.75

0 []

9

9

9

9

0.50

0.25

9

9i D I

550

,

,

600

0.00

,

650

T e m p e r a t u r e (K)

Fig. 2. Light-off curves for MEK combustion over Mn823 oxides series,

I

450

500

550

600

'

I

650

'

I

700

'

750

T e m p e r a t u r e (K)

Fig. 3. Light-off curves for MEK combustion over Mn1375 oxides series.

532

effect, always in comparison to the performance of the corresponding parent manganese oxide. No significant change of the catalytic behaviour has been observed when the citric acid is used as additive. Various ways in which the alkali and acid additives could affect the catalytic performance of manganese oxides have been proposed [15]. They can favour the formation of enolic species and strongly adsorbed aldol condensation products from MEK on the oxide surface, promote changes on the electron donor or acceptor character of the catalysts, and also create new possible pathways for the combustion reaction. In our case, the presence of alkali (sodium or cesium) additives could help the removal of strongly adsorbed species from the catalyst surface, thus reducing the combustion temperature. In the case of sulfate, the presence of strongly bound enolic species and aldol condensation products might be favoured, thus increasing the combustion temperature.

3.2. Supported manganese oxides Both c~-MnzO3 and 13-MnOz, are observed in the XRD patterns of all supported manganese oxide catalysts. The mean manganese oxide crystallite diameters have been estimated from application of the Scherrer equation [22], and the results are summarized in Table 2. XRD line broadening measurements were carried out using the ct-MnzO3 (222) (33.0 ~ in 20) and ]3-MnOz (110) and (101) (28.8 ~ and 37.3 ~ in 20) peaks, respectively. The width at half-maximum of these peaks was corrected for instrumental broadening. Supported manganese catalysts can exhibit a variety of crystalline manganese oxide phases depending mainly on the metal loading, and the temperature and atmosphere of calcination. The distribution of manganese between these phases will depend on several factors related with the preparation method used and the reactivity of the support surface. In the present work, the catalysts have been calcined at 773 K in air for 4 h and the presence of both ct-MnzO3 and f3-MnOz has been detected. This is consistent with the results found by various authors [7,8] and with a bulk-like behaviour of the supported manganese oxide species, suggesting a weak metal-support interaction. This conclusion could also be supported by the relatively high mean a-MnzO3 and [3-MnOz crystallite diameters obtained from line broadening measurements, and there is no clear relationship between these values and the nature of the supports used. The ignition curves for MEK combustion over the supported manganese oxide catalysts are presented in Figures 5 and 6. A significant influence of the nature of the support and the acid or alkali additives on the catalytic performance can be observed. It should be noted that, as judged by the corresponding T50 values summarized in Table 2, the catalytic performance exhibited by the supports is far from negligible. These temperatures are between 80 and 200 K higher than the ones over the supported manganese oxide catalysts. The following order of improving catalytic performance can be established among the supported catalysts: Mn/(MgO) ~ Mn/(a-AlzO3) < Mn/(SiOz) < Mn/(3,-AlzO3) ~ Mn/(SiOz-AlzO3) and Mn/(SiOz)Na < Mn/(SiOz) ~ Mn/(SiOz)S < Mn/(SiOz)Cit < Mn/(SiOz)Cs. These sequences are quite similar to the one obtained with the supports, a fact which suggests that there is an important effect of the support characteristics on

533 the performance of these catalysts for MEK combustion. Nevertheless, the dispersion of the supported active phases, the distribution of the manganese between c~-MnzO3 and [3-MnOz, and the catalysts acid-base properties might also be important factors conditioning the catalytic performance. Table 2 Physicochemical properties and Ts0 values of the supported manganese oxides Sample

SBETa

(mZ/g)

Vp a (cm3/g)

(wt.%)

(nm)

Mn

dMn203

dMnO2 (nm)

Additive (wt.%)

Mn/(cc-AlzO3)

1

0.004

5.91

37.9

. . . .

Mn/(qp-AlzO3) Mn/(MgO)

142 5

0.434 0.031

6.12 8.36

23.4 17.1

14.2 . . . .

Ts0 (K) 608 (811) b

--

581 (659) 619 (822)

Mn/(SiOz-Alz03)

278

0.439

6.33

20.0

12.8

--

575 (719)

Mn/(SiOz)

188

0.767

6.59

13.1

10.6

--

599 (730)

Mn/(SiOz)S Mn/(SiOz)Cit Mn/(SiOz)Na Mn/(SiOz)Cs

192 184 30 74

0.787 0.767 0.069 0.571

6.65 6.60 6.44 6.50

10.6 __ c 52.2 22.1

11.7 16.0 9.7 15.5

0.16 (S) -4.86 (Na) 11.27 (Cs)

595 576 608 535

(751) (738) (772) (683)

a Data from nitrogen adsorption at 77 K. b The Ts0 values of the corresponding supports are given in the brackets. c Diffraction peak was too weak to make an accurate calculation.

v ILl

9 Mn/(a-AI20) 9 Mrl/~ -AI203) Oj o Mn/(MgO) 9 [] Un/(SiO-AI20) [] 9 9 Mn/(SiO) 9O

0.75

0 r

.---

0.50

O O

0.75

9~O [] E I I 9 1O4 9

0.25

Mn/(SiO2) Mn/(SiOz)S Mn/(SiOz)Cit Mn/(SiOz)Na Mn/(SiOz)CS

9

9

in

IPB

o ~o

0.25

~

I o io

o o

0.50

[]

> r

9 9 0 [] 9

o ~

))

1.00

1.00

9

o In

O_In

.,omlo

~,-,~-~mm~D 0.00

i

400

500

,

600

i 700

0.00 800

Temperature (K)

Fig. 4. Light-off curves for MEK combustion over Mn/(support) series,

i

400

500

600

, 700

Temperature (K)

Fig. 5. Light-off curves for MEK combustion over Mn/(SiO2) series.

534 ACKNOWLEDGMENTS Financial support by the Ministry of Science and Technology (MAT2000-0985) and the Department of Education and Culture of the Navarre Government (Orden Foral 143/1998) is gratefully acknowledged. REFERENCES

1. M.S. Jennings, N.E. Kron, R.S. Berry, M.A. Palazzolo, R.M. Parks and K.K. Fidler, Catalytic Incineration for Control of Volatile Organic Compound Emissions, Noyes Publications, Park Ridge, NJ, 1985, p. 36. 2. N. Mukhopadhyay and E.C. Moretti, Current and Potential Future Industrial Practices for Reducing and Controlling Volatile Organic Compounds, Center for Waste Reduction Technologies, AIChE, New York, 1993. 3. J.J. Spivey in G.C. Bond, G. Webb (Senior Reporters), Complete Oxidation of Volatile Organics, Catalysis, Vol. 8, The Royal Society of Chemistry, Cambridge, 1989, p. 157. 4. T. Seiyama, Catal. Rev.-Sci. Eng., 34 (1992) 281. 5. M.F.M. Zwinkels, S.G. J~ir~s, P.G. Menon and T.A. Griffin, Catal. Rev.-Sci. Eng., 35 (1993) 319. 6. W. Feitknecht, Pure Appl. Chem., 9 (1964) 423. 7. B.R. Strohmeier and D.M. Hercules, J. Phys. Chem., 88 (1984) 4922. 8. P.G. Tsyrulninov, V.S. Salnikov, V.A. Drozdov, S.A. Stuken, A.V. Bubnov, E.I. Grigorov, A.V. Kalinkin and V.I. Zaikovskii, Kinet. Catal., 32 (1991) 439. 9. F. Kapteijn, A.D. Vanlangeveld, J.A. Moulijn, A. Andreini, M.A. Vurman, A.M. Turek, J.M. Jehng and I.E. Wachs, J. Catal., 150 (1994) 94. 10. C. Lahousse, A. Bernier, E. Gaigneaux, P. Ruiz, P. Grange and B. Delmon, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 110 (1997) 777. 11. F. Arena, T. Torre, C. Raimondo and A. Parmaliana, Phys. Chem. Chem. Phys., 3 (2001) 1911. 12. M. Paulis, L.M. Gandia, A. Gil, J. Sambeth, J.A. Odriozola and M. Montes, Appl. Catal. B, 26 (2000) 37. 13. L.M. Gandia, M.A. Vicente and A. Gil, Appl. Catal. A, 196 (2000) 281. 14. A. Gil, N. Burgos, M. Paulis, M. Montes and L.M. Gandia, Stud. Surf. Sci, Catal., Elsevier, Amsterdam, 130 (2000) 2153. 15. L.M. Gandia, A. Gil and S.A. Korili, Appl. Catal. B, 33 (2001) 1. 16. P. Courty, H. Ajot, C. Marcilly and B. Delmon, Powder Technol., 7 (1973) 21. 17. V. Perrichon and M.C. Durupty, Appl. Catal., 42 (1988) 217. 18. L. van de Beld, M.P.G. Bijl, A. Reinders, B. van der Weft and K.R. Westerterp, Chem. Eng. Sci., 49 (1994) 4361. 19. G. Busca, E. Finocchio, G. Ramis and G. Ricchiardi, Catal. Today, 32 (1996) 133.

535 20. M. Baldi, F. Milella, G. Ramis, V. Sanchez Escribano and G. Busca, Appl. Catal. A, 166 (1998) 75. 21. G. Busca, E. Finocchio, V. Lorenzelli, G. Ramis and M. Baldi, Catal. Today, 49 (1999) 453. 22. H.P. Klug and L.E. Alexander (eds.), X-Ray Diffraction Procedures, Wiley Interscience, New York, 1974, p. 618.

Studies in Surface Scienceand Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

537

Ni~f~-zeolite catalysts prepared by deposition-precipitation. Rub6n Nares 1'3, Jorge Ramirez 1, Aida Guti6rrez-Alej andre 1, Rogelio Cuevas 1, Catherine Louis 2 and Tatiana Klimova 1 ~UNICAT, Departamento de Ingenieria Quimica, Facultad de Quimica UNAM, M6xico, D.F., (04510); E-mail [email protected].~j_..dg..r_.t.!._n_.a._n_.a.._m..,_x.. 2 Laboratoire de R6activit6 de Surface, URA 1106 CNRS, Universit6 P. et M. Curie, 4, place Jussieu, 75252 Paris Cedex 05, France. 3 Instituto Mexicano del Petr61eo, Eje Central Lhzaro C/vdenas, 152, M6xico, D. F., (07730). In the preparation of Ni/HI3 catalysts by the deposition-precipitation method (DP), nickel hydrosilicates are formed mainly but not exclusively in the external surface of the HI3 zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during the activation of the catalysts (calcination and reduction) and a homogeneous distribution of small nickel particles is obtained. The catalyst prepared by DP showed better catalytic activity in the hydrogenation of naphthalene than the catalyst prepared by cationic competitive exchange. 1. INTRODUCTION The preparation of high loading highly dispersed metal catalysts is a field of great interest in catalysis. Impregnation or ion exchange are the usual methods to prepare metal catalysts. Ion exchange can provide high metal dispersions but limited metal loading. Impregnation on the other hand, can achieve high metal loading but a limited dispersion. To overcome these problems, several novel preparation strategies have been proposed for the different systems. In the case of Ni catalysts supported on SiO2, there have been reports indicating that with the deposition-precipitation (DP) method developed by Geus [1-4] high loading and high Ni dispersion could be achieved. This method, based on the slow basification of the impregnating solution by controlling the temperature of urea hydrolysis, leads to the formation of Ni phyllosilicates, which when activated in hydrogen lead to highly dispersed Ni particles. It appears interesting to extend the application of this method to the preparation of Ni catalysts supported on SIO2-A1203 structures and in particular to zeolites like HBeta. This will allow the preparation of Ni/HBeta catalysts with high loading and dispersion. It has been established previously that Ni/SiO2 materials prepared by cationic competitive exchange [5, 6] and deposition-precipitation [1-4] give rise, after drying to supported layered silicates. In the present work a comparison between the

538 characteristics and hydrogenation catalytic activities of Ni/HI3 catalysts prepared by deposition-precipitation and cationic competitive exchange is presented. XRD, FTIR, TPR, SBET and TEM were used to characterise the changes occurring to the zeolite support and Ni phases during the different stages of the preparation of the Ni/HI3 catalysts. The catalytic activity of the catalysts was compared in the naphthalene hydrogenation reaction. 2. E X P E R I M E N T A L The HI3-zeolite sample was provided by Zeolyst International CP811E-75 (lot 1822-74; SIO2/A1203 = 75, surface area =579 mZg-1). The Ni/H[3 zeolite samples were prepared according to the following procedure: First, 250 ml of a nickel nitrate aqueous solution (0.14M) was prepared. This solution was divided in two parts: one part (40 ml) was used to dissolve urea at room temperature and the other part (210 ml) was used to make a suspension of 1.9 g of HI3 zeolite (pH = 2). Afterwards, the zeolite suspension was heated to 70 ~ and mixed with the urea dissolution. The mixture was taken to 90 ~ to start the deposition-precipitation of nickel onto the HI3 zeolite (Fig. 1). At the end of the chosen DP time (1 to 4 h), the suspension was cooled to 15-20 ~ and filtered and the solid was washed three times with 20 ml of distilled hot water (50-60 ~ Finally, the samples were dried at 110 ~ for 24 hours. Hereafter, the samples will be referred to as Ni/H[31, Ni/H[32, Ni/H[-53 and Ni/H[34, where the numbers indicate the time in hours of depositionprecipitation used in the preparation of the sample. For the preparation of the cationic competitive exchanged sample, a procedure described previously was used [7]. 2.5 g of HI3 zeolite was put in contact with 50 ml of a solution 0.1 M Ni(NO3)z.6H20 and 0.8 M NH4NO3 in a controlled temperature vessel at 25 ~ The suspension was magnetically stirred for 24 h and the pH was adjusted to 9.8 by bubbling gaseous ammonia. The solid was filtered, washed three times with 50 ml of distilled water, and dried in an oven at 110 ~ for 24 h. This sample will be referred as Ni/HI3-CE. Sample Characterization. The BET nitrogen adsorption-desorption isotherms were performed in an ASAP 2000 instrument using N2 at -195 ~ The microporous area was estimated using the correlation of t-Harkings & Jura (T-plot method). The XRD spectra were registered with a Philips 1050/25 diffractometer using CuKa radiation (X = 1.5418 A)" the goniometer scan speed was l~ q. The FTIR spectra were performed at room temperature on a Nicolet Magna IR 760 instrument with 100 scans and a resolution of 4 cm -1. The samples were conditioned in KBr disk method. In this case both the HI3 zeolite and Ni/HI3 samples were finely ground and dispersed in KBr pellet with a ratio of 1:100 of KBr. Self-supported pressed wafers were prepared with 10 mg of sample and a pressure of 10 kg.cm -z. Temperature programmed reduction (TPR) of the calcined samples were performed in a RIG-100 In Situ Research Instruments catalyst characterization apparatus. The TPR experiments were performed in a quartz gas flow reactor, from room temperature to 1000~ with a heating rate of 7.5 ~ -1 under a stream of 5% v/v Ha in argon (total flow rate 25 ml.min -1) at atmospheric pressure. The elemental analysis of the HI3 zeolite and the Ni/H[3 samples prepared at different DP times was performed by SEM with a JEOL JSM-5900LV microscope equipped with an

539 analytical EDX accessory. The nickel concentration in the samples was confirmed by the IMP-QA-206 method (Determination of Ni via EDTA titration with murexide indicator). The reduced Ni particle sizes were examined by high-resolution electron microscopy in a JEOL JEM 2010 electron microscope operating at 200 kV. Catalysts activation. The samples were calcined at 500 ~ with a heating rate of l~ -1 during 5 h. After, the samples were reduced at 450 ~ with a heating rate of l~ -1, under a flow of Ha (70 mlmin -1) during 5 h. Reactions. The hydrogenation reactions were performed in a 300 ml batch autoclave equipped with a magnetic stirrer, rotating at 1000 rpm and operating at 220 ~ and 6.6 MPa. In all the experiments, 34 grams of n-decane were mixed with 6 g of naphthalene (>99%) and 0.1 g of catalyst. The products were identified and quantified with an HP 6899 series chromatograph equipped with an HP-1 capillary column. 3. RESULTS AND DISCUSSION The deposition-precipitation of Ni(II) shows the typical pH versus time behaviour found in high surface area Ni/SiO2 systems [8-11]. The pH-curve displays a maximum; after this value the nucleation and growth of the solid phase start up rapidly and the rate of generation of hydroxyl ion is lower than its consumption leading to a temporary decrease in the pH value (Fig. 1). 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

L

......................

,

0

,

,

,

,

,

,

,

,

,

,

,

40 80 120 160 200 240 280 320 360 400 440 480 Time, (min)

Fig. 1. pH-curve of the Ni/HI3 samples by hydrolysis of urea at 90 ~ as a function of the DP time. Table 1 displays the textural properties and the Ni loading for the catalysts prepared by DP and competitive cationic exchange. With one h DP time a Ni concentration close to that obtained by cationic exchange is obtained. Also, the microporous area of the Ni/H[51 and Ni/HJ3-CE catalysts is similar. The nitrogen adsorption isotherms of the HI3 zeolite and the Ni/H[3-CE samples (Fig. 2) are, as expected, very similar. In the sample prepared by DP, the presence of a hysteresis caused by the formation of a secondary porous system is evidenced in the Ni/H[~2 and Ni/HI34. The shape of the newly formed hysteresis suggests the formation of a laminar type porous structure [21, 22], possibly nickel phyllosilicates.

540 Table 1. Ni loading and textural properties of the HI5 zeolite, Ni/H~5 and Ni/H[5-CE samples. HI5 Ni/HI3 Ni/HI31 Ni/HI32 Ni/HI34 zeolite CE Wt % Ni 3.8 5.4 14 18 Surface area, mZ.g-1 579.4 540.4 584.9 520.2 452 Microporous area, ma.g-1 359.2 280.2 288.1 200 120 Microporous volume, cmg.g-1 0.166 0.125 0.133 0.094 0.053

~

= 100

d r

Z

b

>

o.o'o11 'oi2'ols'o~'ols'd6'dT'ois'019'1.o Relative Pressure, P/Po

Fig. 2. Nitrogen adsorption-desorption isotherm: a) HI5 zeolite; b) Ni/H[3-CE" c) Ni/HI32; d) Ni/HI34. X-Ray diffraction. The XRD patterns for the HI5 zeolite, the Ni/H[5-CE sample and some Ni/H[3 samples prepared at different DP times are shown in Fig. 3. The incorporation of Ni to the zeolite by either of the two methods, cationic exchange or DP, produces a small loss in the crystallinity of the zeolite. However, only the samples prepared by DP show clearly two new asymmetric reflections at 20 = 33.2 and 59.2 ~ with d spacing of 2.66 and 1.544 ,~ respectively. These two new reflections according to published results [8, 9, 12, 13] may be attributed to the formation of Ni hydrosilicates. The intensity of these two reflections increases with DP time indicating an increase in the crystallinity of the responsible phase. TPR of dried samples. The TPR profiles of the HI5 zeolite and the dried Ni/HI3-CE, Ni/H[32 and Ni/H[54 samples, shown in Fig. 4, display an asymmetric reduction peak at 400 ~ whose intensity decreases in the Ni/H[M sample. According to previous work on bulk and SiO2 supported Ni prepared by DP [14, 15], the peak at 400 ~ can be assigned to the reduction of nickel hydroxide or highly disordered Ni phyllosilicate. The high temperature peaks can be assigned to the reduction of 1:1 nickel phyllosilicates with different degrees of crystallisation [8, 9, 16, 17].

541

~= 50

c

o

u+.+

t 0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

2 Theta Degree

Fig. 3. XRD patterns of the a) HI3 zeolite; b)Ni/HI3-CE" c) Ni/H[32; d) Ni/HI34.

~= 100

j

d O cl.

__J

E O

o

:ff a 0

1o 0

2o 0

300 '

4~o 500 . . 600 . . 700. Temperature, ~

800

900

1000

Fig. 4. TPR profiles of dried samples a) Ni/HI3-CE; b) Ni/HI32" c) Ni/HI34. FTIR characterization. The mid-infrared region of 2000-400 cm -1 contains the fundamental framework vibrations of the Si(A1)O4 groupings [18-20]. In the same way, nickel phyllosilicates exhibit in the 1200-400 cm -1 range bands characteristic of stretching and bending of SiO vibrations, and bending vibrations specific of structural OH groups [7]. In the IR spectra of our samples several features are evident. The band that in the HI5 zeolite appears at 618 cm -1 grows and develops with DP time into one broad band with two contributions, one at 641 cm -1, assigned to the presence of Ni(OH)2, and another at 665 -1 cm , assigned to the presence of 1"1 nickel phyllosilicate. It appears that in the Ni/H[3 samples prepared by DP there is a mixture of 1"1 nickel phyllosilicate and nickel hydroxide. For the sample prepared by cationic competitive exchange there is no clear evidence of these bands. The band that in the pure zeolite appears at about 797 cm 1 and that corresponds to symmetrical stretch of SiO4 tetrahedra, diminishes with DP time indicating a probably dissolution process of the siliceous framework during the DP process. This also occurs with the band at about 1086 cm -1. This behaviour supports the formation of Ni hydrosilicates at the expense of the siliceous framework in the DP prepared samples. The

542 sample prepared by cationic competitive exchange shows similar features indicating also the partial dissolution of the siliceous framework of the zeolite, but the bands at 640 and 665 cm -1 are not clearly evident in this case. Fig. 5 shows the IR spectra in the fundamental region of the HI3 zeolite, Ni/HI3-CE, Ni/HI32 and Ni/H[34.

3000

2000 Wavenumbers(cm4)

1000

Fig. 5. IR spectra a) HI3 zeolite, b)Ni/HI3-CE; c) Ni/HI32; d) Ni/HI54.

I=0.5

1090 A

___.__~Ni/HI~4Dp=4h ~ _ _ . _ . . ~ 2193

~

Subtraction

8

/

5

463

\ 792/'---

1049

~,~oo5

_~ /"-

~6o

~

~

I

465

HI5zeolite1629~___~ 2500

2250 20()0 1750 1500 1250 1000 750 Wavenumbers (cm -1)

500

Fig. 6. IR spectra of the HI3 zeolite treated with urea at 90% during 4h, Ni/H[34 and subtraction spectrum In the DP samples, after two hours deposition-precipitation, there is formation of a clear shoulder with DP time at about 1005 cm -1, which according to the literature [7], points to the presence of 1:1 nickel phyllosilicate. This is more clearly seen in the subtraction spectra. It appears that in the case of the cationic competitive exchangeprepared sample the formation of hydrosilicate species is only incipient. In fact, these

543 poorly crystallized species were only detected by TPR. Fig. 6 displays the subtraction of the Ni/HI34 and Hf~ zeolite treated with urea during 4h. Electron microscopy. TEM micrographs of the reduced Ni/H[32 and Ni/H[3-CE samples are shown in Fig. 7. The micrograph of the DP reduced sample (Fig. 7a, b) shows a homogeneous distribution of Ni metal particles. In contrast, the sample prepared by cationic competitive exchange shows a highly inhomogeneous distribution of Ni particles, which in general are larger than the ones found in the DP samples.

Fig. 7. TEM micrographs of a) Ni/H[31 and b) Ni/H[3-CE (bar= 50 nm) Catalytic Activity. The results of catalytic activity in the hydrogenation of naphthalene are well in line with the above findings. By comparing the concentration versus time curves for the Ni/H[51 and the Ni/H[5-CE samples, it is easily observed that the DP method leads to higher catalytic activities. In fact, with the DP catalyst, the total conversion of naphthalene is reached at 120 rain while the catalyst prepared by cationic competitive exchange takes 300 rain to convert all the naphthalene. The amounts of products resulting from the hydrogenation of the second aromatic ring, cis and trans decalins, also make clearly evident the supremacy of the DP prepared catalyst. Fig. 8 (a-b) shows the catalytic activity of Ni/H[51 and Ni/HI3-CE in the hydrogenation of naphthalene.

0.050 T ,J --~ o 0.040

.e

- 0.030 ._

I~

(1)

(3)

ai i

0.050

- 0.030 ._

0.020 8

b

;

0.020

8

c

)

.J o 0.040

8 0.010

0.010

0.000 ~

0.000

0

60

120

180

240

0

Time, (min)

50

1O0

150

200

250

Times, (min)

(1) Naphthalene (2) Tetraline (3) Trans-Decaline (4) Cis Decaline

Fig. 8. Hydrogenation reaction of naphthalene at 220 ~

a) Ni/H[31 and b) Ni/H[3-CE

300

544 4. CONCLUSIONS From the above results one can conclude that the DP preparation method leads to Ni/H[3 catalysts with better Ni dispersion than those prepared by cationic competitive exchange. This result seems to be due to the formation of a stronger support metal interaction (formation of Ni hydrosilicates) in the case of the DP method. The better deposition of Ni achieved by the DP method is clearly reflected in a superior activity in the naphthalene hydrogenation reaction. ACKNOWLEDGMENTS We acknowledge the financial support from the IMP-FIES Program. We are grateful to Mr Ivan Puente for the microscopy work. REFERENCES

1. J.W. Geus, Dutch Patent Applications, 1967, 6705,259, and 1968, 6813,236. 2. J.A. van Dillen, J.W. Geus, L.A. Hermans and J. van der Meijden, In Proceeding of 6th International Congress on Catalysis, London, 1976; G.C. Bond, P.B. Wells, F.C. Tompkims, Eds., Elsevier, Amsterdan, 1977; p 667. 3. L.A.M. Hermans and J.W. Geus. in Preparation of Catalysts II, B. Delmon, P. Grange, P.A. Jacobs, G. Poncelet, Eds., Elsevier; Amsterdam, (1979) 113. 4. J.W. Geus, Preparation of Catalysts III, G. Poncelet, P. Grange, P.A. Jacobs, Eds. Elsevier, Amsterdam (1983) 1. 5. O. Clausen, M. Kermarec, L. Benneviot, F. Villain and M. Che, J. Am. Chem. Soc., 114 (1992) 4709. 6. M. Kermarec, J.Y. Carriat. P. Burattin, M.Che and A. Decarreau, J. Phys. Chem. B 98, (1994) 12008. 7. J.Y. Carriat, M. Che, M. Kermarec, M. Verdaguer and A. Michalowicz, J. Am. Chem. Soc., 120 (1998) 2059. 8. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B 102 (1998) 2722. 9. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B 103 (1999) 6171. 10. J.W.E. Coenen, Appl. Catal. 54 (1989) 65. 11. G. Ertl, H. Kn6zinger and J. Weitkamp, Preparation of Solid Catalysts; Wiley-VCH, Weinheim, (1999) 460. 12. H. Mod6sir and A. Decarreau, Bull. Min6ral. 110 (1987) 409. 13. O. Clause, L. Benneviot, M. Che and H. Dexpert, J. Catal. 130 (1991) 21. 14. B. Mile, D. Stirling, M.A. Zammitt, A. Lovell and M. Webb, J. Catal. 114 (1988) 217. 15. J.P. Espin6s, A.R. Gonzfilez-Elipe, A. Caballero, J. Garcia and G. Munuera, J. Catal. 136, (1992) 415. 16. P. G~nin, A. Delahaye-Vidal, F. Proteger and F. M. Tekaia Figlarz, Eur. J. Solid State Inorg. Chem. 28 (1991) 506. 17. A. Decarreau, H. Mod6sir and G.C.R Besson, Acad. Sci. Paris (S6r. II) 308 (1989) 301. 18. R. Szostak, Molecular Sieves Principles of Synthesis and Identification; Van Nostrand Reihold: New York, (1989) p. 282.

545 19. Zeolite Chemistry and Catalysis; J.A. Rabo, Ed.; ACS Monograph 171 American Chemical Society: Washington, D. C. (1976) p. 80. 20. D.W. Breck, Zeolite Molecular Sieve: Structure, Chemistry, and Use, John Wiley and Son: New York, (1974) p. 415. 21. S.J. Gregg and K.S. Sing, W. Adsorption, Surface Area and Porosity; Academic Press, London and New York, (1967) p. 173. 22. G. Leofanti, M. Padovan, G. Tozzola and V. Venturelli, Catal. Today, 41 (1998) 207.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

547

Sol-Gel A1203 structure modification by Ti and Zr addition. A NMR study J. Escobar a*, J. A. De Los Reyes b and T. Viverosb aInstituto Mexicano del Petr61eo, Tratamiento de Crudo Maya, Eje Central Lfizaro Cfirdenas 152, San Bartolo Atepehuacan, G. A. Madero, M6xico, D. F., M6xico 07730 bArea de Ing. Q., UAM-Iztapalapa, San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, M6xico, D. F., M6xico 09360 In order to assess the effect of various synthesis parameters and the presence of a second oxide (TiO2 or ZrOz) at different concentrations on the AIaO3 structure, the corresponding samples were prepared by low-temperature sol-gel method. The oxides were characterized by N2 physisorption, XRD and 27A1 MAS-NMR. In all samples calcined at Tc_<973 K four-, five- and six-coordinated aluminum were detected. A pseudospinel model was used to evaluate the structural changes in the different oxides. The sol-gel AIzO3 calcined at 1173 K (AlVI/AlIV-5.7, no metastable pentahedral aluminum) was the material that better fitted the proposed model. A1203 oxides calcined at different temperatures were studied in the isopropanol dehydration in order to correlate their performance to the amounts of A1TM. 1. I N T R O D U C T I O N By manipulating different parameters during the sol-gel synthesis the textural, structural and surface properties of metal oxides can be modified. By using this technique, defective AIzO3 matrices where aluminum atoms are coordinated to four, five or six oxygens [1] can be obtained. According to Wang et a[ [1], pentahedral aluminum (A1v) in sol-gel alumina could be originated by both substitution of lattice oxygens in octahedral symmetry by hydroxyl groups or by oxygen defects adjacent to aluminum nucleus. Regarding mixed oxides, Chiaro et aL [2] associated an increase of the proportion of metastable AI v to a more random distribution of metallic cations (higher homogeneity) in AIaO3-TiOa samples. Pentahedral aluminum has also been related to the creation of Lewis acid sites (electron deficient sites) through the promotion of higher amounts of oxygens of electrostatic bond strengths (EBS)<2 [3]. In this work, we investigated the effect of some parameters (type of synthesis additive and calcination temperature) as well as the presence of a second cation (Ti4+ or Zr 4+) on the formation of five-coordinated aluminum and the consequential structure modification in sol-gel alumina-based supports. The samples prepared were characterized by Na physisorption, X-ray diffraction and Z7AI MAS-NMR. The isopropanol dehydration over

548 A1203 samples calcined at various temperatures was studied as tetrahedral aluminum (AITM) concentration could be linked to acidic dehydrating sites on the alumina surface [4]. 2. E X P E R I M E N T A L

2.1. Materials A1203 (A1) and A1203-TiO2 mixed oxides at two compositions (A1/Ti=25 and 2, AT25 and AT2, respectively) were synthesized by a low temperature sol-gel method using aluminum trisecbutoxide and titanium triisopropoxide (Aldrich) as organic precursors. HNO3 (A) or CHaCOOH (C), both from Baker were used as hydrolysis catalysts and complexing additive, respectively [5]. The alcohol/alkoxide (r), HzO/alkoxide (h) and additive/alkoxide (a) molar ratios were: 65, 30 and 0.2, respectively (Method 1). The hydrolysis mixture (water+additive) was added dropwise to a stirred alcoholic solution of metal alkoxides maintained at 4273 K. The obtained gels were aged for 24 h at the synthesis temperature and then vaccum dried at room temperature. A similar technique was applied for the preparation of AI203-ZrO2 samples (A1/Zr=46 and 22, AZ46 and AZ22, respectively) with HNO3. In this case, r, h and a were: 30, 20 and 0.2, respectively (Method 2). The dried solids were calcined at different temperatures (573
2.3. Isopropanol dehydration Isopropanol dehydration over A1203 calcined at different temperatures was studied in a pyrex glass steady state system. The fixed bed (50 rag) tubular reactor was operated at differential regime (% conversion<10), in the 423
3.1. Materials characterization Some properties of the prepared oxides are shown in Table 1. Except the A1203 prepared with acetic acid (AlC) calcined at 1173 K, the rest of the samples were amorphous. No signals of A1203, TiO2 or ZrO2 crystalline phases could be identified in the diffractograms corresponding to these solids (not shown).

549 For all the oxides calcined at Tc<973 K, octahedral (AIvI, 2-10 ppm), pentahedral (A1v, 23-35 ppm) and tetrahedral (AIIv, 54-65 ppm) aluminum were detected, the latter being most abundant (Fig. 1 to 4, Tables 2 and 3). All the signals were not well resolved and relatively wide due to quadrupole broadening. Thus, a exact quantification of the different aluminum species by deconvolution-integration was rather difficult. Nevertheless, the method could provide a good estimate of the amounts of 27A1 in different coordinations [6]. No effect of the synthesis additive was found on the 27A1 speciation in alumina samples. Table 1. Surface area and structure (crystalline phase) of the prepared sol-gel oxides. Oxide Additive Tc Sg (K) (m2 g-l) AIA7 AIA9 AIC5 AIC7 AIC9 AIC11 AT25C AT2C AZ46A AZ22A

HNO3 HNO3 CHaCOOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH HNO3 HNO3

773 973 573 773 973 1173 973 973 773 773

451 407 564 405 338 165 355 216 302 287

Phase Amorphous Amorphous Amorphous Amorphous Amorphous ~/-A1203 Amorphous Amorphous Amorphous Amorphous

By increasing the calcination temperature from 773 to 973 K the AI v present in the alumina samples seemed to gradually transform to Al TM probably due to removal of structural hydroxyls still retained [7]. The oxygen transfer between two metastable Al v to produce one AITM and one AIvI [3] could be discarded because the proportion of the latter did not show appreciable changes. At more severe conditions (1173 K), some of the AI vl was also converted to four-coordinated aluminum. None of the materials showed A1 species distribution close to that of the pseudospinel (AlVI/AlIV-2, no pentahedral species

[3]). The main effect of TiO2 added in incremental amounts in the alumina-titania solids was a progressive increase in both five- and four-coordinated aluminum, as compared to alumina single oxide (samples calcined at 973 K). Other authors [2, 7, 8] have also reported high proportions of AIv in equimolar (AI/Ti=2) sol-gel AI203-TiO2 that could contribute to the enhanced acidity of this binary system [9], property primarily attributed to the formation of hetero-linkages A13+-O-Ti4+ [10]. By ZrO2 addition no important differences were registered in the A1v amounts (samples calcined at 773 K). Moreover, its proportion decreased as ZrO2 in the formulation increased. On the contrary, Miller et al. [7] found high proportions of pentahedral aluminum (i. e., 44% of the total 27A1) in equimolar alumina-zirconia calcined at 773 K. In our case, the influence of ZrO2 at low concentrations was more clearly reflected in an increased amount of tetrahedral aluminum. In order to assess the changes in structure of the solid matrices due to calcination temperature and TiO2 or ZrO2 addition, we used a pseudospinel model even in the case of

550 the mixed oxides considering that Ti4+ cations could enter octahedral sites in spinel type structures by partially substituting A13+ ions, the basic atomic arrangement being retained [11]. Then, the formula would be A12-4x/sTixO3. We supposed that our solids contained 0.111 cationic vacant sites (pseudospinel). Thus, the chemical formula used was A121.33032 [31. Table 2. 27A1 species relative amounts in the solgel A1203 samples studied. Oxide A1Iv A1v A1vI (%) (%) (%) AIA7 A1A9 AIC7 A1C9 AICll

3.97 5.55 3.71 5.33 14.93

2.94 1.16 2.30 0.43 0.0

93.09 93.29 93.99 94.24 85.07 K

973 K "-,...

773 K I

200

I

100

I

I

I

I

0 -100 8 (ppm)

773 K

I

-200

Fig. 1.27A1 MAS-NMR spectra of solgel A1203 prepared with HNO3 and calcined at different temperatures.

200

1;0

0

-1;0

-200

(ppm) Fig. 2. 27A1 MAS-NMR spectra of solgel A1203 prepared with CH3COOH and calcined at different temperatures.

After Chen et al. [3], let Xo, Xp, XT and XD be the probabilities for a cationic site to be occupied by A1vI, Alv, A1TMand cationic vacant sites, respectively, and the oxygen

551 coordination (to four sites) in our materials similar to that of andalusite, a natural aluminosilicate where six-, five- and four- coordinated aluminum are found [12]. Then: Xo + Xp + XT + XD= 1

(1)

Table 3. 27A1 species relative amounts in the sol-gel AI203-TiO2 and A1203-ZrO2 samples studied. Oxide A1Iv A1v A1vl ,

AT25C AT2C AZ46 AZ22

~ (a)

(%)

(%)

(%)

6.18 9.83 5.27 5.80

1.55 5.41 2.42 1.74

92.27 84.76 92.31 92.46

,.

f

r

!

200

100

0

I

!

-100

-200

8(ppm) Fig. 3. 27A1 MAS-NMR spectra of sol-gel A1203-TiO2 calcined at 973 K. (a) A1/Ti=2, (b) AI/Ti=25.

200

I

100

I

I

I

0

b

-10

I

-200

6 (ppm) Fig. 4. 27A1 MAS-NMR spectra of solgel AIzO3-ZrO2 calcined at 773 K. (a) AI/Zr=46, (b) A1/Zr=22.

There will be six possible types of oxygens surrounded by four sites occupied by AI3+ in different coordinations or cationic vacancies, namely: Oa, Ob, Oc, Od, Oe and Of, Table 4. According to Pauling [13], the strength of the electrostatic bond between an anion and an AI3+ cation in octahedral coordination is equal to +0.5 whereas that between an

552

anion and the same cation in tetrahedral position amounts to +0.75. It follows that the corresponding value when a pentahedral aluminum is involved would be +0.6. Thus, the electrostatic bond strengths (EBS) of various oxygens types proposed will be dictated by the number and type of neighbouring oxygens or vacancies, the corresponding values being shown in Table 4. Table 4. A priori probabilities and electrostatic bond strengths of the different types of oxygens present in an AIv modified spinel. Oxygen type Oa Ob Oc Od Oe Of Probability X o 3 X D X o 3 X T X o 2 X D X p XoXDXTXp Xp2XDXT XoZXDXT EBS 1.5 2.25 1.6 1.85 1.95 1.75 If N is a normalization factor, the number of the six types of oxygens will be:

(2)

NOa + NOb + NOc + NOd + NOe + NOr Or:

N(Xo3XD + Xo3XT + Xo2XDXp + XoXDXTXp + Xp2XDXT + Xo2XDXT)--32

(3)

The electrostatic valence rule states that the net charge in a stable ionic structure should be equal or nearly equal to zero [13]. In order to fulfill this requirement, the electroneutrality condition should be:

N[ (1.5) Xo3XD + (2.25) Xo3XT + (1.6) XozXDXp + (1.85) XoXDXTXp + (4)

(1.95)Xp2XDXT + (1.75)Xo2XDXT)]--'64

In a pseudospinel structure (no pentahedral aluminum) just three types of oxygen exist (Oa, Ob and Of, as here defined). For such atomic arrangement the ratio of equation (4) to equation (3), R, will be two. The calculated R values for our materials are showed in Table 5, that corresponding to A1C11 being the closest to two, followed by that of AT2C. The rest of samples exhibited higher deviations suggesting that their structures were very different as to that of the pseudospinel. In agreement with these results, AICll was the only one sample that showed a definite structural order, as detected by X-ray diffraction (Table 1). If AIC11 had a well defined 7-A1203 arrangement its AlVI/AITMratio would be --3 [14]. The higher value found (~5.7) suggested that for the complete transformation to a 7A1203 structure a even more severe treatment is needed. Table 5. R values (pseudospinel model) for the sol-gel oxides prepared. Oxide AIA7 AIA9 A 1 C 7 AIC9 AIC11 AT25C AT2C R 1.68 1.73 1.67 1.73 1.90 1.75 1.82

AZ46A 1.72

AZ22A 1.74

553 Regarding the AlzO3-ZrO2 mixed oxides, the cationic radius of Zr 4+ (0.079 nm) is larger than those corresponding to Ti 4+ and AI3+ in octahedral coordination (0.060 and 0.053 rim, respectively) [15]. Moreover, its oxygen coordination number (seven or eight) is very different to that of the cationic positions in a pseudospinel structure (four- or sixcoordinated). Owing to that, it is not probable that Zr 4+ cations could enter the AlzO3 matrix. The distribution of different oxygen types with respect to the electrostatic bond strengths for a pseudospinel AlzO3 (AlVI/AllV=2, no pentahedral aluminum, R=2) and the two samples with R values closest to two (where the applied model could be considered valid) are shown in Fig. 5. It could be observed that in a pseudospinel ~36% of the oxygens posses an EBS<2 meanwhile in the A1Cll sample this amount was notably increased (~50%). These kind of oxygens, when located on a lattice plane on the surface, could be associated to hydroxyls of lower positive charge (more basic) than those related to oxygens with higher EBS (more acidic). The former species would be more prone to thermal removal [16], leaving an electron deficient site (Lewis acid site). For the AT2C oxide, -60% of the oxygen population had an EBS<2. That property could contribute to the increased Lewis acidity of this system [9]. A possible objection to our approach could be the lack of evidence for a direct relationship between the surface area and the structural properties of the studied oxides. 3.2. Isopropanol dehydration test A1203 calcined at different temperatures was studied in the isopropanol dehydration (propylene only product), the results at TR=473 K being shown in Fig 6. For both series (AIA and A1C) the trends were similar both on per weight and per surface area basis (not shown). Conversely to other authors [4], there was no correlation between the dehydrating activity and the Al TM amount in the oxides (Table 2), the latter often related to Lewis acid sites [17]. Although A1C11 had a four-fold amount of A1TMto that of AIC7, both samples showed similar dehydrating properties. In our case, a concerted acid-base mechanism (E2, [18]) would be more likely to occur.

Fig. 5. Bulk distribution of oxygens with respect to their EBS for the oxides with R values closest to two.

Fig. 6. Isopropanol dehydration rate of sol-gel A1203 prepared with different additives (per weight basis), TR=473 K.

554 4. CONCLUSIONS In sol-gel A1203, AIzO3-TiO2 and AIzO3-ZrO2 samples calcined at Tc<973 K AITM, AIv and AIvI were detected. The five-coordinated metastable species disappeared after high temperature calcination. To assess the structural changes due to various synthesis parameters a pseudospinel model was used. The A1203 prepared with CH3COOH and calcined at 1173 K (AlVI/AlIV--5. 7, no AIv) was the material that best fit the proposed model. No correlation was found between dehydrating activity and A1TMconcentration in A1203 oxides. ACKNOWLEDGMENTS The authors acknowledge CONACYT (J. Escobar also to IMP) for financial support. REFERENCES

1. J.A. Wang, X. Bokhimi, A. Morales and O. Novaro, J. Phys. Chem. B, 103 (1999) 299. 2. S.S.X. Chiaro, J.L. Zotin and A.C. Faro Jr., Stud. Surf. Sci. Catal., V. 118, Preparation of Catalysts VII (B. Delmon et a|., eds.), Elsevier, Amsterdam, (1998) 633. 3. F.R. Chen, J.G. Davis and J.J. Fripiat, J. Catal., 133 (1992) 263. 4. C.R. Narayanan, S. Srinivasan, A.K. Datye, R. Gorte and A. Biaglow, J. Catal., 138 (1992) 659. 5. J. Escobar, J.A. De Los Reyes and T. Viveros, Ind. Eng. Chem. Res., 39 (3) (2000) 666. 6. S. Kureti and W. Weisweiler, Appl. Catal. A: Gen., 225 (2002) 251. 7. J.M. Miller and L. Jhansi Lakshmi, J. Phys. Chem. B, 102 (1998) 6465. 8. T. Klimova, H. Gonz~lez, R. Hern~ndez and J. Ramirez, Stud. Surf. Sci. Catal., V. 118, Preparation of Catalysts VII (B. Delmon et al., eds.), Elsevier, Amsterdam, (1998) 807. 9. J.A. Montoya, T. Viveros, D. Chadwick, J.M. Dominguez, J. Navarrete and I. Schifter, J. Sol Gel Sci. Tech., 2 (1994) 431. 10. M. Toba, F. Mizukami, S. Niwa, Y. Kiyozumi, K. Maeda, A. Annila and V. Komppa, J. Mater. Chem., 4 (4) (1994) 585. 11. A. Guti6rrez, M. Gonz~lez, M. Trombetta, G. Busca and J. Ramirez, Microporous Mesoporous Mater., 23 (1998) 265. 12. C.W. Burnham and M.J. Buerger, Z. Kristallogr., 115 (1961) 269. 13. L. Pauling, The Nature of the Chemical Bond, Cornell Univ. Press, Ithaca, New York, 1960. 14. C.S. John, N.C.M. Alma and G.R. Hays, Appl. Catal., 6 (1983) 341. 15. M. Daturi, A. Cremona, F. Milella, G. Busca and E. Vogna, J. Euro. Ceram. Soc., 18 (1998) 1079. 16. H. Kn6zinger and P. Ratnasamy, Catal. Rev. Sci. Eng., 17 (1) (1978) 31. 17. A.B. Mohammed Saad, V.A. Ivanov , J.C. Lavalley, P. Nortier and F. Luck, Appl. Catal. A: Gen., 94 (1993) 71. 18. B.C. Gates, Catalytic Chemistry, John Wiley and Sons, Inc., New York, 1992.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

555

Promotion of Ru/ZrOz catalysts by platinum A.M. Serrano-Sfinchez, F. Blas-Sufirez, P. Steltenpohl*, M.P. Gonzfilez-Marcos, J.A. Gonzfilez-Marcos and J.R.Gonzfilez-Velasco Department of Chemical Engineering, Faculty of Sciences, University of the Basque Country/EHU. P.O. Box 644, E-48080 Bilbao, Spain. Contact e-mail: [email protected] *Department of Chemical and Biochemical Engineering, Slovak University of Technology, 81237 Bratislava, Slovak Republic. Contact e-mail: [email protected] Physico-chemical and catalytic properties of zirconia supported ruthenium and ruthenium-platinum catalysts were investigated. In order to improve the ruthenium stability a second noble metal, namely platinum, was introduced into the catalyst. Ten catalysts, consisting of zirconia supported Ru and/or Pt were prepared. Investigation of supported ruthenium phase stability was based on atomic absorption spectroscopy (AAS) measurements of the active metal loading variation in catalysts caused by different pre-treatment procedures. Ruthenium stabilisation in the presence of platinum was verified by temperature programmed reduction (TPR) experiments. Finally, the stabilising influence of Pt on Ru catalysts was evidenced using methylcyclopentane (MCP) hydrogenolysis as model reaction. 1. I N T R O D U C T I O N Combined ruthenium-platinum catalysts are prepared with the aim of taking advantage from the corresponding catalytic properties of both metals. Ruthenium, able to dissociate hydrogen molecules, enhances the catalyst activity, and platinum prevents the catalyst from excessive deactivation. Combination of ruthenium and platinum is especially advantageous due to some specific features: (i) the crystalline Ru-Pt particles are formed in the whole metal composition range, (ii) Ru-Pt bimetallic clusters are reasonably stable in both oxidising and reducing atmospheres, (iii) both metals are effective catalysts in many interesting reactions, (iv) Ru and Pt are easily reducible metals, and (v) both metals show a well defined stoichiometry in hydrogen chemisorption [1]. Zirconia is considered a neutral support due to its dual superficial characteristics, oxidizing or reducing and basic or acid behaviour. Moreover, zirconia is a more reactive support than silica and alumina and hence with a stronger metal-support interaction. In this particular work, Ru(NO)(NO3)3 and HaPtCI6"2HzO have been chosen as metallic precursors for their incorporation onto zirconia support. Different ruthenium species are present in solution depending on pH: neutral, anionic or cationic complexes of general

556 formula [Ru(NO)(HzO)x(NO3)s_x] x-z [2,3]. In aqueous solution, the hexachloroplatinic acid suffers hydration and hydrolysis, and behaves like a poor acid of pKa=3.8 [2,4]. TPR analysis allows to study reducibility, nature and number or distribution of different reducible species present in the catalytic precursor sample [5-7]. Thus, metal-support interaction, state in which the supported metals can be found in the catalyst, and mutual effect on their reducibility can be studied [8,9]. The methylcyclopentane (MCP) hydrogenolysis has been employed to study the catalytic surface, due to its sensitivity to metal dispersion and electronic structure [10-14]. Depending on operation conditions and type of catalyst, it is possible to obtain products from ring opening, cracking, and ring enlargement and aromatization reactions in case of acid sites in the catalyst [11,12,15]. Ruthenium and platinum behaves very differently in this reaction. Ruthenium is active at 413 K and shows a high hydrogenolytic capacity. However, platinum cannot activate the hydrogen molecule as well as ruthenium and is active at 493 K. Alloying ruthenium with platinum, a decrease in activity and deep hydrogenolysis can be expected in favour of a selectivity towards C6 [11,12,16]. The objective of this work is to determine the stabilizing effect of platinum on the metallic ruthenium phase when supported on zirconia, instead of more conventional supports, as SiOz, AIzO3 or TiOz. The pre-treatment influence, both for monometallic ruthenium and bimetallic ruthenium-platinum catalysts, is also studied. 2. E X P E R I M E N T A L

2.1. Catalysts preparation The support, zirconia (ISA), was supplied by the Norton Company. The oxide was grounded and sieved to a particle size ranged from 0.16 to 0.25 mm, and calcined at 773 K. Its surface properties, 63.3 m2g-1 of specific surface and average pore diameter of 8.60 rim, were determined from the nitrogen adsorption isotherms. The catalysts were prepared by adsorption from solution and/or impregnation of precursor(s), ruthenium nitrosyl nitrate (Alfa) and hexachloroplatinic acid (Aldrich), onto the support. Being zirconia isoelectric point 6.5 (determined by electrophoresis [17] using a Malvern Instrument Zetasizer 4) the precursors solution pH value was kept sufficiently low to enable the desired adsorption of complex metal anions. The final catalysts were activated by either calcination-reduction or drying-reduction of precursors. Two levels of reduction temperature, 573 and 773 K, were chosen. Thus, the denomination of a catalyst reflects composition and preparation method (A/I=monometallic adsorbed/impregnated and CA/CI=bimetallic coadsorbed/coimpregnated catalysts), pretreatment procedure (C=calcination or D=drying) and activation temperature (5 = 573 K or 7 = 773 K), e.g. CAC7 denominates the bimetallic catalyst prepared by coadsorption, calcinated and then reduced at 773 K. 2.2. Procedures in characterization A Perkin-Elmer model 1100B spectrophotometer was used to determine the actual metal loading by atomic absorption spectroscopy (AAS).

557

TPR of the catalyst precursors was used to determine the presence of metal-metal interaction. Pulse chemisorption of hydrogen was employed to calculate the catalyst dispersion. Both techniques were carried out in an Autochem 2910 Micromeritics apparatus. Before TPR experiments, the catalyst precursor was conditioned in Nz, rising the temperature from ambient to 323 K. TPR data were obtained from 223 to 773 K in a flow of Hz/Ar = 5/95, through a thermal conductivity detector (TCD). Prior to the pulse chemisorption experiments the catalysts were degassed by heating from ambient temperature up to 773 K in At. Then the sample was cooled to 273 K and pulses of hydrogen were injected into the stream of argon. The hydrogen amount adsorbed by the sample was measured by TCD.

2.3. Procedures in reaction The operating conditions have been chosen so as to minimize secondary reactions as well as carbon deactivation: total pressure, P = 6.0 bar; temperature, T = 473-533 K; catalyst weight, We = 0.0319-0.4541 g; feed-stream, FMCP= 0.25 tool h 1" Hz/MCP = 40/1" Nz/MCP = 80/1. The activity measurements were achieved in a fixed bed differential reactor (conversion below 10-15 %), designed to minimize the time necessary to reach the steady state [18,19]. 3. RESULTS AND DISCUSSION

3.1. Catalyst preparation and characterization The thermal instability of ruthenium is well known. Gonzfilez-Velasco et al. [2] observed some metal loss in the form of RuO4 after exposing the ruthenium precursors and catalysts to oxidising or inert atmosphere. This fact was not observed in reducing atmosphere. Nominal metallic content of monometallic catalysts was 1 and 0.5 wt% in ruthenium for adsorbed and impregnated catalyst respectively, while that of bimetallic catalysts was 0.5 + 1 wt% for the coadsorbed, and 0.25 + 0.5 wt% for the co impregnated, in Ru and Pt, respectively. The actual metal loading of the corresponding catalysts as measured by AAS are summarised in Table 1. Ruthenium actual metal loading is similar to nominal in both mono- and bimetallic catalysts, but it does not occur the same with platinum. The results in Table 1 show that, if the two metallic precursors are present in the initial solution, zirconia support reveals preference towards the ruthenium complex. The main factor affecting metal content is the reduction temperature. The higher the level, the higher the loss in metal is observed with respect to precursors. The calcination protocol preserves more the metal loading than the drying protocol, if CAC and CAD series are compared. Comparing the ruthenium loss observed for mono- and bimetallic catalysts, stabilization of adsorbed ruthenium in bimetallic catalysts could be stated. This stabilization should be attributed to the presence of platinum, that interacts with ruthenium. In order to prove this hypothesis TPR experiments were performed. Fig. 1 shows the decomposition of monometallic ruthenium precursors, impregnated (b) and adsorbed (e), and monometallic platinum adsorbed precursors (f) supported on zirconia. These results are compared to those obtained for bimetallic precursors prepared either by

558

mechanical mixture of the two monometallic precursors (a) or by coimpregnation (c) or coadsorption (d). Table 1. Metallic contents of precursor and catalysts, in wt%, results of pulse chemisorption, TOF values and selectivity towards paraffinic fraction at 503 K. Precursor or

Nominal

Actual

catalyst

Ru

Pt

Ru

Pt

Adsorbed

1.0

m

1.02

.

AD5

0.51

AD7

0.62

~

0.54

--

0.51

Impregnated

0.5

ID5 ID7

Pulse chemisorption pmOIu2gcat -1 .

.

TOF,

S -1

L, nm

n-H/2-MP

(503 K)

.

7.89

4.10

0.099

0.35

11.00

3.58

0.041

0.26

--

20.12

1.62

0.250

0.07

23.57

1.05

0.280

0.24

0.39

--

0.50

0.79

CAC5

0.40

0.67

9.29

4.30

0.007

0.32

CAC7

0.40

0.50

11.42

3.16

0.013

0.10

CAD5

0.20

0.62

7.38

3.56

0.005

0.12

CAD7

0.14

0.51

9.66

2.09

0.013

0.08

0.24

0.47

Coadsorbed

0.5

1.0

Coimpregnated 0.25

0.5

CID5

0.24

0.42

7.24

3.31

0.068

O.11

CID7

0.21

0.34

8.85

2.34

0.061

0.11

.2

o

c)

'd) l

273

I

I

I

373 473 573 673 Temperature, K

I

773

Fig. 1. TPR profiles for monometallic and bimetallic catalyst precursors.

Monometallic platinum precursor shows two reduction peaks in Fig. 1 [9,20]. On the other hand, ruthenium precursors show two overlapped peaks corresponding to the reduction of Ru(IV) and to reduction of welldispersed ruthenium [21]; the latter presenting a much higher intensity. Fig. 1 shows clearly that only one hydrogen consumption peak was found for the bimetallic precursor prepared by coadsorption, which has been assigned to hydrogen conjointly consumed during the reduction of both metals due to the formation of a kind of alloy between platinum and ruthenium. The bimetallic clusters thus formed will be richer in platinum for the CAD series than for the CAC series, due to the metal contents shown in Table 1.

559

For the coimpregnated sample, the peaks are situated at lower temperature with respect to those obtained for the monometallic precursors and the mechanical mixture. This is attributable to a synergetic phenomenon between the two metals. However, the peak at 470 K is very intense and it can be related to the reduction of all the platinum with part of the ruthenium, if the signal is compared to that obtained for the impregnated monometallic precursor. Therefore, the strong interaction between the two active phases is less marked in this case. Results of pulse chemisorption experiments are shown in Table 1. Using the standard evaluation method and a stoichiometric ratio Hz/Metal=2 for both ruthenium and platinum, the active metal dispersion was calculated, and from it the mean metal particle size, L. The results agree with those obtained for monometallic precursor TPR profiles in Fig. 1, with a big peak corresponding to well-dispersed ruthenium on the support. However, the most significant variable affecting .~ 8 dispersion is the metal content. [] 9 H/Ru Dispersion diminishes with increasing ~N ~ 6 [] 9 H/Pt metallic contents [21,22]. This trend is ~: 9 o H/(Ru+Pt) presented in Fig. 2, where the H/metal "~ 9 ratio is plotted v e r s u s the actual 4 ::k []m 9 metallic loading. ,-g [] "~ [] oAoA Reduction temperature has a [] 13(3 o o o 2 positive influence on metal dispersion 0 for both monoand bimetallic 0 0.2 0.4 0.6 0.8 1 catalysts. As far as pre-reduction Metal content, % treatment is concerned, when drying treatment is used, slightly more Fig. 2. Cl~misorbed hydrogen as a ft~ction dispersed particles are observed in CA of actualrr~talcontent,

series. Both cases are related to a lower metal content (Table 1) as well. Moreover, when drying protocol is used, the lowest mean particle size can be expected, due to a high mobility of metallic complexes on the support [19]. However, through calcination, the complexes anchoring onto the support occurs via metal oxides such as PtO and RuOx [23], giving worse dispersion values. 3.2. Activity and selectivity Behaviour of the catalysts in MCP hydrogenolysis is presented in Table 1, TOF at 503 K, and Fig. 3, where the areas are additive. An intermediate temperature has been chosen as suitable to study different products. The order of activity for the five catalytic series can be established as: ID > CID > CAD ~ AD >CAC, with an order of magnitude between the most active and the less active catalyst. The high activity shown by the monometallic impregnated catalysts is mainly due to cracking, directly related to the ruthenium metallic phase content and to the mean cluster size. The higher the ruthenium content in the catalyst, the lower the cracking activation energy, as can be seen in Fig. 4. This fact was also observed by Villamil et aL [24] in MCP

560 hydrogenolysis on Ru/MgO, who obtained that cracking activity increased with metal particle size. Platinum incorporation reduced the TOF values with respect to those of single Ru catalysts. Platinum inhibits the deep hydrogenolysis, whose activation energy increases. Similar activity has been reported by other authors for Pt-Ru/AlzO3 [11], Ru-Pt/SiOz [12] and Ru/AlzO3 [15]. However, Bond et al. [25] found that Ru/ZrOz is ten times less active than Ru/SiOz in propane and n-butane hydrogenolysis. There is no temperature reduction level that improves the activity in all catalysts. Adsorbed and coimpregnated catalyts are more active for the lowest reduction temperature, 573 K, while the 7 series is the most active in the rest, specially in the CA Fig. 3. Activity and selectivity in MCP series. The reactive character of hydrogoao lisisat503 K. zirconia is responsible for the fact that higher reduction temperature ". 80 does not necessarily produce a higher metal dispersion. ~ CAC5 9 9 CID5 The preferred pre-reduction ~ 60 CAC7 9 protocol for bimetallic catalysts is ~ 9 CID7 CAD7 drying, where a lower metal content .-~ 40 ID7 9 9 9 AD7 gets similar values of TOF, to ~= CAD5m calcination, although there is more ~ ID5 platinum present in the active phase. ~~" 20 AD5 Also it is remarkable that this activity is due to a greater extent to 0 , , 0 deep hydrogenolysis in CAD than in 0 0.2 0.4 0.6 CAC series. Thus, more segregation C o n t e n t in Ru, % of particles of ruthenium from bimetallic clusters occurs with Fig. 4. Relationship between cracking and drying, ruthenium loading. Concerning selectivity, the most interesting parameter is n-H/2-MP, because it gives the ratio between the lineal paraffin and the isoparaffin, and hence the ratio between non-selective (NSM) and selective mechanism

561 (SM) in hydrogenolysis of MCP [10-12]. In an hydrogenolysis process at random, the ratio n-H/2-MP, is 1.0. This value is theoretical, but in any non-selective process a value slightly lower than 1.0 will be obtained, due to the steric hindrance in the n-H breaking. This fact increases slightly the likelihood to obtain 2-MP. In preliminary assays to establish the optimal reaction conditions, it was determined that ruthenium, at 573 K, is capable of cracking n-H, more stable than 2-MP and 3-MP isoparaffins. However, the ruthenium behaviour in bimetallic catalysts is different in the reaction temperature range chosen, 476-533 K, as the fraction of n-H in the distribution of products does not changed appreciably with the preparation procedure (see Fig. 3). So, in these conditions, the cracking products are considered to arise from isoparaffinic products through consecutive reactions [12]. 2-MP is the most abundant product obtained, specially for the impregnated and co impregnated catalysts, and this 40 selectivity is linked to the mean metal "T' particle size, as can be seen comparing 9 [] Fig. 3 and Table 1. As a general trend, n the lower the crystal size, the higher the selectivity towards 2-MP. Lower 20 metal content in the catalysts leads to a higher dispersion and hence a lower mean metal particle size, with better selectivities towards 2-MP. Thus, its activation energy increases with the metal content. Fig. 5 shows this 0 0.4 0.8 1.2 relationship. Metallic content, % This tendency is against the Fig. 5. Dependence of 2-MP activation energy adlineation concept, developed by with actual metallic loading. Kramer and Zuegg, and observed in Pt/AI203 [10] and Pt/SiO2 [14,26]. The adlineation concept suggests that n-H fraction produced increases with dispersion as the number of interfacial sites increase. This behaviour is characteristic of NSM. In CAC and CID catalysts the fraction of 2-MP in the products is similar, but production of other C6 is higher in CAC. So, again, this behaviour can be attributed to the formation of some type of bimetallic alloy or a strong interaction between the two metals, promoted by the calcination activation protocol with the creation of dual sites Pt-Ru [12]. Also this synergetic effect was observed in the coimpregnated catalysts, but the selectivity towards cracking was slightly higher, what is related to a weaker interaction and more amount of free ruthenium [11], as was seen in TPR experiments. As far as the global selectivity, the ratio of the methylated products (3-MP+2-MP) to the MCP consumption is concerned, the bimetallic catalysts showed selectivities much higher than the theoretically expected for a non selective behaviour, 0.6 (Figure 3). AD series shows a good selectivity towards isoparaffinic products, and ID series presents the lowest m

m

[]

9,

"v"

i

i

,

562

value. This fact could be associated with cracking processes, which are less important for a higher reduction temperature, producing isoparaffins, as it can be seen in Figs. 3 and 4. 4. C O N C L U S I O N S Platinum promotes Ru/ZrOz through the formation of intermetallic alloys, or a strong interaction between the two metallic phases for the two bimetallic precursors, although this phenomenon is less marked in the coimpregnated sample. The incorporation of platinum reduces TOF values and the selectivity to products of deep hydrogenolysis with respect to monometallic catalysts. The order of activity can be established for the five catalytic series as: ID > CID > CAD = AD >CAC. Bimetallic catalysts showed selectivities much higher than the theoretically expected for a NSM. AD series showed a good selectivity towards isoparaffinic products, and ID serie presented the lowest value from the theoretical. In CAC and CID catalysts, the fraction of 2-MP remained constant, but that of other C6 was higher in CAC. with the creation of dual sites Pt-Ru. However, it can be concluded that the impact of platinum on ruthenium dispersion is small. Reduction temperature seems to have positive influence on metal dispersion for both mono- and bimetallic catalysts. There was no reduction temperature that improved the activity in all catalysts. Cracking or deep hydrogenolysis was directly related to higher ruthenium content and to higher mean cluster size. Catalysts reduced at 773 K are more suitable to produce isoparaffins. ACKNOWLEDGMENT The authors acknowledge the Universidad del Pals Vasco/EHU for the financial support (UPV 069.310-G40/98) to this research and for a FPI Grant to one of the authors (A.M.S.S.). REFERENCES

1. H. Miura, T. Suzuki, Y. Ushikubo, K. Sugiyama, T. Matsuda and R.D. Gonzfilez, J. Catal., 124 (1984) 194. 2. J.R. Gonzfilez-Velasco, M.A. Guti6rrez-Ortiz, J.A. Gonz~ilez-Marcos, P. Pranda and P. Steltenpohl, J. Catal., 187 (1999) 24. 3. L. Maya, J. Inorg. Nucl. Chem., 41 (1979) 67. 4. G.H. Van Den Berg and H.Th. Rilnten, Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon. P. Grange, P. Jacobs and G. Poncelet (eds.), p. 265, Elsevier, Amsterdam, 1979. 5. J.L. Falconer and J.A. Schwab, Catal. Rev.-Sci. Eng., 25 (1983) 141. 6. N.W. Hurst, S.J. Gentry, A. Jones and B.D. McNicol, Catal. Rev., 24 (1982) 233. 7. A. Baiker, AIChE, 25 (1985) 30. 8. A.S. Sass, N.A. Antonova and N.M. Popova, Kin. Catal., 37 (1996) 105. 9. M.U. Kilsyuk and V.V. Rozanov, Kin. Catal., 36 (1995) 13.

563 10. F.G. Gault, Adv. Catal., 30 (1981) 1. 11. G. Diaz, F. Garin and G. Maire, J. Catal., 82 (1983) 13. 12. G. Diaz, F. Garin, G. Maire, S. Alerassol and R.D. Gonzfilez, Appl. Catal. A: General, 124 (1995) 33. 13. Y. Zhuang and A. Frennet, Appl. Catal. A: General, 134 (1996) 37. 14. Y. Zhuang and A. Frennet, Appl. Catal. A: General, 177 (1999) 205. 15. B. Coq, A. Bittar and F. Figueras, Appl. Cata|. A: General, 59 (1990) 103. 16. M.J. Dees, M.H.B. Bol and V. Ponec, Appl. Catal. A: Gen., 64 (1990) 279. 17. J.M. Brunelle, Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon. P. Grange, P. Jacobs and G. Poncelet (eds.), Elsevier, Amsterdam, 1979. 18. J.R. Gonzfilez-Velasco, J.A. Gonzfilez-Marcos, M.A. Guti~rrez-Ortiz, J.I. Guti6rrezOrtiz and S. Amaiz, Spanish patent No. 9.500.364 (1995). 19. S. Amaiz, Ph.D. Thesis. University of Basque Country/EHU (1997). 20. D.L. Hoang, H. Berndt and H. Lieske, Catal. Lett., 31 (1995) 165. 21. P. Betancourt, A. Rives, R. Hubaut, C.E. Scott and J. Goldwasser, Appl. Catal. A: Gen., 170 (1998) 307. 22. J.G. Goodwin, J. Catal., 68 (1981) 227. 23. G. Del/imgel, V. Bertin, P. Bosch, R. G6mez and R.D. Gonzfilez, New J. Chem., 15 (1991) 643. 24. P. Villamil, J. Reyes, N. Rosas and R. G6mez, J. Mol. Catal., 54 (1989) 205. 25. Bond, G.C. and Hooper, A.D., Appl. Catal. A: General, 191 (2000) 69. 26. Bond, G.C. and Pafil, Z., Appl. Catal. A: General, 86 (1992) 1.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

565

Catalysts based on RhMo6 heteropolymetallates. Bulk and supported preparation and characterization. C. I. Cabello .1, I. L. Botto 2, M. Mufioz 1 and H. J. Thomas ~. ~Centro de Investigaci6n y Desarrollo en Procesos Cataliticos, CINDECA-CONICETUniversidad Nacional de la Plata, (1900) La Plata, Argentina. 2Centro de quimica Inorgfinica CEQUINOR- CONICET-Universidad Nacional de La Plata, (1900) La Plata, Argentina.

The ammonium salt of Rh(III) Anderson type heteropolymolybdate [RhMo6024H6] 3- has been prepared and characterized by powder X-ray diffraction, spectroscopic [FTIR-Raman, DRS (UV-visible)] and SEM-EDAX electron microscopy techniques. The water soluble salts were used in the design and preparation of 7-A1203 supported catalysts. The varied Mo:Rh ratio of both olution and solid samples was measured by AAS technique. The supported oxidic system was characterized by DRS spectroscopy and SEM-EDAX microscopy. The HDS and HYD activity for different bimetallic catalysts was measured in a high-pressure reactor. In addition, some conventional catalysts and some CoM06 and combined supported systems [(RhMo6 + AIM06)] have been tested for comparative purposes. The discussion about the performance of the new catalysts is made on the basis of the structural and physicochemical heteropolyanion properties as well as the preparation conditions. 1. I N T R O D U C T I O N Heteropolyoxometallates are polymeric species formed by the condensation of more than two different oxoanions. There is a great variety of these structures [1,2]. The compounds with this type of anions containing molybdenum are very interesting for chemistry and particularly for hydrotreating catalytic processes [3,4]. Although practical applications of some of these phases, especially those belonging to the Keggin structure, have been tried for a long time, it is only recently that the relationship between the crystal structure, the physicochemical properties and the catalytic functions has been carefully investigated [2]. Our recent research on a large series of hexametallates named Anderson [XM6024H6] 3-, [1] [with M = Mo; W or Mo(6_x)Wx and X = Co(III); Cr(III); Rh(III); Fe(III); Ni(II); Cu(II); Te(VI) etc.] [5,6,7] enabled the design and preparation of a variety of mixed bi- or tri-metallic phases which show interesting structural and redox properties, and can be used especially in heterogeneous catalysis. These planar heteropolyanions are precursors that provide welldispersed bi- or tri-metallic ensembles on supports and have been proved successfully on some catalytic processes such as hydrotreating, ammoxidation, etc. [3,8,9]. * Corresoondinu author. Member of the research staff of CICPRA. Fax" +54-221-42q4277 F.-mnil"

566 Particularly, Mo(VI)-Co(III) and Mo(VI)-Ni(II) containing species (here in CoM06 and NiM06 monolayer supported phases) have shown to be important precursors for HDS and HYD reactions [3,4]. In this sense, it seems important to prepare and characterize the RhM06 species as both bulk and 7-A1203 supported phase (by equilibrium impregnation with aqueous solution of RhM06 ammonium salts) in order to analyze the catalytic behaviour. In the last years, rhodium-based catalysts have shown a relatively high activity and selectivity for NO reduction to N2. The highest selectivity distinguishes the Rh catalyst from other Pt- and Pd-based ones in the ability to promote N-pairing in adsorbed NO molecules before the N-O bond is broken. This property is related to the highly dispersed supported Rh catalysts used in practice [10]. Hence, clean combustion (CO elimination and NOx reduction) can be another promising use of the catalysts based on RhM06, in addition to the hydrotreating processes. The present work shows the preparation and characterization of RhMo6/7-AI203 based catalysts and the preliminary results for hydrotreating (HDS of thiophene and HYD of cyclohexene) tests, discussed on the basis of the relationships between catalytic activity and spectroscopy and structural behaviour of RhM06 heteropolyanion. In addition, similar tests have been performed with [RhMo6+A1Mo6]/7-AI203 combined system; RhMo6/7-A1203 pretreated catalyst in reducing atmosphere; CoMo6/7-AI203 and Co-Mo or Rh/7-A1203 commercial catalyst for comparative purposes. 2. EXPERIMENTAL

2.1. Synthesis and characterization of the pure phases (NH4)3[RhMo6Oz4H6]. 7H20 was prepared by solution reaction from Rh(III) chloride hydrate and ammonium heptamolybdate (AHM), as previously described [11, 12]. The pure samples were characterized by X-ray powder diffraction analysis (DRX), using a Philips PW 1714 diffractometer (Cu Ka radiation, Ni filtered), Fourier transform infrared spectroscopy, using a Bruker IFSS 66 FT-IR equipment (KBr pellet technique); Raman spectroscopy using a Spex-Ramalog 1403 double monochromator spectrometer, equipped with a SCAMP data processor (excitation line: 514.5 nm of an Ar-ion laser), Scanning electron microscopy (SEM) using a Philips 505 with an EDAX 9100 for the electron probe microanalysis and Diffuse Reflectance spectroscopy (DRS) by a UV-VIS Varian Super Scan3 Spectrophotometer with a diffuse reflectance chamber with integrating sphere. The range covered was 200-800 rim, using BaSO4 or ~-AlzO3 as reference for pure and supported phases respectively. 2.2. Catalysts preparation The supported catalysts were prepared by equilibrium impregnation of ~,-AlzO3 with aqueous solutions of RhMo6 ammonium salts in the Mo range of 20-100 ~tmolMo/ml. The ~,AlzO3 powder used had a specific surface area of 226mZ/g, a pore volume of 0.36 cm3/g and a grain size of 200 ~m. All impregnations were performed at room temperature employing an excess of solution with continuous stirring for 24 h. The solid was then separated by centrifugation and dried at 350 K. Chemical analyses of the solutions before and after impregnations were made using an IL-457 atomic absorption spectrometer (AAS). This enabled calculation of the values of C~ (initial concentration of the impregnating solution) and Cf (final concentration of the impregnating solution) expressed as ~tmolMo/ml solution. From these values and the use of a simple material balance equation, it was possible to calculate Ca (the concentration of adsorbed metal, i. e. Rh, Co, and Mo) expressed as g metal/g support. In

567 addition, the [RhMo6+A1Mo6]/7-Al203 combined system was prepared by using a mixed solution of both rhodium and aluminium heteropolymolybdates 0.5:0.5 ratio) in attempts to analyse the synergic effect of rhodium, (since the Al(III) species is inactive for hydrotreatment processes). Some RhMo6 samples were pretreated by TPR up to 473 K before being introduced in the catalytic reactor.

2.3. Adsorption isotherm The adsorption isotherms for the RhMo6 Anderson phase were measured at room temperature using the Ci, Cf and Ca values determined for M=Mo. When the concentration of Mo adsorbed (Ca) expressed as monolayer (%) was plotted against the Mo concentration in solution at equilibrium (Cf), the shape of the resulting curve was found to follow the Langmuir model [13]. Hence, by plotting the linearised form of the Langmuir equation, i. e. Cf/Ca = [1/(Kad S)]+ Cf/S

(1)

and extrapolating the subsequent straight line obtained, it was possible to calculate the total number of active sites present on the surface (S) expressed in gMo/g 7-A1203. The equilibrium adsorption constant (Kad), expressed in ml/g Mo could be obtained from the slope of the line.

2.4. Catalytic activity The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts were measured in a high pressure reactor. The experimental conditions for the activity tests were a feedstock of thiophene (15000 ppm), cyclohexane (90%) and cyclohexene (10%), flow rate 0.353 ml/min, total pressure = 26 Kg/cm2 and LHSV=52 1/h. Additional experiences with toluene (90%) and cyclohexene (10%) were also carried out. The operative conditions for the hydrotreating test were selected according to the recent experience about the CoMo6 [3]. The products were analysed by gas chromatography by means of a Varian Start 3400 gas chromatograph, with FID detector. 3. RESULTS AND DISCUSSION

3.1. Morphological and structural features Structurally, the [XMo6024H6] group is made up of a compact package of six MoO6 octahedra surrounding one XO6 polyhedron in a planar hexagonal configuration, with approximate overall D3d symmetry [14]. In general, the ionic radius of the metallic central ion or heteroatom is 0.5 to 0.7A, so that the preparation of a great variety of phases is possible, with X(III): AIMo6; CoMo6; CrMo6; RhMo6; FeMo6, with X(II): CuMo6, NiMo6 and with X(VI): TeMo6. It is also possible to prepare solid solutions, by partly replacing the heteroatom by any other of the above mentioned ions by means of a stoichiometric mixture of the corresponding nitrates or sulfates with ammonium heptamolybdate, e.g.: Cr0.25A10.75Mo6 or Cr0.75A10.25Mo6 [6]. Moreover, the method enables the synthesis of phases having W, which can partially or totally substitute the Mo, producing phases of [XMonW(6-n)O24H6] type. From the crystal-chemical point of view, the phases containing X (III) are isomorphic, therefore the X-ray powder diagram of the Rh phase is similar to that corresponding to Co, Cr and Al. Also, scanning electron microscopy (SEM-EDAX) is a good method to characterize these phases, as the obtained microphotographs show the same crystalline characteristics (square plates) for all the series of phases [5]. On the other hand, the semi-quantitative chemical

568 analysis by microsound (EDAX), showed Rh and Mo contents of 1 5 . 9 1 % and 84.09 %, comparable to the theoretical values of 15.16 % and 84.83 %, respectively, in pure samples. 3.2.Characterization by FTIR and Raman spectroscopy. Fig. 1 shows the FTIR spectra for the RhMo6 phase in the region 4000-400 cm -1 55

55

50

(a)

50

45

45

40

40

o~ 35

/

o~' 35

~" 30 25 ~ v ~

~ ~

20 15 4000

(b)

~" 30 25

"

~ '

~ 3500

20 ' v

3000 [cm -1]

'

2500

'

2000

15 2000

'

'

1500 v

1000

'

' 500

[cm -1]

Fig. 1: FTIR spectra of (NH4)3[RhMo6Oz4H6].7H20 in the range (a): 4000 to 2000 cm -a and (b): 2000 to 400 cm -1. In general, the spectra for an ammonium salt can be divided into the following typical regions: 3600-2800 cm -1 (O-H and N-H stretchings), 1650-1400 cm -1 (OH and N-H bendings), 1000-800 cm -1 (Mo-Ot vibrations), 750-550 cm -1 (fundamentally Mo-Ob modes) and below 450 cm -1 (Mo-Oc and some other modes). This low spectral region is rather difficult to assign. Between 550-500 cm -1, it is possible to observe some bands attributable to water librations. Below 450 cm -1, the bendings of the Mo-Ot and Mo-Ob bonds can be mixed with the Mo-Oc stretching vibrations. In relation to these last modes, only the Mo-Ot vibrations can be considered as pure stretching. [15]. On the other hand, it is possible to observe from the crystallographic information [16], that the heteropolyanion size does not differ significantly from the X-ionic radius. The molybdic framework of this type of phases usually presents a great capability for a rearrangement when some atomic replacement occurs. This can lead to slight changes in the Mo-O-Mo angles, but not noticeable variations in the heteropolyanion-size. This effect was also observed in some Keggin phases [17,18]. In some previous vibrational studies for Anderson phases, the observed bands seem to be independent from the heteroatom types [12]. However, by careful measurements between 950-200 cm 4 of the Raman spectra for all members of this series, it is possible to notice that the bands associated to the Mo-Ot bonds are the most affected by the X change. The vibrational modes associated to the Mo-Ob and Mo-Oc bonds are less affected by the change of heteroatom, undoubtedly because they cannot be considered as pure stretchings and show some bend- character [15]. The Raman spectra, showing very sharp bands, make the shift clearer. Generally, Raman spectra of compounds with an octahedral coordination exhibit main lines in the high frequency range (800-1000 cm -l) [19]. Table 1 shows Raman frequencies for the RhMo6 and some other Anderson phases for comparative purposes.

569

Table 1 Raman spectra of ammonium salts of RhMo6, CrMo6, CoMo6 and A1Mo6, (registered between

1000- 180 cml). RhM06

CrM06

CoMo6

AIM 06

947 s 947 vs 945 vs 906 m 900 s 900 s 572 w 569 w 570 vw low 480 vw 481vw resolution 363 m 363 m Ref.: vs: very strong, s: strong, m: medium, w: weak, vw: very weak

944 vs 902 m 578 w 482 vw 364 m

Assign ation VsymMo-Ot VasymMo-Ot

vX-O vMo-Oc 6Mo-O, 6X-O

The most intense Raman line clearly shows that the Mo-Ot symmetric stretching is affected by X substitution. The general trend is a weakness of the Mo-O terminal bonds when the X(III) -size decreases (rAl(III) = 0.530A, r Co(re)lowspin -- 0.545/~, r Cr(III) = 0.615A, r Rh(In) = 0.665 A). This can be attributed to the X(III) polarizing power which produces an inductive effect, affecting the mentioned type of Mo-O bonds. On the other hand, the existence of the hydrogen bonds, proceeding of the NH4 + ions and the H20 out of the XMo6 ring, seems to contribute to this effect.

3.3. Characterization by diffuse reflectance spectroscopy Fig. 2 shows the diffuse reflectance spectrum of the RhMo6 phase, which is compared to CoMo6 and CrMo6 isomorphous phases. It is known that there are more than two types of chromophores in the polyoxometallates [20]. The charge- transfer transitions of the Mo-O bonds usually appear in the highest energy region of the DRS spectra (~250 nm for the Mo-O terminal bonds and between 250-350 nm for the Mo-O-Mo bridge bonds related to polymeric species) [21]. In the lower energy region, some other bands can be observed (usually due to the d-d transitions). The charge transfer transitions seem to be independent from the heteroatom. This behaviour is similar to that observed in other related compounds such as the Keggin and Dawson phases [21]. For the rhodium(III) compound, there are two bands at 511 and 409 which are assigned to the 1Tlg ~-lAlg and 1T2g <--1Alg transitions, characteristic of both low spin Rh(III) and Co(III) species. The latter shows bands at 610 and 418 nm. Besides, the spectrum of the Cr(III) phase shows bands at 545 and 396 nm (4T2g <---4A2g and 4Tlg <--4A2g) spin- allowed transitions for the typical octahedral environment [22].

,-.-?

o~ <

I

'a;o'4oo

,

5;o

i

6;o

Wavelength, x [nm] Fig. 2. DRS spectra for (a) RhMo6 and (b) CrMo6

i

I

7oo

i

8oo

570

3.4. A d s o r p t i o n i s o t h e r m

Fig. 3 shows the adsorption isotherm in "f-A1203 of RhM06 obtained at room temperature and using Ca (gMo/100gT-A1203) values but expressed as (%) of monolayer in function of Cf (pmol Mo/ml). Similarly to other Anderson phases recently studied, the bend shape corresponds to a Langmuir model and, by using its linearized equation (1), it was possible to obtain adsorption parameters such as number of S active sites and adsorption constant Kad [7].

12010080>" 60

m

O

tO

2;

40

20

0

~

1'0

2'0 3'0 4'0 Ct [gmolMo/ml] Fig. 3 Adsorption isotherm of 7-Al203-supported RhM06

5'0

60

The adsorption parameters from the corresponding isotherm are given in Table 2, including the data of AHM/ 7-A1203 (AHM:ammonium heptamolybdate), CoMo6/7-A1203, CrMo6/7A1203 and AIMo6/]t-A1203 for comparative purposes. Table 2. Adsorption parameters for 7-Al203-supported XM06 and AHM systems a. 102~ (atoms Mo / Phase Kad (ml/g Mo) S (g Mo/g support) g support) RhMo6 1460 0.07 4.30 CrMo6 1430 0.08 5.00 CoMo6 1374 0.09 5.60 AIMo6 840 0.09 5.60 AHM 290 0.10 6.30 a CrMo6, CoMo6, AIMo6 and AHM/]t-A1203 included for reference purpose, Kad: molybdenum adsorption constant, S: number of active sites on the alumina surface. Recent studies on the adsorption in equilibrium of several Anderson phases on ~'-A1203, showed that this process strongly depends on the heteroatom, and the adsorption constant Kaa varies according to the following order: RhMo6>CrMo6>CoMo6 >TeMo6-AIMoa>NiMo6 [7]. The adsorption parameters obtained for the system containing Rh get closer to the values corresponding to Cr and to Co. Although the number of active sites is smaller than those observed in other phases and AHM, the fact that Kaa values for all Anderson phases are higher by several size values than AHM is even more significant. The presence of trivalent heteroatoms of ionic radius between 0.50 y 0.66 A leads to a good deposition on ]t-A1203 support due to a possible X-AI interchange. Likewise, Raman results have shown that a lower heteroatom size is associated to higher Mo-Ot stretching frequencies. However, the Kaa calculated values present an inverse ratio which can be attributed to a different M o - O -

571 support interaction, while the active sites decrease when the heteroatoms increase due to possible geometrical considerations. All these facts surely play a very important role in the adsorption process of the planar species which contribute to the global impregnation process, characterized by a series of homogeneous and heterogeneous equilibria (anion deposition, A1 dissolution of the support, cationic interchanges, diffusion, etc). Such reactions have been recently characterized by means of spectroscopic and thermal methods [7, 23]. The characterization of Rh phases supported on alumina has been carried out by DRS. Although the spectra present less intense bands than those of pure phases, it is possible to observe the bands of load transference of octahedral Mo ( ~300 nm) as well as a band of wide transition and lower resolution attributed to Rh(III), around 400 nm. The surface of catalytic systems based on CoMo6/3,-AlzO3 and NiMo6/3,-AIzO3, sulfided and untreated, has also been recently studied by XPS and EXAFS [4, 24], and the corresponding RhMo6/7-A1203 study is in course. 3.5. Catalytic activity The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts have been measured in a high pressure reactor, the operative conditions having been selected according to the recent experience about the CoMo6 and NiMo6 based systems because of the common structural and physical-chemical properties [3, 4]. Table 3 shows chemical data and conversion obtained for selected catalysts based on RhMo6. In addition, the data for CoMo6 Anderson, CoMo commercial and Rh commercial 3,-A1203 supported catalysts are included for comparative purposes. Table 3 Chemical data and conversion of thiophene and cyclohexene for different RhMo6 catalytic and reference systems a, b 7A1203

supported Catalysts

X(%)

Mo(%)

HYD(%)

SHYD

CoMo6 0.80 8.00 30.00 3.75 CoMo a 1.80 9.30 15.55 0.86 RhMo6 1.00 6.00 84.68 8.47 RhMo6 b 1.12 6.80 66.80 5.96 [RhMo6 + AIMo6] 0.50 6.00 54.98 10.00 Rh a 5.00 64.96 1.30 Note: X(%) and Mo(%) are total metal concentration in gM/100g support. a Commercial catalysts. b Catalyst after TPR conditions treatment at 473 K. SHVDand S riDS = HYD/X and HDS/Mo

HDS(%)

SHDS

70.81 71.00 72.40 13.70

8.85 7.63 12.06 2.01

50.14 22.38

8.35 -

From the analysis of results shown in Table 3, it can be deduced that: 9 The comparison between the catalytic behaviour of Co and Rh Anderson phases reveals that, although the Mo content is lower for the latter, the HDS activities are similar. Hence, the synergic effect of Rh on the Mo is higher than that observed for the Co system. This is in agreement with literature for pure sulfides. This behaviour is more clearly observed from the comparison among the specific activities (Sm,Dand S HDS).

572

9 In HYD, the heteropolymetallate systems also show a better performance, especially the catalyst based on RhlMo6 in which the activity is enhanced more than five times. 9 The reducing thermal pre-treatment of RhMo6/7-AI203 produces a substantial decrease in HDS activity and not so noticeable in HYD. The presence of Rh induces the Mo(VI)-Mo(IV) reduction at lower temperature. Thus the Mo(IV) species, poorly sulfided, affect the catalytic performance, as it is well known. 9 The dilution of the impregnating solution RhM06 with A1M06 (which is poorly active for hydrotreatment processes) shows that the catalyst behaviour is function of Rh content. CONCLUSIONS Taking into account the specific activities a better performance of heteropolymetallate systems than commercial catalysts is clearly observed. The bifunctionality of the RhMo6 system is an important feature. This advantage seems related to the adsorptive interaction process of the planar anion that offers a good site distribution and involves a synergic effect. REFERENCES

1. M.T. Pope, "Heteropoly and Isopolyoxometalates", Springer-Verlag, Berlin, New York, (1983). 2. M.T. Pope and A. M/.iller, "Polyoxometalate Chemistry from topology via self-assembly to applications", Kluwer Academic Publishers, London (2001). 3. C.I. Cabello, I.L. Botto and H.J. Thomas, Appl. Catal. A: General, 197 (2000) 79. 4. I. Pettiti, I. L. Botto, C. I. Cabello, S. Colonna, M. Faticanti, G. Minelli, P. Porta and H.J. Thomas, Appl. Catal. A: General, 220 (2001) 113. 5. C.I. Cabello, I. L. Botto and H. J. Thomas, Thermochim. Acta, 232 (1994) 183. 6. I. L. Botto, C. I. Cabello, H. J. Thomas, D. Cordischi and P. Porta, Mater. Chem. Phys., 62 (Z000) 254. 7. C. I. Cabello, I. L. Botto, F. Cabrerizo, M. G. Gonzfilez and H. J. Thomas, Adsorption Sci. Tech., 18(7) (2000) 591. 8. I. L. Botto, C. I. Cabello and H. J. Thomas, Mater. Chem. Phys., 47 (1997) 37. 9. T. Liu, K. Asakura, U.Lee and Y. Iwasawa, J. Catal., 135 (1992) 367. 10. M. Shelef and G. W. Graham, Catal. Rev. Sci. Eng., 36(6) (1994) 433. 11. R.D. Hall, J. Am. Chem. Soc., 29 (1907) 692. 12. K. Nomiya, T. Takahashi, T. Shirai and M. Miwa, Polyhedron, 6 (1987) 213. 13. a) C.H. Giles, D. Smith and A.J. Huitson, J. Coll. Interf. Sci. 47 (1974) 755. b) C.H. Giles, H. P. D'Silva and I.A. Easton, J. Coll. Interf. Sci. 47 (1974) 766. 14. Jr. H. T. Evans, J. Am. Chem. Soc., 70 (1948) 1291. 15. C. Rocchiccioli-Deltcheff, M. Fournier, R. Frank and R. Thouvenot, Inorg. Chem., 22, (1983) 207. 16. A. Perloff, Inorg. Chem., 9 (1970) 2228. 17. H. Kondo, A. Kobayashi and Y. Sasaki, Acta Crystallogr., C45 (1980) 661. 18. R. Thouvenot, M. Fournier, R. Frank and C. Rocchiccioli-Deltcheff, Inorg. Chem., 23 (1984) 598. 19. T. L. Barr, Ch.G., F. Cariati, J.C.J. Bart and N. Giordano, J. Chem. Soc. Dalton Trans., (1983) 1825. 20. K. Nomiya, Y. Sugie, K. Amimoto and M. Miwa, Polyhedron, 6 (1987) 519. 21. H. So and M. T. Pope, Inorg. Chem., 11 (1972) 1441.

573 22. A.B.P. Lever, "Inorganic Electronic Spectroscopy"(Sec. Edition), Elsevier (1984) 23. X. Carrier, J.B. D'Espinose de la Caillerie, J.F. Lambert and M. Che, J. Am. Chem. Soc., 121 (1999) 3377. 24. P. Porta, G. Minelli, G. Moretti, I. Petitti, I. L. Botto, C. I. Cabello and H. J. Thomas, J. Mater. Chem., 4 (1994) 1641.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

575

Metallosilicate mesoporous catalysts prepared by incorporation of transition metals in the MCM-41 molecular sieves and their catalytic activity in selective oxidation of aromatics (styrene and benzene) V. PgtrvulescuI and B. L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium MCM-41 analogue metallosilicates containing the redox active metals V, Co, Cr and Ni or bimetals VCo and CrNi have been synthesized from aqueous gel by hydrothermal treatment. The mono- and bimetallic molecular sieves were characterized by various techniques, such as XRD, N2 adsorption-desorption, SEM, TEM and FTIR spectroscopy. The catalytic activity in the selective oxidation of styrene and benzene with H202 has been evaluated. The catalytic properties and the characteristics of the structure were correlated with the synthesis conditions, the nature and concentration of the metal, the association of two metals and their molar ratio in the bimetallic incorporated silicates and the oxidation state of the metal cations in the precursors. 1. INTRODUCTION The newly discovered hexagonal mesoporous molecular sieves such as MCM-41 offer new opportunities for transition metal incorporation into silica framework in order to generate potential catalytic activity [1-5]. Transition metals containing MCM-41 mesoporous redox molecular sieves have shown a high activity and selectivity in the oxidative transformation of organic molecules [6-12]. The properties of these high selective catalysts named as "mineral enzyme" can be tailored over a broad range, allowing the easy preparation. Transition metal ions can be introduced into molecular sieves in three different ways: ion exchange, impregnation [13, 14] and direct introduction by hydrothermal synthesis [9, 10, 15]. In the first two cases, extra-framework metal ions dispersed on the internal surface of channels were in general observed and only very small amount of metal ions were found to be incorporated in the framework. While in the last case, lattice metal ions in principal, but extra- lattice metal ions in small quantity may be found, depending on the metal content. In this work we present our results on the synthesis parameters and characterization of a series of MCM-41 molecular sieves containing active metals (V, Cr, Co and Ni) and bimetals (V-Co and Cr-Ni). The synthesis conditions and the effects of nature and content of the metals on the ordered hexagonal structure and morphology of the mono- and

1On leave from: Institute of Physical Chemistry I.G. Murgulescu, Spl. Independentei 202, Bucharest, Romania, SSTC and DGRE-DRI-RWresearch fellow * Corresponding author ([email protected])

576 bimetallic mesoporous samples were discussed. The catalytic activity in selective oxidation of styrene and benzene is evaluated. 2. EXPERIMENTAL 2.1. Synthesis The synthesis of mesoporous molecular sieves by hydrothermal treatment was carried out using two synthesis procedures. First, V (1.6 and 3.2%), Cr (1.6%), Co (1.8-9.2%), Ni (1.8-3.6%), V-Co (Co/V molar ratio was fixed at 1 for VCol-MCM-41, 0.3 for VCo2MCM-41, and 3 for VCo3-MCM-41) and Cr-Ni (Cr/Ni molar ratio = 1.0 and 3.4 % in total metal contem) containing-MCM-41 catalysts were synthesized from sodium silicate, as the silica precursor, cethyltrimethylammonium bromide as the surfactant and VOSO4-5H20, Cr(NO3)3-9H20, CrO3, Co(NO3)2-6H20, Ni(CH3COO)2-4H20, as the metal precursors. The pH was adjusted to 11 with H2SO4 solution. These mixtures were heated at 373K for 3 day. In the second procedure, the V (1.6 %) and Ni (1.8 %) mesoporous molecular sieves were synthesized with tetraethylorthosilicate (TEOS), VOSO4.5H20, or Ni(CH3COO)24H20, ethanol, 2-prophanol and an acidified mixture of CTMAB in water. The gels with pH 13 were loaded into teflon-lined steel autoclave and heated at 373K for 3 days. The pH of the gels was ft~her modified to 11 with H2SO4. The gels were then subjected to another 3 days hydrothermal treatment at 373K. The solid products dried in air at 373K and calcined at 773K. 2.2. Characterization The obtained materials were characterized by XRD (Phih'ps PW 170 diffractometer), N2 adsorption-desorption (Tristar, Micromeritics), SEM (Philips XL-20 microscope), TEM (Philips Tecnai microscope) and FTIR (Spectrum 2000, Perkin Elmer) techniques. The oxidation of aromatic hydrocarbons (styrene and benzene) was carried out in the thermostated glass reactor with magnetic stirring in the presence or absence of the solvent (acetonitrile). The reaction temperature and time were 343K and 24h, respectively. The molar ratio of hydrocarbon/solvent/hydrogen peroxide was 1/-/3 for benzene and 1/1.8/3. After reaction, the catalyst was separated by centrifugation and the oxidation products were chromatographically analyzed. 3. RESULTS AND DISCUSSION

Using the procedures described in the experimental section, the mono- and bimetallic ions incorporated mesoporous structures, V-MCM-41, Co-MCM-41, Cr-MCM-41, NiMCM-41 VCo-MCM-41 and CrNi-MCM-41 could be generated with different M/Si and Co/V molar ratios (0.02-0.1 and 0.3-3 respectively). The synthesis conditions, the nature and concentration of the metal in monometallic silicates, the association of the metals and their molar ratio in the bimetallic silicates and the oxidation state of the metal cations in the precursors can affect the regularity of hexagonal arrangement of the MCM-41 structure, the incorporation of the metal into these structures and the physico-chemical and catalytic properties of the obtained solids.

577 3.1. Effect of nature and precursors of metal ions, M/Si molar ratio, MI/M2 molar ratio and silica sources on the synthesis of mesoporous molecular sieves

3.1.1. Nature and oxidation state of the metal cations The XRD patterns (Fig. 1) of all the mono- and bi-metallic (the ratio of V/Co and Cr/Ni is equal to 1) samples show 4 diffraction lines at the small angle range, characteristic of well ordred MCM-41 type mesoporous materials with a regular hexagonal array of their cylindrical channels. N2 adsorption-desorption isotherms are type IV (see Fig. 6), typical for mesoporous materials. The pore size distribution (see Fig. 6) obtained from BJH method give a monodal peak centred at around 2.7 0.2 nm. The regular hexagonal arrangement of cylindrical channels of our materials can be clearly viewed by TEM technique. Fig. 2 depicts the TEM images of one mono-metallic incorporated and one bimetallic ions incorporated samples. When electron beam of TEM is in the parallel direction to the channel axis of our materials, a honeycomb structure can be observed while when the electron beam of TEM is perpendicular to the channel axis of our materials, layer-type materials with equal distance between layers can be visualized. All the above technical analysis results such as XRD, N2 adsorption-desorption isotherms, pore diameter distributions

Fig. 1. X-ray diffractograms of the mono- and bimetallic mesoporous molecular sieves Fig.2. TEM images of the molecular sieves

578

Fig.3. SEM images of the monometallic samples: (a)" CrlN4, (b): Cr2N4 and (c): NiN4 and TEM (Fig. 2) confirm strongly the highly structured mesoporous materials with well regular hexagonal array of cylindrical channels of our mono- and bi-metallic materials. It has to be noted that the higher ordered and stable hexagonal phases are obtained when the oxidation state of the cation, introduced during synthesis, is favourable for a tetrahedral coordination, i.e. RM(n+)/Ro(2.)ratio in the range of 0.3 and 0.5. The XRD diffractogram of sample synthesized with CrO3 (Cr2N4) is better resolved compared with that of sample synthesized with Cr(NO3)3 (CrlN4) (Fig. 1), showing higher order in the mesoporous structure of Cr2N4 than that of Cr~N4. The initial oxidation state of metal Fig.4. SEM images of NiCrprecursor is thus important for the regularity of final MCM-41 structure of materials. The morphology of aggregates formed by a large number of small spherical particles observed by SEM for most of monoand bi-metallic ions incorporated samples such as Cr2-MCM-41, Ni-MCM-41 (Fig. 3) and CrNi-MCM-41 (Fig. 4) samples, except Cr~-MCM-41 (Fig. 3a), is typical for metals modified MCM-41 as reported previously [9, 10]. A "sandy-rose" morphology is visualized for Crl-MCM-41 (Fig. 3a). It is evident that the metal precursors can modify the ordered structure and the morphology. The characteristics of the mesoporous structure of our mono- and bimetallic molecular sieves are listed in Table 1. From this table, it can be concluded that the above described mesoporous silicates have heteroelements in the framework. The unit cell parameter ao confirm incorporation of the metal cation in the framework of our materials. It is observed that for a defined metal and metal source, the unit cell parameter ao increases with increasing the metal content and then decreases when the metal content is too high. As the bond length of M-O is higher than that of Si-O, the increase in unit cell parameter with increasing the metal content suggests the incorporation of metal ions in the framework of mesoporous structures. The loss in unit cell parameter observed when the metal content is too high is due to the loss, at least a part, of the mesoporous structure. The

579 decrease in surface area and the loss in the intensity of XRD peaks support our explanation. IR-spectra of the transition metal containing mesoporous silicas are similar to that of amorphous silica. Only an supplementary absorption band at around 940 nm is present in all the spectra of the mono- and bimetallic incorporated samples but its intensity is variable. This band can be attributed to the M-O-Si bond and often used as a prove of the incorporation of metal ions in the framework of silicas. Table 1. Characteristics of the mono- and bi-metallic ions incorporated mesoporous MCM-41 molecular sieves

Catalyst

M %

SBET O ( m 2 / g ) (nm)

ao Catalyst (nm)

M %

SBET O ( m 2 / g ) (nm)

ao (nm)

VN4 1.6 1055 2.5 4.65 NimN4 1.8 945 2.8 3.97 VN1 1.7 1032 2.8 4.55 NizN4 3.6 828 2.6 4.70 CrgN4 1.8 958 2.6 4.48 Ni3N4 5.5 777 2.6 4.60 CraN4 1.8 862 2.7 4.56 N4N4 7.4 654 2.7 4.30 Co1N4 1.8 990 2.8 3.99 NsN5 9.2 568 2.7 4.20 Co2N4 3.6 905 2.8 4.70 VCo1N4 3.4 948 2.8 4.70 Co3N4 5.5 644 2.7 4.70 VCo2N4 3.4 1013 2.7 4.60 Co4N4 7.4 568 2.6 4.80 VCo3N4 3.4 973 2.9 4.70 Co5N4 9.2 681 2.5 3.92 CrNiN4 3.4 914 2.7 4.32 NimN1 1.9 1021 2.8 4.51 N=MCM-41, N4: synthezised from sodium silicate, N l: synthesized from TEOS, CrlN4 synthesized from Cr(NO3)3 and Cr2N4 from CrO3; Col-CO5: different metal content and obtained from Co(NO3)3 and Nim-Nis: different metal content and synthesized from Ni(CH3COO)a-4H20

3.1.2. Effects of the M/Si molar ratio The variation of the M/Si molar ratio, between 0.02 and 0.1, for Co-MCM-41 and NiMCM-41 can induce a modification in the ordered hexagonal structure and surface area. Xray diffractograms (Fig. 5) show a less ordered hexagonal structure when M/Si >0.06. Surface area and unit cell parameter decrease with increasing M/Si molar ratio (Table 1 and Fig. 5). For a high quantity of the metal, the incorporation of the metal in the framework of mesoporous structure is difficult. The surface area decreases remarkably when the metal content is higher than 3.5 %. Above this value, the regular hexagonal arrangement was progressively lost. Nitrogen adsorption-desorption isotherms of the samples with different Co/Si molar ratio (Fig. 6) are characterized by a very steep increase at the relative pressure P/P0 of around 0.38, indicating the homogeneity of our bimetallic incorporated mesoporous molecular sieves.

580

800

'~

"

500 . 2

.

Col

. 4

.

6

8

II lllIl~ll~r

-

10

2

i

I

I

I

2

4

6

8

> 10

2 Fig.5. X-ray diffractograms of the Co-MCM41 and Ni-MCM-41 samples with a variable metal content

0 .... 0.0

,

,

,

,

0.2

0.4

0.6

0.8

1.0

RelativePresane (P/P0) Fig. 6. N2 adsorption-desorption isotherms and pore size distribution of a series of Co-MCM41 samples with a variable metal content

This value decreases only slightly from 0.38 to 0.34 with increasing metal content. The adsorption capacity decreases also with metal content. The BJH pore size distribution

581 shows a very narrow monomodal peak for all the M/Si molar ratio values and the average pore diameter decreases insignificantly (Fig. 6 and Table 1). The decrease in surface area and adsorption capacity with increasing metal content suggest that not all the metal ions introduced in the synthesis gel can be incorporated into the mesoporous framework. A part of the metal ions introduced in the synthesis gel will be present as extra-framework species and dispersed on the internal surface of mesopores. These extra-framework species can be easily removed by leaching since they are less strongly linked with internal surface of mesopores. SEM (Fig. 7) images show the typical morphology of aggregates of small spherical particles, characteristic of mesoporous metallosilicates. It has to noted that a notable quantity of the amorphous phase was observed in the samples with a high metal content (Co4N4 and Co5N4) while the particle size decreases with increasing Co/Si molar ratio. Highly organized mesoporous structures were observed with low M/Si ratio by TEM (Fig. 8) and lower ordered structure was viewed in the samples with high M/Si ratio. 3.1.3. Effects of the C o N molar ratio

The M1/M2 molar ratio can modify the structure, the morphology and the catalytic activity of the mesoporous redox molecular sieves. A highly ordered hexagonal arrangement (Fig. 9) is evidenced for Co/V molar ratio of 0.3 (VCo2-MCM-41). This sample has the high surface area (Table 1). It is very interesting to note that the calcination ~00_ increases the intensity of the first Bragg peak and calcined metallosilicates have a highly ~000ordered pore system with a high porosity. It is possible that the calcination can allow a better .~ 4OO0 penetration of metal ions into the framework and "a a reorganization of structure into a well ordered " ~00 framework. TEM (not shown here) images confirm the 0 higher ordered pore systems for V-C02 and VC03 samples. SEM (Fig. 10) micrographs show the typical morphology of mesoporous Fig.9. X-ray diffractograms of the metallosicates. VCo-MCM-41 samples 3.1.4. Effects of the synthesis method

Transition metal containing mesoporous silicas were obtained by hydrothermal treatment from two different silica sources: sodium silicate (VN4 and NiN4) and tetraethylorthosilicate (VN1 and NiN1). The sol of TEOS is treated in autoclave in basic medium to obtain highly hydrolysed and reticulated silica species during the condensation. The condensation of silica species in the presence of the metal cations favors their incorporation into framework of silica. These silica species with metal cations are ordered around the well self-organized micelles of CTMABr at pH 11. All the samples obtained from TEOS have a very high ordered structure (Fig. 11), a very high surface area (Table 1) and are a very stable structure. TEM and SEM images show a highly ordered hexagonal structure for all the samples synthesized both with sodium silicate and TEOS. However, it

582 is observed that the particle size evidenced by SEM is smaller for the samples prepared with TEOS.

Fig.10. SEM images ofthe VCo-MCM-41 with V/Co molar ratios: (a): 0.3, (b)" 1.0, (c): 3.0 3.2. Evaluation of catalytic activity in selective oxidation of styrene and benzene Metalosilicates synthesized here with MCM-41 structure are very active in oxidation of styrene and benzene in liquid phase (Table 2). The selectivity tD to benzaldehyde and phenol are very high for all the catalysts. For Ni-series of catalysts, in oxidation of styrene, the activity increases, however, the selectivity decreases with increasing the metal 2 content. In oxidation of benzene, the Fig. 11. X-ray diffractograms of the V and Niconversion is not very attractive (4MCM-41 samples synthesized with TEOS (VN1 11%) although the selectivity is high. and NiN1) and with sodium silicate (VN4 and For Co-series of catalysts, in oxidation NiN4) of styrene, the catalyst with lowest metal content gives the highest conversion and selectivity. The conversion is then practically insensitive to the metal content while the selectivity decreases significantly with increasing metal content. In oxidation of benzene, the variation in conversion is opposite. The conversion increases sharply with metal content. Comparing the two Cr-modified catalysts synthesized with different metal precursors, although the selectivity for benzaldehyde and phenol from styrene and benzene, respectively, remains quite high, the variation in conversion in these two reactions is opposite too. The sample with less well organized structure CrlN4 synthesized with Cr(NO3)3 gives a higher conversion for styrene oxidation while CrEN4 synthesized with CrO3 shows a higher activity for benzene oxidation. The oxidation state and the location of metal ions and the regularity of structure can play an important role in benzene and styrene oxidation reaction. Comparing the samples

583 synthesized with different silica sources, VN1 and VN4, NilN1 and NilN4, the results given in Table 2 demonstrate that the catalytic activity observed for N1 samples synthesized with TEOS do not differ strongly from those observed for N4 samples synthesized with sodium silicate under same reaction conditions. However, the efficiency of the hydrogen peroxide and the stability under reaction conditions of the redox molecular sieves are higher when the samples are synthesized by TEOS (N1). For mono-metallic samples, for a given metal content (1.7-1.9%), the conversion varies in an order of VN 1
Catalyst VN4 VN1 CrlN4 Cr2N4 COLN4 Co2N4 Co3N4 Co4N4 Co5N4 NilN1

Styrene C, ~ SBzA.,% 5.6 78.4 4.8 84.5 51.6 76.4 13.4 82.3 60.3 84.2 28.8 76.5 29.2 75.4 34.5 60.4 31.5 51.4 8.6 78.4

Benzene Catalyst C, % Sph.,% 22.7 84.5 NilN4 10.2 85.6 Ni2N4 2.4 80.1 Ni3N4 77.4 84.0 N4N4 1.4 78.4 NsN5 10.5 84.5 VColN4 26.7 86.2 VCoEN4 85.6 92.4 VCo3N4 80.4 61.2 CrNiN4 10.5 84.2

Styrene Benzene C, % SBzA.,% C, % Sph.,% 12.8 62.8 11.4 48.2 21.3 86.2 4.6 86.5 54.6 36.2 8.2 88.6 74.5 24.3 10.6 89.0 80.2 6.3 3.6 56.2 2.3 87.2 50.6 86.2 3.8 78.2 43.2 84.0 24.5 64.2 17.2 85.4 79.5 92.7 62.6 89.4

4. CONCLUSION Highly ordered mesoporous mono- and bi-metallic incorporated catalysts with high porosity and surface area are very active and selective in oxidation reaction of aromatic hydrocarbons such as styrene and benzene. Their framework composition can be modified by incorporation of various transition metals in the single or associated forms. The ordered hexagonal arrangement, the morphology, the surface area, and the catalytic activity and selectivity can be modified by method of the synthesis, nature and content of the metal and the molar ratio of the metals. Very active catalysts were obtained by incorporation of chromium and nickel in the hexagonal ordered structure.

584 ACKNOWLEDGEMENTS

This work was performed within the framework of PAI-IUAP and a bilateral scientific cooperation between the R6gion Wallonne of Belgium and Romania. The VP thanks the SSTC (Federal scientific, technological and cultural office of Premier Minister, Belgium) and DGRE-DRI ofR6gion Wallonne, Belgium, for the research scholarships. REFERENCES

1. 2. 3. 4.

A. Corma, Chem. Rev., 97 (1997) 2373. A. Tuel, Microporous Mesoporous Mater., 27 (1999) 151. W.A. Carvalho, P. B. Varaldo, M. Wallau and U. Schuchardt, Zeolites, 18 (1997) 408. N. Yao, C. Pinckney, S. Lim, C. Pak and G.L. Haller, Microporous Mesoporous Mater., 27 (1999) 151. 5. C.T. Kresge, M.E. Leonowicz, W.J.Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 6. W.Zhang, J. Wang, P.T. Tanev and T.J. Pinnavaia, Chem. Commun., 1996, 979. 7. M. Dusi, T. Mallat and A. Baiker, Catal. Rev.Sci. Eng., 42 (2000) 213. 8. S. Biz and M.L. Occelli, Catal. Rev.Sci. Eng., 40 (1998) 329. 9. V. Pgtrvulescu, C. Dascalescu and B.L. Su, Stud. Surf. Sci. Catal. 135 (2001) 4772 10. V. Parvulescu and B.L. Su, Catal. Today, 69 (200 l) 315. 11. C.W. Lee, D. H. Ahn, B. Wang, J.S. Hwang and S-E. Park, Microporous Mesoporous Mater. 44-45 (2001) 587. 12. M. Stockenhuber, R.W. Joyner, J.M. Dixon, M.J. Hudson and G. Grubert, Microporous Mesoporous Mater. 44-45 (2001) 367. 13. G. Grubert, J. Rathousk~, G. Schulz-Ekloff, M. Wark and A. Zukal, Microporous Mesoporous Mater., 22 (1998) 225. 14. D. Brae and K. Serf, Zeolites, 17 (1996) 444. 15. H.Y. He, S.L. Bao and Q.H. Xu, Stud. Surf. Sci. Catal., 105 (1997) 85.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

C o n t r o l l e d surface modification of alumina-supported sulfides by surface organometallic chemistry

585

Mo

and

Co-Mo

Jae-Soon Choi, Carine Petit-Clair, and Denis Uzio* IFP, 1 & 4 Avenue de Bois-Pr6au, 92852 Rueil-Malmaison Cedex, France Abstract: Surface organometallic chemistry (SOMC) was applied to the selective incorporation of cobalt or tin atoms to the active phase of molybdenum-based sulfides supported on alumina. The nature of involved reactions was investigated by analyzing the gaseous species evolved during the synthesis. The resulting materials were characterized by various techniques, like elemental analysis, electron probe microanalysis, X-ray diffraction, transmission electron microscopy, energy-dispersive spectroscopy and infrared spectroscopy study of adsorbed CO. The influence of cobalt or tin doping on the catalytic performance of sulfide was evaluated in a model reaction of FCC gasoline hydrodesulfurization. Under the appropriate synthesis conditions, SOMC appeared to be an efficient method to selectively modify the sulfide phase, leading to significant change in catalyst selectivity hydrodesulfurization/olefin conversion. 1. I N T R O D U C T I O N Cobalt-promoted molybdenum sulfides supported on alumina are widely used in petroleum refining for sulfur removal [1]. However, there is now increasing necessity to further improve their performance (activity, selectivity, and stability), due to more and more stringent legislation on sulfur contents in transportation fuels [2]. It is thus primordial to understand the detailed structure and catalytic behavior of these catalysts. In this respect, controlled preparation of catalysts with desired composition and structure has great importance in permitting fundamental study of structure-performance correlation. Surface organometallic chemistry (SOMC) has shown high potential for the preparation of supported metal catalysts with desired composition and good dispersion [3]. For example, the controlled hydrogenolysis of tetra-n-butyltin (Sn(n-C4H9)4) on the surface of group VIII metals leads to well-defined bimetallic catalysts [3-6]. In SOMC on metal supported on oxide, judicious selection of reaction conditions (temperature, initial complex concentration etc.) allows the reaction to occur preferentially between organometallic complexes and metal surface [3,5,6]. As the edge sites of sulfide slabs are generally known as active sites in hydrotreating reactions [1], it seemed thus possible to us that reactions take place preferentially between sulfide edge sites and organometallic complexes, if placed in contact each other. Selective modification of edge sites could then be made by different metals like cobalt or tin. In this study, we applied SOMC to the modification of the conventional hydrotreating catalysts: alumina-supported molybdenum sulfides with or without cobalt promoter. The Co and Sn Corresponding author. E-mail address: [email protected].

586

deposition procedures by SOMC and characterization of resulting catalysts will be detailed in this paper. 2. E X P E R I M E N T A L 2.1. Cobalt and tin doping by surface organometallic chemistry Modification of sulfides using SOMC was realized by reaction of different organometallic complexes, such as bis (2,4-pentanedionato) cobalt (II) (Co (C5H702)2), bis(cyclopentadienyl)cobalt (Co(C10H10)2), tetraallyltin (Sn(C3H5)4) and tetra-n-butyltin (Sn(n-C4H9)4), with the surface of freshly prepared sulfides. First, oxide precursor like 10 wt% MoO3/A1203 or 3 wt% COO-10 wt% MoO3/A1203 was sulfided in flowing 15 vol% HzS/H2 under atmospheric pressure. The reaction temperature was raised from room temperature (RT) to 673 K at 300 K h -1 and maintained for 2 h. Then the temperature was lowered to 473 at 300 K h -1, at this temperature the gas flow was switched to pure H2 and the reaction was further maintained 2 h. At the end of the reaction, the reactor was rapidly cooled to RT in flowing H2. The as-prepared sulfide was then transferred, without air contact, to the reactor designed for SOMC treatment. This reactor, containing reaction solution (complex + solvent), was being purged by Ar flow before catalyst introduction. The reaction was done at RT in flowing H2 for 2 h. After reaction, the catalyst was washed with pure solvent (n-heptane or toluene) and dried, if necessary, under Ar atmosphere at RT. To ensure complete decomposition of anchored complexes, a post-treatment was performed either by reduction under pure H2 flow at 473 or 573 K or re-sulfidation at 673 K. The composition of the gas flow leaving the reactor was analyzed with a gas chromatograph (HP 5890 Series II) equipped with a flame ionization detector (FID), during the preparation of Sn-doped sulfides. 2.2. Catalyst characterization Elemental analysis was carried out either by X-ray fluorescence (XRF, Philips, PW 2404) for Mo, Co, Sn and S titration, or by high temperature combustion with infrared detection (Horiva, EMIA8200) for carbon. The distribution of elements over catalyst pellets was determined using JEOL JXA-8800R electron probe microanalyzer (EPMA). A Philips PW1710 automatic diffractometer with a CuK~ monochromatized radiation source was used for the X-ray diffraction (XRD) measurements. Electron microscopy was performed with a Philips Tecnai F20 electron microscope to examine any change in particle size and phase distribution caused by Sn incorporation (TEM) and also to locate the deposited metals (EDS, STEM mode). The adsorption of carbon monoxide was monitored by infrared spectroscopy (IR(CO)). Before the measurement, sulfide catalyst, pressed into a self-supporting disc (10 mg, 2 cm 2) in air, was re-sulfided in situ in the IR cell. CO chemisorption was done at 100 K and the spectra were registered with a Nicolet Magna 550 FT-IR spectrometer. The spectrometer operated at 4 cm -1 resolution collecting 512 scans. 2.3. Evaluation of catalytic performance Catalytic performance of the prepared sulfides was evaluated in an autoclave with a model reaction mixture of FCC gasoline under hydrogen pressure. The reaction mixture consisted of 3-methylthiophene (Fluka, > 98%), 2,3-dimethyl-2-butene (Fluka, > 97%) and n-heptane (Fluka, > 99%, HPLC grade) as a solvent, leading to a 1000 ppm total sulfur and 10 wt% olefin contents. Fresh catalyst was introduced under Ar flow into the reactor, which was also previously purged by Ar flow to prevent any air contact of the catalysts. The reaction was

587 performed at 473 K with a total pressure of 2.5 MPa. The composition change of the reaction mixture with time was followed by analyzing the liquid samples using a gas chromatograph (Varian, CP-3800) equipped with a FID. The absence of mass transport limitation was checked by a linear increase of reaction rate with increasing mass of catalyst. First-order and zero-order rate constants were obtained, respectively, for 3-methylthiophene and 2,3-dimethyl-2-butene conversions, in order to compare catalyst activities. 3. RESULTS AND DISCUSSION a 9

"-" e~

!~

b

I0

!,g

~s

!.8

Lg

8.'

,

l.s

l.O '

,

,

,

I

~,.R

.5 '

'

t

2.s

~.,8 '

,

,

'

I

~.

Z.5 ,

'

I

.

.

.

.

.

1 '

.

~

.

" '

'

'

'

I

' '

'

,~ '

'

'

'

!'

:] '

'

'

I

u Pellet

radial

distance

(x 1000

tam)

Fig. 1. Electron probe microanalysis profiles of Mo/AlaO3 sulfides (spherical form) modified by SOMC of(a) C0(C5H702)2 and (b) Co(CloHlo)2. 3.1. Cobalt doping of Mo/AbO3 sulfide by SOMC First, SOMC was applied to Co promotion of Mo/A1203 sulfide. As shown in Fig. 1, the choice of complex was important for uniform distribution of Co over catalyst pellets. Probably due to its too high reactivity, bis(2,4-pentanedionato)cobalt(II) (Co(C5H702)2) resulted in over-concentration of Co in outer region of the pellets (Fig. l a), while bis(cyclopentadienyl)cobalt (Co(C10H10h) led to a uniform distribution over the entire catalyst pellets at the same concentration of Co (Fig. l b). Selective reaction between Co-containing complexes and Mo sulfide edge sites should give more efficient Co-promotion. More local analyses like EDS and IR(CO) should be done to verify if, by use of Co(C10H10)a, closer association was really achieved between sulfide slabs and incorporated Co. 3.2. Tin doping of CoMo/Ai203 sulfide by SOMC As described in the experimental section, the Sn (or Co) incorporation by SOMC proceeded in two steps: liquid phase reaction of organometallic complexes and sulfide (anchoring) and then thermal decomposition of surface organometallic complexes (grafting). Fig. 2 show the concentration evolution of gas evolved during the incorporation of Sn to CoMo/A1203 sulfide using Sn(C3Hs)4.

588 523

,-:,.

A

~e "r

:3

473 ~.

::3

/

ng

~

Grafting

423 .,., ~= 373

E 9

0

I

i

I

50

100

150

9

D.

E

323 19. 0

Reaction time (mini

,

~

,-

273

50

100

150

Reaction time (mini

Fig. 2. Propene gas evolution during the Sn doping of CoMo/A1203 sulfide by SOMC of Sn(C3Hs)4, under H2 flow. Except some traces of propane, the only gas product was propene. In the same way, butane was the only gas phase product of tin doping by Sn(n-C4H9)4 (not shown). It indicates that the preferential hydrogenolysis of C-Sn bond of complexes was involved for the anchoring and grafting of Sn. This phenomenon is well known for metal catalysts [3,5-6]. The quantities of produced gas were almost equivalent for the anchoring and grafting steps (Fig. 2). Considering the residual C content of about 10% after grafting, the Sn doping processes can be summarized as follows. First, the two allyl ligands of a Sn(C3H5)4 complex were removed, as propene, forming a surface tin complex. Then the thermal post-treatment further hydrogenolyzed the remaining two ligands leaving on the catalyst surface ligand-free Sn. For the sake of comparison, the same experiment was done for A1203 alone, which was pretreated under the same conditions as for the sulfidation of CoMo catalyst. Again, the only product was propene and a similar gas evolution was observed during the reactions (Fig. 3). The final product contained also about 10 % of residual C. Previous studies of SOMC on metal showed that the oxide supports (activated by H2 or 02 treatment) alone are capable of grafting metal atoms via organometallic complex hydrogenolysis [3,5-6]. It is thus primordial to select reaction conditions such that selective hydrogenolysis occurs preferentially on metal surfaces. 523 A

=--C

nchoring

473

o

Grafting

==

423 = E

373 ~. 323

_

0

I

I

50

100

Reaction time (mini

150

0

50

;

,-

100

150

E

273

Reaction time (mini

Fig. 3. Propene gas evolution during the Sn doping to "presulfided" A1203 by SOMC of Sn(C3Hs)4, under H2 flow.

589

as

Fig. 4 represents relationship between initial Sn quantity introduced to the reaction solution Sn(C3Hs)4 or Sn(C4H9)4 and fixed Sn content to the catalysts.

3.0

Center of

2.5 ~'2.0 ~'1.5 1.0

pellet

A

I

=-

8

0.5 0.0 I n t r o d u c e d Sn 1• 2 2 3

l~mol g.1)

Fig. 4. Relationship between the initial quantity of Sn and the Sn content fixed to CoMo/A1203 sulfides by SOMC of Sn(C3H5)4 or Sn(CaH9)4.

0

500

1000

1500

2000

2500

Pellet radial distance (l~m)

3000

Fig. 5. Electron probe microanalysis profiles of CoMo/A1203 sulfide (spherical form) modified by SOMC of Sn(C3Hs)4. Fixed Sn content was 1.6 wt%.

At a given concentration of Sn, Sn(C3H5)4 showed higher fixed Sn content than Sn(C4H9)4, possibly because of its weaker C-Sn strength than Sn(C4H9)4. Indeed, an ally ligand has a C=C bond whose C atoms are more electronegative that C atoms of C-C (7). In the case of Sn(CaHs)4, more electronegative C (C=C), attached to the C atom of C-Sn, would weaken the C-Sn bond by polarization. It is also to be noted in Fig. 4 that an increase in initial Sn quantity resulted in an increase in fixed Sn content. In the case of more active Sn(C3H5)4, the fixed Sn content almost reached a plateau indicating a monolayer formation. According to XRD measurements, alumina-supported CoMo/A1203 sulfides showed no change in diffractograms obtained before and after the SOMC treatment (not shown). In addition, TEM analyses gave similar particle size and phase distributions (not shown), indicating also that Sn was very finely dispersed. EPMA informed uniform distributions of Sn over the catalyst pellets. Representative distribution profiles are presented in Fig. 5. However, this technique gave analyses too broad (order of m) to locate Sn at the active site scale. As shown hereafter, EDS and IR(CO) analyses helped to tackle this problem of Sn location. First, EDS analyses (Table 1) revealed that Sn, Co and Mo concentrations and Sn/Mo ratio had tendency to increase at the domains where sulfide slabs were observed. On the contrary, in the regions without observation of sulfide slabs, these elements were titrated with smaller concentration and Sn/Mo decreased. The detection of these elements on the support was believed to be due to the very small particles observed uniformly on the support but difficult to identify or/and in part to finely dispersed each metals. Global analyses by EDS and XRF showed similar results each other and concentrations are generally lower than those obtained at slab-containing regions. Somewhat high Sn/Mo ratio obtained by global EDS analysis was believed to be due either to the imprecision of the measurement (cf. XRF result) or to the over-concentration of Mo at sulfide slab-containing regions. In smammry, from EDS

590 analyses, it could be deduced that a part of Sn was deposited to the support or to unidentified small particles. But more important, part of Sn appeared to be in interaction with CoMo sulfide slabs. Table 1. EDS analysis of CoMo/AI203 sulfide modified by SOMC of Sn(C3Hs)4 Sn/A1 Co/A1 Mo/A1 Sn/Mo Single slaba

0.03 (0.02) a

0.05 (0.02)

0.11 (0.05)

0.27

Multi-slaba

0.06 (0.08)

0.17 (0.14)

0.26 (0.31)

0.23

Support + small 0.02 (0.01) particlesa Global (EDS) b 0.01 (0.00)

0.05 (0.01)

0.14 (0.04)

0.14

0.02 (0.00)

0.03 (0.0 l)

0.33

Global (XRF) c

0.02

0.04

0.22

0.01

a Using nanoprobe (0.2 to 2 nm). b Obtained on the domains of 100 to 200 nm. r Atomic ratios obtained by elemental analysis (XRF). d Figures in parentheses show standard deviation. CO/Co-promoted Mo

Fig. 6. Infrared spectra of CO adsorbed (1 Torr) at 100 K on CoMo/ml203 sulfides without and with 1.6 wt% Sn incorporated by SOMC of Sn(C3Hs)4.

!o...

co,o. COIAI 3.

2250

2200

i /~

COIMo / i /

2150

/ i

21 O0

W a v e number

\

! i

2050

~

\

Sn-doped

CoMo IAI

2~ 3

2000

(cm -I)

Infrared spectroscopy studies of chemisorbed CO provided further detailed information on the positions of deposited Sn atoms (Fig. 6). The spectrum of CO adsorbed on CoMo/ml203 without Sn doping, corresponds to typical IR(CO) spectra obtained for Co-promoted Mo/A1203 sulfides [8-10]. First, two characteristic bands (CO) of alumina could be observed. The bands at 2156 cm1 and 2190 cml correspond, respectively, to CO in hydrogen bond with hydroxyl groups and to the coordination of CO to the Lewis acid sites of alumina [10]. Lower wave number region is characteristic of sulfide phase. The band at 2070 cml is due to the Co-promoted sites [8-10]. The shoulder around 2055 crn1 could indicate the presence of Co not participating in Mo sites promotion (8,10). It is interesting to note that the band at 2108 crn-~ which is characteristic of non-promoted Mo sites presents very weak intensity. It means that almost all Mo edge sites were in interaction with Co atoms. The

591 spectrum of Sn-doped catalyst has the same kind of bands as the non-doped catalyst, but Sn led to the intensity decrease of some bands. Indeed, the presence of Sn brought about a decrease in the number of A13§ sites of alumina (1/3 left) whereas the band corresponding to the hydroxyl groups was not affected. On the sulfide phase, the number of edge sites titrated by CO (promoted and non-promoted) was clearly diminished in the presence of Sn (1/3 left). The electronic structure of sulfide phase did not appear to be changed by Sn doping, as no significant band shift was observed. The presence of Sn on the support might imply that the capacity of Sn doping of sulfide phase was exceeded at 1.6 wt% of Sn. That is, while about 1/3 of edge sites were still accessible to CO after grafting step (Fig. 6), bulky Sn complexes could not reach these residual sites during the anchoring step, due to the steric hindrance (cf. surface Sn complexes still had two ligands). Consequently, some of Sn complexes reacted with support AB+ sites. On the supported metal catalysts, Sn reacts with support when the number of complexes is greater than that of surface metal atoms [3,5,6]. It will be interesting to estimate the number of edge and comer sites (active sites) of sulfide and compare it with that of fixed Sn atoms. A geometrical model [11 ] and slab length measured by TEM can permit reasonable estimation of sulfide active sites. 3.3. Influence of Sn and Co doping on the catalytic performance Table 2. Relative activities of (Co)Mo/AI203 sulfides without and with Sn or Co doping by SOMC Composition of Relative activitya

catalyst (by XR~) Co/Mo

Sn/Mo

0.64

0.00

1.00

1.00

0.64

0.21

0.30

0.59

0.00

0.00

0.13

0.60

0.25 0.00 0.89 (SOMC catalyst) a With respect to the activity of conventional CoMo/A1203.

0.94

CoMo/A1203 (conventional catalyst) SnCoMo/AI203 (SOMC catalyst) Mo/A1203 (conventional catalyst)

CoMo/A1203

3MT

23DM2BN

The results of catalytic tests on different sulfides are presented in Table 2. Cobalt enhanced significantly 3-methylthiophene (3MT) conversion, while 2,3-dimethyl-2-butene (23DM2BN) conversion was promoted to smaller extent. It is remarkable that CoMo/AI203 prepared by SOMC method was almost as active as the conventional catalyst, which had 2.6 times more Co than SOMC catalyst. SOMC was thus more efficient than conventional impregnation method to "decorate" the edge sites of Mo sulfide slabs. It was previously reported that reaction between Mo sulfides and Co carbonyls leads to more effective use of the promoter metal [9,12]. On the contrary, tin suppressed dramatically 3MT conversion, but much less olefin conversion. This activity decrease was due to the site blocking without significant change in electronic structure, as evidenced by IR(CO). It is noteworthy that the activi~ drop in 3MT conversion was similar to the intensity decrease of IR(CO) band at 2070 cm" with

592 Sn-doping. Maug6 et al. [9] reported good correlation between the intensity of this band and the thiophene HDS activity. The observed pronounced difference in sensitivity of 3MT and 23DM2BN conversions to Sn and Co doping, might indicate the existence of different sites for these two types of reactions [13]. But, more detailed analysis of reaction mechanism with varying Sn or Co contems, and their characterization (e.g., IR(CO)) should be done to clarify this problem. 4. CONCLUSIONS Uniform deposition of Co by SOMC on MoS2/AI203 pellets could be achieved by using Co(CloHlo)2 rather than C0(C5H702)2. Tin doping of CoMoS/A1203 by SOMC proceeded by progressive hydrogenolysis of Sn-C bond until near total decomposition. A greater complex concentration or the use of Sn(C3Hs)4 rather than Sn(C4H9)4 led to increase in fixed Sn content until monolayer formation. XRD, TEM and EPMA indicated good dispersion of fixed Sn. The fixed Sn was found to be associated with sulfide slabs (edge sites blocking) but some Sn was also observed on the support, as found by EDS and IR(CO). Cobalt promoted more importantly 3-methylthiophene conversion than 2,3-dimethyl-2-butene conversion. On the contrary, tin suppressed catalyst activities, again more significantly 3-methylthiophene conversion. The SOMC appears to be an attractive method for the selective incorporation of promoter (or poison) metals on sulfide phase, permitting a comprehensive study of the nature of liDS and olefin conversion sites. ACKNOWLEDGEMENT

The authors thank Dr. F. Maug6 of ISMRA in Caen for the IR(CO) experiments and helpful discussion. REFERENCES

1. H. Topsoe, B.S. Clausen and F.E. Massoth, in "Catalysis, Science and Technology" (J.R. Anderson and M. Boudart, Eds.), Vol. 11, Springer-Verlag, Berlin, 1996. 2. T.G. Kaufinann, A. Kaldor, G.F. Stuntz, M.C. Kerby, and L.L. Ansell, Catal. Today, 62 (2000) 77. 3. J.P. Candy, B. Didillon, E.L. Smith, T.B. Shay, and J.M. Basset, J. Mol. Catal., 86 (1994) 179. 4. J.P. Candy, O.A. Ferretti, G. Mabilon, J.P. Bournonville, A. E1 Mansour, J.M. Basset and G. Martino, J. Catal., 112 (1988) 210. 5. B. Didillon, C. Houtman, T. Shay, J.P. Candy and J.M. Basset, J. Am. Chem. Soc., 115 (1993) 9380. 6. F. Humblot, B. Didillon, F. Le Peltier, J.P. Candy, J. Corker, O. Clause, F. Bayard and J.M. Basset, J. Am. Chem. Sot., 120 (1998) 137. 7. T.W.G. Solomons, "Organic Chemistry." John Wiley & Sons, New York, 1996. 8. J. Bachelier, M.J. Tilliette, M. Comae, J.C. Duchet, J.C. Lavalley and D. Comet, Bull. Soc. ChirrL Belg,. 93 (1984) 743. 9. F. Maugd, A. Vallet, J. Bachelier, J.C. Duchet and J.C. Lavalley, Catal. Lett., 2 (1989) 57. 10. Maug6, F., and LavaUey, J. C., J. Catal. 137, 69 (1992). 11. S. Kasztelan, H. Toulhoat, J. Gfimblot and J.P. Bonnelle, Appl. Catal., 13 (1984) 127. 12. T.R. Halbert, T.C. Ho, E.I. Stiefel, R.R. Chianelli and M. Daage, J. Catal., 130 (1991) 116. 13. S. Hatanaka, M. Yamada and O. Sadakane, Ind. Eng. Chem. Res., 36 (1997) 5110.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

593

one step synthesis of c o b a l t (II) phtalocyanine-hydrotalcite catalysts for mercaptan oxidation in light oil s w e e t e n i n g

Novel

I. Chatti a, A. Ghorbel a and J.M. Colin b aLaboratoire de chimie des mat6riaux et catalyse. D6partement de chimie, Facult6 des Sciences de Tunis, Tunisie. b D6partement de la recherche universitaire et coop6rative. Centre europ6en de recherche et de technologic. Total Raffinage Distribution SA. Harfleur, France. MgAI hydrotalcites containing increasing contents of cobalt (II) phtalocyanine were prepared by an improved one-step synthesis. An evolution of both the catalytic effectiveness and the structural and textural properties was observed with the cobalt content. The selected catalyst tested in a pilot modeling the fixed-bed sweetening process shows a very good mercaptan removal confirming that the adequate tuning of basic and oxidizing properties lead to a promising catalyst. 1. I N T R O D U C T I O N Sweetening is an efficient catalytic process for the removal of sulfur present as mercaptans in light oils. Even when present in very small quantities, mercaptans cause foul odor, deterioration of additives in finished products and are corrosive. They are oxidized to innocuous disulfides by air under caustic condition and in the presence of cobalt phtalocyanine catalyst [1]. In industrial process, the use of caustic soda solution leads nevertheless to environmental problems in the refinery. To set up a more environmentally friendly process, it is mandatory to replace the caustic soda by solid bases. Many catalysts such as zeolites [2], alumina [3,4], sepiolite [5] and hydrotalcites [6,7] have been regarded as promising solid basic materials. Hydrotalcites, which are anionic clays, are the subject of this investigation. They are layered double hydroxides with the general formula [M(II)l.xM(III),,(OH)z].[An-x/n].mHzO. The host layers are charged positively by replacement of divalent metal cations usually Mg z+, Ni z+ and Zn z+ with trivalent ones as AI3§ Cr 3§ and Fe 3§ The positive charge is compensated by anions occupying the interlamellar space [8,9] Anions of any nature like simple anions, organic anions and metal complexes can be intercalated between the layers [9-14]. Incorporation of metal phtalocyanine complexes into the inter gallery of hydrotalcite is the subject of many interest owing to their advantageous catalytic properties especially in oxidation reaction [15-18]. These catalysts could be prepared by different methods: direct synthesis [18,19], anion exchange [19,20] and structure reconstruction [21,22]. Pinnavaia et al [21] have tested Co(II) phtalocyanine, intercalated in MgAI hydrotalcite as catalyst in

594 air oxidation of thiols at alkaline pH (9.25). This intercalated Co (II) complex was twice as active as the homogenous complex. However, the thiol oxidation test reaction was carried out at pH-9.25 in the presence of a borate buffer, and the catalyst was obtained by a three steps method (structure reconstruction method) [21 ]. In this paper we report that without alkali medium a very good mercaptan removal was achieved using bifunctional catalyst prepared by an improved one-step preparation method. 2. EXPERIMENTAL 2.1. Catalysts synthesis The catalysts containing increasing contents of cobalt were synthesized by precipitation (Fig. 1) of two solutions, one of Mg (1.4 M) and A1 (0.7M) nitrates, the second of cobalt (II) phtalocyanine tetrasulfonate (1.6 10-3 to 3.2 10-2 M) and NaOH (3.5 M). The first solution was added to the second one while maintaining the temperature at 308 K with vigorous stirring under helium atmosphere to minimize contamination by atmospheric CO2. The addition took nearly 3 h. The precipitates formed were aged in their mother liquor for 18 h at 333 K and then filtered, washed with hot distilled water, and dried overnight at 343 K.

Mg(NO3)2 + Al(NO3)3

Precipitation Inert Atmosphere NaOH + Na4[CoPcTs] Aging ........... -~ [ Crystallized gel Filtration- Washing Drying

:~ [

Catalyst

Fig. 1. Catalysts synthesis The prepared catalysts are directly used in catalytic test reaction without ft~her additional operation, as the mechanical resistance of the solids is high enough. Very

595 resistant materials are obtained, simply broken up and sieved to separate lmm desired particles. 2.2. C h a r a c t e r i z a t i o n

Elemental analysis was carried out by inductively coupled plasma using a TJA IRIS-HR spectrometer after dissolving the compounds in a minimum amount of hydrochloric acid. X-ray diffraction spectra (XRD) were recorded on a Siemens D-500 diffractometer with a graphite-filtered CuKct radiation (1,5405 ,~). ESR spectra were recorded at 77 K using an ER 200 tt Brucker spectrometer. An ASAP 2000 Micrometrics apparatus measured specific surface areas by nitrogen adsorption at 77K after degassing the samples in situ at 353 K for 4h. 2.3. R e a c t i o n p r o c e d u r e

Screenings: the apparatus consist of a fixed-bed reactor containing l g of catalyst. Hydrocarbon feed (carbureactor containing 189 ppm RSH) and air (atmospheric pressure) are passed through the reactor with a space velocity of 1.2 h -1 at room temperature. The residual mercaptan content was determined using the silver nitrate potentiometric method norm [23]. Pilot test: the apparatus consist of a fixed-bed reactor containing 100 g of I mm catalyst particles. Hydrocarbon feed (carbureactor containing 125 ppm RSH) and air (6 bar) are passed trough the reactor with a space velocity of 1.2 h -1 at 313 K. The residual mercaptan content was also determined using the same silver nitrate potentiometric titration method

[23]. 3. R E S U L T S AND DISCUSSION 3.1.Characterization

f

5

l0

i~

20

25

30

35

40

45

50

55

60

65

2 0 (cte~'ee) Fig.2. Evolution of XRD spectra with the cobalt content (10 -3 mol g-l). b" 0.03 (DS1), c: 0.09 (DSz), d" 0.13 (DS3), e: 0.22 (DS4), f" 0.31 (DSs), g" 0.37 (DS6), h: 1.09 (DS7). a: Sample prepared without cobalt (DS0). (*) peaks assigned to NaNO3.

596 Fig. 2 reports the X-ray diffraction patterns of the prepared samples and Table 1 gives the corresponding cobalt content. An evolution of the solid structure with cobalt content is observed. At low cobalt concentration poorly crystallized hydrotalcites are obtained with a precipitation of NaNO3 (Fig. 2b and 2c), while good crystallized hydrotalcites are obtained for higher cobalt concentrations (Fig. 2d to 2g). At least an amorphous product is obtained for a cobalt content of about 1.0910 .3 mol g-1 (Fig. 2h). Table 1, gives the interlayer spacing measured for planes (003). Even working under helium atmosphere to minimize contamination by atmospheric COz, the obtained d003 is about 7.8 .&, as generally reported for hydrotalcites containing CO32- in the interlayer [20,24]. Comparing this result to those obtained by Carrado et al. [19] on copper phtalocyanine tetrasulfonate, it is important to note that the same result was obtained for stochiometric amount of CuPcTs. Nevertheless for a CuPcTs concentration corresponding to twice that necessary to balance the charge created by aluminum in the hydrotalcite framework, Carrado et al. [19] obtained a perpendicular intercalated copper phtalocyanine complex with a d003 reflection at 23 A. However, this component presented an unexpectedly low surface area of about 0.2 mZ/g. In the present work we presume a parallel orientation of the cobalt phtalocyanine between the hydrotalcite sheets. This hypothesis was confirmed by a direct synthesis in which we voluntary omitted the addition of the cobalt phtalocyanine tetrasulfonate. The solid obtained, noted as DS0, was not an MgAI hydrotalcite but NaNO3 (Fig. 2a). This result indicates that CoPcTs 4- contributes certainly to the charge compensation in the hydrotalcite samples (DS3 to DS6). A parallel orientation of a metal phtalocyanine in the observed interlayer spacing is conceivable due to its planar geometry [20], moreover a parallel orientation of a nickel phtalocyanine between a LiA1 hydrotalcite layers was already described by Dutta et al. [25]. For the hydrotalcite samples (DS3 to DS6), elemental analysis results point to AI/Co ratio of about 21/1, However, as the cobalt phtalocyanine anion is tetravalent, the AI/Co ratio was expected to be 4/1.This statement is correct if there is only the metal complex in the interlayers. In our case in addition to cobalt phtalocyanine, a meaningful contamination by CO32- anions takes place. Table 1. Physico-chemical properties Sample

DS1

DS2

DS3

DS4

DS5

DS6

DS7

Co (10 .3 mol g-1)

0,03

0,09

0,13

ICP 0,22

0,31

0,37

1,09

d 003 (A)

7.8

7.8

7.8

DRX 7.8

7.8

7.8

SBET(mZ/g)

2,5

16,46

46,1

N2/BET 43,5

42,5

65,6

8,2

Mean meso pore size (A)

34

49

62

56

50

49

67

597 The XRD study is correlative with the textural study (Nz/BET) and shows as reported in Table 1, an evolution of the surface area with the cobalt content. These results are probably related to the structural evolution of the solids in function of the cobalt content. Furthermore all the samples are mesoporous and exhibit a very low variation of the mesoporous diameter. The dispersion of the cobalt phtalocyanine complex in the MgAI hydrotalcite material was evaluated by electron spin resonance. The ESR spectra (Fig. 3) are complex and result from the superposition of several signals. A signal attributed to iron is observed around g = 2.51 in all the spectra. With the DS6 sample (Fig. 3b), we notice a clearly hyperfine splitting due to 59Co (I=7/2) attributed to magnetically isolated complexes indicating a molecular distribution of cobalt phtalocyanine. With both increasing (Fig. 3a) and decreasing (Fig. 3c) cobalt content, very similar spectra are observed, indicating that the maximum capacity of dispersion of the cobalt complexes is never reached even with a cobalt content of 1.09 10 -3 molg -1 HT (Fig. 3c). These results are most important when compared to those of Iliev et al. [22] showing by ESR an aggregation of the cobalt phtalocyanine complexes for lower cobalt content of about 6.5 10 -5 molg -1 HT. A narrow and symmetrical ESR signal located at g=1.97 is also observed. A similar narrow singlet near g=2.004 has been assigned to cobalt phtalocyanine crystallites with crystal defects [22]. The attribution of the singlet observed to the same crystallites is likely since the same signal is also observed with neat CoPcTsNa4 (Fig. 3d). g=2~

_

.

1

2600

a

|

3000

,

. i .

3400

I

.

t

....|

3800

_

I

.

4200 H (gauss)

Fig. 3. Evolution of ESR spectra with the cobalt content (10 -3 mol g-l). a: 0.13 (DS3), b: 0.37 (DS6), c: 1.09 (DS7). d: neat CoPc(SO3Na)4 complex.

598

3.2. Catalytic

activity

Mercaptans are known [1] to be oxidized into disulfides by air under caustic conditions and in the presence of cobalt (II) phtalocyanine catalyst as below: 2 RSH + 1/2 O2

"

RSSR

+

HzO

An anion - radical mechanism was proposed by Wallace et al. [26]. The authors suggested the activation of mercaptan RSH into RS- species by a base, followed by an oxidation of RS- to R S radical by the cobalt phtalocyanine. The catalytic effectiveness of the well-crystallized catalysts (DS3 to DS6) was evaluated by a simple laboratory test (screenings). Mercaptan removals of 70 to 83 % are obtained. This important catalytic activity is attributed to a) the meaningful contamination by CO32anions in the interlayer domain, which induces the pursued basic character as described by Constantino et al. [27,28] for the decomposition of MBOH to acetone and acetylene by MgA1CO3 hydrotalcite at 353 K, and b) to the dispersion of the cobalt phtalocyanine complex in the interlayer space, since an aggregation of the cobalt phtalocyanine complexes decreases the thiol oxidation effectiveness [22]. The differences observed between the catalysts could be discussed in terms of solid state and cobalt content. DS3, DS4 and DS5 samples having close surface area present different mercaptan conversions (Fig. 4), this result is due to cobalt content, which increases from DS3 to DS5 (Fig. 5). DS6 sample is the most active catalyst due to its superior surface area and cobalt content. 84 82 80 .o 78 > o o T rv CO

76 74 72 70 68

84 o~ 82

DS6 DS5

c-

0

lDs4 DS3~ ,

,

50

1O0

Surface area (rr~/aD

Fig.4. Mercaptan conversion as a function of surface area.

DS6

80

~ 78 9 > 76 co 74 0 -r 72 co ,v 70 68

5 DS4 DS3 i

i

i.

0,1

0,2

0,3

.

.

.

i

0,4

C__.d3dtcontent (mmd/g)

Fig.5. Mercaptan conversion as a function of cobalt content.

599 9 8

E 7ca. 6

@

-

,o,

I

03 5 CK ~4 .x2_ 3

~ j

n,, 2 1

o o

@

9

I

r

i

f

i

1

2

3

4

5

--

6

7

8

9 10 11 12 13 14 15 16 17

Time (day)

Fig. 6. Residual mercaptans as a function of time for DS6 catalyst in a pilot test. The selected catalyst, DS6, was tested over a long period (17 days) with a pilot modeling the fixed-bed sweetening process. Knowing that the actual norm of mercaptan amount in carbureactor product is about 15 ppm maximum, we observed after one day, a residual mercaptan of about 0.8 ppm corresponding to 99.4 % removal (Fig. 6). The amount of RSH increases slowly until 8.4 ppm atter 17 days testing. This low deactivation is probably due to the numerous impurities contained in the industrial hydrocarbon feed. Addition of active charcoal by mechanical crossing in the fixed-bed reactor or incorporated it into the final catalyst could probably remove these impurities and improve the lifetime of the catalyst. 4. CONCLUSION In conclusion, we have shown that without alkali medium a very good mercaptan removal using a bifimctional catalyst was obtained confirming that the adequate tuning of basic and oxidant properties lead to a promising catalyst. This catalyst consists of cobalt phtalocyanine parallel-intercalated MgA1 hydrotalcite obtained by a direct synthesis method. Moreover it exhibited a good mechanical resistance and did not need further post treatment to improve physical properties. ACKNOWLEDGMENT We thank Dr. Anne Davidson and Mr. Bernard Morin from Laboratoire de Rractivit6 de Surface for their help in ESR characterization.

600 REFERENCES

1. B. Basu, Satapathy and A.K. Bhatnagar, Catal. Rev.-Sci.Eng., 35(4) (1993) 571. 2. A. Corma and R.M Martin-Aranda, J.Catal., 130 (1991) 130. 3. J.Muzart, Synth. Commun., 15 (1985) 285. 4. J.Muzart, Synth. Commun., 1 (1982) 60. 5. A. Corma, A. Fornes, R.M Martin-Aranda, H. Garcia and J. Primo, Appl. Catal., 59 (1990) 237. 6. M.J. Climent, A. Corma, S. Iborra and J. Primo, J. Catal., 60 (1995) 151. 7. D. Tichit, M. H. Lhouty, A. Guida, B.H. Chiche, F. Figueras, A. Auroux and D. Bartalini, E.J. Garrone. J. Catal., 50 (1995) 151. 8. E. Narita, T. Yamagishi, K. Tamaza, O. Ichizo and Y. Umetsu, Clay Sci., 9 (1995) 187. 9. F. Cavani, F. Trifiro and A. Vaccari, Catal.Today, 11(1991) 173. 10. S. Miyata, Clays Clay Miner., 31 (1983) 305. 11. W.T. Reichle, Chemtech., 16 (1986). 12. E. Suzuki, S. Idemura and Y. Ono, Clays Clay Miner., 37 (1989) 173. 13. T. Kwon and T.J. Pinnavaia, J. Mol. Catal.,. 23 (1992) 74. 14. M. Del Arco, M.V.G. Galiano, V. Rives, R. Turjillano and P. Malet, Inorg. Chem., 35 (1996) 6362. 15. K.J. Balkus Jr and A.G.Gabrielov, J. Incl. Phenom. Mol. Recog. Chem., 21 (1995) 159. 16. F. Bedoui, Coord Chem. Rev., 39 (1995) 144. 17. N. Herron, Chemteeh., 19 (1989) 542. 18. S. Kannan, S.V. Awate and M.S. Agashe, Recent Advances in Basic and Applied Surface Science and Catalysis., 113 (1998) 927. 19. K.A. Carrado, J.E. Forman, R.E, Botto and R.E. Winans, Chem. Mater.,5 (1993) 472. 20. M.A. Dredzon, Inorg. Chem., 27 (1988) 4628. 21. M. Perez-Bernal, R. Ruano-Casero and T.J. Pinnavaia., Catal. Lett., 11 (1991) 55. 22. V.I. Ileiv, A.I. Ileva and L.D. Dimitrov, App. Catal. A., 126 (1995), 333. 23. Mercaptan sulphur in gasoline, kerosene, aviation turbine and distillate fuels (potentiometrie method). ASTM D 3227-83. 24. W.T.Reichle, J .Catal., 94 (1985) 547. 25. P.K. Dutta and M. Puri, J. Phys. Chem. 93 (1989) 376. 26. T.J. Wallace, A. Schriescheim, H. Hurwitz and M.B. Glaser, Ind. Chem. Prod. Res. Dev., 3 (1964) 237. 27. Constantino V.R.L. and Pinnavaia T., Catal. Lett., 23 (1994) 361. 28. Constantino V.R.L. and Pinnavaia Y., Inorg. Chem., 34 (1995) 883.

Studies in SurfaceScienceand Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

601

Structural and catalytic properties of Zr-Ce-Pr-O xerogels S. Rossignol, C. Descorme, C. Kappenstein and D. Duprez Laboratoire de Catalyse en Chimie Organique, UMR 6503, CNRS et Universit6 de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers, France. Zr0.10(Cel-xPrx)0.90Oz mixed oxides (x between 0 and 0.75) were prepared by coprecipitation (nitrates) or by the sol-gel route. Zirconium n-propoxide and cerium and/or praseodymium nitrates were used as precursors. "Sol-gel" oxides calcined at 900~ were shown to be cubic with a fluorite-type structure. Coprecipated oxides could not be obtained as solid solutions. The BET surface area of these samples rapidly decreases when x > 0.50. A Raman study confirmed that all oxides were cubic and put in evidence the presence of oxygen vacancies. The optimum oxygen storage capacity was obtained for Zr0.10(Ce0.50Pr0.50)0.9002. It appears that the substitution of cerium by praseodymium in Zr0.10Ce0.90Oz mixed oxides leads to a material with improved redox properties. The presence of vacancies, associated with pr3+/pr4+ ions, is thought to be responsible for these enhanced OSCs. 1. I N T R O D U C T I O N Ceria CeOz is a crucial component of modern automotive pollution control catalysts [1]. This oxide is a promoter in the water gas shift reaction and prevents the sintering of the noble metals used as active centers. More important is the ability of this oxide to store oxygen. This phenomenon is associated with a fast Ce4+/Ce 3+ redox process in the solid, also involving anionic vacancies [1-4]. It was shown that up to 50% zirconium could be substituted to cerium into the ceria framework to form solid solutions with facecentered cubic structures. In a previous work, we have evidenced that among the Zr-Ce-O mixed oxides, the compound Zr0.1Ce0.902 prepared by sol-gel procedure displays the highest oxygen storage capacity [5, 6]. Praseodymium is one of the possible "new" additives, which today attracts more and more attention. In fact, it was demonstrated that the oxygen exchange occurs at lower temperature on cerium-praseodymium mixed oxides than on ceria [7]. Furthermore, high temperature pretreatment does not affect the oxygen exchange capacity (OSC) of the mixed oxides. Moreover, high surface area PrOy-ZrO2 materials with a fluorite-type structure were also prepared by the sol-gel way [7, 8]. The main objective of our study was to modify the composition of CeO2 and Zr0.10Ce0.90Oz samples, in order to follow the influence of cerium substitution by praseodymium and to enhance both thermal stability and oxygen storage capacity. The results will be discussed on the basis of DTA, XRD, Raman spectroscopy, BET surface area and OSC data.

602

2. E X P E R I M E N T A L 2.1. Preparation Two series of samples were prepared. Series 1 consists of Zr0.10(Cel-xPrx)0.90Oz samples while, in the second series, oxide samples do not contain any zirconium (Cel_xPrxO2). Preparation methods were described earlier [5]. Solution concentrations were adjusted in order to synthesize 3 g of solid in one batch. All reactants were Aldrich products, 99% purity. They were used without any further purification. The synthesized materials were dried in air at 60~ for 1 h and at 120~ overnight before further calcination in air at 900~ for four hours. 2.2. Characterization Differential Thermal Analysis (DTA) experiments were carried out in dry air (Air Liquide, < 5 ppm impurities) between 25 and 500~ using a Thermal Analyst 2100 TA apparatus. Samples previously dried at 120~ were heated at 5~ -1. Preliminary experiments showed that no DTA peak was observed above 500~ Specific surface areas were determined by N2 (Air Liquide, 30% N2 in He) adsorption at -196~ (one point BET method) with a Micromeritics Flow Sorb II. XRD analysis was performed on a Siemens DS00 powder diffractometer using CuKct radiation (~(CuKctl) = 0.15406 nm). Standard diffractograms recording parameters were: dwell time = 1 s, step = 0.05 ~ 20 range = 10 to 90 ~ divergence slit = 1~ for precise cell parameter determination: dwell time = 10 s, step = 0.02 ~ Raman spectra were recorded using a Perkin Elmer spectrometer: Nd-YAG laser (1064.4 nm, 100 mW), dwell time = 120 s, number of scans = 20. Oxygen storage capacity (OSC) was measured at 400~ under atmospheric pressure. A 20 mg sample was continuously purged with helium (30 cm3.min-1). Successive or alternate pulses (0.265 cm 3) of Oz (Air Liquide, < 5 ppm total impurities) and CO (Air Liquide, N20) were injected every minutes in order to simulate lean and rich operating conditions as those encountered in an Otto engine coupled with a three-way catalytic converter. The Oxygen Storage Capacity (OSC) was calculated from the CO consumption after stabilization of the sample in alternate pulses condition. 3. RESULTS 3.1. Structural characterizations (DTA, XRD,

Raman)

DTA and XRD. After the drying step at 120~ all the samples correspond to complex mixtures containing major M iu (precursor oxidation state) and minor M TM oxides and hydroxides (M = Ce or Pr) and no well-defined phase could be evidenced from the XRD results. Fig. 1 shows the thermograms obtained under air of all series 1 samples and of one series 2 sample (x = 1). All the thermograms consist in endothermic peaks due mainly to the elimination of a small quantity of the residual water molecules and to the transformation of hydroxide into oxide ions. Therefore, the possible exothermic reactions such as the formation of a crystallized phase or the oxidation of Ce Iu or Pr m atoms are hidden by the endothermal events.

603 For x=0 in series 1, the peak at 250~ is characteristic of the appearance of a fluoritetype solid solution Zr0.1Ce0.9Oz [6]. For the other samples, this peak is shifted to higher temperature and additional endothermic peaks appear at lower temperatures. Diffractograms of the samples after calcination at 900~ are displayed in Fig. 2 in the 260-36 ~ 20 range, including the calculated crystallite size. The samples of series 1 (with x between 0 and 0.75) present only symmetrical diffraction peaks in agreement with the presence of one phase identical to Zr0.]0Ce0.9002 compound (x = 0) described in a previous paper [5]. This mixed oxide has been shown to be a solid solution with a fluorite-type structure and lattice parameter a = 5.3962 A. The lattice parameters of some samples are given in Table 1 and we observe a slight contraction of the cell as praseodymium atoms are substituted to cerium atoms, in agreement with a smaller ionic radii for Pr 4+ [9]. For the sample Zr0.10Pr0.9002 (Fig. 2, x = 1, series 1) the distortion of the diffraction peaks indicates the presence of two phases: a major Pr6011 phase and a fluorite-type structure solid solution, in agreement with the work of Sinev et al. [10]. X=I series 2 I

Pr60 ,

x=0.75 series 1

II

'

Particle size l

~

x=1series2

x=O series 1

I

~T = 0.01

100

200

300

1"/~

400

500

. ~

~ ~ ~ j ~ x

=0series 2

600

Fig. 1. DTA profiles of Zr0.1o(Cel_ xPrx)0.9002 oxides prepared by sol-gel (series 1) and Cel_xPrxO2 oxides prepared by coprecipitation (series 2).

20/ degree

Fig. 2. XRD patterns of Zr0.10(Cel_ xPrx)o.90Oz oxides prepared by sol-gel (series 1) and Cel_xPrxO2 oxides prepared by coprecipitation (series 2).

604

In series 2, the oxides CeOz (x=0) and Pr6Oll (x=l) appear to be single phase stable after air calcination at 900~ The non-stoichiometric praseodymium oxide can be better formulated: [prIV4 prnI2][O-IIll (Vo~176 which displays the importance of the vacancies. For both series, the samples containing no cerium (x= 1) display different DTA profiles by comparison with the single phased samples (Fig. 1) and this behavior agrees well with the final presence of the praseodymium oxide phase Pr6Oll and the successive formation of intermediate phases [ 11]. Table 1. Lattice parameters and experimental and calculated OSC. Series

Praseodymium atom. fraction

Lattice

parameter

O S C cal

(OSCexp/OSCcal)

(gmolCO.g-)

O S C exp 1

([amolCO.g -1)

Ratio

(A) Series 1

Series 2

x=0

5.3962

197

180

1.09

x = 0.25

5.3913

272

175

1.56

x = 0.50

5.3937

284

108

2.63

x= 0.75

5.3953

234

67

3.51

x = 0.1

42

Not calculated

x=0

Phase mixture 5.4138

60

34

1.77

x= 1

5.4641

35

6

6.28

Raman spectroscopy. Four Raman spectra only are presented in Fig. 3. In fact, for samples with x higher than 0.25, fluorescence effects due to praseodymium were so important that no spectra could be recorded. The band at ca. 460 cm -1 is attributed to the T2g vibration mode of the metal-oxygen bond in the fluorite-type structure [12-14]. In the case of series 1 samples, the presence of this band confirms that these oxides are solid solutions. In order to explain the shift of this band as a function of x, samples were analyzed again after calcination at different temperatures (Fig. 4). This band is shifted towards higher frequencies as the calcination temperature increases (200, 400, 900~ This phenomenon could be related to structural ordering. On the contrary, a shift towards lower frequencies can be observed as zirconium or praseodymium atoms are substituted to cerium atoms. This shift indicates that the incorporation of Zr or Pr atoms induces ~i perturbation of the M-O bond involving some kind of disordering or structural distortion. Mc Bride et al. [15] gave two reasons for the observed frequency shift: (i) an increase in the oxygen vacancy density and (ii) an expansion or contraction of the cell. In fact the presence of oxygen vacancies in this material could be explained by the presence of reducible praseodymium atoms (Pr4+/pr3+). Moreover, in the case of Ce0.75Pr0.2502 (Fig. 3), a small and broad band appears at about 570 cm -1. The same authors have tentatively attributed this band to the presence of oxygen vacancies in the material. In

605

the other samples, this small band could not be observed. This fact can be linked either to the very weak intensity of the recorded spectra or to the absence of praseodymium atom in the oxide samples.

4

>,

3

X= 0.25 series 1

4-1 X= 0.25 series 2

~ 46~ cm-1

A

~4~4crn'

~

2

l/

5701 cm 1

o

120 ::;

100

X= 0 series 1 465 cm 1

80

.~- 60

. I

1500-1 1200 ] 900

II

II

800

600

1

~176176

3~ 400

X= 0 series 2 466 cm 1

660

200 800

R a m a n shift / cm 1

460

2()0

R a m a n shift / cm 1

Fig. 3. Raman spectra of Zr0.10(Cel-xPrx)0.9002 and Cel.xPrxO2 samples with x = 0 and x = 0.25 after calcination at 900~

470

u U

C

E

465 460 t"" ~ x= 0 series 2 r x = 0 series 1 I x = 0.25 series 2] x = 0.25 series 1}

455 450

~

...

~

~

~

~

o

445

i

0

200

i

i

400 600 Temperature / ~

i

800

1000

Fig. 4. Raman shift as a function of the calcination temperature for the two series of samples with x = 0 and x= 0.25.

606

3.2. Surface characterization and oxygen storage capacity All BET surface areas are reported in Fig. 5. Whatever the series, introduction of Pr results in a decrease of the BET surface area. For series 1 samples, this decrease is even more pronounced for x > 0.5. It seems that the presence of P r 6 O l l could be responsible for this decrease of surface area. Furthermore, the presence of zirconium remains crucial to stabilize cerium-praseodymium oxides: BET surface areas of series 1 samples are much higher than those of series 2, except for x = 1. Praseodymium can stabilize the texture of ceria but its influence in obtaining high surface area oxides appears to be much weaker than in the case of zirconium addition. [2-4, 16]

40

--0- series 2 l # series 1

35

,:,

~ 3o 9 25 u

20

~

15

0

u 111 ~

5

---....... ""

"-0 .

.

.

.

.

.

"-0

J

,

,

i

0,2

0,4

0,6

0,8

1

Fig. 5. Specific surface areas of Zr0.10(Cel-•215 oxides prepared by sol-gel (series 1) and Cea_xPr,,O2 oxides prepared by coprecipitation (series 2) after calcination at 900~ The evolution of the oxygen storage capacity (OSC; in t.tmol-CO.g -1) is given in Fig. 6 as a function of the amount of praseodymium in the solids. For both series, the introduction of praseodymium induces an increase of the OSC up to x = 0.50, whereas above x = 0.50, a decrease is observed. Therefore, an excess of praseodymium has a negative effect on the OSC [17-19]. In order to understand how praseodymium can influence the oxygen storage capacity, "calculated" OSC values were obtained as follows:

OSCcal =. b S Na 2 with:

(1)

- S = BET surface area (m2.g -1) - a = lattice parameter (m) - N - Avogadro number - b = fraction of reducible elements (Ce + Pr) in the unit cell (0.9 for series 1, 1 for series 2)

This calculation is based on the following assumptions: - all the C e Iv o r Pr Iv atoms of the surface monolayer are reduced to oxidation state III"

607

- the surface faces are { 100} faces and contain 2 metal atoms and 4 oxygen atoms per unit surface area a 2 (a = cubic parameter)" therefore one oxygen atom out of four participates to the OSC process as indicated by equation (3); oxygen atoms bonded to reducible elements are the only one to participate to the OSC.

300

";"r,.200250 )d~ ~

' ' ~ serie21 s

0

E 150 o 100 0

, .-"" o

50

.

0

i

i

0,2

0,4

Fig. 6. OSC values measured at 400~ calcined at 900~

x

i

l

0,6

0,8

1

as a function of x for the two series of samples

Calculated and experimental values, expressed in lamol-CO.g -1, are collected in Table 1. The ratio of these two values may represent the number of oxygen layer involved in the redox process. In all cases, the calculated values are lower than the experimental ones. This observation shows that 2 to 3 layers are involved in the redox process. The increase of the OSC along with the amount of Pr shows that the presence of this element induces the creation of anionic vacancies. In fact, in substitution to cerium cations, Pr ions may undergo a redox process (pr3+/pr 4+) whereas it is not possible for zirconium cations. 4. C O N C L U S I O N S We prepared by sol-gel some thermally stable Zr0.a0(Cel-• mixed oxides (x between 0 and 0.75) with a fluorite-type structure. This structure was confirmed by the presence, in the Raman spectrum, of a single band ca. 460 cm -1, characteristic of the M-O vibration in the fluorite-type structure. Moreover, the band position and the shoulder at 570 cm -1 indicate the presence of oxygen vacancies probably associated with praseodymium cations. Consequently, high OSCs appears to be the result of the presence of both cerium and praseodymium atoms. Thus, addition of praseodymium atoms into zirconia-ceria oxides appears to be very promising for the design of new automotive catalysts. R E F E R E N C E S

1. C. K. Narula, J. E. Allison, D. R. Bauer and H. S. Gandhi, Chem. Mater., 8 (1996) 984. 2. T. Bunluesin, R.J. Gorte and G.W. Graham, Appl. Catal. B, 14 (1997) 105. 3. Y. Sun and P. A. Sermon, J. Mater. Chem, 6 (1996) 1025.

608 4. A.Trovarelli, F. Zamar, J. Llorca, C. Leitenburg, G. Dolcetti and J. T. Kiss, J. Catal., 169 (1997) 49. 5. S. Rossignol, F. Gerard and D. Duprez, J Mater. Chem., 7 (1999) 1615. 6. S. Rossignol, Y. Madier and D. Duprez, Catal. Today, 50 (1999) 261. 7. C. K. Narula, L. F. Allard and G. W. Graham, J. Mater. Chem, 9 (1999) 1155. 8. M. D. Krasil'nikov, I.V. Vinokurov and S.D. Nikitina, Fiz Khim. Electrokhim., 3 (1979) 123. 9. N. N. Grenwood and A. Earnshaw, Chemistry of the Elements, 2nd edition, 1997, Butterworth Heinemann, Oxford, UK, p. 1295 10. M. Yu Sinev, G. W. Graham, L. P. Haack and M. Shelef, J. Mater. Res., 11(8) (1996) 1960. 11. See ref. 9, p. 643. 12. S. Mochizuki, Phys. Stat. Sol. (b), 114 (1982) 189. 13. W. H. Weber, K. C. Hass and J. R. Mc Bride, Phys. Rev. B, 48 (1) (1993) 178. 14. G. Groppi, C. Cristiani, L. Lietti, C. Ramella, M. Valentini and P. Forzatti, Catal. Today, 50 (1999) 399. 15. J. R. Mc Bride, K. C. Hass, B. D. Poindexter and W. H. Weber, J. Appl. Phys., 76 (4) (1994), 2435. 16. S. Imamura, J. I Tadani, Y. Saito, Y. Okamoto, H. Jindai and C. Okamoto, Appl. Catal. A: Gen., 201 (2000) 121. 17. G. Balducci, J. Kaspar, P. Fornasiero, M. Graziani and M. Saiful Islam, J. Phys Chem. B., 102 (1998) 557. 18. L. Mubmann, D. Lindner, E.S. Lox, R. Van Yperen, T.P. Kreuser, I Mitsushima, S. Taniguchi and G. Gaff, SAE Technical Paper 970465 (1997). 19. C. Leitenburg, A. Trovarelli, F. Zamar, S. Maschio, G. Dolcetti and J. Llorca, J. Chem. Soc., Chem. Comm., (1995) 2181.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

609

Influence of the precursor (nature and amount) on the morphology of MoO3 crystallites supported on silica D. Navez ~, G. Weinbergb, G. Mestl~, P. Ruiza and E.M. Gaigneauxa• a

Unitt~de catalyse et chimie des mat&iaux divis6s, Universit6 catholique de Louvain, Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium.

b Department of inorganic chemistry, Fritz-Haber-Institut der Max Planck Gesellshafl, Faradayweg 4-6, D- 14195 Berlin, Germany. NanoScape AG, Frankfurter Ring 193A, D-80809 Munich, Germany. The influence of the nature of the precursor and the amount used is investigated in the preparation of MoO3 catalysts supported on a mesoporous silica by a wet impregnation method. Four precursors are compared: ammonium heptamolybdate, peroxopolymolybdate, a citrate complex of Mo and an oxalate complex of Mo. The MoO3 loading is adjusted between 0 and 2 theoretical monolayers of MOO3. Characterization of the samples was performed by N2 physisorption, X-ray diffraction and scanning electron microscopy with as main objective the evaluation of the dispersion of MoO3 on the silica and of the morphology of MoO3 crystallites eventually obtained. At low loadings, MoO3 appears as amorphous material remaining at the outer surface of the silica. At high loadings, MoO3 appears as molybdite crystals. The loading above which MoO3 crystals are obtained depends on the precursor used. The morphology of MoO3 crystals, when obtained, also depends on the precursor used. Isotropic crystals are obtained with Mo oxalate and peroxopolymolybdate while Mo citrate leads to crystals with largely developed basal faces. Ammonium heptamolybdate leads to crystals with an intermediate morphology. Our results thus show that changing the nature and the amount of precursor used to impregnate MoO3 on silica offers a promising possibility to adjust the morphology and, to some extent, the dispersion of MoO3 crystals at the surface of the support. 1. INTRODUCTION AND OBJECTIVES In addition to its dispersion, the morphology of the crystallites of the active phase is another important parameter that influences the performance of supported catalysts. The morphology of the crystallites may become particularly crucial when the supported material exhibits "structural specificities", i.e. when the different faces of the active phase crystals have different behaviors in a given process, either in terms of activity and selectivity or of surface dynamics under the catalytic conditions. MoO3 possesses "structural specificities" in oxidation reactions and related processes carried out in the presence of oxygen (oxidative dehydrogenation, oxygen-assisted dehydration *Corresponding author: E.M.G. is Chercheur qualifi6 (Research associate) for the National Foundation for Scientific Research (FNRS) of Belgium

610

of alcohols and of amides, etc). Typically, on the one hand, the (010) basal faces are weakly active and preferentially perform the total oxidation of olefins, but the (100) lateral faces selectively transform olefins to partially oxygenated compounds [1-3]. On the other hand, the synergetic effects between MoO3 and a-Sb204 or BiPO4 (or any other spillover oxygen donor phase, as defined in the Remote Control Mechanism theory [4]) are much more intense when the MoO3 crystals develop larger (010) basal faces [5,6]. The phenomenon is due to the pronounced ability of (010) faces to reconstruct under the influence of spillover oxygen to new more active structures, namely nanometric pits and steps with walls oriented as (100) facets, while (100) faces do not undergo such modification during the reaction. These differences of behavior most likely originate, at least partially, from a difference of reducibility of the different faces of MoO3 crystals, the later being itself related to a difference in the co-ordinations of the superficial oxygen atoms exposed on the different faces. Namely, the accessible oxygen atoms on the (010) face are doubly linked to Mo atoms (Mo-O), while both doubly linked oxygen and oxygen species bridging two Mo atoms (Mo-O-Mo) are exposed on the (100) face [7,8]. The morphology for MoO3 crystals in an oxidation catalyst should thus be carefully adjusted to an optimum depending on the particular catalytic application to which it is dedicated. For the preparation of selective oxidation supported catalysts using MoO3 as the active phase, it is thus crucial to dispose of synthesis methods that allow efficient tuning of the morphology of MoO3 crystals at the surface of the support. The methods proposed in the literature to do so, e.g. spin-coating [9], thermal evaporation [10], chemical vapor deposition [11], flash evaporation [12], laser deposition [13] and r.f. reactive sputtering [14], are rather scarce and complex. Moreover, they are often more dedicated to the deposition of active phase on flat and/or monolithic supports (to produce model catalysts for surface science purposes) than on powder supports. These methods thus usually only allow the production of samples at a small scale, so that they are often inadequate for the production of pulverulent real catalysts in large amounts. In this contribution we demonstrate that, using an easy wet impregnation method that is suitable for deposition of active phase on powder supports, the morphology of the active phase crystallites of silica-supported MoO3 catalysts can be efficiently adjusted by simply varying the nature and the amount of the precursor used in the synthesis. 2. E X P E R I M E N T A L 2.1. Preparation of the samples The support used for the preparation of the samples was a commercial mesoporous silica (Grace 532). Prior to the impregnation, the silica was activated overnight in air at 773 K. After activation, the surface area was 314 m 2 g 1 and the pore volume was 1.75 cm 3 g-1 with an average pore diameter of 112.9 A. Four different precursors in aqueous solutions were used to impregnate MoO3 at the surface of the silica: 1~ commercial ammonium heptamolybdate (Vel, p.a.), hereinafter noted HEPTA, dissolved as such in distilled water, 2 ~ peroxopolymolybdate, hereinafter noted PEROXO, prepared by reacting a metallic Mo powder (Aldrich, 99.95%) in H202 (Aldrich, 30% wt, diluted to 10% wt in distilled water) following the procedure described in [9], 3 ~ citrate complex of Mo 6§ hereinafter noted CIT, prepared from HEPTA in aqueous solution and an equivalent amount of citric acid (Merck, 99.5%),

611 4 ~ oxalate complex of Mo 6+, hereinafter noted OXA, prepared from HEPTA in solution and an equivalent amount of oxalic acid (Aldrich, 98%). For all the samples, the impregnation procedure was carried out by dispersing the silica in the precursor solution. The solvent was then removed under vacuum from the suspensions maintained at 303 K in a rotavapor. The obtained solids were dried at low temperature. A precalcination at 413 K was applied in the case of the samples prepared from the PEROXO precursor. A precalcination at 573 K was applied for the samples prepared using the CIT and OXA precursors. The solids were finally calcined in air under conditions identical for all the samples. The concentrations and the volumes of the precursor solutions, and the masses of support involved in the impregnation, were adjusted to synthesize samples with loadings of MoO3 on the silica, between 0 and 2 theoretical monolayers (hereinafter abbreviated ML). Calculations of the quantities used for the syntheses were based on the assumption that a [MOO6]6" octahedron occupies 0.17 nm 2 on the support [ 15]. Table 1 summarizes the conditions for the syntheses of the samples and the loadings of MoO3 prepared starting from the four different precursors. Table 1 Experimental conditions for the syntheses of the silica-supported MoO3 catalysts and loadings of MoO3 prepared expressed in number of theoretical monolayers (ML) Precursors HEPTA PEROXO CIT OXA Drying air/22 h/393 K air/22 h/393 K vacuo/16 h/353K air/20 h/393 K air/5 h/413K air/16 h/573 K air/20 h/573K Precalcination Calcination air/90 min/723 K air/90 min/723 K air/90 min/723 K air/90 min/723 K v

Loading (ML)

0, 0.1, 0.25, 0.375, 0.5, 0.75, 1, 1.5, 2

0, 0.25, 0.375, 0.5, 1, 2

2.2. Characterization N2 physisorption was used to evaluate the specific areas of the samples, the volumes of their pores and their mean pore diameters. Specific area measurements were performed with a Micromeritics Flowsorb II 2300 equipment. During the whole analyses, the samples, analyzed as batches of 0.1 g, were flowed with a gas mixture (hereafter termed reference gas) containing 30% vol of N2 in He. Samples were first degassed at 433 K during 90 minutes. Adsorption of N2 was then performed by flowing the reference gas on the samples maintained at 77 K. When the adsorption equilibrium was reached, desorption was finally performed by rising the temperature of the samples back to room temperature. The quantities of N2 having physisorbed at the surface of the samples were measured with a catharometer integrating the defaults of N2 and the excesses of N2 in the gases recovered at the cell outlet during the successive steps of the analyses with respect to the reference gas. Calculation of the specific area was then made considering that the occupation of a uniform monolayer formed by 1 ml of gaseous N2 is 2.84 m 2. Pore volumes and mean pore diameters of the samples were measured with a Micromeritics ASAP 2010 equipment. Prior to the analyses, the samples were degassed at 433 K down to a pressure of 0.1 Pa. Analyses were then based on the determination of the hysteresis between the adsorption isotherm of N2 at 77 K and the corresponding desorption isotherm for partial pressures of N2 between 0.1 Pa and 101325 Pa.

612 Powder X-ray diffraction (XRD) was performed with a Kristalloflex Siemens D5000 diffractometer using the K radiation of Cu ( =1.5418 /~) and equipped with a secondary curved graphite monochromator. The analyses were done in the continuous coupled /2 reflection mode. Two- angles were scanned between 5 ~ and 70 ~ at a rate of 0.45 ~ min1. Scanning electron microscopy (SEM) was done with a $4000 microscope from Hitachi using a field emission gun and interfaced with an energy dispersive X-ray spectrometer (EDX) device. Samples were analyzed without any coating. Observations were made with an accelerating voltage of 5 kV. 3. CHARACTERIZATION RESULTS AND DISCUSSION 3.1. N2 physisorption Existence of two steps in the deposition of MoO3 on the silica Fig. 1 shows the evolution of the specific area of the samples as a function of the amount of MoO3 impregnated at the surface of the silica. Very similar tendencies are observed regardless of the precursor used for the impregnation. As expected, the specific area of the samples decreases with the amount of MoO3 deposited on the support increasing. However, a break in the decrease is observed corresponding to an amount of MoO3 impregnated of about 0.75 ML. For the low loading domain (less than 0.75 ML of MoO3 applied), a fast decrease of the specific area is observed. But for the high loading domain (more than 0.75 ML of MoO3 applied), the specific area decreases more slowly with the amount of MoO3. This observation suggests that the deposition of MoO3 at the surface of the silica proceeds in two distinct steps depending on the amount of material impregnated.

+ PEROXO 9 HEPTA I--10XA • CIT

~t~ 300

o250

O ,,

o,=~ o tD

200

150

;t ,

0

,

,

|

I

0,5

,

,

!

.

.

.

.

i

,

,

,

,

1

1,5 2 MoO 3 loading (ML) Fig. 1. Evolution of the specific areas of the silica-supported MoO3 catalysts as a function of the MoO3 loading and the precursor used in the synthesis.

Further on the mechanism of deposition of MoO3 on the silica A plausible hypothesis to account for this interpretation could be that, at low loadings, MoO3 preferentially deposits inside the pores of the support, while at high loadings, it also

613 deposits in significant amounts at the outer surface of the silica. However, this hypothesis must be discarded considering pore volume and mean pore size data in the case of the samples with low MoO3 loadings prepared from PEROXO as precursor (Table 2). Table 2 Pore volumes and mean pore diameters for the samples prepared using PEROXO as precursor MoO3 loadings 0 MC 0.1 MC 0.25 MC 0.375 MC 0.5 MC Pore volume (cm3 g-l) 1.75 i.68 1.60 1.53 1.46 Mean pore diameter (A) 112.9 112.0 113.4 112.0 113.7 The volume developed by the pores of the silica decreases linearly with the amount of MoO3 deposited on the support. But the mean pore diameter remains unchanged independently of the number of MoO3 monolayers applied on the silica. If the MoO3 material had been deposited inside the pores, one should have observed, regardless of the mechanism by which the pores would get filled, a variation of the mean pore diameter with the quantity of MoO3 impregnated. Precisely, a decrease of the mean pore diameter would suggest that, for increasing amounts of MoO3 impregnated, the pores would get progressively filled, all together and independently of their sizes, through the coating of their inner walls by progressively thicker layers of MoO3 material. On the contrary, an increase of the mean pore diameter would indicate that, for increasing amounts of MoO3 impregnated, the pores would get entirely filled sequentially by increasing order of size. Namely, the narrowest pores would first be entirely filled, then the just slightly wider pores would be filled, and so on until no empty pores remain. None of these hypothetical mechanisms match our observation that the mean pore diameter does not vary with the amount of MoO3 impregnated. Therefore, the most plausible conclusion is that, using PEROXO precursor (and presumably the others also) MoO3 does not penetrate inside the pores of the support, but only deposits at the outer surface of the silica particles, thus covering the mouth of the pores and blocking them, independently of their size.

3.2. X-ray diffraction Existence of two steps in the deposition of MoO3 on the silica Fig. 2 shows the X-ray diffraction patterns of the silica-supported MoO3 samples impregnated with CIT as precursor. Attention is directed to the influence of the amount of MoO3 deposited on the silica. When low loadings of MoO3 are applied, no diffraction peaks are detected and only the signal typical of the fresh silica is observed. When 0.375 ML of MoO3 is deposited on the support, tiny diffraction peaks appear at 20 angles typical of molybdite [16]. For higher loadings, intensities of molybdite peaks then increase with increase of the amount of MoO3 impregnated on the silica. These results indicate that for loadings lower than 0.375 ML, MoO3 remains at the surface of the silica as an amorphous material. Crystallites of molybdite only appear when at least 0.375 ML of MoO3 is applied. The number and/or the size of the crystallites increase when the amount of MoO3 impregnated increases. This interpretation of the X-ray diffraction data clearly confirms the existence of two distinct steps in the deposition of MoO3 on the silica depending on the amount of material impregnated.

614

.q,,~ r.~

=o

d 9

b

9

~

15

25

35

a

45 55 65 2 Theta Angles (o)

Fig. 2. XRD patterns for MoO3 supported on silica prepared with the Mo citrate precursor: a) 0 ML, b) 0.25 ML, c) 0.375 ML, d) 0.5 ML, e) 1 ML and f) 2 ML.

5

15

25

35

45 55 65 2 Theta Angles (o)

Fig. 3. XRD patterns for MoO3 (1 ML) supported on silica prepared with as precursor: a) the peroxopolymolybdate, b) the Mo oxalate, c) the ammonium heptamolybdate and d) the Mo citrate.

This interpretation is fully consistent with the understanding of the system from N2 physisorption data. More precisely, the deposition of amorphous material most likely leads to an efficient blocking of the pores of the support. This would match closely the rapid decrease of specific area observed for increasing amounts of MoO3 impregnated in the low loading domain. Respectively, the slower decrease of specific area observed for increasing amounts of MoO3 impregnated in the high loading domain most likely matches the formation of MoO3 crystals which is likely to not be so efficient in pore blocking.

Dependence of MoO3 deposition on the nature of the precursor The tendencies in the X-ray diffraction patterns observed when impregnating MoO3 starting from PEROXO, OXA or HEPTA precursors are almost identical to those, previously commented, observed when using CIT precursor. However, two differences of behavior must be quoted when comparing samples prepared from different precursors: 1~ the minimum loading of MoO3 for which the diffraction peaks of molybdite are detected slightly depends on the precursor used. When using PEROXO, CIT and OXA precursors, molybdite crystals are formed from 0.375 ML. But, when using HEPTA precursor, the crystals only appear when at least 0.5 ML of MoO3 is deposited.

615 2 ~ the relative intensities of the molybdite diffraction peaks, and thus the morphology of the MoO3 crystals when present, depend on the Mo precursor used for the impregnation. Fig. 3 illustrates the influence of the nature of the Mo precursor used for the impregnation on the morphology of the MoO3 crystals obtained. When CIT precursor is used (except for the sample with 2 ML of MOO3), peaks of the (0k0) series of molybdite are the most intense. This suggests that the corresponding MoO3 crystals possess a morphology with basal faces larger than the lateral faces. On the contrary, when performing the impregnation of the support with PEROXO and OXA precursors, the (110) and the (021) peaks are the most intense, suggesting that the corresponding MoO3 crystals possess a more isotropic morphology with lateral and basal faces exposed in a similar extent. Following the same criterion, the MoO3 crystals obtained when impregnating the silica with HEPTA solutions are suggested to possess an intermediate morphology.

3.3. Scanning electron microscopy Fig. 4 shows scanning electron micrographs of the fresh silica support and of some silica-supported MoO3 catalysts prepared using PEROXO and HEPTA precursors. As can be seen on the micrograph of the sample containing 0.1 ML of MoO3 prepared using PEROXO precursor (Fig. 4 - top right), when low loadings of MoO3 are deposited on the silica, no visual distinction can be made between the fresh silica support (Fig. 4 - top left) and the impregnated samples. For the latter, however, EDX reveals that silica particles are homogeneously covered by Mo-containing material. As a counterpart, as illustrated by the micrographs of the samples containing 1.5 ML of MoO3 prepared using PEROXO and HEPTA precursors (Fig. 4 - bottom), when high loadings of MoO3 are deposited, crystals are observed at the surface of silica particles. The crystals are confirmed by EDX to be made of Mocontaining material. Two differences appear when comparing the formation of crystals as a function of the precursor used for the synthesis of the silica-supported MoO3 samples: 1~ for the syntheses using PEROXO, OXA and CIT precursors, the crystals are observed for the samples containing at least 0.375 ML of MOO3, while when HEPTA precursor is used, crystals only appear when 0.5 ML of MoO3 or more is impregnated on the silica. 2 ~ the morphology of the crystals depends on the precursor used for the impregnation. Crystals observed for the samples prepared using CIT and HEPTA precursors are very similar. Their average thickness is about 100 nm and their basal faces have approximate dimensions ranging from 1.5 gm up to 3 gm (and sometimes more) by about 1 gm (Fig. 4 - bottom left). As a counterpart, crystals typical of the samples prepared using OXA and PEROXO precursors are much more isotropic as their thickness is about 200 nm and the dimensions of the basal faces are about 1 [am by 750 nm (Fig. 4 - bottom right). These observations are consistent with the conclusions drawn from the interpretation of Xray diffraction patterns.

616

Fig. 4. SEM micrographs of the fresh silica support (top-left) and of the silica-supported MoO3 catalysts prepared using PEROXO precursor (top-right: 0.1 ML, bottom-left: 1.5 ML) and HEPTA precursor (bottom-right: 1.5 ML). For the top micrographs, bar lengths correspond to 2 lam; for the bottom micrographs, bar lengths correspond to 5 lam. 4. CONCLUSION All characterization data are fully consistent. They lead to an unambiguous understanding of the deposition of MoO3 impregnated on the silica. Namely, 1o two modes of deposition for MoO3 at the surface of the silica exist: at low loadings, MoO3 deposits as amorphous material and homogeneously covers the support; at high loadings, crystals are formed; 2 ~ the amount of impregnated MoO3 required to induce the formation of crystals depends on the precursor used: for HEPTA precursor, crystals appear for a quantity of deposited MoO3 larger than for the other precursors; 3~ the nature of the precursor used for the impregnation influences the morphology of the crystals: OXA and PEROXO precursors lead to more isotropic crystals than those obtained for the samples prepared from CIT and HEPTA solutions.

617 Changing the nature and the amount of precursor used to impregnate MoO3 on the silica thus offers a promising possibility to adjust the morphology of the crystals obtained at the surface of the support. In addition, the dispersion of MoO3, precisely obtaining a crystalline or an amorphous material for a given amount of active phase impregnated, can also be mastered to some extent by adjusting the same two parameters. ACKNOWLEDGMENT

ESEM analyses were carried out at the Fritz-Haber-Institut der Max Planck Gesellschatt (Berlin, Germany). The FNRS (Belgium) and the "Communaut6 Fran~aise de Belgique" are gratefully acknowledged for financial support for the acquisition of XPS and XRD equipments. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

R.A. Hemandez and U.S. Ozkan, Ind. Eng. Chem. Res., 29 (1990) 1454. K. Briickman, R. Grabowski, J. Haber, A. Mazurkiewicz, J. Sloczynski and T. Wiltowski, J. Catal., 104 (1987) 71. J.C. Volta and J.M. Tatibouet, J. Catal., 93 (1985) 467. L.T. Weng and B. Delmon, Appl. Catal. A: General, 81 (1992) 141. E.M. Gaigneaux, H.M. Abdel Dayem, E. Godard and P. Ruiz, Appl. Catal. A: General, 202 (2000) 265. E.M. Gaigneaux, P. Ruiz and B. Delmon, Stud. Surf. Sci. Catal., 112 (1997) 179. E.M. Gaigneaux, P. Ruiz and B. Delmon, Catal. Today, 32 (1996) 37. E.M. Gaigneaux, P. Ruiz, E.E. Wolf and B. Delmon, Appl. Surf. Sci., 121/122 (1997) 552. E.M. Gaigneaux, K. Fukui and Y. Iwasawa, Thin Solid Films, 374 (2000) 49. N. Miyata, T. Suzuki and R. Ohyama, Thin Solid Films, 281-282 (1996) 218. A. Abdellaoui, G. Leveque, A. Donnadieu, A. Bath and B. Bouchiki, Thin Solid Films 304 (1997) 39. C. Julien, A. Khelfa, O.M. Hussain and G.A. Nazri, J. Cryst. Gr., 156 (1995) 235. Zs. Geretovszky and T. Sz6renyi, Appl. Surf. Sci., 109-110 (1997) 467. A. Gorenstein, J. Scarminio and A. Louren~o, Solid State Ionics, 86-88 (1996) 977. J. Sonnemans and P. Mars, J. Catal., 31 (1978) 209. Powder Diffraction File, Joint Committee on Powder Diffraction Standards, International Center for Diffi'action Data (JCPDS-ICDD), 1996, Card 05-0508.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

619

Single step synthesis of metal catalysts supported on porous carbon with controlled texture N. Job, F. Ferauche, R. Pirard and J.-P. Pirard Universit~ de Liege, Laboratoire de G6nie Chimique, Institut de Chimie (B~t. B6a), B-4000 Liege, Belgium This paper presents an original preparation method of metal catalysts supported on carbon porous material [1]. Carbon porous materials can be synthesised by evaporative drying and pyrolysis of aqueous resorcinol-formaldehyde gels whose operating variables are correctly chosen. The porous texture of these materials is mainly controlled by the precursor solution pH. Texture analysis shows that it is possible to tailor the morphology of these materials : indeed, micro-macroporous, micro-mesoporous, microporous or non porous materials can be obtained by varying the pH value in a narrow pH interval. This texture control is slightly influenced by the introduction of a metal in the gel : the limits of the pH interval can slightly differ when a metal salt is added to the resorcinol-formaldehyde aqueous solution. Carbon supported nickel and palladium catalysts prepared by this method have proven to be active for ethylene hydrogenation. 1. I N T R O D U C T I O N Porous carbon materials prepared by sol-gel process from hydroxylated benzene and aldehyde in a solvent have been extensively studied for the past ten years [2-10] and used in electrochemical applications [11-12]. This type of material is generally obtained from resorcinol-formaldehyde gels synthesised in water in presence of a catalyst (sodium carbonate). After solvent exchange (water to acetone), these gels are dried by CO2 hypercritical drying process in order to avoid material shrinkage due to capillary forces observed when the gels are dried by simple evaporation. The dried gel is finally pyrolysed in order to obtain pure carbon (containing small amounts of sodium). Under appropriate synthesis conditions mainly defined by the Resorcinol/Catalyst molar ratio (R/C), these materials (called 'carbon aerogels') exhibit high specific surface areas and high pore volumes. The procedure can be simplified if acetone is used as synthesis solvent [13] (in this case, the reaction between resorcinol and formaldehyde is catalysed by perchloric acid). This method eliminates the time-consuming solvent exchange step. Nevertheless, this process remains too expensive and difficult to apply at an industrial level. Researches tend to develop other drying processes. The evaporative drying process, which is cheaper and easier to use than hypercritical drying, generally leads to densified non porous materials. Large capillary forces at the liquid-vapour interface cause the gel to shrink and crack. Attempts to overcome this

620 limitation led to the use of solvent exchange technique before evaporative drying : water can be exchanged with acetone, and acetone reexchanged with cyclohexane, which can be removed by simple evaporation [9, 10, 14]. In this case, capillary forces are less severe, so the dried gel remains porous, but this technique is long, complicated and uses large toxic solvent volumes. Freeze drying was also envisaged [15-16], but freezing induces a high volume expansion of the solvent when water is used and the samples never remain monolithic. A porous material can nevertheless be obtained. The present study shows that it is possible to synthesise still largely porous materials by evaporative drying (vacuum drying) of aqueous resorcinol-formaldehyde gels when the gel synthesis conditions are appropriate. This paper presents a synthesis method using an evaporative drying process that leads to hyperporous carbon materials. Synthesis pH is the key operating variable which controls the texture of the carbon material obtained after drying and pyrolysis of resorcinol-formaldehyde aqueous gels. Both pure carbon materials and metal-containing carbon materials can be prepared. The material texture is still controllable when a metal is incorporated to the gel by dissolution of a salt in the precursor solution in order to synthesise carbon supported metal catalysts. To our knowledge, previous attempts to incorporate metals in carbon aerogels (prepared by dissolution of a metallic salt in the resorcinol-formaldehyde solution and hypercritical drying) have failed to control the material texture [17-18]. Carbon supported catalysts could be used in various chemical reactions, and more specifically as fuel cell electrodes, the carbon support being a good electrical conductor. 2. E X P E R I M E N T A L

2.1. Synthesis Organic gels were prepared by polycondensation of resorcinol with formaldehyde in water as a solvent. Three series of carbon samples were prepared : one without addition of metal ; the two others containing nickel and palladium (about 1% weight) respectively. The incorporation of metal was achieved by dissolution of a metal salt: nickel acetate (tetrahydrate) and palladium acetate were used. As resorcinol plays the role of nickel complexant, nickel is easily soluble in resorcinol-formaldehyde aqueous solutions. On the contrary, significant amounts of palladium cannot be dissolved without using an additional complexant. Diethylenetrinitrilopentaacetic acid (DTPA) was then added to the solution. No sodium carbonate as polymerisation catalyst was used. In this work, the pH was adjusted to a chosen value by the use of sodium hydroxide (aqueous solution) and measured by a pH-meter. The synthesis variables are : (i) the Resorcinol/Formaldehyde molar ratio : R/F ; (ii) the Dilution ratio : D = Total Water/Reactants molar ratio. Note that 'Total Water' includes both added deionized water and water contained in the formaldehyde solution. Reactants refers to resorcinol, formaldehyde, metal salt and complexant.

621 (iii)

the theoretical Carbon/Metal molar ratio. This was calculated assuming that no carbon loss occurs. The real metal weight percentage was re-evaluated after pyrolysis. (iv) the Complexant/Metal molar ratio : DTPA/M; (v) the pH of the resorcinol-formaldehyde solution. In the case of metal incorporation, the Carbon/Metal molar ratio was chosen in order to theoretically obtain 1% weight metal samples (without carbon loss during drying and pyrolysis). The complexant/metal molar ratio (DTPA/M) was kept at 1 for palladiumDTPA samples. The obtained aqueous gels were dried by vacuum evaporation. The unsealed flasks were kept at 60~ and the pressure was progressively reduced from 105 Pa to 103 Pa. This step was performed over 5 days. The samples were then heated to 150~ (103 Pa) for three days. After drying, the gels are generally monolithic despite a significant shrinkage (40% vol. for low pH to 60% vol. for high pH). After drying, the gels were pyrolyzed at 800~ under inert gas (N2). The pyrolyzed materials are black, matt (for low pH) or bright (high pH). The samples generally stay monolithic, but pyrolysis also induces shrinkage. The total shrinkage (50% vol. for high pH to 80% vol. for low pH), which must be taken into account for the manufacture of monolithic parts, can be evaluated prior to synthesis. These materials possess a very good mechanical strength.

2.2. Sample characterisation and catalytic tests The porous texture of the dried gels and the pyrolyzed gels was characterised by the analysis of nitrogen adsorption-desorption isotherms, performed at 77 K. The analysis of the isotherms was performed according to the methodology proposed by Lecloux [19]. Samples bulk density was obtained by mercury pycnometry. Infrared and X-ray spectra analysis allowed to obtain data about the elementary composition of the samples and the aggregation state of the metals. Nickel and palladium catalytic activities were tested on ethylene hydrogenation into ethane in an isothermal differential reactor. 2.3. Samples nomenclature The three series of samples are denominated as follows : i) C1-C6 : pure carbon samples; ii) Nil-Ni5 : nickel loaded samples; iii) Pdl-Pd5 : palladium loaded samples. As the index increases (i.e., from C1 to C6), the pH of the resorcinol-formaldehyde solution increases. A 'p' is added for pyrolyzed samples (i.e. Clp). 3. RESULTS AND DISCUSSION

3.1. Texture analysis Fig. 1 shows the nitrogen adsorption-desorption isotherms of pure carbon samples after drying (series C1-C6). As pH increases, the samples evolve from micro-macroporous material to exclusively microporous material. Lower pH than the inferior limit leads to

622 losing the mechanical strength of the material. Intermediate samples contain both microand mesopores. The hysteresis becomes smaller and moves to lower P/p0 values as pH increases. This shows that a pH increase of the precursors solution leads to the decrease of the broader pores size and porous volume. The sharp increase of the adsorbed gas volume at low relative pressure is quite the same for every sample (except C1) : the micropore volume must then be constant. These isotherms show that the dried gel texture can easily be defined by pH control.

800 --~- C l

700

-a9 C2 --~- C3

600

---e- C4

"~ 500

--~- C5

E

-x-C6

,9, ~ 400 I/}

300 200 100 T

0

i

i

i

i

i

0.2

0.4

0.6

0.8

1

p/po

Fig. 1. Nitrogen adsorption-desorption isotherms after drying 9series C1-C6. After pyrolysis, the material exclusively contains carbon (and metal in the case of Niand Pd-gels) and traces of sodium. This was shown by infrared spectrum measurements : no C-H, O-H or C-O bonds could be detected. X-ray spectra showed that nickel and palladium are in metallic state after pyrolysis. Fig. 2. shows isotherms relative to the same series of samples after pyrolysis (Clp-C6p). If the pH is not too high, the effect of pyrolysis is to increase the microporous volume (lengthening of the sharp increase of the adsorbed gas volume at low relative pressure) and to slightly decrease the total adsorbed gas volume and the size of the pores (hysteresis smaller and shifted to lower values of p/p0). On the contrary, for higher pH (C5p and C6p), the adsorbed gas volume roughly drops, and the material becomes non porous. Similar results are found for metal loaded samples. In each case (with or without metal), it is possible to find a pH interval that leads to micro-mesoporous materials after drying and pyrolysis. This pH interval is always very narrow. If the pH is lower than the inferior

623 limit, the obtained material is micro-macroporous and tends to lose its mechanical strength: the sample becomes friable and brittle. The pore size is probably still controllable below this pH limit, but as our goal was to obtain monoliths, this pH range was not investigated. Nevertheless, such materials can be interesting if the mechanical strength of the material is not a key factor. If the pH is greater than the superior limit, the sample is exclusively microporous after drying and becomes quasi non-porous after pyrolysis. Between these two extremes, the sample texture can be adjusted as desired. The position and width of this pH interval are nevertheless slightly influenced by the introduction of metal and complexant.

1200 1000

---~Clp .-a-- C2p --..~ C3p ---~- C4p --x9 C5p

800

E

600 400 200

1

0

0.2

0.4

0.6

0.8

1

p/po

Fig. 2. Nitrogen adsorption-desorption isotherms after pyrolysis 9series Clp-CSp. Tables la and lb show the quantitative results of the nitrogen adsorption-desorption isotherm analysis, i.e., BET specific surface area (SBET), pore volume (Vp) calculated from the adsorbed volume at saturation, and total micropore volume (VDuB) calculated by the Dubinin-Radushkevich equation [19]. Bulk density of each sample is also displayed. Before pyrolysis, specific surface areas (SBET) vary from 280 to 510 ma/g, and pass through a maximum when the pH increases. The microporous volume slightly increases with the pH and seems to reach a step (0.20 to 0.25 cm3/g). The total porous volume (Vp) decreases when pH increases. This pore volume varies from 0.25 to more than 1.30 cm3/g. This last value is very high for materials dried by simple evaporation. Note that the total porous volume of samples containing macropores (i.e., C1, Nil, Pdl and Pd2) is underestimated by N2 adsorption-desorption isotherm analysis because of the technique

624 limitations (pore width < 50nm). The total porous volume, macropores included, probably continues to increase when pH decreases. Concurrently, bulk density increases when pH increases, and varies from 0.49 to 1.01 g/cm 3. Table 1a Texture after drying

Table lb Texture after pyrolysis Sample SBET VDUB v o Sample SBET VDUB (cmVo3/g)dbulk dbulk (mE/g) (cm3/g) (g/cm3) (mZ/g) (cm3/g) (cm3/g) (g/cm 3) C1 330 0.15 1.06" 0.49 Clp 625 0.27 1.44 0.53 C2 435 0.19 0.95 0.63 C2p 635 0.27 0.92 0.68 C3 475 0.22 0.79 0.75 C3p 610 0.27 0.66 0.98 C4 505 0.23 0.59 0.85 C4p 565 0.24 0.41 1.10 C5 510 0.24 0.40 1.01 C5p <40 <0.02 <0.03 1.35 C6 470 0.22 0.27 1.19 C6p <40 <0.02 <0.03 1.32 Nil 405 0.19 1.05" 0.50 Nilp 680 0.29 1.45 0.57 Ni2 440 0.20 1.36 0.58 Ni2p 705 0.30 1.24 0.65 Ni3 425 0.19 1.15 0.64 Ni3p 605 0.27 1.00 0.72 Ni4 475 0.21 0.85 0.74 Ni4p 565 0.25 0.66 0.85 Ni5 465 0.21 0.45 1.03 Ni5p 145 0.07 0.15 1.10 Pdl 280 0.13 0.88* 0.50 Pdlp 605 0.26 1.43" 0.53 Pd2 395 0.18 1.28" 0.56 Pd2p 565 0.25 1.16" 0.53 Pd3 400 0.18 1.30 0.64 Pd3p 545 0.24 1.12 0.67 Pd4 475 0.21 0.86 0.74 Pd4p 510 0.22 0.66 0.95 Pd5 325 0.15 0.25 0.82 Pd5p < 40 <0.02 < 0.03 1.55 *underestimated by the Nz adsorption-desorption isotherm analysis because of the presence of macropores (macroporous volume not taken into account). When the pH is not too high, the effect of pyrolysis is to develop the microporous volume: VDUB increases from 0.15-0.20 cm3/g to about 0.25-0.30 cm3/g in each series ; SBET increases to 600-700 mZ/g. Vp slightly decreases and the maximum pore size decreases too : samples that were micro-macroporous before pyrolysis become micromesoporous. The samples are denser as dbu~k systematically increases after pyrolysis. The main effects of pyrolysis are then to expand the amount of micropores and to shrink the largest pores. When the pH exceeds a determined superior limit, SBET, VDUB and Vp sharply drop and cannot be measured any longer by N2 adsorption-desorption: pyrolysis then leads to almost non-porous materials. The introduction of a metal salt (and a complexant) seems to have only a slight influence on the pH range that leads to micro-mesoporous material. The metal weight percentage was re-calculated, assuming that no metal loss occurs. The mass balance shows that some carbon is lost during pyrolysis, leading to a metal weight percentage value of 1.10 to 1.20%.

625

3.2. Catalytic tests Catalytic tests show (Fig. 3) that nickel and palladium dispersed on carbon supports by this method are active for hydrogenation of ethylene. These tests, performed between 30~ and 80~ after H2 reduction (320~ allowed to measure apparent activation energies : Ea- 53 kJ/mol and Ea = 28 kJ/mol were found for nickel and palladium respectively. These values are in good agreement with those found in the literature [20]. Nickel E = 53 kJ/mol

-2.5

I o Palladium Nickel

r 0

~9

i.

> C 0 0

-3.5 -4

_c

-4.5

-5.5

Palladium E=28 kJ/mol

2.8

2.9

-~~

3.0

3.1 1/'!" xlO ~

3.2

3.3

3.4

Fig. 3. Palladium and nickel catalytic tests. 4. C O N C L U S I O N The porous texture obtained after evaporative drying and pyrolysis of resorcinolformaldehyde aqueous gels is controllable by the pH of the aqueous solution. This texture is still adjustable when a metal (and an organic complexant if necessary) is introduced in the gel. In both cases it is possible to define a pH interval that leads to a micro- and mesoporous carbon material. This pH interval apparently depends on the nature of the metal and/or the nature and the concentration of the organic complexant used. Again, the materials obtained by fixing the pH in this interval exhibit specific surface areas in the range of 500 to 600 mZ/g and pore volume varying from 0.3 to more than 1.5 cm3/g. When the pH is lower than the inferior limit, the obtained sample is a micro- and macroporous material with poor mechanical strength. When the pH exceeds the superior limit, the obtained material is non porous. Nickel and palladium are in metallic state after pyrolysis without needing further treatment. This type of material, synthesised in one single step (gelation) followed by simple evaporative drying and thermal treatment under inert gas, can be used in varied chemical applications as carbon supported metal catalyst. Catalytic activity of palladium and nickel dispersed on micro and mesoporous carbon prepared by this method was measured for ethylene hydrogenation. Apparent activation energies were calculated for nickel (Ea = 53 kJ/mol) and for palladium (Ea = 28 kJ/mol).

626 The monolithic structure, good mechanical strength, high surface area, and electrical conductivity of these carbon materials make them attractive as electrodes for various electrochemical applications. As hydrogen oxidation (or oxygen reduction) catalysts may be incorporated to such porous materials, one specific application to consider is the use of this type of material as alkaline fuel cell electrode. ACKNOWLEDGEMENTS The authors thank the Belgian Fonds National de la Recherche Scientifique, the R6gion Wallonne -Direction G6n6rale des Technologies, de la Recherche et de l'Energie-, the Minist6re de la Communaut6 fran~aise -Direction de la Recherche scientifique and the Fonds de Bay for their financial support. REFERENCES

1. J.P. Pirard, R. Pirard and N. Job, European patent application 01202862.7-1215, July 20, 2001. 2. R.W. Pekala and F.M. Kong, Rev. Phys. Appl., 24 (1989) C4.. 3. R.W. Pekala, C.T. Alviso, J.D. LeMay, L.L. Hench and J.K. West, Eds. Wiley, 1992, 671. 4. R.W. Pekala, C.T. Alviso and J.D. LeMay, J. Non-Cryst. Solids, 188 (1995) 34. 5. Pekala R.W., Alviso C.T., Lu X., Gross J., Fricke J., J. Non-Cryst. Solids, 125 (1990) 67-75. 6. R.W. Pekala, J.C. Farmer, C.T. Alviso, T.D. Tran, S.T. Mayer, J.M. Miller and B. Dunn, J. Non-Cryst. Solids, 225 (1998) 74. 7. R.W. Pekala and C.T. Alviso, Mat. Res. Soc. Symp. Proc., 270 (1992) 3. 8. R.W. Pekala, US Pat. 4 997 804 (1991). 9. C. Lin and J.A. Ritter, Carbon, 35 (1997) 1271. 10. C. Lin and J.A. Ritter, J. Electrocherrt Soc., 146 (1999) 3639. 11. R.W. Pekala et al., US Pat. 5,932,185, WO 9,506,002 (1999). 12. S.T. Mayer et al., US Pat. 5,601,938, WO 9,520,246, (1995). 13. G.M. Pajonk, A. Venkateswara Rao, N. Pinto, F. Ehrburger-Dolle and M. Bellido Gil, Preparation of Catalysts VII, Elsevier Science, Amsterdam, (1998) 167. 14. S.T. Mayer, J.L. Kaschmitter and R.W. Pekala, US Pat. 5,420,168, WO 9,422,943 (1993). 15. B. Mathieu, S. Blacher, R. Pirard, J.P. Pirard, B. Sahouli and F. Brouers, J. Non-Cryst. Solids, 212 (1996) 250. 16. B. Mathieu, B. Michaux, O. Van Cantford, F. Noville, R. Pirard and J.P. Pirard, Ann. Chim. Ft., 22 (1997) 19. 17. F.J. Maldonado-Hodar, C. Moreno-Castilla, J. Rivera-Utrilla, Y. Hanzawa and Y. Yamada, Langmuir, 16 (2000) 4387. 18. F.J. Maldonado-Hodar, C. Moreno-Castilla, J. Rivera-Utrilla and M.A. Ferrero-Garcia, Studies in Surface Science and Catalysts, vol. 130, Elsevier, Amsterdam, 2000, pp. 1007. 19. A.J. Lecloux, Catalysis, Science and Technology, in: J.R. Anderson, M. Boudart (Eds.), Vol. II, Springer, Berlin, 1981, p.171. 20. G.C. Bond, Catalysis by Metals : Academic Press, New York, 1962, p. 241.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

627

Ag/SiO2 and Cu/SiOz cogelled xerogel catalysts for benzene combustion and 2-butanol dehydrogenation S. Lambert a, N. Tcherkassova a, C. Cellier b, F. Ferauche a, B. Heinrichs a, P. Grange b and JP. Pirard a aLaboratoire de G6nie Chimique, B6a, Universit6 de Li6ge, B-4000 Li6ge, Belgium bUnit6 de catalyse et chimie des mat6riaux divis6s, Universit6 catholique de Louvain, Place Croix du Sud, 2/17, B-1348 Louvain-la-Neuve, Belgium Ag/SiO2 and Cu/SiO2 xerogel catalysts were synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Ag and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). The resulting catalysts are composed of completely accessible metallic crystallites with a diameter of about 3 nm located inside silica particles exhibiting a monodisperse microporous distribution centered on a pore size of about 0.8 nm and larger metallic particles with a diameter of 20 to 40 nm located outside the silica network. The activity of Ag/SiQ and Cu/SiOa catalysts has been shown for benzene combustion and 2-butanol dehydrogenation. 1. I N T R O D U C T I O N The sol-gel method has been used by several authors in order to obtain monometallic catalyst particles finely dispersed on a mineral support. Schubert et al. [1] developed a particularly interesting method to homogeneously disperse nanometer-sized metal particles in a silica matrix. These authors used alkoxides of the type (RO)3Si-X-A in which a functional organic group A, able to form a chelate with a cation of a metal such as palladium, silver, copper, nickel, etc., is linked to the hydrolysable silyl group (RO)3Sivia an inert and hydrolytically stable spacer X. The cocondensation of such molecules with a network-forming reagent such as TEOS, Si(OC2H5)4, results in materials in which the catalytic metal is anchored to the SiO2 matrix. Schubert et al. [1] applied this method to the preparation of numerous monometallic samples such as Ag/SiO2, Cu/SiO2, Co/SiO2, Pd/SiOa, Pt/SiO2, etc. Heinrichs et al. [2] used this cogelation method for the preparation of Pd/SiO2 aerogel catalysts. In this study, (RO)3Si-X-A=3-(2-aminoethylamino)propyltrimethoxysilane, NH2-CHz-CHz-NH-(CHz)3-Si(OCH3)3 = EDAS, and the Pd precursor is Pd acetylacetonate, Pd[CH3COCH=C(O-)CH3]2 = Pd(acac)a. It has been shown that the Pd 2+ complex, Pda+[NHz-CHz-CHa-NH-(CHz)3-Si(OCH3)3]2, acts as a nucleation agent in the formation of silica particles. The resulting catalysts are composed of completely accessible palladium crystallites located inside silica particles. It was shown that these samples are sinterproof during hypercritical drying due to the fact that Pd crystallites cannot migrate because they are trapped inside the microporous SiO2 particles.

628 The aim of this work is to explore the applicability of the sol-gel method for the preparation of Ag/SiO/and Cu]SiO2 catalysts and to see whether such a method can yield silver and copper species stabilized by the carrier. Characterization of the catalyst structure by several physical and chemical techniques, including N2 adsorption-desorption isotherms, mercury porosimetry measurements, X-ray diffraction and transmission electron microscopy, has been used to correlate the micro structure of Ag/SiO2 and Cu]SiO2 catalysts with their catalytic performance. The first reaction used is the selective dehydrogenation of 2-butanol to 2-butanone on Cu/SiO2 cogelled xerogel catalysts. In fact, the activity and selectivity of some coppercontaining solids as catalysts for hydrogenation and dehydrogenation processes at moderate temperatures is well established [3]. For example, a Cu/ZnO catalyst is used in industry for methanol synthesis by hydrogenation of carbon monoxide [4] and the selective catalytic reduction of NO over Cu-containing zeolites has been the subject of intense research in the past years, driven by the expectations of large-scale applications for the control of NOx [5]. The second reaction studied is the oxidation of benzene on Ag/SiO/ and Cu/SiO/ cogelled xerogel catalysts. In fact, volatile organic compounds (VOCs) are recognized as major contributors to air pollution, either directly through their toxic or malodorous nature, or indirectly as ozone and smog precursors [6]. When there is no interest in recovering VOCs, they are usually destroyed by deep oxidation. Catalytic incineration of VOCs is preferred over thermal incineration for several reasons. A thermal incinerator operates at higher temperatures (973-1473 K) than a catalytic unit (473-773 K), and thermal incineration often has to be supplemented with fuel to maintain a flame in a dilute VOC waste stream (less than 1000 ppm). Thermal incineration could produce toxic substances and NOx [7]. So fundamental knowledge of catalytic oxidation mechanisms can be valuable for developing effective methods for using catalysts to control air pollution. Though silver and copper are not the best oxidation catalysts, this study is performed in order to correlate catalyst activity to metal dispersion.

2. EXPERIMENTAL 2.1. Catalyst preparation Six Ag/SiO2 and four Cu/SiO2 catalysts were synthesized in ethanol from TEOS, EDAS (NH2-CH2-CH2-NH-(CHa)a-Si(OCH3)3), and a solution of aqueous 0.18N NH3. Metal precursor (Ag(OAc) or Cu(OAc)2) and EDAS were mixed together in half of the total volume of ethanol (molar ratio EDAS/Ag(OAc) = 2 and EDAS/Cu(OAc)2 = 4). The slurry was stirred at room temperature until a colourless solution was obtained for silver, or a clear blue solution for copper (about half an hour). After addition of TEOS, a solution of aqueous 0.18N NH3 in the other half of the total volume of ethanol was added to the mixture under vigorous stirring. The hydrolysis ratio, that is the molar ratio H H20/(TEOS + 3/4 EDAS) (factor 3/4 is due to the fact that EDAS contains only three hydrolysable groups in relation to TEOS which contains four hydrolysable groups) was kept constant at the value of 5 for all samples. The dilution operating variable, that is the molar ratio R = ethanol/(TEOS + EDAS) was kept constant at the value of 10 for all xerogels. The vessel was then closed and heated up to 70~ for 3 days (gelation and

629 ageing). The wet gels were dried under vacuum according to the following procedure: the flasks were opened and put into a drying oven at 60~ and the pressure was slowly decreased (to prevent gel bursting) to the minimum value of 1200 Pa after 90 h. The drying oven was then heated at 150~ for 72 h. The resulting samples are xerogels. Synthesis operating variables of Ag/SiO2 and Cu/SiO2 catalysts are shown in Table 1. For each catalyst, Ag or Cu denote the name of metal followed by the weight percentage of metal in the sample. The conditions of calcination for Ag/SiO2 and Cu/SiO2 catalysts were as follows: the sample was heated up to 400~ at a rate of 120~ under flowing air (1.8 Nl/h) ; this temperature was maintained for 12 h in air (9 Nl/h). The catalysts were then reduced. The Ag/SiO2 catalysts were heated to 350~ at a rate of 350~ under flowing H2 (20 Nl/h) and maintained at this temperature for 3 h (same flow). The Cu/SiO2 catalysts were heated to 400~ at a rate of 350~ under flowing H2 (20 Nl/h) and maintained at 400~ for 3 h (same flow). Table 1 Synthesis operating variables of Ag/SiO2 and Cu]SiO2 catalysts Sample Metal n E D A S nTEOS E/T" nH20 nC2HsOH powder (mmol) (mmol) (mmol) (mmol) (mmol) Ag0.25 0.260 0.48 188 0.003 940 1890 Ag0.40 0.442 0.85 188 0.004 940 1890 Agl.00 1.073 2.13 186 0.011 940 1890 Agl.55 1.608 3.21 185 0.017 940 1890 Ag2.60 2.705 5.39 183 0.029 930 1890 Ag3.65 3.832 7.63 181 0.042 930 1890 Cul.00 1.757 7.20 181 0.040 940 1890 Cul.50 2.716 10.86 178 0.061 930 1890 Cu2.15 3.814 15.31 173 0.088 920 1890 Cu4.50 8.036 32.08 156 0.205 900 1890 aE/T is the molar ratio nEDAS/rtTEOS.

Gel time

Metal loading 0.1%

(min)

(wt%)

195 170 51 50 50 48 69 54 49 40

0.25 0.42 1.02 1.53 2.58 3.65 0.99 1.52 2.14 4.52

2.2. Catalyst characterization and catalytic experiments N2 adsorption-desorption isotherms were determined at 77 K after outgassing for 24 h at room temperature. The mercury porosimetry measurements were performed between 0.01 and 200 MPa after outgassing the sample monolith for 2 h at ambient temperature. The size of silica and metallic particles was examined by transmission electron microscopy (TEM). The composition and size of the metallic particles were examined by X-ray diffraction (XRD). For benzene oxidation on Ag/SiO2 and Cu/SiO2 catalysts, 0.6 Nl/h of C6H6/N2 mixture (2550 ppm of benzene in nitrogen), 0.8 Nl/h of oxygen and 4.2 Nl/h of nitrogen were used. 0.1 g of catalyst in the form of pellets with a diameter between 0.2 and 0.315 mm was diluted in 7 ml of glass beads with the same diameter, previously checked to be inactive. The reactants and products of a possible incomplete combustion were separated by gas chromatography and then quantified using a flame ionisation detector (FID). The conversion for all Ag/SiO2 and Cu/SiO2 xerogel catalysts has been measured at 1 arm and

630 at increasing temperatures from 433 K to 633 K with three measurements for each temperature. For 2-butanol dehydrogenation, catalytic reactions were carried out at atmospheric pressure in a fixed bed flow-microreactor. The feed was 4.2 N1/h of helium with a partial pressure of 8.5 kPa of 2-butanol. Reaction temperatures were from 373 K to 573 K. Concentrations of reactants and products were measured at the reactor outlet by on-line gas chromatography.

3. RESULTS 3.1. Texture of catalysts Fig. 1 shows the evolution of the cumulative volume distributions over the entire pore size range as a function of the ratio EDAS/TEOS for Ag/SiO2 catalysts. These curves were obtained by applying a combination of various methods to their respective validity domains following the Mesopores Macropores lO Micropore, method presented by Ali6 et al. [8]. All Ag/SiO2 and Cu/SiO2 catalysts are characterized by a steep volume increase around 0.8 I nm followed by a plateau. In EDAS/TEOS 0.1 the range of meso- and 9 0.004 (Ag0.40) macropores, all samples O0.017 (Agl.55) & 0.029 (Ag2.60) exhibit a broad distribution. r..) 0.01 These xerogels then contain 90.042(Ag3.65) micropores (width < 2 nm), mesopores (2 nm < width < 50 0.001 nm) and macropores (width 0.1 1 10 100 1000 > 50 nm). The distribution loresize(rim) shifts towards the smaller pores respectively from Fig. 1. Pore size distributions as a function of the Ag0.40 to Ag3.65 (Fig. 1) that EDAS/TEOS ratio for Ag/SiO2 catalysts is when the EDAS/TEOS ratio increases. The same trend is observed for Cu/SiO2 catalysts. Table 2 gives textural properties of Ag/SiO2 and Cu/SiO2 samples. The total pore volume Vv decreases respectively from 5.7 to 2.8 cma/g for samples Ag0.25 to Ag3.65 and from 3.4 to 1.3 cma/g for samples Cul.00 to Cu4.50 when the EDAS/TEOS ratio increases. The specific surface a r e a aBET increases respectively from 180 to 465 na2/g for samples Ag0.25 to Ag3.65 and from 285 to 570 m'/g for samples Cul.00 to Cu4.50 when the content in EDAS increases. The high values of Vv show that xerogels are very porous, but do not prove the accessibility of silver and copper. All samples were examined using a transmission electron microscope to obtain metallic and silica particle sizes. Although TEM gives only a 2D view, a careful examination of cogelled-catalysts TEM pictures /

/

.,.

t

i

i

631 shows that Ag and Cu particles seem to be located inside the microporous silica particles. However for metal loading > 1.5 wt%, there are Ag and Cu cristallites located on the external surface of silica particles and these large metallic particles have a mean diameter of 20 to 40 nm.

3.2. Catalytic results Fig. 2 shows conversion of benzene as a function of time for each temperature for Ag/SiO2 and Cu/SiO2 catalysts. With these experimental conditions, benzene is oxidised in H20 and CO2. For all the catalysts, the conversion increases with the temperature and with the metal loading. However for all the Ag/SiO2 catalysts, it is observed that the conversion decreases above 553 K because Ag/SiO2 deactivates above this temperature. Fig. 3 shows conversion of 2-butanol as a function of time for each temperature for Cu/SiO2 catalysts. With these experimental conditions, 2-butanol is selectively dehydrogenated in 2-butanone and an insignificant amount of butene is produced. For all the catalysts, the conversion increases with the temperature and with the metal loading. However, for Cu4.50 sample, it is observed that the conversion decreases and for Cu2.15, that the conversion stays constant above 533 K. Both Cu/SiO2 catalysts deactivate above this temperature. Table 2 Sample textural properties Sample Vng Vcum<7.5nm Vm Vv aBET dsio2 TEM (cm3/g) (cm3/g) (cm3/g) (cm3/g) (m2/g) (nm) Ag0.25 5.60 0.04 0.07 5.7 180 35.6• Ag0.40 5.35 0.05 0.08 5.5 200 31.4+1.8 Ag 1.00 5.20 0.05 0.10 5.4 260 25.2+ 1.0 Agl.55 4.00 0.05 0.10 4.2 280 22.8+1.3 Ag2.60 2.75 0.06 0.15 3.0 375 17.3+1.7 Ag3.65 2.50 0.08 0.17 2.8 465 15.8+ 1.9 Cul.00 3.30 0.02 0.11 3.4 285 48.2+4.4 Cul.50 2.70 0.03 0.15 2.9 395 34.3+3.7 Cu2.15 2.20 0.06 0.17 2.4 460 23.5+2.6 Cu4.50 1.00 0.09 0.23 1.3 570 16.7+1.1 ling: specific pore volume measured by mercury porosimetry; Vcum<7.5nm: cumulative volume pores of diameter between 2 and 7.5 nm determined by Broekhoff-de-Boertheory; Vm:microporous volume; Vv:pore volume obtained by addition of ling, Veum<75nm and Vm;SBET:specific surface area obtained by BET method; dsio2 : silica particle diameter measured by TEM.

4. DISCUSSION In Ag/SiO2 and Cu/SiO2 xerogel catalysts, in order to reach active sites, reactants must first diffuse through large pores located between aggregates of SiO2 particles and then through smaller pores between those elementary particles inside the aggregates. Finally, they diffuse through micropores inside silica particles. Diffusion in such a 'funnel' structure can be examined at three levels of decreasing size: the macroscopic pellet, the aggregate of

632 100

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Fig. 2. Conversion of benzene as a function of time for (a) Ag/SiO2 catalysts and (b) Cu/SiO2 catalysts silica particles, and the elementary silica particle. It was shown that there is no problem of mass transfer at each of the three levels [9]. In order to explain the results in Figs. 2 and 3, it is necessary to obtain intrinsic kinetic rates for benzene oxidation and 2-butanol dehydrogenation. With the experimental conditions used, the reactor is not a differential reactor because conversion is too high. The kinetic rates are obtained by considering the reactor as an integral reactor with the following assumptions :

633 for 2-butanol dehydrogenation on Cu/SiO2 catalysts, it is checked that conversion from 2butanol to 2-butanone varies linearly with the initial 2-butanol concentration. So it is verified for the first time that the kinetic rate expression is rbut = kb,t [Butanol] ; -for benzene oxidation on Ag/SiO2 and Cu/SiO2 catalysts, the kinetic rate can be expressed as : rbe,, - kben [benzene] n [02] m. For the conditions encountered in most VOC control applications, the concentration of oxygen is always much larger than the concentration of the organic reactant (for a feed of 250 ppm benzene in air, [O2]/[benzene] ~ 840). Thus because the oxygen partial pressure is essentially constant during the reaction, the observed rate depends only on the concentration of benzene. Gangwal et al. [ 10] observed a zero-order behavior for benzene on Pt-Ni/Al~O3 catalysts. Although the catalysts used in this work are different, it is for the first time assumed that the kinetic rate expression is rben=kben.

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Fig. 3. Conversion of 2-butanol as a function of time for Cu/SiO2 catalysts Fig. 4 presents the specific kinetic rates at 533 K and 553 K for benzene oxidation on Ag/SiO2 and Cu/SiO2 catalysts as a function of metal loading and Fig. 5 presents the specific kinetic rates at 513 K and 533 K for 2-butanol dehydrogenation on Cu/SiO2 catalysts as a function of copper loading. Fig. 4 shows, for benzene oxidation, that rben increases when the silver or copper loading decreases. In fact, visual observations by TEM show that in samples Ag0.25, Ag0.40, Agl.00 and Cul.00, it seems that there are no larger silver or copper particles located on silica particles external surface. Thus the metal dispersion values are greater in these samples. So this could be the proof that Ag and Cu particles located inside the silica particles are accessible for benzene in this catalytic system. Fig. 5 shows, for 2-butanol dehydrogenation, that rbut increases when the copper loading increases or when the metal

634 dispersion values decreases. However, in Cul.00 sample, only copper particles inside silica particles are observed by TEM. It seems that these metallic particles are accessible for 2butanol in this catalytic system. A possible explanation for the different behaviour for 2butanol dehydrogenation in 2-butanone could be found in the results of Cunnigham et al. for selective dehydrogenation of isopropanol to acetone on copper catalysts [11]. Those authors found indeed that higher activity is observed with copper catalysts having the lowest dispersions. However the ratio CuO/Cu is also an important factor. For benzene oxidation, Ag/SiO2 catalysts have a greater activity than Cu/SiO2 catalysts. However Ag/SiO2 catalysts are deactivated above T - 553 K. This deactivation could arise from a change of active sites surface or from the disappearance of catalyst porosity. It seems that XRD spectrum of Ag3.65 tested during 12 hours at T = 633 K for benzene oxidation always shows characteristic Ag-metal peaks and no AgO or Ag20 peaks, although in this case, the ratio signal/noise is very low. So there is little transformation of the active sites surface. The specific surface 10~ r

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a r e a ( S B E T ) of fresh Ag3.65 is equal to 470 m2/g and SBET of tested Ag3.65 is equal to 35 m2/g. So after benzene oxidation during 12 hours at T = 633 K, micro- and mesoporosity of silica network disappear preventing benzene from reaching silver particles inside the silica network.

5. CONCLUSIONS The sol-gel method applied to Ag/SiO2 and Cu/SiO2 catalysts allows to stabilize silver and copper species by the silica network. Although metallic particles are located in SiO2

635 particles, their complete accessibility, via the microporosity, has been demonstrated for benzene oxidation and 2-butanol dehydrogenation. In fact, for benzene oxidation, greater intrinsic kinetic rates are observed for catalysts with lower metal loading and higher dispersion. ACKNOWLEDGEMENTS S. L. is grateful F.R.I.A. for a PhD grant. The authors also thank F.N.R.S., the Fonds de Bay, the Fonds de Recherche Fondamentale et Collective, the Minist~re de la R~gion Wallonne and the Minist~re de la Communaut6 Fran~aise (Action de Recherche Concert~e) for financial support. REFERENCES

1. 2. 3. 4. 5. 6.

B. Breitscheidel, J. Zieder and U. Schubert, Chem. Mater., 3 (1991) 559. B. Heinrichs, F. Noville and J.-P. Pirard, J. Catal., 170 (1997) 366. N.W. Cant, S.P. Tonner, D.L. Trimm and M.S. Wainwright, J. Catal., 91 (1985) 197. G.C. Chinken, K.C. Waugh and D.A. Whan, Appl. Catal., 25 (1986) 101. M. Iwamoto and H. Hamada, Catal. Today, 17 (1991) 94. J.A. Horsley, Catalytica Environmental Report E4, Catalytica Studies Division, Mountain View, CA, 1993. 7. P.O. Larsson, A. Andersson, L.R. Wallenberg and B. Swensson, J. Catal., 163 (1996) 279.

8. C. Ali6, R. Pirard, A.J. Lecloux and J.-P. Pirard, J. Non-Cryst. Solids, 246 (1999) 216. 9. B. Heinrichs, J.-P. Schoebrechts and J.-P. Pirard, AIChE J., 47 (2001) 1866. 10. S.K. Gangwal, M.E. Mullins, J.J. Spivey and P.R. Caffrey, Appl. Catal., 36 (1988) 231. 11. J. Cunnigham, G.H. A1-Sayyed, J.A. Cronin, J.L.G. Fierro, C. Healy, W. Hirschwald, M. Ilyas and J.P. Tobin, J. Catal., 102 (1986) 160.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

637

Preparation of zeolite catalysts for dehydrogenation and isomerization of n-butane Megumu Inaba, Kazuhisa Murata, Masahiro Saito, Isao Takahara, Naoki Mimura, Hideaki Hamada and Yohei Kurata a'b National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan aCentral Research Laboratory, SHOWA DENKO K.K., 13-9, Siba Daimon 1-Chome, Minato-ku, Tokyo 105-8518, Japan bJoint Research Center for Harmonized Molecular Materials, Japan Chemical Innovation Institute (JCII), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan In this study, the preparation of zeolite catalysts for dehydrogenation and isomerization of n-butane to produce isobutene was investigated. H-SSZ-35 type zeolite, having one-dimensional cage-type channel structure, was hydrothermally synthesized with cis, cis, cis-N-methyl-hexahydrojulolidinium hydroxide as structure-directing agent (SDA). We found that Cr catalyst supported on H-SSZ-35 showed high activity for the dehydrogenation and isomerization of n-butane to isobutene, and maintained the activity for 6 h. 1. INTRODUCTION Zeolites have micropores with ordered size and solid acidity, and there has been growing interest in the application of zeolites as catalysts as well as molecular sieves. We investigated the synthesis of novel zeolites by using ammonium salts as structure-directing agents (SDAs) [1, 2]. SSZ-35 has an unusual one-dimensional straight channel structure with pore openings that alternate between rings of ten tetrahedral atoms (10MR) and eighteen tetrahedral atoms (18MR), and micropore volume of this zeolite is larger than that of other one-dimensional high-silica zeolites. Wagner et al. [3, 4] reported the hydrothermal synthesis of SSZ-35, in which cyclic and polycyclic quaternized amine molecules were used as SDAs. Catalytic conversion of n-butane has received much attention. In particular, isobutene is

638

an important intermediate for many common petrochemicals and plastics, and, therefore, dehydrogenation and isomerization of n-butane to isobutene have been investigated quite extensively. Wang et al. [5] reported the simultaneous dehydrogenation and isomerization of n-butane to isobutene over Cr-based WO3-promoted catalysts. For zeolite supports, Kishida et al. [6] investigated the direct conversion of butane to isobutene over various zeolite-like materials loaded with Pt or Ga (III) oxide. On the other hand, Byggningsbacka et al. [7] reported the combination of Zn-modified K-ZSM-5 with a ZSM-22 system for the simultaneous dehydrogenation and isomerization of n-butane to isobutene. In this study, zeolites and Cr catalysts supported on zeolites were prepared, and their effects on the conversion of n-butane and selectivity of products were investigated. 2. E X P E R I M E N T A L

2.1 Synthesis of zeolite support H-SSZ-35 support was hydrothermally synthesized with cis, cis, cis-N-methylhexahydrojulolidinium hydroxide (shown in Fig. 1) as SDA. cis, cis, cis-N-methylhexahydrojulolidinium hydroxide was synthesized as follows: 2,6-dicyanoethylcyclohexanone was obtained from cyclohexanone (Wako Pure Chemical) and acrylonitrile (Wako Pure Chemical) as starting materials, followed by cyclization by high-pressure hydrogen atmosphere (hydrogen pressure was 10 MPa, at ca 120~ in autoclaves in the presence of Raney nickel catalyst, to produce hexahydrojulolidine [8]. The obtained transparent oil of hexahydrojulolidine was a mixture of cis, cis, cis-type (used as precursor of SDA), cis, trans, cis-type and cis, cis, trans-type. To remove cis, trans, cis-type and cis, cis, trans-type, the hexahydrojulolidine oil was diluted with ether, refluxed with methyl iodide, and cooled. After removing the precipitated solids by filtration and evaporating the residual solution, transparent oil of cis, cis, cis-type was obtained. This oil was diluted with toluene, followed by stirring at room temperature, adding methyl iodide, and refluxing. After cooling, filtering and drying, the white powder of cis, cis, cis-N-methyl-hexahydrojulolidinium iodide was obtained. The cis, cis, cis-N-methyl-hexahydrojulolidinium iodide was converted to hydroxide by ion-exchange resin. The solution of cis, cis, cis-N-methyl- hexahydrojulolidinium hydroxide was mixed with NaOH (Wako Pure Chemical), colloidal silica (Shokubai Kasei, Cataloid), sodium aluminate (Wako Pure Chemical) and distilled water in a glass beaker, and stirred overnight at room temperature. The ratios were as follows: Si/AIa=40, 100, 500, H20/Si=50, OH-/Si=0.1, SDA/Si=0.2. Afterwards, the mixture was poured into Teflon-lined autoclaves (Parr) and the sealed autoclaves were heated at 170~ for 7 days. The obtained solid of Na-SSZ-35 was filtered, washed with water the dried. Na-SSZ-35 was calcined at 550~ for 6h in air, and converted to H-SSZ-35 by stirring in the solution

639 of ammonium nitrite overnight, followed by filtration and drying. The obtained H-SSZ-35 was calcined in the same condition again. This SDA could supply the crystal of SSZ-35 with high crystallinity in a short time. H-ZSM-12 support was hydrothermally synthesized by using hexamethylene-bis (triethylammonium) hydroxide (Et3N+(CHz)6N+Et3 (OH-)z) as SDA [2]. Hexamethylene-bis (triethylammonium) bromide was prepared by the reflux of 1,6-dibromohexane (Tokyo Kasei) and triethylamine (Tokyo Kasei) in acetone. Precipitated bromide was filtered, air-dried and purified by recrystallization using mixed solvents of ethanol and diethyl ether. The recrystallized diammonium salt was air-dried again after filtration, followed by conversion to hydroxide by using ion-exchange resin. The solution of hexamethylene-bis (triethylammonium) hydroxide was mixed with colloidal silica (Shokubai Kasei, Cataloid), aluminum nitrate nonahydrate (Kanto Chemical), NaOH (Nakarai Chemicals) and sulfuric acid to adjust the total amount of OHin Teflon-lined autoclaves (Parr). The ratios were as follows: Si/AI2=200, 1000, H20/Si=60, OH-/Si=0.2, Na+/Si=0.3, SDA/Si=0.05. The synthesized crystals of Na-ZSM-12 were obtained by heating sealed autoclaves at 160~ for 4 days, followed by cooling, washing with distilled water, filtering and drying. Na-ZSM-12 was calcined at 500~ for 6 hr in air, and converted to H-ZSM-12 by stirring in the solution of ammonium nitrate with heating for a few days, followed by filtration and drying. The obtained H-ZSM-12 was calcined in the same condition again. Other zeolite supports, USY, H-Beta, H-ZSM-5 and H-Mordenite were obtained commercially (Toso). Before use, these zeolites were also calcined at 500~ for 6 hr in air.

H

H

CH3 Fig.1

OH @

cis, cis, cis-N-methyl-hexahydrojulolidinium hydroxide.

2.2 Characterization of zeolite support To measure the solid acidity of supports, NH3-temperature-programmed-desorption (TPD) experiments of zeolite supports were carried out on a special NH3-TPD apparatus (Ohkura Riken, Model ATD700A) interfaced to a personal computer. TPD profiles were obtained under vacuum conditions with the temperature varying from 100 to 600~ at 10~ in the TPD process. Samples were degassed at 500~ for 1 h under vacuum condition before measurement. The surface area and micropore volume of the zeolite

640

supports were measured by nitrogen adsorption a t - 1 9 6 ~ using a Micromeritics volumetric equipment (Nihon Bell, BELSORP 28SA) after degassing at 300~ for 5.5 h under vacuum conditions. 2.3 Preparation of Cr catalysts supported on zeolites Cr catalysts supported on zeolites were prepared by impregnation method. The loading of Cr was 10wt%. Chromium (III) nitrate nonahydrate (Wako Pure Chemical) was used as the source of Cr. After impregnation for a day, the wet catalysts were dried at 100~ in an oven, followed by calcination at 700~ for 3 h in air. Table 1 Physico-chemical properties of H-SSZ-35. Si/AI2 ratio

Total surface area (mZ/g)

Micropore volume (mm3/g)

NH3 adsorption ( # mol/g)

40 100 500

390.5 484.0 486.7

172.3 214.1 222.8

1010.8 844.0 294.1

2.4 Measurement of catalytic activity Catalytic activity was measured with a fixed-bed reactor. Weight of catalyst was l g. Before reaction, the catalyst was reduced at 600~ flowing hydrogen. The effluent gas was analyzed by gas chromatography (GC) with Molecular Sieve 5A and Porapak Q columns. Two detectors were used: TCD to detect hydrogen, and FID to detect hydrocarbons. The reaction gas contained n-Chill0 (20%) and Ar (80%) and the gas flow rate was 60 cm 3 min -1. The reaction temperature was changed from 400~ to 600~ at every 50~ 3. RESULTS AND DISCUSSION

3.1 Characterization of zeolite support Table 1 shows the results of characterization of H-SSZ-35. The image of one-dimensional cage-type channel structure of SSZ-35 is shown in Fig. 2 [9]. From these results, total surface area and micropore volume increased with the increase in Si/AI2 ratio, while NH3 adsorption decreased with the increase of the ratio. Fig. 3 shows the NH3-TPD profile, exhibiting NH3 desorption peaks characteristic of H-SSZ-35 zeolites.

3.2 Catalytic activity Table 2 shows the conversion of n-butane and the selectivity of each products over several kinds of zeolites and Cr catalysts supported on zeolites at 500~ In most cases, the zeolite supports alone were found to be active for this reaction. The n-butane conversion was in the order: H-Mordenite (Si/A12=18.3) > H-ZSM-5 (Si/A12=29) > H-SSZ-35 (Si/AI2=40) >

641 H-Beta (Si/A12=27) > H-ZSM-12 (Si/AI2=200) > H-SSZ-35 (Si/AI2=100) > H-SSZ-35 (Si/Al2=500). However, these zeolite supports showed the predominant formation of C1-C3 hydrocarbons over C4 products and the selectivity of C4 compounds such as isobutene was low. Loading of Cr on the zeolite supports improved the selectivity of C4 compounds. In particular, Cr catalysts supported on H-SSZ-35 (Si/A12=100, 500) showed increased conversion as well as improved selectivity to isobutene (No.4 and 6). Cr/H-SSZ-35 (Si/AI2=40), Cr/H-ZSM-12 (Si/AI2=200), Cr/H-Beta (Si/A12=27) and Cr/H-Mordenite (Si/A12=18.3) catalysts also showed improved selectivity to isobutene; however, they showed decreased conversion of n-butane (No. 2, 8, 10 and 14). On the other hand, in the case of Cr/H-ZSM-5 (Si/A12=29) catalyst, the conversion of n-butane slightly increased, while the selectivity to isobutene decreased (No. 12).

/ Fig. 2. Image of one-dimensional cage-type channel.

"2

--~

Si/A12=40

o

Si/Alz=100 Si/A12:500

6,,

100

200

300

400

500

Temperature / ~ Fig. 3 NH3-TPD profiles of H-SSZ-35

600

642 Table 2 Catalytic performance of the various zeolite catalysts at 500~ No.

Conv. (%)

C1

Cz 24.0 14.2 31.9 14.4 36.4

C3 57.6 39.4 28.7 34.5 15.5

Selectivity (%) Iso-

Iso-C4 1-C4'

(1) (2) (3) (4) (5)

50.4 17.1 8.6 11.1 3.3

5.5 4.1 11.4 7.0 13.9

8.0 18.9 4.6 12.4 0.0

(6)

33.8

15.1

10.5

14.4

(7) (8) (9) (10) (11) (12) (13)

18.8 12.7 48.1 7.7 83.4 86.9 85.8

9.1 6.0 10.6 10.4 10.7 10.5 15.2

32.0 16.4 38.2 21.6 27.9 26.2 39.1

35.8 23.5 42.4 45.6 52.1 56.7 41.7

(14)

61.5

5.7

16.9

61.3

10.8

C4'

trans-

cis-

2 - C 4 ' 2-C4'

1,3-

C4"

0.7 3.8 0.0 4.7 0.0

2.0 9.4 6.6 11.9 11.2

1.3 6.1 5.9 8.0 8.6

0.9 4.2 4.0 6.3 5.7

0.0 0.0 6.9 0.9 8.8

1.4

7.3

16.1

13.6

10.6

11.0

4.3 2.5 3.8 2.3 5.3 4.7 2.4

2.9 8.0 0.7 3.9 0.5 0.3 0.0

7.5 20.6 2.0 7.6 1.6 0.8 0.7

4.9 13.2 1.4 5.2 1.0 0.5 0.6

3.6 9.2 1.1 3.5 0.8 0.3 0.3

0.0 0.6 0.0 0.0 0.1 0.0 0.0

0.2

2.1

1.5

1.4

0.0

(1) H-SSZ-35 (Si/AI2=40), (2) Cr/H-SSZ-35 (Si/AI2=40), (3) H-SSZ-35 (Si/AI2=100), (4) Cr/H-SSZ-35 (Si/AI2=100), (5) H-SSZ-35 (Si/AI2=500), (6) Cr/H-SSZ-35 (Si/Al2=500), (7) H-ZSM-12 (Si/AI2=200), (8) Cr/H-ZSM-12 (Si/AI2=200), (9) H-Beta (Si/A12=27), (10) Cr/H-Beta (Si/A12=27), (11) H-ZSM-5 (Si/A12=29), (12) Cr/H-ZSM-5 (Si/A12=29), (13) H-Mordenite (Si/A12=18.3), (14) Cr/H-Mordenite (Si/A12=18.3). Thus, zeolite catalysts were found to be effective for isomerization and dehydrogenation of n-butane into other C4 compounds such as isobutene. In most cases, the selectivity to C4 compounds, especially isobutene was improved by addition of Cr to the zeolite supports. In the case of H-SSZ-35, the increase of the Si/Alz ratio resulted in a decrease of the conversion of n-butane and an increase of the selectivity to C4, in particular isobutene. This indicated that there could be a relation between the acidity of the support and the dehydrogenation. The effect of Cr addition on the conversion of n-butane was dependent on the support used and the selectivity of dehydrogenated products was improved by addition of Cr. Fig.4 shows the conversion of n-butane and the selectivity to isobutene at 500 ~ over Cr catalysts supported on zeolites. The numbers stand for the Si/AI2 ratio of the zeolite support. Cr/H-SSZ-35 (Si/AI2=500) showed a lower conversion of n-butane (33.8%) than Cr/H-ZSM-5 (Si/Alz=29) and Cr/H-Mordenite (Si/AIz=18.3), but higher than other Cr

643 catalysts supported on zeolites. This catalyst showed moderate selectivity to isobutene (16.1%), much higher than Cr/H-ZSM-5 (Si/AIz=29) and Cr/H-Mordenite (Si/A12=18.3), and lower than Cr/H-ZSM-12 (Si/Alz=200, 1000) and Cr/H-ZSM-5 (Si/AI2=190). Thus, the yield of isobutene (conversion of n-butane x selectivity to isobutene) over Cr/H-SSZ-35 (Si/AIz=500) was 5.44%, the highest among the Cr-supported catalysts used in this study. 30 I 1000

O A

r

20 ,.Q 0

0

>10-

.,.~

. ,...~

o

X u190 & 200 360 0 500 V240 0 100 0 40 027

V 9 X

Cr/H-SSZ-35 Cr/USY Cr/H-ZSM-5 Cr/H-Mordenite Cr/H-Beta Cr/H-ZSM-12

r.,t3

v 18.3 0

I

20 Fig.4

"

'

I

'

I"

'

[3 29 I

4O 60 8O Conversion of n-butane / %

'

100

Conversion of n-butane and selectivity of isobutene over Cr-catalysts

supported on zeolites at 500~ The selectivity to the dehydrogenated C4 products (1-butene, isobutene, trans-2-butene, cis-2-butene and butadiene) over Cr/H-SSZ-35 (Si/Alz=500) was 58.6%. The yield of 19.8% was also the highest. However, the selectivity to the branched C4 products (isobutane and isobutene) over Cr/H-SSZ-35 (Si/Alz=500) was 17.5%. These findings indicate that Cr/H-SSZ-35 (Si/Alz=500) has moderate activity for both the dehydrogenation and isomerization of n-butane. The conversion of n-butane and the selectivity of each C4 product over Cr/H-SSZ-35 (Si/AI2=500) at each temperature between 400~ and 600~ are shown in Fig. 5. The conversion of n-butane increased with increasing temperature (x), while the selectivity to the C4 products decreased (O). This result suggested that a high temperature favors the ecomposition of n-butane over hydrogenation and isomerization of n-butane. With increasing temperature, the selectivity to trans-2-butene and cis-2-butene gradually decreased, while the selectivity to butadiene increased. The selectivity to 1-butene remained unchanged with temperature, and the selectivity to isobutene increased at temperatures below 500~ and decreased above 500~ From these findings, it is suggested that, at high temperature, dehydrogenation predominates over isomerization.

644

Wang et al. [5] reported the dehydrogenation and isomerization of n-butane on Cr-supported WOa-ZrOa and, in his case, the catalytic activity decreased with reaction time by deposition of carbon. In our study, the catalyst performance for n-butane conversion and the C4 products selectivity over Cr/H-SSZ-35 catalyst (Si/Ala=500) remained unchanged at 500~ for 6 h. From these findings on the catalytic activity and lifetime, H-SSZ-35 was expected to be one of the promising supports for the production of isobutene by hydrogenation and isomerization of n-butane.

100 - ~ "-

80

Select. of C4 products

".~

---0-- Select. of isobutane

/

--~ 60

I / / /

=

40

O

Conv. of n-butane

~2x

Select.of 1-butene

[]

Select.of isobutene

,~

Select.of trans-2-butene

j

o= 20

-~

Select. ofcis-2-butene V

0 400

450 500 550 Temperature / Oc

Select.of butadiene

600

Fig. 5. Catalytic performance of the Cr/H-SSZ-35 (Si/AIa = 500, Cr = 10 wt%) catalyst as a function of temperature 4. C O N C L U S I O N S For dehydrogenation and isomerization of n-butane to isobutene, zeolites were effective catalysts. However, the zeolite supports, especially those with low Si/A12 ratio, favor the decomposition of n-butane over dehydrogenation and isomerization because of their strong acidity. Deposition of Cr on zeolite supports enhanced the dehydrogenation and isomerization of n-butane. Among the Cr catalysts supported on zeolites, Cr/H-SSZ-35 (Si/AI2=500) had moderate n-butane conversion and isobutene selectivity and kept catalytic activity at 500~ for 6 h. As a result, the yield to isobutene (cony. of n-butane x select, of isobutene) was the highest among the catalysts used in this study. H-SSZ-35 type zeolites, hydrothermally synthesized with cis, cis, cis-N-methyl-hexahydrojulolidinium hydroxide as SDA, have one-dimensional cage-type channel structure, large micropore

645 volume and broad peak of NH3-desorption in TPD. It is suggested that these characteristics of H-SSZ-35 led to the highest isobutene yield in n-butane conversion. ACKNOWLEDGEMENT This work was supported by the Ministry of Economy, Trade and Industry, for the project on Molecular Harmonized Materials. REFERENCES

1. M. Inaba and H. Hamada, Stud. Surf. Sci. Catal., 125 (1999) 125. 2. M. Inaba and H. Hamada, Extended abstracts of ZMPC 2000 (International Symposium on Zelites and Microporous Crystals), Sendal, 2000, p.124. 3. P. Wagner, S. I. Zones, M. E. Davis and R. C. Medrud, Angew. Chem. Int. Ed., 38 (1999) 1269. 4. P. Wagner, Y. Nakagawa, G. S. Lee, M. E. Davis, S. Elomari, R. C. Medrud and S. I. Zones, J. Am. Chem. Soc., 122 (2000) 263. 5. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki, Catal. Lett., 66 (2000) 13. 6. Y. Takiyama, H. Nagata, M. Kishida and K. Wakabayashi, Sekiyu Gakkaishi, 41 (1998) 80. 7. R. Byggningsbacka, N. Kumar and L.-E. Lindfors, Catal. Lett., 55 (1998) 173. 8. L. Mandell, J. U. Piper and K. P. Singh, J. Org. Chem., 28 (1963) 3440. 9. Y. Kubota, Shokubai, 43 (2001) 615.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

The application of well-dispersed nickel nanoparticles inside mesopores of MCM-41 by use of a nickel citrate chelate as precursor

647

the

Dennis J. Lensveld, J. Gerbrand Mesu, A. Jos van Dillen and Krijn P. de Jong Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB, Utrecht, The Netherlands MCM-41 supported nickel catalysts were prepared with two different nickel precursors, viz. nickel nitrate and nickel citrate. The nature of the precursor significantly influenced the characteristics of the resulting catalysts, such as dispersion, average particle size and particle size distribution. With a weakly interacting precursor (nickel nitrate) only a small amount of nickel was deposited as nanoparticles inside the mesopores of the support material and the majority of nickel had aggregated into large crystallites at the external surface of the MCM-41 support. With a chelating nickel citrate precursor a Ni/MCM-41 catalyst having far more favourable properties was obtained. After incipient wetness impregnation the precursor salt is thought to leave a strongly adhering, thin film of nickel citrate inside the mesopores, which breaks up during the calcination treatment. As a result only very small nickel oxide nanoparticles were formed, which were located inside the mesopores of the support material. Reduction of these nickel oxide catalysts did not influence the characteristics of the support material, nor the dispersion and average particle size of nickel. 1. INTRODUCTION Since the first discovery of MCM-41 [1, 2] and related mesoporous materials efforts have been undertaken to apply metals inside the mesopores, in order to prepare silica-based heterogeneous catalysts with unprecedented high surface areas and unique structure. Nevertheless, only a small number of publications in the open literature have dealt with the application of nickel onto these types of support materials [3-7]. Junges et al. [3] showed that the in-situ incorporation of Ni 2§ during synthesis of MCM-41 is not a viable synthesis method for Ni/MCM-41 catalysts, since already at loadings exceeding 1.5 wt% the distinct capillary condensation step in the nitrogen isotherms is lost. Therefore Hartmann et al. [4] used the ion-exchange technique with NiC12 as a precursor to prepare nickel catalysts supported by both all-silica- and A1-MCM-41. Details about physical characterisations of the catalysts are not given, however. The most frequently applied method to prepare nickel catalysts supported by mesoporous materials is incipient wetness impregnation [e.g. 5-7]. Only for this method nickel catalysts with loadings exceeding 5 wt% have been reported [5-7]. Usually a very Corresponding author, e-mail: [email protected] Tel: +31-30-2536762, Fax: +31-30-2511027

648 simple nickel precursor salt, viz. Ni(NO3)2, is used [5, 6]. Unfortunately, already at loadings as low as 6 wt% the use of this precursor results in the formation of large nickel oxide particles (XRD) and a rather large decrease of surface area and pore volume, both on MCM-41 [5, 6] and KIT-1 [6]. Another method of catalyst preparation is ion-exchange with chelated nickel complexes, e.g. as described by Carriat et al [8] using ethylenediamine nickel complexes. However, for ion-exchange to occur on silica support materials a rather high pH is required. Due to the instability of MCM-41 towards alkaline solutions this is not a viable method for the application of nickel inside the mesopores. Therefore, in order to prepare Ni/MCM-41 catalysts with a high nickel loading and dispersion, we have used a different chelated nickel precursor, namely nickel citrate [7]. This chelated precursor was chosen because previous work in industry and academia has shown that catalyst preparation with some specific chelated metal complexes results in catalysts with a high dispersion of small supported metal (oxide) particles with a narrow particle size distribution [9-19]. The chemical nature of these specific chelate precursors enables them to interact with hydroxyl groups at the support surface via hydrogen bonding (instead of ordinary ion-exchange). Chelating ligands frequently used for these purposes comprise citrates, EDTA's (Ethylene Diamine Tetra-acetic Acid), NTA's (Nitrilo Tri-acetic Acid) and their structural analogues. The mechanisms underlying the fundamental processes during catalyst preparation with these types of precursors have been extensively studied by Geus and Van Dillen and coworkers [ 10, 11 ].

2. EXPERIMENTAL 2.1. Support synthesis All-silica MCM-41 was prepared following a slightly adapted literature procedure [20]. Template (cetyltrimethylammoniumbromide, Acros, 99+%) was dissolved in demineralised water. To the resulting solution a mineralising agent (tetraethylammoniumhydroxide, Acros, 20 wt% in water) and finally silica (Aerosil 380, Degussa) was added. The molar composition of the resulting gel was 1 SiO2 : 0.27 CTABr : 0.19 TEAOH : 40 HE0. This highly viscous gel was stirred for one day at room temperature, after which it was transferred to a teflon-lined autoclave. This autoclave was placed in an oven and the synthesis gel was aged at 150~ under autogeneous pressure for 2-3 days. The product was worked up by repeated washing with demineralised water and filtration, after which it was dried for 12 hours at 120~ and calcined for 6 hours at 550~ (heating rate = 1~ minl), both in air.

2.2. Catalyst preparation Nickel catalysts were prepared by incipient wetness impregnation of the powdered MCM-41 support with solutions of nickel citrate. To prepare these solutions a suspension of nickel carbonate (Baker Analyzed Reagent) in demineralised water was made and after heating to 100~ citric acid (Merck, p.a.) was added until a clear solution had formed (NiCO3 : citric acid 3 : 2). After impregnation the samples were dried for 12 hours at 120~ and calcined for 4 hours at 450~ (heating rate = 1~ minl), both in air.

649 As a reference a catalyst was made by incipient wetness impregnation with a solution of nickel nitrate (Riedel-de-Ha6n, extra pure) in demineralised water. This catalyst was dried and calcined similar to the citrate catalyst. For both catalysts the loadings were 10 wt% Ni. To study the conversion of the resulting nickel oxide catalysts to their metallic counterparts a reduction treatment was performed: a small amount of catalyst sample was pelletised, placed in a fixed bed reactor and hydrogen was passed through the catalyst bed at a rate of 50 ml min 1 at 650~ Next the reactor was cooled to room temperature and the catalyst sample was subjected to a gentle passivation treatment, by injecting small aliquots of air with a syringe into the gas stream (argon). No heat release was measured during this treatment. 2.3. Characterisation

Nitrogen physisorption was performed with a Micromeritics ASAP 2400 apparatus. Measurements were done at -196~ Prior to the measurements the powdered samples were degassed for 24 hours at 300~ in vacuum. Surface areas, pore volumes and pore size distributions were determined with standard BET and BJH theory. X-ray diffraction was performed on an Enraf Nonius PDS 120 powder diffraction system equipped with a position-sensitive detector, using Co KCtl radiation with a wavelength of 0.1788970 nm. Electron microscopy was performed with a Philips CM 200 transmission electron microscope equipped with a field emission gun and operated at 200 keV. For the measurements sample material was ultrasonically dispersed in ethanol and a drop of the resulting, highly diluted suspension was applied onto a holey carbon film supported on a copper grid. 3. RESULTS The textural characteristics of the MCM-41 support material and the nickel catalysts were determined with nitrogen physisorption. Representative isotherms of the materials obtained are given in Fig. 1. All samples show a characteristic capillary condensation pore filling step at a relative pressure of approximately 0.37, which is indicative of the filling of the uniform mesopores of the support. The surface areas and pore volumes of the materials are listed in Table 1. The high values for SBET and pore volumes of the nickel catalysts indicate that no pore blocking has occurred after catalyst preparation with both precursors. This assessment is also apparent from the pore size distributions, which are presented in Fig. 2, since no shrinkage of the average pore diameter is observed, nor a broadening of the pore size distribution. The structural integrity of the nickel catalysts, as well as the support material, was determined with X-ray diffraction. The results are given in Fig. 3. At least three wellresolved diffractions are visible, indicating that an MCM-41 support material with longrange hexagonal ordering had been prepared. Moreover, the structural integrity of the support material is retained upon catalyst preparation with both nickel precursors. Both X-ray diffraction and transmission electron microscopy were used to determine the (approximate) particle size (distributions) for the catalysts e x citrate and e x

650 nitrate. For the catalyst X-ray diffraction

ex

nitrate sharp reflections, assignable to NiO, were observed with Catalyst

ex

nitrate

Catalyst

ex

citrate

I200mlg l f ~ ~ ~~f~M-41

0

support

~ ~

r~

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

P/P0 Fig. 1. Nitrogen isotherms ofMCM-41 and catalysts. Arrows indicate the "onset" of each isotherm. (Fig. 4, upper line), indicating the presence of large nickel oxide crystallites. However, the onsets of the peaks are very broadened, which means that very small crystallites must be present also. A completely different dii~actogram was obtained for the catalyst e x citrate (Figure 4, lower line): NiO reflections were almost absent, the ones remaining having very low intensities and being very broad. As a result the nickel oxide particles present on this catalyst must, without exception, be very small; i . e . << 10 nm. An estimation of the external surface area of the MCM-41 support material of less than 10 m2 g-1 by Voegtlin et at [21] learns that these nickel oxide nanoparticles have to be accommodated inside the mesopores of the support to prevent extensive sintering during calcination.

Catalyst e x nitrate o

-,.._

Catalyst e x citrate ,MCM-41 suppo~

1

2

3

4

5

6

7

pore diameter (nm)

8

9

,

Table 1 Textural properties of the MCM-41 support and resulting nickel oxide catalysts.

10

Fig. 2. Pore size distributions of the MCM-41 support and resulting catalysts; straight line = adsorption, dotted line = desorption.

SBET

MCM-41 catalyst e x citrate catalyst e x nitrate

(m2 g-l) 1,000 860 870

pore volume (ml g-l) 0.98 0.84 0.92

651 (100)

A Catalyst

ex

40

nitrate

50

60

70 02

80

90

100

(110) (200) Fig. 4. X-ray diffractograms of the catalysts, showing NiO reflections.

rJl

Catalyst

ex

citrate

MCM-41 support ]~ ~ 1

2

3

4

(210) 5

6

7

Fig. 3. X-ray diffractograms showing the characteristic reflections of the hexagonal MCM-41 lattice.

02

Fig. 5. TEM image of the catalyst ex nitrate. Very large nickel oxide particles outside the MCM-41 support material are observed.

Fig. 6. TEM image of the catalyst e x nitrate. Some small nickel oxide nanoparticles confined inside the mesopores can be seen.

652 These assumptions were corroborated by transmission electron microscopy, as can be seen in Figures 5 - 7. The catalyst e x nitrate displays very large nickel oxide particles (Fig. 5) as well as nanoparticles that are confined inside the mesopores of the support (Fig. 6). For the catalyst e x citrate only very small nanoparticles have been observed, which are situated predominantly inside the mesopores of the support material (Fig. 7). The results that have been obtained with the catalysts aider reduction and passivation are the same as those aider calcination, i.e. the textural and structural properties of the support material have completely been retained after the treatments (as determined with nitrogen physisorption, X-ray diffraction and transmission electron microscopy). Information concerning the metallic nickel particles has been obtained with X-ray diffraction and transmission electron microscopy. Diffractograms of the catalysts after passivation are shown in Fig. 8. The observed features are exactly the same as for the oxidic systems (Fig. 4): only very broad and low diffractions are visible for the catalyst e x citrate, whereas sharp, intense peaks with a broad onset are observed for the catalyst e x nitrate. Consequently the nickel particles of the catalyst e x citrate have resisted sintering during the reduction treatment, thereby conserving the high dispersion of the catalyst. These results were confirmed by transmission electron microscopy measurements (not shown): only very small nickel nanoparticles situated inside the mesopores were found for the catalyst e x citrate. A more elaborate discussion on the reduction behaviour of the described catalysts, as studied with temperature programmed reduction, can be found elsewhere [7], along with an X-ray photo-electron spectroscopy investigation involving the oxidic catalyst samples.

Fig. 7. TEM image of the catalyst e x citrate. Exclusively very small nickel oxide nanoparticles situated inside the mesopores of MCM-41 are observed.

653

rd~

nitrate

ex

I

40

50

60

t

I

I

t

I

70

80 90 100 110 120 o2 Fig. 8. X-ray diffractograms of the catalysts after reduction and passivation.

4. DISCUSSION 4.1. Catalyst preparation Several characterisation techniques indicate that the characteristic features of the MCM-41 support material have been retained after application of nickel with two different precursor solutions. Surface areas and pore volumes have remained very high after nickel incorporation, indicating the absence of mesopore blocking for both systems. TEM as well as XRD measurements provide explanations for this finding. First of all, very large nickel oxide particles have been found for the catalyst e x nitrate, which have to be situated at or next to the external surface of the support material. Growth of these very large particles inside the mesopores of the support material is very unlikely, because TEM as well as XRD (Fig. 3) indicate that the characteristic, hexagonal structure of the support material is retained after nickel application, thus indicating framework stability. Growth inside the MCM-41 particles would have resulted in extensive damage of the framework and broadening of the pore size distribution, which is not observed. Therefore, these large nickel oxide particles likely have been formed by a different mechanism. The most probable explanation is that after incipient wetness impregnation a large amount of the nickel ions has been entrained out of the mesopores with the solvent flow during the drying treatment. As a result, large nickel nitrate crystaUites have precipitated at or next to the external surface during drying, thus giving rise to the formation of the very large nickel oxide crystallites of this catalyst during calcination. However, a significant quantity of nickel ions has also been retained inside the mesopores after drying, giving rise to the formation of very small nickel oxide particles inside the mesoporous support structure. Therefore, due to the limited interaction between nickel ions and the silica pore walls during the mild catalyst preparation process a catalyst with a very broad particle size distribution is obtained, consequently exhibiting a relatively low dispersion of nickel (oxide). When a chelated nickel citrate precursor is used for catalyst preparation strikingly different results are obtained. Only very small nickel oxide nanoparticles can be observed after calcination, which are situated inside the mesopores of the support material. As a

654 result the nickel oxide dispersion of this catalyst is higher than in the case of a nitrate precursor. The explanation for these observations can be found in the behaviour of the impregnated solution, containing the chelating citrate precursor, during drying and subsequent calcination. The viscosity of such a solution is higher than that of a nickel nitrate solution with the same nickel content. Moreover, upon solvent removal during drying the viscosity of the solution with chelated complex rapidly increases, contrary to the behaviour of a nickel nitrate solution. As a result, the solvent flow out of the mesopores during drying will be much less or even absent for a solution with chelated precursor compound. Furthermore, due to the high viscosity of the impregnated solution diffusion of the chelated complexes inside the mesopores is very much hindered during drying, which results in a homogeneous distribution of precursor compound in the mesopores, where it interacts with silanol groups at the pore surface v i a hydrogen bonding. After drying the chelated nickel precursor is therefore expected to have formed a thin film on the pore surface of the support. Because of the hydrogen bonding this film is tightly adhered to the pore surface. Upon calcination of the thus immobilised catalyst precursor the thin film of nickel citrate breaks up and decomposes due to the combustion of the organic ligand. The resulting catalyst contains only very small nickel oxide nanoparticles situated inside the mesopores. Moreover, the pore volume for this catalyst decreases much more than for the catalyst e x nitrate, indicating that a significantly larger amount of nickel oxide must be present inside the mesopores. It is remarkable, though, that the textural and structural properties of the support material have been completely retained atter calcination of the thin film of nickel citrate precursor, since a rather large adiabatic rise in temperature is generally observed during the combustion of these types of precursor materials. This sudden rise in temperature might very well have damaged the structure of MCM-41. Nevertheless, XRD learns that the structural order of the mesoporous support material is retained and TEM (Fig. 7) and nitrogen physisorption clearly show the presence of the 2.7 nm mesopores after calcination. Furthermore these characterisation techniques do not provide evidence for the formation of additional pores, nor a decrease of long-range order of the materials, indicating that the support structure has not been damaged during the combustion of the organic part of the precursor. Finally, reduction experiments showed that the nickel oxide nanoparticles of the catalyst e x citrate could be converted gently into metallic nickel nanoparticles without damaging the structure of the support material. This finding indicates that despite the intimate interaction between precursor and support material no "mutual" surface compound, exhibiting a nickel phyllo- or hydro-silicate structure, had formed atter calcination, since reduction of these kinds of compounds would result in extensive restructuring of the support material. However, there is a sufficiently large interaction between nickel and the support material to prevent sintering of the active phase during the reduction treatment. The resulting nickel nanoparticles are still confined inside the mesopores of MCM-41, thereby retaining the high dispersion of nickel that had been achieved during the catalyst preparation procedure. Therefore the used preparation method with nickel citrate can be considered as a "mild" technique for the preparation of metallic nickel catalysts exhibiting a high dispersion of active phase, supported by a mesoporous host material.

655

4.2. Prospects for the application of other elements with chelate precursors Various research efforts over the years have demonstrated that solutions of chelated metal ions provide excellent precursors for the preparation of heterogeneous catalysts by means of incipient wetness impregnation [e.g. 10-19]. The film-forming abilities of these precursors on a support material, notably during drying after impregnation, result in homogeneous distributions of active phase over catalyst support bodies. Moreover, the resulting catalysts after calcination (and reduction) contain only very small particles of metal (oxide) supported by such common, "flat" support materials as SiO2, - and -Al203, TiO2 and ZrO2. Also the dispersions of the active phases thus obtained are very high. A list of metals that have been applied as a chelated complex onto various oxidic support materials by means of incipient wetness impregnation is given in Table 2. Table 2 Elements which have successfully been applied onto support materials as chelated complexes to produce heterogeneous catalysts with very small metal (oxide) particles and homogeneous distributions. element Co

chelate EDTA citrate

Cr

Cu

Fe

EDTA

reference Boot [15], Van de Loosdrecht [16] Van de Loosdrecht [16] Van den Brink [17]

EDTA salicylaldehyde acetate citrate

Tijburg [22] Van Wijk [ 13] Van Wijk [14] E. de Wit [22]

citrate EDTA gluconate

Van den Brink [11]

element La

chelate EDTA

reference Tijburg [12]

Mn

citrate EDTA

Van de Kleut [22] Boot [15], Van de Kleut [22]

Ni

EDTA citrate

Van Yperen [22] Takahashi [ 19]

Sn

citrate EDTA formate

Meima [1 O]

Zn

citrate

Ter6rde [18]

Zr

citrate

E. Stobbe [22]

The results described in a previous paper [7] and this one indicate that the use of chelated metal precursors for the preparation of heterogeneous catalysts can suitably be extended to mesoporous support materials. The mechanisms underlying the fundamental processes occurring during catalyst preparation appear to be the same for both types of support materials. Therefore no limitations appear to exist to apply a wide variety of other elements into the pores of several types of mesoporous supports, with retention of the unique textural and structural properties of the support materials. Catalysts thus prepared will feature very high dispersions of active phase as well as very small particles with sizes even smaller than on conventional support materials (due to the limiting size of the pores of mesoporous support materials).

656 5. CONCLUSIONS For the preparation of nickel catalysts supported by mesoporous MCM-41 material the choice of nickel precursor was found to significantly influence the characteristics of the resulting catalysts. With a weakly interacting nickel precursor (nickel nitrate) a very broad particle size distribution was obtained, with both very large nickel particles situated outside the mesopores of the support and very small nickel nanoparticles inside the mesopores. The very weak interaction between nickel ions and the silica pore surface causes a considerable amount of these ions to be entrained with the solvent flow during drying after incipient wetness impregnation, giving rise to a decreased dispersion of thus prepared catalysts. Impregnation with a chelated nickel citrate precursor solution results in catalysts with only very small nickel nanoparticles which are confined inside the mesopores of MCM-41. The viscosity of the impregnation solution during drying prevents the transportation of precursor complexes out of the mesopores, thus giving rise to the formation of a thin film of nickel citrate that is tightly bound to the surface of the mesopores. Combustion of the precursor film during calcination results in the formation of the nickel oxide nanoparticles. Upon reduction the high dispersion of nickel is retained for the catalyst e x citrate. Nickel loadings of thus prepared mesoporous catalysts are relatively high, i.e. 10 wt% Ni, and the favourable and unique structural and textural properties of the MCM-41 support material are completely retained upon catalyst preparation. A close inspection of results obtained with ordinary support materials in the past indicates that no fundamental differences during catalyst preparation are apparent, which offers the possibility for the application of a wide variety of other elements inside the pores of mesoporous materials with chelated precursor complexes. ACKNOWLEDGEMENTS

We would like to thank John Raaymakers t (N2-physisorption), Sandra Kemp and Cor van der Spek (both TEM) for their efforts and enthusiasm during characterisation of the described materials. REFERENCES

1. 2.

3.

4.

C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. U. Junges, S. Disser, G. Schmid and F. SchiJth, in Mesoporous Molecular Sieves, Stud. Surf. Sci. Catal. 117, (L. Bonneviot, F. Bdland, S. Giasson and S. Kaliaguine, Eds.), Elsevier Science B.V., Amsterdam (1998) 391. M. Hartnmma, A. P6ppl and L. Kevan, in 11'" International Congress on Catalysis 40 th Anniversary, Stud. Surf. Sci. Catal. 101 (issue PA&B) (J.W. Hightower, W.N.

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Delgass, E. Iglesia and A.T. Bell, Eds.), Elsevier Science B.V., Amsterdam (1996) 801. J. Cui, Y.-H. Yue, W.-Y. Dong and Z. Gao, in Progress in Zeolite and Microporous Materials, Stud. Surf. Sci. Catal. 105 (H. Chon, S.-K. Ihm and Y.S. Uh, Eds.), Elsevier Science B.V., Amsterdam (1997) 687. Y. Yue, Y. Sun and Z. Gao, Catal. Lett., 47 (1997) 167. D.J. Lensveld, J.G. Mesu, A.J. van Dillen and K.P. de Jong, Mieropor. Mesopor. Mater., 44-45 (2001) 401. J.Y. Carriat, M. Che, M. Kermarec, M. Verdaguer and A. Michalowicz, J. Am. Chem. Soc., 120 (1998) 2059. G.A. Lee and S.N. Isaacs, U.S. Pat. 4367167. G.R. Meima, B.G. Dekker, A.J. van Dillen, J.W. Geus, J.E. Bongaarts, F.R. van Buren, K. Delcour and J.M. Wigman, in Preparation of Catalysts IV, Stud. Surf. Sci. Catal. 31 (B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, Eds), Elsevier Science Publishers B.V., Amsterdam (1987) 83. P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen and J.W. Geus, in Preparation of Catalysts V, Stud. Surf. Sci. Catal. 63 (G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, Eds.), Elsevier Science Publishers B.V., Amsterdam (1991) 527. I.I.M. Tijburg, J.W. Geus and H.W. Zandbergen, J. Mater. Sc~, 26 (1991) 6479. R. van Wijk, O.L.J. Gijzeman, J.W. Geus, E. ten Grotenhuis and J.C. van Miltenburg, Catal. Lett., 24 (1994) 171. E. ten Grotenhuis, J.C. van Miltenburg, J.P. van der Eerden, R. van Wijk, O.L.J. Gijzeman, J.W. Geus and C.H.M. Mar6e, Catal. Lett., 28 (1994) 109. L.A. Boot, M.H.J.V. Kerkhoffs, B.Th. van der Linden, A.J. van Dillen, J.W. Geus and F.R. van Buren, Appl. Catal. A, 137 (1996) 69. J. van de Loosdrecht, M. van der Haar, A.M. van der Kraan, A.J. van Dillen and J.W. Geus, Appl. Catal. A, 150 (1997) 365. P.J. van den Brink and J.W. Geus, Eur. Patent 0409353. R.J.A.M. Ter6rde and J.W. Geus, WO Patent 9732813 and U.S. Patent 6207127. R. Takahashi, S. Sato, T. Sodesawa, M. Kato, S. Takenaka and S. Yoshida, J. Carat, 204 (2001) 259. C.-F. Cheng, D.H. Park and J. Klinowski, J. Chem. Sot., Faraday Trans., 93 (1997) 193. A.C. Voegtlin, A. Matijasic, J. Patarin, C. Sauerland, Y. Gfillet and L. Huve, Micropor. Mater., 10 (1997) 137. J.W. Geus, A.J. van Dillen and co-workers, unpublished results.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation of

Ce-Zr-O composites by a

659

polymerized complex method

T. G. Kuznetsova, V. A. Sadykov, E. M. Moroz, S. N. Trukhan, E. A. Paukshtis, V. N. Kolomiichuk, E. B. Burgina, V. I. Zaikovskii, M. A. Fedotov, V. V. Lunin,* E. Kemnitz**. Boreskov Institute of Catalysis, SB RAS, Lavrentieva 5, Novosibirsk 630090, Russia * - Chemical Department of Lomonosov Moscow State University, Moscow, Russia ** - Institute for Chemistry, Humboldt -Univ., Berlin, Germany. Fluorite-like oxides based on CeOz and ZrOz and Ca and/or F-modified have been prepared by organic polymerized complex method. Dispersed phase-pure cubic fluorite-like mixed oxide phases were obtained provided the synthesis parameters are optimized. The real/defect structure of those phases is characterized by using combination of diffraction and spectroscopic methods. For bulk structure, the main feature is the presence of strong framework distortions due to complex defects including extended ones generated by guest cations and/or anions. For ceria-based solid solutions, the nature of surface Br6nsted and Lewis acid centers, their density and ability to stabilize charged molecular forms of oxygen strongly depend upon the sample composition and preparation route, surface being in general more disordered compared with pure ceria phase. 1. INTRODUCTION Ce-Zr-O solid solutions are important components of promising catalysts for different oxidative processes, in particular, for partial methane oxidation into syngas and three-way car exhaust clean-up [1-2]. Different synthesis procedures have been used for the preparation of Ce-Zr-O solid solutions including sintering of oxides, coprecipitation of mixed hydroxides from nitrate salts, decomposition of mixed acetates or citrates [3-5]. Homogeneous metastable tetragonal solid solutions were also successfully synthesized by the organic polymerized complex method (OPCM) which is a modified version of the polymeric precursor method suggested by Pechini [5]. In this work OPCM was further modified for the preparation of highly dispersed fluorite-like oxide systems including CeO2, Ce-Zr-O solid solutions and partially stabilized zirconias. The aim of the work was to elucidate the effect of the samples composition and specificity of their preparation procedure on the bulk and surface defect structure, which is known to be of importance for catalysis of red-ox reactions.

660 2. E X P E R I M E N T A L Water solutions (WS) of starting salts (nitrates of Ce, Y, Ca; oxychloride of zirconium, ammonium and calcium fluorides) or their solid crystalline hydrates (SCH) (the content of admixed rare earth cations was less than 0.2 wt. %) taken in a required ratio were added at 60 ~ to the solution of citric acid in ethylene glycol (1:4 mole ratio) while the Me: citric acid (CA) ratio was varied from Me:CA=l:2 to 1"10. Solutions were heated up to 140 ~ for the time required to form a highly viscous polymeric brownish gel. This product was further calcined in air up to 700 ~ The degree of oxygen substitution for fluorine was less than 20 at. % or 40 at. % in samples of Ce-Zr series or Ce-Zr-Ca series. Some calcined samples were fluorinated by heating with solid NH4F at 220~1q for 6 h followed by annealing at 500~1~1for 1 h. CeO2 samples were prepared through the OPCM route and by Ce 3+ nitrate decomposition at 700 ~ The X-ray phase analysis of samples was carried out using a HZG-4C diffractometer (Cu I ~ radiation and a flat monochromator) in the range of 20 angles equal to 1-70 ~ The unit cell parameter of ceria and modified composites was determined from the position of 311 diffraction peak. The surface chemical composition was analyzed with SIMS method using a MS7201 mass-spectrometer. The energy of the incident Ar + ion beam was equal to 4 Key, the current density 10-20 mka/cm 2. To prevent sample charging during ion bombardment, samples were supported onto high purity indium foil. Static magnetic susceptibility was measured by the Faraday method. Infra-red spectra of samples in the 220-4000 cm -1 range were recorded using a BOMEN M 102 spectrometer. UV-Vis spectra were recorded using a Shimadzu 8300 spectrometer equipped with a diffuse scattering DRS 8000 cell. Spectra were recorded in the 1000060000 cm -1 range with 4 cm -1 resolution, the number of scans being equal to 50. Samples were loaded into the vacuum cells equipped with CaF2 windows. To decrease the mirror reflection, a sample layer was inclined for 20-25% with respect to the horizontal plane. The surface properties were probed by the IR spectroscopy of adsorbed CO and 02 test molecules (a Fourier-transform IFS 113V Bruker spectrometer). In these experiments, samples were pressed in wafers with densities 4.4-22.7 mg/cm 2 and pretreated in the IR cell first at 100 Torr of 02 and then in vacuum at 400 ~ for 1 h. CO was adsorbed at 77 K first by introducing several doses (each CO dose corresponds to = 4 ptmol), finally setting CO pressure to 10 Torr. 02 (20 Torr) was adsorbed at RT. The data were presented as absorbance normalized to the unit weight of sample. 3. RESULTS AND DISCUSSION 3.1. Phase composition Table 1 lists data for the phase composition and specific surface area of samples prepared and studied here. For pure ceria samples prepared either via nitrate salt decomposition, or OPCM route, the cubic fluorite phase with the lattice parameter a= 5.411 ,~ corresponding to a standard data (JCPDS 43-1002) was revealed. Samples prepared via OPCM route possess higher specific surface areas increasing with the content of CA.

661

Table 1 XRD data of CeO2, ZrO2 and Ce-Zr-O based composites.

No

Chemical composition

Preparation route

Ceo.8Zro.zOxFy

Ce3+ nitrate decomposition OPCM (SCH- Me : C A = I :2) OPCM (WS- Me : C A = I : 6) OPCM (SCH- Me : C A = I :2) OPCM (WS- M e : C A = I :2) OPCM (WS- M e : C A = I : 6) OPCM (WS- Me : C A - 1 : 10) OPCM (SCH- M e : C 1 : 2); GpF

10

Ceo.sZro.5Ox

OPCM (SCH- M e : C A = I :2) NH4F OPCM (SCH- Me : C A = I : 2

11 12

Ceo.sZro.sOxFy

13

Ceo.6Zro.zCao.zOx

CeO2

Ceo.8Zro.zOx

8 9

14 15

16

Ceo.6Zr0.2Ca0.20•

Spec. Surf. area, mZ/g 3 6 19 22 24 33 39 20 11 16

OPCM (WS- Me : CA=I :6) OPCM (SCH- Me : C A = I : 2) NH4F OPCM (SCH- M e : C A = I : 2

69 31

OPCM (WS- Me : CA=I :6) OPCM (SCH- Me : CA=I :2) CaF2

44 15

54

Phase composition (cell parameter, ,~) CeO2 (5.411) CeO2 (5.411) CeO2 (5.411) CeO2 (5.380) CeO2 (5.376) CeO2 (5.368) CeO2 (5.368) CeO2 ( 5.383); CeF3, ZrO2 c*, CeO2 (5.384) CeO2 (5.385), ZrOz c (5.139) CeO2 (5.282) CeO2 (5.274), ZrOzC* Ce02 (5.368), Zr02 c ( 5.134 ); Ce02 ( 5.358 ) CeO2 ( 5.383 ), ZrO2 c ( 5.152 ), CaF2 CeO2 ( 5.358 )

OPCM (WS- Me : CA--1 : 6) 58 NH4F 17 Zro.9Cao.lOx OPCM (WS- Me : CA=I : 6) 60 ZrO2 c (5.121) 18 Zr0.8Ceo.lY0.1Ox OPCM (WS- Me : CA=I : 6) 90 ZrO2 c (5.152) 19 ZrO2 OPCM ( W S - Me : CA=I : 6) 8 ZrO2 M, ZrO2 v OPCM - organic polymerized complex method; SCH - solid crystalline hydrates; WS - water solutions; Me : CA - metal : citric acid ratio; GpF-gas-phase fluorination ; ZrO2 c _ cubic phase, ZrO2M- monoclinic phase, ZrO2z - tetragonal phase; * - trace of cubic phase. When water salts solutions are used in OPCM synthesis, provided the Me:CA ratio is equal to 1:6 in all the composition range studied here (up to Zr relative content equal to 0.9), single phase samples with a cubic fluorite-like structure are formed. Thus, for ceriazirconia samples with zirconia content up to 50 mol. %, ceria-based cubic solid solution is formed. For those samples, the lattice parameter is decreased as compared with that of a pure ceria phase, due to cerium cations substitution by zirconium cations with a smaller

662

cation radius (0.88 ,~ and 0.82 ,~, respectively). For single-phase binary samples containing 20 and 50 mol. % of ZrO2, the lattice parameter a is equal, respectively, to 5.368 ,~ and 5.282 ,~, which is somewhat higher than additive values of 5.35 ,~ 6 5.25 ,~ expected for the ideal solid solutions of cubic ceria and zirconia with the lattice parameters taken to be equal to 5.411 ,~ 6 5.090 ,~ (JCPDS 42-1164), respectively. For samples prepared by using a mixture of solid crystalline hydrates with Me:CA ratio equal to 1:2, deviations from the additivity are even bigger, though in this case, part of zirconium is segregated as a separate zirconia phase. Gas phase fluorination of mixed Ce-Zr oxides is accompanied by a partial decomposition of the solid solution, leading to the segregation of CeF3 (JCPDS 08-0045) and ZrO2 phases. More or less homogeneous fluorine-modified solid solutions are obtained when ammonium fluoride is added to the mixed solution of the starting compounds. In general, the effect of fluorine on the lattice parameter depends upon the zirconium and/or calcium content and degree of precursors mixing in the course of synthesis (Table 1). Thus, when a mixture of solid crystalline hydrates is used, the effect of fluorine is more pronounced in systems with a higher (50 tool. %) content of zirconium. When fluorine is introduced into Ce-Zr-Ca-O solid solution, its lattice parameter is not noticeably affected. 3.2. Bulk structure

Deviation of lattice parameters of homogeneous cubic solid solutions from the additive values could be assigned either to the presence of Ce 3+ cations with a bigger ionic radius, or to distortions of the lattice due to incorporation of guest cations/anions. Magnetic susceptibility data (X298 is in the range of-0.31++1.2, 10 -6 CGSM units) indicate that the relative content of Ce 3+ cations does not exceed 1 % . A small content of those cations implies that their effect on the lattice parameters has to be negligible. Hence, some distortions appear to be responsible for the relative lattice expansion considered here. In general, the lattice distortion can be caused both by point and extended defects generated due to incorporation of modifying cations (Zr, Ca) or anions (fluorine, residual lattice hydroxyls). Extended defects differing from the surrounding lattice by the electron density can be detected by using SAXS [6], while point defects (anion vacancies etc) are usually probed by UV-Vis [7]. The integral intensity of X-ray scattering on the extended regions with modified electron density (typical sizes up to 200 ,~) is listed in Table 2. The relative density of extended defects strongly depends upon the samples composition and their preparation route. Moreover, the size distribution of those defects varies as well. Thus, for pure ceria, extended defects with typical sizes in the range of 20-40 ,~ dominate. Since increasing water content in polymerized precursor increases the integral intensity of scattering, for pure ceria extended defects can be tentatively assigned to microstrains generated by the presence of residual hydroxyls in the lattice (vide infra). Along with those defects, more extended defect regions (typical sizes up to 80-170 ,~) were detected as well. Those defects are usually assigned to intergrain boundaries present in samples with relatively big particle sizes [6]. When up to 20 mol. % of zirconia are added, the size distribution of inhomogeneities changes rather slightly, the share of more extended defects being decreased. Since integral density of scattering for ceria-zirconia samples with a low Zr

663 content strongly depends upon the mode of reagents premixing before adding to citric acidethylene glycol solution, namely, higher integral intensity is observed for less homogeneous samples, it implies that in the case of a poor reagent mixing, a part of extended defects is generated by the incorporated zirconium cations probably present as clusters or microinclusions. When fluorine is incorporated into a sample with a low zirconium content, the integral density of extended defects remains nearly constant, although the relative share of defects assigned to intergrain boundaries somewhat increases. For samples with a relatively high content of zirconium characterized by a higher dispersion (higher specific surface area, Table 1), extended defects in the range of 10-25 ,~ dominate. Those defects can be tentatively assigned to the lattice distortion around incorporated guest cations/anions with associated local charge variation as detected by photoemission and XANES studies [8]. The integral density of those clustered defects tends to increase with addition of zirconium and fluorine. On the contrary, for calcium-containing samples, the density of those extended defects appears to be lower. FTIRS data of lattice modes (not shown for brevity) revealed pronounced distortion of the local coordination sphere of cations in samples synthesized via OPCM route. In ceria samples, along with main band at ~ 400 cm -1, a strong shoulder at ~ 510 cm -1 is observed as well, which is not expected for symmetric cubic phase. This feature can be assigned to the coordination sphere distortion due to the presence of residual hydroxyls. In ceriazirconia samples the intensity of this high-frequency shoulder is increased, while the band is broaden. Since in cubic Ca-ZrO2 sample prepared via OPCM route (Table 1), a band at ~ 505 cm -~ with a strong shoulder at ~ 590 cm -1 is observed, it implies that a similar type of Zr cations coordination exists in CeOz-ZrOz samples, which agrees with the conclusions of Liu et al [8]. Fig. 1 shows UV-Vis spectra of pure ceria and zirconia phases as well as those of fluorite-like solid solutions with a homogeneous distribution of components. For pure ceria and zirconia samples the spectra differ substantially due to the difference of those oxides band gap width equal to 3.1 and 5.0 eV, respectively. Incorporation of Zr into the framework of ceria is accompanied by the appearance of an absorption in the visible range (12000-25000 cm-~), which increases with the content of zirconium. This absorption can be assigned to C e 3+ cations characterized by absorption band at -- 15000 cm -1 [9], and/or to F and V centers such as those revealed in pure zirconia [10]. For samples modified by Ca or Ca + F, the enhanced absorption in the range of 26000-40000 cm -~ can be assigned to bulk/surface peroxide groups Oz z [11]. In the visible range, calcium incorporation increases the absorption apparently due to the generation of anion vacancies. Since the simultaneous incorporation of calcium and fluorine decreases absorption in this range, it implies that anion vacancies are filled by fluorine anions.

664

Table 2. SAXS integral intensity I (a.u) and maxima position of Ce-Zr based composites prepared from solid crystalline hydrates (a) and water solutions of salts (Me : CA=I : 6) (b). Chemical composition I (a.u.)/maxima position, a b CeO2 32/20-40 ~, 80-170 m 164/20-40 S, 80-170 w 35/20-40 s, 80-170 TM

Ceo.sZro.zOx

142/20-40 ~, 80-170 w

Ceo.8Zro.zOxFy

135/35-55 s, 80-170 m

Ceo.sZro.5Ox

335/10-25 s, 40-60 w

Ceo.5Zro.5OxFy

717/10-25 S, 40-60 w

Ceo.6Zro.zCao.zOx

155/10-25 s, 40-60 w

66/10-25 s, 40-60 w

Ceo.6Zro.2Cao.2OxFy

82/10-25 s, 40-60

323/10-25, 40-60 w

Ceo.lYo.lZro.8Ox

378/10-25 s, 40-60 TM

1088/10-25 s, 40-60 s

s - strong, m - medium, w - weak - contribution of intensity. 3.3. Surface properties According to SIMS data (not shown here for brevity), zirconium cations and fluorine anions are rather uniformly distributed across the particle depths, while calcium tends to segregate at the surface, where its content is up to several times higher than in the bulk. TEM data (not shown for brevity) revealed the samples particles to be of a platelet appearance, with the most developed faces being of (111) and (110) types as determined by SED analysis. Intergrain boundaries are present in low surface area samples. In the spectra of CO adsorbed at 77 K on the surface of samples pretreated in oxygen, at low CO coverages (Fig. 2a), bands corresponding to carbonyl complexes of coordinatively unsaturated Ce3+ cations (~2120-2130 cm-1), Ce 4+ cations (--2160 cm -1) and Zr 4+ cations (~2170-2180 cm -~) are observed. The intensity of the strongest Ce3+-CO complexes is rather low, although they appear after the first dose of CO, thus indicating their high bonding strength. At higher CO pressures in the gas phase (10 Torr), bands at ~ 2100 cm -~ appear, which can be assigned to bicarbonyl complexes of Ce 3+ cations having at least two vacancies in their coordination sphere. Although Ce 3+ centers exist at the surface of a pure ceria sample prepared via OPCM, their density increases when Zr is introduced into the structure. This feature certainly correlates with the integral intensity of extended defects estimated by SAXS and enhanced absorption in the visible range of ceria-zirconia samples corresponding to bulk Ce 3+ cations (vide supra). Moreover, Ce0.sZr0.5Ox sample containing an admixture of zirconia-based phase (Table 1), although having a lower specific

665 surface area, has a higher intensity of bands in the 2100-2130 cm -1 range. Hence, at least a part of surface coordinatively unsaturated Ce 3§ cations appears to be located at the interphase boundaries. Fluorination decreases the density of those centers probably due to incorporation of fluorine atoms into vacant positions in the coordination sphere of Ce 3§ cations. Simultaneously, as would be expected, the intensity of bands corresponding to carbonyls of Ce 4+ and Zr 4+ is increased (Fig. 2a, cf. spectra 2 and 3). a)

b) 0.2

4

g o.1 u_

o.o

51

/~ 6

--.----~

12000 14000 16600 18600 20600 22600 Wavenumber, cm1

15 n~" 10

0 84 10000 20000 30000 !

,|

,

,|

40000

,

50600 60000

Wavenumber, cm"1

Fig. 1. UV-Vis spectra of the most homogeneous samples 91 - CeOz, 2 - Ce0.8Zr0.zOx, 3 Ce0.6Zr0.zCa0.zOx, 4 - Ce0.6Zr0.zCa0.zOxFy, 5 - Ce0.sZr0.sO• and 6 - ZrOz. Calcium addition removes the Ce3+-CO band, decreases the intensity of the Me4+-CO band, and shifts it to lower frequencies (cf. spectra 2 and 6, Fig. 2a). Since no appreciable variation in the density of bulk extended defects (Table 1) or Vis range absorption (Fig. 1) is observed, this result can be explained by preferential segregation of calcium cations in the vicinity of surface coordinatively unsaturated sites thus blocking them. Estimation by using known values of carbonyls absorption coefficients [12] revealed that the surface density of Ce 3+ cations is less than 1% of a monolayer. Hence, those sites are clearly defects. For Me 4+ centers including weak ones filled only in the presence of gas phase CO, the integral surface density amounts up to 30-40 % of a monolayer, weakly depending on the sample nature. Hence, the predominant part of those sites can be assigned to 7-coordinated regular cations located on the most developed faces of the (111) type [13]. Pronounced variations with samples composition were also observed in the hydroxyls stretching region (Fig. 2c). For pure ceria sample, low-frequency bands corresponding to bridging hydroxyls bound with three cerium cations (~3638 cm 4) as well as bulk hydrogen bound hydroxyls (band ~ 3570 cm -1) dominate. A shoulder at ~3698 cm -1 can be assigned to bridging hydroxyls bound with two Ce 4§ cations [14]. Incorporation of Zr shifts hydroxyl bands to higher frequency and makes them more intense. It implies that a part of the hydroxyls is bound with zirconium cations, since the hydroxyl band positions is rather close to those observed partially stabilized zirconias [15]. The appearance of bands at - 3780 cm 4 implies the emergence of terminal hydroxyls. For

666

fluorine-containing sample, a band at ~ 3730 cm -1 corresponding to terminal hydroxyls appears, due to fluorine incorporation into the coordination sphere of Ce cations. Calcium cations decrease the intensity of Br6nsted acid sites as well due to surface sites blocking. a)

b) I Ads. CO 4 doze 6 84 .... ~r~ (D

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t~ 100 3787

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60 3400

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,

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,

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Wavenumber, cm4

",

|

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0 10'80

11'00

11'20

11'40

11'60

11'80

Wavenumber, cm -~

Fig.2. FTIR spectra of adsorbed CO (carbonyl region) (a, b), OH groups ( c ) and adsorbed Oz (d); Spectra designation" 1 - CeOz (sample 3), 2 - Ce0.sZr0.zOx (sample 4), 3 Ce0.sZr0.zOxFy (sample 9), 4 - Ce0.sZr0.5Ox (sample 10), 5 - Ce0.sZr0.5Ox (sample 11), 6 Ce0.6Zr0.zCa0.zOx (sample 14). As follows from Fig. 2d, contrarily to the pure ceria sample, samples of mixed fluorite-like solid solutions retain molecular forms of oxygen (Oz [16]). The intensity of those bands is higher for samples containing calcium, fluorine and smaller amount of zirconium. It seems to correlate with the decreased ability of those systems to dissociate molecular oxygen due to a lower density of either highly unsaturated cations or clustered centers.

667 4. CONCLUSIONS OPCM route of ceriamzirconia solid solutions synthesis appears to be reasonably flexible approach allowing to make rather broad substitution in cation and anion sublattices of fluorite-like oxides without appearance of new phases provided homogeneous mixing of starting salts is ensured. Reasonably high dispersion of samples thus obtained and disordered bulk and surface structure makes them promising candidates as catalysts and supports for red-ox reactions. ACKNOWLEDGMENTS. This work is in part supported by RFBR/INTAS grant No IR--97-402. REFERENCES

1. M. Fathi, F. Monnet, Y. Schuurman, A. Holmen and C. Mirodatos, J. Catal., 190 (2000) 43. 2. J. Kaspar, P. Fornasiero and M. Graziani, Catal. Today, 50 (1999) 285. 3. M. Inoe, K. Sato, T. Nakamura and T. Inui, Catal. Lett., 65 (2000) 79. 4. S. Rossignol, F. Gerard and D. Duprez, J. Mater. Chem., 9 (1999) 1615. 5. M. Yashima, K. Ohtake, M. Kakihana and M. Yoshimura, J. Am. Ceram. Soc., 77 (1994) 2773. 6. V.A. Sadykov, S.F. Tikhov, G.N. Kryukova, N.N. Bulgakov, V.V. Popovskii and V.N. Kolomiichuk, J. Solid State Chem., 73 (1988) 200. 7. P. A. Cox, Transition Metal Oxides - An Introduction to their Electronic Structure and Properties, International Series of Monographs on Chemistry 27, Clarendon Press, Oxford, 1995. 8. G. Liu, J.A. Rodriguez, J. Hrbek, J. Dvorak, C.H.F. Peden and A.F. Rodriguez, J. Phys. Chem. B, 105 (2001) 7762. 9. C. Binet, A. Badri and J.C. Lavalley, J. Phys. Chem., 98 (1994) 6392 10. A. Emeline, G. Kataeva, A.S. Litke, A. Rudakova, V. Ryabchuk and N. Serpone, Langmuir, 14 (1998) 5011. 11. C.B. Azzoni and A. Paleari, Phys. Rev. B., 53 (1996) 53. 12. D.A. Seanor and C.H. Amberg, J. Chem. Phys., 42 (1965) 2967. 13. C. Conesa, Surf. Sci., 339 (1995) 337. 14. A.N. Kharlanov, E.V. Lunina and V.V. Lunin, Zh. Phyz. Khim., 71 (1997) 1672. 15. A.N. Kharlanov, E.V. Lunina and V.V. Lunin, Zh. Phyz. Khim., 73 (1999) 898. 16. M. Daturi, E. Finocchio, J-C. Lavalley, F. Fally, V. Perricon, H. Vidal, N. Hickey and J. Kaspar, J. Phys. Chem. B, 104 (2000) 9186.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

669

Sol-gel routes for the preparation of heterogeneous catalysts based on Ru, Rh, Pd supported metals P. Moggi a, S. Morselli a' and G. Predieri b aDepartment of Organic and Industrial Chemistry, University of Panm, Parco Area delle Scienze 17/A, 1-43100 Parma, Italy bDepartment of General and Inorganic Chemistry, Analytical Chemistry, Physical Chemistry, University of Parma, Parco Area delle Scienze 17/A, 1-43100 Parma, Italy 1. INTRODUCTION The synthesis of heterogeneous, sol-gel entrapped metal complexes is becoming very popular as a new, less conventional approach in supported metal catalysts preparation, covering by now many of the major families of catalytic reactions [1-3]. According to this new approach, the intrinsic advantage of sol-gel techniques, i.e. the precise control of the synthetic conditions and hence of factors relevant to the catalytic activity, such as purity, stoichiometry, homogeneity (or controlled heterogeneity), structural stability, are combined with that deriving from the metal complexes used as metal precursors, i.e. nanostructural properties and good reducibility. This allows a fine tuning of the catalysts architecture in terms of composition, metal dispersion and matrix porosity. Very high dispersed metal Ru particles on silica were obtained by us using the sol-gel method starting from a cluster compound, such as Ru3(CO)12, or an anionic cluster, such as [HRu3(CO)ll]" [4]. Ru/SiO2 sol-gel prepared from Ru3(CO)12, free of halogen or alkali ions, resulted a very active catalyst for the Fischer-Tropsch synthesis (FTS) at relatively low temperatures (473-573 K) and at atmospheric pressure [5]. Then, since it was demonstrated that synergistic bimetallic interactions between Co and Ru increase FTS rate and selectivity to higher hydrocarbons on supported catalysts [6], the catalytic activity of sol-gel prepared Ru, Co and Ru-Co bimetallic systems was studied [7]. TEM investigations on Ru3Co samples evidenced the presence of metal nanoparticles, very homogeneously dispersed into the silica matrix. The X-ray fluorescence analysis confirmed a mean Ru/Co 3:1 composition of the metal grains, according to a possibly bimetallic character of any single particle. However, the XPS analyses displayed, for the Ru/SiO2 catalyst, the presence of metallic Ru and, for the Co/SiO2 catalyst, the presence of Co(II) species in larger amount than Co(0). This situation was also observed for the Ru3Co and the other CoRu mixed catalysts. Therefore, it was concluded that the observed metal particles were Corresponding author. Tel. +39-0521-905464 E-mail: [email protected]

670 composed essentially by ruthenium, while the most part of cobalt was dispersed as oxidised species in the matrix around. This was most probably related to the low metal loading, which is known to promote strong interactions between cobalt and support. The effect of cobalt loading was investigated by preparing six catalysts with a different cobalt amount following the sol-gel method [8]. High CO conversions were observed with catalysts having a metal loading higher than 5% by weight. Characterization results highlighted the good metal distribution and the presence of a large amount of metallic cobalt on the surface of both 10% and 30% catalysts. The preparation of Ru supported catalysts by sol-gel method, indeed, was extended to obtain new formulations by changing the type of support. Alkali-promoted RtgMgO systems were prepared starting from magnesium ethoxide, Ru3(CO)12 and a cesium compound [9]. The gels were subjected to an activation/reduction procedure to substantially obtain Ru-CsOH/MgO and then tested as catalysts in the ammonia synthesis at atmospheric pressure. It was evidenced that the sol-gel prepared Cs-promoted Ru/MgO catalysts are much more active, under similar reaction conditions, than the analogous catalysts prepared by the impregnation procedures reported in literature [ 10]. In this work, the sol-gel preparations of monometallic supported systems Ru/Nb2Os, RJl/Nb205, Ru/Ai203, Pd/Al203 and bimetallic PdRu/Al203 are described. The catalytic behaviour of these systems in the NO removal reaction is preliminarly tested.

2. EXPERIMENTAL 2.1. Sol-gel preparation of the catalysts

2.1.1 Preparation of monometallic supported systems 5% Ru/Nb205 and 5% Rh/Nb205 Two 5% Ru/Nb205 catalysts were prepared starting from Ru3(CO)12 and Nb(OR)5 (R = CH(CH3)C2Hs, CH(CH3)CH(CH3)2). To a stirred solution of Ru3(CO)12 in THF (0.18 g in 150 ml), a solution of Nb precursor in ethanol, then a mixture water/ethanol were added dropwise. In both cases, a stoichiometric amount of water was used and a stable orange sol was obtained, which turned into gel within 10 days. The 5% Rh/Nb205 catalyst was prepared from Rh(acac)(CO)2 and Nb(O(CH3)CHC2Hs)5. To a stirred solution of the Rh precursor in ethanol (0.13 g in 50 ml), a solution of Nb sec-butoxide in ethanol, then a mixture water/ethanol were added dropwise. A stoichiometric amount of water was used and a stable orange sol was obtained, which turned into gel within 2 days. The obtained xerogels were activated at 300~ in pure H2, for 2 h. Ru3(CO)12 and Rh(acac)(CO)2 were pure commercial products (Aldrich) and were used as received. Nb(O(CH3)CHC2Hs)5 and Nb(O(CH3)CHCH(CH3)2)5 were prepared from NbCI5 (Aldrich) and the corresponding alcohol. All the preparations were carried out under inert atmosphere using a suitable glove-box and Schlenk glassware.

2.1.2 Preparation of monometallic 1% Ru/A1203 and 1% Pd/A1203 and bimetallic 1% PdRu/A1203 supported systems The 1% Ru/AI203 catalyst was prepared starting from Ru3(CO)12 and AI(O(CH3)CHC2Hs)3. To a stirred solution of Ru3(CO)12 in THF (0.06 g in 30 ml), a solution of A1 sec-butoxide in ethanol, then a mixture water/ethanol were added dropwise.

671 A large excess of water with respect to the stoichiometric amount (5:1) was used to promote the hydrolysis of A1 sec-butoxide and a stable orange sol was obtained, which turned into gel within few hours. The 1% Pd/A1203 catalyst was prepared from Pd(acac)2 and AI(O(CH3)CHC2Hs)3. To a stirred solution of Pd(acac)2 in ethanol (0.06 g in 20 ml), a solution of A1 sec-butoxide in ethanol, then a mixture water/ethanol were added dropwise. A large excess of water with respect to the stoichiometric amount (5:1) was used to promote the hydrolysis of A1 sec-butoxide and a stable yellow sol was obtained, which turned into gel within few hours. The bimetallic 1% PdRu/AI203 catalyst was prepared from Ru3(CO)12, Pd(acac)2 and AI(O(CH3)CHC2Hs)3, the total metal loading being always 1% by weight. To a stirred solution of Ru3(CO)12 in THF (0.01 g in 15 ml), a solution of Pd(acac)2 in ethanol (0.015 g in 10 ml), a solution of A1 sec-butoxide in ethanol, then a mixture water/ethanol were added dropwise. A large excess of water with respect to the stoichiometric amount (5:1) was used to promote the hydrolysis of AI sec-butoxide. A stable yellow sol was obtained, which turned into gel within few hours. The obtained xerogels were activated at 300-350~ in pure H2, for 5 h. Ru3(CO)12, Pd(acac)2 and AI(O(CH3)CHC2Hs)3 were pure commercial products (Aldrich) and were used as received. All the preparations were carried out under inert atmosphere.

2.2. Catalysts characterization The metal supported catalysts were characterized by FTIR spectroscopy, TEM analyses, surface area and metal dispersion determinations. The 5% Ru/Nb205 catalysts, discharged after the activity tests, were subjected to pulse NO chemisorption in a ZnSe DRIFTS cell at 423 and 523 K. FTIR and DRIFTS spectra in the range 4000-400 crnl were recorded by using a Nicolet 5PC spectrophotometer. BET surface areas were measured with a Micromeritics ASAP 2000 analyser using nitrogen at 77K. The Ru dispersion measurements were carried out with a Micromeritics ASAP 2000 analyzer using a pulse-flow chemisorption system in which, following the pretreatment to obtain the reduced catalyst, H2 was adsorbed on the catalyst surface at the temperature of 100~ by pulse injections into the carrier gas upstream. The amount of adsorbed H2 was directly related to the amount of active sites according to the stoichiometric ratio Ru/H 1:1. TEM images were obtained by a Jeol JEM 2000EX instrument, at the MASPEC Institute (CNR, Parma, Italy). 2.3. Catalytic tests The systems 5% Ru/Nb205 and 5% Rh/Nb205 were tested as catalysts for the NO reduction with H2 and CO as reductants. The reaction was conducted in a microreactor, at atmospheric pressure, temperatures from 25~ to 250~ and space velocity of 25 ml/min geat. In a first series of tests, the gas feed composition was 50 % NO and 50% H2, in a second one 50% NO and 50% CO. The systems 1% Ru/A1203 , 1% Pd/A1203 and 1% (Pd+Ru)/A1203 were also tested as catalysts for the NO reduction with H2, CO and propene as reductants, but different operating conditions were adopted. The reaction was conducted in a microreactor, at atmospheric pressure, temperatures from 75~ to 350~ and space velocity of 100 ml/min geat. In a first series of tests, a large excess of reductant was used, the gas feed composition

672 being 3% NO, 9% reductant and 88% He. Then the NO/reductant ratio was increased from 1:3 to 2:3, with a gas feed composition of 6% NO, 9% reductant and 85% He. Finally the NO/reductant ratio was adjusted to 1"1, with a gas feed composition of 9% NO, 9% reductant and 82% He. 3. RESULTS AND DISCUSSION

The xerogels obtained from Ru3(CO)12 or Rh(acac)(CO)2 and Nb precursors were amorphous solids, all characterized by a broad band at 3400 cm1 assigned to the stretchin~ vibration of O-H bonds of water and alcohol entrapped molecules, a band at 1650 cm-1 related to the O-H bending vibration, and a broad band in the region 900-500 cm1 due to Nb-O-Nb and Nb=O stretching vibrations. In addition, twin carbonyl bands, at 2062 and 1996 crn~, were observed in the spectrum of the Ru containing gels. Twin carbonyls bands have been reported by many authors [11 ], being attributable to monomeric [Ru(CO)2] 2+ or polymeric [Ru(CO)E]n m+ species grafted to the support matrix, the latter containing Ru-Ru bonds. In the spectrum of the Rh containing gel, the bands due to the carbonyl ligands were not detected, while two bands at 1557 and 1530 cm-1, related to the carbonyl function in the acetylacetonate group, were observed. After the activation treatment, the bands related to the metal precursors disappeared in the FTIR spectra of all samples, confuaning that the metal precursor had been completely destroyed. TEM images recorded on the activated Rh containing catalysts showed the presence of metal particles on the support matrix either as small and homogeneously dispersed particles or as large aggregates (Fig. 1) and evidenced the polymorph nature of the support, microporous and substantially amorphous in some zones (Fig. 2), more defined in others (Fig. 3). The polymorph nature of niobia was confirmed by XRD analysis. The XRD pattern showed the diffraction peaks characteristic of the monoclinic TT niobia phase together with an amorphous halo (Fig. 4). The pulse NO chemisorption analysis performed at 423 and 573 K, in the DRIFTS cell, on the 5% Ru/Nb20 5 catalysts discharged after the activity tests, showed that the catalysts were able to bind NO. A band attributable to the nitrite group M-O-N=O was in fact observed at about 1400 cm-1. The intensity of this band progressively increased by increasing the chemisorption temperature. According to the literature [12], it can be suggested that the metal-coordinated NO could be easily oxidised to give the nitrite species reported above by the niobia surface oxygen atoms. The three supported systems described above gave quite good results in the NO removal catalytic tests. In the presence of H2 as reductant, the two 5% Ru/Nb205 catalysts converted totally NO to N2 with 100% selectivity at 225-250~ (Fig. 5), while the 5% Rh/Nb205 catalyst was even more active (total NO conversion at 200~ but less selective since high amount of ammonia together with N2 were detected in the reaction products (Fig. 6). In the presence of CO, the 5% Ru/Nb205 catalyst converted totally the gas feed to CO2 and N2 at 240~ (Fig. 7), while the 5% Rh/Nb205 was less active and less selective giving a total conversion of the gas feed only at 300~ and producing high amount of N20 together with N2 (Fig. 8).

673

Fig. 2. TEM image of the 5% Rh/Nb205 catalyst after the activation treatment, evidencing microporous zones of the niobia support. [ 1cm = 20nm]

Fig. 3. TEM image of the 5% Rh/Nb205 catalyst after the activation treatment, evidencing the polymorph nature of the niobia support with the presence of more defined zones. [ 1cm = 50nm]

674

1600 1400 r

g

................................................

1200 1000 8OO

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0 O 0 to

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0 O vO)

0 O 0 ~1"

0 O O) O)

0 C) CO r

0 O r... o)

0 O tD r

0 O it) oO

2teta Fig. 4. X R D p a t t e m o f the 5% Rh/Nb205 catalyst, containing the monoclinic TT-Nb205 phase. 2500000

1200000

~ o

,

1000000

~. 2(X)0(X)O -

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1500000-

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r~ r~

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200

225

~

~

9

ltXX)tX)0 500000

~ 25

250

125 135 150 175 185 200

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--a~N2

Fig. 5. N O removal by reduction with H2. Catalyst: 5% Ru/Nb205 prepared from R u 3 ( C O ) I 2 and N b ( O ( C H 3 ) C H C 2 H s ) 5 . 5000000

* NO + N 2

Fig. 6. N O removal by reduction with H2. Catalyst: 5% Rh/Nb205 prepared from R u 3 ( C O ) 1 2 and N b ( O ( C H 3 ) C H C 2 H s ) 5 . 50000(10

-

~" 4000000 o 3000000 o 2000000 ~ 1000000 r~ 0 150 200 210 227 233 240 250 260 T(*C) r NO+CO + N 2 0 -" CO2 • N2

"-- NH3

":'- 4120(0)00 r~ o

30000~ 200O(0 ltX)oooo o

............. 13o 17o 200 215 230 245 265 300 T(~ r NO+CO -- N20 A CO2 • N2

Fig. 7. N O removal by reduction with CO. Fig. 8. N O removal by reduction with CO. Catalyst: 5% Ru/Nb205 prepared from Catalyst: 5% Rh/Nb205 prepared from Ru3(CO)12 and Nb(O(CH3)CHCH(CH3)2)5. Ru3(CO)12 and Nb(O(CH3)CHC2Hs)5.

675 The results of the measurements of surface area and metal dispersion on the catalysts obtained from Ru3(CO)12, Pd(acac)(CO)2 and the A1 precursor are reported in Table 1. The three systems were highly active in the NO removal reaction. Figs. 9-11 compare the catalytic performances of the three catalysts as a function of the reaction temperature, respectively in the presence of H2, CO or propene in large excess. Table 1 Characterization data for the 1% Ru/AI203, 1% Pd/AI203 and 1% Pd-Ru/AI203 catalysts Catalyst Surface Area (m2/g) Dispersion (%) 1% Ru/A1203 300 78.8 1% Pd/ml203

320

79.0

1% Ru-Pd/A1203

280

73.7

The 1% Pd/A1203 showed the highest activity in the presence of H2 as reductant, converting totally NO to N2 (100% selectivity) at 50~ The 1% Ru/A1203 catalyst showed the lowest activity converting totally NO to N2 (100% selectivity) at about 200-220~ The bimetallic system 1% PdRu/A1203 showed an intermediate activity behaviour, converting totally NO to N2 (selectivity 100%) at 125~ The same catalyst was the most active in the presence of CO as reductant, giving a total NO conversion to N2 (selectivity 100%) at 175~ with NO/CO ratio 1:3. 100

"~" 800 . ....~

~

60-

=

40-

O

J

t,)

0 Z

200

0 100 T(~ - 1% Ru/Al203 + A 1% Pd/Al203

200 300 1% Pd-Ru/AI203

Fig. 9. NO removal by reduction with H2 in large excess with respect to the stoichiometric amount. Catalysts: 1% Ru/A1203 (11), 1% Pd/A1203 (A) and 1% Pd-Ru/A1203 (0). The NO reduction by propene required in general higher reaction temperatures than NO reduction by HE or CO. NO was totally converted to N2 (100% selectivity) at 260-280~ by all the three catalysts, the 1% Pd/AI203 showing a somewhat higher activity than 1% Ru/A1203 and 1% Pd-Ru/A1203 catalysts at lower temperatures.

676 ,.100 "~

80-

o

~ o r

9 Z

6O40-

200

|

70

120 T (~

I

I

170

220

1%Pd-Ru/A1203

-- 1 % R u / A 1 2 0 3

--*- 1% P d / A 1 2 0 3 Fig. 10. NO removal by reduction with CO in large excess with respect to the stoichiometric amount. Catalysts: 1% Ru/A1203 (11), 1% Pd/A1203 (A) and 1% Pd-Ru/A1203 (*).

,-, 1 0 0 "" 0

9~

80-

60 40-

0 Z

200

....

I

I

I

I

100 150 200 T o 250 300 - ~ - 1% PdRu/AI203 (_~C)1% Ru/AI203 A 1% Pd/AI203 Fig. 11. NO removal by reduction with propene in large excess with respect to the stoichiometric amount. Catal: 1% Ru/AI/O3 (11), 1% Pd/A1203 (A) and 1% Pd-Ru/AI203 *). 5. CONCLUSIONS In this work, transition metal (Ru, Rh, Pd) supported systems were prepared by applying sol-gel techniques. High metal dispersions were obtained, particularly with the alumina supported ones. The prepared systems were tested as catalysts in the NO removal reaction by reduction. Hydrogen, CO and propene were the reductants adopted. The best results were obtained with the monometallic 1% Pd/A1203 and bimetallic 1% Pd-Ru/Al203 catalysts. REFERENCES

1. F. Gelman, D. Avnir, H. Schumann and J. Blurn, J. Mol. Cat. A, 171 (2001) 191. 2. J. Blmn, D. Avnir and H. Schumann, Chemtech, 29 (1999) 32.

677 3. A.J. Sander, L.A. van der Veen, J.N.H. Reek, P.C.J. Kamer, M. Lutz, A.L. Spek and P.W.N.M. van Leeuwen, Angew. Chem. Int. Ed., 38 (1999) 509. 4. F. Di Silvestri, P. Moggi and G. Predieri, Stud. Surf. Sci. Catal., 118 (1998) 219. 5. P. Moggi, G. Predieri, F. Di Silvestri and A. Ferretti, Appl. Catal., 182 (1999) 257. 6. E. Iglesia, S. L. Soled, R. A. Fiato and G. H. Via, J. Catal,. 143 (1993) 345. 7. B. Botti, D. Cauzzi, P. Moggi, G. Predieri and R. Zanoni, Stud. Surf. Sci. Catal., 130 (2000) 1091. 8. C. L. Bianchi, F. Martini and P. Moggi, Catal. Lett., 76 (2001) 65. 9. P. Moggi, G. Predieri and A. Maione, Catal. Lett., in press. 10. F. Rosowski, A. Hornung, O. Hinrichsen, D. Herein, M. Muhler and G. Ertl, Appl. Catal., 151 (1997) 443. 11. K. Asakura, K.-K Bando and Y. Iwasawa, J. Chem. Soc., Faraday Trans., 86 (1990) 2645. 12. N. Nakamoto, Infrared Spectra of Inorganic and Coordinated Compounds, J. Wiley & Son, Inc., New York, (1970) 160.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

679

Development of novel heterogeneous catalysts for oxidative reactions" preparation and performance of Co-N~ catalysts in partial oxidation of nbutane and toluene M.L.Kaliya, S.B. Kogan, N. Froumin, M.Herskowitz Blechner Center for Industrial Catalysis and Process Development, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel, e-mail [email protected], Fax 972-86477-678 CoNx on alumina catalyst yielded high performance in the oxidative dehydrogenation of n-butane. 47.6 mol% yield of olefins at 82% butane conversion was reached at 600~ Most of olefins were ethylene and propylene formed via route of oxidative cracking of n-butane. Catalyst was prepared by deposition of sodium salt of octa (4,5-carboxy) cobalt phthalocyanine (NaCoPc) followed by calcination in He at 700~ Decomposition of NaCoPc in inert medium results in formation of Co-Nx structure that was stable in oxidative processes. Comparison of performances on the novel catalyst CoO/A1203 and traditional metal oxides catalysts shows that the novel catalytic system is significantly more active in production of light olefins particularly at low temperatures (400-450~ XRD and XPS studies of the novel system indicate an essential modification of the active component by nitrogen. 1. INTRODUCTION Demand for light olefins, ethylene and propylene, is increasing gradually. The search for novel effective catalysts for oxidative dehydrogenation of hydrocarbons has generated a large number of studies. The yield of olefins was limited to <30% [1-3]. Improvement of oxidative processes such as drastically decreasing of contact time to 2-5 milliseconds at 800-850~ [4], lowering of oxygen partial pressure by multiple feeding [5] or using of membranes [6,7] have also been proposed. The challenges in catalytic oxidation of hydrocarbons are summarized in large body of articles and reviews [1-8,1320] concluding actually that high yield of light olefins have been reached only at high temperatures, >630~ where oxidative dehydrogenation is accompanying by oxidative cracking of paraffins. However, under these conditions, most catalysts are unstable. Our recent studies of cobalt oxides catalyst deposited on y-alumina indicate unexpected activity of Co-O/A1203 in oxidative cracking of n-butane at low temperature. High yield of olefins 30% (mostly ethylene and propylene) was reached at 550~ and WHSV - 4.5h 1 [8]. Furthermore varying of oxygen concentration affects olefins distribution. Such essential features of cobalt oxide catalyst render it interesting as a base for catalyst development.

680 Cobalt porphyrins and phthalocyanines were used as precursors for preparation of carbon supported cobalt catalysts that displayed high activity in reduction of oxygen in fuel cells [9-12]. These samples were prepared by deposition of cobalt phtalocyanine on active carbon following by heat treatment at 650-700~ in an inert atmosphere providing formation of Co-Nx structure deposited on active carbon. The object of the present study was the application of this approach to the preparation of heterogeneous oxidative catalysts using cobalt phtalocyanine as precursor of cobalt deposited on y-alumina and silica that are traditional supports for oxidative catalysts, and test of the catalysts in partial oxidation of n-butane and toluene. 2. E X P E R I M E N T A L The experimental rig consisted of a stainless tubular preheater and reactor in series, 17 mm ID and 250 mm long with 5.5 mm axial thermowell. Electric tapes controlled by Eurotherm heated the reactor and preheater. 1-4 g of catalyst (extrudates 1.5 mm in diameter and 2-3 mm length) diluted with 4-8 g of inert SiC pellets to keep the reactor isothermal, was located between two layers of SiC particles of 3-4 mm in diameter. The preheater, operated at 300~ was filled with SiC. n-Butane, oxygen and nitrogen (>99% purity purchased by "Maxima LTD") were fed from cylinders. Brooks mass-flow meters controlled the gas flow rate. Liquid was condensed in a trap at 3~ and gases flowed to GC for analysis. The analysis of volatile reaction products in butane oxidation was performed on line with GC HP-5890 that contained four columns: 45/60 molecular sieve 13X, 10 ft x 1/8"; A1203,50 m x 0.53 mm; 80/100 Hysep Q, 4 ft x 1/8" and 1 ft x 1/8" with internal switching valves and two detectors TCD and FID controlled by ChemStation analytical software. Analysis of water collected in the condenser displayed only traces of oxygenates. Blank experiment without a catalyst indicated conversion less than 1% at 550~ [5,8]. HPLC analysis was used for the analysis of liquid products of toluene oxidation. Toluene, benzaldehyde and bezoic acid were analyzed on HP-1050, column C-18, Zorbax, feed of eluent containing 60 and 40% of acetonitrile and water was 1 ml/min, ~, = 254 nm. Two supports, u (Norton 6175) and SiO2 (PQ 1030) were selected for catalyst preparation. Water-soluble sodium salt of (4,5-carboxy) cobalt phthalocyanine (NaCoPc) was kindly prepared by Prof. E. Lukyanetz (Organic Intermediates and Dyes Institute, Moscow). Catalysts were prepared by impregnation of the supports in a rotavapor with an aqueous solution containing NaCoPc, 4 g/L at pH 8.3 (NaOH, NH4OH). The catalysts were dried for 3 hr at 100~ and calcined at 700~ for 5 h in the stream of He. Standard cobalt oxide catalyst was prepared by impregnation of y-A1203 with an aqueous solution of cobalt nitrate hexahydrate (Aldrich) followed by drying at 100~ 3 h and calcination at 550~ 6 h. V-Mg-O catalyst was prepared according to the procedure reported elsewhere [ 17]. Analysis of the catalysts is given in Table 1. SEM, XRD, XPS and BET methods were used for catalyst characterization [8].

681 Table 1 L Catalyst characterization Catalyst [ Support Co no.

Content, wt % N Na V

1 dried 1 calcined 1 tested 2 dried

A1203

04s

08

A1203

2tested

A1203

(.4 (.4 (.8 (.9 (.7

06 06 12 11

(.7 2.3

05

4

SiO2 SiO2 Mg M~O *) 3-6- calcined

5.6 0.6 0.5 10. 0.8

Surface area m2/g

Pore volume cc/g

Average D pore nm

199

0.63

13.0

191 190 343 330 45

0.69 0.65 0.46 0.45 -

14.5 13.5 11.5 10.7 -

3. RESULTS AND DISCUSSION

Catalysts 1 and 2, that are 0.4 and 0.8 % CoNx/y-ml203,indicate similarly high performances in oxidative dehydrogenation of n-butane. Even at 400~ the conversion exceeded 40 % (Fig. 1) and yield of olefins reached 25 %. The special feature of this catalyst consists of a high conversion of n-butane yielding mostly light olefins, ethylene and propylene. 90 wt% of olefins formed at 400~ and molar ratio O2/n-butane of 1.5, were C2-C3 olefins;53 wt.% was ethylene and the rest propylene. Therefore, the selectivity to ethylene on the novel catalysts was much higher than over traditional Co-O/A1203 (Fig. 2). Experimental data and comparison of the performance and products selectivity of all the catalysts studied are given in Table 2. Although the highest total olefins selectivity and highest selectivity toward butylenes and butadiene was obtained over the traditional vanadia-magnesia catalysts no. 6, conversion of n-butane and yield of light olefins were significantly higher over samples 1 and 2. It is interesting that total olefin selectivity with promoted samples 1 and 2 displayed little change with an increase of temperature, and was close to 58% at 400-600~ (Fig. 1) while the selectivity on traditional systems is usually high at low temperature and low at high temperatures. Increasing of temperature in the range 400-600~ (Figs. 1,2) significantly affected n-butane conversion which reached 82% at 600~ In spite of the comparatively low value of total selectivity at 58 wt.%, yield of olefins over sample 2 peaks at 47.6 wt.%, with yield of ethylene of 31 wt.%. The specific performance of nitrogen containing samples 1 and 2 was compared with results described in literature for oxidative dehydrogenation of n-butane. A summary of the catalytic data is given in Fig. 3. Most catalysts, based on vanadia, yielded optimal performance toward olefins in the range 520-560~ and yielded preferably butylenes. A general feature of most catalytic systems is the low selectivity to light olefins resulting in yield of ethylene and propylene <10%. Only catalyst Zr-Dy-Li-CI-O displayed higher performance toward ethylene - 18%, but at 650~ Even steam cracking of n-butane at 750~ yielded only 37% of total olefins and 15.3% of ethylene. The performance of the

682 novel catalyst at 600~ is comparable to that of catalyst Pt/AI203, measured at 850~ and contact time 2-5 milliseconds [4]. Table 2 Dehydrogenation of n-butane at T = 550~ WHSV = 5 h1 Selectivity *), wt.% Catalyst O2/C4 ConverTotal no. mol. sion selectivity C2H4 C3H6 C4H8 CO2**) CO **) % wt.% 5.4 1. 0.5 22.3 67.3 8.4 21.7 28.6 17.0 14.9 64.2 12.3 29.4 25.0 9.8 14.6 6.0 1.0 47.4 24.0 34.6 14.8 5.6 13.2 1.5 71.5 55.0 5.5 2. 0.5 24.0 9.6 23.8 26.6 16.3 13.7 4.9 66.7 1.0 49.3 65.7 14.9 32.3 23.8 9.6 12.5 3.7 24.4 36.0 15.9 5.4 12.4 57.3 3.4 1.5 73.7 64.7 1.8 9.9 8.8 46.0 17.6 13.3 3. 0.5 13.3 13.8 27.2 16.2 15.9 14.9 10.2 1.0 31.9 59.3 1.5 51.2 52.8 19.5 31.2 13.3 8.3 18.1 8.1 4. 1.0 24.6 71.1 4.6 21.6 21.2 28.3 14.9 7.9 ,

.

.

.

.

.

.

.

.

.

.

,

5.

1.0

19.3

75.5

4.4

8.1

8.0

59.4

1~2.5.

6.2

6.

1.0

32.2

76.1

3.8

16.1

7.7

52.3

11.2

7.4

*) not contained traces of ethane and propane, **) as carbon. The increase of oxygen to n-butane molar ratio from 0.5 to 1.5 on the novel catalyst strongly increases the conversion of n-butane while it slightly depresses the olefins selectivity. Therefore, the highest yields were obtained at O2/butane molar ratio of 1.5. A further increase of the oxygen lowered the total selectivity below 55%. No deactivation over 35 h at 400-600~ was found. The catalyst is characterized by a coking of < 0.1 wt.%. Replacing alumina with silica in the preparation of CoNx catalysts yielded a decrease of the performance. Nevertheless, comparing catalysts 4 and 5 prepared with the novel and the traditional Co precursors (NaCoPc and cobalt nitrate) indicates that the selectivity to ethylene and propylene was significantly higher with catalyst 4, which was similar to the alumina-based samples (Table 2). A similar decrease of the performance of cobalt-supported catalysts was observed previously [8]. Runs of toluene oxidation depicted in Table 3 also indicate improved activity of catalyst 1. Similar conversion of toluene on promoted and non-promoted catalysts 1 and 3 was reached at 400~ and 500~ accordingly. Selectivity to benzaldehyde was also higher on the promoted catalytic system; however, the total performance of this catalyst was lower than results reported for the vanadia based catalysts [13]. Adsorption of oxygen with generation of active species of catalyst is a key factor in oxidative dehydrogenation and oxidative cracking of paraffins. Characterization of the catalysts supports their difference in performance:

683 1. SEM analysis of samples 1 and 2 before and after treatment at 700~ in a stream of helium indicates that 90% of organic skeleton of NaCoPc being initially deposited on alumina was decomposed and removed from the catalyst, while 90% of the initial amount of cobalt and 75% of the nitrogen remained in the catalyst (Table 1). Moreover, further operation in oxidation of n-butane in the range 400-600~ and at high partial pressure of oxygen had a limited effect on catalyst composition. Therefore, deposition of NaCoPc on alumina followed by its high temperature decomposition in inert medium yielded active species that involved cobalt and nitrogen. Such complex was sufficiently stable in oxidative processes. Atomic ratio between nitrogen and cobalt in those samples was around 6, while reported value was 4 for carbon-supported samples that were used in electrochemical cells [9-12]. Table 3 Performance of catalyst no. 1 in oxidation of toluene WHSV Temperature Toluene Selectivity to h "1 ~ conversion benzaldehyde % % wt 2.5 320 0.2 98 2.5 350 0.5 95 2.5 370 0.9 93 2.5 400 1.1 90 0.5 400 6.4 62 0.3 400 11.5 43 2.5 500 0.8 84 0.5 500 3.6 41 Content of toluene and oxygen 0.8 and 0.1 atm. , ,

2. XRD of sample 2 indicates no crystal phase that relates to interaction between cobalt and nitrogen (Fig. 4). Although calcination temperatures were much higher than for sample 3, the sharp peaks referred to cobalt oxides (2 =19.0, 31.25 and 36.84 ~ present in the plot of sample 3 prepared with cobalt nitrate precursor are not found in sample 2. It demonstrates that promoting by nitrogen results in higher dispersion of cobalt in sample 2. 3. XPS study (Fig. 5) indicates a gradual shill of Co (2p) maximum from 780 eV (COO) to lower energy that is expected for interaction of cobalt with less than oxygen electron-accepting elements like CoB (B.E.=778.0 eV) and CoP (B.E.=778.4 eV) [21]. Moreover significant broadening of Co peaks of samples 1 and 2 compared with sample 3 shows existence of several electron states (Fig. 5a). Concerning oxygen the identical de-convolution of peak O ls was obtained for promoted and non-promoted samples 1-3 (peak of B.E=531 eV is referred to alumina). Enhancement of mobile (low energy) oxygen share for nitrogen containing samples 1 and 2 compared to sample 3. (Fig. 5b, Table 4) agrees with accelerating of destructive activity [22] of novel catalysts.

684 'it30 ......................................................................................................

40 -~..................................................................................... 1A

.

35":

~

3O":

>; 25i

o r

->- 20 3 5

2,4 - cataqst 3 1,2 - Conve.'~iOD

5!

3 , 4 - select;vit>,

,. . 3 , 4 - propy~ene 5 , 6 - CO-.+CO

1.3,5 - cataJyst 2 2.4.6 - cata~,yst 3

4.

0

37,5

425

475

-~-'a5

575

375

625

Temperature:OC

425

4.7 ro.

Fig. 1. Effect of temperature on catalysts performance

575

525

T e m ~ r a t u r e ,

*C

Fig. 2. Selectivity to ethylene propylene and carbon oxides

Table 4 XPS data on oxygen species of Co/AI203 Catalyst no.

1 2 3

Cobalt precursor

Oxygen (O l s) distribution (%) depending on B.E. (eV) 530-530.2 532.1-532.4 58 42 59 41 43 57

NaCoPc NaCoPc

Cobalt nitrate

800-

._~00

\3

r

~oo 200

lo 2 o 3 o 4 o 5 o 6 o 2O

Fig.4. XRD analysis of catalysts 1- pure 7- A1203; ~ phases related to cobalt oxides; 2, 3 - samples 1 and 3.

625

685

b

2

1

800 796 792 788 784 780 776 Binding Energy, eV

538 536 534 532 530 528 526 Binding Energy, eV

Fig. 5. XPS plots, a- cobalt, b - oxygen. 1,2,3 - catalysts numbers in table 1.

REFERENCES

1. F.Cavani and F.Trifiro, Catal. Today, 24 (1995) 307. 2. F.Cavani and F.Trifiro, Catal. Today, 51 (1999) 561. 3. F. Cavani and F. Trifiro, Catal. Today, 36 (1997) 431. 4. M. Huff and L. Schmidt, J. Catal., 149, 1 (1994) 127. 5. M.L. Kaliya, O.V.Malinovskaya, M.V. Landau, M. Herskowitz and P.V. Oosterkamp, Studies in Surface Science and Catalysis, 133 (2001) 113. 6. A. Pantazidis, J. A. Dalmon and C. Mirodatos, Catal. Today, 25 (1995) 403. 7. G. Capannelli, E. Carosini, F. Cavani, O. Monticelli and F.Trif~o, Chem. Eng. Sci., 51, 10 (1996) 1817. 8. S.B. Kogan, M.L. Kaliya, N. Froumin and M. Herskowitz, Proceedings of the DGMK-Conference, "Creating Value from Light O l e f i n s - Production and Conversion", 2001, Hamburg, 219. 9. H. Jahnke, M. Schoenborn and G. Zimmermann, Katal. Phthalocyaninen, Symp. (1973), 71. 10. M. Labouceur, G. Lalande, D. Guay and J. P. Dodelet, J. Electroehem. Soc., 140, 7 (1993) 1974. 11. K. Wiesener, Electrochim. Acta, 31 (1986) 1073. 12. J.A.R. van Veen, H.A. Colijn and J.F. van Baar, Electrochim. Acta, 33, 6 (1988) 801.

686 13. S. Larrondo, A. Barbaro, B. Irigoyen and N. Amadeo, Catal. Today, 64, 3-4 (2001), 179. 14. T. Blasco, J. M. Lopez Nieto, A. Dejoz and M. I. Vazquez, J. Catal., 157 (1995) 271. 15..A. Lemonidou, Appl. Catal., A, 216 (2001) 277. 16. C. Tellez, M. Menendez and J. Santamaria, J. Catal.,183 (1999) 210. 17. M. A. Chaar, D. Patel, M.C. Kung and H. H. Kung, J. Catal., 105 (1987) 483. 19. J.M. Nieto, P. Conception, A. Dejoz, F. Melo, H. KnSzinger and M. I. Vazquez, Catal. Today, 61 (2000) 361. 20. L. Owens and H. H. Kung, J.Catal., 144 (1993) 202. 21. NIST X- Ray Photoelectron Spectroscopy Database, version 2.0, US Department of Commerce, National Institute of Standard and Technology.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

687

Synthesis and modification of basic mesoporous materials for the selective etherification of glycerol. J-M. Clacens**, Y. Pouilloux and J. Barrault Laboratoire de Catalyse en Chimie Organique, CNRS - UMR 6503, ESIP - 40, Avenue du Recteur Pineau, 86022 Poitiers cedex, France. The glycerol etherification was studied in the presence of base modified (impregnated or exchanged) mesoporous catalysts (MCM-41 type). The catalytic results obtained show that the impregnation method gives the most important activity, which must be correlated to an important active species incorporation. Concerning the selectivity, the best values to (di- + tri-) glycerol are obtained over mesoporous solids modified by caesium impregnation or exchange. The re-use of such caesium impregnated catalysts doesn't affect the selectivity to the (di- + tri-) glycerol fraction. Nevertheless, the impregnation methods lead to a leaching of the active species during the reaction which doesn't occur using the exchange method. In the presence of lanthanum or magnesium containing catalysts, the glycerol dehydration to acrolein is very significant whereas this unwanted product is not formed when caesium is used as basic promoter. I. INTRODUCTION In the general context of the development of the use of agricultural products for non food applications, and particularly in the field of the glycerol valorisation I1 ] (coproduct of the triglycerides hydrolysis or methanolysis process), the selective etherification of glycerol was studied. The objective of this work is the selective formation of linear di- and/or triglycerol by direct polymerisation of glycerol (stemming from vegetable oil) in the presence of basic mesoporous solid catalysts while avoiding acrolein formation (Fig. 1). The applications of these polyglycerols concern the fields of lubricants [2] and non-ionic surfactants [3] (from the synthesis of polyglycerol esters). For environmental reasons, it is now recommended to use solid catalysts instead of homogeneous ones to carry out that kind of reaction. The main difficulty encountered in that polar medium reaction is to find a stable basic catalyst. In a previous work, we have studied various modified zeolites and MCM-41 type mesoporous materials with different elements incorporated in their framework [4]. Results obtained with incorporated mesoporous catalysts were not really successful because the selectivity of the reaction was not better than that obtained with homogeneous catalysts. Present address : Institut de Reeherches sur la Catalyse, CNRS- UPR 5401, 2, Av. A. Einstein, 69626 Villeurbanne cedex, France.

688

When using exchanged and impregnated zeolites, the increase of selectivity was not important. It seemed that the reaction occurred mainly at the external surface of the catalyst meaning for example that the pore size of the modified zeolites was too small. It was one of the reasons for which the preparation of basic mesoporous materials was studied specially the exchange or the impregnation of basic elements. OH

Glycerol

OH

OH

Diglycerol + glycerol

I-!20

HaO

Triglycerol [Polyglycerols] Fig. 1. Etherification of glycerol. 2. E X P E R I M E N T A L 2.1. Preparation and characterisation of the catalysts Impregnated Mesoporous Materials are prepared as following: a certain amount (see notation) of alkali, alkaline earth or/and rare earth salt is added to 5 g of pure silica or aluminosilica mesoporous material, prepared (with cetyltrimethylammonium bromide) as previously described [4] and 50 g of methanol. The mixture is agitated at ambient temperature during 2 hours, the solvent is then rapidly evaporated under vacuum and the" solid is calcined under air at 450~ overnight at a heating rate of l~ Notation : Element(amount of elementimpregnated,"10-4 mole.g-1) AI(si/AIratio), example : CszsAlz0 or Csz5 if the mesoporous support does not contain aluminium. Exchanged Mesoporous Materials are prepared as following: to a solution containing 0.32 g of caesium nitrate in 150 ml of hexane, a suspension containing 2 g of pure silica MCM-41 in 50 ml of hexane is added. After 10 minutes of agitation under inert atmosphere (Ar) at ambient temperature, the mixture is kept 24 hours without agitation under Ar.The mixture is then filtered and the resulting solid is washed with hexane and calcined under air at 550~ (heating rate : 5~ Notation : CseSi-MCM-41 The other catalysts used have been described previously [4]. Some of the solids have been recovered after reaction by filtration using hot water and/or ethanol to dissolve the polyglycerols formed, and reused without reactivation (Notation : R after the catalyst code; example : CszsR). Characterisation of the solids were done using X-ray diffraction, N2 adsorption (BET method), transmission electronic microscopy and chemical analysis.

689

2.2. Reaction

Glycerol etherification is carried out at 260~ in a batch reactor at atmospheric pressure under N2 in the presence of 2 wt% of catalyst, water being eliminated and collected using a Dean-Stark system. Reagents and products are analysed with a GPC after silylation [5]. Batch processes are generally used in lipochemistry, especially for the esterification and transesterification reactions (except for the preparation of methyl esters). Different porous solids were used in these studies differing in their surface area, porosity as well as acido-basic properties. 3. RESULTS AND DISCUSSION 3.1. Impregnated mesoporous catalysts

From previous works reported in the literature [6-8] an impregnation procedure was proposed using acetate salts in methanol. As shown in Fig. 2, impregnation of caesium in the structure of the MCM-41 leads to the most selective catalyst; when compared with homogeneous Na2CO3, caesium exchanged X zeolite and lanthanum incorporated MCM-41 (which was the most active "incorporated" catalyst) are more active than Cs impregnated MCM-41. Caesium exchanged X zeolite is also selective but less selective than mesoporous materials due to a smaller size of its channels. I00

m

v

w

Diglycerol

so ~

6o

~

40

r~

20

~x 9 9 9 x

0

, 0

, , -

,,x ,,x,,x

20

Triglycerol i

-

40

Na2CO3 La incorporated MCM-41 Cs exchanged X zeolite Cs impregnated MCM-41

60

i

80

100

C o n v e r s i o n (%)

Fig. 2. Polyglycerols selectivity over various catalysts. By changing the impregnated element in the structure of MCM-41, we observe that caesium leads to the most selective catalyst to diglycerol while lanthanum leads to the less selective one (Fig. 3). Moreover if La and Mg lead to high activity, an important acrolein formation is also noticed so that the use of such catalysts must be avoided. The caesium additive was selected and Fig. 4 shows that the increase amount of Cs impregnated has also a positive effect on the activity of the solid. Physico-chemical characterisations (Tables 1 and 2) showed that the impregnation technique led to a decrease of the specific surface area (depending on the amount of

690 impregnated species), a loss of the typical hexagonal XRD structure, while the porous system was still observed by electron microscopy. So, this modification procedure changed the long range organisation of the framework but not the short range, which could explain why these solids kept a similar selectivity. But it is also important to report that a significant leaching of the active species during the reaction was observed. IOO 80

~9

rol

60

9 Li25A120 9 La25A120

4o

#

Cs25A12o

0 Na25A120 r~

20

•

Mg25A120

.t 9 9 1 4 9t.t; 9u t

0

20

40

60

Convesion

80

100

(%)

Fig. 3. Influence of the impregnated element on the selectivity to di- and triglycerol. 100 ,-,

80

ffi

60

* m A 0

O

o~ r~

40 o

Cs~ooA120 CssoA120 Cs25A120 Cs6A120

2O n

0

i

i

1

5

10

15

i

20

25

Temps (h) Fig. 4. Influence of the amount of impregnated caesium on the glycerol conversion. The use of pure silica or alumina-silica MCM-41 as support for Cs impregnation doesn't clearly influence the activity and the selectivity of the catalyst (Figs. 5 and 6). On the other hand, in both cases, the reuse of the catalysts slightly affects the activity while it does not change their selectivity. The decrease of activity can not be correlated with the Cs leaching (Table 2) and this leaching continues during the second use of the solid.

691 Table 1 Physico-chemical characterisations of different caesium loaded Catalysts BET surface area Amount of Cs (m2.g"1) (x 104mole.g 1) Before After Before After impregnation impregnation test test Cs6A12o 838 620 6,4 0,1 CsE5A12o 972 248 18,8 14,1 CssoAI2o 410 12 28,2 8,9 CslooA12o 410 5 40,7 12,0

catalysts X ray diffraction d* Before impregnatio n 35,7 38,6 36,2 36,2

After impregnation 33,8 37,7 (weak) no peak no peak

* d = 1.155 a0

I00 ,.~

8o

9 Cs25 9 Cs25A12o

60 o W~

40 o

O Cs25R

[] CSEsA120R

20 mr

0

F

I

I

10

20

30

T e m p s (h)

Fig. 5. Conversion of glycerol over catalysts containing or not aluminium; and their reuse. Indeed, the comparison of the selectivity of an usual homogeneous catalyst (Na2CO3) and of an impregnated mesoporous solid clearly showed an increase of the selectivity to diglycerol over the porous catalyst while polyglycerols with a polymerisation degree superior or equal to 4 decreased. The stability of that type of modified catalyst has to be increased because a large part of the impregnated caesium is leached during the reaction. Despite of this leaching and the lost of the XRD structure (Table 2), these catalysts remain selective. The reason is that the mesoporous structure is not completely suppressed as we can observe it by electron microscopy [8]. Another proof of the MCM-41 structural effect on the selectivity has also been recently reported by our group [8]; by adding successively pure MCM-41 and CsOH at the beginning of the reaction, we observed a migration of the Cs species inside the porous system, which gave a similar selectivity than that observed using Cs impregnated MCM-41.

692

~

----~----

I00 " 8O

g

erol

9 Cs25 9 CsEsA120 o Cs25R

"~ 60 9~

40

[] CSE5A120R

J~

20-

_ 0

~

y 20

c

e

r

40 Conversion

0 60

1 80

100

(%)

Fig. 6. Influence of the presence of aluminium in the structure and of the reuse of the catalysts on the selectivity to di- and triglycerol. Table 2 Physico-chemical characterisations of caesium impregnated catalysts and reused catalysts. Amount of Cs XRD Catalysts BET surface area (m2.g1) (x 10-4mole.gq) d (A) . . . . . . . . . . . . . . . . . . . . . . Before After Before After Before After impregnation impregnation test .... test impregnation impregnation Cs25 1031 137 17,3 11,4 39,5 37,0 (weak) Cs25A120 972 248 18,8 14,1 38,6 37,7 (weak) Cs25R 11,4 5,5 No peak CSE5A120R 14,1 4,8 No peak 3.2. E x c h a n g e d m e s o p o r o u s catalysts

The exchange procedure was done using a solution of caesium nitrate in absolute ethanol. The activity of obtained solids was lower than that of the impregnated samples, probably due to the lower number of active species (Tables 2 and 3). Concerning the selectivity (Fig. 7), the diglycerol fraction was lower over the exchanged solids but the selectivity to (di- + triglycerol) was rather similar to that of impregnated samples. Moreover, the stability of the active species in the porous structure was greatly improved because there was no caesium leaching during the reaction (Table 3). The physico-chemical and structural analyses of these exchanged catalysts didn't indicate any deterioration or collapsing of their framework.

693 lOO "

rol

80 ""

60

A Cs impregnated MCM-41 o Cs exchanged MCM-41

.~

9-

40

r~

20

T 0

40 Conversion

60

glycero, ,0

100

(%)

Fig. 7. Polyglycerols selectivity over Cs impregnated and exchanged MCM-41. Table 3 Physico-chemical characterisations of Cs impregnated and exchanged.MCM- 41 Catalysts BET surface area Amount of Cs XRD (m2.g1) (x 10.4 mole.g-i) d (A) Si-MCM-41 0 39,1 Cs~Si-MCM-41 744 7,6 32,9 Cs~Si-MCM-41R 7,6 No peak Cs2s support 1031 0 39,5 Cs2s 137 17,3 37,0 (weak) Cs25R 11,4 No peak 3.3. C h e m i o s e l e e t i v i t y

A comparison of the different isomers of diglycerols and triglycerols obtained with these modified MCM-41 type catalysts showed that their distribution was quite different to that observed with homogeneous systems. Indeed, two "typical" distributions appearing from the chromatographic peaks (Fig. 9) were observed using modified MCM-41 (a) and homogeneous Na2CO3 (b) (when compared at the same glycerol conversion). Table 4 Diglycerol isomers over homogeneous or mesoporous Catalysts Peak 1 Peak 2 (% of total (% of total area) area) 3,6 30,6 Na2CO3 20,4 50,4 CS25 15,1 44,8 Cs25R

catalysts Peak 3 (% of total area) 65,8 29,2 40,1

694 Such results strongly suggested that a chemioselective etherification of glycerol occurred in the porous structure of the catalyst which was not reported before.

Glycerol

a

b

Internal standard (methyl laurate) 2

Diglycerols 2 Triglycerols

,.~,~..

,

,

...4=,

,

,

,

'

Iv

1~

Q

0

!

|

u~.: ~

,

Tetraglycerols

|

,~::,

|

w

.

i

i

Time (minutes)'" Time (minutes) Fig. 9. Chromatograms of the polyglycerols mixtures after the use of Cs25 (a) and Na2CO3 (b). (C: cyclic diglycerol) 4. CONCLUSION From these new preparations, stable active and basic mesoporous catalysts were obtained for the chemioselective conversion of glycerol to linear di- and triglycerol. The impregnation method gives the most important activity, which must be correlated to an important active species incorporation. Concerning the selectivity, mesoporous solids modified by caesium impregnation or exchange lead to the best selectivity and yield to (di- + tri-) glycerol. The most stable catalysts are the exchanged ones but even if they are less stable, the impregnated catalysts can be re-used without major modification of their selectivity to the (di+ tri-) glycerol fraction. ACKNOWLEDGEMENTS The authors gratefully acknowledge support from the European Community "FAIR program". REFERENCES 1. A.J. Kaufman and R.J. Ruebush, Proceedings of the World Conference on Oleochemicals into 21 st Century, T-H- Applewhite Ed., American Oil Chemist Society (199 l) 10. 2. A. Demoulin, OCL, 2(4) (1995), 274. 3. K.S. Dobson, D.D. Williams and C.J. Boriack, J. Am. Oil. CherrL Soc., 70(11) (1993) 1089. 4. J-M. Clacens, Y. Pouilloux, J. Barrault, C. Linares and M. Goldwasser, Stud. Surf. Sci. Catal., 118 (1998).

695 5. M.R. Sahasrabudhe, J. Am. Oil. Chem. Soc., 44(7) (1967) 376. 6. K.R. Kloetstra and H. van Bekkurn, J. Chem. Soc., Chem. Commun., (1995) 1005. 7. K.R. Kloetstra, M. van Laren and H. van Bekkum, J. Chem. Sot., Faraday Trans., 93(6) (1997), 1211. 8. J-M. Clacens, Y. PouiUoux and J. Barrault, Appl. Catal. A, in press.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

697

Carbon nanotubes: a highly selective support for the C=C bond hydrogenation reaction J-P. Tessonnier, L. Pesant, C. Pham-Huu*, G. Ehret t and M.J. Ledoux Laboratoire des Mat6riaux, Surfaces et Proc6d6s pour la Catalyse (LMSPC), UMR 7515 du CNRS, ECPM, Universitd Louis Pasteur, 25 rue Becquerel, BP 08, 67087 Strasbourg Cedex 02, France. Tel: +33 3 90 24 26 75, Fax: +33 3 90 24 26 74, *e-mail: [email protected] f Groupe Surface Interface, Institut de Physique et Chimie des Mat6riaux de Strasbourg, UMR 7504 du CNRS, Universit6 Louis Pasteur, 23 rue du Loess, 67087 Strasbourg, France.

Palladium metal particles with an average diameter of ca. 5 nm were homogeneously dispersed inside carbon nanotubes. Such nanostructured material was an extremely active and selective catalyst for the hydrogenation of the C=C bond of cinnamaldehyde. Thehigh external surface area of the carbon nanotubes could explain the high reactivity of the catalyst despite its relatively low specific surface area, i.e. 20 mE.g1. On the other hand, the high selectivity towards the C=C bond hydrogenation was attributed to the absence of a microporous network and of residual acidic sites in the carbon nanotube catalyst as compared to a commercial activated charcoal. 1. INTRODUCTION Since their discovery in 1991 [1], carbon nanotubes have received great attention due to their unique chemical and physical properties which render them attractive in several potential applications [2]. Among them, the use of nanostructured carbon as catalyst support seems to be very promising according to the last results reported in the literature [3-5]. The carbon nanostructured support provides both a high activity and a high selectivity when compared to what is usually observed on traditional supports such as alumina or activated charcoal. Such catalytic behavior is attributed to the presence of a peculiar electronic interaction between the carbon nanofilaments and the metal which constitutes the active phase. This leads to a metallic site with unexpected catalytic performances [6,7]. In addition, due to their small dimensions, typically of about hundred of nanometers or less, the carbon nanofilaments display an extremely high external surface area which makes them a catalyst support of choice for liquid phase reactions. Due to the low diffusion coefficients of gaseous reactants in liquids, mass transfer phenomena become predominant in the liquid phase. The high external surface area considerably decreases the

698 problem of mass transfer between the gaseous reactants and the active sites. Finally, the complete absence of any porous structure in the carbon nanofilaments should be underlined in contrast to what is usually encountered with traditional porous supports. Such a characteristic allows the rapid desorption of the products from the active site without subsequent secondary reactions leading to the formation of unwanted products, which significantly contributes to the selectivity of the reaction. The aim of the present article is to report the use of carbon nanotubes as catalyst support for a palladium active phase in the selective C=C hydrogenation of cinnamaldehyde in liquid-phase. Such reaction is of interest especially in the fine chemical domain where specific hydrogenation is actively sought. The catalytic performance was evaluated by comparing the observed activity and selectivity with those of a commercial catalyst supported on a high surface area activated charcoal. The influence of the support morphology and microstructure on the hydrogenation activity and selectivity will also be discussed. 2.

EXPERIMENTAL SECTION

2.1. Support and catalyst Multi-walled carbon nanotubes (Pyrograff-III) were supplied by Applied Science Ltd (USA) and had a specific surface area of 20 m2.g1. The average tube inner diameter was in the range 60 to 100 nm with length up to several hundred micrometers. The carbon nanotubes (CNTs) were used as received without any pretreatment. The palladium active phase was deposited on the CNTs by incipient wetness impregnation using an aqueous solution of palladium nitrate. After impregnation the solid was dried at 100 ~ overnight and then calcined in air at 350 ~ for 2 h in order to decompose the palladium salt into its corresponding palladium oxide. The oxide was then reduced in flowing hydrogen at 400 ~ for 2 h and stored in an argon atmosphere before the catalytic use. 2.2. Catalytic test The hydrogenation of cinnamaldehyde was carried out in liquid phase under atmospheric pressure at 80 ~ in a stirred batch reactor with 10 ml of cinnamaldehyde and 0.1 g of catalyst (Pd loading 5 wt. %) in 50 ml of dioxane. The hydrogen (20 ml min1) was continuously fed into the reactor via a mass flow controller. The reaction was carded out under stirring of 500 rpm. The reaction was followed by withdrawing a liquid sample (0.2 ml) at regular intervals. The product distribution was obtained using a gas chromatograph (Varian 3800) equipped with a capillary column (Carbobond with 50 m length) and a FID detector. 2.3 Characterisation techniques The metal loading was measured by inductively coupled plasma-mass spectroscopy analysis. The micro structure and location of the metallic active phase were investigated on a Topcon EM002B transmission electron microscope working at 200 kV accelerating voltage and with a point-to-point resolution of 0.17 nm. The sample was prepared by

699 sonication in ethanol followed by deposition of a drop onto a holey carbon-coated cooper grid. 3.

RESULTS AND DISCUSSION

3.1. Catalyst characteristics Fig. 1 presents the TEM images of the palladium nanoparticles deposited in the CNTs. According to the TEM observations almost all the metal particles were located inside the CNTs with an average particle size centered at around 5 nm. It should be noted that some palladium particles were observed on the outer surface of the nanotube especially next to the tube tip. This could be explained by some of the palladium salt which was located next to the tube tips having been physically transported from the inner tubule to the outer surface during the evaporation process. Such a phenomenon was attributed to the existence of a capillary force which drove the liquid inside the tubule. Previous work has shown that liquids with low surface tension, i.e. < 190 mN m 1 [8] can fill and wet the nanotube. Water has a surface tension of 72 mN m1 and is expected to perfectly wet and fill the nanotube. Using such a method several groups have succeed in filling carbon nanotubes with various foreign elements [9-11]. During the thermal treatments the liquid was slowly evaporated leaving behind the metal salt which was subsequently reduced by hydrogen into its corresponding metal. Similar results have also been observed in the case of silicon carbide nanotubes for selective oxidation of residual Fig. 1. TEM images of the palladium deposited hydrogen sulfide in Claus tail-gases inside the carbon nanotube tubule. The [12]. Recently, Gogotsi et al. [13] have palladium was almost exclusively located reported an in situ observation under inside the tubule of the support and the average TEM irradiation of the behavior of the palladium particle size distribution was around 5nm. liquid water trapped inside carbon nanotubes with diameter similar to the diameter of the tubes of the present study. The authors have found that during the irradiation process the trapped water was rapidly spread along the inner wall of the tube which indicated an hydrophilic character of the inner wall.

700 The relatively high dispersion of the palladium particles inside the CNTs was attributed to the existence of a peculiar electronic interaction between the graphite layer which constituted the inner wall of the support material and the metal itself. The existence of a confinement effect induced by the tubular morphology of the support during the evaporation and decomposition process could also be partly responsible for this. During the heating process both the liquid and the gaseous (steam and nitrogen oxide formed by decomposition of the nitrate salt) products formed were in close contact while the overall temperature and pressure inside the tube were significantly raised, which could have nduced a peculiar crystaUisation of the salt precursor different to that obtained in an open system. High-resolution TEM observations (Fig. 2) have revealed that the metal particles were almost in a round shaped form which indicates the presence of a mild chemical interaction between the active phase and the support surface. Such an observation was consistent with the low reactivity of the graphite basal plane [ 14]. When palladium particles are supported on carbon nanofibers which expose only reactive prismatic planes, the metal particle morphology is highly faceted (not shown) due to the higher interaction with the graphite edges where a strong electron delocalisation occurs. Similar observations have also been reported by Baker's group over nickel supported on a graphite nanofilament support [6,7]: due to the strong interaction with the support the nickel was in a fiat form spread along the axis of the fiber. The palladium dispersion on the activated charcoal (AC) was somewhat different Fig. 2. High-resolution TEM images of the when compared to that observed on the palladium particles supported on carbon CNTs catalyst. On the activated charcoal, nanotubes with the unreactive basal planes palladium was present in agglomerate exposed. The metal particles were in a round shape instead of individual particles as shaped form. observed on the CNTs, which led to a less homogeneous dispersion of the metal particles on the support. However, the average particle size estimated from TEM was similar to that of the palladium supported on the CNTs, i.e. 5 nm.

3.2. Cinnamaldehyde hydrogenation The catalytic activity and selectivity obtained on the Pd-5wt%/CNTs and a commercial Pd-5 wt%/AC catalyst are shown in Fig. 3. The catalytic activity obtained on the two catalysts was similar at the beginning of the test. However, the carbon nanotube-

701 based catalyst showed a slightly higher activity at the end of the reaction despite the large difference in the overall specific surface area between the two catalysts, i.e. 900 mE g-1 for the commercial catalyst compared to only 20 mE g-1 for the CNT-based catalyst, and a similar metallic particle distribution. This high catalytic performance observed on the CNFs based catalyst was quite surprising as the literature reported that a relatively low surface area creates a major drawback for such an application. The high hydrogenation activity of the CNT-based catalyst was attributed to the high external surface area of the support which considerably decreased the mass transfer phenomenon and provided a high surface contact between the gaseous reactants and the catalyst body. For the traditional grain size or powder supports, liquid diffusion can reduce the effectiveness of the catalyst itself. The absence of micropores inside the CNTs material when compared to the activated charcoal (ca. 60 %) could also partly explain the diffusion limitation. Similar results have also been obtained during the hydrogenation of nitrobenzene into aniline in a liquid phase medium (not reported).

.~~>OI~176 oi80_aJ~. I

c=

"i=} f~

E

I

i

I

A

(-.

U

I

CNTs AC

6040-

2o-

r9~

U

0 0

I

I

I

I

5

10

15

20

I

i

25

"--"

I

30

Time on stream (h) Fig. 3. Cinnamaldehyde hydrogenation activity on the Pd/CNTs and Pd/AC at 80 ~ and under atmospheric pressure of hydrogen. The higher degree of crystallinity, i.e. ordered stacking of graphene planes, of the carbon nanofibers compared to the activated charcoal, i.e. turbostratic carbon with no long range order, could also explain the relatively high activity observed. Similar results have been reported by Rodriguez et al. [15] for the Fe-Cu bimetallic catalyst supported on carbon nanofibres and activated carbon during the hydrogenation of unsaturated hydrocarbon. The influence of the catalyst particle location with respect to the support on the catalytic performance should be studied in more detail in order to obtain a better understanding of the mechanism.

702 Fig. 4 shows the reaction pathway involved in the cinnamaldehyde hydrogenation, leading to compounds containing hydrogenated C=C and/or C=O bonds. Cinnamaldehyde

Hydrocinnamaldehyde (3-Phenylpropionaldelyde)

_Cr o

~

OH

Cinnamyl alcohol

3-Phenyl propanol (3-Phenylpropyl alcohol)

Fig. 4. Reaction pathway of the cinnamaldehyde hydrogenation.

The CNTs based catalyst also displays an oustanding performance when compared to the commercial activated charcoal catalyst in terms of product selectivity. A totally different selectivity was observed between the two catalysts: on the CNT-based catalyst, the ratio of the C=C bond hydrogenation product versus the complete hydrogenation product was 80:20 whereas on the commercial catalyst, the selectivity was significantly modified toward the total bond hydrogenation which leads to a ratio of only 45:55 (Fig.

5B). Such a difference in terms of product selectivity was attributed to the complete absence of any acidic sites on the carbon nanotubes surface and also to the absence of micropores which could induce re-adsorption and consecutive reaction [16]. The presence of micropores could artificially increase the contact time and as a consequence, modify the hydrogenation pathway. The influence of the support nature on the electronic properties of the metallic phase could also be put forward to explain these results. Depending on the metal-support interaction, the metal particles could exhibit different exposed faces and as a consequence, significantly modify the chemisorption of the reactant on their surface. According to the interaction between the C=C bond and the faces exposed by the palladium particles, the residence time and the desorption of the intermediate could be different and thus, lead to a different selectivity. The presence of palladium aggregates on the activated charcoal as compared to the individual palladium dispersion on the CNTs could be the illustration of this difference in exposed crystalline faces.

703

100 8O" -o i1) >..

60

~23

4o-

a.

20-

"o o

I

I

A

I

I

= 1

100

I

CNTs

C=Cbond / hydrogen/ation/

~-- 80 "~ i1) >.

t~

"Z3 o

e

0~ 0

I1

5

10

15

20

Time on stream (h)

25

30

I

60

40

I

I

I

I

I

AC

B

C=Cbond hydrogenation

201 ~ ~ Z ~ O ~n d 0~,~---I~, , , hydrogenation 0 5 10 15 20 25 30 Time on stream (h)

Fig. 5. Distribution of the reaction product during the hydrogenation of cinnamaldehyde on the Pd/CNTs and Pd/AC at 80 ~ and under atmospheric pressure of hydrogen.

4. CONCLUSION Carbon nanotubes are efficient catalyst supports for liquid phase reactions, i.e. hydrogenation of cinnamaldehyde. The palladium particles are well dispersed inside the CNTs tubules when using a classical incipient wetness impregnation followed by a mild thermal treatment and reduction. The peculiar morphology of the support and the location of the active phase allows an active and highly selective catalyst for the C=C bond hydrogenation in a,/~unsaturated compounds to be obtained, when compared to the commercial high surface area activated charcoal catalyst. Such results could open a new field of catalytic investigations for fine chemical applications.

REFERENCES 1. S. Iijima, Nature, 354 (1991) 56. 2. J-M. Bonard, L. Forro, D. Ugarte, W.A. de Herr and A. Chfitelain, Eur. Chem. Chronicle, 1 (1998) 9. 3. G. Che, B. B. Lakshmi, C.R. Martin and E. R. Fischer, Langmuir, 15 (1999) 750. 4. F. Salman, C. Park and R.T.K. Baker, Catal. Today, 53 (1999) 385. 5. R . T . K . Baker, K. Laubernds, A. Wootsch and Z. Paal, J. Catal., 193 (2000) 165. 6. C. Park and R. T. K. Baker, J. Phys. Chem. B 102, 5168 (1998). 7. A. Chambers, T. Nemes, N.M. Rodriguez and R.T.K. Baker, J. Phys. Chem. B, 102 (1998) 2251. 8. E. Dujardin, T. W. Ebbesen, H. Hiura and K. Tanigaki, Science, 265 (1994) 1850. 9. S.C. Tsang, Y.K. Chen, P.J.F. Harris and M.L.H. Green, Nature, 372 (1994) 159; P.M. Ayajan, O. Stephan, P. Redlich and C. Colliex, Nature, 375 (1995) 564.

704 10. J. Sloan, J. Hammer, M. Zwiefka-Sibley and M.L.H. Green, Chem. Commun., 347 (1998). 11. J. Sloan, D. M. Wright, H. G. Woo, S. Bailey, G. Brown, A.P.E. York, K.S. Coleman, J.L. Hutchison and M.L.H. Green, Chem. Commun., 699 (1999). 12. C. Pham-Huu, N. Keller, G. Ehret and M.J. Ledoux, J. Catal., 200 (2001) 400. 13. Y. Gogotsi, J.A. Libera, A.G. Yazicioglu and C.M. Megaridis, Mater. Res. Soc. Symp. Proc., Vol. 633 ~ Nanotubes and Related Materials >>A.M. Rao, Ed., p. A.7.4 (2001). 14. R. Schl6gl, in: Handbook of Heterogeneous Catalysis, Eds. G. Ertl, H. Kn6zinger, J. Weitkamp, Wiley-VCH, Weinheim, Vol. 1, p. 138 (1997). 15. N.M. Rodriguez, M.S. Kim and R.T.K. Baker, J. Phys. Chem., 98 (1994) 13108. 16. L. Zhang, J.M. Winterbottom, A.P. Boyes and S. Raymahasang, J. Chem. Technol. Biotechnol., 72 (1998) 264.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

705

Raman studies of the templated synthesis of zeolites P.P.H.J.M. Knops-Gerrits* and M.G.L.J. Cuypers D6partemem de Chimie, Universit6 Catholique de Louvain (UCL), Batiment Lavoisier, Place L. Pasteur n~ B-1348 Louvain-la-Neuve, Belgium T61 : (32)10 - 47 29 39 Fax : (32)10 - 47 23 30 E-mail : [email protected] * To whom correspondence should be addressed. The application of confocal Raman and FT-Raman in the understanding of zeolite synthesis is a goal that can be reached by studying different levels of complexity. First, the Raman properties of TEOS and its polymerization products under acid conditions is investigated. Structural characterization of zeolite framework vibrations is related to their building blocks such as prisms and sodalite cages. The Raman study of zeolite synthesis with structure directing agents permits the study of the geometry of these organic molecules, such as in ZSM-5 synthesis, the tetra-propyl-ammonium ion (TPA+), in EMT synthesis the Na-18-Crown-6 that can direct hypo- (and hyper-)cage formation, Can Li and co-workers performed zeolite X synthesis studies and FAU (X,Y), MFI, MOR and BETA characterization with UV-Raman. Key-words vibrations

9 Raman spectroscopy, TEOS, synthesis, zeolite, templates, framework

1. INTRODUCTION In comparison with this vast amount of infrared spectroscopy work, the field of zeolite Raman spectroscopy seems rather evolving [1-2]. The examination of highly characterized surfaces such as single crystals to which are sorbed sub-monolayers of reactive gases or vapors must obviously be far more likely to yield results of significance on catalytic processes [3-4]. If we wish to carry out a simple transfiectance measurement the sample thickness would be -2 angstroms i.e. -10 -5 of the conventional infrared pathlength. If one planned to try Raman the sample size is absurdly small. So the outlook looks bleak before one starts studying microporous oxide materials but in the presence of zeolites the good sorption capacities allow to study adsorbed or ship-in-the-bottle complexes quite well. Many catalysts are highly porous. Compounds such as silicaaluminas or zeolites have huge areas to which molecules can sorb. In microporous materials the holes may be of molecular size and limit access to sites nearer the surface, but the surface area of these materials can be huge indeed - values of 100 to 1000m2g 1 are not unusual. Sorbing a monolayer to these materials does provide a decent amount of

706 material and so vibrational spectroscopy started with this type of catalyst system. Pioneers such as Norman Sheppard adsorbed methane to silicas and using infrared were able to show how the molecules interacted [3-4]. In the development of zeolite science, IR has been one of the major tools for structure and reactivity characterization. The reason for this choice is that there is considerable difficulty in obtaining Raman spectra with acceptable signal-to-noise ratios from highly dispersed materials such as zeolites [56]. The Raman effect is intrinsically a weak phenomenon. In order to pass about the low sensitivity an increase of the signal can be obtained ( v 4 law) by an increase of the excitation frequency [7-9]. Raman spectra of zeolites are often obscured by a broad fluorescence. Several causes for fluorescence have been identified. (1) small amounts of aromatic, strongly luminescent molecules might be present in the samples. (2) the presence of (reduced) transition metal ions or Fe impurities in the lattice is known to cause luminescence. The latter problem can be overcome via high-purity synthesis, starting e.g. from metallic Al. (3) proton super-polarizability and basic surface OH groups are two other reasons. Dehydroxylation can only be used if the sample resists such a treatment. The advent of Fourier transform Raman (FT-Raman) spectroscopy with excitation in the near infrared (NIR) domain offers new perspectives in reducing background fluorescence. 2. EXPERIMENTAL 2.1.Materials TEOS 98% was obtained from ACROS Organics. Commercial samples of ZSM-5 (PQ Company Si/Al ~ 81, ALSI-PENTA Si/Al ~ 11), MOR (TOSOH with Si/A1 ~ 10), FER (PQ Company Si/Al ~ 10), NaY (Zeocat; Si/A1 ratio of 2.47) were obtained. Intergrowths of FAU/EMT were synthesized using crown-ethers as structure directing agents. The quantification of the FAU and EMT phases is done by independent calibration with pure 1,4,7,10,13,16-hexaoxacyclo octadecane 18-Crown-6 containing EMT and 1,4,7,10,13-pentaoxacyclopentadecane 15-Crown-5 containing FAU. In EMT only Na-18-Crown-6 can direct hypo- (and hyper-)cage formation, whereas in a FAU phase both Na complexes of 18-Crown-6 or 15-Crown-5 can direct supercage formation. [26-27].

2.2.Spectroscopy Raman spectra were recorded on a Renishaw Raman Microscope. The zeolite samples, in powder form, were placed on a glass microscope slide. The power on the sample was about 2mW/mm 2. The collection time varied from sample to sample and was between 15 and 20 minutes. The spectra were background corrected and a Fourier deconvolution procedure, described elsewhere [3], was applied to resolve the overlapping bands in the OH stretching region. FT-Raman spectra were recorded on a Bruker IFS100. The zeolite samples, in powder form, were pressed in metal sample-holders. FT-Raman spectroscopy has been used to characterise FAU/EMT intergrowths. The vibrational properties of the crown-ethers are discussed in the context of their symmetry and complexation. In the CH stretching region planar or puckered coordination around the

707

Na + cations can occur. The conformation of the cation 15-Crown-5 and 18-Crown-6 complexes in the different faujasite structures is discussed. FT-IR spectra were recorded on a Nicolet F-730 spectrometer. 3. RESULTS AND DISCUSSION 3.1. Raman studies of TEOS and the polymerization of silicon oxides. The few bands observed in the Raman spectrum of tetra-ethyl-ortho-silicate (TEOS) led to the assumption of a high symmetry for this compound. A detailed vibrational characterization of TEOS has been reported by Mondragon et al. [12]. They carried out a normal mode analysis to confirm the experimental assignments and to obtain the force field parameters. TEOS has 3n - 6 = 93 normal modes and with double or triple degenerated bands this spectrum thus becomes simpler. The vibrational spectra can be analyzed according to structural models of Dzd and $4 symmetry. The Dad structural models has 43 normal active modes in IR and 67 in Raman, with 20 polarized active Raman modes. The $4 structural model has 47 normal active modes in IR and 70 in Raman, with 23 polarized active Raman modes. If it is considered that the -OC2H5 groups rotate around the Si-O axis they describe revolutional cones, the TEOS structure could be considered as pseudo-tetrahedral Ta, in this case structural models has 17 normal active modes in IR and 34 in Raman, with 14 polarized active Raman modes. For the structural fragment -OC2H5 of C s symmetry, the symmetric and anti-symmetric stretching normal modes of the -CH3 and-CH2- groups, the group frequencies are well known in Raman and are observed at 2964, 2925 and 2881 cm -1. The 6 (O-Si-O) and 6 asym (O-Si-O) are observed at 301 and 391 cm -1. The v (Si-O) can be found at 995 and 1024 cm -1. The attribution of most (intense) Raman bands is given in Table 1. By varying the water alkoxide ratios R (0.5 < R < 2.0) the progressive hydrolysis and polycondensation of tetraethoxysilane (Fig. 2) in acidic medium was investigated (HCl 0.03M). The chemical reactions of hydrolysis and polycondensation are the following : Si-O-CHe-CH3 + He0 "-) Si-OH + CH3CHzOH Si-OH + HO-Si ") Si-O-Si + HzO Si-O-CHz-CH3 + HO-Si ") Si-O-Si + CH3-CHz-OH In order to appreciate the effects of the alcohol on the reactions involved, part of the solutions can be prepared with different amounts of ethanol as has been presented by Dhamelincourt and co-workers [13]. The Raman band characteristic for the monomer TEOS appearing at 650 cm -1 has been assigned to the symmetric SiO4 stretch. In the scheme of the formation mechanism of polymeric silicate species at 40, 60 and 80~ with R - 1 different intermediates can be observed, which are shown in Fig. 3 and Table 2. It can be noted that at 80~ the bands of ethanol are largely absent as ethanol is evaporated at this temperature. At 40~ mainly monomers and dimers are observed, at 60~ mainly monomers, dimmers, trimers, tetramers and polymers are observed, at 80~ mainly dimers and polymers are observed.

708 Table 1. Raman data of tetra-ethyl-ortho-silicate (TEOS). wavenumber (cm- 1)

intensity

attribution

301

w

(O-Si-O)

391

w

asym (O-Si-O)

614

m

not used in the calculations

647

s

sym (Si-O4)

789

m

(Si-O + C-O)

928

m

995

vs

1024

m

1084

s

(C-C) (Si-O) (Si-O) asym (SiO-CO)

1146

w

(CH3)

1176

m

(HCH)(CH2)

1191

m

(HCH)(CH2)

1291

m

(H-C-H) and CH2 twist

1385

w

(O-C-H / C-C-H)

1449

m

asym (H-C-H)

1479

m

(O-C-H)

1597

m

(O-C-H)

2881

m

sym(CHa)

2925

rn

asym(CH2)

2969

w

, asym(CH3),

= elongation ; = in plane angular deformation; = rocking; = out of plane angular deformation; Table 2. Structural Assignmems of mono-, oligo- and polymeric species in the Raman spectra of water/TEOS mixtures with [HC1]=0.03M. Raman (cm") Structural assignments polymer 500-530 530-550 Si(O-Et)3-O-Si(0-Et)2- O-Si(O-Et)2-O-Si(O-Et)3 550- 570 Si(O-Et)3-O-Si(O-Et)2-O-Si(O-Et)3 600 Si(O-Et)2 (OH)-Si(O-Et)3 and Si(O-Et)3-Si(O-Et)3 Si(O-Et)4 et Si(O-Et)3 OH 650

709

,,,. 2500u'l

~2000-

.Q i,,,

m

r 1500-

~ '1000,=..-

r

,~ 500,.=.

O---7

. . . . . . . . . . . . . .

3OOO

i . . . . . . . . . . .

2500

q

+

2OO0

+

~

'

cm -1

-+

i

"

1500

T

1000

500

Fig. l. Raman spectrum ofTEOS (tetraethoxysilane).

.....~00*ILLJI~LIIL

JI ..J~--~ .... I J-~--J i ..i-i-

,.a

.~3000-

~2000I/%

~ 1000-

ira,

. . . . . . .

I

3500

..............

i

3000

. . . . . . . .

i

|

I

2000

t500

........

2500

cm -1

. . . .

I

..................

1000

I

-'

500

Fig. 2. Scheme of the formation mechanism of polymeric silicate species at 40~ (top), at 60~ (middle) and at 80~ (bottom) with R = 1.

710 3.2. Raman studies of zeolite synthesis. Just like IR spectroscopy, Raman can detect small, X-ray amorphous zeolite particles. Therefore Raman has been used to examine both the liquid and the solid phase of zeolite synthesis mixtures [28]. Ex situ methods (with separation of solid and liquid) and in situ methods have been applied. In studying the liquid phase [10-11], one should remember that (i) minimum concentrations for detection of spontaneous Raman from liquids are typically 0.05 - 0.1 M, [15-27, 29] (iO that the cross-section of the AI(OH)4 species is much stronger than e.g. for silicate or aluminosilicate anions [30]. Thus species which are present in low concentration or with variable structures may easily be overlooked in Raman spectra of the synthesis liquors. When zeolites are synthesized starting from a silica sol, the Raman spectrum of the initially formed solid phase resembles that of vitreous silica, with a broad band around 450-460 cm 1 [29, 31,32]. For zeolites A, X and Y, aluminate is available in solution, as shown by a Raman band at 620 cm 1 [30-34]. A1 can be rapidly incorporated into the solid phase. In particular for zeolites X and Y, this process is accompanied by the appearance in solution of monomeric silicate species, such as SiO2(OH)22 (780 cm "1) or dimeric silicates [30,32,34]. There is no Raman evidence for the presence of aluminosilicate ions in solution. Analysis of the 400-550 cm 1 spectrum during heating or eventual aging of the synthesis gel provides information on the building blocks present in the still relatively disordered solid phase. Again this analysis is based on the correlation between ring-size and ~r frequency. Thus for zeolites A and X, the band close to 500 cm -1 indicates the presence of 4MR at the beginning of the gel heating [31 ]. In zeolite Y synthesis, the gel aging causes a shift towards lower frequencies (440, 361 cml), showing the formation of 6MR. During the subsequent heating, sodalite units with 4MR are formed and the 500 cm 1 band gains intensity again [32]. Syntheses from Si sources other than colloidal silica have also been studied [34]. UV Raman spectra of the liquid phase of the synthesis of the framework of zeolite X indicate that AI(OH)4" species are incorporated into silicate species, and the polymeric silicate species are depolymerized into monomeric silicate species during the early stage of zeolite formation. An intermediate species possessing Raman bands at 307, 503,858 and 1020 cm -1 is detected during the crystallization in the solid phase transformation. The intermediate species is attributed to the 13 cage, the secondary building unit of zeolite X. A model for the formation of zeolite X is proposed, which involves four-membered tings connecting to each other via six-membered ring to form 13 cages, then the 13 cages interconnect via double six-membered tings to form the framework of zeolite X [6,8]. Mordenite and ZSM-5 are synthesized at lower pH values, and it is not surprising that in these conditions silicate species are not usually observed in solution [27,29]. In both cases, the solid initially resembles vitreous silica. In the case of ZSM-5, new vibrations in the solid phase are only observed when ZSM-5 crystals appear [27]. More information concerning the intermediate synthesis steps can be retrieved from mordenite synthesis spectra [29]. In mordenite synthesis, a 495 cm 1 band is observed at an early stage in the spectrum of the solid fraction of the gel. In analogy to what is observed for A and X zeolites, this band is ascribed to 4MR aluminosilicate units. Only at a later stage, broader bands appear at 402 and 465 cm 1, indicative of the formation of still rather

711 disordered 5MR mordenite-like units. These bands eventually sharpen into the characteristic framework vibrations of mordenite.

Bim IF

L._J 6'il

am 4

Ai,,m~N,~nc:a~

$O1' a n d 111~ ~ ' * llwia e ~ g e

2 9 E . M D a n d d~14 e m "~ ZB~Ih~ X

Fig. 3. Scheme of the formation mechanism of zeolite X (after Ref. 6 ).

3.3. Template observation via Raman. Observation of the organic template against the weak zeolite background is highly facilitated in Raman in comparison with IR. In some particular cases, the incorporation and conformation of the organic template can be followed. In ZSM-5 synthesis, the tetrapropylammonium ion (TPA § is initially incorporated into the solid amorphous aluminosilicate in the all trans configuration, which it also possesses in solution. The eventual ZSM-5 crystals however contain a high-energy TPA + conformer, in which one of the N-C bonds is rotated [27,35]. The strong non-bonded interactions between-CH2groups of different propyl groups are evidenced by the changed rocking and wagging modes. This conformation has also been proved in XRD studies. In syntheses of LTA zeolites, a tetramethylammonium ion (TMA+) can be incorporated in the sodalite cages. The tightness of this fit induces a frequency increase in the symmetric C-N stretching vibration of the template [33-36]. Raman studies indicate that at a fixed pH, organic cations (e.g. TMA +) and alkali ions stabilize different silicate species [37]. This clarifies the role ofTMA + in synthesizing LTA zeolites with high Si/A1 ratio, e.g., ZK-4. Crown-ether templates are used in the synthesis of cubic and hexagonal faujasites. The Na + forms of the cyclic ethers 18-crown-6 (18C6) and 15-crown-5 (15C5) can be used to direct the synthesis of faujasites towards the cubic FAU or the hexagonal EMT topology [26-27]. FT-Raman spectra of as-synthesized FAU, EMT and a structural intergrowth of both topologies (MIX) are shown in Table 1. The intergrowth was synthesized with a 3 91 18C6 915C5 mixture. The FT-Raman technique yields superior spectra compared to visible excitation, based on other Raman reports on this system [3940]. In the spectra, features of framework and crown ether template are superimposed. The band at 503 crn1 is the framework symmetric bending vibration; most other bands are crown-ether vibrations. There are a few differences between the spectra ofNa+-15C5

712 in FAU and Na+-I 8C6 in EMT. Both ethers have a characteristic intense band between 800 and 900 cml, which has C-O stretching and CH2 rocking character. A sharp band is observed at 1001 crn1 in FAU and to a lesser extent in the intergrowth. This band is lacking in as-synthesized pure EMT, for 18-crown-6 and its complexes, the relation between Raman spectra and the symmetry of a compound (D3d, Ci or CI) has been studied in depth. K+-18C6 always assumes D3d symmetry, and in crystalline (Na+18C6)(SCN) it has an envelope-like C~ structure. For dissolved Na+-I 8C6, the geometry may fluctuate between different conformers (D3d or CI) depending on the solvent. While Na+-18C6 undergoes some distortion in the EMT hypocage, the EMT hypercage provides sufficient space for a more relaxed crown ether conformation, similar to solution structures [39-42]. Table 3. Characteristic differences in the FT-Raman spectra of the FAU/EMT containing the crown-ethers 15-Crown-5 and 18-Crown-6. MIX FAU EMT (15-Crown(18-Crown5) 6) 348 347 (Na-O)lattice 354 355 (Na-O)lattice 503 503 503 (S i-O-S i)sym lattice I 860 860sh 15-Crown-5 (C-O-C)sym 869 869 18-Crown-6 (C-O-C)sym II 1001 1002 15-Crown-5 CH2 rocking complex III 1040 1040 15-Crown-5 (C-O-C)asym 1092 1082 18-Crown-6 (C-O-C)asym 1137 18-Crown-6 CH2 wagging complex IV 1137 15-Crown-5 CH2 wagging complex 1152 1152 1270s 15-Crown-5 CH2 twisting complex V 1264 1294 18-C-6/15-C-5 CH2 twisting 1294sh 1296 18-Crown-6 CH stretching complex VI 2859 ,i

i

ACKNOWLEDGMENTS

PPKG thanks the UCL and ESA PRODEX for a research grant. MC is a DEA fellow of UCL. REFERENCES

1. I.R.Lewis and H.G.M.Edwards, Handbook of Raman Spectroscopy, 2001, Marcel Dekker. 2. N.J. Ortinis, T.A. Kruger and P.J. Dutta, Anal. Applics of Raman Spect. Ed., M.J. Pelletier Blackwell Science Oxford (1999) and refs contained therein. 3. R. Ferwerda, J.H. van der Maas and P.J. Hendra, J. Phys. Chem, 97 (1993) 7331.

713 4. P.J. Hendra. Intemet J. Vib. Spec.[www.ijvs.com] 5, 2 (2001) 4. 5. W. Pilz, Z. Phys. Chem. (Leipzig), 271 (1990) 219. 6. G. Xiong, Yi Yu, Z-C. Feng, Q. Xin, F-S. Xiao and C. Li, Micropor. Mesopor. Mater., 42 (2001) 317. 7. P.-P. Knops-Gerrits, D.E. De Vos, E.J.P. Feijen and P.A. Jacobs. Micropor. Mater., 8 (1997) 3. 8. C. Li and P.C. Stair. Catal. Today, 33 (1997) 353. 9. Y. Yu, G. Xiong, C. Li and F-S. Xiao, Micropor. Mesopor. Mater., 46 (2001) 23. 10. P.K. Dutta and B. Del Barco, J. Phys. Chem., 92 (1988) 354. 11. P.K. Dutta and J. Twu, J. Phys. Chem., 95 (1991) 2498. 12. M.A. Mondragon, V.M. Castano, J. Garcia M., C.A. Telles S., Vibrational Spectr., 9 (1995) 293. 13. J. Gnado, P. Dhamelincourt, C. P616gfis, M. Traisnel and A. Le Maguer Mayot, J.Non-Cryst. Solids, 208 (1996) 247. 14. P.K. Dutta and B. Del Barco, J. Phys. Chem., 89 (1985) 1861. 15. P.K. Dutta and B. Del Barco, J. Chem. Soc. Chem. Commun., (1985) 1297. 16. A. Miecznikowski and J.Hanuza, Zeolites, 7 (1987) 249. 17. P.K. Dutta and M. Puri, J. Phys. Chem., 91 (1987) 4329. 18. P.K. Dutta, D.C. Shieh and M. Puri, Zeolites, 8 (1988) 306. 19. P.K. Dutta, K.M. Rao and J.Y. Park, J. Phys. Chem., 95 (1991) 6654. 20. A.J.M. de Man, W.P.J.H. Jacobs, J.P. Gilson and R.A. van Santen, Zeolites, 12 (1992) 826. 21. P.P. Knops-Gerrits, P.A. Jacobs, A. Fukuoka, M. Ichikawa, F. Faglioni and W.A. Goddard III, J.Mol.Cat., A, 166 (2001) 3, P.P. Knops-Gerrits, M. Witko, W. Goddard and R. Millini Eds. 22. P.-P. Knops-Gerrits, H. Toufar, X.-Y. Li, P. Grobet, R. A. Schoonheydt, P.A. Jacobs and W. A. Goddard III, J. Phys. Chem.A.,104, 11 (2000) 2410. 23. P.P. Knops-Gerrits and W.A. Goddard III, J.Mol.Cat., A, 166 (2000) 127. 24. G. Mestl, P. Ruiz, B. Delmon and H. Kn6zinger, J. Phys. Chem., 98 (1994) 11269, 11276 and 11283. 25. G. Mestl, J. Mol. Cat., A, 158 (2000) 45. 26. A. Kozlov, A. Kozlova, K. Asakura and Y. Iwasawa, J.Mol.Cat., A, 137, (1999), 223. 27. E. Feijen, K. De Vadder, M.H. Bosschaerts, J.L. Lievens, J.A.. Martens, P.J. Grobet and P.A. Jacobs, J. Am. Chem. Soc., 116 (1994) 2950. 28. P.A. Jacobs, E.G. Derouane and J. Weitkamp, J.Chem.Soc. Chem.Commun., (1981) 591. 29. J. Twu, P.K. Dutta and C.T. Kresge, J. Phys. Chem., 95 (1991) 5267. 30. F. Roozeboom, H.E. Robson and S.S. Chan, Zeolites, 3 (1983) 321. 31. P.K. Dutta and D.C. Shieh, J. Phys. Chem., 90 (1986) 2331. 32. P.K. Dutta, D.C. Shieh and M. Puri, J. Phys. Chem., 91 (1987) 2332. 33. B.D. McNicol, G.T. Pott and K.R. Loos, J. Phys. Chem., 76 (1972) 3388. 34. J. Twu, P.K. Dutta and C.T. Kresge, Zeolites, 11 (1991) 672. 35. C. Peuker, W. Pilz, B. Fahlke, E. Loettler, J. Richter-Mendau and W. Schirmer, Z. Phys. Chem. (Leipzig), 266 (1985) 74. 36. P.K. Dutta, B. Del Barco and D.C. Shieh, Chem. Phys. Lett., 127 (1986) 200.

714 37. P.K. Dutta and D.C.Shieh, J. Raman Spectrosc., 16 (1985) 312. 38. K. Nakamoto, IR & Raman Spectra of Inorganic & Coordination Compounds, Wiley (1986) 124. 39. F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites, 10 (1990) 546. 40. S.L. Burkett and M.E. Davis, Microporous Mater., 1 (1993) 265. 41. J.J.P.M. de Kanter, I.E. Maxwell, P.J. Trotter, J. Chem. Soc. Chem. Commun., (1972) 733. 42. C. Br6mard and M. Le Make, J. Phys. Chem., 97 (1993) 9695.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

715

Templateless synthesis of catalysts with narrow mesoporous distribution 9 a* N. Yaoa, G. Xlong , S. Sheng a, M. He b and K.L. Yeung e

a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P. O. Box 110, Dalian 116023, P. R. China bResearch Institute of Petroleum Processing SINOPEC, Beijing 100083, P. R. China c Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, SAR, P. R. China In this study, amorphous silica-alumina nanomaterials with narrow mesoporous distribution can be obtained by two novel sol-gel processes, without the use of any templates. The results of our experiments show that the preparation method has a great influence on the precursor sol structure as well as the specific surface area and mesopore volume of the final product, but has no effect on the pore size distribution. 1.

INTRODUCTION The tendency of the refining industry to process heavier fossil oils has raised the

interest in catalysts which will have pore sizes optimized to cope with large molecules. Microporous zeolites, which are widely used in catalytic processing, are limited in their applications in this area of the treatment of large molecules due to their small pore geometry. Thus, the demand for mesoporous materials has triggered major synthetic efforts [1]. Since the discovery of mesoporous molecular sieve (M41S) in 1992 [2], it has now been well accepted that the formation of such mesoporous materials could occur through several templating pathways by using various types of organic templates [3]. Until now, none has described a method for the synthesis of mesoporous material without the aid of a surfactant.

Corresponding author Email: [email protected]

716 The objective of the present work is to obtain narrow pore-sized, mesoporous materials without the use of surfactants. Two distinct templateless synthesis routes defined as vacuum-sol method and ultrasonic-sol method were developed, respectively. In addition, attempts have been made to identify and evaluate the effect of the synthesis method on the texture of the materials. 2. EXPERIMENTAL

Method A (Ultrasonic-sol method): The pH value of a 50ml water glass solution was adjusted to 10 through the addition of 0.94 M nitric acid. The prepared alumina sol was then added to the water glass solution until the desired Si/A1 molar ratio was attained. The resulting precipitate was peptized using nitric acid and then ultrasonically treated for 1 minute to obtain a stable sol. When the sol finally formed a gel at room temperature, 70ml of NH4NO3 solution (1.2M) was added to the gel to remove the sodium ions. At the end of 24 hours, the solution was drained offby centrifugation, and this procedure was repeated 3 times. The gels were calcined in air at 550 C for 10 hours to obtain the solid samples. In this work, the sol and solid samples prepared by the method A were named as USG samples [4]. Method B (Vacuum-sol method): A 0.96M aluminum nitrate solution was added to 50ml water glass solution until the Si/A1 molar ratio was attained. The precipitate was collected by centrifugation and washed 7 times to remove the sodium ions. After washing, the precipitate was put into 200ml of water and then added a certain quantity of 0.94M nitric acid to peptize the precipitate to obtain the sol sample. The prepared sol was dried at room temperature in a vacuum box until it began to form the gel sample. The gels were also calcined at 550 C in air for 10 hours. The sol and solid samples produced by method B were defined as VSG samples [5]. A N4 plus laser scattering particle meter (Coulter) was used to measure the sol particle diameter distribution at a 90 angle to the light beam. The material structure and chemistry were characterized by X-ray diffraction (XRD, Rigaku D/MAX-RB), atomic force microscopy (AFM, Nanoscope III), transmission electron microscopy (TEM, JEM2010) and N2 physi-adsorption (Omnisorp- 100CX).

717 3. RESULTS AND DISCUSSION 3.1. Effect of preparation method on the precursor soi's properties

Fig.1 and Table 1 summarize the effect of the Si/AI molar ratio and preparation method on the sol samples' pH values and the particle diameter distributions. It is seen that the VSG sol samples have large particle diameter in the range 200nm to 2000nm. Increasing the Si/A1 molar ratio results in a decrease in pH value as well as broadens the particle diameter distribution as shown in Fig. 1A. In contrast, every USG sol sample has small particle diameter and narrow particle diameter distribution (e.g. Fig. 1B). Fig. 2 shows USG1 and VSG1 sol samples' AFM images, respectively. The large secondary particles existed in VSG1 sample are formed by random aggregated of spherical primary particles with size of 10-20ran, while the uniform sized primary particles within 13-25nm disperse well in the USG1 sol sample. It is thus that USG1 and VSG1 sol samples have similar sized primary particles, although they have different particle structure. These results mean that the preparation method take great contribution to precursor sol's structure and particle diameter. The method A favors the formation of monodispersed and stable sol particles in the system. 100 USG1

75

ii

50 25

~50~

20

.40

40

lq~O

60 USG2

9

.

_ _

,

|

,

I

,

,

'

10 0541 411

30

VS~

,,/ A

size(m)

20

40

60

80

ltbO

USG3

20 10

~

B

o

40 60 Size (nm)

80

Fig. 1. Particle diameter distribution of USG and VSG sol samples, respectively.

100

718 Table 1 pH value of every precursor sol sample Sample No. VSG1 VSG2 VSG3 USG1 USG2 USG3

Si/A1 molar ratio 10 7 3 10 7 3

pH value 2112 2.41 2.50 2.55 2.48 2.01

3.2. Effect of the preparation method on the solid's texture

The XRD analysis shows that all of synthesized materials are amorphous. Table 2 reports the solid samples' specific surface area and main pore texture characteristics. The solids have large specific surface area, and this value diminish as decrease of Si/A1 molar ratio in both USG and VSG series. Fig. 3 displays the pore size distribution of USG1 and VSG1 samples, respectively. It is clear from the figure that the both samples have a narrower pore size distribution (i.e., 2- 10nm). Fig. 4 shows a representative micrograph of the solid sample analyzed by TEM. There is no apparent order in the pore arrangement unlike the ordered hexagonal array observed in M41S mesoporous molecular sieves. Also, the sample is made of spherical particles with narrow size distribution from 12nm to 25nm. This result indicates that the present methods enable to produce nanomaterial and the pores existed in the solids may be created through the random packing of spherical particles. Based on the TEM and N2 sorption results, it is concluded that the developed methods are able to synthesize silica-alumina nanomaterials

719 with large specific surface area and narrow mesoporous distribution absent of organic templates.

Fig. 3. Pore size distribution of USG1 and VSG 1 samples, respectively.

Fig. 4. TEM morphology synthesized solid sample.

of

Table 2 Physic chemical characteristics of the synthesized materials Sample Preparation S i / A 1 MEPV" MIPV + Specific surface Pore size No. method molar ratio (ml/g) (ml/g) area (m2/g) distribution (nm) VSG1 B 10 0.244 ' 0.03 542.99 2.12-10.61 VSG2 B 7 0.294 0.00 493.89 2.12-12.08 VSG3 B 3 0.364 0.00 432.96 3.21-12.03 USG1 A 10 0.291 0.01 692.54 2.04-10.20 USG2 A 7 0.355 0.01 648.59 3.24-11.23 USG3 A 3 0.305 0.01 587.24 3.20-11.70 Mesopore volume calculated by BJH method from adsorption branch. + Micropore volume calculated by t-plot method from adsorption branch. i

i

As can be seen in Table 2, it is found that the USG samples have larger specific surface area and pore volume than those of VSG samples, except for VSG3's pore volume. This phenomenon can be ascribed to two reasons. First, as described in experimental section, the drying processes of two methods are totally different. The sample derived from

720 vacuum-sol method should dry under the vacuum condition, otherwise, it is impossible to produce mesoporous materials with narrow size distribution. However, the ultrasonic-sol method does not take any drying action prior to calcination. The gel was formed under the general air pressure and the room temperature. This difference makes a great contribution to the gel's structure. As can be seen in Fig. 5A, many sol particles densely aggregated to form large island-like particles in VSG1 gel sample due to the effect of capillary force appeared in the vacuum drying process. Nevertheless, the sol particles only form an incompact gel network in USG1 gel sample. Obviously, the denser particle aggregation causes more voids collapse in the VSG gel network, which leads to decrease the pore volume and specific surface area.

Fig. 5. (A) AFM image of VSG1 gel sample. (B) AFM image of USG1 gel sample. In order to confirm the above explanation, typical TG analysis was performed and its pattern is displayed in Fig.6. In the TG curves, the weight loss between the 40 C and 150 C can be ascribed to the desorption of physically adsorbed water [6]. In this region, it should be noted that the weight loss of USG1 gel sample is much greater than that of VSG1 sample. That is to say, the shrinkage of USG1 gel sample caused by capillary force is restrained mostly so that its network has much more pore structure to contain the water before the heat treatment. This result is in good agreement with the above

discussion. 110 ,

100

'~

90

80 .~ 70 -" .~ 60 ~ 50 40 30 20 lo o o

:

~

VSG 1

.......

USG1

,! ~

-

~.! ~._.

:

"~'"'~'i,

I

100

,

I

,

Rtgion III I

200 300

|

I

,

i

,

i

400 500 600

Temperature

,

i

700

,

i

800

~C)

Fig. 6. TG curves of USG1 and VSG1 gel samples, respectively.

721 The second possible reason is the role of base-exchange procedure in the ultrasonic-sol method. In fact, such a step is not only to remove the sodium ions in the system, but also to age the gel. The latter takes a significant effect on the gel structure because the amount of shrinkage that occurs during drying is dependent on the stiffness of the gel's network. If the gel is aged under a suitable condition, the network may be strengthened and stiffened so that it can resist compression by capillary forces during the subsequent calcination [7]. Accordingly, it has a profound effect on the decrease of the pore collapse. It is hereby reasonable to find that USG samples have larger pore volume and specific surface area than VSG samples. Another noticeable feature in Table 2 is the pore size distribution. It is seen that materials have similar and controlled mesoporous distribution, although they have different Si/AI molar ratios and are prepared by different methods. A possible explanation can be obtained by analysis of the particle diameter of the precursor sols. It is found in Fig.2 that the VSG1 and USG1 sol samples have primary particles with similar size. After gelation, the primary particles maintain their size and morphology but form regularly packed aggregate clusters. The fluid-filled inter-particle voids are the precursors for the mesoporous channel network in the final material. The size of these void spaces is dictated mainly by the size of the primary sol particles and their packing order. Thus, primary sols of similar size and distribution (cfr. Figs. 2 A and B) should lead to final mesoporous materials of similar pore size distribution (cfr. Table 2). A more detailed explanation about this point has already described in our previous work for every case, respectively [4,5]. Thus, the N2 sorption analysis results mean that the preparation method only affects the specific surface area and pore volume of the materials, but has no relationship to the pore size distribution. The pore size distribution of the final products is only related to the diameter and size distribution of the primary sol pai'ticles. 4. CONCLUSION In the present study, amorphous silica-alumina nanomaterials with controlled mesoporous distribution have been synthesized by two templateless approaches: (1) vacuum-sol process, and (2) ultrasonic-sol process. It is found that the preparation method affects the precursor sol properties and the specific surface area and pore volume of the final materials. Ultrasonic-sol method favors the formation of monodispersed sol particles with narrow size distribution. Because of several base-exchange cycles and absence of drying process prior to heat treatment, the gel derived from ultrasonic-sol method may have enough stiffness to protect the network from pore collapse by capillary force, thus, leading to produce the materials with

722 larger specific surface area and pore volume than vacuum-sol method. Moreover, the analysis results also indicate that the synthesis method has no effect on the pore size distribution of the materials due to the fact that their precursor sols have similar-sized primary particles.

ACKNOWLEDGMENT

The authors are indebted to Ms. Zhang Yan (The Materials Characterization & Preparation Faculty, HKUST) for acquiring JEM 2010 Microscope. The financial support from the Chinese Academy of Sciences, the Research Institute of Petroleum Processing SINOPEC and the National Sciences Foundation of China are also acknowledged. REFERENCES

1. S. Biz and M. L. Occelli, Catal. Rev.-Sci. Eng., 40(3) (1998) 330. 2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 3. Q. Huo, D. I. Margolese, U. Ciesla, P. Feng, T. E. Gier, P. Sieger, R. Leon, P. M. Petroff, F. Schtith and G. Stucky, Nature, 368 (1994) 317. 4. N. Yao, G. X. Xiong, S. S. Sheng, M. Y. He, W. S. Yang and X. H. Bao, Catal. Lett., in press. 5. N. Yao G. X. Xiong, M. Y. He, S. S. Sheng, W. S. Yang and X. H. Bao, Chem. Mater., 14(1) (2002) 122. 6. C. J. Brinker and Ct W. Scherer, Sol-gel Science. The Physics and Chemistry of Sol-gel Processing, Academic Press Inc., Harcourt Brace Jovanovich, 1990. 7. C. J. Brinker and G. W. Scherer, Sol-gel Science. The Physics and Chemistry of Sol-gel Processing, Academic Press Inc., Harcourt Brace Jovanovich, 1990.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Control of pore structures of titanias andtitania/aluminas complexing agents

723

using

M. Toba, S. Niwa, N. Kijima and Y. Yoshimura Research Institute of Green Technology, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan In order to control the surface area, pore volume and pore distribution, titanias were prepared by using complexing agents and the relationship between the structure of complexing agents and pore structures weas examined. The structure around the hydroxy group of the alcohol used as a complexing agent has large influence on the pore structure of titania. The modification of titania sol by using complexing agents was also effective for controlling the pore volume. The specific surface area and pore volume of precipitated titania/aluminas were proportional to those of alumina supports prepared with various complexing agents. 1. I N T R O D U C T I O N Titania and titania containing mixed oxides are extensively used as catalysts and supports. Recently, many preparation techniques have been developed to control their pore structure. However, some of them are not suitable for a systematic control of the pore distributions. In addition, pores usually disappear after calcination at high temperature [1]. To avoid this problem, catalysts were calcined at relatively low temperature [2-4] or an extraction method was used to remove organic residues [2]. Under such mild conditions, it is impossible to remove completely the organic residues which are bonded to a titania precursor. In this work, we prepared titanias using two procedures: sol-gel [5, 6] and modified sol methods using complexing agents which were able to be easily removed. Titania/aluminas were prepared by titania precipitation on alumina supports which were modified by complexing agents. The effect of the structure of complexing agents on their surface areas, pore volumes, pore distributions and crystallinities were examined. 2. E X P E R I M E N T A L The sol-gel titanias were prepared using mono alcohols as complexing agents. Titanium iso-propoxide (TIP) was mixed with a complexing agent and stirred at 393 K for 3 h. After removing 2-propanol which was formed by ligand exchange reaction under reduced pressure at 353 K, water was added to the solution to hydrolyze the various titania complexes formed. The modified titania sols were obtained by neutralization of basic titania sol modified by complexing agent under reflux condition. Titania/aluminas were

724 prepared as follows. Alumina supports prepared by a complexing agent-assisted sol-gel method were calcined at 823 K for 2 h. Titanium iso-propoxide was added to alumina and 2-propanol suspension and then water was added to the suspension to hydrolyze the alkoxide. Ligand exchanged titanium alkoxide was also used as a titania source. The obtained gels were dried at ca. 413 K under reduced pressure. Finally, the dry gels were calcined at 823 K for 2 h. Specific surface areas, mesopore volumes and pore distributions were determined from nitrogen adsorption data at 77 K using a BELSORP 28SA (Nippon BEL. Co.). All the samples were outgassed at 413 K for 3 h before measuring the nitrogen adsorption. The X-ray powder diffraction patterns were obtained on a MAC Science MXP-18 instrument using Cu-Ka radiation with a Ni filter. 3. RESULTS AND DISCUSSION

3.1 Sol-gel titanias Fig. 1 shows the typical nitrogen isotherms of titanias measured at 77 K. The isotherms of the sol-gel titanias are type IV of IUPAC isotherm classification [5]. Effect of the amount of the complexing agent on the rate of ligand exchange, surface area and pore volume is shown in Table 1. The rate of ligand exchange was calculated by the amount of 2-propanol evaporated by ligand exchange reaction. The rate of ligand exchange increased with increasing the amount of alcohol (2-ethyl-l-hexanol; 2-EHA) used as a complexing agent. However, the pore volume increased with increasing 2-EHA/TIP ratio from 1 to 3, and remained constant at 2-EHA/TIP ratio of 3 to 5. The specific surface area slightly decreased with increasing the amount of alcohol. The effect of the amount of complexing agent on the pore distributions is shown in Fig. 2. V/cm3g -1 300

'''

dV/dR/mm3nm-lg -1

I'''

I'''

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'-{

j,/~

250

i00{2,,,,....'....'....'....'....I....,""_{ 80 I

i

.....2-EHA/TIP=I~ 2-EHA/TIP=2 "~ 2-EHA/TIP=3 ] -f"~"j .....2-EHA/TIP=4 ~'.. " Z

200 adsorption desorption

150

( ,

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Z

100 50

"

~

-

f~

,,, l,,, l,, , l , , , l , ,

,

0.2 0.4 0.6 0.8 1 Fig. 1 Typical nitrogen isotherm of titania Complexing agent, 2-ethyl-l-hexanol; calcination, 823 K. 2h.

0

0

5

10 15 20 25 30 35 40 Pore diameter / n m Fig. 2 Effect of the amount of the complexing agent on the pore distributions of titanias. Complexing agent, 2-ethyl-l-hexanol; calcination, 823 K, 2h.

725 Table 2 shows the effect of calcination temperature on the specific surface area and the pore volume of titania. The surface area and pore volume were almost constant between 723 K and 773 K and decreased between 773 K and 823 K. Table 1. Effect of the amount of the complexing agent on the rate of ligand exchange, surface area and pore volume 2-EHA/TIP Rate of ligand Specific surface Mesopore (mol/mol) exchange (%) area (m2/g) volume (cm3/g) 1 2 3 4 5

13.2 34.5 42.3 62.6 71.9

125 123 113 117 113

0.32 0.38 0.41 0.41 0.41

Calcination, 823 K, 2h. Table 2. Effect of calcination temperature on the specific surface area and pore volume of titania Calcination temperature Specific surface Mesopore (K) area (m2/g) volume (cm3/g) 723 773 823 Complexing agent, 2-ethyl-l-hexanol

139 136 117

0.46 0.47 0.41

The rates of ligand exchange, the surface areas and the pore volumes of titanias prepared with various complexing agents are shown in Table 3. The rate of ligand exchange decreased in the order primary alcohol > secondary alcohol > tertiary alcohol. Secondary and tertiary alcohol gave titanias with higher surface area and larger pore volume. For example, the specific surface area and the pore volume of sol-gel titania prepared with 1-phenylethanol are much larger than those of the titania prepared with 2-phenylethanol, while the molecular sizes of 1- and 2-phenylethanol are almost equal. These results mean that the structure around the hydroxyl group of the complexing agent molecule has a large influence on the pore structure of titania. Among the non-branched primary alcohols, the specific surface area and mesopore volume decrease with increasing chain length, while the ligand exchange rates are almost equal to each other. Branched primary alcohol such as 2-ethyl-l-hexanol also shows higher surface area and larger pore volume. Fig. 3 shows the effect of the complexing agent on the pore distributions of titanias. Most of the primary alcohols (Group A) gave titanias with small pores (3~6 nm). Branched primary alcohol (2-EHA), secondary alcohols and tertiary alcohols (Group B) gave titanias with large pores (10 ~13 nm). Comparing three alcohols of Group A, a more bulky alcohol gave a titania with larger pore size. Asimilar tendency was observed in Group B. The ligand exchange rates of primary alcohols are higher than those of secondary and

726 tertiary alcohols. These results indicate that titanium atoms in the alkoxide species formed by ligand exchange reaction with primary alcohol were surrounded by more bulky alkoxy groups. These alkoxy groups seem to prevent the hydrolysis reaction. As expected, precipitates were slowly formed when 1-octanol, 1-decanol, benzyl alcohol and 2-phenylethanol were used as complexing agents. These results also mean that a considerable amount of alkoxy groups bonded to titanium still remains in the gels prepared by using aromatic and straight chain aliphatic primary alcohol after hydrolysis and drying. During calcination, these remaining organics are eliminated at relatively high temperature and the structural change caused by the elimination results in decreasing the surface area and the collapse of mesopore. In contrast, the titanium alkoxide species formed by ligand exchange reaction with secondary and tertiary alcohol easily hydrolyzed and excess amount of organics could be removed during drying. When the ligand exchange was depressed under mild condition, primary alcohol also gave a titania with relatively high surface area and mesopore. For example, when the ligand exchange rate is 51%, the surface area and mesopore volume of titania prepared by using 1-octanol are 96 mZ/g and 0.22 cm3/g, respectively. Therefore, in order to control the pore structure, both control of the ligand exchange rate and selection of optimum complexing agent are important.

.

Table 3. Specific surface areas and pore volumes of titanias prepared by using various complexing agents. Rate of ligand Specific surface Mesopore Complexing agent exchange (%) area ( m a / g ) volume (cm3/g) Primary alcohol 1-Hexanol 74.5 95 0.21 1-Heptanol 74.4 87 0.17 1-Octanol 82.2 7 0.04 1-Decanol 77.3 11 0.03 2-Ethyl- 1-hexanol 62.6 117 0.41 Benzyl alcohol 66.3 33 0.07 2-Phenylethanol 61.6 46 0.07 Secondary alcohol 2-Heptanol 72.7 91 0.43 2-Octanol 43.7 108 0.37 1-Phenylethanol 44.6 122 0.45 Cyclohexanol 72.6 92 0.29 1-Butoxy-2-propanol 53.6 114 0.30 Tertiary alcohol 2-Methyl-2-butanol 49.8 89 0.18 2-Phenyl-2-propanol 39.1 122 0.44 Calcination, 823 K, 2h; alcohol/TIP (mol/mol) = 4. .

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Fig. 4 shows X-ray powder diffraction patterns of sol-gel titanias. Most of samples showed patterns characteristic of anatase form and weak diffraction patterns of brookite

727 titania. The extent of crystallinity decreased in the order (A) > (B) > (C) = (D) = (E). The diffraction peak derived from brookite titania (2[]=31 ~ was getting smaller with decreasing specific surface area and pore volume. These results indicate that crystallinity of titania is influenced by complexing agents. dV/dR/mm 3nm-lg -1 100 F"

80

. . . . 2-Heptanol ...... 1-Phenylethanoi"/'~!'2-P---henyl-2-prop ano~

6O

If-'\

:

~

20 0

~1 5 10 15 20 25 30 35 40 0 Pore d i a m e t e r / n m

0

5

10 15 20 25 30 35 40 Pore d i a m e t e r / n m

Fig. 3. Pore distributions of titanias prepared by using primary alcohol (A), secondary and tertiary alcohol (B). Calcination, 823 K, 2h; alcohol/TIP (mol/mol) = 4. cps 5000

'"'

'

I

' ' ' '

I ' ' ' '

I ' ' ' '

I ' ' ' '

I ' ' '

'

I

' ' ' '

I ' ' '

"

-

4000 3000

ooo

Z

1ooo _0 "

(E) 10

20

30

40

_~.~/~

60

70

so

Fig. 4. X-ray powder diffraction patterns of titanias prepared by using various alcohols. Calcination, 823 K, 2h; alcohol/TIP (mol/mol) = 4. (A), benzyl alcohol" (B), 2-phenylethanol; (C), 2-ethyl-l-hexanol; (D), 1-phenyl ethanol; (E), 2-phenyl-2-propanol

728

3.2 Modified

sol titanias

The specific surface areas and the pore volumes of titanias prepared by a modified sol method are shown in Table 4. The mesopore volume increased with increasing the molecular size of the complexing agent, while the specific surface areas of three samples are almost equal. The pore size of modified sol titania also increased with increasing the molecular size of the complexing agent (Fig. 5). In this method, a complexing agent reacts with the surface hydroxy group of titania sol at the first step. This combined complexing agent has an influence on the structural changes during drying and calcination. These changes seem to result in different pore structures. The modified sol method was also effective for controlling the pore structure. Table 4. The specific surface areas and the pore volumes of titanias prepared by a modified sol method. Specific surface Mesopore Complexing agent area (m2/g) volume (cm3/g) Ethylene glycol Diethylene glycol Triethylene glycol

77 82 78

0.09 0.21 0.24

Calcination, 823 K, 2h; alcohol/Ti (mol/mol) = 4. dV/dP~,~l:0m3nm-lg-1 o u , I'", ...... " I ....

dV/dR/mm3nm -lgq 100

80

400

60

.

300 40 20

200~-

o

lO 0

r

ia

2mO e t

/

20

' .... ""i 89 A1203

-

....

' TiO2/A1203 (2-ME) 2::::: A1203 (2-EHA) TiO2/Al203 (2-EHA~ - - - A1203 ( 1 -PEA) ~ TiO2/A1203 (1-PEA)S_ i=''=" TiO2(1-PEA) /A1203 (1-PEA) __ ! ,-,

'

.

.

.

~t

,~ : ~

k

-,.

o ~ ~ i ~ . . '.':. 9.-r..i~ Fig. 5. Pore distributions of titanias prepared by 0 5 10 15 20 25 30 35 40 Pore d i a m e t e r / n m modified sol method. Calcination, 823 K, 2h; alcohol/Ti (tool/tool) = 4. Fig. 6. Pore distributions of precipitated titania/aluminas, 2-ME, 2-methoxyethanol, 2-EHA, 2-et hyl- 1-hexanol, 1-PEA, 1-phenylethanol. Calcination, 823 K, 2h; alcohol/Al (mol/mol) = 4, Ti/AI=I.

729

3.3 Precipitated titania/aluminas Table 5 and Fig. 6 show the specific surface areas, pore volumes and pore distributions of precipitated titania/aluminas (Ti/AI=I) and alumina supports prepared by using complexing agents. Generally, the specific surface areas, pore volumes and pore sizes of precipitated titania/aluminas were proportional to those of the corresponding alumina supports. However, the mesopore of precipitated titania/alumina prepared by using an alumina (1-PEA) with extremely large pore disappeared. In this case, the titania precursor was filled in the pore on the alumina support during mixing and most of the pore might be covered by titania after calcination. However, pore volume and pore size remarkably increased when ligand exchanged titanium alkoxide was used as a titania precursor. Titania/aluminas with large pore volume and pore size could not be obtained with this method. However, their surface areas are larger than those of pure titanias, depending on the structure and coordination ability of the complexing agents. Table 5. Specific surface areas and Por e volumes of precipitated titania. Mesopore Material Complexing agent Specific surface volume (cm3/g) area (mZ/g) AlzO3 2-Metho xyet ha no I 282 0.58 TiOz/Alz03 2-Methoxyethanol 153 0.29 AIzO3 2-Ethyl- 1-hexano 1 315 0.89 TiOz/AlzO3 2-Ethyl-l-hexanol 183 0.40 AlzO3 1-Phenylethanol 338 1.43 TiOz/Alz03 1-Phenylethanol 142 0.08 TiOz/Alz03 a' 1-Phenylethanol 187 0.45 Calcination, 823 K, 2h (support)+2h(precipitated sample); alcohol/Al (mol/mol) = 4, Ti/AI=I. a) Titanium alkoxide which was prepared from titanium iso-propoxide and 1-phenylethanol by ligand exchange reaction was used as a titania source. ,

,

4. C O N C L U S I O N The preparation of titanias and precipitated titania/aluminas using complexing agents enables a systematic control of their pore structures. This method is effective not only for modification of titanium alkoxide which is used as a titania precursor but also for modification of titania sol.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

K.C. Song et al., J. Colloid Int. Sci., 231 (2000) 289. Y. Miyake and T. Kondo, J. Chem. Eng. Jpn., 34 (2001) 319. S. Cabrera et al., Solid State Sci., 2 (2000) 513. S. Takenaka et al., J. Sol-Gel Sci. Tech., 19 (2000) 711. M. Toba et al., J. Mater. Chem., 4 (1994) 585. M. Toba et al., J. Sol-Gel Sci. Tech., 19 (2000) 695. K . S . W . Sing et al., Pure Appl. Chem., 57 (1985) 603.

This Page Intentionally Left Blank

Studies in Surface Science andCatalysis143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

731

Tungstophosphoric acid immobilized in polyvinyl alcohol hydrogel beads as heterogeneous catalyst Luis R. Pizzio, Carmen V. Cficeres and Mirta N. Blanco Centro de Investigaci6n y Desarrollo en Procesos Cataliticos (CINDECA), UNLP-CONICET, 47 N ~ 257, (1900) La Plata, ARGENTINA e-mail: hds @dalton.quimica.unlp.edu.ar The preparation of tungstophosphoric acid supported on beads of polyvinyl alcohol hydrogel and polyethylenglycol, by means of equilibrium impregnation, was studied. The catalyst was characterized by FT-IR, 31p MAS-NMR, XRD, SEM-EDAX and TG-DTA. The results showed that the acid retains its Keggin structure upon impregnation. Moreover, the anion is firmly attached to the support, as seen by leaching studies. The acidity measurements by means of potentiometric titration with n-butylamine showed that the catalyst presents similar acid strength than the bulk acid, although the number of acid sites is lower. A high selectivity to ester formation was observed in the reaction of esterification of acetic acid with isoamyl alcohol. 1. I N T R O D U C T I O N The strong acidity of heteropolyacids (HPA) make them suitable as catalysts for many acid-catalyzed reactions. Since HPA are less corrosive and produce lower amount of waste than conventional acid catalysts, as sulfuric acid, they can be used as replacement in environmentally benign processes. The use of a support allowing the HPA to be dispersed over a large surface may result in an increase of its catalytic activity. The performance of supported HPA catalysts depends on the carrier, the HPA loading, conditions of pretreatment, among other variables. Acidic or neutral solids such as active carbon, SiO2 and ZrO2 are suitable as supports [1]. But HPA often leaks out of catalyst supports even in vapor-phase reactions. It is important, for practical purposes, to develop supported catalysts which can be applied to several reactions with no leakage of HPA. Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect.

732

Molybdophosphoric acid was also used to prepare HPA-polymer composite film catalysts, using polyphenylene oxide, polyethersulfone and polysulfone as polymers [4]. The membrane-like materials were tested as catalysts in the liquid-phase synthesis of tertbutanol from isobutene and water, showing higher catalytic activity than the bulk acid. On the other hand, polyvinyl alcohol is used in biotechnology for the encapsulation of enzymes or cells, due to its ability to form hydrogels by crosslinking with boric acid or formaldehyde [5]. Its use for the encapsulation of metal catalysts was also reported [6]. Taking into account the above-mentioned reports, we prepared a tungstophosphoric acid (TPA) based catalyst by means of equilibrium impregnation and using beads of polyvinyl alcohol hydrogel (PVA) and polyethylenglycol (PEG) as support. The aim is to obtain materials without TPA leakage. The characterizations were carried out by different physical-chemical techniques. The activity of the catalyst is measured in the esterification of acetic acid with isoamyl alcohol. 2. E X P E R I M E N T A L

2.1. Catalyst preparation The PVA-PEG gel beads (mean diameter: 2 ram) were prepared using the freezingthawing method [7]. The PVA solution was prepared by solving PVA (Mallinckrodt, 2.2 g) in a hot mixture of PEG (Mallinckrodt, 2.0 g) and water (18 g). The solution was slowly dropped into liquid nitrogen. After freezing, the beads were slowly thawed after the nitrogen was evaporated, using a Dewar vessel. The beads (1 g) were impregnated in equilibrium at 20 ~ with an ethanol-water (50 % v/v) solution (0.004 dm 3) of TPA (Fluka) of 130 g W/dm 3 concentration for 72 h and dried at room temperature. Tungsten concentration in the catalyst was calculated on the basis of the decrease of tungsten amount in the solution, by means of a mass balance. The tungsten concentration in the solutions, both before and after contacting the beads, was determined by atomic absorption spectrometry. The calibration curve method was used, with standards prepared in the laboratory. The equipment used was an IL Model 457 spectrophotometer, with single channel and double beam, and monochromator of 330 mm focal distance. The light source was a hollow monocathode lamp. The analyses were carried out at a wavelength of 254.9 nm, bandwidth 0.3 rim, lamp current 15 mA, phototube amplification 800 V, burner height 4 mm and acetylene-nitrous oxide flame (11:14). Catalyst thus obtained was washed with toluene, acetonitrile or chloroform, at 70 ~ for 6 h, in a system with continuous stirring. Finally, catalysts were thermally treated in the same conditions as before washing. 2.2. Catalyst Characterization 2.2.1. Fourier transform infrared spectroscopy Spectra of PVA-PEG beads and TPA-PVA-PEG samples dried at room temperature were recorded. For these analysis, a Bruker IFS 66 FT-IR equipment, pellets in BrK and a measuring range of 400-1500 cm- 1 were used. 2.2.2. Nuclear magnetic resonance spectroscopy The same solid samples studied by FT-IR were analyzed by 31p MAS-NMR. For

733

this purpose, a Bruker MSL-300 equipment with a sample holder of 5 mm diameter and 10 mm in height was employed, using 5 [as pulses, a repetition time of 10 s and a frequency of 121.496 MHz, being the resolution of 3.052 Hz per point and the spin rate 2.1 kHz. The repetition time was 3 s, and several hundred pulse responses were collected. Phosphoric acid 85% was employed as external reference. 2.2.3. X-Ray diffraction XRD patterns of the solid samples were recorded. The equipment used to this end was a Philips PW-1732, with built-in recorder. The operative conditions were: Cu I ~ radiation, nickel filter, 30 mA and 40 kV in the high voltage source and scanning angle between 5 and 55 ~ of 20 at a scanning rate of 1~ per minute. 2.2.4. Scanning electron microscopy The distribution of TPA molecules over the radius of the beads of PVA-PEG was measured using a Philips Model 505 scanning electron microscope with energy dispersive X-ray analysis (EDAX) system. The secondary electron micrographs of selected solid samples were obtained. 2.2.5. Thermogravimetric and differential thermal analysis The TG-DTA measurements of representative samples dried at 70 ~ were carried out using a Shimadzu DT 50 thermal analyzer. The thermogravimetry and differential thermal analysis experiments were performed under argon or nitrogen respectively, using 25-50 mg samples and a heating rate of 10 ~ Quartz cells were used as sample holders with c~A1203 as reference. The studied temperature range was 25-700 ~ 2.2.6. Acidity measurements Acidity of solid samples was measured by means of potentiometric titration. A small quantity of 0.05 N n-butylamine in acetonitrile was added to a known mass of solid suspended in acetonitrile, and agitated for 3 h. Later, the suspension was titrated with the same amine solution at 0.05 ml/min. The electrode potential variation was measured with a digital pHmeter. 2.2.7. Catalytic activity The esterification was carried out in a glass batch reactor at atmospheric pressure. Isoamyl alcohol (0.058 tool) and acetic acid (0.058 tool) were dissolved in toluene (0.114 tool). Then, the TPA-PVA-PEG catalyst (0.100 g) was added and the resulting mixture was heated to reflux. The reaction was followed by gas chromatography, using a thermal conductivity detector. Also, hydrochloric acid was used as catalyst for comparative purpose. 3. RESULTS AND DISCUSSION

3.1. Fourier transform infrared spectroscopy FT-IR spectrum of bulk TPA dried at 70 ~ (Fig. la) shows bands at 1081, 982, 888, 793, 595 and 524 cm -~, which coincide with those referred to in the literature for the acid

734

H3PW12040 [8]. The first five bands are assigned to the stretching vibrations P-Oa, W-Od, W-Ob-W, W-Oc-W, and to the bending vibration Oa-P-Oa, respectively. The subscripts indicate oxygen bridging the W and the heteroatom (a), corner-sharing (b) and edge-sharing (c) oxygens, belonging to WO6 octahedra, and terminal oxygen (d). Fig. lb shows the FF-IR spectrum of TPA-PVA-PEG catalyst (290 mg W/g support). It presents bands at 1081, 982, 897 and 812 cm -1 assigned to [PW~2040] 3- anion, although some of them are overlapped with those of the support (Figure lc). The W-Od band (982 cm -1) exhibits a splitting that may be a result of a direct interaction between the [PWlzO40] 3- anions and C-OH2 + groups of the PVA-PEG support. Similar observations have been reported for the case of W-Od in Cs2.sH0.sPW]2040 [9] and Cul.sPWl2040 [10] salts.

a

23

(1) o

C

E 03

t"-

I--

c

I

1400

,

I

1200

,

I

1000

,

I

800

,

I

600

,

400

Wav enumber (cm -1)

Fig. 1. FT-IR spectra of bulk TPA (a), TPA-VPA-PEG catalyst (b), PVA-PEG support (c). 3.2. Nuclear magnetic resonance spectroscopy 3ap MAS-NMR spectrum of TPA-PVA-PEG beads shows an intense line a t 14.6 ppm which can be attributed to the [PW12040] 3- anion [11]. Neither the [PWllO39] 7lacunar species nor [P2W21071] 6- dimeric anion were detected by this technique. Then, the study of the catalyst by FT-IR and 3]p MAS-NMR allowed us to verify that the species present in the solid coincided with those in the impregnating solution of the support, that is [PW12040] 3- anion [1]. 3.3. X-ray diffraction and scanning electron microscopy The XRD pattern of the TPA-PVA-PEG catalyst did not present lines corresponding to crystalline structures and, as a consequence, it is similar to that of the support. This may

735 be due to a high dispersion of TPA on the support surface and/or to the presence of amorphous TPA. On the other hand, according to the results obtained by energy dispersive X-ray analysis (EDAX), a uniform distribution of TPA along bead radius was obtained. The secondary electron micrographs of the bead inner shows an sponge-like gel structure.

3.4. Thermogravimetric and differential thermal analysis The DTA of the PVA-PEG beads (Fig. 2a) shows three endothermic peaks at 51, 221 and 317 ~ The first one is associated with the loss of physisorbed water. The other two peaks are assigned to evolution of water and other volatile compounds (aldehydes, ketones and ethers), as a result of the thermal degradation of the PVA and PEG [12]. From the TG diagram, it was calculated that the amount of physisorbed water is less than 11% of PVA-PEG bead weight, and the degradation of the support is almost complete at 500 ~ (95 % weight lost of the total amount). The DTA of TPA-PVA-PEG catalysts (Fig. 2b) presents an endothermic peak at 145 ~ associated with the dehydration of H3PW12Oa0.6H20 phase and another exothermic peak at 580 ~ assigned to the Keggin's anion decomposition. According to Mioc et al. [13], Keggin's anions are transformed at about 600 ~ in a new monophosphate bronze type compound PW8026. The TG diagram shows that the decomposition takes place without appreciable weight loss.

3.5. TPA leaching The amount of TPA removed from TPA-PVA-PEG catalyst washed with toluene, acetonitrile or chloroform, was less than 0.5 %. This indicates that [PW12040] 3- anion is firmly attached to the support. This can be explained assuming that the interaction between the tungstophosphate anion and PVA-PEG support groups is of electrostatic nature due to transfer of TPA protons to the OH groups of the support. However, positive charge may be distributed through conjugate bonds giving, as a result, relatively soft cations. Then, the adsorption might not even be purely electrostatic involving, in addition, interactions of covalent nature in variable degree, as in the interaction between soft cations and anions.

3.6. Acidity measurements by potentiometric titration The curves obtained by titration with n-butylamine for bulk TPA, PVA-PEG beads and TPA-PVA-PEG sample are shown in Fig. 3. A criterion for interpreting the results is that the initial electrode potential (Ei) indicates the maximum acid strength of the surface sites and the range where the plateau is reached (meq/g solid) indicates the total number of acid sites [14]. Bulk TPA presents very strong acidic sites (Ei = 631 mV) (Fig. 3a). TPA-PVA-PEG sample shows acidic surface sites with essentially the same strength (Ei = 640 mV), but a lower total number of acid sites (Fig. 3b), as a result of the lower acidity of the protons engaged to OH groups. On the other hand, the PVA-PEG beads present very weak acidic sites (El =-63 mV).

736

a

b

5

5 ,< (5

(5

s

145 ,

o

'

2~o

'

!

4~o

600

Temperature (~

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o

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,

200

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400

I

600

Temperature (~

Fig. 2. TG-DTA of PVA-PEG beads (a) and TPA-PVA-PEG catalyst (b). 3.7. Catalytic activity

The catalytic activity of TPA-PVA-PEG catalyst for the esterification of acetic acid with isoamyl alcohol is presented in Table 1. A high selectivity to the ester was obtained, although traces of 3-methyl-l-butene, 2-methyl-l-butene, and isovaleric aldehyde were detected by means of GCMS. On the other hand, the catalytic activity value obtained with HC1 as catalyst is also shown in Table 1. Besides, a previously prepared catalyst [15], TPA supported on functionalized silica (TPA/SF-T), was tested and the value given in the same table for comparative purpose. a 600

b

_~mm \

mm

|

t

400

200

-200 0,0

i 0,5

,

n,m

~mmmmmmmmm

2o0

Tm ,

\

40o

0

I 1,0

-200 0,0

,

I 0,5

,

, 1,0

....

meq/g TPA

Fig. 3. Potentiometric titration of bulk TPA (a) and TPA-PVA-PEG catalyst (b).

737

Table 1 Catalytic activity of TPA-PVA-PEG, TPA/SF-T and HC1 catalysts Catalyst TPA-PVA-PEG TPA/SF-T

HC1

TON 8.02 10 z 6.96 10 z 4.09 10 z Turn over number (TON): moles of ester formed at 5 hours/moles of H + in the catalyst It is observed that the activity of the TPA-PVA-PEG catalyst is slightly higher than that of TPA/SF-T catalyst. The different acidity of these catalysts may explain the behavior mentioned. Both catalysts have very strong sites, nevertheless the acid strength of the TPA supported on the functionalized silica (Ei = 185 mV) is lower than that of TPA supported on the polymeric material (Ei = 640 mV). Also, the catalysts based on TPA show TON values appreciably higher than that corresponding to HCI. So, the TON value of TPA-PVA-PEG is twice that of the mineral acid. These results are in accordance with the higher acidity of TPA with respect to HCI. On the other hand, the TPA-PVA-PEG catalyst were reused several times without appreciable loss of catalytic activity. It was determined by atomic absorption spectrometry and UV spectroscopy that, during the reaction, leaching of TPA did not occur. Therefore, this type of catalysts can be used in liquid phase reactions without an appreciable loss of TPA, so being attractive for many processes in replacement of conventional homogeneous catalysts at temperatures under the decomposition of support (200 ~ 4. CONCLUSIONS The use of PVA-PEG beads as a support of TPA enables to retain the primary Keggin structure of the heteropolyacid, as seen through the physical-chemical characterizations. A new TPA supported catalyst was obtained, which proved to give a high yield and selectivity in the preparation of isoamyl acetate by means of the liquid-phase catalytic esterification of isoamyl alcohol and acetic acid. A comparison with the yield obtained with a mineral acid, HCI, showed high performance of the TPA-PVA-PEG catalyst. Moreover, it has the advantages of an easy catalyst separation from the reaction medium and lesser problems of corrosion. As a consequence, it leads to an ecofriendly technology for the preparation of isoamyl acetate, which is important for flavor and fragrance industries and another fine chemical products. REFERENCES

1. L.R. Pizzio, C.V. Cficeres and M.N. Blanco, Appl. Catal., 167 (1998) 283. 2. B. Wang and S. Dong, Electrochim. Acta, 38 (1993) 1029. 3. M. Hasik, W. Turek, E. Stochmal, M. Lapkowski and A. Pron, J. Catal., 147 (1994) 544. 4. S.S. Lim, Y.H. Kim, G.I. Park, W.Y. Lee, I.K. Song and H.K. Youn, Catal. Lett., 60 (1999) 199. 5. M. Watase, K. Nishinari and M. Nambu, Cryo Letters, 4 (1983) 197. 6. U. Pr/isse, S. H6rold and K.-D. Vorlop, Chem. Ing. Tech., 69 (1997) 100.

738 7. U. Prfisse, B. Fox, M.F. Bruske, J. Breford and K.-D. Vorlop, Chem. Ing. Tech., 21 (1998) 29. 8. C. Rocchiccioli-Deltcheff, R. Thouvenot, and R. Franck, Spectrochim. Acta, 32A (1976) 587. 9. S. Choi, Y. Wang, Z. Nie, J. Liu and C.H.F. Peden, Catal. Today, 55 (2000) 117. 10. T. Okuhara, T. Hashimoto, T. Hibi and M. Misono, J. Catal., 93 (1985) 224. 11. R. Massart, R. Contant, J.M. Fruchart, J.P. Ciabrini and M. Fournier, Inorg. Chem., 16 (1977) 2916. 12. K.J. Voorhees, S.F. Baugh and D.N. Stevenson, Thermochim. Acta, 274 (1996) 187. 13. J.B. Mioc, R.Z. Dimitrijevic, M. Davidovic, Z.P. Nedic, M.M. Mitrovic and PH. Colomban, J. Mater. Sci., 29 (1994) 3705. 14. R. Cid and G. Pecchi, Appl. Catal., 14 (1985) 15. 15. L. Pizzio, P. Vfizquez, C. Cficeres and M. Blanco, Proceedings ENPROMER 2001, Vol. II (2001) 979.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Functionalized

SiMCM-41

as

739

support for heteropolyacid based catalysts

L.R. Pizzio, A. Kikot, E. Basaldella, P. Vfizquez, C.V. Cficeres and M.N. Blanco Centro de Investigaci6n y Desarrollo en Procesos Cataliticos (CINDECA), Facultad de Ciencias Exactas, UNLP-CONICET, 47 N ~ 257, 1900-La Plata, ARGENTINA. e-mail: vazquez @dalto n. quimica.unlp.edu, ar Different characterization techniques were used to evaluate the immobilization and acidic properties of Keggin structured tungstophosphoric and molybdophosphoric acids impregnated on an amine-functionalized SiMCM-41 support. The characteristics of prepared catalysts were correlated to the activity in the esterification of acetic acid with isoamyl alcohol to obtain isoamyl acetate. 1. I N T R O D U C T I O N Nowadays, significant changes are taking place worldwide in the area of classical fine chemicals. As a consequence, the development of profitable production of already established fine chemicals can only be achieved with innovative methods that have ecological and economical benefits. In comparison with heterogeneous catalytic systems, homogeneous catalysts often show attractive selectivities under mild conditions and are generally better understood on a molecular level [1]. Nevertheless, their disadvantages, due to the difficult product recovery and environmental pollution as a consequence of harmful wastes, among others, lead to the use of heterogeneous catalysts as a possible and desirable solution. The development of catalysts obtained by means of heteropolyacids (HPA) and related compounds is a very important growing field. Since HPA are less corrosive and produce lower amount of wastes than conventional acid catalysts, they can be used as replacement in ecofriendly processes. Within the HPA, there is a special interest in those which present a Keggin type structure. A disadvantage of HPA as catalysts lies in their relatively low thermal stability. It has been tried to stabilize them by supporting the HPA on several carriers as silica, alumina, titania [2, 3] and functionalized silica [4]. Nevertheless, the leaching of supported heteropolycompounds cannot beforehand be excluded when catalysts are used in heterogeneous liquid reactions. In order to avoid such phenomenon, the behavior of SiMCM-41 supported tungstophosphoric (TPA) and molybdophosphoric (MPA) acids, with Keggin structure, is studied. This material was selected as the host structure because it has been shown to be an excellent support for preparing bifunctional catalysts due to its high surface area coupled with its regular hexagonal array of uniform pore sizes within the mesoporous region. The SiMCM-41 surface was also modified by covalent attachment of primary amino groups through the grafting of 3-aminopropyltriethoxysilane to the siloxy groups.

740

For a better understanding of these supported HPA catalysts, the characterization of the adsorbed species nature by alp NMR, FT-IR and XRD techniques and their acidity by potentiometric titration were performed before and after the leaching with ethanol/water. Besides, the catalytic activity in the esterification of acetic acid with isoamyl alcohol to obtain isoamyl acetate is discussed.

2. EXPERIMENTAL 2.1. Catalyst Preparation Mesostructure synthesis. Pure siliceous, SiMCM-41 (MS) support (SBET: 915 mE/g) was hydrotermally synthesized at pH = 9-10 in our laboratory according to the methodology described in [5]. A solution of commercial waterglass (SiO2, 26.8 % w/w; Na20, 9.2 % w/w; H20, 64 % w/w) was used as the silica source, employing cetyltrimethylammonium bromide (CTABr, 98 %, Aldrich), as the framework structure director. The molar ratio of the starting mixture was 1.89 SiO2:l CTABr:0.738 Na20:0.267 H2SO4:160 H20. The resulting gel was stirred for about 1 h, then it was transferred in a teflon container, and placed in an oven at 100 ~ for 4 days. To provide a control for the pH, the synthesis was carried through to completion with addition of appropriate 1M H2SO4 solution each 24 h. The solid product was recovered and washed by filtration on a Buchner funnel, and dried in air at room temperature (r.t.). The surfactant was subsequently removed from the mesostructure by calcination at 650 ~ for 6 h. Functionalization. MS functionalization was performed by addition of 3aminopropyltriethoxy-silane to a suspension of organic free MS in refluxing toluene and stirred for 5 h. The solid (MS-F) was filtered, washed in a Soxhlet apparatus with diethylether and dichloromethane and dried at 120 ~ [6]. Catalyst Preparation. MS and MS-F were impregnated using the equilibrium adsorption technique at 20 ~ The support (1 g) was contacted for 72 h with 4 ml of a solution obtained by dissolving the corresponding HPA in an ethanol/water (e/w) solvent. The solution concentration was 110 g W(Mo)/1, using Fluka H3PW(Mo)IEO40.nH20 as precursors. The solid was separated from the solution by centrifugation and dried at r.t., thus obtaining samples HPA-MS and HPA-MS-F. In order to evaluate the HPA retention in the mesoporous structure, these samples were leached in e/w, with continuous stirring, for two periods of 24 h (hereinafter named HPA-MS-L and HPA-MS-F-L).

2.2. Characterization Textural properties. The specific surface area (SBET), the pore volume and the mean pore diameter of supports and catalysts were determined by nitrogen adsorption/desorption technique using a Micromeritics Accusorb 2100E equipment. A Bruker IFS 66 equipment, pellets in BrK and a measuring range of 400-4000 cm -1 were used to obtain spectra of solids. A Bruker MSL-300 equipment linked to a "SOLIDCYC.DC" pulse program was utilized to obtain 31p MAS-NMR spectra of solids. The mesostructure supports were characterized by small-angle X-ray scattering (XRD), using a Phillips PW-1714 diffractometer. A small quantity of 0.1 N n-butylamine in acetonitrile was added to a known mass of solid, and shaken for 3 h. Later, the suspension was potentiometrically titrated with the same base at a flow of 0.05 ml/min. The electrode potential variation was

741 measured with an Instrtmaentalia S.R.L. digital pHmeter. Catalytic activity. The esterification was carried out in a three-neck flask (100 ml) equipped with a water-cooled condenser, a thermometer and a glass tube to extract the solution. The reactants were acetic acid (0.1162 mol) and isoamyl alcohol (0.1162 mol); toluene (0.2317 mol) was used as solvent. After the catalyst (0.200 g) was added to the solution, the resulting mixture was heated to reflux and the reaction followed by gas chromatography, using a thermal conductivity detector. The specific conversion was calculated as the molar ratio of the formed ester and the amount of HPA present in the solid.

3. RESULTS AND DISCUSSION 3.1. Support characterization XRD. XRD patterns corresponding to SiMCM-41 before and after functionalization (MS and MS-F, respectively) are shown in Figure 1. It can be seen that MCM-41 was the only phase formed. The pattern shows the strong reflection for the (100) plane of MCM-41 and also well --resolved secondary peaks. The secondary peaks indicate long-range .~" ordering of the MCM-41 structure. Additionally, it can be seen in Fig. 1 -= that organosilane grafting to the mesostructure (MS-F) causes a significant decrease in peak MS intensities. These results could be MS-F attributed to the occurrence of contrast matching between the silica 2 4 6 8 10 framework and the grafted organic 0 2 tetha groups [8]. Fig. 1. XRD patterns of SiMCM-41 before FT-IR. Fig. 2 shows the FT-IR (MS) and after functionalization (MS-F). spectra of MS, MS-F and 3aminopropyltriethoxysilane (ANH). The main difference between the MS-F s~ectrum and that of the MS is due to the presence of small shoulders in the 3000-2750 c m region of the MS-F spectrmn, which can be assigned to the C-H2 carbon-hydrogen stretching of ANH [9]. The ANH used in the graining process presents another strong band in the region 1110-1050 cm1, assigned to the Si-O-C aliphatic groups [9], which is overlapped with a band of MS. It is present as a small shoulder in the functionalized support. r

'

I

'

l

'

I

'

I

'

I

742 On the other hand, the surface area decreases from 915.5 to 307.3 m2/g by the grafting of the MS surface. These results suggest that a noticeable attachment of the basic amino groups and the silanol groups takes place at the working conditions.

Fig. 2. FT-IR spectra of MS, MS-F and ANH. ai

3.2. Catalyst characterization FT-IR. The FT-IR spectra of bulk and supported HPA are shown in Fig. 3. The main characteristic features of bulk TPA are observed at 1081 (P-O), 982 (W=O), 888 and 793 (W-O-W) cm-l (Fig. 3a) and at 1064 (P-O), 962 (Mo=O), 869 and 787 4000 3500 3000 2500 2000 1500 1000 500 (Mo-O-Mo) cm-1 for bulk MPA (Fig. 3b). wavenumber (cm-1) In the HPA/MS and HPA/MS-F spectra, a support band masks the HPA band placed at the 1100-1050 cm-1 zone. Anyway, information can still be obtained from the less masked regions. For MPA/MS, an intensity increase of the band placed at 962 cm1 and a small nonoverlapped band at 869 crnl is observed (Fig. 3b), thus confirming the presence of the undegraded heteropolyanion. TPA/MS catalyst shows the bands at 982 and 793 cm 1 as an increase in transmittance of support bands, while the band at 888 crn-1 is observed without overlapping (Fig. 3a). For TPA/MS-F and MPA/MS-F samples, small bands and shoulders are observed in the 800 - 950 cm-1 zone, which could be possibly assigned to the [PW(Mo)IIO39] -7 lacunar species. The FT-IR spectra of the catalysts, after leaching with ethanol/water (TPA(MPA)/MS-L; TPA(MPA)/MS-F-L) do not present important changes, when they are compared with the spectra before leaching (Figure 3c and 3d). NMR. Fig. 4 illustrates the 31p MAS NMR spectra for MPA and TPA supported on MS. Previously reported bulk acids chemical shills are between -14.8/-15.3 ppm for TPA and 2.9/-4.8 ppm for MPA [2, 3]. In our samples, obtained spectra exhibit one wide line with maximum at -3.6 ppm for MPA/MS and at -15.1 ppm for TPA/MS. These measurements confirm the presence of the acids, in accordance with the FT-IR results.

743

, /.-

TPA. y

" ~ ........

/I\/

-',.

\

MPA

,,.y, '",\ //-, :

//

. ................

-

,

:

i~.

\/

~/

\

1-

k

"7:;" -~'......... -'x

t\ /~J~

.....

I

/ \/

5

a

(1) O

1200 ' 12'00 ' 10~00 ' 8()0 ' 6;0

E (#}

TPNMS-F-L

E fill

'

/ '',,

L

14'oo

400

'

1200

1000

.

\

\.........

':.j'

:./'

\-

\\

.:

'

~2'oo

400

/ \/

I

\ :t

L

/'

C /4'00

600

MPNMS-F-L

i-

TPA/MS-L

800

'

lo'oo

'

8~o

'

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'

!

,

1400

400

-

\

1200

/

1000

800

~venumber

waven umber (cm -1)

600

400

(cm -1)

Fig. 3. FT-IR spectra of MS and MS-F supported TPA (MPA), and the bulk acids (a, b) and the catalysts after leaching with ethanol/water (c, d).

-15.1

-3.6 ppm

MPNMS

,

,

.

.

-100'-80-60-40-20

.

.

0

.

.

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.

chemical shift (ppm)

Fig.

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40

.

.

.

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spectra

.

.

.

.

.

.

.

.

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chemical shift (ppm)

for

TPA

and

MPA

supported

100

on

MS.

744 Leaching in ethanol/water. MPA and TPA contents of the catalysts, before (CT) and after (CL) leaching with e/w at 20 ~ are shown in Table 1. Table 1 TPA(MPA) contents in the catalysts before and after the leaching Catalyst MPA/MS MPA/MS-F TPA/MS CT 0.25 0.21 0.16 CL 0.23 0.19 0.13 CT, eL: concentrations in mmol TPA(MPA)/g cat

TPA/MS-F 0.15 0.14

The results show that there is a decrease of the HPA amount for catalysts on MS-F support with respect to MS. For MPA-MS, CT is nearly 18 % higher than in MPA-MS-F; this difference is smaller, around 7 %, for TPA catalysts. This behavior could be due to MS surface area undergoes a high decrease during grafting process, thus diminishing the contact of MPA, or TPA, with MS surface. In addition, the MPA amount in both supports is higher than the corresponding to TPA. In a previous paper, we have reported that the MPA interaction with silanol type groups is stronger than that of TPA [4]. On the other hand, TPA and MPA solubility (So) during the leaching is shown in Table 2. The TPA amount removed from both supports is higher than that of MPA, though for MS-F, the difference between the two So values is lower. These low values are very attractive because they imply that these solids are promissory heterogeneous catalysts to be used in liquid phase. Table 2. TPA(MPA) solubility during the leaching Catalyst MPA/MS MPA/MS-F %So 10.73 7.16 O-foSo--(CT-CL/CT)1O0

TPA/MS 16.74

TPA/MS-F 8.72

Potentiometric titration. The catalyst acidity measurements by means of potentiometric titration with n-butylamine enable the evaluation of the total number of acid sites and their acidic strength [3]. The titration curves obtained for the catalysts, before and after leaching, are shown in Fig. 5. MS and MS-F were titrated in order to compare their acidities, although both carriers exhibit weak sites, MS shows higher acidity than MS-F (Fig. 5a). It is evident from this result that the grafting process attached the amine groups of 3aminopropyltriethoxysilane to the acidic sites of MS. Additionally, MPA and TPA present similar acid strength values when they are supported on MS, 1062 mV for TPA/MS and 1017 mV for MPA/MS (Figs. 5b and 5c). These results can be attributed to the presence of Keggin structures that remain unaltered onto the MS surface. The acidity considerably decreased for TPA when it is supported on MS-F (Fig; 5b). For MPA/MS-F, though the acid strength slightly decreased with respect to the system MPA/MS, the acidity is lower (Fig. 5c). This behavior could be due to different interaction of HPA on the functionalized support with respect to MS as a consequence of the different superficial groups. Another factor would be the Keggin structure degradation to less acidic lacunar species in the catalyst based on MS-F. In addition, this difference in acidity could be assigned

745 to a change of the HPA proton positions. The protons would be localized on the most highly charged oxygen atoms, they could migrate from bridged to terminal oxygens. The acidity remains practically unchanged for the MS supported catalysts after the leaching with e/w (Fig. 5d). A similar behavior was observed for the samples MPA/MS-F and TPA/MS-F. 1250.

a

50

1000

o

100-

~ o

-

%

750 -. 0

150-

o 0

500 200 250-

250 -

~

- 350

0 - Cl

MS-F

300 .

'

.

.

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.

.

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.

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"~:~-~'"~

-250

'

'

T PA/M S-F '

'1

'

I

1250

LU 1250

ooo

1000.,

750

t~

150 -

MPA/MS-L

500-

o4/

\

-[~

250

'1

] /

I

0,0

'

I 0,5

cb~,.,~_

MPA/MS

"~ MPA/MS-F '

I 1,0

meq/g

'

I 1,5

. '. . .

~_50-

oi :# ~pA/lVlS_F_LT PA/MS-F-L - ~ ~ ~ r P / ~ IVlS"EI' 25

2,0

' 0,0

I 0,5

'

I 1,0

rneq/g

'

I 1,5

"

' 2,0

Fig. 5. Potentiometric titration of MS and MS-F (a), supported MPA (TPA) (b, c) and the catalysts after leaching with e/w (d).

3.3. Catalytic activity The catalyst behavior was tested in liquid medium reaction for the esterification of acetic acid with isoamyl alcohol to obtain isoamyl acetate. This compound is an important product for flavor and fragance industries. The isoamyl acetate can be used as for instance flavor in mineral waters and syrups, perfume in diverse products, as shoe polish, and in the manufacture of materials like artificial silk and other textiles, among other uses.

746 Table 3 Specific conversion of the catalysts Catalyst MPA/MS-L MPA/MS-F-L SC 1.99 1.35 SC (Specific conversion)" tool ester/mmol MPA(TPA)

TPA/MS-L 2.47

TPA/MS-F-L 2.08

The specific conversion (SC) values for MPA(TPA)/MS-L and MPA(TPA)/MS-F-L are shown in Table 3. The TPA catalysts present a SC higher than the corresponding to the MPA system. Nevertheless, the acidity of these systems (Fig. 5d) is similar. This behavior can be explained taking into account that the [PW12040] 3- heteropolyanion presents a higher softness than [PMo12040]3-. In addition to the acidity, the softness of the heteropolyanion is an important characteristic in catalysis and greatly influences the catalytic behavior in organic solution [4]. On the other hand, the catalysts obtained from MS-F, after leaching, showed a slightly lower conversion than those on MS, also washed with e/w. This fact is correlated with the acidity measured by potentiometric titration (Fig. 5a). However, it is important to point out that these catalysts show low TPA(MPA) solubility during the leaching, as above-mentioned. 4. CONCLUSIONS The MPA and TPA based catalysts supported on a new amine-functionalized SiMCM-41 carrier showed important activity in the esterification reaction of acetic acid with isoamyl alcohol. On the other hand, low TPA (MPA) solubility during the leaching was observed. Then, these catalysts are very attractive for their use in liquid medium reactions. REFERENCES 1. M. Belier, Stud. Surf. Sci. Catal., 108 (1997) 1. 2. L. Pizzio, C. C~ceres and M. Blanco, Appl. Catal. A: General, 167, (1998) 283. 3. P. V~zquez, M. Blanco and C. C~ceres, Catal. Lett., 60, (1999) 205. 4. L. Pizzio, P. V~zquez, C. C~ceres and M. Blanco, Proceedings ENPROMER 2001, II (2001) 979. 5. K. Edler and J. White, Chem. Mater., (1997) 1226. 6. M. Lasp6ras, T. Lloret, L. Chaves, I. Rodriguez, A. Cauvel and D. Brunel, Stud. Surf. Sci. Catal., 108 (1997) 75. 7. W. Zhang, M. Froba, J. Wang, P. Tanev, J. Wong and T. Pinnavaia, J. Chem. Am. Soc., 118 (1996) 9164. 8. J. W.Cooper (ed.), Spectroscopic Techniques for Organic Chemists, John Wiley & Sons, New York, (1980).

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

747

Influence of the preparation method on the surface properties and activity of alumina - supported gallium oxide catalysts Alice Luminita Petre, a Bernard Bonnetot, b Antonella Gervasini e and Aline Auroux a a Institut de Recherches sur la Catalyse, CNRS, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France; e-mail: [email protected]

b Laboratoire des Multimat6riaux et Interfaces, UMR CNRS 5615, UCB Lyon I, 69622 Villeurbanne Cedex, France c Dipartimento di Chimica Fisica ed Elettrochimica, Universit~ di Milano, via Golgi 19, 20133 Milano, Italy As alumina seems to be the best support for gallium species in terms of activity in the reaction of NOx reduction by hydrocarbons, alumina-supported gallium oxide catalysts presenting similar surface areas were prepared by two different methods, an innovative pseudo sol-gel method and for comparison a conventional impregnation method. The bulk and surface compositions of the samples as well as their structural and textural features confirmed the better dispersion of the pseudo sol-gel sample. The acid-base properties of the samples were determined by adsorption microcalorimetry of ammonia and sulfur dioxide. The sample prepared in non-aqueous mixture showed a more homogeneous distribution of the acid sites, accompanied by an increase of the acid strength and a decrease of the number of basic sites. Moreover the catalytic activity for NOx reduction by ethene in oxygen excess was superior for the sol-gel sample. It is likely that this better activity could be associated with the higher dispersive effect of this synthesis method. 1. INTRODUCTION Gallium-containing materials have received a sustained attention over the past two decades as catalysts for aromatization and/or dehydrogenation of light alkanes. It is only recently that the high activity of supported gallium oxide catalysts in the NO reduction by hydrocarbons has been discovered, making them one of the most effective catalytic technologies for the abatement of NOx in diesel and lean burn engine exhausts [1,2]. The influence of the Ga203 content added to various supports on the catalytic activity for NO reduction by ethene or propene in the presence of high amounts of oxygen has been recently for a series of conventional commercial supports (A1203, SiO2, TiO2) [3]. It has been revealed that Ga203/A1203 exhibits high activity and selectivity with a good tolerance against water in the NO reduction [3,4].

748

The preparation method is decisive for the creation of highly dispersed and coordinatively unsaturated gallium species covering completely the alumina support; these species should play the role of active sites. Moreover, the support can alter the electronic and geometric configuration of Ga sites according to the acidity of the oxide surface binding the metal phase in different ways. It is expected that the physico-chemical properties and the activity of gallium oxide surfaces should be dependent on the type of preparation method. Ga203/AIzO3 prepared by a sol-gel method exhibits a much higher activity for NO reduction by propene than samples prepared by an impregnation method [5]. However, the important differences in surface area of the samples as well as the peculiarities of the preparation methods (co-precipitation or addition of gallium oxide precursor into the aluminium boehmite sol solution) make it difficult to compare the samples and understand the role of dispersion and/or of the presence of a composite oxide [5-71. As alumina was found to be the best support for gallium species in terms of activity towards NO reduction, we prepared alumina-supported gallium oxide catalysts presenting similar surface areas by two different methods, an innovative pseudo sol-gel method and for comparison a conventional impregnation method. In the pseudo sol-gel synthesis, the deposition of gallium a|koxide was obtained from gallium (III) acetylacetonate {Ga(CH3COCHCOCH3)3 or Ga(acac)3}, which was decomposed under strong basic conditions in a non aqueous multidentate Lewis base using published methods [8,9]. In this work the surface properties (dispersion, acidity) of thesupported gallium oxide catalysts and A1203 are presented comparatively, in relation with the preparation method and catalytic activity in deNOx. 2. E X P E R I M E N T A L A series of gallium oxide supported catalysts was prepared by incipient wetness impregnation of a "~-A1203 (surface area 108 m2.g-1, non porous, reference Oxid C from Degussa) with appropriate amounts of an aqueous solution of Ga(NO3)a,9H20. A second type of samples was prepared by a pseudo sol-gel method using the same alumina support. The gallium oxide was obtained from gallium acetylacetonate (Ga(acac)3) decomposed in a non aqueous medium. 5 g of dried ~,-alumina were added to a mixture of 150 mL of dried tetraglyme (methylether of tetraethylene glycol) and 50 mL of 2-propanol in a three necks glass flask under argon atmosphere. The amount of Ga(acac)3 required to obtain the expected GaaO3/A1203 ratio was dissolved in 50 mL of tetraglyme and added to the alumina suspension. A mixture of 6 mL of hydrogen peroxide (50 % solution in water) and 3 mL of concentrated ammonia hydroxide (28 % NH3 in water) was added to the reaction before heating. The mixture was refluxed under argon (the temperature was raised from 378 K to 443 K in order to eliminate the volatile species) for 48 hours under vigorous stirring and then cooled down to room temperature. During the reaction, the acetylacetonate was displaced by ammonia and an exchange of ligand took place leading to a gallium alcoholate with 2-propanol. The crude catalyst was separated from the mixture of solvents by filtration, and the remaining organic parts were removed by washing twice the solid with 50 mL of toluene. No gallium acetylacetonate was remaining in the organic solution. This reaction can be considered as a pseudo sol-gel method, because no gel of

749 gallium isopropanolate could be characterized and because the high temperature reached by the reflux at the end of the synthesis favoured the formation of oxide from alcoholate. The solid was then dried under vacuum up to a temperature of 373 K and until the pressure reached 10"l hPa (around 48 h). Both series of samples were subjected to a further calcination at 773 K under nitrogen flow (6 h) and then oxygen flow (6 h). The bulk and surface compositions of the samples as well as their structural and textural features have been determined by chemical analysis, BET, XRD, TEM and XPS measurements. The acid-base properties of the samples were investigated using adsorption of appropriate probe molecules, namely ammonia and sulfur dioxide, monitored by microcalorimetry. The microcalorimetric studies were performed at 353 K for sulfur dioxide adsorption and at 423 K for ammonia adsorption in a heat flow calorimeter of Tian-Calvet type (Setaram C80), linked to a conventional volumetric apparatus. Before each experiment the samples were outgassed overnight at 673 K. The Ga203-supported catalysts were tested in NO reduction by ethene under lean conditions (NO-C2H4-O2). The catalytic tests were carried out with about 0.1 g of sample placed in a quartz tubular microreactor (5 mm ID). The reactant stream was provided from a set of mass flow controllers (Bronkhorst, Hi-Tee) supplying 3000 ppm of NO and of C2H4 and 40,000 ppm of 02 in helium at a total flow rate of 85 cm 3 minl, with the reactor at close to atmospheric pressure. The contact time was maintained constant at 70 g s~ L -~, corresponding to a space velocity of about 50,000 h "~. The interval of reaction temperature from 473 K up to 823 K was investigated. The exit gas stream from the reactor flowed through an FT-IR gas cell (path length 2.4 m, multiple reflection gas cell) in the beam of a spectrometer (FT-IR from Bio-Rad with DTGS detector). The spectrometer provided analyses for NO, N20, and NO2 for the N-containing species, and C2H4,CO and CO2 for the C-containing species. The measurements were carried out at a resolution of 0.50 crn"~ with an accuracy of + 10 ppm for NO, and + 4 ppm for N20 and NO2, using bands centered at 1876, 2225, and 1619 cml , respectively. The sample was placed in the reactor between plugs of quartz wool and initially pretreated in a 20 % O2/He flow while raising the temperature in stages up to 623 K and maintaining it for 4 h. NO conversion was measured with respect to its initial effective concentration in the gas stream as determined from FT-IR analysis. The amount of NO flowing into the reaction line differed from that introduced in the mixture, due to the presence of high amounts of 02 that transformed part of the NO into NO2. The effective feed gas composition flowing into the reaction line was 2380 pm of NO, 600 ppm of NO2 and some amount of N20. Conversion of NOx (NO plus NO2) corresponded to N2 production. 3. RESULTS AND DISCUSSION

The main physicochemical characteristics of the alumina and of the differently prepared supported gallium oxide catalysts are reported in Table 1. The impregnated sample presented a small decrease in surface area compared to the alumina support. For the theoretical geometry monolayer coverage (approx. 20 wt % Ga203), the surface area was about 99 m2.g~. On the contrary, the surface area of a similar

750 sample prepared by our sol-gel method was higher than that of the support, about 124 m2 g-1. The XRD patterns were the same for the supported samples as for the alumina support. This showed that gallium oxide was deposited in an amorphous phase but did not allow to differentiate the quality of the dispersion. The better dispersion of the sol-gel sample in comparison with the impregnated sample was confirmed by TEM and XPS measurements. Using different probe molecules such as ammonia and sulfur dioxide, the acid-base properties of the samples were determined by adsorption microcalorimetry.

Table 1. Physicochemical characteristics of the alumina and of the differently prepared supported gallium oxide catalysts . . . . . . Sample Ga203 SBET Theoretical Chemisorption uptake weight % /m2.g1 coverage Acidity Basicity /% layer ~tmolNH3.g"l ~tml S02.~."1 7-A1203 108 105 209 Ga-A1 (i)

22.3

99

1.10

123

184

Ga-A1 (sg)

16.7

124

0.83

138

164

(i) impregnation; (sg) pseudo sol-gel

Fig. l a. Differential heats of ammonia adsorption versus coverage for the alumina and the supported gallium oxides. Fig. l a shows the differential heats of adsorption of NH3 at 423 K for the alumina support and for the two differently prepared supported gallium oxides. The alumina and Ga-A1 (i) sample show similar and very high initial heats of adsorption around

751 200 kJ mo1-1. As the coverage of these two samples increases, the initial heats decrease rapidly to reach values of =150 kJ moli and then more slowly but continuously, a result indicative of heterogeneous acidity. The plots of Qd~rvalues v s . coverage for sample Ga-A1 (sg) show the appearance of a plateau, and this profile can be related mainly to a significant homogeneity of the gallium acid sites. The number of acid sites corresponding to a given strength interval is represented in Fig. lb. The energetic distribution of acid sites in Ga-A1 (sg) leaves almost no room for doubt that more strongly acidic sites appear in comparison to Ga-A1 (i).

Fig. 1b. Strength distribution of the acid sites for the alumina and the supported gallium oxides. The increase in acidity upon deposition of gallium oxide on alumina is also confirmed in Table 1 by the increase in NH3 chemisorption uptakes, while the basicity, as measured by the acidic probe SO2, concomitantly decreased. The differences in sulfur dioxide adsorption behaviour between the two samples can be evidenced in Figs. 2a and 2b which represent, respectively, the differential heats of SO2 adsorption v s . coverage and the strength distribution of basic sites. For both samples, the total number of basic sites decreased approximately in the same manner as the acidity increased, the greatest differences in comparison with alumina being observed for the sol-gel sample. The sol-gel preparation allowed intimate contact of the gallium oxide with the support, resulting in a maximized strength (and hence energy) of the interaction. Ga203 deposition contributed to decrease the number of basic sites and increase the number of acid sites of the amphoteric alumina support. The isomorphism between alumina and gallia renders a comparative study difficult, but the observed differences in heats of adsorption for the two samples show that the nature of the interaction between active phase and support, and thus the dispersion, are different, a situation for which the preparation method should be responsible. Indeed, in aqueous solution the gallium oxide deposition occurs preferentially on the strongest OH

752 groups of the alumina support, while in non-aqueous mixture the deposition is more homogeneous on the alumina sites. The sol-gel samples also displayed a greater activity in the catalytic reduction of NO by C2H4 in oxygen excess. The reaction of selective reduction of NOx (SCR) was carried out with C2H4 as reducing species in high oxygen atmosphere (NO-C2H4-O2) working at very high space velocity. Conversions of both NO and NO2 (formed by the homogeneous oxidation of NO in the feed mixture) to N2 were followed together with conversion of C2H4 to CO and CO2.

Fig. 2b. Distribution of the strength of the basic sites for the alumina and the supported gallium oxides.

753 As expected, the two catalysts (Ga-A1 (i) and Ga-A1 (sg)) are active towards NOx reduction at high temperatures [3]. Therefore, we did not observe the typical volcanoshaped curve for NO conversion to Nz that is observed for de-NOx catalysts working at lower temperatures, such as copper-based catalysts [10]. The NOx conversion to N2 as well as the C2H4 conversion were continuously increasing with temperature. Quantitative conversions of NOx as well as of C2I-I4 were not observed even at the highest reaction temperature. Under the severe conditions employed in terms of concentrations and contact time, both catalysts were highly active and selective towards N2 formation. Starting from about 3000 ppm of NOx in the feed, N2 yields of 53 and 7 1 % were obtained at 823 K for Ga-Al (i) and Ga-A1 (sg), respectively. This corresponded to very high specific activities for N2 formation: 4.4 10.4 and 7.7 10.4 mOtN2, s'lmol-]Ga for Ga-A1 (i) and Ga-A1 (sg), respectively. Formation of N20 was detected only at temperatures higher than 723 K, the maximum amount formed being around 140 ppm for both catalysts. It is interesting to calculate the so-called SCR selectivity (SscR, %), defined as the ratio between the amount of C2H4 consumed to reduce NO to N2 and the total amount of C2H4 consumed. The SscR was around 20 % at 723 K and decreased to 14 % at 823 K for both catalysts. These values are among the highest reported in the literature and indicate that gallium oxide species remain unable to oxidize hydrocarbons even at very high temperatures, a very interesting behaviour for deNOx catalysts. Fig. 3 compares the NOx and C2H4 conversions over the two Ga-based catalysts. The results have been plotted as the conversion of NOx to N2 as a function of the extent of the C2H4 conversion, considered to be an index of the extent of the reaction. C2I-I4 is indeed the common species simultaneously able to reduce NOx to N2 while it can also be oxidized by 02 in the parallel side reaction. In this representation, the curve of Ga-A1 (sg) lies above that of Ga-A1 (i). Fig. 4 shows the NO and NO2 concentrations versus reaction temperature for the two catalysts. Starting from 673 K, the NO concentration decreased in a marked way as well as that of NO2, leading to N2 formation. The curve of NO concentration is steeper for Ga-A1 (sg) than for Ga-A1 (i), indicating the superior activity of Ga-A1 (sg). N2 production over the two catalysts is also reported in Fig. 4. 100 --.

80

o :

60

~-

4o

0 z

20

- e - Ga-AI (i) - ~ - Ga-AI (sg)

0

20

40

60

80

100

C2H4 conversion 1%

Fig. 3. Conversion of NOx to N2 as a function of the extent of C2H4 conversion

754 4. CONCLUSION The difference in deNOx activity between the two differently prepared samples further highlights their differences in structure and surface chemistry. The catalytic activity of supported gallium oxide is likely to be governed by the surface concentration ratio of acidic/basic functional groups. The differences in the heats of adsorption of NH3 and SO2 allow some insight into the nature of the interaction and hence the type of surface functional groups with which Ga/O3 interacts; namely in the impregnation method the strong acid sites of the alumina and in the pseudo sol-gel method a much larger population of acid sites.

2500

E

! 80

2000

60

o.

-A-Ga-AI (sg) ; NO

c 1500

-O- Ga-AI (i) NO2 9

50

~. e-

0=

-~-Ga-AI (sg)" NO2 -X-Ga-AI (i)" N2 (%)

40

o

o

L_

c 1000

o

:3

o 30 a.

-X-Ga-AI (sg)" N2 (%)

O C

o o

~

20 z

5O0-

10 0 300

,

,

400

500

0 600

700

800

900

Temperature I K

Fig. 4 - NO, NO2 concentrations and N2 production as ftmctions of reaction temperature for gallium-containing catalysts.

REFERENCES

1. 2. 3. 4. 5. 6. 7.

T. Maunula, Y. Kintaichi, M. Inaba, M. Haneda, K. Sato and H. Hamada, Appl. Catal. B, 15 (1998) 291. M. Haneda, Y. Kintaichi, T. Mizushima, N. Kakuta and H. Hamada, Appl. Catal. B, 31 (2001) 81. A.L. Petre, A. Auroux, A. Gervasini, M. Caldararu and N.I. Ionescu, J. Thermal Anal., 64 (2001) 253. K. Shimizu, A. Satsuma and T. Hattori, Appl. Catal. B, 16 (1998) 319. M. Haneda, Y. Kintaichi, H. Shimada and H. Hamada, J. Catal., 192 (2000) 137. K. Shimizu, M. Takamatsu, K. Nishi, H. Yoshida, A. Satsuma, T. Tanaka, S. Yoshida and T. Hattori, J. Phys. Chem. B, 103 (1999) 1542. Yu.N. Pushkar, A. Sinitsky, O.O. Parenago, A.N. Kharlanov and E.V. Lunina, Appl. Surf. Sci., 167 (2000) 69.

755 R.C. Mehrotra, R. Bohra and D.P. Gaur, "Metal -Diketonates and Allied Derivatives" Academic Press, London 1978. S. Kawagushi, Inorg. Chem. Concept, Vol 11, Springer Verlag (1988), p 89-90. 10. P. Camiti, A. Gervasini, V.H. Modiea and N. Ravasio, Appl. Catal. B, 28 (2000) 175. .

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation and properties influence of catalyst reduction

of bimetallic

757

Ru-Sn sol-gel catalysts:

J. Hajek 1, N. Kumar 1, H. Karhu 2, L. Cerveny3, J. Vayrynen2, T. Salmi I and D. Yu. Murzin 1. 1Laboratory of Industrial Chemistry, Process Chemistry Group, Abo Akademi, Turku/Abo, Finland ZDepartment of Applied Physics, Laboratory of Electron Spectroscopy and Surface Physics, University of Turku, Finland 3Department of Organic Technology, Institute of Chemical Technology in Prague, Czech Republic Ru-Sn/SiOz catalysts were prepared by the sol-gel method. The influence of the reduction procedure and modification with sodium was investigated. The properties of the chemically reduced catalysts were compared to non-reduced and sodium-modified catalysts. The influence of Ru/Sn metals ratio was also studied. Physical characterization and liquid-phase hydrogenation of cinnamaldehyde demonstrated the high importance of the chemical reduction in the preparation of tested sol-gel catalysts. The highest selectivity to cinnamylalcohol was achieved on catalysts of 5%Ru-2.5%Sn/SiOz type (70%). Sodium modification of catalysts decreased the formation of acid catalysed side products and increased the yield of saturated aldehyde. The hydrogenation properties were dependent on Ru/Sn ratio. 1. INTRODUCTION Selective preparation of unsaturated alcohols from corresponding unsaturated carbonyl compounds is a difficult task to achieve with heterogeneous catalysis. Thermodynamically preferred reduction of double C=C bond can be restricted only with difficulties. Industrial relevance of unsaturated alcohols [1,2] in conjunction with economically expensive methods based on chemical reduction of unsaturated carbonyl compounds calls for the development of new catalysts for highly selective preparation of unsaturated alcohols. Specific hydrogenation features of ruthenium are widely exploited [3,4]. Good intrinsic selectivity for unsaturated alcohols makes ruthenium very attractive for the abovementioned task. Modification of conventional ruthenium catalysts can increase selectivity to the desired alcohols. In the series of reactions, the best result in the formation of unsaturated alcohols was proved over tin doped ruthenium catalysts [5,6]. Corresponding author. E-mail: [email protected]

758 Sol-gel chemistry is a versatile tool for the preparation of more active and selective catalysts [7,8]. Sol-gel technique was applied for the preparation of a variety of catalysts, including also metals supported on silica and alumina [9,10]. Several papers were dedicated to catalytic hydrogenation over sol-gel catalysts [11,12]. Utilisation of sol-gel technique with respect to the preparation of supported bimetallic catalysts allows to produce catalysts with homogeneous distribution of finely dispersed metals. Another advantage includes improved thermal stability of the metals, higher surface areas, well-defined pore size distribution and ability to control the microstructure of the carrier. Concerning the preparation, the sol-gel catalysts properties can be easily effected by a number of contributing factors comprehended in a synthesis route. One of the steps in the catalyst synthesis is chemical reduction of prepared catalysts. This work is focused on the investigation and improvement of sol-gel Ru-Sn/SiO2 catalysts. Main attention is dedicated to study the chemical reduction benefits involving metals reduction and chemical and physical stabilization. Influence of Ru/Sn ratio and sodium modification on catalysts activity and selectivity will be also discussed. For clarification of the reduction effect, the catalysts were characterized with a wide range of physical methods (BET, PSD, XRF, EA, XPS). Catalysts hydrogenation properties, namely selectivity and activity were compared to Ru/SiO2 catalyst and evaluated during liquid-phase hydrogenation of 3-phenyl-2-propenal (cinnamaldehyde). 2. E X P E R I M E N T A L 2.1. Preparation of sol-gel 5%Ru-5%Sn/SiO2 catalysts A solution of a pertinent amount of ruthenium chloride (RuCl3.x HzO (x < 1), Aldrich) and tin precursor (SnCl2, Aldrich) was stirred for 30 minutes at 333 K in 1,2-ethanediol (p.a). Added molar amount of 1,2-ethanediol was 2/1 compared to carrier precursor (Si(OC2H5)4) molar amount. Tetraethoxysilane (98%, Aldrich) was inserted to the cooled solution of metal precursors under stirring at a room temperature. Acquired mixture was heated to 343 K and stirred at this temperature for 3 hours. Then, a stoichiometric excess of distilled water (90 ml) was added to the solution and the solution was further stirred at 343 K until a gel formed. The produced gels were left to mature for 12 hours. To remove water, solvent and organic residues from preparation, aged gels were dried at decreasing pressure. At the first stage, drying was realized in a water-rotary evaporator. Water bath temperature was at pressure of 1.9 kPa slightly increasing (20 K/hour) to 363 K. The temperature of 363 K was kept for 12 hours. Final stage of gels drying was performed at a pressure of 0.5 kPa (oil-vacuum pump) for 2 hours. Drying temperature at this stage was 473 K. Prospective reduction of catalysts was carried out with 10%-solution of NaBH4 in distilled water. The amount of NaBH4 (97%, Fluka) was selected to apply the following relation: nNaBH4/ (nRu + nsn) = 10. Reduced catalysts were washed several times with small amounts of distilled water and finally with small amounts of ethanol. Washed catalysts were dried for 2 hours under inert atmosphere (N2 4.0, Linde Technoplyn, CR) at the temperature of 473 K.

759 Sodium modification of non-reduced catalysts was carried out with ethanolic-NaOH. Crushed and sieved catalysts were inserted into intensively stirred sodium hydroxide dissolved in ca 50 ml of ethanol. The amount of NaOH was tuned to match the value of nNaOH / (nRu + nsn) = 50. The stirring followed for a couple of minutes, until the coagulation of the catalyst powder was observed. Subsequently the catalyst was carefully washed several times with small amounts of ethanol. The obtained catalyst was dried in nitrogen atmosphere for 2 hours at 473 K. Prior to hydrogenation, prepared catalyst were activated in hydrogen flow at the temperature of 473 K for a period of 2 hours. 2.2. Characterization of

catalysts

Surface areas were determined from nitrogen adsorption-desorption isotherms (Sorptomatic 1900, Carlo Erba Instruments) at 77 K. For the calculation of the specific surface areas the BET method was used. Pore size distribution was obtained by the DollimHeal method. Electrochemical coulometric method was used [13] for the determination of active metal surface. In this method, metals in catalysts were transformed to their oxides by oxidation in air at 473 K before the surface determination. The method is able to determine only a relative surface of metals since it is not possible to verify the values of measured surfaces by another conventional method. XRF analysis was used for the determination of ruthenium, tin, sodium and chlorine content. Solid catalysts samples were examined by automatic sequence RTG spectrometer (ARL 9400 XP). Elements loadings were evaluated by UNIQUANT analyzer. The program used a universal calibration method. Elementary analysis measurements were carried out on an automatic CHN-analyser (Perkin-Elmer 2400, USA). The chlorine content was determined by AgNO3 titration. The exact amount of carbon and chlorine residues in the catalysts was determined from three independent measurements. The activated catalyst sample was examined by ESCA spectrometer (Perkin Elmer PHI 5400, USA). During the analysis an energy pass of 35 eV was used, with pressure lower than 2.7.10 -9 kPa. Binding energy calibration was based on carbon impurity peak at 284.6 eV. Shirley background removal method was applied to remove the background of inelastically scattered photoelectrons. Liquid-phase hydrogenation (H2 4.0, Linde-Technoplyn) experiments were carried out in a 300 ml batch reactor in kinetic regime under 7 MPa of total pressure at 433 K. Typically 3.0 g of cinnamaldehyde (98%, Aldrich) and 0.5 g of catalysts were stirred in 2propanol (p.a.). Total liquid phase volume in autoclave was 200 ml. The products were identified with GC-MS and analyzed by gas chromatography. Hydrogenation samples were analyzed with a gas chromatograph HP-5890 Series II Plus (Hewlett-Packard, USA). Chromatograph was equipped with FID detector and capillary column HP 20M. Content of individual components in the reaction mixture was determined by the Internal Standardization Method (n-decane, Aldrich, USA).

760

3. RESULTS AND DISCUSSION

3.1. Catalysts characterization The catalysts surfaces were strongly dependent on the catalyst type and on metal content. As shown in Table 1, the highest surface areas were by the exhibited non-reduced catalysts. The lowest surfaces areas were observed for the sodium modified catalysts. Generally, the surface area decreased with increased tin content; 5%Ru-5%Sn/SiO2 and 5%Ru-10%Sn/SiO2 showed almost identical results. Pore size distribution of non-reduced and chemically reduced catalysts was well defined. Non-reduced catalysts showed narrow pore distribution of mesoporous character with maximum of pores in the range of 1.5-2.5 nm. Chemically reduced catalysts exhibited a microporous structure with pores of maximum 1 nm. Surprisingly, the pore distribution did not depend on tin content. In the case of sodium modified catalysts, their low surface areas made the evaluation of pore size distribution meaningless. Table 1. Surface area of catalysts. Catalyst

Non-reduced

Reduced

Sodium Modified

5%Ru/SiO2

411

436

121

5%Ru-2.5%Sn/SiO2

333

202

19

5%Ru-5%Sn/SiOz

254

156

<15

5%Ru-10%Sn/SiO2

267

187

<15

Surface areas in [mZ/gcat.] Ruthenium surface determination of non-reduced and reduced catalysts demonstrated that reduced catalysts had higher ruthenium surface areas. Table 2. Surface area of active metal. Catalyst

S [mZ/gRu] Catalyst

Non-reduced

S [mZ/gRu]

Reduced

5%Ru/SiO2

17

5%Ru/SiO2

64

5%Ru-2.5%Sn/SiO2

19

5%Ru-2.5%Sn/SiO2

49

5%Ru-5%Sn/SiO2

21

5%Ru-5%Sn/SiO2

53

5%Ru-10%Sn/SiO2

12

5%Ru-10%Sn/SiO2

38

XRF analyses were used mainly for the determination of the ruthenium and tin loading in catalysts. As reported, the determination of the real content of metals is of high importance especially in the instance of sol-gel catalyst [14]. The genuine loading of

761 metals in catalysts prepared by the sol-gel method can be considerably different from that expected. However, the obtained metal loadings were in accordance with anticipated values. Only the ruthenium content was slightly lower than expected. The XRF analyses performed also showed that during the catalysts reduction, the elements from NaBH4 solution are impregnated into the catalyst. XRF results regarding chlorine content were in good correspondence with the data from elementary analysis. The determination of carbon and chlorine content confirmed higher content of both elements in non-reduced catalysts. The results showed that a high temperature is needed for the removal of the organic residues. Among the carbon-containing substances resulting from the preparation, ethanediol has the highest normal boiling point (ca 471 K). Even applying a vacuum (0.5 kPa) drying procedure at 473 K was not sufficient to remove this residue. Moreover, the measured pore size distribution confirmed the formation of uniform sol-gel catalysts with narrow meso- and micropores. Narrow pores can more easily entrap substances present during first preparation steps from reaction mixture. On the basis of the results obtained, it can be concluded that the synthesized catalysts presumably encapsulated starting chemicals and preparation residues in their internal structure. Thus, the removal of the used chemicals was complicated. Table 3. Carbon and chlorine content in catalysts. Catalyst

Carbon Content [%]

Chlorine Content [%]

Non-reduced

Reduced

Non-reduced

Reduced

5%Ru/SiO2

9.3

1.2

2.3

0.0

5%Ru-2.5%Sn/SiO2

10.4

1.1

4.5

0.0

5%Ru-5%Sn/SiO2

7.9

0.9

3.7

0.0

5%Ru- 10%Sn/SiO2

9.1

2.9

5.0

0.0

As demonstrated by elementary analysis, the chlorine and carbon content of the reduced catalyst is lower compared to non-reduced catalysts. It is supposed that the chemical reduction procedure in connection with catalysts washing is highly efficient in removing the remaining carbon and chlorine residues from catalysts. Most probably, the chemical reduction decreased the chlorine content and washing of the catalysts decreased the carbon content. The XPS analysis was used for the characterization of reduced 5%Ru-5%Sn/SiO2 catalyst. Ruthenium binding energy was found lower than Ru ~ which is explained by interaction with alkali metal [15]. Tin exists in two states, which may present Sn(II) in SnO, and Sn(IV) in SnO2. Boron is probably in 3+ state. Weak (noisy) B ls peak at 189 eV was noticed. 3.2. Hydrogenation properties The hydrogenation curve of cinnamaldehyde depended on the type of catalyst. Note that hydrogenation properties of Ru/SiO2 were different from the properties of ruthenium-

762 tin modified catalysts. Especially, hydrogenation of cinnamaldehyde on non-reduced conventional sol-gel Ru/SiO2 catalysts showed profound differences. In this case, the aromatic ring hydrogenation was observed, as the presence of cyclohexane derivatives was revealed in the reaction mixture. The detected presence of ethylbenzene and other hydrogenolytic products also confirmed hydrogenolytic reactions. During the hydrogenation of cinnamaldehyde over non-reduced Ru/SiO2 catalyst, the presence of all reaction by-products was confirmed. Modification of non-reduced catalysts with tin suppressed the formation of hydrogenolytic products. Chemically reduced Ru/SiO2 catalyst exhibited very low hydrogenation of aromatic ring. In this case, after tin modification, the only reaction products were 3-phenyl-2-propen-l-ol (COL), 3-phenyl-2-propanal (PPAL) and 3-phenylpropan-l-ol (PPOL). Sodium modified non-reduced Ru/SiO2 catalyst had hydrogenation properties very similar to reduced Ru/SiO2 catalyst. Bimetallic Ru-Sn catalysts after modification with sodium exhibited the same properties as chemically reduced Ru-Sn catalysts, showing higher selectivity to saturated aldehyde (PPAL). On the basis of acquired kinetic data, a general reaction network for reduced and sodium modified Ru-Sn/SiO2 catalysts is proposed in Scheme 1. The initial activities of catalysts are compared in Table 4. The highest activities were achieved with conventional ruthenium catalyst. Among the catalysts prepared by the different methods, the chemically reduced ones had the highest activity. It follows from Table 4 that there is no apparent relationship between tin content and catalyst activity. A small amount of tin (2.5%) decreased the catalyst activity compared to 5%Ru/SiO2. However, further introduction of Sn did not result in a clear decrease of the catalyst activity. Sodium modification decreased significantly the activity of the catalysts with a higher tin content (5 and 10% of Sn).

r~ ~

~ O H 3-phenyl-2-propen-1-ol " ~

~ O 3-phenyl-2-propenal r2X~

(CAL)

~ O H

(COL)

~

o

r/~4

3-phenylpropan1-ol (PPOL)

3-phenyl-2-propanal (PPAL) Scheme 1. Proposed hydrogenation network. The selectivity to the unsaturated alcohol (COL) was calculated as the molar ratio of the product to the total formed. Selectivity as a function of conversion is presented in Fig. 1. Amazingly, non-reduced catalysts exhibited high selectivity to unsaturated alcohol (COL), achieved a selectivity comparable to chemically reduced catalysts. The obtained results can be explained by low yields of reaction side-products (PPAL & PPOL). High chlorine content resulted in formation of acetals. The promoting effect of chlorine in

763

5%Ru- 10%Sn/SiO 2 Reduced

45

m 30

Non-reduced 5 %Ru-10 %S n/ S i0 2

-----m J

25

,

,

50

75

100

Conversion [%]

hydrogenation of unsaturated aldehydes was previously demonstrated on cobalt catalysts [16]. In this case it was assumed that the residual chlorine content influenced the reduction step, leading to the formation of favorable metal particle size, and depressed the C=C bond hydrogenation.

Fig. 1. Selectivity to COL as a function of conversion in hydrogenation over 5%Ru-10%Sn/SiO2 and 5%Ru/SiO2 Table 4. Initial activity of catalysts. Initial Activity [mmol/min.gcat] Non-reduced

Reduced

Sodium modified

5%Ru/SiO2

7.32

5.12

3.00

5%Ru-2.5% Sn/SiO2

1.02

3.16

2.18

5%Ru-5%Sn/SiOz

1.16

2.56

0.64

5%Ru- 10%Sn/SiO2

1.04

3.70

0.78

Catalyst

A more common idea assumes chlorine to act in various systems merely as a poison [17,18]. In the case of selective hydrogenation of crotonaldehyde over Pt/ZnO catalyst, it was speculated that C1- can polarize Zn ~§ increasing the interaction between Zn 8§ and C=O bond, leading to an increase of the selectivity to unsaturated alcohol [19]. As it is known, sol-gel Ru-Sn/SiO2 catalysts reduced at 473 K are X-ray amorphous. Thus, most probably, in the non-reduced Ru-Sn/SiO2 catalyst system, chlorine acts partially as a poison (decreased activity of non-reduced catalysts). Polarization of Sn similar to polarization of Zn in the Pt/ZnO catalytic system can also be considered, having negative effect on catalyst selectivity. It must be mentioned that chlorine effect was not proved during hydrogenation of crotonaldehyde over Ru-Sn/C [20]. The presence of chlorine also did not influence benzene hydrogenation [21]. The negative chlorine effect was described during the hydrogenation of citronellal over Ru/SiO2 and Ru/TiO2 [22]. Compared to non-reduced catalyst, chemical reduction decreased the catalyst acidity (see also chlorine content of the catalysts), and acetals were not detected. On the other hand, the

764

formation of unwanted side products (PPAL, PPOL) was higher. Modification of nonreduced catalysts by sodium preferably increased the formation of saturated aldehyde. The mentioned trends are clearly perceptible on ruthenium reduced and modified catalysts. Tin modification and increased tin loading of reduced and modified catalysts increased the formation of the desired alcohol by decreasing the formation of saturated aldehyde. Higher tin content in non-reduced catalysts decreased the yield of unsaturated alcohol, simultaneously increasing the yield of the final reaction product (saturated alcohol). As it is visible from Fig. 1, the selectivity towards COL increased with conversion. This was most clearly observed in non-reduced catalysts. Previously [23], it was demonstrated that in the liquid-phase hydrogenation of a similar molecule (e.g. crotonaldehyde), there was a clear increase of the selectivity as a function of conversion. This kinetic pattern did not obey the general behavior characteristic for parallelconsecutive reactions, thus calling for the introduction [23] of the reaction-induced formation of active sites. One can thus speculate that, also in cinnamaldehyde hydrogenation, during the reaction there is in situ formation of sites responsible for the hydrogenation of the carbonyl group (formation of unsaturated alcohol). 4. CONCLUSIONS Liquid-phase hydrogenation of cinnamaldehyde over prepared sol-gel Rn-Sn/SiOz catalysts showed that the highest selectivity (70%) to unsaturated alcohol cinnamylalcohol was achieved over 5%Ru-2.5%Sn/SiOz. Chemically (NaBH4) reduced catalysts exhibited the highest activities. It was also found that the hydrogenation properties are dependent on metals ratio. The higher the tin content of the catalyst, the higher the catalyst selectivity and the lower its activity (reduced and sodium modified catalysts). Non-reduced catalysts provided highly acidic properties, changing the hydrogenation products distribution and increasing the yield of the unsaturated alcohol. Basic differences between reduced, nonreduced, and sodium-modified catalysts were confirmed by characterisation methods. Sodium modification did not improve the performance of the catalysts but decreased the formation of undesired acid catalysed side products. During hydrogenation, in situ formation of active sites responsible for the hydrogenation of the carbonyl group was observed. ACKNOWLEDGMENT This work is a part of the Graduate School of Chemical Engineering activities at the ,~bo Akademi Process Chemistry Group, within the Finnish Centre of Excellence Programme. This work was also supported by the Czech Grant Agency (Grant N ~ 104/00/1004). REFERENCES

1. L. Marcadante, G. Neri, C. Milone, A. Donato and S. Galvagno, J. Mol. Catal., 105 (1996) 93. 2. D.G. Blackmond, R. Oukaci, B. Blanc and P. Gallezot, J. Catal. 131 (1991) 401.

765 3. N.M. Gupta, V.S. Kamble and R.M. Iyer, J. Catal., 88 (1984) 457. 4. B. Arena, J. Appl. Catal., 87 (1992) 219. 5. H. Yoshitake and Y. Iwasawa, J. Catal. I25 (1990) 227. 6. B. Coq, P. S. Kumbhar, C. Moreau, P. Moreau and M.G. Warawdekar, J. Mol. Catal., 85 (1993) 215. 7. Yu.M. Rodionov, E.M. Slyusarenko and V.V. Lunin, Usp. Khim., 65 (1996) 865. 8. D.A. Ward and E.I. Ko, Ind. Eng. Chem. Res., 34 (1995) 421. 9. A. G. Sault, A. Martino, J. S. Kawola and E. Boespflug, J. Catal., 91 (2000) 474. 10. T. Lopez, L. Herrera, J. Mendez-Vivar, P. Bosch, R. Gomez and R.D. Gonzalez, J. Non-Cryst. Solids, 147 (1992) 773. 11. M. Toba, S. Tanaka, S. Niwa, F. Mizukami, Z. Koppany, L. Guczi, K.-Y. Cheah and T.-S. Tang, Appl. Catal. A: Gen., 189 (1999) 243. 12. C.M.M. Costa, E. Jordao, M.J. Mendes, O.A.A. Santos and F. Bozon-Verduraz, React. Kinet. Catal. Lett., 66 (1999) 155. 13. I. Paseka, Appl. Catal. A: Gen., 207 (2001) 257. 14. C.K. Lambert and R.D. Gonzalez, Appl. Catal A: Gen., 172 (1998) 233. 15. V.M. Deshpande, W.R. Patterson and C.S. Narasimhan, J. Catal., 121 (1990) 165. 16. Y. Nitta, Y. Hiramatsu and T. Imanaka, J. Catal., 126 (1990) 235. 17. T.E. McMinn, F.C. Moates and J.T. Richardson, Appl. Catal. B, 31 (2001) 93. 18. A. Golebiowski, Z. Kobinski, Z. Spiewak and K. Stolecki, Przem. Chem., 77 (1998) 367. 19. M. Consonni, D. Jokic, D. Yu. Murzin and R. Touroude, J. Catal., 188 (1999) 165. 20 B. Bachiller-Baeza, A. Guerrero-Ruiz and I. Rodriguez-Ramos, Appl. Catal. A: Gen., 192 (2000) 289. 21. G. Marques da Cruz, G. Djega-Mariadassou and G. Bugli, Appl. Catal., 17 (1985) 205. 22. A.A. Wismeijer, A.P.G. Kieboom and H. Van Bekkum, Appl. Catal., 25 (1986) 181. 23. J.L. Margitfalvi, I. Borbath and A. Tompos, Stud. Surf. Sci.Catal., 118 (1998) 195

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

A

767

new insight into molybdate/boehmite interaction

D. Minoux a'*, F. Diehl a , P. Euzen a, Jean-Pierre Jolivet b, Edmond Payen c a - IFP, 1-4 av de Bois Pr~au, 92852 RUEIL-MALMAISON C~dex, France b- Laboratoire de Chimie de la Mati~re Condens6e, Universit~ Pierre et Marie-CurieUMR CNRS 7574, 4 Place Jussieu, 75005 PARIS, France c-Laboratoire de Catalyse h&~rog~ne - Universit~ des Sciences et Technologies de Lille Brit. C3, 59655 VILLENEUVE D'ASCQ C~dex, France A possible way to increase the amount of active phase deposited on hydrotreatment catalysts may consist of reacting the active metal solutions directly with boehmite (AIOOH), the precursor of ~-alumina carrier. Experiments carried out on well defined systems such as boehmite sols provide a more precise insight into molybdenum/boehmite interaction. Therefore, this work gives some keys for modulating the catalytic properties in realistic systems. 1. I N T R O D U C T I O N As environmental matters become more and more of a central concern, the specifications of petroleum products get more and more stringent. To improve the performance of their desulphurization units, refiners will have to use more and more active catalysts in the future. The active phases of these catalysts consist of molybdenum disulfidenanocrystallites, promoted by cobalt or nickel, and well dispersed on a carrier with a high specific area. These active phases are obtained through sulfidation of an oxidic precursor generally prepared by impregnation of the carrier with an aqueous solution of the elements to be deposited. Generally, ammonium heptamolybdate is used as a precursor of Mo. One way to improve the catalytic performance, is to increase the amount of active phase on the carrier. Unfortunately, the major drawback of the conventional preparation process is precisely that the amount of Mo deposited by incipient wetness impregnation is limited by the solubility of the metal salts in the impregnation solution, the volume of which is restricted by the total porous volume of the alumina. Therefore, one practical solution may consist in reacting the active metal solutions directly with the alumina precursor (boehmite) prior to the extrusion, i.e. during the shaping step of the alumina support, called kneading. Previous works have already dealt with this appealing way of preparation[I-2]. They have shown that the final kneaded catalyst significantly differs from the conventional one, on both the textural and catalytic aspects. However, these studies do not allow one to explain these results by a simple follow-up of each step of synthesis. So, to better capture

768 the influence of the kneaded element on the final catalytic activity of the material, it is important to understand how the transition metal oxides (molybdenum in our case) and oxide support material (boehmite A1OOH) interact, because their way of interaction could ultimately determine the sulfidability of the metal (oxides), the dispersion, the morphology and the catalytic activity of the metal sulfides. 2. EXPERIMENTAL 2.1. Sample preparation The oxidic precursors have been synthesized as follows: boehmite sol (A1OOH Pural SB3, from Condea, 4g/1 in water) was prepared according to a procedure reported elsewhere [3]. The pH of the sol was adjusted at 4.8 (in order to limit the dissolution process) by addition of HNO3. Increasing amounts of ammonium heptamolybdate aqueous solution (0.05 M obtained by dissolving (NH4)rMo7024.4H20 - Merck - called AHM in the following) were added to different samples of the boehmite sol, up to the concentration of [AHM]=I. 10-2 M. The samples were stirred at room temperature for 3 days to make sure getting the equilibrium state, then ultrafiltrated upon a 30kDa membrane. The solid phases were left to dry under air at room temperature. 2.2. Characterization ICP : Elemental analyses of the filtrates were performed by Inductively Coupled Plasma (ICP) to determine the amount of residual molybdenum and dissolved aluminum. The intensity of the molybdenum emission signal is detected at 202.030 nm. The one of aluminum is determined from the line at 394.401 nm. Laser Raman spectroscopy (LRS) : The Raman spectra of the samples were recorded using a Raman microprobe (Infinity from Dilor), equipped with a photodiode array detector. The exciting light source was a YAG laser emitting the 532 nm line and the wavenumber accuracy was _.+2 cm "1. The laser power was around lmW at the samples. The identification by LRS of the oxomolybdate phases present on the boehmite surface has been established by comparison of the Raman features of the oxidic precursor with those of reference solids, their modifications being ascribed to the effect of the interaction with the carrier. It is important to notice that all Raman Table 1. spectra will exhibit in addition to the Boehmite Raman line assignment Symmetry Attribution molybdate stretching modes, additional Stretching line (cm1) Big 8 As (OH) lines, which are due to the boehmite 1110 Ag 6 s (OH) stretching modes [4] as reported in 1053 B2g y OH Table 1. Those lines will not be 733 676 Ag v2 A106 (Eg) discussed further in this paper. Big v5 A106 (F2g) 27A! liquid-state NMR spectroscopy: 639 Ag V2 A106 (Eg) Liquid-state NMR spectra were 497 B2g v5 A106 (F2g) obtained over an Avance 300 454 Ag 3/1 A106 (AI~) spectrometer. For the data acquisition, a 369 B2g v5 A106 (F2g) standard Hahn-Echo pulse sequence has 343 272 BI~ A106 (FI~) been used in concentric

769 tubes with a 10 mm probe, in order to eliminate probe signal. The Larmor frequency was 78.2 MHz, the 90 ~ pulse length 11 ~tsec, the time delay was 2 msec. The recycle time was 1 sec. Computations of (AI, Mo) equilibrium concentrations: We implemented C language program [5], to calculate the concentration of aqueous species in equilibrium from the equilibrium constants taken from literature [6]. All equilibria were computed with no ionic strength, taking into account boehmite dissolution, and also the relevant chemical equilibria as defined by Carrier et al. [7]. 3.RESULTS AND DISCUSSION 3.1. AHM adsorption on boehmite and nature of the adsorbed species From the quantitative determination of residual molybdenum concentrations in the filtrates, the AHM adsorption isotherm has been established as shown in Fig. 1 and exhibits three adsorption branches as already reported by Van Veen et al. [8]: i-the first adsorption branch (up to [AHM]i=9 10.4 M) corresponds to the adsorption of the entire amount of the molybdenum introduced, gaman spectra of the solid reveal a line at 920 cm 1 (Fig. 2-a), characteristic of the Mo=Ot stretching mode of adsorbed monomeric molybdate species according to reference [9]. ii-In the second branch ([AHM]i comprises between 9 10 .4 mol/1 and 17 10 -4 mol/1), a first plateau is reached. The gaman spectra of the adsorbed species exhibit now a broad line in the 900-1000 cm 1 spectral range. In addition to the line characteristic of adsorbed monomers, a new one appears at 952 cm 1 and can be ascribed to the symmetric stretching Mo=Ot of the Anderson-type entities, A1Mo6(OH)60183. The existence of these species is confirmed by the Raman line at 570 cm~, characteristic of the A1-O stretching mode of the Anderson-type structure (Fig. 2-b, Fig. 3 and ref [10]). Simultaneously, the aluminum dissolution occurs and seems indeed to interfere with the adsorption experiment. iii-In the third branch ([AHM]i > 17 10.4 mol/1), the same gaman lines (952 and 570 cm ~) are still present but sharper and well defined (Fig. 2-c). No Mo-containing species other than the Anderson-type structure have been detected by LRS. 3.2. Adsorption mechanism Adsorption can result either from electrostatic interactions between molybdates and boehmite surface or from chemical interactions, i.e. from the formation of a iono-covalent bond through a chemisorption mechanism. The interaction mode is governed by the boehmite hydroxyl surface groups as well as by the solution molybdate species. The determination of the nature and concentration of the molybdenum species involved in the experiments (before and after the adsorption equilibrium) has been carried out by computer simulations (cf w 2.2). Concerning hydroxyl surface groups, we referred to MUSIC modeling [11,12] as well as to the work of gaybaud et al. [13], who performed DFT studies on boehmite and so determined boehmite morphological and structural surface properties.

770

3.2.1. Low molybdenum loading 9[AHM]i < 9 1 0 -4 mol/! In solution, at low molybdenum loading and at pH=4.8, the initial AHM entities give rise mainly to the monomer, non or monoprotonated ( M O O 4 2" o r H M o O 4 ) species because of the dilution effect (Fig. 4).

r~

0.

4

0.5

r.t9

04

o

3

c~

<

5

<

0.3

9

2

9

1

0.2 0.1

9

13

13

20

40 60 Introduced [AHM! ~I10 ~ molll)

120

o

~ 9 ~

~'~

Fig. 1. AHM/boehmite adsorption isotherm ; pH=4.8 - The conversion in atMo/nm_ has been achieved taking into account a specific area of 489.8m2/g A1OOH i ' It was checked by dynamic light scattering experiments, that in our water medium, boehmite particles have a monodisperse size, which hydrodynamic radius is centered around 20 nm [5]. Therefore, we have chosen boehmite geometrical surface to describe our systems. 1

~952 ~'

c

9Q2]I!

~

i~.-~

"

*

iii

b

ii -

200 300 4 0 0 500 600 700 800 900 1000 Wavenumber (cm -1)

b

1

Fig. 2. Raman spectra of Mo/boehmite solids for three molybdenum loading: a- [AHM]i=I.7 10 "4 mol/1; b- [AHM]i = 10 10.4 mol/1; c- [AHM]i=50 10.4 mol/1; d- solution of (NH4)3A1Mo6(OH)6018.4H20 * lines ofboehmite

500 6 0 0 7 0 0 Fig. 3 - Zoom of the [5007 0 0 ] c m 1 region of the Raman spectra presented in Fig. 2. * lines ofboehmite

771 Boehmite surface exhibits 2 and 4 coordinated oxo groups (la2-OH and ~1.4-O) o n basal planes and ~ti-OH, ~t2-OH and la3-OH sites at the lateral surfaces. The la4-O groups are strongly polarized and will not participate in the surface acidity; according to MUSIC modeling, the la=-OH groups are almost uncharged. The (010) boehmite basal plane is then very slightly charged. The molybdate adsorption is then expected to occur preferably at boehmite lateral surface [ 14]. On these lateral surfaces, the most available hydroxyl groups are the terminal aquo ~t]-OH=la+ sites [12], positively charged in the experimental conditions. An electrostatic attraction between monomers and boehmite surface is then possible. But the significant shift of the Raman line from 898 (free monomer entity) to 920 cm "l (adsorbed monomeric entity) suggests that the formation of a stronger interaction between molybdate species and the boehmite cartier should occur as previously reported for Mo/A1203 systems [15]. Chemisorption would then arise from the nucleophilic attack of the monoprotonated monomer HMoO4- on lLtl-OH 21/2+ followed by water elimination. Indeed, the monoprotonation of MoO42 lengthens the Mo-OH bond and then reduces the :t-transfer of these bonds [ 16]. Thus in spite of a small decrease of its nucleophilic ability, the OH group of HMoO4 [~)(OH)=-0.25 according to the partial charge model [ 17] might well achieve this reaction, in the same way as for molybdate condensation [6]. Furthermore, supporting this idea, the amount of adsorbed molybdates reaches 1.5 Moat./nm 2 at the plateau; such a value corresponds to the number of p l- OH 21/2+ sites located on the boehmite lateral surfaces as determined by Raybaud et al. [ 13 ].

1

o~ 0

= ..... ~.

1

~_

2

3

pH

.,_~~.......".C.. 4

5

6

Fig. 4. Theoretical molybdate speciation calculated from [Mo]i=12 10"4mol/1 ([AHM]i=I.7 104 mol/1) before adsorption. 100l I-I2M~ ~ ~, I H3MOTO243" HMozOz~/

~6ol

l

\

,>-,, ~,

MoO42-

Fig. 5. Theoretical molybdate speciation calculated from a [Mo]i=70 10 4 mol/1 before adsorption ([AHM]i=10 10-4 mol/1).

.:., /

3( \." V '.I / \ / \ , , , . 7,

[ / ..-'" ,~:4,.:..:.~HMoO4 o 1/ 0 1 2 3 pH 4 5 6 7 ~

~

:

'

_

z

-

.

.

_

N

_ . . -. .

,

,

8

772 Fig. 6. Theoretical molybdate speciation calculated from a [Mo]i=70 10-4 mol/1 after H3M%O243 MoO42- a d s o r p t i o n and equilibrium reached. / aH2M~ [Mo]eq-22.8 10-4 mol/1; [A1]eq=m.19 10-4 X A ,'~.~:"'7/., AIMo6(OH)60~?mol/1 were determined by ICP.

lo

~

s

~/\~.M.o~o~:-/

o'e6 0

/\

I

~ ' ~ / _ . ~ ,

2

3

pH

4

5

--HMo04 6

7

3.2.2. Intermediate m o l y b d e n u m loading : 9 10 -4 mol/l < [AHMIi <17 10 -4 mol/l

Calculations of molybdate speciation in solution before adsorption (Fig. 5) show that, the main species at pH-4.8 are the non or mono-protonated AHM and the monomer MoO42, the amount of which decreases with increased introduced [AHM]. After adsorption, elemental analyses of the filtrates reveal boehmite dissolution, which exactly coincides with the Anderson-type polyanion formation on boehmite surface. From the quantification of residual molybdenum and the total dissolved aluminum concentration determined by ICP after equilibrium, calculations show that one more entity, the Anderson-type polyanion has to be taken into account (Fig. 6). These computational results are in agreement with 27A1 liquid state NMR experiments (Fig. 7): indeed, the NMR spectrum of the filtrate exhibits a single peak at 16 ppm, characteristic of the 27A1 chemical shift of the central A1 atom in the Anderson-type polyanion structure [18]. The above results suggest that the Anderson-type formation and further adsorption might occur in solution thanks to the removal of some aluminum atoms from boehmite cartier. As previously mentioned, LRS suggests that monomers and Anderson-type entities both coexist on boehmite surface at intermediate molybdenum loading. This allows us to consider a competitive adsorption between monomers and Anderson-type entities, the formation of which requires boehmite dissolution. If the interaction between molybdate monomers and boehmite surface is supposed to be an electrostatic one, there will be no reasons that boehmite dissolution be hindered ; therefore, Anderson-type structure formation and further substantial adsorption would take place and no plateau should then be observed at that intermediate molybdenum loading. An explanation might be found if monomer chemisorption takes place instead. Indeed boehmite dissolution occurs by means of its lateral surfaces [19]; monomer chemisorption on lateral boehmite surface would block surface functional sites and thus inhibit at least partially boehmite dissolution. Therefore, at intermediate molybdenum loading, as monomer concentration in solution is not negligible compared to AHM concentration, monomer adsorption on boehmite lateral surfaces should compete with aluminum extraction. Fig. 8 reports boehmite dissolution kinetics carried out at pH=4.8, for two [AHM] concentrations (intermediate and high molybdate loading). This result could support the fact that aluminum dissolution is slowed down at intermediate molybdenum loading and that no substantial moly-bdate adsorption could take place; a plateau is reached in the adsorption isotherm (Fig. 1).

773 support the fact that aluminum dissolution is slowed down at intermediate molybdenum loading and that no substantial moly-bdate adsorption could take place; a plateau is reached in the adsorption isotherm (Fig. 1). }

Q. Q. (,D

! [

t

. . . . . . . . . . . . . . . .

i

. . . .

i

. . . . . . . .

I

.~<__: . . . . . . . . . .

v--

P_ ;,,o.8 ...

...............................

4d

~

""

v

........

I

oo.,

:,~, .• x

.s

~0.2

2

-2 -6 - 1 0 - 1 4 - 1 8

Fig. 7. 27A1 liquid-state NMR spectrum of the filtrate after the adsorption experiment; the peak at 16 ppm is characteristic of the chemical shift of A/Mo6(OH)6Ols 3".

...............

!

.........

0

(~m)

],~

,:"

!

i

:

.-i ..........

i

i ...........

4000

!

!

i

] ..........

i ...........

!..........

=*-:-="~:M:-::-~a:,o~-,.i~,~.~,.

i

3000

:..........

i

,

~.................................

2000

: ......................

= [AHM] ='iOto" ~o~ , J ,

].........

i

...... .....~ .......... ~...................

s

26 22 18 14 10 6

.:-,,-~-- -'- -'--'. .......... .....,'

. . . .

~-. . . .

4000 T i m e (rain)

i

i ................................

SO00

6000

7000

8000

Fig. 8. Dissolution kinetic of boehmite at pH=4.8 ; [ A H M ] i - 10 10-4 mol/1 and [AHM]i = 28 10.4 tool/1

3.2.3. High molybdenum loading" [AHM]i >17 l0 -4 mol/! Before adsorption, AHM is the main entity present in solution. No AHM chemisorption could take place; unlimited aluminum extraction is likely to occur and leads to the formation of the Anderson-type structure. The comparison between the Raman spectrum of the solid at high molybdenum loading (Fig. 2-c), with the reference spectrum (Fig. 2-d) seems to support the existence of isolated Anderson-type structures on the boehmite surface ; the very small line shift (952 cm 1 instead of 950 cm 1) and the weak line broadening suggest that some interaction with the carrier may exist. Upon calcination at 773K and rehydratation in air, no MoO3 is detected and the isolated Anderson entities are restored [10] (Fig. 9). This suggests that this adsorption regime is not due to a surface precipitation. A closer look to the Raman spectra in the 900-800 cm I region reveals a weak Raman line at 860 cm -~, which is traditionally not observed in Raman spectra and which could be assigned to the A1-O(H)-Mo stretching mode of Anderson-type entity [20] [(NH4)3A1Mo6(OH)6018] (Fig. 10). This Raman line could be then due to a breaking in the selection rules arising from symmetry distortion of the physisorbed species. This may thus indicate the formation of a hydrogen bonding between the non acidic proton of Andersontype species and the boehmite surface. Furthermore, DRIFTS studies of boehmite hydroxyl surface groups presented in detail elsewhere [21], have been carried out; at high molybdenum loading, the disappearance of the OH stretching mode of the groups located at boehmite basal planes has been pointed out [21] and would indicate that adsorbed species interact with boehmite basal planes.

774 At the third adsorption regime, the surface oxomolybdate phase may then consist of isolated Anderson units interacting with boehmite basal planes.

A 6

952

i

a

300

400

500

600

700

800

900

1000

Fig. 9. Raman spectra of adsorbed species at [ m n M ] i - - 50 10.4 mol/1; a) solid dried; b) calcined upon 773K and rehydrated *" lines of boehmite

.~ ........

~: . . . . . . . .

:. . . . . . . . .

~........

~__,,-,~

.....

~. .

.

.

. ,.

.

700 750 800 850 900 950 1000 1050

Fig. 10. Raman spectra of the [8001000] c m "1 region for [ A H ] ~ ] i - - 5 0 10 .4 mol/1

4. CONCLUSIONS This work clarifies the interaction processes between molybdate and boehmite: 9 At low molybdenum loading, tetrahedrally coordinated molybdate monomers preferably adsorb on lateral surface of the boehmite particles. 9 At intermediate molybdenum loading, molybdate monomers and Anderson-type entities coexist on boehmite surface supporting the idea of a competitive adsorption mechanism. A chemisorption mechanism for molybdate monomers could lead to a limited dissolution of boehmite, necessary for the removal of A1 atoms required in the Andersontype structure formation. This would explain the plateau reached in molybdate adsorption. 9Finally, at high molybdenum loading, a third adsorption regime occurs. Adsorbed monomers are not observed by LRS. Assuming unlimited dissolution, is consistent with the fact that Anderson-type polyanion formation and further adsorption on boehmite basal planes occurs. Experiments are now underway to study how molybdenum influences textural and structural properties of the carrier after boehmite/u transformation. ACKNOWLEDGMENTS This work was supported by the IFP, Kinetic and Catalysis Division. We gratefully ackowledge Dr. D. Frot for dynamic light scattering measurements.

775 REFERENCES

1. F. Luck and F. Viez, Stud. Surf. Sci. Catal., 48 (1989) 611-623. 2. R.A. Kemp and C.T. Adams, Appl. Catal. A : General, 134 (1996) 299. 3. F. Mange, D. Fauchadour, L. Barre, L. Normand and L. Rouleau, Colloids and Surface A. 155 (1999) 199. 4. A.B. Kiss, G. Keresztury and L. Farkas, Spectrochim. Acta, 36A (1980) 653. 5. D. Minoux, PhD Thesis, Universit6 Pierre et Marie Curie, (2002) to be published. 6. Mo: K.H. Tykto, and O. Glemser, Adv. Inorg. Chem. Radiochem., 19 (1976) 239. AI: C. Base and R.E. Mesmer, The Hydrolysis of Cations, J.Wiley & Sons. NY (1976). 7. X. Carrier, J.F. Lambert and M. Che, J. Am. Chem. Soc., 119(42 (1997) 10137. 8. J.A.R. Van Veen, P.A.J.M. Hendriks, E.J.G.M. Romers and R.R. Andrea, J. Phys. Chem, 94(13)(1990) 5275. 9. E. Payen, J. Grimblot and S. Kasztelan, J. Phys. Chem. 91 (1987) 6642. 10. L. LeBihan, P. Blanchard, M. Fournier, J. Grimblot J. and E. Payen, J. Chem. Soc., Faraday Trans. 94 (7) (1998) 937. 11. T. Hiemstra, W.H. Van Riemsdijk and G.H. Bolt, J.Colloid Interface Sci., 133 (1989) 91. 12. C. Froidefond, PhD Thesis, Universit6 Pierre et Marie Curie, Paris 2001. 13. P. Raybaud, M. Digne, P. Euzen and H. Toulhoat, J. Catal., 201 (2001) 236 14. J.P. Nordin., D.J. Sullivan, B.L. Phillips and W.H. Casey, Geochim. Cosmochim. Acta, 63(21) (1999) 3513. 15 E. Payen, J. Grimblot and S. Kasztelan, J. Phys. Chem., 91 (1987) 6642. 16 M. Fournier, C. Louis, M. Che, P. Chaquin and D. Masure, J. Catal., 119 (1989) 400. 17. M. Henry, J.P. Jolivet and J. Livage, Structure and Bonding, 77 (1992) 253. 18 L. LeBihan, PhD Thesis- Universit6 Lille 1, Lille (1997). 19 W. Stumm, Chemistry of the Solid-Water Interface, J.Wiley & Sons, 1992. 20 I.L. Botto, A.C. Garcia and H.J. Thomas, J. Phys. Chem. Solids, 53(8) (1992) 1075. 21 T. Armaroli, D. Minoux, S. Gautier and P. Euzen, to be published.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

777

Controlled coating of high surface area silica with titania overlayers by atomic layer deposition J. Ker/inen a'b, E. Iiskola b, C. Guimon c, A. Auroux a'* and L. Niinist6 b alnstitut de Recherches sur la Catalyse, CNRS, 2 av. Einstein, F-69626 Villeurbanne, France bHelsinki University of Technology, P.O. Box 6100, FIN-02015 Espoo, Finland CLPCM, 2 av. du Pr6sident Angot, F-64000 Pau, France Atomic layer deposition (ALD) was applied to prepare highly dispersed titania/silica support materials by chemisorption of volatilized titanium(IV) isopropoxide on silica followed by an oxygen treatment. The practical temperature range for controlled precursor adsorption was observed to be between 110 and 180~ as examined by chemical analysis and inert atmosphere DRIFTS measurements. The surface modification was extented up to a monolayer coverage of titania by consecutive precursor binding - calcination cycles. The ALD TiO2/SiO2 samples maintained the high surface area and porosity as well the amorphous nature of the support even at high loadings. At low coverage, titania was present in isolated tetrahedral units whereas at greater loadings, the coordination of titanium increased up to six-fold as suggested by UV-Vis DRS. Small titania particles (d < 3 nm) were observed by TEM after five ALD cycles (~5.1 Ti atoms/nmasupport). However, no crystallites were detected by XRD even after seven ALD cycles (~7.0 Ti atoms/nmZsupport). Moreover, the uniformity and high relative dispersion of the titania species by formation of strong Ti-O-Si bonds was maintained even after several ALD cycles as probed by XPS. The importance of the preparation mode was illustrated by comparing the ALD-dispersed titania/silica to a TiOa-SiOz mechanical mixture. 1. I N T R O D U C T I O N Highly dispersed titania supported on silica is a material of great interest either as a catalyst [1,2] or as a support for active species [1,3]. The coating of silica by grafting the titanium(IV) isopropoxide precursor in both liquid [1,4-6] and gas phase [2,7] has been shown to lead to enhanced Ti-O-Si interaction and better dispersion of titania. Gao et ak [5] showed that at low coverage (~0.3 Ti atoms/nmZsuppo~t) titania is present in isolated tetrahedral units whereas at monolayer level (~4 Ti atoms/nm2support) mostly twodimensional, polymerised TiO5 species are found on silica support under dehydrated conditions. Ding et aL [2] and Castillo et a[ [4] agreed that the preparation parameters in Corresponding author; fax: +33-47244-5399; email: [email protected]

778

gas and liquid phase modification, respectively, affect substantially the characteristics, e.g. structure, dispersion and crystallinity of supported titania layers. By liquid phase grafting of Ti(OPr')4 on SiOz, some pore plugging was observed at elevated titanium (> 4.3 Ti atoms/nmZsupport) concentrations [4]. However, the chemical vapor deposition (CVD) technique preserved better the texture of the support, the anatase particles being dispersed both on the external and internal silica macropore surface [2]. Lindblad et at [7] have investigated the binding of titanium(IV) isopropoxide and growing of layers of titanium oxide on high surface area silica in an ALD reactor. The atomic layer deposition (ALD) method is a preparation technique based on surfacesaturating gas-solid reactions [8]. Recent reviews are available in the literature describing in detail the principle and applicability of the ALD method on porous supports [8]. In the present study, the atomic layer deposition technique was applied to modify silica surfaces with increasing amounts of highly dispersed titania. The ultimate goal of the study was to prepare high performance titania/silica support materials for metal oxides with interesting surface properties. Firstly, the suitability of a wide reaction temperature range for the isopropoxide precursor was investigated by elemental analyses and inert diffuse reflectance infrared spectroscopy (DRIFTS) measurements. Secondly, in order to achieve a titania monolayer coverage, the deposited amounts were increased by applying several consecutive precursor - oxygen cycles. The texture and dispersion of the TiOz/SiOz samples were examined in detail as a function of the number of ALD reaction cycles by BET surface area/porosity measurements, X-ray diffraction (XRD), and UV-Vis diffuse reflectance (DRS) and X-ray photoelectron spectroscopies (XPS). The microcrystallinity of the samples was studied by transmission electron microscopy (TEM). Bulk TiOz and a TiOz-SiOz mechanical mixture were characterized for comparison. 2. E X P E R I M E N T A L 2.1. Catalyst preparation

The materials and operating conditions used for the growth of titania on silica are listed in Table 1. The precursor-support surface reaction was carried out in vacuum in a heated fixed-bed quartz chamber of a flow-type ALD reactor (F-120, ASM Microchemistry Ltd.) [8]. The adsorption was performed by letting the precursor gas flow react with the silica surface and the solids were further calcined in oxygen. Table 1 Experimental conditions for the deposition of titania on silica by ALD SiO2 a, 5 - 10 g Support in air 600~ h, in reactor 110 - 260~ (160~ b, 2 - 3 h Support pretreatment Ti(OPri)4 c, concentration 2.0 mmol/gsupport Precursor Precursor sublimation temperature 110- 115~ (ll0~ b 5h Deposition temperature, duration 110 - 260~ (160~ 1 - 5 kPa Reactor pressure Nz, 3 L/h Carrier gas 500~ in dry Oz, 11 h Calcination EP10 from Crosfield Ltd.; b optimized conditions for precursor-oxygen cycles; c Acros Organics (98+ %) a

779 The successive and controlled precursor - oxygen cycles were repeated one, three, five or seven times under selected conditions (Table 1). In order to demonstrate the importance of the preparation method for the physical properties of the material, a reference TiO2-SiO2 sample with 13.3 wt-% of Ti was prepared by mixing appropriate amounts of TiO2 (DT51 from Rh6ne-Poulenc, 100 % anatase) and SiO2 supports by magnetic stirring for 8 h and calcination under 02 flow at 500~ for 1 lh.

2.2. Catalyst characterization The titanium content of the catalysts was determined by AES-ICP and the carbon content by total combustion in oxygen at 1050~ followed by an acidimetric coulometry measurement. DRIFT spectra of uncalcined samples were measured in inert nitrogen atmosphere conditions on a Nicolet Impact 400 D FTIR spectrometer within the spectral range of 4000 - 400 cm -1 with a resolution of 2 cm -1 and the number of scans equal to 64. The BET surface area and pore volume and diameter of the catalysts were obtained by nitrogen adsorption at -196~ after heat pretreatment under vacuum for 4h at 350~ The XRD patterns were collected with a Bruker D5005 diffractometer (50 kV, 35 mA) between 3 and 80 ~ 20 using CuK0t radiation. The UV-Vis DR spectra were recorded at ambient conditions with a Perkin-Elmer Lambda 19 spectrophotometer equipped with an integrating sphere reflectance accessory. The measurements were carried out at wavelengths ranging from 200 to 2000 nm, using MgO as a standard white reference. The XPS data were obtained with a SSI 301 instrument using a monochromatic and focused (spot diameter 600pro, 100 W) A1Kct radiation source (1486.6 eV). The analyzer was operated at 50 eV pass energy for the high-resolution spectra and the charging was controlled by the use of a flood gun (5 eV). The binding energies (BE) were referenced to the Si2p or Ti2p 3/2 lines at 103.5 or 458.8 eV, respectively. The experimental spectral lines were fitted to theoretical lines (80 % Gaussian, 20 % Lorentzian) with a least-squares algorithm using a nonlinear baseline. The TEM measurements were carried out with a JEOL JEM-2010 microscope operated at 200 kV. 3. RESULTS AND DISCUSSION

3.1. Binding the titanium precursor on silica Controlled chemisorption of the precursor on silica was achieved between 110 and 180~ as identical surface densities of titanium, 1.25 _ 0.1 atoms/nm2support, were obtained throughout the support bed pretreated at 600~ as seen in Fig. 1. The carbon/titanium (C/Ti) atomic ratio, calculated for the upper bed sample, was between 6 - 7 thus indicating that the surface species formed contained approximately two isopropoxide ligands (ligand/titanium ratio ~2) after a loss of the two others in the binding [9]. The growth was highly reproducible and a homogeneous saturation through the reactor bed was achievable as confirmed with five repeated process runs at 140~ [10]. In practice, the growth experiments were limited to the deposition temperatures greater than 100~ due to the insufficient volatilization of the precursor at lower temperatures. The thermal decomposition of Ti(OPri)4 was observed to occur gradually at reaction temperatures

780 higher than 180~ as observed from the increasing Ti contents and from the fast decreasing C/Ti ratios [8] in Fig. 1. When the reaction temperature increased up to 240~ the decomposition of the precursor occurred already on the walls of the gas feeding tubes thus decreasing the final amount of precursor on silica (Fig. 1). --O-Ti top of reactor --&-Ti bottom of reactor -O-C/Ti top of reactor 5

, 8

4.5-

r

3.5-

5

32.5-

2-

3

o 1.5.~ 1 -

2

o

.~.

~"

0.5-

0

I

)0

130

I

i

1

v

160 190 220 Reaction temperature (~

I

250

Fig. 1. Titanium contents and C/Ti atomic ratios at the top or bottom parts of the ALD reactor bed as a function of the precursor reaction temperature.

r

[--,

V I

3800

3350

I

2900

I

2450 2000 Wavenumber (cm "l)

I

I

1550

1100

650

Fig. 2. Influence of the reaction temperature on DRIFTS patterns: Precursor deposited on silica at (a) 160~ (b) 180~ (c) 220~ (d) and (e) 260~ Samples (a, b, c and d) are taken from the top and sample (e) from the bottom part of the ALD reactor bed. For comparison, pure silica pretreated at 600~ is shown as well (f).

781 The ideal reaction temperature range was verified by DRIFTS measurements under inert conditions (Fig. 2). At reaction temperatures from 110 to 160~ (Fig. 2 (a)), the consumption of the isolated Si-OH groups of support was revealed by the disappearance of the intense peak at 3745 cm q [11], thus indicating the surface saturation. The appearance of intense titanium bound isopropoxide ligand stretching bands was detected at 2974 (l~as(CH3)), 2932 (1)as(CH3 or CH2)) and 2882 cm -10s(CH3))[12-15]. The weak bending bands of methyl C-H arised at 1467 and 1452 cm -1 (Sas(CH3)) and at 1381 and 1367 cm -1 (6s(CH3)) [12-15], respectively. Similarly, a weak peak located at 1332 cm -1 can be identified to methyne C-H bending (5(CH(CH3)2)) [12-14]. The assigned peaks are marked with an asterisk (*) in Fig. 2. At 180~ the exposure of free silanols (band at 3745 cm -1) started (Fig. 2 (b)), and it became more profound further increasing the reaction temperature up to 200, 220 (Fig. 2 (c)), 240 and 260~ (Fig. 2 (d and e)). Furthermore, the intensity of the precursor ligand absorption bands at 2974, 2932, and 2882 cm 1 [12-15] decreased systematically at elevated reaction temperatures, especially for the bottom bed samples (Fig. 2 (e)). Ritala and co-workers [9,16] reported a surface-limited reaction of Ti(OPri)4 with a substrate at 100 - 250~ in their study concentrating on TiO2 thin film growth by atomic layer deposition. However, the temperature range for controlled growth observed in our work (110 - 180~ is in agreement with previous ALD results for porous silica carrier [7], differing only slightly by the upper temperature limit that was suggested to be 200~ The authors reported a concentration of ~ 1.4 Ti atoms/nm2support (ligand/titanium ratio ~2) after the reaction of precursor at the temperature of 160~ with SiO2 EP10 pretreated at 450~ [7]. Considering the number of isolated OH groups, that serve as primary bonding sites for the precursor [8], ~2 and ~1.5 for SiO2 EP10 pretreated at 450 and 600~ [8,11], respectively, one could suggest that in addition to the silica pretreatment temperature, the steric hindrance of precursor ligands is a predominant factor influencing the deposited titanium amount on silica, pretreated at temperatures above 450~ 3.2. Characteristics of titania/silica obtained by c o n s e c u t i v e p r e c u r s o r - o x y g e n cycles Physico-chemical characteristics of the titania/silica samples and silica and titania supports are collected in Table 2. The samples are labelled xTiSi (x = precursor - oxygen cycle number) or mix-TiSi (mechanical mixture sample). When repeating the ALD cycles from one to seven times, the titanium loading increased in a very controlled, linear manner from 1.2 to 7.0 atoms/nm2support (Table 2). Furthermore, the specific surface area (SBET), pore volume and pore diameter of the silica support diminished only very slightly, from 301 to 284 m2/g, from 1.20 to 0.95 cm3/g and from 195 to 190 ,~, respectively (Table 2). However, the corresponding SBET and porosity values for the mix-TiSi sample were much lower, being 245 m2/g, 0.55 cm3/g and 190 ,A,, respectively. The XRD pattern of the mix-TiSi sample showed distinct titania anatase peaks due to the TiO2 component, but no XRD detectable peaks of TiO2 crystallites (d > 5 nm) were observed for the ALD samples, even above the theoretical monolayer coverage of 45.5 Ti atoms/nm2support [4,10]. The deposition of highly dispersed titania only caused intensity masking of the broad amorphous line of silica centered at 20 = 23 ~ [10]. Nevertheless,

782 further transmission electron microscopy studies have revealed that very small TiO2 crystal particles (d < 3 nm) were present after five precursor cycles (5.1 atoms/nm2support) on the highly dispersed and amorphous titania/silica surface [10]. Table 2 Physico-chemical properties of TiO2/SiO2 samples and those of bulk silica and titania Sample Titanitan amount N2 adsorption (wt-%) (atoms/nm2support) SBET(m2/g) Pore volume (cm3/g) Pore diameter (A) SiO2 301 1.20 195 TiO2 101 0.35 95 1TiSi 2.7 1.2 323 1.25 210 3TiSi 6.5 3.1 305 1.15 205 5TiSi 10.2 5.1 297 1.05 200 7TiSi 13.1 7.0 284 0.95 190 mix-TiSi 13.3 7.1 245 0.55 190 These results on the crystallinity differ rather significantly from those of Lindblad et al. [7] who observed a weak anatase XRD signal for their ALD TiO2/SiO2 catalysts already after four reaction cycles (-~4 Ti atoms/nm2support). In general, titania crystallites in addition to a profound (-~10 - 20 %) decrease in the surface area/porosity have been detected for Ti(OPr')a liquid-phase grafted samples starting with a coverage of-5.1 atoms/nm2suppo~tby XRD [4] or UV-Vis DRS [4] and of-~4.0 Ti atoms/nm2support by more sensitive Raman spectroscopy [1,5]. Indeed, the presence of humidity in the ligand removal procedure has been proposed to lead to an agglomeration phenomenon by breaking the Ti-O-Si bonds and further forming crystalline titania [2,11 ].

c) 1/

200

\\\\\

d)

I

I

I

I

250

300

350

400

Wavelength (nm) Fig. 3. UV-Vis DR spectra of titania/silica samples and supports: (a) TiO2, (b) mix-TiSi, (c) 5TiSi, (d) 7TiSi, (e) 1TiSi, (f) 3TiSi and (g) SiO2. For explanations, see also Table 2.

783 Broad transitions at between 240 and 300 nm were observed for the ALD samples by UV-Vis DRS in hydrated conditions (Fig. 3 (c - f)), indicating the presence of isolated TiO4 units (-~220 - 250 nm [5,17]) and of either hexacoordinated Ti with two or more ligands other than framework oxygen or partially condensed titanium-oxygen-titanium (-~300 nm [17,18]). The formation of Ti-O-Ti bonds contributed to the slight delocalisation of the absorption edge positions towards higher wavelengths with increasing titania loading [1,5,17], but no specific absorption due to crystalline TiO2 was detected at 330 nm [17,18] (Fig. 3). However, both bulk titania and the mechanical mixture sample (mix-TiSi) showed identical UV-Vis DRS patterns of Ti in octahedral sites [5], as seen in Fig. 3 (a) and (b), respectively. The binding energy (BE) value of Ti2p3/2 line for the mix-TiSi sample (458.2 eV) was close to that of pure titania (458.8 eV), indicating a weak Ti-O-Si bond interaction and together with a low surface Ti/Si atomic ratio (Fig. 4), a presence of bulk titania phase [4]. In addition, the Ols photoelectron line was divided into two separated (A = 3.3 eV) bands at 532.9 and 529.6 eV due to the contributions of silica (Si-O-Si bond, 93 %) and oftitania (Ti-O-Ti bond, 7 %) [4], respectively, as seen in Fig. 4. The overal O ls pattern of pure titania consisted of two components; TiO2 at 530.0 eV (80 %) and Ti(OH)x at 531.4 eV (20 %).

Fig. 4. Percentage distribution of the components of the Ols band obtained from the XP spectra together with Ti/Si atomic ratios from XPS and chemical (CA) analysis for the titania/silica samples. For the 1 - 7TiSi samples, however, a positive shift of-4).5 eV in the BE values of Ti2p3/2 compared to that of pure titania were detected [4]. The values stayed within the range of 459.3 + 0.2 eV, independently of the surface coverage. When the Ols peak arising from pure silica was fixed at 532.8 eV, the lower peak maxima of the deconvoluted peak decreased, as a function of the titania concentration, from 531.1 eV (contribution 10 %) to 530.8 eV (contribution 35 %) (Fig. 4). The difference between the two Ols peak maxima was smaller, from 1.7 to 2.0 eV, than for the mix-TiSi sample. The results indicate the

784 formation of strong Ti-O-Si bridging bonds [4,5]. In addition, regarding the surface and bulk atomic ratios in Fig. 4, which follow parallel patterns up to a coverage of ~5.1 atoms/nm2support (5TiSi sample), a high relative dispersion of surface titania species can be concluded. This suggestion is supported by the study of Gao et al. [5] where a linear correspondence of the XPS and CA Ti/Si ratios was detected up to the loading of ~4.2 ato ms/nm2support. 4. CONCLUSION A controlled gas-solid phase reaction of titanium(IV) isopropoxide with silica surface pretreated at 600~ was achieved at between 110 and 180~ in an atomic layer deposition (ALD) reactor. Element determinations and DRIFTS measurements suggested the formation of two isopropoxide ligand containing complex on silica with a titanium density of ~1.25 atoms/nmZsupport. At reaction temperatures exceeding 180~ the growth was no longer determined by the surface. The reaction at 160~ was followed by a calcination treatment at 500~ and the consecutive precursor- oxygen cycles were repeated from one to seven times. The deposition of increasing amounts of titania onto silica preserved the high surface area and amorphous character of the material even at monolayer coverage. The coordination of titanium in hydrated environment changed from tetracoordinated up to hexacoordinated upon increasing the loading. XPS measurements confirmed the results concerning excellent dispersion and homogeneity of the ALD deposited titania species and suggested that firm titaniumoxygen-silicon connections were formed. The influence of the preparation mode on the characteristics of TiO2/SiO2 was demonstrated. ACKNOWLEGMENTS The University of Joensuu is thanked for providing access to an inert DRIFT spectrometer. Financial support is acknowledged from the Research Foundation of the Fortum Corporation, the Foundation for Advancement of Technology in Finland and the Association Franco-Finlandaise pour la Recherche Scientifique et Technique.

REFERENCES

1. X. Gao and I. Wachs, Catal. Today, 51 (1999) 233. 2. Z. Ding, X. Hu, P. Yue, G. Lu and P. Greenfield, Catal. Today, 68 (2001) 173. 3. X. Gao, S. Bare, J. Fierro and I. Wachs, J. Phys. Chem. B, 103 (1999) 618. 4. R. Castillo, B. Koch, P. Ruiz and B. Delmon, J. Catal., 161 (1996) 524. 5. X. Gao, S. Bare, J. Fierro, M. Banares and I. Wachs, J. Phys. Chem. B, 102 (1998) 5653. 6. A. Fernandez, J. Leyrer, A. Gonzfilez-Elipe, G. Munuera and H. Kn6zinger, J. Catal., 112 (1988) 489. 7. M. Lindblad, S. Haukka, A. Kyt6kivi, E-L. Lakomaa, A. Rautiainen and T. Suntola, Appl. Surf. Sci., 121/122 (1997) 286.

785 8. (a) S. Haukka, E-L. Lakomaa and T. Suntola, Stud. Surf. Sci. Catal., 120 (1998) 715. (b) E-L. Lakomaa, Appl. Surf. Sci., 75 (1994) 185. (c) S. Haukka, A. Kyt6kivi, E-L. Lakomaa, U. Lehtovirta, M. Lindblad, V. Lujala and T. Suntola, Stud. Surf. Sci. Catal., 91 (1995) 957. 9. A. Rahtu and M. Ritala, Chem. Vap. Deposition, 8 (2002) 21. 10. J. Ker~inen, E. Iiskola, C. Guimon, A. Auroux and L. Niinist6, in preparation. 11. S. Haukka, E-L. Lakomaa and A. Root, J. Phys. Chem., 97 (1993) 5085. 12. R. Urlaub, U. Posset and R. Thull, J. Non-Cryst. Solids, 265 (2000) 276. 13. K. Von Witke, A. Lachowicz, W. Briiser and D. Zeigan, Z. Anorg. Allg. Chem., 465 (1980) 193. 14. M. Schraml-Marth, A. Wokaun and A. Baiker, J. Catal., 124 (1990) 86. 15. S. Srinivasan, A. Datye, M. Smith and C. Peden, J. Catal., 145 (1994) 565. 16. J. Aarik, A. Aidla, T. Uusitare, M. Ritala and M. Leskel/i, Appl. Surf. Sci., 161 (2000) 385. 17. K. Schrijnemakers, N. Impens and E. Vansant, Langmuir, 15 (1999) 5807. 18. S. Bagshaw, F. Di Renzo and F. Fajula, Chem. Commun., 18 (1996) 2209.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

787

Concept of the synthesis of novel platinum catalysts for the selective hydrogenation of unsaturated carbonyl compounds Jacek Kijenski and Piotr Winiarek Faculty of Chemistry, Warsaw University of Technology (Politechnika), ul. Noakowskiego 3, 00-664 Warszawa, Poland. A new method of synthesis of selective platinum catalysts for the hydrogenation of unsaturated carbonyl compounds is presented. Platinum was deposited on the supports tailored with the monolayer of transition metal oxide. Selectivity of these catalysts strongly depended on the type of inorganic support as well as on the type of transition metal in the monolayer. Catalysts were tested in the hydrogenation of furfural, crotonaldehyde and cinnamaldehyde. Selectivity of the synthesis of the appropriate unsaturated alcohols was enhanced when compared with the reactions performed over classical Pt-metal oxide catalysts. 1. I N T R O D U C T I O N In the last decade, extensive investigations have been done in order to find catalysts for the hydrogenation of unsaturated aldehydes and ketones to their corresponding alcohols [1,2]. Application of classical noble metal catalysts in this reaction is strongly limited due to their unsatisfactory selectivity. Non-polar metal surface does not favour the adsorption of a strongly polar carbonyl group. In a properly designed catalyst an active metal phase should be polarized to enhance its interaction with C=O bond and to increase its activity in carbonyl hydrogenation. There are few methods [2] of noble metal catalysts modification in order to force C=O group adsorption, i.e. doping by electropositive metal (alloys with Sn, Fe), deposition on partially reduced supports (TiO2, ZrO2) or shape-selective zeolites, etc. In the early eighties, a new method for the deposition of transition metal ion monolayer on the surfaces of inorganic oxides was developed in our laboratory [3]. The essential step of this method is the selective reaction of the surface hydroxyl groups with transition metal alkoxide molecules [3,4]. The adopted procedure allows the design of catalysts containing a strictly determined number of transition metal monolayers or sandwich-type catalysts with different metals monolayers anchored to the surface of the carrier. In this work, a concept of the modification of platinum catalysts is presented. According to our proposal, the monolayer oxide systems should be the appropriate carriers that can influence the noble metal properties. The monolayer carrier effect depends on its acid-base or electro-nucleophilic properties and the number and capacity of monolayers. This is of great importance in the selective hydrogenation of unsaturated carbonyl compounds. The

788

final activity of metal oxide - monolayer oxide - p l a t i n u m system depends on two effects: strong oxide-oxide interaction (SOOI) between primary carrier and monolayer oxide, and strong m e t a l - support interaction (SMSI) between platinum and monolayer transition metal oxide. 2. E X P E R I M E N T A L

As supports, commercial Degussa silica (Aerosil 300), 7-alumina (Typ C) and titania (P-25) were used. MgO was prepared in our laboratory by precipitation of Mg(OH)z from the aqueous solution of Mg(NO3)2"6 HzO with aqueous ammonia; the precipitate was dried at 393 K and then the hydroxide was decomposed at 823 K. Before the reaction, supports were calcined at 823 K overnight in a stream of dry air and then in the stream of dry and deoxidized nitrogen for 3 hours. Ti(i-OC3H7)4, (i-C4H90)3VO, and Zr(n-OC4H9)4 were used as the corresponding monolayer oxide precursors. A portion of the calcined support (2 g, grains of 0.5-1.0 mm diameter) was placed into the Schlenk vessel, 5 ml of dry n-hexane was added after which an appropriate amount of the precursor solution in water-free nhexane was also added. Only after 24 h was the solution removed, and the catalysts washed several times with the solvent (n-hexane). Impregnated catalysts were calcined in the dry air for 3 h at 623 K. Platinum was immobilized on a monolayer oxide by the dry impregnation of the carrier with the solution of hexachloroplatinic acid HaPtC16xHzO in 2-propanol or platinum acetylacetonate Pt(CsH702)2 in acetone. Elementary compositions of the catalysts were determined using Brucker AAS spectrometer. The concentration of the surface OH groups was measured by the potassium naphthenide titration method. TPR measurements were performed using an Altamira apparatus. The activity of the catalysts was tested in the hydrogenation of crotonaldehyde, cinnamaldehyde and furfural at atmospheric pressure. Before the reaction, the catalysts were reduced in a stream of dihydrogen at 623 K. Hydrogenation reactions were carried out in a standard fixed bed vertical reactor. After the catalyst reduction, the reactor was cooled down to the reaction temperature (423, 470 or 523 K) and unsaturated aldehyde and hydrogen were introduced onto the top of the reactor. The first product sample for analysis was taken after 30 min of reaction (period needed to reach reaction steady state). The identification and analysis of the reaction mixtures were performed by means of GC-MS using HP-50 capillary column. 3. R E S U L T S AND D I S C U S S I O N

As we reported in our former works [3,4] alkoxide molecules underwent selective, stoichiometric reactions with surface hydroxyl groups. As a result of this reaction surface alkoxy complex was formed. \ --Mg--OH

O'/

\ --Mg--OH /

\

~Mg--O + Zr(OC4H9)4

.OC4H9

+ 2 C4H9OH \Zr/ / --~Mg--O ~OC4H9

.~ o / \

(1)

789 It is noteworthy that the anchoring of subsequent layer of zirconium compound is not possible due to the lack of OH groups on the surface of the support modified with transition metal alkoxide. The surface alkoxy complex was then decomposed by calcination of the catalyst in a stream of dry air at 623 K (see eq. 2). During the reaction, the evolution of stoichiometric amount of appropriate alkene was observed. Secondary hydroxyl groups appeared as a result of the reaction (2). Their presence created the possibility of anchoring the subsequent layer of the same or the other transition metal ions. \ ~Mg--O. /OC4H9 O/ \Zr \ /

\oc .9

623 K ~ dry air

\ -~Mg--O\ /OH O Zr \ /

"o.

(2)

+ 2 C4H8

In order to prove the adopted mechanism of the reaction between transition metal alkoxide and surface hydroxyls, it was necessary to determine the concentrations of primary and secondary OH groups and the concentration of transition metal bonded to the support surface. These results are presented in Table 1. Table 1. Properties of the supports modified with transition metal alkoxides. Specific OH groups surface area concentration Support Precursor (m2/g) (mmole/g) .

Transition metal concentration (mmole/g)

SiO2

Ti(i-OC3H7)4 (i-C4H90)3VO Zr(n-OC4H9) 4

300 292 296 295

0.30 0.29 0.32 0.25

0 0.28 0.30 0.28

~/-A1203

Ti(i-OC3H7)4 (i-C4H90) 3VO Zr(n-OC4H9)4

181 175 179 170

0.54 0.45 0.48 0.50

0 0.51 0.50 0.52

MgO

Ti(i-OC3H7)4 (i-C4H90)3VO Zr (r/-OC4H9)4

50 48 45 49

0.23 0.12 0.14 0.19

0 0.10 0.13 0.12

51 46 44

0.21 0.12 0.13

0.08 0.10

TiO2

-

(i-C4H90) 3VO Zr (n-OCnH9) 4

0

790 The following conclusions can be drawn concerning the data collected in the Table 1: (a) specific surface area of the support diminished very little (practically remained unchanged) during the reaction with the transition metal alkoxide; (b) in no case did the concentration of transition metal exceed the concentration of OH groups on the support surface; (c) in the case of catalysts obtained with silica, alumina, and titania, the concentration of transition metal was practically equal to the concentration of surface OH groups, whereas in the case of magnesia, the concentration of surface OH groups was ca. two times higher in comparison with the concentration of transition metal deposited on the surface. Thus, it seems clear that a controlled surface reaction occurred between the surface hydroxyl groups and transition metal alkoxide molecules leading to the formation of uniform monolayer of transition metal oxide "tailored" to the support. The uniform character of the monolayer was confirmed by results of TPR experiments, in which, for example, one relatively sharp peak of V 5§ ions reduction was observed (for polycrystalline V205 three or four peaks are usually present in the TPR). The temperature of maximum hydrogen consumption was different for each support and for VzOs/support systems increased in the order: TiOz < AlzO3 < SiOz < MgO. This difference in the reducibility of vanadia monolayer was the result of strong oxide-oxide interaction (SOOI) occurring in the system. The application of transition metal oxide monolayer/support systems as carriers for platinum allows the possibility of modification of the properties of platinum in a wide range [5]. According to our previous works we adopted an optimal loading of platinum (2 %w). The reaction of platinum compound with OH groups of the support proceeded to the formation of transition metal-O-Pt bond, according to the reaction (3) - a useful tool for obtaining well dispersed noble metal on the surface of inorganic support [6,7]. \ mMg--O. / OH 0/ \Zr + 2 Pt(acac)2 /

\ wMg--O"

/ O--Pt(acac)

"- 0 / \Zr + 2 acacH " / --~Mg--O \ 0 - - Pt(acac)

The reducibility of platinum should be affected both by SOOI effect (see above) and strong metal-support interaction (SMSI) between platinum and transition metal oxide monolayer. Both mentioned effects were observed by means of temperature-programmed techniques. TPR investigations put in evidence that the reducibility of platinum strongly depended on the type of primary carrier as well as on the type of transition metal monolayer. The results of TPR investigations are presented in Fig. 1 and Table 2. The lowest temperatures of Pt reduction were recorded in the case of catalysts deposited on titania, whereas the highest for catalysts deposited on magnesia. An interesting phenomenon was observed in the case of catalysts deposited on silica. The consumption of hydrogen by these systems was very low. This was probably due to the high degree of platinum dispersion on it. The elucidation of this phenomenon will be the subject of a forthcoming paper.

(3)

433K460K 5 2 1 ~

~

1

Pt/TiO2/MgO calcined at 623 K Pt/ZrOe/TiO2 calcined at 823 K Pt/TiO2/MgO calcined at 823 K Pt/Ti02/SiO2 calcined at 823 K '----'~'~'-~--

Pt/TiO2/SiOzcalcined at 623 K

Fig. 1. TPR profiles of Pt/TiOz/support and Pt/ZrQ/support catalysts. Table 2. Results of TPR investigations (catalysts calcined in the air at 823 K) Catalyst

TPR maxima

Pt/Ti02/SiO2 Pt/TiO2/MgO Pt/TiO2/AI203

350 K (weak), 470 K(weak), 530 K(weak) 553 K 520 K

Pt/V2Os/SiO2 Pt/V2Os/TiO2 Pt/V2Os/MgO Pt/V2Os/AIz03

380 K (weak), 468 K(medium) 346 K 509 K 557 K

Pt/ZrO2/SiOz Pt/ZrO2/MgO Pt/ZrO2/AI203 Pt/ZrO2/TiOz

370K(weak), 441 K(weak)

517 K 501 K 433K, 460 K

The aim of this work was to develop and present a new method for the synthes selective platinum catalysts for the hydrogenation of unsaturated aldehydes to unsatuJ alcohols. The method of preparation presented above makes it possible to obtain cata possessing "polar" platinum surface, due to the presence of active centers near the inte~ m e t a l - partially reduced transition metal oxide. This transition metal oxide adsorb, carbonyl oxygen atom whereas adjacent platinum atom interacts with carbonyl ca atom. In such a situation, the C=C bond is quite far from the surface so its adsorption i., favored. There are also some additional parameters which should be taken consideration. The first is the size of platinum crystallites located on the catalyst sur Large platinum crystallites cause planar adsorption of aldehyde molecule; in such adsorption of both double bonds is highly probable. It is known that a more sui

792

catalyst for selective hydrogenation of unsaturated aldehydes should possess rather small platinum crystallites [1,2]. Thus, platinum acetylacetonate should be a more suitable precursor for the preparation of selective catalyst than hexachloroplatinic acid. The second parameter is the adequate strength of aldehyde adsorption. Strong chemisorption of aldehyde (occurring for example on strong acidic or strong basic sites) could be a reason for several unexpected transformations such as condensation, polymerization, decarbonylation, hydrogenolysis reactions. Summing up these considerations, the selective catalyst for unsaturated aldehydes hydrogenation should possess platinum crystallites with low size and not very strong basic or acidic properties. XRD investigation of catalyst samples did not provide additional information concerning either Pt crystal phase or crystal phases of transition metal oxides in the monolayer. The investigations of adsorbed aldehyde species by means of IR spectroscopy were done in order to prove the activation of C=O bond of unsaturated aldehyde. The results are presented in Fig. 2.

/ 1\

" "x

""ff'~3 5 / J"~" "'~'"'>.

T .~

"~\ 1

,,

',

~ ~

I

~

I I

',

,!'

,,,

J j.- ,.

I1

II I | |,

!

', '.;. ~";' I

/

i': "J: ''

'"

'

',, ~'~'

'~ ~l~ V \

,.,,.. "'" ~',"~"~

",

I

i

i

i

!

18(210

!

i

1

1600

i

i

i

i

1400

i

!

i

i

1200

i

i

---

~n~d~hyd~

......

,,

i

cinnamald~hyde

~h~o,~t~on

-1

till

20~

background

phy~sorption

~ ~3 ,"~, ... ,,,..':,,:,~

I I. ,, ' ";' "il

---

i

lC1~0

Fig. 2. IR investigation of cinnamaldehyde adsorption on the surface of Pt/VzOs/SiOz. Adsorbed carbonyl species were observed after the adsorption of cinnamaldehyde on the surface of Pt/VzOs/SiOz catalyst. The bands derived from these species shifted to lower wavenumbers which indicated the diminishing strength of the C=O bonds. On the other hand, the bands originating from C=C bonds practically did not shift after aldehyde chemisorption. The results of unsaturated carbonyl compounds hydrogenation in the presence of metal oxide - monolayer o x i d e - platinum (Pt loading 2 %) catalysts are collected in Table 3 (Pt from HzPtC16) and Table 4 (Pt from Pt(acac)z). In all the cases, the selectivity of the platinum catalysts deposited on supports modified with transition metal oxide monolayers

793 was higher when compared with classic Pt-inorganic oxide systems. The most interesting results were obtained in the presence of Pt/Ti02/SiO2 and Pt/Zr02/SiO2 catalysts. In the hydrogenation of crotonaldehyde and cinnamaldehyde higher selectivities of unsaturated alcohols were obtained when platinum was introduced onto the catalyst surface from Pt(acac)2 solution. Furfural hydrogenation proceeded with satisfactory yields and selectivities in the presence of catalysts derived from HaPtCI6 as well as from Pt(acac)2. The results obtained in this work indicate that the selectivity of platinum catalysts for the hydrogenation of unsaturated carbonyl compounds can be markedly improved by doping the carrier with a monolayer of transition metal ion. It is also worth to note that sandwich-type catalysts (ZrO2/Pt/ZrO2/TiOa) are also active and selective in the studied reaction. We believe that on the surface of this system, the end-on adsorption of aldehyde molecule has been forced. Table 3. Yields and selectivities of unsaturated aldehydes hydrogenation over catalysts derived from HaPtC16 Crotonaldehyde (Tr = 398 K)

Cinnamaldehyde (Wr= 423 K)

Furfural (Tr = 423 K)

Catalyst Yield of crotyl alcohol

Yield of Yield of Selectivity cinnamylalcohol Selectivity furfuryl alcohol

Selectivity

[mote %1

[%1

[mote %1

[%1

[mote %1

[%1

Pt/SiO2

8.7

9.1

0.5

1.9

18.3

24.5

Pt/TiO2

8.1

37.0

10.3

17.4

33.5

41.2

Pt/TiO2/SiO2

32.4

62.9

3.9

37.9

67.5

92.0

Pt/TiO2/MgO

11.2

49.4

10.2

36.5

60.7

87.5

Pt/V205/SiO2

22.7

54.3

4.9

29.9

11.8

84.1

Pt/V2Os/TiO2

32.6

74.2

11.8

38.8

23.5

90.3

Pt/V2Oa/MgO

18.2

57.1

13.9

51.2

41.0

73.5

Pt/ZrO2/TiOz

39.5

68.3

28.3

42.1

59.2

94.0

ZrO2/Pt/ZrO2/TiOz

28.3

76.2

18.4

55.7

58.0

96.7

4. C O N C L U S I O N S Platinum catalysts deposited on the supports modified with transition metal oxide monolayers exhibited high activity and satisfactory selectivity in the hydrogenation of unsaturated aldehydes to the corresponding unsaturated alcohols. Platinum acetylacetonate was a more suitable catalyst precursor than hexachloroplatinic acid in the preparation of the transition metal-O-Pt catalytic system for the hydrogenation of cinnamaldehyde and

794

crotonaldehyde. This was probably connected with the small dimension of the platinum crystallites obtained from Pt(acac)z. The catalytic properties of platinum were modified both by SMSI and SOOI effects. The most suitable supports for the investigated catalysts were silica and titania, probably due to the lack of strong acidic and strong basic sites on their surfaces. Table 4. Yields and selectivities of unsaturated aldehydes hydrogenation over catalysts derived from Pt(acac)z. Crotonaldehyde (Tr = 398 K)

Cinnamaldehyde (Wr= 423 K)

Furfural (Tr = 423 K)

Catalyst Yield of crotyl alcohol

Yield of Yield of Selectivity cinnamylalcohol Selectivity furfuryl alcohol

Selectivity

[mole %]

[%]

[mole %]

[%]

[mole %]

[%1

Pt/SiO2

10.3

12.1

0.8

3.9

14.1

30.8

Pt/TiO2

14.1

35.0

14.2

19.9

23.7

48.7

Pt/Ti02/SiO2

37.2

66.9

4.2

38.3

44.2

95.1

Pt/TiO2/MgO

14.7

46.1

8.1

44.2

39.8

92.2

Pt/V2Os/SiO2

23.5

58.1

5.8

32.1

10.1

88.6

Pt/V2Os/TiO2

35.1

72.5

13.1

40.3

25.9

87.3

Pt/V2Os/MgO

20.1

61.2

15.3

50.8

39.2

76.8

Pt/ZrO2/TiO2

40.8

70.7

29.1

41.7

58.1

96.2

REFERENCES 1. P. Claus, Topics in Catalysis, 5 (1997) 51. 2. P. Gallezot and D. Richard: Catal. Rev. - Sci. Eng., 40 (1998) 81 and references therein. 3. J. Kijenski and M. Glinski, Prep. Catal. III, Stud. Surf. Sci. Catat, 16 (1983) 553. 4. J. Kijenski, A. Baiker, M. Glinski, P. Dollenmeier and A. Wokaun, J. CataL, 101 (1986) 1. 5. J. Kijenski and P. Winiarek, Appl. Catal. A: Gen., 198 (2000) L1. 6. B. Coq, A. Chaqroune, F. Figueras and B. Neiri, Appl. Catal., 82 (1992) 231. 7. B. Coq, A. Tijani, R. Dutartre and F. Figueras, J. Mol. Catal., 79 (1993) 253.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

795

Storage and supply of hydrogen mediated by iron oxide: modification of iron oxides S. Takenaka, C. Yamada, T. Kaburagi and K. Otsuka* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan The redox of iron oxides (Fe304 + 4H2 ~ 3Fe + 4H20) can be applied as a new method of storage and supply of hydrogen. The iron oxides prepared by precipitation of aqueous Fe(NO3)2 with urea consisted of small particles with uniform size. The addition of AI, Sc, Ti, V, Cr, Ga, Zr and Mo cations into iron oxides improved their redox performances. The added cations prevented the sintering of Fe metal and/or Fe304 during repeated redox cycles. 1. INTRODUCTION H2-O2 fuel cells are expected to be utilized at large scale in the near future, because they are clean energy devices emitting neither COx, NO,, nor SOx. Especially, the vehicles powered by solid polymer fuel cells (SPFC) are currently very active in developing towards the commercialization. One of the key technologies to be solved for a wide use of fuel cell vehicles is a safe and simple method for storage and supply of hydrogen. We have proposed a novel technology for storage and supply of hydrogen by utilization of redox of iron oxides [1]. (Step 1:H2 storage) (Step 2:H2 supply)

Fe304 + 4 Ha ~ 3 Fe + 4 H20 3 Fe + 4 H20 "'> Fe304 + 4 H2

(1) (2)

In this method, Fe304 was reduced with hydrogen into Fe metal according to eq. (1). The Fe metal was loaded in the vehicles and contacted with water vapor to form pure hydrogen according to eq. (2). In this method, theoretical amount of hydrogen stored as Fe metal is calculated to be 4.8 wt% of Fe metal, which is relatively high compared to that for hydrogen absorbing alloys easily obtained. However, the problem in this method is that the iron species (Fe metal and Fe304) are deactivated due to sintering during repeated cycles of reduction (eq. (1)) and oxidation (eq. (2)). The principle of our method for storage and supply of hydrogen is analogous to the Steam Iron Process [2]. In the process, iron oxides were reduced with CO and/or hydrogen and the reduced oxides were oxidized with water vapor to form hydrogen at temperatures higher than 1073 K. However, the formation of hydrogen through water decomposition on the vehicles requires the operational temperatures to be as low as possible. In the present study, we explored the effectiveness of iron oxides modified with different metal cations for the storage and supply of hydrogen. We will report that the added metal cations improve the redox performance of iron oxides and suppress the sintering of iron species during repeated redox reactions.

796

2. EXPERIMENTAL Fe203 was prepared by precipitation of an aqueous solution of Fe(NO3)3 by hydrolysis of urea at 363 K or by addition of aqueous NH3. The iron oxide samples with different metal cations were prepared by coprecipitation of a mixed aqueous solution of Fe(NO3)3 and the added metal cations with urea. The amount of the metal cation added (M) into iron oxide was adjusted to be 3 mol% of total metal cations (M/(Fe+M) = 0.03). The precipitates were dried at 353 K for 24 h and calcined at 773 K for 10 h. Reduction of the iron oxide samples with hydrogen and the successive oxidation of the reduced samples with water vapor were carried out with a conventional gas-closed gas-recirculation system with a quartz-made reactor (dead volume = ca. 210 ml). Fe203 with and without additives were used as starting materials. Prior to the reduction, the iron oxide samples, the samples were treated with oxygen at 673 K. The reduction was initiated by the contact of hydrogen (initial pressure = 33.3 kPa) with the samples at 603 K. Water formed during the reduction was condensed in a trap cooled at ca. 200 K which was installed in the gas circulation line just after the reactor. The reduction was continued until the reduction degree of the iron oxide sample reached 80%. The reduction degree was estimated based on the number of oxygen atoms in Fe203, i.e., the reduction degrees of Fe203 and Fe metal were 0 and 100%, respectively. The reduction of the metal ions added in the iron oxide samples was not taken into account for the calculation, even if the ions were reduced. After reduction of the samples with hydrogen, hydrogen remaining in the apparatus was evacuated and the water condensed in the trap was vaporized in the apparatus by warming up at 287 K (partial pressure = 1.5 kPa) to contact with the reduced samples at 653 K. The partial pressure of water vapor was kept constant during the oxidation. The reduction degree of samples was recovered to 30% by the oxidation. After this first oxidation, reduction with hydrogen and oxidation with water vapor were performed repeatedly in the range of reduction degree between 30 and 80%. 3. RESULTS AND DISCUSSION 3. 1. Preparation of iron oxides

Fig. 1.

SEM images of fresh iron oxide samples.

797 Our method for storage and recovery of hydrogen requires the iron oxides to have high reactivities for the reduction with hydrogen (hydrogen storage) and oxidation with water vapor (hydrogen recovery). The activity of iron oxides for reduction and oxidation should depend on the specific surface area of samples, i.e. the activity of samples with larger surface area would be higher. Therefore, we prepared Fe203 samples by precipitation of aqueous Fe(NO3)3 with aqueous NH3 and urea (denoted as Fe-oxide(NH3) and Fe-oxide(urea), respectively). A reagent grade Fe203 from Wako Pure Chem. Co (Fe-oxide(W)) was also used to compare the redox performances of these samples. Fig. 1 shows SEM images of the three samples before the reaction. The images of Fe-oxide(urea) and Fe-oxide(NH3) indicate that the samples were composed of small particles with uniform size, and the average particle size of Fe-oxide(urea) was smaller than that of Fe-oxide(NH3). The image of Fe-oxide(W) shows larger particle sizes and some sintering. The features of the samples shown in the SEM images were consistent with the specific surface areas of the samples and the crystallite size of Fe203 estimated from the full widths at half-maximum of the diffraction line of Fe203 ((104) plane) in the XRD patterns of the samples. The specific surface areas of Fe-oxide(urea), Fe-oxide(NH3) and Fe-oxide(W) were estimated to be 20, 12 and 8 m2g-1, respectively, by BET method using N2 adsorption at 77K. The crystallite sizes of Fe203 in Fe-oxide(urea), Fe-oxide(NH3) and Fe-oxide(W) were estimated by Scherrer equation to be 19.7, 34.8 and 47.7 nm, respectively. These results indicate that the precipitation with urea produces small Fe203 particles of uniform size and large surface area. In the precipitation method with urea, the urea was first hydrolyzed to form NH3 ((NH2)2CO + H20 --> 2 NH3 + CO2). Since the rate of hydrolysis of urea could be easily controlled by temperature, the pH of solution containing Fe 3+ could be changed slowly and gradually. Thus, the pH of solution during the precipitation of Fe(OH)3 could be more uniform when urea was utilized compared with the addition of aqueous NH3. It is well known that a local non-uniformity of pH in the solution makes the size of precipitates large and diverse. Thus, we consider that the particle size of Fe-oxide(urea) became smaller and more uniform than that of Fe-oxide(NH3). Fig. 2 shows the change of the reduction degree of iron oxides duringthe reduction with hydrogen (Step 1) at 603 K and oxidation with water vapor (Step 2) at 653 K. The reduction and oxidation were repeated for three cycles. For the first reduction (Fig. 2 (a)), Fe-oxide (NH3) and Fe-oxide(W) were reduced to a reduction degree of 80% at almost the same time (27 min), while the reduction of Fe-oxide (urea) needed longer time. After the first reduction, the contact of water vapor with the reduced iron oxides resulted in the formation of hydrogen and all the samples were oxidized to reduction degree 30% at almost the same time as shown in Fig. 2 (a). As for the second reduction (Fig. 2 (b)), it took ca. 9 min for all the samples to be reduced up to reduction degree 80%. In the second oxidation with water vapor (Fig. 2 (b)), it is clear that the kinetic curves are quite different among three samples. The Fe-oxide(urea) was oxidized to reduction degree 30% in 34 min although the rate for the second oxidation became slower than that for the first run. On the other hand, the reduction degrees for Fe-oxide(W) and Fe-oxide(NH3) were not recovered to 30% by the oxidation in 60 min. In the third oxidation, all the samples were not oxidized to 30 % for lh. These results suggested that the deactivation of Fe-oxide(urea) due to the repeated redox cycles was most moderate among three samples. This might be due to larger surface area and smaller particle size of Fe-oxide(urea). Thus, we prepared iron oxides by precipitation of Fe(NO3)3 with urea hereafter.

798

3. 2. Effect of addition of different metal ions on the redox p e r f o r m a n c e of iron oxides

80 70 .~ 60

~" ~ 7o

~ 5o ~ 4o

~ 6o

~ 3o ~ 20 -~ 10

8

i

(a') 1st oxidation

'~ 5o

~ 4o As observed in Fig. 2, all the iron oxides were 3 . deactivated for the hydrogen 0 10 20 30 40 50 60 0 10 20 30 40 50 formation through water 80 ~. 80 decomposition with repeated = 70 = 70 cycles. The deactivation of iron oxides must result from = 60 ~ 6o sintering of Fe metal and/or ~-9 50 ~ 50 Fe304 during repeated redox ~ 40 duction cycles. It is likely that the ~~ao 40 addition of other metal 30 cations prevents sintering of 30da'l'.... j . . . . I . . . . iron species as reported in 15 0 10 20 30 40 50 60 0 5 10 iron catalysts for NH3 80 ~ ~ 80 synthesis [3]. In addition, the .~ 70[~ f 1~ 70 metal ions added to iron oxides were expected to .~ 6o ~~,/~ 1~6o activate hydrogen and/or 50 ~5 50 water molecules to promote reduction and oxidation of ~b40~ tc' Jr~ reaucu~ [ -~ ~o 40 iron species. Therefore, we investigated the effects of 3 0 ~ 30 addition of different metal 0 5 10 15 0 10 20 30 40 50 60 time / min time / min cations into iron oxides on the redox performance of Fig. 2. Change in the reduction degree of iron oxides during iron oxides. At first, we added foreign reduction with hydrogen (Step 1) at 603 K and reoxidation with metal cations (M) into iron water vapor (Step 2) at 653 K. /k :Fe-oxide(urea), oxides by two different methods A :Fe-oxide(NH3) and O:Fe-oxide(W). to compare the redox performance of the two samples. One sample was prepared by coprecipitation of a mixed aqueous solution containing Fe(NO3)3 and the metal cations (M) with urea (denoted as M-Fe-oxide(urea)), and the other was prepared by impregnation of Fe-oxide(urea) with an aqueous solution containing the metal cations (denoted as M-Fe-oxide(imp.)). Fig. 3 shows the changes in the reduction degree of two iron oxide samples modified with Cr ions as a function of reaction time for three cycles. Cr cation was one of the most effective promoters for redox cycles among all the additives tested in this study as described hereafter. As for the first reduction with hydrogen (Fig. 3 (a)), the rates for iron oxides added with Cr cations, especially that for Cr-Fe-oxide(urea), were slower than that for Fe-oxide(urea). However, the reduction rates for both samples with Cr cations became faster with repeated number. The addition of Cr cations into iron oxides improved not only the reduction with hydrogen, but the oxidation with water. The addition of Cr ions into iron oxides enhanced the rate of the first oxidation compared to that of Fe-oxide(urea), and the ,I

~

....

I ....

I ....

I ....

799 oxidation rates for both samples with Cr cations cycles. In high addition, the Cr_Fe_oxide(urea)Were kept during 3were oxidation rates for

~.. 80 70 ~ ~ ~e=60~50

""

(a) 1st I ~'&80 n I "~'~6 0 7 0 ~ ~

always higher than those ~ 40 for Cr-Fe-oxide(imp.)for ~ 30 ~ 1 ~ 50~- ' ~ "~ all the cycles. These ~o~20 ~ ] ~ 40 [ "1~'7~.i~ results suggest that the .~ 10~i~ [~o ~ %~t3"~ addition of Cr cations by coprecipitation was more 0 ~ 30[-. ........ , . ' ~ . . ~ . . , , . . . effective for the redox 0 100 z00 300 400 500 0 5 10 15 cycles of iron oxides ~ 80~ /~ . d i ~ ~ ~ 7t] ~ 80~/~A!b) Zndoxidation ] compared to that by =o 70r " " ~ [ .~ 701~ "-'~lfi~. ] impregnation. Thus, we ~ 60 60 prepared the iron oxides N with foreign metal cations ~50~ ] ~ 50~ ~ . -"ix by coprecipitation with ~ (b) 2nd reduction urea. ~40~ [ "~~40 I XN~Ir~ I The XRD patterns of "~ Cr-Fe-oxide(urea) and 30N..................... I,..., .... I 30 h ....... "~...'~.... , ......... I 0 5 10 15 Cr-Fe-oxide(imp.) before 8005101520253035 801~ . . . . reduction with hydrogen ~ (c') 3rd oxidation showed only the ~ 70 diffraction lines due to "~ 60 FezO3, indicating that Cr species would be highly ~ 50 dispersed in iron oxides. In addition, the full ~4 ~40 I ~ _ widths at half-maximum "~ 3 30 g........ .'~..-%..., ......... of the diffraction lines due 0 5 10 15 to FezO3 became larger in 0 5 101520253035 time / min the following order, time / min Cr-Fe-oxide(urea)> Fig. 3. Change in the reduction degree of iron oxides during Cr-Fe-oxide(imp.)= reduction with hydrogen (Step 1) and reoxidation with water vapor Fe-oxide(urea). It is likely that the addition of Cr ions (Step 2). 9 9Cr-Fe-oxide(urea), C)" Cr-Fe-oxide(imp.)and into iron oxides by /x :Fe-oxide(urea). coprecipitation with urea decreases the crystallinity of host oxide. We examined the redox performances of the iron oxide samples added with various metal ions by coprecipitation with urea. Fig. 4 shows average rates of hydrogen consumption in Step 1 at 603 K (left) and those of hydrogen production in Step 2 at 653 K (right) during the repeated cycles. The amounts of added metal cations (Mg, A1, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo and Ce) were adjusted to 3 mol% of the total metal cations. The rate of hydrogen consumption and of hydrogen production were estimated from the change in reduction degree from 35 to 50% and from 70 to 55%,

800 respectively. As for the samples with AI, Sc, Ti, V, Cr, Ga, Y, Zr, Nb, Mo and Ce cations, the rates of hydrogen consumption in Step 1 became faster with repetition number, although the rates in the first reduction were slower by the addition of these ions compared to that of Fe-oxide(urea) (unmodified sample). For the reduced samples added with AI, Sc, V, Cr, Ga and Mo, the rates of hydrogen production (Step 2) were generally higher in all the cycles than those of Fe-oxide(urea) and the rates were kept high during 3 cycles. The rates of hydrogen production at the first cycle for the samples added with Ti and Zr were slower than that for Fe-oxide(urea). However, the rates for the samples with Ti and Zr became higher after the second cycle. On the other hand, the addition of Co, Ni and Cu ions into iron oxides increased the rate of the first reduction significantly. However, the reduction rates were reduced considerably with repeated cycles. The rates of hydrogen production (Step 2) for the samples added with Co, Ni and Cu ions also decreased with repeated cycles. On the basis of the results described above, we concluded that the iron oxides added with AI, Sc, Ti, V, Cr, Ga, Zr and Mo ions are effective mediators for storage and recovery of hydrogen.

oxidation at 653 K

reduction at 603 K

lll 1 st cycle 3 rd cycle 2 nd cycle

l

none +Mg

I ,, "IilIII/I//A

+A1

I

I

+Ca +Sc

I/II/)A

//////////~A

+Ti Nil

+v +Cr

"111111///1 , , I/I/////////J

+Mn +Co +Ni

/////A'

I

.1

I

I l/ll//l/l/i

+Ga

I

I

/II/III

"///////A

+Cu +zn

,

I

"7///'//////////J

+v +Zr +Nb +Mo +Ce

"111/111/i//I ~ ~ , ~ 1 , , , I , , ,

-~,,I,,,,I,,,,I

I

,,, ,I, ,,,

0 0.02 0.04 0.06 0.08 0.1 0 0.05 0.1 0.15 0.2 0.25 rate of 142consumption rate of I42 production /mol (mol-Fe)-I min-1 /mol (mol-Fe)-I min-1 Fig. 4. Average rates of hydrogen consumption in Step 1 and hydrogen formation in Step 2 for the iron oxide samples with different metal ions.

3. 3. Effect of addition of different metal ions on the structure of iron oxides Fig. 5 shows the SEM images of Fe-oxide(urea), AI-Fe-oxide(urea) and Cr-Fe-oxide(urea) before the first reduction and after the third oxidation. Al and Cr cations are effective additives for redox performance of iron oxides, as described earlier. The SEM images of the samples before the first reduction showed that all the samples consisted of small particles with uniform size and the particles sizes of AI-Fe-oxide(urea) (Fig. 5 (b)) and Cr-Fe-oxide(urea) (Fig. 5 (c)) were smaller than that of Fe-oxide(urea) (Fig. 5 (a)), suggesting that the addition of Al and Cr cations into iron oxides divides the oxides into small particles. In the SEM image of the Fe-oxide(urea) after the third oxidation (Fig. 5 (a)), larger particles with different size were observed. Therefore, the iron species in Fe-oxide(urea) were sintered during redox cycles. On the other hand, the SEM images of

801

Fig. 5. SEM images of the iron oxide samples. (a), (b) and (c): Fe-oxide(urea), AI-Fe-oxide(urea) and Cr-Fe-oxide(urea) before the first reduction. (a'), (b') and (c'): Fe-oxide(urea), AI-Fe-oxide(urea) and Cr-Fe-oxide(urea) after the third oxidation. I

I

Fig. 5 (b~ and (c') indicated that the particle sizes Fe-oxide ] of iron species in AI-Fe-oxide(urea) and (urea) I! Cr-Fe-oxide(urea) were kept small even after the third oxidation. The changes of particle sizes AI-Fe-oxide ~ J during redox cycles could be also suggested from (urea) ~ " the changes in specific surface areas of the _ samples. Fig. 6 shows specific surface areas of the iron oxide samples before the first reduction Cr-Fe-oxide ' ] and after the third oxidation. The specific surface (urea) ~ ~ I I I, I areas before the first reduction for 0 10 20 30 40 50 A1-Fe-oxide(urea) and Cr-Fe-oxide(urea) were specific surface area / rnz g-1 larger than that for Fe-oxide(urea), which was consistent with the results of SEM images shown in Fig. 5. The redox cycles decreased the specific Fig. 6. Specific surface areas of the iron surface areas of all the samples. However, the oxide samples. I-'-1. before the first surface areas after the third cycle for reduction and m after the 3rd oxidation. AI-Fe-oxide(urea) and Cr-Fe-oxide(urea) were significantly higher than that for Fe-oxide(urea). Thus, we concluded that Cr and A1 cations prevented sintering of iron species during the redox cycles, which is one of the reasons why the redox activities of the iron oxide samples with A1 and Cr cations were kept high during repeated cycles. However, we can not explain the favorable effects of these ions only from suppression of sintering, because the rates of reduction and/or oxidation of the samples with A1, Sc, Ti, V, Cr, Ga, Zr and Mo cations were enhanced with repeated cycles while the specific surface areas became smaller. The addition of these metal ions into iron oxides may produce amorphous compound oxides which activate water and/or

802

hydrogen molecules to accelerate the redox reactions. If the compound oxides between the added ions and iron oxides were formed gradually during the repeated redox cycles, the rates of the redox should be improved with repeated cycles. At the moment, the formation of the compound oxides were not confirmed by XRD analyses of the iron oxides added with AI, Sc, Ti, V, Cr, Ga, Zr and Mo cations during repeated cycles. Further studies about the structure and the state of added metal ions are needed to clarify the effects of these ions on the redox performance of iron oxides. REFERENCES 1. 2. 3.

K. Otsuka, A. Mito, S. Takenaka and I. Yamanaka, Inter. J. Hydrogen Energy, 26 (2001) 191. P.B. Terman and R. Bijetina, Coal Process Technol., 5 (1979) 114. M.E. Dry, J. A. K. du Plessis and G~M. Leuteritz, J. Catal., 6 (1966) 194.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

803

Catalytic activity of bulk and supported sulfated zirconia Ivo J. Dijs, Leonardus W. Jenneskens, and John W. Geus Debye Institute, Department of Physical Organic Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands To assess whether Lewis acid sites are present on zirconium sulfate, we prepared waterfree bulk zirconium sulfate. Furthermore, a water-free silica-supported zirconium sulfate catalyst was prepared by deposition-precipitation of zirconia on silica and subsequent gasphase reaction with SO3. The activity of these catalysts was compared with that of two conventionally prepared sulfated zirconia catalysts. The different catalysts were extensively characterized. XPS indicated that the conventionally prepared sulfated zirconia catalysts contained sulfuric acid. The activity of the catalysts was determined with the gas-phase trans-alkylation of diethylbenzene with benzene and the solvent-free liquid-phase addition of acetic acid to camphene. Both water-free zirconium sulfate catalysts did not exhibit a significant activity; Lewis acid sites are therefore not active in these sulfated zirconia catalysts. Upon exposure to water vapor the initially water-free catalysts were active. The stability of the conventional sulfated zirconia catalysts appeared to be determined in the gas-phase by the volatilization of sulfuric acid. As a result, a highly porous catalyst was more effective than a catalyst based on zirconia of a relatively low porosity. With liquid-phase reactions extraction of sulfuric acid proceeds leading to an acid liquid, which is catalytically active also after separation of the solid catalyst from the reaction mixture. 1. INTRODUCTION The presence of catalytically active Lewis acid sites in sulfated zirconia catalysts is much debated [1-5]. The conventional preparation of sulfated zirconia catalysts inx~olves reaction of freshly precipitated zirconium hydroxide with diluted sulfuric acid or impregnation of zirconium hydroxide with sulfuric acid or ammonium sulfate [6,7]. The final solid acid catalyst results by calcination at a temperature of 723 to 873 K. Provided thermodynamic equilibrium has been reached, all water and free sulfuric acid should have evaporated upon calcination at 673 to 873 K and only chemically bonded sulfate groups remain [8]. Above 890 K, bulk anhydrous Zr(SO4)2 decomposes [1 ]. When uptake of water by the calcined catalyst is prevented or after loading of the catalyst in the reactor physisorbed water is removed by thermal treatment, only Lewis acid sites are present. Since it is difficult either to prevent the uptake of water vapor or to remove adsorbed water completely, it is difficult to attribute the acid activity of sulfated zirconia catalysts unambiguously to Lewis acid sites.

804 In view of the fact that complete removal of water vapor cannot be readily achieved, we prepared water-free bulk and silica-supported zirconium sulfate. The bulk anhydrous Zr(SO4)2 was obtained by reaction of zirconium tetrachloride with oleum [1]. The silicasupported zirconium sulfate resulted from deposition-precipitation of zirconium hydroxide on silica, calcination at 723 K and subsequent reaction with gaseous sulfur trioxide. The catalytic activity of the sulfated zirconia's was measured in the gas-phase trans-alkylation of benzene (1) with diethylbenzene (2) to ethylbenzene (3, reaction 1) [8,9] and the liquidphase hydro-acyloxy-addition reaction of acetic acid (4) and camphene (5) to isobornyl acetate (6, reaction 2) [8,10]. With the trans-alkylation we used an amorphous silicaalumina catalyst as a reference. For comparison purposes we prepared and investigated also two different sulfated zirconia catalysts prepared conventionally [6,7]. One catalyst was prepared by reaction of 0.5 M H2804 with freshly precipitated zirconium hydroxide and calcination at 773 K [H2SO4/ZrO2(prec.)], the other in a similar way but with calcined zirconia [H2SO4/ZrOz(Gimex)] [8]. It is interesting that it has been concluded from their infrared spectra that addition of water vapor leads to formation of sulfuric acid [5]. As the volatility of sulfuric acid and its constituents is higher than that of metal sulfates, the transport of water and sulfur oxides out of the porous structure of the zirconia is important.

2

*

1

2

(1) 3

H3C~ jCH3 HO..qlfCH3

CH3

+

O 4

?vc. ~

~

~../~O...ff..CH3

~H2CH3 5

(2t

O 6

The difficult transport of strongly adsorbing molecules out of a porous system may give an explanation for the result mentioned in the literature that the reaction of zirconium hydroxide with sulfuric acid leads to a fairly active catalyst, whereas reaction of calcined zirconia with sulfuric acid providing the same sulfur content did not result in an active catalyst [11 ]. Accordingly, the removal of the constituents of sulfinSc acid out of calcined zirconia will proceed much more smoothly than that out of zirconium hydroxide. The activity of catalysts prepared from zirconium hydroxide thus may be due to water and sulfilfic acid remaining in the catalyst providing Bronsted acid sites. The sulfated zirconia catalyst based on non-porous calcinied zirconia [H2SO4/ZrO2(Gimex)], on the other hand, will loose readily most of its sulfuric acid during thermal treatment. Furthermore, we investigated the effect of water on the activity of the above-mentioned sulfated zirconia catalysts and the observed activities were compared. We have extensively characterized the different catalysts by XPS, physical adsorption, analytical electron microscopy, and thermogravimetry.

805 2. EXPERIMENTAL

2.1. Catalyst preparation The water-free bulk zirconium sulfate Zr(SO4)2 was prepared by treatment of ZrCI4 with oleum as published earlier [1 ]. The preparation of the silica-supported, water-free sulfated zirconia catalyst started with the deposition-precipitation of zirconia on silica. In a reaction vessel (2 L) equipped with a pH-meter, thermometer, baffles and a stirrer (1000 rpm), 13.5 g (0.225 mol) SiO2 [Aerosil OX50 (Degussa-Htils), 50 m2/g] was suspended in 750 mL water. Under stirring, both 4.0 M HC1 and 4.0 M NH3 were separately injected via narrow tubes (i.d. 1.0 mm) ending below the level of the liquid using two Gilson Minipuls III peristaltic pumps. Whereas the 4.0 M HC1 was injected at 0.25 mL/min, the injection of the 4.0 M NH3 solution was automatically regulated to maintain a pH of 4.5. When a constant pH of 4.5 was reached, the 4.0 M HC1 was replaced by a 4.0 M HCI (250 mL) solution containing 3.92 g (12.2 mmol) ZrOC12.SH20. After addition of these solutions at a pH of 4.5, the pH was raised to 6.5 with the 4.0 M NH3 solution. The wet residue was re-suspended three times in water (300 mL) for one day followed by filtration in order to remove remaining NH4C1 impurities. The final residue was dried at 393 K for one day and subsequently sieved; the 500-850 m fraction was isolated. Calcination was performed under carefully controlled conditions using a quartz fixed bed reactor (i.d. 10 mm) equipped with a K-type thermocouple. The 500-850 m sieve fraction was calcined at 723 K for 10 h (heating/cooling rate: 5 K/min) in a dry air flow (50 mL/min). On top of the sieve fraction, glass beads (d 1.0 mm) were placed to achieve effective preheating of the air. The next step involved the sulfation of the small zirconia particles. For the oxidation of SO2 to SO3 a layer of a finely powdered 2 wt% Pt/SiO2 catalyst was installed at half-height of the glass bead layer. Helium was used to maintain a gas flow of 50 mL/min. The temperature was raised to 675 K at 5 K/min. Subsequently 5 vol.% of SO2 and 5 vol.% of 02 were added to the helium flow, which was maintained for 5.5 h. Via a heat-traced tube (423 K) the gas-flow leaving the reactor was passed through a stirred suspension of Ca(OH)2 in water. Next, the temperature was lowered to 573 K and the SO2 and 02 were switched off, after which the temperature was decreased to room temperature at 5 K/min. A literature procedure was used for the preparation of calcined H2SO4-impregnated ZrO2 catalysts [6,7]. Two different ZrO2 sources were used. (1) Precipitated ZrO2:20.23 g (62.8 mmol) ZrOCI2.SH20 was dissolved in water (160 mL) and 25 wt% NH3 was added dropwise under vigorous stirring until pH = 8. Next, the NH4C1 was removed re-suspending the wet residue by four times in water (300 mL) for one day followed by filtration. The ZrO2 residue was dried at 433 K for 16 h. (2) Commercial ZrO2 (Gimex B.V., The Netherlands, surface area 60 m2/g, monoclinic). Sulfation of both zirconia's was performed by stirring 4.0 g (32.46 mmol) of each zirconia sample with 20 mL 0.5 M H2SO4 for 3 h and drying at 433 K for 16 h (no filtration). Sieve fractions (500-850 m) were calcined in a flow of dry air (50 mL/min) at 773 K for 3 h (heating/cooling rate: 10 K/min). The catalysts were isolated from the reactor as well as stored under dry air prior to analysis and application.

806

2.2. Catalyst characterization XPS analysis was performed on a Vacuum Generators (Fisons Instruments) MT-500 with a non-monochromatic A1 X-ray source (Ka 1486.6 keV) and a CLAM-2 hemispherical analyzer for electron detection. The samples were supported on carbon adhesive tape. Spectra were corrected for charging using the Si(2p) peak and scaled on the Si(2s) peak. For the determination of the binding energies a background correction was applied. Thermostabilities were determined by analyzing the relative loss of weight in a dry N2 flow (50 mL/min) as a function of temperature and time with a PC-controlled PerkinElmer TGS-2 TGA apparatus, autobalance AR-2. Temperature program: 1 h at 323 K, heating rate 10 K/min to 1123 K followed by 15 rain at 1123 K. Samples of ca. 3.5 mg were used. Transmission electron microscopy was performed with a Philips EM420 and a Philips CM200 equipped with a field-emission gun and an EDAX detector for elemental analysis. Ground and ultrasonically dispersed (in dry n-hexane) samples were brought on copper grids covered by a thin polymer film on which carbon was deposited. SEM analysis was performed with a Philips XL 30 FEG equipped with an EDAX detector for elemental analysis. The samples were supported on carbon adhesive tape and covered with a carbon layer by vapor-deposition. 2.3. Gas-phase trans-alkylation of benzene (1)and diethylbenzene(2) The gas-phase trans-alkylation reaction was performed in an automated micro-flow apparatus containing a quartz fixed-bed reactor (i.d. 10 mm) at 105 Pa [ 16 vol% benzene (1, p.a., dried on molsieve), 3.2 vol% diethylbenzene (2, consisting of 25% ortho, 73% meta, 2% para isomers, dried on molsieve), N2 balance (50 mL/min), WHSV = 1.5 h-1] with 2.0 mL of the tube reactor filled with catalyst particles (500-850 ~tm sieve fraction, typically 1.4 g). Two separate saturators were connected to the inlet of the reactor for the supply of 1 and 2. The partial vapor pressure of 1 and 2 was controlled by adjusting the temperature of the saturator-condensers and the N2 flow rate. After equilibration for 30 min at the applied reaction temperatures (473 K and 673 K, heating rate 10 K/min) within a dry N2 flow (50 mL/min), benzene (1) and diethylbenzene (2) were passed through the reactor. To prevent condensation of both reactants and products prior to GC analysis [Hewlet Packard 5710 A, column: CP-sil 5CB capillary liquid-phase siloxane polymer (100% methyl) 25 m x 0.25 mm, 323 K, cartier gas: N2, FID, sample-loop volume: 1.01 ~tL], tubes were heat-traced (398 K). FID sensitivity factors and retention times were determined using ethene (99.5 %, dried over molsieve) and standard solutions of 1, 2, and ethylbenzene (3, 99%) in methanol (p.a.). The conversion of 2 was measured as a function of time [8]. 2.4. Liquid-phase hydro-acyloxy-addition of acetic acid (4) to camphene(5) A mixture of glacial acetic acid (4 p.a., 0.70 tool), camphene (5 95%, 0.70 tool) and acetic anhydride (p.a., 9.05 mmol)was mechanically stirred (1500 rpm)ovemight at 328 K under a N2 atmosphere. Subsequently, 2.5 g of catalyst was quickly suspended in the reaction mixture. The composition of the soluble fraction of the reaction mixture was analyzed by capillary GC as a function of reaction time; samples were prepared as follows: 1.00 mL of the reaction mixture was added to water (25.00 mL) followed by an extraction with n-heptane (25.00 mL). 1.00 mL of the n-heptane fraction was diluted with n-heptane

807 to 25.00 mL in a volumetric flask. 1.0 txL of the diluted solution was injected into the GC [Varian 3400, column: DB-5 capillary liquid-phase siloxane polymer (5% phenyl, 95% methyl), 30 m • 0.323 mm, temperature program: 5 min at 333 K, 10 K/min to 553 K, 10 min, carrier gas: N2, FID]. In the case of hydro-acyloxy-addition reactions performed in the presence of water, 320 ~tL (17.78 mmol) 1-120 was added instead of acetic anhydride. To establish whether leaching occurs, the insoluble catalyst particles were removed from the reaction mixture by filtration with a double-ended glass filter under a dry N2 atmosphere (before equilibrium had established), whereas the composition of the reaction mixture was further measured as a function of time. Since no solid residue remained after evaporation of the reaction mixture to dryness in vacuo, removal of the solid particles was complete. 3. RESULTS AND DISCUSSION 3.1. Catalyst characterization The chemical composition of the different catalysts investigated are collected in Table 1. Also the sulfur contents calculated for complete conversion to Zr(SO4)2 are indicated. The experimental sulfur contents are lower than the calculated values. The reaction of the silica-supported zirconia with gaseous sulfur trioxide is therefore not complete and the reaction of zirconium hydroxide and zirconia with sulfuric acid involves only a limited fraction of the zirconia. As to be expected, the specific surface area of the catalyst prepared from zirconium hydroxide is much larger than that of the other catalysts. The catalyst based on calcined zirconia exhibited the X-ray diffraction pattern of zirconia and the catalyst based on zirconium hydroxide showed broadened reflection of zirconia. The bulk water-free zirconium sulfate did not display an X-ray diffraction pattern; after exposure to ambient air (relative humidity 50 to 60%) for two weeks the sharp X-ray diffraction pattern of Zr(SO4)2.4H20 appeared [ 1].

Table 1. Quantitative analysis of elements by ICP-AES and Flash-combustion GC for the 10 wt% ZrO2/SiO2 subjected to gaseous SO3 and for the conventional H2SOa/ZrO2 catalysts. Residual atom (%)" oxygen. Sample

S03/ZI02/Si02 H2SO4/ZrO2(prec.) H2SO4/ZrO2(Gimex)

Element

Atom (%)

S Zr Si S Zr S Zr

Calc. ICP-AES / (100% Flash-combustion sulfated) GC 3.0 1.7 1.5 1.3 27.6 23.7 7.3 4.0 23.9 23 9 7.3 18 23.9 24.2

BET area (m2/g)

50

217 50

808 Deposition-precipitation of zirconium hydroxide on silica at a constant pH level of 4.5 leads to very finely divided zirconia. Fig. 1 shows an electron micrograph of the resulting catalyst precursor. Tiny zirconia particles have been deposited onto the non-porous silica spheres. Fig. 1. Transmission electron micrograph of 10 wt.% ZrO2/SiO2 prepared by pH-static deposition-precipitation onto silica [8]. Large light-gray spheres: silica support (Aerosil OX50, Degussa-Htils) Dark dots: zirconia.

Thermogravimetry indicated a continuous weight loss of the sulfated silica supported zirconia of only about 3 %. The thermogravimetric data on the catalysts prepared by reaction of zirconia with sulfia'ic acid were more informative (see also ref. [12]). When the temperature was raised with 10 K/min, the catalyst prepared by reaction with calcined zirconia showed a much more smooth weight loss, which set on already at about 350 K. Apparently, it is much more difficult to remove the constituents of sulfia'ic acid out of the much more porous structure of the zirconium hydroxide. The bulk anhydrous Zr(SO4h catalyst exhibited a ratio of the areas of the S(2p) and the Zr(3ds/2,3;2) peak of 0.50. Employing XPS atomic sensitivity factors of S(2p) = 0.54 and Zr(3dsa,3a) = 2.1, the S/Zr atomic ratio is 2.0, which agrees with the bulk chemical composition. The energy of the S(2p) peak of the silica-supported zirconia after treatment with sulfur trioxide was at 170.0 eV, which agrees nicely with that exhibited by anhydrous bulk zirconium sulfate, which was at 170.3 eV [1 ]. Bulk anhydrous zirconium sulfate has the Zr(3dsa,3t2) peak at 185.6 eV and zirconia at 183.3 eV. The partial conversion of the supported zirconia into zirconium sulfate is not only evident from the chemical analysis, but also from the energy of the Zr(3dst2,3/2) peak, which was at 184.1 eV. Together with the broadening of the peak, which was 0.3 eV, the sulfur-to-zirconium peak ratio being 0.25 instead of 0.5 as measured with the bulk zirconium sulfate, indicates the incomplete reaction of the tiny zirconia particles. It is significant that the zirconia catalysts prepared by reaction of sulfuric acid with zirconium hydroxide exhibit a S(2p) binding energy of 169.3 eV, which is nearly identical to that of liquid H2SO4 (169.4 eV [13]). The Zr(3dst2,3t2) peak of the catalysts is broadened and is positioned at an energy lower than that measured for bulk anhydrous zirconium sulfate. The XPS results therefore point to the presence of

809 sulfuric acid adsorbed on a zirconia surface that has reacted at most to a limited extent to the sulfate.

3.2. Gas-phase trans-alkylation The anhydrous bulk zirconium sulfate preparation did not display any activity in the

trans-alkylation of benzene (1) and diethylbenzene (2) to ethylbenzene (3). At 473 K the silica-supported, gas-phase sulfated zirconia showed a very small activity, which rapidly dropped to a negligible level (Fig. 2). The conclusion is that Lewis acid sites are not active with sulfated zirconia catalysts. The low activity of the silica-supported catalyst is due to adsorption of some water leading to Bronsted acid sites. Desorption of water at 473 K leads to the decrease in activity with time. Pre-hydration of the supported catalyst brings about a slightly higher activity as apparent from Fig. 2; the activity drops again due to the loss of water. Conversion (%)

Fig. 2. Conversion of 2 in the transalkylation of benzene (1) and diethylbenzene (2) at 473 K on SO3/ZrO2/SiO2 catalyst; !"7 before hydration and A after hydration (2 h, 2 % H 2 0 , 50 mL/min).

5

A A 0

A 0

0

A

Zk ~ A

100

/k A

/~

200

300

400

500

600

Time (min) Conversion (%)

Conversion (%)

15 o

15! O

o O O o O

9

[]

O0

0

O O O 0

10

5 []

o []

0

~

0

~

~

|~

100

~

~

~|

~

200

~

~, ~

~

300 Time (min)

~ |~

400

~

~ |~

500

~

~

!

600

~

0

0

100

~

~

~

200

300

400

500

600

Time (min)

Fig. 3. Conversion of 2 in the trans-alkylation of benzene (1) and diethylbenzene (2) on HzSO4/ZrOz(prec.) (left-hand side) and on HzSO4/ZrOz(Gimex) (right-hand side) 1"7 473 K and 0 673 K. Fig. 3 represents the catalytic activity of the two catalysts prepared by reaction with liquid sulfuric acid. The activity has been measured at 473 and at 673 K. In agreement with the result mentioned in ref. [10] that calcined zirconia does not exhibit activity upon reaction with sulfuric acid and calcination, the activity of the HzSO4/ZrOz(Gimex) catalyst

810 at 473 K is low. The activity rapidly decreases with time on stream. At 673 K the catalyst did not show any activity. The catalyst prepared by reaction of precipitated zirconium hydroxide with sulfiafic acid and calcination [H2SO4/ZrO2(prec.)], on the other hand, exhibited a significantly high conversiQn, which did not drop with time on stream. At 673 K, however, the latter catalyst also showed a very low activity. Since an activity decreasing with temperature is unusual, we compared the behavior of the sulfated zirconia catalysts with that of an amorphous silica-alumina catalyst. Fig. 4 shows the conversion of the silica-alumina catalyst at 473 and 673 K. Fig. 4 also indicates that the amorphous silica-alumina catalyst displays the expected dependence of the temperature; at higher temperature the conversion is higher. The higher activity is partly due to reaction to ethene; at 473 K the selectivity to ethylbenzene (3) is 100 %, but 50 % at 673 K. The anomalous behavior of the sulfated zirconia catalysts is due to the loss of sulfuric acid at elevated temperatures. The catalyst prepared from calcined zirconia looses its sulfuric acid at 673 K and, consequently, is not active at this temperature. As a result, decreasing the temperature of the catalyst to 473 K does not restore the activity. Also the more highly porous catalyst prepared from zirconium hydroxide releases sulfuric acid, but in narrow pores some sulfia-ic acid is lett. The loss of sulfitdc acid at 673 K is obviously irreversible. When the catalyst prepared from zirconium hydroxide is, however, kept at 473 K, the transport of water out of the porous structure is thus low that a stable activity is exhibited. Pre-hydration of the bulk anhydrous zirconium sulfate does not provide an active catalyst. That no catalytic activity is induced in this case is due to the fact that bulk anhydrous zirconium sulfate readily reacts to a stable tetrahydrate, viz., Zr(SO4)2.4H20 [1 ]. As a result the hydrolysis of the sulfate by water vapor is suppressed. Conversion (%)

Fig. 4. Conversion of 2 in the transalkylation of benzene (1) and diethylbenzene (2) on amorphous silicaalumina. 1"7 473 K andO673 K.

15

o o 0 0 0 O 0 r 1 6 2

[]

0

I~

o

o |

100

o

o

~ |

200

o

o

o

,

o

0 r

o

o

300

o

,

n

400

o

n

o

irl

500

r

O

[]

n

,

600

Time (min)

3.3. Liquid-phase hydro-acyioxy addition Neither the anhydrous bulk zirconium sulfate nor the silica-supported, sulfated zirconia were active in the addition of acetic acid (4) to camphene (5). The lack of activity is due to the fact that addition of acetic anhydride removes water completely from the reactants. Also the liquid-phase reaction thus demonstrates that Lewis acid sites are not active in our catalysts. Addition of water leads to a well measurable activity with both catalysts. Fig. 5 represents the activity of the silica-supported sulfated catalyst after pre-hydration. The

811 activity is considerable, but a homogeneous catalyst, such as, sulfuric acid or BF3 in acetic acid, is more active raising the conversion to about 70 % in 1500 min. It is interesting that filtration of the catalyst did not stop the reaction. Apparently some sulfuric acid has been formed by reaction with the water, which has been released by the catalyst into the solution. Yield 6 (%) 30

[]

\ 9

.

10 0

0

i

2000

4000 6000 Time (rain)

i

i

8000

10000

Fig. 5. Course of the reaction to isobomyl acetate (6) by the hydro-acyloxy-addition of 0.70 mol acetic acid (4) to 0.70 mol camphene (5) at 338 K (stirred tank reactor, N2 atmosphere). O 2.5 g SO3/ZrO2/SiO2 without H20 and !-i 2.5 g SO3/ZrO2/SiO2 and 320 ml (17.78 mmol) H20. Fig. 6 shows the activities of the sulfated zirconium hydroxide and the sulfated calcined zirconia catalyst. In contrast to the activities displayed in the gas-phase reaction, the calcined zirconia catalyst now shows the higher activity. Since the mass transport in the liquid is much slower, the rate of the reaction is now more strongly transport-limited with the catalyst prepared from the more porous zirconium hydroxide. Upon removal of the solid catalyst by filtration the reaction continues also with these two catalysts. Yielc 6 (%) 3O

0

20 / ~ 10 l

[]

Removalof solid

pirticlesbyfiltration

0

,

0

Fig. 6. Course of the reaction to isobornyl acetate (6) by the hydro-acyloxy-addition of 0.70 mol glacial acetic acid (4) to 0.70 mol camphene (5) at 338 K (stirred tank reactor, N2 atmosphere). El 2.5 g H2SO4/ZrO2(Gimex) and 0 2.5 g HESO4/ZrOE(prec.).

500

,

1000

Time (rain)

|

1500

,

|

2000

812 4. CONCLUSIONS The sulfated zirconia catalysts prepared and investigated in this research do not exhibit activity due to Lewis acid sites both in a gas-phase and in a liquid-phase reaction. The positive effect of water as well as the XPS evidence together with infrared results from the literature suggests that sulfated zirconia catalysts are actually zirconia-supported sulfuric acid catalysts. The fact that sulfi,ic acid is the active component leads to a drawback of sulfated zirconia catalysts. In gas-phase reactions at temperatures where the vapor pressure of the constituents of sulfuric acid is considerable, de-activation of the catalyst has to be taken into account. A highly porous structure can significantly slow down the loss of the active constituent of the catalysts. In the liquid-phase dissolution of sulfuric acid can lead to corrosive properties and to contamination of the reaction products. Furthermore deactivation of the catalyst will eventually result. References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13.

I.J. Dijs, IL de Koning, J.W. Geus and L.W. Jenneskens, Phys. Chem. Chem. Phys,. 3 (2001) 4423. K. Tanabe, M. Misono, Y. Ono and H. Hattori, New Solid Acids and Bases, Their Catalytic Properties, Elsevier, Amsterdam, 1989. G.D. Yadav and J.J. Nair, Microporous Mesoporous Mater. 33 (1999) 1. X. Song and A. Sayari, Catal. Rev.-Sci. Eng., 38 (1996) 329. F. Babou, B. Bigot, G. Coudurier, P. Sautet and J.C. V6drine, Stud. Surf. Sci. Catal., 90 (1994) 519. K. Arata and M. Hino, Mater. Chem. Phys., 26 (1990) 213. A. Corma, A. Martinez and C. Martinez, Appl. Catal. A, 144 (1996) 249. I.J. Dijs, J.W. Geus and L.W. Jenneskens, J. Phys. Chem. B, submitted for publication. T.-C. Tsai, S.-B. Liu and I. Wang, Appl. Catal. A, 181 (1999) 355. J.O. Bledsoe, Terpenoids, in: J.I. Kroschwitz, M. Howe-Grant (Eds.), Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Wiley, New York, 1997, Vol. 23, pp. 833-882. F.R. Chen, G. Coudurier, J.-F. Joly and J.C. Vedrine, J. Catal., 143 (1993) 616. S. Ardizlone, C.L. Bianchi and E. Grassi, Coll. Surf. A, 135 (1998) 41. D.H. Fairbrother, H. Johnston and G. Samorjai, J. Phys. Chem., 100 (1996) 13696.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

813

New one-step synthesis of superacid sulfated zirconia L. Zanibelli, A. Carati, C. Flego, R. Millini EniTecnologie SpA- Via Maritano, 26 - 1-20097 S. Donato Mil. (MI) - Italy email:[email protected]

A new one-step synthesis of Sulfated Zirconia (SZ) is proposed. After calcination in the range 500-600~ the resulting SZ shows a pure tetragonal phase with S content close to the theoretical value for the monolayer coverage of the surface hydroxyls. The superacidity of SZ has been confirmed by FT-IR analysis in presence of pyridine. After Pt impregnation on SZ, a new bifimctional catalyst is found for isomerisation of n-heptane, active also at low temperature. I. INTRODUCTION Sulfated zirconia (SZ) materials are usually prepared by multi-step synthesis [1,2], involving the preparation of an amorphous hydrous zirconia powder and its contact with a source of sulfated groups, followed by calcination at temperatures varying from 500 to 650~ More recently several new preparation routes of SZ involving one-step synthesis have been described, including a single step in acidic medium [2]. According to Arata and Hino [3], SZ exhibits an acidity 104 times stronger than 100% sulfinqc acid, as determined by Hammett acid function, therefore it is recognised as a superacid. Even if the presence of superacidity can be debated [4], several studies have been devoted to the synthesis of SZ with the aim to study new catalytic applications. The crystal structure of zirconia and the catalytic properties of SZ generally depend on the synthesis method and thermal treatment adopted. In particular zirconia crystallises in three different polymorphs characterised by monoclinic, tetragonal and cubic symmetry. Among them only the tetragonal SZ phase displays significant catalytic properties [5-7]. Unfortunately, the synthesis of the pure tetragonal polymorph is difficult and, in the absence of promoted oxides [8], it could be stabilised only through an accurate control of the synthesis parameters, with particular attention to the thermal treatments. In this work a sol-gel synthesis of sulfated zirconia (SZ) in basic medium is presented. The new synthesis route, claimed in patents [9, 10], permits to obtain in a single step SZ materials that, after calcination in the range 500-600~ show a pure tetragonal phase stabilised by the nano dimension of crystallites. From the earlier sixties the application of SZ to the isomerisation of hydrocarbons (C5-C6 cut) has been extensively investigated [ 1,11-13]. In refinery, with isomerisation processes, normal C5-C6 paraffins are isomerised into their higher-octane branched isomers, then blended into the

814 gasoline pool. The new gasoline requirements for reducing aromatics and benzene, obtained primarily from reforming of C7-C9 fractions, are pushing towards the isomerisation also of C7 cut added to C5-C6 paraffins. With conventional catalysts, the main problem is due to the high tendency towards cracking of paraffins while increasing the molecular weight. The catalyst described in this work shows good performances in n-heptane isomerisation. 2. EXPERIMENTAL

2.1. SZ synthesis SZ1 is considered as a reference of this series. All the other materials have been prepared modifying one synthesis parameter each with respect to SZ1. SZ1 synthesis. A transparent sol is formed by adding 66.0 g of Zr(OC3H7)4 (70 wt % in nPrOH) and 10.0 g of tetrapropylammonium hydroxide (TPAOH, 40 wt % in water) to 364.0 g of n-PrOH. After 2 hours of ageing under stirring, the sol is added with 50 g of aqueous solution of 0.44 M H2SO4. The dense slurry obtained is stirred for 4 hours at room temperature, followed by 4 hours at 60 ~ The sample is dried for 8 hours at 100 ~ under vacuum and calcined at 550 ~ for 5 hours. SZ2 synthesis. The effect of the presence of organic base is studied performing the synthesis of SZ1 without TPAOH. An opalescent sol is formed from the solution of 66.0 g Zr(Of3H7)4, 364.0 g of n-PrOH and 6 g of water. The synthesis continues as for SZ1. SZ3 synthesis. The effect of the presence of complexing agent (acetylacetone, AcAc) during hydrolysis is studied. 0.14 g of AcAc are added to the solution of 364.0 g of n-PrOH and 66.0 g of Zr(OC3H7)4 (70 wt % in n-PrOH). The synthesis continues as for SZ1. SZ4 synthesis: The effect of increasing S content is studied. In the synthesis of SZ1, an aqueous solution 1.32 M HzSO4 is added. Pt-SZ catalysts preparation. 10 g of SZ samples are impregnated using wet imbibition technique with 16 ml of an aqueous solution of HzPtCI6 containing 0.031 g of Pt per ml. The resulting products, named Pt-SZ, are dried at 100~ and calcined at 550 ~

2.2. Physico-chemical characterisation Textural analysis was performed by Nz physisorption at-196~ with a Carlo Erba 1990. The surface area of calcined samples was calculated with B.E.T. method and the pore distribution with Dollymore-Heal model. XRD (X-Ray powder Diffraction) analysis was performed with a Philips X'PERT diffractometer equipped with a secondary monochromator; data were collected in the 15<20<90 ~ angular region, with 0.03 ~ 20 step and 20 s/step accumulation time; the CuKa radiation (L=1.54178 .A,) was used. TG-DSC (ThermoGravimetric-Differential Scanning Calorimetric) experiments of pure dried powders were performed from 25 to 900~ in Nz and air flow (0.40-1.50 ml/sec) at 10~ with a TG-DSC 111 (Setaram).

815 Acid sites and hydroxyl distribution were evaluated by FI'-IR (Fourier Transform-InfraRed) spectroscopy (mod. 2000 Perkin-Elmer). The spectra were recorded in transmittance at 21~ after in situ treatments of pure wafers of calcined samples. The acid sites distribution was determined by pyridine adsorption at 200~ and stepwise desorption (in the 200-500~ range, 1 h, dynamic vacuum 2.10 .3 mbar) on pure pellets, after evacuation (500~ 1 h, dynamic vacuum 2-10 -3 mbar). The density of the acid sites was obtained from the intensity of the IR signals at 1545 and 1455 cm ~ applying the extinction coefficient of [ 14].

2.3. Catalytic tests The catalytic tests were performed in a fixed bed apparatus feeding n-heptane, at P=50 bar and Hz/HC molar ratio equal to 18, at 75
3.1. Synthesis and characterisation In the described one-step synthesis procedure, the high ratio alcohol/water in the sol prevents precipitation. The formation of small particles is favoured by the very low water content in the sol, that permits a slow and controlled hydrolysis [15]. The following addition of sulphuric acid causes a fast sulfatation of the zirconia surface and consequently favours a reduced agglomeration of the particles. To obtain small size of the crystallites is very important in order to stabilise the tetragonal form of zirconia in calcined samples. The calcination temperature range of the as-synthesised amorphous gel constitutes one of the critical parameter. In fact, the tetragonal form of pure ZrO2 is metastable at room temperature and its stabilisation mainly depends on the size of the crystallites. Furthermore, a high calcination temperature may cause a partial S removal, modifying the acidic properties of the final product. So the minimum calcination temperature of the assynthesised gel was evaluated by combining TG and XRD analyses on the SZ1 sample. Figure la shows the TG pattern of the sample, which is characterised by different steps of weight loss (see the differential DTG pattern). At low temperature (< 150~ Tmax in the DTG pattern = 71~ A~ = -15.1 wt%) the elimination of adsorbed H20 and alcohol is observed. The decomposition of the TPA ions occurs in two steps between 150 and 470~ (Aw = -18.5 wt%). In the first step (150 - 270~ Tmax= 198~ the Hoffman degradation of the cations is observed; while the second one (270-470~ corresponds to the decomposition of TPA ions and their derivatives, trapped in the gel.

816 This interpretation is supported by the fact that the TPA/Zr molar ratio computed on the basis of the TG data (0.14) is close to that derived from the composition of the reaction mixture (0.13). In the region 550 - 600~ a sharp weight loss of 3.7 wt% is observed ( T m a x 573~ For the interpretation of this phenomenon, two samples were prepared by interrupting the TG analysis at 500 and 600~ respectively. XRD analysis of these samples shows that the , 0.000

(b) - -0.002

~- -0.004

i -0.006

~ -0.008

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i

,

,

,

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,

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,

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Fig. 1" (a) TG (B) and DTG (.... ) patterns of as-synthesised SZ1. XRD patterns of the products heated at 500 (b), 600 (c) and 900~ (d) in the TG furnace. material at 500~ is still amorphous (Figure l b), while at 600~ it is transformed in the tetragonal form of ZrO2 (Figure l c). So the weight loss can be assigned to the elimination of H20 from the partially hydrated ZrO2 (molar ratio Zr/H20 -~ 2.5). Between 600 and 850~ another broad step of weight loss of 2.8 wt% is observed, assigned to the elimination of the sulfate groups and the consequent dehydroxylation of the zirconia surface (Fig. l a). It is interesting to note that the tetragonal symmetry is maintained up to 900~ (Fig. ld). The crystallinity of the sample is high and the reflections very sharp. That allowed to exclude the presence of even trace amounts of the monoclinic phase.

817 Taking into account these data, it is evident that the transition amorphous ~ crystalline occurs at 573~ This value, however, is influenced by the heating rate (10~ The same analysis was carried out with a heating rate of l~ in order to reduce the dynamics component in the determination of the minimum calcination temperature. The pattern obtained is very similar to that obtained at higher heating rate, but shifted 2 0 - 30~ lower, confirming the validity of the experimental condition applied (calcination at 550~ The main physico-chemical properties of some representative SZ samples prepared by the different synthesis routes are reported in Table 1. SZ1 sample, reference materials of this work, is constituted by pure tetragonal ZrOz, with an average crystal size (determined with the classical Scherrer equation) of 85 A (Table 1). The stabilisation of the tetragonal ZrOz depends on the presence of TPAOH in the reaction mixture. In fact, variable amounts of monoclinic phase are detected in the products synthesised in the absence of organic base (SZ2 sample, Table 1). This is probably related to the pH of the sol, but this hypothesis should be confirmed. Basic medium usually enhances the condensation reactions [15], but at the same time the ability of sulphates to form complexes with Zr 4§ precursors up to pH = 11 [16] and the low water content in the sol prevent the growth of zirconia particles. Acetylacetone (AcAc) reacts with alkoxides as a chelating ligand: its role should result in slowing down condensation reactions and in formation of only small oligomers [15]. The product obtained (SZ3) is constituted by small size particles of tetragonal ZrOz, similar to those obtained without AcAc (SZ1). Unexpectedly, the catalytic performances of this materials are improved with respect to SZ1 (see after). Table 1 Preparation parameters and physico-chemical characteristics of calcined SZ samples Sample Modified S Crystal Size Crystalline phase SSA .... parameter (wt %) (,~) (mZ/g) SZ1 reference 1.7 85 T 103 SZ2 no TPAOH 1.7 100 T(96%), M(4%) 93 SZ3 AcAc 1.5 90 T 74 SZ4 3 X HzSO4 1.6 90 T 84 S = wt % from chemical analysis; T = tetragonal ZrOz; M = monoclinic ZrO2 The S content after calcination is not influenced by the amount introduced in synthesis (compare samples SZ1 and SZ4 in Table 1) but seems more related to the surface area and close to the theoretical value [17] for the monolayer coverage of the surface hydroxyls. So all the samples display a high density of sulfate groups on the surface, allowing a good comparison among them. The acid nature of SZ samples is evaluated from the shift to lower wavenumbers after pyridine adsorption of the IR band at 1399-1386 cm-1, attributed to the asymmetric stretching of S=O group (Table 2).

818 Table 2 Acidity characteristics, as determined by pyridine adsorption method Sample total Total B/L ratio sulfate IR A Bronsted Lewis position (cm-1) (Ftmol/g) (Ftmol/g) .... (cm-l) SZ1 42 71 0.6 1399 -60 SZ4

37

121

0.3

1386

-44

strong Bronsted (lam01/g) 13

strong Lewis (lamol/g) 2

0

22

The original position of this IR signal is in agreement with the coverage higher than half-monolayer and the presence of isolated [SO4]2- groups in tri-brigded structure, as proposed by [18]. The analysis of the IR hydroxyl region of SZ materials shows two IR signals: bridged ZrOH groups (at 3630 cm-1), which represent the larger amount of OH groups, and terminal ZrOH species (at 3763 cm 1) [19]. With respect to the IR spectrum of one conventional SZ material, the hydroxylation of the sulfated surface is higher in these materials, presumably due to this peculiar synthesis route. The shift (A) is considered a measure of the tendency to reduce the ligand order (n) of SO bond and has been correlated to the catalytic activity of these materials [16]. After pyridine adsorption, SZ1 shows the largest shift (A=-60 cm-1), that corresponds to a change in the ligand order from 1.88 to 1.78 and a partial charge to O from 0.12 to 0.22. The total density of the acid sites (corresponding to the sites able to retain pyridine after desorption at 200~ is reported in Table 2, together with the amount of strong acid sites (corresponding to the sites able to retain pyridine after desorption at 500~ and the position of the sulfate IR signal before and after pyridine adsorption. According to [19], the strong acidity (or "superacidity") of SZ materials is originated from the presence of both Lewis and Bronsted sites, the strength of the latter being enhanced by the neighborhood of the former. For this reason, the optimum of catalytic performances is found in the samples with a Bronsted/Lewis ratio (B/L) near to 1 [19]. SZ1 sample approximates this value.

3.2. Pt-SZ catalysts in n-heptane isomerisation The reactions involved in n-heptane isomerisation can be represented in the following scheme: n-heptane

mono-branched C7 ~ kcl

di-branched C7 - ~ Ikc2

tri-branchedC7 kc3/

cracking products So far while conversion increases, the selectivity towards isomerisation products decreases in favour of cracking products, obtained by a consecutive reaction. So the operative conditions and the catalyst have to be chosen to maximise the yield towards isomers, minimising the contribute of cracking. From the thermodynamic equilibrium the highly

819 branched isomers, which increase the octane number of the product, are favoured at low temperature: the preferred catalysts are therefore those highly active at low temperature. Fig. 2 reports the selectivity towards iso-C7 isomers obtained by Pt-SZ1 catalyst in the temperature range 75-150 ~ in comparison with data reported for literature catalysts [11] at higher temperature (nearby 200~ Higher isomerisation selectivity is obtained with Pt-SZ1 catalyst, being able to work at lower temperature. Fig. 3 reports the distribution of iso-C7 compounds (expressed as yield of the different families of branched C7 isomers) versus conversion for Pt-SZ1 catalyst at 100 and 130~ to verify the influence of reaction temperature towards formation of highly-branched isomers. It is observed that the yield towards Di- and Tri-branched isomers (DB and TB) versus Monobranched (MB) does not depend on reaction temperature. So the success of the catalyst is based on the high iso-selectivity, in particular in the range of 50-70% conversion, at low temperature and not on the capacity to increase the ratio (DB+TB)/MB isomers, that depends on the conversion. The effect on activity of the presence of complexing agent (AcAc) in SZ synthesis is shown in Fig. 4. The catalytic tests of Pt-SZ1 and Pt-SZ3 were performed at the same operative conditions. A strong increase of n-heptane conversion is observed for Pt-SZ3, obtained with AcAc. The physico-chemical characterisation presently performed on this material is not able to give reason to this improved catalytic performances. Further detailed work is going on, especially to study the surface properties of SZ3 (e.g. defectivity and acidity distribution).

100 * Pt-SZ1 I Literature SZ

80-

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I

ID

4

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M

20-

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100

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.

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l

40 60 80 Conversion (%) Fig. 3: Distribution of C7 isomers .

~86 . ~

n

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100

.

~ Pt-'sz

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0

5

n

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15

20

1/WHSV (l/h) Fig. 4: Effect ofcomplexing agent on activity 4. CONCLUSION The new one-step synthesis in basic medium [9,10] gives rise to sulfated zirconia (SZ samples) in pure tetragonal phase. This phase is formed after calcination at relatively low temperature (500-600~ and is stabilised by the small size of the particles. Their formation is favoured by both the controlled hydrolysis of zirconia precursor in the sol and the following fast sulfatation of zirconia oligomers. The Pt-SZ catalysts obtained from these samples show improved

821 catalytic performances with respect to conventional catalysts: high conversion at low temperature and high selectivity to branched C7 products. REFERENCES

1. G.D. Yadav and J.J. Nair, Microporous Mesoporous Mater., 33 (1999) 1. 2. X. Song and A. Sayari, Catal. Rev.-Sci. Eng., 38 (1996) 329. 3. IC Arata, Adv. Catal., 37 (1990) 165. 4. R.S. Drago and N. Kob, J. Phys. Chem. B, 101 (1997) 3360. 5. M. Hino, S. Kobayashi and K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. 6. F.R. Chen, G. Coudurier, J.-F. Joly and J.C. Vedrine, J. Catal., 143 (1991) 199. 7. W. Stichert, F. Schiith, S. Kuba and H. Kn6zinger, J. Catal., 198 (2001) 277. 8. C. Miao, W. Hua, J. Chen and Z. Gao, Catal. Lett., 37 (1996) 187. 9. S. Peratello and A. Carati, EP 860208 (1998). 10. S. Peratello and A. Carati, WO 0003801 (2000). 11. E. Iglesia, S.L. Soled and G.M. Kramer, J. CataL, 144 (1993) 238. 12. U.B. Demirci and F. Gafin, Catal. Lett., 76 (2001) 45. 13. R. Le Van Mao, S. Xiao and T. Si Le, Catal. Lett., 35 (1995) 107. 14. J. Take, T. Yamaguchi, K. Miyamoto, H. Ohyama and M. Misono, Stud. Surf. Sci. Catal., 28 (1986) 495. 15. J. Livage, Stud. Surf. Sci. Catal., 85 (1994) 1. 16. P.J. Squattrito, P.R. Rudolf and A. Clearfield, Inorg. Chem., 26 (1987) 4240. 17. P. Nascimento, C. Akratopoulou, M. Oskagyan, G. Coudurier, C. Travers, J.P. Joly and J.C. Vedrine, Proc. 10 th Int. Congr. on Catalysis, L. Guczi et al. Eds, Elsevier Amsterdam, (1993) 1185. 18. C. Morterra, G. Cerrato and V. Bolis, Catal. Today, 17 (1993) 505. 19. K. Tanabe and T. Yamaguchi, Stud. Surf. Sci. Catal., 44 (1988) 99. .~

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

823

Elaboration and characterization of a model Phillips catalyst for ethylene polymerization P.G. Di Croce 1, F. Aubriet z, P. Bertrand z, P. Rouxhet 3, P. Grange I 1Unit6 de catalyse et chimie des mat6riaux divis6s, Universit6 catholique de Louvain, Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium ([email protected]) 2 Unit6 de physico-chimie et de physique des mat6riaux, Universit6 catholique de Louvain, Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium 3 Unit6 de chimie des interfaces, Universit6 catholique de Louvain, Croix du Sud 2/18, B1348 Louvain-la-Neuve, Belgium

A model Phillips catalyst for ethylene polymerization has been prepared by spin coating of a Cr(III) precursor (Cr(acac)3) on a fiat silicon wafer (100) covered by amorphous silica. The spin coating parameters were chosen in order to obtain a homogeneous film. The model catalyst was submitted to an activationprocess. The surface concentration of Cr decreased from about 0.8 to 0.4 Cr atom/nm as the temperature increased from 150 to 550~ Direct information concerning the surface molecular species and the environment of Cr was provided by ToF-SIMS and XPS. At 350~ the catalyst precursor was decomposed; Cr species were in the form oxide and surface-anchored chromates. Upon final activation at 650~ for 6 h, Cr species were below the XPS detection limit; however the model catalyst was active for ethylene polymerization at 160~ and 2 bar pressure.

1. I N T R O D U C T I O N Cr is present in many catalysts used in hydrogenation or dehydrogenation, oxidation, isomerization, aromatisation, and deNOx reactions. The most important industrial application is the Phillips Cr/SiO2 catalyst for the production of polyethylene [1]. Phillips catalysts are commonly prepared by impregnation of a high surface area silica with an aqueous solution of Cr(VI) or Cr(III) salts. The typical loading is about 1 wt % of Cr but it is estimated that only one tenth provides active sites for the polymerization. After drying, the catalyst is activated under a high temperature oxidative treatment, possibly followed by reduction (CO, Hz), and used in a slurry process conducted in a loop reactor (Phillips Particle Form Process) [2].

824 Although extensively studied, the Phillips catalyst is still poorly known [3]. Much uncertainty concerns the mechanism and kinetic control of polymerization. A strong point of debate concerns the molecular structure of Cr(VI) species which, during the activation step, can be anchored on the support as monomeric or polymeric chromate [4-6]. The chemical state of Cr depends on parameters such as the nature and properties of the oxide support, the preparation step (precursor, loading), the activation step (temperature, time, calcination rate, atmosphere), the chemical treatments (reduction by CO, HE, dehydration by CS2) and the reaction progress [7]. Several techniques [8] such as XPS, DRS, FTIR, ESR, Raman, provided direct information on the chemistry of Cr. However, some could not be exploited to their full potential. The active phase of industrial catalysts is hidden in the porous network of the support, which reduces seriously the intensity and/or the resolution of techniques such as XPS and SIMS. The study of a flat model catalyst mimicking industrial catalysts [9-12] is an alternative way to understand the behaviour of such a system Niemantsverdriet and coworkers have investigated a realistic model catalyst for the Phillips catalyst [13]. Spin coating a flat silicon wafer with an aqueous Cr(VI) solution provided a control of the amount of the deposited material and simulated the conventional pore vohmae impregnation widely used in the preparation of industrial catalysts [14,15]. Using a combination of surface techniques (XPS, RBS and SIMS), Thiine et al. [16] came to the conclusion that the Cr(VI) species are anchored to the surface mainly as monochromate. They found evidence that these monomeric species are very stable at high temperature, while polymeric species, present at higher loadings, evaporate with increasing temperature. Atter ethylene polymerization reaction, AFM investigations [17,18] evidenced the semicrystallized structure of polyethylene produced at different reaction times. ToF-SIMS is mainly referenced in the characterization of polymers and organic additives and blends [19-26]; only few investigations of catalysts are reported in the litterature [27-29]. However, inorganic applications of ToF-SIMS were illustrated by Aubriet et al. [30] who studied the behaviour of the first row transition metal oxides and proposed, for each of them, a reliable methodology of speciation. In this work, a Phillips model catalyst was prepared by spin coating of a THF solution of Cr(acac)3 on an oxidized silicon wafer. This study was mostly focused on a molecular description of the activation process in conditions similar to those used for industrial catalysts. This model catalyst was characterized by means of SEM, ToF-SIMS and XPS, and tested for ethylene polymerization.

2. MATERIALS AND METHODS 2.1. Model catalyst preparation p-type silicon wafers with (100) surface orientation were used as substrates and pretreated as follows [13]. Square pieces (1,5"1,5 cm2) of wafers were calcined at 750~ in air for 24 h in order to develop an amorphous SiO2 layer with a hydrophilic siloxane surface. Then they were cleaned at 70~ with a mixture of H202 (30%) and NH4OH (30%) in a ratio 3/2 v/v. After rinsing several times with milli-Q water (HPLC grade produced by a Milli-Q plus system from Millipore), the wafers were placed for 30 min in boiling water,

825 with the aim to hydroxylize the surface. They were kept in water until the impregnation process to avoid contamination from the atmosphere. Spin coating was performed in air with a CT60 spin coater from Karl Suss Technique. The water film was first removed by spinning 3 min at 5000 rpm. Then the wafer was totally covered with three drops of the chromium acetylacetonate (C15Hz1CrO6, for synthesis, Merck) precursor solution noted here Cr(acac)3 in THF (99 %, Aldrich) at different concentrations. Unless stated otherwise, the spinning proces s was conducted at 3000 rpm during 60 s. The evaporation of the solvent was monitored with the Newton rings which, in all cases, disappeared before the end of the spinning.

2.2. Activation and polymerization procedure The spin coated samples were activated in a quartz reactor, under a 30 ml/min flow of dry air (99.98 %, Indugas) purified over molecular sieves (3 A,, Alltech). The temperature was increased by steps of 100~ at a rate of 10~ Thermal equilibration of the reactor was ensured by holding each plateau for 1 h. The final activation step was at 650~ during 6 h, reflecting the industrial activation process. The activated wafer was then used in a polymerization reaction. This was carried out in 2 bar ethylene (30 ml/min), purified over molecular sieves, at 160~ during approximatively 30 rain. These reaction conditions were chosen to reduce the induction period prior to polymerization. The reaction was then stopped by switching to purified He flow (30 ml/min) at 160~ for 2 h, before cooling the reactor to room temperature.

2.3. ToF-SIMS characterization Positive and negative ToF-SIMS measurements were performed with a spectrometer from Charles Evans and Associates. The sample was bombarded with a pulsed primary 15 keV 65Ga+ liquid metal ion beam. Secondary ions were first accelerated to 3 keV by applying a bias on the sample and on the extraction lens. The spreading of the initial energies of the secondary ions was compensated by 270 ~ deflection using three electrostatic analysers (TRIFT). To increase the detection efficiency of ions at high m/z values, a post-acceleration of 7 keV was applied to the detector entry. A 600 pA d.c. primary ion beam was pulsed at 11 kHz with a pulse width of 22 ns, electrodynamically bunched down to 1 ns to increase mass resolution. The analysed area was 130"130 ~m 2. With a data acquisition time of 10 min on the 0 to 1000 amu mass range, the total ion dose was about 1012 ions.cm 2, which ensures static conditions. Charge effects were compensated by means of a 24 eV pulsed electron flood gun. Under these experimental conditions, the mass resolution (m/Am) was about 2800 at m/z 100. Two kinds of experiments were conducted, with and without sputter cleaning of the surface with an ion fluence of 10 TM ions.cm -2 before the analysis. This sputter cleaning was carried out to eliminate organic contaminants. The analyses were performed on four different areas of the same sample and were duplicated with samples prepared independently. For the following presentation, the relative SIMS intensity of a peak means the ratio I / Z Ii where I is the area of the considered peak and Z Ii is the sum of the areas of all ions chosen in the spectrum. In the negative mode, the following ions were taken into account : [H], [C], [CH]-, [O]-, IOn]-, [Ca]-, [C2H], [Si], [Oa]-, [Cl]-, [C3]-, [HCO2]-, [NO2]-, [C4H], [SiO2]-, [NO3]-, [SiO3], [HSiO3]-, [(acac)-O]-, [CrO2]-, [acac]-, [CrO3]-, [CrO4], [HCrO4]-,

826

[Si205]-, [HSi2Os], [SiCrO4]-, [CrO2(acac)-H]-, [Cr205]-, [HSi307]-, [Cr206]-, [Cr(acac)2OH]-, [Cr(acac)2-H]-, [CrO(acac)2-H]-, [Cr(acac)3-H]-. In positive mode, the considered ions were: [H] +, [CH2]+, [CH3]+, [Na] +, [C2H2]+, [A1]+, [C2H3]+, [Si] +, [K] +, [C2H30] +, [Cr] +, [CrO] +, [HCrO2] +, [Cr20] +, [Cr(acac)-O] +, [Cr(acac)] +, [CrO(acac)] +, [CrOH(acac)] +, [Cr(acac)2] +, [CrH(acac)3-O2] +, [CrH(acac) 3-O] +, [Cr(acac) 3]+, [CrH(acac)3] +, [CrE(acac)3] +, [CraO(acac)3] +, [Cr2(acac)4] +, [Cr2OU(acac)4] +, [Cr30(acac)5] +, [Cr2(acac)5] +, [Cr30(acac)6] +, [CrE(acac)6] +, [Cr30(acac)7] +. 2.4. SEM characterization Scanning electron microscope (SEM) photographs were taken on a DSM982 Gemini microscope with a field emission gun operated at 1 kV voltage. Samples were characterized without any metallic coating. The occurrence of the observed features was checked at different sites on the samples, so that SEM micrographs presented below are representative of the whole sample. 2.5. XPS characterization The XPS analyses were performed on a Kratos Axis Ultra spectrometer (Kratos Analytical- Manchester- UK) using an aluminium anode (Al K a = 1486.3 eV) powered at 10 mA and 15 kV; the pressure was around 10 .6 Pa. The samples were fixed on a stainless steel holder by using double sided conductive tape. The angle between the perpendicular to the surface normal and the axis of the lens was 0 ~ The X-ray bombarded area was approximately 2000 x 800 ~m 2. The hybrid lens magnification mode, a combination of magnetic and electrostatic lenses, was used with the slot aperture (10 x 3.7 ram) and the iris drive position set at 0.5 inch (8.4 mm diameter) resulting in an analysed area of 700 x 300 [am2. The constant pass energy of the hemispherical analyser was set at 40 eV. In these conditions, the energy resolution determined by the full width at half maximum (FWHM) of the Ag3d5/2 peak is about 1.0 eV. Sample charging was stabilized by a charge neutralisation system consisting of a filament (current set at 1.8 A) and an electrode plate (voltage set at -2.3 V and bias at -1.01 V) mounted at the bottom of the electrostatic input lens system. The following sequence of spectra was recorded: survey spectrum, Si2p, Cr2s, Cls, Ols and Si2p again to check for charging stability and absence of sample degradation during the analyses. The binding energies were calculated with respect to Si2p peak fixed at 103.5 eV. The spectra were decomposed with the least squares fitting routine provided in the manufacturer Unix based Vision 2.1.2. software, with a Gaussian/Lorentzian ratio of 70/30 and after subtraction of a linear baseline. The elemental concentration ratios were calculated using peak areas normalised on the basis of acquisition parameters, sensitivity factors provided by the manufacturer (based on experimental Wagner sensitivity factors and transmission factors).

3. RESULTS 3.1. Model catalyst The spectra of a sample spin coated at 3000 rpm with a Cr(acac)3 10 -3 M solution are displayed in Fig. 1. In the positive mode, the spectrum was dominated by [Cr(acac)x] +

827 and Cr+ ions. Numerous other clusters, e.g. [Cry(acac)x]+, [Cry(aca)xOH] + and [Cry(aCa)xO]§ were also detected. In the negative spectrum the main contribution was [acac]" besides [O] and [OH]', attributed to surface hydroxyls of the Si wafers. The detection of an ion at 84 m/z was attributed to the [CrO2]-. Another ion at 99 m/z was assigned to [acac]'. Other weakly detected ions were ions corresponding to abstraction of oxygen, hydrogen or water from the precursor. Fig. 2 shows that the spinning rate did not influence the distribution of the different ions. Therefore a spinning rate of 3000 rpm was adopted for the rest of the work.

.

u

9

i

l

,

g

v+

+,s.

_,. J,,L..,L

0

.

. i

250

0

t

m/z

_j>-'

I;~;

9 :'

2

,

..~

r~

;

l,,l 0

500

~

,

750

.L_

I

Il

~

~

-~

9

--

;

i s '~176176+_

..

250

0

m/z

500

Fig. 1. Positive and negative ToF-SIMS spectra of Si wafer after spin coating with a 103 M Cr(acac)3. 10

,

10 El 8 0 0 0

,<

rpm

B 3000 rpm

8

Q 5000 rpm A=

1000

a 3000 rpm

8 ,11

IC :

J

S

>0

33

s

A=IO

i= ,,.,,

A=IO0

4

n

,,4

:gl

m m

#

2

9

[si] §

[cr]+ Positive ions

[Cr(acac)zl

+

O

+

...t....... I

_ _ _l~x~xxx"~i

[SiT

[acac]-

[CrOft

Negative ions

Fig. 2. Intensity of characteristic ToF-SIMS peaks of the model catalyst obtained with 10~ M Cr(acac)3 solutions using spinning rates of 3000 and 5000 rpm.

7:

828 The influence of Cr(acac)3 concentration is shown in Fig. 3. For both modes, the abundance of ions containing both Cr and acac increased slowly with the concentration until 10-2 M and more markedly from 10-2 M to 101 M. The intensity of the surface wafer Si was constant until 10-2 M, after which it decreased. 10 +1

1~1 10-1

ISz'l+

4t"I

Is~]"

C

=9 10 .2 ,,,,,

_m

laear

at 10

lO.e

10-s

10-4

10 4

10-2

Concentration (mol/I)

10-1

Iv

I

10 "s

I

10-4

1'0"z

1;u2

1'0"1

Concentration (mol/I)

Fig. 3. Evolution of characteristic ToF-SIMS peaks of the model catalyst as a function of the Cr(acac)3 concentration ; spinning rate 3000 rpm. SEM images for the 10q M and 10"3 M samples provided morphological information on the deposited precursor at the micrometer scale. The 101 M sample, presented in Fig. 4, showed a heterogeneous morphology, with silicon substrate dark regions appearing as holes or fissures in the Cr precursor layer. In contrast, a relatively homogeneous morphology (Fig. 4) was observed for the 10-3 M sample, indicating that the substrate was uniformly covered by a film of precursor.

Fig. 4. SEM photographs of a model catalyst spin coated at 3000 rpm with 10-3 M and 10~ M Cr(acac)3 solution.

829

3.2. Activation of the model catalyst The activation process was investigated by ToF-SIMS and XPS, using samples prepared with a 10 -3 M Cr(acac)3 solution and a spinning rate of 3000 rpm. Table 1 presents the chemical composition determined by XPS, considering that the analyzed zone is homogeneous as a function of depth. Alternatively, the Cr surface concentration may be evaluated by relation (1), considering that Cr species are atomically dispersed at the surface and that the Cr2s photoelectrons are not subject to inelastic collision [32,33] : Ic----r-~= c----------------z--~ i Crd Isi

(1)

isiCsiLcr

where I and i are the measured intensity and the correspondin3g sensitivity factor, respectively; Csi is the silicon concentration in silica (0.038 mol/cm ); dcr is the surface concentration of Cr; kcr is the mean free path that the Cr2s photoelectrons would have in a silica matrix. Considering a specific weight of 2.32 g/cm 3 for silica, application of the expression proposed by Tanuma [34] leads to kCr values of 2.5 nm when using the A1 anode. During the activation process (from 150 to 550~ the surface concentration of Cr decreased from 0.79 to 0.43 Cr atom/nm z (Table 1). For a conventional Phillips catalysts, with a Cr loading of about 1 wt % and a carrier surface area of the order of 300 mZ/gsilica, the Cr surface concentration is about 0.4 Cr/nm z. This indicates that the Cr loading of the model catalyst is representative of industrial catalysts. Table 1. Apparent surface composition measured by XPS (mole fraction in %, excluding hydrogen) after activation at increasing temperature. Estimation of the surface density of Cr. Temp.

(oc)

150 250 350 450 550 650

Si 30.0 29.5 29.8 30.0 31.2 30.9

O 58.7 58.2 60.2 61.6 60.1 61.8

C 10.8 11.9 9.8 8.1 8.5 7.2

Cr 0.4 0.3 0.3 0.2 0.2 < D.L.

O/Si 2.0 2.0 2.0 2.0 1.9 2.0

C/Cr 26.3 36.0 36.2 35.4 37.0 -

dcr (atom/nm9 0.79 0.65 0.52 0.44 0.43

In Fig. 5, intensities of [Cr(acac)l,z] § peaks are shown to decrease drastically from 150 to 350~ Heating the model catalyst under dry air led to a progressive oxidative destruction of Cr(acac)3, which was completed at 350~ The evolution of the peak intensity of [Cr] § [CrO] § [CrOz]- and [CrO3]-was observed from 150 to 250~ while at 350~ a maximum was reached. The peak of ions with masses 128 and 144 amu (Fig. 5) also decreased from 150 to 250~ and reached a maximum at 350~ [Si] § peak intensity increased slowly with the temperature. The negative spectra obtained after activation at

830

350~ (Fig. 6) exhibited Cr oxides such as [Cr204 to 6] and [Cr305 to 7], which were not found for other activation temperatures. A progressive decrease of the Cr concentration occurred as the temperature increased up to 550~ and dropped below the detection limit atter 6 h at 650~ (Table 1). 16

8

%

6-

:

'0f

[Cr(acac)]+

- - = - - [Cr(acac)2]+

e~

4-

9-

12

E

=

o...

8

m

tl

.=.

~

4

t

I

+ I

i

I

t

I

i

I

1

150 250 350 450 550 650 Activation t e m p e r a t u r e (oc)

150 250 350 450 550 650 Activation t e m p e r a t u r e (~ 16

=

[CrOZ]-

- - = - - [CrO3]-

e~ e~

~=~ 12

,~,

- - s - - Mass 128

"~

~

8

~

4

C

o,,.

m

1

g, i

i

I

i

i

150 250 350 450 550 650 Activation t e m p e r a t u r e (oc)

Fig. 5. Evolution of ToF-SIMS peaks during activation.

i

I

i

I

i

I

J

I

i

I

i

150 250 350 450 550 650 Activation t e m p e r a t u r e (~

831

,..,

,.-,

~

'-~

/

,.

.a,

.~

[.

o

,,~

|

,K

0

~i .~. :

50

o" o~a

g

"~

-~: /

100

:-I

I

i :I I i

i I i

150

250

200

300

m/z Fig. 6. Negative ToF-SIMS spectra of the model catalyst activated at 350~ under air atmosphere. 8

+

~

6

-l-

\

o

+~ +~ +~. + ~

o

r~:

~4

~

~

,.. ~

9

~ !/// +

ff

'

:

x~

~i1 " ~0

50

m/zlO0

150

Fig. 7. ToF-SIMS spectra of the activated model catalyst after polymerisation of ethylene and sputtering.

832 3.3. Polymerization reaction The model catalyst, once activated under dry air flow at 650~ for 6 h, was tested in polymerization of ethylene at 160~ for 30 min and then kept under He at 160~ for 2 h before cooling down to room temperature. ToF-SIMS positive spectra provided direct evidence for polyethylene formation. After cleaning by sputtering, the spectra still exhibited (Fig. 7) cluster patterns of [CxHy]+ fragments, as reported in the literature [1921,3 ! ] for polyethylene. Moreover, they revealed the presence of fragments at m/z of 159, 185 which were assigned to [SiO2CrC2H3]+ and [SiO2CrC4Hs] +, respectively (Fig 7). 4. DISCUSSION Most of the studies dedicated to the development of a Phillips model catalyst are based on a CrO3 precursor [13,16-18]. Industrially, the Phillips catalysts can be prepared by impregnation of organic or inorganic Cr salts. Prior to reaction, the catalysts are activated in the presence of oxygen at high temperature to remove the precursor and to anchor chromium species on the support. In the present work, the model catalyst was prepared starting from a Cr(acac)3 precursor, using spin coating to deposit the precursor. This technique gives control over Cr loading and morphology, and mimics the conventional pore volume impregnation. A description of the activation process, which is a crucial aspect of Phillips catalysts, is presented. The degradation of the acac ligands, followed by ToF-SIMS, was complete after heating in dry air near 350~ Many investigations reported a complete oxidation of Cr(acac)3 deposited or adsorbed on silica near 340~ [35-38]. At 150~ [Cr] +, [CrO] + and [CRO2.3] are attributed to fragments of the Cr(acac)3 precursor: the decrease of their peak intensities occurred from 150 to 250~ following indeed the same trend as the precursor fragments. Ions of mass at 128 and 144, observed at 150~ are ascribed to an unidentified inorganic compound or to a gas phase recombination between Cr and Si species. As Cr is not oxidized under these conditions, there is no reason to ascribe them to anchored chromate species. At 350~ the maximum of the [Cr] +, [CrO] + and [CrO2,3]" peak intensities can be related to a chemical modification of the Cr species: the thermolysis of the supported complex in an oxidizing atmosphere is known to form Cr oxide [35]. The spectrum shows the presence of clusters, namely [Cr204 to 6] and [CrsO5 to 7] ions, only after activation at 350~ Cr species are reported to migrate and associate on the surface at high temperature [42,43]. The oxidative destruction of acac ligands is accompanied by an oxidation of Cr(III) to Cr(VI) which may be anchored to the support via reaction with OH groups of silica [39-41]. At 350~ anchored Cr(VI) surface species may be responsible for the detection of the ions of mass 128 and 144, identified as [CrSiO3] and [CrSiO4]-, respectively. These results are comparable to those reported by Thiine et al [16] who detected [CrSiO4,5] ions on a comparable system. At 450 and 550~ the ToF-SIMS signals of the Cr ions (single, clustered, containing Si) decreased drastically while the XPS data showed only a moderate decrease of chromium concentration, from about 0.8 to 0.4 Cr atom/nm2. In these conditions, chromate is anchored on the support [40,44] but other forms of Cr(VI) may desorb [16].

833 The decrease of the ToF-SIMS signal is thus attributed in part to a decrease of sputter yield and in part to a loss of chromium. The last step of the activation process, 6 h at 650~ reflecting industrial conditions, induced also modifications on the surface of the model catalyst. The Cr species were below the XPS detection limit. This can possibly be attributed to two phenomena: (i) desorption due to a decrease of chromate stability, as a result of dehydroxylation and strain induced in the surface Si-O bond; surface reorganization of chromate species [45,46], forming CrzO3 aggregates, which are known to be inactive versus ethylene polymerization [7]. Even if the surface Cr loading is weak, the activated model catalyst exhibited catalytic activity by polymerizing ethylene at 160~ The sputter cleaning allowed to detect particular fragments, [SiOzCrCzH3] +, [SiOCrC4H9] § and [SiOzCrC4Hs] +, which were not reported before. At this stage it is not possible to conclude whether they are due to chemical species involving Si, Cr and polyethylene at the catalyst polymer interface or to a recombination under sputtering. 5. C O N C L U S I O N An active Phillips model catalyst has been successfully prepared starting from a Cr(III) precursor. The active phase was deposited homogeneously on a silicon wafer by spin coating, and the model was submitted to the activation process usually applied to real Phillips catalysts. Using complementary surface science techniques, molecular information on the modifications of the state of the Cr during activation was provided. The Cr density varies from about 0.8 and 0.4 Cr atom/nm z between 150 and 550~ After removal of acac ligands of the precursor at 350~ Cr oxide have been detected while chromate species are also anchored to the silica surface. After 6 h activation under dry air flow at 650~ the Cr density falls under the XPS detection limit. Nevertheless, the model catalyst is active for polymerization of ethylene at 160~ under 2 bar pressure. From these observations, it is concluded that the model catalyst prepared from Cr(III) precursor behaves like its industrial counterpart; it contributed to a better understanding of its activation process at a molecular scale. ACKNOWLEDGEMENT The financial support of the R6gion Wallonne (convention n~ is gratefully acknowledged. We thank Dr G. Debras and Dr P. Bodart (Atofina Research) for valuable discussions and F. Hamadache and C. Poleunis for experimental help. REFERENCES

1. 2. 3. 4. 5.

J.P. Hogan, J. Polym. Sci., Part A : Polym. Chem., 8 (1970) 2637. B.M. Weckhuysen and R.A. Schoonheydt, Catal. Today, 51 (1999) 215. H.L. Krauss, J. Mol. Catal., 46 (1988) 97. M.P. McDaniel, J. Catal., 67 (1981) 71. D. Hardcastle and I.E. Wachs, J. Mol. Catal., 46 (1988) 173.

834 6. C.E. Marsden, Plastics, Rubber and Composites Processing and Applications, 21 (1994) 193. 7. M.P. McDaniel, Adv. Catal., 33 (1985) 47. 8. B.M. Weckhuysen, I.E. Wachs and R.A. Schoonheydt, Chem. Rev., 96 (1996) 3327. 9. C.R. Henry, Surf. Sci. Rep., 31 (1998) 231. 10. C.T. Campbell, Surf. Sci. Rep., 27 (1997) 1. 11. G.A. Somorjai, J. Phys. Chem. B, 104 (2000) 2969. 12. P.L.J. Gunter, J.W. Niemantsverdriet, F.H. Ribeiro and G.A. Somorjai, Catal. Rev.-Sci. Eng., 39 (1997) 77. 13. P.C. Th/ine, C.P.J. Verhagen, M.J.G. van den Boer and J.W. Niemantsverdriet, J. Phys. Chem. B, 101 (1997) 8559. 14. J.W. Niemantsverdriet, A.F.P. Engelen, A.M. de Jong, W. Wieldraaijer and G.J. Kramer, Appl. Surf. Sci., 144-145 (1999) 366. 15. R.M. van Hardeveld, P.L.J. Gunter, L.J. van Ijzendoorn, W. Wieldraaijer, E.W. Kuipers and J.W. Niemantsverdriet, Appl. Surf. Sci., 84 (1995) 339. 16. P.C. Th/ine, R. Linke, W.J.H. van Gennip, A.M. de Jong and J.W. Niemantsverdriet, J. Phys. Chem. B, 105 (2001) 3075. 17. P.C. Th/ine, J. Loos, P.J. Lemstra and J.W. Niemantsverdriet, J. Catal., 183 (1999) 1. 18. J.Loos, P.C. Th/ine and J.W. Niemantsverdriet, Macromolecules, 32 (1999) 8910. 19. P. Bertrand and L-T. Weng, Mikrochim. Acta [Suppl.], 13 (1996) 167. 20. W.J. van Ooij, and R.H.G. Brinkhuis, Surf. Interf. Anal., 11 (1988) 430. 21. A.A. Galuska, Surf. Interf. Anal., 25 (1997) 790. 22. D. Briggs, Surf. Interf. Anal., 15 (1990) 734. 23. A. Benninghoven, Surf. Sci., 299 (1994) 246. 24. D. Stapel and A. Benninghoven, Appl. Surf. Sci., 174 (2001) 261. 25. A. Rossi, B. Elsener, G. Hahner, M. Textor and N.D. Spencer, Surf. Interf. Anal., 29 (2000) 460. 26. J.B. Lhoest, P. Bertrand, L.T. Weng and J.L. Dewez, Macromolecules, 28 (1995) 4631. 27. L-T. Weng, P. Bertrand, G. Lalande, D. Guay and J.P. Dodelet, Appl. Surf. Sci., 84 (1995) 9. 28. L-T. Weng, P. Bertrand, O. Tirions and M. Devillers, Appl. Surf. Sc., 99 (1996) 185. 29. F. De Smet, M. Devillers, C. Poleunis and P. Bertrand, J. Chem. Soc., Faraday Trans., 94 (1998) 941. 30. F. Aubriet, C. Poleunis and P. Bertrand, J. Mass Spectrom., 36 (2001) 641. 31. A.A. Galuska, Surf. Interf. Anal., 25 (1997) 1. 32. P.O. Scokart, A. Amin, C. Defoss6 and P.G. Rouxhet, J. Phys. Chem., 85 (1981) 1406. 33. C. Changui, A. Doren, W.E.E. Stone, N. Mozes and P.G. Rouxhet, J. Chimie Physique, 84 (1987) 275. 34. S. Tanuma, C.J. Powell and D.R. Penn, Surf. Interf. Anal., 21 (1994) 165. 35. P. Van Der Voort, K. Possemiers and E.F. Vansant, J. Chem. Soc., Faraday Trans., 92 (1996) 843. 36. J.C. Kenvin and M.G. White, Langmuir, 7 (1991) 1198. 37. V.J. Ruddick and J.P.S. Badyal, J. Phys. Chem. B, 101 (1997) 9240. 38. V.J. Ruddick, P.W. Dyer, G. Bell, V.C. Gibson and J.P.S. Badyal, J. Phys. Chem., 100 (1996) 1062.

835 39. I.V. Babich, Y.V. Plyuto, P. Van Der Voort and E.F. Vansant, J. Coll. Interf. Sci., 189 (1997) 144. 40. A. Hakuli and A. KytSkivi, Phys. Chem. Chem. Phys., 1 (1999) 1607. 41. M.P. McDaniel and M.B. Welch, J. Catal., 82 (1983) 98. 42. S.M. Augustine and J.P. Blitz, J. Catal., 161 (1996) 641. 43. M.P. McDaniel, J. Catal., 67 (1981) 71. 44. B.M. Weckhuysen, B. Schoofs and R.A. Schoonheydt, J. Chem. Soc., Faraday Trans., 93 (1997) 2117. 45. P.G. Harrison, N.C. Lloyd and W.J. DanieU, J.Phys Chem. B, 102 (1998) 10672. 46. N.E. Fouad, H. Kn/~zinger and M.I. Zaki, Z. Phys. Chem., 186 (1994) 231. 47. B. Liu and M. Terano, J. Mol. Catal. A : Chem., 172 (2001) 227.

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Studies in SurfaceScienceand Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation grafting

of new basic

mesoporous

837

silica catalysts

by a m m o n i a

Hisao Yoshida, a Yoshitaka Inaki, a Yoshiyasu Kajita, a Kenji Ito b and Tadashi Hattori a Department of Applied Chemistry, Graduate School of Enginering, Nagoya University, Nagoya 464-8603, Japan. a

b Department of Molecular Design and Engineering, Graduate School of Enginering, Nagoya University, Nagoya 464-8603, Japan. New basic mesoporous silica catalysts were successfully prepared by the method consisting of two steps: i) Mesoporous silica, FSM-16, calcined was pre-treated at given temperature in 02 and evacuated at the same temperature (pre-activation), ii) then exposed to 30 Tort gaseous NH3 at given temperature in a closed system, followed by evacuation at the same temperature (NH3-treatment). The NH3-grafted FSM-16, FSMN, exhibited catalytic activity in some base catalysed reactions. The catalytic activity in Knoevenagel condensation was affected by the pre-activation temperature and the NH3-treatment temperature. The highest activity was observed on the FSMN sample pre-activated at 1073 K and NH3-treated at 923 K. FT-IR study revealed that the basic active sites are Si-NH2. Furthermore, two formation mechanisms were confirmed; NH3-treatment at low temperature (473 K) produced the pair site of Si-NH2 and Si-OH, while that at high temperature (923 K) produced the single site of Si-NH2. The former showed the higher turn over frequency than the latter. This would be attributed to the presence of neighbouring Si-OH with the basic sites, Si-NH2. 1. I N T R O D U C T I O N Mesoporous silica materials such as FSM-16 [1] and MCM-41 [2] developed in the last decade have high surface area and regular nano-meter sized pores. They are favorable for catalytic support because they provide the suitable fields for the functional guest components and the large reaction space. On the other hand, the development of solid base catalyst is desired to replace the liquid base for the production of fine chemicals in industrial processes, because solid base is easy to recycle and environment friendly [3,4]. Thus, the development of mesoporous base catalyst has recently become active [5]. The amino functionalised mesoporous silica catalysts were obtained by incorporation during synthesis of the mesoporous silica [6,7], or by silylation on the surface hydroxy groups [8,9]. Other modification method to obtain base catalysts were also reported e.g., thermal treatments at above 1023 K in a flow of NH3 for amorphous aluminophosphates [10,11] or zeolite [12], leading to the formation of-NH2 and/or-NH- sites, while there are no reports concerning mesoporous silica.

838 As for the preparation of silicon nitride, the modification of the silica surface by NH3 is well investigated: NH3 can chemisorb to produce Si-NH2 at moderate temperature (> 673 K), silazane Si-NH-Si at higher temperature (> 873 K), and nitridated silica surface (silicon-oxinitride, SiaN20) at more higher temperature (> 1473 K) [13]. On the other hand, it is also reported that the strained siloxane bridge on silica activated by evacuation at high temperature can easily react with NH3 to form Si-NH2 [14]. We expected that the sites such as Si-NHz formed through NH3 adsorption at moderate temperature on the activated sites function as base sites in catalysis. In our previous study, it was revealed that the strained siloxane are easily generated on the mesoporous silica, especially FSM-16, by evacuation at high temperature above 473 K [15,16]. In the present study, we examined the new simple preparation method to obtain the basic mesoporous silica catalyst and evaluated their catalytic activities [17]. 2. E X P E R I M E N T A L 2.1. Catalysts preparation Mesoporous silica, FSM-16, was prepared in the same way as the previous study [11]. The structure was confirmed by XRD pattern and N2 adsorption isotherm. BET surface area was 1025 m 2 g-a. The basic mesoporous silica FSM" 16 calcined at 873 K ] catalyst was prepared as shown in scheme 1. The FSM-16 calcined was (1) in 100 Torr 02 at 673 K - 1073 K for 1 h., treated in Oz at given temperature for (2) evacuation at the same temperature for 1 h 1 h and evacuated for 1 h at the same temperature ] Pre-activated FSM-16 [ Subsequently, was carried out; the pre-activated FSM-16 was exposed to 30 Torr gaseous NH3 (1) in 30 Torr NH3 at 473 K or 923K for 30 min. for 30 min at 473 or 923 K in a (2) evacuation at the same temperature for 30 min. closed system, and evacuated for 30 min at the same temperature. Thus ] FSMN (NH3-grafted sample) ] obtained NH3-grafted FSM-16 was referred to as FSMN. Scheme 1 Preparation of FSMN sample

I Pre-activation

(pre-activation). NH3-treatment

NH3-treatment

2.2. Reaction and characterisation The Aldol condensation and Knoevenagel condensation (eqn. 1) was carried out as follows. The powder catalyst was added to toluene (2.5 ml) solution of benzaldehyde (0.7 mmol, 1) and reactant (0.5 mmol, 2), then the reaction mixture was stirred at 323 K. Products 3 were analyzed by gas chromatography employing dodecane as an internal standard. For the measurement of FTIR spectra, sample powder was pressed (3 MPa) into the self-supported disk of 5-10 mg cm -z. Temperature programmed desorption (TPD) was carried out for FSMN at the rate of 5 K min -1 in a flow of He (60 ml min-1).

839

Ph ~=

0

/ H2C

+

Zl

\

H

Ph

- H20 ~

Z2

1

Z1 (1)

- -

H

Z2

2

3

3. R E S U L T S A N D D I S C U S S I O N

3.1. Base-catalysed reactions over F S M N catalyst The catalytic activity of the NH3-grafted mesoporous silica, F S M N , was examined in some base-catalysed condensations (eqn. 1). The results were listed in Table 1. The F S M N catalyst used here was F S M N - 5 that was prepared by the pre-activation at 1073 K followed by NH3-treatment at 973 K. The Aldol condensation of benzaldehyde and acetone did not proceed in this condition (entry 1). The Knoevenagel condensation of benzaldehyde and diethyl malonate (entry 2) did not occurred. On the other hand, the reactions with malononitrile (entry 3) and with ethyl cyanoacetate (entry 4) were catalysed by the FSMN-5. This shows that the NH3-grafted mesoporous silica would function as base catalyst. Table 1. Results of base-catalvsed condensations a over F S M N - 5 b Entry Catalyst Reaction Reactant 2 '7 /g time / h Z1 Z2 1 2 3 4

0.3 0.1 0.1 0.1

18 0.5 0.5 0.5

H COzEt CN CN

COMe COzEt CN COzEt

pKa c

Yield (%) of 3

20 13 11 >9

0 0 80 35

a Mixture (2.5 ml toluene as solvent, 0.7 mmol benzaldehyde 1 and 0.5 mmol reactant 2) was stirred with catalyst at 323 K. See also eqn. 1. b See Table 2. c From ref. [19,20]. Table 2. Results of the Knoevenagel condensation a Product Amount of S iNHz b Pre-activation NH3-treatment Yield / % / p,mol g-1 TF / h -1 Entry Sample temperature / K temperature / K 1 FSMN-1 673 473 4.0 0.528 568 c 2 FSMN-2 873 473 12 7.60 158 3 FSMN-3 1073 473 16 11.2 143 4 FSMN-4 923 923 30 107 28.0 5 FSMN-5 1073 923 35 140 25.0 6d FSMN-5 1073 923 19 140 27.1 7e FSM-16 1073 .... 0.0 0.0 0.0 a Mixture (2.5 ml toluene as solvent, 0.7 mmol benzaldehyde, 0.5 mmol ethyl cyanoacetate and 0.1 g catalyst) stirred at 323 K. Reaction time 30 min. b Values estimated using an absorption coefficient of SiNHz band at 1553 cm 1. Coefficient calculated as molar amount of desorbed NH3 in NH3-TPD above 923 K per decrease amount of the integrated intensity of SiNHz band for FSMN-5 above 923 K. Listed values are refined from values previously published in ref. [17] c Very high TF would be due to underestimation of SiNHz band. d Half amount of FSMN-5 catalyst (0.05g) used. e Reaction time 18 h. .

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840 When it is considered that the base-catalysed reaction starts with the formation of carboanion on the methylenic group by the abstraction of proton, the acid dissociation constant (pKa) would be one of the indexes for the difficulty of the reaction. Although there are no good relationship between the product yield and the pKa value, it is clear that this catalyst could not promote the reaction with reactant 2 of high pKa value. The tendency of catalytic property for these four condensations (entry 1-4) is similar to that over 3-aminopropyl-functionalised silica gel catalyst prepared through silylation [18]. 3.2. Effect of p r e p a r a t i o n condition of F S M N on the catalytic activity Table 2 shows the results of the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate on the FSM-16 and the FSMN samples prepared in various conditions. The reaction occurred on all FSMN samples (entry 1-5), while reaction did not occur over unmodified FSM-16 (entry 7). 1H-NMR and GC did not detect any by-products, confirming the progress of the selective reaction. Moreover, it was confirmed that basic species would not elute during the reaction, since the filtrate after the reaction over active catalyst did not exhibit the activity. The catalytic activity of FSMN increased with increasing the pre-activation temperature for FSM-16 prior to NH3-treatment at 473 K (entry 1-3) or 923 K (entry 4, 5). These results suggest that the sites on which NH3 molecules are stabilised would be generated by evacuation at higher temperature. NH3-treatment temperature much influenced the catalytic activity. Increase of the treatment temperature from 473 K (entry 3) to 923 K (entry 5) enhanced the activity significantly. In the present study, the most active catalyst was FSMN-5 prepared through the pre-activation at 1073 K and the NH3-treatment at 923 K.

~

3600 3400 Wavenumber / cm-1

f I A'T ~ [

16'00 15'00 Wavenumber / cm-1

\\\

'

891[

950 900 850 Wavenumber/ cm-1

Fig. 1. [A] FTIR spectra of FSM-16 after evacuation at 1073 K (a), followed by NH3-treatment at 473 K (b) and 923 K (c). [B] Difference spectra (b) and (c) obtained by subtraction the spectrum Aa from Ab and Ac, respectively. [C] The same curves as [B]. The intensity was normalised by the sample weight.

3.3. The generation of base sites The generation of the base sites on the evacuated FSM-16 was confirmed by FT-IR spectroscopy as shown in Fig. 1. Evacuation at high temperature such as 1073 K gave rise to the absorption band at 891 cm -1 with a small band at 910 cm 1 (Fig. 1Aa). These bands

841 were assigned to the strained siloxane bridge formed by dehydroxylation of the isolated hydroxyl groups (eqn. 2) [14]. Intensities of these bands on FSM-16 increased with increasing the evacuation temperature above 673 K (not shown). Evacuation at high temperature is required for dehydoxylation of isolated hydroxyl groups on the silica surface. This surface strained siloxane was known as reactive sites towards some molecules including NH3 [14]. OH

OH

I

Si

.o

_ H20

J

(2)

'-

Si

Si"

Si

After NH3-treatment at 473 K, the strained siloxane bands at 891 and 910 cm -1 completely disappeared (Fig. lAb), and new absorption bands were generated at 1553 cm 1 (Fig. 1Bb), 3444 cm -1 and very weak band at 3525 cm -1 (Fig. 1Cb). These new bands could be assigned to bending, symmetric stretching and asymmetric stretching vibration of Si-NH2, which were formed by dissociative chemisorption of NH3 on the strained siloxane bridges (eqn. 3) [14]. In the spectrum, no other N-H bands than the SiNH2 bands were observed. At the same time, the intensity of Si-OH band at 3740 cm -1 increased (Fig. 1Bb) by NH3-treatment, indicating that the reaction of eqn. 3 occurred during NH3-treatment at 473 K. ,O

NH 3

NH2

OH

(3)

/

Si"

Si

473 K

Si

Si

When the NH3-treatment was done at room temperature, some bands due to physisorbed NH3 were observed in addition to the bands due to the Si-NH2. In the present study, NH3-treatment was done at 473 K to exclude the physisorption of NH3 that might elute into the solvent during the reaction. Higher temperature NH3-treatment at 923 K resulted in generation of three intense bands at 1553, 3452 and 3540 cm -1 (Fig. 1Bc and 1Cc). These could be also assigned to Si-NH2 bands [14]. Even Si-N stretching of Si-NH2 at 932 cm -1was also observed, which was probably due to the huge amount of Si-NH2 (Fig. 1Ac). It was reported that another reaction would occur to form Si-NH2 (eqn. 4) at high temperature such as 923 K [13, 14]. In the present case, much stronger SiNH2 bands were formed than the case at 473 K. However, Si-OH band intensity did not increase so much (Fig. 1Cc), and the strained siloxane band at 891 cm -1 remained partially (more than a hal0 (Fig. 1Ac). These results suggest that the reaction of eqn. 4 should mainly occur at 923 K than that of eqn. 3. OH

NH 3

]

Si

NH 2 ~

923 K

+

H20

(4)

Si

In these ways, by the NH3-treatment at the both low and high temperature, the Si-NH2 species were formed. However, the wavenumbers of the N-H stretching vibration mode for Si-NH2 obtained at 473 K (3525 and 3444 cm -1 in Fig. 1Cb) were shorter than those

842

obtained at 973 K (3540, 3452 cm -1 in Fig. 1Cc). This would be originated from some interaction between the Si-NH2 and Si-OH moieties in the pair sites formed at 473 K as described in eqn. 3. These FT-IR studies indicate that the reaction of eqn. 3 occurred by NH3-treatment at 473 K, while the reaction of eqn. 4 should mainly occur at 923 K. This means that we can produce two kinds of Si-NHz sites; one is the pair site of Si-NHz and Si-OH, and another is the single Si-NH2 site. Fig. 2 shows the X-ray diffraction patterns of FSM-16 sample and FSMN-5 sample prepared by the pre-activation at 1073 K and NH3-treatment at 923 K. The FSMN sample showed almost the same clear XRD pattern as that of FSM-16, meaning that the structure was maintained even after the treatment at high temperature.

1

20000

Q.. o

_ (b)

(3)

r,o

..= (a) 2

4 6 20 / degree

8

Fig. 2 XRD patterns of FSM-]6 (a) and FSMN-5 (b).

0

50 100 Amount of SiNHz / l~nol

150

Fig. 3 Product yield for Knoevenagel condensation against the amount of Si-NHz sites. Numbers in the graph correspond to the entry number in Table 2.

3.4. Two kinds of base sites

The amounts of Si-NH2 sites were estimated by using NH3-TPD and the intensity of the band at 1553 cm -1 (see footnote text in Table 2) and listed in Table 2. The yield in Knoevenagel condensation increased with increasing the amount of Si-NH2 sites (Table 2), suggesting that Si-NH2 would be the basic active sites catalysing this reaction. Fig. 3 shows the plot of the product yield in Knoevenagel condensation against the amount of Si-NH2. It is noteworthy that the plot gives two straight lines (Fig. 3). NH3-treatment temperature for each line was different: The one with steep slope was obtained by NH3-treatment at 473 K (entry 1-3), while another line was obtained at 923 K (entry 4-6). The activity per amount of Si-NH2 (TF) was apparently larger for the former than for the latter as listed in Table 2. As mentioned above, the pair site of Si-NHz and Si-OH was formed by NH3-treatment at 473 K (eqn. 3), while single Si-NH2 site was formed at 923 K (eqn. 4). The higher TF for FSMN treated at 473 K (entry 2-3) would be attributed to the presence of neighbouring Si-OH with the basic site Si-NH2 (eqn. 3).

843 3.5. High activity of the pair site The pair sites on the FSMN catalyst prepared at low temperature showed higher specific catalytic activity. The role of the Si-OH moiety in the pair site is discussed here. There are alternative possibilities, the direct or indirect participation of the Si-OH moiety to the catalysis. The indirect one is that the Si-OH enhances the base property of the Si-NH2. However, at present, it is difficult to consider a suitable model that could enhance the base property. The direct one, which is claimed in the present study, is the concerted mechanism where the neighboring Si-OH takes part in the catalytic reaction as an acid site as shown in Scheme 2. Angeletti et al. [18] also proposed a similar mechanism to the latter one, i.e., the Knoevenagel condensation over propylamine catalyst bounded on the surface of amorphous silica was promoted by participation of residual surface silanols. In the scheme 2, the surface hydroxy group as a weak acid site activates the carboxyl group in the benzaldehyde. The surface amino group as a base site abstracts the proton from methylenic group of ethyl cyanoacetate to form carboanion. These two activated molecules react more easily to produce the product. In this way, the pair sites would function cooperatively as acid and base sites in the reaction. This might be one of the merits of using the heterogeneous catalyst in comparison with homogeneous system. _

H

\COzEt

?H NHz

/

O

/

\

OzEt

H

CO2Et

~ CO~E~

I I I I I Si I I I I I I I I I Si IIIIII

(~-~/

Pair sites

IIIIIIIIIIIIIIIIIIIIIIII

Si

iH 3

Si

Scheme 2 Proposed reaction mechanisms of Knoevenagel condensation on the pair site consisting of the Si-NH2 and the neighboring Si-OH. The neighboring Si-OH functions as an acid site to activate the carbonyl group.

Both types of FSMN catalysts, prepared by pre-activation at 1073 K followed by NH3-treatment at 473 K and 923 K, had hydroxy groups on the surface with very low density; 0.72 nm -2 and 0.60 nm -z, respectively, which were estimated from the data in the previous paper [15]. On the other hand, the densities of the surface amino groups coexisting on the surface were further low; 0.015 nm -z and 0.098 nm -2, respectively. This means that both catalysts had larger amount of hydroxy groups than the amino groups. However, on the FSMN prepared at high temperature, the Si-NH2 sites and the Si-OH sites are isolated each other since the Si-NHz was generated by substitution reaction (eqn. 4) with the Si-OH isolated originally, in other words, the distance between the Si-NHz and Si-OH is long. On the other hand, the Si-NH2 on the FSMN prepared at low temperature is paired with Si-OH, as expected from both the formation scheme (eqn. 3) and the shift in FT-IR spectrum (Fig. 1Cb). Thus, the higher TF on the FSMN prepared at low temperature would be originated from the shorter distance between the Si-NH2 and Si-OH moieties of the pair site in comparison with those on the catalyst prepared at high temperature. This means that the acid site separately located from the base site could not effectively enhance this catalysis reaction, while the neighbouring acid site would be effective. This would be one of the merits of the pair site.

844 4. CONCLUSION The NH3-grafted mesoporous silica, FSMN, exhibited the base catalytic activity for some reactions. The catalytic active site was clarified to be the surface Si-NH2 group. The highest activity in Knoevenagel condensation was obtained on the FSMN sample that pre-activated at 1073 K and NH3-treated at 923 K since the catalyst possessed the largest amount of Si-NHz. The pair sites of Si-NHz and Si-OH were formed on the FSMN catalyst prepared by the NH3-treatment at low temperature (473 K) while the Si-NHz single site was formed on the catalyst prepared by the NH3-treatment at high temperature (923 K). The former pair sites exhibited higher TFs in Knoevenagel condensation. This FSMN of high TF was prepared by the simple grafting method using surface reaction between molecules and activated silica surface. It is expected that this simple grafting method could be widely applied to the preparation of other functionalised mesoporous or amorphous silica catalysts. REFERENCES

1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 680. J.S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. H. Hattori, Chem. Rev., 95 (1995) 537. K. Tanabe and W. E H61derich, Appl. Catal., A, 181 (1999) 399. J. Weitkamp, M. Hunger and U. Rymsa, Microporous and Mesoporous Mater., 48 (2000) 255. D.J. Macquarrie, Chem. Commun., (1996) 1961. D.J. Macquarrie and D. B. Jackson, Chem. Commun., (1997) 1781. D. Brunel, Microporous and Mesoporous Mater., 27 (1999) 329. B.M. Choudary, M L. Kantam, P. Sreekanth, T. Bandopadhyay, F. Figueras and A. Tuel, J. Mol. Catal. A, 142 (1999) 361. M. J. Climent, A. Corma, V. Forn~s, A. Frau, R. Guil-L6pez, S. Iborra and J. Primo, J. Catal., 163 (1996) 392. S. Delsarte, A. Auroux and P. Grange, Phys. Chem. Chem. Phys., 2 (2000) 2821. S. Ernst, M. Hartmann, S. Sauerbeck and T. Bongers, Appl. Catal. A, 200 (2000) 117. E.F. Vansant, P. Van Der Voort and K. C. Vrancken, Stud. Surf. Sci. Catal., 93 (1995) 383. B.A. Morrow, I. A. Cody and L. S. M. Lee, J. Phys Chem., 80 (1976) 2761. Y. Inaki, H. Yoshida, K. Kimura, S. Inagaki, Y. Fukushima and T. Hattori, Phys. Chem. Chem. Phys., 2 (2000) 5293. Y. Inaki, H. Yoshida and T. Hattori, J. Phys. Chem. B, 104 (2000) 10304. Y. Inaki, Y. Kajita, H. Yoshida, K. Ito and T. Hattori, Chem. Commun., (2001) 2358. E. Angeletti, C. Canepa, G. Martinetti and E Ventullo, J. Chem. Soc. Perkin Trans. I, (1989) 105. R. G. Pearson and R. L. Dillon, J. Am. Chem. Soc., 75 (1953) 2439. S. Singer and E Zuman, J. Org. Chem., 39 (1974) 836.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

845

Titania-silica catalysts prepared by sol-gel method for photoepoxidation of propene with molecular oxygen Chizu Murata, Hisao Yoshida, and Tadashi Hattori Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Photooxidation of propene to propene oxide (PO) by molecular oxygen was performed over TiO2-SiO2 binary oxides prepared by sol-gel method (TS(s)) and TiO2/SiO2 supported oxides prepared by impregnation method (TS(i)). On both the TS(s) and TS(i) systems, the selectivity to PO increased with decreasing Ti content. Diffuse reflectance UV spectra showed that the samples of low Ti content consisted of the isolated tetrahedral Ti species predominantly, while the [TiOz]n clusters were also formed and became larger with increasing Ti content. Thus, it was concluded that the isolated tetrahedral Ti species were the active sites for PO production. The TS(s) sample of low Ti content exhibited much higher selectivity to PO (60 %) than the corresponding TS(i) sample (40 %) although the dominant Ti species seemed to be the same in both samples. It was confirmed that the consecutive reaction of PO was much suppressed on the sample prepared by the sol-gel method. 1. INTRODUCTION The direct gas phase epoxidation of propene by molecular oxygen has been desired, and has been attempted by many researchers [1-4]. As a new approach, 'photoepoxidation' of propene using only Oz has been investigated over several systems such as TiO2, Ba-Y type zeolite, Nb205/SiO2, MgO/SiO2 and SiO2 [5-10]. But these activities were still low and it was not clear whether the reaction proceeded catalytically or not. In our previous screening study of silica-supported metal oxides for propene photoepoxidation, TiO2/SiO2 showed the highest PO yield [11]. In the present study, we prepared titania-silica catalysts by two kinds of preparation method, the two-stage sol-gel method [12] and the conventional impregnation method, and compared their propene photooxidation activity. 2. EXPERIMENTAL TiOa-SiOa mixed oxide samples were prepared by the sol-gel method consisting of two-stage hydrolysis procedure, by which Ti species are expected to be highly dispersed in silica since the aggregation of Ti is suppressed during the hydrolysis process [12]. A mixture of Si(OCaH5)4, CaH5OH, HaO and HNO3 (0.5, 1.9, 0.5 and 0.043 mol, respectively) was stirred at 353 K for 3 h to hydrolyze Si(OC2H5)4 partially, and the obtained sol was cooled down to room temperature. A 2-propanol (20 ml) solution of

846 titanium isopropoxide (0.0005-0.05 mol) was added to the sol and stirred for 2 h. Then, an aqueous HNO3 solution (H20 and HNO3 were 0.5 and 0.043 mol, respectively) was added to the sol and stirred until the gelation was completed (about 3 - 14 days). The gel was heated up to 338 K at 0.2 K m-in-1 and dried for 5 h. After additional drying for 5 h at 373 K, it was calcined at 773 K in a flowing air for 8 h. Ti content was determined by inductively coupled plasma (ICP) measurement. Titania-silica samples thus prepared by the sol-gel method were denoted as xTS(s), where x (mol% of Ti) = NTi/(NTi + Nsi)xl00. TiOz/SiO2 supported oxide samples were prepared by the conventional impregnation method; amorphous silica (1.0 g) was impregnated with an aqueous solution (50 ml) of (NH4)2[TiO(C204)2]" 2H20, then dried at 383 K for 12 h and calcined at 773 K in flowing air for 5 h. Amorphous silica was prepared from Si(OC2H5)4 by another sol-gel method followed by calcination in a flow of air at 773 K for 5 h [11]. Titania-silica samples prepared by the impregnation method in this way were denoted as xTS(i) similarly to above. The TiO2 sample employed was a Japan Reference Catalyst (JRC-TIO-4; equivalent P-25). The BET surface area of the samples was determined by N2 adsorption. Prior to each reaction test and spectroscopic measurement, the sample was treated with 100 Tort oxygen (1 Torr = 133.3 N m -2) at 673 K for 1 h, followed by evacuation at 673 K for 1 h. The photooxidation of propene was performed with a conventional closed system (123 cm 3). The sample (200 mg) was spread on the flat bottom (12.6 cm 2) of the quartz vessel. Propene (100 l.tmol, 15 Torr) and oxygen (200/Ltmol, 30 Torr) were introduced into the vessel, and the sample was irradiated by a 200 W Xe lamp. After collecting the products in gas phase, the catalyst bed was heated at 573 K in vacuo to collect the products adsorbed on the catalyst by a liquid nitrogen trap. These products were separately analyzed by GC. The results presented here are the sum of each product yield. The reactivity of PO over titania-silica catalysts was tested in two ways. (a)Thermal isomerization of PO; PO (20 [.tmol) was introduced to the reactor in the dark, then the sample was heated at 573 K to collect the products. (b)Photooxidation of PO; PO (20 ptmol) and O2 (200 ~mol) were introduced to the reactor, then the sample was irradiated for 1 h. After collecting products in gas phase, the sample was heated at 573 K to collect the adsorbed products. Diffuse reflectance UV spectra of the samples in vacuo were recorded on a JASCO V-570 spectrophotometer at room temperature. The crossing-point between the base line and the tangential line of the inflection point was employed as the wavelength of absorption edge. Temperature-programmed desorption of NH3 (NH3-TPD) was examined to estimate the amount of surface acid sites. The pretreated sample (200 mg) was exposed to NH3 at 373 K for 0.5 h, and evacuated at 373 K to remove weakly adsorbed NH3. The temperature of the sample increased linearly by 5 K min -1 in a He carrier stream. The amount of desorbed NH3 was monitored with the mass spectrometer at m/e = 16. 3. RESULTS

3.1. Photooxidation of Propene Table 1 shows BET surface area of the SiO2, TiO2, titania-silica samples and the results of photooxidation of propene over them. The specific surface area of each

847 titania-silica and silica samples were similarly high (400-550 m2g'l). The SiO2 sample showed a low conversion as previously reported [10], while the TiO2 sample showed high activity for the complete oxidation of propene to CO2. Partial oxidation of propene took place over all the titania-silica samples, both the TS(i) and TS(s) samples, under photoirradiation. The major products were propene oxide(PO), ethanal, propanal, CO, CO2, acetone and acrolein; and small amount of2-propanol, ethene and butene were also formed. It was confirmed that propene was not converted on the titania-silica in the dark, as typically shown over the 0.34TS(s) sample. Over both the TS(i) and TS(s) systems, the conversion of propene increased with an increase of Ti content, while the yield of PO increased with increasing Ti content up to about 1 mol % and then decreased. Fig. 1 depicts the propene conversion and the selectivity in the photooxidation of propene over the TS(i) and TS(s) systems from Table 1. In the TS(i) system (Fig. l a), the samples containing small amount of Ti (0.01-0.1 mol %) showed high selectivity to PO such as 40 %. Increasing Ti content, the selectivity to PO decreased and the selectivity to COx and propanal increased. Also in the TS(s) system (Fig. lb), the distribution of products changed according to Ti content similarly to that over the TS(i) system. However, comparing at the same content of Ti, the TS(s) system always showed higher PO selectivity than the TS(i) system. Especially, the TS(s) sample containing small amount of Ti (0.08 mol %) showed the best selectivity to PO such as 60 % among all the samples tested in the present study and the previously reported photoepoxidation systems [5, 8-11 ]. Each TS(s) sample showed lower selectivity to ethanal and acrolein than TS(i) sample (Table 1). Table 1. Results of the photooxidation of propene over the SiO2, TiO2 and titania-silica samples. SA a Conv. b PO ~ Selectivity / C% Sample /mEg q / C % /C% PO d propanal acetone acrolein ethanal HC e CO t. SiO2 558 0.7 0.2 22.3 3.5 25.8 15.2 18.1 10.0 5.1 TiO2 34 59.5 0.0 0.0 0.4 0.2 0.0 0.1 0.4 98.8 0.0ITS(i) 544 1.9 0.7 37.0 3.0 15.1 7.6 26.8 2.7 4.9 0.1TS(i) 478 9.1 3.7 40.8 3.3 11.3 4.2 24.5 2.3 10.1 0.5TS(i) 495 17.8 5.4 30.1 11.0 12.4 4.2 20.0 1.8 16.4 1.5TS(i) 473 24.4 4.7 19.2 17.6 9.3 8.7 21.5 2.4 14.1 5.0TS(i) 453 23.8 1.1 4.8 10.2 12.2 2.6 26.4 3.7 37.0 0.08TS(s) 387 4.4 2.6 60.2 2.9 10.4 1.1 17.7 3.0 3.9 0.34TS(s) 423 9.2 5.3 57.5 2.7 5.8 1.2 21.1 5.1 6.6 1.0TS(s) 535 12.5 6.3 50.5 6.2 8.3 1.7 22.1 3.3 7.4 4.1TS(s) 483 21.0 4.5 21.5 19.1 11.4 2.4 24.1 3.4 14.4 8.3TS(s) 416 32.1 1.8 5.7 24.5 13.6 3.1 26.4 2.9 19.1 0.34TS(s) g 423 0.5 trace 5.5 1.6 2.9 0.0 70.0 20.0 0.0 Catalyst 0.2 g, propene 100 tool, 02200 moi, reaction time 2 h. aBET surface area. b Conversion based on propene, c PO yield, d Propene oxide, e Ethene and butenes, f CO and CO2. g In the dark for 2 h at 323 K.

848 40

40

60~TS(s)

601 (a) TS(i) 30~

30 ~40

~ 9

~-

"

2 4 6 Ti content / mol %

o

20"~

R2 ~___.x'-"_~_ . . . . _Y. . . . . . . . . . . . .

0

20 ~~D

N 20 0

8

- >

0 " r -~

0

,

2

,

,

,

4 6 8 Ti content / mol %

Fig. 1 Results of photooxidation of propene over the TS(i) (a) and TS(s) (b) samples. Conversion (C)), and selectivity to PO (O), propanal (A), ethanal (Y) and COx (•

60

40~

40

30-~,, ~ O ~.4020 "~ -~ ,

> .,..q

ID

60. (b) 4.1TS(s) / ~ ~ ~

!40~...

> Io)r

~" i"

30 "~, j

O

- 20

t~

20

10 "~

~20

o 10 "~

ID

0

2 4 6 8 Irradiation time / h

0

10

0 ~ 40 ~

~D

0

:

0

1 2 3 4 Irradiation time / h

0

0 ~

(c) 0.1TS(i)

60

o

- 30

~40

-*..4

>

".~

20

/"

0

O r 20 ~ =o 10 "~ "r

,

~

"Y,,

......---X" . . . . . . . _~

08 10

ethanal (v) and COx (• are plotted.

4 6 8 Irradiation time / h Fig. 2 shows time courses of propene photooxidation over the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples. Over all these samples, the conversion increased with an increase of irradiation time. With increasing conversion, the PO selectivity decreased slightly, and the selectivity to ethanal and propanal much decreased, while the selectivity to COx increased. This indicated that ethanal and propanal were consecutively oxidized to COx more easily than PO was. Over the 0.34TS(s) sample the PO yield reached 15.7 % at 10 h, while over the 0.1TS(i) sample it reached 9.2 % at 8 h (Fig. 2). Even if all the Ti atoms were assumed to 0

2

-

Fig. 2. Time course of photooxidation of propeneover the 0.34TS(s) (a), 4.1TS(s) (b) and 0.1TS(i)(c) samples. Conversion (o), PO yield ( , ) , and selectivity to PO ("), propanal (zx),

849 be the active sites, the turnover number, TON = (the amount of produced PO) / (the amount of active sites), was 1.4 and 2.8, respectively. This means that the photoepoxidation of propene over titania-silica proceeds catalytically [13]. In all the runs mentioned above, most of products except for CO2 were collected by heating at 573 K in Vacuo. Thus, we examined the desorption temperature of each product over the 0.34TS(s) and 4.1TS(s) samples by heating the catalysts stepwise at 323 K, 373 K, 473 K and 573 K. Over both of the samples, most of PO and ethanal were collected by heating up to 373 K. These results suggest that PO and ethanal weakly adsorbed on the catalysts. On the contrary, propanal was mainly collected by 573 K heating. Propanal adsorbed on the catalyst more strongly than PO and ethanal. 3.2. The reactivity of propene oxide (PO) To know the possibility of the conversion of produced PO, the thermal isomerization of PO and the photooxidation of PO were examined over the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples (Table 2). By the thermal isomerization of PO (Table 3 (a)), PO was converted mainly to propanal without irradiation. A very small amount of PO was converted over the 0.34TS(s) and 0.1TS(i) samples, while 18.8 % of PO was converted over the 4.1TS(s) sample. By the photooxidation of PO with molecular oxygen (Table 3 (b)), ethanal, acrolein and COx were obtained in addition to propanal over all the samples. These results suggested that propanal was produced by the thermal isomerization of PO in the dark predominantly, and others such as ethanal and CO,, were mainly produced by the photooxidation of PO. The amount of PO converted by the photooxidation, which would correspond to the difference between (b) and (a), was 2.1%, 20.3 % and 11.7 % over the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples, respectively. Table 2. Reaction of propene oxide (PO) over the representative titania-silica catalysts. Yield/C% b Method a Sample

propanal acetone acrolein ethanal alcohols HCCCOx d

Total yield / C%

(a) 0.34TS(s) 1.7 0.1 0.1 0.1 0.0 0.2 0.1 2.3 (a) 4.1TS(s) 14.3 0.7 0.0 0.0 0.0 1.1 2.7 18.8 (a) 0.1TS(i) 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.2 (b) 0.34T S (s) 1.3 0.2 0.0 1.4 0.0 0.2 1.3 4.4 (b) 4.1TS(s) 18.4 2.6 2.4 6.1 3.5 1.3 4.8 39.1 (b) 0.1TS(i) 1.5 0.6 1.5 2.8 0.7 0.5 4.3 11.9 Catalyst 0.2 g, propene oxide 20 lamol, a (a) The thermal isomerization of PO and (b) the photooxidation of PO. See text. bBased on PO. c Propene, ethene and butenes, d CO and COz.

850

3.3 Characterization

of the titania-silica

samples

Fig. 3 shows diffuse reflectance UV spectra of the samples. The TiO2 sample showed a large absorption band below 380 nm. The SiO2 -,,, \ sample scarcely showed absorption. The titania-silica samples of low Ti content less than 0.4 mol %, in both \ ,, \,, ,.,,,, \ the TS(i) and TS(s) systems, \ ',\', ,\, exhibited a narrow absorption band ~ 2. ~,, below 250 nm, which was assigned to the LMCT (ligand-metal charge transfer) from O to Ti of isolated tetrahedral Ti species [14, 15]. The samples containing more than 0.4 ' 315 ' I ' ~1 mol % of Ti in both the TS(i) and 2q)0 250 waveleng~... / nm 350 400 TS(s) systems showed the additional Fig. 3 Diffuse reflectance UV spectra of SiO2 (---), absorption in the region above 250 TS(i)(----), T S ( s ) ( ~ ) and TiO2(. . . . ). The samples nm. The absorption edge was shifted were evacuated at 673 K. SiO2 (a), 0.0ITS(i) (b), to longer wavelength with increasing 0.08TS(s) (c), 0.1TS(i) (d), 0.34TS(s) (e), 1.0TS(s) ~, Ti content and became close to that 0.5TS(i) (g), 4.1TS(s) (h), 1.5WS(i)(i), 8.3TS(s) ~,'), of the TiO2 sample. The absorption in 5.0TS(i) (k) and TiO2 (/). the 250 - 330 nm region are assigned to the [TiOz]n clusters, and it is generally known that the absorption edge is shifted to longer wavelength as the size of the [TiOa]n clusters become larger [14, 15]. From these results, it was indicated that both the TS(s) and TS(i) samples containing less than 0.4 mol o% of Ti consisted of the isolated tetrahedral Ti species predominantly. In the titania-silica samples containing more than 0.4 mol % of Ti, the [TiOz]n clusters were also formed, and the size of the [TiOz]n clusters become larger with increasing Ti content. Fig. 4 shows the relationship between the Ti content and the absorption edge in UV spectra. In the region of more than 0.4 mol % of Ti content, the TS(s) system showed the absorption edge of shorter wavelength than the TS(i) system. This indicates that the TS(s) system have more dispersed Ti species than the TS(i) system when they were compared at the same Ti content. Fig. 5 shows the NH3-TPD profiles of the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples. The 4.1TS(s) sample showed 54.7 ~tmol g-1 of desorbed NH3, which corresponds to about 8 % of Ti atoms in this sample. On the other hand, the 0.34TS(s) and 0.1TS(i) samples showed very small amount of desorbed NH3, which means that both of them have almost no acid site.

,,,

851

400

J

E q) ca3

0.8 ea3 o

350

0.6

= 300 .2

=~" 0 . 4 -

.o

(b)

.,.a

o 250

o 0.2

,.o

200

0

2 4 6 8 Ti content / mol % Fig. 4. Relationship between the Ti content and the absorption edge in UV spectra of TS(s) (a) and TS(i) (b).

Z

400

500 600 700 Temperature / K Fig. 5. NH3-TPD profiles of 0.34TS(s) (a), 4.1TS(s) (b) and 0.1TS(i) (c).

4. DISCUSSION 4.1. The structure of Ti species and the acidity over titania-silica samples When Ti content was very low (< 0.4 mol %), the titania-silica samples consisted of the isolated tetrahedral Ti species predominantly, regardless of the preparation method (Fig. 3). In the titania-silica samples containing more than 0.4 tool % of Ti, the [TiO2]n clusters were also formed, and the clusters became larger with increasing Ti content (Fig. 3). The TS(s) samples contained more dispersed Ti species than the TS(i) samples when they were compared at the same Ti content (Fig. 4). By the sol-gel method employed in the present study, the aggregation of Ti atom would be reduced in comparison with the conventional impregnation method, as expected. The 0.34TS(s) and 0.1TS(i) samples predominantly having the isolated tetrahedral Ti species possessed almost no acid site, while the 4.1TS(s) sample containing the [TiOz]n clusters 60~oCk(a had acid sites (Fig. 5). It is known that Ti-O-Si ~" ) bridges where the Ti atoms reside in ~ 40 pentahedral or octahedral sites cause the charge i~ A"~ I imbalance and generate Bronsted acid sites [14] 9 ~5 Thus, the acid property of the 4.1TS(s) sample ~ 20 would be due to the presence of [TiO2]n clusters, which contained the octahedral or pentahedral 0 I I - ~ Ti moieties 9 200 250 300 ~ 3 5 0 400 absorption edge / n m 4.2. The effect of Ti content on the Fig. 6. Relationship between the PO photocatalytic activity selectivity and the absorption edge in UV On both the TS(s)and TS (i) systems, the spectra of the TS(s) (a) and TS(i) (b) PO selectivity increased and the selectivity to samples. propanal and COx decreased with decreasing Ti

852 content (Fig. 1). As mentioned above, the dispersion of Ti species varied with Ti content. Fig. 6 shows the relationship between the PO selectivity and the absorption edge of UV spectrum of each sample. The selectivity to PO increased when the absorption edge was shifted to shorter wavelength. This means that more dispersed Ti species show higher PO selectivity. The 4.1TS(s) sample showed higher selectivity to propanal, ethanal and COx than the 0.34TS(s) sample at similar conversion (Fig. 2). This result would be explained by the result of PO conversion shown in Table 2. PO was not so much converted over the 0.34TS(s) sample, while a large amount of PO was converted into propanal over the 4.1TS(s) sample in the dark (Table 2(a)). Generally, it is known that PO is isomerized into propanal on acid sites, and into acetone on basic sites [16]. Thus, it was suggested that a part of PO produced on the 4.1TS(s) sample in the photooxidation of propene cannot be desorbed as PO, and was converted into propanal due to the acid sites arising from the [TiOz]n clusters. In addition, much larger amount of PO converted to ethanal and COx over the 4.1TS(s) sample than over the 0.34TS(s) sample under photoirradiation in the presence of molecular oxygen (Table 2(b)). The [TiOz]n clusters promoted not only the thermal isomerization of PO to propanal, but also the consecutive photooxidation of PO during the propene photooxidation. As a conclusion of this section, it is found that the isolated tetrahedral Ti species on SiO2 are effective for propene photoepoxidation, while the [TiOz]n clusters for other products such as propanal, ethanal and COx production. 4.3. The effect of the preparation method on the photocatalytic activity The TS(s) system showed higher selectivity to PO than the TS(i) system at any Ti content (Fig. 1). In Fig. 6, the plots of the TS(s) and TS(i) samples seem to be on the common curve in the region from 270 to 380 nm of the absorption edge. This means that PO selectivity would be determined by the dispersion of Ti species regardless of the preparation methods in this region. On the other hand, in the region less than 250 nm of the absorption edge, although the samples in the both systems consisted of the isolated tetrahedral Ti species predominantly, the plots of the TS(s) system showed higher PO selectivity than the TS(i) system (Fig. 6). The 0.1TS(i) sample showed higher selectivity to ethanal, acrolein and COx than the 0.34TS(s) sample (Table 1). This result agrees with the result of photooxidation of PO; more PO converted to ethanal, acrolein and COx over the 0.1TS(i) sample than over the 0.34TS(s) sample (Table 2(b)). The yield of propanal converted from PO was similar over both samples (Table 2), and the 0.1TS(i) sample showed almost no acid sites similarly to the 0.34TS(s) sample (Fig. 5). Therefore, the conversion of PO over the 0.1TS(i) sample should not be attributed to the acidity arising from [TiOz]n clusters. Although no evidences were obtained to clarify the structural differences between the TS(s) and TS(i) samples containing a small amount of Ti, some possibility can be considered. One is that the local structure of the isolated tetrahedral Ti species may be different. For example, Lamberti et al. [17] suggested the existence of two kinds of the isolated tetrahedral Ti species in TS-1; [Ti(OH)(OSi)3] and [Ti(OSi)4]. As a conclusion of this section, it should be noted that the sol-gel method gave more selective catalysts for propene photoepoxidation than the impregnation method.

853 5. CONCLUSION The titania-silica catalyst of low Ti content prepared by the sol-gel method exhibited the highest selectivity to PO in the photooxidation of propene by molecular oxygen. The selectivity to PO decreased with increasing Ti content. It was concluded that the isolated tetrahedral Ti species were active sites for propene photoepoxidation, while the [TiOz]n clusters for the formation of propanal and CO2. Comparing samples TS(s) and Ts(i) which exhibited similar UV absorption band attributed to the isolated tetrahedral Ti species, the TS(s) sample showed much higher selectivity to PO, up to 60 %, than the TS(i) sample. It was found that the sol-gel method could provide effective titania-silica catalysts for the photoepoxidation of propene by molecular oxygen. ACKNOWLEDGEMENT This work was supported by a grant-in-aid from the Japanese Ministry of Education, Science, Art, Sports and Culture, and by Nippon Sheet Glass Foundation for Materials Science and Engineering. REFERENCES 1. Y. Wang and K. Otuka, J. Catal., 157 (1995) 450. 2. T. Hayashi, K. Tanaka and M. Haruta, J.Catal., 178 (1998) 566. 3. G. Lu and X. Zuo, Catal. Lett., 58 (1999) 67. 4. K. Murata and Y. Kiyozumi, Chem. Commun., (2001) 1356. 5. E Pichat, J. Herrmann, J. Disdier and M. Mozzanega, J. Phys. Chem., 83 (1979) 3122. 6. E Blatter, H. Sun, S. Vasenkov and H. Frei, Catal. Today, 41 (1998) 297. 7. Y. Xiang, S. C. Larsen and V. H. Grassian, J. Am. Chem. Soc., 121 (1999) 5063. 8. T. Tanaka, H. Nojima, H. Yoshida, H. Nakagawa, T. Funabiki, and S. Yoshida, Catal. Today, 16 (1993) 297. 9. H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki and S. Yoshida, Chem. Commun., (1996) 2125. 10. H. Yoshida, T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki and S. Yoshida, J. Catal., 171 (1997) 351. 11. H. Yoshida, C. Murata and T. Hattori, J. Catal., 194 (2000) 364. 12. R. Lange, J. Hekkink, K. Keizer and A. Burggraaf, J. Noncryst. Solids, 191 (1995) 1. 13. H. Yoshida, C. Murata and T. Hattori, Chem. Commun., (1999) 1551. 14. X. Gao and I. E. Wachs, Catal. Today, 51 (1999) 233, and references therein. 15. S. Bordiga, S. Colucia, C. Lamberti, L. Marchese, A. Zecchina, E Boscherimi, E Buffa, E Genomi, G. Leofanti, G. Petrini, and G. Vlaic, J. Phys. Chem., 98 (1994) 4125. 16. Y. Okamoto, T. Imanaka and S. Teranishi, Bull. Chem. Soc. Jpn., 46 (1973) 4. 17.C. Lamberti, S. Bordiga, D. Arduino, A. Zecchina, E Geobaldo, G. Span6, E Genoni, G. Petrini, A. Carati, E Villain and G. Vlaic, J. Phys. Chem. B, 102 (1998) 6382.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

855

Preparation of large surface area MnOx-ZrO2 for sorptive NOx removal M. Machida, M. Uto and T. Kijima Department of Applied Chemistry, Faculty of Engineering, Miyazaki University Gakuenkibanadai-nishi, Miyazaki 889-2192 Japan

Surfactant-controlled coprecipitation was applied to the preparation of large surface area MnOx-ZrO2. Coprecipitation in the presence of cationic surfactants, CTAB or CTAOH, and subsequent heating in air yielded homogeneous noncrystalline mixtures of Mn203 and ZrO2. The resulting porous materials with large surface areas in the range 200-300 m2/g were found to be useful for oxidative adsorption of NO. For this application, precursors and surfactants containing halide ions (C1- or Br-) should be avoided, because these species are strongly bound to surface sites for the oxidative NO adsorption. 1.

INTRODUCTION

A number of metal oxides show various interactions with NO, which result in adsorption, absorption and solid-gas reactions [1]. Recently, binary oxides containing Mn and tetravalent ions (Zr or Ce) have been reported to show the high ability to adsorb gaseous NO in the presence of Oa at lower temperatures [2-7]. The binary oxides are effective in promoting oxidative NO adsorption to form nitrate and/or nitrite species on the surface. In the previous reports, we have pointed out that the NOx adsorbing materials are applicable to not only adsorptive NOx removal, but also catalytic deNOx by loading noble metals. For the Pd-loaded MnOx-CeOz, hydrogen activated on Pd is readily spilt over onto the MnOx-CeO2 surface and reduce NOx (NO2-or NO3-species) adsorbed thereon to Na [8,91. Because the amount of NOx adsorption on these materials is less than surface monolayer, preparation of larger surface area materials is requested to increase the NO uptake as well as the catalyst efficiency. MnO~-ZrQ is more promising in this regard. The larger surface area, ca.150-180 m/g, was obtained for noncrystalline MnOx-ZrO2 materials, compared to ca. 60-70 m2/g for MnOx-CeO2 with a fluorite-type crystalline phase. Due to the noncrystalline nature, the surface area of MnOx-ZrO2 is strongly sensitive to the preparation route. One possible method to achieve large surface areas is surfactant-template precipitation. In the present work, preparation of MnOx-ZrO2 fine particles was studied by the use of coprecipitation in the presence of cationic surfactants, which would cause electrostatic interactions with Mn/Zr hydroxide ions. The thermal stability and NO adsorbability of as prepared oxides were also evaluated. 2. E X P E R I M E N T A L 2.1

Sample preparation MnOx-ZrOz was prepared from corresponding metal chlorides or nitrates as shown

856 surfactants, CTAB (cethyltrimethylammonium bromide) or CTAOH (cethyltrimethylammonium hydroxide), were dissolved in distilled deionized water. To the solution was added an aqueous solution of NaOH or NH4OH under vigorous stirring. The pH of the resultant suspension was controlled from 5 to 13. After subsequent aging at 75-90 ~ a solid product was centrifuged and washed with water, and finally calcined at 450 ~ in air. The sample prepared using different sources and surfactants is designated as X-Y, where X is NO3 (nitrates) or C1 (chlorides), and Y is B (CTAB) or OH (CTAOH) as listed in Table 1. MmOx-ZrO2 was also prepared by conventional coprecipitation from an aqueous nitrate solution (NO3-1). An aqueous solution of dinitrodiamine platinum nitrate was impregnated onto as prepared binary oxides and calcined at 450 ~ for 5 h (1.0 wt% loading as Pt). As prepared powder samples were pressed and crushed into 20 mesh granules before use for adsorptive NOx removal.

I

Chlodde or nirate Na0H or NH3OH CTAB or CTAOH stirring

I

I,Mn-Zr, hydr~

:1

rinsing

I,

dr~ng under reduced pressure ~calcination in air

! Mn~176

!

Fig. 1. Diagram of surfactantcontrolled coprecipitation of 2.2 Characterization MnOx-ZrO2. Crystal structure of calcined samples was determined by powder X-ray diffraction (XRD, Table 1 Samples prepared in this study Shimadzu XD-D 1) using monochromated CuKa sample source surfaetant base radiation (30 kV, 30 mA). The BET surface area was obtained by measuring N2 adsorption NO3-1 nitrate none NH 3 isotherms a t - 1 9 6 ~ The XPS measurement CI-Br chloride C T A B NaOH was performed on a Shimadzu-Kratos AXIS-HS NO3-Br nitrate CTAB NaOH spectrometer with a magnesium anode ( M g K ) NO3-OH nitrate C T A O H NaOH operated at 15 kV and 10 mA. DRIFT spectra of NO3-OH-2 nitrate CTAOH NH3 NOx species adsorbed onto MnOx-ZrO2 were ' recorded on a Jasco FT-IR610 spectrometer. A temperature-controllable diffuse reflectance reaction cell (Jasco DR600A) was connected to a gas flow system and a vacuum line. The sample was heated in a stream of 20vol%O2/He at 400 ~ for 1 h and then exposed to the reaction gases containing 0.08vol%NO, 2vo1%O2, and He balance at 25 ~ for 30 min. After the treatment spectrum was recorded in a flowing He at ambient temperature.

2.3 Adsorptive NO removal The sorptive NO removal was carried out in a conventional flow system at atmospheric pressure. Gas mixtures of 0.08 vol% NO and 10% 02, balanced with He were fed to the granular sample (0.2 g) at W/F=0.24 s-g" cm ~. The effluent gas was analyzed by an on-line gas chromatography (TCD) with molecular sieve-5A and Porapak-Q colunms, and a chemiluminescence NOx analyzer.

857 3. RESULTS AND DISCUSSION 3.1

Phases of MnOr-ZrOz

Coprecipitation is the most widely used procedure for preparing a precursor of mixed oxides. Its application to the present system caused noticeable difference in the phase of as calcined oxides with different compositions. Fig. 2 shows powder X-ray diffraction patterns of (n)MnOx-(1-n)ZrO2 prepared by coprecipitation from nitrate sources and subsequent heating at 450 *C. The diffraction patterns at n=0 and 1.0 consisted of tetragonal/monoclinic ZrO2 and Mn203, respectively. However, the increase of n converted the crystalline phases into noncrystalline products at 0.20.6 caused significant crystallization of Mn203, which was accompanied by considerable loss of surface area. When the corresponding samples were o Zr02 monoclinic prepared by coprecipitation in the presence of ~ ...._ . _ n=0.2 CTAB (CI-Br-2 in Table 2), the resulting XRD patterns (Fig. 4) were different from those in Fig. 2. ! 9ZrO~tetragonal ~0 A n=O.O The crystalline Mn203 in this case was not formed up to n=0.8, i.e, the noncrystalline region was extended in a wider range of 0.2
3.2

Effect of surfactants The microstructures of the two samples (n=0.4), NO3-1 and CI-Br-2, are compared in Fig. 5. NO3-1 was mainly composed of submicron particles, but large agglomerates were also formed. By contrast, CI-Br-1 was composed of highly dispersed fine particles with uniform size less than 0.1 mm. These are noncrystalline mixtures as evident from Fig.4. The amorphous-like nature of MnOx-ZrO2 appears to be originated from the growth process of hydroxides in the coprecipitation process. With an addition of base (OH) to the aqueous solution of Mn(NO3)2, hydroxide [Mn(OH)6] 4"clusters formed would be bound together to grow into Mn(OH)2 crystallites.

858

Table 2 Surface area of 0.4MnO,-0.6ZrO2 prepared by using cationic surfactant sample

precursor

NO3-1

nitrate

CI-Br- 1 CI-Br-2 CI-Br-3 CI-Br-4

chloride chloride chloride chloride

NO3-Br nitrate NO3-OH nitrate NO3-OH-2 nitrate

surfactant -

base

pH

aging temp./~

aging surfacea~ Mn/(Mn+Zr)b) time/h area/mZg-1

NH3

12.8

-

-

158.0

0.4

CTAB CTAB CTAB CTAB

NaOH NaOH NaOH NaOH

5.2 7.0 12.7 10.0

75 75 75 90

24 12 24 90

65.0 224.8 231.6 270.9

0.06 0.19 0.36 -

CTAB CTAOH CTAOH

NaOH NaOH NH3

10.2 10.6 11.0

90 90 90

90 90 90

300.0 273.9 243.4

0.36 -

a)After calcination at 450~ b)Determined by EDX measurement. In the present coprecipitation process, the interaction between hydroxide clusters of Mn and Zr would suppress each other to grow into large crystallites. This is a possible reason for the small particle size and thus the large surface area of the MnOx-ZrO2 system. The coprecipitation in the presence of cationic surfactant would lead to more complex interactions with hydroxide clusters. Suib et al. [11] reported that electrostatic interactions would occur between the Mn hydroxide cluster and CTAB to form an ordered hexagonal mesostructure. The present system is not the case, because such a long-range periodicity was not observed in the XRD patterns. However, it is plausible that the negatively charged building blocks of Mn and Zr hydroxides are bound to the positively charged surfactant ammonium group rather than to each other. Consequently, this interaction would further limit the size of Mn/Zr hydroxide particles and improve chemical homogeneity. Owing to the high homogeneity, the transformation of the quasi-stable mixture to crystalline MnzO3/ZrOzcan be suppressed. 1.0 Table 2 shows the properties of MnOx-ZrO2 samples (n=0.4), which 0.8 were prepared under different coprecipitation conditions. N 0.6 Coprecipitation at pH<7 (CI-Br-1) "~ 0.4 produced a low surface area product. Incomplete precipitation of Mn resulted 0.2 in a lower Mn content in the precursor, which is not enough to inhibit the 0.0 i , i 018 1.0 crystallization of a monoclinic ZrO2 0.0 0.2 0.4 0.6 phase. Nearly stoichiometric oxides n in (n)Mn0,-(1-n)Zr0~ (C1-Br-3,4) with larger surface areas Fig. 3. Surface composition of ranging 220-270 mZ/g were successfully (n)MnOx-(1-n)ZrOz as a function of bulk yielded at pH>10 by aging at higher composition temperatures (90 ~ The largest surface area in the present study, 300 mZ/g, was obtained by using nitrate sources in the place of chlorides (NO3-Br). We believe that this is one of largest values for Mn-oxide materials reported so far. It was found that CTAOH is also applicable to obtain the large surface area products (NO3-OH). This is t-

859 important because the halide-flee synthesis is essential for the preparation of NOx adsorbing material as described below. 3.3

Thermal stability of MnOx-ZrO2 Since the large surface area of MnOx-ZrO2 is due to their noncrystalline fine particles of Mn203 and Zr02, thermal stability should be a key point to be evaluated before practical uses. The effect of calcination temperature on the C1-Br-2 product was examined. Fig. 6 represents the XRD diffraction patterns after calcination at elevated temperatures. Clearly, significant crystallization of tetragonal ZrO2 took place at <500 ~ On the other hand, the crystalline Mn203 appeared after calcination at 800 ~ Fig. 7 shows the corresponding change of surface area of C1-Br-2. The surface area increased at elevated temperatures up to 450 ~ because the removal of organic components gave rise to micropores in the products. However, further heating at >450 ~ caused the considerable sintering as a result of crystallization of ZrO2. The sintering of this material caused complete deterioration ofNOx adsorbability [3]. Since a similar result was obtained for the oxides synthesized by conventional coprecipitation (NO3-1), the thermal stability of MnOx-ZrO2 cannot be improved by modifying the preparation route. The use of this material must be limited below 450 ~ 3.4

NO adsorption property The sorptive NO uptake of as prepared MnOx-ZrO2 is summarized in Fig. 8. On contrary to the improved surface area, both CI-Br and NO3-Br samples exhibited small NO uptakes. Since the solid-gas reaction in this case should be limited on the oxide surface, the XPS measurement was conducted to detect surface contaminations for these samples. The spectra suggested the presence of chloride or bromide anions, which were totally absent on the surface of NO3-1. These residual halide species originated from metal sources and/or surfactants would be strongly bonded to the NO adsorption site. This is consistent with that NO3-OH samples, k_...... n-0.8 which were prepared from halide-free precursors ............-- " ~ ............ ~................ and CTAOR showed much larger NO uptake. The use of halide anions must be avoided for the n-0.6 preparation of NO adsorbing materials. The NO uptake was further improved by loading 1 wt% Pt n=0.4 via wet incipient impregnation of a Cl-free ~-~--___.-_---_---- - ~ ' ~ ' - - - ............. - : . . . . . platinum complex. A Zr02 tetragonal

Structure of adsorbed NO In situ DRIFTS measurement was --:~_.... _ _o o _ _ ~ conducted to evaluate the structure of NOx adsorbed on MnOx-ZrO2. Fig. 9 compares the , , , , , , , , spectra taken from MnOx-ZrO: (NO3-OH) (a) and o lo 20 30 40 so eo 70 Pt/MnOx-ZrO2 (b) after NO adsorption at 25 ~ 2 /deg The admission of NO immediately yielded several a)~.at,a ,.,. b)Detcnnined by EDX measul'emcstt different bands in the range 1200-1600 cm1, Fig. 4. Powder XRD patterns of depending on the 02 concentrations in the gas feed. (n)MnOx-(1-n)ZrO2 prepared by The band at ca. 1270 cm] observed for unloaded coprecipitation in the presence of NO3-OH in the absence of 02 is due to nitrite CTAB and subsequent calcination at 450 ~ pH7.0-7.4 .__...

. . . . . . . . .

;..

9 ._...,~

__

o Zr02 monoc/inic

~,m.m4mvu

~..*Jr

it,,,..~

_1,.~_

-_-.,,,~

9

-:~:--,---.,.

n=0.2 3.5 . . . .

860 (NO2) type species. The intensity of this band decreased with increasing 02 concentration as a result of the conversion of nitrite to nitrate (NO3, 1450 cm'l). These bands are quite similar to those observed for Mn203 [7]. The manganese oxide is an active catalyst for NO oxidation to NO2, but the single phase Mn203 could not adsorb large amount of NO because of the low surface area. In this respect, the NO3-OH is quite adequate as an NOx-adsorbing material. The Pt-loaded NO3-OH showed more intense bands at ca.1560, 1450, and 1360 cm~. The bands at 1560 cm~ may be assigned to nitrate adsorbed on ZrO2, which was formed by NO oxidation over Pt and subsequent adsorption. As pointed out in our previous report [12], the oxidative adsorption onto ZrO2 requires a catalyst, such as Pt and Pd, to promote the NO oxidation. In accordance with this effect, the Pt loading intensified the nitrate bands in the oresence of 02.

Fig. 5.

SEM photographs of NO3-1 CI-Br-2 after calcination at 450 ~

and

~,~

400"C

9Zr02 tetragonal

0

I

10

I

20

A

I

550oc

I

30 40 2#/deg

I

50

I

60

Fig.6. Powder XRD patterns ot CI-Br-2 prepared by coprecipitation in the presence ot CTAB and subsequent calcination.

I

70

861

Fig.8. Temperature dependence of NO uptake of MnOx-ZrO2 prepared by using different metal sources and surfactants 0.08% NO, 10% 02, He balance.

[~ ~~ Fig.9.

in situ DRIFT spectra of and b) 1wt~ after NO adsorption at 25 ~ 0.04 % NO, 0-10 % O2 He balance, 30min.

a)MnOx-ZrO2

,!7/

.~, ,~ 2000

150(

1000

Wavenumber/cm'

2000

500

I000

V~venumber/cm -~

862 REFERENCES

1. M.Machida, in Catalysis, Vol.15, The Royal Society of Chemistry, Cambridge, 2000, p.73. 2. M.Machida, Catal. Survey (2002) in press. 3. K.Eguchi, M.Watabe, S.Ogata and H.Arai, Bull. Chem. Soc. Jpn, 68 (1995) 1739. 4. K.Eguchi, M.Watabe, M.Machida and H.Arai, Catal. Today, 27 (1996) 297. 5. K.Eguchi, M.Watabe, S.Ogata and H.Arai, J. Catal., 158 (1996) 420. 6. M.Machida, M.Uto, D.Kurogi and T.Kijima, Chem. Mater., 12 (2000) 3158. 7. M.Machida, D.Kurogi and T.Kijima, J. Chem. Mater., 11 (2001) 900. 8. M.Machida, D.Kurogi and T.Kijima, Chem. Mater., 12 (2000) 3165. 9. M.Machida, D.Kurogi and T.Kijima, Stud. Surf. Sci. Catal., 138 (2001) 267. 10. Phase Equilibria Diagrams, ver.2.1, Am. Ceram. Soc. (1997). 11. Z.R.Tien, W.Tong, J.Y.Wang, N.CtDuan, V.V.Krishnan and S.L.Suib, Science, 276 (1997) 926. 12. M.Machida, A.Yoshii and T.Kijima, Int. J. Inorg. Mater., 2 (2000) 413.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation of C u O x - T i O 2 intercalated layered structure

nano-composite

863

photocatalysts

from

M.Machida*, S.Nagasaki and T.Kijima Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Gakuenkibanadai-nishi, Miyazaki 889-2192, Japan

Nano-composite photocatalysts, CuOx-TiO2, were synthesized from Cu(OAc)2intercalated fibrous layered titanates by thermal decomposition in different atmospheres (Na, air, and Ha). The structural characterization using XRD, UV-vis, XPS, and SEM implied that the composite of partially reduced CuOx and anatase-type TiO2 in a waffle-like texture would be a reason for the excellent photocatalytic activity for Ha production from CH3OH/H20 mixtures. 1. INTRODUCTION The pillared layered materials are well-known nano-composites, which can be prepared via intercalation of large polymeric inorganic cations or metal alkoxides into ion-exchangeable layered hosts [1-9]. The structure of composites obtained after heat treatment is characterized by periodic alternative stacking of a host layer and a guest oxide. One interesting feature of such composites is a wide variety of possible chemical combinations between hosts and guests. In particular, many researchers are interested in the layered titanate, niobates, and tantalates, because of their excellent photocatalytic property [10-14]. We have previously reported that the photocatalytic activity of SiOz-pillared layered titanates is influenced by partial replacement of transition metals for Ti [13]. However, no attempt was made to incorporate various transition metal oxides into the interlayer. In the present study, a series of composite photocatalysts were synthesized by stepwise ion exchange and intercalation of transition metal acetates into fibrous layered tetratitanate microcrystals (K2Ti409) and subsequent calcination in different atmospheres. The highest photocatalytic activity obtained for the CuOx-composites was studied in relation to the microstructure. 2. EXPERIMENTAL

2.1. Sample preparation Fibrous potassium tetratitanate (K2Ti409) powders were supplied from Otsuka Chemical Co., Ltd. They were obtained from K2MoO4 flux containing mixtures of KzCO3 and TiO2 as was first reported by Fujiki et al. [15]. The pillared samples were prepared by a stepwise exchange process. The synthetic route followed was a modification of the method adopted by Yan et al.[9]. First, the sample was exchanged in lmol/1 HCI at room temperature for 1 week to give protonated derivatives. Intercalation of hexylamine was carried out by mixing the protonated sample in an aqueous solution of 2 mol/l n-C6HlaNH2

864 at room temperature for 1 week. This was followed by the treatment with 3 mol/l tetramethylammonium chloride for 3 days. The solid product was next treated with 0.1 mol/l aqueous solutions of transition metal acetates at 60 *C or at room temperature for 3 days. The resulting solids were centrifuged, washed with distilled water and air-dried at 60 ~ Finally, the products were heated at elevated temperatures (300-500 ~ for 2 h in a stream of Nz, Ha, or dry air. We denote the synthesized composites as M-X-Y, where M=Cr, Mn, Fe, Co, Ni, or Zn, X=heating temperature (~ and Y=heating atmosphere (N2, Ha, or air). 2.2. Characterization The crystal structure of as prepared samples was identified by using a powder X-ray diffractometer equipped with CuK~ radiation (30kV, 20mA) and a monochromator. An infrared spectrometer was used for the chemical structure analysis. Chemical composition of samples was determined by EDX analysis. To determine the content of organic species in the composites, thermal gravimetric (TG) analysis was carried out at a heating rate of 10 ~ in air. The BET surface area was determined by measuring N2 adsorption isotherms at 77 K. The microstructure of samples was observed by FE-SEM. Diffuse reflectance spectra were recorded with a UV-vis spectrometer. 2.3

Photocatalytic reaction The photocatalytic Ha evolution was conducted in an inner irradiation Pyrex cell, which was connected to a closed gas-circulating system consisting of a circulation pump, a pressure sensor, gas sampling valves, and stainless steel tubing. As calcined powder nano-composite (0.2 g) was suspended in 4mol/1 CH3OH (200 cm ~) in the cell by use of a magnetic stirrer. Prior to the reaction, the mixture was deaerated by evacuation and then

Fig. 1.

Crystal structure and SEM photograph of KzThO9.

flushed with Ar gas (20 kPa) repeatedly to remove Oz and COz dissolving in water. The reaction was carried out by irradiating the mixture with light from a 400W high-pressure Hg lamp. Gas evolution was observed only under photo-irradiation, being analyzed by an on-line gas chromatograph.

865 3. RESULTS AND DISCUSSION 3.1

Nano-composites derived from intercalation

Fig. 1 shows a SEM (200) photograph and the crystal 0.83nm structure of KzTi409 microcrystals H2Ti409 employed in the present study. The 2.23nm powders consist of well-developed fibrous microcrystals with smooth surface, the geometrical surface area of which is close to the measured BET surface area (11 mZ/g). From electron diffraction measurement in TEM observation, 1.12nm it was confirmed that the fiber axis 06H13NH3-Ti409 is parallel to the b axis of the 2.18nm monoclinic structure, which is composed of TiO6 octahedral sheets stacking along the a* direction. As shown in Fig. 1, the crystal structure is built up from a unit of four T i O 6 octahedra (CH3)4N-Ti409 arranged in a line by edge sharing [16,17]. These units are joined to 1.64nm t 11.17nm similar blocks above and below to ! I 079nm . . . . . . . . .___ .... ~ form zigzag strings. The strings are combined by sharing corners of I . I I J i . I I 10 20 30 40 50 60 octahedral to form staggered 0 28 / deg sheets. The structural change of the Fig. 2. Change of XRD patterns of HzThO9 layered titanates during stepwise by stepwise reactions with C6H13NH2, exchange to pillared derivatives was (CH3)4NCI, and Cu(OAc)2. studied by XRD (Fig. 2). Here, the (200) reflection corresponds to the interlayer distance between two adjacent TiO6 sheets. The interlayer distance of the protonated phase, H2Ti409, was increased from 0.83 nm to 2.23 nm by intercalation of C6H13NH2. The corresponding interlayer spacing of ca.1.5 nm is much lager than the chain length of n-hexylammonium ions (ca.1.0 nm), suggesting the bilayer configuration of alkyl chains as was pointed out by Hou et al. [7]. Mixing the hexylamine-intercalated phase with an aqueous solution of copper (II) acetate led to the decreased interlayer distance (1.64 rim). According to FT-IR measurement, this structural change was accompanied by the appearance of bands due to vco at 1450 and 1550 cm-1. The reaction was also conducted using aqueous solutions of other transition-metal acetates in the same manner. As shown in Table 1, the M/Ti ratio for the as prepared composites was in the range of 0.10-0.47, suggesting that the bulk-type reaction took place between metal acetates and the layered titanate. No deposition of metal acetate was confirmed on the surface of the fibrous microcrystals by the SEM for all as prepared samples before calcination. These results demonstrated that metal acetates were accommodated in the interlayer.

(CH3COO)2C_u-Ti,0?

866 Table 1 Chemical composition and BET surface area of M-400-N2 M M/Ti Surface area/m2g '~ " Cr 0.35 100.5 Mn 0.41 24.1 Fe 0.10 32.1 Co 0.35 26.4 Ni 0.34 44.5 Cu 0.33 36.6 Zn 0.47 23.3 ............

M=Cr -,

.

.

.

.

,

-

-

-

.

.

.

.

.

.

.

.

.

.

3.2. Structure and photocatalytic ,A M n O ~ 9 Mn property of nano-composites after calcination Fig. 3 shows the XRD patterns u Fe~03 after calcination of as prepared [] u Fe solids at 400 ~ in N2 (M-400-N2). The disappearance of basal reflections at 20<10 deg suggests the lack of periodicity in the layer Co stacking. The products for M=Mn, Fe, and Zn exhibited diffraction peaks due to oxides of M, Ni suggesting that part of intercalated metal acetates was precipitated as A Cu oxides. The Cu-400-N2 solely A produced metallic precipitations Ti02 9 9 9 A oo C u in addition to anatase-type TiO2. FT-IR measurement confirmed the o ZnO Zn complete elimination of the o ~ o o organic components (VCH ~- - . . . . . . . . . . . . I I I I i I 2840-2960 cm 4, vco 1450, 1550 cm -1) from these composites. As 0 10 20 30 40 50 60 shown in Table 1, the BET surface 20 / dea areas of M-400-N2 were in the Fig. 3. XRD patterns of M-400-N2 composites range of 23-100 mZ/g, compared to prepared from metal acetate-intercalated 11 m2/g for pristine KaTi409 tetratitanates after calcination at 400 ~ in a microcrystals. The largest surface flowing N2. area was attained by the Cr-400-N2 (100.5 m2/g). As reported by Yen et al. [9], Cr-acetate tends to produce Cr(III) origomers in an aqueous solution that is effective to prop up layers to produce the open pillared structure. Fig. 4 shows SEM photographs of Cr-400-N2 and Cu-400-N2. The Cr-400-N2 retained the fibrous morphology of pristine layered titanate as shown in Fig. 1. Since the deposition of amorphous Cr oxide was not observed, nm-scale slits formed between layers are responsible for the increased surface area. By contrast, fibrous particles of Cu-400-N2

867 were strongly deformed to exhibit winding waffle-like structure due to insertion of oxide/metal particles between layers. This is the reason for the disappearance of reflection from the layered host. Also, on the surface of Cu-400-N2, many submicron particles of Cu metal could be observed in accordance with the XRD result (Fig. 3). Table 2 summarizes the photocatalytic activity of M-400-N2 for H2 evolution from CH3OH/H20 mixtures. This activity was found to be strongly dependent on the transition metals, i.e., the Cu compound exhibited the highest activity. The evolution of CO2 was observed only for this composite as a result of photocatalytic oxidation of CHaOH.

Fig. 4.

SEM photographs of Cr-400-N2 and Cu-400-N2 composites.

Table 2 Photocatalytic H2 evolution from CH3OH/H20 Rate of evolutiona) Composite /~tmol h-1 Ha CO2 Cr-400-N2 0 0 Mn-400-N2 0 0 Fe-400-N2 3.13 0 Co-400-N2 0.02 0 Ni-400-N2 16.7 0 Cu-400-N2 681.2 23.0 Zn-400-N2 2.8 0 .

.

.

.

.

.

.

.

.

.

a) 4mol/l CH3OH 200 cm 3, catalyst 0.2 g

3.3. Structure and photocatalytic property of Cu-titanate nano-composites Fig. 5 shows XRD patterns of Cu-intercalated samples after calcination at elevated temperatures. The products after calcination at 350 ~ in N2 exhibited no diffraction peaks, but the SEM observation suggested that the fibrous crystalline shape was almost retained without deposits on the surface. Thus, the sample probably consisted of CuOx-stuffed layered composites with unequal interlayer distances. The formation of anatase-type TiO2 and Cu from the noncrystalline layered composite was observed after heating at >400 ~ in N2. When the precursor was heated in air (Cu-450-air), however, these phases were scarcely observed. This means that residual organic molecules in the interlayer would play

868 as a reducing agent for Cu acetate to produce the metallic state. The reduction process caused the collapse of pillared layered structure to produce the Cu/TiOz composite, but the fibrous morphology was retained. The photocatalytic activity was strongly dependent on the heating temperature as well as heating atmosphere as shown in Fig. 6. Calcination at <350 ~ in Nz yielded very low activity, whereas heating above 400 ~ significantly increased the activity in accord with simultaneous deposition of anatase-type TiOz and Cu. The activity of Cu-450-N2 was 4500 times as large as that of

300 ~ in N2

L

350 ~

in N2

9 TiO2 (anatase) o Cu

9

9

-~

~_

A

~.............

400 ~ in N2

O

9

9

9

o

450 ~ in N2 9

oOo 9

o

500 ~ in N2 oO oo 9

HzTinO9. Further heating decreased .... the activity probably due to loss of the surface area. Fig. 7 compares UV-vis 450 ~ in air reflectance spectra of samples ~ ............. shown in Fig. 5. A clear absorption I I I I I I i i 30 40 50 60 70 2O edge at ca. 370 nm was observed 0 10 for Cu-450-Nz and Cu-500-Nz, and 20 / deg the corresponding band gap energy, 3.2 eV, was close to anatase TiOz. Fig. 5. XRD patterns of CuOx-composites On the other hand, calcination prepared from Cu(OAc)z-intercalated tetratitanates neither at the lower temperatures in after calcination at elevated temperatures. Nz nor in air produced such a 1800 band-gap transition because of their ~-~_--

negligible amount of anatase. Only a gradual slope centered at ca.530 nm (2.3 eV) was observed as characteristic absorption of the stuffed layered composite. To ascertain the different band gap, the valence band spectra were taken by XPS measurement for Cu-300-Nz

~= 1600 9 =o 1400 ..= 1200 { 1000 ~ 800 ~ 600 > 400

_ ~-

__~_:-

_,,,__ ~ _

. . . . . . . . . .

.

.

.m

o

and Cu-450-N2. The former exhibited a position of the highest filled bands ca. 0.9 eV higher than the latter. However, such an alteration of the valence band was not effective in improving the photocatalytic activity.

200

/ .

0

250

-~

300

~

in Air ,

m

1

i

350

400

A

450

i

500

55O

Calcination temperature / ~

Fig. 6. Photocatalytic H2 evolution from 4mol/l CH3OH solution over CuOx-composites calcined at different temperatures.

869 Table 3 Effects of pretreatment on photocatalytic activity of CuOx-composites Atmosphere Rate of Hz evolved a) / ~tmol.h-1 air(5h) 37 Nz(5h) 1780 air(5h) + Na(5h) air(5h) + Hz(5h)

478 603

Table 4 Photocatalytic activity of unloaded layered titanates Catalyst Rate of a)/~tmol h-1

Cu-loaded gas

evolution

Hz 15.6 44.6 0.4 802.8

KzTi409 26wt%Cu/KzThO9 b) HzThO9 26wt%Cu/Hz Ti409 b)

and

CO2 0 0 0 11.6

All treatment was carried out at 450 ~ a) 4mol/l CH3OH 200 cm 3, catalyst 0.2 g a) 4mol/l CH3OH 200 cm 3, b) Prepared by wet impregnation of Cu(OAc)2 catalyst 0.2 g and subsequent heating at 450 ~ in N2.

ii

300 ~ in N2 300~

in N2

350 ~ in N2 45

II 450 500 ~ in N2

450 ~ in air 980 250

350

450

550

650

750

Wavelength / nm

Fig. 7. UV-vis reflectance spectra of CuOx-composites derived from Cu(OAc)2- intercalated tetratitanates after calcination at elevated temperatures.

850

I

I

I

I

I

970

960

950

940

930

920

Binding energy / eV

Fig. 8. Cu2p CuO• and air.

XPS spectra of after calcination in N2

870 Table 3 exhibits the effect of heating atmosphere on the structure and the photocatalytic activity of Cu-450. Contrary to Cu-450-N2, Cu-450-air exhibited much less activity. However, it was found that the activity of Cu-450-air was much improved by additional heating in N2 as well as H2. Since the treatment in N2 did not give rise to metallic Cu, the reduction of Cu to a metallic state is not essential to the photocatalytic activity. Fig. 8 shows the XPS spectra of Cu2p region. As is evident from a main peak of Cu2p3/2, all samples contained Cu in the divalent state, but a shoulder on the right side of the peak was observed for the exception of Cu-450-air. This means that the metallic Cu as well as partially reduced Cu species (Cu+) was formed after heating in N2. The formation of composites between partially reduced Cu and anatase-type TiO2 should be a key to yield the high photocatalytic activity in the present system. Table 4 shows the photocatalytic activity of CuOx supported on the pristine samples of layered titanates. The loading (26 wt% Cu) corresponds to the Cu/Ti ratio (0.33) for the composite prepared by the intercalation process. The two pristine layered titanates showed very low activity without loading CuOx. In contrast, the activity of the CuOx-loaded samples was quite different; the protonated phase produced ca. 18-times higher rate of H2 evolution. Copper oxides in these materials were deposited only on the surface of the fibrous crystals. However, the protonated phase was decomposed to produce anatase-type TiO2 as in the case of the intercalated composite (Fig. 5). This result also supports that the formation of anatase is essential for the photocatalytic activity. ACKNOWLEDGMENT One of the authors (M.M.) gratefully acknowledges the financial support from Grant-in Aid for Scientific Research from the Ministry of Education, Science, and Culture and Shiseido Foundation for Science and Technology. REFERENCES 1. I.V. Mitchell (ed), Pillared Layered Structures: Current Trends and Applications, Elsevier Applied Science, London, 1990. 2. S. Cheng and T.C. Wang, Inorg. Chem., 28 (1989) 1283. 3. M.W. Anderson and J. Klinowski, J. Am. Chem. Soc., 29 (1990) 3260. 4. M.E. Landis and B.A. Aufdembrink, P. Chu, I.D. Johnson, G.W. Kirker and M.K. Rubin, J. Am. Chem. Soc., 113 (1991) 3189. 5. W. Hou, Q. Yan and X. Fu, J. Chem. Soc. Chem. Commun., (1994) 1371. 6. W. Hou, B. Peng, Q. Yan, X. Fu and G. Shi, J. Chem. Soc. Chem. Commun., (1994) 253. 7. W. Hou, Q. Yan, B. Peng and X. Fu, J. Mater. Chem., 5 (1995) 109. 8. C. Guo, W. Hou, M. Guo, Q. Yan and Y. Chen, J. Chem. Soc. Chem. Commun., (1997) 801. 9. Y. Chen, W. Hou, C. Guo, Q. Yan and Y. Chen, J. Chem. Soc. Dalton Trans. (1997) 359. 10. J. Kondo, S. Shibata, Y. Ebina, K. Domen and A. Tanaka, J. Phys. Chem., 99 (1995) 16043. 11. S. Uchida, Y. Yamamoto, Y. Fujishiro, A. Watanabe, O. Ito and T. Sato, J. Chem. Soc., Faraday Trans., 93 (1997) 3229.

871 12. M. Machida, J. Yabunaka, H. Taniguchi and T. Kijima, Stud. Surf. Sci. Catal., 118 (1998) 951. 13. M. Machida, X.W. Ma, H. Taniguchi, J. Yabunaka and T.Kijima, J. Mol. Catal. A, 155 (2000) 131. 14 M. Machida, J. Yabunaka and T. Kijima, Mol. Cryst. Liq. Cryst., 341 (2000) 249. 15. Y. Fujiki and N. Ohta, Yogyo Kyokaisi, 88 (1980) 11. 16. H. Izawa, S. Kikkaw and M. Koizumi, J. Phys. Chem., 86 (1986) 5023. 17. T. Sasaki, M. Watanabe, Y. Komatsu and F. Fujiki, Inorg. Chem., 24 (1985) 2265.

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Studies in Surface ScienceandCatalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

873

Vanadia-doped titanium pillared clay: preparation, characterization and SCR activity of NO by ammonia L. Khalfallah Boudali a, A. Ghorbel a, P. Grange b and S. M. Jung b Laboratoire de Chimie des Mat6riaux et Catalyse, D6patement de Chimie, Facult6 des Sciences de Tunis, Tunis, Tunisia.

a

b Unit6 de catalyse et chimie des mat6riaux divis6s, Universit6 catholique de Louvain, Croix du sud, 2/17, 1348 Louvain-la-Neuve, Belgium. Vanadia doped sulfated Ti-pillared clays were prepared and characterized by BET, XRD, XPS, TPD-NH3 and compared with sulfated Ti-pillared clays and vanadia-doped unsulfated Ti-pillared clays. When sulfated Ti-pillared clay was doped with vanadia, the BET surface areas decrease whereas the diffraction line (001) are not significantly affected. The acidic properties of sulfated catalysts are higher than vanadia doped sulfated samples. The comparison of the activity of the catalysts in the selective catalytic reduction (SCR) of NO by ammonia in presence of oxygen show that vanadia doped sulfated Tipillared clay were highly active for the SCR NO. Therefore sulfated Ti-pillared clay appears as a good support for vanadia catalysts for the SCR reaction. 1.

INTRODUCTION

The selective catalytic reduction (SCR) of NO with ammonia is an industrial process for the abatement of NO from gaseous emissions [1,2]. Among the large number of catalysts active in such reaction, vanadia-based solids have been described as the best ones. Various methods for their preparation have been studied [3, 4]. Many supports have been reported to be active for this reaction, such as vanadia supported on AlzO3, SIO2, TiOz/SiOz, etc. [4, 5], but VzOs/TiOz catalysts present the highest activity and resistance to poisoning by H20 and SO2 [5]. It is accepted that TiO2 in the anatase form is the appropriate support [6]. Commercial SCR catalysts are mostly based on vanadia/titania and WO 3 and/or MoO 3 are usually added to VzOs/TiO2, partly as promoters [7]. Both Lewis and Br6nsted acid sites exist on pillared clays. The acidity depends on the exchanged cations, the preparation method and the starting clay [8]. It is known that surface acidity is important for SCR reaction of NO by NH3 [4, 5, 9]. Different pillared clays were synthesized and tested for their activities in the SCR NO [10, 11]. Research on titanium pillared clays was initiated by Sterte [12], who first reported the synthesis of titanium pillared montmorillonite using TIC14 solution in hydrochloric acid. Bernier et al.

874 [13] later studied the influence of temperature on titanium pillared clay. Khalfallah Boudali et al. [14] prepared titanium pillared montmorillonites by intercalating polymeric cationic species via hydrolysis of TiCI4 with HC1 and H2SO4. Smaller BET surface areas but stronger Br6nsted acid sites were observed on sulfated Ti-modified pillared montmorillonites than on the unsulfated ones. It appeared that the nature of the polymeric Ti species depends on the mineral acid used. Del Castillo et al. [15] carried out a detailed study on the experimental conditions needed to synthesize sulfated Ti-pillared montmorillonite. The sulfation of the pillared clays was performed by intercalation of the clay with the titanium species in the presence of (NH4)2SO4 or H2SO4 and by impregnation of Ti-PILC. The catalytic behavior of the Ti-modified pillared montmorillonites in the selective catalytic reduction of NO by NH3 with or without SO2 has been reported by these authors [16]. It has been shown that vanadia-doped pillared clays represent interesting potential catalysts for SCR NO with ammonia and that the catalytic performance of these systems depends on the method of preparation and on the V content [17]. It was also shown that the addition of SO42- species on TiO2 is attractive since sulfated titania exhibits a good activity for high temperature SCR reactions [18, 19]. It has been proposed that the sulfate produce a strong acidic site at high temperature [20-23], since it is a way to tune the acid-redox properties required. It appears interesting to investigate vanadia doped sulfated Ti-pillared clay for the selective catalytic reduction of NO by ammonia and to compare their performance with sulfated Ti-pillared clay and vanadia doped Ti-pillared clay. In this work, all the catalysts were synthesized under identical conditions and were characterized by different techniques. These techniques included surface area measurement, pore size distribution, Xray diffraction, X-ray photoelectron spectroscopy, TPD-NH3 and chemical analysis of Ti retained by the clay. The catalysts were then tested in the selective catalytic reduction of NO by ammonia in the presence of oxygen at different temperatures. 2. E X P E R I M E N T A L 2.1. Catalyst preparation The initial material used for intercalation was a volclay montmorillonite from CECA. The pillaring solution was obtained by slowly adding TiCl 4 into H2SO4 (6 M) under vigorous stirring. Final concentrations of 0.82 M in titanium and 0.2 M in H2SO4 were reached by adding water; H+/Ti value of 0.24 was thus obtained [14]. The fresh pillaring solution was then added dropwise to 500 cc of a suspension containing 2 g of clay to obtain sulfated titanium pillared clay, in such quantities that final Ti/clay ratios of 10, 20 and 40 mmol/g were obtained. The same procedure was employed using HC1 (6 M) for hydrolysis of TiCh to obtain unsulfated titanium pillared clays. After 24 h of stirring the solid fraction was separated by centrifugation and filtration, then washed several times with distilled water and dried at room temperature. Vanadia doped sulfated and unsulfated Ti-PILC, containing 3 % of vanadium, were prepared by impregnation of the supports with a solution of ammonia vanadate (NH4VO3) 0.1 M dissolved in water acidified by oxalic acid. All the samples were then dried at 80~ for 20 h and calcined at 400~ for 3 h. The catalysts are referenced S-xTi, V-xTi and V-S-xTi in which x is the Ti/clay ratio equal to 10, 20 or 40 mmol/g clay.

875

2.2 Characterization The basal spacings d001 of the samples were evaluated by XRD on CGR theta 60 instrument using monochromatized CuK~ radiation at ~ - 1.54 A. The specific surface areas were measured by the BET method from nitrogen adsorption isotherms using a Micromeritics ASAP 2000 equipment. The thermal stabilities were investigated for all samples at the heating rate of l~ The quantities of titanium retained by the clay were determined by colorimetry using a Philips PYE Unicam PU 8650 spectrophotometer after dissolution of the sample in 6 N sulfuric acid at 80 ~ and addition of 5 ml H202 per mg of Ti [24]. The total acidity measurement was evaluated by temperature-programmed desorption of ammonia (TPD- NH3). For these experiments, a sample weight about 0.150 g calcined at 400 ~ was placed in the cell, evacuated at 400 ~ for 2 h and then cooled to 100 ~ Pure ammonia gas (50 cc.min -1) was adsorbed at 100 ~ for 20 min. Thereafter, while increasing the sample temperature to 400 ~ at a constant rate 10 ~ and maintaining the carrier gas flow rate at 50 ccHe/min, the desorbed ammonia was passed through a 20 wt% HBO3 solution in order to check the amount of NH3 evolved. The NH3 amount was obtained by Kjeldhal method. XPS analysis were performed at room temperature with a Surface Science Instruments SSX-100 model 206 spectrometer with a monochromatized AIKa source operating at 10 KeV and 12 mA. Samples calcined at 400 ~ were compressed in a small cup under a 5 Kg/cm 2 pressure for 30 s and supported on a holding carousel. The binding energies (B.E.) of Ti2p, V2p and S2p lines were referenced to the C ls band at 284,8 eV. Activity measurements were performed in a continuous flow fixed bed reactor operating at atmospheric pressure between 100 ~ and 400 ~ The total flow rate was 100 cc/min and feed composition was: NO (0,1 vol. %), NH3 (0,11 vol. %) and 02 (2,5 vol. %), in helium. The catalyst amount of 0,150 g calcined at 400 ~ and the space velocity were kept constant for all experiments. The inlet and outlet gas compositions were measured using a quadrupole mass spectrometer QMC 311 Balzers coupled to the reactor. 3. RESULTS AND DISCUSSION

3.1. Catalyst characterization The BET surface areas, pore volumes and pore size distributions for all catalysts investigated are summarized in Table 1. S-10Ti sample showed high specific surface area. But, according to the increase of Ti/clay ratio, the surface area was decreased. This result indicates that the higher Ti/clay ratio leads to a progressive plugging of the internal structure of the clay. After addition of vanadium (3 %) to sulfated samples, the surface area and the micropore volume decreased sharply, whereas the pore diameter increased. This effect can be explained by the blocking of small pores due to the building up of the vanadia layer. The BET surface areas of vanadia-doped unsulfated Ti-PILC were near 100 mZ/g. The average value is larger than that of vanadia-doped sulfated Ti-PILC. This may be due to the blockage of small pores by both the vanadium oxides and sulfate ion in the case of vanadia-doped sulfated Ti-PILC. The d001 spacings of the pillared clays are also presented in Table 1 and range from 15.1 to 18.6 ,~ with a maximum of 48 % TiO2 retained by the

876

clay. The basal spacings are not significantly affected by vanadia addition. A study by Einaga [25] on Ti4§ and polymerization suggested the existence of a polymeric cationic species, (TiO)8(OH)84§ However, the structure of the complex is presently unknown. Table 1 Physico-chemical properties of the catalysts prepared in the present study (preparation with H+/Ti = 0,24 and Ti/clay = 10, 20, 40 mmol/g). Catalysts S.A. VpTotal Viap d001 TiO 2 NH3 (mZ/g) (cma/g) (cm3/g) (,~) (wt%) (~mol/m 2) S-10Ti S-20Ti S-40Ti

255 137 100

0.146 0.082 0.058

0.060 0.041 0.033

18.6 15.4 16.2

38.9 32.9 40.0

2.5 5.9 9.0

V-10Ti V-20Ti V-40Ti

103 92 98

0.132 0.068 0.125

0.010 0.020 0.001

17.2 15.2 18.5

39.2 48.5 48.4

6.97

V-S-10Ti V-S-20Ti V-S-40Ti

60 91 87

0.083 0.092 0.078

0.003 0.015 0.012

17.7 15.1 15.9

38.9 32.9 40.0

4.41 3.85 2.75

S.A. :Specific surface areas, VpTotal: Total pore volume, VIap" Micropore volume, d001: basal spacing The effect of thermal treatment was investigated on the samples of the 10Ti series. The specific surface areas were measured according to the different temperatures. As shown in Fig. 1, the BET surface areas of sulfated Ti-PILC decreased with the increase of the thermal treatment temperature, while the surface areas for vanadia Ti-PILC increased. At 400 ~ which is the calcination temperature for SCR reaction, all samples showed almost the same value of specific surface area (near 160 mZ/g). In order to evaluate the total acidity, TPD- NH3 was carried out. The desorbed amount of NH3 was also compared in Table 1. In the case of S-xTi series, it is evidenced that the amount of desorbed ammonia increases with the added amount of sulfate ion during the preparation. In other words, the different Ti/clay ratio also induced the different amount of sulfuric acid added. This result shows that the different amount of sulfate intercalated contributes to the increase of the acidity. When vanadia was added to sulfated Ti-PILC, the amount of NH3 desorbed decreased significantly. R. T. Yang [26] reported that the vanadia create more Br6nsted acid sites with the increase of vanadia from 2 to 6%. Thus, the decrease of the acidity observed in our samples may be explained by the assumption that some of the ammonia desorbed from the surface was oxidized by lattice oxygen of the catalysts. It is known that lattice oxygen of VzO5 can oxidize ammonia to Nz and nitrogen oxides at high temperatures (27).

877 350 ~- S-10Ti

300 "

V-10Ti

--x-

250 -

Ti

t~

200 150 r~

100 50-

50

!

!

I

150

250

350

450

Temperature (~ Fig. 1. BET surface areas of catalysts after calcination at increasing temperatures. Table 2 XPS Data of sulfated Ti-PILC and vanadia doped sulfated Ti-PILC. Binding energy (eV) Catalyst

S/(Ti+V) atomic ratio Ti 2p 3/2

V 2p 3/2

S 2p

S-10Ti

459.3

-

169.2

0.163

S-20Ti

459.2

-

169.1

0.306

S-40Ti

459.2

-

169.1

0.432

V-S- 10Ti

459.1

517.3

169.0

0.336

V-S-20Ti

459.3

517.3

169.2

0.339

V- S-40Ti

459.2

517.4

169.3

0.345

The XPS results of Ti 2p3/2, V 2p3/2 and S 2p on the vanadia doped sulfated Ti-pillared clay catalysts are summarized in Table 2. A broad XPS band near 459 eV was observed on all samples investigated. This value is almost the same as the binding energy of 2p3/2 of Ti on TiO2 [28]. The intensity of this band increases with the Ti concentration, which reflects an increase of the surface concentration of Ti species. A band centered near 517 eV was detected on the vanadium containing samples. This indicates that vanadium on the fresh

878

catalysts was present mainly as the +5 valence form, as VzO5 [28]. During calcination at 400 ~ in air, vanadia was oxidized to V205 by oxygen. Contrary to the previous investigations of the VzOs/TiOz catalysts [29], no asymmetry was detected in the V 2p spectra, testifying the absence of V 4§ contribution. These results indicate that compared to VzO5/TiO2 catalysts [29] the state of V 5+ in the Ti-pillared clay is much more stable. The S 2p spectra of both sulfated Ti-PILC and vanadia doped sulfated Ti-PILC exhibit a band with a binding energy near 169 eV which is attributed to sulfate SO42, typical of S 6§ in SO bonds [19, 28]. Therefore, the XPS results lead to the conclusion that the vanadium was present mainly as the +5 valent form and the sulfur-containing species on all sulfated catalyst surfaces is the S 6§ form as sulfate. The amounts of sulfur in the vanadia-doped sulfated Ti-PILC measured by XPS atomic surface composition evaluated as S/(Ti+V) atomic ratio are also shown in Table 2. The increase of the S/(Ti+V) atomic ratio of the sulfated Ti-PILC is proportional to the sulfate concentration in the Ti-pillaring solution without vanadia. For vanadia doped Ti-PILC, the S/(Ti+V) atomic ratios became almost similar which evidences the homogeneous distribution of sulfate after vanadia doping. 3.2. Catalytic activity The present results of SCR NO in presence of oxygen indicate that vanadia-doped sulfated Ti-pillared clay catalysts are highly active for ammonia SCR (Fig. 2). The activity of the sulfated Ti-PILC increases with temperature of reaction up to 400 ~ but NO conversions are small at 200-350 ~ The activity of these samples is essentially related to the presence of sulfates which enhances the acidity of the catalysts and there is a direct correlation between the SCR activity and acidity as has been already reported [4, 5, 9]. It was shown that the introduction of sulfate ions on the Ti-PILC not only increased Lewis acidity but also the strength of acidic Br6nsted sites [14, 15]. Jung [18] reported that the strong acidic sites, especially the strong Lewis sites generated by modifying TiO2 with SO42- are responsible for the higher reactivity of TiO2-SO42- in the SCR at high temperature. When a small amount of vanadia (3%) was added to the unsulfated Ti-PILC, the NO conversion increased significantly with temperature reaction from 200 to 400 ~ The SCR activity of vanadia-doped unsulfated Ti-PILC was higher than that of sulfated Tipillared clay, suggesting that vanadium played an important role in this reaction. This result is confirmed by the catalytic performance of vanadia sulfated Ti-pillared clay. On these latter catalysts, NO conversion was increased at low temperature (< 200~ but declined slightly at high temperatures (> 350 ~ due to the oxidation of ammonia by oxygen. The NO conversion was higher on vanadia-doped sulfated Ti-PILC than on the other catalysts investigated at any temperature between 100 and 400~ The high activity of vanadia-doped sulfated Ti-PILC in the SCR reaction could originate from the requirement of a surface redox site with a surface acid site for this reaction. For all the catalysts investigated, the increase of the Ti concentration (Ti/clay = 10, 20 or 40 mmol/g) does not have a main role in the SCR reaction.

879 100

-" S-10Ti S-20Ti S-40Ti f V-40Ti r + V-S-10Ti / ~ ---o--V-S-20Ti ///

80 "-" ~9 60 g

--~v

40

,~"

/" /f /

I r /

//?

//i/ ~// [1~'

9

20 0 0

100

200

300

400

500

Temperature (~ Fig. 2. NO conversion vs. temperature

4. CONCLUSION Vanadia doped sulfated Ti-pillared clay are highly active for the reduction of NO by ammonia in the presence of oxygen. The results of SCR reaction show that vanadium present mainly astthe +5 valence played an important role in this reaction. SCR activity was found to be correlated to acidity at high temperature. At low temperatures, the redox properties play a key role for the activity. REFERENCES

1. 2. 3. 4. 5. 6.

V.I. Parvulescu, P. Grange and B. Delmon, Catal. Today, 46 (1998) 233. F. Anssen and R. Maijer, Catal.Today, 16 (1993) 157. G.C. Bond and S.F. Tahir, Appl. Catal., 71, (1991) 1. H. Bosch and F. Janssen, Catal. Today, 2 (1988) 369. G. Busca, L. Lietti, G. Ramis and F. Berti, App. Catal. B Environ., 18 (1998) 1. I.M. Pearson, H. Ryu, W.C. Wong and K. Nobe, Ind. Eng. Chem. Prod. Res. Dev., 22 (1983) 381. 7. L.L. Sloss, "Nitrogen Oxide Control Technology Fact Book." Noyes Data Corporation, Park Ridge, 1992. 8. F. Figueras, Catal. Rev. Sci. Eng., 30 (1988) 457. 9. N.Y. Topsoe, J. A. Dumesic and H. Topsoe, J. Catal., 151 (1995) 241. 10. R. T. Yang, J. P. Chen, E. S. Kikkinides, L. S. Cheng and J. E. Cichanowicz, Ind, Eng. Chem. Res., 31 (1992) 1440. 11. R. T. Yang and J. E. Cichanowicz, Pillared clays as catalysts for selective catalytic reduction of NO, US Pat. 5, 415 (1995) 850. 12. J. Sterte, Clays Clay Miner., 34 (1986) 658.

880 13. A. Bernier, L. F. Admaiai and P. Grange, Appl. Catal., 77 (1991) 269. 14. L. Khalfallah Boudali, A. Ghorbel, D. Tichit, B. Chiche, R. Dutartre and F. Figueras, Microporous Mater., 2 (1994) 525. 15. H. L. Del Castillo, A. Gil and P. Grange, Catal. Lett., 43 (1997) 133. 16. H. L. Del Castillo, A. Gil and P. Grange, Catal. Lett., 36 (1996) 237. 17. K. Bahranowski, J. Janas, T. Machej, E. M. Serwicka and L. A. Vartikian, Clay Miner., 32 (1997) 665. 18. a) S.M. Jung and P. Grange, Catal. Today, 59 (2000) 305; b) S.M. Jung and P. Grange, Appl. Catal. B, 27 (2000) L11. 19. J. P. Chen and R. T. Yang, J. Catal., 139 (1993) 277. 20. K. Tanabe, M. Misono, Y. Ono and H. Hattori, Stud. Surf. Sci. Catal., 51 (1989) 199. 21. H. Hino and K. Arata, J. Chem. Soc. Chem. Comm., (1979) 1148. 22. Y. Tsutomu, Appl. Catal., 61 (1990) 1. 23. J. R. Sohn and H. J. Jang, J. Catal., 136 (1992) 267. 24. G. Charlot "Chimie analytique quantitative" Vol 2, Masson Ed, Paris (1974). 25. H. Einaga, J. Chem. Soc. Dalton Trans., (1974) 1917. 26. R. Q. Long and R. T. Yang, Appl. Catal. B Environ., 24 (2000) 13. 27. Y. Kosaki, A. Miyamoto and Y. Murakami, Bull. Chem. Soc. Jpn., 52 (1979) 617. 28. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Mullenberg (Eds) "Handbook of X-ray Photoelectrons Spectroscopy", Perkin-Elmer, Eden Prairie, 1979. 29. J. Ph. Nogier and M. Delamar, Catal. Today, 20 (1994) 109.

Studies in Surface ScienceandCatalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

881

Advanced preparation by sol-gel method of the encapsulated P d / A I 2 0 3 catalysts for methane combustion S. Fessi a, A. Ghorbel a, A. Rives b and R. Hubaut b aLaboratoire de Chimie des Mat6riaux et Catalyse, D6partement de Chimie, Facult6 des Sciences de Tunis, Campus Universtaire 1060 Tunis, Tunisie. bLaboratoire de catalyse H6t6rog6ne et Homog6ne, URA CNRS 402, Universit6 des Sciences et Technologies de Lille, 59655 Villneuve d'Ascq, France.

In this work, the influence of the preparation method and the thermal treatment on the textural, structural and catalytic activity of Pd/AlzO3 catalyst is investigated. The palladium occluded into the alumina support appears on the surface when the encapsulated catalyst is calcined at 700~ Compared to the catalysts prepared by impregnation method, higher resistance to sintering during calcination up to 800~ and during ageing under reaction mixture at 500~ for 72h is observed on the encapsulated catalyst. The nature of the metalsupport interaction seems to be the reason. Furthermore, the catalytic activity in methane combustion is observed to be dependent both on palladium particle size and metallic surface area. Moreover, during the catalyst ageing under reaction mixture at 500~ less reactive catalytic sites are obtained. The large palladium particles are more inhibited than the small ones. 1. I N T R O D U C T I O N The supported palladium are the most effective catalyst in the combustion process [1]. However sintering is a serious problem causing catalyst deactivation [2]. Many studies were devoted to the thermal stability improvement of the support, either by the help of additives (Ba, La and Si oxides),[3-5] or by the preparation of a thermally stable support, using an adequate synthesis procedure, such as the sol-gel method [5-8]. Unfortunately, this was insufficient to maintain a good metal dispersion at high temperatures. Many studies suggest that a strong metal-support interaction should be among the required properties needed to improve the supported palladium catalysts [5,7]. For these reasons, in this work, two different sol-gel method are used to improve the thermal stability of alumina supported palladium catalysts. Moreover, since the catalysts are applied to function at around 700~ during the ignition step of the methane combustion process [2], the thermal treatment effect on the texture, the structure and the activity of the catalysts are investigated in this paper.

882

2. EXPERIMENTAL 2.1. Catalyst preparation Sol-gel alumina, (AI203-SG): a mixture of aluminium-sec-butoxide (Across, 97 %), (AsB) and sec-butanol (Across, 99 %), (sB) is aged for 20 mn, ([AsB] =IM). Then, acetic acid (Across, 99.8 %), (AcA) is added as AsB modifier and water producer in-situ with the concentration molar ratio [AcA]/[AsB] = 4. The obtained gel is finally oven dried at 70~ for 24 h and calcined in flowing oxygen (1.8 l/h) at 800~ for 2 h.

Sol-gel alumina supported palladium catalysts: Method A, (SG catalyst): a mixture of AsB and sB is aged for 20 mn, ([AsB] =IM), followed by addition of palladium acetylacetonate, Pd(acac)2, (Fluka, 34 wt.-% Pd), with a designed metal loading of 2 wt.-%. The mixture is then kept for an appropriate time under constant stirring. After, AcA is added, ([AcA]/[AsB] = 4). The obtained gel is finally oven dried at 70~ for 24 h, calcined in flowing oxygen (1.8 l/h) at 500, 700 or 800~ for 2h and reduced in flowing hydrogen (1,2 l/h) at 500~ for 1 h. Method B, (SGI catalyst): the sol-gel alumina calcined at 800~ is stirred for 24 h with an acetone solution containing dissolved Pd(acac)2. The designed metal loading is 2 wt.-%. The solvent is then evaporated and the residual solid is dried in oven at 110~ for 24h, calcined in flowing oxygen (1.8 l/h) at 500, 700 or 800~ for 2 h and reduced in flowing hydrogen (1,2 l/h) at 500~ for 1 h. Reference alumina supported palladium catalyst, (I catalyst): A commercial alumina (oxide C, from Degussa) is impregnated with palladium(II) chloride solution acidified with hydrochloric acid to facilitate the dissolution of PdC12 (Fluka, 60 wt.-% Pd) as [PdCl4] 2-. The designed metal loading is 2 wt.-%. Excess water is then removed by evaporation, followed by drying in oven at 110~ for 24h. The catalyst is finally calcined in flowing oxygen (1.8 l/h) at 500, 700 or 800~ for 2h and reduced in flowing hydrogen (1,2 l/h) at 500~ for lh.

2.1. Characterization methods Specific surface area was determined by the BET method from the nitrogen adsorption at 77 K, using an automatic Micrometrics ASAP 2000. Palladium metal dispersion was determined by dynamic pulsed hydrogen chemisorption. The metallic average particle size of palladium was examined by transmission electron microscope (JEOL 100 CX) with a resolution of 0.3 nm. Chemical analysis allowing the determination of palladium and chloride contents were performed respectively by inductively coupled plasma spectrophotometry and by potentiometry. XPS analyses were performed on a Leybold-Heraeus LHS10 spectrometer using a non-monochromatized Al Ka source (hv = 1486.6) operated at 300 W (15kV, 20 mA). The energy resolution of the instrument was 0.80 eV. The standard deviation of the analysed result is 0.02 eV. The error from the charge effect was calibrated using the binding energy of the Al 2p photopeak as a internal standard assuming a value of 74.70 eV. Catalytic activity for methane combustion was determined over the reduced catalyst (fresh catalyst, 100 mg), in a dynamic microreactor. The flow of 1 vol.-% methane, 4 vol.-% oxygen and balance helium were mixed. The total flow rate was regulated at 6 l/h and admitted at 500~ The effluent gas was led directly to the six port gas sampling valve of a gas chromatograph (Intersmat IGC 120 ML) for analysis using a

883 thermal conductivity detector and a porapak Q column heated at 100~ under stream reaction was fixed at 72 h.

The ageing time

3. R E S U L T S AND DISCUSSION 3.1. BET Surface area measurements The nitrogen physical adsorption results show that SG-500 catalyst has a higher BET surface area SGI-500 and 1-500 samples (Table 1). When the catalysts are calcined at 700 or 800~ when they are aged under the reaction mixture at 500~ for 72 h, no significant BET surface area loss is observed for samples SGI and I. However, a considerable decrease of the surface area is observed for SG-500. In addition, it is important to note that the SG samples still show the highest BET surface areas for the different calcination temperatures and after the ageing step.

3.2. X-ray diffraction measurements The XRD measurements show an amorphous phase for the SG-500 catalyst and a mixture of 3' and 6-alumina for SGI-500 and 1-500. When the calcination temperature increases to 700 and 800~ the same alumina phases are conserved in SGI and I. However, transformations to ~,-alumina and to a mixture of ~,and 6-alumina are observed, respectively, for SG-700 and SG-800. After ageing under the reaction mixture, only the amorphous alumina of sample SG-500 changes to the ~/phase. According to these results, a good correlation between the textural and the structural characterisation is observed. In fact, the surface area drops are related to the formation of a thermal stable alumina structure. Moreover, the sol-gel method appears to favour the thermal stability of the alumina support, since the surface area of the SGI catalysts are higher than that of the I samples. In addition, the introduction of Pd(acac)2 during the first step of the SG catalyst preparation seems to have also a positive effect on the alumina resistance to sintering, since the surface area of the SG solids is higher than that of the SGI samples. 3.2. Metal dispersion measurements The hydrogen chemisorption measurements performed on the catalysts calcined at 500~ show a relatively high palladium dispersion for the SGI-500 catalyst (D -36%), and similar values for the SG-500 and 1-500 samples (around 20 %, Table 1). However, in spite of the similar palladium contents (around 2 %) of SG-500 and 1-500, the TEM results show larger average particle diameter (5.3 nm) on the 1-500 catalyst than for the SG-500 sample (2.7 nm). In addition, compared to the SGI-500 and 1-500 catalysts, only the average palladium particle diameter of the SG-500 catalyst deduced from TEM micrographs is not in accordance with the value determined from hydrogen chemisorption measurements. This can be explained by the fact that the fraction of palladium unable to chemisorb hydrogen is not only in the bulk of the SG palladium particles, but also in unreachable regions of the support. This metallic fraction could be inserted in inaccessible pores or surrounded by alumina. Similar results were reported by Khelifi et al. [9] and Balakrishnan et al. [8] when supported palladium or platinum alumina catalysts were prepared by the metal incorporation during the first step of alumina synthesis by a sol-gel method.

884 When the calcination temperature increases, the metal dispersion on catalysts I and SGI decreases. However, it increases from SG-500 to SG-700 and then decreases in SG800. The palladium particle size distributions deduced from TEM characterisation show principally small palladium particles on SG-500 sample (1 to 10 nm) with a narrow metallic particle size distribution and an average particle diameter of 2.7 nm (Fig.l), and larger palladium particles on SG-800 sample (1 to 50 nm), with a wide metallic particle size distribution and an average particle diameter of 7.6 nm. Nevertheless, in spite of the mixture of small and large particles on SG-700 catalyst (1 to 40 nm), the corresponding metallic particle size distribution is narrow and the average particle diameter (1.9 nm) is lower than that obtained for SG-500. This apparent contradiction can be explained by the increase of the small number of palladium particle caused by the re-dispersion of the occluded palladium particles during the calcination from 500 to 700~ This is illustrated by the intensification of the peak of 1 nm particle size distribution of the SG-700 (Fig.l) and by the increase of the metal dispersion on this catalyst. Moreover, the accordance between the average particle diameter determined from hydrogen chemisorption and from TEM micrographs of sample SG-800 suggests that practically the total amount of palladium (2 %) has appeared on the alumina surface. Table 1 BET Surface area, metal dispersion, average particle diameter and chloride content of fresh and aged catalysts. Sample

S BET (mZ/g)

D (%) Ha-chemisorp.

Average particle diameter (nm)

500 700 800

500 700 800

SG fresh state aged state

337 215

21 18

45 41

15 14

5.4 2.5 6.3 2.8

7.5 8.0

SGI fresh state aged state

148 131 146 127

132 137

36 25

16 14

5 5

3.1 7.0 4.5 8.0

22.5 22.5

96 94

20 12

10 9

4 4

5.6 9.4

192 167 194 152

500 700 800

C1 Pd (wt-%) (wt-%) 500 800

500 1.95

-

-

2.05 -

I

fresh state aged state

106 108

97 96

11.3 28.2 0.74 0.67 1.90 12.5 28.2 <0.05 <0.05 -

Furthermore, it is also important to note that a pronounced metal sintering occurs over the I and SGI catalysts compared to SG. Indeed, compared to the samples calcined at 500~ the 1-800 and the SGI-800 catalysts kept, respectively, only 20 % and 14 % of the initial metallic dispersion values, whereas 7 1 % of the initial metallic dispersion is kept for the SG-800 catalyst (Table 1). This result suggests a relatively higher resistance to sintering for the SG catalyst, which is also observed during the ageing step under reaction mixture.

885 Indeed, the aged 1-500 and SGI-500 catalysts kept, respectively, 60 % and 69 % of their initial metallic dispersion values, whereas 86 % of the initial metallic dispersion was kept on the aged SG-500 catalyst (Table 1). In order to investigate the metal-support interaction suspected to be responsible of this improved thermal stability, XPS measurements were performed on the supported palladium catalysts.

3.3. XPS analysis The XPS characterisation of the Pd/Al203 catalysts shows that the binding energy values are typical of the oxide palladium [10-11] (Table 2). Nevertheless, the Pd 3d peak of the SG-800 catalyst is at lower energy position than in the SGI-800 and 1-800 catalysts. The binding energy shift to lower values of about 0.8 eV. Barr [12] suggests that the formation of a mixed oxide AjOkBt from different simple oxides, AxOy and BmOn is associated with a shift in the binding energy of the respective peaks compared to the binding energies of the source oxides peaks. According to these results, the shift of the Pd 3d5/2 lines for the SG-800 catalyst can be interpreted by a strong palladium-alumina interaction compared to SGI-800 and 1-800. This result can explain the improved resistance to sintering of the SG samples. In addition, the analysis of the intensity ratio (Ipa/ IA~) listed in Table 2, and obtained from the integrated areas of the Pd 3d5/2 and the A1 2p peaks after a gaussian deconvolution, shows that the higher ratio is obtained for the SG800 catalyst (0.25). This confirms that the palladium agglomeration is less pronounced on this sample compared to SGI-800 and 1-800 catalysts.

Fig.1. Average particle size distributions of the SG catalysts.

886 Table 2 Binding energy of the Pd 3d5/2 and intensity ratio (Ipd/ IAl) of the alumina supported palladium catalysts calcined at 800~ Sample

Pd 3ds/z (eV)

IPd/IAl

SGI-800 1-800 SG-800 SG-800-aged

337.3 337.3 336.5 336.5

0.16 0.09 0.25 0.27

3.4. Catalytic activity The catalytic activity for methane combustion increases with the calcination temperature for the SG and the I catalysts, but for the SGI samples, the activity increases from SGI-500 to SGI-700 and then decreases for SGI-800 (see Table 3). For the catalysts calcined at 500 and 700~ the SGI solids show better activity than the SG ones, which also show higher activity than the I samples. However, when the catalysts are calcined at 800~ the activity order is modified and the SG-800 sample shows the best activity, followed by the SGI-800 and the 1-800 samples. Moreover, an increase of the TON (turn-over number) with the palladium particle size, which confirms the sensitivity of the methane oxidation reaction to the structure [13-14], is observed. On the other hand, in spite of the relatively high TON of the SGI-800 catalyst, the catalytic activity shows a drop on this sample, and in spite of the relatively low TON of the SG-700, high methane conversion is obtained over this catalyst. These results can be explained by the catalytic activity dependency both on palladium particle size and metallic surface area. Indeed, for a given metal content, an optimum metal particle size seems to be needed to maximise the catalytic activity. This conclusion can be illustrated by the highest activity obtained over the SG-800 and the SGI700 samples, which show an average palladium particle size around 7 nm. A similar conclusion was advanced by Mar6cot et al. [15] in the study of the propane and the propene oxidation on alumina-supported palladium and platinum catalysts. For propane oxidation, the optimum palladium particle size corresponds to near 5.6 nm. These results are in accordance with previous works on hydrocarbons oxidation [13,16], and are generally explained by the competitive adsorption between hydrocarbon and oxygen on the same catalytic sites and by the reactivity difference of the adsorbed oxygen. It is also important to note that, in spite of the larger average palladium particles obtained for the 1-500 catalyst (5.6 nm) compared to SG-500 (2.7 nm) and to SGI-500 (3.1 nm), a lower TON is obtained on this catalyst (Table 3). As was reported [15,17-20], the poisoning effect of chloride was advanced to be the cause for these results. Nevertheless, higher TON (2577 h -1) is obtained on the 1-800 catalyst compared to SGI-800 (2063 h-l). This result cannot be explained only by the difference between the particle size of the two catalysts, but also by the decrease of the negative chloride effect on the 1-800 catalyst. According to the chemical analyses results, similar chloride amounts are obtained for the 1500 and the 1-800 catalysts (Table 1). Consequently, the decrease of the poisoning effect

887 cannot be explained by the chlorine removal when the calcination temperature increases. However, the decrease of the inhibition effect with the growth of the palladium particle size or/and the migration of chloride to the alumina support could explain these results. Obviously, more investigation should be performed to confirm these suggestions. Furthermore, when the catalysts are aged at 500~ under reaction mixture, they show an activity decrease. The activity order of the aged catalysts is changed compared to the fresh ones, with the highest methane conversion being obtained over the SG-700 sample. These results are also correlated to the dispersion values determined on the aged catalysts. The similar TON obtained on the aged catalysts can explain this result. Indeed, with similar activity of the catalytic sites, the metallic surface area remains the determining factor for the activity of the catalysts. Yet, the activity drop cannot be explained only by the loss of the metallic surface area. Indeed, despite the absence of metal sintering in the catalysts calcined at 800~ and aged at 500~ a considerable activity loss and low TON are obtained. Moreover, the ratio of the TON obtained on the fresh catalyst to that obtained on the aged one increases with the palladium particle size (Table 3). These results suggest that less reactive catalytic sites are obtained during ageing, and that the large palladium particles are more inhibited than the small ones. Table 3 Methane conversion and turnover number (TON) calculated at 325~ catalysts calcined at 500 and 800~ Catalyst

Methane conversion (%)*

TON (h-1)

of flesh and aged

fresh TON / aged TON

Calcination temperature (~

SG fresh state aged state SGI fresh state aged state

500

700

800

500

700

800

74 12

95 26

100 8

472 89

283 85

893 77

96 19

99 9

81 4

340 97

788 82

2063 102

13 9

55 7

75 4

89 103

756 107

2577 137

I

flesh state aged state

500

700

800

5.3

3.3

11.6

3.5

9.6

20.3

0.9

7.0

18.8

*All the methane conversions are determined over 100 mg o f catalyst.

Many works have been devoted to investigate the deactivation causes of the supported palladium catalysts in methane oxidation [13,21-23]. The inhibition of the catalytic sites by water through the formation of palladium hydroxide [21], the change in the distribution of the particle sizes [13], the extensive oxidation of the palladium oxide [22]

888 and its interaction with the support [22] are described in the literature as deactivation causes. According to our previous work, the in-situ XPS characterisation of the SG catalyst treated under reaction mixture has shown that the carbonated species deposed during the ageing step seems to be the deactivation cause [23]. Consequently, with the absence of a solution to avoid the inhibition of the catalyst under reaction mixture and according to the advanced results, it seems important to favour the small palladium particles on the aluminasupported palladium catalyst in order to maximise the catalytic activity of the aged state. So, the sintering resistance improvement remains an efficient tool to reach this goal. Accordingly, the preparation of the encapsulated Pd/AI203 catalyst by sol-gel method, as described in this work, seems to be a promising way for the improvement of the thermal stability and catalytic activity, mainly when the palladium re-dispersion phenomenon is well controlled. 4. C O N C L U S I O N S The following conclusions can be drawn from this work: - The palladium fraction occluded into the encapsulated Pd/AIaO3 catalyst prepared by sol-gel method can appear on the alumina surface when the catalyst is calcined at 700~ which leads to an increase of the metallic surface area compared to the sample calcined at 500~ - Compared to the catalysts prepared by an impregnation method, higher resistance to sintering during calcination at 800~ and during ageing under reaction mixture at 500~ is observed for the encapsulated Pd/AI203 catalyst prepared from a uniform mixture of aluminium and palladium organic precursors. The nature of the metal-support interaction favoured by this preparation method is strongly suspected to be the reason. - The catalytic activity for methane combustion is dependent on both palladium particle size and metallic surface area. For a given metal content, an optimum metallic particle size is, indeed, needed to maximise the catalytic activity. - The poisoning effect of chloride observed for the catalyst prepared by impregnation with the PdCI2 precursor and calcined at 500~ decreases when the solid is calcined at 800~ The decrease of the inhibition effect of chloride with the growth of the palladium particle size and/or the migration of chloride to the alumina support seems to be the cause. During the catalyst ageing under reaction mixture at 500~ less reactive catalytic sites are obtained and the large palladium particles are more inhibited than the small ones. -

REFERENCES 1. 2. 3. 4.

Y. F. Yu Yao, Ind. Eng. Chem. Prod. Res. Dev., 19 (1980) 293. H. Lee Joo and D. L. Trimm, Fuel Process. Techn., 42 (1995) 339. M. Machida, K. Eguchi and H. Arai, J. Catal., 103 (1987) 385. H. Schaper, D. Amesz, E. Doesburg and L.Van Reijen, App. Catal., 9 (1984) 129.

889 5. Y. Mizushima and M. Hori, Appl. Catal., 88 (1992) 137. 6. K. Masuda, T. Sano, F. Mizukami, T. Tukezaki and K. Kuno, Appl. Catal., 4 (1994) 187. 7. T. Lopez, I. Garci and R. Gomez, Mat. Chem. Phys., 36 (1994) 187. 8. Y. Balakrishnan and R.D.J. Gonzalez, J. Catal., 144 (1993) 395. 9. L. Khelifi, E. Garbowski, M. Primet and A. Ghorbel, J. Chim. Phys., 94 (1997) 2016. 10. C. Xiaoping, C. Lili, Y. Wenqing and Y. Xiaoyan, Surf. Interface Anal., 24 (1996) 662. 11. B. Laszlo, N. Istvan, S. Zoltan and G. Laszlo, Appl. Catal., 147 (1996) 95. 12. T.L. Barr, Modern ESCA (CRC Press, Boca Raton, 1994). 13. R. F. Hicks, H. Qi, M. L. Young, and R. G. Lee, J. Catal., 122 (1990) 280. 14. C. F. Cullis and B. M. J. Willatt, Appl. Catal., 83 (1983) 267. 15. P. Mar6cot, A. Fackche, B. Kellali, G. Mabillon, M. Primet and J. Barbier, Appl. Catal. B, 3 (1994) 283. 16. Y.F. Yu Yao, J. Catal., 87 (1984) 152. 17. K. Otto, J.M. Andino and C.L. Parcks, J. Catal., 131 (1991) 243. 18. H. Lieske, G. Lietz, H. Spindler and J. Volter, J. Catal., 81 (1983) 8. 19. J. Barbier, D. Bahloul and P. Marecot, Catal. Lett., 8 (1991) 327. 20. D.O. Simone, T. KenneUy, N.L. Brungard and R.J. Farrauto, Appl. Catal., 70 (1991) 87. 21. C.F. Cullis and T.G. Nevell, Proc. Roy. Soc. Ser. A, 349 (1976) 523. 22. Se.H. Oh, P.J. Mitchell and R.M. Siewert, J. Catal., 132 (1991) 287. 23. S. Fessi, A. Ghorbel, A. Rives and R. Hubaut, Stud. Surf. Sci. Catal., 130 (2000) 3795.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

891

Non-ionic surfactant templated synthesis of mesoporous silica in the presence of platinum salts M.A. Aramendia, V. Borau, C. Jim6nez,* J.M. Marinas, F.J. Romero,* and F.J. Urbano Department of Organic Chemistry, University of C6rdoba, Campus de Rabanales, Edificio C-3, Ctra. Nnal. IV, Km 396,m 14014 C6rdoba, Spain

Various mesoporous silica materials were synthesized using a non-ionic surfactant as template in the presence of different platinum salts in the synthetic gel. The salts were found to influence the structure and porosity of the solids obtained. Thus, our results show that platinum salts promote the hydrolysis of tetraethylorthosilicate (TEOS) and the presence of (NH4)zPtCI4 or HzPtCI6 leads to the formation of materials with smaller pore sizes and less condensed than those obtained in the absence of platinum salts or in the presence of (NH3)4PtCIz. 1. I N T R O D U C T I O N Ever since it was first reported by researchers at Mobil [1,2], the synthesis of mesoporous solids has aroused vast interest in various fields by virtue of their high potential as catalysts [3], sorbents [4], and optical or conducting materials [5]. Mesoporous solids are synthesized by using surfactant assemblies as templates; this ensures the formation of ordered mesostructured materials [6]. As a rule, these materials possess some advantages such as high thermal stability, pore size uniformity and surface area. In this work, we used an electrically neutral polyethylene oxide surfactant and a neutral inorganic precursor to obtain solids of the so-called MSU-X type [7]. This type of surfactant promotes framework assembling through hydrogen bonding between the hydrophilic (EO)n segments and the silanol groups of the neutral inorganic precursor. These materials exhibit a wormhole structure that lacks regular channel packing order; however, they possess uniform channel diameters over a range comparable to M41S materials. The low cost and ready biodegradation of the surfactant are two major advantages of this synthetic procedure. The influence of a number of factors including surfactant type [7], temperature [8], hydrogen and Na + ion concentrations [9], and the presence of fluoride [10] on the synthesis of mesoporous silica has been examined by using non-ionic surfactants as templates. Our research group has studied the effect of using platinum salts in the synthesis of mesoporous silica with a view to the preparation of Pt-supported silica solids [11], which possess a high catalytic potential. In addition, this methodology could be extended to the synthesis of other metals supported on alternative materials compatible with the sol-gel route.

* To whom correspondence should be addressed.

892 However, whether or not the metal is incorporated into the support, including these salts in the synthetic gel causes substantial morphological and porosity changes in the resulting solids. This paper reports on the structural and porosity effects of different platinum salts included in the synthetic gel used to synthesize various types of mesoporous silica solids in the presence of a polyglycol ether (non-ionic) surfactant as template. 2. E X P E R I M E N T A L

2.1. Synthesis A reference mesoporous silica, MSU-1, was prepared by following a previously reported procedure [7] that involves the use of TEOS as silica source and Tergitol 15-S-12 [CH3(CH2)ln(OCH2CHa)12OH] as template. All other solids were obtained by a similar procedure but including 1 mol% Pt (with respect to Si) in the synthetic medium. The platinum salts tested, viz. (NH4)2PtCh, H2PtCI6 and (NH3)4PtCI2, yielded the solids named MSU-I-1, MSU-1-2 and MSU-1-3, respectively. After addtion of the Pt salt, the pH of the synthetic medium was about 4, 2 and 6, respectively. The Si/Pt/surfactant/H20 mole proportion in the initial mixture was 10:0.1:1:560. By way of example, the synthetic procedure for one of the solids is described. Thus, solid MSU-I-1 was obtained by pouring 10 mL of TEOS over a solution containing 0.171 g of (NH4)2PtCh in 46 mL of 0.1 mol L -1 Tergitol 15-S-12 at room temperature. Following stirring for 24 h, the suspension was allowed to stand for 3 days and the solid was filtered, air-dried and calcined at 873 K for 3 h. All other solids were obtained by using a similar procedure and the appropriate platinum salts.

2.2. Characterization X-ray diffraction patterns were recorded on a Siemens D5000 diffractometer using C u K a radiation. Thermogravimetric and differential thermal analysis curves were recorded on a Setaram Setsys 12 thermal analysis station by heating in an argon atmosphere from 25 to 1200 ~ at a rate of 5 ~ min -1. Samples were used untreated. The Pt content was determined by the Service Central d'Analyse, CNRS (Vernaison, France) and the microanalyses (C, H) were performed at Complutense University (Madrid, Spain). N2 isotherms were determined on a Micromeritics ASAP 2000 analyzer. 1H MAS NMR, 29Si MAS NMR and ~3C CP MAS NMR spectra were recorded at 400.13, 79.49 and 100.61 MHz, respectively, on a Bruker ACP-400 spectrometer at room temperature. An overall 1000 free induction decays were accumulated. The excitation pulse and recycle time for 1H MAS NMR spectra were 5 ~ts and 3 s, respectively, those for 29Si MAS NMR spectra 6 pts and 60 s and those for ~3C CP MAS NMR spectra 6 pts and 2 s. Chemical shifts were measured relative to a tetramethylsilane standard. Prior to measurement, if necessary, samples were dehydrated in a stove at 423 K for 24 h.

893 3. RESULTS AND DISCUSSION The XRD patterns obtained exhibited a low angle peak (d~00), with a d spacing between 37 and 66 ,~ (Table 1). The second-order peaks obtained at higher incidence angles were broad and short. These are typical of mesostructured materials with a sponge-like or worm-like pore channel structure, which are devoid of any regular long-range order, as expected for materials assembled using neutral or non-ionic surfactants [7,8]. The low angle dl00 peak was shifted to a higher angle upon calcination, thus indicating gradual contraction of d-spacing in the lattice upon removal of the surfactant. 1H MAS NMR and 13C CP MAS NMR spectra (Fig. 1) confirmed the presence of the surfactant molecules in the pore channels of these materials. The surfactant used exhibits several peaks, namely: two at 3.6 and 3.4 ppm due to CHz groups in the ethylene oxide units; two others at 1.5 and 1.3 ppm due to CHz groups in the alkyl chain; and one at 0.9 ppm due to the CH3 end group in the alkyl chain. None of these signals were found in the materials subjected to the thermal treatment. Noteworthy is the absence of a signal at 1.8-2.3 ppm, which corresponds to non-acidic (silanol) OH groups. It is indeed present after calcination of the as-synthesized materials at 600 ~ [12]. Table 1 Physicochemical properties of mesoporous silica samples Material dl00 lattice dl00 lattice BET surface spacing (~)a spacing (,~,)b area (mz g-l) MSU-1 55 644 MSU-I-1 41 38 851 MSU-1-2 43 37 606 MSU-1-3 66 56 707 a As synthesized, b Calcined in air at 873 K for 3 h.

Pore volume (mL g-l) 0.96 0.42 0.30 0.87

BJH pore size (.~) 37 19 19 38

Table 2 gives the C percent weights and the corresponding C/H ratios for the different solids studied. All of them showed a C/H ratio lower than expected since they contain occluded residual water in addition to surfactant molecules (C39H80013), which possesses a C/H ratio of 5.81. The surfactant content for each sample could be calculated from the C content, giving values of 56.9, 43.3, 44.7 and 38.4 % for materials MSU-1, MSU-I-1, MSU1-2 and MSU-1-3, respectively. Table 2 Properties of silica samples prepared in the presence of platinum salts Material C content C/H ratio Pt content Qn/Q3ratio a (wt%) (wt%) MSU- 1 35.20 5.36 2.3 MSU-I-1 26.82 5.05 2.08 1.0 MSU-1-2 27.68 4.82 0.36 1.0 MSU-1-3 23.80 5.01 0.23 2.1 a As synthesized, b Calcined in air at 873 K for 3 h.

Q4/Q3ratio b "3.9 1.9 2.9 3.6

894 1H MAS NMR

13C CP MAS NMR

Surfactant

i

.VISU-1-3

MSU-1-2

MSU-I-1

U I MSU-1

I

i

10

'

i

'

5

i

0 ppm

'

i

-5

80 70 60 50 40 30 20 10 0 ppm

Fig. 1. IH MAS NMR and 13C CP MAS NMR spectra for the as-synthesized solids.

895 Fig. 2 shows the TGA and DTA curves for the solids studied. The total weight losses exhibited by solids MSU-1, MSU-I-1, MSU-1-2 and MSU-1-3 were 63, 51, 51 and 43 %, respectively. The first loss, below 100 ~ was associated to an endothermal peak and assigned to water desorption [13,14]. The other losses gave endothermal peaks between 180 and 550 ~ and were assigned to the desorption and decomposition of the surfactant. The weight loss around 300 ~ has been ascribed to the departure of micellar surfactant [9]. The main peaks are shifted to higher temperatures for solids MSU-I-1 and MSU-1-2 indicating a stronger interaction between the surfactant molecules and the silica walls. Taking into account the surfactant content and the thermogravimetric analyses, the silicon to surfactant ratio could be calculated. Ratios around 8, 14, 14 and 19 are obtained for solids MSU- 1, MSU- 1-1, MSU- 1-2 and MSU- 1-3, respectively.

0

120 -

-10

l~

':,t

100

:' !ii

80

o~ -30

~ 60

-50

..--.-..._

-60 j

-

-70

t 0

t

t

20 t

i

I

200 400 600 800 10001200 T (~

I.'

40

.

/

0

i 0

200

'

i 400

'

I

I

600

800

T (~

Fig. 2. TGA (left) and DTA (right) curves for as-synthesized materials: MSU-1 (.... ), MSU-11 (--), MSU-I-2 (...), MSU-1-3 ( ~ ) . Table 1 also gives the surface areas, pore diameters and pore volumes of the solids. As can be seen, all possess a high specific surface area (above 600 m 2 g-l). The N2 adsorptiondesorption isotherms are of Type IV (i. e. typical of mesoporous solids) for MSU-1 and MSU1-3 (Fig. 3), with a broad but well-defined adsorption step at P/Po ratio of 0.35-0.70 and hysteresis in the desorption branch over the same pressure ratio range. The pore size distribution of these solids fell in the mesoporous range (35-50 and 32-42 ,~). The hysteresis cycles may be of the H1 and H2 types (Fig. 4), which can be ascribed to the presence of open cylindrical capillaries [15]. The presence of hysteresis loop above p/po- 0.8 is suggestive of textural mesoporosity. Also, MSU-I-1 and MSU-1-2 exhibit a Type I isotherm, with the step at a relative pressure around 0.2-0.4 due to the adsorption of framework confined pores. Nitrogen isotherms of this kind are typical of pores with sizes in between the micro and meso ranges [16]. The micropore distribution in solid MSU-I-1 as determined by the HorvathKawazoe method confirms the presence of pores in this size range - most of size about 7 ,~ (Fig. 4) [17].

896 350 700 300 -

600 ~o 500 -

,t:x0 250 -

.~fi 4 0 0 -

E 200r

i

r

300-

.L)

O

0

o,]

"~ 2 0 0 r

0.0

j

_

..

__

"

_ A

15010050-

Z 1000

(a) . . . . . .

~ 0.2

i 0.4

i 0.6

i 0.8

0 1.0

0.0

i 0.2

i 0.4

P/Po

J 0.6

i 0.8

1.0

P/Po

Fig. 3. N2 adsorption (solid line) and desorption (dotted line) isotherms for mesoporous materials (left): MSU-1 (a) and MSU-I-1 (b), and for materials between the micro and meso ranges (right): MSU-1-2 (a) and MSU-1-3 (b).

I

I

I

6

8

10

r (,~)

1

20

40

60

80

100

r (,~)

Fig. 4. Horvath-Kawazoe pore diameter (r) distribution of MSU-I-1 (left) and BJH pore diameter distribution of MSU-1 (right). The solid-state 298i MAS NMR spectra for the solids (not shown) exhibited several signals centred at a b o u t - 1 1 0 , -101 and -91 ppm that can be assigned to the Q4 [(SiO4)Si], Q3 [(SiO3)SiOH] and Q2 [(SiO)2Si(OH)2] sites, respectively, of the framework. Table 2 gives the Q4/Q3 ratios - determined by deconvolution analysis of the spectra - for the solids studied. The Q4/Q3 ratios for as-synthesized materials MSU-I-1 and MSU-1-2 are lower than those for MSU-1 and MSU-1-3; this suggests a higher degree of polymerization and cross-linking -

897 and hence a greater content in OH groups per gram of catalyst - in the latter two. Calcination of these solids yields more condensed materials, as can be inferred from their increased Q4/Q3 ratios; differences continue to exist among them, however. Solid MSU-I-1 is that exhibiting the highest proportion of QZ units (10.5%), which, however, decreases upon calcination (to 4.5%). The as-synthesized solids MSU-1, MSU-1-2 and MSU-1-3 have a proportion of QZ sites of 4, 5 and 3 %, respectively. While the Q4/Q3 ratio for solid MSU-1 is consistent with previously reported values [8,18], our results clearly show that the addition of a platinum salt to the synthetic gel results in less condensed materials - particularly with (NH4)zPtCI4 and HzPtCI6. The effect is of the opposite sign to that of fluoride, which promotes the polymerization of silica and hence mesoporous materials condensed to a higher extent are obtained [ 10]. Taking into account that the QZ and Q3 sites are located at the surface of these materials, it would be possible to deduce the number of silicon atoms interacting with each surfactant molecule. Thus, the surface silicon atoms to surfactant molecules ratio is about 3, 8, 7 and 6 for solids MSU-1, MSU-I-1, MSU-1-2 and MSU-1-3, respectively. These results indicate that platinum salts, specially (NH4)zPtCI4 and HzPtCI6, where platinum is forming anionic complexes, induce extensive hydrolysis of TEOS. Several anions (e.g., halides, SO4z-, NO3) have been found to increase the rate of hydrolysis of the TEOS [19]. Monomeric silica species interact with the surfactant headgroups affording surfactantinorganic complexes. If the condensation between silicic units (Equation 2) is slower than the hydrolysis of the silica precursor (Equation 1), the rate of formation of nucleus able to grow decreases, giving rise to larger particles, as observed previously by SEM for solids MSU-I-1 and MSU-1-2 [12]. If the hydrolysis rate is low in comparison to condensation rate, the easy condensation between silanol groups on the surface of different micelles brings about a more rapid nucleation and therefore the formation of smaller particles [20]. -Si-OEt + HzO ~ =Si-OH + Et-OH -Si-OH + HO-Si = ~ =Si-O-Si-+ HzO

(1) (2)

As can be seen from Table 2, the amount of platinum incorporated into the silica was only significant when (NH4)zPtCh was present in the synthetic medium, which implies that this salt is strongly interacting with the surfactant-inorganic complexes during the gelling process. The complexation of small cations by the EO groups of polyethylene oxide surfactants [21], in a similar way to crown ether surfactants [22], is well known. In our case, these interactions would lead to the formation of complexes between the surfactant headgroups and the ammonium cations, which would be surrounded by the counter anions PtCh z-. The uncomplexed EO oxygen atoms of the surfactant would form hydrogen bonds with the silanol groups. 4. C O N C L U S I O N In summary, we synthesized various types of silica by using a non-ionic surfactant as template in the presence of different platinum salts in the synthetic gel. The salts were found to induce substantial structural and porosity changes in the materials obtained. In fact, they promote the hydrolysis and condensation of the silicon precursor; the degree of cross-linking

898 achieved, however, depends on the particular salt employed. The procedure can theoretically be extended to the synthesis of other materials obtained by hydrolysis of a precursor; in the presence of suitable salts of platinum or other noble metals, this should allow one to prepare different materials by properly altering other reaction variables (e.g. pH, presence of fluoride). ACKNOWLEDGMENTS The authors would like to thank Spain's Direcci6n General de Investigaci6n, Ministry of Science and Technology, for funding this research within the framework of Project BQU20012605, and Junta de Andalucia for additional financial support. REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992), 710. 2. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Corma, Chem. Rev., 97 (1997) 2373. 4. L. Mercier and T. J. Pinnavaia, Adv. Mater., 9 (1997) 500. 5. F. Marlow, M. D. McGehee, D. Zhao, B. F. Chmelka and G. D. Stucky, Adv. Mater., 11 (1999) 632. 6. C. G61tner and M. Antonietti, Adv. Mater., 9 (1997) 431. 7. S. A. Bagshaw, E. Prouzet and T. J. Pinnavaia, Science, 269 (1995) 1242. 8. E. Prouzet and T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl., 36 (1997) 516. 9. L. Sierra, B. Lopez, H. Gil and J.-L. Guth, Adv. Mater., 11 (1999) 307. 10. P. Schmidt-Winkel, P. Yang, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Adv. Mater., 11 (1999) 303. 11. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas and F. J. Romero, Chem. Commun., (1999) 873. 12. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas, F. J. Romero and F. J. Urbano, Acta Mater., 49 (2001) 1957. 13. Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schfith and G. D. Stucky, Chem. Mater., 6 (1994) 1176. 14. C. Chert, H. Li and M. E. Davis, Microporous Mater., 2 (1993) 17. 15. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouqu6rol and T. Siemieniewska, Pure Appl. Chem., 57 (1985) 603. 16. U. Ciesla, S. Schacht, G. D. Stucky, K. K. Unger and F. Schtith, Angew. Chem. Int. Ed. Engl., 35 (1996) 541. 17. G. Horvath and K. Kawazoe, J. Chem. Eng. Jpn., 16 (1983) 470. 18. R. Richer and L. Mercier, Chem. Commun., (1998) 1775. 19. S. A. Bagshaw, Chem. Commun., (1999) 1785. 20. F. Di Renzo, F. Testa, J. D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater., 28 (1999) 437. 21. W. Zhang, B. Glomski, T. R. Pauly and T. J. Pinnavaia, Chem. Commun., (1999) 1803. 22. Ch. Danumah, S. M. J. Zaidi, G. Xu, N. Voyer, S. Giasson and S. Kaliaguine, Microporous Mesoporous Mater., 37 (2000) 21.

Studies in Surface Science andCatalysis143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

899

Synthesis and acid-base properties of catalysts based on magnesium and sodium-magnesium mixed phosphates M.A. Aramendia, V. Borau, C. Jim6nez,* J.M. Marinas, R. Roldfin, F.J. Romero,* and F.J. Urbano Department of Organic Chemistry, University of C6rdoba, Campus de Rabanales, Edificio C-3, Ctra. Nnal. IV, Km 396, 14014 C6rdoba, Spain Various solids consisting of magnesium and sodium-magnesium mixed orthophosphates were synthesized and used as catalysts in the transformation of 2-hexanol. The solids obtained were characterized by using various elucidation techniques. In particular, acid-base properties were determined by several methods. The solids were found to be active in the dehydration and dehydrogenation of the alcohol studied, and to be highly selective towards the dehydrogenation product in some instances. In addition to surface properties, the structure and composition of the catalysts -particularly their sodium content- appear to play essential roles in their catalytic behaviour. 1. I N T R O D U C T I O N Over the past 30 years, metal orthophosphates have been used as catalysts for a variety of organic processes [1]. Most are active in the gas-phase dehydration of alcohols and some such as Cal0(OH)z(PO4)6 [2], Zn3(PO4)2 [3] and calcium nickel phosphate [4] exhibit a dehydrogenating ability in the conversion of secondary alcohols into ketones. More recently, mixed orthophosphates including NaZnPO4 [5] and NaMgPO4 [6] have been used as catalysts for the dehydrogenation of alcohols. The dehydration of an alcohol usually competes with its dehydrogenation; the proportion of each type of product obtained depends on the particular catalyst and the reaction conditions used. The selectivity of the catalyst is dictated by its acid-base and redox properties [7,8], which are acquired during the synthetic process. In fact, the transformation of alcohols is a test reaction for determining catalyst acidity and basicity. The dehydration is believed to be catalysed by acid sites and the dehydrogenation by both acid and basic sites, via a concerted mechanism [9] which, however, has been questioned [10]. This paper reports the synthesis and characterization of various magnesium and sodiummagnesium orthophosphates and their use as catalysts for the dehydration-dehydrogenation of 2-hexanol.

* To whom correspondence should be addressed.

900 2. E X P E R I M E N T A L

2.1. Synthesis Solids MgPc and MgOc were obtained by calcination of commercially available Mga(POn)2"SH20 and Mg(OH)2, respectively. Solid NaMgP-1 was synthesized by adding a 3 N NaOH solution over one containing 81.3 g of Mg(NOa)2"6H20 and 14.3 mL of HaPO4 in 250 mL of water, placed in an ice bath, until pH 9 was reached. After 24 h of standing, the solid was filtered and air-dried. Solid MgP was prepared similarly to the previous one except for additional washing with distilled water (20 mL/g) for 30 min, followed by standing for 24 h and air-drying. Solid NaMgP-2 was prepared by using the following procedure. A 3 N NaOH solution was added dropwise to an aqueous solution containing 232 g of MgC12"6H20 and 115 g of Na2HPO4 up to pH 9. The precipitate thus formed was allowed to stand and was then filtered and air-dried. An amount of 25 g of the solid was then suspended in 200 mL of water at 70 ~ and 100 mL of a saturated Na2CO3 solution was added dropwise. The solid obtained after 24 h of standing was filtered and air-dried. The procedure used to synthesize solid NaMgP-3 involved placing 12.3 g of Mg(OH)2 and 20 g of Na2HPO4 in a flask and suspending the mixture in 200 mL of distilled water. Following refluxing for 24 h, the suspended solid was filtered while hot and air-dried. All solids were subjected to the same thermal treatment at 200, 300, 400 and 500 ~ for 1 h. Prior to use, calcined solids were all sifted through 200-250 mesh.

2.2. Characterization The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuI~ radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of pKa 10.6 and 9.9, and ~.max 226 and 271.6 rim, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa- 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford).

2.3. Catalytic experiments Reactions were conducted in a glass tubular reactor of 6 mm ID that was fed at the top by means of a SAGE propulsion pump the flow-rate of which was controlled via a nitrogen flowmeter. The reactor was loaded with 0.175 g of catalyst, over which 1 g of glass beads intended to act as a vaporizing layer was placed. The temperature was controlled via an externally wrapped heating wire that covered the height of both the catalyst bed and the vaporizer, and was connected to a temperature regulator. Evolved gases emerging from the reactor outlet were passed through a condenser onto a collector that allowed liquids to be withdrawn at different times. No diffusion control mechanism was detected [12], nor was any of the reactor elements found to contribute to the catalytic effect under the reactor's operating conditions

901

(viz. feeding rate 1.0-1.5 mL h-1, T 200-550 ~ flow-rate 30-40 mL min-1, amount of catalyst 0.1-0.3 g) in blank runs. All experiments were carried out at atmospheric pressure. Collected samples were analysed by gas chromatography using an SPB-5 60 m x 0.25 mm ID phenylsilicone column and raising the temperature from 100 to 180 ~ at 15 ~ min-1. The products obtained were identified by comparison with standards and their structures confirmed by mass spectrometry. 3. RESULTS AND DISCUSSION Table 1 lists the major phases found in the catalysts studied and summarizes their surface chemical properties. As can be seen, the catalysts consist of various crystal phases. Solid MgPc consists of Mg3(POn)z, which is amorphous on calcination at 500 ~ but becomes crystalline above 650 ~ This is also the main phase found in solid MgP. Such phase could also be synthesized according to previously described procedures [13]. Solid NaMgP-1 was found to be a complex mixture of species: Mg3(POn)z, MgO and sodium-magnesium mixed phosphates, such as NaMgPO4 and NaMg4(PO4)3. It has been reported the formation of these phases by reactions in the solid state [6,14]. Solids NaMgP-2 and NaMgP-3 are basically a mixture of NaMgPO4 and MgO. Finally, solid MgOc possesses a periclase MgO structure that results from calcination of Mg(OH)z above 390 ~ The composition of the solids (Table 2) is consistent with their structures. The most salient outcome is probably the presence of sodium due to the formation of sodium-magnesium mixed orthophosphates. Chlorine was also present as NaCI in solid NaMgP-2. Table 1 Structure, surface chemical properties and primary phases present in the solids Catalyst Phase SBET (m2 g-l) Acidity (~mol g-l) MgPc Mg3(PO4)2 7 24 MgP Mg3(PO4)2 34 70 NaMgP-1 Mg3(PO4)2, NaMgPO4, 6 18 NaMg4(PO4)3, MgO NaMgP-2 NaMgPO4, MgO, NaCI 8 12 NaMgP-3 MgO, NaMgPO4 16 49 MgOc MgO 241 123

Basicity (~tmol g-l) 6 45 13 5 36 254

Most of the solids studied possess a very low specific surface area; it exceeds 35 m2 g-1 in none of the magnesium phosphates. Only the magnesium oxide MgOc, obtained by calcining commercially available Mg(OH)2 at 500 ~ has a fairly high surface area. Solid MgP has a greater surface area than solid MgPc, as a consequence of their different initial structures before calcination, Mg3(PO4)z'22H20 for the former and Mga(PO4)z'8H20 for the latter [13]. Consistent with their low surface areas, magnesium orthophosphates contain few acid and basic sites (Table 1). All of them contain more acid sites than basic sites on their surface, as determined by the retention isotherms of cyclohexylamine and phenol, respectively. Only in the solid MgOc the population of basic sites is larger than that of acid sites.

902 Table 2 Elemental composition (at. %) of solids, calcined at 773 K Catalyst Mg P O MgPc 22.3 13.6 64.1 MgP 22.0 9.5 67.4 NaMgP-1 19.8 9.5 63.2 NaMgP-2 15.6 8.2 64.2 NaMgP-3 19.3 7.9 60.6 MgOc 39.8 60.2

Na 0.9 7.5 9.5 12.2 -

C1 2.5 -

The population of acid sites was also measured by temperature-programmed desorption of pyridine and cyclohexylamine. Table 3 gives the amount of these bases adsorbed on the catalysts studied. As can be seen, pyridine is not able to discriminate among them, since most of the catalysts exhibited very small differences in total acidity. A wider range of values was obtained by using cyclohexylamine as probe molecule. Cyclohexylamine is a stronger base than pyridine and so the former can titrate weaker acid sites. Thus, solids consisting of Mg3(PO4)z showed the higher population of acid sites. These results are in contrast with those obtained in the liquid phase, where MgOc was revealed as the most acid solid. This fact is related to the different conditions used in both experiments. In the liquid phase titration, all solids can have a higher degree of hydration than in the temperature-programmed experiments where catalysts are calcined before adsorption and kept under an inert atmosphere. By other hand, the MgO surface is easily hydrated to Mg(OH)z, thus increasing the number of weak Br6nsted acid sites [15]. For that reason, these experiments which are carried out under conditions closer to those of the catalytic reactions should be able to provide better results. Table 3 Population of surface acid sites determined by temperature-programmed desorption of pyridine and cyclohexylamine Catalyst Pyridine (~tmol g-l) Cyclohexylamine (~tmol g-l) MgPc 4.5 43.8 MgP 4.6 97.3 NaMgP-1 3.6 14.2 NaMgP-2 4.1 10.9 NaMgP-3 2.9 12.0 MgOc 3.6 17.5 Figs. 1 and 2 depict the TPD curves for pyridine and cyclohexylamine, respectively. As can be seen, all profiles are quite similar in Fig. 1. The desorption temperature can give some indications of the relative strengths of the acid sites for the different solids. The maximum temperature of the pyridine peaks for solids MgPc and MgP was the highest (> 280 ~ and so both solids showed stronger acid sites than the others.

903

6000

6000

700 4000

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,

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40

60

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o

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300 ..-- <-, -::..o. 200

;/

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100 0

/

[.-,

100

80 1 0 0 1 2 0 1 4 0

0

20

Time (min)

40

60

80 1 0 0 1 2 0 1 4 0

Time (min)

Fig. 1. Pyridine TPD profiles for solids. Left: MgPc ( ~ ) , MgP (---), NaMgP-1 (-..), and NaMgP-2 (---). Right: NaMgP-3 (---) and MgOc (---). All solids were calcined at 500 ~ As discussed above, temperature-programmed desorption of cyclohexylamine allowed to observe more differences among the catalysts. In this case, solids MgPc and MgP are the most acidic; also, its population of acid sites largely exceeds that of the catalyst MgOc. The rest of the solids possess similar number of acid sites. In any case, all these solids contain few acid sites on their surface. For comparison, a zeolite USY (Si/A1 ratio = 30) can adsorb 254 ~tmol g-i of pyridine under identical experimental conditions. 3000

3000

2500

t.

700

~ x >

/

2000

600 500 400

..~ 1500

2500

G

700

/

~" 2000

300

t 1000

500

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L) L.,

400 300

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1000

200

100 500

600

100 500

0

20

40

60

80 1 0 0 1 2 0 1 4 0

Time (min)

0

20

40

60

80 1 0 0 1 2 0 1 4 0

Time (rain)

Fig. 2. Cyclohexylamine TPD profiles for solids. Let~: MgPc ( ~ ) , MgP (-.-), NaMgP-1 (.--), and NaMgP-2 (---). Right: NaMgP-3 (---) and MgOc ( ~ ) . All solids were calcined at 500 ~

Table 4 shows the activity and selectivity in the transformation of 2-hexanol in the gas phase using the studied solids (previously calcined at 500 ~ The main reactions promoted by

904 these catalysts are dehydration of the alcohol to the corresponding alkenes and dehydrogenation to the carbonyl derivative (2-hexanone). All solids except MgPc and MgP proved highly selective towards the dehydrogenation reaction, so 2-hexanone was invariably the major product. In other words, all the solids that were assumed to contain sodiurlv-magnesium phosphates favoured dehydrogenation over dehydration. The dehydration-dehydrogenation of alcohols is a model reaction for studying the acid-base properties of solid catalysts [16]. Some authors have ascribed the dehydrating ability of a solid to its acidity [17] and its dehydrogenating ability to its basicity [18]; others, however, believe that the dehydrogenation reaction is catalysed by both types of site (acid and basic) via a concerted mechanism [19]. Based on this assumption, magnesium orthophosphates must be considered acid solids and the rest of them essentially basic solids. As noted earlier, however, the population of acid sites was larger than that of basic sites, so the sites titrated by cyclohexylamine or phenol in the liquid phase are not strictly the same as those responsible for catalytic activity, as discussed above. Consequently, measurements of this type in solids containing so few sites and of such a low strength rathe highest acid strength estimated for magnesium orthophosphate is H0 = +4.8 [20]-- must be used with caution. This is also probably the reason why temperatureprogrammed desorption of pyridine did not give good results. Table 4 Total conversion and selectivity towards hexanone (HONE) and hexenes (HENES) in the transformation of2-hexanol on the catalysts studied" Catalyst XT (mol%) SUO~E SHE~S MgPc 97.1 0.02 0.98 MgP 44.3 0.24 0.75 NaMgP-1 14.0 0.90 0.06 NaMgP-2 13.0 0.99 0.01 NaMgP-3 41.6 0.92 0.05 MgOc 41.3 0.69 0.15 " Reaction conditions: N2 flow rate, 40 mL minq; catalyst weight, 0.175 g; feed rate, 388 mL h-l; Tre,c= 500 ~ t~ac= 120 min. Acidity values obtained from TPD experiments with cyclohexylamine can be well correlated with the selectivity in the transformation of 2-hexanol. Thus, solids MgPc and MgP, which are selective towards the dehydration reaction are also the most acidic. The less acidic solids, NaMgP-1, NaMgP-2 and NaMgP-3, are very selective towards the dehydrogenation reaction. Catalyst MgOc is slightly more active in the dehydration than those solids containing sodium-ma~esium mixed phosphates, in virtue of its higher acidity. MgO is a typically basic solid, but it possesses some Lewis acidity due to the Mg 2+ cations [21]. Its dehydrogenating activity has been ascribed to its surface basicity. The presence of strong basic sites on its surface gives rise to side reactions, particularly condensation reactions which produce high molecular weight compounds, decreasing the selectivity to both dehydration and dehydrogenation. Catalysts MgPc and MgP possess an Mg3(PO4)2 structure and hence Br6nsted sites of the P-O-H type [22], which are responsible for the dehydrating activity. On the other hand, the basicity of sodium-magnesium mixed phosphates has been ascribed to the basic character of the

905 oxygen atoms in Na-O-P bond sequences [23], which can act in isolation or in a concerted manner with acid surface sites to yield 2-hexanone. Solid NaMgP-3 is so active as MgOc in the transformation of 2-hexanol but more selective towards the dehydrogenation. Catalyst NaMgP-3 is also more active than NaMgP-1 and NaMgP-2, probably because of its higher basicity and surface area (Table 1). The activity and dehydrogenating activity of sodiummagnesium mixed orthophosphates in other catalytic systems has also been demonstrated elsewhere [6,24]. The different catalytic behaviour between solids MgPc and MgP can be ascribed to the presence of a small amount of sodium in the latter (Table 2). This could indicate the existence of some residual sodium-magnesium mixed orthophosphates dispersed on the surface of Mg3(PO4)z. It would contribute to its higher basicity and therefore to its dehydrogenating ability, which is absent on solid MgPc. This would also explain the increased total conversion for solid MgPc. On the other hand, this solid exhibits a higher density of acid sites (6.3 ~tmol m -z) as compared to solid MgP (2.9 lamol m z) and, in addition, the influence of the structure of the surface sites cannot be discarded. 4. C O N C L U S I O N In summary, the magnesium orthophosphate catalysts prepared in this work are structurally complex; however, they exhibit interesting activity and selectivity in the transformation of 2-hexanol. Their specific surface areas are low, and so are their acidity and basicity. The selectivity towards the alcohol dehydration and dehydrogenation depends on the particular catalyst. Those catalysts consisting of Mg3(PO4)z are essentially acidic and are highly selective towards the hexenes. However, sodium-magnesium mixed orthophosphates, which comparatively contain few acid sites, are selective for the hexanone. Structural and compositional factors have also been discussed in relation to the acid-base properties of these catalysts. Among the different methods to measure the acidity of the samples, in general that based on the temperature-programmed desorption of cyclohexylamine gives the best results. Thus, TPD experiments are more suitable to establish structure-activity relationships, since the acidity is determined under experimental conditions closer to those carried out at room temperature. ACKNOWLEDGMENTS The authors would like to thank Spain's Direcci6n General de Investigaci6n, Ministry of Science and Technology, for funding this research within the framework of Project BQU20012605, and Junta de Andalucia for additional financial support. REFERENCES

1. 2. 3. 4. 5.

J.B. Moffat, Catal. Rev. Sci. Eng., 18 (1978) 199. C.L. Kibby and W. K. Hall, J. Catal., 31 (1973) 65. A. Tada, H. Itoh, J. M. Kawasaki and J. Nara, Chem. Lett., (1975) 517. M. Ruwet, P. Berteau and S. Ceckinwicz, Bull. Soc. Chim. Belg., 96 (1987) 281. M . A . Aramendia, V. Borau, C. Jim6nez, J. M. Marinas and F. J. Romero, J. Catal., 155

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10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

(1995) 44. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas and F. J. Romero, J. Colloid Interface Sci., 179 (1996) 290. K. Tanabe, M. Misono, I. Ono and N. Hattori, Stud. Surf. Sci. Catal., 51 (1989). A. Ouqour, G. Coudurier and J. C. Vedrine, J. Chem. Soc., Faraday Trans., 89 (1993) 3151. M. Ai, Bull. Chem. Soc. Jpn., 49 (1976) 1328. D. C. Tomczak, J. L. Allen and K. R. Poeppelmeier, J. Catal., 146 (1994) 155. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas and F. Rodero, Colloid Surf., 12 (1984) 227. R. M. Koros and E. J. Novak, Chem. Eng. Sci., 22 (1967) 470. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas and F. J. Romero, J. Colloid Interface Sci., 217 (1999) 288. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas and F. J. Romero, React. Kinet. Catal. Lett., 58 (1996) 183. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas, F. J. Romero, J. A. Navio and J. Barrios, J. Catal., 157 (1995) 97. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas, A. Porras and F. J. Urbano, J. Catal., 161 (1996) 829. F. Figueras-Roca, L. De Mourgues and Y. Trambouze, J. Catal., 14 (1969) 107. Y. Matsumura, K. Hashimoto and S. Yoshida, J. Catal., 117 (1989) 135. A. Gervasini and A. Auroux, J. Catal., 131 (1991) 190. T. Imanaka, Y. Okamoto and S. Teranishi, Bull. Chem. Soc. Jpn., 45 (1972) 1353. M. A. Morrow, Stud. Surf. Sci. Catal., 57A (1990) 202. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas, F. J. Romero and J. R. Ruiz, J. Solid State Chem., 135 (1998) 96. A. Tada, Bull. Chem. Soc. Jpn., 45 (1975) 1391. M. A. Aramendia, V. Borau, C. Jim6nez, J. M. Marinas, F. J. Romero and F. J. Urbano, J. Colloid Interface Sci., 240 (2001) 237.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

907

Preparation of Pd-Ce/Zr02 catalysts for methane oxidation L. S. Escand6n, S. Ord6fiez, F. V. Diez and H. Sastre Department of Chemical and Environmental Engineering, University of Oviedo, C) Julihn Claveria s/n, 33071-Oviedo, Spain. Phone: 34-985 103 508, FAX: 34-985 103 434. E-mail: fds@sauron, quimica.uniovi.es

Two different cerium oxide promoted zirconias were prepared and tested as supports for Pd catalysts for the catalytic oxidation of methane, alone and in presence of a strong catalyst poison (SO2). The introduction of cerium oxide was carried out by incipient wetness of zirconium hydroxide or zirconium oxide, followed by calcination. Both catalysts present very different properties, the first method producing a catalyst with better performance, and thermal stability markedly higher than the unmodified zirconia support. However, the addition of cerium does not lead to any enhancement of the catalyst performance in presence of SO2. I. INTRODUCTION Methane is present in the emissions from many processes such as coke ovens, farms and biological processes used for solid wastes and wastewater treatment. In the last years, methane emissions are of great environmental concern, mainly because of its implication in the greenhouse effect [1]. Thermal incineration (the conventional technique) requires very high temperatures (up to 1000~ producing noxious by-products such as NOx. Catalytic combustion is considered as an interesting alternative for the abatement of methane in diluted emissions. The main advantage of this technique is the lower temperature needed, saving energy and reducing the risk of formation of NOx [2]. However, methane is the hydrocarbon which is most difficult to oxidize. Most studies on catalytic oxidation have been carried out with more reactive VOC, and hence the thermal stability of the support is not as important as in the case of methane. It is known that supported palladium catalysts are the most active for the total oxidation of methane [3], and there are many studies focusing on the alumina supported ones [4 and references cited therein] However, alumina is not stable at the temperatures commonly used for methane oxidation. To avoid this problem, other authors [5] have suggested the use of zirconia-based supports, which are considered as more thermally stable. In this way, these supports were found to present very different properties, depending on the synthesis method and the presence of additives.

908 On the other hand, cerium has been shown to be an effective oxygen reservoir, enhancing the activity of many oxidation catalysts. Due to this property, cerium oxide is also considered to potentially enhance the thioresistance of the catalysts. This aspect is of great practical importance, since the use of palladium catalysts is hindered by the poisonous effect of sulphur compounds, often present in off gases. Most works dealing with ceria-zirconia catalysts have been carded out with catalysts prepared by coprecipitation methods, whereas in this work an alternative procedure, based on the incipient wetness technique is used to incorporate ceria to the zirconia support. The aim is to maintain the advantages of zirconia supports, especially the thermal stability. In the present work, the performance of different palladium catalysts supported on cerium-modified zirconia has been studied for the oxidation of methane in air, alone and in the presence of SO2. This compound has been chosen because it is very commonly present in gaseous emissions. On the other hand, in previous works it was found the effect of other reduced sulphur compounds on Pd catalysts was very similar [6].

2. EXPERIMENTAL 2.1. Catalyst preparation The catalyst support used in this work was zirconium hydroxide, supplied by MELCAT, with 318.2 m2.g1 surface area, measured by nitrogen physisorption. Particles between 100 and 250 ~tm were selected. Cerium was added to the support by two different procedures, both based in the incipient wetness method. In the first procedure, zirconium hydroxide was impregnated with aqueous cerium (IV) nitrate in order to get 1 % wt of cerium in the support (support A). The second procedure consisted of calcining the hydroxide at 700 ~ for 4 h to obtain the oxide and then impregnation with the same cerium solution (support B). In both cases the incipient wetness technique was used to impregnate the solid. Zirconium oxide with no cerium added was prepared by calcining the hydroxide at 700~ Palladium was added to unmodified zirconium oxide and supports A and B by incipient wetness, using an aqueous solution of Pd(NO3)2 as precursor, in order to obtain solids with 1% wt palladium in all the cases. The solids produced according to this procedure will be called here catalysts U, A and B, respectively. PdCI2 was not used as precursor, as chlorine anions are considered poisons for the catalysts, so a wash step is necessary to eliminate them when PdC12 is used as precursor. On the other hand, it is known that Pd(NO3)2 leads to catalysts with higher crystallite size and lower metal dispersion, although they are more active for the oxidation of methane [7]. The catalysts thus prepared were dried overnight at 100~ after impregnation, and then calcined in air at 550~ for 2 h. In previous works of our group [8] it has been observed that non-reduced palladium catalysts are more active than the reduced ones. For this reason, the calcined catalysts were used in the reactor without reduction.

2.2. Catalyst characterization Surface area measurements of fresh and used catalysts were carried out using a Micromeritics ASAP 2000 apparatus with nitrogen as the adsorbed gas. All the samples were outgassed at 200~ before analysis.

909 Temperature programmed reduction analyses were carried out using a Micromeritics TPD/TPR 2900 Analyzer, operating at ambient pressure. The reactor was a quartz glass vertical tube and the catalyst sample (0.02 g) was kept in position by plugs of quartz wool. This reactor was placed inside an electric furnace, temperature being controlled by a PID controller (Eurotherm). The gas flow rate was measured by rotameters and the gas leaving the reactor was analysed by a TCD detector. The flow rate of the carrier gas (5% hydrogen in nitrogen) was 50 mL/min. The temperature was increased at 10~ from room temperature to 900~ The hydrogen consumed was detected by a TCD. Powder X-ray diffraction patterns were recorded in a D-5000 Siemens diffractometer, using nickel-filtered CuKa as monochromatic X-ray radiation. The patterns were recorded over a range of 20angles from 20 to 70 ~ and crystalline phases were identified using JCPDS files. 2.3. Reaction studies Experiments were carried out in a laboratory fixed-bed catalytic reactor, consisting of a stainless steel tubing of 400 mm length and 9 mm internal diameter. The reactor was loaded with 0.30 g of catalyst, mixed with 1 g of inert glass as dilutant. The catalyst was placed in the middle section of the reactor, and the upper part was filled with additional inert material. The reactor was placed inside an electric furnace, the temperature being controlled by a PID controller (Honeywell) connected to a thermocouple situated inside the reactor, in the zone charged with catalyst. The system was provided with 5 additional thermocouples that measured the reactor wall temperature at different positions. The reactor feed consisted of 5000 ppmV methane in N-50 synthetic air (Air Liquid). To study the poisonous effect of sulphur compounds, 40 ppmV of SO2 were added to the feed in some experiments. Gas flow rate (1 NL/min) was controlled by a mass flow controller (Brooks 5850 TR), and the exhaust gas was analysed by gas chromatography (Hewlett Packard HP 5890 Series II). CO was not detected in any experiment. Methane and SO2 concentrations have been selected because they are values representative of industrial emissions, such as coke oven emissions. 3. R E S U L T S AND DISCUSSION 3.1. Fresh catalyst characterisation The fresh catalysts were analysed by BET, XRD and TPR techniques. BET analyses show clear differences in the surface area, pore volume and pore size distribution between cerium catalysts (A and B) (Fig. 1). Pore distribution and morphological parameters of catalyst B are similar to those of pure ZrO2 (23 m2/gfor ZrO2 and 22 m2/g for support B), whereas the surface area of catalyst A is markedly higher (71 m2/g). Concerning to the TPR profiles (Figs. 2 and 3), in the case of catalyst B, a clear reduction peak of cerium oxide is observed at 495~ which is not detected for catalyst A. Catalyst A presents the lowest Pd reduction temperature, followed by catalysts U and B. The presence of negative peaks at low temperatures (75~ in the case of catalysts A and U can be tentatively explained by adsorption-desorption of hydrogen on the catalyst surface. Comparing the low temperature zone of the TPR profiles, it is observed that catalysts U and A show similar behaviour (reduction temperatures and hydrogen release), whereas the

910

behaviour of catalyst B is completely different (no hydrogen release produced). These differences suggest that, in the case of catalyst B, there is a strong interaction between Pd and cerium oxide.

0.05 ..B -A

et0

0.04 o

0.03

E o

>

0.02

o

0.01 0.00 10

1000

100 log (D (A))

Fig.1. Pore size distribution of cerium-modified catalysts

m

----B ~A

228

=i 246

h~

50

250

450 Tempemture (~

650

Fig. 2. TPR profiles of zirconia catalysts

850

83

150

--,,

i

l

i

i

100

150

200

250

Temperature

(*(2)

300

Fig. 3. Detail of TPR profiles of zirconia catalysts

911 A strong reduction peak appears for catalyst B, caused by the reduction of cerium oxides. However, this peak does not appear for catalyst A. This result suggests that cerium oxide is present as a separate phase in catalysts B, whereas there is a strong interaction between Ce and Zr oxides in catalysts A. A marked influence of the procedure for the addition of cerium was observed in XRD profiles. These profiles suggest that the proportion of zirconium oxide with tetragonal structure, considered as the most stable phase, is higher for procedure A. Important changes with respect to unmodified zirconia were not observed in the case of procedure B. No cerium-containing phases were detected, probably because of the low cerium content of the catalysts. 3.2. Reaction experiments The performance of the three 1% PdO catalysts for the combustion of methane was tested both obtaining the light-off curves and with ageing experiments. The catalytic activity of the supports with no Pd loaded was checked at the reaction conditions of these experiments, methane conversions attained being negligible. Light-off curves were obtained after 15 h on stream at 450~ increasing then the temperature at 2~ from 150~ to 550~ Light-off curves were obtained both for 5000 ppmV methane and methane and SO2 (5000 ppmV CH4 and 40 ppmV SO2). The light-off curves were characterised by the parameter Ts0, defined as the temperature needed for achieving 50% conversion. The values of this parameter are shown in Table 1. Table 1. Parameters obtained from light-off curves Absence of SO2 Catalyst Ts0 (~ k 450~c (s -1) Ea (kJ/mol) A 354 2.91 78.0 B 387 1.15 62.4 U 406 0.79 40.1

. . . .

T50 (~ 470 --484

Presence of SO2 k 4s0-oc (s-i) Ea 0.20 0.05 0.24

(kJ/mol) 112.0 68.0 88.0

On the other hand, the kinetic-controlled region of the light-off curves (conversion below 80 %) was fitted using a pseudo-first order kinetic model. Although it is accepted that the best kinetic mechanisms to model the oxidation of methane over palladium catalysts are Mars-Van Krevelen ones, in a given range of conditions pseudo-first order models can be accurate enough and suitable for comparison purposes. So, considering zero order with respect to oxygen (due to its high concentration), order 1 with respect to methane, and Arrhenius dependence for the kinetic constant with respect to temperature, the kinetic equations are the following: (1)

(2)

912 (-r) being the reaction rate for methane, P the partial pressure of methane, k the kinetic constant, R the gas constant, T the absolute temperature and k0 and Ea the kinetic parameters that will be fitted to the experimental data. The values obtained for these parameters are given in Table 1. Ageing experiments were carried out at 450~ (Fig. 4). The operation temperature was chosen because in previous experiments it was observed that self-deactivation of palladium catalysts is important at this temperature. Space time was 4.5 g-h/mol CI-I4 in all the experiments. The gas feed consisted of 5000 ppmV methane in synthetic air. In order to study the poisonous effect of sulphur compounds, 40 ppmV SO2 were added to the feed in some experiments.

100

o

.~.,q

=o

j

., "~4q,94,ve,e4q,ee,e,e4q4,~4j,e e 4 ~ milL--

0

0

2

_ _

-

-

In,

L

99 --

i

i

i

!

i

i

4

6

8

10

12

14

16

tine ~) Fig. 4. Ageing experiments in absence of m B, 9 U).

8 0 2 (0

A, [] B, o U) and in presence of

8 0 2 (#

A,

As it can be observed in Fig. 4, when only methane is added to the feed, catalyst A shows the best behaviour, followed by catalysts B and U. According to this, the addition of cerium to the zirconium hydroxide increases the activity of the zirconia-supported palladium catalysts. Comparing the performance of the cerium-containing catalysts, it is remarkable that catalyst B presents a poorer performance, than catalyst A (slightly lower initial conversion and faster deactivation). This result suggests that the interaction between Pd and Ce, revealed in the TPR experiments, does not enhance the activity of the active phase (Pd). In contrast, the interaction Ce-Zr in catalysts A increases the thermal stability, considered as the main factor for preventing catalyst deactivation in these reactions [9]. On the other hand, in the experiments carried out in presence of SO2, catalyst A shows a higher resistance to deactivation and higher conversions than catalyst B. However, both perform worse than the unmodified zirconia catalyst (Fig. 4). So, it can be concluded that the thioresistance of zirconia-supported palladium catalysts is not increased by the addition of cerium.

913 In previous works of our group [8], it was observed that an important factor to the thioresistance of palladium catalysts is the ability of the support to adsorb SO2, higher adsorption capacities leading to higher thioresistance. According to this, it seems that the addition of cerium decreases the capacity of the catalyst to adsorb SO2. At this point, it is important to remark that the characterisation of deactivated catalysts (XRD, BET) does not reveal any important change in the catalyst morphology and crystalline structure. ACKNOWLEDGEMENTS

This research was financed by the Environmental Research Programme of the European Union (ENV4-CT97-0599). REFERENCES

1. S. Vigneron, J. Hermia and J. Chaouki (eds.), Characterization and Control of Odours and V OC in the Process Industries, Amsterdam, 1994 2. E.C. Moretti and N. Mukhopadhyay, Chem. Eng. Prog., 7 (1993) 20 3. J.H. Lee and D.L. Trim, Fuel Process. Tech., 42 (1995) 339 4. K. Narui, K. Furuta, H. Yata, A. Nishida, Y. Kohtoku and T. Matsuzaki, Catal. Today, 45 (1998) 173. 5. S. Yang, A. Maroto-Valiente, M. Benito-GonzAlez, I. Rodriguez-Ramos and A. Guerrero-Ruiz, App. Cat. B, 28 (2000) 223. 6. P. Hurtado, S. Ord6fiez, H. Sastre and F.V. Diez, Proceedings of the 7th North American Catalysis Society Meeting, Toronto, Canada (2001) 7. O.D. Simone, T. Kennelly, N.L. Brungard and ILL Farrauto, Appl. Catal., 70 (1991) 87 8. L.S. Escand6n, S. Ord6fiez, H. Sastre and F.V. Diez, Proceedings of the 5th European Congress on Catalysis, Limerick, Ireland (2001) 9. J.J. Spivey and J.B. Butt, Catal. Today, 11 (1992) 465

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

T h e effect of c e r i u m introduction on v a n a d i u m - U S Y

915

catalysts

C.R. Moreira 1, M. Schmal z* and M.M. Pereira 1 1 Institute of Chemistry, Federal University of Rio de Janeiro (UFRJ), Ilha do Fund~o, CT - Bloco A, sala 637, CEP: 21941-590, Rio de Janeiro, RJ, Brazil, [email protected]. z Chemical Engineering P r o g r a m - COPPE/NUCAT/UFRJ, [email protected]

USY modified by cerium and vanadium tolerance of theses catalysts were studied. Three methodologies were used for cerium introduction: aqueous precipitation at 25 and 90~ wetness impregnation and ion exchange. The results suggest that the ratio of total cerium and cerium exchanged is very different for each catalyst, decreasing from impregnated to exchanged cerium introduction methodologies. These differences become much more evident in the presence of vanadium. All characterization, DRS in situ, DRX, TPR, BET after steam and cracking activity are in agreement that cerium and vanadium interacted. For cerium located on zeolite it is not possible to detect vanadium in the oxidation state V § by DRS in situ. On the other hand, increasing cerium into the framework the vanadium in oxidation state V § increased. In this way, the impregnation methodology leads to the most vanadium tolerance. 1. I N T R O D U C T I O N The fluid catalytic cracking unit (FCCU) is used for vacuum distillates and residues into olefinic gases. The great demand in processing heavy feedstocks and the high amounts of metals in Brazilian oils, forced to develop novel catalysts that are more resistant to metal contamination. Since the FCCU is a cyclic process, the catalyst passes through reduction and oxidation conditions. Indeed, the reductive atmosphere and coke observed in the riser favor the deactivation and reduce the life time of the catalysts. The burn off in the regenerator releases steam, CO, NOx, SOx, and other compounds. The catalyst is recycled in the reaction-regeneration (reduction-oxidation) between 10000-50000 times and, therefore, the change in the metal environment is very complex, affecting the metal oxidation state, which is an important parameter. Although the reaction conditions are well known in the literature, there are few reports concerning the environment of reduced vanadium species. Vanadium is the most important deactivation compound in FCC catalysts. In steam atmosphere, the zeolite framework is completely destroyed, and therefore the rate of make-up catalyst in the unit is very high [1,2]. The low melting point of V205 (690~ and the possibility of formation of acid species, as reported in the literature, are responsible for this deleterious effect [3,4]. The control of vanadium

916 migration and oxidation state are important to preserve the FCC catalyst. Rare earth elements have been used as metal traps, but there are still fundamental questions concerning the rare earth zeolite interactions and vanadium oxidation states. The objective is to study the effect of cerium introduced in USY zeolite and the resistance of these modified catalysts to vanadium in steam. 2. E X P E R I M E N T A L

2.1. Catalyst preparation The ultra-stable (USY) zeolite (SAR=13) was exchanged twice with a NHnNO3 aqueous solution at 343 K for 1 h, reducing the sodium content to less than 0.5% [5]. After a calcination at 873 K for 2 h in a muffle, the zeolite (HUSY) was modified by addition of cerium using three different methods: precipitation of an aqueous solution of cerium (III) chloride at 298K(PP25) and 363K (PP90); by wetness impregnation (IMP), and ion exchange (EX) as reference. A 1.5 M solution of cerium (III) chloride was poured together with a 1 M ammonium hydroxide solution in the HUSY suspension at pH 8, forming a precipitate of a cerium (III) hydroxide which, after calcination at 873K for 2 h, was transformed in cerium(IV) oxide. In the second procedure, the required amount of cerium (III) chloride was dissolved in ethanol and added to the zeolite at 268K. After drying overnight, it was calcined in a muffle at 873K for 2 h. In the ion exchange method, an aqueous solution of cerium (III) chloride was added to a HUSY suspension at 353K. After 1 h at this temperature, the system was washed with deionized-water and then calcined at 873K for 2 h. In the impregnation method, vanadium was introduced using vanadyl octanoate in toluene, followed by drying and calcination at 873K [6]. All the catalysts were submitted to a hydrothermal treatment for 3 h at 1073K using water at partial pressure of 0.3 atm. The metal contents were determined by Inductive Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Perkin Elmer 1000), after dissolving 100 mg of the sample in 5 drops and 5 ml of hot HNO3 and HF, respectively.

2.2. Textural properties The BET surface areas of the catalysts were determined in a Gemini 2360 Micromeritrics equipment. Samples were first calcined at 873K for 1 h and then transferred at high temperature to a vacuum unit at 50 mTorr for 1 h and cooled to 473K. Finally, the sample was transferred to the equipment for N2 adsorption. Isotherms were taken in a range of relative pressure of 0.06 to 0.21atm.

2.3. Temperature Programmed Reduction (TPR) The TPR analysis was carried out in continuous flow system, as described elsewhere [7]. Before reduction the catalyst was heated with an argon flux at 873K for 2 h to eliminate water. After cooling to 373K, the catalyst was reduced in a 1.53% H2/Ar flow (30 ml/min) at 10K/min up to 1273K. The hydrogen consumption was measured using a

917 thermal conductivity detector.

2.4. X-ray diffraction (DRX) The X-ray diffraction patterns were obtained using a Rigaku Miniflex equipment with Cu K a radiation of 154.18 pm. The angular interval scans were carried out over the range 14-35~ in 0.5 ~ steps and counts of 1.5 s per step.

2.5. Cracking activity The catalysts were evaluated by measuring the activity and the conversion with time on stream by using cracking of cyclohexane. Before reaction, the catalysts were reduced under flowing 10% Hz/Nz at 60 ml/min, rising the temperature at 10K/min up to 773K and held at this temperature for 5 min, according to the literature [8]. The reaction was performed at 703K, using a saturator with cyclohexane at a constant temperature of 284K, and pure hydrogen as carrier gas (20 ml/min). The reaction products were analysed by on-line chromatography (Shimadzu GC-17 A) with a packed Chrompack column (60 m length and 0.32 nm diameter) at 453K.

2.6. UV-VIS Spectroscopy in diffuse reflection mode (DRS) ~in sitm> The analysis was performed in a Varian Cary 5 spectrophotometer equipped with a Harrick diffuse reflectance chamber. The first spectra were taken after drying at 473K for 1 h, then after a reduction at 773K for 1 h passing a 20% H2/N2 flow at 10K/min. The spectra were taken at room temperature in the range 200-2000 nm, using zeolite HUSY as reference [9]. 3. RESULTS

3.1. Cerium-zeolite catalysts The preparation method and the main properties are presented in Table 1. The loss of surface area is presented as the ratio of BET area after and before the hydrothermal treatment. The surface area of the samples prepared by ion exchange and impregnation, respectively, EX and IMP, are very similar to the surface area of zeolite HUSY. However, the precipitated catalysts (PP25 and PP90) presented a lower surface area, which can probably be attributed to some blocking of pores. After hydrothermal treatment with steam, the loss of surface area of the reference zeolite was 28%, while the loss of surface area of the catalysts containing cerium was not more than 21%. The PP25 sample presented a small loss of surface area, and was therefore more resistant. The EX and IMP catalysts presented similar losses of surface area, around 20-21%. The reduction profiles catalysts are presented in Fig. 1. The PP25 and EX catalysts exhibited similar reduction profiles with a broad peak with a maximum at 836K and at 786K. On the other hand, the PP90 sample showed a profile with two reduction peaks at 897K and at 1059K. Note that the temperature of precipitation is an important parameter in this method. The impregnated (IMP) sample showed a different behavior with no significant hydrogen uptake.

918 Table 1. Properties of cerium-zeolite catalysts and the lost of area after hydrothermal deactivation by steaming. Catalyst Ce Method of cerium introduction BET area Loss of area (%) (m2/g) after steam (%) HUSY 648 28 EX 2.2 Ion exchange 631 20 PP25 3.0 Precipitation T = 25~ 612 12 PP90 3.5 Precipitation T = 90~ 609 17 ................!M P 2,0 ~ Wetness Impregnation 639 .......................................21

(o)

__ 624~

563~

~~--~--._.__..~_..

(b)

513oc

(a)

26o

~o

~

Temperature(~

~o

,doo

Fig. 1. Temperature Programmed Reduction (TPR) profiles [8]" a) EX; b) PP25; c) PP90, d) IMP.

3.2. Vanadium-cerium-zeolite catalysts Table 2 presents the main properties of these samples, the loss of surface area after hydrothermal treatment and the surface area after a TPR analysis. The H2 consumption was calculated based on the metal content (~mol H2/~tmol Ce + V). In addition, a ratio of extrapolated and observed H2 uptake from the TPR profile (H2 ext/obs) was calculated, taking in account the H2 consumption of isolated cerium and vanadium oxides in the bimetallic system, in order to see if interaction between both occurred or not, or if they are isolated particles. As observed, there is a loss of the surface area after steam treatment, which markedly depends on cerium addition method. The PP25V catalyst after steam treatment led to a higher damage of the zeolite structure. Although the TPR results of the PP25V and EXV samples indicate some similarity, the BET surface areas suggest noteworthy differences in their morphology, due to the cerium location. Indeed, the impregnated sample (IMP) was the only one that protected the zeolite structure and presented the lowest loss of surface area (25%).

919 Table 2. Properties of cerium-vanadium-zeolite catalysts including: BET area after TPR measurement, H2 consumption during TPR analysis and cracking activity of cyclohexane. Catalysts V Loss of area Area after TPR ~mol H2/ C6H12 Ha (ppm) after steam (mZ/g) ~mol Ce + V (ext/obs) (ext/obs) (%) activity HUSYV EXV PP25V PP90V IMPV

3000 2000 3900 2600 2000

33 31 45 28 25

455 33 239

1.15 0.78 0.48 0.57 0.28

4.23 3.35 2.82 2.75

0.5 1.0 1.2 3.2

However, the BET surface area of the catalysts after TPR experiment presented a drastic crystalline damage of the EXV catalyst and some crystallinity retention for the IMPV catalyst. These results agreed with the X-ray diffraction pattern after TPR measurements, as shown in Fig. 2. The crystalline difference for both catalysts was 20 %. Vanadium reduction was strongly affected on the modified zeolites. The amount of Hz consumption in the TPR analysis related to the metal content (~tmol Hz/~tmol Ce + V) decreased largely. Moreover, the ratio of the extrapolated and observed Ha consumption (Hz ext/obs) was higher on the IMPV catalyst compared to the others. This behavior was supported by the UV-VIS spectroscopy data (Figs. 2 and 3), showing similarity of both profiles, the dry catalyst and after the reduction treatment. Both did not present absorption bands attributed to d-d transition in the 800-1800 nm region. These results suggest that almost all the vanadium is in the oxidation state V +5 in the IMPV catalysts. On the other hand, the ratio H2 (ext/obs) < 1 of the EXV catalyst indicates an increase of the amount of hydrogen consumption. The UV-VIS spectra on this catalyst (Fig. 3) support this result, showing a large band in the of d-d transitions region. According to the literature [3,10], the peak at 800 nm corresponds to the octahedral vanadium species in oxidation state V +4. The catalytic activity, C6Hlz (ext/obs), decreases depending on the preparation method (EXV > IMPV) of the catalysts, in opposition to the increase of H2 (ext/obs) ratio. The highest activity observed on the EXV catalyst indicates that, in this case, the acid sites of the zeolite were better protected by the presence of rare earth than on the IMPV catalyst. The activity of the cyclohexane cracking are presented in Table 2. The C6H12 (ext/obs) activity is presented as the ratio between the extrapolated and the observed activity. The extrapolated activity was calculated assuming isolated metals without interaction, while the observed value was obtained experimentally, meaning that the extrapolated activity is the sum of the isolated activity, pondered to the metal content in the catalyst.

920 0,10.

~3.

..-,~ 0,05

(~ o,1 am

(a) c~o 800

1000

1200

1400

V~de-cJh(n~

1630

18~0

2[]30

i 800

,

i 1000

,

! 1330

,

! 1400

,

i 1600

,

i 1800

,

2000

Figs. 2. UV-VIS Spectroscopy in situ after Fig. 3. UV-VIS Spectroscopy in situ after dry (a) and reduction (b) treatment for dry (a) and reduction (b) treatment for IMPVcatalyst. EXV catalysts. 4. DISCUSSION 4.1. State of cerium in the zeolite The results have shown that the introduction of a rare earth element in the zeolite really depends on the preparation method that directs the location of this metal in the zeolite. As expected, when cerium oxide is introduced by impregnation, the surface area decreases due to formation of particles blocking the pores. On the other hand, the ion exchange method would favor the introduction of cerium in the zeolite framework and, therefore, it does not affect the surface area but favors the dispersion of cerium. Noteworthy is the influence of the temperature in the precipitation method during the introduction of cerium in the zeolite. As seen, the BET surface area of the modified zeolites with cerium at 298K (PP25) and 363K (PP90), decreases slightly compared to HUSY, or to ion exchanged catalysts(EX). Therefore, the best method is when cerium is introduced in the zeolite framework and, therefore, a better dispersion of cerium is expected. The TPR results have shown very similar profiles for EX and PP25 catalysts. It supports the previous results that cerium on PP25 catalyst was exchanged in the framework. However, in this case, after steam treatment the destruction in the zeolite framework was very large. It suggests that the cerium environment in the PP25 catalyst is different compared to the EX catalyst. On the other hand, the reduction of the PP90 catalyst showed a profile which is very similar to the reduction of cerium (IV) oxide, and this suggests the presence of superficial cerium species and bulk species [11,12,13]. Therefore, there are larger cerium aggregates on the external surface of the zeolite and

921 isolated cerium species. On the contrary, the impregnation of cerium evidences the reduction of external particles and the existence of different easily reducible cerium bulk species. The presence of different cerium species would probably influence the vanadium environment and the catalytic behavior of these catalysts containing both elements. Indeed, the Hz(ext/obs) ratio which is a measure of the reduction degree and therefore indicates if there is an interaction with the zeolite or between cerium and vanadium, exhibited different values, depending on the way of introduction and species formation. The catalyst treated with steam, EXV, presented a low H2 (ext/obs) ratio, which indicates a better reduction. On the other hand, the impregnated catalyst (IMPV) presented a high H2 (ext/obs) ratio, and thus low reduction. This could explain the indication that an interaction occurred during the treatment, with the formation of bimetallic or alloys or even the formation of aluminum silicate-metal interaction. DRS measurements support the TPR results. The impregnated catalysts and steam treated (IMPV) did not show the presence of V +4 after the reduction. Probably, the hydrogen consumption in the TPR profile is due (a) to the reduction of cerium. The band (b) in the d-d transition can be attributed to the formation of alloys like cerium vanadate, according to the literature [14]. Baugis et al. [15] reported that the presence of vanadate with rare earth decreases the diffusion of vanadium in the zeolite structure [14]. The existence of these compounds Fig 4. X-ray diffraction patterns of a) EXV may affect the oxidation state, the and b) IMPV catalysts after TPR dispersion, morphology and location measurements. of cerium species in the catalyst. The DRS spectrum of the EXV catalyst after reduction showed the presence of vanadium in V § oxidation state. Based on thermodynamics redox results, it is expected that when vanadium and cerium present some interaction, the last one should present an easier reduction. The reduction of cerium is favored because of its higher potential (1.64 eV), that should maintain vanadium in the V § oxidation state [16]. Therefore, the formation of rare earth vanadate is favored. On the IMPV catalyst, where probably cerium is dispersed over the zeolite, the reduction process would be preserved. On the other hand, for the EXV catalyst, the reduction of cerium exchanged in the presence of vanadium leads to an easier reduction of both components. But it is not possible here to distinguish and to quantify the formation of V +4 and cerium in a Ce +3 oxidation state. The DRX diffractograms after TPR suggest a possible model of crystalline destruction of the catalysts using their reduction potentials. The highest crystalline damage is observed on the IMPV catalyst compared to the catalyst containing exchanged cerium (EXV) that is completely amorphous (Fig. 2). This sustains the proposed model that the introduction of cerium by wetness impregnation leads to more cerium species outside the

922 zeolite structure. If cerium species are exchanged in the framework, the catalyst should stay amorphous like the other one, due to the formation of vanadates inside the zeolite framework. The cracking activity suggests that the amount of active sites of the zeolite poisoned by vanadium depends on the cerium location in the zeolite. The higher the activity value of C6H12 (ext/obs), the lower is the poisoning effect of vanadium. (Table 2). Therefore, the most active catalysts are those that protect the zeolite better, and this was observed on those catalysts that contain exchanged cerium, resulting in a higher proximity between cerium and vanadium. This is because the cerium species exchanged in the acid sites would not allow that vanadium interact with the acid sites, keeping vanadium close to them. On the other hand, on the IMPV catalyst, part of the actives sites of the zeolite would be affected by vanadium. The literature has reported methodologies to quantify the oxidation state of vanadium species using TPR [ 17] and EPR/DRS [10]. In summary, this work shows for the first time a marked influence of cerium species on the vanadium reducibility. Since rare earth elements in FCC catalysts depend on different preparation morphologies, it is necessary to develop a model to quantify the oxidation state of vanadium. 4.2. Resistance of zeolite under steam

The BET results after steaming show that the catalysts containing cerium exhibited higher hydrothermal stability. The literature reported that rare earth exchanged in zeolites enhance the thermal and hydrothermal stability [5]. After the introduction of vanadium, it is possible to verify that the catalyst containing cerium introduced by impregnation (IMPV) protected the zeolite structure. Therefore, the protection depends on the dispersion of cerium species on the zeolite surface, decreasing the vanadium mobility. However, the mechanism which explains the damage of zeolite by vanadium is unclear [1,3]. The higher cerium contact increases the framework damage. Probably the steam effect on PP25V catalyst could explain the non-homogeneous distribution of cerium exchange in the framework as it is in the EXV catalyst. The heterogeneous cerium distribution leads to a high local damage and a higher effect of steam. In this way, it is possible to observe that the preservation of the zeolite structure depends very much on the cerium location in the catalyst. Probably vanadium introduction first localizes vanadium outside the zeolite framework, as expected in real cracking catalyst, which increases the probability of formation of cerium-vanadium compounds on the IMP catalyst. 5. CONCLUSIONS The different ways of cerium introduction lead to different morphology and location on the zeolite. The ratio of total cerium and cerium exchanged into zeolite decreases from cerium impregnated to cerium exchanged. The precipitated catalysts lead to intermediary systems between IMP and EX catalysts, showing exchanged and superficial cerium species. After vanadium introduction, different behaviors in the vanadium reduction/oxidation capacity were observed depending on the cerium location and the

923 morphology, but cerium exchanged and dispersed on zeolite presented high interaction with vanadium. The last modified zeolite did not show, after the reduction treatment, vanadium in low oxidation state, by in situ UV-Vis spectroscopy. On the other hand, V +4 was observed when cerium was exchanged in the zeolite. Finally, cerium provides better thermal resistance to the zeolite and, after hydrothermal treatment with and without vanadium, the crystallinity of the zeolite depends on the cerium species in the catalyst. Zeolite modified with cerium are good models compounds to study the vanadium oxidation state. ACKNOWLEDGMENT

CNPQ and CTPETRO are gratefully acknowledged for financial support. REFERENCES o

2. 3. 4.

5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17.

M. Torrealba, and M.R. Goldwasser, Appl. Catal. A: General, 90 (1992) 35. J. Biswas and I.E. Maxwell, Appl. Catal., 63, (1990) 197. C.A. Trujillo, C.A. et al., J. Catal., 168 (1997) 1. G. Martino, Stud. Surf. Sci. Catal., 130 (2000) 83. M.L. Occelli, Catal. Rev.-Sci. Eng., 33 (3-4) (1991) 241. B.R. Mitchell, Ind. Eng. Chem. Res. Dev., 19, (1980) 209. L.T. Santos et al, Stud. Surf. Sci. Catal., 139 (2001) 343. J. Abbot, J. Catal., 123 (1990) 383. M.A. Bafiares, M.A. et al., S. Surf. Sci. Catal., 130 (2000) 3125. G. Catana et aL, Phys. Chem. B, 102 (1998)8005. A. Piras, A. Trovarelli and G. Dolcetti, Appl. Catal. B: Environ., 28, (2000) L77 B. Ernst, L. Hilaire and A. Kiennemann, Catal. Today, 50, (1999) 413. F. Giordano, J. Catal., 193 (2000) 273. R. Zhuo, F. Wang and W. Wu, in: 215th National Meeting American Chemical Society, Dallas, 1998 A. G.L. Baugis et al., in: 11~ Congresso Brasileiro de Catfilise e 1~ Congresso de Catfilise do Mercosul, 2 (2001) 916. J.G. Nery et al., Zeolites, 18 (1997) 44. E.F. Souza-Aguiar et al, Zeolites, 15 (1995) 620.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

925

Rh-Co mordenite catalysts for the selective reduction of NO by methane C.E.Quincoces, M. Incollfi, A. De Ambrosio, M.G.Gonzfilez 1 CINDECA (CONICET, UNLP), 47 Nro 257, 1900 La Plata, Argentina

The performance of RhCoMOR catalysts for the NO selective reduction by methane in the presence of excess oxygen has been determined. Incorporation of 1% w/w of Rh and 5% w/w of Co to mordenite provides a solid that is active and selective for SCRCH4 reaction showing the highest activity between 400 and 500~ with a CH4/NO ratio=l and 2% of oxygen in the feed, and a GHSV= 20,000 h 1. The Co-catalysts were prepared by ionic interchange of Na Mordenite (MOR) with aqueous solution 0,025 M of Co acetate and, after calcination, Rh was added to the CoMOR sample by incipient wetness impregnation at room temperature. The Co mono- and bimetallic catalysts showed a good performance for the reaction of NO reduction by methane. The addition of 1% of Rh improved the activity of SCR-CH4 on CoMOR catalyst, decreasing the temperature of maximum NO conversion. The TPR characterization of RhCoMOR catalyst showed a signal that could be attributed to intermetallic compounds, and another signal at high temperature, attributed to Co § which would be responsible for the catalytic activity. 1. I N T R O D U C T I O N The selective reduction of nitrogen oxide with methane is one of the most promising technologies to control the NOx, both in stationary emission sources, where it represents an alternative for the use of ammonia as reducer, and in mobile sources that use natural gas. Methane offers the benefits of low cost and wide availability compared to other hydrocarbons and is much less corrosive than ammonia. The use of methane as NO reducer offers the opportunity of reduced capital and operating costs compared to other reducing agents. This technology may allow to find a solution to exhaust from natural gas fired power plants and lean-burn natural gas engines. In both cases, methane may be available at low level in the exhaust or can be easily injected into exhaust downstream of the burner from on-hand supply of natural gas. It has been shown by Li and Armor [1,2] that the selective reduction of NOx by methane is produced with high selectivity on zeolitic materials interchanged with Co. Even though the presence of zeolites plays an important role by enabling a high dispersion of Co, its disadvantage is that it is very sensitive to the presence of water vapor. The catalyst systems for the SCR of NOx with methane that will have the greatest impact and 1 Corresponding author: Fax:+54-221-425-4277. e-mail: [email protected]

926

applicability must be both highly active and stable. Bicomponent systems, generally a transition metal and noble metal [3-8] have been suggested to solve inconveniences resulting from the presence of water vapor and higher temperatures. This paper studied Co catalysts interchanged on mordenite to which Rh is added over the activity. An analysis of Rh effect on the activity and selectivity for the selective reduction of NO by methane in oxidizing medium is presented. 2. E X P E R I M E N T A L The CoMOR catalysts were prepared by means of ionic exchange of NaMordenite (NORTON 2900, Si/AI = 5.9) with 0,025 M aqueous solution of Co acetate by stirring 24 h at different temperatures (25 and 70~ in order to modify the metallic content. The interchanged material was washed and dried for 24 h and then calcined at 400~ for 14 h following the heating pattern of 2~ min -1, with 2 isothermal intervals of 90 min at 110 and 210~ according to [8]. Part of the interchanged material was treated at 25~ in a second stage, previous to calcinations, with a fresh solution for another 24 h at room temperature. Bimetallic samples, having between 0.1 and 1% of Rh, were obtained from the monometallic catalysts by incipient wetness impregnation with aqueous solution of CI3Rh.2H20 at room temperature. After adding Rh, the solid was calcined at 350~ for 2 h with a heating rate of 2.5~ min 1. The samples were denominated Rh(m)Co(n)MOR, where m and n represent the percentage of the elements contained in the catalyst. The monometallic and bimetallic solids were characterized by adsorption of Nz, energy-dispersive X-ray analysis (EDX), temperature programmed reduction (TPR) and Xray diffraction (XRD).. The BET surface area and pore volume distribution were measurement by nitrogen adsorption in an Accusorb 2100E Micromeritics analyzer. The chemical analysis using EDX were carried out on a DXPRIME 10 system connected to the scanning electron microscope. The TPR experiments were carried out in a conventional equipment. The samples (0.06g) were heated from R.T. to 1000~ at a rate of 10~ min -1 in a 10 % H2-N2 stream at a rate of 20 ml min -1. Sample diffractograms were obtained with a Philips PW 1732/10 equipment and CuKct radiation at a rate of 20/min The measurements of catalytic activity were carried out in a fixed bed reactor having 0.18 g of catalyst, in the temperature range 200-650~ and at a GHSV of 20000 h -1. The feed mixture to the reactor consisted of 1000 ppm of methane, 1000 ppm of NO, 2% of Oz and He as balance. The feed and effluent were analyzed by an "on-line" gas chromatograph using a column CTR1 at 40~ and a thermal conductivity detector. The conversions of NO and hydrocarbon were calculated from the amount of N2 and CO2 produced. 3. RESULTS AND DISCUSSION During the preparation stage, the effects of temperature and the number of interchanges on the Co amount added to zeolite were determined. Table 1 shows the Co

927 content in the different samples. The Co content was determined by EDAX, and the ion exchange level was calculated, assuming that one divalent cobalt ion is exchanged for two monovalent Na cations. It is observed that, as long as the successive interchanges scarcely affect the content of Co interchanged, the temperature has a distinct effect, allowing the doubling of the percentage of active component. In agreement with different authors, Co(5)MOR was selected to prepare the bimetallic RhCoMOR solids. Table 1. Catalysts composition Catalysts Ion exchange Temperature(~ Co(4)MOR 25

Ion exchange steps .... 1

Co(w/w%) 4.3

Ion exchange level 90

Co(5)MOR

25

2

5.1

106

Co(10)MOR

70

1

9.8

203

The monometallic and bimetallic catalysts showed to be active and selective for the reaction of NO reduction by methane. CoMOR was active at higher temperatures than RhCoMOR catalysts. All the samples show good stability during a test of 24 h at 500~ The NO and methane conversions for the different samples are shown in Figs. 1 and 2.

35 30 25 A

0 =" x

20 15 10

200

400 T ern

600 p eratu

ra

800

(~

Fig. 1. Nitric Oxide conversion as a function of temperature on: (A) Rh(1)Co(5) MOR, (11) Co(4)MOR,(O) Co(5) MOR,(O) Rh(1)Co(5)MOR reduced

928 The methane was oxided only to CO2 and H20 over the catalysts; CO was not detected as reaction product. At the maximum NO conversion, the methane oxidation was near 100%. Bi- and monometallic samples achieved the maximum methane conversion at 400 and 600~ respectively. The results obtained in this study over CoMOR samples are in general agreement with the data reported by several authors [2,7]. Gutierrez et al [7] reported that Co(2)MOR gives 30% of NO conversion at 450~ but with only 6500 h-1 GHSV.

100 80

o'-?,

60

I

o X

40

20 0 4-,,._,l~-~____~r~ i v , , , , '='` ~

200

~

300

.r ..

I

I

400

500

600

700

T e m p e r a t u r e (~ Fig. 2. Methane combustion as a function of temperature on: (A) Rh(1)Co(5) MOR, (11) Co (4) MOR, (O) Co(5) MOR,(O) Rh(1)Co(5)MOR reduced The addition of Rh improves the activity of Co/MOR catalysts, increasing the NO conversion to Nz and decreasing the temperature of maximum conversion. At 430~ the NO reduction increases from 19 to 32% by the addition of 1% of Rh to the monometallic catalysts. The reduction of bimetallic catalyst decreases considerably the NO conversion and enables the methane combustion. At the opposite to observations of Petunchi et al. [9] for PtCoMor catalysts, the reduction does not increase the catalyst activity of RhCoMOR sample. The bimetallic catalysts submitted to Hz-He stream at 400~ during 2 hours show a great drop in the NO conversion without modification of the temperature window for maximum activity. At low temperatures, the reduced and unreduced samples present the same behaviour for the NO conversion. At temperatures higher than 400~ the NO conversion decreased for the reduced catalyst. This behavior can be associated with the increment of methane combustion at high temperatures over the reduced sample, which would affect the selectivity of the reaction. In order to analyze the effect of the Rh content, bimetallic samples containing 5% of Co and between 0.1 and 1% of Rh were prepared and tested for the SCR-CH4 reaction.

929 It is observed in Fig. 3 that the performance of the bimetallic samples is affected by the amount of Rh incorporated to CoMOR. Whereas the addition of 1% Rh increased the conversion of NO to Nz without modifying the selectivity, the incorporation of smaller amount of Rh to CoMOR decreased the catalytic activity (Table 2) and increased the temperature of maximum NO conversion. These results suggest that Rh has a promoting effect on the activity of CoMOR for a content of 1%.

100 c

80

.o 03

60

>

40

o

20

I,.,.

tO

"

- - ~ , - - r - ' - -r-

0

--=l

~

200

i~

j

J

400

600

800

Temperature(~ Fig. 3. Effect of Rh content on the NO (black symbols) and CH4 ( white symbols) conversion on (0) Rh(1)Co(5)MOR, (0) Rh (0.5)Co(5)MOR, (A) Rh(0.1) Co(5)MOR. Table 2. Co and RhCo Mordenite Catalytic Behavior a Catalyst

XNO (%)

Tmaxb

XCH4 ( % ) c

Selectivityd

Co(4)MOR

29.1

500

53.8

0.54

Co(5)MOR

30.0

490

54.6

0.55

Rh( 1) Co (5) MOR

33.0

438

61.5

0.536

Rh(0.5)Co(5)MOR

15.0

453

82.1

0.18

Rh(0.1)Co(5)MOR

10.5

454

49.2

0.20

Rh (1)Co(5)MOR red.

12.0

435

84.3

0.14

a: reaction conditions: GHSV=20000 h-1, NO=1000 ppm, CH4=1000 ppm, 02=2%. b:Maxima NO conversion temperature. C" CHnto COz conversion, d 9NO/CH4 selectivity

930 At about 440~ the NO conversion to Nz is near 35% for the Rh(1)Co(5)MOR, suggesting a promoter effect of Rh. MonometaUic samples containing the same % of Co as bimetallic ones show a drop of NO conversion near to 20%, when evaluated under the same conditions. In order to understand the effect of the active sites on the SCR-CH4 reaction, the different samples were analyzed by TPR technique. Fig. 4 shows the reduction thermograms of the different samples. For comparison, the TPR profiles of Na mordenite and mordenite impregnated with Rh are also included in the figure. The TPR profiles of mono and bimetallic solids show a peak whose maxima is observed about 1000~ This signal can be assigned to the presence of interchanged CO +2 which would be responsible for the formation of NO2. The NOz reacts with the adsorbed methane in the acid places, beginning the SCR-CH4 reaction [8,9]. These Co +2 ions at exchange sites may be considered as the active sites. The peak at 400~ in the Rh(1)Co(5)MOR catalyst, compared with Rh/MOR sample, is attributed to the reduction of Rh. The shoulder at the higher temperature side present in the bimetallic sample, can be attributed to intermetallic compounds. This intimate contact of cations would contribute to the promoting effect of Rh on CoMOR activity. During the calcination step, the formation of CO304 is quite probable, but the peak at lower temperature (230 and 390 ~ assigned by Wang et al. [11] to the reduction of Co oxo-ion and Co304, is not observed. This result suggests that there is no formation of Co +3 during the preparation step of the catalysts.

E 0.4

~

m c

c d

~ o.a CJ

e

~ I 0.2

0

I

500 Temperatura (oC)

I

1000

Fig. 4. TPR patterns of a) Rh(1)Co(5)MOR, b) Rh/NaMOR, c) Co(5)MOR, d) Co(4)MOR, e)NaMordenite The diffractograms obtained for the mono- and bimetallic samples were similar to those of the original Na-mordenite. No signal corresponding to Co and Rh species were observed in the diffractograms of calcined and reduced samples, which indicates that the metallic particles have a size below 40 A, detection limit of the instrument. Surface area measurements and pore volume confirm that the mordenite structure is not affected during the calcination treatment.

931 4. CONCLUSIONS The results obtained indicate that the addition of a small amount (1%) of Rh to the CoMOR catalysts improves its performance for the reduction of NO to Nz in excess of Oz, increasing significantly the NO conversion to N2 and decreasing the temperature of maximum conversion. Intermetallic compounds and Co +z at exchange positions would be responsible for the improved performance of the catalysts. These solids could be an alternative in the development of effective catalysts for the SCR-CH4 reaction in excess of O2 ACKNOWLEDGMENTS. The authors thank CONICET and UNLP for financial support for this research project.

REFERENCES 1.Y. Li and J.A. Armor, Appl. Catal. B, 1 (1992) L31. 2. Y. Li and J.A. Armor, Appl. Catal. B, 2 (1993) 239. 3. H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito and M. Tabata, Appl. Catal., 75 (1991) L1. 4. E. Kikuchi et al., Catal. Today, 27 (1996) 35. 5. H. Ohtsuka and T. Tabata, Appl. Catal. B, 21 (1999) 133. 6. M. Ogura, Y. Sugiura, M. Hayashi and E. Kikuchi, Catal. Lett., 42 (1996) 185. 7. L. Gutierrez, A. Ribotta, A. Boix and J. Petunchi, in 11th International Congress on Catalysis, J.W. Hightower, W.N. Delgas, E. Iglesia and A.T. Bell (eds.) Elsevier, Amsterdam, 1996, 631. 8. A.V. Boix, M.A. Ulla and J.O. Petunchi, J. Catal., 162 (1996) 239. 9. J-Y. Yan, H.H. Kung, W.M.H. Sachtler and M.C. Kung, J. Catal., 175 (1998) 294. 10. L. Gutierrez, A. Boix and J. O. Petunchi, J. Catal., 179 (1998) 179. 11. X. Wang, H-Y. Chen and W.M.H. Sachtler, Appl. Catal.B, 26, (2000) L227.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

933

Surface characterization of WO3-TiOz/Alz03 catalysts and reactivity o n selective catalytic reaction of NO by NH3 Silvia Egues a, Neuman S. de Resende a and Martin Schmal a* aNUCAT/PEQ/COPPE - UFRJ, Cidade Universitfiria, Rio de Janeiro, Brasil. Caixa Postal 6 8 5 0 2 - CEP 21945-970. E-mail: [email protected] In this work, the structural and physico-chemical properties of WO3/TiO2, WO3/A1203 and WO3-TiOz/A1203 catalysts and their activity in the SCR of NO and NH3 were investigated. The catalysts were prepared with titanium content between 7 and 13 % (w/w), keeping tungsten at 7 % (w/w). The catalysts were characterized by BET, XRD, UV-Vis DRS, Raman spectroscopy, and the catalytic activity was observed by TPSR. The results suggested the presence of well dispersed tungsta and titania phases on the support forming amorphous phases or microcrystallites. DRS spectra showed that the WA sample has only tetrahedral tungsten species, but the incorporation of TiO2 to the support leads to a higher content of octahedral tungsten species. Raman spectra of mixed supports indicated that TiO2 forms a surface layer on alumina, which has a polymeric structure. The TPSR profiles showed that the catalytic activity increases in the following order WO3/TiO2 > WO3TiOz/AI203 > WO3/m1203 and that the presence of oxygen increases the NO reduction rate. 1. I N T R O D U C T I O N Commercial oxide catalysts for the selective catalytic reduction (SCR) of NOx by NH3 usually consist of TiO2 (anatase) as support, V205 as active component, and WO3 (or MOO3) as promoter. Vanadia is responsible for the activity and selectivity of the catalyst in NOx reduction but also favors the transformation of anatase to rutile, leading to sintering, loss of surface area and to undesired oxidation of SO2 to SO3 in the case of sulfur fuels [1]. Accordingly, the V205 content is generally low, 0.3-1.5% w/w. WO3 is employed in large amounts (--10% w/w) increasing the working reaction temperature window, the thermal and mechanical stability. An additional advantage is that WO3 stabilizes the titania structure and hinders both surface area loss of anatase and its transformation to rutile [2]. Therefore, the use of tungsta as active phase seems a good alternative for SCR catalysts. Attention has been paid to titania-alumina supported catalysts, which use high surface area and thermal stability of inert alumina to sustain the structure of titania, enhancing the textural properties of the catalysts [3,4]. The literature presents a number of studies dealing with the characterization of WO3-AIzO3 and WO3-TiOz catalysts; however, little has been reported on the properties of the WO3-TiO2/AI203 system. It was found that the support composition and the physico-chemical properties determine the tungsten species at the surface, presenting tetrahedral species on alumina, and octahedral species on titania [5].

934 It seems interesting to study the changes that occur in the tungsten surface species as the TiO2/A1203 support changes with variable amounts of titania until monolayer, and the relation between the type of tungsten surface species with the catalytic activity in SCR reaction. Therefore, this work reports the preparation of TiO2/A1203 and WO3-TiO2/A1203 with various TiO2 contents, and their characterization by different techniques (BET, X R , DRS, Raman spectroscopy). Also, an attempt has been made to relate the results from these characterizations to the changes of the catalytic activity in NO reduction with NH3 observed by TPSR. 2. EXPERIMENTAL

2.1. Supports and catalysts preparation The TiO2/Al203 supports were prepared by dry impregnation method with variable amounts of TiP2 (7.2%, 9.7% and 13.5% w/w) over alumina calcined at 600~ [4]. Titanium isopropoxide (Ti(OC3H7)4) from Aldrich was used as precursor. Titanium dioxide (anatase) was obtained by slow hydrolysis of the isopropoxide. The supports were named 7TA, 10TA and 13TA according to the TiP2 content. The series of W catalysts containing 7% w/w WO3 were prepared using ammonium paratungstate ((YH4)6H2W12040) as precursor in wet impregnation. Later on, all the solids were dried at 120~ for 17 h and calcined at 500~ for 5 h. The catalysts were named W7TA, Wl0TA and Wl3TA. 2.2. Supports and catalysts characterization The specific surface area and pore volume distribution were obtained with nitrogen physisorption using BET method in a Micromeritics ASAP 2000. X-Ray diffraction (XRD). A Philips PW 1410 powder diffractometer with CuK~ radiation (40 KV, 30 mA) plus a Ni filter was used with an angular range varying between 19 to 80 ~ in a 0.02 ~ per step and 0.8 seconds counting per step. Diffuse reflection spectroscopy (UV-DRS). UV-Vis diffuse reflectance spectra were recorded in a Varian Cary 5 spectrophotometer (Harrick Scientific) with a diffuse reflectance accessory of Praying Mantis geometry. The samples were set in holds with 2 mm thickness. The spectra were recorded in the 200 nm and 800 nm range with a scanning speed of 1800 mm.min1 at room temperature against supports as reference. Laser Raman spectroscopy (LRS). The spectra were recorded on a Nicolet 950 FTRaman spectrometer instrument, equipped with a nitrogen cooled Ge detector. A Nd:YAG laser (1064 nm) was used as excitation source. The measurements were performed with a power at the sample of 100-200 mW in order to avoid decomposition and thermal effects. The samples were rotated to provide a noncontinuous irradiation of any given spot on the samples. The spectral slit width was typically 4 cm -1. Temperature-programmed surface reaction (TPSR). Measurements were performed in a flow apparatus with a BALZERS quadrupolar mass spectrometer to monitor effluent products versus temperature. Samples of 300 mg were used in each measurement. The gases employed were He (99.9%) and mixtures 4% NH3/He, 1% NO/He and 5% O2/He. TPSR measurements were obtained after NH3 adsorption at 70~ under continuous flow until base line stabilization. The reactor was heated to 500~ at 20~ and kept at this

935 temperature for lh. Concentration of desorbed compounds were determined from peak intensities, as follow: NH3 (m/e = 15), NO (m/e = 30), H20 (m/e = 18), N2 (m/e = 28), 02 (m/e=32) e N20 (m/e = 44). 3. R E S U L T S AND DISCUSSION

3.1. Textural properties The structural and morphological properties of the catalysts were investigated. Table 1 presents the specific surface area (SBET), pore volume (Vp), TiO2 content of the mixed supports and the WO3 content at the surface of the catalysts. Assuming a monolayer capacity of 0.083 %w/w TiO2/m 2 [6], the titanium coverage (0wi) was calculated considering TiO2 content and specific area of the samples. As shown in Table 1, the theoretical TiO2 monolayer (10.4 [amol Ti/m 2) was not exceeded in any mixed support samples. The results of the surface area measurements of the supports present a small decrease relative to alumina, as follows: A1203 (SBET--197 m2/g) and 13TA (SBET--180 m2/g). The addition of WO3 caused only a slight drop of the surface area of the catalysts relative to mixed supports, as seen for W13TA (SBET--174 m2/g). However, higher surface areas were obtained for Ti-A1 mixed oxides than for pure TiO2 (101 m2/g). All the catalysts showed a monomodal pore volume distribution curve, which gives an indication of the absence of segregation of the different oxide supports in the catalysts formulation. 3.2. X-ray diffraction (XRD) The diffractograms of the catalysts are shown in Fig. 1. The XRD patterns of the WO3/ZiO2 and WO3/A1203 catalysts present peaks characteristic of anatase and alumina, respectively. The diffraction lines of the catalysts WTA showed peaks related to T-alumina phase. Only the W13TA sample showed an incipient peak of anatase phase (20 = 25,2 ~ corresponding to a slight segregation of crystallized titania. No diffraction lines of any tungsten phase were detected in the different samples. The above observations indicate that, at this measuring scale, both titania and tungsten phases were well dispersed over the different catalysts, forming either an amorphous phase or microcrystallites not detectable by XRD. Table 1 Textural properties of the catalysts samples Sample SBET Vsp TiO2
0Ti 0.50 0.64 0.93

WO3 (a) lamol/m 2 1.9 2.8 1.9 1.8 1.8

0w 0.17 0.26 0.18 0.16 0.17

936 I

I

I

L WA .A,,.uu~ ~ ~'~ .................. ,..W7~A . . . . . . ~

"

I

'

I

"

I

'

I

"

~

,r ~ ....... ,

~,.. 9- .^ " , - '

l ~ ~ h ~ .

..............

T

WT

10

20

30

40

50

60

70

80

Angulo de Bragg (20) Fig. 1. X-ray diffractograms of the WTA catalysts.

3.4. Raman spectroscopy The Raman spectra for the TiO2 sample showed bands at 145, 197, 397, 516, 638 cm -1 characteristic of anatase, and a shoulder at 448 cm -1, indicating a small portion of rutile. The ~,-alumina support showed no Raman active modes [5]. The Raman spectra of 7TA, 10TA and 13TA support samples exhibited weak bands due to anatase and the feature of ?-alumina. The futile form was not detected for any mixed support. The Raman spectra also showed a weak band in the 800-900 cm -1 region which is indicative of Ti-O-Ti bonds [7]. Bands in the 9501000 cm 1 region were not observed and this suggests that surface titanium oxide species do not contain Ti=O moieties. Thus it is concluded that TiO2 forms a surface layer on alumina which has a polymeric structure.

3.3. UV-Vis diffuse reflectance spectroscopy (DRS) The literature reports that tetrahedral and octahedral tungsten species appear in 250-275 nm and 340 nm, respectively [5,8]. The DRS spectra of tungsten catalysts presented in Fig. 2 showed only an intense single band at 252 nm for WA sample, characteristic of W 6+ in tetrahedral coordination. The WT spectrum presents also a single band at 369 nm, where the contribution of octahedral tungsten in WO3 appears. Fig. 3 shows electronic spectra of WTA catalysts, using the corresponding support as reference. As can be observed, as the content of TiO2 in the support is increased, the intensity of the bands decreases and the maximum is shifted to higher wavelength. The maximum in the absorption bands of W7TA and W10TA does not correspond to either tetrahedral 0~=275 nm) or octahedral (~=340 nm) well-defined tungsten species, but for the W13TA catalyst, a single band in the region of octahedral tungsten species is evident. The W10TA and W13TA catalysts present also a shoulder in the spectral region of tetrahedral tungsten species. It appears from these observations that in the WA sample, only tetrahedral species are present; however, the incorporation of TiOz to the catalyst support leads to a higher content of octahedral tungsten species and, in the monolayer titania W l 3 T A sample, mainly tungsten in octahedral coordination seems to be present.

937 |

,

|

!

i

,

c~

0,10 s.

I

I

l

I

,-.,I ,5

'~

1

I

LL

0,05

l

~'P

W2 l

I

1,0

I I

0,5

200

. A ......

X

WT

0,0 0 ,95

W7TA

0,00 %

WIOTA

~,

'

%,

-0,05 ,

I

250

,

I

300

,

I

350

C o m p r i m e n t o de o n d a (nm)

Fig. 2. Diffuse reflectance spectra of the WA and WT catalysts.

,

,

400

200

|

I

250

|

i

300

i

i

350

,

400

C o m p r i m e n t o de o n d a (nm)

Fig. 3. Diffuse reflectance spectra of the WTA catalysts.

3.5 TPSR experiments TPSR experiments were performed to investigate the reactivity of preadsorbed ammonia with gaseous NO, both in the absence (Fig. 4) and in the presence of gaseous oxygen (Fig. 5). The figures show the consumption profile measured when NO is fed over the NH3 preadsorbed WT, WA, W10TA and W13TA catalysts. The figures also show the formation of N2 and the ammonia profile, which was completely consumed in all the experiments. Over the WT sample, the consumption of NO started at 300~ with parallel evolution of N2 and water (not shown in the figure), indicating that SCR reaction has occurred. The Nz desorption peak at 178~ is not related with SCR reaction, but probably due to ammonia oxidation, as has been observed for VzO5/TiO2 and V205-WO3/TiO2 catalysts [9]. Similar results have been obtained over the Wl0TA and Wl3TA samples. However, for these catalysts, the NO consumption is monitored at higher temperatures, starting from 350~ in the case of Wl3TA, and 400~ over Wl0TA. The WA catalyst was also tested and did not show the SCR reaction. The consumption of NO was not observed, along with NH3 desorption in a large range of temperature (150-500~ These observations indicate that the reactivity of the catalysts probably increases in the following order: WT > Wl3TA > W l 0 T A > WA. As discussed in the literature [10,11], it is likely that the occurrence of the SCR reaction requires the participation of the catalysts framework oxygen, resulting in catalyst reduction, according to the stoichiometry: 2NH3 + 2NO + O2- --> 2N2 + 3HzO + 2e-

(1)

Based on this, the initial temperature for SCR reaction would be related to the availability of the catalyst structure oxygen atoms, that is due to catalyst reduction. This suggests that the TiO2 incorporation leads to a higher catalyst reducibility compared to pure alumina supported catalyst. In agreement with this assumption, the DRS results show that increasing titanium loading on mixed supports increased the population of octahedral tungsten species, which are easily reducible [5]. When the reaction is performed in the presence of oxygen (Fig. 4), an important increase in NO reduction occurs for all the samples. The SCR reaction is observed over all the catalysts, with complete consumption of adsorbed ammonia. The N2 formation at low temperature, 140~ due to ammonia oxidation is apparent in all the experiments and it is increased by the presence of oxygen.

938

[

WT

. . . . . .

NO' [

N__00 N2

W13TA ......... "......... .

I NO N2

......................

1

1

W10TA

4

NO /"i

9~

_

WA

100 200 300 400 500 Temperatura -~

N z

.NH~ .[

Isot6rmico

Fig. 4. TPSR profiles of NH3 and NO over WO3/TiO2, WO3-13,5% TiO2/ml203 and WO3/m1203.

"~

100 200 300 400 500 Tempertura (~

NO

Isot6rmico

Fig. 5. TPSR profiles of NH3 and a mixture of NO+O2 over WO3/TiO2, WO3lO%TiOa/A1203 and WO3/AI203.

Over the WT sample, NO is consumed starting at 100-300~ and more intensely at 394~ The reaction is already completed at 430~ due to the depletion of adsorbed ammonia surface species. The W10TA catalyst starts up the SCR reaction at 375~ and it is extended up to 500-~ however the reaction rate is slower than that presented by WT catalyst. Over the WA sample, the reduction of NO occurs between 381 and 490~ These results confirm the reactivity order pointed out in the case of TPSR experiments in the absence of oxygen. The presence of gaseous oxygen in the feed enhances the rate of NO removal and shifts the start up of the reaction to lower temperatures. This behavior is in agreement with the redox mechanism proposed in literature for the SCR reaction over oxides, where gaseous oxygen accelerates the reaction associated with catalyst surface reoxidation. 4. CONCLUSIONS DRX measurements indicated that both titania and tungsten phases were well dispersed on the different catalysts. The DRS spectra showed that the TiO2 addition to catalyst support leads to a higher content of octahedral tungsten species and, in the monolayer titania sample, mainly tungsten in octahedral coordination seems to be present. The Raman spectra of supports exhibited only bands indicative of Ti-O-Ti bonds, suggesting that TiO2 forms a surface layer on alumina with a polymeric structure. TPSR experiments showed that the activity of the catalysts increased slightly and linearly with the titanium loading of the support. This difference in activity could be due to changes

939 induced by the support on the dispersion of the tungsten phase, as the tungsten octahedral species are more easily reduced. ACKNOWLEDGEMENTS To CNPq for the scholarship (S. Egues) and Pronex, PADCT, FINEP for financial support. REFERENCES 1. M. Bafiares, L. Alemany, M. Jim6nez, M. Larrubia, F. Delgado, M. Granados, A. MartinezArias, J. Blasco and J. Fierro, J. Solid State Chem., 124 (1996) 69. 2. G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal. B, 18 (1998) 1. 3. N. Resende, M. Schmal and J.-G. Eon in Proceedings of the International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Belgium, Vol. 1 (1995) 1059. 4. N. Resende, J.-G. Eon and M. Schmal, J. Catal., 183 (1999) 6. 5. J. Ramirez and A. Guti6rrez-Alejandre, J. Catal., 170 (1997) 108. 6. G. Bond, S. Flamerz and L. van Wijk, Catal. Today, 1 (1987) 229. 7. B. Scheffer, J. J. Heijeinga and J. A. Moulijn, J. Phys. Chem., 91 (1987). 8. M. Vuurman and I. Wachs, J. Phys. Chem., 96 (1992) 5008. 9 M. Reiche, P. Hug and A. Baiker, J. Catal., 192 (2000) 400. 10. L. Lietti, J. Alemany, P. Forzatti, G. Busca, G. Ramis, E. Giamello and F. Bregani, Catal. Today, 29 (1996) 143. 11. L. Casagrande, L. Lietti, I. Nova, P. Forzatti and A. Baiker, Appl. Catal. B, 22 (1999) 63.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

941

Catalytic materials for the synthesis of hydrofluorocarbons P. Cuzzato a, V. Giammettab, R. Trabace b, F. Triflrb b "Ausimont SpA, Via della Chimica 5, 30175 Porto Marghera (VE), Italy ~Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy HFC-125 (pentafluoroethane) can be synthesized by reaction in gas phase of HCFC-124 (1,1,1,2-tetrafluoro-2-chloroethane) with HF over chromium oxides supported on AIF3. The use of HF submits problems of corrosion and high toxicity: a pulse reactor, with HF generated in situ, allows to overcome these problems in the study of this reaction at laboratory scale. The AlE 3 support shows catalytic activity in the halogen exchange reaction but also in the production of CFC-1113 (1,1,2-trifluoro-2-chloro-ethene) by 1,2 HF elimination and in the production of oligomers adsorbed on the catalyst. The introduction on the support of amorphous or microcrystalline phases of chromium oxide determines a progressive increase of the catalytic activity and a reduction of the yield in the olefinic byproduct CFC-1113. To limit the production of oligomers, the composition of the catalyst is modified by introduction of elements like magnesium and calcium both by coimpregnation and by sequence impregnation. 1. INTRODUCTION For their physical properties, high stability and low toxicity, chlorofluorocarbons (CFC's) have found an extensive use, accompanying the development of various industrial fields. About forty years after their discovery in the early thirties [1], the hypothesis was advanced that, because of their stability, these compounds dispersed in the atmosphere arrived in the stratosphere where they caused a series of reactions generating chlorine and leading to an abnormal consumption of the ozone layer [2]. The Protocol of Montreal and its successive amendments have therefore placed limits to the production and the use of CFCs proscribed in industrialized countries since 1996 and from 2010 in the underdeveloped nations. One technical approach to solve this environmental problem is to eliminate the chlorine content of CFCs, primarily by replacing any chlorine atoms with fluorine atoms in these molecules. Hydrochlorofluorocarbons (HCFC's) do not have impact zero on the ozone layer and although they have been used in this period, they are already currently subject to restrictions and will be proscribed from 2025. Hydrofluorocarbons (HFC's) have ODP (Ozone Depletion Potential) equal to zero and low GWP (Global Warming Potential) and they are therefore designated as substitutes to CFCs in the field of refrigeration [3].

942 Despite the use of these compounds, also in mixture or azeotropes, it has not been possible, however, to obtain products with properties equal to those of CFCs since small differences in the physical properties cause great differences in terms of application. Also for this reason, till now the increase in the use of substitutes has been much lower than the decrease in the production of CFCs: the HFCs have replaced less than a third of the CFCs while the HFCs for approximately 10% [4]. The syntheses of HFCs are commonly carried out by reaction of hydrofluoric acid with trichlorethene (TCE) or tetrachloroethene (PCE) in gas phase with heterogeneous catalysis

[5]. HF + CCIz=CCIz ---) CHCIzCF3 + CHCIFCF3 + CHFzCF3 PCE HCFC-123 HCFC-124 HFC-125

(1)

After addition of HF to the chlorinated olefinic reagent, the reaction can proceed through successive CI/F halogen exchanges or by formation of olefinic intermediate by elimination of HCI and successive hydrofluorination: studies carried out with use of DF have allowed to discover that the first pathway is prevalent [6]. The outline of reaction (1) is very simplified; a series of isomerization and disproportionation reactions are known [7] but these can be limited using appropriate operating conditions and adapted catalysts. Oxides, oxyfluorides and fluorides of trivalent metals among which aluminium, chromium and iron, are generally used as catalysts, also supported or in mixture with other metals, usually bivalent metals [8]. In particular chromium oxide and fluorinated chromium oxide have found industrial applications [9] but the study relative to the catalytic mechanism and to the catalytic nature of the species involved is still under way. Studies carried out by use of labelled fluorine 18F [10] have shown that during the reaction of halogen exchange there is a slow and progressive substitution of Cr(III)-O bonds with Cr(III)-F bonds and there are different kinds of fluorinated species: in correlation with TPR/TPO studies [1], the hypothesis is that species characterized from oxidation state larger than 3 are involved in the generation of the catalytically active sites. Actually, the nature of these sites is still not clear, but the analysis of the thermodynamic data shows that the role of trivalent chromium can be correlated to the fact that from CrzO3 the formation of CrF3 (AG~ and CrC13 (AG~ =-56KJ/mol) is nearly equally favourite; it was postulated that metal fluorides are not directly involved in the catalytic cycle but a chromium oxyfluoride (III) of empirical composition CrOxFy [12], generated also by reductive processes, can have and open structure and Lewis acidity [13] to favour the CI/F exchange as regards the other reactions. The hypotheses relative to the mechanism of halogen exchange agree on mechanisms such as Langmuir-Hinshelwood [14,15] or modified Langmuir-Hinshelwood [16]. Our research concerns the study and optimization of catalysts for the synthesis of the HFCs; in particular catalysts for the synthesis of pentafluoroethane (HFC-125), appropriate substitute of CFCs as refrigerants (also in mixture with other HFCs) and, in particular, the last step of the synthesis, the halogen exchange reaction on 1,1,1,2-tetrafluoro-2-chloroethane (HCFC- 124):

943 CF3-CHC1F + HF --->CF3-CHFz + HCI HCFC-124 HFC-125

(2)

Beside the main reaction, at least two side reactions are observed: the formation of CFC-1113, CFz=CFCI, by HX release from a saturated precursor, and the oligomerization of this compound and other olefinic intermediates to form oligomeric "heavies" that adsorb very strongly on the surface of the catalyst. Thus, we studied the effect of different catalyst formulations on the relative yields of these products. 2. E X P E R I M E N T A L

2.1. Catalyst preparation and characterization The synthesis of the catalysts has been carried out by incipient wetness impregnation: the support (A1F3, Ausimont) is treated with a volume of impregnating solution (solutions of metal chlorides) equal to the total volume of the pores and every impregnation is followed by drying (120~ 1 h) in order to remove the water. The samples prepared are reported in Table 1. Tablel isample '' AIF3 Crli Cr2i Cr5i Crl 0i Crl0Ca0,05c Crl0Ca0,1c Crl0Ca0,2c Crl 0Ca0,5c Mgli Cal i MglCrl0is CalCrl0is

Preparaiion ..... . . . . . / impregnation impregnation impregnation impregnation coimpregnation coimpregnation coimpregnation coimpregnation impregnation impregnation Sequence impregnation Sequence impregnation

Solution; Sufleort ' / CrCIs'6 HzO; AIF3 CRC13-6HzO; AlE3 CRC13-6 HzO; AlE3 CRC!3-6 HzO; AIF3 CRC13-6HzO/CaCI2"2 HzO; AIF3 CRC13-6 HzOICaClz'2 HzO; AIF3 CRC13-6 HzO/CaClz'2 HzO; AIF3 CRC13"6HzO/CaClz'2 HzO: AIF3 MgClz-6 HzO; AIF3 CaClz-2 HzO; A1F3 CRC13"6HzO; Mgli CRC13"6HzO; Cali

(}omposition % W/w / 1% Cr 2% Cr 5% Cr 10% Cr 10% Cr, Ca 0,05% 10% Cr, Ca 0,1% 10% Cr, Ca 0,2% 10% Cr, Ca 0,5% 1% Mg 1% Ca 10% Cr, Mg 1% 10% Cr, Ca 1%

We prepared samples with different contents of chromium and with chromium and magnesium or calcium: the latter were introduced either by a single impregnation with a solution of both chromium and calcium (or magnesium) chlorides (coimpregnation, c) or in two steps, first with calcium (or magnesium) and then with chromium (sequence impregnation, is). After each impregnation, the catalyst was treated at 360~ in a nitrogen flow for 4 hours. We studied the sequence impregnation in order to identify the interaction of the elements already introduced with the chromium introduced subsequently and to verify how the interaction between the phases of chromium and the support can be modified.

944 In order to investigate the structure of the catalysts, various characterization techniques are used since these can provide complementary information. The FT-IR-absorption spectra have been recorded in transmittance using a Perkin Elmer 1750 Infrared Fourier Transform, in a range between 400 and 4000 cm -1. To analyse the samples, the tablets are prepared with potassium bromide. The diffuse reflectance UV-Vis spectra have been obtained between 190 to 900 nm using a Perkin Elmer UV/VIS/NIR Spectrometer Lambda 19. The X-ray diffractograms have been obtained with a diffractometer Philips PW 1050/81, provided with a PW 1710 unit (wavelength 1,5406 nm, CuKa emission). The total surface area of the catalyst was determined using a Carlo Erba Sorpty 1750 by applying BET method. The pores volume and size distribution were established with a Sorptomatic instrument 1900 Carl Erba, using the nitrogen desorption isotherm of the sample, assuming cylindrical pores and physical adsorption of the gas (theory of Wheeler).

2.2 Catalytic tests The tests have been carried out with use of a pulse microreactor. To minimize the hazards involved in handling anhydrous HF, this acid was produced in situ by thermal decomposition of NaHF2 and carried to the reactor by means of a nitrogen stream. In order to run the reaction, pulses of the reagents (10 lal) were fed. The reaction occurred between adsorbed hydrofluoric acid, or fluorinated species on the surface of the catalyst, and the organic reagent. This technique allows a "preliminary screening" using relatively small amounts of catalyst (0,6 g); the obtained data are not directly compared with those obtained from a continuous operation reactor, but they allow to compare the activity of the various catalysts prepared. The data of catalytic activity are about constant after every pulse of reagent: it confirms that the adsorbed HF is sufficient and small amounts of reagent leads to a pseudo stationary state. After the thermal treatment, the catalyst was loaded in the reactor at 350~ at this temperature the hydrofluoric acid, produced in situ in a second reactor (reaction (3)), is transferred for 1 hour (60 ml/min) on the surface of the catalyst by means of carrier of nitrogen. NaHFzt~) ~ NaFt~) + HF~)

T=215~

(3)

The same nitrogen carrier is used during the catalytic tests in order to carry out the pulses of the organic reagent and, in continuous, the off-gases are directly analysed via GC (Carlo Erba Fractovap 2350, packing column, I=420 cm, active phase: SP 1000 1% on Carbopack 60/80). The hypothesis is that the losses in the carbon balance, after verifying the seal of the experimental system, are correlated to the formation of heavy products adsorbed on the surface of the catalyst and, therefore, they are not revealed from the gaschromatographic analysis; for this reason, the data relative to the yield of oligomers is obtained from the data of the carbon balance.

945 3. RESULTS AND DISCUSSION 3.1 Study of the AIF3 support and the introduction of chromium in the composition of the catalytic material. The support is constituted of mixed phases of A1F3 and is synthesized industrially from 7-A1203. The IR spectrum of the support has shown some characteristic absorption bands of AIF3 hydrate [17]. counts~ 4000~

XO

3500-

Fig 1. X-ray diffractogram of the support A1F3:

30002500~

(X) 7*-A1F3, (+) 6A1F3, (O) [3-A1F3

2000~ 1500XO

1000

++

500-. 0

10

+ ''''

+

I ....

I ....

20

I''

' ~

30

40

....

I ' ' ' '

50

I ....

I ' ' ' ' I ' ' ' '

60

~

70

,.

'l ne A-Kay Olrrractogram trig. 1) snows me presence or various crystaitme phases of AIF3, besides other diffuse reflections attributable to the amorphous phases. In particular the 7*AIF3 phase, or phase of Christoph [18] (to distinguish it from the codified 7-AIF3 phase from JCPDS) is present as the main component. There are also peaks with small intensity relative to 6-AIF3 and [3-AIF3. The diffuse reflections are centred in correspondence with the main one of the various aluminium fluoride phases and can be attributed to amorphous phases of the same ones. After introduction of chromium on A1F3, FT-IR spectrum did not show substantial changes relative to the single support: on the other hand these spectra have been obtained in transmittance with use of KBr pellets. This technique allows the characterization of the bulk, but it is very difficult to characterize the chromium species on the surface. The characterization of the samples obtained after the various preparation steps of the catalyst through UV-Vis spectroscopy in diffuse reflectance is reported in Fig. 2. The spectra have allowed us to verify the introduction of chromium in the composition of the catalytic materials. After impregnation (Crl0iN), the absorptions are measured at wavelengths that do not much differ from the absorption of chromium (III) chloride in octahedral coordination, characterized by two bands of ligand-metal charge transfer between 210 and 280 nm and two bands relative to the d-d transitions with absorptions between 450 and 630 rim. The thermal treatment (Crl0iT) has caused a widening of the second band (around 300 nm) attributable, from the data of the literature [19], to the

946 formation of chromium species with valence larger than three, probably formed by the presence, even if limited, of oxygen in the carrier of nitrogen.

13,8

~21 ~

I~r~b

-~

1L'. ~/A~-"~ ) ,---'~ 91~2.!,~/\~os\ '~ /" "N~Crl01T 87~/~ ] ~ ~ \ 4~ / Crl0iS ~\ .t}tt v \ ~ L~_~ / ~ ~\ 5~12~7 \ ~ ~\~..//// ~ ! "~k \ ~i

Fig 2. UV-Vis spectra of the samples containing 10 wt% of chromium aider drying (Crl0iN), after thermal treatment (Crl0iT) and . unloaded after catalytic (Crl0iS)

0,1 190,0

250

300

350

400

450

$00

550

Rl0

650

700

750

800

850

900,0

J.~vJ.

We can also observe a shift of the absorption bands at elevated wavelengths, probably by interaction between the support and the active phases formed during the preparation of the catalyst; the shift of these bands is caused by the variation of the chemical neighbourhood of chromium with a consequent variation of the energetic levels involved in the d-d transitions. The sample unloaded from the reactor after the catalytic test (Crl OiS) shows a shift towards smaller wavelengths, especially of the second d-d band (around 645 nm): this can be explained by the introduction of fluorine in the chemical neighbourhood of chromium which determines a greater splitting of the d orbitals and a consequent increase of the energy corresponding to the relative absorption of this electronic transition. It is not possible, however, to identify exactly the chromium phases because of the simultaneous presence of various ligands like chlorine, oxygen and fluorine on the surface of the catalyst. The X-ray diffractograms of these samples (Fig. 3) do not show the appearance of further crystalline phases regarding the support, and the relative intensities do not change during the various stages of the preparation: this indicates that the chromium phases introduced are amorphous or microcrystalline. The role of the support is to disperse the active phases containing chromium and, inhibiting their crystallization, to give greater surface area and catalytic activity. In the literature, however, there are examples of catalytic materials with active phases containing chromium partially crystalline, characterized by lower surface area [20]. The UV-Vis absorption spectra of the catalysts with increasing content of chromium, after thermal treatment, do not show shifts of the various bands but only a progressive increase of the intensities of the absorptions. The comparison of X-ray diffractograms of

947 these samples aider thermal treatment does not show the appearance of reflections different from those relative to the support A1F3. It is confirmed that the introduction of chromium between 1 and 10 wt% with respect to the support, by this procedure of synthesis, leads to the formation of amorphous or microcrystalline phases. counts~

OI .... 10

i"

' ' '"i .... 20

~ ....

~ .... 30

i'"''i

.... 40

i ....

i .... 50

! ....

w .... 60

i ....

I 70

Fig 3. X-ray diffractogram of the sample containing 10 wt% chromium

It was very interesting to determine the pore volume and size distribution through N2 porosimetry. Fig. 4 shows the distribution of the pores radius of the AIF3 support and of the sample containing 10 wt% chromium thermally treated (Crl0iT). Note that the former is characterized by a wide distribution of the pore radii, while the latter has a more homogeneous distribution, with a value of about 20 A. It can be assumed that the mixed phase comaining chromium covers the porosity of the support, determining the porosity of the solid alter thermal treatment (Fig. 5). The support has its own catalytic activity (see Fig. 6), not only in the halogen exchange reaction, but also in the production of the olefinic by-product, CFC-1113, and in the formation of heavy products. The average catalytic data of the samples with different contents of chromium (Fig. 6) are characterized by an increasing activity (Conv124) corresponding to the higher surface area.

948 l

~

e

s

~

l!meu

,004

,006

9

'

......

~

d

i

~

...... .

~

~"~ '...

"'i

.......... .

. . . . . . .

Crl0iT

~,'D~

(c=3/g)/A 9OOZ

_ L

(~31g) tJk

[

.0031.

....

i

.

.

n~,~(l)

.

.

.

.

.

.

.

looo

0

. . . . . . . .

1

!

~

~us(J)

t

.

.

.

.

.

_

1ooo

Fig. 4. Distribution of the pore radius of A1F3 and of the sample with 10 wt% chromium thermally treated (Cr 10iT)

Fig. 5. Morphological evolution of the catalyst The introduction of chromium leads to a progressive increase of the selectivity to the halogen exchange product (HFC -125) and, regarding the support, it is obvious that it is reduced towards the CFC-1113 product. It can be assumed that the active phase containing chromium, introduced on the surface of the support, is characterized by sites of different nature, only selective for the reaction of halogen exchange: it is therefore necessary to cover completely the surface of the support, evidently not inert. The increased activity leads to an increment of the yield of "heavy products" adsorbed on the surface of the catalyst.

949

Fig. 6. Average catalytic data of the sample with increasing content of chromium

3.2 Effect of calcium

In order to solve the problem of the formation of heavies, an alkaline element like calcium that modifies the acid nature of the catalytic material has been introduced in the composition of the catalyst. The UV-Vis spectra of the samples obtained by coimpregnation to achieve various amounts of calcium and 10% by weight of chromium, are very similar to the 10% weight one (Crl0iT). This can be interpreted considering that the calcium does not have absorptions in this range of wavelengths and it does not possess such size as to modify the structure. The X-ray diffractograms do not show the appearance of new phases.

Fig. 7. Average catal~ic data of the sample prepared by coimpregnation Cr/Ca

950 The catalytic activity of the catalysts prepared by coimpregnation is similar (Fig. 7): the hypothesis is that clusters of calcium can be formed and the effect of this poorly dispersed element is independent from its concentration. The introduction of calcium determines only a small increase of the catalytic activity compared with the sample containing chromium, but the product distribution shifts to the formation of heavy products. Moreover, all the data confirm that the presence of chromium reduces the formation of CFC-1113.

3.3. Sequence impregnation The catalytic data (Fig. 8) show that the activity of the samples prepared by impregnation of AIF3 with 1% calcium or 1% magnesium is similar to that of the support. The data confirm the hypothesis that the formation of the olefirtic by-product CFC-1113 is connected to the sites present on the surface of the support: the introduction of chromium, covering such sites, diminishes the selectivity towards this by-product.

Fig. 8. Average catalytic data of the sample with magnesium and calcium The introduction of magnesium and calcium before impregnation with chromium does not modify the UV-Vis spectra, neither before nor after catalytic test. The introduction of 10% chromium in this sample already impregnated determines a surface area similar to that of Crl 0iT; this means that chromium is probably homogeneously distributed on the surface. The catalytic data of the samples obtained by sequence impregnation (Fig. 9) demonstrate the effective influence of the element introduced on the support before impregnation with chromium. The catalyst MgCrl0is, prepared by impregnating with chromium the sample with 1% Mg (Mgli), has an activity comparable to that of the sample containing only chromium, but a lower yield in the product HFC-125 and a higher yield in oligomers.

951

Fig. 9. Average catalytic data of the sample prepared by sequence impregnation The effect of this kind of preparation is more obvious for the sample CaCrl0is, prepared by impregnating with 10% of chromium the sample containing 1% Ca (Cali). This catalyst shows in fact that the product distribution has shifted from heavy products towards HFC-125, but the catalytic activity is low. This result is interesting because, by m o d ~ g the relative content of calcium and chromium, a more active and selective catalyst could be obtained. All the data confirm that the presence of chromium strongly reduces the formation of the CFC- 1113 product. 4. CONCLUSIONS The use of a pulse microreactor, without continuous use of HF, has allowed the study of catalysts for the halogen exchange reaction in gas phase. The AIF3 support, characterized by different allotropic phases, has a wide distribution of the pore radii. Incipient wetness impregnation method has allowed the introduction on the support of chromium phases with amorphous or microcrystalline structure. The N2 porosimetry has shown that these phases cover the surface of the support, determining a more homogeneous pore radius distribution of the catalytic material. The catalytic tests showed that the AIF3 support was active not only in the synthesis of HFC-125 but also in the formation of the olefmnic by-product CFC-1113, and in the formation of "heavies" (oligomers, adsorbed on the catalyst). The presence of chromium improved the activity of the catalyst and the selectivity to hydrofluorocarbon, and limited the formation of oligomers. The effect of chromium on the formation of CFC-1113 is proportional to the Cr loading: thus, it appears that the support is the main (or sole) responsible for the dehalogenation reaction (by 1,2 elimination), which is repressed when the chromium phase covers the support surface and blocks the access to the dehalogenation sites. However, the formation of heavies remained important. So, we tried to modify the catalyst by introducing calcium. This was extremely interesting: when mixed with chromium (sample Crl0Ca0,1c), Ca had the sole effect of promoting the production of heavies; but

952 when Ca was introduced before chromium (sample CalCrl0is), the formation of heavies was minimised, to just a small reduction of the yield to HFC-125; it is thus readily apparent that, in the latter case, the effect of calcium was to modify the interaction between the chromium (III) active phase and the AlF3support. REFERENCES

1. T. Midgley Jr., Ind. Eng. Chem., 29 (1937) 239. 2. M.J. Molina and F.S. Rowland., Nature, 249 (1974) 810. 3. G. Webb and J.M. Winfield, Chemistry of Waste Minimization, ed. J.H. Clark, Blakie Academic and Professional, (1995), pp 222. 4. A. McCulloch, J. Fluorine Chem., 100 (1999) 163. 5. Z. Ainbinder, L.E. Manzer and M.J. Nappa, Handbook of Heterogeneous Catalysis (Wiley, VHC), vol. 1, (Environmental Catalysis), (1997) 1677. 6. D.M.C: Kavanagh, T.A. Ryan and B. Mile, J. Fluorine Chem., 64 (1993) 167. 7. L.E. Manzer and V.N.M. Rao, Adv. Catal., 39 (1993) 329. 8. E. Kemnitz and D.H. Menz, Prog. Solid St. CherrL, 26 (1998) 97. 9. P. Cuzzato, L. Bragante and F. Rinaldi, US Patent 5,981,813 (1999) Ausimont SpA. 10. J. Kijowsky, G. Webb and J.M. Winfield, Appl. Catal., 27 (1986) 181. 11. J. Barrault, S. Brunet, B. Requieme and M. Blanchard, J. Chem. Soc., CherrL Commun, (1993) 374. 12. J.M. Rao, A. Sivaprasad, P.S. Rao, B. Narsaiah and S.N. Reddy, J. Catal., 184 (1999) 105. 13. A. Hess and E. Kemnitz, J. Catal., 149 (1994) 449. 14. D.R. Coulson, J. Catal., 142 (1993) 289. 15. A. Farrokhia, B. Sakanini and K.C. Waugh, J. Catal., 174 (1998) 219. 16. E. Kemnitz, G. Hansen, A. Hess and A. Kohne, J. Mol. Catal., 77 (1992) 193. 17 C. Morterra, G. Cerrato, P. Cuzzato, A Masiero and M. Padovan, J.Chem.Soc., Faraday Trans, 88 (1992) 2239. 18. M.F.J. Christoph Jr. and G. Teufer, FR 1383927 (Du Pont) 1964. 19. H. Lee, H. S. Kim, H. Kim, W. S. Jeong and I. Seo, J. Mol. Catal., 136 (1998). 20. K. Tsuji and T. Makajo, EP 0641598 (Showa Denko Kabushiki Kaisha) 1994.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

953

Preparation, characterization and r e a c t i v i t y in m - c r e s o l m e t h y l a t i o n of new heterogeneous materials having basic properties F. Cavania, C. Fellonia, D. Scagliarinia, A. Tubertini a, C. Flego b, C. Perego b a Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy. Tel. & fax: +39-051-2093680. E-mail: [email protected]

b EniTecnologie SpA, Via Maritano 26, 20097 S. Donato Milanese (MI), Italy Tri-component mixed oxides, Mg/A1/La, Mg/AI/Ce and Mg/AI/Zr, were prepared with the co-precipitation technique, with the aim to improve the basic properties with respect to the reference Mg/A1 mixed oxides, obtained from Mg/AI hydrotalcite-like precursors. For samples containing Ce and Zr, the basic properties (as determined by COz adsorption and TPD) were considerably improved with respect to Mg/AI/O, whereas for the La-containing system a decrease in basicity was observed. These materials were tested in m-cresol methylation, in both liquid- and gas-phase reactions. A relationship was found between basic features and catalytic performance. 1. INTRODUCTION The development of heterogeneous basic catalysts, to replace homogeneous systems commonly used in the production of fine chemicals, specialties and pharmaceuticals, is one target of current research in catalysis [1]. The benefits of heterogeneous systems are wellknown and concern not only easier catalyst separation but also lowering of the environmental impact by reduction of the amount of inorganic salts in effluents. Several different classes of solid bases have been used, including Mg/Al mixed oxides obtained starting from hydrotalcite-like precursors (HT) [2]. These materials have been reported as catalysts for many different reactions, such as the Claisen-Schmidt condensation [3] and the Knoevenagel condensation [4]. Mg/A1 mixed oxides obtained by HT decomposition show the presence of medium basicity [5], while pure MgO possesses strong basic sites [6,7]. The basic properties of calcined hydrotalcites are a function of the Mg/AI ratio [2]; when the A1 amount increases, the total number of basic sites decreases and the proportion of the stronger ones increases [8]. Several preparation parameters may play an important role in improving the basic properties and the catalytic features: changing the divalent or the trivalent elements, addition of a third metal ion, and variation of the crystal size [9,10]. The aim of the present work was to prepare tri-component materials (Mg/A1/La/O, Mg/AI/Ce/O and Mg/A1/Zr/O) by co-precipitation, a procedure analogous to the preparation of Mg/AI/O hydrotalcite-like precursors. The catalytic properties of these materials were studied in the reaction of m-cresol methylation. The distribution of products

954 in this model reaction can give useful information on the basic and acid properties of the materials [11]. 2. E X P E R I M E N T A L

Mg/AI mixed oxides were prepared following the conventional procedure of coprecipitation, as described elsewhere [2]. Mg/A1/Me/O systems (Me=Ce, La, Zr) and the corresponding reference oxides (Mg/O, La/O, Ce/O and Zr/O) were prepared with the same procedure, using La(NO3)3"6HzO, Ce(NO3)3"6HzO and ZrO(NO3)z'6H20 as La, Ce and Zr sources, respectively. The dried samples were calcined at 450~ for 8 hours. The catalysts were characterized by means of (i) X-ray diffraction (Philips PW 1050/81), (ii) surface area measurement (single point BET, Sorpty 1700 Carlo Erba), (iii) adsorption and thermal-programmed-desorption (TPD) of CO2 (PulseChemisorb 2705, Micromeritics), and (iv) atomic absorption spectroscopy (Philips PU 9100). In the TPD experiments, the overall density (~tmol/g) of the basic sites was evaluated from the adsorption of CO2 at room temperature. The presence of physisorbed CO2 was avoided by carrying out the adsorption under a helium flow. The basic strength distribution was evaluated from the capacity of the material to retain the probe during desorption at increasing temperature. A deconvolution procedure was applied to the experimental TPD profiles; curve fit quality was measured by the correlation coefficient (R2), standard deviation (o) and the Levenberg-Marquardt algorithm (X2). The quality of the deconvolution was high in all experiments: Rz>0.997, 6<0.006 and ;~2<1.036. The liquid-phase catalytic tests were carried out in a 300 mL Parr 4560 autoclave reactor loading 32 g of m-cresol, 19 g of methanol (molar ratio m-cresol/methanol 0.5) and 5.7 g of catalyst. The initial molar ratio between m-cresol and methanol at reaction conditions in the liquid phase was equal to 2.3 (in the gas phase 0.31). The reaction temperature was 300~ the initial autogenous pressure was 38 atm and the final pressure was around 35 atm. The reaction was carried out for six hours and the stirring rate was 700 min -1. The testing procedure can be summarized as follows: the catalyst is loaded first, the reactor is sealed and evacuated, and the m-cresol/methanol mixture is then loaded. Heating up is started, and the beginning of the reaction time is taken as being when the temperature reaches 300~ The gas-phase catalytic tests were carried out in a continuous-flow reactor operating at atmospheric pressure in a temperature range between 250 and 450~ A mixture of mcresol and methanol (1/5 molar ratio) with a molar flow of 0.03 mol/h (F) was injected using a syringe pump in a Nz flow of 60 mL/min (Nz/reactants = 5/1 molar ratio). The catalyst bed contained 1.5 g catalyst (30-60 mesh) which results in a W/F ratio of 50 g.h.mol 1. The reaction products were collected during 60 min time-on-stream. The reaction mixture (liquid and gas phases) was analyzed using a gas chromatograph equipped with a HP-5 capillary column. The activation of the catalysts before reaction was carried out at 450~ for 3 h in a gas-chromatograph under N2 flow.

955 3. R E S U L T S AND DISCUSSION 3.1. Characterization of the materials Table 1 reports the experimental composition of calcined samples, as determined by atomic absorption, together with the values of specific surface area. The surface area of the tri-component samples is lower than that of the Mg/Al mixed oxide having the same Mg/A1 ratio, especially in the case of the La-containing samples. This could be due either to the formation of mixed oxides with morphological features different from that of Mg/A1/O, or to the blocking of Mg/Al/O pores by Me/O crystallites.

Table 1. Main features of the samples prepared. Nominal composition of the Real composition, oxides, atomic ratios atomic ratios Mg La Ce Zr Mg/AI 2/1 1.8/1 Mg/AI 4/1 3.7/1 Mg/AI/La 4/1/1 4.3/1.3/1 Mg/AI/Ce 4/1/1 5.3/1.8/1 Mg/AI/Zr 4/0.5/1.5 4.7/0.5/1.7

Specific surface area,

mZ/g 206 <10 21 <10 185 209 85 120 126

Both the Mg/AI/La/O and Mg/A1/Ce/O precursors give XRD patterns corresponding to the presence of a hydrotalcite-like structure and a Me hydroxy carbonate phase (Me = La, Ce). The La containing material is much better crystallized than the Ce containing one. The Mg/AI/Zr/O precursor is characterised by a pattern similar to that of the Zr/O precursor, indicating the absence of any crystalline precipitate. In this case, therefore, the presence of Zr hinders the development of the crystalline hydrotalcite-like precursor. The X-ray diffraction patterns of the calcined Mg/A1/O and of tri-component systems are shown in Fig. 1. In the case of the former samples, a MgO-like phase is observed. By increasing the A1 amount, a shift towards higher 20 values is found due to a decrease in the volume of the crystallographic cell of MgO (periclase), in agreement with the progressive isomorphic replacement of Mg z+ cations with the smaller A13§ cations (Mg z+ = 0.65 ,~; AI 3§ = 0.50 ,~). In the Mg/A1/O 4/1 pattern, additional reflections are present, which can be attributed to the presence of the spinel-like phase MgAlzOn. In the tri-component systems the characteristics of the third metal ions influence the pattern of the calcined samples. In the case of the Mg/AI/Ce/O sample, the dominant reflections are those of CeOz, coincident (in position, shape and relative intensity of the peaks) with those of the reference Ce/O sample. Less intense reflections of Mg/AI mixed oxide are also present" this suggests that A1 is present in the MgO lattice. Similarly, in the Mg/A1/La/O system the highly crystalline pattern (the same as that observed for the reference La/O sample) is superimposed on the broad reflections of Mg/AI mixed oxide. The 20 values seem to be coincident with those of

956 the reference compound Mg/AI/O 4/1. Therefore, in these materials it is probable that segregation of Mg/AI/O and of Me/O (Me = Ce, La) occurs, since the two oxides maintain the same morphological features of the corresponding pure oxides. In Mg/AI/Zr/O pattern the main reflections relative to Mg/A1/O are still present and only weak and very broad reflections attributable to ZrOz at 20 50-52 ~ and = 60 ~ are observed, while the main reflection of ZrOz (at 20 = 30 ~ in the reference single oxide) is shifted towards higher angles (20 32-33 ~) and considerably broadened. In the same region, a broad reflection typically is observed for Mg/AI/O systems, attributed to extended cationic defects, as due to the introduction of excess positive charges [12,13]. In Mg/A1/Zr/O the attribution of the broad reflection at 20 32-33 ~ either to MgZ+-doped ZrOz or Zr4+-doped Mg/AI mixed oxide is not possible, but nevertheless indications exist of the interaction between cations in this tri-component system. The COz-TPD profiles for the tri-component systems and for the reference Mg/AI/O 2/1 and 4/1 samples are shown in Figure 2. The complex desorption profiles are clearly related to the presence of basic sites with different strengths. For all the samples the profiles consist of: i) one low temperature peak, with maximum desorption at temperatures lower than 100~ and attributed to the interaction with sites having weak basic strength, ii) one desorption peak with a maximum in the range 140-170~ related to desorption of COz from sites at medium basic strength, and iii) one broad desorption area which covers the temperature range from 200 to 450~ attributed to COz desorption from sites with strong basicity. The results of the deconvolution procedure for all samples are summarized in Table 2, together with the overall amount of COz adsorbed at room temperature, and the overall amount of COz desorbed, as determined from the desorption profile. When a good correspondence exists between the two values, this means that no COz is retained by the sample at the end of the desorption experiment.

* MgAI204 spinel o MgO-likephase x CeO2 cenanite /

~

,

Countsls

U

(A.U.)

~

[A

~

/i X

.j/

x

J.

'

~b .... '

x

\, / ~

,

1'0

l

I~

MglAIILa/O

t j ~ ~ ~ U t . . ~

~

.

,~....j"\

j

~.

'

.]t.~

',....,.j

4'0

x

./~,....,.,

/'\.~.

~, ....

9

'

Mg/AI/Ce/O

5~ .... '

6~

'

Mg/AI/O4/1 ,~

~'0 .... '

~

80

Fig. 1. X-ray diffraction patterns of Mg/AI/O samples and of the tri-component systems Mg/AI/Ce/O, Mg/A1/La/O and Mg/A1/Zr/O.

957 Table 2. Results of the COz adsorption and of the deconvolution procedure of TPD profiles. Total amount of Total amount of Weak Sample Medium Strong (oxides) adsorbed COz desorbed COz, basic sites, basic sites, basic sites, 9mol/g Fmol/mz ~tmol/g lamolCOz/g lamolCOz/g lamolCOz/g Mg 356 1.73 346 56 25 265 Ce 62 2.95 59 4 0 58 Zr 6 0.60 6 1 0 5 Mg/A1 2/1 263 1.42 274 50 148 77 Mg/AI 4/1 144 0.69 146 25 68 52 Mg/AI/La 122 1.43 85 14 8 62 Mg/AI/Ce 350 2.92 350 27 150 173 Mg/AI/Zr 429 3.40 441 18 76 347 The very low density of the basic sites of single oxides other than MgO (which has many basic sites) is due to the very low surface area. In Mg/AI/O systems the presence of A1 in the periclase lattice modifies the strength distribution with respect to MgO, and the medium-strength basic sites become the dominant fraction [11]. In Mg/AI/La/O, the considerable difference between the number of adsorbed and desorbed COz molecules suggests the presence of strong and very strong basic sites. However, the overall number of basic sites is low, if compared to that of the other multi-component samples, in agreement with the low surface area. These results can be explained by the presence of lanthanum oxide (or carbonate) located on the surface of the sample. The addition of Ce increases the overall number of basic sites with respect to Mg/AI/O samples, notwithstanding the lower surface area; also, an increase in the relative amount of the strongest basic sites occurs. The presence of Zr in the Mg/A1/Zr/O system causes the most significant improvement in the basicity. The overall number of basic sites increases considerably with respect to both Mg/AI/O samples and to Mg/O as well, and sites with strong basicity become predominant. This effect can not be simply attributed to the basicity of zirconia, and suggests the presence of a strong synergy between the components, as also indicated by the XRD characterization. It is possible to hypothesize a partial dissolution of Zr4§ in the Mg/A1/O lattice, with generation of an unbalance in positive charges, compensated by the formation of cationic vacancies and thus of coordinatively unsaturated and strongly nucleophilic O z. ions.

3.2. Reactivity in the liquid-phase m-cresol methylation The catalytic performances in the liquid-phase methylation of m-cresol are summarized in Table 3. For each catalyst the m-cresol conversion after six hours is given, together with the following selectivity parameters: (i) O/C ratio, i.e., the ratio between the selectivity to 3-methylanisole (the product of O-alkylation) and that to dimethylphenols (the products of C-alkylation); (ii) o/p ratio, i.e., the ratio between the selectivity to 2,3-dimethylphenol plus 2,5-dimethylphenol (the products of ortho-C-alkylation) and that to 3,4-dimethylphenol (the product of para-C-alkylation); (iii) 2,5/2,3DMP ratio, i.e., the ratio between the selectivity to 2,5-dimethylphenol and that to 2,3-dimethylphenol (both products of ortho-C-alkylation); and (iv) selectivity to the products of side-chain alkylation (e.g. thymol). Besides these products, heavier polyalkylated compounds were also found. All these parameters are

958 meaningful since the reaction scheme consists of parallel reactions for the formation of the main products, and thus the parameters are not a function of m-cresol conversion (at least in a moderate range of reactant conversion) [11]. 500 Mg/AI/TJ'/O

Temperature

_ 450 _

400 350 300 250 ~" o 200 150 lO0 50

0

,

,

5

10

,

~,

,,

15

j

J

20

25

0 30

Time (rain)

Fig. 2. CO2-TPD profiles of Mg/Al/O samples and of the tri-component systems.

Table 3. Summary of reactivity of samples in m-cresol methylation. Sample Conv., % O/C ratio a o/p ratio a 2,5/2,3DMP a (oxides) Mg 2.9 7.3 4.0 1.2 La 2.0 17.1 4.2 1.4 Ce 4.2 0.9 2.5 2.6 Zr 2.1 10.1 2.1 1.8 Mg/A12/1 12.9 4.7 28.2 1.0 Mg/A14/1 4.3 6.1 16.6 1.0 Mg/AI/La 2.7 10.3 4.1 1.2 Mg/AI/Ce 2.4 2.0 2.0 1.8 Mg/AI/Zr 2.4 6.7 10.2 1.2 aSee text for explanation.

Side-chain, Heavy, sel.% a sel.% 0.5 0.0 2.1 12.9 18.3 12.7 14.4 13.5 0.3 0.0 0.7 0.0 9 10.7 1.3 8.2 4.8 0.0

In alkylation of phenol derivatives, the O/C ratio usually decreases when the catalyst acid or basic strength increases [11, 14-16]. The situation becomes more complex when the strong basic sites deactivate under reaction conditions, as usually happens in liquid-phase reactions. The o/p ratio is the highest (up to values higher than 30) for basic catalysts and the lowest (~'~2)for acid catalysts, but is also affected by the "softness" or "hardness" of the cation bound to the basic oxygen [17]. A soft cation may favour the interaction with the

959 aromatic ring and hence the planar adsorption of the molecule, making possible the attack by the alkylating agent on both ortho and para positions. On the contrary, with hard cations the repulsion between basic sites and the aromatic ring favours the vertical adsorption of the molecule and hence the alkylation in the ortho position, leading to a high o/p ratio. Also the 2,5/2,3DMP ratio depends on the m-cresol adsorbed state; as the interaction between the aromatic ring and the catalyst surface increases, the steric hindrance of the methyl substituent in the meta position hinders a fully planar coordination of the ring, making the ortho position adjacent to the methyl less favoured for C-methylation; in this case the 2,5/2,3DMP ratio then becomes closer to 1. All samples show a low m-cresol conversion, mainly because of the reaction conditions (low temperature and poor solubility of the methanol in the liquid phase). The conversion is particularly low for both samples with a low surface area and samples having a low number of medium basic sites. In the latter case catalyst deactivation occurs because formaldehyde (precursor of heavy compounds) is formed by methanol dehydrogenation on strong basic sites; the latter then soon deactivate. The most active catalyst is the Mg/A1/O 2/1 sample, characterized by the highest number of medium-strength sites. A comparison of the O/C ratios with the density of medium-strength basic sites (Fig. 3) indicates that the C-alkylation becomes preferred when the number of these sites increases [11]. For instance, their absence in the La-containing sample is evident in the value of the O/C ratio, the highest of all samples. Also the Mg/O, Zr/O and Mg/AI/Zr/O samples show a high O/C ratio, in agreement with the presence of a low fraction of medium-strength sites. Such a strict influence on the catalytic performances is not found for weak and strong basic sites. Mg/Al mixed oxides show intermediate values of O/C ratio and represent the best compromise between sites of different strength.

Fig. 3. Comparison of the O/C ratios, o/p ratios and 2,5/2,3DMP ratios with the concentration of medium-strength basic sites. The trend of the o/p ratio follows that of the medium-strength basic sites, except for the Mg/AI/Ce/O system (Fig. 3). A high ratio indicates preferred vertical coordination of the molecule on these catalysts, and thus predominance of the catalyst basicity effect over a

960 Lewis acid-like effect of interaction with the aromatic ring (leading to a more planar coordination of the aromatic ring). For the Ce-containing sample, the low o/p ratio, together with the highest 2,5/2,3DMP ratio, indicates predominance of the coordinative properties of cerium: because of its high ionic radius Ce interacts with the aromatic ring causing a more horizontal-like adsorbed state for m-cresol. This is likely also favoured by the spreading of Ce/O over the Mg/A1 mixed oxide. The lowest 2,5/2,3DMP ratios and the highest o/p ratios are exhibited by the Mg/AI/O samples, which contain the hardest cations; in these cases the vertical coordination of the molecule is preferred. The performance of the Mg/AI/Zr/O system is the closest, in terms of the distribution of the products, to that of the Mg/AI/O system. The features of Zr, in terms of"hardness", are the closest to those of the host matrix, but are sufficiently different to give rise to strong basic sites, and to enhance the coordinative properties, finally leading to a lower o/p ratio and a higher 2,5/2,3DMP ratio with respect to the Mg/AI/O systems.

3.3. Reactivity in the gas-phase m-cresol methylation The main drawbacks of the liquid-phase m-cresol methylation (low conversion and low influence of strong basic sites) may be overcome by carrying out the reaction at higher temperature. The results of tests carried out in the gas phase for the Mg/AI/O 2/1 and Mg/AI/Zr/O samples are shown in Fig. 4. In the gas phase, the reactions of C-alkylation are largely preferred with respect to those of O-alkylation. The difference in activity between the two samples is maintained even under these conditions, but the o/p ratio of the Mg/AI/Zr/O system becomes considerably higher than that of the Mg/AI/O systems at temperatures higher than 400~ confirming a possible contribution of strong basic sites under these conditions.

Figure 4. Conversion (continuous line), O/C ratio (drawn line) and o/p ratio (dotted line) for the Mg/AI/O 2/1 ( 9 ) and Mg/AI/Zr/O (--) samples in the gas-phase m-cresol methylation.

961 4. CONCLUSIONS Mg/AI/Me/O systems with basic characteristics different with ,respect to those of the reference Mg/A1 mixed oxide were prepared by the co-precipitation technique. While with the Mg/AI/Ce/O and Mg/AI/La/O systems no real mixed oxide was obtained, in the Mg/A1/Zr/O system the possible formation of a solid solution led to a considerable enhancement of the basic properties. In the Mg/A1/Ce/O system, nevertheless, an increase in surface basicity was found and attributed to the spreading of Ce/O over Mg/AI/O, representing a way to support a basic oxide with low surface area, and to improve its basicity. A relationship was found between basic features and catalytic performance in mcresol methylation. The improved basicity in tri-component oxides affected the distribution of products, the effect being more relevant under gas-phase reaction conditions than in the liquid phase. ACKNOWLEDGEMENTS EniTecnologie SpA is gratefully acknowledged for financial support. REFERENCES 1. K. Tanabe and W.F. H61derich, Appl. Catal. A, 181 (1999) 399. 2. F. Cavani, F. Trifir6 and A. Vaccari, Catal. Today, 11(2) (1991) 173. 3. A. Guida, M.-H. Lhouty, D. Tichit, F. Figueras and P. Geneste, Appl. Catal., A, 164 (1997) 251. 4. A. Corma, V. Forn6s, R.M. Martin-Aranda and F. Rey, J. Catal., 134 (1992) 58. 5. W.T. Reiche, J. Catal., 94 (1985) 547. 6. T.M. Jyothi, T. Raja, K. Sreekumar, M.B. Talawar and B.S. Rao, J. Mol. Catal. A, 157 (2000) 193. 7. H.L. Pernot, F. Luck and J.M. Popa, Appl. Catal., 78 (1991) 213. 8. T. Nakatsuka, H. Kawasaki, S. Yamashita and S. Koijiya, Bull. Chem. Soc. Jpn., 52 (1979) 2449. 9. A. Vaccari, Catal. Today, 41 (1998) 53. 10. J. P6rez-Ramirez, F. Kapteijn and J.A. Moulijn, Catal. Lett., 60 (1999) 133. 11. M. Bolognini, F. Cavani, D. Scagliarini, C. Flego, C. Perego and M. Saba, Catal. Today, in press 12. M. Gazzano, W. Kagunya, D. Matteuzzi and A. Vaccari, J. Phys. Chem. B, 101 (1997) 4514. 13. A.R. West "Solid State Chemistry and its Applications", John Wiley & Sons, 1984. 14. S.C. Lee, S.W. Lee, K.S. Kim, T.J. Lee, D.H. Kim and J.C. Kim, Catal. Today, 44 (1998) 253. 15. M. Marczewski, J.P. Bodibo, G. Perof and M. Guisnet, J. Mol. Catal., 50 (1989) 211. 16. S. Velu and C.S. Swamy, Appl. Catal., A: General, 162 (1997) 81. 17. A.E. Palomares, G. Eder-Mirth, M. Rep and J.A. Lercher, J. Catal., 180 (1998) 56.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

963

The effect of glycols in the organic preparation of V/P mixed oxide, catalyst for the oxidation of n-butane to maleic anhydride S. Albonetti", F. Cavani a, S. Ligi a, F. Pierelli a, F. Trifir6a, F. Ghelfi b, G. Mazzoni b aDipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy. [email protected] bLonzagroup, Via E. Fermi 51, 24020 Scanzorosciate (BG), Italy.

V/P/O catalysts have been prepared using the organic procedure, by reduction of V205 with isobutanol in the presence of phosphoric acid and of a glycol, chosen amongst 1,2ethandiol, 1,3-propandiol, 1,3-butandiol and 1,4-butandiol. The addition of the glycol considerably affected the morphological features of the vanadyl orthophosphate hemihydrate, the V/P/O catalyst precursor, the organic compounds being retained in the precursor. The thermal treatment of the precursor, aimed at the development of the vanadyl pyrophosphate, led to the removal of the organic fraction. When the treatment was carried out in the absence of molecular oxygen, bulk oxygen was involved in the combustion of organic residues. Consequently a fraction of the V 4+ ions was reduced to V 3+ species, and the compound which finally developed was in part amorphous. In some cases it was possible to transfer the morphological features of the precursor up to the vanadyl pyrophosphate, and thus to finally affect the catalytic performance. 1. INTRODUCTION Vanadyl pyrophosphate (VO)2P207is amongst the most widely studied catalysts in scientific literature. The reason for this is that it is very effective in catalyzing the complex transformation of n-butane to maleic anhydride with good conversion level and selectivity [1,2]. The compound is prepared by dehydration of a "precursor", VOHPOa.0.5H20, vanadyl orthophosphate hemihydrate. The main features of the vanadyl pyrophosphate and the reason why it displays so unique features and catalytic performance have been deeply investigated, and agreement exists on the general features of the V/P/O system (surface Lewis acidity, multifunctional properties [3], presence of isolated V sites able to perform specific redox cycle on the one hand, isolation of selected V sites ensembles for the 14electrons transformation on the other hand [4], and ability to generate oxygen species having peculiar characteristics), and on the specific characteristics the V/P/O must exhibit in order to best exploit these properties. In regard to this, the most important aspect is a crystallite morphology with preferential exposure of those faces relative to the crystallographic plane containing the active sites, the (200) plane, and for this reason the {FWHM [042] / FWHM [200]}, "aspect ratio", is used to establish relationships between

964 crystallomorphic features and catalytic performance [5]. It is worth mentioning that this characteristic may be important in affecting the catalytic performance during ageing, but that finally all catalysts tend to develop the same morphological features (probably close to those characteristics of the "thermodynamic" crystal [6]) and that therefore all catalysts finally behave similarly, regardless of their starting characteristics (apart from the presence of dopants which may affect the redox properties) [7]. But since the equilibration can take long time, during which the performance is considerably affected by those features, many efforts have been devoted to finding methods for modifying the properties of the fresh catalyst. One way is to control the morphology of the precursor, trying to develop crystals which preferentially exhibit the {001 } face; in fact the pseudomorphic transformation (sometimes referred to as topotactic transformation) of the precursor into the vanadyl pyrophosphate leads to a compound characterized by the preferential exposure of the { 100} face, since the (200) crystal plane of the vanadyl pyrophosphate is structurally related to the (001) plane of the vanadyl orthophosphate emihydrate. Amongst the different ways to do this (e.g., addition of dopants [8], tuning of the P/V ratio [9,10]), one of the most successful is the preparation of the precursor made using organic compounds able to intercalate in the interlayer spacings of the layered structure of VOHPOa.0.5H20 [ 10-20]. However, if we look at the performance reported in the literature for catalysts starting from precursors prepared in such a way, the results are sometimes poor; it seems that one crucial step in the preparation of these catalysts is the thermal treatment of the precursor, but little information exists on this step. The aim of the present work was to develop a preparation for the vanadyl orthophosphate hemihydrate which makes use of glycols as the organic compounds able to affect its morphology; these have been used as additional components, besides isobutanol, in the reducing medium. A further aim of the present work was to understand which are the key aspects in the thermal treatment through the control of which is it possible to transfer morphological features of the precursor to the vanadyl pyrophosphate, and thus to finally affect the catalytic performance. 2. EXPERIMENTAL The catalyst precursor, V O H P O 4 . 0 . 5 H 2 0 , w a s prepared by the "organic procedure", using a mixture of 80% vol. isobutanol as the reducing agent for V205 in the presence of H3PO4 and of 20% vol. glycol, chosen amongst 1,2-ethandiol, 1,3-propandiol, 1,3- and 1,4butandiol. The precipitate was filtered, washed with isobutanol and dried at 125~ for 5 h. The compounds obtained were thermally treated by a pre-calcination in air at 300~ (6 h) and a final treatment in nitrogen flow at 550~ Catalysts obtained after this treatment were characterized by means of: (i) X-ray diffraction (Phillips diffractometer PW 3710, with CuKtx radiation); (ii) chemical analysis by means of volumetric titration; (iii) analysis of the C content, using a ELTRA FKV instrument, with combustion of organic residues at 1400~ in 02 flow and measurement of CO2 concentration by TCD; (iv) SEM micrographs, taken with a LEO 435PV Instrument, equipped with a Link Isis probe from Oxford Instruments for microanalysis. DSC and TG curves were recorded simultaneously using a Netzsch STA 409 instrument. Catalytic tests of n-butane oxidation were carried out in a stainless-steel laboratory flow reactor operating at atmospheric pressure, loading 3 g of

965 catalyst shaped in particles. The following reaction conditions were used: feedstock composition 1.7 vol.% n-butane, 17% oxygen, remainder nitrogen. The W/F ratio was equal to 1.3 g s ml1. The products were collected and analyzed by means of gas chromatography. 3. R E S U L T S A N D D I S C U S S I O N

Table 1 reports the main characteristics of the samples prepared: the type of glycol used for the preparation of the precursor and its amount (the remaining being isobutanol), the surface area of the precursor after drying, and the amount of C retained in the precursor. Also, the weight and the number of moles of glycol retained per unit weight of precursor are reported, as calculated starting from the hypothesis that the glycol is the only compound which has been retained in the precursor. Table 1. Main characteristics of the compounds prepared. Sample Glycol,vol.%a Amount of organic compound retained a

1 2 3 4 5

b

c

0.6 0.9 0.012 1,2-ethandiol, 20 2.3 5.9 0.095 1,3-propandiol, 20 2.8 5.9 0.078 1,3-butandiol, 20 1.6 3.0 0.033 1,4-butandiol, 20 1.6 3.0 0.033 a: Amount of organic compound retained, wt. % C b: Amount of glycol retained, wt. % c: Amount of glycol retained, mol./gcat % d: Remaining is isobutanol

Surface area m2/g 7 2 9 2 13 2 nd 11 2

FWHM[220] FWHM[001] 0.70 0.05 0.46 0.05 0.32 0.05 0.65 0.05 0.57 0.05

The X-ray diffraction patterns of samples are reported in Fig. 1, while the corresponding aspect ratio FWHM [220] / FWHM [001 ] is given in Table 1. The weight of organic compounds retained in the precursor, possibly intercalated in the interlayer spacing (which in the vanadyl orthophosphate is 5.6 A wide), is the same for the two shorter-chain alcohols 1,2-ethandiol and 1,3-propandiol, while is lower (and the same) for the two butanediol isomers. Thus it seems that the molecule bulkiness has some role in determining the maximum amount of glycol which can diffuse in the interlayer spacing, or be entrapped during the development of the precursor crystal, in agreement with literature results obtained with other types of alcohols [10,16,17]. The role of molecule bulkiness is also demonstrated by the much lower amount of alcohol which is retained when only isobutanol (a branched-chain compound) is used in the standard preparation procedure. A relationship exists between the C content and the "aspect ratio" of the crystal. Increasing amounts of retained organic compound disturb the stacking of crystallographic planes along the c direction, and only [hkO] reflections remain sharp for increasing C content in the precursor. Correspondingly, the aspect ratio changes considerably, indicating the development of crystals having different morphology. Other differences are observed in the XRD patterns of samples 2 and 3 with respect to that of reference sample 1;

966 additional weak, broad reflections are observed in the 10-20 ~ 20 range, suggesting the development of another, less crystalline compound. Differences in the morphology of crystals agglomerates of also evident in SEM micrographs, as shown in Fig. 2 for selected samples. A more plate-like morphology is observed in samples 1 and 5, and moreover the corresponding primary particles are less agglomerated and more rosette-like. P,mJ.qts/s

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Fig. 1. XRD patterns of samples 1-5, prepared with different mixtures of isobutanol/glycol (see Table 1 for details). When the organic preparation is used for the vanadyl orthophosphate, implying the use of isobutanol as the reductant for V205, various additional organic compounds can be used to affect the morphology of the precursor [10-20]. The effect can be different, but substantially an anisotropic line broadening occurs, with [hkO] reflections noticeably sharper than reflections having the 1 component, in the same way as observed with our samples. This effect is observed by intercalation of benzyl alcohol (often used in the standard organic preparation for V/P/O) [10-12], of linear and branched-chain aliphatic alcohols [ 13,14], and of primary n-alkylamines [ 15], which affect the interlayer spacing by developing a close-packed interlayer structure with the amino groups bonding to P-OH groups. The same effect can be obtained starting from VOPOa2H20 [16,17], or in the preparation of vanadyl alkylphosphonates, where the P-OH group is replaced with P-R group [18-20] and where the spacing increases on increasing the carbon chain length. Precursors have then been treated at increasing temperatures (heating rate 1~ final temperature 300~ in a nitrogen flow, and the products obtained by desorption and transformation of retained compounds have been analyzed by GC in correspondence of different temperatures (Thermal Programmed Reactive Desorption technique). The results are summarized in Table 2, which for each sample reports the nature of main products obtained (besides isobutanol and the original glycol, which anyway were always found in the outlet stream), the formation of carbon oxides and the temperature at which they began to form, as well the total weight loss as determined in TG tests (Tmax500~ Fig. 3 shows the corresponding TG and DSC profiles, both carried out in flowing air.

967

Fig. 2. SEM pictures of samples 1 (top left), 2 (top right), 3 (bottom left) and 5 (bottom right). In sample 2, the only products of the ethylene glycol ('Feb 198~ transformation were those which formed by condensation of two molecules, in the temperature range 200250~ while no formation of carbon oxides was observed up to 300~ This means that the glycol leaves the catalyst without abstracting oxygen ion from the framework, and that probably no weak chemical interaction has developed between the precursor and the glycol. In fact, if a chemical bond had established, the glycol would have probably left the catalyst at higher temperatures, or products of partial or total oxidation would have formed. This does not exclude the possibility that a strong chemical interaction has occurred, leading to products of transformation only for temperatures higher than 300~ Table 2. Results of TPRD tests (in Nz flow), and TG tests (in air flow), and main characteristics of the samples after the pre-calcination and after the final thermal treatment. Sample Products of TPRD tests TG loss, C content, C content, n in wt.O~ wt.o~a wt.o~b vn+ b 1 nd 11.2 0.5 0.3 4.07 2 1,4-dioxane 14.9 1.2 0.1 nd 3 Acrolein, propanal, COx (170~ 14.7 1.8 nd 3.82 5 Tetrahydrofuran, COx (250~ 12.5 1.1 0.4 3.75 aat~er the pre-calcination step; barter the final thermal treatment.

968 Different is the case for sample 3, because 1,3-propandiol (Teb 213~ was transformed to acrolein and propanal already at 150-200~ and to carbon oxides as well. This means that the glycol has developed some kind of interaction with the precursor, and leaves the latter at temperatures lower than those of precursor transformation to vanadyl pyrophosphate. Finally, in sample 5 the only product of selective transformation was tetrahydrofuran (originated by intramolecular condensation of 1,4-butandiol, Teb 230~ but the existence of a chemical interaction between the glycol and the precursor is demonstrated by the appearance of carbon oxides at around 250~ These tests demonstrate that even in the absence of air, the precursor is able to oxidize the organic

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969 highest, and similar in the two cases, in agreement with the analysis of the C content, while for sample 5 the value of weight loss was intermediate between those of samples 1 and samples 2-3. The organic compounds were lost in a temperature range 200-400~ before the dehydration of the precursor. DSC profiles of samples 2 and 3 show the convolution of a broad, low-temperature endothermic peak due to the release of low-boiling compounds (including water and the glycol), and a broad exothermal peak due to the combustion of the organic compounds, well evident in the temperature range between 250 and 400-450~ This overlaps with the endothermic transformation of the precursor into vanadyl pyrophosphate; this transformation is represented by two distinct peaks at 360-380~ and at 420-430~ but the relative intensity of them was not the same in all the samples. The exothermal peak relative to the combustion of organic compounds even occurs in the case of sample 2 (for which no COx formation was observed in TPRD tests in N2 flow). This means that indeed a chemical interaction between the glycol and the precursor has occurred, but that this interaction is so strong that it is not possible to remove them in N2 flow (at least up to 300~ and that an air flow is necessary to burn them Based on the analysis of the transformations occurring during the structural evolution of the precursor, we decided to make use of a thermal treatment which included a precalcination step in air up to 300~ aimed at the desorption and oxidation of as much as possible organic compounds without damage or over-reduction of the vanadyl orthophosphate, followed by a thermal treatment in N2 in order to complete the transformation of the precursor into the vanadyl pyrophosphate. The first step must occur avoiding the dehydration of the precursor; in fact, when the latter occurs in air and during the combustion of the organic fraction, with considerable development of heat, an amorphous, oxidized V 5+ phosphate preferentially develops instead of the vanadyl pyrophosphate [21 ]. The analyses of the C content after the pre-calcination and the final thermal treatments for each sample are reported in Table 2. It is shown that the pre-calcination removes only a fraction of the organic compounds retained, and that the removal is completed during the final step at high temperature. This high-temperature removal therefore occurs with reduction of V 4+ to V 3+. The XRD patterns (Fig. 4) show that all the calcined catalysts contained a significant fraction of amorphous compound; in sample 2, the compound was fully amorphous. Moreover, weak reflections relative to V(PO3)3, a V 3+ compound, were presem samples 3 and 5. The electronic spectra (not reported here) also showed the presence of a considerable fi'action of V 3+ in samples 3 and 5, as it was also confirmed by volumetric titration (Table 2). Therefore, the residual organic compounds which are left in the precursor after the pre-calcination treatment in air are sufficient to reduce the vanadyl pyrophosphate during the final treatment in N2. On the contrary, in sample 1 the catalyst appears has having good crystallinity features, as usually obtained when the conventional calcination procedure is applied to a precursor having low amount of retained organic compound. The chemical analysis confirms the absence of reduced V 3+ species, and the presence of a small fraction of V 5+. Also in the industrial preparation of the ALMACAT catalyst, the former fluidized-bed catalyst commercialized by Lonza, a considerably reduced catalyst formed [22,23], which then took a long time to equilibrate under industrial reaction conditions (which, moreover, for fluidized-bed reactors are more reducing than for

970 fixed-bed reactors), and for this reason a new thermal treatment under hydrothermal-like conditions was developed [24]. It appears that the most dramatic changes occurring during the thermal treatment are in sample 2. This agrees with the existence of a stronger chemical interaction than that which develops with samples 3-5, with a consequent release of the organic compound only at temperatures higher than 300~ thus during the thermal treatment in N2 flow; this causes an extensive reduction and amorphization of the sample. This is also confirmed by comparison of the C contents before and after the final thermal treatment in N2 (Table 2). The amount of C present in samples 2 and 5 after the pre-calcination (i.e., before the final treatment) was similar, but the amount finally left was different in the two cases, since it was higher for sample 5 than for sample 2. Thus the amount lost during the final treatment in N2 was the highest for sample 2, and this justifies the worse features finally obtained in this case. counl~s

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~

60

Fig. 4. XRD patterns of samples 1, 2, 3 and 5 after the thermal treatment. It appears that the overall amount of organic compounds (i.e., glycols in the specific case) which are retained in the precursor structure are fundamental in affecting the chemical-physical features of the precursor itself and of the final compound, and that the nature of the interaction affects the range at which these compounds leave the catalyst. For relatively high contents of C, a procedure like the one here illustrated is not sufficient to avoid a non-negligible extent of morphological and structural transformations. It is also clear that it is possible to imagine carefully designed procedures which can be used to clean the catalyst without inferring modifications to the vanadyl pyrophosphate [24]. A treatment only in air would lead to catalyst over-oxidation [21 ], and a treatment in inert gas is possible only provided the amount of organic compound retained in the precursor is low. Catalysts have then been loaded in the lab reactor, and allowed to run under reaction conditions for several hundreds hours. This "pseudo-equilibration" time was necessary, since the performance considerably changed before reaching an almost-stable performance. Fig. 5 plots for samples 1, 3 and 5 the curves relative to the selectivity to maleic anhydride as a function of n-butane conversion, under almost-steady performance. Table 3 gives the

971 corresponding main chemical-physical features. It is possible to note that samples 3 and 5 had a better performance (a few points more of selectivity to maleic anhydride) than sample 1. This agrees with the slightly lower aspect ratio for the corresponding unloaded catalysts, that is a morphology with a preferred exposure of the face containing the active sites; therefore, in the case of samples 3 and 5 it was possible to transfer, at least in part, the aspect ratio of the corresponding precursor (lower than that of sample 1) up to the final catalyst, notwithstanding the features of the catalyst after the final thermal treatment. However, a possible role played by other factors (e.g., the presence of V ions having valence state other than 4+, the presence of amorphous compounds not yet transformed to the vanadyl pyrophosphate) can not be excluded. 80

70

~ 60 er t~

o 0 m

50

~: 4 0

~d O O

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

........................................................................................................................................................

20

,,

0

'

'

'

20

40

60

n-buta~

conversion,

,

'

80

1O0

%

Fig. 5. Selectivity to maleic anhydride as a function of n-butane conversion (the latter was varied by varying the reaction temperature) for samples 1 (0), 3 (&) and 5 (0).

Table 3. Main characteristics of the catalysts unloaded after reaction. Sample Unloaded samples FWHM (042) Surface area, n+ in V n+ FWHM

(200)

0.55 0.05 0.50 0.05 0.45 0.05

m2/g

24 24 30

4.02 3.99 3.93

4. CONCLUSIONS The known effect of organic compounds intercalation in the structure of the vanadyl orthophosphate hemihydrate, in the present case glycols having different number of C atoms used in mixture with isobutanol, has been confirmed. The removal of the retained organic fraction during the thermal treatment may lead either to fully amorphous compounds, or to compounds which exhibit some peculiar features of the precursor, e.g. a

972 defined value of the aspect ratio. The characteristics of the compound obtained are a function of the amount of organic fraction originally retained in the precursor, and of the nature of glycol used in the preparation~ It is possible to transfer the peculiar features transferred from the precursor up to the final, operating catalyst by making use of a complex thermal treatment procedure (pre-calcination in air plus high-temperature thermal treatment in nitrogen flow) provided (i) the retained organic fraction is relatively low, and (ii) no strong chemical interaction between the precursor and the glycol develops. This is possible when either 1,3-propandiol or 1,4-butandiol are used in the precursor preparation. The final catalyst exhibits a better selectivity to maleic anhydride than the reference sample prepared with isobutanol only [25]. REFERENCES

1. F. Cavani and F. Trifirb, in Catalysis Vol 11, J.J. Spivey and S.K. Agarwal, Eds., Royal Soc. Chem., Cambridge, 1994, p. 246. 2. G. Centi (Ed.), Catal. Today, 16 (1993) 1. 3. F. Cavani and F. Trifirb, Stud. Surf. Sci. Catal., 110 (1997) 19. 4. P.A. Agaskar, L. DeCaul and R.K. Grasselli, Catal. Lett., 23 (1994) 339. 5. E. Bordes, Catal. Today, 16 (1993) 27. 6. J. Ziolkowski, E. Bordes and P. Courtine, J. Mol. Catal., 84 (1993) 307. 7. C. Cabello, F. Cavani, S. Ligi and F. Trifirb, Stud. Surf. Sci. Catal., 119 (1998) 925. 8. F. Cavani, A. Colombo, F. Trifirb, M.T. Sananes Schulz, J.C. Volta and G.J. Hutchings, Catal. Lett., 43 (1997) 241. 9. G. Busca, F. Cavani, G. Centi and F. Trifirb, J. Catal., 99 (1986) 400. 10. E. Kestenmn, M. Merzouki, B. Taouk, E. Bordes and R. Contractor, Stud. Surf. Sci. Catal., 91 (1995) 707. 11. G. Busca, F. Cavani, G. Centi and F. Trifirb, J. Catal., 99 (1986) 400. 12. L.M. Comaglia, C.A. Sanchez and E.A. Lombardo, Appl. Catal., 95 (1993) 117. 13. H.S. Horowitz, C.M. Blackstone, A.W. Sleight and G. Teufer, Appl. Catal., 38 (1988) 193. 14. T. Myiake, T. Doi, Stud. Surf. Sci. Catal., 110 (1997) 835. 15. J.B. Benziger, V. Guliants, S. Sundaresan, Catal. Today, 33 (1997) 49. 16. G.J. Hutchings, M.T. Sananes, S. Sajip, C.J. Kiely, A. Burrows, J.I. Ellison and J.C. Volta, Catal. Today, 33 (1997) 161. 17. C.J. Kiely, A. Burrows, S. Saijp, G.J. Hutchings, M.T. Sananes, A. Tuel and J.C. Volta, J. Catal., 162 (1996) 31. 18. J.W. Johnson, A.J. Jacobson, J.F. Brody and J.T. Lewandowski, Inorg. CherrL, 23 (1984) 3842. 19. J.W. Johnson, A.J. Jacobson, W.M. Butler, S.E. Rosenthal, J.F. Brody and J.T. Lewandowski, J. Am. Chem. Soc., 111 (1989) 381. 20. V. Guliants, J.B. Benziger and S. Sundaresan, Chern. Mat., 7 (1995) 1493. 21. S. Albonetti, F. Cavani, F. Trifirb, P. Venturoli, G. Calestani, M. Lopez Granados and J.L.G. Fierro, J. Catal., 160 (1996) 52. 22. S. Albonetti, F. Budi, F. Cavani, S. Ligi, G. Mazzoni, F. Pierelli and F. Trifirb, Stud. Surf. Sci. Catal. 136 (2001) 141.

973 23. F. Cavani, S. Ligi, T.Monti, F. Pierelli, F. Trifir6, S. Albonetti and G. Mazzoni, Catal. Today, 61 (2000) 203. 24. G. Mazzoni, G. Stefani and F. Cavani, Eur. Patent 804,963 B1 (1997), assigned to Lonza SpA. 25. S. Ligi, F. Cavani, S. Albonetti and G. Mazzoni, WO 0072963 (2000), assigned to Lonza SpA

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Synthesis and characterization

975

of nanostructured Mo2C on carbon

m a t e r i a l b y c a r b o t h e r m a l h y d r o g e n reduction Changhai Liang, Zhaobin Wei, Qin Xin and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Nano-structured ~-MozC on ultrahigh surface area carbon material (>3000 m2/g) was prepared by carbothermal hydrogen reduction method. The MoO3 precursor and MozC have been characterized by X-ray diffraction, nitrogen adsorption, high-resolution transmission electron microscope (HRTEM) and temperature-programmed reduction-mass spectroscopy (TPR-MS). The data show that nanostructured 13-MozC can be formed on the ultrahigh surface area carbon materials by carbothermal hydrogen reduction at 700 ~ The particle sizes of [5-MozC increase with the increase of the temperature of carbothermal hydrogen reduction. The carbothermal hydrogen reduction includes two successive steps: reduction of MoO3 precursor by hydrogen and reaction between partially reduced molybdenum oxides and surface carbon atoms of carbon materials. 1. INTRODUCTION The transition metal carbides have received considerable attention as advanced materials, especially as catalytic materials, because of their unique physical and chemical properties [1, 2]. Many methods including gas-phase reactions of volatile metal compounds, reaction of gaseous reagents with solid state metal compounds, pyrolysis of metal complexes and solution reactions, have been developed for the preparation of high surface area carbides [2, 3]. These carbide materials show exceptionally high activity in hydrogen-involved reactions [4-10]. It is also found that the catalytic properties of carbide materials strongly depend on their surface structure and composition, which are closely associated with the preparation methods. Among those preparation methods, the typical one is the temperature-programmed reaction between oxide precursors and flowing mixture of hydrogen and the carbon-containing gases, such as CH4 [11-13], C2H6 [14, 15], C4H10 [16], and CO [17]. However, there exist also some problems in this method of preparation: the carburization and passivation processes must be carefully controlled; and the resultant carbide surface is usually contaminated by polymeric carbon from the pyrolysis of the containing-carbon gases. The carbon is blocked in the pores, covers the

976 active sites, and is difficult to remove. Recently, Ledoux and coworkers developed a novel synthesis route to preparing high surface area carbides which can avoid the formation of carbon residues on the surface [18-20]. It involves the reaction of solid carbon with vaporized metal oxides at very high temperatures, even above 1000 ~ Activated carbon was used as the carbon source and the final carbides appear to remain the characteristic of porous structure of the activated carbon. Mordenti et al. [21] modified this method and prepared activated carbon-supported MozC samples under moderate temperatures. But this method is still in an early stage of development. A broad range of supports and metal compounds need to be tested and studied in detail because metal compounds can be highly dispersed on carbon materials, and the carburized materials are potential catalysts in hydrogenation reaction. In the present work, we have tried to prepare nanostructured MozC using ultrahigh surface area carbon material (>3000 mZ/g) [22], a kind of novel carbon material with uniform pore sizes, as carbon source and template by carbothermal hydrogen reduction. 2. EXPERIMENTAL Ultrahigh surface area carbon material is made by a direct chemical activation route in which petroleum coke is reacted with excess KOH at 900 ~ to produce carbon materials containing potassium salts. These salts are removed by successive water washings. The surface area of the carbon materials measured by BET method is about 3234 mZ/g and the pore volume is about 1.78 m3/g (Table 1). Table 1. Surface area and porosity of HSAC, Mo/HSAC and the samples with carbothermal hydrogen reduction Sample

BET surface area (mZ/g)

Pore volume (cm3/g)

HSAC

3234

1.78

Mo/HSAC-RT

2446

1.31

Mo/HSAC-600

2505

1.38

Mo/HSAC-700

2341

1.35

Mo/HSAC-800

2180

1.30

The ultrahigh surface area carbon material was impregnated in a rotary evaporator at room temperature with aqueous solution of ammonium heptamolybdate. The molybdenum content is 10 wt. %. After evaporating and drying in air at 120 ~ overnight, the materials were transferred to a quartz reactor inside a tubular resistance furnace controlled by temperature programmer. The amount of the sample was about 4 g/batch. Pure hydrogen was passed through the sample at a flow rate of 200 cm3/min. The temperature was

977 increased at a linear rate of 1 ~ to the final temperature, which was held for 1 h. The samples were quenched to room temperature at flowing argon, then passivated by 1% O2/Na mixture. X-ray diffraction analysis of the samples was carried out using a Rigaku D/Max-RB diffractometer with Cu K ct monochromatized radiation source, operated at 40 KV and 100 mA. Temperature-programmed reduction (TPR) of the sample was carried out in a stream of 95% argon and 5% hydrogen with a flowing rate of 30 cm3/min. The catalyst bed was heated linearly at 20 ~ from room temperature to 950 ~ Mass spectroscopy was used as detector. Transmission electron microscopy (TEM) studies were carried out on a JEOL 2000 electron microscope. High-resolution transmission electron microscopy (HRTEM) and electron diffraction were performed on a JEM-4000EX electron microscope with an acceleration voltage of 200KV. Nitrogen adsorption and desorption isotherms at 77K were measured using Micromeritics 2010. Surface areas were calculated from the linear part of the Brunaure-Emmett-Teller (BET) plot. 3. RESULTS AND DISCUSSION Surface area and porosity of the ultrahigh Surface area carbon and the samples after carbothermal hydrogen reduction are complied in Table 1. After impregnation of the carbon material, its surface area decreases from 3234 mZ/g to 2446 mZ/g and its porosity from 1.78 m3/g to 1.30 m3/g. It can be assumed that molybdenum precursor fills and blocks a fraction of the pores. The surface area and porosity of the samples shows a decrease, to

m

0

44 m , o

......

20

30

40

HSAC

~ .....

50 2-theta

60

70

80

(~

Fig. 1. XRD patterns of HSAC, Mo/HSAC precursor and the samples with carbothermal hydrogen reduction

978 some extent, with the increase of the temperatures of carbothermal hydrogen reduction. The changes of the surface area and porosity may be attributed to the reaction of carbon materials with hydrogen and molybdenum precursors, and the changes of molybdenum species. The phases and dispersion of molybdenum compound were also measured by XRD after the supported samples were reduced in hydrogen. Fig. 1 shows XRD patterns of the samples with carbothermal hydrogen reduction at different temperatures, together with ultrahigh surface area carbon and the supported Mo precursors. XRD pattern of the ultrahigh surface area carbon shows clearly that the carbon material is non-graphitizable. XRD pattern of the supported molybdenum precursor sample does not show any diffraction peaks of molybdenum precursors, indicating that molybdenum species is well dispersed on the carbon material and the particle size is smaller than 4 nm. The result is consistent with that obtained using high-resolution TEM by Mordenti et al. [21].

Fig. 2 TEMs of samples with carbothermal hydrogen reduction: a obtained at 700 ~ b obtained at 800 ~ XRD pattern of the sample with carbothermal hydrogen reduction at 600 ~ does not show any diffraction peak due to the MoO2 phase, indicating highly dispersed MOO2. But this result is different from that of molybdenum precursor supported on activated carbon [21], where MoO2 was detected by XRD. The good dispersion of MoO3 precursor and MoO2 may be due to the ultrahigh surface area and pore volume of the carbon material used in..this study. XRD pattern of the sample with carbothermal hydrogen reduction at 700 ~ shows a diffraction peak at 39.4 o, which is due to [3-Mo2C with hexagonal close-packed structure. With further increase of the carbothermal hydrogen reduction temperature up to 800 ~ the typical diffraction peaks due to fS-Mo2C clearly show up at

979 39.4, 37.8, 34.3, 61.7, 52.0, 69.6, 74.5, and 75.7 o 20. The above XRD results also imply that the particle sizes of 13-Mo2C increase with the increase of the temperature of carbothermal hydrogen reduction. In order to estimate the particle sizes of 13-Mo2C, TEM and high-resolution TEM analyses were carried out. The TEMs of the samples with carbothermal hydrogen reduction at 700 and 800 ~ are shown in Fig. 2. It can be seen that the particle sizes of 13-Mo2C at 700 ~ are remarkably uniform, about 10 nm in diameter, dispersed on the outer surface of the carbon materials (Fig. 2a). The particle sizes of 13-Mo2C at 800 ~ are non-uniform, with about 25 nm diameter (Fig. 2b), thus larger than at 700 ~

Fig. 3 HRTEM of sample with carbothermal hydrogen reduction at 700 ~ HRTEM picture of Fig. 3 shows the molybdenum carbide containing a high density of planar defects and dislocations after synthesis and passivation at room temperature. The EDS analysis performed on this phase indicated the presence of Mo, C, and O. The existence of O is due to formation of passivated layer, which avoids a violent oxidation of the sample when it is exposed to air. To understand the carbothermal hydrogen reduction process, the reactant and principal products were monitored with mass spectroscopy in real time. The sample was first dried at 150 ~ in a stream of nitrogen for 2 h, then cooled to room temperature in order to avoid the desorption peak of water at about 100 ~ Fig. 4 shows the synthesis traces of masses (M) 2, 16, 18, 28 and 44 with the increase of temperature. The signals at M=2 and 16

980

represent hydrogen and methane, while the signals at M=18, 28 and 44 represent water, carbon monoxide, and carbon dioxide, respectively. The TPR-MS traces show that the carbothermal hydrogen reduction reaction proceeds in three stages in the range of temperatures studied. First, there was a considerable amount of hydrogen consumption and water formation at about 400 ~ From it, one can conclude that MoO3 was reduced into Mo oxide with low chemical valence. But the actual Ha consumption is higher than needed for the following reaction. MoO3 + Ha <---)MoO2 + H20 Second, a CO mass spectrometer signal appears when the temperature is above 530 ~ further increases with the increasing temperature, and reaches a maximum at 680 ~ Meanwhile, a decrease of the hydrogen mass spectrometer signal accompanied by simultaneous water evolution is observed. CO formation is mainly due to carbothermal reaction between reactive carbon atoms or groups on the carbon material [23] and Mo compounds in the presence of hydrogen. MoO2 + 1.5 C* + H2 <---)MoC0.5 + CO + H20 C* is a surface carbon atom or group of carbon material. In addition, CO can be also formed from carbon material and water produced by reduction of the oxides, and from the decomposition of C-O groups. C* + H20 <---->CO + Ha Finally, the signal of methane appeared when the temperature increased up to 900 ~ Simultaneously, a peak for hydrogen consumption was also observed. So, it can be

:i

A

"'" m" m'm,%m.,.m m m m m m m m m m

~o

Oooooo,,m, ~ l:

"m

~OOOOOOOOOOOO0

d

w w (I

:E

M=18

~176176176176 A

mm

0 L

m m m m m m m m m mM =m2 m m'mmlmm

~AAAAAAAAAAAAAAAAAAAA~

~.AAAAAAAAAAAM=28 M=~ rVVVVVVVVVVVVVVVVVVVVVyVVVVVVVVVVVyyVVy M=16 .# 200

,

I 300

n

I ~0

9

u 500

9

n 600

Temperature

,

n 700

.

l 800

,

n 900

(OC)

Fig. 4. TPR-MS traces of MoO3/HSAC sample

.

1000

981 assumed that hydrogen reacted with carbon material, or, carbide was hydrogenated according to the following reactions. C* + 2H2 4-->CH4 MozC + 2H2 4-->2Mo + CH4 4. CONCLUSION Ultrahigh-surface-area carbon material with narrow distribution of pore sizes was used as carbon source and templates during the process. Uniform fA-M02C particles with about 10 nm were obtained after carbothermal hydrogen reduction at 700 ~ and the particle sizes of [3-M02C increased with the increase of the reaction temperature. The carbothermal hydrogen reduction included two successive steps: reduction of molybdeum precursor by hydrogen, and reaction between partially reduced molybdenum oxides and carbon materials. The method of carbothermal hydrogen reduction is a useful and simple way to synthesize nanostructured carbides under mild conditions. ACKNOWLEDGMENTS

This study was made possible by financial support from State Key Project for Basic Research & Development (Grant No. G2000048003). One of the authors (Dr Changhai Liang) is very grateful to Dr Wenping Ma for TPR-MS measurement. REFERENCES

1. L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. 2. S.T. Oyama (Ed.), The Chemistry of Transition Metal Carbides and Nitrides, Blackie Academic & Professional, Glasgow, 1996. 3. S.T. Oyama, In Handbook of Heterogeneous Catalysis (G. Ertl, H. Kn6zinger, J. Weitkamp, Eds.), Wiley-VCH, 1997, pp132-138. 4. R.B. Levy and M. Boudart, Science, 181 (1973) 547. 5. H. Abe and A.T. Bell, J. Catal., 142 (1993) 430. 6. C.W. Coiling and L.T. Thomapson, J. Catal., 146 (1994)193. 7. M. Nagai and T. Miyao, Catal. Lett., 15 (1992) 105. 8. J.S. Lee, M.H. Yeom, K.Y. Park, I.S. Nam, J.S. Chung, Y.G. Kim and S.H. Moon, J. Catal., 128 (1991) 126. 9. V. Keller, P. Weher, F. Garin, R. Ducros and G. Maire, J. Catal., 153 (1995) 9. 10. L. Leclercq, M. Provost and G. Leclerq, J. Catal., 117 (1989) 384. 11. L. Volpe and M. Bourdart, J. Solid State Chem., 59 (1985) 348. 12. J.S. Lee, L. Volpe, F.H. Ribeiro and M. Bourdart, J. Catal., 112 (1988) 44. 13. J.S. Lee, S.T. Oyama and M. Bourdart, J. Catal., 106(1987)125.

982 14. J.B. Claridge, A.EE. York, A.J. Brungs and M.L.H. Green, Chem. Mater., 12 (2000)132. 15. A.P.E. York, J.B. Claridge, C.V. Williams, A.J. Brungs, J. Sloan, A. Hanif, H. A1-Megren and M.L.H. Green, Stud. Surf. Sci. Catal., 130 (2000) 989 16. T.C. Xiao, A.P.E. York, C.V. Williams, H. AI-Megren, A. Hanif, X. Zhou and M.L.H. Green, Chem. Mater., 12 (2000) 3896. 17. J. Lemaitre, B. Vidick and B. Delmon, J. Catal., 99 (1986) 415. 18. M.J. Ledoux, J.Hantzer, J. Guille and D. Dubots, U.S. Patent NO. 4914070 (1990). 19. M.J. Ledoux, J. Hantzer, C. Pham-Huu, J. Guille and M.P. Desaneaux, J. Catal., 114 (1988) 176 20. F. Meunier, P. Deporte, B. Heinrich, C. Bouchy, C Crouzet, C. Pham-Huu, P. Panissod, J.J. Lerou, P.L. Mills and M.J. Ledoux, J. Catal., 169 (1997) 33. 21. D. Mordenti, D. Brodzki and G. Djega-Mariadassou, J. Solid State Chem., 141 (1998) 114. 22. G.C. Grunewald and R.S. Drago, J. Am. Chem. Soc., 113 (1991) 1636 23. L.R. Radovic, In Surfaces of Nanoparticles and Porous Materials (J.A. Schwarz, C.I. Contescu, Eds.), Marcel Dekker Inc., New York, 1999, pp 529-555.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

983

Carbon composite-based catalysts: new perspectives for low-temperature H2S removal. J.M. Nhut a, R. Vieira ~b, N. Keller~+, C. Pham-Huu a, W. Bollc and M.J. Ledouxa Laboratoire des Mat6riaux, Surfaces et Proc6d6s pour la Catalyse, ECPM - Louis Pasteur University, UMR 7515 CNRS, 25 rue Becquerel, BP 08, 67087 Strasbourg Cedex 2, France. Tel. +33 3 90 24 26 75, Fax. +33 3 90 24 26 74, + E-marl: [email protected] a

b Instituto Nacional de Pesquisas Espaciais, Rodovia Presidente Dutra, Km 40, CP 01, 12630-000 Cachoeira Paulista, SP, Brasil. c Lurgi OI-Gas-Chemie GmbH, Lurgi Allee 5, Frankfurt/Mainz, Germany. Graphite felt supporting 40nm diameter carbon nanofibers was prepared and successfully used as support for a nickel-based catalyst for the low temperature (60~ selective oxidation of HaS into elemental sulfur. The carbon nanofiber synthesis was performed by the catalytic decomposition of an ethane-hydrogen mixture at 700~ on a graphite felt supported nickel catalyst. The use of this new composite as NiS2 support led to a significant increase in performance, both in terms of activity and resistance to the solid sulfm- deposition onto the material, as compared to a very efficient SiC supported analogue. The macroscopic design of such a carbon composite-based catalyst avoids the drawbacks of handling the very powdery nanofibers for industrial fixed-bed reactions. I. INTRODUCTION Removal of hydrogen sulfide (H2S) from waste acid gases generated by petrochemical refineries or natural gas plants is essential for the protection of the environment. It needs ever more efficient catalysts and processes, in order to meet the ever increasing standards of efficiency required by the environmental pressures [1-4]. Previous work has shown that a NiS2-based catalyst supported on medium surface area silicon carbide (SIC) was very efficient as compared to nickel catalysts supported on high surface area alumina generally used in the Sulfreen-like processes developed by Elf and Lurgi [5,6], and allowed the reaction to be performed at very low temperatures such as 60~ [79]. The use of tubular nanostructures has been recently reported to increase the performance of such NiS2-based catalysts [10]. Their industrial applications nevertheless remained scarce for fixed-bed uses because of the fine powder form of their primary structures. It is consequently of interest to find a new catalyst support material, which could combine the advantages of nanostructures and the easy handling of macroscopic forms. The aim of the present article is to report the synthesis of a new graphite felt

984 supported carbon nanofiber material and its application as catalyst support for the low temperature selective oxidation of HaS into elemental sulfur. Performances will be compared to a silicon carbide supported catalyst.

2. EXPERIMENTAL SECTION 2.1. Support and catalyst preparation The carbon nanofiber/graphite felt composite is based on a macroscopic felt supplied by Carbone Lorraine Com., composed of entangled microfibers. Table 1 summarizes the characteristics of the starting carbon microfibers. Nickel (1 wt.%) was deposited by incipient impregnation of the graphite felt with nickel nitrate Ni(NO3)2.6H20 (Merck) using ethanol as solvent.The solid was dried overnight at 110~ and calcined in air at 350~ for 2h to form the corresponding nickel oxide. After reduction at 400 ~ under hydrogen for lh, the hydrogen flow was replaced by a mixture of ethane and hydrogen and the carbon nanofibers were grown on the graphite felt by the catalytic decomposition of ethane on the nickel particles at 700~ for 2 h. Table 1 Characteristics of the entangled graphite microfibers which compose the carbon felt Carbon Ash Volatile compounds Sulfur Average diameter Length BET surface area

94-97 wt.% 0.1-0.3 wt.% 1.5-4.0 wt.% 0.3-0.5 wt.% 10-15 m Up to several dozen millimeters 1-2 rn2/g with no microporosity

Medium surface area silicon carbide (grains 0.4-1 mm) used as support was synthesized at the CRV-Pechiney, according to the Shape Memory Synthesis developed by Ledoux and co-workers, involving the gas-solid reaction between SiO vapours and solid carbon under dynamic vacuum around 1200-1300~ [11-13]. A calcination was then performed at 700~ for 3 h to stabilize the textural characteristics and to bum of the remaining unreacted carbon. No microporosity was present in the SiC. The supported Ni catalysts (5 wt.%) for H2S oxidation were prepared by incipient wetness impregnation of the two supports, i.e. SiC grains and graphite felt coated with carbon nanofibers, with an aqueous solution of Ni(NO3)2.6H20 (Merck). After drying overnight at 120~ the catalysts were calcined at 350~ for 2h in order to decompose the nitrate salt and to form the nickel oxide. The corresponding sulfidic catalysts were obtained by sulfidation of NiO by reaction with a H2S/He flow at 300~

2.2. Catalytic tests The catalyst was placed on a silica wool in a Pyrex heated fixed-bed reactor. The flow rate of 02 and H2S was monitored by Tylan FC280A flowmeters linked to a Tylan RC280 control unit, whereas steam was provided by a saturator kept at the required

985 temperature to obtain the desired partial pressure of steam. The reactant mixture contained H2S (2000 ppm vol.), 02 (3200 ppm vol.), H20 (20 % vol.) and balance He. The total flow rate corresponded to a Weight Hour Space Velocity (WHSV) of 0.027 h1, i.e. weight of H2S per weight of catalyst per hour. The analyses of the inlet and outlet gases were performed on-line using a Varian CX-3400 gas chromatograph equipped with a Chrompack GSQ capillary column (allowing the separation of 02, H2S, 1-120 and SO2), a catharometer detector and a calibrated six port loop (500 L). Regeneration was carried out in flowing helium at 300~ for 2h in order to vaporize the sulfur deposited on the catalyst during the test and to calculate the sulfur mass balance of the reaction. 2.3. Characterisation techniques Surface area measurements were performed on a Coulter SA-3100 porosimeter using N2 adsorption at -196~ The morphology of the materials was observed by Scanning Electron Microscopy (SEM) using a Philips Model JSM-840 operating at 20kV and a JEOL JSM6700-NT operating at lkV, and by Transmission Electron Microscopy (TEM) using a Topcon Model EM200B working at 200kV with a point-to-point resolution of 0.17 nm. Thermal Gravimetry (TG) was performed using a Balzer thermo-analyser (heating rate of 5~ using a 20 % (v/v)

02/N2 rfftxture). 3. RESULTS AND DISCUSSION 3.1. Material characterisation After growth at 700~ for 2 h, the starting carbon microfibers forming the carbon felt (Fig. l a, b) were coated by the carbon nanofibers, leading to a considerable increase in diameter and in turn in the specific volume of the material (Fig. l c,d). Fig. l d shows a naked part of the composite, showing the complete coverage of the carbon microfiber with a several micrometer thick layer, composed of entangled carbon nanofibers. Very few microfibers or parts of microfibers remained naked after reaction as could be seen on a statistical sample of high resolution SEM images. The high density of the synthesized nanofibers can be observed in Fig. l e. Transmission Electron Microscopy also revealed the total selectivity towards nanofiber formation, since no amorphous carbon, onion-like carbon or nanoparticles were detected. This is in good agreement with the literature, which reported the

986

Fig. 1. SEM images of (a,b) the support composed of carbon microfibers and (c-e) the supported carbon nanofibers synthesized. Fig. If shows a TEM image of a supported herringbone carbon nanofiber. advantages in terms of fiber selectivity for the catalytic decomposition of hydrocarbons as compared to conventional high temperature synthesis methods such as carbon-arc production or plasma decomposition, where a significant amount (20-70 wt.%) of such impurities is observed together with carbon nanotubes or nanofibers [14-16]. The carbon nanofibers displayed the well known fishbone or herringbone structure, with stacked graphene sheets forming a close angle and sometimes connected at the middle of the fibers, according to the results reported in the literature for nickel-based catalysts [ 10,17-19]. The carbon nanofibers exhibited a mean diameter of about 40 nm and lengths up to several micrometers. They had an interplanar distance between two graphene sheets of ca. 0.34 ran, close to what is observed for the (002) plane in graphite, i.e. 0.335 nm. The prismatic faces exposed at the outer surfaces of the nanofibers are preferential sites for carbon oxygen groups, giving also hydrophilic properties to the material [19,20]. Such

987 hydrophilic oxygenated surface groups allow the easy anchorage of the supported phases on the composite support, compared to what is generally reported in the literature with the less reactive basal planes of graphite or carbon nanotubes (nickel particles in the present study, not reported). It led to highly dispersed supported phases even with high loaded catalyst (for example, narrow particle size distribution centered at 2 nm diameter for a 30 wt.% of iridium metal [21 ]). Fig. 2 shows the behavior of the carbon nanofiber composite under an oxidizing atmosphere. It evidences the formation as a function of time on stream of a second carbon species, i.e. carbon nanofibers at about 630-650~ which presented a high reactivity as compared to that of the starting graphite felt. They showed a maximum reactivity toward oxygen identical to that observed for similar pure carbon nanofibers [19], and were more reactive than the better crystallized graphite micro fibers. This decrease in the oxidation temperature could be explained by (i) the presence of the prismatic exposed planes, known to be highly reactive toward oxidation and/or (ii) the high external surface area of the nanofibers, which favors diffusion inside the nanofibers compared to the larger carbon micro fibers. The oxidation behavior of the composite after 2 h of growth allowed the calculation of the carbon nanofiber stoechiometry inside the composite used as catalyst support for the low temperature H2S oxidation, -i.e. about 20 wt.%. The carbon nanofiber synthesis

100 80

E

.................

A:F

"

60

/il-

40 20 0 '

I

'

I

'

I

'

I

'

I

'

I

'

100 200 300 400 500 600 700 800 100 .

.

.

.

80

._~

,

60

'

-

40 20 0

'

I

'

I

'

I

'

!

'

I

'

I

'

l O0 200 300 400 500 600 700 800 100

q .

.

.

.

.

.

"J" --'i.

-

80

-=

60 - ~ ..o 40

/ ",,

20 ~-"'~'v'~ ,"~

~t t

0 ' I ' I ' I ' I ' I ' I 100 200 300 400 500 600 700 8()0 Temperature (~

Fig. 2. TG analysis of the carbon felt support (a), the composite after respectively 2 h (b) and 6 h (c) of growth. Weight loss in % (left side) and heat flow released (fight side).

988 led to an increase in the surface area of the material, from 1-2 mE/g up to about 50 mE/g for the composite after 6 h of growth. The surface area was lower than that of pure carbon nanofibers [19], because of the remaining low surface area carbon, located in the core of the composite. Such an increase could be explained by the presence of graphite prismatic edges, which could act as adsorption sites and also to the relatively low crystallinity along to the nanofiber axis as compared to more ordered carbon nanotubes. The composite displayed a strong mechanical resistance, neither formation of fine powder nor nanofiber loss being observed after sonication treatments (weight loss lower than l wt.%). This indicates the strong anchorage of the carbon nanofibers on the macroscopic graphite felt, which is required to resist the industrial conditions of use.

3.2. The low-temperature selective oxidation of H2S It has been reported in previous work that the NiS2 catalyst supported on SiC grains was totally efficient (100% selective for a conversion of 100%) at 60~ in the presence of 20 vol.% of water for a WHSV of 0.007 hl [8-10]. The desulfurization activity expressed in terms of conversion and the sulfur weight deposit obtained on the supported NiSE-based catalysts at 60~ in similar reaction conditions, but with a higher WHSV of 0.027 h "1 a r e reported in Fig. 3. In both cases, the selectivity into elemental sulfur remained total and stable on stream (not reported), no trace of SO2 ever being detected at the outlet of the reactor. This was attributed to the use of a low reaction temperature, in close agreement with the literature, which reported that SO2 formation only occurred at higher temperatures [8,9,22-25]. Previous results published by Steijns et al. [26] showed that between 100~ and 200~ the rate of formation of SO2 by the successive oxidation of sulfur by oxygen was 100 times lower than the rate of oxidation of H2S, due to the very large difference between the corresponding experimental activation energies for oxidation. The catalyst supported on SiC grains required an activation period of a few hours on stream before reaching its maximum of activity, corresponding to 84% of conversion. This activation period was attributed to the time required by the NiS2 phase to be superficially transformed into a new active phase, i.e. formation of an oxysulfide phase by oxygen and sulfur exchange, as has been reported at lower temperature (40~ in previous work. Deactivation occured after 5 h on stream, probably due to the pore blockade by the solid sulfur deposit, and the SiC grain-based catalyst was totally inactive after 50h on stream with about 50% of solid sulfur stored on the surface, as shown for lower WHSV [ 10]. In contrast, the composite supported catalyst was totally efficient from the beginning of the test. Slower deactivation only occurred after about 20 h on stream and the HES conversion remained at about 50% even with a solid sulfur loading reaching 150%. The lack of any activation period observed on the test carried out over the composite supported catalyst was attributed to its higher external surface area as compared to the SiC grainbased catalyst. It was worth noting that very large sulfur particles were observed on different zones of the surface, large parts of nanofibers remaining accessible for the reactants (Fig. 4). The formation of large sulfur aggregates was probably due to the presence of water, cleaning the surface during the reaction, as has been observed with SiC support presenting both hydrophylic and hydrophobic surfaces. The anchorage of the sulfur particles nevertheless remained relatively weak, and only few podes mixing sulfur and carbon nanofibers were observed (insert). This kind of pode seemed to anchor the solid

989

Composite

-100 t~~ 0

O

.mm

r~ Zm

O CP~

-50

f

O u.

Grains

20-

0-

'

0

'

'

I

20

'

'

'

I

'

'

'

40

I

'

60

'

'

I

'

80

Time on stream (h)

Fig. 3. H2S conversion and wt.% solid sulfur deposit as a function of time on stream with a W H S V of 0.027h ~ at 60~ in the presence of 20 vol.% of water obtained on the composite (squares)- and SiC grain (circles) -based catalysts.

Fig. 4. S EM images of the sulfur loaded composite catalyst (sulfur loading of 150 wt.%).

sulfur deposit on the carbon nanofiber composite during the reaction. The stronger storage capacity of the composite-based catalyst can be explained by its large void volume. The composite support had a much larger void volume between each carbon nanofiber and also between each carbon microfiber, than the volume outside the pores of a conventional meso- and macroporous material such as SiC grains. Hovewer, it should be remembered that SiC nanotubes had an even larger storage capacity than this composite (200 wt.% without deactivation) [10].

990 4. CONCLUSION A new macroscopic support made of 40 nm diameter carbon nanofibers supported on a graphite felt was prepared by catalytic decomposition of an ethane and hydrogen mixture at 700~ over a graphite felt supported nickel catalyst. This felt, built of entangled 10-15 m diameter graphite microfibers, was completely coated after growth by a high density network of entangled carbon nanofibers. These nanofibers exposed prismatic planes on the outer surfaces, allowing an easy anchoring of highly dispersed active phases for use in heterogeneous catalysis. This new carbon composite material was successfully used as support for a NiS2-based catalyst for the low temperature (60~ selective oxidation of H2S into elemental sulfur and increased both desulfinfzation activity and resistance to the solid sulfur loading on the catalyst, compared to a traditional SiC grainbased catalyst. The total activity of the catalyst at high WHSV was explained by the high external surface area of the material. The improvement of the resistance to the sulfur deposition could be attributed to a peculiar mode of sulfur deposition on the catalyst surface, involving the presence of water and probably specific zones of the material. Work is going on to obtain further insight into the mode of sulfur deposition and its transport on the composite surface. The macroscopic design of 40 nm diameter carbon nanofiber-based catalyst avoids the traditional drawbacks of industrial handling observed for both carbon nanofiber and nanotube-based catalysts because of their too small dimensions. ACKNOWLEDGMENTS

Prof. J. Guille and Dr. C1. Estourn~s (IPCMS, UMR 7504 CNRS) are gratefully acknowledged for performing respectively SEM analysis and the thermo-gravimetric work. RV would like to thank the INPE for financial support. REFERENCES

1. 2. 3. 4. 5.

J. Wieckowska, Catal. Today, 24 (1995) 405. Sulphur, Keeping Abreast of the Regulation, 231 (1994) 35. Sulphur, Controlling Hydrogen Sulfide Emissions, 250 (1997) 45. Sulphur, Approaching the Limit: 99,9+% Sulphur Recovery, 257 (I 998) 34. G. Bandel and W. Willing, "The Doxosulfreen claus tail gas processes- meeting enhanced sulphur emission standards", Sulphur 96, loose paper. 6. S. Savin, J.-B. Nougayr6de, W. Willing and G. Bandel, "New Developments in Sulfur Recovery Process Technology", The Int. J. of Hydrocarbon Eng., (1998) 54. 7. M.J. Ledoux, J.-B. Nougayr6de, S. Savin-Poncet, C. Pham-Huu, N. Keller and C. Crouzet, French Pat. Appl. 97-16617, assigned to Elf Aquitaine Production, 1997. 8. N. Keller, C. Pham-Huu, C. Crouzet, M.J. Ledoux, S. Savin-Poncet, J.-B. Nougayr6de and J. Bousquet, Catal. Today, 53(4) (1999) 535. 9. M.J. Ledoux, C. Pham-Huu, N. Keller, J.-B. Nougayr6de, S. Savin-Poncet and J. Bousquet, Catal. Today, 61 (2000) 157. 10. C. Pham-Huu, N. Keller and M.J. Ledoux, J. Catal., 200(2) (2001) 400.

991 11. M.J. Ledoux, S. Hantzer, C. Pham-Huu, J. Guille and M.P. Desaneaux, J. Catal., 114 (1988) 176. 12. M.J. Ledoux, S. Hantzer, J.L. Guille and D. Dubots, U.S. Patent No 4 914 070, assigned to Pechiney Recherche, 1990. 13. N. Keller, C. Pham-Huu, S. Roy, M.J. Ledoux, C. Estoum6s and J. Guille, J. Mater. Sci., 34(13) (1999) 3189. 14. P.M. Ayajart, Chem. Mater., 11 (1999) 3862. 15. In Carbon Nanotubes: preparation and properties (Ed. T.W. Ebbesen), CRC, Boca Raton, 1997. 16. P.M. Ajayan and O.Z. Zhou, in Carbon Nanotubes. Synthesis, Structure, Properties and Applications, Topics in Applied Physics, Vol. 80 (Eds. M.S. Dresselhaus, G. Dresselhaus and Ph. Avouris), Springer, Heidelberg, 2001, pp.391-425. 17. N.M Rodriguez, J. Mater. Res., 8 (1993) 3233. 18. N.M Rodriguez, A. Chambers and R.T.K. Baker, Langmuir, 11 (1995) 3862. 19. C. Pham-Huu, N. Keller, V.V. Roddatis, G. Mestl, R. Schl6gl and M.J. Ledoux, Phys. Chem.-Chem. Phys., 4(3) (2002) 514. 20. N. Keller, N.I. Maksimova, V.V. Roddatis, M. Schur, G. Mestl, Yu.V. Butenko, V.L. Kuznetsov, and R. Schl5gl, Angew. Chem., in press. 21. R. Vieira, C. Pham-Huu, N. Keller and M.J. Ledoux, submitted. 22. Primavera, A. Trovarelli, P. Andreussi and G. Dolcetti, Appl. Catal. A: General, 173 (1998) 185. 23. J. Klein and H.-D. Henning, Fuel, 63 (1984) 1064. 24. A.I. Chowdhury and E.L. Tollefson, Can. J. Chem. Eng., 68 (1990) 449. 25. A.K. Dalai, A. Majumdar, A. Chowdhury and E.L. Tollefson, Can. J. Chem. Eng., 71 (1993) 75. 26. M. Steijns, F. Derks, A. Verloop and P. Mars, J. Catal., 42 (1976) 87.

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Studiesin Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

993

Active carbon surface oxidation to optimize the support functionality and metallic dispersion of a Pd/C catalyst. V. Dubois*, Y. Dal and G. Jannes Institut Meurice, Physical Chemistry and Catalysis, Avenue Gryzon, 1 - B-1070 Brussels, Belgium, e-mail : vincent.dubois @meurice.heldb.be We have oxidized active carbon to improve its properties as a support to prepare 5% Pd/C catalysts by precipitation or by incipient wetness impregnation. The oxidizing agent is nitric acid. Temperature of the slurry, contact time and nitric acid concentration enable to control the number and the type of acidic groups created on the support surface. Long and severe oxidation leads to a sudden destruction of the physical structure of the active carbon, losing in that way its interest as a support. Soft or short severe oxidation allows preparing well-dispersed catalysts. Incipient wetness is recommended as a preparation method if the surface acidity is intended to play a independent catalytic role during the reaction. During the precipitation process, on the contrary, the surface acidity created by the oxidizing treatment is neutralized. 1. I N T R O D U C T I O N Precious metals are widely used in catalytic hydrogenation, and peculiarly in the production of fine chemicals, pharmaceutics or, more generally, molecules bearing several reducible groups. In this frame, platinum and palladium are obviously most frequently used and extensively studied in literature. Noticeably, this work only deals with palladium catalysts: a parallel research focused on platinum is still under progress. The origin of this work is a paradoxical kinetic phenomenon observed when bare active carbon is added to the reaction medium during hydrogenation of a model nitroaliphatic compound. This effect has been extensively described elsewhere [1-3]. The presence of this bare active carbon deeply affects the reaction rate in a negative way for a few cases only or in a positive way for the other cases, and this effect cannot be explained by impurities trapping. Systematic studies, during the last ten years, of the effects of supports in fine chemical catalysis have revealed the essential role they may play in some reactions [4]. Considering a catalytic transformation, it is therefore essential to choose the most appropriate support because it may influence the structure and the electronic state of the final catalyst. In this respect, it would be a mistake to consider carbon as an inert matrix [5-7]. We have thus started this study to try to shed light on the effect of active carbon, used as a support, in a given reaction (i.e. hydrogenation of 2-methyl-2-nitropropane).

994 2. E X P E R I M E N T A L

2.1. Modifications of the support Support (active carbon type 1 from Johnson Matthey) may be used as received, or after washing (water, methanol) and/or oxidation with a liquid reactant (HNO3). Temperature (20 to 105~ time duration (1 min to 24 h) and concentration of the oxidizing agent (0.05 to 14 M) may be used to control treatment severity. Support is dispersed in a nitric acid solution of the desired concentration, according to a ratio 1:10 (wt/vol.). The slurry is stirred, and heated if needed. After desired contact time is reached, slurry is filtered, and active carbon paste is rinsed with water until any trace of nitric acid has been removed. The solid is then washed with methanol at a ratio 1:10 (wt/vol.). Finally, the active carbon is dried in air or under vacuum. 2.2. Catalysts preparation Two standard ways of catalyst preparation are considered. The first one proceeds through the precipitation of palladium hydroxide from palladium dichloride according to a simple method described in [8]. The support (1 g) is stirred with 15 ml of water. Palladium chloride (0.08 g) in 10% hydrochloric acid solution (5 ml) is added and the suspension is kept stirred during 15 minutes (pH is between 0 and 3). Then soda 20% (around 2.5 ml) is slowly added until pH reaches 12. Formaldehyde (2 ml) is added to reduce palladium hydroxide under stirring at constant pH value (pH = 12) for 1 h. The slurry is then heated to 90~ during 1 minute, pH is adjusted at 5, mixture is rapidly cooled down, filtered and washed with water. ICP analysis of the filtrate is performed to ensure that the palladium is well on the support and to obtain an accurate value of metal loading of the catalyst. Catalysts prepared according to this method are called P catalysts. The second way to prepare Pd/C catalyst is the incipient wetness method. Typically, 0.5 ml of palladium chloride solution (containing 0.05 g of palladium) is slowly added in several steps to 1 g of active carbon. Between each addition, the active carbon is carefully mixed. The catalyst is then dried (helium, 4 h, 130~ and reduced (hydrogen, 1 h, 150~ The impregnation volume of palladium salt solution is kept at a constant value, typically lower than the porous volume of the support. Such catalysts are called I W catalysts. 2.3. Characterization methods "Boehm titrations" [9-10] aim at quantifying the acidic functions present on the solid surface and classifying them in different types by titrations with bases of different strengths. High care is needed for this characterization because of the small amount of solid, the high dilution of the solution used for the titrations and possible soda carbonation. Yet in this work, total acidity only is often measured by titration of the solid with soda. "Johnson Matthey titration" is a simple pH titration of the support by hydrochloric acid (for the basic functions) and by soda (for the acidic functions). This procedure is well described in [11]. It does not give direct quantitative information on the number of each type of acidic function. According to [12], a mathematical differential treatment of the experimental data may give the pKa of acidic functions present on the carbon surface. Nevertheless, the complexity and the wide variety of acidic functions present on the active carbon make it impossible to present very accurate quantitative information. From the

995 shape of the curves meq NaOH or HCI/100g of support vs. pH, we derive qualitative information on the types of acidity. FTIR measurements are performed to identify the oxygenated groups created on the surface of the solid by the oxidizing treatments. The active carbon or the catalyst is mixed (0.25% wt.) to KBr. After grinding and drying (2 h, 120~ the mixture, disks are prepared (10 9 Pa, 15 min) with a thickness around 0.5 mm and a mass about 0.12 g. Each disk is scanned 16 times, with a resolution of 4 or 16cm -' [2,13]. This infrared analysis is performed with a Perkin Elmer 1600 FTIR or 1720-X IFTS, both used in the transmission mode. Absorbance peaks or bands due to specific surface function are very small because of the opacity of the carbon. To get useful information from FI'IR analysis, the spectrum has to be recalculated by the computer, reconstructing a flat baseline. Afterwards, each defined peak or band can be integrated. The surface (S) of a band transforms into a relative peak surface (Sret) taking into account the mass (m) and the carbon loading (r) of the KBr disk. The relative peak surface is hence independent of the sample preparation procedure. S S re I - m.'c

Surface area of the bare support, modified supports or catalysts have been determined by nitrogen adsorption and desorption isotherms at 77K recorded on a Micromeritics ASAP 2010. Considering the appropriate range of relative pressure vs. amount of adsorbed nitrogen, we are able to calculate the Langmuir surface area (P/P0 from 0.01 to 0.05) or the BET surface area (P/P0 from 0.02 to 0.06), for the microporous carbon we are studying. This characteristic can also be determined according to the single point BET method as well with a Micromeritics TPD-TPR 2900 as with the ASAP 2010. The sample is flushed with helium (25ml/min) at 150~ for 1 h. It is then cooled down to 77 K and exposed to a nitrogen and helium flow (in a well-known ratio) until equilibrium is reached. Determination of the physisorbed nitrogen leads to BET surface area values. Metallic dispersion (D) is measured by CO chemisorption performed with a Micromeritics Pulse Chemisorb 2700. The catalyst is first dried and reduced under hydrogen flow (1 h, 150~ then flushed with helium (2 h, 200~ to avoid chemisorbed or dissolved hydrogen. A series of CO pulses is passed at room temperature on the sample (between 0.03 and 0.06 g). When the amount of unadsorbed gas reaches a constant value, this saturation value and the loop volume lead us to the total volume of CO chemisorbed. Stoichiometry of CO chemisorption is supposed to be 1:1, as it is very often recognized in the literature [ 14-16]. 3. RESULTS AND DISCUSSION FFIR spectra of studied active carbon show five absorption bands at 1710cm -1, 1570 c m 1, 1435 c m -1, 1384 c m -1 and 1100 cm 1, this last one being too large to give accurate information. These bands have been assigned : for 1710 cm -1 to lactonic and carboxylic functions [17-20]; for 1570 cm I to quinonic functions [17,19,20] or carboxylate functions [21]; for 1435 cm -1 to phenolic or carboxylic acid functions [17-19];

996

for 1384 cm -1 to carbonate functions [20]" for 1100 cm 1 to lactonic, phenolic or carboxylic functions [17-20]. A band at 1570 cm -1 is sometimes assigned to carboxylate functions, but we rejected this assignment in the present work because they are only created by a basic treatment. Table 1 summarizes the relative areas of the integrated absorbance bands for the support and a panel of oxidizing treatment and catalysts. Table 1 Relative band area for support, treated or not treated, for a few catalysts. Treatments are performed at room temperature during 24 hours. Solids Support : C (raw) C - HaO C - HNO3 0.01 M C - HNO3 0.05 M C - HNO3 0.25 M Pd/C (P20) Pd/C - HNO3 0.25 M (P21) Pd/C (IW22) Pd/C - HNO3 0.25 M (IW23)

1710 cm1

1570cm-1

1435cm-1

1384cm-1

1 1 0 0 c m -1

0 0 0 0 0 2.3 3.1 0 0

13.5 10.2 1 2 18.5 26.4 17.8 22.8 19.6

7.1 1.8 0.45 0 0.8 0 0 0 0

0 0 0.2 0.4 8.0 0 0 0 0

123 80 83 80 86 90 94 123 94

Even if the inaccuracy of the large band at 1100 cm -1 prevents any quantitative conclusion, this band seems to be more important when the solid has not been put in aqueous slurry, either during the treatment of the support or during the catalyst preparation. This is demonstrated with catalyst IW22 that is prepared by incipient wetness on a raw support. A lower relative surface of this band without any increase of another band strongly suggests that this decrease be due to the elimination of surface organic impurities more than to the hydrolysis of a surface function. The fine band at 1384 cm -1 has sometimes been assigned to carbonate structure [20]. We rather consider that this band is due to the presence of nitrate groups, more or less bonded on the surface, because : 9 inorganic nitrate (NaNO3) shows an absorbance band at 1384 cm -1 while inorganic carbonate (NazCO3) absorbs at 1462 cm-1; 9 treatment with another strong oxidizing agent (HC104) does not induce this band, but rather another one centered on 1114 cm -1 which is in the absorption range of HCIO4; 9 if the nitric treated supports are further washed with water, the surface area and the height of this band (1384 cm -1) slowly decrease. If this peak is no longer present on the spectrum of the catalysts, it is probably because the responsible nitrate disappeared during the precipitation process (for P catalysts) or during the high temperature reduction for the IW catalysts. The total acidity of the solids is calculated from soda titration. Results are presented in Table 2 for Boehm titrations, where the number of acidic functions is expressed in meq/100 g of solid, and in Figs. l a and l b for the Johnson Matthey titration.

997

Table 2 T o t a l s u r f a c e a c i d i t y e x p r e s s e d in m e q / 1 0 0 g o f s o l i d . Solids

Solids

Acidity

Severe oxidation

Acidity

Soft oxidation

C - HzO

54

C - H N O 3 - 0.25 M - 2 5 ~

- 24 hrs

80

- 24 hrs

106

C - H N O 3 - 14 M - 1 0 5 ~

- 1'

163

C - H N O 3 - 0.25 M - 6 5 ~

C - H N O 3 - 14 M -

105~

- 10'

241

C - H N O 3 - 0.25 M - 1 0 5 ~

C - H N O 3 - 14 M - 1 0 5 ~

- 25'

330

C - u n t r e a t e d (P)*

C - H N O 3 - 14 M - 1 0 5 ~

- 45'

484

C - H N O 3 - 0.25 M - 2 5 ~

C-

105~

- 52'

519

P d / C - H N O 3 - 14 M -

C - H N O 3 - 14 M - 1 0 5 ~

- 65'

552

C - H N O 3 - 14 M - 1 0 5 ~

- 90'

583

P d / C - H N O 3 - 0.25 M - 6 5 ~

C-

- 120'

604

Pd/C-

H N O 3 - 14 M -

H N O 3 - 14 M -

105~

- 24 hrs

125 52

- 24 hrs ( P)*

105~

- 90'

55 420

Catalysts IW H N O 3 - 14 M -

105~

- 24 hrs - 90'

90 420

(P)* 9 means that the support has undergone the blank process for preparation of a catalyst by the precipitation method, except that the palladium salt was not used. From

the Johnson

Matthey

t i t r a t i o n s , it a p p e a r s t h a t s o f t o x i d a t i o n a n d s h o r t s e v e r e

o x i d a t i o n first i n c r e a s e t h e n u m b e r o f w e a k a c i d i c f u n c t i o n s . I n c r e a s i n g t h e s e v e r i t y o f t h e treatment or the time of a severe oxidation leads to transform the so-created

weak acidic

functions into stronger ones, probably through a classical oxidation process.

100

NaOH

80 (:D

o

60

-u -r ~

40

20

o

i or z

0 20

!

_(

1"-

4

14

o

a- 40 E

60

HCI

80 pH = C-H20 " -.,,l--C-severe 9 ox-25 r a i n

C-severeox-1 rain J C-severe ox-90 minI

F i g . 1. a. J o h n s o n M a t t h e y t i t r a t i o n a f t e r s e v e r e o x i d a t i o n ( 1 4 M , 1 0 5 ~ 90 min) and washing with water.

a n d 1, 2 5 o r

998 60 NaOH o o

40

T"I

O 20 -l-

k,,.

o

-1-

0

0 4

z

~

o

6 P

8

10

12

20

E 40 60

HCI pH --m--C-H20 C-soft ox-65~

.t '

C-soft ox-25~ C-soft ox-105~

Fig. 1. b. Johnson Matthey titration after soft oxidation (0.25 M, 24 hrs and 25, 65 or 105 ~ and washing with water.

Boehm titrations clearly indicate that severe oxidation (even during 1 minute) creates more acidic functions on the support than soft oxidation, even during a long time. The blank precipitation process makes these acidic functions disappear. The precipitation process is still shorter than the titration, either in the Boehm method (24 h contact time between solid and soda) or in Johnson Matthey method (24 h between each pH measurement). Results obtained after the precipitation process are close to those obtained with raw active carbon. It means that the intrinsic acidity of the support is not neutralized, contrary to the acidity created by the oxidizing treatment. Further, it demonstrates that the oxidizing pre-treatment of an active carbon to prepare catalyst by precipitation only makes sense if this surface acidity allows differences in the metallic dispersion after reduction. If the oxidizing treatment aims at reaching a modified support surface of the working catalyst, it should be performed after precipitation, or the catalyst has to be prepared in non-neutralizing conditions (e.g. by incipient wetness). Preparation of catalyst by incipient wetness method leads to catalysts containing more acidic functions than the raw carbon, but less than the corresponding treated support (-15 to -25%). It could be an indication that palladium crystallites are localized on acidic functions. But it could also just result from pore blocking by the palladium crystallites. In this case, we would expect lower surface area for the catalyst than for the corresponding support. Concerning the surface area of the solids, we must first validate the BET single point method for the rapid determination of the surface area of a microporous solid. Therefore, we have compared values obtained following this procedure either with the ASAP 2010 or with the TPD-TPR 2900 with the Langmuir value and the BET value calculated from the complete isotherm. Converging values for Langmuir, BET and BET single point surface area are obtained if the appropriate range of relative pressure is considered. If the classical

999

range

(from relative p r e s s u r e

o f 0.05

to 0.30)

is c o n s i d e r e d

as default value, e r r o r

m e a s u r e m e n t can reach 15%. Results o f B E T single point surface area are listed in T a b l e 3. Table 3 B E T single point surface area in Solids

ma/g

o f s u p p o r t , t r e a t e d or not, and catalysts.

Area (mZ/g)

C - HzO

Solids

725

Soft oxidation

Severe oxidation CCC CCCC C-

Area (mZ/g)

HNO3 HNO3 HNO3 HNO3HNO3 HNO3 HNO3 HNO3-

14 14 14 14 14 14 14 14

MMMMM M M M-

105~ 105~ 105~ 105~ 105~ 105~ 105~ 105~

- 1' 10' - 25' ' - 52' - 65' - 90' 120'

645 615 540 417 287 90 49 25

C - HNO3 - 0.25 M - 25~ - 24 hrs C - HNO3 - 0.25 M - 65~ - 24 hrs C - HNO3 - 0.25 M - 105~ - 24 hrs

713 710 715

Catalysts IW Pd/C Pd/C Pd/C Pd/C-

HzO HNO3 - 14 M - 105~ - 1' HNO3 - 14 M - 105~ - 45' HNO3 - 14 M - 1 0 5 ~ 90'

609 619 375 37

Soft o x i d a t i o n d o e s not c h a n g e the surface area, e v e n if it is p e r f o r m e d for a very long time. A f t e r ten days, surface area is still higher than 710 ma/g. O n the contrary, s e v e r e o x i d a t i o n directly influences the surface area o f the s u p p o r t , leading to a c o m p l e t e d e s t r u c t i o n o f the s t r u c t u r e o f the active carbon: 95 % o f the surface area can be lost. T h e solid b e c o m e s v e r y difficult to r e c o v e r by filtration. In Fig. 2, w e can see that the d e s t r u c t i o n o f the s u p p o r t is non-linear vs. time. T h e beginning o f the dislocation o f the a r o m a t i c sheets o c c u r s b e t w e e n 45 and 65 minutes, c o r r e s p o n d i n g to a s l o w d o w n o f the c r e a t i o n o f surface acidic functions (also r e p r e s e n t e d in Fig. 2). 800 700 600 500 400 300 200 100 0

I

i

t

i

i

I

20

40

60

80

1O0

9 Surface area (m 2/g)

A !

120

k Total surface acid ity (m eq/100g)

Fig. 2. S u r f a c e area and n u m b e r o f acidic functions o f the s u p p o r t as a f u n c t i o n o f time d u r a t i o n o f a s e v e r e oxidation.

1000

It is obvious that palladium crystallites are blocking some pores of the active carbon, confirming our hypothesis. All catalysts develop a surface area lower than the surface area of the corresponding support. Nevertheless, this does not exclude the possibility of a partially selective deposition of the palladium on acidic groups. If this phenomenon occurred during the preparation process, we would obtain catalysts exhibiting higher dispersion when prepared on a functionalized support. Indeed, if palladium is anchored on the surface but also and more specifically on the acidic functions, the number of anchored crystallites should increase with the acidity of the support. In this way, dispersion should increase, average size of the crystallites should be lower, and the blocked part of the porosity should decrease. The small difference between IW catalysts P d / C - HzO and Pd/ C - HNO3 - 14 M - 105~ - 1' (Table 3) may suggest so, but we are still unable to confirm it. Dispersion could be a second argument. Table 4 summarizes the metallic dispersion results obtained for our catalysts. Table 4 Metallic dispersion (%) of our catalysts Catalysts

Dispersion (%)

By precipitation cl)

Catalysts

Dispersion (%)

By incipient wetness

P1

12.1

IW0 : P d / C - HzO

32.8

P2

15.3

Severe oxidation (HNO3,14 M, 105~

P3

7.4

IWl : P d / C - 1'

30.7

P9

14.9

IWl0 : P d / C - 25'

27.3

P10

15.6

I W l l : P d / C - 65'

17.7

P31 : Pd/C - HNO3 - 0.01 M ce)

19.8

IW 5 : Pd/C - 120'

2.8

P32 : Pd/C - HNO3 - 0.05 M c2)

17.6

Soft oxidation (HNO3,0.25 M, 24 hrs)

P33 : Pd/C - HNO3 - 0.25 M c2)

19.9

IW7 : Pd/C - 25~ 35.2 IW 8 : P d / C - 65~ 31.1 (1) From P1 to P10 : modification of the time to add soda and the heating time during the preparation (2) Treatment of the support is performed at room temperature during 24 h

For the catalysts obtained after precipitation, varying the time during which the slurry is kept at 90~ only slowly affects the dispersion of the final catalysts if heating is kept over one minute (P1 and P2 = 1 min, P9 = 5min, P10 =1 h, P3 = no heating). Adding soda at once or dropwise does not affect the metal dispersion (P1 and P3 at once, P2: in 2 min, P9 and P10: in 25 min). But when catalysts are prepared on softly oxidized supports (P31 to P33), dispersion increases to 20%. Incipient wetness produces catalysts with higher dispersions than precipitation. The metal dispersion decreases with the severity of the oxidation. This treatment also decreases the available surface area. It is hence logical to observe lower metal dispersions for more severe oxidations. Soft oxidation seems to be the best compromise between creation of acidic functions on the support and high dispersion values.

1001 4. C O N C L U S I O N S Before considering testing the catalysts to evaluate their activity and selectivity in the hydrogenation of fine chemicals, for example a nitroparaffin, we may conclude on the steps we have studied in this paper: oxidation of the active carbon used as a support and preparation of the catalyst either by incipient wetness or by precipitation. Oxidation of the support with nitric acid creates acidic functions on its surface. The number and type of acidic groups created can be controlled by temperature, concentration of oxidizing agent, and contact time. In the case of severe oxidation, strong acidic functions are very quickly created. Increasing the contact time leads to increase the number of these strong acidic functions, but also to destroy the physical structure of the active carbon. The surface area of the support is drastically decreased and the solid loses its interest as a support. Soft oxidation mainly leads to the creation of weak surface acids; the structure and surface area of the solid remain unchanged, even after a very long time. Soft oxidation of the support also leads to a slight increase of the metal dispersion if the catalyst is prepared by precipitation. However, during this procedure, the previously created acidity is neutralized, but it probably plays a role in anchoring the palladium on the support surface, before its precipitation under hydroxide form. When incipient wetness impregnation is considered to prepare catalysts, very high metal dispersions are obtained if the support is not or is softly oxidized, or severely oxidized but for a very short time. Increasing the severe oxidation of the support leads to decrease the metal dispersion, probably through the drastic decrease of the surface area. As for the surface characterization of the catalysts, FFIR is not yet a quantitative method to evaluate the number of several oxygen-containing surface groups of a support or a catalyst. As a consequence, FTIR is unable to distinguish a catalyst prepared by precipitation from another one prepared by incipient wetness impregnation. We have also shown that the absorption band observed at 1384 cm -1 after oxidation is more probably due to adsorbed nitrate groups than carbonate or carboxylate functions. ACKNOWLEDGMENTS We very much appreciated the generous gift of active carbon from Johnson Matthey. We acknowledge the contribution of A. Villeval, R. Buggenhout and B. Renard in conducting a part of the experimental work and we thank D. Desmecht for his valuable help in preparing the manuscript. REFERENCES 1. V. Dubois, G. Jannes and P. Verhasselt, in "Heterogeneous Catalysis and Fine Chemicals IV" (H.U. Blaser, A. Baiker and R. Prins, eds.), Elsevier, Amsterdam, Stud. Surf. Sci. Catal., 108 (1997) 263. 2. V. Dubois, PhD Thesis, University of Louvain, Louvain-la-Neuve, 2000. 3. V. Dubois, G. Jannes, J.L. Dallons and A. Van Gysel, in "Catalysis of Organic Reactions" (J.R. Kosak and T.A.Johnson, eds.), Marcel Dekker, New York (1994), p 1. 4. M. Spiro, Catal. Today, 7 (1990) 167.

1002 5. A.S. Lisitsyn, P.A. Simonov, A.A. Ketterling and V.A. Likholobov, in "Preparation of Catalyst V" (G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, eds.), Elsevier, Amsterdam, Stud. Surf. Sci. Catal., 63 (1991) 449. 6. D.J. Suh, T.J. Park and S.K. Ihm, Ind. Eng. Chem. Res., 31 (1992) 1849. 7. P. Albers, K. Deller, B.M. Despeyroux, A. Sch/iffer and K. Seibold, J. Catal., 133 (1992) p 467. 8. R.J. Peterson, Hydrogenation Catalysts, Noyes Data Corporation, Park Ridge- NJ (1977), 35. 9. H.P. Boehm and M. Voll, Carbon, 8 (1970) 227. 10. J.S. Bandosz, J. Jagiello and J.A. Schwarz, Anal. Chem., 64 (1992) 891. 11. D.S. Cameron, S.J. Cooper, I.L. Dodgson, B. Harrison, and J.W. Jenkins, Catal. Today, 7 (1990) 113. 12. S. Biniak, G. Szymanski, J. Siedlewski and A. Swiatkowski, Carbon, 35 (1990) 1799. 13. F. Rositani, P.L. Antonucci, M. Minutoli, N. Giordano and A. Villari, Carbon, 25 (1987) 325. 14. J.J.F. Scholten, A.P. Pijpers, and A.M.L. Hustings, Catal. Rev. Sci. Eng., 27 (1985) 151. 15. J.L. Lemaitre, P.G. Menon and F. Delannay, in "Characterization of Heterogeneous Catalysts" (F. Delannay, ed.), Marcel Dekker, New York (1984) 299. 16. N. Krishnankutty and M.A. Vanice, J. Catal., 155 (1995) 327. 17. J. Zawadzki, Caron, 16 (1978) 491. 18. C. Ishizaki and I. Marti, Carbon, 19 (1981) 409. 19. S. Shin, J. Jang, S.H. Yoon and I. Mochida, Carbon, 35 (1997) 1739. 20. D.J. Suh, T.J. Park and S.K. Ihm, Carbon, 31 (1993) 427. 21.G.R. Heal and L.L. Mkayula, Carbon, 26 (1988) 803.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1003

n-Butane isomerization over Al-promoted sulfated z i r c o n i a s . I n f l u e n c e of the sulfate content J. A. Moreno and G. Poncelet Unit6 de catalyse et chimie des mat6riaux divis6s, Universit6 catholique de Louvain, Croix du Sud, 2/17, 1348 Louvain-la-Neuve, Belgium n-Butane isomerization in the presence of hydrogen has been studied over Al-promoted sulfated zirconas (SAZ). In particular, the influence of the sulfate loading has been examined over SAZ catalysts with 6 tool% AlaO3. An optimum performance was found for 7.5 nominal wt% sulfate. This catalyst has a high Bronsted acidity content (NH3-IR) and a sulfate density which, in terms of surface coverage, is close to that necessary for achieving a complete monolayer. 1. I N T R O D U C T I O N Iso-butane is a highly demanded chemical in the refinery industry for the production of alkylates (by alkylation with butenes), and methyl tert-butyl ether (MTBE) (from isobutene and methanol), both important additives for reformulated gasolines, n-Butane isomerization is performed over platinum supported on chlorinated alumina. The chlorine compound which is continuously supplied to the feed in order to maintain the activity [1] is harmful to the environment. Sulfated zirconias, nowadays a well established class of acid solids first reported by Holm and Bailey [2], and systematically studied by Arata [3] and Tanabe et aL [4], are considered as potential alternative catalysts for the skeletal isomerization of n-butane. These catalysts have recently found a commercial application (Par-Isom Process of UOP) for the isomerization of light naphtha (C5-C6), but since they are less active than Ptchlorinated aluminas, there is a real interest for improving their catalytic performance [5]. Al-promoted sulfated zirconias, more active and stable than non-promoted sulfated zirconia catalysts, have been reported in the last years. Addition of A13§ appears to stabilize the sulfate species at the oxide surface, thereby increasing the number of acid sites with intermediate strength [6], and favoring the tetragonal structure [7]. Catalytic results over a series of Al-promoted sulfated zirconias (0.75-6.0 mol% AIzO3) showed that the catalyst with 2 mol% AIzO3 was the most active: the tetragonal phase was most developed and the surface coverage by sulfate was close to a monolayer (assuming a cross-section area of 0.25 nm 2 per SO42 group) [8]. In this contribution, the influence of the sulfate content on the catalytic performances of Al-promoted sulfated zirconia with 6 mol% AlzO3 is examined.

1004 2. E X P E R I M E N T A L 2.1 Catalyst preparation A mixed solution of ZrOClz and the appropriate weight of aluminum nitrate in order to have 6 mol% AlzO3 was brought at pH = 8 by drop-wise addition under stirring of concentrated ammonia solution. Stirring was maintained for an additional 20 min, and then the precipitate was washed free of chlorides. After drying at 100~ the solid was impregnated with 0.5 M sulfuric acid solution, and the solid was dried at 100~ and then calcined in static air 3 h at 650~ using a ramp of 5~ min -1. Catalysts with nominal sulfate loadings of 2.5, 5, 7.5, and 10 wt% (with respect to the ZrOz mass) were prepared. 2.2 Characterization methods The catalysts were characterized by Nz adsorption-desorption isotherms, thermogravimetric analysis (TGA), temperature-programmed desorption of ammonia (NH3TPD), X-ray diffraction (XRD), Raman spectroscopy, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS). The procedures and experimental conditions have been detailed elsewhere [9]. 1 O0

2.3 Catalytic activity measurement 60 Isomerization of n-butane was carried out 8O in a continuous-flow microreactor operated at .~ 50 atmospheric pressure. The catalyst (0.5 g, 0.2- .~_ 40 60 ._~ 0.315 mm fraction)) was activated at 450 ~ .w .__. in flowing dry air for 1.5 h and then cooled at ~ 30 0 250~ (reaction temperature). The reactor o 4O ~ID was purged with helium prior to admission of ~ 20 the feed mixture, which consisted of 1 ml min ~ 10 2O 1 of n-butane (99.95%) and 10 ml min -1 of H2 (99.90%). The WHSV was 0.3 h -1. The 0 , 0 different gases (from Air Liquide) were passed 0 1 2 3 4 5 6 7 through 'Hydro-purge' and 'Oxy-trap' AI203 (mol %) (Alltech Associates) in order to remove water Fig. 1. Steady conversion and selectivity and traces of oxygen. Analysis of the reaction vs Al content (10 wt% SO42-) products was made on-line in a gas chromatograph equipped with TC detector and 50 m long HP-PLOT AlzO3/KC1 capillary column to separate the products. Conversion, yields, and selectivity were calculated on a molar basis. 3. R E S U L T S AND DISCUSSION Fig. 1 shows the variation of the steady conversion as a function of the catalyst AI content, expressed as mol% of AlzO3. Clearly, the catalyst with 2 mol% AIzO3 is the most active one. The catalysts with different promoter contents were impregnated with a similar

1005 nominal amount of sulfate (10 wt%). The sulfate retained on the different catalysts increased between 5.45 and 8.63 wt% for aluminum content increasing from 0.75 to 6.0 mol% AlzO3. For promoter contents above 3 mol%, the amount of sulfate exceeded a full monolayer coverage (namely, a density above 4 SO4z nm-Z). These catalysts exhibited lower activities. In order to study the influence of the sulfate content on the isomerization activity, a catalyst with 6 mol% AIzO3, which typically showed a low activity, was chosen in 6rder to optimize the sulfate loading. The nominal sulfate loading was varied between 2.5 and 10 wt%. 3.1 Characterization Characterization results of Table 1 show that the sulfate retained on the mixed oxide is proportional to the nominal sulfate content, with nearly total sulfate retention for nominal sulfate loadings up to 5 wt%. The S/Zr atomic surface ratio (determined by XPS), the surface area, and the surface coverage regularly increase with the amount sulfate determined by chemical analysis.

Table 1 Characterization results of SAZ catalysts (6 mol% AIzO3) with different sulfate contents Sample Wt% 8042(S / Zr) Surf. area Coverage Nominal Chem. anal. xps (m z g-l) (%) Sz.sA6Z SsA6Z S7.sA6Z S10A6Z

2.5 5.0 7.5 10.0

The DTG curves of Fig. 2 show a first weight loss with minimum at about 70~ due to the removal of hydration water, which is proportional to the sulfate content. The high temperature weight loss peak shows distinct profiles according to the sulfate loading, with only one minimum at about 960~ for the catalyst with 2.5% sulfate, and two minima at 715-720~ and 810-820~ for the catalyst with 10 wt% nominal sulfate. The presence of components with distinct decomposition temperatures is indicative of different sulfate species on the solids. The sulfate species

2.7 4.8 6.4 9.0

0.090 0.130 0.135 0.165

88 94 98 100

48.1 79.8 102.5 140.4

~

'~ ...."',.-~--2.5 9 "\\

5 wt%

"~ 2

7.5 wt%

/

wt%

lO wt%

lJ / . LJ :

i

r

O.OLO % oc-1

2s

'

4()0

'

6()0

'

8()0

'

10'00

Temperature (~ Fig. 2. DTG curves of the different catalysts previously calcined at 650~ stored at ambient conditions, and with different sulfate contents

1006 which decompose at 720~ corresponds to the sulfate in excess of the monolayer (overlayer sulfate). The XRD patterns (Fig. 3) of the catalysts exhibit the reflections of the pure tetragonal structure. However, the decreasing intensity and peak broadening noticed at increasing sulfate content suggest a decrease of the crystallinity and/or smaller crystallite size. Raman

10%

7.5 %

i

I

'

10

I

20

I

'

I

9

30

f

40

'

I

50

I

I

'

60

20 Fig. 3. XRD patterns of the catalysts calcined at 650~ with increasing sulfate content VS - O spectroscopy confirmed these 0.5 1386 : ~ 1 0 % observations. ; A Fig. 4 shows the DRIFT spectra in ,,,---,,i the 1700 - 900 cm -1 region after in situ activation in air at 450~ (as prior to the catalytic measurements). There is a continuous upward shift of the S=O stretching band, from 1382 to 1393 cm -1 when the sulfate content increases up to 7.5 wt%, suggesting 1000 ~6'oo ~4oo ~2'oo ' an increasing covalent character of the sulfate species. The position of Wavenumber (cm-1) this band in sulfated zirconias has Fig. 4. DRIFT spectra in the sulfate region recorded at been considered, indeed, as an 450~ (in air) over SAZ (6 mol% A]203) with indication of the covalent character increasing nominal sulfate content

1007 of the sulfate species with the higher the frequency, the more covalent the character, and the higher the catalytic activity [10]. From 7.5 to 10 wt% sulfate (coverage above a surface monolayer), the Vs=o band broadens and shifts back to 1386 cm 1. This weakening of the covalent character of the sulfate species is probably related to the higher hydration degree of the more amorphous zirconia matrix, which is reflected in the OH stretching region by a diffuse absorption massif between 3600 and 3000 cm -1. Perhaps also, an excess of sulfate might bring about a perturbation of the catalytic active sulfate species with high covalent character (with a Vs=o band near 1400 cm-1), probably being transformed into less active sulfate species (with a Vs=o band at about 1386 cm-1). On the other hand, a broad band develops at 1200 cm -1 for sulfate contents above 2.5 wt%, and the Vs-o band centered at 1022 cm -1 broadens. A band near 1200 cm -1 has been ascribed by Morterra et aL [11] to complex poly-sulfate species, typically observed in samples with sulfate in excess of a monolayer. Olindo et aL [12] also observed a band at this frequency for an amorphous Al-promoted SZ catalyst with 9 mol% A1203, and a surface coverage less than a monolayer, which was merely attributed to sulfate groups with a strong reciprocal perturbation owing to the disordered (amorphous) nature of the precursor system. In this study, there is a parallelism between the band at 1200 cm -1 and the overlayer sulfate (sulfate characterized by a DTG minimum near 720~ DRIFF spectra were recorded after ammonia adsorption over the solids previously activated in air at 450~ and cooled at 250~ followed with a purge of helium. As illustrated in Fig. 5 some differences are observed for the catalysts with 2.5 and 7.5 wt% sulfate. These samples, as seen in Fig. 6, exhibit the lowest and highest catalytic activity, respectively. (a) 2.5 wt % SO42I

0.4

~

S . :.. i NH4 i : 1430 l! x ~ ' ' J ~

/

./

//

i ',JN'~ -a's,

:: ' . .. ,~jn,t,a,

(b) 7.5 wt % SO42I ' 0.4

:: 1430

6NH+~

1400

1200

1000

Wavenumber (cm-1)

i:

6'oo

X~initial

! i

i

I

'

'

NH3-ads.

~

5 NH3

1600

1300

4oo

'

|

i

ooo

Wavenumber (cm-1)

Fig. 5. D R I F t spectra in the sulfate region recorded at 250~ (in He) before and after ammonia adsorption at 250~ over SAZ (6 mol% A1203) with nominal sulfate content: (a) 2.5 wt % SO42-, and (b) 7.5 wt % SO42Ammonia adsorption causes a significant decrease of intensity and a downward shift of the Vs=o band with, as shown in Table 2, shifts of 23 and 39 cm -1 for the catalysts with 2.5 and 7.5 wt% sulfate, respectively. According to Jin et aL [13], there would be a relation

1008 between the shift of the frequency and the catalytic activity, which as shown below, is consistent with our catalytic results. The broad band near 1430 cm -1 is assigned to the deformation band of NH4 § (NH3 on Bronsted acid sites). In addition, a small band at 1610 cm 4 (deformation mode of NH3 on Lewis acid sites), and a broad band at 1300 cm -1 also appeared. The latter band is barely distinguished for the sample with 2.5 wt% sulfate. This broad band is complex and probably contains contributions of a sulfate species with ionic character and perhaps also the symmetric bending of NH3 on Lewis sites. For Morterra et at [14], a ligand displacement (i.e. sulfate groups with highly covalent character) occurring upon adsorption of the base would explain the appearance of the broad band centered at 1300 cm 4 and the simultaneous decrease of the intensity of the Vs-o band near 1400 cm 4. Komarov and Sinilo [15] attributed similar spectral modifications upon ammonia adsorption to an electrostatic interaction of the negatively charged oxygen ion of the S=O group with the positively charged complex formed by the base molecule coordinated on the acid center. Table 2 Ammonia adsorption over SAZc-6 catalysts. Effect of sulfate loading. Sample

Wt % SO42Nominal Chem. anal.

Vs=o Before

After

AV•s=o

Integrated NH4+ band

Sz.sA6Z

2.5

2.70

1377

1354

23

2.48

SsA6Z

5.0

4.79

1386

1352

34

6.11

87.sA6Z

7.5

6.41

1390

1351

39

8.55

S10A6Z

10.0

8.96

1383

1350

33

7.40

The integrated areas of the deformation band of NH4 + are given in Table 2. The population of Bronsted sites increases with the nominal sulfate content up to 7.5 wt%, and decreases for the sample with the highest sulfate content. No reliable figures could be estimated for the Lewis bands at 1610 and 1200-1300 cm -1 (~)as and as NH3) due to the overlapping with the deformation band of HOH or H 3 0 § species, and the sulfate bands with ionic character, respectively. In the high frequency region (spectra not shown), some differences were also observed between the two catalysts. The relative intensity of the VN-H bands at

160

IO0 9O

S'electi~t

140

8O

120

7O

100 80

~ so 40

n'

"q"

~

60 r0

30

40

20

., / f i ~

10 0

,

0

|

,

2

i

4

Steady conversion ,

i

6

,

,

8

,

0

10

S042" content (wt%) Fig. 6. Initial, steady conversion, selectivity, and surface coverage as a function of the sulfate content over SAZ catalyst (6 mol % AlzO3)

1009 3364 cm -1 (asymmetric stretching of NH3 on Lewis acid sites) with respect to that at 3276 cm -1 is higher for the catalyst with 2.5 % sulfate, suggesting a higher amount of Lewis sites in the sample with the lower sulfate content. It has been shown in the literature that the L/B ratio decreases with increasing sulfate content [16, 17]. 3.2 Catalytic activity 40 The conversion - time curves (not shown here) of the different catalysts ~' show a loss of conversion = 30 (deactivation) in the first hours of the .9 reaction. This loss is insignificant for | 9 20 the catalyst with the smallest sulfate o O content which is nearly inactive, but >, "O it increases with the sulfate content ~ 10 as shown in Fig. 6 where the initial ~o and steady conversions are plotted as 0 a function of the sulfate content 0 2 4 6 8 10 determined by chemical analysis. Up Br6nsted acidity (a.u) to 6.4 wt% (7.5 nominal %), the conversion linearly increases with the Fig. 7. Relationship between the steady sulfate content, and decreases at conversion and the Bronsted acidity higher contents. The increase of (NH3-IR) conversion is accompanied by a small decrease of the selectivity, with ethane, propane, and iso-pentane (below integration limit) being the main by-products. Fig. 6 also shows that the sulfate coverage (expressed as percentage of monolayer) runs parallel with the increase of the steady conversion. The most active catalyst has a sulfate content which corresponds to a full monolayer (density close 4 SO42 nm2), thus indicating the importance of this parameter on the catalytic activity. Fig. 7 shows the variation of the steady conversion as a function of the integrated areas of the NH4 § deformation band. The linear relationship verifies that the Bransted acidity is directly ruling the isomerization activity of the catalysts. Although no reliable values could be established for the Lewis acidity, the bands in the N-H stretching region suggest a relative decrease of the Lewis/Bransted acid ratio with increasing sulfate content. ,

i

,

i

,

i

,

i

,

4. CONCLUSIONS The sulfate retention over Al-promoted zirconia (6 mol% AlzO3) is proportional to the nominal sulfate amount used at the impregnation step. Such a proportionality is only partial in non promoted sulfated zirconias. This difference reflects the stabilizing effect of the promoter on the sulfate, and, subsequently on the tetragonal structure. The Bronsted acidity and the catalytic activity increase parallel with the surface coverage, with both being highest when the sulfate content of the catalyst is close to that necessary for achieving a complete monolayer. The results demonstrate that the sulfate content plays an important role on the catalytic activity, whereas overlayer sulfate is detrimental. The excess sulfate is less tightly

1010 bonded at the surface of the oxide and decomposes at a lower temperature than the sulfate species participating in the formation of the monolayer. This overlayer sulfate is characterized by an IR band at 1200 cm-1. ACKNOWLEDGMENT J.A.M. gratefully acknowledges Colciencias (Colombia) for financial support. REFERENCES

1. C. Travers, in: "Le raffinage du p6trole. 3. Proc6d6s de transformation." (P. Leprince, Ed.), Editions Technip, Paris, 1998. 2. V.C.F. Holm and G.C. Bailey, U.S. Patent No. 3,032,599 (1962). 3. K. Arata, Adv. Catal., 37 (1990) 165. 4. K. Tanabe, H. Hattori and T. Yamaguchi, Crit. Rev. Surf. Chem, 1 (1990) 1. 5. S.A. Gembicki, in "Studies in Surface Science and Catalysis", Vol. 130, p. 147, Elsevier Sci., Amsterdam, 2000. 6. Z. Gao, Y. Xia, W. Hua and C. Miao, Topics Catal., 6 (1998) 101. 7. P. Canton, R. Olindo, F. Pinna, G. Strukul, P. Riello, M. Maneghetti, G. Cerrato, C. Morterra and A. Benedetti, Chem. Mater., 13 (2001) 1634. 8. J.A. Moreno and G. Poncelet, in "International Symposium on Acid-Base Catalysis IV", Matsuyama (Ed.), Book of Abstracts, 2001, P2. 9. J.A. Moreno and G. Poncelet, J. Catak, 203 (2001) 453. 10. C. Morterra, G. Cerrato, F. Pinna and M. Signoretto, Catal. Lett., 41 (1996) 101. 11. C. Morterra, G. Cerrato, G. Meligrana, M. Signoretto, F. Pinna and G. Strukul, G., Catal. Lett., 73 (2001) 113. 12. R. Olindo, F. Pinna, G. Strukul, P. Canton, P. Riello, G. Cerrato, G. Meligrana and C. Morterra, C., in "Studies in Surface Science and Catalysis", Vol. 130, p. 2375, Elsevier Sci., Amsterdam, 2000. 13. T. Jin, T. Yamaguchi and K. Tanabe, J. Phys. Chem, 90 (1986) 4794. 14. C. Morterra, G. Cerrato, F. Pinna and G. Meligrana, Topics CataL, 15 (2001) 53. 15. V.S. Komarov and M.F. Sinilo, Kinet. Katal., 29 (1988) 701. 16. P. Nascimento, C. Akratopoulou, M. Oszagyan, G. Coudurier, C. Travers, J.F. Joly and J.C. Vedrine, in "Studies in Surface Science and Catalysis", Vol. 75, p. 1186, Elsevier Sci., Amsterdam, 1993. 17. D.A. Ward and E.I. Ko, J. Catal., 150 (1994) 18.

Studiesin Surface ScienceandCatalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1011

Influence of preparation procedure on physical and catalytic properties of carbon supported Pd-Au catalysts. P. Canton ^, F. Menegazzo ~ M. Signoretto ~ F. Pinna ~ P. Riello ^, A. Benedetti ^ and N. Pernicone* Department of ~ and ^Physical Chemistry, Universit~ di Venezia, Dorsoduro 2137, 30123 Venezia and Via Torino 155/b, 30170 Venezia-Mestre, Italy; *Consultant, Via Pansa 7, 28100 Novara, Italy

The influence of preparation variables on the physical and catalytic properties of the PdAu/C system, taking benzaldehyde hydrogenation as a probe reaction, has been investigated. Catalysts were prepared by wet impregnation (simultaneous and consecutive). TPR, CO chemisorption and WAXS were used for catalyst characterization, in addition to catalytic tests. The chemisorption data point to the absence of Au surface enrichment in the fresh samples. The metal dispersion decreases upon heating at temperatures up to 500~ independently on Au presence. WAXS data show that coimpregnation gives rise to a PdAu alloy, while consecutive impregnation gives progressive alloying only after heat treatment. An appreciable amount of very small Pd particles is likely to be present in all the samples. For monometallic Pd samples the turnover frequency appears to increase with Pd particle size, but this does not occur in the presence of Au. Au addition to Pd catalysts does not increase their activity in the hydrogenation of aromatic aldehydes. 1. I N T R O D U C T I O N The elucidation of the physical-chemical nature of supported bimetallic catalysts and of its relationship with preparation technology is a present challenge in heterogeneous catalysis. Palladium-gold catalysts find industrial application in the selective hydrogenation of various organic compounds and in the production of vinylacetate [1]. More recently, they have been studied for the hydrodechlorination of chlorofluorocarbons [2]. Most studies until now regarded Pd-Au on oxidic supports (silica, alumina). Complete alloying between Pd and Au has been demonstrated [3]. However, the extent of alloying frequently depends on the preparation procedure. Moreover, surface enrichment must be also taken into account. Contradictory data are found on this aspect, as some authors report Au surface enrichment [4], while others [5,6] report the same for Pd. We deemed interesting to investigate Pd-Au supported on active carbon, in view of the possible use of Au for modifying the catalytic properties of the numerous Pd/C catalysts applied both in fine chemistry and petrochemistry. In fact, the presence of Au could change both site geometry and electronic properties of supported Pd, with practical consequences not only

1012 for the activity and selectivity, but also for the resistance to sintering and poisoning. Therefore, we have studied the influence of preparation variables on the physical and catalytic properties of the Pd-Au/C system, taking benzaldehyde hydrogenation as probe reaction. 2. E X P E R I M E N T A L 2.1 Preparation A commercial granular coconut active carbon (1200 m2/g) was used as support. H2PdCh and HAuCh have been employed as metal precursors. Monometallic Pd/C (Pd 0.4-0.5%) and bimetallic Pd-Au/C catalysts, with the addition of 0.10-0.13% Au, have been prepared by wet impregnation in three different ways: i) coimpregnation from a mixed solution of the Pd and Au precursors; ii) consecutive impregnation: first Pd precursor and then Au; iii) consecutive impregnation: first Au precursor and then Pd. Reduction was performed by sodium formate at 80~ Both mono and bimetallic samples have been subjected to a thermal treatment in inert atmosphere at three different temperatures (300, 400 and 500~

2.2. Characterization The quantitative determination of Pd and Au was performed by atomic absorption spectroscopy. CO chemisorption measurements were performed at 25~ using a home-made pulse flow system. Prior to the measurements, the samples were subjected to a pretreatment involving exposure to hydrogen flow for 1 h at 25~ followed by He purge for 2 h at the same temperature of reduction. TPR experiments were done in a home-made equipment: samples (100 mg) were heated at 10~ from 25 ~ to 820~ with a 5% H2/Ar reducing mixture (40 ml (STP)/min). X-ray diffraction (XRD) patterns were recorded using a Philips X'Pert vertical goniometer, connected to a highly stabilized generator [7]. Cu-K0t Ni-filtered radiation, a graphite monochromator and a proportional counter with a pulse-height discriminator were used. Data were collected for 60 s/step with a step size of 0.05 ~ in 20 on the 25~ ~ range. All the measurements were carried out under a continuous nitrogen flow in a sample holder suited for gas fluxing [8]. Before XRD measurements, the sample was treated under hydrogen flow to transform the PdO into PdH, and then purged with nitrogen to transform the palladium hydride into palladium.

2.3. Catalytic measurements Benzaldehyde hydrogenation was carried out in a glass batch reactor fitted with a reflux condenser, a mechanical stirrer and an external thermostating jacket [9]. The catalyst (corresponding to 4 mg Pd) was suspended in the solvent (100 ml ethanol) and pretreated in H2 flow (30 ml/min) at 80~ for 1 h. After cooling to the reaction temperature (20~ benzaldehyde (1.0 ml) and n-octane (0.3 ml), used as internal standard, were added through a serum cap fitted to one arm of the reactor. The reactor was stirred at 1500 rpm and operated at atmospheric pressure in H2 flow. The progress of the reaction was

1013 followed by gas-chromatographic analysis of samples withdrawn from the reaction mixture. The absence of external diffusion limitations was verified by preliminary runs performed under different stirring conditions. 3. RESULTS AND DISCUSSION 3.1. TPR and chemisorption data The TPR profiles of catalysts Pd-1, Pd-Au-2, Pd-Au-3, Pd-Au-4 prepared by wet impregnation are reported in Fig. 1. All profiles show a negative peak centered at about 70~ due to a hydrogen evolution from 13-hydride decomposition [10] and a broad band in the range 500-800~ related to the support and in particular to the reduction of the oxygenated species present at the carbon surface [11]. The very small peak sometimes present around 200~ is likely due to residual oxychlorides. Table 1 Physical characterization and reaction data of catalysts Catalyst Particle Size (A) CO/Pd CO WAXS Pd-1 Pd-1 wet impregnated Pd-l-300 Pd=0.50 wt% Pd-l-400 Pd-l-500

30 56 63 125

Kobs (xl03mol/s gPd)

61 81 87 96

0.17 0.09 0.08 0.04

1.23 0.83 0.85 0.66

Pd-Au-2 coimpregnated Pd=0.45 wt% Au=0.11 wt%

Pd-Au-2 Pd-Au-2-300 Pd-Au-2-400 Pd-Au-2-500

51 61 93 102

0.13 0.08 0.05 0.05

1.29 0.92 0.91 0.55

Pd-Au-3 cons. impreg. 1~ Pd=0.42 wt% 2 ~ Au=0.11 wt%

Pd-Au-3 Pd-Au-3-300 Pd-Au-3-400 Pd-Au-3-500

94 69 70 13

0.17 0.13 0.10 0.06

1.19 1.19 1.19 0.69

Pd-Au-4 cons. impreg. 1o Au=0.13 wt% 2 ~ Pd=0.51 wt%

Pd-Au-4 Pd-Au-4-300 Pd-Au-4-400 Pd-Au-4-500

100 n.d. n.d. 90

0.16 0.10 0.08 0.04

1.14 1.10 0.81 0.38

Disappearance, or strong decrease, of the Pd hydride decomposition peak in TPR experiments gives good evidence for Pd alloying, when Pd dispersion is not high [12]. This criterion could be applied to our samples, because the ratio between chemisorbed CO molecules and total Pd atoms never exceeded 0.17. So any change of the Pd hydride peak can be related to alloy formation. In fact, our TPR data show that Pd-Au alloying occurred only for the sample prepared by coimpregnation. The occurrence of alloying in similar samples after coimpregnation was recently evidenced by WAXS [13].

1014

v

O

~

o Q_

E o o

0

200

400

600

800

1000

T (~ Fig.1. TPR profiles: (a) Pd-]" (b) Pd-Au-2; (c) Pd-Au-3; (d) Pd-Au-4. Chemisorption data are reported in Table 1. The average Pd particle size has been calculated by the procedure recently developed by our group [7] for the monometallic Pd sample. For bimetallic samples, in the absence of any information about a possible influence of Au on Pd/CO chemisorption stoichiometry, only the experimental CO/Pd values have been reported. As expected, Pd dispersion decreases with the increase of treatment temperature, but no evident role seems to be played by Au neither on Pd sintering nor on Pd dispersion. This allows to exclude the occurrence of Au surface enrichment and also strong electronic interactions in the alloy.

3.2. X-ray diffraction data Deconvolution of instrumental line broadening was done before applying Fourier analysis to the 111 Bragg peak. In the case of Pd-Au-2 series, the crystallite sizes reported in Table 1 are those of the 111 reflection of the Pd-Au alloy, while for the other series, the Pd crystallite sizes are indicated. X-ray diffraction patterns for the series Pd-1 reported in Fig.2 show the presence of pure Pd with no formation of PdO. With the increase of calcination temperature, growing of Pd crystallite size occurs, as shown in Table 1. The crystallite sizes of the four Pd-1 samples are larger than those previously reported [7] for similar samples. This difference could be due either to different preparation techniques (an industrial catalyst was investigated previously) or to very small Pd particles undetected in the present study (Rietveld and quantitative analysis were not performed yet).

1015

80 r O

r

60

t

.

v

. m

t"

40

.4.--,

r

20 3'6'3'8'4'0'4'2 20 ~ Fig. 2. XRD patterns for Pd-1 series. Pd-l" continuous line; Pd-l-300: closed circles; Pd-l-400: open square plus continuous line; Pd-l-500: closed triangles.

,,l,.a

t':::3 O

6O

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t

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-,, Pd

AA 9

9

I ~,

o

v

t-

,,

20

|

3'6

'

I

38

4'0

'

42

20 ~ Fig. 3. XRD patterns for Pd-Au-2 series. Pd-Au-2: continuous line; Pd-Au-2-300: closed circles; Pd-Au-2400: open square plus continuos line; Pd-Au-2-500: closed triangles.

X-ray diffraction patterns for the series Pd-Au-2, Pd-Au-3 and Pd-Au-4 are reported in Figs. 3, 4, 5, respectively. From the XRD patterns it is possible to see that different preparation methods result in completely different samples. In fact the sample Pd-Au-2,

1016 prepared by co-impregnation, resulted in the formation of a Pd-Au alloy together with some pure gold. The heat treatment at different temperatures allowed the alloying of the residual pure gold, but for the sample calcined at 500~ it is possible to note the presence not only of the alloy but also of pure Pd (which was not visible in the other samples). This can be explained considering that the samples Pd-Au-2, Pd-Au-2-300, and Pd-Au-2-400 were very dispersed, and the same certainly holds for the residual pure Pd, thus preventing the Pd peak from being visible. In fact, previous studies on similar samples showed the presence of an appreciable amount of metal particles with size lower than 20 A [14]. The heat treatment at 500~ favored the sintering of the small Pd particles, so that the Pd peak is now evident. Completely different is the behavior of samples Pd-Au-3, Pd-Au-3-300, Pd-Au-3-400 and Pd-Au-3-500, prepared by subsequent impregnation of palladium and gold. In this case (see Fig. 4) the starting sample, Pd-Au-3, is a mixture of pure Pd and a Pd-Au alloy very rich in gold. After heating, the alloy becomes richer and richer in Pd, since the 111 Bragg peak of the alloy moves towards the Bragg position of the 111 Pd reflection. At the same time there is also the sintering of Pd crystallites. Fig. 5 reports the XRD patterns of samples Pd-Au-4 and Pd-Au-4-500, also prepared by subsequent impregnation, but starting with gold and then adding Pd. In this case, the "fresh" sample is formed by the pure metals but, after calcination at 500 ~ C, two Pd-Au alloys with different metal compositions are formed. Also in this case (as for the Pd-Au-3 series), it is possible to observe the sintering of the Pd crystallites with the increase of the calcination temperature.

Pd

Au .

5o

4O ~

30 20 3 6'

'

3 8' '

'

4" 0

'

42

2O ~

Fig. 4. XRD patterns for Pd-Au-3 series. Pd-Au-3: continuous line; Pd-Au-3-300: closed circles; Pd-Au-3400: open square plus continuous line; Pd-Au-3-500: closed triangles.

1017

"-" C O

50

t

Au

,, Pd

..;. ,

40

'

:s.. I

;

"

9 "

%,.,

9

I

~'

9

C

30 9

. ..~:'

20

a6

a's

4o

42

2O ~

Fig. 5. XRD patterns for Pd-Au-4 series. Pd-Au-4: continuous line; Pd-Au-4-500: closed circles. 3.3. C a t a l y t i c a c t i v i t y

The first-order rate constant of benzaldehyde hydrogenation to benzyl alcohol [9], referred to Pd mass, is reported in Table 1. For monometallic Pd samples, the specific surface activity, or turnover frequency (TOF), roughly calculated as ratio between Kobs and CO/Pd, appears to increase with Pd particle size, a behavior that was suggested in our previous paper on the effect of different carriers [9]. When Au is present, this behavior seems to disappear (TOF roughly independent on Pd dispersion), but presently no straightforward explanation can be suggested. As a possible one, a progressive Au surface enrichment on heating could occur. From a practical point of view, Au addition to Pd in carbon-supported catalysts does not give any advantage as to activity in hydrogenation of aromatic aldehydes. Possible effects on the resistance to deactivation are presently being investigated. 4. CONCLUSIONS TPR and WAXS data showed that, when Pd and Au have been coimpregnated on active carbon, almost complete alloying occurs. When impregnation of the two metals is consecutive, partial alloying, with the formation of different Au-rich alloys, occurs only after suitable thermal treatment. CO chemisorption data showed that Pd dispersion does not change after the addition of Au, whose surface enrichment is therefore to be excluded. Au proved to have no effect on the Pd activity (per gram) in benzaldehyde hydrogenation, but the increase of turnover frequency with Pd particle size has been confirmed.

1018 REFERENCES

1. Handbook of Heterogeneous Catalysis (G. Ertl, H. Kn6zinger and J. Weitkamp Eds.), Vol. 5, VCH, Weinheim, 1997. 2. M. Bonarowska, A. Malinowski, W Juszczyk and Z. Karpinski, Appl. Catal. B, 30 (2001) 187. 3. G. Meitzner and J.H. Sinfelt, Catal. Lett., 30 (1995) 1. 4. R.J. Wu, T.Y. Chou and C.T. Yeh, Appl. Catal. B, 6 (1995) 105. 5. S.N. Reifsnyder and H.H. Lamb, J. Phys. Chem. B, 103 (1999) 321. 6. Y. Mizukoshi, T. Fujimoto, Y .Nagata, R. Oshima and Y.Maeda, J. Phys. Chem. B, 104 (2000) 6028. 7. G. Fagherazzi, P. Canton, P. Riello, N. Pernicone, F. Pinna and M. Battagliarin, Langmuir, 16 (2000) 4539. 8. P. Canton, P. Riello, A. Furlan, A. Minesso, L. Bertodlo, F. Pinna and A. Benedetti, Catal. Lett., 69 (2000) 17. 9. F. Pinna, F. Menegazzo, M. Signoretto, P. Canton, G. Fagherazzi and N. Pernicone, Appl. Catal. A, 219 (2001) 195. 10. F. Pinna, M. Signoretto, G. Strukul, S. Polizzi and N. Pernicone, React. Kinet. Catal. Lett., 60 (1997) 9. 11. F. Pinna, M. Signoretto, G. Strukul, A. Benedetti, M. Malentacchi and N. Pernicone, J. Catal., 155 (1995) 166. 12. G. Fagherazzi, A. Benedetti, S. Polizzi, A. Di Mario, F. Pinna, M. Signoretto and N. Pernicone, Catal. Lett., 32 (1995) 293. 13. A. Benedetti, L. Bertoldo, P. Canton, G. Goerigk, F. Pinna, P. Riello and S. Polizzi, Catal. Today, 49 (1999) 485. 14. P. Riello, P. Canton, A. Minesso, F. Pinna and A. Benedetti, Stud. Surf. Sci. Catal., 130 (2000) 3273.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation of mesoporous highly dispersed Pd-Pt catalysts

1019

for deep

hydrodesulfurization

Xiaoding Xu a, P. Waller b, E. Crezee a, Z. Shan b, F. Kapteijn a and J.A. Moulijn a aReactor and Catalysis Engineering, Delft ChemTech, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands bLaboratory for Applied Organic Chemistry and Catalysis, Delft ChemTech, Faculty of Applied Sciences, Delft University of Technology Several Pd-Pt (Pd:Pt=4:l) catalysts for deep hydrodesulfurization (HDS) were prepared using a commercial amorphous silica-alumina (ASA), a novel mesoporous silica (MPS) and a novel mesoporous silica-alumina (MPSA) as carriers with dispersions of > 50%. They were characterized by various techniques and their performance in deep HDS of 4-ethyl, 6-methyl-dibenzothiophene measured at 633 K and 6.0 MPa in the presence of carbazole and HaS. The Pd-Pt catalysts showed good activity as compared to typical conventional HDS catalysts. Their activity decreased with a decreasing A1 content in the support (thus support acidity): ASA > MPAS > MPS. Enhanced support acidity seems to favor the stabilization of the dispersed metal particles against sintering. 1. I N T R O D U C T I O N Organic sulphur- and nitrogen-compounds in motor fuels are a source for acid rain and harmful to the environment. Moreover, they are poisonous to the auto exhaust catalysts. To meet new developments in EU regulations on the S-concentration, a commonly applied one-step hydrodesulfurization (HDS), using conventional catalysts, e.g. Co-Mo/7-A1203, is insufficient. A second HDS step, viz. a deep HDS step, can be more economical to reduce the S-content to the currently allowed European level of 350 ppm. This level will be reduced further to 50 ppm in 2005 [1]. In the first HDS step, often the heavy organic sulfur-containing polyaromatics survived, such as dibenzothiophene (DBT) and (4-, and/or 6-) alkylated DBTs [2,3]. They are the most refractory. In crude oils, there are also aromatic N-compounds, which suppress the performance of the HDS catalysts. Hence, a model feed for representative HDS-activity measurements should contain characteristic S- and N- compounds for practical relevance. Previous studies by Reinhoudt et al. [4-7] using a model feed, showed that PdPt/ASA catalysts are suitable for deep HDS. It was reported that at the same total loading of Pd and Pt, the Pd/Pt atomic ratio also influences the performance [8]. Pd-Pt

1020 catalysts for deep HDS on three carriers, viz. ASA, MPS and MPSA, were studied due to their interesting textural properties and different Al-contents, hence acidic properties. 2. E X P E R I M E N T A L

Three supports, viz. a novel mesoporous silica (MPS), a mesoporous silica-alumina (MPSA, Si:Al ratio of 6) [9-11] and a commercial amorphous silica-alumina (ASA, Grace Davison HA) were used to prepare Pd-Pt catalysts with a Pd/Pt atomic ratio of 4 [8]. The MPS and MPSA carriers were prepared according to [9-11]. The Pd-Pt catalysts were prepared by ion-exchange [12,13] of 3.5 g carrier using one liter of aqueous solution containing 0.001 M Pd(NH3)4C12 and 0.00025 M of Pt(NH3)4CI2 (both from Alfa Aesar) under vigorous stirring. After one hour the pH was adjusted to 9.5 by 12.5 N NH4OH. Then, it was filtered and dried at 353 K overnight. Similar procedures were used to prepare Pd or Pt on MPS catalysts. This procedure is a modification of the method described in [12,13], which makes the preparation more simple and costefficient. The catalysts and their precursors were characterized using various techniques, viz. TGA (TGA/SDTA 851 e from Mettler TOLEDO), CO chemisorpiton, nitrogen physisorption (Quantachrome Autosorb-6B), Temperature Programmed Reduction (TPR, at 10 K/min in a flow of 7.6% H2 in Ar), elemental analysis (ICP-OES) and High Resolution Transmission ElectroMicroscopy (HRTEM), to optimise the procedures of preparation, calcination and pretreatment. For chemisorption, the sample was reduced in 100 % Hz (50 ml/min) at 573 K for 3h, followed by evacuation at 573 K for 2 h. Then, the temperature was lowered to 308 K and CO chemisorption started. The amount of CO chemisorption was calculated from the difference in CO uptake between two successive measurements (with an interim evacuation). Transmission Electron Microscopy (TEM) was performed using a Philips CM30T electron microscope with a LaB6 filament as the source of electrons operated at 300 kV. Samples were mounted on microgrid carbon polymer supported on a copper grid by placing a few droplets of a suspension of ground sample in ethanol on the grid, followed by drying at ambient conditions. The catalytic performance was determined in a micro batch reactor at 633 K and 6.0 MPa, using 200 mg catalyst (particle size 100 - 200 ~tm) and a model diesel fuel containing 150 mg 4-ethyl, 6-methyl-dibenzothiophene and 27.5 mg carbazole in 100 g n-hexadecane with n-octadecane as internal standard [5]. The catalyst was reduced in H2 flow (50 cm3/min) at 0.1 MPa at a heating rate of 2 K/min up to 573 K and kept at the temperature for 2 h. After reduction and cooling to about 300 K, the catalyst was introduced into the batch autoclave reactor without any air contact. The reaction mixture and the catalyst were heated under hydrogen to 633 K and pressurized to the desired level. The reaction was started by stirring at 2500 rpm using a capillary swing stirrer [14]. The samples were analysed by a gas chromatograph (Chrompack CP 9001) with a flame ionisation detector and a fused silica capillary column (CPSIL-8, 60 m, 0.22 mm internal diameter and a 0.25 ~m dimethylsiloxane film thickness). Samples for GC analysis were taken at intervals of ca. 30 min.

1021 3. R E S U L T S AND DISCUSSION CO chemisorption at 308 K was used to measure the metal dispersion and a stoichiometry ratio of M/CO (M = Pd or Pt) = 1:1 was assumed in the calculation for both Pt and/or Pd. Due to the possible formation of Pd hydride and the uncertainty on the metal to hydrogen ratio, hydrogen chemisorption was not used. Our previous results on Pd/MPS catalysts showed that the hydrogen concentration during reduction has little influence on the dispersion obtained. The metal dispersions of some Pd and/or Pt catalysts are presented in Table 1. Table 1 Pd and/or Pt dispersion of some reduced catalysts Pd wt% Pt wt% % Dc Support Preparation 2.75 0 0.7 MPS impregnated 2.75 0 6.4 MPS impregnated 2.75 0 43 MPS ion-exchanged 2.75 0 54 d MPS ion-exchanged 4.6 0 53 d MPS ion-exchanged 0 2.5 53 d MPS ion-exchanged 0 2.5 33 MPS ion-exchanged 2.56 1.16 56 MPS ion-exchanged 2.56 1.16 51 MPS ion-exchanged 2.5 1.1 51 ASA ion-exchanged 1.76 0.76 54 MPSA ion-exchanged a. At 623 K in air for 2 h. b. At 573 K in air for 2 h. c. Measured method, d. particle size of 1-2 nm observed in HREM.

Calcination Tred/K uncalcined 453 calcined a 453 calcined a 573 calcined a 453 calcined a 453 uncalcined a 548 uncalcined 673 uncalcined 573 calcined b 573 calcined b 573 calcined b 573 by CO chemisorption

The dispersion of properly reduced Pd/MPS catalysts, prepared by the ion-exchange method, is substantially higher than the corresponding ones, prepared by impregnation. It follows that impregnation is not a good method for preparing Pd on MPS catalysts. Hence, Pt and Pd-Pt catalysts were prepared by ion-exchange method only. Table 1 showed that calcination prior to reduction is favorable for Pd/MPS catalysts, but not for the Pd-Pt catalysts. The results of Pd-Pt/MPS showed that air calcination at 573 K for 2 h prior to the reduction reduces the dispersion from 56% to 51%, so a prior calcination is not necessary for the Pd-Pt catalysts as for the Pt catalysts. High dispersions (51-53%) are obtained for the properly reduced Pd-Pt catalysts on all the three carriers, prepared by the ion-exchange method, similar to the corresponding Pd/MPS catalysts, which are higher than the Pt or Pd catalysts prepared by ionexchange, described in [12,13]. These dispersions are also higher than those reported by Navarro et al. [7], though their Pd-Pt/ASA catalyst has a lower total metal content (Pd: 0.27wt% and Pt: 0.94 wt%). Note that their catalyst was prepared by co-impregnation. This confirms that ion exchange is a better method to prepare the Pd-Pt catalyst than impregnation. Moreover, it appears that a too high reduction temperature is unnecessary. For example, 573 K is proper for the Pd-Pt catalysts and 453 K is enough

1022 to reduce the Pd/MPS catalysts. The lower dispersion of the 673 K-reduced Pt/MPS catalyst by ion-exchange as compared to a 548 K-reduced one (33% versus 53%), is thought to be caused by a too high reduction temperature, which is above the H/ittig temperature of Pt (THtittigPt ~ 608 K), leading to possible sintering of the reduced Pt particles. TEM/HRTEM measurements of some samples were performed and Fig. 1 shows some HTREM images of Pd-Pt catalysts

Fig. 1. High resolution transmission microscopy image of Pd-Pt/MPS catalysts a: after chemisorption and recalcination (left) and b: spent (right). The metal particle sizes observed for three MPS supported Pd or Pt samples marked with a in Table 1 are between 1-2 nm. As CO chemisorption data showed that the Pd-Pt catalysts have similar dispersion, it may be expected that these have similar metal particle sizes. A calculation of Pd and Pt particle sizes at 100% dispersion using a spherical model yield 1.12 and 1.13 nm, respectively. It follows that the dispersion measured by CO chemisorption may represent a lower limit, the actual dispersion could be higher than those presented in Table 1. For Pd on silica or alumina, Moss [15] used a CO:Pd stoichiometric ratio of 0.6 and a value of 0.82 was used by Navarro for Pd-Pt on ASA [7]. Were these ratios used in calculating the chemisorption data, it would indeed lead to a higher dispersion. HRTEM results show that no metal-containing particles have been found for uncalcined samples, the only material visible in the HRTEM images in uncalcined PdPt/MPSA is the MPSA. Pd-Pt samples on MPS (after reduction, CO chemisorption and recalcination), MPSA (calcined) and ASA (calcined) were measured. The former contain mainly larger metal oxide particles of about 5-15 nm in diameter, Pd-Pt/MPSA showed only small particles of 1-5 nm, whereas Pd-Pt/MPS showed many small particles (1-5 nm) next to a few large clusters of large particles. HRTEM photos of a spent Pd-Pt/MPS sample showed Pd-Pt particles ranging from a few nm up to ca. 200 nm, indicating that at least part of the noble metal particles agglomerated in the HDS reaction [7] and the fast sintering of this active phase might be the cause for the quick deactivation. Obviously, the lower activity of Pd-Pt/MPSA as compared to that on ASA is not caused by a lower dispersion.

1023 Table 2 Textural properties* Sample dpmax(nm) SBET (mZ/g) Vp (cm3/g) Vmicro (cm3/g) Al wt% *: reduced catalyst.

and AI content of Pd-Pt catalysts Pd-Pt/MPS Pd-Pt/ASA 13.3 5.8 270 440 0.97 0.7 0.024 0 0 22.7

Pd-Pt/MPSA 2.4 520 0.31 0 5.63

The textural data in Table 2 shows that the reduced catalysts based on all the three carriers have mainly mesopores. The BET surface area of the reduced Pd-Pt catalysts decline in the following order of the carriers: MPSA > ASA > MPS, whereas the maximum pore diameter in pore distribution and pore volume follow the opposite sequence. Note that the dispersion of the reduced Pd-Pt catalysts has the following order of the carrier: MPSA > MPS - ASA. Fig. 2 shows the pore distribution of the three catalysts.

~, 9

30

a

25 0

E

20

8 7"6

15

4

).0

0.2

0.4

p/pO

0.6

0.8

1.0

Fig. 2. Adsorption isotherms of the reduced Pd-Pt catalysts on: a. MPSA, b. ASA and c. MPS. Here, for sample a, the peak at p/p0 value of 0.5, corresponding to 3.5 nm is caused by tensile stress effect, thus an artifact. It is clear that they are all dominated by mesopores. Table 3 gives the catalytic HDS performance of the catalysts, expressed as a first order reaction rate constant in h-lgcat-1. Those of some typical relevant HDS catalysts are also presented for comparison. For the Pd-Pt catalysts (at similar Pd/Pt ratio), it decreases in the followin.g order of the support" ASA >> MPSA > MPS. This is also the decreasing order of A1 content in the catalyst. Note that the total content (wt%) of Pd and Pt for Pd-Pt/MPSA is less than those on MPS or ASA, although the same preparation procedure was used. This may be caused by a lower amount of surface hydroxyl groups on the surface of MPSA, available for ion-exchange.

1024 Table 3 First order reaction rate constants (hqg-lcat) in deep HDS and the metal dispersions Catalyst Base Case With carbazole % Dispersion Co-Mo/AlzO3 (commercial) 0.34 0.14 0.38 0.24 Ni-Mo/AlzO3 0.72 0.24 Ni-W/AIzO3 Pd-Pt/ASA [4] 2.82 1.68 46 a Pd/Pt=2, 2 wt% Pd+Pt 3.44 51 c Pd-Pt/ASA (3.5wt% Pd+Pt) b 0.77 (3.9 d) 51 c Pd-Pt/MPS (3.5wt% Pd+Pt) b 0.92 53.6 c Pd-Pt/MPSA (Si:AI=6, 2.5 wt% Pd+Pt) b a. by Hz chemisorption, b: Atomic ratio of Pd/Pt = 4:1. c: by CO chemisorption. d: initial activity. The first order reaction rate plot of Pd-Pt catalysts on ASA and MPS are given in Fig. 3.

~3 o

,- 2

n

e--

m

0

0

~

200 Reaction time / min

400

0

0

~

200 Reaction time / min

400

Fig. 3. First order reaction rate plot of various catalysts, a. Pd-Pt/ASA and b. Pd-Pt/MPS (Table 3). For Pd-Pt catalysts on ASA and MPSA (not shown here), linear regression gives a straight line passing through the origin. For Pd-Pt catalyst on MPS, at the beginning, the rate was very high, later on it slowed down and followed a line, which did not pass through the origin. This indicates a fast deactivation at the beginning of the reaction, in accordance with a fast growth of the metal particles in the spent catalyst, as observed by HRTEM. Table 3 shows that the Pd-Pt catalysts are more active than the commercial catalysts, e.g. Co-Mo/7-AlzO3, using the model feed. Although the Pd and Pt loading in our PdPt/ASA catalyst is 75% higher than that used in [5] (3.5 versus 2.0 wt%), its rate constant is about twice as high. This may be partially attributed to its higher dispersion and/or to its different Pd:Pt ratio (4 instead of 2) [8].

1025 Although all three Pd-Pt catalysts are mesoporous and have similar initial dispersions, their performance differs enormously. The activity decreases in the decreasing order of their A1 content, viz. ASA > MPSA > MPS. The replacement of Si by AI in the silica skeleton and its subsequent charge unbalance are often used to explain the acidity in silica-alumina. It is expected that acidity will decrease in the same order, i.e. ASA > MPSA > MPS. This shows the impact of acidity on noble metal catalyzed deep HDS [5,6,8]. The texture is of secondary importance to the performance. For the Pd-Pt/MPS catalyst, a high initial activity was observed, which declined fast (Table 3). Apparently, the active sites on MPS are less stable as those on MPSA or ASA, they sinter fast according to our HRTEM observation. Although this catalyst has a higher pore volume, larger pore diameters and a higher Pd or Pt content than those of Pd-Pt/MPSA, its activity is lower. Calculations show that the adsorption of the substrate on the active sites cannot explain this initial high activity. This catalyst does not contain any AI. This implies that strong acidic sites may not necessarily be essential for the active sites for deep HDS. It follows that the presence of AI (thus strong surface acidity of the carrier) contributes to the stability of the noble metal active sites in deep HDS. Besides the stabilization of small Pd-Pt particles, it may also affect the metal-sulfur bond strength and the nature of the active sites, via creating electron-deficiency, leading to changes in the reaction kinetics [5], but in view of the high initial activity of the MPS sample this seems less probable. ACKNOWLEDGEMENTS We wish to thank Dr. P.J. Kooyman of the National Center for HREM, Delft University of Technology for HRTEM measurements and helpful discussions, Ing. J. Groen of DCT/TNW for nitrogen physisorption and CO chemisorption measurements, and ABB Lummus for financial support. REFERENCES 1. European Standard 590. 2. P. Waller, Reaktivit/it organischer Schwefelverbindungen beim Hydrotreating von Gas61en unterschiedlicher Herkunft, Ph.D. Thesis, 1997, University Karlsruhe (TH), Germany. 3. H. Schulz, W. Bohringer, P. Waller and F. Ousmanov, Catal. Today, 49 (1999) 87. 4. H.R. Reinhoudt, R. Troost, A.D. van Langeveld, S.T. Sie, J.A.R. van Veen and J.A. Moulijn, Fuel Proc. Tech., 61 (1999) 133. 5. H.R. Reinhoudt, The development of novel catalysts for deep hydrodesulfurisation of diesel fuel, Ph.D. Thesis, 1999, Delft University of Technology, The Netherlands. 6. H.R. Reinhoudt, R. Troost, S. van Schalkwijk, A.D. van Langeveld, S.T. Sie, J.A.R. van Veen and J.A. Moulijn, Fuel Proc. Tech., 61 (1999) 117. 7. R.M. Navarro, B. Pawelec, J.M. Trejo, R. Mariscal and J.L.G. Fierro, J. Catal., 189 (2000) 184. 8. H. Yasuda and Y. Yoshimura, Catal. Lett., 46 (1997) 43. 9. Z. Shan, Th. Maschmeyer and J.C. Jansen, WO 00/15551 (2000).

1026 10. Z. Shan, Th. Maschmeyer and J.C. Jansen, US Patent Appl. no. 09/390276 (2000). 11. J.C. Jansen, Z. Shan, W. Zhou, L. Marchese, N. van de Puil and Th. Maschmeyer, Chem. Commun., (2001) 713. 12. P.C. Aben, J. Catal., 10 (1968) 224. 13. H.A. Benesi, R.M. Curtis and H.P. Studer, J. Catal., 10 (1968) 328. 14. S. Tajik, P.J. van der Berg and J.A. Moulijn, Meas. Sci. Technol., 1 (1990) 815. 15. R.L. Moss, Catalysis (Specialist Periodic Report), The Chemical Society, London, 4 (1981) 31.

Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1027

Preparation of highly ordered CMI-1 and wormhole-like DWM mesoporous silica catalyst supports using C16(EO)10 as surfactant Alexandre L6onard #, Jean-Luc Blin and Bao-Lian Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium phone" +32-81-72-45-31, Fax" +32-81-72-54-14, e-mail" [email protected] Concentrations of polyoxyethylene alkyl ether surfactants located in the hexagonal H1 micellar domain typically afford disordered mesoporous structures of wormhole-like channels after polymerization of the inorganic precursor in solution. The effect of hydrothermal treatment duration and temperature on the characteristics of the obtained disordered DWM (Disordered Wormhole-like Mesostructure) compounds has been investigated. Thermal stability of these samples has also been studied and appears to be superior to that of MCM-41 type materials. If the weight percentage of surfactant in solution is however decreased, well-ordered hexagonal CMI-1 can be directly obtained. In this case, a co-operative mechanism leading to spherical particles of organized channel structures has been proposed. Influence of the surfactant concentration on the arrangement has been discussed in detail. 1. INTRODUCTION Since their appearance in 1992, ordered mesoporous silicas have been widely used, due to their high surface area and uniform pores sizes, as catalyst supports, to graft a series of catalysts or to disperse metallic or semi-conductor clusters [1-5]. However, the pore size control as well as the reduction of the synthesis costs remain still great challenges. The latter can be reduced significantly if easy-to-use, non-toxic and biodegradable polyoxyethylene alkyl ethers are employed as templating agents. Besides, the micelles have bigger sizes than the "conventional" ionic templates and the recovery as well as the re-use of the surfactant are made possible because of the H-bonding type interactions that exist between surfactant and inorganic species [6-8]. The materials obtained however possess a disordered structure of wormhole-like channels (MSU or DWM-type compounds) for concentrated micellar solutions unless working in strong acidic media like reported by Stucky et al. [9] or by removing the methanol released from tetramethoxysilane hydrolysis from the synthesis medium [10]. The present paper reports a systematic study of the influence of hydrothermal treatment on the final features of such disordered materials obtained with concentrated micellar solutions. The thermal stability of these compounds has also been evaluated. Moreover, Mobil researchers showed that highly ordered mesoporous could be obtained with CTMABr concentrations well below CMC-2 (Critical Micellar Concentration), composition at which micelles adopt a hexagonal ordering in solution. In that way, we studied the effect of decreasing the non-ionic C16(EO)10 surfactant weight percentage. This will lead directly to well-ordered CMI# :FRIA fellow

1028 lmaterials with a hexagonal empilement of their channels. These structures can be advantageous if they are to be used as encapsulating hosts for nanowires or as nanoreactors for the production of low-branched polyethylene fibres, for instance [4-5]. On the other hand, the 3-dimensional access to the reaction centres of the disordered MSU or DWM frameworks seems to be more suitable for catalytic purposes, so the study on the influence of the treatment on the final texture of such compounds is of considerable interest if the pore diameters are to be well-controlled for size seleetivities for example. Different characterization techniques (SEM, TEM, XRD and nitrogen adsorption-desorption analysis) have been used to shed light on the morphological, structural, and textural features of the prepared DWM and CMI-1 compounds. 2. EXPERIMENTAL 2.1. Synthesis The first series of micellar solutions leading to disordered DWM compounds were prepared with 50 wt.% C16(EO)10 (Brij 56~) by dissolving the surfactant at 70~ in an aqueous solution. The pH of the micellar solution was then adjusted to 2.0 with H2SO4. After homogenization at 70~ during 3 hours, the silica source, tetramethoxysilane (TMOS), was added dropwise, the surfactant / silica molar ratio being equal to 1.5. After further stirring for 1 hour, the synthesis gel was poured into teflon cartridges sealed in stainless steel autoclaves. Hydrothermal treatment was then performed during 0 to 15 days at 60, 80 and 100~ Surfactant was removed from the inorganic framework by ethanol extraction with a soxhlet apparatus and calcination at 550~ under nitrogen and oxygen. The same procedure was applied for the preparation of ordered CMI-1 materials. In this case, less C16(EO)~0 was dissolved in the same aqueous volume, still for 3 hours. The surfactant / silica molar ratio was still equal to 1.5 and hydrothermal treatment was performed for 1 day at 80~ 2.2. Characterization To investigate the structure of the samples, XRD at low angles was performed on a Siemens D-5000 diffractometer and transmission electron microscopy on a Philips Techna~" 100 kV microscope. The powdery samples were embedded in an epoxy resin before being sectioned with an ultramicrotome and deposited on carbon coated copper grids. The textural properties of our compounds were assessed by nitrogen adsorption-desorption measurements. Analysis took place over a wide range of relative pressures on a Micromeritics ASAP 2010 or Tristar 3000. The pore diameters and the pore size distributions were determined by the BJH method. Morphological features have been investigated with the use of a Philips XL-20 scanning electron microscope. For conductivity purposes and in order to enhance the yield in secondary electrons, powders were first covered by a thin layer of gold.

3. RESULTS AND DISCUSSION 3.1. Influence of hydrothermal treatment on the features of disordered DWM materials Fig. 1 reports the XRD patterns of materials obtained at 60~ for different treatment durations. At this temperature, only one peak located at 13.0, 11.6 and 16.4 nm for heating times of 2 (Fig.lb), 3 (Fig.l c) and 11 days (Fig.ld) respectively, is detected. The presence of only one reflection line indicates that the compounds belong to the MSU or DWM family. The structure of these molecular sieves consists of regularly sized wormhole-like channels. The single broad peak arises from the average pore-to-pore separation in the disordered

1029 wormhole framework, which presents a lack of long-range crystallographic order. However, according to the phase diagram, a hexagonal H1 phase of micelles is present in aqueous solution at such a surfactant concentration (50 wt.%) before adding the silica source [11] and thus, mesoporous possessing this structure could be expected. However, the results obtained by ~-~ = ~~-- 11.6mn XRD and TEM are contrary to our expectations. We suggest that the interaction between the hydrophilic head of decaoxyethylene cetyl ether and TMOS as well as the methanol released during hydrolysis of the latter could disturb the hexagonal micellar array preformed in solution, thus leading to a disordered framework. The TEM picture of one of our typical s m i l e s reported in Fig. 2 confirms the wormhole-like structure of our materials and its adherence to MSU (or DWM) family. If the heating temperature is increased to 80 or 100~ (diffractograms are not reported), no peak is detected any more after 2 days of hydrothermal treatment. This results from the fact that, as will be discussed further, the pore size distributions are very likely to broaden and to become more or less inhomogeneous for the 2 4 8 8 io 12 higher temperatures. The regular size of the channels could then 2 (o) be partly lost. It should be noted that for the sample which has Fig. 1 : XRD patterns of not been subjected to a hydrothermal treatment, one reflection compounds synthesized at located at 7.5 nm (Fig. l a) is present on the XRD pattern. So, 600C for a :0, b :2, c : 3, when tetramethoxysilane is added to the prepared micellar d: 11 days. solution and pH is adjusted, the condensation and polymerization of the silica directly take place around the micelles of surfactant. This is suggestive of the formation of mesoporous materials even before the hydrothermal treatment. The latter will then affect the textural properties and in particular the pore diameter of the final compounds. The textural characteristics of the samples as a function of heating temperature and duration are listed in Table 1. From these data, it appears that the increase of heating temperature favours the formation of large pore mesoporous molecular sieves. Too long heating at higher temperature, 100~ for Fig. 2 9TEM micrograph example, will however lead to the destruction of the of a typical DWM sample. compounds. Their X-ray diffraction pattern will not show any peak any more and the pore size distributions broaden a lot. Moreover, specific surface area decreases indicating a partial collapse (surface remains at about 400 m2/g) of the mesoporous framework. For the shorter durations of treatment, homogeneous mesoporous materials, showing pore sizes from 4.0 nm up to 13.4 nm with very high specific surface areas of around 950 m2/g have been synthesized.

1030 It is observed that the pore diameter strongly depends on the heating time and temperature (Table 1). This is in agreement with Heating Pore size (nm) aider heating at" results reported previously with time (days) C13(EO)6 [12] and Cls(EO)10 [13, 60~ 80~ 100~ 14] surfactants and observed by Pinnavaia et al. [15]. Two main 0 1.7 tendencies can be drawn from Table 0.5 1.7 5.2 7.5 1. First, with increasing 1 1.7 4.7 8.0 temperature, the pore diameter 2 9.4 4.0 13.4 grows as mentioned above. 3 6.7 Secondly, for a given temperature, 4 5.8 12.9 15.8 in particular for 60 and 80 ~ the 5 5.2 pore diameter first sharply increases 6 4.5 9.9 17.1 and then decreases when 8 4.8 12.1 14.6 hydrothermal treatment is 11 7.9 12.1 prolonged. Concerning the first - : no data tendency, it appears that decaoxyethylene cetyl ether can be solubilized in water in different ways. Under acidic conditions, large amounts of hydrogenbonded water molecules are present around the hydrophilic heads of the surfactant and the equilibrium between the contracted and extended conformation of C16(EO)10 is strongly shitted towards the contracted form. A low treatment temperature would not alter this conformation and interaction of the ethoxy oxygens with the silanol groups of the silica would not be favoured. The effective cross-sectional area of the hydrophilic headgroups of the surfactant is important and, according to Kunieda et at [16], micelles with a high curvature and a small diameter are therefore formed. If the heating temperature is raised, due to the thermal motion, the water molecules tied around ethoxy oxygens through hydrogen bonds progressively disappear and the oxyethylene head can extend. The curvature of the micelles will then decrease and this conformation allows for more interactions with silica. This results in an increase in the pore diameter. Finally, if the heating temperature is further raised, the surfactant - silica interface becomes less important and the size of the mesopores increases with the stretching of the surfactant molecules. This growth in length of surfactant chain or size of micelles can induce a breakdown of the wall separating adjacent pores. Larger-sized channels are therefore formed, but porosity and relatively high specific surface area can be maintained. This phenomenon of wall breakdown was previously reported by Sayari et al. [17] to explain the process of pore size enlargement of pure siliceous mesoporous materials during a post-synthesis treatment, using an amine as swelling agent. Pairs or triplets of adjacent pores can transform into single pores in a similar way. However, when thermal treatment is prolonged over a wider period of time, for a given temperature, the pore diameter decreases (Table 1). Indeed, it is observed that between 2 and 6 days of hydrothermal treatment at a heating temperature of 60~ the pore diameter decreases from 9.4 to 4.5 nm instead of increasing as expected. The possibility of reorganization of the micelles should be considered in this case. Some molecules leave the formed micelles and, in order to maximize the hydrogen bonding interactions between the molecules of surfactant and water molecules or silica source, a rearrangement of the micellar Table 1. Variation of the pore diameter determined by the BJH method with synthesis heating temperature and time

1031 system in the gel occurs, which leads to the formation of more micelles, but with a smaller size. For durations inferior to 6 days, the materials result from a compromise between the reorganization of the formed micellar phase to get more stable micelles and the stretching of the surfactant molecules with increasing hydrothermal duration. 3.2. Thermal stability of the disordered mesoporous materials obtained

This study was performed using three samples (a, b and c) synthesized under different hydrothermal treatment conditions (1 day at 100~ 1 day at 80~ and 6 days at 60~ respectively). The pore diameters of these compounds are respectively 8.0, 4.7 and 4.5 nm. The materials were calcined at 550, 600, 700, 800, 900 and 1000~ and the variation of specific surface area was monitored as a function of this calcination temperature. A large pore MCM-41 (sample d) with 7.5 nm pore size is also reported in Fig. 3 for comparison. From Fig. 3, it appears that the specific 1000 c surface area of the disordered DWM materials t:l b ... ~.. remains very high till 800~ Beyond this ~ 800d "~"~-,,.{ temperature, for sample B for instance, its value decreases from 800 to 20 m'/g if calcination temperature is raised from 800 to 1000~ 400 Whatever the hydrothermal treatment conditions of 200 the synthesis are, the thermal resistance of the .=. materials is similar. Even at 1000~ the recovered *~~' 0500 660 760 860 "960 1000"1100 materials exhibit a type IV isotherm, characteristic Calcination temperature (~ of mesoporous compounds. A part of mesoporosity Fig. 3. Specific surface area vs. is thus maintained with quite a narrow pore size calcination temperature for disordered distribution, but the maximum adsorbed volume is DWM structures (a, b and c) and for a MCM-41 type compound (d). sharply reduced. This is in accordance with the very broken appearance of the particles observed by SEM. Thermal stability of these disordered materials is, however, very superior to MCM-41, whose structure does not resist beyond 600~ This behaviour can be related to the different preparation method that affords compounds with a different structure and also thicker walls. 3.3. Obtention of ordered hexagonal CMI-1 materials

Fig. 4 shows the XRD patterns of compounds prepared at surfactant weight percentages of 40 (A) and 10 (B), respectively. For samples synthesized at a low concentration of surfactant, in addition to the sharp 100 reflection line, two weaker peaks, suggestive of a hexagonal organization of the channels can clearly be pointed out. \__ A However, these secondary reflections vanish with increasing 2 4 6 8 10 12 surfactant concentration and finally completely disappear at 20 (o) weight percentages above 30. Beyond this composition, only Fig. 4. XRD patterns of A: one peak, characteristic of the regular repetition of the pore disordered compound and diameter, can be observed, suggesting that the compounds B: highly ordered CMI-1 evolve towards a disordered array of very uniformly sized sample. channels like MSU materials. TEM pictures show a disordered wormhole-like framework at surfactant weight percentages above 30 (Fig. 5A) whereas a very highly ordered hexagonal structure can be evidenced for lower concentrations (Fig. 5B and C). This is also confirmed by the inserted FFT of the micrograph which clearly

1032

Fig. 5 : TEM pictures of: A : a disordered wormhole-like structure, B : a regular hexagonal framework and C : a ~t f'mgerprint >>-likearrangement of channels. exhibits a sixfold symmetry. Interestingly, in addition to this honeycomb-like arrangement, a fingerprint-like channel array (Fig. 5 C), which could be related to the morphology, (see belo w) can be observed. As explained above, the hexagonal micellar mould is only present for the micellar solutions that contain between 30 and 65 wt.% of surfactant. The addition of TMOS probably disturbs this array, leading to a disordered framework. However, in the case of less concentrated micellar solutions, where only individual micellar rods exist, it appears that the addition of the silica source plays the key role in the obtention of very regular hexagonally arranged channels. Such a co-operative mechanism could be analogous to the LCT pathway initially proposed by Mobil researchers [2]. From nitrogen adsorption-desorption measurements (Fig. 6), we can observe that all the samples exhibit a type IV isotherm characteristic of mesoporous compounds. However, there is an evolution of hysteresis loop from type H2 (IUPAC), usually encountered for disordered structures, towards H1 commonly observed for MCM-41 when the weight percentage of template decreases below 30 wt.%. For the lower concentrations, the adsorption branch of the isotherm becomes much steeper and the pore size distributions (PSD) become much narrower (see inserts) and centred on about 4.4 nm. Specific surface area also increases 1600~_

,

j

~2oo~=J\

//

....

~ool

/ /

400

2o

C

0|.

0.0 ;~

0.2

0.4

0.6

0.8

Relative pressure P/Po

1.0

0.0 0.2 O.4 0.6 0.8 Relative pressure P/Po

1o0

0.0

,

,

,

0.2

0.4

0.6

"

,

0.8

'

1.0

Relative pressure P/Po

Fig. 6 : Nitrogen adsorption-desorption isotherms of: A: disordered wormhole-like sample, B: highly organized CMI-1 compound and C: sample prepared at 30 wt.% showing secondary mesoporosity. to values higher than 1000 m2/g. A more peculiar behaviour is noted at 30 wt.% as the sample shows secondary mesoporosity (Fig. 6C). This could arise from the competition between the ordered CMI-1 and wormhole-like DWM structures that appear at this composition.

1033

Fig. 7 9SEM pictures representing" A" sample prepared at a concentration above 30 wt.%, B 9 the spongy particles obtained at 30 wt.% and C: spheres corresponding to the lower concentrations of template. As can be seen from Fig. 7A, the compounds prepared at high surfactant loadings (>30%) exhibit particles of different sizes and shapes. At 30 wt.%, large spongy particles, which are very likely to emerge from a competition between the spheres and the irregular edge-shaped particles can be found. However, as the concentration of template decreases beyond this limit, the morphology remains unchanged and can be described by an assembly of very small spheres (1-2~m) and the section of such a sphere could explain the ~ fingerprints >> observed by TEM (Fig.7C). At this low surfactant concentration, it is also possible to obtain some "exotic" gyroidal, toroidal and rope-like morphologies like already encountered by Ozin et al. [18-20]. Thus, not only the channel structure but also the morphology of MCM-41 can be successfully reproduced with a non-ionic surfactant. All these results led us to postulate two different synthesis mechanisms [21 ]. First, for the higher weight percentages where, according to the phase diagram, a hexagonal liquid crystal phase is present in the micellar solution, it appears that the polymerization of the silica source as well as the methanol released during its hydrolysis disturb the hexagonal array. The interface interactions between the hydrophilic heads of the template and the hydroxyl groups of the silica and the polymerization of the inorganics will provoke a slight gliding of the channels in order to minimize steric and electrostatic energies. In this case, the prepared compounds possess a disordered wormhole-like structure. Irregularly-shaped particles with a high specific surface area are then obtained. On the contrary, when the surfactant concentration is lowered, only isolated micelles exist in solution. The silica can easily polymerize around the single micelle-rods present in solution and a complete condensation would imply that these rod-like supramolecular assemblies join in order to form the regular hexagonal framework. We propose that in this case, a co-operative mechanism analogous to Mobil's LCT pathway affords the regular empilement. Thus, the addition of the silica source favours the formation of highly ordered CMI-1 mesoporous molecular sieves leading to spheres with very high surface areas and very narrow PSD's. This co-operative mechanism is also compatible with the obtention of ropes, toroidal, gyroidal and spherical shapes as the Interaction at the Single rod-micelle PEO-silica interface .

Eo ~-,/~, Eo 9

~~o~::~

~"

"~

o,,

:-~0,,

. . . . Surfactant.TMOS,o , interaction ~.~ #%"0~~

Silica-covered rod-micelles agglomerate owing to a cooperative pathway ,:oE.o~o

~"

TMOS . ~, ~o~ .'f~ Hydrothermal polymerization ~"~[~,~o~' treatment,extraction, Eo~ " to~ . t ~_,~u~o o calcination

Fig. 8 : Cooperative pathway leading to the ordered CMI-1 materials.

1034 change in morphology occurs when the amount of added silica is varied at a constant low weight percentage of template. 4. CONCLUSION This work has shown that hydrothermal treatment strongly affects the texture of disordered wormhole-like mesoporous catalyst supports, and in particular the ability of the hydrophilic oxyethylene head of the non-ionic surfactant to adopt different conformations with temperature. These compounds show a remarkable thermal resistance up to 800~ suitable for a series of catalytic reactions. On the other hand, ordered hexagonal CMI-1 can be directly obtained by lowering the surfactant concentration below the domain of existence of hexagonally ordered micelles in solution. ACKNOWLEDGEMENTS

Alexandre L6onard thanks FNRS (Fonds National de la Recherche Scientifique, Belgium) for a FRIA scholarship. We thank Prof. P.A. Jacobs and Prof. P. Grange for giving access to their XRD diffractometer. REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. and Schlender, J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Sayari, Chem. Mater., 8 (1996) 1840. 4. K. Kageyama, J.I. Tamazawa and T. Aida, Science, 285 (1999) 2113. 5. K. Moiler and T. Bein, Chem. Mater., 10 (1998) 2950. 6. G.S. Attard, J.C. Glyde and C.G. G61tner, Nature, 378 (1995) 366. 7. S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science, 269 (1995) 1242. 8. S.A. Bagshaw and T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl, 10 (1996) 1102. 9. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. 10. N.R.B. Coleman and G.S. Attard, Microporous Mesoporous Mater., 44-45 (2001) 73. 11. D.J. Mitchell, G.J.T. Tiddy, L. Waring, T. Bostock and M.P. McDonald, J. Chem. Soc. Faraday Trans., I, 79 (1983) 975. 12. J.L. Blin, A. Becue, B. Pauwels, G. Van Tendeloo and B.L. Su, Microporous Mesoporous Mater., 44-45 (2001), 41. 13. J.L. Blin, G. Herrier, C. Otjacques and B.L. Su, Stud. Surf. Sci. Catal., 129 (2000) 57. 14. G. Herrier, J.L. Blin and B.L. Su, Langrnuir, 17, 14 (2001) 4422. 15. E. Prouzet and T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl., 36 (1997) 516. 16. H. Kunieda, K. Ozawa and K.L. Huang, J. Phys. Chem., 102 (1998) 831. 17. A. Sayari, M. Kruk, M. Jaroniec and I.L. Moudrakovski, Adv. Mater., 10 (1998) 1376. 18. H. Yang, N. Coombs and G.A. Ozin, Nature, 386 (1997) 692. 19. H. Yang, G. Vovk, N. Coombs, I. Sokolov and G.A. Ozin, J. Mater. Chem., 8 (1998) 743. 20. G.A. Ozin, H. Yang, I. Sokolov and N. Coombs, Adv. Mater., 9 (1997) 662. 21. J.L. Blin, A. L6onard and B.L. Su, Chem. Mater., 13 (2001) 3542.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1035

Synthesis and characterization of nanostructured mesoporous zirconia catalyst supports using non-ionic surfactants as templating agents J.L. Blin, L. Gigot, A. L6onard # and B.L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium The formation of nanostructured porous zirconia catalyst supports using non-ionic surfactants as templating agents has been studied in order to optimize the synthesis conditions without addition of structure stabilizing agents such as sulfate or phosphate anions. The effect of the quantity of added zirconium source with respect to the surfactant concentration in solution has been studied. We also examined the role played by the surfactant during the synthesis. Further investigations have shown that the pore diameters could be increased towards the mesoporous domain if heating time and temperature are raised. The present work reveals that the control of the balance between the precipitation rate of zirconia and the interaction of zirconium source will be fatal in the formation of nanostructured porous zirconia. 1. I N T R O D U C T I O N Among the non-silica mesoporous oxides, zirconia is of particular interest due to its large field of applications ranging from catalysis to ceramics [1]. However, the surface area of crystalline zirconia is usually rather low, in the order of 20-50 mVg. Thus, to develop particular catalytic properties such as high conversion and selectivity, the synthesis of ordered mesoporous ZrOz with high specific surface area and narrow pore size distribution is of capital importance both from the scientific and industrial application point of view. In previous work [2,3], we have shown that it was possible to obtain mesoporous zirconia using the CTMABr-ZrOClz.8H20 system. However, the packing of the channels was not well-ordered as expected even by varying the weight percentage of template or the surfactant/zirconium molar ratio. On the basis of TEM, SEM XRD and N2 adsorptiondesorption results, a synthesis mechanism has been proposed. It is observed that at low temperature or for short durations at higher temperatures, the obtained materials are first supermicroporous, then a breakdown of the walls separating adjacent pores allows the transformation to mesopores with increasing hydrothermal treatment time. The obtained materials have a uniform pore size and their surface can reach 300 mZ/g. But the channel array is, at least part of samples, wormlike. However, if the hydrothermal treatment is performed at too high temperature or for too long durations, mesoporous compounds are no longer obtained. Indeed, the final phases observed correspond to tetragonal and monoclinic crystalline zirconium oxides with very low specific surface area. However, the major drawback that has been encountered in the synthesis of these materials is the removal of the surfactant. Without addition of phosphate or sulfate anions [4, 5], which # : FRIA Fellow * : Corresponding author

1036 delay the crystallization of the amorphous ZrO2 into the tetragonal or monoclinic form, it is very difficult to remove the template in order to free the pores. This step of the synthesis is facilitated when the template belongs to the family of polyoxyethylene alkyl ether [Cm(EO)n]. In this case, the removal of the surfactant can easily be achieved by solvent extraction, using ethanol for instance, because of the weaker interactions between the entities (H-bonds). The synthesis of mesostructured oxides, that is difficult or impossible by electrostatic assembly can be performed through this N~ ~ process. Moreover, these templating agents are cheaper, more biodegradable and less toxic than their ionic analogues. Thus, based on the experience acquired in the synthesis of mesoporous silicas using non-ionic [Cm(EO)n] templates [6-9], we have established a synthesis protocol of porous zirconia through a neutral C13(EO)6-Zr(OC3H7)4 assembly pathway. The effect of different physico-chemical parameters such as heating time and temperature on the synthesis of pure nanostructured porous zirconia has been investigated, in order to shed some light on the synthesis mechanism and to determine the important physicochemical variables on the preparation. 2. E X P E R I M E N T A L

2.1. Synthesis A 50 wt.% micellar solution of C13(EO)6 was prepared by dissolving the surfactant at room temperature in an aqueous solution during 3 hours. The obtained medium was further stirred for three hours at room temperature before adding drop by drop the inorganic source : zirconium propoxide [Zr(OC3H7)4]. The surfactant / zirconia molar ratio was varied from 0.5 to 10. The obtained gel was sealed in Teflon autoclaves. Hydrothermal treatment was performed during 2 days at 60~ in a first time. The surfactant was removed by ethanol extraction.

2.2. Characterization The XRD patterns were obtained with a Philips PW 170 diffractometer, using CuKct (1.54178 ,A,) radiation, equipped with a thermostatisation unit (TTK-ANTONPAAR, HUBER HS-60). The transmission electron micrographs were taken using a 100 kV Philips Techna'i microscope. For TEM observations, sample powders were embedded in an epoxy resin and then sectioned with an ultramicrotome. The thin films were supported on copper grids previously coated by carbon to improve stability and reduce the accumulation of charges. The morphology of the final phases was studied using a Philips XL-20 Scanning Electron Microscope (SEM) with conventional sample preparation and imaging techniques. Nitrogen adsorption - desorption isotherms were obtained at -196~ over a wide relative pressure range from 0.01 to 0.995 with a volumetric adsorption analyzer ASAP 2010 or TRISTAR 3000 both from Micromeritics. The samples were further degassed under vacuum for several hours before nitrogen adsorption measurements. The pore diameters and their distribution were assessed by the BJH (Barret, Joyner, Halenda) method [10].

1037 3. R E S U L T S AND D I S C U S S I O N 3.1. Role of the surfactant: Effect of the variation of the C 1 3 ( E O ) 6 [ Zr ( O C 3 H 7 ) 4 molar ratio The XRD patterns of some samples C C prepared without template (A) and with different C 1 3 ( E O ) 6 / Z r ( O C 3 H 7 ) 4 molar ratio (B) are depicted respectively in Fig. 1A and B. The concentration of Zr(OC3H7)4 in the absence of template corresponds to the same fictive surfactant / "7. b zirconium molar ratio. Except a broad band, located in the range of 250<20<40 ~, analogous to the one observed for oH amorphous silica, no peak is detected on the XRD patterns. This indicates that these ZrO2 materials are amorphous. From the TEM micrographs, represented in Fig. 2, it is obvious that the compounds have a disordered structure with a large number of 0 10 20 30 40 50 60 10 20 30 40 50 60 wormhole-like channels lacking a long range packing order as reported for DWM 20 e) Fig. 1 9XRD patterns of samples prepared A 9 (Disordered Wormlike Mesostructure) without template and zirconium concentration of compounds [6]. As no small angle a 90.13, b 90.013 and c: 0.006 mol/l; B 9with diffraction peak is detected, the compounds 50 wt.% C13(EO)6 a n d a C13(EO)6 / Z r ( O f 3 H 7 ) 4 belong to the family of DWM-2. molar ratio of a 90.5, b 95 and c 910. Comparing to DWM-1 or MSU, this kind of materials exhibits inhomogeneity in their pore sizes, which involves the lack of reflection in the small angle region. Working with a weight percentage located in the H1 domain [11], we could expect that the recovered materials should have a hexagonal array of their channels, similar to the "micellar mould", like MCM-41 or CMI-1 type materials. However, the XRD and TEM results show the contrary and that the compounds belong to DWM-2 family. This led us to conclude that the addition of the zirconium source disturbs the micellar array formed in solution. It is well known that zirconium alkoxydes have a high reactivity. Indeed, in a paper dealing with the precipitation rate of zirconium propoxide, Grange et al. [12] have shown that if the ratio (h) between the concentration of water and zirconium is higher than 4, the condensation of zirconium propoxide is very fast. In the present study, the value of h is located in the range 12-250, so we can conclude that condensation of Zr(OC3H7)4 occurs as soon as it is incorporated into the micellar solution, involving a complete destruction of the hexagonal array of micelles. The resulting synthesis gel will thus consist of dispersed isolated molecules, spherical and cylindrical micelles surrounded by the inorganic source. Comparing the X-ray diffractograms represented Fig. 1A and B, it is obvious that the addition of template does not conduct to

A

B

1038 the formation of well-structured materials with hexagonal micellar shapes like CMI-1 and MCM-41. Fig. 3 shows scanning electron micrographs, representing the morphologies of materials obtained with (a) and without (b) surfactant. For all of the compounds, irregular edge-shaped large particles with variable sizes and forms are obtained. At the microscopic scale, i.e. micrometer range, the surface of most of samples appears to be very porous. In some case, macropores are detected. However, further investigations have to be made in order to better understand the origin of these macropores.

Fig. 3 : SEM micrographs of compounds prepared with C13(EO)6 / Zr (OC3H7)4molar ratio 1.5 (a) and without surfactant (b).

Fig. 2 9TEM micrograph of a typical sample.

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Relative pressure p / p0 Fig. 4 : Nitrogen adsorption isotherms of sample prepared A : without template and with a zirconium concentration of a : 0.13 , b : 0.013 and c: 0.006 mol/l; B : with 50 wt.% C13(EO)6andC13(EO)6 / Zr (OCxHT)a molar ratio of a : 0.5. b : 5 and c : 10.

The nitrogen adsorption isotherms of compounds synthesized with (Fig. 4B) and without (Fig. 4A) surfactant are not well defined in the way that they exhibit a linear region from p/p0 = 0.1 to 0.4 before reaching a plateau until a relative pressure p/P0 equal to 0.85. This kind of isotherm is situated between the type I, related to microporous materials, and the type IV, characteristic of mesoporous materials. The analysis of the pore size distribution shows that these materials possess supermicropores, i.e. pore sizes centered at about 1.7 nm. According to Dubinin [13], these materials belong to supermicroporous family. Super microporous materials form an

1039 interesting category of materials, between microporous materials such as zeolites, and mesoporous, for example M41S materials, which can open a wide field of applications such as the treatment of molecules requiring only the compounds containing supermicroporosity. For relative pressures superior to 0.85, the adsorbed volume of nitrogen increases significantly instead of remaining constant due to saturation. This means that the compound contains an appreciable amount of secondary mesoporosity or macroporosity as observed on the SEM micrographs. The condensation of nitrogen in these large mesopores or macropores very likely occurs in addition to the adsorption inside the channels. This complementary textural mesoporosity and macroporosity makes the transport of reagents to framework reaction centers more efficient and thus favors the catalytic activity. In a paper dealing with applications of mesoporous molecular sieves with wormhole framework structures, Pinnavaia et al. [14] show that the catalytic activity of HMS compounds for a sterically demanding condensed phase reaction, such as the alkylation of 2, 4-di-tert-butylphenol with cinnamyl alcohol, is strongly enhanced by the textural mesoporosity. From Fig. 5, we can see that samples prepared without template have a lower specific surface area than those obtained with a surfactant / zirconium molar ratio lower than 5. A beneficial effect of the template on the specific surface area is thus observed. All these results led us to propose the mechanism represented in Fig. 6. When. zirconium propoxide is added to the micellar solution, its rapid polymerization will disturb the micellar array. Furthermore, the large amount of propanol produced during the hydrolysis of Zr(OPr)4 will provoke a change of organisation of the surfactant molecules in solution. This variation in organisation of the surfactant molecules associated with the high

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Fig. 5 9Variation of the specific surface area with A 9the zirconium concentration and without template, B: the C13(EO)6 / Zr molar ratio, [C13(EO)6] - 50 wt.%. reactivity of Zr(OPr)4 will lead to the destruction of the H1 phase. Isolated, spherical and cylindrical forms of micelles can coexist in solution. Hydrogen-bonded interactions will take place between the zirconium propoxide and the ill-aggregated entities of the template. But, as the reactivity of zirconium is predominating over these interactions, no cooperative mechanism, which can lead to the formation of highly ordered mesoporous [6], can occur and only supermicroporous molecular sieves are formed after surfactant removal.

1040

3.2 Effect of the hydrothermal treatment on the pore diameter From the nitrogen adsorption-desorption isotherm we can observe a hysteresis loop similar to H2 type when the heating duration is increased (Fig. 7A). It may arise from the same types of open capillaries as are responsible for H1 type hysteresis, characteristic of MCM-41 kind of materials. This indicates that the effective radii of the bodies of the pore of our samples are more or less inhomogenously distributed but the effective radii of narrow entrances are all of equal size. H2 type hysteresis loop is typical for wormhole structured MSU-type materials. Simultaneously, the maximum of the pore size distribution shifts toward higher values and it is located in the mesoporous range after 4 days at 60~ Thus increasing the hydrothermal duration involves the formation of mesoporous zirconia instead of supermicroporous ones. This phenomenon is also favored by increasing temperatures, for example the pore diameter varies from 1.7 to 2.3 nm if the hydrothermal treatment is performed during 2 days at 80 instead of 40~ In a previous paper, we have reported that at low heating temperatures, the hydrophilic part of the surfactant could adopt a more contracted conformation because of large number of the hydrogen-bonded water molecules present around the hydrophilic oxyethylene heads. According to Kunieda et al. [15], micelles with a high curvature and a small diameter are therefore formed. If the

Fig. 6 9Proposed mechanism for the synthesis of porous zirconia.

1041

heating temperature or time is raised, due to the thermal motion, the water molecules bonded around the ethoxy oxygens through hydrogen bonds disappear progressively and a more extended conformation is expected. Micelles with a smaller curvature and therefore with larger diameter will be trapped by the zirconia. This results in an increase in the pore diameter of the porous zirconia recovered after solvent extraction. 0.020]

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Fig. 7 9 Nitrogen adsorption isotherms (A) and pore size distribution (B) of samples prepared with surfactant and obtained at 60~ after a 91, b 96, c 98 and d 911 days. 3.3. Thermogravimetric analysis. From the curve of DSC (Fig. 8), 3 important weight loses in the range 25-240, 240380 and 380-450~ can be clearly observed in the DSC curves. They can be assigned as follows: Weight loss in the range 25-240~ : This weight loss is accompanied by an endothermic effect. It can be attributed to the desorption of water molecules physisorbed at the surface of zirconia.

1042

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Fig. 8 : Thermograms of samples prepared with (a) and without (b) surfactant and washed with ethanol. Weight loss in the range 240-420~ : The loss of weight is accompanied by exothermic effects (peaks 2, 3 and 4 in DSC curves). They can be attributed either to the oxidation of the template or to the oxidation of ethanol. Indeed it was reported in literature that the surfactant could be calcined from 250 to 500~ [16]. However, the first possibility can be rejected, as these peaks are also detected for the sample prepared without template (Fig. 8b). Moreover, it was r observed by infrared spectroscopy that all the surfactant was removed after 30 hours i of ethanol extraction. In addition, if the sample prepared without C13(EO)6 is not washed with ethanol, these exothermic peaks are no longer detected (thermogram 10 20 30 40 50 60 not represented). 20( ~) % The peak 5 at around 470~ observed in Fig. 9 : X-ray diffractogram of zirconia DSC curve, corresponds to an exothermic after the thermogravimetric analysis * : phenomenon without any loss of weight. It monoclinic, i tetragonal zirconia. can thus be attributed to the crystallization of the amorphous zirconia. This is confirmed by the X-ray diffraction pattern obtained after the thermogravimetric analysis (Fig. 9). According to del Monte et al. [17] the peaks located at 20 - 28 and 31.5 ~ are characteristic of the monoclinic zirconia whereas those situated at 20 = 30, 34.5 and 50 ~ belong to the tetragonal structure.This study showed that nanostructured porous zirconia has a low thermal stability. In the preparation of the stable and efficient catalysts for the complete oxidation of aromatics, this low thermal stability will be taken into account. i

.

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1043 4. CONCLUSION A synthesis protocol of porous zirconia catalyst support, through a neutral C13(EO)6-Zr(OC3H7)4 assembly pathway has been developed. Our studies evidenced the role played by the surfactant. It has also been observed that the increase of hydrothermal treatment time and temperature have a benefical effect on tailoring the pore sizes. The synthesized materials will be used in preparation of Au / ZrO2, Au / VOx / ZrO2 catalysts, which will be tested in the benzene oxidation reaction. Thermogravimetric analysis shows that the recovered zirconia present a relatively low thermal stability. Then the structure collapses due to the crystallization to more stable tetragonal and monoclinic phase. ACKNOWLEDGMENT This work has been performed within the framework of PAI/IUAP 4-10. Alexandre L6onard thanks FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship. REFERENCES

1. T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima and T. Maki, Appl. Catal. A, 4 (1992) 149. 2. J.L.Blin, R. Flamant and B.L. Su, I. J. Inorg. Mater., 3 (2001) 959. 3. J.L.Blin, R. Flamant and B.L. Su, Prep. - Am. Chem. Soc., Div. Pet. Chem., 46 (2001) 23. 4. Y. Inoue and H. Yamazaki, Bull. Chem. Soc. Jpn., 60 (1987) 891. 5. A. Clearfield, Inorg. Chem., 3 (1964) 146. 6. J.L. Blin, A. L6onard and B.L. Su, Chem. Mater., 13 (2001) 3542. 7. J.L. Blin, A. L6onard and B.L. Su, J. Phys. Chem. B, 105 (2001) 6070. 8. J.L. Blin, A. Becue, B. Pauwels, G. Van Tendeloo and B.L. Su, Microporous Mesoporous Mater., 44-45 (2001) 41. 9. G. Herrier, J.L. Blin and B.L. Su, Langmuir, 17 (2001) 4422. 10. E.P. Barret, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 37. 11. D.J. Mitchell, G.J.T. Tiddy, L. Waring, T. Bostock and M.P. Mc Donald, J. Chem. Soc. Faraday Trans. I, 79, (1983) 975 12. L. Davies, L. Daza and P. Grange, J. Mater. Sci., 30 (1995) 5087. 13. M.M. Dubinin in : Progress in Surface and Membrane Science, 9 (D.A. Cadenhead, ed) Academic Press, New York, (1975), p. 1. 14. T.R. Pauly, Y. Liu, T.J. Pinnavaia, S.J.L. Billinge and T.P. Riecker, J. Am. Chem. Soc., 121 (1999) 8835. 15. H. Kunieda, K. Ozawa and K.L. Huang, J. Phys. Chem. B, 102 (1998) 831 16. P. Tres, M.J. Hudson and R. Denoyel, J. Mater. Chem., 8(9) (1998) 2147. 17. F. Del Monte, W. Larsen and J.D. Mackenzie, J. Am. Ceramic Soc., 83 (2000) 1508.

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Studies in Surface Science and Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1045

Effect of preparation parameters on the catalytic activities of sulfated ZrO2-SiO2 obtained by sol-gel process R. Akkari, A. Ghorbel Laboratoire de Chimie des Mat6riaux et Catalyse, D6partement de chimie, Facult6 des Sciences de Tunis, Universit6 E1-Manar, Campus universitaire, 1060 Tunis, Tunisie.

The preparation by sol-gel process of sulfated ZrO2-SiO2 mixed oxides using TEOS, Zr(acac)4 and in situ sulfation is described. Some parameters were varied in order to study the effect of the preparation conditions on the physico-chemical and catalytic properties of the solids. Mainly, two different procedures for drying the gels were adopted: one of them gives rise to a mesoporous series of catalysts. These catalysts exhibited interesting performances in the n-hexane isomerization reaction. Furthermore, both Si/Zr molar ratio and drying procedure are able to influence the physico-chemical characteristics (textural properties, catalytic behaviour) of mixed oxides. Mesoporous samples exhibited a good thermal stability with high surface areas even at a temperature of 650~ 1. I N T R O D U C T I O N Superacid catalysts have received considerable attention in the last years. Their utilization in a wide range of reactions especially in alkane isomerization [1-4] increased their importance in the field of catalysis. Sulfated zirconia constitutes one of the most interesting superacid catalysts [5,6,19]. However, the number of active sites of sulfated zirconia is probably limited by the available surface area [5,7] and it appears useful to combine the acid properties of sulfated zirconia with a high surface area and thermal stability of a support such as silica [5,7]. The textural and catalytic properties of silica supported sulfated zirconia samples are influenced by their preparation parameters [8,9]. It has been found [10-12] that sol-gel chemistry offers a high degree of homogeneity of the mixed oxides; this technique is believed to give solids with high surface areas [5]. Only few papers are concerned with the preparation using sol-gel process of silica-supported sulfated zirconia catalysts [5] and in these works, sulfation was usually done by impregnating the dried gels of ZrO2-SiO2 with a solution of sulfuric acid (5). To our knowledge, our work is the first attempt at using H2SO4 with a double role: to catalyse the TEOS hydrolysis and to generate sulfate ions in the solution containing zirconium acetylacetonate as a precursor. By varying the drying procedure, we have tried to modify the porosity of the solids and to obtain mesoporous catalysts in order to improve the catalytic activity.

1046

2. E X P E R I M E N T A L

2.1. Catalyst preparation Tetraethoxysilane (TEOS) and zirconium acetylacetonate Zr(acac)4 were used as precursors to prepare gels with molar ratios Zr/Si = 0.05 or 0.1. These samples were compared to the reference with Zr/Si = 0. The samples were prepared as follows : H2SO4 combined with water was mixed with 1-propanol. TEOS was then added dropwise under continuous stirring. Concentration of TEOS was 2 mol/l and the amount of aqueous H2SO4 was adjusted in order to obtain a molar ratio a = HzSO4/Si of 0.06 and a hydrolysis ratio h = HaO/TEOS = 4. Finally, the molar ratio Z = Zr/Si was fixed at : 0.05 and 0.1. Sample with Zr/Si equal to 0 was used as a reference. Drying procedure After gelation, each sample was separated in two batches and dried with two different procedures: overnight in oven at 393 K: the so-prepared samples are named xerogels (X). , by high temperature supercritical drying of the alcogel in an autoclave, which allows to obtain highly divided materials (aerogels). Calcination step Xerogels and aerogels were calcined under oxygen flow at different temperatures (T). The solids obtained were denoted as follows: Z Z r - Y- T where : Z : Zr/Si molar ratio Y : - A for aerogel - X for xerogel T: calcination temperature

2.2. Experimental techniques The effects of Zr/Si ratio, drying procedure and calcination temperature on the properties of these solids were studied by nitrogen physisorption at 77K, X-ray diffraction and infrared spectroscopy. Surface areas and pore size distributions were obtained from N2 isotherms determined at liquid nitrogen temperature with a Micrometrics ASAP 2000 sorptometer. The samples were previously outgassed for about 6 h at 4 7 3 K . IR spectra of the solids were performed using a Perkin-Elmer FTIR Spectrometer. XRD was used to study the effect of calcination temperatures on the structure of aerogels. The diffractograms were recorded on a Philips equipment, using Cu K a radiation. The bifunctional catalytic n-hexane isomerization was performed in a microflow reactor under atmospheric pressure, using a mechanical mixture (1w/w) of catalyst and a standard Pt/Al203 reforming catalyst (0.35 wt% Pt). The latter was previously reduced at 723 K for 4 h. The catalytic test was performed as follows: The mixture Pt/A1203 + catalyst was pretreated first under He flow at 723K for 2 hours and then under Hz for 30 min at 493K.

1047

Hydrogen saturated with n-hexane at 273K was passed over the catalyst. The analysis of the products was done on-line using FID gas chromatography. 3. RESULTS AND DISCUSSION 3.1. Effect of the Zr/Si molar ratio on the gelation process Gelation times and gel aspects are summarized in Table 1. As shown in this table, an important decrease in the gelation time (tg) is recorded when a small amount of Zr(acac)4 is added (case of the molar ratio Zr/Si = 0.05). A similar behaviour is observed if a larger amount of zirconium acetylacetonate is used (Zr/Si = 0.1). However, for this ratio the gel is harder. The presence of acetylacetonate ligands in the solution could take a prominent part in gelation time, and the increase of their number influences especially the gel aspect, due to their chelating character [13]. Table 1 Effect of the Zr/Si molar ratio on the gelation process. Zr/Si 0 0.05 0.1 tg : gelation time.

tg 14h 2h 10min 2h

Gel aspect and hardness transparent Translucent, fairly hard Slightly opaque, hard

3.2. BET analysis The effect of the Zr/Si molar ratio is studied both for aerogels and xerogels, and a comparison allows to determine the effect of solvent evacuation procedure on the textural properties of silica supported sulfated zirconia samples. Aerogels The effect of Zr/Si molar ratio on the BET surface area of aerogels calcined at 773K is shown in Table 2. Table 2 Effect of zirconium content on surface area for aerogels calcined at 773K. Sample 0 Zr-A-773 0.05 Zr-A-773 0.1 Zr-A-773

BET surface area (ma/g) 930 740 417

The BET surface decreases area when the Zr/Si molar ratio increases, which is in a good agreement with previous observations [7,10]. This result could be explained in terms of reduction of the division state for these samples when the zirconium content is increased. Besides, it is well known that Zr-rich gels are more dense than gels with low Zr contents [15]. These three aerogel samples calcined at 773K show type IV isotherms which are characteristic of mesoporous solids (Fig. 1). However, the hystereses observed are not of

1048

the same type. In fact, silica (Zr/Si = 0) and 0.05Zr-A-773 show the same H2-type of hysteresis, whereas Hl-type hysteresis is noticed when the zirconium content is more important (Zr/Si = 0.1). Hl-type hysteresis is observed with rigid bulk particles having uniform sizes, while when the distribution of the pores and the particle sizes are less defined, intergranular porosity is characterized by H2-type hysteresis [14]. On the other hand, two types of mesopores are found for aerogels prepared with a low Zr content (Zr/Si = 0.05) and for silica aerogel (Zr/Si = 0). On the contraty, for a higher amount of zirconium (Zr/Si = 0.1) a monomodal distribution of pore size is observed. Futhermore, the increase of the Zr/Si molar ratio and probably of the content of acac ligand, favours the disappearance of the lowest mesopore diameters (about 35*) at the advantage of the larger ones, for which the size increases with Zr/Si molar ratio: 55,~ when Zr/Si = 0 and 0.05, and 95,~ for Zr/Si = 0.1. Fig. 2 illustrates the pore size distributions of the three aerogels calcined at 773K. 8OO

i

2,4 600 400

o,8

;~ 200

m

i

0

i

0,5

1

10

100 Pore diameter (A)

Relative pressure (P/P0)

Fig. 1. Isotherms of aerogels O 0 Zr-A-773

li

l.a

Fig. 2. Pore size distributions for aerogels

~ 0.05 Zr-A-773

a 0.1 Zr-A-773

i Xerogels Also in this case, the sulfated zirconia-silica xerogels exhibit a decrease of the BET surface area compared to the original support with molar ratio Zr/Si=0, which agrees with the results obtained for the aerogels and with literature data [7,10,15]. Table 3 Effect of zirconium content on surface area for xerogels calcined at 773 K. Sample 0 Zr-X-773 0.05 Zr-X-773 0.1 Zr-X-773

BET surface area (mZ/g) 390 231 333

1049 Nevertheless, the BET surface area increases with the zirconium content, which may be due to the fact that zirconium stabilizes the surface against the damages that might cause the non-controlled drying. These xerogels calcined at 773K show type I isotherms characteristic of microporous solids. The importance of the drying procedure is then obvious. For the xerogels, drying causes cracking because of the large capillary forces generated in the tiny pores. This problem is avoided with hypercritical drying, which eliminates the liquid-vapor interface and then prevents the development of capillary stresses [16,17]. Thereby, xerogels exhibit interesting properties such as a very high porosity and surface area, a low density and they are known to have a very good textural stability during heat treatment at high temperatures. Fig. 3 illustrates the thermal stability of the xerogels and aerogels of silica-supported sulfated zirconia catalysts. For the xerogels, a large decrease of surface area is observed upon calcination at 923K, which reflects sintering. Aerogels show a good stability even at this temperature.

1 000

!

T 800 + ~

- . - 0.05 Zr-X - m - 0.05 Zr-A

m---------__n__________m_

600

" I 400

200 T 0!

773

~

. I

~

. I

823

~ I

I

873

923

Temperature (K) Fig. 3. Comparison of thermal stability for xerogels and aerogels prepared with a molar ratio Zr/Si = 0.05. 3.3. IR spectoscopy IR spectra of aerogels and xerogels prepared with the different Zr/Si molar ratios are shown in Fig. 4. In the spectrum of pure silica (spectrum a), five features are observed: Asymetric Si-O-Si "network" stretches at 1089 and 1228 cm -1, the corresponding symmetric stretch at 801 c m ~ and Si-O-Si or Si-OH at 980 cm -1 [5,10,12]. The band at 459 c m -1 corresponds to the bending mode of silica. For the xerogels (spectra b, c), the main Si-O-Si band moves toward lower wavenumbers as the zirconia content of sulfated mixed oxide increases. These spectral changes are consistent with a weakening of the silica network by zirconium atoms [5,12]. This shift to lower frequencies is sometimes attributed to the formation of Zr-O-Si bonds [5]. For the aerogels (spectra d, e), the Si-O-Si band shifts to lower frequencies when a small amount

1050 of zirconium is added (Zr/Si=0.05); however, when the zirconium content increases, this band moves toward higher wavenumbers. This observation might be attributed to a different degree of order [18] when the Zr/Si molar ratio raises. In fact, the drying procedure is carried out in this case at a relatively high temperature (536K) and the gel structure could perhaps be different. At this level, a comparison of the effect of the zirconium content on the crystallinity of the aerogels will perhaps give information on the order degree of the structure. _

i

,

9

1<

,

I,

t

C

d

/ f ......

kJ

/,,'"

lc

/,,"

~b"~.

~'-',3 /

/[,' "d

/

la

I

'~'

-'~

1079

./

~ ~1

;

iv1 l

k\~!l

f!l

-~",,, \

1049

\'\

~",.

1228~

/

35oo

"~ r

"t\ /

~J

~\\i I

1089 ------N\ !

25b0

' ~500

W ave number

' 5oo

(cm -1)

Fig. 4. Infrared spectra of calcined aerogels and xerogels: (a) :0 Zr-A-773 / 0 Zr-X-773 (b): 0.05 Zr-X-773; (c): 0.1 Zr-X-773; (d): 0.05 Zr-A-773; (e): 0.1 Zr-A-773

3.4. XRD analysis Fig. 5 shows the diffraction patterns corresponding to aerogels of silica-supported sulfated zirconia with Zr/Si molar ratios of 0.05 and 0.1 and calcined at different temperatures. The crystallization is improved when the Zr content and calcination temperature increase, which is in agreement with results reported by many research groups, e.g. [4].

1051 However, in all the samples, tetragonal ZrOz was detected by XRD as the main crystalline phase (20 = 30, 50 and 60~ The intensity of the corresponding peaks increases with calcination temperatures. It has been reported that the nature of the zirconia phase could give informations on the crystallite size [7]. The tetragonal form exists in microcrystallites at temperatures below 973K, while the monoclinic phase indicates an increase of the crystallite size [7]. On this basis, these findings suggest that zirconia is well distributed within the sulfated ZrOz-SiOz aerogel mixed oxides studied.

5

15

25

35

45

55

65

75

20

Fig. 5. Diffraction patterns of (a) 0.05 Zr-A-773; (b) 0.05 Zr-A-873; (c) 0.05 Zr-A-923; (d) 0.1 Zr-A-773; (e) 0.1 Zr-A-873 and (f) 0.1 Zr-A-923 3.5. Catalytic activity Table 4 Comparison between aerogel and xerogel in n-hexane isomerization reaction at 493K.

0.1Zr-X-873 0.1Zr-A-873

Isomerization activity (mol. g-1. s-l), 106 1.00 2.42

Isomerization selectivity (%) 98.9 97.4

2,3-DB selectivity (%) 0 0.78

n-hexane activity (mol. g-1. s-l), 10 6 1.01 2.48

The comparison between aerogels and xerogels suggests the importance of the textural properties of this class of solids on their catalytic behaviour. In fact, the interesting

1052 properties for aerogels, namely, high surface areas, mesoporosity, and a good thermal stability improve their activity in the n-hexane isomerization. The selectivity to the isomerization products especially 2,3-dimethylbutane (2,3-DB), is enhanced relative to the xerogel calcined at the same temperature. This product is obtained only in the presence of strong acid sites. Table 4 gives the results obtained over the two catalysts 0.1Zr-A-873 and 0.1Zr-X-873. The difference between aerogels and xerogels is important: Although the selectivities are similar, the n-hexane activity of the aerogel is higher owing to the presence of mesopores. 4. CONCLUSION The results of this study show that the catalytic activity depends on the textural properties and thus, on preparation conditions. The drying procedure is a very important parameter which allows to obtain solids having different textural properties. A hypercritical drying leads to mesoporous materials with high surface areas. As a consequence of these interesting properties, aerogels are much more active than xerogels. Increasing the Zr content lowers the surface area for the aerogels. However, for the xerogels, when the Zr/Si molar ratio raises from 0.05 to 0.1, the BET surface area increases, which may be due to the stabilization of the surface against damages caused by the non controlled drying. REFERENCES

1. W. Stichert, F. Sch/ith, S. Kuba and H. Kn6zinger, J. Catal., 198 (2001) 277. 2. D. Farcasu, J. Qi Li and S. Cameron, Appl. Catal., 154 (1997) 173. 3. Y.Y. Huang, B.Y. Zhao and Y.C. Xie, Appl. Catal., 173 (1998) 27. 4. R. Barthos, F. Lonyi, J. Engelhardt and J. Valyon, Topics Catal., 10 (2000) 79. 5. T. Lopez, J. Navarrete, R. Gomez, O. Novaro, F. Figueras and H. Armendariz, Appl. Catal., 125 (1995) 217. 6. K. Tanabe and T. Yamaguchi, Stud. Surf. Sci. Catal., 44 (1988) 99. 7. S. Damyanova, P.Grange and B. Delmon, J. Catal, 168 (1997) 421. 8. J.R. Sohn and H.J. Jang, J. Mol. Catal., 64 (1991) 349. 9. J.B. Miller, S.E. Rankin and E.I. Ko, J. Catal., 148 (1994) 673. 10. J. A. Anderson, G. Fergusson, I. R. Ramos and A. G. Ruiz, J. Catal., 192 (2000) 344. 11. F. Babou, G. Coudurier and J.C. Vedrine, J. Catal., 152 (1995) 341. 12. J. B. Miller and E. I. Ko, J. Catal., 159, (1996) 58. 13. C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-Cryst. Solids, 100 (1988) 65. 14. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure. Appl. Chem., 57 (1985) 603. 15. R. Pirard, D. Bonhomme, S. Kolibos, J.P. Pirard and A. Lecloux, J. Sol-Gel Sci. Technol., 8 (1997) 831. 16. G.M. Pajonk, Appl. Catal., 72 (1991) 217. 17. G.W. Scherer, J. Non-Cryst. Solids, 87 (1986) 199. 18. J. C. Debsikdar, J. Non-Cryst. Solids 86, (1986) 231. 19. L. Ben Hamouda, A. Ghorbel and F. Figueras, Stud. Surf. Sci. Catal., 130 (2000) 971.

Studies in Surface ScienceandCatalysis143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1053

Non-aggressive way using zirconium acetate for preparation of zirconium pillared clay developing high sulfur thermal stability over 830 ~ S. Ben Chaabene a, L. Bergaoui a, A. Ghorbel a and J.-F. Lambert b A Laboratoire de chimie des mat6riaux et catalyse, Facult6 des Sciences de Tunis, Universit6 El Manar, Campus Universitaire, 1060 Le B61ved6re, Tunis, Tunisie B Laboratoire de r6activit6 de surface, UMR 7609 CNRS, Universit6 Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France Pillared zirconium clay modified by sulfate constitutes a new class of materials. Nevertheless, the classical procedure consisting of impregnating these solids with sulfate solutions presents some disadvantages such as low surface area and a poor sulfur thermal stability. In a previous work, we have developed a new in situ sulfation preparation method using zirconyl chloride as zirconium precursor. However, the use of this salt gives an intercalation solution with a very low pH which can affect the clay layers. Furthermore, the addition of sulfate ions to the ZrO2 solution is limited by Hauser salt or polymeric phase precipitation. In this work, zirconium acetate solution was used in order to increase the pH of the pillaring solution and the content of sulfate ions introduced. Different preparation parameters and their effect on the structural and textural properties have been investigated. The resulting materials present a best zirconium-sulfate intercalation with higher sulfate rate and develop very high thermal stability of sulfur even at about 830 ~ 1. I N T R O D U C T I O N The isomerization of light paraffin using superacid solid catalysts is a clean way to increase the octane number of hydrocarbons. On this basis, sulfated metal oxides have attracted the attention of many research groups owing to their high activity in acid catalyzed reactions [1]. Sulfated zirconia was found to be a promising catalyst in this field and at the industrial level [2]. Recently, several works study the preparation of nano scale acid solids constructed between the interlayers of smectite clay minerals [3]. Some attempts consisting of adding ammonium sulfate to the pillared montmorillonite suspension have been reported [4,5]. These preparation methods show that adding sulfate ions decreases considerably the basal spacing without developing high acidity. Other studies attempted to prepare zirconium sulfate pillared clays by adding in situ sulfate. However, the solids obtained have a low surface area, low thermal stability [6] and low sulfate to zirconium ratios [7]. Earlier, we have prepared a zirconium sulfate pillared clay by in situ

1054

sulfation using zirconyl chloride octahydrate salt [8]. The use of zirconyl chloride solutions favors the polymerization of zirconium sulfate complexes in addition to the severe preparation conditions (low pH value) which can partly destroy the clay layers. Recent studies have demonstrated that the use of commercial solution of zirconium acetate allows a higher microporosity and thermal stability. Furthermore, this precursor has the property to be non aggressive for the clay [9,10]. The present study deals with the incorporation of zirconium sulfate hydroxyl complex in Na-montmorillonite using zirconium acetate as a precursor. The effect of the preparation parameters on the textural and structural properties of the resulting materials will be discussed. 2. E X P E R I M E N T A L

Starting material The clay used is an industrial treated montmorillonite Clarsol KC2 (France). Before use, the clay was purified by taking (using sedimentation) the fraction less than 2 lam. By means of cation exchange, using a 1 mol/L sodium chloride solution, homoionic montmorillonite was obtained after free ions elimination. The clay obtained has a surface area of 50 m2/g, an exchange capacity of 79 meq/100g of clay and a basal spacing of 12.7 A.

Catalysts preparation. Intercalation solutions were prepared by addition of an ammonium sulfate solution to a fresh zirconium acetate one. The mixture was adjusted to the desired pH using a HC1 solution. In another beaker, Na-montmorillonite suspension was adjusted at the same pH than the intercalation solution. Na-montmorillonite suspension was added dropwise to the fresh pillaring solution with different clay to zirconium ratios. The mixture was magnetically stirred for different times at 15 ~ The dispersed montmorillonite was separated by centrifugation and washed by dialysis. The resulting clay was dried overnight in oven at 120 ~ Potentiometric study, pH measurements were effectuated using pH stat TITRINO titrimeter with a combined glass electrode saturated with KCI.

X-ray diffraction. A SIEMENS X-Ray D 500 powder diffractometer using Cu K a radiation was used to determine the basal spacing. Samples were prepared by air drying of a clay slurry on a glass plate.

BET surface area and microporous volume determination. Surface area measurements were performed by nitrogen physisorption at 77 K using the static volumetric apparatus (Micromeritics ASAP 2000 adsorption analyzer) with application of the BET method. All measurements were performed after treatment of solids under vacuum at 110 ~

Thermogravimetry. Thermogravimetric analysis (TGA/DTG) was performed on a Seiko instrument. The samples were heated in platinum crucibles up to 1000~ with a heating rate of 3 ~ in 60 mL/min under flowing dry air.

1055

Mass spectrometry. Thermal treatments of the intercalated solids from room temperature to 1000 ~ were carried out in quartz reactors under helium, with a flow rate of 3 cc/min. A Hidden Analytical HPR 20 Mass spectrometer (MS) was used for the analysis of the gases evolved upon thermal treatment, in the mass range 1-200 a.m.u. A capillary leak maintained at 170~ was used to divert a fraction of the gas flow to the analysis chamber. Thermal treatment. The samples were placed in a thin bed configuration in a quartz tube swept by air (flow 100 mL/mn). The temperature was raised by l~ and maintained for 5 hours at the desired level. 3. RESULTS AND DISCUSSION

3.1. Intercalation solution study It is known that a zirconyl chloride solution is not stable [11]. The pH of a fresh 0.1 mol/L ZrOCl2 solution decreases from 2.1 to 1.9 after 10 minutes. To study the stability of the zirconium acetate solution, the pH was measured periodically. It was found to be equal to 3.76 and a very weak decrease was observed after two days. These properties could give, besides the non aggressive medium for clay layers, a better reproducibility than the zirconyl chloride solution. The effect of the addition of 2 mol/L ammonium sulfate to the 0.1 mol/L zirconyl chloride solution or to the 0.1 mol/L zirconium acetate solutions on the pH values is plotted in Fig. 1. This plot shows two important results. The first one is the absence of precipitation occurring for high SO4:Zr ratios for the acetate solution. The second one is the increase of pH of the zirconium acetate solution when the sulfate quantity increases. This increase of pH is probably due to the release of acetate groups in solution substituted by sulfate ions, which is more important than the weak acidity of the ammonium ion. 3.2. Resulting material 3.2.1. Intercalation time The diffraction patterns of dried zirconium sulfate pillared clay obtained after different intercalation times are presented in Fig. 2. These samples were prepared using a 0.1 mol/L zirconium acetate solution with an SO4:Zr ratio = 0.5. The clay concentration was 10 g/L, with Clay : Zr =10 g/mol, at pH = 4.00. The intercalation was performed at 15 ~ For a short reaction time the XRD pattern shows a large and non symmetric peak centered at 12 ,~ and a shoulder at lower 0 angles. The samples exhibit a high surface area (170 m2/g) and a microporous volume of about 0.018 mL/g. The large diffraction peaks and the enhancement of the surface area prove that the intercalation is partial. Fig. 2 also shows that the solids change with intercalation time, to give an amorphous phase. This could be explained by the polymerization of zirconium sulfate species governed by the low kinetic polycondensation of these species. No significant changes of the BET specific surface area and microporous volume are observed.

1056

zirconium acetate

~Z 3

Precipitation

I Zirconyl chloride

0

1 2 3 SO4:Zr molar ratio

4

Fig. 1. Variation of pH as a function of sulfate to zirconium ratio for 0.1 mol/L zirconyl chloride and zirconium acetate solution at 20 ~

3.2.2. Effect of suspension pH The XRD patterns of solids prepared using similar conditions described above, with sulfate to zirconium ratio equal to 0.5 and using different pH values, are presented in Fig. 3. In addition to the phenomena discussed above, a weak shift of the maximum of the d001 peak to higher angle with decreasing pH is observed. This result is probably due to the partial exchange of Na + by proton. Table 1 gives the surface area and the microporous volume of these samples. The surface area increases with the pH but the microporous volume does not follow this tendency. Table 1 Specific surface area and microporous volume of intercalated clays prepared at different pH. pH 3.50 4.00 4.50 161 183 196 Surface are (mZ/g) 0.021 0.016 0.019 microporous volume (cm3/g) Thus, the increase of the pH does not seem very important when the zirconium acetate is used. It is not the case for zirconyl chloride where a change of the pH induces a very important change of the texture of the obtained solids [12].

1057

13.7/~ 140 hours 12.8 .A

90 hours 42 hours

12..2 ,~.

23

4.56

12.5 A 30

19A

3.50

0 hour '

I

I

t

t

I

4

8

12

16

20

20 Fig. 2. DRX patterns for different reaction times, 3.2.3.

Effect

of SO4:Zr

"

I

i

I

I

I

4

8

12

16

20

20 Fig. 3. DRX pattern of samples prepared at different pH values.

ratio

All samples were prepared using 0.1 mol/L zirconium acetate solution at 15 ~ with 42 hours intercalation reaction. The clay suspension pH was fixed at the same value as that of the intercalation solution. The X-ray diffraction patterns of zirconium sulfate pillared clays prepared with different sulfate to zirconium ratios are shown in Fig. 4. This figure shows a shoulder at about 19.5 ,~ for the sample prepared using a SO4:Zr ratio equal to 0.125. The 19.5 ,~ spacing corresponds to the intercalation of non sulfated zirconium polycations [13]. Non-drastic loss in surface area is observed when the sulfate to zirconium ratio increases. But the use of zirconyl chloride solution shows a drastic loss of surface area when the SO4:Zr ratio increases (more than 50 %) [13]. Table 2 Specific surface area and microporous volume data for intercalated clays prepared with different zirconium concentrations. [Zr] (mol/L) 0.1 0.05 0.025 Surface mZ/g 196 133 137 microporous volume cm3/g 0.019 0.020 0.023

1058

13.5 A I

r - - 12.4A

~2.8

A

12.2 A

~ - - - . 13.7 .,~ 0.025

19.6 ,~ 12.8 ,~

O.05mol/L

0.50 0.125

0.1 mol/L

4

8

20

12

16

20

Fig. 4. DRX pattern of solids prepared with different SO4:Zr ratios

4

8

12 20

16

20

Fig. 5. DRX pattern of solids prepared with different Zr concentration

3.2.4. Effect of zirconium acetate concentration The XRD patterns of zirconium sulfate pillared clays obtained after 90 hours of intercalation with different zirconium acetate concentrations using 0.5 as sulfate to Zr ratio and the same clay concentration as used earlier are presented in Fig. 5. The diffraction data show the appearance of two first order reflections. The first one is at 23.4 ,~ for the lowest zirconium concentration and appears as a shoulder at the same distance for 0.05 mol/L concentration. The second reflection is observed at approximately 12.3 ,~ for the lowest concentration and at 13.7 ,~ for 0.1 mol/L zirconium acetate. The first one results from the intercalation of sulfated zirconium species. Those species are more voluminous than the non sulfated one which gives a distance spacing at only 19.6 ,&. The better intercalation of sulfated zirconium species at low Zr concentration is probably due to the slow progress of polycondensation reactions. This process reduces the number of different zirconium species and gives a better cristallinity of the solid. Table 2 summarizes the textural properties of samples prepared with different zirconium concentrations. The decrease of the surface area with the decrease of the Zr concentration is probably due to the increase of the sodium clay layers by comparison with the intercalated layers. The microporous volume increases when the Zr concentration decreases. The higher microporosity is due to the important basal distance of this sample.

1059

3.3. Thermal stability study In this part, the effect of the preparation parameters on the thermal stability of the solids is studied.

3.3.1. Thermogravimetric analysis Fig. 6 shows the weight loss of the sample obtained with 0.025 mol/L zirconium concentration after 140 hours reaction using the same preparation conditions as in 3.2.4. Three major weight loss regions are identified for sulfate zirconium pillared clay. The first weight loss of about 12 % is observed between room temperature and 200 ~ The second of about 8 % is observed between 300 ~ and 700 ~ The third weight loss occurs at high temperature (maximum at 800 ~ The first one is due to the departure of water molecules adsorbed in the interlayer spaces and on the external surfaces. The second loss is related to several phenomena: the departure of water molecule strongly associated with hydroxyzirconium sulfate cations, the water which comes from the dehydroxylation of Zr-OH-Zr bridge and, finally, the water and carbon dioxide resulting from the combustion reaction of the acetate groups. The third stage must correspond to the sulfate loss.

3.3.2. Mass spectrometry analysis Zirconium-sulfate pillared clay prepared with reaction time of 140 hours and using 0.1 mol/L zirconium acetate solution has been analyzed with mass spectrometry. When the sample was heated in air, masses of 17, 18, 44, 48, 64 and 80 were recorded, corresponding, respectively, to OH +, H20 § CO2 § SO § SO2 § and SO3 § Figs. 7, 8 and 9 show the variation of the different masses with heating temperature. The CO2 peak shown in Fig. 8 is due to the oxidation of acetate groups. This peak, centered at 400 ~ in the CO2 mass loss is accompanied by a similar one in the same region in the water mass curve (Fig. 7). This fact indicates, as expected, that combustion of acetate gives carbon dioxide and water. Fig. 7 also shows the departure of a large quantity of physisorbed water at 150 ~ The secondary maximum at 500~ can arise from dehydration and dehydroxylation processes of the zirconium-containing pillars. Water evolution stops only at very high temperatures because of the condensation of hydroxyl groups of the clay. As shown in Fig. 9, the weight loss at about 760 ~ could be attributed to the evolution of gases that produce peaks with masses of 48 and 64. However, mass spectrometry intensities corresponding to SO3 (mass 80) are not observed (Fig. 9). This fact indicates that the sulfate decomposition gives sulfur dioxide and certainly oxygen. However, when air or oxygen is used as carrier gas, it is not possible to measure the oxygen evolution. Nevertheless, previous works using helium showed that the SO2 departure is accompanied by oxygen [8,14,15]. The use of zirconyl chloride as zirconium precursor in the preparation of zirconium sulfate pillared clay [8] shows that the sulfate loss occurs at 680 ~ So, the use of zirconium acetate precursor allows a better thermal stability of sulfates in the zirconium sulfate pillared clay than zirconyl chloride.

1060

0

0.4 -5

~i

0.3

il~

1 o.2-

-15

.~

~ 0.1

OIT -20

0 200

400 600 800 Temperature, ~

200

1000

Fig. 6. D T G / T G curves of 0.025 mol/L zirconium concentration.

r a$

400 600 800 Temperature, ~

1000

Fig. 7. OH + and H 2 0 + mass curves as a function of temperature.

r

ra~ ~D r~

.F..~

SO2 +

o+ .... .... .... 200 400 600 800 1000 Temperarature, ~ Fig. 8. CO2 + mass, curve as a function of temperature.

500

600

. . . .

700 800 900 Temperature, ~

1000

Fig. 9. SO + and SO2 + and SO3 + mass curves as a function of temperature.

1061 Fig. 10 displays the SO2 weight loss evolution (mass 64) as a function of temperature for different Zr concentrations and SO4:Zr ratio = 0.5. This figure shows that the SO2 peak shifts to higher temperature when the Zr concentration decreases. In the case of 0.025 molZr/L, the major sulfate loss occurs at 830 ~ This indicates that the sulfates linked to the pillars which give a d001 at 23.4 ,~ develop the best thermal stability. However, for samples prepared with higher zirconium acetate concentration, the departure of sulfur occurs at lower temperatures. As those samples contain a polymeric phase, this Zr-SO4 phase probably gives a less stable sulfate.

3.3.3. Thermal stability of textural properties In this part the effect of zirconium concentration on the textural thermal stability is investigated. Fig. 11 shows the variation of the microporous volume as a function of temperature for two Zr concentrations (0.1 mol/L and 0.025 mol/L). As shown, the Zr concentration seems to be important to the thermal stability of the microporosity. In fact, the microporosity of the sample prepared with 0.025 mol/L develops a higher thermal stability than that prepared with 0.1 mol/L zirconium. The polymeric phase deposited on the clay layers appears to be responsible for this loss of microporosity. An insignificant increase in the surface area between 120 ~ and 400 ~ is observed for the sample prepared with Zr concentration of 0.025 mol/L. However, the decrease of surface area is very important in the samples prepared with high concentrations of zirconium acetate (from 195 at 120 ~ to 140 m2/g at 400 ~ The presence of amorphous phase exerts a great effect on the thermal stability of the solids. The dehydroxylation of the zirconium complex seems not to be the only factor responsible for the loss of surface area, as reported by Figueras et al. [16]. 4. C O N C L U S I O N Zirconium acetate seems to be a very interesting zirconium precursor for the preparation of zirconium sulfate pillared clay because of its high and stable pH level and its low sensitivity to the precipitation when the sulfate was added. Nevertheless, the progress of the polymerization reaction is very rapid at high Zr concentrations, even at reduced intercalation temperature (15 ~ When the Zr concentration and the SO4:Zr ratio increase, the formation of a polymeric phase is accentuated. For low SO4:Zr ratio, the pillars in the intercalated clay fraction consist of non sulfated polycations. But when the zirconium concentration is low (0.025 mol/L) and the SO4:Zr ratio is high, the intercalation of sulfated polycations occurs. These preparation conditions give a solid with a high textural thermal stability. Sulfate ions linked to Zr-pillars seem to be more stable than those incorporated in the polymeric phase. Those sulfate ions dispersed in the solid could generate an interesting catalytic behaviour that merits further study.

1062

II

#

0.025 mol/L I

e

0.100 mol/L I

|

rb 3

~2 g~ 0.1

O gh

mol/L

0.05 mol/L _ ~ 0~025 m o l / L ~ I ....

I ....

I ....

I ....

I ....

I

700 800 900 1000 Temperature, ~ Fig. 10. SO2+ mass curve as a function of temperature.

0 400

500 600

500 600 700 Temperature, *C

800

Fig. 11. Variation of the microporous volume as a function of temperature (0.025 and 0.1

mol/L).

REFERENCES 1. K. Arata, Adv. Catal., 37 (1990) 165. 2. D. Y. Ganabaty and J. J. Nair, Microporous Mesoporous Mater., 33 (1999) 1. 3. R. Mokaya and W. Jones, J. Catal., 153 (1995) 76. 4. J. W. Johnson, J. F. Brody, S. L. Soled, W. E. Gates, J. L. Robbins and E. MarucchiSoos, J. ofMolec. Catal., A: Chem., 107 (1996) 67. 5. E. M. Farfan-Torres, E. Sham and P. Grange, Catal. Today, 15 (1992) 515. 6. E. M. Farfan-Torres and P. Grange, Bulletin de la classe des sciences 6+me S6rie Tome 1 4-5 (1990) 113. 7. M. Katoh, H. Fujisawa and T. Yamaguchi, in : H. Hattori, M. Misono, Y. Ono (Eds.), Acid-Base Catalysts II, Studies on surface and science Catalysis, 90, Elsevier-Kodansha. Japan. 1194. p. 263. 8. S. Ben Chaabene, L.Bergaoui, A. Ghorbel and J.F.Lambert, To be published. 9. L. M. Gandia, R. Toranzo, M. A. Vicente and A. Gil, Appl. Catal. Gen., 183 (1999) 23. 10. J. W. Johnson, J. F. Brody, R. M. Alexander, L. N. YacuUo and C. F. Klein, Chem. Mater., 5 (1993) 36. 11. M./~berg and J. Glaser, Inorg. Chim. Acta, 206 (1993) 53. 12. E. M. Farfan-Torres and P. Grange, J. Chim. Phys., 87 (1990) 1547. 13. E. M. Farfan-Torres and P. Grange, Catalytic Science and Technology, 1 (1991) 103. 14. A. F. Belido and K. J. Klabunde, J. Catal., 176 (1998) 448. 15. R. Srinivansan, R. A. Keogh, D. R. Milbum and B. H. Davis, J. Catal., 153 (1995) 123. 16. F. Figueras, A. Mattrod-Bashi, G. Fetter, A. Thrierr and J. V. Zanchetta, J. Catal., 119 (1989) 91.

Studiesin SurfaceScienceand Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1063

Influence of the preparation conditions on the structure of the active phase and catalytic properties of Ni-Co-molybdate propane oxydehydrogenation catalysts M. M. Barsan, A. Maione*, F.C. Thyrion Unit~ des Proc~d~s, Voie Minckeleers 1 *Unit~ de Chimie des Mat~riaux Inorganiques et Organiques, Place Louis Pasteur 1 Universit6 catholique de Louvain, 1348 Louvain-la-Neuve, Belgium

The effect of the pH conditions on the structure and composition of the active phase in Ni0.sCo0.sMoO4 propane oxy-dehydrogenation (ODH) catalyst is investigated by using different characterization techniques: XRD, XPS, laser Raman spectroscopy, chemical composition and BET-surface area. Increasing precipitation pH from 6.0 to 8.5 produces the following effects: i) BET surface area progressively increases, ii) Mo loading decreases, iii) all samples show higher Co loading than Ni in the bulk, iv) the Ni content increases strongly from pH 6.0 to 7.5 and remains almost constant between 7.5- 8.5 pH, v) Co content is almost constant in the pH range 6.0-7.5, but strongly increases at pH 8.5, vi) surface composition varies as follows: at pH 6.0: Mo >Co >Ni, at pH 7.5: Mo ___-Ni > Co, at pH 8.5: Ni > Co _= Mo. MoO3 phase was detected only in the sample prepared at pH 6.0. XRD patterns showed, for the catalysts prepared at pH 6.0 and 7.5, diffraction lines characteristics of both [3-NiMoO4 and [3-CoMoO4-1ike phases. The catalyst prepared at pH 8.5 is a solid solution of [3-NiMoO4 and [3-CoMoO4 in a common molybdate structure and probably NiO and Co304 separated phases. Activity measurements conducted at different space-time indicated that the sample prepared at pH 8.5 is the most effective for the propylene production. Keywords: nickel molybdate, cobalt molybdate, nickel cobalt molybdate, propane oxidative dehydrogenation, propylene production 1. INTRODUCTION The production of propylene from the oxydehydrogenation of propane is an exothermic process and thermodynamically limited. For this reason, a large number of catalytic systems have been studied for increasing propylene production. The majority of the catalysts investigated in the literature are based on vanadium compounds. They show propylene yield between 5% on vanadium-magnesium-oxide [1] and almost 30% on Vsilicalite [2].

1064 The most noteworthy propane ODH catalysts, not based on the vanadium, are those containing Co and/or Ni molybdate. Mazzocchia [3] has investigated ct and [3-NiMoO4 catalysts and concluded that the [3-phase is more selective to propylene that the a-phase at the same conversion and temperature. For instance, [3-NiMoO4 catalyst shows propy|ene yield and selectivity of about 13% and 63% respectively. Yoon and coworkers [4] have reported that Co0.95MoOx (~-CoMoO4 and MoO3 phases) exhibit 60% selectivity to propylene at 20% propane conversion. Stern [5, 6] achieved a comprehensive study on single and binary metal molybdates supported on silica. NiMoO4/SiO2 and Ni0.sCo0,5MoO4 /SiO2 catalysts show a maximum propylene yield of 16% at propane conversion of 27% and 34%, respectively. The catalytic properties of the Ni-Co mo|ybdate system have been correlated to the structure and composition of the active phase. This aspect takes a high importance because several metal oxide phases, such as c~ or [3-NiMoO4, CoMoO4, Ni-Co-molybdate solid solutions have been identified as active phases for the propane ODH reaction. The formation of pure MOO3, Co304 and NiO oxides was also observed in the Ni-Co molybdate system and it strongly depends on the catalyst preparation parameters such as the pH of the precipitating solution, the nature, the concentration, the order of addition of the metal solutions and the precipitation temperature. Thereby, a rigorous control of the different parameters involved in the preparation step is crucial. However, very few works have been published concerning these catalyst preparation parameters and their influence over the structure of the active phases and the activity behavior. The present work is part of a research program aimed at a systematic investigation of the different parameters controlling the preparation of Ni-Co molybdate propane ODH catalysts. The goal of this paper is to study the influence of the co-precipitation pH conditions on the nature of the active phase and the activity properties of unsupported NiCo-Molybdate catalyst. 2. EXPERIMENTAL 2.1. Catalyst preparation The Ni-Co-molybdate samples were prepared by co-precipitation of solutions containing Ni and Co nitrates (Merck) and ammonia heptamolybdate (Fluka) at pH between 6.0 and 8.5 followed by steps of filtering, washing, drying (110~ 12 h) and calcination (500~ 3 h in air). The amounts of metals used for all preparations correspond to the following stoichiometry Ni0.5Co0.5MoO4. The mixed Co and Ni nitrate solution was added stepwise to the ammonium heptamolybdate solution, both heated at 60~ and then the co-precipitation was realized for 8 h. Ammonium hydroxide solution was used to reach and maintain a constant pH at 6.0, 7.5 and 8.5 in successive runs. The catalyst was crushed and sieved in particle sizes of 200-250 ~m for catalytic tests.

2.2. Physical-chemical characterization BET surface area was measured by nitrogen adsorption using a FlowSorb 2300 equipment. The samples were outgassed at 150~ for 2 h prior to the measurement.

1065 The chemical composition of the catalysts was determined by inductively coupled plasma (ICP) atomic emission spectroscopy using a Thermo Jarrell Ash Iris Advantage equipment. X-ray diffraction patterns were recorded at room temperature on a Siemens Diffractometer D5000 using a Cu K a radiation (X = 1.5418 .A,). A scan rate of 0.058 degree min-1 with a step size of 0.01 degree was used for data collection. The identification of the crystalline phases was carried out by using references from JCPDS files [7]. X-ray photoelectron spectroscopy (XPS) measurements were performed using a SSX100 model 206 Surface Science Instrument Spectrometer operated at 10 kV, 12 mA with a monochromatized A1 K a radiation (1486.6 eV). The catalysts were pressed into the samples holders of 6 mm and then introduced into the preparation chamber of the spectrometer. The C ls, MO3d, Cozp, Nizp and Ols lines were recorded for each sample. All binding energies were referenced to the Cls level at 284.8 eV. Surface composition was determined from the peak intensities and the Scofield sensitivity factors provided by the instrument software. For spectrum deconvolution, a Shirley baseline was used and peaks were considered Gaussian/Lorentzian ratio of 85/15. Confocal laser Raman spectroscopy was performed with a Dilor LABRAM Spectrometer equipped with a He-Ne laser (632.81 nm, 10 mW laser power at the sample surface). Spectra were recorded in the 100-1100 cm -1 range.

2.3. Catalytic activity measurements Activity tests for the propane ODH reaction were carried out at atmospheric pressure in a fixed bed reactor. The tests were conducted under the following conditions: reaction temperature 450~ feed composition consisting of 10% vol. propane, 10% vol. oxygen and 80% vol. helium, total gas flow 30 ml/min, propane space-time (W/Fc3H8) between 0.074-0.43 g's/lamol, time on stream 9 h. The reactor effluent was analyzed by online gas chromatography, using TCD and FID detectors. Separations were performed using a Silica Gel 60/80 (18 ft xl/8 ") column for He, Oz, CO, CO2, and Porasil B (2 m x 1/8 ") for hydrocarbons. In the following discussion, conversion is defined as the molar flow of propane or oxygen transformed divided by the initial molar flow. Selectivity is the ratio between molar flow of product and the molar flow of propane consumed. The stoichiometric coefficients of the primary and secondary reactions are accounted in the definition of the selectivity. Blank runs showed the absence of the homogeneous reaction under the experimental conditions used in this work. 3. RESULTS AND DISCUSSION 3.1. Influence of the precipitation pH on the physico-chemical properties of catalysts Specific surface area measurements corresponding to fresh Ni0.sCo0.sMoO4 catalysts prepared at different pH and the loss of surface area after 9 h of reaction are presented in Table 1. It can be observed that increasing the pH from 6.0 to 8.5 the surface area 2 . . . . progressively increases from 23 to around 103 m / g , indicating that the size of the catalyst particles decreases. After 9 h of reaction, all samples undergo a surface area loss. The samples prepared at pH 7.5 and 8.5 are the most affected by the reaction conditions.

1066 Table 1 Surface area of the sample prepared at different pH pH SBET Surface Area (m2/g) Loss of precipitation Fresh (%) pH=6.0 pH=7.5 pH=8.5

2'3 60 103

22 42 49

Chemical composition analysis showed changes in metal composition when the Ni0.sCo0.sMoO4 is prepared at different pH conditions. The results of Table 2 show the following trends when the precipitation pH increases from 6.0 to 8.5: i) for all samples, the amount of Co is higher than Ni, ii) Ni content increases strongly in the 6.0 -7.5 pH range while it remains almost constant between 7.5-8.5 pH, iii) Co content remains almost constant between 6.0 and 7.5 pH units while it increases strongly in the 7.5-8.5 pH range. A maximum Ni/Co ratio is reached for the sample prepared at pH=7.5, iv) Mo content decreases progressively and therefore, the Ni+Co /Mo atomic ratio increases gradually. The higher Co content with respect to Ni observed for all samples is consistent with thermodynamic data, which indicate that CoMoO4 can be more easily formed than NiMoO4 phase due to its higher negative enthalpy of formation (-1032 kJ/mol and -948 kJ/mol, respectively). Table 2 Chemical composition analysis corresponding to the different Nio.5Coo.5MoO4samples pH of Ni Co Mo Ni/Co Ni+Co/Mo precipitation wt. (%) wt. (%) wt. (%) pH=6.0 pH=7.5 pH=8.5

4.5 13.9 14.3

17.0 15.7 23.4

48.3 40.9 33.2

0.26 0.88 0.61

0.73 1.19 1.83

The strong increases of the Ni content observed when pH rises in the 6.0-7.5 range can be explained by the fact that the precipitation of Ni(OH)2 phase occurs at pH >_ 7.0 [8]. However, in this pH range, Co atoms are present as CoMoO4 or NixCoyMoO4-1ike phases. It is only at pH > 7.5 that Co(OH)2 species precipitates [8]. The latter explains the increase of Co content in the sample prepared at pH 8.5. It was observed that for the sample prepared at pH 6.0, the Ni+Co/Mo atomic ratio is lower than 1.0. This suggests that both Ni and Co are present in the catalyst under the form of NiMoO4, CoMoO4 or forming a Nio.21Coo.79MoO4 mixed phase (Ni/Co=0.26) and a part of Mo precipitates as a separate MoO3-1ike phase. The formation of MoO3 phase during the preparation of NiMoO4 or CoMoO4 compounds at pH < 7.0 has been observed by several authors [4, 9]. This phase can be formed from the [M07024]6- polymeric species, which are in chemical equilibrium with [MOO4]2- monomers at pH < 7.0 [10]. At pH > 7.5, the

1067 Ni+Co/Mo atomic ratio is higher than 1.0 which suggests the formation of separate NiO and Co304 phases from their respective metal hydroxides. Surface composition, as determined by XPS, is shown in Table 3. Table 3. Surface atomic ratio for fresh catalysts pH of Ni Co Mo +5 precipitation at. (%) at. (%) at. (%) pH=6.0 pH=7.5 pH=8.5

3.4 11.3 13.1

5.87 5.95 7.50

5.55 2.66 0.64

Mo +6 at. (%)

Ni/Co

Ni+Co/Mo

11.04 8.0 5.97

0.57 1.90 1.75

0.56 1.61 3.12

As a result of increasing the precipitation pH from 6.0 to 8.5 the following trends are observed: i) Nickel loading rises very strongly in the pH range of 6.0-7.5 followed by a slight increase in the pH interval 7.5-8.5, ii) Cobalt content shows a very slight increase in the pH interval of 6.0-7.5, followed by an important increase in the pH range of 7.5-8.5, iii) Molybdenum content (both Mo 5+ and Mo 6+ species) decreases gradually, indicating that at low pH, the catalyst surface is richer in Mo. The lower value of the ratio Ni+Co/Mo (pH 6.0) is in agreement with the presence of MOO3, and iv) Surface composition varies as follows: at pH-6.0: Mo >Co >Ni; at pH= 7.5: Mo ~ Ni > Co; at p H - 8.5: Ni > Co -_- Mo, indicating that, depending of the pH, the surface is preferentially enriched by Mo or Ni. As it can be seen, the relative XPS intensities among the different metals follow the same trends as shown by the chemical composition results. However, as XPS technique is sensitive to the superficial atomic layers, therefore the XPS surface composition values obtained do not exactly correspond to the bulk analysis results. Additionally, the samples present differences in specific surface area (Table 1), which affect the surface composition estimation. XPS spectra corresponding to fresh and used Ni-Co molybdate catalysts (Fig. 1) showed for all samples, the characteristic signals of Mo 6+ species (for example in catalyst prepared at pH 6.0:236,5 eV and 233,4 eV). In the case of the sample prepared at pH 6.0, the binding energy values at 235,7 eV and 232,5 eV suggest the presence of reduced Mo +5 species (which is already present before the catalytic test even if in smaller amount). It was reported in the literature that the reduction of MOO3, NiMoO4 and CoMoO4 could be performed at temperatures higher than 400~ in the presence of reducing agents (Ha or hydrocarbons). In our case, the reaction temperature was 450~ so the reduction of theses molybdenum phases can be favored. In the used catalysts, it can be observed that the amount of Mo 5+ species increases while Mo 6+ decreases. It has been observed that the most active catalyst, prepared at pH 8.5, is characterized by the lowest MoS§ and the highest Mo6+/Mo atomic ratio intensity, before catalytic test. Likely explanation for this highest activity observed is that a high oxidation state (Mo6*) favors the propane ODH reaction. The diffractograms of all Ni0.sCo0.sMoO4 samples are reported in Fig. 2.

1068

a

. ,.,-4

pH6.0 I .......

242.0

I

233,.5 Binding Energy (eV)

li

,,,

226,5

Fig. 1. Mo3d photoelectron spectra of Ni0.sCo0.sMoO4 samples (a) pH 6.0 used, (b) pH 6.0 fresh, (c) pH 8.5 fresh, (d) pH 8.5 used

9"

" l

u

....

I

. . . . . . .

I

.

.

.

.

22 24 26 28 30 32 34 36 38 40 20 (degree) Fig. 2. Comparison of X-ray diffraction patterns of fresh Ni0.sCo0.sMoO4 samples prepared at pH 6.0, 7.5 and 8.5

XRD pattern corresponding to Ni0.sCo0.sMoO4 sample prepared at pH 6.0 indicates characteristic lines of ~-CoMoO4 (3.50 and 3.366 ,~,) and ~-NiMoO4 (2.664 ,~). The catalyst prepared at pH 7.5 shows characteristic diffraction lines of 13-NiMoO4 (3.350, 3.801 and 2.654 ,~) and [3-CoMoO4 (3.271 ,~). The formation of NixCoyMozO4 solid solution is possible (Table 4). In the case of the catalyst prepared at pH 8.5, the application of Vegard's law leads us to conclude that this mixed molybdate is a solid solution of [3NiMoO4 and ~-CoMoO4 in a common molybdate structure. In addition, for this catalyst characteristic lines of ~-NiMoO4 and ~-CoMoO4 were observed. In spite that nickel and cobalt hydroxides precipitate at pH of 7.0 and 7.5 respectively, no signals corresponding to separated NiO and Co304 phases were observed. From the results of the chemical composition, we were able to calculate the theoretical amount of each phase in the catalysts. For instance, in the catalyst prepared at pH 7.5 we have calculated an amount of 4.9% NiO. A MoO3 phase was suggested by ICP in the samples prepared at pH 6.0 and confirmed by Raman spectroscopy. Table 4. X-ray diffraction Sample ~-NiMoO4 [7] ~-CoMoO4 [7] MoO3 [7] pH 6.0 pH= 7.5 pH= 8.5 vs, very strong.

patterns of Ni0.sCo0.sMoO4 and reference materials: d-values d-values 3.805 3.483 3.354 3.351vs 3.277 3.243 3.81 3.49 3.360vs 3.270 3.240 3.81 3.463 3.26vs 3.812 3.50 3.471 3.366vs 3.264 3.801 3.475 3.350vs 3.271 3.242 3.812 3.484 3.356vs 3.276

3.132 2.782 3.140 2.787 3.138 2.791 3.135 2.781 3.139 2.785

1069 The results of Raman spectroscopy, summarized in the Table 5, provide a clear evidence of the presence of MoO3 in the sample prepared at pH 6.0 (994, 664 cm -1 bands). The Raman spectrum presents characteristic bands of the molybdenum atoms tetrahedral coordinated (945, 935, 884, 823 cm -1) [11] as X-ray diffraction has previously indicated. Owing to the fact that the characteristic lines of ~-NiMoO4 and ~-CoMoO4 are very close, the Raman technique does not allow to give more evidence than X-ray diffraction concerning these crystallographic phases. Table 5. Raman band positions of Nio.sCoo.5MoO4samples and reference relative intensities -1 Sample cm [3-NiMoO4 945 935 884 823 365 [3-CoMoO4 943 935 882 818 363 MoO3pure 994s 821vs 664s 381m pH6.0 994s 940 927s 870m 821vs 664s 381m pH7.5 944 933vs 880s 815s 365 pH 8.5 927vs 870s 820s 361s vs, very strong; s, strong; m, medium; w, weak.

materials (cm -1) and

330 330 339m 339m 335 333s

281s 289s 155s 281s 289s 155m 277w 276w

Fig. 3. Conversion of propane vs. space-time over Ni0.sCo0.sMoO4 samples 3.2. Catalytic activity in propane oxydehydrogenation All tested catalysts showed a product distribution limited to propylene, carbon dioxide, carbon monoxide, traces of methane and ethylene. No oxygenated products such as aldehydes or acids were detected in the exhaust from the reactor.

1070 3.2.1. Influence of the space-time Figs. 3 and 4 show the effect of space-time (W/Fc3H8) on propane conversion and propylene selectivity over the tested set of catalysts. The selectivity of propylene decreases when propane conversion increases due to the consecutive transformation of the propylene formed to COx products. For a space-time of 0.074 g's/lamol, the catalyst prepared at pH 8.5 is the most active with 14.9% propane conversion, two times more active than the catalyst prepared at pH 7.5. Due to this enhancement in propane conversion, the selectivity to propylene decreases from 65% to 60.1%. In addition, it can be observed that at the space-time mentioned above, the propane conversion and propylene selectivity for the catalyst prepared at pH 7.5 and 6.0 are almost the same. Increasing the space-time to 0.22 g.s/~mol the catalyst prepared at pH 8.5 remains the most active with a 22.4% propane conversion, followed closely by the catalyst prepared at pH 6.0 with 19.5%. Selectivities to propylene for these two catalysts are almost identical. 3.2.2. Behavior of the catalysts tested at iso-conversion Owing to the fact that the conversion of propane varies inversely with selectivity to propylene, for comparative purposes, we have decided to test the whole series of catalysts at almost the same conversions. So, the catalysts were tested at low (near 7%) and high propane conversion (almost 20%). The activity results corresponding to the different NiCo-molybdate samples tested at high conversions are shown in Table 6. At similar propane conversion, (+ 20%), small differences in propylene selectivities and yields are observed for all samples. However, the catalyst prepared at pH 8.5 showed the highest productivity in propylene.

Fig. 5. Relative activities as a function of time on stream for Ni0.sCo0.sMoO4 catalysts

1071 Table 6. Conversion of propane, oxygen and selectivities in propylene, COx over Ni0.sCo0.sMoO4 samples pH of precipitation W W/Fc3H8 Conversion Selectivities C3H6 (%) (%) Productivity (gcat) (g-s/pmol) C3H8 pH=6.0 0.450 0.22 19.5 pH=7.5 0.880 0.43 20.3 pH=8.5 0.276 0.135 19.5 Feed composition: 10% vol. propane and 10% ml/min, T=450~

02 C3H6 63.1 42.3 67.5 40.0 67.6 44.4 vol. oxygen in He,

COx (lamol/g's) 53.2 0.37 60.3 0.19 59.2 0.64 total gas flow 30

The last aspect examined was the behavior of the catalysts as a function of time on stream. The loss of activity in time was quantified as relative activity i.e the current conversion divided by the initial conversion. As it can be observed from Fig. 5, the catalyst prepared at pH 7.5 has the most important loss of activity followed by the catalyst prepared at pH 8.5 and pH 6.0, respectively. This loss of activity in time can be correlated with the loss of the surface area observed after the catalytic tests. The catalysts prepared at pH 7.5 and 8.5 are characterized by the most important loss of surface area, 42% and 49%, respectively. This loss of surface could be explained by the reduction of the NiO and Co304, which occurs at 425~ [12] and 317~ [13], respectively. On the contrary, the absence of these metal oxides in the sample prepared at pH 6.0 can be responsible for the lesser losses of surface (22%) and activity found with this catalyst. In this last case, the loss of surface area can be due to the chemical reduction of MoO3 phase, which begins at 400~ 4. C O N C L U S I O N S Our work is a tentative to rationalize the influence of the preparation parameters over the composition, active phase and catalytic activity. Modifying the pH of precipitation between 6.0 and 8.5, the surface area, the bulk, the surface composition and the active phases of the catalysts were modified significantly. Raman spectroscopy has revealed the presence of MoO3 phase only in the catalyst prepared at pH 6.0. From the literature data, we know that the precipitation of the Ni(OH)2 and Co(OH)2 occurs at pH 7 and 7.5, respectively. At pH higher than 7.5 our calculations suggest the presence of nickel or/and cobalt oxides but these phases were not detected with any of the characterization techniques used. For comparison reasons, the catalysts were tested at the same values of conversion (near 7% and 20%). For a fixed value of conversion, the catalysts showed almost the same level of selectivity to propylene, but working at very different space-time. Therefore, it can be concluded that the preparation pH plays a very important role over the catalytic properties. This study illustrates that an optimum value of precipitation pH of 8.5 provides a minimum space-time to obtain an equally active and selective catalyst (Table 6).

1072 ACKNOWLEDGMENTS The financial support of the R6gion Wallonne (Belgium) in the flame of an 'Action de Recherche Concert6e' is gratefully acknowledged. The authors thank the Fonds National de la Recherche Scientifique (Belgium) for financing the laboratory equipment used in this research. Dr. R. Prada Silvy and Dr. P. Ruiz (Unit6 de catalyse et chimie des mat6riaux divis6s, Universit6 catholique de Louvain) are thanked for fruitful discussions of structureactivity relationships. REFERENCES

1. P.M. Michalakos, M. C. Kung, I. Jahan and H. H. Kung, J. Catal., 140 (1993) 226. 2. G. Bellussi, G. Centi, S Perathoner and F. Trifir6, in Catalytic Selective Oxidation, American Chemical Society, Washington, DC, 1992, 281. 3. C. Mazzocchia, C. Aboumrad, C. Diagne, E. Tempesti, J. M. Herrmann and G. Thomas, Catal. Lett., 10 (1991) 181. 4. Y. -S. Yoon, N. Fujikawa, W. Ueda, Y. Moro-oka and K. -W Lee, Catal. Today, 24 (1995) 327. 5. D. L Stern and R. K. Grasselli, J. Catal., 167 (1997) 550. 6. D. L Stern and R. K. Grasselli, J. Catal., 167 (1997) 560. 7. JCPDS -International Centre for Diffraction Data, (1999). 8. Chariot G., Les r6actions chimiques en solution aqueuse et caract6risation des ions, 7~me 6d., Masson, Paris (1983). 9. U. Ozkan and G. L. Schrader, J. Catal., 95 (1985) 120. 10. J.-P. Jolivet, De la solution h roxyde; Collection Savoirs Actuels, Inter6ditions/CNRS 6ditions, Paris (1994). 11. S. Sheik Saleem, Infrared Phys., 27 (1987), 309. 12. J. L. Brito and J. Laine, J. Mat. Sci., 24 (1989) 425. 13. P. Arnoldy and J. A. Moulijn, J. Catal., 93 (1985) 38.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1073

Oxidized d i a m o n d as a new catalyst support Toshimitsu Suzuki a, Kiyoharu Nakagawa a'b, Na-oki Ikenaga a, Toshihiro Ando b a Department of Chemical Engineering, Kansai University, 3-3-35 Yamate, Suita, Osaka 564-8680, Japan b Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Corporation (JST) and National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan 1. I N T R O D U C T I O N Diamond has long been regarded as an inert material for chemical reactions. Recently, one of the present authors elucidated that the surface of diamond can be modified by chemical reactions, such as hydrogenation, oxidation, chlorination, etc. [1]. Activated carbon, composed of sp a carbon atoms, has been utilized as a support material for catalyst. However, no carbon materials consisting of sp 3 carbon atoms have been used as catalyst supports. Silicon oxide (or silica) is also a very popular support material to catalysts. Carbon belongs to the same group of elements as silicone, but no carbon oxide solid phase exists. Oxidized diamond (Fig. l b) can be prepared by oxidation of hydrogen treated diamond (900 ~ with hydrogen gas; Fig. l a) with molecular oxygen at 450~ and it contains functional C-O-C, C-OH, and C=O groups on the top surface of solid diamond. This can be regarded as pseudo carbon oxide solid phase. We have first proposed the use of oxidized diamond as a support to Cr203 for the dehydrogenation of ethane, and found that oxidized diamond behaved as an excellent support for the dehydrogenation of ethane in the presence of carbon dioxide [2]. This paper deals with the detailed studies on oxidized diamond-supported catalyst in the dehydrogenation of ethane and partial oxidation of methane with oxygen. Scope and limitation of the Fig.1 Hydrogenated (above) and new support material is exploited, oxidized (below) diamonds 2. E X P E R I M E N T A L Commercial powdered diamond (<0.5 9m) was washed with HNO3. Dried diamond was treated with hydrogen flow at 900 ~ for lh, and oxidized in Ar and O2 (4:1) flow for 4h. Several metal salts in aqueous solution were deposited on the oxidized diamond according to a usual impregnation method. After drying under reduced pressure, the solids were calcined at 400 ~ for 4 h. Oxidized diamond exhibited IR absorptions at 1800 and

1074 1100-1400 cm-l,which are evidence of C=O, and C-O-C functional groups. Catalytic reactions were carried out using a micro fixed bed flow type reactor. 60 to 400 mg of oxidized diamond-supported catalyst was used. The effluent was analyzed by an on-line micro gas chromatograph. 3. RESULTS AND DISCUSSION 3.1 Dehydrogenation of alkanes with oxidized diamond supported Cr203 and V205 catalysts. Several reactions were tested with oxidized diamond-supported metal oxide catalysts. Dehydrogenation of ethane was carried out in both ethane and CO2 mixed gas and ethane and Ar flow. The results are shown in Fig. 2. C2H6 < C2H6 + CO2

> C2H4 + H2 > C2H4 +

H20

Oxidized

(1) (2)

+ CO

diamond

I

7..-.,.%,~.~ Cr203 loaded catalysts (12.6) -I exhibited high activity, and Zn (15.6) ethylene yield of 22 % in ~-,_-,.v,.,t F e (15.9) the presence of CO2 was i observed at 650 ~ The Mn (18.8) _ [] Ar ethylene yield decreased to C e (17.8) [] C0z 6% in an Ar atmosphere. Very high promoting effect _ Ga (11.6) of carbon dioxide is the Mo (11.7) characteristic feature of / V (10,9) Cr203 loaded-diamond _ oxide catalyst. VzO5 loaded 7..-.,:~_x_x-~:.,:.o,_x:~ x "~l'~.xx_x:~--,:-.:.,:-,_x%%xxxxx%x%xxxxxxxxx~ C r (12.0) catalyst showed moderate none dehydrogenation activity, I I I I but only a small promoting 0 5 10 15 20 effect of CO2 was observed. R2 H 4 y i e l d (%) GaaO3-10aded on TiOz Fig. 2. Dehydrogenation of C2H6 in the presence of exhibited the highest CO2 or Ar activity in dehydrogenation Temperature: 650 ~ Reaction time: 0.5 h; C2H6" of ethane, but a moderate CO2(Ar) = 5:25 (mL/mL); Catalyst = 200 mg; SV = activity was observed when 9000 h mL/g-cat; ( ): surface area (m~/g); M it was loaded on oxidized oxidized diamond = 5:95 (wt%) diamond. These results indicate that promoting effect of CO2 on the dehydrogenation reaction does not seem to be general to the oxide used, and specific to the combination of loaded oxides and support materials. Fig. 3 shows the temperature dependence of ethane dehydrogenation in the presence or absence of COa. Ethylene yield in uncatalyzed runs did not depend on the atmosphere. Over oxidized diamond-supported Cr203 catalyst, in the presence of CO2, the ethylene yield increased linearly with an increase of the reaction temperature, and reached 40% at 700 ~ In contrast, the ethylene yield decreased with increasing reaction temperature in the absence of CO2 and above 675~ it increased again with an increase of the reaction

1075 temperature, similarly to the noncatalyzed run. This inverse relation between reaction temperature and yield of ethylene seems to be due to the rapid deactivation of the catalyst in the absence of COz. In the presence of COz, CO was produced during the dehydrogenation of ethane over the oxidized diamond-supported CrzO3 catalyst. The effect of Cr203 loading of oxidized diamond was examined, and it was found that 2.5 wt% of Cr203 afforded the highest ethylene yield of 23.8% at 650 ~ The yields of ethylene were not significantly increased for loadings between 2.5 and 7.5 wt%. 40

35

35

100

30_o

75

>" 25-

~ 2

o

~-20-

~ o 0

50_~

20~

15 10-

A

X

15-

.~ 10-

L.~

5-

5 0 600

625

650

Temperature

I

I

675

700

(*C)

Fig. 3. Effect of temperature on the conversion of Call6 over Cr (2.5 wt%)/oxidized diamond catalyst Reaction time: 0.5 h; C2H6:CO2 (Ar) = 5 : 25 (n'~L/mL); Catalyst = 200 mg; SV = 9000 h-" mL/g-cat

0

0

100

200

300

9

C2H 4 yield

9

C2H 6 conversion

9

C2H 4 selectivity

400

500

600

N

o

700

Partial pressure of C02/mmHg

Fig. 4. Effect of partial pressure of C O 2 on the Call6 conversion. CzHG4 yield and Call4 selectivity over Cr(2.5 wt%)/oxidized diamond. Temperature" 650 ~ Reaction time: 0.5 h; Catalyst: 200mg; S V - 9000 h -1 mL/min; C2H6 95 mL/min; Total flow rate = 30 mL/min

The effect of the partial pressure of carbon dioxide on the ethane dehydrogenation over oxidized diamond-supported CrzO3 catalyst was examined. The results are shown in Fig. 4. Ethane conversion and ethylene yield markedly increased at a low partial pressure of COz, and increased with increasing partial pressure. However, the ethylene selectivity slightly decreased with increasing partial pressure. The following reaction with COz, in the dehydrogenation of ethane, would be possible: CO2 + Hz < CO2 + C <

> CO + H20 > 2CO

(3) (4)

In the dehydrogenation of ethane, CO and H20 were produced under COz. Thus, one possible reason for the promoting effects is to shift the dehydrogenation-hydrogenation equilibrium by removing H2 from the products. However, this contribution does not seem to be significant, since ethane conversion is below equilibrium at the temperature examined. As mentioned above, the removal or suppression of carbon deposition seems to

1076 be the important roles of COz To investigate amount and type of deposited carbon, catalysts after the reaction were examined with Raman spectroscopy. The Raman spectra of used oxidized diamondsupported Cr203 catalysts in the presence and absence of CO2 were recorded. Two bands were observed in the spectra of carbon deposited both in the presence and absence of COz. One appeared at around 1350 cm -1 and the other at 1600 cm -1. The intensities of these absorptions in the spectra of the catalyst tested in the absence of COz were much stronger than those of the catalysts tested in the presence of CO2. This indicates that the amount of carbon deposited in the absence of CO2 was larger than that in the presence of CO2. X-ray photoelectron spectra of fresh and used oxidized diamond-supported CrzO3 catalyst showed that for the catalyst dehydrogenated under Ar atmosphere, the binding energy of Cr 2p2/3 shifted at lower value. The catalyst used in the presence of COz, on the contrary, shifted to higher binding energy. This clearly indicates that CO2 played a role of oxidant in the dehydrogenation of ethane. From these results, several small but significant promoting effects of COz synergetically activated CrzO3-1oaded oxidized diamond catalyst. These results indicate that the CO2-promoted dehydrogenation could be a kind of oxidative dehydrogenation. 3.3 Dehydrogenation of propane and i-butane The same catalyst was used for the dehydrogenation of propane and i-butane, in both COz and Ar atmosphere. The results are shown in Table 1. As it is seen, dehydrogenation of propane required a higher temperature than that of ethane, and the yield of propylene was considerably lower than that of ethane. In the dehydrogenation of ethane, CrzO3-10aded catalyst exhibited much higher activity than VzOs-loaded catalyst. However, in the dehydrogenation of propane, the differences between the VzOs-loaded and CrzO3-10aded catalyst became small. The promoting effect of CO2 was observed in both catalysts. Table 1. Dehydrogenation of propane and butane using oxidized diamond supported catalysts Run 1 2 3 4

alkane

catalyst

Temp

propane

Cr203 600

600

5 6 7 8

9 10

Vz05 i-butane

Cr203

V205

COz 650 650 650 650 600 600 600 600

atmosphere yield of alkene ~ % Ar 4.3 7.8 Ar 6.5 COz 13.6 Ar 5.8 CO2 10.7 Ar 12.0 COz 10.7 Ar 10.8 COz 13.8

Catalyst 400 mg, SV= 4500 h1 mL/g-catalyst, alkane : COz(Ar) = 5 : 25 (mL/mL) oxide loading level 5 wt % In the dehydrogenation of i-butane, a higher yield of i-butene was obtained at 600 ~ compared to the dehydrogenation of propane. In this case, the oxidized diamondloaded Cr203 catalyst did not exhibit a higher yield of i-butene in COz. VzOs-loaded oxidized diamond showed slightly higher yield of i-butene in the presence of CO2. The promoting effect of CO2 decreased with an increase of the carbon number of the alkane

1077

together with the selectivity to alkene. Differences in the dehydrogenation activity in the different alkanes are difficult to understand at present. Both propylene and butene contain allylic hydrogen, which have much smaller bond dissociation energy than the C-H bond dissociation energy of the parent alkanes. Thus, hydrogen abstraction from allylic position may easily occur compared to the parent alkanes. As a result, the conversion of alkane decreased. Also, the possible formation of surface n-allylic complex cannot be ruled out, for the low conversion of higher alkanes. 3.4 Partial oxidation of methane to synthesis gas with oxidized diamond loaded Ni catalyst As shown above, oxidized diamond exhibited considerable activity in the oxidative dehydrogenation of alkanes, hence further studies on the oxidized diamond supported catalysts were exploited. Nickel-loaded alumina is generally used for the partial oxidation of methane (reaction 5). However, carbon deposition onto the nickel is the major problem in the commercialization of this process. CH4 + 1/2 02

>

CO + 2H2

(5)

We have previously shown that Ir-loaded TiO2 exhibited excellent catalytic activity without carbon deposition in a prolonged run [3]. The reaction was proposed to proceed through a two-step process, that is complete oxidation of methane to give CO2 and H20, followed by reforming reactions. At a lower temperature of 600 ~ Ni-A1203 catalyst is not active. The use of oxidized diamond-supported catalyst was examined under oxidative conditions. Diamond is believed to be inert to chemical reactions. However, in the presence of oxygen diamond can be oxidized to carbon dioxide. If the oxidized diamond support could be utilized under oxidative conditions, application of diamond support could be expanded to many catalytic reactions. Fig. 5 shows the effect of various supports of nickel-loaded catalysts and reaction temperature on the methane conversion, in the partial oxidation of methane. At methane to oxygen ratio of 5 : 1, the maximum conversion of methane is 40 %, when reaction (5) proceeded, and 10% when complete oxidation proceeded. Only the oxidized diamondsupported Ni catalyst exceeded 10% conversion above 550 ~ indicating that the synthesis gas formation proceeded. Ni-loaded La203 catalyst afforded considerable methane conversion above 450 ~ but the product is mainly CO2. Other supports to nickel showed no or only slight catalytic activity in the partial oxidation of methane. These results clearly show that oxidized diamond has excellent properties in the partial oxidation of methane at a low temperature, giving synthesis gas. Fig. 6 shows the effect of temperature on the product distribution, in the partial oxidation of methane. Above 550 ~ Ha and CO were produced, and below 500 ~ only complete oxidation occurred. The H2 to CO ratio should be 2 according to the stoichiometry. However, 3.2 and 2.8 were obtained at 550 and 600 ~ respectively. In the course of the reaction, catalytic activity of Ni-loaded oxidized diamond catalyst slightly decreased (Fig. 7), giving a small amount of carbon at 600 ~ However, at 700 "(2, carbon deposition was not observed during the reaction and H2 to CO ratio reached nearly 2.0. Changes in the mechanism of the reaction seem to be considered: above 700 ~ the reaction seems to proceed via a two-step path, where complete oxidation occurs to give H20 and CO2. Thus, carbon deposition would be eliminated. On the other hand at 600 ~ the first step is methane decomposition to carbon and Ha, followed by the oxidation of carbon on the nickel.

1078

Fig. 5. Effect conversion over Catalyst: 0.06 (CH4/O2 = 5.0);

of temperature on the CH4 Ni-loaded catalysts. g; Flow rate: 30.0 mL/min Ni loading level = 5 wt%.

Fig. 6. Effect of temperature on the product distribution over Ni(5wt%) /oxidized diamond catalyst. CH4:O2 = 5:25 mL/mL; S V - 30000 h -] NI (21)3/2) : 853 eV I

11111

90 ODC o

..

O

O

O

O

C

,;,

~.

>9 80

8 7o 0 0

8

6O

c

411

~C3 0 -'q .2 w '- 20

O.

C

o o 10

I

0

I

0 0

1

" I

2

o 700"CI

600"C I

3

I

4

I

5

I

6

I

7

I

8

I

9

10

Tlme-on-stream / h

Fig. 7. Effect of time-on-stream on the CH4 conversion and CO selectivity over Ni (3 wt%)/O, diamond, SV = 30000 h -] mL/g-catalyst; CH4:Oz = 5:1; Catalyst: 60 mg

856

854 852 Binding Energy (eV)

850

Fig. 8. XPS spectra of Ni 2p3/2 on Ni/O-diamond catalyst. a) fresh catalyst; b) catalyst after reaction with CH4/Oz at 873 K for 2h; c) Hz reduced catalyst at 873 K for lh

1079 X-ray photoelectron spectra of Ni-loaded oxidized diamond catalysts were examined, and spectra are shown in Fig. 8. The fresh catalyst exhibited Ni 2p3/2 peaks at 853.1 and 854.5 eV. When this catalyst was used for the partial oxidation at 600 ~ for 1 h, the peak at 853.1 eV shifted to a lower binding energy, 852.7 eV and its relative intensity increased considerably, with a decrease of the relative intenSitoY of the peak at higher binding energy. When the fresh catalyst was reduced with Ha at 600 C, a single peak appeared at 851.6 eV, due to formation of metallic nickel. These results indicate that the catalytically active species is a slightly reduced nickel oxide on oxidized diamond. Nickel oxide loaded on oxidized diamond can easily be reduced with methane at a temperature above 550 ~ to form active species to give synthesis gas. These results show that on oxidized diamond, the interaction of nickel oxide and the support is weak, compared to other support such as alumina or titania. Such a weak interaction between support and loaded metal or metal oxide is one of the most characteristic feature of oxidized diamond support. REFERENCES 1. T. Ando, K. Yamamoto, M. Ishii, M. Kamo and Y. Sato, J. Chem. Soc. Faraday Trans., 89 (1993) 3635. 2. K. Nakagawa, C. Kajita, N. Ikenaga, T. Kobayashi, M. N-Gamo, T. Ando and T. Suzuki, Chem. Lett., (2000) 1100. 3. K. Nakagawa, C. Kajita, N. Ikenaga, T. Kobayashi and T. Suzuki, Appl. Catal., A, 169 (1998) 281. 4. K. Nakagawa, C. Kajita, K. Okumura, N. Ikenaga, M. Nishitani-Gamo, T. Ando, T. Kobayashi and T. Suzuki, J. Catal, 203 (2001) 87.

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Studies in Surface Science and Catalysis143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1081

Preparation of catalytic membranes, micro-capsules and fabrics active in immobilized Fenton chemistry J. Fernandez, V. Nadtochenko, A. Bozzi, T. Yuranova and J. Kiwi Laboratory of Photonics and Interfaces, Department of Chemistry, Swiss Federal Institute of Technology, Lausanne 105, Switzerland.

New and up to-date materials have been developed in our laboratory to carry out Fenton photoassisted processes efficient in the decoloration/degradation of organic pollutants that show: stability against radical attack, do not allow the leaching of Fe-ions into the solution and intervene with suitable kinetics in the degradation processes. These new materials comprise Feions supported on: Nation membranes, Nation-glass mats composites, alginate beads, amorphous polycrystalline fused copolymers and silica woven fabrics. 1. INTRODUCTION Suitable H202 decomposition kinetics with meaningful yields of .OH-radicals (eq(1)) were recently attained in photo-assisted Fenton reaction[l]. Fe-sludge disposal and/or regeneration after the Fenton reactions is a serious problem during Fenton photo-assisted processes in homogeneous media since in large volumes, it implies the use of important amounts of chemicals and manpower. In order to overcome this problem, we undertook the present study. This new approach will be applied to a non-biodegradable azo-dye Orange II as a model compound found commonly in the effluents of the textile industry. In the search for suitable supports for photoassisted Fenton processes we have examined a variety of suitable supported Fe-systems as reported below in this write up. 2. EXPERIMENTAL The preparation of the supported catalytic Fe-clusters have been recently reported by our laboratory for Fe/Nafion membranes [1,2,3]; for Fe/Nafion/glass-mats [4,5]; micro-encapsulated Fe-alginate beads [6]; Fe-amorphous polycrystalline thin film fused copolymers [7] and finally Fe/silica woven fabrics [8]. Experiments were conducted with Nation perfluorinated cation transfer membrane. Photolysis experiments were carried out by means of a Hanau Suntest Lamp with tunable light intensity equipped with an IR filter to remove infrared radiation. Light

1082

irradiation reached the Nafion/Fe membrane which was positioned immediately behind the wall of the reaction vessel (60 ml volume containing 40 ml solutions) as the only light absorber in this two phase system. The detection of Orange II in solution was carried out via an HPLC (Varian 9065 Diode Array) provided with a Phenomenex C-18 inverse phase column. The azo-dye peak was detected at )~=486 nm with a retention time of 11.1 min. The decrease in the total organic carbon was monitored with a Shimadzu 500 instrument. The characterization of the supported clusters was carried out by UV-Vis, FTIR, Raman and M6ssbauer spectroscopies. EM was employed to determine the size of the Fe clusters and XPS, the degree of Fe-oxidation before and after reaction. 3. RESULTS AND DISCUSSION 3.1 Degradation of Orange II on catalytic Fe/Nafion membranes. Static and reactor processes Photo-assisted degradation of non-biodegradable Orange II is shown to be catalyzed by Nation cation-transfer membranes exchanged with Fe-ions in the presence of H202 [1]. The Nation membranes have sulfonic groups in their inside cavities and the Fe3+-ions ion-exchanged with the H + of the sulfonic groups were found to attach strongly due to the 2+ electrostatic excess charge of the encounter-pair. The Nation membranes in the oxidative media degrade Orange II (taken as a model pollutant) with a similar kinetics as found in the case of the homogeneous Fe3+/H202 photo-assisted processes. The immobilization of Fe3+-ions on membranes avoids the drawbacks of the homogeneous treatment such as sludge formation and the separation of the sludge at the end of the treatment. The activity of the membranes during the Orange II decomposition was tested for 1500 hours and remained fairly stable within this period. The Fe/Nafion membranes consisted initially of Fe(HI) 78% composites (BE 710.7 eV) and Fe(II) 22% composites (BE 709.6 eV). After degradation of Orange II the Fe(II) composites in the nation were seen to increase to 86% with concomitant decoloration of the membrane since Fe(II) species are not colored as found by X-ray photoelectron spectroscopy (XPS)). The size of the Feparticles in the Nation was investigated by transmission electron microscopy (TEM) and turned out to be 37+4 ,~ in agreement with the dimensions of 40-60 A reported by Gierke for Nation cavities [2]. These Q-sized Fe-particles on the Nation also absorbed directly the light energy during the degradation of colorless long lived intermediates after the initial Orange II had disappeared. A simplified reaction mechanism for Orange 1I decomposition is suggested below in Fig. 1 consistent with the experimental findings for solutions up to pH 4.8. T h e recycling on the Fe3+/2+/Nafion membrane involve the reactions: Nafion/Fe(II) + H202 ......... > Nafion/Fe(III) + .OH + OH-

(1)

and probably not reaction (2) as the main pathway for Orange II degradation Nafion/Fe(III) + H202 ........ > Nafion/Fe(II) + HO2" + H +

(2)

1083 since during Orange II degradation the solution pH was seen to increase from 2.8 to ~ 3.2. This corresponds to an increase by a factor of --4 for the amount of OH- radicals in the solution. That the Orange II degradation occurs mainly through the step shown by eq(1) is substantiated further by three observations: a) methanol (0.26 M) precluded the abatement of the azo-dye observed in Fig. 1 due to its .OH radical scavenging properties, b) the shift in absorption in the Nafion/Fe membrane during the dye abatement process from red-brown to beige due to the formation of FeZ+-ions and c) if the superoxide radical HO2. (pKa= 4.8) would be responsible for Orange II degradation eq(2), this is improbable since this radical has a considerably lower one electron standard reduction potential (HOz/O2.-) of E~ V vs NHE than the OH radical (OH/OH-) E~ V vs NHE. The fast oxidation of Orange II cannot be accounted in terms of this kinetically slower and less energetically HO2 radical.

Orange II

~

(OrangeII-OH)(oxidation)

H+OH ._o Z o

h'v

._.o O

-~

~

H202

Fig. 1. Degradation mechanism of Orange II on Nafion/Fe membranes under light.

In photo-reactors provided with a Fe/Nafion membrane, the degradation up to bio-compatibility of Orange II proceeds at an acceptable kinetic rate only up to pH 4.7 [3]. The long-lived intermediates of the degradation were observed to be bio-degradable compared to Orange II which is not bio-degradable.

1084

3.2 Fe/Nafion/glass-mats as heterogeneous photocatalysts for Orange II abatement. The azo-dye Orange II has been taken as a model azo-dye to monitor the decoloration and degradation in photo-Fenton reactions on Nafion/Fe 3+ membranes [1]. These processes were possible up to pH 4.6. More recently [4], Fe-ions were exchanged on Nation droplets that were later attached to glass-mats. The Nation oligomer containing the Fe-ions achieved a suitable dispersion on the glass-mats and the degradation of the model azo-dye was possible up to pH 8. Harmer et al. [5] have shown that acid groups of the Nation are more accessible in Nation-glass composites. The accessibility of these sulfonic groups leads to an improved ion-exchange capacity at the interface between Nation and the glass-mat. Therefore, different Fe-clusters may be present on the Nation-glass composites compared to Nafion/Fe 3+ membranes. The decoloration / degradation of Orange II proceeded at a rate close to the one observed on Nafion/Fe 3+ [1] and the catalyst could be recycled without loosing his efficiency many times showing the stable nature of the process under study. The recycling of the catalyst is shown in Fig. 2 below

Fig. 2. Mechanism of Orange II photocatalytic degradation mediated by Fe/Nafion/glass mats under visible light irradiation during the degradation of Orange 1I.

1085 Mineralization of Orange II under Suntest irradiation on Fe3+/Nafion was observed to proceed up to pH 4.8, attaining -~50% mineralization of the Orange II [1]. On Fe3+/Nafion/glass mats, the discoloration was observed in the pH range 2.8-9.0. Acidification is needed to treat many effluents from the textile industry which have an inherent pH value of 6-9 via Fenton reactions. But a pH drop during the degradation of Orange H was observed when starting at pH values close to 8.0 when using the Fe3+/Nafion/glass mats due to two factors: a) mineralization of Orange II produces HSO4- and predominantly NH4+ [1-4] acidifying the solution and b) intermediate Fecomplexes consisting of Fe-chelates are formed leading to carboxylic acids during the degradation of Orange II. Oxalic, formic and acetic and smaller concentrations of other acids have been reported during the mineralization of Orange II. The pH of the solution at the end of the degradation corresponds to the pKa of the short carboxylic acids already mentioned The degradation of Orange II involves the formation of Fe-organic complexes is noted as [RCOO-Fe] 2+ + hv ~ [R] + CO2 + Fe 2+

(3)

where the LMCT complex in eq(3) is the precursor step leading to the abatement of Orange II. Eq(3) proceeds concomitantly to the radical reaction contributing to the organic compound degradation RH + HO ~ (HO2~ ---, [R] + RHOH or 1-120

(4)

3.3 Evidence for Fenton photo-assisted processes mediated by encapsulated Fe-ions at biocompatible pH-values. Fe-alginates in aqueous solutions are capable of degrading organic compounds at biocompatible pH values [6]. This material consists of encapsulated Fe-cross-linked alginate. Alginic acid is a naturally occurring polysaccharide that will crosslink in the presence of di- and trivalent cations. The preparation of the Fe-alginate was carried out starting from alginic acid sodium salt (Sigma). A 3% (w/v) solution of sodium alginate in distilled water was prepared one hour before use. Elementary analysis of the alginate beads revealed an Fe-concentration of 8.5x 10-2 M and a Na-concentration of 8.2xl 0-2 M. This indicates that alginate chains are cross-linked by Fe3+-ions and that each Fe 3§ replaces about two Na +. During the preparation of Fe-alginate, the Fe3+-ions exchange significantly with the Na+-ions of the functional carboxylic groups of the alginate strands. The decoloration of Orange II by Fenton and photo-Fenton reactions on Fe-alginate proceeds in the presence of H202 under visible light irradiation at initial pH values of 5.6 and 7.8. With Fe-alginate as an oxidative catalyst at pH 7.8 a time of degradation was observed for Orange II (0.1 mM) close to the time observed in homogeneous media at pH values < 4.5 in the presence of 1-1202 (4.85 mM) and the same Fe3+-ion concentration as in Fe-alginate. No pH adjustment is necessary to proceed with subsequent biological treatment starting from pH 6-8.

1086 Further aliquots of Orange II were added adjusting each time the initial pH to a value of 7 or 8. The results for the decoloration were catalytic over many cycles. The amount of Fe(III) in solution was analyzed by complexation with thiocyanate but no Fe(III) could be detected. No Fe(II) could be detected adding o-phenanthroline. By EM, darker more dense Fe-species are seen at the border of the Fe-alginate beads. Also Fe-clusters were seen to be scattered in the bulk of the alginate beads. These particles do not present sharp edges reflecting metal character and these clusters were observed in the range of 0.5 nm. From these two preceding observations it can be suggested that Fe-clusters are present as Feoxy-hydroxides since metallic character begins to appear >2 nm for Fe-particles. 3.4 Modified block-copolymers thin films loaded as innovative catalysts. . . . . More recently [7], Fe 3 + , Fe203 and T102 have been lmmoblhzed on low cost polyethylene@ modified copolymers films containing maleic anhydride anchoring groups. The observed rates of degradation of Orange II and halocarbons were only slightly below the rates observed during homogeneous Fenton photo-assisted degradation or with TiO2 suspensions. Polyethylene is known to be the second most inert Dupont polymer after Teflon, a C-F polymer of widespread use. IR spectroscopy suggests that Fe 3+/Fe203 interacts with the negatively charged conjugated carboxylic groups through -COO-Fe 3+. In the case of the copolymer-Fe3+, the bands in the IR spectrum at 1523 and-1557 cm "1 correspond to two types of iron-carboxyl species. The strong attraction between the negative anhydride groups and the Fea+-ion leads to formation of metal to ligand transfer charge bond (MLTC) between the negatively anhydride group and the Fe-ion on the copolymer thin films. This accounts for the lack of leaching of metal ions into the solution. It is also the reason for the attachment of a one layer of complexed species on the thin film surface that will not come out of the polymer surface due to mechanical factors. During the last decade it has been repeatedly invoked that homogenous Fenton processes involve three reactive intermediate species: a) the formation of oxidative radicals like OH ~ HO2~ and other oxidative radicals due to the H202 added in solution b) the possible formation but controversial solution species (Fe(IV) and finally c) Fe-chelates intermediates like oxalates, maleates, pyruvates that decompose to CO2 with accelerate kinetics in Fenton photo-assisted reactions and with slow kinetics in dark reactions. Fig. 3 shows the mechanism of the reaction intermediates in the dark and under for many organic compounds The importance of the [R-COO] as a preferred reaction pathway in the decomposition of pollutants on the surface [polyethylene-COO--Fe3§ is rationalized by the observation that these copolymer thin films should be attacked by OH ~ HO2~ and other oxidative radicals available in solution. But this was not the case for [polyethylene-COO-Fe 3§ even when used over long times (300 hours) and repeated recycling below the polyethylene flowing temperature (80~ Moreover, the observation reported recently [8] that degradation of organic compounds is able to take place during Fenton photo-assisted treatment in the presence of 3000 ppm of Cl-ion

1087

Fig. 3 3.5 T r e a t m e n t of azo-dyes and textile effluents by immobilized Fenton photo-assisted reactions on inorganic silica membranes. Fe-clusters on silica fabrics have been prepared in our laboratory | [9] and used in the degradation of textile waters and waters of the SMDK treating station for toxic industrial waters in K611iken, Switzerland. The pH of the waste waters as received was in the range 7-9. Treatment with HzO2 without pH adjustment in the dark was ineffective in the reactor. Under UV light irradiation the TOC was seen to decrease by less than 5% with the latter treatment. But in the presence of silica/Fe fabric under 254 nm (75 W) light irradiation, the reflecting immersion concentric photo-reactor decreased the initial COD values of the textile waste waters with pH 9 by about 75% in less than 25 minutes. More surprisingly, compared with homogeneous photo-Fenton processes the degradation was the same in terms of COD reduction. But the BOD increase when using the silica/Fe fabric was more significant than in the case of the homogeneous photo-Fenton treatment. Adsorption of toxic organic compounds on from the waste waters was occurring on the Fe-silica fabrics and turned out to be beneficial in the latter case.

1088 ACKNOWLEDGMENT The financial support of KTI/CTI TOP NANO 21 (Bern, Switzerland) under Grant N ~ 5320.1 TNS is appreciated.

REFERENCES

1. J. Fernandez, J. Bandara, A. Lopez, Ph. Buffat and J. Kiwi, Langmuir, 15 (1999) 185. 2. T. Gierke, G. Munn and F. Wilson. J. Polym. Sci. Polym Phys., 19 (1981) 1687. 3. A. Lopez and J. Kiwi, J. Eng. Ind. Chem. Res., 40 (2001) 1852. 4. M. Dhananjeyan, J. Kiwi. P. Albers and O. Enea, Helv. Chim. Acta, 84 (2001) 3433. 5. A. Harmer, E. Farneth and Q. Sun, J. Amer. Chem. Soc., 118 (1996) 7708. 6. J. Fernandez, M. Dhananjeyan, J. Kiwi, Y. Senuma and J. Hilbom, J. Phys. Chem. B, 104 (2000) 5298. 7. M. Dhananjeyan, E. Mielczarski, K. Thampi, Ph. Buffat, M. Bensimon, A. Kulik, J. Mielczarski and J. Kiwi, J. Phys. Chem. B, 105 (2001) 12046. 8. J. Kiwi, A. Lopez and V. Nadtochenko, Environ. Sci. Technol., 34 (2000) 2162. 9. A. Bozzi, T. Yuranova, P. Lais and J. Kiwi, article in preparation 2002.

Studiesin SurfaceScienceand Catalysis 143 E. Gaigneauxet al. (Editors) 9 2002 ElsevierScienceB.V. All rightsreserved.

1089

Preparation of vanadium-based catalysts for selective catalytic reduction of nitrogen oxides using titania supports chemically modified with organosilanes H. Kominami, a M. Itonaga, a A. Shinonaga, a K. Kagawa, b S. Konishi, a and Y. Kera a aDepartment of Applied Chemistry, Faculty of Science and Engineering, Kinki University, Kowakae, Higashiosaka, Osaka 577-8502, Japan bTechnical Research Center, The Kansai Electric Power Co. Inc., Nakoji, Amagasaki, Hyogo 661-0974, Japan A titania (TiO2) sample with a large surface area (300 mZg-1) was chemically modified with 3-aminopropyltrimethoxysilane (APS) under a reflux of toluene. The thermal stability of the modified TiOz (APS-TiO2) and the adsorptivity of metavanadate anion (VO3-) on APS-TiO2 from an aqueous solution were investigated. Modification with APS suppressed crystal growth and transformation of anatase crystallite to rutile upon calcination, and the anatase phase was preserved even after calcination at 1000~ while transformation to rutile in the unmodified TiOz samples was observed at around 800~ Since there was little crystal growth in APS-TiO2, it possessed a large surface area of 205 mZg-1 after calcination at 700~ The amount of VO3- adsorbed on APS-TiO2 was ca. 1.5-times larger than that on unmodified TiO2 due to a strong affinity caused by acid-base interaction between VO3-and the amino group of APS-TiO2. A supported V205 catalyst that was prepared by decomposition of a VO3--adsorbing APS-TiOz sample had a large surface area despite the large V205 loading and exhibited higher activity than that of a catalyst prepared from an unmodified TiO2 sample. 1. I N T R O D U C T I O N Vanadia/titania (VzOs/TiO2) catalysts are widely used as commercial environmental catalyst for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia [1-3]. Due to current strict regulations, a high level of NOx removal is now required, and one of the most effective approaches for the removal of a large amount of NOx is to use a TiO2 support that has a large surface area. Generally, a large surface area is required for a catalyst support to disperse a catalyst material effectively and increase the number of active sites in the catalyst. Since a catalyst is usually used at high temperature, a high degree of thermal stability is also important. We have shown that chemical modification of metal oxide supports, such as silica (SiO2) and TiO2, with silane-coupling agents (organosilanes) having amino groups was effective for dispersion of a catalytic material such as phosphododecatungstate and

1090 phosphotetradecavanadate [4-8] and that the thus-obtained catalysts exhibited higher activities than those of catalysts without chemical modification in some reactions such as partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of isobutyric acid to methacrylic acid [5, 7]. We applied this technique to the preparation of VzOs/TiO2 catalysts and here report the effect of organosilane on the thermal stability of a TiO2 support with a large surface area, the adsorption properties of metavanadate anion (VO3-), and the SCR activities of supported V205 catalysts prepared from VO3--adsorbing TiO2 supports. 2. EXPERIMENTAL

2.1. Modification of TiO2 with organosilane TiO2 powder with a large surface area (ST01) was kindly donated from Ishihara Sangyo, Osaka, Japan. Organosilane, 3-aminopropyltrimethoxysilane (H2N-(CH2)3-Si(OCH3)3; APS), was supplied from Shin-Etsu Chemical Co., Ltd., Tokyo, Japan and was used without further purification. Chemical modification of the ST01 TiO2 with APS was carried out according to the procedure previously reported [4-8]. The TiO2 powder was dried in advance at room temperature under reduced pressure for 24 h. Two grams of the TiO2 powder (2 g) was then suspended in a toluene solution (20 cm 3) of APS (22.1 mmol) and heated at ll0~ for 2 h. After completion of the reaction, the powder was filtered, washed with toluene, diethylether and finally methanol, and then allowed to stand in a desiccator for 30 min. Finally, the sample was dried at l l0~ for 1 h. Hereafter, the TiO2 powder modified with APS is designated APS-TiO2. 2.2. Adsorption of VO3" on modified and unmodified TiO2 Ammonium metavanadate (NH4VO3) (Kanto Chemicals, Tokyo, Japan) was dissolved in H20 to give solutions (0.04-2.00 • 10.2 mol din-3). A hundred milligrams of unmodified TiO2 or APS-TiO2 was added to 30 cm 3 of NH4VO3 solution and stirred to make VO3- adsorb at room temperature for 24 h. The samples were then collected by filtration under suction and dried at l l0~ for 1 h. The amount of VO3- adsorbed was determined using a UV-vis spectrometer (UV-2400, Shimadzu, Kyoto, Japan) from the difference in the concentrations of the solution before and after addition of TiO2. 2.3. SCR of NOx with NH3 over a V205[TiO2 catalyst Unmodified TiO2 and APS-TiO2 samples adsorbing VO3- were decomposed at 550~ for 1 h in air to form supported V205 catalysts. Two V205 catalysts were used for deNOx reaction. The catalytic reaction (catalyst weight: 80 mg) was carried out in a fixed bed flow reactor at temperatures ranging from 300 to 400~ (flow rate: 1.2 dm3 min -1) using a model gas containing 200 ppm NO, 240 ppm NH3, 500 ppm SO2, 33 ppm CO, 3% 02 and 12% H20. The concentration of NOx in the outlet gas was determined by using a NOx meter. 2.4. Characterization Powder X-ray diffraction (XRD) (MultiFlex, Rigaku, Tokyo, Japan) was measured using CuKct radiation with a carbon monochromater. Crystallite size (d101) was calculated

1091 from the half-height width of the 101 diffraction peak of anatase using Scherrer's equation. The value of the shape factor, K, was taken to be 0.9. The specific surface area was calculated using the BET single-point method on the basis of nitrogen (Na) uptake measured at 78 K at the relative pressure of 0.3. Before the N2 adsorption, each sample was dried at 403 K for 30 min in a 30% Na-helium flow. Differential thermal analysis (DTA) (TG-8120, Rigaku) was carried out at a rate of 10~ min -1 in air flow. The morphology of the sample was observed under a JEOL JEM-3010 transmission electron microscope (TEM) operated at 300 kV. Chemical analysis of the APS-TiO2 was performed at Galbraith Laboratory Inc., TN USA.

(a)

IU

(b)

1000~C

b

10001:,C 9001~C

9001:,C

800~C

..~.

-..

8001=C 7001~C .....

7001~C

-

~

.

_. - , , -

5501~C

5501:,C 'before calcination ,

20

I

30

I

40 20 /

,

I

50

,

degree

I

60

I

,

70

20

I

30

I

I

t

I

I

I

40 50 60 20 / degree

t

70

Fig. 1. XRD patterns of samples obtained by calcination of (a) unmodified TiO2 and (b) APS-Ti02 at various temperatures. 3. RESULTS AND DISCUSSION 3.1. Effect of modification with APS on thermal stability of TiOz In the DTA curve of APS-TiO2, a large exothermic peak was observed at around 300~ due to combustion of the aminopropyl group of APS-TiO2. Chemical analysis

1092 revealed that the degree of modification in the APS-TiO2 sample, D(APS), was 2.21 mmol-APS.g-TiO2 -1 and that the Si/Ti ratio was 0.177. Since the initial amount of APS in the toluene solution was 22.1 mmol for 2 g of TiO2, the degree of deposition of APS onto TiO2 was calculated to be 20%. The specific surface area of unmodified TiO2 (bare ST01), S(TiO2), was determined by the BET method to be 300 m2g-1, which is in good agreement with the value reported by the supplier. From D(APS), S(TiO2) and Avogadro's number, the number of APS per nm 2 of TiO2 was calculated to be 4.4, which is almost equal to the numbers (4.5-4.9) of surface hydroxyl groups per nm 2 of anatase TiO2 [9]. The surface coverage of APS on TiO2, 0 (APS), can be estimated by the following formula: 0 (APS)

= D(APS)

9 or (APS) / S(TiO2),

where cr(APS) denotes the cross-sectional area (m 2 mo1-1) of APS. By applying the values for D and S given above and cr = 7.8 • 104 m 2 mo1-1 by estimation from the value given by the supplier (minimum covered area, 436 m 2 g-l), 0(APS) of the present APS-TiO2 sample was estimated to be 0.58.

60

300 (a)

E

~,~ 2OO

4O

"~ .

(b) 9

9

I-U.l rn

m

20

0

I

~

I

,

i

~

!

,

I

200 400 600 800 1000 Tcal /13C

100

0

~

w

200 400 600 800 1000 TcaI/13C

Fig. 2. Changes in (a) crystallite size and (b) BET surface area of TiO2 samples upon calcination at various temperatures. Circles and squares show data of unmodified TiO2 and APS-TiO2, respectively.

The XRD patterns of samples obtained by calcination of unmodified TiO2 and APS-TiO2 at various temperatures are shown in Fig. 1. Transformation of the anatase phase in the unmodified TiO2 sample to the rutile form occurred at 800~ and was almost completed at 900~ (Fig. 1(a)), whereas the anatase crystallite in the APS-TiO2 sample was stable even after calcination at 1000~ (Fig. l(b)). Changes in dl01 and BET surface area of the unmodified TiO2 and APS-TiO2 samples upon calcination at various temperatures are shown in Fig. 2. Unmodified TiO2 exhibited a small dl01 of 8 nm before calcination and, as

1093 mentioned above, possessed a large surface area of 300 mZg-1. However, calcination of the TiOz sample at 550~ drastically increased d101 to 17 nm and decreased the surface area to 91 mZg-1, indicating low thermal stability of the original TiO2 powder. Modification of the TiOz powder with APS inhibited the growth of the anatase crystallite as well as its transformation to the rutile crystallite, as shown in Fig. 2. For example, the sample obtained by calcination at 700~ showed the almost same dl01 as that before calcination and retained a large surface area (>200 mZg-1) due to the inhibition of crystal growth. As can be seen in Fig. 3, TEM observation clearly showed a difference between the behaviors of crystal growth of unmodified and modified TiO2 upon calcination. It had been reported that silica-modified TiO2 samples synthesized from a mixture of titanium alkoxide and silicate ester by glycothermal reaction and thermal decomposition in toluene showed a high degree of thermal stability [10, 11]. Most of the silicon (silica) was incorporated into the anatase structure of both products and suppressed crystal growth and transformation of the TiOz phase. On the other hand, in the present APS-TiOz, silicon originating from APS was distributed only on the surfaces of TiOz crystals (particles). It should be noted that surface modification of TiO2 with APS results in significant improvement of the thermal stability of the original TiOz if the original TiOz has a large surface area.

Fig. 3. TEM photographs of samples after calcination of (a) unmodified TiO2 and (b) APS-TiO2 at 550~ 3.2. Effect of modification with APS on adsorption properties of TiOz Fig. 4(a) shows the amounts of VO3- adsorbed on unmodified TiO2 and APS-TiO2 (Ca~) as a function of equilibrium concentration of VO3-(Ceq). Both TiO2 samples gave Langmuir-type isotherms. From linear plots (Ceq vs. Ceq/Ca~s) (Fig. 4(b)), the limiting

1094 amounts of Cads (maXCads) were estimated to be 0.91 and 1.4 mmol g-1 for unmodified and modified TiOz, respectively, indicating that adsorptivity of VO3- onto TiO2 was improved by modification of TiOz with APS. Since the methoxy group of APS is reactive to HzO or moisture, pKa of APS can not be determined directly in the presence of HzO. Here, pKa of propylamine (10.7) is regarded as that of the amino group of APS-TiOz. The large pKa indicates that the amino group of APS-TiOz is protonated in the NH4VO3 solution (pH 6.8). Therefore, the strong interaction between VO3- and the protonated amino group in APS-TiO2 is attributable to the larger amount of VO3- adsorbed. From the results of chemical analysis and maXCads, the ratio of VO3- adsorbed to APS was calculated to be 1.0, indicating that all of the amino groups of APS-TiOz interacted with VO3-. Therefore, despite the higher loading, greater dispersion of VO3- on the APS-TiOz support is expected because one VO3- anion interacts with the protonated amino group of APS. 1.5

(a)

=

"o,

20 l

(b)

~ 15 0

0~

E E co

0

"o

0.5 5

0 1

0

=

0

5 10 15 20 0 5 10 15 20 Fig. 4. (a) Isotherms of adsorption of VO3- (Ceq-Cads plot) and (b) Ceq-Ceq/Cads plot. Circles and squares show data of unmodified TiO2 and APS-TiOz, respectively.

3.3. Properties and deNOx activities of supported V205 catalysts The unmodified and modified TiOz samples adsorbing VO3- in the most-concentrated NH4VO3 solution (2.00 • 10.2 mol dm -3) were decomposed at 550~ for 1 h in air to form two supported V205 catalysts. The VzO5 catalyst, prepared from the VO3--adsorbing unmodified TiO2, had a small surface area (81 mZg-1), as expected from the low thermal stability of the unmodified TiOz. It is well known that an excess loading of VzO5 on a TiO2 support generally induces sintering and pore-mouth plugging of the support, resulting in a decrease in surface area of the catalyst. Therefore, the amount of VO3- loading (1.0 mmol g-i) on the unmodified TiOz from the NH4VO3 solution exceeded the limited amount of the support for effective dispersion of V205 on it. On the other hand, the other V205 catalyst, prepared from the VO3--adsorbing APS-TiOz sample, retained a sufficient surface area (144 mZg-1) as expected from the improved thermal stability of the APS-TiO2 support. This indicates that VzO5 species were effectively dispersed on the modified TiOz support despite

1095 the large amount of V205 loading. Thus the modification of a TiO2 support with a large surface area with APS enables preparation of V205 catalyst with both a large surface area and a large amount of V205 loading.

60

~

40

> 0 E •

0

20

Z

I

I

I

300 350 400 Temperature/bO Fig. 5. Removal of NOx over V205 catalysts. Circles and squares show results of the catalysts prepared from VO3-adsorbing unmodified and APS-modified TiO2 samples, respectively. Reaction conditions are described in the Experimental section. These two V205 catalysts were used for SCR of NOx with ammonia in a temperature range from 300 to 400~ and the results are shown in Fig. 5. With elevation in the reaction temperature, the amount of NOx removed increased. Due to the large amount of highly dispersed VzO5 species, i.e., large number of active sites, V=O, the V205 catalyst prepared from APS-TiO2 exhibited higher activity than that of a catalyst prepared from unmodified TiO2. 4. CONCLUSIONS Chemical modification of a TiO2 support with a large surface area with organosilane having an amino group suppressed the growth and sintering of anatase crystallite, resulting in improved thermal stability of the support. The adsorption properties of VO3- were also improved by the strong interaction with VO3- and the protonated amino group in the modified TiO2. A supported V205 catalyst prepared from VOa--adsorbing modified TiO2 satisfies both the requirements of large surface area and large V205 loading and, in SCR of NOx with ammonia, exhibits higher activity than that of a catalyst prepared from unmodified TiO2.

1096 REFERENCES

1. S. Matsuda and A. Kato, Appl. Catal., 8 (1983) 149. 2. M. Inomata, A. Miyamoto and Y. Murakami, J. Chem. Soc., Chem. Commun., (1980) 233. 3. E Luck, Bull. Soc. Chim. Belg., 100 (1991) 781. 4. M. Kamada, H. Nishijima and Y. Kera, Bull. Chem. Soc. Jpn., 66 (1993) 3565. 5. Y. Hanada, M. Kamada, K. Umemoto, H. Kominami and Y. Kera, Catal. Lett., 37 (1996) 229. 6. M. Kamada, H. Kominami and Y. Kera, J. Colloid Interface Sci., 182 (1996) 297. 7. M. Kamada, H. Kominami and Y. Kera, Nippon Kagaku Kaishi, (1996) 300 [in Japanese]. 8. Y. Kera, M. Kamada, Y. Hanada and H. Kominami, Composite Interfaces, 8 (2001) 109. 9. H.P. Boehm, Disc. Faraday Soc., 52 (1971) 264. 10. S. Iwamoto, W. Tanakulrungsank, M. Inoue, K. Kagawa and E Praserthdam, J. Mater. Sci. Lett., 19 (2000) 1439. 11 H. Kominami, M. Kohno, Y. Matsunaga and Y. Kera, J. Am. Ceram. Soc., 84 (2001) 1178.

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1097

Design, preparation and testing of effective FeOx/SiOz catalysts in methane to formaldehyde selective oxidation F. Arena a,,, F. Frusteri b, L. Spadaro b, A. Venuto a, A. Parmaliana a a. Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Universit~ degli Studi di Messina, Salita Sperone 31, 1-98166 S. Agata (Messina), ITALY b Istituto CNR-TAE, Via Salita S. Lucia 39, 1-98126 S. Lucia (Messina), ITALY A preparation method of highly effective methane to formaldehyde selective oxidation (MPO) FeOx/SiO2 catalysts, based on "adsorption-precipitation" of Fe II ions under controlled conditions (ADS/PRC), is reported. The performance of ADS/PRC catalysts in the MPO reaction at 650~ has been compared with that of conventional systems prepared by the "incipient wetness" of silica carriers with aqueous solutions of Fe Iu (INC/WET). The ADS/PRC method, enabling a higher dispersion of the active phase, provides very effective MPO catalysts featuring CH4 turnover frequency (TOF) and HCHO productivity (STYHcHO) values larger than those of the counterpart INC/WET systems. 1. INTRODUCTION A large number of research papers devoted during the last decade to the catalytic partial oxidation of methane to formaldehyde (MPO) allowed the peculiar functionality of the silica surface in driving HCHO formation to be highlighted [1-3]. The preparation method affects the catalytic performance of silica samples, according to the following activity scale: "precipitation > sol-gel > pyrolysis" [1,4], as it determines the density of "strained siloxane bridges" early claimed to be the active sites in MPO [1,4]. In fact, recent studies dealing with the origin of the catalytic functionality of commercial silicas [5-7], really pointed to constitutional Fe 3+ ions incorporated into the matrix during preparation as the active centres enabling the occurrence of redox cycles under MPO conditions [5-11]. Direct relationships between the concentration of "isolated" Fe 3+ species in a rhombic-like coordination and either reaction rate or HCHO productivity (STYHcHo) have been outlined, whereas Fe203 clusters, bearing Fe 3+ ions in an octahedral symmetry, mostly drive COx formation owing to a high availability and mobility of lattice oxygen ions [5-7]. Such findings undoubtedly pointed to dispersion as the key-factor affecting the performance of the FeOx/SiO2 system in MPO [5,6], stimulating further research efforts aimed to disclose effective preparation methods enabling a high dispersion of the active phase [7,11]. Actually, due to a relatively high level and dispersion of iron impurities [5,6], we found that a "precipitated" SiOz sample (Si 4-5P grade, Akzo product) features the best "activityselectivity" pattern and highest HCHO productivity (STYHcHo) in the range 650-750~ * e-mail address: [email protected]

1098

[3,12]. Such promising catalytic performance prompted us to investigate in some detail also the kinetics and mechanism of the MPO reaction on the "precipitated" silica catalyst [8-10] putting the scientific basis for further catalyst improvements [8]. Therefore, this contribution is aimed at providing basic evidences into the suitability of the preparation method based on the "adsorption-precipitation" of Fe H precursors to attain highly dispersed FeOx/SiO2 systems featuring a superior catalytic performance in MPO.

2. EXPERIMENTAL 2.1. Catalysts A series of silica supported iron catalysts (FS-x) was prepared by the "adsorptionprecipitation" (ADS/PRC) method [7,11] according to the following procedure using either Si 4-5P (Akzoproduct) or F5 (Akzoproduct) "precipitated" silicas as carrier [11]. 10 g of a powdered silica carrier were put into 0.3 g of distilled water (pH=3) and the suspension was vigorously stirred under a nitrogen flow for lh at r.t. to remove any dissolved oxygen avoiding any air admission. Then, an amount of FeSO4, corresponding to the designed Fe loading, was added to the stirred suspension, raising progressively the pH to a value of 7.07.5 by adding a 10% NH4OH solution. The suspension was kept at the final pH value for lh to attain a complete adsorption of the precursor, with an efficiency higher than 98%. Thereafter, the catalyst was filtered, washed, dried at 100~ and then calcined at 600~ in air for 16 h. For the sake of comparison, a series of silica supported iron catalysts (x-FS) was prepared by the "incipient wetness" (INC/WET) method, using powdered silica samples and aqueous solutions (pH =3) of Fe(NO3)3. After impregnation, the catalysts were dried overnight at 100~ and then calcined at 600~ (16 h). The list of the studied catalysts is reported in Table 1. Table 1 List of SiO2 and FeOx/SiO2 catalyst samples Code SiO2 Preparation method - Fe precursor

S.A.BET (m2.g"1) 385 607

S.L. (Feat.nm"2) 0.011 0.003

Si 4-5P F5

Si 4 5P F5

precipitation precipitation

Fe loading (wt%) 0.020 0.015

1-FS 2-FS 3-FS

F5 Si 4 5P Si 4 5P

INC/WET - FeIII INC/WET - FeIII INCAVET- FeIII

0.095 0.10 0.43

593 402 398

0.018 0.027 0.118

FS-1 FS-2 FS-3

Si 4 5P F5 F5

ADS/PRC- FeII ADS/PRC - FeI1 ADS/PRC - Feu

0.35 0.09 0.37

399 601 597

0.096 0.081 0.018

support

2.2. Catalyst Characterization The redox properties of FeOx/SiO2 catalysts were probed by Temperature Programmed Reduction (TPR) measurements in the range 200-800~ using a 6% H2/Ar reducing mixture flowing at the rate of 30 stp cma'min-1 and a heating rate of 20 K-min -1.

1099 Quantitative calibration of peaks area was made by monitoring the reduction of known amounts of CuO.

2.3. Catalyst Testing Catalyst testing in MPO was performed by a specifically designed recirculation batch reactor operating at 650~ and 1.7 atm total pressure with a flow rate of 1,000 stp cm 3rain-1 (He:N2:CH4:O2=6:1:2:1) and a catalyst sample of 50 mg unless otherwise specified [5-11]. 3. RESULTS and DISCUSSION 3.1 Theoretical basis to FeII "adsorption-precipitation" preparation route The marked tendency of Fe 3+ ions to form the insoluble hydroxide (Kps Fe(OH)3, 1.2-1038) in aqueous solution prompted since 1969 the use of Fe 2+ precursors to attain a high dispersion of the active phase, basically, across the ZSM-5 zeolite support [13,14]. That is, conventional impregnation methods based on aqueous impregnation (e.g., "incipient wetness", "ion exchange" or "electrostatic adsorption") of Fe III precursors, preventing an effective interaction of the positive Fe-hydroxo-ions with hydroxyl groups of acidic oxide carriers at pH>6, cannot m principle enable a high dispersion of Fe species in supported systems [7,11,13]. On the other hand, special preparation methods like gas-phase grafting of Fe 3+ precursors (i.e., FeC13), although enabling a high dispersion, implies the deposition of foreign ions (i.e., C1-) on the support requiring further treatment steps for their removal [13,14]. Then, the "adsorption-precipitation" of Fe 2+ ions (ADS/PRC) in aqueous solutions, under N2 atmosphere and controlled pH conditions [13], has been exploited to attain a higher dispersion of MPO FeOx/SiO2 catalysts [7,11]. Such a preparation route, preventing the formation of insoluble hydroxides (Kps Fe(OH)2, 1.8-10 -12) also at high pH values (pH>6), allows for a progressive interaction of the negatively polarised silica support surface with Fe 2+ ions leading thus to a selective tailoring of "isolated" Fe 3+ moieties onto the matrix [7,11]. .

.

.

.

.

.

III

9

3.2 Catalyst performance A first evidence for a different effectiveness of ADS/PRC and INC/WET preparation routes in promoting the dispersion of Fe uI on the silica carriers stems from a different colour of the two series of catalysts, mostly at Fe loadings higher than 0.1 wt%. Namely, while a typical brownish dye of INC/WET catalysts, the intensity rising with loading, likely signals the presence thereon of Fe203 clusters at any loading (0.09-0.43 wt% Fe), ADS/PRC samples resulted colourless up to the highest investigated Fe loading (0.37 wt%). Then, the effectiveness of the ADS/PRC method in promoting the dispersion of the supported phase and consequently the reactivity of FeOx/SiO2 system in MPO, has been evaluated by comparing the performance of catalysts prepared using either Si 4-5P or F5 silica carriers by INC/WET and ADS/PRC methods, respectively. Activity data at 650~ of the various catalysts in the MPO reaction, with reference to Si 4-5P and F5 silica carriers, are presented in Table 2 in terms of hourly CH4 conversion (XcH4,h, O~), product selectivity (Sx, %) and reaction rate (rate, ~molcu4"S-l'g-1), while the relative space time

1100

yield (STYHcHO, g'kgcat-l'h -1) and CH4 turnover frequency (TOF, atmcH4-1.s-1) values, calculated on the basis of the actual Fe loading (Table 1), are compared in Fig. 1. According to our previous findings [5,6], Fe-addition always results in a marked promoting effect on the activity of whatever silica support, though it is evident that the ADS/PRC route is considerably more effective than conventional INC/WET one in enhancing the catalytic performance of the FeOx/SiO2 system in MPO at any loading [7,11]. In particular, the bare Si 4-5P silica support features a hourly CH4 conversion of 3.5%, corresponding to a methane TOF of 3.0 atmcH4-1-s-1, along with a SHCHOvalue of 80% (Sco, 14%; Sco2,6%) which account for a STYHcHO of 310 g'kgcat-l"h-1 (Fig. 1). In spite of its larger surface area (Table 1), the F5 silica carrier displays both lower activity (XcH4, h, 2.9 %) and SHCHO(64%) with respect to Si 4-5P sample, resulting in a STYHcHO of 195 g'kgcatl'h-l. Yet, while a TOF value of 3.0 atmcHn=l-s-~ (Fig. 1), equal to that of the Si 4-5P silica catalyst, proves that the decrease in activity is the consequence of the lower Fe content (see Table 1), the drop in SHCHO, counterbalanced by a specular rise in Sco, reflects a higher tendency of such system to drive the consecutive oxidation of the primary oxidation product [1-3,5-7], likely because of the larger development in surface area [11]. Table 2 Activity d a t a of SiO2 a n d FeOx/SiO2 c a t a l y s t s in M P O at 650~ Wcat XCH4,-h SHCHO Sco Sco2 rate Sample (mg) (%) (%) (%) (%) (~tmolcH4"Sl'g 1) Si 4-5P 50 3.5 80 14 6 3.6 F5 50 2.7 65 27 8 2.8 1-FS 2-FS 3-FS

50 50 50

8.6 13.4 13.2

55 56 39

28 22 30

17 22 31

8.8 13.8 13.6

FS-1 FS-2 FS-3

50 50 50

37.2 14.9 34.2

33 64 35

29 22 28

31 12 27

38.3 15.0 35.4

FS-1

5.0

4.6

55

23

22

47.3

Addition of ca. 0.10 wt% of Fe to the Si 4-5P silica sample by INC/WET (2-FS) implies a ca. four-fold rise in XCH4,h (13.4%) and rate (13.8 txmOlcH4"Sl"g 1) along with a lowering in SHCHOfrom 80 to 56% (Table 2). With reference to the Si 4-5P silica carrier, the above figures account for a decrease in TOF from 3.0 to 2.1 atmcH4-1"s-1 but in a marked rise of STYHcHO from 310 to 830 g'kgcat-l"h-1, respectively (Fig. 1). A comparable amount of Fe (0.095 wt%) added to F5 silica carrier still by INC/WET (1-FS) attains a lower promoting effect on the activity (Xcn4, h, 8.6%), while the product distribution (SHcHO, 55%) looks similar to that of the previous sample. These catalytic data reflect in lower reaction rate (8.6 ~tmolcH4"Sl"g1) and STYHcHO (525 g'kgcat-l'h -1) values which, on the basis of a TOF equal to 1.5 atmcHn-l's q (Fig. 1), is ascribable to a poorer dispersion of

1101 the active phase [11]. On the other hand, the FS-2 system, bearing 0.095 wt% of Fe loaded by the ADS/PRC method, displays the highest activity (XcH4,h, 14.9%,) and an even larger SHCHO (64%) accounting for TOF and STYHcHO values equal to 2.7 atmcH4-1"s-1 and 1,040 g'kgcat-l"h-1 respectively; these being considerably larger than those of both 1-FS and 2-FS catalysts (Fig. 1). Compared to the 2-FS system, addition of 0.43 wt% of Fe III to Si 4-5P carrier by INC/WET (3-FS) has, yet, a negative impact on all the catalytic evaluation parameters (e.g., SncHo, TOF, STYHcHO) [11]. Indeed, while a XCH4,.h of 13.2%, practically equal to that of the 2-FS sample, and a lower SHCHO (39%) accounts for a STYHcHO of only 570 g'kgcat-l"h-1, the rise of both Sco (30%) and Sco2 (31%) and the concomitant marked decrease in TOF (0.5 atmcH4-1"s-1) altogether are diagnostic of a weak catalytic functionality consequent to a drop in FeO,, dispersion [7,11]. Despite the comparable Fe loading (0.35 wt%), the FS-1 sample, obtained by ADS/PRC of Fe 2+ ions on Si 4-5P silica, features an impressive activity (Xcu4,.h, 37.2%) larger by ca. one order of magnitude than that of the relative silica carrier and ca. three times higher than that of the counterpart 3-FS catalyst, though SHCHO (33%) keeps at a comparable level of the former system. Considerably higher TOF and STYHcHO values, equal to 1.9 atmcH4-1-s-1 and 1,360 g'kgcat1.h-I respectively, indicate that the FS-1 catalyst is ca. four times more active than the counterpart 3-FS sample, though it maintains a satisfactory functionality towards HCHO formation.

Fig. 1. MPO on silica and FeOx/SiO2 catalysts at 650~ Methane turnover frequency (TOF, atmcH4-1-s-1) and space time yield (STYHcHO,g'kgcat-l"h-1) values.

1102 Herewith, such findings undoubtedly prove that the ADS/PRC method confers to FeOx/SiOz catalysts a considerably superior activity in MPO with respect to the conventional INC/WET route, also irrespective of the silica carrier. In fact, considering the FS-3 sample (0.37 wt% Fe), prepared by ADS/PRC on F5 silica carrier, an activityselectivity pattern quite similar to that of the homologous FS-1 system (XcH4, h, 34.2%) is noticed, while also the TOF and STYHcHO values, resulting equal to 1.7 atmcH4-1"s-1 and 1,340 g'kgcat-l"h-1 respectively, well compare with the values of the latter sample (Fig. 1). Since the high value of CH4 conversion (ca. 13.5%) of the above two system which could result in reaction kinetics controlled by the 02 conversion level [8-10], by tuning a timely decrease in contact time (Wcat, 0.005 g), the activity-selectivity pattern of the FS-1 catalyst at a CH4 conversion level comparable with that of the Si 4-5P silica carrier has been evaluated. Such data, also included in Table 2, outline a XCH4, h of 4.6% corresponding to a TOF equal to 2.3 atmcn4-1"s-1, larger by ca. 20% than the value (1.9 atmcn4q.s -1) at higher z. A SHCHOlevel of 55% coupled to such high specific activity allows for a steep rise in STYHcHO to a value of 2,810 g'kgcat-l'h -1. Notably, such a figure results one order of magnitude greater than that of the related bare silica sample and, really, represents a breakthrough in view of a potential industrial exploitation of the MPO reaction [7,10-12].

3.3. Structure and redox properties of FeOx/SiO2 catalysts In order to ascertain if the catalytic pattern of the studied systems is fairly related to the dispersion of the active phase, the redox properties of representative FeOx/SiOz catalysts have been probed by TPR measurements with the aim to shed lights into the reduction pattern of the various Fe 3+ species and getting insights into their surface structures. Then, to highlight the influences of the Fe loading and preparation method, the TPR profiles of F5 silica and differently loaded (0.015-0.43 wt% Fe) INC/WET FeOx/SiOz catalysts are compared in Figure 2A, while those of Si 4-5P silica, FS-3 and 1-FS catalysts are shown in Figure 2B. Moreover, for the sake of comparison the TPR profile of bulk Fe203 (d) is reported, the values of the peak maxima (TMi) and the extent of H2 consumption being summarised in Table 3. Since an analogous very low Fe loading level (Table 1), the bare F5 (Fig. 2A, a) and Si 4-5P (Fig. 2B, a) silica samples feature an analogous TPR profile, spanned in the range 400-800~ being characterised by a main reduction peak with a broad TM2 maximum at 540-550~ and an unresolved one at ca. 600~ (TM3), evidently skewed on the high T side. This spectrum likely denotes the reduction of highly dispersed Fe 3+ moieties, in the form of "isolated" ions in a strong interaction with the silica matrix [5,6]. Indeed, according to literature data, pointing to a hard reduction of supported Fe 3+ ions strongly interacting with carriers [15], a Hz/Fe ratio close to 0.5 accounts for the reduction of Fe 3+ ions to Fe z+.

1103

Fig. 2. TPR profiles of silica and FeOx/SiO2 catalysts. Legend: (A) F5 silica (a), FS-2 (b) and FS-3 (c), Fe203 (d); (B) Si 4-5P silica (a), 3-FS (b); FS-1 (c); Fe203 (d).

Table 3 T P R of SiO2 a n d FeOx/SiO2 c a t a l y s t s

(*C)

(*C)

TM3

Hz consumption

Sample

TMI

TMZ

(*C)

(Hzmoi/Feat)

Fe203

419

541

680

1.48

Si 4-5P F5

420

530 558

583 604

0.45 0.48

3-FS

399

FS-1 FS-2 FS-3

423 409 421

0.57 510 479

570 651 -

0.51 0.49 0.56

1104

Addition of 0.095 wt% Fe by ADS/PRC to the F5 silica carrier (FS-2) implies a corresponding rise in the intensity of peaks area, while no significant differences in qualitative terms are noticed with respect to the spectrum of the related support. In particular, besides to a systematic shift of the reduction maxima to lower T (Table 3), a relatively larger intensity of the TM3 peak is recorded, while the TM1 component still looks as an unresolved shoulder of the main TMZ peak (Fig. 2A, b). At higher Fe laoding (Fig. 2A, c), the TPR profile of the FS-3 catalyst still outlines a resolved TM2 maximum, further shifted to lower T (479~ revealing also a concomitant increase in the intensity of the TM1 component and an almost total disappearance of the TM3 shoulder. Taking into account the TPR profiles of the highly loaded 3-FS and FS-1 catalysts (Fig. 2B) some peculiar differences connected with the different preparation method, are observable. Actually, the 3-FS catalyst (Fig. 2B, b) features one sharp and symmetric reduction peak centered at 399~ being virtually absent any other component at higher T. By contrast, the FS-1 catalyst (Fig. 2B, c) outlines a broader and asymmetric peak centered at 423~ clearly skewed on the high T side because of the contribution of other unresolved components. Quantitative data indicate for all the studied FeOx/SiO2 catalysts (Table 3) a Hz/Fe ratio ranging between 0.49 (FS-2) and 0.57 (3-FS). The TPR profile of the bulk FezO3 (Fig. 2A and B, d) practically spans the same T range of supported species (Fig. 1A and B), indicating minor differences in the reducibility of the various Fe 3§ moieties. However, quantitative data signal a quite different behaviour, since at 800~ a stoichiometric reduction of Fe In to Fe ~ (e.g., Ha/Fe=l.48) is attained in bulk Fe203 [15], clearly in contrast to that found for all silica-supported systems (Table 3). However, the related qualitative TPR features provide basic clues towards the identification of the various Fe 3§ moieties in supported systems taking into account also the general effects of loading and preparation method on dispersion of the active phase in supported systems. In particular, according to the TPR features of bare silica samples, the reduction of "isolated" Fe 3§ ions gives rise to the broad TMZ peak because of their distribution all over the various defect sites of the silica surface [5,6]. Accordingly, the low loaded (0.095% Fe) FS-2 sample presents the largest concentration of isolated species and Fe 3+ ions strongly interacting with the silica matrix (TM3) [5-7,11,15], while the specificity of the ADS/PRC method prevents an extensive formation of more easily reducible "small Fe203 clusters" [5-7,11], monitored by the TM1 peak. Notably, the same TM1 value recorded for both bulk Fe203 and supported catalysts (Table 3) points to the same origin of such a component. On the FS-3 catalyst (0.37% Fe) a further rise in the extent of isolated species is paralleled by an increased concentration of small FezO3 clusters which enhance the reduction kinetics at lower T [5,6], well evidenced by the larger intensity of the TM1 peak. Accordingly, the spectrum of the 3-FS sample (Fig. 2B, b) confirms that the INC/WET route, at such a "high" loading, leads more favourably to the formation of small Fe203 aggregates, whereas the broader TPR profile of the FS-1 system (Fig. 2B, c), signals a more "homogeneous" distribution of Fe Iu ions across the silica matrix allowing foe the formation of both "isolated moieties" and "small FezO3 clusters" [5,6,15].

1105

4. CONCLUSIONS A versatile preparation method of effective FeOx/SiO2 catalysts via the "adsorptionprecipitation" (ADS/PRC) of FeII precursors from aqueous solution is outlined. The ADS/PRC method ensures a high dispersion of the active phase, conferring a considerably higher performance to FeOx/SiO2 catalysts in MPO in terms of specific activity, selectivity and HCHO productivity. Basic evidences into the surface structures, redox properties and catalytic pattern of FeOx/SiO2 catalysts are provided. Formaldehyde productivity values as high as 9-10 kgncno'kgcat-l'h-l('atmcn4-1), ensured by "ADS/PRC" FeOx/SiO2 catalysts, represent a breakthrough in view of a potential exploitation of the MPO reaction on an industrial scale.

ACKNOWLEDGEMENTS This work has been realized in the framework of a research contract between SOD CHEMIE Ag (Miinchen, GERMANY) and University of Messina (Messina, ITALY). REFERENCES

1. 2. 3. 4.

Q. Sun, R.G. Herman and K. Klier, Catal. Lett., 16 (1992) 251. M.M. Koranne, J.G. Goodwin and G. Marcelin, J. Catal., 148 (1994) 378. A. Parmaliana and F. Arena, J. Catal., 167 (1997) 57. K. Vikulov, G. Martra, S. Coluccia, D. Miceli, F. Arena, A. Parmaliana and E. Paukshtis, Catal. Lett., 37 (1996) 235. 5. F. Arena, F. Frusteri, J.L.G. Fierro and A. Parmaliana, Stud. Surf. Sci. Catal., 136 (2001) 531. 6. A. Parmaliana, F. Arena, F. Frusteri, A. Martinez-Arias, M. Lopez-Granados and J.L.G. Fierro, Appl. Catal. A: General, 000 (2002) 000-000. 7. A. Parmaliana, F. Arena, F. Frusteri and A. Mezzapica, Get. Patent n. GEM 174 filing number 100 54 457.6 in the name of SOD CHEMIE AG (November, 2000). 8. F. Arena, F. Frusteri and A. Parmaliana, J. Catal., 207 (2002) 00-00. 9. F. Arena, F. Frusteri and A. Parmaliana, Appl. Catal. A: General, 197 (2000) 239. 10. F. Arena, F. Frusteri and A. Parmaliana, AIChE J., 46 (2000) 2285. 11. F. Arena, T. Torre, A. Venuto, F. Frusteri, A. Mezzapica and A. Parmaliana, Catal. Lett., (2002) accepted for publication. 12. A. Parmaliana, F. Arena, F. Frusteri and A. Mezzapica, Stud. Surf. Sci. Catal., 110 (1998) 665. 13. X. Feng, and W.K. Hall, Catal. Lett., 41 (1996) 45. 14. X. Feng, and W.K. Hall, J. Catal., 166 (1997) 36. 15. S. Yuen, J.E. Kubsch, J.A. Dumesic, N. Topsoe and H. Topsoe, J. Phys. Chem., 86 (1982) 3022.

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Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1107

New Fe-Mo-Ti mixed oxides prepared via the sol-gel method: comparison of the textural properties with solids obtained by impregnation S.R.G. Carraz~n*, C. Martin, C.M. Pedrero and J. Saunders Departamento de Quimica Inorg~inica. Universidad de Salamanca., 37008-Salamanca, Spain

Iron-molybdenum-titanium oxides were prepared via the sol-gel method. Either (NH4)6Mo7Ozn'4HzO, and ferric nitrate (FEN), Fe(NO3)3"9HzO, or FeC13 and MoOCh were used as precursors together with Ti-isopropoxide. These solids were characterised by chemical analysis and N2 adsorption. The sol-gel samples developed higher surface areas (c.a. 100 mZg-1), except for the sol-gel solid prepared with Fe and Mo chlorides as precursors, than those prepared by impregnation. 1. I N T R O D U C T I O N The main industrial processes based on iron molybdate-containing catalysts are the production of formaldehyde from methanol [1,2] and the oxidation of toluene to benzaldehyde [3,4]. Since the first work of Adkins and Peterson [5], the growing industrial importance of Fe-Mo oxide catalysts has led to a large number of investigations, but controversial opinions has been found with respect to the relation between the method of preparation, the solid properties of the catalyst and its catalatytic behaviour. Several researchers have found that the catalytic behaviour of iron molybdate mostly depends on the Mo/Fe atomic ratio [6-10]. Stoichiometric iron molybdate (FezMo3Olz) is likely to be the active component of these catalysts. Boreskov [6] and Okamoto [11] showed that an excess of MoO3 in Fe-Mo-O catalysts promoted the formation of stoichiometric ferric molybdate, while other researchers [8,10] believe that the active sites of Fe-Mo-O catalysts are associated with Mo atoms in octahedral coordination, such coordination being only present in FeMo mixed oxides with an excess of Mo. Introduction of a support (i.e. TiO2) to these systems can induce positive effects on the active phase (FeMo), as to avoid an excessive sintering of the particles during the thermal treatment and/or modification of the reduction capacity and the acid properties. The synergy of iron molybdate and the support is therefore another way for improving the catalytic performance of these solids. Such benefitial effects have been detected in bismuth-molybdenum-titania mixed oxides prepared via sol-gel, in addition these solids resulted to be amorphous materials with a unique morphology and extraordinary dispersion of the active phase [12]. These results encouraged us to extend this field to iron molybdenum oxide catalysts.

1108 The aim of this work is to develop new Fe-Mo containing mixed oxides highly dispersed in a titania matrix, prepared by the sol-gel method, and to compare these materials to those of iron molybdate prepared by conventional methods (i.e. impregnation). Here, we report the preparation of sol-gel derived iron molybdenum titanium mixed oxides. The bulk composition and the textural properties of these materials are investigated by elemental chemical analysis and N2 adsorption, respectively. 2. EXPERIMENTAL

2.1. Sample Preparation Distilled water and analytical grade reagents (Fluka, Switzerland) were used in all preparations. Bulk Fe2Mo3012 (FM) was prepared by coprecipitation of ammonium heptamolybdate (AHM), (NH4)6Mo7024"4H20, and ferric nitrate (FEN), Fe(NO3)a'9HzO [13]. This oxide was calcined in the same conditions as the sol gel materials (see below). FeMoO catalysts were included in a TiO2 matrix using three methods, simultaneous impregnation (FMT-I samples), consecutive impregnation (FMT-IS samples) and the solgel (FMT-SG samples). 2.1.1. Samples prepared by impregnation: samples FMT-I and FMT-IS have been prepared by simultaneous or consecutive implregnation, respectively, of the titania support (Degussa P25, ca. 80% anatase, ca. 50 m g-, previously calcined at 500~ in air for 3 h) with aqueous solutions of ammonium heptamolybdate (AHM), and ferric nitrate (FEN. The precipitates were dried at 100~ and calcined at 500~ for 3 h after each impregnation. The relative amounts of AHM, FeN and titania were chosen to yield solids with a geometric monolayer of MoO3 after calcination, and molar Mo/Fe ratios of 3/2 for samples FMT-I and FMT-IS1. 2.1.2. Samples prepared by the sol-gel method: In Diagram 1, the steps followed for the preparation of the FMT solids via the sol-gel method [12] are indicated. First, a solution of titanium (IV) isopropoxide (11.5 ml) was prepared in methanol (40 ml). Then, the appropriate amounts of the Fe and Mo compounds were introduced by syringe along with 0.1 ml of concentrated HNO3 acting as an hydrolyzing agent. The H20:alkoxide:acid molar ratio was 5:1:0.01 and the molar Mo/Fe ratio was 3/2 (sample FMT-SG1). A second solid (FMT-SG2) containing an excess of MoO3 has also been prepared, with a Mo/Fe molar ratio of 6/2. Gelation always occurred within seconds at ambient temperature. The gels were stirred at 500 rpm for 20 h before being dried in vacuo at room temperature. A third solid (FMT-SG3) has also been prepared by the sol-gel method, but using Mo and Fe chlorides as precursors. In this case, an aqueous acid solution was preliminarily added to the Ti-alkoxide. After gelation has occurred, the solution containing FeCl3 and MoOC14 in isopropanol was injected via a syringe. The molar Mo/Fe ratios was also 3/2 for this sample.The gel was redispersed after the addition of the solution by vigorous stirring at 500 rpm for 20 h and the isopropanol was removed in vacuo at room temperature.

1109

A

B

Ti(i-OC3H7)4 in isopropanol

Fe(NOa)3.9H20

Ti(i-O~H7)4HNO3 + H20

(NHa)6Mo7024.9H20 (Mo/Fe =3/2) HNO3 + H20 (HzO:alcoxide:acid = 5:1:0.01)

]

Gelation, gmin

I

I Drying in v a c u o at r.t.

I

MoOC14 + FeC13 in isopropanol

(Mo/Fe =3/2)

]

Aging, 24 h Stirred at 500 r.p.m

Aging, 24 h Stirred at 500 r.p.m

Calcination in 02 atm.

] Gelation ]

I !

I Drying in vacuo at r.t.

I FMT Xerouel

Calcination in 02 atm.

[

I

Diagram 1. Experimental steps followed to obtain the FMT solids via the sol-gel method using, A) FeN and AHM and B) Fe and Mo chlorides, as precursors All the solids were calcined in flowing 02 at a heating rate of 5 C min -1 up to 250~ this temperature was maintained for 30 min in order to remove the organic residues. Then it was raised at 5~ min -1 up to 500~ and held for another 30 min. The solids were then cooled to ambient temperature whilst still in the oxygen flow. 2.2. Sample characterisation

Elemental chemical analysis (ECA) of iron, molybdenum and titanium were carried out by atomic absorption in a Mark-2 ELL 240 apparatus at Servicio General de Anfilisis Quimico Aplicado (University of Salamanca, Spain). The nitrogen adsorption-desorption isotherms for specific surface area and porosity assessment were recorded at -196-~ in a Gemini instrument from Micromeritics. The specific surface areas were determined by the Brunauer-Emmett-Teller (BET) method. The pore size distributions were obtained from the desorption branch, and the micropore volume was determined by the t-plot method, using literature software [14].

1110

3. RESULTS AND DISCUSSION

3.1. Elemental chemical analysis (ECA) Table 1 lists the results of ECA for some of the samples studied. The solids were not filtered during the synthesis procedure, and consequently the percentage of Fe, Mo and Ti determined by chemical analysis practically coincide with the expected values. Considering the sol-gel materials, the atomic ratio of 3/2 has been preserved throughout the preparation procedure only when iron nitrate and ammonium heptamolybdate were used as precursors (sample FMT-SG1). When using Fe and Mo chlorides precursors (sample FMTSG3), a loss of molybdenum was observed and the Mo/Fe atomic ratio was 1.28 instead of 1.5. Tablel. Bulk composition of the samples determined by ECA Sample [Bulk Mo/Fe(at/at)]exp FM 1.55 FMT-I 1.52 FMT-SG1 1.52 FMT-SG3 1.28

3.2. Textural properties The specific surface areas, calculated following the BET method are given in Table 2. The adsorption capacities of the samples prepared via sol-gel method are noticeably larger than for samples obtained by impregnation, and consequently the specific surface areas of the FMT-SG samples is ca. twice that of the samples prepared by impregnation. A different behaviour can be observed, however, for sample FMT-SG3; in this case, the calculated specific surface area is of the same order as for the impregnated samples. Although the origin of this difference is not clear at all, it is also true that the nature of the precursor used to prepare this sample has an important influence on the type of the pores developed in the sample and also on its surface area. Table 2. Summary of textural properties X* Sample --FM FMT I 1.44 FMT IS 1 1.44 FMT SG1 1.44 FMT SG2 1.38"* FMT SG3 1.44 *percentage molar fraction of FezMo3012 ** percentage molar fraction of MoO3 =4.14

SBEX (mZg-1) 10 47 47 99 111 47

1111 Nitrogen adsorption-desorption isotherms of the the mixed oxides obtained by the impregnation and sol-gel methods exhibit very different shapes, showing the great influence of the preparation method on their textural properties (Figs. 1 and 2).

16s0

A |

O)

o

, g

0

E

:=L .,_4

o.

"0

~=

f-

-FMT-ISI_ ,,1:) .0,, ~

,= ,, . . . . .

,~ ~,

.o.o.

o

o

O ~=0,.

o,=

FMT-I 6) .o= ~

FM

~.p. 0

...... ~ '=e''~

~, p ~ . p . 0.2

~ o

0"

.o- ~'

~

= p ~.?. 0.4

= f,.o.p 0.6

,, ~ - o - j 0.8

c~

p/pO

1.0

Fig. 1. Nitrogen adsorption-desorption isotherms a t - 1 9 6 ~ impregnation

of samples prepared by

For comparison the isotherm of FM is also shown in Fig. 1. The isotherm for bulk iron molybdate is completely reversible, but the adsorption capacity is extremely low, the specific surface area being about 10 m2 g-1. The isotherms for samples prepared by impregnation (both simultaneous and consecutive) belong to type II in the IUPAC classification [15]. In all cases, a type H3 hysteresis loop, originated by particle aggregates forming slit-like pores, is observed. Samples FMT-SG1 and FMT-SG2 (not shown) exhibit, however, type IV isotherms, with type H2 hysteresis loops (Fig. 2). A different behaviour can be observed, however, for sample FMT-SG3; in this case, the shape of the isotherm again corresponds to type II, with a type H3 hysteresis loop. It should be also noticed that micropores do not develop in any of the samples, as concluded from the t-plots (not shown), which correspond to straight lines passing through the origin upon extrapolation. Pore size distribution curves for the samples prepared by impregnation shows the major presence of pores with diameters ranging from 3 to 4 nm, while larger pores (ca. 4 nm diameter) should be present in samples prepared by the sol-gel method (Figs. 3 and 4).

1112

l soo o

o o. I-' '

FMT-SG3

A 01

~.o.q

~' , p . O .

.o.o.. o

.o.O.

m

0

E

6

o

o

o

o

o

9

9 ~

9~

o

o

F M T - S G 1~

0

"

,o , o 9G o

i

I

I

I

0,2

0.4

I

I

0.6

0.8:

1.0

p/p0

Fig. 2. Nitrogen adsorption-desorption isotherms at -196~ of samples prepared by sol-gel

13 E r o

E

"o

l

~

o ~,Lg, . t l

~,~ 9

~--~ ' , ~ . . . . . . . . . . . N

-"N

ffl

FMT-IS1

~'~~r-~,

.",'--,'--~

,

FMT-I

FM . . . . . . . . "71"1

"= -

~N"

d (nm) Fig.3. Pore size distributions of samples prepared by impregnation

1113

[10

E c

E <

<

FMT-SG3

.

0

4.0

8.0

12 0

.

16 0

.

.

20.0

d (nm) Fig. 4. Pore size distributions of samples prepared by sol-gel

4. CONCLUSIONS It has been shown that titania-based iron molybdates oxides can be synthesized by solgel techniques. The bulk properties and textural properties of the xerogel can be tuned in a broad range, varying some important parameters of the sol-gel preparation as the precursors salts. ACKNOWLEDGMENTS Financial support from Junta de Castilla y Le6n (Consejeria de Educaci6n y Cultura, grant SA71/99) is greatly acknowledged. REFERENCES

1. G. Reuss, W. Disteldorf, O. Grundler and A. Hilt, in: Ullmans's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A11, VCH, Weinheim, 1992, p. 619. 2. B. Stiles and T.A. Koch, in: Catalyst Manufacture, 2 na Edition, Marcel Dekker, New York, 1995, Chapter 20, p. 197. 3. J.E. Germain and R. Laugier, C.R. Acad. Sci. Paris Ser. C, 276 (1973) 1349. 4. Hui-liang Zhang, Zong-chang Li, Xian-cai Fu, Jian-Piang Huang and Yu-chang Zhang, J. Mol. Catal. (China), 3 (1989) 139. 5. H. Adkins and W. R. Peterson, J. Am. Chem. Soc. 53 (1931) 1512. 6. G.K. Boreskov, G.D. Kolovertnov, L.M. Kefeli, L.M. Plyasova, L.G. Karakchiev, V.N. Mastikhin, V.I. Popov, V.A. Dzis'Ko and D.V. Tarasova, Kinet. Catal. (Engl. Transl.), 7 (1) (1965) 125. 7. G. Fagherazzi and N. Perniconi, J. Catal., 16 (1970) 321. 8. F. Trifir6, S. Notarbartolo and I. Pasquon, J. Catal., 22 (1971) 324. 9. Yu.V. Maksimov, M.Sh. Zurmuktashvili, I.P. Suzdalev, L. Ya. Margolis and V. Krylov, Kinet. Catal. (Engl. Transl.), 25 (4) (1984) 809.

1114 10. M.R. Sun-Kou, S. Mendioroz, J.L. Fierro, J.M. Palacios and A. Guerrero-Ruiz, J. Mater. Sci., 30 (1995) 496. 11. Y. Okamoto, Y. Oh-Hiraki and S. Teranishi, J. Chem. Soc. Chem. Commun., (1981) 1018. 12. M.D. Wilberger, J. Grunwaldt, M. Maciejewski, T. Mallat and A. Baiker, Appl. Catal. A: General, 175 (1998) 11. 13. C.G. Hill, Jr. and J.H. Wilson III, J. Mol. Catal,. 63 (1990) 65. 14. V. Rives, Adsorption Sci. Technol., 8 (1991) 95. 15. K.S.W.Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R. Pierotti, J. Rouquerol and T. Sieminiewska, Pure Appl. Chem., 57 (1985) 603.

1115

Author Index

Aika, K.-I., 361 Akita, T., 345 Akkari, R., 1045 Albonetti, S., 963 Alifanti, M., 177 Alifanti, M., 337 Alini, S., 131 Ali6, C., 25 Alkhawaldeh, A., 217 Alper, H., 295 Ando, T., 1073 Anthony, R.G., 217 Aramendfa, M.A., 891,899 Arena, F., 1097 Arends, I.W.C.E., 277 Aubriet, E, 823 Auroux, A., 227, 747, 777 Avila, R, 111 Baeva, G.N., 509 Baron, G.V., 159 B arradas, S., 55 Barrault, J., 687 Barr6, L., 453 Barsan, M.M., 1063 Basaldella, E., 739 Basile, F., 131 Baute, D., 287 Bedetti, A., 1011 Bedilo, A.F., 353 Begag, R., 331 Beketov, G., 25 Ben Chaabene, S., 1053 Bergaoui, L., 1053 Bertrand, P., 823 Bitter, J.H., 201 Blanco, C., 499 B lanco, J., 111 Blanco, M.N., 731,739 Blas-Smirez, E, 555

Blin, J.L., 1027, 1035 Boll, W., 983 Bologna, A., 131 Bonnetot, B., 227, 747 Borau, V., 891,899 Botto, I.L., 565 Bozzi, A., 1081 Brei, V.V., 387 Brueva, T.R., 509 Buchmeiser, M.R., 305 Bujor, A., 67 Burgina, E.B., 659 Busch, O., 93 Cabello, C.I., 565 C~iceres, C., 731,739 Canton, P., 1011 Carati, A., 813 Caricato, E.A., 55 Carrazan, S.R.G., 1107 Casci, J.L., 17 Cauzzi, D., 149 Cavani, E, 953, 963 Cellier, C., 627 Cerveny, L., 757 Chatti, I., 595 Choi, J.-S., 585 Cibrian, S., 463 Cinibulk, J., 443 Clacens, J.-M., 687 Colin, J.M., 595 Collignon, E, 185 Constantin, C., 67 Crezee, E., 1019 Cseri, T., 239 Cuevas, R., 537 Cuypers, M., 705 Cuzzato, P., 941 Dal, Y., 993 De Ambrosio, A., 925

1116 de Jong, K.P., 201,647 de Lang, M., 277 de Los Reyes, J.A., 547 de Resende, N.S., 933 de Rivas, B., 463 Descorme, C., 601 Devillers, M., 149 Di Croce, P.G., 823 Di Serio, M., 77 Diaz-Aufion, J.A., 295 Diehl, F., 767 Dfez, EV., 907 Dijs, I.J., 803 D6bber, D., 489 Draelants, D.J., 159 Dubois, V., 993 Dubromez, V., 481 Duprez, D., 601 Efendiev, A.A., 313 Egues, S., 933 Ehret, G., 121,697 Elaloui, E., 331 Escand6n, L.S., 907 Escobar, J., 547 Euzen, P., 767 Fauchadour, D., 453 Fedotov, M.A., 659 Felloni, C., 953 Ferauche, E, 619, 627 Fernandez, J., 1081 Fessi, S., 881 Flego, C., 813, 953 Fournier, M., 481 Frusteri, E, 1097 Gagea, B.C., 177 Gaigneaux, E.M., 609 Galiasso Tailleur, R., 321 Gandia, L.M., 517, 527 Gervasini, A., 747 Geus, J.W., 803 Ghelfi, F., 963 Ghorbel, A., 595, 873, 881, 1045, 1053 Giammetta, V., 941 Gigot, L., 1035

Gil Llambfas, EJ., 111 Gil, A., 517, 527 Gonzalez, E, 499 Gonz~ilez, M.G., 925 Gonz~ilez-Marcos, J.A., 555 Gonz~ilez-Marcos, M.P., 555 Gonz~ilez-Velasco, J.R., 463, 555 Grange, P., 337, 627, 823, 873 Grobet, P.J., 185 Guil, J.M., 227 Guimon, C., 777 Guisnet, M., 369 Guttierrez-Alejandre, A., 537 Guti6rrez-Ortiz, J.I., 463 Hajek, J., 757 Hamada, H., 637 Hanefeld, U., 277 Hao, X., 45 Harl6, V., 141 Haruta, M., 167 Hattori, T., 837, 845 He, M., 715 Heinrichs, B., 25, 627 Hensen, E.J.M., 257 Herskowitz, M., 679 Hilaire, L., 121 Hubaut, R., 881 Hultman, H.M., 277 Ichikawa, S., 345 Iiskola, E., 777 Ikenaga, N.-O., 1073 Ilyin, V.G., 425 Imamura, H., 397 Inaba, M., 637 Inaki, Y., 837 Incolla, M., 925 Isaeva, V., 435 Ishiguro, A., 361 Ito, K., 837 Itonaga, M., 1089 Ivanov, A., 435 Ivanova, A.S., 353 Izumi, Y., 361 Jacobs, P.A., 185

1117 Jannes, G., 993 Jenneskens, L.W., 803 Jim6nez, C., 891,899 Jir~itov~i, K., 121 Job, N., 619 Johann, T., 93 Johansen, K., 1 Jolivet, J.-P., 767 Jung, S.M., 873 Kaburagi, T., 795 Kagawa, K., 1089 Kajita, Y., 837 Kaliya, M.L., 679 Kantzer, E., 489 Kappenstein, C., 601 Kapteijn, E, 1019 Kapustin, G.I., 509 Karhu, H., 757 Kasztelan, S., 239 Keller, N., 193,983 Kemnitz, E., 659 Kera, Y., 1089 Ker~inen, J., 777 Khalfallah Boudali, L., 873 Kiessling, D., 489 Kijenski, J., 787 Kijima, N., 723 Kijima, T., 855, 863 Kikot, A., 739 Kim, J.G., 45,377 Kirschhock, C.E.A., 185 Kiwi, J., 1081 Klimova, T., 267, 537 Knops-Gerrits, P.P.H.J.M., 705 Kogan, S.B., 679 Kolenda, E, 209, 453 Kolomiichuk, V.N., 659 Kominami, H., 1089 Konishi, E., 345 Konishi, S., 1089 Korili, S.A., 517, 527 Koryabkina, A., 257 Kouda, N., 397 Kozlova, L., 435

Kremer, S.RB., 185 Kropf, A.J., 45 Kruse, N., 25 Kumar, N., 757 Kurata, Y., 637 Kustov, L.M., 509 Kuznetsova, T.G., 659 Lambert, J.E, 1053 Lambert, S., 25, 627 Lamonier, C., 141 Lapeyrolerie, R., 369 Lebeau, B., 209 Ledoux, M.J., 193, 697, 983 Lensveld, D.J., 647 L6onard, A., 1027, 1035 Leroi, P., 193 Levchuk, M.M., 387 Li, C., 975 Li, Yongdan, 101 Liang, C., 975 Ligabue, O., 149 Ligi, S., 963 Lin, W., 471 L6pez-Agudo, A., 267 Lopez-Fonseca, R., 463 Louis, C., 537 Lunin, V.V., 659 Lynch, J., 141 Machida, M., 855, 863 Magnoux, P., 369 Maione, A., 1063 Marchand, K., 239 Marinas, J.M., 891,899 Martens, J.A., 185 Martin, C., 141, 1107 Martin, J.A., 111 Maschmeyer, T., 277 Mayr, B., 305 Mayr, M., 305 Mazzoni, G., 963 Melezhyk, O.V., 387 Menegazzo, E, 1011 Mestl, G., 609 Mesu, J.G., 287, 647

1118 Mijoin, J., 369 Miller, J.T., 45 Millini, R., 813 Mimura, N., 637 Minato, T., 361 Minoux, D., 767 Mirodatos, C., 89 Mishin, I.V., 509 Miyao, T., 415 Moggi, P., 149, 669 Montanari, T., 131 Moreno, J.A., 1003 Moroz, E.M., 353, 659 Morselli, S., 149, 669 Moulijn, J.A., 1019 Mufioz, M., 565 Murata, K., 637 Murata, C., 845 Murzin, D.Yu., 757 Myroniuk, T.V., 425 Nadtochenko, V., 1081 Nagasaki, S., 863 Naito, S., 415 Nakagawa, K., 1073 Nakajima, T., 361 Nares, R., 537 Navez, D., 609 Nehasil, V., 25 Nhut, J.-M., 983 Niinist6, L., 777 Niwa, S., 723 Normand, L., 239, 453 Nowotny, M., 277 Okumura, M., 345 Ord6fiez, S., 907 Orlyk, S.N., 425 Otsuka, K., 795 Pajonk, G.M., 331 Pakhomov, N.A., 353 Park, E.H., 377 Parmaliana, A., 1097 Parvulescu, A.N., 177 Parvulescu, V., 67, 177, 575 Parvulescu, V.I., 177, 337

Patarin, J., 209 Patryliak, K.I., 387 Paukshtis, E.A., 659 Paul, S., 481 Payen, E., 141,767 Pedrero, M., C, 1107 Perdigon-Melon, J.A., 227 Perego, C., 953 Pereira, M.M., 915 Pernicone, N., 1011 Pesant, L., 193, 697 Pesquera, C., 499 Petit-Clair, C., 585 Petre, A., 747 Pham-Huu, C., 193, 697, 983 Pierelli, E, 963 Pinard, L., 369 Pinna, F., 1011 Pirard, J.P., 25, 619, 627 Pirard, R., 619 Pizzio, L.R., 731,739 Plazenet, G., 141 Pommier, B., 331 Poncelet, G., 337, 1003 Popescu, G., 67 Pouilloux, Y., 687 Predieri, G., 149, 669 Prihod'ko, R., 257 Prins, R., 247 Prinsloo, F.F., 201 Prudius, S.V., 387 Quincoces, C.E., 925 Ram/rez, J., 267 Ramirez, J., 537 Ramos Moreira, C., 915 Rebollar, M., 407 Regalbuto, J.R., 45 Riello, P., 1011 Rives, A., 881 Rold~in, R., 899 Rom~in-Martfnez, L.C., 295 Romero, EJ., 891,899 Rossignol, S., 601 Rouleau, L., 453

1119 Rouxhet, P., 823 Ruiz, P., 149, 609 Ruiz, R., 499 Rymes, J., 121 Sadykov, V.A., 659 Saito, M., 637 Sakamoto, Y., 415 Sakata, Y., 397, 397 Salinas-Marffnez de Lecea, C., 295 Salmi, T., 757 Santacesaria, E., 77 Sastre, H., 907 Saunders, J., 1107 Scagliarini, D., 953 Schmal, M., 915, 933 Schmidt, W., 93 Schrier, M., 45 Schiith, E, 93 Schwickardi, M., 93 Seelan, S., 167 Serrano-S~inchez, A.M., 555 Shakhtakhtinsky, T.N., 313 Shan, Z., 1019 Shannon, M.D., 17 Sharf, V., 435 Sheldon, R.A., 277 Sheng, S., 715 Shinonaga, A., 1089 Shishido, T., 35 Sicard, L., 209 Signoretto, M., 1011 Sinha, A.K., 167 Sohn, J.R., 377 Solis, D., 267 Sorrentino, A., 77 Spadaro, L., 1097 Spieker, W.A., 45 Stakheev, A.Yu., 509 Steltenpohl, P., 555 Stinner, C., 247 Struzhko, V.L., 425 Su, B.L., 67, 575, 1027, 1035 Suarez, S., 111 Suzuki, T., 1073

Sychev, M., 257 Takahara, I., 637 Takehira, K., 35 Takenaka, S., 795 Tarret, M., 239 Tcherkassova, N., 627 Telegina, N.S., 509 Tesser, R., 77 Tessonier, J.-P., 697 Thomas, H.J., 565 Thyrion, EC., 1063 Tielen, M., 185 Toba, M., 723 Toebes, M.L., 201 Trabace, R., 941 Trifiro, E, 941,963 Tromp, H.J., 287 Trukhan, S.N., 659 Tsubota, S., 167 Tsuji, M., 415 Tubertini, A., 953 Urbano, EJ., 891,899 Uto, M., 855 Uzio, D., 585 Vaccari, A., 131 Valenzuela, M.A., 407 van Berge, P.J., 55 van de Loosdrecht, J., 55 van Dillen, A.J., 201,647 van Faassen, E.E., 287 van Santen, R.A., 257 van Veen, J.A.R., 257 Vanhove, D., 481 Vayrynen, J., 757 Vazquez, P., 739 Venuto, A., 1097 Vicente, M.A., 517 Vieira, R., 193,983 Visinescu, C.M., 337 Vit, Z., 443 Viveros, T., 547 Volodin, A.M., 353 Wakatsuki, Y., 361 Waller, P., 1019

1120 Weber, T., 247 Weckhuysen, B.M., 287 Wei, J.-Y., 471 Wei, Z., 975 Weinberg, C., 609 Wendt, G., 489 Wine, G., 193 Winiarek, P., 787 Wu, Dongfang, 101 Wu, X., 217 Xie, Y.-C., 471 Xin, Q., 975 Xiong, G., 715 Xu, X., 1019 Yamada, C., 795 Yao, N., 715

Yates, M., 111,407 Yeung, K.L., 715 Yoshida, H., 837, 845 Yoshimura, Y., 723 Yuranova, T., 1081 Zaikovskii, V.I., 659 Zair, L., 481 Zanibelli, L., 813 Zeinalov, N.A., 313 Zeng, L., 471 Zhang, Y., 159 Zhao, H., 159 Zhao, X.-D., 471 Zhu, Y.-Z., 471 Zuzaniuk, V., 247

1121 STUDIES IN SURFACE SCIENCEAND CATALYSIS Advisory Editors: B. Delmon, Universit~ Catholique de Louvain, Louvain-la-Neuve, Belgium J.T.Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1

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Preparation of Catalysts I.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, RA. Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts I1.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedingsofthe Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, R Grange, RJacobs and G. Poncelet Growth and Properties of Metal Clusters.Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci~t~ de Chimie Physique, Villeurbanne, September 24-28,1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11,1980 edited by B. Imelik, C. Naccache,Y. BenTaarit, J.C.Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium,Antwerp, October 13-15,1980 edited by B. Delmon and G.E Froment New Horizons in Catalysis. Proceedings ofthe 7th International Congress on Catalysis,Tokyo, June 30-July4,1980. PartsA and B edited by T. Seiyama and K.Tanabe Catalysis by Supported Complexes by Yu.l.Yermakov, B.N. Kuznetsov andV.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. L=tzniEka Adsorption at the Gas--Solid and Liquid-Solid Interface. Proceedings of an International Symposium,Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, R Meriaudeau, R Gallezot, G.A. Martin and J.C.Vedrine Metal Microstructures in Zeolites. Preparation - Properties-Applications. Proceedings of aWorkshop, Bremen, September 22-24,1982 edited by RA. Jacobs, N.I. Jaeger, R Jirb and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. B6nard Vibrations at Surfaces. Proceedings of theThird International Conference, Asilomar, CA, September 1-4,1982 edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets

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Volume 32 Volume 33 Volume 34 Volume 35

Preparation of Catalysts III. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings oftheThird International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, R Grange and RA. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J.Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13,1984 edited by RA. Jacobs, N.I. Jaeger, RJi~,V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings ofthe 9th Canadian Symposium on Catalysis, Quebec, RQ., September 30-October 3,1984 edited by S. Kaliaguine andA. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25--27,1984 edited by B. Imelik, C. Naccache, G. Coudurier,Y. BenTaarit and J.C.Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29,1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure,Technology andApplication. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8,1984 edited by B. Dr~aj, S. HoEevar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings ofthe International Symposium on Future Aspects of Olefin Polymerization,Tokyo, July 4-6,1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings ofthe Fourth International Conference, Bowness-on-Windermere, September 15-19,1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerven~ New Developments in Zeolite Science andTechnology. Proceedings of the 7th International Zeolite Conference,Tokyo, August 17-22,1986 edited by Y. Murakami,A. lijima and J.W.Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis andAutomotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11,1986 edited by A. Crucq andA. Frennet Preparation of Catalysts IV. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings ofthe Fourth International Symposium, Louvain-laNeuve, September 1-4,1986 edited by B. Delmon, R Grange, RA. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by RWissmann Synthesis of High-silicaAluminosilicate Zeolites edited by RA. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1,1987 edited by B. Delmon and G.E Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine

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Volume 45 Volume 46

Volume 47 Volume 48 Volume 49 Volume 50

Volume 51 Volume 52 Volume 53 Volume 54

Methane Conversion. Proceedingsof a Symposium on the Production of Fuels and Chemicals from Natural Gas,Auckland,April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.E Howe and S.Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by RJ. Grobet,W.J. Mortier, E.EVansant and G. Schulz-Ekloff Catalysis 1987.Proceedings of the 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22,1987 edited by J.W.Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a.Ts.,Apri126-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11,1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17,1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. P6rot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Patti Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium,W~irzburg, September 4-8,1988 edited by H.G. Karge and J.Weitkamp Photochemistry on Solid Surfaces edited by M.Anpo andT. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16,1988 edited by C. Morterra,A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14,1989. PartsA and B edited by RA. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting,Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G.Anthony New SolidAcids and Bases.Their Catalytic Properties by K.Tanabe, M. Misono,Y. Ono and H. Hattori RecentAdvances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19,1989 edited by J. Klinowsky and RJ. Barrie Catalyst in Petroleum Refining 1989. Proceedings ofthe First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8,1989 edited by D.L.Trimm, S.Akashah, M.Absi-Halabi andA. Bishara Future Opportunities in Catalytic and SeparationTechnology edited by M. Misono,Y. Moro-oka and S. Kimura

1124 Volume 55

New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22,1989 edited by G. Centi and F.Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium Volume 56 on Recent Developments in Olefin Polymerization Catalysts,Tokyo, October 23-25,1989 edited by T. Keii and K. Soga Volume 57A SpectroscopicAnalysis of Heterogeneous Catalysts. Part A: Methods of SurfaceAnalysis edited by J.L.G. Fierro Volume 57B SpectroscopicAnalysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Introduction to Zeolite Science and Practice Volume 58 edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. P6rot, R. Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium Volume 60 on Chemistry of Microporous Crystals,Tokyo, June 26-29,1990 edited by T. Inui, S. Namba andT.Tatsumi Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Volume 61 Conversion, Oslo, August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Characterization of Porous Solids II. Proceedings of the IUPAC Symposium Volume 62 (COPS II),Alicante, May 6-9,1990 edited by E Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of CatalystsV. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings ofthe Fifth International Symposium, Louvain-la-Neuve, September 3-6,1990 edited by G. Poncelet, RA. Jacobs, R Grange and B. Delmon NewTrends in COActivation Volume 64 edited by L. Guczi Catalysis andAdsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, Volume 65 August 20-23,1990 edited by G. Ohlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, BalatonfQred, September 10-14,1990 edited by L.I. Sim~tndi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings ofthe ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, Apri122-27,1990 edited by R.K. Grasselli andA.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26,1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13,1991 edited by RA. Jacobs, N.I. Jaeger, L. Kubelkov~ and B.Wichtedov~ Poisoning and Promotion in Catalysis based on Surface Science Concepts and Volume 70 Experiments by M. Kiskinova

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Volume79 Volume80 Volume81 Volume82

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Volume 85 Volume 86 Volume87

Catalysis andAutomotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC2), Brussels, Belgium, September 10-13,1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developmentsin Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by P.Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28,1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission.Theory and CurrentApplications edited by S.D. Kevan New Frontiers in Catalysis, PartsA-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, E Solymosi and RT6t6nyi Fluid Catalytic Cracking: Science andTechnology edited by J.S. Magee and M.M. Mitchell, Jr. NewAspects of Spillover Effect in Catalysis. For Development of HighlyActive Catalysts. Proceedings of theThird International Conference on Spillover, Kyoto, Japan, August 17-20,1993 edited by T. Inui, K. Fujimoto,T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals II1. Proceedings of the 3rd International Symposium, Poitiers, April 5- 8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. P~rot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, RW.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals ofAdsorption, Kyoto, Japan, May 17-22,1992 edited by M. Suzuki Natural Gas Conversion I1. Proceedings of theThird Natural Gas Conversion Symposium, Sydney, July 4-9,1993 edited by H.E. Curry-Hyde and R.E Howe New Developments in Selective Oxidation II. Proceedings of the SecondWorld Congress and Fourth EuropeanWorkshop Meeting, Benalmddena, Spain, September 20-24,1993 edited by V. Cort6s Corberdn and S.Vic Bellbn Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan,August 22-25,1993 edited byT. Hattori andT.Yashima Zeolites and Related Microporous Materials: State of theArt 1994. Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22,1994 edited by J.Weitkamp, H.G. Karge, H. Pfeifer andW. H61dedch Advanced Zeolite Science andApplications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids II1.Proceedings ofthe IUPAC Symposium (COPS III), Marseille, France, May 9-12,1993 edited by J.Rouquerol, E Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger

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Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5,1994 edited by B. Deimon and G.F.Froment Volume89 Catalyst Design forTailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design forTailor-made Polyolefins, Kanazawa, Japan, March 10-12,1994 edited by K. Soga and M.Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Volume90 Acid-Base Catalysis II, Sapporo, Japan, December 2-4,1993 edited by H. Hattori, M. Misono andY. Ono Preparation of CatalystsVl. Scientific Bases for the Preparation of Volume91 Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8,1994 edited by G. Poncelet, J. Martens, B. Delmon, RA. Jacobs and R Grange Science andTechnology in Catalysis 1994. Proceedings of the SecondTokyo Volume92 Conference on Advanced Catalytic Science andTechnology, Tokyo, August 21-26,1994 edited by Y. Izumi, H.Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface Volume 93 by E.F.Vansant, RVan DerVoort and K.C.Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Volume94 Hungary, July 9-13,1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys Volume95 by V. Ponec and G.C. Bond Catalysis andAutomotive Pollution Control III. Proceedings of Volume96 theThird International Symposium (CAPoC3), Brussels, Belgium, April 20-22,1994 edited by A. Frennet and J.-M. Bastin Zeolites:A RefinedTool for Designing Catalytic Sites. Proceedings of Volume97 the International Symposium, Quebec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials Volume98 to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22,1994 edited by H.G. Karge and J.Weitkamp Adsorption on New and Modified Inorganic Sorbents Volume 99 edited by A. Dqbrowski andV.A.Tertykh Volume 100 Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26,1995 edited by M.Absi-Halabi, J. Beshara, H. Qabazard andA. Stanislaus Volume 101 11th Intemational Congress on Catalysis -40th Anniversary. Proceedings ofthe 11th ICC, Baltimore, MD, USA, June 30-July 5,1996 edited by J.W. Hightower, W.N. Delgass, E. Iglesia andA.T. Bell Volume 102 RecentAdvances and New Horizons in Zeolite Science andTechnology edited by H. Chon, S.I.Woo and S. -E. Park Volume 103 Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by RV. Kamat and D. Meisel Volume 104 Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzifiski,W.A. Steele and G. Zgrablich Volume 105 Progress in Zeolite and Microporous Materials Proceedings ofthe 11th International Zeolite Conference, Seoul, Korea, August 12-17, 1996 edited by H. Chon, S.-K. Ihm andY.S. Uh

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Volume 119

Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19,1997 edited by G.E Froment, B. Delmon and R Grange Natural Gas Conversion IV Proceedings ofthe 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23, 1995 edited by M. de Pontes, R.L. Espinoza, C.R Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings ofthe 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12,1996 edited by H.U. Blaser,A. Baiker and R. Prins Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings ofthe International Symposium,Antwerp, Belgium, September 15-17,1997 edited by G.E Froment and K.C.Waugh ThirdWodd Congress on Oxidation Catalysis. Proceedings oftheThirdWorld Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons Catalyst Deactivation 1997. Proceedings ofthe 7th International Symposium, Cancun, Mexico, October 5-8,1997 edited by C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings of the 4th International Conference on Spillover, Dalian, China, September 15-18, 1997 edited by Can Li and Qin Xin RecentAdvances in Basic andAppliedAspects of Industrial Catalysis. Proceedings ofthe 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India,April 2-4,1997 edited by T.S.R. Prasada Rao and G. Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings of the 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11,1997 edited by T. Inui, M.Anpo, K. Izui, S.Yanagida andT.Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideds Catalysis andAutomotive Pollution Control IV. Proceedings ofthe 4th International Symposium (CAPoC4), Brussels, Belgium, April 9-11,1997 edited by N. Kmse, A. Frennet and J.-M. Bastin Mesoporous Molecular Sieves 1998 Proceedings of the 1st International Symposium, Baltimore, MD, U.S.A., July 10-12,1998 edited by L.Bonneviot, E B61and,C. Danumah, S. Giasson and S. Kaliaguine Preparation of Catalysts VII Proceedings of the 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4,1998 edited by B. Delmon, RA. Ja 9 R. Maggi, J.A. Martens, R Grange and G. Poncelet Natural Gas ConversionV Proceedings ofthe 5th International Gas Conversion Symposium, Giardini-Naxos, Taormina, Italy, September 20-25,1998 edited by A. Parmaliana, D. Sanfilippo, E Frusteri, A.Vaccari and EArena

1128 Volume 120A Adsorption and its Applications in Industry and Environmental Protection. Vol I: Applications in Industry edited by A. D~browski Volume 120B Adsorption and its Applications in Industry and Environmental Protection. Vol I1:Applications in Environmental Protection edited byA. Debrowski Volume 121 Science andTechnology in Catalysis 1998 Proceedings oftheThirdTokyo Conference in Advanced Catalytic Science and Technology, Tokyo, July 19-24,1998 edited by H. Hattori and K. Otsuka Volume 122 Reaction Kinetics and the Development of Catalytic Processes Proceedings ofthe International Symposium, Brugge, Belgium,April 19-21,1999 edited by G.F.Froment and K.C.Waugh Volume 123 Catalysis: An Integrated Approach Second, Revised and Enlarged Edition edited by R.A. van Santen, RW.N.M. van Leeuwen, J.A. Moulijn and B.A.Averill Volume 124 Experiments in Catalytic Reaction Engineering by J.M. Berry Volume 125 Porous Materials in Environmentally Friendly Processes Proceedings of the 1st International FEZA Conference, Eger, Hungary, September 1-4,1999 edited by I. Kiricsi, G. PdI-Borb~ly, J.B. Nagy and H.G. Karge Volume 126 Catalyst Deactivation 1999 Proceedings ofthe 8th International Symposium, Brugge, Belgium, October 10-13,1999 edited by B. Delmon and G.F. Froment Volume 127 Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 2nd International Symposium/7th European Workshop, Antwerpen, Belgium, November 14-17, 1999 edited by B. Delmon, G.F. Froment and RGrange Volume 128 Characterisation of Porous SolidsV Proceedings ofthe 5th International Symposium on the Characterisation of Porous Solids (COPS-V), Heidelberg, Germany, May 30- June 2,1999 edited by K.K. Unger, G. Kreysa and J.R Baselt Volume 129 Nanoporous Materials II Proceedings ofthe 2nd Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30, 2000 edited byA. Sayari, M. Jaroniec andT.J. Pinnavaia Volume 130 12th International Congress on Catalysis Proceedings of the 12th ICC, Granada, Spain, July 9-14, 2000 edited byA. Corma, F.V.Melo, S. Mendioroz and J.L.G. Fierro Volume 131 Catalytic Polymerization of Cycloolefins Ionic, Ziegler-Natta and Ring-Opening Metathesis Polymerization byV. Dragutan and R. Streck Volume 132 Proceedings of the Intemational Conference on Colloid and Surface Science, Tokyo, Japan, November 5-8, 2000 25th Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan edited byY. Iwasawa, N. Oyama and H. Kunieda Volume 133 Reaction Kinetics and the Development and Operation of Catalytic Processes Proceedings of the 3rd International Symposium, Oostende, Belgium,April 22-25, 2001 edited by G.F.Froment and K.C.Waugh Volume 134 Fluid Catalytic CrackingV Materials and Technological Innovations edited by M.L. Occelli and R O'Connor

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Zeolites and Mesoporous Materials at the Dawn of the 21st Century. Proceedings of the 13th International Zeolite Conference, Montpellier, France, 8-13 July 2001 edited by A. Galameau, E di Renzo, E Fajula and J.Vedrine Natural Gas ConversionVI. Proceedings ofthe 6th Natural Gas Conversion Symposium, June 17-22, 2001, Alaska, USA. edited by J.J. Spivey, E. Iglesia and T.H. Fleisch Introduction to Zeolite Science and Practice. 2"dcompletely revised and expanded edition edited by H. van Bekkum, E.M. Flanigen, RA. Jacobs and J.C. Jansen Spillover and Mobility of Species on Solid Surfaces. edited byA. Guerrero-Ruiz and I. Rodriguez-Ramos Catalyst Deactivation 2001 Proceedings of the 9th International Symposium, Lexington, KY, USA, October 2001. edited by J.J. Spivey, G.W. Roberts and B.H. Davis Oxide-based Systems at the Crossroads of Chemistry. Second International Workshop, October 8-11,2000, Como, Italy. Edited by A. Gamba, C. Colella and S. Coluccia Nanoporous Materials III Proceedings of the 3rd International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002 edited by ~ Sayari and M. Jaroniec Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium Proceedings of the 2ndInternational FEZA (Federation of the European Zeolite Associations) Conference, Taormina, Italy, September 1-5, 2002 edited by R. Aiello, G. Giordano and F.Testa Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the 8th International Symposium, Louvain-la-Neuve, Leuven, Belgium, September 9-12, 2002 edited by E. Gaigneaux, D.E. De Vos, P. Grange, P.A. Jacobs, J.A. Martens, P. Ruiz and G. Poncelet