Zeolite Chemistry and Catalysis: Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13, 1991 (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 69

ZEOLITE CHEMISTRY AND CATALYSIS

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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 69

ZEOLITE CHEMISTRY AND CATALYSIS Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-1 3 , 1 9 9 1

Editors P.A. Jacobs Laboratorium voor Oppervlaktechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 8-3030 Leuven (Heverlee), Belgium

N.I. Jaeger Universitat Bremen, Forschungsgruppe Ange wandte Katalyse, Postfach 330440, D-2800 Bremen, Germany and

L. Kubelkova and B. Wichterlova J. He yrovskp Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, DolejSkova 3, 182 23 Prague 8, Czechoslovakia

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CONTENTS Preface Acknowledgements Organizing and Scientific Committee Financial Support Hydrocarbon Transformations over Analogues and Derivatives of Zeolite Y (plenary lecture) Dwyer J . , Dewing J . , Karim K., Holmes S., Ojo A.F., Garforth A.A., Rawlence D.J.

XI XI I XI11 XIV 1

Isomorphous Substitution in Zeolitic Frameworks: Procedures and Characterization (plenary lecture) Vedrine J. C.

25

Introduction of Cations into Zeolites by Solid-state Reaction (plenary lecture) Karge H . G . , Beyer H.K.

43

Zeolite-hosted Metals and Semiconductors as Advanced Materials (plenary lecture) Schulz-Ekloff G.

65

Isomorphous Substitution in Zeolites: a Route for the Preparation of Novel Catalysts (plenary lecture) Bellussi G . , Fattore V.

79

Zeolite Synthesis with Metal Chelate Complexes Balkus Jr. K.J . , Kowalak S., Ly K.T., Hargis D.C.

93

Synthesis of Ferrous Cyanide Complexes inside Zeolite Y Bresinska I., Drago R.S.

101

Genesis of Gallosilicates with ZSM-5 Structure. Insertion of Ga and Zeolitic Properties at Various Steps of Crystallization Kosslick H., Richter M., Tuan V.A., Parlitz B , Szulzewsky K., Fricke R.

109

Studies on the Phosphorus Substituted Zeolites Prepared by Secondary Synthesis Reschetilowski W . , Einicke W.-D., Meier B., Brunner E.. Ernst H

119

Synthesis of Zeolite Beta in Boron-Aluminium Media Derewinski M., Di Renzo F. ,Espiau P., Fajula F . , Nicolle M. -A.

127

VI On the Possibility of Generation of Brrensted Acidity by Silicon Incorporation in Very Large Pore Alp04 Molecular Sieves Martens J . A . , Balakrishnan I . , Grobet P.J . , Jacobs P.A.

135

Crystallization of Porous Alurninophosphates and Metal Substitutions Lechert H . , Weyda H., Hess M . , Kleinworth R., Penchev V . , Minchev Ch.

145

Factors Affecting the Crystallization of Zeolite ZSM-48 Giordano G., Dewaele N., Gabelica Z., Nagy J.B.,Nastro A , , Aiello R . , Derouane E.G.

157

Synthesis and Characterization of Cr-modified Silicalite-1 Cornaro U . , J i r g P., Tvarfiikova Z.,Habersberger K.

165

Synthesis, Characterization and Catalytic Activity of V-ZSM-5 Zeolites Fejes P., Marsi I . , Kirisci I . , Halasz J., Hannus I . , Rockenbauer A , , Tasi Gy., Korecz L . , Schoebel Gy.

173

A Study of Acid Sites in Substituted AlPO-5 Gorte R.J . , Kokotailo G.T . , Biaglow A. I., Parrillo D., Pereira C.

181

Structure and Photocorrosion of NaX Hosted Q-Size Metal Sulfide Particles Wark M . , Schulz-Ekloff C . , Jaeger N . I . , Zukal A .

189

Faujasite-Hosted Methylene Blue: Synthesis, Optical Spectra and Spectral Hole Burning Hoppe R., Schulz-Ekloff G., Woehrle D., Ehrl M., Brauchle C.

199

Preparation and Characterization of Zinc-ZSM-5 Catalyst Liang J . , Tang W., Ying M.-L., Zhao S.-Q.,Xu B.-Q., Li H.-Y.

207

The Formation of Well Defined Surface Carbonyls of

215

Ru and Ir with Highly Dealurninated Zeolite Y as

Matrix Burkhardt I . , Gutschick D., Landmesser H., Miessner H.

VII A Comparative Study of State and Reactivity of Copper Ions Embedded in Various Molecular Sieve Materials Wendlandt K.-P., Vogt F., Moerke W., Achkar I.

223

Acidity, Redox Behaviour and Stability of CoAPO Molecular Sieves of Structure Types 5, 1 1 , 34 and 16 Kraushaar-Czarnetzki B. , Hoogervorst W.G.M. , Andrea R.R., Erneis C.A., Stork W.H.J.

231

State of Iron and Catalytic Properties of AkaliMetal-Exchanged Ferrisilicate Zeolite Molecular Sieves Kan Q., Wu Z.,Xu R., Wei Q., Peng S., Xiong G., Sheng S., Huang J.

241

Framework and Extraframework Ti in TitaniumSilicalite: Investigation by Means of Physical Methods Zecchina A , , Spoto G . , Bordiga S . , Ferrero A , , Petrini G . , Leofanti G . , Padovan M.

25 I

Studies on the State of Copper and the Formation of Its Oxidic and Metallic Phases in Zeolite CuNaY Piffer R., Hagelstein M., Cunis S., Rabe P., Foerster H . , Niemann W.

259

ESCA Study of Incorporation of Copper into Y Zeolite Jirka I., Wichterlova B., Mary5ka M.

269

Preparation of Ga-Doped Zeolite Catalysts via Hydrogen Induced Solid-state Interaction between Ga 0 and HZSM-5 Zeolite

277

2 3

Kanazirev V . , Price G.L., Dooley K.M. Comparison of Kydrosulfurization Zeolite Catalysts Prepared in Different Ways Onyestyak Gy., Ka116 D . ,Papp J.,Jr.

287

Effect of the Introduction of Ni(I1) on the Catalytic Properties of SAPO-5 Molecular Sieves Mavrodinova v., Neinska Ya., Minchev Ch., Lechert H., Minkov V., Valtchev V . , Penchev V.

295

Study of Broensted and Lewis Acid Sites in Phosphates, Silicates and Silica Gels with Molecular Sieve Properties Kustov L.M., Zubkov S.A , , Kazansky V. B., Bondar L.A

303

VIII Influence of Framework Phosphorus on the Acidic Properties of Faujasite Type Zeolite Briend M., Lamy A., Dzwigaj S., Barthomeuf D.

313

Zn-Doped HZSM5 Catalysts for Propane Aromatization Guisnet M., Gnep N.S.,Vasques H . , RamBa Ribeiro F.

32 1

Sulfided Ni-Mo-Y Zeolites as Catalysts for Hydrogenation and Hydrodesulfurization Reactions Laniecki M., Zmierczak W.

331

Reduction of SO on Molybdenum Loaded Y Zeolite Soria J. , Gonzafez-Elipe A. R. , Conesa J.C.

339

Contribution of Metal Cations to the Para-Selectivity of Small Crystals of H-ZSM-5 Zeolite in Toluene Alkylation with Ethylene Cejka J . , Wichterlova B., Krtil J . , Ki-ivanek M., Fricke R.

347

NO Decomposition on Cu-Incorporated A-Zeolites under the Reaction Condition of Excess Oxygen with a Small Amount of Hydrocarbons Inui T., Kojo S., Shibata M., Yoshida T., Iwamoto S.

355

A

Comparison of the Catalytic Properties of SAPO-37 and HY Zeolite in the Cracking of n-Heptane and 2,2,4-Trimethylpentane Lopes J.M., Lemos F., RamBa Ribeiro F., Derouane E.G.

365

Cracking of Light Alkanes over MeAPO-5 Molecular Sieves Meusinger J . , Vinek H. , Dworeckow G . , Goepper M. , Lercher J.A.

373

Promoting Effect of Pt Supported on Galliumsilicate in n-C4HI0 Aromatization Dmitriev R.V., Shevchenko D.P., Shpiro E . S . , Dergachev A. A. , Tkachenko 0.P. , Minachev Kh.M

381

Conversion of Ally1 Alcohol to Oxygenated Products over Zeolite Catalysts Hutchings G.J . , Lee D.F.,Williams C.D.

389

Cation Exchange Influence on the Activity of Zeolites in Reactions between Alcohols and Hydrogen Sulphide Ziolek M . , Hildebrand-Leksowska K.

397

Possible Intermediates during C3H8 aromatization over Ga-HZSM-5 Catalyst Meriaudeau P., Naccache C.

405

IX Dehydrocyclodimerization of Short Chain Alkanes on Ga/ZSM-5 and Ga/beta Zeolites Corma A., Goberna C., Lopez Nieto J.M., Paredes N., Perez M.

409

Bifunctional Cobalt-ZSM-5 Catalyst for the Synthesis of Hydrocarbons from the Products of Biomass Gasification Krylova A , , Lapidus A., Rathousky J . , Zukal A , , JanCalkova M.

417

Shape Selective Reforming: Possible Reaction Pathways on Platinum-Containing Erionite/Alumina Catalysts Kalies H., Roessner F., Karge H . G . ,Steinberg K. -H.

425

Framework Ordering2+n Aluminophosphate Molecular Sieves Studied by A1 Double Rotation NMR Chmelka B.F., Wu Y . , Jelinek R . , Davis M.E., Pines A.

435

A Computer Analysis of ESR Powder Spectra of Silver and Sodium Clusters in Molecular Sieves Uytterhoeven M. G., Schoonheydt R.A.

443

Magic-Angle-Spinning Nuclear Magnetic Resonance and Infrared Studies on Modified Zeolites Brunner E., Freude D., Hunger M., Pfeifer H., Staudte B

453

129Xe NMR Study of Intra- and Inter-Crystallite Diffusion of Cations in Faujasite Zeo 1i tes Fraissard J . , Gedeon A , , Chen Q . , Ito T.

461

Intracrystalline Diffusion of Benzene in Ga-Silicate Zikanova A,, Struve P., Buelow M., Wallau M., KoCiiik M., Micke A , , Tissler A , , Unger K.K.

469

Intracrystalline Diffusivities of HZSM-5 Zeolites Hashimoto K., Masuda T., Murakami N.

477

New Porous Materials from Layered Compounds (plenary lecture) Clearfield A . , Kuchenmeister M . , Wang J . , Wade K.

485

Author Index

499

Subject Index

505

Studies in Surface Science and Catalysis (other volumes in the series)

511

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XI

Preface

The International Symposium entitled "ZEOLITE CHEMISTRY AND CATALYSIS", held in Prague from September 8 to 1 3 , 1991 and elected by the International Zeolite Association as a local IZA Symposium, is one of a series of European Symposia which has been organized during the past decade. As the field of zeolite science is continually growing, each of the previous locally organized European Zeolite Symposia (Szeged, Villeurbanne, Bremen, Prague, Siofok, Nieuwpoort, Wurzburg, Leipzig) has focussed on a particular area in zeolite science and technology. The present Symposium emphasizes the effect of modifying components on the structure and reactivity o f molecular sieves. The plenary lectures and contributed papers concentrate

on the problem of isomorphous substitution in a zeolitic framework; on the occlusion and the structure of metal, metal oxide, and metal sulfide clusters and complexes in the intracrystalline void volume of molecular sieves and zeolites as well as in the interlaminar space of layered compounds. Attention has been paid to synthesis, structural characterization and the mobility of charged encapsulates in such phases or their mixtures. Moreover, not only has the impact of such modifications on catalysis been examined, but also the use of such materials as active components in photo or chemical sensors. New developments are to be expected from the recent growth of knowledge in traditional areas of zeolite applications and from the recent progress which has been made with such systems in material science. The use of zeolitic materials as hosts for specific chemical entities and their application in SUPRAMOLECULAR chemistry and catalysis looks particularly promising.

XI1 We expect that the Prague 1991 Symposium and its Proceedings will become a milestone in this evolution and will stimulate not only the use of molecular

sieves in new research areas but also in applications involving new and sophisticated experimental and theoretical methods. The Peer review system to which all the contributed papers were subjected, guaranteed the high quality of the zeolite science papers in the present volume.

Prague, June 1991

Peter A . Jacobs Nils I. Jaeger Ludmila Kubelkova BlankaWichterlova

Acknowledgements The Organizing Committee of the International Symposium on "Zeolite Chemistry and Catalysis" held in Prague from September 8 to 13, 1991 highly appreciated the efforts of all the participants who contributed to the Scientific Program of the Symposium and presented their results in plenary lectures, contributed papers and in recent research reports. We thank all the organizations and companies which sponsored this Symposium, thereby enabling, especially, our younger colleagues to participate. The work of the Scientific Committee in accomplishing the difficult task of selecting the contributed papers deserves a special mention. The Editors would also like to thank the authors for their careful preparation of the camera ready manuscripts and the reviewers who conscientiously evaluatedthese papers in short time.

XIII Organizing Committee

R. Zahradnik (chairman) J. V. BosaEek J . Cejka J. K. Habersberger J. I. Jirka L. P. JirPl J.

Kapieka

Z . Tvarfiikova

Koubek

B, Wichterlovh

Krtil

N. iilkova

Kubelkova Novakova

Scientific Committee

H. van Bekkum (Delft University, The Netherlands) H.K. Beyer (Academy of Sciences, Budapest, Hungary) A.C. Corma (Institute of Chemical Technology, Valencia, Spain)

D. Barthomeuf (University P.& M. Curie, Paris, France) J. Dwyer (UMIST, Manchester, Great Britain) G.J.

Hutchings (Liverpool University, Great Britain)

P.A . Jacobs (Catholic University, Leuven, Belgium) N.I. Jaeger (Bremen University, Germany)

H G. Karge (Fritz-Haber-Institute,Berlin, Germany) A. Kiss (Degussa AG, Hanau, Germany) J.A . Lercher (Technical University, Vienna, Austria)

W. Mortier (Exxon Chemical Holland B.V.,Rotterdam, The Netherlands) G. Schulz-Ekloff (Bremen University, Germany) A . A. Slinkin (Academy of Sciences, Moscow, USSR) D. E.W .

Vaughan (Exxon Research and Engineering Co. , Annandale, USA)

J . C . Vedrine (Research Institute for Catalysis, Vil eurbanne, France

J. Vblter (Central Institute of Physical Chemistry, Berlin, Germany)

XIV Financial support

BP International Ltd., Sunbury on Thames, Great Britain Chemical Works, Litvinov, CSFR Czechoslovak Academy of Sciences, Prague, CSFR Degussa, AG, Frankfurt, FRG

Dow Benelux, Terneuzen, The Netherlands Eniricerche S.p. A . Milano, Italy Exxon Chemical Holland B. V., Rotterdam, The Netherlands Grace GmbH, Worms, FRG International Zeolite Association (IZA) Sudchemie AG, Munich, FRG

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 1991 Elsevier Science Publishers B.V., Amsterdam

J D q e r , J Dew-, D J Rawlence (a)

1

K Karhn, S Holmes, A F eo, A A Garforth and

UMIST Chemistry Saclrville Street, MANcHEsTER M60 lQD, UK

(a)Crosfield cfieshire, UK

Catalysts,

4

Liverpool Road,

WARRINGION WA5

lAB,

zeolites of type Y are prepared by either primary or secondary synthesis. S t r u a e s include zeolite Y in both the cubic and hexagonal forms, SAFC-37 and faujasitic frameworks Containing Ga or Zn. These materials are characterised using solid state NMR, X-ray powder diffraction, infrared spectroscopy, surface analysis and sorption. Catalysts are then evaluated for the conversion of n-hexane, cyclohexane andgas-oil. Resultsare interpretedin terms of the effectiveness of catalytic sites in alkane activation and in the effect of both density and distribution of active sites.

lTmamaTm The extensive use of zeolite Y as an acid catalyst for hydrccarbn conversion has had a mjor impad on the petroleum refining industry. Zeolite Y has the faujasite f m w o r k structure (1) consisting of SOddLite u n i t s ( p cages) linked through rings of six tetrahedra, via OF-, to generate layers of scdalite units linked by double-six r T s (hexagonal prisms). The layers are also linked by hexagonal prisms in an ABc seqgence to generate a tetrahdral array of scdalite u n i t s having cubic symmetry and the same space group (Fd3m) as diamond. In fact the structure of zeolite Y is readily derived from that of diamond if carbon atcnns are replaced by scdalite units and C-C bonds by double-six rings. This arrangement of sodalite units generates wide pores of 12 linked tetrahedra, with free diameter 7 - 8A, which provide entrances into larger supercages of 11 - 12 A diameter. The supercages and linked tetrahedrally viathe 12 rings to form an openthree dimensional pore system (Fig 1). analogue of zeolite Y, sconetimes called hexagonal Y, has also been synthesised recently (2). In this structm layers of linked scdalite units are linked in the sequence ABAB, by mtatirg every second layer, to produce hexagonal syrmnetry (Fig 1). These framework structures represent end members with other intermediate structures for example ZSM-20 ( 2 ) represented by varying stacking sequences. In the hexagonal framework there are five, 12-ring openings in each supercage, two of which (in the 001 direction) are planar, the other three being elliptical (Fig 1).

An

2

the last decade or so heteratom have been intrcduced into the faujasitic framework, and a phosphate-based material (SARI - 37) has also been synthesised with the faujasitic framework (3).

rxlring

?Lpically, zeolite Y is synthesis&, in aqueous alkaline media, with a framework ccanposition Si/Al I 3 and in order t o enhance thermal/hydrothennal stability and t o modify catdlytic function, postsynthesis modifications are employed. Typical post-synthesis procedures for siliceous zeolite Y involve hydrothermal treatment ( 4 ) or dealmination by chemical methcds. chemical dealunination can u t i l i s e ccanplexing agents such as EDTA (5) which remove a l m i n i m w i t h very limited replacement of silicon in the vacated site, or they u t i l i s e secondary synthesis prooeCtures i n which a second source of s i l i c o n is available for "healingll framaJork vacancies prcduced by extraction of aluminium. Secondary synthesis may involve gas/solid reactions for example reaction of zeolites w i t h Sic14 (6) or solid/ solution reactions for exanple reaction of zeolites and aqueous (MIq)2SiF6 ( 7 ) . Both of these approaches canbe usedto h q m r a t e heteroatms into zeolite frameworks (8) ( 9 ) . The present lecture describes recent work by the authors, on siliceous forms of zeolite Y (cubic and hexagonal), SAFO-37, and gallated Y zeolites. The materials used are well-characterised and are evaluated

catalytically using the mnversion of n-hexane, cyclohexene and gas-oil t o camment on (i) site activity (ii) site density (iii) site distribution and (iv) the role of gallium i n FCC catalysts.

cl~emically s t a b i l i s e d y zeolites (CSY) w e r e prepared by reacting a (Si/Al = 2.5) a t 70 C and buffered a t pH = 6.5 w i t h an aqueous solution of (MIq)2Ss6 ( 9 ) . Heteroatm Y zeolites w e r e prepared i n a similar way but using fluorides of gallium and zinc in place of silicon hexafluoride (12). A semi-batch reactor was used and in a l l cases products w e r e washed free of fluoride.

slurry of W Y

zeolite Y w a s also synthesiseddirectly i n b o t h c u b i c and hexagonal forms (2) using the templated aqueous fluoride system and the intergrowth zm-20 (10) w a s dlso synthesis&. SAFO-37 was

siliceous

synthesised using published procedures (3).

olaraderisation procedures The XRD data w e r e obtained using an XIlS 2000 SCINTAG diffractcm&er (Cu Kcr radiation) over a range of 2 - 60" 2 Theta. The unit cell dimensions were determined using silicon as internal standard fo1lmh-g AS'IM methcds. Nitrcgen sorption i s o t h m and surface areas w e r e

3

~ l g u r e1 ,

770‘

J 10

I

‘243

20 30 0 10 NUflBER OF FRAflEWDRK ALUMINIUM SUBSTITUTED BY m

20

Substitution of m (m: Si,Ga.Zn) intu the faujasitic framework a Changes in frequency of symmetric stretch IIR) b Changes in unit cell parameters (XRDI

Figure 2 .

A 158 316 L13 631

‘

FLUENCE pA mrn Icrn’l 7 9 0 941 11001260

$161

Figure 3.

Hexagonal faqarih framework

Cubic faujarite framevork

i8

2

.

Depth profiling of faujasitic zeolites B [Ga/AIIY2 prepared by fluorogallate ACUB-Y prepared using crown ether template NaY ex crusfield catalysts

A 0CS-Y prepared by reaction of NH,-Y and INHI, SiF,

30

4

deterrmned ' using a Micromeretics ASAP 2400 porosimeter at Crosfield Catalysts. ’IheIRspectroscopic studywasperformed usinga Mattson Cysnus 100 FTZR spectrcweter. Framework-region absorptions were recorded using JBr discs while the hydroxyl spectra were obtained using self-supporting zeolite wafers activated at 400 C/10’5 torr/4 hours. 29Si, 27Al and 7 k a Solid S t a t e NMR spedra were recorded at 59.6, 78.2 and 91.4 MHZ respectively, using a Varian VXR 300 niltinuclear spectrameter at theuniversity of Ilxham. Detailed procectures are described elsewhere (11). The surface ccanpositional depth profiles were obtained using a VG SINS spectmmter. An Ar+ ion beam of 10 W energy and 20 @ current w a s used to etch the surface. catalysis

Catalytic evaluation was made using an intennittent micro-flow reactor (11)(13)(14) a continuous micro-flaw reactor and a conventional MAT unit. catdlysts enployed w e r e either as 100% zeolite or as 25% zeolite bound with silica and formed into a typical FCC matrix. For the latter tests, catalyst were elutriated, preheated at 590 C for 3 hours and fluid bed steam de-activated at 760 C for 5 hours in 100 Cprior to catalyst evaluation.

The substitution of Si for framework Al is well reported (4) (6) (7) arid it is also reported (9) that Fe, Ti, Cr and Sn may be substituted. m e unit cell size changes progressively (12) with substitution of si or ~a and probably Zn for Al (Fig 2). Solid state N M R confirm the substitution of Ga into the framaJork (Fig 4) and IH - 29Si cross polarisation indicates that few silanol defectsare prcdud since relative intersities of 29Si signals are not significantly affected by cross polarisation. ?his is confirmed by FTIR analysis of the hydroxyl region (14). Prcduc3.s are highly crystalline (XRD) with good sorption capacity (Table 1). kpth profiling using sm shows that camposition gradients are not extmae at the levels of substitution considered but as substitution of either Si or Ga inrreases there is enri-t of the substituting a m in layers closer to the surface (Fig 3 ) suggesting some diffusion limitation (shrinkixq core model) at higher levels of substitution.

5

’T

F w p e 4.

'9S~-'H (crosspolansation)

Sold state MASNMR spectra for H-lGa/AII-YI Zeolite Figure 5 29S1 MASNMR spectra of (a) CU0.V (SllA1=38

( b ) ZSM-20 ( S I A1;3

Silicon

NMR

So

s1

Parameter S25SL

Si/Al

25 70 5 0 0

2 6 53 21 0 0

12 50 33.5 0

Figure 6 .

3

Fauysite structure Centres of symmetry D6R links

6 ) and (c) CSY (SlIAl

4)

I

6 TABLE 1 Conparison o f Physical Properties of Faujasite-type Z e o l i t e s

Zeolite

Unit C e l l

Sanptes

S i z e (A)

Crystallinity (X )

Si/Al (Bulk)

Si/AI

(NMR)

%/A1

(XRD)*

Surface Area (cm'g-') BET

Langrmir

Micropore Volm

(c2g-l) NH4Y CUB-Y WB-Y ZSM-20 CSY 1 CSY 2 CSY 3 CSY 4 [Gal A l Y 1

[Gal ALY2 [Gal A l Y 3 [Gal ZSM-20 [GalCSY 4 [Zn/All Y

*

24.69 24.62 24.57 24.57 24.40 24.45 24.58 24.50 24.74 24.77 24.79 24.72 24.53 24.73

100 110 115 110 89 92 102 95 109 109 105 110 92 98

2.60 3.20 3.50 3.70 7.35

2.50 3.10 3.80 3.60 7.0

2.33 2.88 3.40 3.40 6.9

6.9 4.37 4.30

5.3 4.70 4.40

5.4 4.40 4.87

2.94 3.58 4.91 4.55 3.1

2.66 2.74 4.36

-

-

860 859 869 894 584 817 820 750 607 792 779 829 776

894 915 945 930 670 848 860 767 820 793 867 790

Gallium or Zinc Oxide (ut%)

0.32 0.33 0.34 0.31 0.28

0.31 0.30

0.32 0.31 0.29 0.27 0.31 0.32

2.01 3.62 5.95 7.95 3.3 2.3

Calculated from Breck-Flanigen equation

a) Activity.

Ccmersicn of n+xxme

Fig (7) shaws a plot of conversion against contact t b (W/F) for the reaction of n-hexane over CSY2 (Si/Al = 5.3) at 400 C. A clear induction period isapparent. R e s u l t s m y b e d e m i b e d b y t w o rate constants, kl at lawer conversion and k2 a t higher conversion. In order to minimise effects dueto reactor geometry, whichmight lead t o spurious results at higher flaw r a t s and lower conversion, results are cbtained using various experimental approaches. Conversion is varied by chanqiq flaw rate of n-haare aver fixed amounts of catalyst and by varying the amount of catalyst in a bed of fixed volune, a t constant f l m rate of feed. In all cases the experimental trials are randcrmised and same trials consist of single-point determinations on fresh c a t a l y s t to rninimise any affects due t o catalyst deactivation. The concordance of results suggests that the pattern evident in Fig (7) does not result frcnn geometric factors nor f r m deactivation. The probable explanation for these and similar results (11) is that the

Figure7.

Rate of n-hexane cracking Over X [Ga/AIIYZ and 0 CSYZ at 400°C.

15

05

25

35

45

55 CMTACT TlME/rec

Figure 8 .

Rate of n-hexane cracking over CSYl zeolite (framework S i / A I d )

( ) 1 ’ -

2425

Flgure 9

24 35

I

ZL 15

i

2455

24 65 UNIT CELL SIZE

2475

1/11

Isomerisation/hydrogen transfer as a function o f Zeolite unit Cell size (1) NHLY OCSY x ZSMZO x CSY-S OSAPO-37(38) REUSY Kheng J Catai ,19891

8

slower i n i t i a l rate largely reflects the activation of hydrmwbon molecules w i t h generation of sorbed active species, presumably carhnium ions or t h e i r precursors w h i c h are then involved i n propagation of the reaction. Tkis view is supported by the effect of adding mall amounts of olefins to the feed, which can enhance the i n i t i a l rate of reaction (14), o r by the presence of h e t e r o a t m w h i c h can pmvide active sites for n-hexane activation. The effect of gallium, w h i c h is discussed subsequently, is also seen in Fig (7). Fig (8) shows the effect of teqxmture on C S Y l (Si/Al= 6.9) in the range 350 C t o 450 C (temperatures less than 350 C can result i n problems associated with sorption (15)). E s t i m a t e s of activation ensuggest that the i n i t i a l rate is associated with a higher activation (Ea - 102 k J mol-I) than the subsequent rate (E - 85 k J mo1-l) hplying that a t higher temperatures the rate of generation of c a r b a t i o n Separate precursors is enhanced relative t o the subsequent reactions. pulse studies (14) using results a t higher conversion (15 - 30%), corresponding to the second stage of reaction gave an activation energy (E,) of (E 85 k J nr0l-l) in agreement w i t h results for k2 from Fig (8). A recent paper (15) reports activation energies a t hiqher conversion, of 88 and 85 k J m1-l for the reaction of n-hexane over zeolite Y (LZY -82; 41 Al/vC) in nitrogen or helium respectively.

-

b)

Selectivity in n-Hexam Coenrersim

The activation of hydmcarbns over zeolites is widely held t o result f m direct protmnation a t C-C or a t C-H bonds (16) (17) as proposed for reaction in superacid media (18) (19). mesent results (14) are exemplified by Fig (11) and Table 2. Frrrm the l i m i t i n g slopes of plots of weight selectivity against conversion (20) the p r d u c t s a t zero conversion may be estimated (Schm 1).

75

c1 + c5 0.05 (0.04)

n

c2 + c2= + c4 0.02 (0.23)

+ c4=

c3 + c3= 0.7 (0.67)

- hexane

H2 4- c6= 0.05 (0.05)

and C, signify olefins and alkanes respectively ard the n W r s refer t o weight selectivities determined from limiting slopes of selectivity/conversion plots. The figures in brackets are weight selectivities calculated by extrapolation of results in Fig (11) t o zero conversion.

Cn=

9 Table 2

Z e o l i t e CsY2 ( S i / A L = 5.3) * Reaction Tenp 400°C Feed: n-Hexane (mole o f Product per 100 moles o f n-hexane converted)

CT U s e c

0.021 8.2 0.21

U/F (ghlmol)

Conversion (%) Hydrogen Methane Ethane Ethene Propane Propene iButane n-Butane tran-2-butene 1-butene iso-butene c is- butene Butene Pentane Pentene i- hexane Hexene Heptane Heptene Aromatics P/O C/H (Total) iC4/nC4 c3/c3= c2/c2= CMR

0.00 11.1 18.2 10.5 59.9 82.0 4.53 5.13 3.68 3.68 1.47 8.86 0.00 2.22 0.00 4.36 0.00 0.0

(w)

0.049 11.19 0.491

0.085 16.34 0.588

0.16 26.07 0.698

0.18 28.92 1.290

0.00 6.377 13.9 7.36

10.00 6.30 13.4 7.38 54 80.5 5 .8 5.6 2.84

7.5 4.5 9.5 6.7 67.2 80.0 6.8

4.90 2.58 10.3 0.16 2.7

9.87 6.05 12.95 7.61 59.63 84.20 6.14 6.16 2.19 0.66 2.85 1.76 7.47 0.96 1.92

8.4

4.7

48.7 78.03 5.0 5.3 3.43 2.49 1.25 7.2 0.0 2.7 0.0 3.2 0.0

0.83 0.43

0.83 0.43

0.837 0.623 1.i3 8.81

0.944 0.62 1.89 5.29

0.78 0.44 1.032 0.66 1 .a1 4.67

0.87 0.43 0.99 0.70 1.70 4.33

0.32 31.472 5.45

1.66 142.40 18.98

2.36 0.99 5.6 5.90 1.86 5.11 3.41 0.17

1.44 0.79 1.68 5.68 69.9 31.5 23.0 7.55 1.24 0.69 2.01 0.89 4.83 10.60 1.35 8.8 1.32 0.26

0.41

2.10

0.97 0.43 1.10 0.83 1.41 3.04

1.41 0.42 1.93 1.24 0.67 1.03

2.74

6.1 1.91 1.19 2.39 1.43 6.95 2.20 1.98 3.58

2.68 1.75 3.86 5.74 72.9 58.63 10.9 5.7 1.40 0.79

0.42 3.07 2.21 0.295 0.35

iC4 Total Mass (%)

99.9

99.9

98.4

99.8

99.8

100

59.9

Total Mass

8389

8392

8266

8386

8390

8405

8395

*

C a l c u l a t e d f r o m NMR

10

Extrapolation produces scam= uncertainty but provides an approximation to the initial prcduct distribution. ’Ihe initial product distribution (Table 3 ) derived from results in Table 2 can be rationalised in t e r m s of a possible reaction network (Fig 10) w h i c h suggests that the overall reaction involves a 90% contribution f m momlecular protolytic attack at C-C andC-H bonds, the reminder involving bklecular secondary processes particularly bblecular hydmgen transfer. Table 3

I n i t i a l ProrLrt Selectivity of n-llexane Cracking

(Moles/lM Moles of

A

-

-

-

_

13

-

11

50

50

-

C

7

-

Total

7

13

20

11

11 10 - 1 4 50 85

EXPT

14

13

20

11

50

B E&D

ovw CSYZ ( S i / A l = 5.3) at

W ' C

% Converted)

-

20

-

-

_

85

-

-

-

10 10

10 10

2

9

11

2

2

-

5

5 5

300 78 180 42 600

1400

64lO

1412

This mechanl’stic network, which is based on accounting and is not proven, Suggests that, at the low conversions observed at 4OO0C, moncanolecular processes d&te the reaction of n-hexane over Y zeolites enriched in silica. As activegaseous andsurface species becaw available at higher conversion, the reaction proceeds more extensively by bhlecular processes producing the induction period seen in Fig (7). This view is supported by results for the CMR values (21) shown in Fig (12) which hcrease very markedly as conversion approaches zero. Estimations of activation energies indicate that, as temperature is increased, mnmlecularprccesses tend todmhate the overall conversion. A recent detailed study of n-hexane cracking at 500 C over zeolite H-Y ( M e SK40), based on extrapolation using theonstream theory (22), reports that primary products are consistent with direct pmtonation at C-C bonds. Frcduct distributions (22) differ frmn t h e reported (Fig 11) here in that no hydrqen, methane, or hexane are observed as primary praducts at 500 C but isohexane is detected ( 2 2 ) . Apart f m differences in activation treatments the discrepancy between results at 500 C (22) and thcse reported here for 400 C may reflect differences in the types of catalyst used since the present results refer to the u s e of siliceous Y zeolites which are much stronger acids than typical Y zeolites. Figure 10 suggests the following order of protolytic crackh-q in n-hexane at 400 C (Table 4 ) , over the cSY2 zeolite.

at bonds

11

17 0

=PRODUCT

L-REACTlON M--NoOF

No MOLES

Figure10,

Schematic of n-hexane converslon over Y type zeolite (Si/A1=53 CSY2 ,Temp 400OC)

t

! % CONVERSION

Figure 11

% CONVERSION

Conversion of n-hexane over CSYZ zeolite (framework Si/AL=53) at 400°C- intermittent flow reactor

12

Table 4

mitial

scissicgl in n-kxam cxackiq over S - Y ( S i w = 5.3) at

400°C.

Bond

wlative Scission

The preference for attack at C-C rather than C-H (other than tertiary Cis reprtd for paraffin reactions in superacid solutions ( 2 3 ) and Mind0/3 calculations ( 2 4 ) support this. Olah ( 2 3 ) gives the order of protolytic attack in superacids at lm temperature as:

H)

(tertc-H)

> C - C >

(secc-H)

>> (prirraryC-H)

Although there is scnne cxnnplication arising frcnn a -1 contribution frum secondary reactions at 4 0 0 ° C it is noted that on H-ZSM-5, a stronger acid than conventional Y zeolite, h y m e n is observed ( 1 6 ) and is attributed to protolytic attack at C-H. Moreover, recent studies on conversion of both n o m l and iso-butane over H-ZSM-5 shm cleavage of C - H bonds at low conversion ( 1 3 ) .

The preference for attack on the centre C - C bond (Table 4 ) is expect& i n linear paraffin cracking ( 2 5 ) . The absence of ism-hexane in our study and its presence in conversion over H-Y ( 2 2 ) is more difficult to explain. In the present work, amounts of ism-hexane at low conversion were never more than impurity levels in the feed. If these are taken into account in previcus studies ( 2 2 ) differences may relate to temperature and acid strength. Iso-butxe/n-butane ratios are close to equilibrium values as mnversion decreases to zero and iso-butene is never more than 50%of the total C4 olefins (the equilibrium value at 400 C) suggesting that the butenes are close to equilibrium. There is uncertainty because 1-butene is not detected but, s h at equilibrium only around 10% of the butenes is present as 1-butene at 400"C, this is probably due to difficulty in detection. These C4= results suggest that is0 C4= and i s o C 4 are not extensively produoed by cracking of oligcnners and this view is supported by the absence of c7+ material in the product stream. since the isamerisation of the n-C4 carbem’um ion is energetically unfavourable it that consideration should he given to the possible isomerisation of the c6 HIS+ carbonium ion to give isoC6 H15'. This ion may then crack directly, or lose hydrcgen to given an iso-cg H13+ carbenium ion which, in turn, may crack by an unfavourable (D-cracking)p-Scission (%heme 2 ) .

13

& c c *

Similarly in scheme (2a) primary carknium ions are included. These are energetically unfavourable species in m i u m ion p-scission but the suggestion from these and previous results (13) is that the generation of primary w i u m ions by scission of carbonium ions may be less unfavourable. At higher conversion increased a m u n t s of iC4 are observed which, presumbly, are readily produced by oligamerisation to give %+ (n > 7) carbenium ions followed by isamerisation and cracking (Scheme 2b)

.

overall the present results are consistent with an induction period largely involving primary (mnmlecular) activation processes which generate surface precursors for carbenium ion processes. stabilisation of these carbnium ion precursors results in an increased rate of nhexane conversion with increasing contribution from bhlecular processes at higher converson, lmer temperatures and longer contact t i m e s . The balance of involvement of monamolecular versus bimolecular processes depends upon temperature, as reflected in activation energies, and upon strength and proximity of acid sites. The strength of acid sites strongly influences the equilibrium concentrations of surface carbenium ions which, for stronger acids, have longer surface lifetimes and can undergo secondary bimolecular processes providm that neighbow% sorption sites are available (13, 26). Consequently prcduct distributions can usually be explained by ptulathq different proportional contributions of primary and secondary reactions.

14

me

qui1ibriumbetwee.n olefinsand s0rbedcark.m’urn ions is a key feature of the overall reaction.

.mis reaction is endothesmic ard psh& to the right when temperature is raised. This shortens the lifetime of the carbenium ion and its surface as does reaction a t short contact time and l m concentration,

conversion, limiting its participation in oligcmrisation and hydrosen transfer - hence the increasing contribution from the primary m&am'SmS as temperature hmeases and conversion decreases. The effect of acid strength is ccanplicated by the fact that strength is UflldLly increased by reducing the framework aluminium content and therefore the proximity of sites ami the surface polarity ( 2 7 ) . Stronger acids stabilise the durn ions enhancing t h e i r lifetime, but the reduced proximity of sites can make bimolecular hydrosen transfer less favourable, a s discussed sukquently, and changes i n polarity can affect relative sorption of olefins versus paraffins (27). In the range of ccmrpositions studied here, the strorqer acids (more siliceous zeolites) shm an increase in the r a t e s o f both primary andsecondary processes as compared t o the parent Y zeolite. In scheme 2 we depict the activation of n-hexane involving direct protoMtion t o the cxaimnium ion. Hmever, this is not t o inply that these species are stabilised on zeolitic surfaces tplt rather t o provide passible recham'stic pathways. In fact, a t this tim, the nature of sorb& activat&hydmca&onspecies i s n o t k n m n andforexanple, a role for radicals o r radical ions cannot be excluded (28) nor can the involvement of Lewis acid sites w h i c h continue to a t t r a c t interest

-

(29)

DlENsITy

OF ACITW SITJB IN l?AIKR&mC

F o l l a w i ~ the ~ ~ generation of initial reactive surface species from hydmcarbn feedstocks the prcduct distrilxltion is largely governed by the balance of moncanolecular processes such a s isomerisation and Beta scission and b h l e c u l a r processes (for example oligoanerisation and b h l e c u l a r hydrogen transfer). A measure of the relative contribution of b b l d a r versus mncanolecular carbeniutn-ion like processes can be p m i d e d from the distribution of prcducts from cycloheene conversion (30). The basis for distindion may be seen from Scheme ( 3 ) .

15

c r a c k i n g t olkylation

Y - transfer

,

/+CHE

MI: PA

Readion of cyclohexene over acidic zeolites: CHE, cyclahexene; MCPA methylcyclocpentane; aIA cycl0heXi;me; MCPE, methylcyclopentene.

lm conversion over H-Y zeolits a t 250"C, the major prcducts are aIA, MBE and MCPA. consequently, by considering initial selectivities t o these products it :is possible t o define the relative rates of ismerisation (monmlecliLar:~and bimolecular hydrosen transfer

A t very

as:

Isamerisation Hydrqa-l Transfer

._ .-

I (MBE) + I (MBA) I (CNA) + I (MCPA)

Where 1 refers t o the i n i t i a l selectivities taken as the slope of the appropriate yield/conversion (x) 1 x u v r 3 a t the origin (x -> 0)

.

This function is platted against unit cell s i z e in Figure 9, w h i c h is related to the rnnober of framework a.luminiums and hence to site density (31). Previous results (30) dem~nstratet h i s effect for more siliceous ZSM-5 and zeolite Y materials and it is reported that mre than one site per large cage is r e q u i r d f o r b h o l e c u l a r hydrosentransfer in H-Y (32). T h e present results suggest I - h t the effect of site density is also evident i n the range of ompcx;itions studied here. Unit cell parameter is used for sirrp?le correlation and is adequate for zeolite Y and the CSY materials but is not s t r i c t l y adequate for ZSM-20 which is an i n of hexagonal an3 cubic Y nor for SARI-37 w h i c h although faujasitic i n strudure is not dir13ztly related by cell s i z e to the almincsilicate faujasites. Nevertheless the effect of site density aplpears to be detectable also i n SAPQ-37. Table (5) shows that a slight hcrease in silica content, w h i h can increase the density of acid sites, results in an increase i n the iscanerisation/hydmgen transfer function.

16

Table 5 Catalytic Cunmsicn of

2MIm

W Y -37 A sAK+37 B

%

si 6.6 3.6

w & m c e m ewer = s~1+37 ard Zeolite Y a t MCPE 0.49 0.34 0.30

MCPA

0.236 0.245 0.254

CHA

0.29 0.41 0.50

250°C

ISOMERISATION/ H-TRANS??ER

1.40 0.90 0.70

couzse, the relative sorption p a n n ~ t e r sfor alkanes and olefins can influence the balance of cracking o r isclmerisation versus b h l e c u l a r hydrogen transfer, particularly a t very high Si/Al (27), and t h i s effect is under investigation for these lower Si/M materials. Of

DEXREWTIPI OF ACITVE SITES IN F'AUIXITIC !ZEDIZl!ES

Previous Camments concern the activity and average density of active sites. It is of interest to know whether variation in the distribution of aluminium sites in faujasites having the same framework ccarposition (Si/AL) canbe achieved. In thepresent workthis isattempted by producing cubic faujasitic zeolites by synthesis i n the presence of fluoride anions (2) for amparison with prcducts of similar ccanpositon made by secondary synthesis using m4siF6 ( S Y zeolites). Details are given in Table (1). Solid state NMR can hxtkectly provide infornation on aluminium distribution and the relative intensities of the si(nAl)configurations (Fig 5) fllggest interesthq differences in the distribution of "T1' atcans in synthetic products asccanpared to those produced by secondary synthesis. The relative populations of S i ( W ) are higher and those of Si(0Al) are lmer for zeolites prepared by primary synthesis than for zeolites produced by secordary synthesis (11). Wt the enhance3 si (W) peak contains no s y f i c a n t contribution froan silanol grovps (sim) is clear frcnn the 1~ 9si cross-polarisation spectmm w h i c h shms negligible tof thesi(lA1) peak suggesting a very low concentration of silanols. ?his is confirmed by FTlR spectra of the hydroxyl region (14).

mereas

it is possible t o calculate the 2 9 ~ NMR i parameters for a faujasite structure tihere the aluminium orderirq is kncwn, it is not generally possible to define aluminium ordering f m experimental NMR spectra. Aluninium onkrhq i n zeolites m y be based on selection of unique structures (33) (34) or on mre detailed statistical models (35) (36) ( 3 7 ) . Althcugh it is simplistic to presume that 29Si spectra can be simulated by a single &asen aluminim ordering scheme it is

17

IL

6

20 '

Aromatar 2

3

4

CMIVERSION 19 1 . Figure12.

5

6

7

CARBDI WtiBER

Cracking mechanism ratm ICMRI for n-hexane conversion +ZSH-5 ZSM-11 3cCSY2 0 H-Y X CUB-Y 0 ZSM-20 A CUB-Y

F w r e 14.

Pmduct selectivhes far n-hexane conversion l1%1 aver 0 lGa/AIlY2. I NHLY at LW0C

-1s

5 a

. 1

z a 20.5

-

0

0

1

2

3

L

5

6

7

CaWERSION 19 1 .

O

O

l

L

t

CONVERSlON 1%1

0

c m s m DL1 Figura 15.

Ratio of prad~ctsfor n-hexane tonversmn aver NH'Y, x 1Ga/AUYZ, o tGa/AIIY3, at 400 C

Figure 13.

02

04

06 08 CONTACI TIME

10

li'l

n-hexane cracking wer CSY ISiIAI-LLI CUB-Y WAl-361 and ZSM-20 (Si/A1=35)

18

instructive to use specific distributions for ccenparative purposes (33). the synt2eticmateridls used i n t h i s study are close in ccmposition (Si/Al = 3.8 and 3.1) to values for centm-qmmetric stmctures (34) w i t h 40 and 48 alurniniums per unit cell (Si/Al = 3.8 and 3.0 respectively). Ihe faujasitic structures represented by two p cages (Fig 6) give calculated 29Si NMR parameters w h i c h are reasoMbly close to expervalues (11). The NMR results, therefore, suggest strongly thatthe alurniniu150-% i s d i f f e r e n t i n the synthetic materials asccanparedto thcsemadeby secondarysynthesisand i t i s then of considerable interest to knmwhether this is reflected in catalytic properties. 'Ihere is a f a i r l y widely held view that isolated aluminiumS provide the most active framework sites and these appear t o be mre numerous (for a given Si/Al) in the products made by primary synthesis since si(lA1) u n i t s a r e increased and Si(0Al) units are

TWO of

deQ-eased.

Fig (13) shms results for hexane cracking a t l m conversion over CUB-Y and CSY zeolites of similar ccanposition. It does appear that the synthetic pruc3uct.s are mre active and it is tapting to ascribe this to diffin alrmciniUm ordering. Hawever, a t this t h e this conclusion should betaken a s t e n t a t i v e s i n c e otherfactorSsuch as traces of s c d i u m ions can markedly affect rates and, although extensive ion exchangeprocedures areused, theanalysis f o r t r a c e sodim is subject to error and further work on t h i s aspect is i n progress.

'MEEEFECl!OFGALUCJM

T h e foreyohq discussion is confined to active sites generated in the Y strucbm by. aluminium boded via oxygen to silica. mere is wnsiderable interest i n themle of hetematom in zeolites and particularly i n the m l e o f g a l l i u m which isused i n the Qclar technology (38) i n association w i t h H-ZSM-5. In this present paper we discxlss briefly the effect of g a l l i u m incorpOration into zeolite Y, to

pruduce H-[&/All-Y oil. a)

zeolites, on the conversion of

n-hexane and gas

w€kxar~<3nnrersian

Introducing Ga into the frmamrk produces profound changes in reactivity. The slcwer kiuction period &served for CSY zeolites is replaced by a rapid initial r e a c t i c m (Fig 7 ) . Both this initial rate and the subsequent rate increase with increase in Ga content up t o a t least six Ga atoms per unit cell, the maxinnrm substitution examined in the present work. ?his rapid initial rate is associated w i t h the generation of considerable amountS of hydrcgen and very olefinic gasfxxls produds. In ccsnparison w i t h the CSY zeolites the [Ga/Al]-Y mterials shm deposition of %eke".

-

19

initial rapid process associattd, with the hydrcgen generation involves dehydmgenation of n-hexane,, presumably on Ga oxide species formed by dislogdement of Ga f m the framework during pretreatwnt and catalysis. De.hydroqa=tion is report& t o be rapid on acid zeolites (39). containing -20-3 and scheme (4) is p r c ~ d

The

-

(4) C6H13+

CGHf4 +

I Gap03

H' H2

Ga203

I

Hi

I

GaXL O?-G;iX'

+

CsHt3+ 2-

Carbenium ion stabilised on the zeolite

Gaseous product distributions (exc:tudhg hydrogen) for H-[Ga/Al]-Y2 (6 Ga/UC) and H-Y (Si/Al = 2.55) are shown in Fig (14) Even a t this l a conversion there is evidence of 0:LigonHisatin and cyclisation for the gallated catalyst. clear differences are also seen i n the increased olefinicity of the gaseous products ancl the l o w e r C3/C4 r a t i o when g a l l i u m is present. This r a t i o is fcuncl to be structurally dependent (40) for the cracking of Cn (n = 7, 8,, 10) hydrocarbns but is clearly d e m e n t on other factors for c6 (mnvwsion, when hetematcnns are present.

.

The high concentrations of hydrosen do rtot r e s u l t i n correspondingly cxmcmtmtions of c6= presumbly krause of further reactions of c6= either by ca&e.m'um ion processes or in further dehydrcgenation. An additional feature of the gallatexi ~ i t a l y s t s is the considerable inrrease in iso-cq/"c4 dLkane r a t i o (l'ig 15). Relative yields of iso-c4 alkanes are w e l l above equilibrium va:tues w h i c h is consistent with the generation of iso-Cq by cracking of b~ranchedoligconers which are readily gerierated on the gallated catalyst [see Sch2b). It is also clear frum the present work that the ga1:Latecl zeolites made by secondary synthesis appear to be more active than 1-Y zeolites containsimilar amounts of g a l l i u m introduced by hpnqnat.ion (14).

high

b)

oopnrersionof Gasoil

In the -p ' section the p-ct changes, for n-hexane cracking are briefly discussed. It is of particular interest t o see i f these changes are r e f h c k d i n the conversion of gas o i l . Recent legislation t o m e lead fram gasoline requires o c h n e enhancement without affectthe yield of other desired products. Results for n-hexane

20

conversion over the gallated Y zeolites shm increased i-C4/n-C4 alkane r a t i o and im=reased ammatics content. Increases in both isomer r a t i o and arclmatics in the produds f m gas-oil cracking should result i n an enhanced octane g a s o l h . 'Ihe zeolites, formed into Fee catalysts and activated, are used t o convert a Iclrwait waxy distillate gas o i l using a conventional MAT reactor a t 516 C w i t h varying catalyst/oil ratio. Liquid and gaseous

products a r e c o l l e c t d i n w n v e n t i o n a l m r e c e i v e r s andanalysd by

glc(41)

.

TABLE 6

GIIS-OIL C I U M I N G

Ut% OF FEE0

HY

Conversion Coke Gasoline LPG c 1 - C ~Dry Gas Hydrogen N-Butane Iso-Butane Iso-Butene

65.0 1.66 46.0 15.5 1.70 0.07 0.65 2.85 1.71

65.0 2.32 46.4 14.4 1.52 0.19 0.67 3.28 1.68

C5-Cio Iso-Paraf f ins C5-Cio Iso-Olefins c6+0 Aromatics Gasoline Olefin/Paraffin

14.2 6.1 11.1 0.50

13.5 7.1 11.3 0.65

H[ W A L I Y

'Ihe main features abserved for the conversion of n-hexane

a t lower conversion appeartoke r e f l e c t e d i n r e s u l t s f o r g a s - o i l m c k i n g a t much higher conversion. I n particular hmeases in gasoline yield, isoolefins, hydrosen and coke are obsenred and the i-C4/nC4 r a t i o is higher than for the parent H-Y. 'Ihe o l e f h i c i t y of the gasoline i s also increased w i t h H-[&/All - Y ccaopared to H-Y (olefin/paraffin r a t i o of 0.65 versus 0.50).

21

Althcugh the faujasite framework s t r u c t u r e has provided themjor catalytic acanponent of catalysts for ,camxcial crackirq of hydrocarbons for many years, it seems likely that further i m p r o v m t s i n processing are possible using recently-develapedimalogues and derivatives of zeolite Y.

control of monamolecular versus b h l e c u l a r processes in terms of the nature of the active sites, their strenqtli, proximity and distribution provides a basis for improved hydrocarbons processing.

W e thank s

m for support under the catilysis i n i t i a t i v e and the scheme (Grants GR/F 99380 and GR/F 22777) and the University of Ixlrham Industrial Research Laboratories for solid state NMR. W e also thank past and present students and co-workers.

L Broussard and D P Schoemaker, J Am.Chem.Soc.,82

1.

(1960)

1041.

J Guth, F Delpeato, L D e l m D t t e and L H w e , Zeolites, 10 2. (a) (1990) Sept. 546 J ciric, US Patent 3,972,983. (b) 3 . (a)

(b) 4.

B M lhk, E M F l h q e n , S T Wilsm, C A Messina and T R Cannon, in G D stucky (Ed.), ACS Symposium Series, 218, Intrazeolite chemistry, 1983, p79. A F Ojo, J DrJyer, J Wing and K Karh, press (JCS, Fmday Trans. 1991). C V M c D m i e l and P K Maher; US Patent 3,292,192 (1966);

3,449,070 (1969). 5.

G T Kerr, J Fhys.Chm., 72 (1969), 2594.

6.

H K Beyer and I Belenzkaya in B Imelik (Ed.), Zeolites, Elsevier, ~ ~ ~ t e r d a1980, m , p203.

catalysis by

22

G W Skeels and D W Breck in D Olson and A Bkio (Ed.) , m-oceeding of 6th Int. Zeolites conference, Butterworth Ltd

7.

UK, 1984, pp 87-96.

R M Dessau,

C T

W Chu, G T Kfx-r, J N Miale, Eur Pat-

134849

(1983)

G W Skeels and E M Flanigen; presented in Part at the National ACS Meet-, b mdes, CA; symposium on Advancesin Zeolite Synthesis. Sept 26-30, 1988. G W Skeels and E M F l d g a , in P A Jacobs et a1 (Ed.). Facts, Figwas, Fbture. Stud.Surf.Sci and Catal., 49A, E l s e v i e r , An&e?&m, 1989 pp 331-343. 10.(a)

(b)

S Emst, G T Kokotilo and J Weitkamp, Zeolites, 7 (1987) 180. J Dwyer, D Millward, A Araya and A Corms, J.Chem.Soc. Faraday Trans, 86,6, (1990) 1001. Dwyer, (1991)

K K a r i m a d R K Harris, in press (J.phys.Chem)

11.

J

12.

J Dwyer, K Karim, JCS Chem. Cormnun., Accepted for publication (1991).

R K Harris, R Skigeisi, in press

13.

J

14.

K K a r h andJ Dwyer. Workto bepublished. K K a r i m F A . D mesis, UMIST (1990).

15.

P D H a p k i n s , C L Marshall, J T M i l l W andL BRaska, in catalysis 1987, J W Ward (Ed). Elsevier (1988) 281-295.

16.

W 0 H a g and R M

Dwyer,

A A Garforth,

J.Catal. , (1991).

D e s s a U , m-oC.

8th

Int. cOngr.catal., Berlin,

2 (1984) pp 305. 17.

e ~Sanchez-mrin , and F Tamas, J.Cdtd1, 93 A Corms, J p l a ~ ~ A l J (1985) 30.

18.

G A Olah, Y Halpern, J Shen and K Mo-Y, J Amer.Chen~.SoC. 95 (1973) 4960; (b) 93 (1971) 1251.

19.

D M 179.

20.

(a)

BKOLWC%and H Hoq~~een, prOg.phyS.~.Ch~., 9 (1972)

B W Wojciechawski, prOg.Reaction Kinetics, 12 (1983) pp 201212.

21.

A F H Wielers, M Vaarkamp and M F M post, J Cat&., pp 51-66.

127 (1991)

23

B W wojciechclwski and J Nbbot, J

22.

Qn. Chm. Eng.,66

(1988)

825-830.

G A Olah, J R De Meltker & ,J Shen, J. Amer. Chem. Soc., 95

23.

(1973) 4952.

J Planelles, S Sanchez-Marin and F Tanas, 'Iheochem, 17 (1984)

24.

15. 25.

J Abbot and B W Wojciechowski, J. Cam., 115 (1989) 1.

26.

E A LcanbardO and W K Hall, tJ.CaI-d.,

27.

A Corn,

112 (1988) 565.

M Faraldos and A Mifwl, -1.

Cam., 47 (1989) 125-

133.

G B McVicker, G Kramer and J Zidak, J. Catalysis, 83 (1983)

28.

286. 29.

V L Zholobenko. L M Kustav, V B Kazansky, E Loeffler U LDhse and G O e h h a m , Zeolites, (1991) 132-134.

30. (a)

W C Chq,

(b) 31.

A W EeterS and K Rajagopalan, J.Catal., 122 (1990). W C cheng, A W Peters and K Rajagopalan ACS, Division of Petroleum Chemistry meeting Sep-t. (1989).

J Dwyer, K Karin and A F @ o r JtS, Faraday Trans.,

87

(1991)

873. 32.

E Jacquht, A Mendes, F Ratz, (3 Marcilly, F R Ribeiro and J Caeiro, Appl.catal., 60 (1990) 110-117.

33.

J M ? h a m a s , SRamlas, J K l h m k i , C A Fyfe and J Hartman. J.C!l-mn.Soc, Faraday Trans., 2,78, (1982) 1025.

34.

E Dempsey, J.Cat&l., 33 (1974) 497; 39 (1975) 155; 49

(1977)

115.

35.

A W P&fzrs, llgook of Abstracts11183rd National Meeting of ACS, Washington MJ 1982, 482.

36.

B Beagley, J Dwyer, F R Fitch, 13 Mann, J Walter, J.PhF.

37.

W A Wachter Proc. 6th Int. ZEO~. Conf., D Olson, A Bisio (Ed.). Butterworths Guildford (UK) , (1984) p141.

38.

J A Johnson and G K Miller, Pa]March 1984, San Antonio, (USA).

Chm. , 88 (1984) 1744

.

AM-84-45, NPRA Ann Meet-,

24 39.

P ?ierizucieia w d C N a c c a c h e , 2 ?sol. L36.

40.

C Mircdatcs and D Barthomenf, J Catalysis, 93 (1985) 246; 11 (1988) 121.

41.

D J Rawlence and J DtJyer in Fluid Catalytic Cracking IJ Concepts i n c a t a l y s t Design, M L O c c e l l i (Ed.) ACS Symp. SW.

, 452 p

56-78.

Catal.,

59 (1990) L31

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

25

ISOMORPHOUS SUBSTITUTION ,IN ZEOLITIC FRAME WORKS PROCEDURES AND CHARACTERIZATION

Jacques C. VEDRfNE Institut de Recherches sur r'a Catalyse, CNRS, 69626 Villeunbanne Cidex FRANCE

Abstract Isomorphous substitution of T element in a molecular sieve material is very interesting in order to modify its acidic Dr redox catalytic and shape selective properties. Different ways to perform such a substitution are now well established either during synthesis or post synthesis including solid-solid reaction between the zeolite and another oxide. The substituted element may be strongly or weakly bound to the framework i.e. may remain stable or may give rise to well dispersed metallic oxide particles entrapped in the cavities. This results in different catalytic properties and may even lead to bifunctional catalysis as for (;a-ZSM-5 material. Different characterization methods are used in order to determine whether the element is incorporated at framework or at #cationicexchangeable site position or as tiny metallic oxide particles entrapped in the ports or cavities. Several examples are presented including substitution of B, Ga, Fe, V, ( 3 , Cu in zeolitic frameworks or Si, Co, Mn etc in AlPO-4 molecular sieves. Effect cif such isomorphous substitution on acidic properties is described and discussed. The case of Ti is described in the same book by G . Bellusi and V. Fattore (Milano) while catalytic features are presented by P.A. Jacobs (Leuven). 1. INTRODUCTION

Catalytic properties of zeolites and molecular sieves materials are very much depending on both their chemical and physical features. The former aspect correspond to acidic, metallic, basic or redox-type reaci.ions since it involves active sites whose nature, chemical strength, density and distribution in strength have to be determined. The physical aspect is known to greatly influence shape selectivity (l), confinement (2) and diffusion (1) properties.

26

In the recent years isomorphous substitution of Si by foreign elements (3) as B(4-6), Fe(5), Ga(5), Ti(7), etc has been largely studied and has been shown to result in different catalytic properties. New materials as AlP04-n family has also opened new fields of interests and prospectives with the possibility of isomorphous substitution by many elements (8,9), as Si, Co, Mn and many others. Carbon and sulfide-type molecular sieves have also been synthesized. The purpose of obtaining isomorphous substitution and new molecular sieve materials was obviously to modify the chemical and physical properties of zeolitic type materials and therefore their catalytic properties. Modification of chemical properties of "well established" zeolites may be carried out chemically. Dealumination by hydrothermal treatment followed by acid leaching or dealumination by chemical reactants as EDTA, acetyl acetonate, etc are well known processes. Modification of zeolitic material mainly Y and ZSM-5 type zeolites by halides has been largely studied. The used of BCl3, Sic14 and Tic14 results in isomorphous substitution of T atoms by B(10) Si (11) or Ti (12) although precipitation of small amounts of the corresponding oxides in small particles cannot also be excluded. The effect on catalytic properties and also in crystallinity was also observed with stabilization due to the subsequent dealumination (13) but loss of crystallinity in other cases. NH4F was also used and shown to substitute OH- by Fresulting in a decrease in acidity (13). This also holds true for direct synthesis in fluoride medium as introduced by J.L. Guth and coworkers (14). PCl5 was also used for Y type zeolite (15). The use of Lac13 was at variance shown to neutralize Bronsted sites, i.e. to result in cationic exchange rather than to isomorphous substitution (13). Another type of modification consists in grafting a given compound on the outer surface or in the inner pore of zeolite crystallites by chemical reaction with external or internal OH groups. This results in a decrease in acidic sites density, in OH group amount as evidenced by infrared spectroscopy but in an increase in physical constraints. This may be visualize as an inner pore coating. This was realized with a phosphorous compound (16, 17) in ZSM-5 material or with disilane (18) in mordenite. One may imagine to graft any bulky compound to the OH group on the outer layer of the crystallites or within the pores (internal coating). Many compounds have been used and were shown to decrease acidity either external or internal and to improve shape selectivity feature (internal coating or narrowing of pore mouths favors diffusion of the less bulky reactant or product molecules). IR study of OH groups intensity and diffusion coefficient measurements allows one to define the role of the

27

additive. XPS data allow one also to determine if the additive remains on the outer layer or penetrates the inner pores of the crystallites. Modification may be performed by ilonic exchange of alkaline cations mainly Na+ or K+ by other cations which exhibit different chemical properties as such or after reduction to metallic state forming memllic particles. Steric hindrance may also play a role. At last modification may arise from solid-solid reaction between the zeolite and a given salt or oxide in dry or hydrothermal conditions. This often results in cationic exchange and was studied by many scientists for Cu, Cr, Fe, V, Mn, Mo (19, 20). Cationic exchange was also evidenced using metal chlorides (21). In this presentation our interest is focused on the characterization of modified zeolites samples and the effect in physical and chemical properties. The case of Ti and the effects of modification on catalytic features will be described in other papers of this book by G. Bellusi, V. Fattore (Milano) and by P.A. Jacobs (Leuven) respectively. 2. lMAIN TECHNZQUES USED FOR THE CHARACTERIZATION When a modification either during the synthesis or by post synthesis procedure has been performed the first question which aeveryone is asking himself is to determine if the modifier element has been incorporated in the lattice framework at the lattice position or at cationic sites location (exchangeable sites for instance) or at last entrapped in the cavities or pores as tiny cluster!; or metallic oxide particles or even metallic oxide crystallites as impregnated or deposited near the zeolitic crystallites. 2.1 Use of X ray diffraction technique One of the key technique is the X ray diffraction. Progresses in powder pattern analysis , mainly by the Rietveld refinement prclcedure (13), have greatly improved the characterization since it allows detailed analysis and better accuracy in crystallographic structural parameters values. The idea is then that by incorporating an element of different size the unit cell volume and lattice parameters will be changed as a function of the extent of substitution. This has Been very clearly observed for boron which is a very tiny element resulting in a sjkrinkage of the unit cell volume as shown for ZSM-5 zeolite in ref. 6 and 22 as a fimction of B content with B-0 length of 0.14nm instead of 0.16nm for Si-0. This i!; examplified in fig. 1 and 2 from ref. 6 where the linear dependence of the U.C. volume or CT XRD peak positions (4 peaks in the 2 0 = 45-48" region) versus B content corresponds to the incorporation of B at

28

framework position. For B not in substitutional position the U.C. volume or CT values shifts from the straight line (sample C in fig. 1 and 2).

t All

0 0

P

535

0

0

t

a

Alumhum

or Boron content p.r

U.C.

t

n \\

0.3'5 boron

C . 1.84 *

1..2.

1.6% boron

:

2

1

.'- .

s

4 4

J

~

-

I

L

Boron content pnr U.C.

Fig. I . Variations of unit cell volume values versus B and A1 contents expressed per U.C. f i e dashed line correspond to re$ 22 data. Sample C correspomi3to HZSM-S sample with high B content and obviously non framework B. Fig. 2. Variations of CT XRD peak positions (4 peaks taken in the 2 8 = 45 to 48 ") versus Boron content; the arrows represent the values for H-ZSM-5 samples impregnated with 0.3 and 1.6 wt % boron respectively. One arrives (23, 24) to the following expressionsfor the unit cell volumes of ZSM-5 zeolite : V (nm3) = 5.35 - 2.2271 B V (nm3) = 5.35 - 0.0049 A1

for Boron for Aluminum

29

The latter expression agrees with the finding by Bibby et al (25). Even more when ZSM-5 zeolite is contacted with B2O3 and heated at 50O0C, the XRD pattern analysis allowed us to show from the U.C. bolume or CT value (fig. 2) that part of boron has been incorporated into the lattice uhile ii part presumably remains as B2O3. The same kind of unit cell volume value changes was also mentioned to increase for Ti and Fe-ZSM-5. However in the latter cases the amount of element incorporated and the difference in T-0 bond length are loo small for the XRD data to be really conclusive to me. T-0 length equals 0.175 and 0.184nm for Ti-0 and Fe-0 respectively against 0.16 for Si-0 and 0.17 For A1-0. At variance for Y type zeolite the cell parameters are still used to determine the dealumination extent of such a material. The formula was proposed to relate the unit cell parameter a, (in nm) and the framework All" content as : Si/AlIV

192

= __________________________

for Y zeolite

1124 (a0 - 2.42383) Incorporation of Zr in ZSM-5 matrix (tetravalent but rather big cation) has been tried in direct synthesis using ZrClq in aqueclus or alcoholic (C2H5OH) solutions with TPA' as a template. It was observed (23) that the cristallinity degree was sharply decreasing with Zr content (up to 7 Zr per unit cell) which made the XRD characterization rather difficult. Using the Rietveld type refinement procedure, adapted by B.F. Mentzen one could have some indications as far as the samples have been well equilibrated in water (vide infra). It was observed that the unit cell volume increases with Zr content up to 3wt% (= 0.5 :Zr atom per u.c) from 5.345 up to 5.380 nm 3 . For higher Zr content the U.C.volume slightly decreased, which indicates that Zr is not any more substituted for Si in tht: framework. Moreover hydrophilic properties were observed to be brought about by the presence of substituted Zr at variance to pure silicalite features. Moreover small variations in unit cell volume and crystalline structure of ZSM-5 zeolite were observed to depend on the presence of adsorbates as water, xylene, benzene, etc (26-28). This makes the previous statement to be considered with care when experimental conditions of XRD analysis, particularly water pressure, are not well controlled. As a matter of fact X-ray diffraction data have been used i n a more complex manner by developing the well-known Rietveld method applied to powder X-ray diffraction pattern. By substracting the total integrated Bragg surface intensity to the background measured for a standard as silicon powder one may

30

calculate the crystallinity degree (29). Moreover using a refinement method (30) with the modified DBW 3.2 or DLS 76 programmes (31) and conventional differenceFourier synthesis, in order to get a good fitting between calculated and experimental X-ray diffraction patterns (Rb should be less than 15%), one may determine the topology of the material (P211n space group for MFI, Ima2 space group for AEL, etc), the spatial location of all T atoms and even more the presence of some hosts within the pores or cavities. It was then possible to locate p-xylene molecules within ZSM-5 pores (32), di-n propylamine molecules in as-synthetized SAPO-11 (33), tetrapropylamine template molecules in silicalite (34) and even some non crystalline phase within the pores (33). The technique allows to postulate the presence of occupied sites but unfortunately does not give the exact nature of the occupant host (35,36). Let us take one example to try to visualize more precisely the above discussion. The details of a part of the XRD pattern are shown in fig. 3 for the as synthesized (SYN with tetrapropylammonium template), hydrated (WAT) and four xylene molecules adsorbed per unit-cell (XYL-1) of B-ZSM-5 sample from ref. 35 and 36. The calcined form exhibits a monoclinic structure (P21/n space group) whereas SYN and WAT forms correspond to orthorhombic phases (Pnma space group). The striking differences in XRD patterns are seen in fig. 3. The p-xylene adsorption is known (27) to take place in two steps. The first one, up to four molecules per U.C. and designated XLY-I, corresponds to the formation of a low coverage complex and the second one (XYL-11) to a maximum of xylene adsorption with 8 molecules per U.C. The location of p-xylene could even be determined in the ZSM-5 channel by careful analysis of powder X-ray diffraction patterns using the trail error method (35) as shown in fig. 4. For SAPO-11 type materials it was observed (29) that the as synthesized SYN (with di-n propylamine template) the calcined (CAL), hydrated (WAT) and cyclohexane (CYC) saturated samples exhibit different XRD patterns as shown in fig.5. As for other molecular sievesit was important to determine if silicon was incorporated into the Alp04 framework or not and subsequently if basic or acidic properties were induced depending if Si4+ was substituting A13+ or $+, respectively. The X-ray diffraction pattern analysis allowed us to show that A1 and P atoms are strictly alternate and that the material crystallizes in the non centro symmetric Ima2 space group with a = 1.867 (2), b = 1.3373 (2) and c = 0.84220 (9) nm (33). The channels along c axis are elliptical and are occupied by the di-n propylamine molecules. Disordered domains were also evidenced presumably

31

1327

2e 9 .s

-I6

.

29 22 .s

.S

16

Fig. 3. Details of the XRD patterns for S W , WAT and X K - 1 forms sample (takenfrom re$ 35,36).

Of

22.5

B-ZSM-5

Fig. 4. Location of p-xylene in MFI framework deduced from XRD pattern analysis (Rietveld type refinement) @om reJ 35, j i g . 9). 389 I

SYN

CYC

Fig. 5. Details of the XRD patterns for SYN,WAT and CYCforms of SAPO-11 sample (takenfrom re$ 35).

32

occupied by Si02 clusters with formation of defects. The formulation was then determined to correspond to Sio.82 Pi8 o 1.18 A120080, 1.18 [SO21 ; 2DPA. This shows that Si substitutes for P at least partly. It was further evidenced that calcination to remove the template in air at 550°C results in a better crystallization and to an acidic (and non basic) material as shown by IR and TPD of ammonia (29). Interpretation of X-ray powder diffraction spectra by using profile refinement techniques allowed us to determine the positions of P, A1 and 0 in AEL - type materials (AlPO4-11, SAPO-11) and the effedt of hydration on the geometry of the AEL framework (37). The as-synthesized sample and its 550°C calcined form crystallize in the acentric Ima2 space group with 3P 3A1 sites per symmetric unit. Upon rehydration the material adopts the acentric P n g l space group with 5P 5A1 sites per asymmetric unit. The same procedure of refinement by Rietveld techniques and determination of water location within the 18-member ring pore were performed on VPI-5 (MCM-9) molecular sieve (38). The water molecules were found to form weakly associated layers within the pores in a manner not unlike that of liquid water. The VPI-5 structure was determined as hexagonal with space group P63cm and unit cell parameters of 1.89777 (3) and 0.81155 (1) nm with an U.C. volume of 2.53 14nm3. In MAS-NMR spectroscopy a broad 31P peak was observed centered near -30.7 ppm for the calcined SAPO-11 sample. For the hydrated form two peaks centered at -23 and -29 ppm and designated B and C in the following were obtained with a ratio close B:C to 1:4 but decreasing with Si content as shown in table 1 from ref. 37. In addition a very broad signal (-20 to -23ppm) was also observed for some samples and is designated A in the following.

+

+

Table 1. Ratios of B to C MAS-NMR peak intensities in fig. 6 as a finction of Si content of SAPO-11 samples. from re$ 3 7. Samples

1 2

3

Si Content wt%

A

B (6 in PPm)

C

C:B ratio

0 2.2 4.2

-20.5

-23 -23 -22.7

-28.5 to -3 1.O

3.2 3.7 8.3

-23

33

CAL

WAT

Fig. 6. Decomposition of M A S - M R spectra for three samples of SAPO-11 with Si contents of 0, 2.2 and 4.2 wt% respectively @om re$ 37fig.5).

n

0 0 0

n

framework oxygen framework ? frarnework A 1

Fig. 7. Dimers of water molecules localized by precise X ray powder dipaction analysis of a pattern recorded step by step (0.02' 29) for the hydratedform of SAPO11 @om ref. 37,fig. 2)

34

Decomposition of 31P spectrum into 5 components (fig. 6) has been carried out since XRD analysis allows to identify 5 different locations of P atoms. Moreover the low intensity peak B near -23 ppm is highly enhanced by cross polarisation which is obviously due to the proximity of protons as hydroxyl groups or water molecules. XRD pattern analysis allowed us to locate water dimers within the elliptical channels as shown in fig. 7. The broad peak A at = -20 to - 23 ppm was observed to be part of the spectrum and was assigned to an amorphous AlPO4 phase. Moreover the decreases in intensity of the B peak with Si content (table 1) allowed us to suggest that the corresponding P atom (Pl) is substituted for Si in the SAPO-11 framework. For Si content increasing acidity as determined by Temperature Programmed Desorption of Ammonia (TPDA) was observed to increase and then to decrease (37). It is quite probable that Si substitutes first P at P1 atom position creating acidic site and then substitutes nearby A1 resulting in Si substituted as pairs and then in a decrease in acidity. Such a substitution on site P1 was also proposed for Mn AlP04-5 materials (39). 2.2. Infm-red spectroscopy IR spectroscopy constitutes also a key technique for the characterization of modified zeolites either in the vibrational mode region (400 --> 1300c1n-~)or in the hydroxyl group region (3500-375C~m-~). One may obviously expect hydroxyl groups, particularly the acid groups, to decrease in intensity upon modification by a foreign element able to be attached or to exchange the protons. Moreover, the shift in the acid hydroxyl group band is a good criterion for the substitution of trivalent cations. For instance the bands at 3605cm-1 for A1 H-ZSM-5 is shifted to 3630 for Fe, 3660 for Ga and 3700 for B substitutions. Not only this is a good criterion to the substitution but also on the acid features. As a matter of fact the shift toward higher frequency values is a good indication of a decrease in acidic strength. Vibrational bands are also typical of a given zeolite and may therefore be used as a criterion. For instance crystallinity degree of ZSM-5-type material may be estimated from the 550:450cm-l band (40) ratios since the 550cm-1 band is typical of pentasil while the 450cm-1 band as observed for Si@ and all silicates and zeolites. When a foreign element is added new bands appear as for boron at 1380, 920, 700 and 67Ocm-l. It is tempting to assign these new bands to framework new T element. However such a criterion appears to be ambiguous. For instance the appearance of a band near 900-960cm-1 is often postulated as a good criterion but it is observed for B, Ti, Fe, ... substituents. It may therefore be considered cautiously. It may be quite

35

possible that this band correspond to T-OH vibrational mode shifted from T-0 mode and thus related to hydroxyl attached to the new T element (46) or to T-0 (Si). An interesting analysis consists in determining the v OH shift upon adsorption of weakly basic probe molecules as H2S, CO, C2H4 or c6H6 as shown in table 2, the shift increasing with acid strength.

Table 2. Ships of v o ~ f i e q u e n qupon adsorption of some weakly basic probes, expressed in em-1. Samples

VOH

VOH

co

H2S 23°C HY HY HY H-FeZSMS H-AlZSM5 H-SAPO-5 AlPO4-5

3640 3640 3640 3630 3605 3625 3680 3800

-196°C 275

Ref c2H4 23°C

c@6 23°C

350 300 440b 370 390 330

6Wa 284 3 16

300 280-320

47 48 49 48 47,48 48 48 48

330 350 325 240 180

a a peak was detected at 2480 c m - l / 2 6 0 ~ measured at -63°C

2.3. The use of ESR technique The technique is very sensitive to environmental symmetry of transition metal cations as far as they are paramagnetic. It may then constitue a hopefully reliable technique since substituted elements have obviously different environments from that of exchanged cations or metallic oxide particles within the pores or external to the zeolite matrix. Elements as V4+ or Ti3+ (d’ ions), Fe3+ (d5 ion), (d3 ion) have been extensively studied by ESR. The cation can be incorporated either in the synthesis medium or post synthesis via VCl5, VOCl3, PCl5, TiC14, ... attack. A detailed ESR study for Fe-ZSM-5 complemented by Mossbauer and UV-vis techniques allowed us to characterize three types of Fe3+ environnments (50) as : site1

r’

Site I1

0

0 NFe,o

Site I11

@./ 0 ’

‘ 0

Fe 0’ ‘ 0 0

A0

O, 0

a/

/Fe\

O\ /

0 0

Si

/

\ 0

36

Site I : geff = 4.28 6 = 0.34, QS = 0.2 to 1mm.s-' Site I1 : geff = 2 to 3.3 6 = 0.18 , Q S = 0 mm.s- 1 Site I11 : geff = 2.0 6 = 0.18 , QS = 0 mrn.s- 1 Site I corresponds to the usual tetracoordination of a trivalent cation resulting in acidic OH group (3630 cm-' band). Site I1 is rather peculiar since it corresponds to lattice Fe3+ ion but is sensitive to adsorbate as H20, NH3 or any hydrocarbon resulting in tetracoordinated species. Reversibility was demonstrated by ESR with geff value varying with the adsorption of water or NH3 from 3.3 down to 2.0. The site III corresponds to highly symmetric Fe3+ cation i.e. with geff = 2.00 as expected for a symmetric d5ion. The geff = 2.00 value may also be enterpreted in terms of high symmetry of Fe3+ cations like Fe3+ at exchangeable cationic sites. This holds also true in many cases, which makes the geff = 2.0 signal non unambiguously assigned. This arises from the fact that Fe3+ ions occupy a 6S5/2 ground electronic RusselSaunders state for high spin d5 ion with the spin Hamiltonian as follows :

8 s

= g A H. S,

+ D [ Szz - (S (S+1)/3] + E (SX' - Sy')

D correspond to fine structure parameter and E t o out-of axial symmetry factor of the fine structure tensor. For Fe3+ in high spin state g = ge = 2.0023 S = 5/2 and one writes :

For high symmetry environments one expect the ground state to be five fold degenerate and to give an isotropic peak at g = 2.002. When distorsion takes place, inducing charge disymmetry, one has one symmetry axis (so called "axial symmetry") with D # 0 and E = 0. The transition between to spin state (AMS = f 1) correspond to hv = gef@H, geff including fine strucure parameters but the true g value is still equal to 2.002. For high symmetry one has E/D = 1/3 and one gets an "isotropic" peak at geff = 30/7 = 4.286. For E = 0 one has gII = 2 and = 6,i.e. a powder spectrum spread over a broad magnetic field range. The above conclusion concerning framework iron (site 11) is unusual but has also be met for boron ZSM-5 which was shown to greatly depend on adsorbates by MAS-NMR technique mainly (6 and ref. therein). It corresponds to surface defect (unsaturated coordination). Reduction by H2 for Fe-ZSM-5 at high temperature

37

(500°C) or dissolution by a chemical reactant supports such a conclusion since the ESR signal I1 was not observed to decrease. ESR technique may also be used to try to identify metallic oxide particles. As a matter of fact when magnetic ions constitute an oxide (ferromagnetic, antiferromagnetic, etc) it is usually not detectable by ESR as for Fe2O3, V2O4, Cr2O3 because of the too strong interaction between spins. In such a case magnetic susceptibility determination from magnetization measurements is the only way. When the particle size decreases magnetization decreases and tends to superparamagnetic behaviour which could be observable by ESR. This holds true for tiny crystallites of Fez03 and Cr2O3 with broad peaks (500 > - 1500G) and geff values in the 2.1-2.5 range. The change in ESR peak shape and intensity with temperature may then be very useful to follow the crystallite size changes. Unfortunately the theory is not advanced enough to get precise information on the particle size.

In a study of Fe and Cr zeolites particularly ZSM-5 such a characterization has been carried out by many authors (23,50-55 and many others). Hydrothermal treatment post synthesis was studied by means of ESR and magnetic susceptibility measurements (56) and was shown to give Fez03 and Cr2O3 particles. In the same vein ESR technique has been used to try to characterize solid-solid reaction between a given salt or oxide and a zeolite matrix particularly by Kucherov, Slinkin and others (19, 20, 57) with elements as Cu, Cr, Fe, V on HZSM-5 (19) and on H-Ga ZSM-5 (20) and Fe and Mn on ZSM-5 (57). Usually the interpretation consists in assigning the interaction to cationic exchange of H+ or Na+ by the added elements. This holds probably true but as mentionned above for B2O3 impregnated on HZSM-5, part of the element may be isomorphously substituted as T element in the framework. Exchange of cations as Cr3 -t and their characterization was already studied on Y-type zeolite in the early 70 ies (60,61). 24. The use of NMR spectroscopy

Some uses have already been mentionned as a complementary technique to XRD technique. For instance, MAS-NMR of 31P atom allowed to differentiate different types of framework P in SAPO-11 matrix (37). Static and MAS-NMR of "B element was very useful to determine if B was incorporated substitutionaly in the

38

framework and to evidence the role of adsorbates (6, 42, 62) even on substitutioned element. In a recent work (63) X -type zeolite was modified by Cs in order to get a good catalyst for toluene side chain alkylation by methanol giving styrene and ethyl benzene. Cs+ ion has been introduced in cationic exchange position or as tiny Cs20 particles. A mechanistic study has shown that such a reaction follows a bifunctional mechanism and necessitates Cs' cation both in cationic sites and in Cs20. A MASNMR study of 29Si and 27Al has shown that Cs incorporation did not modify the material. 133Cs Nh4R gives a peak nea.r 40 ppm relative to CsCl 1 M solution, which is due to hydrated cation Cs (H20)'. Upon dehydration a broad peak was observed with a chemical shift depending on Cs content. The MAS-NMR study showed that CsNaX exchanged zeolite exhibits one peak while exchanged and impregnated CsNaX sample exhibits two peaks as reported below. The first peak corresponds to Cs' exchanged cation and the second one to Cs20.

Samples

Cs-NaX Cs2O/CsNaX

Cs(wt%) exchanged impregnated

26.3 26.3

0 7.7

Chemical shift (ppm) hydrated dehydrated Partly dehyd. MAS 40

40

-81 -67

-75 -70,-108

These results show that water strongly affects Cs cations whatever in cationic positions or in Cs20 particles and the NhfR allows one to easily differentiate both types of CS+ cations. At last the use of MAS-NMR for characterization Si/A1 ratios is well recognized at present. Note that the use of changes in 29Si peaks intensity ratios versus isomorphous substitution by elements as Fe (paramagnetic), Ti etc, is not valuable because of both low 29Si and T element contents. The uses of 12'Xe introduced by J. Fraissard (64)is also of interest to detect the presence of amorphous phases within the pores as far as their amount is large enough.

39

CONCLUSIONS

1. 11.

...

111.

iv . V.

vi .

Different procedures of modification of zeolite-type materials can be used : Direct synthesis by adding a given salt in the basic medium. Hydrothermal treatment to extract relatively unstable T element from the lattice as Al, Ga, Fe, etc. Uses of halides to substitute T elements by the new elements as for Si, B, V, Ti, Sn, P, etc. Solid-solid reaction at high temperature (e.g. 500°C) between a given salt and the zeolite. This usually was shown to result in cationic exchange. Exchange with any cations which size and chemical features have some effect. Impregnation with a given salt which results in inner pore or outer crystallite neutralisation, i.e. changes in acidic and physical constraints properties.

XRD Diffraction Technique using Rietveld refinement approach and infrared spectroscopy of hydroxyl groups and vibrational mode appear to be the key methods to determine if a foreign element has been incorporated at substitutional sites of the framework. MAS-NMR, ESR, UV-vis and Mossbauer techniques appear also to be of great value when applicable and are anyway of great interest. Diffusivity measurements may also give some information if steric hindrance has been created. However in any case the presence of amorphous oxide phase as tiny particles entrapped in the pores and cavities cannot be totally excluded. This was shown long time ago for Ca and Mg exchanged Y zeolites (65) where small clusters (Mg0)n or (Ca0)n with n=2 to 10 were evidenced after calcining the Mg and Ca exchanged Y zeolite at 600°C. This was also shown for Fe (56), Ga (66), Cs (63) which induces bifunctional properties in catalysis. For instance Fe or Ga substitution induces acidic properties, strength being less than for A1-ZSM-5 while the presence of iron or gallium oxide particles induces hydrogenation or dehydrogenation properties typical of the oxide. The proximity of both sites within a zeolitic pore may strongly favor bifunctional catalytic properties. The case of Cs is original and has been considered. As a general conclusion it can be said that modification of zeolite by multiple ways is a fascinating and promising scientific challenge since by controlling it one may monitor any zeolite material to make it more efficient for a given reaction.

40

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

P.B. Weisz, Pure Appl. Chem. 52 (1980) 2091, J.M. Csicsery in "Zeolite Chemistry and Catalysis", J.M. Rabo, ed.,Monograph ACS 171 (1976) 680. E. G. Derouane, J.M. Andre and A.A. Lucas, J. Catal., 110 (1988) 58. R.M. Barrer, in "Hydrothermal Chemistry of Zeolite", Academic Press, London, UK, 1982 p419. M. Taramasso, G. Perego and B. Notari, in "Proceed. of 5th Intern. Zeol. Confer., Napoli", L.V.C. Rees, ed.,Heyden, London, UK, (1980). C.T.W. Chu and C.D. Chang, J. Phys. Chem., 89 (1985) 1569. G . Coudurier and J.C. Vedrine, Pure Appl. Chem. 58 (1986) 1389. M. Taramasso, G. Perego and B. Notari, US Patent 4, 410, 501 (1983) assigned to ENI. E.M. Flanigen, B. M. Lok, R.L. Patton and S.T. Wilson, Pure Appl. Chem. 58 (1986) 1351. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Sannan and E.M. Flanigen, J. Am. Chem. SOC.104 (1982) 1146. E.G. Derouane, L. Baltusis, R. M. Dessau and K. D. Schmitt in "Catalysis by acids and bases", B. Imelik et al (ed) Stud. in Surf. Sci. and Catal., series Elsevier, Amsterdam 10 (1985) 135. H.K. Beyer and I. Belekaja in "Catalysis by Zeolites", B. Imelik et al (ed) Stud. in Surf. Sci. and Catal., series, Elsevier, Amsterdam, 5 (1980) 203. B. Kraushaar-Czametzki and J.H.C. Van Hooff, Catal. Letters 1 (1988) 81 and 2 (1989) 43. C. Ferrini and H.W. Kouwenhoven, in "New Developments in Selective Oxidation", G. Centi and Trifiro (Ed), Stud. in Surf. Sci. and Catal., 55 (1990) 53. R.S. Li, W.Y. Zhang, D.R. Lai and Q. Wei. Appl. Catal. 71 (1991) 185. J.L. Guth, H. Kessler and R. Wey, Pure and Appl. Chem. 58 (1986) 1389, J.L. Guth, H. Kessler, J.M. Higel, J.M. Lamblin, J. Patarin, A. Sieve, J.M. Chezeau and R. Wey in "Zeolite Synthesis" (M.L. Occelli and H.E. Robson, ed.)ACS Symp. Ser. 398 (1989) 176. M. Kojima, F. Lefebvre and Y. Ben Tarrit, J. Chem. SOC. Faraday Trans. 86 (1990) 757. J.C. Vedrine, A. Auroux, P. Dejaifve, V. Ducarme, J. Hoser and S.B. Zhou, J. C a d . 73 (1982) 147. H. Vinek, G. Rumplmary and J.A. Lercher, J. C a d . 115 (1989) 291. Y. Yan, Y. Verbiest, P. de Hulsters and E.F. Vansant, J. Chem. SOC., Faraday Trans. I85 (1989) 3087 and 3095. A.V. Kucherov, and A.A. Slinkin, Zeolites 6 (1986) 175, 7 (1987) 38 and 7 (1987) 43. A.V. Kucherov, A.A. Slinkin, G.K. Beyer and G. Bordely, J. Chem. SOC., Faraday Trans I, 85 (1989) 2737. G . Bordely, H.K. Beyer, L. Radics, P. Sandor and H.G. Karge, Zeolites 9 (1989) 428. N.A. Kutz, in "Proceed 2nd Sympos. of industry - University Cooperative Chem. Program", Texas A et M Univ. Press, 1984 p.121. S. Chakib, PhD dissertation, University of Lyon (F) (1989) 91-89. B.L. Meyers, S.R. Ely, N.A. Kutz and J.A. Kaduk, J. Catal. 91, (1985) 352. D.M. Bibby, L.P. Aldridge and N.B. Milestone, J. Catal. 72, (1981) 373. E.L. Wu, S.L. Lawton, D.H. Olson, A.C. Rohrman and G.T. Kokotailo, J. Phys. Chem. 83 (1979) 2777.

41

27 28 29 30 31 32 33 34 3s 36 37 38 39 40 41 42 43 44 4s 46 47 48 49

so 51 52 53 54 55

56

D.H. Olson, G.T. Kokotailo, S.I., Lawton and W.H. Meier, J. Phys. Chem. 85 (1981) 2238. C.A. Fyfe, G. Kennedy, C.T. De Schutter and G.T. Kokotailo, J. Chem. Soc., Chem. Comm. (1984) 541. R. Khouzami, G. Coudurier, B.F. Mentzen and J.C. VMrine in "Innovation in Zeolite Materials Science", P.J. Grobet et a1 (ed) Stud. in Surf. and Catal. Series, Elsevier, Amsterdam 37 (1988) 351. B.F. Mentzen, C.R. Acad. Sci. Paris 303 (11) (1986) 1299. D.B. Wiles and R.A. Young, J. Appl. Cryst. 14 (1981) 149. B.F. Mentzen, J.C. VMrine and R. Khouzami C.R. Acad. Sc. Paris 304 (11) (1985) 1017. B.F. Mentzen, J.C. VMrine and R. Khouzami C.R. Acad. Sc. Paris 304 (11) (1987) 11. C. Baerlocher, in "Proceed. of the 6th Int. Zeol. Confer.", D.H. Olson and A. Bisio (ed), Butterworths, London, UK, (1984) 823. J.C. VWrine, G. Coudurier and B.F. Mentzen, in "Prospective in Molecular Sieve Sciene", W.H. Flank and T.E. Whyte, ed., ACS Sympos. Ser., Washington, 368 (1988) 66. J.C. Vtdrine, G. Coudurier and B.F. Mentzen, in "Proceed. XI Symposio iberoamericano de Catal., Guanajuato, Mexico, F. Cossio, 0. Bermudez, G. del Angel and R. Gomez ed.,IMP, Mexico, 1988, 473 R. Khouzami, G. Coudurier, F. Lefebvre, J.C. VCdrine and B.F. Mentzen, Zeolite 10 (1990) 183. P.R. Rudolf and C.E. Crowder, Zeolites 10 (1990) 163. J.J. Pluth, J.V. Smith and J.W. Richardson, J. Phys. Chem. 92 (1988) 2734. G. Coudurier, C. Naccache and J.C. VMrine, J. Chem. SOC.,Chem. Comm. (1982) 1413. M.S. Sayed, A. Auroux and J.C. VWrine, Appl. Catal. 23 (1986) 49 and J. Catal. 116 (1989) 1. G. Coudurier, A. Auroux, J.C. Vdrine, R.D. Farke, L. Abrams and R.D. Shannon, J. Catal. 108 (1987) 1. M. Taramasso, G. Perego and B. Notari, US Patent 4, 410, SO1 (1983) assigned to ENI. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrini, in "Structure and Reactivity of Surfaces". C. Morterra et al (ed), Stud. in Surf. Sci. and Catal. Series, Elservier, Amsterdam 48 (1989) 133. J.S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal. 58 (1990) L1. G. Coudurier, Personnal communication. L.M. Kustov, V.B. Kazansky and P. Ratnasamy, Zeolites 7 (1987) 79. S.G. Hedge, P. Ratnasamy, L.M. Kustov and V.B. Kasansky, Zeolites 8 (1988) 137. A. Macedo, PhD dissertation, Paris VI (F) IFP 36875 (1988) D.H. Lin, G. Coudurier and J.C. Vdrine in Zeolites : Facts : Figures, Future", P.A. Jacobs and R.A. van Santez (ed), Stud. In Surf. Sci. and Catal., series, 49 (1989) 1431. R. Szostak and T.L. Thomas, J. Catal. 100 (1986) 555, and J. Chem. SOC. Chem. Comm. (1986) 113. K.B. Borade, Zeolites 7 (1987) 398. O.V. Bragin, L.M. Kustov, T.V. Vasina, E.G. Khelkovskaya-Sergeeva and V.B. Kazansky, Kin. i Kat. 29 (1988) 1393. R. Kumar and P. Ratnasamy, J. Catal. (1990) 89. G.J. Kim and W.S. Ahn, Appl. Catal. 71 (1991) 55. R. Szostak, V. Nair, D.C. Shieh and T.L. Thomas, Mat. Res. SOC.Symp. Proc. 111 (1988) 289.

42

57 58 59 60 61 62 63 64 65

B. Wichterlova, S. Beran, S. Bednarova, K. Nedomova, L. Dudikova and P. Ji N,in "Innovation in zeolite Material Science". P.J. Grobek et al (ed), Stud. in Surf. Sci. and Catal., series, Elsevier, Amsterdam, 37 (1987)199. B. Wichterlova and P. Jiru, React. i Kin. Catal. Letters. 13 (1980)197. B. Wichterlova, Zeolites 1 (1981)181. J.P. Hemidy, F. DehveMat and D. Comet, J. Chem. Phys. 70 (1973) 1716. J.P. Hemidy and D. Comet, J. Chem. Phys. 71 (1974)739. K.F.M.G.J. Scholle, A.P.M. Kentgens, W.S. Veeman, P.F. Frenken and G.P.M. Van der Velden, J. Phys. Chem. 88 (1984)5. D. Archier, PhD dissertation, University of Lyon (F), 154-89(1989). T. Ito, M.A. Springuel-Huet and J. Fraissard, Zeolites 9 (1989)68. P. Meriaudeau and C. Naccache, 3. Mol. Catal. 59 (1990) L31,Appl.Catal.

73 (1991)L13. 66

A. Abou-Kais, C.Mirodatos, J. Massardier, D. Barthomeuf and J.C. VMrine, J. Phys. Chem. 81 (1977)397.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

43

IN'I'KODUCTION OF CATIONS INTO ZEOLITES BY SOLID-STATE KEACTION

Hellmut G. Karge and Hermann K. Beyer Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, 1000 Berlin 33, FRG

ABSTRACT A brief review is given of early observations of cation incorporation into zeolites through solid-state reaction. The general procedure and techniques suitable for investigation of solid-state ion exchange in zeolites are described. Results of systematic studies on introduction of alkaline, alkaline earth, rare earth, transition metal and noble metal cations into hydrogen, ammonium and sodium forms of zeolites are reported. In these studies, halides or oxides of the in-going cations are preferentially employed. Particular attention is paid to the stoichiometry of the solid-state ion exchange. It is shown that in several cases a one-step solid-state reaction leads to a 100% cation incorporation, whereas such a high degree of exchange is difficult to obtain by conventional methods. As is illustrated by a few selected examples, solid-state ion exchange might be an interesting way to prepare active acid and bifunctional catalysts. INTKODUCTION In the early seventies some interesting results were reported which showed th a t ion exchange in zeolites might take place not only by interaction of the solid zeolite phase (containing the cation I to be replaced) and a solution (containing the cation II to migrate into the zeolite structure and replace cation I). Rather, it turned out that a reaction of two solid phases, for instance of a zeolite with cation I and a salt of cation It,may also result in the zeolite containing cation II. Thus, Rabo et al. [l-31 found th a t proton-containing samples of zeolite Y reacted with sodium chloride under evolution of hydrochloric acid. By means of IR spectroscopy i t was demonstrated that this reaction eliminated acidic OH groups and thus, removed residual Brprnsted centres from the zeolite structure which are active in butene isomerisation [ll. This observation was in line with the early finding that cracking catalysts were deactivated by interaction of the zeolite component with the matrix. Obviously, in these cases cations from the matrix (clays, alumina) migrated into the zeolite structure and eliminated acidic hydroxyl groups. Clearfield and co-workers [4] were the first to apply ESR spectroscopy in order to confirm solid-state ion exchange. They introduced transition metal cations such as Cu2+, Zn2+, Ni2+ and Cr3+ into zeolites A, X and Y via reaction of salts of those cations and deammoniated ammonium forms of the zeolites. Occupation of the cation

44

sites in the zeolite structure by transition metal cations was indicated by the appearance of the typical ESR spectra. These spectra were identical to those obtained from conventionally exchanged zeolites. Surprisingly, for a relatively long time the studies on solid-state ion exchange were not continued. Only in the mid-eighties did three research groups start systematic work in this area, viz. the groups of Kucherov and Slinkin, Karge and Beyer, and Wichterlova and Beran. Since then considerable activity in this field has emerged. A special case of solid-state ion exchange was found by Fyfe et al. [5] who investigated the cation transfer between two zeolite pairs, for instance L i - m a - A , L i - m a - Y or Li-AINa-MORby 29Si MAS NMR and XRD. Obviously, there is a steadily increasing interest in the phenomenon of solid-state ion exchange. This is partly due to possible advantages of solid-state ion exchange with respect to several applications. In contrast to conventional ion exchange, solid-state ion exchange does not require handling and disposal of large volumes of salt solutions. Moreover, in many cases solid-state ion exchange proceeds more easily than the conventional exchange in suspension. This is especially true when, in aqueous solution, the in-going cation is strongly hydrated and prevented by its hydration shell from penetrating into small cavities and narrow channels of the zeolite structure. Examples are the incorporation of lanthanum, nickel or platinum via solid-state reaction. In view of the many interesting aspects of solid-state ion exchange, recent developments in this field will be reviewed. General Procedure a n d Experimental Techniques The usual procedure for carrying out solid-state ion exchange is simple. Finely dispersed powders of the zeolite and a salt or oxide of the in-going cation are carefully mixed and heated, either in a stream of inert gas or in high vacuum. In fact, the temperature required for achieving a certain degree of exchange frequently depends on the nature of the cations and anions involved. In some cases a fraction of the cations is exchanged even during grinding of the salt-zeolite mixtures a t ambient temperatures. The effect of the duration of heat treatment is less pronounced. In most cases the exchange is fast in the initial period of the solid-state reaction and then levels off. Examples will be given below. Solid-state ion exchange may be carried out in stoichiometric mixtures of salts (or oxides) and zeolites, related to the A1 content (in case of aluminosilicates) of the framework. However, i t also works with under-stoichiometric mixtures or mixtures containing a n excess of the in-going cation. In the latter case, the excess may be removed by brief extraction of the solid-state reaction product with water; also, salt occlusion may occur during heat treatment [2-31. Thus, with many important salt(oxide)-zeolite

45

systems a desired degree of exchange, even a 100% exchange, can be achieved in a onestep procedure. There are quite a number of experimental techniques to prove qualitatively, and determine quantitatively, the extent of solid-state ion exchange. In the first instance, electron spin resonance (ESR) and infrared (IR) spectroscopy should be mentioned. The experiments employing these techniques are carried out in essentially the same way a s frequently described in zeolite research [6-lo]. Due to its high sensitivity, especially in the case of many transition metals, ESR is appropriate for deciding whether or not solid-state ion exchange has taken place. Because of calibration problems, however, i t is frequently less suitable for determining the degree of exchange. With IR both qualitative and quantitative analysis is feasible. If the solid-state ion exchange is conducted with hydrogen or ammonium forms of zeolites, the change in absorbance of the OH stretching bands caused by the consumption of OH groups upon ion exchange is measured. To indicate the cations themselves, appropriate probe molecules such as pyridine, CO, CH,CN etc. may be used. Furthermore, the reaction of hydrogen or ammonium forms of zeolites with halides is easily monitored via titration of the hydrohalic acid evolved. This works particularly well with chlorides. Similarly, if gases such as hydrogen chloride or water are evolved due to solid-state reaction of hydrogen forms of zeolites with chlorides or oxides, mass spectrometry and gas chromatography are frequently employed. These techniques enable u s not only to detect the gases and thus confirm the fact of solid-state reaction but also provide means to evaluate the rate of exchange and the amount of exchanged cations. The loss of weight, which originates from the reaction of a halide or oxide with a hydrogen form of a zeolite, i.e. from evolution of hydrohalic acid or water, can be measured by a balance. This thermogravimetric analysis may be combined with GC or MS measurements or with continuous titration. In most instances, however, zeolites and zeolite-like materials are synthesized a s sodium forms. Thus, when sodium forms are the starting materials for solid-state ion exchange, magic angle spinning-nuclear magnetic resonance of 23Na (23Na MAS NMR) is advantageously employed to monitor that process. Examples are the exchange of sodium for other alkaline metal cations and the incorporation of lanthanum cations (vide infra). Solid-state Ion Exchange with Alkaline Metals Systematic studies on solid-state ion exchange with alkaline metals were carried out in the systems M'Cl/H-ZSM-5 and M'Cl/NH,-Y, where MI represents Li, Na, K,

46

Rb and Cs [ll-121.Also, the solid-state replacement of Na+ by Li+ or K + in Na-Y was investigated [ 131. As the first example, Figure 1 shows a sequence of IR spectra obtained with NaCVHZSM-5, where NaCl was admixed in excess (NaCVAl= 1.9). These spectra provide evidence for the following points. (i) Solid-state ion exchange took place upon heating the NaCl/H-ZSM-5 mixture in high vacuum (HV) a t 723 K (change of the absorbance of the OH bands). (ii) Not only the strong acidic OH groups (band a t 3605 cm-') but also the weak ones (band a t 3740 cm-l of so-called silanol groups) were involved (the latter band was weakened). (iii) The Na+ 0'-Si f groups formed by reaction of NaCl with silanol OH'S were easily hydrolyzed, since upon treatment with H,O the OH band a t 3740 cm-l was fully restored whereas the band at 3605 did not reappear; i.e. Na+ cations, which had replaced protons of the bridging, strongly acidic OH groups, were irreversibly held. (iv) No lattice destruction due to HCl evolution (vide infra) had occurred, since conventional re-exchange with NH,Cl solution and subsequent deammoniation resulted ina complete recovery of the original OH band a t 3605 cm-'. In this context it should be stressed that the structural integrity of the product of the solid-state ion exchange (NaZSM-5) was also confirmed via comparison of its X-ray diffraction pattern with th a t of the starting material (H-ZSM-5). No change in the intensities of the zeolite lattice reflections was detected. However, a significant decrease in the NaCl reflections was observed which is a further proof for the occurrence of a solid-state reaction. The same result holds for all the other examples of solid-state ion exchange reported here. The IR results of the NaCVH-ZSM-5 reaction were in excellent agreement with the stoichiometric data obtained from AAS analysis and titration. These data are listed in Table 1.

3605 I

3800

I

3600

W A V E N U M B E R [crn-']

Fig. 1. IR spectra of the OH stretching region after evacuation at 723 K (2h, 10" pa). a, HZSM-5; b, NaCl/H-ZSM-5 mixture (NaCl/Al= 1.89) calcined at 900 K (1 h); c, (b) washed with water; d, (b) twice exchanged with 1 N NH, C1 solution (Ref. [I11, with permission).

47

Table 1 Mass balance* for solid-state ion exchange in the system NaCl/H-ZSM-5

Zeolite H-ZSM-5 (1) H-ZSM-5 (2)

(1) SiiAl

(2) A1

155 23

0.107 0.691

0.01

0.808 1.306

(5)

(6) C1' extracted

(7) Na+ extracted

(8) Na+ non-extracted

mixture

HC1 evolved

NaCl/H-ZSM-5 (1) NaCVH-ZSM-5 (2)

0.549 0.828 =(4) - (6)

*

0.260 0.478

(3) Na+ -_

0.707 0.670

(4)

NaCl

0.101 0.636 (4) - (7)

all data in m m o l per gram zeolite fired a t 1273 K

(1) to (3) data of the starting zeolites; (4) added to the zeolite; (5) evolved during calcination a t 900 K for 1 hour; (6) and (7) extracted from the heat-treated mixture by

washing with water; (8) not extractable with water; compare with Al, column (2).

One recognises from Table 1 that the stoichiometry is excellent: (i) the amount HC1 evolved agrees well with the difference between employed and extracted Cl-, see columns (4) and (6); (ii) the amount of non-extractable N a + , i.e. the difference between N a + employed and extracted, see columns (4) and (71, coincides with the A1 content. Since these H-ZSM-5 samples did not contain essential amounts of extra-framework Al, each A1 corresponded to one bridging OH group. Therefore, the data of column (8) in comparison with column (2) show that a 100%or almost 100%exchange of H + for N a + occurred. With the same system, i.e. NaCUH-ZSM-5,it is illustrated in Figure 2 how the solidstate reaction upon temperature-programmed heating is monitored by thermal gravimetric analysis TGA and simultaneous continuous titration of HCl evolved (TPE). Curve (a) indicates the weight-loss of the NaCl/H-ZSM-5 mixture, curve (b) the amount of HC1 released. Both curves exhibit three steep steps. The first one (at about 400 K) is mainly caused by dehydration and only to a minor extent by evolution of HCl (see small step in curve (b)). The second step in curve (a) centered around 830 K coincides with the strongest evolution of HCI (curve (b)). Finally, at very high temperatures decomposition of the excess NaCl occurred and gave rise to steps a t ca. 1200 K i n both curves.

48 1.10

-

c

2

I

E

w W

' w

Y

n W

1.05

> A

-I

0

a.

I

>

Q

-

w

VI

U

I

1.oo

300

500

700

900

1100

T E M P E R A T U R E [K]

Fig. 2. a, TGA and b, continuous titration of evolved HC1 upon temperature-programmed heating of a NaCV HZSM-5 mixture (Na/Cl= 1.89)i heating rate 2.5 K min(Ref, [ll], with permission).

-

Investigation of solid-state ion exchange with the help of mass-spectrometrically monitored evolution of HCl and NH, was carried out with the system M'CVNH,ZSM-5. Results are presented in Figure 3. Two regions, viz. a low-temperature (LT) and a high-temperature (HT) regime of solid-state reaction, can be distinguished. With LiCl

r

1

c

5

TEMPERATURE

[K]

Fig. 3. Temperature-programmed evolution (TPE) of HCl upon heating M'CY NHA-ZSM-5mixtures; MI, Li(--); Na. (---); K (-*-a); Rb (-*--*=); c s (-**--*--); Li refers t o right scale (Ref. [Ill,with permission).

49

the LT reaction is eminently prevailing. The exceptional ease of solid-state ion exchange with LiCl was observed with other zeolites as well (vide infra). In the case of the other M'C1 salts, the contribution of the HT reaction to the overall process is significantly higher than that of the LT reaction. The temperature maxima both of the LT and the HT peaks decrease in the sequence T(Na) > T(K) > T(Rb) > T(Cs). Very similar results, including the exceptional behaviour of LiCl and the sequence of the temperature maxima, were obtained with mixtures of M'Cl/NH,-Y. The sequence of the peak temperatures corresponded to the decrease in the lattice energies of the alkaline chlorides. Thus, i t seems t h a t the solid-state ion exchange between alkaline chlorides and hydrogen forms of zeolites proceeds more easily the lower the lattice energy of M'C1. This suggests t h a t a low lattice energy would facilitate the separation of M'C1 entities from the M'C1 structure which then can migrate to and into the zeolite structure in order to react there with the OH groups. Note, however, t h a t such a relationship between the lattice energy of the compound of the in-going ion and the ease of solid-state ion exchange was not established with, e.g., Mn compounds and H-ZSM-5 (see below and Ref. [14]). Thus, parameters other than lattice energies may be operative as well. Cation exchange with alkaline metal cations can also be achieved by solid-state reaction between M'C1 and sodium forms of zeolites [13]. This was proven, inter alia, via 23Na MAS NMR. For the sake of comparison i t seems useful first to look at the 23Na MAS NMR signals of sodium in various surroundings (Figure 4). In the 23Na MAS NMR spectra, sodium chloride is used as a reference. Thus, the signal of crystalline NaCl appears a t 0 ppm. Sodium cations in aqueous solutions give rise to a sharp line at - 12.5 ppm whereas the sodium cations in hydrated Na-Y are indicated by a broad signal around - 8 ppm. When the solid-state reaction is carried out with LiCl and Na-Y a dramatic change i n the 23Na MAS NMR spectrum occurs even when the reaction temperature is as low as ambient. As is demonstrated i n Figure 5, the signal typical of Na in Na-Y disappears. Instead, the line typical of crystalline NaCl is developed and, furthermore, a broad signal appears a t about - 12.5 ppm where sodium cations in aqueous solution are indicated (compare Figure 4, line b). However, the linewidth of the 12.5 ppm band in Figure 5 is very large compared to that of Figure 4. The most likely explanation for these observations is t h a t a fraction of t h e sodium cations, which were expelled from the cation sites by Li cations, form tiny NaCl crystallites at the outer surface of the zeolite particles and give rise to the 0 pprn signal (simultaneously, i n the X-ray pattern of this sample the reflections of crystalline NaCl were detected). Another fraction of the sodium cations remains in the intracrystalline water of Na-Y. But in the cavities of Na-Y the mobility of the water molecules is restricted, and a large variety of coordination states are available for the sodium cations. These

50

factors may give rise to the observed broadening of the - 12.5 ppm signal, via a homogeneous (relaxational) and/or inhomogeneous (chemical shift) effect on the 23Na resonance.

I

I

l

l

I

1

I

I b, Nain

NaCl soh.

b

0

I

I

I

5

0

-5

I

-10

C H E M I C A L SHIFT,

-15

-20

6Nacl.cryrt.

I

-25

[ppml

Fig. 4. 23Na MAS NMR spectra. a, crystalline NaC1; b, saturated NaCl solution; c, Na-Y, without pretreatment (Ref. 11, with permission).

I0

I

I

0

I

I

-10

I

I

-20

C H E M I C A L 5 H I FT,

I

I

-30

6NaCI.cryrt.

I

I

-40 [ppm]

Fig. 5. 23Na MAS NMR spectra of Na-Y, without pretratment and LiCU Na-Y mixture, ground and intimately mixed (Ref. 14, with permission).

Again, LiCl reacts much more easily than other alkaline chlorides. Most probably, this is due to a particularly large decrease in the free energy when Li+ is transferred from crystalline LiCl or aqueous LiCl solution to the zeolite where the small Li+ cations are strongly "solvated" by the oxygens of the framework [ 151. The ease of exchange with LiCl is of great interest with respect to the important dealumination method using SiCl, [16-171, which yields silicon-rich, hydrophobic zeolites, i.e. valuable adsorbents. Sulikowski et al. 1181 have found that, when Li-Y is dealuminated instead of Na-Y, no self-inhibition of the dealumination by SiC1, occurs. This is due to the fact that, in contrast to NaA1C14, the product LiAlC1, decomposes a t the reaction temperature, and thus plugging of the zeolite pores is avoided.

51

Solid-state Ion E x c h a n g e with Alkaline E a r t h Metals Results were reported on the systems MgC12/H-MOR,CaC12/H-MOR or NH,-MOR [19] a s well a s on CaCl,/Na-FAU and BeC12/Na-FAU [ 131.

The stoichiometry of solid-state ion exchange was checked in detail with CaC12/ €1 MOR and CaCl,/NH,-MOR. Again, when an excess of CaC1, was used, part of the weak silanol-type OH groups also reacted. But they were easily hydrolysed. Excellent agreement between the aluminium content (bridging OH groups) and the amount of irreversibly held calcium cations was found.

1.40

3

-

m

Y

+

1.30

I

W W

3 1.20

1

1.10

1.oo

,+, LUV

*++.

, ,

%+-

wuu

TEMPERATURE [“C]

I

+

,+-+-+-+-+-+-

-

Fig. 6 a, TGA and b, continuous titration of evolved HCl upon heating of a CaCl / H-MOR mixture (Ref. r191. with permisssion)

isothermal heating at 55OoC

However, the exchange with CaCl, proceeded less easily than with alkaline metal chlorides. Temperature-programmed heating of the CaC12/H-MOR mixture (Ca/Al= 0.51, similar to the case of alkaline metal chlorides, provided two steps of HC1 evolution corresponding to a low-temperature and a high-temperature process (Figure 6). While the LT exchange gave rise to a sharp step between 400 and 575 K, the HT exchange resulted in a less pronounced evolution of HCl. Only after repeated heating and a second isothermal treatment a t 875 K was i t completed. Then the reaction ceased and a total of 2.5 mmol HCl per gram zeolite was evolved. This compared satisfactorily with the A1 content (2.2 mmol per gram zeolite). The agreement between HC1 or NH,Cl evolved and A1 content was perfect in the case of CaCl,/NH,-MOR (Ca/A1=0.5), viz. 2.54 and 2.52 mmol per gram.

52

Thermogravimetric analysis showed only one steep decrease in weight of the CaC12/ H-MOR mixture due to release of H 2 0 and HCl. The system initially contained significant amounts of water because the hydrated chloride, CaC12 * 2H20, was employed. Thus, the LT exchange was not, in a strict sense, a solid-state ion exchange and was probably facilitated by the presence of water molecules in the zeolite pores. However, as will be pointed out later, the presence of water is not a n indispensable condition for solid-state ion exchange to occur. Rather, the solid-state exchange can be conducted even in ultrahigh vacuum, i.e. in the absence of any traces of water. Solid-state ion exchange with CaC12/H-MORand MgCl,/H-MOR was also monitored by IR. Figure 7 demonstrates, as a n example, the removal of acidic OH groups (spectrum 2a) in H-MOR due to replacement of the protons by Ca2+. Subsequent pyridine adsorption gave rise to a sharp band a t 1446 cm-*typical of pyridine coordinatively bonded to calcium cations and a smaller band a t 1455 cm-l which is due to pyridine attached to "true" Lewis sites (spectrum 2b). Most likely, both types of pyridine coordination contributed to the small signal a t 1610 cm-l. Note, however, that only a very weak pyridinium ion band at 1540 cm-' was observed. Another sample, which had been prepared via solid-state ion exchange (spectrum 2a), was contacted with small amounts

I

I

I

/

I

I

iI

Y

U

z

a

II-

r

v)

z a CT

c

3av I

I

I

4000

3500

3000

W A V E N U M B E R [cm-'I

Fig. 7. IR spectra of mordenite samples. l a , lb, spectra of H-MOR after activation (775 Pa); 2b, same treatment a s Pa); 2a, CaCl /H MOR after heating at 775 K, (2a), subsequent pyrikne adsorption and removal of excess pyridine (475 K, loe5Pa); 3a, same treatment as (2a) and brief contact with H,O, followed by degassing at 775 K ( Pa); 3b, same treatment as (3a), subsequent pyridine adsorption and removal of excess pyridine (475K, Pa).

53

of water vapour and again degassed. This resulted in spectrum 3a showing a strong OH stretching band around 3618 cm-l. Obviously, interaction of H 2 0 molecules with Ca2+ introduced by solid-state ion exchange generated acidic OH groups according to the Hirschler-Plank mechanism [20-211.In fact, subsequent pyridine adsorption produced a strong band a t 1540 cm-' indicating pyridinium ions. While the ion exchange with alkaline metal cations does not result in catalysts active in acid-catalysed hydrocarbon reactions and, in contrast, may be carried out to remove any residual activity [l],the incorporation of alkaline earth cations by solidstate reaction should lead to active catalysts. I t was shown, however, that the solidstate ion exchange had to be followed by contact with water vapour in order to obtain calcium or magnesium mordenites which are sufficiently active in, for instance, disproportionation of ethylbenzene. This is in full agreement with the IR spectroscopic results which indeed showed that only upon interaction of the heat-treated CaC12/H-MOR mixture with water vapour were acidic OH groups generated. Also, in the case of alkaline earth chlorides some investigations were undertaken using sodium forms of zeolites instead of hydrogen forms as starting materials for solidstate ion exchange. These experiments, however, were conducted a t ambient temperature and moisture. 23Na MAS NMR was used to evaluate the degree of exchange when, for instance, BeC1, or CaC12was contacted with Na-Y. The results were similar to what was found in the case of LiC1. The 23Na MAS NMR spectra suggested complete exchange. The line indicating N a + in Na-Y completely disappeared. Instead, the signal of Na+ in crystalline NaCl and the broad band a t -12.5ppm due to N a + in intracrystalline water developed. The exchange between BeC1, and Na-Y was also studied by IR, using pyridine a s a probe. Before solid-state reaction, pyridine indicated Na+ on cationic sites through a band a t 1444 cm-I.After solid-state reaction N a + was completely replaced by Be2+ and the corresponding band of pyridine coordinatively bonded to Be2+ was observed a t 1453 cm-I. Interestingly, i t was important to avoid heating of the BeC12/Na-Y mixture higher than 400 K, because a t temperatures higher than this, partial re-exchange of N a + for Be2+ occurred. Solid-state Ion Exchange with Hare-earth Metals In view of catalyst preparation, the solid-state ion exchange with lanthanum or rare-earth cations is of particular interest. Rare-earth-containing zeolites are widely used a s cracking catalysts. However, special measures are required [22-231to obtain a degree of exchange higher than about 75% of, e.g., La3+ or Ce3+ when the exchange is carried out with the conventional technique, i.e. by suspending the zeolite powder in a solution of the in-going trivalent ion. The reason is that in solution the highly solvated

54

M3+ cation cannot enter the sodalite cages. To achieve higher degrees of exchange, usually the conventional exchange procedure with suspensions is repeated several times and the exchange product intermittently heated to push the, a t least partially, desolvated La3+ cations into the sodalite cages. It turns out, however, that solid-state ion exchange may lead to an almost 100% exchanged La-Y zeolite in a one-step procedure and provide highly active catalysts [24-251. Similar to the results already described, also with LaC13/NH4-Y the stoichiometric measurements were very instructive (Table 2).The data of Table 2 show the results of asolid-state reaction between LaCl, and NH4,Na-Y. The zeolite had a degree of exchange of 89% NH4+ for Na+. The LaC13/NHq,Na-Ymixture contained 1 La3+ per 3 A1 of the zeolite structure. The experimental results were obtained by titration of the evolved gases and chemical analysis of both the water-extracted product and the extract solution. From these data it is evident that (within the limits of error) the amount of La3+ introduced by solid-state reaction (4.95meq./g) corresponded exactly to the amount of framework aluminium (4.83meq./g) or maximum of bridging OH groups. However, even the main fraction of Na+ of the starting material (1.61 meq./g) was removed from the zeolite. Only about 0.7 meq. Na+ and 0.8 meq. Cl- remained in the structure, corresponding to roughly one NaCl molecule per P-cage. It is assumed th a t this NaCl is occluded in the structure. According to Rabo's study [2-31this would enhance the thermal stability of the exchange product. Tabl e 2.

Solid-state Ion Exchange*, LaCl,/NH,,Na-Y (LafAl =0.33); thermal treatment at 850 K La3

c1-

Na+

NH4+

A1

_-

--

1.61

3.29

4.83

1.61

4.83

+

Starting zeolite Salt admixed Evolved gas (as NH,Cl)

3.29

Extracted with K,O

0.06

0.72

0.94

irreversibly held

1.60

0.82

0.67

4.80

*

3.29

__ __

4.83 4.83

data in mmol per gram zeolite, with the exception of the last line (meq./g)

The results were exactly the same when an excess of LaC13, e.g., a ratio of 2 La3+ per 3 A1 of the zeolite structure, was employed. The only difference was that, after completion of the reaction, a higher amount ofLaC1, was extracted [251.

55

In the same way, a 100%La-Y sample was obtained by stoichiometric solid-state ion exchange when the starting zeolite was a 100% exchanged NH,-Y and a ratio La/A1= 0.33 employed. In contrast to the conventional procedure, the solid-state reaction yielded the 100% ion-exchanged La-Y by one exchange step [25]. Solid-state incorporation of La3+ into Y-type zeolite was also studied by IR. Figure 8 exhibits a set of spectra in the OH and NH stretching region obtained upon thermal treatment of a LaCl3/NH,,Na-Y(89) mixture a t successively higher temperatures (spectra a-c) and after contact of the exchange product with water vapour followed by degassing (spectrum d). At 475 K deammoniation was still incomplete; however, the prominent high frequency and low frequency OH bands of hydrogen Y a t 3640 and 3542 crn-l, respectively, were already partly developed. Upon heating to higher temperatures, solid-state reaction of LaCl, with the OH groups eliminated the OH bands almost completely. Subsequent treatment with H,O vapour and degassing produced the OH bands typical of La-Y (spectrum d). 3738 3560

I

ll I l l

CII

I 1

3800

l

h

I

Y

3738

3738 3530

c

d'

I

I

I

.u

I

725 K.30 min, HV

I If1 I /I I I /I I 3400 3000"38003400"3800 3400 " 3800 W A V E N U M B E R [cm'l

I

3400

I

3000

Fig. 8. IR spectra of the OH stretching region of a LaCl /NH -Y mixture. a, b and c, after heating a t 475, 625 and 725 K, res ectively h a , 2%);d,gfter (c) and brief contact with H,O (0.6 kPa, 3 min) followefby evacuation (725 K, 10- Pa, 30 min). Adsorption of pyridine subsequent to spectrum c gave rise to a band a t 1452 cm-l typical of pyridine coordinatively bonded to lanthanum cations; only a tiny pyridinium ion band a t 1542 cm-l was observed. When, however, generation of spectrum d (i.e. after H,O contact of the sample) was followed by pyridine adsorption, the band of acidic OHgroups at 3630 cm-' was completely removed and a strong band a t 1542 cm-l (pyridium ions) appeared.

56

The intensities of the bands were similar to those of La-Y samples obtained by conventional ion exchange. TPD of ammonia showed that also the strength of the acidic sites were essentially the same as found with conventionally exchanged La-Y. Thus, the nature, density and strength of acidity of La-Y prepared via solid-state ion exchange are comparable to those of the conventionally obtained products and, therefore, similar catalytic behaviour in acid-catalysed reactions was expected. This was, indeed, found when La-Y was employed as a catalyst for ethylbenzene disproportionation.

I

w

z w

N

I

'

l

~

l

'

1

~

1

~

1

1

1

~

1

-

16-

0

v U

T I M E O N S T R E A M [hl

Fig. 9. Selective disproportionation of ethylbenzene at 425 K (1.3 vol % EB in He, 5 ml min-', m[catl=O.25 g). A, over a La-Y (98) catalyst obtained by solid-state ion exchange; B, over a conventionally prepared La-Y (96)catalyst. Finally, La-Y zeolites were obtained by solid-state ion exchange between LaCl, and the sodium form of Y-type zeolites as well [25]. This was proven by chemical analysis, IR, X-ray diffraction (XRD) and a test reaction. Details of the preparation and characterisation are outlined in Ref. [25]. Chemical analysis gave evidence for a partial replacement of sodium by lanthanum cations. IR showed the formation of acidic OH groups. Finally, XRD demonstrated, via the appearance of reflections of crystalline NaC1, that obviously Na + cations were expelled from the interior of the zeolite crystals by in-going La3+. Outside the zeolite particles they had formed small NaCl crystallites.

~

l

51

Even though a 100% exchange in the system LaC13/Na-Y was not yet achieved by solid-state ion exchange, the product obtained showed catalytic activity i n acid-catalysed ethylbenzene disproportionation similar to that of conventionally obtained La-Y with a n exchange degree of 74%. Solid-state Ion E x c h a n g e with Transition Metals As mentioned in the Introduction, Clearfield et al. [4] were the first to incorporate transition metal cations into zeolites via solid-solid reaction. These authors reacted partially (i.e. to 36 - 58%) NH4-exchanged and subsequently deammoniated forms of Na-A, Na-X or Na-Y with transition metal compounds such a s CuC12, ZnCl,, NiCl,, CoCl2, CrC1, or MnC1,. The extent of solid-solid reaction was monitored by titration of HCl evolved. Depending on the reaction conditions, up to 100%exchange of the zeolite protons by metal cations could be achieved. Investigation of transition metal cations and their reactions fall in the realm of ESR spectroscopy. Such studies benefit by the high sensitivity of that spectroscopic technique. Moreover, ESR spectroscopy of cations in zeolites provides in many cases deeper insight into their coordination state. Thus, Clearfield et al. [41 were also the first to successfully apply ESR spectroscopy to prove the incorporation of transition metal cations into a zeolite structure by solid-solid interaction. In Figure 10, ESR spectra of Cu,Na-Y

Fig. 10. ESR spectra of heat-treated Na-Y and Cu(II)-containing Y-type zeolite s m ples. A, 15% of original N a + replaced through solid-state reaction between CuCl, and H,Na-Y; B, 15% of original N a + replaced by conventional exchange in aqueous suspension; C, original Na-Y (after Ref. [51, with permission).

58

obtained via solid-state exchange (line A), conventionally exchanged Cu,Na-Y (line B) and the starting zeolite Na,H-Y (line C) are compared. In both Cu,Na-Y samples about 1 5 6 of the original Na+ cations were replaced by Cu2+. It is evident from comparison of spectra (A) and (B) that very similar Cu2+-containing zeolite Y samples were obtained from solid-state and conventional aqueous exchange. Both spectra (A) and (B) exhibit 8 lines indicative of two different environments for the Cu2+ cations. The first set of signals is characterised by g,,=2,35 and g,=2,06 whereas the g-values of the second set are g,,= 2,30 and gl= 2,06.

ESR spectroscopy was extensively used by the group of Kucherov and Slinkin in their systematic work on solid-state reaction between high-silica zeolites and transition metal compounds. As zeolites they preferentially employed hydrogen mordenites, hydrogen forms of ZSM-5 or highly dealuminated H-Y. In their experiments, the transition metal compounds reacted with the zeolites are mostly oxides such a s CuO [261 or CrO,, Cr20,, MOO, or V,O, [27]. However, salts were also applied, e.g. MoCl, [271, CuCl,, CuF2, Cu3 (PO,),, Cu2S [261 or FeCl3[281. Finally, the studies were extended to simultaneous or successive reaction of two oxides (CuO + CrO,, CuO + V20& with HZSM-5 [29]. In particular, the system CuO/H-ZSM-5 was studied in great detail. An interesting result was that Cu2+ introduced by solid-solid reaction of CuO and H-ZSM-5 at 825 K o r 1075 K exhibited ESR spectra which were completely identical with those obtained from Cu,H-ZSM-5 prepared via conventional ion exchange in aqueous solution (Ref. [301, Figure 11).The g-values and hyperfine splitting constants for samples prepared via either route showed full agreement, hence the authors concluded that also the

I

Fig. 11. ESR spectra of Cu(II)-containing MFI-type zeolite samples. A, CuCl,/ H-ZSM-5 mixture heat-treated in vacuum a t 10 75 K; B, Cu,H-ZSM-5 obtained by conventional exchange, calcined in air at 1075 K and evacuated a t 300 K (after Refs. [261 and 1301, with permission).

59

isolated Cu2+ ions introduced by solid-state reaction were present in two different coordination states, viz. in a square planar environment (g,,=2.32, g,=2.045, A,,= 17 mT, Al= 2.9 mT) and a fivefold coordinated state (g,,=2.32, gL= 2.06-2.07, A,,= 14 - 14.2 mT, Al= 1.8 mT). The latter coordinatively unsaturated state, however, was completely absent when CuO was reacted with H,Na-ZSM-5 (40% Na+ exchanged for H + ) instead of H-ZSM-5 (95% N a + exchanged for H f ) . The H,Na-ZSM-5 starting zeolite was prepared by burning-off of the organic template without any further exchange of the remaining N a t cations. Therefore, on the basis of the above ESR observation i t was suggested that, upon synthesis, N a + and organic cations have been located in the ZSM-5 structure not randomly but, rather, in a somehow ordered manner. The Cu2+ cations introduced by solid-solid reaction were easily accessible for adsorbates such as 0, a s was evidenced by the dramatic but reversible change, i.e. loss of intensity, broadening and removal of hyperfine splitting of the ESR signals upon oxygen admission. The amount of Cu2+i which could be introduced into H-ZSM-5 by solid-state reaction, increased linearly with the A1 content of the framework. Similar results were obtained with H-MOR. However, when Na-MOR was calcined in a mixture with CuO no appearance of the Cu2+ ESR signal was observed. Solid-state reaction also occurred between H-ZSM-5 and CuC12, CuF, or Cu3(P0,),. In these cases a n effect of the anions was realised. The Cu2+ cations introduced were a s a ligand. With Cu,S i t coordinated not only to the framework but also to F' or is supposed that also Cut cations migrate. The Cu2+ ESR spectrum appeared upon oxidation of the thus introduced Cut cations residing on cation sites. It is stressed that reaction of CuF, with H-ZSM-5 caused no destruction of the lattice. X-ray diffraction patterns obtained after solid-state reaction gave evidence for the integrity of the structure. Thermal treatment of mixtures of hydrogen zeolites with CrO,, MoCl, and V205 gave rise to ESR spectra typical of isolated Cr(V), MOW)and V(W) cationic species [271, respectively. The authors do not comment on the reduction of Cr(VT) or V(V). Most probably this occurred due to the presence of organic contaminants such a s residual template. Because of the large distance between the framework A1 atoms (or protonic sites) in the highly siliceous hydrogen zeolites used, the authors assume that isolated complex cations such as CrO,+, MoC14+, VO(OH)+ were introduced by solid-state reaction, rather than Cr5+,Mo5+ or V4+. Interestingly, Kucherov and Slinkin observed a "superhyperfine splitting" in the case of ESR spectra of Cr(V) or V ( N ) obtained after solid-state reaction [27,31]. The additional splitting (see Figure 12) is ascribed to a n electronic interaction of 53Cr (nuclear spin I=3/2) or (I=7/2) with adjacent framework A1 (I=3/2) [27,31]. Yang et al. [32], who also investigated by ESR the introduction of Cr or Mo into H-ZSM-5, arrived a t similar results.

60

There is also one publication by Kucherov et al. [33]concerning the introduction of Cu, Fe, Cr or V cations into the gallium analog of H ZSM-5. In this work it was found that, in line with the well-known lower acidity strength in H-[Gal ZSM-5, the cations introduced are less stabilized in the Ga form than in the A1 form of ZSM-5 type zeolite.

Fig. 12. ESR spe tra of isolated vanadyl cations tS1V (IV)) introduced into H-ZSM-5 (Si/Al = 35) by solid-state reaction with V 0, a t 1025 K (after Ref. 1311, wizh permission). Polyvalent cations could be co-introduced, e.g. by reacting H-ZSM-5 with CuCrO,. This provided ESR spectra identical to those of isolated Cu(II) and Cr(V) randomly distributed over cation positions. When CuO was reacted with the product of solid-state exchange between CrO, and H-ZSM-5, the signal intensity of Cr(V) considerably decreased, indicating exchange of Cr(W cations for Cu(II) species. Hence the Cu(lI) cations are more strongly bound than Cr(V) cations. Similarly, V(IV) introduced by solid-state reaction could be replaced by successive introduction of Cu(II). No solid-state exchange was observed upon thermal treatment of mixtures of FeO or Fe304 with H-ZSM-5 (but see Ref. [341 later in the text). However, Fe(lII) cations were successfully introduced into H-ZSM-5 or H-Y (Si/Al= 25) through reaction with FeC1, [281. Typical ESR signals (gl =4.27; g2= 5.65; g3 = 6.25) were most intense after stepwise oxidation of the reaction product in air at 575 and 825 K. The signals indicated that Fe(lTI) species were located in strong crystal fields of low symmetry. Again, the authors assume that FeC12+ or FeO+ is incorporated into the zeolite rather than Fe3+. The ESR spectra were compared with those of ferrisilicate which also exhibit a prominent line a t g, =4.25 (with additional signals a t g2=5.2 and g3 =7.9). However, the extra-framework Fe(III) species were distinguished from framework Fe: In contrast to framework Fe, (i) the Fe(lTI) cations (introduced by solid-state reaction or by aqueous

61

ion exchange) exhibit a n anomalous temperature effect, i.e. the intensity of the signal with g=4.27 was a t least 200 fold increased by cooling the sample to 78 K; (ii) the Fe(III) species were accessible for sorbates such as O,, NH, and pyridine which caused a dramatic change of the ESR spectrum (disappearance of the low field lines a t g2 = 5.25 and g3 = 6.25); (iii) the Fe(III) species on cationic sites could be replaced by subsequent reaction with CuO (vide supra). Solid-state ion exchange of Fe3+ into H-ZSM-5 was also investigated by Wichterlova et al. [34]. These authors used, besides ESR, temperature-programmed desorption of ammonia (TPDA) to monitor solid-solid reaction. Their results obtained with mixtures of Fe203 and H-ZSM-5 showed that significant amounts of Fe3+ were incorporated. Fe3 ions introduced in this way gave rise to a n ESR signal with g=4.27 and were easily reduced. In the reduced form the samples were tested for methanol conversion and toluene disproportionation. Due to a reduced density of acidic sites, the activity was lower than that of the starting zeolite H-ZSM-5. The yield of aromatics in methanol conversion, however, was about the same as in the case of Fe,H-ZSM-5 obtained by conventional exchange. Wichterlova e t al. [35] and Beran et al. [14] also studied the solid-state ion exchange of nickel and manganese compounds (chlorides, sulphates, acetates and oxides) with KZSM-5. Again, IR, ESR, TPDA, TPE, but also X-ray photoelectron spectroscopy (XPS)were employed a s techniques for investigation. With NiC1, the exchange reaction was optimum; 100% of the Ni2+ applied could be introduced a s long as no excess of NiCl, was used. In a stoichiometric mixture all of the acidic OH groups were consumed, i.e. one Ni2+ replaced two bridging hydroxyls upon 6 h calcination a t 770 K in a stream of dry oxygen. Such a high degree of exchange with Ni2+ is difficult to achieve by the conventional aqueous solution method. Reduction in H, a t 720 K or re-exchange with NH,N03 solution and subsequent deammoniation restored the original density of OH groups. Under identical conditions reaction with NiSO, was less efficient. No ion exchange was obtained with Ni(CH3 COO), or NiO. After reduction, zeolites obtained through solid-state ion exchange in the system NiCl,/H-ZSM-5 exhibited an activity in hydrogenation of ethylene comparable to that of samples prepared by standard Ni2+ ion exchange in aqueous medium and subsequent reduction. When mixtures of Mn(NO,), and H-ZSM-5 were thermally treated for 2 h a t 870 K, XPS revealed a significant decrease in the surface concentration of manganese [351. Further evidence for introduction of Mn2+ into H-ZSM-5 was provided by the ESR spectra of MnO/H-ZSM-5 and MnCl,/H-ZSM-5 mixtures after heat-treatment a t 870 K and 770 K followed by rehydration a t ambient temperature [14,351. The spectra showed a signal with six hyperfine lines typical of Mn2+ in cation sites with 0, symmetry. Also, TPDA, TPE and IR (consumption of acidic OH groups, decreased pyridinium ion formation upon pyridine admission to the heat-treated mixture, increased density of

62

pyridine attached to Mn2+ cations) proved solid-state ion exchange between Mn compounds and H-ZSM-5. Under identical conditions the degree of exchange obtainable decreased in the sequence MnC12>Mn304>MnS04. In contrast to the case of NiCl,, however, even with MnC1, no complete elimination of the acidic OH'S was achieved. Moreover, a fraction of the manganese compound admixed always remained unreacted. In the case of MnC12/H-ZSM-5the solid-state ion exchange was studied as a function of reaction time and temperature. Figure 13 clearly demonstrates the effect of temperature. Raising the reaction temperature from 570 K to 770 K resulted in a significant increase in the degree of exchange. Most of the manganese ions were introduced during the first stage of reaction (within 1 h), and then the further reaction proceeded very slowly.

-

1

Y

I

I

0-

P / O - O

50-

670K

3

0 W e* 0 P 25-

-

\;I.-- I I

770 K

&?

cn

1

o-

W

I 3

z

0 U

-

0Heat treatment in vacuum

I

I

0

I

I

I

5 10 15 R E A C T I O N T I M E [hl

I

20

I

Fig. 13. Solid-state ion exchange with manganese chloride. Number of bridgin OH groups consume via solid-state exchange in a mixture of MnCl, and H-Z8M-5 (SiiA1=13.5, M$+iOH=0.33) as a function of reaction time a t 570, 670 and 770 K (after Ref. [141,with permission). Modification of H-ZSM-5 zeolites through solid-state reaction with ZnO was described by Yang et al. [32]. On the basis of XPS results they reported that, upon heattreatment of a ZnO/II-ZSM-5 mixture, Zn ions migrated from the outer surface into the channels of the zeolite. This finding was supported by TPDA, IR (decrease of acidic Br#nsted sites upon solid-state reaction between ZnO and H-ZSM-5) and temperatureprogrammed reduction (TPR).The latter showed increased uptake and reducibility after thermal treatment of ZnO/H-ZSM-5 compared to ZnO. Zeolites Zn,H-ZSM-5 exhibited, after reduction in H,, pronounced selectivity in propane aromatization. More recently, Karge et al. [361 have shown that also noble metals can be easily introduced into zeolites via solid-state reaction. Various zeolites, such a s NH4-Y, US-Y,

63

H-MOR and H-ZSM-5, and noble metal compounds (PdCl,, Pd(NO,),, PdO, PtCl,, orPtC1,) were used. It was demonstrated with the help of several techniques (IR, TPDA, TPE etc.) that the noble metal cations upon solid-state reaction occupy cation sites inside the zeolite structure. After reduction in H, the thus-obtained materials possessed hydrogenation properties. Provided a suitable balance between the acid function (residual acidic OH groups) and the hydrogenation function (noble metal aggregates) was established, these catalysts were efficient in hydroisomerisation of, for instance, ethylbenzene. Role of Water i n Solid-state Ion Exchange In most cases solid-state ion exchange in zeolites was conducted in the presence of ambient moisture or residual water vapour. However, it was shown that this type of exchange also occurs whenever traces of water are carefully excluded [ 121. Moreover, solid-state ion exchange into zeolites was also achieved with compounds insoluble in water, e.g. with AgCl o r Hg,Cl,. This suggests that the presence of residual water is not necessarily a prerequisite for the solid-state ion exchange in zeolites to occur, even though small amounts of water such as the crystal water might facilitate the low-temperature solid-state reaction (vide supra). However, more subtle details of solid-state ion exchange in zeolites as, for instance, the particular mechanism of ion migration remain a mystery, and their clarification needs further experimental work. Acknowledgment Financial support by the Bundesminister fur Forschung und Technologie (BMFT, Project No. 03C 257 A7) is gratefully acknowledged.

K K F E KENCES 1 J.A. Rabo, M.L. Poutsma and G.W. Skeels, in J.W. Hightower (Editor), Proc. 5th Int. Congress on Catalysis, Miami Beach, Flo., USA, August 20-26, 1972, NorthHolland Publishing Co., New York, 1973, pp. 1353-1361. 2 J.A. Rabo and P.H. Kasai, Progress in Solid State Chemistry 9 (1975) 1-19. 3 J.A. Rabo, "Salt Occlusion in Zeolite Crystals", in J.A. Rabo (Editor), "Zeolite Chemistry and Catalysis", ACS Monograph 171, Am. Chem. SOC.,Washington, D.C., USA, 1976, pp. 332-349. 4 A. Clearfield, C.H. Saldarriaga and R.C. Buckley, in J.B. Uytterhoeven (Editor), Proc. 3rd Int. Conference on Molecular Sieves; Recent Progress Reports, Zurich, Switzerland, Sept. 3-7, 1973; University of Leuwen Press, 1973, Leuwen, Belgium, Paper No. 130, pp. 241-245. 5

6 7 8

9 10

C.A. Fyfe, G.T. Kokotailo, J.D. Graham, C. Browning, G.C. Gobbi, M. Hyland, G.J. Kennedy and C.T. DeSchutter, J. Am. Chem. SOC.108 (1986) 522-523. J.H. Lunsford, Adv. Catal. 22 (1972) 265-344. H.G. Karge, S. Trevizan de Suarez and I.G. Dalla Lana, J. Phys. Chem. 88 (1984) 1782-1784. J.B. Uytterhoeven, L.G. Christner and W.K. Hall, J. Phys. Chem. 69 (1965) 21172126. M.L. Hair, "Infrared Spectroscopy on Surface Chemistry", Marcel Dekker Inc., New York, 1967. H.G, Karge, Z. Phys. Chem. [NF] 122 (1980) 103-116.

64

11 H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79-82. 12 H.G. Karge, V. Mavrodinova, 2. Zheng and H.K. Beyer, in D. Barthomeuf, E.G. Derouane and W. Holderich (Editors), "Guidelines for Mastering the Properties of Molecular Sieves", NATO AS1 Series, Series B, Physics Vol. 221, Plenum Press, New York, 1990, pp. 157-168. 13 G. Borbely, H.K. Beyer, L. Radics, P. SBndor and H.G. Karge, Zeolites 9 (1989) 428-431. 14 S. Beran, B. Wichterlovh and H.G. Karge, J. Chem. SOC.Faraday Trans. I 86 (1990)3033-3037. 15 R. Schollner, P. Nobel, H. Herden and G. Korner, in P. Fejes (Editor), Proc. Symp. on Zeolites, Szeged, Hungary, Sept. 11-14,1978, Acta Universitatis Szegediensis, Acta Physica e t Chemica, Nova Series 24 (1978) 293-298. 16 H.K. Beyer and I. Belenykaja, in B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud (Editors), Proc. Int. Symp."Catalysis by Zeolites", Ecull (Lyon), France, Sept. 9-11, 1980; Elsevier, Amsterdam, 1980; Stud. Surf. Sci. 5 (1980)203-210. 17 H.K. Beyer, I.M. Belenykaja, F. Hange, M.Tielen, P.J. Grobet and P.A. Jacobs, J. Chem. SOC. Faraday Trans. I81 (1985) 2889-2901. 18 B. Sulikowski, G. Borbely, H.K. Beyer, H.G. Karge and I.W. Mishin, J. Phys. Chem. 93 (1989)3240-3243. 19 H.G. Karge, H.K. Beyer and G. Borbely, Catalysis Today 3 (1988)41-52. 20 A.E. Hirschler, J. Catal. 2 (1963) 428-439. 21 C.J. Plank, in W.M. Sachtler, G.C.A. Schuit and P. Zwietering (Editors), Proc. 3rd Congress on Catalysis, Amsterdam, The Netherlands, July 20-25, 1964, NorthHolland Publ. Comp., Amsterdam, 1965, pp. 727-728. 22 D. Keir, E.F.T. Lee and L.V.C. Rees, Zeolites 8 (1988) 228-231. 23 S. HoEevar and B. Dr&aj,in L.V.C. Rees (Editor), Proc. 5th Int. Conf. Zeolites, Na les, Italy, June 2-6,1980,Heyden, London, 1980, pp. 301-310. 24 H.8. Karge, G. Borbely, H.K. Beyer and G. Onyestyhk, in M.J. Philips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis,Calgary, Ottawa, Canada, June26-July 1,1988, Chemical Institute of Canada, Ottawa, 1988, pp. 396-403. 25 H.G. Karge and H.K. Beyer, in DGMK-Berichte-Tagungsbericht 9101, DGMKFachbereichstagung "Cl-Chemie - Angewandte Heterogene Katalyse - C4Chemie", Leipzig, FRG, Febr. 20-22, 1991, ISBN No. 3-928164-07-4, ISSN No. 0938-068 X, pp. 191-206. (English Version to be published in Erdol & Kohle, Erdgas, Petrochemie). 26 A.V. Kucherov and A.A. Slinkin, Zeolites 6 (1986) 175-180. 27 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38- 42. 28 A.V. Kucherov and A.A. Slinkin, Zeolites 8 (1988) 110-116. 29 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987)43-46. 30 A.V. Kucherov, A.A. Slinkin, D.A. Kondrat'ev, T.N. Bondarenko, A.M. Rubinstein and Kh.M. Minachev, Zeolites 5 (1985)320 - 324. 31 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 583-584. 32 Y. Yang, X. Guo, M. Deng, L. Wang and Z. Fu, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders-Applications and Innovations",Wiirzburg, FRG,Sept. 4-8,1988;Elsevier, Amsterdam, 1989; Studies Surface Sci. Catalysis 46 (1989) 849-858. Faraday 33 A.V. Kucherov, A.A. Slinkin, H.K. Beyer and G. Borbely, J. Chem. SOC. Trans. I, 85 (1989) 2737-2747. 34 B. Wichterlovh, S. Beran, S. BednaFovh, K. NedomovB, L. Dudikovh and P. J iru in P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff (Editors), Proc. Int. Symp. "Innovation in Zeolite Materials Science", Nieuwpoort, Belgium, Sept. 13-17,1987, Elsevier, Amsterdam; Studies Surf. Sci. Catalysis 37 (1988) 199-206. 35 8. Wichterlovh, S. Beran, L. Kubelkovh, J, NovhkovB, A. SmiegkovA and R. Sebik, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders - Applications and Innovations", Wurzburg, FRG, Sept. 4-8, 1988, Elsevier, Amsterdam, 1989; Studies Surf. Sci. Catalysis 46 (1989)347-353. H.G. Kame. Y. Zhanp and H.K. Bever. Dublication in DreDaration.

catalysis

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

65

Zeolite-hosted metals and semiconductors as advanced materials G. Schulz-Ekloff

I n s t i t u t f u r Angewandte und Physi kalische Chemie, Universitat Bremen, 0-2800 Bremen 33, FRG

Abstract Preparations and characterizations o f zeolite-hosted materials, 11 k e metals, semiconductors o r dyes are described. Photophysical and photochemical properties are reviewed. Potential applications, e.g. i n optical switching, microwave absorption, optical data storage, microsensor devices o r dispersion electrolysis, are summarized.

1.

INTRODUCTION

The regularly s t r u c t u r e d pores and cages o f molecular sieves represent host systems which can accomodate guest particles o r molecules i n a colloidal dispersion and , thus, o f f e r t h e properties of a solid solvent. The fixation of colloids i n zeolitic ceramic materials o f f e r s advantages f o r practical applications due t o t h e h i g h chemical, mechanical and thermal stability of a molecular sieve s u p p o r t and t h e isotropic access o f t h e colloid f o r guest molecules. The preparation of s t r u c t u r e d cluster dispersions exhibiting tailored particle sizes i n t h e nanometer range is o f practical importance f o r t h e development o f advanced quantum-confined electronic, optical o r opto-electronic devices, e.g. f o r lasers, switches, t r a n s i s t o r s o r information storage, since t h e low dimension o f quantum-dot (QD) systems w i l l make electron excitation o r t r a n s f e r processes faster and more energy selective and will exhibit novel photophysical properties. The development o f zeolite-based host-guest systems as heterogeneous or immobilized homogeneous catalysts is a permanent a t t r a c t i v e challenge, whereas t h e design o f advanced molecule separation materials o r t h e creation of molecule sieving sensor devices seem t o be r a t h e r a t t h e beginning. Other new fields are photochemistry, photocatalysis and applications r e q u i r i n g composite systems on a nanometer scale. Many of t h e f u t u r e potential applications o f zeolite-based advanced materials have been mentioned i n recent review articles. I n t h e following, some problems w i l l be pointed o u t i n connection with t h e application of zeolite hosts f o r t h e preparation of well-defined colloidal dispersions o f guests, which have t o be overcome f o r a breakthrough in t h e development of new materials f o r advanced usages.

66 I

-

NONINlIRCONNECTIN(I CHANNELS

a.

c. d.

P

P 7

.FtRRlElllTF

;::lK

It. INTERCONNECTING CHANNEL5

\ l l . INTERCONNECTING C H A N N t L S AND C A G E S

Figure 1. I l l u s t r a t i o n of zeolite void f i l l i n g guest particles f o r various s t r u c t u r a l t y p e s o f channel networks (from E.G. Derouane i n "Intercalation Chemistry" (M.S. Whittingham and A.J. Jacobson, Eds.) Academic Press, New York 1982, p. 101)

2.

ZEOLITE-HOSTED

TRANSITION M E T A L S

2.1 Application of metal-loaded zeolites in catalysis Zeolite catalysts containing reduced metals are broadly applied i n t h e upgrading of l i g h t gasoline fractions and hydrocracking of heavy Beyond t h e established usages in petrochemical feedstocks [l-41. processes a variety o f prospective catalytic properties o f transition metal containing zeolites are investigated intensively, Ii k e oligomerization, hydrogenation, dehydrogenation, selective oxidation [5,6], carbonylation [7], hydroformylation [8], low temperature water gas s h i f t [9] o r syngas conversion [lo]. The aromatization o f lower alkanes, which appears to be of great importance f o r economic and ecological reasons, seems t o be close t o an industrial realization [ll-131. The break-even point is already reached f o r t h e application o f titanium silicalite i n t h e selective oxidation The o f hydrocarbons using hydrogen peroxide as oxidant [14-151. applications o f transition metal containing zeolites f o r t h e catalyzed synthesis o f chemical intermediates and f i n e chemicals have been reviewed recently [16,17].

2.2 Preparation of zeolite accomodated metal dispersions Incorporation o f metals i n a zeolite void system o f f e r s advantages as well as shortcomings as compared t o dense ceramic carriers, l i k e silica o r alumina. The open crystalline polyanion framework enables controlled reproducible stoichiometric metal loadings v i a ion exchange from aqueous solutions. Solid-state ion exchange can be applied i n some cases [18,191. The reduction by hydrogen produces, however, protons f o r charge compensation, which might f a v o r undesired side-reactions, like coke

61

a

b

Figure 2. Transmission electron micrographs o f iridium crystallites hosted i n NaX (a) and platinum crystallites hosted in ZSM-5 (b). The phase contrast imaging o f t h e zeolite lattices represents an internal scale.

deposition o r removal of framework atoms, o r affect t h e selectivity o f conversions. Alternative loadings can be obtained v i a carbonyls o r nitrosylcarbonyls [lOc, 20-241 as well as b y organometallics l i k e rr-allyls, metallocenes or phosphines [25-271 from vapor phase o r solution, f a v o r i n g t h e incorporation of zerovalent metals. The generation o f zeolite-hosted metal c r y s t a l s o r large clusters, respectively, r e q u i r e s subsequent steps of calcination and reduction. The reducibility o f t h e metal and t h e resulting dispersion depend sensitively on t h e parameters f o r these treatments, e.g. zeolite t y p e and acidity, manner and extent of loading, calcination medium, reducing agent, temperature programs, additionally exchanged unreducible cations o r presence of noble metals [28-331. Dispite t h e large number o f parameters influencing t h e preparation of the zeolite-supported metals, the preparation o f monodisperse metal phases having narrow particle size histograms and being located exclusively i n t h e zeolite void is achieved repeatedly.

2.3 Characterization For any judgement on possible correlations between t h e state o f a zeolite-hosted metal phase and i t s physical o r chemical properties in classical o r novel applications detailed informations about location, dispersion, geometric and electronic s t r u c t u r e under working condition are needed. I n fact, there is a scarcity o f knowledge about s t r u c t u r e p r o p e r t y correlations f o r most of t h e described systems. I n many cases a

68

Figure 3. I l l u s t r a t i o n o f a supra-supercage c r y s t a l hosted i n a mesopore formed by zeolite framework fragmentation d u r i n g guest partical growth.

lack of information exists f o r t h e location of t h e metal phase, i.e. inside t h e channels and cages o r at t h e external surface o f t h e zeolite crystals. Presumably, an exclusive internal accomodation o f metal crystallites can be proved by photoelectron spectroscopy only [34-361. With few exceptions no histograms o f metal dispersions based on electron micrographs are given. Usually, rough estimates are made from X-ray diffraction and/or probe molecule chemisorption, o r averaged values are drawn from magnetization measurements [37]. Charged clusters are internally located i n lattice positions and can exhibit distinct sizes, e.g. (Ag)6+ [38-411. Location and coordination o f metal atoms o r small aggregates i n lattice positions o r well-defined sites can be identified b y XRD [421 o r EXAFS [43, 441. Electronic s t r u c t u r e s might be studied favorably by means of t h e reactivity o f probe molecules [45, 461 o r by photoelectron spectroscopy [471. Origin and catalytic effect of a partial positive charge on metal clusters are not yet f u l l y understood [48, 491. New silicon o r aluminum containing phases o r alloys, which might result from s t r o n g metal-support interactions i n analogy t o observations on dense s u p p o r t s [50-521, have not yet been found with metal loaded zeolites. A prospective way f o r t h e design of tailored cluster sizes might be t h e anchoring o f reducible metal at non-reducible transition metal ions [53, 541. Resulting bimetal clusters can exhibit interesting novel catalytic properties [13, 551. However, f o r t h e interpretation o f bimetal catalysis with zeolite supports, effects o f distribution, acidity o r stability have t o be taken into account [56]. The metal dispersion effect on a c t i v i t y and

69

F i g u r e 4. Model o f a platinum c r y s t a l l i t e i n a faujasite supercage exhibiting a host-guest orientation-relationship due t o s t r u c t u r a l accomodation. selectivity i s well-established [57,58]. The influence o f t h e dimensions o f t h e zeolite channels and cages on t h e metal dispersion, as depicted i n Fig. 1, is not clear. There is no doubt, t h a t metal crystallites can grow beyond these dimensions under zeolite framework fragmentation and reorganization (Fig. 2) [59-601. The zeolite framework removal around t h e growing metal c r y s t a l s results i n t h e generation o f halos (Fig. 3) indentified as mesopores from t h e evaluation o f hysteresis loops i n adsorption isotherms [61]. The zeolite fragments can be reorganized b y i n-situ recrystal Iization forming secondary micropores as identified f r o m an appropriate adsorption isotherm analysis [62]. The zeolite framework fragmentation decreases with increasing Si/AI ratio, resulting in a corresponding limitation of metal crystal growth [58a]. The embedding o f metal c r y s t a l guests i n a zeolite host can result i n a host-guest orientation relationship, i.e. a parallel orientation of crystal axes of host and guests [59]. This effect is related t o a s t r u c t u r a l accomodation, i.e. an optimum f i t t i n g o f metal crystallites into t h e supercage space (Fig. 4). This orientation relationship w i l l be maintained d u r i n g f u r t h e r growth, since a particle is no longer f r e e to rotate once it has reached t h e confinement o f t h e supercage [ 6 3 ] .

2.4 Potential novel applications For s i l v e r sodalites a variety of physical and chemical properties, e.g. photochromy, barochromy o r thermochromy, are summarized which could f i n d possible utilization i n h i g h resolution imaging o r high density data storage [64]. The effects are observed following t h e reduction o f silver

70

ZEOLITE-SUPPORTED

ULTRAMICROELECTRODES

Figure 5. Schematic representation of a dispersed-particle electrode as contained between feeder electrodes with an exploded view o f faujasitesupported platinum (from ref. 69).

ions and/or t h e formation of silver clusters, i.e. are related t o nonreversible processes. Corresponding effects are expected f o r other zeolite-hosted redox systems which can be influenced b y d i f f e r e n t k i n d s o f energy impact under change o f optical o r dielectric properties. I n general, t h e fabrication o f conductor o r semiconductor nanostructures in zeolite channels o r cages could be of importance f o r nonlinear optics o r ultimate microelectronic v e r y large scale integration. Nanoparticulate metals o r semiconductors o f colloidal dispersion i n ceramic supports exhi b i t three-dimensional confinements o f charge carriers, resulting i n changes o f t h e s t r u c t u r e o f electronic levels and alterations o f t h e complex dielectric functions [65,66]. A prospective usage in optical signal processing is based on t h e possible complete absorption saturation f o r interband transitions i n t h e small discrete level system of a QD. The particle size dependent change in absorption and/or r e f r a c t i v e index could be v e r y large as compared t o b u l k materials. Furthermore, t h e refractive index could be changed by feedback effects on local fields surrounding a QD. This effect could be used f o r optical storage devices. The size-induced metal-insulator transition f o r QDs from metal atoms could be applied f o r t h e engineering of resistors o r microwave absorbing materials 167,681. Up t o now, size-induced metal-insulator transitions have not yet been studied at zeolite-hosted metal crystallites. I n t e r e s t i n g app I ications o f metal-loaded zeolites as intracrystal Ii ne electrodes are proposed. The use of feeder electrodes and dispersions of metalated zeolites results i n electrode functions without a direct electrical contact (Fig. 5). I t was demonstrated t h a t dispersion electrolysis can be achieved on platinum-loaded NaY zeolites [69].

71

3. ZEOLITE-HOSTED SEMICONDUCTORS 3.1 Preparation

I n analogy t o t h e experiences with metal dispersions inside a zeolite matrix t h e preparation of corresponding semiconductor colloids with quantum-sized particles i n t h e range 1-10 nm should be possible. The generation of zeolite-hosted dispersions has been described repeatedly f o r CdS [70-751, PbS [70,74] and ZnS [71, 751. The sulfidation o f t r a n s i t i o n metal ion-exchanged zeolites can be achieved b y treatment o f t h e dehydrated samples with H S [70, 72-75] o r by adding Na2S t o an aqueous s l u r r y [71]. However, t h e sulfide formation remains incomplete since t h e metal ion w i l l competitively interact with H S and t h e zeolite lattice [76]. Furthermore, t h e tendency f o r sulfide formation decreases with decreasing sulfide cluster size due t o t h e corresponding increase of t h e solubility constants o f quantum-sized sulfide [77,78]. The charge zeolite compensating protons formed during su I f idat ion faci Iitate framework fragmentation around t h e growing particle in analogy t o metalloaded zeolites [59-611, thus, resulting i n mesopores accomodating suprasupercage sulfide aggregates [76]. Precipitation of metal hydroxides inside t h e zeolite c r y s t a l o r a t i t s external surface d u r i n g ion exchange has t o be considered. The extent o f t h i s side-reaction depends on t h e solubility p r o d u c t o f t h e metal hydroxides, t h e alkalinity of t h e zeolite and t h e acidity o f t h e salt solution [79]. The tailored preparation and characterization o f transition metal oxide clusters i n zeolite channels and cages has not been investigated, up t o now. A possible way could be t h e conversion o f transition metal clusters, e.g. prepared v i a zeolite-accomodated carbonyls o r vapor-deposited volatile metal compounds, t o encaged oxide cluster under appropriate conditions. The incorporation of semiconductor oxide clusters, e.g. Ti02 o r SnO2, v i a ion exchange s u f f e r s from s t r o n g h y d r o l y s i s and polymeric cation formation i n t h e exchange s l u r r y . The incorporation o f transition metals b y isomorphous substitution d u r i n g molecular sieve synthesis and subsequent aggregation i n extra-framework positions v i a removal from Tsites would be an alternative approach.

3.2 Characterization EXAFS has been used f o r t h e analysis o f location or s t r u c t u r e o f zeolite-hosted CdS and CdSe clusters [73]. The persuasive power of t h i s method suffers, however, from t h e lack o f well-defined reference s t r u c t u r e s i n model compounds needed f o r exact data analysis procedures, i.e. f o r precise values o f t h e bond length and t h e coordination number [SO]. Always convincing are optical spectra g i v i n g t h e blue-shifted absorption edge o f a quantum-sized particle as expected theoretically from an increasing band gap with decreasing particle size [81, 821. Blueshifted absorption edges were identified f o r zeolite-hosted CdS [70, 71, 741, PbS [70, 741 and ZnS [75]. A less pronounced blue-shift is obtained upon several measures, l i k e increasing loading o f t h e zeolite host with t h e semiconductor guest, calcination procedures o r sulfidation of non73a, 741 and is interpreted by cluster dehydrated samples [71, aggregation. The particle sizes deduced from t h e absorption edge positions exceed the supercage dimensions by far. However, photocorrosion and photoreaction measurements point t o an internal location of t h e aggregates [71, 741. Different aggregation mechanisms have

12

a

b

F i g u r e 6. Models (a) o f a (CdS)4 cluster i n a sodalite cage and ( b ) of occupied adjacent sodalite cages (from ref. 73a).

been proposed f o r these zeolite-accomodated sulfide agglomerates, i.e. ( i ) interconnection of clusters in adjacent supercages [71], (ii) interconnection o f filled sodalite cages (Fig. 6) [73a] o r ( i i i ) growth o f sulfide particles to supra-supercage size under zeolite lattice fragmentation and mesopore formation [74,75]. The l a t t e r mechanism is supported by adsorption isotherm analysis. The application of transmission electron microscopy gives a s t r i c t correlation between cluster sizes gleaned from t h e optical absorption edge position and those drawn from t h e electron micrographs [74]. The increase o f particle size is accompanied b y characteristic colour changes (Table 1). The observed increase of t h e rate of photocorrosion with decreasing sulfide particle size [75, 761 is i n agreement with theoretical expectations. Small semiconductor particles are characterized b y a high extent of lattice defects affecting bond lengths [83] and solubility [77,78] and leading t o surface states energetically located within t h e band gap [84]. The surface states t r a p electrons and, therefore, suppress t h e recombination of lightinduced electron-hole pairs. The holes remain available f o r photocorrosion, i.e. t h e oxidation of sulfide ions belonging t o t h e lattice of t h e semiconductors. I t i s an open question, whether photocorrosion can be used t o eliminate certain fractions i n cluster size distributions i n o r d e r t o prepare single-size particles.

3.3 Potential Applications The use o f o f QD semiconductors f o r optical switching are suggested repeatedly [65, 661. The suggestions are based on the experimental finding, t h a t t h e excitation absorption is bleached d u r i n g t h e presence of a trapped electron-hole pair and recovers as t h e exciton pair decays [go, 911. The effect is r e f e r r e d t o a s t r o n g reduction of t h e excitation oscillator s t r e n g t h in t h e presence o f a surface-trapped electron-hole pair [91]. More sophisticated applications, l i k e cellular automation computers, are i n early stages o f development [85]. Contacting o f QDs requires t h e development o f quantum wires. F i r s t attempts are reported f o r t h e preparation o f zeolite-hosted selenium chains [86, 871. The inclusion of conducting polymer chains in zeolite channels is under investigation [88, 891.

73 Table 1 Colour changes o f size-tuned (nm) zeolite-supported PbS clusters 0.5

1 greenish l i g h t yellow beige

2 dark beige

4

yellow earth

6 8 red r u s t y brown

10 dark r u s t y brown

> 10 black brown

The use of zeolite-hosted semiconductor oxides as chemical sensors towards oxidizing o r reducing gases might be attractive. Since t h e alteration of t h e conductivity depends on changes of t h e oxide s h o r t e r diffusion distances i n smaller clusters stoichiometry [93,94], should r e s u l t i n shorter response times of t h e sensors. Fast response is a prerequisite f o r t h e application of sensors based on changes of t h e bulk composition, e.9. in air/fuel ratio control devices. Furthermore, t h e application of t h e molecule sieving effect o f zeolites f o r t h e development o f molecule selective sensors i s studied. One route of preparation aims t o coat t h e sensor material b y a glassy t h i n film hosting zeolite c r y s t a l s as molecule sieving gates [95, 961. The shortcoming o f t h i s technique is t h e limited thermal and mechanical stability of t h e glassy t h i n film. It seems t o be more prospective t o use zeolite single c r y s t a l s hosting t h e sensor materials. The changes i n t h e stoichiometry o f t h e sensor oxides by interaction with gas molecules can be detected b y methods which do not r e q u i r e electric contacts, e.g. optical absorption, dielectric p e r m i t t i v i t y o r phonon absorption. A t present only a few studies on t h e application o f zeolite-hosted semiconductors i n photochemistry [71, 741 exist. The potentials of nanocomposite systems i n a zeolite host, comprising photosemiconductor, catalyst, photosensitizer, sacrificial donor and acceptor in optimum spatial arrangement mediated by t h e void s t r u c t u r e o f t h e host, give reasons for f u r t h e r intensive investigations. The applicability o f zeolite-based molecular multicomponent systems f o r light-induced charge separation o r photochemical H2 evolution has been demonstrated impressively [97].

4.

MISCELLANEOUS ZEOLITE-HOSTED

SYSTEMS

4.1 Optical processing systems Optical processing systems are proposed based on relative intensity changes of second harmonic generation i n sorbate complexes of p n itroan i I i ne and 2-meth y I-p-n itroan i Ii ne i n molecu lar sieve hosts [98]. Optical data storage would be one o f a variety o f potential electro-optical applications. I n addition, molecular optical effects b y t h i r d harmonic generation are suggested f o r optical data storage based on t h e possible formation o f bistabilities f o r t h e local fields [65,991. 4.2 Zeolite-hosted dyes

Zeolite-hosted dyes can be prepared by ion-exchange o f cationic dye molecules o r b y incorporation d u r i n g hydrothermal crystallization [loo]. Optical absorption bands o f molecular sieve-accomodated dyes can be

broadened, presumably, due t o t h e variety o f d i f f e r e n t possible interactions based on t h e d i s t r i b u t i o n of t h e aluminum atoms in t h e zeolite framework. This effect favours optical data storage by spectral methylene blue shows a hole b u r n i n g [ l o l l . Faujasite-encapsulated decoupling o f t h e guest electron excitation and t h e host phonon movement, which is a prerequisite f o r high temperature hole b u r n i n g [loo]. A f u r t h e r advantage of t h e zeolite host i s its relatively h i g h thermal, chemical and mechanical stability as compared t o polymer hosts. Reversible optical data storage based on reversible changes o f molecular structures, .e.g. cis-trans-isomerization o f thioindigo [102], is possible.

5.

ACKNOWLEDGEMENT

I am grateful t o Prof. D r . N.I. Jaeger f o r f r u i t f u l cooperation and critical reading o f t h e manuscript, t o D r . R. Lamber f o r electron micrographs t o A. Kleine f o r molecular modelling, t o M. Wark f o r PbS particle size t u n i n g and t o Academic Press and American Chemical Society f o r permission o f p i c t u r e reproduction. Financial s u p p o r t by t h e Bundesmi n i s t e r f u r Forschu n g u nd Technolog ie (BMFT-423-4003-03C 2583, BMFT-NT 20 606) and t h e Max Buchner-Forschungsstiftung (MBFStKennziffer 1557) are gratefully acknowledged.

6

REFERENCES

1

A.P. Bolton, i n "Zeolite Chemistry and Catalysis" (J.A. Rabo, Ed.), ACS Monograph 171, American Chemical Society, Washington 1976, p. 552. E. Gallei, Chem.-1ng.-Techn. 52 (1980) 99; and references therein. H. Heinemann, i n "Catalysis: Science and Technology" (J.R. Anderson and M. Boudart, Eds.), Springer, Berlin 1981, vol. 1 , p. 1. K. Becker, H. John, K.-H. Steinberg, M. Weber and K.-H. Nestier, in "Catalysis on Zeolites" (D. Kallo and Kh. M. Minachev, Eds.), Akademia Kiado, Budapest 1988, p. 515. C. Naccache and Y. Ben Taarit, in "Zeolites: Science and Technology" (F.R. Ribeiro, A.E. Rodrigues, L.D. Rollmann and C. Naccache, Eds.) Martinus Nijhoff, The Hague 1984, p. 373. Kh. M. Minachev, V.V. Kharlamov and V.I. Garanin, in "Catalysis on Zeolites" (D. Kallo and Kh. Minachev, Eds.), Akademia Kiado, Budapest 1988, p. 489. B.K. Nefedov, A.A. Slinkin, A.V. Kucherov, N.S. Sergeeva and Ya. T. Eidus, I z v . Akad. Nauk (SSSR), Ser. Khim. (1974) 2119. B.K. Nefedov, N.S. Sergeeva and L.L. Krasnova, Izv. Akad. Nauk (SSSR), Ser. Khim. (1977) 614. (a) J.J. Verdonck, P.A. Jacobs and J.B. Uytterhoeven, J. Chem. SOC., Chem. Commun. (1979) 181. ( b ) M. Niwa, T. I z u k a and J.H. Lunsford, J. Chem. SOC., Chem. Commun. (1979) 684.

2 3 4

5 6 7 8

9

75 10

11

12

13 14

15

16

17 18 19

20

21 22 23 24

25 26 27 28

29 30 31

(a) H.H. Nijs, P.A. Jacobs and J.B. Uytterhoeven, J. Chem. SOC., Chem. Commun. (1979) 1095. ( b ) D. Fraenkel and B.C. Gates, J. Am. Chem. SOC. 102 (1980) 2478. (c) T. Bein, G. Schmiester and P.A. Jacobs, J. Phys. Chern. 90 (1986) 4851. O.V. Bragin, A.V. Preobrazhensky and A.L. Liberman, I z v . Akad. Nauk (SSSR), Ser. Khim. (1974) 2751. O.V. Bragin, E.S. Shpiro, A.V. Preobrazhensky, S.A. Isaev, T.V. Vasina, B.B. Dysenbina, G.V. Antoshin and Kh. M. Minachev, Appl. Catal. 27 (1986) 219. O.V. Bragin, T.V. Vasina, A.V. Preobrazhensky and Kh.M. Minachev, I z v . Akad. Nauk (SSSR), Ser. Khim. (1989) 750. B. Notari, i n "Innovation i n Zeolite Materials Science" (P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff, Eds.), Elsevier, Amsterdam 1988, Stud. Surf. Sci. Catal., vol. 37, p. 413. (a) U. Rornanov, A. Esposito, F. Masperi, C. Neri and M. Clerici, i n "New Developments i n Selective Oxidation" (G. Centi and F. Trifiro, Eds.), Elsevier, Amsterdam 1990, Stud. S u r f . Sci. Catal., vol. 55, p. 33. ( b ) P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti and P. Gervasutti, ibid., p. 43. ( c ) C. F e r r i n i and H.W. Kouvenhoven, ibid., p. 53. W.F. Holderich, i n "Zeolites: Facts, Figures, F u t u r e " (P.A. Jacobs and R.A. van Santen, Eds.), Elsevier, Amsterdam 1989, Stud. Surf. Sci. Catal., vol. 49A, p. 69. W.F. Holderich, Angew. Chem. 100 (1988) 232. H.G. Karge, H.K. Beyer and G. Borbely, Catalysis Today 3 (1988) 41. B. Wichterlova, S. Beran, S. Bechnarova, K. Nedomova, L. Dudikova and P. Jiru, i n "Innovation i n Zeolite Materials Science" (P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff, Eds.), Elsevier, Amsterdam 1988, Stud. Surf. Sci. Catal., vol. 37, p. 199. D. Ballivet-Tkatchenko, G. Coudurier and N.D. Chau, i n "Metal Microstructures i n Zeolites" (P.A. Jacobs, N.I. Jaeger, P. J i r u and G. Schulz-Ekloff, Eds.) Elsevier, Amsterdam 1982, Stud. Surf. Sci. Catal., vol. 12, p. 123. R.K. Unger and M.C. Baird, J. Chem. SOC., Chem. Commun (1986) 643. D.K. Lee and S.K. Ihm , J. Catal. 106 (1987) 386. W.R. Hastings, C.J. Cameron, M.J. Thomas and M.C. Baird, I n o r g . Chem. 27 (1988) 3024. L.F. Rao, A. Fukuoka, N. Kosugi, H. Kuroda and M. Ichikawa, J. Phys. Chem. 94 (1990) 5317. D.R. Corbin, W.C. Seidel, L. Abrams, N. Herron, G.D. Stucky and C.A. Tolman, I n o r g . Chem. 24 (1985) 1800. T.N. Huang and J. Schwartz, J. Am. Chem. SOC. 104 (1982) 5244. A. Mahay, G. Lemay, A. Adnot, I.M. Szoghy and S. Kaliaguine, J. Catal. 103 (1987) 480. J.A. Rabo, V. Schomaker and P.E. Pickert, in "Proc. 3 r d I n t . Congr. Catal." (W.M.H. Sachtler, Ed.), North-Holland, Amsterdam 1965, vol. 2, p. 1264. R.A. Dalla Betta and M. Boudart, i n "Proc. 5th I n t . Congr. Catal." (J.W. Hightower, Ed.), North-Holland, Amsterdam 1972, vol. 2, p. 1329. P. Gallezot, Catal. Rev.-Sci. Eng. 20 (1979) 121. M. Briend-Faure, J. Jeanjean, M. Kermarec and D. Delafosse, J. Chem. SOC., Faraday Trans 1 , 74 (1978) 1538.

76 32 33

34

35 36 37 38 39 40 41 42 43 44 45

46 47 48 49 50 51 52 53

54 55 56

57 58

59

60

J.T. Richardson, J. Catal. 21 (1971) 122. P.A. Jacobs, i n "Metal Microstructures i n Zeolites" (P.A. Jacobs, N.I. Jaeger, P. J i r u and G. Schulz-Ekloff), Elsevier, Amsterdam 1982, Stud. Surf. Sci. Catal., vol. 12, p. 71. Kh. M. Minachev, G.V. Antoshin, E.S. Shpiro and Yu. A. Yusifov, i n "Proc. 6th Int. Congr. Catal." (G.C. Bond, Ed.), Chemical Society, London 1977, vol. 2, p. 621. J.H. Lunsford and D.S. Treybig, J. Catal. 68 (1981) 192. G. Schulz-Ekloff, D. Wright and M. Grunze, Zeolites 2 (1982) 70. W. Romanovski, Z. anorg. allg. Chem. 351 (1967) 180. D. Herrnerschmidt and R. Haul, Ber. Bunsenges. Phys. Chem. 84 (1980) 902. L.R. Gellens, W.J. Mortier and J.B. Uytterhoeven, Zeolites 1 (1981) 11. G.A. Ozin, M.D. Baker and J. Godber, J. Phys. Chem. 88 (1984) 4902. J.R. Morton and K.F. Preston, Zeolites 7 (1987) 2. G. Bergeret, P. Gallezot and B. Imelik, J. Phys. Chem. 85 (1981) 411. K. Moller and T. Bein, J. Phys. Chem. 93 (1989) 6116. Z. Zhang, H. Chen, L.-L. Sheu and W.M.H. Sachtler, J. Catal. 127 (1991) 127. P. Gallezot, J. Datka, J. Massardier, M. Primet and B. Irnelik, in "Proc. 6th I n t . Congr. Catal." (G.C. Bond, Ed.), Chemical Society, London 1977, vol. 2, p. 696. L.L. Sheu, H. Knozinger, W.M.H. Sachtler, J. Am. Chem. SOC. 111 (1989) 8125. E.S. Shpiro, R.W. Joyner, Kh.M. Minachev and P.D.A. Pudney, J. Catal. 127 (1991) K.-P. Wendlandt, H. Bremer, F. Vogt, W.P. Reschetilowski, W. Morke, H. Hobert, M. Weber and K. Becker, Appl. Catal. 31 (1987) 65. G. Larsen and G.L. Haller, Catal. Lett. 3 (1989) 103. B.C. Beard and P.N. Ross, J. Phys. Chem. 90 (1986) 6811. T. Sheng, X. Guoxing and W. Hongli, J. Catal. 111 (1988) 136. R. Lamber, N.I. Jaeger and G. Schulz-Ekloff, J. Catal. 123 (1990) 285. N.I. Jaeger, P. Ryder and G. Schulz-Ekloff, i n " S t r u c t u r e and Reactivity of Modified Zeolites" (P.A. Jacobs, N.I. Jaeger, P. Jiru, V.B. Kazansky and G. Schulz-Ekloff, Eds.), Elsevier, Amsterdam 1984, Stud. Surf. Sci. Catal., vol. 18, p. 299. M.S. Tzou, H.J. Jiang and W.M.H. Sachtler, Appl. Catal. 20 (1986) 231. L.F. Rao, A. Fukuoka and M. Ichikawa, J. Chem. SOC., Chem. Commun. (1989) 458. M. Dufaux, M. Lokolo, P. Meriandeau, C. Naccache and Y. Ben Taarit, i n "New Developments in Zeolite Science and Technology" (Y. Murakami, A. I i j i m a and J.W. Ward, Eds.), Elsevier, Amsterdam 1986, Stud. Surf. Sci. Catal., vol. 28, p. 929. N.I. Jaeger, G. Schulz-Eklaff and A. Svensson, cf. ref. 56, p. 923. (a) N.I. Jaeger, G. Schulz-Ekloff and A. Svensson, i n "Catalysis and Adsorption by Zeolites" (G. dhlmann and H. Pfeifer, Eds.), Elsevier, Amsterdam 1991, Stud. Surf. Sci. Catal., vol. 65, p. 327. (b) N.I. Jaeger, A. Jourdan and G. Schulz-Ekloff, J. Chem. SOC., Faraday Trans.1, 1991 in press. (a) A. Kleine, P.L. Ryder, N. Jaeger and G. Schulz-Ekloff, J. Chem. SOC., Faraday Trans. 1, 82 (1986) 205. ( b ) D. Exner, N. Jaeger, A. Kleine and G. Schulz-Ekloff, J. Chem. SOC., Faraday Trans. 1, 84 (1988) 4097. G. Schulz-Ekloff and N. Jaeger, Catalysis Today 3 (1988) 459.

61 62 63 64 65 66 67 68 69 70 71 72

73

74 75

76 77 78 79

80

81 82 83 84

85 86

N.I. Jaeger, J. Rathousky, G. Schulz-Ekloff, A. Svensson and A. Zukal, cf. ref. 16, vol. 498, p. 1005. J. Rathousky, A. Zukal, N.I. Jaeger and G. Schulz-Ekloff, in preparation. A. Kleine a n d P.L. Ryder, Catalysis Today 3 (1988) 103. G.A. Ozin, A. Kuperman and A. Stein, Angew. Chern. 101 (1989) 373. S. Schrnitt-Rink, D.A.B. Muller a n d D.S. Chemla, Phys. Rev. B 35 (1987) 8113. G.D. S t u c k y and J.E. Mac Dougall, Science 247 (1990) 669. P. M a r q u a r d t , L. Borngen, G. Nimtz, H. Gleiter, R. Sonnberger and J. Zhu, Phys. L e t t e r 114 A (1986) 39. G. Nirntz, P. M a r q u a r d t and H. Gleiter, J. C r y s t a l Growth 86 (1988) 66. D.R. Rolison, Chern. Rev. 90 (1990) 867. Y. Wang and N. Herron, J. Phys. Chem. 91 (1987) 257. M.A. Fox a n d T.L. Pettit, Langrnuir 5 (1989) 1056. (a) R.D. Strahrnel, T. Nakamura and J.K. Thomas, J. Chern. SOC., Faraday Trans. 1, 84 (1988) 530; ( b ) X. L u i and J.K. Thomas, Langmuir 5 (1989) 58. (a) N. Herron, Y. Wang, M.M. Eddy, G.D. S t u c k y , D.E. Cox, K. Moller and T. Bein, J. Am. Chem. SOC. 1 1 1 (1989) 530; ( b ) K. Moller, M.M. Eddy, G.D. S t u c k y , N. H e r r o n and T. Bein, J. Am. Chem. SOC. 111 (1989) 2564. M. Wark, G. Schulz-Ekloff and N.I. Jaeger, Catalysis Today 8 (1991) 467. M. Wark, G. Schulz-Ekloff, N.I. Jaeger and A. Zukal, in "Zeolite Chemistry and Catalysis" (P.A. Jacobs, N.I. Jaeger, L. Kubel k o v a and B. Wichterlova, Eds.), Elsevier, Amsterdam 1991; Stud. S u r f . SCi. Catal., t h i s book, vol. , p. M. Wark, G. Schulz-Ekloff and N.I. Jaeger, Mater. Res. SOC. Symp. Proc.; Materials Research Society, P i t t s b u r g h 1991, in press. M.I. Vucemilovic, N. Vukelic and T. Rajh, J. o f Photochem. and Photobiol. 42 A (1988) 157. S. Gallardo, M. Guiterrez, A. Henglein and E. Janata, Ber. Bunsenges. Phys. Chem. 93 (1989) 1980. W. Lutz, H. Fichtner-Schmittler, M. Bulow, E. S c h i e r k o r n , N. Van Phat, E. Sonntag, I. Kosche, S. Arnin and N. Dyer, J. Chem. Soc., Faraday Trans. 1, 86 (1990) 1899. D.E. Sayers and B.A. Bunker, i n "X-ray Absorption" (D. Koningsberger a n d R. Prins, Eds.), Wiley, New York 1988; Chemical Analysis, vol. 92, p. 211. H.M. Schmidt and H. Weller, Chem. Phys. Lett. 129 (1986) 615. L.E. Brus, J. Phys. Chem. 90 (1986) 2555. Y. Wang, A. Suna, M. Mahler and R. Kasovski, J. Chern. Phys. 87 (1987) 7315. (a) A.N. Chestnoy, T.D. Harris, R. Hull a n d L.E. Brus, J. Phys. Chern. 90 (1 986) 3393. (b)Y. Wang and N. Herron, J. Phys. Chem. 92 (1988) 4988. E. Corcoram, Spektrum d e r Wissenschaft, Januar 1991, p. 76. (a) V.N. Bogomolov, S.V. Kholodkevich, S.G. Rornanov, L.S. Agroskin, Solid State Cornrnun. 47 (1983) 181; ( b ) 0. Terasaki, K. Yamazaki, J.M. Thomas, T. Ohsuna, D. Watanabe, J.V. Sanders and J.C. B a r r y , Nature 330 (1987) 58.

78 J.B. Parise, J.E. Mac Dougall, N. Herron, R. Farlee, A.W. Sleight, Y. Wang, T. Bein, K. Moller and L.M. Moroney, I n o r g . Chem. 27 (1988) 221. 88 P. Enzel and T. Bein, J. Phys. Chem. 93 (1989) 6270. 89 J.V. Caspar, V. Ramamurthy and D.R. Corbin, J. Am. Chem. SOC. 113 (1991) 600. 90 (a) A. Henglein, A. Kumar, E. Janata and H. Weller, Chem. Phys. Lett. 132 (1986) 133. ( b ) M. Haase, H. Weller and A. Henglein, J. Phys. Chem. 92 (1988) 4706. E. Hilinsky, P. Lucas and Y. Wang, J. Chem. Phys. 89 (1988) 3435. 91 92 Y. Wang, A. Suna, J. McHugh, E. Hilinsky, P. Lucas and R.D. Johnson, J. Chem. Phys. 92 (1990) 6927. 93 D. Baresel, W. Gellert, W. Sarholz and P. Scharner, Sensors and Actuators 6 (1984) 35. 94 D. Kohl, Sensors and Actuators 18 (1989) 71. 95 T. Bein, K. Brown and C.J. Brinker, cf. ref. 16, vol. 49 B, p. 887. 96 T. Bein, K. Brown, G.C. F r y e and C.J. Brinker, J. Am. Chem. SOC. 111 (1989) 7640. 97 (a) Z. Li, C. Lai and T.E. Mallouk, I n o r g . Chem. 28 (1989) 178. ( b ) J.S. Krueger, J.E. Mayer and T.E. Mallouk, J. Am. Chem. SOC. 110 (1988) 8232. (c) L. Persaud, A.J. Bard, A. Campion, M.A. Fox, T.E. Mallouk, E.S. Webber and J.M. White, J. Am. Chem. SOC. 109 (1987) 7309. 98 S.D. Cox, T.E. Gier, G.D. Stucky and J. Bierlein, J. Am. Chem. SOC. 110 (1988) 2986. 99 Y . Wang and N. Herron, J. Phys. Chem. 95 (1991) 525. 100 R. Hoppe, G. Schulz-Ekloff, D. Wohrle, M. Ehrl and C. Brauchle, cf. ref. 75 ( t h i s book). . 101 W.E. Moerner (Ed.), Persistent Spectral Hole Burning: Science and Applications, Topics i n C u r r e n t Physics, Springer, New York 1988, p. 251. 102 R. Hoppe, G. Schulz-Ekloff and D. Wohrle, submitted. 87

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

I9

ISOMORPHOUS SUBSTITUTION IN ZEOLITES: A ROUTE FOR THE PREPARATION OF NOVEL CATALYSTS G. Bellussi and V. Fattore ENIRICERCHE S.p.A., Via F.Maritano 26, 20097 San Donato, Milan, Italy Abstract In the recent years a growing interest in the preparation, characterization and utilization of Titanium-silicalite has been observed. New interesting applications of TS-1, such as the low temperature oxidation of paraffins to alcohols and ketones, were reported. In this paper we will review the state of the knowledge concerned with the preparation of titanium-containing zeolites and the characterization of lattice Ti-sites. We will try to depict the emerging routes for research activities in this field. 1. Introduction With the growth of the interest in the utilization of zeolites as catalysts, more attention was devoted to the introduction of various elements as substitute for lattice silicon or aluminum. Prof. Barrer, some years ago, classified four types of isomorphous replacement in zeolites [l]: 1- One guest molecule by another (i.e.: substitution of sodium chloride by sodium sulphate transforms Sodalite into Nosean). 2- One cation by another (i.e.: the more common method is the treatment of a zeolite with an aqueous solution of the salt containing the different cation; it is the base of water sweetening). 3 - One element by one of its isotopes (i.e. : mainly hydrogen, oxygen and silicon). 4 - One element in tetrahedral position by another (i.e.: substitution of silicon or aluminum with sterically compatible elements). Type one substitution has low relevance from the point of view of zeolite applications. Type two substitution is important because through it is possible to obtain active catalysts and to eliminate certain cations contaminating water or solutions. Type three substitution could be useful to investigate the zeolite synthesis and for Characterization purposes. Studies on type four substitution were initiated many years ago. Goldsmith [ 2 ] referred in 1952 his success in the synthesis of

80

Thomsonite samples in which Si was replaced by Ge in the zeolite framework. Again in the fifties appeared the remarkable work of Barrer et al. [ 31 : Thomsonite, zeolite A, Faujasite and Harmotome wereobtained havinggalliumand/or germaniuminthe lattice. These works were followed by a worldwide scientific effort to identify zeolites in which isomorphous substitution could give rise to: - new structures, - new chemical compositions, - new properties and, consequently, new applications. This paper will deal with type four substitution taking in consideration one of the most interesting case: the replacement of Ti for Si in Silicalite-1. The synthesis of titano-silicates considered to exhibit zeolitic properties was described the first time in the patent literature by D.A. Young in 1967 [4]. The information given in the Young’s patents were not sufficient to draw clear conclusions about the crystalline structure of that compounds. More than ten years later, again in the patent literature, Taramasso et al. describedthe synthesis of a titanium containing silicalite [5]. In the following years, several other patent applications claimed the possibility to prepare Ti-containing zeolites, but clear evidences for the presence of titanium in framework positions were not reported until 1986 when Perego et al. described in a paper the synthesis and the characterization of the Titanium-silicalite-1 (TS-1) [6]. The presence of titanium in the silicalite structure gave to the TS-1 original properties in oxidation reactions with hydrogen peroxide [6-131. The peculiar TS-1 catalytic activity, never observed before with other Ti-containing materials, open new and very interesting perspectives for industrial applications of shape selective catalysis. In the recent years, several papers dealing with the synthesis, the characterization and the catalytic activity of the TS-1 have been published, nevertheless many questions about its preparation, activation and characterization of active sites remain still open. 2. Physical-chemical characterization of TS-1

Only for few elements the possibility to substitute Si or A1 in zeolitic structure has beendemonstrated. In factonlycationswith specific steric requisites can fit in the tetrahedral positions of the zeolite lattice. K.G. Ione et al. [14], by applying the Pauling criterion, concluded that only the cations for which the r a t i o g = rc/ro, where rc andr,arethe radiiofthe consideredcationan the oxygen respectively, is in the range 0.414>p>0.225 should be stable in a tetrahedral surrounding. Then we have to expect that for cations for which p is out of that range, the substitution should be impossible or it could take place only on a limited scale. In the latter case it appears very difficult to demonstrate whether an element is really sitting in the lattice and not only supported

81

in an extraframework position. In the case of titanium,p is equal to 0.515; this can explain the difficulties encountered to recognize its presence in the TS-1 lattice. Many different physical-chemical methods where used to carry out such kind of characterization. X-ray diffraction analysis was the more effective technique. For samples havinga Ti molar fractionx =Ti/(Si +Ti) lower than 0.025 a clear increase of the unit cell volume was observed with the increasing of x [ 6 ] . The monoclinic lattice symmetry, characteristic of Silicalite-1, is preserved up to x = 0.01; for higher values of x, orthorombic symmetry was observed. The unit cell volume variation was correlated to the higher length of the Ti0 bond with respect to the Si-0 bond. The cell volume was related to the framework composition through the equation:

is the cell where V(,i)is the cell volume of the silicalite, V volume of TS-1, x is the Ti molar fraction, dTi andxAsi represent the Ti-0 and Si-0 bond length respectively. By applying the above equation to the TS-1, a value of 1.79 A was obtained forthe Ti-0 bond length. This valuewas in agreement with Ti-0 bond distances measured for BaZTiOq in which Ti displays a tetrahedral coordination. Since this equation does not consider T-0-T angle variations, the presence of framework T atoms in a distorted tetrahedral symmetry can affect the T-0 bond distance values derived from it. As a difference with the silicalite, TS-1 shows a characteristic IR band at 960 cm-l. Adsorption of water shifts upwards the 960 cm-l peak to 970-975 cm-lj151. By increasing the Ti molar fraction in TS-1 from 0 to 0.025 a linear increase of the above mentioned IR band intensity with x was observed [15]. This means that the 960 cm-l IR band is in someway related to the presence of lattice titanium. SEM-EDX analysis of TS-1 showed an homogeneous distribution of Ti through the TS-1 crystals [ 6 ] . Evidence in favour of the structure homogeneity of TS-lwas achieved a l s o by FABMS analysis [161. More recently, A. Tuel et al. demonstrated the possibility to reduce the tetravalent titanium in TS-1 giving rise to the formation of Ti3+ species [ 171. The signals on EPR spectra of the reduced TS-1 sample were attributed to Ti3+ in tetrahedral coordination; from that the authors concluded about the framework sitting of the precursor tetravalent Ti. All the above mentioned experimental results and the peculiar TS-1 catalytic activity, lead to conclude about the presence of titanium atoms in framework position but as we will see in one of the next paragraphs, several different hypothesis can be thought to describe the environment of such titanium atoms.

a2

3 . Direct synthesis of TS-1

The synthesis of TS-1 is particularly difficult, probably because of the higher tendency displayed by Ti4+ to assume the octahedral coordination with respect to the tetrahedral in compounds with the oxygen. In the patent of Taramasso et al. [5] two methods for the hydrothermal synthesis of TS-1 are described. In the first method the reaction mixture is prepared by hydrolysis of tetraethylsilicate and tetraethyltitanate, while in the second it is prepared from colloidal silica and tetrapropylammonium peroxotitanate. One of the critic point of these synthesis is the presence of alkali cations, even in trace amounts, in the reaction mixture. Several authors [18-211 proved that the presence of sodium or potassium can prevent the insertion of titaniuminto the silicate framework. These cations are contained as impurity in most of the commercial solution of tetrapropylammonium hydroxide. The addition of increasing amount of NaOH to a solution of high purity tetrapropylammonium hydroxide, was showed to produce an increase of the amount of Ti detected in the solid recovered at the end of the synthesis and a d e ~ r e a s e o f t h e 9 6 0 c m ~ ~ I R b a n d[21]. At the highest level of sodium, the presence of anatase beside the silicalite crystals, was revealedby X - r a y d i f f r a c t i o n a n a l y s i s . The presence of sodium in the reaction mixture seems to favour the formation of insoluble titanium-silicate species which reduce the amount of titanium available for the formation of the TS-1 crystals. In few cases, the synthesis of a titanium containing silicalite prepared in the presence of sodium was reported in the literature [22-241. Starting f r o m a g e l h a v i n g t h e f o l l o w i n g m o l a r c o m p o s i t i o n : Si02, 0.004-0.04 TiO2, 0.14 Na20, 0.1 TPA-Br, 23.5 H20, Kornatowsky et al. [22] reported the formation of very large silicalite crystals (160-180 pm) containing 1 Ti atom every 60 Si atoms. The influence of the molar SiOZ/Ti02 ratio, in the starting reaction mixture, on the Ti content of TS-1 is reported in Fig. 1. For these experiments we used a synthesis procedure similar to thatdescribedinthe exarnplelof [5]. The syntheseswereperformed in the absence of alkali cations, by using tetraethyltitanate and tetraethylsilicate as sources of Ti02 and Si02 respectively. The TS-1 results always with a higher SiOz/TiO2 ratio compared to the starting solution. At the higher concentration of titanium in the reaction mixture, the formation of bulky Ti02 was observed. Crystallization temperature has a strong influence on the TS-1 composition. Fig. 2 shows how variable is the Si02/Ti02 ratio in the TS-1 with the variation of the crystallization temperature. It is relevant that from 100 " C to 200 " C no anatase is observed; its formation starts at higher temperatures. The maximum amount of framework Ti obtained in these experiments was correspondent to 2 Ti atoms per elementary cell.

83

Si02/Ti0 10 20 50

13

(reagent mixtures)

100

300

110

42 59

325

Si02/Ti02 (products) Figure 1. Influence of the reaction mixture composition on Ti contents in TS-1.

SiO /Ti0 20

2

23 30

(reagent mixture)

2

40

60

97

Si0,/Ti02 (products) Figure 2. Influence of the crystallization temperature on Ti contents in TS-1. An interesting method for the synthesis of TS-1 was recently reported by Padovan et a1 [ 251. A sample of dried microspheroidal silica was impregnated up to incipient wetness with an aqueous solution obtained by hydrolizing Ti isopropoxide in aqueous tetrapropylammonium hydroxide. The sample was then sealed in a glass tube and kept at 448 K for several hours. A well crystallized titanium containing silicalite was obtained after 10 hours. At longer crystallization times, the formation of octahedralTi02was revealed by UV-Vis. spectroscopy. J.M. Popa et al. described the preparation of a titanium containing silicalite from a reaction mixture having a very low pH (6.5 - 7.5) andcontaining fluoride anions [26]. The crystalline product obtained by this method had a monoclinic symmetry. No evidence was given to support the presence of titanium in lattice position. Several efforts has been devoted to the synthesis of silicalite

.

84

crystals containingtitanium and atrivalent element. The interest in this kind of material is not only related to the possibility of having catalysts active in both oxidizing and acid catalyzed reactions, but also to investigate forthe presence of synergysms between the two sites. Several patents describe the synthesis of silicalites containing Ti and A1 [27-291, Ti and Fe [30], Ti and Ga [31], Ti and B [32]. In many of them, only applications in acid catalyzed reactions were reported. We demonstrated in a recent paper the possibility to insert Ti and A1 or Ga or Fe in lattice position in the Silicalite-1 [21]. The amounts of framework titanium and trivalent element are limited to a restricted range of compositions. In fact, when the amount of framework trivalent element is increased above a certain value ( Si02/M203 - 150), the amount of lattice titanium begins to decrease. We observed a different catalytic behaviour between TS-1 and Al(Ga,Fe)-TS-1 in the butene epoxidation with H20e2. While TS-1 is very selective toward the formation of the epoxide, the other catalysts, because of the presence of acid sites, are more selective toward the formation of glycols. The rate of H 2 0 2 conversion was lower when the selectivity to glycols was higher. This was explained by considering that the slow diffusion rate through the silicalite channels of glycols and polyglycols formed on the acid sites hinders the diffusion of H 2 0 2 and of the olefins. The difference in the rate of H 2 0 2 conversion accounts for the presence of lattice Ti and Al(Ga,Fe) in the same crystals. The direct synthesis of titanium-containing zeolites with a framework topology different from the MFI was also investigated. The preparation of a TS-2 (titanium-silicalitewith MEL framework topology) has been reported [33-341. The TS-2 can be prepared in the same way as the TS-1 but substitutingthe tetrapropylammonium cation with the tetrabutylammonium during the synthesis. As already observed in the case of the corresponding aluminosilicates [35-361 and boro-silicates [37], the use of mixed alkylammonium cations allow the formation of titanium-silicalites witha structure intermediatebetween that of MFI andMEL structure type 1341 * The synthesis of titanium containing Mordenite, Sodalite and ZSM-12 is also reported in the patent literature [28]; the informations given are not sufficient to drawn conclusions about the situation of titanium. Apart from the MFI and MEL structure type, there are not yet clear evidences about the possibility to insert titanium atoms in other zeolite structures by direct synthesis. Recently D.M. Chapman et al. described a method for the synthesis of crystalline microporous titanium-silicates. These materials are different from the zeolites since their lattice is constitued by tetrahedral SiO, and octahedral Ti02 units [ 3 8 ] . Xuznicki reported in a patent the preparation of a crystalline microporous titanium-silicate with pore size of approximately 8 A [39]. It is not clear if this new material is really constitued of Si02 and Ti02 tetrahedra or if the TiO, units are in octahedralcoordination as in the case mentioned above.

85

4. Secondary synthesis of TS-1 A procedure able to modify the lattice composition of a preformed zeolite leaving the framework topology relatively unchanged is indicated with the terms "indirect synthesis" or "secondary synthesis". The method consists in contacting the zeolite crystals with a suitable compound of the element to be inserted in the framework. This procedure has been mainly used to substitute silicon for aluminum atoms in Y zeolite [40]. Several examples describing the indirect synthesis of titanium containing zeolites are reported in the literature (Table 1). The treatment of the zeolite crystals with titanium compounds are made either through the contact with a liquid phase [41], or witha gas phase [42-441.The titanium salts used forthe secondary synthesis are gaseous TiC14 or an aqueous solution of (NH,),TiF,. Skeels and Flanigen [41] reported the preparation of several titanium containing zeolites: Faujasite, Phillipsite, Omega, L, ZSM-5. They put in contact the zeolite crystals with a solution of (NH4)2TiF6 at 75 - 95 " C . The reaction with the ammonium f l u o r o t i t a n a t e p r o d u c e s t h e r e m o v a l o f acertainamountof aluminum and the deposition of a relevant quantity of titanium on the solid phase. In spite of the high amount of titanium detected on the treated samples no new IR band, attributable to the presence of titanium, was observed in the region between 900 and 1000 cm-l. Table 1. Examples of secondary synthesis of Ti-containing Zeolites Precursor

SiOz/ A1 7 0 -

Pretreatment

Fauj asite Omega L Phi11ip ZSM-5 ZSM-5 ZSM-5 ZSM-5 Faujasite Beta Bor-C Bor-C

5.08 6.62 5.80 3.77 30.82 46.49 50.00 25.00 (el (el 73(f) 73(f)

NH4+ NH4+ NH & NH4+ NH 4+

.

form form form form form

--

dealum. -_

H+ H+

form form

--

--

Ti salt

Si02/ SiO,/ A1 0, TiOz (2) (c)

Ref,

7.76 7.55 8.04 8.82 7.36 12.74 4.88 6.00 58.34 13.22 62.20 90.00 2000 s 80 25.00 36.00 (el (el (el (el 86(f) 40.00 99(f) 27.00

41 41 41 41 41 41 42 43 43 43 44 44

(a) zeolite composition (molar ratio) before the treatment with the titanium salt. (b) zeolite composition (molar ratio) after the treatment with the titanium salt. (c) (NH4),TiF6 in aqueous solution. (d) TiC1, in the gas phase. (e) not reported. (f) Si02/B203 molar ratio.

86

T h e p r e p a r a t i o n o f t i t a n i u m c o n t a i n i n g ZSM-5 bycontacting a ZSM5 sample with TiC1, in the gas phase at 200 - 500 ' C was described byKraushaarandVanHooff I 4 2 1 a n d l a t e r b y F e r r i n i a n d K o u v e n h o v e n [43]. In the first case the secondary synthesis is performed on a sample of dealuminated ZSM-5 and the substitution presumably takes place on the sites made vacant from the removal of aluminum. In the second case the synthesis is performed on a ZSM-5 not dealuminated and the treatment with TiC14 does not reduce the aluminum content; in this case, if titanium replace some lattice aluminum, extraframework aluminum oxide must be formed in the zeolite crystals. In both the above mentioned works samples treated with TiC1, have an IR absorbance band at 960 cm-’. Ferrini observed the appearance of such a band also on Ti-containing Beta and Y samples prepared by secondary synthesis, We tried the secondary synthesis of a titanium containing silicalite by having as a precursor a sample of MFI structure-type boro-silicate (Bor-C) [44]. The treatment with TiC1, produced a decrease of the boron content and the deposition of titanium on the Bor-C crystals. In the samples s o prepared an IR absorption band at 960 cm-I was detected. Analyzing samples after different times on stream of gaseous TiC14 by XPS spectroscopy, we observed at the beginning the formation of a Ti(2p) signal at the same binding energy as in the case of a TS-1 prepared by direct synthesis. Proceeding the treatment with TiCl,, a second Ti(2p) signal at a different b.e. appeared. The b.e. of this signal was equal to that observed for Ti4+ species in bulk titanium oxide compounds. Fromthese experimentwe c o n c l u d e d t h a t a t t h e b e g i n n i n g of the treatment a part of boron atoms, probably near to the crystals surface, could be substitutedbytitanium, but very soon, because of the steric hindrance of the TiCl,, the reaction of this compound with surface -OH groups gave rise to the formation of extraframework titanium oxide. 5 . The Titanium environment in TS-1 lattice

If a cation M4+ is a substituent for Si,’ in a zeolite framework, it is expected to be bonded to four silicon atoms through bridging oxygens in tetrahedral coordination. In the case of TS-1, doubts arose about the real environment of Ti4+, mainly because of the possible different attributions for the characteristic TS-1 IR peak at 960 cm-l. In the same region of the IR spectrum are located the absorption lines for the following groups: (SiO), Si-OH ; (SiO), Si-OTi ;

(SiO), Ti=O

[15].

An extensive spectroscop’ic characterization of TS-1 was carried out by Boccuti et al. [15]. The authors concluded that the 960 cm-’ IR peak is due to the Si-0 stretching of the polarised Si-Od---Ti (IV) bond. This hypothesis was supported on the ground of the following: a) the electronic transitions of the titanyl group (expected at

87

25000-35000 cm-l) are not present in the U.V.-Vis. reflectance spectrum of TS-1; b) the peak at 960 cm-I does not show any tendency to exchange with 1802 even at high temperature (700 C); c) the peak at 960 cm-I is totally insensitive to the reduction in molecular H2 at 700’C. A strong transition band at 48000 cm-’ observed in the UV-Vis spectrum of TS-1 was attributed to a transition having charge transfer character involving the Ti (IV) sites schematized in the following:

0

\

\

/ Ti / \

/

Si / \ 0 0

0

0

\

\

/

/

/

Si / \

Si / \

H H

H H

H H

00 \ /

00 \ /

00 / \

Ti / \ 0 0

0 0

/

Si / 0 0

\

\

Si / \ 0 0

/

Si / I

Ti / \ 0 0

Other authors, on the ground of results obtained in olefins epoxidation [18] and paraffins oxidation [12] suggested the following situation for the titanium site:

H \

H

fTil

/O

Si /

\

/ 0 0

0 \

/

Si \

/ 0 0

\

In this case the TS-1 IR peak at 960 cm-I should be attributed to the (SiO),Si-OH or to the Ti=O stretching. A recent study on interactions at low temperature (20-60 "C) of H2170, H2180, and D,O with TS-1 adds new information related to this argument [45].The interaction of H2017 with large Silicalite-1 crystals ( s l mm) does not produce variation on 170-MAS-NMR spectra while the interaction of H2017 with small Silicalite-1 crystals ( 0.1 nun) produces the appearance of peaks due to the exchange of -OH group with H2170 on Si-OH defective sites. The same was observed for large crystals and small crystals TS1 with the exception that for both these samples a new signal appeared at 370 ppm chemical shift. The latter was probably due to the interaction of H2170 with titanium sites. The IR band at 960 cm-l was shifted to lower frequencies after exchange with H 2 1 7 0 or with H2180 as expected, but no variation was observed upon exchange with D20. From this, the attribution of the 960 cm-’ IR band to the group (SiO), Si-OH must be excluded. Moreover, the above mentioned results indicate that even at low temperature there is an exchange of water in proximity of the titanium sites. The exchange can be represented by the two schemes:

88

SiO

OSi \

I

/ SiO

\

Ti

+ H20*

Sio O*H \ / / Ti + HO-Si/

OSi

\

SiO

\

-

Sio o*Si \ / Ti + H20 /

-

OSi

SiO

\

OSi

O*H \

Ti=O

+

/

\

H20*

\ Ti=O* + H20 /

/

Ti / \ OH

B y c o n s i d e r i n g a l l t h e p o s s i b i l i t i e s , the following overall scheme

can be drawn to describe all the possible interactions of the Tisites with water: SiO

SiO

OSi

\

HOSi

/

HOSi

\

HOSi Ti-OH + I \ SiO OSi

H20 / /

SiO

OH HOSi

The possible existence of these equilibria even at room temperature, can make very difficult to state about the absence of one or another of the above depicted intermediates. 6 . TS-1 interaction with hydrogen peroxide

By wetting the TS-1 microcystalline powder with a solution of hydrogen peroxide, its color turns from white to bright yellow. The treatment causes also the appearance of a band in the visible spectrum at 425 nm [12]. These experimental results are in agreement with the formation of titanium peroxoderivatives. The interaction of TS-1 with H202 causes the disappearance of the 960 cm-I IR band; by heating the sample this band reappears. This indicatesthat thedecompositionofthe peroxo-derivative restores the original site [12]. According to literature data [46] two different peroxo forms can be thought:

89

HO-Si SiO

SiO

0

1

\

\Ti/ \ SiO 0 HO-Si /

and

OSi /

Ti / ’0-OH SiO HO-Si

In a recent paper we reported the results of our investigation on the peroxo-derivatives of TS-1 (hereafter referred.as TS-1-0,) [45]. TS-1-02 shows an acidity that is much higher than that of TS-1 or Silicalite-1 or Silicalite-1/H202: the first one in fact hydrolizes much more faster the trans-2-3-epoxybutane than the latter three. The acidity of TS-1-0, may come from one of these two situations:

H\ /"i sio O o

sio

0'

/si

o

Si0

SiO

0

SiO

$ + H +

Ti

)l
\

0

*

0-d

-

A Ti-0-OH unit is able to form a five membered cyclic structure with a donor hydroxyl moiety coordinated on Ti as represented in the scheme (11). The cyclic structure can produce a significant increase of both s t a b i l i t y a n d a c i d d i s s o c i a t i o n . We observed also that the acid activity of TS-1-0, is solvent dependent. The hydrolysis rate of trans-2-3-epoxybutanedecreases in the order: CH30H > C2H50H > H20. This behaviouris consistentwith the presence of cyclic complexes like: R *H SiO-Ti / \ 0-O~H sio

H and

SiO-Ti / \ SiO 0-O’H

Although, according to literature data [ 4 6 1 ,groups IV-VI t r a n s i t i o n m e t a l p e r o x o c o m p o u n d s have the structure (I), theabove mentioned experimental results strongly suggest the presence of the hydroperoxo form ( 1 1 ) in the TS-1 peroxo derivative. Another question still open concerns whether the oxidation of organic substrates goes through an homolytic or an heterolytic

90

pathway . The recent discovery of the TS-1 capability to oxidize paraffins and the TS-lactivityin aromatic hydroxilation could indicate the involvement of radical species in the oxidation mechanism. This has been shown to be most likely in the oxidation of paraffins [ 12131. On the other hand, in the epoxidation of olefins higher than C3, no isomerization products has been observed: cis-epoxide is formed only from cis-olefins [7]. The stereoselectivity in this reaction is more consistent with the heterolytic pathway. 7 . Conclusions

The substitution of Ti4+ for Si4+ in zeolitic structure is a certainly new and interesting field of study for researchers involved in zeolite synthesis. Although strong research effort is still required for a full u n d e r s t a n d i n g o f t h e T i s i t u a t i o n inTS-landofthe transformation it undergoes upon interaction with different molecules, it is already possible to state that Ti in TS-1 produces a high performance, high flexible and high stable catalytic site. This three properties allowed to carry out several oxydation reactions with unexpected activity and selectivity. The possibility to prepare Titanium-silicalites containing also lattice trivalent elements beside titanium open new opportunity for the application of these materials in heterogeneous catalysis. There is still anotherwide space for research activities related to titanium containing zeolites: the synthesis of large pores Tizeolites, the activationof TS-lwith oxidants different fromHZOZ, the comprehension of the mechanism of reactions catalyzed by TS1. These are just some of the more actractive perspectives for future research in this field. 8 . References

R.M. Barrer, inD. OlsonandA. Bisio, Proc. of 6th Int. Zeolites Conf., Reno July 10-15 1983, Butterworth Ltd UK, (1984) 870. 2 J.R. Goldsmith, Min. Mag., 29 (1952) 952. 3 R.M. Barrer, J.W. Baynham, F.W. Bultitude, W.M. Meyer, J. Chem. SOC., (1959) 195. 4 D.A. YOUng, US Patent No 3 329 481 (1967). 5 M. Taramasso, G. Perego, B. Notari, US Patent No 4 410 501 (1983) ; M. Taramasso, G. Manara, V. Fattore, B. Notari, US Patent No 4 666 692 (1987). G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo, Stud. in Surf. Sci. and Catal., 28 (1986) 129. U. Romano, A. Esposito, F. Maspero, C. Neri, M.G. Clerici , Stud. in Surf. Sci. and Catal., 55 (1990) 3 3 . P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti, P. Gervasutti, Stud. in Surf. Sci. an Catal., 55 (1990) 43. 9 M.G. Cleric:, G. Bellussi, U. Romano, J. of Catal., in press. 1

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10 A. Thangaraj, R. Kumar, P. Ratnasamy, Applied Catal., 57 (1990) Ll-L3. 11 T. Tatsumi, M. Nakamura, S. Negishi, H. Tominaga, J.Chem. SOC., Chem. Comm., (1990) 476. 12 D.R.C. Huybrechts, L. De Bruycker, P. Jacobs, Nature, 345 (1990) 240. 13 M.G. Clerici, Applied Catalysis, 68 (1991) 249. 14 K.G. Ione, L.A. Vostrikova, M. Mastikhin, J. Mol. Cat. 31 (1985) 355. 15 M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti, G. Petrini, Stud. in Surf. Sci. and Catal., 48 (1989) 133. 16 A.G. Ashton, J. Dwyer, I.S. Elliott, F.R. Fitch, G. Qin, M. Greenwood,J. Speakman, in D. Olson and A. Bisio, Proc. of 6th Int. Zeolites Conf., Reno July 10-15 1983, Butterworth Ltd UK, (1984) 704. 17 A. Tuel, J. Diab, P. Gelin, M. Dufaux, J.F. Dutel, Y. Bee Taarit, J. of Mol. Catal., 63 (1990) 95. 18 B.Notari, Stud. in Surf. Sci. and Catal., 37 (1987) 413. 19 J. El Hage A1 Asswad, J.B. Nagy, Z. Gabelica, E.G. Derouane, inJ.C. JansenandL.MoscouEd.s,Proc. 8thInt. ZeolitesConf., Amsterdam July 10-14 1989, Recent Research Reports, (1989) 475. 20 B. Notari, Proc. of Int. Symp. onChem. of Microporous Crystals, Tokyo June 26-29 1990, Elsevier, in press. 21 G. Bellussi, A . Carati, M.G. Clerici, A . Esposito, Proc. of 5th Symp. on Scient. Bases for the Preparation of Het.Catalysts, Louvain-la-Neuve September 3-6 1990, Preprints (1990) 201. 22 J. Kornatowski, M. Malinowski, J.C. Jansen and L. Moscou Ed.s, Proc. 8th Int. ZeolitesConf., AmsterdamJulylO-14 1989, Recent Research Reports, (1989) 49. 23 X. Ruren, P. Wenquin, Stud. in Surf. Sci. and Catal., 24 (1985) 27. 24 R.Y. Saleh, Eur. Pat. Appl. No 132 550 (1985). 25 M. Padovan, F. Genoni, G. Leofanti, G. Petrini, G. Trezza, A. Zecchina, Proc. of 5th Symp. on Scient. Bases for the Preparation of Het. Catalysts, Louvain-la-Neuve September 36 1990, Preprints (1990) 221. 26 J.M. Pope, J.L. Guth, H. Kessler, Eur. Pat. Appl. No 292 363 (1988). 27 H. Baltes, H. Litterer, E.I. Leupold, Eur. Pat. Appl. No 77 522 (1982). 28 B.M.T. Lok, K.M. Bonita, E.M. Flanigen, US Patent No 4 707 345 (1984). 29 G. Bellussi, A. Giusti, A . Esposito, F. Buonomo,Eur. Pat.App1. No 226 257 (1988). 30 G. Bellussi, M.G. Clerici, A. Giusti, F. Buonomo, Eur. Pat. Appl. No 226 258 (1988). 31 G. Bellussi, M.G. Clerici, A. Carati, A. Esposito, Eur. Pat. Appl. No 266 285 (1988). 32 K.L.S.L. Kee, Eur. Pat. Appl. No 104 107 (1983). 33 J.S. Reddy,R.Kumar,P. Ratnasamy, AppliedCatalysis, 58 (19901 Ll-L4. 34 G. Bellussi, A. Carati, M. G. Clerici, A. Esposito, R. Millini,

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F. Buonomo, Bel. Pat. No 1 001 038 (1989). 35 G.R. Millward, S . Ramdas, J.M. Thomas, M.T. Barlow, J. Chem. SOC. Faraday Trans. 11, 79 (1983) 1075. 36 G.A. Jablonski, L.B. Sand, J.A. Gard, Zeolites, 6 (1986) 396. 37 G. Perego, G. Bellussi, A. Carati, R. Millini, V. Fattore, in M.L. Occelli and H.E. Robson, Zeolite Synthesis, ACS Symposium Series 398, (1989) 360. 38 D.M. Chapman, A.L. Roe, Zeolites, 10 (1990) 730. 39 S.M. Kuzniki, US Patent No 4 853 202 (1989). 40 G.S. Skeels, D.W. Breck, in D. Olson and A. Bisio, Proc. of 6th Int. Zeolites Conf., Reno July 10-15 1983, Butterworth Ltd UK, (19841 87. 41 G.W. Skeels, R. Ramos, D.W. Breck, WO Patent No 85/04854 (1985). 42 B. Kraushaar, J.H.C. VanHooff, Catal. Letters, 1 (1988) 81; ibid. 2 (1990) 43. 43 C. Ferrini, H.W. Kouvenhoven, Stud. in Surf. Sci. and Catal., 55 (1990) 53. 44 A,Carati, S . Contarini, R. Millini, G. Bellussi, ACS Symposium on Synthesis and Properties of New Catalysts, Boston 26 Nov.1 Dec. 1990, Mat. Res. SOC. Ext. Abstract (EA-24), (1990) 47. 45 G. Bellussi, A. Carati, M.G. Clerici, G. Maddinelli, R.Millini, submitted to J. of Catalysis. 46 J.A. Connor, E .A.V. Ebsworth, in H. J. Emeleus and A.G. Sharpe Ed.s "Advances in Inorganic Chemistry and Radiochemistry", Academic Press, New York, Vol. 6 (1964) 279. 47 M.G.Clerici, G.Bellussi, Eur.Pat. Appl. No 315248 (1989).

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

93

ZEOLITE SYNTHESIS WITH METAL CHELATE COMPLEXES K. J. Balkus, Jr.,

S.

Kowalak, K. T. Ly, and D. C. Hargis

Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688, United States

Abstract

X type zeolites have been synthesized in the presence of metallophthalocyanines (MPc, where M = Fe, C o , Ni, Cu) resulting in partial inclusion of these complexes. Synthesis variables such as metal loading, order of mixing, and aging will be discussed.

1. INTRODUCTION

The synthesis of zeolites and molecular sieves often requires a template or directing agent. Generally, these templates are cationic or nuetral organic molecules. The manner in which such templates affect zeolite crystallization is not straightforward. The organic additives may function as a structure-directing agent, gel modifier, buffer, and void filler [l]. Some zeolites such as synthetic faujasite type X can be prepared in the absence of organic additives. In this case the alkali metal ions may function as directing agents. If zeolites and molecular sieves crystallize around cationic and nuetral organic molecules as well as metal ions, then it is reasonable to expect that cationic and nuetral metal complexes might also act as directing agents. Surprisingly, the previous reports of zeolite synthesis in the presence of metal complexes appears limited to a few patents [2-41. We recently reported the preparation of X and A type zeolites [ 5 , 6 ] as well as A1P04-5 [7] in the presence of cationic and nuetral metal chelate complexes. Although, zeolites X and A are easiliy prepared in the absence of a template there is evidence that the metal complexes modify the crystallization. A feature of these syntheses is the partial encapsulation of the metal complexes. There are many examples of organic additives that become occluded in the zeolite during crystallization such as tetrapropylammonium ion in ZSM-5 [ 8 ] .

94

Nuetral templates may not be bound to the zeolite surface but simply trapped. This is the basis of our z e o l i t e s y n t h e s i s method for the preparation of ship-in-a-bottle metal complexes, ie crystallization of the zeolite around a metal chelate complex. Zeolite encapsulated metal complexes have many applications, ranging from shape selective catalysis [9] to magnetic resonance imaging contrast agents [lo]. X type zeolites are well suited for the preparation of encapsulated complexes by virtue of the large supercage (12A in diameter) and the restricted openings (7.4A) to the supercage. Several strategies have been explored for the entrapment of metal chelate complexes in synthetic faujsite type zeolites including the f l e x i b l e 1 i g a n d and t e m p 1 a t e s y n t h e s i s approach. The first method involves reacting a metal exchanged zeolite with a flexible chelate that can diffuse into the zeolite whereupon complexation becomes too large to exit. The tetradentate Schiff base N , N ' (salicyla1dehyde)ethylenediimine or SALEN has been used to prepare metal complexes in X and Y type zeolites [ 11-14]. In the t e m p l a t e s y n t h e s i s method the ligand precursors diffuse into the zeolite where they form the chelate and complex around a metal ion template. For example intrazeolite metallophthalocyanines (figure 1)have been prepared by the

Metallophthalocyanine (MPc)

condensation of four dicyanobenzene molecules inside a metal ion exchanged zeolite [15-241. The resulting complex is much too large to escape through the zeolite pores. In comparison to the z e o l i t e s y n t h e s i s approach there are many disadvantages associated with the preparation of intrazeolite complexes by the f l e x i b l e l i g a n d and t e m p l a t e synthesis methods. The complexes are difficult to characterize, especially if the ligand has multiple coordination modes available and some of the target metal ions may remain uncomplexed which will complicate any reactivity studies. Additionally, there are limitations to the types of metal complexes that might be encapsulated in a zeolite. The only criteria for incorporating metal complexes

95

during zeolite crystallization is that the complex must be stable at high pH and moderate temperatures. In addition to the variety of complexes that might be encapsulated during crystallization there are now many zeolites and molecular sieves that might be modified with metal complexes. For example we reported the crystallization of A type zeolites around copper (11) phthalocyanines [5]. The 4.1A apertures to the large cavity in A type zeolites are too small for dicyanobenzene to enter which precludes the template synthesis method for preparing the intrazeolite MPc. The potential exists for encapsulating metal chelates in a whole host of different zeolite and molecular sieve structures. In this paper we report the results for crystallization of X type zeolites in the presence of MPc complexes where M = Fe, Co, Nil and Cu (MnPc decomposed under synthesis conditions). The crystallization is affected by both the amount of complex added and the order of mixing. Additionally, the amount of complex encapsulated depends on the metal ion. 2.

EXPERIMENTAL

Silica gel and aluminum isopropoxide were purchased from Aldrich. Metallophthalocyanines were obtained from Strem Chemical and were used without further purification. Freshly prepared silicate and aluminate solutions were combined in the ratio 1 A 1 2 0 3 : 3.2Si02: 4Na20: 155H20 to produce an X type zeolite. The metal complexes were added in various amounts to either the silicate solution, aluminate solution or the initial gel mixtures combined in a ratio close to the X type recipe. The mixtures were crystallized in polypropylene bottles at 90 C. The resulting zeolites were washed with copious amounts of distilled water then Soxhlet extracted with pyridine and dried at 1OOC. Surface adsorbed metal complexes were removed by vacuum sublimation at 450-5OOC. FT-IR spectra were recorded as KBr pellets on a Nicolet 5DX spectrophotometer. X-ray powder patterns were obtained with a Scintag XDS 2000 diffractometer. Elemental analyses were performed by Galbraith Laboratories, Knoxville. 3. RESULTS

and DISCUSSION

The results for the synthesis of an X type zeolite in the presence of FePc, CoPc, NiPc, and CuPc are shown in table 1. In all cases here the metal complex or metal complex solution was added to the aluminosilicate gel immediately after mixing the silicate and aluminate solutions. The mixture was magnetically stirred for 15 minutes before heating. The resulting crystals were washed with water extracted with pyridine and sublimed. The surface MPc complexes can not be removed by solvent extraction. However, vacuum sublimation appears to be completely effective for removing non-intrazeolite complexes. The product zeolites were various shades of blue but became a very pale blue-green after

96

sublimation. The characteristic electronic spectra for phthalocyanines have been used previously to characterize the entrapped complexes. However, in our samples the level of encapsulation varies from -5-25% or between 1 MPc in 20 unit cells to 1 in 4 . At these l o w loadings interpretable spectra were not obtained. Except for MnPc the intact complexes were recovered from extractions and sublimations which provides additional evidence for stability during synthesis and purification. A highly crystalline X type zeolite can easily be prepared in the absence of organic additives in less than 4 hours. Slight variations in this recipe such as the amount of water does not have a dramatic effect on the synthesis. The addition of small amounts of metallophthalocyanine (-2% by weight relative to the silica gel) nearly doubles the crystallization time. Generally, phthalocyanines are Table 1 Results for X crystallization with MPc added to initial gel Si02:Na20:H20:MPca

Hrs

3.2: 4: 155 3.1: 3.7: 141:O. 007 3.0:3.6:138:0.007 3.1:3.7:141:0.005 3.0:3.5:138:0.007 3.1:3.7:141:0.006 3.0:3.6:139:0.007 3.1:3.7:141:0.005 3.0:3.5:138:0.009

10 10 10 10 10 10 10 8

4

T (C) Zeoliteb %M 90 90 90 90 90 90 90 90 90

-0.034 <0.014 0.022 0.051 0.042 < O . 014 0.020 0.035

Complex Only FePc, H20 COPC, H20 NiPc, H20 CUPC, H20 FePc, Py CoPc, Py NiPc, Py CUPC, Py H20

a. per mol A1203 b. determined by XRD and FT-IR insoluble in water but the gels into which the solid MPc complexes have been added appear homogeneous. Prior dissolution of the MPc complex in pyridine does not alter the appearance of the initial gels. The addition of pyridine (-lmL per lOmg MPc) does not appear to affect the crystallization. However, after several hours there is a phase separation with a large percentage of the metal complex located in the top pyridine layer. There is no clear trend in the amount of metal complex encapsulated relative to the presence of pyridine. The CuPc samples produced the highest level of complex encapsulation in the water only case. However, in the presence of pyridine the FePc samples achieved the highest loading. In a l l cases the CoPc samples contained the lowest amount of intrazeolite complexes. This may be the result of dimerized CoPc species which would be too large for entrapment.

97

The low loading of metal complexes in the synthesized zeolites may be considered a positive result from a reactivity viewpoint. If all the supercages were filled with MPc complexes one might expect diffusional problems as well as a number of inaccessible active sites. However, we were interested in the effect of metal concentration during zeolite crystallization. Table 2 lists the results for several zeolite syntheses using higher MPc concentrations. In the case of FePc increasing the complex concentration more than 3 times that of the samples in table 1 results in a poorly crystalline mixture of X and P type zeolites. The P type zeolite consists of a small pore 8 ring system [25] which is a phase that grows in if the X crystallization time is too long. Even by altering the time we were not able to obtain a pure X phase with this concentration of FePc and the presence of pyridine did not change these results. In the case of NiPc a mixture of X and A type zeolites was obtained. Table 2 Effect of MPc concentration on crystallization Si02:Na20:H20:MPca 3.1:3.7:141:0.023 3.1:3.7:141:0.026 3.1:3.8:141:0.024 3.1:3.8:141:0.023 3.1:3.7: 141:O. 048 3.1:3.7:177:0.022 3.0:3.7:191:0.048 3.0:3.7:177:0.022 3.0:3.7:177:0.022

Zeoliteb

-

Complex FePc, . FePc, NiPc, NiPc, NiPc, NiPc, NiPc, COPC, COPC,

H90 Py H20 Py Py H20 H20

H20 Py

a. per A1203 b. determined by XRD and FT-IR c. poor crystallinity The presence of zeolite A is somewhat surprising since the A type synthesis generally requires much less water and a different pH. Since the A phase was not oberved with the lower concentrations or no MPc, this must be some gel modifying effect. If pyridine is added then a pure X phase 1 s observed with a higher level of inclusion (0.057% Ni) than the with the lower concentration (0.020% Ni) Encouraged by these results the initial concentration of NiPc was increased further but a mixture of X I P and A phases was obtained. Likewise increasing the amount of water then increasing the amount of water and MPc concentration both produce mixtures of X I P and A zeolites. The results for CoPc were similar to FePc except the A phase was observed in the sample containing pyridine. Clearly this series of MPc complexes affects the crystallization differently. Increasing the initial gel

.

98

concentration of complex also modifys the synthesis. Further study of this reaction may provide the conditions that would allow one to prepare two different pure phases simply by adjusting the concentration of metal complex. We have also investigated the effects of aging and the mixing procedure on the crystallization. The addition of the metallophthalocyanine to the aluminate or aluminosilicate gel does not appear to affect the crystallization. However, if the MPc complex is added to the silicate solution then mixed with the aluminate solution then a much higher amount of MPc becomes encapsulated into the zeolite. For example, using the same recipe for NiPc as in table 1 except for the order of mixing results in 0.084% Ni versus 0.022% Ni. We attempted to further enhance the level of encapsulation by stirring the mixtures at room temperature for 24 hours prior to heating. However, aging in this fashion does not have an effect on the crystallization regardless of the order of mixing. We are continuing our study of these synthesis variables. 4. CONCLUSIONS

The crystallization of zeolites and molecular sieves with metal complexes represents a fresh strategy for the synthesis of these materials as well as a novel method for encapsulation of metal chelate complexes. We have shown that several first row tranistion metal phthalocyanines complexes can be encapsulated in X and A type zeolites by synthesizing the zeolite around the metal complex. Preliminary results indicate the concentration and type of phthalocyanine complex modify the crystallization of X type zeolites. The extension of this method to other metal chelate complexes and molecular sieves is currently under investigation. 5. ACKNOWLEDGEMENTS

We thank the National Science Foundation (Grant No. the R. A. Welch Foundation and the Petroleum Research Fund administered by the ACS for their support of this work. CHE-9016705),

6. REFERENCES

R. Szostak, Molecular Sieves, Van Nostrand Reinhold, New York, 1989. 2 B. P. Pelrine, U. S. Patent No. 4 259 306, (1981). 3 L. A. Rankel and E. W. Valysocsik, U. S. Patent No. 1

4 388 285, (1983). 4

L. A. Rankel and E. W. Valysocsik, U. S. Patent No.

5

K. J. Balkus, Jr., S. Kowalak and K. T. Ly, J. Am. Chem. SOC., Submitted.

4 500 503, (1985).

99

6

7 8 9 10

11 12 13 14

S. Kowalak, K. T. Ly, D. C. Hargis and K. J. Balkus, Jr., in Synthesis and Properties of New Catalysts: Utilization of Novel Materials Components and Synthetic Techniques, E. W. Corcoran, Jr. and M. J. Ledoux (eds.), Materials Research Society, Pittsburgh, 1990. K. J. Balkus, Jr. and S. Kowalak, J. Am. Chem. SOC., Submitted. G. D. Price, J. J. Pluth, J. V. Smith, J. M. Bennett, R. L. Patton, J. Am. Chem. SOC. 104 (1982) 5971. For example N. Herron, G. D. Stucky and C. A. Tolman, J.C.S, Chem. Comm. (1986) 1521. K. J. Balkus, Jr. and S. Young, Manuscript in Preparation. N. Herron, Inorg. Chem. 25 (1986) 4741. K. J. Balkus,Jr., A. A. Welch, and B. E. Gnade, Zeolites 10 (1990) 722.. C. Bowers and P. K. Dutta, J. Catal. 122 (1990) 271.

17

K.J. Balkus,Jr. and S. Kowalak J.C.S., Chem. Comm., In Press. A. D. Gabrielov, A. N. Zakharov, B. V. Romanovsky, 0. P. Tkachenko, E. S. Shpiro, and Kh. M. Minachev, Koord. Khim. 14 (1988) 821. A. N. Zakharov, B. V. Romanovsky, D. Luca,and V. I. Sokolov, Metalloorg. Khim. 1 (1988) 119. T. Kimura, A. Fukuokaand M. Ichikawa, Shokubai 30

18

A. N. Zakharov and B. V. Romanovsky, J. Inclus. Phenom.

19 20

B. V. Romanovsky, Acta Phys. Chem. 31 (1985) 215. G. Meyer, D. Worhle, M. Moh1,and G. Schulz-Ekloff, Zeolites 4 (1984) 30. H. Diegruber, P. J. Plath, and G. Schultz-Ekloff, J. Mol. Catal. 24 (1984) 115. N. Herron, J. Coord. Chem. 19 (1988) 25. A. N. Zakharov, A. G. Gabrielov, B. V. Romanovsky, and V.I. Sokolov, Vestn. Mosk. Univ., Ser. 2: Khim 30 (1989)

15 16

(1988) 444. 3 (1985) 389.

21 22 23

234. 24 25

K. J. Balkus,Jr., A. A . Welch and B. E. Gnade, J. Inclus. Phenom., In Press. W. M. Meier and D. H. Olson, Atlas of Zeolite Structure Types, Butterworths, London, 1988.

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P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

101

SYNTHESIS OF FERROUS CYANIDE COMPLEXES INSIDE ZEOLITE Y I. Bresinska’ and R.

S.

Dragob

a A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznaii, POLAND b University of Florida, Department of Chemistry, Gainesville, FL 32611, USA

Abstract

We report the preparation of ferrous-cyanide complexes in the zeolite framework. The treatment of Fe(I1)-Y zeolite with a methanolic solution of cyanide ions results in the formation of two anionic species distributed into the cavities. The preliminary results of thermal stability and characterization of genarated complexes are reported. 1. INTRODUCTION

In recent years a new method of zeolite modification consisted of the anchoring or immobilization of transition metal complexes in the zeolite cages continues to attract scientific interest. Depending on whether or not lattice oxygens are in the first coordination sphere, the zeolite can be considered as an anionic ligand or as an anionic solvent. The previously reported transition metal complexes synthesized inside a zeolite framework have been either cationic or neutral [l-71. We recently reported the preparation and characterization of an anionic cobalt(I1) cyanide complex inside zeolite Y [ 8 ,91. The Co (11)-Y zeolite reacted with cyanide solutions to form entrapped cobalt cyanide species. This reaction is unique since it requires a negatively charged ligand to enter into a negatively charged framework and form an anionic complex. To our knowledge it represented the first case in which a negative ion has been synthesized inside a zeolite, opening the possibility for preparation of a whole new class of trapped anionic complexes. The positive results with the Co-Y zeolites encouraged us to study the encapsulation of other transition metal cyanide complexes in zeolites. We wish to report here preliminary results for the encapsulation of Fe(I1) cyanide complexes in zeolite Y.

102 2 . EXPERIMENTAL

The synthesis procedure of ferrous-cyanide complexes in the zeolite Y was adopted from earlier studies [ 8 , 9 ] . Generally, the method involes the treatment of a ferrous ion exchanged NaY zeolite with methanolic solutions of cyanide. Linde LZY-52 Nay zeolite was used as the starting material. It was slurried in 0.25 M NaCl solution, washed until no C1- was detected in the filtrate and dried at lOOC under a vacuum prior to use. Unless otherwise stated, all reactions were carried out with precautions to exclude air under argon atmosphere using Schlenk techniques. All solvents were deoxygenated by bubbling argon. Fe (11)-Y samples were prepared by exchanging Na+ for Fez+ ions in an aqueous solution of Fe(NH4) z ( S O . 4 ) 2~6H2O at room temperature. After exchange, the solids were washed with water until no Sod2- was present in the filtrate. The resulting light green solids were rinsed with methanol. The next step consist of treating the ferrous zeolite with a methanolic solution of sodium cyanide at room temperature for 2 days. Fe (CN)-Y ( 1), Fe (CN)-Y (2) and Fe (CN)-Y ( 3 ) were prepared by reacting the Fe-Y zeolites with various amounts of cyanide ions corresponding to lO:l, 4:1, and 1:l of CN/Fe molar ratio based on the iron amount used for the exchange Fe (CN)-Y (4), Fe (CN)-Y ( 5) and procedure , respectively Fe (CN)-Y (6) were prepared in the same manner except without precautions to exclude air. In these cases the Fe-Y zeolites were dried under a vacuum at 200C prior to treatment with cyanide ions. Chelate and NaCl treatment was carried out by strirring the samples with aqueous 0.05M Na4EDTA or 0.1 M NaCl at room temperature. The solids were washed with water and dried at 80C under vacuum. Elemental analysis for nitrogen, carbon and hydrogen was carried out by the microanalysis laboratory of the University of Florida. Iron and sodium were determined by the atomic adsorption method of the zeolites dissolved in strong acids. Samples of Fe (CN)-Y (1) , Fe (CN)-Y (2) and Fe (CN)-Y (4) as well as K4Fe(CN) 6 and Fez[Fe(CN) 6 1 were treated with 10% solutions of H 2 0 2 at room temperature. The solids excepte for K4Fe(CN)s were filtered, washed with water and dried at 1OOC. The K4Fe(CN)6 was recovered from solution by precipitation with acetone. Samples of the Fe(CN)-Y(1) , Fe(CN)-Y(2) and Fe(CN)-Y(3) zeolites were heated under a vacuum at 100, 150, 200, 250, 300, 350, and 400C then analyzed by IR spectroscopy. The samples for IR measurements were prepared in a glove box as nujol mulls. The IR spectra were obtained with a Nicolet 5DXB FT-IR spectrophotometer.

.

3 . RESULTS and DISCUSSION

For NaY zeolite stirred with cyanide or Fe(CN) 6 4 solutions no bands for the CN stretching vibration in the IR

103

spectrum can be observed. However, after treatment of the Fe-Y with either CN- or Fe(CN) 6 4 - ions the IR spectra indicate the presence of cyanide species. The Fe, [ Fe (CN)6 ]-Y zeolite is blue and displays one band in the v c N region at 2080 cm-I similar to results obtained by Scherzer [4]. The same band is observed for the solid Fe2[Fe(CN) 6 1 Complex synthesized from Fe(NH4) (Sod) and Na4[Fe(CN) 6 1 solutions. Figure 1 shows the IR spectra of Fe-Y zeolites after treatment with the various amounts of cyanide A (lO:l, 4:1, 1:l of CN/Fe molar ratio). The resulting solids display green-olive, green-blue and blue color B respectively. The IR spectrum for each sample consists of combinations of two bands in the VCN region, at -2115 and 2050 C cm-l, that indicates the formation of new type species different form those described above. When the synthesis is repeated using Fe-Y zeolites dried under a vacuum at 200C prior to treatment with 2400 cyanide, the band at -2115 t cm-l is more pronounced 2115 2052 especially for the Fe (CN)-Y (4) sample that was 2100 1900 prepared at the highest CN/Fe ratio of 1O:l. W A V E N U M BE R S [cm - '1 The presence of two bands in the IR spectra Figure 1 IR spectra of Fe(CN)-Y suggest that at least two types of species are zeolites. ( A ) Fe (CN)-Y ( 1), ( B ) Fe(CN)-Y(2), (C) Fe(CN)-Y(3).generated. The major band at -2050 cm-l is consistent with literature data concerning ferrous-cyanide complexes [lo-141 containing 2 to 6 cyanides per Fe ion. As follows from figure 1 the major band is accompanied by the band at 2115 cm-l. It could suggest the presence of Fe(II1)-cyanide species that according to the literature [lo-121 absorbs in the IR above 2100 cm-l. However, we were not able to detect any traces of Fe3+ either in the filtrate after the exchange procedure or in the FeY zeolite (SCNtest). Even when the synthesis is carried out in an air atmosphere, no traces of Fe3+ are detectable, due to the high stability of Fe(NH4) z(SO4) employed as the source of Fe(I1) ions. Therefore, it is reasonable to attribute the second band at -2115 cm-I to the presence of other Fe(CN), species but in different environments. Moreover this species seems to

i,

104

be a precursor to the species characterized by the IR band at -2050 cm-’. A s the concentration of cyanide employed in the syntheses increases from 4 to 10 of CN/Fe molar ratio, the intensity of this band increases while the band at 2115 cm-’ Similar behavior was previously decreases (figure 1) observed for Co(I1) cyanide complexes in NaY [8,9] and for cyanide-silver or copper- complexes [lo]. In these complexes species containing less cyanides per tranistion metal ion were characterized by the IR bands at the higher wavenumbers. Therefore, it is reasonable to assume in the case of Fe(CN)-Y that the band at -2115 cm-I arises from the presence of species containing smaller amounts of cyanide per Fe compared with those represented by the band at -2050 cm-l. In an attempt to determine the complex compositions, the Fe(CN)-Y zeolites were treated with aqueous EDTA or NaCl solutions in order to extract uncomplexed iron. Chemical analysis results for selected samples are listed in table 1. Since charge balance inside the framework

.

Table 1 Chemical Analysis of Fe (CN)-Y Material Fe (CN)-Y (1) Fe(CN)-Y(1)EDTA Fe(CN)-Y(3) Fe(CN)-Y(3)EDTA

Fe content wt% per unit cell 2.09 1.94

2.49 2.24

5.7 5.3 6.8 6.1

N content wt% per unit cell

N/Fe

Na+/Fe

2.53 1.93 2.07

27.5 21.2 22.5

4.83 4.00 3.31

4.53

1.70

18.5

3.04

4.16 1.84 1.53

must be maintained after formation of anionic species it should be expected to find the additional amount of sodium cations. Indeed, the elemental analyses in table 1 show that Na+ ions are incorporated. It should be noted that these numbers do not reflect the accurate single complex compositions. It only shows the overall N or Na+ concentration per Fez+ present in the zeolite. However, these results do allow us to draw some conclusions as to the complex composition. The N/Fe ratio for the sample prepared under high CN/Fe ratio is close to the value of 5. For samples prepared under lower CN/Fe ratios that show the pronounced IR band at -2115 cm-I the N/Fe and Na/Fe ratios are lower and do not exceed the value of 4. These facts suggest that the species responsible for the IR band at 2050 cm is a higher coordinated one. This is further suppported by elemental analysis of samples treated with EDTA as well as the corresponding IR spectra. We expected that the treatment of Fe (CN)-Y zeolites with a chelating agent would remove the uncomplexed ferrous ions and the lower coordinated complexes similar to previoulsy reported Co (CN)-Y zeolites [ 91 Such treatment increased the

.

105

N/Fe ratio and eliminated the IR bands at higher wavenumbers. However, when Fe (CN)-Y samples were given the same treatment only a small amount of iron is removed with a decrease in the N/Fe and Na/Fe ratios (table 1). The IR spectra for chelate treated samples still consist of the two bands that were observed for the original samples but now vary in intensity. The absorbance ratios of the IR bands at 2115 and 2050 cm-I increase as shown in table 2. These facts suggest that the higher coordinated species is removed form the zeolite by EDTA treatment. To resolve the problem of whether or not effect is due to the chelating ability of EDTA or water solubility, the Fe(CN)-Y(1) and Fe(CN)-Y(2) zeolites were stirred with a NaCl solution. In each case the absorbance ratio of IR bands at 2115 and 2050 cm-’ increases several times whereas the C and N content decreases (table 2). From these results it appears that a decrease in the intensity of the IR band at 2050 cm-’ can account for the solubility of higher coordinated species in Table 2 Influence of EDTA and NaCl treatment on Fe(CN)-Y samples Material Fe (CN)-Y (1) (1)EDTA (1)NaCl Fe (CN)-Y (2) (2)EDTA (2)NaCl

Synthesis CN/Fe

Content wt % H N

C

10

2.43 1.91

4

2.35 1.72 1.69

2.00

1.27 1.26 1.32 1.32 1.44 1.40

2.53 1.95 1.94 2.22 1.66 1.64

Absorbance ratio A 2 1 1 5/A2 0 5 0 0.108 0.207 0.349 1.090 2.280 3.590

water. However, we can not rule out the possibility that the species associated with the band at 2115 cm-I is soluble in water as well. This complex may not be removed by washing because of the different intrazeolite location. If this complex is formed in the i3 cages then it may be trapped inside because of the 2.2A apertures. The complex Na4Fe(CN)6, which has a reported IR band at 2050 cm-I [lo-121 might be one of the higher coordinated species. In further attempts to better define the nature of the intrazeolite complexes, samples of Fe(CN)-Y(l), Fe(CN)-Y(2) and K4[Fe(CN) 6 3 as a reference were treated with a hydrogen peroxide solution. The first two samples after reaction with H 2 0 2 display a deep blue color and the filitrate after separation of solids contains Fe(II1). The IR spectra consists of a band at -2080 and 1945 cm-’. The Kd[Fe(CN) 6 ] and Fe2[Fe(CN) 6 1 samples remain unreacted after eroxide treatment. The presence of Fey+ in the solution and the IR band at 1945 cm-I originates from NO coordinated to the iron-cyanide moiety [lo-121 which results from CN oxidation. This clearly demonstrates that zeolites containing ferrous complexes

106

undergo decomposition and oxidation. The blue color of the samples and the IR band at 2080 cm-’ can account for the formation of a double iron salt precipitated in the zeolite similar to the results obtained by Sherzer [4]. On the basis of these results the presence of a hexacyano complex in the zeolite can be ruled out. The zeolite Y containing ferrous cyanide species were gradually heated under a vacuum up to 400C and the IR spectra recorded. Heating the Fe (CN)-Y ( 1) sample up to 200c (figure 2 86 2, spectrum A) does not I 1 IR band affect the 2400 2100 1900 positions, except for the shift of the major band WAVENUMBERS [crn-l] from 2044 to 2056 cm-l. Figure 2 IR spectra of Fe(CN)-Y(l) However, when the sample is zeolite heated under a vacuum. exposed to moist air and ( A ) 100 C, (B) 300 C and (C) 400 C. the zeolite rehydrated this band shifts back to 2044 cm-l. Above 200c decomposition is observed (figure 2, spectrum B) and a new band develops at 2186 cm -I. As the temperature is increased the new band at 2186 cm-l continues to grow in as the bands at 2117 and -2050 cm-I decrease in intensity. Finally, at 350C the IR band at 2117 cm-’ is completely gone. The brown zeolite obtained after heating at 400C under a vacuum displays two bands (figure 2, spectrum C) at 2186 2052 2056 and 2186 cm-l. L Elemental analysis of this 2400 2100 1900 sample indicates both carbon and nitrogen are WAVENUMBERS [crn.'] present but compared with Figure 3 IR spectra of Fe(CN)-Y(2) the sample before heating the concentrations are zeolite heated under a vacuum. halved. ( A ) 250 C, (B) 400 C and (C) after treatment with CN-. Figures 3 and 4 show the

t

1

107

IR spectra for samples Fe (CN)-Y (2) and Fe (CN)-Y (3) heated under a vacuum. These zeolites also show evidence of decomposition above 200C. The IR spectra for these samples are silimilar to that observed during decomposition of Fe(CN)-Y(l) and are shown in figure 3 (spectrum A) and figure 4 (spectrum A ) . The zeolite obtained at the final temperature of 400C displays two IR bands at -2050 and 2186 cm-I (figures 3 and 4, spectrum B). As follows from spectrum C in figure 3 the treatment of this residue with cyanide ions does not restore the original spectrum. For sample Fe (CN)-Y (3) the same changes were observed. However, one additional product, characterized by the IR band at 2230 cm-I can be observed (figure 4 , spectrum B). According to the literature the thermal degradation of cyanoferrates is gradual and begins between 250 and 300C [13-171. It has been reported that MFe [ Fe (CN)6 3 , Fe2 [Fe(CN) 6 ] , and KCN can be detected during decomposition of hexacyanoferrates. The IR spectra for such compounds should display a V C N -2090 cm-l. For our zeolite samples we do not observe any bands in this region. 4 2052 Our decomposition products are characterized mainly by a band at 2186 cm-I with 2400 2100 190° shoulders at -2176 and 2230 WAVENUMBERS [cm-'] cm-l. These bands probably can be assigned to cyanate Figure 4 IR spectra of Pe(CN)-Y(3) species. The band at 2230 zeolite heated under a vacuum. cm-’ is similar to that ( A ) 250 C and (B) 350 C. reported for NaOCN [lo, l11 while the band at 2186 cmmay be an iron-cyanate r l o.l .as comDlex. Nakamoto . reported the IR spectrum for Fe(CN0)i2- which has a band at 2182 cm-l. The most probable source of oxygen seems to zeolitic water that is usually released above 200C. At this temperature we observed a decrease in the band at 2117 and the formation of decomposition products. It should be noted that not all of the intrazeolite cyanide undergoes oxidation to CNO-. Elemental analysis of the zeolites after heating at 400C shows a reduced amount of carbon and nitrogen by as much as a factor of 2-3 compared with the unheated samples. This indicates that part of the C and N is lost as volatile compounds during heating under vacuum. The cyanates, because of poor solubility in methanol are still present even after treatment with cyanide solutions. The IR spectra f o r these samples display bands for both CN-

-

108

and CNO- ions ( figure 3, spectrum C). Further characterization of the intrazeolite complexes and species after thermal as well as chemical treatment is currently in progress. 4. CONCLUSIONS

Cyanide ions react with green Fe (11)-Y zeolites resulting in the formation of two distinct anionic species. It is speculated that these complexes are also coordinated to the framework through a lattice oxygen and distributed within the cavities. The preparation of intrazeolite Fe (11) cyanide complexes does not depend on the exclusion of air. We believe that a water soluble, thermally stable species characterized by the IR band at 2050 cm-l is located in the supercage. This complex is also hydrated and is proposed to contain a CN/Fe ratio between 4 and 5. The other intrazeolite complex is characterized by an IR band at 2115 cm-l. This complex is thermally unstable and reacts with water above 200C to form cyanates. It is proposed that this complex resides in the sodalite cages and has a CN/Fe ratio between 3 and 4. These preliminary results for the zeolite encapsulation of anionic metal complexes in NaY are quite promising and we are currently extending this work to other systems. 5. REFERENCES

1 R.F.Howe and J.H.Lunsford, J. Phys. Chem., 79 (1975) 1836. 2 W.J.Mortier and R.A.Schoonheydt, Prog. Solid State Chem. 16 (1985) 99. 3 B.V.Romanovsky, Acta Phys. Chem., 31 (1985) 215. 4 J. Scherzer and D.Ford, J. Catal., 71 (1981) 111. 5 J.H.Lunsford, Molecular Sieves-11, ACS Symp. Ser., 40 (1977) 473. 6 J.H.Lunsford, Rev. Inorg. Chem., 9 (1987) 1. 7 K. Klier, Langmuir, 4 (1988) 136. 8 R.S.Drago, I.Bresinska, J.E.George, X.J.Balkus,Jr. and R.J.Taylor, J. Am. Chem. SOC., 110 (1988) 304. 9 R.J.Taylor, R.S.Drago and J.E.George, J. Am. Chem. SOC., 111 (1989) 6610. 10 K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Interscience, 1970. 11 R.A.Nuquist and R.O.Kage1, Infrared Spectra of Inorganic Compounds, Academic Press, 1971. 12 A.G.Sharpe, The Chemistry of Cyano Complexes of the Transition Metals, Academic Press, 1971. 13 J. Burgess and R.F. Haines, J. C. S . , Dalton Trans., (1978) 1447. 14 K.J.Brewer, Inorg. Chem., 6 (1987) 3376. 15 A.Horvath, B.Mohai and F.Miko, J. Therm. Anal., 32 (1987) 927. 16 D.DeMarco, A.Marchese, P.Migliardo and A.Bellomo, J. Therm. Anal., 32 (1987) 927. 17 J.J.Kunrath, C.S.Muller and E.Frank, J. Therm. Anal., 14 (1978) 253.

109

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

Genesis of Gallosilicates with ZSM-5 Structure. Insertion of Ga and Zeolitic Properties at Various Steps of Crystallization H. Kosslick, M. Richter, Szulzewsky and R. Fricke

Vu

Anh

a Tuan ,

B.

Parlitz,

K.

Institute of Physical Chemistry, 0-1199 Berlin, Germany a National Centre for Scientific Research of Vietnam, Hanoi, Vietnam Abstract The crystallization of Ga-ZSM 5 (Si/Ga = 50) in dependence on the synthesis time is examined using XRD, IR spectroscopy, TG/I)TA and ammonia TPD. During the rapid crystal growth only a

part of the gallium present in the sample is incorporated into the framework. During the following recrystallization process the crystal lattice becomes partially reconstructed where the residual gallium is being fully inserted into framework positions. The catalytic properties are characterized by the conversion of ethylbenzene and the m-xylene isomerization. 1. INTRODUCTION

The modification of

acidic properties of zeolite catalysts

through the isomorphous substitution of trivalent species into tetrahedral framework positions has generated great interest recently 11-31. Gallium analogues of ZSM-5, in particular, have act as acid catalysts f o r several reactions 14-bJ. However, the knowledge about the temporal course or the crystallization process is still incomplete. The present paper reports on the influence of the crystallization time on the synthesis of gallosilicates (Ga-ZSM 5) and on their catalytic

been

shown

properties.

to

110

2. EXPERIMENTAL

Gallosilicate samples (Si/Ga ratio = 5 0 ) with different crystallization times ( 5 - 9 6 hrs.) were synthesized using in each case a starting gel composed of 15 NaZO * 1 Ga203 * 100 S i O z * 13 TPABr * 4400 H 0 . The template-assisted hydrothermal 2 crystallization was carried out in a teflon-coated autoclave under autogeneous pressure at 4 4 3 K . The products were filtered, washed and after N H 4 + ionic exchange activated at 823 K for 2 h r s . The characterization of the samples is based on XRD, IR, DTA/TG, NH3-TPD and chemical analysis. Note that the term crystallinity is used in two ways. During the period of the crystal growth it mainly refers to the zeolite content whereas during the recrystallization period it corresponds to the ordering of the crystal lattice. Catalytic properties were examined in a fixed bed continuous flow microreactor at atmospheric pressure with H2 resp. N2 as carrier gases (flow rate 10 1 h-l) applying the conversion of ethylbenzene and the isomerization of m-xylene as probe reactions on one gram of the binder-free zeolite (0.35 - 1.0 mm). The shape selectivity was tested as described elsewhere L71.

3. RESULTS AND DISCUSSION 3. 1. Crystallization of Ga-ZSM 5 and incorporation of Ga into

the framework The crystallinity and phase purity were examined by XRD. The XRD patterns of all samples resemble those of pentasil-type zeolites with respect to position and intensity of reflexes. No other crystalline by-products could be detected within the experimental accuracy of the method. The narrow peaks and their high intensity reflect the good crystallinity of the samples. ’rhe chemical analysis reveals that the amount ot gallium present is the same as in the starting synthesis mixture. A s not all silica is incorporated into the zeolite the Si/Ga ratio of the samples is finally smaller than that of the recipe. The insertion of gallium has been characterized by the

111

following methods. Thermoanalysis. A large high temperature shift of the peak characterizing the template decomposition/oxidation is observed in comparison to silicalite (Fig. 1). Since the peak maximum position is known to be dependent on the lattice charge [81 the observed shift indicates the incorporation of gallium into the tramework of peak

and

the zeolite. The low width-to-high ratio of

the

appearance of

high-temperature

side

is

only

a

weak

shoulder

characteristically

for

on

the the

highly

crystalline products. The shoulder (773 - 873 K ) originates from the exothermal oxidation of coke which is formed from decomposition products of the template. This shoulder sensitive to acidity as well as crystal perfection.

is

Temperature I K

5W

700

900

;:i Silitoliie

II 200

400

600

Temperature I ’ C

Figure 1. Characterization of

Ga-ZSM-5

.

I I

1200 1000 800 600 Wave nurnber/cm-’

the template decomposition for

silicalite and Ga-ZSM 5 by DTA (5 K min-l, air). Figure 2. IR spectrum of silicalite and Ga-ZSM 5. Infrared spectroscopy. The IR spectra of Ga-ZSM 5 (Fig. 2) also provide evidence for the incorporation of gallium into the zeolite framework during crystal growth. The asymmetric T-0-T stretching vibration band is shifted to lower wave numbers by

112

about -20 But, also the reduced intensity ratio E of the double ring (D5R) and deformation vibration band at 550 cm-l -1 and 450 cm , resp., indicates the gallium incorporation. This

E ratio decreases from 0.7 to 0 . 6 due to the greater disorder of the lattice. Additionally, the shape of structure-sensitive vibration bands is affected. In comparison to silicalite the band assigned to the D5R vibration is broadened. The shoulders arising at 620 and 585 cm-l in the silicalite spectrum disappear in the case of Ga-ZSM 5. The same is observed for the vibration band at 1220 cm-l which was found to be very sensitive to the crystallinity of ZSM-5 zeolites. lhermodesorption of ammonia. The NH3 T P D spectrum exhibits a high temperature peak (htp) (Fig. 3 ) which evidences the location of gallium in the framework. The assignment of the htp 3+ to ammonia desorbed from Si-0 (H)-Me bridges is generally accepted. On the basis of the peak area it is possible to estimate semi-quantitatively the concentration of acid sites assuming that one ammonia molecule is located at each acid site. Two points are worth mentioning. Despite the low crystallinity after 10 h crystallization time (see Fig. 4 ) the concentration of strong acid sites is comparatively high. On the other side, a significant increase of strong acid sites is

I

Go-ZSM-S(Si/Ga =50)

400

600

800

Temperature iK

Figure 3 . Ammonia TPD curves of Ga-ZSM 5 zeolites. Influence of the crystallization time at a fixed Ga content.

113

revealed between 48 and 96 h crystallization time although the crystallinity is only marginally affected (see Fig. 4 ) . 3. 2. The crystallization-recrystallization process and inser-

tion of gallium into the framework Besides the methods applied before crystallization

was

followed

by

the

the

weight

course

change

of

of the

precipitated solid (amorphous gel and zeolite). The percentage of Ga-ZSM 5 present in the synthesis product was estimated from (sum of peak intensities between 23 < = 2 8 < = 2 5 )

XRD data resp.

from IR spectra using the E ratio relatively short induction period of

(Fig. 4 ) . After a about 5 h the

crystallization o f the zeolite proceeds very rapidly. Already after 14 h more than 90 % crystallinity is achieved. XRD and IR results differ since the appearance of a XRD pattern needs crystals of approximately uniL cell dimensions whereas IK spectra can be obtained already from subunits. During the period of crystal growth between 5 and 14 h the weight change passes a minimum suggesting that at least partly the

4

a

12 24 Crystalliitii time /h

40

96

20

40 60 Bo Crystallboti time I h

100

Figure 4. Crystallization oi Ga-ZSM 5 in dependence on the crystallization time according to XRD ( 0 ) and IK spectroscopy (XI r e s p . weight of the precipitated solid (amorphous gel and zeolite) related to the silica content of the starting gel ( 0 ) . Figure 5. Influence of the crystallization time on the total Ga content (x) according to chemical analysis and on the framework Ga content ( 0 ) estimated from ammonia TPI), expressed as Ga atoms per unit cell. ( - - - ) Ga content of the starting gel.

114

crystalli-zationoccurs in the liquid phase (Fig. 4). After a crystallization time of 24 h the zeolite content reaches its maximum. A further prolongation of the crystallization time causes a loss of crystallinity (cf. Fig. 4). The changes are thought to be associated with a recrystallization process of the zeolite proceeding in the alkaline synthesis gel. During this process small crystals are obviously dissolved in favour of the growths of larger ones. The overall yield of the synthesis changes only marginally. Surprisingly, the increase in crystal size is not accompanied by a distinct increase of the intensity of XRD reflexes. Whereas a growth of crystal size often causes a distinct intensity increase, the insertion of gallium into the framework is expected to decrease the intensity due to a loss of lattice ordering brought about by the greater atomic diameter of gallium compared to silicon. NH -TPD reveals that the crystallization process involves a 3 post-synthesis insertion of gallium atoms into the zeolite framework (Fig 3b). After a crystallization time of 24 h o n l y about 50 % of the gallium present in the sample is actually incorporated into the framework (Fig. 51, although the transformation of the silica into the zeolitic structure is terminated at this point (cf. Fig. 4). Therefore, the other half of the gallium is present in form of nonframework species with unknown nature. Between 24 h and 96 h, however, nonframework gallium is being progressively transformed into framework species as infers from NH 3 -TPD. After 10 h the TPD curve shows a htp which originates from framework gallium. This proves that already during the first stage of synthesis a gallosilicate crystallizes (and not silicalite). Nevertheless, the low resolution of the TPD curve confirms the existence of considerable amounts of nonframework galliuiii. The broad shoulder of the low temperature peak at T > 513 K is assigned + to the coordinative interaction of ammonia with GaO or Ga20j species (Lewis-acid-base interaction). With increasing time of crystallization the htp becomes more intense since nonframework gallium is increasingly inserted into the framework. This

115

process is considered to be terminated after 9 6 h. 3. 3. C a t a l y t i c properties

Both periods of Ga-ZSM 5 synthesis, that is the crystallization between 5 h and 24 h where the percentage of the zeolite within the solid reaches 100 % and the subsequent recrystallization process which leads to further insertion of Ga in framework positions affect the catalytic properties (Fig. 6). In case of the m-xylene isomerization it should be noted that the m-xylene conversion is restricted to about 48 % f o r thermodynamical reasons. This equilibrium is practically reached for samples after 24 hcrystallization time. In case of the ethylbenzene it can be seen that the conversion continues to increase even after 24 h crystallization time. Obviously, the refinement of the zeolite structure with a successive transformation of nonframework gallium species in framework species enhances the activity. The degree of activity increase, however, does not completely parallel the temporal

Figure 6. Conversion of ethylbenzene (a) and m-xylene (b) v e r s u s crystallization time for 623 K (x), 673 K (U) and 723 K (0). Catalyst weight: 1 g zeolite without binder, flow rate: 1 0 l/h (1 V o l . % aromatics), carrier gas: hydrogen.

116

course of gallium insertion as follows by comparison with Figure 5. This discrepancy should be attributed to the uncertainty connected with the discrimination between framework and nonframework gallium on basis of the NH3-TPD curves. Figure 7 finally illustrates how the shape selectivity is influenced by the crystallinity of the samples.

'6 -

4-

i 3-

"XXk o

.

-

m

'h a\&

2-

y-%;",. < = \a 0-0

0

1-

0

\A

% -.

%a-x

Figure 7. Influence of the crystallization time on the shape selectivity of the samples. p-/o-Xylene ratio at 573 Ii versus m-xylene conversion. Crystallization time: 5 h ( O ) , 8 h ( X ) , 10 h (A), 24

It is well known that the diffusivity of p-xylene in the narrow pentasil channels considerably exceeds that of the bulkier o-xylene. This difference leads to an enrichment of the p-isomer in the main stream outside the zeolite and consequently to higher p-/o-xylene ratios than predicted by thermodynamics. This ratio additionally depends on the conversion level in most cases. Figure 7 illustrates that for the gallosilicates this ratio and, therefore, the shape selectivity abruptly changes between 10 and 24 h crystallization time. Surprisingly, the shape selectivity is less pronounced for the well-crystallized sample (lower ratio) and higher at earlier stages of synthesis (higher ratio). Obviously the ZSM-5 structure, if properly formed, exhibits less severe transport barriers as in presence of amorphous material. Nonframework gallium species should be, however, of

117

m-inor importance since for crystallization times > 24 h no further modification of the shape selectivity is observed although the transformation of gallium into framework positions continues. The amount of: nonframework species is too low to cause a visible effect. 4 . CONCLUSION

During the course of crystallization, but especially in the subsequent recrystallization process the physico-chemical properties of the samples vary to a considerable extent with respect of the nature of the gallium, the crystal size and shape, and the perfection of the lattice, although the overall composition ot the Ga-ZSM 5 samples as well as their structure remains essentially unchanged. The manufacturing of catalysts with desired properties on the basis of Ga-ZSM 5 has to take into account these circumstances. 5. REFERENCES 1. C. T. W. Chu, G. H. Kuehl, R. M. Lago, C . 2. 3. 4.

5.

6. 7. 8.

D. Chang, J.

Catal., 93 (1989) 451. G . Choudurier, A . Auroux, J. C . Vedrine, R. D. Farlee, L . Abrams, R. D. Shannon, J. Catal., 108 (1987) 1. R . Szostak, V. Nair, T. L. Thomas, J. Chem. SOC., Faraday Trans. I, 83 (1987) 487. 0 . K. Simmons, R. Szostak, P. K. Agrawal, T . L. Thomas, J. Catal. 106 (1987) 787. H . K. Beyer, G. Borbely, in New Developments in Zeolite Science and Technology, Y. Murakami et al. (Eds.), Kodansha, Tokyo 1986, 867. T. Kanai, N. Kawata, Appl. Catal. 55 (1989) 115. M. Richter, W. Yiebig, H.-G. Jerschkewitz, G. Lischke, G. Ohlmann, Zeolites 9 (1989) 238. L. M. Parker, D. M. Bibby, J. E. Patterson, Zeolites 4 (1984) 168.

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P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

119

STUDIEG ON THE PHOSPHORUS SUBSTITUTED ZEOLITES PREPARED BY SECONDARY SYNTHESIS W. Reschetilowski, W.-D. Einicke, B. Meler. E. Brunner and

H.

EfnSt Karl MarX University Leipzig, Department of Chemistry and Physics, Linndstr. 3-5, 7010-~eipzig, Federal Republic of Germany

Abstract A study was made on the phosphorus substitution on pretrea-

ted ZSM-5 zeolites, The phosphorus lnsertion is demonstrated by means of MAS NMR, X-ray diffraction and I R investigat ions.

1. INTRODUCTION

The possibility of isomorphous substitution of aluminium and/or silicon by other chemical elements in pentasil zeolites and the consequences concerning the properties of these zeolites have been recently lnvestigated (refs. 1, 2 ) . The formation of such products is possible by means of hydrothermal synthesis and also by the post-modification of the parent synthesis products with aqueous solutions of different elements to insert them into the zeolltic lattice (refs. 3-51. own experiments concernlng the realumination of dealuminated pentasils have shown that also the secondary synthesis of lsomorphously substituted pentasils by the treatment of templat-free synthesized zeolites wlth aqueous solutions of different compounds at higher temperatures seems to be useful (ref. 5). This work deals with systematic studies on the phosphorus modified ZSM-5 zeolites prepared by secondary synthesis to demonstrate relations between the structural, surface-chemical and adsorptional properties of these materials.

120

2. EXPERIMENTAL

The insertion of phosphorus was investigated for ZSM-5 zeolite (Si/A1=15) from template-free synthesis. which were modified by thermical, combined thermlcal-mechanical and ultrasonic pretreatment. For this reason the parent zeolite was calcinated for 16 hrs at 6OO0C and taken into a vibration box for 10 hrs. Furthermore a sample treated with ultrasound was also used. For the experiments of phosphorus insertion a certain amount of phosphoric acid (87 wt-X) was diluted by water and contacted with all zeolites mentioned above. In each case the phosphorus content in the suspension was calculated as six phosphorus atoms per zeolitic unit cell at solld/liquid ratio of 0.1. The mixtures were treated hydrothermally at 70 OC in an autoclavic vessel. The products were filtered, washed very carefully and dried on air. BY means of the X-ray diffraction pattern the products were identified as ZSM-5 zeolites with a crystallinity of about 93 % (Leuna standard sample = 100%). The Si/Al ratios were determined by 27Al MAS NMR investigations on a home-made puls spectrometer at 70.3 MHz in comparison to a standard reference sample HZSM-5 (Si/A1=15). 3 1 MAS~ and 3 1 CP ~ MAS NMR experiments were carried out at 121.4 M H Z with proton decoupling on a Bruker spectrometer MSL 300 with phosphoric acid (60 wt-%) as reference for the chemical shifts. For the determination of the phosphorus content the free induction decay extrapolated to time zero were compared with those of the standard N H 4 H 2 P 0 4 . For the 31P CP MAS investigations a radio frequency field corresponding to a n / 2 puls of 6 /-ISwas used. The reference samples were AlPO molecular sieve and NH4H2POq. For further characerization pellets prepared by the KBrtechnique (zeolite:KBr=lr400, pressure =15 MPa) were lnvestigated in the lattic vibration region on a SPECORD M 80 (Carl Zeiss Jena).

121

The adsorption properties were determined by the measurements of nitrogen isotherms at 77 K on a SATORIUS balance. The designation of the zeolites prepared by secondary synthesis. the phosphorus content per unit cell (P/u.c.) the Si/A1 and P/(P+Al) ratios, the volume of the unit cell determined from X-ray data (v/u.c.) ,the 1550/1450 ratio from I R spectra and the nitrogen adsorption capacity at 0.1 MPa are shown in table 1. I

TABLE 1 Sample characterization

.

v/u c. 1550/1450 preparation Sl/A1 P/U.C. P/(P+Al) 7 N r v conditions nm h h h 3 h 4 h 5 h ’6 7++12 h 1 2

3 6 12 16 24 12

3 70 70 70 3 C

at 70 at 70 oC

at at at

C

at 70 at 70

+ultrasonic

OC

OC

and

19 19 20 19 20 17 20

0.025 0.65 1.25 1.46 4.70 0.30 1.92

0.052 0.119 0.215 0.233 0.569 0.053 0.296

5.3783 5.3809 5.3820 5.3859 5.3937 5.3778 5.3892

0.713 0.710 0.714 0.724 0.741 0.708 0.729

mg,g

133.80 131.90 129.68 128.80 121.05 137.30 128.93

++combined thermical-mechanical pretreatment

3. RESULTS AND DIsCussI:ON

The 3 1 M~A S NMR spectra of some P-contalning ZSM-5 zeolites prepared after a thermical pretreatment for 16 hrs at 600 C by the secondary synthesis are given in fig 1 a-c. The time of zeolite treatment with phosphoric acid was varied from 3 to 24 hrs. A s shown by the spectra, a narrow line of the chemical shift in the region from -19 to -25 ppm appears which should be connected with phosphorus atoms In tetrahedrical positions in the zeolitic lattice. Furthermore i t is demonstrated a significant correlation between the time of secondary synthesis and the number of inserted phosphorus atoms. The increase of the secondary synthesis time from 3 to 24 hrs leads to an Increase of the inserted P-atoms per unit cell from 0.025 to 4.7 in comparisan to 6 atoms offered in the synthesis mixture (see

122

0

-1 8.9

1

- 25.3

,

50

50

0

-29.6

-50 -100 d/pprn

* I .

0

-50

-100

- C/ppm

Flgures 1 and 2

3 1 MAS ~ NMR spectra of the samples 4Cb). 5 (a) and samples 6(b) 7(a)

2~c) I

table 1). With a higher number of inserted phosphorus atoms in the lattice the chemical shift of the P-signal tends t o higher values. For the sample with 4.7 P-atoms the chemlcal shift with about -25 ppm agrees well with the results of Blackwell and Patton (ref. 6) for SAP0 zeolites. The position of the Psignal in the 3 1 MAS ~ N M R spectra at -25 ppm is not caused by the number of pho8phorus atom8 in tetrahedrical coordination as shown by fig. 2 a-b for the spectra of samples prepared by secondary synthesis after the pretreatment of the Parent Zeolite carried out a s combined thermical and mechanical and by means of ultrasound. The degree of the phosphorus insertion into the zeolitic lattice depends on the tlme of the seconary Synthesis. The number of the inserted P-atoms of both zeoli-

123

tes do not achieve the values obtalned after 24 hrs with 4.7 P/u.c.. Certainly i t can be seen that the mechanical pretreatment in combinatlon with the calclnation has a positive influence on the phosphorus insertlon. This behavlour could not be detected in the case of the ultrasonlc pretreatment. The value of the chemical shift of both samples of -29 and -27 ppm is higher than for the thermicelly pretreated zeolites and agrees well with those of A l P O molecular sieves (ref. 7 ) . Further information concerning the state of phosphorus in the lattice are avallable from selected X-ray Patterns. The different ionic radii of A13+ (0.051 nmII Si4+ (0.041 nm) and P5+ (0.035 nm) should lead to a change of the lattice parameters with an increased number of inserted phosphorus atoms after the secondary synthesis. W i t h the assumption that the phosphorus atoms were inserted into lattice positions. the lattice constants should decrease with increasing phosphorus content. In opposition to this assumption the higher the tetrahedrlcally coordinated P-atoms in the lattice the higher the volume of the unit cell as ehown in table 1. The increase of the lattice constants a5 a result of a realumination can be excluded, because the Si/Al ratios of the prepared zeolites are nearly the same. From the experimental results i t becomes clear that for the products of the secondary synthesis the isomorphous substltution of aluminium and/or silicon by phosphorus can play only a subordlnate Part. I t seemsl that the increase of the volume of the unit cell should be due to a phosphorus insertion into "activated" regions of the zeolitic surface. For the constructlon of the post-synthesized phosphorus containing ZSM-5 zeolltes we want to submit the followlng structure mentioned by Lercher (ref. 8 ) in another connection.

Ho\ /OH H \

/

O

/

A1

+

H3P04

-

\ /

A1

61

/ \

/

\

+

H20

124

on the basis of our recent investigations concerning the realumination of dealuminated ZSM-5 zeolites in alkaline medium, the provable phosphorus insertion is coupled wlth the presence of lattice defects in the activated zeolites. The concentration of lattice defects and their distribution depends on the aluminium distribution n the Parent zeolite and the kind of pretreatment. The structural change of the zeol tes during the process of the secondary synthesis can be determined by means of I R spectra of the zeolites. Particulary the intensity of the absorptlon band at 550 cm is sensitive for lattice defects and lattice reconstructions. This behaviour la shown in table 1 for the post-synthesized Phosphorus containing ZSM-5 zeolites by the ratio of the intensities of the bands at 550 and 450 cm- . I t is demonstrated that wlth increasing Phosphorus content the ratio 1550/1450 is di8tinCtlY increasing which is due to the reconstruction of the lattlce defects during the phoephorus Insertion. A s shown by the nitrogen adsorption in table 1 this process leads to a significant decrease of the adsorption capacity of the samles with the highest phosphorus content, because the geminal hydroxyl groups on the phosphorus species can block a part of the free volume of the zeolitic channels. Further information on the chemical state of phosphorus in the zeolites are available from the 31P CP MAS NMR investigations (ref. 9). By means of this method phosphorus in the neighbourhood of protons can be detected which allows a differentia of phosphorus in lattice position and in the state suggested above. In the CP experiments the intensity of the P-signal can be increased at the most on the factor I c p / I F I D = YH/Yp

x NH

(NH

+

Np)

For the given strucural model this value is 1.67. Therefore the lntensities of the free induction and the cross polarlzatlon should not differ considerably. In the case of phosphorus atoms In lattlce positions the values tend to zero cau-

125

sed by the large distance of the phosphorus atoms to the protons and should give no contribution to the NMR signal. The spectra for the FID MAS and CP MAS lnvestlgations for sample 2 and 4 are shown in the flge. 3 a-b and 4 a-b.

I I A _ . . .

20

0 -20 -40

Figures 3 and 4

-60 dlppm

-

20

0

3 1 CP ~ (a) and F I D (b) MAS les 2 (left) and 4 (right)

-20 -LO -60 dippm

-

NMR

spectra of samp-

For these experiments the zeolites were transfered into their Proton forms by cation exchange with 0.1 N NH4NQ3 and calcination at 55OoC. While the CP intensity for sample 2 is about six-fold higher as the FID intensity, sample 4 shows a n

126

opposite behaviour, beCaUBe the FID intensity is about fiveteen-fold higher than the CP intensity. From the results i t can be drawn, that in sample 2 the phosphorus is inserted into regions of the ’activated" surface, while the P-atoms in sample 4 are positioned in the zeolitic lattice. This conclusion can be supported by the corresponding chemlcal shifts. In the MAS spectra for the free induction of sample 4 a sig- nal at -30.2 ppm appears, which is connected to phosphorus atoms in the lattice (ref. 7). In the case of sample 2 this signal is absent in both MAS experiments. The signal at -19.5 ppm should be correspond to the phosphorus on the *activated" zeolitic surface. The signal at 0.8 ppm should be assumed as phosphoric acid which was built during the tranfer of the zeolites into the proton form. From a comparison of the FID spectra i t can be seen that phosphorus was migrated during the proton exchange Into lattice positions. 4. REFERENCES

1 2 3 4

5

9

M. Tielen, M. Geelen and P.A. Jacobs, Proc. Int. Symp. Zeolite Catal., Bzeged 1985, Acta Phys. Chem. Szeged. , 1985, P. 1 K.G. Ione and L.A. Vostrikova, usp. Khim. 88 (1987) 393 G.W. Skeels and E.M. Flanigen, ACS Symp. Ser. (M.L. Occelli and H.E. Robson, Eds.) 388 (1989) 420 8 . Sulikowski and J. KlinOwski, ACS Symp. Ser. (M.L. occelli and H.E. Robson, Eds.) 398 (1989) 393 w. Reschetilowski, W.-D. Einlcke, B. Meier, E. Brunner and H. Ernst, Zeocat 90 8 ’Catalysis and AdSOrptiOn by Zeolltes’, Leipzig, 1990 C . S . Blackwell and R.L. Patton, J . Phys. Chem. 92 (1988) 3965 D. MUller, E. Jahn, 8. Fahlke, G . Ladwis and U . Haubenreisser , Zeolites 5 (1985) 53 J.A. Lercher, G. Rumplmayr and H. NOller, Proc. Int. S m p . zeolite Catal.. Szeged 1985, Acta Phys. Chem. Szeged., 1985, p. 71 H. Mehring, ’Principles of High Resolution NMR in Solids". Springer-Verlag Berlin, Heidelberg, New York. 1983

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991Elsevier Science Publishers B.V., Amsterdam

127

Synthesis of zeolite beta i n boron-alurmnium media Miroslaw DEREWINSKIa, Francesco DI RENZOblC, Pierre ESPIAUb, Francois FAJULAb and Marie-Agnes NICOLLEb alnstitute of Catalysis and Surface Chemistry, polish Academy of Science, NieZdpOminaJeK 1 , 3 0 2 3 9 Krakow, Poland bLaboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees, URA 418 du CNKS, ~ c o l eNationale Superieure de Chimie, 8 rue de 1 Ecole Normale, 34053 Montpellier, France Cto whom all correspondence should be sent

Abstract E m o n and a l m m u m compete in the crystallization of zeolite beta. A l m n i w n incorporation is faster than boron incorporation Non-linear effects of the B/A1 ratio on the (B+Al)/Si ratio are observed m e particle size is affected by the composition of the synthesis gel, the bigger crystals being f o m d fromUUghly boric gels.

1.

INTRODUCTION

Zeolite beta was m n g the first zeolites m c h underwent successful replacement of boron for alurmmum ( 1 ) . The main ground for inserting boron in zeolitic frameworKs is the modulation of the strength of the acid sites ( 2 - 5 ) , but structural boron proved to be less stable tnan alwnium in the activation trea-nts, especially in hydrothermal conations (6, 7). W s drawback may be turned into advantage when a networlc quite unstable under dealurmnating conations is concerned, as in the case of zeolite beta (8). ?he mlder conations required for deboration are likely to affect to a lesser extent the lattice stability. B-beta could then represent a suitable precursor of the activated form of the zeolite ( 9 ) . Moreover the afferent Kinetics of incorporation of boron and alurmmum are llKely to influence other properties of the solid, like the size andhabit of the crystals and the defect patterns (10-12).

128

The reagents usedhave been precipitated silica (Zeosil 175MP from RhOne Poulenc, Na 0.9%, A1 0 . 4 L , H20 6.5 %, gram size 2-20 p, pore volunr? 0.08 rnlig), tetraethylammmum (TEA) hydroxide solution (Aldrich), sodlum alumnate (Car10 E2-U W ) , so&un tetraborate decahydrate, sodmm hydroxide (Frolabo RP N o r m a p ) , deionized water. The reagents were m x e d under stirring in the following order: alKaline solution, organic agent, alumnate anWor borate, silica The mxture was stirred for 4 hrs at room temperature before the beginning of the synthesis. The crystallization experimnts have been carried out at 150°C in 120 ml stainless steel autoclaves, without stirring. For all gels the (Al+B)/(Si+Al+B)mlar ratio WaS 0. 07, TEA/(Si+Al+B) 0.35, CH-/Si02 0. 35, H20/Si02 17. ?he B/(B+Al) ratio was 0.00, 0.36, 0.48, 0.64, 0.77and 0. 88 for experlmnts from 1 to 6, respectively. Tne solid fraction has been recovered by filtration, washed with deionized water up to FH 9 and dried at 70 O C in air. The proctucts have M e n characterized by powder X-ray dlffraction, scanrtlng electron mcroscopy (Canibridge S1W instrument), atormc absorption spectroscopy and nitrogen adsorption. Characterization By llBand 27Al MAS-NMR spectroscopy and thermal gravlrretry have been already reported ( 7 ) . 3. RESULTS AND DISCUSSION

Zeolite beta was obtained as a pure D s e from all experiments. A conplete crystallization, as indlcated by XF3, w a s attamed after 24 hours for experlments from 1 to 4, whereas 72 hours were needed for experlrrents 5 and 6. On longer crystallization time other phases appeared beside zeolite beta. Quartz was formed after 6 days in the concZltions of experiment 1, zeolite 2'3%-12 after 3 days in the condltions of experiment 4, and zeolites ZSM-5 and ZSM-12 after 6 days in the condltions of the experiments 5 and 6. The longer time needed to obtam full crystallization in the experIm2nts at hgher boron content suggests that alurmnlum features an efficiency of incorporation h@er than boron A more detaled evidence comes from the yields of crystallization (ratio between the amount of element recovered in the crystals and the amount intromced in the synthesis pel) of the tetrahedron-fomng elements, as reported in table 1. m ' e yields of alummum and silicon are nearly constant for all t h e experients, m e yield of alumnlum being slightly hlgher. The yield of incorporation of boron, instead, is very low when the synthesis gel is almnium-rich (exp. 2-4). Boron and almnium are incorporated at t h e s a m extent only when the available boron largely exceeds alurmniun (exp. 5 , 6). W s behaviour can be accounted for by a corrpetition between the kinetics of incorporation of borosilicate and alumnosilicate units in the zeolite. No rate constants can be established without a Knowledge of the partition coefficients of the elen-ents between the liq-ud and solid phases of the synthesis gel. Anyway, the lugher crystallization efficiency of the

129

Table 1 G e l m l a r comsitlon, zeolite crystallization yielcts and sodrm content in the products B Yield

B/ (B+A1)1 gel

A1 yield

2

0. 00 0. 36

0. 92 0. 88

0. 06

0. 73

0.003

3

0. 48

0. 92

0. 08

0. 82

0. 004

4

0. 64

0. 88

0. 32

0. 78

0.005

5

0. 77 0. 88

0. 90

1.00 0. 86

0. 84

0. 007

0. 78

0. 005

experiment # 1

6

yield

-

0. 95

Na/ (Si+Al+B)

Sl

0. 83

0. 002

almnosilicate units is evident. The changes in the yield ratios between the tetrahemon-fomng elements correspond to a non-linear evolution of the zeolite composition, as shown in figure 1, where the B/(B+Al) and (B+Al)/(B+Al+Si) ratios in the products are reported as functions of the B/(B+Al) ratio in the syntnesis gel. When boron replaces a part of the alurmnium in the gel, a trivalent-poor, mre silicic zeolite is oDtained (exp. 2-4).At bgher boron concentration, the m l e fraction of trivalent elements climbs again to the values of t h e silicoalurmnate zeolite ( e m . 5, 6). It can be observed that the experiments from 1 to 3, in m c h very few bOrosi1icate units are incorporated in the zeolite, present a fairly linear correlation between the alurmnium content in the zeolite and the alurmnium/silica ratio in the synthesis gel. The change of the boron content of the gel from notlung to an m u n t equal to the amount of alurmniumhardly affects the Si/Al ratio. Hence the polymerization degree of the silica in the silicoalurmnate growth units should depend only on the Si/Al ratio in the gel, independently of the boron concentration A sirmlar behaviour in the case of the borosilicate units can easily explain why the trivalent content of the

0.06

'

Figure 1.

Composition of the zeolite as a +unction of the composition of the gel.

(7

0.2

II 0

L

0.2

0

'

m

0

I

0.6

B/B+Al ( G e l )

I

1

130

zeolite increases again when the boron concentration in the gel increases (exp. 5, 6). The mole fraction of s o d ~ u min the crystals is also reported in table 1. The slight increase of the sodmnn content with the insertion of boron could suggest that the more boric solids present a lower cristallinity or a mgher defect concentration. The lattice paramters of zeolite beta as a function of the mole fraction of almnium in the solid are reported in figure 2. A fairly linear shrinl(ing of the unit cell is observed when the alurmmum content decreases. The Si/A1 ratio can account for the @x?nomnon, independently from the m u n t of boron incorporated in the solid As a consequence, the shrinliing of the m t cell alone could not be considered as an evidence of the insertion of boron in the framworK, at least when alummum is competing for incorporation llB MAS-NMR evidences appear m c h m r e suitable (7, 12-15). From the figure 2 it can also be inferred that the &latation module of paramter c at increasing alumllvum content is sligTkly Wgher than the &latation module of parameter a. The crystal size cbstributions for all expermnts are reported in figure 3. The average crystal size as a function of the composition of the synthesis gel is reported in figure 4. WE crystals f o m d in boron-rich m&a (exp. 4-6) are sigmficantly larger than the silicoalurmnate crystals (exp. I ) , in good WeeIEnt w i m results concerning other Kinds of Zeolites (10, 16). However, the correlation is f a r from monotone, as testified by the data reported in figure 4. The crystal size presents a deep m n l m when small m u n t s of boron substitute for alurmmum (exp. 2). AS a consequence, no mrect correlation between nucleation rate and alurmmum or boron content can hold over the whole crystallization field. Very liKely, the size of the crystals is strongly affected by the myslcal properties Of the synthesis gel. The slrmltaneous presence of borate and almnate ions m y indeed influence in a non-strwgth omardway the agglomeration of the mrphous silica (17, 18).

12.4

Figure 2. Lattice parameters as functions of the mole fraction of a l m m m

I

0.02

I

I

1

I

0.1 0 .0 6 Al/tet. ( C r y s t . )

131

20 10

20 10

30

20 10

1

2

I

I

3 I

L

I

Ilt

30 b2

20

G: 4

u

.d ci

10

L4

a

14 d.3

30 20 10

30

20 10

1

2 Size (

Figure 3. Particle size experiments from 1 to 6 .

3 I.I )

a s t r i b u t i o n of zeolite beta From top to bottom:

132 2

Figure 4. Average crystal size as a function of the gel composition.

1

m

0.2

0.6

1

B/B+Al ( G e l )

The non-linear influence of boron on the crystal size confirms that the lughest caution is needed when the interpretation of any crystallization experiment carried out in Pyrex vessels is attempted (15, 1 6 ) . In figure 5 the mcrograpx of s o m samples of zeolite beta are reported. The more alurmruc samples (exp. 1 , 2) feature flattened smeroids with irregular surfaces (figure 5a). When some boron substitutes for alununium (em, 3, 4) almnd-llKe crystals with a four-fold axis are obtaned (figure 5b). The mre boron-rich crystals (exp. 5, 6) feature a s m l a r habit, but some flat faces are present at the outer run of the square almonds (figure 5c). The angle that the flat faces f o m with the axis of the crystal corresponds to the orientation of the face (101) of zeolite beta Ihe slupe of the crystals probably does not depend arectly on the composition of the solid, but it is a function of the crystallization rate. A slower crystallization brings to m r e developed flat, low-index faces. As an example, a solid obtained at lower allcaliruty is depicted in figure 5d 4. CONCLUSlQNS

Partial substitution of boron for alurmnium in the synthesis is a mtable tool to control not only the composition, but also the crystal size of the zeolite beta ?he nucleation flow of the zeolite is a nonlinear function of the composition of the parent gel. The composition of the crystals, Instea& depends on the relative rate of sticking of independent borosilicate and alurmnosilicate species.

133

Figure 5. Micrograghs of zeolite beta. Top left (a): experimnt 1 (no boron present). Top right (b): experiment 4 (B/(BtAl) 0.38). Bottom left ( c ) : experiment 6 (B/(BtAl)0,87). Bottom right (d): solid crystallized at oH-/s102 0.20.

134 5. 1

M. Taramasso, G. Perego and B. Notari, in L.V. Rees (Ed ) , E m c . 5 t h Int. Zeolite Conf., Napoli, June 2-6, 1980, Heyden, London, 1980, pp. 40-48.

2

W. Holderich H, Eichhorn, R. Lehnert, L. Marosi, W. Mross, R. Reinke, W. Ruppel and H. Schlinpr, in D. Olson and A. Bisio (Eds. ) , Proc. &th Int. Zeolite Conf., Reno, July 10-15, 1983, Buttemorths, Guilclford, 1984, pp. 545-555.

3

4

EG. Derouane, L. Baltusis, R.M. Dessau and K. D. Schrmtt, St. S’urface Sci. Catal., 20 (1985) 135-146. A. Auroux, G. Coudurier, R. Shannon and J.C. Vedrine, Cal. Anal. T h e m , 16 (1985) 68-75.

G. Coudurier and J.C. Vedrine, Pure Awl. Chem , 58 (1986) 1389-1396. Sayed, J. Qlem Soc., Faraday l'rans. I, 83 (1987) 1751-1759. M. Derewinslu, P. Massiani and F. FaJUla, in J.C. Jansen, L. Moscou and M. F,M. Post (Eds.) I Recent Research Reports 8th Int. Zeolite Conf. , Amsterdam, July 10-14, 1989, pp. 103-104. N.A. Bsiscoe, J.L. Casci, J . k Daniels, D.W. Johnson, M.D. Shannon and 8 A. Stewart, St. QSrf. Sci. Catal., 49 (1989) 151. C.D. Chang and P. B. Weisz, US Pat. 4,701, 313 (1987) to Mobil Oil Co. 9 10 P. modart, J. B. N a y , 2. Gabelica and EG. Derouane, Applied Catal., 24 5 6 7

M.B.

(1986) 315-318. 11

J.C. Jansen, C.W.R. Fngelen and H.

van BewcUm

ACS

Sm.

Ser., 398

(1989) 257-273. 12

T. R. Gaffney, R. Pierantozzi and M. R. Seger, ACS Symp. Ser., 398 (1989) 374-392.

K.F.M.G.J. Scholle and W,S. V e e m , Zeolites, 5 (1965) 118-122. H. Kessler, J.M. Chezeau, J.L. G u m H. S t r u b and G. Coudurier, 14 Zeolites, 7 (1987) 360-366. 15 2. Gabelica, J. E l Nagy, P. Bodart and G. Debra, chem Lett., 1984, 13

1059-1062. 16

A. Cichoclu, Zeolites, 5 (1985) 26-30.

17

R.K. Iler, m e Chemstry of Silica, Wiley, New York, 1979, pp. 13, 190, 381.

18

6.

C. J. Brinker and G.W. Diego, 1990, p. 225.

Scherer, Sol-Gel Science, Acadermc Press, San

ACKN-

Many thanks are due to the staff of the Service Central d’Analyse du CNRS in Solaize for elemental analysis and to Roger Dutartre for electron rmcroscopy.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

135

On the possibility of generation of Bransted acidity by silicon incorporation in very large pore AlP04 molecular sieves J.A. Martens, I. Balakrishnana, P.J. Grobet and P.A. Jacobs Centrum voor Oppervlaktechemie en Katalyse, KU Leuven Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium a, on leave from National Chemical Laboratory, Pune 411008, India Abstract Si-VPI-5 is synthesized according to a novel method using aluminium isopropoxide as source of aluminium and using a recipe from literature. The samples are compared with SAPO-5 and SAPO-11 using 27Al and 29Si MAS NMR, thermoanalysis and the decane catalytic test reaction. The Si for P isomorphic substitution mechanism, which generates Bronsted acidity in SAPO-5 and SAPO-11 is not active in Si-VPI-5. An explanation for the fundamental difference between SAPO-n materials and Si-VPI-5 is offered.

1. INTRODUCTION

In some of the AlPO4-n molecular sieves discovered in 1982 [l], it is possible to substitute part of the P and Al framework elements with Si [2]. In the resulting SAPO-n materials, isolated Si atoms occupy P sites, while patches of Si atoms replace localIy P as well as Al atoms [3,4]. The degree of Si substitution and the substitution mechanism depend on the topology of the framework and on the synthesis method [4,5]. While the synthesis method does not seem to be critical for the incorporation of traces of Si in SAPOJ and SAPO-11, extensive Si incorporation in these structures can be achieved only by using very specific synthesis recipes [4,6]. Silicon-rich crystals of SAPO-5 and SAPO-11, e.g., can be prepared by using aluminium isopropoxide as a source of aluminium and specific templates, viz. dipropylamine for SAPO-11 and cyclohexylamine for SAPO-5 [4,6]. There are indications in literature that it is possible to incorporate silicon during the crystallisation of very large pore AlP04 molecular sieves of the type VPI-5 [7] and MCM-9 [S]. We report now on attempts of Si incorporation in VPI-5 using synthesis methods which have proven to be succesful with SAPO-5 and SAPO-11. The generation of acidity following such incorporation was probed by catalytic testing of SAPO-8, the thermal transformation product of Si-VPI-5. Such material seems to have 14-membered ring pore openings [9].

136

2. EXPERIMENTAL 2.1. Techniques The conversion of decane was performed in a fixed bed, tubular microreactor. The H2/decane molar ratio in the feed was 100. The pressure in the reactor was 0.35 MPa and the space time of decane 0.5 kg s mmol-1. Powder X-ray diffraction patterns were recorded on a Siemens instrument, equipped with a McBraun position sensitive detector. TG-DTA patterns were recorded on a Setaram TG-DTA92 thermobalance. 27Al, 29% and 31P MAS NMR was performed on a Bruker 400 MSL instrument, using the following operational parameters:

Parameter MAS frequency (MHz) Pulse length (ms) Pulse angle (O ) Repetition time (s) Spinning rate (Mz) Number of scans Chemical shift reference

29~i

27Al

31P

79.5 4.0

104.2 0.6 15 0.1 5-15 3,000 AlCl3

161.9 3.0

45

5 3 10,000 TMS

45

60 15 8 H3P04

2.2. Synthesis of Si-WI-5 Si-VPI-5(1) was obtained as follows. To a mixture of 32.1 g of aluminium isopropoxide (Janssen Chimica) in 34 g of water, 17.7 g of phosphoric acid (85%) (Janssen Chimica) diluted with 12 g of water was added under stirring. Then, 4.6 g of colloidal silica (Ludox AS-40, DuPont) was added, and finally 10 g of dibutylamine (DBA)(Janssen Chimica). This final mixture was stirred to obtain a homogeneous gel with composition

DBA.0.4SiO~.Al203.P205.4OH20. The gel was transferred to a stain ess steel autoclave with a capacity of 150 ml and heated statically at a temperature of 423 K for 8 h. Si-VPI-5(2) was prepared according to a synthesis procedure reported by Davis et al. [7] using pseudobeuhmite (Vista), orthophosphoric acid (85%, from Janssen Chimica), colloidal silica (AS-40 from DuPont)) and dipropylamine (DPA) (Janssen Chimica) as reagents. The gel composition was: DPA.SiOp?l203.P205.40H20. The method involves two ageing steps and hydrothermal treatment at a temperature of 415 K [7]. After the hydrothermal treatments, the autoclaves were cooled to room temperature, the contents centrifugated, and the solids washed and dried in air at a temperature of 313 K. The XRD pattern of SI-WId(1) and Si-VPI-5(2) are shown in Fig.1. That of SiW I d ( 1 ) is in agreement with the one of VPI-5 reported in literature [lo]. The Si-WI5(2) sample contains crystalline impurities of the AlPO4-H3 or MCM-1 type. SAPOJ and SAPO-11 were the SAPO-11/1 and S A P 0 - 5 / 2 samples used in previous work [4]. The Si/(Si+Al+P) fraction in the SAPOJ and SAPO-11 samples is 0.15 and 0.04, respectively.

137

Si-WI-5( 1) Si-VPI-5(2)

15

5

25

45

35

5

15

25

35

45

28

26)

Figure 1. XRD pattern of Si-VPIJ(1) and Si-VPI-5(2). 2.3.Preparation and activation of catalysts The Si-VPI-5 samples were loaded with platinum by impregnation of 1 g of sample with 8.5 mg of Pt(NH3)4Cla dissolved in a minimum quantity of water. The powders were shaped into pellets hamng a diameter of 0.3-0.5 mm by compressing, crushing and sieving. A 200 mg sample of the pellets was charged into a reactor tube having an internal diameter of 1cm. Pt/SAPO-S(1-0) was obtained by calcination of Pt/Si-VPI-5( 1) in flowing oxygen. The temperature was increased from 291 K to 773 K at a rate of 6 K per minute. After 1 h of calcination at 773 K, the catalyst was purged with nitrogen and cooled to 573 K. A flow of hydrogen was conducted over the catalyst for another hour at 573 K to reduce the platinum ions. Pt/SAPO-8( 1-V) and Pt/SAPO-S(Z-V) were prepared by evacuation of Pt/Si-VPI5(1) and Pt/Si-VPI-5(2) under vacuum (15 mPa) at a temperature of 291 K during 12 h. Subsequently, the temperature was increased with a rate of 1 K per minute to 653 K. After 12 h of calcination, the sample was cooled to 291 K. The samples were subjected to an oxygen/hydrogen activation as described for Pt/SAPO-8(1-0). The phase transition of Si-VPI-5 into SAPO-8 was verified with XRD and 3IP MAS NMR on the rehydrated used catalysts. Due to the presence of platinum, XRD lines at low angles were scattered. Fig.2 shows the XRD pattern of Pt/SAF'O-8 (1-0) and Pt/SAPO-8 (2-V). I

I

2.’

20

Pt/SAPO-8( 1-0)

8

.

25

'

'

1

'

30

'

35

'

I "

40

.

45

1

L "

20

'

'

25

'

" '

'

30

29

Figure 2. XRD pattern of Pt/SAF'O-8( 1-0)and Pt/SAPO-8(2-V).

'

35

.

*

I

'

'

40

28

' 45

138

These XRD patterns are in agreement with that for the AlPO4-8 topology [l].The 31P MAS NMR spectrum of Pt/SAPO-8 (1-0) shown in Fig3 is also representative of the AlPO4-8 topology [ 111.

-20

PPH

-40

Pt/SAPO-5 and Pt/SAPO-11 were prepared by impregnation of calcined samples with Pt(NH3)4C12, followed by oxygen/hydrogen activation.

RESULTS AND DISCUSSION The three-dimensional structures comprised in the AlP04-n phases of Wilson et al. [l] have been classified by Bennett et al. as (i) aluminophosphate molecular sieves, (ii) semi-dense phases and (iii) hydrates [ 121. The latter two categories were distinguished from the first one by the presence of Al atoms in five and six coordination with oxygen atoms, four of which belonging to the framework and the additional ones to water molecules or hydroxyl groups [12]. In the early work [3,5,12-141, aluminium in tetrahedral coordination was thought to be essential in order to obtain molecular sieving properties. Meanwhile, it has been shown that in several of these molecular sieves, the coordination number of part of the aluminium can change from IV to VI after the removal of the template and adsorption of water [15,16]. Changes in the aluminium coordination can be monitored conveniently with highfield 27Al fast spinning MAS NMR and DOR NMR [17,18]. In the 27Al MAS NMR spectra of SAPO-5, SAPO-11, Si-VPI-5(1) and Si-VPI-5(2) shown in Fig.4, signals of Alw appear at chemical shifts ranging from 37 to 41 ppm. The 27Al NMR signals at ca. 10 ppm and in the range from -10 to -20 ppm represent Alv and Alm respectively [17]. As-synthesized SAPO-5 and SAPO-11 contain small amounts of Alv and Am.In the Si-VPI-5 samples, ca. one third of the aluminium atoms have the Alw coordination. A similar Alm content is found in VPI-5 [17,18]. The Si-VPI-5(2) sample contains traces of Alv. After calcination and hydration, the Alw signal in SAPO-5 has increased significantly and the Al distribution in calcined hydrated SAPO-5 is AlIV (58%), Alv (9%) and Alm (33%). Meinhold and Tapp reported that in calcined AlPO4-5 up to 40% of the AlIv can be converted reversibly into Alm u on adso tion of water 1151. At the magnetic field of 4.7 T used in that work [15], and Al% signals were not resolved. In calcined hydrated SAPO-5, a comparable share of the Al atoms is found now to be coordinated to one or two water molecules (Fig.4). AlPOg-11 and SAPO-11 undergo a reversible phase transition upon calcination and hydration [19], which is responsible for further structural inequivalency among the

Alt

139

AlIv crystallographic sites, and splitting of the AlIv signal (Fig.4). The contribution of A l v in calcined hydrated SAPO-11, estimated by integration of the 27Al signals of Fig.4, amounts to ca. 22%. Alv is not observed in calcined hydrated SAF'O-11 (Fig.4).

SAPO-11 AS.

11111111111!IIII 50

PPM

0

11111111111111(1 50

SAPO-11 C.H.

Si-VPI-5(2) A.S

11111111))111111 50

0

PPM

0

IIIIIIIIIII(IIII

50

0

SAPO-5 A.S.

1111111111111111 50

PPM

0

SAPOJ C.H.

11111111111111)1 50

0

PPM PPM PPM Figure 4. 27Al MAS NMR spectra of Si-VPI-5(1), Si-VPI-5(2), SAPO-5 and SAPO-11; A.S. stands for as-synthesized, C.H. for calcined hydrated. The TG-DTA results on Si-VPIJ( 1) are shown in F i g 5 Endothermic weight losses due to water desorption are observed at temperatures of 353 K and 400 K. The absence of weight losses associated with exothermic reactions indicates that the micropores do not contain organics. Similar observations were made previously with W I - 5 [7]. Assynthesized VPI-5 is silent in I3C MAS NMR [18] and Duncan et al. have shown that organics are not essential in the synthesis [20]. The absence of organics in the pores of Si-VPI-5 after synthesis explains its similarity in 27Al MAS NMR with calcined hydrated SAPO-5 and SAPO-11 samples (Fig.4).

140

EX0 ENDO

\1.

-

-10

-

-20

I-I

PR,

atmosphere, using a flow rate of 50 ml per minute and a heating rate of 10 K per minute.

29Si MAS NMR spectra are shown in Fig.6. The most intense 2% resonances in SAPO-5 are at chemical shifts of -93 ppm and -111 ppm. These signals represent Si(4Al) and Si(4Si) environments, respectively [4]. The -1 11 ppm signal representing the Si(4Si) environment predominates in the SAPO-11sample (Figd).

n Si-VPI-5 (1)

--

~~

-100

PPM

A

-150

-100

PPM

Si-WI-5(2)

-100

PPM

- 150

1'1

-150

SAPO-11

1 1 1 1 1 1 , 1 1 1 1 1 -100

PPM

-150

141

The 29Si resonance of the Si-VPI-5 samples exhibits a maximum at ca.-111 ppm, indicative of the presence of Si(4Si) environments. These 29Si MAS NMR spectra do not allow to decide on the presence of other Si environments. The conversion of decane over the different catalysts at increasing reaction temperatures is shown in Fig.7. Based on the conversion curves of Fig.7, the activity of the catalyst decreases in the order: Pt/SAPO-5, Pt/SAPO-11 > > Pt/SAPO-8(1-V) > Pt/SAPO-8(1-0) > Pt/SAPO-8(2-V) , Pt/AlP04-5. This activity sequence reflects the Bransted acidity of the samples. The Si, Al, P composition is generally not homogeneous throughout the individual SAPO-n crystals [4]. SAPO-n crystals contain aluminosilicate domains (SA), where the silicon is concentrated, and silicoaluminophosphate (SAPO) domains. The Bransted acid sites of the SAPO-5 crystals of this work are located in the SAPO domains [21]. The SA domains do not contain aluminium and are catalytically inactive [21]. The Si(4M) environment generates the Bransted acidity in SAPO-5. It represents 4% of the Si+Al+P atoms in this particular sample [21]. In the SAPO-11 crystals used in this work, the Brmnsted acid sites are located in the SA crystal domains and at the interface of SA and SAPO domains [21]. Si(nAl) environments responsible for the Brcinsted acidity of SAPO-11 are not resolved from the 29Si resonance envelop (Fig.6). Their amount was estimated at ca. 1% of the Si+Al+P atoms [21]. From the high reaction temperatures, necessary to render the SAPO-8 samples active in decane conversion, it can be concluded that the number of Bransted acid sites in these samples is substantially lower than in SAPO-5 and SAPO-11. The catalytic activity of SAP0-8(2-V) is comparable to that of AlPOq-5. It can be concluded that the silicon in this sample doesnot give rise to Brmnsted acidity. Pt/SAPO-8(2) is more active and calcination in vacuum results in a higher activity compared to activation in oxygen (Fig.7). The refined constraint index, CIO, is the 2-methylnonane/5-methylnonaneproduct ratio at 5% isomerisation conversion of decane [22]. Large pore zeolites have CIO values between 1.0 and 2.2. The CIO criterion does not allow to distinguish 12-membered ring zeolites from structures with larger ring sizes. For 10-membered ring zeolites, CIO is larger than 2.7. The 10-membered ring properties of SAPO-11 are reflected in its CIO value of 3.5 (Fig.8). SAPO-5 with a CIO value of 1.5 fits into the family of 12-membered ring structures (Fig.8). SAPO-8(1-0) and SAP0-8(2-V) have CIO values of 2.1 and 2.2, respectively, classifying them among the open structures. SAPO-8(l-V), which is the most active SAPO-8 sample (Fig.7) has a CIO value of 3.5. This 10-membered ring characteristic could arise from a trace of SAPO-11 impurity in this particular catalyst batch, or else from a trace of SAPO-11 by-product in the Si-VPI-5 into SAPO-8 transformation. From the very low catalytic activity of the SAPO-8 samples, it can be concluded that Si for P substitution does not occur in SAPO-8, and probably neither in its precursor, Si-VPIJ. A plausible reason is that in the absence of organic amines in the micropores and in the absence of alkali cations, no species are available to play the role of charge compensating agent. Si for P substitution in AlP04 molecular sieves requires filling of the micropores with template [4,6].

142

The 2% MAS Nh4R spectra suggest the occurrence of substitution of Si atoms for P and Al atom pairs, a mechanism which doesnot create net negative framework charges. However, if present, the silica patches cannot be large as it is expected that micropores in siliceous domains of the crystals are filled with organics. h

8

100

Figure 7. Conversion curves of decane on Pt/SAPO-n catalysts.

U

-d 0

m LI dl

*d

6 r

't

v

-

60

SAPO- 1 I

40

-.-

20

--- SAPO-8 (I-V)

SAPO-5

0 450

550

650

750

"+

SAPO-8 (2-V)

---

SAPO-8 ( 1 - 0 )

-a-

ALPO-5

Reaction temperature (K)

CI" Figure 8. CIO values of the Pt/SAPO-n catalysts.

4 10-MR

3 .

J.12-MR

2 1 0 -

SAPO-8 SAPO-8 SAPO-8 SAPO-11 SAPO-5 (1-0)

(1-V)

(2-V)

CONCLUSIONS Si for P substitution during the synthesis of aluminophosphates generates net negative framework charges and potential Br~nstedacidity. In SAPO-5and SAPO-11 materials, these framework charges are compensated by organic cations filling the micropores. Si for P substitution in VPI-5 is suppressed since during synthesis the micropores of as-synthesized Si-VPI-5 do not contain organic molecules nor alkali cations. The catalytic activity of Si-WI-5 is much lower than that of SAPO-5 and SAPO11 and is probably due to traces of impurities. The 2% MAS NMR spectra suggest the occurrence of Si for Al+P pairwise substitution. The absence of organics in Si-VPI-5 precludes the presence of siliceous crystal domains. In as-synthesized Si-VPI-5, part of the Al atoms have a coordination number of VI.Al coordinationnumbers larger than IV are found in SAPO-5 and SAPO-11 only after evacuation of the organic template from the micropores and hydration.

143

ACKNOWLEDGMENTS

J.A. Martens and P.J. Grobet acknowledge the Flemish National Fund for Scientific Research for research positions as Research Associate and Senior Research Associate, respectively. This work has been sponsored by the Belgian Government in the frame of "Geconcerteerde Onderzoeksakties". REFERENCES 1. S.T. Wilson, B.M. Lok and E.M. Flanigen, US Patent No. 4 310 440 (1982). 2. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent No. 4 440 871 (1984). 3. S.T. Wilson, R.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. h e r . Chem. SOC.104 (1982) 1146. 4. M. Mertens, J.A. Martens, P.J. Grobet and P . k Jacobs, in Guidelines for Mastering the Properties of Molecular Sieves, D. Barthomeuf et al. (eds.), Plenum Press, New York, 1990, p. 1. 5. E.M. Flanigen, R.L. Patton, S.T. Wilson, S.T. Wilson, Stud. Surf. Sci. Catal. 37 (1988) 13. 6. J.A. Martens, M. Mertens, P.J. Grobet and P.A. Jacobs, Stud. Surf. Sci. Catal. 37 (1988) 97. 7. M.E. Davis, C. Montes, P.E. Hatthaway and J.M. Garces, Stud. Surf. Sci. Catal. 49A (1989) 199. 8. E.G. Derouane, L. Maistriau, Z. Gabelica, A. Tuel, B. Nagy and R. von Ballmoos, Appl. Catal. 51 (1989) L13. 9. R.M. Dessau, J.L. Schlenker and J.B. Higgins, Zeolites 1990, 10,522. 10.M.E. Davis, C. Saldarriaga, C. Montes, J. Garces and C. Crowder, Zeolites 8 (1988) 362. ll.J.A. Martens, H. Geerts, P.J. Grobet and P.A. Jacobs, to be published. 12.J.M. Bennett, W.J. Dytrych, J.J. Pluth, J.W. Richardson, Jr. and J.V. Smith, Zeolites 6, 1986,349. 13.R.M. Lok, C.A. Messina and E.M. Flanigen, Proceed. 6th Int. Zeolite Conf., Ed. D. Olson and A. Bisio, Buttenvorths, Guildford, 1984, 97. 14.E.M. Flanigen, B.M. Lok, R.L. Patton, S.T. Wilson, Proceed. 7th Int. Zeolite Conf., Y. Murakami A Lijima and J.W. Ward, eds., Kodansha, Elsevier, 1986,103. 15.R.H. Meinhold and N.J. Tapp, J. Chem. SOC.Chem. Commun. (1990) 219. 16.M. Goepper, F. Guth, L. Delmotte, J.L. Guth and H. Kessler, in Zeolites: Facts, Figures, Future, ed. P.A. Jacobs and R.A. van Santen, Stud. Surf. Sci. Catal. 1989, 49B, 857. 17.Y. Wu, B.F. Chmelka, A. Pines, M.E. Davis, P.J. Grobet and P.A. Jacobs, Nature 346 (1990) 550. 18.P.J. Grobet, J.A. Martens, I. Balakrishnan, M. Mertens and P.A. Jacobs, Applied Catal. 56 (1989) L21. 19.R. Khouzami, G. Coudurier, F. Lefebvre, J.C. Vedrine and B.F. Mentzen, Zeolites 10 (1990) 183. 20.B. Duncan, R. Szostak, K. Sorby and J.G. Ulan, Catal. Lett. 7 (1990) 367. 21.J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Catal. 126 (1990) 299. 22.P.A. Jacobs and J.A. Martens, Pure Appl. Chem., 58(10) (1986) 1329.

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P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991Elsevier Science Publishers B.V., Amsterdam

145

CRYSTALLIZATION OF POROUS ALUMINOPHOSPHATES AND METAL SUBSTITUTIONS

H. LECHERT*, H. WEYDA**,M . HESS*, R. KLEINWORTH*, v. PENCHEV***

***

AND CH. MINCHEV

*

**

of

Institute

of

Physical Chemistry of t h e University

Hamburg,

Bundesstrasse 45, 2000 Hamburg 1 3 , Germany SUD-CHEMIE AG, Katalyse-Labor

,

Waldheimer S t r

. 13,

8206 Bruckmiihl/Heu f e l d , Germany

***I n s t i t u t e

of Organic Chemistry, Bulgarian Academy of Sciences

S o f i a , Bulgaria ABSTRACT phenomena of a series of t h e molecular s i e v e s MeAP04-5

Crystallization and

MeASPO-5

have

been s t u d i e d under t h e

influence

of

Me-

different

components. A s M e components Be, Mg. Zn, N i and Fe have been used. These components

been o f f e r e d i n some excess t o study t h e e x t e n t of t h e

have

incorporation

which

was determined by SEM-EDAX a t t h e product c r y s t a l s .

For Be and Mg almost no i n f l u e n c e on t h e k i n e t i c s can

be

observed

compared

s t r u c t u r e with about 0.12 M@ shows

a

phosphate,

reduced The

with a pure

of

crystallization

batch.

f o r one A1203 P2O5 u n i t .

c r y s t a l l i n i t y and a

ZnO

ALP04-5

Mg

enters

A batch with

cocrystallization

of

content of t h e c r y s t a l s shows about t h e

an

the Zn

unknown

same

molar

r a t i o as t h e MgO. Extended experiments with N i O show t h a t t h e N i i s not incorporated i n t o ALP04-5 and SAPO-5 c r y s t a l s . I n t h e presence of N i O pure ALP04-5 and SAPO-

5 c r y s t a l l i z e which undergo r e c r y s t a l l i z a t i o n t o a c r i s t o b a l i t e

like

phase. Fe i s incorporated i n t o t h e c r y s t a l s f o r both valency states t a k i n g

about

Generally

the

presence

of an excess

of

the

Me-ions favours some

r e c r y s t a l l i z a t i o n t o denser phosphate phases o r t o c r i s t o b a l i t e o r

mite

.

up

0,l Fez03 f o r one A1203 P2O5 u n i t i n t h e s t r u c t u r e . tridy-

146 INTRODUCTION Porous aluminophosphates and silicoaluminophosphatehave been

first

described by E.M. Flanigan et al. (ref 1-5). Later on this family has been expanded introducing the so-called metalloaluminophosphates, containing an additional metal component (ref. 6-13). Excellent reviews about these substances have been given by WILSON and FLANIGEN (refs. 14,15). In a preceding paper we have thouroughly studied the kinetics of the crystallization of ALP04-5 and SAPO-5 in dependence on all relevant parameters (ref. 17). For

a variation

of the ratio Al/P

a distinct maximum

crystallinity can be observed always near a value of Al/P = 1.0.

of the Starting

with a general batch composition of for

A1203 * P2O5 * s Si02 * r TP * 50 H20 the incorporation of Si the following typical values can be observed

f o r the formation of 100% crystalline samples.

< s < 0.5 , r > 0.5 , TP = 0.4 and a temperature of 473 K

SAPO-5: 0 For

batches with s =

Pr3N the templates P q N ,

Et3N and Et2NH cause nearly the same induction time of about two hours. The crystallization time is about 2 hours €or the triamines and about 4 hours for the Et2NH. Pr4NOH shows an induction period of 4 hours and a very short crystallization time of less than 2 hours. In the following, f o r a further study of the incorporation of ions into the ALP04-5 structure kinetic experiments shall be reported. At first Be and Mg have been studied. According to the literature both give welldescribed MeAPO-5 structures, which have been characterized by solid state NMR (refs. 15.16). This holds also for Zn (see also ref. l 9 ) . As a contrast, a series of experiments with Ni has been carried out. Ni occurres in its ionic compounds almost only in sixfold coordination. In the literature no Ni-containing ALP04-5 o r SAPO-5 has been reported. Kinetic studies with Fe have been done with both valencies from which

the

respective ALP04-5 analogues are wellknown. Generally, the experiments were carried out with some excess of the Me compounds in the batch, to study the maximum content of the incorporation beside the influence on the kinetics. As far as possible the incorporation of the ions into the crystal shall be studied by SEM-EDAX experiments.

147

EXPERIMENTAL METHODS Svnthesis The

aluminium s o u r c e was always a pseudoboehmite phase obtained

CONDEA-Chemie.

Fumed

silica,

85%-orthophosphoric a c i d and

the

from sodium

hydroxide were products from Merck, as w e l l as t h e template tripropylamine

(Pr3N). For a l l r e a c t i o n mixtures d i s t i l l e d water was used. The

was

crystallization

steel

stainless

temperatures

autoclaves.

under

carried out i n

teflon

bottles

The mixtures were heated

autogeneous

pressure.

For t h e

to

placed

the

in

reaction

preparation

of

the

b a t c h e s two b a s i c r e a c t i o n mixtures were used: an aluminophosphate gel and another

gel,

reaction

t h e s i l i c a and

containing

the

template.

The

mixture t o p r e p a r e a SAPO- (AlPO4-) s t r u c t u r e had t h e

'standard' following

molar composition:

*

A1203

TP denotes t h e template,

P2O5

*

s Si02

*

r TP

f o r example Pr3N;

*

50 H20

A s r e a c t i o n temperature 473 K

was choosen throughout t h e experiments. A t s u i t a b l e t i m e i n t e r v a l s samples were taken,

filtered,

washed with d i s t i l l e d water t o near n e u t r a l i t y and

d r i e d o v e r n i g h t a t 460K.

For t h e experiments with t h e metals u s u a l l y batch compositions A1203

*

P205

* 0.4

Me0

*

r TP

*

50 H20

were used. Characterization For t h e a n a l y s i s of t h e products X-ray powder d i f f r a c t i o n p a t t e r n s were taken u s i n g a d i f f r a c t o m e t e r ISO-DEBYEFLEX 1000 with copper K The

X-ray

fluorescence

spectrometer.

measurements were c a r r i e d

out

with

radiation.

a

Philips

The method used t o o b t a i n t h e % c r y s t a l l i n i t i e s i s based on

a

of

the

The SEM-EDAX experiments have been c a r r i e d o u t a t a EDAX terminal

PV-

the

peak

areas

i n t h e i n t e r v a l 20=5"-40" a f t e r

subtraction

background. 9900 i n connection with a P h i l i p s scanning microscope SEM 515/D806.

RESULTS The g e n e r a l composition o f t h e batches used for t h e

experiments

was

given by A1203 This

*

P2O5

*

0.4 Me0

*

2 Pr3N

*

50 H20

means t h a t i n case of a f u l l i n c o r p o r a t i o n of t h e metal about

each

148

100

-

-

80

-

U

w 1 ) .

60 -

c

b

.A

40 -

m c,

cn

20 -

u

-

0

Fig.

I

1

A

I

I

1. Kinetics of the crystallization of a batch

* P205 * 0.4 Be0 * 2 Pr3N * 50 H20 u denotes the presence of an unknown phosphate phase A1203

I

100 w

80

\ ) .

2 60 c

.A

e

nJ

40

c,

L

20

u

0

10

0 Fig.

20

30

time /

40 hours

50

2. Kinetics of the crystallization of a batch A1203

*

P2O5

*

0.4 MgO

*

2 Pr3N

* 50 H20

u denotes the presence of an unknown phosphate phase

149

fifth

A 1 o r P should be replaced by a metal atom.

amounts

have

been o f f e r e d t o study t h e e x t e n t of

The

relatively

incorporation

large

to

and

obtain d i s t i n c t e f f e c t s f o r the kinetics.

1 and 2 show t h e r e s u l t s of t h e experiments with B e and Mg. Both

Fig.

elements are added t o t h e batch as oxides. There is no d i f f e r e n c e of t h e course of t h e c r y s t a l l i z a t i o n with t h e pure ALP04-5 and t h e SAPO-5 with s = 0.4.

compared

The i n d u c t i o n time

n e a r two hours and t h e c r y s t a l l i z a t i o n i s completed i n another two

is

hours.

The Be-containing sample c o u l d n ' t be analyzed by SEM-EDAX. The Mg c o n t e n t was a t about 0.1 MgO f o r one A1203

*

which

P2O5 u n i t

i s i n accordance t o t h e c o n t e n t s r e p o r t e d i n t h e l i t e r a t u r e ( r e f s . 15,16). After

about

one

day by X-ray some p e r c e n t of an unknown

observed i n t h e X-ray diagrams,

phase

can

be

t h e c o n c e n t r a t i o n of which remains almost

c o n s t a n t f o r a long t i m e . This phase cannot be i d e n t i f i e d i n t h e SEM. In

t h e r e s p e c t i v e experiments with ZnO t h e mentioned

phase

appears the

a l r e a d y a f t e r s i x hours i n a l a r g e r q u a n t i t y and t h e c r y s t a l l i n i t y of ZnAPO-5 phase does n o t exceed 60 %. The Zn-content i s n e a r 0.1 ZnO f o r A1203 In

*

a

P2O5 u n i t . comparison,

some Ni-containing samples

have

been

studied.

Ni-

c o n t a i n i n g materials with ALP04-5 s t r u c t u r e have never been r e p o r t e d .

In Fig.

are

4a

series of experiments with d i f f e r e n t N i - and S i

summarized.

contents

It can be seen t h a t a f t e r 6 hours a batch with 0.2

and 0.2 N i O h a s a c r y s t a l l i n i t y of more than 90 % compared with batch with 0.4 Si02. is s t a b l e f o r t h e

a

Si02 SAPO-5

It can be seen t h a t t h e sample c o n t a i n i n g only

Si02

48 hours o b s e r v a t i o n t i m e . The Ni-containing phases show The Si02 i n c r e a s e s t h e l i f e t i m e

a recrystallization t o cristobalite.

of

t h e m e t a s t a b l e SAPO-5 phase. Because

of t h e p o s s i b l e i n t e r e s t of t h e s e substances

experiments

for

catalysis,

f o r t h e f u r t h e r c h a r a c t e r i z a t i o n of t h e s t a t e of t h e N i

have

been c a r r i e d o u t . P r i m a r i l y EDAX experiments have been done. t h e g e n e r a l p r o p e r t i e s of t h e Ni-ions, the

nickel

is never i n t h e c r y s t a l s ,

b e s i d e t h e c r y s t a l s . UV-VIS-spectra coordination. the

Ni

Thus,

b u t always i n an

the

amorphous

phase

show, t h a t t h e Ni2+-ion has o c t a h e d r a l

a c r y s t a l l i z a t i o n o f pure ALP04-5 occurs

completely o u t of t h e c r y s t a l s .

accelerates

from

A s i t may be expected

t h e s e experiments show c l e a r l y t h a t

change t o c r i s t o b a l i t e .

The presence

of

NiO

Using Ni-acetate

leaving obviously for

the

150

100 2 s

' 80 =c,

c

.A

60

.A

A

ro

A

40

c,

cn

L

20

0

0

10

0 Fig.

20

30

40

time / hours

50

3. Kinetics of the crystallization of a batch A1203

*

P2O5

* 0.4

ZnO

*

2 Pr3N

*

50 H20

u denotes the presence of an unknown phosphate phase

100 be

'80 ) .

5 60 c .A

m

40

c, Ln

p 20 0

0

151 crystallization The

time.

experiments t h e c r y s t a l l i n i t y remains f o r a longer

crystallinity

decreases,

but

no

cristobalite

can

be

observed.

a t t h e BET-data f o r a sample without S i and 0.4 N i a f t e r 6 hours

Looking

crystallization

time

48

m2/g.

decreases t o

261 m2/g can be found.

A f t e r 52 hours

this

value

For t h e pure SAPO-5 with s = 0.4 310-330 m2/g

are

observed. For t h e Si-containing samples t h e decrease of t h e pore volume i s much slower.

a

The preceding experiments with t h e Ni-containing batches show t h a t characterization whole

5 and 6 show experiments with an Fe-containing

Figs. be

by X-ray accompanied only by a chemical a n a l y s i s of

the

sample may be sometimes misleading. batches.

It can

seen t h a t t h e c r y s t a l l i z a t i o n occurres f a s t e r than i n t h e presence

of

N i . S i m i l a r t o t h e N i c o n t a i n i n g batch a r e c r y s t a l l i s a t i o n t o c r i s t o b a l i t e

is

observed ( F i g . 5 ) .

EDAX experiments show t h a t i n c o n t r a s t t o

t h e Fe i s p r e f e r a b l y i n s i d e t h e c r y s t a l s r e g a r d l e s s

product

has been two- o r t h r e e v a l e n t .

Fe-source

This agrees with

the

Ni-

whether results

the which

have been obtained from MoBbauer experiments ( r e f . 18). The

comparatively

c r y s t a l l i n i t y i n t h e case of

low

Fez03

is

probably

the

SEM-EDAX

caused by t h e low s o l u b i l i t y of t h i s substance. The c r y s t a l composition was throughout t h e experiments n e a r A1203 which

means

that

*

*

1.1 P205

within

0.12 Fez03

t h e l i m i t s of t h e

accuracy

of

determination t h e aluminium about 10% of t h e A 1 have been replaced by Fe. In

the

X-ray diagram and i n t h e colour of t h e c r y s t a l s and

v a l u e s no d i f f e r e n c e s could be observed using Fe2+ o r Fe3'

in

the

BET

compounds a s an

Fe source. From preliminary experiments could be seen t h a t s i m i l a r t o t h e with

Ni

longer

t h e samples with Si02 c r y s t a l l i z e slower and a r e time.

To

study t h e amount of t h e i n c o r p o r a t i o n of

samples

stable the

for

Fe,

a the

following batches have been used. A1203 with n = 0.1:

*

P2O5

*

n Fe2O3

0.2; 0.3; 0.5;

*

0.2 SiO2

. As

*

2 Pr3N

*

50 H20

Fe-source Fe3+-acetate was used. The

c r y s t a l l i z a t i o n temperature was again

473 K.

By X-ray and by SEM-EDAX t h e following compositions could be obtained.

It could be shown t h a t a f t e r about 10 hours almost 100% c r y s t a l l i n i t y

can be obtained which i s s t a b l e f o r a long time. Most of t h e c r y s t a l s have t h e shape of hexagonal prisms which a r e surrounded by some amourphous

152

100 w

80

1 ) .

2 60 c

.A

40

m

c,

cn

=-. 20 L

u

0 10

0 Fig.

20

30 40 t i m e / hours

50

5. Kinetics of the crystallization of a batch

*

A1203

P2O5

*

0.4 FeC12’

2 Pr3N

* 50

H20

and the recrystallization to cristobalite (open circles)

100 w

80

\

>

Z, 60 c

-I+

d

40

m

c,

cn

=- 20

L

0

0 0 Fig.

10

20

30 40 t i m e / hours

50

6. Kinetics of the crystallization of batches A1203 * P2O5 * 0.4 Fe R * 2 Pr3N * 50 H20 with different Fe sources Fe R R

(CH3COOH)2 ( rn ) ; R = C12 ( 0 ) , FeR = 1/2 Fe203 ( A

153 material

and some s q u a r e p l a t e l e t s with t h e a molar r a t i o Fe/P

of

about

0.8 and about A1/P of about 0.2. I n t h e s e c r y s t a l s only very few S i can be found

From

. 1 can be s e e n t h a t t h e Si02-content is reduced t o

Table

compared with t h e b a t c h composition.

formulae t h e v a l u e s i n l a s t column can be found.

n

=

0.3

these

data

about

are i n good agreement

1/2 these

C a l c u l a t i n g t h e n e t charge o f

Apart from t h e v a l u e f o r

with

those

found

in

the

l i t e r a t u r e ( see e . g . r e f . 1 4 ) . The balance of t h i s charge may be p o s s i b l y given by some a c i d sites o r t h e r e s p e c t i v e c a t i o n of t h e template. Composition of t h e FeASPO samples with d i f f e r e n t Fe

Table 1.

contents

i n t h e batch. Fez03 c o n t e n t

Composition o f hexagonal prisms

Net charge of a

of t h e batch

FeASPO c r y s t a l s i n t h e product

(Fe,Al,P,Si)O2unit

A1203

n = 0.1: n = 0.2:

A1203

n = 0.3:

A1203

n = 0.5:

A1203

* * * *

1 . 2 P2O5

*

0.12 Fe203

1.1 P2O5

*

0.09 Fez03

1.1 P2O5

* *

0.16 Fez03

1.1 P2O5

0.21 Fez03

* * * *

0.10 Si02

-0.01

0.16 Si02

-0.03

0.12 Si02

’0.18

0.08 Si02

-0.09

CONCLUSIONS If

an

incorporation

of t h e a metal

into

an

ALPO-5

structure

is

observed, t h e i n d u c t i o n period and a l s o t h e r a t e of c r y s t a l l i z a t i o n i s n o t i n f l u e n c e d a p p r e c i a b l y compared with t h e d a t a observed f o r ALP04-5. The amount of i n c o r p o r a t i o n of t h e Mg, appreciable Fe3'

this

variation

Zn, Fe2+ and Fe3'

and lies g e n e r a l l y between

amount i s n o t changed a p p r e c i a b l y ,

shows no

5 and 1 0 %. For t h e

i f additionally

Si

is

offered i n t h e batch. Pure s u b s t a n c e s can always obtained remaining i n t h e batch below

these

values. N i i s not incorporated i n t o the c r y s t a l s .

I n t h i s c a s e only ALP04-5 o r

SAPO-5 c r y s t a l l i z e s . The presence of c a t i o n s and t h e compound i n which they are t h e b a t c h have, SAPO-5 phase.

added

to

g e n e r a l l y , i n f l u e n c e s on t h e s t a b i l i t y of t h e ALPO4-5 and

154 phosphates of t h e incorporated metals a r e u s u a l l y

The

water.

The c r y s t a l l i z a t i o n o f t h e s e phosphates o r of an

insoluble

in

aluminophosphate

with h i g h e r d e n s i t y may t a k e p l a c e i n d i f f e r e n t ways. It can be amorphous, crystallize

in

d i f f e r e n t s t r u c t u r e s on o r b e s i d e t h e ALPO4-5

and may cause a r e c r y s t a l l i z a t i o n c r i s t o b a l i t e - o r

structure

or

SAPO-5

tridymite-.

l i k e aluminophosphate. Which of t h e s e cases i s p r e s e n t i n a p e c u l i a r system must

be

studied

separately. ACKNOWLEDGEMENTS

W e thank t h e "Deutsche Forschungsgemeinschaft" f o r t h e generous support of

our work. REFERENCES

1 E.M. Flanigan, B.M.

Lok, R. Lyle P a t t o n and S.T. Wilson, Pure and Appl. Chem.,

2

B.M.

3

B.M. Lok, C.A.

Lok, C.A.

E.M. Flanigan, U S P a t . E.M.

4 S.T. 5 B.M.

4 440 871 (1984)

Messina, R.L.

106 (1984) 6092

Wilson, B.M. Lok, E.M. Flanigan, EP 0 043 Lok, C.A.

Messina, R.L. P a t t o n , R.T. EP 0 103

562 (1984)

Gajek, T.R. Cannan and

117 (1986)

Lok, R . Lyle P a t t o n and S.T. Wilson,

E.M. Flanigan, B.M.

EP 0

7 8 9

P a t t o n , R.T. Gajek, T.R. Cannan and

Flanigan. J . A m e r . SOC.,

E.M. Flanigan,

6

58 (1986) 1351

Messina, R . L . P a t t o n , R.T. Gajek. T.R. Cannan and

158 976 (1985)

B.M. Lok. L.D. Vail and E.M. Flanigan, EP 0 B.M.

Lok. L.D. Vail and E.M.

B.M.

Lok, B.K. Marcus and E.M. Flanigan, EP

Flanigan, EP 0

158 348 (1985) 161 491 (1985) 0 161 490 (1985)

10 B.M. Lok, B.K. Markus, C . A . Messina, R . L . P a t t o n , S.T. Wilson and E.M. Flanigan. EP 0

11

158 349 (1985) 158 350 (1985) S.T. Wilson, EP 0 158 977 ( 19851

B.M. Lok, B.K. Marcus and E.M. Flanigan, EP 0

12 B.M.

Lok, B.K.

Marcus, C.A.

Messina and

13 B.M.

Lok, B.K.

Marcus, L.D.

V a i l , E.M.

S.T. Wilson, EP 0

159 624 (1985)

Flanigan, R.L. P a t t o n and

155

14 R.

Khouzami, G . Coudurier, B.F. Mentzen and J . C . Stud. S u r f . S c i . ,

15 E.M.

Vedrine,

37 (1988) 355

Flanigen. B.M. Lok, R.L. P a t t o n and S.T. Wilson i n

"New Developments i n Z e o l i t e Science and Technology" Y . Murakami, A. Iima. J.W. Ward ( E d s . ) , Kodansha, E l s e v i e r

Amsterdam, Oxford, New York. Tokyo (1986) , p . lo3 16 S.T. Wilson and E.M. Flanigen i n " Z e o l i t e S y n t h e s i s " . M.L. O c c e l l i and H.E. Robson (Eds.) ACS Symposium S e r i e s 398 (1989) 329

17 H. Weyda and H. Lechert, Z e o l i t e s 10 (1990) 251 18 Hong-Xin L i , J . A . Martens, P.A. Jacobs, S. Schubert, F. Schmidt, H.M.

Ziethen and A.X.

Trautwein, i n "Innovations i n Z e o l i t e Materials

Science" P . J . Grobet e t a l . (Eds.) S t u d i e s i n S u r f . Science and Catal.

37 (1988) 75

19 G.C. Bond. M.R.Gelsthorpe, K.S.W. Sing and C . R . Theocharis. J . Chem. SOC. Chem. Commun., 1056 (1985)

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

157

FACTORS AFFECTING THE CRYSTALLIZATION OF ZEOLITE ZSM-48 G. Giordanoa, N. Dewaeleb, 2. Gabelicab, J. B.Nagyb, A. Nastroa, R. helloa and E.G. Derouaneb b Laboratory of Catalysis, FacultCs Universitaires N.D. de la Paix, rue de Bruxelles, 61, B-5000 NAMUR (Belgium) a Dipartimento di Chimica, Universita della Calabria, Arcavacata di Rende,

1-87030 RENDE (CS), (Italy)

Abstract Zeolite ZSM-48 was synthesized in the presence of either hexamethonium ions or octylamine admixed with tetramethylammonium ions. It was observed that at high temperature and in presence of ammonium ions, the hexamethonium ions are decomposed into hexyltrimethylammonium and trimethylammonium ions, when ZSM-48 is formed, while no decomposition occurs when EU-1 crystallizes from the same but slightly Al-richer reaction mixture. The relative amount of defect groups can be rationalized if one supposes that the hydroxyl nests resulting from missing T atoms must be created in the framework as to better accomodate the trimethylammonium terminal groups of the hexamethonium ions located along the linear ZSM-48 channels. 1. INTRODUCTION

In the last few years, the preparation of a series of structurally similar zeolites involving one-dimensional channels, namely ZSM- 12, ZSM-22, ZSM-23, ZSM-48, KZ-2, EU-1 and NU-10, h a s been reported in the literature[ 1- 111. The high-silica zeolite ZSM-48 can be synthesized from aqueous silica hydrogels, with or without alkali cations or aluminium, in presence of a variety of N-containing organic molecules (mono or diaminoalcanes) [4, 12- 171 b u t also from mixtures containing quaternary imidazole compounds [51 in presence of ethyleneglycol, glycerol or butanol as solvents [181 or even in non-aqueous systems 1161. The structurally related zeolite EU- 1 was claimed to crystallize for SiOz/A1203 ratios of 120 or lower in systems containing hexamethonium cations [2,4,12,13], but recently, the synthesis of high-silica EU-1 was realized when benzyl dimethylamine and benzylchloride were used as templates [lo].

158

On the other hand, it was recognized that the presence of ammonium ions in hydrogels leading to high-silica zeolites, such as ZSM-5 plays an inhibiting role on the nucleation process [ 191. Finally one should bear in mind that, because of the high alkalinity of the synthesis mixtures, a rather high amount of defect groups (= SiOX, where X = H, alkali or organic cation) are formed in the final zeolite crystals. Van Santen et al. 1201 proposed a hypothesis to explain the presence of these groups in the silicalite-1 and also supposed the existence of "hydroxyl nests" created by the missing tetrahedral sites in the MFI framework. The relative amount of the various kinds of defect groups could be easily determined by using 29Si-NMR [21,22]. A series of preliminary investigations [13] revealed that the presence of ammonium ions in the starting hydrogel induced a partial decomposition of hexamethonium ions into the corresponding hexyltrimethylammonium and trimethylammonium fragments that were found incorporated in the final ZSM-48 framework and that this decomposition noteworthyly influenced the final amount of defect groups in the zeolite lattice. In order to investigate the actual role played by ammonium ions on the synthesis of ZSM-48 and on its final "defected" structure, we have investigated more systematically the influence of a series of synthesis variables (nature of the organic guests, presence of alkali hydroxides, and of Si- and Al- bearing ingredients in various concentration) on the behaviour of ammonium ions during synthesis. 2. EXPERIMENTAL

2.1. synthesis

Two series of hydrogels having the following molar composition have been investigated: Gels of type 1: xNa2O-yHMBra -z(NH4)20-wAl203-6OSi02 -3000H20 where HMBr2 stands for hexamethonium bromide and 05 x 510, 01 y 110. 01 z 110, 05 w 51.5. The syntheses were run in static conditions under autogeneous pressure at 200 f 2°C in 60 ml Teflon-lined Morey-type autoclaves for variable periods of time. Gels of type 2: 1 5 N a 2 0 - 15.6~MABr-80.40cNH2-0.48Al20310.8H2S04-6OSi02-3258H20

where TMABr stands for tetramethylammonium bromide and OcNH2 for n-octylamine. The syntheses were performed under autogeneous pressure in stirring conditions at 160 5 2OC in the same autoclaves utilized for the type 1 synthesis. The detailed synthesis procedures for type 1 and 2 systems were reported elsewhere [4,13].

159

2.2. Characterization

The identification of the solid phases and the determination of their crystallinities were carried out by X-ray powder diffraction. The alkali and Al contents were determinated by PIGE [23], while the amount of organic and water molecules was evaluated by thermal analysis. The amount of defect groups in the zeolite framework was calculated from solid state MAS 29Si-NMR spectra [21]. Finally, the identification of the decomposition products of hexamethonium ions were performed by combining TG-DTA analysis and MAS 13C-NMR [ 141.

3. RESULTS AND DISCUSSION A detailed study of the systems of type 1 allows to define adequately the influence of organic and inorganic cations, on the synthesis course. The nature of the crystalline products so-obtained as a function of the relative a m o u n t s of sodium hydroxide, ammonium ions and hexamethonium ions, is reported in Table 1.

Table 1 Nature and crystallinity of the products obtained from the system: xNa~O-yHMBr~-z(NH~)~O-O.5Al~0~-60Si0~-3000H~O at 200°C. ~

~~

Sample

~

~

mole in hydrogel X

1

2 3 4 5 6 (a) 7 8

0 0.5

5 5 5 5 5 5

Y

5 5 5 0.5 0 0 5 5

Synthesis Nature [crystallinity] time (days) of the products Z

10 5

5 5 5 5 0,5 0

5and9 5and9 2.7 7 5 16 2.7 2.7

amorphous amorphous ZSM-48 [63%] ZSM-5 [25%]+a-quartz ZSM-5 [20%l+a-quartz ZSM-5 [50?'0] ZSM-48 [8O%l ZSM-48 [81%]

(a)Syntheses carried out at 170 "C. In these systems zeolite ZSM-48 does not form if HM++ions are absent (samples 5 and 6) or if their relative concentration is low (sample 4). Under such conditions the reaction leads to the formation of ZSM-5, probably because the formation of 5-1 SBU is favoured in a high-silica hydrogel containing both Al and Na+ [13,24,25]. The presence of a-quartz admixed with ZSM-5 is caused problably by the high synthesis temperature. Indeed if the synthesis temperature decreases no dense phases are detected (sample 6). In absence or with a low NaOH content even in the presence of NH4+ ions, the starting hydrogel does not crystallize, showing that the presence of alkali cations is indispensable for

160

the nucleation to proceed (samples 1 and 2). Low NH4+ concentrations neither influence the crystallinity. nor the crystallization rate of ZSM-48 (samples 7 and 8). For a better understanding of the role played by the NH4+ ions on the crystallization rate of ZSM-48 the ammonium contents was vaned from 0 to 5 moles in the hydrogel of type 1 (Table 1 samples 3. 7 and 8). It can be seen that the crystallinity of ZSM-48 decreases with increasing NH4+ content. Similar behaviour was observed for the nucleation and growth of ZSM-5 [19]. Dodwell et al. 1121 also observed a decrease of the crystallization rates of ZSM-48 and EU- 1 with increasing ammonium content. A decrease of the crystallization rates due to the presence of NH4+ ions was also observed during the formation of silicalite-2 in presence of F- ions [261. The presence of NH4+ ions, in type 1 hydrogels, favours the partial decomposition of HM++ ions in hexyltrimethylammonium (HTMAm+)and trimethylammonium ions (TMAm+).The results of this investigation are reported in Table 2. Table 2 Nature and crystallinity of the products obtained from the system: 5 N a ~ O - 5 H M B r ~ - x ( N H ~ ) ~ O - y A l ~ O ~ - 6 O S i Oas ~ -a3 O function O O H ~ Oof synthesis conditions. Sanple

mole

x 9 8

0 0

y 0 05 0

Synth.

synth

time, (d-1

temp.

("a

2 200 2.7 200 10(d 5 25 180 ll(d 5 025 33 180 11 5 025 2.7 200 3 5 05 2.7 200 12 5 1 6 200 13(b) 5 15 7 200 (a): under stirred conditions. (b):using 10 Na2O and 10 HMBr2.

Nature[crystalluutyl oftheprodmts ZSM-48 [97%] ZSM-48 [819'01 ZSM-48 [95%] ZSM-48 [88?!] ZSM-48 [8!3?!] ZSM-48 [63%] ZSM-48 [26%] EU-1 [78%]

Decomposit. of HM ions NO NO NO NO YES YES YES NO

In a previous work, we have shown that the decomposition products are also incorporated in the ZSM-48 framework 1141. The degradation of HM++ ions was shown to occur by nucleophilic substitution of the trimethylamine groups by NH3 [ 141. The decomposition is essentially influenced by the synthesis temperature and not by the actual Al content in the hydrogel (sample 11). On the other hand the channel dimension of zeolite ZSM-48 also seems to play an important role in the decomposition of the HM++ ions. Indeed, in the case of sample 13 which turns to be zeolite EU-1, the HM++ ions were found intact. We believe that the terminal trimethylammonium groups of each HM++ entity can be easily

161

accomodated in the side pockets of EU-1 channels so that their decomposition, probably otherwise not very favourable energetically, does not need to occur. Table 3 gives the chemical composition of the various of ZSM-48 samples. The amount of incorporated water per unit cell is similar in all samples, its low value confirming the hydrophobicity of the high-silica zeolite. Considering the low value of Na/u.c.. it can be supposed that the alkali cations do not play a major role in the crystallization of ZSM-48 [12,13,15,27]. In fact, when diamines (15,271 or even HM++ ions [12] are used, ZSM-48 crystallizes easily in absence of alkali cations. The alkali cations probably play a role on the nucleation rate, on the morphology and on the crystal size by neutrlizing the Si-0- defect groups along with the (Si-0-Al)-negative charge, when available. In all syntheses carried out in presence of HM++ ions (samples 8 and 9) the organic content per unit cell is close to one (Table 3). Most likely the organic cations stabilize the framework by a pore filling action and neutralize the (Si-0-All-negative charges and a part of the Si-0- defect groups. On the other hand the role of HM++ ions as counterions and as pore fillers is confirmed by the observation that in the presence of NH4+ ions, although the HM++ ions are decomposed, ZSM-48 is obtained. The presence of OcNH2 also leads to the formation of zeolite ZSM-48 (sample 14). In this case, it is supposed that the TMA+ ions, added to the hydrogel, along their possible pore filling action, essentially act as counter cations together with the Na+ ions, to the framework negative charges 14,151. The HM++ions insure a good filling of the intracrystalline pore volyme of zeolite ZSM-48, because the length of hexamethonium, is. 14.05 A, is close to the channel length of one unit cell of ZSM-48 (16.8 A). MorFover, considering that the diameter of one hydrated sodium ion is 4.6 A and that these ions are located in the channels one obtains a good pore filling. Sample 3 differs from the other samples (pore filling 75%), since in this case the contribution of the NH4+ ions present in the pore volume has not been taken into account. Higher values of pore filling are obtained for the ZSM-48 synthesized in presence of OcNH2 and TMA+ ions, since the dimension of the TMA+ ions insure an even better filling of the zeolitic channels. In the syntheses carried out in the presence of HM++ions only quite a number of defect groups are created, most likely due to the more bulky terminal trimethylammonium groups that have a larger dimepion (6.9 A) than the average ZSM-48 channel diameter (about 5.3 x 5.6 A) [131. To explain how the HM++ ions are responsable of the creation of framework defect groups, it must be supposed that the presence of each terminal trimethylammonium bulky groups generates a missing tetrahedral site in the zeolitic framework. This consequently corresponds to 4 defect groups (hydroxyl nests) per trimethylammonium group. For sample 8 that contains about one HM++ion per unit cell, 8 hydroxyl nests are created in the framework by the terminal ends of the template, the

Table 3 Crystallinity and chemical composition of various ZSM-48 zeolites. mole in hydrcgel

Sample

9 8(a) 3(a) 14

%Cryst.

Na

R

NH4+ Al

5 5 5 15

5 5 5 96

0 0

0 97% 0.5 75%

5

0.5 0.4

0

73% 85%

H20 (2)

Na (1)

Al (1)

HM* (394)

1.80 2.25 2.05

n.d 0.24 0.26 0.20

n.d 0.72 0.94 0.95

0.98 1.00

2.05

0.23

-

Composltionperunitcel HTMAm' TMAm' OcNH2 TMA' (3) (3) (2) (2)

0.54

0.26

-

0.20

1.90

Pore filling SiOR

Yo

(5) 11.1 10.1 5.1 2.0

82% 84%

68% 90%

R= HMBr2 for samples 3, 8 and 9, and OcNH2 + TMABr for sample 14. System of sample 3 . 8 and 9 : x Na2O y HMBr2 z (NH4)2O w A1203 60 Si02 3000 H2O System of sample 14: 15 Na2O 15.6 TMAI3r 80.4 OcNH2 0.48 A1203 10.8 H2SO4 60 SiO2 3258 H20 syntheses carried out at 160 f 2 OC under stirring conditions [ 151. (1): Evaluated by PIGE 1231 (2):Evaluated by TG-DTA 1141 (3):Evaluated by TG-DTA and 13C-NMR [14] (4):For sample 8 and 9 evaluated by TG-DTA and ammonia titration (131 (5):Evaluated by 29Si-NMR [21] (6):Percentage of filling as calculated by considering the total channel length of one ZSM-48 unit cell equal t.0 16.8 A and the length of each organic molecule respectively: HM++ = 14.05 A; HTMAm+ = 11.91 A: TMA+=TMAm+= 6.60 A and OcNH2 = 12.90 A [131 (a):The difference in the crystallinity percentage of samples 3 and 8, compared to the same samples in Tables 1 and 2, is due to the different synthesis times.

163

remaining 2.1 defects, as measured by 29Si-NMFt, are probably those that are usually statistically created throughout the framework at high synthesis pH values. Oppositely, in syntheses ran in the presence of NH4+ ions, a lower number of defects is detected (e.g. sample 3). Indeed, in that case, part of the HM++ ions is decomposed by NH4+, HTMAm+ yielding the equivalent a m o u n t of hexyltrimethylammonium a n d trimethylammonium "fragments". Because of their small steric dimension, these latter probably do not induce additional defects. The remaining "bulky" organics, namely 0.23 HM++ and 0.54 HTMAm+ present in the unit cell, respectively generate 0.23 x 2 x 4 = 1.84 and 0.54 x 4 = 2.16, thus a total of about 4, hydroxyl nests per unit cell. As for the preceding case, the remaining 1.1 defects stem from the synthesis condition and are statistically distributed along the framework. Finally, ZSM-48 crystallized from systems involving OcNH2 and TMA+ ions, do not show a large number of defects (Table 3, sample 14). Indeed, these molecules are not bulky enough to generate missing T sites, their actual dimension corresponding well to that of the average channel diameter, therefore ensuring their good fitting. On the other hand this synthesis requires a long synthesis time to obtain the crystalline ZSM-48 zeolite [151. The resulting ZSM-48 therefore contains a low amount of defects, this explaining well the relatively long crystallization time necessary to obtain a highly crystalline material [15], as also observed in the case of ZSM-5 [221. 4. CONCLUSION

The presence of NH4+ ions in the hydrogel involving HM++ ions reduces the cristallization rate of ZSM-48 zeolite in agreement with a previously publication [12]. On the other hand, ammonium ions also favour the partial decomposition of hexamethonium ions at high temperature. The decomposition occurs if Z S M - 4 8 is formed, while no decomposition is observed if the reaction mixture leads to EU- 1. It seems therefore that the zeolite plays a catalytic role in the decomposition of hexamethonium ions. I t is observed that a much larger relative amount of defect groups is formed when hexamethonium ions are incorporated in the ZSM-48 zeolitic structure. The results can be rationalized, if it is supposed that each terminal trimethylammonium groups leads to one missing tetrahedral site in the zeolitic structure, therefore inducing the formation of 4 hydroxyl nests. ACKNOWLEDGEMENTS

The authors are indebted to Mr. G . Daelen for his skillful help in obtaining the N M R spectra. They also thank Dr. P. Ratnasamy and Dr. R. Kermar for fruitful discussions.

164

5. 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

A. Araya and B.M. Lowe, Zeolites, 4 (1984) 280. J.L. Casci, Stud. Surf. Sci. Catal., 28 (1986) 215. B. Marler. Zeolites, 7 (1987) 393 N. Dewaele, 2. Gabelica, P. Bodart, J. B.Nagy, G. Giordano and E.G. Derouane, Stud. Surf. Sci. Catal., 37 (1988) 65. S.I. Zones, Zeolites. 9 (1989) 458. S . Emst, P.A. Jacobs, J.A. Martens and J. Weitkamp, Zeolites 7 (1987) 458. C. Pellegrino, R. Aiello and 2. Gabelica, in M.L. Ocelli and H.E. Robson (Eds.). Zeolite Synthesis (ACS Symp. Series 3981, Am. Chem. SOC.. Washington DC, 1989, p 161. R.A. Lefebre, H. Kouwenhoven and H. van Bekkum, Zeolites 8 (1988) 60. S. Emst, J. Weitkamp, J.A. Martens and P. Jacobs, Appl. Catal.. 48 (1989) 137. G.N. Rao, P.N. Joshi. A.N. Kotasthane and P. Ratnasamy, Zeolites, 9 (1989) 483. C.A. Fyfe, G.T. Kokotailo. H. Strobl, C.S. Pasztor, G. Barlow and S. Bradley. Zeolites 9 (1989) 531. G.W. Dodwell. R.P. Denkewicz and L.B. Sand, Zeolites, 5 (1985) 153. G. Giordano, J. B.Na@. E.G. Derouane, N. Dewaele and 2. Gabelica, in M.L. Ocelli and H.E. Robson (Eds.), Zeolite Synthesis (ACS Symp. Series 398), Am. Chem. SOC.. Washington DC, 1989, p 587. G. Giordano, N. Dewaele, 2. Gabelica, J. B.Nagy and E.G. Derouane, Appl. Catal., in press. G. Giordano, Z . Gabelica, N. Dewaele. J. B.Nagy and E.G. Derouane, Proc. Int. Symp. Chemistry of Microporous Crystals, Tokyo, J u n e 26-29, 1990. in press, and references cited therein. X. Wenyang, L. Jianquan and L. Guanghuan, Zeolites 10 (1990) 753. J. Quingzhu and P. Wenqin, Huaxue Xuebao, 48 (1990) 761, (C. Abstr.) 113, 2146986, 1990. H. Qisheng, F. Shduhua and X. Ruren. J. Chem. SOC. Chem. Commun. (1988) 1486. 2. Gabelica, N. Blom and E.G. Derouane, Appl. Catal., 5 (1983) 227. R.A. van Santen, J. Keijsper, G. Ooms and A.G.T.G. Kortbeek, Stud. Surf.Sci. Catal., 28 (1986) 169. J. B.Nagy, P. Bodart, H. Colette, J. El-Hage Al-Asswad. 2. Gabelica, R. Aiello, A. Nastro and C. Pellegrino. Zeolites, 8 (1988) 209. J.M. Chezeau, L. Delmotte, J.L. Guth and Z . Gabelica, Zeolites in press. G. Debras, E.G. Derouane, J.P. Gilson, 2. Gabelica and G. Demortier. Zeolites, 3 (1983) 37. A. Nastro, C. Colella and R. Aiello. Stud. Surf. Sci. Catal., 24 (1985) 39. G. Bellussi, G. Perego, A. Carati, U. Cornaro and V. Fattore, Stud. Surf.Sci. Catal., 37 (1988) 37. R. Mostowicz , personal comunication to one of the authors. A. Araya and B.M. Lowe, J o u m . Catal., 85 (1984) 135.

PA. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991Elsevier Science Publishers B.V., Amsterdam

165

SYNTHESIS AND CHARACTERIZATION OF Cr-MODIFIED SILICALITE-1

U.Cornaroa, P.Jirub, Z.Tvaruzkovab

and K.Habersbergerb

a Snamprogetti, Via Maritano 26, S.Donato Mi., Italy The Heyrovsky Institute of Physical Chemistry, Dolejskova 3, Prague 8 , Czechoslovakia Abstract

A series of Cr-modified silicalites-1 was prepared and characterized. Acidic and dehydrogenating properties were observed. Electron-acceptor Cr(II1) species were detected by IR adsorption experiments. 1.INTRODUCTION

Few atoms are recognized as compatible with a Silicon based MFI framework topology: All Gal Fe, B, Ti, Ge among others, with different degrees of stability [1,2,3]. Cr2O3 [ 4 ] and Cr-based materials are well known catalysts in alcohol [5], alkane [ 6 ] dehydrogenation and olefins polymerization [7]. Cr-zeolites have already been recognized as interesting materials [ a ] . Cr(II1) is very likely to be unstable in framework tetrahedral coordination [ 9 ] , although isomorphous substitution has also been claimed [lo]. Aims of this work are: - To synthetize a series of Cr-modified Si-MFI zeolites, with attention paid to obtain homogeneous, XRD-single phase materials. - To characterize the nature of chromium sites using model chemical reaction and physico-chemical methods. 2.EXPERIHENTAL 2.1

Syntheses and preliminary characterization

Zeolites syntheses were carried out by mixing: Si(OC2H5), A1 free (Huls); N(Pr0p)qOH (aq.so1. 14% wt), alkaline ion free: Cr(N03)3 (Carlo Erba), ethanol and water. After hydrolysis of alkoxide the reactions mixtures were aged overnight and hydrothermally crystallized in static 250 ml autoclaves at 150 " C for 7 days. The products filtered and washed were dried at 120 "C. Thermal treatment in air at 550 "C burns out the organic ternplating agent. Samples in C form were obtained in this way. Calcined samples were ionically exchanged with NH4+ (1 mol/l, 10 cc/gr zeolite). By thermal treatment in air at 550 " C

166

samples in H form were obtained. Chromium content was determined by inductively coupled plasma analysis, silicon content gravimetrically. Structure characterization was carried out via MID-FTIR (KBr pellets, 0.3 % ) . Phase purity was also confirmed via XRD measurements. Morphology of crystals was observed via optical microscopy. 2.2 Physico-chemical characterization

The adsorption of NO and CD3CN were studied by FT-IR (Nicolet Mx-1E). Heatable, high vacuum IR cuvette with samples in the form of self supporting pellets(zl0 mg/cm2) were used. Before measurements, all samples were evacuated at 350 C overnight. In all the experiments the equilibrium amount of either NO (2 Torr) or CD3CN ( 1 Torr) was adsorbed at 25 C for 15 minutes and spectrum recorded. The normalized absorbance of the respective bands was measured after desorption at room temperature and 100 "C; its variation was selected to characterize the stability (strength) of the bond between the adsorbate and the sample. 2.3 Catalytic characterization

Ter-butanol dehydration and n-hexane cracking were studied on samples in H form, isobutane dehydrogenation was studied on dried samples activated in situ. Reactions were carried out on st.stee1 or pyrex integral, fixed bed, plug flow reactors at atmospheric pressure. Catalyst (1-2 cc) was crushed to 2 0 - 4 0 mesh size. On-line chromatographic analyses were carried out. Experimental conditions are outlined in Table 1. Kinetic constants were evaluated by applying eq.(l). Table 1 Catalytic tests i n flow reactors: Experimental corditions Acidic a c t i v i t y Activation : Conditions :

1. Ter-Butanol dehydratim

2. n-Hexane cracking

500 C'CI, Nitrogen flow T= 140-180 [ T I ; PtButOH" 0.5 CAtml;

500 pC1, Nitrogen flow T= 450-575 [ T I ; p n - ~ a =0.5 [Atml; PtOt= 1 Wtml; T= 2.3 + 3 [ g r * s e c * ~ c - ~ l

pH20=0.05 [ntd Ptot= 1 [ntml; T= 0.3+3 [gr*sec*cc-lI

Dehvdrogenatins a c t i v i t y 3. lsobutane dehydrogenation : P form, 3 0 4 0 msh Catalyst Activation : 550 [ ' C I Air flow, 650 [ " C I CH4 flow. Conditions : l= 580 ["Cl, pi^^= 1 tatml T= 9 Isecl

k7 = l n I l / ( l - X ) l 7

(1) apparent contact tim

tgr*sec*cc-l~

k kinetic constant [gr-l*sec'l*ccl X

reactant conversion

Ethylene oligomerization was investigated with a McBain balance in static arrangement at the temperature of 80 "C and pressure of 40 Torr. The mass increase of the sample was determined as a function of time, and from the data obtained the initial rate ra of oligomerization was calculated. The experimental details were published previously [ll].

167

3.RESULTS AND DISCUSSION 3.1 synthesis and preliminary characterization A series of Cr,Si-MFI zeolites was prepared from reaction mixtures with composition as reported in Table 2. Attention had to be paid in order to avoid polymeric Cr-oxide species formation [12]. Syntheses were carried out in the absence of alkaline metals and at a pH as low as possible. A pH varying from 9.2 in the absence of Cr (scpl0) and 8.2 ( Si/Cr =loo, scp4) was observed. Preparations at the lower pH values were scarcely reproducible and amorphous phases (scp42) were occasionally obtained. The structural characterization of synthetized materials was carried out via Mid-FTIR and XRD techniques. MFI structures were observed as evidenced by integrated absorbance ratio ( 5 5 0 / 8 0 0 cm-1) (Table 2 ) . No IR bands related to the presence of Cr were evident, nor were extraphases detected in XRD diffraction patterns. A monoclinic elementary cell was observed for samples in H form. Large hexagonal prismatic crystals were observed. (Table 2 ) . A comparison of Cr content for samples in C (calcined)and H form evidences Cr leaching in the ionic exchange step, suggesting a weak zeolite-Cr interaction (Table 2).

.

Table 2 Zeolites synthesis and preliminary characterization Preparations

Chemical analysis Si/Cr

sample Si/Cr TPAISi HzO/Si

pH

form C

form H

Characterization Cryst. Yield

[cl (548/797)

IAR

[dl form form

XRD

Cryst a 1s [el Morphology l u

C H [a1 [bl _ - - - - - _ _ _ _ - _ _ _ _ - - _ _ - ~ - - - - - - - - - - - - -- -.- _ _ _ _ _ _ _ _---_ _ --_ - _ _ _ _ _- _--scpl0 scpl3 scp7

00

0.05

500 300

scp4 scp42

100 100

0.05 0.05 0.05 0.05

40 40 40 40 40

9.2 8.8 8.3 8.1 8.2

00

00

2803 556 85 110

6348 928 248 146

77 75 81

1.5 1.6 1.5

m c m c

40 n.m.

1.1 0.1

or mc amorphous

or mc mmc

60 20 40 20

25 7 15 5

[a]: Reaction mixture molar ratios; [bl: (Zeolite ueight form C) / (Si02+ Cr2~),eact.mixt. *loo; Ccl: Integrated Absorbance R a t i o 1R band 550 crn-'/ 800 an-'; [dl: Symxtry of elementary c e l l . Orthorhorrbic (or), Monoclinic (mc); [el: Crystals dimensions [XI. Length (1). Width (u)

3.2

Physico-Chemical characterizations

(i) NO adsorption The IR spectrum of NO adsorbed on Cr,Si-MFI sample (scp4H) at 25 " C exhibits a weak band at 1757 cm-l (Fig.1). In the case of pure silicalite (where no NO adsorption takes place) , such band was not observed. From this it follows that in the case of Cr,Si-MFI sample this band The corresponds to a weakly bonded NO com lex with Cr(II1) normalized absorbance of the 1757 cm-y band ( 0 . 2 8 ) decreases after the desorption at 25 C by about 50%, after further desorption at 100 C by about 70%. A similar band, only shifted

.

168

towards higher wavenumbers (1780 cm-l), thus corresponding to a higher bond strength, was found after the adsorption of NO on Cr exchanged Si.Al-MFI zeolite (HCrZSM5), prepared and characterized according to [13]. We suggest in this case the formation of a similar surface complex Cr(III)--.NO [13], only more weakly bonded. This result seems to indicate that: - The predominant part of Cr(II1) present in the Cr,Si-MFI is accessible to the interaction with molecules in the gaseous phase in the same way as in the Cr exch,Si.Al-MFI sample, with Cr in cationic positions. - No Cr (or only a very small part of it) is inserted in the zeolite skeleton. Besides that, the presence of a minor part of Cr in a higher valency state (V,VI) (which does not form adsorption complexes with NO) cannot be excluded. Fig.

A

r-

I

Lu U

Fig. 2

1

ul I-

z

a

m

$In m

a

I

1700

1900

-1 crn

...

SO0

*

2000.

2500

zoo0

1

cm' Fig.1. I R spectra o f NO adsorption on Cr-SiLicalite: (1)adsorption of 2 (2) desorption o f NO a t 25'C. Fig.2.

15 min. (3) Oesorption of NO a t lOO'C,

IR spectra o f CD3CN adsorption on Cr,Si-MFI (scplrH),(A); and

TOW

of NO at 25-C.15 min.

1 hour. S i - M F I (scplOH),(B).

(1) Adsorption of CD3CN a t 25°C. 15 min, g l l o r r . (2) Oesorption o f CD3CN a t 25 " C , 15 min. (3) desorption o f CD3CN a t 100 "C, 15 min.

(ii) CD?CN adsorDtion The IR spectra of CD3CN adsorbed on a Cr,Si-MFI (scp4H) as well as those after its desorption at 25 and 100 "C, respectively, are presented in Fig.2. The corresponding spectra obtained with pure silicalite (scpl0) are also reported for comparison. In the spectrum three bands were found in the wavenumber range characteristic for adsorption complexes of CD3CN with proton-donor and electron-acceptor sites [14,15,16]. The band at 2300 cm-1 corresponds to the interaction of CD3CN with electron acceptor centers exhibiting a good thermal stability: the value of the normalized absorbance remains the same after the desorption at 25 C. The shift of the corresponding band towards lower wavenumbers (by

169

21 cm-1) indicates the minor strength of the electron acceptor centers when compared with those on HZSM5 [15]. The lower value of the normalized absorbance (0.15) of 2300 cm-1 band indicates also their lower concentration in comparison with the HZSM5 zeolite (0.75). On pure silicalite after the adsorption of CD3CN no bands in the 2300 cm-1 region were found. These centers are therefore connected with the presence of Cr in Cr-silicalite. In the case of the Cr-silicalite the other bands in the range 2260-2240 cm-1 correspond to the non- specific sorption of CD3CN and the VC-D vibration. Both bands are thermally unstable in the course of the desorption at both 25 and 100 C. No bands corresponding to the proton donor center were found in the spectrum. The conclusion drawn from NO-adsorption experiments may be completed by the statement that the active sites represented by the part of cr(II1) accessible to the interaction with NO and CD3CN have electron-acceptor properties. 3.3 catalytic Characterization

[i) Ter-Butanol dehydration Dehydration reaction was studied in order to evidence the occurrence of weak acidic sites. Expected first order kinetics was experimentally observed when plotting In (1/1-x) versus apparent contact time. Silicalite and Cr,Si-MFI samples were studied. Arrhenius plots Fig 3(a) allowed a determination of activation energy, summarized with kinetic constants ratios,(kcatalyst/ksc 10,at 180 "C) in Table 3. When comparing Cr,Si-MFI and silicayite catalysts? the same activation energy and an activity correlation with Cr content are observed. The higher loaded scp4 sample exhibits a decline in observed kinetic constant due to a decreased specific activity or number of active sites. No differences are observed in activation energy values thus reinforcing the latter hypothesis. Acidic activity is related to Cr sites whose strength cannot be differentiated from weak sites on silicalite by means of the easy dehydration reaction. As Cr loading increases,sites pairing is likely to occur, this resulting in a lower overall kinetic constant.

(ii) n-Hexane crackinq cracking reaction was studied to evidence the occurence of sites capable of a more acid strength demanding reaction. A first order kinetics widely used in the literature [17] allowed us to obtain kinetic constants and the activation energy. Results are summarized in Arrhenius plots Fig.3(b) and in Table 3 . Activation energy is reduced in the presence of Cr sites, their involvement in catalytic reaction is also evident when comparing correlation with (kcat/ksil)550 and Cr content. A Cr sites pairing effectl as previously discussed,is also evident in this case Products distribution analysis evidences two main effects (Table 3): - Aromatics selectivity increases with Cr content, thus suggesting the presence of dehydrogenating species related to Cr sites.

.

170

- Enhanced

p-xylene isomers selectivity compared to the thermodynamic one is observed, suggesting that the reaction takes place mainly in the pore system.

3

-2

2

1 Y

0

3-1 -2

-3

:zq\ \ ,

-4

u

fi -6

0

Fig.3.

SWIO

0

A

-8

Arrhmius p l o t s . (a) Ter-butanol dehydration. (b) n-Hexane cracking.

Table 3 Catalytic characterization: Acidic Properties. t-butanol dehydration Catalyst pmlCr/gr

Nature

Ea [a1 [Kcal/moll (Rk)18oaC

Ea tKcal/moll

n-hexane cracking [a1 Arm. (Rk)18o'C

Yield

p/o Xylme i s a x r a t i o [bl

--__-_-_______-_-_______________________------~--------------------------------------------------~-----scplOH scpl3H scp 7H scp 4H [a]: tbl:

0 3 18 67

Si-MFI Cr,Si-MFI

Cr,Si-MFI Cr,Si-MFI

23 28 26 27

1 4.4 16.4 11.1

45 26 23 20

1 1.2 6.8

7.2

.3 1.1 2.4 10.7

__

2.6 6.7 10.1

(Rk)TeC = ( k catalyst/ k scplo), k i n e t i c constant evaluated a t the tenperature T; (p/o Xylene)5500~, thenodynamic e q u i l i b r i u n r a t i o =0.91

(iiil Iso-Butane Dehvdroqenation Dehydrogenation activity was further characterized by comparing isobutane reaction on silicalite (scpl0): Cr,Si-MFI (scp7,scp4): cr/siO2 (scp42) and on a traditional Cr/A1203 (ca) catalyst, prepared according to [18]. Results are outlined in Table 4. Cr modification of silicalite results in an increase of both isobutane conversion and isobutene selectivity, thus evidencing Cr involvement in dehydrogenation reaction. Effects of crystalline versus amorphous matrix in determining acidic versus dehydrogenating properties appear when comparing Cr,Si-MFI and cr-sioz samples. Higher yield ratios, both for cracking and skeletal isomerization products on Cr,Si-MFI than in Cr/SiO2 or Cr/A1203 are observed. Appearance of isomerization products suggests the presence of acidic sites, also enhancing cracking reaction. Higher conversion and isobutene yields per pmol of Cr

171

(Table 4 ,X/pmol; Y/pmol) are observed on Cr,Si-MFI samples compared with the conventional catalyst. Table 4 lsotutane dehydrogenation.

scp42P ca

150 2589

Cr/Si02 Cr,K/AL2$

17.7 48.9

83.4 91.3

0.11 0.10

0.09

0.06

0.16

0.01

0.01

0.09

[a1 : lsobutane conversion / w o l of Cr; [bl: lsotutene Yield / p o l of Cr; [cl: (Linear C4 y i e l d ) / lsobutene yield; [dl : (Cl+C2+C3) yield/ Isotutene y i e l d

[iv) Ethvlene oliqomerization The values of mass increase of samples during the oligomerization as a function of time are presented in Fig.4 in the case of CrfSi-MFI(scp4H), pure silicalite, (scpl0) and the Cr exchanged Si,A1-MFI (HCrZSM5) The values of ro for the individual curves are also given. For Cr,Si-MFI the value ro=1.0*10-2 [min-l] is significantly higher than for pure silicalite where practically no C2Hj oligomerization takes place (r*<<0.1*10[min-ll) The centers for C2H4 oligomerization on Cr-silicalite are electron acceptor sites represented by the accessible Cr (111), as also evidenced with NO and CD3CN adsorption experiments. With Cr exch,Si.Al-MFI, a significantly higher value of ra=8.3*10-2 [min-l] was found, this value being clearly due to the effect of active sites represented by the Cr (111) together with the electron-acceptor and min. proton-donor centers of the original Si.A1-MFI [ 13 3

.

.

.

Fig.4 Zeolite ueight increase Cut%) i n t i m e dependence on ethylene i n t e r a c t i o n u i t h z e o l i t e (at 80°C and 40tOrr): (l),scp4H (2). HCrZSM5 (3)

4.CONCLUSIONS

Syntheses of crystalline, XRD-single phase, Cr,Si-MFI materials have been achieved from l o w pH, Na free, reaction mixtures. Cr(II1) framework substitution seems unlikely. No effects of Cr content on unit cell symmetry or on IR spectra compared with the silicalite are evident. Accessible Cr (111) cationic species have been detected by IR adsorption experiments. Cr

172

sites clustering within the zeolites channels are not considered to be relevant because of the absence of XRD visible extra phases and shape selective effects observed in n-hexane cracking. Chromium modified zeolites exhibit acidic-dehydrogenating bifunctional catalytic properties. The dehydrogenation activity is well known to be related with coordinatively unsaturated Cr(II1) species [6]. Accordingly, both from NO and CD3CN adsorption, we have experimental evidence of the presence of Cr(II1) sites with electron acceptor properties. The nature of sites active in acid catalyzed reactions remains open. Coordinatively unsaturated Cr (111) species can act as Lewis acid in n-hexane cracking via hydride ion abstraction. Bronsted sites are probably easily formed in dehydration reaction due to the presence of water. Effects of Cr (VI) species cannot be excluded at this point. The decline of acidic activity with Cr loading has been attributed to a site pairing effect. Aknowledgments Thanks are due to Dr.R.Iezzi (Snamprogetti) for suggestions , Dr.G.Bellussi (Eniricerche) for comments and Dr R.Millini (Eniricerche) for XRD measurements. 6.BIBLIOGRAPHY 1 2 3

4

M.F.M.Post, T.Huzinga, C.A.Emeis,J.M.Nanne and W.H.J.Stork, Stud.Surf.Sci.Cata1.; 46 (1989) 365 C.T.W.Chu and C.D.Chang, J.Phys.Chem., 89 (1985) 1569 G.Perego, G.Bellussi, C.Corno, M.Taramasso, F.Buonomo and A.Esposito, Stud.Surf.Sci.Cata1, 28 (1986) 129 R.L.Burwel1, G.L.Haller, K.C.Taylor and J.F.Read, Advan.Catal.Re1at.Sub-j.:

5 6 7 8 9

20 (1969) 1

K.Yamashita, S.Naito and K.Tamura, J.Catal., 94 (1985) 353 S.Carra and L.Forni, Catal.Rev.- Sci.Eng., 5 (1972) 159 M.P. Mc Daniel, Advan.Catal.Relat.Subj., 33 (1985) 47 M.R.Klotz, U . S . Patent 4,299,808, Standard Oil, 1981 K.G.Ione, L.A.Vostrikova and M-Mastikin,J.Mol.Catal., 31 (1985) 355

10 11

T.S.R. Prasada Rao and R.B.Borade, Chem.Expr., 1 (1986) 709 Z.Tvaruzkova and B.Wichterlova; J.Chem.Soc.Faraday Trans.1,

12 13

P.Shu,Proc.Am.Chem.Soc.,Div.Petr.Chem.,Toronto

(1983) 1591

14 15 16

17 18

(1988) p.43

B.Wichterlova, Z.Tvaruzkova, L.Krajcikova and J Novakova, Stud.Surf.Sci.Catal., 18 (1984) 249 C.L.Angel1 and M.V.Howel1, J.Phys.Chem, 73 (1969) 2551 Z.Tvaruzkova, K.Habersberger and P.Jiru, Abstracts Conf.Zeocat 90, Leipzig (1990) p. 272 P.Jiru, Z.Tvaruzkova and K.Habersberger, Stud.Surf.Sci.Catal., 55 (1990) 317 J.N.Miale, N.Y.Chen and P.B.Weisz, J.Catal., 6 (1966) 278 F.Buonomo, R.Iezzi and M.Kotielnikov, U . S . Patent 4,746,643, Snamprogetti-Niimsk (1988)

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

SYNTHESIS, CHARACTERIZATION AND CATALYTIC ACTIVITY OF V-ZSM-5

P.

Fejes,

173

ZEOLITES

I. Marsi, I. Kiricsi, J. Halasz, I. Hannus, A. Rockenbauerx ,

*

Gy. Tasi, L. Korecz and Gy. SchBbel Applied Chem. Dept., Jdzsef Attila University, Szeged, Hungary

*Central

Kesearch Inst. of Chem., Hung. Acad. Sci., Budapest, Hungary

Abstract

A V-ZSM-5 sample with a Si/V 42 was synthetized outgoing from VO(COO), and 4-brand sodium silicate using TPA-Br as template. ESR spectroscopy proved that vanadium(1V) ions in the zeolitic framework exhibit a distorted square planar symmetry. Upon heat treatment a part of the framework vanadium ions migrate to extra-framework positions. After dehydration no Bronsted acidity was found. Treatment in oxygen and hydrogen above 570 K revealed the redox character of the V-ZSM-5 sample. In oxidation of n-butane (as catalytic test reaction) the V-ZSM-5 zeolite exhibits selective dehydrogenation and aromatization activity. INTRODUCTION

Materials with zeolitic structures can be grouped into t h r e e distinct categories on the basis of net electric charge on their frameworks: (i) no charge (like silicalites or ALPO-s of ideal composition); (ii) common zeolites have excess negative charge which is compensated by cations; (iii) net positive charge compensated by anions. "Nature" seems to prefer classes (i) and (ii) and dislikes structures in (iii). There are several ways to endow frameworks with net positive charge from direct synthesis to structure modifications in different media. Most promising to achieve this goal by outgoing from zeolitic structures falling into categories ( i ) and (ii) which contain framework ions with several (but at least two) oxidation states. The more o r less stable as synthesized form can be brought into an "out of balance" condition by artificially changing the oxidation state of the (mostly transition-) metallic component. In this paper results obtained in the course of synthesis and characterization of V-containing ZSM-5 zeolites are discussed.

174 EXPERIIMENTNA V4+(f)-ZSM-5

sample was synthesized, t h e Si02/V02 r a t i o o f which

was equal t o 42 by X-ray

fluorescence analysis. This value was very close

t o t h a t ( S i / V = 40: reported by A . Miyamoto e t a l . The synthesis was c a r r i e d out a t pH = 9.1, and @brand sodium s i l i c a t e , using TPA-Br seeds ( 4 mass% o f t h e product).

[ 11. outgoing from

VO(COO)2

as template and by adding ZSM-5

The batch composition o f the synthesis

slurry was: 1.0 Si02, 0 . 2 V02, 39.2 H20, 0.3 TPA-Br and c r y s t a l l i z a t i o n continued f o r 10 days a t 433 K under vigorous mixing. The product was f u l l y c r y s t a l l i n e by X-ray d i f f r a c t i o n and had ZSM-5 s t r u c t u r e i n both the "as synthesized"

(A.S.)

and "burned o f f " (8.0.)

forms .

For c h a r a c t e r i z a t i o n we used I R and ESR spectroscopy. ESR spectra 2+ were recorded on a JEOL-FE-3X type spectrometer i n the X-band u s i n g Mn i n MgO m a t r i x f o r f i e l d c a l i b r a t i o n . For I R s t u d i e s KBr p e l l e t and z e l f supporting wafer techniques were u t i l i z e d . Adsorption o f

N2

and solid-gas

(02,H2) interaction: were i n v e s t i g a t e d i n a h i g h vacuum equipment supplemented w i t h MS gas analysis. As teEt r e a c t i o n , the o x i d a t i o n o f n-butane was used and i t was

car-

r i e d o u t i n an impulse r e a c t o r ; the r e a c t i o n products were analyzed by gas-chromatography;

conversion and product y i e l d s were used t o characte-

r i z e the c a t a l y t i c p r o p e r t i e s .

RESULTS AND DISCUSSION ESR investigations The f i r s t question t o d e a l with concerns t h e symnetry o f V4+,

('do2+)

i o n together with i t s l i g a n d s i r r e s p e c t i v e o f the f a c t whether t h i s e n t i t y is

part

o f the

F4+(ex)].

As t h e

z e o l i t i c framework

V4+(f)

[V4'(f)]

spectra show a x i a l

or i s i n exchange p o s i t i o n symmetry t h e r e is only one

p o s s i b i l i t y , t h a t o f square planar arrangement f o r t h e c e n t r a l V4+ i o n and

(or C 4v ) as proposed by Kucherov This suggestion i s , however, i n c o n t r a d i c t i o n t o t h e w e l l

i t s (0) ligands, i . e . the symmetry i s D4h e t al.

[

2,3].

known f a c t t h a t i n z e o l i t i c frameworks the e l e c t r i c a l l y n e u t r a l {V4+(f)

.O

175 o u i l d i n g elements have t e t r a h e d r a l (Td)

symmetry.

I n e x p l a i n i n g t h e ESR s p e c t r a o f V4+,

(VO

2+ ) i o n s i n d i f f e r e n t "sol-

use i s made of t h e e i g e n v a l u e s of the t i m e independerit H a r n i l t o n i a n

vents'',

which can be expressed as [41: h x 9 = g x p % H + &mI

(1)

where h x 3 : t h e microwave energy; p l i n g tensors;

H: t h e

d.c.

9 and

t h e n u c l e a r quantum number ( f o r 51V

(VO

A:

magnetic f i e l d ; n u c l e i mI

t h e g- and t h e h y p e r f i n e cou-

/3:

t h e Bohr magneton and m:I

= 7/2,.

. . ,-7/21. V4+,

Some e a r l i e r r e s u l t s and o u r d a t a on t h e ESR parameters o f 2+ . ) i o n s o b t a i n e d i n d i f f e r e n t media a r e summarized i n T a b l e 1.

Table 1 ESR parameters of V - c o n t a i n i n g m a t e r i a l s Compound

Solvent

1 '

AH A, <97 (A7 (gauss) ( g a u i s ) 1.962 118.0

[4]

Ref.'

vo2+

A

'11 -

VO - a c e t y l ace t o n a t e

8

1.944

1.996

191.2

68.1

1.979

103.1

[5]

V4+-ZSM-5

-

1.93

2.02

198.0

83.0

1.99

121.3

[2]

V4+-HY -3

-

1.938

1.977

192.0

73.0

1.?64

112.7

[6]

2+

-

V02+(ex)-ZSM-5 V4+(f)-ZSM-5 V02+(ex) -Nay

(A.S.) -

-

1.963

2.007

190.8

75.4

1.992

113.9

[x]

1.949

1.990

185.G

72.5

1.976

110.3

[%]

1.963

2.007

190.8

75.4

1.992

113.9

[*I

A: w a t e r ; 8: 60% c h l o r o f o r m + 40% t o l u e n e ; [x]

t h i s publication

:

As concerns z e o l i t e s , as " s o l v e n t s t t , t h e e a s i e s t way t o b e a b l e t o d e c i d e whether t h e V

4+,(V0 2+) i o n s were

i n t h e framework

or i n exchange

p o s i t i o n was t o exchange Na+ i n a Na-ZSM-5 z e o l i t e o f s i m i l a r S i / A l r a t i o f o r V02+ i o n s and t o compare t h e V02+(ex) and V 4 + ( f ) I n t h e ESR s p e c t r a o f

VO2'(ex)-ZSM-5

and

spectra. V4+(f)-ZSM-5

zeolites

(which were r e g i s t e r e d a t ambient t e m p e r a t u r e ) n o t a b l e d i f f e r e n c e s c o u l d

be observed i n t h e p a r a l l e l components o f t h e g t e n s o r and i n t h e elements on t h e main d i a g o n a l s o f t h e h y p e r f i n e c o u p l i n g t e n s o r s ( s e e T a b l e 1). these caused a d i s p l a c e m e n t o f t h e V4+(f)-ZSM-5 as can be seen i n F i g . 1.

All

spectrum t o h i g h e r f i e l d s

176

i

!

Figure 1. Term schemes o f V4+(ex) and V

4+

(f)

The s h i f t o f the 1 - s t p a r a l l e l l i n e s was 51.9 gauss which diminished t o 15.5 gauss f o r the 8 - t h l i n e s due t o the greater width o f the V02+(ex)-

ZSM-5 spectrum. The reduced values o f t h e

9 and

A tensors

f o r the V 4 + ( f )

specimen can

be i n t e r p r e t e d e i t h e r by a lesser separation o f the vanadium d o r b i t a l s

or

by the weakening o f t h e covalent character of the V-0 bonds. Both e f f e c t s should be reckoned with i f the c e n t r a l V i o n leaves the O4 p l a i n (pyramid a l d i s t o r t i o n ) , o r the

O4 p l a i n experiences a t e t r a h e d r a l d i s t o r t i o n . Ne-

vertheless, the measure of t h i s i s s u r p r i z i n g l y small. The h y p e r f i n e coupling constants (by the r e d u c t i o n o f the Fermi cont a c t term) substantiate t h i s view.

51V

MAS-NMR spectra r e v e a l by a well-resolved absorbtion peak a t

1344 pprn t h a t a s u b s t a n t i a l p a r t o f the vanadium i s i n framework p o s i t i o n ( f u r t h e r d e t a i l s are t o be published l a t e r ) . I n summing up, we can conclude t h a t V4+ i o n s i n z e o l i t i c frameworks e x h i b i t a s l i g h t l y d i s t o r t e d square planar symmetry. Whether the d i s t o r t i o n is pyramidal or t e t r a h e d r a l cannot be answered y e t .

Oxidation-reduction properties The A.S.

V4+( f)-ZSM-5

samples have reddish-yellow colour which t u r n s

t o white a f t e r burning o f f the template above

770 K i n a i r o r oxygen. The

177

s p i n concentration i n the A . S .

specimen i s about 2 . 3 3 ~ 1 0g~ - l~ which 1s

reduced t o p r a c t i c a l l y n i l i n the B.D.

samples.

ESR spectra taken on specimens which were incompletely oxidized and s t i l l i n contact with oxygen show the c h a r a c t e r i s t i c spectrum o f 0;. r a d i c a l w i t h g = 2.0027 l i n e s unresolved),

( t h e ,g ,

ion

s e t p a r t l y r e s o l v e d , t h e gxx and g

i n other words the {V4+(

f ) .D2{ framework-elements

YY in-

t e r a c t with gaseous O2 l i k e :

; s o r t o f degradation of t h e 0

A t higher temperatures a

r a d i c a l takes

p l a ce. The n e t p o s i t i v e charge of t h e framework caused by t h e presence of v5+ . ions can be compensated by 0’- ions onl y . I n c o n t a c t w i t h water vapour they undergo hydration:

[Z

{V5+(f).D2}].02-

+ H20

-

2

{ V5+(f).D2)+.0H-

(3)

causing a change i n t h e colour from white t o y e l l o w . The e f f e c t o f hydrat i o n i s c l e a r l y recognizable i n the I . R .

s p e c t r a o f these specimens (see

l a t e r ) . S t i l l , t h e behaviour of these oxidized and hydrated samples i s en i gmatic: th e OH- ions cannot be exchanged

for o t h e r anions, i . e . they ex-

h i b i t o n l y a & ( - ) p a r t o f t h e elementary n e g a t i v e charge. S i m i l a r l y , i f t h e o x i d a t i o n o f V4+

i o n i s c a r r i e d out with halogens, l i k e bromine, i n

aprotic solvents as medium:

and t h e r e a f t e r water i s added t o the dry, o x i d i z e d sample t h e d i r e c t i o n o f e quation (4) reverses and release of f r e e , elementary bromine takes place. UV-visible spectra,

r e g i s t e r e d by

Miyamoto e t a l . [l]

,

show t h a t

h y d r a t i o n s h i f t s the main t r a n s i t i o n s f r o m the UV r e g i o n t o l o n g e r wavel e n gths and the specimen shows the complementary c o l o u r which i s y e l l o w .

I t means i n o ther words t h a t the yellow c o l o u r i s caused by h y d r a t i o n and

not by t h e o x i d a t i o n s t a t e o f vanadium i o n s . Upon dehydration o f 8 . 0 . V(f)-ZSM-5

z e o l i t e s ( y e l l o w ) a t 570 K or a-

bove i n vacuo, the colour o f the samples t u r n s t o grey, t h a t i s an i n d i c a t i o n f o r fo rmation of a mixed o x i d a t i o n s t a t e o f same

time COX

(by t h e

o x i d a t i o n of

vanadium i o n s . A t t h e

trace amounts o f carbon l e f t behind

178

from t h e template) appears i n t h e mass spectrum of t h e g a s phase, followed by a slow r e l e a s e of H2 ( a t h i g h e r t e m p e r a t u r e s ) . The white c o l o u r ' of t h e samples can be r e s t o r e d by h e a t i n g e i t h e r i n H2 or i n O2 a t 670 K or above. These t r e a t m e n t s l e a d probably t o a homogeneous o x i d a t i o n s t a t e of V . By our results t h i s c o l o u r change seems t o be reversible. The sequence of e v e n t s can be v i s u a l i z e d a s follows: t h e r e l e a s e of oxygen i s caused by a well-known phenomenon i n z e o l i t e c h e m i s t r y , t h e s o c a l l e d " a u t o r e d u c t i o n " , d e s c r i b e d f i r s t by Beyer e t a l . [7] :

The u n s t a b l e {V3'(f).0]+

e n t i t y g e t s pushed o u t from t h e framework

with a r e l e a s e of l a t t i c e s t r a i n . The mobile {V3'(f).$*OH-

s p e c i e s form

clusters ( r e c o g n i z a b l e i n t h e r e s p e c t i v e ESR s p e c t r a ) i n t h e v i c i n i t y of still e x i s t i n g V5+(f) s i t e s c o n t r i b u t i n g t o t h e development of grey ( b l a c k ) c o l o u r ( o v e r l a p of o p t i c a l t r a n s i t i o n s ) .

The r e l e a s e of H2 a t

h i g h e r temperatures o r i g i n a t e s from t h e homolysis of w a t e r ; t h e p a r a l l e l 3+ , V4+ s p e c i e s reduces t h e number of c o l o u r

o x i d a t i o n of extra-framework V centers.

F i g . 2 shows an ESR spectrum of a sample exhibiting grey colour. The magnified p a r t s ( a t low and high f i e l d s ) d i s c l o s e t h e presence of a V4+ s p e c i e s i n extra-framework p o s i t i o n i n s m a l l q u a n t i t i e s . As V3+ i o n s have very s h o r t

spin-lattice

r e l a x a t i o n time even a t l i q u i d n i t r o g e n tempera-

Figure 2. ESR spectrum of V(f)-ZSM-5 a t 77 K

179

ture,

t h i s f e a t u r e of t h e

spectrum is

c e r t a i n l y not c a u s e d by V

3+ . ions.

Due t o t h e g r e a t e r w i d t h o f t h e s p e c t r u m f o r t h i s extra-framework paramagn e t i c s p e c i e s t h e s e q u e n c e of l i n e s is r e v e r s e d a t h i g h f i e l d s .

IR studies Only t h e band of s i l a n o l g r o u p s a b s o r b i n g a t 3740 cm-l c o u l d b e c l e a r l y o b s e r v e d on s a m p l e s

which were a c t i v a t e d

i n vacuo o r i n O2 a t 673 K .

A f t e r r e h y d r a t i n g t h e s a m p l e s t h u s p r e t r e a t e d t h e n e v a c u a t e d a t 723 K show a broad OH band c e n t e r e d a t 3600 cm-l i n a d d i t i o n t o t h e SiOH band. The ac i d i t y tests, i n which p y r i d i n e was used a s p r o b e , showed p r a c t i c a l l y o n l y

Lewis a c i d i t y .

The band due t o

B r o n s t e d a c i d i c sites a t 1550 cm-l

was

d e t e c t a b l e b u t n o t e v a l u a b l e . From t h e s e r e s u l t s , it can b e c o n c l u d e d t h a t t h e c o n c e n t r a t i o n of B r o n s t e d a c i d i c s i t e s is n e g l i g i b l e i n t h e d e h y d r a t e d s a m p l e s i n d e p e n d e n t l y of t h e p r e t r e a t m e n t c o n d i t i o n s .

Adsorption studies The s p e c i f i c s u r f a c e a r e a of t h e V(f)-ZSM-5

sample i s less by a b o u t 2 m / g ) t h a n t h a t o f a s i m i l a r s t a n d a r d H-ZSM-5 specimen (440.2 mL/g). The N2 i s o t h e r m a t l i q u i d n i t r o g e n t e m p e r a t u r e e x h i b i t s a s l i g h t l y 20% (307.0

s i n u s o i d c h a r a c t e r a t a b o u t p/po

0.5 r e l a t i v e pressure.

No hydrogen o r oxygen u p t a k e c o u l d b e o b s e r v e d below 570 K .

A t 670 K

( c o n t r a r y t o K u t c h e r o v ' s f i n d i n g s [3]) t h e hydrogen a d s o r p t i o n of t h e ( a t 3 t h e same t e m p e r a t u r e ) p r e o x i d i z e d sample ( 2 . 2 1 cm /g S.T.P.) was n e a r l y 3 twice t h a t of oxygen (1.10 cm /g S . T . P . ) i n d i c a t i n g a r e a l r e d o x behav3 i o u r . On t h e b a s i s of s p i n c o n c e n t r a t i o n a v a l u e of 4.33 cm /g S.T.P. c o u l d b e c o n t e m p l a t e d f o r H2 a d s o r p t i o n r e v e a l i n g t h a t a b o u t 50% of t h e vanadium i o n s a r e i n v o l v e d i n t h i s redox r e a c t i o n .

Oxidation of n-butane As c a t a l y t i c

t e s t of t h e V - c o n t a i n i n g z e o l i t e s t h e o x i d a t i o n of n-bu-

t a n e was used w i t h t h e i n l e t g a s - c o m p o s i t i o n of 10% n - b u t a n e , 10% 02, and 80% N2. A s u r v e y of t h e c a t a l y t i c a c t i v i t y is t o b e s e e n i n F i g u r e 3/a f o r

s a m p l e s c o n t a i n i n g V i n d i f f e r e n t p o s i t i o n s . (VC13(ex)-ZSM-5 sample w i t h " s o l i d exchange" u s i n g a n h y d r o u s VC13.)

means h e r e a

180 1

I

-I 204

b

I

C

.\*

F i g u r e 3: a : conversion of n-butane over d i f f e r e n t c a t a l y s t s (0: V205; 8 : H-ZSM-5;

0:

V(f)-ZSM-5;

b: y i e l d over V(f)-ZSM-5;

h: a r o m a t i c s ;

0:

dehydrogenates;

n:

0:

VC13(ex)-ZSM-5; 0 : V(ex)-ZSM-5)

c : y i e l d over VC13(ex)-ZSM-5 oxygenates; 8: c r a c k prod. ; v: COX)

The c a t a l y t i c a c t i v i t y of V(f)-ZSM-5 is r e l a t i v e l y low, however, t h e s e l e c t i v i t y f o r dehydrogenates (n-butenes, b u t a d i e n e ) and a r o m a t i c s (rnainl y xylenes) i s q u i t e good, a s shown i n F i g . 3/b.

VClj(ex)-ZSM-5,

a s ESR

s p e c t r a convincingly show, c o n t a i n s V4+ i n both framework and exchange pos i t i o n s . It is capable t o a c t i v a t e r e a c t a n t s with e a s e ( F i g . 3 / c ) , but t h e product d i s t r i b u t i o n is broad, and a t h i g h e r t e m p e r a t u r e s t o t a l o x i d a t i o n p r e v a i l s r e s u l t i n g i n COX and H20. ACKNOWLEDGMNT

The g r a n t (OTKA No. 693/86) of t h e Hungarian Academy of S c i e n c e s is g r a t e f u l l y acknowledged, The a u t h o r s e x p r e s s t h e i r s i n c e r e thanks t o Professor 3. 8. Nagy

( U n i v e r s i t y Notre-Dame de l a P a i x , Narnur, Belgium) f o r h i s h e l p i n regist e r i n g t h e 51V MAS-NMR s p e c t r a . REFERENCES

1 A . Miyamoto, Y. Iwamoto, H . Matsuda and T. I n u i , Stud. S u r f . S c i . C a t a l . , 49 (1989) 1233. 2 A.V. Kucherov and A.A. S l i n k i n , Z e o l i t e s 7 (1987) 583. 3 A . V . Kucherov and A . A . S l i n k i n , Z e o l i t e s 7 (1987) 38. 4 R . N . Rogers and G.E. Pake, J . Chem. Phys., 33 (1960) 1107. 5 H . R . Gersrnann and J.O. Swalen, J . Chern. Phys., 36 (1962) 3221. 6 M. Huang, S. Shan, C . Yuan, Y. L i and Q. Wang, Z e o l i t e s 10 (1990) 772 7 H . K . Beyer, I. Bica and P.A. Jacobs, Magy. K6m. F o l y . , 83 (1977) 34.

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

181

A STUDY OF ACID SITES IN SUBSTITUTED AIPO-5 R.J. Gorte, G.T. Kokotailo, A.I. Biaglow, D. Parrillo, and C. Pereira

Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA Abstract We have examined temperatured-programmed desorption (TPD) and thermogravimetric analysis (TGA) of isopropylarnine on a series of Si-, Co-, and Mgsubstituted AIPO-5 samples. The TPD-TGA results on the substituted samples show ammonia and propene desorbing in a well-defined feature between 575 and 650K, a feature not observed on pure AIPO-5. The results suggest that TPD-TGA measurements of isopropylamine may be useful in determining the framework concentrations and acid site densities for SAPO-5, CoAPO-5, and MAPO-5. 1. INTRODUCTION The acidity of metal-substituted,aluminophosphate molecular sieves is not well understood [l]. There is no clear picture of what site concentrations are present in these materials, how strong the sites are, or whether one should even think in terms of discrete sites. The effects of various calcination and pretreatment conditions are also difficult to quantify. In this paper, we have tried to address these questions by examining the adsorption of simple amines in materials formed by the substitution of Mg, Co and Si into an AIPO-5 structure. The main technique that we have used to study adsorption is simultaneous temperature programmed desorption/thermogravimetric analysis (TPD-TGA). In previous work on the adsorption of amines in H-ZSM-5 and other high-silica zeolites [2-41, it was shown that well-defined adsorption complexes, in a coverage of one/framework Al, could be identified in the TPD-TGA results. For all of the simple amines except methylamine [4], the 1 :1 complex decomposed to ammonia and an olefin, while molecules adsorbed at a coverage in excess of one/Al desorbed unreacted. The decomposition to an olefin and ammonia appears to be similar to the Hofmann elimination reaction which is observed for quaternary ammonium salts, suggesting that reaction of the amines occurs through an ammonium ion which, in turn, would require the presence of a strong Brsnsted site. Since the activity of H-ZSM-5 increases linearly with Al content for several important reactions [5], it appears that this high-silica zeolite has discrete acid sites in a concentration of one/Al.

182

Our hypothesis for studying the substituted AIPO-5's was that these materials might be similar to the high-silica zeolites. It is known that pure AIPO-5 is not acidic and is analogous to ZSM-5 with no Al. The replacement of Al+3 by Co+2 and Mg+2 should result in an ion exchange site capable of holding a proton. The addition of SP4 is slightly more complex due to the fact that Si can substitute for P+5 to give an exchange site or a pair of Si atoms can exchange for an ALP pair, resulting in a neutral framework. If this simple picture gives an accurate description of the acid sites, the TPD-TGA results for simple amines should exhibit well-defined features and the number of molecules corresponding to these desorption features should vary with the framework concentration of the substituted atoms. This is indeed what we found for low concentrations of substituted atoms. In addition to demonstrating that discrete acid sites are formed upon substitution into the AIPO-5 framework, we made two other important observations. First, the calcination conditionk used for preparing all of the materials was critical. This had been reported previously for CoAPO-5 [6], but we found that it was equally true for SAPO-5. Second, materials prepared with high concentrations of either Co+2 or Mg+2 tended to have many defects. These materials exhibited reasonable pore volumes as measured by 0 2 uptakes; however, larger molecules were unable to enter the structure. 2. EXPERIMENTAL

The equipment and procedures have been described in detail elsewhere [2-41. The apparatus consists of a microbalance which is mounted inside a high vacuum chamber that can be evacuated to a background pressure of 10-7 torr. A quadrupole mass spectrometer, interfaced to a microcomputer, was used to sample the desorbing products. Approximately 15 mg of sample were spread in a thin layer over a flat sample pan in order to minimize bed effects in desorption [7,81.The TPD-TGA experiments were performed with a heating rate of 10Wmin. lsopropylamine was the molecule which was studied in most detail, and all of it was found to desorb completely during the experiment. The samples were synthesized under hydrothermal conditions with a tetraethylammonium hydroxide template. Each sample was characterized by x-ray diffraction and only peaks corresponding to the substituted AIPO-5 structures were observed. The concentrations of Co, Mg, and Si for each sample were determined by ICP and are reported in Table 1, along with the gel concentrations, as Me/(Me+AI+P). All samples were calcined in dry 02, since calcination conditions were found to greatly affect results. Calcining the CoAPO-5 and MAPO-5 samples in air essentially destroyed the samples. Even for SAPO-5, calcination in air decreased the number of acid sites measured by TPD-TGA by more than half and substantially decreased the pore volumes which were measured by 0 2 adsorption at 78K and 64 torr (P/P0=0.4), even though x-ray diffraction patterns remained excellent and exhibited no measureable changes in peak widths. While some of the pore volumes reported in Table 1 are actually higher than the theoretical value of 0.146 cm3/g, the conditions used for the measurements were the same as those used in previous

183

studies [land provide a useful comparison between samples. Pore volumes above 0.1 46 cm Ig were never observed for samples calcined in air, suggesting that those samples contained significant amounts of noncrystalline material.

B

Table 1 Properties of molecular sieves which were examined.

Porosrtv gel AIPO-5 SAPO-5(0.5) SAPOd( 17) CoAPO-5(1) CoAPO-5(5) MAPO-5(1) MAPO-5(5)

0.0 0.48 16.8 1.o 5.0 1.o 5.0

ICP

cm31g

0.25 14.5 1.2 6.0 1.3 5.6

0.153 0.1 71 0.150 0.1 32 0.126 0.1 30 0.117

One complication associated with the CoAPO-5 samples is that the Co could be present in the framework in either the +2 or +3 oxidation state. It has been reported that samples are blue if Go is +2 and green if Co is +3, the oxidation state which was observed following calcination [6]. We also found that the samples turned from blue to green following calcination; however, exposure to a few torr of isopropylamine prior to performing a TPD-TGA measurement was sufficient to turn the samples blue again. They remained blue even after the amine had desorbed. We were unable to detect the hydrocarbon products which must have formed when the Co was reduced, but it is apparent that the +2 oxidation state is more stable and that the sample was in this state during our measurements.

3. RESULTS The TPD-TGA results on each of the substituted samples differed from that obtained for a pure AIPO-5 in that some of the isopropylamine desorbed as propene and ammonia between 575 and 650K on each of the substituted materials. This is shown in Figure 1 for the SAPO-5(17) and AIPO-5 samples, but all of the substituted AIPO's exhibited TPD curves which were qualitatively similar to that for the SAPO-5. On all samples investigated, unreacted isopropylamine desorbed as a broad peak centered at -400K, with a shoulder tailing to about 600K. On the substituted samples, an additional feature was observed leaving the sample as propene and ammonia between 575 and 650K. It is interesting that the TPD-TGA curves for the substituted AIPOs appear to be essentially identical to those reported for H-ZSM-5, for which it has been argued that the decomposition feature is due to strong, Br~nstedsites associated with framework Al atoms [2]. By analogy, the decomposition feature on the MeAPO's must be due to B r ~ n s t e dsites associated with framework Si, Mg, or Co atoms. The isopropylamine which desorbs unreacted below -550K appears to be unrelated to

184

0

0 IF

0 0

a

In

0 0

Tf

0 0 0 0 rr)

185

strong acid sites since this desorption feature was observed even on pure AlPO-5. It is likely that the low temperature peak is associated with defects in the AIPO-5 structure [9]. One difficulty which appeared with the CoAPO-5 and MAPO-5 samples was that these appeared to have restricted pores when higher Co or Mg concentrations were used. For the MAPO-5(5) and CoAPO-5(5) samples, it was not possible to adsorb isopropylamine to any significant extent. The uptake of isopropylamine in these samples was very slow at 300K and only a limited amount of the amine could adsorb. It was found that ethylamine, as well as 0 2 , would adsorb in these samples, implying that the effective pore diameter must be small. Attempts to ion exchange extraframework Co from the CoAPO(5) using NH4CI at 300K and at 370K had no effect. While we have not yet thoroughly investigated this problem to see if it is a general feature of these systems, our results have been reproduced on several samples, suggesting that this may be a common problem in MAPO-5 and CoAPO-5. We also examined the adsorption of ethylarnine and n-propylamine on the SAPO-5 samples. Again, the results were very similar to those obtained on H-ZSM-5 in that some of the amine desorbed as ammonia and the corresponding olefin between 625 and 700K [4]. Of particular interest was the fact that, for a particular sample, the number of moles which reacted for each amine was the same. This implies that each sample contains a discrete number of acid sites and that each amine samples the same sites. Using this as a measure of the acid site concentration, it is interesting to compare the site concentration to the concentration of substituted metals. This is shown in Table 2 for isopropylamine on the SAPO-5 samples and on MAPO-5(1) and CoAPO-5(1). For this comparison, we used the gel concentration as a measure of the framework metal ion content. Table 2 Acid site concentrations. Me ions (10-5moVg) AIPO-5 SAPO-5(0.5) SAPO-5(17) CoAPO-5(1) MAPO-5(1)

_7.9 275. 16. 16.

Sites (10-5mo~g) <1.o 7.0 24. 18. 12.

Sites/Me 0.89 0.087 1.1

0.75

In the SAPO-5 samples, the number of acid sites generated per Si appears to change dramatically with the Si concentration. For the dilute case, SAPO-5(0.5), there appears to be approximately one site/Si. This sample was also the most sensitive to calcination conditions, with the concentration of sites measured following calcination in laboratory air being only a tenth of that reported above. Both of these facts suggest that the Si are entering the framework as isolated sites, with most of the Si atoms replacing framework P. For the more concentrated case,

186

SAPOd(l7), the number of acid sites generated is only a small fraction of the number of Si atoms. This sample was also much less sensitive to calcination conditions, exhibiting approximately half the sites when calcined in laboratory air. Most of the Si appear to be entering the framework as pairs, displacing an AI-P pair. These Si islands generate fewer acid sites and are likely less sensitive to the presence of water during calcination. The tendancy to form Si islands in SAPOs has been reported previously [lo]. For the MAPO-5 and CoAPO-5 samples, the concentration of acid sites appears to be very close to onelmetal ion. While we have not yet examined the effect of nonframework Co or Mg on the TPD results, it seems highly unlikely that nonframework species would generate a strong Bransted site which would cause the decomposition of isopropylamine at exactly the same temperature found for H-ZSM-5 [4]. (Notice that the decomposition temperature for isopropylamine in TPD-TGA does not appear to be sensitive to the strength of the acid site so long as the site strength is above a certain minimum i l l ] . ) This would suggest that most of the Co and Mg ions are present in the framework of these two samples, generating one acid site for every Al displaced from the lattice. Since we were unable to examine samples with high Co and Mg concentrations, it remains to be seen whether this relationship holds for high concentrations. However, the present results imply that TPD-TGA measurements of simple amines may be very useful for measuring the framework concentrations in these materials. It should be pointed out that the choice of probe molecule is very important in the measurement of acid site concentrations. In TPD-TGA ammonia and 2-propanol, we found it difficult to distinguish between the AIPO-5 and SAPO-5(17) samples [12]. For 2-propano1, significant quantities remained in the AlPO-5 sample after evacuation and most of it decomposed to propene and water near -450K. While more 2-propanol reacted on SAPO-5(17), it was impossible to separate the alcohol reacting at Si sites from that reacting on the AlPO portion of the sample. Similarly for ammonia, desorption features present on the pure AIPO-5 overlapped with similar features on SAPO-5(17). Significant differences were observed with propene which showed no adsorption or reaction on AIPO-5 between 300 and 400K. Similar to results on H-ZSM-5 [13], propene was observed to oligomerize at 300K and fill most of the pore volume of the SAPO-5. Due to the fact that more than one propene is present at each site when oligomers are formed, it is not possible to use this result to count sites. As discussed in this paper, the simple amines appear to be ideal probes. 4. ACKNOWLEDGEMENTS This work was supported by the NSF, Grant CBT-8720266. Some facilities were provided by the NSF, MRL Program, Grant DMR-88216718. We would also like to thank Mark Davis for very helpful discussions and for a CoAPO-5 sample which was used for comparison.

187

5. REFERENCES 1. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, and E.M. Flanigen, ACS Symp. Ser., 218 (1983) 109. 2. T.J.G. Kofke, R.J. Gorte, and W.E. Farneth, J. Catal., 114 (1988) 34. T.J.G. Kofke, G.T. Kokotailo, R.J. Gorte, and W.E. Farneth, J. Catal., 115 (1989) 3. 265. 4. D.J. Parrillo, A.T. Adamo, R.J. Gorte, and G.T. Kokotailo, Applied Catalysis, 67 (1990) 107. 5. W.O. Haag, and N.Y. Chen, in "Catalyst Design: Progress and Perspectives", L.L. Hegedus, ed., Wiley, New York (1987) pg. 180. 6. C. Montes, M.E. Davis, B Murray, and M. Narayana, J. Phys. Chem., 94 (1990) 6425. 7. R.J. Gorte, J. Catal., 75 (1982) 164. 8. R.A. Demmin, and R.J. Gorte, J. Catal., 90 (1984) 32. 9. H. Thamm, H. Stach, E. Jahn, and B. Fahlke, Adsorp. Sci. & Technol., 3 (1986) 217. 10. J.A Martens, P.J. Grobet, and P.A. Jacobs, J. Catal., 126 (1990) 299. 11. T.J.G. Kofke, R.J. Gorte, and G.T. Kokotailo, J. Catal., 116 (1989) 252. 12. A.I. Biaglow, A.T. Adamo, G.T. Kokotailo, and R.J. Gorte, to be published. 13. T.J.G. Kofke and R.J. Gorte, J. Catal., 115 (1989) 233.

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P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

189

Structure and photocorrosion of NaX-hosted Q-size metal sulfide particles M. Wark’, G. Schulz-Ekloffl, N.I. Jaeger’,

A. Zukal2

’Institute o f Applied and Physical Chemistry, University o f Bremen, 2800 Bremen 33, F.R.G.

2J. Heyrovsky I n s t i t u t e of Physical Chemistry and Electrochemistry, Czechoslovak Academy o f Sciences, 18223 Praha 8, CSFR

Abstract Highly dispersed phases o f CdS and ZnS are prepared i n a NaX matrix b y ion-exchange and gas phase sulfidation. The growth o f sulfide particles exceeding supercage size is accompanied b y mesopore formation. The rate o f photocorrosion increases with increasing dispersion of t h e semiconductor phase.

1.

INTRODUCTION

Recently zeolites were found t o be suitable c a r r i e r s f o r t h e preparation and stabi I ization of highly dispersed semiconductor materials l i k e CdS (refs.1-6), PbS (ref.l,6), CdSe (ref.7) o r ZnS (ref.5). The zeolite matrix allows t h e preparation of dispersions with distinctly d i f f e r e n t and narrow particle size d i s t r i b u t i o n s i n a range where size dependent electronic properties can be studied (quantum size particles). Quantum-mechanical calculations suggest t h a t t h e energy level of t h e f i r s t excited state of t h e exciton increases with decreasing particle size of t h e semiconductors in correspondence with t h e experimentally observed blues h i f t of t h e optical absorption edge (refs. 8-10). Q(quantum)-size semiconductors are o f interest, e.g. f o r novel photonic or electronic applications (refs. 11,121 o r as photocatalysts, i.e. f o r t h e conversion of solar energy i n t o chemical energy. However, t h e l a t t e r application is complicated namely in t h e case o f sulfides due t o t h e i r easy suscept i b i Ii t y t o p hotocor rosion. The present s t u d y r e p o r t s about t h e characterization and t h e sizedependence o f t h e photocorrosion of Q-size sulfides encapsulated within a f a u jasite.

190 2.

EXPERIMENTAL

2.1 Metal Sulfide Loaded Zeolites As described in a previous paper (ref.6) a NaX zeolite (Si/AI=1.2) was ion-exchanged using Cd2+ and Zn2+ acetate solutions (<0,05 molar) at r m m temperature for 5 hours. The obtained degrees of metal exchange were determined by measuring the concentrations of the exchange solutions before and after the exchange procedure by atomic absorption spectroscopy (Pye Unicam 9200). Sulfidation of the Cd2+ and ZnZ+ loaded samples was carried out with H z S from the gas phase at 293 K. The HzS was supplied in excess of at least 10% over the stoichiometric amount needed for the formation of the sulfides. The reaction was carried out in a closed reactor. The samples were deposited on a frit in a shallow bed and either dried in vacuum ( 1 Pa) at 623 K o r used in the non-dehydrated state for the sulfidation procedure. The excess H z S was removed after a reaction time of 1 h by evacuation at 423 K followed by flushing with the d r y nitrogen.

2.2 Adsorption isotherms I n order t o evaluate the formation of secondary pores within the zeolite structure in the course of the sulfidation and the growth of sulfide particles adsorption and desorption isotherms of cyclopentane vapours were measured at 293 K (Micromeritics Accusorb 2100 E). Prior t o the adsorption studies each sample was evacuated f o r 30 h at a Pa was attained. temperature of 623 K until a pressure of Adsorption-desorption cycles were followed up to relative pressures p/po = 1.

2.3 Characterization The quality of the samples was checked by scanning electron microscopy (IS1 loo), Debye-Scherrer X-ray diffraction and nitrogen physisorption at 77 K. The optical absorption spectra were recorded with a UV-VIS spectrometer (PE Lambda 9), equipped with a diffuse reflectance attachment. The absorption is expressed in units of the Kubelka-Munk function. The samples were diluted with barium sulfate and pure sodium faujasite was used as a reference.

2.4 Photocorrosion The photocorrosion of the zeolite encapsulated CdS at room temperature in air was followed by recording optical reflectance spectra after various times of illumination with a Xe-high-pressure lamp (150 mW cm-2). Two samples with the same degree of ion-exchange ( a = 20%) either dehydrated or not dehydrated prior to sulfidation were used in the p hotocor rosion ex per iments.

191

3. R E S U L T S 3.1 Optical spectra Fig. 1 shows t h e reflectance spectra o f ZnS containing faujasite samples i n dependence on t h e degree o f ion-exchange o f Zn2+ ions. The points o f inflexion f o r all t h r e e samples are blue-shifted compared t o bulk ZnS. The blue s h i f t was found t o be more pronounced f o r smaller degrees o f ion-exchange. For higher degrees o f exchange a t a i l i n g on t h e r e d flank is observed, which is probably due t o sulfide formation from zinc hydroxide, which might be deposited on t h e external surface of t h e zeolite ( r e f . 13). The maximum a t 230 n m is onty observed following t h e sulfidation o f t h e samples. An absorption due t o Zn2+ ions can therefore be excluded. The points of inflexion can be used f o r calculating t h e band gap E g The results are listed i n Table 1 in comparison with t h e values E g f o r t h e bulk sulfides ZnS, CdS and PbS. All samples were prepared with t h e same degree o f ion-exchange (a = 20%) and were dehydrated p r i o r t o t h e sulfidation procedure. The smaller t h e band gap of t h e b u l k materials t h e more pronounced t h e observed increase i n t h e band gap E Q - E g f o r t h e Q-size particles.

3

ZnS 2

Degree of ion exchange

_-__ .- - - - -

F(R)

5 % 10

- 20

58 %

1

0 L

F i g u r e 1. Diffuse reflectance spectra of NaX encapsulated ZnS with d i f f e r e n t degrees of Zn2+-ion-exchange. The arrow marks t h e point of inflexion (Table 1).

192 Table 1: Band gap widening of faujasite encapsulated sulfides ( a

Eg(bul k ) a E Q (observed)b EWE9

a) ref. 14;

b,

ZnS

CdS

PbS

3.60 eV (345 nm) 282 nm 4.40 eV 0.80 eV

2.42 eV (513 nm) 359 nm 3.46 eV 1.04 eV

0.41 eV (3025 nm) 480 nmc 2.59 eV 2.18 eV

= 20%)

from points of inflexion (Figs. 1 and 3); c, ref. 6

3.2 Adsorption isotherms Evidence f o r t h e growth o f sulfide particles within t h e zeolite lattice t o sizes considerably exceeding t h e dimension of t h e supercage can be obtained from adsorption isotherms o f cyclopentane. The creation o f mesopores due t o a local destruction o f t h e zeolite lattice around t h e growing particles results in a hysteresis loop i n t h e adsorption desorption cycle. This is depicted i n Fig. 2 f o r a Cd-X sample with a degree of ion-exchange o f a =20%. I n Table 2 t h e pore volumes are given f o r t h e samples (a) Cd-X p r i o r t o sulfidation, ( b ) a f t e r sulfidation without p r i o r dehydration and (c) after subsequent calcination of sample b at 623 K f o r 24 h i n an Ar-flow. The adsorbed amount a j corresponds t o t h e relative pressure p / p ~ 0.32, i.e. t o t h e junction o f both branches o f t h e hysteresis loop. The value of

a

2.5

2.0

0.25

0.50

0s75 P/P,

1.0

Figure 2. Adsorption (open symbols) and desorption (full symbols) isotherms o f Cd-X samples (degree of ion-exchange: 20%) before sulfidation (a), after sulfidation without dehydration ( b ) and after additonai calcination f o r 24 h (c). Adsorbed amount a [mmol/gl.

193 a. represents t h e total adsorbed amount at t h e relative pressure p / p o = 1. The volume o f t h e supercages is vmi = aj-v, v i s t h e molar volume o f l i q u i d cyclopentane a t 293 K. The volume o f t h e mesopores is Vme = (ao-aj)-v. The values i n Table 2 show t h a t sample a (Cd-X) is a microporous zeolite with a typical value f o r t h e micropore volume. The micropore volume is substantially decreased i n sample b. The formation o f t h e metal s u l f i d e particles leads to t h e appearance o f mesopores in t h e zeolite matrix of t h e sample. A f t e r calcination (sample c ) t h e volume of t h e mesopores decreases compared t o sample b. The hysteresis loops range from p/po = 0.3 t o p/po = 0.85 correponding t o average pore size diameters o f 1.5 - 10 nm (ref. 15).

Table 2: Micro (Vmi)- and mesopore (Vme) volumes sample a b C

V mi( cm3.g -1)

0.273 0.226 0.245

V md. cm3-g-1)

0.000 0.01 2 0.005

3.3 Photocorrosion The photocorrosion of Q-size sulfides i n t h e presence o f a i r leads t o t h e corresponding su Ifates, which cou Id be identified qua1itativel y by precipitating Bas04 The process can be followed by recording t h e decrease o f t h e intensity of absorption with time d u r i n g illumination . For CdS particles around 2 nm in diameter (Fig 3a-c) t h e intensity o f absorption i s found t o decrease at t h e same rate f o r all wavelengths. Neither t h e points of inflexion nor t h e shape of t h e extinction bands are changed d u r i n g illumination. The preparation o f a CdS dispersion b y sulfidation of t h e nondehydrated Cd2+ loaded sample results i n sulfide agglomerates 6-8 n m i n diameter (ref. 6), which do not show t h e detailed s t r u c t u r e in t h e absorption spectrum compared t o t h e small particles. I n t h i s case t h e photocorrosion i s considerably suppressed u n d e r t h e same illurnination conditions (Fig. 4a-c).

4.

DISCUSSION

4.1 Optical spectra The experimental conditions used led t o sulfide particles with diameters around 2 nm. This can be deduced from t h e points of inflexion b y comparison with previous observations f o r CdS and PbS (refs. 16-18). I t is assumed t h a t t h e point of inflexion at around 280 nm i n t h e case o f Z n S / X represents s u l f i d e particles o f a similar size.

194

CdS illuminated in air

Figure 3. Diffuse reflectance spectra o f NaX encapsulated CdS (particle diameter around 2 nm) (a) before, ( b ) after 60 minutes and (c) a f t e r 180 minutes of illumination (150 mW cm-2) i n air. The arrow marks t h e point o f inflexion (Table 1).

Figure 4. Diffuse reflectance spectra o f NaX encapsulated C d S (particle diameter around 6-8 nm) (a) before, ( b ) a f t e r 60 minutes and ( c ) after 180 minutes o f illumination (150 mW cm-2) i n air.

195 Particles of t h i s size could n o t be identified by electron microscopy and X-ray d i f f r a c t i o n due t o t h e noise from t h e zeolite matrix. The widening o f t h e energy gap E Q - E g showed t h e expected sequence ZnS < CdS < PbS (Table 1). Semiconductors with a narrower band gap in t h e bulk material l i k e PbS show a more sensitive change of t h e i r optical properties with decreasing particle size due t o t h e small values f o r t h e effective masses of t h e electrons and holes (ref. 19). A change i n t h e particle size from 8 nm t o 2 n m f o r PbS samples prepared by d i f f e r e n t methods can be recognized by a drastic change o f t h e color from black v i a red t o yellow. Increased metal loading o r a variation o f t h e preparation methods led t o a r e d s h i f t i n t h e absorption spectra and t o particle sizes considerably exceeding t h e dimension of t h e supercages, which could be observed i n t h e transmission electron micrographs. The s t r u c t u r e d absorption spectrum i n t h e case o f CdS (Fig. 3a) indicates t h e existence o f a narrow d i s t r i b u t i o n o f t h e particle sizes. The maxima in t h e absorption spectra represent excitations o f an electron from t h e valence band t o t h e f i r s t and t h e second excited state (ref. 20).

4.2 Adsorption isotherms For samples which were n o t dehydrated p r i o r to t h e sulfidation, PbS and CdS dispersions with particle sizes around 6-8 nm have been identified by optical absorption spectroscopy i n agreement with results from transmission electron microscopy (ref. 6). The mesopores detected b y cyclopentane adsorption (Fig. 2 and Table 2) f o r t h e faujasite embedded CdS c r y s t a l s have average diameters comparable t o t h e mean size o f t h e sulfide particles. This indicates t h a t t h e grown sulfide c r y s t a l s are surrounded b y holes o f f r e e space representing t h e mesopore volume. The data can be i n t e r p r e t e d b y a growth mechanism f o r particles exceeding t h e dimension o f t h e supercage, which requires a local fragmentation o f t h e zeolite framework around t h e growing sulfide aggregate. This process is facilitated b y protons formed d u r i n g t h e sulfidation process and leads t o t h e formation o f t h e observed mesopores. An analogous growth mechanism has been reported f o r zeolite embedded metal c r y s t a l l i t e s ( r e f . 15). After calcination t h e observed mesopore volume decreased. This might be explained b y a f u r t h e r growth o f t h e sulfide particles v i a Ostwald ripening and b y partial recrystallization of fragments o f t h e zeolite lattice in t h e mesopores. The l a t t e r is indicated b y t h e increase i n t h e volume o f t h e micropores.

4.3 Photocorrosion Small semiconductor particles are characterized by a h i g h extent o f lattice defects affecting bond lengths (ref. 10) and solubility ( r e f . 21) and leading t o surface states energetically located within t h e band gap (refs. 22, 23). The surface states t r a p electrons and therefore suppress t h e recombination o f l i g h t induced electron-hole pairs. The holes remain available f o r photocorrosion, i.e. t h e oxidation o f sulfide ions belonging t o t h e lattice o f t h e semiconductor. The rate o f photocorrosion i s expected t o depend on t h e diameter of

196 t h e particle. For larger particles more electron-hole pairs recombine before reaching t h e surface (ref. 24). This can be clearly recognized by comparing 2 nm (Fig. 3a-c) and 6-8 nm (Fig. 4a-c) CdS particles. I n t h e case of t h e rapidly corroding 2 nm particles t h e s t r u c t u r e of t h e absorption spectrum is maintained without a significant s h i f t of t h e absorption edge. This indicates a mechanism of successive complete corrosion o f t h e particles s t a r t i n g i n t h e o u t e r shell o f t h e zeolite crystal.

5.

CONCLUSION

(1) For low degrees o f ion-exchange t h e sulfide phase can be hosted exclusively within t h e NaX matrix. (2) Sulfide particles o f supra-supercage size are accomodated i n mesopores formed u n d e r zeolite lattice fragmentation d u r i n g particle growth. (3) For high metal loadings some hydroxide precipitation at t h e external surface o f t h e zeolite c r y s t a l s and subsequent formation of t h e sulfide has t o be considered. (4) The rate o f photocorrosion depends on t h e size of t h e semiconducting sulfide particles.

6.

ACKNOWLEDGEMENT

Financial s u p p o r t o f t h i s research by t h e Max-BuchnerForschungsstiftung (MBFSt-Kennziffer 1557) and by t h e Bundesminister fur Forschung und Technologie (BMFT-NT 20 606) is gratefully acknowledged. We thank D r . W. Lutz (Berlin) and D r . J. Rathousky f o r helpful discussions.

7.

REFERENCES

1 2

Y. Wang, N. Herron, J. Phys. Chem., 1987, 91, 257. N. Herron, Y. Wang, M.M. Eddy, G.D Stucky, D.E. Cox, K. Moller, T. Bein, J. Am. Chem. SOC., 1989, 111, 530. R.D. Strahmel, T. Nakamura, J.K. Thomas, J. Chem. SOC. Faraday Trans. I,1988, 84, 1287. X. Lui, J.K. Thomas, Langmuir, 1989, 5, 58. M.A. Fox, T.L. Pettit, Langmuir, 1989, 5, 1056. M. Wark, G. Schulz-Ekloff, N.I. Jaeger, Catalysis Today, 1991, 8, 467. K. Moller, M.M. Eddy, G.D. Stucky, N. Herron, T. Bein, J. Am. Chem. SOC., 1989, 111, 2564. H.M. Schmidt, H. Weller, Chem. Phys. Lett. 1986, 129, 615. L.E. Brus, J. Phys. Chem. 1986, 90, 2555.

3 4 5 6 7 8 9

197 10

11 12 13

14 15

16 17 18 19 20 21 22 23 24

Y. Wang, A. Suna, M. Mahler, R. Kasowski, J. Chem. Phys. 1987, 87, 7315. G.D. Stucky, J.E. Mac Dougall, Science 1990, 247, 669. G.A. Ozin, A. Kuperman, A. Stein, Angew. Chemie 1989, 101, 373. W. Lutz, H. Fichtner-Schmittler, M. Bulow, E. Schierhorn, N. Van Phat, E. Sonntag, I. Kosche, S. Amin, A. Dyer, J. Chem. SOC. Farad. Trans. 1990, 86, 1899. Handbook o f Chemistry and Physics, 63. Edition, CRC Press, 1982. N.I. Jaeger, J. Rathousky, G. Schulz-Ekloff, A. Svensson, A. Zukal i n Zeolites: Facts, Figures, F u t u r e (P.A. Jacobs, R.A. van Santen, eds.), Elsevier Publishers, Amsterdam 1989, Stud. Surf. Sci. Catal., vol. 49B, p. 1005. H. Weller, U. Koch, M. Guiterrez, A. Henglein, Ber. Bunsenges. Phys. Chern. 1984, 88, 649. R. Rossetti, J.L. Ellison, J.M. Gibson, L.E. Brus, J. Chem. Phys. 1984, 80, 4464. S. Gallardo, M. Guiterrez, A. Henglein, E. Janata, Ber. Bunsenges. Phys. Chem. 1989, 93, 1980. L.E. Brus, J. Chem. Phys. 1984, 80, 4405. J.M. Nedelkovic, M.T. Nenadovic, D.I. Micic, A.J. Nozik, J. Phys. Chem. 1986, 90, 12. M.I. Vucemilovic, N. Vukelic, T. Rajh, J. o f Photochem. and Photobiol., 1988, 42A, 157. Y. Wang, N. Herron, J. Phys. Chern. 1988, 92, 4988. N. Chestnoy, T.D. Harris, R. Hull, L.E. Brus, J. Phys. Chern. 1986, 90, 3393. G.T. Brown, J.R. Darwent, J. Phys. Chem. 1984, 88, 4955.

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199

Faujasite-hosted methylene blue: synthesis, optical spectra and spectral hole burning R.Hoppel,

G.Schulz-Ekloff

l,

D.Wohrle2, M.Ehrl3 and C. Brauchle3

' I n s t i t u t f u r Angewandte und Physikalische Chemie, Universitat Bremen, D-2800 Bremen 33, Germany

q n s t i t u t f u r Organische und Makromolekulare Chemie, Utiiversitat Bremen, D-2800 Bremen 33, Germany

q r i s t i t u t f u r Physikalische Chemie, Universitat Munchen, D-8000 Miinchen 2 , Germany

Abstract Faujasite-hosted methylene blue samples are obtained by ion exchange of t h e dye ion i n aqueous solution, o r by addition o f t h e dye t o t h e reaction mixture f o r t h e hydrothermal synthesis o f t h e NaY zeolite. The coloured c r y s t a l l i n e NaY products contain 1 0 - 6 - 10-5 moles d y e per gram zeolite. The possibility of persistent spectral hole burning is demonstrated f o r a host-guest system o f supercage-encapsulated dye molecules.

1.

INTRODUCTION

Polymer-hosted dyes are intensively studied as prospective systems I t was f o r optical data storage v i a persistent spectral hole b u r n i n g [l]. shown t h a t t h e use o f inorganic c a r r i e r s for anchoring of dyes can r e s u l t i n reduced phonon side band formation i n t h e hole b u r n i n g process, due t o reduced linear electron phonon coupling [23. The decoupling i s a prerequisite f o r " h i g h temperature" ( T 2 77 K ) hole b u r n i n g which would make t h e use o f frequency domain optical data storage more a t t r a c t i v e [l]. Zeolite-hosted dye systems are expected t o o f f e r several additional new advantages t o high temperature hole burning. There are strongly inhomogeneously broadened absorption bands, due t o d i f f e r e n t chemical environments caused b y local Si/Al r a t i o fluctuations it1 t h e zeolite r e s u l t i n g i n d i f f e r e n t orientations and interactions o f t h e encaged molecules. The host-guest interactions are d i f f e r e n t from chromophore glass systems and might allow to burn narrow holes at

200 increased temperatures on a broad spectral range, t h u s showing a h i g h multiplexing factor even at increased temperatures. The relatively high thermal, chemical and mechanical stability o f t h e zeolite c r y s t a l s is a f u r t h e r advantage f o r t h e technical realization of optical data storage systems. I t has been reported that methylene blue (MB) reacts readily with clay minerals by ion exchange and electrostatic bonding [3]. This mechanism should also hold f o r t h e found adsorption o f MB a t t h e external surface of zeolite crystals [43. The incorporation o f MB i n t o zeolite voids has not yet been reported. I n the following t h e accomodation o f MB in NaY by either ion exchange o r d u r i n g zeolite synthesis is described. F i r s t evidence o f persistent spectral hole b u r n i n g in t h e described system is given.

2.

EXPERIMENTAL

2.1 Materials A zeolite NaY (Si/Al = 2.9) is synthesized in analogy t o proven recipes [5], i.e. using an aluminate solution with a molar r a t i o AIOZ- : OH1 : 1

and a sodium water glass solution ( D A B 6, Merck). The aluminate (48 g) is added t o t h e water glass solution (175 g) under s t i r r i n g at 361 K. A s l u r r y (16 g) o f NaX seeds (5 g) is added following t o t h e gel formation, i.e. a f t e r ca. 15 min, under heavy s t i r r i n g . The formation o f t h e NaY crystals is finished a f t e r one week. The quality of t h e charge i s checked by scanning electron microscopy, Debye-Scherrer X-ray diffraction, nitrogen physisorption at 77 K and reversible water uptake. Methylene blue (MB) is supplied by Riedel-de Haen as chloride (3,7b is( d imeth y lamino)-p henolth iazi n i umch loride). I t is used without f u r t h e r purification.

2.2 MB ion-exchange Dye ion exchange is carried o u t using s l u r r i e s o f 2 g zeolite i n 25 ml of 1.5 vmole aqueous solutions of MB at 298 K under constant mechanical shaking f o r up t o 30 days. Following t o f i l t r a t i o n t h e coloured residue is extracted with ethanol i n a Soxhlet u n t i l t h e boiled o v e r solvent is colourless. The dried residue is characterized by X-ray analysis and Nz physisorption and i s analyzed by optical spectroscopy.

2.3 MB template The hydrothermal synthesis o f NaY i n t h e presence of MB is carried o u t according t o t h e recipe given above. ME is added in t h r e e d i f f e r e n t ways: moles MB are added t o t h e sodium water glass solution (sample 1) o r t o the aluminate solution (sample 2) o r t o t h e s l u r r y containing t h e NaX seeds (sample 3). The coloured NaY c r y s t a l s are subjected t o t h e extraction procedure, characterizations and optical spectroscopy l i k e t h e ion-exc han ged samples.

201

F i g u r e 1. Scanning electron micrographs o f NaY (a) and NaY samples loaded with methylene blue d u r i n g hydrothermal crystallization: (b) Sample 1, i.e. dye added t o waterglass; (c) sample 2, i.e. dye added to aluminate; ( d ) sample 3, i.e. dye added with NaX seeds.

2.4 UV-VIS s p e c t ~ p y Absorption spectra o f t h e ME-loaded zeolites are taken in t h e range 300 - 800 nm. The spectrometer (PE Lambda 9) i s equipped with a diffuse reflectance attachment. Sodium zeolites are generally used as diluent o r reference standard. Holmium oxide is used f o r t h e calibration of s l i g h t d r i f t s i n t h e recording s t a b i l i t y of t h e instrument, thus, enabling quantitative photometric evaluations of reflectance spectra.

2.5 Persistent spectral hole burning The MB ion-exchanged faujasite sample (200 mg) i s filled in a glass cuvette ( V i t r o Dynamics), evacuated at room temperature t o 0.01 Pa f o r 1 h, sealed i n vacuo and positioned i n a l i q u i d helium bath c r y o s t a t (Cryovac) a t reduced pressure. Spectral hole b u r n i n g i s carried o u t using the l i g h t of a 150 W Xe(Spex 1402). The arc-lamp (Oriel) dispersed by a monochromator temperature o f 1.6 K is measured by a calibrated Si-diode (Lake Shore). The hole is burned a t t h e excitation wavelength o f 700.0 nm using a b u r n i n g power of 5 gW/crn2 f o r 4000 seconds r e s u l t i n g i n a b u r n i n g energy of 20 mJ/cm2.

202

3. R E S U L T S 3.1 Crystallinity The X-ray analysis gives t h e reflexes o f Nay. A small fraction (ca. 2%) o f NaP is detected f o r samples 1 and 2. An increased background scattering is found i n t h e Debye-Scherrer diagrams f o r sample 3. The scanning electron micrographs (Fig. 1) give NaY c r y s t a l s o f 1-3 pm size. Sample 3 exhibits nonregular edges and faces f o r t h e c r y s t a l s (Fig. l d ) . The analysis o f t h e c r y s t a l l i n i t y o f t h e MB-template loaded zeolite c r y s t a l s by nitrogen physisorption at 77 K gives n o reductions of t h e Nz uptake capacity valid f o r NaY (ca. 170 ml N2 (NTP)/g d r y faujasite) if t h e MB was added t o t h e aluminate o r water glass solutions (samples 1 and 2) p r i o r t o t h e gel formation. A drastic (ca. 40%) reduction o f t h e NZ uptake capacity i s found if t h e MB is added with t h e NaX seeds (sample 3) a f t e r t h e gel formation. The water uptake capacity o f samples 1-3 is comparable t o t h e value f o r dye-free Nay, i.e. ca. 25 wt%.

3.2 MB content Dye contents (mole MB p e r g Nay) a r e determined photometrically. Calibrated MB/NaY samples are obtained b y difference measurements o f t h e MB containing solutions p r i o r and a f t e r ion exchange. Since t h e absorption maxima o f t h e analysed and t h e calibrated samples do not coincide, only o r d e r s o f magnitude can be given f o r t h e MB contents. The amount of MB taken u p by ion exchange under t h e applied conditions (6.10-4 molar solutions) is found t o be ca. 10* mole/g. Higher loadings r e q u i r e exchange solutions having higher MB concentrations. The MB content o f t h e as-synthesized samples 1-3 i s i n t h e range 10-6 - 10-5 mole/g, i.e. corresponds t o t h e concentration present i n t h e synthesis solution. The extended extraction by ethanol can leach o u t up t o 50% o f t h e dye. I t follows t h a t u p t o 2% of t h e supercages are filled b y MB if t h e dye is incorporated d u r i n g NaY synthesis, whereas 0.3 % o f t h e supercages can be filled by ion exchange under t h e applied conditions.

3.3 Optical spectra Optical absorption spectra of solid, dissolved and Nay-hosted MB are given i n Fig. 2. The solid MB (Fig. 2a) shows absorption i n t h e U V region like many o t h e r rr-electron systems o r dyes. More characteristic is t h e system of MB in neutral aqueous solution (Fig. 2f), giving a band around 660 nm, related t o t h e 0-0 electronic transition, and a shoulder around 620 nm, which i s r e f e r r e d t o t h e 0-1 vibronic transition [3]. The band of t h e MB dimer i s also located around 610 nm [3a]. The influence o f t h e pH value on dissoived M B is demonstrated b y a spectrum taken a t pH 14 (Fig. 29). Presumably, t h e spectrum is dominated b y dimers ( = 580 nm) and trimers ( = 550 nm) [31. The bands o f t h e Nay-hosted MB incorporated d u r i n g synthesis as template (Fig. 2b-d) are strongly broadened. Differences i n band shape and maximum positions seem to reflect t h e pH values which t h e Synthesis mixtures exhibited at t h e point of MB addition, i.e. pH 14 o f t h e aluminate solution (Fig. 2c), pH 13 o f t h e mixture p r i o r t o gel formation (Fig. 2b) and pH < 13 a f t e r gel formation (Fig. 2d). Reversible s h i f t s o f 10-20 nm

203 of t h e bands are observed upon addition o r removal of water, i.e. a blue s h i f t occurs i n a vacuum and a r e d s h i f t by rehydration. The suspension of t h e MB-loaded NaY samples in aqueous HCI solution s t r o n g l y s h i f t s the maxima t o t h e red side and leads t o t h e appearance o f bands related t o t h e v i b r o n i c transition o f t h e protonated MBH2+molecule. The spectrum o f t h e ion-exchanged MB (Fig. 2e) d i f f e r s from those o f t h e MB template, i.e. it i s narrower and exhibits two d i s t i n c t maxima, located at wavelengths which are characteristic f o r monomers and dimers.

3.4 Spectral hole burning Fig. 3 shows t h e obtained Gauss-fitted hole i n t h e range from 699.0 to 701.0 nm with a hole width o f 1.5 cm-1, i.e. 0.074 nm o r 45 GHz. The hole depth is 4.2%. i t s width represents the spectral resolution of t h e monochromator.

4.

DISCUSSION

A1 Optical spectra The MB-loaded NaY obtained by ion exchange (Fig. 2e) exhibits a s t r o n g dimer absorption maximum around 610 nm i n addition t o t h e monomer signal around 650 rim. Since an intact faujasite supercage does not allow t h e accomodation o f a dimer f o r sterical reasons [61, i t has t o be assumed that a fraction of damaged supercages exist, allowing t h e encapsulation o f a (MB+)z. Dimer signals are not found f o r MB adsorbed at t h e external surface of ZSM-5 t y p e zeolite c r y s t a l s [4b]. The spectrum o f t h e MB incorporated i n t o t h e faujasite d u r i n g synthesis i s complicated by several effects: ( 1 ) The MB can aggregate by interaction with a mineral c a r r i e r resulting in a metachromasy [3,71, i.e. an increase o f absorption at the blue flank. (2) Base catalyzed demethylation t o trimethylthionine (TMT) [3a] at pH values > 12 can take place. (3) A spectrum o f d i f f e r e n t chemical environments, caused b y a high number of possible permutations o f t h e local Al distribution, results in a great number o f d i f f e r e n t possible orientations and interactions of t h e molecule and, thus, inhomogeneous broadening of t h e absorption band. The samples exhibit blue-shifted absorption bands which might be related t o t h e formation of t r i m e r s (MB+)3 as well as t o some TMT formation [3a]. The assumption o f a base-catalyzed TMT formation is supported b y t h e facts ( i ) t h a t t h e intensity o f absorption at t h e blue flank is increased with increasing pH value, which t h e dye experienced d u r i n g t h e hydrothermal zeolite crystallization, and ( i i ) t h a t t h i s blue flank absorption decreases faster d u r i n g extraction than other regions of t h e absorption band. The neutral TMT molecule is expected t o be eluted more r a p i d l y than t h e charged MBf ion. The addition o f hydrochloric acid (0.1 M) gives t h e expected red s h i f t o f t h e absorption bands o f MB and TMT, which can be reversed b y addition o f NHOH. The appearance of a s t r o n g band a t 750 nm at l o w pH values is characteristic f o r t h e protonated MBH2+ ion [a]. This indicates t h a t at least a large fraction o f MB is preserved, i.e. i s not demethylated. Obviously, t h e demethylation process i s impeded f o r zeolite-hosted MB.

204

Wavelength (nm)

Figure 2. Optical spectra of (a) solid methylene blue (MB), ( b ) MB/NaY template (sample l ) , (c) MB/NaY template (sample 21, ( d ) MB/NaY template (sample 3), (e) MB/NaY ion-exchanged, (f) MB i n aqueous solution at pH = 7, ( 9 ) MB i n aqueous solution a t pH = 14.

205

4

e CI

C v)

m

4

14295.5

14275.5

WAVENUMBERS

(cm-')

F i g u r e 3. Trace of a burned hole i n t h e r e d flank o f t h e absorption band of an ion-exchanged MB/NaY system.

4.2 Irregular crystal Iization The i r r e g u l a r crystal shape obtained i f t h e MB is added d u r i n g synthesis with seeds (sample 3, Fig. I d ) might be related t o t h e special t y p e of dye uptake by t h e faujasite seed crystal. I f accomodation of t h e MB i n a shell o r layer close t o t h e external surface of t h e seed crystal is assumed then t h e f u r t h e r growth o f t h e NaY c r y s t a l s t a r t s at nuclei exhibiting a distortion due t o t h e accomodated dye molecules. This distortion might be preserved i n t h e f u r t h e r growth resulting i n c r y s t a l s having a h i g h fraction o f defects and an i r r e g u l a r shape. The h i g h density of defects i s responsible f o r t h e decreased uptake of Nz, whereas t h e f u l l c r y s t a l l i n i t y i s indicated by t h e unaffected uptake o f water. 4.3 Spectral hole burning The described hole b u r n i n g experiment gives evidence f o r t h e possibility o f spectral hole b u r n i n g i n Nay-hosted MB. To o u r knowledge, t h i s is demonstrated f o r zeolite-hosted dye systems f o r t h e f i r s t time. Detailed information about s t r u c t u r e and dynamics o f zeolite-encapsulated dye systems and t h e i r possible use f o r frequency domain optical data storage is available from spectral hole b u r n i n g i n case o f using an excitation l i g h t source with a high spectral resolution, e.g. a d y e laser o f about 1 MHz o r smaller. This work is in progress.

206

5.

ACKNOWLEDGEMENT

Financial supports by t h e Bundesminister f u r Forschung u n d Technologie (BMFT-NT 20606) and b y t h e Max-Buchner-Stiftung (MBSt Kennziffer 1557) are g r a t e f u l l y acknowledged.

6.

REFERENCES W.E. Moerner (Ed.), Persistent Spectral Hol Burnin : Science and Applications, Topics i n C u r r e n t Physics, Springer, New York 1988, p. 251. a) Th. Basche and C. Brauchle, Chem. Phys. Lett., submitted; b ) Th. Basche and C. Brauchle, Appl. Phys. Lett., submitted. a) K. Bergmann and C.T. O’Konski, J. Phys. Chem. 67 (1963) 2169; and references t h e r e in b ) J. Cenens and R.A. Schoonheydt, Clays and Clay Minerals 36 (1988) 214; and references therein. a) M. Susic, N. Petranovic and B. Miocinovic, J. inorg. nucl. Chem. 34 (1972) 2349. b ) G.P. Handreck and T.D. Smith, J. Chem. SOC. Faraday Trans. 1, 84 (1988) 4191. c ) J.R. Anderson, Y.-F. Chang and A.E. Hughes, Catalysis Letters 2 (1989) 279. H. Kacirek and H. Lechert, J. Phys. Chern. 79 (1975) 1589. R. Hoppe, G. Schulz-Ekloff, D. Wohrle, L. Uytterhoeven, i n preparation. G. Holst, Z. physik. Chern. A 182 (1938) 321. G.N. Lewis and J. Bigeleisen. J. Amer. Chern SOC. 65 (1943) 1144.

.

P.A.Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

207

PREPARATION AND CHARACTERIZATION OF ZINC-ZSM-5 CATALYST Juan Liang, Wei Tang, Mu-Liang Ying, Su-Qin Zhao, Bo-Qing Xu and Hong-Yuan Li Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian 116023, China

Abstract Zinc-ZSM-5 catalysts which contained zinc either in the framework ([Zn]ZSM-5) or in the extraframework (ZnZSM-5) positions were prepared by a gas-solid (ZnClz(g) + HZSM-5) reaction o r by ion exchange. Characterizalion using XRD, IR, TPDA, and 27Al MAS NMR showed that isomorphous substitution of the framework aluminium for zinc occurred during the gas-solid reaction at 773 K. A loss of strong acidity and a generation of weak acidity were observed as a result of the substitution. Zinc insertion into the vacancies formed by aluminium migration out of the framework was assumed to be responsible for the substitution. In contrast to ZnZSM-5, H[ZnlZSM-5 showed no activity in the hydrogenation of benzene and in promoting aromatization of propylene.

1. INTRODUCTION

Recently substitution of the framework aluminium of synthetic high-silica zeolites by solid-gas and solid-liquid reactions has created a new method for the preparation of metal zeolites. Combined with the direct synthesis method, this has led to a rapid development of a new zeo1it.e catalyst [1,21.Zinc-containing ZSM-5 zeolite has proven to be an important catalyst for the synthesis of aromatics from lower alkanes [3-51. However-, few results have been given in the literature to show the variation in the properties of zinc-ZSM-5 in relation to its zinc content and lhe differences between the framework zinc"Zn1ZSM-5) and cationic zinc(Zn2SM-5). In the present study, two series of zinc-containing ZSM-5 catalysts were prepared. One is [ZnlZSM-5, which contained different amounts of zinc in the framework, while the other is ZnZSM-5, in which zinc exists as a cation in the zeolite pores to maintain the charge balance. The properties of these zinc-ZSM-5 catalysts are characterized by the XRD, NH3-TPD (TPDA), IR and "A1 MAS NMR techniques and catalytic reactions.

2. EXPERIMENTAL

HZSM-5, which was synthesized in our laboratory by using TPA as the template, was used as the starting material.Unless otherwise specified, the

208

ratio of SiOz/A1203 in the starting material is 50. [ZnlZSM-5 was prepared by a gas-solid reaction between ZnClzcg) and HZSM-5(s) at 773 K. The content of zinc in [ZnlZSM-5 was controlled by varying the reaction time. All the [ZnlZSM-5 was changed into their proton forms by ion exchange with 0.1N HC1 solution before use. This ion-exchange procedure ensures removal of extraframework aluminium and zinc species that may be residual in the zeolite pores. The composition of the H[ZnlZSM-5 samples is given in Table 1 . ZnZSM-5 catalysts were prepared by ion exchange with 0.05N Zn(N03)2 solution. XRD, IR, and MAS NMR measurements were performed on a Rigaku D/max-rbl2kw diffractometer, a Hitachi-270 spectrophotometer, and a XL-200 nuclear magnetic resonance spectrometer at the Wuhan Institute of Physics. Table 1 Crystalline unit cell composition of the samples Samplea’

unit cell formula

HZSM-5 H[ZnlZSM-5(2) H[ZnlZSM-5(4) H[ZnlZSM-5(6) H i Zn1 ZSM-5 ( 8)

A13.7Si92.30192 Al2.6Zno.46Si92.90192 A~a.45~no.88Si92.70192 Al2.38Znl.39Si92.20192 A 1z.asZn1.79Si91.80192

"1. '1.

Al/u. c. 3.7 2.67 2.45 2.38 2.33

Zn/u. c. 0 0.46 0.88

1.39 1.79

the figures in parenthesis denote the time of the gas-solid reaction in hours. the residual sodium ion content in the samples is confirmed to be less than 0.04 per unit cell.

The catalytic properties of the samples were characterized by the hydrogenation of benzene and aromatization of propylene. The reactions were carried out in a pulse microreactor 161.

3. RESULTS AND DISCUSSION

1. Isomorphous substitution of the framework aluminium of ZSM-5 for zinc The gas-solid reaction of anhydrous ZnCl2 vapor with HZSM-5 was studied at 773 K. The retention of the crystallinity of the zeolite after the reaction was confirmed by XRD results. Figure 1 shows the change in the volume of the crystalline unit cell, VU.C., and the frequency of the framework-sensi tive IR absorption near 1100 cm-’ during the reaction. The result that the product zeolite of the reaction has an VU.C. value greater than that f o r the initial HZSM-5 indicates that the zinc atom is inserted into the zeolite framework since the bond of Zn-O(1.96 A) is longer than A1-O(1.75 A ) and Si-O(1.61 A). The linear relationship between VU.C. and the reaction time suggests that the insertion of zinc is proportional t o the reaction period. The continuous shift in the framework-sensitive IR absorption frequency towards lower wavenumbers provides further evidence

209

M

I

0 4

I

I

I

I4

I

5.6

\

E

1102

m

5,5

I

0

b

1098

d X

4

=-'

W '

5.4 1094

a, 3 tr a,

&

5.3 I

0

I

1090

2 4 6 8 Reaction time / h r

Figure 1. Variation of VU.C. and the framework frequency during the gas-solid reaction.

sensitive

IR absorption

4

W

2 f

3

\

I

\ A

4

0

0

2

4 6 8 Reaction time /hr

Figure 2. Variation of Zn/u. c. and AI/u. c. during the gas-solid reaction.

210 for the insertion of zinc into the framework. Figure 2 shows the relationship between the content of framework zinc, expressed by the number of zinc-atom per unit cell (Zn/u.c. 1 , and the reaction time. A linear increase of Zn/u.c. in this figure corresponds well to the value of VU.C. in Figure 1. thus demonstrating that insertion of zinc into the framework occurs during the gas-solid reaction. Changes in the three crystalline parameters, a, b and c, are given in Table 2. It is shown that all three parameters increase simultaneously during the reaction. This implies that the increase in the unit cell dimension, and then the insertion of zinc, is directionally random. Table 2 Crystalline unit cell parameters Samplesa

HZSM-5 H[ZnlZSM-5(2) H [ZnlZSM-5 (4 H [ZnlZSM-5 ( 61 H [ZnlZSM-5 (8

unit cell parameters

(A3)

a(A)

b(A)

c(A)

vu.c.

20.104 20.098 20.251 20.367 20.425

19.905 19.912 19.975 19.982 20.151

13.399 13.386 13.424 13.436 13.510

5361.88 5357.36 5430.19 5468.09 5560.50

"1. see the footnote to Tab. 1 Zinc insertion into the framework of ZSM-5 was affected by the content of aluminium in the zeolite. It is found that the sample having a higher content of aluminium shows a more rapid VU.C. increase during the gas-solid reaction. Variation of the framework aluminium during the reaction was 27 measured by A1 MAS NMR. A continuous decrease in the value of A l / u . c . (Fig.2) reflects aluminium migration out of the famework. In adition, a rapid decrese in Al/u.c. during the first 2 hours may suggest that some of the framework aluminium can relatively easy migrate out of the framework under the reaction condition. In contrast, the linear relationship between Zn/u.c. and the reaction period should suggest that the insertion of zinc occurs smoothly during the gas-solid reaction. Since the rate of z i n c insertion is slower than the migration rate of the framework aluminium in t h e first 2 hours (Fig.21, it may be assumed that the insertion of zinc should occur at the vacancies resulting from aluminium migration from the framework. The fact that the sum of Al/u.c. and Zn/u.c. in H[ZnlZSM-5(5) (4.12) i s greater than Al/u.c. in the initial HZSM-5 (3.71,see also Table l., is considered to support the assumption because some aluminium vacancies may already exist in the framework of the initial sample.

2. Variation of the surface acidity The acidities of [Zn]ZSM-5 and HZSM-5 are characterized by TPDA. Two peaks of NH3 are observed; that appearing at lower temperature (ca. 553 K, Peak-I) is attributed to NH3 desorption from the weakly acidic sites while that at higher temperature (ca. 723 K, Peak-11) corresponds to the strongly acidic sites. The acidity evaluated from the area of TPDA peaks is given in

211 Figure 3. It is apparent that the total acidity (S-curve) of [ZnlZSM-5 is less than that of HZSM-5. A maximum reduction rate in the acidity seems to exist in the first 2 hours of the substitution reaction during which abouL 0.46Zn/u.c. is inserted into the framework. When more zinc, at least up to 2.0 Zn/u.c. is inserted, a continuous regeneration of the acidity is evident. Curves I and 11, respectively, reflect changes in the acidities connected with the weak and strong acid sites. The parallel relationship between curves I and S is noticeable when more than 0.46 Zn/u.c. is inserted into the framework. It may be assumed that the acidity regeneration results from the formation of a weak acid site by zinc insertion into the framework. Curve I1 in Figure 3 seems parallel to the curve of the aluminium content in Figure 2, which indicates that the strongly acidic site is connected with the framework aluminium. The nature of the acidic sites is characterized by the IR spectra of adsorbed pyridine (Fig.4). It is apparent that the acidic sites on HZSM-5 are strongly Bronsted acid in nature, and can retain the pyridinium ion species (1550 cm-l) at above 723 K. On the sample of H[ZnlZSM-5(8), however, strong absorption at 1450 cm-I is observed due to pyridine adsorbed acid sites. These Lewis acid sites are mainly weakly acidic since the 1450 cm-I band is almost eliminated by evacuation to 723 K. Consequently the substitution of the framework aluminium for zinc modifies not only the acid strength but also the nature of the acid sites. 3. Catalytic behaviour of [ZnlZSM-5 and ZnZSM-5 Dehydroganation of benzene at 473 K and propylene conversion aromatic? at 773 K are used to compare the catalytic properties of [Zn]ZSM-5 and ZnZSM-5 in a pulse microreactor. The results shown in Table 3 indicate that

the framework zinc catalyst (H[ZnlZSM-5) does not exhibit activity for the hydrogenation reaction of benzene. In contrast, a hydrogenation product, cyclohexane (c-C6H12), is observed over the cationic zinc catalyst (ZnZSM-5). The fact that an increase in the zinc content in ZnZSM-5 leads to an increase in the activity for the hydrogenation reaction suggests that

Table 3 Hydrogenation of benzene over [ZnIZSM-S and ZnZSM-5 catalysts “ 1 Catalyst b Z n h .c. 1

HZSM-5

H[ZnlZSM-5

0

0.46 0.88 1.39 1 . 7 9

Product distribution / C6H6

C3H8 C-C6H12 ”1.

98.1 1.9

-

96.3 3.7

-

ZnZSM-5 0.49

0.88 1 . 0 6

C) 1.39

wt% 95.9

3.5 0.6

93.4

5.8 0.9

93.7

5.5 0.8

91.2 6.4 2.4

79.8 5 4 . 2 7.0 7.3 13.238.5

90.9 6.9 2.3

reaction temperature: 473 K values of Zn/u.c. for the samples of H[ZnlZSM-5 refer to the framework zinc, while those of ZnZSM-5 to the cationic zinc in the pores. “1. a sample obtained by steaming H[ZnlZSM-5(Zn/u.c.) at 873 K for 4 hr.

b).

212

0

J

Reactiom time / h r 2 4 6 8

0.5

0

1,o Zn/u. c .

1.5

Figure 3. Acidity evaluated from the area of the TPDA peaks

1700

1500

1300

1700

1500

1300

F r e q u e n c y /crn-' Figure 4. IR spectra of pyridine adsorbed o n HZSM-S(A) and H[ZnIZSM-S(XI. (a), (b), and (c) refer to the spectra after evacuation at 473, 623, and 723 K, respectively.

213 zinc cations in the pores of ZSM-5 have a hydrogenation function while those in the framework have not. This point seems to be supported further by the fact that 2.3 percent of the benzene is hydrogenated to cyclohexane when H[Zn]ZSM-5(8) is steamed at 873 K for 4 hours since the steam treatment is believed to cause some of the framework zinc migrate into t h e zeolite pores, as has been reported for [GalZSM-5 [71. Table 4 Conversion of propylene over [ZnIZSM-5 and ZnZSM-5 catalysts "1 Ca ta 1yst Zn/u.c. b,

H[ZnlZSM-5 1.79

ZnZSM-5 1.06

15.70

2.66 19.26 43.60 2.40 10.11 15.92

5.30 -

5.05 -

7.40 12.56 4.67 27.79 35.08 8.35 4.12

30.03

31.08

HZSM-5 0

Product distribution / wt% 2.68 CH4 19.23 CzH4+CzH6 44.78 C~H~+C~HB c4+

C6H6 ( B ) CH3C6Hs(T (CH3)2C6H4(X) C9+(aromatics) Total aromatics

3.49 9.03

75.44

" 1 . reaction temperature: 773 K '1. see the footnote to Tab. 3.

Recently, it was shown that the formation of aromatics from propane a n d propylene over ZSM-5 zeolite can be greatly increased by the addition of a dehydrogenation function (Zn, Ga, P t , etc. 1 to the acidic zeolite [3-71 The rule of exchanged zinc cations in this catalyst was attributed to a ) modifying the surface acidity of the zeolite 151; and b ) . promoting the aromatization of the olefin as a dehydrogenation catalyst [3,6,71.Table 4 compares the results of propylene conversion over H[ZnlZSM-5 and ZnZSM-5 The yield of aromatics over ZnZSM-5 is 75 wt%, while that over both H[Zn]ZSM-5 and HZSM-5 is only 30 wt%. The low aromatics yields over the latter two catalysts again supports the suggestion that the framework zinc has lost its property as a metallic catalyst.

4. CONCLUSIONS

The substitution of the framework aluminium for zinc is carried out by the gas-solid reaction of ZnClz(g) and HZSM-5. This substitution results in a generation of weakly acidic sites and a loss of strongly acidic sites. I t may be assumed that the substitution occurs via zinc insertion into the aluminium vacancies formed by aluminium migration from the framework. Zinc inserted into the framework exhibites no activity for the hydrogenation reaction of benzene and for promoting the aromatization of propylene.

214 4. REFERENCES

C. D. Chang, C. C-W. Chu, J. N. Miale, R . F. Bridger and R. B. Calvert, J . Amer. Chem. SOC., 106 (1984) 8143. 2 . M. W. Anderson, and J. Klinowski, J. Chem. SOC., Faraday Trans. I, 82 (1986) 1449. 3. T. Mole and J. R. Anderson, J. Catal., 17 (1985) 141. 4. S.-Q.Zhao, R.-H. Wang, J . Liang, W.-G. Guo, M.-L. Ying, and B.-Q. Xu, in Proc. 1st Shanghai Int. Symp. Tech. Petroleum & Petrochemical Industry, Shanghai, May 16-20, 1989, Shangai, China, 1989, pp. 1-6. 5 . S.-Q.Zhao, R-H. Wang, J. Liang, W.-G. Guo, and H.-Y. Li, Ci Chemistry & Chemical Industry (Chinese), 4 (1988) 24. 6. J . Liang, H.-Y. Li, S. -Q. Zhao, W.-G. Guo, R.-H. Wang. and M. -L. Ying, Appl. Catal., 64 (1990) 31. 7 . D. K. Simmon, R. Szostak, P. K. Agrawal, and T. L. Thomas, J. Catal., 106 (1987) 287. 1.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

215

THE FORMATIONOF WELL DEFINEDSURFACECARBONYLSOF Ru AND IR WITH HIGHLY DEALUMINATED ZEOLITEY AS MATRIX *

I.Burkhardt, D.Gutschick, HLandmesser and H-Miessner Zentralinstitut f u r Physikalische Chemie Rudower Chaussee 5, 0-1199 Berlin, Germany

ABSTRACT FT-IR spectroscopy has been used t o s t u d y t h e surface species formed during

the

interaction

of

dealuminated zeolite (US-Ex).

CO

with

Ru

and

Ir

supported

on highly

With both oxidized a n d reduced Ru/US-Ex a

well defined Ru tricarbonyl could be identified with bands at 2152, 2091 and 2086 cm-l indicating a slightly distorted C3v structure.

Evacuation at

473 K r e s u l t s i n t h e formation of a Ru dicarbonyl with bands at 2092 a n d 2031 cm-l.

Ir(CO)2/US-Ex with bands at 2108 and 2037 cm-’

a f t e r an interaction of CO with oxidized Ir/US-Ex.

was obtained

Different t o Rh and Ru,

reduced Ir did not form a well defined surface carbonyl on US-Ex.

TPR-TPO

w a s used to explain t h e s e differences a n d t h e increasing temperature (Rh
INTRODUCTION

It h a s been shown by FT-IR spectroscopy t h a t t h e interaction of CO with Rh introduced into highly dealuminated zeolite Y (US-Ex) results in t h e formation

of

(F.W.H.M. 5 5

a

ern-')

well

defined

Rh

dicarbonyl

with

unusually

sharp

carbonyl stretching bands at 2118 and 2053 cm-’[lI.

Sharpness and intensity of

t h e s e bands allowed t h e

detection of

satellite bands even with 1 3 C 0 in natural abundance (1.1%)[1,21.

I3C

Low

temperature adsorption experiments and t h e interaction of Rh/US-Ex with NO

or COtNO demonstrated t h e r a t h e r rich surface chemistry of Rh carbonyls a n d

216

nitrosyls bonded t o t h e zeolite framework 13,41. These unique properties of US-Ex as a matrix for the formation of well defined surface carbonyls have been used to extend t h e study t o t h e surface carbonyl chemistry of Ru and Ir.

For supported Ru, t h e formation of

several different surface carbonyls has been proposed in t h e literature on the basis of carbonyl bands with wavenumbers from 2140 to 1950 cm-l. The assignment of these bands to definite surface complexes is far from being clear. Two high-frequency (HF) bands at ca. 2140 and 2080 cm-l has been assigned t o a Ru dicarbonyl [5-131 or, alternatively, t o a tricarbonyl L13-181 with Ru in a positive oxidation state (t1 - t3). The HF bands have been also assigned t o different monccarbonyls of positively charged Ru atoms in contact with oxygen. 119-211. A carbonyl band at ca. 2040 cm-l has been usually assigned to CO linearly adsorbed on Ru m e t a l particles. On the other hand, dicarbonyl species have been proposed on t h e basis of band dubletts with wavenumbers varying from 2100-2050 and 2040-1970 cm-l [7,9,13-15,21-241. The surface complexes of Ir are not as intensively studied as those of Rh o r Ru.

Besides t h e linearly and bridge bonded CO on Ir particles, t h e

existence of a n Ir dicarbonyl has been proposed on t h e basis of I R absorption bands a t 2110-2070 and 2040-1995 cm-l [25-281. By using US-Ex as support w e hoped t o obtain more insight in t h e surface chemistry of well dispersed Ru and Ir.

It will be shown that it was

possible to identify Ru tri- and dicarbonyls as well as an Ir dicarbonyl on the surface of dealuminated Y zeolite US-Ex as support.

EXPERIMENTAL M/US-Ex (1 w t % M, M=Ir,Ru, Si:Al=95) was prepared as follows.

US-Ex

obtained by thermochemical treatment of NHIY and subsequent extraction of the non-framework aluminium species with dilute hydrochloric acid [291 w a s treated with aqueous solutions of [Ir(NH3)5C11(C1)2 and RuC13tNH4C1, respectively. The samples w e r e dried at 383 K f o r 3 h and calcined in air a t 533 K for 3 h. The structure of t h e zeolite, checked by I R spectroscopy using the KBr wafer technique, was not changed by t h e pretreatment and t h e adsorption experiments. Transmission I R studies were performed with a conventional cell made

217

from glass with K B r windows, connected to a vacuum and a gas dosing line for in situ pretreatment. The samples were pressed into self-supporting wafers wi t h a "weight of ca.10 mg/cm'. The spectra were recorded usually at r o o m temperature (r.t., 300 K) after t h e interaction of t h e pretreated samples with 5-10 Torr CO as indicated in the text. The pretreatment Torr) at r.t. for consisted of an evacuation of t h e calcined sample at least 30 min o r in a reduction with hydrogen at 573 K and subsequent evacuation a t t h e same temperature. The spectra were recorded with an FT-IR spectrometer IRF-180 (ZWG) at a resolution of 2 To obtain a good signal t o noise ratio, 100 scans were accumulated. The spectra shown in this paper are corrected for t h e background and t h e contribution of t h e windows. The TPR experiments were carried out with a flow apparatus, using a 5% H2 in N2 mixture at a gas flow r a t e of ca. 20 ml/min and a catharometric detection. 200 m g of t h e calcined sample were cooled t o 195 K in a N2 flow. A t that temperature t h e gas stream w a s switched to t h e H2/N2 mixture and, after removing of t h e dry ice bath, t h e hydrogen consumption was recorded during t h e increase of t h e temperature up to r.t. The temperature w a s then ramped with a heating rate of 3.5 K/min. After reoxidation with

ern-'.

5% O2 in A r at t h e desired temperature a subsequent second TPR r u n w a s

performed. RESULTS AND DISCUSSION Figure 1 shows t h e results of t h e interaction of CO w ith both oxidized (calcined) and reduced Ru/US-Ex. With both samples w e found well resolved s h a r p bands at 2152, 2091 and 2086 cm-' indicating that t h e band usually found a t 2 0 8 0 ~ 1 0cm-' is, at least in the case of US-Ex as support, a composite one and should be assigned together with t h e band at 2152 cm-' to a Ru tricarbonyl with slightly distorted C3v symmetry. The Ru tricarbonyl is assumed t o be localized in t h e vicinity of t h e remaining A1 atoms in t h e zeolite framework with Ru in a positive oxidation state. Besides t h e absorption bands due t o t h e tricarbonyl, a broader band is observed at 2020-2030 cm-', which can be assigned to CO linearly adsorbed on Ru m e t a l particles. A s shown in Figure l b t h e Ru tricarbonyl is also formed with reduced samples. This could be explained by a disruption of Ru-Ru bonds during t h e

218

l

A

l

J"J L

0. O f 2200 2100 2000 1900

2200

2100

.2000

a

1900

WAVENUMBERS crn-l

WAVENUMBERS cm-'

Figure 1. Infrared spectra in t h e carbonyl stretching region of Ru/ US-Ex, oxidized (a) and reduced (b), after interaction with 10 Torr CO at 473K and subsequent evacuation at 300 K.

Figure 2. Infrared spectra in t h e carbonyl stretching region of Ru/ US-Ex as in Fig.lb (a), subsequent evacuation at 473 K for 10 min (b), for 20 min (c), and interaction with 10 T o r r at 423 K (d).

interaction with CO and a subsequent oxidation by t h e surrounding hydroxyl groups [111. Obviously, only a part of t h e reduced Ru may be transferred into a surface complex of Ru ts This behavior might be connected with the tendency of Ru to form bulk Ru oxid and m e t a l particles at the outer surface of t h e zeolite during subsequent oxidation-reduction treatments 1303. This is in agreement with our TPO-TPR results, that show the formation of bulk like Ru oxide. An additional argument f o r t h e assignment of the H F bands to a tricarbonyl is the behavior of the complex during t h e evacuation at higher temperatures (Figure 2). During desorption at 473 K t h e HF triplett transforms into a doublett with carbonyl stretching bands at 2092 and 2031 cm-l. This doublett is consequently assigned t o a dicarbonyl Ru(C012.

.

219 The

decarbonylation is

reversible

and t h e tricarbonyl can be obtained again by interaction of the dicarbonyl with CO at 423 K (Figure 2d). Figure 3 shows t h e results of t h e interaction of CO with oxidized and reduced Ir/US-Ex at 523 K. With t h e oxidized sample t h e formation of a well defined dicarbonyl of Ir is observed with carbonyl stretching

at 2108 and 2037 cm-' and weak satellites at 2091 and 2006 an-'. The assignment of these bands t o t h e dicarbonyl with t h e corresponding 1 3 C 0 satellites has been proven by a ligand exchange with isotopically labelled CO. The bands

b

0.0 2200

2100 ~

2000

1900

WAVENUMBERS c m - l

Figure 3. Infrared spectra in the carbonyl stretching region of Ir/ US-Ex, oxidized (a) and reduced (b), after interaction with CO at 523 K and subsequent evacuation a t 300 K.

results

of

a calculation in an force field [311 are

shown in Table 1. Thus, t h e interaction of Ir/US-Ex with CO results, similar to Rh/US-Ex, in the formation of a well defined dicarbonyl. There are, on

Table 1. Observed and calculated wavenumbers

13 Ir( C 0 l 2

kCO= 1734 Nm-',

c0-co.--

i

(ern-')

for I r ( C 0 I 2

2108

2037

2107

2036

2091

2006

2090

2006

2058

1990

2060

1991

59 Nm-'

220

the other hand, some significant differences: At first, t h e wavenumbers and, consequently. t h e force mnstants ot the ( 3 0 stretching are at lower values indicating a higher extent of 11-back bonding as compared with t h e Rh

[ll.

dicarbonyl (kCO= 1757 Nm-l)

Secondly, t h e formation

dicarbonyl needs more severe conditions ( T

which is formed already a t r o o m temperature. able to obtain

an

Ir

dicarbonyl

of

the Ir

> 473 K) than for Rh dicarbonyl, And, finally, w e were not

starting from

the

reduced

sample.

The infrared spectrum (Figure 3b) shows only a broad absorption band at ca. 2020

ern-',

which could be assigned to CO Linearly adsorbed on t h e Ir

surface. To explain t h e different behavior of oxidized and reduced Rh, Ru and Ir on US-Ex during t h e interaction with CO, w e performed TPR-TPO experiments. Rh is

Figure 4. shows t h e reduction profiles for t h e calcined samples.

reduced a t temperatures much lower than those necessary to reduce Ru and Ir.

A s the formation of well defined surface carbonyls by interaction of

t h e oxidized samples with CO needs primarily a reduction of t h e m e t a l ions by CO itself, it s e e m s t o be logically to expect higher temperatures for t h e formation of t h e surface carbonyls in t h e order Rh

< Ru < Ir.

The

other point is t h e oxidative disruption in t h e case of Rh and Ru but not Ir to

form

well

defined

surface

carbonyls

with

the

reduced

samples.

Subsequent TPO-TPR experiments showed t h a t after a reoxidation to 473 K ca. 60% of Rh and 90% of Ru had been oxidized, whereas only 10% of reduced Ir/US-Ex

had been oxidized by TPO at 548 K.

These results indicate t h e

difficulty to oxidize Ir on US-Ex as support and may explain t h a t we could

TPR profiles Figure 4. of M/US-Ex (M=Rh,Ru,Ir). The first peak near 200 K is due t o the fast desorption of physically adsorbed N2 after removing of the d r y ice bath.

4

221

I

not obtain any Ir ( C 0 l 2 on t h e reduced sample. It should be noted that Solymosi et al. [281 observed t h e formation of IrI( C 0 l 2 during t h e interaction of CO with reduced Ir on A1203 as support.

This different

behavior might be due t o a different particle size dependend on t h e support (the oxidative disruption is only expected f o r highly dispersed metals) o r to a different character of t h e hydroxyl groups, which are involved in t h e oxidation. The results of t h e present study are summarized in Table 2. For comparison reasons, t h e well defined carbonyls and nitrosyls of Rh on US-Ex

are also included. As in the case of Rh surface complexes 11-41 US-Ex act as a kind of unique matrix for t h e formation of well defined surface carbonyls.

The following properties of US-Ex might be responsible for

these effects: the remaining amount of Al atoms as centres for the a SkAl ratio of lccalization of cationic carbonyl species is small (at At t h e same time, also t h e amount of other cations (as Na' in t h e case of NaX or Nay) and adsorbed molecular w a t e r is rather small in US-Ex. The carbonyls formed in US-Ex are, therefore, isolated from each other and free of interaction with other

ca.100 only about 2 per unit cell).

species in t h e supercages of the zeolite framework.

Table 2.

W e l l defined surface compounds on US-Ex

Ru(CO12,

Ir(co)2,

c2v c2v

Rh1(CO)2,

c2v

Rh1(C0I3, Rh'(CO)4,

c3" c2v

Rh1(NO)2, Rh'(CO)(NO)2

c2v

I

I

Rh(CO)2(NO)

2092

2031 2108 2037

this work this work

2118 2053

1

2119 2083

3 3

2151 2135 2124 2102

2160 2128

2111

1855 1780 1770 1707 1785

3 3 4

222

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 28 29 30 31

I.Burkhardt, D.Gutschick, U.Lohse and HMiessner, J.Chem. Soc., Chem.Commun., (19871, 291. H.Miessner, I.Burkhardt, D.Gutschick, A.Zecchina, C.Morterra and G.Spoto, J.Chem.Soc.,Faraday Trans., 1, 83 (1989) 2113. H.Miessner, I.Burkhardt, D.Gutschick, A.Zecchina, C.Morterra and G.Spoto, J.Chem.Soc.,Faraday Trans., & 6 (1990) 2321. H.Miessner, 1.Burkhardt and D.Gutschick, J.Chem.Soc., Faraday Trans., 86 (1990) 2329. zA.Davydov and A.T.Bell, J.Catal., 49 (1977) 332. H.Yamasaki, Y.Kobori, S.Naito, T.Onishi and K.Tamaru, J.Chem. Soc., Faraday Trans., 1,D (1981) 2913. A.Zecchina, F.Guglielminotti, A-Bossi and M.Camia, J.Catal., 74 (1982) 225. J.T.Kiss a n d R.D.Gonzales, J.Phys.Chem., 88 (1984) 892. S.Uchiyama and B.C.Gates, J.Catal., 110 (1988) 388. S.Dobos, I. Boszormenyi, J.Min k and L. Guczi, Inorg. Chim.Acta, & I (1988) 37. F.Solymosi and J.Rasko, JXatal., 115 (1989) 107. G.D.Lei and L.Kevan, J.Phys.Chem., 24 (1990) 6384. E.Guglielminotti and G.F.Bond. J.Chem.Soc.. Faraday Trans., (1990) 979. V.L.Kuznetsov, A.T.Bell and Y.I.Yezmakov, J.Catal., 65 (1980) 374. H.KnMnger, YZhao, B.Tesche, R.Epstein, B.C.Gates and J.P.Scott, Faraday Disc. Chem.Soc., 22 (1981) 53. L.D'Ornelas, A.Theolier, A.Choplin and J.-M.Basset, Inorg.Chem., 27 (1988) 261. G.H.Yokomizo, C.Louis a n d A.T.Bell, J.Catal.,lXl (1989) 1. J.L.Robbins, J.Catal., 115 (1989) 120. M.F.Brown and R.D.Gonzales, J.Phys.Chem., 80 (1976) 1731. H.-W.Chen, Z.Zhong and J.M.White, J.Catal., 90 (1984) 119. J.Evans and G.S.McNulty, J.Chem.Soc., Dalton Trans., (1984) 1123. J.J.Verdonck, R.A.Schoonheydt and P.A.Jacobs, J.Phys. Chem.. 82 (1983) 683. K.Asakura, K.-K.Bando and Y.Iwasawa, J.Chem.Soc., Faraday Trans., (1990) 2645. T.Mizushima, K.Tohji, Y.Udagawa a n d A.Ueno, J.Phys.Chem., @ (1990) 4980. P.Gelin, G.Coudurier, Y.Ben T a a r i t and C.Naccache, J.Catal., 70 (1981) 32. K.Tanaka, K.L.Watters and R.F.Howe. J.Catal.,E (1982) 23. P.Gelin, A.Auroux, Y. B e n Taarit and P.C.Gravelle, Appl. Catal., 46 (1989) 227. F.Solymosi, E.Novak and A.Molnar, J.Phys.Chem., 94 (1990) 7250. H.Stach, U,Lohse, H.Thamm and W.Schirmer, Zeolites, 6 (1986) 74. J.J.Verdonck, P.A.Jacobs, M.Genet and G.Poncelet, J.Chem.Soc., Faraday Trans.1, 76 (1980) 403. P.S.Braterman, Metal Carbonyl Spectra, Academic Press, London, 1975.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

223

A coiparative study of s t a t e and reactivity of copper

ions embedded i n various wlecular sieve materials K.-P. Wendlandt, F. Vogt. W. Morke and I. Achkar Department of Chemistry. Technical University "Carl Schorlemmer" Leuna-Merseburg, Merseburg 4200, Germany

Abstract The coordination. localization. stabilization and reduction of copper ions embedded in various zeolitic and oxidic materials were studied using EPR and TPR. Coordination polyhedra and kind of ligands in the copper complexes after thermal treatment at 250 and 50OoC has been evaluated and their relation to TPR results is shown. The covalent bond character of the copper-matrix interaction increases with increasing Si02/A1203 mole ratio in zeolites.

1. IliTRODUCTIOH

Coordination , localization and stabilization of transition metal ions in zeolitic materials, apart from activation conditions, depend on the structure and composition of the zeolite matrix. To evaluate the extent of such a matrix effect CuZ+ions are used as a metal ion probe in this work , and copper ion-matrix interaction in different zeolites and oxides has been studied by EPR and TPR . The CuZ+ion is especially suited as an ion probe, since the d9 electron configuration enables a direct EPR characterization and because the redox behaviour o f the system Cu2+/Cu+/Cuo. which can be followed by TPR and IR investigations, is very sensitive to ligand effects. Besides the use as a model system, copper exchanged zeolites themselves are of technical interest. e.g. as hydrocracking catalyst components [l]. The copper ion redox behaviour is important for cocatalytic effects during catalyst activation (reduction of Fe-zeolites ) and catalyst regeneration.

224

2. EXPERIHEHTAL

Materials Samples containing 0 , 5 wt % copper were prepared by ion exchange (2h, 98 O C , 0.05M C u ( N 0 3 ) ~ solution) with zeolites (NaA-2.1, Nay-5.3, HY-5.3. NaM-10.0. HM-10.0. NaZSM-5-23.5. HSZM-5-23.5) and by impregnation of oxides and zeolites ( f - A l ~ 0 ~ . S i O ~ . S i O z / A l ~ 0 ~ -NaZSN-5-23.5. 0.5 HZSM-5-23.5) using a 0.C5 M aqueous solution of Cu(N03)~.The numbers behind the sample designation characterize the SiOz/A1203 mole ratio. EPR measurements EPR spectra were recorded at -150 C using ERS 220 (ZWG Berlin) after dehydration at different temperatures (25OoC, 5OOOC) in a stream of argon. TPR measurements Temperature Programmed Reduction (TPR) was 7OOOC in a stream of argon containing 8 vol heating rate of 8OC min-I.

%

studied up H2 with a

to

3. RESULTS AIID DISCUSSIOH

3.1 EPR Investigationst Theoretical

background in modified zeolites always give rise to the same groups of g,,values ( g , , = 2.32. 2.35. 2.38) [2-31 .these values can be attributed to very similar [CU(pSlO),(HzO)b(OH).] species in the different zeolites. On account of the pronounced d x - p~ interaction between silicon and oxygen a predominance of the T-acceptor properties of the ( p s i - 0 - ) fragment is to be expected. Following the "rule of medium environment" [4] the g,, values should decrease already at a partial substitution of the ligands H20.OH- by (=Si-0-). Thus the [Cu(eSi-O-)a(H20,0H)x] species can be classified and the indices x.y derived from g,,if the gII values of the both border cases are known, namely x=6. y=O ([Cu(H~0)6]2+) and x=O,y=4 (Cu(=Si-O-)a), and as long as holoedric symmetry is preserved. Within the frame of crystal field theory it is possible to derive additional information on the coordination number x of CuLx polyhedra from the quotient 4 g r / l! g,. I which attains the values 4 for octahedral, 5.4 for quadratic pyramidal and 6.9 for quadratic coordinations [5]. Further discrimination between the ligands H20 and OH- is based on the reasonable assumption that g , , [ C U ( G ~ - O ) ~ H Z O>] gll[Cu(=Si-O-)aOH]. The extend of an hemiedric deformation can be evaluated from the quotient g , , / A I I [6] .as the Cu4p-function (of the 3da4p configuration) is involved in the A t , value [7-81. From equation (1) follows that with increasing tetrahedral distortion A n decreases and g , , / / A , , f (left term in equ. (1)) possesses an A s CuZ+ ions

225

indication opposite to the d electron based A,,component (right hand term in equ.(l))

where Exy - E x 2 - y 2 , A L E Exz.yz - E x 2 - Y 2 r h and h , = spin orbit coupling constant of Cu2+ ions with

A,,=

the electron configuration 3d9 and 3d94p. respectively, corresponding to - 829 cm-1 and -925 cm-1 . Pd and P p = dipol term of the free ion with configuration 3d9 and 3d9 4p [S]. K = isotropic part of the hyperfine interaction of the CuZ+ion (K = 0.43) 191. The degree of covalency ( a 2 ) has been described by KIVELSON and NEIMAN [9]:

evaluated

from

A,,

3 . 2 R e s u l t s d e r i v e d from EPR measurements

EPR spectra parameters and conclusions derived on the coordination of the copper polyhedra are summarized in table 1 and 2. Results obtained with samples pretreated at 5OO0C (table 1) should have a closer connection to the TPR investigations (same pretreatment temperature) than the results compiled in table 2 which were obtained after pretreatment at 250OC. However, at that medium temperature only part of the water ligand molecules in the copper polyhedra are substituted by the zeolite and consequently differences in the ligand behaviour of different zeolites are easier to be followed. The g,,values (table 2 ) can be divided into the following groups:1,2.3,4 and 6 (column 3). which are graduated: 1>6>2>3>4. All g,I values ( with the exception of g,, = 2.474group 1) characteristic of Cu2+ ions in tetrahedral coordination [ l o ] are lower than those for group 6 corresponding to [CU(Hz0)6]2+ ions in untreated samples. The sequence of g , ,values (6 ~ 2 ' . 2" > 4. 4’.4") can be explained by dehydration and ligand exchange (water vs. zeolite framework). A s described above, propositions for copper coordination (column 5) were derived from g,, and A g , , /@gP Additional information on the distortion of coordination

226

Table 1: EPR and FMR results for Cu2+ containing zeolites and oxides pretreated at 5OOOC sample C uN aY

4II +0.005

g, tO.O1

A l l- 1 0 - 4 [cm-11

2.362 2.317 2.380 2.322 2.383 2.368 2.359 2.338 2.328 2.306 2.466 2.340 2.331 2.313 2.372 2.320 2.340

2.06 2.06 2.07 2.07 2.07 2.07 2.07 2.07 2.06 2.06 2.11 2.08 2.06 2.06 2.07 2.06 2.06

133.2 179.6 136.4 177.1 167.9 167.9 120.2 172.4 165.5 173.4 111.2 143.3 168.4 151.0 143.5 165.4 146.3

2.362 g<2 2.320 Cu/Si02 / 2.362 A1203 2.331

2.06 2.06 2.06 2.06

163.5 173.9 142.2 168.4

CuHY CuNaZSM-5 CuHZSM-5

CuNaA CuNaM CuHM Cu/SiOz Cu/Alz03

L

=

Coordination

TPR-peak maximum ["CI

Cu(=Si-O-)2L2- 3 330 Cu(=Si-O-)4 481 Cu( =Si-0- )L3 391 C~(=si-o-)~L 391 Cu(=Si-O-)L4 400 Cu( =Si-0-)2L3 441 Cu(sSi-O-)zLz 557 414 Cu(=Si-O-)3L2 CU(=S~-O-)~L 414 C~(aSi-o-)~ 447 CU(=S~-O-)~L 521 C U ( ~ S ~ - O - ) ~ L ~ 375 Cu(=Si-O-)4L 426 Cu(1Si-0-)4 486 305 CU(=S~-O-)~L~ 443 Cu(=Si-0-)4 347 CU(=S~-O-)~L~ Cu silicate T>600 338 CU(=Al-O-)zL2 462 CU(=Al-O-)aL 393 Cu(=Al-O-)2LZ 393 CU(=S~-O-)~L~

OH-, 0 2 - formed by hydrolysis of water

polyhedra was obtained from the quotient g,,/A,l(column 8 in tab. 2). This ratio is a characteristic value for tetrahedral ( between 200 and 700 cm) and tetragonal (between 1 0 5 and 135 cm) coordination of Cu2+ ions 161. The tetrahedral distortion increases with the rise of gll/ A , , ( < 180 cm [ll] ) . From table 2 can be seen that the proposition or group 1 corresponds to this classification . All signals of groups 2’ and 2" exhibit a gIl/AIl ratio close to those of [CU(Hz0)6]2+ ions. consequently there is no essential tetrahedral distortion in these cases. In the oxidic matrizes the tetrahedral distortion of the Cuz+ complexes increase with increasing silica content (Cu/Si02 > Cu/SiOz/ A1203> Cu/A120, ) , whereas in the zeolites it is only in CuZSM-5 that a tetrahedral distortion can be observed. The covalence parameters a * are given in the last column of table 2 . If there is a given kind of polyheder in all samples (and this is true for the pyramidal complex with 4 (=Si-0-) groups as ligands), from a2 values the covalent character of the copper-zeolite interaction can be evaluated f a 2 = 1 for ionic bond, a 2 = 0 . 5 for covalent bond [ll]).

221

Thus the following sequence of a 2 is obtained indicating an increase of covalent bond character with increasing silica content from CuNaA t o CuNaZSH-5: CuNaA
Table 2 : EPR parameters and assignation for Cuz+ containing zeolites and oxides pretreated at 25OOC Sample

gfJ

Cu(NH4)izAI) CuNaY

Ag,,/AsL Coordination Group

2.474 2.390 2.371 2.325 CuHY 2.385 2.320 CuNaZSH-5 2.374 CUHZSH-5 2.335 2.325 CuNaH 2.321 CuHH 2.382 Cu/Alz03 2.320 cu/sio2 2.340 Cu/SiQz/Alz03 2 . 3 3 5

6 2' 4 2' 4' 2' 3 4 4" 2" 4'' 3' 3

4.74 4.9 5.3 5.4 5.5 6.4 5.3 5.2 5.4 5.1 5.4 5.3 5.2 5.2

CuNaA

2.486

1’

6.9

1.982

5

-

1)

1

9,1/AtJ

a2

1-1 240 170 171 130 166 134 161 145 142 139 170 134 160

0.85 0.85 0.83 0.88 0.85 0.86 0.85 0.85 0.87 0.85 0.79 0.82 0.81

145

0.82

180 305

0.93

-

parameters from [ l o ]

3.3 TPR Invcatigationa

I n fig.1 TPR curves measured after sample pretreatment at are given together with the corresponding reduction degree (Cuz+ t o C u * ) obtained up t o 700OC. With the exception of Cu/SiOz a reduction degree of (100% is mainly due to autoreduction during the pretreatment and consequently, after a pretreatment with oxygen ( 2 5 0 ° C . 2 h ) the reduction degree amounts t o 100%. The possible appearance of significant amounts of Cu+ during the reduction of the investigated samples could also be excluded by parallel IR investigations on the same samples [ 1 2 ] . 5OOOC

228

7 L *I.

347

Cu /Si 02 100'1

/

I

J

VI c

._ c

414

\\

Cu /Si02 /A120,

3

n

-

L

0

W

x 0 L

a

3 I n

I I

) (

CuHNaY

100

300

Temperature,

500 'C

L2%

-

The low t e m p e r a t u r e p e a k (201OC -CuHZSM-5/1; 208OC due to -CuNaZSM-5/1) i s a oxidic reduction of clusters containing antiferromagnetically coupled C u z + i o n . The f o r m a t i o n o f clusters just in these samples could have been expected because i n t h e s e samples (unlike to the other zeolite samples) copper h a s been introduced by impregnation. T h i s peak is a b s e n t a f t e r introducing Cu2+ into ZSM- 5 by careful ion exange (pHr4. 98OC). i s shown i n f i g . 2 . which 300 O C The TPR p e a k s a t in t h e z e o l i t i c samples correspond to the reduction of c o p p e r i o n s in different positions and coordination polyhedra, respectively. F i g u r e 1: TPR c u r v e s f o r 0 . 5 w t % Cu2+ c o n t a i n i n g z e o l i t e s and o x i d e s pret r e a t e d a t 5OOOC ( r e d u c calculated consumption)

from

H2

F o r p r e c a l c i n e d CuHNaY s a m p l e s t h e p r e f e r e n t i a l o c c u p a t i o n o f S l p o s i t i o n s by t h e Cu2+ i o n s h a s b e e n shown [13]. which i s i n a c c o r d a n c e w i t h t h e a p p e a r a n c e of o n l y one r e d u c t i o n peak a t t h i s sample. Cu2+ i o n s s t a b i l i z e d a t t h e s u r f a c e o f S i 0 2 a n d A 1 2 0 3 g i v e r i s e t o r e d u c t i o n p e a k s a t 300-380OC. The r e d u c t i o n o f s u r f a c e s p i n e l i n C u / A 1 ~ 0 3 i s i n d i c a t e d a t 462OC . The r e d u c t i o n o f c o p p e r s i l i c a t e i s n o t f i n i s h e d e v e n a t 700OC.

229

In fig.2. TPR curves of acidic CuHand nonacidic CuNasamples are compared. From the different reduction degrees follows that the autoreduction is more intensive in the case of the acidic samples corresponding to a higher covalency of the CuO(zeo1ite) bonds in the more acidic samples.

UHZSM-5 /I

uNoZSM-5 /I

hNaZSM-5 ZuHZSM -5 :uH M

Figure 2: TPR curves for 0 . 5 wt % Cu2+ containing acidic and nonacidic zeolites pretreated at 5 0 O O C (I = impregnated samples )

CU NQ M

Cu H NoY CuNoY CuNaA

100

300

Temperature,

500

-

700

"C

Relation between EPR and TPR results There is a good correspondence between the g,:parameters and the number of TPR reduction peaks (cf. table 1 column 2 and 6 ) leading to the supposition that both , g,,and reduction temperature , are governed by the same factors, especially by the kind of coordination polyhedra and the copper matrix interaction (covalency).This could be affirmed by additional experiments following the reduction process by EPR. On this basis , for the 5OOOC precalcined samples, g!. parameters can be attributed to reduction intervals as follows: 220 - 2aooc: g.. =2.38 - 2.34 380 - 45OOC: g: =2.34 - 2 . 3 2 4 5 0 - 5 0 0 0 ~ :g =2.32 - 2.30 > 500 C:tetrahedral coordinated Cu copper in silicate (Cu/SiOz ; g -Z 2 ):>600 C. This assignation implies that the reduction temperature increases with increasing number of (aSi-0-) ligands. The most easily reduced species are oxidic clusters which

230

contain antiferromagnetically coupled copper ions. Copper ions which occupy tetrahedral coordinations are remarkably stabilized in this samples. Such coordination symmetry is found in CuHZSM-5 (g,,= 2 . 3 5 9 ) and in CuNaA ( g , , = 2 . 4 6 6 ) .

4.

COlDCLUSIONS

From EPR parameters conclusions on the coordination polyhedra and kind of ligands in copper complexes in various zeolites and oxides have been derived. Different g,, values can be associated with different reducibilities of the copper ions. From EPR results the following sequence of increasing covalent bond character of the zeolite-Cuz+ interaction is obtained: CuNaA
5

K.-P. Wendlandt, H.Bremer. F.Vogt, W.Reschetilowski. W.Morke. H-Hobert. M.Weber and K.Becker. Appl. Catal. 3 1 (1987) 65. M.W. Anderson and L.Kevan, J.phys.Chem. 9 1 ( 1 9 8 7 ) 4 1 7 4 . A.V. Kucherov and A.A. Slinkin. Zeolites 6 ( 1 9 8 6 ) 1 7 5 . H.L. Schlafer and G. Gliemann. Einfuhrung in die Ligandenfeldtheorie Akademische Verlagsgesellschaft Geest & Portig KG. Leipzig 1 9 6 7 . p . 8 7 . W. Morke. F . Vogt and H. Bremer. Z.anorg.allg.Chem. 4 2 2

6

U. Sakaguchi and A.W. Addison, J.chem.Soc..Dalton

7 8 9 10

M. Sharnoff. J.chem.Phys. 42 ( 1 9 6 5 ) 3 3 8 3 . G.F. Kokoszka. C.W. Reirnann. and H.C. Allen,Jr.. J.phys.Chem. 7 1 ( 1 9 6 7 ) 1 2 1 . D. Kivelson and R.Neimann. J.chem.Phys. 3 5 ( 1 9 6 1 ) 1 4 9 . M . Narayama and L. Kevan. J.phys.Chem.,Solid State Phys.

11

0. Cannistraro. G. Giugliarelli, G. Onori and

1

2 3 4

(1976) 273. Trans.1979,

600.

16 ( 1 9 8 3 ) 3 6 1 .

12 13

E. Rongoni, Biophys-Chem. 2 2 ( 1 9 8 5 ) 1 0 7 . F . Vogt. I. Achkar. K.-P. Wendlandt. H. Hobert and M. Meier, Z.Chem. 30 ( 1 9 9 0 ) 1 1 2 . P.Gallezot. Y . Ben Taarit and B. Imelik. J.Cata1. 2 6 (1972) 295.

P.A.Jacobs et al. (Editors),Zeolite Chemistry and Catalysis

231

01991 Elsevier Science PublishersB.V., Amsterdam

ACIDITY, REDOX BEHAVIOUR AND STABILITY OF CoAPO MOLECULAR SIEVES OF STRUCTURE TYPES 5 , 11, 34 AND 16 Bettina Kraushaar-Czarnetzki, Wilma G.M. Cees A. Emeis and Wim H . J . Stork

Hoogervorst, Ronald

R. Andrea,

Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), Badhuisweg 3 , 1031 CM Amsterdam, The Netherlands

Abstract In CoAP04-5, -11, -34 and -16, a (Al+Co)/P ratio greater than 1 indicates the presence of non-framework cobalt and/or aluminium. The amount of framework Co(I1) and, hence, the number of potential Bronsted acidic sites, can be determined by means of electron absorption spectroscopy. The maximum amount o f cobalt that can be incorporated depends on the structure type and decreases in the order CoAP04-5 > -11 > -16 > - 3 4 . Framework Co(II1) redox reactions, which are cobalt can undergo Co(I1) <-> accompanied by changes in the net framework charge, in the cation exchange capacity and in the number of acidic sites. In the case of CoAP04-5 and -11, these redox reactions are completely reversible, whereas CoAP04-16 and -34 lose crystallinity and framework cobalt during each redox cycle.

1. INTRODUCTION Bronsted acidity in aluminophosphate molecular sieves re uires isomorphous substitution of lower valency elements for A13+ or +’P in [A102]- and [PO2]+ building units of the Alp04 framework. MeAPOs, in particular, are derivatives of Alp04 molecular sieves in which divalent metals such as magnesium, zinc, cobalt, manganese etc. are considered to replace part of the aluminum [l-31. Elemental analysis of MeAPOs should, therefore, reveal an ideal ([A102]-+ [Me02]*-)/[PO2]+ molar ratio of 1, where the amount of Me should indicate the framework charge and, hence, the number of Bronsted acidic sites. In fact, "real" MeAPOs usually exhibit ratios greater than 1, giving rise to the question how much Me and/or A1 is actually incorporated. In zeolites, tetrahedral framework aluminium can be distinguished from non-tetrahedral extra-framework species by means of 27Al MAS NMR. However, some AlPOs are known to contain 5- or 6-coordinated A1 in the framework, which complicates a quantitative determination 14-61. Quantitative methods for the monitoring of substituting metals Me are, therefore, required. In the case of transition metal ions, possible changes in the oxidation state must also be taken into account, since the charge n of [Me02In- building units should directly affect the number of Bronsted acidic sites. This paper is concerned with the characterization of CoAPO molecular sieves of different structure types. The purpose is to distinguish and determine the framework and the extra-framework cobalt, to monitor changes in oxidation state and associated acidic properties, and to study the stability of the different structure types in repeated redox reactions.

TABLE 1. Specification of the AlPO and CoAPO samples under investigation. Sample

Sample Type

A1203 P2O5

No.

1 2 3

4

5 6 7 8 9 10

11 12 13

14 15 16 17 18 19

M -N

Product Composition’)

Gel Composition

1 P2O5 * 1 Pr2NH3) * 45 H20 AlP04-l12) 1 A1203 * 1 P2O5 * 1 Pr2NH3) * 45 H20 CoAP04-11 0.03 COO * 1 A1203 * 1 P2O5 * 1 Pr2NH3) * 45 H20 CoAP04-11 0.05 COO * 1 A1203 * 1 P2O5 * 1 Pr2NH3) * 45 H20 CoAP04-11 0.075 COO * 1 A1203 * 1 P2O5 * 1 Pr2NH3) * 45 H20 CoAP04-11 0.1 COO * 1 A1203 * 1 P2O5 * Pr2NH3) * 45 H20 ". CoAP04-11 0.17 COO * 1 A1203 * 1 P2O5 * Pr2NH3) * 45 H20 CoAP04-5 0.03 COO * 1 A1203 * 1 P2O5 * 1 Pr3N4) * 45 H20 CoAP04-5 0.1 COO * 0.86 A1203 * 1 P2O5 * 1 Pr3N4) * 45 H20 CoAP04-5 0.17 COO * 1 A1203 * 1 P2O5 * 1 Pr3N4) * 45 H20 CoAP04-16 0.03 COO * 1 A1203 * 1 P2O5 * 1 Q5) * 45 H20 CoAP04-16 0.05 COO * 1 A1203 * 1 P2O5 * 1 Q5) * 45 H20 CoAP04-16 0.1 COO * 1 A1203 * 1 P2O5 * 1 Q5) * 45 H20 CaAP04-16 0.1 COO * 1 A1203 * 1 P205 * 1 Q5) * 45 H20 CoAP04-34 0.05 COO * 1 A1203 * 1 P2O5 * 1 TEAOH6) * 40 H20 CoAP04-34 0.075 COO * 1 A1203 * 1 P2O5 * 1 TEAOH6) * 40 H20 CoAP04-34 0.1 COO * 1 A1203 * 1 P2O5 * 1 TEAOH6) * 40 H20 CoAP04-34 0.15 COO * 1 A1203 * 1 P2O5 * 1 TEAOH6) * 40 H20 CoAP04-34 0.15 COO * 1 A1203 * 1 P2O5 * 1 TEAOH6) * 40 H20 AlP04-ll

1 A1203

*

-

COO [moll

[wt.-%]

1.0

1.0

0.0

0.0

1.0

1.0

0.10

2.47

1.06

1.0

0.03

0.60

0.93 0.91

1.0

0.04

1.11

1.0

0.06

1.52

1.01

1.0

0.10

2.26

1.07 1.05

1.0

0.19

4.04

1.0

0.03

0.65

1.0 1.02 1.0

1.0

0.11

2.40

1.0

0.18

1.0

0.05

3.89 1.05

0.98

1.0

0.09

2.02

1.13

1.0

0.12

2.47

0.95

1.0

0.15

3.33

1.15

1.0

0.07

1.38

1.15

1.0 1.0 1.0 1.0

0.11

2.27

0.12

2.45

0.16

3.10

0.15

3.22

1.17

1.1 1.08

1) on anhydrous base, after removal of the template; 2) impregnated with Co2+; 3) Pr2NH = tripropylamine; 5) Q = quinuclidine; 6) TEAOH teraethylammonium hydroxide.

4 ) Pr3N

total Col)

=

dipropylarnine;

233 2. EXPERIMENTAL

2.1. Sample preparation CoAPO samples of structure types 5, 11, 34 and 16 as well as samples of ~ 1 ~ 0 ~ -were l l synthesized according to the procedures described in the patent literature [l-31. Calcinations were performed by heating in air at 823 K for 3 h. The oxidation of CoAPOs was achieved in the same way. CoAPOs were reduced by exposure to liquid methanol at room temperature, followed by washing with water and drying at 393 K. A sample containing exclusively extra-framework cobalt was prepared by calcination of AlPOh-ll and subsequent pore-volume impregnation with an aqueous solution of cobalt acetate. 2.2. Characterization All samples were analyzed by means of X-ray diffraction and elemental analysis. Additionally, the cobalt distribution throughout the samples was studied by means of scanning electron microscopy in combination with electron microprobe analysis. Prior to the measurements, the samples were imbedded in resin, cut and subsequently polished with diamond paper. Diffuse-reflectance electron absorption spectroscopy was performed on a spectrometer equipped with a diffuse-reflectance Perkin-Elmer 320 W-VIS accessory. Infra-red spectra of self-supporting wafers were recorded on a Digilab FTS-15/90 Fourier transform instrument after in-situ drying at 750 K. Pyridine adsorption was performed by contacting with pyridine vapour at 300 K for 15 min. Subsequently, physisorbed pyridine was removed by pumping off and increasing the temperature to 425 K. The infra-red band intensities of absorbed pyridine were determined from the difference between the spectra taken before and after pyridine adsorption. 31P solid-state NMR measurements were performed at 109.55 MHz using excitation pulses of 5 p s and relaxation delays of 10 s . The magic-angle spinning rate was 4.2 kHz. 3 . RESULTS AND DISCUSSION

3.1. Chemical composition and cobalt distribution The X-ray diffraction patterns of the A D O 4 and CoAPOh samples indicate pure phases, except for some samples of CoAP04-34, which might contain amorphous material. However, in view of the product compositions (Table 1) it must be concluded that many samples are not pure since the (Al+Co)/P ratio is considerably greater than 1. In these cases, part of the aluminium and/or cobalt must be present in extra-framework species. In particular, samples prepared with high cobalt concentrations in the gel and samples of CoAP04-34 exhibit unfavourable element ratios. These samples are also often less homogeneous in that they can contain white particles, whereas the major phase exhibits a blue colour. Several CoAPOs have been investigated by means of scanning electron microscopy in combination with electron microprobe analysis. Figure 1 shows representative element maps of a CoAP04-5 sample with a metal/phosphorus ratio close to one (sample 9 , (Al+Co)/P 1.05) at different magnifications. Clearly, there are no separate particles of alumina or aluminium phosphate, but rather crystals in which the elements aluminium, phosphorus and cobalt are accompanied by each other. The cut through a large single crystal, moreover, shows that cobalt is homogeneously distributed. Comparable results were obtained from other samples with

-

234

Figure 1. Element maps of as-made CoAP04-5 (sample 9) at two different magnifications.

Figure 2. 31P MAS NMR spectra of (A) as-made CoAP04-11, sample 6; (B) AlPO4-ll after calcination and cobalt impregnation, sample 2; ( C ) CoAP04-11 (sample 6) after calcination. I

150

'

~

~

~

100

l

'

'

50

"

l

'

'

0

~

'

l

-50

~

'

'

~

-100

I

"

"

-150

I

'

~

PPM

'

'

I

'

235

metal/phosphorus ratios close to one. In contrast, the element maps of a CoAP04-34 with (Al+Co)/P = 1.18 (sample 15, not shown) indicated species with relatively low phosphorus and cobalt contents besides other particles enriched in cobalt. In the case of CoAPOs possessing a three-dimensional framework with four-connected TO2 building units such as CoAP04-5, -11, -34 and -16, the metal/phosphorus ratio can indicate the presence of non-framework species. Care should be taken, however, if the material under investigation has a layered or an unknown structure. Moreover, elemental analysis cannot tell whether both aluminium and cobalt or only one of these elements forms non-framework species. The quantitative monitoring of cobalt in the framework remains a requirement in order to determine the number of acidic sites.

3.2. How to distinguish between framework and non-framework cobalt In a recent paper by Montes et al. it was suggested that isomorphously substituted paramagnetic cobalt(I1) causes strong dipolar interactions and a chemical shift anisotropy that can be monitored by means of 27A1 and 31P NMR [7]. We performed 31P MAS NMR measurements on as-made and calcined CoAPO 11 as well as on Co(I1)-impregnated AlPO4-ll. All spectra (Fig. 2) 4- numerous, strong spinning sidebands, showing that paramagnetic exhibit Co(1I) whether in framewQrk or in non-framework positions causes strong dipolar interactions. Moreover, the 31P chemical shifts of the template-free materials are the same. The only difference between CoAP04-11 and Co(I1)-loaded ~ l P o ~ - lconcerns l the spectral resolution. However, a broadening of NMR signals can also arise from structural defects and does not give evidence of isomorphous substitution. In contrast to Montes et al., we conclude that 31P MAS NMR (and most probably also 27Al MAS NMR) is no appropriate technique to prove framework substitution of cobalt. Our results clearly show that the phosphorus nuclei "feel" no difference between framework and non-framework cobalt, since the latter strongly interacts with the framework. The intense blue colour of as-made CoAPOs as compared to the pale purple colour of Co(I1)-loaded AlPOs suggests that the electronic state of cobalt and, hence, its co-ordination and oxidation state can be characterized by means of electron absorption spectroscopy. We evaluated the potential of electron absorption spectroscopy as a quantitative tool to determine the amount of framework Co in CoAPOs of different structure types. Figure 3A shows the W-VIS spectrum of a representative as-made sample of CoAP04-11, exhibiting a strong absorption at 500-650 nm. This absorption must be ascribed to a d-d electron transition of tetrahedrally co-ordinated Co in a d7 configuration (81. The three sub-maxima at 540, 580 and 626 MI can be explained by spin-orbit coupling during the spin-allowed 4A2 -> 4T1 transition and/or by Jahn-Teller distortions. For comparison, the spectrum of AlPO4-ll impregnated with cobalt is also shown (Fig. 3B). The absorption band is broader, less intense and does not exhibit the fine structure, which can be seen in the spectrum of CoAP04-11. Although framework and non-framework Co(I1) clearly show different spectral features in W-VIS, it appears that non-framework Co2+ ions in the impregnated sample give rise to an absorption band of similar position and shape This indicates a nearly tetrahedral co-ordination and, in line with the 3iP MAS N M R measurements, a strong interaction with the AlP04 framework. The electron absorption spectra of CoAPOs of different structure types (5, 11, 34 and 16) are shown in Fig. 4. In spite of the different local environments of cobalt in these framework types, the spectral features are

236

Figure 3. Electron absorption spectra of (A) as-made CoAP04-11, sample 6; ( B ) AlP04-11 after calcination and cobalt impregnation, sample 2.

800

600

400

2 00

wavelength/nrn

1.5

10 c

.0. II L

0

n Y) 0

0.5

Figure 4 . Electron absorption spectra of as-made CoAPOs of structure types 5 (sample 9 ) , 11 (sample 6), 16 (sample 13) and 34 (sample 18). 0

800

600

400

waveleng th/nrn

-200

237 identical. The differences in intensity cannot be ascribed to specific structural properties but rather to differences in the amount of Co(I1) in the framework. All as-synthesized CoAPO samples (see Table 1) have then been measured and the integrated absorption intensities were plotted against the total cobalt content. The plots (Fig. 5) show the expected linear increase in absorption intensities at low cobalt concentrations. At higher cobalt concentrations, however, the curves become flatter and approach a structure-specific maximum level. Our results suggest that the maximum amount of cobalt that can be incorporated into an AlP04 framework depends on the structure type. It is not surprising that the samples giving the most pronounced deviation from the linear correlation in Fig. 5 also exhibit the most unfavourable metal/phosphorus ratios (greater than 1.10; see chemical compositions in Table 1). Within the experimental error of diffuse-reflectance measurements it is, therefore, possible to estimate the amount of framework cobalt and, hence, the number of catalytically active sites per mass unit. It is not possible, however, to give cobalt per unit cell or framework element ratios since this would require the quantitative determination of non-framework alumina and cobalt-free AlP04 material.

160-

120/

+A

Figure 5 . Integrated absorption intensities of various as-made CoAPOs of different structure types versus total cobalt content. The electron absorptions were fitted and integrated in the range from 710 to 400 n m .

238 3 . 3 . How acidity changes upon redox reactions of framework cobalt

Recently, it was suggested from data obtained by UV-VIS spectroscopy that calcination of as-made CoAPOb-5 can result in oxidation of cobalt(I1) [ 9 ] . Figure 6 shows the corresponding W-VIS spectra of to cobalt(II1) as-made (A) and calcined CoAP04-11 (B), the latter having a green colour. The spectrum of the calcined sample exhibits the well known band at 500-650 tun and additionally a strong absorption band around 350 n m which must be assigned to a ligand-to-metal charge transfer transition from an electron of the framework to Co(II1) ions. Deconvolution of the absorption bands and determination of the difference in integrated intensity between the low-energy absorption in the spectra A and B enables the calculation of the oxidation degree after calcination. Thereby, it can be shown that 60-70% of Co(1I) in the as-made sample 6 has been oxidized towards Co(II1). CoAPOs of structure types 5, 34 and 1 6 show very much the same spectral changes upon calcination as CoAP04-11. A methanol treatment of the calcined CoAPO samples results in a colour change from green to blue, indicating the reduction of Co(II1) towards Co(I1). The electron absorption spectra of

31 5

I

626

I Figure 6 . Electron absorption spectra of ( A ) as-made CoAP04-11, sample 6 ; (B) CoAP04-11 (sample 6 ) after calcination; (C) cobalt-impregnated AlPO4-ll, sample 2; (D) cobalt-impregnated A1PO4-ll (sample 2) after subsequent calcination.

800

600

400

wavelength/ nrn

200

239 reduced samples of CoAP04-5 and -11 again exhibit the 500-650 nm band with the original intensity, giving evidence of the full recovery of Co(I1) in the framework. Spectra C and D in Figure 6 show the electron absorption spectra of cobalt-impregnated A1P04-ll before and after calcination. The broad, featureless spectrum and the absence of the band at 350 nm (D) clearly different demonstrate that non-framework cobalt shows a completely behaviour upon calcination. The corresponding sample exhibits a dark-grey colour. A methanol treatment does not affect the spectral features, indicating that non-framework cobalt species, once calcined, cannot be reduced under these conditions. Most probably, calcination of non-framework cobalt results in the formation o f a stable oxidic phase. Infra-red spectroscopy in combination with pyridine adsorption as well as sodium ion exchange experiments were applied to CoAP04-5 and -11 (samples 9 and 6) in order to monitor changes in the number of acidic sites during oxidation and reduction of framework cobalt. Our results (Fig. 7) clearly show a considerable increase in the number of Lewis and Bronsted acidic sites and in the cation exchange capacity upon reduction of Co(II1) to Co(I1). In particular, the cation exchange capacity is almost quantitatively in line with the amount of framework Co(I1) as determined by means of W-VIS spectroscopy (Fig.6).

7 1 COAPO-11

0-4

C

0.3

0.2 4

calcined

.-v) 3

.a!

0.1

0 Figure 7. Lewis and Bronsted acidic sites (determined by means of infra-red spectroscopy in combination with pyridine adsorption) and sodium ion exchange capacity o f CoAP04-5 and CoAP04-11 (samples 9 and 6) after calcination and after subsequent reduction by methanol treatment.

240

CoAPOs of structure types 5 and 11 can be exposed to repeated calcinations and methanol treatments without undergoing considerable structural changes, the oxidation state and the acidity thereby changing. In contrast, CoAP04-16 and, more pronounced, CoAP04-34 suffer from each redox reaction. Both calcination and methanol treatment result in a decrease in crystallinity and a loss of framework cobalt, as can be shown by means of X-ray diffraction and electron absorption. CoAP04-16 loses about 10% of its crystallinity during each redox cycle. CoAP04-34 suffers 30% crystallinity loss during the first calcination and becomes almost amorphous after two redox treatments. The samples under investigation show decreasing stability in the order COAP04-5 2 CoAP04-11 > CoAP04-16 >> CoAP04-34.

4. CONCLUSIONS Framework charge and, hence, cation exchange capacity and acidity in CoAPO molecular sieves are a function of the concentration of incorporated cobalt(II), which can be monitored quantitatively by means of electron absorption spectroscopy. The maximum framework cobalt acceptance as well as the stability depends on the structure type and decreases in the same order: CoAP04-5 > CoAP04-11 > CoAP04-16 > CoAP04-34. Less stable structure types such as CoAP04-16 and -34 suffer from a loss in crystallinity and framework cobalt during calcination and reduction experiments, whereas in stable structures framework cobalt can reversibly undergo redox reactions In conclusion, CoAPOs have potential for both redox Co(I1) <-> Co(II1). catalysis and acidic catalysis. In the latter case, however, fluctuating acidic properties due to changes in the oxidation state can be a serious restriction in many operations. 5 . REFERENCES

S.T. Wilson, B.M. Lok and E.M. Flanigen, US Patent No. 4 310 440 (1982). B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, US Patent No. 4 440 871 (1984). C.A. Messina, B.M. Lok and E.M. Flanigen, US Patent No. 4 544 143 (1985). J.J. Pluth and J.V. Smith, Acta Cryst. C40 (1984), 2008. J.M. Bennett, 3.M. Cohen, G. Artioli, J.J. Pluth and J.V. Smith, Inorg. Chem. 24 (1985), 1 8 8 . Y. Wu, B.F. Chmelka, A. Pines, M.E. Davis, P.J. Grobet and P.A. Jacobs, Nature 346 (1990), 550. C. Montes, M.E. Davis, B. Murray and M. Narayana, J. Phys. Chem. 94 (1990), 6425. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, John Wiley, New York, 1 9 8 2 , p. 725. R.A. Schoonheydt, R. de Vos, J. Pelgrims and H. Leeman, in P.A. Jacobs and R.A. van Santen (Eds.), Zeolites: Facts, Figures, Future , Elsevier, Amsterdam, 1989, p. 559.

241

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

STATE OF IRON AND CATALYTIC PROPERTIES FERRISILICATE ZEOLITE MOLECULAR SIEVES

OF

ALKALI-METAL-EXCHANGED

Qiubin Kana, Zhiyun Wua, Ruren Xua, Quan Weia, Shaoyi Penga, b Guoxing Xiongb, Shishan Shengb and Jiasheng Huang a Department of Chemistry, Jilin University, Changchun, China

Dalian Institute of Chemical Physics, Academia Sinica, China Abstract Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used for investigations of the state of iron and structural stability of alkali-metal-exchanged ferrisilicates, such as Li-FeZSM-5, Na-FeZSM-5 and K-FeZSM-5. With increasing electrostatic potential of the exchanged alkali metal cations, the Fe ( 2 p ) binding energy of framework Fe(II1) in ferrisilicates increased and the reduction temperature of Fe(II1) in the zeolite lattice decreased. The reduction of framework Fe(II1) in ferrisilicates resulted in breaking of the zeolite structure. The temperature-programmed and pulse reaction of ethylbenzene over ferrisilicates revealed the effect of cations in zeolites on the active centers of ethylbezene dehydrogenation. 1. INTRODUCTION

There is considerable interest in isomorphous substitution of aluminium in the zeolite framework by other elements and some papers have described the synthesis of MFI zeolites containing boron, gallium, titanium and iron as lattice elements (ref.1-3). The replacement of A1 ions with the ions of another element can modify both the acidity and pore size features of the zeolite (ref.4, 51, resulting in modification of the catalytic property of zeolite catalysts (ref.6-8). Iron substitution for Si has also been studied frequently and shown to occur for Y zeolite, MFI zeolite and other zeolites (ref.9-12). Some characterizations and catalytic abilities related to the acidic properties of ferrisilicates with MFI structure have been elucidated (ref.13, 14). H-FeZSM-5 has shown higher selectivity for the conversion of methanol to light olefins, in the synthesis of high Octane gasoline from light olefins (ref.151, xylene isomerization and benzene alkylation (ref. 16, 17) and may be of more interest for naphta reforming in the recycling mode (ref.18) than H-ZSM-5; this can be explained by weaker acid strenght in H-FeZSM-5 than that in H-ZSM-5. No special attention has been paid to other catalytic reactions and the effects of metal cations. In our previous investigations (ref. 19, 201, we reported that ferrisilicates had high catalytic activity for dehydrogenation of ethylbenzene (EB) and explored the action of framework iron in zeolite. In order to obtain more information on the state of ferric ions, the effects of alkali metal cations and the role of

242 exchanged cations in the dehydrogenation of EB, we investigated a series of alkali-metal-exchanged ferrisilicates with MFI structure, such as Li, Na, K-FeZSM-5, by X-ray photoelectron spectroscopy, X-ray diffraction, temperature-prorammed techniques and catalytic conversion of ethylbenzene. 2. EXPERIMENTAL

Ferrisilicate with MFI structure was synthesized using 1,6-hexanediammine as a template by the rapid crystallization method (ref.13, 16). The details of the synthesis will be published elsewhere. After synthesis, the materials were dried at 523 K for 1 hour and then calcinated in a He stream at 823 K for 4 hours followed by a further calcination in an air stream at 823 K for 4 hours. The calcinated ferrisilicates were converted into the hydrogen forms by repeated ion-exchange with 1 M NH4C1 solution at 357 K for 2 hours, washing with water, drying at 393 K and calcinating at 813 K. H-FeZSM-5 samples were exchanged five times at 367 K in 1 M solution of LiC1, NaCl and K C I , respectively, to yield Li-FeZSM-5, Na-FeZSM-5 and K-FeZSM-5. The ferrisilicates subjected to hydrothermal treatment at 823 K for 4 hours yielded the steamed sample. The Fe203/Fe-AlZSM-5 sample was obtained by exchange of 2.0 g of ZSM-5 (Si/Al=30) with 1.0 g of FeSOs . 7Hz0 in a deaerated aqueous solution, pH 4, under flowing Ar at 353 K for 24 hours, washing with water, drying and then impregnating with 2 ml of 1.0 % Fe(N03)3 solution, calcinating at 473 K for 1 hour and at 773 K f o r 2 hours in air. The compositions of zeolite catalysts, determined by atomic absorption and X-ray fluorescence spectroscopy, are given in Table 1. Table 1 Chemical compositions of the zeolite samples Chemical compositions Samples Fe (wt.%) H-FeZSM-5 Li-FeZSM-5 Na-FeZSM-5 K-FeZSM-5 Fe203/Fe-AlZSM-5

2.4

2.4 2.4 2.4 1.5

alkali metal/Fe -

0.71 0.70 0.75

Si/Fe 31.5 31.4

31.5 31.2

XPS measurements were performed in a MARK-I1 ESCA spectrometer with a pretreatment cell without exposure to the air. Hydrogen reduction of the sample was carried out at 723 K in the pretreatment cell. Binding energy measuremensts were related to the C ( 1 . s ) photoelectron line at 285.0eV as a reference. Structural analysis of the zeolite samples was completed in a Shimadzu XD-3A diffractometer. TPR studies were performed in a conventional apparatus equipped with a water trap, using hydrogen diluted in argon (Hz : Ar=1:9). Hydrogen consuption was followed with a catharometric detector as a function of the temperature increment of 6 K. min-l.

243 Temperature-programmed desorption (TPD) and surface reaction (TPSR) were carried out at 450 torr with a temperature increment of 15 K.min-l. A 5988 GC-MS mass spectrometer was used for detect.ion of the desorbed species. The sample was calcinated in a quartz glass reactor at 823 K in a He stream for 1 hour, cooled to 423 K and ethylbenzene was adsorbed. After evacuation at 423 K for 30 min the sample was cooled to 300 K and TPD/TPSR started. The ethylbenzene dehydrogenation reaction was carried out using the pulse method. The catalyst weight (60 80 mesh) was 150 mg. The carrier gas was highly purity N2 with a velocity of 35 ml.min-I at 293 K.

-

3. RESULTS

AND DISCUSSION

3.1. X-ray Photoelectron Spectroscopy Measurements Representive core-level spectra were measured for several samples. The binding energy data are summarized in Table 2. The data show that only one O ( 1 . s ) peak maximum at 533.0 eV is present in the XPS spectra of ferrisilicates including Li-FeZSM-5, Na-FeZSM-5 and K-FeZSM-5, while two peaks at 533.0 eV and 530.1 eV are observed for Fe203/Fe-AlZSM-5 containing both occluded Fez03 and Fe3+ located in cationic sites. The Fe (2.~3312) binding energy for Na-FeZSM-5 is 711.8 eV, higher than that of 711.1 eV for FeaOs/Fe-AlZSM-5 (the latter peak is asymmetrical, obtained by fitting the peaks at 712.0 and 710.9 eV). A 2p3/2-2pl/z splitting of 13.0 13.6 eV is observed for every sample.

-

Table 2 Binding energy data for zeolite samples (e\O Samples Li-FeZSM-5 Na-FeZSM-5 K-FeZSM-5 Fez03/Fe-AlZSM-5 Steamed Na-FeZSM-5 Steamed Na-FeZSM-5 (H2 reduction,723K,0.5h)

712.1 711.8 711. 4 712.0/710.9 712.0/710.9 712.0/710.9 709.5

725.7 725.3 725.0 724.3 -

-

533.0 533.0 533.0 533.0/530.1 533.0/530.2 -

-

All the above results are similar to those reported by Stencel et al. and Borade (221, in which the O ( 1 . s ) peak with lower value was associated with the oxygen of added Fez03 and the other O ( 1 . s ) peak with higher value was correlated to the zeolite lattice oxygen, so that the iron ions in ferrisilicates investigated are mainly situated in the lattice sites and are in the trivalent state. After hydrothermal treatment, Na-FeZSM-5 exhibits two Fe(2pw2) binding energies at 712.0 and 710.9 eV and two O ( Z s ) values at 533.0 and 530 2 eV because of partial iron migration from the framework site towards the zeolite surface. The Fe(II1) core-level spectrum of steamed Na-FeZSM-5 reduced by hydrogen at 723 K contains a new peak maximum at about 709.5 eV, accompanied by a decline in intensity of the peak at 710.9 eV, but no

244 change in the intensity of the peak near 712.0 eV. According to the conclusions regarding the spectra of iron oxide surfaces codified by Wandelt (231, that Fe(II1) has a Fe(2p3n) binding energy of 711.2 eV in all iron oxides and Fe(I1) has a binding energy of 709.7 eV in FexO, i t is considered that nonframework Fe(II1) in ferrisilicate zeolites is partially reduced to Fe(I1) by hydrogen at 723 K for 0.5 hour which is not true for the framework Fe(II1) in ferrisilicate zeolites. Three different values of Fe(2p) binding energy are observed for three kinds of alkali-metal-exchanged ferrisilicates in Table 2. The Fe(2p) binding energy of these zeolites decreases in the order Li-FeZSM-5 > Na-FeZSM-5 > K-FeZSM-5, in agreement with the decreased order of the electrostatic potentials ( e / r ) of the exchanged alkali metal cations. The exchanged cations probably interact with the framework iron in zeolite. 3.2. Temperature-programmed reduction and X-ray diffraction measurement

Plots of the rates of the uptake of hydrogen versus temperature can be seen in Figure 1, in which the first peak for the adsorption of hydrogen at about 320 K is not shown. For Li-FeZSM-5, five maxima are found approximately at 643 K, 723 K, 943 K, 984 K and 1002 K. While the former two maxima are also found with Na-FeZSM-5 at about 643 K and 723 K, the latter maxima are observed at 993 K, 1087 K and 1105 K. K-FeZSM-5 exhibits four peaks at 643 K, 723 K , 1109 K and above 1200 K. FeaOs/Fe-AlZSM-5 has only two clear peaks at 643 K and 723 K. Peaks at about 643 and 723 K are very small for all the ferrisilicates studied, while the area of the two peaks at about 643 and 723 K clearly increases followed by an addition of the peak at 1105 K and peak shift towards high temperature after hydrothermal treatment. If the TPR for Na-FeZSM-5 is performed on the sample reoxidized at 823 K (curve e), only strong peaks at lower temperature are observed and the peaks at higher temperature almost disappear. X-ray diffraction results in Figure 2 show good and comparable patterns for Na-FeZSM-5 before and after hydrgthermal treatment, except for a small decrease in peak height in 28=24.38 for the steamed sample; however the framework structure of Na-FeZSM-5 is destroyed after the procedure in TPR. Because of absence of framework Fe(II1) in Fe203/Fe-AlZSM-5 zeolite and no reduction of framework Fe(II1) in ferrisilicates below 773 K (ref.241, the two peaks at 643 and 723 K for all samples belong to the reduction of nonframework Fe(II1). The stronger peaks at higher temperature f o r alkali-metal-exchanged ferrisilicates characterize the reduction of framework Fe(II1) in zeolites lattice sites. Based on the work of Lin et al. (14) and Kan (251, different Fe[III) sites in zeolite lattice were present and some framework Fe (111) had migrated out of the lattice above 973 K for Li-FeZSM-5 and 1023 K for Na-FeZSM-5. DT-TG analysis indicated framework collapse for all the ferrisilicates investigated above 1273 K in the air. Therefore, with Li-FeZSM-5 and Na-FeZSM-5, the peaks at 943 K and 993 K are due to reduction of the thermal removal of Fe(II1) from framework sites and the maxima at 984 and 1002 K for Li-FeZSM-5 and those at 1105 and 1087 K for Na-FeZSM-5 are assigned to reduction of framework Fe(II1) in different lattice sites. The reduction temperature of framework Fe(II1) in K-FeZSM-5 is above 1200 K. Once framework Fe(II1) in ferrisilicates is reduced, the original charge balance in the zeolites is destroyed and the cell volume of the zeolites expands, resulting in breaking of the zeolite structure. Hydrothermal treatment of Na-FeZSM-5 at 823 K does not apparently destroy the zeolite

245 structure in spite of migration of Fe(II1) out of the lattice, increasing the content of nonframework Fe(II1) and the intensity of the peaks at 623 and 723 K in the TPR procedure. 3 . 3 Temperature-programmed surface reaction and catalytic dehydrogenation

of ethylbenzene The rates for the desorption of unreacted ethylbenzene and reaction products, styrene and benzene, are plotted in Figure 3. For all zeolite catalysts investigated a significant amount of ethylbenzene, retained at the surface of catalysts after evacuation at 423 K, desorbs in one or two steps at the same temperature at which dehydrogenation of EB to S T and cracking to benzene start. It was observed that the temperature of t h e maximum of the desorption rate of the reactant or reaction products in the first step is almost identical for all the samples, which may point to the presence of the same kind of adsorptive and active centers. In the second step, however, the difference in the desorption and reaction of EB over these samples is apparent.

5 373

6-73

973

1273

10

15

20

25

30

DEGREES, 2 THETA

TEMPERATURE, K

Fig. 1 TPR profiles of Li-FeZSM-5(a), NaFeZSM-5(b ) , K- FeZSM- 5 ( c ) , FezOJ FeAIZSM-5(d), Na-FeZSM-5 reoxidated after TPR(e), steamed Na-FeZSM-5(f).

2 XPD patterns of Na-FeZSM-5 (a) before hydrothermal treatment ( b ) , after hydrothermal treatment and (c) after TPR. Fig.

With H-FeZSM-5 no desorption of EB and ST occurs above 638 K and only a desorption peak of benzene is observed in the range of 638 to 923 K. Tho maximum of this peak is at about 753 K. With alkali-metal-exchanged ferrisilicates both the desorption of EB and dehydrogenation of EB to ST

246

a

2

b C

d 283

453

623

793

283

453

623

793 T(K)

T(K)

(styrene)

( ethylbenzene)

a

H-FeZSM-5

b Li-FeZSM-5

c Na-FeZSM-5 d K-FeZSM-5

Ic d 283

453

623

(benzene)

793 T(K)

Fig. 3 Temperature programmed desorption and reaction of ethylbenzene

occur; the peak area of benzene desorption near 753 K, observed over H-FeZSM-5, dramatically decreases for Li-FeZSM-5 and Na-FeZSM-5 and disappears for K-FeZSM-5. The temperature at which the maximum of the rate of desorption of EB or ST is secondly reached f o r alkali metal exchanged ferrisilicates deceases in the o r d e r Li-FeZSM-5, Na-FeZSM-5 and K-FeZSM-5 equal to 606 K, 595 K and 558 K, respectively. The difference between the first and the second maximum of EB or ST desorption decreases in the same order, so that the two desorption peaks of EB and ST cannot be resolved. The results of conversion and selectivity f o r dehydrogenation of EB to ST over H-type and various ferrisilicates modified by alkali metal are summarized in Table 3. The main product is styrene accompanied by several by-products, e.g. benzene, toluene, ethylene and methane, over- alkali metal exchanged ferrisilicate catalysts. The conversion and ST yield apparently increase and the selectivity of dehydrogenation decreases slightly with the

247

electrostatic potential of the exchanged alkali metal cation. H-FeZSM-5 shows high activity for cracking of ethylbezene and yields higher ST yield and selectivity than conversion of EB, but lower alkali-metal-exchanged ferrisilicates. Table 3 Conversion and selectivity of ethylbenzene dehydrogenation over zeolite catalysts Catalysts H-FeZSM-5 Li-FeZSM-5 Na-FeZSM-5 K-FeZSM-5

conv.% 82.9

57.6 49.2 39.3

ST select.% 32.4 94.1 96.3 97.x

ST yield %

Benzene yield

26.9 54.2

38.6

47.4 38.4

1.1 0. 6

%

2.1

Based on the above results and analyses, we confirm the influences of exchanged cations in ferrisilicates on the adsorption, desorption and reaction properties of EB over ferrisilicate catalysts. I t is interesting to note that an increase in the electrostatic potential of exchanged cations in ferrisilicates leads to a shift towards higher temperature of the maximum of the desorption rate of EB and ST, which suggests increasing interaction of aromatic hydrocarbons with zeolites and activation energy of desorption of EB and S T over the zeolite catalysts, and increases of the conversion and yield of dehydrogenation in spite of the slight decrease in the selectivity of dehydrogenation under the conditions of reaction at 523 K over ferrisilicate catalysts. An increa,se in the electrostatic potential of the exchanged cations increases the polarization of the Fe-0 bond, reducing the energy of activation for reaction of EB to S T (ref. 25,261, of EB over and therefore the activity for dehydrogenation alkali-metal-exchanged ferrisilicate catalysts increases with increasing electrostatic potential of the exchanged cations. The slight decrease in the selectivity of dehydrogenation in the order K-FeZSM-5 > Na-FeZSM-5 > Li-FeZSM-5 is correlated to the increasing Lewis acid strength of alkali metal cation in the same order. ’, however, a strong Bronstcd If the cation position is occupied by H acid center is formed, resulting in the strong intraction between the aromatic hydrocarbon with the Bronsted acid center and cracking of EB. Tt. is concluded that increasing the electrostatic potential of the exchanged cations is benefiacial to the dehydrogenation of EB over ferrisilicate catalysts, but only if strong acid centers are not present. 4. CONCLUSIONS

Exchanged alkali metal cations had a significant effect upon the chemical state of the framework Fe(II1) j.n ferrisilicates. An increase in the electrostatic potential of the exchanged cations caused a moderate shift towards higher values of the F e ( 2 p ) binding energy and a decrease in

248 the reduction temperature of the framework Fe(II1) in ferrisilicates. Although Fe(II1) was removed from the lattice of the ferrisilicate zeolite afte hydrothermal treatment at high temperature, no framework collapse occured. The reduction of framework Fe(III1 in ferrisilicates, however, resulted in breaking of the zeolite structure and therefore the structural stability of the ferrisilicates in hydrogen stream, lower than that in air-flow, decreased in the order K-FeZSM-5 > Na-FeZSM-5 > Li-FeZSM-5. The presence of H+ in ferrisilicate zeolite induced a strong interaction between EB and zeolite, resulting in cracking of EB. A moderate interaction of EB with alkali-metal-exchanged ferrisilicates occurred and mainly lead to the reaction for dehydrogenation of EB to ST. The conversion yield of EB to ST greatly increased and the ST selectivity slightly decreased with increasing electrostatic potential of the exchanged alkali metal cations compared to ferrisilicate catalysts. 5. REFERENCES

1.

2. 3.

4.

5. 6. 7.

8. 9. 10. 11. 12.

13. 14. 15.

16.

M. Taramasso, C. Perego and B. Notari, Proc. Fifth Int. Conf. on Zeolites, Naples, L. V. C. Ress Ed., London Heyden and Sons, 1980, p. 40. E. Moretti, S. Contessa and M. Padovan, Chim Ind., 67, 1985, 21. P. Ratnasamy, R. B. Borade, S. Sivasanker, V. P. Shiralker and S. G. Hedge, Proc. of Int. Symp. on Zeolite Catalysts, Siofok (Hungary), 137, 1985. R. M. Barrer, "Hydrothermal Chemistry of Zeolites", Academic Press, London, UK, 1982, p. 419. C. T. W. Chu and C. D. Chang, J. Phys. Chem., 89 (19851, 1569. N. A. Klotz, US Patent 4 268 420. 1981. T. Inui, 0. Yamase, K. Fukuda, A . Itoh, J. Tarumoto, N. Morinaga, T. Hagiwara and Y. Tekegami, 8th Intern. Congr. Catal., Berlin, 1984, Vol. 1 1 1 , p.569. B. R. Gane and A. H. P. Hall, Eur. Pat. Appl. 171 981, 1986. B. D. McNicol and G. T. Pott, J. Catal., 25 (19721, 223. R . Szostak and T. L. Thomas, J. Catal., 100 (19861, 555. G. C. Alis, P. Frenken, E. de Boer, A . Swolfs and M. A. Hefni, Zeolites, 7 (19871, 319. B. Wichterlova, S Beran, S. Bednarova, K. Nedomova, L. Dudikova and P. Jiru, in "Innovation in Zeolite Materials Science", Stud. Surf. Sci. Catal., Elsevier, Amsterdam 37 (19871, 199. T. I n u i , H . Matsuda, 0. Yamase, H. Nagata, K. Fukuda, T. Ukawa and A. Miyamoto, J. Catal., 98 (19861, 491 501. Dong Hui Lin, G. Coudurier and J. C. Vedrine, Stud. Surf. Sci. Catal., Elsevier, Amsterdam 49B (19891, 1431. T . Inui, A. Miyamoto, H. Matsuda, H. Nagata, Y. Makino, F. Fukuda and F. Okazumi, Proc. 7th Int. Zeolites Conf., Kodansha, Tokyo, 1986, p.859. B. Ratnasamy, R. B. Borade and S. B. Kulkarni, Eur. Pat. ,

-

160136 (1985). 17. L . E. Iton, R. B. Beal and D. T. Hodul, J. Mol. Catal., 21 (19831, 151. 18. S. Sivasanker, K. J. Waghmare, S. R. Padalkar, P. Ratnasamy and K. R. Murthy, Appl. Catal., 39 (19881, 127. 19. Q. Kan, Z . Wu, S. Qiu, W. Pang and S. Peng, 8th Int. Zeolite conf.,

249 Recent Research Report, Amsterdam, 1989, 168. 20. 2. Wu, Q. Kan, W. Pang, S. Qiu and S Peng, 8 th Int. Zeolite conf., Recent Research Report, Amsterdam, 1989, 185. 21. J. M. Stencel, J. R. Diehl, L. J. Douglas, C. A . Spitler, J. E. Crawford and G. A. Melson, Colloids Surface, 4 (19821,337. 22. R. B. Borade, Zeolites, 7 (19871, 398. 23. K. Wandelt, Surface Sci. Rept 2 (1982) 1. 24. L. M. Kustov, V. B. Kazansky and P. Ratnasarny, Zeolites, 7 (19871 79. 25. Qiubin Kan, Thesis for Ph. D, Jilin University, China, 1990, p. 53,73. 26. Takenori Hirano, Appl. Catal., 26 (19861, 65, 81.

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P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catulysis 1991 Elsevier Science Publishers B.V., Amsterdam

251

FRAMEWORK AND EXTRAFRAMEWORK TI IN TITANIUM-SILICALITE: INVESTIGATION BY MEANS OF PHYSICAL METHODS

A. Zecchina’, G. Leofanti’

G. Spotol, S. Bordigal, A . Ferrero’, and M. Padovan’

G. Petrini2,

1) Dipartimento di Chimica Inorganica, Chimica Fisica e dei Materiali, Universita di Torino, via P. Giuria 7 , (Italy) 2) ENIMONT ANIC, units di Ricerca, via S. Pietro 50, 28100 Bollate (MI) (Italy)

ABSTRACT

The application of UV-Vis diffuse reflectance, I R transmission, Raman and ESR spectroscopies to the determination of extraframework and framework Ti in Titanium-Silicalite is discussed in detail. The four methods give complementary information. In particular the utility of the W - V i s and Raman spectroscopies to detect extraframework Ti is definitely established, while IR proves to be more useful for framework Ti. ESR spectroscopy can be also utilized on reduced samples to distinguish between the two types of Ti. 1. INTRODUCTION

Ti-Silicalite, a zeolite of the pentasil family containing a very small percentage of Ti substituting Si (1-2 % in atoms) is an important, efficient and selective catalyst for oxidation and ammoximation reactions with H202. (ref.1-3) Although a few contributions have already appeared in the literature concerning its characterization by means of physical methods (ref.4,5), the growing importance of this catalyst requires further efforts concerning the standardization of the methods. In this contribution we compare the results obtained by the simultaneous application and comparison of the following physical methods: a) UV-Vis diffuse reflectance spectroscopy; b) Raman spectroscopy; c) IR (transmission) spectroscopy and d) ESR spectroscopy. The choice of the previous spectroscopic methods is suggested by their widespread presence in the industrial laboratories, by their complementary nature and by the potential capacity to give information either on framework and extraframework Ti.

.

2.

EXPERIMENTAL

The Silicalite and Titanium-Silicalite (TS1 and TS2) samples

252

investigated in this contribution have been synthesized in the ENIMONT ANIC laboratories following the method described in ref.2. The UV-Vis spectra were obtained on a Perkin Elmer Lambda 15-DRS spectrometer, using a suitably designed quarz cell allowing in situ outgassing procedures under high vacuo at 673 K to eliminate adsorbed species (mainly H20). The IR spectra have been obtained on a Bruker IFS 4 8 FTIR spectrometer. The samples were in form of thin film deposited on KBr or Si plates inserted in a suitable cell allowing in situ outgassing procedures at 673 K. Special care has been taken to obtain a thoroughly dispersed, homogeneous film through sedimentation of the powder previously dispersed in isopropyl alcohol by means of ultrasonic waves. The Raman spectra were made on a Jobin Ivon Raman Spectrometer, using a Kripton laser source. No difference was observed among the spectra made in air or in controlled atmosphere, probably because the powerful laser irradiation is sufficient to eliminate the adsorbed species even in the presence of air. Finally the ESR spectra at 77 K were obtained on a Varian E 109 spectrometer, by using a silica cell suitable for outgassing and reduction procedures. Several reduction methods have been used i.e: i) reduction with H2 and CO at 673-873 K; ii) reduction with H2 at 300 K under X-ray irradiation; iii) reduction in Na vapours at 623673 K and iv) reduction at 250-300 K with a very diluted NaNH3 solution first prepared in a side arm of the ESR cell through condensation of NH3 on the Na metal and then allowed to contact the sample inside the cell without exposure to the external atmosphere. 3. RESULTS AND DISCUSSION

W-Vis reflectance svectroscovv I a sensitive method for both framework extraframework Ti. Following Jorgensen classification (ref.6) and our own previous considerations (ref.4,5), the electronic transition with ligand to metal charge transfer character involving isolated framework Ti (IV) in tetrahedral coordination is expected at 48,000 cm-l, while that involving isolated Ti (IV) in octahedral environment is expected at about 42,000 cm-l. Non-isolated octahedrally coordinated Ti (IV) in oxidic clusters absorbs at lower frequency. For instance in anatase the absorption edge is observed (fig.1) at 30,500 cm-l. In fig.1 the reflectance spectra taken under vacuo of two TS samples (hereafter TS1 and TS2) previously outgassed at 673 K are illustrated. On the basis of the previous considerations it is immediately evident that in TS1 the Ti is mainly in framework (tetrahedral) position, while in TS2, an abundant fraction is extraframework anatase-like phase.In order to roughly estimate the sensitivity of the method, we have taken the spectra of many intimate mixtures of pure silicalite and anatase of

253

variable concentration. We can safely conclude from the results that a limiting concentration of anatase in silicalite corresponding to = 0.03 wt % can be detected. In conclusion the W - V i s diffuse reflectance spectroscopy proves to be a simple and powerful technique for the determination of both extraframework (30,000-42,000 cm-l) and framework (48,000 cm-l) titanium.

-I

1

18

,

15

1

,

28

1

.

25

1

,

38

1

,

35

,

,

4e

1

,

45

I

~

se com-l

I

x lo3

Fig. 1. UV-Vis diffuse reflectance spectra (Kubelka Munk function vs wavenumber) of: 1) --- anatase; - - TS1 and TS2. In the inset the intensity of the 30,500 cm-I band of different mixtures of anatase and silicalite are reported versus Ti concentration.

....

Raman sPectroscoX)Y: a tool for extraframework Ti. As illustrated in fig.2aI the Raman spectrum of anatase is characterized by several peaks. The remarkably intense band at 140 cm-I is characteristic of the anatase phase only (ref.7) and can be used to evaluate the amount of extralattice Ti present in TS preparations. The results obtained for TS1 and TS2 samples are illustrated in fig.2b (in the same figure the silicalite spectrum is reported or sake of comparison). The sample TS2 which resulted to be contaminated by TiO, when studied by means of UV-Vis reflectance spectroscopy, shows the presence of the characteristic 140 cm-I Raman peak of anatase.

254

In order to obtain an estimation of the sensitivity of the method, the Raman spectra of intimate mixtures of anatase and silicalite characterized by various anatase content have been measured.

RCM-1

Fig. 2. Raman spectra of anatase (a); TS1, TS2 and Silicalite (b) on the basis of many determinations we can state that, despite the high scattering of the data, the minimum content of anatase which can be revealed by Raman spectroscopy is approximately 0.5 wt %, i.e. a figure which is at least one order of magnitude higher than that obtained by means of W-Vis reflectance spectroscopy. Despite this limitation, the technique can be usefully utilized in junction with other spectroscopies because it is specifically sensitive to anatase (other forms of Ti02 in small concentration being substantially silent in Raman spectroscopy)

.

IR sDectroscoDv: a semiauantitative method sDecific for framework Ti. Before taking in consideration the possibility to derive any

255

information about structural Ti via IR spectra, it seems useful to clarify that no data can be achieved on extraframework Ti using the same technique. In fact IR spectra of both TS1 and TS2 films treated under vacuo in the same way, give undistinguishable spectra. This means that the small amount of Ti02 (fraction of percent), easily revealed by W and Raman spectra, cannot be revealed by IR spectroscopy, because the relevant broad absorption at 700-400 cm-I (8-10) is overshadowed by the strong IR mode of the silicalite skeleton.

1 65

Fig. 3. IR spectra recorded at 300 K of TS1 (TS2) (I), Silicalite ( 2 ) and Na-ZSM5 (Si/A1=14) (3)

t (n

3 z

f

0.825

a m

IL 0 (n

c rn

0.0 1

WRVENUMBER CM-1

As it is well known, the IR spectra of TS (fig.3) is characterized by the presence of a well defined peak at 960 cm-’ which, being not associated with any form of Ti02 phase (ref. 8-10), is considered a fingerprint of framework Ti. As already discussed in ref.5, The presence of Ti atoms in the silicalite structure can account for the 960 cm-I peak essentially in two ways (arb), which depend upon the equivalence or unequivalence of the Ti-0 and Si-0 bonds and Ti-0-Si bridges. a) eauivalence of the Ti-0 and Si-0 bonds. The [Si04] and [Ti04] units are essentially equivalent as far as the force constant of the SiO and Ti0 bonds are concerned. In such case the 9 6 0 cm-’ peak is simply the B mode of the [Ti04] unit shifted from 1120 cm-l ([SiO,] essentially because the mass effect). The absence of any local [A104] mode in Na-ZSM5

256

(spectrum 3 ) is in favour of this hypothesis, because the mass effect in this case is practically absent. b) uneuuivalence of Ti0 and SiO modes. The [Sio,] and [Ti04J units are not equivalent as far as the force constants are concerned because the Ti-0 bonds are much more polar than the SiO bonds. In such a case the 960 cm-l is better assigned to a local mode of the [SiO,] unit adjacent to the Ti atom. In the limit of a fully ionic Ti-0 bond, the 960 cm-l band becomes simply the stretching mode of a dangling -rSi-O- bond. It is evident from this short discussion that a distinction among the two models is troublesome. Especially when only small differences in the bond polarity are present. In such case the dem ’ l peak in terms of a stretching mode of scription of the 960 c a single tetrahedral unit is intrinsically useless the stretching modes of the cluster of the all 5 tetrahedra disturbed by the presence of the titanium atom must be considered. Concerning the use of the 960 cm-’ band to estimate the concentration of framework Ti, a first warning is represented by the fact that Ti-free silicalite and even high surface area silica have an absorption in this range attributable to the stretching modes of surface silanols (ref.11). A second warning is represented by the fact that the 960 cm-l is perturbed by the presence of adsorbates in the channels (ref.4): consequently the IR spectra of TS films must be registered after in situ outgassing (ref.4,5). Moreover we have noticed that the intensity of the peaks are very much influenced by the homogeneity of the film (homogeneous films giving the strongest peak). This fact can be explained as follows. In non homogeneous films, IR radiation passing through the holes (inhomogeneities) of the film gives a small residual transmittance even in the IR region where the sample is strongly absorbing (like for instance at 1100 cm"). This has the consequence to selectively depress the absorbance of the strongest peaks and hence to alter the intensity ratio between modes of different specific intensity. The accurate preparation procedure adopted in this investigation has partly eliminated this artefact (compare these results with IR spectra of less homogeneously deposited films) (ref.4,5). This intrinsic complication, makes IR spectroscopy of powdered materials an essentially semiquantitative tool. In principle this effect could be diminished by taking the IR reflectance spectra of very diluted mixtures of TS and KBr. However, in this case, the contamination by foreign molecules (like H 0 2 i known to influence the intensity and the shape of the 960 cmpeak (ref.5) and KBr itself) cannot be avoided. In conclusion, although the 960 cm-I peak is definitely associated with the presence of framework Ti, its use for a truly quantitative estimation of the Ti content in the silicalite framework is debatable, especially in the lowest concentration range.

257

ESR sDectroscopy: a qualitative method discriminatinq between framework extraframework titanium. Ti3+ (dl) is ESR active and its spectrum in various coordination states is well known (ref.12). We have so examined the effect of different reduction procedures with the aim to differentiate the Ti3+ centres deriving from framework and extraframework precursors. Of course this methodology can have some validity only if the reduced structures retain a structural memory of the precursor, which in turn can happen only if the reduction is made under the mildest conditions.

1945

1945

1.968 1948

i\-i

1912

1909

50

G

Fig. 4 . ESR spectra at 77 K of TS1 (a) and TS2 (b) in presence of NH3 (1) and after outgassing at RT (2). Some reduction can be obtained at very high temperature (>= 800 K) in H2 and CO: however the intensity and reproducibility of the ESR signal is always low. Na vapors are more efficient: but the still high T required to run the experiment (>= 520 K) suggests some caution in the interpretation of the results. Moreover a spurious signal associated with excess sodium in form of (Na), clusters trapped in the channels, is always observed. Reduction in H2 at 300 K under X-ray irradiation (but other radiation sources can be utilized as well) gives well reproducible results. However as they are not substantially

258

different from those obtained with a very diluted solution of Na in liquid ammonia, we shall describe only the latter results (which, by the way, correspond to the mildest reduction proceThe results for the two samples (TS1 and TS2) are shown dure) in fig.4. In presence of NH3, the samples (TS1 and TS2) give the same, essentially isotropic, spectrum centered at g=1.920 corresponding to Ti3+ in octahedral and or tetrahedral coordination with low contribution of tetragonal field. Elimination of weakly adsorbed NH3, induces a main modification of the spectrum with appearance of a new gzz component at g = 1.968 which is definitely stronger on TS2, i.e. the sample with extraframework Ti. The modification is totally reversible. The explanation is as follow: framework Ti3+ gives an isotropic signal which is not substantially affected by the removal of the NH3 ligands present in the channels. On the contrary extraframework Ti3+ changes its coordination state passing from a fully coordinated (octahedral) situation in presence of NH3, to a lower coordination state characterized by a larger g factor This effect has been alanisotropy, after NH3 elimination ready documented for similar Ti02-SiO, systems (ref.12). In conclusion ESR spectroscopy confirms that framework Ti is much more abundant on TS1, while some reduced Ti3+ species deriving from an extralattice precursor is definitely more abundant on TS2.

.

.

5.

BIBLIOGRAPHY

1) W. Holderich, M. Messe and F. Naumann, Angew. Chem. Int. Ed Eng. 27 (1988) 26. 2) C . Neri, A . Esposito, B. Anfossi and F. Buonomo, Eur Pat. 100, 119. 3) C. Neri, M. Taramasso and F. Buonomo, U. K. 102, 665.

4) M. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Structure and Reactivity of Surfaces, Elsevier, Amsterdam, (1989) 133. 5) A . Zecchina, G. Spoto, S. Bordiga, M. Padovan, G. Leofanti and G. Petrini, Zeocat 90, Leipzing August 90 Proccedings Elsevier, Amsterdam, in press. 6) C. K. Jorgensen, Prog. Inorg. Chem., 12 pp. 101 S. J. Lippard ed. Intersci. Pub., John Wiley N. Y. 1970. 7) I. R. Beattie and T.R. Gilson, Proc. Roy. SOC. A307 (1968) 407.

T. Ohsaka, F. Izumi and Y. Fujiki, J. Raman Spectr. 1 (1978) 321. 9) N. T. Mc Devitt and W. L. Baun, Spectrachimica Acta, 20 (1964) 799. 10) C. U. Ingemar Odenbrand, S . Lars T. Andersson, Lars A. H. 8)

Andersonn, Jan G. M. Brandin and Guido Busca, J. of Catal. 125 (1990) 541. 11) F. Boccuzzi, A. Chiorino, G . Ghiotti, C. Morterra, A. Zecchina, J. Phys. Chem., 82 (1978) 1298. 12) V. A. Shvets and V. B. Kasanskii, Kinet. Katal., Vol 12, N.4, (1971) 935.

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

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Metallic

S t u d i e s on t h e S t a t e o f Copper anq t h e F o r m a t i o n o f i t s O x i d i c and Phases i n Z e o l i t e CuNaY R . P i f f e r ' , M. H a g e l s t e i n 2 , S . C u n i s 2 , P . Rabe',

I n s t i t u t e o f P h y s i c a l Chemistry, U n i v e r s i t y 0-2000 Hamburg 13, FRG 'Fachhochschule O s t f r i e s l a n d ,

H. F o r s t e r ' and W . Niemann3

o f Hamburg, B u n d e s s t r . 45,

C o n s t a n t i a p l a t z 4, D-2970 Emden,

FRG

3 H a l d o r Topsoe R e s e a r c h L a b o r a t o r i e s , DK-2800 Lyngby, Denmark*

Abstract The o x i d a t i o n s t a t e o f Cu i o n s i n z e o l - i t e Y depends on t h e p r e t r e a t m e n t p r o c e d u r e . A f t e r a c t i v a t i o n f o r more t h a n 8 h o u r s a t 675 K phases s i m i l a r t o CuO o c c u r . A d m i t t i n g h y d r o g e n a t 575 K, a r e d u c t i o n t o C u ( 1 ) b u t n o t t o Cu(0) m e t a l l i c c l u s t e r s i s observed. P r o b a b l y a r e a c t i o n w i t h extraframework oxygen t a k e s p l a c e . However, a r e d u c t i o n t o r a t h e r s m a l l m e t a l l i c c o p p e r c l u s t e r s i s o b s e r v e d a f t e r c o - a d d i t i o n o f w a t e r v a p o u r and h y d r o g e n . T h i s demonstrates t h e i m p o r t a n t r o l e o f s t r o n g l i g a n d s l i k e w a t e r i n t h e r e d u c t i o n mechanism, e n a b l i n g a c o n t r o l l e d f o r m a t i o n o f c o p p e r c l u s t e r s .

INTRODUCTION Copper-exchanged z e o l i t e s w i t h f a u j a s i t e s t r u c t u r e have f r e q u e n t l y been b y means o f d i f f e r e n t t e c h n i q u e s , t o e n l i g h t e n t n e i r the object o f studies, r e d o x as w e l l as t h e i r c a t a l y t i c p r o p e r t i e s [I-51. E s p e c i a l l y t h e o x i d a t i o n s t a t e and t h e c r y s t a l l o g r a p h i c e n v i r o n m e n t o f t h e c o p p e r i o n s a r e o f g r e a t i n t e r e s t . The i n t e n t i o n o f t h i s work was t o s t u d y t h e i n f l u e n c e o f t h e p r e t r e a t m e n t c o n d i t i o n s . We a p p l i e d X-Ray A b s o r p t i o n Near Edge S t r u c t u r e (XANES) and E x t e n d e d X-Ray A b s o r p t i o n F i n e S t r u c t u r e (EXAFS) as a s i t e s p e c i f i c p r o b e . The o x i d a t i o n s t a t e c a n be d e r i v e d f r o m t h e t h r e s h o l d e n e r g y . The c o - o r d i n a t i o n s p h e r e o f t h e c o p p e r i o n s i s g i v e n b y t h e r a d i a l d i s t r i b u t i o n f u n c t i o n which i s d i r e c t l y d e r i v e d from t h e F o u r i e r transform o f t h e EXAFS o s c i l l a t i o n s . The r e d u c t i o n b y h y d r o g e n and t h e i n t e r a c t i o n w i t h w a t e r a t d i f f e r e n t t e m p e r a t u r e s w i l l be e x p l o r e d . The f o r m a t i o n o f a c i d i c h y d r o x y l g r o u p s and s o r p t i o n c o m p l e x e s o f w a t e r has been s t u d i e d b y I R spectroscopy.

*

P r e s e n t address: P h i l i p s Research L a b o r a t o r i e s , V o g t - K o l l n - S t r . D-2000 Hamburg 54, FRG

30,

260

EXPERIMENTAL CuNaY z e o l i t e s were prepared by i o n exchange o f z e o l i t e NaY w i t h an aqueous 0.03 M s o l u t i o n o f Cu(NO,), ( Merck, p r o a n a l y s i ) a t 300 K. A f t e r washing and d r y i n g a t ambient temperature, t h e composition determined by AAS was Cu,, ,Na,, ,Y. The samples were pressed i n t o s e l f - s u p p o r t i n g wafers o f t h i c k n e s s f o r I R and X-Ray measurements, r e s p e c t i v e l y . about 8 o r 80 mg They were dehydrated i n vacuo up t o 12 h a t temperatures between 625 and 675 K . Water, p y r i d i n e (Merck, p r o a n a l y s i ) and hydrogen (Messer Griesheim, 99.999%) were i n t r o d u c e d w i t h o u t f u r t h e r p u r i f i c a t i o n . The I R s p e c t r a were recorded a t ambient temperature ( i f n o t e x p r e s s l y s t a t e d o t h e r w i s e ) on a F o u r i e r t r a n s f o r m spectrometer DIGILAB FTS 20E w i t h r e s o l u t i o n s between 1 and 4 cm-’. The EXAFS and XANES d a t a were o b t a i n e d a t t h e Cu K edge on t h e E4 and X beam l i n e s a t the s y n c h r o t r o n r a d i a t i o n source o f HASYLAB/DESY i n Hamburg, a t a r i n g energy o f 5 . 3 GeV w i t h a r e s o l u t i o n o f 2-4 eV by means o f S i - 1 1 1 and Si-400 (DEXAFS) monochromators and e l e c t r o n c u r r e n t s o f 20-40 mA. F o r t i m e - r e s o l v e d s t u d i e s e n e r g y - d i s p e r s i v e XAS a l l o w s t o o b t a i n a s i n g l e XAS spectrum w i t h i n about t h r e e seconds a t t h e D i s p e r s i v e EXAFS (DEXAFS) spectrometer [ 6 ] . F o r t h e examination o f t h e o x i d a t i o n s t a t e o f copper, t h e XAS s p e c t r a o f Cu,O ( R i e d e l de Haen) as w e l l as o f CuO (Merck, p r o a n a l y s i ) and Cu metal f o i l (Goodfellow) were examined under i d e n t i c a l c o n d i t i o n s . A l l s p e c t r a were recorded i n s i t u i n t h e same a l l metal sample c e l l equipped w i t h KBr windows f o r t h e I R range and w i t h b e r y l 1ium windows f o r t h e XAS measurements.

RESULTS AND D I S C U S S I O N Hydrated, A c t i v a t e d and Rehydrated Samples The r a d i a l d i s t r i b u t i o n f u n c t i o n o f t h e hydrated sample e x t r a c t e d by EXAFS proves t h e e x i s t e n c e o f a s i n g l e c o - o r d i n a t i o n s h e l l w i t h n e a r l y 4 oxygen neighbours a t a d i s t a n c e o f about 200 pm. T h i s s h e l l must be a t t r i b u t e d t o a copper-aquo complex f l o a t i n g i n s i d e t h e supercages w i t h o u t any bonding t o z e o l i t e oxygen, o t h e r w i s e f u r t h e r c o - o r d i n a t i o n spheres should be observable. T h i s i s supported by comparable measurements o f hydrated Cu(NO,), showing n e a r l y t h e same EXAFS and t h e r e f o r e the same Fourier transform. A f t e r d e h y d r a t i o n a t temperatures between 625 and 675 K i n h i g h vacuo f o r about 12 h, a l l copper i o n s should l o o s e t h e i r s u r r o u n d i n g water molecules. Therefore, t h e i o n s s h o u l d occupy d i s t i n c t c r y s t a l l o g r a p h i c s i t e s w i t h i n t h e z e o l i t e , c o - o r d i n a t e d by t h e z e o l i t e oxygen [ 1 , 7 ] . Indeed, t h e simultaneous decrease o f the f i r s t c o - o r d i n a t i o n sphere and the b u i l t - u p o f h i g h e r s h e l l s d u r i n g d e h y d r a t i o n i n d i c a t e s a c a t i o n movement t o these s i t e s ( F i g . 1, l o w e r and i n t e r m e d i a t e spectrum). The corresponding EXAFS i s n e a r l y i d e n t i c a l w i t h t h e EXAFS o f CuO c l u s t e r s which are s m a l l e r t h a n 800 pm i n diameter [ 8 ] . The Cu-0 d i s t a n c e t u r n s o u t t o be n e a r l y 196 pm, approaching t h a t o f CuO w i t h a peaks s l i g h t l y decreased c o - o r d i n a t i o n number. The l a c k o f any CuO o r Cu,O i n t h e X-ray d i f f r a c t i o n p a t t e r n s i n d i c a t e s t h e absence o f any m a c r o c r y s t a l l i n e copper o x i d e phases. A l a r g e r amount o f Cu(1) o x i d e a s w e l l as Cu(0) may be excluded from t h e comparison w i t h t h e r e s p e c t i v e EXAFS o s c i l l a t i o n s and i s supported by XANES measurements. The i o n s seem t o be b r i d g e d by extraframework oxygen under f o r m a t i o n o f

261 I

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1

I

,

I

(

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0

2

1

3

4

atomic distance

5

6

(A)

F i g . 1. F o u r i e r t r a n s f o r m o f s i g n a l X ( k ) : - h y d r a t e d sample ( a ) - a c t i v a t e d sample ( b ) - r e h y d r a t e d sample ( c ) window: B e s s e l , w e i g h t i n g : k 2 , k - r a n g e : 2 . 5 - 7 . 5

A- 1

300

0 0 c

F i g . 2 . IR s p e c t r a o f adsorbed pyridine i n the - a c t i v a t e d CuNaY sample ( a ) - H2-reduced CuNaY ( b ) - H2/H20-reduced CuNaY ( c )

c

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Cu-0-Cu complexes. From EPR r e s u l t s CHAO e t a l . have suggested them t o be a p o s s i b l e c o n f i g u r a t i o n among o t h e r s [ 91. During the d e h y d r a t i o n process some o f t h e w a t e r molecules d i s s o c i a t e under f o r m a t i o n o f an i n c r e a s i n g amount o f Bronsted a c i d OH groups i n the supercages and t h e s o d a l i t e u n i t s [ 8 ] . However, t h e amount o f these a c i d s i t e s i s small, as has been proved by p y r i d i n e a d s o r p t i o n . From I R s p e c t r a bands a t 1390, 1488, 1542 and 1632 cR’ should be expected [ l o ] . F i g u r e 2 ( l o w e r spectrum) proves t h e almost t o t a l absence o f these bands. On t h e o t h e r hand the s t r o n g bands a t 1455, 1490 and 1605 c m i n d i c a t e t h e presence o f Lewis a c i d s i t e s e . g . Cu c a t i o n s [ l o ] . The r e s p o n s i b l e band a t 1455 cm-’ s p l i t s i n t o a d o u b l e t i n d i c a t i n g two d i f f e r e n t Lewis a c i d s i t e s which probably a r e due t o t h e f r e e C u ( I 1 ) c a t i o n s and t h e t r u e Lewis a c i d s i t e s o r r e s i d u a l Na’ i o n s . Treatment o f t h e Dehydrated Sample w i t h Hydrogen Admission o f hydrogen a t 575 K t o a dehydrated sample causes a s t r o n g i n c r e a s e o f b o t h a c i d i c OH groups due t o t h e r e d u c t i o n o f C u ( I 1 ) i o n s t o Cu(1) accompanied by t h e f o r m a t i o n o f p r o t o n s ( F i g . 3 ) . Simultaneously t h e amount o f Lewis a c i d s i t e s decreases, observable b y t h e corresponding I R bands o f adsorbed p y r i d i n e ( F i g . 2, i n t e r m e d i a t e spectrum). The lowfrequency band o f t h e 1455 cm-’ d o u b l e t i s reduced t o a shoulder, i n d i c a t i n g a loss o f accessible cations, probably the Cu(I1) ions. The XANES and EXAFS prove t h e immediate i n t e r a c t i o n between hydrogen and the copper i o n s a t 575 K ( F i g . 4 ) . The a b s o r p t i o n edge s i g n i f i c a n t l y s h i f t s towards lower photon e n e r g i e s and a change i n t h e EXAFS i n d i c a t e s a d i f f e r e n t l o c a l environment. Both, XANES and EXAFS o f t h e sample i n t h e o r copper f i n a l s t a t e d i d n o t show any resemblance t o those of CuO, Cu,O metal ( F i g . 5 ) . A comparison o f t h e XANES w i t h s e v e r a l c r y s t a l l o c r a p h i c a l l y well-known compounds c o n t a i n i n g copper [ 111 and t h e observed chemical s h i f t i n d i c a t e t h e f o r m a t i o n o f Cu(1) i o n s . Rather s i m i l a r phases appear when butadiene was used as t h e r e a c t i v e gas a t 575 K [ 8 ] . Reduction o f a Rehydrated Sample w i t h Hydrogen Completely d i f f e r e n t r e s u l t s were o b t a i n e d w i t h samples a c t i v a t e d f o r o n l y one hour a t 500 K. A f t e r repeated o x i d a t i o n - r e d u c t i o n c y c l e s m e t a l l i c copper c l u s t e r s were o b t a i n e d [12]. W i t h t h e assumption t h a t under these a c t i v a t i o n c o n d i t i o n s t h e s t r o n g l y bound water c o u l d n o t be c o m p l e t e l y removed, t h i s r e s u l t i n d i c a t e s t h e i m p o r t a n t r o l e o f water f o r r e d u c t i o n . To e n l i g h t e n t h e mechanism o f t h e r e d u c t i o n a new sample was a c t i v a t e d a t 570 f o r 8 h i n h i g h vacuo. A f t e r having reached ambient temperature again 3 . 1 0 Pa water vapour was i n t r o d u c e d . Having heated t h e sample up t o 575 K under t h i s atmosphere, no dramatic change i n t h e l o c a l environment o f t h e copper i o n s c o u l d be observed ( F i g . 1, upper spectrum). The i n t r o d u c t i o n o f an excess o f hydrogen y i e l d e d copper m e t a l . Time-resolved XAS measurements a l l o w f u r t h e r c l a r i f i c a t i o n o f the r e d u c t i o n process. The v a r i a t i o n o f the XANES i n d i c a t e s a two s t e p r e a c t i o n . Immediately a f t e r a d d i t i o n o f hydrogen t h e XANES changes i n t h e same way as f o r t h e c o m p l e t e l y dehydrated sample ( F i g . 4 ) , b u t i s f o l l o w e d by a f u r t h e r change o f t h e XANES r a t h e r s i m i l a r t o t h a t o f copper metal ( F i g . 6 ) . A t f i r s t the hydrogen r e a c t s w i t h t h e b r i d g i n g oxygen under f o r m a t i o n o f w a t e r . T h i s i s supported by I R measurements which show small b u t d i s t i n c t bands a t 1640 and near 3300 cm-l, due t o adsorbed w a t e r ( F i g . 3 , lower spectrum).

5

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F i g . 3 . IR d i f f e r e n c e spectra o f CuNaY a f t e r reduction compared t o the activated sample : - a f t e r H2 ( a ) - a f t e r H2/H20 ( b ) reduction

0

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energy (keV) F i g . 4 . Cu K-edge XANES measured d u r i n g t h e r e d u c t i o n o f C u ( I 1 ) c a t i o n s i n CuNaY. The r e a c t i o n proceeds from t h e f a t t o t h e dashed spectrum. F o r comparison see t h e XANES o f a Cu f o i l ( d o t t e d ) .

264

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1.5 F i g . 5 . Compari t i ve Cu K-edge XANES of - CuO ( a ) - C U ~ O( b ) - Cu metal ( c ) - H2-reduced CuNaY ( d )

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Fig. 6. CLI K-edge XANES measured d u r i n g t h e r e d u c t i o n o f C u ( I 1 ) c a t i o n s i n r e h y d r a t e d CuNaY. The r e a c t i o n proceeds w i t h t h e s h i f t t o l o w e r energy, r e a c h i n g t h e f i n a l s t a t e (dashed). F o r comparison t h e Cu K-edge o f a Cu f o i l r e f e r e n c e ( d o t t e d ) .

265

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F i g . 7 . F o u r i e r transform of EXAFS signal ~ ( k ) :The r e d u c t i o n o f Cu(I1) s p e c i e s t o m e t a l l i c Cu c l u s t e r s s t a r t s with the bottom graph ( r e h y d r a t e d s a m p l e ) . For compzrison s e e the FT of a C u f o i l r e f e r e n c e spectrum ( d o t t e d ) .

F i g . 8 . IR d i f f e r e n c e s p e c t r a o f the H2-exposed CuNaY v s . tne rehydrated saxpie recorded a t 575 I(: . 1mmed;ately a f t e r hydroge!] adrrlisslon ( a ) - a f t e r 30 m n ( b ) - a f t e r 60 r i i n ( c ) .

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1600

266

I n t h e second s t e p t h i s makes t h e C u ( I ) i o n s a c c e s s i b l e t o w a t e r m o l e c u l e s so t h a t f u r t h e r r e d u c t i o n may o c c u r . The c o r r e s p o n d i n g radial d i s t r i b u t i o n f u n c t i o n o f t h e f i n a l s t a t e shows a s t r u c t u r e identical t o t h a t o f a Cu m e t a l f o i l ( F i g . 7 ) . I n comparison t o t h e l a t t e r t h e f i r s t c o o r d i n a t i o n s h e l l d i s p l a y s a s m a l l e r h e i g h t , w h i c h can be e x p l a i n e d by a reduced number o f n e a r e s t copper n e i g h b o u r s i n agreement w i t h r e c e n t f i n d i n g s i n t h e l i t e r a t u r e [ 1 3 ] . T h i s i n t e r p r e t a t i o n i s s u p p o r t e d by t h e comparison o f t h e XANES o f t h e f i n a l s t a t e ( F i g . 6 ) w i t h t h a t o f s m a l l copper p a r t i c l e s [14,15]. Comparative I R s t u d i e s show t h e e f f e c t o f i n t r o d u c i n g hydrogen a t 575 I< t o t h e w a t e r t r e a t e d sample. I m m e d i a t e l y a f t e r exposure t h e bands o f adsorbed w a t e r a t 1640 and around 3500 cm-’, a t t r i b u t e d t o t h e bending and the s t r e t c h i n g vibrations, r e s p e c t i v e l y , disappear. Simultaneously a small b u t d i s t i n c t band a t 3650 cm?, due t o a c i d i c OH g r o u p s i n t h e supercage, i n c r e a s e s ( F i g . 8, l o w e r s p e c t r u m ) . T h i s i s caused by t h e g r o w i n g amount o f p r o t o n s c r e a t e d d u r i n g r e d u c t i o n . T h i r t y and s i x t y m i n u t e s a f t e r a d m i s s i o n o f hydrogen a s i g n i f i c a n t i n c r e a s e o f t h e absorbance i s observed i n d i c a t i n g t h e p r o g r e s s i v e c r e a t i o n o f copper c l u s t e r s ( F i g . 8, i n t e r m e d i a t e and upper spectrum). Preceding the r e d u c t i o n t o m e t a l l i c c l u s t e r s , a vanishing o f the amount o f adsorbed w a t e r i s observed. P y r i d i n e a d s o r p t i o n shows a d r a m a t i c i n c r e a s e o f B r o n s t e d a c i d c e n t e r s p a r a l l e l e d by t h e decrease o f Lewis a c i d c e n t e r s ( s e e u p p e r spectrum o f F i g . 2 ) . T h i s i s i n agreement w i t h t h e o b s e r v a t i o n o f t h e g r o w i n g amount o f reduced Cu p a r t i c l e s accompanied b y a l o s s o f Lewis a c i d s i t e s . From o u r e x p e r i m e n t s a c o r r e l a t i o n between c l u s t e r s i z e and t h e w a t e r and hydrogen p a r t i a l p r e s s u r e must be assumed. CONCLUSIONS The r e d u c t i o n of t h e copper i o n s i n a c t i v a t e d CuNaY z e o l i t e depends on t h e p a r t i a l p r e s s u r e o f b o t h hydrogen and w a t e r . A r e d u c t i o n by hydrogen t o copper m e t a l o n l y o c c u r s i f t h e p r e s s u r e o f w a t e r exceeds a c e r t a i n l i m i t . O t h e r w i s e t h e r e d u c t i o n w i l l be i n c o m p l e t e and s t a b l e Cu(1) phases show up. The i n c r e a s e d m o b i l i t y o f copper i o n s due t o t h e a v a i l a b i l i t y o f s t r o n g l i g a n d s seems t o be i m p o r t a n t . Only t h e s e i o n s a r e a b l e t o be reduced t o copper atoms by m o l e c u l a r hydrogen f o l l o w e d by an a g g l o m e r a t i o n t o c l u s t e r s . ACKNOWLEDGEMENT A p a r t o f t h i s p r o j e c t i s s u p p o r t e d by t h e BMFT under c o n t r a c t No. DAI.

05419

REFERENCES 1 P . G a l l e z o t , Y . Ben T a a r i t and B . I m e l i k , J . C a t a l . , 26 (1972) 295. P.A. Jacobs, W . de Wilde, R . A . Schoonheydt, J.B. U y t t e r h o e v e n and H . Beyer, JCS F a r a d . Trans. I, 72 (1976) 1221. 3 I.E. Maxwell, J . J . de Boer and R.S. Downing, J . C a t a l . , 61 (1980) 493. 4 R . A . Schoonheydt, J . Phys. Chem. S o l i d s , 50 (1989) 523. 5 I . E . Maxwell and J . J . de Boer, J. Phys. Chem., 79 (1975) 1874.

2

267 6

7 8 9

10 11 12 13 14 15

M. Hagelstein, S . Cunis, R . Frahrn, W. Niemann and P . Rabe, Conf. P r o c . , V o l . 25, 2nd European C o n f . P r o g r . X-Ray S y n c h r o t r o n R a d i a t i o n Res., A . B a l e r n a , E. B e r n i e r i and S. M o b i l i o , Eds., SIF, Bologna, (1990) 407. W.J. M o r t i e r and R . A . Schoonheydt, P r o g r . S o l i d S t a t e Chern., 16 (1985) 105. R . P i f f e r , H . F o r s t e r and W. Niemann, C a t a l . Today, i n p r e s s . C.C. Chao and J.H. L u n s f o r d , J. Chem. Phys., 57 (1972) 2890. H. Karge, Z . Phys. Chem., 76 (1971) 133. L.S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahn, K.O. Hodgson and E.I. Solomon, 3 . Am. Chem. SOC., 109 ( 1 9 8 7 ) 6433. M. H a g e l s t e i n , S . Cunis, R . Frahm, W . Niemann, R . P i f f e r and P. Rabe, XAFS V I , 1990, P r o c . Conf., i n p r e s s . S . Tanabe and H. Matsumoto, B u l l . Chem. S O C . Jpn., 63 (1990) 192. Shenoy, E.E. Alp, W. S c h u l z e and J. Urban, Phys. P.A. Montano, G.K. Rev. L e t t . , 56 (1986) 2076. G.N. Greaves, P . J . Durham, G. Diakun and P . Q u i n n , Nature, 294 (1981) 139.

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P.A. Jacobs et al. (Editors), Zeolite Chemistry and Cafalysis 0 1991 Elsevier Science PublishersB.V., Amsterdam

269

ESCA STUDY OF INCORPORATION OF COPPER INTO Y ZEOLITE Ivan Jirkae Blanka Wichterlovaa and Martin Maryskab

aJ. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 182 23 Prague 8 , Czechoslovakia ’Institute of Chemical Technology, Department of Silicates, 166 28 Prague 6, Czechos1ovakia Abstract Both low and high temperature mode of contact interaction between Cu2O and NH4-Y zeolite in a mechanical mixture has been observed by means of XF'S and XAES spectroscopies. Moreover, hydration of this mixture significantly increases extent of this interaction.

1. INTRODUCTION

It has been shown that solid-state (or contact) interaction can occur among various metal compounds and zeolites in mechanical mixtures, resulting in deaggregation of the metal compound phase and migration of metal ions into the zeolite channels [l-61. Generally, both the low temperature and high temperature modes of solid-state interaction can take place 131. The detail mechanism of this interaction is not known being affected by the type of a zeolite and a metal compound in the mixture /4-61. It has been found previously that a solid-state ion exchange occurs in the mixture of Cu oxides and NHs-Y or H-ZSM-5 zeolite after heating above 670 K [61. This information belongs to the changes in the bulk of zeolite crystals. I t seems probable that some changes in the Cu20/NH4-Y (CuzO/H-ZSM-5) interface may occur at much lower temperature. The electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM) have been used to investigate the changes in the Cu20 - zeolite interface resulting from heating and hydration of the mixture. 2. EXPERIMENTAL

The mixture was prepared by mechanical grinding of Cu20 (Merck) and NH4-Y zeolite in an agate mortar for 60 minutes. The chemical composition of NH4-Y was (wt.%): SiO2 = 67.72, A1203 = 22.21, Na2O = 1.41 and (NH4)20 = 8.65. The concentration of CuzO in the mixture was 112 mg/g of zeolite, corresponding to a Cu/OH (bridging) molar ratio equal to 0.5. In some cases the mixture was exposed to water vapour (p(H20)- 440 torr) in a static air atmosphere at 358 K for 0.5, 11.0 and 20.5 hours. The spectra of the mixtures were measured without any heat treatment and after vacuum heat treatment at 420, 620 and 770 K for 1 hour. The photoelectron and Auger lines were measured on an ESCA 3 Mk I1

270

spectrometer at ambient temperature and at a base pressure typically lower than lo-* torr. The AlKa ( E = 1486.7 eV) and MgKa (E = 1253.4 eV) lines were used to excite the photoelectrons. The Cls line (Eb = 284.4 eV1 was employed to calibrate the energies of spectra. The error of Eb (Ek) estimation was typically 0 . 3 eV. An analytical information from the electron spectra may be obtained from their intensities, binding energies of the photoelectrons and kinetic energies of the Auger electrons of a given atom. As investigated mixtures were substantially heterogenous (see bellow), the Cu concentrations estimated by ESCA were only semiquantitative. The simplest equation was used: Cu/Si = I(CuZp)~(SiZp)/I(SiZp)u(Cu2p)

(1)

where Cu/Si is a copper-silicon atomic ratio, I(Cu2p) and I(Si2p) are intensities of Cu 2p3/2 and Si 2p photoelectron lines, respectively, and cr(Cu2p) and u(Si2p) are photoionization cross-sections of Cu2pw2 and Si2p levels, respectively [71. Scanning electron microscopy (SEMI was done on a JEOL JEM 1008. Accelerated voltage was 40 kV. The surface charge of the sample was compensated by evaporated layer of Pd/Au alloy. 3. RESULTS AND DISCUSSION 3.1. ESCA of Cu ions

The estimation of the location of Cu ions in the mixture is based on a fingerprint method, i.e. on comparison of copper core level binding energies Eb and kinetic energies Ek of Auger electrons of copper with standard values of Eb and Ek of Cu compounds. This method may be complicated by charging effects resulting from the emission of electrons from insulating materials like zeolites. The results then should be carefully checked, whether they are in accordance with recent interpretations of Eb and Ek values. It is known that Eb of Cu 2p3/2 line of Cul’compounds do not substantially differ each other while their kinetic energies Ek of an Auger Cu CVV transition depend on the type of Cu compound. Both the core level E b and Auger Ek values of Cu2+are dependent on the type of a compound. These effects can be explained by screening theory proposed in literature IS]. Two channels are available for the screening - local and non-local one. Two lines are then observable in the core level photoelectron spectrum of divalent copper. Lower energy main line, screened by 3d9 electrons of copper and by another electron from a ligand localized during screening in the Cu 3d orbital and a higher energy satellite screened by Cu 3d9 electrons only. According to the interpretation of van der Laan et al. [Sl the main line is for the case of divalent copper sensitive on its chemical surroundings due to screening mechanism, while the energy of a satellite is almost independent on the chemical surroundings. F o r monovalent copper only 10 one screening channel is available (3d configuration which exclude any charge transfer from the ligands) and so this line should be, according to the above interpretation, insensitive to the chemical surroundings of the Cul+ ion. The use of the Eb values of Cu 2p3/2 line of copper enable to distinguish changes in the coordination of Cu2+ ion in the investigated mixtures. Moreover, knowing the origin of the satellite in the Cu 2p3/2

271

TABLE 1 The Cu 2p3/2 binding energies Eb (eV) and Cu L3M4,5M4,5 Auger kinetic energies Ek (eV) of Cu compounds and Cu ions in zeolites. Compound

Ref.

Eb

Ek

cu20 Cu20 (dispersed) cuc1 cul+-y

932.2 932.7 932.6 932.4

916. a 915.9 915.0 913.2

this work

CUO CUCl2 Cu (OH12 Cu"-Y

933.5 934.4 935.1 936.2

917.9 915.5 913.1

this work

9 10 11

8 10 11

TABLE 2 The Cu/Si. 10’ratio of CuzO/NH4-Y ( A ) unhydrated, hydrated for (B) 0.5 h., (C) 11 h., (D) 2 0 . 5 h., and heated at temperature T (K) in situ.

A

B

(4.5)

(7.5)

-

3.7 3.8 3.3

-

4.6 4.1 3. 6

C 12.2 10.7 10.6 7.1

D

T

(28.7) 23.8 19. a

293 293 420 620 770

-

2.3

number in brackets - Cu/Si ratio after 5 . 5 minutes of measurement (see the text) TABLE 3 Binding energies Eb(eV) of the Cu 2p3/2 lines of copper in hydrated Cu20/NH4-Y mixture and their atomic ratios of Cul /Si and Cu2+/Si estimated from eq. ( 1 ) af-ter 5 . 5 minutes of measurement- see the text). Eb(CU1+) 931.4 932.2 931.8

Eb(CU")

Cul+/Si.10'

934.5 934.9 935.0 935.1

2.4 2.0 3. a

-

Cu2+/Si.lo2 Hydration (h) 2.1 5.5 9.4 28.7

0.5 11.0 20.5

272

spectrum (no satellite is observable in this spectrum for CulC), we can estimate the oxidation state of Cu in the mixture. Similar screening effects influence two hole Auger final states. It has been shown that kinetic energy Ek of L3M4,5M4 5 transition of both Cu2+ (3d9 and 3d8 Auger final states) and Cu1+(3d8’ Auger final state) depends on chemical surroundings of the copper ion. Our previous data on the ion exchanged Cu-Y and Cu-ZSM-5 zeolites are in accordance with the above theory. The E b of Cu 2p3/2 line of Cul+ ion in zeolites do not substantially differ from the values which belong to other cuprous compounds. On the contrary, the Ek values of the Auger Cu CVV transition of Cul+ ions in zeolites are much lower in comparison with any other Auger Cu CVV kinetic energy (see Table 1). In accordance with the screening theory the Eb of Cu 2$1*3/2line of Cu2+ in Y zeolite is much higher than that of the other Cu compounds. It follows that the Eh of Cu 2p3/2 line of Cu2+ species and the E k of Cu L3M4,5M4,5 (Cu CVV) line of both Cul+ and Cu2’ species may be used to distinguish qualitatively the coordination and, therefore, location of Cu in the mixture. 3.2. CU O/NH -Y 2 4

The SEM reveals that the unhydrated mixture was composed from the grains with a diameter of about 1 pm and from the agglomerates with a diameter of about 3 - 6 pm. The hydration of the mixture at 358 K resulted in a disappearance of these agglomerates. The structureless spots with a diameter of 10 pm appeared in the mixture hydrated for 20.5 hours. As no changes induced by hydration were observed for pure Cu20, the disappearance of agglomerates and the presence of structureless spots in the heavily hydrated mixtures may be explained by the deaggregation of CuaO caused by the zeolite induced hydrolysis. This was also indicated by the dependence of the Cu/Si intensity ratios on the time of hydration estimated by ESCA (Table 2 ) . A pronounced increase of this ratio with the time of hydration confirms deaggregation of copper oxide phase Further details on the copper oxide-zeolite interaction were gained from the binding energy values and shapes of the Cu 2p3/2 photoelectron spectra (Figure 1). Two lines abbreviated as line I and I1 (at -932 and -935 eV, respectively) with a satellite at a higher binding energy were resolved by a fitting procedure for unhydrated mixture and for that hydrated for 0.5 and 11.0 hours. The high energy Cu 2p3/2 line and a satellite disappeared during the measurement (after -240 minutes 1. The accumulation time of the Cu 2p3/2 spectra was thus minimized (5.5 minutes). The only one Cu 2.~312 line with a satellite was observed for the mixture hydrated for 20.5 hours at a binding energy Eh = 935.1 eV (not shown in Figure 1). Line I belongs to cuprous species and its Eh was slightly lowered in comparison with standard Eb values. However, this deviation (except of unhydrated mixture) was about what is expected from experimental error. A lowering of Eb of line I which belongs to unhydrated mixture was most probably caused by wrong calibration (see discussion of this problem in [ 1 2 1 ) . Alternative explanation of this effect as a consequence of charge donation from the zeolite to Cul+ species is in disagreement with the results of discussion presented below. The high energy Cu 2p3/a line with a satellite corresponds to the cupric ions bonded most probably in Cu(0H)a and no Cu2+ ions were observed by ESCA to be exchanged into the zeolite by hydration. This follows from a comparison of the binding energy Eb value

-

273 (Table 3 ) with that of the standard compounds (see Table 1). The longer time of hydration, the higher was observed concentration of Cu(0H)z. This effect seemed to be quantitative for heavily hydrated mixture, as no 1+ substantial concentration of Cu was found. Migration of some copper species into the zeolite channels occurred under vacuum during spectra measurement. This follows from a decrease in the Cu/Si ratio with time of measurement (see Table 2 and Figure 2 ) . A further decrease in the Cu/Si ratio was caused by heating of the samples at 770 K. Again, this effect was most pronounced for the heavily hydrated mixture (Table 2 ) . It follows from the above discussion that the Cu/Si ratio estimated by XPS was influenced by two effects - deaggregation of the copper oxide phase, which increases the Cu/Si value, and subsequent migration of copper species into the volume of the zeolite crystals, decreasing, on the contrary, the Cu/Si ratio. The more hydrated the copper oxide phase (corresponding to a higher initial concentration of Cu(0H)z in the mixture), the greater extent of incorporation of copper species inside the zeolite was found. The conclusion on the deaggregation and diffusion of (at least part of) the copper species into the zeolite channels is supported by discussion of the shape and energy of Auger CVV spectrum of copper. The numerical values of Ek presented here are only rough estimations being discussed here only qualitatively. Their exact values can be obtained, in principle, by curve fitting of Auger spectra. However, this procedure cannot be unfortunately used because of a lack of information on the line shapes of the fitted components, which may be very different

A

91 9

A

934

B

Eb( e v ) 949

FIGURE 1 ( A ) Cu 2p lines of copper in CuzO/NH4-Y mixture (a) unhydrated; hydrated for (b) 0.5 h, (c) 11.0 h ( B ) Typical fit of the Cu 2p line

274

B A

lop

0

0

0

0 0

0 0

0

IL . 0

I

100

., % I 200

0

0

100

200

t (min ) FIGURE 2 ( A ) Dependence of Cul+/Si (open points) and Cu2+/Si (full points) of the and hydrated ( 0 1 for 11.0 h on the time t (min) sample unhydrated ( of measurement. ( B ) Dependence of Cu/Si ratios (Cu/Si = Cu’+/Si + Cu2+/Si) of the unhydrated mixture ( c) and the sample hydrated for 11.0 h ( 0 ) and 20.5 h ( A on the time t (min) o f measurement.

a)

Figure 3 depicts the Cu CVV Auger lines of copper in the unhydrated mixture and in the samples hydrated for 11.0 and 20.5 hours. For the unhydrated mixture and f o r that hydrated for 0.5 (not shown in Figure 2) and 11.0 hours the Cu CVV spectra were composed of two lines at a kinetic 917 eV (close to the value observed for Cu20, 917.4eV) and at energy Ek Ek 913 eV (close to the value of Cul’ion exchanged in the Y zeolite, 913.2 eV). The interpretation of an additional line (at lower kinetic energy) found in this spectrum, also observed in a pure zeolite, is not yet clear. The intensity of the Auger line at 913 eV increases with the time of hydration and decreases with the heating of the mixture (Figure 2). Only one broadened Cu CVV line was observed for heavily hydrated sample 917 eV. Heating of this sample at 770 K caused a substantial line at Ek shape change and shift to 913.5 eV. These effects can be explained in terms of migration of at least part of the Cu species into the zeolite channels, even at ambient temperature (see discussion of the Cu/Si ratio above). More extensive incorporation of Cu species into the zeolite channels due to heating of the sample increases with the time of hydration, In the sample hydrated for 20.5 hours followed by heating at 770 K, all the copper species can be assumed to be incorporated into the zeolite channels (no Cu CVV line of Cul+in Cu20 was observed - see Figure 3).

-

-

-

275

I

I

I

I

1

1

I

1

915.8

912.9

Ek(eV)

916.8 912.8

Ek(eV)

I

FIGURE 3

Auger Cu CVV spectra of copper in Cu20/NII4-Y mixture ( 1 ) unhydrated; ( 2 ) and (3) unhydrated followed by heating at 620 and 770 K, resp.; (4) hydrated for 11.0 h; (51 hydrated for 11.0 h followed by heating at 770 K; ( 6 ) hydrated for 20.5 h; (7) and ( 8 ) hydrated for 20.5 h followed by heating at 420 and 770 K, resp. 4. CONCLUSIONS

The core level and Auger shifts of copper ion exchanged in zeolites compared to those in various Cu compounds may be explained by the screening theory proposed in literature. The core level Eb of Cul’ions depends only weakly on their chemical surroundings. The kinetic energy of the Auger Cu CVV line of the Cul’ion exchanged in zeolite may be used as a fingerprint^ value due to nonlocal screening of the Auger final state of the Cul+ ion. In the case of Cu2+both the core level E b and Auger E k values may be used due to the nonlocal screening of final states. The first step of contact interaction of CuzO with NH4-Y zeolite in their mechanical mixture is deaggregation of Cu20 and its oxidation to Cu(0H)z. A part of Cu species is incorporated into the zeolite channels during measurement of photoelectron spectra likely as a consequence of sample heating during measurement and/or by photodissociation of the Cu(0H)a thin layer. Heating of the mixture already at 420 K causes further migration of Cu species into the zeolit,e channels. This migration was previously indicated for the zeolite bulk after heating of the mixture

216

above 620 K. Oxidation of CuzO and a low as well as high temperature migration of Cu species into the zeolite are substantially increased by pre-exposure of the mixture to water vapour. This effect is explainable by an increased fraction of Cu bonded in Cu(0H)a due to hydration. As a lattice energy of cupric hydroxide is lower than that of cuprous (cupric) oxide, dissociation of a former compound is energetically more favourable and expectably an easier incorporation of Cu into the zeolite channels takes place.

REFERENCES 1. 2. 3. 4.

5.

D.W. Breck in Zeolite Molecular Sieves - Structure, Chemistry and Use, Wiley, New York, 1974, pp 588 - 592. A . V . Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38. H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79. B. Wichterlova, S. Beran, L. Kubelkova, J. Novakova, A. Smieskova and R. Sebik, Stud. Surf. Sci. Catal. 46 (1989) 347. S. Beran, B. Wichterlova and H.G. Karge, J. Chem. SOC. Faraday Trans. 86 (1990) 3033.

6. B. Wichterlova and H.G. Karge, submitted for publication. 7. J.H. Scofield, J. Electron Spectroscopy, 8 (1976) 129. 8. G. van der Laan, C. Westra, C. Haas and G.A. Sawatzky, Phys. Rev.B 23 (1981) 4369.

I. Jirka, Thesis, J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Prague 1989. 10. Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy (D. Briggs and M.P. Seah eds. 1, John Wiley, New York, 1983. 11. I. Jirka and V. Bosacek, Zeolites 11 (19911, 77. 12. T.L. Barr and M.A. Lischka, J. Am. Chem. SOC.,108 (1986) 3178. 9.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science PublishersB.V., Amsterdam

277

PREPARATION OF Ga-DOPED ZEOLITE CATALYSTS VIA HYDROGEN INDUCED SOLID STATE INTERACTION BETWEEN Ga203 AND HZSM-5 ZEOLI’TE b

V.Kanazireva,G. L.Priceb and K. M. Dooley

armtitUte of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria bDepartment of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, USA

Abstract Ga-doped catalysts were prepared by mechanical mixing of 6 Ga203 and HZSM-5 zeolite. Several physical methods (XRD, XPS, IR, TPR, TEM/EDAX) unanbigously evidence that in the presence of Hz Ga203 reduction process occurs leading to gradual depletion of the Ga203 crystalline phase and simultaneous transfer of the gallium species into the zeolite The formation of GaaO is considered as the first step of the reduction process The zeolite acidity seems to be the most important factor in facilitating this process via trapping in cationic zeolite positions of gallium species in a lower (probably Ga oxidation state. The role of the hydrogen reduction in creating of a new active state of Ga-containing zeolites is confirmed by the dramatic enhancement of propane and n-pentane aromatization activity as well as by the strong increase in the ethylbenzene conversion.

1. INTRODUCTION

Ga containing zeolites have received extensive current interest since their utilization in the Cyclar process for- light paraffin aromatization [ll. Ion exchanged [21 or impregnated gallium [31 as well as gallosilicates [41 and even mechanical mixtures of GaZ03 and HZSM-5 zeolites [51 have been shown to exhibit catalytic activity in this new reaction. Examination of several Ga zeolite catalysts preparations has disclosed that all of the common synthesis techniques can conceiveably generate an intimate mixture of a gallium oxide or hydroxide with the zeolite 161. Moreover, recent a bifunctional reports [5,7,81 in the literature have focused on (Ga~Os)/acidiczeolite) mechanism for the catalytic conversion process. We have found, however, that mechanical mixtures of Ga20UHZSM-5 undergo chemical transformation upon treatment with hydrogen or via treatment with propane feedstock [9,1O] and have suggested that the bifunctional theory needs to be re-evaluated to include the possibility of a reduced galliurn oxidation state [111. This paper reports some additional evidence, which confirms that under suitable temperature conditions, the hydrogen induces gallium transfer from the GaZ03 crystalline phase into the HZSM-5 zeolite.

The increased catalytic activity of the resulting Ga-modified zeolite catalyst is corroborated in the conversion of both n-pentane and C8 aromatic hydrocarbons.

2.

EXPERIMENTAL

Catalysts were prepared by mixing powdered Ga203 (4N5-grade, Ingal International Co) and HZSM-5 zeolite (Union Carbide,Linde Div.) in a stainless steel ballmill .The manufacturer reported composition of the HZSM-5 zeolite is 3.73% A1203, 94,95%Si02 and 0.03% Na2O by weight. We refer to these catalysts as Ga/HZSM-5 catalysts with a numerical prefix indicating the gallium loading by weight. The catalysts were pelletized and crushed to 40-60 mesh prior to use in reduction or catalytic experiments. A Scintag PAD-V X-ray diffractometer equipped with a C u K a radiation source operated at 1.6 KW and Kevex Peltier-cooled solid state silicon detector was used to characterize the catalysts as described in ref. 10. The content of the gallium oxide crystalline phase in "fresh","reduced" or "used" ( i n catalytic experiments) catalyst samples was determined by using a M-80 Carl-Zeiss spectrometer and the KBr-pellet technique. XPS measurements were carried out in the analysis chamber of an ESCALAB MK I 1 (VG Scientific) electron spectrometer. The spectra were excited with A1Ka radiation (KV=1486eV). The powder samples were pressed into stainless steel sample holders and then introduced into the preparation chamber and pumped down to 10-8-10-’ombar. After transferring the samples in the analysis chamber of the XPS spectrometer, several photoelectron and Auger lines were recorded and Ar ion bombardment was used for depth profiling. The procedure applied is described in detail in [12]. The IR spectra were recorded on a DIGILAB FTS 20E Fourier transform spectrometer. The sample was pressed into a self-supporting wafer and dehydrated in vacuo at 783 K. 300 Torr of highly pure hydrogen was used as a reducting agent. After introduction of the hydrogen the temperature was raised again to 783 K and kept at this temperature for 1.5 h including a short evacuation and renewed admission of 300 Torr H2. A transmission electron microscope (PHILIPS EM 420) equipped with an X-ray spectral analyzer (EDAX) was used to obtain the selected area electron diffraction pattern and for microanalysis of the catalyst. Speciments for microanalysis were prepared by dispersing the powders in ethanol, placing a drop of this suspension on a thin carbon support net and allowing the solvent evaporate. The temperature programmed reduction (TPR) experiments were performed in a SETARAM TGDTA 92 microbalance. After evacuation of the sample to lo-’ mbar, the sample was equilibrated at room temperature with a 65 ml/min pure argon flow.The temperature was then raised with rate of 10 K/min up to 823 K and kept at this temperature for 1 h. Then the sample was cooled down to 360 K and 50 ml/min of the argon flow was replaced by the equal amount of hydrogen (80 kPa H2 in the total flow). Finally the sample was purged in hydrogen at 360 K for 15 min and TPR was performed at a scan rate 10 K/min up to 1073 K. The catalytic experiments for propane, n-pentane and C8 aromatics conversion were carried out in fixed bed type reactors operated in an inert gas (He or N2) stream at atmospheric pressure. The catalyst amount varied from 0.1 to 0 . 8 g depending on the selected experimental conditions and the

279

particular reaction investigated. The analysis of the feed and reactor products was performed with HP-5880 and HP-5890 gas chromatographs equipped with high performance fused silica capillary columns (Supelco SPB-1 and HP-PONA). The detailed procedure for the catalysts testing is described in

[lo].

3. RESULTS AND DISCUSSION

3.1 Gallium state in hydrogen treated GaXI3 catalysts

The relatively simple technique of temperature programmed reduction (TPR) was employed to elucidate whether the mixed Ga203/HZSM-5 catalysts undergo a chemical transformation in the presence of hydrogen as a reducing gas. The TG and DTA curves in Fig. 1 show a process of weight l o s s taking place in hydrogen flow whereas there are almost no changes in the sample weight when the same experiment is conducted in an inert gas atmosphere. In both cases, DTG and TG curves are calculated by the computer facility dividing the data for the pure HZSM-5 sample gathered in a separate experiment from those for the 5Ga/HZSM-5 catalysts. This approach significantly increases the accuracy of the weight loss estimation. The DTG band at 853 K clearly indicates a fast process of hydrogen reduction of the Ga203 leading to a weight loss of approx. 0.9%. This process is followed by a slower one, which accounts for approximately one third of the total weight loss. The above results confirm our previous observation [ l o ] that the gallium oxide reduction with H2 can be greatly facilitated in the presence of an intimate admixed acidic ZSM-5 zeolite. Due to the higher H2 partial pressure used in the present investigation, the DTG reduction band is shifted to a lower temperature than those reported in ref 10. Moreover, the applied procedure of monitoring difference TPR spectra allows verification that the weight loss effects observed are not due to processes such as dehydration and dehydroxylation of the zeolite component of the mixed catalyst.

l..'---:I:lGE I., Nlu

'

bn

U/minl

-0.00

_---

0.3

0 00%

+ t -

.

580

,\

-0.1

\

-0.10 -0.m

ARGON

-

\

-0.m

\

'\O

7

1-

t-O'O

-0.90

HVOROGEN

\

...

-1.7

.sax

80

-0.m

*

-TuE 3w

490

roo

roo

700

Figure 1. TG and DTG curves of 5Ga/HZSM-5 in Hz and

N2

El

-0.m

280

We have shown previously, that the two HZSM-5 zeolite XRD bands at about 24.2’ and 29.2’ 26 and the two lines of the 6 Ga203 at about 39.6’ and 35.2’ do not interfere, therefore, these bands were used to measure the 13 Ga203 crystalline phase content of the mixed catalyst. The XRD examination of the 5Ga/HZSM-5 sample before and after reduction with hydrogen at S30 K shows that, after the reduction of the sample in the microbalance, only traces of gallium oxide crystalline phase are present. The process of Ga203 IR depletion can be more conveniently recorded by using a simple KBr-pellets technique as it is ilustrated in Fig. 2. From this figure i t can be seen that one of the most intense IR bands of Ga203 appears in the 600-800 cm-’ region of the IR spectrum of HZSM-5 zeolite. Examination of several Ga20UHZSM-5 mechanical mixtures confirmed the intensity ratio of the peaks at 698 and 456 cm-’ as a suitable measure of the gallium oxide content even in the case of catalysts containing less than lwt% Ga203. In application of the approach described above for investigation of the 5Ga/HZSM-5 sample after the TPR experiment (Fig.1) as well as for characterization of a number of other "reduced" and "used" gallium catalysts, we did not observe any detectable amount of remaining Ga203 phase in the 2GaIHZSM-5 and 5Ga/HZSM-5 samples pretreated with hydrogen at a temperature equal to or higher than 763 K. In contrast, the degree of reduction of the 10Ga/HZSM-5 sample treated under the same experimental conditions does not exceed 50%. Therefore, we assume that only a limited amount of gallium can be effectively reduced with H2 in the presence of an acidic zeolite component. Finally, we should note that both XRD and I R methods used for gallium oxide determination produce well-correlated

456

548

A

Ga(3d) :,i: oj2s) :,: I I

.I.

5 Figure 2. I R spectra of the mixed catalyst and its components

B.E.,eV

35

Figure 3. XPS spectra of 5GaIFZSM-5 a-before and b-after Ha reduction

281 results. Due to the small sample size used in the microbalance and catalytic experiments, however, the described IR approach was found to be more suitable and even solely applicable in the determination of the GaaO3, crystalline phase in these samples. The transmission electron microscopy TEM coupled with EDAX microanalysis along with X-ray photoelectron spectroscopy (XPS) helped to establish the processes occurring in the mixed Gaz03/HZSM-5 catalysts after their treatment with hydrogen. Transmission electron micrographs of the SGa/HZSM-5 sample before the hydrogen reduction clearly indicate two kinds of partic1es:O.l-0.3 pm zeolite prisms and spherical particles ranging from 0.05 to aprox. 0.3 pn in size and containing gallium oxide. After its reduction in the microbalance, several micrographs of the same sample show that the gallium oxide particles have almost disappeared. Selected data from EDAX microanalysis of a number of zeolite particles listed in Table 1 provides convincing evidence that, during the TPR experiment shown in Fig.1, a process of gallium species transfer into the zeolite has occured. It is difficult to decide on the basis of the data in Table 1 whether the variations in the composition reflect a non-random gallium distribution among the zeolite particles. Nonetheless, there is no doubt that the major part of the gallium oxide crystalline phase undergoes a degradation process and the resulting gallium species are distributed into the zeolite microcrystallites. Neither gallium foreign phases on the zeolite crystallites nor gallium enrichment of their surface are observable. Table 1 EDAX microanalysis of selected zeolite particles ~

Composition wt%

A 1203 Si02 Ga203

~

Partical number (5Ga/HZSM-5 sample) before 1 4.238 95.762 0.000

reduction 2 4.010 95.990 0.000

after 3 4.571 90.944 4.485

reduction 4 4.838 90.042 3. 120

5 4.542 90.944 3.598

Additional important evidence on the process of gallium transfer into the zeolite was obtained by XPS. As one can see from Fig. 3 the reduction of the 5GaIHZSM-5 sample with hydrogen causes an increase in the Ga (3d) intensity by factor of about ten. This phenomenon can be well understood assuming that the hydrogen reduction leads to spreading of the Ga over the zeolite, which greatly increases the effective surface area of the Ga-containing material. On the other hand, the removal of 15-20 monolayers of the sample by Ar’ bombardment does not change more than 1-2% of the Ga (3d) intensity as measured from the total peak area of Ga (3d)+ + 0 ( 2 s ) lines, which clearly shows that Ga is transferred into the bulk of the zeolite. Finally, the Ga(3d) peak of the reduced sample shifts to a binding energy that is 0 . 7 eV lower compared to the non-reduced mechanical mixture, which can be interpreted as a lowering of the gallium formal oxidation state. However, the determination by XPS of the normal Ga oxidation state of the mixed catalysts is rather complicated for several reasons and a separate publication [121 is devoted to this subject.

282

In a previous paper [lo1 we assumed that the zeolite acidity is the driving force, that allows a substantial lowering of the reduction temperature of the admixed gallium oxide phase. Recently we proved the involvement of the acidic zeolite OH groups in the process of gallium transfer into the zeolite [131. As it can be seen from Fig 4, the narrow band at 3610 cm-* due to the acidic OH groups of the HZSM-5 zeolite greatly decreases in intensity after reduction of the 5Ga/HZSM-5 sample with hydrogen. A rough estimation shows that approximately 50% of the initially present OH groups are lost despite the mild conditions (static apparatus and relatively low temperature) of the reduction experiment. In contrast, numerous investigations e.g.[14, 151 failed t o confirm any significant changes in the OH group content of the HZSM-5 zeolite, when the gallium was introduced by the common wet techniques of ion exchange or impregnation. This effect can be readily explained by taking into account the large size of the hydrated Ga3+ ion as well as the constraints due to electrostatic disbalance when three isolated ne ative charges of the zeolite framework have to be compensated by one Ga3’ ion. Furthermore, the IR data reported in the literature are not related to Ga/ZSM-5 zeolites, that have been subjected to H2 treatment at elevated temperatures. We suggest that the hydrogen reduction plays a key role in facilitating the introduction of gallium species in the bulk of the zeolite even if the gallium source is present in the form of a separate crystalline phase. It seems that the reduction to a lower oxidation state of polyvalent cations such as Ga3+ helps to avoid the above mentioned constraints and assure 15 the introduction and random 0 0 distribution of these cations in W the HZSM-5 zeolite. The gallium U B suboxide appears to be the first ' product of the reduction process, 10 a 0 which is readily trapped by the VI m acidic sites and further a 5distributed in the cationic A positions of the zeolite framework, probably as a Ga’ cation. ’3;OO’ ' * . 3600 ' . ' . * 3700 ' WAVENUMBERS [IICM] Figure 4.,IR spectra of 5Ga/HZSM-5 sample A-before and B-after hydrogen reduction

-

F

5

3.2 Catalytic properties of Ga203/HZSM-5 catalysts

Catalytic experiments reveal the hydrogen reduction of Ga203/HZSM-5 mixed catalysts as a process which strongly affects the catalytic properties of these catalysts. As has been reported elsewhere [10,111, the aromatization of propane is greately enhanced after reduction of the catalysts which either hydrogen o r propane reactant. The data listed in Table 2 show that there is a gradual increase of both total conversion and aromatic selectivity of the 5GaIHZSM-5 catalyst with the time on stream at the same time a process of Ga203 reduction by the propane reactant or hydrogen envolved during the reaction takes place. The IR examination of

283

the "used" catalyst after the catalytic run mentioned above showed almost complete disappearance of the gallium oxide phase. No development of catalytic activity,however,was observed when pure HZSM-5 and Gaa03+HZSM-5 separated by a 0.5 cm long quartz wool bed were used as catalysts. From Table 2 it can also be seen that, after 1500 minutes on stream with propane the aromatic selectivity is enhanced by a factor of about 100 and there is a threefold increase in the total conversion compared with the HZSM-5 sample containing no Ga. While the methane yield does not change significantly, the content of saturated hydrocarbons increases steadily, reflected in the olefin/paraffin ratios 2. Characteristic changes also occur in the shown in Table isobutane/n-butane ratio. Table 2 Propane conversion on HZSM-5 and 5Ga/HZSM-5 catalysts Sample Time on stream, min

HZSM-5 90

5Ga/HZSM-5

40

90

180

400

1500a

Total conversion, wt%

7.49

11.26

13.99

16.53

18.28

20.20

Yield of products, wt% Methane Ethy1ene Ethane Propylene Butenes Bytanes C5 aliphatics

2.24 3.72 0.30 1.02 0.12 0.04 0.02

2.70 4.69 0.48 2.06 0.41 0.14 0.03

2.93 4.82 0.59 2.70 0.52 0.27 0.04

2.89 4.77 0.75 3.40 0.57 0.36 0.04

2.89 4.50 1.00 3.38 0.60 0.44 0.04

2.70 4.29 1. 29 3.48 0.60 0.49 0.05

Total aliphatics Total aromatics

7.44 0.03

10.51 0.75

11.87 2.12

12.78 3.75

12.85 5.43

12.88 7.32

Aromatic selectivity, %

0.4

6.7

15.1

22.7

29.7

36.2

12.4 3.0 0.61

9.8 2.9 0.63

Selected product ratios Ethylene/Ethane ButenedButanes Isobutaneh-Butane ~

8.2 1.9 0.70

6.4 1.6 0.76

4.5 1.4 0.71

3.3 1.2 0.64

~~

Conditions: Temperature 803 K; WHSV 1 h-l, Propane partial pressure 14 kPa Total pressure 123 kPa ( H e as a diluent gas) a Catalytic testing at a higher temperature (833 and 858 K) was performed between 400 and 1500 rnin of the experiment. The effect of Ha reduction of the 2Ga/ZSM-5 sample is demonstrated in Tables 3 and 4 for n-pentane and c8 aromatics conversion respectively. After a short catalytic testing in Nz the same catalyst sample was purged with pure nitrogen at 773 K, then reduced with pure Ha at this temperature for 1.5 h and finally subjected again to the catalytic testing in nitrogen atmosphere.

284 Table 3 Pentane conversion at 693 K, WHSV 1.8

Table 4 Conversion of C8 aromatics

Catalyst

Cata 1yst

HZSM-5 2GaZSM-5 A

Conversion wt% Selectivity % : Aromatics Aliphatics Distribution of aliphatics %

c1 c2

c3 c4

24.1

36.5

5.4 6.8 95.6 93.2

13.6 86.4

1.9 1.9 27.8 27.6 49.3 48.9 21.0 21.6

2.7 26.7 45.6 25.0

1.59 0.55 0.91 0.53

1.60 0.53 0.93

0.55

2Ga/HZSM-5

B

24.6

Ethylene/ethane Propylene/propane Butenes/Butanes Isobutane/n-butane

HZSM-5

1. 11 0.40 0.35 1.57

Product Composition wt% Benzene 1.29 Toluene 1.77 Ethylbenzene 15.87 Xy 1enes 78.20 3.01 Cg aromatics Ethylbenzene 19.3 Conversion %

A

B

1.66 2.51 1.95 2.54 16.06 13.8s 77.70 77.00 2.60 4.09 18.3

29.4

A-before and B-after H2 reduction WHSV 1.5 h-I, Nz/feed=3, T=553 K Feed composition wt%: Toluene 1.41 Ethylbenzene 19.66,Xylenes 78.SO

A-before and B-after H2 reduction As shown in Tab. 3 an increase in the aromatization activity is observed after the hydrogen reduction. The hydrogen reduction also strongly influences both isobutaneln-butane and olefidparaffin product ratios whereas the production of methane and aromatics distribution do not change as much.It seems that the Ha reduction introduces into the Ga/HZSM-5 catalysts an increased ability to produce more saturated and aromatic hydrocarbons converting a paraffinic feedstock.This feature of the reduced catalyst is likely related to the catalyst capability to greatly facilitate hydrogen transfer reactions as compared to both HZSM-5 and nonreduced Ga/HZSM-5 zeolite.This assumption is in agreement with the results in Tab. 4 for the conversion of c8 aromatics. Indeed, the hydrogen reduction of the 2Ga/HZSM-5 sample leads to an enhanced ethylbenzene conversion and to an increased rate of the disproportionation and transalkylation reactions Since such reactions require a bimolecular transition state, i t is reasonable to assume that the new active catalyst state contributes to acceleration of the hydrogen transfer processes. On the basis of the catalytic results it is rather difficult to determine the reason for such dramatic changes in the catalytic properties of the Ga/HZSM-5 catalysts after hydrogen treatment. Along with the above mentioned changes in the gallium state,the zeolite acidity is also altered considerably by the process of gallium transfer. The replacement of a part of the acidic OH groups by new Lewis sites containing cationic gallium may be one of the most important factors contributing to the specific catalytic action of the Ga203IHZSM-5 catalysts.

285 4. CONCLUSIONS

The increased reducibility of the gallium oxide in the presence of both hydrogen and acidic ZSM-5 zeolite causes a process of gradual depletion of the gallium oxide and simultaneous transfer of cationic gallium species into the zeolite.The solid-state reaction proceeding through galljum suboxide as an intermediate represents a new route to high quality catalysts,which requires no wet operation for the catalyst preparation.The theories pointing to a bifunctional gallium oxide/acidic zeolite mechanism of the light paraffin aromatization need to be re-evaluated to include the possibility of a reduced gallium oxidation state, as well as the impact of gallium on the zeolite acidity.

5. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of the Bulgarian Academy of Sciences and the Exxon Foundation.The authors a r e indebted to Drs. V. Mavrodinova, C. Tyiliev, V. Valtshev, Mrs. L. Kosova and M. Stojanova for their helpful1 assistance and stimulating discussions. V. K. thanks Prof. H. Forster (Hamburg University, Germany) for providing the FTIR facilities.

6. REFERENCES

1 J.R.Mowry, R.F.Anderson and J.A. Johnson,Oil Gas J., (1985) 128 2 H.Kitagama, Y.Sendova and Y.Ono, J.Catal., 101 (1986) 12

J.Y.Doyemet, A.M.Seco, F.Ramoa Ribeiro and M.Guisnet, Appl.Catal., 43 (1986) 155 J.M.Thomas and Xiu-Cheng Liu, J.Phys.Chem., 90 (1986) 4843 N.S.Gnep, J.Y.Doyemet and M.Guisnet, J , M o l . Catal., 45 (1988) 281 G.L.Price, K.M.Dooley and V. Kanazirev, submitted for publication T. Inui, Y.Makino, F.Okazumi, S.Nagano and A.Miyamoto, Ind.Eng.Chem.Res.,

3 N. S. Gnep,

4 5 6 7

26 (1987) 647

8 P.Meriadeau and C. Naccache, J.Mol.Catal., 59 (1990) L31 9 V. Kanazirev, G.L.Price and K.M. Dooley, J.Chem.S O C . ,Chem.Commun., 9 (1990) 712

G.L.Price and V.Kanazirev, J.Catal., 126 (1990) 267-278 G.L.Price and V.Kanazirev, in press V. Kanazirev, G.L.Price and G.Tyuliev, submitted for publication V.Kanazirev, R.Piffer and H.Forster, submitted f o r publication 14 V. B. Kazansky, L.M.Kustov and A. Yu.Khodakov, Stud.Surf.Sci.Catal. 10 11 12 13

49 (1989) 1173

15 V. I. Yakerson, T.V. Vasina, L. I.Lafer, V.P.Syntyk, G.L.Dikh and 0.V. Bragin in Abstracts of ZEOCAT 90, Leipzig, p.55

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P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

281

COMPARISON OF HYDROSULFURIZATION ZEOLITE CATALYSTS PREPARED IN DIFFERENT WAYS

Gy. Onyestyak, D. Ka116 and J. Papp, Jr. Central Research Institute for Chemistry, Hungarian Academy of Sciences, 1525 - Budapest, P. 0. Box 17, Hungary

Abstract

Transition metal ions can be introduced into zeolites by solid state ion-exchange for preparing catalysts to be used in hydrosulfurization. Chlorides of Co, Mn, Zn, Cd, Ca were contacted with NH4-Y; CdClz with NH4-MOR and NH4-X; and different compounds of Cd with NHs-Y. Ion-exchange was followed by i. r. spectroscopy: decrease of OH band intensity and bands generated by dissociative HzS adsorption were detected. The rearrangement of adsorbed bases after HzS adsorption observed in i. r. spectra indicated more extensive dissociative adsorption of H2S on Cd-faujasites prepared in solid state than in liquid phase ion-exchange. Dehydrosulfurization tests also revealed this recognition.

1. INTRODUCTION

Hydrosulfurization of olefins, i e , the addition of Has to C=C double bond resulting in the formation of thiols and thioethers can selectively be catalysed by transition metal-forms of different zeolites ( 1 ) . I t has been found that on the same catalysts thiols and olefins are converted to thioethers, furthermore, thioethers with HzS transform into thiols (2). Catalysts were prepared with conventional ion-exchange carried out in an aqueous solution of the corresponding metal salt (1). For transition metal ions this method involves some limitations. At low pH values, required to avoid hydrolysis of the salt, the zeolite lattice may be damaged, moreover, deep ion-exchange for transition metals can hardly be achieved. Solid state ion-exchange seems to be promising, as it proved to be suitable for the introduction of alkali, alkaline earth and earth metal ions as well ( 3 - 6 ) . According to these investigations NH4- and H-forms readily react with chlorides and the desired cationic form is produced. Incorporation of Mn and Fe ions into the cationic sites of H-ZSM-5 has been observed when solid state reactions proceed between the zeolite and the corresponding salts or oxides at elevated temperatures ( 7 ) . In this way exhaustive ion-exchange can be achieved. The resulting cationic forms are mostly of higher activity than those prepared with usual ion-exchange, as it has been found for

288

partly hydrated La-Y in ethylbenzene disproportionation (51 and for Fe- and Mn-ZSM-5 in methanol transformation to aromatics and toluene disproportionation (7) owing to the deeper ion-exchange. We wish to examine (i) whether solid state ion-exchange can be effectively carried out with transition metal compounds, and (ii) how these samples behave in comparison with samples prepared in usual aqueous ion-exchange.

2.

EXPERIMENTAL

Ma ter ia 1s Na-Y and Na-MOR were produced in Wolfen/Germany, Na-A by Bayer AG/Germany and Na-X by BDH Chemicals Ltd/Great-Britain. 100 g of Na-zeolite in 2 1 1 N aqueous solution of NH4C1 was refluxed for 5x7 hours, then washed with distilled water. The degrees of ion-exchange f o r NH4 were NH4-X 85%

NH4-Y 96%

NH4-MOR 90%

NH4-A 98%

Ion-exchange with 2 1 of 0.2N solution of metal chlorides was performed for 5x7 hours under refluxing. The degrees of io~-exchangewere: CdX 94%

CdY 87%

CdMOR 79%

CdA 99%

ZnY 94%

MnY 77%

Solid state ion-exchange was carried out as follows: generally stoichiometric amounts of metal compounds were mixed with 2 g of NH4-zeolite in an agate mortar and stored in a glass flask. HzS was a Linde product of 99.6% purity, ethanethiol (EtSH) and diethyl sulfide (EtzS) (Fluka), ammonia (Matheson) and pyridine (reanal/Hungary) were of GC purity. Methods I. r. spectra of samples pressed into wafers were recorded usually after heat treatment at 5OO0C at lo-' Pa f o r 1 hour; adsorption of water: 20 Pa for 10 min. at 35OoC, evacuation: 10 min. 35OOC; adsorption of HzS: 6.6 kPa for 10 min. at 2 5 O C , evacuation: 10 min. 25OC. X. r. d. patterns of the samples investigated did not show any loss in crystallinity. Decomposition of EtSH and EtzS was tested by ethylene formation i n a pulse reactor. 100 mg catalyst was inserted and pretreated at 500°C in Nz-flow. At 377 C first 4 pulses of HzS ( 2 5 0 p1 each), than 10-10 pulses of 1 1.11 EtSH and EtzS were injected in Nz flow.

289 3. RESULTS AND DISCUSSION

Indication of ion-exchange Solid state ion-exchange can be favourably performed when the process results in the formation of volatile compounds. NH4-zeolites are preferred since the ammonium salts formed are removed easily at elevated temperatures (3). During the heating of thoroughly mixed powders of a metal salt and NH4-zeolite, ammonium salt (or its dissociation product) is released in the extent of the ion-exchange. (We found, e. g . , that NH4Y violently reacts with NazS even at room temperature with the evolution of NH3 and H2S and Na-Y is formed stoichiometrically.) The sites not exchanged are deammoniated at higher temperatures and acidic OH groups are formed. Formation of these groups indicate the incompletness of introduction of the desired cations. The i. r. spectra of well mixed NH4-Y and CdClz wafers treated (i) at Pa for 1 hour at each increasing temperatures between 200 and 500’C temperatures), (ii) at 400’C at 1,0-2 Pa for 1-12 hours, (iii) with excess Pa for 1 hour) show that the of Cd salt (pretreatment at 300 C, intensity of the LF band (OH vibration band at 3540 cm-*I decreases and thereafter the intensity of the H F band (OH vibration band at 3640 cm-’1 diminishes. After disappearance of the LF band, the HF band also disapears, pointing to perfect ion-exchange. Since in the strong electrostatic field of multivalent cations water dissociates heterolytically, resulting in the formation of acidic OH groups (81, corresponding absorption bands appear after adsorption of water on Cd-Y prepared with solid state ion-exchange (Fig. la HF band of the heat treated sample reflects incomplete exchange f o r Cd , while its intensity increase and the apearance of the LF band after the adsorption of water indicate Cd ions in cationic sites. For comparison the behaviour of solid state ion-exchanged La-Y is shown in Fig. lb. It has been proved (1) that the adsorption of H2S introducing hydrosulfurization proceeds similarly to the adsorption of water (figs. lc and d): both HF and LF bands are generated on Cd-Y and La-Y. However, H 2 S adsorption on the catalytically most active Cd-Y (1) is larger than on La-Y showing low activity in hydrosulfurization; the situation is reversed compared to water adsorption. H2S adsorption on Ca- or Na-Y does not generate detectable OH bands. An intense OH band is recorded at 3580 cm-I for NHs-MOR after deammoniation at 500’C (Fig. 2, curve 1). When CdClz is admixed, the same treatment results in a weak OH band (curve 2 ) owing to ion-exchange for Cd+2. H2S adsorption increases the intensity of a diffuse OH-band (curve 3j, and simultaneously a band appears at 2520 cm-’, which is assigned to SH stretching vibration ( 9 1 . Appearance of the SH band indicates the dissociative chemisorption of H2S on the introduced cations. The SH groups formed in H2S dissociation could not be fully removed even at 2OO0C (curve

+&

4).

Factors affecting solid state ion-exchange In the first run of experiments chlorides of bivalent metals were used and solid state ion-exchange capability of dufferent metals was thus compared (Fig. 3). Increasing order of ion-exchanges based on the intensity decrease of the

290

NHLY Lac13

3630

la1

3520

Ib1

NH4Y+LaCI3

3640 3530

10

3500

1

10

3500

3000

WAVENUMBER ICM-’)

Fig. 1. I. r. spectra of NH4-Y with admixed CdCla and Lac13 after standard pretreatment for 3 hours (upper curves), after water and H2S adsorption (lower curves).

LOO0

3Si0, 3500

I

I

3000

2500

00

WAVENUMBER (CM-’)

Fig. 2. I. r. spectra of NH4-MOR; 1: after standard pretreatment; 2: with admixed CdCla, standard pretreatment for 3 hours; 3: followed by adsorption of H2S; 4: pumped off at 2OO0C for 30 rnin.

HF band of deammoniated NHI-Y is CoiMniCd-Zn
291

+

CdCl

+ZnCI

I

10

2500

WAVENUMBER (CM-’]

Fig. 3. I. r. spectra of NH4-Y without and with admixed metal chlorides; (a): after standard pretreatment; (b) and (c): after HzS adsorption.

WAVENUMBER lCM-’I

Fig. 4. As Fig. 3 but with admixed Cd compounds.

be attained in each case when the time of treatment is long enough. Ion-exchange rates can be influenced by lattice energies, water solubilities, ion hydrations, ion diffusivities, etc. (10). For NH4-X and NH4-A OH bands are not clearly detectable especially after solid state ion-exchange. The introduction of cations can be checked, therefore, by the successive adsorption of NH3 and pyridine (vide infra). Comparison of aqueous phase and solid state ion-exchanges X- and Y-faujasites were studied. Cd-forms were prepared by ion-exchanges carried out in solution (samples Cd-X, Cd-Y) and in solid state (samples NH4X+CdClz, NH4Y+CdC12). NH3 as well as pyridine was adsorbed on samples pretreated under standard conditions (Fig. 5, curves 1, 3; Figs. 6a, b, curves 1). Comparison of spectra 1 and 3 in Fig. 5 shows no significant differences for X or Y samples prepared in different ways. After H2S adsorption, however, the difference between Cd-X and NHsX+CdC12 and similarly between Cd-Y and NH4Y+CdC12 (cf. curves 2 and 4 in Fig. 5 ) is rather striking: HzS adsorption results in a much higher intensity increase of the band at 1440 -1 cm (assigned to ammonia adsorbed on BrGnsted sites) for samples prepared with solid state ion-exchange than for Cd-X or Cd-Y aqeous exchanged samples. When pyridine adsorption is followed by H2S adsorption, the intensity of the band at 1548 cm-I (for X) or at 1550 cm- (for Y), characteristic of pyridinium, increases (Fig. 6a, b curves 2). Intensity increase is greater for NH4X+CdCla or NH4Y+CdClz than for Cd-X or Cd-Y (cf. spectra 1 and 2 in Figs. 6a and b), i. e. more Bronsted sites are generated by H2S adsorption

292

x

I

I. r. spectra of Fig. 5 . Cd-faujasites after standard pretreatment at 45OoC; 1 and 3: after NH3 adsorption at 6.6 kPa, 25OC, 10 min., evacuated at 200 C for 30 min.; followed by H2S adsorption and evacuation at 20O0C, 30 min. (curves 2 and 4); samples were ion-exchanged in liquid phase (curves 1 and 2 ) and in solid state (curves 3 and 4). 1440

u I 1600 ILOO 1600 ILOO WAVENUMBER (CM-')

on samples prepared via solid state ion-exchange than on .those ion-exchanged in solution. On comparing band intensities at 1447 cm-’ (for X) or at 1450 cm-’ (for Y) characteristic of pyridine bound to a Lewis site (probably to Cd2+) we find that H2S adsorption removes more pyridine from these centers in the case of NH4X+CdC12 o r NH4Y+CdC12 than for Cd-X or Cd-Y. Presumably, solid state ion-exchange results in higher numbers of Cd2+, which adsorbs H2S forming CdSH’ species, than observed f o r liquid phase ion-exchange. It is interesting to note that adsorbed bases rearrange when H2S is adsorbed. Dissociative adsorption of H2S ’seems to be more pronounced on Cd-faujasites prepared with solid state ion-exchange than on samples ion-exchanged in liquid phase. This conclusion is confirmed in Figs. 7a and b. Reverse reaction of hydrosulfurization, i. e. the decomposition of EtSH and EtzS was determined on Cd-faujasites, which provides the highest activity among zeolite catalysts. Relative conversions are in accordance with above findings. Because of limitations of the pulse technique, further evalution of conversion data is rather uncertain.

4. ACKNOWLEDGEMENT

We ackowledge the financial support granted by the National Scientific Research Foundation (OTKA Project No. 1-600-2-89-1-674 MTA).

/y

NH,+X+CdC12

I 1600

1400

1600

WAVENUMBER KM-lI

1400

WAVENUMBER lCM-'1

Fig. 6. I. r. spectra of Cd-faujasites ion-exchanged in liquid phase (a: Cd-X, Cd-Y), in solid state ( b : NH4X+CdCla, NH4YcCdC12) after standard pretreatment at 45OoC; curves 1: after pyridine adsorption at 1.1 kPa, 200 C, 10 min., evacuated at 2OO0C for 30 min.; followed by H 2 S adsorption and evacuation at 200 C for 30 min. (curves 2)

100

100

50

5C

E! W

’5,

aJ

c W

L + W

0

f r o m EtSH

f r o m EtzS

1 1 1 1 1 1 I I I I

I I I I I I I I L I

1

5

10 1

5

Number o f pulses

f r o m EtSH 10

c

f r o m Et2S

I I I I I I I I I I

1

5

I I I I I I I I I I

10 1

5

10

Number of pulses

Fig. 7. Decomposition of EtSH and E t 2 S on Cd-forms of X (a) and Y (b) prepared in different ways.

294 5. REFERENCES 1.

2.

3. 4. 5.

6. 7.

8.

9. 10.

D.Kall6, Gy. Onyestyak and J. Papp, Jr., in Proc. 6th Intern. Zeolite Conf. (Eds.: A. Bisio and D. H. Olson). Butherwoth, Guilford. 1984, p. 444. D. Kallo, Gy. Onyestyak, J . Mol. Catal. 62 (1990) 307 H. K. Beyer, H. G. Karge and G. Borbely, Zeolites, 8 (1988) 79 H. G. Karge, H. K. Beyer and G. Borbely, Catal. Today 3 (1988) 41 H. G. Karge, G. Borbely, H. K. Beyer and Gy. Onyestyak, in Proc. 9th Congr. on Catal. (Eds.: M. J. Phillips and M. Terman). Chem. Inst. of Canada, Ottawa, 1988, Vol. 1, p . 396 G. Borbely, H. K. Beyer, L. Radics, P. Sandor and H. G. Karge, Zeolites, 9 (1989) 428 B. Wichterlova, S. Beran, S. Bednarova, K. Nedomova, L. Dudikova and P. Jiru, in Innovation in Zeolite Materials Science (Eds.: P. J. Grobet, W. J. Mortier, E. F. Vansant, G. Shulz-Ekloff) Elsevier, Amsterdam, 1988, p. 199 J . W. Ward, in Zeolite Chemistry and Catalysis (Ed.: J. A . Rabo), Am. Chem. SOC.,Washington D. C . , 1976, pp. 124-136 A . V. Deo, I. G. Dalla Lane and H. W. Habgood, J. Catal. 21 (1971) 270 S. Beran, B. Wichterlova, H. G. Karge, J. Chem. SOC.Faraday Trans. 86 (1990) 3033

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

EFFECT OF THE INTRODUCTION OF Ni(I1) SAPO-5 MOLECULAR SIEVES

-

ON

THE

295

CATALYTIC

PROPERTIES

OF

V. Mavrodinovaa, Ya.Neinskaa, Ch.Mincheva, 14. Lechertb, V. Minkova, V.Valtchev’and V. Pencheva

a

Institute of Organic Chemistry, Sofia 1040, Bulgaria Institute of Physical Chemistry, 2 Hamburg 13, Germany Institute of Applied Mineralogy, Sofia 1000, Bulgaria

Abstract The addition of different Ni compounds (NiO, Ni acetate) during the hydrothermal synthesis of SAP0 leads to the formation of Ni-modified SAPO-5 materials with enhanced catalytic acid activity in m-xylene isomerization and toluene disproportionation. Both pure SAPO-5 and NiO-impregnated SAPO-5 samples are used as references. The hydrogen treatment of the NiO-containing materials results in the appearance of a Ni metal phase possessing a strongly expressed hydrogenolytic activity in a hydrogen of stream. Physico-chemical characterization reveals the presence extra-lattice Ni in the form of NiO, Ni metal and other Ni-containing species depending on the method of preparation and the thermal treatment conditions of the samples. 1.

INTRODUCTION

The incorporation of both metal (Me) and silicon (Si) into the microporous Alp04 framework offers additional opportunities for regulation of the catalytic and adsorptive properties of these new materials, denoted as MeAPSO [1,21. According to the substitution mechanism proposed by Flanigen et al. [21, the Me is incorporated exclusively into the A1 sites (mechanism A), while Si can occupy both A1 and P sites. By analogy with MeAPO, other authors 131 have also not excluded the possibility that MeAPSO contains some Me present in extra-lattice positions (mechanism B). When a transition metal is used for this modification, it can result not only in generation of new acidic sites [ 1 , 2 1 , but also bifunctional metal oxide- or metal-containing catalysts may be obtained [4,51, depending on the preliminary thermal treatment. Thus, the catalytic behaviour of MeAPSO molecular sieves can be significantly varied by the type and amounts of modifying metal corresponding to mechanisms A and B, and the conditions of thermal activation of the material. The present work was carried out to ivestigate the effect of various nickel compounds, added to the gel used for synthesis of SAPO-5, on the

296

catalytic activity of the samples obtained. This method differs essentially from the preparation of nickel-loaded AlPO [6,71where calcinated ALPO-5 is used for modification. The effect of the introduced nickel has been studied in the conversion of m-xylene or toluene in the presence of nitrogen or hydrogen. The use of different carrier gases allows estimation, under suitable experimental conditions, of the acidic and/or mettalic function of the catalyst obtained. Various physico-chemical methods have been employed in order to elucidate the nature of the active sites formed.

2.

EXPERIMENTAL

2. 1 . Sample preparation SAPO-5 (sample 1) was synthesized from a gel with the following molar composition: 2.0 Pr3N*O.4 SiOa*A1a03*PaOs*50HzO as described in refs. 8 and 9. In the case of the modified samples, 0.4 mol NiO was added as NiO (sample 2) or as Ni acetate (sample 3) to the above reaction mixture [lo]. With the exception of pseudoboehmite from Condea-Chemie, all the remaining chemical reagents used in the synthesis were obtained from Merck. The crystallization was conducted at 423-473K in teflon bottles placed in autoclaves for 24 h. For comparison, a sample containing about the same amount of Ni was prepared by impregnation of calcinated SAPO-5 with Ni nitrate (sample 4). The final thermal treatment of all the samples " 0 . 1 - 4 ) was performed at 873 K in the air for 6 h in a muffle oven. The content of Al, Si and P was determined by conventional chemical analysis. Atomic absorption was used for Ni analysis.

Instrumental X-ray diffraction patterns were recorded with a Isodebyeflex 100 or Dron-1 powder diffractometer using CuKa or CoKa radiation, respectively. The method used to estimate the crystallinities of the samples is based on comparison of the peak areas in the interval 28=5-40 degrees after substraction of the background [9].The XRD data on the pure SAPO-5 (sample 1) are in very good agreement with the literature [9,111.The crystallinity of the "as-made’’Ni-containing samples 2 and 3 is about 90% of t at of -P were "pure" SAPO-5 [lo]. Infrared patterns in the region 1400-300 cm obtained with a Specord M 80 IR spectrophotometer using the KBr technique. Scanning electron microscopy was carried out using a SEM 519/D 806 Philips apparatus combined with a PV-9900 EDAX-Terminal. The samples were coated a with an evaporated Au film. Nitrogen adsorption was carried out on Sorptomatic unit from Carlo Erba at liquid nitrogen temperature. The samples (400 mg) were degassed under vacuum at 573 K for 18 h. The chemisorption of oxygen on the Ni-containing sample was measured by the method of gas chromatographic titration of the metal with pulses of oxygen in a helium flow, after reduction with hydrogen at 723 K. 2. 2.

2. 3. Catalytic measurements The catalytic experiments with m-xylene were performed in the flow-type apparatus coupled "on line" with GC. The preliminary treatment of the catalysts was carried out for 5 h in nitrogen at 773 K, followed by establishment of the temperature at 673 K and introduction of m-xylene from the saturator. The analysis of the reagent mixture and the reactor effluent

297 was carried out by GC. The toluene disproportionation was performed in a continuous flow reactor at 723 K-Ffter the same preliminary treatment of the samples. The WHSV was 0.84h and the nitrogen /toluene molar ratio equals to 10. In order to evaluate the metallic function of the samples in both catalytic reactions, comparative studies in the presence of hydrogen were carried out after reduction of the catalyst at 673-723 K with hydrogen. 3 . RESULT AND DISCUSSION

Physico-chemical characterization of calcinated samples X-ray data and IR-vibrational spectra (Table 1) show that the crystallinity of the samples is preserved after the removal of the template (samples 1-3) and after the decomposition of nickel nitrate (sample 4). The X-ray powder diffraction patterns exhibit only a negligible decrease in the intensity of some reflections. The ratio of the peak intensities at 28=12.9 and 14.9 degrees varies after calcination, as already described [lll. The preliminary thermal treatment in hydrogen (Table 1,Sample 2a) also leads to no substantial changes in the crystalline structure. 3. 1.

Table 1 IR Spectroscopy data on the cacinated samples N 1 la 2

2a 3 4

Sample SAPO-5 SAPO-5 NiO’SP NiO’SP NiAc*SP NiO/SP

-1

Absorption bands #, cm 1112vs 738vw 112Dvs 740vw 1106~s 1108~s lll0vs 1107~~-

720vw 720vw -

-

702m 705m 702m 704m 702m 706m

632w 633w 631w 634w 632w 632w

560ms 559ms 558ms 560ms 560ms 562ms

463s 463s 467s 472s 476s 468s

# - vs=very strong; s=strong; ms=rnedium strong; m=medium; w=weak; vw=very weak; la - SAPO-5 as given in ref.13; 2a - sample 2 after the catalytic experiment; SP=SAPO-5 everywhere.

The scanning electron micrographs show that the crystallite morphology of the nickel-modified samples is inhomogeneous. Both large hexagonal prisms (5-10 pm in diameter and 15-30 prn in length) and a considerable amount of smaller particles, mostly spherical (Fig.1 ) are observed. I t is interesting to check the presence of nickel compounds in the modified samples. The X-ray powder diffraction patterns (Fig. 2 A and B) of sample 2 (NiO’SP) and sample 4 (NiO/SP), in contrast to those of pure SAPO, exhibit pronounced changes in the signal at 28=43.3 and 50.7 degrees (CoKa radiation), which obviously correspond to the NiO (111) and (200) reflections, respectively. The XRD pattern of sample 3 (NiAc*SP) does not indicate the presence of a NiO phase (Fig.2C). Evidently, there are essential differences in the state of nickel in the samples investigated and elucidation of this problem needs additional data obtained by other methods. Special attention should be paid t o sample 3 where no NiO has been found.

298

The following assumptions can be made concerning the state and the location of nickel: a) isomorphous incorporation in the framework, b ) location in the channels of the molecular sieve as cations [compensating for the negative charge of the lattice) or highly dispersed species, and cl location on external surface of the crystallites as agglomerates of another nickel containing phase. The data obtained so far permit a more definite conclusion about the presence and state of extra lattice nickel. The results from chemical analysis (Table 2 ) show that, for samples 2 and 3 the sum of the atomic fractions of Ni and A1 is higher than that of Si and P.

I

Figure 1. Scanning electron micrograph of sample 3 ; (lcm=30pm) Magnification x326

2 THETA Figure 2. XRD patterns of modified SAPO-5. A=Sample 4; B=Sample 2; C=Sample 3.

In view of the substitution mechanism proposed by Flanigen et al. [1,21, this result may be considered an indication of the presence of significant amounts of extra-lattice Ni. For analogous relative concentrations of the also accepted the respective elements in CoAPSO L31, the authors have

299

in the possibility that Co occupies hypothetical phosphorus sites framework. The pore volume measurements have also been used to demonstrate the state and location of the nickel in SAPO-5. The adsorption capacity of the modified materials decreases to 67-87% of that of pure SAPO (Table 21, which probably confirms the presence of considerable amounts of extra-lattice nickel in all the samples. This effect may be due to the occlusion of highly dispersed nickel species in the channels and/or to the Table 2 Chemical composition, adsorption capacity and chemisorption of 0

2

N 1 2 3

4

Sample

Chem.composition Ads. capacity (Ni Si A1 P)Oz A % X

SAPO-5 0.07 0.51 0.42 NiO*SP 0.07 0.09 0.48 0 . 36 NiAc*SP 0 . 0 8 0.07 0.49 0.36 NiO/SP -

100 78 67 76

Oxygen XX ( pu 1ses1 11

2 11.5

x calculated as % of Nz adsorption capacity of sample 1; xx chemisorption of 02, measured after reduction in Hz at 723 K. blocking of some pore entrances by nickel-containing phases. Taking also into account the X-ray data (Fig.21, it can be stated for samples 2 and 4 that the predominant part of the nickel forms large NiO particles on the outer crystal surface. Due to steric hindrance, these particles cannot be located within the channels, because the crystallinity of the samples is preserved after the modification. In the case of sample 3, the more pronounced decrease in the adsorption capacity and the XRD data (Fig.2C) confirm the assumption that the nickel species are occluded in the pore structure. It is more difficult to estimate the proportion of the extralattice nickel located on the external o r internal surfaces of the crystals. At present, preliminary examination of the chemical homogeneity of the samples by EDAX shows that the single crystals have a considerably smaller nickel content than the other particles and aggregates [141. 3. 2. Catalytic studies According to the physico-chemical characterization there are essential differences in the individual samples investigated. Two catalytic test reactions and both nitrogen and hydrogen as carrier gases have been applied to provide some information about the effect of nickel-modification on the catalytic properties. Table 3 presents data for the catalytic patterns of the samples investigated in the m-xylene reaction. It is evident that m-xylene isomerization and disproportionation products typical of acidic catalysts appear when nitrogen is used as a carrier. This result indicates that the nickel-modification does not in principle change the reaction selectivity The increase in the catalytic activity of the pure SAPO suggests that the introduction of nickel compounds during the hydrothermal synthesis of the SAPO-5 may effect the crystallization process, leading in this case to a catalyst with more expressed acidic properties. The selectivity remains

300 practically unchanged for all the samples. The replacement of nitrogen by hydrogen stream during the thermal pretreatment of the catalysts as well as in the reactor causes a dramatic effect in the catalytic properties of the nickel-containing samples 2 and 4. The conversion of the rn-xylene gives almost completly products of the metal hydrogenolysis reflecting the strong catalytic action of the component. It should be noted that the high hydrogen partial pressure facilitates to a great extent such hydrogenolytic reactions which helps to distinguish between two basic reaction routes: acidic, yielding C6-C9 aromatic hydrocarbons and metallic, leading to methane and light gases. If the metallic function is more pronounced, then the conversion of the Table 3 Conversion of m-xylene, T=673 K N Sample

Carrier gas, nitrogen Total conversion,%

1 SAPO-5

2 NiO*SP 3 NiAc*SP 4 NiO/SP

23 30 28

p:zyL:~g o-xylene

Carrier gas, hydrogen Light gases,%

0.36 0.35 0.38 -

5

Total conversion,%

0

11

0 0

98 24

0

99

p:zyLg25

Light

o-xylene

gases,X

0.60

0

-

92 0.2 99

0.58

-

m-xylene feed is directed to paraffinic products via reaction steps of hydrogenation of the aromatic hydrocarbons and fast cracking and hydrogenolysis reactions. In the case of sample 3 (synthesized using Ni acetate) the situation is opposite. M-xylene is transformed into products of isomerization and disproportionation and only traces of light gases are observed (Table 3). The catalytic acid activity of the sample is almost the same as in the case using a nitrogen stream. Table 4 Disproportionation of toluene, T=723 K, WHSV=O. 84 h-’

N Sample

Carrier gas, nitrogen Total conversion,%

1 SAPO-5 2 NiO SP

3.6 33.1

3 NiAc*SP 4 NiO/SP

20.7 18.4

!E”:ns

xylenes

Carrier gas, hydrogen Total conversion,%

1. 0 5.5 1.5

25.6

4.6

41.9

1.6 14.3

be_nzens xylenes 1.0

5.9 2.3 18.8

Analogous differences in the catalytic behaviour of the samples investigated are also observed during toluene conversion. The changes in

301 the disproportionation and dealkylation activities of the catalysts in the presence of nitrogen or hydrogen after pretreatment in hydrogen have been studied (Table 4). In this case again, the modified samples exhibited a higher activity than pure SAPO-5. This is due to the enhanced acidic and metallic activities of the samples, which is evident from the data on the benzene/xylene ratio. The NiO-modified samples and especially the sample In obtained by impregnation show a pronounced hydrogenolytic activity contrast, sample 3 exhibits enhanced activity mainly in the disproportionation reaction. At a relatively high total conversion, thc benzene/xylene ratio is 1.5 and is comparable with that for pure SAPO. The above differences in the catalytic activities of the metal are j n good agreement with the data on oxygen chemisorption (Table 21, which indicates that the surface of the metallic nickel is comparable for samples 2 and 4, and about 6 times larger than that of sample 3. The result on the latter sample evidences that the nickel is not easily reducible, which explains very well the negligible activity of the metal in this catalyst.

4. CONCLUSION

Ni-modified SAPO-5 molecular sieves have been prepared by addition of Ni acetate to the reaction mixture used for hydrothermdl crystallization of SAPO-5. The crystallinity and thermal stability of the materials obtained are comparable with those of pure SAPO-5. The modified samples have a higher catalytic acid activity in the isomerization of rn-xylene and the disproportionation of toluene as compared to pure SAPO-5. The selectivity is close to that of SAPO-5 containing no Ni. Reduction of NiO-containing samples with hydrogen leads to the formation of metallic nickel possessing a pronounced hydrogenolytic activity in a hydrogen stream. The physico-chemical studies (XRD, EDAX, nitrogen adsorption and oxygen chemisorption) have revealed the presence of extra-lattice Ni depending on the preparation method and the reduction of the samples. The sample obtained in the presence of Ni acetate, where no NiO admixtures are detected by X-ray studies, deserves special attention. The specific catalytic and physico-chemical properties of this sample do not exclude the possibility of partial incorporation of Ni ions into the crystal lattice which should be checked separatelly. This will be a subject of future investigations [141.

NiO or

5 . ACKNOWLEDGEMENTS

Thanks are due to the Bulgarian Academy of Sciences, Ministry of scicncr and education and the Deutsche Forschungsgemeinschaft for support of this work. Ch. M. and V. P. are also obliged to the Deutsche Akademischc Austauschdienst and Alexander von Hurnboldt Stiftung, respectively, for financial support.

302 6 . REFERENCES

1

E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, Stud. S u r f .

Sci. Catal., 28 (1986) 103-112 2 E. M. Flanigen, R. L. Patton and S. T. Wilson, ibid., 37 (1988) 13-28 3 S . Ernst, L. Puppe and J. Weitkarnp, ibid., 49 (1988) 447-458 4 R. Szostak, T. Thomas, R. Kuvadia, N. Rohatgi, R. Csencsits, V. Nair and W. Powell, ibid., 52 (1990) 5 V. Penchev, H. Lechert, Ya. Neinska and V. Minkov, in preparation 6 B. Kraushaar-Czarnetzki and J. H. C. van Hooff, Stud. S u r f . Sci. Catal., 49 (1989) 1063-1069

Ji Ming, Su Guifa, Sun Decun, Fu Xiancai, Cuihua Xuebao, 10 (19891 282-8 Ch. Minchev, V. Kanazirev, V. Mavrodinova, V. Penchev and H. Lechert Stud. Surf. Sci. Catal., 46 (1989) 29-38 9 H. Weyda and H. Lechert, Zeolites, 10 (1990) 251-258 10 H. Weyda, Thesis, Univ. Hamburg, 1989 11 EP 103117 12 V. Penchev, N. Davidova, V. Kanazirev, C. Minchev and Y. Neinska, Advances in chern. ser., 121 (1973) 461-468 13 R. A . van Nordstrand, D. S . Santilli and St. I. Zones, ACS Syrnp. ser.,

7 8

368 (1988) 236-245

14 Ch. Minchev, V Valtchev, Y. Neinska, V. Mavrodinova, V. Minkov, H. Lechert and V. Penchev, in preparation

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

303

STUDY OF BROENSTED AND LEWIS ACID SITES I N PHOSPHATES, SILICATES AND SILICA GELS WITH MOLECULAR SIEVE PROPERTIES L-M-KUSMV, S.A.ZUBKOV, V.B-KAZANSKY and L-A.BONDAR'

N.D.Zelinsky Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow, Lenineky prom- 47, 117334, USSR

ABSTRACT Broensted and Lewis acid sites in crystalline phosphates, silicates and in gels with molecular sieve properties were studied by IR-spectroscopy. Two types of bridged hydroxyls were found in SAFO-5 which were accessible to adsorbed molecules and were able to interact with ethylene. Lewis sites in metallophosphates, zirconosilicate and in Ti-containing silica gel were observed which did not interact with weak bases (03, hydrogen) but formed strong complexes with acetonitrile. They were supposed to be framework metal ions in tetrahedral coordination. INTRODUCnON

"he recent break-through in zeolite chemistry is connected with the synthesis and investigation of new types of zeolite-like materials. Among them crystalline metallosilicates containing Ti, Ga and other ions C11 as well as aluminophosphates, silicoaluminophosphates and more complex materials C21 are the most interesting and promising from the point of view of their application in catalysis. A few examples of modified silica gels exhibiting molecular sieve properties have also been reported in the literature C3,41. Although the structure and catalytic properties of different zeolite-like materials are described in detail, little infomation is still available about the nature and properties of active sites. This paper is devoted to the study of the following questions: 1) the nature, localization, accessibility to adsorbed molecules and acidic properties of OH-groups in SAPO-5 and MeAFQs.

304

2) the nature and properties of Lewis acid sites in different crystalline pho3) the state and properties of Me"4 ions in crystalline zirconosilicate and in silica gels modified by Ti+4 ions.

EXPERIMENTAL The crystalline phosphates of AlPD-5, VPI-5(P:Al=l), SAFO-5 ( Si:Al:P = 0.2:0.8:1) and Me-5 types ( Me=Co, Mg, Fe, Ti, Be, Me:Al:P=O.l0.2:0.8:1), as well as zirconosilicate (Zr/Si=35) with the pentasil structure were studied. Silica gel8 containing Ti+4 ions (2-5wt.%) introduced during the synthesis which exhibited molecular sieve properties (surface area 700-8OOm2/g) were also investigated. Before the spectroscopic measurements the samples were evacuated at 720-87OK for 5h. Diffuse reflectance IR-spectra were measured usirg "Bedrman Acta M-VII" and "Perkin-Elmer 580B" spectrophotometers C51. (30, HZ, CD3CN and C2H4 were used as probe-molecules for identification of Lewis sites o r testing the reactivity of Broensted sites. W-spectra were recorded by "Hitachi 340" spectrophotometer supplied with DR-unit- ESR spectra were measured using "ERS-220"(X=3.2cm) spectrometer.

RESULTS AND DISCUSSION 1. BROENSTKD ACID SITES IN PHOSPHATES. The IR-spectra of OH-groups in AlPO-5(a), WI-5(b) Wd(M=Mg)(c) and SAW-5(d) are shown in Pig.1. There are only the low intensity bands at 3680, 3740 and 379Ocm-’ in the spectra of AlFO-5 and VPI-5 which were earlier assigned C61 to weakly acidic POH and AlOH groups a t the outer surface of aluminophosphates and broad bands at ~<360Ocm-~ due to H-bonded hydroxyls. For MAFQ-5 o r BeAPO-5 no additional bands appear in the spectra a8 might be expected due to introduction of bivalent ions into the framework. However the intensity of the band of WH-groups at 3680 cm-’increases for MeAWs a8 compared with A1W-5. It may be explained by the appearance of additional terminal P-OH groups as a result of incorporation of Me" ions into distorted tetrahedral positions nhere an interaction between cations and WH-groups seems to be rather weak:

305

These bands are also present in the spectrum of SAPO-5. In addition two high-intensity bands at 3625 and 3520 cm-’ ascribed previuusly [6J t o the bridged hydroxyl groups are observed. !l?icty origirLat.r h e to the isomorphous substitution of silii-wn f m phnsphorua in the framework (let us denote them as HP and LF-hydroxyls respectively). As shown in E61 HP-hydroxyls exhibited rather strong acidic properties comparable to or somewhat stronger than those of HY-zeolite. This estimation w a s made using adsorption of weak bases like benzene [el. It w a s found that unlike HF-hydroxyls, LF-hydroxyls were hardly accessible to benzene molecules. This was explained by localization of these protons in inaccessible sites of the framework by analogy with LF-hydroxyls in HY-zeolites. Here we investigated the interaction of LF-hydroxyls with adsorbates of a smaller size. Fig.1 s h o w s the IR spectrum of SAPO-5 after adsorption of CD3crJ at 300K. As seen from the spectrum (e) both HF and LF-hydroxyls disappear after interaction with acetonitrile and a new broad band at "29OOcm-’ appears due to H-bond formation. The similar pattern was observed when other small molecules like CH31 or N20 were adsorbed. Thus, LF-hydroxyls are able to interact with small molecules, and therefore they should be taken into account when acidic properties and catalytic behaviour of SAW-5 are concerned. 300K may be used aa an appropriate test both

!-3680 *-< Figure 1. IR-spectra of AlPD-5 (a), VPI-5 (b), MAW-5 (c) and S M - 5 before (d) and after (e) acetonitrile adsorption.

306

for acidic properties of the bridged OH-grou~ein zeolites and for their activity in catalytic reactions C7,81. The IR-spectra of SAW-5 measured in 0.2, 1.0 and 24h after adsorption of ethylene and deuteroethylene are presented in Fig.2. Interaction of both HF and LP-hydroxyls with C2D4 results in formation of H-bonded complexes (=3300cm-1) and simultaneously in H-D exchange which proceeds with a moderate rate- It converts both types of the bridged hydroxyl groups into corresponding deuteroxyls which are isolated (v(HF)=2670cm-l), (S(LF)= Z S S O C ~ - ~or ) hydrogen-bonded (d=240O~m-~)The reaction with ethylene is accompanied with rather slow growth of the bands attributed to species containing commensurable concentrations of CH3 (142970,2870cm-l) and CHz-groups ($=2930,Z~SOC~-~). However, the intenaities of these bands in the spectra measured in 1 and 24h after ethylene admission are practically the same. It follows from these data that the reaction of ethylene on SAPOs proceeds not so deeply as on HZSM-5 zeolites and it Beems to stop at the early stages. It is known [7,81 that acidic OHgroups of HZSM-5 zeolite are able to catalyze fast oligomerization of ethylene whereas those of HY-zeolite are inactive because of weaker acidic strength. Thus, in agreement with our previous data C61 acidic strength of OH-groups in SAPO-5 is intermediate between those of OH-groups in HY and HZSM-5 zeolites. The data on ethylene transformations on SAPO-5 could be explained taking into consideration the concerted mechanism of oligomerization proposed earlier [81. According to this mechanism, first molecule of ethylene forms an H-complex with a bridged OH-group and after that it could be transformed into ethoxy group and further into oligomer chains:

T si- Bi This scheme explains also the H-D exchange occuring in a complex of CzD4 with bridged OH-groups via the same transition state - "carbenium ion"-like species with elongated C-O bond, a8 the reaction of ethoxy groups formation

307

does C93. Further interaction of ethoxy species with second olefin molecule via similar concerted mechanism would lead to linear oligomersAa waa mentioned this scheme seema to be valid for HZSM-5 zeolites. In our opinion for SAPO-5 which exhibit weaker acidity than HZSM-5 this mechanism is stopped at the stage of ethoxy groups formation and further oligomerization does not take place. It is in good agreement with the observed

Figure 2. IR-spectra of SAPO-5 measured in 0.2 (l), 1 (2,3) and 241 (4) after C2H4 (1,2,4) o r C2D4 adsorption at 300K. in the IR-spectra CHs/CH2 ratio which is close t o 1 and with the growth of these IR-bands at the early stages of interaction of C2H4 with SAPO-5 and their fast saturation. Finally, occurance of rather fast H-D exchange in Hcomplex also confirms the above scheme. When discussing the concerted mechanism of ethylene transformations on zeolites ZSM-5 and SAPO-5, not only the acidic strength of bridged OH-groups should be taken into account, but also the basic properties of oxygen anions of the framework which would participate in this mechanism. Cumparing ZSM-5 and SAPO-5 frameworks (Scheme 1) one may come to a conclusion that anion O2 in SAPO-5 exhibits weaker basicity due to the presence of positively charged phosphorus than the corresponding oxygen atom in ZSM-5 structure does (where T=Si). 2. LEWIS ACID SITES IN CRYSTALLINE PHOSPHATES, ZIRCDNOSILICATE AND IN Ti-CONTAINING SILICA GELS.

Aluminophosphates are the molecular sieves formed by A104 and Po4 tetrahedra so that each A104 tetrahedron is surrounded by four PO4 tetrahedra. Such a neutral structure should contain neither Broenated nor Lewis acidic

308

sites. The absence of bridged OH-groups is confirmed by the IR-spectra of phosphates (Fig.1). To study Lewis sites different probes were used. The IR-spectra of hydrogen adsorbed on A1W-5 and Mcw3-5 are shown in Pig.3a. The low-intensity bands at 4100-4130~m-~could be attributed to weak complexes of hydrogen with OH-groups C51. No bands at ~<41OOcm-~ which might be ascribed to complexes with Lewis acid sites were found- The spectra of adsorbed CO also did not reveal any low-coordinated ions which would usually result in the appearance of the bands at v>Zl7Ocm-l. Similar spectra were observed for WI-5, SAW-5 and amorphous A1W4Use of stronger baaes like acetonitrile shows a completely opposite pattern. The spectra of CD3CN adsorbed on AlW-5, MAPO-5 and SAEQ-5 are presented in Fig.3b. The bands at "2280 cm-l are assigned to physically adsorbed molecules and to complexes with OH-groups. There is also an additional band at 232Ocm-’ which should be ascribed to complexes with Lewis sites of moderate strength. Desorption at 300K results in the disappearance of the lowfrequency bands whereas the intensity of the band at 232Ocm-’ remains mchanged. The concentration of Lewis sites of this type estimated by acetonitrile adsorption is -0.35mol/g. Unlike AlW-5 where adsorption even of small amounts of acetonitrile enables observation of the band at 232Ocm-l, for SAEQ-5 the band at 229Ocm-l due to complexes with OH-groups appears first and predominates in the spectra. Only after saturation of this band the line at 2320cm-l could be observed (Fig.%). These data show that in phosphates Lewis acid sites exist in high concentration which are not able to interact with weak bases but are capable of

9.

b.

Figure 3. IR-spectra of hydrogen (a) and acetonitrile (b) adsorbed on AlPO5 (11, MAPO-5 (21, ZrSil (31,Ti/Si02 (4) and SAW-5 ( 5 ) .

309

complex formation with rather strong bases. In our opinion, framework A104 tetrahedra play the role of such centres- Indeed, their concentration in the phosphates under study is very high even though not all of them are accessible to adsorbed molecules h e to differences in crystallographic positions. The obtained value of 0.35mmol/g is only 3 times lower than the amount of hydrocarbon molecules which may fill the pore volume of AlW-5 ("O.lcc/g o r h o l / g ) [l]. These sites do not formally contain exposed COordinatively unsaturated metal ions which may interact as Lewis acids with weak bases. However these tetrahedral aluminum ions are able to form complexes with stronger bases clue to additional coordination, for instance, via transformation of tetrahedra into distorted trigonal bipyramide or octahedra. This assignment is in good agreement with recent 27Al MAS NMR data [lo] showed the presence of 5- or 6-coordinated Al+3 ions in hydrated Alms which after dehydration reversibly transformed to AlIV spcies. Unlike A1W-5 in SAW-5 Broensted acid sites are the main type of sites which interact with the base. That is why only one band at 2290cm-’ is observed at low coverages. However the concentration of silicon in SAPO-5 is lower than that of aluminum, therefore, the bridged OH-groups should be divided by aluminophosphate domains. Thus, after saturation of OH-groups interaction of A104 tetrahedra belonging to domains with CD3CN becomes possible- The bhaviour of MeAEQa toward adsorption of acetonitrile is very similar to that found for AlPO-5. The characteristic band at 2305cm-1 appears in the spectmm of MAPO-5 after admission of the base. The following maxima were also found for different MeAPOs (in cm-’) : 2325 (Me=Ti), 2325 (Be), 2320 (Zn), 2318 (Sn), 2307 (Co) and 2301 (Fe). All these complexes could not be destroyed by evacuation at 300K Let us now discuss the state of Zr+4 and Ti+4 ions in the frameworks of zirconosilicate and Ti-containing silica gel exhibiting molecular sieve properties. As might be predicted from the chemical composition these samples should contain no Broensted centres because there is no difference in charges of Si or Me ions. In agreement with this assumption only the band at 374Ocm-1 which may be assigned both to terminal SiOH and MeOH groups is observed in the spectra. Furthermore similar to AlFO-5 these samples do not contain either Lewis sites capable to interact with weak bases like CO, H2. Unlike weak bases adsorption of acetonitrile reveals Lewis acid sites responsible for the IR-band at 2310-2305 cm-’ (Fig.3b). The appearance of

310

a.

Figure 4. a) W-spectra of Ti-silica gel before (1) and after (2) adsorpti on of acetonitrile. b) KSR-spectra of photoredwed Ti-silica gel (1) and Zr-silicate (2). these bands may be explained as in the case of AlPO-5 by a change of coordination of Me" ions due to additional interaction with adsorbed molecules which transforms tetrahedra into distorted trigonal bipyramids or octahedra. This hypothesis is supported by W-spectroscopic data obtained after adsorption of acetonitrile on Ti/Si02 (Fig.4a). Before adsorption the band at 44000~m-~ due to charge transfer in Ti04 tetrahedra is observed in the W-spectrum which is shifted to 41000~m-~ after adsorption of the base. According to C113 this should be considered as an indication of increasing coordination number of Ti+4 ions in the framework. ions could be detected by ESR spectroscopy It is known that Ti+3 or 3’rZ but the data available in the literature concern mainly octahedral complexes of these paramagnetic ions. For symmetric tetrahedral c o n ~ p l c xthe ~ ~ gvalues for d1-ions are assumed to Ic.increased hut they also should be below 2.0. Ibwc.ver for distorted tetrahedra the observed BSR signals may be shifted to higher g-values. We prepared the samples containing Ti+3 and Zr+3 ions and investigated them by ESR-spectroscopy. Instead of thermal reduction in order to avoid the removal of titanium or zirconium ions from the framework the samples of Ti-containing silica gel and zirconosilicate pretreated at 720K in vacuum were photoreduced at 300K using mercury Wsource in the presence of hydrogen (2OTorr). The characteristic signals with g1=2.0314, g2=2.0152, g3=2-0094appeared in the spectrum of the photoreduced Ti-silica gel and those at g1=2.0144, g2=2.0076 and g3=2.0048 for Zr-silicate. In our opinion they could be ascribed to Me+3 ions in distor-

311

ted tetrahedral coordination.

CONCLUSION The data obtained show that in phosphate-based molecular sieves as well as in metallosilicates or modified silica gels with molecular sieve properties, unusual Broensted and/or Lewis acid sites may exist which exhibit different properties toward weak o r strong bases. This should be taken into account when the behaviour of such zeolite-like materials in catalytic reactions and adsorption phenomena is discussed.

REFERENCES 1 R-Szostak,Molecular Sieves, van Nostrand Reinhold, New York, 1989.

2 E.M.Flanigen, B.M.Lok, R-L-Pattonand S-T-Wilson,in Y.Murakami et al. (eds.), New Developments in Zeolite Science and Technology, Elsevier, Amsterdam, 1986, p.103. 3 L-M-Kustov,L.A.Bondar’, V.Yu.Borovkov, 1.E.Neimark and V.B. Kazansky, Kinetics and Catalysis, 27 (1986) 1392-1397. 4 A.Corma, J-Perez-Pariente,V-Fornes,F-Rey and D-Rawlence,Appl. Catal., 63 (1990) 145-164. 5 V-B-Kazansky,V.Yu.Borovkov and L.M.Kustov, in Pmc. 8th Intern. Congr. Catalysis, Dechema, Berlin, 1984, v.3, p.3-14. 6 S.G.Hegde, L-M-Kustov,P-Ratnasamyand V-B-Kazansky,Zeolites, 8 (1988) 137-144. 7 J-Novakov4 L-Kubelkov;, Z.Dolejsek and P-.JL&, Coll. Czhechoal. Chem. Commun.. 44 (1979)3341-3348. 8 L.M.Kustov, V.Yu.Borovkov and V.B.Kazansky, in P.A.Jacobs et al.(eds.), Structure and Reactivity of Modified Zeolites, Elsevier, Amsterdam, 1984, p.241-249. 9 I-N-Senchenyaand V-B-Kazansky,Catal Letts., subolitted for publication. 10 N. J-Tapp, N-B-Milestone, M.E.Bowden and R.H.Meinhold, Zeolites, 10 (1990) 105-110. 11 A-Zecchina,G.Spoto, M-Padovan,G-Leofantiand G-Petrini,in J.C.Jansen et al.(eds.), Zeolites for the Nineties, Proc. 8th Intern. Zeolite Conf., Amsterdam, 1989, p.235-237.

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INFLUENCE OF FRAMEWORK PHOSPHORUS ON THE ACIDIC PROPERTIES OF FAUJASITE " P E ZEOLI'lX M. BRIEND, A. LAMY, S. DZWIGAJ*, D. BARTHOMEUF Laboratoire de mactivit6 de Surface et Structure, URA 1106 CNRS, Universit6 P. et M. Curie, 4 place Jussieu, 75252, Paris CBdex 05

* On leave from the Institute of Catalysis and Surface Chemistry, Krakow, Poland Abstract Two SAPO-37 samples are compared to HY and t o a dealuminated Y zeolite. Thermoprogrammed oxidation experiments show that only a part of TPA' are involved in the exchange with NH; in SAPO-37. Infrared spectroscopy and TPD of NH, and pyridine are used to study the acidic properties generated in SAPO-37 upon the organic templates decomposition. The comparison with the two faujasites indicate that the distribution of acid strengths in SAPO-37 is narrower than in faujasites. The few very strong protonic sites existing in SAF'O-37 could belong to hydroxyls vibrating in the sodalite cage.

INTRODUCTION SAPO-37 molecular sieve which has the crystalline structure of faujasite differs from this zeolite by the presence of phosphorus in the structure (1). It was shown that this element increases the thermal and hydrothermal stability of the structure (2). With regards to acidity, the SAPO-37 materials have acidic properties (1,3,4) with two OH groups very similar t o those of faujasites (1,4). It was also observed that the SAPO-37 materials have besides acid centers of medium strength a small number of protonic sites stronger than in HY o r even than those of an ultrastable LZY-82 (4). The present paper is devoted to a better characterization of the acidity of SAPO-37 with regards to the number of sites, their strength and their stability upon cation exchange.

314

EXPERIMENTAL Materials Two samples were prepared with different Si contents. A material, Si-0.13, with the formula (Sio.13Alo.~Po.~)Oz was obtained following (5) example 43. The silicon richer sample, Si-0.16, with the formula (Si,,,~o.aePo.,s)O, was prepared according t o (6). Both materials were synthesized using hydroxides of tetramethyl ammonium and of tetrapropyl ammonium as templates (TMAOH and TPAOH respectively). An acidic HY zeolite containing 10 Na' per unit cell was used. The dealumination of its NH4 form by (NH4),SiF6 gave after decomposition of the ammonium ions a dealuminated zeolite with 24 AVu.c. and referred to as HYD (7). All the samples were highly crystalline.

Cation exchange Exchanges in aqueous salt solutions were performed using NaCl, La(NO,), and NH4N03.Two types of conditions were used. For concentrations lN, the exchange was performed at room temperature for 15h. At lower concentration (0.1N) the exchange was carried out at 370 K for lh.

Infrared measurements Infrared studies were performed on self supported wafers (15 mg with 18 mm diameter). The templates were decomposed upon evacuation at 875 K. After a 2h treatment under 0, at the same temperature in static conditions, the wafers were evacuated for 6h. The infrared spectra were recorded a t room temperature. Pyridine was then adsorbed as previously described (4) and progressively evacuated at increasing temperatures for 6h.

mandpyridineTPD The samples (10 mg for pyridine and 20 mg for NH,) were placed in a U tube on a fritted glass. They were pretreated at 875 K for 6h under an air flow and then 6h under vacuum at the same temperature in order to decompose the template. NH, or pyridine were adsorbed at room temperature for l h and the excess was evacuated for 6h at 385 K for NH, and 425 K for pyridine. The thermodesorption was performed at a rate of 5" per min.. The gas evolved was analyzed by a quadrupole mass spectrometer Quadruvac PGA 100 LeyboldHeraeus.

TPO The thermoprogrammed oxidation of the templates was followed using the same quadrupole mass spectrometer as for TPD with the same heating rate.

315

RESULTS AND DISCUSSION Cation exchange The study was conducted on sample Si-0.16. The experiments with NaCl or La(N03)3solutions at room temperature or 400 K give either no exchange or amorphisation of the materials. After exchange with a NH4N03solution at a pH of 4 the X ray diffraction does not detect a change in the sample crystallinity. The unit cell parameter is decreased from 24.76 (k 0.01) A to 24.65 (k 0.01) A. A similar modification was already observed after the template decomposition upon heating at 875 K (2). In order to check the exchange effectiveness the initial and NH4N03 treated materials were studied by thermoprogrammed oxidation. Figure 1 reports the results for three mass numbers 42, 40 and 16. A detailed study showed that mass 42 is due to propene (8,9). It gives in the starting sample Si0.16 (figure 1A) a broad peak with several maxima. They correspond very likely to the TPA desorption from acid sites of increasing strength. The mass 40 (curve b) results from both a fragment of propene (peaks at 615 and 695 K) and from acetonitrile or methylisocyanide (10)(peak at 815 K). The last peak is related to the decomposition of TMA' located in the sodalite cage (8,9). The mass 16 (curve c) represents mainly NH, , a fragment of NH, issued from the decomposition of TPA' or NHf. A part of it arises from a fragment of CO, formed during the combustion.

.

aL u

A

500

700

900

B

I

I

500

700

I

900

Fig. 1. TF'O for samples Si-0.16 (A) and Si-0.16 exchanged with NH,NO, (B), a, a' : mass 42, b,b' : mass 40, c,c' : mass 16

316

With regards to TPA', the comparison of figures 1A and 1B shows that the exchange with NH: ions leaves for the formation of propene (mass 42 and 40) only the peak at 695 K (curves a' and b). The peak at 615 K disappeared. The

curves c and c' indicate that the NH, fragment issued from the desorption of TPA+ from the strongest sites is unchanged (peak at 695 K). The results suggest that TPA' which due to its size is located in the supercage is partly exchanged by NH;. This is confirmed by the presence of the peak at 455 K in the exchanged sample (curve c' figure 1B) which results from the NJ3; ion decomposition (8).The TMA' ion mainly characterized by the peak at 815 K (curves b, b') present in both samples is not disturbed by the exchange. This is in line with the fact that it is located in the sodalite cage (11)and is too big to leave this cavity without its own decomposition. The decrease of 0.11 in the unit cell parameter mentioned above, upon the exchange is similar to the one observed after TMA’and TPA' are removed by heating (2). This suggests that the change is for its largest part related to TPA' and not much t o TMA'.

Tnfrared study The infrared spectra in the hydroxyl range for SAPO-37 has already been reported (1,4).It gives two OH bands very similar t o those of faujasites (12). The wavenumbers obtained for the sample Si-0.13, i.e. 3640 and 3575 cm1(4), are identical to the ones recorded for Si-0.16. After adsorption of pyridine and evacuation at 425 K the band at 3640 cm-1 assigned t o OH in the supercage completely disappeared and the one at 3575 cm-1due to OH in the sodalite cage partially vanished (4). Upon desorption of pyridine for 6h at increasing temperatures the OH groups are restored. Figure 2 shows for sample Si-0.13 the increase i n absorbance of the two OH groups as a function of the evacuation temperature of pyridine. Simultaneously the absorbance of the pyridinium ion band at 1542 cm-1 decreases. It is seen that the 3640 cm-1 band recovers its full intensity more rapidly than the 3575 cm-1 band and that the hydroxyls in the supercage reach their initial status after evacuation at around 675 K. A t the same temperature some pyridinium ions are still present in the sample as seen in curve c. These ions are not detected at desorption temperatures of 725 K or higher, The 3575 cm-1 hydroxyls are still slightIy increasing in intensity after evacuation at higher temperatures, up t o around 775 K. The OH recovered between the temperatures 725 and 775 K are acidic since they were neutralized by pyridine. The corresponding pyridinium ions are not detected in the infrared spectra very likely owing to their very small number. They should be very strong. The increase in the 3575 cm-1 band for a constant 3640 cm-1band above 675 K suggests that the strongest protonic sites in the SAPO-37 material belong t o hydroxyls which in the absence of pyridine are in the sodalite cage, the proton being attached to O3 oxygen. The results are in contrast t o what is obtained for the dealuminated Y, HYD, which has also strong acid sites (7). Figure 3 shows that pyridinium ions are not detected for an evacuation temperature of 675 K or higher. The hydroxyls in the sodalite cage vibrating at 3555 cm-1 already reach their full

317

P W

wl

0 W

W

D

z 0 m I

< 0 ;D

0

x < r wl

Fig. 2

. ,

Fig. 2. Sample Si-0.13. Recovery of the 3640 cm-1(a) and 3575 cm-1 (b) OH bands upon evacuation of pyridine at increasing temperatures. Decrease in the 1540 cm-l pyridinium ion band (c). Fig. 3. Sample HYD. Recovery of the 3638 cm-1 (a) and 3555 cm-1 (b) OH bands upon evacuation of pyridine at increasing temperatures. Simultaneous decrease in the 1540 cm-1 pyridinium ion band (c). recovery at around 625K while the 3638 cm-1 band is not yet completely recovered a t 675 K. The HYD zeolite, dealuminated with ammonium hexafluorosilicate, does not have extraframework aluminium. The protonic sites arise only from the two OH groups 0 , H and O,H which also exist in SAPO-37. The results of figure 3 suggest that the stronger sites in HYD regenerated after the desorption of pyridine at the highest temperature belong to the 0,H hydroxyls vibrating in the supercage. Comparing the desorption of the pyridinium ions for the two samples, the derivatives of the curves c of figures 2 and 3 give figure 4. The ordinate reflects the energy of pyridine desorption in equilibrium conditions at each temperature. It shows that SAPO-37 has a more homogeneous distribution of acid strengths than HYD, most of the sites desorbing between 525 and 570 K (figure 4a). For the dealuminated material (figure 4b) the broad acid site distribution is reflected in the rather constant energy of pyridine desorption between 475 and 600 K. The changes reported in figures 2 and 4 can be deduced for other SAPO-37 from the results described in (4). Other dealuminated Y lead

318

also to conclusions similar to the present ones (7). This indicates a systematic difference between the two types of materials. Such a feature is also found in the thermodesorption results of ammonia and pyridine.

Fig. 4. Derivatives of the curves c (pyridinium ion bands) in figure 2 and 3. a : SAPO-37, b : HYD.

~andpyridineTPD The thermodesorption of bases has often been used to study the acidity of zeolites (13-15). The figures 5 and 6 reports the results obtained for two SAPO37, Si-0.13 and Si-0.16 and for H Y for ammonia and pyridine respectively. Pyridine desorption has also been studied for HYD. The two SAPO-37 materials give curves with only one peak at around 500 K for NH,and 665 K for pyridine. The higher temperature for pyridine is in line with what is usually obtained with this base which is more strongly held in zeolites than ammonia. The HY (and HYD for pyridine) give two peaks at 455 and 520 K for NH, and 545 and 735 K for pyridine. The existence of two peaks for such faujasite samples has already been reported (14). They both correspond to protonic sites since the bases are released from Lewis sites at quite higher temperatures (13,15). Their presence is in line with the IR results for the desorption of pyridine (figure 4) indicating a broad range of temperatures for the removal of the pyridine adsorbed. Compared to the static desorption in infrared studies, the evacuation of the bases in the dynamic TPD conditions points out two main

319

a.u.

Fig. 5.TPD of N H 3 for samples for samples a : HY, b : Si-0.13, c : 53-0.16 a.u.

400

600

800

(K)

Fig. 6. TPD of pyridine for samples a : HY, b : HYD, c : Si-0.16, d : Si-0.13 sub-ranges of energies of desorption included in the overall phenomenon. Of course the static equilibrium experiments locate the desorption a t lower temperatures than the non equilibrium dynamic ones. The presence of only one peak for SAPO-37 near 665 K for pyridine or 500 K for NH, is also in agreement with the infrared results of the pyridinium ions removal (figure 4a) showing a sharp pyridine desorption near 550 K. This

320

confirms that SAPO-37 has a quite more homogeneous and narrow distribution of acid strengths than Y and modified Y zeolites and that the average strength of the acidity is lower in SAPO-37. These sites are very likely the isolated Si atoms in the framework surrounded by four Al atoms as first neighbours and 9 P atoms in the second shell. The TPD technique is not sensitive enough to detect the very small number of strong sites evidenced in infrared on SAPO-37. In conclusion, despite the low stability of SAPO-37 in aqueous solutions, a part of the organic templates can be replaced by ammonium ions without structural damage. The acid sites generated upon decomposition of the templates have a more homogeneous acid strength than those existing in faujasites. Few protons with very strong acidity could be related to hydroxyl groups vibrating in the sodalite by contrast with faujasites. All the results show that the presence of P in the framework induces a distribution of -Si-0-Al- species different from that in Si-Al faujasites.

ACKNO-GMENTS The authors thank D. Delafosse and M.J. Peltre for helpful discussions.

REFERENCES E.M. Flanigen, R.L. Patton, S.T. Wilson, in Innovation in Zeolite Materials Science, (P.J. Grobet et al., eds), Stud. Surf. Sci. Catal., 1988,3Z, 13. M. Briend, A. Shikholeslami, M.J. Peltre, D. Delafosse, D. Barthomeuf, J. Chem. SOC.,Dalton Trans., 1989, 1361. L.S. de Saldarriaga, C. Saldarriaga, M.E. Davis, J. Am. Chem. SOC.,1987, m,2686. S. Dzwigaj, M. Briend, A. Shikholeslami, M.J. Peltre, D. Barthomeuf, Zeolites, 1990, lQ,157. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US. Pat. 4.440.871 (1984). G.C. Edwards, J.P. Gilson, V. Mc Daniel, US Pat. 4.681.864 (1987). Z. Baizoumi, Thesis Paris 1987 and D. Barthomeuf, Z. Baizoumi, unpublished results. A. Lamv, Thesis Paris 1990. ' M. Briekd, A. Lamy, M.J. Peltre, P. Man, D. Barthomeuf, in preparation. 10 E.L. Wu, T.E. Whyte, P.B. Venuto, J. Catal., 1971,21,384. Chem. C o r n . , 1984, 414. 11 R.H. Jarman, M.T. Melchior, J. Chem. SOC., 12 T.R. Hughes, H.M. White, J. Phys. Chem., 1969, 71,2192 and J.W. Ward, J. Phys. Chem., 1969, B, 2086. 13 C. Mirodatos, B.H. Ha, K. Otsuka, D. Barthomeuf, Proceed. Fifth Intern. Zeol. Conf. on Zeol. (L.V. Rees ed.), Heyden, London, 1980, p. 382. 14 A. Macedo, F. Raatz, R. Boulet, A. Janin, J.C. Lavalley, in Innovation in Zeolite Materials Science, (P.J. Grobet et al, eds), Stud. Surf. Sci. Catal., 1988, 375. 15 H.G. Karge, V. Dondur, J. Phys. Chem., 1990,94, 765.

a,

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

321

Zn-DOPED HZSMS CATALYSTS FOR PROPANE AROlVITIZATION

M. GUISNEP, N.S. GNEPa, H. VASQUESb, F.R. RIBEIROb aURA CNRS DO 350, Catalyse en Chimie Organique, Universite de P o i t i e r s , 40, avenue du Recteur Pineau, 86022 P o i t i e r s Cedex, France b I n s t i t u t o Superior Tecnico, Lisboa, Portugal Abstract Propane and propene transformations were c a r r i e d out a t 530'C on ZnHZSM5 samples i n t o which zinc was introduced i n the form of zinc chloride. Zn species are probably ooxychlorides formed during the treatment under a nitrogen flow a t 530 C and deposited o r g r a f t e d inside the pores o r on the outer surface o f the c r y s t a l l i t e s . ZnHZSM5 c a t a l y s t s are more a c t i v e and more s e l e c t i v e than HZSM5 i n the production o f Cg-c8 aromatics. Propane aromatization occurs through a b i f u n c t i o n a l process : Zn species catalyze the dehydrogenation o f propane, t h e acid s i t e s the 01igomerization o f propene and the c y c l i z a t i o n o f 01 igomers, the naphtenes formed being dehydrogenated i n t o aromatics on Zn species. D i f f u s i o n l i m i t a t i o n s created by Zn species are responsible f o r an increase i n the s e l e c t i v i t y f o r benzene, toluene and p-xylene, i.e. the 1ess bulky aromatics

.

1. INTRODUCTION HZSM5 z e o l i t e s catalyze the transformation o f propane i n t o C Cg aromatic products. This aromatization requires various steps : formafjon o f propene, 01igomerization o f propene, cycl i z a t i o n o f 01 igorners and hydrogen t r a n s f e r from naphtenes t o o l e f i n i c compounds w i t h formation o f aromatics. The cracking o f C Cg oligomers leads t o Cz-Cg o l e f i n s which, 1 i k e propene, p a r t i c i p a t e i n %e formation o f aromatics.

React ional scheme

322

However on the acid s i t e s step 1 occurs slowly and moreover steps 1 and 5 are not selective. Indeed step 1 which occurs v i a carbonium intermediates leads not only t o propene but also t o ethylene and t o non-reactive methane. Hydrogen tra n s fe r (step 5) gives not only aromatics but also l i t t l e react ive alkanes [1,2]. To increase the r a t e and the s e l e c t i v i t y o f propane aromatization it i s therefore i n t e r e s t i n g t o associate hydrogenating s i t e s t o the acid s i te s . On these b i f u n c t i o n a l c a t a l y s t s the hydrogenating s i t e s would catalyze steps 1 and 5 and the acid s i t e s steps 2-4. Various types o f hydrogenating components were t r i e d . Thus the introduction o f platinum i n t o HZSM5 zeol i t e s increased considerably t h e i r a c t i v i t y f o r propane aromatization [2-41. Unfortunately t h i s p o s i t i v e action o f platinum was accompanied by the production o f methane and o f ethane through hydrogenolysis o f alkanes and o f alkylaromatics. Moreover the s t a b i l i t y o f PtHZSM5 was not good. The association o f g a l l i u m o r zinc species t o HZSM5 increased also t h e r a t e o f propane transformation and t h i s whatever th e mode o f introduction : i o n exchange [1,5-91, isomorphic subst it ut ion [lo], mechanical mixture [11,12]. Moreover the s e l e c t i v i t y towards aromatics improved considerably, steps 1 and 5 ocurring select ively and th e hydrogenolysis being very slow. Ue show here t h a t zinc species introduced i n th e form o f ZnCl2 increases also the a c t i v i t y o f HZSM5 zeol i t e s f o r propane aromatization and moreover improves t h e i r shape selective properties. 2. EXPERIMENTAL

ZSM5 samples were prepared from a HZSM5 z e o l i t e synthesized according t o Mobil patents [13]. The S i / A l r a t i o obtained by chemical analysis (CNRS, Vernaison, France) was 49 and t k e c r y s t a l l i t e size about 6 pn. The HZSM5 z e o l i t e was f i r s t treated a t 285 C (sublimation temperature o f zinc chloride) under a mixed fl o w o f nitrogen and zinc c h l o r i d e (p = 0.001 bar) during d i f f e r e n t t i p s : 2, 10 and 70 hours and then under a f l o w of nitrogen and water a t 80 C during 2 hours. The samples thus obtained were calcinated a t 530 C under nitrogen fl o w during 10 hours. Propane and propene transformations were Carrie$ out i n a f l o w reactor. The operating conditions were as follows : 530 C ; propane o r propene pressure, 1 atm, WUH (weight o f reactant injected per u n i t weight o f c at alyst per hour) between 10 and 1000 i n order t o obtain product d i s t r i b u t i o n s f o r a wide range o f conversions. The procedure f o r each run was the following : introduction o f th e reactant during a f i x e d period o f time, a t the end o f which th e reaction products were analyzed on l i n e by gas chromatography ; during the analysis (about 70 min) the introduction o f the reactant was interrupted, the c a ta l y s t being kept under a nitrogen flow. Liquid products were also recovered and analyzed by GC. The conditions o f th e GC analysis are given i n r e f 2. 3. RESULTS

The zinc content o f ZSM5 samples i s given i n t a b l e 1. Calcination a t 530 C decreased the Zn content.0n th e samples treated by zinc c h l o r i d e the atomic Cl/Zn r a t i o was equal t o 2 before and a f t e r treatment by water vapor while on calcinated samples it was close t o 1. I n the calcinated samples, zinc was p r e f e r e n t i a l l y on the outer surface o f the z e o l i t e c r y s t a l l i t e s : indeed the Zn/Si atomic r a t i o measured by XPS was 4 t o 10 times greater than the t o t a l r a t i o (Table 1).

323

Table 1 Physicochemical c h a r a c t e r i s t i c s o f the ZnHZSM5 samples

Catalyst

Zn

(Zn/S i) a t .

( w t %)

global

Adsorption c pa i t y l i q u i d cm g N2 m-xylene

J

surface

-E

0

0

0

0.17

0.07

a b c

1.5 1.4 1.3

0.013

0.115

0.16

0.065

a b c

3.3 1.8 1.6

0.0165

0.189

6.2 ZnHZSM5 c

6.2

0.064

0.227

0.14

0.055

HZSM5 1.3 ZnHZSM5

1.6 ZnHZSM5

The adsorption capacity o f the calcinated samples f o r nitrogen and f o r mxylene decreased when the zinc content increased, which shows there was a p a r t i a l blockage o f t h e pore access. However no change was observed i n the c r y s t a l l i n i t y ( s i m i l a r X Ray D i f f r a c t i o n Spectra) nor i n the a c i d i t y o f the z e o l i t e (The ammonia thermal desorption spectra o f HZSM5, 1.3 and 1.6 ZnHZSM5 were almost

7

I

_..._ 255. -

Ol

m I

z

.--> A

0ffl

E

-

c I

-

0

1 .

"C

Figure 1. Ammonia thermal desorption : amount o f a m n i a desorbed a t d i f f e r e n t temperatures (-) HZSM5 ; (--) 1.6 ZnHZSM5.

324

3.1. Propane conversion A l l the ZnHZSM5 samples were more a c t i v e than HZSM5 and had also a good s t a b i l i t y (Table 2). Table 2 Prooane transformation. I n i t i a l a c t i v i t y Ao s t a b i l i t y S t o f the catalysts. S t : A1 hour/AO Catalysts

Ao

St

h-lg-l)and

(%I

100 95 96 100

18.5 70 77.3 59

HZSM5 1.3 ZnHZSM5 1.6 ZnHZSM5 6.2 ZnHZSM5

mole

(

On a l l the c a t a l y s t s the main products observed were C C7 o l e f i n i c and saturated hydrocarbons and c6-c aromatics. Methane, eihene and propene appeared as primary products, t i e other hydrocarbons as secondary ones. Great differences were observed between t h e product d i s t r i b u t i o n s on HZSM5 and on the ZnHZSM5 c a t a l y s t s : i)Zn increased the i n i t i a l r a t e o f propene formation by 6-9 as well as the i n i t i a l s e l e c t i v i t y f o r propene (Figure 2a). ii) Zn increased s i g n i f i c a n t l y t h e s e l e c t i v i t y f o r Cfj-C, aromatics (Figure 2b) and the r a t e o f t h e i r production. The s e l e c t i v i t y f o r a l l the other products and p a r t i c u l a r l y methane and ethylene (Figure 2c and d) decreased. It must also be noted t h a t the secondary transformations o f ethylene, propene and butenes were f a s t e r on ZnHZSM5 than on HZSM5.

c;

(wt%)

2c

a 15 12 8

4

d

0 10

20

30

40

50 X%

4,

2 10

20

30

40

50

X%

325 16

c,

(wt%) C

12

8 I

4

10

20

30

40

50 X%

10

20

30

40

50 X%

Figure 2. Propane conversion HZSMS ( 0 ) and on ZnHZSM5 c a t a l y s t s : 1.3 ( 0 ), 1.6 ( * ), 6.2 ( A ). Percentages o f the products ( w t %) as a function of the t o t a l conversion (X %) a) propene b) Cs-Cg aromatics c) methane d) ethene.

iii) The production o f hydrogen estimated from the d i f f e r e n c e between the hydrogen content o f t h e reaction products and o f the reactant was more s i g n i f i c a n t on ZnHZSMS samples than on HZSMS (Figure 3 ) . About 4.5. moles o f hydrogen per mole o f aromatic hydrocarbon were formed on ZnHZSM5 samples as against about 2 moles on HZSMS.

:7 ( H 2 / A r ) mol

U

10

20

30

40

50 X%

0

5

.

10

15

20 XAr

Figure 3. Propane conversion on HZSM5 ( 0 ) and on ZnHZSM5 c a t a l y s t s : 1.3 ( O ) , 1 . 6 . ( * ), 6.2 ( A ) a) Hydrogen y i e l d (H2) as a function o f the t o t a l conversion X(%) b) Hydrogen/aromatic molar r a t i o (HZ/Ar) as a function o f the conversion i n t o aromatics X A r (%).

326

i i i i ) The C c8 aromatic hydrocarbon d i s t r i b u t i o n was d i f f e r e n t on HZSN5 and on ZnHZ&5 samples. On HZSM5 toluene was more favored than benzene and c8 aromatics while on ZnHZSM5 samples benzene and toluene were formed i n equimassic amounts (Table 3 ) . Table 3 Propane transformation. D i s t r i b u t i o n o f the Cg-ca aromatics ( w t %) f o r a conversion o f 10 o r 20 % ( ) Catalysts

Benzene

To1uene

HZSM5 1.3 ZnHZSM5 1.6 ZnHZSM5 6.2 ZnHZSM5

22 43.5 42 43

52 37 38.5 37.5

(21) (40) (39.5) (39.5)

c6-c8 aromatics

(51) (40.5) (42) (39.5)

26 19.5 19.5 19.5

(28) (19.5) (19.5) (21)

3.2. Propene conversion On a l l the c a t a l y s t s the r a t e o f propene conversion was much greater than t h a t o f propane. ZnZSM5 seemed t o be s l i g h t l y more a c t i v e than HZSM5. Deactivation was always very slow. The products were t h e same as those obtained from propane but the d i s t r i b u t i o n s were d i f f e r e n t . I n p a r t i c u l a r the production o f butenes and o f c 5 - C ~non aromatics (C5+) was greater whereas t h a t o f methane, ethane and aromatics was smaller. The percentage o f ethylene, butenes and C5+ went through a nnvximum showing t h a t these compounds underwent secondary reactions. The product d i s t r i b u t i o n s were d i f f e r e n t on HZSM5 and on ZnHZSM5 : i) Zn cause a d e f i n i t e increase i n the production o f aromatics, o f methane and o f ethane (Figure 4a-c). 2

50

40

1,s 30

d /

6

I 20 10

0

20

40

60

80

100 X%

0

20

40

60

80

100 X%

327

0

20

40

60

80

100 X%

Figure 4. Propene conversion on HZSM5 ( A ) and on ZnHZSM5 c a t a l y s t s : 1.3 ( ), 6.2 ( 0 ). Percentages o f the products ( w t %) as a function o f the t o t a l conversion(X %) a) c6-c8 aromatics b) methane c ) ethane.

*

ii) Zn d i d not modify the i n i t i a l s e l e c t i v i t y f o r ethene, butenes and C + b u t increased t h e i r secondary transformations. The maximum y i e l d o f tzese hydrocarbons decreased when the Zn content increased. Thus the maximum y i e l d i n ethylene was equal t o 7 w t % on HZSM5 and t o 5 % on 6.2 ZnHZSM5, t h a t o f butenes equal t o 22 % on HZSM5 and t o 13 X on 6.2 ZnHZSM5, t h a t o f C5+ t o 11 % on HZSM5 and t o 6 % on 6.2 ZnHZSM5. The s e l e c t i v i t y f o r propane and butanes was not modified by Zn. iii)The y i e l d i n hydrogen, close t o zero on HZSM5, was s i g n i f i c a n t on the ZnHZSM5 samples f o r high conversion l e v e l s . Whatever the conversion l e v e l , about 3 moles per mole o f aromatic hydrocarbon were formed. iiii)Zn caused a s l i g h t increase i n the percentages o f benzene and toluene i n the C -c8 aromatics. There were more d i f f e r e n c e i n the aromatics : Zn increased the s e l e c t i v i t y f o r distribution of paraxylene (59 % on HZSM5, 73 % on 6.2 ZnHZSM5) and f o r ethylbenzene (10.5 % on HZSM5, 16 % on 6.2 ZnHZSM5). Moreover the para/meta ethyltoluene r a t i o equal t o 2.1 on HZSM5 was about 9 on 6.2 ZnHZSM5.

4

4. DIXUSSIW

Zn increased the a c t i v i t y o f HZSM5 f o r propane conversion and the s e l e c t i v i t y f o r aromatic products. This e f f e c t o f zinc was n o t due t o a change i n the a c i d i t y o f HZSM5. Indeed there was p r a c t i c a l l y no m o d i f i c a t i o n i n t h e amnonia thermal desorption spectrum (Figure 1). The nature o f Zn species and t h e i r r o l e i n propane aromatization are discussed. 4.1. Nature of Zn species Before c a l c i n a t i o n under nitrogen a t 53OoC, t h e Cl/Zn atomic was equal t o 2, which indicates t h a t there i s prqbably no chemical i n t e r a c t i o n w i t h the z e o l i t e . A f t e r c a l c i n a t i o n a t 530 C, t h i s r a t i o became equal t o 1. This decrease d i d not seem t o be due t o a s u b s t i t u t i o n o f Zn2+ f o r H+ o f HZSM5 s i n c e Zn d i d not change t h e a c i d i t y of t h e z e o l i t e . T h i s decrease was

328

therefore probably due t o a reaction o f ZnCl w i t h water, which l e d t o oxychlorides such as Zn(0H)Cl , Zn5(OH)gCl2.. .?14]. Reactions could also occur between zinc c h l o r i d e o r oxychlorides groups and OH o f the z e o l i t e w i t h g r a f t i n g o f Zn species : {n-Cl 0

H

e.g.

bI

Z

+

I

ZnC12 - - - - - - > Z

+

HC1

Although the size o f zinc c h l o r i d e molecules i s smaller than t h a t o f the z e o l i t e pores, XPS study showed t h a t the deposit o f Zn species o r the g r a f t i n g occurred p r e f e r e n t i a l l y on the outer surface o f the c r y s t a l l i t e s : the Zn/Si atomic r a t i o on the outer surface was 4 t o 10 times greater than t h e t o t a l r a t i o (Table 1). 4.2. Role o f Zn species Zn species caused a s i g n i f i c a n t increase i n the conversion o f propane i n t o propene. These species act therefore as dehydrogenating s i t e s transforming propane i n t o propene only w h i l e on the acid s i t e s o f HZSM5 the formation o f propene occurs through a carbonium ion mechanism w i t h simultaneous formation o f a s i g n i f i c a n t amount o f methane and ethylene. The r a t e o f propene formation on ZnHZSM5 c a t a l y s t s was about 6-9 times greater than on HZSM5 which means t h a t the formation o f propene through a carboniurn ion intermediate i s p r a c t i c a l l y n e g l i g i b l e . Zn species increased also t h e s e l e c t i v i t y f o r aromatics and therefore intervened i n the dehydrogenation o f naphtenes i n t o aromatics (step 5). On ZnHZSM5 c a t a l y s t s the formation o f aromatics through hydrogen t r a n s f e r on the protonic s i t e s o f t h e z e o l i t e was n e g l i g i b l e compared t o t h i s dehydrogenation on Zn species. Indeed f o r propane conversion on ZnHZSM5 the production o f one mole o f aromatic was accompanied by the formation o f 4.5 moles o f hydrogen against about 2 moles on HZSM5,for propene conversion by the formation o f about 3 moles against p r a c t i c a l l y no hydrogen on HZSM5. These values are q u i t e close t o those calculated f o r t h e transformations o f propane and o f propene i n t o the mixture o f c6-cS aromatics obtained on ZnHZSM5 : 4.7 and 2.1 respectively. Zn species have also a s i g n i f i c a n t e f f e t on the d i s t r i b u t i o n o f aromatics. Whether i t be from propene o r from propane, the production o f the less bulky compounds (benzene, toluene, paraxylene) i s more favored o n ZnHZSM5 than on HZSM5. This can be r e l a t e d t o d i f f u s i o n l i m i t a t i o n s created by the Zn species and shown by the adsorption o f nitrogen and o f m-xylene (Table 1). 5.

cowcLusIoN

Zn species introduced i n t h e form o f zinc chloride i n ZSM5 z e o l i t e s increase t h e i r a c t i v i t y and t h e i r s e l e c t i v i t y f o r propane aromatization. On ZnZSM5 c a t a l y s t s propane aromatization can be considered as a b i f u n c t i o n a l process, zinc species catalyzing propane a c t i v a t i o n (step 1) and naphtene dehydrogenation (step 5) and the acid s i t e s o f HZSMS catalyzing steps 2-4. The increase i n s e l e c t i v i t y f o r aromatics i s p a r t i c u l a r l y s i g n i f i c a n t because on zinc species step 5 occurs without

329

the simultaneous production o f alkanes as i s the case through hydrogen t r a n s f e r on the a c i d s i t e s . Moreover Zn species l i m i t i n g the d i f f u s i o n o f the b u l k i e r aromatics i n t h e z e o l i t e pores, the s e l e c t i v i t y f o r benzene, toluene, ethylbenzene and paraxylene i s greater on ZnHZSM5 than on HZSM5. Zn species are probably oxychlorides deposited or grafted inside the pores or on t h e outer surface o f the r e o l i t e c r y s t a l 1 i t e s .

6. REFERENCES

1 H. Kitagawa, Y. Sendoda and Y. Ono, J. Catal. 101 (1986) 12-18. 2 N.S. Gnep, J.Y. Doyemet, A.M. Seco, F.R. Ribeiro and M. Guisnet, Appl. Catal . 35 (1987) 93-108. 3 C.W.R. Engelen, J.P. Wolthuizen, J.H.C. van Hoof and H.W. Zandbergen,

4 5

6 7 8 9 10 11 12

13 14

i n Y. Murakami, A. L i j i m a and J.W. Ward (eds.), New Developments i n Zeol i t e Science and Technology, Kodansha, Tokyo and Elsevier, Amsterdam, Oxford, New-York, 1986, p. 709-716. T. I n u i and F. Okazumi, J.Cata1. 90 (1984) 366-367. N.S. Gnep, J.Y. Doyemet, A.M. Seco, F.R. Ribeiro and M. Guisnet, Appl. Catal. 43 (1988) 155-166. P. Meriaudeau, G. Sapaly and C.Naccache, i n P.A. Jacobs and R.A. van Santen (eds.), Zeolites * Facts, Figures, Future, Elsevier, Amsterdam, Oxford, New-York, iokyo, 1989, pp. 1423-1429. L. P e t i t , J.P. Bournonville and F. Raatz, i n P.A. Jacobs and R.A. van Santen (eds.), Zeolites Facts, Figures, Future, Elsevier, Amsterdam, Oxford, New-York, iokyo, 1989, pp. 1163-1171. 1. Mole, J.R.Anderson and 6. Creer, Appl. Catal. 17 (199985) 141-154. M.S. Scurrell, Appl. Catal. 41 (1988) 89-98. T. I n u i , i n 1. I n u i (ed.), Successful Design o f Catalysis, Future Requirements and Development, Elsevier, Amsterdam, Oxford, New-York, Tokyo, 1989, pp. 189-201. J. Kanai and N. Kawata, J. Catal. 114 (1988) 284-290. N.S. Gnep, J.Y. Doyemet and M. Guisnet, i n H.G. Karge and J. Weitkamp (eds.), Zeolites as Catalysts, Sorbents and Detergent Builders, Elsevier, Amsterdam, Oxford, New-York, Tokyo, 1989, pp. 153-162. R.G. Argauer and G.R. Landol t, U.S. Pat., 3.702.886 (1972). B.J. A y l e t t , i n A.F. Trotman and Dickenson (eds.), Comprehensive Inorganic Chemistry, Pergamn Press, London, 1973, pp. 187-253.

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P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

SULFIDED Ni-Mo-Y ZEOLITES AS CATALYSTS FOR HYDRODESULFURIZATION REACTIONS

331

HYDROGENATION AND

M.Laniecki and W.Zmierczak Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6 60-780 Poznan, Poland Aber tract Sulfided Mo-Y and Ni-Mo-Y catalysts were tested in thiophene hydrodesulfurization and hydrogenation of pentene-1 and cyclopentene. Catalysts were prepared by thermal decomposition of supported Mo(CO), encaged in Y and stabilized Y zeolites. Cracking ability in both reactions is related to the surface acidity of catalysts but is not parallel to their HDS activity. H,S generates protonic acidity over NaY and KY zeolites. Synergetic effect between Ni and Mo sulfided species in HDS reaction was observed. The presence of extra-lattice aluminum in stabilized forms of Y-zeolites favours selectivity towards formation of isopentane and cyclopentane during hydrogenation. 1. INTRODUCTION The need to produce liquid fuels from low-quality hydrocarbon feedstocks has stimulated numerous investigations over the supported and non-supported transition metal sulfides (TMS) used as catalysts in hydroprocessing (ref. 1). Co-Mo and Ni-Mo sulfides supported on alumina are widely used as hydrogenation (HD) or hydrodesulfurization (HDS) catalysts. Application of zeolites as the supports for these sulfides seems to be promising due to the dual functionality of such systems (ref.2) Studies performed in previous years (ref.3-5) over molybdenum loaded zeolites in HDS reaction showed that reactivity of these catalysts depends on the type of zeolite used, concentration of transition metal and the way of preparation. However, preparation of molybdenum based catalysts applying ammonium heptamolybdate usually results in low dispersion of molybdenum sulfides and relatively low activity. Recent studies showed that saturation of Y-zeolites with molybdenum carbonyl can produce catalysts with high molybdenum dispersion (ref.6, 7). Subsequent sulfidation of these catalysts leads toward highly dispersed, supported sulfided molybdenum species (ref. 8 , 9 ) exhibiting high reactivity in HDS and water-gas shift reaction (ref. 9,101. In the present study, Y-zeolite supported Ni-Mo-sulfides prepared from Mo(CO), were tested in hydrodesulfurization of thiophene and hydrogenation of pentene-1 and cyclopentene.

332

2. EXPERIMENTAL A. Catalvst preparation NaY from Katalistiks (Si/A1=2.56) has been used for the preparation of ammonium, potassium and cesium exchanged Y-zeolites. Three series of NH,NaY zeolites differing in the degree of exchange h a v e been prepared by the multiple exchange of NaY with NH,C1 solutions and denoted hereafter as 1, 2 , 3 . Stabilized forms of Y-zeolites (USY) were obtained from 1, 2 and 3 by deep-bed hydrothermal treatment at 875 K for 4.5 hours under 100% steam. In order to obtain Ni-containing zeolites powdered samples of Nay, KY, CsY and USY were exchanged with Ni(NO,), solution. Nickel containing and nickelfree supports (grains 0.5 - 1.0 mm) were activated in catalytic reactor at 675 K for two hours in the stream of highly purified hydrogen. After cooling, molybdenum carbonyl has been sublimed at room temperature onto the activated zeolitic support (0.59 ) in the stream of hydrogen for 15 hours. Samples were next decarbonylated at temperatures ranging from 425 till 575 K. All samples after partial (425 K ) or complete (575 K) decarbonylation were exposed to air at room temperature and finally presulfided at 675 K for two hours with the mixture of H,S (10 vol.%) and hydrogen.

B. Characterization Sulfided samples were characterized with XRD, BET surface area, NO sorption capacity, ESR and FTIR spectroscopy. The details concerning the characterization procedures as well as certain properties of USY based samples can be found elsewhere (ref. 9 , 10). The ammonia adsorption capacity of sulfided and non-sulfided catalysts and supports was measured from the desorption peak obtained during-,the temperature programmed desorption (heating rate 30 K min ) . Each sample (0.1 g) after activation or sulfidation was saturated with ammonia (a series of 1 cm NH, injections) at 375 K until full saturation was achieved. This was monitored as a sharp GC peak detected by thermal conductivity detector. Next, sample was purged 1 hour weakly in purified helium at 375 K to remove the excess of held ammonia and TPD started.

C. Catalvtic activitv Sulfided Ni-Mo-Y zeolites were tested by hydrodesulfurization (HDS) of thiophene and hydrogenation (HD) of pentene-1 and cyclopentene. A continous-flow reactor operating under atmospheric pressure was applied in both studied reactions. The HDS reaction was performed at 675 K but HD reactions were mainly studied at 450 K. The experimental details of HDS reaction are given in the ref. 9 . During hydrogenation reaction, the hydrogen stream was passing through the saturator filled with pentene or cyclopentene kept at 196 K. Both during HDS or HD reactions the concentration of H,S in the feed was steady and equal to 2 vol.%.

333

3 . RESULTS AND DISCUSSION

Hydrodesulfurization of thiophene Most of molybdenum loaded Y-zeolites contain 2 Mo atoms per supercage, what gives approximately concentration of 10 wt.%. The amount of nickel for non-stabilized supports was always adjusted to 3 wt.% , what provides about 30% of exchange. Lower loadings, both for Mo and Ni, were obtained for stabilized Y-zeolites (for details see reference 10). A variety of hydrocarbons has been formed during the HDS reaction over the series of studied catalysts. The amount of C, hydrocarbons, however, were considered for HDS reaction and regarding the distribution the major products were butane, butene-1 and izobutene. The lighter hydrocarbons, mainly ethylene and propylene, were always detected at the initial stage of reaction. The only exception was for CsY support where only HDS products and non-reacted thiophene were observed. The calculated specific pseudo-first order reaction rate constants for HDS activity are shown in Fig. 1 and Fig. 2. Whereas Fig. 1 shows the results concerning non-stabilized supports, Fig. 2 presents the HDS activity for the series of stabilized Y-zeolites. All studied catalysts show a rather rapid activity decay during the first hour of run time, followed by slow deactivation. The highest stability among the nickel-free supports was found for CsY. NaY and KY supports, indicating higher activity than CsY, show however lower stability and faster deactivation. ~~

A

-

+ ,L .A

B

N ~ Y KY CSY

7'

m

LOO

A-A-A

NaNiY KNiY

PI’

-5 300 y

tZ

200 v)

Z 0 0

100

W

k 1 TIME ON STREAM ( h )

m

o

I

1 TIME ON STREAM ( h )

Fig.1. Sulfided molybdenum loaded Y-zeolites in thiophene hydrodesulfurization at 675 K. A. Sulfided Mo-Y catalysts (open symbols) B. Sulfided Ni-Mo-Y catalysts (filled symbols) Supports activated at 675 K; Mo(CO), decarbonylated at 425 K.

334

-._ 100

TC'

E

*-*--a

USY-1 USY-l.Ni &--&-A UM-2 U M -21 Ni D - O - . Q USY - 3 CCI USY-3.Ni

Fig. 2 . Hydrodesulfurization of thiophene over the series of stabilized Y-zeolites with supported, sulfided Mo and Ni-Mo species. Temp. react. 6 7 5 K.

Whereas drop of activity for CsY support was leas than 2% after three days of reaction for NaY support the activity after one day indicated only 6 0 % of the initial one. 0 1 2 It was expected that the TIME ON STREAM ( h l effect of deactivation is related to the increased acidity of the support, which can appear after the contact with H,S, even though Karge and Rasko applying IR technique (ref.11) claim the absence of dissociative adsorption of H,S over the 2.5. The TPD faujasites with Si/A1 ratio higher than measurments of ammonia desorption over pure supports and molybdenum containing catalysts before and after interaction with H,S confirmed our assumptions (see data in table 1). The lowest acidity was found for molybdenum loaded CsY (0.137 mmol g ) but KY and NaY showed very,similar values for ammonia adsorption: 0.861 and 0. 8 4 0 mmol g , respectively. Moreover, experiments with molybdenum-free NaY and KY revealed that amounts of NH, adsorbeed over the sulfided samples were always higher of about 3% than for those activated only in helium. These results as well as calculations applying Sanderson’s electronegativity model (ref.12) confirmed the expected acidity of alkaline zeolites after interaction with H,S.+ The dissociative adsorption of H,S generates HS and acidic H ions. The latter is involved in olefin oligomerization (ref.13). coking and finally MoY catalyst deactivation. The introduction of nickel yields catalysts with higher acidity (Table 1) and activity (Fig.lB). This is consistent with the known increase of Bronsted acidity of exchanged zeolites with divalent ions. As in the cas9,of Ni-free zeolites, catalysts based on CsNiY (0.605 mmol g NH,) show relatively low activity but they are more stable than those originating from NaNiY or KNiY zeolites. A comparison between the molybdenum catalysts containing nickel or Ni-free samples with their activity and acidity shows that initial acidity of the support (or generated v i a H,S dissociation) can play an important role at the initial stage of HDS reaction. For example the results of activity of NaY(Mo)and NaNiY(Mo) and their NH, sorption capacity supports earlier suggestion. Moreover, data presented in Fig. 1A and 1B indicate that catalysts composed both from Ni and Mo are more active then those containing only Mo. Sulfided NaNiY, cqtalyqts (Mo-free) showed very low initial activity (k= 20 cm g min ) , This suggests the existence of synergy between the sulfided

335

species of Ni and Mo. The inreased acidity of the applied supports strengthen the synergetic effect. Catalysts prepared from stabilized forms of Y-zeolites, loaded with Mo show significantly lower activity and very rapid deactivation (Fig.2). The presence of different types of acid sites in USY supports instantly decreases activity due to very fast coking. The acces of thiophene into the zeolites cavities is very much limited. The effect of plugging is especially visible for Ni-containing USY zeolites. IR (see ref.10) and MAS NMR spectra in relation to HDS activity indicate that extra lattice aluminum is responsible for plugging and that reaction runs on the external surface of zeolite (similarly a5 over the catalysts prepared from ammonium heptamolybdate (ref.9). HYdroaenation of pentene-1 and cvclopentene Hydrogenation activity over the sulfided catalysts is shown in Table 1 and Fig. 3. Whereas hydrogenation of pentene-1 provided a wide spectrum of saturated hydrocarbons, hydrogenation of cyclopentene yielded mainly cyclopentane with traces of methylcyclopentane. In the case of pentene-1 the main products were isopentane and pentane. Due to the cracking and isomerization reactions methane (for Ni containing zeolites) butane and isobutane were also found among the products. As in the case of hydrodesulfurization of thiophene all catalysts indicate deactivation in time. Ni- and Mo-free NaY supports show additionaly the formation of sulfur containing organic compounds. Table 1 Hydrogenation of pentene-l at 450 K

Catalyst

Conversion after 2 h (%)

Yield ( % ) of isopentane n-pentane after 10 min. 2 h 10 min. 2 h

NaY NaY (Mo) NaNiY NaNiY (Mo)

14.3 92.3 94.7 94.6

3 23 52

0 3 17 16

USY-1 USY-I (Mo) USY-1-Ni USY-l-Ni(Mo)

94.8 99.7 95.1 96.2

5 20 56 56

USY-3 USY-3 (Mo) USY - 3-Ni USY-3-Ni(Mo)

97.4 93.6 92.8 98.2

24 27 55 53

0

0 3 2

2 4

4

5

4 5 19 18

0 4 6

1 2 4

6

4

15 7 10

4 2

4 2

5 5

3 6

30

0

Amount of NH, adsorbed -1

(mmol g

0.832 0.840 1.134 1.390 0.585 0.548 1.151 1.446 0.708 0.902 1.163 1.368

)

336

---___ -- - -=-- - _ _ _ - _ _

0

1 TIME O N STREAM ( h )

2

0

1

2

TIME ON STREAM ( h )

Fig.3. Hydrogenation of cyclopentene over molybdenum-free Y-zeolites at 450 K. Open symbols represent nickel-free zeolites; filled symbols represent nickel-exchanged zeolites. Similar effect as for pure NaY support but with much higher yiel3 of sulfur containing compounds was always observed for acidic catalyst6 after their deactivation (usually after 2 hours) in hydrogenation of pentene-1. It was easy to observe this effect for the supports characterized by relatively weak Bronsted acidity, e.g. nickel-exchanged Nay, or with relatively large amount of sodium ions e.g. USY-1. Experiments in which NaNiY support was impregnated with very diluted solution of Na,CO, confirmed the assumption that only basic sites are involved in organoeulfur compounds formation. The absence of hydrogenation products in this case confirm this suggestion. Moreover, adsorption of ammonia over the coked catalysts showed lowered (about 40%) adsorption capacity. Kallo et al. (ref.14) established that acidic sites do not play any role in the hydrosulfurization of ethylene over X and Y-type zeolites. Ziolek et al. (ref.15) found that alkaline metals in zeolites play an important role during formation of thiols from S and alcohols, whereas Bronsted acidity is involved in formation of organic sulfides. The obtained results indicate that hydrogenation of linear olefin occurs over the catalysts containing either extra-lattice aluminum (see USY-3 in table 1) or both transition metal sulfide and extra-lattice aluminum ions. The presence of acidic sites causes oligomerization of olefin what further result in poor access of olefin particles into the active sites for hydrogenation.

337

Results presented in Table 1 and those obtained during hydrogenation of cyclopentene indicate that at the experimental conditions supported molybdenum sulfide is rather inactive in hydrogenation. For this reasons the description of the results concerning hydrogenation of cyclopentene will focus on molybdenum free catalysts. The most intriguing effect was found for USY supports, because dealumination v i a steaming caused the increase of hydrogenation ability (see Fig.3B-open symbols). Whereas a significant increase of hydrogenation activity in the presence of sulfided nickel species can be easily understood, the high activity of stabilized Y-zeolites was rather unexpected. The yield of cyclopentane increases in time together with the degree of dealumination, while overall activity decreases. This can be explained by initial deposition olefin-originating species over the studied samples. Discrepancy between conversion and the yield of cyclopentane is caused by the transformations of olefins to heavier hydrocarbons (Nay-Fig.3A) In order to clarify unusual hydrogenation phenomenon over USY the experiments with acid leached and USY steamed at 1075 K were perf0rmed.h both cases any hydrogenation activity was detected.This indicates that extra-lattice aluminum, extracted at rather mild conditions, is responsible for hydrogenation. 2 7 A l MAS NMR experiments (see also ref .lo) indicated the absence of octahedrally coordinated A1 atoms, what suggests formation of bohemite-like structure inside the supercages (ref.17). Application of industrial 7-alumina (AERO-1000, American Cyanamid Co.,99.99% pure) in cyclopentene hydrogenation resulted in cyclopentane formation with a yield of 50%. Bowman and Burwell (ref.18) found that y-alumina can catalyze propylene hydrogenation even below 425 K. It was expected that contamination of USY obtained froin commercial NaY or industrial alumina with Fe+3 can influence the hydrogenation activity. ESR experiments with USY applying high amplifications detected two wery weak signals with g-values of 4.34 and 2.05 ascribed to differently coordinated iron ions. On the other hand, hydrogenation performed with very pure alumina (obtained v i a hydrolysis of doubly sublimed aluminum isopropoxide) indicated a complete inertness this material toward hydrogenation. Steaming of this alumina at 875 K, however, resulted in the appearance of hydrogenation activity (about 4%). ESR spectra as well as additional experiments with very pure alumina (steaming, treatment with diluted acids, interaction with hydrogen sulfide) indicate that at least two factors can influence hydrogenation activity of USY zeolites obtained v i a steaming: contamination of parent material with Fe*3 ions and/ or formation of new active phase of alumina inside supercages. The catalytic sites are probably coordinatively unsaturated surface species, which are poisoned by carbonaceous deposits formed as the result of prolonged interaction with olefins. Further works are required to elucidate the possible mechanism of hydrogenation over sulfided or non-sulfided stabilized Y-zeolites.

338

4. CONCLUSIONS (i)

Stability in HDS reaction is related to the acidity of the support. The following sequence of stability was observed: CsY>KY>NaY>CsNiY>KNiY>NaNiY>USY. (ii) Supported Ni and Mo sulfided species indicate the synergetic effect in HDS activity. (iii) Interaction of hydrogen sulfide at 675 K with stabilized and non-stabilized Y-zeolites increases their acidity. Presence of newly generated protonic acid sites in alkaline Y-zeolites is responsible for slow deactivation of studied catalysts both in HDS and HD reactions. (iv) Dealumination of Y-zeolites v i e mild steaming increases their activity in hydrogenation reactions. Cyclopentene hydrogenation proceeds selectively over nickel containing stabilized Y-zeolites.

5. REFERENCES 1 2 3

4

5 6 7 8 9 10

11 12 13 14 15 16 17 18

R.R. Chianelli, Catal.Rev.-Sci.Eng. 26 (1984) 361-393. C.S. Brooks, 6th North-American Meeting of Catalysis

SOC. Chicago, March 18-22, 1979, D-5. N. Davidova, P.Kovacheva and D. Shopov, Zeolites, 6 (1986) 304-306. N. Davidova, P. Kovacheva and D. Shopov, in Y. Murakami, A. Iijima and J.W. Ward (Eds.), Proceed. 7th 1nt.Zeolite Conf., Tokyo, August 17-22, 1986, Kodansha-Elsevier 1986, Tokyo-Amsterdam, pp.811-818. R. Cid, F. Orellana and A . L. Agudo, Appl. Catal. 32 (1987) 327-336. T. Komatsu and T. Yashima, J. Mol. Catal., 40 (1987) 83-92. M. Laniecki, in H.G. Karge and J. Weitkamp (Eds.), Studies in Surface Sci. and Catalysis, vo1.46, Elsevier, Amsterdam 1989, pp.259-269. Y. Okamoto, A. Maezawa, H. Kane and T. Imanaka, J. Mol. Catal., 52 (1989) 337-348. M. Laniecki and W. Zmierczak, Zeolites, 11 (1991) 18-26. M. Laniecki and W. Zmierczak, in G. Ohlmann, H. Pfeifer and R. Fricke (Eds.), Studies in Surface Sci. and Catalysis, Proceed. Int. Symp. ZEOCAT 90, Leipzig, August 20-23, 1990, Elsevier, Amsterdam, in press. H.G. Karge and J. Rasko, J.Co1l.Interface Sci.,64 (1978) 522 W.J. Mortier, J. Cata1.,55 (1978) 138-145. D. Eisenbach and E. Gallei, J. Cata1.,56 (1979) 377-389. D. Kallo, G. Onyestyak and J. Papp Jr., in D. Olson and A . Bisio (Ede.), Proceed. 6th 1nt.Zeolite Conf. Reno, USA, Butterworth 1983, pp.444-453. M. Ziolek,H.G. Karge and W.Niessen. Zeolites, 10 (1990) 662. J. Leglise, A. Janin, J. C. Lavalley and D. Cornet, J. Catal., 114 (1988) 388-397. R.D. Shannon, K.H. Gardner, R.H. Staley, G. Bergeret, P. Gallezot and A. Auroux, J. Phys. Chem., 89 (1985) 4778-4781. R.G. Bowman and R.L. Burwell Jr., J. Cata1.,88 (1984) 388.

339

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991Elsevier Science Publishers B.V., Amsterdam

Reduction of SO, on molybdenum loaded Y z e o l i t e J. Soriaa, A.R. Gonzilez-Elipe’ and J.C. Conesaa "Instituto de Catalisis y Petroleoquimica, C.S.I.C., Madrid, Spain

Serrano 119, 28006-

’Instituto de Ciencia de Materiales, C.S.I.C., Sevilla, Spain Abstract The effect of SO, adsorption at different temperatures has been studied by ESR on three molybdenum-loaded zeolites with and without cobalt or H+ ions. In comparison with the outgassing-only treatments, the presence of SO, lowers the temperature needed t o reduce the molybdenum and leads to the generation of different radicals (SO,-, SO,-, S,' and O,-). The formation of these radicals depends on the outgassing pretreatment and the temperature of interaction with SO,. The generation of sulfur radicals, observed when the sample is outgassed at room temperature and heated in the presence o f SO, at T 2 523 K, i s related to the molybdenum ions and the acidity of the sample, suggesting an increased stabilization of these species on protonated molybdenyl groups. 1. INTRODUCTION

In recent studies by ESR on the interaction of SO, with different transition metal oxides, it was observed that thermal treatments under SO, lead to the reduction of the oxides and, in most cases, to the formation of SO,- radicals by electron transfer from electron donor sites to the adsorbed molecule [I-41. In the case o f MoOdAl,O,, although the reduction of MOO, could be confirmed by the presence of Mo ions, the only signal of adsorbed species observed in the ESR spectrum [3] presented g-values very close to those reported for ;0 radicals bound to Mo6+ ions [5]. The formation of 0,--like species, without contacting the sample with oxygen and in presence of Mo5+ ions, suggested a deeper transformation of the SO, molecule than in other systems, leading to stable complexes between Mo ions and sulfur-containing molecules, which might contain an O,--like fragment. In order to obtain more information on the interaction o f SO, and molybdenum ions we have studied, by ESR, the effect of SO, treatments on molybdenum loaded zeolites. On this support the molybdenum can be in a more dispersed condition than on A1,0, and occupy different positions in the zeolite [6,7]. These positions can be modified by outgassing treatments or by the introduction of a second cation. With the purpose of studying the influence of these modifications on the adsorption of SO,, the molybdenum Y zeolite was also ion exchanged with cobalt, a cation used with molybdenum in hydrodesulfurization catalysts, and with protons, in order to check the effects of acidity changes.

340

2. EXPERIMENTAL

The starting material was a NaY zeolite (Linde SK-40, Si/A1 = 2.5). Co and/or Mo were loaded by impregnating NaY zeol ite powders with aqueous solutions of cobalt nitrate and/or ammonium heptamolybdate at pH = 6 and T = 309 K in a rotary evaporator. The resulting samples were dried at 383 K overnight, followed by heating in air at 623 K for 3 hr and calcination at 823 K for 4 hr. The catalysts contained 12% MOO, (sample I) and 12% MOO, and 5% COO (sample 11). Another sample I 1 1 was prepared by the same procedure, but performing the impregnation, with Mo only, on a zeolite previously exchanged with protons (using NH,NO, solution and calcination) to reach a 70% exchange level. For all samples, the calcination treatment resulted in a partial loss of crystallinity; the surface areas were 340, 480 and 550 m’g-’ for samples I, I 1 and 111, respectively. Electron spin resonance spectra were obtained with a Bruker ER 200D spectrometer operating in X-band. The spectra were recorded at 77 K, unless stated otherwise. A DPPH standard (9 = 2.0036) was used to calibrate the magnetic field. Samples of about 0.050 g were placed in a quartz vacuum cell with greaseless stopcocks, and then outgassed at the selected temperature T, during 1 hr, a specimen in such state is designated S-T, (S = I, I1 or 111, T in K ) ; the outgassed samples were contacted at 295 K with SO, at a pressure close to 2 K Nm-’ and heated stepwise under SO, during 1 h at increasing temperature T., Most specimens presented in the spectra a sextet of sharp lines with spacing = 90 G, which can be ascribed to a Mn2+ impurity. Computer substractions were made in order to eliminate these peaks; this succeeded only partially, as the thermal treatments changed somewhat their shapes so that complete cancellation was not achievable. 3. RESULTS ESR signals corresponding to different paramagnetic species were detected upon heating the samples under vacuum or SO,. Their shapes and parameters (the latter being summarized in Table 1, together with their assignations discussed below) were ascertained through comparison of many different spectra o f which only a selection is presented in the figures given.

a) Outgassing treatments The three samples were treated under vacuum at different T, in the 373773 range. No significant ESR signal was originated by outgassing at T, < 473 K, but a broad signal A was observed for each sample at T, = 573 K. Signal A had shape corresponding to centers with axial symmetry and g-values close to g,= 1.93, g z 1.90. This signal, as obtained for each sample, is presented in figures la, 3a and 4 a . Higher outgassing temperatures produced a small increase of the signal intensity and a broadening that makes the lineshape more symmetric. In most of the spectra obtained for T, 2 573 K a narrow (AHpp = 46) and symmetric signal B with g = 2.002 was also obtained. Its intensity grows with T,. b) Treatments with SO, For sample 1-295 the ESR spectrum shows, for T, = 423 K , the formation of a signal A , narrower than that observed by mere outgassing at T, = 573 K and

thus with better resolved parameters (9, = 1.934, g, = 1.883); its intensity reaches a plateau for T, = 473-573 K, figures lb and l c , and shows a sharp

341

= A u

(Y

0 Y 2

x1.6

9 \I

IT\

x 2.5

, (

x 2.5

.

50 G

Figure 1. ESR s p e c t r a of sample I : ( a ) outgassed a t 573 K; (b) outgassed a t 295 K and SO, adsorbed a t 523 K; ( c ) 573 K , and (d) 623 K.

Figure 2 . ESR s p e c t r a of sample I outgassed a t 373 K and SO, adsorbed a t ( a ) 523 K; (b) 623 K , and ( c ) 723 K . decrease f o r T, = 623 K , while a new signal A’ with g, = 1.944 i s observed, f i g u r e Id. An axial signal C with g,, = 2.009, g1 = 2.002 i s observed f o r T, = 473 K, i t reaches an i n t e n s i t y maximum f o r T, = 573 K , and i s s t i l l present,

342

together with small signals A and A’, a t T, = 723 K. For T, = 523 K an orthorhombic signal D appears with g, = 2.043, g, = 2.030 and g3 = 1.999. The i n t e n s i t y of t h i s signal increases f o r T, = 573 K, f i g u r e l c , b u t i t desappears a t T, = 623 K. Another orthorhombic signal E appears as shoulders a t T, = 573, w i t h g1 = 2.014, g, = 2.010 and g3 = 2.002, reaching a maximum i n t e n s i t y f o r 623 K, figure Id. The spectra obtained f o r a specimen 1-373 are formed mainly by an A-type signal and a symmetric one F with g = 2.004 and AH = 86. The i n t e n s i t y of both s i g n a l s , which f i r s t appear a t T, = 473 K , incpeases w i t h T, and signal A becomes more symmetric, figures 2a-c. For sample 11-295 (Fig. 3 b-c), progressively heated under SO,, signal A appears a t T, = 473 K, reaches a maximum a t T = 523 K and then decreases, showing a very small i n t e n s i t y f o r T, L 675 K. Signal C i s observed, overlapping signal F, f o r T = 473-523, figure 3b and 3c, and (more weakly) 723 K. In the T, = 573-673 (range signal F overshadows signal C, f i g u r e 3d. Signal D i s only observed with very low i n t e n s i t y f o r T, = 523 K , f i g u r e 3c, and signal E i s not observed. For sample 11-373 signal A i s observed a t T, = 473 K and reaches a maximum f o r T = 573 K; f o r higher 1, i t s i n t e n s i t y i s very low and only a small signal if‘ i s observed. Signal C i s observed in the T, = 423-573 K range. A t T, > 573 K i t i s probably overshadowed by a strong signal F. In the case of sample 111-295 signal A i s formed i n the T, = 423-473 K range, reaching an i n t e n s i t y plateau in the T, = 473-523 K range, f i g u r e 4b, and showihg a marked decrease above t h a t temperature. Signal C i s observed, more o r l e s s isolated i n the 1, = 423-523 K range and overlapping with signal F i n the T, = 573-623 K range and w i t h signal E f o r T, 2 623 K. Signal 0 i s observed f o r T, L 573 K, f i g u r e 4c, reaches a maximum f o r T, = 673 K, f i g u r e 4d, and i t i s s t i l l observed f o r T, = 723 K (the maximum T, used i n t h i s study) For sample 111-373 signal A i s f i r s t observed a t T, = 423 K, reaches a maximum a t T, = 473 K and decreases sharply f o r T, = 523 K, showing a marked broadening; above T, = 523 K the signal i s no longer modified. Signal C i s observed without interference in the T = 423-473 K range and T, = 773 K, but i s overlapped by signal F in t h e T, = s23-623 range and by signal E i n the T = 623-723 K range. Signal D i s observed a t T, = 523 K, reaches a maximum af: T, = 623 K and disappears a t T, = 773 K. We examined a l s o a sample 111-473 K , the main differences observed in r e l a t i o n with 111-373 are t h a t now signal E i s not observed, and t h a t signal D, which appears w i t h lower i n t e n s i t y in the T, = 523-623 K range, is no longer observed f o r T, 2 673 K.

.

Table 1 Parameters and assignment o f the ESR s i g n a l s Signal A A’ 6

C D E F

g-Val ues

Assignment

91 = 1.934, 911 = 1.884 g, = 1.944, g,, = unresolved g = 2.002 (AH = 46) gl, = 2.009, 91 = 2.002 g, = 2.043, g, = 2.030, g3 = 1.999 g1 = 2.014, g = 2.010, g3 = 2.002 g = 2.004 (A$= 86)

MO~+

MO~+

Trapped electrons

so -

Sutfur r a d i c a l s (S,-) 0-

sb3-

343

r5 \

x2.

m

a-

Figure 3. ESR spectra o f sample 11: (a) outgassed at 573; (b) outgassed at 295 K, and SO, adsorbed at (b) 473 K, (c) 523 K and (d) 573 K.

Figure 4. ESR spectra o f sample 111: (a) outgassed at 573 K; (b) outgassed at 295 K and SO, adsorbed at (b) 523 K, (c) 573 K and (d) 623 K.

344

All t h r e e samples were examined a l s o f o r higher temperatures T, of preoutgassing. Similar behaviour was obtained i n a l l cases: A and F s i g n a l s were generated f o r T, 2 423 K 4. ASSIGNMENT OF

THE SIGNALS

The l a r g e l i n e width of signal A i n d i c a t e s t h a t i t i s not due t o r a d i c a l s b u t t o t r a n s i t i o n metal ions; here i t must be due t o Mo5+ ions. Signals with s i m i l a r g values have been observed f o r Mo5+ ions in MoNaY z e o l i t e s [6] and assigned t o such s p e c i e s belonging t o Mo,O, clusters produced during the preparation treatments ( i s o l a t e d Mo5+ g i v e s sharper peaks, s e e r e f . 6 ) . Modifications in t h e environments of t h e Mo5+ ions can produce changes i n t h e g-values as in t h e case of signal A’. Signal B i s often observed a f t e r thermal treatments under vacuum in many c a t a l y s t s ; i t is assigned generally t o e l e c t r o n s trapped i n oxygen vacancies or t o carbon impurities. Considering t h a t t h e samples have been previously calcined a t 823 K the f i r s t assignment seems more l i k e l y . As t o t h e other symmetric signal F , although i t i s located c l o s e t o t h e p o s i t i o n o f signal B and i s only a l i t t l e broader than i t , i t s c o n s i s t e n t s h i f t t o higher g-value i n d i c a t e s t h a t i t must have a d i f f e r e n t o r i g i n . I t s parameters a r e s i m i l a r t o those of s i g n a l s assigned by d i f f e r e n t authors t o SO,- s p e c i e s [8], we will adhere t o t h i s assignment. Species s i m i l a r t o our signal C have been observed a f t e r adsorption of SO, on vacuum t r e a t e d Y z e o l i t e s [9] and many other compounds [ l o ] ; following these works, i t can be assigned t o SO,- r a d i c a l s . As t o signal E i t s g values a r e not much d i f f e r e n t from those obtained f o r 0,- bound t o Mo6’ ions [5]. We would not d i s c a r d , however, t h e p o s s i b i l i t y t h a t i t corresponds t o an 0-, fragment bonded t o a chain of s u l f u r atoms as suggested in [ l l ] . In any case, t h e unpaired e l e c t r o n d e n s i t y would have t o be concentrated on the 0 atoms, in view of t h e small deviation of i t s g values from g,. F i n a l l y , signal D i s q u i t e s i m i l a r t o t h a t found upon adsorption of HS, on MoO,/A1,0, samples [12], wh5;e i t could be c l e a r l y ascribed, on t h e b a s i s of i s o t o p i c s u b s t i t u t i o n with S and 95M0, t o symmetric S-, s p e c i e s bonded t o Mo ions. I t i s t o be noted t h a t s u l f u r chain r a d i c a l s present r e l a t i v e l y s i m i l a r g parameters [13]; without discarding such a p o s s i b i l i t y , we consider more l i k e l y the assignation t o S,-, e s p e c i a l l y s i n c e t h e d e v i a t i o n (9,-g,) < 0 presented by signal D i s shown by S-, b u t n o t by S., 5. DISCUSSION

The f i r s t observation t o be made from t h e presented s p e c t r a i s t h a t Mo5+ ions a r e thermally generated more e a s i l y by i n t e r a c t i o n with SO, than under vacuum. SO, can obviously a c t as reductant; t h e overall basic process f o r t h i s i n our case (where Mo5’ i s generated) would be 2 Mo6+ t 2 OH- t SO,

-

2 Mo”

t HO ,

t SO,

(1)

o r s i m i l a r . SO, can, however, a c t a l s o as e l e c t r o n acceptor ( i . e . o x i d a n t ) , which makes p o s s i b l e t h e generation of SO,- by r e a c t i o n o f SO, w i t h an e l e c t r o n donor as Mo” i t s e l f

345

Mo5+ t SO, + Mo6+ t

(2)

SO,-

It is to be noted that a combination of (1) and ( 2 ) would amount to a disproportionation of SO, (with formal redox state t 4 for sulfur) into product with S in formal redox state higher (6t for SO,) and lower (3t for SO,-). The implication of Mo in this process is clear, since the parent zeolite does not produce comparable quantities of SO,- under these mild conditions. The formation of SO,- can be explained either as an intermediate step in (1): Mo6+ + 2 OH-

t SO,

-

-

Mo5+ t HO,

t SO,-

(3)

or as a result from an electron transfer similar to reaction ( 2 ) : Mo5+ t SO,

Mo6+ t SO,-

(4)

favoured by the oxidizing character of the SO, molecule. In any case, this radical appears in substantial amounts specially when the sample has been previously outgassed at temperatures of 373 K and higher; this suggests that its stabilization i s achieved by insertion into coordination vacancies produced on Mo ions by outgassing. More remarkable is the observation of S-, radicals; this corresponds to a level of reduction of SO, deeper than SO,-; i .e. elemental sulfur and beyond. The overall process for this could be described as: 4 (Mo5+ - OH-) t SO,

-

4 (Mo6+ = 0) + 2 HO,

t 1/2 S,(5)

although instead of this a direct auto-reduction of SO, can be also formulated: 3 SO, t 2 OH-

-

2 H SO,-

t

1/2 S,

(6)

Any of these reactions would be then followed by stabilization o f the sulfur radical on Mo itself: MO~+

-

H

t

s, +Mo6+

. ..

S,

(7)

Such a particular stability of monoanion radicals in a cation-exchanged zeolite has been also observed in the case of 02- formed on Ce-loaded Y zeolite [ 1 4 ] . In that case, this was ascribed to the presence o f a Ce4+ (OH-) (0,J complex, with the Ce-(OH) group stabilized by the particular coordination constraints imposed to the cation by the rigid zeolite framework. In our case, it may well be that the final product of (7) is particularly stable if an (OH-) group is also bonded to the Mo6+ ion, resulting in a Mo6+ (OH-) ( S J species. Then, this would explain that this species is particularly stable in proton-exchanged zeolites since in the absence o f H+, [Mo6+ = OI4+ species, will dominate, and their smaller positive charge rather than [Mo6+ - (OH)-]'+, will be less stabilizing for the coordinated Sz- anions. Also, the excess of protons will be eliminated (desorbed as H,O) upon outgassing at increasing temperatures and this agrees with the fact that the S-, species disappears at lower temperatures in pre-outgassed samples. Since the acidity of the samples is lowered by the exchange with Co [ 1 5 ] , the lower thermal stability of the Sz- species in the Co-containing samples can be explained with the same arguments. On the other hand, the reason why S,- is formed at lower temperature in the less acidic samples is less clear; it must be related to the details

346

of t h e p a r t i c u l a r elementary s t e p s which sum up t o r e a c t i o n s (3-4). A s i m i l a r consideration can be made about t h e 0,--type s p e c i e s ; they a r i s e probably from t h e process in which SO, l o s e s oxygen t o give f i n a l l y S,-type species. A p o s s i b l e mechanism might be:

2 SO,-

SO,oos0,-

SO,-

-

H'

[0, S-S0,]2-

- -

t (0,s-S0,H)-

so,

t (S-S0,H)-

+

SO,-

(0, S-SO, H)-

t (OS-SOZH)'

+

(8)

(0-OS0,)- t (S-S0,H)-

(9)

t 0-,

SO,

t S,- t

OH-

but many o t h e r r e a c t i o n schemes can be devised, where b a s i c a l l y SO, o r SO,a b s t r a c t oxygen from a species where t h e S-S bond has been already formed; in a l l c a s e s , c a t a l y s i s of some s t e p s by Mo ions would not be excluded. Since some of t h e a b s t r a c t i o n r e a c t i o n s could r e s u l t in formation of 0-0 bonds (as in (9) above), t h e generation of 0,--type intermediates could be explained. In t h e end, a f t e r heating under SO, a t high temperatures, only those r a d i c a l s will remain which a r e able t o become s t a b i l i z e d on Mo ( o r perhaps Co) c a t i o n s ; seemingly, t h i s a p p l i e s t o S,- only in proton-rich samples, otherwise only SO,o r SO,- withstand the treatment. Thus, t h e d i f f e r e n t behaviour observed w i t h i n t h i s s e t of samples seem t o depend mostly on t h e amount of H,O and/or protons i n them. No s p e c i f i c e f f e c t of Co i s revealed beyond those which can be explained in terms of t h e reduction in a c i d i t y induced by the exchange w i t h co In conclusion, SO, a c t s on Mo-loaded z e o l i t e s b o t h a s reductant (giving Mo5+ and SO, o r SO,-) and as e l e c t r o n acceptor, giving SO,- and/or S,- r a d i c a l s , which become s t a b i l i z e d on Mo ions i n t h e presence of Bronsted a c i d i t y . 02-type r a d i c a l s a l s o appear in t h e process, being probably a s i g n a t u r e of t h e mechanisms which a b s t r a c t 0 atoms from SO, t o g i v e f i n a l l y S,- o r s i m i l a r .

.

6. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

A.R. Gonzalez-Elipe and J . S o r i a , Z . Phys. Chem. N.F. 132 (1982) 67. A.R. Gonzalez-Elipe and J. S o r i a , J . Catal. 51 (1983) 235. A.R. Gonzalez-Elipe and J. S o r i a , J.C.S. Faraday 182 (1986) 739. A.R. Gonzilez-Elipe and J. S o r i a , J . Catal. 103 (1987) 506. M. Che and A.J. Tench, Adv. Catal. 32 (1983) 1. J.L.G. F i e r r o , J.C. Conesa and A. L6pez Agudo, J . Catal. 108 (1987) 334. R.F. Howe and H. Minming, Proc. gth Intern. Congr. Catal. 4 (1988) 1585. Y. Ono, H. Takagiwa and S. Fukuzumi, J.C.S. Faraday 1 7 5 (1975) 1613. Y. Ono, H. Tokunaga and T. Keii, J. Phys. Chem. 79 (1975) 752. R.A. Schoonheydt and J.H. Lunsford, J . Phys. Chem. 76 (1972) 323. M. S t e i j n s , P. Koopman, B. Nieuwenhuijse and P. Mars, J . C a t a l . 42 (1976) 96. A.K. Kolosov, U . A . Shvets, M.D. Chuvylkin and V.B. Kazansky, J. Catal. 47 (1977) 190. R. Steudel, J. Albertsen and K. Zink, Ber. Buns. Phys. Chem. 93 (1989) 502. J.C. Vedrine, G . Wicker and S. Krzyzanowski, Chem. Phys. Let. 45 (1977) 543. R. Cid, F. Orellana and A. Ldpez Agudo, App. Catal. 32 (1987) 327.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

341

CONTRIBUTION OF METAL CATIONS TO THE PARA-SELECTIVITY OF SMALL CRYSTALS OF H-ZSM-5 ZEOLITE IN TOLUENE ALKYLATION WITH ETHYLENE J. Cejka, B. Wichterlova, J. Krtil, M. Krivanek and R. Fricke

1

The J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Dolejskova 3 , C S - 1 8 2 2 3 Prague 8 , CZECHOSLOVAKIA ’Central Institute of Physical Chemistry, Rudower Chaussee 5 , D-01199 Berlin, FEDERAL REPUBLIC OF GERMANY Abstract

The alkylation of toluene with ethylene yielding a mixture of ethyltoluenes, coke formation and its reoxidation were investigated on small crystals of H-ZSM-5 zeolites containing Fe, Mn and A1 cations. The cations were located either in the zeolite channel intersections or attached to the zeolite surface, where strong acid OH groups were annihilated by silylation. It appears that the "surfacettmetal cations contribute to the lower p-ethyltoluene selectivity of the zeolite and, therefore, enhance isomerization of ethyltoluene mixture. In contrast, the cations located in the channel intersections increase the zeolite para-selectivity as a result of steric hindrances for transport of bulkier isomers and likely due to para-selectivity of the initial alkylation step. Even though the metal cations as electron acceptor sites slightly enhance the deactivation by by coking, their redox properties contribute significantly to coke removal in the regeneration process. 1. INTRODUCTION

For application of zeolites as catalysts in industrial processes, high activity and easy removal of coke deposits are required. To meet these requirements, small crystals of zeolites (0.5-1.0 um) should be advantageously used. On the other hand, the para-shape selectivity of zeolites in alkylaromatic transformations is connected especially with large crystals of the ZSM-5 zeolite structure, modified by silicon, boron, magnesium and phosphorus [1-5]. However, no definite conclusion has been drawn on the contribution of various species to restricted transport of the bulkier isomers through the zeolite crystals, selectivity of the initial

348

alkylation step and subsequent isomerization of para to meta and ortho isomers in the zeolite channels and/or on the outer zeolite surface [l-81. Further, it has been found that depending on the number of strong acid bridging OH groups and reaction conditions, even small crystals of ZSM-5 zeolites can exhibit an over equilibrium concentration of para-isomer in toluene alkylation with ethylene [ 9 ] . Moreover, the presence of Na and K cations and A 1 0 species in the zeolite channels X Y contribute to the increased zeolite para-selectivity likely through the initial alkylation step [lo]. This paper deals with the activity and para-selectivity of small crystals of H-ZSM-5 zeolites modified by various metal cations located at different sites. The effect of Fe, Mn (which can be expected to enhance coke removal) and A 1 cations, modelling an electron acceptor site without redox properties, located mostly in the zeolite channels or on the zeolite outer surface, has been investigated. Attention has been paid to the alkylation of toluene with ethylene, including coke formation and its removal by oxidation. The surface deposition of silicon is discussed to explain some effects of the metal cation location in the zeolite on its para-selectivity. 2. EXPERIMENTAL AND ZEOLITE CHARACTERIZATION

H-ZSM-5 zeolites (prepared from Na forms by ion exchange with 0.5 M HN03) with Si/Al ratio from 22.5 to 600 and crystal size in the range of 0.5 to 1.5 pm were supplied by the Research Institute for Oil and Hydrocarbon Gases,Czechoslovakia The ion exchange of Fe3+ , Mn2+ and A13+ into H-ZSM-5 (abbrev. MeH-ZSM-5) was carried out at 330 K using 0.5M FeC13, A1(N03)3, and MnC12 under conditions (pH 3-4) avoiding precipitation of hydroxo-oxidic species. The solid-state interaction of Mn304 oxide and H-ZSM-5 zeolite, carried out in a nitrogen stream at 770 K f o r 6 hours yielded Mn304H-ZSM-5 zeolite containing surface Mn ions in the Mn304 phase and some Mn 2+ in the cationic sites. The location of Mn 2+ in the latter sites was reflected in a decrease in the number of strong acid zeolite OH groups compared with the parent H-ZSM-5 zeolite and

349 2+

in the ESR signal of isolated Mn ions; for details see ref. [11].The surface silylated zeolites were prepared by suspending the H-ZSM-5 or FeH-ZSM-5 zeolite in n-hexane into which a calculated amount of tetraethyl orthosilicate was added to obtain addition of 1.5 wt. % of Si in the final product (abbrev. SiHZSM-5 or SiFeH-ZSM-5). n-Hexane was evaporated and the zeolites were dried and calcined in an oxygen stream at 770 K for 5 hours. To add some Fe cations to the surface of silylated zeolites, the SiH-ZSM-5 was introduced into a FeC13 solution, filtered and dried (abbrev. FeSiH-ZSM-5). The characteristics of the parent and modified zeolites are given in the Table. The alkylation of toluene with ethylene was carried out in a vapour phase continuous flow microreactor at atmospheric pressure. Nitrogen as a carrier gas was saturated with toluene to 18.5 vol. % I the toluene to ethylene molar ratio was 3 . 8 . The reaction products were analyzed by an vlon-linevvgas chromatograph (Hewlett-Packard 5890) with MS and FID detection.

3. RESULTS AND DISCUSSION

It has already been reported that the conversion of toluene in toluene alkylation with ethylene on pure H-ZSM-5 zeolites with different Si/Al ratios is proportional and the para-selectivity is inversely proportional to the number of zeolite strong acid bridging OH groups [9,10]. MeH-ZSM-5 zeolites, containing Fe, Mn and A 1 mainly in the cationic sites in the inner channel intersections and with a lower number of OH groupsI exhibit correspondingly lower conversion in comparison with the parent H-ZSM-5 zeolite (Fig. 1 and the Table). It indicates that no significant contribution of metal cations to the zeolite activity in these zeolites was observed. However, the para-selectivity of MnH- and FeH-ZSM-5 was slightly higher than corresponded to the conversion vs. para-selectivity relationship for the pure H forms of ZSM-5 zeolites (see Fig. 1). The investigation of the reaction in time-on-stream (T-0-S) at relatively high space velocities (WHSV = 20 h-l) revealed that zeolites containing metal cations at the cationic sites were

350 Table Characteristics of zeolites and their conversion and para-ethyltoluene selectivity in the toluene alkylation witlh ethylene after 15 minutes of time-on-stream (T 620K,WHSV 10 hT/E = 3.8). Zeolitea OH groupsb metal (mmol/g) cation

f

conversion

(%I

(wt.% )

p-ETf coke amount’ selectivity (mg/g) (wt.%)

H-ZSM-5

0.72

-

25.4

33.4

1.34

FeH-ZSM-5

0.63

0.242

24.1

48.4

3.34

A1H-ZSM-5

0.67

0.2oe

23.5

36.1

2.37

MnH-ZSM-5

0.63

0.293

24.7

39.6

3.47

25.2

32.0

18.8

83.1

Mn304H-ZSM-5 0.64

10.80

S iH-ZSM-5

0.58

FeSiH-ZSM-5

0.58

19.9

58.2

SiFeH-ZSM-5

0.51

20.6

88.4

H-ZSM-5A

0.30

16.8

55.3

H-ZSM-5B

0.02

6.7

85.0

%i/Al

d

1.50e

= 22.5 (H-ZSM-5 and related zeolites), 45 (H-ZSM-5A), 600 (H-ZSM-5B) bestimated from temperature programmed desorption of ammonia (see.ref.9) Cmg coke per g of a zeolite after the alkylation of toluene with ethylene for T-0-S of 200 minutes dcalculated value from the chemical analysis emeans the amount of A1 or S i added to the H-ZSM-5 zeolite ’for simplicity toluene conversion and p-ET selectivity are presented complete aromatic product composition was (wt.%): B 0.2, T 68.5, EB 0.47, pX 0.26! mX 0.25, OX 0.07, PET 9.46, mET 17.34, oET 0.7, diEB 0.6, Clo 0.9

351

- 75 - 50 I-

- 25

W

n

i 0.5

0.25

0.75

OH groups

20

10

(mmo~/~)

Conversion

30

(%)

Fig. 1 Alkylation of toluene with ethylene at WHSV 10 h-l, 6 2 0 K, after 15 minutes in T-0-S on ZSM-5 zeolites. A ) Dependence of toluene conversion and p-ET selectivity on the number of strong acid OH groups, B) Dependence of p-ET selectivity on toluene conversion H-(O,.), MnH-(O,.), FeH-(A,4), AlH-(V,v), Mn304H- (0,4), SiH- (@,el, FeSiH- ( 0 , O ) , SiFeH-(0,O). A

30 -

B

-

w

IT\

3

33 c

8

-75 u

- _ h

3

u

s20-

Y

.-C0In

/ a - Q

L aJ

>

g

0

10-

r

a I

-50

--

a W

2

U

-YaJ

0

-25

U

v,

t; CL I

I

1

I

I

I

I

I

I

I

Time-on-stream (min)

Fig. 2 Alkylation of toluene with ethylene at WHSV 10 h-l, 6 2 0 K, in dependence on T-0-S on Z S M - 5 zeolites. A l Toluene con) , FeH-( 8 ) , version,-B) p-ET selectivity. H-( e ) , FeSiH-( SiFeH-( @ ) . SiH-( ),

a

6

352

deactivated to a larger extent in comparison with the H-ZSM-5 zeolite. The following toluene conversion decrease within T-0-S of 200 minutes has been observed for H-ZSM-5 (79.8-77.6), FeH-ZSM-5 (79.5-72.5) , MnH-ZSM-5 (74.2-64.1) and A1H-ZSM-5 (78.6-74.1). This was in agreement with a higher amount of Ifcokedeposits" formed with MeH-ZSM-5 zeolites compared to that formed with H-ZSM-5 in the course of reaction under the same conditions and with similar conversion values (the Table). However, despite of the higher amount of coke deposits on MeH-ZSM-5 zeolites, no significant decrease in the conversion was observed at WHSV = 10 h-l (Fig. 2). A substantially higher para-selectivity was observed for silylated SiH-ZSM-5 zeolite (Fig. 1). However, a lower conversion of toluene was found than would correspond to the number of strong acid OH groups (estimated by ammonia desorption) present in the silylated SiH-ZSM-5 zeolite (Fig. 1). Because of the large molecule of tetraethyl orthosilicate used for silylation, only the "surface" OH groups (or those in the mouth of the zeolite channels) were captured by silicon. A s the number of strong acid OH groups (ca 15 % lower than the value for the parent H-ZSM-5 zeolite) is higher than would correspond to the toluene conversion value, it can be assumed that some of the bridging OH groups accessible to ammonia are not accessible to reactants. Therefore, even though a very low amount of Si was added, the silylation most likely caused plugging of some zeolite pore openings and/or considerable decrease in the free diameter of the zeolite channel mouths. Similar results were obtained for the SiFeH-ZSM-5 zeolite, where the surface strong acid sites were poisoned by subsequent silylation of the Fe ion-exchanged zeolite. Similarly, the para-selectivity of SiFeH-ZSM-5 was substantially increased compared with the FeH-ZSM-5 zeolite. The combined effect of higher para-selectivity in the initial alkylation step (due to the presence of Fe ions) and suppression of isomerization reaction on the outer surface on the resulting ethyltoluene para-selectivity is likely (see Figs. 1, 2 and the Table). On the other hand, when some Fe cations were attached to the zeolite surface covered by Si

353

(assuming that the Fe ion-exchange did not occur to a larger extent inside the zeolite) the para-selectivity of this sample was considerably decreased in comparison with SiH-ZSM-5. Similarly, the Mn304H-zeolite containing Mn cations in the channels at the cationic sites as well as on the outer surface exhibits significantly lower p-ethyltoluene selectivity than MnH-ZSM-5 having Mn2+ only at the cationic sites. It follows from the above results that the presence of metal cations mostly in the zeolite intersections (MeH-ZSM-5) increases the zeolite para-selectivity, while their contribution to the zeolite alkylation activity is not significant. On the other hand, the metal cation presence in the "surface" sites (FeSiH-ZSM-5, Mn304H-ZSM-5) substantially enhances the zeolite isomerization activity. A s the para-selectivity of the MeH-ZSM-5 zeolites is higher than that of the pure H-ZSM-5, the metal cation location in the channel intersections should cause steric hindrances owing to the diameter of the metal cations (Fe3+ 0.64 8 , Mn2+ 0.80 8 and A13+ 0.50 2); this effect (which is also necessarily affected by the number of cations present in the zeolite) was not found with the relatively small A 1 cation. Then the contribution of the metal cation to the transport limitation of the bulkier isomers and/or to the selectivity of the initial alkylation step should exceed the contribution of the cations to the isomerization activity. The presence of isolated metal cations in the zeolite channels also plays an important role in the oxidation of coke formed during the alkylation of toluene with ethylene. The facility of coke burning was characterized by the initial temperature at which the coke started to be removed as CO and C 0 2 and the temperature for the CO and C 0 2 concentration maxima. It appears that Mn and especially F e cations enhance significantly coke burning. The initial temperature for coke oxidation and the concentration maxima for CO and C 0 2 evolution were 470, 590 and 780 K, resp., for FeH-ZSM-5 and 5 2 0 , 765 and 8 3 0 K, resp., for pure H-ZSM-5. On the other hand, A1 cations apparently slightly retard coke oxidation (CO is evolved at higher temperatures) likely owing to steric hindrances.

354 4. CONCLUSION

It can be summarized that the presence of metal cations in zeolites, exhibiting redox properties, increases both the zeolite para-selectivity and the burning off coke deposits, however, their electron acceptor properties enhance slightly coke formation during the alkylation reaction. Depending on the location of the metal cations in the zeolite structure, they may strongly affect the zeolite para-selectivity and contribute much more to ethyltoluene isomerization, than to the alkylation reaction. When metal cations are lodged on the zeolite surface they substantially enhance the ethyltoluene isomerization reaction. On the other hand, their location in the zeolite channel intersections, causes greater steric hindrances because of their larger diameter in comparison with protons, resulting in a higher para-selectivity of the zeolite. Finally, it can be stated that even small crystals of H-ZSM-5 zeolites, when properly modified both by metal cations at cationic sites with redox properties and by subsequent silylation, can give a catalyst exhibiting the paraselectivity exceeding 9 5 % at a high conversion level and, moreover, enabling coke removal at relatively low temperatures. 5. REFERENCES

1.

W.W. Kaeding, C. Chu, L.B. Young and S.A. Butler, J.Catal.67,

2. 3.

W.W. Kaeding, L.B. Young and C. Chu, J.Catal. 89 ( 1 9 8 4 ) 2 6 7 . W.W. Kaeding, C. Chu, L.B. Young and S.A.Butler, J.Cata1. 69

4.

L.B.Young, S.A.Butler, W.W.Kaeding, J.Cata1.X ( 1 9 8 2 ) 4 1 8 . W.W. Kaeding, G.C. Barile and M.M. Wu, Catal.Rev.Sci.Eng. 26

(1981)

(1981)

5.

(1984)

,

159.

392.

597.

J. Wei, J.Cata1. 76 ( 1 9 8 2 ) 4 3 3 . I. Wang, C. Ay, B.Lee M. Chen, Appl. Catal. 54 ( 1 9 8 9 ) 2 5 7 . P. Ratnasamy and S.K. Pokhriyal, Appl. Catal. 55 ( 1 9 8 9 ) 2 6 5 . J. Cejka, B.Wichterlova, S.Bednarova, Appl. Catal., in press. J. Cejka, B. Wichterlova and G.L. Raurell, Stud. Surf. Sci. Catal., in press. 11. S. Beran, B. Wichterlova and H.G. Karge, J.Chem.Soc., Faraday Trans. I, 86 ( 1 9 9 0 ) 3 0 3 3 .

6. 7. 8. 9. 10.

355

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

NO DECOMPOSITION ON CU-INCORPORATED A-ZEOLITES

UNDER THE REACTION

CONDITION OF EXCESS OXYGEN WITH A SMALL AMOUNT OF HYDROCARBONS

Tomoyuki I N U I , IWAMOTO

S h i n i c h i KOJO,

Masashi SHIBATA, Takashi YOSHIDA,

Department o f Hydrocarbon Chemistry, U n i v e r s i t y , Sakyo-ku, Kyoto 606 (Japan)

and S h i n j i

F a c u l t y o f Engineering,

Kyoto

SUMMARY Copper c o n t a i n i n g A - t y p e z e o l i t e s w h i c h c o n t a i n e d copper w i t h c o n s i d e r a b l y h i g h c o n c e n t r a t i o n s were s y n t h e s i z e d t h r o u g h c r y s t a l 1 i z a t i o n . I t was c o n f i r m e d t h a t Cu' i o n s i n t h e c r y s t a l s c o u l d be s t a b l y m a i n t a i n e d compared w i t h t h o s e i n t h e c o p p e r - l o a d e d s a m p l e s p r e p a r e d b y an i o n NO d e c o m p o s i t i o n a c t i v i t y o n t h e CU-A c a t a l y s t e x c h a n g e d method. c o r r e s p o n d e d t o t h e c a p a c i t y o f r e d o x response. Even u n d e r an e x c e s s oxygen c o n d i t i o n t h e NO d e c o m p o s i t i o n progressed smoothly a t around 300 350°C b y t h e a d d i t i o n o f a v e r y s m a l l e x p l a i n these unusual n o n - l i n e a r r e a c t i o n phenomena, M i c r o s c o p i c S e q u e n t i a l R e a c t i o n mechanism was p r o p o s e d and t h e n e c e s s a r y c o n d i t i o n s t o r e a l i z e t h i s mechanism were discussed. INTRODUCTION D i r e c t d e c o m p o s i t i o n o f NO w i t h o u t u s a g e o f a n y r e d u c t a n t has been an o u t s t a n d i n g t a s k o f c a t a l y s t s t u d y f o r NO r e m o v a l f r o m e x h a u s t gases, e s p e c i a l l y w h i c h comes f r o m d i e s e l engines. oxygen;

Some k i n d s o f reduced m e t a l -

can decompose NO t o n i t r o g e n and

o r p a r t l y reduced m e t a l - o x i d e - c a t a l y s t s

however, t h e oxygen formed i s i m m e d i a t e l y adsorbed on t h e s u r f a c e

of t h e c a t a l y s t and d e a c t i v a t e s i t (ref. t h e r e a c t a n t gas,

7).

When oxygen i s c o n t a i n e d i n

t h e c a t a l y s t i s e a s i l y o x i d i z e d and deactivated.

The c a t a l y t i c r e d u c t i o n o f NO w i t h NH3 i s w i d e l y adopted f o r NO removal i n t h e s t a t i o n a r y generators.

I n t h i s r e a c t i o n , c o e x i s t e n c e o f oxygen

w i t h a p r o p e r c o n c e n t r a t i o n r a t h e r enhances t h e NO r e d u c t i o n :

however,

a

l a r g e excess o f oxygen s t i l l d e a c t i v a t e s t h e c a t a l y s t and t h e NH3 i s a p t t o b u r n b e f o r e t h e r e a c t i o n w i t h NO.

Therefore,

e v e n t h e NH3 r e d u c t i o n

m e t h o d c a n n o t b e a d o p t e d f o r t h e r e m o v a l o f NO i n t h e e x h a u s t gas f r o m d i e s e l e n g i n e s , i n w h i c h a l a r g e e x c e s s oxygen, as h i g h as 13%, r e m a i n s

356 unconsumed.

i t was reported (ref.

Recently,

H-ZSM-5

2) t h a t an excessively Cu-ion-exchanged

e x h i b i t e d an NO decomposition a c t i v i t y under 02-absent c o n d i t i o n

w i t h o u t s i g n i f i c a n t deactivation: concentration 02 i n t h e feed gas,

however,

w i t h coexistence o f even a low

t h e a c t i v i t y could n o t be exerted.

I n t h i s study, i n o r d e r t o overcome t h e s e d i f f i c u l t problems, a n o v e l r e a c t i o n mechanism,

M i c r o s c o p i c S e q u e n t i a l R e a c t i o n mechanism (MSR The MSR mechanism was b u i I t f r o m t h e v i e w

mechanism), was considered.

p o i n t t h a t t h e e s s e n t i a l p r o p e r t y o f t h e r e a c t i o n on t h e s o l i d c a t a l y s t must be non-1 i n e a r phenomena i n v o l v i n g m i c r o s c o p i c a l l y sequential r e a c t i o n processes,

which are d i f f e r e n t from the conventional Langmuir-Hinshelwood

r e a c t i o n mechanism based on l i n e a r phenomena. mechanism and achieve t h e NO decomposition,

To r e a l i z e t h e MSR

novel metal c o n t a i n i n g z e o l i t e

c a t a l y s t s were prepared and t h e r e a c t i o n c o n d i t i o n s were investigated. EXPERIMENTAL Catalyst Four study.

k i n d s o f c o p p e r c o n t a i n i n g z e o l i t i c c a t a l y s t s were used i n t h i s The a b b r e v i a t i o n and b r i e f explanation o f t h e c a t a l y s t s are l i s t e d

i n T a b l e 1. Characterization Behaviors o f redox treatments a t 250°C and temperature programmed r e d u c t i o n (TPR) f o r t h e p r e - o x i d i z e d samples were measured by a TG-DTA Shi madzu Thermal Analyzer DT-30.

For t h e measurements o f redox responses,

a 20 mg p o r t i o n o f t h e sample was p l a c e d i n a sample pan, and 5% 02 o r 5%

H2 d i l u t e d w i t h N p was allowed t o f l o w w i t h a feed r a t e o f 40 ml/min.

The

amounts o f O2 o r H2 supplied was s u f f i c i e n t l y excess f o r t h e o x i d a t i o n o r The TPR was measured f o r t h e same amount of

r e d u c t i o n o f t h e samples.

samples w i t h a constant h e a t i n g r a t e o f 10"C/min. Reaction The c a t a l y s t i n powder form was t a b l e t e d w i t h a t a b l e t machine. crashed and s i e v e d t o 1 5

-

24 mesh t o p r o v i d e t o t h e r e a c t i o n .

It was

A 0.5 g

(ca. 0.7 m l ) p o r t i o n o f t h e c a t a l y s t was packed i n a quartz t u b u l a r r e a c t o r o f 8 mm i n n e r diameter.

The c a t a l y s t - b e d

l e n g t h was 1.4 cm.

The

357 TABLE 1 Copper c o n t a i n i n g z e o l i t i c c a t a l y s t s used. Catalyst

Abridged notation

Description

No. Cat. 1

Cu/H-ZSM-5

H-ZSM-5 having S i / A 1 a t o m i c r a t i o 40 prepared b y t h e r a p i d c r y s t a l l i z a t i o n method (ref. 3) was by u s i n g 0.5 m o l a q u e o u s ion-exchanged s o l u t i o n o f Cu n i t r a t e a t room temperature. The Cu l o a d i n g was 1.00 w t % w h i c h c o r r e s p o n d e d t o 80% ion-exc han ged.

Cat, 2

Cu-silicate

MFI-type C u - s i l i c a t e prepared by t h e r a p i d c r y s t a l 1 i z a t i o n method. The Cu c o n t e n t was 0.57 w t % .

Cat. 3

Cu/NaA

The c o m p o s i t i o n o f m i x e d g e l was S i / A 1 r a t i o = 1, N a / A 1 r a t i o 5. I t was h y d r o t h e r m a l l y c r y s t a l l i z e d a t 85°C f o r 6 h, c a l c i n e d a t 430°C f o r 1.5 h. I t was i o n - e x c h a n g e d b y Cu n i t r a t e aqueous s o l u t i o n a t r o o m t e m p e r a t u r e . The Cu l o a d i n g was 18.2 w t % w h i c h c o r r e s p o n d e d t o 84% ion-exchanged.

Cat. 4

Cu-NaA

Added Cu n i t r a t e t o t h e p r e p a r a t i o n p r o c e d u r e f o r NaA d e s c r i b e d on Cat. 3. The Cu c o n t e n t was 8.6 w t % .

c a t a l y s t was u s e d f o r t h e r e a c t i o n a f t e r d r y i n g i n an He f l o w a t 400°C. The r e a c t i o n gas was i n t r o d u c e d a t t e m p e r a t u r e range f r o m 200 t o 500°C w i t h an SV r a n g e f r o m 500 t o 2500 h-’.

The r e a c t i o n gases and p r o d u c t s w e r e

analyzed by u s i n g a gas chromatograph equipped w i t h an i n t e g r a t o r . RESULTS AND DISCUSSION Comparison

of c a t a l y t i c

p r o p e r t y between Cu-ion-exchanged-ZSM-5

and Cu-

i n c o r p o r a t e d MFI-type s i l i c a t e Cu/H-ZSM-5

(Cat.

as t h a t o f H-ZSM-5, oxides.

and no i n d i c a t i o n f o r t h e e x i s t e n c e o f i s o l a t e d copper

NO conversion was measured on b o t h c a t a l y s t s under t h e 02-absent

condition. h-l

1) and C u - s i l i c a t e (Cat. 2) gave t h e XRD p a t t e r n s same

4% NO d i l u t e d w i t h N2 was f e d t o t h e r e a c t o r w i t h a SV 2000

a t 500°C and C h a t t e m p e r a t u r e was m a i n t a i n e d f o r 8 h.

c o n v e r s i o n s a t t h e s t e a d y s t a t e on Cats. respectively.

The

NO

1 and 2 w e r e 42% and 12%.

The i n t e g r a t e d amount o f NO c o n v e r t e d t i l l 8 h on s t r e a m

358 1 and 2 were 85 t i m e s and 54 t i m e s o f t h e Cu q u a n t i t i e s ,

f o r Cats.

r e s p e c t i v e l y , i n d i c a t i n g t h a t t h e s e c a t a l y s t s had t h e NO d e c o m p o s i t i o n acttvity.

Under an 0 2 - p r e s e n t c o n d i t i o n , b o t h c a t a l y s t s a c c e p t e d t h e

o x i d a t i o n o f copper,

and no NO c o n v e r s i o n a c t i v i t y was e x h i b i t e d any

more. I n order t o c o n f i r m t h e d i f f e r e n c e i n s t a b i l i t y o f both catalysts,

the

CO o x i d a t i o n t e s t according t o t h e Forced-Oscillating r e a c t i o n method (ref.

4) was adopted.

By t h i s method,

t h e redox processes o f c a t a l y s t surface

d u r i n g t h e r e a c t i o n can be r e a l i z e d forcedly, therefore,

through examining

t h e redox response repeatedly, t h e s t a b i l i t y o f c a t a l y s t s can be evaluated. As a r e s u l t , t h e r e d o x c y c l e s f o r Cat. 2 were shown v e r y r e p r o d u c i b l y , i n d i c a t i n g t h a t t h e c o p p e r p a r t can be s t a b l e t h r o u g h t h e r e d o x c y c l e s . On t h e o t h e r hand, as f o r Cat. 1 a w i d t h o f t h e h y s t e r e s i s became n a r r o w w i t h an i n c r e a s e o f number o f t h e r e d o x cycle.

This indicates t h a t the

s i n t e r i n g o f t h e i r copper p a r t would progress.

Further,

the temperature

dependence o f CO conversion on Cat. 2 was much sharp compared w i t h t h a t on Cat. 1, b o t h i n t e m p e r a t u r e r i s i n g and l o w e r i n g .

T h i s corresponds t o a

h i g h d i s p e r s i o n o f Cu and a very narrow Cu-particle d i s t r i b u t i o n range i n Cat. 2.

This was supported by t h e d i f f e r e n c e o f e f f e c t i v e pore-diffusion

c o e f f i c i e n t between Cats.

1 and 2 , i.e.,

t h e l a t t e r was t w i c e o f t h e

former. C h a r a c t e r i s t i c s o f Cu-containing z e o l i t e s S i n c e i t was supposed t h a t a z e o l i t e , w h i c h c o n t a i n s l a r g e r amount o f Cu,

would have a h i g h e r p o t e n t i a l f o r NO decomposition,

f o r v a r i o u s k i n d s o f z e o l i t e s was i n v e s t i g a t e d .

Cu i n c o r p o r a t i o n

As a r e s u l t , 8.6 w t X Cu

c o u l d be u n i f o r m l y i n c o r p o r a t e d i n t o t h e c r y s t a l s o f z e o l i t e s A(Cat. 4). The NO decomposition a c t i v i t y o f Cat. 4 a t 350 C was the same as Cat. 2 a t 500°C.

The a c t i v i t y p e r Cu i n v o l v e d i n Cat. 4 was l o w e r than t h a t o f Cat.

2; however, t h e a c t i v i t y p e r c a t a l y s t volume o f Cat. 4 was 5.5 t i m e s t h a t o f Cat. 2. Since the s t a b i l i t y o f i n t e r m e d i a t e o x i d a t i o n - s t a t e o f Cu i s one o f t h e key p o i n t s t o r e a l i z e t h e NO decomposition as shown above, TPR response f o r The Cu-ion-exchanged NaA (Cat. 3) showed 270 C h a v i n g a s h o u l d e r a t 240°C. On t h e o t h e r hand, t h e C u - c o n t a i n i n g NaA (Cat. 4) showed t w o d i s t i n c t peaks appeared a t 160 various c a t a l y s t s were compared. one peak a t around

359 and 250°C.

The h i g h e r t e m p e r a t u r e one was near t o t h e s i n g l e peak o f Cat.

3; h o w e v e r , t h e l o w e r one was f a r f r o m t h e s h o u l d e r o f Cat. 3.

The t w o

p e a k s o f Cat. 4 c o r r e s p o n d e d t o t h e c h a n g e f r o m CuO t o Cu20 and f r o m Cu20 t o Cu, r e s p e c t i v e l y .

I n case o f t h e CuO supported by ion-exchange method

(Cat. 3 ) i t was somewhat d i f f i c u l t t o r e d u c e compared w i t h Cat.4, r e d u c t i o n s h i f t e d t o h i g h e r temperature:

however,

and t h e

once r e d u c t i o n began i t

progressed s u c c e s s i v e l y f r o m CuO t o Cu w i t h o u t showing a s t a b l e Cu20 state. These p r o p e r t i e s r e f l e c t t h a t i n t h e c a s e o f t h e i o n - e x c h a n g e m e t h o d Cu f o r m s c o n s i d e r a b l y l a r g e r c l u s t e r s o r t h a t t h e Cu p a r t i c l e s b l o c k t h e p a r t o f p o r e c h a n n e l s , and r e t a r d t h e d i f f u s i o n o f h y d r o g e n and f o r m e d w a t e r , r e s u l t i n g t h e d e l a y o f hydrogen r e d u c t i o n . Effect

of

hydrocarbon a d d i t i o n on NO c o n v e r s i o n under

an

excess

oxygen

condition The s e n i o r a u t h o r e t a l . ( r e f . 5 ) r e p o r t e d p r e v i o u s l y t h a t t h e o r d e r o f r a t e c o n s t a n t f o r hydrogen r e d u c t i o n o f t h e p r e o x i d i zed s u p p o r t e d c o p p e r o x i d e w i t h v a r i o u s k i n d s o f r e d u c t a n t s were: CO > H2 > CH4 > C3H8, and t h i s o r d e r was t h e same as t h e o r d e r o f t h e r a t e o f O2 a d s o r p t i o n t o t h e reduced s u r f a c e s w i t h these reductants.

T h i s means t h a t t h e s t a t e s o f t h e reduced

s u r f a c e s a r e changeable w i t h t h e k i n d s o f r e d u c t a n t s s u g g e s t i n g t h a t t h e i m p o r t a n c e o f t h e m i c r o s c o p i c change o f t h e s u r f a c e state. Furthermore,

-

we have a l r e a d y s t u d i e d t h e c a t a l y t i c combustion r a t e o f C1

C14 s t r a i g h t c h a i n s a t u r a t e d h y d r o c a r b o n s on a s u p p o r t e d Pt-Ce02

catalyst.

I t was f o u n d t h a t t h e c o m b u s t i o n r a t e s o f c a r b o n number C7

C10 h y d r o c a r b o n s w e r e maximum among them,

-

and t h a t t h o s e o f above C8

hydrocarbons g r a d u a l l y decreased w i t h an i n c r e a s e o f carbon number o w i n g t o t h e i n c o m p l e t e combustion ( r e f . 6).

I t was a l s o found t h a t t h e h y s t e r e s i s

i n t h e f o r c e d o s c i l l a t i n g r e a c t i o n t e s t l a r g e l y d i f f e r e n t f r o m each other. I t i s c o n s i d e r e d t o b e n e c e s s a r y t h a t t h e a m o u n t o f h y d r o c a r b o n s added

s h o u l d d i s t r i b u t e i n a c a t a l y s t bed b e f o r e t h e c o m b u s t i o n a s w i d e l y as possible t o play the role effectively. hydrogen and carbon monoxide,

The o t h e r r e d u c t a n t s s u c h as

w h i c h combust t o o e a s i l y ,

give l i t t l e effect

t o t h e o b j e c t i v e r e a c t i o n because t h e s e r e d u c t a n t s a r e consumed j u s t a t t h e e n t r a n c e o f t h e c a t a l y s t bed. According t o t h i s c o n s i d e r a t i o n , t h e NO decomposition i n t h e presence o f e x c e s s 02 o n Cu-NaA(Cat.

4) was s t u d i e d w i t h an a d d i t i o n o f l o w

360 c o n c e n t r a t i o n s t r a i g h t c h a i n s a t u r a t e d C2

-

n-C,

hydrocarbons.

The

t e m p e r a t u r e dependence o f NO c o n v e r s i o n r o u g h l y c o r r e s p o n d e d t o t h e temperature dependence o f the combustion r a t e o f hydrocarbon added. From these r e s u l t s , chosen,

i t was expected t h a t when a proper hydrocarbon was

t h e NO decomposition c o n d i t i o n and t h e r e d u c t i o n c o n d i t i o n o f an

o x i d i z e d c a t a l y s t surface could be adjusted, $6

and then n-C8,

n-ClO,

and n-

saturated hydrocarbons were selected as t h e hydrocarbons t o be added

here.

The amount o f added hydrocarbons was s e t a t about 0.6 molar r a t i o

o f complete combustion stoichiometry.

As shown i n Fig. 1, t h e o r d e r o f

m a g n i t u d e o f t h e NO c o n v e r s i o n was n-cs < n-$O 350°C each NO conversion a t t a i n e d maximum. (n-C16)

addition,

< n-CI6.

and around 300

-

Especially, i n case o f cetane

t h e c o m p l e t e NO c o n v e r s i o n was a c h i e v e d a t t h a t

temperature range, and moreover, even a t temperature range above 350"C, t h e degree o f decrease i n t h e NO conversion was v e r y l i t t l e compared w i t h o t h e r cases.

As shown i n Fig. 2 t h e conversions o f hydrocarbons t o C02 and

H20

d u r i n g t h e NO conversion were detected above ca. 200°C and these increased e x p o n e n t i a l l y up t o ca. 300°C. s u d d e n l y s l o w e d down.

and above t h a t t e m p e r a t u r e t h e i n c r e a s e

I t i s n o t e w o r t h y t h a t t h e o r d e r o f NO c o n v e r s i o n

was i n v e r s e o f t h e o r d e r o f h y d r o c a r b o n c o n v e r s i o n f o r t h e k i n d o f As shown i n Fig. 3, when NO was n o t i n v o l v e d i n t h e

hydrocarbons added.

r e a c t i o n gas t h e c o n v e r s i o n o f each h y d r o c a r b o n i n c r e a s e d e x p o n e n t i a l l y w i t h an increase o f t h e r e a c t i o n temperature and reached a t 100% conversion u n t i l 300

-

350°C.

and above t h a t t e m p e r a t u r e t h e t o t a l c o n v e r s i o n was

maintained, t h a t was d i f f e r e n t from t h e case o f coexistence o f NO. As can be u n d e r s t o o d f r o m t h e c o m p a r i s o n between Figs. 1 and 2, t h e increase o f NO conversion up t o 300°C was markedly l a r g e r than t h e increase o f hydrocarbon conversion,

a1though t h e temperature range f o r increase o f

each c o n v e r s i o n c o i n c i d e d .

T h i s suggested t h a t t h e NO c o n v e r s i o n

progresses e f f e c t i v e l y w i t h coexistence o f a l e s s amount o f hydrocarbon on a considerably small

number o f a c t i v e s i t e s .

The d e c r e a s e i n NO

conversion a t higher temperature must be a t t r i b u t e d t o t h a t t h e o x i d a t i o n of the c a t a l y s t surface progresses predominantly and the a c t i v e s i t e s f o r

NO conversion diminish. I n case o f t h e c e t a n e a d d i t i o n , products such as aldehydes, 2-ketones,

s m a l l amounts o f p a r t i a l o x i d a t i o n a-a1 kylfuranes, etc.,

were detected.

I t i s c o n s i d e r e d t h a t t h e s e p r o d u c t s s t r o n g l y adsorbed on t h e c a t a l y s t

361

I

t

I

300

I 400

I

I

500

Temperature ("C) F i g . 1. E f f e c t o f k i n d o f h y d r o c a r b o n s added o n NO c o n v e r s i o n u n d e r an e x c e s s o x y g e n c o n d i t i o n . Cat. : Cu-NaA(Cat.4). NO 9600ppm, 02 11.0%, SV, 0 : n-CgH18 6500ppm. a: n-C10H22 4100ppm. m : n-C16H34 2600pprn. 2500 h-',

I

300 Temperature

400

I

I

500

("C)

F i g . 2. E f f e c t of t e m p e r a t u r e on h y d r o c a r b o n c o m b u s t i o n d u r i n g t h e NO c o n v e r s i o n shown i n Fig. 1.

362

s u r f a c e and r e t a r d e d a d e e p o x i d a t i o n o f t h e c a t a l y s t s u r f a c e , consequently,

t h e decrease o f t h e NO c o n v e r s i o n would be moderate even a t

t h e h i g h e r t e m p e r a t u r e range. S i n c e i t was s u g g e s t e d as m e n t i o n e d above t h a t a v e r y s m a l l amount o f h y d r o c a r b o n s was s t i l l e f f e c t i v e f o r t h e NO c o n v e r s i o n , t h e n we t r i e d t o reduce t h e c o n c e n t r a t i o n o f cetane added, 4.

and t h e r e s u l t s a r e shown i n Fig.

The m o l a r r a t i o s o f c e t a n e added t o t h e c o m p l e t e c o m b u s t i o n

s t o i c h i o m e t r y were, 0.56, 2600,

700,

and 190ppm,

0.15, and 0.04 respectively.

f o r t h e concentration o f cetane The d e g r e e o f d e c r e a s e i n NO

c o n v e r s i o n was v e r y 1 i t t l e , c o n s i d e r i n g t h e d e c r e a s e o f t h e c o m b u s t i o n s t o i c h i o m e t r y , and even 190ppm cetane a d d i t i o n , s t i l l 50% NO c o n v e r s i o n was realized. C o n s i d e r a t i o n on t h e r e a c t i o n mechanism The r e a c t i o n mechanism i n w h i c h NO c a n b e decomposed e v e n u n d e r t h e c o e x i s t e n c e o f excess oxygen w i t h t h e c a t a l y t i c combustion o f a v e r y s m a l l amount o f h y d r o c a r b o n o f a c o n s i d e r a b l y l a r g e c a r b o n number,

can be

c o n s i d e r e d as f o l l o w s ;

By c o n s i d e r i n g t h e successive o c c u r r e n c e o f t h e c o n s e c u t i v e r e a c t i o n s (1-1). (1-2) a n d (2-1). (2-2). and p a r a l l e l r e a c t i o n s (1-3) and (3-1). t h e whole e x p e r i m e n t a l r e s u l t s can be reasonably understood.

363

Temperature ( " C ) Fig. 3. C a t a l y t i c combustion of n-cg, n-Cl0, and n-C16 hydrocarbons. Cat. : Cu-NaA(Cat.4), S V 2500 h-', O 2 12.0%, 0 : n-CgH18 4600ppm. 0: n-C10H22 4800ppm, M: n-C16H34 3400ppm.

w v

0

I

200

I

I

300

I

I 400

-I e n o e r a t u r e ("C)

I

I 500

Fig. 4. E f f e c t of cetane c o n c e n t r a t i o n on NO conversion. Cat. : Cu-NaA(Cat.4). NO 9600ppm, O2 11.0%, SV 2500 h-'. Cetane; 0 : 2600ppm. 0 : 700ppm, A: 190pprn.

364 I n o t h e r words,

a p r o p e r hydrocarbon,

w h i c h adsorbs on t h e oxygen-

adsorbed c a t a l y s t s u r f a c e , combusts w i t h consuming t h e oxygen on t h e surface explosively.

Successively,

t h e combustion products.

C02 and HZO.

d e s o r b and t h e a c t i v e s i t e s f o r NO d e c o m p o s i t i o n a r e recovered. recovered a c t i v e s i t e s would be o x i d i z e d by t h e oxygen:

however,

The

NO can be

adsorbed and decomposed on t h e a c t i v e s i t e s a t a p r o p e r t e m p e r a t u r e . T h i s r e a c t i o n mechanism i s based on t h e u n d e r s t a n d i n g o f t h e non-1 i n e a r phenomena l i k e t h e o s c i l l a t i n g r e a c t i o n on t h e s o l i d c a t a l y s t surface (ref. 7).

Therefore,

we propose t o c a l l t h i s mechanism Microscopic Sequential

R e a c t i o n mechanism smoothly,

(MSR mechanism).

F o r t h i s mechanism t o o p e r a t e

t h e f o l l o w i n g c o n d i t i o n s are necessary.

(i)m e t a l oxides,

w h i c h a r e c o n s i d e r a b l y easy t o be o x i d i z e d and

reduced, are supported w i t h a h i g h l y d i s p e r s i o n b u t s t a b l y on a microporous crystal. (ii) the r e a c t i o n r a t e s o f

NO

decomposition and t h e combustion r a t e o f

hydrocarbons added are comparable a t around 300

- 400°C.

I n order t o s a t i s f y these c o n d i t i o n s i n t h e l i g h t o f t h e new mechanism i t i s expected t h a t many o t h e r new c a t a l y s t s w i l l be able t o be developed.

REFERENCES H. Niiyama, K. Sasamoto. S. Yoshida, and E. Echigoya, J. Chem. Eng. Jpn.. 14(4) (1981) 301-306. 2 M. Iwamoto, H. Yahiro, T. Yoshioka, and N. Mizuno, Chem L e t t . 1990(11), 1967-1 970. 3 T. I n u i , Mechanism o f Rapid Z e o l i t e C r y s t a l l i z a t i o n s and i t s A p p l i c a t i o n t o C a t a l y s t S y n t h e s i s , in: M. L. O c c e l l i and H. E. Robinson (Ed.), Z e o l i t e Synthesis :ACS Symp. Series, Vol. 398, 1989, pp. 479-492. 4 T. I n u i , H. Wakita, and H. Fukuzawa, A n a l y s i s on C h a r a c t e r i s t i c s o f Supported Pd, P t , and Rh i n Methane Combustion by the Forced O s c i l l a t i n g R e a c t i o n Method, i n : Y. Morooka (Ed.), MRS I n t e r n a t i o n a l M e e t i n g on Advanced Materials, Vol. 2, 1989, pp. 271-176. 5 T. I n u i , T. Ueda, and M. Suehiro, J. Jpn. Chem. SOC., 1977(7), 934-940. 6 T. I n u i , Y. Adachi, T. Kuroda, M. Hanya, and A. Miyamoto. Chem. Express, 1( 4 ) ( 1986) 255-258. 7 T. I n u i and T. Iwana, A n a l y t i c a l S t u d y o f an O s c i l l a t i n g R e a c t i o n on Copper C a t a l y s t s and i t s S i m u l a t i o n , i n : S. K a l i a g u i n e and A. Mahay (Ed.), Studies i n Surface Science and C a t a l y s i s 19, Elsevier, Amsterdam, 1984, pp. 205-212. 1

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

A

comparison o f t h e

365

and HY z e o l i t e i n

c a t a l y t i c p r o p e r t i e s o f SAPO-37

t h e c r a c k i n g o f n-heptane and 2,2,4-trimethylpentane J.M.

Lopesa, F. Lemosa, F. Ramaa R i b e i r o a and E.G.

Derouaneb

a Grupo de Estudos de Cata”l i s e Heterogknea, I n s t i t u t o S u p e r i o r Te‘cnico,

Av. Rovisco P a i s , ‘1096 L i s b o a Codex, P o r t u g a l Faculte‘s U n i v e r s i t a i r e s N.D.

de l a P a i x , L a b o r a t o r y o f C a t a l y s i s ,

Rue de B r u x e l l e s , 61, B-5000-Namur,

Belgium

Abstract The c a t a l y t i c a c t i v i t y o f t h e p r o t o n i c forms o f SAPO-37 and HY z e o l i t e were

compared

in

the

cracking o f

n-heptane

and

2,2,4-trimethylpentane.

HY z e o l i t e p r e s e n t s a h i g h e r i n i t i a l a c t i v i t y w h i c h i s i n agreement w i t h its

higher

o f ammonia. r e l a t i ve

acidity

characterized

by

temperature

programmed

desorption

T h i s i s c o n f i r m e d by t h e f a c t t h a t SAPO-37 e x h i b i t s a h i g h e r

cracking

activity

(2,2,4-trimethylpentane/n- heptane f

than

HY

zeo I it e . Cracking product d i s t r i b u t i o n s a r e very simi l a r f o r both c a t a l y s t s : C3 and C4 hydrocarbons

i n quasi

e q u i m o l a r amounts c o n s t i t u t e more t h a n

90% o f t h e c r a c k i n g p r o d u c t s and t h e i s o / n C q r a t i o always p r e s e n t s h i g h values.

1. INTRODUCTION S i licoaluminophosphates

crystal line

microporous

and phosphorus 11-31.

(SAPO’s)

molecular

framework

the

species,

of

SiIv

sieves,

a

containing

novel

class

silicon,

of

aluminurn

SAPO-37 i s i s o s t r u c t u r a l t o f a u j a s i t e , and has l a r g e

p o r e s and an a n i o n i c presence

constitute

12-41. The n e g a t i v e charge a r i s e s froin partially

substituting

Pv

in

a

neutral

366 aluminophosphate

framework

structure.

Thus,

i t i s possible t o

generate

Bronsted a c i d s i t e s upon c a l c i n a t i o n o f SAPO-37 i n t h e f o l lowing manner: t h e o r g a n i c c a t i o n s used a s t e m p l a t e agents d u r i n g t h e s y n t h e s i s ,

and

which remain i n t h e s t r u c t u r e compensating some o f t h e framework charges, a r e decomposed and generate

a c e r t a i n number o f p r o t o n i c s i t e s

a subsequent ion-exchange w i t h

+ NH4

cations

followed

by

14,5l;

calcination

can

lead t o an o c c u p a t i o n o f c a t i o n i c s i t e s a l m o s t e x c l u s i v e l y by protons. The

acidity

of

this

material

spectroscopy e x a m i n a t i o n o f

the

has

hydroxyl

been

confirmed

region

12,41

by

infrared

and a l s o t e s t e d

b y the n-butane c r a c k i n g r e a c t i o n 11,2/, showing an a c t i v i t y i n t e r m e d i a t e between t h a t o f aluminophosphates and z e o l i t e s . we w i l l

I n t h e p r e s e n t work,

compare t h e c a t a l y t i c p r o p e r t i e s o f

t h e p r o t o n i c forms o f SAPO-37 and Y z e o l i t e f o r t h e c r a c k i n g o f n-heptane and o f 2,2,4-trimethylpentane.

C a r r y i n g o u t these two r e a c t i o n s w i I I g i v e

us

SAPO-37’s

a

better

knowledge

characterization

wi I I

of

also

be

made

acid

by

strength.

ammonia

Acid

temperature

strength programed

d e s o r p t i o n (TPD). The

data

presented here

for

Y

zeolite,

and which

i s given f o r

comparison purposes, concerns NaY (LZY-52 f r o m Union Carbide) and a p r o t o n i c form HY.

2. EXPERIMENTAL 2.1.

P r e p a r a t i o n and c h a r a c t e r i z a t i o n of t h e c a t a l y s t s SAPO-37 was synthesized by t h e s i n g l e phase method f r o m a g e l h a v i n g

the

f o I Iowing mo I a r composition:

(TMA)20:50H20

A 12O3:0.9P205

:O. 4Si 02:0.86( TPA)20:0.0250

based on example 43 o f t h e p a t e n t o f Lok e t a I.

is d e s c r i b e d e I sewhere I7 I

.

The m o l a r c o m p o s i t i o n o f SAPO-37

i s about 50% A l ,

I61 , and

sample used i n t h e p r e s e n t s t u d y

40% P and 10% S i as T atoms. The ammonium f o r m o f SAPO-

37 was o b t a i n e d by t h r e e i o n i c exchanges w i t h a 1 M s o l u t i o n o f amnonium n i t r a t e a t room temperature. i

HY was prepared b y NH4

ion

exchange

of

a

NaY

sample

( f r o m Union

Carbide) by t r e a t i n g t h e z e o l i t e w i t h a 2 M s o l u t i o n o f ammonium n i t r a t e 3 times a t 20°C

and 5 times a t 100°C

( l o g z e o l i t e p e r 40 cm3 s o l u t i o n ) ;

367

the degree of ion exchange was 92%. After exchange i t was washed, then dried a t 120OC f o r 8 h , and calcined a t 5OOOC under a low flow of dry a i r t o obtain t h e protonic form. The Si/P81 r a t i o was about 2.6. Samples were characterized by X-ray d i f f r a c t i o n and shown t o present a good level of c r y s t a l l i n i t y . Acidity was characterized by ammonia TPD; the c a t a l y s t s were submitted t o a pretreatement a t 45OoC f o r 12 h under a flow of dry helium (60 m ' l / m i n ) . NH3 adsorption was performed a t 90°C, a f t e r which the temperature was raised from 90°C t o 5OO0C a t a r a t e of 1(IoC/min.

Reaction Conversions of n-heptane and 2,2,4-trimethylpentane were c a r r i e d out in a flow r e a c t o r a t 350oC, a t a tota.1 pressure of 1 bar w i t h a nitrogen-to-hydrocarbon r a t i o equa.1 t o 9 and a WHSY (weight of a.lkane per hour per u n i t weight of zeo.lite) equii'l t o 6.9. Previously, the cata.lysts were pretreated in s i t u a t 450% f o r 12 ti under a flow of dry nitrogen. Since t h e SAPO-37 s t r u c t u r e i s degraded by moisture 15,81, the protonic form was generated from the ammonium one by t h i s i n situ 2.2.

pretreatement.

After

the

cata.lytic t e s t s the c r y s t a l l i n i t y was checked

by Xray d i f f r a c t i o n , and t h e r e were no g1oba.l .losses detected. The

reaction

products

were

separated

and

identified

by

Gas

Chromatography ( G C ) on a 50 rn PLOT c a p i H a r y column coated w i t h alumina deactivated by KC.1. The coke content of the c a t a l y s t s a f t e r 5 h reaction was determined by therrnogravimetric combusti on.

3. RESULTS AND DISCUSSION In Figure 1 we present ammonia TPD data both f o r the protonic form of SAPO-37 and f o r HY. As can be readi ly seen, t h e r e iire s i g n i f i c a n t d i f f e r e n c e s in t h e high temperature region. The protonic form of SAPO-37 c l e a r l y has a lower acid strength than z e o l i t e HY a s evidenced by the absence of NH3 desorption above 40OoC.

368 I n agreement w i t h these r e s u l t s ,

SAPO-37 was found t o be much l e s s

a c t i v e than HY z e o l i t e f o r t h e c r a c k i n g o f n-hepane (Table 1). T h i s r e a c t i o n requests t h e f o r SAPO-37

presence o f s t r o n g a c i d s i t e s .

2,2,4-trimethylpentaneY

of

weaker a c i d t =

Nevertheless,

i s s i g n i f i c a n t l y h i g h e r than t h a t f o r Nay. sites,

a

SAPO-37

reaction that

can

be performed w i t h much

p r e s e n t s a reasonable i n i t i a l a c t i v i t y ( f o i -

5 min), a l t h o u g h q u i t e lower than t h a t observed f o r

90

300

the a c t i v i t y

For t h e c r a c k i n g

T("C)

F i g u r e 1. Thermoprogrammed d e s o r p t i o n

HY.

500

of

ammonia

on

(-)

SAPO-37

and

HY z e o l i t e (---).

Table 1 1 n i t i a . l a c t i v i t i e s o f HSAPO-37,

C 7 ) and 2,2,4-trimethylpentane

SAPO- 37

n-C7 Zy2,4-tmC5

cracking

.

(mol h - l . 9 - l )

(t=5min)

I n i tia.1 a c t i v i t y

~____

HY and NaY a t 350oC i n t h e n-heptane (n-

(2,2,4-tmC5)

HY

NaY

2.3~10-4

8.4~10-3

5.4~10-~

1.4~10-2

3.6~10-~

4. 5x10q4

_~

369 If we compute t h e r e l a t i v e i n i t i a l a c t i v i t i e s ( 2 , 2 , 4 - t r i m e t h y l p e n t a n e

/ n-heptane

cracking va.Iue

with

cracking)

SAPO-37

(60.9)

for

than

both

with

catalysts, (4.3),

HY

we

in

obtain

a

agreement

higher

with

the

e x i s t e n c e o f s t r o n g e r a c i d s i t e s on HY z f d i t e . 0vera.I.I 37

HY:

and

cracking product d i s t r i b u t i o n s are at

the

beginning o f

the

very simi.lar f o r

n-heptane

SAPO-

r e a c t i o n C3+C4 p r o d u c t s

c o n s t i t u t e r e s p e c t i v e l y 95% and 91% o f t h e p r o d u c t s . F o r t h e 2 , 2 , 4 - t r i m e t h y l pentane

reaction,

catalysts.

C4 c o n s t i t u t e

Iso/nCq

c r a c k i n g and

distribution,

cracking

2,2,4-trimethylpentane).

for

significant

A

the

products

r a t i o s p r e s e n t always h i g h v a l u e s (5-6

36-58

t h e c l a s s i c a . 1 carbenium i o n 19,101.

90% o f

6-scission

difference

Thus,

for

both

f o r n-heptane

it i s dear

that

c r a c k i n g mechanism i s d o m i n a t i n g

is

the

symmetry

of

the

product

measured by t h e C4/C3 r a t i o , which i s a b o u t 1.3 f o r HY and

v e r y c l o s e t o u n i t y f o r SAPO-37. ( i n c l u d i n g coke f o r m a t i o n )

T h i s i n d i c a t e s t h a t secondary r e a c t i o n s

do n o t a f f e c t t h e p r i m a r y d i s t r i b u t i o n i s s u e d

b y t h e 6 - s c i s s i o n mechanism.

1,

From T a b l e

with

those

o b t a i n e d w i t h NaY z e o . l i t e . The ' l a t t e r has a n e g . l i g i b . l e a c i d i t y and,

thus,

its

n-heptane

reactions. the

can

cracking

The

cracking

we

higher products

a.lso compdre

activity

activity

of

distribution

SAPO-37

activities

corresponds

practically

SAPO-37

n-heptane

which

for

i s observed,

to

thermal

cracking,

confirms

and

to

the

conc'l u s i o n t h a t t h e SAPO-37 a c t i v i t y corresponds t o c a t a l y t i c c r a c k i n g . and HY d e a c t i v a t e

Both SAPO-37 coke d e p o s i t i o n .

However,

the

coke

very

r a p i d l y as

c o n t e n t o f SAPO-37

a

consequence

of

obtained w i t h

n-

heptane c r a c k i n g i s u n e x p e c t e d l y h i g h ( a p r o x i m a t e l y 15% whi l e f o r HY i t was 13%), d e s p i t e t h e .lower c r a c k i n g a c t i v i t y of SAPO-37. between

cracking

and

coking a c t i v i t i e s

was

a.lso

This discrepancy

observed e a r - l i e r w i t h

p a r t i a l l y exchanged RENaY z e o l i t e 1111. I n fact, sites

we t h i n k t h a t f u r t h e r

i n v o l v e d i n SAPO-37

performance w i t h a Y

i n f o r m a t i o n about the kind o f a c i d

can be o b t i i i n e d i f we

compare

zeo.lite having a s i m i l a r a c t i v i t y .

i t s cata'lytic The

comparison

o f SAPO-37 w i t h a fu1.I HY f o r m i s r a t h e r u n f a i r s i n c e t h e f o r m e r has o i l y mi.Id a c i d s i t e s .

The same '1eve.I o f c a t X I y t i c a c t i v i t y has,

however,

been

a c h i e v e d w i t h PrNaY z e o . l i t e s w i t h a r e l a t i v e l y .low .level o f P r 3 + c a t i o n s Tab'le 2 shows a comparison of

introduced

1101.

n-heptane

cracking

for

SAPO-37

( PrU. 16Na0.52)A'I 02( s i 0212.36.

and

a

Pr3+

t h e main parameters f o r exchanged

NaY

zeo.lite

370

Table 2 Comparison o f HSAPO-37 and PrNaY a t 350% i n n-heptane cracking. Values taken a t 5 min TOS: Cracking a c t i v i t y , C4 t o C3 r a t i o (C4/C3), propane t o propylene r a t i o (C3-/=) and coke c o n t e n t a f t e r 5 hours TOS.

. tl

Acti (mo I vih- g - l )

SAPO-37 PrNaYa

0.23 0.36

c4/c3

c3-/=

I .o

0.4 0.4

1.3

Coke (wt.%)

15 15

aValues taken f r o m r e f . 10. A c t i v i t y computed from d e a c t i v a t i o n parameters. As can be seen f r o m these r e s u . l t s t h e r e a r e g r e a t s i m i l a r i t i e s between these

two

catalysts.

which

shoild

have

comparable amounts o f

protonic

s i t e s . SAPO-37 has about 0.1 H+/T atom, whi.le t h i s PrNaY should have between

0.05 and 0.1 H+/T atom depending on t h e f o r m o f t h e P r 3 + c a t i o n s . A

significant difference,

d i s t r i b u t i o n f o r SAPO-37 f r o m a simple

however,

i s t h e C4/C3 r a t i o :

i s v i r t u a . l . l y symmetrical,

6 - s c i s s i o n mechanism,

the product

as one would expected

w h i l e Y z e o l i t e s u s u a l l y g i v e an

asymmetrical d i s t r i b u t i o n w i t h a C4/C3 r a t i o g r e a t e r t h a n one.

T h i s means

t h a t s i d e r e a c t i o n s o c c u r a t much 'lesser e x t e n t i n SAPO-37 than i n PrNaY. S t r u c t u r e t y p e and pore dimensions a r e s i m i l a r f o r SAPO-37

and Y

zeo.lite. Thus, t h e observed d i f f e r e n c e i n a c i d i t y must on.ly be a consequence of

different

framework

charge

and composition.

For z e o . l i t e s t r u c t u r e s ,

i t i s g e n e r a l l y accepted t h a t t h e a c i d s t r e n g t h o f a s i t e i n c r e a s e s as

t h e number o f d o s e A l neighbors decreases

1151:

t h i s corresponds t o an

i n c r e a s e i n p r o t o n charge w i t h A l c o n t e n t r e d u c t i o n . such an e f f e c t ,

a l l Si(nA.1) c o n f i g u r a t i o n s (n=O-4) Si(4A.l)

Taking i n t o account

t h e above s i t u a t i o n i s favoured on Y z e o l i t e which has

i n c o n t r a s t t o SAPO-37 which o n l y has

s i t e s a s can be seen b y 2 7 A l ,

and 29Si-NMR

141. However,

the

comparison w i t h PrNaY shows t h a t f o r a comparable number o f a c i d s i t e s , b o t h materia.ls behave i n much t h e same way. The c a t a . l y t i c p r o p e r t i e s o f severa.1 SAP0 m a t e r i a l s have been i n s p e c t e d by s e v e r a l o t h e r a u t h o r s ,

11 I ,

xylenes

methylation

isomerization 1131.

f o r t r a n s f o r m a t i o n s such as n-butane c r a c k i n g 1121,

Most o f them,

propy-lene o l i g o m e r i z a t i o n and and s p e c i f i c a l l y SAPO-37,

a c i d c h a r a c t e r , s i m i . l a r t o t h e one r e v e a l e d i n t h i s study.

toluene

presented m i l d

371

A c i d i c c n a r a c t e r i s t i c s o f these matelria I s are, however, much dependent on

composition,

catalyst

as

shown

containing

in

recent

species

Pt

was

work

1141.

compared

A

to

bifunctional for

Pt-HY

SAPO-37

the

decane

conversion. The r e s u l t s g e n e r a l l y showed a c a t a l y t i c a c t i v i t y f o r HY much higher

than

that

SAPO-37.

of

However,

with

materials.,

has

an

enriched

Si

SAPO-37,

a c t i v i t i e s became compa r a b I e.

4. CONCLUSION SAPO-37,

as

other

SAPO

t o some forms o f Y z e o l i t e s .

Nevertheless,

acidic

p r o p e r t i e s simi l a r

t h e usual s y n t h e s i s does n o t

produce a m a t e r i a l h a v i n g t h e s t r o n g e,cid s i t e s r e q u i r e d f o r demanding r e a c t i o n s , namely n-heptane c r a c k i n g . These s i t e s a r e found i n HY, SAPO-37's i s o s t r u c t u r a l analogue.

5 . ACKNOWLEDGEMENTS T h i s work was p a r t i a ' l ' l y supported by Junta Nacional de I n v e s t i g a c a o C i e n t i f i c a e Tecnol6gica under r e s e a r c h c o n t r a c t no. 856.86.160.

The SAPO-

37 m a t e r i a l was prepared by Mrs. N. Dumont and L. M a i s t r i a u a t t h e Facu.lte's Uni v e r s i t a i r e s de Namur.

6. REFERENCES 1 B.M.

Lok,

C.A.

Messina,

R.L.

Flanigen, J. Am. Chern. SOC.,

2 t.M. E.F.

F l a n i g e n , R.L.

P a t t o n and S.T. Schultz-Ekloff

Vansant and G.

Science,

Studies

Patton,

R.T.

Gajek,

T.R.

Cannan and E.M.

106 (1984) 6092-6093. Wilson, i n P.J. (Eds),

Grobet, W.J.

Mortier,

Innovation i n Zeolite Materials

in Surface Science and C a t a l y s i s No.

37,

Elsevier,

i n Y.

Murakami,

Amsterdam, 1988, pp. 13-27. 3 L.M. A.

Flanigen, Lijima,

Technology, Amsterdam,

J.W.

B.M.

Lok,

Ward

R.L.

(Eds),

P a t t o n and S.T. New Developments

Wilson,

i n Zeolite

S t u d i e s i n Surface Science and C a t a l y s i s No. 1986, pp. 103-112.

Science and 28,

Elsevier,

312

4

L.S.

Saldarriaga,

S a l d a r r i a g a and M.k.

C.

Davis,

J.

Chem.

Am.

SOC.,

109 (1987) 2686-2691. 5 N. Dumont, T. I t o and E.G.

Derouane, Appl. Catal.,

6

R.L.

Lok,

B.M.

Messina,

C.A.

Patton,

Flanigen, U.S.

P a t e n t 4 440 871 (1984).

7

L.

N.

8

M. Briend, A. Shikholeslarni, M.J.

Maistriau,

Dumont,

J.B.

R.T.

Nagy,

Z.

54 (1989) Ll-L6.

Gajek,

T.R.

Cannan,

Gabelica and E.G.

E.M.

Derouane,

Zeo lit e s , 10 (1990) 243-250. J . Chem.

9

SOC. D a l t o n Trans.,

(1989) 1361-1362.

B.W.

Wojciechowski

Inc.,

New York, 1986, ch. 5, pp. 127-194.

10 F.

Lemos,

and A.

P e l t r e , D. Delafosse and D. Barthomeuf,

Corma.

Lopes and F.

J.M.

C a t a l y t i c Cracking,

Ramda R i b e i r o ,

Marcel Dekker,

J. Mol. Catal.,

53 (1989)

265- 273. 11 F. Lemos, Ph. D. Thesis, Univ. Tec. Lisboa, 1989.

12 D.R.

Pyke,

Whitney and H.

P.

Houghton,

Appl.

Catal.,

18 (1985) 173-

190. 13 R.J. J.W.

Pel l e t ,

Long and J.A.

G.N.

Ward (Eds),

Rabo,

i n Y.

Murakami,

L i j i m a and

A.

New Developments i n Z e o l i t e s Science and Technology,

S t u d i e s i n Surface

Science and C a t a l y s i s No.

28,

Elsevier,

Amsterdam,

1986, pp. 843-849. 14 J.A.

Martens,

i n P.A.

C.

Janssens,

Jacobs, R.A.

S t u d i e s i n Surface

P.J.

Grobet,

H.K.

Beyer and P.A.

Jacobs,

van Santen (Eds), Z e o l i t e s : f a c t s , Figures, f u t u r e , Science and C a t a l y s i s No.

49,

E l s e v i e r , Amsterdam,

1989, pp. 215-225. 15 U.

Barthorneuf,

i n f.

C.

Naccache (Eds),

E,

80,

Ramda R i b e i r o , A.E. Zeolites:

Science and Technology,

Martinus N i j h o f f Publishers,

pp. 317-345.

Rodrigues,

The Hague,

L.D.

Rollmann and

NATO AS1 SERIES

Boston,

London, 1984,

P.A. Jacobs e t al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

373

Cracking of light alkanes over MeAPO-5 molecular sieves J. Meusinger', H. Vinelt, G. Dworeckow', M. Goepperb and J.A. Lercher' Institut f i r Physikalische Chemie und Christian Doppler Laboratorium fir Heterogene Katalyse, Technische Universitat Wien, Getreidemarlct 9, A-1060 Vienna, Austria

a

mole Nationale Supeneure de Chimie de Mulhouse, 3, Rue Alfred Werner, 68093 Mulhouse, France Abstract The catalytic activity and selectivity of SAPOS, MgAPO5, CoAPO5 and ZnAPOS for cracking and dehydrogenation of n-butane was investigated. At 773 K the turnover frequencies of cracking according to the monomolecular pathway are constant for all SAP05 samples (1.21 +/-0.17*10" molec./p+].s). The turnover frequencies for MgAPOS and CoAPOS were considerably higher (2.96and 3.2*105 molec./~+].s).ZnAPOS did not show appreciably cracking activity along the monomolecular pathway. The higher turnover frequencies of MgAPO5 and CoAPOS are not due to a higher strength of the acid sites but should rather be caused by lateral interactions of n-butane close to the accessible metal cation.

INTRODUCTION The changes of the acid - base properties of metal substituted aluminophosphate based molecular sieves (MeAPO) as function of the chemical composition and the crystal structure are proposed to be complicated and to be substantially different compared to zeolites (1,2). Three mechanisms for the incorporation of Si or other metals (Me) into ALP05 frameworks have been proposed: Substitution of A1 by Si (Me), substitution of phosphorus by Si (Me) and the simultaneous (formal) substitution of A1 and P with Si or other tetravalent cations. The consequences of these substitution depend upon the ionic charge of the two partners in the substitution. If Si is substituting phosphorus strong Bronsted acid sites should be produced, if it is substituting aluminum strong basic sites should be produced and if two silicon are substituting aluminum and phosphorus the neutral charge of the framework should not change. Jacobs et al. (3) demonstrated that large domaines of silica lattices can be incorporated in this way. High concentrations of either one of the components (Si, P or Al) may lead to extraneous material partially blocking the molecular sieve pores (23. Because the concentration of tetrahedrally coordinated metal cations of a kind is frequently not constant throughout the crystal, it might be difficult to rationalize the catalytic activity or the acid - base properties as function of the overall lattice charge or the overall composition of the material (3). This complicated situation is also reflected in widely varying values for the n-butane cracking rate constants for even a material of one given kind, e.g.

374

for SAP05 (1,6). Note that no apparent correlation between the chemical composition, the intensity of OH bands and other indications were used to assess the acid strength and the concentration of acid sites for this molecularsieves (7). In contrast, Halik et al. (5) showed that the specific rates for cracking light n-alkanes (number of cracked molecules per second and strong acid site) is approximately the same for SAP05 samples synthesized with one template. Recently Jacobs et al. (3) reported identical rates of hydrocracking of n-decane on four Pt-loaded SAPO5-samples. In order to probe these differences further, we investigated a series of SAP05 molecular sieves synthesized with different templates and samples of the same structure but containing: Mg (MgAPOS), Co (CoAPO5) and Zn (ZnAPOS). The conversion of n-butane was used as test reaction and t.p.d. of pyridine to assess the strength and the concentration of acid sites.

EXPERIMENTAL Temperature-programmed desorption (t.p.d.) T.p.d. was carried out in vacuum (p = lo6 mbar) using a temperature increment of 10 Wmin. The sample was calcined in siru at 873 K for 1 hour, cooled to 293 K and contacted with 5 mbar pyridine for 30 min. Subsequently, the system was evacuated at 433 K, 493 K and 553 K, in order to determineweakly, moderately and strongly chemisorbed pyridine (4). Cracking of n-butane The conversion of n-butane was studied in continuous flow mode. 50 to 110 mg molecularsieve powder were mixed with different amounts of washed and calcined quartz (MERCK) to achieve the same sample volume for all experiments. For activation, the temperature was increased with an increment of 10 Wmin up to 873 K in He flow (10 mllmin). After 30 min. at 873 K He was replaced with air for further 30 min. to remove any carbonaceuous residues. The rates of reaction of 2 mol% n-butane in He were measured between 733 and 833 K in intervalls of 20 K. The conversion was kept below 5 mol % . The absence of thermal cracking of n-butane was confirmed with blank experiments. Material The synthesis procedures followed the description given in ref. (8). The removal of various templates from the molecular sieves was achieved by calcination in air at 873 K for 1 h. After calcination, the only crystalline phase detected in all samples by XRD was SAP05 (MeAPO5). The size of the crystals was determinded by scanning electron microscopy. The chemical composition was determined by electron microprobe. analysis. The chemical composition, the templates used for synthesis and the crystal size are listed in Table 1.

RESULTS Temperature programmed desorption (t.p.d.) of pyridine The variations of the rates of desorption of pyridine from SAPO5-1 and SAPO5-3 during t.p.d. can be seen in Figs. la and lb. For all samples, but ZnAP05, three maxima in the rate of desorption were observed and attributed to desorption from weak, moderate and strong Br6nsted acid sites. We have no indication that Lewis acid sites did contribute significantly to the strongest of these sites. The concentration of the acid sites and the upper limit of the concentrations of strong Brijnsted acid sites expected from the bulk chemical composition are compiled in Table 2. It should be noted that the values found experimental-

375

ly were considerably lower than those estimated from the overall composition and that the low values indicate isolated strong Bronsted acid sites.

Table 1 Chemical composition, template used and crystal size of MeAPO-5 samples Sample

Template

composition

Crystal size

SixA$P,Oz

Si (Me)

A1

P

(pm)

SAPOS-1

TEA

0.12

0.50

0.38

4

SAP05-2

DEOLA

0.10

0.50

0.40

7

SAPO5-3

DEOLA

0.33

0.42

0.25

3

SAP05-4

TEOLA

0.32

0.41

0.27

2

MgAPO5

TEA

0.02

0.48

0.50

9

CoAPO5

TEA

0.04

0.46

0.50

5

ZnAPO5

TEA

0.06

0.44

0.50

TEA DEOLA TEOLA

15

tetraethylamine diethanolamine triethanolamine

Table 2 Concentration of acid sites determined by t.p.d. of pyridine per 100 Si,A$P,O, Sample

acid

sites

T

09'

strong

moderate

weak

expected

SAPOS-1

2.42

0.48

0.56

12

733

SAP05-2

1.OO

0.25

0.06

10

703

SAP05-3

0.93

0.38

0.53

17

703

SAP05-4

1.30

0.18

0.29

14

703

MgAPOS

0.59

n.d.

n.d.

2

643

CoAPOS

0.69

n.d.

n.d.

2

723

ZnAPOS

0.00

n.d.

n.d.

6

1

n.d.

__-

Temperature of the maximum of the rate of desorption of pyridine from strong acid sites not determinded

376

Cracking of n-butane The product distribution, the rates of total conversion of n- butane, the rates for monomolecular cracking and for dehydrogenation as well as the turnover frequency (TOF) for cracking are compiled in Table 3. For SAP05-3 the selectivity as a function of the reaction temperature is shown in Fig. 2, respectively. For the SAP05 samples the TOF for cracking was approximately constant, irrespectively of the concentration of the Briinsted acid sites. Note that the rates based on the sample mass varied considerably which corresponds to the variations reported in the literature (1,6). The apparent energies of activation varied between 138 and 150 kT.mo1'. With the exception of SAPOS- 1 the rates of dehydrogenation were significantly lower than those of cracking indicating a minor importance of this reaction pathway at 773 K. The catalytic activity of the MeAPOS samples varied strongly as a function of the metal cation. The samples with Mg and Co exhibited rates of cracking similar to those observed with the SAP05 samples while ZnAPOS was virtually inactive for cracking. The rates for dehydrogenation varied between 0.3*10-'and l.2*lO" rnol.g-'.s-'. ZnAPO5 showed primarily activity for dehydrogenation under our experimental conditions.

Table 3 Selectivity (mol %), rates (mol.g-'.s-') and turnover frequencies (molecules . Irr+]"s-') for reactions of n-butane at 773 K SAPO5-1

SAPO5-2

SApO5-3

SAP054 MgAPOS

COAPOS

W

0

5

methane

19.0

22.8

19.0

19.1

23.2

17.7

0.0

ethane

11.5

13.7

10.2

11.5

10.7

5.5

0.0

ethene

24.6

34.6

36.1

36.9

25.9

35.4

17.5

0.0

0.0

propane

0.0

0.0

1.7

0.0

0.0

propene

24.6

26.8

30.7

29.9

26.3

39.0

47.1

butene

16.6

2.1

2.2

2.5

13.8

2.4

35.4

i-butane

3.9

0.0

0.0

0.0

0.0

0.0

0.0

rate (total)' *lo9

8.03

2.73

3.20

3.92

4.9

8.52

1.40

rate (cracking)' *lo9

5.49

1.94

2.03

2.25

2.89

3.47

0.00

rate (dehydr.)' *lo9

3.19

0.11

0.09

0.18

1.17

0.35

1.01

TOF (~racking)~ *los

1.38

1.16

1.30

1.04

2.96

3.2

0.00

' '

,

rate (cracking, total) = 114 (rc + 2rc + 3rc ,) rate (cracking, monomolecular) = r (methane) + r (ethane) rate (dehydr.) = r @utene) rate (cracking, monomolecular) normalized for the concentration of strong Brcinsted acid sites

317

DXSCUSSION The strength and the concentration of acid sites T.p.d. of pyridine indicates that the acid sites of the samples investigated could be classified into weak, moderate and strong on the basis of the maximum of the rate of desorption. Each of these desorption maxima corresponds to OH groups as sites for the adsorption of pyridine. From separate i.r. measurements we have no indication that large a Concentration of pyridine are desorbing from metal cations (Lewis acid sites). In the light of the large difference between the concentration of sites measured and that expected as the maximum value (Table 2), we conclude that relatively large domaines of pure silica structure should exist in our samples. This is confirmed by the relatively low intensity of the SiOH and POH bands in comparison with the intensity of the bands of SiOHAl groups and is in agreement to the literature (3,9). If we accept that an upper limit of Si incorporation in the SAP05 phase exists (3) the silica rich crystalline phase (or amphorphous phase) must contain alumina and thus should exhibit Bronsted acidity. It is interesting to note that neither the t.p.d. of pyridine nor the cracking of n-butane indicates a significant contribution of these sites to the acidity and the catalytic activity, respectively.

Fig. 1 Rates of desorption of pyridine during t.p.d. from SAPOS-1 (a) and SAP05-3 (b) Although it should only be used with great caution (lo), the similar temperature of the maximum attributed to desorption of pyridine from strong Bronsted acid sites indicates a similar strength of sites for all samples (see table 2). Note that this agrees very well with our previous conclusions (4,5). The subtly higher temperature of this maximum found with SAPOS-1 in comparison with the other SAP05 samples is concluded to be caused by the higher concentration of acid sites in the former sample. With HZSMS, the variation in the concentration of the acid sites without the change of the heat of adsorption was demonstrated to give a similar effect (11). While CoAPOS showed a maximum in the rate of desorption at a temperature of maximum (723 K) close to the values found for SAPOS, MgAPO5 exhibited the maximum at considerably lower temperatures (643 K) indicating somewhat weaker acid strength. It is not clear at present whether the absence of strong Bronsted acid

378

sites is an intrinsic property of ZnAP05 or if our sample did not contain any Zn2+in the zeolite lattice. It should be emphasized, however, that the sample did not show any peak due to the possible desorption of pyridine from accessible Znz+ cations which was found with ZnO at approximately 783 K. Based on this indirect evidence we suggest that amorphous impurities containing Zn” are not important in the sample studied.

Cracking of n-butane Two possibilities exist to crack n-butane via an ionic intermediate: (i) via the formation of a carbonium ion or (ii) via the formation of a carbenium ion. According to Haag et al. (12) cracking via the carboniumion is a monomolecular reaction. The proton is added to a saturated hydrocarbon and the carbon - carbon bond adjacent to the carbon atom bearing the positive charge is broken. In this case for each molecule butane cracked one molecule of methane and propene or of ethane and ethene is formed. In addition, to the cleavage of the carbon - carbon bond, the cleavage of two C-H bonds (dehydrogenation) might be possible. In the case of the route via the carbenium ion a hydride ion is abstracted from the saturated molecule, either by the surface or by an adsorbed carbenium ion (hydride transfer). The carbenium ions usually cleave the C-C bond next nearest to the carbon atom bearing the positive charge (B-rule). This is not to likely for n-butane, because the mechanism requires either a primary carbonium ion or a methyl carbenium ion in the reaction pathway. Furthermore, it was pointed out that a low partial pressure of the hydrocarbon, low concentrations of acid sites, high temperatures and narrow zeolite pores favor the monomolecular pathway. Except for the pore size all other parameters are adjusted to favor primarily cracking via the carbonium ion route.

Selectivity [mol%]

40 35 30

*

25

% Methane % Ethane

.x % Ethene

20

0 % Propane 15

.X % Propene

10

\

f % Butene

0 5

730

740

750

760

770 780 790 800

810 820

Temperature [Kl

Fig. 2 Product selectivity for reactions of n- butane over SAP053

379

The selectivity of the conversion of n-butane over MeAPOS materials differs from the product distributions found with e.g. HZSMS. As it can be seen in Fig. 2, at reaction temperatures around 730 K the reaction products are dominated by unsaturated hydrocarbons in excess to the cracking products formed. Only at reaction temperatures as high as 800 K, the product distribution is that expected for the monomolecular pathway. At 730 K more than 50 % of ethene and propene formed are concluded to be produced via the monomolecular pathway of cracking. At present, it is impossible for us, however, to asses unequivocally the reaction pathway by which these unsaturated hydrocarbons were formed. We would only like to point out that in general the apparent energies of activation for cracking (138 - 150 ldlmol) were higher than for dehydrogenation (45 - 90 kl/mol) indicating a higher energy barrier for the cleavage of the carbon - carbon bond than for the carbon - hydrogen bonds. Furthermore, the apparent energies of activation for formation of methane and ethane were similar, that of ethane being slightly higher. Because this suggests similar rate determining steps for the formation of both products and because ethane can only be formed from butane via monomolecular (protolytic) cracking we conclude that the monomolecular pathway dominates. The rates of cracking of n-butane showed a direct proportionality to the concentration of strong Bronsted acid sites of the SAP05 samples. Thus, the catalytic activity per strong Bronsted acid site and hence the acid strength of these sites are identical for all of the SAP05 molecular sieves investigated. This is in good agreement with the conclusions drawn by Halik et al. (5) and by Jacobs et al. (3), but we can now extend this for several other templates and site concentrations. Thus, the preparation with different templates leads to samples of the same or very similar intensive acid - base properties. All correlations of this and other studies (13,14,15) indicate that only Br6nsted acid sites contribute to the catalytic activity. Because it was proposed earlier that an upper limit of approximately 6 mol% of Si incorporation in SAP05 exists, the question arises whether we probe only the acidity of the SAP05 phase or also and indifferentiable that of a (crystalline or amorphous) silica - alumina phase. Neither the i.r. spectra nor the t.p.d. of bases gives indication of acid sites of appreciable strength in the silica - alumina phase. Thus, also the catalytic activity is supposed to be low. This is certainly in part due to the presence of phosphorus acid during the preparation which has shown to decrease the acid strength of high silica zeolites remarkably (16,17,18). The MeAPOS samples that exhibited strong Br6nsted acid sites had a higher turnover frequency than that of any of the SAP05 samples. We conclude that this is not due to a higher acid strength of the hydroxyl group, Neither the calculation of the partial charge at the proton according to Sanderson (19) nor the position of the t.p.d. maxima suggest acid strengths higher than those observed with SAP05 samples. Therefore we are inclined to speculate that the presence of larger metal cations tends to modify the environment around the Bronsted acid site, i.e. primarily the charge at the oxygens. This should increase the strength of interaction of the hydrocarbon with the zeolite by lateral interactions (20) which migth compensate the lower density of strong acid (in comparison to the SAP05 samples) and have a positiv influence upon the reaction rate by increasing the transition state entropy. In order not to overemphasize this effect, it should be noted that these turnover frequencies (2.96 and 3.2*10-’ molec./[H+]/s)are still significantly lower than that found with HZSM5 3*104 molec./[H+]/s. In contrast to MgAPO5 and CoAPO5, ZnAPOS was not active for monomolecular

380

cracking of n-butane. It should be emphasized that we did not observe strong Bronsted acid sites with this sample. Thus, the absence of cracking confirms quantitatively the direct correlation of cracking with the presence of the strong Bronsted acid sites. It also shows that the presence of (more accessible) metal cations alone does not suffice for cracking of hydrocarbons and that the proton of the strong Bronsted acid site is indispensable. As the rate of dehydrogenation (the second highest of all samples investigated) was not affected by the lack of Bronsted acid sites, we would like to speculate that dehydrogenation uses at least in part different catalytically active sites than cracking.

ACKNOWLEDGEMENT The supply of SAP05 samples by Dr. L.Puppe, Bayer AG and the financial support of the Christian Doppler Society are gratefully acknowledged.

REFERENCES 1

2 3 4

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

E.M. Flanigen, B.M. Lok, R.L. Patton, R.T. Gajek and S.T. Wilson, Stud. Surf. Sci. Catal., 28 (1986) 103. E.M. Flanigen, R.L. Patton and St-T. Wilson, Stud. Surf. Sci. Catal., 37 (1987) 13. J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Catal., 126 (1990) 299. C. Halik and J.A. Lercher, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 4457. C. Halik, S.N. Chaudhuri and J.A. Lercher, J. Chem. Soc., Faraday Trans. 1, 85 (1989) 3879. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 106 (1984) 6092. X. Quinhua, Y. Aizhen, B. Shulin and X. Kaijun, Stud. Surf. Sci. Catal., 28 (1986) 835. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, US Patent No. 4 440 871 (1984). N.J. Tapp, N.B. Milestone and D.M. Bibby, Stud. Surf. Sci. Caral., 37 (1987) 393. R.J. Gorte, J. Catal. 75 (1982) 164. G.I. Kapustin, T.R. Brueva, A.L. Klyachko, S. Beran and B. Wichterlova, Appl. Cat. 42 (1988) 239. W.O. Haag and R.M. Dessau, Proc. 8th Int. Congr. CataI. (Verlag Chemie Weinheim, 1984), 2 (1984) 305. R.B. Borade, S.G. Hegde, S.B. Kulkarni andP. Ratnasamy, Appl. Cat., 13 (1984) 27. D.H. Olsen, W.O. Haag and R.M. Lago, J. Catal., 61 (1980) 390. J.G. Post and J.H.C. van Hooff, Zeolites, 4 (1984) 9. H. Vinek, G. Rumplmayr and J.A. Lercher, J. Catal., 115 (1989) 291. J.A. Lercher and G. Rumplmayr, Appl. Cat., 25 (1986) 215. A. Jentys, G. Rumplmayr and J.A. Lercher, Appl. Cat., 53 (1989) 299. R.T. Sanderson, Chemical bonds and bond energy, Academic press, New York, 1976. A. Jentys, G. Mirth, J. Schwank and J.A. Lercher, Stud. Surf. Sci. Cat., 49 (1989) 847.

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier SciencePublishersB.V., Amsterdam

381

PROMOTING EFFECT OF Pi. SUPPORTED ON GALLIUMSILICATE IN n-C4HI0 AROMATIZATION

R. V. Dmitriev, D. P. Schevchenko, E. S. Shpiro, A. A. Dergachev, 0. P. Tkachenko and Kh. M. Minachev

N. D. Zelinsky Institute of Organic Chemistry of USSR Academy of Sciences, MOSCOW B-334, USSR Abstract

Catalytic properties of both Ga-silicate and Pt/Ga-silicate in n-CaHio aromatization have been investigated in a wide range of reaction conditions (at temperatures of 573-773 K, space velocities of 300-10000 h-l, total pressures of 0.1-0.5MPa, partial pressures of butane and hydrogen of 5-100 kPa and 5-400 kPa, respectively. The introduction of platinum into the Ga-silicate was shown to result in dramatic effects on butane aromatization: a decrease of the reaction temperature, an increase in the reaction rate and a drastic change in the product distribution. Platinum was found to accelerate both the initial paraffin and alicyclic intermediate dehydrogenation and suppressed the cracking process. 1. INTRODUCTION

The A1 isomorphous substitution by Ga in zeolites with ZSM-5 structure permits the preparation of crystalline galliurnsilicates with Ga in the framework tetrahedral positions [ l l . Such material exhibits rather higli catalytic activity and selectivity for lower alkane aromatization [ 2 , 3 1 The activity of galliumsilicate was related to the Broensted acidity produced by the Ga framework ions as well as to Lewis acidity which results from Ga extraframework species [41. Recently [5,61, a strong platinurn promoting effect on the aromatization activity of Ga-silicate has been found. The main platinum functions were proposed to be the enhancement of the initial paraffin dehydrogenation activity, decrease of coking and facilitating of the regeneration process due to the effects of both hydrogen and oxygen spillover [71. To elucidate the Pt promoting effect in more detail we have investigated various catalytic features of n-butane aromatization with Ga- and Pt/Ga-silicates. XPS and TEM characterization of the catalysts studied was also performed. 2. EXPERIMENTAL

The starting sample was Ga-silicate with ZSM-5 structure, SiO2/Ga2Os=GO, pretreated in a Nz stream at 823 K. According to MASNMR 59Ga most of the Ga is in tetrahedral coordination in the zeolite framework [41. The 0.5%

Pt/Ga-silicate was prepared by impregnation of the starting material with [Pt(NH3)41C12 solution. The catalysts were treated in the air at 723 K and then in hydrogen at 773 K, the heating rate being 1 K/min. The n-butane aromatization was performed, using an automatic flow unit, in a quartz microreactor (the charge was 0.5g). The reaction conditions were varied in a wide range: temperatures of 573-773 K, space velocities of 300-10000 h-l, total pressure of 0.1-0.5 MPa, butane partial pressure in He stream of 0.5-100 kPa. Some runs were made in hydrogen atmosphere, the hydrogen partial pressure was varied from 5 to 400 kPa. To avoid the flow gradient caused by butane condensation the feed was injected as a liquid at 1.5 MPa into a mixer heated to 673 K. The use of separate flow lines f o r catalyst activation and for the feedstock make it possible to measure the conversion and product distribution immediately after feedstock injection. The probes for analysis were taken automatically every 3-5 min. Normally, stable activity was reached after 5-10 min and remained practically constant over 4 hours. The Ga-silicates and Pt/Ga-silicates were investigated by XPS according to the procedure described in [&I. Pt dispersion in the reduced samples was determined by TEM [91. 3. RESULT

AND DISCUSSION

1. Catalyst Characterization

The Table listed XPS data obtained for the initial samples and after their calcination and catalytic reaction followed by regeneration. I t should be noted that, in contrast to Ga impregnated ZSM-5 [ S ] , n o substantial surface enrichment with Ga was observed after redox treatment. Thus if even the Ga is released from the framework during calcination and/or catalytic reaction [41, it is located mainly inside the zeolite channels. Pt has no significant effect on the Ga distribution. Platinum reduction in the Pt/Ga-silicate occurred during calcination which was a l s o observed with Pt/HZSM-5 [91. The amount of Pto remained the same after the reaction and regeneration but it seems to be higher in the prereduced sample, which w a s confirmed by TEM data. They indicate three types of Pt particles in the Ga-silicate: large crystals with sizes exceeding 100 A (minor fraction). hemispherical particles of 20-50 A in diameter (major fraction) located on the surface and very dispersed clusters less than 10 A i n size located inside the zeolite channels. 2. Catalytic Properties 2.1.

Temperature dependence

Fig.1 shows the butane conversion and selectivity to aromatics in dependence of the reaction temperature for both Ga- and Pt/Ga-silicates. Pt introduction substantially increased Ga-silicate activity. This is manifested as a tctal conversion increase and dramatic enhancement of the aromatics selectivity. These effects are much more pronounced at lower reaction temperatures. When Pt is included the minimal temperature of aromatics formation drops by 50-70 K with respect to Ga-silicate. separate hydrocarbon selectivities The temperature dependences of demonstrated additional differences between the PtKa-silicate and the Ga-silicate (Fig.21, which are again more distinct in the low temperature

383 region. Under these conditions cracking and isomerization are the main reactions with Ga-silicate while Pt/Ga-silicate yields more olefins. T h e Table

XPS spectra parameters of Ga- and Pt/Ga-silicates

B.E. , eV

Samples

Atomic ratios

SL___-

si_ _ _ _

?t

-

40

40

-

21.5

-

-

40

-

1118.7

21.6

-

53

48

-

103.7

1118.5

21.1

72.9

50

50

-

103.7

1118.4

21.2

71.7a;73.0

59

-

0.003

103.8

1118.5

21.5

71.5a;73.0

53

59

0.001

(treatment )

Si 2p

Ga 2p

Ga 3d

Ga-silicate (initial) Ga-silicate (talc. Ga-silicate (reac.+regen.) Pt/Ga-silicate (initial) Pt/Ga-silicate (calc.) Pt/Ga-silicate (reac.+regen.1

103.7

1118.6

21.4

103.6

1118.5

103.6

Pt 4f

Ga(2p) Ga(3dl Si

"1 Pto fraction is equal to 45%

Conversion,

Selectivity to aromatics,

"I 80

573

673

773

T, K

Fig.1 Temperature dependence of n-C.rHio conversion ( 0 ) and selectivity to aromatics ( 0 ) for Pt/Ga-silicate (solid line) and Ga-silicate ( b r o k e n line). WHSV = 2000 h-’

maximum olefin selectivity on Pt/Ga-silicate was observed at the lowest temperature studied (573 K) while, on the Ga-silicate, the maximum is shifted to 673 K. The aromatic products on the Pt/Ga-silicate are enriched

384

in xylene (50%) especially at low temperature whereas on the Ga-silicate, the relative xylene content does not exceed 25% and remains constant in the whole temperature range. 2.2. Contact t i me dependence Fig.3 presents the dependece of the total conversion and aromatics yield on the relative contact time for several temperatures. Conversions on the Ga-silicate comparable with the Pt/Ga-silicate can be obtained at contact

Selectivity,

Selectivity, ( a )

wt.

(b)

wt. %

40

:

o z20

20

1

0 573

673

773

Temperature. K Fig.2 Temperature dependence of different hydrocarbon selectivities for Pt/Ga-silicate (a) and Ga-silicate (b) : 0 - aromaticsi 0 - C4H8, a - C3H6, x - C4Hio, A - ZCi-C3 WHSV = 2000 h-

times that are an order of magnitude longer than on the Pt/Ga-silicate. Fig.4 shows the dependence of the different hydrocarbons yield on contact time for two catalysts at 673 K. C3-C4 paraffins yield increased linearly with contact time and aromatics content rose also gradually. C3-C4 olefins yield rapidly reached steady values with Ga-silicate while a distinct maximum in their concentration was observed with the Pt/Ga-silicate. The analysis of these trends confirmed suggestions made in the literature [ l o ] that aromatization process involves several parallel and consecutive stages of both initial paraffin and intermediates transformations, which can depicted as the following scheme: 1

n-C4Hio

n-CaH8 Ca-Cs-olefins Ci-Cs-paraffins -c

I

13

385

To elucidate the Pt promoting effect in the Pt/Ga-silicate we have comparethe reaction product distribution at similar conversions f o r Ga-silicate and Pt/Ga-silicate. Comparable conversions have been obtained at the same temperatures by varying contact time (see the diagram on Fig.5). Again at lower temperature we can see prominent differences between the two catalysts. The following main conclusions have been drawn from the analysis of these features: (i) higher ( 2 times) butene fraction over Pt/Ga-silicate than on Ga-si licate; (ii) lower fraction of C I - C ~ cracking products over Pt/Ga-silicate; Conversion, ( a ) %

Aromatic yield, (b)

wt.%

II

I 40

-. -

_- --

---I'

20

0

4

8

12

4

16

8

12

16

Contact time (relative unit) Fig.3 Contact time dependence of n-CaHio conversion f a ) and yield of aromatics ( b ) for Pt/Ga-silicate (solid line) and Ga-silicate (broken line) at several temperatures: 623 K - A , 673 K - 0 , 773 K - 0

(a)

Yield, wt.%

Yield, wt.%

I

I

I

I

(b)

1

./ n

"C 4

8

-

t

12

16

l k E 3 .

U ( , P

0

4

1

I

8

12

16

Contact time (relati\re unit) Fig.4 Contact time dependece of different hydrocarbons yield Pt/Ga-silicate ( a ) and Ga-silicate ( b ) at 673 K: 0 - aromatics, 0 - C4H8, a - C3H6, X - i-C4H10, A - Cl-c3.

for

386 (iii) similar total Cs-yields but higher C3Hc/C3H8 ratio for Pt/Ga-silicate at lower temperatures. The CS/C3 ratio on this catalyst is equal to 1 : l at all temperatures while on Ga-silicate it varied from 1:5 (623 K) to 1: 1.5 (773 K); (iv) higher yields of Cs-hydrocarbons over Pt/Ga-silicate. At 623 K the Cs-yield reached 14-1677 while f o r Ga-silicate it did not exceed 3-4%; (v) the aromatic products on Pt/Ga-silicate are enriched in Cs-aromatics;

2.3.

Hydrogen Effect

The product distribution determined at equal conversions in dependence of the hydrogen partial pressure demonstrates significant difference between the two catalysts (Fig.6). When partial pressure of hydrogen varied from 0 to 0.4 MPa, the C3/C5 ratio on Pt/Ga-silicate changes more than 30 times while this ratio on Ga-silicate decreases by only 2-2.5 times. The butene fraction also decreased more strongly over the Pt/Ga-silicate. Aroma: i c e C

623K (1)

Conv.=32.1%

(2) Conv.=27.3% 673K Conv.=56.9% (2) Conv.=50.0% Fig.5 Product distribution (C-wt%) for Pt/Ga-silicate ( 1 ) and Ga-silicate (2) at equal conversion of n-CeHio Nevertheless, despite the strong hydrogenation platinum activity, the butene fraction remained higher over the Pt/Ga-silicate. The above data indicate that at a low hydrogen partial pressure and rather high butane yield aromatization is completely suppressed. This evidenced that, first of all, hydrogen influenced intermediate chemical transformations rather than butene formation. The stronger platinum hydrogenation activity found in these experiments is likely to facilitate coke precursor hydrogenation in real reaction mixtures, where the hydrogen partial pressure is rather low. Based on the data obtained and the described trends, we could consider a plausible mechanism of platinum promoting action in Pt/Ga-silicate. Inui [71 ascribed to platinum the role of strong dehydrogenation agent for starting paraffins which, in turn, increases the intermediate concentration. The higher butene yield obtained with Pt/Ga-silicate confirmed this suggestion. But our data clearly shown that platinum is

387

likely to accelerate another important step in aromatization - alicyclic hydrocarbon dehydrogenation. This follows from the aromatics distribution and propane/propene ratio for two catalysts. Since the main dirner product in butane aromatization should be CsHi6 the enrichment of aromatics with Cs-hydrocarbons indicates greater Pt activity in the alicyclic hydrocarbon dehydrogenation:

On galliumsilicate, particularly at lower temperatures, aromatics a r e formed via hydrogen transfer:

(y+3c=c-c - 3c-c-c

+

@f

This explains why, at equal conversions and propane concentrations, the propene fraction on Ga-silicate is much lower than on Pt/Ga-silicate. This route of arene formation is catalyzed by acidic centers and it competes with cracking. At higher temperatures, gallium became more active in alicyclic hydrocarbon dehydrogenation [7,81 and selectivities for the both catalysts became similar. 679K

pH L

Pt/Ga-silicate

Conv . 36 - 7%

Ga-silicate

0

.o

Conv . 34.2%

-4

26.6%

C

I

iC H 4

0

Arom.

'

10

50

100

0

50

Fig.6 Products distribution [wt%) in dependence of pressure at equal conversions.

100 partial hydrogen

The difference in Cs-hydrocarbon concentrations observed for Pt/Ga-silicate and Ga-silicate can be related to the Pt reactivity in hydrogen spillover. Since Cs-hydrocarbons are likely to be Cs-dimer cracking products produced over acidic sites, they can also be involved in subsequent oligomerization reactions over acidic sites, too. Cs-hydrocarbon reactions are more probable than C3-hydrocarbon reactions, because Cs-hydrocarbons are more volatile. However, Cs-unsaturated intermediates are rapidly hydrogenated over Pt/Ga-silicate by hydrogen which was activated on platinum and spills over the acidic sites. Consequently, they did not participate in further conversion and their concentration remained constant. The platinum hydrogenation activity was confirmed by the data on the hydrogen effect on the activity for Pt/Ga-silicate and Ga-silicate.

388 4. CONCLUSIONS

Platinum introduction into Ga-silicate resulted in an increase of b o t h the activity and selectivity in lower paraffin aromatization. This effect is very strong in the temperature range where galliumsilicate has no appreciable aromatization activity. Platinum promoted the dehydrogenation of both the initial paraffin and alicyclic intermediates. Platinum provides lower olefins and catalyzes direct a higher concentration of dehydrogenation of aromatic precursors. Pt and Ga synergic action cannot be ruled out at least for the highly dispersed Pt fraction located inside the channels. These effects can be of great importance to provide higher aromatization activity in the medium temperature region a s well as to improve the catalyst stability due to Pt efficiency in hydrogen spillover and backspillover processes. 5. REFERENCES

C. T. - W. Chu and C. D. Chang, J.Phys.Chem.,89 (1985) 1569 D. K. Simmons, R. Szostak, P. K. Agrawal and T. L. Thomas, J.Cata1, 106 (1987) 287 Kh. M. Minachev, V. B. Kazansky, A, A. Dergachev, L. M. Kustov, 3 T. N. Bondarenko and A. Yu., Khodakov, Bull.Acad.Sci.USSR, 1 (1990)311 4 A. Yu. Khodakov, L. M. Kustov, T. N. Bondarenko, A. A. Dergachev, V. B. Kazansky, Kh. M. Minachev, G. Borbely and H. K. Beyer, Zeolites, 10 (1990) 603 T. Inui, 0. Yamase, K. Fukuda, A. Itoh, J. Tarumoto, N. Morinaga, 5 T. Hagiwara and Y. Takegami, Proc. 8th Intern. Cong. Catal., Berlin, 1984, Vol. 1 1 1 , p.569 T. Inui, Y. Makino, F. Nagano and A. Miyamoto, Ind. Eng. Chem. Res., 6 26 (1987) 647 7 T. Inui, Y. Ishihara, K. Kamachi and H. Matsuda, Stud. Surf. Sci. Catal. 49 (1989) 1183 0. P. Tkachenko, E. S. Shpiro, T. V. Vasina, A. V. Preobrazhensky, 8 0. V. Bragin and Kh. M. Minachev, Bull. Acad. Sci. USSR, (1991) in print E. S. Shpiro, G. J. Tuleuova, V. 1. Zaikovskii, 0. P. Tkachenko, 9 T. V. Vasina, 0. V. Bragin and Kh. M. Minachev, Zeolites as Catalysts, Sorbents and Detergent Builders, 1989, Amsterdam, p. 143 10 N. S. Gnep, J. Y. Dovement, A. M. Seco, F. Ramoa Ribeiro and M. Guisnet, Stud. Surf. Sci. Catal., 43 (1988) 155 11 Kh. M. Minachev, V. B. Kazansky, A. A . Dergachev, L. M. Kustov and T. N. Bondarenko, Bull. Acad. Sci. USSR, 303 (1989) 412

1 2

389

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

Conversion of a l l y l alcohol t o oxygenated products over zeolite catalysts Graham J . Hutchings, Darren F . Lee and Craig D. W i l l i a m s Leverhulme Centre f o r Innovative C a t a l y s i s , Department U n i v e r s i t y of Liverpool, PO Box 147, Liverpool L69 3BX

of

Chemistry,

Abstract The r e a c t i o n of a l l y 1 a l c o h o l over z e o l i t e s H-ZSM-5, Na-ZSM-5 and H-Y i s d e s c r i b e d and discussed. Over t h e a c i d i c forms of t h e z e o l i t e s s i g n i f i c a n t are s e l e c t i v i t i e s of C, oxygenates ( p a r t i c u l a r l y CH,CHCHO and CH,CH,CHO) observed. I n p a r t i c u l a r , under s o m e r e a c t i o n c o n d i t i o n s i n v e s t i g a t e d acetone c a n become a s i g n i f i c a n t product. The mechanism of t h e r e a c t i o n i s i n v e s t i g a t e d using m o d e l compounds as r e a g e n t s and it i s proposed t h a t p r o t o n a t i o n of t h e carbon-carbon double bond i s t h e i n i t i a l r e a c t i o n s t e p l e a d i n g t o t h e formation of C, oxygenated p r o d u c t s .

1. INTRODUCTION The conversion of a l c o h o l s t o hydrocarbons u s i n g z e o l i t e c a t a l y s t s forms t h e b a s i s of a number of commercial o r near commercial production p r o c e s s e s , eg. t h e methanol t o g a s o l i n e process ( r e f . l), t h e methanol t o o l e f i n s p r o c e s s and a s s o c i a t e d production of g a s o l i n e and d i s t i l l a t e ( r e f . 2 ) , as w e l l as a number of o t h e r v a r i a n t s ( r e f . 3 ) . Since t h e r e a c t i o n w a s i n i t i a l l y described i n s o m e d e t a i l by Chang and S i l v e s t r i ( r e f . 4 ) it has a t t r a c t e d s i g n i f i c a n t r e s e a r c h e f f o r t from both i n d u s t r i a l and academic l a b o r a t o r i e s ( r e f . 1 ) . The mechanism of formation of t h e i n i t i a l products, methane and ethene remains a c o n t r o v e r s i a l t o p i c , although t h e r e i s a g e n e r a l consensus t h a t t h e i n i t i a l s t e p i n t h e r e a c t i o n involves t h e formation of a s u r f a c e methoxyl i n t e r m e d i a t e v i a a methylation process (ref. 1,5). More r e c e n t l y , t h i s o b s e r v a t i o n h a s been used t o develop p r o c e s s e s f o r t h e s y n t h e s i s of l i n e a r m i n e s using methanol as a co-reagent However, t w o major with ammonia over s m a l l p o r e z e o l i t e s ( r e f . 6 ) . problems p e r s i s t f o r t h e r e a c t i o n of a l c o h o l s over z e o l i t e c a t a l y s t s . F i r s t , t h e products are almost e x c l u s i v e l y hydrocarbons with e t h e r s being t h e o n l y s i g n i f i c a n t oxygen c o n t a i n i n g product; second, s e l e c t i v i t y t o a s p e c i f i c product i s t y p i c a l l y very low, p a r t i c u l a r l y f o r high carbon number p r o d u c t s . The e x c l u s i v e loss of oxygen i s a consequence of t h e previously c i t e d methylation mechanism, s i n c e t h e a l c o h o l OH group i s i n i t i a l l y protonated by t h e Bronsted a c i d s i t e of t h e z e o l i t e and as a consequence Considerable advantage would be water i s e l i m i n a t e d from t h e molecule. achieved if t h e oxygen could be r e t a i n e d i n t h e product, p a r t i c u l a r l y i f high s e l e c t i v i t y t o oxygenated p r o d u c t s could b e obtained. T o d a t e , t h i s a s p e c t of a l c o h o l conversion has received l i t t l e o r no a t t e n t i o n . We have now addressed t h i s area and i n t h i s paper w e r e p o r t our preliminary f i n d i n g s f o r t h e conversion of a l l y l a l c o h o l over z e o l i t e c a t a l y s t s , which demonstrate t h a t oxygenated products can be formed i n high s e l e c t i v i t y .

390 2. EXPERIMENTATA Z e o l i t e H-ZSM-5 ( S i O , / A l , O , = 35) was prepared according t o t h e method of Howden ( r e f . 7 ) . The ZSM-5 prepared by t h i s method was converted i n t o t h e hydrogen form (H-ZSM-5) by i o n exchange. ZSM-5 ( l o g ) was s t i r r e d i n aqueous ammonium n i t r a t e ( 1 0 0 m 1 , 0.1M) under r e f l u x f o r 4h. The s o l i d was recovered by f i l t r a t i o n and t h e procedure was repeated twice. The s o l i d was then c a l c i n e d a t 65OOC f o r 3h. The sodium form of t h e z e o l i t e (Na-ZSM-5) was prepared using a s i m i l a r ion exchange method using aqueous sodium n i t r a t e . Z e o l i t e Y w a s purchased from Union Carbide i n t h e a c i d form (Zeolite HZY-82). Both z e o l i t e samples were calcined a t 660°C f o r 3h p r i o r t o use a s c a t a l y s t s . The c a t a l y t i c r e a c t i o n s were c a r r i e d o u t using a microreactor i n which a l l y l alcohol was vaporised i n a stream of dry nitrogen a t a c o n t r o l l e d flow r a t e t o achieve t h e required WHSV of 0 . 5 h - l . The a l l y l alcohol vapour was then r e a c t e d over t h e z e o l i t e c a t a l y s t (0.59) The products i n a t h e r m o s t a t i c a l l y c o n t r o l l e d microreactor ( i . d . = lorn). were analysed by gas chromatography. I n a d d i t i o n , products were c o l l e c t e d i n a low temperature t r a p and analysed using g . c . m a s s spectroscopy (VG7070E with DEC PDP 11-24 d a t a system). Blank thermal r e a c t i o n s i n t h e absence of c a t a l y s t were found t o be n e g l i g i b l e and s a t i s f a c t o r y mass balance was obtained f o r a l l d a t a presented.

3. RESULTS AND DISCUSSION 3.1 Conversion of a l l y l alcohol over H-Z91-5 The r e s u l t s € o r t h e conversion of a l l y l alcohol over H-ZSM-5 a t 25OOC I n i t i a l l y t h e products comprise mainly of a r e shown i n Figure 1. hydrocarbons a s would be expected from t h e conversion of an alcohol over an a c i d i c z e o l i t e c a t a l y s t . However, as t h e conversion decreases with time on CH,CHCHO and stream, t h e s e l e c t i v i t y t o C, oxygenated products (CH,COCH,, CH,CH,CHO) becomes s i g n i f i c a n t . Product i d e n t i f i c a t i o n was confirmed by 80% with lesser amounts of g.c.m.s. and s e l e c t i v i t i e s t o a c r o l e i n of propanal and acetone could be achieved. A t 100°C t h e s e l e c t i v i t y t o t h e s e oxygenated products was higher a t t h e expense of conversion (Figure 2 ) . Although t h e r e a c t i o n conditions have yet t o b e optimised, it i s c l e a r t h a t s i g n i f i c a n t s e l e c t i v i t i e s t o oxygenates can be obtained from t h i s r e a c t i o n .

3.2 Conversion of a l l y l alcohol over H-Y The r e s u l t s f o r t h e conversion of a l l y l alcohol over z e o l i t e H-Y a t A t 25OoC t h e products 250°C and 350OC a r e shown i n Figures 3 and 4 . comprise mainly hydrocarbons i n i t i a l l y , b u t as t h e conversion decreases due t o c a t a l y s t d e a c t i v a t i o n , t h e s e l e c t i v i t y t o C, oxygenates increases s t e a d i l y . However, t h e maximum s e l e c t i v i t y achieved with z e o l i t e H-Y f o r C, oxygenates i s lower than t h a t f o r z e o l i t e H-ZSM-5 under comparable conditions. A t a higher r e a c t a n t f e e d r a t e and higher temperature (Figure 4) t h e s e l e c t i v i t y t o C, oxygenates can be s i g n i f i c a n t l y enhanced, i n d i c a t i n g t h a t t h e r e e x i s t s considerable scope t o optimise t h i s r e a c t i o n f o r t h i s z e o l i t e . I t i s c l e a r from t h e d a t a t h a t t h e c a t a l y s t l i f e t i m e f o r both ZSM-5 and z e o l i t e H-Y a r e s h o r t f o r t h e conversion of a l l y l alcohol. This s h o r t l i f e t i m e w a s due t o t h e formation of coke during t h e r e a c t i o n 4% carbon a f t e r r e a c t i o n f o r 3h. and t h e c a t a l y s t s t y p i c a l l y contained The rapid formation of coke was probably due t o a l d o l condensations occurring f o r t h e C, oxygenated products of t h e r e a c t i o n .

391 100

80

60

40

20

0 0

w

lw

too

Time on Stream (minutes)

F i g u r e 1. Conversion of a l l y 1 a l c o h o l over H-ZSM-5 a t 25OoC, WHSV = 0 . 5 h - l . ethene, .f propene, 13 b u t e n e s , i( pentenes, 0 C, oxygenates (CH,COCH,, CH,CHCHO, CH,CH,CHO), I conversion of a l l y l a l c o h o l .

+

100

0

7

0

20

40

60

80

HWI

120

NO

11)O

Time on Stream (minutes)

F i g u r e 2. Conversion of a l l y l a l c o h o l over €3-ZSM-5 a t 100°C, WHSV = 0.5h”; Key as i n F i g u r e 1, except ethene, 2-propanol

x

3 . 3 Conversion of allyl alcohol over Na-ZSM-5

and effect of added water

The r e s u l t s f o r t h e conversion of a l l y 1 a l c o h o l over Na-ZSM-5 are shown 5. It is c l e a r t h a t t h e production of C, oxygenates i s s i g n i f i c a n t l y lower f o r Na-ZSM-5 when compared w i t h H-ZSM-5 a t comparable conversion and r e a c t i o n c o n d i t i o n s . This i n d i c a t e s t h a t Bronsted a c i d s i t e s a r e important f o r t h i s r e a c t i o n . I n t e r e s t i n g l y , a d d i t i o n of 3% water t o t h e a l l y l a l c o h o l r e a g e n t i n c r e a s e s the s e l e c t i v i t y for C, oxygenates o v e r Na-ZSM-5 ( F i g u r e 6 ’ ) , whereas a similar e f f e c t is n o t observed with H-ZSM-5 under comparable c o n d i t i o n s ( F i g u r e 7 ) . However, for H-ZSM-5 t h e a d d i t i o n of water a l s o d e c r e a s e s c a t a l y s t d e a c t i v a t i o n , which is c o n s i s t e n t from coke formation being t h e r e s u l t of a l c o h o l condensation r e a c t i o n of t h e C, oxygenated p r o d u c t s .

i n Figure

392

i

40

n

10

2

4

n v

0

-

100

60

160

200

TOL (mind

Figure 3. Conversion of allyl alcohol over zeolite H-Y at 25OoC, WHSV = 0.5h-l, I# ethene, -4- propene, butenes, C l dimethyl ether, X C, oxygenates (CH,COCH,, CH,CHCHO, CH,CH,MO), 2-propano1, .$ unconverted allyl alcohol; TOL = Time on Line

*

70

60 50

x

40

30 20 10

-

n 0

60

100

160

200

TOL (mins)

Figure 4 . Conversion of allyl alcohol over zeolite H-Y at 350QC, WHSV 1.6h-’. propene, t methanol, % C, oxygenates, X 2-propanol, 13 unconverted ally alcohol; TOL = Time on Line

=

3 . 4 Reaction of model reactants

The conversion of 1-propanol over H-ZSM-5 or H-Y was not found to yield any C, oxygenated products for a range of reaction conditions and the products are mainly propene and butenes. This confirms that the introduction of the carbon-carbon double bond into the reactant molecule significantly affects the reactivity. Conversion of 2-propanol over H-ZSM-5 was found to give significant selectivity to acetone at low f l o w rates and this indicates that this could be a possible reaction intermediate. In addition, reaction of propene oxide over H-ZSM-5, under comparable conditions to those utilised for allyl alcohol, produced significant selectivities of both acetone and allyl alcohol.

393 rv

60

60 40

x

30 20 10 0

0

60

100

160

200

TOL (mind

Figure 5. Conversion of allyl alcohol over Na-ZSM-5 at 25OCC, WHSV = 0.5h-l; ethene, + propene, .# dimethyl ether, 131 butenes, X C, oxygenates, 0 unconverted allyl alcohol; TOL = Time on Line

80 70

I

0

60

100

160

200

TOL (mins)

Figure 6. Conversion of allyl alcohol/3% water over Na-ZSM-5 at 25OoC, WHSV = 0.Sh-l; B methane, t ethene, X propene, X C, oxygenates, 0 2-propanol, C'l unconverted allyl alcohol; TOL = Time on Line 3.5 CoIwents on the reaction mechanism

Ally1 alcohol possesses two functional groups that could be protonated by the Bronsted acid sites of the zeolite. Protonation of the OH group would lead to loss of water via an elimination mechanism (Figure 8) resulting in the formation of hydrocarbons as exclusive products. This is demonstrated both by the reaction of 1-propanol and by the initial reaction of allyl alcohol of H-ZSM-5. However, protonation of the carbon-carbon double bond leads to oxygen retention via the formation of a carbenium ion intermediate, which could yield acetone via a 1,2 oxygen shift. A reaction mechanism consistent with the observed reaction of allyl alcohol, 1-propanol, 2-propanvl and propene oxide over zeolite catalysts is given in Figure 8 .

394

40 U

30

20 10 0

0

60

100

160

200

TOL (mind

Figure I . Conversion of allyl alcohol/3% water over H-ZSM-5 at 25OoC, WHSV = 0.5h-l; il ethene, T propene, % butenes, 17 C, oxygenates, $2-propanol, 'x unconverted allyl alcohol; TOL = Time on Line

Figure 8. Proposed reaction mechanism for the conversion of a l l y l alcohol over zeolite catalysts. It is possible that the intermediate formed from initial loss of water via elimination could also be important in the formation of C, oxygenates. Reaction of this intermediate with water could be expected to lead to the formation of C, oxygenates with oxygen at either the primary or secondary carbons. However, this possibility can be discounted, since the addition of water as a co-reagent significantly decreased the selectivity to C, oxygenates when H-ZSM-5 was used as catalyst. The observation that the reaction of allyl alcohol/3% water over Na-ZSN-5 produces significant selectivities of the C, oxygenates requires further comment. In the absence of co-fed water, Na-ZSM-5 is not particularly selective to C, oxygenates. The interaction of Na’ and H,O within zeolite pore systems has been well studied (ref. 8) and it is possible that polarization of the solvation shell of Na’ within the zeolite pore may be sufficient to induce the required acidity f o r this reaction.

395 The r e s u l t s of t h i s p r e l i m i n a r y study have shown t h a t t h e i n t r o d u c t i o n of a carbon-carbon double bond i n t o an a l c o h o l r e a g e n t can l e a d t o t h e formation of oxygenate p r o d u c t s i n high s e l e c t i v i t y and t h i s may be of s i g n i f i c a n c e f o r t h e u s e of z e o l i t e s f o r t h e s y n t h e s i s of f i n e chemicals.

W e thank t h e I n t e r f a c e s and C a t a l y s i s I n i t i a t i v e , SERC, f o r f i n a n c i a l support and Alan M i l l s f o r o b t a i n i n g t h e g . c . m.s. r e s u l t s .

5 . REPERENCES

7 8

C.D. Chang, Stud. S u r f . S c i . C a t a l . , 36 (1988) 127. S.A. Tab& and S . Yurchak, Catal. Today, 6 (1990) 307. L.V. McDougall, C a t a l . Today, i n p r e s s . C.D. Chang and A . J . S i l v e s t r i , J. C a t a l . , 4 9 (1977) 247. G . J . Hutchings and R. Hunter, C a t a l . Today, 6 (1990) 279. R.G. Copperthwaite, G . J . Hutchings and T. Themistocleous, ' C a t a l y s t s f o r t h e p r o d u c t i o n of methyl m i n e s ' , S . African P a t e n t A p p l i a t i o n 1990. M.G. Howden, CSIR Report C.Eng 4 1 3 ( C S I R , P r e t o r i a , South A f r i c a , 1982). J . W . Ward, J . C a t a l . , 17 (1970) 355; 22 (1971) 237.

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P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

397

C A T I O N E X C H A N G E I N F L U E N C E O N T H E A C T I V I T Y OF ZEOLITES

IN REACTIONS B E T W E E N ALCOHOLS A N D H Y D R O G E N SULPHIDE

M . Z I 6 t E K a n d K.HILDEBRAN0-LEKSOWSKA F a c u l t y of C h e m i s t r y , A . 6 0 - 7 8 0 Poznar5, P o l a n d

Mickiewicz U n i v e r s i t y ,

SUMMARY The i n f l u e n c e o f t h e a l k a l i c a t i o n e x c h a n g e i n f a u j a s i t e t y p e z e o l i t e s on t h e z e o l i t e a c i d i t y and e l e c t r o n e g a t i v i t y i s p r e s e n t e d . C o r r e l a t i o n s b e t w e e n t h e c h a n g e s of t h e s e p a r a m e t e r s and t h e a c t i v i t y and s e l e c t i v i t y of z e o l i t e s i n t h e h y d r o s u l p h u r i z a t i o n of a l c o h o l s a r e d i s c u s s e d . I t was s t a t e d t h a t f o r t h e s e p r o c e s s e s i n w h i c h t h e d i s s o c i a t i v e l y a d s o r b e d H2S t a k e s p a r t , t h e i n c r e a s e of t h e z e o l i t e e l e c t r o n e g a t i v i t y c a u s e s t h e decrease i n the activity.

INTRODUCTION Most of t h e c a t a l y t i c r e a c t i o n s w h e r e o n e o f t h e r e a c t a n t s i s H2S e x h i b i t s i m i l a r f e a t u r e s , s u c h a s ( r e f . 1 ) : - h i g h e r a c t i v i t y of X-type

z e o l i t e s t h a n Y-type

- h i g h e r a c t i v i t y of a l k a l i metal c a t i o n exchanged z e o l i t e s than acidic z e o l i t e s - t h e i n c r e a s e of t h e a c t i v i t y w i t h t h e i n c r e a s e of t h e

alkali cation radius. T h e s e f e a t u r e s were o b s e r v e d for s u c h r e a c t i o n s a s : h y d r o g e n s u l p h i d e o x i d a t i o n , r i n g t r a n s f o r m a t i o n of r - b u t y r o l a c t o n e into (-thiobutyrolactone

and r e d u c t i o n o f n i t r o compounds w i t h

hydrogen s u l p h i d e i n t o amines. T h e aim of o u r s t u d y was t o p r e s e n t t h e c h a n g e s i n p r o p e r -

t i e s of z e o l i t e s a f t e r a l k a l i c a t i o n e x c h a n g e a n d t h e i r i n f l u e n c e on t h e h y d r o s u l p h u r i z a t i o n of a l c o h o l s .

398 EXPERIMENTAL Catalysts Z e o l i t e s NaX L i n d e ( L o t No. 2 1 2 4 9 8 / 5 8 2 ) NaY Leuna w i t h S i / A 1 = 2 . 5 6

with S i / A l = 1 . 1 3 and

were used a s p a r e n t m a t e r i a l s .

M o d i f i e d f o r m s were p r e p a r e d by an i o n - e x c h a n g e w i t h 0 . 1 M s o l u t i o n s o f t h e r e s p e c t i v e a l k a l i m e t a l c h l o r i d e s . The f o l l o w i n g c a t a l y s t s were o b t a i n e d (degree o f exchange i n b r a c k e t s ) : Li,NaX K,NaY

( 2 8 % ) ; K,NaX

( 6 0 % ) ; Cs,NaY

( 6 1 % ) ; Cs,NaX

(20%); L i , N a Y ( 4 3 % ) ;

(46%).

A l l z e o l i t e s w e r e s t u d i e d b y I R ( i n t h e 300-1500

c m - l re-

g i o n ) and X-ray methods.

o f p y r i d i n e was

The t e m p e r a t u r e - p r o g r a m m e d d e s o r p t i o n ( T P D ) used f o r c h a r a c t e r i z i n g t h e Y-type z e o l i t e s .

A w a f e r o f 1 0 mg

o f z e o l i t e was a c t i v a t e d u n d e r vacuum ( w 4 P a ) a t 673K f o r 2h. A f t e r c o o l i n g t o 473K, t h e s a m p l e was e x p o s e d t o p y r i d i n e and lh-outgased.

P y r i d i n e d e s o r p t i o n was m i n i t o r e d w i t h a B a l z e r s

QMG 3 1 1 mass s p e c t r o m e t e r .

R E A C T I O N CONDITIONS The c o n t i n u o u s f l o w t e c h n i q u e was u s e d t o m e a s u r e t h e c a t a l y t i c a c t i v i t y of z e o l i t e s i n t h e r e a c t i o n between methanol o r e t h a n o l and h y d r o g e n s u l p h i d e ( s e e r e f . 2 ) . a c t i v a t e d i n h e l i u m a t 674K f o r 4 h o u r s .

The c a t a l y s t s w e r e The r e a c t i o n was

c a r r i e d o u t a t 523K by u s i n g a r e a g e n t m i x t u r e c o n t a i n i n g Merck r e s e a r c h g r a d e H2S ( 5 % v o l . ) ,

a l c o h o l (2,5% v o l . )

and

h e l i u m as a c a r r i e r gas and on l i n e G C a n a l y s i s . RESULTS C h a r a c t e r i z a t i o n of z e o l i t e s The s t r u c t u r e o f X - t y p e z e o l i t e s i s m o r e s e n s i t i v e t o t h e c a t i o n exchange and r e a c t i o n c o n d i t i o n s t h a n t h a t o f Y - z e o l i t e s (ref.3,Y).

Therefore,

were Y-type z e o l i t e s ,

t h e m a j o r c a t a l y s t s i n our s t u d y

d e s p i t e t h e f a c t t h a t X z e o l i t e s show

h i g h e r a c t i v i t y i n t h e r e a c t i o n s w i t h H2S c o n t r i b u t i o n . X-ray

IR a n d

s t u d i e s of a l k a l i c a t i o n exchanged Y z e o l i t e s i n d i c a t e d

t h a t no s t r u c t u r a l changes o c c u r e d a f t e r t h e m o d i f i c a t i o n o f

399

w

Fig. 1. MS/TPD spectra of pyridine preadsorbed on alkali cation exchanged Y-type zeolites; temperature of activation: 6 7 3 K .

313

17 3

57 3

673

TEMPERATURE C K 1

Y-zeolite. The acidity of zeolites can be estimated on the basis of the strength of pyridine chemisorption. Fig.1 shows the results of TPD of pyridine from Me IY zeolites. They confirmed the well known fact that pyridine i s adsorbed only on cations. The highest strength of cation acid sites i s observed for Li,NaY. With the increase of the alkali cation radius, the acidic strength of zeolites decreases, except for the Cs,NaY zeolite. The cesium form shows the maximum at a little bit higher desorption temperature than the maximum of pyridine desorption from K,NaY. Generally, alkali cation exchanged faujasite type zeolites are considered to be basic catalysts. However, it i s important to stress their acidity to explain selectivity changes in the reaction between alcohols and hydrogen sulphide. One of the parameters which are changed a s a result of the cation exchange in zeolites is their electronegativity. Fig.2 presents the electronegativity of Me I X and Me I Y zeolites in comparison with the electronegativity of the hydrogen sulphide molecule, which i s one of the reagents in the described reaction. The electronegativity of zeolites was calculated using the equation presented by Mortier (ref.5). The electronegativity of Cs,NaX i s higher than K,NaX because of a lower

400

3.1

3.2

3.3

3.4

3.6

3.5

3.1 I

I I

3.8

electronegativity K Na Li

I

I

I ]I

Y

I

K

Cs Li

cs

1

I I t

Na

H2S Fig. 2. The electronegativity of used faujasi.te type zeolites and H 2 S molecule.

degree of exchange of cesium ions than of potassium. All zeolites have lower electronegativity than hydrogen sulphide rnolecule. The differences between the zeolite electronegativity and H2S electronegativity i s higher for X type zeolites than for the Y-type. Activity and selectivity of zeolites The activity and selectivity of Me I Y zeolites in the reaction between methanol and H2S are showed in Fig.3. T h e increase of the zeolite electronegativity causes the

- 100

100

- 90 <7

rl

I0

00

f

70

o

60

,

so =

",

50

40

40 U I

30 - 20

10

E L ECTR 0 NEGAT I V I T Y

Z 2

Fig. 3. The influence of the zeolite electronegativity on the alcohol conversion and the selectivity in the reaction between methanol and H2S.

401

i n c r e a s e of t h e s e l e c t i v i t y t o w a r d s h y d r o c a r b o n s a n d t h e d e c r e ase i n t h e s e l e c t i v i t y t o m e t h a n e t h i o l . D i m e t h y l s u l p h i d e appea r e d i n t h e f i r s t s t a g e of t h e r e a c t i o n o n l y o v e r Cs,NaY.

The

c o n v e r s i o n o f m e t h a n o l d o e s n o t show a l i n e a r c o r r e l a t i o n w i t h the z e o l i t e electronegativity. T a b l e 1 shows t h e y i e l d of t h i o l s i n t h e r e a c t i o n s o f m e t h a n o l and e t h a n o l w i t h hydrogen s u l p h i d e and t h e e l e c t r o n e g a t i v i t y of t h e X-type

z e o l i t e s s t u d i e d e a r l i e r (ref.6).

s o n o f t h e y i e l d of t h i o l s o v e r K,NaX

and Cs,NaX

The c o m p a r i -

zeolites

s t r e s s e s t h e r o l e of t h e z e o l i t e e l e c t r o n e g a t i v i t y depending n o t o n l y on t h e t y p e o f c a t i o n s b u t a l s o on t h e degree o f exchange. TABLE 1 F o r m a t i o n o f m e t h a n e t h i o l and e t h a n e t h i o l i n t h e h y d r o s u l p h u r i z a t i o n o f m e t h a n o l and e t h a n o l ( r e s p e c t i v e l y ) Zeolite

Electronegativity

Y i e l d of t h i o l s ,

MeSH L i ,NaX NaX K,NaX C s , NaX

3.231 3.226 3.141 3.195

mol%

EtSH

12

10

12 87 58

25 52

42

DISCUSSION The r e a c t i o n b e t w e e n a l c o h o l s a n d h y d r o g e n s u l p h i d e c a n p r o c e e d n o t o n l y v i a one r e a c t i o n p a t h w a y . The r e a c t i o n p a t h w a y d e p e n d s o n t h e t y p e of z e o l i t e s .

I t was s t a t e d t h a t o v e r h y d r o -

gen f o r m s o f f a u j a s i t e t y p e z e o l i t e s t h e B r t l n s t e d a c i d s i t e s p l a y t h e r o l e of a c t i v e c e n t r e s and t h e r e a c t i o n p r o c e e d s b e t w e e n c h e m i s o r b e d a l c o h o l a n d p h y s i s o r b e d H2S (ref.2). H o w e v e r , i n t h e a b s e n c e of B r l l n s t e d a c i d s i t e s t h e c a t i o n s sEem t o p l a y t h e r o l e o f a c t i v e c e n t r e s ( r e f . 7 ) .

I t was s u g g e s -

t e d t h a t o n c a t i o n i c forms o f z e o l i t e s t h e r e a c t i o n t a k e s p l a c e b e t w e e n d i s s o c i a t i v e l y a d s o r b e d h y d r o g e n s u l p h i d e and a l c o h o l . The r e s u l t s o b t a i n e d i n t h i s w o r k c o n f i r m t h i s s u g g e -

402

stion. Karge and Rasko (ref.8) estimated the dissociative adsorption of H2S over faujasite type zeolites with different Si/Al ratio and stated that with the increase in Si/A1 ratio i.e. the increase in the zeolite electronegativity the dissociative adsorption of H 2 S decreases. Mortier (ref.5) stated that during the formation of an adsorption complex, the equalization principle predicts an intermediate electronegativity for the entire compound. If the zeolite has a higher electronegativity, a n equalization can be promoted by a proton transfer from the zeolite to the molecule. Of course, if the molecule has higher electronegativity, the proton transfer to the zeolite lattice occurs. In this equalization of electronegativity, the difference in the value of electronegativity of zeolite and adsorbed molecule limited the possibilities and the rate of proton transfer. Therefore, in the case of H 2 S adsorption over zeolites with low electronegativity the differences between electronegativity of the sample and H 2 S molecule are big and the dissociative adsorption o f H2S proceeds very easily. As Fig.2 shows all X-type zeolites modified with alkali cations have electronegativity low enough for dissociation of H 2 S to occur. The electronegativity o f Y type zeolites i s more close to that for H2S molecule and therefore the dissociative adsorption of H 2 S over these samples i s difficult (ref.91, and in the case of Li,NaY it practically does not occur. In the view o f the above discussion, one can state that the fact that the increase of the zeolite electronegativity causes the decrease of the activity in the investigated reactions, proves that the dissociative adsorption of H 2 S is the step limiting the reaction rate. Thus, the correlation between the electronegativity o f zeolites and their activity and selectivity presented in Fig.3 indicates that in the formation of thiols the dissociatively adsorbed H 2 S takes part in the reaction pathway. The formation of hydrocarbons increases with the increase in the zeolite electronegativity. Usually, the decomposition

403

of alcohols towards hydrocarbons is influenced by the acidity of zeolites. Therefore the correlation between the hydrocarbons formation and the acidity of zeolites should be considered. The maximum of pyridine desorption observed in TPD spectra (Fig.1) i s a compilation of the desorption from sodium and the other alkali cations. In the case of Cs,NaY the degree of cation exchange is low, and, therefore the pyridine desorption from sodium cations influences strongly the position of the TPD band. The increase in the selectivity towards hydrocarbons o n K,NaY in comparison with Cs,NaY (Fig.3) indicates that the acidic strength of cations showed by TPD maxima does not play the most important r o l e in the formation of hydrocarbons. One should note that the generation of acidic hydroxyl groups as a resu1.t of the dissociative adsorption of H2S (ref.8) a l s o influences the selectivity towards hydrocarbons. No simple correlation between the electronegativity of zeolites and the conversion of methanol i s observed (Fig.3). It i s caused by two competitive processes which occurs during the reaction b e t w e e n ROH and H 2 S i.e. transformation of alcohols towards hydrocarbons (increasing with the increase of the zeolite electronegativity) and hydrosulphurization of alcohols towards thiols (decreasing with the increase of the electronegativity). The comparison of the results in Fig.3 and Table 1 indicates the higher yields of thiols over X-type zeolites than over Y-zeolites. I t gives the additional proof of the contribution o f dissociatively adsorbed H 2 S in the reaction pathway. CONCLUSIONS The electronegativity and acidity changes in zeolites,resulting from the cation exchange, influences the selectivity in the reaction between ROH and H 2 S . The contribution of the dissociatively adsorbed H 2 S in the reaction pathway was stated on the basis of the dependence of the thiols formation on the Me I -FAU zeolite electronegativity.

-

-

404

-

The i n c r e a s e i n t h e z e o l i t e e l e c t r o n e g a t i v i t y r e s u l t e d f r o m t h e a l k a l i c a t i o n qxchange causes t h e decrease i n t h e t h i o l s formation.

ACKNOWLEDGMENTS The a u t h o r s a r e i n d e b t e d t o D r . H . G . K a r g e

TPD f a c i l i t y .

f o r providing the

T h i s w o r k was f i n a n c e d f r o m t h e g r a n t a f f o r d e d

b y t h e N a t i o n a l R e s e a r c h Work C o m m i t t e e ( K B N ) . REFERENCES 1 Y.Ono, i n B . I m e l i k e l a l . ( E d s . ) , P r o c . I n t . S y m p . C a t a 1 y s i s b y Z e o l i t e s , E c u L l y ( L y o n ) , S e p t e m b e r 9-11, 1 9 8 0 S t u d i e s i n S u r f a c e S c i e n c e and C a t a l y s i s , Vo1.5), E l s e v i e r , Amsterdam, 1 9 8 0 , p . 1 9 . 2 M . Z i 6 l e k a n d I . E r e s i h s k a , Z e o l i t e s , 5 ( 1 9 8 5 ) 245. 3 M.Zi64ek and Z.Dudzik, React. K i n e t . C a t a l . L e t t . , 1 2 ( 1 9 7 9 ) 213. 4 M . Z i 6 l e k and Z.Dudzik, React. K i n e t . C a t a l . L e t t . , 22 ( 1 9 8 3 ) 455. 5 W . J . M o r t i e r , J . C a t a l . , 55 ( 1 9 7 8 ) 1 3 8 . 6 M . Z i 6 4 e k , D.Szuba a n d R . L e k s o w s k i , i n P . J . G r o b e t e t a l . ( E d s . ) , P r o c . I n t . Symp. I n n o v a t i o n i n Z e o l i t e M a t e r i a l s S c i e n c e , N i e u w p o o r t , September 13-17, 1987 ( S t u d i e s i n S u r f a c e S c i e n c e and C a t a l y s i s , Vo1.371, E l s e v i e r , Amsterdam, 1988, p . 4 2 7 . 7 M . Z i 6 l e k , 1 . B r e s i f i s k a a n d H.G.Karge, i n P . F e j e s a?d D . K a l l o ( E d s . ) , P r o c . I n t . Symp. o n Z e o l i t e C a t a l y s i s , S i o f o k , May 1 3 - 1 6 , 1 9 8 5 , P e t B f i Nyomda, K e c s k e m e t , A c t a P h y s i c a e t Chemica S z e g e d i e n s i s , 1 9 8 5 , p . 5 5 1 . 8 H.G.Karge a n d J . R a s k o , J . C o l l o i d I n t e r f a c e S c i . , 64 ( 1 9 7 8 ) 5 2 2 . 9 H.G.Karge, J . L a d e b e c k a n d N.K.Nag, i n S.E.Wanke a n d S . K . C h a k r a b a r t l y ( E d s . ) , P r o c . 7 t h C a n a d i a n Symp. o n C a t a l y s i s . Edmonton, 1 9 8 0 , C a n a d i a n S o c i e t y o f C h e m i c a l E n g i n e e r i n g , 1 9 8 0 , p . 223.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 1991Elsevier Science Publishers B.V., Amsterdam

405

POSSIBLE INTERMEDIATES DURING C3Hg AROMATIZATION OVER Ga-HZSM-5 CATALYST

P. MERIAUDEAU and C. NACCACHE lnstitut de Recherches sur la Catalyse, Unit6 Propre du C.N.R.S., conventionn66 21 I'Universit6 Claude Bernard LYON I, 2 avenue Albert Einstein, 69626 - Villeurbanne Cedex France

SUMMARY The reaction pathway for propane aromatization over Ga-HZSM-5 has been established by comparing the reactivities of the different possible intermediates. It appears that dienes (Cg-cg) are intermediates. INTRODUCTION HZSM-5 catalysts, modified by Ga(1)(2), Zn(3), or Pt(4)are efficient to transform propane into aromatics. Concerning the mechanism of the reaction few studies have been reported and some reaction steps are still unclear. The various reaction steps for the C3H8 aromatization which have been proposed are the following (1) (5) :

c5=

C6= cg= C7= cg=

H+ +

Ga +

Ga alcyclics diolefins

+

H+ +

aromatics (1) Ga ? cyclic + (11) olefins

Pathways (I) and (11) have been respectively postulated in (2) and (5). In order to determine which of the steps (I) or (11) are the most probable we have studied, over GaHZSM-5 the rate of benzene (or toluene) formation starting from molecules considered as possible intermediates during the propane aromatization. EXEPER!MENTAL ZSM-5 samples were synthetised according to the literature patent in presence of TPA Br as structure directing agent in an autoclave at 428 K ; Si/Al value of 30 was obtained. Sample was first calcined under H2 at 773 K overnight and then under N2. HZSM-5 form was obtained by exchanging Na form with NH4CI followed by calcination at 773 K, Qn HZSM-5, 2 w t % sf GP wae depesited by wet imprewatien tnohnique

406

(Ga(N03)3 8 H20 was used as starting salt). After impregnation, sample was calcined at 773 K under 0 2 overnight. Three treatments (02 1 hr and H2 1 hr) were performed before useReactions were studied at 773 K with low contact times and low pressure in order to have low conversion. For a series of experiments, the carrier gas used was N2 and for another series of 1/1 mixture N2 + H2 was employed. Reactants and products were analyzed on line with two chromatographs equipped with Bentone and Unibeads columns. RESULTS Let us consider the general reaction pathway described previously : when the higher olefins (Cg-Cg) are formed there are two possibilities for obtaining aromatics, as indicated by reaction pathways I and II : for example, c g olefin could be transformed on an acid site into methycyclopentane (MCP) and then, via successive dehydrogenation and isomerization steps into benzene ; on the other hand (way II) C6 olefin could be dehydrogenated on Ga center giving a diolefin which in turn could be converted in methycyclopentene (MCPe) over an acid site or dehydrogenated again into triene before cyclisation. So, for the CgHg production, it is of interest to compare the reactivities of MCP. cyclohexene (CHe) hexadiene (Hde) and for toluene that of methycyclohexane (MCH) ethycyclopentane (ECP) and methycyclohexene (MCHe). For all of these reactants, the benzene (or toluene) formation rate were measured in the presence of N2or N2 + H2 (1/1 mixture). N2 was choosen in order to simulate a low propane conversion (eg. .5 torr of C3would be transformed mainly into C3H6, C1 and C2H4(2) and consequently the H2 among the products will be lower than . 5torr) ; N2 + H2 was choosen in order to simulate high propane conversion (e.9. if 500torr of propane are converted over GaHZSM-5, the major product is hydrogen since the selectivity towards aromatics is high (60 %, carbon basis) (5) and since starting from C3Hg the formation of one aromatic molecule corresponds to the formation of 5 molecules of H2 : 2 C3H8 + CgHg + 5 H2). The experimental results are reported in table I. TABLE I Rate of benzene or toluene formation for different reactants, T = 773 K, PHC = 0.12 torr. Reactant rate of CsH6 Or C7H8 formation (u.a.)

Carrier gas N2 N2+H2

MCP

nHe

CHe

MCPe Hde

MCH

ECP

nHpe MCHe

2080 3240 4450

5380 6340 660

2680

2900 4380

800 3280

4200 4000 415

1400

1400 4000

800

DISCUSSION With Np as carrier gas It appears that MCP is the less reactive molecule to form benzene. Since n hexene is transformed into benzene with a higher reaction rate than MCP, it is concluded that

407

MCP is not the intermediate for benzene formation from hexene on these catalysts ; same conclusion is reached concerning the toluene from n-heptene : MCH and ECP are probably not intermediates. So it is concluded that at low propane conversion the alicyclics are not intermediates for the formation of the aromatics. It is concluded that Cg-Cg olefins are dehydrogenated into dienes before their cyclisation. Concerning the CgHg formation from the corresponding diolefin it appears that hexadiene 1-5 is transformed into CgHg with a higher rate than CHe or MCPe suggesting that triene could be well the reaction intermediate : such an intermediate will give CHde, by gas phase and/or acid catalized cyclisation, which will be in turn dehydrogenated into CgHg. To summarize the above discussion, at low propane conversion the reaction pathway for the aromatics formation would be the following :

Such a mechanism, via diene and triene was proposed for the cyclisation of nhexane over Cr203-K/A1203 catalyst, evidence of these elementary steps being obtained by using 3C labelled molecules (6). With N7 + H2 (1/1) as carrier gas In presence of hydrogen in the gas phase, the reaction pathway proposed previously is no more valid since it is observed (table I) that MCP and nHe are transformed into CgHg with the same rate ; it could be postulated that e.g., C6 Olefin is cyclised into MCP and then dehydrogenated on Ga centre but in such a scheme, this would imply that the cyclisation rate of the nHe over HZSM-5 is greater or equal than the rate of nHe over GaHZSM-5. This assumption is not true since it has been evidenced that the rate of MCP from nHe over HZSM-5 is much smaller than the rate of benzene formation from nHe over GaHZSM-5 ; this result suggests that in presence of H2, the first of the aromatization step proposed previously is still valid : olefins are dehydrogenated into diolefins on Ga centers. For the next step (transformationof the diene) things are less clear since MCPe and Hde are converted into benzene with approximatively the same rate ; in addition it is known that the equilibrium between Hde and Htriene is greatly in favor of Hde. For those reasons it is likely that trienes would not be intermediates. Coming back to the diolefins, particulary for c6, it would be of interest to know how cyclisation proceeds, through MCPe andlor through a c 6 ring. Results of Hde and CHe indicate that CHe dehydrogenate into benzene at a slower rate than Hde is transformed into benzene suggesting that CHe is not the intermediate for the transformation of Hde into benzene. Thus, MCPe would be the most likely intermediate in the Hde transformation into benzene. The next step, transformation of MCPe into benzene would be the MCPde formation ; MCPde would be transformed into benzene via isomerisation into CHde on acid sites and subsequent dehydrogenation on Ga centre. To Summarize up the results, it is proposed that at low propane conversion (and

408

consequently low hydrogen pressure in the gas phase) c6-c9 olefins are dehydrogenated into dienes and trienes before cyclisation into cyclodiolefines. For higher propane conversion (and consequently with high hydrogen pressure in gas phase) the trienes are no more intermediates but dienes are still formed and are transformed into cyclic olefins on acid centers before dehydrogenation on Ga centres. Similar step (diene formation) was proposed recently for the n-hexane cyclisation on gallo silicates (7). It has to be pointed out that the above conclusions have been established for a given Ga loading (2 wt %) on a given HZSM-5 (Si/AI = 30) ; since Ga-HZSM-5 catalyst acts as a bifunctionnal catalyst, the steps we proposed are valid for a given balance between acid and dehydrogenating functions, for a complete different balance (e.g. very low Ga loading on HZSM-5 having small Si/AI ratio and consequently high acidity) steps could be different.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

M. Kitagawa, Y. Dendoda and Y. Ono, J. Catal., 101, (1986), 12. N.S. Gnep, J.Y. Doyemet, A.M. Seco, F.R. Ribeiro and M. Guisnet, Appl. Catal., 43, (1988), 155. T. Mole and J.R. Anderson, Appl. Catal., 17, (1985), 154. C.W.R. Engelen, J.P. Wolthuizen and J.C. Van Hoff, Appl. Catal., 19, (1 985), 153. P. Mbriaudeau, C. Naccache in "Zeolites : facts, figure, future", P. Jacobs and R. Van Santen Eds., Elsevier Amsterdam, (1989), p. 1423. B.A. Kasanshy, V.V. Isagulyants, M.I. Rozengart, Y.G. Dubinsky and J.I. Kovalenko, Proceedings of the 5th Int. Cat. Cong. J.W. Hightower Ed., North Holland Amsterdam, (1973), p. 1277. J. Kanai in "Successful design of Catalysts" T. lnui Ed. Elsevier Amsterdam, (I 988), p. 21 1.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

409

DEHYDROCYCLODIMERIZATION OF SHORT CHAIN ALKANES ON Ga/ZSM-5 AND Ga/BETA ZEOLITES A.

COMA", C.GOBERNA", J.M. LOPEZ NIETO", N. P A R E D E S ~ , M. PEREZ~

"Instituto de Tecnologia Quimica, UPV-CSIC, Camino de Vera, s/n, 46071 Valencia, Spain bCEPSA S . A . , Spain

Poligono

Industrial

San

Fernando de Henares,

Abstract The reaction of different C,- and C,-hydrocarbons and a light straight run ( L S R ) on a Ga/H-ZSM-5 zeolite and the reaction of propane on a Beta-zeolite, with or without gallium,has been studied. The differences observed in the activity and selectivity are explained taking into account the relative stability of the different possible carbocation intermediates. The influence of the zeolite structure is also discussed. 1. INTRODUCTION

Since the introduction of the Cyclar process by BP( 1 ) , much work has been carried out on the influence of the procedure for incorporation of dehydrogenating components (Ga or Zn) into the ZSM-5 zeolite, as well as on the mechanism of the conversion of light alkanes to aromatics (2-7). The differences observed among the different catalyst preparations are related to the dispersion and the stoichiometry of the metal oxide. Indeed changes in selectivity to aromatics with time on stream (8) or with a pretreatment of the catalyst with H,, have been associated with the formation of lower valences of Ga as the active dehydrogenating species (6). The mechanism of formation of aromatics from short chain alkanes on the above catalysts is believed to proceed via the corresponding olefins. These when formed, oligomerize and cyclate to give as the final step aromatics, especially the BTX fraction (2-8). However, there is still discussion on the first step of the reaction, i.e. the formation of olefins. Some authors (5) claim that the olefin is formed by dehydrogenation on the metallic species of the catalyst, while others ( 9 ) believe that the main initiating step is the cracking of the paraffin feed, to give a shorter olefin which oligomerizes in a consecutive step.

410

From an industrial point of view, the Cyclar process looks like an attractive way for producing aromatics, starting from relatively low cost raw materials. However this process presents, two major drawbacks: The problems associated with the decay-regeneration of the catalyst, and the high amount of dry gases formed, which imply costly gas separation. Moreover, the new policy for producing lead free gasoline has generated a new refinery stream, i.e. light straight run ( L S R ) , that because its low octane number can not be afforded to be added to the gasoline pool, and has to be processed. One way of doing this is by catalytic isomerization of the LSR, which can increase the octane number from 63 to 83. It also can be fed to stream cracking units to produce ethene, but it also appears that LSR could be fed into a cyclar process to produce aromatics. In this work we have studied the mechanism of alkane oligomerization, establishing a network for the primary and secondary reactions occurring on Ga ZSM-5 catalysts. Normal C, and normal and branched C, alkanes and alkenes and a LSR have been used. The influence of the zeolite structure on the conversion of propane is also discussed. 2. EXPERIMENTAL Ga/H-ZSM-5 zeolite (Si/Al= 20) and Ga/Beta-zeolite (Si/Al =la), with a Ga content of 2% wt, were prepared by *’wet"impregnation method with an aqueous solution of gallium nitrate, and calcined at 550 " C . Beta-zeolite was prepared by know methods (10). The catalysts were loaded in a stainless-steel tubular reactor (20 mm i.d.) and prior to use pretrated in helium atmosphere at 550 OC. The experiments were carried out in the

80

40

0

20

40

WIF

Fig.1. Influence of the contact time (W/F, in g h/mol) on the hydrocarbon conversion: propane ( 0 ), n-butane ( 0 ), isobutane ( A ) and LSR ( x ) , at 530 OC; propene ( 0 1 and l-butene ( W 1, at 425 "C.

411

TABLE 1. Initial selectivities’ of products in the reaction of propane and n-butane on a Ga/ZSM-5 zeolite.

SELECTIVITY(%) Reactant

CH, -

propane

30

20

10

-

15

16

9

5

23

12

40

7

8

5

n-butane

C,

C, -

C,-C, -

Benzene Toluene ~

Xylenes

’Initial selectivities are calculated for zero conversion, at 530 OC. 400-550 OC range, at atmospheric pressure, and at NJhydrocarbon ratio of 0.48. The products of the reaction were analyzed on line with a Varian-3400 gas cromatograh equipped with a FID detector, cryogenic oven, and automatic gas sampling valve injection, with a capillary column (DB-5, 60 m).

3. RESULTS AND DISCUSSION The influence of the contact time (W/F) on the conversion of different hydrocarbons is given in Figure 1. In the reaction of propene or 1-butene, lower temperatures have been used. The selectivity to the different reaction products obtained at different levels of conversion are given in Figure 2 . The differences in the composition of C,-C, and C,-C, or C,-C, fractions are detalled in the specific discussion for every reaction. Reaction of Propane The selectivity to the different reaction products obtained at different levels of conversion when feeding propane, are given in Figure 2 . From the shapes of curves and the initial slopes, it can be deduced that methane and benzene are primary plus secondary stable products, while ethene, propene toluene and xylene are primary and unstable products. Ethane is a secondary stable product. Moreover, from the values of the initial selectivities given in Table 1, it can be seen that the initial selectivity f o r C, is similar to the initial selectivity to ethene. These results indicate that in the first step, propane can be cracked to give methane and ethene. On the other hand, from the behavior of the selectivity curves, it can be deduced that methane is not only formed by cracking of propane, but also can be generated by dealkylation of the alkyl aromatics. The dealkylation reaction would explain the unstable character of toluene and C, alkylaromatics, together with the secondary character of benzene. The oligomerization of ethene

412

I-

t W

30.

z

w

N

(0

z

<20.

W

m

0

10

0

50

16

0

50

CONVERSION ( % I Figure 2. Variation of the selectivities to different reaction products with the conversion of C, and C, hydrocarbons. Symbols and reaction conditions as Fig. 1. formed during the cracking of propane can also contribute to the observed excess of C, with respect to C, hydrocarbons. When the same reaction is carried out on pure H-ZSM-5, the same products than those presented in Table 1 are obtained, although, the yields and selectivities for aromatics were lower. Meanwhile, when the reaction was carried out using two catalyst beds (the Y-A1,0,, and the second one with first one containing Ga,O, on the pure H-ZSM-5 zeolite), total conversion and selectivity to This, together with the results aromatics increased (11) obtained when feeding propene (Fig.l), clearly shows that the

.

413

controlling step of the process is the formation of the olefin, and that the role of Ga,O, is to dehydrogenate the alkane, increasing the partial pressure of the olefin in the reaction media. In a first aproximation, a reaction scheme has been proposed (Scheme 1).

When the intermediate carbocationic (C,-C12)oligomers have been formed on the acid sites of the zeolite they can go through different reactions: (i) To accept a hydride ion by hydrogen transfer and ulterior desorption as longer chain hydrocarbons; (ii) to recrack; and (iii) to cyclize. In the case of the propane reaction, the fact that aliphatic hydrocarbons with more than four carbon atoms have not been detected, and the amounts of C, hydrocarbons found were very small (Fig.2), indicates that very little, if any, desorption of the oligomers occurs, while the cyclization and dehydrogenation reaction should be very fast, avoiding the recracking to occur in a significant extent. Then, we should not expect that C, and C, can be formed through recracking of the oligomers. It is worth to remark that benzene is the major product in the BTX fraction, even if no Ga,O, is used on the ZSM-5, while during cracking of n-alkanes (12,-C,) and gasoil little benzene is observed in the BTX fraction. Moreover, the reaction of different C,-C, hydrocarbons (1-hexene, 1-heptene, methylcyclohexene) on Ga,O, produces isomerization or aromatization but not cyclization (5). From these results it has been concluded that oligomerization and cyclization are produced on the acid sites of zeolite. The most probable way of forming benzene is from a C, oligomer, obtained from C, condensation, but not through primary carbocations. The critical step would be the formation of the protonated cyclobutane ring, however, the four member ring cycloalkane has been claimed to occur as an intermediate during the isomerization of alkanes on bifunctional zeolite catalysts (12).

When oligomers of more than six carbons are formed then benzene and alkylaromatics can be obtained through intermediates involving secondary carbenium ions. The cycloalkenes formed by oligomerization would rapidly dehydrogenate either on Ga20,, or by hydride transfer on the zeolite since either on

414

metals or on zeolites when the first dehydrogenation has occured on cycloalkanes the process follows very rapidly up to the aromatic (13). Reaction of n-Butane When n-butane is fed, this can crack to give methane and propene, and ethane plus ethene. It can also dehydrogenate to give butene, which will oligomerize with itself, and with propene and ethene formed by cracking. In this case, it is clear that most of the oligomers formed will have seven and eigth carbon atoms. These oligomers can recrack to give C,, C, and C, and some C, and C, hydrocarbons. They can also cyclise and dehydrogenate to give a higher proportion of C, and C, alkylbenzenes in the BTX than when propane was fed. Results in Figure 2 and Table 1, agree with this, and also with the assumption that C, and C, alkane and alkene products should be found. Moreover, due to the higher facility of nbutane, both to dehydrogenate and also to crack, a higher conversion is obtained with n-butane than with propane, being also observed a higher selectivity to aromatics. From all this, it becomes apparent that the size of the intermediate oligomer is what controls the product distribution in the BTX fraction. This size, inturn, depends on the length of the initial olefins, and on its partial pressure. Indeed, since oligomerization is a bimolecular reaction, increasing the partial pressure of the olefin will increase the size of the oligomers. Following this idea, if one feeds propylene instead of propane, then amounts of oligomers containing nine or more carbon atoms should increase, these could either cyclise or recrack (the longer the chain, the easier the recracking versus the cyclization). In any case, if a C, oligomer recracks, it will give C,, C,, C,, and C, products (mainly branched ones). These, by reoligomerization will give a higher proportion of C, and C, oligomers, that will produce toluene and xylene in higher proportions. To check this assumption, propene was fed together with N,. As can be seem in Figure 2, C,, C, and C, alkanes and alkenes are obtained in the products (the ratio of iso- to normalhydrocarbon is higher than that expected from thermodynamic equilibrium). Meanwhile, the ratio of toluene plus xylene, to benzene has considerably increased with respect to the case of propane, as was predicted above. This ratio is even higher when 1-butene is fed. Notice that due to the high reactivity of the olefins a lower reaction temperature was used when feeding propene and 1-butene ( 4 2 5 "C). When feeding olefins, hydrogen transfer is an important reaction and the corresponding alkane is formed. It appears then that at lower temperatures and in the presence of hydrogen acceptor molecules, an important fraction of the aromatics formed from the cycloalkanes will be formed by hydrogen transfer on the acid sites. On the other hand, at higher temperatures the dehydrogenation of the cycloalkanes will occur mostly on the metal. Reaction of isobutane In branched alkanes there is a tertiary carbon and a

415

TABLE 2. Catalytic properties of Beta and Ga/Beta-zeolites in the propane dehydrocyclization SELECTIVITY ( % ) '

xt2 Catalyst B-zeol. Ga-B-zeol.

(%) ~ _

8.8 27

CH, _

_

C, _

C, -

-

C,-C, -

-

-

B _

T

X

N

_

37

55

-

6.5

0.5

0.8

0.2

-

56

29

-

3.1

5.2

3.2

0.8

2.7

Raction conditions: T= 530 OC, W/F= 30 g h/mol xylenes, N= naphthalenes

' B= benzene, T= toluene, X = ' propane conversion

terciary hydrogen. the presence of a tertiary carbon in isobutane will favor the formation of methane by protolytic cracking, while in n-butane, the formation of methane implies the formation of primary carbocation. Moreover, the dehydrogenation of isobutane either on Ga,O, or on acid sites is also easier than that of n-butane. Then, the high amount of tertiary carbocations and olefins will strongly favour formation of branched oligomers. Results from Fig. 1 confirm that conversion of isobutane and the yield C,-C, oligomers is much higher than with n-butane. The fact that selectivity to methane is lower on isobutane would indicate that the rate of dehydrogenation and of hydride transfer is much faster than the rate of cracking. Reaction of LSR When LSR is fed the C, and C, hydrocarbons can crack and dehydrogenate, and then oligomerize. Nevertheless, the long oligomers which could directly be formed from the C, and C, components of the feed, will have a high tendency to recrack or if cyclized to dealkylate. If this is so, one should expect when feeding a LSR stream to produce C,, C,, C,, C, and C,.products all of them postreacting giving a high yield of aromatics. However, in this case one should expect a similar (toluene plus xylene), to benzene ratio than when n-butane was fed. Results in Figure 2 show the comparable reactivity of propane, n-butane and LSR, showing that the later gives a high yield of C, and C,-products (with a paraffin to olefin ratio 2). The selectivity to benzene and toluene is lower than for nbutane, while for xylenes it is practically the same. These results show that the C, and C, present in the LSR do not directly dehydrocyclize, but mostly crack and then oligomerize. In any case the LSR appears as an interesting feed giving even better results than with n-butane. Influence of the zeolite structure The influence of the zeolite structure was studied by

416

carrying out the reaction on another zeolite tridirectional with a high Si/A1 ratio but with larger pore diameter such as the Beta-zeolite. The conversion of propane on a Beta zeolite with a Si/A1=18 ratio, with and without gallium, is presented in Table 2. The catalyst without gallium is less active than with gallium, and decays proportionally much slower. The selectivity to aromatics is higher on the gallium containing Beta-zeolite. When the results are compared with those obtained with GaZSM-5 zeolite, the Beta-zeolite is less selective to BTX, and more selective to naphthalene and methylnaphthalenes. This is a consequence of the biger dimension pores. Another point to be remarked is the high selectivity to C, and C, which would indicate, that on this zeolite the relative rate of cracking to dehydrogenation is higher than on the Ga-ZSM-5. Moreover, while in the case of the ZSM-5 the CJC, ratio was sensibly higher than 1 (specially at higher temperatures), in the Beta-zeolite this ratio is closer to one. The presence of relativelly higher amounts of ethane, indicates a higher hydrogen transfer ocurring on the Beta-zeolite. Finally, it has to be pointed out that on Beta-zeolite a strong catalyst decay ocurrs; in one hour of reaction an 80% of the activity has been lost. 4 . REFERENCES

1. R. Mowry, R.F. Anderson, and J.A: Johnson, O i l G a s J . 128 (1985). 2. N. S. Gnep, J. Y. Doyemet, A. M. Seco, F. Ramoa Ribeiro, and M. Guisnet, A p p l . C a t a l . , 43, 155 (1988). 3. H . Kitagawa, Y. Sendoda, and Y. Ono, J . C a t a l . , 101, 12 (1986). 4 . a)L.M. Thomas and X. Lin, J . Phys. Chem. 90, 843 (1986). b) T. Inui, J . Jpn. P e t r . I n s t . , 33, 198 (1990). 5. N.S. Gnep, J. Y. Doyemet, and M. Guisnet, in "Zeolites as Catalysts, Sorbents and Detergent Builders", H. G. Karge and J. Weitkamp (Eds.), Elsevier, p.153 (1989). 6. G. L. Price, and V. Kanazirev, J . Catal., 268 (1990). 7. T. Mole, J.R. Anderson, G. Creen, A p p l . C a t a l . , 17, 141 (1985). 8. V. Kanazirev, G.L. Price and K.M. Dooley, J. Chem. SOC., C h e m . C o m u n . , 712 (1990). 9. Y . Ono, H . Nakatani, H . Kitagawa and E. Suzuki, in "Successful Design of Catalysts". T. Inui (Ed.) ,Elsevier, p.279 (1988). 10. J. PBrez Pariente, J.D. Martens and P.A. Jacobs, A p p l . C a t a l . 3l,35 (1987). 11. J.D. Martens and P.A. Jacobs, J. C a t a l . , 124, 357 (1990). 12. A. Corma, J. M. Lbpez Nieto and N. Paredes, to be published. 13. J.F. Garcia de la Banda, A. Corma and F. Melo, A p p l . C a t a l . , 2 6 , 103 (1986); A. Corma, G. Koermer, F. Mocholi, and V. OrchillBs, A p p l . C a t a l . , 67, 307 (1991).

417

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

BIFUNCTIONAL COBALT-ZSM-5 CATALYST FOR THE SYNTHESIS OF HYDROCARBONS FROM THE PRODUCTS OF BIOMASS GASIFICATION b A. Krylovaa, A . Lapidusa, J. Rathousky , A. Zukalb and b M. Jancalkova aN.D.Zelinsky Institute of Organic Chemistry, MOSCOW, USSR bJ. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Dolejskova 3 , 182 23 Prague 8, Czechoslovakia

Abstract The Co-MgO-ZSM-5 catalyst was prepared as a physical mixture of cobalt hydroxycarbonate, MgO and NaZSM-5. It was found to be active in the hydrocarbon synthesis from the products of biomass gasification. The addition of C02 caused a decrease of the growth factor while that of N2 an increase

.

1. INTRODUCTION

Metal-zeolite catalysts made by combining a Fischer-Tropsch component with ZSM-5 give the possibility of producing mixtures of liquid hydrocarbons from syngas with high yields 114 1 . Cobalt-ZSM-5 catalysts were found 151 to be active in the synthesis of liquid hydrocarbons from syngas. Their catalytic activity and liquid hydrocarbon selectivity decrease with increasing calcination temperature. The aim of the present study is to show that a dried cobalt-ZSM-5 catalyst can be successfully applied for the synthesis of liquid hydrocarbons from the products of biomass gasification consisting of syngas di-

418

luted with C 0 2 and N2

.

2. EXPERIMENTAL

The catalyst was prepared by kneading of a physical mixture of precipitated cobalt hydroxycarbonate, MgO and NaZSM-5 (Si02/A1203 = 3 8 ) with water used as a binder. The paste was dried at 12OoC. It contained 32 wt.% Co, 3 wt.% MgO and the rest was the ZSM-5 zeolite. Adsorption isotherms of cyclopentane vapours were measured at 2OoC. Temperature programmed reduction profiles were measured by the continuous flow technique with a thermal conductivity detector. Simultaneous TG, DTG and DTA were carried out in an argon atmosphere at the heating rate of 10 K.min-’. Catalytic measurements were performed in a continuous flow integral fixed bed reactor under 0.1 MPa at 20OoC. The inlet CO/H2 ratios ranged from 0.5 to 2.0, the space velocity was 100 h-I. Before catalytic experiments, catalyst samples were reduced in situ in H2 at 45OoC for 5 hours.

3. RESULTS AND DISCUSSION

3.1. Adsorption measurements

The adsorption isotherms of cyclopentane on the catalyst and on ZSM-5 are shown in Fig. 1. A pronounced hysteresis loop in the isotherm on the catalyst may be attributed to interstitial capillary condensation between the particles of cobalt oxides, MgO and ZSM-5. The isotherm on the ZSM-5 was used as a standard isotherm for assessment of the catalyst microporosity with a modified method proposed in / 6 / . The amount adsorbed on the catalyst was plotted against the adsorption on the ZSM-5 (Fig. 1). From this linear plot for aZ
419

part corresponds to the content of ZSM-5 in the catalyst. This result proves that the adsorption capacity of the zeolitic component is preserved.

10

Fig.

1.

20

P

30

1,5

0,5 aZ

Adsorption isotherms of cyclopentane and the corresponding plot of ac vs. az’ act aZ (mrno1.g-l), amounts adsorbed on Co-MgO-ZSM-5 (0,o) and ZSM-5 (Q) , respectively; p (kPa), pressure.

3.2. Temperature programmed reduction In Fig. 2, TPR profile of the unreduced dried catalyst is compared with the profile of a sample prepared from this catalyst by calcination at 5OO0C in air. The reduction peak at 32OoC in the profile of the calcined sample may characterize the reduction of CojOq 171 while that at 47OoC is caused by the reduction of mixed oxides,e.g. xCoO.yMgO.The peak at 32OoC in the catalyst profile originates very probably from C02 evolved during the decomposition of cobalt hydroxycarbonate (compare with Fig. 3b). The peak at 37OoC may be ascribed to the reduction of cobalt oxides originating from the decomposed hydroxycarbonate. From the comparison of both TPR profiles it clearly follows that the composition of unreduced forms differs and that the metallic phase originated from different precursors.

420

Fig. 2. TPR profiles of the catalyst samples pretreated by (a) drying at 12OoC, (b) calcination at 50OoC. T ( O C ) , temperature. 3.3. Thermal analysis

During the thermal decomposition of dried Co-MgO-ZSM-5 in argon (Fig. 3) , the weight losses took place in several consecutive stages. The distinct DTG peak at 3OO0C corresponded to the decomposition of cobalt hydroxycarbonate. Both water and C02 were detected in the decomposition products in several peaks up to very high temperatures. The most prominent C02 and water peaks at ca. 3OO0C are in agreement with the respective DTG and DTA peaks which characterize the decomposition of cobalt hydroxycarbonate The intensity proportions of the respective C 0 2 and water peaks changed and depended on temperature.

.

3.4. Catalysis

The composition of the products of biomass gasification depends on the type of the material and on the gasification method used. E.g. , by the air gasification of wood a mixture of the following composition can be produced: 25-30% CO, 12-15% H2, 5-7% C 0 2 , 50% N2 and 1-8% H20 whereas when agricultural plants are used the composition changes to 10-15% CO, 15-20% H2, 13-15% C02, 50% N2 and max. 10% H20.

421

a

-40 I

.

.

400

I

.

.

800

.

.

I

1200 T

b 101

I

400

800 1200 T

Fig. 3. Simultaneous TG-DTA analysis of Co-MgO-ZSM-5 in argon: (a) TG (1), DTG ( 2 ) , DTA (3); (b) MS analysis of the decomposition products. M (%I, mass; T ( O C ) temperature; AT (UV) temperature difference; I (A.10-8) , intensity. MS analysis: TG (1), C 0 2 ( 2 ) , H 2 0 ( 3 ) . At first, the effect of only one additive on the hydrocarbon synthesis was investigated (Tab.1). When the total hydrocarbon yield was related to the CO+H2 content in the reaction feed it decreased with increasing C02 concentration but was not practically influenced by the N 2 concentration. C 0 2 may be suggested to act as an inhibitor while N 2 only dilutes the gaseous mixture Also the chain growth factor depends on the feed gas composition. The addition of 18% C 0 2 caused its decrease from 0.86 to 0.80 while a 16% N2 admixture increased its value to 0.90. These variations are in agreement with the respective roles of both additives. N 2 causes an increase of

.

422

the liquid hydrocarbon selectivity (the action of an inert gas). The C02 additive in a low concentration increases the methane yield and selectivity and decreases those of liquid hydrocarbons. At a higher C02 concentration, however, the yields of both methane and liquid hydrocarbons decreased substantially. The action of the zeolitic component manifests + itself in a high proportion of isoalkanes in the C5 products (Tab.1). Tab. 1. The effect of the addition of N2 or C02 on the production of hydrocarbons Reaction temperature: 2OO0C, space velocity: 100 h-’. The product analysis was performed after the catalyst activity attained its stable value (after about 60h). Hydrocarbon production (g.m-3 of CO+H2 in the feed)

Feed gas composition (2)

CO 33

27 24 14 28

22 16

H2 67

CO

(2)

37

80

28

57

8

16

0

20 32 24

C= 8

28

44

8

23

69

0

12

10

32

7

29

64

16 34 52

15

13

90

9

13

78

15 17

14

94 87

14

20

13

26

67 61

2 N2 0 0

55

18

0

47

29

27

59

56

0

44 32

0 0

Composition of + fraction c5

c1

c2-c4

17

c;

n-C 16

i-C 76 76

C= ,alkenes; n-C ,n-alkanes; i-C ,i-alkanes When both C02 and N2 were present in the feed gas mixture the both additives acted in accordance with their individual functions (Tab.2, lines 1-3). Tab.2 (lines 4-6) also demonstrates that the Co-MgO-ZSM-5 catalyst surpasses several typical Fischer-Tropsch catalysts (including the industrial catalyst Co-5%Zr02-Si02) in the total hydrocarbon yield and in the liquid hydrocarbon selectivity.

423

Tab. 2. The effect of the feed gas composition on the production of hydrocarbons over supported cobalt catalysts Reaction temperature: 2000C, space velocity: 100 h-I. The product analysis was performed after the catalyst activity attained its stable value (after about 60h). Catalyst

Feed gas composition

(%I Co-3%MgO-ZSM-5

Co-Si02 Co-2%Pt-Sio2 Co-5%Zr02-Sio2

Hydrocarbon production (g.m-3 of CO+H2 in the feed)

CO

H2

C02 N2

10

20

20

50

37

23

93

50(a) 50(b)

30

18

90

4

0

64

50

37

27

33 (c)

50 (c) 67 (c)

15

20

15

30

15

5

10

20

20

:c

4 ’ 2 ’

10

20

20

50

40

53

10

20

20

50

43

23

( a ) models the products of the air gasification of agricultural plants; (b) models the products of the air gasification of wood; (c) taken from Ref.8.

4.

CONCLUSIONS

The Co-MgO-ZSM-5 catalyst was found to be suitable for the synthesis of liquid hydrocarbons from the products of the air gasification of wood and agricultural plants, its activity and liquid hydrocarbon selectivity being higher than those of classical Fischer-Tropsch cobalt catalysts without zeolitic component

.

424

5. REFERENCES 1 V.U.S. Rao and R.J. Gormley, Catal. Today, 6 (1990) 207 2 L.A. Bruce, G.J. Hope and J.F. Matthews, Appl. Catal., 9 (1984) 351

R.L. Varma, N.N. Bakshi, J.F. Matthews and S.H. Ng, Can. J. Chem. Eng., 63 (1985) 612 4 K. Fujimoto, M. Adachi and H. Tomonaga, Chem. Lett., (1985) 3

783

5 Nguen Kuang Guin, N.E. Varivonchik, E.S. Shpiro, A . L .

Lapi-

dus and Kh.M. Minachev, Kinet. Katal., 28 (1987) 717 Sing, Surface area determination, D.H. Everett and R.H. Ottewill (eds.), Butterworths, London, 1970, pp. 25-42 7 A. Lapidus, A. Xrylova, V. Kazanskii, V. Borovkov, A . Zaitsev, J. Rathousky, A . Zukal and M. Jancalkova, Appl. Catal., in press 8 A.Yu. Krylova, A.L. Lapidus, F.T. Davlyatov and Ja.M. Paushkin, Dokl. Akad. Nauk SSSR, 304 (1989) 162 6 K.S.W.

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 1991Elsevier Science Publishers B.V., Amsterdam

Shape-Selective Reforming: Possible Reaction Pathways on Platinum-Containing Erionite/Alumina Catalysts HEIKO KALES,

FRANK ROESSNER~,HELLMUT G. KARGE~

and KARL-HERMANN STEINBERG' Universitat Leipzig, Fachbereich Chemie, Institut fiir Technische Chemie, LinnbtrafJe 3-4, 0-7010 Leipzig, F.RG. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, W-1000 Berlin 33, F.R.G. ABSTRACT The effect of admixing the narrow-pores zeolite erionite with a conventional platinum / alumina reforming catalyst on its shape-selective reforming behaviour is described. A mixture of model hydrocarbons was converted on the erionite-containing catalysts in a fixed-bed plug-flow microreactor at 24.5 bar. 1.r. measurements were carried out to clarify the assignment of the OH vibration bands and the accessibility of the acidic Bronsted sites in the erionite pore system for some hydrocarbons being involved in the conversion. The results are interpreted in terms of Hougen-Watson reaction models regarding the particular effects of the erionite on the classic reforming reaction network. INTRODUCTION The efficiency of the catalytic reforming process is determined by the relationship between the octane number (ON) and the liquid yield. For improvement of the ON of reformates the n-alkanes can be hydrocracked shape-selectively on narrow-pores zeolites, i.e. in case of the Selectoforming process (ref. 1) on metal containing H-erionite. During the past 15 years efforts were directed towards the integration of the shapeselective catalyst into the reforming unit (ref. 2). During the industrial application of a Pt/H-erionite catalyst in three reforming units in Germany, starting in 1978 (ref. 3), it has been observed that besides the intended shape-selective hydrocracking of the n-alkanes, an additional formation of cyclic compounds took place. This was surprising, since due to the size of the pore-openings (0.36

x 0.52 nm) of the erionite no hydrocarbons except n-alkanes are expected to be able to enter the erionite super cages (Fig.1gives the actual crystallographic data available for erionite). The major part of the additionally-formed cyclic compounds consists of cyclopentane derivatives. Cyclopentane derivatives are valuable fuel components, es-

425

426

I

pecially in view of environmental demands, toxicity and front octane number. This study of the conditions and reasons for the particular and unexpected effect of erionite as a reforming catalyst double B-rlna unlt

will attempt to clarify the accessibility of active centres inside the

canwlnlte cage nuper cage (owmen 8-W owning, 0.31 x a62 nm) a 1.981 nm c = 1.616 nm

-

erionite, the decisive steps in the shape-selective reforming process and the basis for further model-

ling of the rather complex proFig.] Model of the erionite structure

cess.

EXPERlMENTAL Catalvtic b e r i m e n t s NaK-erionite [Na4.11&.89(A10J9(SiOJn] was supplied by CKB Bitterfeld (F.RG.). The material was exchanged 3 times with NH4N0, solution under intermittent calcination in

air. It was then mixed and extrudated with boehmite. For the i.r. measurements, a particular series of ammonium exchanged erionites (37, 7 , 9 3 and 97 % exchange degree) was prepared. Except for the 0.37 NH4-erionite, all samples were calcined at 720

K between two exchange steps. Platinum was introduced by impregnation with an aqueous solution of HPtCl,. The microreactor used for this study and all the associated peripherals were described previously (ref. 4). The reaction products were quantitatively analysed by an on-line Siemens SiCHROMAT 2 capillary gas chromatograph equipped with a 50 m OV-1 fused Silica column and an FID. Data were stored via a RS 232 interface in archive files. Data handling and interpretation were performed by a particulary designed program package [on IBM-AT 2861 (ref. 5). The composition of the feed for all catalytic experiments (except those of Fig.6, vide infra) was 34.24 wt% n-heptane, 39.62 wt% methyl-cyclohexane and 25.87 wt% toluene. 1 g of the catalyst (grain fraction 0.3150.400 mm) was activated in situ at normal pressure according to an automatically controlled oxidation/reduction program (refs. 4,23). After this pretreatment the catalyst was exposed for 8 hours to the feed/hydrogen mixture to achieve a state of relative stability, taking into account the significant differences of the initial activity and the activity near steady state conditions. The hydrocarbon conversion was

427

carried out with a hydrogen/ hydrocarbon molar inlet ratio of 5.69. The conditions were varied as follows,

TABLE 1 Variation of temwerature, space velocify and content of erionite in the catalyst

I

varameters ~~~

I

unit

I

1I

temperature

K

673, 693, 713, 733

space velocity

h’

2, 2.6, 4, 6, 8

erionite content

wt%

0, 5, 10, 15, 25

RESULTS AND DISCUSSION 1.r. Spectroscopic Studies Fig. 2 shows the i.r. spectra of ammonium-exchanged erionite activated at 670 K. OH vibrations are observed at 3740,3690, 3660, 3610 and 3560 cm-’depending on the

ion exchange degree. The intensity of the band around 3610 cm-’increases with increasing ion exchange degree, whereas the band at 3690 cm-’decreases in the same order. After hydrothermal treatment caused by the intermittent calcination steps a band at 3660 cm-' appears in the spectra of the zeolites with an exchange degree higher than 77 % . Its intensity increases as well. Furthermore, a weak band of residual NH,-ions is

observed at 3260 cm-' . The assignment of the bands is difficult because of the complex composition of the system consisting of erionite, zeolite P and amorphous components (ref. 7). The band around 3610 cmd represents acidic bridged OH groups (ref. 7). The similarity of the building units of erionite and Y zeolite (single and double six rings, large cavities) allows the assignment to O(1)H groups located in the supercage (refs. 8, 9,lO). The presence of OH bands shifted to lower wavenumbers (3560 cm-') hints at perturbed vibration. Jacobs and Mortier (ref. 11) reported that such hydroxyl groups should be located in six ring units, i.e. they could be placed in the cancrinite cage forming 0(2,3,4)H groups similar to Y zeolites or they are bonded to O(5) or O(6) oxygens and

428

point into the supercage of the erionite structure.

- I I

A band near 3700 cm-' appears only in the spectra of synthetic (ref. 12) but not in that of natural erionites (refs. 8,13). This seems to indicate that the 3700 an-’band

represents OH groups of other phases formed during the (hydrothermal) synthesis. During thermal treatment these groups are irreversibly removed via dehydroxylation.

With respect to the band at 3660 cm-’contradictory explanations were published (refs. 7,9). However, recently published results on the dealumination of zeolites have shown that bands in the region 3650 - 3700 an-’indicate hydroxyl groups at octahedrally coordinated extra-framework aluminium (refs. 14,15). Taking into account that all samples with an ion exchange degree higher than 70 76 underwent a (hydro)thermal treatment it seems reasonable to attribute the band at 3660 an-’to OH groups associated 100

4000

with extra-framework aluminium. The presence of non-decomposed ammonium (3260 m") is further evidence for the formation of additional, strong acidic sites as proved by

N.M.R (ref. 9) and NH3-t.p.d. (ref. 7). Fig2 Formation of erionite hydroxyl groups with respect to exchange degree a) 37 %, b) 77 %, c) 93 %, d) 97 %

In order to check the accessibility of the OH groups, i.e. of the potential catalytic sites, different probe molecules were adsorbed. In Fig. 3 the spectra of the hydrocarbons

adsorbed on H-erionite are displayed. The adsorption of cyclohexa-1,4diene (dotted line) does not affect the intensities of the hydroxyl groups except the silanol groups at 3740 cm-'. Assuming that these groups are located at the external surface of the crys-

tallites a weak adsorption of cyclohexa-1,4-diene at these sites is understandable. The low intensities of the corresponding C-H stretching vibrations corroborate this interpretation. The adsorption of a linear molecule like hex-1-ene changes the intensities both of external and internal hydroxyl groups and intense C-H bands at 2870, 2940 and 2977 cm-' appear. If cyclopentene is adsorbed, the peak around 3610 cm-’ is shifted to 3560 cm-' and a broadening of the band indicates hydrogen bonds between the olefins

and the OH groups. The H-D isotope exchange between adsorbed molecules and the surface also provides information about the accessibility of the OH groups. OH groups were completely converted into OD groups absorbing at 2630, 2660, 2720 and 2750 crn-'

429

by exchange with 40. Simultaneously the bands in the OH vibration range disappeared (Fig.4, curves a). After exposure of CYCLOHEX l,+DIENE

the erionite to cyclopentene, the OH groups were restored and only a smdl amount of OD groups remained, with varied intensity ratio. This is another proof for the accessibility of sites inside the erio-

HEX.l-ENE

nite structure for cyclopentene. In particular, the band at 2720 cm-' vanished

-

and a shoulder at 3660 cm" attributed to

CYCLO P,ENTENE

OH groups at extra-framework aluminium L

10 wavenumber [

3400 Cm-'

4000

1

positions reappeared (Fig.4, curves b). Only the silanol groups remained unex-

Fig.3 Accessibility of hydmxyl groups for hydrocarbons

changed.

Catalvtic Studies Lumping of single hydrocarbons to pseudo-components (PC, for symbols see notation) is a possible way to simp19 and to manage the obscure reaction network of complex multi-component systems. As expected, the selective n-heptane cracking was promoted by the addition of erionite to the catalyst system and by this the formation of C6--hydrocarbons. The cracking reactions took place in particular in the pore system of the zeolite leading preferentially

Lu

9

to propane and n-butane production. The

U

C4/C3 molar ratio in the cracking pro-

d a

9 a

ducts, which was significantly smaller than

d

that obtained at the erionite-free catalyst 2200

3000

3000

wavenumber

40C

em-’ I

(FigJ), may be regarded as an accidental confirmation for the "energy gradient selectivity" suggested by hlirodatos and

F i g 4 1.r. spectra of a) 091H-ERI twice deuterated at 570 K and pDlo= 900 Pa and for 30 min, b) 0.91H-ERI, 30 min after adsorption of cyclopentene (p = 160 Pa) at 300 K

Barthomeuf (ref. 16). Small pores, strong field gradients and energetic inhomogeneities create a special "constrain-

430

673

~~

~

733

673

673

733

733

~

~

673

733

673 733 Temperature [K] ~~~

Fig.5 C4/C3 molar ratio of cracking products as function of catalyst erionite content ERI [wt%],temperature and space velocity [h-'1

ing environment" within the erionite pore system, not only from the geometrical but also from the electrostatical point of view. However, what is more important and particulary noteworthy, erionite produced considerable amounts of cyclopentane derivatives (Fig. 6). If only n-heptane is used as a feed, in general larger amounts of cyclic

j

c

hydrocarbons are yielded with erionite compared to the conventional platinum/ in,

5N7 (m01961

alumina catalyst (0 wt% erionite), as shown in Fig.7 (data: ref. 17). Froment et al. have investigated the reforming of C6- and C7-hydrocarbons over Pt/alumina-, Pt-Re/alumina- and sulphided Walumina-catalysts (refs.

18,19,20,21) and have formulated and dis673

=

493 n3 temperature [El

733

cussed for these systems reaction networks of the pseudo-component type. These

OERI

5ERI

could be applied sucessfully to the modelling of the process. Based on experiments

1 Fig.6 Amount of cyclopentane derivates ( 5 ~ 7 as ) function of catalyst erionite content ERI [wta] and temperature [space velocity = 4 h-’1

of initial selectivities a reaction scheme Was found in which the formation Of 6ring naphthenes and aromatics from nheptane takes place via 5-ring inter-

431

mediates. Cracking products result according to this scheme from single- and multi-branched iso-paraffins (ref. 19). The question arises how the reaction network is modified by the erionite contained in the catalyst. erionite cantent of the catalyst [wtS.] E M A

B

W

C

Our i.r. spectroscopic and catalytic experiments prove that (i) erionite-containing catalysts crack much more than

Fig.7 Formation of cycles by erionite addition, feed n-heptane. A: 733 K, 8 h-' 8: 693 K, 8 h 1 C:733 K, 2h-I

erionite-free catalysts, first of all to propane and n-butane (ref. 4); (ii) erionitecontaining catalysts produce a much

larger amount of 5-ring naphthenes from methyl-cyclohexane than erionite-free catalysts; (is) erionite containing catalysts can produce, under comparable conditions, a higher total of cyclic hydrocarbons than erionite-free catalysts; (iv) there are catalytically active, strongly acidic Brensted sites inside the erionite supercages which are accessible for linear and cyclic C5-hydrocarbon molecules, but not accessible for iso-paraffins and cyclic C6-hydrocarbons. These observations give rise to the question wether there exists a correlation between the formation of C3 and C4 fragments inside the pore structure of erionite and the observed formation of cyclic compounds. El Tanany et al. (ref. 22) have investigated the n-heptane hydroconversion at 428 K and 1013 mbar on H-erionite. According to Haag et al. (ref. 26) and their results the formation of C3/C4 species should take place via a pentacoordinated carbonium ion by protolytic cracking. In a following chain process higher hydrocarbons may result. As our i.r. results show, n-heptane is able to reach the strongly acidic Brensted sites in the erionite supercage via the 8-ring pore opening (see Fig.1) where the cracking to C3/ C4 fragments occurs. Under the geometrically and energetically "constraining

environment" of the erionite pore structure the fragments are in a very close contact during the process of bond weakening, scission and new bond formation. This should facilitate new bond formations. The Brensted sites may stabilize possible 5-ring intermediates. By this route 1,2dimethyl-cyclopentaneand ethyl-cyclopentane may form in the erionite pore system. Outside the erionite, all well-known reaction pathways of bifunctional catalysis are open leading finally to toluene (Fig.8). However, the major part of the C3/C4 species should leave the catalyst as propane

432

j.

and n-butane. A direct n-heptane

b

cyclization to 5-ring naphthenes can not be excluded. The modifi-

@&

cation of the generally accepted

network of reforming reactions

"\

for the shape-selective reforming is shown in Fig. 9. The relations n-heptane o 5-ring naphthenes and 5-ring naphthenes o 6-ring naphthenes are still valid, but the

Fig.8 Bifunctional mechanism of cyclization from C3/C4fragments by erionite [me: metallic site ac: acidic site] underlying reaction mechanisms

are changed. These changes effect the form and parameters of the related reaction rate expression. The rate expressions for reforming with erionite-free catalysts (ref. 19) were modified in this sense and several variations corresponding to our hypotheses for the 5-ring formation were formulated in Hougen-Watson terms (ref. 23). The microreactor was described by the most simple mathematical model: the onedimensional pseudohomogeneous reactor model. The different rate expressions were tested for use by calculation. The initial value problem

dY = dz

f(y,T,a) ,z:=[0,11

w i t h y ( 0 ) =yo, y(1) = ?

(1)

was solved (for symbols see Notation). For the examination of the resulting system of differential equations under non-isothermal conditions a combination of an efficient integration procedure (Runge-KuttaFehlberg 4.5)(ref. 24) and of an algorithm developed by Nagel and Wolff for non-hear parameter optimization (based on Levenberg-Marquardt algorithm) were used (ref. 25). By variation of the rate expressions and by parameter optimization the differences between measured (mes) and calculated (cal) values of the pseudo-component concentration in the product stream were minimized

The results of the model calculations confirm the rate expressions obtained by van Trimpont et al. (ref.19) for the erionite-free catalysts.

433 The best fit with the erionite containing catalysts was obtained using the following modifications. (i) The desorption of 5-ring naph-

thenes controls the rate of 5-ring formation from both n-heptane and methyl-cyclohexane; the corresponding rate equations are:

I I

/I

nP7-5N7-6N7-

A

MB7

Fig.9 Reaction network shape-selective reforming of C7hydrocarbons over platinum-erionite/alumina catalysts

5N7 o 6N7

FfcLis the product of the adsorption-rate coefficient K for the adsorption of 5-ring-naphthenes and the concentration of free surface sites cp] The importance of the desorption step in the reaction network may originate from the specific interaction of the 5-ring hydrocarbon molecules with the erionite pore system. The shape-selectivity of the erionite component strongly influences the equilibrium reactions n-heptane o 5-ring naphthenes o methyl-cyclohexane o toluene because only n-heptane and the cyclopentane derivatives are able to reach the active Bransted sites. In addition, the mobility of these hydrocarbons in the erionite should be quite different. (ii) The cracking products (first of all propane and n-butane) are mainly formed from n-heptane. SUMMARY There are several significant differences between the shape-selective and the classical reforming process. It is observed that not only the cracking of n-heptane, but also the formation of cyclopentane derivatives and other cyclic hydrocarbons are strongly promoted by addition of the erionite component. From i.r. results we can conclude that the bands at 3560 and around 3600 cm-’( O D 2630, 2670 cm" ) represent structural OH groups and that at 3660 cm-' ( O D 2730 cm-') represents non-structural OH-groups being accessible to linear and cyclic C5-hydro- carbon molecules and interacting with them, i.e. they are potential catalytic centres. Furthermore, H-D exchange takes place on the sp2hybridized carbon atom and involves only acidic hydroxyl groups. These differences and results suggest that over platinum-erionitelalumina catalysts a synergism of the

434

well known n-heptane cracking in the internal pore system and the observed formation of cyclopentane derivatives is probably operative. ACKNOWLEDGEMENT Choice, implementation and tests for use of the algorithm for integration and optimization were carried out in close cooperation with Dr. H.G. Rex from the Departement of Mathematics at the University of Leipzig. NOTATION vector of parameter estimates ac ERI erionite k K equilibrium constant for reaction me and adsorption, respectively n P Pressure PC r reaction rate T y vector of observed values of the dez A pendent variable y (pseudocomponent partial pressures) nP7 C3 propane or hydrocarbon fragment C4 with 3 carbon atoms C6cracked products MB7 587 single-branched heptanes 5N7 6-ring naphthenes with 7 carbon atoms 6N7 a

acidic site reaction rate coefficient metallic site number of measuring points number of pseudo-components temperature normalized reactor length toluene n-heptane butane or hydrocarbon fragment with 4 carbon atoms multi-branched heptanes 5-ring naphthenes with 7 carbon atoms

REFEXENCJ3 /I/ /2/ /3/ /4/

/5/ /6/ /7/ /8/ /91

/lo/ /ill /12/ /131 /14/ /15/ /16/ /17/ /18/ /19/

/m/ /21 1 /22/ /Wl /241

/25/ /26/

N.Y. Chen, J. Maziuk, A.B. Schwartz, P.B. Weisz, Oil Gas J., & (1968) 154 N.Y. Chen, T.F. Degnan, Chem. Eng. Progr., 2 (1988) 32 K.-H. Steinberg, K-H. Nestler, K Becker, Acta Phys. Chim. Univ. Szeged, 2 (1985) 41 A. Mosch, J. Meusinger, K.H. Steinberg, Chem. Techn., -4l 1 (1989) 17 J. Meusinger, A. Mosch, K.H. Steinberg, Chem. Techn., 2 (1989) 76 H. Kalies, W. Kunze, program package (1990,1991) H. Karge, Z. phys. Chem. NF, 3 (1971) 133 H.G. Karge, Z. phys. Chem. NF, 122 (1980) 103 A. Kogelbauer, J.A. Lercher, K.H. Steinberg, F. Roessner, A. Soellner, R.V. Dmitriev, Zeolites, 2 (1989) 224 D.F. Best, R.W. Laffion, C.L. Angell, J. Phys. Chem., 77 (1973) 2183 F. Roessner, K.H. Steinberg, D. Freude, M. Hunger, H. Pfeifer, in H.G. Karge, J. Weitkamp (Eds.), Zeolites as Catalysts, Sorbents and Detergent Builders, Elsevier Sci. Pub., Amsterdam, 1989,421 A. Cichocki, J. Chem. Soc. Faraday Trans. I, 76 (1980) 1380 P. Jacobs, W.J. Mortier, ZEOLITES, 2 (1982) 226 G.P. Cinckaladze, M.K. Carhiani, G.V. Cicishvili, Z.V. Gjaznova, A.R. Nefedova, Z. fiz. chim., 59 (1985) 385 L.P. Aldrigde, C.G. Pope, New Zealand Journal of Science, 2 (1981) 263 U. Lohse, E. Liiffler, M. Hunger, J. Stijckner, V. Patzelova, ZEOLITES, 2 (1987) 11 A. Coma, V. Fornes, F. Rey, Appl. Catal., 59 (1990)267 C. Mirodatos, D. Barthomeuf, J. Catal., 93 (1985) 246 J. Meusinger, A. Mosch, Diss. A, Univ. Leipzig, 1986 G.B. Marin, G.F. Froment, Chem. Engn. Sci.,-73 5 (1982) 759 P.A. van Trimpont, G.B. Marin, G.F. Froment, Appl. Catal., 24 (1986) 53 P.A. van Trimpont, G.B. Marin, G.F. Froment, Ind. Eng. Chem. Res., 2 (1988) 51 P.A. van Trimpont, G.B. Marin, G.F. Froment, Ind. Eng. Chem. Fundam., ZS(1986)544 A. El Tanany, G.M. Pajonk, K.H. Steinberg, S.J.Teichner, Appl. Catal., 39 (1988) 89 H. Kalies, Diss. A, Univ. Leipzig, 1991 G.E. Fotsythe, M.A. Malcolm, C.B. Moler, Computer Methods for Mathematical Computations, Prentice-Hall, Englewood Cliffs, 1977, 129 G. Nagel, W. Wolff, Biom. Z., & 6 (1974) 431 W.O. Haag, R.M. Dessau, Proc. 8th Int. Congr. Cat., Berlin-West, F.R.G. July 2-6, 1984, Verlag Chemie Weinheim, Vol. 11, 1984, 305

-

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

435

Framework Ordering in Aluminophosphate Molecular Sieves Studied by 27Al Double Rotation NMR B.F. Chmelka, Y.Wu, R. Jelinek, M.E. Davis?, and A. Pines Materials Sciences Division, Lawrence Berkele Laborato and Department of Chemistry, University of California, Berkeley, Ca ifornia 94%0 U.S.A.

Y

?Chemical Engineering, California Institute of Technology, Pasadena, California 91125 U.S.A.

Abstract Aluminum-27 Double Rotation NMR spectroscopy (DOR) has been used to investigate framework ordering in the aluminophosphate molecular sieves VPI-5, AlP04-5, and AlPO4-8. Well-resolved peaks in the 27Al DOR spectra of both hydrated and dehydrated VPI-5 allow isotropic shifts to be correlated with local framework structure. More distorted aluminum environments are reflected by broader lines in 27Al DOR spectra of AlPOq-5 and AlP04-8.

1. Introduction Framework aluminum atoms play a crucial role in establishing the adsorption and reaction properties of zeolite and aluminophosphate molecular sieves. It is, consequently, important to understand how molecular sieve structure correlates with the locations and interactions of guest molecules adsorbed at lattice sites within porous aluminosilicate or aluminophosphate matrices. While 27Al NMR is known to be a sensitive probe of solid microstructure [i-51, it has previously experienced limited application to the study of framework ordering in molecular sieves, largely because of troublesome broadening from anisotropic second-order quadrupolar interactions that dominate the NMR spectra of 27Al species in many polycrystalline solids. By using the technique of Double Rotation NMR (DOR) [6,7], however, this broadening can be eliminated completely to provide new insight into bonding arrangements of framework atoms in molecular sieves. High-resolution 27Al DOR can, thus, be used to examine subtle structural changes in molecular sieves produced by adsorption of guest molecules or by thermal treatment at elevated temperatures. We have previously published a preliminary report on interactions between the porous aluminophosphate VPI-5 and adsorbed water [8]. We focus here on the

436

different framework ordering properties of closely related VPI-5,A1Po4-5, and AlPO4-8 molecular sieve structures.

2. Experimental Dehydration of 0.30 g each of hydrophilic aluminophosphates VPI-5, Ap04-5, and APO4-8 was carried out at torr as described below: VPT-5was evacuated at room-temperature for 72 h; Ap04-5 was evacuated at room-temperature for 48 h, followed by 5 h at 473 K; AlPO4-8 was evacuated at room-temperature for 48 h. The AlP04-8 sample was prepared initially by thermal decomposition of nascent VPI-5at 373 K for 4 h. Schematic diagrams of the dehydrated framework structures for VPI-5 [9,10], AP04-5 [11,12], and AlPO4-8 [13,14] are shown in Figure 1. Xray diffraction data confirmed that all samples were highly crystalline with no impurity phases present. The dehydrated samples were transferred in a dry nitrogen glove box to air-tight sample spinners for use in the DOR experiments. After DOR experiments were performed on the dehydrated materials, the samples were rehydrated in air overnight, after which DOR experiments were again performed.

Figure 1: Diagrams of the dehydrated framework structures of large pore aluminophosphates (a) VPI-5,(b) AlPO4-5, and (c) AlP04-8. The lines represent bridging oxygen atoms bonded to aluminum and phosphorus atoms which alternate at the vertices.

437

36 PPm

V -18

I

60

I

I

40

I

I

20

1

I

0

1

I

I

-20

PP" Figure 2: 27Al DOR spectra at 9.4 T of (a) dehydrated and (b) hydrated VPI-5. Spinning sidebands are present on each side of the centerbands at integer multiples of the spinning frequency (800 Hz) of the large outer rotor used in the DOR experiments.

All DOR experiments were performed at room temperature on pulsed NMR spectrometers operating at either 104.23 MHz or 130.29 M H z , using home-built DOR probes that have been described elsewhere [15]. DOR spinning speeds of 5 kHz for the inner rotor and 800 Hz for the outer rotor were obtained routinely. Spectra were obtained using a 1-s delay between 45" pulses with 1000-2000 acquisitions. All isotropic 27Al shifts have been referenced to AI(NO& in aqueous solution.

438

3. Results and Discussion A104 and PO4 tetrahedra in VPI-5 are arranged with a high degree of order, so that under conditions of double rotation, high-resolution 27Al Nh4R spectra are obtained for both the dehydrated and hydrated materials. As shown in Figure 2(a), the 27Al DOR spectrum of dehydrated VPI-5 at 9.4 T contains two peaks, at 36 ppm and 33 ppm, that are attributed to different tetrahedrally-coordinated 27Al species. The linewidths of the peaks are approximately 200 Hz. Based on the dehydrated structure for VPI-5 in Figure l(a), the 2:l intensity ratio of the 27Al signals permits assignment of the peak at 36 ppm to tetrahedral aluminum sites in the six-membered oxygen rings and the peak at 33 pprn to tetrahedral sites in the center of the double four-membered rings [8]. Addition of adsorbed water dramatically alters the aluminum microstructure of VPI-5, as reflected by the markedly different 27Al DOR spectra for the hydrated and dehydrated materials in Figure 2. The peaks at 36 ppm and 33 ppm in Figure 2(a) have disappeared completely in Figure 2(b), as the presence of adsorbed water imparts a new microstructural configuration to framework aluminum atoms, which nonetheless retain a high degree of order. Two partially resolved peaks associated with tetrahedral 27Al environments are present at 41 ppm and 40 ppm (100 Hz linewidths) in Figure 2(b), together with a somewhat broader upfield peak at -18 pprn in the range ascribed to octahedral 27Al sites [l]. The integrated intensities of the three peaks in Figure 2(b) occur in a 1:l:l ratio. Certain aluminum atoms in the framework, thus, acquire octahedral coordination in the presence of chemisorbed water molecules, while others retain modified tetrahedral configurations [8]. High-resolution 27Al DOR spectra reveal ordered aluminum microstructures in both dehydrated and hydrated forms of VPI-5. The structure of AlP04-5 [Fig. l(b)] is closely related to that of VPI-5, though the six-membered oxygen rings of the former are separated by single (instead of double) four-membered rings, resulting in smaller main channel dimensions. The 27Al DOR spectrum of dehydrated AlP04-5 in Figure 3(a) contains a single peak at 36 ppm with a linewidth of approximately 200 Hz. The narrow linewidth indicates ordered framework aluminum positions within the dehydrated AlP04-5 lattice and is consistent with the lone tetrahedral aluminum environment expected from the structural configuration in Figure l(b). Adsorption of water molecules, however, has a much different effect on aluminum ordering in Ap04-5 than in VPI-5. The high degree of framework order in hydrated VPI-5, as measured by the well-resolved 27Al peaks in Figure 2(b), is in contrast to significantly less-ordered aluminum environments in hydrated AlP04-5. The 27Al DOR spectrum of hydrated ALP0 5 in Figure 3(b) has features that can be attributed to tetrahedral and octahedral 4;7Al species at 39 ppm and -14 ppm, respectively, similar to DOR results for hydrated VPI-5. However, after eliminating

439

36 PPm

39

)I 100

50

PPm

-14

0

-50

Figure 3: 27Al DOR spectra at 11.7 T of (a) dehydrated and (b) hydrated AlP04-5. The shoulders in (a) are spinning sidebands which occur at integer multiples of the spinning frequency (800 Hz)of the large outer DOR rotor. broadening contributions from fiist- and second-order interactions, including chemical shift anisotropy, dipole-dipole effects, and second-order quadrupolar effects, these peaks remain significantly broader than those observed in the 27Al DOR spectrum of hydrated VPI-5 [Fig. 2(b)]. A small, broad feature is additionally

440

39 PPm

40

I

75

I

I

50

I

I

25

I

I

0

I

I -25

PPm Figure 4: 27Al DOR spectra at 11.7 T of (a) dehydrated and (b) hydrated AlP04-8. present near 7 ppm in a region of the spectrum where lines from penta-coordinated aluminum species have been observed in the aluminophosphate AlP04-21 [16]. These observations reflect a distribution of aluminum environments in hydrated AlP04-5, consistent with a highly strained framework configuration or a random distribution of water molecules adsorbed within the sieve channels. The presence of double four-membered oxygen rings apparently allows the VPI-S/H20 system to adopt an ordered configuration which is not possible in AlP04-5. The absence of

44 1

such double four-membered rings in the AP04-5 lattice, where only single fourmembered oxygen rings separate the hexagonal six-rings [Fig. l(b)J, produces a hydrated aluminum microstructure that is much less ordered than in VPI-5. Similar hydration effects have been observed in the aluminophosphate sieve AlP04-25, which also possesses a structure containing single four-membered rings positioned between hexagonal six-rings [17]. The features in the 27Al DOR spectrum of hydrated AlP04-25 at 11.7 T [ 161 are essentially identical to those present in the DOR spectrum of hydrated AlP04-5 shown in Figure 3(b). Changes in aluminophosphate sieve structure produced by treatments at elevated temperatures can similarly be followed by 27Al DOR. For example, thermal treatment of VPI-5 at 373 K dramatically alters the aluminophosphate framework, inducing an irreversible phase transformation to AlP04-8, a material containing five crystallographically distinct tetrahedral A1 sites in its dehydrated form [Fig. l(c)]. The 27Al DOR spectra of AlP04-8 in Figure 4 are significantly different from those of VPI-5 in Figure 2, reflecting major modification of the VPI-5 structure after heating. Moreover, the broad 27Al lines in Figure 4(a) indicate substantial disorder in the aluminum environments of the dehydrated AlP04-8 framework, especially when compared with the well-resolved 27Al peaks of dehydrated VPI-5 [Fig. 2(a)] and dehydrated Ap04-5 [Fig. 3(a)]. This behavior is not unexpected, since AlP048 formed from thermal transformation of VPI-5 contains a high degree of stacking disorder [ 181. Upon rehydration, sharpened peaks appear at 40 and -7 ppm in Figure 4(b), reflecting a more ordered aluminum arrangement in AlPOq-8 with both tetrahedrally- and octahedrally-coordinated A1 species present. Nevertheless, the broad feature connecting these two peaks indicates a continuous distribution of aluminum environments spanning the range between these two relatively ordered sites. The sharp subsidiary peaks on either side of the peak at 40 ppm and also upfield of the peak at -7 pprn appear not to be spinning sidebands and may reflect additional ordering of the AlPO4-8 lattice. Adsorption of water, thus, imparts an additional degree of order to aluminum sites in the AlP04-8 framework.

4. Conclusions Resolution of peaks from distinct aluminum sites in 27Al DOR spectra of VPI-5 and dehydrated AlP04-5 permits isotropic shifts of the various framework 27Al species to be determined. This allows local nuclear structure to be correlated with perturbations of the molecular sieve lattice induced either by adsorption of guest molecules or by thermal modification. Highly ordered aluminum environments in both hydrated and dehydrated VPI-5 suggest intriguing adsorbate interactions with the host framework. In the case of AlP04-5, the highly ordered framework of the dehydrated material is replaced by disordered aluminum environments following incorporation of water into the pore spaces. In contrast, the disordered aluminum

442

microstructure of dehydrated AU’0,-8 is modified by adsorption of water molecules at framework sites, which apparently imparts additional order to the sieve structure. It is clear that ordering of framework aluminum atoms is highly dependent on local symmetry and bonding characteristics of the aluminophosphate matrix, both of which can be modified appreciably by interaction with adsorbed molecular guests. In circumstances where guest-induced changes in local lattice structure are significant, perturbations of the aluminophosphate framework are likely to have a substantial impact on the macroscopic adsorption and reaction properties of these materials. Acknowledgments. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. B.F.C. is a NSF post-doctoralfellow in chemistry.

5. References [l] C.S. Blackwell and R.L.Patton, J. Phys. Chem. 88 (1984) 6135. [2] R. Oestrike, A. Navrotsky, G.L. Turner, B. Montez, and R.J. Kirkpatrick, Am. Mineral. 72 (1987) 788. [3] L.B. Alemany, H.K.C. Timken, and I.D. Johnson, J. Mugn. Reson. 80 (1988) 427. [4] H.D. Morris and P.D. Ellis, J.Am. Chem. Soc. 111(1989) 6045. [5] S.F. Dec and G.E. Maciel, J. Mugn. Reson. 87 (1990) 153. [6] A. Samoson, E. Lippmaa, and A. Pines, Molec. Phys. 65 (1988) 1013. [7] B.F. Chmelka, K.T.Mueller, A. Pines, J. Stebbins, Y. Wu, and J.W. Zwanziger, Nature 339 (1989) 42. [8] Y. Wu, B.F. Chmelka, A. Pines, M.E. Davis, P.J. Grobet, and P.A. Jacobs, Nature 346 (1990) 550. [9] C.E. Crowder, J.M. Garces, and M.E. Davis, Adv. X-rayAnal. 32 (1989) 503. [lo] J.W. Richardson Jr., J.V. Smith, and J.J. Pluth, J. Phys. Chem. 93 (1989) 8212. [ l l ] J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth, and J.V. Smith, ACS Symp. Ser. 218 (1983) 109. [12] J.W. Richardson Jr., J.J. Pluth, and J.V. Smith, Actu Crystullogr. C43 (1987) 1469. [13] R.M. Dessau, J.L. Schlenker, and J.B. Higgins, Zeolites 10 (1990) 552. [14] E.T.C. Vogt and J.W. Richardson Jr., J. SoZidStute Chem. 87 (1990) 469. [15] Y. Wu, B.Q. Sun, A. Pines, A. Samoson, and E. Lippmaa, J. Mugn. Reson. 89 (1990) 297. [16] R. Jelinek, B.F. Chmelka, Y. Wu, P.J. Grandinetti, A. Pines, P.J. Barrie, and J.A. Klinowski, J. Am, Chem. Soc., in press. [17] J.W. Richardson Jr., J.V. Smith, and J.J. Pluth, J. Phys. Chem. 94 (1990) 3365. [18] K.Sorby, R. Szostak, J.G. Ulan, and R. Gronsky, Cutul. Lett. 6 (1990) 209.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

443

A COMPUTER ANALYSIS OF ESR POWDER SPECTRA OF SILVER AND SODIUM CLUSTERS IN MOLECULAR SIEVES

M.G. Uytterhoeven, R. A. Schoonheydt K.U.Leuven, Centrum voor Oppervlaktechemie en Katalyse (COK), Kardinaal Mercierlaan 92, B-3001 Heverlee

Abstract An iterative computer program for the optimization of ESR powder spectra is described. Its applicability to metal clusters is demonstrated. Spectra of silver and sodium clusters in zeolites are generally recognized to be composed of two isotropic signals: metallic and ionic. An accurate iterative refinement requires introduction of a third underlying signal and dependence of the linewidth on the quantum number M,.

1. INTRODUCTION

In the literature one finds ESR spectra of (1) lattice ions: Co [ 11, Fe [2],Mn [3],V [4]; (2) exchangeable ions and their complexes: Cr [5], Cu [6], Fe [2], V [7]; (3) metal clusters: Ag [8,9], Na [lo, 1I], Cs [ 111 in molecular sieves. Interpretations are often based on an intuitive understanding or comparison of spectra after different treatments. The most advanced interpretations use spectral simulations based on a trial and error approach. Quantum chemical calculations on metal clusters in zeolite A [12] and semi-empirical ligand field interpretations of spectroscopic data of transition metai ions f6]have proven to be successful in structural characterizations of molecular sieves and their guest species. The present tendency in catalysis towards a more fundamental approach justifies the expectation that ESR, combined with other spectroscopic techniques, will become important. However, this requires an accurate and unambiguous parameterization of the ESR spectra. The parameter set thus obtained forms a firm basis for a theoretical investigation of the coordination environment of the paramagnetic entity. Some of the trial and error simulations have produced fittings of remarkable quality [6]. However, a good fit by mere observation does not necessarily prove the underlying assumptions. Moreover, powder spectra of molecular sieves are generally very complicated. This is mainly due to three reasons: (1) the spectra consist of several overlapping signals, associated with paramagnetic species on different sites; (2) the site symmetry in molecular sieves is low; (3) many effects - dipolar interactions, quadrupole

444

interactions, superhyperfine structure of the framework ions, line broadening due to field inhomogeneities - can distort the spectra. An iterative computer program for the optimization of ESR powder spectra is described. Its applicability to metal clusters is demonstrated. This article presents a working strategy to extract in a most accurate and objective way a maximum of information from the ESR spectra of paramagnetic species in molecular sieves.

2. THEORETICAL BASIS An ESR spectrum is usually presented as the first derivative of the absorption intensity with respect to the magnetic field as a function of that magnetic field. This derivative intensity I’(Bi) at magnetic field Bi is calculated as: I

I

X

In this formula, I is the nuclear quantum number, I’(Bi) the first derivative lineshape function, B, the resonance position and P the transition probability. 8 and b are the Euler angles expressing the orientation of the magnetic field vector B with respect to the principal axes of the tensors. Integration is needed since in powder samples, the crystallites take all possible orientations with respect to the magnetic field. Since the principal tensor axes and the crystal axes are assumed to be coincident, integration can be restricted to one octant of the unit sphere. The lineshape function L can be chosen to be Gaussian (LG’),Lorentzian or a linear combination of both (Lc):

(c)

L;

- L~ + m I

)

Variation of the linewidth r with the quantum number M, can be calculated by the empirical formula:

M,’ sums the quantum numbers MI of the nuclei of the cluster. A superposition of signals can be calculated:

j- 1

with s the number of composing signals and frj the fractional amount of the jth species.

445

The most general system is characterized by the spinhamiltonian:

quantifying Zeeman and hyperfine interactions respectively. The summation runs over all the nuclei of the cluster. Based on this spinhamiltonian, the resonance position B, is calculated using second order perturbation theory of non-degenerate systems. This method is justified when the Zeeman interaction dominates the hyperfine interaction. Isotropic, axially symmetric and non-axially symmetric systems can be handled. The transition probability P is calculated by the perturbation formula of Bleaney [13] corrected for the Aasa Vanngard factor [14,15]. The program contains an option for filtering of the noise [16,17]and for correction of the baseline for shift and drift by a modified method of Wyard [18] and Loveland et al. 1191. The program can either be used for simulations with all parameters fixed, or for optimizations, this is iterative refinement of the ESR parameters. The parameters are then varied until that combination is found for which the deviation between the calculated and experimental spectrum is minimal. The criterium function F to be minimized is defined as:

- C [Zh(Bi)-ZkBi)]2 P

F

i-1

where p is the number of sampling points, IYc(Bi)and I'E(Bi) are the calculated and experimental first derivative intensities respectively. To concentrate the optimization on spectral details, the spectrum can be fitted in segments or the criterium function F can be multiplied by a constant factor over a specified field range. The iterative routine is based on the conjugated vector method of Powell [20], with several modifications to ensure convergence [21].The parameters that can be varied are: Zeeman (g), hyperfine interaction (A), linewidth (rl,r2,r3),lineshape (m) and relative intensity (fr). Calculations were performed on a Digital VAXstation 3200. The preparation methods of the silver clusters in zeolite A and sodium clusters in zeolite X are described in reference [8] and [lo]respectively, together with the experimental specifications on the scanning of the ESR spectra.

4. RESULTS 4.1. Silvercluster in zeolites A

A visual inspection of the spectrum (figure 1) reveals an isotropic Ag,"+signal: the seven hyperfine lines are due to the interaction of the electron with six equivalent silvernuclei with nuclear spin I = 1/2. The parameters estimated from the plot are g = 2.025, A = 6.9 mT and r, = 2.5 mT. Starting from this parameter set g, A and rl were varied iteratively. The lineshape was chosen to be Gaussian. Although the fitting thus obtained was satisfactory, formula (3)

446

was used in a secondary refinement, now optimizing rZ and r3together with g, A and rl starting from the parameters obtained in the previous run. Finally, the lineshape parameter m was fitted too. With this procedure an excellent fit is obtained. The resulting parameters are: g = 2.0227, A = 6.890 mT, r, = 2.545 mT, rz = O.OO0 mT, r3 = -0.033 mT and m = 0.670. The criterium function was reduced by a factor 173.

Figure 1. Experimental (upper) and theoretical (lower) spectrum of Age+ in LTA 4.2.Sodium clusters in zeolite X The spectrum of figure 2 was interpreted as due to isotropic Nqyf clusters, containing one unpaired electron delocalized over four equivalent sodium nuclei with I = 3/2. The parameters g = 2.002,A = 2.6 mT and r, = 1.7 mT were read from the plot. Starting from these values and assuming a Gaussian lineshape, the program optimizes the linepositions with reasonable accuracy. However, the intensities and the linewidth do not match satisfactorily. By introducing formula (3) and fitting the linewidth parameters iteratively, the criterium function is further reduced by a factor 4. Variation of the lineshape parameter m gives no substantial improvement: the ionic signal has a Gaussian character. The parameters g = 2.0001, A = 2.895 mT, I?, = 2.371 mT, rz = 0.005 mT, r3= -0.023 mT and m = 1.O correspond to the almost perfect fit of figure 2.

441

915.00

325.00

335.00

345.00

355. 00

965.00

MSNETIC F J U lmT1

Figure 2. Experimental (upper) and theoretical (lower) spectrum of Na4Y+ in zeolite X Figure 3 is a superposition of two signals: a sharp intense signal of Na: with g = 2.000 and r, = 1 mT and a Na4Y+ signal with g = 2.000, A = 2.8 mT and rl = 1.2 mT. The intensity of the ionic relative to the metallic signal is set to fr = 1.0. With these parameters as a starting set for the iterative refinement and with a Gaussian lineshape for both types of clusters, a reasonable fit is obtained (fig. 34. The parameters are: g = 1.9966, A = 2.613 mT, rl = 2.088 mT and fr = 21.4 for the ionic clusters; g = 2.0002 and rl = 0.810 mT for the metallic one. The criterium function concentrates mainly on the central field range, dominated by Na.: The sensitivity of the criterium function for the outer hyperfine lines and thus for the ionic signal is low. To orient the fitting towards the Na4Y+ signal, the criterium function was multiplied by 100 between the field ranges 0.318 T to 0.330 T and 0.345 T to 0.357 T. All the parameters of the metallic and the ionic signal were varied, except for the lineshape parameter m which was fixed on m = 1.00. In that way the ionic signal can be optimized perfectly (fig. 3b) with the parameters: g = 1.9968, A = 2.650 mT, fr = 14.161, rl = 2,122 mT, rz = -0.001 mT, r3 = -0.019 mT for the ionic cluster; g = 2.0000 and rl = 0.801 mT for the metallic cluster. A third underlying signal with an appreciable intensity is present. A lobe of positive intensity is clearly situated near 0.333 T (gcff= 2.016) as evidenced by the anomalously high intensity of the sixth hyperfine line. Since the exact form of the signal can not be derived from the spectrum, a simple axially symmetrical signal with g, = 2.016 and g, = 2.000 was superimposed. The final optimization (fig. 3c) gives: g = 1.9966, A = 2.644 mT, fr = 16.24, rl = 2.061 mT, r2 = -0.001 mT and r3 = -0.019 mT for the ionic cluster; g = 2.0002, fr = 1.00 and rl = 0.786 mT for the metallic cluster; g, = 2.0181, g, = 1.9910, rn = 1.326 mT, rl, = 3.260 mT and fr = 1.5 for the third signal.

448

, 3iB.00

1

1

325.60

341.40

333.60

349.20

357.00

MSNETIC F I M (&TI

Figure 3. Metallic and ionic sodium clusters in zeolite X. (a) Experimental. Fitted: (b) from initial guess; ( c ) concentrating on the outer lines; (d) including third signal. Table 1: Parameters of ESR spectra of sodium clusters (fr quantifies the relative amount of metallic versus ionic cluster) Metallic cluster

Ionic cluster

g 2.0001 1.9966 1.9989 1.9969 1.9957

A(mT) 2.895 2.644 2.909 2.923 3.329

(mT) 2.371 2.061 2.362 2.614 2.735

rl

(mT) 0.005 -0.001 -0.002 -0.004 -0.008

r2

(mT) -0.023 -0.019 -0.024 -0.020 -0.034

r3

2.0002 2.0025 2.0010 1.9957

0.786 1.046 1.007 1.578

0.062 0.006 0.009 0.070

449

Spectra with various ratios of intensities of the ionic and metallic cluster can be produced in zeolite X. They can all be fitted along the same lines as explained before. In table 1 the parameter sets of the signals for the different cases are summarized.

5. DISCUSSION

5.1. Program The successful strategy for the refinement of the powder ESR spectra is as follows: (1) Initial refinements are made as simple as possible, varying only a limited number of parameters. Further refinements can thereafter be performed, starting from the best result obtained. The complexity of the systems is raised gradually introducing secondary effects until no further substantial reduction of the criterium function F is obtained. (2) It can be necessary to concentrate the optimization on specific spectral details. This is done by optimizing the spectrum in smaller segments or by multiplying the criterium function F by a contant factor over a specified field range, as was done for the sodium clusters of figures 4. The optimizations are always performed, starting from the parameters estimated from the plot with no preliminary trial and error calculations. The criterium function is in many cases reduced by a factor 100 or even more. Therefore, our program appears to be fairly insensitive to the initial parameter estimation. 5.2. Interpretation A short discussion of the parameters of the A&,"+and Na4Ytclusters is appropriate at this time. The isotropic parameters of A g t + and the g-value very close but somewhat above that of the free electron value (Ag = 0.0204) point to an octahedral cluster. The 6 5s orbitals span the representations algtlueg.The positive g-shift is explained by the electron configuration a l l t,: e: or a, e: ,:t both corresponding to the cluster Ag6'. In the subject to a Jahn-Teller effect, An isotropic spectrum latter case the ground-state is oints to a dynamic Jahn-Teller effect. The former configuration yields the states 'E,, 'TZuand 4A,u, the latter being the groundstate. Due to octahedral symmetry the zero field splitting parameter D is zero and a single isotropic line is expected. Morton and Preston [9] favour the last explanation. Liquid Helium measurements (to convert the dynamic Jahn-Teller effect to a static one) are necessary to distinguish between both possibilities. The cluster Na4Y+also has isotropic parameters with (g) = 1.9976 and (A) = 2.9 mT or 82 MHz. This is explained as a tetrahedral cluster. The 4 3s orbitals span the representations a, and tz. Of the two possible configurations, .,It: and a:ti, the former is in agreement with the ESR parameters and the room temperature spectral observations. Thus, the cluster is written as Na?' with groundstate 'A,. Na,' is discarded because the groundstate 'T, is subject to a Jahn-Teller effect and ESR spectra will probably only be observed at low temperature. Again, liquid Helium measurements are necessary to proof unambiguously our conclusions.

' 'fig,

k"?

450

The average g-value of metallic Na,
ACKNOWLEiDGEMENTS

A research grant from the IWONL is gratefully acknowledged by M.G.U. The authors acknowledge financial support from the ministerie voor wetenschapsbeleid. (G.O.A. nr 86/91-98) and the I.I.K.W. (nr 4.0010.89). The authors thank Prof. P. Grobet and M. Gielen for making ESR spectra available.

1. 2. 3. 4.

L.E. Iton, I. Choi, J.A. Desjardins, and V.A. Maroni, Zeolites, 9 (1989) 535 A.V. Kucherov and A.A. Slinkin, Zeolites, 8 (1988) 110 D. Goldfarb, Zeolites, 9 (1989) 509 A. Miyamoto, Y. Iwamoto, H. Matsuda and T. h i , in P. A. Jacobs and R.A. van

45 1

Santen (Eds.), Studies in Surface Science and Catalysis, Vol. 49B, Zeolites: Facts, Figures, Future, Elsevier, Amsterdam, (1989), 1233 5. M. Huang, Z. Deng and Q. Wang, Zeolites, 10 (1990) 272 6. R.A. Schoonheydt, J. Phys. Chem. Solids, 50 (1989) 523 7. M, Huang, S. Shan, C. Yuan, Y. Li and Q. Wang, Zeolites, 10 (1990) 772 8. P.J. Grobet and R.A. Schoonheydt, Surf. Sci., 156 (1985) 893 9. J.R. Morton and K.F.Preston, Electronic Magnetic Resonance of the Solid state, J.A. Weil (Ed.), (1987) 295 10. M. Gielen, Alkali-metaalclusters in zeolieten, Thesis, K.U.Leuven, (1989) 11. K.W. Blazey, K.A. Miiller, F. Blatter, E. Schumacher, Europhys. Lett., 4 (1987) 857 12. R. A. Schoonheydt, M.B. Hall, J. H. Lunsford, Inorganic Chemistry, 22 (1983) 3834 13. B. Bleaney, Philos. Mag., 42 (1951) 441 14. R. Aasa, T. Vanngard, J. Magn. Reson., 19 (1975) 308 15. J.R. Pilbrow, J. Magn. Reson., 58 (1984) 186 16. M.U.A. Bromba, H. Ziegler, Anal. Chem., 55 (1983) 1299 17. A. Savitsky, M.J.E. Golay, Anal. Chem., 36 (1964) 1627 18. S. J. Wyard, J. Sci. Instrum., 42 (1965) 769 19. D.B. Loveland, T.N. Tozer, J. of Physics E, 5 (1972) 535 20. M.J.D. Powell, Comp. J., 5 (1962) 147 2 1. R.P. Brent, Algorithms for Minimization without Derivatives, Prentice-Hall, Engelwood Cliffs, New Yersey, (1973) 22. L.R.M. Martens, P.J. Grobet, W.J.M. Vermeiren, P.A. Jacobs, in Y. Murakami, A. Iijima, J.W. Ward (Eds.), New developments in Zeolite Science and Technology, Kodansha, Tokyo, (1989), 935 23. L.R. Gellens, W.J. Mortier, R.A. Schoonheydt, J.B. Uytterhoeven, J.Phys.Chem., 85 (1981) 2783 24. Unpublished results

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P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V.. Amsterdam

453

Magic-angle-spinning nuclear magnetic resonance and infrared studies on modified zeolites E. Brunner, D. Freude, M. Hunger, H. Pfeifer and B. Staudte

Sektion Physik der Universitat Leipzig, LinnhtraJe 5 , 0-7010 Leipzig, Germany

Abstract

llB MAS NMR yields quantitative information about the incorporation of boron into zeolite frameworks. lH MAS NMR and IR spectroscopy show that OH groups introduced into the framework by boron substitution are nonacidic. 2D proton spin diffusion measurements of the zeolite SAPO-5 reveal that defect OH groups are adjacent to acidic bridging hydroxyl groups and do not exist in an amorphous phase. Strongly adsorbed water molecules in mildly steamed zeolites H-Y can be explained by Lewis sites.

1. INTRODUCTION The study of zeolite adsorption sites capable of donating protons to or accepting electron pairs from molecules adsorbed on these sites is one of the most important areas in heterogeneous catalysis. In this paper recent results of MAS NMR and infrared studies on modified zeolites will be presented. The application of 2D 'H MAS NMR gives new information about the Bronsted and Lewis sites.

2.ZSM-5-TYPE BOROALUMINOZEOLITES ZSM-5 zeolites can be synthesized without an organic template by hydrothermal synthesis but only with relatively high A1 contents (Si/A1 = 10 - 20) [l]. We have studied this synthesis with a batch composition with 4 Na,O (1-x)Al,O, xB,O, YSiO, 0 I x I0.6, 20 Iy I40 to determine the coordination state of boron atoms in zeolites. llBMAS NMR measurements were generally carried out on rehydrated samples, which were kept in a desiccator for 48 h over aqueous NH,Cl. For 'H MAS NMR and NIR samples, shallow-bed like activation conditions were utilized in a glass tube of 5.5 mm inner diameter and with 10 mm bed-depth of zeolite. The temperature was increased at a rate of 10 K per h. The samples were kept at the final

454

activation temperature of 673 or 873 K under a pressure below Pa for 24 h and then cooled and sealed. A modified BECKMAN photospectrometer and a Bruker MSL 300 NMR spectrometer equipped with a home made MAS probe were used. As shown in Figure 1, llB MAS NMR spectra demonstrate the boron incorporation into the zeolite framework during the synthesis. The narrow line at -3.6 ppm is due to tetrahedrally coordinated framework boron and the broader line at ca. -2.0 ppm is caused by boron atoms in the amorphous part of the sample. For a crystallization time t k > 7 h the boron is completely incorporated into the framework.

c

=

34% F = 2 8 %

7h

5

0

-5

Figure 1 . IIBMAS NMR spectra of boroaluminozeolites for various crystallization times $. C denotes the crystallinity of the sample determined by X-ray diffraction and F is the fraction of boron incorporated into the framework which is determined by I'B MAS NMR.

In addition, we have studied the hydrogen form of a zeolite ZSM-5, containing 0.72 boron atoms per unit cell. The results of 'H MAS NMR, NIR and llB MAS NMR studies are given in Figure 2. Three types of hydroxyl groups can be found in the spectra of boron containing zeolites in the hydrogen form: SiOH groups on the surface of zeolite crystallites and at defect sites giving rise to a signal at 2.0f0.2 ppm (line a), SiOHB groups causing a line at 2.5f0.2 ppm (line a') and bridging SiOHAl groups giving rise to the signal at ca. 4.0 ppm (line b). This shows that the SiOHB groups are much less acidic than the bridging hydroxyl groups. Lines a and a' cannot be resolved in Figure 2. The chemical shift of the centre of gravity of the overlapping signals

455

(2.3 ppm in Figure 2A) increases with the concentration of boron in the framework. The concentration of SiOH groups is independent of the boron content. Concerning the concentration of Bronsted sites we found that for the hydrogen form of the zeolites, the number of bridging hydroxyl groups is in good agreement with the number of framework aluminium atoms determined by 27Al MAS N M R of the hydrated samples.

6Mlpprn

6 4

2

0

4400 4700

Vlcrn-'

d,lpprn

0

-5

Figure 2. 'H MAS N M R (A), NIR (B) and "B MAS N M R (C) spectra of a zeolite H-ZSM-5 containing 0.72 boron atoms per unit cell.

The NIR spectrum in the OH combination vibration region shows two peaks: the band due to bridging OH groups at 4655k 10 cm-l and the band of the nonacidic SiOH groups at 4545& 10 cm". The signal of the SiOHB groups is (as in the lH MAS N M R spectra) superimposed on the signal of the SiOH groups. An additional band appears at 4385f50 cm-' due to non-acidic AlOH groups. The llB MAS N M R spectrum of the hydrated sample shows one line at -3.3f0.1 ppm which is assigned to boron atoms incorporated into the zeolite framework. Other samples which are not completely crystalline contain boron in the amorphous part, giving rise to a broad line at about -2.0 ppm. Therefore, the intensity of the line at -3.3ppm may be used to determine the concentration of framework boron atoms.

3. SAF'O-ZEOLITES The preparation and measurement of the samples SAPO-5 and SAPO-11 [2-31 were carried out in the same way as described above. The spectra of 'H MAS N M R and NIR measurements are shown in Figures 3 and 4. Four lines or bands of OH groups appear: Two N M R signals (line b at 3.9 ppm and line c at 4.9 ppm) and two bands in NIR (4677cm-I and 4608 cm-l) are caused by OH groups with and without interaction with lattice oxygens, respectively. The line at 2.1 ppm (line a) is connected with SiOH and possibly with POH groups. The band at 4380 cm-l due to non-acidic AlOH groups corresponds to a signal at about 3 ppm, which could not be resolved in the 'H MAS N M R spectrum.

456

sj -I; -B

dnlpprn 6 4 2 0

4400 4700

’Jlcm-’

Figure 3. ‘H MAS NMR (A) and NIR Q3) spectra of SAPO-5 pretreated at 673 K.

Figure 4. N M R (A) and NIR (B) spectra of SAPO-11, pretreated at 873 K.

The 2D proton spin diffusion spectrum (NOESY experiment [5]) in Figure 5 shows cross peaks due to lines a and b, hydroxyl groups at defect sites and bridging OH groups, respectively. These cross peaks are caused by spin diffusion during the mixing period of 50 ms. Since line b is caused by bridging hydroxyl groups in the main channels of the SAPO-5 framework, the adjacent defect OH groups which are included in the spin exchange process must be located in these channels.

I

I

I

I

I

I

- 4

-

*

i l -

I

I

8

I

I

4

I

I

O

I

6”IPPrn

Figure 5. Contour plot of the 2D spin diffusion spectrum of the hydroxyl protons of SAPO-5. S (tl, 5) has been recorded with a d2-tl-~12-r-d2-t2 experiment, a mixing p e r i a of r =50 ms and a MAS frequency o f l kHz. Asterisks denote cross peaks of lines a and b.

457

4. DEALUMINATED ZEOLITES H-Y The ammonium form of the Y zeolite was prepared by an 88+_3% cation exchange in an aqueous solution of ammonium nitrate starting from Na-Y (Zeosorb Y, Chemie AG Bitterfeld-Wolfen, Si/A1=2.6). The hydrothermal treatment was carried out in a tube of 5 mm inner diameter with 8 mm maximum bed depth containing ca. 2 g of the zeolite H-Y. At first, the temperature was increased up to 810 K at a rate of 10 K min-' in a water-free nitrogen stream of 1 dm3 min-'. Finally, the sample was steamed at this temperature for 20 h under a vapour pressure of 4 kPa, adjusted by the temperature of the water bath through which the nitrogen was flowing. For the ’H MAS NMR measurements all samples were pretreated as described above at 673 K. Before being sealed, the samples were loaded at room temperature under vacuum with doubly distilled water. A pulse length of 4 p s for the a/2 pulse, a recycle delay of 10 s and a rotational frequency of 2500+5 Hz were used for the 'H MAS NMR and 2D experiments. The 27Al NMR pulse length of 1 ps corresponds to a a/12 pulse for non-selective excitation.

E

G _ ( 1

~ , , I D o ~ 60

0

Figure 6. 'H MAS NMR spectra of the hydrothermally dealuminated zeolite (DeY), activated under 670 SB conditions: unloaded (A), rehydrated with up to 10 H20 per unit cell (B) and (C), ca. 40 H20 per unit cell (D) and after full (ca. 240 H 0 per unit cell) rehydration (E), and 27kl N M R and "A1 MAS NMR spectra of samples (D) and (E); (F) and (G), respectively. Asterisks denote MAS sidebands.

458

Figure 6 shows 'H MAS NMR spectra of the hydrothermally dealuminated zeolite H-Y (DeY) with a framework Si/Al ratio of 5.0k0.5.The spectrum of the dehydrated sample (Figure 6A) consists of two signals, a peak at 1.8k0.2 ppm due to 17+3 silanol groups per unit cell, on the external surface of the crystallites and at crystal defects (line a), and a shoulder at 4.0+0.2 ppm due to 5.6* 1.5 bridging OH groups per unit cell (line b). As shown in Figure 6B, after the adsorption of water, a line at 6.5f0.2 ppm appears, not shifted in frequency up to a loading of 10 H,O per unit cell (Figure 6C). The maximum concentration of water molecules giving rise to this line is 2 H20 per unit cell. The shoulder due to bridging OH groups is not affected by a small loading (Figure 6B). With a higher loading a broad line appears at ca. 4 ppm which is caused by physically adsorbed water molecules (Figure 6D). In Figures 7 and 8 the 2D proton exchange spectrum of the hydrothermally dealuminated zeolite H-Y loaded with ca. 40 H,O per unit cell (identical to the sample in Figure 6C) is shown as contour and stacked plot, respectively. A homonuclear 2D ROESY pulse sequence [5] and a spin lock pulse of 2 ms were used. The absence of cross peaks between the signals at 1.8k0.2 ppm and 6.5k0.2 ppm shows that there is no chemical exchange in the time scale of one millisecond and that the latter signal is caused by protons of strongly adsorbed water molecules probably located at Lewis sites [4].

-

d"/PPrn

- 0

- 4

-

8

Figure 7. Contour plot of the of hydrothermally dealuminated zeolite H-Y loaded with ca. 40 H,O. S(t,, t,) was recorded with a homonuclear d2-t -rSL- experiment (ROESY), spin lock pu se of

?

459

n

F

Figure 8. Stacket plot of the 2D proton exchange spectrum of hydrothermally dealuminated zeolite H-Y loaded with ca. 40 H,O. S(t,, tz) was recorded with a homonuclear

5. ACKNOWLEDGEMENT We are grateful to Dr. E. Jahn (Berlin) and Dr. W. Schwieger (Halle) for providing the zeolites.

6. REFERENCES 1 W. Schwieger, K.-H. Bergk, D. Freude, M. Hunger, H. Pfeifer, ACS Symp. Ser. 398 (1989) 274. 2 D. Freude, H. Ernst, M. Hunger, H. Pfeifer, E. Jahn, Chem. Phys. Lett. 143 (1988) 477.

3

U. Zscherpel, B. Staudte, E. Loffler, C. Peuker, E. Jahn, Z. Phys. Chem. 270 (1989) 207. 4 M. Hunger, D. Freude, H. Pfeifer, J. Chem. SOC.Faraday Trans. 87 (1991) 657. 5 R.R. Ernst, Chimia 41 (1987) 323.

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V.. Amsterdam

461

129Xe NMR study of intra- and inter- crystallite diffusion of cations in faujasite zeolites J.Fraissard; A.Gedeon; Q.Chen and T.lto Laboratoire de Chimie des Surfaces , associe au CNRS URA 1428, Universite Pierre et Marie Curie, 75252 Paris Cedex O5,France

Abstract Xenon NMR spectroscopy was used to characterize xenon in Ca, Mg, Ni, Ag, La, Ce and Rb - exchanged Y and X zeolites. We report here some examples concerning the location of these cations and their inter - and - intra crystallite diffusion INTRODUCTION Since the first i29Xe NMR study of xenon adsorbed on a zeolite, this technique has been shown to be of interest for the investigation of the distribution and the size of supported metal particles, the quantitative distribution of phases chemisorbed on these particles, the dimensions of the void spaces of zeolites, the detection of structure defects, the location of cations and the effect of electric fields they create [I ,2 1. We report here some typical applications related to the study of intra-and inter-crystallite diffusion of cations in faujasite zeolites.

Chemical shift of Xe adsorbed in a pure zeolite

As in the gas phase [3], the shift of xenon adsorbed in a pure zeolite is the sum of terms corresponding to the different perturbations to which it is subjected :

6

ref

is the reference,generally the chemical shift of gaseous xenon extrapolated to

zero pressure. 6 is characteristic of the interactions between the xenon and the walls of the cages and channels; it depends on the dimensions of the zones where the xenon is adsorbed and on the ease of its diffusion ; 8~and aM express the electrical and magnetic effects, respectively, due to the cations.bx, is associated

462

with Xe-Xe interactions ; it is proportional to the local xenon density. The amount of Xenon adsorbed is expressed as the number of Xe atoms,N, per gramme of anhydrous solid or N’, per supercage.

- CRYSTALLITE EXCHANGE OF CATIONS 1.Na or H - faujasite

I. INTRA

Let us consider first the case of Na - faujasite denoted Nay, where x represents the Si/AI ratio (12 8 < x < 54.2). The signal shift increases linearly with the adsorbed xenon concentration [Xe] but is practically independent of the value of x, therefore of the number of Na+ cations [l] . This result proves that in the Y supercages the time-average electric field < 6 p due to these cations is negligible at 25°C. The results relative to HY, are similar to the previous ones. At very low [Xe] the motion of each atom is disturbed only by cage walls. Consequently the chemical shift Ss (58 k 2 ppm) obtained by extrapolation of the line 6 = f[Xe] to [Xe] = 0 can be considered as characteristic of the zeolite with respect to xenon adsorption.The increase of 6 with [Xe] results from mutual interactions between Xe atoms. 2.CaY and MgY zeo1ites:Electrical effect of cations Let us imagine now that there are in the supercages of a Y zeolite M2+ cations such as Ca2+or Mg*+ which interact with xenon much more strongly than Na+, O-HE+or the cage walls. In this case and particularly at low xenon concentration, each Xe atom will have a relatively long residence time on these M2+ centres. The corresponding chemical shift S will be much greater than in the case of pure NaY. When Xe increases, S must decrease if there is exchange of these atoms adsorbed on M2+ with those adsorbed on the other sites (Na+,walls), and then increase with the number of Xe - Xe collisions.This is exactly what is found at 25°C for Mg,Y

and Ca,Y

zeolites,where h denotes the degree of exchange with Na+.

When M2+ cations are in the sodalite cage or in the hexagonal prisms ( h < 55%) 6 is a linear function of Xe, identical with that for Nay, whatever the extent of dehydration of the sample.When some M2+ cations are situated in the supercages ( h > 55%), one observes for MY (dehydrated under vacuum above 500°C ) variations in 6, compared to Nay, which are greatest when h is high, especially at low [Xe] ( Fig.1 ), and which correspond to the electric field effect in the supercages (GE#O).More precisely the experimental value of 6 for [Xe] = 0 , SS,M,is proportional to the square of the electric field at the Xe nuclei of Xe atoms adsorbed on M2+ cations [4]. The effect of the dehydration and rehydratation of M,Na,_,Y zeolites on

463

the influence of M2+ cations can also be studied by this technique [4]. 100

6

PPm

80

150 60 Nx 10 I

I

I

1

1

1

I

I

5 110

Figure 1. Dependence of the chemical shift of xenon on MghY zeolites.

L

N x 10

J

1

-5

3. Paramagnetic cations :Ni2+Y zeolites The problem is naturally more difficult in the case of paramagnetic cations, especially when the extent of exchange is so high that the magnetic term ZiM in equation becomes large, as has been shown by Gedeon et al.[5] and Bansal and Dybowski.[G]. Since this case has been treated in detail [ 5 ] we shall only summarize as an example the results for the Nil,Y sample. Figure 2 shows in turn the effect 0f:dehydration (1 , 2 ) ; the number of isolated Ni(H20)62+(3); the destruction of Ni(OH)+ and the migration of the cations outside the supercages (4, 5). 4 . AgX zeolites : Ag

- Xe complex

Fig.3 shows the adsorption isotherms of xenon in NaX and in AgX following various pretreatments. In comparison with NaX, the adsorption of xenon in dehydrated AgX as well as in the material treated in oxygen is strongly enhanced, especially at low pressures. After reduction with hydrogen at 100 and 300°C xenon adsorption decreases,yielding adsorption isotherms slightly above and distinctly below that of NaX, respectively. Compared to NaX with the linear 6 vs. N dependence of monovalent - ion - exchanged X and Y type zeolites, the shifts in

464

dehydrated and oxidized AgX are distinctly lower over the range of concentration studied.(Fig.4). Most remarkably, the shifts decrease with concentration down to negative values in the range -40 to -50 ppm at low xenon concentration (negative, i.e.low frequency with respect to the reference). In comparison with the dehydrated AgX the shifts for the oxidized form of AgX are lower by about 10 ppm. In our opinion, the unusual lZ9Xe NMR chemical shifts of the silver exchanged X zeolite are due to specific xenon / silver - cation interactions [ 7 1.

1.

0

1

N Xx 10-3 N 10-3

5-

-

10 P Pa

Figure 3. Xenon adsorption isothermes at 26°C

Figure 4. Chemical shift versus xenon concentration

[O NaX; rn AgX; VAgX (Ox.); XAgX( Red, 100°C); +AgX ( Red, 300"~)] This shielding indicates the formation of an unstable, short - lived Ag - Xe complex, due to a d, - d, donation from Ag+ to Xe involving the silver 4d - and 5d orbitals. The existence of this strong interaction is confirmed by the weak evolution of 6 for 0.1 IN I1. Buckingham et al.[ 8 ] have shown that in the case of Xe - 0, or Xe-NO contact shift can also be detected even if the lifetime of these complexes is short. Similar phenomena have been recently observed for the 77Se resonance in the presence of silver cations[ 91. The effect (chemical shift and adsorption isotherms) of treating the completely dehydrated AgX with oxygen at 400°C is strong indication that the low frequency shift from NaX is due to the interaction of Xe with Ag+ ions within the zeolite. Indeed, it is known [ 10 ] that after oxidation, the small AgOx aggregates formed during the vacuum dehydration process are dissolved in the zeolite yielding Ag+ ions which provide additional centers for strong adsorption of xenon increasing the xenon adsorption capacity of the zeolite (Fig .3) and, at the same time, further low frequency shift (Fig . 4). The greater the concentration of Ag+ cations in the supercages, the higher the Xe adsorption capacity and the more negative the chemical shift.ln contrast to these results,reduction at low temperature (100°C) leads to a high frequency chemical shift showing thus that the

465

interaction of xenon with silver clusters is not the source of strong adsorption and low frequency shifts. The relative low value of the chemical shift for AgX (100 Red.) at low xenon concentration indicates that after reduction at 100°C, some Ag+ cations are still present in the supercages. In the rapid exchange limit (which certainly is realised because of the single resonance line detected) the observed averaged shift is due to xenon atoms interacting with Ag+ sites (negative shift), and the Ago sites and the framework (positive shift).A higher reduction temperature in H 2 (300°C) results in high frequency xenon chemical shift from NaX.This 6 = f (N ) variation has the classical form for zeolite -supported metals : 6 which is high at low N ( strong metal - Xenon interaction ), decreases as N increases, due to rapid site exchange, then a new increase in 6 when the Xe - Xe interactions become sufficiently important. The very low xenon adsorption of this sample shows that the zeolite lost most of its crystallinity . However, it is known [ 11 ] that the decationized type X,lose their crystallinty after reduction at 300°C. The chemical shift observed in this case is due to the adsorption of xenon on the metal silver particles in a defect structure. 5.LaY and CeY zeolites : Migration of

6

Rare Earth cations

6 100

100

60

60

N x ’01 0

1

0 1

5

5

Figures 5 and 6 .Chemical shift versus xenon concentration for La12Na20Y(figure 5 left ) and for Ce13.zNal6.4Y (figure 6. right ) pretreated at T°C:060;A75;a95; 0150; r220;$300;~400;~00 and , for NaY sample pretreated at ToC:+60;X400;e 600

466

Figures 5 and 6 show the 6 - N variation in the case of Lay and CeY zeolites. When the water concentration is high the rare earth cations are surrounded by water molecules which prevent any strong interaction with Xe atoms;6 therefore varies goes from 2 to 1, ( DREis linearly with N. When 150 < Tt < 200 , that is when uILa

= f [ N ] curves show a the number of water molecules per rare earth cation ) the minimum , characteristic of the xenon interaction with a strong centre which can only be ( LaOH )2+. When nILa< 1, (Tt > 200°C ), B N - , ~should increase and the minimum should

be more pronounced if the La3+ cations stayed in the supercages. However, the chemical shifts are seen to decrease, the minimum weakens and shifts to smaller N =0 values, while the 8La = f [ N ] variation becomes again a straight line for (Ttz6OO0C). This evolution shows that in the range 200
ILINTER

- CRYSTALLITE EXCHANGE OF CATIONS

6 .Study of ion exchange between RbNaX and NaY

Figure 7 gives the 129Xe-NMRspectra obtained from xenon adsorbed at 300 K on a sample containing NaY and RbNaX zeolites. These zeolites have the same framework structure and have been studied extensively by Ito et al.[ 4 ] The two peaks in Fig. 7-(a), (I) 144 and (11) 89 ppm, correspond to xenon adsorbed in RbNaX and NaY zeolites, respectively. RbNaX has a higher xenon chemical shift than NaY at the same xenon equilibrium pressure due to its higher cation concentration inside the zeolite and, more importantly, to much higher polarizability of Rb+ than Na+ cations. Upon mechanical mixing at 300K, (b) , the intensities of the two signals decrease and a third broad signal (111) at about 141 ppm appears ( Fig. 7(b) ). After treatment at 673 K and 10-5 torr, (c), the spectrum 7(b) changes to (c) with a much greater decrease of the intensities of RbNaX and NaY signals , the disappearance

467

of signal (Ill),and the detection of two additional broader signals at 128 ( IV ) and 109 ( V ) ppm. These changes can be an indication either of cation exchange between these two zeolites, as for LiA and NaA zeolites under hydrated conditions [13 ] or of rapid exchange of adsorbed xenon atoms between different crystallites which causes coalescence of the NMR signals. To elucidate this point, low temperature experiments were conducted on sample (b) and (c).Fig. 8 shows the low temperature i29Xe-NMR spectra of xenon adsorbed on (b). It is clear that the two xenon resonance signals corresponding to the two pure zeolites can be restored upon cooling the sample to 200 K, which proves that the third signal in Fig. 7(b) is really due to coalescence of two signals. The mixture obtained directly in the tube by shaking is very inhomogeneous as regards the distribution of the crystallites of the two zeolites. There are three types of zone, namely pure RbNaX, pure NaY and RbNaX-NaY mixture. Because of diffusion, the three corresponding lines are distinguished at 300 K. Consequently,the information obtained by Xe-NMR at 300 K is characteristic of macroscopic zones, i.e. containing several crystallites. The extent of these zones depends on the mean life time of xenon atom in the crystallites compared to the NMR time scale. These zones can be reduced by lowering the temperature of the NMR experiment so as to obtain more and more localized information. The rate of exchange of xenon atoms between one crystallite and another must depend also on the crystallite size and above all on the barrier for diffusion to the external surface which is related to the size of the windows of the cavities and channels, other things being equal. The low temperature i29Xe-NMR spectra of xenon adsorbed on (c) ,show that, in contrast to the case of (b), down to 200 K, there are still four well resolved peaks with two corresponding to the original RbNaX and NaY zeolites. The other two signals located between those of RbNaX and NaY should correspond to Rb58-xNa24+xXand RbxNa56-xYzeolites, resulting from ion exchange between the two original solids .

C

I

Figure 7 . 12gXenon - NMR spectra of xenon adsorbed at 300K and 600 Torr :(a) before mixing; (b) after mixing and (c) retreated at 673 K under 10-4Torr.

Figure 8. Low temperature 129Xe - NMR on sample b: RbNaX + NaY mixture

468

CONCLUSION

The 1z9 Xe technique has been successfully applied to the study of the location of cations within the zeolites, and their inter- and intra- crystallite diffusion. It must be noted that when the 129 Xe - NMR technique is applied to large - pore zeolites, such as faujasite, the adsorbed xenon atoms exchange rapidly between zeolite crystallites. It is therefore necessary to choose the experimental conditions according to the nature of the sample , especially when the zeolite is mixed with a matrix. REFERENCES 1 T. Ito and J. Fraissard, J. in Proceedings of the 5 th Int. Conf.on Zeolites, Naples, June, (1980) 510. 2 J. Fraissard and T. Ito, Zeolites, 8 (1988) 350, and references therein. A.K. Jameson, C.J. Jameson and H.S. Gutowski, J. Chern. Phys., 59 3 (1973) 4540. 4 T. Ito and J. Fraissard, J. Chem. SOC.,Far. Trans I, (1987) 451. A. Gedeon, J.L. Bonardet, T. Ito and J. Fraissard, J. Phys. Chem., 93 5 (1989) 2563. 6 N. Bansal and C. Dybowski, J. Phys. Chern., 92 (1988) 2333. 7 A. Gedeon, R. Burmeister, R. Grosse, B. Boddenberg and J. Fraissard, Chem. Phys. Lett. (in Press). A.D. Buckingham, P.A. Kollmann, J. Phys. Chem. (1972) 65. a R. Diedon, L. Hevesi, Xth Meeting ISMAR, Morzine, France, 1989. 9 M.D. Baker; G.A. Ozin and J. Godber, J. Phys. Chem. 89 (1985) 305. 10 J.A. Rabo, RE. Pickert, D.N. Starnirus and J.E. Boyle, Second Int. Cong. 11 on Cat. ; Paris (1960) 2055. 12 Lanth. D.N. Breck. Zeolite Molecular Sreves. John W, ley, New York (1974) 549 C.A. Fyfe, G.T. Kokotailo, J.D. Graham, C. Browning,G.C.Gobbi, M.Hyland 13 G.J. Kennedy and C.T. DeShutter, J. Am. Chem. SOC.108 (1986) 522.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

469

Intracrystalline diffusion of benzene in Ga-silicate

a b A. Zikanova , P. Struveb, M. Biilow , M. Wallauc, M. Kocirik? d C A. Micke , A . Tisslere and K. K. Unger aJ. Heyrovsky Intitute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague, CSFR bZentralinstitut fiir physikalische Chemie, Berlin, FRG C Johannes Gutenberg Universitst, Mainz, FRG dKarl-Weierstrass-Institut fur Mathematik, Berlin , FRG eVereinigte Aluminium-Werke, AG, Bonn, FRG

Abstract

The sorption kinetics of benzene in large Ga-MFI crystals was investigated under constant volume- variable pressure conditions. A complete analysis of the uptake curves has been performed using solution of a nonlinear Volterra equation which describes the interaction of uptake process with the apparatus. Within the time scale of uptake measurements (103 -10 4 s) the uptake curves were found to be consistent with the solution of the second Fick’s law. Corrected diffusion coefficients were found to be essentially independent of loading within the loading range investigated and in contrast to the system benzene-HNaZSM-5 [1,2] their temperature dependence is much stronger.

1. INTRODUCTION

Ga-MFI zeolites have been reported to exhibit catalytic activity in conversion of light paraffins to aromatics [3] and therefore the diffusion data of aromatics are of interest to explain the mechanism of the catalytic reaction.There has been done much of both theoretical and experimental investigations on the motion of aromatics (benzene, toluene, xylene etc.) in ZSM-5 zeolite and silicalite crystals cf.e.9.

470 [1,2,4-81. Thus a study of sorption in Ga-MFI is desirable to bring a deeper insight into the intracrystalline mass transport of aromatics within MFI structure. It is the first time in the present paper that the uptake curves from a piezometric apparatus have been simulated by the solution of the Volterra integral equation [9,10] which reflects in detail the interaction of the sorption kinetics with the apparatus. This approach enabled us to get kinetic data with a high accuracy.

2.

EXPERIMENTAL.

Apparatus : Sorption uptake curves were measured by a constant volume variable pressure apparatus operated in a differential concentration mode The apparatus used is shown schematically in Figure 1.

.

Figure 1. Schema of the volumetric apparatus. V, p, T denotes the volume, sorbate pressure and temperature,

respectively. Indices d, v, s refer to the dosing vessel, sample vessel and to the sorbent, respectively. In all the measurements Td was kept equal to Tv and the pressure pd(t) was monitored by the membrane manometer Baratron. The vessels have been separated from each other by a valve with a

471

capacity K=K(t, pd, p,)

which had been determined in blank

experiments. Materials : Zeolite : Ga-MFI of the composition Si02/Ga203 = 92 was synthesized by M. Wallau. The habitus and size of the crystals can be seen from the electron micrograph shown in Figure 2. Thus the shape of the crystals is approximately that of a parallellepiped with the edge lengths La = Lb = 29.6 Dm and Lc = 1.8 La = 53.2 pm. Benzene : research grade supplied by BDH Chemicals Ltd. , England.

Figure 2. Scanning electron micrograph of Ga-MFI. Procedure : Prior to the kinetic measurements, the Ga-MFI crystals were detemplated in pure O2 stream under heating-Temperature was increased by 20 C/h up to 12OoC, then kept at 12OoC for 12 hours. Then the temperature was increased to 45OoC with the same heating rate. The sample was kept at 45OoC for 3 days. The uptake measurements were performed in three series at temperatures T = 343.2 K (with 26 pressure steps), T = 323.2 K (with 23 pressure steps) and at T = 303.2 K (with 16

pressure steps). The values of initial pressure pd(0) ranged from 5 Pa to 1 kPa. Prior to any series of experiments the sample (about 22.95 mg in mass arranged into a crystal monolayer) was heated under vacuum at 673 K for 24 hours. 3. RESULTS AND DISCUSSION

Equilibrium data : Adsorption isotherms evaluated from the three series of experiments are presented in Figure 3. From the equilibrium data the Darken factor aln p/aln a has been estimated. Isosteric adsorption heat qstamounts about -1 62.69kJ.mol-1 for loading a = 0.35 mmo1.g

.

1

200

400

600

800

Figure 3. Adsorption isotherms :

a . . .T =

303.2 K

;

A . ..T = 323.2 K;

o...

T

=

343.2 K

Kinetic data : The analysis of the uptake curves has shown that the intrinsic sorption kinetics are consistent with the solution of the second Fick’s law for a two-dimensional parallellepiped. The values of diffusion coefficient D were estimated by fitting the experimental uptake curves pd vs t by the solution of corresponding Volterra equation which

473

describes the interaction of the kinetics with the apparatus [9, 101. This has been carried out using the software package IIZEUS’I (Zeolite Uptake Simulator) developed by A . Micke from the Weierstrass-Institut f’;r Mathematik in Berlin. Examples of the fitting the uptake curves are shown in Figures 4 and 5.

41'0r i l-

T1

l-- T

-r7

I r

35.0

29.O b

23.0 . 2 . a

17.0

11 . oo b

5.0

5.00

AI p";"

10.00 10 .oo 1 5 . 0 0

. o o 25.00 3 0 . 00 20.00 20 $2 / p/

Figure 4. Uptake curve pd vs t1l2:T

=

343.2 K, pd(0) = 4 0 . 4 7 . .

Pa, pv(0) = 2.05 Pa, a(m) = 0.132 mmo1.g-l h experimental points, full lines pd vs t1l2(upper curve) , p, vs t1/2 (lower curve) simulated by ZEUS. The diffusion coefficients D and the corrected diffusion coefficients obtained as Do = D (aln p / a l n a) (1) are summarized in Figure 6 . It can be seen that within the loading range investigated the D values increase slightly with increasing a while Do values are essentially independent of a. The results are compared with Do for benzene - HNaZSM-5 [ 1,2]. The dashed lines represent the values of Do for the system benzene - Ga-MFI calculated using the upper branch

474

Figure 5. Uptake curve p vs t1/2;T = 303.2 K, d pd(0) = 53.17 Pa, pv(0) = 0 , a(m) = 0.113 mmo1.g[I experimental points, full lines pd vs t1i2 (upper curve), vs t1/2 (lower curve) simulated by ZEUS.

PV

of the Arrhenius plot of Do shown in Figure 7. It can be seen from Figure 7 that : a) there is a much stronger temperature dependence of Do for benzene - Ga-MFI as compared with that for the system benzene - HNaZSM-5 (dot-and-dashed line) b) Arrhenius plot of the system benzene-Ga-MFI seems to exhibit a nonlinear behaviour. In view of high accuracy of the present data we are inclined to beleave that there exist physical reasons for this nonlinear behaviour. The activation energy EA of selfdiffusion or the system benzene - Ga-MFI (calculated as the average value for both the branches of the Arrhenius plot) amounts about 78 kJ.rno1which is about three times higher than EA for the system benzene - HNaZSM-5 with EA = 26 kJ.mol-l.

I

Ga-MFI 363K -----------------

0

0.2

0.4 a(m)

0.6 irnrno~.gl/

Figure 6. Dependence of diffusion coefficients D and corrected diffusion coefficients D vs loading. 0

System benzene - Ga-MFI : D, Do at 303.2 K ( 0 , m ) 323.2 K ( A ,A ) ; 343.2 K (0 , @ )

---

D

0

calculated using E

A

= 78

;

kJ.mol -1

system benzene - HNaZSM-5 : D~ at 3 3 3 K ( + )

: 363 K ( X ) .

476

-21

t

uk$-e%i1000/T; T/K/

Figure 7. Arrhenius plot coefficients D 0 for the system -.-.-.Dependence of Do on - HNaZSM-5 at a loading a(m) = 4.

of the corrected diffusion benzene - Ga-MFI 1000/T for the system benzene0.4 mmo1.g -1 .

REFERENCES.

l.A. Zikanova, M.Biilow and H. Schlodder, Zeolites, 7 (1987), 115.

2.M. Bclow, J. Caro, B. Rxhl-Kuhn and B. Zibrowius, in Zeolites as Catalysts, Sorbents and Detergent Builders,H.G.Karge,J. Weitkamp,eds.,Elsevier, Amsterdam,l989, p.505. 3 . P. Meriaudeau, G. Sapaly, C. Naccache, in Zeolites : Facts, Figures, Future, P.A. Jacobs and R.A. van Santen, eds., Elsevier, Amsterdam 1989,p.1423. 4. B. Zibrowius, J. Caro, H. Pfeifer, J.Chem.Soc., Faraday Trans 1, 84, (1988), 2347. 5. V.R.Choudhary and K.R. Srinivasan, J.Cata1. 102, (1986), 328. 6. H.J. Doelle, J. Heering, L. Riekert and L. Marosi, J.Cata1.71,(1981), 27. 7. P. Wu, A . Debebe and Y.H. Ma, Zeolites 3 , (1983), 118. 8.K. Beschmann, S. Fuchs and L. Riekert, Zeolites 10,(1990), 798. 9. M.

Kocirik, G. Tschirch, P. Struve and M. Bihlow, J.Chem.Soc.,Faraday Trans. 1, 84, (1988), 2247. 10. A. Micke, M. Kocirik, M. Bihlow, P. Struve and G. Tschirch, unpublished resuls.

411

P.A.Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier SciencePublishersB.V., Amsterdam

Intracrystalline Diffusivities

or

HZSM-5 Zeolites

Kenji Hashimoto, Takao Masuda and Naoko Murakamj Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Kyoto 606, JAPAN Abstract The intracrystalline diffusivities of IIZSM-5 zeolite were directly measured for several hydrocarbons at higher temperatures ( 3 7 3 - 7 7 3 K) by the constant volume method. High silicious HZSM-5 zeolite, which has no activity for reactions, was used as the adsorbent. Aromatics: benzene, xyleneisomers (orthe, meta- and para-xylene) and toluene, and paraffins: nhexane, ppentane, n-octane and iseoctane, were used as adsorbates. Intracrystalline diffusivities of aromatics markedly depended on the minimum size of the aromatics and that of paraffins depended on the carbon A method was developed for number (molecular weight of the paraffins). predicting diffusivity in terms of pore diameter and molecular properties of hydrocarbons. This method was found to well represent the experimental results.

1.INTRODUCTION HZSM-5 zeolite catalysts show high shape selectivity, because they have very fine micro pores within its crystallite [ l ] . The pore diameter i s almost equal to sizes of mono-aromatic molecules [ Z ] . HZSM-5 catalysts are typical solid-acid catalysts, and their acid sites are distributed not only within but also on the outer surface of the crystallite [ 1 , 3 1 . l’herefore, the shape selectivity of HZSM-5 zeolite is affected strongly by the size of the crystallite, the intracrystalline diffusivities of hydrocarbons and acidic properties within and on the outer surface of the crystallite [ 4 - 7 1 . Catalytic reactions are usually carried out at temperatures higher than 5 7 3 K. Although the data of the diffusivity at lower temperature than 3 7 3 K have been published for the adsorption process, few diffusivities at such higher temperatures have been reported and there is no method f o r predicting the diffusivity [ 8 ] . The main objective of this work is to develop a method to predict the diffusivities of hydrocarbons within the crystallite of HZSM-5 zeolite and evaluate its validity using experimental results. High silicious HZSM-5 zeolites were synthesized from silica glass, and intracrystalline diffusivities of eight kinds of hydrocarbons within these zeolites in the temperature range of 3 7 3 - 7 7 3 K were directly measured by A method was developed for predicting the the constant volume method. diffusivity on the basis of the thermodynamics of an impure metal atom diffusing among metal atoms which form crystal lattices. Its validity was experimentally examined.

418 2 . EXPERIMENTAL

2.1.Catalyst High silicious HZSM-5, which has no activity for catalytic reactions ( e . g . cracking) of the adsorbates, was hydrothermally synthesized from silica gel in the range of temperature from 433 to 473 K. The prepared zeolites were found to have the same characteristics of the HZSM-5 zeolite by X-ray diffraction analysis. The crystallites were found t o have a cubic shape by a scanning electron microscopy photo of the crystallites and these sizes were uniformly 2 . 1 urn.

2.2.Measurement o f intracrys tall irie Diffusivi ty Several kinds of hydrocarbons with different molecular properties listed in Table 1 were used as adsorbates. Table 1 llydrocarbons used as adsorbates in diffusion experiments. hydrocarbon (8) benzene toluene (T) p-xylene (p-X) in-xylene (m-X) o-xylene ( 0 - X ) n-hexane (n-C6) n-heptane (n-C7) n-octane (n-C8) iso-octane(i-C8)

molecular weight H [g/moll 78.11 92.13 106.16 106.16 106.16 86.18 100.21 114.23 114.23

minimurn

size d m [nml 0.575 0.575 0.575 0.625 0.625 0.421 0.421 0.421 0.581

molecular a c t i v a t i o n energy E [kJ/moll _ _- -___-.___...___ tykf;;] t h i s work ref.12 0.75 27.00 2554 25.59 0.81 21.90 0.89 30.49 0.85 29.05 0.84 19.95 1.02 17+4 19.68 1.14 32.41 1.26 0.90

The intracrystalline diffusivities of the hydrocarbons were measured under the conditions of the temperature range of 373-773 K and the pressure range of 0-1.33 kPa by the constant volume method [9]. The apparatus and the procedure are the same as employed by Hashimoto e t al. [4,5]. Change in the total pressure caused by adsorption was recorded by use of a piezometric sensor with a transducer, the response of which is first enough to measure accurately the pressure change. To eliminate the influence of several factors (such as mass conductivity between the sorbate and the pressure sensor) on the pressure change, the blank tests were conducted without zeolites. Comparing these data obLained with those wilh zeolilcs, an uptake curve of the amount adsorbed was obtained. The uptake curve of the amount adsorbed usually includes the amount adsorbed in the zeolite crystallite and that on the outer surface of the crystallite [ l o ] . Hence, these two kinds of the amounts adsorbed were evaluated seperately in calculating the diffusivity from the uptake curve by a non-linear least square method. The magnitude of the amount adsorbed on the outer surface was about 5-30 % of that in the crystallite. Hence, in order to avoid several effects (such as the amount adsorbed on the outer surface of the crystallite and the temperature rise at the early period of the adsorption [ l l ] ) , the diffusivity was calculated using all data of the uptake curve by use of a theoretical equation (Eq.[l]) [ 9 ] .

MtIM e = 1 - c ll.1

2a(l-a)

l t a ta 29;

0s;

exp[ - - tl L2

479 14

I

12

-

10

-

I

I

1

adsorbent: h i g h silica

-

HZSM-5

-

p-xyl ene

8

-

6

-

4

-

2

-

0

0

200

400

600

800

t

I000

equi I i b r h r n p r e s s u r e [Pal

Fig.1 Adsorption isotherm of pxylene (Henry’s type).

Fig.2 Independency of the diffusivity on each adsorbed amount

~ . I ~ K S U LAND T S DISCUSSIONS

3.1.Intracrystalline Diffusivities The value of the diffusivity is usually dependent on the amount adsorbed at lower temperatures than about 3 7 3 K, because of the large amount adsorbed [12,13]. However, when the temperature is higher than 373 K, the amount adsorbed is small, resulting in a small interaction among adsorbate molecules. Therefore, the relationship between the adsorbed amount (Me) and the equilibrium pressure (P,) was found to be linear under the conditions of pressures lower than 1.33 kPa and temperatures higher than 373 K, as typically shown in Fig.1. Hence, the diffusivities (D) are independent of the adsorbed amount and equal to the "self-difrusivities" (D,) defined by Darken’s equation (Eq.[2]) [ 1 4 1 .

Actually, the diffusivities were actually independent of the equilibrium adsorbed amount, as shown in Fig.2. Figure 3(a) and 3(b) respectively are the Arrhenius plots of the diffusivities of aromatics and paraffins. The diffusivity of benzene reported by Zikanova et al. [15] was 1 . 9 ~ 1 0 - 1 4m 2 . s - 1 at 333 K, which was calculated from the data of the uptake curve at initial period using the square root of time-law. The diffusivity was recalculated as 8.6~10-16m2.s from their all data according to the method employed in this work. Both this data (key, in Fig.3[aJ) and those in this work lie on a straight line. The data for each hydrocarbon was well expressed by the Arrhenius equation, and the activation energies of the sorbates were listed in Table 1. The values of the activation energies were in good agreement with those reported by other investigators 121, indicating that the diffusivities were accurately measured. The diffusivities of the aromatics: benzene, toluene and pxylene are nearly equal to one another, because they have the same minimum molecular

I

I

I

I

(b)paraf f I n s

\

1

2

ll/Tlx 103 [1/K1

3

,

1

2

I I / T l x lo3

3

[l/KI

Fig.3 Arrhenius plot of diffusivity for each adsorbate. size the value of which is nearly equal to the pore diameter of IIXSM-5 zeolites (about 0.55 nm). There is large resistance to the diffusion of e and m-xylenes, because their minimum molecular sizes are larger than the pore size of the zeolite. Hence, the diffusivities of G- and mxylenes are smaller than that of p-xylene (about 10%). The diffusivities of paraffins are not dependent on the minimum molecular size, but are influenced by the carbon number of the paraffin, namely, molecular length. The longer the hydrocarbon molecules are, the smaller the diffusivities are. Thus, it can be concluded that the diffusivities are dependent on the pore size of HZSM-5 zeolites and the weight, length and minimum diameter of the diffusing hydrocarbon molecule. However, there is no available method to predict the diffusivities within the IIZSM-5 zeolite crystallite. Hence, a estimation method is developed in the next section. 3.2.Method t o P r e d i c t the Diffusivities The magnitude of the diffusivity is affected by several factors, such as the electrostatic field in the crystallite, the interaction among adsorbate I f the electrostatic field is molecules and the steric hindrance [8]. dominant, the value of the activation energy for the diffusion might be proportional to the inverse value of the molecular weight. Ilowever, t h i s relation was found experimentally to hardly hold. Furthermore, the interaction among the adsorbate molecules was found to be negligibly small, as discussed in the chapter 3.1. Hence, the steric hindrance was considered to be dominant. Since the minimum molecular size of the adsorbate is almost equal to the size of micro pore within the crystallite [9,16], molecules of hydrocarbon diffuse by widening the pore openings of the zeolite crystallite. This diffusion process is similar to that of a n impure metal atom diffusing I n case of among metal atoms which form crystal lattices [ 1 7 ] (Fig.4[1]). this diffusion of an impure metal atom, an impure metal atom diffuses by

481

metal

1-

[pore structure

e t a 1 atom f o r m i n g lattice

[I I1

channel of ZSM-5 z e o l i t e ] change in f r e e e n e r g y caused by diffusion

Fig.4 Diffusion processes of an impure metal atom in metal crystal and a hydrcarbon molecule within zeolite crystallite. widening a narrow space between lattices. The change in the Gibbs free energy (AG,) during the diffusion process from (a) to (c) in Fig.ll(1) is shown in Fig.4(III). The distance (a,,) between adjacent lattice points in the diffusion of an impure metal atom could be regarded as that between adjacent intersections of straight and zig-zag channels within the zeolite crystallite. Hence, the method to predict the diffusivity within the zeolite crystallite is formulated using analogically the thermodynamics of the diffusion of an impure metal atom. The diffusivity, D, is expressed according to the random walk theory

(E¶.[31).

D = 7 a& (3) where 7 is a geometric constant, a. is the distance between adjacent intersections of the straight and zig-zag pores within the crystallite (Fig.rl[I]) and is the probability o f a .jump of R hydrocarbon molocule p c r unit time which is expressed in terms of the Gibbs free energy of the diffusion.

Here, Y is the frequency of a jump. AG, is nearly equal to the energy needed to widen the micro pore. The hydrocarbon molecules undergo a stress from the pore in the diffusion. When the diffusing molecule is assumed to be a sphere, this stress , p is the coefficient of elasticity, and 6 i s is represented by ~ 6 where the strain [ 1 8 ] . However, the shape of a hydrocarbon molecule is markedly different from a sphere, since the molecule within the pore touches the pore surface at some points. Therefore, the following assumptions has been made: the shape of the hydrocarbon molecule is regarded as a series of spheres linked together, and the diameter of each sphere is equal to the minimum size (d,) of the hydrocarbon molecule. T h r number of thr sphrrrs is represented by (LJd,,,), where L, is the length of the hydrocarbon molecule. Under these assumptions, the hydrocarbon molecule undergoes the

482

stress of ~6(L,/d,) from the pore. the diffusion is represented by

Usiiig this value, the energy needed for

where 6, is the representative strain value of the pore in the diffusion and is schematically shown in Fig.4[III]. The 6, value can be considered to be proportional to the difference between the minimum molecular size (d,) of the hydrocarbon and the effective pore opening (x) of the diffusion path.

where d, is the diameter of the pore within the zeolite crystallite and is is dJd,and [ is x/d,. about 0.55 nm, is When the thermal energy of a hydrocarbon molecule (the enthalpy AH,) used for the translational energy, the parameter Y can be expressed by [18,191 Y

= (AHJZMa2)1/2

(7)

where M is the molecular weight of the hydrocarbon and a is the distance of the molecule jumps. AG, is also related to the enthalpy AH, and entropy AS, of the diffusion.

A G, = A H,-TA S ,

(8)

The magnitude of the second term of Eq.(8) is almost 7% of the enthalpy AHm value in case of an impure metal atom diffusing among metal lattices [17,18]. It was assumed that this situation could be applied to the diffusion of hydrocarbon molecules within the zeolite crystallite. Under this assumption, the term of TAS, in Eq.(8) can be neglected in comparison with the term of AH, on analogy of the diffusion of an impure metal atom.

AG, = AH, (9) Substituting Eqs.(5), (6) and ( 9 ) into Eq.(7), a formulated frequency Y can be obtained. Using this Y value and Eqs.(4) and ( 5 ) , equation ( 3 ) could be rewritten by where

D = D,.exp[-E/(RT)]

K, = 7 aOZg 1/2k/( 2a ) ( 1 3 ) , K, = j1k2/2 (14) Thus, the intracrystalline diffusivity i s formulatcd as an Arrhenius Lype equation, in which the frequency factor nnd the activnLion energy nrc cxpressed in terms of the pore size of the crystallite (d,) and the molecular properties of hydrocarbon ( e . g . weight (M), minimum diameler (d,) and length (L,) of hydrocarbon molecule). Ilowcver, thcrc are unknown pnrnme-

483

180

30

150

-

LD 120 d

z. x ,-

0

15

90

10

60

5

30

B

(3

-

0

0

n 0

5

10 15 20 25-

1O"X d , ( d ~ - f ) [ m l

Fig.5 Correlations of activation energy and frequency factor to strain.

Oc lm2/¶I

Fig.6 Comparison of diffusivities calculated by proposed method with experimental data.

ters, K,, K, and l , in Eqs.(ll) and (12). Equations (11) and (12) can be rewritten by

’dm

(-)1/2

= K,l/Z(#

-[ )d,

Lm

(16)

The values of D,(Md,/L,)1/2 and (Ed,/L,)l/2, $ and d, in Eqs. (15) and ( 1 6 ) are known for each hydrocarbon molecule listed in Table 1. Hence, varing ( value, the correlation between the values o f Lhc lert hand sides o f Eqs.(l5) and (16) and the (#-[)d, value was examined, and the value which gives the well correlation was estimated. 3.3.Examination o f t h e Proposed MeLhod Figure 5 shows the good correlations between the (#-[)d, value arid Lhe values of the left hand sides of Eqs.(l5) and ( 1 6 ) for several hydrocarbons including paraffins and aromatics. These correlations were obtained using the (=x/d,) value of 0.745, which is smaller than unity. This is evidence that the hydrocarbon molecules and atoms forming the zeolite framework (lattice) vibrate [ l 6 ] and the apparent pore opening (x) is seem t o be less than d, (0.55 nm). We found that the intercepts of the correlations in Fig.5 are larger than the value (zero) expected in Eqs.(l5) and ( 1 6 ) . T h i s is because in the derivation of the model the Gibbs’ free energy of diffusion was considered t o be nearly equal to the energy needed to widen the lattice. Thus, the frequency factor and activation energy of the intracrystalline diffusivity were found to be well expressed by the modification of Eqs.(lO) and (ll), which were obtained from Fig.5, as follows:

Do= { (4.5~10-2)d,fp -0.745) t2.4~10-12)(L,/Md,) E = [ (4.lxlOll)dz(# -0.745) +77)2( L,/d,)

Figure

6 shows the comparison of the diffusivity predicted by

Eqs.(lO),

484 (17) and (18) with the experimental data. Open keys are the data used f o r the correlations in Fig.5, whereas closed keys were not used. The predicted values were found to be in good agreement with the experimental data within the error of 50%. When the reaction proceeds under conditions of diffusion control, the overall reaction rate is proportional to the square Hence, the 50% error would be corresponding root of the diffusivity [ 2 0 ] . to only 22% error in estimating the overall reaction rate. Thus, the diffusivity can be well estimated by Eqs.(lO), (17) and (18).

4.CONCLUSION ( 1 ) The intracrystalline diffusivities of five aromnLics and four paraffins within HZSM-5 zeolite crystallite were directly measured at high temperatures (373-773 K), at which the catalytic reactions were usually performed. (2) The diffusivities of aromatics are markedly dependent on the minimum size of a hydrocarbon molecule, whereas that of paraffins is closely relaLed to the molecular length (corresponding to carbon number) of the hydrocarbon. ( 3 ) A method to predict the diffusivities within IIZSM-5 zeolite crystallite was developed on the basis of the thermodynamics of an impure metal atom diffusing among metal atoms forming crystal lattices. The diffusivities predicted by the method were in good agreement with experimental data.

5.REFERENCES 1 N.Y.Chen and W.E.Garwood, Catal.Rev.Sci.Eng., 31(1989)385. 2 S.L.Meise1, J.P.McCullough, C.H.Lechthaler and P.B.Weisz, Chemitech,

9

10 11 12 13 14 15 16 17 18 19 20

6(1976)86. J.A.Rabo and J.Gajda, Catal.Rev.Sci.Eng., 31(1989)385. K.Hashimoto, T.Masuda and M.Kawase, Stud.Surf.Sci.Catal., 46(1989)485. K.llashimoto, T.Masuda and Y.Ilariguchi, J.Chem.Soc.Jpn., No.3(1989)575. W.T.Mo and J.Wei, Chem.Eng.Sci., 41(1986)703. W.O.Haag, R.Lago and P.B.Weisz, Farad.Disc.Chem.Soc., 7 2 ( 1 9 8 1 ) 317. D.M.Ruthven (ed.), Principles of Adsorption & Adsorption Process, John Wiley & Sons, New York, 1981. J.Crank (ed.), The mathematics of Diffusion, Clarcridorl Press, Oxford, 1975. I.Suzuki, S.Oki and S.Namba, J.Catal., 100(1986)219. H.J.Doelle and L.Riekert, Angew.Chem.Int.Ed.Engl., 18(1979)266. V.D.Begin, L.V.C.Rees, J.Caro and M.Buelow, Zeolites, 9(1989)287. M.Buelow, J.Caro, R.Kuhn and B.Zibrowius, Stud.Surf.Sci.Catal., 46 (1989)505. M.Buelow, P.Lorenz, W.Mietk, P.Struve and N.N.Samulevic, J.Chem.Soc. Trans. I , 79(1983)1099. A.Zikanova, M.Buelow and H.Schlodder, Zeolites, 7(1987)115. A.Miyamoto, K.Kawamura, M.Kubo and T.Inui, Shokubai, 32(1990)400. K.Fueki and H.Kitazawa, Kotainai no Kakusan, Korona-sha, Tokyo. C.Zener, in W.Shockley (ed.), Imperfection in Nearly Perfect Crystals, J.Wiley, New York, 1952. K.J.Laidler (ed.), Chemical Kinetics, 2nd ed., McGraw-Hill, New Delhi, 1978 0.Levenspiel (ed.), Chemical Reaction Erigiriecring, 2nd e d . , John Wi l e y & Sons, New York, 1972.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

485

New Porous lbterials h m Layered Compounds Abraham Clearfield, Mark Kuchenmeister, Jiandang Wang and Karen Wade Department of Chemistry, Texas A&M University, College Station, Texas 7784.3

Abstract The pillaring of clays has become an established technique of preparing a new class of porous materials. While their promise as cracking catalysts has not been fulfilled, never-the-less they exhibit interesting catalytic behavior. We have extended the technique t o the pillaring of many other types of non-clay layered compounds which include phosphates, oxides and layered double hydroxides. This extension broadens the field to the point where one can choose the degree of acidity or basicity and framework metals required for specific catalytic processes. 1. INTRODUCTION

There are almost a hundred known synthetic and natural zeolites, but only a handful of them are used on a commercial scale. Their versatility and complexity, however, suggest that new uses will arise from intensive study of zeolite properties and further utilization of different types will result. Zeolites are either naturally occurring or synthetic crystalline aluminosilicates which have three-dimensional open-structured frameworks. The frameworks enclose cavities o r channels which run throughout the structure. The introduction of zeolites as petroleum cracking catalysts revolutionized oil refining technology as it resulted not only in higher yields of gasoline, but allowed refiners to target product streams. Thus, when a particular product is in high demand, a zeolite can be added as part of the catalyst mix t o preferentially produce that product. The remarkable performance of zeolites is due to a combination of factors within their cavities, the large range of cations and metals which can be placed within the cavities and their shape selectivity. This latter property refers t o the fact that molecules which can readily traverse the channels and fit into the cavities are selectively transformed to molecules of size and shape determined by the zeolite cavity-channel geometry.

486

A serious !imitation of zeolite? is the relatively modest size of the channels (-8 A) and cavities (-13 A being the largest cavity size). Thus, they are not very effective in cracking the gas oil fraction of petroleum because of the large size of the molecules in this fraction. Clays are also aluminosilicates which have been used as catalysts but, in contrast to zeolites, have a two-dimensional or layered structure. The distance between layers is too small to allow gas-oil type molecules to fit between the layers. Thus, all the cracking occurs on the external surface. It was therefore surmised that if inorganic polymers could be inserted between the layers to prop them open, the internal surface would become available for catalysis, greatly increasing the clay's effectiveness. This has now been accomplished t o some extent, but only with clays called smectites which spontaneously swell in water. In the swollen state the interlayer distance is large enough t o admit free access of inorganic polymers such as the aluminum Keggin ion, [ A l ~ ~ O ~ ( O H ) ~ ~ ~or 1 2the H ~tetrameric O I ~ + zirconium species, [Zr(OH)2.4H20]48+. We shall return t o the subject of pillared layered compounds, but first will describe some interesting developments in the general search for new porous materials. A milestone was achieved with the discovery of aluminum phosphate molecular sieves [l]. These compounds have neutral frameworks of very low acidity. Thus, their use as catalysts and to some extent as sorbants is limited. However, it was subsequently shown that silicon could be incorporated into the aluminum phosphate framework [3-41 (SAPO's). This discovery was followed in short order by introduction of many metals (Mg2+, Mn2+, Fe2-, Fe3+, Co2+, Zn2+) [5-61 into the framework by isomorphous substitution for Al3+. These metal substituted products were termed MeAPO's. A notable advance in this field of porous materials occurred with the synthesis of 18-ring aluminum phosphates 17-81. There now appear t o be two related phases in this system, one phase which is thermally unstable (Hl) [9-10]and contains excess aluminum and the other with a 1:l A1:P ratio (VPI-5) [ll] which is thermally stable. We have carried out framework substitutions in these phases as have others [121 in an effort to improve the acidity and catalytic properties. Whether these efforts will be rewarded remains to be seen. 2. SEARCH FOR NEW POROUS MATERIALS

The discovery of aluminum phosphate molecular sieves has fueled the search for new sieve like materials. This search has been amply rewarded. Newsom and Vaughan [131 were able t o prepare new gallosilicate phases and gallophosphates have also been prepared [141. One of the most interesting and startling discoveries is that of zincophosphate and beryllophosphate molecular sieves [151. They can be prepared under very mild conditions [16] and have structures which closely resemble those of aluminosilicate zeolites [16-171. These compounds have frameworks [(ZnOz)(P02)]-which are isoelectronic with the aluminosilicate framework

487

where the &:A1 ratio is 1:l. It is remarkable, therefore, that an analogue of zeolite-X has been prepared in this system [171. Haushalter and coworkers [18-201 have prepared a number of porous molybdenum phosphates using amine templates under hydrothermal conditions. These frameworks have both octahedral molybdenum and tetrahedral phosphate groups. Very recently, a new framework structure was simultaneously prepared by Haushalter [191 and by Peascoe and Clearfield [211. The framework composition is [ M o ~ O ~ ( P O ~ ) ~ Hand ~ PisO ~ I related t o the MoOPO4 [221 and (VOS04)2.H2S041231 structure types. This framework is very flexible towards the size and shape of the occluded cation when prepared without a template. However, small templates, such as methylamine and di-methylamine have also been used in its preparation. Removal of the amine allows other small molecules t o be sorbed [24]. Another interesting porous compound, shown i n Figure 1, was prepared by refluxing zinc chloride with H3P03 and evaporating t o near dryness t o collect the needle-shaped crystals 251. It has the composition Zn(H2P03)2-3H20. The cavities are about 6 in diameter and lined with protons which are hydrogen bonded to water molecules which fill the space. Addition of alkali metal ions results in a total rearrangement of the framework t o strongly coordinate the metal ions. [25-261. Another fruitful area where porous compounds may be prepared would appear to be the iron (111)phosphates of which cocoxenite is a good example [271.

B

3. POROUS MA’lERIALs BY DESIGN; PILLAR;ED CLAYS The original objective in synthesizing pillared clays (PILCS, o r pillared inorganic layered compounds) was to obtain pores larger than those available in zeolites for the cracking of the gas oil fraction of petroleum. Pillared clays are prepared by swelling the clay i n water and then exchanging the interlayer sodium or calcium with a large polymeric inorganic cation. The process is illustrated in Figure 2. Since the ion exchange capacities of clays are low (0.5-1.5 mequiv. per g), spaces are left between the pillars. These spaces are further enlarged by thermally condensing out the water and hydroxyl groups of the pillaring cation. A host of cations have now been used for this purpose including the aluminum Keggin ion, [Al1304(OH)24(H20)1217+, the zirconyl ion, [Zr(OH)2-4H20148+,chromia and titania polymers, silica and iron oxides. Vaughan et al. [29-301 reported pore size distributions for two clays pillared by thg aluminum hydroxy cation. The majority of the pores were Iess than 14 A in diameter with significant numbers of pores between 14 and 20 A and 40 and 60 A. Sorption data for straight chain hydrocarbons have been measured by Occelli et al. 131-321. However, the height and thickness of the aluminum polymer can be varied by hydrothermal treatment with presumably a change i n its charge C331. Use of such polymer solutions for pillaring may change the size and shape of the pores. Much additional work along these lines is required for a proper assessment t o be made.

488

Figure 1. Extended view of the Zn(H~P03)2-3H20 structure in the ab plane. The structure is notable for its large channels running parallel to the c-axis containing the water molecules which are not shown (from Inorg. Chem. 28, (1989) 2608. with permission).

/ -

+

I-

+m z !n + + Y

V

+m

489

490

I

CI

--= s

I 0

491

Calcination at about 500 "C is required to cross-link the pillars with the layers. The overall process is quite complicated [34-351. In essence the pillaring cation dewaters with loss of charge. However, protons are generated from coordinated water molecules during the dehydroxylation process to satisfy the layer charges. These protons appear to tunnel up into the octahedral region of the clay layer at high temperature in montmorillonite type clays, thus reducing their Bronsted acidity. In contrast beidellite clays, in which isomorphous substitution occurs at the silicate tetrahedra, keep the protons on the layer's outer surface and exhibit higher Bronsted acidity. At about 500 "C, interaction of the protons with clay hydroxyl groups may eliminate water, again drastically reducing the Bronsted acidity. Thus, at the high temperatures required for cracking, the pillared clays are essentially Lewis acid catalysts. In spite of the limitations cited above, the pillared clays do exhibit a high level of gas oil cracking with high gasoline yields [361. The problem arises from extensive coking of the catalyst. A t the temperatures required to remove the coke the clay layers are destroyed. Much effort has recently been expended in producing more stable structures with some success [37381. However, the original incentive for preparing pillared clays is no longer operative due t o the changing requirements of the petroleum industry. Never-the-less, these materials remain as highly active catalysts with a broad spectrum of catalytic behavior. Attention should be turned to examining the many other reactions that can be catalyzed by these materials. Modification of their properties, for example, by exchange of catalytically active species into the pores appears to be a viable means of behavior alteration 1391. 4. PILLARED LAYERED COMPOUNDS (PLC) OTHERTHAN CLAYS 4.1. Layered Double Hydroxides Layered double hydroxides (LDH) are complex hydroxides of general formula Mn'+M'+(OH)2+2,X, where n = 2-5 and X is a charge balancing anion. The anions are located in the interlamellar space but do not fill it. A schematic drawing of an LDH is shown in Figure 3. Variable amounts of water reside in the pores or cavities created by the anion positioning. The LDH may be considered as the inverse of the swelling smectite clays because the layers are positively charged, balanced by anions, whereas the clays have negatively charged layers balanced by cations in the interlamellar region. Only in the case of Mg-Al and Ni-Al LDH can the full range of n values be attained. In most other systems n = 2 or in some cases 3 also. There are two attractive reasons for wanting to pillar the layered double hydroxides. First, they have already been shown to possess interesting catalytic behavior [40-411 and second, there are an enormous number of large anions which can be used for pillaring. These include the isopoly- and heteropolyanions 1421. Drezdon [431 reported pillaring Mg2Al(OH)&- by first replacing the halide ion by phthalate ion. He was then able to exchange out the organic anion with V100286-. Shortly thereafter, Pinnavaia et al. [441 were able t o show that the LDH

492

Zn2A1(OH)&l was able t o directly exchange the V100286- ion. Further studies by these workers demonstrated that the same LDH was able t o exchange heteropoly anions such as [H2W1204016-and a-[SiV3Wg04017-but not polyoxymetallates of lower charge [45]. Surface areas of 63-155 m2/g with pore volumes of 0.023 - 0.061 cm3lg were reported. In a most recent communication Dimotakis and Pinnavaia [461 reported a new route for the pillaring of LDH. M ~ ~ A I ( O H ) ~ ( C O ~ ) O was . ~ .first ~H~O heated at 500 "C to obtain the mixed oxides. The oxide was in turn stirred in water for 30 h t o reconstitute the layered double hydroxide with the hydroxide ion as the anion. By adding glycerol to the mix, the layers swell and are able to incorporate large organic acids as anions. These i n turn were exchanged out for polyoxometallate anions. The field of pillared LDH would appear t o hold great promise for future applications [40,471. However, a more general method of pillaring them is required. We have foun! that the LDH layers, which normally have a basal spacing near 8 A, remain swollen t o larger values of this interlayer separation if they are not dried. As a consequence, we have been able to insert a variety of Keggin ions into the interlamellar space by addition of a heteropoly salt solution to the swollen LDH. For example, we have obtained Ni2A1(OH)6(PV3Wg040)o.16.xH20and M g ~ A 1 ~ O H ~ ~ ~ ~ P V ~ 16 Win g Othis ~o)o. way. On dehydration at 200 "C under reduced pressure, the nickel aluminum compound gave a surface area (BET N2 sorption) of 31 m2/g while the magnesium aluminum product surface area was 96 m?g. This increase in surface area apparently stems from the reduced layer charge per metal atom in the Mg5A1 LDH. By control of the layer charge and pillar charge it is theoretically possible to obtain pores of specific sizes and we are working towards this end. 4.2. Layered Phosphates

Group 4 and 14 phosphates with the a-structure are layered [481 and The general formula is bear a resemblance t o clay layers. M(IV)(HP04)2.H20 and the layers consist of a central octahedrally coordinated metal layer sandwiched between tetrahedral phosphate layers. The major difference from the clays is the fact that the phosphate groups are inverted. Three oxygens of each phosphate group bridge across metal atoms and the fourth points into the interlamellar space. The ion exchange capacity of these solids depends upon the mass of the metal. For example, the exchange capacity of the Z r compound is 6.6 meq./g, a value 4-12 times that of a clay. All of this charge is concentrated in the oxygens not bonded to metal which results in covalent bonding with the protons. The water molecule resides in cavities between the layers, but is not a hydronium ion 149-501. Exchange of Na+ for the protons occurs with some expansion of the layers, but in general no swelling occurs for the crystalline phases [511. Because of the lack of swelling of this class of compounds, they did not lend themselves t o pillaring reactions. However, it was demonstrated [521 that large charged species or complexes could be exchanged into zirconium phosphate if first the layers were spread apart by amine intercalation. Shortly thereafter, we succeeded in pillaring both zirconium and titanium

I

I

0

2

In

2

I

I 7

0

0

m 0

7

I In

8

+

493

494

I

I

I

I

I

I

I

I

I

0

-9 0 cu 0

Y

-9 co 0 7

0

-9

0

T

d

-9

Y

0

-: 0

7

0

-9

0

-9 co 0

-9 co

0

d

-9 0

(\I

-9

0

495

phosphates with the All3 Keggin ion, but publication 1531 was delayed to preserve patent rights. Strangely, titanium phosphate (Tip) yielded porous products with surface areas of 35-237 mVg while the zirconium phosphate (ZrP) was nonporous. Figure 4 shows the uptake of different sized molecules by a TiP sample with a surface area of -200 m2/g in which 25% of the pores are in the micro range. This difference has been traced to loss of phosphate by Tip during pillaring. This loss lowers the layer charge and allows less pillar to be inserted. Tin phosphate has also recently been pillared with chromium polymers [54]. Here also it is suspected that phosphate hydrolysis may be responsible for the porous nature of the products. MacLachlan and Bibby [55] have pillared a-zirconium phosphate by first preparing the halfexchanged sodium phase. This phase has an expanded interlayer spacing of 11.8 A and in this condition could be pillared by a basic chromium acetate polymer. As with pillaring by the aluminum Keggin ion, the interlamellar space was stuffed, i.e., non-porous. However, in our laboratory we were able to also pillar amine intercalated ZrP with chromium acetate [561 and achieve a surface area of 119 m2/g. In recent work we have used Jeffamines t o good effect. These products are polyetheramines marketed by Texaco. Addition of these amines t o the phosp6ates produces a swollen colloidal suspension in which the layers are 30-70 A apart depending upon the particular Jeffamine used. We then add the pillaring agent and break the colloid by evaporation t o a small volume. A portion of the recovered zirconium phosphate pillared product had a surface area of 250 m2/g, and a narrow pore size distribution with an average diameter of -13 A (Figure 52. The chromium pillared zirconium phosphate yielded pores in the 10-15 A diameter range. These results give promise that a wide variety of layered materials such as other phosphates and ternary oxides can be pillared in similar fashion. Thus entire new classes of porous materials are in the offing. 5. ACKNOWLEDGMENTS

Our work on pillared layered materials has been supported by NSF grant DMR-88012283 and the State of Texas through the Advanced Research Program for which grateful acknowledgement is made. The impetus and some continuing support has been supplied by Amoco Chemical Company and Texaco, U.S.A. Our thanks t o Dr. Rayford Anthony of the Chemical Engineering Department at Texas A&M University for supplying the sorption data in Figure 4.

1

2

S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flanigen, J. Am. Chem. SOC.,104 (1982) 1146. S. T. Wilson, B. M. Lok and E. M. Flanigen, US Patent No. 4 310 440 (1982).

496

3

4 5

6 7 8

9 10 11 '12

l3 14

15 16 17 18

19

a3 21 22 23 24 25

26 27 28 29

B. M. Lok, C. A. Messina, R. L. Pattern, R. T. Gajek, T. R. Cannan and E. M. Flanigen, J. Am. Chem. SOC.,106 (1984) 6092. B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, US Patent No. 4 440 871 (1984). E. M. Flanigen, Pure and Appl. Chem., 58 (1986) 1351. C. A. Messina, B. M. Lok and E. M. Flanigen, US Patent No. 4 544 143 (1985). M. E. David, C. Saldarriaga, C. Montes, J. M. Garces and C. Crowder, Nature, 331 (1988) 698. M. E. Davis, C. M. Saldarriaga, C. Montes, J. M. Garces and C. Crowder, Zeolites, 8 (1988) 362. J. 0 Perez, N. K. McGuire and A. Clearfield, Catal. Lett., 8 (1991) 145. B. Duncan, R. Szostak, K. Sorby and J. G. Ulan, Catal. Lett., 7 (1990) 367. M. E. Davis, C. Montes, P. E. Hathaway, J. P. Arhancet, D. L. Hasha and J. M. Garces, J. Amer. Chem. SOC.,111(1989) 3919. M. E. Davis, C. Montes, P. E. Hathaway and J. M. Garces, in P. A. Jacobs and R. A. Santen, (eds.), Zeolites: Facts, Figures, Future, Elsevier, New York, 1989. J. M. Newsom and D. E. W. Vaughan, New Developments in Zeolite Science and Technology, Elsevier, Amsterdam, 1986, p. 457. T. Wang, G. Yang, S. Feng, C. Shang and R. Xu, J. Chem. SOC. Chem. Comm., (1989) 948. G. Harvey and W. M. Merer, in P. A. Jacobs and R. A. VanSanten (eds.), Surface Science and Catalysis,Elsevier, Amsterdam, 1989, pp. 411-420. T. E. Gier and G. D. Stucky, Nature, 349 (1991) 508. W. T. A. Harrison, T. E. Gier, G. D. Stucky, J. M. Nicol and J. J. Rush, Chem. Mater., in press. R. C. Haushalter, K. G. Strohmaier and F. W. Lai, Science, 246 (1989) 1289. R. C. Haushalter, H. E. King, Jr., L. A. Mundi and K. G. Strohmaier, J. Solid State Chem., in press. R. C. Haushalter, L. A. Mundi and K. G. Strohmaier, J. Am. Chem. SOC.,112 (1990) 8182. R. Peascoe and A. Clearfield, J. Solid State Chem, in press. P. Kierkegard and M. Westerlund, Acta Chem. Scand., 18 (1964) 2217. M. Tachez and F. Theobald, Acta Crystallogr., Sect B, B37 (1981) 1978. L. A. Mundi, K. G. Strohmaier and R. C. Haushalter, Inorg. Chem., 30 (1991) 153. C. Y.Ortiz-Ada, P. J. Squattrito, M. Shieh and A. Clearfield, Inorg. Chem., 28 (1989) 2608. M. Shieh, K. J. Martin, P. J. Squattrito and A. Clearfield, Inorg. Chem., 29 (1990) 958. C. Palache, H. Berman and C. Frondel (eds.), Dana's System of Mineralogy, Vol. 2, Wiley, New York, 1951, p. 997. See for example Catalysis Today, 2 (1988)185-279. D. E. W. Vaughan and R. J. Lussier, 5th Int. Conf. on Zeolites, Naples, Italy, Heyden, London, 1980.

497

30 31 32

33 34 35

36 37 38 39 40 41 42

43 44 45 46 47 4x3

49 50 51 52

53 51

55 56

D. E. W. Vaughan, R. J. Lussier and J. S. McGee, Jr., US Patent No. 4 271 043 (1981). M. L. Occelli, R. A. Innes, F. S. S. Hure and J. W. Hightower, J. Appl. Catal., 14 (1985) 69. M. L. Occelli, V. Parulekar and J. Hightower, Proc. 8th Int. Congr. Catal., Berlin, 1984. J. Sterte,Catal. Today, 2 (1989) 219. J. J. Fripiat, Catal. Today, 2 (1988) 281. D. T. B. Tennakoon, J, M. Thomas, W. Jones, T. A. Carpenter and S. Ramdas, J.Chem. SOC.,Faraday Trans. 1,82 (1986) 545. M. L. Occelli, Catalysis Today, 2 (1988), 339. D. E. W. Vaughan, Catal. Today, 2 (1988) 187. J. W. Johnson, private communication. K. A. Carrado, S. L. Suib, N. D. Skoularikis and R. W. Coughlin, Inorg. Chem., 25 (1986) 4217. W. T. Reichle, Chemtech, Jan. 1986, p. 58. W. T. Reichle, J. Catal., 94 (1985) 547. M. T. Pope, Heteropoly and Isopoly Oxometallates, Springer-Verlag, New York, 1983. M. A. Drezdon, Inorg. Chem., 27 (1988) 4628. T. Kwon, G. A. Tsigdinos, and T. J. Pinnavaia, J. Am. Chem. SOC., 110 (1988) 3653. T. Kwon and T. J. Pinnavaia, Chem. Mater., 1(1989) 381. E. D. Dimotakis and T. J. Pinnavaia, Inorg. Chem., 29 (1990) 2393. W. Jones, and M. Chibwe in I. V. Mitchell, (ed.), Pillared Layered Structures, Elsevier, Amsterdam, 1990. A. Clearfield, Comments Inorg. Chem., 10 (1990) 89. A. Clearfield, G. D. Smith, Inorg. Chem., 8 (1969) 431. J. M. Troup, A. Clearfield, Inorg. Chem., 16 (1977) 3311. A. Clearfield, W. L. Duax, G. D. Smith, J. R. Thomas, J. Phys. Chem., 73 (1969)3424. A. Clearfield, R. M. Tindwa, Inorg. Nucl. Chem. Lett., 15 (1979) 251. A. Clearfield, B. D. Roberts, Inorg. Chem., 27 (1988) 3237. P. Maueles-Torres, P. Olivera-Pastor, E. Castellon, Rodriguez, A. Lopez-Jimenez, A. A. G. Tomlinson, in I. V. Mitchell (ed.) Pillared Layered Structures, Elsevier, New York, 1990, p. 137. D. J. MacLachlan and D. M. Bibby, J. Chem. SOC., Dalton Trans., (1989)895. M. Kuchenmeister, A. Clearfield, work in progress.

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499 AUTHOR INDEX A

Achkar I. Aiello R. Andrea R.R

223 157 231

B Balakrishnan I. Balkus Jr. ,K.J. Barthorneuf D. Bellussi G . Beyer H.K. Biaglow A. I. Bondar L. A . Bordiga S. Brauchle C. Bresinska I. Briend M. Brunner E. Buelow M. Burkhardt I. C eejka J. Chen Q. Chmelka B.F. Clearf ield A . Conesa J. C. C o r m A. Cornaro U. Cunis S.

135 93 313 79 43 181 303 251 199 101 313 119,453 469 215

347 46 1 435 485 339 409 165 259

D Davis M.E. Derewinski M. Dergachev A . A Derouane E.G. Dewaele N. Dewing J. Di Renzo F. Dmitriev R.V. Dooley K.M. Drago R . S .

Dworeckow G. Dwyer J. Dzwigaj S.

435 127 38 1 157,365 157 1 127 38 1 277 101 373 1 313

E Ehrl M. Einicke W. -D. Erneis C.A.

Ernst H. Espiau P.

199 119 231 119 127

500

F Fajula F. Fattore V. Fejes P. Ferrero A . Foerster H. Fraissard J. Freude D. Fricke R. G Gabelica 2 . Garforth A . A. Gedeon A . Giordano G. Gnep N.S. Goberna C. Goepper M. Gonzalez-Elipe A . R Gorte R.J. Grobet P.J. Guisnet M. Gutschick D. H Habersberger K. Hagelstein M. Halasz J. Hannus I. Hargis D.C. Hashimoto K. Hess M. Hildebrand-Leksowska K Holmes S . Hoogervorst W. G.M. Hoppe R. Huang J. Hunger M. Hutchings G.J.

127 79 173 251 259 46 1 453 109,347

157 1 46 1 157 32 1 409 373 339 181 135 321 215

165 259 173 173 93 477 145 397 1

231 199 24 1 453 389

1

Inui T. Ito T. Iwamoto S.

355 46 1 355

J

Jacobs P.A. Jaeger N. I. JanEalkova M Jelinek R. Jirka I. Jiru P.

135 189 417 435 269 165

501

K Kalies H. Kallo D. Kan Q. Kanazirev V. Karge H.G. Karim K. Kazansky V. B. Kirisci I. Kleinworth R. KoEiFik M. Kojo S. Kokotailo G.T. Korecz L. Kosslick H. Kowalak S. Kraushaar-Czarnetzki B. Kiivanek M. Krtil J. Krylova A . Kuchenmeister A . Kustov L.M.

425 287 241 277 43,425 I

303 173 145 469 355

181 173 409 93 231 347 347 417 485 303

L Lamy A . Landmesser H. Laniecki M. Lapidus A . Lechert H. Lee D.F. Lemos F. Leofanti G. Lercher J.A . Li H.-Y. Liang J. Lopes J.M. Lopez Nieto J.M. Ly K.T.

313 215 331 417 145,295 389 365 251 373 207 207 365 409 93

M Marsi I. Martens J. A . Maryska M. Masuda T. Mavrodinova V. Meier B. Meriaudeau P. Meusinger J. Micke A . Miessner H. Minachev Kh.M. Minchev Ch. Minkov V. Moerke W.

173 135 269 477 295 119 405 373 469 215 38 1 295 145,295 223

502

Murakami N.

477

N Naccache C. Nagy J. B. Nastro A . Neinska Ya. Nicolle M. - A . Niemann W.

405 157 157 295 127 259

0 Ojo A.F.

Onyestyak Gy.

1 287

P Padovan M. Papp Jr. J. Paredes N. Parlitz B. Parrillo D. Penchev V. Peng S. Pereira C. Perez M. Petrini G. Pfeifer H. Piffer R. Pines A . Price G.L.

25 1 287 409 109 181 145,295 241 181 409 25 1 453 259 435 277

R Rabe P. RamBa Ribeiro F. Rathousky J. Rawlence D.J. Reschetilowski W. Richter M. Rockenbauer A . Roessner F.

259 321,365 417 1 119 109 173 425

S

Schoebel Gy. Schoonheydt R.A . Schulz-Ekloff G. Sheng S. Shevchenko D.P. Shibata M. Shpiro E.S. Soria J. Spoto G. Staudte B. Steinberg K.-H. Stork W.H.J. Struve P.

173 443 65, 1 8 9 , 1 9 9 24 1 38 1 355 38 1 339 25 1 453 425 231 469

503

Szulzewsky K.

109

T Tang W. Tasi Gy. Tissler A . Tkachenko 0.P. Tuan V . A . Tvaruikova Z .

207 173 469 38 1 109 165

U Unger K.K. Uytterhoeven M. G.

469 443

V Valtchev V. Vasques H. Vedrine J . C . Vinek H. Vogt F.

295 32 1 25 373 223

u Wade K. Wallau M. Wang J. Wark M. Wei Q. Wendlant K.-P. Weyda H. Wichterlova B. Williams C.D. Woehrle D. wu Y. wu z.

485 469 485 189 24 1 223 145 269,347 389 199 435 241

X Xiong G. XU B. -Q. Xu R.

241 207 241

Y Ying M.-L. Yoshida T.

207 355

z Zecchina A . Zhao S.-Q. Zikanova A. Ziolek M. Zmierczak W. Zubkov S . A . Zukal A .

251 207 469 397 331 303 189,417

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505 SUBJECT INDEX

A

A type zeolite Acidity

355 1,35,173,231,259,303,313,365, 373

Adsorption of - acetonitrile - co - hydrocarbons - hydrogen - Xe Alkaline earth metal ions Alkaline metal ions Alkylation Allylalcohol AlPO-5 AlPO-8 Aromatization

303 215,303 425,469 303 461 43 43,397 347 389 181,435 435 173,277,321,381,405,409,417

B 9-silicalite BeAPO-5 Benzene Beryl 1ium Beta zeolite Boron Butane (-n)

119,127,453 145 207,469 145 127,409 119,127,453 173,373,381

C

co co2 Cadm i um Calcium Carbonyls Chelate complexes Chromium Clusters of metal oxides Clusters of metal sulphide Clusters of metals COAPO-11 COAPO-16 COAPO-34 COAPO-5 Coba 1t Copper Cr-silicalite Cracking Crystallization Cyanide complexes of Fe Cyclization

303,417 417 65,189,287 461 215 93 165 25,43,79,277,287 65,477 65,135,259,443 231 231 231 231,373 93,231,287,373 43,93,223,259,269,355 165 1,165,365,373 25,127,145,157,199 101 277,417

506

D Dealuminated zeolites Decane (-n) Defect groups Dehydrogenation Diffusion Double-hydroxides

1,215,223,313,331 135 157 165,207,277,341,373 461,469,477 485

E ESR spectra of -Ag -cu -Na

-Mo -Ti

-V EXAFS Electronegativity Er ioni te Ethylbenzene Ethyltoluene

443 43,223 443 339 79,251 43,173 65,259 397 425 43 277,347

F Fe-silicalite Framework ordering Framework stability

341 435 1,79, 109,119,295,305,341

G Ga-faujasite Ga-silicalite Gal 1 ium

381,469 1,277,405,409,469

1

H Heptane (-n) Hexane (-n) Hosted dyes Hydrocarbon mixture Hydrodesulphurization Hydrogen Hydrogen sulfide Hydrogen transfer Hydrogenation Hydrosulphurization Hydroxylation

365 1,165 65 425 331 1,303,417 397 1,277 331 287,397 79

507

I IR spectra of -acetonitrile

-co -carbonyls -Fe cyanide complexes

-NO -OH -pyridine -Ti silicalite Inclusion complexes Iridium Iron

Isomerization

43,303 215 215 101 165 1,25,43,79,165,173,259,277, 287,347,425,453 43,231,259 79,251 65,93,199,223 215 43,93,101,145,341,347 277,295,347

K Kinetics of adsorption

469

L Lanthanum Layered compounds Layered material

43,461 485 485

M MAS NMR spectra of -27Al -31B -lH

-71Ga -31P

-29Si Magnesi urn Manganese MeASPO-5 Mesopore structure Metallophatocyanine Me thano 1 Methylene blue MgAPO-5 Microscopic sequential reaction mechanism Molybdenum Molybdenum sulphide Mordenite

1,135 453 453 1 119,231 1,43,135, 145,373,461 43,287,347 145 65 93 397 199 145,373 355 331,339 339 43,79,223,287

N NMR 129 Xe NMR double rotation NO decomposition Nickel

46 1 435 355 93,145,295,331,461

508 0

Olef ins Oligomerization Oxidation of paraffins

1 , 165,347 165,347 79

P Paraffin transformations Phosphates Photo-properties Photocorrosion Phtalocyanine Piezometric measurement Pi1laring Platinum Propane Propene

135,277,365,373,409 485 65 65,477 93 469 4s5 65,135,381,425 277,321,405,409 321

Q Quantum size particles

65,477

R

Raman spectra of Ti-silicalite Rare earth metal cations Reforming Rhodium Ruthenium

25 1 43 425 215 215

S

SAPO-5 SAPO-8 SAPO-11 SAPO-37

SEM SEM-EDAX

SIMS Semiconductors hosted in zeolites Shape selectivity Si-HZSM-5 Si-VPI -5 Silicalite Silver Solid-state reaction Sulphur radicals Syngas Synthesis of molecular sieves

T TEM TG-DTA Temperature-programmed reduction

135,295,303,373,453 135 135 313,365 127,231,269,295 79,145 1 65 25,277,321,347 347 135 165,277,303,341 43,443,461 25,43,207,269,277,287,347

339 417 1,5,25,79,109, 127,135,145,157, 173,207,287

381 135 215,223,277,295,341,417

509 Temperature-programmed oxidation Temperature-programmed desorption of ammonia Temperature-programmed desorption of pyridine Ti-silicalite Titanium Trimethylpentane ( 2 , 2 , 4 - ) U UV-VIS spectra of -cobalt -methylene blue -sulphide clusters -titania complexes V V-silicalite

VPI-5 Vanadium

313 181,347,417 313,379,397 65,79,251, 79,251,303 365

231 199 189 79,251

173 435 43,173

X X type zeolite X-ray diffraction XAES XPS Xylenes

Y Y type zeolite

43,93,199,287,397,443,461 1 , 7 9 , 127,135,145,277 259,269 269,381 277,477

1 , 2 5 , 4 3 , 7 9 , 1 0 1 , 1 8 9 , 199,223,256, 313,331,339,365,389,461

z ZSM-5 ZSM-48 Zeolite admixing Zeolite hosted Zinc Z i r coni um Zn oxychloride Zn-faujasite ZnAPO-5 Zr-silicalite

25,109,119,207,223,277,321,341 389,405, 4 0 9 , 4 1 7 , 4 7 7 157 425 199

1,65,109,145,189,207,321,373 303 32 1 I

145 303

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STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universith Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A.

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Preparation of Catalysts I. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417, 1975 edited by B. Delmon, P.A. 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 8. Delmon Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications t o Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-1 1, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, 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.F. Froment N e w Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyie, September 29-October 3, 1980 edited by M. LazniEka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 2 1-23, 198 1 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 14-1 6, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. JirO and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz

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HeterogeneousCatalytic Reactions Involving Molecular Oxygen by G.I. Golodets Preparation of Catalysts Ill. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, LyonVilleurbanne, September 12-1 6, 1983 edited by G.M. Pajonk, S.J. Teichnerand J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-1 3, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jirb, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by 6. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit 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 and Application. Proceedings of an InternationalSymposium, Portoroi-Portorose, September 3-8, 1984 edited by B. Driaj, S. HoEevar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the 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 of the Fourth InternationalConference, Bowness-on-Windermere, September 15-1 9, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cervenq New Developments in Zeolite Science and Technology. Proceedingsof the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakarni, A. lijirna and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H.Knozinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth international Symposium, Louvain-la-Neuve, September 1-4,1986 edited by B. Delrnon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P. Wissmann Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by 6. Delmon and G.F. Froment

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Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Methane Conversion. Proceedings of 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.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International ’ Symposium, Nieuwpoort, September 13-1 7, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings of the 10th North American Meeting of the 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., April 26-29, 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 7-1 1, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-1 7, 1988 edited by M.Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes b y E.G. Christoffel, revised and edited by 2. Paal Catalytic Processes under U nsteady-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. lnui 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, Wurzburg, September 48,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. 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. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17- 19, 1989 edited by J. Klinowski and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara

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Future Opportunities i n Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura 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 Volume 56 Olefin Polymerization Catalysts. Proceedings of the InternationalSymposium on Recent Developments in Olefin PolymerizationCatalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 576 Spectroscopic Analysis of Heterogeneous Catalysts. Part 6:Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction t o Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd InternationalSymposium, Poitiers, October 2-6, 1990 edited by M. Guisnet. J. Barrault. C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 6 0 Chemistry of Microporous Crystals. Proceedings of the InternationalSymposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui. S. Namba and T. Tatsumi Volume 6 1 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-1 7, 1990 edited by A. Holmen. K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPS 11). Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso,J. Rouquerol. K.S.W. Sing and K.K. Unger Volume 63 Preparation of Catalysts V. Proceedings of the Fifth International Symposium on the Scientific Bases for the Preparation of HeterogeneousCatalysts, Louvain-laNeuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon Volume 6 4 New Trends in CO Activation edited by L. Guczi Volume 65 Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990 edited by G. Ohlmann, H. Pfeifer and R. Fricke Volume 66 Dioxygen Activation and HomogeneousCatalytic Oxidation. Proceedings of the fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonfured, September 10- 14, 1990 edited by L.I. Simandi Volume 67 Structure-Activity and Selectivity Relationships in HeterogeneousCatalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Volume 68 Catalyst Deactivation 1991. Proceedings of the fifth International Symposium, Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Volume 69 Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13. 1991 edited by P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlovi