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Studies in Surface Science and Catalysis 82 NEW DEVELOPMENTS IN SELECTIVE OXIDATION II

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Studies in Surface Science and Catalysis82 Advisory Editors: B. Delrnon and J.T. Yates

Vol. 82

NEW DEVELOPMENTS IN SELECTIVE OXIDATION II Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, September 20-24,1993

Editors

V. CortesCorberan lnstituto de Catalisis y Petroleoquimica, CSIC, Campus UAM Cantoblanco, 28049 Madrid, Spain

S. Vic Bellon Centro de lnvestigacion de Repsol Petroleo S.A., Embajadores 183, 28045 Madrid, Spain

ELSEVIER

Amsterdam -London -New York -Tokyo

1994

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box211,1000AEAmsterdam,The Netherlands

L i b r a r y o f Congress C a t a l o g i n g - i n - P u b l i c a t i o n

Data

New d e v e l o p m e n t s i n selective o x i d a t i o n I 1 p r o c e e d i n g s o f t h e second w o r l d c o n g r e s s and f o u r t h E u r o p e a n w o r k s h o p m e e t i n g , B e n a l r n i d e n a . S p a i n , S e p t e m b e r 2 0 - 2 4 , 1993 / e d i t o r s . V . C o r t e s C o r b e r i n . S . V i c Bellon. p. cm. -- ( S t u d i e s 1n s u r f a c e s c i e n c e and c a t a l y s l s 82) I n c l u d e s b l b l i o g r a p h i c a l r e f e r e n c e s and i n d e x . ISBN 0-444-81552-X 1. Oxidation--Congresses. I. C o r b e r a n , V . C o r t e s . 11. B e l l o n , S . Vic. 111. S e r i e s . TP156.09N483 1994 660'.2993--dc20 9 4 - 10954 CIP

.

ISBN 0-444-81552-X

0 1994 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521,1000AM Amsterdam,The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified.

No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands

PREFACE This Volume contains a collection of the papers presented at the 2nd World Congress and 4th European Workshop Meeting "New Developments in Selective Oxidation", held in Benalmhdena, Spain, September 20-24, 1993. The Congress was organized under the joint auspices of the Grupo Especializado de Cathlisis (Specialized Group on Catalysis) of the Real Sociedad Espaiiola d e Quimica (Royal Spanish Society of Chemistry), and the Instituto de Cathlisis y Petroleoquimica (ICP), of the Consejo Superior d e Investigaciones Cientificas (CSIC). This Congress folIows previous World Congress in Rimini (Italy) in 1989 (Studies in Surface Science and Catalysis, Volume 55, 1990) and the I11 European Workshop in Louvain-la-Neuve (Belgium) in 1991 (Studies in Surface Science and Catalysis, Volume 72, 1992). The objective of the meeting was the presentation of new topics and recent advances as well as the discussion of new aspects concerning fundamental and applied aspects of partial selective oxidation reactions in heterogeneous and homogeneous catalysis. The topics of the symposium were the following: -

-

-

-

New processes and obtention of fine chemicals by catalytic partial oxidation. Recent developments in surface chemistry of oxide catalysts. Novel catalytic systems and preparation methods. Heterogeneized homogeneous oxidation catalysts. Selective oxidation and oxidative dehydrogenation of alkanes. New industrial developments based on catalytic oxidation reactions. Bio-, Photo-, and Electro-catalytic oxidation. Oxidation by other agents than dioxygen. Bifunctional metal-on-metal oxide catalysts for selective oxidation.

The meeting was attended by over 190 researchers from 30 countries, with a very important participation of researchers coming from the major industries working in the field. The programme of the Congress consisted in nine monographic sessions for extended and oral communications and a poster session. At a variance of previous volumes devoted to the meetings on selective oxidation, the organization of the topics in this volume has been arranged by grouping the papers by type of reaction. We hope that this will bring the reader an overall view of the current trends in the research in each field of the selective catalytic oxidation. The Editors are much indebted to the Authors for the quality of their presentations and their contribution to this Volume. Special thanks are also extended to the Scientific Committee and all the researchers participating in the reviewing procedure for the time and effort devoted to ensure the high scientific level of this Volume and to all the Chairmen of the Sessions for providing their time and expertise to lead efficiently the discussions.

VI

The Editors also thank the members of the Organizing Committee whose efforts made possible the realization of the Congress, and to all the sponsoring companies, Spanish Institutions and the Commission of the European Communities for their financial contributions. Finally, our thanks further go to all the members of the research groups of the Editors for their understanding and support during the preparation of both the Congress and this book. Special thanks are addressed to Ms. Nuria Raboso PBrez for her assistance in the preparation of the book. V. CortBs Corberh and S. Vic Belldn

vii

ORGANIZED BY Grupo Especialhado de CatAlisis, Real Sociedad Espaiiola de Quimica (R.S.E.Q.) Instituto de Catilisis y Petroleoquimica, Consejo Superior de Investigaciones Cientificas (CSIC)

SCIENTIFIC COMMITTEE B. Delmon (Belgium) M. Baerns (Germany) R.K. Grasselli (U.S.A.) J. Haber (Poland) G. HCcquet (France) O.V. Krylov (C.I.S.) H. Mimoun (Switzerland) M. Misono (Japan) R.A. Sheldon (Netherlands) F. Trifirb (Italy) S. Vic Belldn (Spain)

ORGANIZING COMMITIEE S. Vic Bellbn, REPSOL, Spain J. Molina Marsans, INTERQUISA, Spain F. Melo Faus, ICP (CSIC), Spain J.M. Campelo, University of Cbrdoba, Spain G. Centi, University of Bologna, Italy J.L.G. Fierro, ICP (CSIC), Spain J.P. G6mez, REPSOL, Spain J.M. Ldpez Nieto, ITQ (UPV-CSIC), Spain S. Mendioroz Echeverria, ICP (CSIC), Spain J.J. Rodriguez, University of Mglaga, Spain P. Ruh, Catholic University of Louvain, Belgium V. CortCs Corberin, ICP (CSIC), Spain

...

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SPONSORING The organizing Commitee gratefully acknowledges financial support from:

- A R C 0 Chemical Co. (USA) - Ayuntamiento d e Benalmgdena (Spain) - Bacardi y Cia. (Spain) - BP America (USA) - CEPSA (Spain) - Commission of the European Communities (European Union)

- Consejo Superior de Investigaciones Cientificas (CSIC) (Spain) - Costa del Sol Convention Bureau (Spain) - DSM Research (The Netherlands) - EXXON Chemical International (The Netherlands)

- F. Hoffman - La Roche Ltd. (Switzerland) - Mitsui Toatsu Chem. Inc. (Japan) - Real Sociedad Espafiola d e Quimica (Spain) - RhBne-Poulenc Chimie (France) - San Miguel, S.A. (Spain)

- Secretaria d e Estado d e Universidades e Investigacibn (Spain)

- Universidad d e Mglaga

(Spain).

ix

CONTENTS Preface

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

Organization

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

V

VII

C,-C, Olefins oxidation Modelling of propylene oxidation in a circulating fluidized-bed reactor (extended communication) G.S. Patience and P.L. Mills. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Selective oxidation over structured multicomponent molybdate catalysts Joe Y. Zou and Glenn L. Schrader. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19

Relationships between the catalytic activity and the composition of various uranium-antimony mixed oxide catalysts in the selective oxidation of olefins F. Gama Freire, J.M. Herrmann and M.F. Portela. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Synergy in the Fe-Mo-Sb-0 niultiphnre system L.E. Cadus, Y.L. Xiong, F.J. Gotor, D. Acosta, J. Naud, P. Ruiz and B.Delmon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41

Role of tellurium oxide in the selective oxidation of Bobutetie to methacrolein: a-Sb,O, - TeO, catalysts P. Oelker, L. Cadus, D. Forget, L. Daza, C. Papadopoulou, F. Gil Llambias, J. Naud, P. Ruiz and B. Delmon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55

Propylene selective oxidation as studied by oxygen-I8 labelling on well-defined MOO, catalysts M. Abon, M. Roullet, J. Massardier, P. Delichbre and A. Guerrero-Ruiz. . . . . . . . . . . . . 67 Selective hydrocarbon oxidation at vanadium pentoxide surfnces: ab initio cluster model studies M. Witko and K. Hermann. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75

Oxidative dehydrogenation of alkanes Comparison between gamma-alumina and aluminium niobate supported vanadium oxides in propane oxidative dehydrogenation J.-G. Eon, P.G. Pries de Oliveira, F. Lefebvre and J.-C. Volta. . . . . . . . . . . . . . . . . . . . .

83

X

Dispersion of V'+ions in a SnO, rutile matrix as a tool for the creation of active sites in ethane oxydehydrogenation S. Bordoni, F. Castellani, F. Cavani, F. Trifiri, and M.P. Kuldarni. . . . . . . . . . . . . . . . . 93 Oxidative dehydrogenation of ethane over chrornia-pillared montinorillonite catalysts P. Olivera-Pastor, J. Maza-Rodriguez, A. JimCnez-Lbpez, I. Rodriguez-Ramos, A. Guerrero-Ruiz and J.L.G. Fierro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

103

Oxidative dehydrogenation of propane and n-butane on V-Mg based catalysts A. Corma, J.M. Ldpez Nieto, N. Paredes, A. Dejoz and I. Vazquez. . . . . . . . . .

113

Oxidative dehydrogenation of tlze C& parafins over vanadium-containing oxide cataZysts R.G. Rizayev, R.M. Talyshinskii, J.M. Seifullayeva, E.M. Guseinova, Yu.A. . . 125 Panteleyeva and E.A. Mamedov. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidelzydrogenation of propane on gallium oxide-faujasite catalysts B. Sulikowski, J. Krysciak, R.X. Valenzuela and V. CortCs CorberAn. . . . . . . . . . . . . . . 133 Characteristics of alumina boria catalysts used in ethane partial oxidation G. Cucinieri Colorio, B. Bonnetot, J.C. Vedrine and A. Auroux. . . . . . . . . . . . . . . . . . . 143 Effect of potassium addition to V,O,/TiO, and MoOJTiO, catalysts on their physicochemical and catalytic properties in oxidative dehydrogenation of propane B. Grzybowska, P. Mekss, R. Grabowski, K. Wcislo, Y . Barbaux and L. Gengembre. . . 151 Catalytic reduction of carbon dioxide by hydrocarbons and other organic compounds O.V. Krylov, A.Kh. Mamedov and S.R. Mirzabekova. . . . . . . . . . . . . . . . . . . . . . . . . . .

159

C,-C, paraffins oxidation

A concise description of the bulk structure of vanadyl pyrophosphate and implications for n-butane (extended communication) M.R. Thompson, A.C. Hess, J.B. Nicholas, J.C. White, J. Anchell and J.R. Ebner

.....

167

A study of the (surface) structure of V-P-0 catalysts during pretreatment and during activation

R.A. Overbeek, M. Versluijs-Helder, P.A. Warring, E.J. Bosma and J.W. Geus. . . . . . 183 The oxidation of n-butane 011 vanadyl pyrophosphate catalysts: study of pretreatment process B. Kubias, M. Meisel, G.-U. Wolf and U. Rodemerck. . . . . . . . . . . . . . . . . . . . . . . . . .

195

Activation of vanadium plzosplzorus oxide catalysts for alkane oxidation oxygen storage and catalyst pet$onnarrce Y. Schuurman, J.T. Cleaves, G. Golinelli, J.R. Ebner and M.J. Mummey. . . . . . . . . . . . 203 Vanadium phospltate catalysts prepared by the reduction of VOPO, , 2H2O G.J. Hutchings, R. Olier, M.T. SananCs and J.C. Volta. . . . . . . . . . . . . . . . . . . . . . . . . .

213

xi

Production of maleic and phthalic anhydrides by selective vapor phase oxidation with vanadium oxide based catalysts C. Fumagalli, G. Golinelli, G. Mazzoni, M. Messori, G. Stefani and F. Trifirb. . . . . . . . 221 A new commercial scale process for n-butane oxidation to maleic anhydride using a circulating fluidized bed reactor R.M. Contractor, D.I. Garnett, H.S. Horowitz, H.E. Bergna, G.S. Patience, J.T. Schwartz and G.M. Sisler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

233

Separation of catalyst oxidation and reduction - A n alternative to the conventional oxidation of n-butane to maleic anhydride? 243 G. Emig, K. Uihlein and C.-J.Hacker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . On the catalyst features affecting selectivity in n-C, hydrocarbon oxidation and oxidative deliya?ogeriation. FT-IR studies G. Busca, V. Lorenzelli, G. Oliveri and G. Ramis. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

253

New reaction: n-Butane direct catalytic oxidation to tetrahydrofuran V.A. Zazhigalov, J. Haber, J. Stoch, G.A. Komashko, A.I. Pyatnitskaya and I.V. Bacherikova, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

265

Oxidation and ammoxidatiort of propane over tetragonal type n.f'OP0, catalysts (extended communication) Ikuya Matsuura and Naomasa Kimura. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

271

Structure and Stability during the Catalytic Reaction of Unsupported V-Antimonate Catalysts for the Direct Selective Ammoxidation of Propane to Actylonitrile (extended communication) 281 G. Centi, E. Foresti and F. Guarnieri. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amnioxidation of propane over vanadium-antimony-oxide catalysts. Role of phase cooperation effects R. Nilsson, T. Lindblad, A. Anderson, C. Song and S. Hansen. . . . . . . . . . . . . . . . .

..

293

Selective oxidation ofpropane in the presence of bismuth -based catalysts J. Barrault, L. Magaud, M. Ganne and M. Tournoux. . . . . . . . . . . . . . . . . . . . . . . . .

..

305

Methane activation A study of the catalytic oxidative oligomerization of methane to aromatics (extended communication) A.P.E. York, J.B. Claridge, M.L.H. Green and S.C. Tsang. . . . . . . . . . . . . . . .

3 15

Investigation of molten cobalt halidelsodium metavanadate mixtures as redox catalysts for the oxidative coupling of methane J.B. Claridge, M.L.H. Green, R.M. Lago, S.C. Tsang and A.P.E. York. . . . . . . . . . . .

327

The active oqgen species in oxidative coupling of methane over LilCaO and NalCaO using N,O and 0,as oxidants A.G. Anshits, V.G. Roguleva and E.V. Kondratenko. . . . . . . . . . . . . . . . . . . . . . .

337

xii

Spectroscopic characterization of surface oxygen species on barium-contaitling methane coupling catalysts M.P. Rosynek, D. Dissanayake and J.H. Lunsford. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

345

Selectivity control by oxygen pressure in methane oxidation over phosphate catalysts M.Yu. Sinev, S. Setiadi and K. Otsuka. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

357

Isotopic labeling studies on oxidative coupling of methane over alkali promoted molybdate catalysts S.A. Driscoll and US. Ozkan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

367

Methane coupling over Sm-A1 mixed oxides P. Malet, M.J. Capitan, M.A. Centeno, J.J. Benitez, I. Carrizosa and J.A. Odriozola.

..

377

Catalytic reactor engineering for the oxidative coupling of methane. Use of a fluidized bed and of a ceramic membrane reactor A. Santos, C. Finol, J. Coronas, D. Lafarga, M. MenCndez and J. Santamaria. . . . . . . . 387 Bi,O,-M,O, catalysts for oxidative coupling of methane: relationship between structural features and catalytic behaviours E.A. Mamedov, N.T. Shamilov, V.P. Vislovskii, P.N. Joshi and S. Badrinarayan. . . . . . . 395 Oxidative conversion of methane over MgOIZSM-5 catalysts P. Kovacheva. N.Davidova and A.H. Weiss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

403

Surface characterization and catalytic behaviour of LilMgO in oxidative coupling of methane C.L. Padr6, W.E. Grosso, G.T. Baronetti, A.A. Castro and O.A. Scelza. . . . . . . . . . . . . 411 Solid-gasphase interface aizalysis on ZrO; correlation between CH,oxidation activity and work function measurements D. Bouqueniaux, L. Jalowiecki-Duhamel and Y. Barbaux. . . . . . . . . . . . . . . . . . . . . . . .

419

The rble of structural defects and oxygen migration in La,O, for the oxidative coupling of methane M.S. Islam and D.J. Ilett. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

427

Kinetic simulation of oxidative coupling of methane in the gas phase V.I. Vedeneev, O.V. Krylov, V.S. Arutyunov, V.Ya. Basevich, M.Ya. Goldenberg and M.A.Teitel’boim. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

435

Oxidative coupling of methane over Ti-La-Na catalysts S.T. Brandao, L. Lietti, P.L. Villa, S. Rossini, A. Santucci, R. Millini, 0. Forlani and D.Sanfilippo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

443

...

Xlll

Oxidation by supported metals Gas Phase Oxidation of Benzene to Phenol using PdlCu Salt Catalysts. Effect of Counter Anion in Copper Salts Kazuo Sasaki, Tomoyuki Kitano, Toshihiro Nakai, Miko Mori, Sotaro Ito, Masahiro Nitta and Katsuomi Takehira. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

451

Low temperature gas-phase selective oxidation of 1-butene to 2-butanone on supported Pd/V20,catalysts G. Centi, M. Malaguti and G. Stella. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

461

The adsorption of oqgen on A g and Ag-Au alloys: Mechanistic implications in ethylene epoxidation catalysis D.I. Kondarides and X.E. Verykios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

471

Detailed modeling of transport-kineticsinteractions of ethylene epoxidation at high vacuum and atmospheric pressures G.D. Svoboda. J.T. Gleaves and P.L. Mills. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

481

Doping effects in ethylene epoxidation over potassium promoted silver catalysts V. Lazarescu. M. Stanch and M. Vass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

495

A temperature programmed surface reaction study of the catalytic epoxidation and total oxidation of ethylene on silver C. Henriques, M.F. Portela and C. Mazzocchia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

499

HRTEM and TPO study of the behaviour under oxidizing conditions of some RhlCeO, catalysts S. Bernal, G. Blanco, J.J. Calvino, G.A. Cifredo, J.A. PCrez Omil, J.M. Pintado andA.Varo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

507

Liquid phase oxidations

Redox molecular sieves as heterogeneous catalysts for liquid phase oxidations (extended communication) R.A. Sheldon, J.D. Chen, J. Dakka and E. Neeleman. . . . . . . . . . . . . . . . . . . . . . . . . . .

515

Influence of the synthesis procedure and chemical composition on the activity of titanium in Ti-Beta catalysts M.A. Camblor, A. Corma, A. Martinez, J. PCrez-Pariente and S. Valencia. . . . . . . . . . . 531 Selective oxidation of ammonia to lzydroxylarnine with hydrogen peroxide on titanium based catalysts M.A. Mantegazza, G. Leofanti, G. Petrini, M. Padovan, A. Zecchina and S. Bordiga. . . 541 Palladium catalyzed oxidation of benzene to phenol using molecular oxygen U. Schuchardt, A.T. Cruz, L.C. Passoni and C.H. Collins. . . . . . . . . . . . . . . . . . . . . . . .

551

XIV

Partial oxidation of ciiinarnyl alcohol on bimetallic catalysts of iinproved resistance to self-poisoning T. Mallat, 2. Bodnar, M. Maciejewski and A. Baiker. . . . . . . . . . . . . . . . . . . . . . . . . . .

561

Novel tungsten catalysts grafted onto polymeric materials: a comparison with phase transfer catalysis (extended communication) J.M. BrCgeault, R. Thouvenot, S. Zoughebi, L. Salles, A. Atlamsani, E. Duprey, C.Aubry,andF.Robert. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

571

Selective oxidation of cyclopenteiie and cycloliexeiie by hydrogen peroxide catalyzed by heteropolyacidr Kwan-Young Lee, Koshi Itoh, Masato Hashimoto, Noritaka Mizuno and Makoto Misono. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

..

583

Highly selective epoxidation of olefins on mono-transition-metal-substituted Kegqin-type heteropolytungstates by molecular oxygen in the presence of aldehyde N. Mizuno, T. Hirose and M. Iwamoto. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

..

593

Selective oxidation of cyclohexene with niolecular oxygen catalyzed by transition metal substituted polyoxometalates Dujie Qin, Goujia Wang and Yue Wu. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . 603

Novel catalysts for olefin cleavage using hydrogen peroxide A. Johnstone, P.J. Middleton, W.R. Sanderson, M. Service and P.R. Harrison. . .

. . 609

Novel one -pot synthesis of indigo from iiidole and organic hydroperoxide Y. Inoue, Y. Yamamoto, H. Suzuki and U. Takaki. . . . . . . . . . . . . . . . . .

. . 615

Selective oxidations with short-lived manganese (V) E. Zihonyi-Bud6 and L.I. Simindi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

..

Catalytic oxidation of polyols: new example of nonradical mechanism of oxygenation A.M. Sakharov. I.P. Skibida. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . 629

Copper-catalyzed oxidative decarboxylation of aliphatic carboxylic acidr F.P.W. Agterberg, W.L. Driessen, J. Reedijk, H. Oevering and W. Buijs. . . . . . .

623

. .. .

639

..

647

Cyclohexane oxidation by the GOAGG" system: fomiation of iron (1iydr)oxideparticles and reactivation U. Schuchardt, C.E.Z. Krahembuhl and W.A. Carvalho. . . . . . . . . . . . . . . . . . . . .

Oxidation of cyclohexane catalyzed by polyhalogenated and perhalogenated manganese po'piyrins P.Battioni, R. Iwanejko, D. Mansuy and T. Mlodnicka. . . . . . . . . . . . . . . . . . . . . . . . . . 653 Polymer supported iron catalysts for the oxidation of cyclohexane Ki-Won Jun, Eun-Kyung Shim, Seong-Bo Kim and Kyu-Wan Lee. . . . . . . . . . . . . . . . . 659 Selective oxidation of 2-mercaptobenzothiazole M. Hronec, M. Stolcovh and T. Liptay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

667

xv

Bio-, electro- and photo-oxidations Selective oxidation of gaseous hydrocarbons by microbial cells G.A. Kovalenko, V.K. Sokolovskii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

675

Selective enzymatic oxidations by using oxygen as oxidizing agent: immobilization and stabilization of FNR, a NADP' regenerating enzyme T. Bes, R. FernBndez-Lafuente, C.M. Rosell, C. Gbmez-Moreno and J.M. GuisAn. . . . . 685 ESR study of photo-oxidation of phenol at low temperature on polyctystalline titanium dioxide M.J. Lbpez-Muiioz, J. Soria, J.C. Conesa and V. Augugliaro. . . . . . . . . . . . . . . . . . . . . 693 Partial oxidation of benzene over the carbon wlzkker cathode added with iron oxide and palladium black during 0,-H, fuel cell reactions Kiyoshi Otsuka, Mitsuhiro Kunieda and Ichiro Yamanaka. . . . . . . . . . . . . . . . . . . . . . .

703

Influence of operational variables on the photodegradation kinetics of Monuron in aqueous titanium dioxide dispersions V. Augudiaro, L. Cavallaro, G. Marci, L. Palmisano and E. Pramauro. . . . . . . . . . . . . 713 Heterogeneous photocatalytic oxidation of liquid isopropanol by TiO, , ZrO, and ZrTiO, powders J.A. Navio and G. Colbn. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

721

Gas-phase aromatics oxidation Effect of the state of vanadiunz on the properties incorporation titanium phosphate-based catalysts for oxidation of toluene J. Soria, J.C. Conesa, V. Villalba, A. Aguilar ElguCzabal and V. CortCs CorberBn. . . . . 729 Quantum-chemical description of the oxidation of alkylaromatic molecules on vanadium oxide catalysts J. Haber, R. Tokarz and M. Witko. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

739

Characterisation of VzO,-FezO,-CsJO, catabsts for the gm-phase oxidation of fluorene to 9-fluorenone F. Majunke, S. Trautmann, M. Baerns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

749

Gas-phase catalytic oxydehydrogenation of ethylbenzene on AIPO, catalysts F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and R.A. Quirbs. . . . . 759 Alcohols oxidation Selective gas-phase dehydrogenation of cyclohexanol with magnesium orthophosphates M.A. Aramendia, J. Barrios, V. Borau, C. Jimknez, J.M. Marinas, F.J. Romero, J.R. R u u and F.J. Urbano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

769

xvi Effect of K-doping on 2-propanol adoption, desotption and catalytic oxidation over vanadia-titania G. Busca, V. Sanchez Escribano, P. Forzatti, L. Lietti and G. Ramis. . . . . . . . . . . . . . . 777 Selective oxidation of methanol on iron-chromium-molybdenum oxide catalysts D. Klissurski, V. Rives, Y. Pesheva, 1. Mitov and R. Stoyanova. . . . . . . . . . . . . . . . . . . 787 Creation of new selective sites by spill-over oxysen a-Sb,O, in the oxidation of ethanol R. Castillo, P.A. Awasarkar, Ch. Papadopoulou, D. Acosta, and P. Ruiz. . . . . . . . . . . . 795 Effect of titania on the properties of alumina supported molybdena catalysts F. Requejo, N. Quaranta, J.M. Coronado, J. Soria and H. Thomas. . . . . . . . . . . . . . . . 803 Selective dehydrogenation of ethanol over vanadium oxide catalyst N.E. Quaranta, R. Martino, L. Gambaro and H. Thomas. . . . . . . . . . . . . . . . . . . . . . . .

811

Gas-phase oxidation of other compounds Cesium promotion of iron phosphate catalyst and influence of steam on the oxidative dehydrogenation of isobutyn’c acid to methacrylic acid J. Belkouch, B. Taouk, L. Monceaux, E. Bordes, P. Courtine and G. Hecquet. . . . . . . . 819 Iron hydroxysilicates: New selective and active isobutyric acid oxidative dehydrogenation catalysts P. Bonnet, J.M.M. Millet, J.C. Vedrine and G. Hecquet. . . . . . . . . . . . . . . . . . . . . . . . .

829

Thermolysis of heteropolyacid HQMo,,O, and catalytic properties of the thermal decomposition products in oxidation of acrolein to acrylic acid T.V. Andrushkevich, V.M. Bondareva, R.I. Maksimovskaya, G.Ya. Popova, L.M. Plyasova, G.S. Litvak, A.V. Ziborov. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

837

Selective oxidation of aldehydes over V-Mo-OJSiO, catalysts J. Machek, J. Svachula, J. Tichy, L.J. Alemany, F. Delgado, J.M. Blasco.

845

..........

Diacetyl syrzthesb by the direct partial oxidation of methyl ethyl ketone over vanadium oxide catalysts E. McCullagh, N.C. Rigas, J.T. Gleaves and B.K. Hodnett. . . . . . . . . . . . . . . . . . . . . . .

853

Selective oxidation of hydrogen sulfide on a sodium promoted iron oxide on silica catabst R.J.A.M. Terorde, M.C. de Jong, M.J.D. Crombag, P.J. van den Brink, A.J. van Dillen and J.W.Geus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

86 1

Effect of motphology of honeycomb SCR catalysts on the reduction of NO, and the oxidation of SO, A. Beretta, E. Tronconi, L.J. Alemany, J. Svachula, P. Forzatti. . . . . . . . . . . . . . . . . . . 869 Author index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subject index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

877 881

V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation If 0 1994 Elsevier Science B.V. All rights rescrved.

1

Modelling of propylene oxidation in a circulating fluidized-bed reactor G. S. Patience and P. L. Mills DuPont Company, Experimental Station P.O. Box 80262, Wilmington, Delaware 19880-0262 USA The performance of a circulating solids fluidized-bed riser reactor for the partial oxidation of propylene t o acrolein has been analyzed using a detailed reaction engineering model. The model accounts for the complex interaction between gas-solid hydrodynamics, heat and mass transfer, and intrinsic kinetics using a core-annular model as the basis for the gas and solid phase flow patterns. Key hydrodynamic model parameters are obtained by interpretation of gas and solid tracer experiments under cold-flow conditions in a pilot-scale reactor. The model predictions give insight into the factors that affect the riser performance, and demonstrate that it has potential as an alternate reactor type for the commercial scale production of acrolein. 1. INTRODUCTION The selective oxidation and ammoxidation of propylene to acrolein, acrylic acid, propylene oxide, and acrylonitrile are important commercial processes that use multicomponent metal oxide catalysts (Snyder and Hill, 1989). Fluidized-bed reactors are generally preferred over conventional multitubular reactors for these classes of highly exothermic reactions since they have superior heat transfer characteristics, which results in nearly isothermal operation with better temperature control. However, operation in the conventional bubbling fluidization regime can result in a lower production rate per unit reactor volume and an increased yield of by-products when compared to multitubular fixed-beds (Arntz et al., 1982) since the catalyst flow pattern in this regime generally approaches perfect backmixing (Kunii and Levenspiel, 1992). One method used for reducing the degree of gas backmixing for fluidized-beds has involved the addition of internals (cf, Avidan et al., 1986). Detailed studies on the effects of various types of internals on the hydrodynamics of laboratory and pilot-scale cold-flow units have been recently reported (cf, Dutta and Suciu, 1992; Jiang et al., 1991; van der Ham et al., 1992). While the use of internals may exhibit some attractive benefits in terms of increased product yield, they may also result in some disadvantages, such as increased pressure drop and catalyst particle attrition. Contractor and Chaouki (1991) have recently advocated the use of circulating fluidized-bed reactors (CFB) as an alternate reactor type for selective oxidation reactions in lieu of conventional multitubular reactors and fluidized-bed reactors. Some advantages of CFB reactors include: (1) separate zones for

2

catalyst reduction and re-oxidation that can be independently controlled which leads t o higher product yields, (2) ability to use hydrocarbon concentrations in the feed gas that lie above the upper explosion limit in air, for example, 11.1 % versus 2 % in the case of propylene, which translates into higher corresponding concentrations of product, (3) reduced catalyst inventory, (4) nearly isothermal operation, (5) high heat and mass transfer rates, ( 6 ) high turndown ratio, and (7) simplified methodology for catalyst addition and removal. A key disadvantage is the need to develop an attrition resistant catalyst that can be readily fluidized while maintaining the required activity and selectivity for practical commercial-scale operation. Development of attrition resistant catalysts is often a major obstacle that limits the commercialization of fluidized-bed selective oxidation and ammoxidation processes. The traditional approach for imparting attrition resistance is based on spray-drying the active catalyst precursor so the matrix contains 30 to 50 wt % silica. Recent technology developed by DuPont (Bergna, 1989) for VPO catalysts used in n-butane oxidation t o maleic anhydride encapsulates the active catalyst in a porous silica shell. The pore openings are large enough so reaction species can readily diffuse into and out of the inner region of the particle without affecting the maleic anhydride selectivity. A 100 million pound per year DuPont tetrahydrofuran plant that uses a CFB riser in the first step of the process to produce maleic anhydride from n-butane with an attrition resistant VPO catalyst is scheduled t o start-up in 1995 in Asturias, Spain (Stadig, 1992). The primary objective of this paper is t o set forth a detailed reaction engineering model for the partial oxidation of propylene to acrolein in a CFB reactor, and to demonstrate its utility in analyzing reactor performance for selected process variables. Another objective is t o briefly summarize gas and solids tracer experiments used to obtain some of the key hydrodynamic parameters used in the model. A final objective is to point out the importance of lattice oxygen on the observed reactor performance, and the possible utility of the CFB reactor for this type of reaction. 2. EXPERIMENTAL

The hydrodynamic model given in a later section for simulating the performance of a commercial scale propylene oxidation CFB reactor is based upon gas and solid phase tracer experiments and suspension density measurements performed in a pilot-scale CFB reactor under cold-flow conditions. Key aspects of this reactor system and the associated experiments are described below. 2.1. Reactor System

A schematic of the CFB reactor system, which consisted of the riser tube, a cyclone, hopper, standpipe, and L-valve, is shown in Figure 1. The riser was a section of stainless steel pipe having an inner diameter of 83 mm and an overall height of 5 m. The solids were introduced into the riser from the L-valve just above an orifice plate distributor and entrained in the upward flowing gas. The gas-solid mixture at the riser exit was separated in the cyclone with the solids being introduced into the hopper and gas exiting through the exhaust.

3

The solids circulation rates were determined by measuring the pressure drop in the horizontal section between the riser and the cyclone. This method has been shown to be sensitive to both the solids mass flux and the gas velocity (Patience et al., 1990). Other particulars about the experimental setup and parameters used are indicated in Figure 1and are given by Patience (1990).

7 -+EXHAUST

7

I

CYCLONE

HOPPER

STANDPIPE

*AERATION

Value Air Solids Sand 277 pm dP 2630 kg/m3 PP Tracer Ar-41 Detectors Nal Scintillators Forcing Function Pulse 4 to 8.5 m/s UG 20 to 139 kg/m2-s Gs 5m Lr 0.083 m dr

GAS

Figure 1. Experimental riser reactor used for the gas-phase tracer, solidphase tracer, and suspension density measurements. (After Patience and Chaouki, 1993). 2.2. Gas-Phase Tracer Experiments The gas-phase flow pattern in the riser was determined using Ar-41 as the tracer. The tracer was produced by irradiating a 9 milligram sample of Ar-40 in the fast neutron flux using the nuclear reactor facility at Ecole Polytechnique de Montreal. NaI scintillators, which were collimated with lead, were used as detectors. They were positioned as shown in Figure 1 at heights of 1 m, 4 m, and at the horizontal section between the riser and the cyclone. The tracer was introduced using a special-purpose syringe which simultaneously activated a computerized data acquisition system. To minimize errors associated with the response times of the tracer injection-sampling system, the two-point method was used t o determine the gas-phase impulse response. Additional details are given elsewhere (Patience and Chaouki, 1993).

4

23 Solid-PhaseTracer Experiments The solid-phase flow pattern in the riser was determined using radioactive sand as the tracer. The sand used for the tracer was taken from the same lot used in the riser experiments. Three fractions having mean diameters of 109 pm, 275 pm, and 513 pm, respectively, were used. These diameters were selected as representative samples of the bed material to determine the effect of particle size on the measured tracer responses. The terminal velocities of these particular samples were calculated t o be Vt = 0.9, 1.9, and 4.0 d s , respectively. A 10 gram sample of each size fraction was irradiated as described above for the Ar-40, except Si-28 was converted to Al-28 which has a half-life of 2.24 minutes and emits high energy gamma rays. The sample was introduced using a special-purpose valve in which compressed air was used t o sweep the loop without significantly affecting the local internal pressure in the vicinity of the injection point. The responses of the radioactive sand were measured using the same NaI scintillators described above for the gas-phase tracer experiments. 2.4 SuspensionDensity

The variation of the gas-solid density in the axial coordinate was determined from measurements of the pressure drop at eleven locations over the length of the riser. Electronic pressure transducers with a time constant of 0.5 millisecond with maximum pressures of 2 and 10 kPa were used. The data was collected using a computerized data acquisition and processed off-line.

3. KINETICMODEL The intrinsic kinetics and reaction network for propylene oxidation to acrolein used in the riser model given below in Section 4 are based upon the work of Tan et al. (1989). The catalyst consisted primarily of a-Bi2MogO supported on a silica carrier. The reaction mechanism given below by eqns. 1 - 4 assumes that the formation of acrolein, acetaldehyde, and carbon dioxide occurs by oxidation of propylene on the re-oxidized sites X t o form reduced sites R. The re-oxidation of the reduced sites R occurs dissociatively through gas-phase oxygen. The justification for this mechanism is given by Brazdil et al. (1980) and Grasselli and Burrington (198 1). ki

CH3CHCH2

+

CH2CHCHO

2X+

+

H20

+R

( 1)

k2

CH3CHCH2

+

9X

+ 3 COz -+ 3 H20 + R

(2)

k3

CH3CHCH2

+

3l2X j 312 CH3CHO

+R

(3)

ka

If202

+R +X

(4)

5

The standard heats-of-reaction (-A€€:) for eqns. 1 - 3 are 191, 1443, and 270 kJ/mole, respectively. The formation of C 0 2 by the complete combustion of acrolein was not included in the final reaction network by Tan et al. (1989) since it was negligible when compared t o the amount of CO2 produced by the combustion of propylene. Similarly, the formation of acrylic acid by the partial oxidation of acrolein was not reported so it must have been negligible. The kinetic rate expressions for the reaction of propylene to acrolein, CO2, and acetaldehyde with the re-oxidized sites X and the re-oxidation of the reduced catalyst sites R based upon the above simplified redox mechanism are defined below by eqns. 5 and 6.

ri = ki C, 6, r4

= k, Chi2 e,. - k1 C, 6,

- 912 k2 C, ex- 314 k3 C,

0,

where 19, and 6,. denote the fraction of oxidized and reduced sites, respectively. The subscript i assumes values of i = 1, 2, and 3 corresponding to acrolein, C02, and acetaldehyde, respectively. The kinetic rate parameters k, and the ki (i = 1, 2, and 3) in eqns. 5 and 6 correspond to k, k12, k13, and k14 in Tan et al. (1989). The frequency factors and activation energies were obtained by nonlinear regression of the k versus T values given in Table 3 of Tan et al. (1989). The results are summarized in Table 1. Table 1 Frequency factors and activation energies Frequency Factor Activation Energy [kJ/kmolel

ka 3.996 x 104

kl 6.880x 102

k2 3.500 x 104

k3

-88,000

-77,500

-1,312

-72,630

3.280 x lo1

[kJ = (kmol rn3)1/2/kg-s [ki] = (m3kg-s where i = 1, 2, and 3 4. RISERMODELING

The hydrodynamic model given below was used to interpret the tracer data and suspension density measurements. The corresponding parameters were then used in the reaction engineering model for the riser, which also incorporates other transport effects and the kinetic model described above.

6

4.1 Riser Hydrodynamic Model

Previous experimental studies [see Patience (1990) for a summary] have shown that the CFB riser hydrodynamics are characterized by a dense turbulent region at the bottom where the catalyst is introduced from the standpipe, which becomes leaner as the flow of solids develops and the particles accelerate t o their steady net upward velocity. Visual observations show that the existence of a lean suspension of solids in the gas flowing upward in the center of the riser, with a denser down flow of solids at the wall(cf., Basu et al., 1991). These form the basis for the so-called core-annular model which is illustrated below. Detailed descriptions of the model parameters and their significance is omitted here since this is given by Patience and Chaouki (1993). UP

t

Wall Region

Figure 2. Core-annular model for the CFB riser gas-solid hydrodynamics. Typical agreement obtained between the model predictions and the experimental gas and solids normalized tracer response data, from which the parameters I$and k, are determined, is shown in Figure 3. U

= 8.05 m / s 2

U

1 16 k g / m s k S = 0.057 m / s 9 = 0.69 eg Input, 2=0.8rn 0 Response, 2 = 4

Gg=

0.20

0.08

=

8.0 m/s 140 kg/m s

Input.

0.06

.) 0.04

Simulation

0 0

-

Z=1.75 r n

Response, Z=4.0m Simulation

i

0.02

0.05 0.00 0.0

G:=

L-0

_Li

0.5 Time (s)

1 .o

1.5

0.00 0.0

0.5

1

Time

.o

1.5

(5)

Figure 3. Experimental and model-predicted gas and solid tracer responses. Gas-phase tracer (Figure 3a) and solid-phase tracer responses (Figure 3b).

Correlations for Q and k, have been established using the parameters obtained in this work as well as literature data. The final forms of these are

Sh, = 0.25 Sc1I2

(up/ugy4

(8)

In eqns. 7 and 8, the various dimensionless numbers are defined as Fr = Ug/(gD)l/2,Sh, = k,D&/pDv, Sc = p/pD,, and Re, = DU,p/&p. 4 2 Riser 'I'ransporbKineticsModel

The riser performance in the presence of transport effects and reaction can be described by material and energy balances using the core-annular model as the basis for the gas and solids flow patterns. The model equations given below apply to the fully developed zone where the solids and gas have accelerated t o their steady-state velocities beyond the initial acceleration zone. The length of the entry region is small since the riser is assumed to have a smooth inlet. Application of mass and energy conservation laws eventually leads t o the following set of model equations:

Mass Balance in the Core Region:

vijri(C,,Tl = 0 for i = I,,,.,ns

Mass Balance in the Annular Region:

Oxygen in the Core Solids:

(9)

8

Oxygen in the Annular Solids:

Energy Balance for the Gas-Solid Homogeneous Suspension:

The parameter r/ that appears in eqns. 11 and 12 represents the fraction of the total solids that fall in the annular region. A precise knowledge of this parameter is lacking, but estimates and available data suggest that the orderof-magnitude should be about 0.1. The boundary conditions for eqns. 10 - 13 are the Dirichlet type corresponding t o specified concentrations and suspension temperature where the subscript i denotes a particular species.

at z =

o , cC,i= c0,i

for

o c r c ~1/;6

Cqo; = c0,o; for ~c r -=R&

T = T o for O< r c R

at z =I,,

c ,=~cC,ifor ~ 4 jr

Ca,&=Cc,o; for R h c r c R

( 14)

Equations 9 - 14 were solved by a finite-differencetechnique for an assumed set of model parameters, or using various correlations. Simulations over various ranges gave various predicted quantities, such as propylene conversion, acrolein selectivity and yield, reaction gas composition, suspension temperature, and acrolein production rate. 5. RESULTS AND DISCUSSION

Various simulations were performed using the above model t o determine the initial feasibility of using a CFB for the partial oxidation of acrolein on a commercial-scale, and to assess the sensitivity of the model t o selected parameters, including the catalyst mass flux and the gas superficial velocity.

9

5.1 Design Bask The simulations described below assume a riser diameter of 0.94 m and a height of 40 m. This particular choice for the riser diameter is identical t o the one used in the commercial FCC riser of Martin et al. (1992). At a mass flux of 1090 kg/m2-s and a superficial gas velocity of 25 m/s, they reported a suspension density of 121 kglm3, which corresponds t o a slip factor, y~,of 2.9. This value for the slip factor is higher than the one predicted from the correlation proposed by Patience et al. (1992). To account for the higher slip factor, the correlation was modified by adding a constant of 0.8 to the first term in their expression. The final correlation for the slip factor then becomes:

The ranges for U, and G, used in the simulations were 8 to 25 m/s and 700 to 1500 kg/m2-s, respectively. The remaining parameters were: (1)Y ~ 3 , i n= 10 to 20 %, (2) Yc02,in = 10 %, (3) Y N =~ balance, (4) Tin = 395 O C , (5) dp = 75 pm and p p = 1500 kg/m3. Since oxygen is entrained with the catalyst from the regenerator, its' inlet concentration varies with the solids mass flux and riser gas operating velocity, which vaned from 1 to 5 %. 5.2 Simulation Results

Figure 4 shows a perspective view of the combined effects of G, and U, on the suspension density and the annual acrolein production rate where the latter is expressed in units of millions of pounds per year and has 8000 hours per year as the basis. Both the suspension density and the annual production rate appear t o be directly proportional to the circulation rate. Production rates greater than 80 million pounds per year can be readily obtained. This rate exceeds the total US. 1991 production, which was about 72 million pounds (Etzhorn et al., 1991).

G,kg/m -s

160

Figure 4. Effect of solids mass flux and superficial gas velocity on suspension density (Figure 4a) and annual production rate of acrolein (Figure 4b).

10

Typical profiles for the propylene conversion, acrolein selectivity and yield, and gas composition are illustrated in Figure 5 as a function of the riser height. The conversion reaches a maximum of about 26 % while the selectivity t o acrolein stays relatively constant at about 62 %, which results in a single pass acrolein yield of nearly 20 %. Figure 5b shows that the oxygen composition approaches zero after about 20 m above which the C02 composition stays relatively constant. 40

40

30

30

20

E. 2 0

E N

N

10

10 0

0

20

0

40

% X, %

60

80

0

5

15

10

20

mole %

S, and % Y

Figure 5. Propylene conversion, acrolein selectivity, and acrolein yield versus riser height (Figure 5a), and gas coinpositions versus riser height (Figure 5b). The variations of the suspension temperature and the concentrations of catalyst lattice oxygen in the core and annulus are shown in Figure 6. The temperature rise of the gas-solid suspension varies from the inlet temperature of 395 O C t o 415 OC corresponding t o 20 OC, which is quite moderate as expected since the degree of conversion is relatively low. A comparison between the percentages of catalyst oxygen consumed in both the core and annulus relative t o the inlet value of 100 % follows the expected trend that the greatest usage occurs in the annulus where the solids have the highest concentration.

40

40

30

30

E- 2 0

20

N

10

10

0 390

0 41 0

400

T.

C

420

25

75

50

100

%Q

Figure 6. Suspension temperature (Figure 6a) and percentage of catalyst oxygen consumed in the core and annulus (Figure 6b) with riser height.

11

6. SulMMARY AND CONCLUSIONS

A reaction engineering model based upon a core-annular flow pattern has been developed for the simulation of a CFB riser for the partial oxidation of propylene t o acrolein. The key hydrodynamic parameters include the core radius, solids holdup, and core-to-annulus mass transfer coefficient. These parameters were determined from interpretation of experimental tracer data for the gas and solids, and measurements of pressure drop versus solids circulation rate. The kinetic model used in the model was based upon a single site redox mechanism involving series-parallel reactions for the partial and total oxidation of propylene to acrolein, acetaldehyde, and CO2. Use of the model t o predict the performance of the CFB riser indicated that the correct trends were obtained in terms of the expected gas phase species compositions, suspension temperature, and other related reactor variables. It was shown that the riser performance was most affected by the suspension density, which can be controlled by solids circulation rate and gas superficial velocity. The concentrations of lattice oxygen and the lattice oxygen capacity of the catalyst are also important since the intrinsic reaction rates are dependent on these parameters. Development of catalytic processes for propylene oxidation t o acrolein and other related partial oxidations and ammoxidations using a circulating fluidized-bed reactor contains many challenges since fundamental information on the kinetics and transport effects is lacking when compared to fixed-bed and fluidized-bed systems. For example, detailed models for the intrinsic oxidationreduction kinetics are essential since reduction of the catalyst in the riser and its' subsequent re-oxidation in the regenerator must be precisely balanced to maintain constant activity. Furthermore, very high space-time yields are obtained in this work, but if the global rate of re-oxidation is slow relative to the global rate of reduction, the yield could be limited by the section of the process devoted to catalyst regeneration. Unfortunately, nearly all of the kinetic models developed so far, including the one utilized in this work, are based upon fixedbed or recycle reactor experiments where the catalyst is exposed to a net oxidizing reaction gas mixture so that the catalyst oxidation state essentially remains constant. Application of these models t o riser reactor modeling might be subject t o error since the kinetic parameters have not been determined under typical riser reactor conditions. A knowledge of the magnitude of various transport resistances is also critical since these may limit the maximum possible conversion. For example, the gas-to-solid mass transfer resistance and the rate of solids exchange between the core and annular regions may be significant under certain conditions, but has been quantified in a few limited cases. Also, evaluation of oxygen diffusion rates in the catalyst bulk and its' effect on the observed rate of reaction has not been widely studied o r quantified. The results of this study suggest that the use of circulating fluidized-bed reactor for propylene oxidation to acrolein has noteworthy advantages over conventional fluidized-bed and fixed-bed reactor technologies, especially in the area of process flexibility. This flexibility provides many opportunities for the development of the next generation of catalytic processes for partial oxidations, ammoxidations,and other related process technologies.

12

Amtz, D., Knapp, K., Prescher, G., Emig, G., and Hofmann, H., in Chemical Reaction Engineering - Boston, ACS Symp. Ser. Vol. 196 (J.Wei and C. Georgakis, eds.), 3, American Chemical Society, Washington, D.C., 1982. Avidan, A. A., Gould, R. M., an d Kam, A. Y., i n Circulating Fluidized Bed Technology (P. Basu, ed.), Pergamon Press, Toronto, 287 (1986). Basu, P., Horio, M. and Hasatani, M., CFB III, Pergamon, Oxford, 1991 Bergna, H. E., Chapter 7 in Characterization and Catalyst Development - An Interactive Approach, ACS Symp. Ser. Vol. 411 ( S . A. Bradley, M. J. Gattuso and R. J . Bertolacini, eds.), 56, American Chemical Society, Washington, D.C., 1989. Brazdil, J. F., Suresh, D. D., and Grasselli, R. K., J. Catal., 66 (1980), 347. Contractor, R. M. an d Chaouki, J., in Circulating Fluidized Bed Technology III (P. Basu, M. Horio, and M. Hasatani, eds.), Pergamon Press, Oxford, 39 (1991). Dutta, S. and Suciu, G. D., J. Chem. Eng. Japan, 25 (1992) 345. Etzhorn, W.A., Kurland, J.J., and Nielsen, W.D., i n Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edn. (J. I. Kroschwitz, ed), Wiley, New York, 1991. Grasselli, R. K. and Burrington, J . D., Adv. Catal., 30 (1981) 133. Jiang, P., Hsiaotao, B., Jean, R.-H., and Fan, L. -S. AIChE J., 37(1991), 1392. Kunii, D. and Levenspiel, O., Fluidization Engineering, Krieger, Malabar, 1969. Martin, M.P., Turlier, P., Bernard, J.R., and Wild, G., Powd. Tech., 70 (19921, 249. Patience, G. S . , Circulating Fluidized Beds: Hydrodynamics and Reactor Modeling, Ph.D. Dissertation, Ecole Polytechnique, Montreal, Canada, 1990. Patience, G.S., Chaouki, J., and Grandjean, B.P.A., Powd. Technol., 61 (1990) 95 Patience, G.S., Chaouki, J., Berruti, F. and Wong, R., Powd. Technol. 72 (1992) 31. Patience, G. S. and Chaouki, J., Chem. Eng. Sci. 48 (1993) 3195. Snyder, T. P., and Hill, Jr., C. G., Catal. Rev. - Sci. Eng. 31 (1989) 43. S t a h g , W., Chem. Proc. (August, 1992) 27. Tan, H. S., Downie, J., and Bacon, D. W., Can. J. Chem. Eng., 67 (1989) 412. van der Ham, A. G. J., Prins, W., van Swaaij, W. P. M., poster presented a t thi AIChE Annual Meeting, Session No. 116, Miami, Fl., November 1992.

13

F. Trifiro (University of Bologna, Bologna, Italy) : You propose a core-annular model for the hydrodynamic regime in selective propylene oxidation. However, the velocity of gas and the flow rate is typical of a coal combustor. I think that for a catalytic process, when a core-annular regime is operating, the efficiency of gas-solid contact will be low and also scale-up problems will arise owing t o the radial gradients. I have some doubt that in this case the CFB reactor has advantages over the fluidized bed reactor. I would like a comment from you.

G. S. Patience (DU POW, Wilmington, Delaware, U.S.A.) : Coal combustion in CFB reactors are carried out at solids circulation rates less than 100 kg/mZ-s and gas velocities between 3-9 m/s, which result in solids densities below 40 kg/m3. The gas velocities assumed in our work correspond to typical FCC risers, which operate at maximum exit velocities of 27.5 m/s (King, 1992), solids mass fluxes in excess of 1200 kg/ma-s and densities up t o 160 kg/m3. Despite these high mass fluxes and suspension densities, core-annular flow has been observed (Martin et al., 19921, which does negatively impact gas-solids contact efficiency. Scale-up problems may arise when operating in the core-annular fluid regime, but mass transfer between core and annulus can be increased by increasing the mass flux (although this may also augment the gas bypassing). However, Contractor et al. (1993)have recently reported gas residence time distribution data that show plug flow of gas at densities between 150 to 300 kg/m3 and gas velocities typical of coal combustors. Clearly, the high density fluid regime described by Contractor et al. (1993) would be preferable for a catalytic process from the point of view of contact efficiency.

References Contractor R., Patience, G. S., Garnett, D. I., Horowitz, H. S., Sisler G. M., and Bergna, H. E., in Proceedings of the 4th International Conference on CFB, Engineering Foundation, New York, 466 (1993). King, D., in FZuidization VII (0.E. Potter and D. J. Nicklin, eds.) Engineering Foundation, New York, 15 (1992). Martin, M. P., Turlier, P., Bernard, J. R., and Wild, G., Powder Tech., 70 (1992) 249.

Grasselli, R. K. (Mobil Research and Development, Princeton, New Jersey, U.S.A.): First, let me congratulate you on a fine paper and also its delivery. I have two questions and/or comments: 1) I note that in your Figure 5 that selectivity to acrolein increases as one moves up the riser. Is this observed selectivity increase due to depletion of dioxygen with riser height and a concurrent decrease of inter-particle gas phase combustion of hydrocarbon or one of its intermediates, or is it due to a decrease in propylene concentration with riser height, because propylene to acrolein is first order in propylene, however, byproduct formations are higher order reactions and thus more sensitive to propylene partial pressure than is acrolein

14

formation? Or is it the lower oxidation state of vanadium of the catalyst with riser height, or a combination of all of these? 2) As you might know our original concept at Sohio in the mid 1950's was to use a fluid bed oxidant reactor, utilizing the lattice oxygen of BigPMo12052 (hardened by LUDOX-Si02) t o produce acrolein from propylene, taking the acrolein overhead and lifting the partially reduced catalyst from the bottom of the reactor continuously to a second vessel where the reduced catalyst was going to be regenerated by oxygen or air to its original oxidized state, and the so regenerated catalyst was going to be continuously returned back to the reactor and so on. We reported on this fact in the open literature at a much later date for obvious reasons, but from an historic perspective, it might still be worth reading - I will send you a reprint (Callahan et al., 1970; Grasselli and Burrington, 1981).

We also considered variations t o this original scheme, namely, a transfer line reactor, which is essentially a riser reactor, but we were stopped with the commercialization of both of these concepts because we had t o circulate 200 kg of solids per kg of acrolein produced. We were in the solids moving business rather than acrolein production business. Thus, we opted for a fixed bed in the production of acrolein from propylene, co-feeding propylene and air; and for two consecutive fixed beds using two different catalysts in each of the two reactors with no interstage separation, for the production of acrylic acid from propylene. We and o u r licensees commercialized both of these processes. A similar situation prevailed for the production of methacrolein from iso-butene, and methacrylic acid, respectively. As you know, for the production of acrylonitrile from propylene, ammonia and air we commercialized the ammoxidation process utilizing a single catalyst in a single fluid bed reactor. The process operates in more than 45 plants worldwide. While I readily admit that there are now much better catalysts than our original BigPMo12052, I wonder what is the amount of catalyst (solid oxygen carrier) that you need to transport in order t o produce a given quantity of acrolein? It is a figure which I should like to know put it into perspective with the history of this fascinating catalytic process.

G . S . Patience :We would like to thank you for the interest in our discussion. At the same time we recognize your early, pioneering contribution t o this field and regret our inadvertent neglect of your important communications. The increased selectivity with riser height, that you have noted, is a consequence of the low activation energies reported for the byproduct formation of CO2. However, from a practical aspect, we would not expect increases in selectivites by either combustion or a reduced catalyst oxidation. Vapor phase oxidation of propylene would probably not occur because the solids effectively quench free radical formation. Furthermore, as you showed in your earlier publication (Callahan et al., 1970), changing the air-to-propylene ratio does not change propylene conversion or product distribution. Also in that paper, you indicated that high oxidant to propylene ratios were required t o maintain the equilibrium

15

oxidation state and selectivity; presumably an over-oxidized catalyst is less selective. Oxidant-to-propylene ratios above 300 were suggested as the minimum amount required, vis-a-vis catalyst stability, which is well above the minimum theoretical requirement of 53 kg of oxidantikg propylene feed converted. We assumed much higher oxidant-to-propylene ratios in our work. At a production rate of 36 MM kg/yr of acrolein with a selecitivity of 60%, the oxygen consumption is 700 molhr of which 200 moVhr comes with the catalyst as interstitial gas (assuming a mass flw of 1200 kg/m% and a circulation rate of 2.5 MM kg/hr). This corresponds to an oxidant-to-propylene ratio of 553; excluding the gas phase contribution results in a oxidant-to-propylene ration of 775, which is about double the rates you reported were required for catalyst stability (Callahan et al., 1970). The impact of high mass fluxes on plant design and process economics is minimal. The most critical economic factor is catalyst inventory, hence reactor volumes, and not catalyst circulation rates. High mass fluxes are obtained by increasing the height of the reactor so that sufficient head is developed in the standpipes t o circulate the catalyst. Many commercial FCC reactors circulate solids at rates as much as three times the rates assumed in this study. The advantage of this reactor concept lies in its high acrolein yield per kg of catalyst inventory and because of the inherent scale advantages (Garnett and Patience, 1993).

References Callahan, J. L., Grasselli, R. K., Milberger, E. C., and Strecker, H. A., Ind. Eng. Chem. Prod. Res. & Dev., 9 (1970) 134. Garnett, D.I. and Patience, G. S., Chem. Eng. Prog., 8 (1993) 76. Grasselli, R.K. and Burrington, J. F., in Adu. Catal., Vol. 30, Acad. Press, New York, 133 (1981).

M. Baerns (Ruhr-Universitat Bochum, Germany) : The state of oxidation of the catalyst changes along the reactor axis. Does this affect the catalytic reaction with respect to activity and selectivity?

G. S . Patience : I presented a graph, which corresponds t o Figure 6b in the paper, that shows the percentage of catalyst oxygen consumed in the core and annulus versus riser height. Another graph (Figure 5) shows the overall propylene conversion and acrolein selectivity versus riser height and gas compositions versus riser height. Our model does not contain the detail needed to evaluate the local oxidation state of the catalyst. In addition, we did not study what happens to the product distribution if the reactor is operated so that all of the catalyst oxygen is consumed. Results indicate that the acrolein selectivity increases slightly over the riser, while the global reaction rate of propylene decreases. A more detailed redox kinetics model would be needed before we

16

could fully assess the precise effect of catalyst oxidation state on activity and selectivity.

E. Bordes (Universite de Technologie de Compikgne, France) : My question is about the catalytic oxidation state profile. What would be the difference between annulus and core (vs 0) in the case of very amount of oxygen in the feed. Would it be larger or smaller?

G. S. Patience : The calculations were performed using 1to 5 mol% of oxygen in the gas phase at the reactor inlet to simulate the effect of oxygen carryover from the interstitial gas in the regenerator. The gas composition versus riser height profiles that I gave in my presentation showed that the concentration of gas phase oxygen is nearly depleted after about one-half the distance up the riser, which corresponds t o 20 m of the 40 m of height. The fraction of available catalyst oxygen monotonically decreases in the annulus and core with increasing riser height or contact time, with the greatest usage occurring in the annulus where the catalyst concentration is the highest. The difference between the available catalyst oxygen in these two zones appears t o decrease more rapidly with respect to height once the gas-phase oxygen is depleted. Our results show that not all of the available catalyst oxygen is consumed for the particular reaction conditions used, however. G. Centi (University of Bologna, Bologna, Italy) : You have based your model of circulating fluidized-bed reactor on kinetic data based on a catalyst optimized for fixed-bed or fluidized-bed applications. In these reactors are simultaneously present the hydrocarbon and oxygen, on the contrary t o riser reactor. This suggests that the optimal catalyst in terms of surface area, oxygen donor properties and so on, may be probably different for applications in the riser reactor with comparison to a catalyst for fixed-bed or fluidized-bed reactors. Can you derive from your model of the reactor which characteristics of the catalyst should be mainly changed to optimize the CFB performances? G. S . Patience : You are absolutely correct in pointing out that the optimal catalyst for a riser application will probably be different than one for a fixed- or fluidized-bed reactor. The model presented here accounts for the oxygencarrying capacity of the catalyst and finites rates for reduction and re-oxidation of the active sites using the kinetic model of Tan et al. (19891, which is based upon interpretation of fixed-bed microreactor data. Its use for riser modelling is actually an extrapolation, since it does not cover the expected ranges for species concentrations and oxidation states that occur under riser conditions, to name a few. A kinetic model that is directly applicable t o this mode of operation for propylene oxidation has not been reported, at least to our knowledge. If one were available, we would be properly equipped t o determine which particular properties of the catalyst should be optimized for a riser application.

17

J. Haber (Polish Academy of Sciences, Krakow, Poland) : Which is the efficiency of the use of catalytic particles? In your conditions it must be very low, probably limited to the outermost layer. G. S. Patience : It can be shown that diffusion of the reaction gases in the catalyst pores will be controlled by bulk diffusion if parameters are used that are typical of commercial catalysts. For the small catalysts (d,.= 75 pm) used in our study, the characteristic time for intraparticle diffusion wdl be on the order of tenths of a millisecond. Using this information and the known reaction rates, it can be shown that catalyst effectiveness factors are nearly unity so that the concentration of the reaction gases in the catalyst pores will be equivalent to the local values that exist in the core and annulus gas. This suggests that the entire particle volume is effectively utilized. Solid-state diffusion of oxygen in the lattice was not considered as part of this analysis, and this is probably important. E. Mamedov (Institute of Inorganic & Physical Chemistry, Baku, Azerbaijan) : According to your conclusions, the redox kinetics is important for operating the process. What do you mean saying "the redox kinetics"? Does it relate to the overall reaction or to separate steps? If it relates t o separate steps (reduction, oxidation), can one use their kinetics for the overall catalytic reaction? G. S. Patience : We are referring to the reaction rates of the elementary steps in the mechanism where oxidation and reduction of the active sites are described in terms of the appropriate concentrations, temperatures, and activation energies o r heats-of-adsorption. If the pseudo steady-state assumption (PSSA) is invoked for the concentrations of the oxidized and reduced sites, the classical form of the reaction rate equation for the net disappearance of propylene will emerge by following the usual analysis. The kinetic constants for the elementary steps are contained in this form, except they often cannot be recognized in many publications as such, due to algebraic manipulations. It is important to recognize that the validity of the PSSA for riser and regenerator modeling depends on the characteristic time scale for transport of the gas and solids relative to the time scale required for significant changes to occur in the concentrations of the oxidized and reduced sites. This was not investigated in the current paper, however. H. P. Neumann ( BASF-AG, Ludwigshafen, Germany) : I wondered that the only side-product you discuss is the formation of acetaldehyde. If you do the propene oxidation in a fixed-bed reactor, you will find acrylic acid to be the prominent side product. Did you find any acrylic acid during your investigations and if yes, how much? G. S. Patience : The kinetic scheme upon which our riser reactor model is based uses the one developed by Tan et al. (1989). In their study, the formation of acrylic acid via the oxidation of acrolein was not reported so one must presume it was negligible. We recognize that in commercial fixed-bed processes, acrylic acid is indeed a major by-product. Since the riser operates in the hydrocarbon-rich

18

mode, it is probable that acrylic acid formation would be more significant when compared to normal fixed-bed operation.

D. D. Suresh (BP RESEARCH, Cleveland, Ohio, U.S.A.) : At BP, we have catalysts providing 2 90% acrolein selectivities in conventional commercial reactors. What is the highest selectivity to acrolein you have obtained using circulating fluidized-bed reactors?

G . S . Patience : The objective of our CFB modeling presented here did & include identifying reaction conditions and reactor configuration that optimized the acrolein selectivity. If we were to perform such a study, we would probably focus on yield optimization. The results given in my presentation show that propylene conversion and acrolein selectivity are approximately 30% and 65%, respectively. However, I want t o remind you that these results are based upon the kinetics reported in the literature by Tan et al. (19891, and do not necessarily reflect our in-house data.

V. Cortds Corberin and S. Vic Bcllon (Edirors), New Develuplnents i n Selective Oxidation 11 0 1994 Elsevier Science B.V. All rights reserved.

19

Selective oxidation over structured multicomponent molybdate cata1y st s Joe Y. Zou and Glenn L. Schrader Department of Chemical Engineering, Center for Interfacial Materials and Crystallization, and Ames Laboratory-USDOE at Iowa State University, Ames, Iowa 5001 1 USA

ABSTRACT NiMo0,-MOO, catalysts prepared by reactive sputtering were evaluated for the conversion of 1-butene to furan and maleic anhydride. As for precipitated catalysts, individual phases of NiMoO, or MOO, yielded very little of these products; however, thin films (150-300 A) of NiMoO, on MOO, (010) surfaces were highly active and selective. The sputtering technique has significant advantages for preparation and characterization of multicomponent catalysts compared to precipitation or impregnation routes.

1. INTRODUCTION Multicomponent metal oxide catalysts are used in industrial processes for the production of intermediates such as acrylonitrile; related catalysts have also been shown to be active for maleic anhydride (from 1-butene) and methacrolein (from isobutene). The catalytic properties of individual metal oxide phases, such as the bismuth molybdates, have been examined by many researchers; the complex nature of multicomponent catalyst systems has also been addressed more generally (1). Various interactions are possible between the components involved in these systems, and developing a fundamental understanding of the physical and chemical nature of these materials and their relation to catalysis is an important goal of our current research approach. In our earlier work on molybdate systems, we demonstrated that the activity and selectivity of precipitated and impregnated "nickel molybdate" catalysts for I-butene conversion to maleic anhydride were enhanced when both NiMoO, and "excess" MOO, were present (2-4). Further research has extended these observations to other multiphase molybdate systems (5-11). The enhancement of the catalyst performance was discussed in terms of a "job distribution" between the phases: the interactions between the phases were believed to include a physical blocking of sites by a surface coverage of one phase on another. In other systematic investigations of catalysts for allylic oxidations and oxidative dehydrogenation, the synergistic effects observed for multiphase catalysts have been discussed in terms of a remote control mechanism (1,12). Multiphase systems are considered to consist of donor and acceptor phases, and oxygen is believed to dissociate to form a surface-mobile species which could spillover from one phase to another. This spillover species is proposed to modify the properties of the acceptor by maintaining the oxidation state, by preventing the destruction of acceptor structure (for example, through

20

reduction), or by inhibiting the deposition of carbonaceous deposits. These (and other) interpretations of the enhancement of the catalytic properties have been rather widely discussed, although the performance of some materials may not be easily evaluated by current experimental approaches. We have developed new catalyst preparation techniques which provide an opportunity to study specific aspects of the enhancement effects for multicomponent catalyst systems. A particularly important goal of this work has been to establish better control of the "contact" between the phases: this is difficult to achieve with precipitated or impregnated catalysts where a uniform dispersion of a single phase on another material (an overlayer) may be important. It is also difficult to control the thickness and structure of the overlayer. With these goals in mind, we have employed an advanced materials processing technique, reactive sputtering, to produce catalysts with controlled compositions and structures: specifically, thin film materials with defined "architectures" have been constructed. An additional advantage of utilizing sputtered samples is that characterization using several important techniques is more readily performed. 2. EXPERIMENTAL METHOD 2.1. Reactive sputtering system Thin films of MOO,, NiMoO,, and combinations of these phases were deposited on silicon wafers having a native growth of silicon oxide (referred to as "SiO, samples") or on glass beads (3 mm, Pyrex). A reactive sputtering system (Plasmatron Inc.) having three planar magnetron guns with RF or DC power sources was used with Ni or Mo targets. The sputtering chamber was evacuated prior to deposition to below 1 x torr. Flow rates of the "working gas" (Ar) and the reactive gas (02)were fixed by mass flow controllers (MKS). The substrates were heated by induction using a rotating graphite susceptor; temperatures could be controlled from room temperature to 1000°C. Pressures in the sputtering chamber were regulated between 2 and 50 mtorr. A residual gas analyzer was used to monitor the vacuum and sputtering environment. Deposition rates, composition, and structure were determined to have a complex relationship to several processing parameters, such as current or power delivered to the guns, flow rates, system pressure, temperature, etc. The deposition processing parameters reported represent "optimal" conditions for producing the desired composition and structures. 2.2. Characterization Characterization was primarily performed using materials prepared on SiO, since these studies could be performed quite readily: however, some characterization was also performed directly on the glass bead samples. a) X-ray diffraction (XRD1. XRD was performed using a Scintag 2000 diffractometer with Cu K, radiation. b) Laser Raman spectroscopy (LRS). LRS were obtained using a Spex 1877 Triplemate monochromator, a Spectra Physics 164 Ar ion laser operating at 531.0 nm and 50-100 mW (at the source), and a CSMA data acquisition system (Princeton Instruments, Inc.). c) Scanning electron microscopy (SEMI. SEM photographs were obtained by a JEOL 840A scanning electron microscope operating at 15 keV. d) X-ray photoelectron spectroscopy (XPS). XPS was performed with a Physical Electronics Industries 5500 multi-technique surface analysis system operated with AI-K, radiation. All spectra were referenced to a carbon 1s binding energy of 284.0 eV.

21

2.3. Reactor studies For the reactor studies (13), glass bead samples were used in order to have a relatively large total surface area present; about 185-195 beads were loaded into the reactor corresponding to a total surface area of 5.2-5.5 x lO-,rn2. The reactor was a passivated 1/2" OD stainless steel tube. The reaction was performed in continuous flow conditions at 450°C using 3% 1-butene (Matheson, industrial grade) in a 20% 0, (Air Products)-in-He (Air Products, zero grade) mixture. The reactor effluent was analyzed by an on-line quadruple mass spectrometer (UTI, 1OOC) interfaced to the reactor system by a glass single stage molecular jet separator (SGE). Steady-state performance was observed after 30 mins; the catalysts were stable over the 4-h (or more) measurement.

3. RESULTS 3.1. Sample preparation and characterization The deposition parameters for specific samples are given in Table 1. Although Mo can be effectively sputtered using DC or RF techniques, sputtering of Ni was preferentially performed using the RF source. In some cases, the system pressure during sputtering was adjusted to insure that the desired composition (stoichiometry) and structure for the deposited material was obtained. The substrate temperature also was critical in determining phase formation and growth. Table 1 Sputtering process parameters Sample W (RF) /target

mA (DC) /target

Substrate T ("C)

System P (mtorr)

Deposition time (min)

A

MoO,/SiO,

1OO/Mo

__

415

5.0

30

B

NiMoO,/SiO,

140/Ni

75/Mo

550

10.0

45

C

MoO,/glass beads

100IMo

__

415

5.0

30

D

NiMoO,/glass beads

140/Ni

75/Mo

550

10.0

30

E

NiMoO,/MoO,/ glass beads MOO, layer NiMoO, layer

100/Mo 140INi

__ 75/Mo

415 550

5.0 10.0

30 7

NiMo04/Mo03/ glass beads MOO, layer NiMoO, layer

100IMo 140lNi

75/Mo

415 550

5.0 10.0

30 15

F

__

22

Extensive characterization was possible for the SiO, samples. t XRD results (Figure la) for Mo03/Si02 (sample A) indicated formation of a-Mo03 with a strong (010) orientation. (The peak at 32.9" [28] is due to the I Si substrate.) The laser Raman spectrum (Figure 2a) with peaks at 667, 820, and 995 cm-' and at lower wavenumbers confirmed the presence of a-Mo03. The peak at 521 cm-' and the broad feature at 900-1000 cm-' are attributed to the substrate (Si and SiO,). The intensity of the low wavenumber Raman bands were low, and some interference from the substrate was clearly evident; an effect due to film thickness may also be possible. However, peaks observed at 116 and 130 cm" (assigned to a - ~ o lattice ~ , phonons) indicated that the material probably has a structure similar to a bulk crystalline Figure 1. XRD of (a) MoO,/SiO, (sample A) showing phase. This crystallinity was also a strong (010) orientation and @) NiMo0,/Si02 indicated by XRD. Both the (sample B). XRD and Raman characterization results did not indicate the presence of any other phases or structures. XPS detected bonding energies at 232.3 eV (Mo 3d3,,) and 235.7 eV (Mo 3d5,,) which are also characteristic of Moo3. The surface composition based on an elemental analysis using XPS for MOJO was 1/2.8. SEM microscopy (Figure 3a) revealed a smooth surface having a small grain size ( - 0.4 pm) as indicated by an observable basal plane structure. Based on thickness measurements using SEM, the growth rate of MOO, at these processing conditions was estimated to be about 40 A/min. XRD results (Figure lb) for NiMoO,/SiO, (sample B) indicated the formation of a-NiMo0,; however, there was no preferred orientation as observed for the MOO, films. The laser Raman spectrum (Figure lb) with peaks at 706, 915, and 959 cm-' also identified the material as a-NiMoO, (2,14), but a shoulder was clearly observed on the 959 cm-' band. Some distortion in the coordination of the molybdate coordination may be present. The Raman bands below 700 cm-' (also attributable to a-NiMo0,) were observed to be of rather low intensity, again probably due to film thickness and interference from the substrate. No other phases were observed by the XRD or LRS characterization. XPS determined that the bonding energies corresponded to the known values for NiMoO, (15):

23

232.9 eV (Mo 3d3,,), 235.9 eV (Mo 3d5,,), 855.8 eV (Ni 2p3/2), and 873.5 eV (Ni 2p,,,). The surface composition using XPS information for Ni/Mo/O was 1/1.2/3.8. SEM microscopy (Figure 3b) revealed a regular polycrystalline morphology with a grain size of about 0.2 pm. Growth of the NiMoO, film appeared to be columnar. The growth rate of NiMoO was determined to be about 24 /min. Thin films of MOO, and/or NiMoO, (samples C-F in Table 1) were deposited on glass beads for reactor studies. The processing conditions were selected on the basis of the previous studies using SiO, substrates. For multicomponent films, a bilayer structure was prepared by a sequential process. MOO, was deposited initially; overlayers of NiMoO, were then produced at sputtering times of 7 and 15 minutes. Although it was possible to obtain some other characterization data for the glass bead samples, Raman characterization was readily performed (Figure 2c29. Formation of MOO,, NiMoO, or both materials (for bilayer films) could be detected. A broad feature extending over 300-600 cm-' and the generally higher "background" were due to the glass beads.

l

%,

i f 200

3.2. Catalytic activities and selectivities Reactor studies for the I-butene oxidation using materials deposited on glass beads are summarized in Table 2. The conversion of 1-butene ranged between 6 % to 16%. The selectivities for the products (as defined in Table 2)

400

600

800

1000

Wavenumber Figure 2. Laser Raman spectra of (a) Mo0,/Si02 (sample A), (b) NiMoO,/SiO, (sample B), (c) Mo03/glass beads (sample C), (d) NiMoO,/glass beads (sample D), (e) NiMoO, (7 mins deposition)/Mo03/glassbeads (sample E), (f) NiMoO, (15 mins deposition)/Mo03/glass beads (sample F).

24

were significantly different for the various films. Samples C and D produced very little maleic anhydride and low yields of furan. Although Mo03/glass bead catalysts (sample C) had high conversion rates, the yields of CO, and CO were also high. NiMoO,/glass bead catalysts (sample D) exhibited the highest selectivity for 1,3-butadiene, resulting probably from the relatively low conversion rate. In comparison, the highest selectivities for maleic anhydride were observed for the specific multiphase or bilayer thin films of NiMo0, on Moo3 (for this particular report). The conversion rate of the NiMoO, thin film deposited for 15 min (sample E) was greater than that of the NiMoO, thin film deposited for 7 min (sample F); the selectivities for furan, maleic anhydride, and CO, or CO were similar.

b Figure 3. Electron micrographs of (a) Mo03/Si0, (sample A) and (b) NiMo0,/Si02 (sample B). Table 2 Product selectivities and rates Sample

Product Selectivities* (%I 1,3-Butadiene Furan Maleic anhydride CO,, co Rate of disappearance of 1-butene (g-moles/min-m2)

*Selectivity for A =

17.9 7.8 <1 63.7 1.10

10”

47.8 3.6

19.5 7.3 26.3 24.8

<1

36.2 8.02

10‘~

moles of A produced moles of 1-butene consumed

1

‘ -

y

4.88

lo-,

18.4 10.9 29.2 22.1 1.38

10”

. 100% where y is the ratio

of C atoms in the reactant to the number of C atoms in the product.

25

4. DISCUSSION

The activities of the catalytic materials produced by reactive sputtering for these studies were remarkably high. The rates of disappearance of 1-butene, as expressed on a unit surface area basis, were higher by at least an order of magnitude compared to those determined for precipitated or impregnated catalysts (2). Even the single component phases (Moo3 and NiMoO,) converted significantly larger amounts of 1-butene despite the fact that the reaction temperature was lower (450°C 41s. 480°C). The Moo3 films were identified as a-phase material with a strong (010) orientation [multiple (OkO) reflections in the XRD data]; furthermore, based on SEM characterization the film appeared uniform, continuous, and smooth. The higher activity of this material, however, must be recognized as resulting in a higher conversion to combustion products. The activity and selectivity of various surfaces of a-MOO, has been investigated experimentally and theoretically by a large number of researchers (16-23). The role of the (010) plane has been discussed in terms of complete oxidation or oxygen insertion (partial oxidation); some studies have also suggested that this surface is inactive. Oyama has indicated some of the difficulties in quantifying such results for multigranular ("particulate") samples (24). The higher activity of the sputtered MOO, catalysts may be attributable to the greater presence of surface defects which may activate oxygen for total combustion. This may be due in part to the sputtering process itself or due to the lower preparation temperature compared to other studies (415 vs. 500°C or even as high as 700-1000°C for solid state syntheses). However, it should be observed that a relatively high amount of a partially oxidized product - furan - is also produced (compared to our previous studies on crystalline a-Mo03) (2). Significant modifications in selectivity may, therefore, be attributable to the sputtering process itself. NiMoO, was produced as a polycrystalline material, apparently formed by a columnar growth mechanism. The sputtering process parameters for producing this material were extensively investigated (25). Preparation of an oriented NiMoO, material (such as for the MOO, sample) is not reported: thus, the NiMoO, is rather similar to the precipitated material characterized in our previous work ( 2 ) . LRS revealed that NiMoO, was formed, but there was some evidence perhaps of a distortion of the molybdenum-oxygen coordination (2,14). As for the MOO, materials, this may be due to the sputtering process itself or due to the relatively low deposition (synthesis) temperature which may incorporate more defect sites. Compared to our previous studies with precipitated catalysts, the most significant difference in selectivity for the single component NiMoO, catalysts is the high yield of 1,3butadiene: this again may be attributable to defects, perhaps at Ni-0-Mo sites. The results for the multicomponent catalysts support previous observations that high activity and selectivity for partial oxidation products can be achieved if both NiMoO, and MOO, are present (2-4). These samples produced amounts of maleic anhydride at significantly higher levels than those reported for precipitated catalysts (an increase of about SO-100%) (3). The largest increase, however, was observed for furan production: yields increased from around 1% to as high as 10%. (It is interesting to note that this was true even for the single component materials, MOO, and NiMoO,). The rates of the multicomponent catalysts were comparable to the other samples although the NiMoO, film deposited for 15 min appeared to be superior. There is no indication from the XRD data of the presence of another phase or structure, and there is no evidence of the existence of a solid solution, although such determinations are difficult for films.

26

In this paper we present only the results for a very specific multicomponent catalyst synthesis: polycrystalline cr-NiMo04 on a-MoO3 (010) surfaces. Our results indicate that the bilayer "architecture" results in an enhancement in catalytic performance. However, different materials and active sites may be present in our sputtered catalysts, and it is possible that other models should be considered for the enhancement in activity and selectivity observed for these multicomponent catalysts (26). 5. ACKNOWLEDGEMENT

This work was performed with support from the Ames Laboratory which is operated for the U.S. Department of Energy by Iowa State University under Contract W-7405-ENG-82. Sputtered samples were prepared in the Center for Interfacial Materials and Crystallization. XPS data were obtained with the assistance of Dr. James Anderegg of the Ames Laboratory.

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.

L.T. Weng and B. Delmon, Appl. Catal. A, 81 (1992) 141. U. Ozkan and G.L. Schrader, J. Catal., 95 (1985) 126. U. Ozkan and G.L. Schrader, J. Catal., 95 (1985) 137. U. Ozkan and G.L. Schrader, J. Catal., 95 (1985) 147. U. Ozkan and G.L. Schrader, Appl. Catal., 23 (1986) 327. U.S. Ozkan, R.C. Gill, and M.R. Smith, J. Catal., 116 (1989) 171. U.S. Ozkan, R.C. Gill, and M.R. Smith, Appl. Catal., 62 (1990) 105. U.S. Ozkan, E. Moctezuma, and S.A. Driscoll, Appl. Catal., 58 (1990) 305. U.S. Ozkan, M.R. Smith, and S.A. Driscoll, J. Catal., 123 (1990) 173. U.S. Ozkan, S.A. Driscoll, L. Zhang, and K. Ault, J. Catal., 124 (1990) 183. U. Ozkan, M.R. Smith, and S.A. Driscoll, Studies in Surface Science and Catalysis, 72 (1992) 363. L.T. Weng, P. Ruiz, and B. Delmon, Studies in Surface Science and Catalysis, 72 (1992) 399. M.E. Lashier and G.L. Schrader, J. Catal., 128 (1991) 113. P. Dufrense, E. Payen, J. Grimblot, and J.P. Bonnelle, J. Phys. Chem., 85 (1981) 2344. J. Stoch and A. Capecki, Surface and Interface Analysis, 15 (1990) 206. J.C. Volta and J. Portefaix, Appl. Catal. 18 (1985) 1. M. Abon, J. Massardier, B. Mingot, J.C. Volta, N. Floquet, and 0. Bertrand, J. Catal., 134 (1992) 542. J. Haber in Structure and Reactivity of Surfaces (C. Morterra, A. Zecchina, and G. Costa, eds.) Elsevier, 1989, p. 447. K. Briickman, J. Haber, and T. Wiltowski, J. Catal., 106 (1987) 188. J. Haber and T. Mlodinicka, J. Mol. Catal., 74 (1992) 131. R.A. Hernandez and U.S. Ozkan, Ind. Eng. Chem. Res., 29 (1990) 1454. J. Ziolkowski, J. Catal., 80 (1983) 263. J. Ziolkowski, E. Bordes and P. Courtine, J. Catal., 122 (1990) 126. S.T. Oyama, Bull. Chem. SOC.Japan, 61 (1988) 2588. J.Y. Zou and G.L. Schrader, manuscript submitted. J.Y. Zou and G.L. Schrader, manuscript in preparation.

21

DISCUSSION J. J. LEROU (DuPont & Co., Wilmington, USA): Sputtering on a flat silicon surface is relatively easy, but how do you insure that the glass beads are homogeneously covered with the sputtered material? Could you comment on scaling up this method?

G. L. SCHRADER (Iowa State University, Ames, USA): The glass beads were rotated during sputtering using a special substrate platform. XPS characterization indicated that the coverage of the glass surface was virtually complete (very little Si detected). SEM indicated that MOO, layers were uniform and continuous. However, the coverage by the polycrystalline NiMoO, layers may not be completely continous. Sputtering can be used to produce fairly large-volume materials and products. The applications for thin-film production include microelectronics devices, protective coatings, and commercial glass.

J. HABER (Inst. of Catalysis and Surface Chem., Krakow, Poland): It is known that MOO, "wets" the surfaces of transition metal molybdates and easily spreads over the surface to form a monolayer. Do you have any indication in your film-type catalysts that MOO, from the bottom layer did not migrate through the noncontinuous NiMoO, layer to the surface - the outermost layer determining the catalytic properties? G. L. SCHRADER: We have also observed "wetting" effects for precipitated catalysts (2-5), but this phenomenon was not observed for the bilayer molybdate materials prepared by sputtering. The elemental characterization from XPS depth profile analysis indicated that the Mo-to-Ni ratio was constant throughout the top NiMoO, layers. For precipitated catalysts, there probably are two contributing factors for "wetting" (during calcination): (1) the presence of water in the system, and (2) the high temperatures required by calcination. Because the sputtering process does not involve either of these two conditions, the MOO, apparently did not migrate to the outer surface of the NiMoO, layer.

J. C. VOLTA (Inst. de Recherches sur Catalyse, CNRS, Villeurbanne, France): What is the experimental evidence for a continuous covering of NiMoO, on the MOO, (010) surface? G. L. SCHRADER: SEM micrographs (edge-view) of sputtered films on SiO, showed a bilayer structure of NiMoO, on MOO,. The NiMoO, grain size depended on the sputtering pressure. At low deposition pressures, dense (small grain size) NiMoO, layers were produced, resulting in a very continuous and uniform coverage of the MOO, surface. Nevertheless, the structure is polycrystalline with observable grain boundaries. It was more difficult to obtain edge view micrographs for materials deposited on glass beads. Surface views tended to show that the coverage of NiMoO, on MOO, was probably nearly continuous, but we have not excluded that some exposure of MOO, exists.

E. BORDES (Univ. Technologie de Compiegne, Compiegne, Cedex, France): You observed growing columns of NiMoO, over Moo3. Did you determine the faces which are involved: for example, can you say if (110) faces (usually observed with CoMoO, and NiMoO,) are in contact with the MOO, (010) surface?

28

G. L. SCHRADER: Such information would be highly interesting (as you and others have clearly shown for multicomponent catalysts), and TEM studies are in progress. Our XRD results indicated a strong (010) orientation for the sputtered Moo3, but no preferred orientation for the sputtered NiMoO,. The most intense XRD reflections for the NiMoO, layer were (101) and (1 10). G. CENT1 (Dip. Chim. Ind. e dei Materiali, Bologna, Italy): According to your mechanism in order to explain the results, one can expect an increase in the rate of reaction with a decrease in the thickness of the NiMoO, phase on MOO,. On the contrary, you observe the opposite effect. How do you explain this result? G. L. SCHRADER: Your proposition may be correct for a catalyst consisting of MOO, covered by relatively thick layers of NiMoO,. In the reactor studies for I-butene oxidation over NiMo04/Mo03/glass beads, the NiMoO, layer is thin (approximately 150-300A), and there is evidence that the coverage of NiMoO, may not be continuous for samples examined in the reactor studies. For materials with low coverages (sample E) compared to samples with thicker coverages (sample F), the conversion rate increased with the amount of NiMoO, deposited on MOO,; the yield of maleic anhydride was higher and the yield of CO, and CO was lower. The increase in activity and selectivity may be attributable to an increase in "contact" between the phases, especially if the NiMoO, coverage is not continuous. 0. V. KRYLOV (Inst. Chem. Physics, Moscow, Russia): In your experiments the selectivities of furan and maleic anhydride change in the opposite direction. Does this mean that they are formed on different active sites or phases?

G. L. SCHRADER: The results reported in this paper indicated an overall enhancement of catalytic activity for bilayers of NiMoO, and MOO,. The selectivity for maleic anhydride increased for these materials, but the trends for furan may be more complicated. The specific role of each phase or different sites for the production of furan vs. maleic anhydride is being further investigated. J. VEDRINE (Institut de Recherche sur la Catalyse, Lyon, France): During the oral

presentation, you compared precipiated catalysts with the sputtered NiMo0,-MOO, materials according to selectivities for furan and maleic anhydride which altogether is less than 40%, i.e. other products are more than 60%. It could be possible then that the selectivity is due to a change in the relative intrinsic rates (if they occur on different sites) due to the change in "other sites" activity. Can you discuss this idea and give the intrinsic rate for each product. G. L. SCHRADER: Our reactor studies have indicated that the sputtered materials have significantly higher activity than precipitated or impregnated catalysts. The selectivities for 1,3-butadiene and furan also were higher, but the selectivity for maleic anhydride was comparable. The differences in product selectivity may reflect changes in the intrinsic properties of the materials and the nature of the active sites. However, this information is not readily obtained from the results reported in this study, due in part to the complexity of the reaction pathway.

29

B. GRZYBOWSKA-SWIERKOSZ(Inst. Catalysis and Surface Chem., Krakow, Poland): If we assume as it is usually done, a multicenter mechanism of selective oxidation, which (in your opinion) is the phase responsible for the butene activation and which is for oxygen incorporation (in the NiMoO,/MoO, system)? G. L. SCHRADER: For the sputtered materials, our results showed that the conversion rate of I-butene and the selectivities for CO and CO, were high on MOO,; NiMoO, had a lower conversion rate and a higher selectivity for 1,3-butadiene. The bilayer materials had high conversion rates and high selectivities for partial oxidation products. Considering the multicenter mechanism of selective oxidation over molybdates, it may be proposed that for the NiMo0,-MOO, catalysts the activation of I-butene is primarily carried out on NiMoO, sites and the oxygen incorporation occurs on the MOO, sites. These results are consistent with our earlier results for precipitated catalysts (2-5). However, the sputtered materials may allow further investigation of the interface where the existence of other compositions or structures may require a new mechanistic interpretation. L. MARGOLIS (Inst. of Chem. Physics, Moscow, Russia): What is your opinion about the nature of the active sites for the multicomponent catalysts prepared by your method? Does it agree with the model of Professor Delmon for this catalyst (acceptor and donor)? G. L. SCHRADER: There may be several possible explanations for the catalytic properties of multicomponent selective oxidation catalysts. It has been suggested (1) that the NiMo0,MOO, system might not clearly fit in a representation of acceptor-donor models. However, we anticipate that because of the ability to more directly control and characterize the structure and composition of sputtered materials some aspects of this model can be addressed. However, there is evidence that the sputtered materials differ from precipitated catalysts. B. DELMON (Univ. Catholique de Louvain, Louvain la Neuve, Belgium): In our own work, we also show synergies explained by the remote control picture when two phases are in contact. We show that spillover oxygen gives selectivity to nonselective surfaces or suppresses nonselective sites. This is apparently what occurs with the (010) face of MOO,, when spillover produced by NiMoO, irrigates that Moo3. This is probably linked to the fact that spillover oxygen keeps the (010) face in a high oxidation state. G. L. SCHRADER: We are anticipating that our approach using "structured" materials may assist in addressing several aspects concerning the nature of catalysis by multicomponent systems. The remote control mechanism and other results by researchers who identify the existence of solid solutions and oriented phase boundaries are areas of strong interest and discussion. R. K. GRASSELLI (Mobil Research and Development, Princeton, USA): You have taken an interesting approach in attempting to shed additional light on the workings of multicomponent catalysts. And, your cautious explanations appear prudent. Although your methods of characterization do not identify any solid solutions or interphase doping, such cannot be ruled out and particularly not on the surface of the multiphase catalytic material. Similarly, you cannot rule out epitaxy which is certainly very important in sophisticated

30

multicomponent, multiphase catalysts with phase cooperation. And here I refer you to our previously published work (1,2,3) which seems to have been forgotten by investigators more recently attracted to this fascinating field of multiphase cooperative catalysis. You also do not refer to the pioneering structural work of Courtine et al. (4)which is very basic to the issues at hand. Some points which you make are interesting indeed, namely, the strong (010) orientation of your MOO, films, columnar growth of NiMoO,, possible surface defect increase due to sputtering, and some LRS evidence of molybdenum-oxygen coordination distortions. All of these bespeak of surface modification and intersurface (epitaxial) interaction of phases. Another point which I might like to raise, and would be interested in your response, is the possible contamination (doping) of catalyst surface, i.e., the surface exposed to the reacting gases by alkalies or earth alkalies coming to the surface from the glass support because of your sputtering technique. It would take but a few atoms of these elements to strongly alter the surface properties of the catalyst(s). Could this be one of the causes that all of your films (MOO,, NiMoO,, MOO, NiMoO,) give unusually high furan selectivities?

+

G. L. SCHRADER: In this limited report of some of our recent studies, we have shown that bilayer materials - if prepared as specific, defined structures - possess high activity and selectivity; and these catalytic properties are comparable to or exceed those of precipitated or bulk phase catalysts, which have been emphasized in much of the previous work. Since multicomponent catalysts are complex, we believe that fundamental studies of specific combinations of materials as thin films would be useful. There are a number of excellent previous studies which deal with solid solution behavior, interphasic orientation, or the remote control mechanism: it is likely that the explanations for particular catalytic activity - for all of the many possible combinations of multiphase material - will be diverse. Our purpose has been to focus on a material which can be structured in a controlled manner and to seek a specific explanation for the catalytic properties. We find the previous literature to be highly informative (25,26), but the number of studies dealing the with NiMo04-Mo03 system is remarkably small. As we have stated, however, there also may be some differences in the basic nature and catalytic properties of sputtered materials (defect concentration, stoichiometry, etc.). We have found no evidence using XPS for the presence of Na or other alkali metals on the surface of materials deposited on glass beads. In other studies we have also found that contamination from SO,, Al,O,, and other oxide surfaces can be minimal at specific sputtering conditions. 1. R.G. Teller, J.F. Brazdil and R.K. Grasselli, J . Chern. Soc., Furaduy Tram. 1, 81, 1693

(1985). 2. (a) R.K. Grasselli, Appl. Cutul. 15, 127 (1985). (b) J.F. Brazdil, M. Mehicic, L.C. Glaeser, M.A.S. Hazle and R.K. Grasselli, ACS Symp Series 288, M.C. Deviney and J.C. Gland (eds.), 3, 27 (1985). (c) J.F. Brazdil, L.C. Glaeser and R.K. Grasselli, J . Phys. Chem. 87, 5485 (1983). (d) R.G. Teller, M.R. Antonio, J.F. Brazdil and R.K. Grasselli, J . Solid State Chem. 64, 249 (1986). 3. R.K. Grasselli, G. Centi and F. Trifiro, Appl. Cutul. 57, 149 (1990). 4. (a) A. Vejux and P. Courtine, J. Solid State Chem. 23, 93 (1978). (b) E. Bordes and P. Courtine, J . Cutul. 57, 236 (1979).

31

Relationships between the catalytic activity and the composition of various uranium-antimony mixed oxide catalysts in the selective oxidation of olefins F . Gama Freire" . J.M. Herrmannb and M. F. Portelan

"GRECAT - Grupo de Estudos de Catalise Heterogenea, Chemical Engineering Department, IST, Technical University of Lisbon, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal bEcole Centrale de Lyon, U. R. A. au CNRS - Photocatalyse, Catalyse et Environnement, BP 163,69 13 1 Ecully Cedex, France

The two pure phases USbOs and USb3010 of the U-Sb mixed oxides system and an equimolar mixed phase were prepared. Their properties and catalytic behavior in 1-butene oxidation to butadiene are compared. The data point out that different reaction mechanisms are involved for the USb05 and USb3010 catalysts. For the mixed phase a synergism was observed leading to improved activity.

1. INTRODUCTION

The uranium-antimony mixed oxides are catalysts used in the oxidation of propylene into acrolein and in the direct ammoxidation of propylene to acrylonitrile [ 11. The catalysts are also active in the oxidative dehydrogenation of butenes [2]. The U-Sb-0 system has two stoichiometric phases, USbOs and USb3010, with similar structure, but distinct activity and selectivity for the oxidation of olefins [ 3 ] . On the other hand important points of the mechanisms of such catalytic reactions are still a matter of debate [4,5]. To shed light on such aspects three U-Sb-0 phases were prepared: USbOs, USb3010 and a mixed phase with a U/Sb ratio equal to 1/2. They were characterized by means of various physical and chemical techniques. In particular they were characterized by electrical conductivity since these solids are known as semiconductors [ 7 ] .

2. EXPERIMENTAL 2.1. Preparation

Unsupported catalysts were prepared by coprecipitation in nitric solution [ 3 ] with Sb/U atomic ratios of 1/1, 3/1 and 2/1 corresponding to the stoichiometric compounds USb05, USb3010 and to a mixed phase. The catalysts activation involved a calcination in air at 1200K to form the stoichiometric phases. The catalysts were characterized by BET adsorption, mercury porosimetry, UV-Vis, XRD, X P S , and SEM-EDX spectroscopies.

32 2.2. Electrical conductivity measurements The electrical conductivity of these uranium-antimony mixed oxides was investigated using a cell specially designed to study electronic interactions between powdered samples and various gaseous atmospheres [ 6 ] . 2.3. Catalytic activity The catalytic activities for the oxidative dehydrogenation and degradation of 1-butene were studied in the range 550 - 700 K by the continuous flow method with feed compositions of 5 to 20% oxygen and 0.1 to 20% 1-butene at near atmospheric pressure and with contact times of about 0.5 s. A tubular reactor was used operating at low conversions (differential regime) to eliminate the effect of the reaction products. The catalytic beds operated in near gradientless temperature conditions and the absence of transport effects was checked. The amount of catalyst was aprox 3 - 5 g, the total flow was higher than 60 I hr-l, and nitrogen was used as diluent.

3. RESULTS

3.1. Textural and morphological characterization The catalysts were prepared as little pellets. The color of the catalysts is brown and becomes darker with increasing uranium content. The solids are microcrystalline and macroporous with low surface areas (Table 1). Table 1 Textural properties of the fresh and used catalysts Surface area (m2 g-1) Sample (SWU) porosimetry 1/1 1.4 21 1 2.8

N2 adsorption 1.35 2.75

Pore volume

Average pore diameter

Crystallite diameter

(cm3g-l) 0.42 0.44

(pm) 0.7 0.6

(w) 1.2 1 .o

The catalysts have similar textural properties, with a near gaussian distribution of pores sizes with mean value corresponding to the intercrystallite space measured by SEM. 3.2. Structure and chemical composition After activation, X-ray powder diffraction showed that catalysts had crystalline structure similar to the industrial catalysts characterized by Grasselli et al. [3]. Figure 1 evidences the structure of the three phases: USb05 is pure; USb3010 contains ca. 5% USb05 and the solid with a ratio Sb/U equal to 2/1 is approximately formed by an equimolar mixture of the two stoichiometric phases. In Table 2, the surface and bulk compositions are compared with the composition used for preparation. A surface enrichment in antimony is observed when comparing the atomic ratios (SWU) of the precursors and of the fresh catalysts obtained by calcination.

33

3.3. Electrical characterization The major results of the electrical studies are summarized in Table 3 . The USbnOlo phase is the less conductive catalyst. It is an intrinsic semiconductor in oxygen (or air) that behaves like a semiconductor of the n-type in I-butene reducing atmosphere. USbOs is at least ten times more conductive than USb3010 but it is of the p-type. The mixed phase is also of the p-type similarly to the USbO5 phase and its conductivity is of the same order of magnitude.

10

20

50

30

60

IlSb3010 10

20

30

40

50

60

10

20

30

28

40

50

60

28

Figure 1 X-ray powder diagrams of the three U-Sb-0 catalysts Table 2 Surface and bulk compositions of the precursors and of the fresh catalysts by XRD, XPS and EDX (atomic ratios) Sample SbLJ atomic ratios (SbkJ) Treatment Preparationa E D X XPS XRD 1/1 Precursor 1 12 11 (amorphous) 3/ 1

fresh catalyst Precursor fresh catalyst

3

26 27

25

31

65

21 1 fresh catalyst 2 a Ratio used for the preparation of the catalysts

36

Table 3 Electrical behavior and activation energy of conduction alog(a) Type of semiconductivity Sample PO, 1 in 1-butene

a w

USbnOio Sb/U=2/1 USbOs

0 0 0

USbOs (amorphous) USb3010 +res USbOs USbOs+USb301o

36

Activation Energy of Conduction (kJ mol-1)

n

94

P P

60 49

Since the conductivity of USb30 10 is independent of oxygen pressure, this solid can be considered in pure oxygen as an intrinsic semiconductor, whose band-gap energy EG is equal

34 to twice the activation energy of conduction, (EG=~Ec), corresponding to an absorption threshold at h=633 nm, which is in fairly good agreement with the UV-Vis spectrum of Fig 2

I

SbllJ=2/ I Ch/Il=lil

I

100

so0

700

900

I100

1300

. 11111

Figure 2. UV-Vis spectra of the catalysts The two other solids behave as p-type semiconductors with acceptor centers within the band-gap. The electrical behavior of the mixed phase (Sb/U=2/1), is determined by the presence o f t h e more conductive phase, USb05, which masks the presence of USb3010, since the percolation threshold is overcome (minimum percentage above which the more conductive phase impose its own conductivity [9]). 3.4 Catalytic activity

Only butadiene and C 0 2 were observed as products during the catalytic tests with feed mixtures of I-butene and oxygen. It is noteworthy that 2-butenes and CO were never detected, conversely to what is observed with other oxidation catalysts. Figure 3 provides a typical representation of the obtained data. The data show that the (Sb/U=2/1) phase is significantly more active for the selective formation of butadiene than the other phases practically in all studied conditions. The reaction behaves as first order in butene for all the studied phases (Fig. 3a). However with respect to oxygen considerable differences occur (Fig. 3b). For the USb3010 phase, the reaction is first order in oxygen, but shows an order higher than one for the USb05. For the (Sb/U=2/1) phase, the rate of butadiene formation increases as a hyperbolic hnction of the partial pressure of oxygen (apparent order in oxygen less than one).

35 Fhc butadiene selectivity increases strongly with the partial pressure of butene for all the phases An increase is also observed with the oxygen pressure Such increase is strong for USbO5, but only moderate for USb3010 and small with the (Sb/U=2/1) phase The butadiene selectivity decreases with temperature for all the studied catalysts (Fig 4) The decrease is strong for (Sb/U=2/1) phase and weak for USb3010 in all range of reactants partial pressures

p(B 1)=17 kPa

p(02)=17 kPa 90 a)

90 b,

70

70

T6 r3 0.0004 i c

-

s

6 v 50

G

s

G

G

0

e

2

50

(A

E L 0

30

30

0

10

0

20

10

20

P ( W (kP4 T=673 K Figure 3 Rates of formation (\>- USb05, .-Sb/U=2/1 and E- USb3O10) and selectivities ( - USbO5, -Sb/U=2/1 and 1 1 - USb3010) for butadiene vs 1-butene a) and oxygen b) partial pressures and curves (- - - - USb05, - - - - Sb/U=2/1 and -USb3010) computed from equations of Table 4 P(B1)

Wa)

550

600

650

700

T (K) p(B 1)= 17 kPa p(02)=20 kPa Figure 4 Butadiene selectivities vs temperature for the catalysts (Q- USb05, .-Sb/U=2/1 and 1 1 - USb3010) and curves(- - - -USbO5,- - -Sb/U=2/I and - - USb3010) computed from equations of Table 4

36 4. DISCUSSION AND CONCLUSIONS

After the study of the transient behavior of these catalysts [S], and the recorded values for the stationary state, it seems that for the three catalysts, the lattice oxygen participation in the catalytic reaction is quite different. Only for USb3010 it is possible to correlate the 1butene activation to the lattice oxygen (active oxygen may be identified as surface anions of USb3010). For the other phases it seems that there is a complex participation of several oxygen species, especially in the case of USb05, for which oxygen participation is quite complex. The quasi-independence of the electrical conductivity u versus p o 2 means that the USbO5 has no surface structural defects in equilibrium with gaseous oxygen Additionally, there are no oxygen ionosorbed species which can trap electrons. On the other hand, the influence of the 1-butene partial pressure shows that the main charge-carriers are positive holes and that the solid has an excess of lattice oxygen which is consumed during the reduction by butene. These p -type character can be explained either by a slight excess of anionic oxygen associated with cationic vacancies or by the existence of acceptor centers in the solid. These acceptors centers might be accounted for by the existence of Sb3+ ions in substitutional position of Sb5+ ions. Such electrical behavior and the kinetic results suggest for USbO5 a butadiene formation mechanism that is neither redox type (Grasselli [4]) neither hydroperoxide type (Keulks [5]), but a mixed mechanism, according the following scheme: Oxygen adsorption on the oxidized catalytic sites &O,

1-butene adsorption,

Formation of butadiene by two successive hydrogen abstractions, with reduction of the catalytic site,

(;H,012/0

-

C4H6 +H#I

MI2

(3)

Water desorption

regeneration of catalytic site

Assuming that the step ( 3 ) controls the rate in the overall reaction and that the other steps occur at near-equilibrium conditions, such scheme leads, for steady state conditions, to the following butadiene formation rate equation:

37

where $

=[/I 4/01+[('4H840]

is the total number of catalytic sites If.

3 I KIK2K5P(>:P(jHg << +K41' 0 2

(7)

equation (6) can be written as,

It was checked that the correlation coefficient (0 989) is the same for (6) and (S), what can be seen and understood as validating condition (7) For USb3010 , taking mainly into account ( I ) the kinetic data, ( ( I ) the n-type semiconductivity ( w ) the intrinsic semiconductor behavior with E G = ~96 eV in good agreement with the UV-Vis absorption threshold, that implies a surface stoichiometry identical to that of the bulk, with no surface deficit in anionic oxygen, and (w)the observation that in conductivity redox cycles the rate of reoxidation is higher than the rate of reduction, a reaction mechanism of the Mars and Van Krevelen type is proposed for the formation of butadiene *First and second hydrogen abstractions on I-butene to produce butadiene on the oxidized catalytic site (:H8 + 2 / 0 + ( ; H 6 +2HOC

(9)

Catalyst dehydroxylation

2 HOI t -'*- H,O

+t +((I

(10)

Reoxidation ofthe reduced catalytic site (filling of anionic vacancies)

X O ? +/

; I 0

(1 1 )

For the steady state, when step (9) controls the rate in the overall reaction and the others steps occur at near-equilibrium conditions, the rate equation for the butadiene formation on USb3010 becomes

s2

K9K:,PO2 PC& 'IHd =-

[

~

2

1+ K I P 4

where S = [ I ]

4PO]

is the total number of catalytic sites, and if

I

KIIP(]i equation (1 2) can be written as, rBd =mOzPCj H g

It was checked that the correlation coefficient (0.975) is the same for (12) and (14) The synergetic effect which is observed for the Sb/U=2/1 catalyst can be explained, on the grounds of the above mentioned reaction schemes, by a cooperation between the two

38 mechanisms, leading to surface concentrations of I-butene and of oxygen higher than on the stoichiometric phases By contrast, for propylene oxidation, it seems that this cooperating effect does not work and that the 0210 species lead to degradation, justifying that for such reaction the best composition is about SbIU=4/1 [3]. It was found that the kinetic data for Sb/U=2/I catalysts fit well the empirical rate equation:

The parameters of the rate equations (S), ( 14) and (1 5 ) were computed using nonlinear regression analysis and are presented in Table 4. Table 4 Rate equations for butadiene formation Catalyst r(Bd) (mol hr-1 g-1) 0 92 * ,(-82a)'-'/R7

)

Sb/U=l/l

Sb/U=2/I

Sb/U=3/ I

=I0 4*e(-X7500/K7 1 *

PNI * Po2

Activation energies in J mol-l, partial pressures in kPa In conclusion, two pure uranium-antimony mixed oxides have been tested. USb3010 is a n-type semiconductor during catalysis, whereas USb05 is of the p-type. The most active and the most selective catalyst is a 1/1 (molar ratio) mixture of both oxides. This is indicative of a synergy effect.

REFERENCES I . R. K. Grasselli and J. D. Burrington, Adv. Catal. 30, (1981) 133. and references therein. 2. C. J. Simons, P. N. Houtman and C. A. Schuit, J . Catal. 23 (1 97 1) 1 3 . R. K. Grasselli and J.L. Callahan, J. Catal. 14 (1969) 93. 4. J. D. Burrington, C. T. Kartisek and R. K. Grasselli, J. Catal. 87, (1984) 3 6 3 . R. K. Grasselli, App. Catal 15, (1985) 127. 5 . E. V. Hoefs, J. R. Monnier, G. W. Keulks, J. Catal. 57, (1979) 33 I 6. J M. Herrmann in "Les Techniques Physiques &Etude des Catalyseurs", Ed. Technip ( 1988) 7. S.E. Golunski, T.G. Novel1 and D.J. Hucknall, J. Chem. SOC.Faraday Trans. I , 81 (1985) 1121. 8. F. G. Freire and M. F. Portela, EUCHEM Conference, Orenas (1991). F. G. Freire and M. F. Portela, XI11 Simposio Iberoamericano de Catalise, Segovia (Spain), (1992) 791. 9. A. Ovenston and J.R. Walls, J. Phys. D. l8, 1859 (1985) and references therein.

39

X. E. VERYKIOS(University of Patras, Patras, Greece): Your results of electrical conductivity as a function of oxygen pressure indicate minimal or no interaction of the solids with oxygen (no variation of c with PO2).The kinetic results, of course, show a strong interaction. How do you reconcile this discrepancy. Does it imply that under working conditions the structure of the solids is significantly different ?

- Photocatalyse fkully, France): The absence of (T variations as a function of oxygen pressure is indicative of the absence of surface defects, such as anionic vacancies, which are in electronic interactions with gaseous pure oxygen. By contrast, the kinetic results indicate a strong influence of oxygen. These two opposite behaviors are not contradictory. Under working conditions, there are no structure modifications of the solids confirmed by X-ray analysis. However, in the presence of hydrocarbons such as 1-butene, there is a surface reduction with creation of anionic vacancies, at least in the case of US&O,,. If there is deficit in surface oxygen, there is a decrease in activity. Actually, in working conditions, it is more the redox state of the surface of the catalyst which is responsible for activity rather than a modification of structure.

J. M. HERRMA" (CNRS

M. BAERNS(Ruhr-Univ. Bochum, Bochum, Germany): How did you determine the electrical conductivity, was there any 02-conductivity measured. If there was no 02-conductivity is this the reason there was no reaction in the absence of gas-phase oxygen? J.M. HERRMANN: The electrical conductivity of the samples was measured with dc-current

under tension of 1 volt. The temperature range was comprised between 300 and 50O0C, including the values of the reaction temperatures (300 - 425 "C). In this temperature range, the conductivity is essentially electronic, i.e. there is no ionic conductivity by 02migration. The absence of reaction in the absence of gas phase oxygen indicates that redox process cannot work without gaseous 0, which spontaneously regenerates the active oxidative species at the surface. B. DELMON(Univ. Catholique de Louvain, Belgium): The 3/1 and 1/1 oxides obviously possess both donor and acceptor properties, (in the Remote Control terminology). The orders with respect to 0, suggest 3/1 is a bit more donor, 1/1 a bit more acceptor. Synergy in such cases have been ObSeNed e.g. between phosphorous - poor (donor) and phosphorous - rich (acceptor) vanadium phosphates. Usually, acceptors tend to reduce during catalysis. 1) Did you check the stability of the phases. Is there some segregation of the phases? 2) I suggest that a-Sb20, (present in large excess in practical catalysts), first mechanically mixed, would diminish the reduction you observe by XPS, and segregation, if any takes place in your experiments. M. F. PORTELA (Inst. Super. Tknico, GRECAT, Lisboa Portugal): The stability of the phases was checked with tests one week long under air at 773 K followed by XRD and XPS analysis. No changes were observed. The synergy observed may in fact be due to the donor and acceptor properties of the invoIved phases or to the effects of an epitaxial growth of the 3/1 phase on the top of the 1/1 phase as suggested by Dr. Grasseli in his comment.

40

R. K. GRASSELLI(Mobil R. & D., Princeton,U.S.A.):Your statement that the Sb/U=I/I composition is the most active and the most selective catalyst for the oxydehydrogenation of butene-1 to butadiene needs some clarification. First of all, the 1/1 catalyst has a surface area which is almost twice that of the 1/1 and 1/3 phases. And, that has not been taken into account as far as I can see. Secondly, in you Figure 4, one can readily see that the selectivity to butadiene versus reaction temperature lines cross each other. At lower temperature, the 2/1 and 1/1 composition appear to be more selective and at the higher temperature the 3/1 phase is more selective. Taking these results and references 1 and 2 which you did not quote into account, one can give the following interpretation to your observation. As you know, butene-1 to butadiene reaction is not a demanding reaction as are those of propene to acrolein or propene to acrylonitrile. Therefore, the demand on the catalyst is much less in the case of butene-1 and the reaction can generally be camed out under much less severe conditions. You also know from our work (1,2 and your reference 3 of our work), that the two phases l / l and 3/1 are structurally very similar, their main difference is the fact that in the 3/1 phase all U sites are isolated from each other by Sb imparts the 3/1 phase its high selectivity in the propene to acrolein and propene to acrylonitrile reactions. Conversely, the 1/1 phase is in these just mentioned reactions very active but unselective. And it has the ability to dissociate dioxygen to lattice oxygen (02-) more effectively and transport this 02-more effectively than the 3/1 phase. The perceived synergy, which you accord the 2/1 composition, is, in my opinion, a phenomenon similar to the one we describe in reference 2 on page 285. And this is entirely consistent with your findings of Sb enrichment on the surface of all compositions studied. What I believe you are observing is an epitaxial growth of the 311 phase on the top of the 1/1 phase. This gives you the phase cooperation effect, namely that the 3/1 phase is doing the majority of the hydrocarbon catalysis while the 1/1 phase is doing the bulk of the reincorporation of dioxygen from the gas phase via 0 2 - generation on the surface and transfer of the 0 2 - to the catalytically most selective 3/1 phase. This is again borne out in your Figure 4. At low reaction temperature the 2/1 composition appears most selective, the reason is that the 3/1 phase does not regenerate well at the low temperature, but an overgrowth of 3/1 over l i l will give the most desired result. At high reaction temperature the 3/1 phase regenerates well by itselc needs no help from the 1/1 phase. And at the higher temperature, the 2/1 and 1/1 phases are less selective, because the lack of site isolation (i.e. U site isolation) hurts them, giving more waste (i.e. COX). Since butene-1 is not a demanding reaction, these effects are less pronounced than they would be with the oxidation or ammoxidation of propene or isobutene to the corresponding unsaturated aldehydes and nitriles. The surface area question, however, still stands, in my opinion, as to the observed activity of various compositions. 2. You mentioned that you had difficulty in making pure 3/1 phase. Here in the audience is Dr. D. D. Suresh from BP-America (formerly Sohio), and one of my valued collaborators while we worked together at Sohio, he is the world’s expert in making pure 3 Sb/lU phase. I suggest you get some tips from him as to how to prepare this fascinating catalytic phase pure I . R. K. Grasselli, D.D. Suresh and K. Knox, J. Catal. l8, 356 (1970). 2. R.K. Grasselli and D.D. Suresh, J. Catal. 25,273 (1 972). M. F. PORTELA: The effect of different surfaces areas was duly taken into account. Your references 1 . and 2 . were really not quoted but their contents are in your review (our ref. [I]) and were taken into account in our work. We quite agree with the other comments and thank the kind information about the help Dr. D. D. Suresh can provide.

V. Cortes Corbcran and S. Vic Bcllon (Editors), New Developments in Selective Oxidation ff

0 1994 Elsevier Science B.V. All rights reserved.

41

SYNERGY IN THE Fe-Mo-Sb-0 MULTIPHASE SYSTEM L. E. Cadus', Y.L. Xiong2, F. J. Gotor3, D. Acosta4, J. NaudS, P. Ruiz and B. Delmon Unit6 de Catalyse et Chimie des MatCriaux DivisCs, UniversitC Catholique de Louvain, Place Croix du Sud 2, boite 17, 1348 Louvain-la Neuve, Belgium. 1.- On leave from INTEQUI, Universidad Nacional de San Luis, San Luis, Argentina. 2.- On leave from Xiamen University, Xiamen, China. 3.- On leave from Instituto de Ciencias de Materiales de Sevilla C.S.I.C., Univ. de Sevilla, Sevilla, Spain. 4.- On leave from Instituto de Fisica, UNAM, Mexico. 5.- Laboratoire de GCologie et MinCralogie, UniversitC Catholique de Louvain, Louvain-laNeuve, Belgium.

ABSTRACT The calcination of a mixture of Fe2(Mo04)3 and a-Sb2O4 yields a complex mixture containing, in addition to the starting phases, FeSb04, Moo3 and possibly some contaminated phases.We give an overview of the results obtained in attempting to analyse the various synergetic cooperations occumng between these phases in the oxidation of isobutene to methacrolein. The work is based on results of catalytic activities with phases contaminated artificially by impregnation and with mixtures of two pure or contaminated phases. The remote control seems to play a major role in the effects observed. The overall result of calcination is the obtention of catalysts with higher activities and selectivities.

1. INTRODUCTION Our results in the last years show that the remote control mechanism plays an important role in selective oxidation. Phases not active or poorly active by themselves can modify the catalytic activity of other phases by the action of spill-over oxygen. In this way a synergy takes place between different oxides (1,2). In the studies proving the existence of a remote control, the catalysts were mechanical mixtures of two oxide phases prepared separately. Physico-chemical characterization showed that the oxide components were not modified during the reaction. The model catalysts used in these investigations were principally Moo3 + S b O 4 or Sn02 + Sb204 (3,4). In general, however, and especially in the case of industrial catalysts, one could expect various changes during the catalytic work: phases may disappear or new phases may be formed and some mutual contamination may occur. In such cases, the observed synergy can in principle be explained by another mechanism than remote control. The present work has been undertaken in this context. Table 1 allows us to introduce the objective of our investigation. Fe2(Mo04)3 is active in the oxidation of isobutylene, but is not selective in the formation of methacrolein. A mechanical mixture of separately prepared Fe2(Mo04)3 and a-SbO4 oxides works synergetically in the same reaction. The synergy was observed when both the yield and the selectivity in methacrolein were considered. When a mechanical mixture formed by the same oxide phases is calcined during 6 days in air at 773'K the synergy increases significantly.

42

Table 1. Catalytic activity results of mechanical mixtures of a-Sb204 and Fe2(Mo04)3 The values in parentheses are those that would be obtained by the addition of the values of the pure catalytic phases as defined in 2.4. Sample

Conversion (%)

Yield (%)

Selectivity (%)

a-sb204

inert

inert

inert

Fe2(Mo04)3+ U-Sb2O4 Rm=0.6 (non calcined)

19.6 (18.7)

5.7 (3.1)

31.0 (16.3)

Fe2(Mo04)3

31.2

5.1

16.3

Fe2(Mo04)3+a-SbO4 with Rm4.6

38.0 (18.7)

9.4 (3.1)

51.5 (16.3)

(Calcined for 6 days)

Comparison of fresh and calcined mixtures ( 6 ) showed the formation of new compounds. This reaction is accompanied by an increase in the BET surface area (from 5 to 7.8 m2/g after six days of calcination). In the fresh samples only Fe2(MoO& and a - S b 0 4 were observed. Calcination leads to a segregation of small particles formed by Sb and Fe. Two new phases are observed: FeSb04 and MoO3. Their concentration increases with time of calcination. Both Fe2(Mo04)3 and a - S b O 4 are not completely decomposed and an enrichment of antimony on the surface is observed during calcination. The final architecture of the calcined samples corresponds to a complex multiphase catalyst formed principally by the non transformed Fe2(Mo04)3 and a - S b O 4 , and new phases such as FeSb04 and MoO3. It is not excluded that antimony contaminated phases such as Sb/MoO3 and Sb/FeSb04 or molybdenum contaminated phases such as Mo/Fe2(Mo04)3 and Mo/FeSbOq could also be formed. The calcined a-Sb2O4 + Fe2(Mo04)3 system thus appears as a complicated system in which synergetic effects of different nature could take place. For understanding the high catalytic performance of this complex multiphasic catalyst, we investigated methodically the cooperative effects between two or several phases present in the mixture. A previous paper (5, 9) presented the results obtained with fresh mixtures of Fe2(Mo04)3 and a-SbO4. Little or no contamination had taken place (7). A synergy was observed. The solid state reactions between Fez(MoO& and a-Sb204 have been the object of investigations presented in a second paper ( 6 ) . The role played by possible surface contamination of one oxide phase (Fe2(MoO4)3 or a-Sb2O4) by metals contained in the other was also investigated using two approaches: physico-chemical characterization of surfaces and comparison with phases contaminated artificially by impregnation (5,7). The objective of the present paper is to give an overview of the principal effects taking place as a consequence of solid state reactions between the catalyst components. Using mechanical mixtures of powders having the same composition as the new phases formed and of artificially contaminated phases, we shall show that other synergetic cooperations can take place. The synergetic effects in catalysis observed with mixtures after they have been calcined are considerable. We shall show that some phases play a major role in this synergy. The work also shows that the remote control mechanism can explain satisfactorily the high catalytic performances observed in the complex multiphase catalysts formed with iron, molybdenum and antimony combined in various oxide phases. 2. EXPERMENTAL 2.1. Sample preparation Temperatures of calcination of catalysts were chosen according to the composition and the nature of the samples in order to decompose the precursor to get the pure or the mixed

43

phases, to decompose the impregnated layer, or to facilitate or not the solid state reaction between the pure phases. 2.1.1. Pure oxide phases Fe2(Mo04)3: was prepared by dissolving Fe(N03).9H20 and citric acid in distilled water. (FiH&MO7024.4H20 was added to the solution. After evaporation of the solvent at 313 OK under reduced pressure, the solid obtained was dried at 373 OK overnight in a vacuum oven and then calcined at 773 O K for 20 hr (9). a-Sb2Oq: was prepared by calcination of Sb2O3 in air at 773 O K for 20 hr (3,4,9). Moo3 was obtained by thermal decomposition of (NH4)6Mo7024.4H20 in air at 773 OK for 20 hr (4, 10). FeSb04: SbC15 and Fe(N@).9H20 were dissolved in an aqueous solution containing HCI and citric acid. The solvent was evaporated at 313 OK under reduced pressure and the solid obtained was dried at 373 OK under vacuum during 16 hr and calcined at 773 OK in air during 24 hr (8). 2.1.2. Impregnated catalysts Samples are denoted as M/Oxid, in which M indicates the type of ions deposited on the surface of the oxide support (Oxid). Sb/Fe2(Mo04)3: Sb(OC4Hg)3, was dissolved in butanol and then added to an alcoholic suspension containing the Fez(Mo04)3 powder. A small amount of water was added. The solvent was evaporated at 313 OK under reduced pressure and the solid obtained was dried at 383 O K overnight and calcined at 673 O K for 2 hr. Samples containing 1%, 3%, 6% and 12% (weight percent of Sb2O4) were prepared (7). FeMo/Sb2Oq: Sb2O4 was added to a solution containing Fe(N03).9H20 and citric acid. The required quantity of (N&)&fo7024.4H20 was added (Fe/Mo atomic ratio of 2/3). The solvent was evaporated at 313 O K under reduced pressure and the solid obtained was dried at 383 O K overnight and then calcined at 773 O K for 6 hr. Samples containing 1%, 5%, 10% and 20% (weight percent of iron molybdate) were prepared (7). SbFeSb04: FeSb04 was immersed in a solution containing equal amounts of SbC13, SbCIs and HCI in CHC13 used as solvent. The solvent was evaporated at 313 O K under reduced pressure and the solid obtained was dried at 403 OK for 20 hr and then calcined at 633 OK for 5 hr. A sample containing 2.34% in weight of S b O 4 was prepared (8). MoFeSb04: FeSb04 was immersed in an aqueous solution of (NH&jMo7024.4H20. Water was evaporated at 313 OK under reduced pressure and the solid obtained was dried and calcined as above. A sample containing 1.1% in weight of Moo3 was prepared (8). Mo/Sb204: a - S b O 4 was added to an aqueous solution of ( N H & ~ ~ ~ O ~ O U . Water ~H~O. was evaporated and the solid dried at 393 O K overnight and calcined at 703 OK for 1 hr. A sample containing 0.15% in weight of Moo3 was prepared (4, 11). Sb/MoO3: Moo3 was added to an equimolar solution containing SbC13 and SbCI5 in CHC13. The solvent was evaporated at 313 O K under reduced pressure and the solid obtained was dried at 393 OK for 15 hr and calcined at 450 OK for 1 hr. A sample containing 0.64% in weight of S b O 4 was prepared (4, 11). 2.1.3. Biphasic catalysts They were prepared by dispersing under mechanical agitation and ultrasonic dispersion in n-pentane for 10 min equivalent amounts of two oxide catalysts such as those prepared in 2.1.1 and 2.1.2 (and designated by A and B for generality). The n-pentane was evaporated under continuous agitation at 298 OK under reduced pressure. The solid was dried in air at 353 OK overnight. No further calcination was camed out on the corresponding mechanical mixtures. The composition of the mixtures of catalyst A and B was expressed as the mass ratio Rm=(weight of catalyst A)/(weight of catalyst A + catalyst B). With a-Sb2O4 and Fe2(Mo04)3 mechanical mixtures with Rm=0.25; 0.5 and 0.75 were prepared. All the other mixtures were prepared with a Rm=0.5.

44

2.1.4. A multi(comp1ex)phase catalyst It was prepared by calcining a mechanical mixture with R m d . 6 at 773 OK during 6 days in air (6). 2.2. Characterization Bulk and surface techniques were used to characterize fresh, calcined and used samples (4,6,7, 8,9, 10, 11). A Siemens D-500 diffractometer using nickel-filtered Cu-Ka radiation was used for XRD analysis. BET surface areas were measured with a Micromeritics 2000 equipment using krypton adsorption at 77 OK,after evacuation for 2 hr in vacuum at 373 OK. XPS analyses were performed on a Surface Science Instruments, SSI-XPS equipped with a flood gun. The excitation radiation was MgKa (1253.6 eV). The C l s peak at 284.6 eV was used as reference. For most of the analyses the concentrations were calculated using a comparison with the signal of an external reference, Si02. CTEM and AEM analyses were realized in a Jeol Temscan 100 CX electron microscope and a Kevex 5100 energy dispersive spectrometer. Selected area electron diffraction (SAED) was also carried out. Mossbauer Spectroscopy was realized in a Mossbauer spectrometer equipped with a 1OmC57Co/Rh source maintained at room temperature. A Northern NS-900 multichannel analyser was used for taking spectra. The isomer shift was calculated based on NCB reference (d=0.275). Electron Spin Resonance (ESR) spectra were recorded by an X-band Varian-El2 spectrometer using a 100 kHz modulation frequency and 20 mW incident microwave power. 2.3. Catalytic test (3,5,7,8,9, 10, 11) Catalytic measurements were performed in a conventional fixed-bed reactor system. The reactor was made of a Pyrex U-tube of 8 mm internal diameter into which the catalyst was packed. The catalyst (300 mg) was made from the powders (pure or mixtures) using a fraction between 500 and 800 pm. The standard reaction conditions were as follows: partial pressure of isobutylene 76 Torr, partial pressure of oxygen 152 Torr, total pressure 760 Torr, the diluent being nitrogen. The total feed rate was 36 d m i n and the reaction temperature 693 OK. Some experiments were realized at lower oxygen partial pressure. Analysis of reactants and products were realized by gas chromatography. 2.4. Expression of synergetic effects The magnitude of the increase of the yield in methacrolein in mixtures is expressed by the synergy in yield, defined as AY=Ym - YA+B.In this expression, YA+Bis the yield obtained in the absence of synergetic effect and defined as RmYA + (1-Rm)YB. YAB,YA and YB are, respectively, the yields of the mechanical mixtures, catalyst A and catalyst B being measured under the same conditions. The effect is expressed in % as AY/(YA+B)x~OO.A similar equation can be obtained for the conversion (C). The absolute value of the synergy in the selectivity is defined as AS= SAB-SA+B,where SAB is the selectivity of the mixtures and SA+Bis the selectivity which would be observed in the absence of any synergetic effect defined, for a mixture with Rmz0.5, as (YA+YB)/(CA+CB),CA and CB being the isobutene conversion of catalysts A and B. The effect is expressed in % as A S / ( S A + B ) X ~ ~ ~ . 3. RESULTS

Results are presented in Tables 1 , 2 and 3. The most salient results are summarized in the following sections. 3.1. Non calcined Fez(MoO4)s + Sb2O4 mixtures (Table 1) Results were already indicated in the introduction. Pure a-Sb204 is inert. Pure Fe2(Mo04)3 is active, but its selectivity is very poor. The mechanical mixture with Rmz0.6 exhibit a high synergy in the yield in methacrolein. The synergy, as indicated by the increase in methacrolein selectivity, is conspicuous.

45

3.2. Catalytic activity of calcined Fe2(Mo04)3 + a-SbzO4 sample (multi(comp1ex)-phase catalysts) (Table 1) The comments in the introduction can be refined in the sense that all indicators of synergy, namely conversion, yield and selectivity, increase in a spectacular way. 3.3. Catalytic activity of other pures mixed oxide phases (Table 2 ) F e S b 0 4 and M o o 3 are very active but not particularly selective. Their effect in mixtures will be summarized in 3.5.

3.4. Catalytic activity of impregnated oxide phases (Tables 2 and 3) Sb/Fe~(M004)3, S b M o O 3 , Sb/FeSbO4, Mo/FeSb04: the impregnation leads t o an increase in the yield and in the selectivity in methacrolein and no change or a decrease of the conversion of the support phases. Table 2. Catalytic activity results obtained with mechanical mixtures In parentheses for mechanical mixtures, values obtained by the addition of the values of the pure catalytic phases (defined in 2.4) and for impregnated catalysts, values of the supported phases. References to the corresponding papers are also indicated. Sample

ref.

Conversion (%)

Yield (%)

Selectivity (%)

46.0

7.2

15.6

8

FeSbOq

36.0

5.5

15.3

8

FeSbO4+n-Sb04

32.0 (23.0)

16.1 (3.6)

50.3 (15.6)

8

Ma3 SbFeSbO4

48.5 (46.0)

11.5 (7.2)

23.6 (15.6)

8

FeSbOq + MoO3

29.0 (4 1.O)

13.3 (6.3)

46.0 (15.4)

8

MO/FeSbO4

45.0 (46.0)

13.2 (7.2)

29.0 (15.6)

8

Sb/FeSbO4+MoOg

30.0 (42.3)

15.4 (8.5)

52.0 (19.4)

8

Sb/FeSbOq +Mo/FeSbO4

38.6 (46.8)

13.0 (12.3)

34.0 (26.3)

8

Sb/Fe2(MoO4)3

25.0 (62.0)

17.5 (8.0)

62.0 (13.0)

7

FeMoI a-Sb204

48.0 (inert)

25.0 (inert)

52.0 (inert)

1

Table 3. Catalytic activity results with mechanical mixtures In parentheses values obtained by the addition of the values of the pure catalytic phases (defined in 2.4) Sample Moo3 MOO3+a-SkO4

Conversion (%)

Yield (%)

Selectivity (%)

Ref.

41.6

6.25

15.0

11

26.0 (21)

7.3 (3.1)

28.0 (7.5)

11

(Rm=0.5) Sb/M003

24.0 (4 1.6)

7.2 (6.25)

30.0 (15.0)

11

Mo/ a-SbO4

5.8 (inert)

0.75 (inert)

13.0 (inert)

11

Mo/a-Sb204 and FeMo/Fe2(MoO&: support, which is inert.

the impregnation increases the catalytic activity of the

46

3.5. Catalytic activity of mechanical mixtures (Table 2) FeSb04 + a - S k O 4 , Moo3 + a - S h O 4 , FeSb04 + MoO3, SbFeSb04

+ Moo3 and S b F e S b 0 4 + Mo/FeSb04: in all cases an increase in the yield and in the selectivity in methacrolein is observed. 3.6. Physico-chemical characterization of samples Results are summarized in tables 4,5 and 6. Table 4. Mechanical mixtures in which no formation of new phases, no decomposition of phases and no indication of contamination between phases were observed. Mo03+ a-Sb204 (SBET, XRD, X P S , ESR,AEM, ISS) (11) Fe2(Mo04)3+a-Sb204(non calcined, non low oxygen concentration) (SBET, XRD, X P S , AEM, ISS, Mossbauer spectroscopy) FeSbOq+MoOj(SBET, XRD, X P S )

(7)

FeSbOq+a-Sb;?Oq(SBET, XRD, X P S )

(8)

(8)

Table 5. Impregnated catalysts where (i) a detachment of the impregnated layer and formation of aggregated particles were observed or (ii) the concentration of impregnated ions on the surface remains unchanged. (i)

Sb/Mo03 (XPS, ISS, CTEM, AEM) Mo/a-Sb04 ( X P S , ISS, C E M , AEM) FeMo/a-Sb04 (XPS, ISS, CTEM, AEM)

(ii)

SbFeSbO4 (after calcination, XPS) Mo/FeSbOq (after calcination, XPS)

(8) (8)

Table 6. Mechanical mixtures or impregnated catalysts where formation of a new phase was observed Sb/Fez(Mo04)3(aggregationof the antimony as a-SkO4 and simultaneous formation of FeSbOq and M a 3 after calcination or/and catalytic reaction, SBET, XRD, XPS, ISS, AEM, SAED, CTEM) (7) Fe2(Mo04)3+a-Sb204 (formation of FeSbO4 and Mo03 after calcination,SBET, XRD, X P S , AEM, SAED) (6)

Fe2(Mo04)3+a-SbO4 (formation of FeMoOq after reaction at low concentration of oxygen. Pure Fe2(Mo04)3. in the same conditions, gives a deeper reduction with formation of FeMQ and Mo03-x. XRD, XPS, SAED, Mossbauer spectroscopy) (9) 4. DISCUSSION

A higher surface area could explain higher conversion or yield, but not higher selectivity. Even if this role of surface area exists, another factor must come into play for explaining the effect. We are thus led to focus the discussion on the last possibility. The new phases formed in calcined mixtures have poor selectivities: FeSb04 and Moo3 have selectivities of 15.6% and 15.3% respectively. Another explanation of the synergy observed could be (mutual) surface modifications of the two phases in contact (contamination by volatilization, for example). This possibility seems to be excluded. The characterization of samples, before and after reaction, using very sensitive techniques such as ISS, XPS and AEM, fails to detect any modification during reaction. On the contrary, a clear indication of the decontamination was observed. On the other hand, if contamination occurs by volatilization and condensation, the synergy should increase during reaction, which is contrary to our results. If some contamination of phases by elements coming from other phases took place, this would lead to situations similar to those obtained by impregnation by the corresponding elements, namely Sb/FeSb04, Sb/Fe2(Mo04)3, Mo/FeSbOq, Sb/MoOj and Mo/a-Sb04. These artificially contaminated oxides also have poor selectivities (23.6, 41.O, 29.0, 30.0 and 13.0% respectively). This observation excludes that the presence of new phases or

41

contaminated phases alone could explain the high selectivity obtained in the complex systems we studied. A third explanation could be a phase cooperation, allowing a-Sb204 crystallites to be oriented on the surface of FeSb04 and thereby creating catalytically active sites not found in either of the isolated phases (13, 14). On the other hand, in these studies (13, 14) catalysts were aimed at maximizing mutual contamination. The conditions were completely different from ours: adding an aqueous solution of Fe3+ to an Sb solution, evaporating, drying, firing at 698"K, firing at 1073°K and at 1173°K. However, it is interesting to note that even when following these drastic preparation methods, the authors observed clearly crystallites of a-Sb204 on the surface of FeS b04, which suggests that starting from an homogeneous (well contaminated!) precursor, the natural tendency is the segregation of a - S t ~ O 4and FeSb04 phases. Thus the experiments of ref 13 and 14 rather suggest a tendency to decontamination when special preparation techniques, such as those used in the cited work, create a metastable mixed oxide. Anyway, it is difficult to accept that a very simple mixture, as in our experiments, of two powders prepared separately could allow a "very precise" matching of two "very precise" faces in an amount that would explain the significant increase in selectivity. On the other hand, yield (together sometimes with conversion) increases after calcination. The increase in the yield is explained by an increase in the total number of selective sites. It ensues that the increase in selectivity is explained by a change in the nature of the active sites, namely by suppression of non selective sites toform selective sites. Results presented in tables 1, 2 and 3 give arguments to demonstrate the creation of selective sites. (i) In mechanical mixtures, no change of surface area takes place. If more selective sites are working in such mixtures, this must be due to the creation of sites or the conversion of non-selective to selective sites. This obviously takes place in the following mixtures: FeSb04+a-Sb204. FeSbOq+MoOj, Sb/FeSbOq+MoOg and M o Q + a - S b 0 4 . The synergetic effects on the yield calculated from the expression given in paragraph 2.4 are impressive: 347%, I l l % , 81% and 135%respectively. (ii) Yaking into account the suppression of non-selective sites, this corresponds to a still more spectacular selectivity: 224%, 198%, 168%and 273% respectively. (iii) Impregnation of non selective phases such as Fe2(Mo04)3, FeSb04 or Moo3 with antimony or FeSbOq with molybdenum gives rise to an important synergetic effect in the selectivity: 215%, 51.3%, 100% and 85% respectively and in the yield loo%, 59.7%, 15% and 83.3% respectively. The characterization results summarized in Tables 4, 5 and 6 show that these mechanical mixtures and impregnated catalysts were always formed by two phases. No new phase was formed, no decomposition or contamination of the phases forming the mixtures was observed after the catalysts had worked in the catalytic reaction. The only change observed in impregnated catalysts, after they had been used, was that the impregnated layer tended to segregate on the surface forming a well characterized biphasic catalyst. This strongly supports the conclusion that the synergetic effects are due to a cooperation between distinct phases and not to contaminated phases. In conformity with the explanation given in other cases, it is proposed that this cooperation is realized via the remote control mechanism. The sample Sb/Fe2(MoO4)3 is the only one showing a different behaviour. In this case, at the same time as the aggregation of a-Sb2O4 occurs, formation of FeSb04 and Moo3 is observed. However, taking into account the low selectivities of FeSb04 and MoO3, the formation of these phases cannot explain the improvement in catalytic performance. It is rather the synergetic cooperation of the new phases with the old ones that explains the synergy. The conclusion of this analysis is that a cooperation berween separate phases is essential in explaining synergy in calcined samples. The explanation of the synergy observed then resides principally on the remote control theory and more precisely on the donor-ncceptor role of the oxide phases (1-4). Previous studies done in our laboratory allow us to propose a new classification of the different oxides or mixed phases used normally as oxidation catalysts (1, 2).

48

According to this classification, oxides that can form the mobile oxygen species play the role of "donor". Other oxides, that can receive these oxygen species and the surface of which can react with it, play the role of "acceptor". The "acceptor" phase contains numerous potentially active sites, but these can only work selectively in the presence of the "donor" phase. A thud type of oxides concerns those that can play both functions. One of these functions may predominate depending on the oxide with which they are in contact. The following couples of oxide phases can contribute principally to the high selectivity of the calcined mixtures : FeSb04 + Sb2O4. FeSb04 + MoO3, SbFeSb04 + Moo3 and Sb2O4 + MoO3. It is not excluded that other couples also play a role, namely FeSb04 + Fe2(Mo04)3, SbFeSb04 + Fe2(Mo04)3, Fe2(Mo04)3 + Moo3 or SbFeSb04 + MoFeSb04, but we have no solid argument for or against this possibility. Examining the whole group of results, it can be concluded that the increase in the conversion in multiphase catalysts can be explained by two effects. First the particles forming the new phases generated by the solid state reactions (particularly FeSb04 and MoO3) are very small and expose a high surface area, thus increasing the number of potential active sites. Second, the synergetic effect in the conversion is positive for FeSb04 + a-Sb204 (+39%) and Moo3 + a-Sb204 (+23%) which indicates that these mixtures could also play an important role in the increase in the conversion. During calcination the surface ratio between acceptor and donor are changed, the small crystallites of the new phases form numerous contacts with the other phases (high interdispersion). On the other hand, the quality of the contacts between the phases are better than in the fresh mixture. These changes give a new architecture to the catalyst, thus facilitating and multiplying the cooperative effects (in the remote control sense), giving a higher conversion with high selectivities. On the whole, it seems that the major effects are due to the donor. It has been proposed in the literature (12) that a catalyst formed by FeSbOq enriched by antimony (probably forming a layer of F e s b o 6 ) shows a high selectivity in propene oxidation. The role of the surface layer should be to provide surface oxygen that oxidizes the olefin selectively. The results with Sb/FeSb04 of table 2 do not exclude this possibility. But the effects of impregnation, although positive, are not impressive. This cannot explain all of the phenomena observed. We can interpret the present results principally by the donor properties of SbO4, and possibly SbFeSb04, and the acceptor properties of Fe2(MoOq) and MoO3. FeSb04 (and probably Fe2(Mo04)3) plays both roles simultaneously: acceptor in contact with a-SkO4 and donor in contact with MoO3. The increase in conversion is accompanied by the creation of selective sites. The lower conversions observed in FeSb04+ Moo3 and Sb/FeSb04+ Moo3 mixtures are explained by a significant inhibition of non-selective sites situated on Moo3 thanks to the donor character of FeSb04 and SbReSb04.The mechanism by which the new selective sites are created is discussed in (2).Spillover oxygen prevents the nucleation of reduced phases on the acceptor. Spill-over emitted by the donor keeps the surface of the acceptor in a higher oxidation state during the catalytic reaction. The results obtained with the mixtures Fe2(Mo04)3 + a-SkO4 ( 6 , 9), Moo3 + a-Sb204 (4) and Sn02 + a-Sb204 under low concentrations of oxygen confmn this suggestion. CONCLUSIONS The addition of a-SbO4 on the Fe2(Mo04)3 oxide and calcination increase significantly the catalytic performance of Fe2(Mo04)3. The initial biphasic system can be transformed to a multiphasic one during the course of the catalytic reaction. The principal phases observed in the final catalyst after simulation of this effect by calcination are: Fe2(Mo04)3. Sb2O4. Moo3 and FeSb04. Other contaminated phases such as Sb contaminated Fe2(MoOqh, FeSbOq and MoO3, or Mo contaminated FeSbO4 and Moo3 could also be formed. The high catalytic performance of multiphase catalysts can be explained by a cooperation between external phases via the remote control mechanism thanks to the donor-acceptor role of the different oxide phases present after calcination. The high interdispersion of the multiphase system facilitates this cooperation.

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OUTLOOKS Our results show that in iron-molybdenum-antimony multiphasic catalysts, the remote control plays an important role in maintaining selectivity during the selective oxidation of isobutylene. This implies that, in the preparation of selective oxidation catalysts, the presence of good donors and acceptors, with optimal surface area ratios and with numerous and effective contacts between them, is crucial to obtain an active and selective catalyst. The optimization of these parameters have been realized with success for the synthesis of a very performant multicomponent catalyst for the oxidation of isobutylene to methacrolein (15) (yield 79% and selectivity 96.8%). ACKNOWLEDGMENTS The authors are grateful for the financial aid received from the CONICET and Univ. Nac. de San Luis, Argentina (L.E. Cadus), Univ. de Sevilla and CSIC, Spain (F.J. Gotor) and the support of the CommunautC Francaise de Belgique, in the frame of a Concerted Action, and that of the Office of the Prime Minister - Science Policy Programme "Catalysis" (Belgium). REFERENCES 1. L.T Wini; P. Ruiz and B. Delmon, Studies in Surface Science and Catalysis "New Developments in Selective Oxidation by Heterogeneous Catalysis" (P. Ruiz and B. Delmon, eds.), Elsevier, vol. 72, pp. 399-413. 2. L.T. Weng and B. Delmon, Appl. Catal. A., 81, 141, (1992). 3. L.T. Weng, N. Spitaels, B. Yasse, J. Ladribre, P. Ruiz and B. Delmon., J. Catal., 132, 319, (1991). 4. B. Zhou, E. Sham, T. Machej, P. Bertrand, P. Ruiz and B. Delmon, J. Catal, 132, 157, (1991). 5. Y.L. Xiong, L.T. Weng, B. Zhou, B. Yasse, E. Sham, L. Daza, F. Gil-Llambias, P. Ruiz and B. Delmon, in Preparation of Catalysts V (G. Poncelet, ed), Stud. Surf. Sci. Catal., Vol. 63, Elsevier, Amsterdam, 1991, p. 537. 6. L.E. Cadus, F.J. Gotor, D. Acosta, J. Naud, P. Ruiz and B. Delmon, 12th International Symposium on the Reactivity of Solids, Madrid, Spain, September 24-30, 1992. 7. Y.L. Xiong, L.T. Weng, J. Naud, P. Bertrand, J. Ladribre, L. Daza, P. Ruiz and B. Delmon, in preparation. 8. L.E. Cadus, P. Ruiz and B. Delmon, in preparation. 9. Y.L. Xiong, R. Castillo, L. Daza, P. Ruiz and B. Delmon, Vth International Symposium on Catalyst Deactivation, June 24-26, 1991, Evanston (IL), U.S.A. Studies in Surface Science and Catalysis "Catalyst Deactivation 1991", (C.H. Bartolomew and J.B. Butt, eds.), 68,425, 1991. 10. P. Ruiz, B. Zhou, M. Remy, T. Machej, F. Aoun, B. Doumain and B. Delmon, Catal. Today, 1, 181, (1987). 11. L. T. Weng, B. Zhou, B. Yasse, B. Doumain, P. Ruiz and B. Delmon, 9th ICC, Calgary, Canada Vol. 4, 1609 (1988) and Ref. (3). 12. N. Yamazoe, I. Aso, T. Amamoto and T. Seyama, 7th International Congress on Catalysis, "New Horizons in Catalysis", Tokyo, Studies in Surface Science and Catalysis, N"7, Part B (T. Seijama and K. Tanabe Eds), 1980,1239. 13. R.G. Teller, J.F. Brazdil and R.K. Grasselli, J. Chem. Soc.,Faraday Trans. I, 81, 1693, (1985). 14. R.K. Grasselli, G. Centi and F. Trifiro, Appl. Catal., 57, 149, (1990). 15. N. Blangenois, P. Ruiz and B. Delmon, Brevet Belge no 09200290 (1992), European Patent (March 24, 1993). ANSWERS TO QUESTIONS R.K. Grasselli, (Mobil Research & Development, New Jersey, U.S.A.) : You call it "cooperation of phases" and we have called it "phase cooperation" (1). We are probably speaking of the same phenomenon, at least I think so. Although our interpretations of the phenomenon are different from yours, there is certainly opportunity for a friendly discourse.

50

For this to occur we need to refresh our memory of work that preceded your current investigation and put it into perspective. If we assume and agree that phases can cooperate as we have shown in our work referred to above. We are of the opinion that for this to happen the phases need to be in close proximity, not remotely situated, and in order for them to cooperate an advantageous property common to both is epitaxy. We have shown this for Fe-Sb-oxides (1) as well as for multicomponent, multiphase Bi-molybdates/Fe-molybdates(2a, particularly p. 130; 2b, particularly p. 32), and Ce-Bi-molybdates (3, particularly p. 5488). Another important factor is some mutual solubility of key catalytic elements, and that is almost always the case, at least on the surface and under reaction conditions. Still another point in question is that metal oxides such as MoO3, Sb2O3, Te04, etc., are rather volatile, particulary in the presence of steam which is always present in redox reactions since we form H 2 0 as one of the necessary products of reaction. The above metal oxides all form volatile species, e.g. Mo03.H2O, which readily migrate throughout the reactor bed, interacting and reacting with the catalytic phases which might be the constituents of the catalyst in the reactor (4, particularly pp. 155-156). In this respect I should like to refer you to the pioneering work of Buiten from DSM (5), who was the first to my knowledge to demonstrate the dramatic effect on catalytic behavior of Sn02 (in the reactor) as it interacts with hydrated Moo3 externally fed to the reactor. The effect is dramatic and instantaneous. It is a classical paper well worth reading and referencing. Having said that, one could envision that the effect of phase cooperation which you observe when you add a-Sb2O4 to Fe2(Mo04)3 is that these phases interact in the reactor under reaction conditions and form FeSbOq which you observe, some MoO3, and the remainder stays as Fe2(Mo04)3. FeSbOq (particularly if Sb enriched on the surface) is a much better isobutene to methacrolein catalyst than Fe2(Mo04)3 will ever be. Conversely, Fe2(Mo04)3 (particularly once slightly reduced on the surface to give FeMoOq) is an excellent dioxygen dissociator to 0 2 - and transporter of 02-. The phase cooperation and synergy thus results from the fact that the in situ generated FeSbOq becomes the isobutene to methacrolein catalyst, enhanced by FeMoOq oxygen transfer phase, probably cooperating through an epitaxial intergrowth, or overlayer of Sb enriched FeSbOq on top of Fe2(M004)3/FeMoOq. In terms of molecular level mechanism, in our view (6), the Sb3+-O is responsible for the ahydrogen abstraction and the Sb5+-O for the oxygen insertion into the allyl, to yield methacrolein. The Fe2+/3+ couple of the molybdate (having a higher reduction potential than the oxygen inserting element) provides for the efficient dioxygen dissociation and 0 2 transfer to the Fe-Sb-oxide active phase. Since small amounts of Mo can incorporate into the FeSbO4 phase (by analogy of Mo incorporation into a-Sb204, ref. 7), and particularly at the surface, then a feasible explanation of the observed synergy might reside simply in that all key catalytic elements (i.e. Sb, Fe and Mo) are combined in a single super phase or super surface phase, with the remaining phases being mere spectators, but also important reservoirs of those key elements which are lost from the active catalytic phase through evaporative processes caused by the redox process during the reaction. These spectator reservoirs continously replenish the lost elements of the active phase, they are Red Cross of selective oxidation catalysis. References : 1. R.G. Teller, J.F. Brazdil and R.K. Grasselli, J. Chem. SOC.,Faraday Trans. 1, 1693 (1985). 2. (a) R.K. Grasselli, Appl. Catal. 15,127 (1985). (b) J.F. Brazdil, M. Mehicic, L.C. Glaeser, M.A.S. Hazle and R.K. Grasselli, ACS Symp. Series 288,M.C. Deviney and J.C. Gland (eds.), 3,27 (1985). 5485 (1983). 3. J.F. Brazdil, L.C. Glaeser and R.K. Grasselli, J. Phys. Chem. 4. R.K. Grasselli, G. Centi and F. Trifiio,Appl. Catal. 149 (1990). 5. J. Buiten, J. Catal. 14, 188 (1968). 6. R.K. Grasselli, J. Chem. Ed. 216 (1986). 7. R.G. Teller, M.R. Antonio, J.F. Brazdil and R.K. Grasselli, J. Solid State Chem. @, 249 (1986).

u,

x,

a,

a,

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L. Cadus (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): Our results show that pure FeSbOq and pure Fe2(MoOq)3 have similar catalytic performances, which is contrary to your suggestion. Concerning the role played by Fe2(MoO& and FeMo04, we have shown before (our ref. 7,. 9, Catalysis Today. "Catalysts Deactivation" Vol. 11, No4,22 January 1992 (J.J. Spivey, ed.), p.455.) that, when alone, Fe2(Mo04)3 is decomposed, by reduction, into FeMoOq and Mo02, which are less selective than Fe2(MoO&. This indicates that the presence of FeMo04 has a negative effect, contrary to your suggestion. The presence of a-SkO4 prevents the formation of FeMo04 and keeps initially the catalysts more selective and stable. Then it is not necessary to call to the presence of FeMoO4 to explain high selectivities. FeSb04 plays the same role as a-Sb2Oq (that is as a donnor phase). We have no information (we do not know if it exists in the literature) if Sb enriched FeSb04 is able to match epitaxially with Fe2(Mo04)3/Fe/FeMoOq . Please see other comments which have been included in the final text of the article, as published above. As concerns the incorporation of Mo into a-SkO4 (your ref. 7), it should be emphasized that your experiments in this reference correspond to coprecipitation of Mo and Sb, and a very high calcination temperature, and consequently formation of p-SkO4, instead of the a-phase. It has been amply demonstrated that oxides associating Mo and Sb easily decompose to the individual oxides Moo1 and a-SkO4 (.K. Nassau, H.J. Levinstein, and Gr. Louicono, J. Phys. Chem. Solids 26, 1805 (1965); K. Nassau, J.W. Shiever, and E.T. Keve, J. Solid State Chem. 3, 411 (1971); M. Parmentier, A. Courtois and C. Gleitzer, Bull. Soc. Chim. Fr., 1,275 (1974); M. Parmentier, C . Gleitzer and R.J. Tilley, J. Solid State Chem., 31, 305 (1980)). By preparing your catalysts by grinding the oxides together or coprecipitating the "Sb2O5 sol" with a solution of ammonium molybdate before firing at 800°C (your reference 7) , you observe exactly the same result which indicates that the natural tendency of the system is to segregate the two oxide phases and not to Contaminate them. Normally, indeed, the precipitation method should facilitate the contamination. The formation of a "super phase", as you propose it, is speculative, and, unless you give experimental data, not supported experimentally. There is some inconsistency in your proposal, as, in one of your papers (ref. 13), you show that starting with a very contaminated iron-antimoniate precursor, there is a segregation of antimony towards the surface. On the other hand well characterized samples (our ref. 4 and others included in this reference) show unquestionably that the tendency of the system Sb-Mo is the decontamination by formation of pure oxides and not the formation of a mixed phase. The present results confirm the above results: starting with artificially contaminated phases the contamination layer is broken during calcination or catalytic reaction. Your assumption that, in the duration of our test (6 hours), there is a loss of metals, is speculative. We never observed that, nor contamination after test. Anyway, if this occured, the synergy should increase as function of time, what is contrary to our results in which the synergy is independent of time. We always pay much attention to possible loss of an element, as proven by our other paper "Creation of new selective sites by spill-over oxygen in the oxidation of ethanol," R. Castillo et al.). J.Vedrine (IRC, CNRS, Villeurbanne, France) : As general idea of donorlacceptor concept to explain synergy effect I think that it is one important parameter but not the only one (electrical conductivity is also important). My concern deals mainly with the distance between donorlacceptor phase which means that distance as nm is involved. Why do you exclude the possibility of contamination where in such a case even keeping in mind your ideas of oxygen anion spill over may make the concept more acceptable ? L. Cadus (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): The aim of our work was to detect any form of reaction between the phases involved, including Contamination. We have no evidence of contamination. We can explain the effects and support the explanation by additional experiments. We do not see any reason to take into

52

account a non-proven speculation. If the effect you mention can be substantiated, we shall be happy to include it in the picture we give. M. Misono, (The University of Tokyo, Faculty of Engineering, Tokyo, Japan) : It seems very difficult (or almost impossible) to exclude experimentally the possibility of slight contamination (or new phase formation). So I wish to ask two questions from a different viewpoint. What is the concrete image of the structure of "the very selective sites", the number of which increases by the remote control (or donor-acceptor) model ? It would be better to try to demonstrate directly by in-situ measurement the increase of these selective sites for the settlement of controversy. What do you think of this possibility ?

L. Cadus (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): With respect to the nature of the active site, the direct information we have is that it should be a fully oxidised species. For molybdenum, according to all classical mechanisms of selective oxidation proposed in literature (Grasselli and Brazdil, Haber et al.) and theoretical representations (Allison and Goddard 111, Solid State Chemistry in Catalysis [Grasselli and Brazdil, eds], Amer. Chem. SOC.,Washington, 1985, p 23), this should be a molybdenum carrying one or two (Mo=O) functions. This is also probably true for molybdates, but here a cooperation with the other metal is to be expected (classical mechanisms mentioned above). It is not sure that such surface species could be identified easily by in-situ physico-chemical techniques. We have found a proportionality between the density of Bronsted sites on Moo3 (as probed by NH3) and the activity of Sb204-Mo03 catalysts, but the question is whether these hydroxyls are directly involved in the catalytic sites or just species the concentration of which is proportional to that of the catalytic sites (L.T. Weng, B. Delmon, Appl. Catal. A, 81 (1992) 141). There are other pieces of information. For example, we have made some "in situ" FTIR measurements. Methacrolein adsorbs on Sn02 and transforms to strongly held precursors of complete oxidation products at 300-400°C. In presence of a-Sb204, no such effect is observed. An identical effect is observed when adsorbed species produced by the contact of isobutene and oxygen with the catalysts are studied. This results are an indication of the modification (creation of selective sites on Sn02) in such a way that the adsorption strength of intermediate oxidation products of methacrolein is depressed (ref.: Journal of Molecular Catalysis 72 (1992) 307-313). We are in agreement with you that more in situ measurements are necessary. J.C. Volta, (IRC, Villeurbanne, France) : How can you exclude the possibility of the modification of the local surface structure in the course of the reaction, particularly with water which is generated and this should explain your synergetic effects. I will give two examples to support this view : i) it is well known that Moo3 can be transported by water as MoO(CH)2 and redisperses the Moo3 structure on a second phase. ii) we have recently shown that in the VPO system at the moment of the activation under butane/air atmosphere of the VOHP04.0,5H20 phase, VOPO4 structures are generated and redispers on the (VO)2P207 structure as V5+ sites on pyrophosphate matrix (see 1st European Meeting on Catalysis - Montpellier -Sept. 1993).

L. Cadus (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): The remote control theory is precisely based on the role of spillover species to modify local structure and coordination of surface atoms. We are trying to have access to surface local structure and their modification in the course of the reaction, and to compare the effects when spillover oxygen is present or not. If there is transport of molybdenum by water as you proposed this should be observed by the physico-chemical characterization. A very complete programme of characterization has been carried out in the case of the system Mo03+a-Sb04, under the experimental conditions used in our studies (temperature, time, composition of the reactional

53

mixtures). We have not observed any indication of contamination, excluding then this possibility (4). For that, we used very sensitive surface techniques as: ISS, X P S , AEM. On the other hand, if contamination as you propose occurred during reaction, the synergy should increase during reaction, what is contrary to our results. Please see answer to question 1.

G. Busca (Instituto di Chimica, Facolti di Ingegneria, Universiti di Genova, Italia): Even supposing no contamination, that is very difficult to prove, you could explain the synergy effect without necessarily invoke oxygen spill-over. In fact, your materials are semi-conductors. When two semi-conductor phases are in contact, electrons can flow from one to the other. So, one phase can have effect on the oxidation state of the other. This could explain synergy between pure" phases, perhaps better than oxygen spill-over. What is your opinion on this? "

L. Cadus (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): A flow of electrons would not explain the migration of 1 8 0 from donor to acceptor that we observe in experiments reproducing as nearly as possible the situation during catalysis (L.T. Weng, P. Ruiz, B. Delmon, D. Duprez, J. Mol. Catal. 52, 349 (1989); G . Mestl, P. Ruiz, B. Delmon, H. Knozinger, submitted). Krylov, Moro-Oka and Matsuura, although not agreeing completely with our interpretations, think oxygen is moving from one phase to another. But one cannot exclude that electron flow could also play a role.

F. Trifiro (Bologna University, Italy) : I have two questions. 1) Did you observe a decrease of time necessary for the observation of synergetic effect during time on stream (without any previous calcination). 2) How the synergetic effect you observed depend from the ratio donor-acceptor ? Did you observe two maxima with composition of the catalyst as must happen according the Komatsu Jander theory in the case the number of contacts between the two solids are important ? L. Cadus (UniversitQCatholique de Louvain, Louvain-la-Neuve, Belgium): No, in the present case the synergetic effect remains constant during time on stream. However in other case the synergy increases as function of time, as in the case of impregnated catalysts by example. The explanation in this case is a progressive restructuration of the impregnated layer, which gets broken during reaction giving a two phase catalysts. Of course, the maximun synergetic effect depends of the ratio of surface areas between acceptor and donor and the number and quality of contacts between these two phases. If the specific surface area of either the donor or acceptor is changed, the position of the maxima is changed also. The number of contacts depends on the interdispersion of the particles of the two phases provided no aggregates are formed. The number of contacts will increase with the decrease of the particle size. The quality of contacs depends on the preparation methods used to synthesize the catalysts.

J. Haber (Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland) : In your model of the remote control you assume that the oxide playing the role of a donor sends a message to the surface of an acceptor oxide, the message being carried out by oxygen. As this process is assumed to take place before the catalytic reaction, there must be a driving force for the oxygen migration, i.e. there must be a negative change of Gibbs free energy. By applying the classical thermodynamic reasoning one arrives at a conclusion that the partial pressure of oxygen over the donor oxide must be higher than the partial pressure over the acceptor oxide. Did you check that this relation holds for your systems ? L. Cadus (Universit6 Catholique de Louvain, Louvain-la-Neuve, Belgium): The remote control theory emphasizes the fact that the message (Oso)is sent continuously to the acceptor during the catalytic processes. Of course, thermodynamics rule all the phenomena, but kinetics also play a role and the generation of Ow is a kinetic phenomenon.

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In this respect, we cannot forget two facts : a) Kinetics: many acceptors (remote control terminology), e.g. Fe2(Mo04)3, do not activate 0 2 very rapidly, and can get reduced. Spill over oxygen, from a-SbO4 which dissociates 0 2 rapidly, helps keep Fe in Fe+3 state. This is the reason for the efficiency of Sb204. b) Thermodynamics : one simplifies the picture when speaking of "lattice oxygen". One has different species, with different reactivities on surfaces: i) spill-over oxygen, mobile, probable 0 2 - , but also ii) dioxo M=O and iii) bridging oxygen in many cases, which are believed to play the crucial role in the concerted mechanism described for allylic oxidation (Grasselli and Brazdil on the basis of kinetics and labelled experiments; Goddard with Quantum Mechanical calculations, both for allylic oxidation). The remarkable effect of Om (highly mobile, probably rather "free" 02-) produced by sbo4 is obviously due to either a higher free energy level, or to its higher mobility, compared to dioxo or bridging oxygen. T. Mallat (E.T.H. Zurich, Switzerland) : What is the lowest temperature at which you observed some interaction between different surface phases ? Is this interaction possible at temperatures below lW0C ? L. Cadus (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): We have not made a study as function of the temperature. The mechanical sample was calcined during two, four and six days at 500OC. After two days, changes in the solid state are already observed (6). Taking into account other results obtained in our laboratory(1, 2) it seems that there is no change below 100°C. Please see answer to question 2 for more details.

V. Cortks Corberan and S. Vic Bellon (Editors), New Developments in Seleclive Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

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ROLE OF TELLURIUM OXIDE IN THE SELECTIVE OXIDATION OF ISOBUTENE TO METHACROLEIN: a-Sb204 TeO2 CATALYSTS

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P. Oelker, L. Cadus', D. Forget, L. DazaZ, C. Papadopoulou3, F. Gil Llambias4, J. NaudS, P. Ruiz and B. Delmon Universitc? Catholique de Louvain, Unit6 de Catalyse et Chimie des Matkriaux DivisCs, Place Croix du Sud 2/17,1348 Louvain-la-Neuve (Belgium) On leave from INTEQUI, Universidad Nacional de San Luis, San Luis, Argentina

* On leave from Instituto de Catalisis y Peaoleoquimica, CSIC, Madrid, Spain On leave from Chemical Department, University of P a m , Greece

Facultad de Ciencias, Departamento de Quimica. Universidad de Santiago de Chile, Chile Labratoire de Gtologie et Mintralogie, Universit6 Catholique de Louvain, Belgium

Antimony and tellurium oxide catalysts were synthesized by (i) mechanically mixing the oxides (ii) coprecipitation and (iii) impregnation of one of the oxides by the metallic element of the other. Some oxides or mixtures were subjected to further thermal treatments. Samples were characterized before and after catalytic test by BET surface area measurement, X-ray diffraction, X P S , C E M and AEM, carbon content and isoelectric point measurements. Catalysts were tested in the oxidation of isobutene to methacrolein. Neither the formation of Sb2Te209 nor contamination explain the synergy observed. The synergy seems to be due to a remote control where Te02 plays the role of a relatively weak oxygen spill-over acceptor. 1. INTRODUCTION

Two or several oxide phases cooperate for enhancing selectivity and/or yield of the desired products (1). Tellurium and antimony are among the elements most often used in industrial catalysts, especially in allylic oxidations and ammoxidations. They enhance selectivity, as indicated by many authors in the course of years (2-4). a-Sb2O4, which is inert, maintains active the surface of other oxides keeping it in a high oxidation state thanks to its ability to form oxygen spillover species, Oso.This Oso migrates to the oxide phase carrying the sites active for selective oxidation. Its role is to create more active sites or to regenerate active sites (1, 5, 6). a-SbO4 is called a Donor and the other phase an Acceptor (Remote Control mechanism). Scales of Donor and Acceptor properties have been established (1,6). All the samples or compound oxides we have investigated can be placed unequivocally on these scales, except tellurium oxide. Tellurium appears sometimes as a Donor, sometimes as promoting Acceptor properties. The present work has been undertaken in this context. Literature concerning tellurium seems to point to a direct participation in the active sites (Acceptor properties). Although reactions between TeOz and Moo3 are possible, an Acceptor role of Te02 is likely in Te02Moo3 catalysts (1). Our first attempt has been to further study these Acceptor properties. For that, we took antimony oxide as a partner for tellurium oxide. Antimony oxide, u - S ~ O ~ , indeed, is the most typical Donor we have found. In addition, it turns out that it does not frequently react with other oxides to make compounds, the catalytic activity of which could blur the picture. In this work, we shall study catalysts composed of a-SbO4 and Te02

56

prepared in different manners (facilitating contamination or formation of a new phase or minimizing interactions) and try to understand the cooperation, if any, between these oxides. In order to be able to detect more easily solid state transformation, catalysts were also submitted to particular treatment under air or under reaction conditions at high temperatures and for a long time. Catalysts were characterized in detail before and after the reaction (or treatment). 2. EXPERIMENTAL

2.1. Sample preparation 2.1 .I. Preparation of pure oxides i) a-Sb204: by precipitation of antimony hydroxide from an SbC13 (5.19 g of 99.5% Aldrich) aaueous solution (100 m l l The solid was filtered. washed (with ammonia), dried at 110°C/i6h'and calcined at 500°C/20h. ii) a-Sb204 (11) by calcination (Sb203 U.C.B. p.a.) at 500°C /20h. i) Te02 was prepared in the same way as a-Sb204 starting from TeC14 (20 g of 99.5% Aldrich). ii)Te02(II) by precipitation by adding ammonia (25%) to an aqueous solution containing 28.71 g of telluric acid. The solid was filtered, washed (with ammonia), dried at 110°C/12h and calcined at 500"C/20h. 2.1 2 . Preparation of mired oxide phases by coprecipitation i) Fresh Te-Sb+3: an aqueous solution containing 14.66 g of telluric acid was mixed with a solution containing 14.55 g of SbC13. Ammonia (25%) was added till a precipitate was obtained (pH 8.5). The solid was filtered, washed (with ammonia), dried at 110°C d12h. ii) Te-Sb+3: part of "fiesh Te-Sb+3" was calcined at 600OC during 20h. 2.1 3.Impregnation i) Sbfle02 : by deposition of an antimony hydroxide gel over 5 g of Te02. The gel was obtained by adding dropwise an antimony butoxide (Sb(OC&Ig)3) solution in n-hexane to an aqueous suspension of Te02. After letting the gel develop overnight, the product was dried at 110°C/12h, calcined at 500°C/20h. Samples containing 0, 1, 5 and 10% (wt) of Sb2O4 were prepared and denoted as OSbD"e02, lSbD"e02, 5SbDe02 and 10Sb/Te02, respectively. ii) Te/a-Sb204 : telluric acid was added to an aqueous suspension containing 5 g of a-Sb204. Tellurium was precipitated by addition of an aqueous solution of ammonia. After resting overnight, the solvent was evaporated and the product dried at 110°C/12h, calcined at 500"C/20h. Samples containing 0, 1 , 5 and 10% (wt) of Te02 were obtained and denoted as OTe/Sb204, lTe/Sb204, 5Te/Sb204 and 10Te/Sb204 respectively. iii) Sb(+3)/Te02 (II) : 3 g of Te02 (11) powder (0.55 m2/g) was immersed in a solution of SbC13 in CHC13 containing the amount of antimony necessary to form about one monolayer of Sb2O4 (1.25 wt% of Sb204). Evaporation of the solvent was performed slowly under reduced pressure. The solid was dried at 110°C/16. iv) Sb(+S)/Te02(II): the same procedure as indicated in (iii) was used; the solution was SbC15 in CHC13. 2.1.4. Mechanical mixture Mechanical mixtures with different compositions were obtained by mixing together powders of samples described above in n-pentane under mechanical agitation and ultrasonic dispersion for 10 minutes; n-pentane was evaporated under continuous agitation at 25°C under reduced pressure. The solid was dried at 80°C overnight. The starting oxides were submitted to the same procedure. The composition of the mixture is expressed by the massratio Rm = mass of oxide phase A/(mass of oxide phase A + oxide phase B). 2.2. Other treatments realized on samples i l ) Fresh Te-Sb+3 sample was calcined in air under one of the following conditions: 230°C for 5h, 420°C for 20h, 500°C for 20h or 40h. Samples are denoted as Te-Sb+3/230/5, Te-Sb+3/420/20, Te-Sb+3/500/20 and Te-Sb+3/500/40. i2) Te-Sb+3 was submitted to a treatment under reaction conditions at 500°C for 20h (Te-Sb+3/reac/500/20). ii) Sb(+3)/Te02

-

57

(11) and Sb(+S)/Te02 (11) samples were calcined in air at 350°C 420°C or 500°C for 20h (denoted as Sb+3/Te02/350/20, Sb+3fI'e02/420/20 and Sb+3/500/20). Similar notation for Sb+S/Te@ (II) samples. iii) a-Sb204 (11) + Te02: the mechanical mixture with Rm4.5 was calcined in air at 500°C during 2 or 4 days. Samples were designated as MM/500/2d, MM/500/4d. A sample was subjected to reaction conditions at 500°C for 20h (MM/reac/500/20). 2.3. Sample characterization i) XRD analyses were realized in a Kristalloflex 805 Siemens diffractometer using CuK a radiation and a nickel filter. ii) BET surface areas were measured with a Micromeritics ASAP 2000 using krypton adsorption at 77K. iii) XPS measurements took place in a Vacuum Generator ESCA-3 MK I1 spectrometer equipped with a Tractor Northern TN 170 accumulator. Binding energies were calculated taking as reference the Cls (284.8 eV) peak of carbon contamination. Surface concentrations were calculated using the normalized intensities measured by XPS and the sensitivity factors given by Wagner (7) and based on the Si2p peak intensity from the external reference Si02 (CTe/CSi, CSb/CSi). iv) Conventional transmission electron microscopy was made in a JEOL Temscan 100 C X equipment with a KEVEX 5 100 C energy dispersive X-ray microanalyzer, which permits analytical electron microscopy measurements (AEM). v) Carbon contents were determined by oxidation, absorption of C 0 2 in a solution of barium perchlorate and titration by coulometry. vi) Isoelecmc point measurements (IEP) were performed in a ZM-77 Zeta meter. Experiments were carried out using 20 mg of about 2 p powder samples suspended in 200 ml of a solution of M KC1 adjusting the pH value with a 10-3 M KOH or HCl solution. 2.4. Catalytic activity measurements The selective oxidation of isobutene to methacrolein was carried out in a conventional fixed bed reactor system. Fractions of the samples, with particle size between 500 and 800 micrometers, were used. For each series of experiments, the weight (800 to 1500 mg, according to cases) were the same. The standard reaction conditions were: partial pressure of oxygen 152 Ton; diluant was helium; total pressure 760 Torr; total feed rate 30 mvmin; reaction temperature was 42OoC, 440°C and 460°C. The reactants and reaction products were analysed by an on line gas chromatograph. 2.5. Synergetic effects Synergetic effect for mechanical mixtures is expressed as the increase of conversion, or yield, compared to the properly weighed average values obtained for the components of the mixture when alone (zero order assumption). For selectivity, the following formula is used : DS = SAB - SA+B, where SAB is the selectivity of the mixture and SA+B is the selectivity which would be observed in the absence of any synergetic effect, defined for a mixture with Rm, as (Rm YA + (l-Rm) YB)/(Rm CA + (l-Rm) CB), in which CA and CB are the isobutene conversion and YA and YB the yield in methacrolein of catalysts A and B respectively.

3. RESULTS 3.1. Catalytic activity measurements 3.1 .I. Pure oxides and their mechanical mixtures (Table I ) At 420"C, there is no synergy in the conversion and only a weak synergy in the yield. Although relatively modest, some synergetic effect is observed on conversion, yield or selectivity at 460°C. Selectivity is maximum for Rm4.75. 3.1 2 . Te-Sb+3 and its mechanical mixtures with pure oxides (Table 2 ) Compared to pure Te02, pure Te-Sb+3 gives lower conversion and yield, but a higher selectivity. When Te-Sb+3 is mixed with Te02, conversion and yield decrease compared to the expected addition of effects due to the components, but selectivity indicates a synergy. When mixed with a-Sb204, there is a synergy in the conversion, in the yield and in the selectivity. In the other series of measurements, at 44OoC, the most significant effect is an

58

important synergy in the selectivity: in both mixtures, selectivities of above 95% were observed. Table 1. Catalytic activity of a-StJ2O4, TeO2 and their mechanical mixtures at 460°C. In parentheses, theoretical values in the absence of synergy Sample

Conversion (%)

Yield (%)

Selectivity(%)

a-Sb204 (Rm=Q.O)

5.6

0.90

15.7

Rm=0.25

8.9 (7.1)

3.8 (2.4)

43.1 (33.8)

Rm = 0.50

11.9 (8.65)

6.4 (3.9)

54.2 (45.1)

Rm=0.75

13.3 (10.2)

8.9 (5.5)

67.3 (53.9)

TeO;! (Rm=1.O)

11.7

7.0

59.8

Table 2. Catalytic activity of Te-Sb+3 and its mechanical mixtures with TeO2 or a - S b O 4 at 460°C. In parentheses theoretical values in the absence of synergy Sample

Conversion (%)

Te-Sb+3

5.5

Yield (%)

Selectivity(a)

5.0

91.0

U-S~QO~ 5.6 Mechanical mixture of Te-Sb+3 9.2 (5.6) and a-Sb20.4(Rm=0.5)

0.90

15.7

5.7 (2.9)

62.0 (51.8)

TeO?

7.O

59.8

4.3 (6.0)

86.0 (69.8)

11.7

Mechanical mixture of Te-Sb+3 5.0 (8.6) and Te02 (Rm=0.5)

Table 3. Catalytic activity results of impregnated S b f I ’ e and Te/a-StJ204catalysts at 460°C Sample

Conversion (a)

Yield (a)

Selectivity(8)

OTe/a-Sb04

7.1

1.8

24.9

1Te/a-StQO4

15.3

5.8

38.8

5Te/a-SbO4

28.8

16.3

56.4

1OTe/a-SbO4

30.7

17.3

56.3

OSb/reCQ

17.9

9.9

55.6

lSb/re@

21.9

13.1

58.6

SSbDeO;!

16.6

8.7

52.5

10SbDa

10.9

5.2

48.6

3.1.3. Impregnated catalysts (Table 3 ) i) Te/a-Sq204: Compared with pure a-SkO4 (OTe/a-SkO4), impregnated samples display higher conversion and yield and are more selective. The performance of the catalysts improves with increase in tellurium content. ii) Sb/TeO2: Compared to pure Te@, lSbfleO;! gives a higher conversion, yield and selectivity, 5Sb/Te@ is nearly unchanged and 10Sb/Te@ shows less conversion and yield and a slightly lower selectivity.

59

3.2. Physico-chemical characterization 3.2.1. Pure Te02 and a-Sb2O4 and their mechanical mixtures The XRD spectra correspond to those of pure Te02 (paratellurite) and pure a-Sb.204 (cervantite) or their superposition. No new phase was detected after catalytic test. The BET surface areas for fresh mechanical mixtures (Table 4) are the sum of those for pure oxides. After reaction, the BET surface area for a-SkO4 remains unchanged, whereas it decreases for pure Te02 and the mechanical mixtures (about 50% for pure Te02).

Table 4. BET specific surface area and X € S surface atomic concenuation ratios of mechanical mixtures of a-SkO4 and T@

Mechanical mixtures

Rm4.25

Rm4.O

Fresh SBET (m2/g,

1.3

CTdCSi

-

CSb/CSi

0.140

Used 1.4

0.180

Fresh

Used

Rm=0.50 Fresh

Used

Rm= 1.O

Rm4.75

Fresh

Fresh

Used

Used

1.5

1.1

1.2

0.8

0.9

0.7

0.8

0.4

0.031

0.044

0.069

0.086 0.052

0.101

0.095

0.120 0.051

0.155

0.091

0.087 0.085

0.081

-

XPS analyses show that the binding energy values of S b 3 d 3 ~and Te3dy2 are identical in all samples, before and after catalytic reaction, and correspond to those of pure aSb2O4 and pure Te02 (540.1 +/- 0.1 and 576.0 +/- 0.2 eV respectively) (8, 9, 10). Results of the surface concentration measurements are shown in Table 4. For pure T e a as well as for the mechanical mixtures, the relative XPS surface CTe/CSi atomic ratio decreases after test. For Rm=0.25 it increases slightly. The XPS CSb/CSi atomic ratio remains unchanged in all cases. CI'EM analyses show that fresh tellurium oxide is constituted of particles with a wide variety of shapes and sizes and that it tends to form aggregates after test. The fresh mechanical mixture with Rm4.50 is composed of distinct particles of pure T e a (about 2000 nm) and pure a-Sb204 (about 500 nm). After test, Teo;! particles tend to form aggregates of larger size (about 6000 nm). There are clear indications of sintering with deformation of particles at their border and at the places of contact. This phenomenon is only observed when the particles of Te02 are not in close contact with a-SbO4. No indication of mutual contamination was detected by AEM. Carbon content measurements do not show any appreciable difference between pure TeO2 and mechanical mixtures (Rm4.5). IEP values for pure a-SkO4 are unchanged after test, while they decrease significantly for pure T e a . For fresh and used mechanical mixtures, the IEP values are slightly lower than those of pure a-Sb204. 3.2.2. Te-Sb+3 and its mechanical mixtures with Te02 (II) and a-SkO4 (II) Pure Te-Sb+3 mixed oxide shows the presence of an unidentified amorphous phase, Te02, a-SbO4 and Sb2Te209. Binding energies correspond to 576.6 (eV) for Te3d5n and 540.1 (eV) for S b 3 d 3 and ~ the CSb/CTe atomic ratio concentration is of 0.89. The fresh mechanical mixtures with TeO2 and a-SkO4 show the same cristalline phases. No new phase is formed after test. T e 3 d 5 ~binding energies are 576.2 (eV) in mixtures of Te-Sb+3 with Te02 and 576.5 (eV) in mixtures with a-SbO4. S b 3 d 3 binding ~ energy is 540.1 in all cases. Binding energies remain unchanged after test. The XPS surface SbRe atomic ratio remains unchanged after test for mixtures with T e a (0.41) and decreases slightly in mixtures with a-Sb-204 (from 3.44 to 3.05).

60

3.23. Impregnated catalysts (Table 5 ) XRD analyses show, in addition to the support, the phase of the deposited oxide (probably S b O 3 in the case of antimony, or T e a ) for weight contents of 5% or more. The catalytic tests do not modify the spectra. Impregnation increases the BET surface area of the 10Te/a-St~04and the whole Sb/Teo;? series, but this decreases after the catalytic test. Table 5. BET specific surface area and X P S surface atomic concentrationratio of impregnated samples Sample :Sb/Teo2

0%

%wt SbO4 fresh

SBET (m2/g) cT&Si

5%

1%

used

fresh

used

0.7

0.5

1.6

0.6

1.5

0.5

0.163

1.1 0.096 0.038

0.5

0.146

0.125

0.064

0.068

0.058

0.109

0.029

0.118

0.028

0.144

0.046

cSb/cSi

fresh

10%

used

fresh

used

Sample :Te/Sb204

0%

% wtT@

SBET (m2/g)

used

1.8

1.7

1.6

1.45

1.8

1.5

4.3

1.4

-

0.033

0.011

0.084

0.027

0.081

0.035

0.129

0.117

0.105

0.107

0.097

0.065

0.098

0.126

fresh

used

fresh

10%

fresh

cT&Si CSbIcSi

5%

1%

used

fresh

used

The XPS binding energies are those observed in the pure oxides, both in the fresh and used catalysts. For Te/a-SbO4 the CTe/CSi surface concentration ratio decreases significantly after test. The CSb/CSi ratio decreases slightly, except for 10Te/Sb204 in which it increases. For SbKe02, CTe/CSi ratio increases while CSb/CSi decreases. In both cases, the signal of the element deposited on the other oxide decreases after the catalytic test. There.is no significant difference in the carbon content of the samples after test.

3.2.4. Catalysts having undergone addirional treatments Tables 6 and 7 summarize the results of characterization concerning samples subjected to additional treatments. 4. DISCUSSION All the results presented above indicate the existence of a synergy between a-SbO4 and phases containing tellurium. However, the existence in one series of samples of a mixed phase, namely Sb2TgO9, apparently stable under the conditions of the catalytic test, and the Occurrence of physico-chemical changes during the test, demand a rather detailed discussion before any conclusion can be reached with respect to the nature of this synergy. Indeed, essentially three explanations could be given a priori, namely: i) formation of a new oxide phase associating Sb and Te, which would be more active and selective than the pure oxides, ii) surface contamination of one oxide by the metallic element contained in the other, iii) cooperation between phases via spillover species, according to a remote control effect. We shall discuss these different possibilities in the following.

4.1. Possible role of phases associating antimony oxide and tellurium oxide The amorphous phase detected in the Te-Sb+3 catalyst is not stable, even at temperatures as low as 230°C, as shown by the increase of crystallinity (section 3.2.4).

61

Sb2Te209 is a better candidate. The selectivity of Te-Sb+3 is much higher than that of any other oxide studied in this work. However, it should be noted that a synergy between TeSb+3 and both a-SkO4 and Te02 appears when selectivity is considered (Table 2). This cannot be attributed to a contamination by antimony of the phases present in Te-Sb+3, the antimony already exists i n large quantities in Te-Sb+3. This does not exclude a contamination, beneficial to catalysis, of a-Sb2O4 by tellurium from Te-Sb+3. This point will be discussed in section 4.2. Without completely excluding this possibility, the discussion will show this is not very likely. Although the explanation by contamination of T e 0 2 by antimony cannot be completely excluded, a more likely interpretation is that the various tellurium containing oxides in Te-Sb+3 benefit from the contact with a - S k O 4 . The fact that the yield also increases, in spite of the loss of surface area that tellurium oxide suffers, could suggest that as a whole the mixture of oxides in Te-Sb+3 possesses some Acceptor properties, in the sense of the remote control. The results of table 2 d o not exclude that they also possess some Donor propemes : this is expected, as Te-Sb+3 contains a-Sb2.04. Although it is not possible to isolate the behavior of S k T e 2 0 9 contained in Te-Sb+3 from that of the companion oxides (a-SkO4 and Te@), it is certain that this phase has no outstanding activity. It ensues that, if present in minute proportions, it could not very much modify the behavior of mixtures in which it would be contained. There are indeed two proofs that SkTe209, if present, only exists in minute quantities. First, it is not detectable by XRD, neither in a-Sb204 + T e e mixtures nor in the impregnated samples after the catalytic test. Second, X P S binding energies of tellurium in these samples (table 6 ) compared to those of Te-Sb+3 (Table 7), could suggest the presence of some Sb2Te209 in calcined Sb(+3)/re02 and Sb(+5)/TeOz, although the difference with pure Te02 is very small (0.3 eV). But the characterization of the impregnated samples seems to rule out the formation of Sb2Te209: although impregnation and the high dispersion of the deposited oxide should strongly favour the formation of the mixed phase, the binding energies observed are then exactly those of pure Te@. even after the catalytic test. All evidence thus makes unlikely that, in our catalysts, the formation of a mixed phase with a measurable impact on catalytic performances could occur. Table 6. SBET, X P S binding energies and XPS atomic ratios of mechanical mixtures of a-Sb204 with T e a and of impregnated samples subjected to alternative treatments (* indicates non mesurable values, peak too low). SBET of a-sbo4(II) = 0.59 m2/g and SBET of T e a (II) = 0.29 m2/g Sample

SBET m2/g

Sb3d3n

Te3d5n

CSdCTe

Fresh MM

0.44

539.8

576.2

O.%

MI500rZd

0.36

539.9

576.3

1.19

MMl6OOl4d

0.3 1

540.1

576.4

0.82

MM/reac/SOOf20

-

539.7

576.2

2.60

Sb(+3)/reO2/350120 Sb(+3)/Teo2/420/20 Sb(+3)/Teo2/500/20

0.29

540.2

576.3

0.094

0.20

8

576.3

0.0056

0.17

*

576.3

0.0075

Sb(+3)I”e@ (11) and Sb(+S)/Te@ (11)

Sb(+5)/reU2/350/20

0.43

540.3

576.3

0.165

Sb(+S)/reo2/420/20

0.25

540.2

576.2

0.M2

Sb(+5)/reo2/500/20

0.17

540.3

576.3

0.023

62

Table 7. SBET, XPS binding energies and X P S atomic ratios of mixed oxide phase prepared by COprecipitation and subjected to alternative treatments. XRD indicates a very amorphous solid which crystallises as function of temperature of calcination, giving S h T e 2 9 , a-Sb204 and T@ for Te-Sb+3/6ooRO Sample

SBET m2/v

Sb3d3n

Te3d5n

cSb/cSi

cTe/cSi cSb/cTe

Te-Sb+3/230/5

-

540.3

577.1

0.14

1.28

Te-Sb+3/420/20 Te-Sb+3/500/20 Te-Sb+3/500/40 Te-Sb+3/600/20 Te-Sb+3/reac1500/20

9.7

540.0

576.5

540.1 540.2 540.1 539.9

576.5 576.6 576.6 576.5

0.31 0.28 0.23

1.03

8.2

0.18 0.32 0.30 0.25 0.16 0.26

0.18 0.12

8.2 0.2 -

1.07

1.08 0.89 2.17

4.2. Possible role of a contamination The impregnated catalysts are those in which surface contamination should be the most intense. Their results are those on which the discussion should rest in the first place. In Te02 impregnated with antimony, the dispersion of antimony oxide is initially high, as indicated by the increase of surface area and the relatively intense signal in XPS (Table 5). But the Sb signal increases less than proportionally to the Sb content and considerably diminishes after the catalytic test. This shows that antimony has little affinity with the surface of T e e and detaches spontaneously. This is confirmed by the behavior of Te-Sb+3 upon treatments. After an initial increase, an overall decrease of the X P S signals is observed (CSb/CSi and CTe/CSi). It shows a segregation of Te and Sb and an overall sintering. This implies that Sb, probably in the form of S k O 4 , gets segregated. The XRD analyses confirm the fact that after calcination, a-Sb204 and T e e segregate. In addition, under reaction conditions, the S b r e surface ratio increases considerably. The fact that, under the reaction conditions, the c S b / c S i ratio is higher compared to the cTe/cSi ratio is explained by the significant sintering of TeO2 and not by a contamination of Teo;! by antimony. The same increase in the SbDe surface ratio is observed in a-SkO4 + T e e mechanical mixtures under reaction conditions. Rather than a hypothetical contamination of Te02 by Sb, this rather shows that Sb has some tendency to detach from Te02 if forced to spread on it. The intervention of a contamination of Sb on Te02 in working catalysts is thus excluded. The situation is less clear with respect to a possible contamination of a-SkO4 by tellurium. The surface areas of the samples with a low tellurium content (lTe/Sb204 and 5Te/Sbz04) are almost identical to those of a-StQO4 (Table 5). Tellurium oxide seems to spread evenly on the surface of a-SbO4. But the decrease of CTeICSi and the increase of CSbICSi at high Te contents indicates a counteractive tendency of tellurium to crystallise and to leave free part of the surface of a-SkO4. This shows that, even if we cannot completely exclude some contamination of a-SbO4 by Te, this does not seem to be very stable, and therefore, should only play a minor role. 4.3. Cooperation between phases according to the remote control concept Although minor contributions due to a mixed Sb-Te-0 phase or a probably unstable contaminating layer of Te on a-SbO4 cannot be excluded, the major effect seems to come from an interaction between essentially pure phases, a-StQO4 and TeO2. This is directly apparent when mixtures of a-SkO4 + Teo;! are considered. This is confirmed by the good performances of T e e impregnated by Sb, where small antimony oxides form spontaneously. This is also confirmed by the good performances of TeO2-impregnated a-SbO4 (Table 3). This last set of results can even be analysed further. The amount of Te02 necessary to form a monolayer on the surface of a-Sb2O4 can be estimated on the basis of the BET surface area of a-SbO4 (1.8 m2/g) and the unit cell of tetragond T e a (0.1 156 nm2, calculated with radius of tellurium and oxygen of 0.2 nm and

63

0.14 nm respectively (8)). This amount corresponds to about 4x10-3 g of TeQ by gram of a-SbO4, namely 0.4% in weight of T e e . In a-SkO4 impregnated by tellurium catalysts, the conversion and the yield increase proportionally to the tellurium content and beyond the minimal amount necessary to form the monolayer. This indicates that the formation of crystallites, probably of Te02, is fundamental to explain the catalytic results. Sample 10Te/Sb~O4containing 150 mg of Te02 gives a conversion of 30% and a yield of 17%, which are significantly higher than for all mechanical mixtures containing more T e a . These results could be explained by the presence of a high number of crystallites of Te02 formed on the surface and the better contacts between the crystallites of Te02 and the support. Generally, good performances are observed with impregnated catalysts in which the impregnated phase has formed minute crystallites in good contact with the suppomng oxide (e.g., 1SbDeO2, lOTe/a-Sb04, Table 3). This is an effect that we have already observed (1 1, 12, 13). Although this does not alone demonstrate the Occurrence of a remote control, this is what the remote control theory would predict.

4.4. Speculation on the role of Tellurium in multiphase catalysts for the oxidation of isobutene If we accept the conclusion that a remote control explains the largest part of the synergy observed, we can attempt to give an answer to the question asked at the beginning, namely the possible role of Te@. The first remark is that the effects observed when TeO2 is in contact with a-SkO4 are relatively modest. Thus, Te02 is an Acceptor but does not seem to be a strong one. This does not decide whether it might also be a Donor: special experiments with mixtures of Te02 with, instead of a Donor as in the present work, an Acceptor are necessary for testing this hypothesis. Changes in the selectivity are explained by the creation of new selective sites. The exact reason for the creation of these sites is outside of the aim of the present study, hovewer the mechanism by which they are created is probably one of those discussed in (1,6, 10, 14, 15). Although this is still speculative, the present results together with the ambiguous results concerning the role of T e e obtained in our previous experiments (1) lead us to make the hypothesis that the Te02 phase is indeed a Donor, but rather weak. The fact that it seems to behave as an Acceptor in certain cases should then be linked to the formation of binary or other compound oxides by reaction of Te02 with other elements contained in the catalysts. The real Acceptors would be these new compounds (tellurates for example).

ACKNOWLEGMENT The support of the Region Wallonne in the frame of an Action ConcertCe is gratefully acknowledged. The stay of L.C. in Louvain-la-Neuve was made possible thanks to the financial support of CONICET and Univ. Nac. de San Luis, Argentine. L.D. benefitted from a fellowship of the European Community. Both institutions are gratefully acknowledged. REFERENCES 1. L.T. Weng and B. Delmon, Appl. Catal. A. 81, 141 (1992). 2. V.M. Zhiznevskii, E.V. Fedevich, O.M. Pikulyk, V.Ya. Shipailo, D.K. Tolopko, Kinet. Katal. 13 (6), 1488 (1972). 3. B. Grzybowska, A. Mazurkiewicz, J. Sloczynski, Appl. Catal. 13,223 (1985). 4. R.K. Grasselli, G. Centi, F. Trifiio, Appl. Catal. vol. 57, 149 (1990). 5. L.T. Weng, D. Duprez, P. Ruiz and B. Delmon, I. Mol. Catal. 52, 349 (1989). 6. L.T. Weng, P. Ruiz and B. Delmon, Studies in Surface Science and Catalysis "New Developments in Selective Oxidation by Heterogeneous Catalysis" (P. Ruiz and B. Delmon, eds.), Elsevier, vol. 72, pp. 399-413. 7. C.D. Wagner, L.E. Davis, M.V. Zeller, I.A. Taylor, R.H. Raymond and L.H. Gale, Surf. Interface Anal. 3,21 (1981). 8. J.C.J. Bart,G. Pemni, N. Giordano, Z. Anorg. Allg.Chem. 412,258 (1975). 9. A.J. Ricco, H.S. White, M.S. Wrighton, J. Vac. Sci. Technol. A 2,910 (1984).

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10. L.T. Weng, N. Spitaels, B. Yasse, J. Ladrike, P. Ruiz and B.Delmon, J. Catal. 132, 319 (1991). 11. L.T. Weng, B. Zhou, B. Yasse, B. Doumain, P. Ruiz and B. Delmon, 9th ICC, Calgary, Canada, vol. 4,1609 (1988). 12. L.T. Weng, B. Yasse, J. Ladritre, P. Ruiz and B. Delmon, J. Catal. 132, 343 (1991). 13. Y.L. Xiong, L.T. Weng, B. Zhou, B. Yasse, L. Daza, F. Gil-Llambias, P. Ruiz and B. Delmon in: Preparation of Catalysts V (G. Poncelet, ed.), Stud. Surf. Sci. Catal., vol. 63, Elsevier, Amsterdam, 1991, p. 537. 14. L.E. Cadus, Y.L. Xiong, F.J. Gotor, D. Acosta, J. Naud ,P. Ruiz and B. Delmon. These Proceedings. 15. R. Castillo, P.A. Awasarkar, Ch. Papadopoulou, D. Acosta and P. Ruiz . These Proceedings.

ANSWER TO QUESTIONS

M. Baerns (Rhur University, Bochum, Germany): Could traces of your compound (cation) migrate into the lattice of the other compound forming a solid solution and hereby changing the electronic properties of the catalytic material?

P. Oelker (UniversitCCatholique de Louvain, Louvain-la-Neuve, Belgium): We do not exclude at all the possibility of contamination. For example, it is clear in our results that the mixed Te-Sb+3 oxide works synergetically with u - S ~ O As ~ . TeSb+3 is already contaminated, it has no meaning to explain this synergy by further contamination. In the other cases, we leave the question open. G.L. Shrader (Iowa State University, Ames, Iowa, U.S.A.): Is it possible for your multiphase catalysts to relate the interaction (or absence of interaction) to a physical chemistry property of the phase such as surface tension (energy), solid state reaction (compound formation), etc ... ? P. Oelker (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): Indeed, this is possible. The formation of the interaction between two phases in contact is governed by thermodynamics. Surface free energy is directly related to surface tension. When the energy of cohesion of one of the oxides is smaller than the energy of its adhesion to surfaces of other oxides, it will spontaneously migrate over these surfaces (namely with the other oxide). For formation of solid solution it is necessary that the solute atoms (or ions) be comparable in size with those of the host lattice. In addition, temperature must be. high enough to permit that the self-diffusion of the atoms takes place and a proper charge balance must be preserved. The formation of interactions could be related to conductivity and magnetic properties of the solids, but it is very difficult to isolate these properties from the bulk. Surface properties are very sensitive to the formation of small amounts of new structures on the surface. (Journal of Catalysis 95,520-526 (1985)).

B. Grzybowska (Institute of Catalysis, Krakow, Poland): Which is in your opinion the oxygen form (species) which is involved in the oxygen spill-over, postulated in your theory of "remote control" in selective oxidation on mixed oxide systems? Bearing in mind high value of stabilisation energy of 0 2 - surface ions in the oxide lattice, it is difficult to envisage the migration of this species. On the other hand, 0- species seem quite mobile. What kind of experiment would, in your opinion, decide between various oxygen species involved in the oxygen spill-over?

65

P.Oelker (UniversitC Catholique de Louvain, Louvain-la-Neuve, Belgium): Activation of molecular oxygen on the surface of an oxide catalyst may lead to the formation of different anionic oxygen species, namely 0 2 - and 0- which are electrophilic and 02- which is nucleophilic. The presence of each one of these species on the surface of some oxides has been demonstrated, the type of anion adsorbed depending on the temperature (1). For example, 02-is present on the surface of Moo3 supported catalyst at temperatures above 450°C. Besides, i t has been suggested (2) that surface 0 2 - ions may act as nucleophilic ligands entering the molecules of the reagent during the catalytic oxidation process. These elements suggest that the beneficial oxygen spill-over responsible for the improved catalytic performances is the 0 2 - anion. This view is nicely supported by effects based on the NEMCA phenomenon (NonFaradaic Electrochemical Modification of Catalytic Activity) (5). Solid electrolyte cells are of the type: gaseous reactants, metal catalyst/Zr02/Metal, 0 2 . The metal catalyses the reaction 0 2 + 4e-= 202- and serves as a means of supplying or removing @-. It was found that the catalytic activity and selectivity of some catalysts can be altered dramatically and reversibly by supplying or removing 0 2 - produced by these electrolyte cells. This proves that this 02species behaves identically to spillover oxygen, and strongly suggests it might be of the same chemical nature. Other experiments that would help decide which kind of oxygen species is involved in the oxygen spill-over could be based on surface potential measurements (your work, (3)) and EPR (4). It seems that the 0 2 - species is predominant on the surface of the oxides used at the temperature where the catalytic reaction occurs. References 1. Y. Barbaux, A. Elamrani and J.P. Bonnelle, Catal. Today 1, 147 (1987). 2. A. Bielanski and J. Haber in: "Oxygen in Catalysis", Dekker eds. (1991). 3. J.M. Libre, Y. Barbaux, B. Grzybowska and J.P. Bonnelle, React. Kinet. Catal. Lett. 30,249 (1982). 4. M. Che and A.J. Tench, Adv. Catal. 31,77 (1982). 5. C.G.Vayenas. Solid State Ionics 28-30, 1521 (1988). R.K. Grasselli (Mobil Research & Development, New Jersey, U.S.A.) : You have certainly chosen to study a complicated and rather unstable system under catalytic conditions. I congratulate you on tackling such a difficult problem. We ran into similar difficulties with FeSe-Tellurates (1). May I offer a possible explanation for the synergistic effect which you are observing. We both agree, I hope, that the Te02-Sb204 system is a dynamic system continuously restructuring under reaction (redox) conditions. I am not surprised that T e e "detaches", as you call it, from a-ShO4, or that you see only scant evidence of Sb2Te209 because Tek oxides are rather volatile under reaction conditions and will continue to leave the catalyst's surface and out of the reactor. The observed synergism is probably due to surface incorporation of some Te into the a-ShO4. And this Te is continously being incorporated and again lost from the a-Sb204 phase under reaction conditions. That is why you need a pool of spectator Te02 (or better said TeO,) in addition to the a-SbOq to experience the observed catalytic synergism. In my opinion, the, Te4+-O is a better a-H abstracting species than Sb3+0, and the Te4+/6+ helps to push more of the antimony into the S@+ state, with Sbs+-O being the oxygen inserting species of the catalyst. Thus, I think the Te4+-0-Sb5+-0- moiety which exists on the surface of Te doped a - S t ~ O 4is a superior catalyst to either Sb3+-0-Sb5+-0 of aSb2O4 or Te4+-O-Te6f-0 of TeO,. An alternative possibility would be that the improved catalyst is a result of the function of Sb3+-0-Te6+-0moieties (i.e. Sd+-0,a-H- abstracting, and Te6+-0 oxygen inserting). But, I believe the first suggestion makes more sense based on inorganic solid state chemistry. Reference : 1. R.K. Grasselli, J.F. Brazdil, J.D. Bumngton, Appl. Catal. 25, 335 (1986).

66

P. Oelker. We cannot formally exclude your mechanism, although it seems complicated and involves two steps of exactly opposite free energy changes. This is not easily compatible with thermodynamic laws. We have no experimental evidence of such a mechanism. If there was really some volatilisation and condensation of tellurium on U - S ~ Othe ~ ,selectivity should increase as fonction of reaction time, which is contrary to our observations. If volatilisation takes place, some indication of condensation of Te02 should be observed outside of the reactor. This is not the case, even after the long test reaction 5OOoC/2Oh (normalIy the reaction temperature used in this study was 46OOC). In addition, please note that mixed oxides calcined at 500 OC during 20 and 40 h, show exactly the same SBET and X P S surface ratio (CSb/CSi, CTe/CSi, CSb/CTe) (Table 7). This is hardly compatible with evaporation condensation processes. On the other hand, contrary to your suggestion, under our experimental conditions, a-Sb2O4 is only negligibly active. It is very difficult to assign the role of a - H abstracting or oxygen inserting to species Sb+3 or Sb+5. There is no proof of such an effect in literature, in spite of the many studies devoted to a-Sb;?O4. It seems that volatilization occurs when Te is reduced, namely with bad catalysts. Oxygen spill-over avoids reduction as well as volatilization. CTEM analysis seems to support these hypothesis but it needs more confirmation.

V. CortCs Corberan and S. Vic Bell6n (Editors), New Developments in Selective Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

67

Pro ylene Selective Oxidation as Studied by Oxygen-18 Labelling on WellDetned Moo3 Catalysts M. Abona, M. Roulleta, J. Massirdies, P. Delich&r$ and A. Guerrero-Ruizb aInstitut de Recherches sur la Catalyse, CNRS - 69626 Villeurbanne CMex France bDepartamento de Quimica Inorghica, Facultad de Ciencias, UNED, 28040 Madrid, Spain

ABSTRACT The propylene oxidation reaction has been studied using 1 8 0 on well defined Moo3 crystallites. Results show that the formation of products -acro?ein and carbon dioxideinvolves the lattice oxygen. The different 180-labelling, according to selective or total oxidation products, agrees with the structure sensitivity of this reaction. Post reaction analysis (by LRS, SIMS, LEIS) support a redox equilibrium which determines the 180 concentration in the surface layers. 1. INTRODUCTION

We have previously studied the structure-sensitivecharacter of the oxidation of propylene (1-4) on Moo3 crystallites. It has been shown that mild oxidation to acrolein occurs on (120) faces, i.e. the stepped-like lateral faces, whereas total oxidation to CO takes place on the basal (010) faces. This approach has been possible thanks to the p r e p i o n of [lo01 oriented MOO plates with crystallites characterized by a more important development of the exposed laiiral faces (3). The activation of dioxygen and the nature of oxygen species (lattice oxygen or adsorbed oxygen species) involved in the formation of products are important steps of the mechanism. In the present work, these processes have been investigated using 180 The l 8 0 labelling has been measured for the main products of the reaction: acrolein andEO This labelling has been also studied on the catalysts using Laser Raman Spectroscopy (LR$ and surface sensitive techniques: Secondary Ion Mass Spectroscopy (SIMS) and Low Energy Ion Scattering (LEIS). The relevance of oxygen-labelling techniques to study the propylene oxidation mechanism has been previously established by Keulks et al. (5-7) and other studies (8-11) on bismuth molybdate catalysts.

2. EXPERIMENTAL The propylene oxidation using 1802 (Eurisotop, purity = 98%) has been perform 420°C in a closed circulating reactor (volume: 158 cm3) in order to reduce the 1 0at 2 consumption. The gas phase was analysed with an on-line mass spectrometer connected to the reactor through a metering leak valve. In blank experiments, it has been checked that no reaction occurs in our experimental conditions, discarding then the homogeneous oxidation reaction. The initid gas phase composition was usually l80 /C3H6/He: 100/38/622, at atmospheric pressure. The reaction mixture was circulated for30 minutes prior to reaction to ensure mixing. The Moo3 catalysts were first pretreated at 450°C under oxygen ('602) for an hour. Oxygen was then evacuated after cooling down to room temperature. A dry ice/acetone cold trap (-77OC) placed just after the reactor in the circulating loop removed all condensible products, that is mainly acrolein and water. The presence of this cold trap

-3

68

was necessary because, in preliminary experiments, it was observed that the rate of acrolein oxidation was about 3 times higher than the rate of mild oxidation of propylene. The analysis of the condensible products required to warm up the trap, the reactor being then by-passed. The circulation ensured a VVH of 360 OOO h - l for a mass of l . l g of catalyst. The high VVH and the presence of a cold trap allowed to discard the secondary reactions such as the formation of C02 by acrolein oxidation. It was also checked that no exchange between 1802 and 160-lattice oxygen occured under our experimental conditions. Such an exchange was only evidenced at much higher temperatures, near 600°C.Carbon dioxide (9, 12, 13) and even more water (8,14) are known to favour the oxygen isotopic scrambling on oxides but this phenomenon must be reduced since H20 is eliminated by trapping. Regarding the influence of C02, it was observed that the relative amounts of labelled and unlabelled C02 (amu 48, 46, 44) remained nearly constant for several hours at the end of the raction, suggesting that the isotopic scrambling can be neglected. The two types of [OlO] and [loo] oriented MOO catalysts compared in the present work have been previously characterized by XRD, S E d HREM, XPS analysis (3). Their main physical properties are recalled in Table I. Besides a different proportion of exposed faces, these catalysts allowed a preferential inspection, by surface sensitive techniques, of either the (010) faces or the stepped lateral (100) faces, i.e. the (120) faces (3, 4). Table 1 Physical characterization of the Moo3 catalysts A(BET) (m2.g-1)

Mean size of crystallites Olm)

Exposed (120) faces (96)

Exposed (010) faces ( W )

~~

M&3[W

0.5

6x2~6

20

60

Moo3[0101

0.05

350x25~1000

7

90

3. RESULTS

3.1. Study of the 180-labelling of products In the static circulation reactor at 420°C, with a dry ice cold trap, the main products are acrolein and C02 in agreement with previous studies in a differential microreactor. After calibration of the mass spectrometer, the selectivity pattern is also comparable to that previously measured (4). The Moo3 [l,OO] catalysts gives an amount of acrolein twice larger than C02 whereas the reverse ratio of selectivity is measured for the Moo3 [OlO] catalyst. In the presence of 1802, the percentage of 180 in acrolein (a ) and in C02 (a ) can be compared as a function of the time of reaction for Moo3 [l& in Fig.1 and &r Moo3 [OlO] in Fig.2. a is defined by the following equations: I(58)

a*( %) =

I(56)

+ I(58)

x 100

+ I(46) x loo 2[1(48) + I(46) + I(44)] 21(48)

a@)=

with I(x): intensity of the amu x peak measured by mass spectrometry (amu: atomic mass unit). As shown in Fig.1 and 2, aAand a first increase with time b t then come to a nearly on the surface of the constant value suggesting a quasi-equil%rium between 180 and

1$

69

ca yst C160 mu 44) is mainly detected at the beginning of the reaction but in eed C g O l i O and 61@02(amu 46 and 48 respectively) rapidly grow with time and C1 02 becomes then prevalent. Note that ac is always higher than aA. Moreover about the same aA is obtained on both Mo03 samples, the e remark is also valid for ac. These results correspond to the same relative pressures ofyP0 (100 Ton) and propylene (38 Ton). For an initial mixture with l802/C Hg: 11 Torr/4 t o n , aA = 40 and a - 50 have been obtained. On the other hand, wdh a much larger pressure of reac %62/C&H6 = 200 Torr/76 Ton, we have measured a - 70 and a - 85. The 1 0 iabelling en clearly depends on the initial pressure o f reactants an2 more generally on the experimental conditions. For mixtures richer in propylene with 180 /C3H6 = 100 Torr/77 Ton, QA and ac first increase with time as already observed but Aen reach a maximum and decrease as shown in Fig.3.

$

vb

Figure 1. l 8 0 labelling of the products vs reaction time on Moo3[ 100](1802/C3H6: 100/38)

5

10

15 time (h)

Figure 2. 180 labelling of the products vs reaction time on Md3[010](1802/C3H6: 100138)

20

40

60

time ( h )

Figure 3. 180 labelling of the products vs reaction time on M003[010](1802/C3Hg: 100/77)

70

3.2. Study of the

lb labelling of M a 3 catalysts

3.2.1. Post-reaction analysis by LRS Changes in LRS spectra have been recorded only when the Mo03 crystallites are first finely ground. Such a treatment indeed increases the ratio surfacidvolume of the samples characterized by a very small surface yea (Table I). Fig. 4 co pares the LRS spectra obtained after propylene oxidation with 6 0 2 (Fig. 4a) and with IF02 (Fig. 4b). The three main bands characteristics of MOO (15) have been observed in both cases at 996 cm-1 (Mo = 0), 820 cm-1 (Mo-0-Mo) and 666 cm-1 (three-bonded oxygen). After reaction with 1802 (Fig. 4b), two additional features have been recorded at lower frequencies as expected (16): a weak band at 948 cm-1 attributed to Mo = 180 and a shoulder in the Even after a careful shape analysis, no change medium band near 790 cm-l (Mo-~~O-MO). be detected in the third band at 666 cm-1. The relative intensity of the additional Mo = 'i"gb band with respect to the Mo = l60 band is less than 1%. The same ratio is found for the band near 790 cm-1 with respect to the band at 820 cm-1. 3.2.2. Post-reaction analysis by S M S One of us has performed a SIMS analysis on the Moo3 catalysts after reaction with 1802 (17). The more significant result is reported here in Fig. 5 showing the ratio (180/160) as a function of the sputtering depth on MoO [OlO] and Mo03 [ I F ] . This ratio has been a uall measured by the r lative in ensity C?I the secondary cluster ions [100M0180]+ and f3M0160]+ typical of l%O and 6O content, respectively. It may be observed that the 8 0 amount is larger on the (010) faces than on the (120 faces mainly exposed on the surface of [ 1001 oriented MOO? crystallites. Moreover the 1 0 concentration first decreases with depth down to a nearly co"nt&t value.

i

d

I

h

-.xu -

lo00

600 wave number (cm

Figure 4. LRS spectra on grounded MOO (a) after reaction with 1602, (b) ader reaction with 1802.

Sputtering Depth (am.)

- 3

')

Figure 5. 180/160 ratio measured by SIMS after reaction on (a) Mo03[010], (b) Mo03[100l

71

3.2.3. Post reaction LEIS analysis

This technique is the most sensitive one to the topmost surface lay r md the experimental conditions -4He+ (1 kev) with low current density (3O.SpA.rnm-?)- strongly reduce the sputtering damage. Post-reaction LEIS spectra are shown in Fig.6 for MoO [loo] and in Fig.7 for Mo03 [OlO]. In Fig. 6a and 7a, the analysis was directly performd whereas in Fig. 6b and 7b the spectra were recorded after heating the sam les at about is clearly 400°C for 5 minutes under ultra-high vacuum (UHV). The presence of detected n the urface of both samples with a ratio 180/l60 = 3/2 on MOO [lo01 (F. 6a) and psO/Of6 = 2/3 on Moo3 [OlO] (Fig. 7a). After heating under U aV , the 18gd signal vanished on Mo03 [loo] (Fig. 6b) and is sharply reduced on Mo03 [OIO] (Fig.%).

Ib

4. DISCUSSION

As shown in Fig. 1 and Fig.2, the oxygen-18 labelling of acrolein is very similar on Moo3 [loo] and Moo3 [OlO]. The same behaviour is observed for the C02 labelling a . This observation is in agreement with the structure-sensitivityof C H6 oxidation on M d 3 (1, 2, 4), phenomenon further confirmed in the present work: (12Offaces active in selective oxidation and (010) faces active in combustion are both exposed on the two samples. Fig. 1 and Fig. 2 also point to the participation of surface lattice oxygen (16&-) to the formation of both acrolein and carbon dioxide. Indeed aA and 01 increase in the course of the run, especially in the be inning, but never reach lOO%.%ote that the reaction conditions strongly favour the labelling of the products owing to the very small surface area of the samples. Takin as usual 10x10-6 mol. of 1602/m2, one could only obtain a few percents of C3H41 0 (less than 3%) from the initial surface la tice oxygen in the best case, that is on the catalyst with the larger surface area (0.5 m2! /g) and assuming the sole formation of acrolein. Note also th t the 180-labelling is significantly lower (aA = 40, a = 50) for low initial pressure of * 180 /C3H6 = 11 Torr/4 Ton. The pmcipation lattice oxygen is also evidenced in Ag.3 wiere a decreases with time when the reactant mixture becomes poor in 180 . The participation of lattice oxygen has been already well established on bismuth m o l y d t e (5-11) oxides at least for acrolein formation whereas the que ti n remained debated for C02 production. In this respect the formation of C1602 and c1&?80 is informative. The nearly constant a value reached by the 18 labelling of products would be a kind of fingerprints of the quasi-equilibrium of the 180/ 6 0 ratio on the catalyst surface. The fact that 'YA< ac strongly suggests that the 180 concentration is lower on (120) faces active in acrolein formation. This seems to be in line with the fact that the combustion reaction requires more surface oxygens that the selective reaction to acrolein. The nearly constant a values further suggest a quasi equilibrium in agreement with a redox mechanism as also evidenced by a post-reaction XPS analysis showing Mov species (Fig.8). The replenishment of surface oxygen vacancies created by the formation of products would occur via two different sources:

A0

8

5

1$

P

- 1802 activation leading to surface lattice oxygen 1802- with a possible diffusion from the surface towards the bulk. - Bulk lattice oxygen 1602- diffusing towards the surface, thus explaining that the 180 labelling do not quickly reach about 100%. Owing to the lamellar crystal structure, the diffusion of oxygen ions could well be anisotropic, favoured in directions normal to the [OlO] axes and therefore towards the (120) faces. Such an equilibrium will indeed depend on the experimental conditions such as the relative 1802 pressure as illustrated in Fig.3. Furthermore a has been shown to depend on the absolute pressure of reactants as already reported. We will now discuss the post-reaction analysis of the catalysts. The additional LRS ands associated with lattice 180 are so weak that it can be infered that the amount of 1 0 is

k

72

-

-

250-

_ _ _ _ _ _ _ ~ _ _ _ _7

---

-

--

150

340

. -~~ ~

400

520

460

340

400

E(eW

E(eW

Figure 6. LEIS anal sis on MoO-[lOO

400°C under UHV J60 near 4&V,

520

460

(a) after reaction, (b) after reaction and heating at

J80 near 45OeV).

140JK A -

,

-

.

' 7 7' 3 =m 250:

i

180*

I

l00C

150t

-

60

50

20

340

400

460

E(eV)

520

340

400

520

460 E(eW

Figure 7. LEIS analysis on Mo03[010], (a) after reaction, @) after reaction and heating at 400°C under UHV.

238

236

234

232

Binding Energy / ev

230

238

236 234 232 Binding Energy / cV

230

Figure 8. Post reaction XPS analysis of the Mo 3d levels on (a) Mo03[010], (b) Mo03[100]. The appearance of an additional doublet attributed to Mov is indicated by arrows.

73

small and restricted to a few surface layers, in agreement with SIMS analysis. However, it is interesting to note that only terminal and bridging oxygens appear to be involved in the formation of products. In bismuth molybdate catalysts, the preferential insertion of terminal (18, 19) or bridging oxygens (7) in acrolein is still debated. The SIMS experiments support the conclusion that 180 is present on the surface after reaction. $" amount of 180 would be higher on the basal (010) face 'n agreement with the isotopic 0 labelling of C 0 2 9 alr dy discussed. The decreasing I d 0 concentration with depth agrees with an anisotropic diffusion determined by the lamellar structure of MOO . L d S experiments clearly show th presence of 180 on both post reaction samples. However, the relative concentration 180,160 appears to be higher on MOO [lo01 than on Moo3 [OlO], at least prior to heating at 4o0"t under UHV conditions. Af?er this thermal treatment at the temperature of reaction, 1 0 is only detected on Mo03 [OlO]. As hydrocarbon residues including oxygen specie have been detected by XPS analysis of the C level, the sharp decrease of the surface 1$ signal after heating can be attributed very Ii&y to a combustion surface reaction. At a less extent other phenomenons can be invoked: dehydroxylation, desorption of weakly bonded adsorbed oxygens and anisotropic diffusion towards the bulk. The conflicting results gained by LEIS with respect SIMS concerning the 180/160 ratio show that comparative quantitative measurements by these surface sensitive techniques are difficult. Note that owing to their small size (table I), the analysis on [lo01 oriented crystallites is not defined as well as on a large (010) face of a single crystal. According to the precise geometry of the experiment, the analysis will include more or less various crystal faces in addition with the expected (120) faces. Other reasons inherent to the techniques can also be invoked: sputtering and definition of the initial surface state (mainly in SIMS experiments), depth of analysis and shadowing effects depending on the crystal orientation, different level of surface contamination by carbon species.. . 5. CONCLUSIONS

We have investigated the 1 8 0 activation and its insertion into the products (acrolein and CO in the oxidation reaction 0% propylene on [lo01 and [OlO] oriented Mo03 crystallites. The 80 labelling is in agreement with the structure sensitivity of this reaction confirmed in the present work: the labelling of acrolein and C02, respectively, is nearly the same on the two Moo3 catalysts as expected if each product is formed on a specific crystal face, (120) for acrolein and (010) for C02 (1-4). The participation of lattice o x y p in both reaction products has been clearly evidenced: preferential formation of C3H4 0 and C1602 in the beginning of the run and further establishment of a quasi constant 180 labelling. This quasi equilibrium implies a redox mechanism with replenishment of oxygen surface vacancies by two sources: activation of gaseous 1802 and diffusion of bulk lattice 160 ions towards the surface. Moreover the Raman spectra suggest that the active oxygens would be both terminal and bridging oxygens. As terminal Mo=O groups are typical of the basal (010) face, it can be tentatively proposed that such oxygens are active for total oxidation. About the question relative to a preferential oxygen activation according to the crystal faces, the higher 180 labelling for CO compared to that of acrolein supports that gaseous oxygen is preferent'ally activated on (OfO) faces in agreement with SIMS analysis on Moo3 samples. Surface 180 has been also clearly evidenced by LEIS on post reaction catalysts. The sharp decrease of the 180 signal after heating has been explained by a surface reaction with carbonaceous residues detected by XPS. Thanks to the uses of 1802 combined with complementary analysis of the products labelling and of the 1 8 0 concentration on the surface of well defined Moo3 crystallites, it has been possible to get a significant insight into important steps of the propylene oxidation reaction.

'1

74

REFERENCES 1. M. Abon, B. Mingot, J. Massardier and J.C. Volta, in "New Developments in Selective Oxidation", G. Centi and F. Trifiro'(Eds), Elsevier (Amsterdam) (1990) 747. 2. M. Abon, B. Mingot, J. Massardier and J.C. Volta, in "Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis", R.K. Grasselli and A. W. Sleight (Eds), Elsevier (Amsterdam) (1991) 67. 3. B. Mingot, N. Floquet, 0. Bertrand, M. Treilleux, J.J. Heizmann, J. Massardier and M. Abon, J. Catal. 118 (1989) 424. 4. M. Abon, J. Massardier, B. Mingot, J.C. Volta, N. Floquet and 0. Bertrand, J. Catal. 134 (1992) 542. 5. E. Hoefs, J. Monnier, G. Keulks, J. Catal. 57 (1979) 331. 6. D. Krenzke and G. Keulks, J. Catal. 61 (1980) 316. 7. G.W. Keulks and T. Matsuzaki, in "Adsorption and Catalysis on Oxide Surfaces" M. Che and G.C. Bond (Eds) (1985) 297. 8. R. Wragg, P. Ashmore anti J. Hockey, J. Catal. 22 (1941) 49. 9. K. Sancier. P. Wentrek and H. Wise, J. Catal. 39 (1975) 141. 10. L. Glaezer, J. Brazdil, M. Hazle, M. Mehicic i d R.K. Grasselli, J. Chem. Soc., Faraday Trans.I, 81 (1985) 2903. 11. T. Ono, T. Nakajo and T. Hironaka, J. Chem. Soc.,Faraday Trans. I, 86 (1990) 4077. 12. I. Brown and W.R. Patterson, J. Chem. Soc.,Faraday Trans. I, 79 (1983) 1431. 13. W.R. Patterson, J. Mol. Catal. 65 (1991) LA1. 14. J. Novakowa and P. Jiru, J. Catal. 27 (1972) 155. 15. I.R. Beattie and T.R. Gilson, J. Chem. SOC. A (1969) 2322. 16.U.S. Ozkan, M.R. Smith and S.A. Driscoll, in "New Developments in Selective Oxidation by Heterogeneous Catalysis", P. Ruiz and B. Delmon (Eds), Elsevier (Amsterdam) (1992) 363. 17. A. Guerrero-Ruiz, J.M. Blanco, M. Aguilar, I. Rodriguez-Ramos and J.L.G. Fierro, J. Catal. 137, (1992) 429. 18. F. Trifir6 and I. Pasquon, J. Catal. 12 (1968) 412. 19. J.D. Bumngton, C.T. Kartisek and R.K. Grasselli, J. Catal. 63 (1980) 235.

ACKNOWLEDGMENTS The authors are indebted to Prof. R. Olier (ECL) for the Raman study of the catalysts. They also thank Drs. J.C. Volta and J.C. Varine and Mr. V. Ducarme for fruitful discussions.

V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation I /

0 1994 Elsevier Science B.V. All rights reserved.

75

Selective Hydrocarbon Oxidation at Vanadium Pentoxide Surfaces: Ab lnitio Cluster Model Studies Malgorzata Witkoa *

and Klaus Hermanna

a Fritz-Haber-Institutder MPG, Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany bInstitute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek, 30239 Cracow, Poland

1.

INTRODUCTION

Vanadium oxide based materials are widely used as catalysts in the s e lective oxidation of hydrocarbons (aliphatic and aromatic) proceeding according to both electrophilic and nucleophilic mechanisms (see [1,2] and references therein). The electrophilic oxidation starts with a preliminary activation of the oxygen species resulting in activated electrophilic forms of oxygen (singlet 02, 02-, 0 - ) which attack organic molecules in the region of highest electron density. The reaction leads to either peroxy and epoxy complexes, which can play the role of precursors of organic oxides, or to intermediates which, under conditions of heterogeneous catalytic reactions, undergo degradation of their carbon skeletons. The nucleophilic oxidation begins with a preliminary activation of the hydrocarbon molecule [ 3 ] in the presence of the nucleophilic oxygen species ( 0 2 - , O Z 2 - ) . These species have no oxidizing properties themselves (the cation of the catalyst acts as oxidizing agent) but can be inserted into the activated hydrocarbon molecule. The reactions result in oxygenated products which desorb from the surface of the catalyst leaving an oxygen vacancy behind. Each elementary step of the nucleophilic oxidation reaction takes place at a different active center of the catalyst. Simple oxides, like V205, contain in their stationary state cations of the same kind which are, however, at different oxidation states (for example V5+, V4+, and V3+). As to the role of these cations in the surface reaction two interpretations have been given[l]. First, only one type of cation (together with its environment) reacts as an active center while the others influence its properties. Second, all cations together with their oxygen surroundings serve as active centers but different centers may be involved in different steps. Both types of oxidation reactions (electrophilic and nucleophilic) may be further systematized according to the number of elementary transformations affecting the reacting molecule. The present study aims at characterizing details of the nucleophilic oxidation of hydrocarbons at vanadium pentoxide based catalysts. In this process the nucleophilic lattice oxygen reacts with the allyl-like complex

76 formed as a result of the activation of the organic molecule via hydrogen abstraction. As an example, Fig. 1 shows the oxidation reaction scheme of propene. After hydrogen abstraction from propene the allylic species, C3H5, transfers an electron to the metal cation where it is adsorbed reducing the metal. The cation [CH2-CH-CH2]+ can then react with a nucleophilic lattice oxygen 02-. In a concerted reaction another hydrogen is abstracted and the lattice oxygen incorporated into the organic species. The resulting acrolein molecule can desorb from the surface leaving an oxygen vacancy behind. Alternatively, the addition of another oxygen to the surface acrolein will lead to acrylic acid. Gaseous oxygen can then serve to fill the surface vacancies and to reoxidize the reduced metal cations. H

I

02-

Mn+

02- Mn+ 02-

-

[CH2-CH-CH2]-

--_--__ -

Mn+

02-

7)

(o Mn+

02-

-

[CH CH-CH,]'

-2:---02-

.(n-l)+

CH2-CH-CH2

-M(n-l)+

+

I

02-

Figure 1. Reaction scheme of the propylene oxidation. The present geometry optimizations based on ab initio Hartree-Fock LCAO cluster models are performed to study the reactivitity of the different surface oxygen sites of vanadium pentoxide concerning elementary steps of the above reaction. First, we consider the interaction of hydrogen with inequivalent oxygen centers of V209 surface clusters. This can give information about which oxygen site is preferred for binding a hydrogen that is abstracted from an organic species at the surface. Second, we examine the relative stability of different surface oxygen sites withlwithout adsorbed hydrogen in V2080H and V209 clusters. The simultaneous optimization of H and 0 positions can describe details of the concerted surface reaction which eventually leads to the oxygenated organic species. Section 2 describes computational details of the cluster model and of the electronic structure calculations while Section 3 discusses the results. Finally, Section 4 summarizes our conclusions stressing those features which are pertinent for a comparison with catalytic processes.

77

2. COMPUTATIONAL DETAILS

The crystallographic structure of bulk vanadium pentoxide, V 2 0 5 , is described by an elementary cell consisting of a distorted octahedron[4,5] with V - 0 distances varying between 1.58 8 (vanadyl groups) and 2 . 7 9 d (between the clevage planes). A s a consequence, the (010) surface of V205 can be characterized as containing edge and corner sharing distorted square pyramids. This plane contains three types of structurally different oxygen

0

Figure 2. Structure of the (010) surface of vanadium pentoxide. The surface clusters, V 2 0 q , used in this study together with the different surface oxygen sites O(A), O(B). and O(C) are marked. centers, the vanadyl oxygens (coordinated to one vanadium atom), O(A), the oxygens bridging two vanadyl groups, O(B) and the oxygens bridging two bare vanadium centers O(C), (the latter two centers are coordinated to two or three vanadium atoms), see Fig. 2. The three different oxygen sites are modelled by a V 2 0 9 cluster of Czv symmetry built of two corner linked square pyramids as marked in Fig.2. In the calculations the vanadyl bond distance is taken from bulk data[4] while the other V - 0 distances are assumed to be 1 . 8 3 A , the average value in bulk V 2 0 5 . In a first sequence of calculations the V 2 0 9 cluster unit is kept fixed at its bulk geometry and a hydrogen is added perpendicular to the surface for each of the three oxygen sites. This yields respective interaction potentials and equilibrium positions where the VZOgH ( V 2 0 9 t H ) cluster is required to conserve its symmetry described by point groups Cs ( O ( A ) site) and C2v (O(B), O ( C ) sites). This is also true for the subsequent calculations where both the adsorption site oxygen and the hydrogen in V20gH ( V 2 0 8 t 0 t H ) are allowed to relax in order to model the oxygen rearrangement and possible OH abstraction as a result of the chemisorption reaction.

78

Electronic wavefunctions and properties of the clusters are obtained from ab initio Hartree-Fock (HF) LCAO calculations[6] using flexible basis sets of contracted Gaussian-type orbitals. For vanadium the Ar core is replaced by an effective-core potential[7] and only 3d,4sp electrons are described explicitly by a 3s,Zp,Sd contracted to 3s,Zp,3d valence basis set. For oxygen an all-electron 9s,5p set contracted to 4 s . 3 ~and optimized for 0 - is used[8] while the hydrogen basis set consists of 4s,lp contracted to 2s,lp [ 9 ] . These basis sets have been successfully applied in previous ab initio cluster studies on the interaction of H and OH species with the vanadium oxide surface[lO]. The present Vz09 and V20gH cluster models have also been used in semiempirical studies[ll,lZ]. It was found that the electronic properties of the different oxygen sites remain unchanged if the substrate cluster is increased beyond V Og. Further, saturation of peripheral oxygen bonds by hydrogens in the clusters has only a minor influence on the oxygen sites[l2].

3. RESULTS AND DISCUSSION

Previous ab initio HF and semiempirical INDO calculations performed for differently charged Vz09 clusters, Vz09q, find the negative ( 9 = cluster most stable[lO-121. Further, these studies show that the electronic environment about the different surface oxygen sites depends only weakly on the cluster charge q . All orbitals which are relevant for binding adsorbates near the O ( A ) and O(B,C) sites are accounted for already by the neutral V209 cluster[lO]. Thus, binding of additional hydrogens near these sites is expected to depend only weakly on q . Studies on the hydrogen approaching the different surface oxygen sites[lO] (with the V209 cluster unit kept fixed and the H position varied perpendicular to the surface) show that H can stabilize at all three sites. The strongest bonding occurs at the O(C) site where oxygen bridges two vanadium atoms. Subsequent desorption of OH surface groups from the V 0 H cluster (V208 t OH, with the 2. 9 perpendicular to the surface) Vz08 and OH geometries kept fixed, OH moving depends on the surface site. It is found to proceed with only small or no activation barrier for the bridging oxygen sites, O(B), O(C), whereas for the vanadyl oxygen site, O(A), desorption is energetically very costly. Calculations were also performed for fixed hydrogen positions in the clusters and the adsorption site oxygen moving towards the hydrogen[lO]. They indicate that only the bridge site oxygens, O(B), O(C), can become mobile due to the presence of the adsorbing hydrogen.

-1)

The model calculations performed so far do not account for dynamical processes involving simultaneous 0 and H movements near the surface[lO]. However, it is evident that H adsorption, OH desorption and 0 vacancy formation are interrelated processes and can even result in surface relaxation or reconstruction. Therefore, we extend the calculations by allowing surface relaxation as well as simultaneous position changes of hydrogen and oxygen centers in the model cluster. In order to check whether the surface oxygen centers undergo relaxation with respect to their bulk positions geometry optimizations are performed for the Vz09 cluster where the appropriate oxygens, O ( A ) , O(B), or

79

O ( C ) , are allowed to move perpendicular to the surface. As a result, the vanadyl bond distance dv-o(A shortens considerably (dv-o A) = 2 . 5 4 a.u.) with respect to its bulk va)lue (dv-o(Al = 2.99 a.u.). $his indicates a stronger vanadyl surface bond as compared to the bulk bond. In contrast, the bridging oxygens, O(B), O ( C ) , move only by 0.12 a.u. above/below their bulk positions leading to rather small V-0 bond shortening (0.03 a.u.).

The presence of adsorbing hydrogen near the oxygen sites affects the oxygen surface bonds. This is seen in geometry optimizations of the V OgH (V208 + 0 + H ) cluster where the adsorption site oxygen and the adsorging 6.00

'

a ) vanadyl

site O(A)

p'

?

0 v v)

c

..-c0 co

0

a I

d 0

2

4

6

8

1

0

1

2

1

4

optimization step 6.00

'

b) bridging site O(C)

p'

2

v v)

c

0 ..-c v)

0

P

I

d 0

2

4

6

8

1

0

1

2

1

4

optimization step Figure 3. Position snapshots from the optimization procedure for the vanadyl site O(A), Fig. 3a, and for the bridge site O ( C ) , Fig. 3b, in the V2OgH cluster. The 0 and H positions are given with r e spect to the oxygen site at bulk equilibrium (starting geometry). The OH equilibration is marked by a dashed line. hydrogen stabilize simultaneously. As an illustration we show in Fig. 3 a sequence of position snapshots from the optimization procedure for the vanadyl site O(A), Fig. 3a, and for the bridge site O ( C ) , Fig. 3b, in the

V 0 H cluster. Here the hydrogen is placed at about 5 a.u. above the oxygen 2 9 (initial geometry, step 0) at the V209 equilibrium geometry and both cen-

ters are allowed to move perpendicular to the surface according to respective forces determined by total energy gradients. Fig. 3a shows the formation of surface OH at the vanadyl site. O(A). The OH equilibrium distance do-H = 1.79 a.u. is reached after 10 steps with the hydrogen approaching the surface but the oxygen remaining almost fixed in position. This is due to the rather strong vanadyl bonding at the surface. The results are different f o r the bridging oxygen site, O(C), see Fig. 3b. Here surface OH formation is combined with substantial position changes of both 0 and H. The stable OH geometry (do-H = 1.79 a.u.) is achieved after 6 steps with the oxygen at 1.6 a.u. above its bulk equilibrium. The subsequent OH movement towards the surface (steps n 5 6 ) is due to the electrostatic attraction between the (negatively charged) OH species and the surface. The formation of surface OH described in Fig. 3 is characterized by different OH - surface bond strengths for the different oxygen surface sites. As shown previously[lO], OH forms a rather strong bond with the underlying vanadium at the vanadyl site. In contrast, the adsorption of hydrogen forming OH at the bridging oxygen site in effect weakens the V-0 bond and as a consequence, OH may easily desorb from the surface at this site.

4. CONCLUSIONS

The results from the present calculations can give useful information concerning the basic mechanism of nucleophilic hydrocarbon oxidation at vanadium oxide surfaces. First, hydrogen abstracted from organic surface species (hydrocarbon or aldehyde-like precursors, see Fig.1) can interact and stabilize at different surface oxygen centers, vanadyl as well as bridging sites. This leads to surface OH species where both the formation process and the OH - surface bond strength differ for different sites. At the vanadyl site the OH species is bound rather strongly and not available for further reactions. In contrast, the OH species formed at the bridging oxygen site binds much more weakly and can easily desorb from the surface. It can. thus, participate in subsequent reaction steps like formation of surface aldehyde species. This represents a concerted reaction involving simultaneous H abstraction from the organic species, OH formation, and OH incorporation into the organic molecule, see Fig.1. It should be mentioned that the present calculations do not account for reconstruction of the surface connected with the above reactions. These effects can be described only by more complex geometry optimizations in much larger clusters.

ACKNOWLEDGEMENT

One of the authors (M. W.) thanks the Alexander-von-Humboldt-Foundationfor a fellowship. This work was supported by NATO Research Grant No. 900031.

81 REFERENCES

B. Grzybowska-Swierkosz and J. Haber (eds.), "Vanadia Catalysts for Processes of Oxidation of Aromatic Hydrocarbons", Polish Scientific, Warsaw, 1 9 8 4 . A. Bielanski, J. Piwowarczyk, and J. Pozniczek, J. Catal., 113 (1988) 334.

J. Haber in R. K. Grasselli and J. F. Brazdil (eds.), "Solid State Chemistry in Catalysis", ACS Symp. Series, No. 2 7 9 , Washington, 1985, p.3.

H. G. Backmann, F. R. Ahmed, and W. H. Barnes, 2 . Kristallogr. Kristallgeom. Kristallphys. Kristallchem., 115 ( 1 9 8 1 ) 1 1 0 . H. Hanke, R. Bunert, and H. G. Jetschekewitz, 2 . Anorg. Allg. Chem.,

109 (1975)

414.

The Hartree-Fock cluster program CLUSTER developed by K. Hermann based on the MOLECULE integrals program by J. Almlof was used. Further, geometry optimizations are carried out with the HONDO package developed by M. Dupuis et al.. P. J. Hay and W. R. Wadt, J. Chem. Phys.,

(1985) 270.

P. S . Bagus and U. I. Wahlgren, Mol. Phys., 33 ( 1 9 7 7 ) 641. C. W. Bauschlicher, P. S . Bagus, and H. F. Schaefer 111, IBM J. Res. Develop., 2 ( 1 9 7 8 ) 2 1 3 .

[lo]

M. Witko and K. Hermann, J. Mol. Cat.,

[ll]

M . Witko, R . Tokarz, and J. Haber, J. Mol. Cat.,

66

(1991) 205.

[12]

M . Witko, R . Tokarz, and J. Haber, J. Mol. Cat.,

66

(1991) 357.

(1993) 279.

This Page Intentionally Left Blank

V. Cork% Corbcrin and S. V I C Bellon (Edilors), New Developments in Selective Oxidalion If 0 1994 Elsevier Science B.V. All rights reserved.

83

Comparison between gamma-alumina and aluminum niobate supported vanadium oxides in propane oxidative dehydrogenation J.-G. Eona, P. G. Pries de Oliveirab, F. Lefebvrec and J.-C. Voltac "Instituto de Quimica, Departamento de Quimica Inorganica and NUCAT/IJFRJ, Bloc0 A63 1, Ilha do Fundao, 21945-970 Rio de Janeiro, Brazil bInstituto Nacional de Tecnologia, Avenida Venezuela 82, Praca Maua 2008 1, Rio de Janeiro, Brazil %stitut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France

The catalytic properties of vanadium oxides supported by alumina and aluminum niobium oxides (AINbO) are compared in propane oxidative dehydrogenation. VO,/AlNbO are less active than VOx/y-A1203, but more selective in the transformation into propene. For the first system, selectivity is maximum at an intermediate calcination temperature of the supports, corresponding to an amorphous AlNbO structure. Bridging oxygens (V-0-V) from the two dimensional V 0 4 array are suggested to be active sites on both catalysts. However, it is not possible to correlate the catalytic properties of superficial vanadium oxides only with their degree of condensation; it is suggested that the structure and the acid-base properties of the support also define the redox function of supported vanadates.

1. INTRODUCTION

The activation of the C-H bond in propane oxidative dehydrogenation is a challenging question in the fascinating route for the conversion of alkanes into chemical utilities. The nature of catalytic surface sites is important since it defines the reactivity of the corresponding systems. In the case of the magnesium vanadate catalysts, it has been shown by electrical conductivity measurements, that the 0 2 - entities are responsible for the oxidation of propane. The activity and selectivity of the different VMgO polyvanadates have been attributed to their respective degree of condensation (ortho-, pyro- and meta-)( 1,2). The structure, redox properties and reactivity of vanadium oxide surface compounds have previously been studied on different oxide supports such as TiO2, AI2O3, Z r 0 2 and Si02 (36). It has been shown that it is possible to change the dispersion of vanadia, depending on the nature of the support. The structure and the acid-base properties of the support were considered as the important parameters to control this dispersion. Recently, we have studied (7,s) the properties of aluminum-niobium oxides of composition Al:Nb:O = 1 : 1:4 (abbreviated as AlNbO in this paper) as supports for vanadium oxides. From FTIR and X P S studies of these

84

oxides, it had been suggested (7) that small areas of alumina (and maybe niobia) exist at the surface of the amorphous AlNbO support. It is thus interesting to compare the catalytic properties of catalysts VOx/y-A1203 and VOx/AINbO in order to clarifjl the role of the alumina areas in the latter. In this communication, we compare AlNbO supports to y-Al2O3, for vanadia dispersion and propane oxydehydrogenation . It is confirmed that the support determines the dispersion of superficial vanadates, but it is suggested that it also affects the redox properties of these oxide species.

2. EXPERIMENTAL Part of the results has already been published (7,8), therefore the experimental details will be abbreviated. y-Al2O3 supported vanadium oxides have been prepared following the continuous adsorption method. The superficial loading and the nature of the precursor species were monitored by varying the pH of the 0.05M ammonium vanadate impregnation solution. Three samples have been prepared at pH 7.0, 4.5 and 2.5, corresponding respectively to V30g3-, v 4 0 1 2 ~ - , V1oO2g6- and V10028H5- solution species (9), using a non-porous (200 m2g-I) y-alumina support. Aluminum niobate supported vanadium oxides have been prepared by grafting VOCl3 in inert atmosphere on AlNbO oxides calcinated at different temperatures. The AlNbO oxides have been prepared from oxalates precursors (7). All catalysts have been calcinated under air flow at 500OC. The catalysts have been tested with propane oxidative dehydrogenation in a flow system. The catalyst (100-350 mg) was deposited on a fixed bed in a quartz microreactor (Utube, 13 mm diameter) operating under atmospheric pressure. The gas mixture containing propane (2 vol %), 0 2 (19.6 vol %) and N2 (78.4 vol %) was fed at a flow rate of 50 ml/min. The conditions for on-line chromatographic analysis have been described elsewhere (8). The vanadium coordination was investigated at ambient conditions by UV-visible, Raman, V NMR and ESR spectroscopies after preparation and catalytic testing. In situ characterization of gamma-alumina supported vanadium oxides has also been performed in a Raman cell under dry air.

3. RESULTS 3.1. Characterization after calcination Table 1 summarizes the results of chemical analysis of the two families of catalysts. It must be noted that the AlNbO supports in samples AN500, AN600 and AN650 are amorphous to Xray diffraction. Sample AN750 corresponds to crystallized AlNbO4. It is seen that the vanadium superficial coverage on the four AlNbO supports varies within a narrow range (0.18 to 0.26), well below the values obtained on y-Al2O3 (0.28 to 0.65). This has been associated with the low density of surface hydroxyl groups on AlNbO oxides when compared to y-Al2O3.

85

UV-visible DRS spectra of the catalysts after calcination show a strong asymmetrical band in the 350-420 nm domain. The position of the band for all solids is reported in table 1. This range is characteristic of the ligand-metal charge transfer observed with V5+ ions. No signal is observed in the 700-800 nm zone, corresponding to the V4+ d-d transition. It is difficult, on the basis of UV experiments to assign a definite coordination to the superficial V species, as the spectrum is not related to that of bulk reference compounds with tetrahedral and octahedral coordination, Table 1 Comparison of some features of the two catalysts series Catalyst

BET area (m2g-1)

[V](at/nm2)

8

h,,,,

AN500 AN600 AN650 AN750

79 73 57 39

1.6 1.4 1.1 1.5

0.26 0.23 0.18 0.25

393 396 396 400

A2

200 200 200

3.9 3.3 1.7

0.65 0.54 0.28

420 3 80 350

A4 A7

G(ppm) -580, -548, -541, -486,

-1984 -1915 -1911 -1903 -502 -530 -544

AN: VO,/AlNbO. The associated number in the name indicates the calcination temperature of the support. A: VOxly-Al2O3. The number indicates the pH of the adsorption solution. [V]: vanadium superficial density. 8: vanadium superficial coverage (calculated from the unit area: A(VO,)=0.165 nm2 ). h,,,: position of UV-visible band. 6(pprn):S1V NMR chemical shift in reference to VOC13.

Table 1 shows also the 51V N M R chemical shift, in reference to VOC13, observed in the two families after calcination. On the basis of the anisotropy of the N M R signal, the sharp line observed (7) at -1900 ppm for the vanadium oxide species supported by AlNbO oxides has been attributed to an isolated vanadate with tetrahedral coordination. The wide, asymmetrical band, observed at approximately -500 ppm is associated with V species with distorted tetrahedral coordination. In contrast with the former, it is assumed that these species are condensed vanadates, forming linear chains or bidimensional arrays. It is seen that the proportion of the two kinds of tetrahedral vanadates (isolated or condensed) on AlNbO supports, depends on the calcination temperature of the material. A Raman scattering study was only possible for VOx/y-A1203 samples, because of a strong absorption by the Nb-0 vibration modes of the AlNbO support in the same region as the V-0 bands, As observed by other authors (lo), the nature of the alumina supported vanadates is influenced by the water pressure above the sample. The spectra have thus been collected at ambient temperature before and after in situ calcination (400OC under dry air). Only results after dehydration are shown in fig. 1.

86

.-a C

fa

800

1000 cm-1

1200

Figure 1. Raman spectra of VOx/y-A1203 (A2, A4 and A7) and alumina (A) A wide band is observed in the three samples A7, A4 and A2 in humid and dry conditions in the 960-995 cm-l domain; after drying, a shoulder develops above 1000 cm-l. This can be attributed to a V=O species forming three bonds with the support: this species is not stable in presence of water, forming hydroxylated, mono-oxo species which absorb in the same region as polycondensed vanadates (960-995 cm-1). W e therefore conclude that both types of vanadates (isolated and condensed mono-oxo vanadates) are present at the surface of the alumina support in the conditions of the catalytic reaction. The shift of the band to lower frequencies from A2 to A7, in hydrated samples, has been interpreted (1 1 ) as a decrease in lateral interactions between linear or branched vanadate chains as their superficial density decreases. 3.2. Propane oxydehydrogenation

Catalytic results for propane oxidative dehydrogenation at approximately isoconversion are shown in table 2.

Table 2 Catalytic results for propane oxydehydrogenation Catalyst

Mass (mg) T(0C) Conversion (“A) TON(s-1) C?Hh

A2 A4 A7

100 100

350 350 350

13.5 8.8 <1.

A7

100

400

7.5

AN500 AN600 AN650 AN750

200 200 350 125

500 500 500 500

11.8 9.2 10.4 11.5

100

7 . 7 lo4 ~ 6 . 0 lo4 ~ <1.3 x lo4

2.1 x 2.0 10-3 2.1 x lo-’ 7.0 10-3

Selectivity(“) co CO:,

c

28 16

--

17.5 14 --

__

-_

58.4

12.1

--

70

12 65 62 44

87 2 16 18

__ 33 21 25

99 100 99 87

41 50.4

87 80

C= selectivity sum (carbon balance); TON= turn over number (propane molecules transformed per vanadium atom and per second) The two families of catalysts present different behavior. VOx/y-A1203 catalysts are much more active than VO,/AlNbO catalysts so that it has not been possible to compare the two series at the same temperature. Results for sample A7 are shown at 400oC because practically no conversion is observed at 350oC. This sample is thus far less active than A4 or A2.This point is more obvious from their turnover numbers (TON). The propene selectivity pattern is however consistent for the three samples (1 1). VO,/AINbO catalysts on the other hand present a higher selectivity to propene at an intermediate calcination temperature of the support (AN600 and AN650). Sample AN500 shows a very low propene selectivity and a higher C 0 2 selectivity. Sample AN750 shows a selectivity compatible with alumina-supported vanadates; its activity (TON) is also the highest in the VOx/AINbO series. 3.3. Characterization of VO,/y-A1203 after catalytic testing

The UV-visible DRS spectra performed on alumina supported vanadium oxides show a new band at 700-800 nin which is attributed to the d-d transition of V4+ ions in octahedral symmetry. It is observed that the intensity of the band increases strongly with the vanadium coverage. On the other hand, the charge transfer band at 300-400 nm has not been significantly modified when compared with the calcinated catalyst. 51V NMR spectra of the same solids are also very similar before and after reaction. ESR spectra measured at ambient temperature show identical qualitative features for the three catalysts, but with different intensities. The ESR parameters are reported in table 3, where they are compared to the parameters of reduced amorphous V2O5 obtained from (12). A reasonable fit is observed between the two sets of parameter. It is interesting to note that the structure of amorphous V2O5 has been proposed to be built up from branched tetrahedrally coordinated mono-oxo vanadates (12).

88

Table 3 Spin Hamiltonian Parameters Spin parameters

VOX/y-Al203(after testing) 1.919 1.982 190 Gauss 70 Gauss

amorphous V205( 12) 1.913 1.985 176 Gauss 66 Gauss

4. DISCUSSION 4.1. VOx/y-A1203 UV-visible spectroscopy clearly establishes the presence of only V5+ species dispersed at the surface of the support after the calcination of the catalysts. l V N M R spectroscopy shows that the major part of vanadium ions are tetrahedrally coordinated and form linear chains or bidimensional arrays. Thus, the two techniques do not show any clear difference between the three samples. Raman spectroscopy indicates that mono-oxo polyvanadates are formed with a higher degree of lateral interaction as superficial coverage increases. Tables 1 and 2 show that the turnover number for these solids strongly increases with their vanadium content. This observation suggests that dispersed species are not very active sites for propane oxydehydrogenation. As indicated by Raman spectroscopy, the extent of lateral interaction between vanadate chains is the only factor differentiating the three catalysts after calcination so that higher activity of vanadate species might result from these interactions. UV-visible and 51V NMR spectroscopy show that little alteration occurs in the vanadium V structure after partial reduction. These results on one hand, and the reasonable similarity of the ESR spectra in reduced VOx/y-A1203 catalysts and reduced amorphous V2O5, on the other hand, lead us to suggest that the active site in propane oxidative dehydrogenation is part of a branched vanadate chain in which vanadium is tetrahedrally coordinated. 4.2. VOx/AINb04 Two different vanadium environments have been characterized by 51V N M R spectroscopy. The first one is preferentially observed when the AlNbO support is calcinated at low temperatures and is characteristic of condensed tetrahedrally coordinated vanadium sites observed on y-Al2O3. As the calcination temperature of the support is increased, this species is substituted by a more symmetrical one which has been assumed to correspond to an isolated vanadium site. Table 2 shows that the turnover number is approximately the same in the three samples, AN500, AN600 and AN650. AN750 is roughly three times more active. These results show that the symmetrical (isolated) vanadate on AlNbO4 is more active than the distorted (condensed) species on amorphous AlNbO oxides. Propene selectivity on the other hand is the

89

best at intermediate calcination temperature (i.e. amorphous AlNbO oxides with reduced superficial hydroxyl concentration). At lower calcination of the support ( A N S O O ) the catalyst gives very poor propene selectivity. The activity and selectivity pattern of these AlNbO supported vanadates is thus not clear. 4.3. Comparison of t h e two systems

As too many parameters are involved in the comparison of the two series (nature and surface area of the support, vanadium loading, reaction temperature) this will be restricted to the analysis of the selectivity differences. Indeed, the curve selectivity-conversion for VOx/yA203 catalysts is not significantly affected by vanadium loading and reaction temperature (1 1). Only two variables will thus be considered to analyze the catalytic results summed up in table 2. These are the degree of vanadium condensation and the chemical properties of the support. The vanadate species supported by amorphous AlNbO oxides may be compared to the alumina supported vanadates as their 51V NMR spectra are quite similar. W e have concluded for VOx/y-A1203 solids that an increase in lateral interactions leads to more active catalysts for propane oxidative dehydrogenation. Thus, it is not easy to understand, on this basis, why the new symmetrical (isolated) species, formed on the crystallized AlNbO4 support, should be more active than the distorted (condensed) one formed on amorphous AlNbO supports. We therefore must imagine that redox properties of the vanadate species depend on the bonds formed with the support. In a previous work (7), we have determined the acidity of the hydroxyl groups as a function of the calcination temperature of the support, by measuring their turnover number for isopropanol dehydration. It appeared that OH groups on crystallized AlNbO4 oxide are much more acidic than corresponding amorphous AlNbO-bonded groups. It therefore seems reasonable to admit that some field effect, induced by the linkage of niobium to aluminum sites, should also modify the electronic distribution in the V - 0 bond. It is obvious that some modification occurs from the 51V NMR spectrum: indeed the strong chemical shift (-1900 ppm) characterizes an unusual electronic density around the vanadium nucleus. It is also interesting to note that the basicity of the vanadium bonded oxygen, measured by oxidation of isopropanol to acetone, increases with the concentration of the new species. Also very striking is the lack of selectivity observed in sample AN500. If we assume that the vanadate species are the same as in the catalyst AN600, and VOx/y-A1203, as indicated by l V NMR spectroscopy, we must imagine that some influence from the support modifies the catalytic properties of the species. It might be possible that acidic functions of the support coupled with redox sites, for example, strongly influence the selectivity of the catalyst. In conclusion, the comparison between the two families of catalysts reported in this work suggests that the reactivity of supported vanadium oxides is determined by the degree of condensation of the vanadate species and the acid-base properties of the support. It seems that the challenge in controlling the catalyst reactivity will be mainly resolved when both steps, support preparation and vanadium dispersion, are strictly controlled to yield a strongly homogeneous and well defined superficial structure. REFERENCES 1. D.Siew Hew Sam, V.Soenen and J.C. Volta, J. Cata1.,123, 417 (1988) 2. A. Guerrero-Ruiz, I. Rodriguez-Ramos, J. L. G. Fierro, V. Soenen, J. M. Hermann and

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3. 4. 5.

6. 7.

8. 9. 10. 11. 12

J.C. Volta, in New Developments in Selective Oxidation by Heterogeneous Catalysis, in Surface Sciences and Catalysis, vo1.72, p203, P. Ruiz and B. Delmon Eds., Elsevier(l992) J.Haber, A.Kozlowska and R.Kozlowski, J.Catal., 102, 52 (1986) I.E.Wachs, J.Catal.,124,570(1990), U.Scharf, M. Schraml-Marth, A. Wokaun and A. Baiker, J. Chem. Soc. Faraday Trans, 8., 3299, (1991) J.M.Jehng and I.E.Wachs, Catal. lett.,13, 9, (1992) P.G.P.de Oliveira, F.Lefebvre, J.G.Eon and J.C.Volta, J. Chem. Soc., Chem. Com., 1480,2 1( 1990) P.G.P.de Oliveira, J.G.Eon and J.C.Volta, J.Cata1.,137, 257 (1992) M.T.Pope, Heteropoly and Isopoly Oxopolymetalates, Springer-Veda&( 1983) G.T.Went, S.T.Oyama and A.T.Bel1, J.Phys.Chem.,4240,94,(1990) , J.G.Eon, R.Olier and J.C.Volta, J. Catal., to be published A. Mosset, P. Lecante, J. Galy and J. Livage, Philos. Mag. (8) 46 (2), 137 (1982)

91

J. EMBER (Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland): Some time ago we have shown that the type of vanadium oxide species present at the surface depends on the water pressure and coverage. In the humid atmosphere decavanadate anions are favored, whereas on careful outgasing either isolated V04 tetrahedra appear or chains of these tetrahedra forming a metavanadate type structure. Which of these exist in the conditions of your oxydehydrogenation and which is then an active phase?

J. G. EON (Instituto de Quimica, Rio de Janeiro, Brazil): We have shown (11) that decavanadate anions are not stable on the alumina surface. After calcination at 5OO0C, even in humid atmosphere, we have characterized mainly tetracoordinated species forming a chain or array structure. Isolated V04 species might coexist; as the specific activity increases greatly with vanadium concentration from A7 to A2, we suggest this species is not the active one on alumina supports. G. CENT1 (Dip Chimica Industriale e dei Materiali, V. Le Risorgimento , Bologna, Italy): In the samples prepared by supporting vanadium on alumina you observe an increase in the activity in the A7-A2 series that was attributed to an increase in the lateral interaction between vanadate species. However, these samples have a different vanadium content. My question is therefore how change the specific activity per vanadium site in these samples and how this specific activity can be correlated to the change of surface species of vanadium. J. G. EON: What we observe is indeed an increase in specific activity per vanadium site (turnover number) in the series A7, A4, A2. We suggest that lateral interactions between vanadate chains stabilize the superficial vanadyl formed after reduction by propane (1 1).

J. S. R U E (Inst. Catalisis, Madrid, Spain): The table with the ESR results is not well written. The g-values should appear as g,, and gL. They should be determined by computer simulation. The hyperfine splitting constants indicate that the sites are different, not similar and should also differentiate between the perpendicular and parallel components. J. G. EON: We agree with these comments. However, the ESR parameters of V4+ in alumina supported vanadates are very different from the values observed in crystallized V2O5 or VOPO4 and strikingly in reasonable agreement with those of amorphous V2O5. Although the sets are clearly not identical, we use the spectrum as a finger-print, suggesting that the structure of vanadate layers on alumina should be compared to that of amorphous V2O5 where vanadium is also tetracoordinated.

G. BUSCA (Instituto di Chimica, Faculta di Inorganica, Universita di Genova, Genova, Italy): In your lecture you denoted the active sites on VOx/A1203 as tetrahedral with Td symmetry. Previous data from several laboratories agree showing that active sites on vanadia-alumina in dry conditions are tetracoordinated but not tetrahedral. Vibrational spectroscopies in fact show that these species have one short V=O bond and probably, three

92

longer V - 0 bonds. They are consequently tetracoordinated, but with C3v symmetry. Tetrahedral centers (Td symmetry) are instead typical of vanadate species in basic environments. In your opinion, your data support a tetrahedral (Td) symmetry or a tetracoordinated symmetry (CjV)?

J. G. EON: We think it is impossible to attribute symmetry labels from our data to alumina supported vanadates. Our spectroscopic data show that these species are mono-0x0, tetracoordinated vanadates.

J. F. BRAZDlL (BP Chemicals, Warrensville Research Center, Cleveland, Ohio, USA): Since aluminum niobate crystallizes with the rutile structure and vanadium is known to dissolve readily into rutile structures, does the isolated vanadium species you see arise from formation of solid solution between vanadium and aluminum niobate? J. G. EON: This is an interesting point. However, aluminum niobate does not crystallize with the rutile structure but is isomorphic to Ti02(B)(7). Moreover, vanadium dissolves in rutile as the reduced V4+ cation. Since we have observed only V5+ species by UV-visible spectroscopy, we did not consider this possibility.

V. Cortes Corberin and S. Vic Bellon (Editors), New Developments in Selective Oxidation I1 0 1994 Elsevier Sciencc B.V. All rights reserved.

93

Dispersion of V4+ions in a Sn02 rutile mptrix as a tool for the creation of active sites in ethane oxydehydrogenation S. Bordoni, F. Castellani, F. Cavani, F. Trifirb and M.P. Kukarni' Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy On leave from the National Chemical Laboratory, Pune, India, owner of a UNDO fellowship

Abstract VISdO mixed oxides were prepared by solid state reaction at 700C between tin oxohydrat s and vanadium pentoxide. The resulting material contained different vanadium species: i) in a VxSnl-xy-type solid solution, where vanadium ions replaced tin in the rutile lattice, ii) amorphous V oxide spread over the solid solution, and iii) crystalline V205. The mixed oxides were tested as catalysts for the oxidative dehydrogenation of ethane. The reaction network was found to consist of parallel reactions for ethylene, CO and C02 formation; co secutive reactions of ethylene and CO overoxidation also occurred. Increasing amounts of in the solid solution increased the Sn02 activity in ethane conversion. The selectivity to ethylene also was affected by the value of x. For ~ ~ 0 . the 0 2selectivity was greater than that of pure SnO;?, while higher values lowered the selectivity. Independent of the value of x, however, the selectivity at 500C was close to 40%, higher than that obtained with SnO2. The V5' oxide was instead detrimental for selectivity, because its presence led to the oxidation of ethane to CO.

?+

+

vb:

INTRODUCTION Vanadium oxide is known to exhibit a specific reactivity towards certain molecules depending on the nature of the phase it forms. For instance, (VO)P207 is active and selective in the oxidation of n-butane to maleic anhydride and of n-pentane to phthalic and rnaleic anhydride, but exhibits a poor reactivity towards other hydrocarbons (1,2). Vanadium oxide spread over Ti02 is selective in o-xylene oxidation to phthalic anhydride, but is absolutely unselective in paraffin oxidation (2). VSb04 works in the ammoxidation of propane to acrylonitrile (3), magnesium vanadates are able to oxidatively dehydrogenate n-butane and propane, but do not attack ethane to form ethylene (4), while the latter reaction works with high selectivity on a V/Nb/Mo/O catalyst, which is unselective on other paraffins (5). Thus, it seems that the nature of the catalyst structure is fundamental in determining the specific reactivity of the vanadium sites, and the possibility of selectively forming the desired product. From this point of view, the surface arrangement and distribution of the active sites addresses the pathway towards the formation of either partially oxidized products, or combustion products. The objective of the present work was to study the catalytic performance of V/Sn/O mixed oxides in the oxidative dehydrogenation of ethane to ethylene. These materials are known to catalyze the oxidation of olefins and the ammoxidation of akylaromatics (6-9); their reactivity in paraffin oxydehydrogenation has, however, never been tested. The catalysts were

94

prepared by solid state reaction between tin oxohydrate and vanadium pentoxide, and then characterized in order to identify the different vanadium species formed. The reactivity of the various species identifed was then studied. The catalytic properties of these systems are discussed in relation to the nature of the vanadium species formed. The reaction was carried out under conditions in which the presence of heterogeneously initiated, homogeneous-type reactions are negligible, thus at reaction temperatures not higher than 550C. In such conditions the phenomena observed can be entirely ascribed to the catalyst itself.

EXPERIMENTAL The V/Sn/O catalysts were prepared by solid state reaction at 700C between SnO(OH)2, precipitated from a SnC4 alcohol solution and dried at 120C, and v205. The samples were characterized by means of several techniques: chemical analysis of vanadium ions, XRD, m-IR, EPR and surface area measurements (BET method). The chemical analysis was carried out with a pr cedure analogous to that reported for the V T / O system (lo), by volumetric titration of V8+ ions with a KMn04 0.1N solution, and of V with a Fe(NH4)2(S04)2.6HzO 0.1N solution. Preliminary treatment of the sample with diluted ammoniacal solution dissolved the vanadium oxide species that do not chemically interact with the insoluble tin oxide. The latter was then attacked with hot concentrated H2S04, to determine the vanadium species bound to the tin oxide. Catalytic tests were carried out using a laboratory stainless steel, flow reactor, at atmospheric pressure. Products were analyzed on line, by gas chromatography. Ethylene and ethane were separated and quantified using a Carbosieve D column, with the oven temperature maintained at lOOC, and a FID. Carbon oxides were analyzed using a Carbosieve S column, with the oven temperature programmed to increase from 40 to 220C, and TCD. The following experimental conditions were utilized (unless otherwise specified): residence time 0.25 or 1 s, catalyst volume 0.67 mL (weight 1.1 g); feed composition: 50% ethane, 15% oxygen, remainder helium. +

RESULTS Catalyst characterization The characterization of V/Sn/O samples prepared with increasing vanadia contents led to the identi ication of three different vanadia species (11): 1) a species, stabilized in the reduced state, and insoluble in dilute aqueous ammoniacal solution. The absolute amounts of this species, determined by chemical analysis, are shown in Figure 1, for increasing amounts of overall vanadia contents. The stabilization of vanadium in the reduced state indicates a strong chemical int raction with the tin oxide. Figure 2 plots the trend of the S 0 2 tetragonal cell volume with content. The decrease in cell volume as the increase indicates amount of of the vanadium ions in the rutile structure, with replacement of ions by having smaller radius. The trend almost follows the one predicted by Vegard's law. The maximum dissolution of vanadium in the rutile lattice was appro 'mately 9-10 atomic % with respect to tin. EPR spectroscopy indicated that higher than 2% (thus, for x>0.02 in VxSni-x02) the intensity of the EP for amounts of signal was lower than for x<0.02. This suggests a strong interaction between neighbouring V ions, o c c y g at relatively high 9 ' concentrations. 2) a V oxide species, spread over the solid solution, either amorphous or microcrystalline. 3) crystalline V2os. This species was observed only for overall amounts of vanadium oxide higher than 20 atomic %. The two latter species could be selectively removed by the treatment with an aqueous ammoniacal solution.

J+

v6:

S3+ ?+

v&.

d!

95

V

0.1

4+

/(V

4+

cell volume, A Y ~

+Sn), at. ratio

I-

0

0.1

0.2

0.3

0.4

0.5

Vtot /(Vt0, +Sn), at. ratio

9 'as a function of the

Figure 1. Amount of overall vanadia content.

0

I

I

I

I

I

0.02

0.04

0.06

0.08

0.1

0.12

v4?(v4kn), at.ratio

Figure 2. Volume of the te agonal cell of Sn@ as a function of the content.

4

Catalytic tests The catalytic tests of ethane oxidative dehydrogenation were carried out ither on the V/Sn/O samples as such, or on the samples treated to selectively remove the VR oxide and bulk V205. In the latter case the reactivity of the VxSni-xO2 solid solution was checked. The effect of residence time on catalytic performance is shown in Figure 3, for a catalyst overall amount of vanadia of 2.9 atomic %, 1.8% of which wy+found to be dispersed in the rutile matrix, and the remainder as soluble V oxide. It is shown that the conversion increased all over the entire range of residence time examined. At 12% ethane conversion, the oxygen conversion was close to 80%. The only products found were ethylene, CO and C02. No traces of other partially oxygenated products, such as acetaldehyde or acetic acid, were detected. The selectivities to CO and ethylene clearly were not zero when extrapolated to 0% conversion. This suggests that CO and ethylene formation occurs via parallel reactions. The selectivity to C02, when extrapolated to 0% conversion, does not seem to be nil, even though the extrapolation is not unambiguous; most likely, also in this case, a parallel reaction of C02 formation can be hypothesized. The selectivity to ethylene and to CO decreased with increasing conversion, indicating the existence of consecutive reactions upon these products; on the other hand, the formation of C 0 2 increased. These data favour the following reaction network:

E:sztf ?+

96 ethane conversion, %

conversion, yields, %

selectivity, % I

1

5

C

8

0

6

W

I4t 12 10 86-

4-

3 2-

0 0

0.5

I

1.5

2

"

0

4

8

12

16

residence time, s

oxygen conc., vol. %

Figure 3. Ethane conversion and selectivities as a function of residence time. T 430C; catalyst V/Sn/O with V/(v+Sn) at. ratio =2.

Figure 4. Ethane conversion and selectivities as function of 02 conc. Res. time 0.25 s , T 480C; catalyst as in Figure 3.

Most likely the parallel reactions have a common intermediate, formed by activation of the ethane, i.e. an ethyl radical species (12).the formation of which is the rate determining step of the reaction, that is then converted either to ethylene or to carbon oxides. Under the conditions employed, the maximum selectivity to ethylene was 25%. Figure 4 reports the effect of oxygen concentration on conversion and yields. Lowering the oxygen concentration led to a remarkable decrease in Co;! selectivity, in favour of both CO and ethylene; the conversion however decreased. The formation of CO and ethylene showed a similar order of reaction with respect to oxygen, lower than 1, and lower than that of C02 formation. The effect of reaction temperature is shown in Figure 5. The selectivity to ethylene initially decreases, then increases for temperatures higher than 44OC.The selectivity to C02 displays the opposite trend, while that to CO continuously decreases. The same tests were carried yt over the catalyst after treatment with the ammoniacal solution, in order to remove the V oxide species. The effects of the oxygen concentration in the feed and of the temperature for these catalysts are shown in Figures 6 and 7, respectively. The treatment changed the catalytic behavior: the overall activity was only slightly decreased, but the distribution of the products was remarkably changed. The selectivity to CO decreased considerably, while a corresponding increase was found for the selectivity to CO:! and, to a lesser extent, to ethylene. Under these conditions, the maximum selectivity to ethylene obtained was the 40%, at 520C,with total oxygen convers'on (14% ethane conversion). Finally, samples containing increasing amounts of in solid solution, treated with the basic solution to remove the soluble phases of vanadium oxide, were tested. Figure 8 compares the ethane conversion with temperature for the different samples, while Figure 9 reports the dependence of the selectivity to ethylene on ethane conversion. Maximum conversion was approximately 14%, reached for total oxygen conversion. The values of apparent activation

'

vb'

91

conversion, %

conversion, yields, %

selectivity, %

t

111 conv C2H6 12 10 86-

42-

340 2 0/ o

380

420

460

500

"

12

16

oxygen conc., vol %

temperature, C

Figure 5. Ethane conversion and selectivities as a function of temperature. Res. time 1 s; catalyst as in Figure 3. conversion, %

a

4

0

Figure 6. Ethane conversion and selectivities as a function of the 02 conc. Res. time 0.25 s, T 480C;catalyst V0.018Sn0.98202. ethane conversion, %

selectivity, %

14 -

12 10

-

8-

6-

42-

340

I

I

I

I

380

420

460

500

temperature, C

Figure 7. Ethane conversion and selectivities as a function of temperature. Res. time 1 s; catalyst as in Figure 6.

0'

320

I

360

I

400

I

440

I

480

temperature, C

Figure 8. Ethane conversion as a function of temperature, for VxSni-x02 catalysts at increasing values of x. Res. time 1 s.

98

energy calculated for the four samples, as well as the selectivities to ethylene at 400C and 520C,are reported in Table 1. an intrinsic activity, in agreement with indications in the The undoped SnO;! itself increased the activity, even though the latter was not literature (13). The content in the rutile lattice. The sample with x=0.018 was strictly proportional to the slightly more active than pure Sn02, while a strong increase in activity was achieved on the sample with x=0.034. The sample with x4.057 showed an activity comparable to that of the sample with x4.034. The apparent activation energy was the highest on pure S n a , decreased on4ihe sample with x=O.O18, and further decreased on the two samples with the highest V content. The selectivity at low conversion was the highest on pure SnO;! and on the sample with x=O.O18 (close to 20%); on the other samples it was lower than 10%. As the ethane conversion increased (thus approaching the total oxygen conversion) the selectivity to ethylene remained approximately constant on Sn02, but increased on all the vanadium- containing samples. At total oxygen conversion the selectivity to ethylene obtained was inversely proportional to the x value. Nevertheless, at 520C all vanadium-containing samples reached a similar selectivity to ethylene, close to 40%.

v$.

x I"

"$7

.p2

VxSnl-xO;! catalysts at increasing x values. Res. time 1 s.

+O +

0 0

0.018

3

6

9

12

15

ethane conversion, %

Figure 9. Selectivity to ethylene as a function of ethane conversion, for VxSni-xO;! catalysts at increasing values of x. Res. time 1 s.

DISCUSSION SnO;! is an n-type, wide-gap, semiconducting oxide; its bulk oxygen is known to exhibit reactivity towards hydrocarbons, converting them to oxidized products and giving rise to partially reduced tin oxide phases (13,14). S n a has a rutile-type crystalline structure, that can form solid solutions or mixed phases with other isostructural metal oxides. The data reported indicate that the solid state reaction between tin oxohydrate and v205

99

form solid solutions or mixed phases with other isostructural metal oxides. state reaction between tin oxohydrate and V205 The data reported causes a partial reduction of , with migration of the latter into the SnO2 during th formation of the rutile crystalline structure. Additional vanadium oxide phases present are a . spread over the rutile surface, and crystalline V2O5. The system is therefore similar V oxide to the V/Tii/O system, with T i m in the rutile form (15). Analogous to that occuring in the latter system, the dissolution of V4+ is limited, no higher than 9-10 atomic %. The improvement in activity decrease in the apparent activation energy indicate that the dispersion of small amounts in the rutile lattice modifies the reactivity of the latter, either by creating new active centers, or, more likely, by modifying the reactivity of the tin oxide itself (for instance, by modifying the conducting properties of Sn02). In the latter case, vanadium acts as a dopant for the tin oxide, interacting either electronically or chemically and modifying the specific reactivity of the tin oxide centers. The highest improvement in activity was found for the sample with ~ 3 . 0 3 4 while , the activity of the sample with x=0.018 was only slightly higher th n that of pure Sn02. The effect of the on the selectivity is more complex. The addition of small amounts of ?' (x=O.O18) does not vary the selectivity at low ethane conversion, while for the highest values of x the selectivity is decreased. The presence of \p' is in any case beneficial at high temperature (i.e. 520C), where all V/Sn/O samples reach comparable values of selectivity (38%, remarkably higher than that obtained on Sn02), even though the selectivity at total oxygen conversion (reached at temperatures lower than 500C) is maximum for x=O.O18 (see Figure 9). Therefore, vanadium plays a positive role at high temperature, directing the reaction pathway towards the formation of ethylene rather than carbon oxides. The results obtained may indicate the existence of different types of V4" arrangements, depending on the value of x in the VxSnl-xO2 solid solutions. Followin EPR indications, it ions are randomly, can be hypothesized that at low values of x (say, lower than 0.02), the homogeneously distributed inside the rutile mamx. For higher values of x, the vanadium ions are likely preferentially segregated into microdomains of V02-like clusters. The differe t vanadium arrangement leads to different catalytic behaviors with respect to ,51102. Isolated sites slightly activate SnO2, and substantially improve the selectivity at high 0 2 conversion. Clustered vanadium ions improve the activity considerably, and a direct contribution of these sites in ethane activation can be hypothesized. On the other hand, these centers are less selective to ethylene at tempe atures lower than 500C. The spurious phase of Vf' oxide has instead a detrimental effect in the entire range of temperature. It has been reported in the literature that supported vanadium oxide works as a catalyst for the oxydehydrogenation of ethane, and that the nature of the interaction between the support and the vanadiup:oxide determines the catalytic performance (16). In the present case, the small amount of V oxide spread over the solid solution is less active than the solid solution. On the other hand, it affects the dishibution of the products, being responsible for most of the CO formed, probably through partial combustion of the intermediate formed on ethane activation, or of some partially oxygenated compound. The selective removal of this species makes the formation of CO almost disappear, with a consequent increase in the two reactions of Co;? and ethylene formation. The increase in selectivity with increasing temperature, shown in Figures 5 and 7, has been observed for a number of different catalytic systems active in ethane oxydehydrogenation (12,17). It is a general feature that occurs both on systems that operate at relatively low temperatures (that is the case of the catalysts based on transition metal oxides) and on those that need higher temperatures to activate ethane (i.e., Li/MgO-based systems). In the first case the reaction mechanism is practically completely heterogeneous, thus the reaction is initiated and completed on the catalyst surface. The catalyst also provides the oxygen necessary to form the ethylene through hydrogen abstraction, and the slow step of the reaction is the formation of an adsorbed ethyl radical species. In this case the increase in selectivity with increasing

f+

4'

g+

?+

100

temperature is attributed to the preferred decomposition of the intermediate (the ethyl radical itself, or perhaps an ethoxy species formed by reaction of the latter with oxygen) to ethylene. At lower temperatures instead the intermediate is rather stable, and its longer lifetime makes an unselective attack by oxygen more likely. CONCLUSIONS The solid state reaction between tin oxohydrate and v205 leads to the dispersion of \P’ ions inside the rutile lattice, with formation of VxSnl-x@ solid solutions. The latter is more active than pure Sn@ in ethane oxydehydrogenation, and the maximum selectivity to ethylene also is increased. Additional phases of vanadium oxide are instead detrimental for the selectivity. The VxSni-x@ system, if compared to the catalytic systems described in the literature, appears to be active, able to operate at temperatures lower than 500C (thus at conditions at which the contribution of homogeneous reactions is relatively low) even in the presence of high concentrations of ethane. Complete oxygen conversion (the limiting reactant) can be achieved with a residence time of 0.25 s, at 480C. ACKNOWLEDGEMENTS The work was sponsored by the Minister0 dell’Universiti3 e della Ricerca Scientifica. Prof. M. Guelton (Univ. of Lille) is gratefully acknowledged for the EPR measurements. REFERENCES

1) G. Centi and F. Trifirb, Catal. Today, 3 (1988) 151 2) C. Fumagalli, G. Golinelli, G. Mazzoni, M. Messori, G. Stefani and F. Trifib, Preprints I1 World Congress & IV European Workshop Meeting “New Developments in Selective Oxidation”, Benalmadena, Spain, sept. 1993 3) G . Centi, R.K. Grasselli and F. Trifirb, Catal.Today, 13 (1992) 661 4) O.S. Owen, M.C. Kung and H.H. Kung, Catal. Lett., 12 (1992) 45 5 ) E.M. Thorsteinson,T.P. Wilson, F.G. Young and P.H. Kasai, J. Catal., 52 (1978) 116 6) W.M.H. Sachtler, G.J.H. Dorgelo, J. Fahrenfort and R.J.H. Voorhoeve, Proceed. Fourth Int. Congress on Catal., Moscow, 1968,p.454 7) A. Andersson and S.T. Lundin, J. Catal., 65 (1980) 9 8) P.J. Pomonis and J.C. Vickerman, Faraday Disc. Chem. SOC.,72 (1981) 247 9) T. Ono and Y. Kubokawa, Bull. Chem. SOC.Japan, 55 (1982) 1748 10) F. Cavani. E. Foresti, F. Parrinello and F. Trifiib, Appl. Catal., 38 (1988) 31 1 11) S . Bordoni, F. Cavani and F. Trifirb, La Chimica & L’Industria (Milan), 74 (1992) 194 12) E. Morales and J.H. Lunsford, J. Catal., 118 (1989) 255 13) R. Burch and E.M. Crabb, Appl. Catal. A:General, 97 (1993) 49 14) P. Harrison and B. Maunders, J. Chem. SOC.Faraday Trans. I, 81 (1985) 1311 15) F. Cavani, G. Centi, E. Foresti, F. Trifirb and G. Busca, J. Chem. SOC.,Faraday Trans. I, 84 (1988) 237 16) J. Le Bars, J.C. Vedrine, A. Auroux, B. Pommier and G.M. Pajonk, J. Phys. Chem., 96 (1992) 2217 17) E.M. Kennedy and N.W. Cant, Appl. Catal., 75 (1991) 321

101

J. SORIA (Instituto de Catalisis y Petroleoquimica, Madrid, Spain): We hav studied the \P'

9'

incorporation in Ti02 (rutile). We have observed that at low concentration is dispersed in TiO2, but at a certain concentration V4+ ions are paired and shear plane are observed by TEM. Did you observe the formation of shear planes in your catalysts when intensity decreases ?

9'

F. CAVANI (Dipartimento di Chimica Industriale e dei Materiali, Bologna, Italy): We also have indications that for x values in VxSnl-xO;! higher than 0.03 vana ium ions clustering ions. We did not likely form rather than an homogeneous solid solution with isolated characterize the samples by TEM.

$+

J. VEDRINE (Institut de Recherches s

la Catalyse, CNRS, Villeurbanne Cedex, France): When you prepare solid solutions as xSni-x@ the surface is known to be V enriched. When you dissolve the "spurious" vanadium species as you mentioned by che 'cal treatment some V species remain. Do you have an idea of these surface species o p*lv'? isolated ?). As catalytic reaction occurred on the catalyst surface catalytic properties may be rather related to these surface species than to bulk solid solutions. Can you comment about this.

?

F. CAVANI: The chemical treatment removes all the vanadium oxide species that are spread on the surface and are not chemically anchored to the surface itself. XPS analysis (1) of treated samples shows a very weak signal of vanadium that cannot be unambiguosly attributed to either v4' or v'+ species. Obviously catalysis occurs on the surface, but it is known that the surface reactivity is affected by the bulk properties. The presence of ?' in the Sn@ lattice does affect the ,51102 chemical physical properties, and this may as well reflect on the surface reactivity of the tin oxide itself. Of course, it cannot be excluded that vanadium ions emerging at the surface of the solid solution can directly participate in ethane activation. 1. A. Santucci, D. Ghisletti, A. Bartolini, F. Cavani and F. Trifiib, in preparation.

B. GRZYBOWSKA (Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, P land): How does the reducibility of V/Sn02 system compare with that of in rutile T i e ? This latter system has been found not selective in solid solutions of o-xylene oxidation and in other oxidation reactions, I believe due to active \P' sites in non-reducible matrix. What are the active centers in

9'

F. CAVANI: We make the hypothesis that V4+ ions in SnO;! matrix affect the bulk and surface properties of tin oxide itself, so modifying its reactivity towards ethane, but it cannot be excluded that surface vanadium ions directly participate in ethane activation. In the case of o-xylene oxidation our system was found to be active but quite unseetive; it is likely that in order to afford oxygen insertion onto activated alkylaromatic bulk V oxide is needed; this is not the case for ethane oxydehydrogenation. We did not measure the reducibility of v4' ions in our systems.

A. ANDERSSON (Dept. of Chemical Technology, University of Lund, Sweden): Have you observed diffusion of vanadium from the bulk up to the surface during use in your reaction ?

F. CAVANI Under reaction conditions VxSni.&

are stable (no diffusion of vanadium up to the surface occurs) only when the initial value of x in calcined catalyst is lower than approximately 0.03. For higher values of x the solid solution is not stable, and during reaction

102

segregation of bulk vanadium oxide occurred.

J.R. EBNER (Monsanto Co., St. Louis, USA): What is the effect of O2/C2H6 ratio on reaction selectivity ? Is there any role that surface carbon might be playing in this reaction chemistry ? F. CAVANI Oxygen concentrations lower than the 10-15% (corresponding to the operation with 30-50 % ethane and air) increased the selectivity to ethylene, but also the ethane conversion remarkably decreased. No carbon was detected by means of XPS in spent catalysts.

V. Cortes Corbcriin and S. Vic Bcll6n Editors), New Developmenfs i n Selecfive Oxidafion If 0 1994 Elsevier Science B.V. All rights reserved.

103

Oxidative dehydrogenation of ethane over chromia-pillared montmorillonite catalysts P. Olivera-PastoP, J. Maza-Rodriguez", A. JimBnez-Lbpez", I. Rodriguez-Ramosb, A. Guerrero-Rufz" and J.L.G. Fierrob

"Departamento de Qufmica InorgAnica, Cristalograffa y Mineralogfa, Universidad de MAlaga, Apdo. 59, 29071 MAlaga, Spain bInstitutode Catiilisis y Petroleoquimica, C.S.I.C., Campus UAM, Cantoblanco, 28049 Madrid, Spain "Departamento de Quimica Inorginica, UNED, Senda del Rey s/n, 28040 Madrid, Spain

Thermally stable chrornia-pillared montmorillonite catalysts have been prepared by ionexchange with Cr 111 polyhydroxo acetate oligomers and subsequent calcination in ammonia. These catalysts have been tested in the oxidative dehydrogenation of ethane in a flow reactor at atmospheric pressure. Both specific activity and ethylene selectivity have been found to depend on the extent of chromia intercalation between the montmorillonite sheets and by pretreatment conditions. At typical reaction temperatures, i.e. 773 K, the catalysts with low chromia contents are rapidly deactivated, while those with high chromia contents display sustained activity for long periods on-stream. The activity decline in the oxidative dehydrogenation appears to be related more with chromium ion mobility than with the extent of coke deposition because the stabilization pretreatments in oxidant or reducing atmospheres strongly influence catalyst deactivation.

1. INTRODUCTION

The oxidative dehydrogenation of lower alkanes has became of major industrial importance in recent years because it represents a simple route to obtaining olefins from the low cost saturated hydrocarbons produced in oil refineries. The selectivity to olefin is generally poor because of the low reactivity of the alkane compared with the olefin product. The reaction appears, however, attractive for paraffins above C,, and several processes for the production of butenes have been described (1). Ethane is much more refractory than butane, and its catalytic conversion to ethylene in the absence of a halogen has generally proved inefficient. Among the varied catalysts proposed for this reaction, vanadium oxide-based catalysts (2-9) remain prominent. Very recently, it has been shown that chromium oxide supported on zirconia or incorporated into layered zirconium phosphates display rather high ethylene selectivities (10,ll). In the search for new catalysts for the same purpose, pillaring of montmorillonite clay by Cr 111 polyoxocations was found to maintain a wide permanent opening of the layer structure (12). In particular, when the Cr 111 polyhydroxo acetate oligorners have been intercalated into the

104

montmorillonite by ion-exchange, followed by calcination in an ammonia atmosphere, the resulting materials display high surface areas and high thermal stability up to 900 K . Against the above background the following objectives were pursued in the present study: (i) analysis of the catalysts properties by pyridine adsorption and X-ray photoelectron spectroscopy; (ii) investigation of the effects of varying the proportions of chromium oligomers intercalated into the montmorillonite sheets, and of the calcination temperature on the product distribution of the ethane oxidative dehydrogenation; (iii) relating the catalytic results to certain properties of the catalyst, i.e. acid-base properties and chromium dispersion.

2 . EXPERIMENTAL

A montmorillonite sample of Tidinit (Morocco) with unit cell formula (Si,,,AI,,)(Al,,~F%,~'''Mg~,~)O,(OH),and cation exchange capacity of 1.25 meq/g was used. Chromia-pillared montmorillonite catalysts were prepared as described elsewhere (12). Basically, an aqueous suspension of 1 g of montmorillonite in its Na-form was refluxed with increasing amounts of chromium 011) acetate. The solids were separated by centrifugation and washed with plenty water, and then calcined in ammonia atmosphere at 673-873 K for 4 h. The BET surface areas of the catalysts were determined using a conventional volumetric apparatus at 77 K, degassing overnight at lC4Torr (1 Torr = 133.3 N m-') and 473 K, using 0.162 nm2 as the cross sectional area of the adsorbed nitrogen molecule. The X-ray diffraction (XRD) profiles were recorded on a Siemens D501 diffractometer using Cu K a radiation equipped with a graphite monochromator. The infrared spectra were recorded on a Nicolet 5ZDX Fourier transformed spectrophotometer with a resolution of 4 cm". Self-supporting wafers of the samples with thickness ca. 10 mg cm-' were placed inside a vacuum cell provided with greaseless stopcocks and KBr windows. The samples were outgassed in situ at 673 K for 1 h, and subsequently exposed to 2 Torr of pyridine vapour at room temperature for 5 min, and then outgassed at 393 K for 1 h. The X-ray photoelectron spectra (XPS) were recorded with a Fisons ESCALAB 200R spectrometer equipped with a Mg Ka X-ray excitation source and hemispherical electron analyser. The samples were turbopumped to ca. lCr5 Torr before they were moved into the analysis chamber. The residual pressure in this ion-pumped chamber was maintained below 1.5 x lo9Torr during data acquisition. Although surface charging was observed in all samples, accurate binding energies ( k 0 . 2 eV) (BE) could be determined by charge referencing with the adventitious C l s peak at 284.9 eV. Catalysts were tested in a 0.6 cm O.D. fixed-bed tubular quartz reactor working at atmospheric pressure. The catalyst temperature was measured by a thermocouple placed in the outer wall of the catalyst bed. A catalyst charge of 0.05-0.15 g without dilution was used in each experiment. The individual flows of reactants were adjusted by mass-flow controllers (Brooks). The effluents of the reactor were analysed by on-line gas chromatography (Varian 3400) provided with a thermal conductivity detector. Porapak Q and molecular sieve 5A packed columns using a column isolation analysis programme were employed for product separation. Prior to the reaction, the catalysts were pretreated at 773 K in air (or He) flow for 2 h. Typically the reaction was studied at 773 K with a total flow rate of 70 cm' m i d and the gas mixture composition CZH,:O,:N, = 30: 15:55.

105 3. RESULTS AND DISCUSSION 3.1. Structural and textural analysis

Montmorillonite takes up large amounts of chromium in excess of the cation exchange capacity upon reaction with chromium (111) acetate. After a maximum uptake, higher additions (> 70 meq Cr(II1)lg) lead to intercalation compounds with lower contents. The intercalated species are Cr(II1) polyhydroxo acetate oligomers (12,13). Assuming a layer thickness Of 0.95 nm for montmorillonite, basal expansions between 0.7 and 1.1 nm are obtained at room temperature (Table 1). Calcination of these materials above 473 K in air or N, yields collapsed structures with segregation of chromium oxide. Instead calcination under ammonia leads to a series of pillared materials stable up to 900 K with basal spacings in the range 1.23-1.53 nm and high surface areas (13) (Table 1). Ammonia not only retains the protons released during calcination (14) but also the presence of NH,+ in the interlayer region stabilizes Cr(III), preventing its oxidation to CrO,, thereby avoiding its segregation from the pillars. Five materials calcined in ammonia were chosen, designated hereafter as Cr-xS, where S represents 1.25 meqlg of Cr(II1) added. Although materials 15S, 20s and 50s do not differentiate very much in chromium content it appeared of interest to compare their catalytic properties, since oligomers with different charge and size are intercalated in layered host depending on the chromium concentration in solution and chromium/montmorillonite ratio (12,13). These three materials retain basal spacings higher than 1.4 nm, a threshold value to be considered a pillared layered structure. In any case, the specific surface areas of these sample increased substantially with respect to the Na-montmorillonite resuspended in water (62 m2/g).The N, isotherms of pillared samples are characteristic of microporous solids (micropore volume = 0.1 mllg), but with a significant contribution of mesopores. Interestingly, although the maximum basal expansion is about 0.6 nm the majority of pores are in a narrow range between 0,75 and 2.0 nm of radii, making possible the access of small molecules such as ethane to the active sites. To study the nature of the acid sites, infrared spectra of pyridine adsorbed on the surface of Cr-xS samples were examined. Figure 1 compiles the spectra of Cr-xS samples pretreated in ammonia atmosphere at 873 K (spectra a-c). Also for comparative purpose, the spectrum of the 0 - 1 5 s sample pretreated in ammonia at 773 K is included (spectrum b'). There were major peaks at ca. 1549, 1490 and 1450 cm" identified following evacuation at 393 K. In agreement with literature findings (15,16) the peak at 1549 cm-' is characteristic of pyridinium ions (PyH+), which were formed on the Bronsted acid sites. The other set of two bands at 1490 and 1450 cm" contain contributions from pyridine coordinatively bonded to Lewis acid sites. An inspection of the spectra of Fig. 1 reveals that Lewis acidity, as measured by the absorbance of the band at 1450 cm-', increases with Cr content. However, the Bronsted acidity, as measured by the band at 1549 cm-', is at a minimum for the sample Cr153873. This finding appears to be very interesting because protons catalyse undesired side reactions with the olefin product. As will be described below, this sample is the most selective to ethylene. Both fresh and spent catalysts were analysed by photoelectron spectroscopy. The BEs values of the most intense Cr2p3,, peak are summarized in Table 1. The BE of Cr2p,,, peak in the fresh samples are typical of Cr(II1) species and are higher than those observed on chromia-alumina catalysts (17). This finding can be interpreted as due to a strong interaction of chromia pillars with montmorillonite sheets, specially in samples of lower Cr-contents. The same tendency is maintained for the spent catalysts, in which large amounts of coke were also detected by this technique. The chromium-to-silicon intensity ratios changed markedly after time on stream for the samples calcined

106

Figure I . intrCir-c.dspectrii of adsortwcl pyritline on several Cr-pillared montmorillonite catalysts: (a), Cr-50S873; ( b ) , Cr-15S873; (c) Cr-5S873. For compative purpose, the spectrum of the Cr-15s sample pretreated in ammonia at 773 K is included (bpect rum b’).

in NH, below 873 K. In particular, for the sample Cr-15s calcined at 773 K, this ratio decreases when it is pretreated in air and increases when pretreatment is carried out in helium. Therefore. pretreatments at T < 873 K to a great extent determine the surface characteristics of the catalysts. I n addition to a segregation of Cr in small proportion (lo), pretreatment in air also causes a partial degradation of the silicate layer, whereas in a non-oxidising atmosphere partial segregation of Cr is predominant. The XPS data also indicate that a certain amount of Cr,O, is deposited on the montmorillonite surface after treatment with large amounts of Cr3-. Thus, the sample Cr-SOS, with chromium content slightly higher than Cr-1% and Cr-20S, presents an unusually high Cr:Si ratio o n the surface and displays the BE of Cr2p3,?peak close to that reported for chromia (18).

3.2. Effect of chromium content Catalytic activity data for the oxidative dehydrogenation of ethane are listed in Table 2. Activity data for a Cr3+-exchanged montmorillonite sample is also included for comparison. All catalysts were tested under similar experimental conditions using ethane and air flow rates of 21 and 50 cm3 min?, respectively. Only data obtained after 200 min on-stream were selected for comparison. The temperature of the reactor was maintained at 773 K. Ethylene, carbon monoxide and carbon dioxide were the only detected carbon-containing products. To avoid the possible dependence between ethane conversion and ethylene selectivity, ethylene yields are also compiled in the last column of Table 2. Initially, ethane conversion levels, specific activities and yields increase with increasing the chromium content, up to a maximum value of 18.2% (catalyst Cr-1.5s). Then the values of these

Table 1 Characterisation data of fresh Cr-pillared montmorillonitesamples

Cr-2S673 Cr-5S873 Cr-5S873' Cr-15S773 Cr-15S773I Cr-15S7732 Cr-15S873 Cr-20S873 Cr-50S873

' Spent catalyst;

4.9 13.8 13.8 18.2 18.2 18.2 18.2 19.2 21.1

1.63 1.65 1.65 1.89 1.89 1.89 1.89 2.04 1.77

1.23 1.23 1.23 1.53 1.53 1.53 1.53 1.52 1.44

186 274 274 333 333 333 333 353 349

0.102 0.109 0.369 0.209 0.570 0.329

577.8 577.8 577.4 577.6 577.6 577.7

0.795

577.2

Pretreated in helium at 773 K and used in reaction

Table 2 Catalytic behaviour for the oxidative dehydrogenation of ethane Catalyst

Cr3 -Montmor Cr-2S673' Cr-5S673 Cr-5S873 Cr-IOS673 Cr-15S773 Cr-15S7733 Cr-15S873 Cr-20S773 Cr-20S873 Cr-50S773 Cr-50.5873 +

XT

Activity'

Selectivity (%)

(%I

bmohLts)

C2H4

co

co,

(%)

6.9 7.0 6.7 2.6 10.4 35.4 36.0 37.0 32.6 36.0 21.3 33.1

14.8 10.8 16.5 8.9 28.5 117.7 117.2 93.3 106.1 117.0 66.5 101.2

22 13 I1 23 12 25 21 26

40 33 31 37 38 28 25 24 22 24 7 20

38 54 58 40 50 47 53 51 63 53 88 63

1.52 0.91 0.74 0.60 1.25 8.85 7.56 9.62 4.89 8.28 1.07 5.63

15

23 5 17

Yield

'Reaction conditions: 773 K, 200 min on-stream, air flow of 50 cm3/min,ethane flow of 21 cm3/min; Temperature of the pretreatment in ammonia; Pretreated in He at 773 K for 2 h

108 catalytic parameters decreased in catalysts with higher chromium content. For a given temperature. there is no clear relationship between the selectivity and chromium content. Rather, the selectivity appears to be increased with the dispersion degree of chromium on the silicate layers. Inasmuch the basal spacing does not increase much with the chromium content is to be expected that the pillar density is higher in catalysts with higher chromium content, in which, furthermore, a part of chromium is deposited on the external surface. Therefore, the dispersion degree decreases with chromium content. This can explain that the catalyst Cr-lSS, still maintaining a high dispersion degree of chromium, displays the highest selectivities and yields at different pretreatment conditions. The relative activity for the oxidative dehydrogenation of ethane at 773 K of samples calcined in an ammonia atmosphere at 873 K and pretreated in air prior to the catalytic test was determined as a function of the time on-stream. These data are displayed in Fig. 2. As regards the variation of the catalytic activity with time on-stream for Cr-xS samples, three different trends can be discerned depending on the chromium content: (i) the activity is maximum at the beginning of the experiment for Cr-SS and then shows a continuous marked decay; (ii) the activity increases slightly during the first 60 min on-stream and then levels off for the samples with high chromium content (Cr-1% and Cr-20s); (iii) the activity is essentially constant from the beginning for the catalyst Cr-50s. On the whole, ethylene selectivity was essentially constant regardless of the chromium content.

80

v

100 h

5

90

d

9

4

2 80 0

100 200 Time on-stream (min)

300

Figure 2 . Effect of chromium content on catalyst deactivation of several Cr-xS873 catalysts: (m), x = 5 ; (A), X = 15; (O), X = 20; k),X = 50. Figure 3. Effect of pretreatments on the deactivation of catalyst 0 - 1 5 s : (@, ammonia at 773 K and then air prior to reaction; (A), ammonia at 773 K en then helium prior to reaction: (o), ammonia at 873 K and then air at 773 K prior to reaction.

109

3.3. Influence of the pretreatments pretreatments of the samples prior to catalytic run seem to alter to a great extent their stability in the reaction. The specific activity for ethane dehydrogenation at 773 K on a catalyst subjected to various pretreatments is displayed in Fig. 3 as a function of time on-stream. Although these data refer to Cr-15s catalyst, they are representative of the behaviour observed on all Cr-xS catalysts. Before testing, the Cr-15s sample was calcined in an ammonia atmosphere at 773 and 873 K, and pretreated in air or helium at 773 K within the reactor. An inspection of the data shows that: (i) catalyst calcination in an ammonia atmosphere at the highest temperature (873 K) leads to a catalyst with sustained activity, (ii) the air-pretreated catalyst appears to be deactivated after 3 h on-stream, (iii) pretreatment in helium does not deactivate the catalyst. In agreement with the data in Fig. 2, it can be noted again that the relative activity increases slightly, in particular for the sample pretreated at 773 K in ammonia, during the first 50 min on-stream. Different parameters relative to the stabilization of the catalyst have been considered. As previously reported (13), preparation conditions such as C?+/montmorillonite ratio, Cr3+ concentration in solution, atmosphere and temperature of calcination, control to a large extent the surface characteristics of these materials. The most thermally stable materials are those prepared under ammonia at 873 K; under these conditions the OH- groups, believed to be responsible for destabilization of the pillars, are practically removed. Additionally, stabilization pretreatments are determinant for the catalyst activity. As indicated by XPS measurements, for materials with high Cr content, pretreatment at 773 K in an oxidising atmosphere causes a certain degradation of the silicate layer, decreasing the Cr:Si ratio on the catalyst surface whereas in a reducing atmosphere only a partial segregation of Cr is observed, thus the Cr:Si ratio increases in sample Cr-15s. As demostrated by pyridine adsorption, these catalysts show both Lewis (L) and Bronsted (B) acid sites, the latter being strongly influenced not only by chromia content but also by catalyst pretreatments. Irrespective of the small change of the textural parameters, the minimum Bronsted acidity and the highest LIB ratio found on the Cr-15s sample pretreated at 873 K corresponds with the largest activity and ethylene selectivity in similar range of Cr composition. Furthermore, the comparison of the catalytic behaviour of the Cr-pillared materials with that of the parent CP+exchanged montmorillonite indicates clearly that pillaring of the silicate layers with appropriate amounts of chromium oligomers leads to formation of effective catalysts for the oxidative dehydrogenation of ethane.

Acknowledgement This research was supported by the Comisi6n Interministerial de Ciencia y Tecnologia, Spain (Grant MAT90-0298). Thanks are due to Dr. J.A. Anderson for critical reading of this manuscript.

References 1.

2. 3.

G. Centi and G. Golinelli, J. Catal., 115 (1989) 452. D. Patel, M.C. Kung and H.H. Kung, J . Catal., 105 (1987) 483. D. Patel, M.C. Kung and H.H. Kung, in M.J. Phillips and M. Ternan (Eds.), Proceedings of 9th International Congres on Catalysis, Calgary, 1988, The Chemical Institute of Canada,

110

4. 5. 6. 7.

8.

9. 10. 11.

12. 13. 14. 15.

16. 17. 18.

Ottawa, Vol. 4, p. 1554. R. Burch and R. Swarnakar, Appl. Catal., 70 (1991) 129. A. Erdohelyi and F. Solymosi, J. Catal., 123 (1990) 31. D.S.H. Sam, V. Soenen and J.C. Volta, J. Catal., 123 (1990) 417. M. Merzouki, B. Taouk, L. Monceaux, E. Bordes and P. Courtine, in P. Ruiz and B. Delmon (Eds.), New Development on Selective Oxidation by Heterogeneous Catalysis, Stud. Surf. Sci. Catal., Vol. 72, Elsevier, Amsterdam, 1992, p. 165. M. Merzouki, B. Taouk, L. Tessier, E. Bordes and P. Courtine, in L. Guzci, F. Solymosi and P. Tetenyi (Eds.), New Frontiers in Catalysis, Vol. A, AkadCmiai Kiad6, Budapest, 1993, p. 753. J . le Bars, J.C. Vedrine, A. Auroux, S . Trautmann and M. Baerns, Appl. Catal. A: General, 88 (1992) 179. S . Cheng and S.Y. Cheng, J. Catal., 222 (1990) 1. M. Loukah, G . Coudurier and J.C. Vedrine, in P. Ruiz and B. Delmon (Eds.), New Development on Selective Oxidation by Heterogeneous Catalysis, Stud. Surf. Sci. Catal., Vol. 72, Elsevier, Amsterdam, 1992, p. 191. P. Maireles-Torres, P. Olivera-Pastor, E. Rodrlguez-Castell6n, A. JimCnez-Mpez and A.A.G. Tomlinson, J. Mat. Chem., 1 (1991) 739. A. JimCnez-Mpez, J. Maza-Rodrlguez, P. Olivera-Pastor, P. Maireles-Torres and E. Rodrlgue~-Castell6n,Clays and Clay Min., 41 (1993). D.E.W. Vaughan, Catal. Today, 3 (1988) 187. M.C. Kung and H.H. Kung, Catal. Rev.-Sci. Eng., 3 (1985) 425. G. Connell and J.A. Dumesic, J. Catal. 105 (1987) 285. O.F. Gorriz, V. CortCs Corberiin and J.L.G. Fierro, Ind. Eng. Chem. Res., 31 (1992) 2670. C.D. Wagner, W.M. Riggs, L.E. Davis, and G.E. Muilenberg (Eds.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin Elmer Corporation, Minnesota, 1978.

111

DISCUSSION J.C. Vedrine (Institut de Recherches sur la Catalyse, Villeurbanne, France): When your pillar clay material the question arises whether the Cr species as pillar play a role or if external Cr,O, are the active sites. This is important for the ODH reaction which may occur also in the homogeneous phase after having been initiated on the catalyst surface. Moreover, porous materials are detrimental for oxidation reaction because of the large residence time of the reactant and product molecules. Can you tell us if the reaction occurs in the outer surface of crystallites or within the interlayer spacing? What is the homogeneous phase part?

P. Olivera-Pastor (Universidad de Mdlaga, Malaga, Spain): Although pillaring conditions avoid as much as possible segregation of chromium oxide, it is evident from XPS analysis that the amount of chromium on the surface increases with increasing additions despite that the overall chromium content does not significantly increases for catalyst Cr-15s. However, evidence of the presence of a fraction of chromium oxide deposited on the external surface is only detected for catalyst Cr-50.5, which exhibits a high Cr/Si ratio and the the BE of the Cr2p3,, peak similar to that of chromia. Although the results presented in this study do not allow us to draw out a definite answer, there are facts indicating that, at least, a fraction of the interlayer surface is involved in the reaction. It is significant that the activity, selectivity and yield for this reaction are drastically reduced in the catalyst Cr-50s as compared with the other catalysts, where chromium is well dispersed at the outer surface. Moreover, the free height of the pillared materials should not be considered as a real width of the pores since the pillar density influences in the pore size too. Interestingly, although the maximum basal expansion reached is about 0.6 nm, most of pores in chromiamontmorillonite samples are in a narrow range between 0.75 and 2.0 nm of radii, so that an important of the catalyst surface is accesible to the small ethane molecules.

Trifiro (University of Bologna, Bologna, Italy): Chromium oxide is known to be active in pure dehydrogenation. Do you have indications that you have an intermediate dehydrogenation step in the reaction you have investigated?

F.

P. Olivera-Pastor: Chromium oxide is a rather well studied catalyst in the pure dehydrogenation and also the ODH of light alkanes. The occurrence of one of these reactions depends strongly on the experimental conditions used. Without doubt, the most important difference between the two reactions is that the oxidative dehydrogenation can only be conducted if oxygen (or air) and hydrocarbon are simultaneously fed into the reactor. As these experimental conditions were used in this study the pure dehydrogenation seems to be very low. In favour of this is the very low activity, and still lower selectivity to ethylene, observed in a blank experiment; large carbon amounts being detected. Accordingly, although the intermediate dehydrogenation step may be involved, its extent does not appear to be large enough to explain the high ethylene selectivities measured.

This Page Intentionally Left Blank

V. Cortks Corberan and S. Vic Bellon (Editors), New Developments in Seleclive Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

113

Oxidative dehydrogenation of propane and n-butane on V-Mg based catalysts A. Corma", J.M. Lopez Nieto", N. Paredes", A. Dejozband I. VaZquezb "lnstituto Tecnologia Quimica, UPV-CSIC, Universidad Politknica, c/ Camino de Vera s/n, 46071-Valencia (Spain) bDepartamento de Ingenieria Quimica, Universidad de Valencia, Burjasot (Spain) Abstract The catalytic properties of vanadium supported catalysts for the oxidative dehydrogenation of propane and n-butane has been studied. Natural Sepiolite and magnesium oxalate were used as starting support materials. Large differences in the type of crystalline phases were detected on the calcined catalysts depending on the vanadium content and nature of the support. The active and selective sites for the oxidative dehydrogenation of propane and n-butane, are related with isolated v5' with a tetrahedral coordination. The presence of vanadium species with coordination higher than 4 negatively influence the selectivity to dehydrogenation products. 1. INTRODUCTION

The oxidative dehydrogenation (ODH) of lower alkanes is an interesting alternative to the conventional dehydrogenation of LPG, owing to the possibility of working at lower reaction temperatures. However, since the formation of carbon oxides is more favorable thermodinamically than the formation of olefins, it is necessary to intercept the desired products kinetically. For this reason, catalysts must increase the formation rate of the desired products. V-Mg-0 catalysts present high activity and selectivityfor the ODH of different alkanes (1-6), although the reaction mechanism and the nature of the selective sites on the catalysts is under discussion. On vanadium supported catalysts, the acid-base character of the support and the preparation procedure of the catalysts strongly influence both the nature of vanadium species on the catalyst and the catalytic properties. A basic character of the support favors the formation of v5' tetrahedral species and increase the selectivity to dehydrogenated products (6). In this way, Sepiolite-supported vanadium catalysts present high selectivityto propylene for the ODH of propane (7). On the other hand, it has been observed that the preparationprocedure of VMg-0 catalysts also influencestheir catalytic properties (8). In this way it has been

114

presented a new preparation procedure of V-Mg-0 catalysts which produces high surface area catalysts, with high catalytic activity (8). In this paper we present a comparative study on the influence of the nature of the vanadium species present in V/Sepiolite and V/MgO catalysts on their catalytic properties for the oxidative dehydrogenation of propane and n-butane. 2. EXPERIMENTAL

2.1. Catalyst preparation The catalysts, with different vanadium contents, were prepared by impregnation of natural Sepiolite or magnesium oxalate using aqueous vanadyl oxalate solution, then they were dried at 100 OC, overnight, and calcined at 550 "C for 3 h (7,8). The catalysts refereed as V/Sep and V/MgO are preceded by a number that indicates the wt% of vanadium given as V,O,. 2.2. Catalyst characterization Specific surface areas were obtained by the BET method using N, adsorption. X-ray diffraction (XRD) patterns were obtained by means of a Philips PW-1100 diffractometer using Ni-Filtered CuKa radiation (h= 0.15406 nm). TPR results were obtained in a Micromeritis apparatus. Samples (10 mg) were first treated in argon at room temperature during 1 h. The samples were subsequently contacted with a HJAr (molar ratio of 0.15, total flow of 50 cm3 rnin-') and heated, at a rate of 10 "C min-', to a final temperature of 900 OC. 2.3. Catalytic testing Catalysts were tested in a fixed tubular reactor equipped with a coaxial thermocouple for temperature profiling. The catalyst (particle size 0.27-0.42 mm) were mixed with SIC (particle size 0.59) at a volume ratio, catalyst/SiC= 1/2. The feed consisted of a mixture of propane/oxygen/helium and n-butane/oxygen/helium of 4/8/88 and 5/20/75, respectively. In order to compare selectivities at the same level of conversion, different contact times, W/F, were used. A more detailed experimental procedure has been described previously (6,7). Blank runs showed that under the experimental conditions used in this work the homogeneous reaction can be neglected. Conversion and selectivity was defined taking into account the number of carbon atoms in each product molecule. The initial rates (in mol h-' m-') for the oxidative dehydrogenation of alkanes were obtained from the variation of the alkane conversion with the contact time (at an alkane conversion level < 10%). 3. RESULTS AND DISCUSSION

The influence of the vanadium content on the catalysts surface areas on both V/Sep and V/MgO catalytic systems are shown in Figure 1. In the V/Sep series, the

115

Figure 1. Influence of the vanadium content in V/MgO (0)and V/Sep (v) series on the surface area of the catalysts.

0

20

40 V2O5,wt %

higher the vanadium content the lower the surface area is. This effect can be explained on the bases of the deposition of vanadium on the surface. However, for the V/MgO catalysts, an influence of the vanadium content on the catalyst surface area was only observed for catalysts containing more than 25 WO of vanadium (reported as V,O$. In V-Mg-0 catalysts, prepared by different methods, a small influence of the vanadium content on their catalyst surface area has been observed. In addition to this, from the XPS results for the V/MgO catalysts presented in this paper, a homogeneous distribution of vanadium along the support has been proposed (8).

0

20 V205,

40 wt z

0

20

40

V2 0 5 , w t %

Figure 2. Dependence of the appearance of crystalline phases dith the vanadium content of the catalyst, obtained for V/MgO (a) and V/Sep (b) series. Symbols: ( ) Mg,V,O,; ( 0 ) a-Mg,V,O,; ( W ) isolated VO, tetrahedron; ( ) WO;)"; ( A ) MgV20,; ( A ) v205.

+

116

From the XRD and raman Laser spectroscopy data, differences in the crystalline phases of vanadium between V/Sep and V/MgO series were observed (7). In Figure 2 it is shown the influence of the vanadium content on the appearance of the crystalline vanadium phases obtained in the V/MgO and V/Sep series. In the V/MgO catalysts (Fig. 2a), the presence of MgO in addition to Mg,V208 (at low vanadium contents) or Mg,V208+ a-Mg,V20, (at high vanadium contents) were observed (8). For the V/Sep catalysts (Fig.2b), the presence of isolated VO, tetrahedron (at low vanadium content) or MgV20, and V,O, (at high vanadium content) has been observed (7). We must point out that modifications of the Mgsilicate structure are also produced when increasing the vanadium content. These results could be explained by the different magnesium accessibility and therefore the different Mg-V interactions in both catalytic systems, being the interaction smaller in the V/Sep series. In V/MgO series, the magnesium oxalate decomposes during the calcination step forming MgO that reacts with vanadium forming only magnesium vanadates (8). On the other hand, the different vanadium-containing phases, present on V/MgO or V/Sep series, strongly influence the catalytic properties for the oxidative dehydrogenation of alkanes. In Figure 3 it is shown the influence of the vanadium content of the catalysts on the initial rate (in mol h-' m-2)for the oxydehydrogenation of propane (Fig.3a) and n-butane (Fig.3b). It can be seen that higher activities are obtained on V/MgO catalysts, being the 20V/MgO catalyst the most active one. The different catalytic activities can be explained on the bases of the different reducibility of the catalysts.

N

'E c

i

0

20

40

v,

0, ,wt %

0

20

40 %

V205,Wt

Figure 3. Influence of the vanadium content in V/MgO (0)and V/Sep (v) catalysts on the initial rate for the oxidative dehydrogenation of propane (a)and n-butane (b), obtained at a reaction temperature of 5UU "C.Experimental conditions in text.

117

Figure 4. TPR results obtained on V/MgO catalysts: (a) N/MgO; (b) 2OVYiWgO; (c) 34VYiWgO; (d)48WMgO.

t 100

a 300 500 700 Temperature , "C

In Figure 4 the TPR patterns of the V/MgO catalysts are shown. In general, all catalysts present two peaks: the first one at about 360 OC and the second one at about 550 OC. Only the 48V/Mg0 catalyst presents a higher reduction temperature, shown by two peaks in the temperature interval of 650-700 "C. The peak at 360 OC corresponds to the more reducible sites, and its area is proportional to the number of the more reducible vanadium species. From the comparison between the area of the peak at 360 "C and the initial rates (Fig 3a), it can be concluded that the vanadium sites that present lower reduction temperature are responsible for the catalytic activity in the ODH of propane and n-butane. We must indicate that the XPS measurements of V/MgO catalysts show a homogeneous dispersion of vanadium along the support (8). As for the 20V/MgO catalyst a M g N surface atomic ratio of 10 was observed, it can be concluded that isolated v5' species are present on the surface of the catalyst. These vanadium species show the higher activity and the higher reducibility. For this reason, at low vanadium contents, both isolated species and activity increase when increasing the vanadium contents. However, at higher vanadium contents, in which the crystallinity of the Mg-vanadates increases, a lower activity and a lower reducibility has been observed. In the case of the V/Sep catalysts, the greater the vanadium content the higher the specific activity is. Except in the case of low vanadium contents, the presence of v5' species, with a coordination higher than 4, are observed. The formation of these species increase when increasing the vanadium content, forming crystalline VO , , that it is also active for the transformation of alkanes. On the other hand, greater differences in the selectivity to dehydrogenated products can be obtained depending on the vanadium content, the support and the alkane.

v'

118

60 -

0

20

40 V20,,wt %

0

20 40 V20,,wt %

Figure 5. Influence of the vanadium content in V/MgO (a) and V/Sep (b) catalysts on the selectivity to dehydrogenated products obtained during the ODH of propane (v) and n-butane (0)at 550 OC and a conversion level of alkane of 20%.

In Figure 5 it is shown the influence of the vanadium content of the catalysts on the selectivity to olefins, obtained during the oxidative dehydrogenation of propane and n-butane on both V/MgO (Fig.%) and V/Sep (Fig.5b) series. We must indicate that dehydrogenated products and carbon oxides were the main products. Methane and ethane were obtained only at high conversion level, and their selectivity was lower than 2%. Partial oxygenated products were not detected. In V/MgO series, the higher selectivities to dehydrogenated products are obtained during the transformation of n-butane, being the 34V/MgO catalyst the most selective one. In the V/Sep series, a different influence of the catalyst vanadium content on the selectivity to dehydrogenated products were obtained in the oxidative dehydrogenation of propane and n-butane. However, with both alkanes the higher selectivity is obtained on catalysts containing about 10% of vanadium. The lower selectivity in the ODH of n-butane, obtained on V/Sep catalysts with high vanadium content, can be explained by the presence of V-0-V pairs and/or V-0 double bonds on the catalyst, as it can be concluded from the presence of crystalline V,O, (Fig.2b). These V-0-V pairs can attack more easily the C,-intermediates producing carbon oxides. Similar conclusion has been recently proposed (5,6). On the other hand, a different distribution of the C, olefins are observed on V/MgO that on V/Sep. In Table 1 are shown the selectivities to the main products for the oxidative dehydrogenation of n-butane on the more selective catalysts. In general, greater selectivities to 2-butenes (cis- and trans-) were observed for V/Sep

119

Table 1. Influence of the reaction temperature on the Selectivity to C,-olefins, obtained at a n-butane conversion level of 20%.

si (%) Catalyst 7V/Sep

35VlMg0

W/F(')

T

ec,

1B

2tB

2cB

BD

co/co, ratio

40.2

500

8.6

9.6

8.9

11.3

0.79

26.8

525

11.5

11.2

9.1

11.8

0.86

18.0

550

13.0

10.0

9.0

13.0

0.95

20.5

500

11.6

4.2

4.7

12.5

0.32

7.9

525

16.5

6.5

7.4

17.9

0.33

4.0

550

18.5

7.1

8.4

21.0

0.34

(a) W/F in g,$(mol-CJ'; BD= butadiene.

lb)

1B= 1-butene; 2tB= 2-t-butene; 2cb= 2-c-butene;

catalysts, while greater selectivities to 1-butene and butadiene were obtained on the V/MgO series. This difference can be due to the different vanadium species observed on both catalytic systems. However, the presence of acid sites in VISep catalysts can also favor the isomerizationof 1-butene to 2-butene and the formation of carbon oxides. The reaction temperature shows no influence on the selectivity for the oxidative dehydrogenation of propane, in accordance with previously reported data (3,4,7,9). Kung et al. (1,2) have also proposed that the selectivity to C,-olefins is not influence by reaction temperature. However, as it can be seen in Table 1, for the oxidative dehydrogenation of n-butane the selectivity to dehydrogenation products increases when increasing the reaction temperature. This effect has been observed in all catalysts studied in this paper. We must remarked, that an inverse trend between alkane conversion and selectivity to dehydrogenated products is observed. For this reason, the selectivity comparison must be carried out at the same alkane conversion level, as has been done in Table 1. Differences in the activation energies for each reaction, in addition to stability of the olefins (the selectivity to butadiene increases when increasing the reaction temperature) could explain the different effect of the reaction temperature on the ODH of n-butane and propane. In conclusion, the catalytic properties of V/MgO and V/Sep are influenced by the different dispersion of vanadium on the support. In this way, higher dispersion of vanadium and the formation of Mg-vanadates with a lower VIMg ratios are

120

obtained on V/MgO than on VISep catalysts. Isolated VO, tetrahedron, that are detected on our more selective catalysts, may be proposed as selective sites for the ODH of alkanes. The importance of the tetrahedral v5‘ species for the oxidative dehydrogenation of alkanes on different catalytic systems, such as: V/MgO (6,9), V/SiO, (lo), V-silicalite (11) and VAPO-5 (12) has been recently proposed. On the other hand, the presence of V-0-V pairs on the catalysts produces a decrease in the selectivity, mainly by the simultaneous attack, in more them one point, to the adsorbed alkane or to the reaction intermediates (6). In this case, the ODH of n-butane must be more influenced by the longer length chain, as it has been recently proposed (5). Indeed, a lower selectivityto dehydrogenated products is obtained for the oxidative dehydrogenation of n-butane on V/Sep catalysts with high vanadium contents, in which V,O, is observed. ACKNOWLEDGMENTS

The financial support by Comision lnterministerial de Ciencia y Tecnologia (Proyect MAT 607/91) is gratefully aknowledged. REFERENCES

1. 2. 3. 4. 5.

6. 7.

8. 9. 10. 11. 12.

H.H. Kung and M.A. Chaar, U.S. Patent 4,772,319, (1988). M.A. Chaar, D.Patel, M.C. Kung, and H.H. Kung, J. Catal., 105 (1987) 483. D. Siew Hew Sam, V. Soenen and J.C. Volta, J. Catal., 123 (1990) 417. M.A. Chaar, D. Patel and H.H. Kung, J.Catal.,lOS (1988) 463. P.M. Michalakos, M.C. Kung, I. Jahan and H.H. Kung, J. Catal., 140, (1993) 226. A. Corma, J.M. Lopez Nieto, N. Paredes, M. Perez, J. Shen, M. Cao and S.L. Suib, in “New Developments in Selective Oxidation by Heterogeneous Catalysis” (P. Ruiz, 6. Delmon, Eds.),Elsevier, Amsterdam (1992) p.213. A. Corma, J.M. Lopez Nieto, N. Paredes and M. Perez, Appl. Catal., 97 (1993) 159. A. Corma, J.M. Lopez Nieto and N. Paredes, Appl. Catal., 104 (1993) 161. A. Corma, J.M. Lopez Nieto and N. Paredes, J. Catal., 144 (1993) in press. J. Le Bas, J.C. Vedrine, A. Aroux, S. Trautmann and M. Baerns, Appl. Catal., 88, (1992) 179. G. Bellusi, G.Centi, S. Perathoner and F. Trifiro, A.C.S. Symposium on Catalytic Selective Oxidation, Washington D.C., (1992), p. 1242. P. Concepcion, J.M. Lopez Nieto and J. Perez Pariente, Catal. Letters, 19 (1993) 333.

121

J. Barrault (L. Catalyse, Poitiers, France): In Figure 5b we can observed that the selectivity in ODH of propane goes to 40 % while the selectivity in ODH of n-butane presents a maximun for a V,05 content of about 10 %. Does it mean that active sites in ODH of propane are not active in ODH of n-butane when vanadium oxide is supported on sepiolite. If so, could you comment?

J.M. Lbpez Nieto (I. Tecnologia Quimica, Valencia, Spain): According to Kung et al. ( l ) , the different catalytic properties between Mg-vanadates for the ODH of propane and n-butane can be explained on the bases of the V-0-V distance and the size of the paraffins. In our case, the different vanadium species obtained on VMg-0 and V/Sep catalysts (specially the presence of V205and MgV20,) can explain the different behaviour between both catalytic systems. J.C. Volta (I. de Recherches sur la Catalise, Villeurbanne, France): Did you try to work at different alkane/O, ratio as compared to the catalytic conditions you propose. If so, do you have the same clasification V-MgO and V-sepiolite as you propose?

J.M. L6pez Nieto (I.Tecnologia Quimica, Valencia, Spain): For the ODH of propane and n-butane on MgO-supported vanadium catalysts, order 1 and 0 for alkane and oxygen, respectively, have been reported (2). Similar results have been obtained on our catalysts in the ODH of propane (3,4). For this reason, altough the experimental conditions are not the same, a comparison between the results obtained during the ODH of propane and n-butane can be considered.

F. Trifir6 (Univ. Bologna, Italy): There are some discrepancies in literature between the nature of active phase in Mg-V system and of the MgN surface ratio (papers of Kung and al. and of Volta and al.), the presence of ortovanadate against that one of pyrovanadate. May you tell us what is your position between these controversies. J.C. Volta (I. de Recherches sur la Catalise, Villeurbanne, France): Thank you Prof. Trifiro to put in advance the discrepancies between our results (5) and the results of Prof. Kung (6). In order to solve these discrepancies and the connection with your

results, it should be important to measure by XPS the superficial MgN ratio in order to see if in some phases there should be an eventual superficial contamination of one phase by a second one. J.M. Lbpez Nieto (I. Tecnologia Quimica, Valencia, Spain): Mg,V20, (6) or aMg2V,0, (5) has been proposed as active and selective Mg-vanadate phases for the ODH of alkanes. According to our XPS results on MgO-supported vanadium catalysts, the catalyst preparation procedure determines both the V/Mg surface

122

atomic ratio and the catalytic properties (7). We have also observed that the higher selectivity to propene is obtained on catalysts with an MgN surface atomic ratio of 15-20. For this reason it has been concluded that isolated VO, tetrahedron are the active and selective sites for the ODH of alkanes. Tetrahedral v5' species can be also proposed on pure ortho and pyrovanadates. In addition, XPS obtained on pure Mg-vanadates also show a Mgenrichment at the surface (8). J.C. Vedrine (Institut de Recherches sur la Catalise, Villeurbanne, France): You propose as Dr. Grasselli that isolated vanadium species are active for ODH of C, and C, alkanes. I do agree with such an approach. However there is another effect that you do not consider: this is the basic property of the excess MgO compound (as you have shown by XPS study) which may favor olefin desorption (because of basic character of olefins). Do you have some suggestion about this parameter? J.M. U p e z Nieto (I. Tecnologia Quimica, Valencia, Spain): From the comparison of the catalytic results obtained on V/MgO and V/Sep series it can be concluded that the presence of acid sites (as occurs on V/Sep) favor the formation of 2-butene and carbon oxides. In this way, it is clear that a correlation between the acid-base properties of catalysts and the basic chacater of the olefin may be proposed. Thus, a higher difference in the selectivity to butenes than propene were obtained.

J. Haber (I. Catalysis and Surface Chemistry, Krakow, Poland): You are ascribing the difference in the behaviour of vanadium oxide-based catalysts in oxidative dehydrogenation and oxigenation to the presence of isolated VO, tetrahedron and V-0-V bridges at the catalyst surface. One may however correlate the difference in the behaviour of V-Mg system which dehydrogenates and V-P system which oxidizes butane to maleic anhydride to the difference in acidity. The intermediate butene is a base and therefore it is held enough long the acidic surface to become oxidized, whereas if rapidly desorbs from basic surface. Similar arguments may be involved in case of sepiolite which is acidic, and Mg salt which is basic. J.M. Upez Nieto (I. Tecnologia Quimica, Valencia, Spain): The selectivity of a catalyst may be controlled by two characteristics: 1) The nature of active sites responsible for the formation of a determined product; 2) The stability of this product (the thermal stability and the appearance of consecutive reaction). With respect to the first point, V=O double bonds are necessary to obtein partial oxygenated products, while V-O-Me pairs are responsible of the H-abstraction. Thus, on V-P0 catalysts maleic anhydride or acetic acid from n-butane or ethane, respectively, are obtained. However, on V/MgO catalystsonly the corresponding olefin and carbon oxides are obtained from alkanes. For this reason, we are proposed that isolated VO, tetrahedron are the selective species for the ODH of alkanes. We agree that in addition to this, the interaction of olefins with the catalysts, as a consecutive reaction, can be influenced by the acid-base character of both the olefin and the

123

catalysts. Thus, the selectivity to olefins could, indeed, be optimize by modifing the acid/base character of the catalysts.

P.Ruiz (Univ. Catholique de Louvain, Belgium): Pyro and orthovanadates need high temperature to be synthetized. Which are your arguments to affirm that at 55OoC/3 h you have observed pyro and orthovanadates? J.M. L6pez Nieto (I. Tecnologia Quimica, Valencia, Spain): According to previously reported data (9), the obtention of pure vanadates from physical mixtures of MgO and V,O, occur at long time of calcination. However, in our case, TG-DTA results show that the formation of Mg-vanadates occurs at the same time taht the decomposition of starting compounds, which justify the fact that Mg-vanadates are formed at low time of calcination (8). In addition to this, our catalysts are prepared at relatively low vanadium contents which can also favor the interaction between vanadium and supports.

References 1. 2. 3. 4. 5.

6. 7. 8.

9.

P.M. Michalakos, M.C. Kung, I. Jahan and H.H. Kung, J. Catal., 140, (1993) 226. M.A. Chaar, D. Patel and H.H. Kung, J.Catal.,lOS (1988) 463. A. Corma, J.M. Lopez Nieto, N. Paredes and M. Perez, Appl. Catal., 97 (1993) 159. N. Paredes, Ph. D. Thesis, Universidad Autonoma, Madrid, 1993. D. Siew Hew Sam, V. Soenen and J.C. Volta, J. Catal., 123 (1990) 417. M.A. Chaar, D. Patel, M.C. Kung, and H.H. Kung, J. Catal., 105 (1987) 483. A. Corma, J.M. Lopez Nieto and N. Paredes, J. Catal., 144 (1993) in press. A. Guerrero-Ruiz, 1. Rodriguez Ramos, J.L.G. Fierro, V. Soenen, J.M. Herrman and J.C. Volta , in “New Developments in Selective Oxidation by Heterogeneous Catalysis” (P. Ruiz, B. Delmon, Eds.),Elsevier, Amsterdam (1992) p. 203. G. M. Clark and R. Morley, J. Solid State Chem., 16 (1976) 429.

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V. CortCs Corberdn and S. Vic Bcllon (Editors), New Developmenis in Seleciive Oxidation I1 0 1994 Elsevier Science B.V. All rights reserved.

125

OXIDATIVE DEHYDROGENATION OF THE C4-Cs PARAFFINS OVER VANADIUMCONTAINING OXIDE CATALYSTS

Rizayev, R.M. Talyshinskii, J.M. Seifullayeva, Guseinova, Yu.A. Panteleyeva and E.A. Mamedov

R.G.

E.M.

Catalysis Division, Institute of Inorganic and Physical Chemistry, 29 Azizbekov Avenue, 370143 Baku, Azerbaijan Abstract A series of binary and ternary vanadium-containing oxides were tested as catalysts for oxidative dehydrogenation of butane and isopentane. The most effective one was found to be aluminasupported Ni-V-Sb oxide system. Its activity and selectivity as well as the reaction kinetics significantly depend on a nature of the support pretreatment which influences the distribution of active components on the surface of the support.

1. INTRODUCTION

Many complex oxides, such as phosphates, molybdates, phosphomolybdates, ferrites and stannates of various metals, are known to accelerate the reactions of oxidative dehydrogenation of the C4-C5 paraffins. These types of catalysts are well studied and repeatedly reviewed [1,2]. Last years it has been shown that some vanadates, like Mg-V [3-51, Nd-V [5,6] and Mo-V [7-91, are also capable of catalyzing the oxidative dehydrogenation of alkanes. On their basis, more complicated catalytic systems were developed [9-131. This paper reports a new family ofvanadium-containing oxide catalysts which demonstrate a good activity and selectivity in oxidative dehydrogenations of n-butane and isopentane. 2.

EXPERIMENTAL

2.1.

Catalyst preparation

All catalysts were prepared by simultaneous impregnation of a commercial y-alumina spheres (1.8-2.0 mm diameter) with solutions of ammonium metavanadate and chlorides or nitrates of other metals in tartaric acid. The alumina had a specific surface area of 200 m2g-’and a pore volume of 0 . 5 0 cm3g-’. The samples were dried at 120 C for 2 h and then calcined in air in three steps at 200, 400 and 800 C f o r 2 h at each temperature. The total amount of supported components did not exceed 28 wt.%. In some cases, before contacting with the impregnating solutions the support was calcined at 900 C for 6 h and/or treated at 50 C for 1 h with a solution of acid (HC1, H3P04! or base (NH,OH) , followed by drying and calcining as described

126

above. The treating procedure as well as the impregnating one were carried out in a specially built installation equipped with a spectrophotometry and pH-metry techniques. Characterization Catalysts were characterized before and after reaction by argon thermodesorption surface area measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy (IR) 2.2.

.

Catalytic tests The catalytic activity and kinetic measurements were carried out in a gradientless flow reactor with a vibro-fluidized bed of catalyst at atmospheric pressure. The feed was a mixture of paraffin and oxygen diluted in nitrogen and water vapour with a paraffin/oxygen and paraffin/water molar ratios ranging from 0.2 to 2.0 and from 8 to 30, respectively. The paraffin space velocity (GHSV) ranged from 50 to 800 h-’. The reaction temperature varied from 580 to 650 C. Under these conditions, the empty reactor showed very poor activity; for instance, the hydrocarbon conversion at 650 C did not exceed 5%. The absence of diffusion control was checked by varying the catalyst particle size as well as by changing the linear flow rate. The reaction products were analyzed by on-line gas chromatography. A Chrom-5 gas chromatograph equipped with a thermal conductivity detector was used. Helium was the carrier gas. A switching valve directed a pulse of reaction products into the gas chromatograph. Two columns were used in parallel; a 4-m column packed with 15% Tvin-80/Polysorb-l was used at 90 C to separate the hydrocarbons and carbon dioxide, and a 1.5-m molecular sieve NaX column was used at room temperature to separate oxygen and carbon monoxide. Experiments on catalyst long exploitation were carried out in a pilot scale installation using a vibro-fluidized reactor as well as a fixed-bed reactor with sectional feeding of oxygen. 2.3.

3.

RESULTS AND DISCUSSION

A l u m i n a - s u p p o r t e d v a n a d i u m p e n t o x i d e catalyzesthe reactions of one-stage oxidative dehydrogenation of n-butane and isopentane to butadiene and isoprene with a selectivity not exceeding 25 %. Both higher selectivity and yield of these products are characteristic of binary and ternary oxides listed in Table 1. Among these catalysts, the most effective one is aluminasupported Ni-V-Sb oxide which was studied in more detail. Its optimum composition was found [14] to be (wt.%): NiO - 9.0, Sb,O, - 11.0, V05 - 4.5 and A1,0, 75.5. The freshly prepared Ni-V-Sb catalyst is a polyphasic solid, consisting of nickel vanadate, antimony vanadate, nickel antimonate, and nickel and vanadium oxides. Nickel antimonate and antimony vanadate were found to be unstable under the reaction conditions, decomposing to the individual oxides. During the catalytic work, the formation of additional amount of the nickel vanadate was also observed.

-

127

Table 1 Catalytic properties of alumina-supported oxides in oxidative = dehydrogenations of n-butane and isopentane (C,H,,+,/O,/H,O 1/1/20; C,H,,, GHSV = 1 5 0 h-’) Butane,

650

C

Isopentane,

605

C

Catalyst Yield of C4H6

Selectivity ( % )

C4H6

Sn-V Mo-V Ni-V co-v Sb-V Bi-V-Sb Sn-V-Sb Co-V-Sb Ni-V-Sb

Yield of Selectivity ( % ) C5H, ( % I

(‘1

C5H8

C5H10

4.8 5.7 6.5 6.6 7.0

27.0 26.0 30.0 30.0 33.0

23.0 21.0 20.0 20.0 20.0

12.6

48.5 47.6

19.7 12.9

C4H8

8.0 9.0 8.0 8.5 12.0 13.6

25.0 30.0 27.0 27.0 33.0 35.6

25.0 20.0 27.0 33.0 32.3 31.4

16.4

42.0

30.5

11.0

Behaviour of the Ni-V-Sb/A1203 catalyst considerably depends on the state of the support surface before impregnation of active components. This conclusion comes from the data on catalytic properties of the samples supported on differently treated alumina (Table 2 ) . Six catalysts were prepared using the alumina pretreated as follows: - Sample 1: untreated; - Sample 2 : treated with a HC1 solution and not calcined; - Sample 3: untreated and calcined at 600 C; - Sample 4 : treated with HC1 and calcined at 600 C; - Sample 5: treated with H,P04 and calcined at 600 C; - Sample 6: treated with NH40H and calcined at 6 0 0 C. Table 2 Textural and catalytic properties ( 6 2 0 C; CnH10/OZ/H20= 1 / 0 . 5 / 2 0 ; C4HloGHSV = 150 h-’)of the Ni-V-Sb oxide supported on variously treated alumina

Sample

1 2

3 4 5 6

Surf.area (rn2g-’)

90 85 95

ao 95 85

Pore volume ( cm3g-’ )

0.65 0.63 0.68 0.64 0.70 0.65

Yield of C4H6 ( % )

13.3 17.8 14.8 16.7 14.6 13.7

Selectivity

(%)

C4H6

C4H8

36.9 47.1 40.2 45.0 41.2 40.5

43.3 24.9

31.1 29.5 39.8

41.8

128

One can see that the alumina pretreatment with a solution of hydrochloric acid (samples 2 and 4) essentially enhances both the yield and selectivity toward butadiene, and practically does not change the extent of butane conversion which varied from 3 4 to 38%. Similar results were obtained for oxidative dehydrogenation of isopentane carried out over the same catalysts. The influence of the support pretreatment on catalytic properties of Ni-V-Sb oxide cannot be explained by a change of its surface area and/or pore volume which, as seen from Table 2, do not show any dependence on the nature of the pretreatment procedure. Also, it cannot be caused by structural changes since XRD and IR analysis did not reveal any difference in phase compositions of the catalysts 1-6, excepting small broadening of the SbVO, and NiV,O, lines observed for samples 2-4 that may be due to a higher surface dispersion of these phases. When using differently pretreated alumina to prepare a Ni-VSb/Al,O, catalyst, there are differences in the distribution of active components on the surface of support. Figure 1 represents the profile distribution, across the diametral section of the alumina sphere, for nickel, vanadium and antimony elements obtained through SEM technique.

0-

Radial

position ( m m )

Figure 1. Profile distribution of nickel, antimony and vanadium elements in the sphere of differently pretreated alumina.

129

For all catalysts, V element is distributed homogeneously in the support, whereas Ni and Sb elements show some heterogeneities. It seems that the ammonium vanadate mainly impregnates alumina while nickel and antimony chlorides are adsorbed on it competing for the same adsorption sites. This idea has been confirmed by studying the adsorption of these compounds fromtheir solutions in tartaric acid using spectrophotometry and pH-metry techniques. According to the results of these measurements presented in Figures 2 and 3 , active elements have different capability to be adsorbed on y-alumina which changes as follows: Ni2' > Sb3+ >> V5+. So, upon impregnating alumina, vanadium is homogeneously distributed in the support whilst nickel and antimony are preferentially located at the edge of the alumina sphere. Taking into account this circumstance and the XRD data, it is reasonable to assume that in the calcined catalyst vanadium-containing phases, including Vz05, uniformly cover the inner surface whereas the NiSb206 phase is mainly concentrated at the edge of the effect , appearing sphere. The extent of such a llchromatographiclg during the adsorption of active elements on alumina, depends on the nature of support pretreatment being enhanced when treating it with a HC1 solution. One can suggest that this kind of pretreatment gives more optimum distribution of active phases in the support, thus providing higher selectivity toward oxidative dehydrogenation of the C,-C, paraffins to corresponding dienes. Another explanation could be related to the presence of chloride ions in the catalyst which are well known to increase the selectivity of partial oxidation reactions, including the oxidative dehydrogenation of ethane [15]. The enhanced selectivity is eventually lost completely after several hours on stream as a result of the chloride ions consumption. Although we did not determine the chlorine content of the catalyst before and after the reaction, this idea seems to be unlikely because our catalysts showed hardly any change in selectivity during several days of operation. The kinetics of oxidative dehydrogenations of n-butane and isopentane over the samples 1, 4 and 6 listed in Table 2 is described by the same model based on the step-wise scheme of redox mechanism of Mars - van Krevelen type. The rates of paraffin oxidative dehydrogenations to olefin and diene obey the expression ri=kiPlP,0.5 (P1 and P, are the paraffin and oxygen partial pressures), in which the value of the rate constant depends on the catalyst genesis, showing the maximum for the sample 4 . Smaller variations have been found for the kinetic rate constant kj in the equation rj=kjPIP, describing the paraffin total oxidation to carbon oxides. On the basis of kinetic data, mathematical modelling and optimization of processes were carried out. From a reactor design point of view, two modes of operation were predicted to have a good outlook. These are (i) a cascade of Eixed-bed reactors with a separate feeding of oxygen into each reactor, and (ii) a fluidized-bed reactor. Table 3 represents the results of the Ni-V-Sb/Al,O, catalyst activity measurements carried out for oxidative dehydrogenation of n-butane in the three-sectional fixed-bed reactor. This mode of operation provides a 22-23% yield of butadiene per single pass. When using a recycling technology, it increases up to 3 0 % . Similar results were obtained for oxidative dehydrogenation of isopentane in the four-sectional

130

1.7 f.6

-

A l

A

A

L\

1.5 -

k 7.4Q

0

-

-

- 3

I

I

/"

T 4

0.4

I V

-

d

I

I

4

c,

2

p '

Figure 2. Evolution of the p H of the NH4V03 (1), NiC1, ( 2 ) and S b C 1 3 (3) solutions when contacting to alumina.

Figure 3. Adsorption of NiCIZ (1), S b C 1 3 (2) and NH4V0, (3) on alumina as a function of their concentration in solutions.

131

fixed-bed reactor. Table 3 Oxidative dehydrogenation of n-butane to butadiene in the threesectional adiabatic reactor with a fixed bed of Ni-V-Sb/A1,03 catalyst (C,Hl,/H20 = 1/ (25-30): butane GHSV=300h-') Temperature in sections ( " C ) I 610 610 610 610 615 610

I1

I11

620

625 625

620 625 620 618 620

C,H,, conO,/C,H,, molar ratio in sections version ( % ) I1

I11

0.50 0.48

0.40 0.38

628

0.50

0.30

0.31 0.33 0.33

625 625 625

0.40

0.31 0.35 0.39

0.28 0.30 0.34

I

0.44

0.47

38.3 39.6 40.8 38.1 40.8 38.8

C4H6 selectivity ( % )

59.0

55.9 57.4 56.9 57.6 56.7

When using one-sectional fluidized-bed reactor, a higher total selectivity to butenes and butadiene at considerably lower C,H,,/H,O molar ratios was observed. In this case, however, there were some problems with a mechanical strength of catalyst spheres. As for the stability of catalytic activity, it did not change during a 500 h pilot scale run. 4. CONCLUSION

Vanadium, antimony and nickel (cobalt) mixed oxides supported on alumina seem to be promising catalytic systems for the oxidative dehydrogenation of alkanes. Their performance considerably depends on the nature of the support pretreatment which effects the distribution of active phases in the support sphere. The concentration of gas-phase oxygen is another key parameter affecting the selectivity since the intrinsic reaction rates exhibit different dependences on this parameter. To keep it optimal along the catalyst bed, a sectional feeding of oxygen can be used. This mode of operation has provided a 22-25% yield of butadiene (isoprene) per single pass. ACKNOWLEDGMENT

We gratefully acknowledge the help of Dr A.Shkarin during the SEM measurements. REFERENCES 1 2 3

V.K. Skarchenko, Russian Chem. Rev., 46 (1977) 1411. T.G. Alkhazov and A.E. Lisovskii, Oxidative Dehydrogenation of Hydrocarbons, Khimiya, MOSCOW, 1980. D. Patel, M.C. Kung and H.H. Kung, in M . J . Philips and M.

132

4

5

Ternan (eds.), Proc. 9th Inter. Congr. Catal., v01.4, p.1555, Chemical Institute of Canada, Ottawa, 1988. M.A. Chaar, D. Patel and H.H. Kung, J. Catal., 109 (1988) 463 D. Patel, P.J. Andersen and H.H. Kung, J. Catal., 125 (1990 132.

6 7 8 9 10 11 12 13

M.C. Kung and H.H. Kung, J. Catal. , 128 (1991) 287. E.M. Thorsteinson, T.P. Wilson, F.G. Young and P.H. Kasai, J Catal., 52 (1978) 116. R. Burch and R. Swarnakar, Appl. Catal., 70 (1991) 129. Y. Han, W. Lu, X. Gao and C. Hui, Shiyou Huagong (Chinese Petrochem. Technol.), 20 (1991) 309. F.G. Young and E.M. Thorsteinson, USA Patent 4250346 (1981). J.H. McCain, USA Patent 4524236 (1985). J.H. McCain, USA Patent 4568790 (1986). R.M. Manyik, J.L. Brockwell and J.E. Kendall, USA Patent

.

4899003 (1990) 14 V.S. Aliyev, R.G. Rizayev, R.M. Talyshinskii et al., USA Patent 4198586 (1980). 15 R. Burch, G.D. Squire and S.C. Tsang, Appl. Catal., 46 (1989) 69.

of Bologna, Bologna, Italy): Is it possible that your catalysts are active in pure (non-oxidative)dehydrogenation and they oxidize molecular hydrogen, thus shifting the equilibrium?

F. TRIFIRO (U.

E . MAMEDOV (I. of Inorganic and Physical Chemistry, Baku, Azerbaijan): Our catalysts show poor activity in pure dehydrogenation which rapidly decreases as the reaction proceeds. As for the oxidation of molecular hydrogen, its rate is much lower than the rate of oxidative dehydrogenation of paraffin. DELMON (Catholic University of Louvain, Louvain-la-Neuve, Belgium): Why did you select Y-Al,O, as a support (here NiO segregates, and its action impairs the selectivity of your catalyst)? Did you compare with other supports?

B.

E . MAMEDOV: We checked a series of commercial supports, and the best one was found to be y - A 1 2 0 3 . Its pretreatment with a HC1 solution reduces the segregation of NiO and facilitates the formation of nickel vanadate and nickel antimonate.

(I. of Chemical Physics, Moscow, Russia): What do you think about the possibility of gas-phase radical-chain reactions initiated by the catalyst surface, and about possible influence of porous structure?

0 . KRYLOV

MAMEDOV: Under the testing conditions, the empty reactor showed very poor activity (the paraffin conversion did not exceed 5%). As for the heterogeneous-homogeneous reactions and the influence of catalyst porous structure, we did not study these questions.

E.

V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation 11 0 1994 Elsevier Science B.V. All rights reserved.

133

Oxidehydrogenation of propane on gallium oxide-faujasite catalysts B. Sulikowski", J. Krysciak", R.X. Valenzuelab and V. Cortes Corberanb a

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 1, 30-239 Krakbw, Poland lnstituto de Catalisis y Petroleoquimica, C.S.I.C., Campus U.A.M.-Cantoblanco, 28049 Madrid, Spain

ABSTRACT The effect of calcination and reduction pretreatments on gallium oxide-faujasite mechanical mixtures, and the influence of the gallium content on the oxydehydrogenation of propane over these catalysts were studied. Under reduction conditions, gallium migrated into the faujasite up t o an amount close t o the number of defects in the zeolite framework. This incorporation did not modify the intrinsic activity but increased the initial selectivity t o propene.

1. INTRODUCTION Gallium has been successfully introduced into numerous zeolite frameworks [ I 1. Gallium-containing zeolites are of interest from catalytic point of view due to their combined redox and acid properties. Most of research has been focused on the Ga,O,-MFI system [2-51. The isomorphous substitution of gallium into aluminosilicate zeolites results in enhanced selectivity towards aromatic hydrocarbons. For example, the gallium analogue of ZSM-5 is useful in the Cyclar process, in which C,-C, alkanes are transformed into aromatics [2]. It is interesting that not only deliberately synthesized [Si,Ga]-ZSM-5 molecular sieves containing framework gallium were the active catalysts. Excellent dehydrogenation properties were also induced by mechanical mixing of the standard [Si,AI]-ZSM-5 zeolite and Ga,O, [3]. Such mixtures after a proper pretreatment were the active catalysts for dehydrogenation of lower alkanes [4]. On the other hand, limited information is available on the interaction of Ga,O, with other zeolites. We wish t o report on the dehydrogenation properties of catalysts based on a zeolite having the pore system wider than ZSM-5. For this purpose zeolite Y, a 12-membered ring aluminosilicate, has been chosen. The acid-extracted ultrastable form of zeolite Y was used due t o its known resistance against temperature. The conversion of propane in the presence of oxygen using gallium (111) oxide, a pure zeolitic matrix, and the matrix loaded with different amounts of Ga,O, was investigated.

134

2. EXPERIMENTAL 2.1. Catalyst preparation. The parent material used was zeolite Na-Y, with Si/AI = 2.47 and Na/AI = 1.09, as determined by wet chemical analysis. Zeolite Na-Y was twice ammonium exchanged by stirring with 1 0 w t % NH,CI at 368 K for 1 h. The 0 . 7 6 NH,,Na-Y form was heated in a horizontal quartz reactor at 81 3 K for 4 h under self-steaming conditions. The zeolite was re-exchanged with NH,CI and calcined at 1093 K as before. The ultrastable zeolite (US-Y) had Si/AI and Na/AI bulk molar ratios of 2.55 and 0.003, respectively. The US-Y form was treated further with 1 dm3 of 0.1 M HCI per 100 g of sample at 3 6 4 K for 1 h and washed with water. The acid treatment was repeated using 0.2 M HCI solution, washed with water and dried. The acid extracted ultrastable zeolite (US-Y-ex) had Si/AI = 8.91 and Na/AI = 0.003. Zeolite US-Y-ex was ground in an agate mortar with spectroscopically pure P-Ga,03 wt%; In, Mg, Mn, (Bi, Cr, Zn, Sn, Al, Co and Fe - the content of each metal < Cu and Pb < 5.10-5 wt%; BET, = 27.5 m2/g). The catalysts containing 4.8 t o 44.4 w t % of ,B-Ga,03 were calcined in air or reduced in H, at 473 to 873 K. The samples for reduction were heated at a rate of 3 K/min in a N, flow (30 cm3/min). The reduction with H, (20 cm3/min) was performed for 2 h followed by cooling the samples t o ambient temperature in a dry N, flow. Mixed samples will be labelled hereafter for brevity as x GaZ, where x denotes the initial amount of P-Ga,O, (wt YO)in the sample and 2 represents the zeolitic matrix US-Y-ex.

2.2. Characterization Powder XRD patterns of the fully hydrated samples were acquired on a Seifert XRD 3000P diffractometer equipped with a vertical goniometer using Ni-filtered Cu KO radiation. Argon sorption at 77 K was measured in a volumetric sorption unit of standard design. The samples were outgassed at 623 K before sorption studies. The IR spectra in the zeolite framework vibration region were obtained with a Nicolet 800 FT spectrometer (resolution 1 cm-’) using the KBr technique. Laser Raman FT spectra of the hydrated samples were collected with a Raman module of the Nicolet 800 spectrometer. A Nd:YAG laser was used as an excitation source ( A = 1046 nm), and the beam power o n the sample was adjusted as necessary (50-600 mW). The use of the Nd:YAG laser significantly reduced the fluorescence that so hampers conventional excitation sources. The sample was placed in a Raman cell and the scattered light was detected with a liquid-nitrogen-cooled germanium detector. The correction of background due t o the Rayleigh scattering and correction for the white light was performed by a Nicolet software. 2.3 Catalytic tests The conversion of propane was studied at temperatures 773-873 K under the atmospheric pressure. Catalyst particles (ca. 0.1 g, 0.25-0.42 mm) were loaded into a tubular, down-flow quartz reactor. SIC chips were placed above the catalyst particles up t o a total bed volume of 1 cm3. The catalyst was preheated in a flow of dry helium (100 cm3/min) at 773 K for 1 h. A propane (99.9 %)-oxygen (99.99 %)-helium (99.999%) mixture (2:1:13.5 molar ratio) was fed into the reactor at a total flow of 100 cm3/min and W/F = 4 g cat. h/gmol C,. Reactants and products

135

were analyzed on-line with a Varian 3400 GC with a TCD detector. Two columns with Porapak QS and molecular sieve were used for analysis. Special cautions have been undertaken t o minimalize the homogeneous reaction that was studied at the same conditions by substituting the catalyst particles by Sic chips. By redesigning the catalytic set-up and applying the above test conditions we were able to reduce it down to 0.2-2.2 mole ?LOat 773-873 K. The conversion level was kept below 1 0 mole YOin order t o observe the primary products of the reaction.

3. RESULTS AND DISCUSSION The characterization of the starting materials and the Ga-containing samples is given in Table 1. The BET surface area of the Ga-loaded catalysts was reduced after the reduction in H,. The intensity of the XRD patterns (i.e., a sum of the several zeolite peaks) of 28.6 GaZ was 100, 90, 108 and 71 YOfor the parent material, calcined at 773 K and reduced at 673 and 873 K, respectively. Thermal treatment of the sample up t o 673 K did not affect the crystallinity of the matrix. For the sample 28.6 GaZ reduced at 873 K (Table 1) the decreased crystallinity of the matrix would correspond t o the decreased BET surface area of 425 m2/g. Experimentally found value was 403 m2/g, therefore restricted or blocked access to the internal pore system of the sample by bulk gallium oxide species was excluded. Table 1. Characterization of the parent materials and gallium-loaded zeolitic catalysts. Lattice parameters were determined for the fully hydrated samples. Sample

BET (Ar) (m2/g)

L!ttice (A)

parameter

SiIAI"

Al, per unit cell

Na-Y

24.690(2)

2.47

55.6

us-Y

24.51 (1)

2.55

33.2

8.91

20.1

US-Y-ex

599

24.373(5)

4.8 GaZ

432""

24.382(5) - as prepared

28.6 GaZ

403"*

24.53(1) 24.56(2) 24.54(2) 24.50(2) -

* **

as prepared after calcination at 773 K after reduction at 673 K after reduction at 873 K

by wet chemical analysis. after the reduction with hydrogen at 873 K for 2 h.

The amounts of B-Ga,O, in the matrix after various treatments were estimated from XRD patterns. The sum of peak heights at 28 values of about 30.28", 33.34" and 37.26" for P-Ga,O, divided by the sum of peak heights of zeolite with hkl indices of 331, 440, 533 and 642 was proportional t o the ratio P-Ga,O,/zeolite. In

136

this way the ratios of 0.28 and 0.05 were found for the 28.6 GaZ and 4.8GaZ samples, in a very good agreement with the amount of Ga,O, loaded into the catalysts. For the sample 28.6 GaZ after the calcination in air and the reduction with H, at 673 K the ratio decreased to 0.15and 0.20, respectively (the calculated values were corrected for the loss of crystallinity of zeolite). Thermal treatment removed therefore a considerable amount of the bulk b-Ga,O, from the samples. Part of the gallium was used to fill the vacancies of the matrix. After thermal treatment of 28.6 GaZ (both in air and hydrogen) the development of minute amounts of an unknown phase was observed. The number of Al atoms in the framework of zeolites X and Y affects their unit cell parameters as well as most of the mjd-IR freque;cies, largely because Si-0 bonds are shorter than AI-0 bonds (1.60A and 1.75A). Empirical formulas were therefore derived that allow estimation of the number of Al atoms occupying framework sites in faujasite. The average number of Al (Table 1 ) was calculated from XRD [61and IR [71measurements. As seen, approximately 13 Al atoms per unit cell were extracted from US-Y by HCI, and hence the same number of defects was introduced into the US-Y-ex matrix (the matrix was not healed under hydrothermal conditions prior t o loading with gallium oxide). We observed the expansion of the lattice parameter of the matrix after the loading of /3-Ga,03. It is known that incorporation of Ga into the faujasite structure led t o the enlargement of the unit cell parameters, due to the longer Ga-0 bonds (as compared t o AI-0 bond). Such an expansion was observed by Kuhl t81 and Szostak [9]. The plot of the lattice parameter a, of hydrated synthetic Ga-FAU as a function of the number of Ga atoms per unit cell shows t w o breaks, even more pronounced that found for AI-FAU, but the relationship within each region is linear [8].In the region < 64 Ga/u.c. the linear regdession analysis of the data by Kuhl gives the expansion of the unit cell 0.01508 A/1 Ga. The experiTentally observed expansion of a, for 28.6GaZ was in the range of 0.157-0.187A and thus corresponded t o the introduction of 10.4-12 Ga per unit cell. On the other hand the expansion of the unit cell of the sample 4.8GaZ corresponded t o the insertion of 0.6 Ga/u.c. only. It should be however noted that for highly siliceous gallosilicates the plot becomes non-linear so the estimation of number of Ga for low amounts by XRD is inaccurate. The lattice of 4.8GaZ slightly expanded upon the reduction at 873 K and only traces of the bulk ,B-Ga,O, were found in this sample by XRD. This confirmed that most of Ga,O, in the precursor disappeared upon reduction. The amount of gallium oxide in the precursor corresponded t o 6.8 Ga/u.C., assuming that all Ga,O, would be accommodated by the matrix. As XRD has shown only traces of B-Ga,03 after the reduction, thus about 6 Ga/u.c. were found in the catalyst. 3.1. Laser Raman spectroscopy The Raman spectrum of the zeolitic matrix is shown in Fig. 1. Contrary to Na-X, Na-Y and other zeolites, the t w o strongest bands were observed at 506.5 and 488.8 cm-’ in the acid-extracted US-Y-ex sample. In sodium faujasite the single band at 515 for Na-X and 505 cm-’ for Na-Y is assigned t o the motion of the 0 atom in the plane perpendicular t o the T-0-T bond [lo]. There is a shift of 10 cm-’ upon increasing the Si/AI ratio in a faujasite substitutional series (zeolites X and Y). In the case of US-Y the strongest Raman signal was splitted into the t w o bands.

137

n e

d

C

b

1

I

1200

1000 800 600 W A V E N UMBER ( c m

Wl

1200

loo0

800

lt00

600

200

200

Figure 1 . FT laser Raman spectra of US-Yex (a); and 28.6 GaZ as-prepared (b) and after reduction a t 673 K (c), 773 K (d) and 873 K (e).

WAVENUMBER

(cm-'

1

Figure 2. FT Laser Raman spectra of: a) B-Ga,O, (100% pure phase); b) after the reduction of a stoichiometric mixture of B-Ga,O, + Ga a t 873 K under vacuum.

The position of a band around 500 cm-' is a function of the average T-0-T angles for different zeolites [ I 11. Accordingly, the band at 506.5cm-' corresponds t o the 142.0"and the band at 488.8cm-' to the 146.9' T-0-T angle. In faujasite there are four nonequivalent oxygen atoms, and hence each Si atom has four different Si-0-T angles (T=Si,AI). In natural faujasite [I 21 these are: 138.6,139.7, 145.3 and 147.4', while in modified faujasite [I31the four angles are: 136.1, 142.4,146.0 and 149.8'. There are thus t w o pairs of similar angles, with the smaller and the larger angles. The average angle of each pair is 139.2' and 147.1'. The splitting of the band around 500 cm-' is consistent with the Si-0-T angles of faujasite. Obviously, due t o the width of the bands the separate signals of each of the four angles in faujasite were not observed. It follows that the Raman spectroscopy is a very sensitive tool and can provide accurate structural information on the subtle changes in the structure of modified faujasite, in addition t o X-ray 1121 or neutron diffraction studies [I 31. The other much weaker bands were seen at 299, 356, 665 and 815 c m ~ ' . Upon mixing with gallium (Ill)oxide both bands of the oxide and those of the matrix were found. The bands related t o the bulk Ga,O, were found (Fig. 1 b) at 347,41 6, 653 and 766 cm-'. After the reduction in H, the spectrum appearance changed significantly. The features characteristic for the matrix disappeared. However, the zeolite retained its crystallinity as seen from IR and XRD data. The lines characteristic for the bulk ,&Ga,O, (416,653 and 766 cm-') also disappeared completely. The signal at ca. 350 cm-' was broaden. The new broad and weak signals appear

138

in the spectrum upon reduction at 568, 827 and 957 cm-’. A separate TPR experiment has shown that the bulk P-Ga,O, was non-reducible by H, at 873 K. However, the same features as found for reduced sample 28.6 GaZ were observed for Ga,O, reduced deliberately by pure gallium under vacuum at 873 K . The Raman spectra of the parent and reduced gallium oxide are shown in Fig. 2. The bands of P-Ga,O, at 346, 416, 476, 630, 653 and 766 cm-’ are seen. The bands in the range of 300-600 cm-’ correspond to bending vibrations, while the bands above 600 cm-’ are due t o the Ga-0, tetrahedra stretching modes 1141. The signals of the reduced oxide are seen at 354, 565, 831 and 948 cm-’. The appearance of the bands is the same as in 28.6 GaZ reduced with H, (cf. Fig. I c - e l . Gallium oxide loaded onto ultrastable faujasite was therefore amenable to the reduction by H,, in a way resembling the behavior of the Ga,O,/ZSM-5 system [15, 161. The influence of calcination in air upon the Raman spectra was studied for the samples containing 5.21 t o 40.3 w t % of gallium oxide (spectra not shown). The mild calcination at 473 K induced changes in the sample 5.21 GaZ. The lines at 41 7 and 766 diminished, and the broad features characteristic for samples reduced in H, appeared (at 836 and 947 cm-’1. The calcination at higher temperatures essentially did not affect the spectra, while the samples with 9.88 GaZ and 28.6 GaZ underwent very similar transformations upon heating. The higher loading with P-Ga,O, was reflected in the higher amount of the unchanged bulk gallium oxide phase (417, 653 and 766 cm-’1.

3.2 FT IR spectroscopy The 28.6 GaZ sample gave a typical IR spectrum with the frequencies corresponding both to the matrix and to the oxide (Fig. 3 a). Upon reduction the signal of the bulk ,6-Ga70, at 668 cm-’ decreased, while the main asymmetric stretching vibration of the zeolite at 1072 cm-’ was shifted to 1070 and 1053 cm-’ after the reduction at 673 K and 873 K, respectively (Fig. 3 b,d). Such a shift t o lower wavenumbers is expected upon the formation of the less siliceous framework [71. The shift of the main stretching frequency might be interpreted as isomorphous substitution of some gallium into the framework. The ratio of intensities of the frequency for P-Ga,O, at 668 cm-’ t o the matrix frequency at 832 cm-’ changed from 1.4 t o 0.5, which confirmed further the loss of some bulk gallium oxide in the reduced sample and transport of gallium (probably as Ga,O [ I 51) to faujasite cages. FT IR spectroscopy confirmed therefore the conclusions drawn from the XRD results. 3.3 Oxidehydrogenation of propane The oxidehydrogenation of propane was studied on the spectroscopically pure /3-Ga70,, the zeolitic matrix, and on the mixed GaZ samples. All the catalysts were reduced in H, before the tests. Representative results for catalytic tests are shown in Fig. 4. The activation energy on every catalyst was in the range of 24.2-34.8 Kcal/mol, thus diffusion effects did not affect the results. The main products on every catalyst were propene, CO, and CO, with minor amounts of ethene and methane. The total conversion of C,H, on pure /3-Ga70, in the presence of oxygen was lower than those reported for anaerobic conditions [ I 71, and about t w o times that corresponding t o the homogeneous reaction. The activity of pure US-Y-ex ( 2 )was relatively high under the conditions used,

139

n

12,

-c

S 6

.i 2

I

I

I

I

I

1400

1200

1000

800

WAVENUMBER

I

600

I

400

(ern-')

Figure 3. FT IR spectra of 28.6 GaZ asprepared (a) and after reduction by H, at 673 K (b), 773 K (c) and 873 K (d).

760 785

810 835

1

I

860

885

Temperature (K)

Figure 4. Conversion of propane on: x , Z; 0, 4.8 GaZ; @, 28.6 GaZ; w , p-Ga,O,. Dotted line: homogeneous reaction.

but the selectivity t o propene was lower than on Ga,O,. Contrary t o ZSM-5 at these conversion levels, not even traces of aromatics were found in the products. It is clear that the matrix contained the Brmsted acid sites of intermediate strength, thus the aromatization of ethene and propene did not take place. These consecutive reactions on MFI zeolites lead t o the formation of aromatics. The activation of propane on Z is due t o Brernsted acid sites, in agreement with other studies [181. We have recalculated catalytic results for Z in order t o compare with ZSM-5 [19]. The estimated conversion of propane on Z in the conditions used with ZSM-5 was 27 YOas compared t o 35 % on the latter, thus the activity of faujasite was slightly lower than that of ZSM-5. The loading of Z with gallium decreased the overall activity (cf. Fig. 4), but this could be explained as due to lower BET areas of 4.8 GaZ and 28.6 GaZ (cf. Table I ) , because the specific activity was roughly the same for the three samples. Again, no aromatics were found in the products on GaZ samples. Addition of 4.8 wt YO gallium to the zeolite enhanced the initial selectivity towards propene (measured at a conversion level of 1 YO)from 42 YOt o 58 YO.Addition of further amounts of gallium up t o 28.6 wt YO(sample 28.6 GaZ) decreased initial selectivity t o 50 YO.Thus, the most selective catalyst was 4.8 GaZ, which contained gallium dispersed in the framework of faujasite, and on which traces of the bulk Ga,O, were found after the reduction. Introduction of further amounts of gallium up to 101 2 Ga/u.c. (sample 28.6 GaZ) did not increase activity nor selectivity. We conclude that the presence of small amounts of dispersed gallium into the faujasitic matrix (ca. 6 Ga/u.c) and essentially lack of the bulk Ga,O, were responsible for the enhanced selectivity t o propene.

140

CONCLUSIONS The main findings are summarized briefly: (i)the interaction o f pure gallium (Ill) oxide with t h e US-Y-ex matrix results in the insertion o f Ga into t h e defects o f the matrix. The maximum amount o f gallium which can still be accommodated b y t h e matrix (i.e., 1 0 - 1 2 Gah.c.1 is close to t h e number o f defects in Z (ca. 13/u.c.); (ii) gallium oxide mixed with the zeolite is amenable t o the reduction b y hydrogen; (iii) t h e highest conversion level was observed for pure US-Y-ex zeolite. The matrix itself is very active in the conversion o f propane. Contrary t o H-ZSM-5/Ga,03 system n o aromatics were found in the reaction products under the conditions studied; (iv) loading o f ultrastable zeolite with /3-Ga,03 gives rise t o decrease o f the overall conversion level o f propane due t o t h e reduction o f t h e specific area, while the selectivity t o propene is enhanced; (v) the presence o f Ga dispersed in the matrix is essential for the increased selectivity t o propene. The gallium-containing catalysts based o n t h e ultrastable zeolite Y would be therefore o f interest for a selective transformation o f light alkanes t o olefins.

Acknowledgements Support f r o m the Committee of Scientific Research, Warsaw, Poland (grant no. 2.P303.149.04), the C.I.C.Y.T., Madrid, Spain (Project M A T 9 2 - 0 1 8 9 ) and t h e Polish Academy o f SciencesK.S.1.C. Interchange Agreement is fully acknowledged.

REFERENCES 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

6. Sulikowski and J. Klinowski, J. Chem. SOC.,Chem. Commun., (1989) 1289 - and references therein. J.R. Mowry, R.F. Anderson and J.A. Johnson, Oil Gas J., 83 (1985) 128. V. Kanazirev, G.L. Price and K.M. Dooley, J. Chem. SOC.,Chem. Commun., (1990) 71 2. V. Kanazirev, R. Dimitrova, G.L. Price, A.Yu. Khodakov, L.M. Kustov and V.B. Kazansky, J. Mol. Catal., 70 (1991) 11 1. E. Lalik, X . Liu and J. Klinowski, J. Phys. Chem., 96 (1992) 805. L. Kubelkova, V. Seidl, G. Borb6ly and H.K. Beyer, J. Chem. SOC., Faraday Trans. I, 84 (1988) 1447. 6. Sulikowski and J. Klinowski, J. Chem. SOC.,Faraday Trans., 86 (1990) 199. G.H. Kuhl, J. Inorg. Nucl. Chem., 33 (1971) 3261. R. Szostak, "Molecular Sieves", Van Nostrand Reinhold, New York 1989, p. 214. P.K. Dutta and D.C. Shieh, J. Phys. Chem., 90 (1986) 2331. P.K. Dutta, D.C. Shieh and M. Puri, Zeolites, 8 (1988) 306. D.H. Olson and E. Dempsey, J. Catal., 13 (1969) 221. P.J. Barrie, L.F. Gladden and J. Klinowski, J. Chem. SOC.,Chem. Commun., (1991) 592. D. Dohy, G. Lucazeau and A. Revcolevschi, J. Solid State Chem., 45 (1982) 180. R. Le Van Mao, R. Carli, J. Yao and V. Ragaini, Catal. Lett., 16 (1992) 43. G.L. Price and V. Kanazirev, J. Catal., 126 (1990) 267. P. Meriaudeau and C. Naccache, J. Mol. Catal., 50 (1989) L7. G. Buckles, G.J. Hutchings, and C.D. Williams, Catal. Lett., 8 (1991 ) 1 15. N. Paredes Quesada, Ph.D. Thesis, Universidad Autonoma, Madrid 1993.

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DISCUSSION CONTRIBUTIONS G. Centi (Dip. Chimica Industriale, Univ. Bologna, Italy): We have shown in the past [ I Ithat the presence of oxygen enhances the low temperature transformation of C,-C, alkanes t o aromatics on H-ZSM-5 and we interpreted the effect as due t o an activation of oxygen on defect sites of the zeolites. The fact that you observe a decrease of the activity in your samples increasing the Ga amount in the ultrastable Y-zeolite may be interpreted in a similar way as a progressive decrease in the number of defects created during the extraction procedure for ultrastabilization of the zeolite. On the other hand, the homogeneous oxydehydrogenation of propane t o propylene is usually very selective; we observed selectivities up t o 8090 YO.It may be thus possible that the increase with selectivity you observed decreasing the activity of the zeolite is due to an increase of the contribution of homogeneous activity. Furthermore you do not observe the formation of aromatics. We observed aromatics even using silicate which has very low acid strength. It may be possible that in your case the absence of aromatics is thus only due t o the fact that you made the tests only at very low conversion? 1. G. Centi, G. Golinelli; J. Catal., 115 (1989) 452 B. Sulikowski (I. Catalysis, Krakow, Poland): Your suggestion that the activation of oxygen takes place on the defect sites of zeolite is indeed interesting, also in view of an earlier work by Beyer and Borbely [2] who observed the catalytic activity of the defect silicalite lattice formed from [B,Sil-ZSM-5 precursor in some reactions of hydrocarbons. In our case, however, the decreased of activity of gallium-loaded zeolites is primarily due t o the decreased surface area. The defects may play role in the conversion on pure zeolitic matrix. The in situ measurements of the catalysts by FT-IR will shed more light on this problem. On the other hand, we compare initial selectivities at a constant conversion level, that is reached at around 773 K. In our experimental conditions, the noncatalytic reaction test gave a conversion of 0.15 mole YO with a selectivity t o propene of 21 % at this temperature; so, the observed increase in the selectivity may not be ascribed t o an increased contribution of homogeneous reaction. Finally, in additional experiments not reported here we reached conversions up t o 3 0 mole % and no aromatics were detected in any case, although this does not exclude the possibility of aromatics formation at much higher conversion levels. 2. H.K. Beyer, G. Borbely ; "New Developments in Zeolite Science Technology", Studies in Surf. Sci. and Catal., 28 (1986) 867. B. Grzybowska (I. Catalysis, Krakow, Poland): In the text you speak about the "initial selectivity". What is meant by it and to which conversion of propane your selectivity values are referred? B. Sulikowski: By "initial" selectivities we meant the values of selectivity measured at very low conversion levels of propane. A s it has been incorporated into the text in the revised version, we compared the selectivity of the catalysts at a conversion level of propane of 1 mole YO.

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F. Trifiri, (Dip. Chimica Industriale, Univ. Bologna, Italy): You reported that the catalyst is active in absence of gallium, and the role of gallium is just to increase the selectivity. What kind of activity is presented by the catalyst without gallium?. B. Sulikowski: The pure zeolitic matrix displays the activity due to Brernsted acid sites and defect sites. Oxygen may be activated at the latter (cf. the remark by Prof. Centi). The dehydrogenation activity of traces of iron impurities in ultrastable zeolite can not be excluded.

J. Vedrine (IRCKNRS, Villeurbanne, France): You have suggested that Ga takes framework location in the Y-type zeolite which may change acidity strength. Another possibility could be that Ga atoms are at cationic sites or as Ga,O, particles and favor redox type mechanism rather than acid type reaction, which could explain the improved selectivity. By the way, what are the other products and what are the selectivities in propene for your different catalysts?. B. Sulikowski: The picture which emerged from XRD, FT-IR and Raman studies is consistent with the insertion of gallium into framework sites, as long as the amount of available gallium does not exceed the number of defect sites in zeolite. Further loading with gallium oxide leads t o the insertion of gallium in cationic sites and/or finely dispersed oxide phase in the supercages of faujasite. Besides propene, the other products of reaction in the conditions used, were carbon oxides (CO and CO,) and cracking products (methane, ethane and ethylene). Initial selectivities t o propene on each catalyst are indicated in text. As conversion increased, these values decreased on each catalyst down t o around 2 0 mole %.

J.C. Volta (IRCKNRS, Villeurbanne, France): According to you, how the activation of propane proceeds and what is the respective role of gallium sites and the structure of the zeolite?. B. Sulikowski: The detailed mechanism of propane conversion is not yet known. As shown here, both bulk gallium oxide (Ill)and pure zeolitic matrix catalyze the transformation of propane t o propene. According t o Gnep et al. [31, propane may be activated by nucleophilic oxygen species, by transferring electrons on the C-H antibonding orbitals. The dual catalytic sites mechanism, suggested recently [41, is based on the (Ga3+,0 ; ) ion pair and Brernsted acid sites interaction and may be involved in the conversion of propane.

3. N.S. Gnep, J.Y. Doyemet and M. Guisnet; J. Mol. Catal., 45 (1988) 281. 4. E.G. Derouane, I. Ivanova, S.B. Abdul Hamid, N. Blom and P. Zeuthen, 1st European Congress on Catalysis EUROPACAT-I , September 12-17, 1993, Montpellier, France, Paper A-9.

V. CortBs Corbersn and S. Vic Bell6n (Editors), New Developments in Selective Oxidation If 1994 Elsevier Science B.V.

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Characteristics of alumina boria catalysts used in ethane partial oxidation G .Cucinieri Colorie, B .Bonnetotb, J.C. Vedrine and A. Aurouxa

"Institut de Recherches sur la Catalyse, C.N.R.S., 2 avenue Albert Einstein, F 69626 Villeurbanne Cedex, France bhboratoire de Physico-Chimie Minkrale, Universitk Claude Bernard Lyon I, 43 Boulevard du 11 Novembre 1918, F 69622 Villeurbanne Cedex, France Abstract

Boron oxide supported catalysts of different compositions (from 10 to 30 wt% &03) prepared by impregnation of porous and non porous aluminas were tested in et ane oxidative reaction using a continuous flow method. The surface characterization was investigated using various techniques including X-ray Photoelectron Spectroscopy (XPS), and microcalorimetric adsorption measurements with probe molecules. The catalytic activity and selectivity were studied in the 470 "C to 550 "C temperature range. The formation of ethylene and acetaldehyde was observed to increase when the boria content increased, while the selectivity for the formation of carbon oxides decreased. A high selectivity was observed for each conversion whatever the amount of boron oxide and the type of support. The stability of alumina-boria catalysts and the role of the support were also determined. Keywords: alumina-boria catalysts, ethane partial oxidation 1. INTRODUCTION

The selective oxidation of alkanes into alkenes and other partial oxidation products such as C,-oxygenates is an important reaction both in fundamental and industrial catalysis. Several types of catalysts were proposed for partial oxidation of ethane using oxygen [l-31 or N20 gas [4-61 as oxidant under atmospheric pressure. The selective transformation of ethane over alumina-boria catalysts has been studied, to our knowledge, only by Y.Murakami et al.[7-81.These catalysts were also used for other catalytic reactions such as vapor phase Beckmann rearrangement of cyclohexanone oxime [9-111, n-butene [12] or xylene isomerizations [ 131 and toluene disproportionation [ 131. The alumina-boria catalysts are interesting for a fundamental point of view because of the lack of redox property, since the usual mechanism involving a change of the oxidation state of boron cannot take place and thus the role of acido-basicity of oxides should be important. In previous papers [14-151 the acidity and the basicity have been studied by adsorption microcalorimetry of probe molecules such as ammonia, pyridine and sulphur dioxide. In this paper we have investigated both the structural and catalytic properties of the deposited boron oxide and we have tried to correlate them with the acidity characterization.

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2. EXPERIMENTAL 2.1. Preparation of catalysts Catalysts were prepared using two different supports: a porous alumina SPH 557 A from RhBne-Poulenc with a given pore volume of 47 cm3/100g measured by CCld and a non porous alumina oxide C from Degussa, whose characteristics were described elsewhere ~41. An appropriate amount of y-alumina was impregnated with an q u a u s solution of boric acid containing the desired amount of boron and corresponding to boron oxide loadings from 10 to 30 wt%. After the impregnation, the water was evaporated under stimng and vacuum by heating at 40 "C overnight and coated alumina was thus obtained. The catalysts were then calcined under air flow at 600°C for 3 h. 2.2. Catalyst characterization The catalysts were characterized by X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction Spectroscopy(XRD). The surface composition of selected catalysts was determined by X-ray Photoelectron Spectroscopy using a VG-Scientific Escalab 200R spectrometer equipped with an A1 Ka source (1486.6 eV). The sample were outgassed in the spectrometer chamber without any other treatment. Sample charging effect was corrected with the C (1s) peak as an internal reference taken at 284.5 eV. Peak areas were smoothed with Gaussian convolution curves. The peak areas of the B (Is), A1 (2s,2p), 0 (1s) signals were used to determine the relative surface concentration of boron, aluminium and oxygen. X-ray Diffraction analyses were carried out by means of a Philips PW 3710 diffractometer with the monochromatized CuKa radiation (X=O. 15418 nm), operating in the step-scan mode (0.02" 2 8 per step and counting for Idstep). The scanning was performed from 10" to 75" 2 8 to cover the range around boron oxide and alumina lines. Powder samples were manually compacted on windowed PVC holders. EDX-SEM analyses were performed on a HITACHI S800 at 15kV. The experimental conditions for the determination of chemical amount of boron oxide, surface area of the samples and microcalorimetricexperiments were reported elsewere [14]. 2.3. Catalytic testing The investigations concerning the catalytic properties of alumina-boria catalysts were performed in a fixed bed continuous flow quartz microreactor. Ethane and oxygen gases With a constant ratio (103 P d I O 3 Pa ) were diluted with helium gas as a carrier and sent to the catalyst under atmospheric pressure and controlled using Brooks mass flowmeters. The products were analyzed with an Intersmat gas chromatograph. Hydrocarbons such as CzH6, C2H4, CH4 and acetaldehyde were separated in a Porapak Q column (4m, 100°C) and analyzed using a flame ionization detector. Oxygen and carbon oxides were eluted on a Carbosieve G column (3m, 60°C) and analyzed with a thermal conductivity detector. During each catalytic experiment the reactor temperature was changed at random using an automatic program. The change of temperature was performed in a range of about 100°C. Conversion and selectivity to products were calculated on carbon atom basis balance. The results were expressed as mol % of ethane transformed to ethane fed, and of each product to total ethane transformed. 3. RESULTS 3.1. Catalyst Characterization The dehydration of boric acid which covered the alumina was a delicate part of the catalysts preparation in order to obtain crystalline boron oxide. The transformation of boric acid into boron oxide involves a complex phenomenon in which some intermediates can be

145

in liquid state. If the sample was heated too quickly during the calcination, the melting of metaborates occured and a glass boron oxide was obtained. Table 1 gives the specific surface areas of the supports and of the different samples, the amount of boron oxide and the number of acidic and basic sites as determined by adsorption microcalorimetry of basic and acidic probe molecules. The surface area of catalysts decreased when R03content increased. Despite a decreasing in the initial heats [14], the total acidity as determined by ammonia adsorption increased in number and strength as a function of the content of boron oxide. The volumes of ammonia irreversibly adsorbed on the surface at 80°C increased regularly with boron oxide content but drashcally with 30 wt % B203 samples on non porous alumina and 25 wt % on porous support. The volume of pyridine irreversibily adsorbed was also dependent on the boron oxide content. These results should be supported by the electron microscopy wich has shown, on high percentages boron oxide samples, the presence of a mixture of alumina supported boria and crystalline %03.The small molecular cross-sectional area of boron oxide may result in a sterical hindrance for the pyridine probe (0,284 nmZ/molec) compared to ammonia (0,14 nm2/molec). This should also explain the larger amount of ammonia irreversibly adsorbed than pyridine on the high loading samples. The volume of S02, acid probe molecule, was highly dependent on boron oxide content, surface area and type of the support. On porous alumina supported catalysts it was necessary to reach a composition of 20 wt% boron oxide to observe the vanishing of all the basic sites coming from alumina, covered by boron oxide. On the non porous support with 10 wt% &03no basic sites were observed anymore but some physisorption occured. Table 1 Physico-chemical characteristics and acidity and basicity results from calorimetric adsorption data of the samples Samples

&03wt% Theoretical Chem. Anal.

Porous Alumina

Non Porous Alumina

Surface Area m2g-1

0 10 20 25 30

0 9 19 25 29

325 287 192 75 70

0 10 20 25 30

0

11 18 21 25

103 105 107 91 58

Porous NH3 Pyridine so2 VolumeN2 Virr Virr Virr cm3g-1 pmol/m2 pmol/m2 pmol/m2 314.6 306.8 182 90.4

1.29 1.69 2.32 5.12 6.55

1.67 1.oo 2.02 3.06 2.02

1.41 0.16 0 0 0

1.63 1.83 2.86 2.58 5.31

1.74 1.41 1.16 1.17 1.68

1.50 0 0 0 0

The results of X-ray diffraction spectra of samples with different contents of boron oxide showed that the XRD pattern attributed to crystalline B203 [16] was observed for the sample containing more than 20 wt 96 of boron oxide on porous alumina and 10 wt % on non porous alumina. The intensities of the diffraction lines in the spectra increased with the boron oxide content. There was no peak showing the formation of any known mixed oxide between boron oxide and alumina [17]. XPS analysis showed that the binding energies (BE) of boron (B (1s) 193.1 eV characteristic of boron in boron oxide [18]), and of aluminium (A1 (2p) 73.7 eV attributed to y-alumina [ 191) remained nearly unchanged for all samples (Table 2).

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However, the boron to aluminium atomic ratios calculated from XPS spectra increased non progressively with increasing boron oxide loading. For the 25 wt% of boron oxide the B/A1 atomic ratios determined by XPS analysis and Chemical Analysis (referred to the initial hydrated samples) were very similar. At lower percentage (10 wt%-20wt%) the B/A1 atomic ratios by XPS Analysis were higher by a factor of two than those measured by Chemical Analysis. Table 2 XPS Data of Alumina-Boria Catalysts Samples Porous Alumina

Non Porous Alumina

B(ls) BE(eV)

AVP) BE(eV)

B/A1 XPS

B/A1 C.A.

10 20 25 30

192.9 192.9 192.9 193.1

73.7 74.6 73.8 74.6

0.32 0.71 0.67 1.1

0.19 0.37 0.67 0.65

10 20 25 30

192.7 192.8 193.1 193.0

74.6 74.6 74.2 74.5

0.48 0.84 0.56 0.73

0.23

&03wt%

0.35 0.53 0.68

3.2. Catalysis Alumina-boria catalysts subjected to the catalytic conditions of a mixture of ethane, oxygen and helium as diluent gave rise mainly to ethylene, methane, carbon oxides, acetaldehyde and water as products. The stability of the sample with 25 wt % B20j on the porous support was studied during one week. The conversion of ethane and selectivity for the products were constant during 80 hours of reaction. After this time the catalyst was still a white powder as before the test without any coke deposition. Figure 1 shows the catalytic behaviour at 550°C of the samples prepared with non porous alumina as support. In presence of an excess of oxygen (103 Pa), ethane was converted over pure alumina ( 12.3% conversion ) and the main products were C2H4, CO and C 0 2 . When 10 wt% of boron oxide was added, the formations of CO and C02 were drastically decreased. On one hand, the selectivities for the formation of ethylene, methane, CO, C02 and acetaldehyde changed weakly with %03 content. Above 10 wt% &03on the other hand, ethane conversion increased progressively when B2O3 amount increased in the whole range. Under the same conditions on the porous alumina as support (figure 2) a maximum of activity (36% of ethane conversion) was obtained with 25 wt% of boron oxide. The results of the catalytic test with the 25 wt% of boron oxide on porous alumina versus the reaction temperatures are presented in figure 3. Ethane conversion in ethylene was not observed under 470 "C. At higher temperature, the reaction led mainly to ethylene, although a high concentration of oxygen (103 Pa) in the mixture with respect to the stoichiometry. Above 525°C the selectivity in ethylene was greater than 95%. Besides the conversion of ethane increased slowly with the temperature and then drastically at temperature higher than 550°C. It has been checked that no diffusional regime occured. At this temperature the selectivity for carbon oxides formation also increased; the yield in acetaldehyde is weak.

147

Selectivity ( % ) Conversion (%) 5o

Selectivity ( % ) Conversion ( % ) '00/40

80

- 30 - 20

- 10

-0

-0

6

20 26

30 3 6 Boron Oxide Content (wt%) 10

16

a0

Boron Oxide Content (wt%)

.

>

-30

I

/

00.

i

/

- 20

/-

40.

I

/'

,

0

20.

470

490

- 10

0

510

530

550

Temperature ('C)

Figure 3. Conversion of ethane and selectivity in various products : o C,H,; CH3CH0;o CO; A CO,; vs temperature. Sample: 25 wt% Boron Oxide content on porous alumina

* CH4;

-t

148

4. DISCUSSION

Assuming a molecular cross-sectional area of boron oxide about 0.17 nm2, the calculated theoretical amount of a boron oxide monolayer over alumina may be evaluated respectively to 20 wt% of boron oxide over porous alumina (325 m2g-1) and 7 wt% of boron oxide over non porous alumina (103 m2g-1). With non porous alumina, the amount of boron oxide was in all cases much superior to a theoretical monolayer and the active sites of alumina (basic and acidic) should be completely covered and probably a growth of crystalline boron oxide occurred with formation of agglomerates. The XPS (Table 2) and Xray diffraction results confirmed that at low content of oxide (10 wt%- 20 wt%) there was some segregation of %03on the surface. BET analysis and calorimetric results with SO2 as probe have shown that the surface did not change and there were not anymore free basic sites of alumina. However there was no evidence for a full coverage of the acidic sites of alumina. At higher content of boron oxide (25 wt%-30wt%) the samples showed similar B/A1 ratios both by XPS and Chemical Analysis which accounted for a more homogeneous system. The decrease in surface area of the samples with the rise in boron oxide content and Xray diffraction results confirmed the formation of crystalline %03on the surface of alumina. In the case of porous alumina, boron oxide entered and filled easily the pore volume and led to a large decrease in surface area above 10 wt% which can reach 45% loss for the 30 wt% % 0 3 sample. Besides a concomittent growth of crystalline boron oxide has been shown by XRD and XPS (Table 1). Assuming that a monolayer of boron oxide has a thickness of 0.47 nm, a maximum of six monolayers (a:3nm) of boron oxide on alumina can be reached with 30 wt% of boron oxide. The XPS technique is able to characterize the binding energies of boron and aluminium up to ten monolayers (=5nm), so even when the surface of alumina was completely covered by boron oxide, aluminium was still detected but with a lower intensity. Murakami et al.[7] using X P S analysis have proposed an asymetric spectra of B(1s) for B203 and have attributed the main peak (192.3 eV) to a crystalline boron oxide and the shoulder peak (189.9 eV) to the oxide of a cluster type consisting of a lower oxidation state of boron than that in a pure boron oxide. The latter type of boron oxide showing a lower oxidation state of boron could be formed on electron-donating sites on the surface of alumina, e.g. basic sites. In our case we have not seen the asymetric spectra from XPS analysis and as explained above the basic sites of amphoteric alumina were totaly covered by 10 wt% of boron oxide on non porous alumina and by 20 wt% of boron oxide on porous alumina assuming that SO2 titrated most of the basic sites. In the reaction conditions the samples were very stable. However the catalytic activity was highly depending on boron oxide content, indicating that an increase in number of acid sites of boron oxide corresponded to an increase of catalytic activity. On non porous support the surface of the samples was totally covered by k O 3 at all the studied percentages and the selectivity for the formation of products was not depending on B2O3 content. So the amount of boron oxide necessary to form a monolayer over alumina was sufficient enough to completely poison the deep oxidation sites on the alumina. Ammonia, as a strong base was shown to adsorb on all kinds of sites from strong to weak acid sites. On the contrary, pyridine as a weaker probe was shown to dose at 80°C only the stronger sites of the samples which stay nearly constant after a coverage by boron oxide reaching the monolayer [15]. On porous alumina, the sample with 10 wt% boron oxide presented still some basic sites from uncovered alumina and a small number of acid sites determined by adsorption of pyridine (1.0 pmol/m2). So not only the selectivity of carbon oxides was high but also the catalytic activity was lower than on pure alumina. At higher percentages of boron oxide the results of catalysis were very similar for samples on porous and non porous alumina. Still on porous alumina the most active catalyst appeared to be the sample with 25 wt % boron oxide more dispersed on the surface (cf. XPS analysis) than the 30 wt% boron oxide sample and the accessibility of sites determined by adsorption of pyridine were shown to be greater

149

for this sample. Concerning the C,-oxygenates the selectivity for the formation of acetaldehyde reached a maximum at 550°C (3%). Comparatively, Murakami et al.[7J who have studied the reaction in the same conditions found a similar selectivity for acetaldehyde (2.7%). 5. CONCLUSION

These results led to several conclusions: One observes both RO3 crystallites as detected by XRD and Boron oxide species deposited on alumina support and neutralizing the acidic and basic sites of amphoteric alumina. This conclusion was supported by EDX-SEM analysis. The activity of the catalyst for the selective oxidation of ethane is mainly related to the number of sites which were determined by adsorption microcalorimetry of pyridine. The boron oxide neutralizes first the basic sites of amphoteric alumina and at low content lets still some uncovered acid sites of the support. These acid sites are responsible for deep oxidation, leading to CO and C02 formation. A segregation of %03to the surface and possibly remaining uncovered alumina are consistent with XPS results. At high %03 content the system is more homogeneous, which agrees with the good fitting between XPS and Chemical Analysis B/A1 ratio and the catalytic formation of mainly C2H4 and CH3CHO. Nevertheless, the real constitution of the catalysts is not completely known. Then, further investigations are needed in particular to distinguish the acid sites of boron oxide from those of alumina at low content of boron oxide. The catalysts were very stable at the reaction temperatures and the conversion rates reached can easily be compared with usual oxidation catalysts results. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

E.M. Thorsteinson, T.P. Wilson, F.G. Young, J.Catal. 52 (1978) 116. A.Argent, P.G. Harrison, J.Chem.Soc. Chem. Commun. (1986) 1058. J. Le Bars, J.C. Vedrine, A. Auroux, B. Pommier and G.M. Pajonk, J.Phys. Chem. 96 (1992) 2217. M.Iwamoto, T.Taga, S.Kagawa, Chem. Lett. (1982) 1469. L.Mendolivici and J.H.Lunsford, J.Catal. 94 (1985) 55. K.Aika, H.Isobe, K.Kido, J.Chem. SOC. Faraday Trans. 1 83 (1987) 3139. Y.Murakami, K.Otsuka,Y.Wada and A.Morikawa Bull. Chem. SOC. Jpn. 63 (1990) 340. Y.Murakami, K.Otsuka,Y.Wada and A.Morikawa, Chem. Lett. (1989) 535. T.Curtin, J.B.McMonagle, B.K.Hodnett, Appl.Catal.93 (1992) 75. S.Sato, H.Sakuri, K.Urabe, and Y .Izumi, Chem. Lett. (1985) 277. S.Sato, S.Hasebe,H. Sakurai, K.Urabe and Y.Izumi, Appl. Catal. 29 (1987) 107. A.Ozaki, K.Kimura, J. Catal. 3 (1964) 395. Y.Izumi,and T.Shiba, Bull. Chem. SOC. Jpn. 37 (1964) 1797. G.C.Colorio, A.Auroux, B.Bonnetot, J. Thermal Anal. 38 (1992) 2565. G.C.Colorio, A.Auroux, B.Bonnetot, J. Thermal Anal. 40 (1993) 1267 Joint Committee on Powder Diffraction Standard ref.6-0297. Joint Committee on Powder Diffraction Standard ref.34-752, 9-158, 26-8, 23-759. J.A. Schreifels, P.C. Maybury and W.E. Swartz, J.Catal. 65 (1980) 195. C.D. Wagner, H.A. Six, W.T. Jansen and J.A.Taylor, Appl. Surf. Sci 9 (1981) 203.

Acknowledgments. The financial support from CEE, "Human Capital and Mobility" Project.

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V. CortCs Corbcrdn and S. Vic Bcll6n (Editors), New Developments i n Selecrive Oxidation / I 0 1994 Elscvier Science B.V. All rights rescrved.

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EFFECT OF POTASSIUM ADDITION TO V2O5ITiO2 AND MoO3ITiO2 CATALYSTS ON THEIR PHYSICOCHEMICAL AND CATALYTIC PROPERTIES IN OXIDATIVE DEHYDROGENATION OF PROPANE B. GtzybowskaA, P. MekJs*, R Grabowski*, K.Wcislo* ,Y . BarbauxB, L. GengembreB *Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakbw, 30-239 Poland. BLaboratoire de Catalyse, Universid des Sciences et Techniques de Lille, Villeneuve d'Ascq, 59655 France. ABSTRACT Addition of potassium to V2O5 / Ti02 and Mo03/Ti02 catalysts leads to the increase in selectivity in oxidative dehydrogenation of propane to propene, the effect being particularly distinct for catalysts with high specific surface area Ti02. The amelioration of the selectivity can be related to the modification of some physicochemical properties of the catalysts in the presence of K. It has been shown, that the potassium addition brings about the decrease in acidity, as inferred from the decrease in isopropanol dehydration to propene, lowers surface potential (work function) and hinders the formation of electrophilic 0- species.The coverage of Ti02 with VO, and MOO, species is higher in the presence of K, as suggested by X P S measurements. INTRODUCTION Potassium is one of common promoters mentioned in numerous patents on oxide catalysts of selective oxidation. The fundamental works concerning its effect on performance of oxide catalysts are scarce [l-31 and show a great complexity of the problem, especially at high K content. In the case of Ti02 supported oxides the effect of K impurity, present in many commercial Ti02 supports on catalytic properties of deposited V2O5 in oxidation of various hydrocarbons have been reported [4-61, the effect being strongly dependent on the type of hydrocarbon oxidized. In the present work the effect of addition of K to Moo3 and V2O5 dispersed on anatase titania of low (7m2/g) and higher (41m2/g), spmrface area, on their catalytic performance in oxidative dehydrogenation of propane has been studied. The literature data [7-91 report some activity of V205iTiO2 system in this reaction.The catalysts were also characterized by the isopropanol decomposition - a probe reaction for acidic and redox centers[ 101- and by surface potential , SP and X P S techniques. *On leave from Institute of Organic Synthesis,Latvian Academy of Sciences,Riga,Latvia

152

EXPERIMENTAL Preparation of catalysts. The catalysts were prepared by impregnation with ammonium metavanadate or heptamolybdate solutions at pH=6 of two Ti02 -anatase supports:A-7 (7m2/g, Chemical Works, Police Poland) and A-41 (41m2/g, Eurotitania, Tioxide), followed by drying at 12O0c and calcination in a stream of air at 450OC for 5h. A-7 was washed with hot water (8OOC) to remove the K and P impurities, The X P S analysis of A- 41 and of A-7 after washing did not show the presence of K or P on the surface of the supports. The content of V or Mo in the samples corresponded to 5 monolayers of V205 or Moo3 calculated with the assumptions that one mnl of VO, contains 10 V atoms per nm2 and that of MOO, -6.5 atoms of Mo/nm2. The catalysts are denoted further in the text by symbols Me/A-11 or by Me/A-41 where Me=V or Mo , A-11 or A-41 correspond to different anatase supports.The catalyst promoted with potassium were obtained by impregnation of the above described samples with 0.01 N solution of KOH, evaporation, drying at 120OC and calcination again at 450OC for 5h. The ratio W e , where Me=V or Mo was 0.1.The XRD analysis showed the presence of the anatase form of Ti02 in all the samples.No crystalline Moo3 or V205 were found.The specific surface areas of the samples were, within l o % , equal to those of the support. Methods (1)Catalytic activity in oxidative dehydrogenation of propane was measured in a fixed bed flow apparatus in the temperature range 250-SOOOC. The reaction mixture contained 5% of propane in air, the contact time being 1 sec. Analysis of products was performed by on-line gas chromatography; C&, CO and C02 were found as main reaction products, the amount of oxygenates was below l%.The blank tests, without a catalyst, showed the absence of homogenous reaction up to 540OC. (1I)Decomposition of isopropanol, iso-PrOH to propene, diisopropylether and acetone was studied at 170OC with the pulse method using dried helium as a carrier gas. 0. l g of the sample and 2 p l pulses of iso-PrOH were used, the total flow rate of helium being 30mUmin. Before the experiment the samples were standardized in a stream of helium at 250 OC for 2h. (1II)Surface potential, SP was measured with a vibrating condenser method under a flow of AdOz mixture in an apparatus described in ref.[ 111. The reported values, V are relative to graphite reference electrode, the increase in V indicating that the surface becomes more negatively charged. The SP technique was also applied to identification of charged oxygen species on the catalyst surface in equilibrium with gaseous oxygen, by following the changes in V at a given temperature on changing oxygen pressure, p02 over the catalyst. As shown previously [ 121 the dependence of V on p02 can be described by the equation (1); V=kT/ne InpO2 , values of n=1,2 or 4 corresponding respectively to 0 2 - , 0 - and 0 2 - species.

153

(IV) X P S sDectra of the fresh samples were taken with an A.E.I. ES 200 B type spectrometer. The values of B.E. referred to the 0 1s level (530 eV) were: TiZp=456.7*0.2eV, v2p3,2=5 16,910.l e v , Mo3d5/2=232.4f0.2 eV, K2p=292.6*0.2eV, indicating the presence of respectively Ti'+, V5+, Mo6+ and K+ ions. The BE values of the main elements, were within k0.2 eV the same for samples with different anatase, and did not vary on addition of K to the samples. The atomic ratios of the elements on the surface nA/nB were calculated from the intensity ratios IA/IB with the formula:

values of

(3

being taken after Scofield

RESULTS AND DISCUSSION

For all of the studied catalysts the increase of the total conversion of propane with the increase in the reaction temperature, accompanied by the decrease in the selectivity to propene has been observed .At the same time the selectivity to carbon oxides increased with the reaction temperature. Such the course of the temperature changes, typical for most of the oxidation reactions, is usually ascribed to the increase with conversion of the rate of consecutive steps of the reaction , leading to total oxidation of the intermediate, selective oxidation products (of propene in the studied reaction). For illustration Fig .1 shows the changes with the reaction temperature of total conversion and selectivities to propene and carbon oxides [CO +C02 J for catalysts VIA - 4 1 and VIA - 4 1 + K. As seen the undoped sample shows high activity already at relatively low temperatures , the selectivity to propene being however low and decreases rapidly with the temperature (with the conversion) . The K doped catalyst is less active but more selective .Similar behaviour has been observed for other catalysts under study Comparison of different catalysts is presented in Table 1, which gives temperature of 10% conversion, Tlo, taken as a measure of total activity, selectivity to propene , values of maximum selectivity to propene, at conversion at which at 10% conversion, S ~ Oand it was observed S,,,/Conv . As seen the total activity of the catalysts decreases on addition of K (Tlo increases). For all the studied catalysts, addition of K leads to the increase in the selectivity to propene, this effect being more distinct for preparations on high specific surface area Ti02. In most of the studied cases the selectivity to propene is higher on Mo0,iTi02 catalysts as compared with VO, / T i 0 2 preparations, the only exception being udoped catalysts of high specific surface area ( S ~ of O VO,/TiO, > Slo of MoO,/TiO2). In the same Table the results of isopropanol decomposition are also given: the amounts of products per pulse given correspond to the average values obtained in 3 successive pulses after the stationary state of activity has been established,

154

1oc V/A

I

8(

120

- 41

-

100 30

6(

50

4c 10

8 .

h0

20

>-^ !=

>

I

180

I

220

260

300

Q i = 100

z:

J

w 15

80

c/)

60

10 40

5 0 ~

20 0

Fig. 1 .Variations of coiiversioii aiitl selectivities to difTereiit products \villi tllc I cnctioii tciii1)ciatiiic lbi. V/A - 4 I niirl VIA -1I -1 K cat:ilysls.I I - to1;il coiivcisioii 01 I)IOI)~IIIC'. + - selectivity to properie, -selectivity to COX (Scot+Sco).

155

Table 1 Catalytic activity of V20j I Ti02 and Moo3 I Ti02 catalysts

Catalyst

Oxidative dehydrogenation of propane Tlo, OC

VIA-7 V/A-7+K VIA-4 1 VIA-4 1 +K MoIA-7 Mo/A-~+K MOfA-4 1 MoIA-4 1+K

290 475 210 480 500 510 350 450

S ~ O , SmaxIConv. Yo 28 55 15 50 38 70 9 65

3615 6218 2413 8212 4518 8717 2014 8712

Isopropanol decomposition 106rnoIeIm2 pulse diiso Pr ether

0.23 0.05 0.16 0.06 1.o 0.3 0.2 0.1

C$&j

1.2 0.3 1.0 0.2 4.5 2.0 3.7 0.5

acetone

1.9 1.3 1.2 1.0 0.9

0.4 0.3 0.5

The addition of K leads also to considerable decrease of the rates of propene and diisopropyl ether formation in the decomposition of isopropanol, which indicates the decrease of the acidity of the K-promoted catalysts. The rates of the acetone formation (a measure of concentration of basic or redox centers) are much less affected. Most probably potassium eliminates the Bronsted acidic centers present on the surface of V2051Ti02 and Mo031Ti02 catalysts [ 13-151 which can be active in total oxidation of hydrocarbons [ 101. Fig. 2 shows the changes of the SP values in air with temperature for VIA-7 and VIA-i'+K samples. As seen in the case of the sample without K the SP values increase with the temperature indicating the increase in the negative charge of the surface: such the changes have been ascribed previously to transformation of the chemisorbed oxygen species from less to more negatively charged forms e.g. 0-+e- --> 02-[16], they can be also due to the increase in total amount of chemisorbed oxygen. The SP values of the sample with K are lower and do not change with temperature. Similar behaviour has been observed for other studied catalysts, as can be seen the differences in V at 400 and 250 OC, from Table 2 in which the SP values at 4000, V~OO, (AV400/25())[a measure of extent of electron transfer processes] are given. In column 4 the type of oxygen species in equilibrium with gaseous oxygen for different catalysts derived from eq. (1) is also given: for K free samples 0- species at low temperatures and 0 2 - at higher temperatures have been identified, whereas on samples with K only 02- form has been observed.

156

mV

'

1750 1730

0

-.-

ll6OI 1140

-

200 300 400 TEMPERATURE, "C Fig.2.Changes of surface potential of VIA-7 , and V/A-7+K catalysts with temperature. Table 2. Surface potential data for V205iTi02 and MoO,/TiO, catalysts Catalyst

v400

Av400/250

[mVI

Oxygen species [temp,OCI

VIA-7

1750

25

VIA-7 + K MoIA-7

1160 1850

5 40

02-(350-4000) 0-(3000) 02-3 500

MOIA-7 + K MOIA-4I MO/A-41 + K

540 1600 880

190 40

0-(320-3800) 02-(360-4000)

0-(3500) 02-3 500

157

It could be then suggested that the presence of K on the surface facilitates the electron transfer leading to formation of 02-species: the electrons from the solid have to overcome lower energy barrier in this casehother explanation could be that K blocks the centers of oxygen adsorption in form of ionosorbed 0- species. Higher selectivity of K-containing catalysts in oxidative dehydrogenation of propane can be then partly related to the absence of electrophilic oxygen forms on the surface, suggested previously as species responsible for total oxidation of hydrocarbons[ 17].The lowering of the total activity with addition of potassium to VOXiTi02 and M0OX/Ti02 catalysts could be on the other hand ascribed to the suppression of the direct, parallel to oxidative dehydrogenation, reaction of total oxidation of propane .This latter reaction is dominating on undoped catalysts at low temperatures , at which the electrophilic oxygen species are present . The explanation of the decrease in the selectivity to propene with the increasing reaction temperature (and the increasing at the same time conversion), observed on both doped and undoped samples, requires detailed kinetic studies, now in course.The preliminary results indicate that the rate of propene oxidation to carbon oxides is about 10 times higher than that of propane oxidative dehydrogenation [8,18], which would account for the selectivity decrease. Table 3 gives the ratio of various atoms on the surface of the catalysts obtained from the X P S measurements. As seen in all the cases the ratio nM/nTi,Where M=V or Mo is higher for samples with K, indicating better dispersion of the deposited oxide phases on titania surface. This effect would also lead to higher selectivity of the catalysts, the fraction of the uncovered support, active in total combustion being smaller in the catalysts with the K addition. Table 3 Ratio of elements on the surface of VzO, I Ti02 and Moo3 I Ti02 catalysts Catalyst VIA-7 V/A-7+K VIA-4 1 VIA-4 1+K

nVInTi 0.15 0.28 0.21 0.34

0.17 0.23

nKhV

Catalyst MoIA-7 MoIA-7+K MoIA-4 1 MOIA-4 l+K

nhIo/nTi

nK/nMo

0.15 0.18 0.21 0.25

0.22

0.17

Acknowledgements. The participation of Mrs I. Gressel in isopropanol decomposition measurements is gratefilly acknowledged.

REFERENCES 1.G.K.Boreskov and A.A.Ivanov, React. &net. Catal. Lett., 3 (1975) 1. 2.J.Zhu and S.Lars T.Anderson, J. Chem. Soc.,Faraday Trans., I, 85 (1989) 3629. 3.D.B.Dadybuqor, S.S.Jewur and E. Ruckenstein, Catal. Rev. Sci Eng., 19, (1979) 293. 4.A.V.van Hengstum, J.G.van Ommen, H.Bosch and P.J.Gellings, Appl. Catal., 8 (1983) 369 5.A.V.van Hengstum, J.Pranger, J.G.van Ommen and P.J.Gellings, Appl.Catal.,l 1 (1984) 317. 6.S.Lars T.Anderson, J. Chem. SOC.,Faraday Trans 82 (1986) 1537.

158

7.A.Corma, J.M.Lopez-Nieto, N.Pareoles, M. Perez, Y.Shen, Studies in Surface Science and Catal., 72 (1992) 213. 8.N.Boisdron, Thesis, University of Lille, 1991. 9.V.Soenen-Lebeaq Thesis, University of Claude Bernard, Lyon, 1991. lO.B.Grzybowska-Swierkosz, Materials Chemistry and Physics, 17 (1987) and references therein. 1l.Y.Barbaux, J.P.Bonnelle and J.P.Beaufils, J. Chem. Research, 1979 (S) 48, (M) 0556. 12.J.M.Libre,Y.Barbaux, B.Grzybowska and J.P.Bonelle, React. Kinet. Catal. Lett., 30 (1983) 249. 13.G.Busca, Langmuir, 2 (1986) 577. 14.H.Miyata, K.Fujii and T.Ono, J. Chem. SOC.,Faraday Trans.1, 84 (1988) 3121. 15.R.A.Rajadhyakshaand H.Kndzinger, Appl. Catal., 51 (1989) 81. 16.B.Grzybowska,Y.Barbaux and J.P.Bonelle, J.Chem. Research, 1981 (S) 48, (M) 0650. 17.A.Bielanskiand J.Haber, Catal.Rev.-Sci.Eng., 19 (1979) 1. 18.R.Grabowski, B.Grzybowska, J.Sloczynski, K.Wcislo : in preperation.

V. CortCs Corberin and S. Vic Bcll6n (Edilors), New Developments in Selective Oxidation I/ 1994 Elsevier Science B.V.

159

CATALYTIC REDUCTION OF CARBON DIOXIDE BY HYDROCARBONS AND OTHER ORGANIC COMPOUNDS O.V. Krylov", A.Kh. Mamedovb, S.R. Mirzabekovab "N.N. Semenov Institute of Chemical Physics, Academy of Sciences of Russia, 117334, Moscow, ul. Kosygina, 4 bInstituteof Petrochemical Processes, Academy of Sciences of Azerbaidjan, 370025, Baku, ul. Telnova. 30

SUMMARY Reaction of oxidative transformation of organic compounds of different classes (alkanes, alkanes, alcohols) with non-traditional oxidant, carbon dioxide, are studied on oxide catalysts Ni-0, Cr-0, Mn-0 and on multicomponent systems based on manganese oxide. Supported manganese oxide catalysts are active, selective and stable in conversion of C&+C02 into synthesis gas and in oxidative dehydrogenation of higher hydrocarbons and alcohols. Unlike metal catalysts Mn oxide based catalysts do not form a carbon layer during the reaction. INTRODUCTION The processes using alternate sources of non-petroleum raw materials excite recently a high interest. Alkanes from natural gas and carbon dioxide can be such raw materials. We proposed earlier [l-31 a new selective reaction of C,-C3alkanes conversion with C 0 2 into olefines and synthesis gas. It was established that manganese containing catalysts are effective both for methane transformation into synthesis gas and for C2H, and C,H, dehydrogenation into C,-C3 olefins. These catalysts were effective also for C 0 2 interaction with organic compounds of other classes. Some examples are given in this paper. Many aspects of the mechanism of organic substances reactions with C 0 2 are yet unclear. This paper is devoted to the discussion of some regularities of these reactions on manganese containing catalysts of different composition and of the role of manganese oxide in these reactions. EXPERIMENTAL The study of catalytic activity was performed in a flow system with a silica reactor of 7 mm diameter, the length of the catalyst bed was 7-8 cm, the amount of the catalyst was 4 ml, the dimensions of grains were 2-4 mm; the space velocity was varied from 900 to 12000 hour-', partial pressure of R-H was varied from 10 to SO kPa, C02 from 10 to 50 kPa

160

with dilution of the reaction mixture by nitrogen. Gaseous mixtures were analyzed by gaschromatography in columns filled with Porapak QS and molecular sieve. Pulse experiments were also performed according to the following scheme: reoxidation of the catalysts by COz pulses, N,-flowing pulses of reaction mixture. TPD experiments of C 0 2evolution in the flow during temperature programmed heating were also done. All the catalysts were prepared by impregnation of Si02 or Al,03 with nitrates of different metals with subsequent drying at 120 "C for 4 hours and heating at 850 "C for 5 hours. Ni-0, Cr-0 and Mn-0 systems, supported on Si02, A1,03 or zeolites were used as catalysts, as well as three- and four component systems based on manganese oxides.

RESULTS AND DISCUSSION Methane conversion. Methane oxidation by carbon dioxide on Ni and other metallic catalysts is well known [4-61 CH,

+ C 0 2 = 2CO + 2H,

(1)

Methane steam reforming proceeds also as a side reaction in CH,

+ H 2 0 = CO + 3H2

(2)

The main difficulty in the use of this reaction for practical process of synthesis gas production is the coking of the presently known nickel catalyst. Modification of nickel by different additives changes its properties in CH4+C0, conversion. From the studied oxides (Cu-, La-, Ce-, Ni-, Co-, Fe-, Mn-oxides) the modification by manganese gives the highest effect [7]. The increase of Mn content leads to the increase of C02 conversion degree. The study of CH, transformation (in the absence of CO,) on reduced catalysts showed that CH4 conversion decreased with increase of Mn content in the Ni catalyst. Therefore, modification of Ni catalysts by Mn allows to change the ration of the conversion rates of C02 and CH4. The further investigation showed that the catalyst can function without any metallic component on the whole. The catalyst 5%Ca-12%Mn-O/A1,O3was the best one. After initial treating of the catalyst precarbonated by CO, with methane at 850 "C, ethane and ethylene are forming on it in non- stationary conditions 2CH4 C,H,

+ [COJads = C2H6 + CO + H2O + [COJsds = CpH4 + CO + H20

(3) (4)

with the yield of 9%. Decrease of pretreating temperature down to 580 "C increases C,hydrocarbons yield up to 13%. When initial oxides Mn,O, and Mn,O, are reduced, C,H, and C,H, formation decreases and only synthesis-gas in stationary conditions is formed, that is, reaction (1) takes place. Increase of CH, conversion practically does not influence the selectivity. This is possibly connected with the fact that water vapour formed by the shift reaction H,

+ CO, * CO + H 2 0

(5)

161

can subsequently react with the formation of H, again (reaction 2). The process on the catalyst 5%Ca-12%Mn-O/Al,O, proceeds in stationary conditions without loss of activity during many days (see Table 1). Table 1. Conversion of methane with CO, in stationary conditions on the catalyst 5 %Ca-12 % MnO/A1,03, space velocity 900 hour-'

T "C

Initial mixture, % CH,

Compostition of the products, %

CO

CO

H,

CO,

49.7 56.4 55.6 56.8

30.6 37.4 40.3 42.4

14.7 3.5 3.6 2.4

Conversion %

CH,

CH,

H* ("1 selectivity, %

CO,

2

43 49 47 43

870 890 920 930

57 51 53 57

5.1 2.3 0.6 -

83.1 91.8 97.8 100

62.7 89.1 89.3 94.6

76.5 80.1 84.1 84.6

A comparison of the H, formation rate in steady state conditions (3.10 ' mol/g.hour) and of the reducing reaction of methane with the catalyst (1.7. l o 3 mol/g. hour) shows, that they are comparable. A study of the regularities of reoxidation of the catalyst showed that the rate of CO, interaction with the reduced surface is higher than the rate of reduction. Thus, unlike the case of metallic catalysts [5,6], methane activation is the rate controlling step on MnOcontaining catalysts but not the CO, activation. The reaction does not seem to proceed by cyclic redox mechanism. According to XRD data the stationary MnO phase is stable during catalysis. The probable scheme of CH, conversion with C02 is the following: 1. CH,

+ MnO

+

MnO ...C

+ 2H,

+ MnO MnCO, 3. MnCO, + MnO ...C 2Mn0 + 2CO

2. CO,

+

+

4. MnCO,

+ H,

5 . MnO ...C

--f

+ H,O

MnO +

+ CO + H 2 0

MnO

+ CO + H,

The kinetic equation for the rate of H, formation according to the scheme (6) is as follows:

1+-- 1

Pi0

k2 k3 Pco,

+

kz Pco2

162

where

pCH,,pCH,pco,

are partial pressures of CH,, CO, C02, correspondingly,

and k,, k,, k,, are rate constants for the reactions in the scheme (6). The expression (7) is confirmed by experimental data and is different from the kinetic equations for metallic catalysts [6]. 1.5%K-5.5%Cr-17%Mn-O/Si02catalyst shows very different properties in methane conversion with CO,. CO prevails in the reaction products according to the reaction (8) CH,

+ 3C0,

+

4CO

+ 2H20

(8)

This is due to deep transformations of both CH, and of H, being formed CO and H,O on the oxidized surface. The process on K-Cr-Mn-O/SiO, proceeds probably by cyclic redox mechanism. Ethane conversion. During ethane conversion with CO, a process without colung also takes place on manganese containing catalysts (see [ 11). Oxidative dehydrogenation of ethane reaction (4)proceeds here. The side reactions are the following ones:

+ 2C02 C,H, + 2C0, C2H, + CO,

C,H6

+ 3H, CO + 3H,O + 3C CH, + CO + H,O

+

4CO

(9)

+

+

Table 2 presents data on ethane conversion on the catalysts with different manganese concentration and on some more complex systems. It is seen that the C2H, selectivity increases with increase of the Mn concentration. 1.5%K-5.5%Cr-17%Mn-O/SiO,was the most active, selective and stable one among the studied catalysts. Table 2 Conversion of ethane with CO,, 800 "C, Space Velocity=3600 hour-', CO,:C,H, = 1.5:l (vol.) Catalyst

Conversion, %

C2H4

C2H4

selectivity, %

yield, %

C,Hh

co~

8 %MnO/SiO,

65.3

42.3

52.5

34.3

13%MnO/SiO,

69.3

46.0

58.4

40.5

17%MnO/SiO,

73.1

49.0

61 .O

44.5

25 %MnO/Si02

75.0

46.2

60.2

45.2

17% MnO/Al,O,

78.4

50.3

46.5

35.5

5.5 %Cr-17%MnO/Si02

72.0

46.2

65.4

47.0

1.5 %K-5.5%Cr-17%MnO/Si02

82.6

52.3

76.8

55.3

163

The data on CO, thermodesorption of the catalyst Mn-O/SiO, pretreated by CO, shows CO, evolution with maxima at 335,425 and 750 "C which can be explained by inhomogeneity of accepting sites. Only one high temperature peak observed in the TPD spectra on the CrMn-O/SiO, catalyst after CO, adsorption; the activation energy is 58 f 2 Kcal/mol. Such a change after transition from Cr-0 to Cr-Mn-0 is probably due to Cr2+ reoxidation by intermediate Mn-containing phase which accepts CO,. Thus, the sites basicity determines the stability of the carbonate structure and, as consequence, the degree of redox sites reoxidation. Basic sites of moderate basicity are necessary for CO, acceptinon and Cr2+ reoxidation. Such sites are situated at the surface of not very basic MnO. The interphase boundary between MnO and CrO makes reoxidation of CrO easier.

+co, MnO.. . C r 2 + 0 2 C r 2 + 0 2 -

> MnO

> MnCO,. ..Cr2+O'-C?+02~

+ CO + 0 2 - C r 3 + 0 2 C ? + 0 2 -

> (10)

The redox mechanism of manganese carbonate decomposition seems to be important here. Ethane and C 0 2 conversion on the catalyst Cr-OiSiO, without manganese is very low. The Cr-Mn-0 composition can be considered as bifunctional: on the one hand, it ensures CO, activation on basic sites, on the other side it carries out hydrocarbons transformation with redox sites participation. The sites adsorbing oxygen and creating nucleophilic oxygen ions are sites of CO, transformation into an activated state with the formation of mobile oxygen particles. They can migrate by spill-over mechanism (through interphase boundary) from Mn carbonate to redox sites of chromium. The potassium addition regulates possibly the basicity of the catalyst. Taking into account that dehydrogenation and deep transformation of ethane proceed on different sites (Mn and Cr sites, correspondingly), the simplified scheme of ethane conversions may be presented in the next form:

> C?H,

'\

I1

z

~

C2H4

> CO, products of cracking

The direction of ethane transformation is determined by the ratio of oxidized and reduced sites (ZO/Z); its increase leads to increase of C,H, selectivity. As the ethane conversion and CO accumulation in the reaction mixture increase, the surface of the catalyst is reduced by CO and the reaction shifts from the route I to the route 11. The rate of ethane dehydrogenation by carbon dioxide is described by the equation

where k,, and krd are rate constants of catalyst oxidation and reduction. This redox equation differs from the corresponding redox equation for alkane oxidation by oxygen, where the

164

denominator is represented by the sum

kox Po,

+

kred

PC,H,

[81.

Propane conversion. Three reactions proceed during propane conversion with CO,, each of them to a different extent on different catalysts: propane dehydrogenation by CO, to propylene (13), selective decomposition with GH, formation (14) and deep conversion into CO and H, (15)

+

+ CO + H 2 0 2 C3Hg + 2C02 -> 3 C2H4 + 2 CO + 2 H,O C3H,

c02

> C3H6

(13)

The best catalyst for selective propane conversion with CO, into olefins is K-Cr-MnO/Si02. The yield to C,-C, olefins at 830 " C is 73% at 96% propane conversion and the ratio CO/(C2H4+C3H,)=1.4. The general trends of propane conversion with carbon dioxide are similar to those observed for ethane transformation. After catalyst reduction by propane the rate of C,-C, olefins formation decreases. The rate of C,H, formation on the preoxidized by CO, catalyst is 2-3 times higher than on the reduced surface. But preoxidation of the catalyst by air gives mainly total oxidation products - CO, CO,. This is explained probably by the formation of higher chromium oxides (up to Cr6+) after catalyst oxidation by air instead of Cr,O, formation after CO, treatment. Mn-containing phase (MnO, according to XRD data in situ) takes part in the oxygen transfer and helps to maintain chromium in a oxidized state C?'. The amount of coke deposited after steady state reaction (C3H,/C0,=1:1.2) is not higher than 3.6% (with respect to the mass of the catalyst). The rate of selective transformation of C3H, with C 0 2 into ethylene is expressed as

- kred &,Ha rC2H4

kred

Pco,

kox PCO, +

kox

Pco

which is very similar to equation (12).

Conversion of isobutane. The most effective catalyst for isobutane interaction with CO, is also a chromium-manganese oxide composition similar to the one used for the reactions of C,C3 hydrocarbons. But a catalyst supported on y-Al,O, is here more active. The i-C,H, selectivity of the Cr-Mn-O/Al,O, at 660 "C and 900 h-' is 7S% for 65% conversion. The dehydrogenation of isobutane into isobutylene with CO, reduction to CO is CO,, but in contrast to ethane and propane characteristic for the reaction: i-C4H,, conversion, pretreatment of the catalyst by hydrogen in reaction conditions does not lead to a decrease of the isobutylene formation rate. A comparison of the reaction product composition obtained during C,-C, alkane transformation with that observed during the reaction of C, showed, that for ethane and propane the ratio CO/H, was 7-8:1, as compared to the value of this ratio for isobutane

+

165

conversion equal to 1 for all the studied catalysts in a wide temperature range (600-900 "C). This corresponds to the total reaction 2 i-C,H,,

+ CO, -> 2 i-C,H, + H, + CO + H,O

(17)

At lower temperatures (600-700 "C) direct isobutane dehydrogenation and equilibrium reverse shift reaction can proceed

+ H, > H,O + CO

i-C,H,, -> i-C4H, H,

+ CO,

<=

(18) (19)

At higher temperatures steam reforming is also probable

+ 4 H,O

i-C,H,,

=4

CO

+ 9 H,

(20)

Let us note that oxidative cracking of another higher hydrocarbon, heptane, carried out with the help of CO, showed a similar regularity: the formation of lower hydrocarbons and CO H, in equimolecular amounts. For example, heptane with CO, on the catalyst 1 %Ida8 %Mn-O/Al,O, is subjected to oxidative cracking

+

C7H,,

+ CO:, -> C,H, + C2H4+ CO + H,

(21)

The hydrogen, formed upon isobutane or heptane homogeneous dehydrogenation is not oxidized completely: only about one half of the hydrogen participates in the subsequent CQ reduction in accordance with the equilibrium of the reaction (19). Ethylene conversion. The redox character of manganese-containing catalysts action is discovered also in ethylene oxidation. During ethylene conversion with CO, mainly ethylene transformation to CO takes place on the K-Cr-Mn-O/SiO, catalyst, probably in its oxidized form, at 750-800 "C C,H,

+ 2 CO,

= 4 CO

+ 2 H,

(22)

At the same time the catalyst Cr-O/SiQ without Mn and K shows a change of its behaviour; ethylene is transformed to butadiene and propylene, the CO concentration is lower

2 C2H4

+ CO,

=

C4H6

+ CO + H2 + (CH4) + (C3H.J + (C,Hg)

(23)

The butadiene yield at 820 "C is 12-13%. The conversion of ethylene on Cr-W-O/SiO, catalyst is equal to 30% with C,H, selectivity about 50%. The free energy change for the reaction (23) is positive but CH,, C,H, and C4H, are also formed. It is probably necessary to search more complex thermodynamic relations for the total process description. Methanol conversion. Mn oxide catalysts function by the redox mechanism also in oxidation of organic compunds of other classes for example, alcohols. Methanol in the presence of CO, on the Mn containing catalysts undergoes dehydrogenation to CH,O with reduction of CO, to co

166

CH,OH

+ CO, -> CH,O + CO + H,O

(24)

Cr-Mn-Mo-O/CO, was the most effective one among the catalysts studied. It showed at 580 "C CH,O selectivity 95% and CH,OH conversion 10.4% and at 700 "C the corresponding values were 89% and 36.8%.

CONCLUSION We have shown here the importance of activation of CO, and of combining its reduction with oxidation of hydrocarbons and other organic substances. Mn- containing catalysts are the most effective ones in these reactions. The obligatory condition is the selection of the system which accepts and activates CO,. The acidic properties of CO, require to use catalysts with basic properties. But alkaline and alkaline earth oxides are ineffective because of strong carbonates formation. Oxides of a moderate basicity are necessary and, moreover, their carbonates must be decomposed with CO, reduction. Mn satisfies these requirements 3MnC0, -> Mn,O,

+ 2 CO, + CO

A modification of Mn-containing catalyst by other oxide (for example, Cr, Ca, K) influences both its accepting properties and the degree of surface oxidation.

REFERENCES 1. A.Kh. Mamedov, P.A. Shiryaev, D.P. Shashkin and O.V. Krylov, "New Developments in Select. Oxidation". Proc. I World Congr. (Rimini, Italy, 1989), Elsevier, Amsterdam, (1990), 477. 2. S.R. Mirzabekova, A.Kh. Mamedov, V.S. Aliev, O.V. Krylov, Kinetika i Kataliz, 33 (1992) 59. 3. S.R. Mirzabekova, A.Kh. Mamedov, V.S. Aliev, O.V. Krylov, React. Kinet. Catal. Lett., 47 (1992) 159. 4. J . Rostrup-Nielsen, In "Methane Conversion", eds. D.M. Bibby et al., Elsevier, Amsterdam, 1988, 73. 5 . A.T. Ashcroft, A.K. Cheetam, M.C.H. Green, and P.D.F. Vernon, Nature, 352 (1991) 225. 6. I.M. Bodrov, and L.O. Apelbaum, Kinetika i Kataliz, 8 (1967) 379. 7. A.Kh. Mamedov, Thesis, Inst. Petrochem. Proc., Baku, (1991). 8. O.V. Krylov, Kinetika i Kataliz, 34 (1993) 15.

V. CortCs Corbersn and S. Vic Bellon (Editors), New Developments in Selective Oxidation 11 0 1994 Elscvier Science B.V. All rights rescrved.

167

A Concise Description of the Bulk Structure of Vanadyl Pyrophosphate and Implications for n-Butane Oxidation Michael R. Thompson*a, A.C. Hessa, J.B. Nicholasa, J.C. Whitea, J. Anchella, J.R. Ebnerb aMolecular Sciences Research Center, Pacific Northwest Laboratory', Box 999, Richland, Washington, USA, 99352; bMonsanto Corporate Research Laboratories, 800 N. Lindbergh Avenue, St. Louis, Missouri, USA, 63303 ABSTRACT The evolution of the structure of vanadyl pyrophosphate from its vanadyl hydrogen phosphate precursors occurs with a change in point group symmetry and a transition through an amorphous phase. Based on the crystal structures of these materials, there are no simple topotactic pathways between the precursor and product. An idealized model of the solid-state structure of vanadyl pyrophosphate is introduced and the notion of polytypism discussed with respect to the preparation of vanadiumphosphorus-oxide (VPO) based catalysts. Periodic ub initio Hartree-Fock calculations have been used to compute energy differences between various polytypical vanadyl pyrophosphate crystal structures. These calculations indicate that the experimentallydetermined structures for emerald-green and red-brown crystals

of vanadyl pyrophosphate are expected to be among the most stable for this material. Implications to catalysis relate to the method of synthesis and equilibration of VPO catalysts, and to variation in the expected surface structuresfor vanadyl pyrophosphate. 1. INTRODUCTION

The family of vanadium-phosphorus-oxidespossess a fascinating and complex structural chemistry [2]. Relative to catalysis, the primary focus has been on the vanadyl pyrophosphate phase, (VO)2P2O7,

which exhibits exceptional selectivity in the 14electron oxidation of n-butane to maleic anhydride [3]. The catalytic performance of this phase has been shown to be correlated with crystal morphology and size, and is also strongly influenced by the presence of non-stoichiometric phosphorus and variations in the bulk oxidation state of vanadium [4]. In order to fully understand the structure/performance dependence in this system and the mechanistics of site isolation 1.51 at the activdselective surfaces parallel to (1.0.0) [6], a thorough investigation of the crystallography and variation in the structure of vanadyl pyrophosphate has been necessary. A molecular-level description of the surface structure and surface chemistry of vanadyl pyrophosphate requires an acceptable crystallographic model of the bulk. Unfortunately, a great deal of confusion

* Author to whom correspondence should be addressed

has surrounded attempts to determine the solid-state structure of this material [7]. For example, crystals and crystallites of vanadyl pyrophosphate have been observed to be defected [8]. The nature of these defects cause severe problems with the refinement of the crystallographic model in single crystal X-ray diffraction studies and this has resulted in a lack of confidence in the previous structural assignment. Other points of confusion revolve around the fact that vanadyl pyrophosphate catalysts are known to exhibit a structure sensitivity related to the method of preparation [9] and that differences in catalytic performance are likely due both to the modification of crystal morphology as well as structure. The solid-state dehydration reaction which transforms the vanadyl hydrogen phosphate hemisolvate precursor into the vanadyl pyrophosphate product has been reported to be topotactic [8,10], with an amorphous intermediate phase required to complete the transformation. Based on symmetry arguments alone, it is clear that this reaction cannot proceed as a simple topotaxy if the published crystal structures of VOHP04 . 0.5 HzO I l l ] and (vo)2Pzo7 [7b] are representative of the precursor and product, respectively. The point group symmetry around the face-shared vanadyl dimeric unit in the precursor is C ~ while V that of the edge-shared dimer in the vanadyl pyrophosphate product is C1. It is apparent that there is a considerable reorganization of structure as the catalyst precursors pass through the amorphous intermediate. Experimentally, we have determined that vanadyl pyrophosphate exists in at least two polytypical forms and it is probable that this phase exhibits a broad range of structure. The intent of this paper is to introduce an idealized model of the solid-state structure of vanadyl pyrophosphate which is consistent with experimental studies, to use this model to illustrate the concept of polytypism. and to briefly outline preliminary results from theoretical studies of the bulk structure.

2. RESULTS AND DISCUSSION

2.1 An Idealized Model for the Orthorhombic Structure of Vanadyl Pyrophosphate Large single crystals of vanadyl pyrophosphatevary in color (either emerald-green or red-brown) and possess subtle structural differences relative to variation in the symmetry of the vanadium atom sites within the asymmetric unit [12]. No variation in phosphorus atom positions are indicated in the single crystals, however, there is evidence of phosphorus disorder in catalyst powders [13]. We have developed an idealized model for the bulk structure of vanadyl pyrophosphate based on these experimental observations. For the sake of simplifying the crystallographic description, minor adjustments have been made to the coordinates of the experimental model in order to maximize the apparent symmetry and remove minor variations in bond lengths and bond angles. This idealized model possesses atom connectivity consistent with the experimental structures, and all equivalent vanadium, phosphorus, and oxygen atoms possess identical bonding environments. The coordinates are tabulated below and a general description of the structure and the structural variables are included. The crystal structure of vanadyl pyrophosphate contains two close-packed layers of oxygen atoms which lie parallel to the bc-plane at approximately 114 and 3/4 along the a-axis (‘Fig. la). These layering planes are made up entirely of the basal oxygens of vanadium octahedra and pyrophosphate tetrahedra (Fig. lb). Figure 2 illustrates the close-packed pattern for the basal-plane and the relative positions of the vanadium and phosphorus sites in the octahedral and tetrahedral interstices. The refinement of the crystallographicmodel indicates a degree of non-planarity and distortion of the oxygen basal plane. These

169

+c

b

Figure 1. (a) The close-packed oxygen basal planes for the unit cell of vanadyl pyrophosphate. (b) The relationship between the coordination spheres of vanadium (octahedra) and phosphorus (tetrahedra) . distortions are minor and idealizing the basal layer by forcing the oxygen atoms to lie precisely in the planes at x a . 2 5 and 0.75, simplifies the description and produces a set of coordinates which possess maximum symmetry. Coordinates for all basal oxygen atoms lying within the unit cell are listed in Table I. The vanadium octahedra are square-pyramidally distorted. The vanadium atoms lie approximately 0.33A out of the basal plane oriented toward the vanadyl oxygen (formally V=O). Figure 3a illustrates the coordination geometry about the vanadium atoms, and Fig. 3b the geometry for the phosphorus atoms, each idealized from the experimental model. Four classes of oxygen atoms exist within the structure: double-bridging oxygen (V-0-P), triple-bridging oxygen (P-0-Vz), vanadyl oxygen (V=O), and pyrophosphate oxygen (P-0-P). The double- and triple-bridging oxygens all lie in the basal plane and are listed above in Table I. The coordinates for all of the vanadyl oxygens which lie within the unit cell are listed in Table 11. It is important to realize that the positions of the vanadyl oxygens are invariant to the direction of the vanadyl bond. The directional sense of the vanadyl column relative to the a-axis is determined by the position of the vanadium a t o m in that column. Two positions are possible for each vanadium atom: above or below the basal plane. If the vanadium atoms lie above the basal planes at 1/4 and 3/4, the direction of the vanadyl column will be aligned with the direction of the a-axis, and if they

Oxide Close Packmg Pattern

Octahedral and Tetrahedral Siles

Figure 2. (a) Basal oxygen close-packing pattern (numbers are in accordance with the labeling scheme in Table I). (b) Location of the octahedral and tetrahedral interstices.

170

Table I. Idealized Fractional Coordinates for Basal Oxygen Plane for Vanadyl Pyrophosphate. 90.00 Idealized Lattice Constants: a= 7.710A,b=9.650& c=16.650A, a=k=~~~

~

~

x a . 2 5 , x'd.75

Name 01 02 03

04 05 06

v 0.1600 0.1375 0.1600 0.1600 0.1375 0.1600

Name

z 0.0850 0.2500 0.4150 0.5850 0.7500 0.9150

09 010 011 012 07 08

v 0.3450 0.3625 0,3450 0.3450 0.3625 0.3450

v

z

0.6375 0.6550 0.6550 0.6375 0.6575 0.6575

0.W

019

0.1675 0.3325 0.5000 0.6675 0.8325

020

Name

z

0.3325 0.5000 0.6675 0.8325 0.W 0.1675

013 014 015 016 017 018

Name

021 022 023 024

v

2

0.8400 0.8625 0.8400 0.8400 0.8625 0.8400

0.0850 0.2500 0.4150 0.5850 0.7500 0.9150

1.62A

o 1.6oA

0

(4

@)

Figure 3. Bond lengths for (a) the vanadium coordination sphere, and (b) the phosphorus atoms in the idealized model of vanadyl pyrophosphate. Subscripted oxygen atoms represent double-bridged (V-0-P) positions (Od) and triple-bridged (P-0-V2) positions (03. lie below these planes, then the direction of the column will be anti-parallel to a. Table III lists all possible positions for the vanadium atoms within the unit cell. Unprimed atoms are located below the basal plane, primed atoms above. Note that only one of these two related positions will be occupied, and that the two occupied vanadium sites within a column must be either both primed or both unprimed to construct two chemically reasonable vanadyl moieties (e.g., V1 and V2, or V3' and V4').

Table 11. Idealized Coordinates for the Vanadyl Oxygens Name

y

z

025 026 027

0.W

0.1500 0.3500 0.6500

0.W

0.W

Name 028 029 030

x=O.00. x'=0.50 y z

O.oo00 0.5000 0.5000

0.8500 0.1000 0.4000

Name

y

z

031 032

0.5000 0.5000

0.6000 0.9000

Within every vanadyl column, one vanadium atom will be positioned between any two basal planes of the structure. Similar to the situation for the vanadium atoms, the phosphorus atoms can lie above or below the planes at 1/4 and 3/4 on the a-axis. However, both phosphorus atoms of an individual pyrophosphate group must lie between two adjacent basal layers. Therefore, a column vacancy will necessarily occur in every other layer. There are eight pyrophosphate columns within the unit cell, each of which possess two possible orientations, and these are listed in Table IV. The atom labels in Table IV are unprimed and primed, denoting whether the pyrophosphate group lies below or above the basal plane at x=1/4, respectively.

171

In summary, there are 104 atoms contained within the unit cell of vanadyl pyrophosphate: 48 basal oxygen atoms listed in Table I, 16 vanadyl oxygen atom from Table II, 16 vanadium atoms (8 pairs) from Table III, and 8 pyrophosphates (24 atoms) from Table IV.As an example, the crystal structure reported

Table 111. Idealized Coordinates for the Vanadium Atoms in Vanadyl Pyrophosphate Name

V1 V1' V2 V2' V3 V3' V4 V4'

x

y

0.2075 O.oo00 0.2925 O.oo00 0.7075 O.oo00 0.7925 O.oo00 0.2075 O.oo00 0.2925 O.oo00 0.7075 O.oo00 0.7925 O.oo00

z

Name

0.1500 0.1500 0.1500 0.1500 0.3500 0.3500 0.3500 0.3500

V5 V5' V6 V6' V7 V7' V8 V8'

x

0.2075 0.2925 0.7075 0.7925 0.2075 0.2925 0.7075 0.7925

y O.oo00 O.oo00 O.oo00 O.oo00 O.oo00 O.oo00 O.oo00 O.oo00

Name

z

0.6500 0.6500 0.6500 0.6500 0.8500 0.8500 0.8500 0.8500

x

V9 0.2075 V9' 0.2925 V10 0.7075 V10 0.7925 V11 0.2075 V11'0.2925 V12 0.7075 V12'0.7925

y

Name

z

0.5000 0.1000 0.5000 0.1000 0.5000 0.1000 0.5000 0.1000 0.5000 0.4000 0.5000 0.4000 0.5000 0.4000 0.5000 0.4000

x

y

z

V13 0.2075 0.5000 0.6000 V13'0.2925 0.5000 0.6OoO V14 0.7075 0.5000 0.6000 V14'0.7925 0.5000 0.6OoO V15 0.2075 0.5000 0.9000 V15' 0.2925 0.5000 0.9oOo V16 0.7075 0.5000 0.9000 V16'0.7925 0.5000 0.9oOo

by Linde et al. can be constructed by using all 64 atoms in Tables I and II, the following pairs of vanadium: Vl/V2, V3'/V4', V5'/V6', V7/V8, V9'/VlO, Vll'/V12', V13/V14, V15/V16, and the pyrophosphate groups associated with Pl', P3', PS, P7, P9,P11, P13', P15'.

Table IV. Idealized Coordinates for Phosphorus and Pyrophosphate Oxygen Atoms Name

x

y

z

Name

x

y

z

Name

x

y

z

Name

x

y

z

P1 0.2000 0.2000 O.oo00 P5 0.2000 0.2000 0.5000 P9 0.2000 0.8000 O.oo00 P13 0.2000 0.8000 0.5000 033 O.oo00 0.1850 O.oo00 035 O.oo00 0.1850 0.5000 037 O.oo00 0.8150 O.oo00 039 O.oo00 0.8150 0.5000 P2 0.8000 0.2000 O.oo00 P6 0.8000 0.2000 0.5000 P10 0.8000 0.8000 O.oo00 P14 0.8000 0.8000 0.5000 P1' 0.3000 0.2000 O.oo00 P5' 0.3000 0.2000 0.5000 P9' 0.3000 0.8000 O.oo00 P13' 0.3000 0.8000 0.5000 033' 0.5000 0.1850 O.oo00 035'0.5000 0.1850 0.5000 037'0.5000 0.8150 O.oo00 039 0.5000 0.8150 0.5000 P2' 0.7000 0.uXx) O.oo00 P6' 0.7000 0.2000 0.5000 P10 0.7000 0.8000 O.oo00 P14' 0.7000 0.8000 0.5000 P3 0.2000 0.3000 0.2500 P7 0.2000 0.3000 0.7500 P11 0.2000 0.7000 0.2500 P15 0.2000 0.7000 0.7500 034 O.oo00 0.3150 0.2500 036 O.oo00 0.3150 0.7500 038 O.oo00 0.6850 0.2500 040 O.oo00 0.6850 0.7500 P4 0.8000 0.3000 0.2500 P8 0.8000 0.3000 0.7500 P12 0.8000 0.7000 0.2500 P16 0.8000 0.7000 0.7500 P3' 0.3000 0.3000 0.2500 P7' 0.3000 0.3000 0.7500 P11' 0.3000 0.7000 0.2500 P15' 0.3000 0.7000 0.7500 034' 0.5000 0.3150 0.2500 036'0.5000 0.3150 0.7500 038'0.5000 0.6850 0.2500 040 0.5000 0.6850 0.7500 P 4 0.7000 0.3000 0.2500 P8' 0.7000 0.3000 0.7500 P12 0.7000 0.7000 0.2500 P16' 0.7000 0.7000 0.7500

2.2 The Structures of Emerald-Green and Red-Brown Crystals of (VO)2PzO7

As we have reported [121, the crystallographic models which result from the single crystal X-ray studies of emerald-green and red-brown crystals of vanadyl pyrophosphate are not grossly different from that published by Linde et al., with the exception that the previous authors neglected to account for vanadium atom disorder in the lattice. The refinement of the crystallographicmodel neglecting vanadium disorder results in a woefully inadequate fit to the data. The description of the crystallographicrefinement

172 utilizing a disorded model will appear shortly in the literature. A discussion of the disorder and crystal defects warrant some extra discussion here. Diffraction Streaks,. It is possible to assign the cause of the diffraction streak effects by considering the coordinates tabulated above for each of the building blocks of vanadyl pyrophosphate, and the manner in which each of these contribute to the structure factors for reflections in the two affected parity groups. The structure factor for a reflection (h,k,I) has the form: Fm= fj [COS2n ( hxj + k ~+jlzj ) + i sin 2x ( hxj + ky, + lz, )], where f, is the scattering factor for the j-th atom type; (xj,y,,zj) are the fractional coordinates for the j-th atom; and the sum runs over all j atoms in the unit cell. For each of the oxygen atoms which lie in the basal plane (Table I) with coordinates (x,y,z), there exist identical atoms with coordinates (1/2+x,y,lm). It is simple to show that for the calculated structure factor the sine and cosine terns for the oxygen at (x,y,z) will have the same magnitude but opposite sign to the sine and cosine terms for the parity group. Precisely the same oxygen at (x,y,l/Bz), for any reflection in the even-even-odd (ew) situation exists for any even-odd-odd (eoo) reflection for the basal oxygen atoms at (x,y,z) and (1/2kx,y,z). Therefore, the contribution of the basal oxygen atoms to a computed structure factor for any reflection in either of these two parity groups will be zero. In addition, it can be shown that the phosphorus, bridging pyrophosphate oxygen, and vanadyl oxygen atoms do not contribute to reflections in the two affected parity groups due to similar relationships within the cell. The only atoms which contribute to the eeo and eoo reflections are those of vanadium, and the magnitude of the structure factor is quite sensitive to the site-occupancy-factorswhich relate the relative disorder of the vanadium atoms between the equivalent sites above or below the basal plane. The vanadium disorder occurs in a manner in which the vector between the two related V-sites lies parallel to the a-axis, and the effects of the disorder (line broadening) are evident only in a select subset of reflections. There are other examples of this type of disorder and diffraction streaking [14]. Pattern of Vanadium Disorder: Enantiomomhism. The above discussion provides a basis of understanding the diffraction streaks, but the pattern of disorder for the four independent vanadium atoms is not random. Consider that the space group P c u 1 is non-enantiomorphic. The implication of this is that the two enantiomorphic structures (mirror images) represented by the coordinate sets (x,y,z) and (-x,y,z) cannot be supported together in an entirely ordered lattice. In other words, an ordered lattice with this continuous structure and this space group infers an enantiomorphicallypure crystal. Our observation for the crystal structure of the emerald-green specimens of vanadyl pyrophosphate is that half of the vanadium sites in the structure disorder and half do not. As shown in Fig. 4, the vanadium atoms which lie along a vector parallel to the c-axis with y= ln disorder with site-occupancy-factors reflecting approximately 3:l disorder (but variable from 4:l to 2:l for various crystals), while those along the edge of the cell with y= 0.0 are fully occupied. The explanation of the disorder can be found by generating models for the two enantiomorphs equivalent to the structure reported by Linde et al. If these models are superimposed, all of the oxygen and phosphorus atoms within the structure superimpose, as well do half of the vanadium atoms. Those vanadium atoms with y = l n are not super-impsable. The interpretation of this disorder is that the emerald-green crystals are composed of the two enantiomorphic isomers equivalent to the structure reported by Linde et al. The red-brown crystals of vanadyl pyrophosphate exhibit a pattem of disorder distinctly different from their emerald-green counterparts. All vanadium atoms disorder, however, those which lie along the unit

173

cell edge with y= 0.0 consistently disorder with site-occupancy-factors of 0.5W.03 for the sites above and below the basal plane. Those vanadium atoms with y= 112 disorder in a manner consistent with the emerald-green materials (variable ranging from 4:l to 2:l). While the interpretationof this is not entirely straightforward, we believe that the statistical disorder along the cell edge is caused by a change in symmetry of adjacent vanadium-centereddimers with y= 0.0 (Fig. 5). To construct the crystal structure of

Figure 4. A plot of the crystal structure of vanadyl pyrophosphate projected on the bc-plane. the red-brown material, the following atomic coordinates are used: all 64 oxygen atoms in Tables I and 11; vanadium atom pairs Vl/V2, V3'/V4', V5/V6, V7'/VS', V!Y/VlO', Vll'/V12', V13/V14, V15/V16, and the pyrophosphate groups associated with Pl', P3', P5, P7, P9, P11, P13', P15'. Superpositioned enantiomorphs of this structure disorder all vanadium sites.

Figure 5. Proposed cell edge columnar orientation for (a) emerald-green and (b) red-brown crystals. While both the emerald-green and red-brown crystal structures possess the same space group, P c a ~ ~ , the symmetry of the vanadium atoms which lie inside the asymmetric unit of the cell is different in each case. The mechanism of the transformation which generates the vanadyl pyrophosphate structure from its precursors must provide more than one path to the product, and in this case, it results in subtly different columnar orientations of the vanadyl moieties. The terminology appropriate for this type of structural variation refers to the structures of emerald-green and red-brown crystals as polytvpes [15]. 2.3 Differences in Non-Bonded Contacts, Theoretical Calculations The XRD patterns of vanadyl pyrophosphatecatalysts exhibit significant differences when compared to the diffracted intensities from the single crystals. The most obvious differences are associated with the extreme broadening or extinction of the 000 parity group in the microcrystallinematerials [13]. The most probable explanation is that there is a significant amount of variation in the structure of vanadyl

174 pyrophosphate in the microcrystalline catalysts. The structures of the single crystals are only two of a great number of possible polytypes for vanadyl pyrophosphate. For the observed cell volume, there are 8 columns of vanadyl groups each possessing two possible orientations, and 8 columns of pyrophosphates each with two possible orientations, yielding 216 (65,536) variations. - i h r 1 s. A property of any vanadyl pyrophosphate structure constructed from the coordinates listed above is that irrespective of which vanadium and pyrophosphate coordinates are chosen, the bonding shells of all vanadium, phosphorus and oxygen atoms will be identical with any other coordinate set. However, the symmetries of these structures will be variable, and more importantly, consecutive next-neighbor (non-bonding) shells will change as a function of vanadium and pyrophosphate positions. Next-neighbor distance relationships which vary from sbucture to structure involve: V-V,

V-P, P-P, V--Opop. and P--Opop interactions. Given any two models, differences in interatomic distances are quite small for the first near-neighbor shell, however, these can become very significant for subsequent shells. As an example (Fig. 6). for models with the vanadyl moieties oriented either cis- or trans- across the edge-shared dimer, the first near-neighbor V-V distances differ by less than 0.07A (3.33A vs 3.40A). the distances to the second near-neighbor vanadium atoms are identical in either case ( 3 . 8 6 h however the third near-neighbor V--V shells differ by 0.50A ( 5 . l l A vs 4.62A) for cis- and trans-vanadyl structures. Similar arguments can be made for the variation in non-bonded shells for the phosphorus atoms.

0

0

(4 Figure 6. Illustration of V-V

0

0

(b)

interactions for (a) cis-, and (b) trans-vanadyl dimers.

The importance of these non-bonded interactions in determining the relative energetics of a crystal structure is difficult to assess without resorting to theoretical methods. If the energy differences are small and interconversion possible, many differing structures might be accessible under butane oxidation conditions. Clearly, a thorough quantum chemistry study of the energetics of all variations of vanadyl pyrophosphate is a ridiculous task. However, it is possible to learn something about the relative dependence of the crystal energy on variations of the vanadyl and pyrophosphate networks for a select number of structures. Ab Initio Ouantum Chemistrv of Vanadvl PvroDhosDhate. Wavefunctions for various polytypical vanadyl pyrophosphates have been computed using a WriodiC nb initio Hartree-Fock formalism known as Crystal [16]. These techniques are capable of computing the solutions to the Hartree-Fcck-Roothan equations subject to periodic boundary conditions for a broad variety of crystalline systems, taking full advantage of the space group symmetry. We can use these methods to compute the ground state energy, G, of crystalline vanadyl pyrophosphates evaluated as a function of the nuclear coordinates. Calculations performed on several carefully chosen polytypes can be used to understand the effects on the electronic

175

structure coincident with changes in near-neighbor environments. Since the V - 0 and P-0bond distances and bond angles do not change for any of these structures, the expected energy differences will be due principally to changes in the Coulomb energy. There are a small set of polytypical structures of vanadyl pyrophosphate which possess coordinates for which the c-axis can be halved (c = 8.325A) to create a unit cell containing half the number of atoms (52). Use of these models for theoretical calculations greatly reduces the computational requirements. Within this set of "half-cell" polytypes, structures exist which place the pyrophosphates in various "networked" or "layered" symmetries, as depicted in Fig. 7. For each of the emerald-green and red-brown vanadyl pyrophosphate crystals, a networked structure exists where half of the pyrophosphate groups surrounding a given edge-shared vanadyl dimer are oriented above the basal plane, and half are oriented below the plane, similar to Figure 7a. As for the vanadyl moieties, half-cell structures exist for both cis- and transvanadyl symmetries.

Figure 7. (a) networked and (b) layered structures of the pyrophosphate groups. Wavefunctions have been computed for four polytypcial vanadyl pyrophosphate structures. All calculations were performed with basis sets optimized for the solid 8441+(3-1)d for V; 8-31 for P; and 8-41 for 0. Two polytypes were chosen which possessed identical pyrophosphate networks and differing cis- and trans-orientations orientations of the vanadyl groups. In the same manner, two polytypes with identical vanadyl structures and differing pyrophosphate networks were chosen to evaluate the energy difference due to changes in pyrophosphate structure. The results from these quantum mechanics calculations have been somewhat surprising. For polytypes with identical vanadyl structures (either all cisor all trans-vanadyl groups), only small energy differences exist between layered pyrophosphates and those with networked structures: AEHF 15-20kcals/cell. However, very significant differences in energy exist between structures with variations in vanadyl symmerry: AEm > 250 kcals/cell.

-

Several consistent features appear in these calculations. Consider first the non-bonded near-neighbor distance variations for the phosphorus atoms in networked and layered structures. The closest interaction between two pyrophosphate groups surrounding an individual vanadyl dimer occur at either end of dimeric moiety (Fig. lb). The distance between the phosphorus atoms when they are positioned on the same side of the basal plane is 3.86A. and approximately 0.lOA greater in length when positioned on opposite sides of the plane. While this is a minor difference in the interatomic separation, it should be noted that a van der Waal contact distance for two phosphorus atoms is on the order of 3.8A. Considerable changes in distance

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are apparent for the O-.O interactions between the bridging pyrophosphate oxygens for these two differing orientations. Elongation of critical contacts by shifting the closely positioned pyrophosphate groups to opposite sides of the basal plane are predicted to result in a stabilization of approximately 3-5 kcals/pyrophosphate,based on the electronic structure calculations. The more dramatic results come as a consequence of positioning the vanadyl groups in environments where they are oriented either cis- or transacross the edge-shared dimer. The vanadium and vanadyl oxygen atoms, in particular, are quite sensitive to changes in near-neighbor environments. For each of the structures with cis-vanadyl groups within the dimer, the effective charges (Muliken populations) on the vanadium and vanadyl oxygen atoms are equivalent throughout the structure, as illustrated in Fig. 8a. However, for structures with trans-vanadyl groups, a segregation of approximately one unit of charge occurs between vanadyl groups within the dimer, and a complementary shift of charge occurs within the column (Fig. 8b). The driving force for -0.44

0

0 -09J

-0.94

6

0 .nod

Figure 8. Effective charges on the V and vanadyl oxygen atoms for columns of (a) cis- and (b) trans-. the charge segregation stems from a significant lowering of the Coulomb energy by minimizing the charge on closely placed vanadium atoms and increasing the charge on the next-near-neighbor shells. Under the assumption that the crystal energies for structures of vanadyl pyrophosphate are driven primarily by the Coulomb energy, we have made an attempt to simplistically rationalize the energetics of the 2'6 possible variations in the structure of this material. Given the effective charges for the vanadium, phosphorus, and four classes of oxygen atoms computed from the various models above, Ewald sums (an evaluation of the Coulomb energy for the solid) have been calculated for wide variations in structure, and this is schematically illustrated in Fig. 9. If we define a quantity equivalent to the mole fraction of vanadyl pairs within the structure which are positioned trans- across the dimeric unit relative to the total number of pairs in the cell, we can evaluate the Coulomb energy as a function of the variation in vanadium atom order, while holding the pyrophosphate structure constant, i.e. E (v;p). Likewise, the mole fraction of pyrophosphate groups positioned between x= 0 and x= 114 relative to the number positioned between x= 0 and x= 1/2 can be used to compute, E (p;v). Figure 9 illustrates the trends computed for a total of 1024 hypothetical polytypical structures for vanadyl pyrophosphate. While the results yield only a semiquantitative description of the energetics of the crystal system, the results are very satisfying. The structure with the minimum Coulomb sum it that of the emerald-greencrystals of vanadyl pyrophosphate, equivalent to that reported by Linde et al. Its Coulomb energy is approximately 120 kcalslcell lower than the polytype with all cis-vanadyl groups and a completely layered pyrophosphate structure. The second lowest

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Coulomb energy is computed for the polytype noted above for the structure of the red-brown crystals and it lies approximately 2 kcal/cell above the minimum.

I Green

amax Figure 9. A schematic of the trends in Coulomb energy for polytypes of vanadyl pyrophosphate. 3.

CoNcLusrorvs

The idealized model of vanadyl pyrophosphate has been presented here primarily to illustrate the point that there are many conceivable variations in the structure of this material. There does not seem to be a simple symmetry preserving mechanistic path between vanadyl hydrogen phosphate and the structures of the emerald-green and red-brown crystal, and therefore, we should not be surprised that an amorphous intermediate phase results in the preparation of the catalyst. The crystallography of this system is very complex and riddled with high pseudosymmetry. We note, as we have in the past, that the powder diffraction patterns of the microcrystallinecatalysts, while easily indexed on the crystal structure of vanadyl pyrophosphate, indicate a great deal of structural variation. Intevretation of experimental observables of the bulk material, particularly powder diffraction, should be made with great care. Our preliminary theoretical results point to the fact that the experimental structures may be representative of the most thermodynamicallyfavorable structures for this material (i.e., lowest crystal energy). As to models of the active site and the expected surface topology. we would likewise expect variation. There is a common misconception that the bulk structure of vanadyl pyrophosphate is characterized as a compact solid oxide. Consideration of the symmetry of the vanadyl and pyrophosphate building blocks more appropriately leads to a description of the bulk as a material with a series of interlayer vacancies or pores. The size and location of the interlayer pore is determined by the symmetry of the pyrophosphate network in the solid. Our hypothesis that the surface topology parallel to (1 ,O,O) in vanadyl pyrophosphate must possess three-dimensional character [3] stems from the fact that these surfaces cut across bulk vacancies. If vanadyl pyrophosphate can exhibit variations in its bulk structure, then the sizes and symmetriesof these vacancies at surface terminationwould likewise be expected to be variable.

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4. REFERENCES 1 The Pacific Northwest Laboratory is operated for the United States Department of Energy by the Battelle This research is supported by the Office of Memorial Institute under contract DE-AC06-76RLO-1830. Conservation and Renewable Energy, Advanced Industrial Concepts Division.

2 (a) Bordes, E., Catalysis Today, 1 (1987),499;(b) Hodnett, B.K., Catal. Rev.- Sci. Eng., 27,373, (1985);(c) Centi, G., Trifiro, F., Chim. I d . (Milan), 68 (1986),74;(d) Hodnett, B.K., Catalysis Today, 1 (1987), 477. 3 (a) “Forum on Vanadyl Pyrophosphate Catalysts”, Centi, G. (Ed.), Catalysis Today, 16 (1993). 1-147; Trifiro, F., Centi, G., Ebner, J.R., Franchetti, V.M., Chem. Rev., 88, (1988),55.

4. (a) Comaglia, L.M., Caspani, C., Lombardo, E., Appl. Catal., 74,(1991).15; (b) Yamazoe, N., Morishige, H., Teraoka, Y., Stud. Sug. Sci. Catal., 44,(1989), 15; (c) Garbassi. F., Bart, J.C., Tassinari, R., Vlaic, G., Largarde, P., J. Catal., 98, (1986),317;(d) Hodnett, B.K.. Pemanne, P., Delmon, B.,Appl. Caul.,6,(1983).231; (e) Hodnett, B.K., Delmon, B., ibid., 23,(19841,465.

5. Grasselli, R.K., in: ‘‘ Surface Properties and Catalysis by Non-MetaLs”,Nonnelle, J., and Derouane, E. (Eds.), Elsevier, Amsterdam, (1983), 273. 6.(a) Bordes, E., Catalysis Today, 16 (1993).27;(b) Okuhara, T., Inumaru, K., Misono, M., ACS Symposium Series, American Chemical Society, Washington, D.C., August, 1992,523,157; Busca, G.,Cavani, F., Centi, G., Trifiro, F., J. Catal., 99, (1986).400. (a) Middlemiss, N.E., Doctoral Dissertation, Department of Chemistry, McMaster University, Hamilton, Ontario, Canada, 1978;(b) Linde, S.A., Gorbunova, E., Dolk. Akad. Nauk, SSSR (English Trans.), 245, (1979),584. 8. Bordes, E., Courtine, P., Johnson, J., J. Solid State Chem., 55,(1984).270. (a) Cavani, F., Centi, G., Trifiro, F., J. Chem. Commun., (1985). 492;(b) Centi, G.,Trifiro, F., Busca, G., Ebner, J.R., Gleaves, J.,Faraday Discuss. Chem. Soc., 87,(1989).215;(c) Horowitz, H. Blackstone, C., Sleight, A.W., Tenfer, G., Appl. Catal., 38, (1988), 193.

10. Bordes, E.,Courtine, P., J. Catal., 57,(1979),236. 11. Torardi, C.C., Calabrese, J.C., Inorg. Chem., 23,(1984),1308.

12.Thompson, M.R., Ebner, J.R., ‘Studies in Surface Science and Catalysis”, Ruiz, P, and Delmon, B., (Eds.), Elsevier, Amsterdam, 72, (1992),353. 13.Diffraction peaks in XRD pattern of microcrystallinecatalysts belonging to the odd-odddd parity group are generally broadened by an order of magnitude relative to any other class of reflections. It is possible to show that these peaks are particularily sensitive to phosphorus atom order in the lattice. 14. (a) Miller, K.M., Strouse, C.E., Inorg. Chem., 23, (1984). 2395;(b) Gryder, J.W.; (b) Donnay,

G., Ondik, H.M., Acta. Crystallogr., 11, (1958)38.

15.Wells, A.F., “Structural Inorganic Chemistry”,5th Edition (1987),Oxford University Press, Oxford, p. 10,987. 16. Pisani, C., Dovesi, R., Roetti, C., “Hanree-FockAb Initio Treatment of Crystalline Solids”,

Springer-Verlag,Berlin, 1988.

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DISCUSSION CONTRIBUTIONS F. Trifiro (Department of Industrial Chemistry and Materials, University of Bologna, Bologna, Italy): What are the analogies or correlation's between the defect nature of the calcined catalysts (broadened x-ray lines) and the defects and structural polytypism you propose to exist in well defined crystalline materials?

Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): The defects in the single crystals are caused by the co-crystallizationof both enantiomorphsof vanadyl pyrophosphate within each crystal. A good analogy for this situation can be found in a paper by Charles Strouse and Kathy Miller (reference 14a). I would not expect this to be observable in the typical powder XRD pattern for a catalyst. The reason for this is that you would have to effect the relative ratio of the amount of each enantiomorph generated in the preparation. I do not believe we have much control over this. However, these defects are relevant to the catalysts. As Elizabeth Bordes found, microcrystallites in catalyst powders indicate diffraction spot streaking. These effects are completely analogous to the siluation in our single crystals. There probably exist domains of each enantiomorph within every mature microcrystalliteof the catalyst. The broader question you ask relates to the breadth effects of diffraction lines in catalyst XRD patterns. I think that the most illustrative peak broadening effect in XRD patterns is actually that for the (1.1,l) reflection at approximately 15.8' 2 8 ~ " .For the very mature catalysts which I am familiar, this peak is generally an order of magnitude broader than any other peak of it size in the pattern. Few, in any, other odd-odd-odd reflections are observed. Using arguments similar to those in text of our paper, it can be shown that this parity group (odd-odd-odd) is highly dependent on the phosphorus atom positions in the lattice. This is brought about by the symmetry of the structure. While I realize that the theoretical arguments made in the paper are somewhat obtuse in this brief format, the conclusions are important here. We observe that changing the placement of the phosphorus atoms within the structure of vanadyl pyrophosphate from one symmetry to another costs little in the way of energy to the system (15-20 Kcals). The conclusion that follows is that I might expect a great deal of variability in the phosphorus atom network as a consequence. Variability in phosphorus positions means greater breadth in these peaks. While these effects will be dramatic to the odd-odd-odd reflections, other peaks will be affected for other reasons. Anyone who has built a scale model of vanadyl pyrophosphate can tell you that there exists a great deal of rotational freedom within the basal plane. Would I expect two structures with different pyrophosphate symmetries to be able to pack their basal planes with exactly the same periodicity? The answer to this question is no. The dimensions of the cell (lattice constants) will not be identical and the placement of the peaks will change slightly, adding to breadth. Variability in structure which occurs in this manner will effectively reduce intensity in an exponential manner and the patterns will appear dramatically simple relative to what would be expected for a pattern for a discrete phase. With crystallite dimensionallity, catalyst maturity, and structural variation all playing rolls in broadening peaks, I would not put a great deal of effort into over interpreting these patterns.

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E. Bordes (Departement de Genie Chimique, Universite de Technologie de Compiegne, Compiegne Cedex, France): The question of knowing if the transformation of the hemihydrate is a true topotactic reaction or not could be secondary if this idea would not allow control in the manufacture of the catalyst instead of using a vague recipe. We showed that in a poster at Europa-Cat 1. Now I have two questions. How did you get these crystals and did you observe any morphological differences between them? What would be the selective force determining the formation of MAA: the cis- or trans(vanadyl)? Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): First, I think that the concept of topotaxy in this case is very important. The initial steps of the dehydration likely occur topotactically. and in effect, this begins to “zip” the structure together and places limits on the possible outcomes. But clearly, since the reaction proceeds through an amorphous intermediate with a change in the point group symmetry of the basal layer, a less proscriptive mechanism seems in order. The crystals were generated by carefully controlling the atmosphere and temperature of a mature microcrystalline commercial catalyst placed in a Lindberg oven for periods of up to six or seven days. The temperature program involved heating to approximately 1250’K and slow cooling of approximately 1‘ per hour. We believe that the most important factor was the maturity of the material used: these catalysts had been taken from reactors after more than 5000 hours in the butane oxidation reaction. As to the crystal morphology, crystals which we studied were varied. Green crystals were found which had plate-like dimensions, and others were cube or block shaped. Almost all crystals of the red-brown materials were block shaped. With respect to differences in reaction selectivity that might be exhibited by any of these polytypical vanadyl pyrophosphates, there is no way of knowing. We have hypothesized that the pyrophosphate termination of the crystal can effectively generate isolated active sites and we can also extrapolate that these sites possess different reactive centers depending upon the vanadium site occupancy. However, at this point in time the practical theoretical tools do not exist which would allow us to look at reactions at these surfaces. G. Centi (Department of Indusrrial Chemistry and Materials, University of Bologna, Bologna, Italy): One conclusion that can be derived from your data is that there exists the possibility of a surface reorganization of vanadyl pyrophosphate during the catalytic reaction, because the energetics of transformation between the various possible surface structures corresponding to the different degrees of disorder in the structure are evidently low. This suggests that during the catalytic reaction there is some degree of flexibility of the surface and therefore the actual surface seen by the molecules may change in the course of the reaction. What is your opinion about this question which suggests also a greater role of the conditions of reaction on the in-situ reorganization of the catalyst surface during the catalytic reaction?

Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): The theoretical calculations were performed to assess the differences which

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wouId be expected for the crystal energies of materials as a consequence of changing the nearneighbor environments in vanadyl pyrophosphate. If framed properly. this might tell us something about the phase diagram for vanadyl pyrophosphate, but it does not give us any understanding about the kinetics nor the barriers for transformation.

J.J. Lerou (DuPont CR&D, Experimental Station, Wilmington, DE): There are two parts of the problem of selective oxidation on VPO - the structure of the solid phase is one, but there is also the interaction of the molecules at the surface. When do you expect to be able to model dynamically these interactions? Michael R. Thompson (Molecular Sciences Research Center, Pacific Northwest Laboratory, Richland, WA, USA): Quantum dynamics is a current hot topic in theoretical chemistry, There are people working both in the molecular and molecular cluster regime, and there are those working the methods to approach periodic materials (surfaces). For simple surfaces like MgO with simple reactions (c.f., chemisorption of water), methods will be generally available probably in two years or less depending on how it’s done. Surfaces and reactions as complex as n-butane to MAA on (1.0.0) vanadyl pyrophosphate, far longer if ever. We can look at the properties of static surfaces of (1,0,0) vanadyl pyrophosphate currently. These require hundreds of hours of supercomputer time to converge a single wavefunction. Consider that the consequences of reacting a substrate on a surface generally requires the lowering of the surface symmetry, and hence a requirement to explicitly treat a greater number of atoms. For this system, the computational requirements of the program Crystal scale approximately as n2.5. Hundreds of Cray-hours per energy point quickly escalate to thousands. More efficient means are needed.

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V. CortCs Corberin and S. Vic Bellon (Editors), New Developrnenfs in Selective Oxidation I1 1994 Elsevier Science B.V.

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A study of the (surface)structure of V-P-0 catalysts during pretreatment and during activation R.A. Overbeek, M. Versluijs-Helder, P.A. Warringa, E.J. Bosma, J.W. Geus Department of Inorganic Chemistry, Debije Institute, Utrecht University, P.O. Box 80.083,3508 TB Utrecht, The Netherlands

ABSTRACT The structure, properties, and surface evolution of two vanadium-phosphorusoxide catalyst precursors, prepared from aqueous and from organic solutions, were investigated during calcination and during the catalytic oxidation of n-butane. It was found for both catalyst precursors, that the oxidation of the precursor led to a structural change accompanied by a more rapid removal of the crystal water from the precursor phase. The thus formed oxidized dehydrated phases were both used in the oxidation of n-butane. During reaction these phases transformed to a mixture of phases. This transformation was accompanied by an 'activation behavior', i.e., an increase in selectivity towards maleic anhydride and an increase in activity in the catalytic oxidation of n-butane. It was found that none of the diffraction patterns of the phases active in the oxidation reaction showed resemblance with that of vanadylpyrophosphate, which was identified after catalytic tests. Although both catalysts exhibited the same precursor phase and the same structure after reaction, it was found that they differ greatly in structure during calcination and under catalytic reaction conditions. 1. INTRODUCTION

Vanadium-phosphorus-oxide (V-P-0) catalysts for the selective oxidation of n-butane to maleic anhydride have been extensively dealt with in literature [1,2]. However, what V-P-0 phase is active in this reaction has thus far not been elucidated. Because most studies on V-P-0 catalysts have been performed ex situ, the properties and structure of the catalysts during pretreatment and under reaction conditions are less well known [l-41. It is therefore doubtful whether the structure of the catalyst under reaction conditions actually is vanadylpyrophosphate, the phase that has been identified ex situ after catalytic test reactions [l].In a critical overview, Centi stated recently that one of the main aspects requiring further investigation should be the characterization of the surface topology of the active plane and of its evolution during the catalytic reaction [3]. Therefore an in situ study of the vanadium-phosphorus-oxide catalysts is desired [3-51.

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Since the method of preparation strongly influences the activity and selectivity of vanadium-phosphorus-oxide catalysts, in this study two different V-P-0 catalysts have been examined during calcination and during activation in a n-butanelair mixture. The catalysts were prepared according to the two generally used preparation methods, viz., from hydrochloric acid and from i-butanol solutions. In order to trace the differences in catalytic behavior and to elucidate the structure under reaction conditions, the catalyst precursors prepared in both ways have been examined during pretreatment and finally during reaction using in situ characterization techniques. 2. EXPERIMENTAL SECTION 2.1 Preparation of V-P-0 catalyst precursors The first catalyst precursor, which will be referred to as V-P-OAQ,was prepared in an aqueous medium according to a procedure described by Centi et al. [6]. After reduction of 6.7 g V205 for 16 hours at 100°C in 80 ml37% HCl, 9.3 g 85% o-H3P04 was added to the dark blue V(1V) solution resulting in a P/V ratio of unity. After refluxing the thus obtained dark green solution for one hour, the solvent was evaporated until dryness. The resulting dark green viscous mass was subsequently dried in a nitrogen flow for 10 hours at 125°C. The second catalyst precursor was obtained in an organic solvent according to a procedure described in a patent by Katsumoto et al. [7]. The catalyst precursor thus prepared will be designated as V-P-OOR.15 g V205 was reduced for 16 hours at 120°C in 60 ml of a 1:l (v/v) i-butanol/cyclohexanol mixture. After cooling to room temperature, 21 g 85% o-H3P04 mixed with 30 ml i-butanol was added to the dark green suspension resulting in a P/V ratio of 1.1. After refluxing for 6 hours a blue/green suspension was obtained, which was filtered and subsequently dried in a nitrogen flow for 12 hours at 125°C. 2.2 Catalyst pretreatment and testing Both catalyst precursors were four times diluted with silica powder (DEGUSSA OX50), pressed (4 ton.cm-*, 5 minutes), crushed, and sieved (fraction 0.41-0.72 mm). The sieve fraction was pretreated according to a procedure based on a pretreatment procedure described in a patent of Katsumoto et al. [ 7 ] . According to this procedure 500 mg of the diluted catalyst precursor was heated in argon to 380°C at a rate of 180"C.h-1 (GHSV=625h-1). Subsequently the catalyst precursor was kept for 2 hours at 380°C in a 20% oxygen, 80% argon flow (GHSV=625 h-1). After oxidation the temperature was raised to 480°C in a 1.5%n-butane,20% oxygen and 78.5% argon flow (GHSV=1250h-1). This temperature was maintained for 16 hours. Reactants and products, evolved during the catalytic reaction, were analyzed using a specially developed on-line Balzers QMA420 quadrupole mass-spectrometer, which operates at 150°C to avoid condensation of products. The conversion of the different mass intensities to concentrations of the evolved gases was executed using a selfdeveloped computer program. Carbon mass-balances ranged from 98 to 100%.

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2.3 Characterization Procedures

Specific surface area Nitrogen adsorption isotherms were recorded on a Micromeritics ASAP 2400. Surface areas were calculated according to the BET theory. Thermal analysis In all thermal analysis experiments the temperature was raised with a rate of 300"C.h-1. Thermogravimetric (TG) analyses were performed in a Stanton Redcroft STA-780 thermobalance. Differential Scanning Calorimetric (DSC) experiments were carried out in a Setaram DSC 92. Evolved Gas Analysis (EGA) experiments were executed in the reactor system described above, equipped with an on-line Balzers QMA420 mass-spectrometer. For the TG and DSC experiments, about 50 and 20 mg of sample was used, respectively. For EGA experiments 500 mg of the catalyst precursor was pressed (4 ton.cm-2, 5 minutes), crushed, and sieved (fraction 0.41-0.72 mm). A sample of the sieve fraction was first heated to a temperature of 380"C, kept at this temperature for one hour, and subsequently heated to 750°C. X-ray photoelectron spectroscopy X-ray photoelectron (XPS) spectra were recorded on a VG Microtech XP Clam I1 analyzer using a Mg-source operating at 10 mA. Finely grounded samples were mounted on a sample holder with double sided adhesive tape. All measured binding energies were calibrated at the position of the CIS peak at 284.6 eV. Catalyst samples were compared after calcination at 450°C in nitrogen for 2 hours, after calcination in air at 380°C for 2 hours, and after being subjected to the catalytic reaction for 150 hours. Infrared Spectroscopy Using a Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) environmental cell, samples were heated in steps of 20°C in nitrogen or air at a rate of 300"C.h-1. Infrared spectra were recorded in situ on a Perkin Elmer 1600 F.T.I.R. spectrometer. Because of the strong absorptions of the pure catalyst precursors, both precursors were diluted two times with silicon powder (BDH, p.a.). Backgrounds of pure silicon, recorded as a function of temperature, were subtracted from the spectra with the catalysts. High temperature X-ray Difiaction In situ X-ray diffraction experiments were carried out in an Enraf Nonius Lenne camera (Ka,Fe , h=l.9373A) supplied with a high-temperature diffraction cell. The catalyst precursors were monitored in situ in order to follow crystallographic phase transformations. To study the crystallographic phase transformations during calcination, the catalyst precursors were heated to 580°C in nitrogen or air at a rate of 10"C.h-l. In order to establish the structure under reaction conditions both catalyst precursors were first heated to 380°C in air at a rate of 600"C.h-1 and calcined at this temperature for 4 hours. Subsequently the calcined precursors were heated to 480°C in a 1.7%n-butane-air flow at a rate of 200"C.h-1. The catalysts prepared from the aqueous and the organic solution were kept at this temperature for 23 and 44 hours, respectively.

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After being subjected to the butane/air mixture, an X-ray diffraction experiment at room temperature was also carried out on the catalysts in an Enraf Nonius Guinier Johansson camera (Ku,cu , h=1.5406A). A more detailed description of the X-ray diffraction experiments will be published elsewhere [8,9].

3.

RESULTS & DISCUSSION

3.1 Catalyst pretreatment and testing As can be seen in figure 1, both V-P-OAQ and V-P-OOR show an 'activation behavior' during the first hours of operation. During this stage the selectivity for both CO and CO2 decreased, whereas the selectivity towards maleic anhydride increased. The conversion of n-butane increased also during the sixteen hours of operation. In agreement with literature data, the selectivity towards MA and activity of V-P-OOR is higher than that of V-P-OAQ.The lower activity of V-P-OAQmight be caused by the lower BET specific surface area of the V-P-OAQcatalyst; in this study 3 vs. 9 m2.g-l of VP-OOR.The evolution of the performance of V-P-OAQ,however, was different from that of V-P-OOR. The cause of the difference in evolution during the activation period should become visible by monitoring the catalyst during the catalytic reaction.

a,

0

4

8

12

TimeOnStEYlm(h0LlrS)

l

a,

16

0

I

4

8

12

16

Time on stream (how)

Figure 1. Conversion of n-butane and selectivity towards CO, C02, and MA as afunction of time on stream Of V-P-OAQ and V-P-OOR;T=480°C, GHSV=1250 h r l . 3.2 Characterization Procedures

Thermal analysis Thermal analysis experiments showed a clear difference in behavior during heating of the fresh catalyst precursor samples in respectively an oxidizing and an inert environment. The area under the DTG differential weight loss peaks and under the DSC endothermic heat flow peaks decreased about 20% for both catalyst precursors (figures 2 and 3), although in the EGA experiments the amount of crystal water removed from the precursor phases remained equal (figure 4). Furthermore the position of the DTG and DSC peak maxima shifted to lower temperatures when the precursors were heated in air. In EGA experiments it was found that the complete removal of crystal water from both precursors was more rapid in air. Also it was observed that this removal was accompanied by a consumption of oxygen. The consumption of oxygen was also

187

measured by determination of the average valence of vanadium (AV) in the catalyst precursors during calcination, according to a titration procedure described by Niwa et al. [lo]. From these measurements it was found that at a temperature of about 300°C both catalyst precursors start to oxidize in air, whereas the AV remained constant during calcination in nitrogen. When the catalyst precursors were heated in air at 380°C the AV increased rapidly to about 4.5 in one hour, but remained constant after prolonged heating at this temperature. It was observed in the EGA experiments, the crystal water can be removed from both precursors during prolonged calcination in air at 380°C. The differences in the DTG and DSC peak areas measured in an oxidizing and an inert atmosphere can thus be ascribed to the oxidation of vanadium in the V-P-0 catalyst precursors in air, causing a weight increase and an exothermic heat evolution, respectively, simultaneously with the removal of the crystal water from the V-P-0 precursor phases. __

0

~ _ . _ _ _ _ _ _ _ .

100200300400500600

0

I

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0

T (“C)

T (“C)

Figure 2. Differentialweight loss (left side) and heatflow (right side) of V-P-OAQas a function of temperature in respectively nitrogen (thick line) and air (thin line).

% a 0

100200300400500600

T (“C)

0

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0

T (“C)

Figure 3. Differential weight loss (left side) and heatflow (right side) of V-P-OORas a function of temperature in respectively nitrogen (thick line) and air (thin line). Although both catalyst precursors displayed roughly the same behavior during calcination, for V-P-OAQ the differences between heating in air compared to heating in nitrogen are less pronounced, indicating less dependence of loss of crystal water on oxidation of vanadium, which is in agreement with recent results published by L6pez Granados et al. [ll].In EGA experiments, e.g., calcination of V-P-OAQ in nitrogen at 380°C led to only 21% less evaporation of crystal water compared to calcination in air, whereas calcination of V-P-OOR in nitrogen displayed 41% less evaporation. Also in air

188

the shifts of the DTG and DSC peak maxima to lower temperatures are clearly less pronounced than in the case of V-P-OAQ. 6

I

T..38o"c

T->WC

T->750"C

x

0 1" 0

50

100

-

150

---. 200

m

T i (minutes)

0

50

100

150

200

2.50

Time (minute)

Figure 4. Concentration of water in the evolved gases of V-P-OAQ and V-POORas a function of time during heating in nitrogen (thick line) and air (thin line). Table 1

XPS data for V-P-OAQand V-P-OORafter different treatments.

V-P-0 precursor V-P-OAQ,450"C, N2 2hr V-P-OAQ,380"C, air 2hr V-P-OAQ,after 150hr reaction V-P-OOR, 45OoC,N2 2hr V-P-OOR,38OoC,air 2hr V-P-OOR,after 150hr reaction

P:V surface ratio 2.6 2.2 2.0 2.6 2.6 2.4

0 : V surface ratio V2p3/2 BE (eV) 7.0 517.7 8.7 518.0 12.6 517.2 7.2 517.0 7.9 518.1 7.8 517.1

X-ray photoelectron spectroscopy In table 1 calculated P:V and 0 : V surface ratios are shown as well as the binding energy (BE) of the V2p3/2 peak for both V-P-0 catalyst precursors after different treatments. It was found that both precursors displayed a P:V ratio at the surface of the catalyst higher than the installed P:V ratio. After calcination in air the catalysts showed an oxygen enrichment as well as a shift of the V+3/2 BE to higher values due to the oxidation of the catalyst surface. After reaction the P:V ratio had decreased, possibly caused by migration of phosphate into the bulk of the catalyst or by loss of phosphate during reaction. This effect is also observed by others [12]. It is interesting to note that after reaction the 0 : V ratio of both catalysts was higher than after calcination in nitrogen, which implies a partially oxidized catalyst surface. However, several explanations can be found to discuss the obtained differences in O:V ratios, especially in case of V-P-OAQafter reaction. The high 0:V ratio in that case can not easily be accounted for.

Infrared Spectroscopy Using in situ DRIFTS, it was found for both precursors that during calcination in air at 380°C all the crystal water could be removed from the precursor phases, whereas in nitrogen for V-P-OOR and V-P-OAQthe crystal water was removed only at 480°C and 425"C, respectively. The removal of the crystal water in air was accompanied by a simultaneous oxidation of V4+ to V5+, displayed by an additionally appearing V5+=O

189

vibration at 1012 cm-1. Furthermore, the maximum of the absorption of the stretch vibration of water shifted to higher energy values, indicating the removal of first weakly bonded and subsequently strongly bonded crystal water. It was found that the removal of the strongly bonded crystal water was influenced by oxidation of the precursors in air. It was observed also that the V4+=Ovibration shifted to lower energy values in both an oxidizing and an inert atmosphere (from 976 to 964 cm-I), probably caused by changes in the V-P-0 structure. This shift to lower energy values is also reported in the literature [13]. During this change in structure the vanadium-oxygen distance is enlarged, resulting in a weakened V4+=Obond.

High temperature X-ray difiaction At a temperature of 405°C vanadylpyrophosphate was formed during calcination in nitrogen, whereas in air both catalysts displayed only the formation of V5+-P-O phases. With the knowledge that at 380°C about 50% of the catalysts consisted of V4+ (see Thermal analysis), it is obvious that the remaining V4+-P-Ophase had to be amorphous. In air V-P-OAQdisplayed only one transformation to y-VOP04 at 365"C, and even up to 580°C no other oxidized phases were found. V-P-OOR,however, displayed three phase transformations. The first transformation, at 350"C, was from the precursor phase to an unknown V-P-0 phase, designated as phase 1, in which vanadium probably has a valence of +5. At a temperature of 420°C this phase transformed to tetragonal VOPO4 [8], and later on to the most stable form of vOPO4, p-VOPO4 [14]. After calcination of the precursors in air at 380°C a n-butanelair mixture was introduced in the in situ cell to monitor the phase transformations under reaction conditions. The V-P-OAQphase characterized after 4 hours of calcination in air at 380"C, y-VOPO4, transformed immediatelv in n-butanelair to an unknown phase, to which will be referred to as phase 2. At room temperature in the reaction medium this phase was still existent, together with probably 6VOPO4 [14] and another phase. To this mixture will be referred as phase 2a. A measurement of the catalyst structure after reaction and outside the reaction chamber, to which will be referred to as phase 2b, showed again a different pattern, but consisted mainly of vanadylpyrophosphate. The V-P-OORphase characterized after 4 hours of calcination in air at 380"C, phase 1, disappeared after 10 hours in the reaction medium, and an unknown phase, phase 3, was found. After 36 hours in the reaction medium, phase 3 was transformed into phase 3a. A measurement in the Guinier Johansson camera of the catalyst phase after reaction, to which will be referred as phase 3b, showed again a diffraction pattern different from phase 3a, but it showed strong resemblance to the pattern of earlier mentioned phase 2b of V-P-OAQ.So the same phases were formed ex situ, and consisted mainly of vanadylpyrophosphate. Details and characterization of the above mentioned phases will be published elsewhere [8,9]. 4.

CONCLUSIONS

Although V-P-0 catalysts are extensively investigated, most authors have studied this catalyst ex situ. As is described in this paper, results obtained studying the catalyst in situ are markedly different.

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During calcination in air V-P-0 catalyst precursors prepared from aqueous medium (VP-OAQ)and from organic medium (V-P-OOR)showed an intriguing behavior around a temperature of 350°C. Around this temperature the catalyst precursors were oxidized and lost most of their crystal water simultaneously with a transformation of the precursor phase to different oxidized phases. This (surface)redistribution of the catalyst precursors and change in vanadium oxidation state was confirmed by XPS. The formation of these oxidized phases influenced the evaporation of strongly bonded crystal water from the precursor phase. When the catalyst precursors were heated in an inert atmosphere this removal took place at more elevated temperatures. The above described V5+-P-O phases were transformed to different unknown active phases during the catalytic oxidation of n-butane. The transformation of the catalysts under reaction conditions can also be observed during the first hours of operation in catalytic performance tests for the selective oxidation of n-butane. During this stage, the selectivity for both CO and C02 decreased, whereas the selectivity towards maleic anhydride increased. The conversion of n-butane increased also during these first hours of operation. It is remarkable that none of the diffraction patterns of the phases active in the selective oxidation reaction, measured in situ, showed resemblance to that of vanadylpyrophosphate, which is ascribed to be the active phase. The diffraction patterns of V-P-OOR and V-P-OAQafter reaction and outside the reaction chamber, however, showed great resemblance with vanadylpyrophosphate. This is in agreement with literature data for the V-P-0 phase characterized after catalytic test reactions [1,2]. Although both V-P-0 catalysts exhibit the same ex situ structure and the same precursor phase, their properties and behavior in all (in situ) characterization procedures are remarkably different.

REFERENCES

G. Centi, F. Trifiro, J.R. Ebner, V.M. Franchetti, Chem. Rev. 88 (1988) 55. B.K. Hodnett, Catal. Rev. -Sci. Eng. 27 (1985)373. G. Centi, Catal. Today 16 (1993) 5. Y. Zhang, R.P.A. Sneeden, J.C. Volta, Catal. Today 16 (1993) 39. F. Ben Abdelouahab, R. Olier, N. Guilhaume, F. Lefebvre, J.C. Volta, J. Catal. 134 (1992) 151. 6. G. Centi, C. Garbassi, I. Manenti, A. Riva, F. Trifiro, Preparation of Catalysts 111, B. Delmon, P. Grange, P.A. Jacobs, G. Poncelet (Eds.),Amsterdam (1983) 543. 7. K. Katsumoto, D.M. Marquis, U.S. Patent 4,132,670 (1979). 8. R.A. Overbeek, M. Versluijs-Helder, M. Ruitenbeek, J.W. Geus, to be published. 9. R.A. Overbeek, M. Versluijs-Helder, E.L.J. Vercammen, M.G.A. van den Brink, J.W. Geus, to be published. 10. M. Niwa, Y. Murakami, J. Catal. 76 (1982)9. 11. M. Lopez Granados, J.C. Conesa, M. Fernandez-Garcia, J. Catal. 141 (1993) 671. 12. J. Haas, C. Plog, W. Maunz, Proc. IX*. Int. Congr. on Catal. Calgary (1988) 1632. 13. R.N. Bhargava, R.A. Condrate, Appl. Spectrosc. 31 (1977) 230. 14. E. Bordes, Catal. Today 1(1987) 449.

1. 2. 3. 4. 5.

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x

J. HABER (I. of Catalysis and Surface Chemistr ,Polish Academy of Sciences, Krakow, Poland): Did you try to reinsert the catalyst, w ich you found to be e x situ (V0)2P207, into your camera and find whether it again transforms into the amorphous phase, and what is its catalytic activit ? In order to claim that it is the amorphous phase which is active, ou have to prove Xat the transformation (VO)2P2O7 amorphous phase is reversi le.

z

R.A. OVERBEEK (Department of Inorganic Chemistry, Debije Institute, Utrecht University, Utrecht, The Netherlands): In the future we have planned to monitor vanadylpyro hosphate in situ using XRD, so we think that your suggestion is interesting. $owever, we do not think that the transformation of the catalyst samples in situ has to be reversible, since the cooling down of the sample as well as the exposure to air can cause an irreversible redistribution of the catalyst. Also, based on the fact that the in situ diffraction patterns of the different catalyst sam les are different, we do not think that V-P-0 catalysts have a unique cr stal structure. urthermore we do not claim that the 'active phase is an amorphous p ase, but we think that the V-P-0 catalysts consist of a mixture of cr stalline and amorphous V(IV)- and V(V)-phases, that can easily transform to other pkases. The distribution of the different phases determines the reactivity and the abundance of the oxygen species at the surface of the catalysts, which determine the activity and selectivity in the catalytic reaction.

i:

f

J. EBNER (Monsanto Co. St. Louis, Missouri, USA.): Please publish the XRD patterns, so that your conclusions can be more carefully examined.

R.A. OVERBEEK (Department of Inorganic Chemistry, Debi'e Institute, Utrecht University, Utrecht, The Netherlands): Based on the demand that t e total length of this paper should not exceed 8 pa es in len th, we were (and are) not able to publish the data as yet in the present pu lication. f n the near future an extended paper will be published, with all the above mentioned data as well as some more characterization data. Another a er will also be published concerning crystal structure calculations as well as a detai e V-P-0 catalyst structure investigation. However, the editors gave us the opportunity to publish one examule of our patterns as a discussion contribution, i.e., the in situ XRD patterns of V-P-OoR-during activation in the catalytic reaction mixture according to a procedure described by Katsumoto et al. [ref. 7 in this paper]. These patterns clearly show the changes in the catalyst structure during the catalytic reaction.

h

E

PB

-\

~~:r4ath30~~=calcinati[)n

Figure A

In situ X-ray diffraction patterns Of V - P - 0 0 R at several stages of the activation procedure; (K,,F~, h=1.9373&.

192

F. TRIFIRb (Dipartimento di Chimica e dei Materiali, Bolo a, Italy): The data and the approach you used in this aper are very interesting, but t ey deal only with the first hours of hme on stream. is is a too short time to draw conclusions on the nature of the main active phase. In your presentation you ave a contribution on the understanding of the activation stage of the catalyst. It wi 1 be useful to give more credit to your presentation to see published also the X-ray diffraction patterns of the samples you investigated.

??

.rl:

P

R.A. OVERBEEK (Department of Inorganic Chemistry, Debije Institute, Utrecht University, Utrecht, The Netherlands): In answer to the last remark, I refer to my answer to the previous question of Dr. Ebner. On the first remark we have to state that, although both catalysts did not show vanadylpyrophosphate diffraction maxima durin reaction, they are active and selective in the catalytic oxidation. Consequently, accorcfing to the theories published in the literature, there should be vanadylpyrophosphate present in the catalyst. Since we indeed have characterized vanadylpyrophosphate ex situ, we concluded that the structure of the catalysts ex situ is different from their structure in situ. This study also proves that an in situ study is re uired to compare different catalyst precursors, because the ex situ structures of di erent V-P-0 precursors were very much alike, although the structures in situ were a preciably different. However, we do not claim that the characterized phases are not c anged after prolon ed time on stream. We, based on our results, claim that the proportion of pentava ent vanadium and lower valent vanadium at the surface of the catalyst slowly reaches the equilibrium level. It is obvious that more in situ research has to be done to elucidate all factors influencing the catalytic performance of the V-P-0 system.

x

R

7

J. EBNER (Monsanto Co. St. Louis, Missouri, U.S.A.): Your published catalyst selectivities are <50% at low conversions, whereas most good catalysts (a ueous based ould more as well) have selectivities of >8O% at low conversions. vanadylpyrophosphate improve your selectivities?

d

R.A. OVERBEEK (Department of Inorganic Chemistry, Debije Institute, Utrecht University, Utrecht, The Netherlands): As mentioned in this paper, the catalyst samples used for catalytic testing were four times diluted with inert silica. Therefore the activity is lower than for undiluted Sam les. Furthermore, when the 'activation period' is rolonged, selectivities increased s ightly . Moreover, since the selectivity is not only a Function of conversion, but also of temperature, higher selectivities are obtained at lower reaction temperatures. As stated in this publication, the catalyst samples, which were characterized using XRD after catalytic tests, consisted mainly of vanadylpyrophosphate, which is in agreement with published literature data. However, zn situ III a n-butane-air mixture vanadylpyrophosphate was not characterized using XRD. Summarizing we can conclude that all catalytic test data are in a reement with literature results and that the catalyst Sam les did not behave different y from others, however we were the first to use in situ XR .

7

8

Q

J. EBNER (Monsanto Co. St. Louis, Missouri, U.S.A.): Your data is in contrast to in situ Raman results of Schrader et al. [l] in butane, which clearly shows (VO)zPz07 at

reaction conditions. What's the difference? 1. T.P. Moser, G.L. Schrader, J. Catal. 92 (1985), 216.

R.A. OVERBEEK (Department of Inorganic Chemistry, Debi'e Institute, Utrecht University, Utrecht, The Netherlands): Moser and Schrader [li prepared a model

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(VO) P207 'catalyst' by calcining a V-P-0 recursor in nitrogen at a tem erature of 760'2. Their synthesis procedure was baseion the solid state reaction of H4H2P04 with V205. After formation of this pure phase they studied the structural behavior in a n-butane-air mixture as a function of temperature usin Raman spectroscopy. They showed that the Raman spectra of well crystallized (VO) f 2 0 7 did not alter much. Also they determined that the conversion level of this sampl?e was less than 10% at 475°C. We, on the contrary, synthesized catalyst precursors according to patented and published methods, which were retreated according to a patented procedure. Using this investigation method we stu ied the structure during the pretreatment and during the catalytic reaction and conclude that (V0)2P207 was not found to be present in the working catalyst. So the difference in approach is obvious, and gives a satisfying explanation. However, we think that our approach obtains the best image of the phase active in the catalytic reaction. We are aware that both characterization methods only show the crystalline part of the catalyst, since XRD is only sensitive for crystallinity and Raman also has a much higher sensitivity for cr stalline phases than for amorphous phases. Therefore, the amorphous part as well as t e surface of the catalyst remains invisible.

K

B

K

E. BORDES (U.T.C., BP64S, Compiegne Cedex, France): Your phase 1 could be the anh drous form of the hemihydrate, that is VOHPO . This phase has been evidenced by zmorhs and Beltran-Porter in Valencia (Spain) [2,3f. Did you check that? P. Amoros, R. Ibafiez, E. Martinez-Tamayo, A. Beltrbn-Porter, D. Beltran-Porter, G. Villeneuve, Mat. Res. Bull. 24 (1989),1347. 3. P. Amoros, R. Ibafiez, A. Beltran, D. Beltran, A. Fuertes, P. Gomez-Romero, E. Hernandez, J. Rodriguez-Carvajal, Chem. Mater. 3 (1991),407.

2.

R.A. OVERBEEK (De artment of Inorganic Chemistry, Debije Institute, Utrecht , Utrecht, d e Netherlands): All V-P-0 hases and diffraction patterns publishe in the literature have been carefully checke and no evidence was found that our phase 1 is a known phase. Also we can say, that using DRIFTS we found that OHvibrations are absent in the temperature regime in which phase 1 exists. Furthermore EGA results also prove that all types of crystal water are absent in that temperature regime. Moreover, we have strong indications that the vanadium oxidation state in phase 1 is +5. In the anhydrous form of the hemihydrate, VOHP04, vanadium has a valence of +4.

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V. CortCs Corbcran and S. Vic Bcllon (Editors), New Developmenis in Selective Oxdotion I1 1994 Elsevicr Science B.V.

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The Oxidation of n-Butane on Vanadyl Pyrophosphate Catalysts: Study of the Pretreatment Process B. Kubiasa, M. Meiselb, G.-U. Wale , U. Rodemercka aZentrum fur Heterogene Katalyse bZentrum fur Anorganische Polymere Rudower Chaussee 5, 12484 Berlin, Germany

ABSTRACT The results of the pretreatment behaviour and catalytic properties of sulfate-containing vanadyl pyrophosphate catalysts in the oxidation of n-butane to maleic anhydride (MA) are presented. It has been found that the increase in the MA selectivity during the pretreatment procedure of the fresh catalysts prepared by anaerobic calcination is mainly caused by the decrease in the catalytic activity for MA total oxidation. The change of the catalyst surface composition by oxidation of the surface layer and the site isolation resulting from surface termination in pyrophosphate groups as well as the change of the acidic properties during the pretreatment process using conventional butandair mixtures are assumed to be the reasons for the above finding.

INTRODUCTION (VO)2P207 catalysts are the best suited catalysts for the oxidation of n-butane to maleic anhydride (MA) [ 11. Notwithstanding considerable efforts in basic research, the understanding of this catalytic reaction has been limited up to now. In particular, the results as well as the interpretation of the data concerning selectivity properties of (VO)2P207 catalysts are controversial [2]. In order to shed more light upon these properties the catalytic behaviour of sulfate-doped vanadyl pyrophosphates has been investigated. These catalysts are prepared from sulfatecontaining VOHP04.0.5HzO precursors synthesized in an aqueous solution. In this synthesis inorganic acids and nitrogen compounds are preferably used as reductants thus avoiding the intercalation of organic substances such as alcohols in the lattice of the precursor. The study of these catalysts allows us to compare the catalytic behaviour of oxovanadium(1V)pyrophosphate prepared by anaerobic calcination with the properties of the catalysts after appropriate pretreatment (conditioning, activation).

196

EXPERIMENTAL Preparation and characterizationof catalysts The samples studied in this paper were prepared as follows [3,4]: VOHPO4.0.5H20 (S:V=0.012) was crystallized by evaporating filtered solutions of V205 in oxalic acid and dilute H3P04 at 150 "C, the latter containing H2SO4 corresponding to a molar ratio of H2S04:V205=1: 12. VOHP04.0.5H20 (S:V=0.048 and 0.103) were prepared by evaporating solutions of VOHP04.2H2O in dilute H2SO4 (molar ratios S:V=O.l; 0.5) at 150 "C. The 2-hydrate was obtained from VOHP04.4H2O by treatment with acetic anhydride. All products were washed with dilute HC1, water and acetone and then air-dried. The resulting precursors were calcined for 4 h at 480 "C in a stream of nitrogen (< 1 ppm 02). Thus, the calcination step was strictly separated from the following pretreatment process to be investigated. The compositions of precursors and catalysts were analyzed by gravimetric methods (phosphorus and sulfur). The content of vanadium and its average oxidation state were determined by potentiometric titration (Autotitrator T 100, Schott-Gerate Hofheim) with "/loo CeIV and FeII standard solutions according to a variant of the method of Niwa and Murakami [ 5 ] . Using this method the limit of error of the calculated average oxidation state amounted to k 0.001 at a valence state of vanadium 2 4.0. Based on this high accuracy it is possible to estimate less than one monolayer of Vv in (V1v0)2P207 catalysts. Furthermore, a rough estimation of the number of VII1 layers in catalysts at a valence state of vanadium < 4 is possible. The powder patterns of the bulks of the solids were obtained using a transmission powder diffractometer and a Guinier de Wolff camera. The compositions of catalyst surfaces were estimated by XPS (Leybold Heraeus LHS 10) using MgKa radiation. The spectra were obtained at room temperature. The relative surface concentrations and the (PN), atomic ratios were calculated using the areas under the Ols, V2p3/2 and P2p peaks and after X-ray satellite subtraction. The TG measurements were carried out on a Netzsch STA 409 C and by an analogous procedure described in [6]. After cleaning the catalyst surface at 500 "C for 2 h in a deoxygenated N2 stream, the total mass of adsorbed oxygen was determined using a 20 % 0 2 / 80 % N2 mixture for the adsorption process. The treatment with the 02/N2 mixture was started at room temperature and continued while heating up to 500 "C. After 3 h the gas stream was switched to N, and the measurement was finished after 6 h. The TPD of NH3 was studied by means of a Car10 Erba Quadrupole Thermalprogrammed Mass Detector System according to the following procedure: Usually the catalyst surfaces were cleaned by evacuation at 527 "C for 1 h. One of the conditioned catalyst specimens was pretreated in this way at 477 "C for 14 h in order to complete the cleaning process. After adsorption of NH3 at room temperature, the physically adsorbed NH3 was removed by evacuation and the TPD was performed at a heating rate of 5 K min-l. The specific surface areas of fresh and conditioned catalysts were estimated by BET (5-point method, N2 adsorption).

197

Catalytic tests An integral microreactor was applied for the calcination procedures, the conditioning of fresh catalysts and the investigation of the catalytic behaviour of fresh and conditioned catalysts. Precursors, catalysts and the thinning agent a-Al2Og that was applied to reduce the temperature gradient were used in granulated form, The feed gas contained 1.51 % n-C4H10 (99.5 %) in synthetic air. Pure N2 as used in the calcination process was also passed through the catalyst bed before and after the oxidation runs. The analytical determinations of the composition of the effluents were carried out by online gas chromatography (MA and n-butane) and IR photometry (CO, C02). In cases of fresh catalysts the analytical control was accomplished as early as possible after the start of the reaction in order to be able to estimate the catalytic properties at the beginning of the reaction. RESULTS AND DISCUSSION The catalytic behaviour of (VO)zPzO, catalysts in the pretreatment process For a phenomenological conditioning study fresh catalyst specimens containing a small sulfate content (S:V=0.036) were pretreated by oxidizing a conventional butane/air feed. The initial MA selectivities of such fresh catalysts amount to about 20-30 % under reaction conditions leading to high butane conversion of 85-95 %. Under these conditions (T < 430 "C, T < 1.5 s) the maximum MA selectivity of 55-60 % is not obtained until 70 h time on stream. Suitable enhancement of the temperature during conditioning leads to a faster growth of the catalytic performance. Thus, catalysts pretreated for 2 h at hot-spot temperatures between 470 and 490 "C show a maximum selectivity under optimum reaction conditions and, after some hours on stream, an only slightly decreased and sufficiently constant activity. All "conditioned" catalyst specimens described in this paper were pretreated in this way. The dependence of the MA selectivity of fresh and conditioned catalysts on the butane conversion degree and the change in some properties of the catalyst as an effect of conditioning have been studied to explain the reason for the enhancement of selectivity in the course of conditioning. To illustrate the collection of data for the initial catalytic behaviour of the fresh catalyst specimens some examples of characteristic dependencies of MA selectivity and butane conversion on the time on stream are shown in Figure 1. In Figure 2 both, the extrapolated initial MA selectivities of the fresh catalyst specimens and the MA selectivities obtained for the conditioned catalysts are depicted. The course of the curves illustrates a difference in MA selectivities 5 12 % at a butane conversion degree near 0 %. This difference is increased remarkably with increasing conversion degree of butane and reaches more than 25 % at 80 % butane conversion.

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80 T = 380°C

60 8

60

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40 U

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80 60 8

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+

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a time [min]

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conversion [%] T = 460'C

2 60 2

60

r_

8

c

0

I

40 200

f

10

20

30

=

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40 U ~ 200

time [min] Figure 1. Dependence of MA selectivity and butane conversion degree on time on stream for fresh catalyst specimens w butane conversion A MA selectivity

30

60

8 90

conversion [Yo] Figure 2. Dependence of MA selectivity on butane conversion degree conditioned catalyst A fresh catalyst

These results are interpreted using the well-known triangular reaction scheme of the oxidation of butane for a qualitative approach: 1 C4H10

* C4H203

XA Y COX At a butane conversion degree near 0 % the ratio of the formation of MA and COXis only determined by the ratio of the rates of the reactions 1 and 2. The results demonstrated in Figure 2 show that the selectivities of both the fresh and the conditioned catalyst specimens are not very different at this point. Therefore, this ratio does not seem to be markedly influenced by the conditioning process . At high degrees of butane conversion, reaction 3 must be taken into account. Consequently, the considerable increase in the MA yield due to conditioning may be explained mainly by the decrease in the rate of the decomposition reaction of MA.

199

Characterization results The XRD patterns of the fresh and conditioned catalysts show the reflections of (VO)2P207. Caused by the incorporated sulfate the line positions of some characteristic reflections are shifted and a broadening of the line width is observed in comparison to the corresponding reflections of the sulfate-free oxovanadium pyrophosphate (For an interpretation see [4]). The characterization of the catalysts both in the fresh and in the conditioned state by chemical analyses, XPS, TG and BET leads to the following results: Table 1 Characterization of fresh and conditioned (VO),P?O7 specimens (S:V=0.007) State of (VO)?P307 catalysts fresh bulk comp.: P:V ratio surf. comp.: P:V ratio V-Val. (pot.titr.) ads. oxyg. (TG) [mg g-'1 spec. surf. im2g-11

1.014 & 0.02 1.2 3.96 (3.98)1) & 0.02 5.2 20.5 (19.2)1)

est. number of monolayers: ~ 1 1 (titr.) 1 0-2S2) VV (titr.) 0 1)different specimens 2) assumption: Vrrr is only present at the catalyst surface

conditioned 1.003 k 0.02 1.3

4.005 (4.007)l) f 0.001 6.1 12.5 (8.2)1)

0 0.3-0.5

As shown in Table I , the chemical composition of the catalyst bulk is not markedly influenced by the chemical reaction. In comparison with the bulk composition, the P:V ratio at the surface of the fresh catalyst is slightly enhanced and increased once more in the course of the catalyst pretreatment. Similar P:V ratios in the spent catalysts were also found by other authors [7, 81. The valence state of vanadium in the fresh catalyst is somewhat lower than 4 because of loss of the oxygen during the calcination. After conditioning, the valence state of the catalyst is slightly higher than +4.0. The numbers of monolayers of vanadium V and 111, respectively at the catalyst surface corresponding to the valence states measured by wet analyses were estimated assuming a dominant role of the (1,0,0) plane of (VO)2P207 in the selective oxidation of butane. The calculation is based on a number of 6.6.1018 V-atoms per m2 surface of (VO)2Pz07 crystallites [6]. Further, the sole participation of surface vanadium and oxygen in the calcination process and in the catalytic reaction is assumed (s. also [9]). Accordingly, less than one monolayer of Vv ions is present at the surface of conditioned (VO)2P207 catalysts and up to 2-3 layers of surface VIII ions should be present in the case of the fresh catalyst. The former finding is in good agreement with the recently reported result of Ye et al. [lo].

200

This result is confirmed roughly by TG measurements of oxygen adsorption on both a conditioned and a fresh catalyst specimen. Figure 3 illustrates one of these experiments using a conditioned catalyst with low sulfate content. 600 1 0,6 0,4

-s Y

0,2

1

- ,

0 0

2

0 100

200

300

4 0

t [min]

Figure 3. Thermogravimetric study of oxygen chemisorption on a conditioned (VO)2P,O7 catalyst (S:V= 0.007; 0.lg) The total amount of adsorbed oxygen is 0.61 wt% corresponding to an adsorption of about two molecules 0 2 per vanadium atom at the catalyst surface. The oxygen adsorption value of the fresh catalyst (0.51 wt%, s. Table 1) corresponds to less than one molecule 0 2 per vanadium atom. The course of the TG curve depicted in Figure 3 shows that the measurement is influenced by reactions of carbon-containing residues on the catalyst surface leading to C 0 2 and H20 and thus causing weight loss. This is seen in the decrease of the weight curve after about 120 min and by the continuation of this decrease after switching from the OzLN2 stream to N2 at 180 rnin. C 0 2 formation under these conditions has also been observed by MS in the TPD measurements discussed below and it takes place in the cleaning operation, too. Additionally, a loss of SO2 could be detected. Hence, the accuracy of such TG experiments is limited. These results show a noticeable resistance of conditioned and also of fresh (VO)2PzO7 catalysts, which were prepared in the way described, to bulk oxidation. Thus, they provide additional evidence for the sole participation of the surface layers in the reaction. To get additional information about the conditioning effect on MA selectivity TPD/MS measurements were performed on both the fresh and the conditioned catalysts containing different amounts of sulfate. The ammonia temperature-programmed desorption spectra on the catalyst specimens studied are depicted in the Figures 4 and 5. The TPD spectra show two different peak regions: A low-temperature peak between 100 and 350°C and a high-temperature region with different peaks above 550'C. The first TPD peak can be attributed to ammonia bound on Brgnsted and Lewis sites of different strength [ l l ] . The second peak is caused mainly by SO2 emerging from the catalyst owing to destruction reactions. Desorbed products of NH3 oxidation do not markedly contribute to the peak at about 600°C.

201

I "

0

200

400

600

800

1000

T "CI

Figure 4. TPD/MS total pressure spectra after NH? adsorption at 27 "C Catalyst: (VO)2P207 (S:V = 0.007) a fresh catalyst, cleaned for 1h b conditioned catalyst, cleaned for 1h c conditioned catalyst, cleaned for 14h

Figure 5. TPDMS total pressure spectra after NH?adsorption at 27 "C Catalyst: (VO)2P207 (S:V = 0.069) a fresh catalyst, cleaned for l h b conditioned catalyst, cleaned for 1h

The comparison of the low-temperature peaks obtained from the fresh and the conditioned catalysts reveals that, in the course of conditioning, the acidic properties of the catalyst surface are shifted to a weaker acidity. This has been observed for both catalysts independent on the sulfate content.

Discussion Based on the ideas of Ebner and Thompson [7], the remarkable enhancement in the MA selectivity during the pretreatment process may be interpreted as an effect of the termination of the catalyst surface in pyrophosphate groups. The formation of polyphosphate anions should be unlikely because of the only small surface enrichment in phosphorus. This termination process should lead to a proper isolation of vanadium centers which are assumed to be the active sites and thus diminish the activity for the total oxidation of MA. The increase in the P:V ratio in the course of conditioning supports this interpretation. Owing to the surface termination in pyrophosphate groups, the presence of strong Bronsted sites, which are attributed to surface P-OH groups, as well as of strong Lewis sites must be expected as discussed in [ 111. Such strong sites are known to favour total oxidation reactions too [12]. In the conditioning process the acidity decreases, so this should be another reason for the enhancement of selectivity. The decrease in acidity could be the result of the formation of carbon-containing residues blocking the acidic centers. To support this assumption an attempt was made to restore the "original" surface corresponding to a fresh catalyst surface with higher Bransted acidity by a long-time heating of a conditioned catalyst specimen under vacuum conditions. The attempt was not successful: The catalyst treated in this way shows a behaviour analogous to that of the conditioned catalyst (see Figure 4, curve C). Another reason for the increase in MA selectivity during conditioning should be the change in the valence state of vanadium in the surface layer. After calcination the valence state of at least a part of the vanadium in the surface layer is assumed to be +3. Such a surface should be less capable of carrying out the selective oxidation of butane to MA as we could show by other experiments, too [13]. During the conditioning, the surface V3+ ions are oxidized to VO2+ and VO3+ ions, thus enhancing the selectivity of MA formation.

202

CONCLUDING REMARKS The present study of the catalytic behaviour of sulfate-doped (VO)2P,O,, starting from a catalyst state obtained by anaerobic calcination, shows that this approach gives an interesting insight into this intriguing catalytic system. Future investigations of such catalysts with different sulfate contents are aimed at examining the effect of sulfate doping taking into consideration the sulfate-doping effect on transition metal oxides which is well known from the study of the superacid properties of these compounds. This kind of anion doping has not been studied yet in contrast to the extensive research in the field of doping (V0)2P207 catalysts with cations (see e. g. [ 141).

REFERENCES 1. 2.

3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13.

14.

G. Emig and F. Martin, Catal. Today, 1 (1987) 477 B.K. Hodnett, Catal. Today, 16 (1993) 131 G. Centi, Catal. Today, 16 (1993) 5 F. Trifirb, Catal. Today, 16 (1993) 91 K. Schlesinger, M. Meisel, G. Ladwig, B. Kubias, R. Weinberger and H. Seeboth, DD-WP 256659 (1984) M. Meisel, G.-U. Wolf and A. Bruckner, DGMK Conference "Selective Oxidations in Petrochemistry", Goslar, Germany, September 16-18, 1992, Tagungsbericht 9204 der DGMK, Hamburg, 1992,27 M. Niwa and Y. Murakami, J. Catal., 76 (1982) 9 J. R. Ebner and J. T. Gleaves, Oxygen Complexes and Oxygen Activation by Transition Metals, E. A. Martel and D. T. Sawyer (eds.), New York, 1988, p. 273 J.R. Ebner and M.R. Thompson, Stud. Surf. Sci. Catal., 67 (1991) 31 V.A. Zazhigalov, A.I. Pyatnitskaya, G.A. Komashko, N.D. Konovalova and V.M. Belousov, Kin. i Kataliz, 31 (1990) 1219 M.A. Pepera, J.L. Callahan, M.J. Desmond, E.C. Milberger, P.R. Blum and N.J. Bremer, J. Am. Chem. SOC.,107 (1985) 4883 D. Ye, A. Satsuma, T. Hattori and J. Murakami, Appl. Catal., 69 (1991) L1 G. Busca, G. Centi, F. Trifirb, and V. Lorenzelli, J. Phys. Chem,90 (1986) 1337 M. Baerns, H. Borchert, R. Kalthoff, P. KaDner and F. Majunke, Stud. Surf. Sci. Catal.,72 (1992) 57 B. Kubias, H. Wolf, B. Lucke and B. Voigt, DGMK-Fachbereichstagung "C1-Chemie-Angewandte heterogene Katalyse-C4-Chemiet', Leipzig, Germany, February 20-22, 1991 Tagungsbericht 9101 der DGMK, Hamburg, 1991,49 G.J. Hutchings, Appl. Catal., 72 (1991) 1

This work was financially supported by the Deutsche Forschungsgemeinschaft. This support and the XPS contribution of H.Papp, Leipzig, as well as the contribution in the TPD study of P. Klobes, Berlin, are gratefully acknowledged.

V. CortPs Corberin and S. Vic Bellon (Editors), New Developments rn Selective Oxidation I1

0 1994 Elscvier Science B.V. All rights rcscrvcd.

203

Activation of Vanadium Phosphorus Oxide Catalysts for Alkane Oxidation Oxygen Storage and Catalyst Performance Y. Schuurman and J.T. Gleaves Department of Chemical EngineeringWashington University 1 Brookings Drive, St. Louis, Missouri 63 130 J. R. Ebner and M. J. Mummey Monsanto Chemical Company 800 North Lindbergh Blvd, St. Louis, Missouri 63167 The reaction of n-butatk with "reactor-equilibrated'' (VO)2P2O7 based catalysts activated with different oxygen treatments has been investigated using a combination of high speed transient response and temperature programmed techniques. Results indicate that (VO)2P2O7 has a unique "storage/supply" system capable of adsorbing oxygen and efficiently channeling it to the active catalytic site. It is proposed that storage occurs via the transformation of (VO)2P2O7 into V+5 compounds and the supply mechanism involves the reverse reaction.

1. INTRODUCTION A characteristic feature of the formation process of industrial VPO catalysts is an extended break-in period (typically 200 - 500 hours) during the formation of the stable active catalyst (1-5).During the break-in period, the "non-equilibrated" catalyst is exposed to an air-butane

mixture at reaction temperatures, and undergoes changes in vanadium oxidation state and the relative concentrations of different VPO phases (1,6).Another interesting feature of the VPO system is that "equilibrated" catalysts show temporary improvements in performance when the butane feed is terminated, and the catalyst is held at reaction temperatures in air for a period of time. Clearly, the air-butane mixture plays an essential role in the catalyst formation process and in determining the performance of an equilibrated catalyst. How exposure of a catalyst to gas phase oxygen influences oxygen avalability at the VPO surface is a particularly important question. In light of new nonsteady-state processing approaches to butane oxidation (7,8), it is also of great interest to investigate the process by which the VPO system stores oxygen and if the stored oxygen is utilized selectively. The purpose of this paper is to examine the effect of different oxygen exposures on VPO catalytic chemistry and to determine if selective oxygen can be stored in the VPO lattice. Post-mortem structural studies of "equilibrated" VPO catalysts indicate that the predominant crystalline phase in working VPO catalysts is vanadyl pyrophosphate (VO)2P2O7 (1-5,9).Many researchers have concluded that (VO)2P2O7 is the active selective phase and there is good evidence that butane oxidation to maleic anhydride occurs readily on the (100) plane of (VO)2P2O7. Recent Raman spectroscopic ( I 0 , l l ) studies suggest that other phases (e.g. VOPO4) may also play an important role in the VPO catalyst system, and it has been widely proposed that the presence of some V + 5 (12) is essential for good catalyst performance. There is evidence that (VO)2P2O7 samples predominantly exhibiting the (100) face can be topotactically transformed into the 6 form of VOPO4 at reaction temperatures ( 1 3 ) , but the role of V+5 phases in the catalytic process is not well understood. Since it has been well established that the (VO)2P2O7 phase can be converted to VOPO4 phases by exposure to oxygen, we chose to investigate the effect of oxygen on well equilibrated VPO catalysts which XRD analysis determined were monophasic (VO)2P2O7.

204 2. EXPERIMENTAL 2.1. Catalyst Preparation Vanadium phosphorus oxide catalysts were prepared by refluxing a mixture of 7340 cm3 of isobutyl alcohol, 513.5 gms of V2O5, and 663.97 gms of H3P04 (100%) for 16 hours to give a light blue precipitate. Upon cooling, the precipitate was filtered and dried at ambient temperature under vacuum. The dried precipitate was washed with isobutyl alcohol, dried for 2.5 hours at 418 K and calcined in air for 1 hour at 673 K. The resulting powder was then charged to a 122 cm long, 2.1 cm i.d. fixed bed tubular reactor for performance testing. Tests were conducted at a fixed set of reactor conditions of 1.5% butane, 15 psig reactor inlet pressure and 2000 GHSV. After a sufficient break-in period, the catalyst gave steady-state selectivities of approximately 66% at 78% conversion. Catalysts operated in this manner for approximately 3000 hours were designated "reactor-equilibrated'' catalyst samples and were used without further treatment in the oxygen activation studies. XRD analysis of the reactorequilibrated samples showed that they were monophasic (VO)2P2O7. Chemical analysis gave a P/V ratio of 1.01 and vanadium oxidation state of 4.02. The samples had a BET surface area of 16.5 m2/gm. 2.2. Multifunctional Reactor System A simplified schematic of the microreactor, multivalve feed manifold, quadrupole mass spectrometer detector (QMS), and vacuum chamber of the reactor system (14)used in this study is shown in Figure 1. The vacuum system can be isolated from the microreactor using a 3-position slide valve that can be positioned for vacuum or high pressure operation.

Figure 1 Cross sectional schematic of multifunctional reactor system: 1. microreactor, 2. VPO catalyst, 3. inert paclung, 4. pulse valve, 5. ionization cage of QMS, 6. 3-position slide valve (open position), 7. pin hole leak, 8. external vent line, 9. microreactor sealing flange, 10. microreactor heating element, 11. thermocouple, 12. heat shield, 13. flow valve, 14. vacuum chamber flange.

205

2.3. Procedures Reaction studies were performed with untreated reactor-equilibrated catalyst samples and reactor-equilibrated samples exposed to different oxygen treatments. Three different procedures were applied to oxidize the catalyst. In one case, oxygen was pulsed into the microreactor at vacuum conditions until a constant oxygen pulse size was observed by the QMS. This procedure is referred to as a low pressure oxidation (LPO) treatment. In the second procedure, the slide valve was positioned for high pressure operation and a continuous flow of pure oxygen or air at 100 kPa was introduced to the microreactor for 3 to 10 minutes. This procedure is referred to as a high pressure oxidation (HPO) treatment. In the third procedure, the microreactor was evacuated and the exit was sealed with the slide valve. The reactor was then filled from a reservoir of a known volume and oxygen pressure. In this case, the oxygen uptake could be determined by measuring the pressure drop which was recorded as a function of time. This procedure is referred to as a static pressure oxidation (SPO) treatment. Low pressure, isothermal pulse response experiments were carried out at a variety of temperatures, y l s e intensities, and pulse formats. Typical pulse intensities were in the range of 1013 to 101 molecules per pulse. A standard microreactor charge consisted of 0.2 grams of catalyst with an average particle size of 350 microns. In a typical experimental sequence, the loaded microreactor was evacuated and heated to reaction temperatures (653 - 693 K) while the desorption spectrum was monitored. After the catalyst bed temperature had stabilized a series of reactant pulses from one or both pulse valves was introduced into the reactor. Anaerobic pulse experiments, using a 311 C&I10/ Ne mixture were used to determine the amount of active oxygen. Pump-probe experiments using a 311 C&Ild Ne mixture and a 411 02/Ne mixture were used to determine the relative product pulse shapes after each oxygen treatment. Temperature programmed desorption (TPD) experiments were performed on reactorequilibrated catalyst samples, reactor-equilibrated samples that had been reacted with butane, and reactor-equilibrated samples exposed to various oxygen treatments. Typically, the reactor was operated in the vacuum configuration and ramped from 623 K to 798 K at 20" per minute and held for ten minutes at 798 K. Data was collected in the scan format. 3. RESULTS

When untreated reactor-equilibrated samples were heated to reaction temperatures (653 - 693 K) in vucuo, they emitted water and carbon oxides. When heated above 693 K, they also emitted small amounts of molecular oxygen. The rate of oxygen emission could be increased by raising the temperature, but eventually went to zero when the catalyst was baked for extended periods at 813 K. In low pressure pulse response experiments, butane reacted with reactor-equilibrated samples at 653 K in the absence of gaseous oxygen. The principal oxidation products were furan, maleic anhydride (MA), and carbon oxides. Butane conversion on untreated reactorequilibrated catalysts was typically = 30% at the start of a pulse sequence, and decreased with each butane pulse. The decrease in butane conversion was more rapid for reactor-equilibrated catalysts that were baked at 813 K prior to butane oxidation. Catalyst samples that produced no selective oxidation products in anaerobic butane oxidation experiments were designated "oxygen-depleted" catalysts. Low pressure oxygen treatments did not significantly affect the performance characteristics of untreated reactor-equilibrated samples. Figure 2 shows the oxygen transient response during a LPO treatment at 653 K, applied to a reactor-equilibrated sample previously oxygen-depleted with butane. Initially, no oxygen transient response is observed, indicating that the oxygen pulses are completely absorbed by the oxygen-depleted catalyst. As pulsing continues, the oxygen pulse intensity slowly increases and gradually becomes constant. Reactor-equilibrated catalyst samples exposed to a series of anaerobic reductions and LPO treatments gave the same oxygen transient response after each butane reduction. The total amount of oxygen adsorbed by a oxygen-depleted catalyst was estimated by measuring the oxygen pressure drop in the valve

206

until the oxygen response became constant. For a .200 grn sample, approximately 4 ~ 1 0 1 ~ oxygen molecules were absorbed. This amount corresponds to approximately one oxygen atom for every ten vanadium surface atoms assuming there are z 11 micromoles of surface vanadium per m2 (15). Increasing the total number of pulses by a factor of ten did not measurably increase the amount of active oxygen adsorbed.

Figure 2 Oxygen breakthrough curve at 653 K showing the oxygen transient response when a series of equally intense oxygen pulses are pulsed over a reactor-equilibrated VPO catalyst that has been reduced with butane. Figure 3 shows the MA transient response to a uniform series of 80 butane pulses applied to a catalyst sample that was oxygen-depleted and then reoxidized using a LPO treatment. The MA pulse intensity decreases with each butane pulse indicating that the surface is rapidly oxygen-depleted. Reactor-equilibrated catalyst samples were cycled several times

Figure 3 Maleic anhydride transient response when a series of equally intense n-butane pulses are pulsed over a reactor-equilibrated VPO catalyst at 653 K that has been activated using the low pressure oxygen (LPO) treatment.

between butane reductions and low pressure oxidations and gave the same MA transient response after each oxidation. A constant MA transient response equal to that obtained during the first butane pulse after a low pressure oxidation could be obtained by alternately supplying butane and oxygen in a pump-probe format. If the oxygen pulse was halted, the MA pulse intensity decreased in the same fashion as shown in Figure 3. Similar results were obtained for furan and carbon dioxide. Figure 4 shows the oxygen pressure drop during a typical SPO treatment at 723 K of an oxygen-depleted reactor-equilibrated sample. A rapid decrease of the oxygen pressure from 215 kPa to 140 kPa, corresponding to a total uptake of 1020 oxygen molecules is observed. 220

,

210

200

-O

190

-o

180

-:

170-:

130

!

I

I

I

0

10

20

30

40

Time, ks Figure 4 Oxygen pressure drop versus time due to the oxygen uptake by a reactorequilibrated catalyst at 653 K. Similar experiments indicated that the rate of oxygen uptake is a function of oxygen pressure and catalyst temperature. Maintaining a catalyst in an oxygen pressure of =: 108 kPa for 8 hours at 723 K gave a yellow compound indicative of a V+5 phase, and an oxygen uptake corresponding to one oxygen atom for every vanadium atom. Chemical analysis of a catalyst exposed to a HPO treatment at =: 108 kPa and 723 K for 3 minutes gave a bulk vanadium oxidation state of 4.14, corresponding to an increase from 2% to 14% in V+5 content. Figure 5 shows the MA transient response to a uniform series of 80 butane pulses applied to a catalyst sample that was oxygen-depleted and then reoxidized at =: 108 kPa and 723 K for 3 minutes. In contrast to the MA response after a LPO treatment, the MA production after a HPO treatment does not appear to decrease with butane pulse number. Even after a series of 1000 butane pulses, no decrease in the MA pulse intensity was observed. Similar results were observed for furan and C02. Figure 6 presents a TPD mass spectrum in the 30 to 40 amu range from a reactorequilibrated sample given a HPO treatment at 703 K for 5 minutes. Prior to initiating the TPD experiment the sample was exposed to 80 pulses of butane at 653 K. The MA transient response during the butane injection was constant and had an identical appearance to the response shown in Figure 5. The spectrum presented in Figure 6 shows that molecular oxygen is the primary desorption product when the sample is heated from 650 to 793 K. The emission continues at a high rate if the catalyst is maintained at 793 K. When an oxygen-depleted catalyst was given a HPO treatment at 423 K, negligible oxygen desorption was observed

208

upon heating the catalyst to 813 K. Similarly, when an oxygen-depleted catalyst was activated using a LPO treatment at 653 K, negligible oxygen desorption was observed upon heating the catalyst to 813 K.

Figure 5 Maleic anhydride transient response when a series of equally intense n-butane pulses are pulsed over a reactor-equilibrated VPO catalyst at 653 K, activated using the high pressure oxygen (HPO) treatment.

Figure 6 Oxygen desorption spectrum from a reactor-equilibrated VPO catalyst, activated at 653 K using the high pressure oxygen treatment. Similar TPD profiles were obtained when static oxidation procedures were used. In these cases, the total amount of oxygen that desorbed was found to be equal within experimental error to the total amount of oxygen adsorbed. The TPD spectrum of a reactorequilibrated catalyst oxidized with l8O2 at 703 K using a static oxidation procedure gave

209

desorption peaks at masses of 32,34, and 36. The ratio of the respective peak intensities was 0.833/0.166/0.001. The total amount of oxygen-18 adsorbed was 0.022 of the total (VO)2P2O7 oxygen. Catalyst that were given HPO treatments and then baked at 793 K until no oxygen emission was detected reacted with butane in the same fashion as reactorequilibrated catalysts that were oxygen-depleted and then given a LPO treatment. Figure 7a compares the MA transient response from two pump-probe experiments on a catalyst sample after high and low pressure oxygen treatments. In both cases, the MA response is averaged over 27 butane-oxygen cycles. The MA pulse intensity in both experiments remained constant during the data collection period. Curve 1 is the MA response after a HPO treatment, and curve 2 is the MA response after a LPO treatment. The neon responses from the two experiments were identical. The area of the MA response after the high pressure oxidation was 2.4 times greater than the MA response after the low pressure

i l

l

\

' , 0.0 0.1

1

0.2

'

1

0.3

'

1

0.4

'

1

0.5

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1

0.6

'

1

0.7

'

1

0.8

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1

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Time (sec.) Figure 7 (a) Maleic anhydride transient response curves from butane/oxygen pump-probe experiments at 653 K over (1) HPO treated VPO, (2) LPO treated VPO; (b) Height normalized maleic anhydride transient response curves from butane/oxygen pump-probe experiments at 653 K over (1) HPO treated VPO, (2) LPO treated VPO

210

oxidation. Similarly, the furan and Co;! responses were respectively 2.2 and 2.3 greater after the HPO treatment than after the LPO treatment. Figure 7b shows the same MA transient responses as displayed in Figure 7a normalized to unit intensity. The MA response after the HPO treatment is significantly more narrow than the response after the LPO treatment. Transformation from HPO performance to LPO performance could be achieved by reducing the sample with an extended series (typically several thousand) of butane pulses or by heating the catalyst at elevated temperatures (793 - 813 K) until no oxygen emission was observed. Repeated cycling between the two performance regimes did not produce noticeable changes in the MA pulse response in either regime during butane oxidation experiments. 4. DISCUSSION

A significant body of evidence (1-6)indicates that (VO)2P2O7 is the active catalytic phase in VPO based catalysts. I t has been a major puzzle however, how the (VO)2P2O7 lattice can supply the 7 oxygen atoms needed to convert n-butane to MA without disintegrating. One proposal is that the oxidation involves oxygen adspecies adsorbed at vanadium surface sites, surface lattice oxygen and relatively little bulk lattice oxygen. This idea is confirmed by a number of studies (4,16) measuring the active oxygen on (VO)2P2O7 that indicate a close correspondence between the number of surface vanadium atoms and the amount of active oxygen. Recent studies (8) of the redox properties of supported VPO catalyst systems provide evidence that in some preparations the amount of active oxygen exceeds the amount of surface oxygen. Thermogravimetric analysis of the reduction (by hydrogen and butane) and reoxidation of different VPO catalysts indicate that some subsurface lattice oxygen (= 3 surface layers) may participate in the reaction. It was suggested that differences in oxygen availability between different catalyst preparations may arise from differences in surface area, P/V ratios, or number of defect sites. The HPO oxygen uptake results presented above indicate that oxygen adsorption at reaction temperatures and ambient pressures is relatively facile for the VPO catalysts used in this study. Conversely, the TPD results show that in an oxygen-depleted environment the same oxygenated samples regurgitate molecular oxygen at reaction temperatures. The 1 8 0 2 SPO experiments indicate that incorporated oxygen-18 exchanges with the oxygen in the VPO lattice but that the exchange process involves only a limited amount of the total (VO)2P2O7 oxygen. This suggests that the adsorbed oxygen is localized in the catalyst surface layers. The LPO oxygen adsorption results indicate that the LPO treatments populate only (VO)2P2O7 surface states. This is not surprising since the average pressure during a LPO experiment is approximately 7 orders of magnitude lower than the average pressure during a high pressure oxidation. Since the rate of oxygen uptake is pressure dependent it is reasonable to assume that a LPO treatment does not possess the driving force to disrupt the (VO)2P2O7 surface lattice and form a new V+5 phase. TPD experiments indicate that the oxygen deposited in LPO experiments does not readily desorb as molecular oxygen. It is commonly held that oxygen can dissociatively adsorb on (V0)2P207fonning V+5 surface sites (16).The LPO adsorption and TPD results are consistent with this concept. The MA transient response data presented in Figures 3 and 5 provides dramatic evidence that the oxygen which is stored in the VPO lattice during a HPO treatment is used in the selective oxidation of butane. Comparing the integrated yields for MA production after LPO and HPO treatments = 100 times more MA is produced after a HPO treatment at = 108 kPa and 723 K for 3 minutes. This increase is the same order of magnitude as the increase in the amount of adsorbed oxygen in going from the LPO to the HPO treatment. The close correspondence between the two increases indicates that the stored oxygen is efficiently channeled to the active catalytic site. I t should also be noted that the specific selectivities for MA, furan, and C 0 2 production do not change significantly in going from the low pressure treatment to the high pressure treatment. Consequently, the incorporated oxygen does not significantly change the chemical selectivity of the VPO catalyst.

21 1

Figure 7a shows that the per pulse MA production increases significantly in going from the LPO treatment to the HPO treatment. This increase may be caused by an increase in the turnover rate per site or by an increase in the total number of active sites. Evidence that the former alternative is correct is provided by the MA pulse shapes. A comparison of the heightnormalized curves (Figure 7b) reveals that the HPO pulse is significantly narrower than the LPO pulse. This decrease in pulse width indicates that MA is being produced at a faster rate on a H P O treated surface or is more strongly adsorbed on a LPO surface. The magnitude of the width differences indicates that the latter alternative is unlikely (14) and suggests that the oxygen incorporation process increases the per site turnover rate. The results presented above provide clear evidence that reactor-equilibrated (VO)2P2O7 catalysts have the ability to store oxygen and then supply that oxygen to the active site. The storagekupply process is likely to involve an interconversion of phases and is driven by the oxygen concentration at the catalytic surface. A high concentration of surface oxygen drives the storage transformation and a loss of surface oxygen drives the supply transformation. One potential route for the storage of active oxygen is through the topotatic transformation of (VO)2P2O7 into 6-VOPO4. Previous redox studies (13) indicate that this transformation can occur at reaction temperatures. Thus, it is interesting to consider how oxygen-18 might be distributed in the oxygen desorption spectrum if this transformation were to occur. Equation (1) depicts an oxidation process in which a surface of (VO)2P2O7 I S converted to a surface cluster of 6 -VOP04 molecular units. (1)

l802

+

2(v0)2P207

<-------> 4VOP04 (contains 2

'80)

The stoichiometry of the incorporation of one diatomic oxygen moiety requires four vanadium atoms to provide the four reducing equivalents, and two pyrophosphate groups to provide storage positions. The resulting vanadyl orthophosphate units contain twenty oxygen atoms of which two are oxygen-18. Assuming the transformation provides for the mixing of the oxygen atoms, the reverse reaction would yield a random distribution of isotopes. Considering that there are eighteen oxygen-16s and two oxygen-18s, a simple statistical analysis predicts the followin isotopic distribution for the desorbing molecular oxygen: 160160 (0.806); 160180 (0.189); $80180 (.005).The experimentally observed distribution is in close agreement with the predicted distribution and thus consistent with the equilibrium process described in equation 1.

REFERENCES 1. G. Centi, F. Trifiro, J.R. Ebner, V. M. Franchetti, Ctzern. Rev., 88 (1988) 55. 3. G. Stefani, F. Budi, C. Fumagalli, G.D. Suciu, Chirn. Did. (Milan),72 (1990) 604 3. G. Stefan;, F. Budi, C. Fumagalli, G.D. Suciu, In New Developments in Selective Oxidatiori, G. Centi and F. Trifiro Eds., Elsevier Pub.: Amsterdam (1990) p.537.

4. J.R. Ebner and J. T. Gleaves, In Oxygen Complexes and Oxygen Activation by Transition Metcils, A.E. Martell and D.T. Sawyer, Eds. Plenum Pub.:(1988) p. 373. 5. G. Cent, F. Trifiro, G. Busca, J.R.Ebner, J.T. Gleaves, Farnday Discuss. Chern. Soc., 87 (1989)31s. 6. G. Centi, Catalysis Ibduy, 16 (1993)5.

7. R. M. Contractor, H. E. Bergna, H. S. Horowitz, C. M. Blackstone, U. C h o w d h q , and A. W. Sleight, Catalysis, 38 (1987) 64.5. 8. R. Contractor, J. R. Ebner, and M. J. Mummey, New Developments in Selective Oxid(ition, 55 ( 1990) 553. 9. M. R. Thompson, J. R. Ebner, In New Developments in Selective Oxidation by Heterogeneous Catalysis, P. Ruiz and B. Delmon Eds., Elsevier Pub: Amsterdam (1993) p. 352. 10. Y. Zhang, R.P.A. Sneeden, J.C. Volta, Cntnlysis Today, 16 (1993) 39.

212

11. T. P. Moser, G. L. Schrader, J. Catal.,92 (1985) 216. 12. F. Trifiro, Catalysis Today, 16 (1993) 91. 13. E. Bordes, Catalysis Today, 16 (1993) 27. 14. J.T. Gleaves, J.R. Ebner, T.C. Keuchler, Catal. Rev. - Sci. Eng., 30 (1988) 49. 15. J.R. Ebner, M.R. Thompson, Catalysis Today, 16 (1993) 51 16. M. A. Pepera, J. L. Callahan, M. J. Desmond, E. C. Milberger, P. R. Blum, N. J. Bremer, J . Am. Chem. Soc., 107 (1985) 4883.

V. CortCs Corberrin and S. Vic Bcll6n (Editors), New Developrnenls in Selective Oxidation 11 0 1994 Elsevier Science B.V. All righls rcserved.

213

V an ad i uin Pho sph ate C at a 1y s t s Prepared by the Reduction of VOPO4,2H20. GI-aham J. Hutchings", Ken6 Olierb, Maria Teresa Sanane sc '' and J e;un-C la tide Vo 1t ;ic .

Lev e rh ti I i i i e Ce 11 t re for I i i nov ;I t i v e Cat ii I y s is . De pa rt m e i i t o f Che i i i is t ry . University of Liveipool, PO Box 147,Liveipool, L69 3BX. United Kingdom. bLaboratoire de Physicochimie des Interfaces, CNKS, Ecole Centrale de Lyon, 36 Avenue Guy de Collongue, R P 163, 69131, Ecully Ckdex, France.

Instittit de Reclierchcs s t i r la Catnlyse. CNKS. 2 Avenue Albert Einstein, 69626. Vi Ile urbanne Ckdex. France. V iiii ad i ti 111 ph o h p h ;I I c cat ;I I j'h t 5 p re pa re d b j d i f fe rent methods a re compared and contrasted f o r the xelective oxidation 01' 11-lxitane to maleic anhydride. V O H P 0 4 . 0.5 I120 c;italy\t precui-sors v,xt.e prepared by three methods: (a) using aqueoux HCI ;is ;I rductiiiit f o r V 2 0 5 . ( b ) using isobutanol a s solvent and reductant f o r VzOs ; i n d ( c ) uhing isobutanol ;IS a reducing agent for VOPO4, 2 H 2 0. 'The p re c ti rs o rx I\ e re trans t'o r m ed LI ntle r i den t i c ii I conditions ( 3 85 C, 1 . S % butane i n a i r , 1000 11- , 7 5 11) to three equilibrated catalysts. The morphology of the precursors atid final catalysts were fotiiid to be very d i ffe rciit a n (I. i 11 part i c ti I a r. the c at ;I I 4 \I5 cie ri ve d f r o m red tic t i on of V 0 PO4, 2 H 2 0 exhibited ;I very high catalyst iictivit),. O

1. IN'I'R ODUC'I'I O

K

214

for the preparation of high activity catalysts. Exaiiiples of this method include the use of isobutanol a s solvent and llCl as reducing agent I41 and the use of isobutanol ;is both solvent a n d reduciiig agent 151. Although catalyst prep ;Ira t i on methods have be e n e x t e 11a i v e I y s t ti d i e d , the re have bee 11 few studies in which different methods have been directly compared. W e have now addressed this aspect of vanadium phosphate catalysts and, in this paper, we present our initial results of definitive study comparing catalysts prepared by both tlie aqueous and noii aqueous solvent routes. In particular. we present results for a catalyst prepared by the reduction of VOPO4, 2 H 2 0 which is found to be particularly effective for n-butane oxidation.

2. EXPERIMENTAL, 2.1. Catalyst precursor preparatiun Catalyst precursor P 1 was p r e p r e d by dissolving V 2 0 5 (6.06 g) in aqueous HCI ( 3 S % , 79 nil) ;it reflux for 2 h. HYPO4 (8.912, 8 5 % ) was added and the solution was refluXeti f o r ;I further 2 h . The solution was then evaporated to dryness a n d the reaulting solid w a x refluxed in water ( 2 0 nil H20/g solid) for 1 h , filtered hot. waxhetl with w;iriii ~ a l e xr i d tlriecl in air ( I 10°C. 1611). Catalyst precursor P2 wxs prepai-cd by adding V3O5 ( 1 1 .8 g) to isobutanol (250 nil). H 3 P 0 4 ( 1 h.lc)g, 85'1 ) ) \\:is theii introduced to the mixture which was then refluxed for I 6 11. 'The light blue xiispension ivas then separated from tlie organic solution by t'iltration anti \basliecl with isobutanol (200 ml). and ethanol ( I S 0 nil. 100%).The resulting solid ~ \ i i aref'liixed i n water (9 nil H 7- 0 / g solid), filtered hot and dried i n air ( 1 10°C. I6 11). Catalyst precursor 1'3 was prepireti v i a V O I ' O ~ . 2 H 2 0 . V 2 0 5 ( 12.0 g) and I-131'04 ( 1 15.5 g. 8 5 % ) were rel'luscd in water (21nil M2O/g solid) for 811. The resulting V O P 0 4 , 2 H-3 0 was i-eco\Jeredby filtration iiiid washed with i\ little - (1g ) was rcfliixetl with isobutaiiol ( 8 0 nil) f o r 21 ti, and water. V O P 0 4 . 21170 tlie rcaultiiig solid \\:\a recovered b y filtration and dried in air ( 1 10°C. 16 11). I

.

2.2 Catalyst testing Catalyst precc"iwr\ 1'1 . P? x i t l !.l' \\ere L I S C ~in powder form and evaluated f'o r the o x idat i 011 o t' i i - b ti t aiic i 11 ;I 5 t aiitia Id I abo rntory mi c r o re tic to r. The precur4ora ( 1 g ) \\ere traiisform~'diii b i i u i i i the reactor to the final catalysts denoted c I , ~2 i i i i c i C> uncler itientical conclitioiij ( 3 x 5 " ~ . IOOO 11- 1 , I .s% ii-butatie in air, 7 5 11) before comp;iriiig their catalytic performances. Reactor prod ti c t s were ;iii;i I y ze d 11 s in g on - I iiic gax c h roni a t og rap11y . Carbon miis s bolmcea were typical11 98- 102 '2 t'or all data cited.

215

2.3. Catalyst characterisat ion Catalyst precursors and final catalysts were analysed by a broad range of techniques including: powder X-ray diffraction (XRD), N2 surface area using the BET method, Laser Raman Spectroscopy (LKS) 161, P MAS-NMR I61 ;in d The rim a 1 A 11a I y s i s ( TA ) with s i m 11It a ne o 11 5 me as LI re men t of e v o I v ed gases .

3 . RESULTS AND DISCUSSION 3.1 Characterisation of catalyst precursors The BET surface areas of the preciirsorx P I , P2 and P3 were found to be 7 respectively 3 . 1 1 and 1 2 in-g- I . X-ray tliifraction patterns are shown in Figure I . 1’1 and 1’2 develop principally the hasal (001 ) crystal face of VOHPO4. 0.5 1 1-7 0 which i x in ayreenicnl 11i t h prcvioiix htiidies 121. ‘I’here are differences i n the ratio 01‘ (001) a i d ( 2 2 0 )p1:iiit.x of’ VOI-IP04 0.5 H?O for precnrsors P1 and 1’2. and. a x cxpectctl, p i ~ e c t i r w rPI prcp;iIcd i n the aqueous niediuni develops mainly the b a x a l (00 1 ) plane (set‘ Figtirc I and ‘I‘able 1 ). However, P? shows a c o m p l e t e l ~diflerent ~ spectrum with ;in iiitenxe (220) line and other broad and not particularly intense V O I 1PO4 0.5 14?0 - lines. This indicates a particular niorphology t’or this precursor with ;I high development of the crystallites in the I 1 101 direction and a limited tlevelopment 011 the I001 I one. This corresponds t o thin platelets developing the (001 ) crystal faces. The d morphology are consistent with the BET s t i r t k e iireiis (Table 1). Analysis of the precursors by LIIS was only possible for P1 since P2 and P3 exhibited fluorescence (Figure 2 ) . I

216

20

PI 00 00 Q\

12

4

cm- 1

800

1000

1GO

I;igure 2.LRS \pectriim of precursor PI

Figure 1. X R D spectra of the precursors

3.2. Thermal Anal! sis of catal!,st preciirsors The catalyst p~-ect~rsc)rs \L c:rc thermally decomposed in vacuo with s i 11111 It a ne o ti s t:v o I v tl d gas iiiiii 1 y s i s . 'I 'hc: p re c ti r h o rs e xli i bi t ed very different behavioiir, but in all cases, only walcr was observed to be evolved (Figure 3). Prec~irsorPI decomposed i n ;I single transition with peak temperature at 430°C. P2 decomposed in two stages with peak temperatures 367 and 442°C. P3 decomposed in ;I single stage with peak temperature 327°C. I t is well - occurs in two steps: the known 151, that cleliyclration of the VOHPO4 0.5 H7O first one a t lower temperature (which corresponds to peak temperature a t 330°C) corresponds to the evoltition of the lH?O - molecule of the hemihydrate, when the second o n e (which correxpoiitls to pcuk temperature at 442°C) is ass oc i ;Ite d to the 0 H c ond en s a t i o 11 bet we en the i 11t e r I a ye r s t rii c t ii re. 0bv i o LI s 1y , rhese two steps are intimatly connected m t l their relative distribution gives rence observed information on the morphology of the hemihydrate. The d for the evolution o f water f o r precursors 1'1 -1'3 indicates such differences. This is consistent with the relalive (001) / ( 2 2 0 )intensities ratio as observed in Table 1 .

217

Figure 3 . TA spectrum of the three precursors

200 300 400 500 T("C)

3.3. Char-actcr-isation ot' final catalysts 'I'he B E T s~irf;iceareas of the final catalysts C I . C2 and C3 after in-situ transformation i n the reactor were l'oiincf to be 3 , 13 a n d 43 rexpectivel),. X l i l l patterns are shown in Figure 4. C 1 shows principally the VOPO4 phascs with ;I higher content of cy11 a s compared to y and 6 VOPO4. S o m e ( V O )-? P-? O 7 can be presciit b u i is difficult to index. C2 is poorly ci-ystallized and can be indexed 21s ;I mixture of ( V O ) 2 P 2 O 7 and VOPO4 phases. On the contrary C1 is mainly ( V O )-? P 2 O 7 (all lines can be indexed with thih pliahe) without any VOPO4.

An enlargeiiient of the (200) line is typical of this catalyst, which is significant of crystals of (VO)2P2O7 with t h i n plittelets in the I I001 direction with a high development of the basal ( 100) I'ace. The peculiar moiphology of catalyst C3 is consistent with the special moiphology of the corresponding precursor P3 iiiicl the \\ell k n o w n ( O O I ) VOIHPO4 0 . 5 1-120/ (100) (VO)2P2O7 epitaxial t ran sfo rma t i on. LKS spectra (Figure 5 ) ~ i d I' M A S - N M K spectra (Figure 6) confirmed the attribution of the previous pliahes 161 and the absence of crystallized VOPO4 phases i n catal) st C'J.

218 n b

a

*i 4001

-

-

A

A

,

4 0 01 f

30

20

10

40

50

20(")

Figure 4. X R D spectra of the equilibrated catalysts fj:

1000

800

1200

Figure 5. LRS spectra of the eq u i I i b rat e d c a ta I y st s

" 1 1 VOPO4; .:y VOPO$ a6 V O P O ~ : a : ( V O ) 2 P 2 0 7 -

.

0

.

v)

r - r -

OI Q. 3

c2

60

20

0 -20

Figure 6.

-60

ppm

60

20

Or-20

-60

ppm

P M A S - N h l R hpectra of the equilibrated catalysts

3.4. Catalyst studies Transformation of the precursor t o tlie final catalyst was carried in-situ in tlie reactor and the ev o I iiti on of cat t; 1y s t pe rfo 1-111 an ce with on - 1ine was fo I lowed (Figure 7).It is apparent that tlie perforniance of C I . C2 plateaus after a short

,

exposure to n-butane/air ( 12 11) whereas C 3 demonstrates ;I significantly differentt trend (plateau for butane conversion after 30 h and for M A xelectivity after 38 11).

,Conversion, %

,

219

,A ,:

S e l e c t i v i t y , ?%

60 40

: :

20

20

1 2 2 4 3 6 4 8 6 0 7Time 2 (h) 1 2 2 4 3 6 4 8 6 0 7 2 1;igul-e 7. Catalytic per~oi~iii~i~icc ot'the catalysts x :

C1

.+ 0 : L-'

*

A ;

c3: 0

x:c3"'

The stabilised eqtiililmteti catalyst performance is shown i n Table 2. It is clear that C3 is conxiderably more active mil selective to maleic anhydride when c ~ ~ ~ i i p \\~ irt he dC'I and C? at ~ o ~ i ~ ~ ~coiiilitioiis ~ ~ r i t ~b r li t ~l11-btitiiiie conversions. I his i \ conxi\tciit with the high 131 xtirl'ace area f o r C3. As the final catalysts have x i ~ ~ i i l ' i c ; ~ ilifl'erent ~i~ly \tirl'ace areax. i t is iiecexxary to compare the specil'ic x t i \ it! ;LI the xaniz rcuctioii conclitions. At 385°C and 1000 1 i - l . the specit'ic a c t i \ i t i L * b 0 1 . ~ 1 C? , aiirl c'? are 1 . 2 4 s 1 0 - 5 , 1.3sx 1 0 - 5 and 1.19X I O - 5 -I mole MA.iii---.h- I , respectively. I t is therefore clear that all catalysts have virtually identical specific activities but by virtue of the high surface area, catalyst C3 is ~ t ~ p c r i o rAlthough . the structures are different, they expose the same number of catalytically active sites per unit area. r

7

_~

._

Cat'll\

41

(;I ISV

~ I 1 - 1711t 'I I1e

~

Prod tict se I ect 1v it y

_

_

220

4. CONCLUSIONS

This study shows that morphology of vanadium phosphate catalysts is of crucial importance to control catalytic performance for n-butane oxidation to maleic anhydride and that new routes for the preparation of these catalysts can be discovered. As the transformation VOHP04 0.5 H201 (VO)2P2O7 is epitaxial, a l l improvements of the catalytic performance are linked to a better control of the preparation of the VOHP04 0.5 H 2 0 precursor. We have previously emphasized this point [71 . In this communication, we have shown that it is possible to prepare the VOHP04,0.5 H 2 0 precursor by a method which involves a reduction from the VOP04, 2H20 dihydrate. This precursor presents a quite different morphology as compared to the precursor prepared by the classical method which implies the reduction of V205 both in aqueous and organic medium. Its BET area is much higher which is also the case of the final activated VPO catalyst . Though its specific activity is the same as the other VPO catalysts, it is more active due to its higher BET area.

ACKNOWLEDGEMENT The authors thank the French CIES for financial support.

REFERENCES 1 . G.J. Hutchings, Appl. Catal., 72, (1991), 1. 2. G. Centi (ed), Forum on Vanadyl Pyrophosphate Catalyst, Elsevier, Amsterdam, 1993, Catal Today, vol. 16. 3. J.P. Harrison, US Patent No 3 985 775 (1976). 4. R.A. Sneider, US Patent No 3 864 280 (1975), US Patent N o 4 043 943 ( 1977). 5 . J.W. Johnson, D.C. Johnston, A.J. Jacobson and J.F. Brady, J. Am. Chem. Soc.. 106, (l9X4), 8123. 6. F. Ben Abdeloiiahab, R. Olier, N . Guilhaume, F. Lefebvre and J.C. Volta, J . Catal.. 134, (1 992), 15 1 . 7. N . Guilhaume, R. Roullet, G. Pajonk, B. Grzybowska and J.C. Volta, New Developments in Selective Oxidation by Heterogeneous Catalysis, P. Ruiz and B. Delnion (eds), Elsevier, Amsterdam, Studies in Surface Science, 1992, vol 72, p 25.5.

V. Cortes Corberlin and S. Vic Bcll6n (Editors), N e w Deveioprnents in Seleclive Oxidation I/ 0 1994 Elscvier Science B.V. All rights rescrvcd.

22 1

PRODUCTION OF MALEIC AND PHTHALIC ANHYDRIDES BY SELECTIVE VAPOR PHASE OXIDATION WITH VANADIUM OXIDE BASED CATALYSTS C.Fumagalli2, G.Golinelli', G.Mazzoni2, M.Messori', G.Stefani2 and F.Trifirb

1)Dept. of Industrial Chemistry and Materials v.le Risorgimento,4 40136 Bologna Italy 2)Alusuisse Italia-FtalitalPlant via Enrico Fermi, 51 24020 Scanzorosciate (BG)Italy A study of the oxidation of o-xylene, n-butane, benzene, n-pentane, 1-pentene and a mixture of c 4 - C ~hydrocarbons on three commercial catalysts for the synthesis of anydrides was carried out. Oxidation of o-xylene was the easiest reaction to achieve, obtaining in all cases phthalic anhydride in high amount, while oxidation of n-paraffins to selective oxidation products was the most difficult reaction to achieve, obtaining high amount of maleic anhydride only using vanadia-phosporous based catalyst. V-P mixed oxide is the mostpolyfinctional catalyst because it gave high anhydride yields in the oxidation of all the feedstocks, excepted olefiis. Vanadia-molybdenum based catalyst gave high yield in anhydrides only in oxidation of o-xylene and benzene and some interesting selectivity in oxidation of olefiis. Vanadia-titania based catalyst gave selective oxidation products only in oxidation of o-xylene. It is proposed that differences in selectivity in oxidation of o-xylene and benzene are due to different type and/or reactivity of intermediates . In particular it is suggested that in o-xylene transformation to phthalic anhydride phthalide is an intermediate for a less selective pathway while a dialdehyde species adsorbed on the catalytic surface is the intermediate in selective oxidation of o-xylene. Besides in n-paraffins oxidation V-P gave higher selectivity in anhydrides than V-Mo. This is attributed to different capabilities of the two catalysts to promote dehydrogenation and oxygen insertion reactions. 1. Introduction In the production of maleic (indicated as MA) and phthalic (indicated as PA) anhydrides the vanadia based catalysts are obtained adding the following different partners: - v 2 0 5 supported on Ti@ in the oxidations of o-xylene and naphtalene to phthalic anhydride (indicated as V-Ti) - V205-Mo03 in the oxidation of benzene to maleic anhydride (indicated as V-Mo) - V205-P205 in the oxidation of n-butane to maleic anhydride (indicated as V-P). It seems that the role of these partners as well as of other promoters added in low amount is to stabilize a different valence state of vanadium and that each reaction requires an optimal concentration of the different valences. There is a good agreement in the literature regarding the catalytic phases responsible for the industrial reactions. In the V-P system (VO)2p207 has been identified as the active phase in

222

the oxidation of n-butane to maleic anhydride [l-71. In the V-Ti system v6013 or v307 [8-121 has been proposed as the active and selective phase in the oxidation of o-xylene to phthalic anhydride. In the V-Mo system the proposed active phase in the oxidation of benzene to maleic anhydride is a V205-Mo03 solid solution which after reduction becomes (Vo.66 Moo.33)6013, probably the real active phase [12-141. The aim of this study was to evaluate the catalytic behavior of the three optimized catalytic systems in the oxidation of o-xylene, benzene, n-butane, n-pentane, l-pentene and a c4-C~ mixture of hydrocarbons (deriving by a fraction of the steam-cracking of the naphta) , with the specific objective of accumulating data that could be useful to understand factors that are responsible in selective oxidation of hydrocarbons on mixed oxide based catalysts.

2. Experimental V205-Ti02 has been synthesized according to the method described in patent [15]. Ti02 anatase has been impregnated with a solution of vanadyl oxalate and KC1, obtaining T i W 2 0 5 / K C l mass ratio of 212/11/1 and calcined at 39OoC for 9 hours. V-Mo mixed oxides has been prepared according to the method described in patent [ 161 by evaporation of a solution containing 6 parts (by weight) of ammonium paramolybdate, 13.22 parts ammonium metavanadate, 71.43 parts of hydrochloric acid and Ni, Bi, Na phosphate . The composition of calcined catalyst by weight is: 1 part of Mo03; 2.1 parts of v205; 0.03 parts of P205; 0.04 parts of Na20; 0.04 parts of NiO and 0.06 parts of Bi203 . Vanadyl pyrophosphate, the active phase of the V205-P205 catalyst, was prepared according to the organic method proposed in several papers [l, 17-19]. In a typical preparation V2O5 was reduced in a 2: l=isobutanol:benzyl alcohol mixture under reflux conditions. The solution was cooled and an exact amount of ortophosphoric acid 100% was added under nitrogen flow. The blue suspension obtained was filtered and dried. Finally the sample was calcined in air and then in n-butane/air mixture. The catalysts were initially activated and studied in analogous conditions to industrial applications until steady-state conditions were reached: - v305 supported on Ti@ in the oxidation of o-xylene to phthalic anhydride (surface area 15 m /g). - V205-Mo03 in the oxidation of benzene to maleic anhydride (surface area 17 m2/g). - V205-P205 in the oxidation of n-butane to maleic anhydride (surface area 25 m2/g). The amount of catalysts has been choosen in order to achieve similar conversion of o-xylene at low temperature: l g for V-Ti and 2 g for V-P and V-Mo. Steatite was added to the catalyst in order to minimize the temperature gradient effect. The catalysdsteatite ratio was 1:4 (w/w). The size of both particles were between 4.2 and 6.0 mm. The catalytic experiments were run in a fixed bed reactor loaded with the catalyst/steatite mixture. The reactants were fed in an air flow of 60ml/min. The hydrocarbon concentrations were fixed in the following values: 1.1% for o-xylene, 1.3% for benzene, 1.0% for n-butane, 1.1% for n-pentane, 1.6% for l-pentene and 1.3% for the c 4 - C ~mixture of hydrocarbons. The main constituents of the C4-c~mixture were: 0.48% of 1,3-butadiene, 1.22% of 3-methylbutadiene, 3.88% of isopentane, 3.89% of l,bpentadiene, 6.83% of l-pentene, 4.88% of 2-methyl-l-butene, 9.88% of n-pentane, 9.38% of isoprene, 5.86% of trans-2-pentene, 2.99% of cis-2-pentene, 3.93% of 2-methyl-2-butene, 4.08% of trans-pyperilene, 7.65% of cis-pyperilene and 7.36% of cyclopentene. The products were analyzed using an off-line system consisting of a crystallizer and two

223

acetone absorbers. The products were concentrated for lh and then analyzed using a gas-chromatograph with a FID detector. Permanent gases were analyzed with a TCD gaschromatograph.

3. Results In table 1 the conversion of the o-xylene and the yield in the main selective oxidation products on the three catalysts at different temperatures are reported. In table 2 and 3 the conversions and yields in anhydrides in the oxidation of and C5 hydrocarbons on V-Mo and V-P as function of the temperature are reported. TABLE 1 (l):Catalytic behavior in the oxidation of o-xylene on V-Ti, V-P and V-Mo. Total flow rate: 60 ml/min - catalyst amount: V-Ti=&, V-Mo and V-P=2g.

(1) The remaining in carbon balance is CO+COz [MA=maleic anhydride, PA=phthalic anhydride, Pd=Phthalide, Conv=conversion] TABLE 2 ('I: Catalytic behavior in the oxidation of Cq and C5 hydrocarbons on V-Mo Total flow rate=60 Wmin - catalyst amount=2g.

I 375 1

71

I

0.0

I

100

I

0

I

.

I

(1) The remaining in carbon balance is CO+C02.

In figure 1 the conversion of benzene and the yields in anhydrides for the three catalysts versus temperature are reported. The other products obtained in these oxidations are only total combustion products.

224

Conversion and Maleic anhydride yield (%)

100

1

/-

/

/-

X-

40

CONVERSION% (V-P) 4- YIELD % MA(V-P)

'YIELD% PA (V-P)

' CONVERSION% (V-Mo) YIELD% AM (V-Mo)

20

0

CONVERSION% (V-TI)

L4=

300

320

340

360

380

400

420

Temperature ('C)

Fig. 1: Conversion and yield in anhydrides in the oxidation of benzene on the three catalysts vs. temperature. Total flow rate: 60 ml/min - catalyst amount: V-Ti=lg, V-P and V-Mo=2g.

TABLE 3% Conversion and anhydride yields vs. temperaturein the oxidation of (21and C5 hydrocarbons on V-P catalyst. Total flow rate: 60 mJ/min - catalyst amount: V-P=2g.

(1) The remaining in carbon balance is CO+CO2. 4. Discussion

In order to discuss analogies and differences in catalytic behavior of the three catalysts, in table 4 the temperature at 30% of conversion and in figure 2 the maximum yield in anhydrides with the relative temperature for all investigated reactions have been reported. Besides in table 4 for each reaction the nature of the proposed main step has been also added.

225

Maximum Yields (YO) O0

(I

1

348'C

o-xylene benzene n-butane n-pentane 1 -pentene

C4-C5

Fig. 2: Maximum yields in maleic and phthalic anhydrides in function of the feed and of the catalyst.

TABLE 4: o-xylene, benzene, n-butane, n-pentane and 1-pentene reaction temperatures at 30%of conversion on the three catalytic systems. Total flow rate: 60 W m i n - catalyst amount: V-Ti=lg, V-Mo and V-P=2g. Temperature of the catalyst ("C)

From this table the following scale of activity can be deduced: o-xyleneoxidation V-Ti = V-Mo> V-P V-Mo>> V-P >> V-Ti benzeneoxidation n-butane oxidation V-Mo > V-P >> V-Ti n-pentaneoxidation V-Mo> V-P > V-Ti I-pentene oxidation V-Mo > V-P 2 V-Ti - C5 oxidation V-Mo 2 V-Ti > V-P As reported in a previous part of paper reaction parameters for all the reactions (amount of catalyst, contact time) have been chosen in order to obtain similar activity in oxidation of 0-xylene, the reaction where all three catalysts showed high selectivity in anhydride. From table 4 and figure 2 it is possible to underline the following conclusions:

226 - all three catalysts are active and selective in oxidation of 0-xylene - only V-P and V-Mo catalysts are active in oxidation of n-butane, benzene and n-pentane

- all three catalysts are active in 1-pentene and C4-C5 mixture and they present very low or no selectivity in anhydrides

- V-P is the only catalyst that presents high selectivity on anhydride in the oxidation of n-paraffines - only V-Mo and V-P are active and selective in oxidation of benzene - V-Mo is more selective in the oxidation of n-pentane than in the n-butane’s one.

4.1. Considerations on the nature of active sites The data reported in figure 2 suggest that the oxidation of n-butane which accentuates the differences among the catalysts must be considered as a structure sensitive reaction. On the contrary o-xylene is easily oxidized to phthalic anhydride with all three catalysts examined and can be considered the most facile reaction. Also total oxidation of olefins can be considered as a facile reaction. In addition V-P catalyst, active and selective in synthesis of anhydrides with all feedstocks excepted olefins, can be regarded as the most polyfunctional type of catalyst among those examined. It is interesting to correlate these conclusions with the mechanism reported below for the oxidation of n-butane to maleic anhydride with V-P catalyst, where many steps of different nature has been proposed 1221: butane ---> butene oxidative dehydrogenation allylic oxidation butene --- > butadiene butadiene --- > 2-3 dihydrofurane 1-4 oxygen insertion allylic oxidation 2-3 diydrofurane --> furane electrophylic oxygen insertion furane --- > maleic an. In order to obtain high selectivity in anhydride, it is important that the rate of allylic dehydrogenation must be higher than the rate of oxygen insertion; this decreases the insertion of oxygen in butenes responsible of a parallel reaction to insaturated aldheyde or ketones and combustion products. It is reasonable to assume that a fraction or all sites involved in the selective oxidation of n-butane are also active in the oxidation of benzene and o-xylene. In fact the first attack to benzene is probably similar to 1-4 oxygen addition to butadiene, while sites responsible for the first hydrogen abstraction to methyl group of o-xylene can be the same of allylic oxidation of an olefin (butene to butadiene). Also fiist attack to an olefin can be caused by the same active sites involved in the activation of o-xylene; in fact the two feeds are oxidized in the same range of temperatures. V-Mo and V-P are active with all feedstocks and they must present the same active sites able to promote the reaction above reported in the n-butane oxidation to maleic anhydride; however V-Mo is not selective in the oxidation of n-paraffin. Lower selectivity presented by V-Mo in oxidation of n-paraffins can be related to higher rate of oxygen insertion reaction in comparison with dehydrogenation. In fact the activity in oxidation of benzene is the highest for V-Mo catalyst. 4.2. Considerations on the nature of the by-products. The above discussion is related to macroscopic analogies and differences among studied catalysts, but other important conclusions can be made on the basis of the obtained by-products. For instance in the oxidation of o-xylene V-P and V-Mo gave phthalide, while V-Ti did not

221

give phthalide in detectable amount in all range of temperature examined. In the l o o k oxidation of benzene and n-pentane V-P is the only catalyst which gave as co-products maleic and phthalic X X 60 anhydrides. The phthalic anhydride selectivity as a of the percent function conversion is reported in Figure 3. Data show that selectivities do 20 -not depend on conversion with a V-Mo x V-P * V-Ti all catalysts and this indicates I I 0 that combustion products derive by oxidation of feed or some intermediates. In oxidation of Fig. 3: Phthalic anhydride selectivity as a function of the benzene the selectivity changes conversion in the oxidation of o-xylene on the three with conversionfor all catalysts catalysts, confirming the parallel reactions of combustion as main responsible for decreasing of the selectivity. The same conclusion can be made in oxidations of n-butane with V-P catalyst. Generally in selective oxidation reactions the parallel and the consecutive reactions are responsible for decreasing of the selectivity and make difficult to build a general theory of the factors that influence the selectivity. In this work industrial catalysts for anhydride productions have been used in order to minimize the effect deriving by consecutive and decomposition reactions. It is very likely that parallel reactions are due to the presence of different intermediates which can be more reactive than starting material. This hypothesis is summarized by the following model: A + X + B Phthalic Anhydride Selectivity (%)

I

I

I

1

cox

In the oxidation of 0-xylene the less selective catalysts V-P and V-Mo gave phtalide as by-products (Table 1). In the literature phthalide has been advanced as an intermediate in formation of phthalic anhydride while the formation of phthalic anhydride from benzene on V-P catalyst probably derives by condensation reactions of intermediates adsorbed on the catalytic surface. The difference of selectivity in anhydrides can be explained with presence of different reaction pathways and/or different stability and reactivity of intermediates adsorbed on the catalytic surface. The formation of phtalide probably derives by oxidation of a single methyl of the o-xylene to the corresponding acid and, after oxidation of the other methyl-group, final condensation to phthalic anhydride [20,21]. Only traces of phthalide were detected in oxidation of o-xylene on V-Ti catalyst suggesting contemporaneous activation of both methyl-groups of o-xylene to a diacid and final condensation to phthalic anhydride. In the case where phthalide is an intermediate of reaction probably we are in the presence of an acid decarboxylation that decreases the selectivity. In V-P and V-Mo catalyst the oxidation centre are more dilute than the V-Ti catalyst where Ti02 acts only as support for vanadium sites, and therefore the

228

oxidation of a single methyl group is more probable. Also in the case of benzene the parallel reaction may start from different intermediates or different surface reactivities. It is difficult to think about a different attack by two catalyst on benzene molecule, thus in this case it is reasonable to assume that parallel reaction starts from different intermediates or these have different reactivity. These hypothesis are supported by production of phthalic anhydride on V-P but not on V-Mo Catalysts. The phthalic anhydride formation is probably due to reactions of condensation of different intermediates which may present different stability on the surface of each catalysts. The fact that V-P catalyst is less active and selective in maleic anhydride from benzene than V-Mo can again be explained with its lesser capacity of oxygen insertion. In fact intermediate species not completely oxidated can have higher probability of side reaction of condensation to phthalic anhydride and or to carbon oxides. The same explanation can be advanced for formation of phthalic anhydride in n-pentane oxidation on V-P and not on V-Mo. Taking into consideration that oxidation of butane does not form phthalic anhydride and also on the basis of the high rate of formation o f phthalic anhydride from cyclopentane [22], this intermediate can be cyclopentane or cyclopentene species which derive from consecutive dehydrogenation of n-pentane.

5. References 1. J.W. Jhonson, D.C. Johnston, A.J. Jacobson and J.F.Brody, J. Am. Chem. SOC.,106 (1984) 8123. 2. M.A. Pepera, J.L. Callahan, M.J. Desmond, E.C. Milberger, P.R. Blum and N.J.Bremer, J. Am. Chem. SOC.,107 (1985) 4883. 3. T. Shimoda. T. Okuhara and M.Misono, Bull. Chem. SOC.Jpn, 58 (1985) 2163. 4.E. Bordes and P.Courtine, J. Chem. SOC.Chem Comm., (1985) 294. 5. J.R. Ebner and M.R.Thompson, Catalysis Today, 16 (1993) 51. 6. G . Centi, F. Trifiib, J.Ebner and V.Franchetti, Chem. Reviews, 88 (1988) 55. 7. J. Li, M.E. Lashier, G.L. Schrader and B.C. Gerstein, Appl. Catal., 73 (1991) 83. 8. G.L. Simard, J.F. Steger, R.J. Arnott and L.A. Siegel, Ind. Eng. Chem., 47 (1955) 1424. 9. G . Centi ,D. Pinelli, F. Trifiib, D. Ghoussoub, M. Guelton and L. Gengembre, J.Catal., 130 (1991) 238. 10. A. Anderson. J. Catal., 74 (1982) 144. 11. M.S. WainwrightandN.R. Forster, Catal. Rev. Sci. Eng., 19 (1979) 211. 12. Vanadium Catal. Roc. Oxid. arom. Hydr., B.Grzybowska and J.Haber (Eds.), Polish Scientific Publishers Warsaw Crakow, (1984). 13. D.J. Cole, C.F. Cullis and D.J.Hucknal1, J. Chem. SOC.,(1976) 2185. 14. A. Bielanski, J. Piwowarczyk and J.Pozniczek, J. Catal., 113 (1988) 334. 15. A. Neri, L. Capitanio and G . Stefani, US Patent 4,405,505 (1983). 16. A. Di Cib and A. Vitali, European Patent 037,020 ( 1984). 17. B.K. Hodnett, Catal. Rev. Sci. Eng., 27 (1985) 373. 18. G . Busca, F. Cavani, G. Centi and F. Trifiib, J. Catal., 99 (1986) 400. 19. G . Busca, G. Centi, J.R. Ebner, J.T. Gleaves, F. Trifiib, Farad. Disc. Chem. SOC.,214 (1989) 87. 20. R.Y. Salex and LE. Wachs, Appl. Catal.,31 (1987) 87. 21. J.C. Bond, J. Catal., 116 (1989) 531. 22. F. Trifirb, Catal. Today, 16 (1993) 91.

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E. BORDES @ept. Genie Chimique, Compiegne, France ): What could be the reason for no activation of alkanes by V-Ti oxides? F.TRIFIRb (Dipartimento di Chimica Industriale e dei materiali, Bologna, Italy): First it is necessary to recall that the V-Ti catalyst used in the work presented in this paper was optimized for o-xylene oxidation. In fact it is well known in literature that V-Ti based catalyst are active for paraffin oxidation; however this catalyst is not active. Therefore your question help us to underline an experimental fact that at the temperature and contact time at which a V-Ti catalyst is very selective for o-xylene oxidation is not active in paraffin oxidation. Therefore sites of different nature must be involved for the two different reactions.The catalyst we used is doped with potassium in order to reach the high values of reported selectivities. Therefore low acidity is present in the catalyst. Acid sites have been claimed to be the active ones for paraffin activation. This can be one reason of inactivity in paraffin oxidation of the catalyst we have investigated. However other reasons can be also valid such as: it is necessary to have at the surface the right configuration of vanadium ions (in terms of distances of vanadium ions, presence of V=O bonds, oxygen vacancy on vanadium .VOH species, different vanadium valences). Therefore in order to reply correctly to your question it would be necessary to characterize and compare the surface properties of the catalyst investigated in the work reported in this paper and other V-Ti based catalysts active in paraffin oxidation. Also on the basis of your question and that one put forward by Gryzbowska we can simply conclude that is not enough to say “V-Ti based catalyst“ to define its reactivity . H.P. NEUMA” (BASF - AG, Ludwigshafen, Germany): Prof. Trifirb you said that in the oxidation of o-xylene to phthalic anhydride the formation of phthalide is decreasing selectivity because phthalide is mostly burnt. Could you give an idea in terms of selectivity which amount of phthalide (under industrial conditions) reacts to COXor to phthalic anhydride.

F.TRIFIRd: We have reported the formation of relatively higher amount of phthalide in the lesser selective catalysts for 0-xylene oxidation. This observation might seem a contradiction, in fact higher amount of intermediates (index of lower oxidizing power) are found in lesser selective catalysts. This apparent contradiction was explained by us assuming that the phthalide is forming from a route of single attack of a methyl group, which can take also to the o-methyltoluic acid which will decarboxylate very easily. Phthalide comes from the same intermediate of o-toluic acid, and its presence can be taken as a index of the possibility of this route. When phthlide is formed the hydrocarbon is safe, because it will trasform with high selectivity to phathlic anhydride (more than 95 %). The more active is a catalyst, the easier is the contemporaneous attack to the two methyl groups and formation of stable phthalic anhydride. The lesser active is the catalyst, the more likely the oxidation reaction occurs in steps with possibility of formation of the intermediate acid . It is useful to remember that p- xylene oxidation or toluene oxidation on the same type of catalyst takes essentially to bulk combustion. The quick formation of anhydride ring is a key factor for selectivity.

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R.K. GRASSELLI (Mobil Re&& Dev., Princeton, Usa): You certainly summarized an impressive m a y of catalysts and reactions , and intercompared their respective properties . My question refers to your Fig. 2 , where starting with o-xylene all three of the catalysts you studied give pnmarly phthalic anhydride which is, of course, expected . But ,I note that V-Ti and V-Mo systems yield small amounts of maleic anhydride, while the V-P system does not . This has some interesting mechanistic implications. Do you think that the differences are caused by electronic factors of the various catalysts, structural factors, or the existence of different oxygen moieties on the surface of these catalysts, or a combination thereof ? F.TRIFIR6: There is an apparent contradiction in the formation of maleic anhydride as by product in o-xylene oxidation. In the case of V-Mo based catalyst we can advance the hypothesis that maleic anhydride is forming from the attack to the aromatic ring as this catalyst is very active in benzene oxidation. However with this hypothesis we cannot explain either the absence of maleic anhydride with V-P based catalyst or the formation with V-Ti, which is inactive in benzene oxidation. Therefore another mechanism must operate with V-Ti responsible for the formation of maleic anhydride, alternative to aromatic ring attack. Very likely a different attack to the methyl groups is responsible for the degradation of o-xylene to maleic anhydride. Coming to your question it is possible to propose the formation of maleic anhydride with V-Mo could be due to the oxygen moieties (higher amount of reactive adsorbed oxygen); in the case of o-xylene it is possible to suggest that is due to a different interaction of the methyl groups with the surface of catalyst, that is to both electronic and structural properties.

B. GRZYBOWSKA (Institute of Catalysis, Krakow, Poland): a) Comment. Contrary to your results we observed some selective oxidation (formation of maleic anhydride) of benzene on V205fli02 catalysts at higher than 2 monolayer content of V2O5. At low content total combustion of benzene was observed. B) Question. What, in your opinion, are the exigencies of various molecules, which are oxidized, towards the structure of vanadium -oxygen centers ? Couldn’t we draw some conclusions about relations between the activity and selectivity in hydrocarbon oxidative reactions and the distance between V - 0 centers in various catalysts ? F. TRIFIRb: I have to underline as I have done with the question of Bordes that the catalyst we used is an optimized one for 0-xylene selective oxidation. Therefore it is possible to suggest that a very selective catalyst for o-xylene oxidation must not be active in aromatic ring oxidation. Before to reply to your question it is possible to summarize some of the properties that I can suggest (but they are not exhaustive) a catalyst must have to be active and selective in o-xylene oxidation. 1) it must not decompose phthalic anhydride (the process operates at 99% of conversion). 2) it must not attack aromatic rings (inactive in benzene oxidation as test reaction) in order to minimize parallel reaction of formation of maleic anhydride and carbon oxides. 3) it mnst not oxidize paraffins (this reaction can be a test reaction which indicates the possibility or not to dehydrogenate totally the methyl groups before the oxygen insertion reaction, which

23 1

takes to phthalic anhydride, occurs).

4 ) it must attack contemporaneously the two methyl groups in order to form the anhydride ring as quick as possible. Therefore high selectivity is obtained by the fine-tuning of different surface properties (and is the reason why it is very difficult to find an unifying theory for selective oxidation). Surely as you suggested the distances of V - 0 centers will play an important role, but unfortunately at this moment I have no clear indications of what is the real situation at the surface of the investigated catalyst.

J. HABER (Institute of Catalysis, Krakow, Poland): In the discussion of the differences of the behavior of the three vanadium based catalysts you take into account only the possibility of the formation of various adsorbed hydrocarbon species. However there is also a second partner of the reaction : oxygen, and we know that different forms of oxygen are involved in the different steps of hydrocarbons transformations giving thus various reaction products. The second point which I would like to make is to remind that many years ago Gasior and Gryzbowska showed that on such catalysts as molybdates irrespectively of whether aliphatic or aromatic molecules are oxidized, the reaction sequence is ended on the formation of aldehydes, whereas on vanadium-based catalysts again in both aliphatic or aromatic oxidations acids and anhydrides are formed.

F.TRIFIRb: Your question helps me to reply also to some of the other questions. The aromatic rings oxidative attack very likely is due to reactive adsorbed species and therefore the ability of the surface of catalyst to form these species is another property which differentiates the three catalysts. These types of oxygen species are important for the formation of maleic anhydride both from benzene and from n-butane (oxygen insertion in butadiene) but they are deleterious for the selective oxidation of o-xylene to phthalic anhydride. You are right: all the industrial catalysts of production of acids and anhydride in gas phase with heterogeneous catalysts are based on vanadium (V based heteropolyacids for the synthesis of acrylic acid and metacrylic acid). I think that this property is related to the redox potential of the couples V(V)-V(IV) and V(V) -V(III).

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V. CortCs Corberin and S. Vic Bcllon (Editors), New Deveioprnenls i n Seleclrve Oxidation //

0 1994 Elscvier Science B.V. All rights rescrvcd.

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A New Commercial Scale Process for n-Butane Oxidation t o Maleic Anhydride Using a Circulating Fluidized Bed Reactor R. M. Contractor, D. I. Garnett, H. S. Horowitz, H. E. Bergna, G. S. Patience, J. T. Schwartz, and G. M. Sisler E. I. du Pont de Nemours and Company, Chemicals, P. 0. Box 80262, Wilmington, Delaware 19880-0262,USA

ABSTRACT DuPont has developed a new process for n-butane oxidation to maleic anhydride using a circulating fluidized bed (CFB) reactor. An extensive effort spanning a period of ten years has resulted in a successful demonstration of the process on a large demonstration plant. A novel approach to imparting attrition resistance t o the catalyst for the process has been demonstrated on a commercial scale. The demonstration plant was used to activate the catalyst, optimize hydrodynamics of the CFB, confirm catalytic performance and attrition resistance, and generate data for the design of a very large commercial plant. Construction of the first commercial plant using this technology is scheduled to be completed in 1995.

1. INTRODUCTION The manufacture of most tetrahydrofuran (THF) has been based on the production of 1,4-butanediol from acetylene and formaldehyde using the Reppe type process. DuPont has developed a new process for making THF from nbutane, and has announced that a 100 million lbs/yr THF plant based on maleic anhydride from n-butane will be built in 1995 in Asturias, Spain. The main outlet for THF from DuPont's new plant will be t o make polytetramethylene ether glycol for the production of spandex fibers and copolyester elastomers. Key advances for the new process include the first commercial use of a CFB reactor for a specialty chemicals process, a method of making attrition resistant catalyst for the CFB reactor and the discovery of a catalyst t o hydrogenate crude aqueous maleic acid directly t o THF. A large scale demonstration of the process to produce maleic anhydride from n-butane using a CFB reactor and scale-up of our novel approach to impart attrition resistance t o the oxidation catalyst used in the process are summarized here.

2. BACKGROUND The best catalysts for the oxidation of n-butane to maleic anhydride are based on vanadium phosphorous oxides (VPO), and there is general agreement in the literature for the crystalline (VO)2P2O7predominating in these best catalyst systems [l]. The literature [2-41 also suggests that selective oxidation of n-butane to maleic anhydride involves a redox mechanism of the surface layers of the VPO catalyst. Oxidation of the hydrocarbon is achieved using surface lattice oxygen;

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reoxidation of the catalyst active sites is accomplished by reduction of oxygen at separate sites and the movement of oxide ions through the lattice framework. The use of CFB (or Riser) reactor technology, as has been previously reported by DuPont [5,6],allows decoupling and optimization of these redox reactions. In the CFB reactor, the catalyst is oxidized in a fluidized bed regenerator zone, and n-butane is selectively oxidized by the catalyst in a separate riser reactor zone maintained under reducing atmosphere. The reduced catalyst from the riser zone is separated from the product gas, stripped of any carbonaceous species in a separate stripper zone and returned to the regenerator for reoxidation. The advantages [7] of the reactor concept include: (i) high selectivity to maleic anhydride because of minimal gas back mixing, optimum oxidation state of the catalyst and controlled oxygen concentration in the riser reactor zone; (ii) a highly concentrated product stream resulting in a lower cost product recovery system; and (iii) high throughput rates resulting in significant reduction in catalyst inventory and the overall investment relative to a comparable size plant using a conventional fluidized bed reactor. The VPO catalyst itself is much too weak t o use in a CFB reactor. The standard approach to making an oxide catalyst attrition resistant is t o add colloidal Si02 in quantities of about 50% by weight. Considering the density and surface areas of this Si02, it actually presents the predominant surface to reacting gases, and the butane degradation reactions on its surface are significant. DuPont has reported a novel route [5,8,91 to produce attrition resistant catalyst. Instead of using colloidal silica, this approach uses Si02 as polysilicic acid. In this way, entirely satisfactory attrition resistance is achieved with only 10 wt.% SiOz, forming a protective, porous shell around the weak VPO catalyst.

3.DEMONSTRATION PLANT A demonstration facility was constructed at DuPont's Ponca City site in Oklahoma as a pilot for the very large commercial plant being built in Asturias, Spain. The facility provided DuPont engineers and scientists the opportunity, during a year of operation, to prove-out and optimize the CFB and the attrition resistant catalyst technologies for butane oxidation. A photograph of the facility is shown in Figure 1. The key elements of the reactor system included a 0.15 m. dia. x about 30 m. tall riser reactor zone, an appropriately sized catalyst regenerator, proprietary design catalyst strippers, stand pipes to achieve desired catalyst circulation rates, heat exchangers in different zones of the reactor system, cyclones and filters. The reactor was designed for different modes of operation and for minimum catalyst attrition. Besides modern process control and data acquisition systems and analytical capabilities including on-line MS, IR, UV and other analytical instruments, the facility included product recovery, scrubbing, waste disposal and unconverted butane recycle equipment. The typical range of operating parameters of the reactor system for studying butane oxidation are described in Table 1.

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Table 1 rating r

QDeratinP Darameter

RanPe

Reaction temperature Reaction pressure Cat. circulation flux i n riser Gas residence time i n riser Butane conc.

360-420°C I few atm. I 1100 kg/rn%ec. I 10 sec. I 2 5 mol. %

Figure 1. DuPont's CFB demonstration facility at Ponca City, Oklahoma

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4. CATALYST PRODUCTION

Several commercial size batches of VPO catalyst precursor were produced in organic media according to the method reported previously 15,101 and its variations. The precursor was converted into fluidizable, attrition resistant 20150 pm microspheres by first micronizing it t o about 2 pm particle size and then spray drying it with polysilicic acid [5,8], using commercial equipment for both operations. Extensive monitoring of the catalyst characteristics a t different stages of production was carried out t o insure that catalyst met all specifications including 0x0 capacity [ l l l and attrition resistance. The spray dried catalyst was calcined a t 390°C in the fluidized bed regenerator zone of the demonstration facility, and “active“ catalyst was produced by running the butane oxidation reaction a t high temperature for several hours in the regenerator. Analysis of the activated catalyst samples revealed an average vanadium oxidation state of a little over 4.0 as expected. XRD analysis showed the presence of a single crystalline phase (VO)2P2O7. Attrition resistance of the initial batches in laboratory evaluations was marginal. Further development of DuPont’s approach to imparting attrition resistance resulted in selection of optimum process conditions and commercial scale equipment that produced the desired attrition resistant catalyst. In fact, the catalyst manufactured in commercial scale facilities, under the optimized conditions, was characterized by attrition resistance superior to that ever produced by us in pilot scale spray dryers. Several tons of in-spec catalyst were produced - enough to make multiple charges in the demonstration unit to conform reproducibility of the catalyst performance.

5. RESULTS AND DISCUSSION The demonstration plant was started up in mid-1990 and was operated until early 1992. A large amount of data were generated. Data analysis is still continuing. Initial studies on catalyst circulation rate control and hydrodynamics were conducted using FCC catalyst. Excellent control of the catalyst circulation rate was achieved in the entire range of circulation flux up to 1100 kg/mzsec. Hydrodynamics in the different zones of the reactor system were studied using pressure difference, temperature difference, and radioactive tracer measurements. Some of the data on axial density profiles in the tall riser were recently reported [12]. A typical density profile with FCC at different fluxes and two different gas velocities is shown in Figure 2. Rather high densities are maintained over the entire length of the riser a t these high fluxes. Similar results were obtained with our attrition resistant VPO catalyst as well. The CFB reactor performance for oxidation of n-butane to maleic anhydride was evaluated a t a wide range of process conditions. Laboratory data on butane conversion, maleic anhydride selectivity and yields over these wide ranges have previously been published [7], and were confirmed in the demonstration plant. An earlier paper [ll] evaluated the long term stability of catalytic performance and physical properties of the DuPont attrition resistant catalyst. In that study, a cycled feed, fluidized bed, intended to simulate a CFB, was employed. In the current study, VPO catalyst from the demonstration plant was similarly evaluated in long duration tests by characterizing periodically sampled catalyst, and the the validity of the earlier results [ l l l confirmed. Figure 3 compares XRD patterns of fresh VPO catalyst and a catalyst sample taken from

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the reactor after two months of operation. The patterns are essentially identical, each showing the presence of single crystalline phase (VO)2P2O7only.

Ua-6.9 m/s @ e x i t

U -8.9 9

30

I

m/s

@ exit I

I

I

I

kg/mL

523 671 850 1050

20

10 -

0

0

0

100 200 3 0 0 400 500

0

100 2 0 0 3 0 0 400 500 psusp,k

dm3

Figure 2 Riser suspension density profiles

10

20

30

LO

50

60

Figure 3 X-ray diffraction patterns of VPO catalyst as loaded and after two months service in CFB reactor. Pattern indicates single phase (VO)2P2O7. CuKa radiation used.

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Attrition resistance of the catalyst was evaluated by monitoring catalyst lossedday and by particle size analysis and laboratory evaluation in attrition test mills of periodically sampled catalyst. Attrition resistance in the demonstration plant, where catalyst was circulated at fluxes well within the range used in commercial catalytic crackers, fully met our expectations. In fact, our commercially produced VPO catalyst exhibited an attrition resistance three times greater than commercial FCC catalyst when measured in a standardized attrition test. Figure 4 shows the attrition resistance as a function of time for one long term experiment where catalyst samples were periodically withdrawn from the plant and characterized in a submerged jet attrition test mill [9]. These results indicate the attrition rate decreases with time in agreement with earlier long duration tests conducted in an identical attrition test mill [9]. This apparent improvement in attrition resistance with time can be attributed to two effects: 1) the early loss from the system of the weaker particles and 2) the loss of fines from the system due to the removal, by surface abrasion, of sharp edges and weakly bound small particles o r "satellites" from the catalyst microspheres. Figure 5 illustrates this latter effect that takes place during extended service in the CFB. The results of Figure 4 demonstrate that once the edges have been rounded out and the satellites removed, a very low attrition rate is achieved, indicating that the remaining smooth silica surface is not further abraded with time. The very smooth appearance exhibited by the microspheres after extended service (Figure 5) is merely the result of a mechanical polishing. As shown in Figure 6 , nitrogen porosimetry indicates that while there is an increase in average pore diameter t o a new steady state value, the total pore volume remains unchanged. Thus, access t o the catalytically active, internal surfaces of the catalyst microsphere remains unimpeded.

o.oooL--7 0

---

10

20

1

30

40

50

60

TIME (DAYS)

Figure 4 Attrition rate of VPO catalyst for extended time in the CFB demonstration plant as measured in a submerged jet attrition mill using a 760 ft/sec jet velocity.

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Strippers of proprietary design achieved efficient stripping of process gases at minimum catalyst inventories. Heat transfer coefficient measurements in different zones of the reactor system agreed, in general, with the available information in the literature.

Day 0 Figure 5 Scanning electron micrographs of VPO catalyst as loaded into the CFB demonstration plant and after 2 months of service.

Figure 6 Pore volume (cc/gm) and average pore diameter (A), as measured by nitrogen porosimetry, vs. time of service in the CFB demonstration plant

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6. CONCLUSIONS

The operation of the butane oxidation demonstration plant confirmed overall economic superiority of the process based on a CFB reactor over alternatives. Moreover, this large scale demonstration has provided basic data for the design of a commercial facility large enough to produce the required amount of maleic anhydride in a single CFB reactor to make 100 million lb/yr THF from butane. Finally, DuPont believes that its advances in CFB reactor and catalyst hardening technologies offer economically attractive options for a variety of chemical reactions, especially selective oxidative reactions. Therefore, instead of dismantling the demonstration facility, DuPont has optioned t o keep it for other potential applications. 7. ACKNOWLEDGMENTS

A large number of engineers a t DuPont's Conoco site a t Ponca City, Oklahoma supervised construction and operation of the demonstration plant under the leadership of J . Chorley and D. Jack.

REFERENCES 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12.

G. Centi, F. Trifiro, J. Ebner and V. Franchetti, Chem. Rev., 88 (1988) 5580. J. R. Ebner and J. T. Gleaves in Proceedings of the 5th IUCCP Symposium, "Oxygen Complexes and Oxygen Activation by Metal Complexes", eds. A. E. Martell and D. T. Sawyer, Plenum Press, New York (1988) 273. J. S. Buchanan and S. Sundareson, Appl. Catal;, 26 (1986) 211. M. A. Pepera, J . L. Callahan, M. J. Desmond, E. C. Millberger, P. R. Blum and N. J. Bremer, J. Am. Chem. SOC.,107 (1985) 4883. R. M. Contractor, H. E. Bergna, H. S. Horowitz, C. M. Blackstone, B. Malone, C. C. Torardi, B. Griffiths, U. Chowdhry and A. W. Sleight, Catalysis Today, l(1987) 49-58. R. M. Contractor, U.S. Patent 4,668,802 issued May 26, 1987 t o E. I. du Pont de Nemours and Company. R. M. Contractor and A. W. Sleight, Catalysis Today, 3 (1988) 175-184. H. E. Bergna, U.S. Patent 4,679,477 issued September 6, 1988 t o E. I. du Pont de Nemours and Company. R. M. Contractor, H. E. Bergna, U. Chowdhry and A. W. Sleight in Fluidization VI, eds. J . R. Grace, L. W. Shemilf and M. A. Bergougnou, Engineering Foundation, New York (1989) 589-596. H. S. Horowitz, C. M. Blackstone, A. W. Sleight and G. Teufer, Applied Catalysis, 38 (1988) 193-210. R. Contractor, J. Ebner and M. Mummy in New Developments in Selective Oxidation, eds. G. Centi and F. Trifiro, Elsevier Science Pub., Amsterdam (1990) 553-562. R. M. Contractor and G. S. Patience, a paper, "Density Profiles in a Tall Experimental Circulating Fluidized Bed", presented a t Nov. 1992, AICHE Annual Meeting in Miami.

24 1

G. EMIG (Universitiit Erlangen, Erlangen, Germany): You have a relatively thin eggshell of silica around your fluid bed catalyst. Isn't there a danger of a sudden destruction of the whole particle? Doesn't this become more of a problem as attrition occurs and the shell becomes even thinner? Is there a negative influence of silica (from the catalyst shell) on conversion and selectivity? We observed with a similar catalyst a slight decrease in both values. R. M. CONTRACTOR (DuPont, Wilmington, Delaware, USA): As previously reported [9],the silica shell on our catalyst is abrasion resistant and does not become thinner with time. Silica has low activity and affects selectivity adversely. But the negative effects are minimized in our catalyst by the use of only 10% silica. G. CENT1 (University of Bologna, Bologna, Italy): You have reported that the concentration of n-butane in the demonstration plant was lower than 25%. Are there specific reasons not t o utilize a higher concentration of hydrocarbon, especially when a recycle of n-butane is used? In addition, what is added to the hydrocarbon flow for the balance to be loo%? A second question is about the mechanical properties of the catalyst. You mention that the catalyst is very resistant t o attrition, but can you give a more quantitative indication about the rate of make-up of the catalyst in the demonstration plant? R. M. CONTRACTOR (DuPont, Wilmington, Delaware, USA): After removing condensables (steam and maleic anhydride) and taking a purge, the gaseous product stream containing unconverted butane was recycled in the demonstration plant. A higher % butane in the feed would have resulted in higher losses of butane with the purge stream. Attrition data are normally presented as the attrition rate in a standard test of a given catalyst sample versus a control which may be a commercial fluidized bed catalyst. Our VPO catalyst from the demonstration plant showed greater than three times higher attrition resistance than a commercial FCC catalyst. Data on catalyst make-up rate per unit production rate are not disclosed. M. BAERNS (Ruhr University, Bochum, Germany): From an investment point of view, you rate the recirculating fluidized bed reactor superior to the fixed and

fluidized bed reactor. What is the relative rating with respect to utilities, i.e., in particular, electric power for gas and solids transport? And what is the ratio of catalyst mass recirculated to the mass of maleic anhydride produced? R. M. CONTRACTOR (DuPont, Wilmington, Delaware, USA): We rate overall economics and not just investment of our CFB technology for selective oxidation of butane superior t o the alternatives. Specific information on electric consumption for gas and solids transport o r on the ratio of catalyst circulation to maleic anhydride production for the commercial plant have not been disclosed. Production of 2.5 g maleic anhydride (or 2 g butane conversion at 75% selectivity) per kg catalyst circulation in a CFB reactor has been previously reported [7].

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H. MIMOUN (Firmenich S.A., La Plaine, Switzerland): Could you tell how much oxygen is available for maleic anhydride formation by air? In other words, how many kilograms of catalyst are circulated per kiligram of maleic anhydride produced?

R. M. CONTRACTOR (DuPont, Wilmington, Delaware, USA): See response t o the previous question. F. TRIFIRO (University of Bologna, Bologna, Italy): A core annular model was proposed t o characterize the hydrodynamic regime in a CFB reactor system. It seems to me from your presentation that you don't believe in this model. It is not clear from the data you presented for what reasons, and on the basis of which experimental data, you think the core annular model is not applicable t o the oxidation of n-butane to maleic anhydride.

R. M. CONTRACTOR (DuPont, Wilmington, Delaware, USA): Typical coreannular structure of the CFB in the literature reports are associated with decreasing average bed density with height and relatively low average density because of the low solids mass flux. The high mass flux used in our process results in essentially constant, high average bed density along the entire 27 meter plus length of the riser, which is not compatible with the typical core-annular structure.

V. CortCs Corberin and S. Vic Bell6n (Editors), New Developmenrs in Selective Oxiddon I1 0 1994 Elsevier Science B.V. All rights rcservcd.

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Separation of Catalyst Oxidation and Reduction An Alternative to the conventional Oxidation of n-Butane to Maleic Anhydride ? G. Emig", K. Uihleinb, C.-J. Hackerb Institut fiir Technische Chemie I, Universitiit Erlangen-Nurnberg, EgerlandstraBe 3, D-91058 Erlangen, Germany

a

Insitut fiir Chemische Technik, Universitst Karlsruhe, KaiserstraBe 12, D-76128 Karlsruhe, Germany

Summary The separation of the redox-process in selective oxidation of n-butane to maleic anhydride was simulated by using the pulse-technique. Influences of the reoxidation-temperature and the reaction-temperature were examined separately, also the influences of catalyst-residence-time and duct-concentration. Yields up to 78% were achieved. Problems still arise from the long duration of catalyst reoxidation which reduces the space-time-yield to a barely economic level.

1. Introduction Beside the common reactor-systems for selective catalytic oxidations as fixed-bed tube bundle reactors and fluidized beds a new concept the so-called riser-regeneratorsystem which has been proposed for the ammoxidation of aromatic substances already 1975 (ref. 1) is propagated for the selective oxidation of n-butane to maleic anhydride (MAA) (ref. 2, 3). According to this concept the oxidation of the catalyst and its reduction are separated in two different reactors, a fluidized bed for the oxidation and a riser - well known from petrochemical 111dustry - for reduction of the catalyst (Fig. 1). This separation allows an independent optimization of the conditions for the reductionstep and for the oxidation-step. Also there is no limit for the concentration of butane in

MAA

Re-

Qenerator

Riser

!-

Euiane

dride. The economy of this way is based not only on selectivity and space-time-yield but also

244

on the integral amount of maleic anhydride (Wt.), that corresponds to the ability of the catalyst to store oxygen reversibly (OZint.). This parameter determines the amount of catalyst which must be circulated and therefore the costs of energy. For example: to run a 20.000 jato MAA plant 650 kg of catalyst must be circulated each second assuming the catalyst is able to store enough oxygen for the production of one gram maleic anhydride per each kg of catalyst.

2. Experimental The catalyst: We used a (VO),P,O, catalyst (idendified by XRD- and IR-spectroscopy) prepared in organic medium (iso-butylalcohol/benzylakohol). The P/V-ratio in the solution was 1:l.Zr,(PO,), was added as promotor (ref. 4). Before activation the catalyst was dispersed in a silica acid solution and spray-dried to get a silica shell for high attrition resistance (ref. 5). The catalyst (particles of 40-80 pm) was activated in a butane/air-mixture at 410 "C (1.8% butane in air) using a fluidized bed. Apparatus: The pulse reactor studies used a tubular stainless steel reactor with an internal diameter of 10 mm. The catalyst was mixed with low-surface alumina (0.23 m2/g) to get defined fluiddynamic conditions. Heating of the reactor achieved by a block furnace was controlled by monitoring the temperature at the midpoint of the catalyst bed. A six-port Valco rotary valve is used to introduce reactant pulses into the He-caniergasstream from the calibrated sample-loop. After leaving the reactor the gas-stream is splitted to different columns in the gas chromatography section for quantitative analysis. The catalyst sees some pulses of butane (butane in nitrogen) which is further called to be the reaction or reduction (of the catalyst). Then the catalyst is reoxidized with pulses of oxygen (oxygen in nitrogen) which is further called to be the regeneration or oxidation (of the catalyst). The time lag between two pulses, caused by the long analysis time, is 25 minutes. Gas-residence-time and catalyst-residence-time (= time that the catalyst is in contact with butane) are determined by the gas-stream (70,3 nml/min), the inlet pressure (1,9 bar), the volume of the sample-loop (2 ml) and its temperature (173 "C). The former was 0,45 seconds at a catalyst input of one gram and a reactor temperature of 400 "C, the latter was 1,254 seconds. To keep the influence of the time lag between butane pulses small, the experiments were stopped when the conversion dropped below 10%. Earlier studies showed that the difference between a frequency of 2 pulses per minute and 2 pulses per hour is then negltgible for the last pulse.

For analysis we used a HP 5890 gaschromatograph with FID and TCD as detector systems. For the separation of maleic anhydride, n-butane and butadiene a 5% phenylmethyl-silicon capillary column (HP5) was employed. It was connected with the FID. No other organic sideproducts were detected. Water and CO, were separated on a Porapak Q, CO, 0, and N, on a molecular sieve 5A A TCD was applied. Butane conversion and yield were calculated based on the amount of carbon detected.

3. Results The results can be depicted as function of pulse-number (that corresponds to the catalyst- residence-time in the riser or regenerator: one pulse = 1,25 s). The experiments

245

could be reproduced either applying a number of oxidation-reduction cycles ( m a . 31) or changing the catalyst using the same or even a different ammount of catalyst. The selectivity vaned by 1%,while the ammount of oxygen consumed differed by +I- 5%.

ReductionJReaction (Fig. 2): The strongly oxidized catalyst shows high activity and selectivity which decreases by increasing residence-time/pulse-number (lower oxidation state). (Note: the initial increase of selectivity with pulse-number by using a larger amount of catalyst (Fig. 8) results from high conversion and hence from consecutive reactions). Therefore the curve for the integral amount of maleic anhydride (= MAA'"'. in g-/lc&,.) first rises considerably and then flattens. Additively butadiene could be found in a portion of m a . 0,5% of the amount of formed maleic anhydride. C-balance was in the range of 95 to 100%. There was no correlation between the number of pulse and the C-balance. Especially the C-balance does not decrease with a growing number of pulses. x I%l

O,lg/kgl

loo@

:

0'

1

2

3

,

4

/

5

,

6

pulse

I

7

,

8

,

9

1

'0 0

[-I

Fig. 2: Conversion (X), selectivity (S) and h4AA as function of pulse-number during reduction (T,= 400 "C, Td,=400"C, cbu = 9 vol%, t,, = 18 h, mat, = 0,33 g)

1

6

11

16

21

pulse

1-1

26

12

31

Fig. 3: Oxygen conversion and fixed oxygen during regeneration as function of pulse-number (mat, = 1 g, Tred,= 370 "C, = 9 ~01%) co2 = 15 vol%, cbU(during

OxidationJRegeneratioion (Fig. 3): The reduced catalyst takes up a large amount of oxygen. With increasing oxidation state (higher pulse-number) the rate of oxygen input flattens. slows down and the curve for the integral amount of consumed oxygen (OZint.) Also small amounts of CO and CO, (<0,1 ~01%)could be detected during the first pulses of the reoxidation due to absorbed organic components. Detailed exploration of the influences of various parameters on the redox-process leads to the following results: temporal and spatial separation of the redox-process is possible. By using reaction temperatures lower than 370 "C higher yields can be obtained than by following the conventional method. For example at 310 "C (Fig. 8) a yield of 78% was

246

achieved, a value that exceeds all yields cited in literature (max. 62% (ref. 6)). Simultaneously so the space-time-yield, related to the reduction-step, achieves 0,5 1 kg-/lcg,,/h; a value which is four times higher than using the same catalyst at 100 higher temperatures in the conventional way.

"c

s 1%)

70

. . .. . . . .. .

_A_-!-

60

I

0

0.5

5

50

500

regeneration [rninl

Fig. 4: Influence of regeneration-time (To,.= Tred,=400 "C, mat,= 0,33 g, cbu= 9%)

50

- i'

40

..

..'a.

.

I

a0

loo

-

,

0 0

*-

20

40

60

x

(461

Fig. 5: Dependence of selectivity upon conversion (To%=T,.= 400 "C, mat, = 0,33 g, cbu= 9%)

Ln fact such a catalyst performance requires a regeneration period of several hours, whereby the space-time-yield related to the whole process (regeneration and reaction) drops to a small value. As seen in Fig. 4 the amount of oxygen available for the reaction increases from 1,9 g/kg,, to 6 &gat. when the regeneration period changes from 30 seconds to 18 hours. The amount of maleic anhydride per kg catalyst formed at different regeneration-times emphasizes once more the importance of this parameter. Thus it is possible to form 2-4 gm/k&, by using a regeneration-time of 18 hours in contrast to by working at a regeneration-time of 30 seconds. an amount of 0,3-0,7 g-/k&, Conversion and selectivity for the first pulse are also influenced positively. While activity still rises after hours of regeneration, selectivity achieves a constant maximum. Fig. 5 illustrates the dependence of selectivity upon conversion as indicator for the oxidation state of the catalyst. The amount of disposable oxygen increases not only along with regeneration-time but also with the temperature applied. The way how to reach higher oxygen-loading - by temperature increase or longer regeneration-period - is not important. Higher regeneration temperature does not reduce selectivity. To obtain improved selectivity using a short regeneration period it is in contrast necessary to apply a high regeneration-temperature (Fig. 6). The ammount of oxygen consumed (Fig. 6) and the ammount of oxygen fixed during regeneration (Fig. 3) matches rather properly.

247

-5

%

Butane

9 %Butane -+ 12 % B u t a n e

320

340

360

380

temperature

400

420

440

1Ocl

Fig. 6: Influence of regeneration temperature (Tred.= 370 "c,mat.= 1 g, t,= 30 s, cbu = 9%, c02(duringreg.) = 15 vol%)

0

1

2

3

4

0 y"'. [g/kgl

5

-7 I

'

6

Fig. 7: Influence of butane concentration on catalyst-reduction (T,,= T,,= 400 mat.= 0,33 g, t,= 18 h)

"c,

Reoxidation can be increased slightly by application of higher oxygen pressures. Considerably more important is the influence of n-butane on catalyst-reduction. Fig. 7 proofs that the conversion is independent from butane concentration at the same oxidation state of the catalyst, expressed in terms of the oxygen consumed after reoxidation. That indicates a first-order reaction in butane. (Note: with respect to the change of the oxidation state during one pulse at high conversions an average amount of oxygen consumed was used: OFrage=O 2pulse + 0,5 * ( 0 2 P u k e i + l - OzPulse ')). This is in agreement with the results of Pepera et al. (ref. 7). The time to obtain a certain oxidation state is therefore inversely proportional to butane for pbu = constant (eqn. 1 to 3). rate expression:

t for n = 1: t for n

z

1:

24 8

*'S

400 " C !

I

01

0

1

2

3

4

0tt' [g/kgl

5

6

Fig. 8: Influence of reduction-temperature (TOx=370"C,tox,=18h, mcat,=lg,c,,=9%)

Fig. 9: Influence of reduction-temperature (To,= 370 "C, to%=18 h, mat.= 1 g)

Increasing reduction-temperature diminishes selectivity while activity grows (Fig. 8). The amount of oxygen which can be delivered rises with reduction-temperature. Hence the amount of maleic anhydride which can be formed per kg catalyst shows a maximum at 380 "C. This temperature is valid using our catalyst if it has been re- ------=07 oxidated at 370 "C (Fig. 9). Using identical temperatures for the reduction-step and for the regeneration-step the maximum moves to higher temperatures (410 "C, Fig. 9). Starting with the same oxidation state of the catalyst selectivity is lower with additional gas-phase-oxygen during reduction phase than without additional oxygen. On the other hand it is possible to avoid overreduction of the catalyst by , application of gas-phase-oxygen. Over- 20 0 1 0 20 30 40 50 60 70 reduction occurs if the catalyst has been x I%l exposed to butane over a too long period; overreduced catalyst causes a decrease in Fig. 10: Influence of gas-phase-oxygen selectivity. Thus oxygen addition can have (Tox,=Tred.= 400 "C, t,= 18h, mcat.=0,3g) a positive effect too (Fig. 10). Thermogravimetric studies,' using a Netzsch STA 409, confirm the results gained by pulse experiments. In the region between 360 "C and 410 "C the amount of oxygen consumed appears to be directly proportional to the temperature. Rising temperature in I

249

the sample during reduction-phase as well as during regeneration-phase indicates that both steps are exotherm. Table 1 Results of the XPS-studies stoic ratio

1

2

stoic ratio3

E

Samples

V

Zr

P

0

P/V

OP

[ev]

reduced (410 "C)

1

0,39

2,4

9,6

1,9

4,O

516,7

oxidized (350 "C)

1

0,39

2,4

10,4

1,9

4,4

516,7

oxidized (400 "C)

1

0,39

2,4

10,6

1,9

4,6

516,8

reduced (410 "C)

1

0,39

2,O

7,4

1,5

3,7

515,6

oxidized (350 "C)

1

0,39

2,O

8,2

1,4

4,O

516,8

oxidized (400 "C)

1

0,39

1,9

8,l

1,5

4,4

516,6

XRD-studies of the reduced and of the oxidized catalyst show that there is essentially no change in the bulk structure of the catalyst. Also the energy of the 2pm - electron remains unchanged as XPS-spectras indicate (tab. 1). However a higher OP- ratio for the oxidized catalyst, even after sputtering with Ar-ions, was detected. That means that not only the surface layer but also deeper layers are involved into the redox-process.

4. Economic Aspects The data collected during this work was used as a basis for experiments in a real riserregenerator system, which is already running. The fluiddynamic conditions there are different from those employing a pulse reactor. Within a riser we will have plug-flow of the gas, but regions of recirculation of the catalyst which might influence the selectivity negative (overreduced catalyst particles). The residence time distribution of the catalyst in the fluidized bed during regeneration is vary similar to that of an ideal stirred tank reactor. This will cause reduced activity and selectivity due to particles with a residence time lower than the average residence time; this can not be compensated by the particles with a longer residence time. This effects were not taken into account when estimating the data given in table 2. Based on various states of operating the dimensions of a riser for a 20.000 t/y MAA plant are evaluated and shown in tab. 2; also mentioned is the amount of catalyst which must be circulated, the energy costs (in percent of the production-costs) and the spacetime-yields (STY) related to the riser as well as related to the whole process. These data make evident that a process based on the separation of the redox-process with the catalyst used here seems not to be economic. The limiting step is the reoxidation. For technical applications reaction- and regeneration-times have to be in the same order of magnitude. Working at very short regeneration- and reduction-periods, the space-time-yields might be in the range of the

250

Table 2 Dimensions of a riser for a 20.000 jato ~. plant Parameter

1

2

3

4

5

370

430

370

400

370

370

370

310

400

370

30

30

64800

300

5

9

10

12

7

5

2

2

2

2

2

52

37

17

22

57

1520

940

373

813

2882

16

679

173

376

33

0,17

0,24

0,5 1

0,40

0,16

0,038

0,054

9,4.10”

0,009

0,08

conventional way, but the process would be inefficient because of high energy costs for circulating the catalyst if the oxygen uptake during the regeneration-phase is too low. Results of Hodnett and Delmon (ref. 8) indicate that some simple modifications like variation of the PN-ratio in the catalyst may improve not only selectivity but also the amount of maleic anhydride which can be produced per kg catalyst by working at some hours of regeneration-time. Assuming that these effects don’t change by working at short regeneration-times, a process based on the temporal and spatial separation of the oxidation-step from the reduction-step might become an interesting and economic alternative.

References: 1.

2. 3.

4. 5. 6. 7. 8.

M.C. Sze and A.P. Gelbein Hydrocarbon Processing, 2 (1975) 103 R.M. Contractor U.S. Patent 4,668,802 (1987) R.M. Contractor, J. Ebner, M.J. Mummey New Developments in Selective Oxidation, Elsevier Science Pub. (1990) 553 F.G. Martin Dissertation, Erlangen (1989) H.E. Bergna US. Patent 4,769,477 (1988) G.J. Hutchings Applied Catalysis 72 (1991) 1 M A Pepera, J.L. Callaghan, M.J. Desmond, E.C. Milberger, P.R. Blum, N.J.Bremer J. Am. Chem. SOC. 107 (1985) 4883 B.K. Hodnett and B. Delmon Applied Catalysis 15 (1985) 141

25 1

Discussion Contributions 1.-C. Volta (I. de Recherches sur la Catalyse - CNRS, Villeurbanne Cedex, France): What is your opinion about the role of the promotor?

G . Emig (I. of Technical Chemistry, Erlangen, Germany): The catalytic active material was optimized for the conventional oxidation, where zirconium improved the selectivity but also reduced the activity. One reason for this effect might be the oxygen-ion-conduction. Also the stabilisation of V(1V) should be taken into account. The latter would reduce the reoxidation rate. A catalyst without a promotor - and bad results during conventional oxidation - could therefore lead to superior results when oxidation and reduction are separated. J.J. Lerou (Du Pont CR & D, Wilmington, USA): The long reoxidation times your catalyst requires are unusual. - Did you made a calculation of how much 0: your reduced VPO cat can take up and how does it compare with your experimental data with very long reoxidation times? - Did you investigate the effect of elapsed time between butane pulses? We have found a very noticeable dependence.

G . Emig: - Assuming that per (VO),P,O, unit one oxygen can be taken up, the maximal possible amount of oxygen fixed during regeneration is about 50 goxygenkgcnt. Our catalyst took up 8,5 gonygen/kgcat maximal. On the surface of the catalyst about 2% of all (VO)2P,07 units (= 0,9 goxygc5gcat) are located. Thus also deeper layers must be active in the reoxidationheduction process. - Our studies showed that the difference between the amount of MAA detected per pulse at a frequency of 2 pulses per minute and 2 pulses per hour is negligible (less than 12%), if the experiments are stopped, when the conversion drops below 10%.

This Page Intentionally Left Blank

V. CortCs Corbcran and S. Vic Bellon (Editors), New Developments in Selective Oxidation I/ 0 1994 Elsevier Science B.V. All rights reserved.

253

On the catalyst features affecting selectivity in n-C, hydrocarbon oxidation and oxidative dehydrogenation. Ft-IR studies G. Busca, V. Lorenzelli, G. Oliveri and G. Ramis

Istituto di Chimica, FacoltB di Ingegneria, Universiti di Genova P.le Kennedy, 16129 Genova (Italy)

The interaction of butane, butenes and butadiene, as well as of some oxygenated compounds with M g vanadate, (VO),P20, and V,O,-TiO, catalysts has been investigated by Ft-IR spectroscopy in the 150-900 K temperature range. A common reaction scheme has been identified. The catalytic activity and selectivities to oxidative dehydrogenation products (main useful products on V-Mg-0), to acetic acid and COX(predominant on V-Ti-0) and to maleic anhydride (predominant on V-P-0) are proposed to depend on the efficiency of the different active sites (Lewis acidic V centers acting as alkylic and allylic CH activation sites, OH'S bonded to oxidizing V ions, V=O oxygen insertion sites) observed through surface characterization studies over the different catalyst surfaces.

1. INTRODUCTION The availability of n-butane from natural gas makes increasingly attractive its use as a feedstock for the production of petrochemicals, as a substitute of n-butenes arising from the C, cut from naphta cracking (1). Butane can be dehydrogenated (or oxidatively dehydrogenated) to butenes and either butane or butenes can be oxidized through heterogeneously-catalyzed reactions to useful compounds, like butadiene, furan, maleic anhydride, methyl-ethyl ketone, methyl-vinyl ketone, crotonaldehyde, acetic acid and anhydride, etc. Many catalytic systems finding industrial or potential application for selective oxidation of C , linear hydrocarbons are mixed oxides containing vanadium as the key element (1). Selectivities in heterogeneous catalytic processes depend on reaction conditions and, of course, on the catalyst chemical composition. Trifirb and co-workers recently showed that product selectivity in butane and butene oxidation over different catalysts can be controlled by "modulation" of reaction conditions (like reactant mixture composition, space velocity, temperature, etc.(2)) and through a control of the valence state of the key catalytic element, vanadium (3,4). Oyama and co-workers (5) proposed different adsorbed forms of the hydrocarbon as the key factor for ethane selective and unselective oxidation over vanadia-silica, while Kung and co-workers (6) similarly proposed a role of the "geometry" of the active site in determining selectivity in butane oxidation over V-based catalysts.

254

In the present paper we will compare the surface chemical behaviour of different catalysts involved in C,-hydrocarbon oxidative conversion, as studied by Ft-IR spectroscopy. Attention will be focused on V-based oxides, like (VO),P,O,, the active phase of industrial catalysts for maleic anhydride synthesis (7), Mg-vanadate, proposed for butane selective oxy-dehydrogenation (8,9), V-Ti-0 used today for alkylaromatic selective oxidation (10) but formerly applied also to butane oxidation to acetic acid ( l ) , and the pure oxide V,O,, for comparison. The aim is to further investigate the factors determining selectivity in heterogeneous oxidation catalysis on a chemical basis in relation to catalyst composition. 2. EXPERIMENTAL

The preparation and characterization of (VO),P,O,, 28 mVg, has been reported previously (1 1,12). V20,-Ti0, (48 m2/g, 9.6 V,O, by weight) was prepared by dry impregnation of P25 TiO, from Degussa (78 % anatase). V,O, (18 m2/g) is obtained from Degussa, and has been described previously (13). The Mg-vanadate catalyst (Mg:V 3:2 atomic ratio, 8 mVg) was prepared by impregnation of Mg(OH), with ammonium metavanadate, followed by calcination at 673 K for 2 h. XRD analysis shows that it is amorphous and crystallizes near 873 K into a mixture of Mg,(VO,),, Mg2V,0, and MgV,O,. According to literature data, mixtures of these phases result in catalyst having good performances in oxidative dehydrogenation (14). V,O, is detected neither by IR nor by XRD analyses. Ft-IR spectra have been recorded with a Nicolet 5ZDX instrument using pressed disks of the pure powders put into contact with controlled atmosphere through conventional gas manipulation apparata and IR cells. 3. RESULTS

In Fig. 1,a the spectrum of a pressed disk of Mg vanadate after activation is reported. It shows a strong absorption in the higher frequency region with two maxima at 3615 and

3510 cm.' where a very sharp and weak band at 3750 cm-' is superimposed. These three peaks are due to OH stretchings of surface hydroxy-groups, the last one very likely being due to MgOH on MgO particles. The band at 3615 cm-' compares well with the bands of surface hydroxy-groups on other V-oxide based catalysts, found in the 3700-3600 cm-' range, and can consequently be assigned to nearly free VOH groups.The band at 3510 cm-' is likely due to H-bonded OH'S. The bands in the region 1800-1000 cm-' are harmonics of the fundamental V - 0 stretchings and deformations observed in the region 1000-500 em-' in the spectra of KBr pressed disks. The weak sharp bands at 1932 and 1908 cm-' are due to the overtone and combination bands of V=O stretchings found in the region 980950 cm-' in the skeletal IR spectrum. No change in the spectrum of Mg-vanadate can be observed by interaction with butane from r.t. up to 773 K. Contact at 793 K causes the progressive decrease of the sample transmittance (Fig. 1, b and c) up to its complete opacity, without the detection of any band due to adsorbed species. This process is due to a reduction of Mg-vanadate by butane. As we showed previously (13), reduction of n-type semiconducting oxides including V,O, causes the production of nearly-free electrons whose absorptions obscure the IR

255

1.5-

1

-

st 111

u C

m u .5 u .r.

E

m

K m L 4-l

0 4000

3200

2400

1600

800

wavenumber cm-1 Figure 1. Ft-IR spectra of Mg-vanadate catalyst pressed disks after activation at 673 K (a) and contact with n-butane (100 torr) at 790 K for 15 min (b) and for 30 min (c).

and the visible regions. The behaviour of Mg-vanadate is very different from that observed for the other Vbased oxides in contact with n-butane (Fig. 2). If (VO),P,O, is put into contact with butane at 600 K, adsorbed species are formed, characterized by a broad absorption in the region 1900-1500 cm-'. If the catalyst was previously put into contact with oxygen at the same temperature, the bands due to maleic anhydride at 1840 and 1780 cm-' (C=O stretchings of the O=C-0-C=O system) grow very intense (Fig. 2,a). The intermediate adsorbed species have been identified previously as furan-like and butirolactone-like intermediates (15). If V20,-Ti0, is put into contact with butane at 520 K (Fig. 2,b) adsorbed species are also found. However, in this case, the bands are rather sharp at 1530, 1440 and 1355 cm-', and can be assigned to acetate species by comparison with the spectrum of the adsorbed species arising from acetic acid adsorption. These data very well agree with catalytic data of butane oxidation in flow reactor. In fact, both (V0),P20, and V,O,-TiO, are active catalysts for butane oxidation at 600 K. but in the former case maleic anhydride is produced with high selectivities, while in the latter acetic acid is the main useful product, but COXpredominate (16). Mg-vanadate is much less active and produces mainly dehydrogenation compounds at temperatures of the order of 700-800 K (6,8,9). As we will show below, at these temperatures the adsorbed species are already desorbed, and cannot be observed. The reactivities of Mg-vanadate, (VO),P,O, and V,O,-TiO, with respect to n-butene and butadiene have also been compared. Mg vanadate appears again to be the least reactive. Adsorbed species are observed starting from near 550 K for 1-butene (Fig. 3,a and b)

256

u a)

m C 3 JI 0 5

I

,

/

2000

I

I

1800

I

I

1600

'

I

1400

/

1200

1000

wavenumber cm-I

Figure 2. Ft-IR spectra of the adsorbed species arising from contact of V,0,-Ti02 and (VO),P,O, with n-butane (50 torr): (a) contact of pre-oxidized (VO),P,O, with nbutane at 620 K for 10 min; (b) contact of V20,-Ti0, with n-butane at 520 K for 5 min. (VO),P20, is opaque below 1350 cm-1, because of bulk P-0 stretchings.

1800

1600

1400

1200

1000

w a v e n u m b e r cm-1

Figure 3. Ft-IR spectra of the adsorbed species arising from contact of V,O,-TiO, and Mg-vanadate with 1-butene (50 torr): (a) contact of Mg-vanadate with 1-butene at 550 K for 10 min, and after subsequent contact with oxygen at the same temperature(b) ; (ce) contact of V,O,-TiO, with 1-butene at 300 K for 5 min and outgassed at 300 K (c), 423 K (d) and 473 K (e).

and 450 K for butadiene, giving in all cases species that can be identified predominantly as carboxylate species (rather broad bands at 1500 cm-', stronger, and at 1430 cm", weaker). If contact with oxygen gas is carried out after butene or butadiene adsorption at high temperature, weak bands appear at 1850 and 1780 cm-'. These bands are taken as an evidence of the formation of maleic anhydride, in small amounts. All bands disappear near 700 K. On (VO),P,O, the spectrum of the adsorbed species formed by contact either with butane, or with butene and butadiene are the same, although the temperature at which these species are formed strongly depend on the reactivity of the adsorbate (near 600 K for butane, near 450 K for butene, already at r.t. for butadiene). Moreover, by further contact with oxygen gas at 500 K the bands due to maleic anhydride grow very strong. On vanadia-titania a completely different picture is observed (Fig. 3, c-e). In fact, nbutenes adsorb at r.t. giving sec-butoxy species that by mild heating dehydrate oxidatively giving rise to methyl ethyl ketone. This compound easily enolizes on the surface and starting from 423 K undergoes an oxidative breaking at its C(2)-C(3) bond giving rise to acetate species. On the other hand, as reported previously (17) butadiene strongly adsorbs on V,O,-TiO, giving rise to furan-like species that give maleic anhydride by further

257

a, U

C

m

n

L 0

m m

n

1700

1600

1500

1700

1600

1500

1400

wavenumber cm-I Figure 4. Ft-IR spectra of pyridine adsorbed on (VO),P,O, Mg-vanadate (d).

(a), V,O,-TiO, (b), V,O, (c) and

oxidation in the presence of gas-phase 0,. So, on V,O,-TiO, two different pathways are observed starting from butane and butene on one hand and butadiene on the other hand. It is well known that the catalytic activity in selective oxidation is greatly affected by the catalyst acid-base properties. For this reason the surface acidities of the three V-oxide based catalysts and of V,O, have been compared, using pyridine as a probe molecule (Fig. 4). The broad band near 1534 cm-' is due to the 19b ring stretching mode of pyridinium ions, produced by pyridine protonation by the surface Br$nsted acidic sites. This band, together with the relatively broad adsorption found near 1640 cm-' due to the corresponding 8a and 8b vibrations (frequently unresolved) is observed on (VO),P,O,, V,O,-TiO, and V,O,, where BrQnsted sites are present, but is not observed on Mg vanadate, where Bronsted sites are absent. The sharp band just above 1600 cm-' is due to the 8a vibrational mode of pyridine coordinated over Lewis acid sites and its shift upwards with respect to the value of the liquid (1583 cm-I) can be taken as a measure of the strength of these sites. We observe this band at 1612 cm-' on (VO),P,O,, at 1610 cm-I on V,O,-TiO,, at 1608 cm-' on V,O, and at 1606 cm-' on Mg vanadate, so showing that Lewis acidity decreases in this order. 4. DISCUSSION

Ft-IR experiments of butane, butene and butadiene oxidation over the three V-based catalysts, as well as on other oxide catalysts, allow us to propose the following generalized reaction scheme for heterogeneously-catalyzed oxidation of C, linear hydrocarbons:

25 8

la lb 2 3

= alkylic C-H activation site = allylic C-H activation site = weakly Br9nsted acidic OH'S on oxidizing sites = oxygen insertion sites (V=O)

Alkane activation. Catalysts active in butane selective conversion (oxy-dehydrogenation or oxidation) must expose sites that allow alkane activation at temperatures at which the desired products (olefins or oxygenates) are still relatively stable and can leave the surface without further oxidation. These sites (type la) are thougth to be constituted by transition metal cations (V"') acting as strong electron acceptors. This has been proposed for vanadyl-pyrophosphate catalysts (13,18) on the basis of the relation between catalytic activity and surface Lewis acidity of different preparations. This proposal finds support on the present data that allow to relate the catalytic activity in butane conversion with the Lewis acid strength, both showing the trend (VO)2P,07 2 V,O,-TiO, >> Mg-Vanadate. The selectivities in oxy-dehydrogenation products obtained on Mg-vanadate can only be explained with the very low activity of this catalyst in the successive transformation of olefins and dienes. O n the other hand, also on catalysts active for oxygenate production, like (VO),P,O,, significant selectivities in dehydrogenation products can be obtained at low oxygen concentration, showing that these compounds are intermediates in oxygenates production at least in the adsorbed state, as shown by Trifirb and co-workers (3,4,7). The mechanism of C, hydrocarbon activation on vanadium-based catalysts can be proposed to be similar to that of activation of methyl-aromatics on V,05-Ti0,, investigated previously (19,20). It has been shown that methyl-aromatics are activated in the form of henzyl species by hydrogen abstraction from a methyl group. These species are formed at r.t. because of the high reactivity of benzylic C-H bonds and are stable because they cannot eliminate another hydrogen. So, they can be detected easily by IR at r.t., and react slowly with surface oxide species in the rate-determining step of alkyl-aromatic oxidation.

259

By analogy, it seems reasonable that butane should be activated in the form of secbutyl species by hydrogen abstraction from the more reactive methylene groups. However, due to the lower reactivity of alkyl C-H bonds with respect to benzyl C-H bonds, this reaction is slower and occurs only at higher temperatures. Moreover, the alkyl intermediates cannot be detected by IR because, at the temperature at which they form they more rapidly eliminate a second hydrogen giving rise to an olefin molecule. This agrees with the conclusion that, in the case of butane oxidation, the abstraction of the first hydrogen is the rate determining step ( 2 ) . Paths to oxvpenates.0ur data provide evidence for four diffcrent paths to oxygenates. Weakly BrQnsted acidic surface hydroxy-groups (type 2 sites) can react reversibly with the olefins producing surface sec-butoxyde species. These species can further evolve by oxidative dehydrogenation to methyl ethyl ketone, that, through its enolate form, can later undergo C(2)-C(3)bond oxidative cleavage, giving acetate species, the precursors for acetic acid. This is the predominant path on V,05-Ti0,, where it is also favoured by the presence of steam (21). while it is a minor one on (VO),P,O, , where acetic acid can also be produced in small amounts (7). This path is almost unactive on Mg-vanadate where BrQnsted acidity is not detected. A competitive path involves hydrogen abstraction in the allylic position of adsorbed butene leading to an ally1 intermediate (type l b site) that is rapidly further converted be elimination to butadiene, or, alternatively, to methyl-vinyl ketone. This way becomes predominant on (VO),P,O, probably because the alternative one is slow in relation to the covalency of the P-(OH) bonds, and the weak nucleophilicity of the surface oxide species. Accordingly, olefins over phosphates tend to polymerize more than to form alkoxides. The production of oxygenates like furan and maleic anhydride needs particular surface sites that react very rapidly with butadiene. These sites, that insert oxygen in the C( 1)-C(4) position of butadiene (type 3 sites), are typically vanadyl-species (V=O). These species, with nearly square-pyramidal coordination, are present in the bulk and on the surface of vanadyl-pyrophosphate and on the surface of vanadia-titania (22), and in general when V5+is in acidic environment. On the contrary, vanadyls are absent in the bulk of Mg-vanadates where vanadium takes a nearly tetrahedral coordination with two short V=O bonds. This is the typical behaviour of V5+in basic environments. Type 3 sites are also absent on the surface of catalysts for butene oxy-dehydrogenation, like femtes that, accordingly, adsorb very weakly the desired product butadiene ( 2 3 ) .

Table I. Efficiency of the active sites on V-based catalysts for C, linear hydrocarbon oxidations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

catalyst

la

ME-vanadate weak (VO),P20, strong V,O,-TiO, strong

lb

2

3

useful products from n-butanc

weak strong strong

=

weak strong

very weak butene, butadiene strong maleic anhydride (furan) strong acetic acid (maleic anhydride)

260

5. CONCLUSIONS The selectivities in C, linear hydrocarbon oxidative conversion over V-based catalysts can be explained by the different strength of three types of surface sites, as summarized in Table I. The general pathway is the same for all catalysts but the relative rate of different steps is very different. ACKNOWLEDGEMENTS The authors are indebted with Prof. F. Trifirb and Prof. G. Centi (University of Bologna, Italy) for the collaboration on vanadyl-pyrophosphate catalysts. This work has been supported by CNR, progetto finalizzato Chimica Fine 11. REFERENCES 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18. 19. 20.

21. 22. 23.

J. Schulze and M. Homann, C,-hydrocarbons and derivatives, Springer Verlag, Berlin, 1989. G. Centi and F. Trifirb, Appl. Catal. 12 (1984) 1. F. Cavani, G. Centi, F. Trifirb and R.K. Grasselli, Catal. Today 3 (1988) 185. F. Cavani, G. Centi and F. Trifirb, Chimica e Industria, 74 (1992) 182. S.T. Oyama, A.N. Desican and W. Zhang, in "Catalytic Selective Oxidation", S.T. Oyama and J. Hightower eds., ACS, 1993, p. 16. P.M. Michalakos, M.C. Kung, I. Jahan and H.H. Kung, J. Catal. 140 (1993) 226. G. Centi, F. Trifirb, J. Ebner and V. Franchetti, Chem. Rev. 88 (1988) 55. D. Patel, M.C. Kung and H.H. Kung, Proc. 9th Int. Congr. Catalysis, Calgary, 1988, p. 1555. D. Bhattacharyya, S.K. Bey and M.S. Rao, Appl. Catal. A,General, 87 (1992) 29. H.G. Franck and J.W. Stadelhofer, Industrial aromatic chemistry, Springer Verlag, Berlin, 1988. G. Busca, F. Cavani, G. Centi and F. Trifirb , J. Catal. 99 (1986) 400. G. Busca, G. Centi, F. Trifirb and V. Lorenzelli, J. Phys. Chem. 90 (1986) 1337. G. Busca, G. Ramis and V. Lorenzelli, J. Mol. Catal. 50 (1989) 231. J. Hanuza, B. Jezowska-Trzebiatowska and W. Oganowski, J. Mol. Catal. 29 (1985) 109. G. Busca and G. Centi, J. Amer. Chem. Soc. 111 (1989) 46. G. Busca, G. Centi and F. Trifirb , Appl. Cata1.25 (1986) 265. V.S. Escribano, G. Busca and V. Lorenzelli, J. Phys. Chem., 95 (1991) 5541. G. Busca, G. Centi and F. Trifirb , J. Amer. Chem. Soc. 107 (1985) 7757. G. Busca, J. Chem. Soc. Faraday Trans. I, 89 (1993) 753. G. Busca, in "Catalytic Selective Oxidation", S.T. Oyama and J. Hightower eds., ACS, 1993, p. 168. W.E. Slinkard and P.B. DeGroot, J. Catal. 68 (1981) 423. G. Ramis, C. Cristiani, P. Forzatti and G. Busca, J. Catal. 124 (1990) 574. G. Busca and V. Lorenzelli, J. Chem. Soc. Faraday Trans., 88 (1992) 2783.

26 1

U S . Ozkan (The Ohio State University, Columbus, Ohio, USA): VanadidTitania catalysts have been studied quite extensively, both for o-xylene oxidation and more recently for selective catalytic reduction of NO. These catalysts are known to have monomeric vanadyl, polymeric vanadate and even crystalline vanadium pentoxide species on the surface, even at loading levels that correspond to the monolayer coverage. How confident are you that only species you have on the surface is monomerioc vanadyl species and that you do not have some of the polymeric chains ? G. Busca (Universiti di Genova, Italy) : We agree with your statement that near the "monolayer" coverage monomeric vanadyls, polymeric vanadates and crystalline vanadium pentoxide species are present on the surface of vanadia-titania catalysts. This in fact was the conclusions of our studies (1-3). It seems likely that the most active species are the polymeric metavanadates (probably rafts more than chains), although isolated vanadyls are also certainly active. 1. G. Busca, G. Centi, L. Marchetti and F. Trifirb, Langmuir, 2 (1986) 568. 2. C. Cristiani, P. Forzatti and G. Busca, J . Catal. 116 (1989) 586. 3. G. Ramis, C. Cristiani, P. Forzatti and G. Busca, J. Catal. 124 (1990) 574.

Ch. Schild (Bayer AG, Leverkusen, Germany) : The problem arising from the decrease in IR transmission of the Mg-vanadate catalyst after contact with butane at 790 K (cf. Fig. l), i.c. the impossibility to detect any species adsorbed at the surface, can be overcome by performing FT-IR measurements in diffuse reflectance geometry (DRIFTS). In addition, by applying this technique, it is easily possible to take spectra of the catalyst surface under reaction conditions (in-situ). Thus, valuable informations are provided not only about the final products adsorbed at the catalyst surface, but also about the present reaction intermediates of the catalytic reaction pathways. G. Busca : The DRIFT technique has not advantage with respect to the transmission technique if the sample opacity is due to absorption, but only if opacity is due to scattering (this is not our case). In general, we prefer to work with the transmission technique if scattering is sufficiently small because it allows better sample activation by outgassing and (in our experience) better spectra are recorded. In any case, the lack of detection of adsorbed species on Mg-vanadate after contact with n-butane is not due to the loss of transmission. In fact this phenomenon occurs slowly at 790 K and upon this phenomenon (so when transmittance is still sufficient) no adsorbed species arc detected. This is due to the fact that in these conditions the rate of intermediate translormation and desorption is faster than the rate of their formation. So, their concentration is nil. This is just what happens for in-situ studies that can be easily performed both using the transmission/ absorption technique and the diffuse reflectance technique. In the reaction conditions, in fact, the concentration of both intermediates and products at the surface is generally very small or zero, and they are frequently not detectcd at all by in-situ experiments. T o have a significant concentration of intermediates and/or products and to detect them you must work at lower temperatures with respect to the real catalytic conditions. For this reason

262

we do not think that in-situ experiments are really very informative on the mechanisms of heterogeneously-catalyzed reactions. They generally only show spectator species. J. Haber (I. Catalysis and Surface Science, Krakow, Poland): It seems to me that one should be very cautious in relating intermediate species seen in infrared spectra with the mechanism of the catalytic reaction and the conclusion that in all three investigated catalytic systems the mechanism of butane activation is the same may be dangerous. The oxyhydration is not the only way to split the C-C bond and to form acetaldehyde and acetic acid, and oxidation of butane to rnaleic anhydride must not necessarily proceed through butene and butadiene. The lack of bands of some intermediates cannot be taken as the proof that the given steps proceed very fast. More detailed comparative in-situ infrared and catalytic studies are needed to formulate the mechanism of reactions at different catalytic surfaces. G. Busca: The results summarized in this communication take into account an extensive series of experiments as well as the results of catalytic experiments performed on the same catalysts in other laboratories. The agreement is apparently very good and, in our opinion, definitely supports our conclusions. I agree that other mechanisms could in principle be proposed, but those w e have proposed here are based on our exprimental evidence for V-based catalysts. On other catalysts we provided evidence of different mechanisms (1,2). In particular, all stages of the reaction of different olefins on transition metal oxide catalysts giving rise to the oxidative cleavage of the C=C double bond have been detected directly (3-5). It seems obvious to us that, to detect an adsorbed species, you should chose conditions where this species is stable in the adsorbed form. If you do not observe it, this indicates that this species is not stable in the adsorbed form in the given conditions. On Mg-vanadates, that are very poorly active, to produce the intermediates you need so high a temperature that the resulting species are even faster transformed and desorbed. As for exemple, oxygenated species are observed at lower temperatures in the adsorbed form, but are already desorbed totally at 790 K. A fortiori these species cannot be observed in-situ.

1. V. Sanchez Escribano, G. Busca, V. Lorenzelli and C. Marcel, in "New developments in selective oxidation by heterogeneous catalysis", P. Ruiz and B. delmon eds., Elsevier, Amsterdam, 1992, p. 335. 2. G. Busca, V. Lorenzelli, G. Ramis and V. Sanchez Escribano, in "New frontiers in catalysis", L. Guczi, F. Solymosi and P. Tetenyi eds., Elsevier, Amsterdam, 1993, p. 2661. 3. V. Sanchez Escribano, G. Busca and V. Lorenzelli, J. Phys. Chem. 94 (1990) 8939. 4. V. Sanchez Escribano, G. Busca and V. Lorenzelli, J. Phys. Chem. 94 (1990) 8945. 5 . G. Busca and V. Lorenzelli, J. Chem. Soc. Faraday Trans. I, 88 (1992) 2783. J.C. Volta (IRC, Villeurbanne, France): I think that it should be important to have now a clarification of the three VTiO, VPO and VMgO on their reducibility and to have a comparison with their acid-base properties. Indeed, both redox and acid-base properties are important for the alkane mild oxidation. G. Busca: I agree with you. However, I would like to comment that both acid-base and redox

263

properties o f a catalytic system are an effect of its chemical composition. So, for strictly related systems, they are not independent properties. E. Bordes (UTC, Compikgne, France): How you describe your "alkylic" C-H activation site ( l a ) ? To my opinion it would be an oxygen linked with V4+. But you find very little nucleophilic species. It could be you performed your experiments in theabsence of oxygen. G. Busca: My idea is that activation of C-H bonds occurs through an interaction of the CJ and CJ*C-H orbitals with empty and occupied d orbitals of the active cation, respectively. If nucleophylic sites were substantially involved, the more active catalysts in C-H activation should be the more nucleophylic ones, and this is apparently just the reverse of the observed trend. On the contrary, it seems that the activity in C-H bond activation on this type of catalysts depends on the Lewis acidity of the active V ion. So, it is not simply related to Lewis acidity, but on the Lewis acidity of the oxidizing ion.

G.L. Schrader (Iowa State University, Ames, Iowa, USA): In-situ experiments have shown that P/V ratios and water content can affect the intermediates involved on the surface. Would your experiments be consistent with an understanding acid-base propei-ties of these materials and the larger number of surface species you have observed.

G. Busca: As already said, I do not think that in-situ studics are the best way to investigate reaction mechanisms, because in reaction conditions spectator species are mostly detected. A careful analysis of the published in-situ studies strongly supports this view. In any case, water should certainly have a role, and this could be studied in more detail in the case of VPO catalysts. With respect to this system, we only studied one very active catalyst, with a P/V ratio of 1.1. F. Trifiro (University of Bologna, Italy): In the paper you presented. you have characterized the acidity of different catalysts. Did you observed correlations between acidity (type and strength) with the step of activation of paraffins '? G. Busca: Yes. The activity in alkane activation correlates well with the Lewis acidity of the active vanadium rcdox sites: the stronger the Lewis acidity of vanadium surface sites, the more active is the catalyst in alkane activation.

This Page Intentionally Left Blank

V. CortBs Corberin and S. Vic Bellon (Editors), New Deveiopments i n Selective Oxidation I! 0 1994 Elsevier Science B.V. All rights reserved.

265

New reaction: n-Butane direct catalytic oxidation to tetrahydrofuran V.A. Zazhigalov, J. HabeP, J.Stochb,G.A.Komashk@, A.I.Pyatnitskayaa and 1.V .Bachenkovaa ahstitute of Physical Chemistry, Ukrainian Academy of Sciences, Prospekt Nauki 3 1, Kiev 39. 2520039 Ukraine "Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek, Krakow, 30-239 Poland

Abstract The posibility of direct catalytic n-butane-to-tetrahydrohrane (THF) oxidation has been demonstrated. Influence of the catalyst composition on the selectivity of the reaction was investigated and its mechanism suggested.

1. INTRODUCTION

At present the most promissing THF process is based on n-butane oxidation to maleic anhydride ("Standard Oil" or "DuPont") followed by three step maleic anhydride hydrogenation to 1,4-butanediol and dehydration of the latter (technology " D a y McKee") [ 11:

Manufacturing of THF by direct n-butane oxidation is an attractive alternative. Theoretically, the possibility of THF formation can be deduced from the mechanism of n-butane conversion in the presence of vanadium-phosphorus-oxide catalysts, proposed in ref [2]. This mechanism assumes that first step of the alkane activation consists in proton abstraction from the end CH3 groups. A similar n-butane oxidation mechanism was suggested on the basis of analytical data by the authors of ref. [3,4]. Subsequently, ring closure with surface oxygen atom may take place and THF may be formed. On the other hand quantum chemical calculations [5] of the energy changes on approaching butane molecule to the cluster of vanadium-oxygen polyhedra show, that the result of the reaction may critically depend on the orientation of the approaching butane molecule in respect to the plane of the cluster: parallel approach resulting directly in total oxidation, whereas side-on perpendicular orientation in the case of boat-conformation may lead to simultaneous abstraction of 2 or 4 hydrogen atoms and formation of a dialkoxy-species, which may then desorb either as tetrahydrohran or, in the presence of electrophylic oxygen, as maleic anhydride.

266

However, there are no literature reports to date of the appearence of THF as a product of this reaction. This can be ascribed either to a very high rate of subsequent tetrahydrofuran oxidation or prevailence of the direct oxidation of butane to maleic anhydride.

2. EXPERIMENTAL

Vanadium phosphorus-oxide catalysts (V-P-0 and V-P-Me-0) were prepared by precipitation in an organic solvent [6].A lanthanum additive was introduced by two procedures: (A) addition of lanthanum salt to the solution in the synthesis of V-P-Me-0 catalyst and (B) impregnation of the synthetized V-P-Me-0 compound. The catalysts were used as pellets 4 mm x 1.2 mm x 5 mm. The oxidation of n-butane and tetrahydrofuran was carried out in a flow unit [7].Hydrocarbon -02-He mixtures were used. Products were analyzed by gas-chromatography and mass-spectroscopy methods [S]. The catalysts were examined by X-ray, X P S and SEM techniques [7]. a RESULTS AND DISCUSSION 3.

The investigation of n-butane oxidation on V-P-0 and V-P-Me-0 catalysts revealed that maleic anhydride was the only partial oxidation product comprising four carbon atoms. No tetrahydrofuran could be detected among the reaction products. Analysis of the data concerning the D2-C4H10 isotopic exchange on different oxides indicates that on lanthanum oxide this exchange predominantly proceeds at the terminal CH3 groups [ 9 ] . It could have been thus suspected that addition of this oxide would enhance such activation of C4H10 which favours its direct transformation into firane-type intermediate indeed. The formation of tetrahydrohran during n-butane oxidation on the V-P-Me-La-0 catalyst was established by gaschromatographic analysis. The presence of THF in the products is also confirmed by massspectrometric analysis (Figurel).

:I

1

111

I

I I

I

II

1 I

I

I

I

Figure 1. Mass spectra of products in the n-butane oxidation on V-P-Me-0.05La-0 (1) and V-P-Me-0 (2) catalysts

261

The selectivity of tetrahydrohran formation increases on decreasing oxygen concentration and rising paraffin content in the reaction mixture (Table 1). Table 1 Conversion of n-butane and selectivity to tetrahydrofuran as function of the contact time and composition of the reacting mixture. (Catalyst V-P-Me-0.05La-0) Reagents concentration vol%

Contact time

C4H10

02

S

1.5 1.5 1.5 1.5 2.2 3.2 2.2 2.2

20.5 11.2 5.3 3.3 5.3 5.3 5.3 5.3

2.4 2.4 2.4 2.4 2.4 2.4 1 .o 0.6

n-Butane % conversion

THF O h

selectivity 0 1 4 6 8 6 10 13

56 53 48 40 37 33 22 16

In order to elucidate the role played by lanthanum in THF formation and to find whether it diminishes the probablility of subsequent THF oxidation or favours an appropriate paraffin or oxygen activation, a study of THF oxidation on the lanthanum-doped catalysts was undertaken. The data summarized in Table 2 indicate that introduction of lanthanum into the catalyst sharply increases the conversion of THF. Its contribution to the process of n-butane oxidation must be therefore related rather to the increase of the probability of specific activation of nbutane molecule at the end CH3 groups or to the generation of reactive oxygen species located in appropriate positions. Table 2 Oxidation of tetrahydrofuran on V-P-Me-O and V-P-Me-0.05La-0 catalysts* Reaction temperature OC

300 330 350 380

Tetrahydrofuran conversion,% V-P-Me-0 V-P-Me-O.OSLa-0 10 22 49 82

23 47 88 99-100

Reaction mixture: 0.81 vol% of THF in air Investigation of the properties of catalysts revealed that introduction of small amounts of lanthanum practically does not change the binding energy of P 2p and V 2p electrons (Table 3 ) , but with increasing lanthanum concentration (LaN > 0. I ) the EB of V 2p electrons begins to rise, indicating the oxidation of superficial vanadium atoms; it is noteworthy that the phase composition of the catalysts remains unchanged, all samples being essentially (VO)2P2O7. At the same time introduction of lanthanum into the catalyst independently of its concentration results in a decrease in the binding energy of 0 1s electrons, which signals an increased effective negative charge of oxygen atoms at the sample's surface.

268 Table 3 Binding energies of electrons of elements in catalysts with different lanthanum content Catalysts v-P-0 V-P-Me-0 V-P-Me-La-0 (LaN=0.03) V-P-Me-La-0 (LaN=0.05) V-P-Me-La-0 (LaN=O.lO) V-P-Me-La-0 (LaN=0.20)

La3d

Binding energy, eV P2p V2p 01s

836.5 836.6 836.8 836.7

133.9 134.0 134.0 133.9 133.9 134.1

517.9 517.8 517.9 517.9 518.5 518.9

532.5 532.4 531.8 531.7 531.7 531.8

Selectivity % 0 0

8 10 8 2

The activity of V-P-Me-La-0 catalysts in n-butane oxidation is superior to that of V-P-0 and V-P-Me-0 samples and somewhat increases with nsmg lanthanum content. The amount of THF found in the products attain a maximum at the ratio LaN=0.05 when vanadium atoms on their surface remain still mainly in tetravalent state, but drops on further increasing the lanthanum content, when +5 vanadium atoms begin to dominate at the surface. Raising the vanadium oxidation state (increase in the binding energy of V 2p) leads to the disappearance of THF from the reaction products. In line with the known structure of (VO)2P2O7 and the proposed mechanism the scheme of C4H10 oxidation can be represented as follows:

H3C -CH2-CH2 -CH3 A

0

0

Increase negative charge of oxygen atoms is conductive to the abstraction of proton which is retained on a double-bonded vanadyl oxygen. Negatively charged carbon fragment is bonded to the bridging oxygen (@). This step is followed by the desorption of the reaction intermediate with simulatneous ring closure and tetrahydrofuran formation. Rise in the degree of vanadium oxidation (+5) observed at higher lanthanum concentration entails a much greater electron density transfer from @ to vanadium and correspondingly from hydrocarbon fragment to @;

269

hence the hydrocarbon fragment-catalyst surface bond becomes stronger which increases the residence time of the intermediate and makes its fitrther oxidation to maleic anhydride more probable. THF is not observed any more in the products. This scheme assumes tetrahydrofuran formation through desorption of hydrocarbon fragment from the surface of the catalyst with cleavage of one of the V...@ bonds and incorporation of the other o b in the cycle. In such a model the role of lanthanum would be reduced solely to increasing the effective negative charge on oxygen that promotes the specific activation of the parafiin. However, we have previously demonstrated that the effective negative charge is increased also on introducing alkali and alkali-earth metals into a V-P-0 catalyst [ 5 ] . Nevertheless, no tetrahydrofitran is formed on these catalysts during n-butane oxidation. The specific character of the effect which lanthanum has on THF formation was supported by experiments with V-P-Me-La-0 samples having the same lanthanum content but obtained by different methods (Table 4). Table 4 Electron binding energies and elemental ratio on the surface of the V-P-Me-La-0 catalysts prepared by different methods. Catalysts

Bulk Ratio Binding energy, eV (LW, V2P P2P 01s

Surface Ratio (PW, (LW,

A B

0.05 0.05

1.61 0.36

517.8 134.0 517.9 133.9

531.7 531.7

0.04 0.47

b - bulk. s - surface

As seen from Table 4, the samples A and B exhibit virtually the same binding energies of V 3p, P 2p and 01s electrons (Eb of La 3d is 836.7 eV), i.e. the degree of vanadium reduction and the effective negative charge of oxygen are similar. Also close turned out to be the values of n-butane oxidation rate (the difference does not exceed 10%). However, tetrahydrofuran was detected solely in the case of sample B. Data in Table 4 indicate, that this catalyst is characterized by a high superficial lanthanum concentration [(LaN),] as compared to sample 4 this concentration being much higher than the bulk one [(LaNk]. Conversely, in catalyst A a unlform lanthanum distribution in the bulk of the sample is observed. The elevated lanthanum content on the surface is paralleled by considerable decline in superficial PN ratio. This may signify either that lanthanum is substituted for phosphorus to form surface structures of the lanthanum vanadate type or that on the surface exist fragments of lanthanum oxide monolayer, as was the case with the phosphate films we detected previously [7]. Examination of catalyst A and B by X-ray and SEM techniques has so far not enabled to provide an unequivocal answer to this question. A hypothesis could be thus advanced that the adsorbed hydrocarbon fragment is located in such an orientation in respect to the vanadium plane that the cyclisation can be completed by oxygen, bonded to lanthanum from an adjacent fragment of the monolayer:

210

.I lCH2 - CH2 \

0

0

In conclusion, a possibility of producing tetrahydrofuran via direct n-butane oxidation has been revealed and the mechanism of the reaction and the role played by the surface of the catalysts have been suggested.

Acknowledgment This work is funded by Ukraine‘ s State Committee for Progress in Science and Technology (Basic Research Fund No 3/132). 4. REFERENCES

1. Hams M. and Tuck M.W., Hydrocarbon Process, No 5 (1990) 79. 2. Pyatnitskaya A.I., Komashko G.A., Zazhigalov V.A., Belousov V.M.,

Batcherikova I.V., Seeboth H., Lucke B., Wolf H. and Ladwig G., All-Union Cot?$ on Mechan.Catal. React. ( R i m . ) , Moskva, Nauka, 1 (1978) 286. 3. Ziolkowski J., Bordes E. and Courtine P., New L)evelop.Selective Oxid., Amsterdam, Elsevier, 1990, 625. 4. Ziolkowski J., Bordes E. and Courtine P., J.Catal., 122 (1990) 126 5 . Haber J., Tokarz R. and Witko M., Proc.EUROPACAT-I, Montpellier 1993. 6 . Zazhigalov V.A., Komashko G.A., Pyatnitskaya A.I., Belousov V.M., Stoch J. and Haber J., in Preparation of Catalysts I , Proc.5-th Intemat..p’S Scient.Bases Prep. Catal., Loiivain-la-Neiive, Sept.3-6, 1990, Ed. G. Poncelet, P. A. Jacobs, P. Grange and B. Delmon, Elsevier, Amsterdam, 1991, p. 497. 7. V.A.Zazhigalov V.A., Belousov V.M., Komashko G.A., Pyatnitskaya A.I., Merkureva Y.N., Poznyakevich A.L., Stoch J. and Haber J., Proc.9-th Congr.Catal., Calgary Canada 1988,4 (1988) 1546. 8. Zahigalov V.A., Shabelnikov V.P., Golovaty V.G. et all. , Teoret.Exp.Khimia( R i m . ) 28 (1992) 159. 9. Bird R., Kemball C. and Leach H.T., J. Catal., 107 (1987) 424.

V. Cortes Corberin and S . Vic Bellon (Editors), New Developments i n Selective Oxidation II 0 1994 Elsevicr Science B.V. All rights reserved.

21 1

Oxidation and ammoxidation of propane over tetragonal type Ms+OP04 catalysts h y a Matsuura and Naomasa Kimura Faculty of Science, Toyama University, Toyama 930, Japan

SUMMARY Oxidation and ammoxidation of propane over tetragonal type M5+OP04 catalysts were studied. The selectivity of catalysts for oxidative dehydrogenation of propane to propylene decreases in the order of V > Mo > Nb > Ta. However, the ratios of acrolein in the C3compound produced by oxidation and acrylonitrile in the C3-compound produced by ammoxidation decreases in the order of V > Mo > Nb > Ta. The catalytic activities of M 5 + 0 P 0 4 depends on the Ms+=O= strength in the crystal. From these results a surface reaction model is suggested which refers to the cleavage (001) surface plane of M5+OP04.

1. INTRODUCTION The synthesis of acrolein through propylene oxidation and of acrylonitrile through ammoxidation are important from an industrial viewpoint. In recent years, efforts have been made to synthesize acrylonitrile using propane instead of propylene. Many patents have been reported in connection with this. Brazdil et al.[l] proposed a V-Sb type multi-component oxide as a propane ammoxidation catalyst, and Seely and Friedrich [2] proposed a modified Bi-Mo multi-component oxide, which is a propylene ammoxidation catalyst. Umemura [3] reported that ammoxidation of lean propane produces acrylonitrile with high selectivity using a V-P-0 mixed oxide .Centi et al. [4]discussed the ammoxidation of propane with very large excesses

of oxygen and ammonia over (VO)2P2O7 which is known as an active compound in maleic anhydride synthesis from butane oxidation. However, (VO)2P2O7 did not act as a catalyst in the acrylonitrile formation. Whereas, Brazdil et al. [S] found that ammoxidation of rich propane with a V-W-P-0 mixed oxide produces propylene with high selectivity. We studied oxidation and ammoxidation of rich propane over orthophosphates of the tetragonal type (space group P4/n) Ms+OP04 each comprising a pentavalent metal of V, Mo, Nb and Ta.

212

2. EXPERIMENTAL a-VOPO4 was prepared by the following procedure. VOP04.2H20 produced by adding V2O5 to a H3P04 solution was dried and dehydrated at 600°C. Also, (v0)2P207 was prepared according to the literature [6]. V2O5 was added to an aqueous solution of NH20H.HCl and H3P04, and the mixture was stirred at 80°C until the V2O5 was completely reduced. Next, the solution was evaporated at 170°C and the dried solution was washed with hot water until hydrocloride was removed. This material was calcined at 500°C for two hours. The tetragonal type Ms+OP04, with the isostructure as a-VOPO4, was prepared by the following procedure. A mixture of equimolar amounts of H2MoO4 and 85%-H3P04 was calcinated at 1000°C for 20 minutes to produce MoOPO4. Niobic acid was mixed with 85%H3P04 and calcinated at 800°C for three hours to produce NbOP04, Tantalic acid was mixed with 85%-H3P04 and calcinated at 800°C for three hours to produce TaOP04. The XRD patterns of these compounds are shown in Figure 1. Comparing the XRD results with literature values [7] confirmed that tetragonal Ms+OPO4 had been produced. IR and Raman spectra of these materials are also shown in Figure 2. Oxidation and ammoxidation of propane were carried out in a conventional fixed bed reactor from 400 to 520°C. Mixtures of gases of C3Hg : 0 2 = 2 : 1 for oxidation and C3Hg : 0 2 : NH3 = 3 : 2 : 1 for ammoxidation were passed over the catalyst at a flow rate 30 ml per

minute.

3. RESULTS AND DISCUSSION The results of oxidation and ammoxidation of propane at 500°C using 2g of catalyst are shown in Tables 1 and 2. The orthophosphates of M5+OP04 have a catalytic activity about twice that of (VO)zP207. As for total selectivity for C3-compounds such as propylene and acrolein produced in the oxidation of propane or propylene, acrolein and acrylonitrile produced in the ammoxidation of propane, M5+OP04 has a higher selectivity compared to (VO)2P2O7. Selectivities for C3-compounds produced in propane oxidation over the M5+OP04 catalyst increases in the order of V < Mo < Nb < Ta. The ratio of acrolein in oxidized C3-compounds, however, decreases in the order of V > Mo > Nb > Ta. On the other hand, in the ammoxidation of propane, the selectivity for producing C3-compounds did not differ as much depending on the pentavalent metals. However, the ratio of acrylonitrile produced in C3-compounds decreases in the order of V > Mo > Nb > Ta.

273

20

30

40 50 2 0 (degree)

Figure 1. XRD pattern of Ms+OPOd catalysts. a,-VOP04 ; d (A) = 4.38, 4.1 1, 3.105, 2.998, 2.12 by E. Bordes, Catal. Today, 1 (1987) 499. MoOPO4 ; d (A) = 4.37, 4.29, 3.53, 3.09, 3.06, 2.32, 2.18, 2.15, 2.03, 1.95, 1.93 by P. Kierkegaard and M. Westerlund, Acta Chem. Scand., 18 (1964) 2217. NbOP04 ; d (A) = 4.51, 3.45, 3.19, 3.04, 2.34, 2.25, 2.05, 2.02, 1.89 by J. M. Longo and P. Kierkegaard, Acta Chem., 20 (1996) 72. TaOPO,; d (A) = 4.54, 4.00, 3.396, 3.21, 3.00, 2.505, 2.334, 2.27, 2.03, 1.975, 1.91 by J. M. Longo, J. W. Pierce, A. Kafalas, Mat. Res. Bull., 6 (1971) 1157.

274

& 1

Raman Spectra

m &

1500 1000 v(cm-1)

Figure 2.

500

1: 1

u-VOPO4

80 0

1000

v (crn-1)

IR and Raman spectra of M5+OP04 catalysts.

Table 1 Catalytic activity of M5+OP04for propane oxidation Catalyst

Conversion

AL

P= 10.0 12.5 23.8 33.3 43.1

4.3 8.5 8.8 10.1 10.5

(vO)2p207 a-VOPO4 MoOP04 NbOPO4 TaOP04

AL/P=+AL

Selectivity (%)

("/.I

26.4 28.5 22.8 24.1 23.5

p=+AL 36.4 41.0 46.6 57.4 66.6

(%)

72.5 69.5 48.9 42.0 35.3

Table 2 Catalytic activity of M5+OPO4 for propane ammoxidation Catalyst

Conversion (%)

Selectivity (%) P= 27.4 19.2 31.1 49.3 62.3

AL

0.5 6.6 8.6 2.1 3.1 10.8 2.3 11.6 10.6 3.8 P=; Propylene, AL;Acrolein, AN;Acrylonitrile (vO)2p207 a-vop04 MoOPO4 mop04 TaOP04

AN/P'+AL+AN

AN

P=+AL+AN

(%)

43.8 43.5 36.4 28.8 22.4

71.5 64.8 70.6 81.4 88.5

61.3 67.1 51.6 35.4 25.3

275

It is very probable that propane is initially dehydrogenated to propylene and then ammoxidized to acrylonitrile from acrolein through an allylic intermediate by the following steps. [I]

CH3CH2CH3

+

OL

[II]

CHzzCHCH3

+

2 0 + ~ CH2=CHCHO

+

H2O

[111]

CHz=CHCHO + NHA + CH2=CHCN ( NH3 + OL -+ NHA + H 2 0 )

+

H20

+ CH2=CHCH3 +

H20

During the oxidation of propane to propylene [I], it is important that hydrogen abstraction from propane occurs at the first step. A strong acid site on Ms+OP04 extracts a hydrogen anion, H-, from propane to produce a carbenium ion, CH3C+HCH3, as an intermediate and then propylene is formed by its dehydrogenation.

CH3C+HCH3

+

H-

+

CH2=CHCH3

+

H2

O n the other hand, if M5+OP04 forms a surface oxygen radical with oxygen adsorption,

0-,the surface oxygen radical abstracts a hydrogen atom from propane to form an alkoxide with a surface O=ion of the catalyst.

The crystal structure of a series of M5+OP04 may be described as consisting of chains of comer-shared M5+O6 octahedra running parallel to the c-axis. The chains are coupled by tetrahedra of PO4, so that every M5+O6 octahedron shares comers with four phosphate tetrahedra, each of which shares corners with four octahedra giving a three dimensional network. The schematic drawing of Figure 3 show the linking of octahedra and tetrahedra. The MS+-O bond lengths along the c-axis are 1.79-1.58 A, which also have double bond oxygen character and are 2.22-2.85 A long as listed in Table 3.

216

Table 3 Structural comparison of tetragonal type M5+0P04 compounds Compound Cell parameters M.5+=0= a-vop04 MoOP04 mop04 TaOP04

a(& 6.01 6.18 6.39 6.43

c(A) 4.27 4.29 4.10 4.00

V(A3) 160.4 163.8 167.4 165.2

M.5+-0=

(A)

(4

1.58 1.65 1.78 1.79

2.85 2.68 2.32 2.22

The oxidation and ammoxidation of propane by the tetragonal type M5+OP04 are supposedly carried out on the cleavage of the (001) plane. The (001) surface plane is constructed with coordination-unsaturated M5+ ions and with double-bond oxygen ions alternately combined with M5+ ions as shown in Figure 3. The supports the conclusion that double-bond oxygen ions have an important role in the oxidation of propane. The force constant of the bond between a double-bond oxygen ion and a metal ion was estimated from the wave number of MS+=O=vibration. The force constant f is shown in the following equation.

where V is the wave number, c is the velocity of light and p is the reduced mass of the M - 0 oscillator in g units. From IR and Raman spectroscopy of Ms+OP04, a frequency observed at 900-1000 cm-l is assigned to the MS+=O= stretching. Table 4 lists the force constants of M5+=O' calculated from the frequency of the IR and Raman spectra of M5+OP04.

Table 4

Force constant of Ms+=O=in Ms+OPO4 compounds Catalyst v M=O Force constant M5+=O= ~-VOPO~ MoOP04 NbOP04 TaOP04

( cm-1)

( md/A 1

902.2 928.1 900.9 980.0

5.84 6.96 7.09 8.49

211

Figure 3. The structure of M5+OP04compound. (a) Schematic drawing showing the links between PO4 tetrahedra and MO6 octahedra viewed along (001) in the structure of M5+0P04. (b) Schematic drawing showing the chains formed by MO6 octahedra linked together by sharing comers and also showing the links PO4 tetrahedra and M 0 6 octahedra. (c) Surface reaction model on the cleavage (001) plane M5+OPO4.

278 The order of the force constant of MS+=O=is V < Mo < Nb < Ta. If the oxidation ability of catalyst depends on the Ms+=O=bond strength, the oxidation ability should be V > Mo > Nb Ta. It was clarified that the oxygen additive reaction [II] of propane into acrolein and the ammoxidation reaction [111] of propane to acrylonitrile are controlled by a double bond oxygen in Ms+OP04. The weaker double-bonded oxygen in Ms+OP04 has a higher reactivity for reactions [11] and [III]. Reaction schemes of the oxidation and ammoxidation of propane are shown in Figure 3.

4. REFERENCE

1. J.F.Brazdil,Jr., M.A.Toft and L.C.Glaeser, US Patent 5,008,427 (1991) , assigned to stangard Oil Co.,Ohio. 2. M.J.Seely and M.S.Friedrich, US Patent 4,978,764 (1990), assigned to Standard Oil Co.,Ohio. 3. J.Umemura, Jap. Patent 77, 148, 022 (1977), assigned to Ube Kosan Co. 4. G. Centi, D. Pesheva and F. Trifiro, Appl. Catal., 33 (1987) 343. . 5. J.F.Brazdil,Jr., M.A.Toft and L.C.Glaeser, US Patent 4,918,214 (1990), assigned to stangard Oil Co.,Ohio. 6. M.Ohotake, M.Murayama and Y.Kawaragi, Jap. Patent S56-45815 (1981), assigned to MitsubishiChem. Ind. 7. E. Bordes, Catal. Today, 1 (1987) 499. P. Kierkegaard and M. Westerlund, Acta Chem. Scand., 18 (1964) 2217. J. M. Longo and P. Kierkegaard, Acta Chem. Scand., 20 (1966) 72. J. M. Longo, J. W. Pierce, A. Kafalas, Mat. Res. Bull., 6 (1971) 1157.

219

G. CENT1 (University of Bologna, Italy): In a recent paper on the propane ammoxidation of (VO)2P2O7 (J. Catal., 1993), we reported different results. We observed in fact that after a preadsorption of ammonia on the catalyst, the initial activity and selectivity to acrylonitrile was l o w , but then the rate of propane depletion progressively increase as a function of the removal of adsorbed ammonia species from the catalyst surface. The selectivity to acrylonitrile instead, gases through a maximum as a function of the surface coverage with ammonia species. The catalytic behavior when propane, 0 2 and N H 3 were covered, was similar to the initial behavior after preadsorption of N H 3 . This suggests that the catalytic behavior of (VO)2P2O7 is considerably affected from the amount and relative surface population of adsorbed species (hydrocarbon, oxygen, ammonia) and not only from the surface properties of a cleavage plane of a crystalline phase. The question is therefore if you believe that the different concentration of adsorbed species. I. MATSUURA (Toyama University, Japan): We think propane oxidation and ammoxidation progress on the (100) plane of (VO)2P2O7, On the surface plane of (100) there is not only V=O double oxygen ions also V ion CUS (coordinated unsaturated site) and P = O ions. For the propane ammoxidation, these surface sites may act selectivity or non-selectivity for the formation of acrylonitrile. Different concentration of adsorption species, specially ammonia, might be important f o r the formation of acrylonitrile. J . C . VOLTA (Insitut d e Recherches sur la Catalyse, France): There are t w o phases of VOPO4, a1 and a ~ What ~ . structure did you u s e in your experiments. I. MATSUURA (Toyama University, Japan): We prepared a1 type of VOPO4 and used it as a catalyst. B . DELMON (Universite Catholique d e Louvain, Belgium): You should be cogratulated to highlight the M=O double bond (dioxo). The double bonded oxygen is the key to oxygen insertion in allylic oxidation, and in ammoxidation, as show by the chemical mechanisms of Grasselli and Brrazir, and the theoretical calculation of Goddard. In addition, Goddard emphasize double O = M bonds vicinal to the reacting dioxo ("specious dioxo"). Are there, in the structure of the compounds your considerate basis (e.g. short, electron rich 0 - M bonds) which could play the more role of "electron reserves" o r petulated by Goddard ? A second problem is the stability of the these reactive M=O to be regenerated by reoxidation after oxygen insertion. Reoxidation is usually s l o w , this leading to the creation of a reduced state of the surface in the steady states. I write that, in the formula of the multicomponent catalysts that mentioned, oxygen "Doners" are proposed: S b (presumably as Sb2O4), C u , Ag, etc. One would expect this non-promoted catalysts you tested would may be protected against reduction, and thus reduce. Did you check the oxidation /reduction state of your catalysts after catalytic reaction, I. MATSUURA (Toyama University, Japan): Surface reaction plane of (001) on M5+OPOq is constructed by two sites which one is M 5 + = O double bonded oxygen ions and the other is anion vacancy on M5+ ion ( C U S ) . We propose that the M 5 + = 0 double bonded oxygen ion is affected by bond strength and MS+-CUS might be affected by electron configuration for the propane oxidation and ammoxidation. After the reaction, catalysts we found reduced stste of catalyst as you suggested that. We would like to study of the propane ammoxidation on the mixture catalysts with your "Doners" oxide such as Sb2O4.

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V. CortCs Corhcrin and S. Vic Bcllon (Editors), New Deveiopments i n Selective Oxrdation 11 0 1994 Elsevier Science B.V. All rights reserved.

28 1

Structure and Stability during the Catalytic Reaction of Unsupported V-Antimonate Catalysts for the Direct Selective Ammoxidation of Propane to Acrylonitrile Gabriele Centi, Elisabetta Foresti* and Francesco Guarnieri Dept. of Ind. Chem. & Materials, V l e Risorgimento 4,40136 Bologna (Italy), Fax:+39-51-644-3680 * Dept. of Chemistry "G.Ciamician " and CSFM (CNR), Via Selmi 2, 40126 Bologna (Italy)

V-antimonate catalysts prepared by redox reaction in a solution containing both V5' and Sb3+ and heat treated in air or under vacuum were characterized by chemical analyses of the V and Sb valence state and infrared spectroscopy, X-ray diffraction analysis and scanning electron microscopy before and after the catalytic tests. The results suggest that the V-antimonate rutile phase is a mixed valence oxide containing V3'and ?+with a relative ratio that depends on the preparation conditions, but which evolves to a similar situation (characterizedby a V3+:v4+ratio of around 1:3) after the catalytic tests of propane ammoxidation. Some aspects of the stability of this phase during the catalytic reaction are also discussed.

1. INTRODUCTION There is increasing interest in the development of a process for direct acrylonitrile synthesis as an alternative to the conventional method based on the olefinic feedstock C1,23.Multicomponent metal oxide catalysts based on V, Sb, W, Mo and other elements have been patented for this reaction, but V-antimonate based catalysts seem to be the most promising [3-5and references therein]. Notwithstanding the interest in this catalytic system, very few data on the structural, surface and reactivity characteristics of this system are available in the literature. In particular, important problems not investigated in the literature are the structure of the V-antimonate active phase and its stability during the catalytic reaction. The VSb04 phase, in fact, has a rutile structure [6,71 and a "formal" valence state of three for V and five for Sb, but only indirect data exist about the real valence state of these elements in the V-antimonate rutile phase (especially of V). In addition, various non-stoichiometricrutile phases have been proposed [6,71, but the effect of the preparation conditions and the nature of the changes occurring during the catalytic reaction of propane ammoxidation are not clear. The stability of this phase during the catalytic reaction, especially using a high propane-ammonia to oxygen ratio (condition

282

suggested to be preferable based on kinetic analysis [3,4]) and possible changes during the catalytic reaction are in fact central problems for the development of a process of direct propane ammoxidation. The results of an investigation on the phase composition, structure and valence state of V and of Sb in the V-antimonate rutile phase as well as on the changes after the catalytic tests in propane ammoxidation of unsupported V-Sb-0 catalysts are reported and discussed in the present paper. The samples used in this study were prepared by a redox reaction between N&V03 and SbzO3, according to the modalities reported in the patents [1,21. 2. EXPERIMENTAL

Unsupported V-Sb oxides were prepared according to patent indications [1,21 by a slurry method which involved refluxing for at least 8 hours (usually 24 hours) an aqueous solution containing NI&V03 and Sb2O3 (in such a ratio as to obtain the SbN atomic ratio in the 1-10 range) followed by solvent evaporation in a rotavapour and drying a t 100°C overnight. The solid was then heat treated at 350°C for 4 hours followed by a further step at 500°C (6 hours) (after intermediate grinding and mixing of Tom). the sample) in the presence of air or under vacuum X-ray diffraction (XRD) analysis was carried out using the powder method and a Philips P.W. 1050/80 dfiactometer with CuK, radiation. Ti02 (anatasel added in calibrated amounts was used as an internal standard for quantitative determinations and as the reference for evaluation of the unit-cell parameters. Infrared (IR) analysis was performed using a Perkin-Elmer FT-IR 1750 instrument with the KBr disc technique and calibrated amounts of the samples. The chemical analysis method was developed to determine simultaneously the @+, Sb3+and Sb5+in a mixed oxide taking into account the preamount of V3+,p+, sence of the possible redox reactions between the ions upon dissolution and is based on various preliminary tests to verify the reability and reproducibility of the method [8]. The procedure ofc chemical analysis involves a series of potentiometric redox titrations with Fe2' and KMn04 combined with extraction of v5' in an ammoniacal solution and calibrated addition of @' in some tests in order to prevent the oxidation of V3' upon dissolution of the sample in an aqueous sulphuric acid solution. All details of the method of preparation will be reported elsewhere [81. However, it should ne mentioned that a series of preliminary tests using standard compounds were made in order to find the conditions of dissolution of the catalyst which allow the better agreement between composition determined in solution and in the solid state. The problem of oxidation of V3+, in particular, has been considered by comparing the results of chemical analysis obtained when the sample in dissolved in the presence or not of cal-

2x3

ibrated amounts of V5'. In fact, the reaction between V3' and v5' is more rapid than the oxidation of V3' by dissolved oxygen.

3. RESULTS AND DISCUSSION 3.1 Composition of the Starting Precursor Unsupported V-antimonate catalysts were prepared according to patent indications [1,21 by a redox reaction in a n aqueous ammoniacal solution between ammoniumvanadate and Sb203 (reflux for a t least 8 hours). The chemical analysis of the amount of V5' in solution shows, in fact, that the amount of V5' declines to zero in about 8 hours starting from a V:Sb atomic ratio of 1. In the aqueous solution v5' ions react with Sb3' with a fast two-electron redox reaction forming V3' and Sb5' [91, but the time for the complete reduction of vanadium depends on the slow solubility of Sb2O3 in aqueous solution. Two other chemical reactions compete with this main reaction, namely the redox reaction between v5' and V3' to form two p' ions and the oxidation of reduced vanadium species by the dissolved 0 2 . It is therefore useful to analyze the valence state of vanadium and antimony in P the precursor (a mixed V-Sb hydroxide) obtained after solvent evaporation (in a rotavapour a t 80°C) and further drying a t 110°C for 24 hours. Results show that, for a sample with a V S b ratio of 1.0, all vanadium is present in the valence four state and about half of the antimony is present as Sb3', the remaining being Sb5+. This indicates that the reaction between V5' and V3' to 2V4' effectively competes with the reduction of V5' by Sb3' during preparation and therefore starting from a n equimolecolar V and Sb solution only half of the Sb3' can be oxidized to Sb5'. 3.2 Effect of Heat Treatment

i Jk 0

Fig.1 IR spectra of V:Sb= 1:l samples calcined in air (a)or under vacuum before (b)or after (c) catalytic tests.

At temperatures of around 400"C, the V-Sb mixed hydroxide transforms to the V-antimonate rutile phase, whereas higher temperatures (around 600°C) are required in the case of a mechanical mixture of V2O5 and Sb2O3 [10,113. The formation of V-antimonate phase depends on several factors such a s the rate of redox reaction between ?' and Sb3', the rate of reaction between V and Sb to form the VSb04 phase and

2 84

.. .

10

20

30

40

50

"VSCO," phases 36-1 485 (ASTM) 30-1412 (ASTM)

60

2Theta

Fig. 2 XRD patterns of VSb= 1:l samples calcined in air (a)or under vacuum before (b)or after the catalytic tests (c, only in the inset). In the inset is expanded the region around 28 = 35' and reported some literature values for this reflection (101plane in the rutile structure).

the rate of oxidation of V and Sb during the thermal transformation. The nature of the compound obtained starting from the V-Sb mixed hydroxide precursor is thus affected from the Sb:V ratio and from the atmosphere of thermal treatment. Therefore two samples prepared from the same V-Sb mixed hydroxide with a VSb = 1:l atomic Torr) were studied. ratio, but heat treated a t 500°C in air or under vacuum Compared in Fig. 1 are the infrared spectra in the region of the skeletal vibrations (400-1200 cm-') of these two samples. In the sample calcined in vacuum (spectrum b ) only two main bands are present, namely a t 675 and 545 crn-l, clearly assigned to v1 and v2 vibrations of the antimonate group of VSb04 [12]. Apparently, no other phases are present, contrary to the sample calcined in air (spectrum a ) where the bands indicating the presence of ++-oxide (1010 and 850 em-') and Sb2O3 valentinite (740 and 600 cm'') can be clearly seen. Compared in Fig. 2 are the X-ray diffraction (XRD) patterns of these two samples. In the sample treated in vacuum (diffractogram b), together with the main lines of a rutile V-antimonate phase, only weak lines of a n a-Sb04phase can be detected. The much shorter half-height width of the latter phase indicates the bigger dimensions of the crystals. In the sample calcined in air (diffractogram a), the relative intensity of the lines of the rutile phase is lower and additional weak lines of Sb2O3 (valentinite

285

Table 1 Effect of catalyst composition and type of heat treatment on the composition of the mixed valence VSb04 rutile phase (see text). sb to ratio

Of

Cata[.Tests

Composition

calcination

1

500"C, air

before

v3+0.66 @'

1

500"C, vacuum

before

v3+0.23 @' 0.77 Sb3+ 0.39 Sb5+0.61 O4

2

500"C,air

before

v3+0.57

2

5OO0C,vacuum

before

v3+0.82 @+0.18 Sb3+0.09 Sb5+0.91 O4

1

5OO0C,vacuum

after

v3+0.21 @' 0.79 Sb3+0.40 Sb5+0.60 O4

2

500"C, air

after

v3+0.24

v4'

0.34 Sb3+ 0.17 Sb5+ 0.83 0 4

0.43 Sb3+ 0.21 Sb5+ 0.79 O4

p+0.76 Sb3+0.38 Sb5+0.62 O4

form) can be seen. In both cases lines due to V-oxide phases (V205, V204 and s.o.) are absent. It should be noted that various diffractograms for the "V-antimonate"rutile phase have been reported in the ASTM tables or in the literature (ASTM 16-600, 30-1412, 35-1485, 37-1075 and ref.s [6,7]) with slightly Werent values for the cell dimensions (see also refs [6,71) attributed to the presence of non-stoichiometry and different modalities of preparation. XRD patterns of the various samples are nearly equivalent; in particular, the most intense line at 28=27.35" (110 reflection) falls a t the same position in the various samples, but a marked shift is observed for the reflection at 29=35.6"(101 reflection) in agreement also with a main difference found in the c parameter of the unit-cell dimensions. The region corresponding to this last reflection is expanded in the inset of Fig. 2 and the relative position given in the literature for some "V-antimonate"rutile phases is also reported. The results show that the two VSb=1:1 samples calcined in air (a) or vacuum (b) are different, even though the position of the reflections are inside the range reported in the literature. It also should be noted that the peak for the sample calcined in vacuum is wider suggesting the presence of disorder along the c axis [the half-peak width for the (110) reflection at 27.35", in fact, is the same while that of the (101)reflection is definitely not the same]. The quantitative determination of the relative amount of the rutile V-antimonate phase in the two samples, using a calibrated amount of Ti02 (anatase form) as an internal standard, shows that the amount of V-antimonate phase in the sample treated under vacuum is about three times higher than in the sample calcined in air [(hkl) 101 VSb04 / (hkl) 101Ti02 anat = 0.363 in the sample calcined in air and 1.12 in that treated under vacuum]. The unit cell dimensions (tetragonal cell) also show a difference in the c parameter (3.01 A for the sample calcined in air with respect to 3.07 A for the sample treated under vacuum), whereas the a=b parameter is the same (4.61 A). These values are in agreement with the values reported in the literature [6,71. The chemical analysis of these samples shows that in the sample calcined in air

286

40% of the vanadium is present as ?' that can be extracted using an ammoniacal solution, whereas no V5' is present in the sample treated under vacuum. The chemical analysis of the insoluble residue after this extraction gives the following results: 66% of the vanadium is present as V3' and 34% as p', whereas 38% of the antimony is present as Sb3' and 62% as Sb5'. Assuming that both V3' and v4' are present in a mixed valence V-antimonate phase the chemical composition for the V-antimonate phase reported in Table 1 can be derived on the basis of the charge balance. The amount of antimony exceeding that necessary to form the antimonate phase is present as Sb-oxides (a-Sb2O4 and Sb203 according to XRD analysis). Taking into account that in the a-Sb04the Sb5':Sb3' ratio is 1:1, it can be estimated that 12%of the total antimony is present as a-Sb04 and 28% as Sb2O3. From this result and the quantitative estimation of the amount of Sb04 using Ti02 as an internal standard, it may be estimated that around 5-6% of the antimony is present as a-Sb04 in the sample treated under vacuum. In the sample treated under vacuum chemical f analysis results show that V5' is absent and 23% of the total vanadium is present as V3' and 77% as v4', whereas 39%of the antimony is present as Sb3' and 61% as Sb5+. Thus calcination in air leads t o a higher relative % of V3' in the V-antimonate phase than in the case of samples prepared under vacuum. This may appear to be contradictory, however, the result can be easily explained taking into consideration the presence of a possible redox reaction between and Sb3+to form V3' and Sb5+and that the amount of V-antimonate phase (where V3' is present) is much lower in the sample calcined in air. The composition of the mixed valence V-antimonate phase that can be estimated from these results is shown in Table 1. It should be mentioned that XRD analysis (Fig. 2) shows the presence of about 5 6 % of a-Sb04 (see above) in this compound. In order to justify this observation, the presence of some oxygen 0 vacancies in the rutile phase must be assumed, according to the following composition: Fig. 3 IR spectra of VSb= 1:2 samples calcined in air (a)or ( ~ ~ 3 ~ ~ 7 S b ~ ~ 9 2 S b X ~ 1 - x 0 4-t-(a-Sb204)x 4x)l-x under vacuum (b).Spectrum (c) tests. (where x = 0.05-0.06) which agrees with the change is sample(a) after

287

observed in the unit-cell parameters and the presence of disorder discussed above. The formation of oxygen vacancies in the preparation under vacuum is quite reasonable. Scanning electron microscopy (SEM) data show the presence of macrocrystals of aSb04 and microcrystals of V-antimonate, the latter characterized by a spongy microstructure. EDS microprobe analysis indicates a relatively uniform VSb ratio centred around 1.0 without apparent segregation of V-oxide phases especially in the sample calcined in air. However, the comparison of SEM micrographs of the sample calcined ' with an aqueous ammoniacal solution shows, in air before and after extraction of 9 in the latter sample, the presence of clear holes suggesting that in this sample V5' is present as microdomains embedded in the a-Sb204 matrix due to oxidation of SbzO3 to SbzO4 before the reaction with V5+-oxide. Finally, it should be noted that in the sample treated under vacuum, a consecutive calcination in air for 6 hours at 500°C apparently does not m o d e the above results (for example, no v5' can be extracted by the aqueous ammoniacal solution), but IR analysis shows the appearance of a band centred at 1005 cm-l which suggests the partial surface oxidation of vanadium in this sample.

3.3 Effect of Excess Antimony When an excess of antimony is present with respect to V in the mixed hydroxide precursor, the effect of the heat treatment is different from that discussed above for the V:Sb=1:1sample. In the sample with a V.Sb=1:2 ratio calcined in air, both IR and XRD analyses show the presence, together with the V-antimonate rutile phase, of aSbzO4 and smaller amounts of SbzO3 (in the valentinite form with traces also of senarmontite). The absolute amount of the V-antimonate phase, estimated by XRD analysis using Ti02 as an internal standard, indicates an amount equivalent to that present in the sample with a VSb ratio of 1.0 treated under vacuum (taking into account the different VSb ratio). However, in this case chemical analysis shows that about 16% of the vanadium can be extracted in the aqueous ammoniacal solution as V5' (therefore, much less than in the case of the 1:1 sample, in agreement also with the lower intensity of the IR band at 1010 cm") (Fig. 3, spectrum a). After this extraction, chemical analysis of the residue gives the following results: 57% V3+, 43% 38% Sb3+and 62% Sb5+,from which the composition given in Table 1can be derived for the mixed valence V-antimonate phase. In the case of the sample with a VSb=1:2 ratio treated under vacuum, the results indicate the presence of an additional phase. In fact, the IR spectrum (Fig.3, spectrum b) is characterized by a sharp band a t 980 cm-' attributed to vv=o in a VOz+-cornpound and XRD data show the presence, together with the rutile VSb04 and a-SbzO4

++,

288

phases, of VOSb204 [131. The latter phase probably originates from the following reaction:

Sb04 + V2O3 + Sb2O3 + 2 VOSb204 Therefore, in the presence of excess antimony, the heat treatment in vacuum does not enhance the formation of the rutile VSb04 phase, but rather gives rise to a side reaction. Accordingly, the quantitative determination of the amount of rutile phase indicates an amount around half that found in the case of the equivalent sample calcined in air. Chemical analysis of this sample shows the absence of v5' and gives the following results: 82%V3+, 18% 60% Sb3+and 40% Sb5' from which the tentative composition of the V-antimonate phase given in Table 1 can be derived. However, in this case the contemporaneous presence of VSb04 and VOSb204 does not allow a clear formulation for this phase.

++,

3.4 Nature of the Changes after Propane Ammoxidation

After the catalytic tests in propane ammoxidation, IR spectra of V:Sb=1:1 samples calcined in air or under vacuum are similar and characterized by two main bands at 675 and 545 cm-l [VSbO in V-antimonatel and a weaker band centred at 995 cm-l (Fig.1, spectrum c) suggesting the presence of a small amount of v5' possibly spread on the surface according to the analogy of the position of the band with that observed for v5' on Ti02. XRD data in both cases indicate the presence of VSb04 (in an amount similar to that of fresh 1:l sample treated under vacuum) and a-Sb04(around 6-8% of antimony). Chemical analysis of the 1:l sample treated under vacuum after propane ammoxidation shows the presence of 8% p' that can be extracted in the aqueous ammoniacal solution. Analysis of the insoluble residue gives the following results: 21% V3+, 79% @+, 46% Sb3+and 54% Sb5+.Similar data are also obtained for the sample calcined in air. The composition of the mixed valence V-antimonate phase that can be derived from these data is reported in Table 1. However, the presence of S b 0 4 (evidenced by XRD) cannot be explained, if the presence of some oxygen vacancies in the V-antimonate is not assumed, in agreement with previous results. The formulation that can be proposed for this sample is the following: ( V ~ : 2 1 V ; f ~ g S b ~ ~ o S b s ~ o _ x o ~-t~(.a~axb )2l0- 4x ) x

where x=O.O6-0.07. The position and shape of the (111)reflection of the rutile phase (see dfiactogram c in the inset of Fig. 2) and the analogy of cell parameters with the fresh 1:l sample treated under vacuum justlfy the above assumption. Therefore, regardless of the starting situation in 1:l samples (calcination in air or vacuum; compare also formulations in Table 1) the same final composition for the mixed valence V-antimonate phase after propane ammoxidation can be indicated.

289

Similar results regarding the composition of V-antimonate can also be derived from the analysis of the samples with a higher Sb:V ratio (see Table 1).Chemical analysis, in fact, suggests the presence of a mixed valence oxide with a V3+:V4+=1:3 ratio independently from the preparation by heat treatment in air or vacuum. This suggests that the more stable V-antimonate phase during the catalytic reaction is characterized by the presence of both V3' and ?. Birchall and Sleight [71 also have proposed a non-stoichiometric V-antimonate phase with the presence of V3' and v4' in a ratio of about 1:2.5, even though based only on indirect evidence. However, differently from the 1:l sample after propane ammoxidation, in the 1:2 sample after catalytic tests, ?' is not detected by chemical analysis or IR data (see Fig. 3, spectrum c). This suggests that the presence of excess antimony avoids or limits the formation of a $'+-phase, probably responsable for a lowering of the catalytic performance [ill (lower selectivity to acrylonitrile, higher formation of carbon oxides and higher rate of the side reaction of ammonia conversion to Nz). Finally, it should be noted that when the oxygen concentration is too low or absent, in the presence of NH3 or propane and high reaction temperature (500°C o r above), the V-antimonate may be reduced leading to decomposition of the structure. XRD analysis of the samples after similar tests, in fact, shows the formation of SbO3 and a decrease in the amount of VSb04. When not quickly oxidized to Sb2O4, Sb2O3 sublimes in the presence of NH3 at temperatures of around 500°C as clearly indicated by the thermogravimetric tests. The decomposition of VSb04 thus probably leads to an irreversible deactivation due to loss of antimony from the catalyst. The mixed valence V-antimonate phase therefore can partially compensate for changes in the redox states of V and Sb and can accomodate a certain degree of oxygen vacancies. It should be noted, in fact, that notwithstanding the relatively great change in the composition of various samples examined, the unit-cell dimensions vary in a limited range. This can be easily explained taking into consideration that the V3+N4+ ratio changes inversely to the Sb3+/Sb5+ratio in the mixed valence V-antimonate phase and the ionic radius of these ions. However, probably when the reducing conditions do not allow this proportional change, the rutile phase decomposes into the two constituent oxides. Possibly, a similar situation may occur when the oxidation is too strong. It should finally be noted that the V3+@ ratio affects the Sb3+/Sb5+ratio in the Vantimonate rutile phase and this influences both the stability and the catalytic behavior of this compound. The modification of the V-antimonate phase by a selective introduction of foreign ions is thus a key for improving the stability during catalytic reaction and tuning the catalytic performance in propane ammoxidation of these samples.

290

REFERENCES [l]A.T. Gutmann, R.K. Grasselli, J.F. Brazdil, U.S. Patent 4,746,641and 4,788,317 (1988)assigned to Standard Oil Co. [2]L.C. Glaeser, J.F. Brazdil, U.S. Patent 4,788,173(1988)assigned to Standard Oil CO. [31R. Catani, G. Centi, F. Trifirb, R.K. Grasselli,I d . Erg. Chem.Research 31 (1992) 107. [4] (a)G. Centi, R.K. Grasselli, F. Trifirb,Catal. Today 13 (1992)661.(b)Ibidem Chim. I d . (Milan) 72 (1990)617. [5]G. Centi, R.K. Grasselli, E. PatanB, F. Trifib,in New Deuelopmnts in Selective Oxidution, G. Centi and F. TrXirb Eds., Elsevier Pub.: Amsterdam 1990,p. 515. [61(a) F.H. Berry, M.E. Brett, W.R. Patterson, J. Chem. SOC.Dalton (1983)9 and 13.(b) F.H. Berry, M.E. Brett, Inorg. Chim. Acta, 76 (1983)L205. ( c ) F.H. Berry, M.E. Brett, R.A. Marbrow, W.R. Patterson, J. Chem. SOC.Dalton (1984)985.(d) F.H. Berry, M.E. Brett, J. Catal. 88 (1984)232. [7]T.Birchall, A.E. Sleight, Inorg. Chem. 15 (1976)868. [8]F.Guarneri, Thesis Univ. of Bologna (Italy) 1992;manuscript in preparation. [9]B.B. Pal, K.K. Sen Gupta, Inorg. Chem. 14 (1975)2268. [lo]G. Centi, D. Pesheva, F. Trifirb, Appl. Catal. 33 (1987)343. El13 A. Andersson, S.L.T. Andersson, G. Centi, R.K. Grasselli, M. Sanati, F. W b , in New Frontiers in Catalysis (proc. 10th Int. Congress on Catalysis, Budapest 19921, L. Guczi et al. Eds., Elsevier Pub: Amsterdam 1993,p. 691. [121C. Rocchiccioli-Deltcheff,T.Dupuis, R. Frank, M. Harmelin, C. Wadier, C.R.Acad. Sc. Paris B 270 (1970)541. [131B. Darriet, J. B o d , T.Galy, J. Solid State Chem. 19 (1976)205.

J. HABER (I. of Catalysis and Surface Chemistry, Krakow, Poland): How does the NH3:02 ratio influence the composition of the products? G. CENTI (Dip. Chimica Ind. e Materiali, Bologna, Italy): Increasing the ammonia partial pressure the formation of carbon oxides decreases, but the rate of acrylonitrile formation passes through a maximum due to a n increase in the propylene formation at the higher ammonia concentrations. This is due to a competition of ammonia on the active sites for propylene adsorption and further transformation to acrylonitrile. A complete kinetic analysis on the propane ammoxidation on V-Sb-0 catalysts supported on alumina has been previously reported (ref. 3 of the paper). Kinetic data on the unsupported sample are in agreement with those for supported samples.

B. DELMON (Catalyse et Chimie des Mat. Div., Univ. Cath. Louvain, Louvain-laNeuve, Belgium): Although your presentation takes care to allow for non-stoichiometric V-antimonate and doped a-Sb04,your results, especially characterization after catalysis, would rather suggest that the stable phases during catalysis are stoichiometric VSb04 and non-doped a-Sb04 (plus reduced antimony oxide). What are the arguments which lead you to assume non-stoichiometry and/or doping?

29 1

G. CENTI (Dip. Chimica Ind. e Materiali, Bologna, Italy): The V-Sb-0 system, even though apparently similar to known catalysts like FeSb04 + a-Sb2O4, shows same differences,suchas the presence of multiple possible valence states for vanadium [V3', v4' and V5+] and the possible spreading on the surface of rutile crystals (VSbO4) of an amorphous layer or patches of +'-oxide, which makes more difficult the characterization of the catalyst. In addition, the physico-chemical techniques we used to study these catalysts (XRD, IR, XPS, EPR, Raman) (see also ref. 11of this work) do not allow to clearly define the structural and surface details of the catalysts. The problem of the real structure of V-antimonate phase and the presence or not of nonstoichiometry is thus open. We have reported in this work some data about the characterization of the valence state of vanadium in the VSb04 phase by chemical analysis. Obviously, chemical analysis gives information on the valence state of vanadium after dissolution of the sample which can be different from that in the solid state. We have used some procedures in order to avoid these possible changes, as described in the text, but we agree that chemical analysis gives only indirect information. However, these results by chemical analysis suggest that the V-antimonate phase is a mixed valence compound where both V3+ and V4' ions are present. In order t o compensate the charge, it is thus necessary that i) both Sb3+and Sb5+are also present in the rutile structure or ii) a cation deficency exist. We have analyzed recently by 121Sb-Mossbauer spectroscopy these samples founding that antimony is mainly present in the Sb5+form. If the evidence by chemical analysis is valid for the solid state also, a tentative composition for the VSb04 is as follows: V ( I I I ) O . ~ ~ V ( I V ) O . ~ . P ~ . ~ ~The S ~ (V-antimonate V ) ~ . ~ ~ ~ ~ . phase is thus non-stoichiometric for the presence of cation vacancies. This composition is in agreement with previous indications on these samples (ref. 6 and 7 of this work). We have observed also that before the catalytic reaction (fresh samples) can exist some differences in this composition depending on the modality of preparation and Sb:V atomic ratio. However, after the catalytic tests of propane ammoxidation the above tentative composition was observed in all samples. In conclusion, on the basis of present knowledges we believe that the VSbO4 phase is non-stoichiometric due to cation vacancies and presence of vanadium in both V3' and V4+ oxidation state. It is true, on the other hand, that more data are necessary, especially for what concerns the direct determination of the valence state of vanadium on the solid catalyst. Finally, a comment is necessary also about the problem of doping or not with vanadium of a-ShO4 particles. Even small amounts of vanadium incorporated in the antimony-oxidecrystal lead to a significant change in the XRD pattern that we do not observe in our samples. In this sense, we do not have any macroscopic doping phenomena. However, SEM analysis combined with microprobe analysis of changes in the Sb:V ratio indicates that amorphous vanadium-oxide patches are usually present on the surface of a-Sb204 crystals or, in some cases, encapsulated in the antimonyoxide particles. After the catalytic reaction, some changes occurs, but usually we detect again the patches of vanadium-oxide on the antimony-oxide particles. We do not have observed usually segregation of separate phases of antimony-oxide and vanadium-oxide, but eventually the reaction to form VSb04 phase. In conclusion, our data indicate the formation after the catalytic tests of the rutile V-antimonate phase

292

or the presence of V-oxide on the SbO, crystals which suggest the possible local doping of surface of S b 0 4 crystals. It should be evidenced also that the VSbOB formation is enhanced in several cases during the catalytic reaction, because the reaction requires the presence of reduced vanadium and oxidized antimony. Similarly, the doping with vanadium of a-SbaO4 crystals may require specific reaction conditions. This suggests that the modification of the surface reactivity of Sb04 crystals due to the reaction with vanadium depends on the nature of the atmosphere of reaction.

J.C. VOLTA (I. Recherches s u r la Catal se, CNRS, Villeurbanne, France): What is Sb5+and Sb3+in this very complex according to you the respective role of system? What is the role of water on this redox system?

&,e,

G. CENTI (Dip. Chimica Ind. e Materiali, Bologna, Italy): We have studied this problem using transient catalytic tests recently. In these studies we followed the change in the formation of the various products of reaction as a function of time-on-stream after a step change in the concentration of one or more reagents, maintaining constant the concentration of the other reagents. Conclusions based on these data are still preliminary, but indicate that vanadium is mainly involved in the step of propane oxidative dehydrogenation to propylene and antimony in the second step from propylene intermediate to acrylonitrile. We have also observed that when the surface is completely oxidized mainly carbon oxides are formed, but the progressive reduction of the surface leads consecutively to the formation of acrylonitrile (after a partial surface reduction) and propylene (for a higher level or surface reduction). Higher levels of surface reduction deactivate completely the catalyst, which can be regenerated, however, by treatment in oxygen when the catalyst was not too strongly reduced with decomposition of the V-antimonate phase. These data suggests that antimony is active in the Sb5+oxidation state, but possibly Sb3+is involved in the activation of propylene intermediate. V5' enhances the side reaction of ammonia oxidation to Nz and oxidation of intermediates to carbon oxides. Its presence in large amounts on the surface is thus negative, but its role when present in lower amounts is questionable. As mentioned above, the formation of acrylonitrile passes through a maximum as a function of the "in-situ" reduction of the catalyst surface during the catalytic tests. However, both vanadium and antimony reduces in these conditions. Preliminary IR data, on the other hand, suggest that propylene mainly reduced antimony, whereas ammonia reduces mainly vanadium. However, it is not possible t o exclude a role of V5' in the stage of acryon the surface of the lonitrile synthesis from intermediate propylene. Finally, rutile matrix can be tentatively suggested as the active sites for the selective activation of propane by oxidative dehydrogenation. The role of water, on the contrary, is not particularly noticeable. We have studied the effect of addition of water on the surface catalytic reactivity without founding significant changes, apart from a slight increase in the acrylonitrile selectivity. For very high water concentrations in the feed (above 30-40%),however, water may give a competitive adsorption with a decrease in the rate of propane conversion.

+'

V. CortCs Corbcrlin and S. Vic Be116n (Editors), N e w Deveiopn1enl.s In Seleciive Ox~dallorrIf 0 1994 Elsevicr Scicncc B.V. All rights reserved.

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Ammoxidation of propane over vanadium-antimony-oxidecatalysts. Role of phase cooperation effects R. Nilssona, T. Lindbladb, A. Anderssona, C. SongC and S. HansenC aDepartment of Chemical Technology, bDepartment of Inorganic Chemistry 1, and CDepartment of Inorganic Chemistry 2, University of Lund, Chemical Center, P.O. Bbx 124, S-22100 Lund, Sweden

V-Sb-0 catalysts with different Sb:V ratios were prepared and used for the ammoxidations of propane and propylene. XRD and Raman data show the presence of SbV04/V205 when Sb:V < 1 and of SbVO&-Sb204 when Sb:V > 1. For Sb:V = 1, S b V 0 4 was the predominant phase. The activity data show that a Sb:V ratio above unity is needed to have a catalyst selective for acrylonitrile formation, an effect that primarily is related to the catalyst function for transformation of propylene, an intermediate in propane ammoxidation, to acrylonitrile. XPS data reveal the superior phase to be S b V 0 4 with supra-surface Sb-sites formed as a result of migration of antimony from a-Sb2O4 during the catalytic reaction. According to Raman results, pure SbV04 without the copresence of a-Sb204 has a low capability for the conversion of formed propylene to acrylonitrile due to slow reoxidation of active [Sb-0-Sb] sites.

1. INTRODUCTION Currently there is a great interest in the development of heterogeneous catalysts for use in conversions of alkanes to useful chemicals [ 1,2]. For the production of acrylonitrile, the SOHIO/BP-process using propylene feedstock is the major process being used world wide [3]. Recently, it was announced that B P Chemicals is about to commercialize an alternative process using direct conversion of propane to acrylonitrile [4]. A benefit of the new process is that propane is substantially cheaper than propylene, rendering about 15-20 % lower production cost than that resulting from the addition of a propane dehydrogenation unit to an existing propylene ammoxidation plant [ 5 ] . According to patents [6-81, multicomponent catalysts belonging to the V-Sb-A1-0 system are most promising. Studies of this system have shown it to be very complex, comprising phases like SbV04, AlVO4 and AlSb04, where possibly the latter phase can form a solid solution with SbV04 [9]. However, SbV04 is reported to be a key component in these catalysts [6].A synergistic effect has been observed, especially, a Sb:V ratio higher than one was reported to improve the selectivity for acrylonitrile formation [Y- 1 I]. The role of excess antimony oxide is not yet clear, why the present investigation was performed, aiming at the investigation of phase cooperation effects.

2. EXPERIMENTAL 2.1. Catalyst preparation Catalysts with different Sb:V ratios were prepared by adding Sb2O3 (Merck, p.a.) to a warm solution of NH4VO3 (Merck, p a ) in water, which was then heated under reflux with

294

agitation for 16-18 hours. Evaporation of the bulk of the water from the resulting slurry was performed with agitation on a hot plate, until a thick paste was obtained. The paste was then dried at 110 OC for 16 hours, and subsequently heated at 350 OC during 5 hours. After sieving the material, the fraction of particles in the range 150-425 pm were eventually calcined at 610 O C €or 3 hours in a flow of air. (For catalyst compositions, see Table 1). For comparison, a sample was prepared from powders of V2O5 (Riedel-de Haen, 99.5 %) and Sb2O3 (Merck, p.a.) in the molar ratio 1:1, which were mixed by grinding and subsequently heated at 800 OC for 2 hours in air. The sample was the same as that previously used for crystal structure determination, according to which the product has a cation deficient rutile structure with a composition close to Sb(V)0.92V(III)0,2gV(IV)0,~~0.1604, where 0 denotes metal ion vacancies [12].

2.2. Activity measurements A plug-flow reactor made of glass was used for activity measurements. In order to have isothermal conditions, the catalyst samples were diluted with quartz grains. All measurements reported were performed at 480 OC, and precautions were taken to avoid contribution of homogeneous reactions. Especially, dead volumes were reduced and hot zones in the tubing between the reactor and the analysis equipment were avoided. Nitrogen, oxygen, ammonia, propane (or propylene) and water were dosed using mass flow regulators. The products were analyzed on a gas chromatograph equipped with a sample valve, an FID detector, a Porapak Q column and a methanizer for the analysis of carbon oxides.

2.3. Characterization of catalysts A Micromeritics Flowsorb 2300 was used for the determination of specific surface areas, and X-ray diffraction measurements were performed on a Philips X-ray diffractometer using a PW 1732/10 generator and Cu K, radiation. Scanning electron micrographs and elemental maps were recorded at 20 kV using a JSM840A scanning electron microscope (SEM), a Link AN10000 energy dispersive X-ray analysis (EDX) system and a Mitsubishi video copy printer. Samples for particle size determination were prepared by mounting intact catalyst grains on aluminium stubs with a graphite colloid. The specimens were covered with a layer of gold by sputtering. To prepare samples for X-ray mapping, the catalyst grains were gently crushed, using a glass slide, on Al holders covered with conducting polymer. A carbon coating was then applied by evaporation. XPS measurements were performed with a Kratos XSAM 800 instrument using Mg K, Xray radiation. The sample was attached to the sample holder with a double sided tape. Charging effects were overcome by mixing the samples with acetylene black (Carbon philblack 1-ISAF from Nordisk Philblack AB). Spectra were collected for the spectral regions corresponding to 0 Is, Sb 3d, V 2p, C 1s and Sb 4d. The C 1s signal was adjusted to a position of 284.3 eV. Raman spectra were recorded with a Bruker IFS 66 FTIR spectrometer equipped with an FRA 66 Raman device. A Nd:YAG laser with an excitation line at 1046 nm and a liquid nitrogen cooled Ge diode detector were used. The power was usually set at 100 mW, 1800 backscattering was measured and 4000 scans were averaged. 3. RESULTS 3.1. Amrnoxidation of propane and propylene The total reaction rate for the ammoxidation of propane, measured at low conversions (3 4 5%). and corresponding selectivities for acrylonitrile and propylene formations are shown in Fig. 1 for different Sb:V ratios. The most active catalyst has a ratio Sb:V = 1:2, and the total activity decreases with increasing antimony content. Catalysts with Sb:V ratios in the range

295

2: 1 - 7: 1 are the most selective for acrylonitrile formation (22 %), while the sum of the selectivities for acrylonitrile and propylene formations is about 65 % irrespective of the catalyst composition. 80

60 40

40 a, +

1

A

20

P

0

0.2 0.4 0.6 0.8

0

0.2 0.4 0.6 0.8

0

1

V/( V+Sb)

1

V/(V+Sb)

Figure 1. Ammoxidation of propane at 480 O C . Total reaction rate (left) and selectivities (right) vx. the ratio V/(V + Sb). . Rate; : selectivity for formation of acrylonitrile; and A: the sum of the selectivities for acrylonitrile and propylene formations. Open symbols are for pure Sb0.92V0.9204. Mole ratios: C3Hg/OdNH3/H2O/N2 = 2/4/2/1/5.

+:

Since propylene is an intermediate in the reaction from propane to acrylonitrile [5,9, 101, cf. Fig. 3, the ammoxidation of propylene was also studied. The results are in Fig. 2. Compared with propane ammoxidation, the same activity trend with the Sb:V ratio is observed for propylene ammoxidation (conversions 4 - 8 %). The selectivity for acrylonitrile formation is the highest (40 %) for catalysts with Sb:V ratios between 2: 1 and 7: 1, and so is the selectivity for formation of acrolein (20 %). At higher conversions, it was observed that acrolein reacts further with ammonia to acrylonitrile, giving a selectivity of about 60 % for acrylonitrile formation at conversions between 60 and 80 % [13].

400

7

-P

a,

0

0.2 0.4 0.6 0.8 V/(V+Sb)

1

0

0.2 0.4 0.6 0.8

1

V/( V+S b)

Figure 2. Amrnoxidation of propylene at 480 O C . Total reaction rate (left) and selectivities Rate; A:selectivity for formation of acrylonitrile; and (right) vs. the ratio V/(V + Sb). I: selectivity for formation of acrolein. Open symbols are for pure Sb0.92V0.9204. Mole ratios: C3Hd02INH3IH20IN2 = 1/4/2/1/6.

+:

The yields and selectivities for propylene and acrylonitrile formations in propane ammoxidation were measured as a function of the conversion over the sample with Sb:V = 2:l by

296

varying the amount of catalyst, see Fig. 3. Clearly, propylene is an intermediate in acrylonitrile formation. The highest yield of acrylonitrile per single pass is 11 %, and is achieved at a conversion around 40 %. At a slightly lower conversion, the selectivity for nitrile formation reaches a maximum of 37 %.

" 0

10

20

30

40

50-

Conversion of propane

Figure 3 . Yields and selectivities in propane ammoxidation at 480 O C vs. the conversion of propane. 0 : Yield of acrylonitrile; A: yield of propylene; +: selectivity for formation of acrylonitrile; and A:selectivity for formation of propylene. Total flow: 7 0 mumin (STP); weight of catalyst: 0.2 - 5 g; and mole ratios: C3HS/02/NH$H20/N2 = 2/4/2/1/5. 3.2. XRD The X R D patterns of fresh samples with Sb:V < 1 showed the presence of V 2 0 5 and SbV04. For a Sb:V ratio of unity, besides SbV04, unreacted V2O5 and a-Sb2O4 were identified. The SbVOq reference sample, or more correct Sb0.92V0.9204, was absolutely pure in XRD. Samples with Sb:V > 1 were found to consist of SbV04 and a-Sb2O4. After use in propane ammoxidation for 7 hours, no significant changes were observed in the XRD patterns of the reference sample and of samples with Sb:V >1. For the sample with the ratio Sb:V = 1, no V2O5 lines were apparent after use. However, the lines both from aSb2O4 and from S b V 0 4 remained unchanged. The sample freshly charged as V2O5 was found to be reduced in propane ammoxidation, and showed lines only from v 4 0 9 [ 141 with traces of V6013 [15] after use for 7 hours. For Sb:V = 1:2, the V2O5 lines had disappeared during use, but in this case no lines from reduced vanadia appeared and the lines from SbV04 remained unchanged.

3.3. SEM and EDX

Figure 4. Scanning micrograph and X-ray maps of a freshly prepared, gently crushed a-Sb204/SbV04 catalyst (Sb:V = 4:l). Scale bar equals 5 p.

291

Since the fresh catalysts consist of a-Sb204/SbV04 or V205/SbV04 in different proportions, it is possible to undertake a phase analysis by elemental mapping of the metals. Thus, in Fig. 4 areas emitting X-rays characteristic of V and Sb can b e identified as SbVO4, while particles giving only the S b signal is a-Sb2O4. The scanning electron micrographs reveal that the catalyst grains produced by sieving, which are a few tenths of a mm in size, actually consist of micrometer-sized, more or less well-defined, crystals. The mean particle diameter of each catalyst sample was determined from SEM micrographs by measurement on hundreds of particles, and it was compared with the corresponding diameter calculated from BET data, assuming the catalyst grains are spheres or cubes having the average density o f the constituent phases. In this context, it should be noted that the value of the latter diameter can be influenced by the presence of closed pores, whereas the determination of the diameter by microscopy is complicated by a large variation in particle size. Table 1 shows a reasonable agreement between the diameters determined by the two methods. Consequently, the surfaces observed in SEM are the surfaces where the catalytic reaction takes place, and the observed particles are not microporous or built up from smaller particles, which is often the case with oxides formed in topotactic reactions [16]. Table I

0.800 0.667 0.500 0.333 0.250 0.200 0.167 0.143 0.125

1:4 1:2 1:1

2: 1 3: 1 4: 1 5 :1 6: 1 7: 1

16.4 9.9 10.6 3.6

4.3 2.6 1.4 2.5 1.3

0.09 0.13 0.10 0.29 0.24 0.40 0.74 0.41 0.80

0.15 0.20 0.17 0.25 0.35 0.47 0.7 1 0.48 0.57 ~

aThe BET area for V2O5, Sb0.92V0.9204, and a-Sb2O4 was 7.8, 4.8, and 0.6 m2/g, respectively.

3.4. XPS In Table 2 are the binding energies for the Sb 3 d y 2 and V 2 ~ 3 1 2bands together with the Sb:V atomic ratio as measured with XPS, both for freshly prepared and used samples. The Sb:V surface ratios for the fresh samples are in fair accordance with the nominal ratios, except for Sb:V = 1:4 and for Sb:V = 5:l. Considering the samples containing V2O5 as freshly prepared (Sb:V = 1:4, 1:2 and l:l), it is observed that the Sb:V ratio has decreased during use in propane ammoxidation, i.e., the surface concentration of vanadium has increased. For all samples with Sb:V > 1, on the contrary, the Sb:V ratio is higher after use than it is before. The Sb:V surface ratio is unaffected by use in propane ammoxidation only in the case of the pure Sb0,92V0,~~204 sample. The variations of the binding energies follow some trends. For the catalysts with Sb:V > 1, the binding energy of the Sb 3d3/2 peak increases about 0.2 eV upon use and it decreases with increase in antimony content for both unused and used catalysts. On the other hand, for the samples with ratios of Sb:V S 1, the binding energy of the S b 3d3/2 peak decreases upon use. This is also true for the Sb0.92V0.9204 sample. Generally, the binding energy of the V 2 ~ 3 1 2peak decreases with some 0.1 eV upon use.

298

Table 2 Core line binding energies and Sb:V ratios determined by XPS Sb:V atomic ratio Sb 3d3n (eV)a Nominal XPS, fresh XPS, used Fresh Used 1:4 1:1.4 1:2 1:1.6 1:l 1.1:l Sb0.92V0.9204 1.1: 1 2: 1 2.0: 1 3: 1 2.6: 1 4: 1 3.8: 1 5: 1 12.6:1 6: 1 5.3:1 7: 1 9.7: 1

1:2.7 1:2.2 0.8:l 1.1:l 2.7: 1 2.7: 1 4.8: 1 16.5:l 8.7: 1 18.5:l

540.1 540.1 540.1 540.2 540.0 539.9 539.8 539.8 539.7 539.8

539.8 540.0 540.0 540.0 540.1 540.1 540.0 540.0 540.0 540.0

v 2~312(eVP Fresh Used 517.0 516.9 516.7 516.8 516.7 516.6 516.8 516.6 516.6 516.6

516.8 516.8 516.7 516.5 516.5 5 16.5 516.7 516.6 516.4 516.6

aThe Sb 3dy2 binding energies measured for a-Sb204 and Sb2O5 were 539.9 and 540.3 eV, respectively. bThe V 2~312binding energy measured for V2O5 was 517.1 eV.

3.5. Raman spectroscopy In Fig. 5 the Raman spectra for fresh and used catalysts are shown for Sb:V = 1:2 and 2:l. In the spectrum for the fresh Sb:V = 1:2 catalyst, one can clearly see some of the strongest bands from V2O5 at 995,701, 285, 195 and 145 cm-I [ 171, but these bands have disappeared after use in propane ammoxidation. The broad features between 950 and 750 cm-1 and between 500 and 300 cm-I and the band at 120 cm-l, which are typical for SbV04 [13], also decrease largely upon use. In the spectrum for the freshly prepared catalyst with Sb:V = 2: 1, no strong V2O5 bands are noticed although a small band at 145 cm-1 can be recognized. The strong band present at 199 cm-l is from a-Sb204, and the peak at 401 cm-1 also may be from this phase [18]. The same broad features from SbV04, as are present in the spectrum for the fresh catalyst with Sb:V = 1:2, are clearly seen. After use in ammoxidation all features are less intense, but no other changes are observed. Compared with the spectrum for the used Sb:V = 1:2 sample, the broadband centered around 860 cm-1 remains comparatively strong.

1100

900 700 500 300 Wavenumbers (cm-1 )

100

Figure 5 . Raman spectra for fresh catalysts and after use in propane ammoxidation for 7 hours. Sb:V = 1:2, (1) fresh and (2) used sample. Sb:V = 2:1, ( 3 ) fresh and (4) used sample.

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4. DISCUSSION

Stoichiometric SbV04 cannot be prepared due to the complex redox behaviour with two redox pairs. Heating of equimolar amounts of V2O5 and Sb2O3 in air produces a cation deficient rutile with a composition close to Sb0.92V0.9204 [12,19,20]. When heating the oxides in a sealed gold tube [19] or in strictly oxygen-free nitrogen [20], antimony oxides are formed together with a phase with the approximate composition Sb0.95V1.0504. Mossbauer data [ 191 showed that the antimony in these structures is predominantly in the pentavalent state. Consequently, the vanadium in the bulk is present as V4+ and V3+. The XPS data in Table 2 for Sb:V = 1 :1 and the pure Sb0.92V0.9204 phase show Sb 3dy2 B.E. (binding energy) values for the fresh samples that are intermediate between those for a-Sb204 and Sb2O5, indicating a predominance for Sb5+ surface states, possibly with some Sb3+ states. The B.E. of the V 2~312peak for these samples is 0.3-0.4 eV less than it is for V2O5, but somewhat higher than reported for V 0 2 [21], suggesting the presence of both V5+ and V4+ surface states. The activity data in Fig. 1 reveal two synergy effects. Catalysts with V2O5 and SbV04 as freshly prepared show an activity maximum at the ratio Sb:V = 1:2. According to the XPS data in Table 2, migration of vanadia occurs under the influence of the catalytic cycle. However, the synergy effect is probably not only due to formation of a SbV04 phase enriched in vanadium at the surface layer because such an enrichment also is observed for the Sb:V = 1:l sample, which is less active than the Sb:V = 1:2 sample. That the activity also is lower for Sb:V = 1:4 points to the synergy effect being caused by the grain boundaries between SbV04 and reduced amorphous vanadia. The reduction of vanadia in propane ammoxidation is evidenced by the change in the V 2 ~ 3 B.E. ~ 2 value, and the absence of XRD peaks from reduced and crystalline vanadia is in line with the formation of an amorphous structure. A second synergy effect is clearly present in the antimony-rich region. Figure 1 shows the catalysts with both a-Sb204 and SbV04 to be the most selective for acrylonitrile formation, though the activity decreases with increase in the a-Sb204 content. Other investigators have observed the same effect in propane ammoxidation [ 10,111 and propylene oxidation [22], and there is an agreement with patent claims [6,7]. However, the previous reports have given data only for a few Sb:V ratios and have not emphasized exploring the origin of the synergy effect. According to the present results, crystal growth phenomena can be excluded as an explanation for the synergy effect at high Sb:V ratios. Micrographs, Fig. 4,reveal the crystals to be irregular in shape and not to be anisotropic, why there is no evidence for variation of the surface plane distribution of SbV04 with excess a-Sb2O4. Also, no evidence was obtained for an epitactic growth of SbV04 on a-Sb204. Instead, migration of Sb over SbV04 modifying its surface most likely causes the synergy effect. Comparison of the Sb:V ratios determined by XPS for fresh and used catalysts with Sb:V > 1, Table 2, shows that the ratio increases during the catalytic reaction. The observation that the B.E. of the Sb 3d3/2 peak increases upon use support that migration of S b occurs from a-Sb2O4, with both Sb3+ and Sb5+, to the surface of SbV04, with predominantly Sb5+ states. Migration of Sb from the bulk of SbV04 up to its surface does not occur because no increase in the Sb:V ratio upon use was observed for the two samples with Sb:V = 1:l. The importance of the surface enrichment in antimony for the conversion of the intermediate propylene to acrylonitrile is seen considering that a Sb:V ratio higher than unity is needed for obtaining a high selectivity for acrylonitrile formation, cf. Figs. 1 and 2. For the samples with Sb:V I1, the B.E. of the Sb 3d3/2 peak decreases upon use due to reduction of the antimony in SbV04. This reduction agrees with the features of the Raman spectra for the Sb:V = 1:2 sample in Fig. 5, which show that the band at = 860 cm-l has almost disappeared after use in propane ammoxidation. This band is not typical for metal oxides with a stoichiometric rutile structure, e.g. FeSbOq [23] and T i 0 2 [24], why the band possibly is from a v (metal-oxygen-metal) mode involving a 2-fold coordinated oxygen. Unlike the oxygens in the stoichiometric rutile structure, which all are 3-fold coordinated, 2fold coordinated oxygens are present in the cation deficient structure of SbV04. A bond valence analysis has suggested that OSb20 (0 is a cation vacancy) is the most favourable configuration [ 121. Possibly, the [Sb-0-Sb] site is active for the transformation into acrylonitrile

300

of formed propylene, in agreement with the coupling of a low intensity of the 860 cm-l band after use and the low selectivity for nitrile formation when Sb:V I1. Catalysts selective for nitrile formation, with Sb:V > 1, do not show the same pronounced decrease in the intensity of the 860 cm-l band upon use. The reason can be either a faster reoxidation due to the new supra-surface sites created as a result of Sb migration, or the supra-surface sites have taken over the function of the in-plane [Sb-0-Sb] sites. A more efficient reoxidation of the antimony sites is evidenced by the Sb 3d3/2 B.E. measured after use. It is worthy to note that the Sb 3d3/2 B.E. of the SbV04 surface in the catalysts with Sb:V > 1 may be higher than the values in Table 2 suggest because these have a contribution also from a-Sb204. Concerning the first step in propane ammoxidation, the activation of propane to give propylene, oxygen bonded to vanadium probably is active. The conclusion is supported by the fact that the two catalysts with Sb:V = 1:l are both highly active and selective for propylene formation, cf. Fig. 1. In agreement with the solid state redox reaction between V2O5 and Sb2O3 forming SbV04, where the antimony is oxidized and the vanadium reduced, most likely a V-site is participating in the reoxidation of [Sb-0-Sb] and [Sb-NH-Sb] sites. This mechanistic view, supported by the present results, agrees with that proposed by Grasselli in a general comment on propane ammoxidation [9] and for which there is much support in previous works [2,25].

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.

H.A. Wittcoff, CHEMTECH, 20 (1990) 48. R.K. Grasselli, G. Centi and F. Trifirb, Appl. Catal., 57 (1990) 149. R.K. Grasselli, J.F. Brazdil and J.D. Burrington, in Proc. 8th Int. Congr. Catalysis, Vol. V, Verlag Chemie, Weinheim, 1984, pp. 369-380. Newsletter, Appl. Catal., 67 (1990) N5. G. Centi, R.K. Grasselli and F. Trifirb, Catal. Today, 13 (1992) 661. A.T. Guttmann, R.K. Grasselli and J.F. Brazdil, US Patent Nos. 4 746 641 (1988) and 4 788 317 (1988). M.A. Toft, J.F. Brazdil and L.C. Glaeser, US Patent Nos. 4 784 979 (1988) and 4 879 264 (1989). J.F. Brazdil, L.C. Glaeser and M.A. Toft, US Patent No. 4 87 1 706 (1989). A. Anderson, S.L.T. Anderson, G. Centi, R.K. Grasselli, M. Sanati and F. Trifirb, in Proc. 10th Int. Congr. Catalysis, AkadCmiai Kiad6, Budapest, 1993, pp. 691-705. G. Centi, R.K. Grasselli, E. Patane and F. Trifirb, in G. Centi and F. Trifirb (eds.), New Developments in Selective Oxidation, Elsevier, Amsterdam, 1990, pp. 5 15-526. G. Centi, D. Pesheva and F. Trifirb, Appl. Catal., 33 (1987) 343. S. Hansen, K. Stihl, R. Nilsson and A. Andersson, J. Solid State Chem., 102 (1993) 340. R. Nilsson, T. Lindblad, A. Andersson and S. Hansen, to be published. A. Andersson, J.-0. Bovin and P. Walter, J. Catal., 98 (1986) 204. K.-A. Wilhelmi, K. Waltersson and L. Kihlborg, Acta Chem. Scand., 25 (197 1) 2675. L. Volpe and M. Boudart, Catal. Rev. Sci. Eng., 27 (1985) 515. L. Abello, E. Husson, Y. Repelin and G. Lucazeau, Spectrochim. Acta, 39A (1983) 641. C.A. Cody, L. DiCarlo and R.K. Darlington, Inorg. Chem., 18 (1979) 1572. T. Birchall and A.W. Sleight, Inorg. Chem., 15 (1976) 868. F.J. Berry, M.E. Brett and W.R. Patterson, J. Chem. SOC.Dalton Trans., (1983) 9. S.L.T. Andersson, J. Chem. SOC.Faraday Trans., 75 (1979) 1356. F.J. Berry and M.E. Brett, J. Catal., 88 (1984) 232. M. Carbucicchio, G. Centi and F. Trifirb, J. Catal., 91 (1985) 85. M. Sanati, A. Andersson and L.R. Wallenberg, in Proc. 10th Int. Congr. Catalysis, AkadCmiai Kiad6, Budapest, 1993, pp. 1755-1758. R.G. Teller, J.F. Brazdil, R.K. Grasselli and W. Yelon, J. Chem. Soc. Faraday Trans. I , 8 1 (1985) 1693.

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B. Delmon (Univ. Catholique de Louvain, Louvain-la-Neuve, Belgium): It is surprising that you conclude that Sb-O-Sb sites are active for ammoxidation, as S k O 4 is completely inactive. Your article would rather favour your other hypothesis, namely a cooperation between Sb2O4 and vanadium antimonate. Sb2O4 would promote contaminated or pure SbV04 by some electronic or mobile species transfer. Why do you give preference to Sb-O-Sb sites? A. Anderson (Univ. of Lund, Lund, Sweden): Catalysts with SblV < 1 have low ability to transform formed propylene to acrylonitrile, and our corresponding XPS data for the used catalysts reveal that the reoxidation of antimony sites at the surface of SbV04 is incomplete. Catalysts with both SbV04 and a - S t ~ O 4 on , the other hand, are selective for acrylonitrile formation and our XPS and Raman data show that this primarily can be linked to a more efficient reoxidation process. The superior phase is SbV04 with supra-surface antimony sites, and is formed as a result of migration of antimony from a - S b O 4 during the catalytic reaction. Pure a-Sb204 has low activity and is selective for prop lene formation, but has no ability to insert nitrogen. This possibly is due to reduction of S b L to Sb3+. The highest oxidation state has to be maintained to facilitate nitrogen insertion. Reduction of Sb5+ in Sb2O4 under the influence of the catalytic cycle, possibly is the reason for the migration of antimon since Sb203 is volatile under hydrothermal conditions. It is also well established (1) that St& is the element involved in nitrogen and oxygen insertion in propylene (amm)oxidation over FeS bOq/a-S b204 and USb3010. 1. R.K. Grasselli, G. Centi and F. Trifub, Appl. Catal. 57 (1990) 149.

J. Haber (Inst. of Catalysis and Surface Chemistry, Krakow, Poland): It should be reminded that some 25 years ago the system iron antimonate was studied in oxidation of propene and it was established that the S W S b site is inserting oxygen, whereas iron is necessary for activation of propene and reoxidation of the catalyst. This is in line with your conclusion and shows that you had to add vanadium to activate propane because iron is unable to do it.

A. Anderson: Thank you for your comment in support of our conclusion that antimony sites have the nitrogen inserting function. Also, there is another similarity between the iron antimonate and vanadium antimonate systems for propylene and propane (amm)oxidation, respectively, namely that beside FeSb04 or SbV04 excess 01-SkO4 is required to have a selective catalyst formulation. In case of the iron antimonate system several explanations for the phase cooperation effect have been offered in the literature (2,3 and references therein). However, there is also an important difference in the structures of FeSb04 and SbV04. In the former phase all cation positions are occupied, while the latter phase is cation deficient and some of the oxygens are 2-fold coordinated. The latter species, according to our results, can play a mechanistic role. 2. 3.

R.G. Teller, J.F. Brazdil, R.K. Grasselli and W. Yelon, J. Chem. SOC.Faraday Trans. 81 (1985) 1693. M. Carbucicchio, G. Centi and F. Trifiro, J. Catal., 91 (198.5) 8.5.

E. Bordes (Univ. Technologie de Compikgne, Compikgne, France): It is uncommon to see a catalyst able to activate propylene and propane, with the same structure or composition. The active sites should not be the same. Would it be because several species are available (V5+, V4+, Sb5+, Sb3+) on the surface providing several M-O sites with adequate bond energy (and basic properties) for alkane activation, and/or oxygen vacancies to form the 7c-allylic intermediate from propylene?

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A. Andersson: You are quite correct that the most commonly used industrial catalysts for propylene (amm)oxidation are not good for propane (-)oxidation. We have found that the a-SbO4/SbVO4 system in propylene ammoxidation gives a yield of 55 % for acrylonitrile formation at 90 % of conversion per single pass. This is much lower than the yield obtained over multicomponent molybdate catalysts, which is about 80% at 98 % of conversion. The fact that the vanadium antimonate system functions for both propane and propylene ammoxidation is not surprising because propylene is an intermediate in propane ammoxidation. Certainly, an important reason for this ability is that antimony and vanadium are present in several oxidation states at the surface. Our results indicate that oxygen bonded to vanadium is participating in the activation of propane to form pro ylene. Considering the previous reports (1,2 quoted above), it is reasonable to assume that Sbq+ or V3+ (V4+) sites are involved in the activation of propylene (a-hydrogen abstraction). R.K. Grasselli (Mobil Research & Development, Princeton, New Jersey, U.S.A.): You have indeed presented an elegant piece of work and added significantly to a better understanding of the ammoxidation of propane to acrylonitrile. I certainly concur with your finding that selective ammoxidation catalysts, within your studied systems, are those whose Sb/V > 1. This is the first prerequisite of the site isolation principle (43.And, that the superior catalyst phase is "SbV04 with supra-surface Sb-sites" formed as a result of migration of antimony from aSb2O4 during the catalytic reaction. This is in complete agreement with our studies of Fe-Sboxides, Fe-Te-Sb-oxides, U-Sb-oxides, etc. (1,2 quoted above), and the mechanistic view of propane to acrylonitrile transformation on V-Sb-oxide catalysts which I presented in Budapest, and to which you refer in the last sentence of your paper.

4. J.L. Callahan and R.K. Grasselli, AIChE Journal, 9 (1963) 755. 5. R.K. Grasselli and D.D. Suresh, J. Catal., 25 (1972) 273. A. Andersson: Thank you for your comment. Indeed, the principle of site isolation is also applicable to SbV04, because the vanadium sites are separated by antimony in the structure.

J.-M. Bregeault (Univ. Pierre et Marie Curie, Paris, France): Can you exclude the formation of 0x0-nitrido compounds as active phases in ammoxidation processes? What are the keyreferences on SbNH-Sb sites? A. Anderson: We have not yet studied the ammonia activation step, and to our knowledge there is no report on this matter concerning SbV04. It is our hope that we will learn more about the mechanistic details combining in situ infrared and Raman studies. Our indication on Sb-NH-Sb sites as nitrogen insertion sites is based on previous works on multicomponent iron antimonate catalysts for propylene ammoxidation (6).

6. J.D. Burrington, C.T. Kartisek and R.K. Grasselli, J. Catal., 87 (1984) 363 F. Trifiro (Univ. of Bologna, Bologna, Italy): In previous papers alumina supported V-Sboxide was investigated and the aluminium oxide was not considered a real support. After your careful investigation of unsupported catalysts, I ask you: What are the main differences in physical chemical characterization of alumina free or not based catalysts?

A. Andersson: In the recent work that we have carried out in collaboration with your group, we have found that one of the roles of alumina is to increase the dispersion of the supported SbV04 phase. However, as you suggest, the alumina is not only an inert carrier but can also react with vanadium and antimony oxides to form AlVO4 and AISb04, respectively. Thus,

303

the A1-Sb-V-0 system is multiphase, and the phase composition under reaction conditions is determined not solely by the reaction temperature and the partial pressures of reactants, but is also affected by the conditions of preparation. Consequently, within this system there will be several possibilities for phase cooperation, which are not yet fully explored. However, it seems quite clear from the experimental results that are given in patents assigned to The Standard Oil Company (Ohio) that Al-Sb-V-oxide catalysts give a higher yield for acrylonimle formation than is obtainable over unsupported Sb-V-oxide catalysts.

G. Busca (Univ. di Genova, Genova, Italy): Just a comment on the use of vibrational spectroscopies for solid state characterzation. The vibrational structure of a solid is a complex matter, with Raman active modes, IR active modes and inactive modes. By changing cation ordering and location, as well as the ratio of two cations in the same structure, the symmetry changes and the activity of several modes should change. Without a knowledge of the complete vibrational structure of the solid phases involved, it is difficult to draw reliable conclusions. Moreover, the Raman data should be confirmed by IR spectra, and vice-versa. A. Andersson: We of course agree with you. We have shown that the composition of the SbV04 phase prepared in air is Sb0.92V0.9204 with 8 % of the cation sites vacant. By powder diffraction methods, we found (7) no indication of a lowering of the ideal tetragonal symmetry in rutile, which at least excludes the possibility of complete long-range ordering of Sb, V and vacancies in the investigated samples. However, the occurrence of more subtle types of ordering cannot be ruled out. This phase gives a band in Raman at 860 cm-1 that we assign to a vibration mode involving the 2-fold coordinated oxygen. According to our previous analysis, the SbO-Sb arrangement is the most favourable. This assignment is confirmed by our recent observation that there is no band at 860 cm-l in the Raman spectrum of a similar but reduced rutile phase with no cation vacancies in the structure. In the latter phase, the oxygens are exclusively 3-fold coordinated. 7.

S. Hansen, K. Stbhl, R. Nilsson and A. Andersson, J. Solid State Chem., 102 (1993) 340.

J.M. Millet (Inst. de Recherches sur la Catalyse, Villeurbanne, France): It has been shown earlier that in air, VSb04 was never obtained and that the thermodynamic equilibrium corresponds to a mixture of Sb2O4 and a rutile type structure with an excess of vanadium This was proposed in particular on the compared to antimony: V(III)~~xV(IV)2xSb(V)~~x04. base of Mossbauer spectroscopy data, which evidenced only Sb3+ species corresponding to Sb2O4. What is your opinion on these results? A. Andersson: I suppose that you refer to the work of Birchall and Sleight (8). They reported the formation of a phase with the approximate composition Sb0.95V1.0504 when heating a mixture of Sb2O3 and V2O5 in a sealed gold tube (not in air as you quoted), and when heating the product in air they observed a gain of weight corresponding to the composition Sb0.92V0.9204. Our recent study quoted above (7) has shown the phase prepared in air to have a cation deficient rutile structure with a Sb/V atomic ratio close to one and with Sb(V) and V(III)/V(IV) cations. In our on-going work we have observed, in oxygen-free N2, the formation of a stoichiometric rutile with a composition similar to the one reported by Birchall and Sleight. Generally, our findings are in line with the results reported by other investigators on this system. However, it is very complex and much remains to discover (see, e.g., the preceding paper by G. Centi and co-workers). 8. T. Birchall and A.W. Sleight, Inorg. Chem., 15 (1976) 868

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V . CortCs Corberan and S. Vic Bell6n (Editors), N e w Developments i n Selecllue Oxidorion I / 0 1994 Elsevier Science B.V. All rights reserved.

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Selective oxidation of propane in the presence of bismuth-based catalysts J Barrault and L Magaud Laboratoire de catalyse, URA-CNRS 350, ESP, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex (France) M. Ganne and M. Tournoux Institut des Materiaux de Nantes, Laboratoire de Chimie des Solides, 2 rue de la Houssiniere, 44072 Nantes Cedex 03 (France)

Abstract Oxidative dehydrogenation of propane and partial oxidation of propane to acrolein were studied using lamellar Aurivillius and Sillen phases. Among these solids, bismuthmolybdate was selective for partial dehydrogenation of propane at high temperature. In order to improve the activity of this catalyst, we tried to prepare a similar phase and to disperse it on titanium dioxide. By using a mixed 3Bi-5Mo/Ti02 a significant increase in activity and in acrolein selectivity was obtained

1. INTRODUCTION Because of the global abundance of liquified petroleum gas (LPG), there is a strong, increasing interest in the potential use of light paraffins as sources of the corresponding alkenes or/ and of oxygenated compounds. The oxidation of n-butane to maleic anhydride was carried out successfblly using V-P-0 catalysts [ 11. On the other hand, the partial oxidation of propane is still under study because the performances of the catalysts used in propene oxidation [2-51 or of new materials were not good enough for practical application. Based on recent results available in the literature, V-P-Te oxides are proposed for the direct formation of acrylic acid through the oxidation of propane in the presence of water [6]. B-P-0 mixed oxide catalysts [7], Ce-Te-Mo-oxides associated with a dehydrogenation component CdX, [8] are used for the formation of acrolein. But the best results for acrolein synthesis were those of Moro-Oka & coll obtained with silver doped bismuth vanadomolybdate catalysts, which gave a selectivity to acrolein of over 60% for a 13% conversion of propane [9]. Nevertheless, the definition of active centers is still under investigation.

306

Our objective in this field is to find i) new catalysts and ii) new trends between catalysts composition and activity - selectivity in the partial oxidation of propane In order to progress in this way, we decided to study the catalytic properties of model catalysts containing bismuth and molybdenum, vanadium, tungsten or titanium belonging to SILLEN or AURIVILLIUS phases [ 1 11 The important element concerning these materials of general formula Bi,O,(&.

lB,O,,, is that their composition, their electronic and physical properties can be changed by substitution of A and B elements without any significant modification of the structure Thus, one can expect a change of the active centers and consequently of the catalytic properties In fact, we have already shown that some of these phases were efficient catalysts for the oxidative coupling of methane [ 12,131 In the present paper, we report the first results obtained with these phases compared to those obtained with bismuth molybdenum oxides supported on titania. Indeed, in order to improve the catalytic properties, especially the activity, we have tried by soft chemistry methods to prepare similar phases and to disperse them on a support

2. EXPERIMENTAL 2-1. Synthesis and crystallographic characterization of the lamellar Bi,02(~.1B,,0,1) compounds. These materials were synthetized in air, from stoichiometric mixtures of Bi203, alkaline and alkaline-earth carbonates and transition metal oxides at temperatures ranging from 1223 to 1423 K [12,13]. Some of their characteristics are presented in Table 1. Their specific surface areas are generally low, about 1 m2.g-1 Table I Characteristics of bismuth-based solids

~

~~

(Bi,O,)MoO, (LT) Aunwlhus (Bi,O,)MoO, (HT)

(Bi,O,)WO, (Bi,O,)VO,

orthorhombic

5 502

5 483

16 213

B2cb

Sillen

tetragonal

3 950

3 950

17210

142m

Aunvlllius

orthorhornbic

5 457

5 436

16 427

B2cb

Aundlius

monocllnic (T < 713 K) 0 orthorhombic 713 K
5 613

15 280

16 602

11 628

5 657

15 399

Fmmm

3 988

3 988

15 420

14/m m m

5 450

5 408

32 832

B2ab

Q

ytetragonal

89 76 C2/m(Cm2)

T > 823 K (Bi,O,)B%Ti,O,,

Aunvlllius

orthorhombic

LT low temperature (< 9 1 3 K), HT high temperature (> 9 13 K)

307

2-2. Synthesis of bismuth-molybdenum oxides supported on titania These solids were prepared by impregnation of titania with aqueous solutions of Bi(NO,), and / or MOO, (nitric solution). After drying, the samples were calcined at 600°C for 1 h. Two titanias were used as support : Degussa P25 and titania prepared in our laboratory

from hydrolysis of TiOCI, in the presence ammonia.

2-3. Reaction procedure The catalytic measurements were performed at atmospheric pressure in a fixed bed conventional flow system equipped with a quartz tube reactor containing the catalyst powder (1 g) Before use, the catalysts were heated in helium at 773 K for lh The standard feed

composition was 60 mol % of propane, 20 mol % of oxygen, the remainder being helium The reaction temperature was varied in the range 573-773 K and the space velocity was about 1500 cm3.gcat-' h-l Emerging gases were heated to 423 K and analyzed by on line gas chromatographs

3 CHARACTERISATION OF [Bi - Mo] CATALYSTS.

3-1. Aunvillius and Sillen Phases. XRD and ESCA patterns of these solids in the fresh state, and after reaction were essentially the same and revealed the absence of any other phase or changes in phase during the oxidation reaction.

3-2. Bismuth - Molybdenum - Titania catalysts A standard XRD analysis showed the presence of both anatase and rutile species, anatase being the main phase, but trace of bismuth and/or molybdenum oxides could not be found by this method. X P S results showed that there was a significant surface enrichment into bismuth and molybdenum in the fresh catalyst. During propane oxidation, we also noticed for the (3 4Bi4.8Mo)/Ti02 catalyst an increase of the content of molybdenum, so that the Mo/Bi atomic ratio varied from 2.1 to 2.6 (the bulk ratio was about 1.5) Following electron microscopy characterization, we observed Bi,O, (traces). By high resolution XRD ( W L ) techniques, Bi,Mo,O,, Bi,Mo,O,, and amorphous MOO, were characterized in the fresh catalyst while a new phase Bi,MoO,, appeared during the reaction Finally, even though the materials were not homogeneously dispersed, there was strong evidence of the formation of bismuth-molybdate phases on the titania surface.

308

4 CATALYTIC RESULTS 4-1. Aurivillius and Sillen phases The main catalytic results are reported in Table 2. We can remark that the activity of these phases was rather low even at high temperature. Nevertheless, a significant selectivity for propene (80%) was obtained in addition to a small fraction of oxygenated compounds.The nature of these compounds depended on the catalyst, acrolein being the main product with (Bi202)MoO4 (LT), whereas acetone and ethanal are formed with (Bi202)MoOq (HT) or (Bi202)Bi2Ti3010. At higher temperature, a significant homogeneous reaction took place. Table 2 Partial oxidation of propane on various Aurivillius and Sillen phases Solid

Selectivity (YO)

Conversion (%)

C,H,

0,

C,H,

Experimental coadihons: T = 773 K, P = 0.1 MPa, C,H,/O,/He

CO

= 3/1/1,

CO,

FTW

=

Coxyg.

acrol

1500 crn3.h-l.g-l

In comparison with results recently published on V-Mg-0 [14-161, on catalysts derived from niobium pentoxide [17] or on B-V-0 [18,19], the catalytic properties of Aurivillius and Sillen phases are quite significant. Moreover, when the experimental conditions (space velocity, partial pressure and temperature) were changed, appreciable changes of activity and selectivity were obtained as for (Bi,O,)MoO, (LT) in Table 3 . Indeed, by decreasing the space

velocity and increasing the partial oxygen pressure, there was a significant increase of propane conversion and of acrolein selectivity. The changes in catalytic properties versus oxygen partial pressure or space velocity are in agreement with consecutive reactions as already proposed by Moro-oka [ 101 and Ai [20] CH,-CHOH-CH, CH,-CO-CH, CH,-COOH + COX

'f

-

-

309

Table 3 :Partial oxidation of propane it1 the presence of (Ri,O j M o 0 , (LV; Itifuence of the experimental cotidifions (7’: 773 K). Molar rauo Space velocity Conversion (%) Selectivrty (%) C,H,/O,,He ~~

~ r n ~ . g c a t - ~ . hC,H, -~ ~

~

0,

C,H,

CO

CO,

COxyg

(a)

~

31111

3000

20

5

74

11

35

4

15

74

31111

1500

49

17

67

13

6

6

80

73

31210

3000

30

5

75

8

6

4

7

50

31210

1500

79

30

66

9

10

7

80

50

31210

750

175

60

42

12

16

8

220

21

(a) acoleme/Eoxygenates (%)

4-2. Bismuth orland molybdenum supported on titania The above results show that perovskite type oxides could be selective catalysts for the oxidative dehydrogenation of propane. Moreover, even in quite severe experimental conditions, we observed a significant acrolein selectivity. In order to improve the activity of these solids we tried, using similar preparation processes, to increase their specific surface area without success. We are now looking for new preparations of bulk and supported materials, derived fiom soft chemistry concepts. In the second case, titania was chosen as support, on account of results presented in table 2. In fact we tried to reproduce a (Bi,0,)Bi2Ti,0,, or a (B&O,) MOO, phase on the titania surface. Previous works have evidenced a well dispersed molybdenum oxide (in the presence of alkaline promoters) on titania [21]. The first results obtained with such solids are presented in the following paragraphs:

a) The activity and the selectivity of titania modified with a low content of bismuth or molybdenum are compared in Table 4. Altough the activity was much more significant than the previous ones (see preceeding paragraph), these catalysts show poor selectivity in formation of propene or oxygenated compounds.

b) Whatever the support used, when the molybdenum content was increased, there was also an enhancement in both the activity and the selectivity to oxygenated compounds, as well as a strong decrease in the propane dehydrogenation to propene (table 4). Among the 0compounds, acetic acid was the main product. This result is rather similar to the one obtained after adding a Mo-heteropolyacid to a V-P-0 catalyst as observed by Ai,[20].

310

Table 1 :Propane oxidation on Bi-Mo, TIO,catalysts Catalysts(e)

T (K) Conversion (YO)

Selectivity (YO)

C,H,

0,

C,H,

co

co,

TiO,

573

20

33

47

623

16 61

21

(7 1mZ/g)

72

18

37

45

16Bi/TiO,

573

18

25

10

17

72

(54m2/g)

623

68

90

18.5

14

66.5

2 .2Mo/TiO,

573

39

45

24

31

45

(65m2/g)

623

71

82

24

35

40

5. IMofI'iO, (P25)

598

89

76

6

38

19

(48mz/g)

623

11 7

100

4

39

18

7.8Mo/TiO,

598

81

7

41

(63mZ/g)

623

97

84 100

7

44

29 26

( 1.5Bi+2.1Mo)/TiO,

573 623

20 73

24 82

42 32

18.5 22

39 44

(3.4Bi+4.8Mo)/TiOZ

573

05

3

60

(27mz/g)

623

15

19.5

22

673 698

38

6 20

36

Bi/Mo = 0.71

24

25.5

30.5

98

44

10 6

81

24 28

27

723

15 11

(63mz/g)

coxyg

BdMo = 0.71

Expenmental c o n b o n s C,H,/O,/He

20

33

3/1/1, w lg, F/W 1500 cm3 h-l g

(a), (b) acehc acid or acroleln were respectively the man products (c)

acroleln 15%, acrylic acid 13%

(d)

acroleln 15%, acrylic acid 6 5%

(e)

Bi or /and Mo contents are m atom (%)

The simultaneous addition of bismuth and molybdenum (with a low (Bi -t Mo) content) increased propene selectivity without acrolein formation (Table 4). This is also demonstrated C)

by the results presented in figure 1, where the selectivity is compared at the same propane conversion (So/,). When the (Bi + Mo) content was increased, we observed at low temperature (. I 673 K) a significant acrolein formation and at high temperature (> 673 K), the formation of both acrolein and acrylic acid. Here the addition of bismuth to Mo/TiO, catalyst inhibited

311

the acetic acid formation and favoured the C, oxygenated compounds (step 3 of the above reaction scheme) in agreement with the proposal of Moro-Oka [lo].

(i0

50

k i p r e I : Propane oxidation in the presence of BI-Mo/TIO, catalysts. Product distribution at a 5% propane conversion. Conditions :(;H$OJHe = 3/1/I, F = I,5 I.H-‘

z0 20 10

0

Ti02

1,6Bi

2 2MO

5. CONCLUSION In t h s paper, we showed that the Aurivillius and Sillen phases are selective catalysts for the partial dehydrogenation of propane into propene. However, due to the small surface area, the activity of these materials are rather low and the reaction must be carried out at high temperature In order to improve the catalytic performance of the bismuth-molybdate system, we tried to disperse it on a titanium dioxide support.We then observed that both the activity and the selectivity to oxygenated compounds increased significantly. Moreover, the detailed products distribution depended very much on the Mo andor Bi contents. On the basis of these first experimental results related to the effect of the temperature, the space velocity and the partial pressure, we believe that the reaction proceeds via propene as the reaction intermediate, similarly to the scheme proposed by Moro-Oka [lo] and Ai 1201. Following these results, we are now studying both the preparation of new catalysts and the mechanistic aspects of these reactions.

Acknowledgments The authors would like to thank the consortium “Actane” for the financial and for beneficial discussions.

312

References G. Centi,F. Tnfiro, J.R. Ebner and V.M Franchetti, Chem. Rev., 88 (1988) 55 G.W Keulks, L.D. Krenzke and T.M Notermann, Adv. Catal., 27 (1978) 183 3 R.K. Grasselli and J.D Burrington, Adv Catal , 30 (1981) 133. 4 G.C.A. Schuit and B C. Gates, Chemtech (1983) 683. 5 Z. Bin@. Pei, S. Shishan and G. Xienian, Chem. SOC.Faraday Trans., 86 (1990) 3145 6 M. Ai, J. Catal., 101 (1986) 389. 7 T. Komatsu, Y. Uragami and K. Otsuka, Chem. Lett., (1988) 1903. 8 N. Giordano, J.C.J. Bart, P. Vitarelli and S. Cavallaro, Oxid. Commun., 7 (1 984) 99. 9 Y.C. Kim, W. Ueda and Y. Moro-Oka, Chem. Lett., (1989) 53 1 10 Y.C. Kim, W. Ueda and Y. Moro-Oka, Appl. Catal., 70 (1991) 175. 11 D.J. Buttrey, D.A. Jefferson and J.M. Thomas, Philosophical Magazine A, 53 (1986) 1

2

897. 12 C. Grosset, Thesis, Poitiers, France (1991). 13 J. Barrault, C . Grosset, M. Dion, M. Ganne and hf. Toumoux, Catal Letters, 16 (1992) 203. 14 D.S.H. Sam, V. Soenen and J.C. Volta, J. Catal, 123 (1990) 417. 15 K.T. Nguyen and H.H. Kung, J. Catal., 122 (1990) 415. 16 M.C. Kung and H.H. Kung, J. Catal. 134 (1992) 668. 17 R.H.H. Smits, K. Seshan and J.R.H. Ross, Stud. in Surf Sci. Catal., Elsevier Ed., (1992) 221. 18 A. Cherrak, R. Hubaut, Y. Barbaux and G. Mairesse, Catal. Lett., 15 (1992) 377. 19 A. Cherrak, Thesis, Lille, France, (1993). 20 M. Ai, Catal. Today, 13 (1992) 679.

72

313

B. GRZYBOWSKA (Inst.Catalysis, Krakow, Poland) Have you not observed in your Bi Mo oxide catalysts other phases well known for many years in this svstem such as Bi,(MoO,), (aphase) and Bi,Mo,O, (0phase) 7

J. BARRAULT (Lab Catal , Poitiers, France) The high resolution XRD (INEL) characterization of the 3 4Bi-4 8Mo/TiO, catalyst showed the presence of different phases, Bi,Mo,O,, Bi,Mo,OI5 and amorphous MOO, in the fresh solid and new phase Bi,MoO,, after reaction Indeed some of them (Bi,Mo,O,) are well known and very selective for the partial oxidation of propene but we have no information concemng the catalytic properties of the others J.C. VOLTA (I.R.C,7 Villeurbanne, France) . Did you control the eventual departure of Mo from your catalysts which should occur particularly in presence of water ? J. BARRAULT : All the catalysts presented in this study were not analyzed after the catalytic test but from the blank experiments done with the “empty”reactor after the reaction we did not observe propane transformation. Moreover the chemical analysis of some of the Aurivillius and BiMo/TiO, phases, did not demonstrated the exit of molybdenum. This was confirmed by the ESCA analysis of the solids before and after the propane conversion which did not show any significant change of surface the molydenum species.

F. TRIFIRO (Dpt. Chm. Ind., Bologna, Italy) : Bi Mo oxides based catalyst are active and selective in oxidation of propylene. Bi Mo V oxides based catalysts are active and not too much selective in oxidation of propane. On the basis of which type of consideration you carried your investigation on Bi- Mo-oxides dispersed on TiO, ?

J. BARRAULT : The answer to this question appears in paragraphe 4.2 of our paper. G. CENT1 : (Dpt.of 1nd.Chem. Mater., Bologna, Italy) : You have performed the catalytic tests with a high concentration of propane (about 60%). Using a so high concentration of hydrocarbon, radical mechanisms of oxidation are favoured. The type of oxidation products you observe are in agreement with this hyphothesis. My

question is therefore the reason of using this concentration of alkane especially because you have usually veIy low conversion and what is the possible contribution of homogeneous gas phase reaction (especially in terms of a mixed heterogeneoushomogeneous mechanism). On the behavior you observed, I think that the use of lover hydrocarbon concentration (my be 5-10%) can clearly give usehl indication on this problem.

314

J. BARRAULT : First of all I suppose that the question concerns the results obtained with Aurivillius and Sillen phases which have been used at T 5 773 K. i) With regards to these solids, we investigated the oxidative pyrolysis of propane in a void volume (empty reactor) without observing any significant propane conversion at a temperature below 800 K (Part of the reactor was packed with quartz chips which are very effective radical quenchers). ii) A heterogeneous-homogeneous catalysis depends on the post-catalytic volume. In our equipment this volume was reduced by connecting the post-reactor section and the analysis section with a capillary tube. iii) By changing the residence time (table 3 ) we did not noticed any significant induction period often found in chain reactions (1) In our opinion, a less significant part of the products is formed via homogeneous reactions.

(l)K.T. NguyenandH.H.Kung, J. Catal., 122,(1990),415

V. CortCs CorberBn and S. Vic Bellon (Editors), New Deveioprnenls i n Selective Oxidution 11

0 1994 Elscvier Science B.V. All rights reserved.

315

A Study of the Catalytic Oxidative Oligomerization of Methane to Aromatics Andrew P. E. York, John B. Claridge, Malcolm L. H. Green' and Shik Chi Tsang

Inorganic Chemistrybboratoiy, Universityof Oxford, South Parks Road, Oxford, OXI 3QR, U. K. The formation of aromatics during the reaction between methane and oxygen between 970 and 1220K, at elevated pressure has been studied. High selectivities and yields of the

aromatics, i.e. benzene and toluene, are obtained over certain oxide catalysts, although, aromatics are also formed when no catalyst is present. A manganese oxide catalyst doped with sodium chloride gave the highest yield of aromatics, however, supported nickel or platinum catalysts, which have been reported to be active catalysts for aromatics formation, were found to give lower yields of aromatics than for the gas-phase reaction. The mechanism for the production of aromatics has been investigated.

1. INTRODUCTION

The utilisation of methane, the major constituent of natural gas, has been actively researched by a vast number of research groups, with the objective of producing condensable products at room temperature, thus, reducing the cost of transportation from the large number of remote gas reservoirs situated in areas such as Siberia and the Middle East [ 11. Currently the only economically viable industrial process employed for methane conversion is the steam reforming reaction [2,3], in which methane is reacted with steam at elevated pressure and high temperature over a nickel catalyst to give synthesis gas which is then reacted further in the Fischer-Tropsch synthesis of hydrocarbons or the synthesis of methanol. Of the direct methane conversion routes explored, partial oxidation of methane to methanol and formaldehyde has attracted a lot of attention. However, this seems to be limited to yields of up to 8% [4],and even this has proved difficult to reproduce [S]. This reaction is still unable to compete economically with the conventional methanol synthesis route via synthesis gas. Oxidative coupling of methane to hydrocarbons appears to offer more promise, but a practical limit, for ethene, of about 20% yield seems to have been reached, over a number of different catalyst systems, without the need for excessive dilution or added chlorine both of which can aid enhancement of the C2 yield. We are grateful to the Gas Research Institute for supporting this work and funding S.C.T., and to B.P. Chemicals for a CASE award to A.P.E.Y.

316

High yields of ethyne and aromatics can be obtained by the high temperature pyrolysis of methane [6], and the separation of aromatics fiom methane is trivial. However, in the case of methane pyrolysis, dissociation of methane to hydrogen and, more importantly, carbon at the temperatures employed (> 1450 K) cannot be ignored. It is, therefore, desirable to use an oxidising agent, i.e. oxygen, to drive the reaction at lower operation temperatures so as to avoid high temperature thermo-pyrolysis of methane to disastrous carbon. The number of publications concerned with the synthesis of aromatics fiom methane with the assistance of oxidising agent is limited [6-lo]. Some of the reported results have been sumrnarised in Table 1 . Of note is the report by Exxon Research and Engineering Co. [6] of the synthesis of ethene and benzene from alternate switching of methane and air over mixed platinum and barium catalysts using a MgAI204 spinel blocked alumina as the support. It is proposed that the benzene is formed via a surface carbide species, or a "special form of coke on the catalyst".

Table 1 Some of the Previously Published Results for Aromatics Production from Methane Catalyst

Temp. IK

Pressure Iatm.

GHSV /hrl

CH4 I oxidant

CH4 conv. I

PtlCrlBalMgl

977

1-20

-

switching

% 12.6

29.6*

3.7

963

1

8000

9:1:4(air) (N*O)

28.0 44.4

33.1 >90

9.3

(N30)

0.1

20

Cg+ sel. Cg+ yield 1% I Yo

Al203[61

Ni/A1203[81

Pentasilzeolite[g] ZSMS[*O]

630

1

53570

40

* The selectivity given does not include the carbon formation on the catalyst. However, these other reports maybe the same and have simply neglected to mention carbon formation which may be considerable in most cases. There is also the report of Abasov et al. [8] who used a supported nickel catalyst for the reaction of methane with oxygen at 1 atmosphere resulting in a benzene selectivity as high as 75%, with a methane conversion of 7%. The optimum benzene yield achieved was 9.3%, and the importance of catalyst preparation was strongly emphasised. It was suggested that a strongly sorbed oxygen species on the catalyst was responsible for the benzene formation. Other work [9] outlines the use of Pentasil zeolite catalysts, both with and without metal loading, and nitrous oxide as the oxidant, to give very high aromatic product selectivities, although it is economically more attractive to use oxygen or air as the oxidising agent. Anderson and Tsai [ 101 reported the conversion of methane to aromatic hydrocarbons, again using nitrous oxide as the oxidant, achieving up to 20% selectivity using a ZSM5 zeolite catalyst, but the methane conversion is extremely low.

317 An attempt to reproduce the results obtained by Abasov et al. using the conditions described in their work [8], and under our conditions has been made. Also, the catalyst described by Exxon Research and Engineering Co. [6] has been examined, under our conditions, and a number of other catalysts have been tested and compared to the gas-phase reaction (i.e. no catalyst).

2. EXPERIMENTAL

Reactions were carried out in a silica tube of 4 mm i.d. and 400 mm in length, in a Heraeus tube furnace. Gas flow rates were controlled by Brooks 5850TR mass flow controllers and the reactor pressure by a Tescom back pressure regulator. The products were analysed on a Hewlett-Packard 589011 gas chromatograph fitted with a methanator before the FID, and separation was achieved using a 3m Porapak Q column. The delivery tube to the gas chromatograph was heated to 180OC so that condensation of liquids in the pipework was avoided. A H.P. 5971A mass spectrometer detector was used to aid in product identification, and a Valco 6-port valve was employed in the switching experiment used for investigation into the reaction mechanism. All the catalysts were prepared by an incipient wetness method on silica and alumina supports using the corresponding soluble salt, except the 0.3% NdAl203 which was prepared as described by Abasov et al. [8] and the Exxon catalyst which was prepared as in the patents [6]. The reactant gases were all greater than 99% purity. 3. RESULTS

3.1 Investigation into the Reported Catalysts.

The catalysts tested were the supported nickel catalyst and the supported platinum catalyst made according to the methods of Abasov et al. and Exxon. The results obtained by the respective research groups have already been given in Table 1, and we have tested our catalysts by cofeeding methane and oxygen to the reactor. The conditions used and results obtained in this study are given in Table 2. 0.3% Ni/AI,O,

The results showed that a supported nickel catalyst gave mainly carbon monoxide and carbon dioxide as the principal carbon containing products. There were only traces of benzene (0.3% selectivity) detected at 1170 K at elevated pressure. In addition, a significant amount of hydrogen was detected when the temperature was above 970 K and it can be concluded that synthesis gas is being produced. Indeed, a number of researchers have reported that nickel is a good catalyst for the partial oxidation of methane to synthesis gas [ 11,121. This catalyst gave a lower yield of aromatics than the empty tube reaction (shown in Table 3) under the same conditions and no aromatics were detected when using the conditions described in the literature [8].

318

Table 2 0.3 g catalyst, total flow rate = 60 ml min-'. t reaction at 1 atm. * reaction at 6 atm. Catalyst Temp. CH4102 CH4 /K conv./% Ni/AI?O?t 960 9 7.96 8.56 10 970 Ni/A1;01* 970 10 9.62 Exxon* 10 16.96 1170 Exxon*

CO 48.8 36.5 32.8 87.4

Sel. 1% CO? C7. 50.2 1.0 15.1 47.9 19.9 46.1 3.8 5.3

Cq 0.0 0.5

Ch+

1.2

0.0 0.0 0.0

0.1

3.4

Exxon Catalyst (Pt/Cr/Ba/Mg/A1203) The supported platinum catalyst described by Exxon Research and Engineering Co. [6] gave similar results to those for the nickel catalyst with no aromatics detected at temperatures below 970 K. However, selectivities of 3.4% to aromatics and 3.8% to C2 products were detected at elevated pressures and temperatures. This catalyst also gave high hydrogen to carbon monoxide ratios (1.7 - 2.1) at temperatures greater than 970 K and is, as in the case of the nickel catalyst, producing synthesis gas. Thus, under our reaction conditions, neither the nickel nor the platinum catalysts were able to give the high yields of aromatics reported.

3.2 The Effect of a Number of Different Oxide Catalysts on the Aromatics Yield and Comparison with the Gas-Phase Reactions During our investigation of the catalysts used by Abasov et al. and Exxon, it was noticed that at slightly elevated pressures and temperatures the formation of aromatics was enhanced. As a result, the pure gas-phase reaction for the production of aromatics has been studied and the catalytic results using oxide catalysts at elevated temperatures and pressures were also compared. All the catalytic reactions in this section were camed out with 0.3 g of catalyst, at 6 atm. pressure and with a CH4:02:N2 ratio of 10:1:4. The results are shown in Table 3.

Gas-phase reaction It has been reported in the literature that the influence of gas-phase reactions is relatively important especially at elevated pressures and at the high temperatures [13], and therefore, studies were conducted in an empty tube reactor. The results are assumed to be pure gas phase reactions although, it is likely that gas phase reactions would be affected by the choice of reactor wall. From Table 3, it can be seen that the reaction at 1220 K and 1 bar resulted in a methane conversion of about 12%, complete oxygen conversion, and a low aromatics selectivity of 2.7%, and this low production of aromatics has been noted by ARC0 [ 141. However, the aromatics selectivity and yield increased significantly as the pressure was increased from 1 to 6 atm. As the temperature was increased, in the reactions at elevated pressure, from 970 K to 1270 K, the selectivity to hydrocarbons increased, with the aromatics selectivity particularly affected. When the reaction temperature was at 1270 K, the selectivity to aromatics was about 32%, corresponding to a yield of 5%, but the yield dropped rapidly

319

when the temperature reached 1320 K, due to decomposition of the hydrocarbon products to carbon, and oxidation of the products to carbon oxides. During the reaction a small amount of toluene soluble organic residues and carbon were deposited on the reactor walls. The UVvisible spectrum showed that these deposited organics had a large extinction coefficient in the region associated with polyaromatics, such as naphthalene, anthracene, phenanthrene [ 151. The amount ofresidue deposited at 1220 K was about 0.02 g over 5 hours, which was not significant compared with the yield of aromatics obtained. Table 3 Catalytic Effect on Aromatics Formation. Silica tube packed with 0.3 g catalyst, pressure = 6 atm., CH4:O2:N2 = 10:1:4, flow rate = 60 ml min-1, (* Reaction at 1 bar). Catalyst Temp. CH4 Sel. /'YO /K conv./% CO CO? C2H4 C2fi cq+ C(& C7H8 1120 8.0 60.7 6.0 12.5 15.4 3.5 2.1 0.0 Gas-phase

Voxide ISiO? Woxide ISiO? Croxide /SiO2 WBaCOR

Mn02

NaCl/MnO2

1170 1220* 1220 1270 1320 1120 1220 1120 1220 1120 1220 1070 1120 1170 1220 1070 1120 1170 1220 1070 1120 1170 1220

9.0 12.3 12.6 15.4 16.4 10.3 12.6 10.1 11.8 10.8 10.7 10.2 10.6 11.3 12.9 10.2 10.1 11.1 12.5 10.2 10.7 11.3 13.2

60.8 54.0 46.1 40.7 76.3 54.5 71.5 49.2 67.1 48.1 63.8 37.6 31.0 31.4 39.5 28.1 36.3 48.9 48.5 22.5 26.5 31.7 38.7

6.5 3.7 4.7 4.4 6.0 8.7 10.8 20.4 16.9 10.0 8.2 21.2 27.1 26.7 20.1 37.4 34.5 25.9 18.9 35.1 32.0 26.1 11.6

12.8 30.6 19.8 15.1 1.2 22.0 4.9 18.9 5.5 25.0 13.0 25.2 25.1 21.1 15.3 20.6 16.7 11.3 10.6 25.2 23.4 20.4 17.8

7.2 3.8 5.5 6.7 2.6 7.1 3.9 6.4 2.9 6.0 5.4 8.8 7.0 6.2 5.8 9.4 7.3 4.5 5.2 10.2 8.0 6.4 6.4

2.9 5.2 2.9 1.2 0.1 4.6 0.2 3.5 0.3 6.0 1.1 6.2 5.6 3.8 1.8 3.8 2.8 1.4 1.0 5.9 4.9 3.4 2.1

7.8 2.7 18.5 29.2 12.2 3.1 8.1 1.7 7.3 4.0 8.4 1.0 3.3 8.8 15.7 0.7 2.3 7.0 14.3 1.1 4.2 10.1 20.8

1.9 0.0 2.6 2.6 1.6 tr 0.6 tr tr 0.9 tr tr 0.9 1.8 1.8 0.0 0.0 1.0

1.4

tr 1.0 1.9 2.6

V oxide/Si02, W oxide/Si02, Cr oxide/Si02 The supported vanadium, tungsten and chromium oxide catalysts gave very similar results. These catalysts have been reported to give small amounts of methanol and formaldehyde from methane and oxygen. However, no oxygenated products were detected

320

under our reaction conditions. At 1120 K the selectivity to C2 products was high and the aromatics yield was slightly higher than for the gas-phase reaction. However, at 1220 K the C2 selectivity dropped significantly, and the aromatic selectivity was lower than for the gas phase reaction at the same temperature. Also, the selectivity to carbon oxides was higher than in the gas-phase and it appears that products formed are being hrther oxidised at this temperature. 0.9% K/BaC03 The selectivity to aromatic products over this catalyst was higher than for the supported nickel and platinum catalysts described above and at temperatures below 1170 K the aromatics yield was higher than for the empty tube reactions. The use of potassium doped BaC03 catalysts as effective oxidative coupling catalysts at 1 atm. has been reported [16]. In this study at 6 atm. pressure it is found that quite high selectivities to carbon monoxide and carbon dioxide are obtained as well as reduced yields of C2 products, with relatively high aromatics yields.

Mn02

The Mn02 catalyst gave mainly carbon oxides as the products, with an aromatics selectivity lower than that obtained in the gas-phase reaction at all the temperatures studied. This catalyst may be hrther oxidising the product hydrocarbons to carbon oxides and this has been reported previously [ 171. Under the conditions employed here the Mn02 may be reduced to a lower oxide, but no attempt to identify the phase present in the reactions was made.

7% Na (NaCI)/MnOz The NaCl promoted MnOz gave the highest hydrocarbon selectivities of all the catalysts studied, and in fact problems were encountered with this catalyst due to high boiling aromatic hydrocarbons (e.g. naphthalene) depositing in the back-pressure regulator, a problem which did not occur to any observable extent using the other catalysts. Burch et al. [17] reported the addition of chloride on manganese oxide catalysts to enhance the C2 yield in the oxidative coupling of methane, and this enhancement in C2 yield may in turn be aiding the production of high yields of aromatics. Unfortunately, hydrolysis of the chloride shortens the life of these types of catalyst, and so high aromatics yields could not be sustained. 3.3 Study of the Mechanism A study of the mechanism was conducted by first observing the effect of residence time

(or flow rate) on the aromatics yield and the results from this experiment are shown in Figure 1. It can be seen that, at both the temperatures studied, as the flow rate was increased the selectivity to aromatic products decreased significantly and this trend suggests that the aromatics are likely to be secondary products. A switching experiment was carried out to observe the effect of ethene on the yield of benzene, as it was thought that the aromatics may be being formed from the ethene produced in the reaction. CH4/air was passed through a reactor at 1170 K and the feed stock composition then switched, using the Valco valve, to CzH4/air at the same composition. The products were monitored on the on-line mass spectrometer, and the results showed that the benzene production increased 10-fold on switching to ethene and then returned to its original

32 1

level after switching back to methane. This implies that the benzene can be formed via the ethene produced in the reactor. 14

12 10

$

b

$

.$

8 6

4 2 0 ' 0

50

100

Flow rate / d m i n

Figure 1. Effect of flow rate on the aromatics yield. (0) 1120K;(+) 1220K

0

10

20

30

40

50

Time / mins

Figure 2. Switching experiment to show the effect of ethene on the formation of benzene (m/e = 78).

4. DISCUSSION

We have investigated the 0.3% Ni/AI,O, catalyst under the conditions used by Abasov et al. but no aromatic products could be detected. Under our conditions both the nickel catalyst, and the supported platinum catalyst patented by Exxon Research and Engineering Co. gave much lower aromatic yields than those obtained in the gas phase reaction. Carbon monoxide was the main carbon containing product over Exxon catalyst at 1170 K, and with approximately 17% methane conversion (CH4/02 = lo), this suggests that synthesis gas is being produced instead of total oxidation of methane to carbon oxides and water. Indeed, we previously reported the selective production of synthesis gas over supported noble metal catalysts i.e. nickel or platinum and found that the H2/C0 ratio is about 2 when CH4/02 = 2 at 1050 K. The calculations have shown that they catalysed methane and oxygen reaction to thermodynamic equilibrium giving high yield of synthesis gas at the equilibrium gas composition [ l l ] . This can be explained as these types of catalyst are so active that the partially oxidised products of the reaction, for example benzene, which are more reactive than methane, are hrther oxidised more rapidly. Supported platinum group metals can also catalyse steam and carbon dioxide reforming of methane and other hydrocarbons so that highly selective formation of synthesis gas is achievable, and therefore, this explains the fact that we have observed very low yields of aromatics. The proposals that the benzene is formed via a surface carbide [6] or a strongly sorbed oxygen species [8] seem unlikely given that higher selectivities and yields to aromatics can be achieved in the gas-phase reaction when no catalyst is present. The influence of gas phase reactions should not be ignored. Additional evidence that the products are hrther oxidised after formation can be obtained from the results of the

322

supported chromium, vanadium and tungsten catalysts, where no oxygenates were observed. On increasing the temperature the yields of aromatics and C2 products decreased while the amount of carbon oxides increased to a level higher than that from the gas-phase reaction. The introduction of WBaC03 to the reactor led to an enhancement in the aromatic yield at relatively lower temperatures than in the gas-phase reaction, but at the higher temperatures this catalyst also appears to catalyse the destruction of the hydrocarbon products. The presence of NaClB4n02 resulted in an increase in the aromatic yield at all the temperatures studied and it can be concluded that a catalytic effect does exist. WBaC03 and N a C M O 2 have been reported as excellent methane coupling catalysts for the production of ethene [16,18], and ethene oligomerization to aromatics is known [19]. Therefore, it is possible that the formation of aromatics is somehow related to the oxidative coupling of methane. Experiments showing the effect of residence time on the aromatics yield suggest that the aromatics are likely to be secondary products, and those catalysts which are active for the oxidative coupling reaction at ambient pressure are selective towards aromatics at elevated pressure. The switching experiment showed that by introducing ethene a 10 fold increase in the benzene yield occurred. It is therefore concluded that the benzene is formed from the ethene produced by the oxidative coupling of methane, possibly via some intermediate C4 hydrocarbon or directly via gas-phase radical reactions. It should be also noted that DielsAlder reactions to give aromatics from alkenes may occur under our reaction conditions [20].

REFERENCES 1. 2. 3. 4. 5. 6.

N. D. Parkyns, Chem. Brit., 26 (1990) 841.

G. C. Chinchen, K. Mansfield and M. S. Spencer, Chemtech., Nov. (1990) 692. M. E. Dry, J. Organornet. Chem., 372 (1989) 117. H. D. Gesser, N. R. Hunter and C. B. Prakash, Chem. Rev., 85 (1985) 235. R.Burch, G. D. Squire and S. C. Tsang, J. Chem. SOC.,Faraday Trans. I, 85 (1989) 3561. H. L. Mitchell and R. H. Waghorne of Exxon Research and Engineering Co., U. S. Patents, 4,172,810 ; 4,239,658 ; 4,507,517. 7. S.A. Shepelev and K.G. Ione, React. Kinet. Catal. Lett., 23 (1983) 323. 8. S. I. Abasov, F. A. Babaeva and B. A. Dadashev, Kinet. Katal., 32 (1991) 202. 9. V. D. Sokolovskii, T. M. Yur'eva, Yu Sh Matros, K. G. Ione, V. A. Likholobov, V. N. Parmon and I. Zamaraev, Russ. Chem. Rev., 58 (1989) 2. 10. J. R.Anderson and P. Tsai, Appl. Catal., 19 (1985) 141. 1 1 . P. D. F. Vernon, M. L. H. Green, A. K. Cheetham and A. T. Ashcroft, Catal. Lett., 6 (1990) 181. 12. D. Dissanayake, M. P. Rosynek, K. C. C. Kharas and J. H. Lunsford, J. Catal., 132 (1991) 117. 13. D. J. C. Yates and N. E. Slotin, J. Catal., 1 1 1 (1988) 3 17. 14. A. M. Gaffney, C. A. Jones, J. J. Leonard and J. A. Sofranko, J. Catal., 114 (1988) 422. 15. D. H. Williams and I. Fleming, Spectroscopic Methods in Organic Chemistry (4th Edn.), McGraw-Hill, London, 1989. 16. K. Aika, T. Moriyama, N. Takasaki and E. Iwamatsu, J. Chem. SOC.,Chem. Commun, (1986) 1210.

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17. R. Burch, G. D. Squire and S. C. Tsang, Appl. Catal., 43 (1988) 105. 18. R. Burch, G. D. Squire and S. C. Tsang, Appl. Catal., 46 (1989) 69. 19. I. V. Elev, B. N . Shelimov and V. B. Kazanskii, Kinet. Katal., 25 (1984) 1124 20. D. Nohara and T. Sakai, Ind. Eng. Chem. Res., 3 1 (1992) 14.

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O.V. Krylov ( Institute of Chemical Physics, Academy of Sciences of Russia, Moscow, Russia): In the beginning of your work, you have mentioned Dadashev' paper where they obtained very high yield of aromatics. Can you compare their data with yours? What is the stability of your manganese containing catalysts? S.C. Tsang (University of Oxford, Oxford, U.K.): It is the high yield (>9%) and high selectivity to aromatics (>33%) over Ni catalyst reported by Abasov and Dadashev that prompted us to study aromatics formation from methane. However, we cannot reproduce their results under their reaction conditions nor under our elevated pressures conditions. In both cases Ni/Al,O3 gave a high yield of synthesis gas with no aromatics at the temperatures from 950 K to 1170 K. Abasov and Dadashev have commented on the extremely significant effect of catalyst preparation to the benzene formation and therefore our failure to reproduce their results may be due to slight differences in our catalyst. Nevertheless, our previous studies on methane partial oxidation using group VIII metals indicated that group VIII metals are very active catalyst that can catalyse methane oxidation reaction to thermodynamic equilibrium giving synthesis gas (ref 11). Therefore it is not suprising that Ni subseqent oxidation of any hydrocarbons to carbon oxides occurs under the reaction conditions. On the other hand, sodium chloride doped manganese oxide is the best catalyst we tested for aromatics formation. This catalyst deactivated by loosing small amount of chlorine as reported in the literature (ref 5), however, it maintained its activity during our measurements (about 3 hours). The effect of chlorine on catalyst surface or in the gas phase on aromatics formation is not yet known. K. Otsuka (Tokyo Institute of Technology, Tokyo, Japan): For the formation of aromatics of NaClh4n02 and K/BaC03, which are well known catalysts for oxidative methane coupling reaction, higher temperatures than 1100 K and high pressure (- 6 bar) are necessary according to the results in Table 3 . However, oxidative coupling reaction does not require higher pressures than 1 bar and optimum temperatures are around 1000 K. If the formation of aromatics is related to oxidative coupling of methane and ethylene is precursor of aromatics, why do you need such severe reaction conditions for aromatics formation? S.C. Tsang: It is true that the optimum temperatures of methane coupling reactions giving C2 products are around 1000 K and would not require pressure higher than 1 bar. However, the subsequent reactions of either C , hydrocarbons or their intermeidates to aromatics require more severe conditions. As a result, the optimum conditions for aromatics formation in a single stage reactor will be different from methane coupling reactions. One can also imagine that cyclization of C2 hydorcarbons to aroamtics is likely to be more favourable at higher pressures. E.A. Mamedov (Inst. Inorg. Phys. Chemistry, Baku, Azerbaijan Republic): You observed a formation of certain amounts of C3 hydrocarbons which are known to undergo the process of dimerization and cyclization more easily than C2 hydrocarbons. Why don't you take into account a route of aromatics formation through the C3 hydrocarbons?

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S.C. Tsang: We know that C3 can also undergo cyclization to aromatics under pressure. We cannot disaccount this mechanism of giving aromatics. In fact, we believe some of the aromatics are actually formed by cylization of C3 hydrocarbons. However, the production of C3 hydrocarbons at ambient pressure and 6 atmospheres is only a small fractions of the total hydrocarbons (<6% selectivity to C3 hydrocarbons) and this agrees with the methane coupling mechanism via radical reactions that occurs in the same temperature regime. The strong inverse relation between C2 and aromatics leads us to posulate C2 cyclization to aromatics is more likely to account for high selectivity to aromatics under our reaction conditions. G. Centi (Dept. of Ind. Chem. & Materials, Bologna, Italy): The content of aromatics, especially that of benzene, in the gasoline pool should be decreased to meet regulations. In your opinion what is thus the interest in a process of synthesis of benzene and other aromatics from methane? S.C. Tsang: We are aware that new regulations to further reduce the aromatics content of gasoline are currently being imposed in many contries and we agree that the production of aromatics from methane is not likely for fuel use. However, what we have shown in this paper is that high selectivity to aromatics from methane and oxygen can be obtained at elevated temperatures and pressures. The evidence points to the formation of aromatics from methane via C2 hydrocarbons (or their intermediates) derived from methane oxidative coupling. This clearly demonstrate the concept of condensing C2 hyodrocarbons, especially ethene, from methane coupling reactions to other useful products under increased pressure before it is fhrther oxidised in the catalyst bed. Therefore, it is conceivable that by choosing a suitable catalyst, conversion of methane to other higher hydrocarbons via methane coupling processes silimilar to our aromatic process can be achieved in a single stage reactor. M. Misono (University of Tokyo, Tokyo, Japan): As related to the subject of this meeting, that is, new (or future) developments in selective oxidation, what is most interesting may be how we can improve the performance by using catalysts. So if you have any thoughts for catalyst design for this purpose, I would like to know. S.C. Tsang: We have shown that high selectivity to aromatics from methane can be obtained in a single pass over a catalyst bed, though the mechansim suggests that it is probably via a sequence of reactions. The enhancement of aromatics formation over gas phase reactions by using sodium chloride doped manganese oxide clearly demonstrates that the performance can be improved by using a suitable catalyst. Our intital thought is that a catalyst that promotes methane coupling reactions and C2 cyclization would be likely to be an active catalyst. Therefore, a systematic investigation for catalysts that catalyse both of these reactions could be worthwhile.

M. Sinev (Inst. of Chemical Physics, Moscow, Russia): In the beginning of the 80's in one of the patents it was shown that in separated cycles of treatment of the solids (Pt metals over

326

complex oxides containing Mg, Ba, Mo, W and other ions) with methane and next regeneration the most of aromatic hydrocarbons can be obtained at treatment of "coke"like deposites with steam. What can you say about contribution of this way of aromatics production over your catalysts, which also contain the similar components? S.C. Tsang: We have not investigated our catalyst under redox mode for aromatics formation. Therefore, we do not know whether aromatics will be produced under steam treatment. However, under our co-feed conditions, significant amounts of aromatics can also be produced from methane and oxygen at elevated pressures and temperatures in an empty reactor tube where no catalytic surface is available for special "coke" formation. Although we cannot rule out that a special form of "coke" may have been produced on the surface of the silica tube, the strong inverse relation between C2 and aromatics products and the results from our switching experiment suggest that it is more likely to be gas phase reactions. We envisage that our catalyst generate C2 products from methane and oxygen and under our elevated pressures and temperatures, they are subsequently be converted to aromatics.

V . Cork% Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxrdatiun I1 0 1994 Elsevier Science B.V. All rights reserved.

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Investigation of Molten Cobalt Halide / Sodium Metavanadate Mixtures as Redox Catalysts for the Oxidative Coupling of Methane. J. B. Claridge, M. L. H. Green, R. M. Lago, S . C. Tsang and A. P. E. York Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR9 U. K.

Molten mixtures of sodium metavanadate and cobalt halide have been tested as catalysts for the activation of methane at 75OOC in redox mode. It is found that these melts can convert methane selectively to C2+ products in the absence of molecular oxygen. The initial activity and selectivity of the melt can then be restored after dioxygen treatment. It is also found that the addition of cobalt chloride gives a higher CH4 conversion, C2+ selectivity and C2H4/C2% ratio than pure sodium metavanadate. Surprisingly, addition of cobalt bromide shows a similar enhancement. IR and ESR spectra of quenched catalyst samples showed that the cobalt and vanadium ions are reduced by methane and can be reoxidised by dioxygen 1. INTRODUCTION

The work of Keller and Bhasin [l] concerning the conversion of methane to higher hydrocarbons by using solid reducible oxides conducted in redox mode has attracted widespread interest and many different classes of oxides and mixed oxides have been investigated [2-61. However, there are only few papers reporting the use of molten salts to activate methane, despite the fact that some molten salts are reported to be very selective for hydrocarbon activation [7-171. Reduced oxygen species, namely peroxides and superoxides, possibly active for selective methane activation, have been demonstrated to be stabilised in some molten salt media [7-lo]. Furthermore, the molten salts have some advantages over the solid catalysts such as higher thermal conductivity (dissipation of the reaction heat), the catalyst compositions are more easily reproducible and their contact surface always remains fresh due to the high mobility of the components in liquid form. The operation in redox mode should also improve the product selectivity by avoiding non-selective gas phase reactions with molecular oxygen. In this work cobalt halide/sodium metavanadate molten mixtures were studied in redox mode for the oxidative coupling of methane. It is known that the addition of chloride can lead to a higher ethene to ethane ratio in methane coupling reaction [IS-231. Therefore, we have investigated the effect of different cobalt halides in molten salt systems.

328

2. EXPERIMENTAL The molten salt catalysts were all prepared from high purity reagents. Components were mixed, ground, and melted in a hrnace at 800°C for 2h. 2 g of the resulting mixture was used for the catalytic tests. The catalytic experiments were carried out in a silica tube reactor of 120 mm length and 8 mm i d . and reactant gases were bubbled to the molten salt via a thin stainless steel tube, The reaction products were monitored either by quadrupole mass spectrometer (HP5971A) or a Pye-Unicam G.C. All the exit tubes were heated at 120°C to avoid any product vapour condensation. For the experiments in redox mode pure dioxygen at 5 ml min-l was bubbled through the melt for 10 minutes and then the system was purged with nitrogen at 10 ml min-l for 10 minutes (until no residual oxygen could be detected by the mass spectrometer) and finally, methane was bubbled through the melt at 10 ml min-'. The initial product gases were sampled 2 minutes after methane was switched on. After the reaction the molten mixtures were quenched to room temperature and the samples transferred under inert atmosphere for IR and ESR spectroscopic studies. The ESR spectra were obtained in a Varian E-line Century series operating in the X-band with 100 kHz modulation frequency and calibrated with DPPH (a,a-diphenyl-Ppicrylhydrazyl radicals), g value 2.0036. The IR spectra were obtained in a FTIR Mattson Polaris. 3. RESULTS

3.1. A study of various molten salt/ cobalt chloride mixtures Molten sodium metavanadate, sodium molybdate, sodium tungstate and boron oxide with and without cobalt chloride have been investigated for the reactions with methane and oxygen at 750°C (Table 1). Pure sodium metavanadate, molybdate and tungstate gave about 50% selectivity to higher hydrocarbons. It is interesting to find that molten B,O, gave very high hydrocarbon selectivity ( 80 %). However, all the molten salts tested gave very low methane conversions under redox conditions. Homogeneous molten mixtures were obtained by adding cobalt chloride to each of the molten salts at the Co/M molar ratio of 0.33 (M=Mo, W and B).

-

Table 1 The oxidative coupling of methane in redox mode at 750°C using CoC12 molten mixtures. CH4 Product select. 19'0 system conv./% CO C02 c7Hq 2c5€( others NaV03 2.0 1.3 48.4 22.8 6.7 20.7 (C4) 26.1 56.8 CoC12/NaV03 9.6 3.8 8.1 4.3 (C3) N2MoO4 0.8 13.4 32.5 31.0 22.6 0.5 19.3 -CoC12/Na2MoOq 2.3 4.0 55.8 20.9 17.1 -Na2W04 0.9 17.5 33.0 32.4 CoC12/Na2W04 4.4 7.9 32.9 39.2 15.4 4.5 (C3) B203 0.6 9.0 9.8 52.8 8.9 19.0 (C4) COC12/B70~ 2.3 3.8 58.1 18.1 16.4 3.4 (C?)

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Table 1 shows that the inclusion of cobalt chloride to the molten salts generally leads to an increase in the methane conversion. However, the different molten mixtures containing the same concentration of cobalt chloride have very different activities and selectivities for the formation of higher hydrocarbons. Cobalt chloride and sodium metavanadate gave the highest methane conversion (9.6%) and highest ethene selectivity (56.8%). This clearly indicates cobalt chloride has different effects with different molten salts. 3.2. The concentration effect of cobalt chloride in sodium metavanadate The data in Figure 1 shows that the addition of CoC12 to sodium metavanadate increased the total hydrocarbon selectivity. Higher CoC12/NaV03 ratios gave higher methane conversions. The methane conversion (C(CH4)) increased from 2.0% up to about 9.6% at Co/V molar ratio of 0.33. However, only partially molten mixtures can be obtained when the molar ratios of C O N were higher than 0.33 . It is interesting to note that there is no significant change in total C2 selectivity when CoJV molar ratio reached about 0.1, however, higher C2H4 selectivity can be obtained at the expense of C2& at higher ratios, and presumably ethane conversion to ethene was more favourable at higher cobalt chloride concentrations. 60

T

0

0.1

0.2

0.3

0.4

C O N molar ratio

Figure 1 . Effect ofthe CoC12 concentration on the oxidative coupling of methane using CoC12/NaV03 molten redox catalyst at 750°C.

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3.3 The redox properties of cobalt chloride in sodium metavanadate Figure 2 clearly shows the methane conversion decreases with time as methane was continuously bubbling into the molten mixtures. It can be envisaged that the active oxygen species of the molten mixtures is depleted when it is heated under a flow of methane. However, it is also found that the Cz selectivity decreased simultaneously with the decrease in methane conversion. The methane conversion and hydrocarbon selectivity for cobalt bromidelsodium metavanadate was very similar to the pure sodium metavanadate melt after it was heated under methane flow for more than 30 minutes. It should be noted that at lower methane conversion, higher hydrocarbon selectivity would be expected, provided that the same species are involved for methane activation.

5 h

50

--

40

--

-

b

IE 30 8

--

6

C0 A

20

0

--

k&=km-d 0

10

20

30

40

Time under methane flow (min)

(0) C(CH4) ; (0) co ; (4co2 ; (X) c2Hq ; (+I c2wj Figure 2. Deactivation of the CoBr2NaV03 molten redox catalyst at 750°C as fknction of methane flow.

It is also found that when reduced molten mixtures were heated in the presence of oxygen for more than 10 minutes, the initial activity and selectivity were restored.

33 1

3.4 Effect of halides in the molten mixtures Table 2 shows the initial methane conversions and product selectivities for cobalt containing sodium metavanadate molten catalysts using different halides. Higher C2+ yield and especially C2H4/C2& ratio (c.a. 7) associated with the addition of chloride are observed. The chloride effect for enhancing ethene selectivity has been extensively studied in methane activation over solid catalysts, and is normally observed for chloride only [18-231. However, it was surprising to find that C,H,/C2H, ratio in melts containing bromide (c.a. 6.3) was almost as high as for the chloride catalysts.

Table 2 Molten cobalt halidekodium metavanadate mixtures tested for the oxidative coupling of methane in redox mode at 750OC. CH4 Product sell YO c2H4 systema conv./% CO CO2 C2H4 C,& C?+ C?& CoCI2/NaVO3 9.6 3.8 26.1 56.8 8.1 4.0 7.0 CoBri/NaVOT 9.5 7.8 17.9 62.1 9.9 2.3 6.3 1.2 CoF2/NaVO7 1.9 0.8 50.8 26.4 22.8 Co(OH)?/NaV03 2.3 7.0 49.4 19.8 20.8 2.7 1.0 a C O N molar ratio approximately 0.33 In addition, higher methane conversions (>9.5%) were obtained only when chloride and bromide were present in the molten mixtures. Continuous monitoring of the reaction products by a mass spectrometer showed that small amounts of HCI and methyl chloride were detected over the molten chloride catalysts, and traces of methyl bromide but no HBr were detected over bromide containing molten catalysts. 3.5 IR and ESR spectroscopic studies of quenched catalyst samples The IR spectra of samples of quenched catalysts of cobalt halides/sodium metavanadate show that when methane and oxygen were bubbled through the melt alternatively, some associated infrared absorbing species appeared and disappeared (Figure 3). The IR spectrum of quenched CoBrz/NaV03 after treatment with dioxygen shows the sodium metavanadate bands at around 961 (usV03), 939, 838 (uasVO3), 690 and 481 cm-1 [24]. The bands at 690 and 838 cm-1 gradually disappear and a strong absorption at 790 cm-1 (typical for a V-0-V bond [25]) arises after methane is passed through the melt for more than 10 minutes. ESR studies suggest that V (V) was reduced to a V (IV) species when the oxidised melt was heated under methane flow as shown in Figure 5 (9, = 1.9737, A,, = 175 G and g, = 1.9732, A, = 75 G). This indicates that metavanadate is being reduced with the formation of V-0-V bond. No ESR signal for cobalt species could be detected under our conditions. However, ESR data for manganese, copper and iron halide/sodium metavanadate quenched mixtures indicate that the transition metal is involved in the catalytic redox cycle (not shown). For instance, MnCI,/NaV03 mixtures, which is a very active and selective melt (with CH4 conversion of 12.0%

332

and C2 selectivity of 65%), gave a broad ESR signal with g value around 2.0 after short exposure to oxygen at 750°C.

1200

loo0

800

600

cm-1

Figure 3. IR spectra of CoBr2/NaVO3 quenched samples after treatment with methane for 1.5 min at different temperatures.

(X=F, C1, Br and OH) and FeC13/ NaV03 after treatment with methane at 750°C for 15 min.

333

This Mn(I1) ESR signal increased 4 times when the same sample was exposed to methane at the reaction temperature and then reverted back to the weaker signal when the reduced sample was retreated with oxygen. These observations suggest the broad and intense ESR signal is due to Mn(I1) (d5) species which is oxidised by oxygen to a higher oxidation state giving weaker ESR signal (less unpaired electrons). 4. DISCUSSION

It is generally accepted that over heterogeneous catalysts high temperature reactions of methane proceed by the formation of gaseous methyl radicals. These are thought to arise by the interaction of methane with either adsorbed or lattice oxygen. Ethane is formed by recombination of the methyl radicals. There is no evidence to dispute this mechanism may occur in our molten metavanadate mixtures. The inclusion of cobalt halides to sodium metavanadate increased the methane conversion and hydrocarbon selectivity suggesting that they have created an active species for the selective methane activation in sodium metavanadate melt . It is also confirmed that cobalt halides and sodium metavanadate mixtures can be reduced or oxidised in the presence of methane or oxygen, respectively. The chloride and bromide containing molten mixtures gave higher methane conversions and ethenelethane ratios than the other anions implying that these anions have an important role in the methane conversion. The role of chloride for enhancing the ethene/ethane ratio over heterogeneous catalysts [ 18-23] is still disputed and the chloride effect may be due to the involvement of gas phase halogen radicals or the creation of a specific surface species. Our results suggest the active species are present in the molten mixtures since the rates of halogen radical gas-phase reactions are significantly different for CI and Br (for example the rate constants for hydrogen abstraction from ethane by chlorine and bromine radicals are 1.86 x 10-18 cm3 molecule-l s-l [26] and 7.00 x 10-1' cm3 molecule-1 s-l [27] at 373 K, respectively), thus it is unlikely that the two halogen radicals in the gas phase will produce the comparable effects we have observed. The IR and ESR spectra showed that the pure sodium metavanadate and cobalt halide/sodium metavanadate molten mixtures all gave V(1V) species and V-0-V bonds by reduction of the metavanadate species by methane. Therefore, we suggest the active species formed in the transition metal halideshodium metavanadate can give a higher initial activity and hydrocarbon selectivity. However, when these active species become depleted then the reduction of V(V) species to V(IV) will be significant and less selective methane oxidation takes place. This explains the fact that methane conversion and hydrocarbon selectivity decreased at the same time as the molten mixture is heated under methane flow for a prolonged period of time.

Acknowledgements We wish to thank the Gas Research Institute for supporting S.C.T. and the Conselho Nacional de Ciencia e Tecnologia for support to R.M.L.

334

REFERENCES [ l ] G.E. Keller and M.M. Bhasin, J.Catal., 25 (1982) 9. [2] J.S. Lee and S.T. Oyama, Catal. Rev. Sci. Eng., 3 (1988) 249. [3] J.H. Lunsford, Catal. Today, 6 (1990) 235. [4] Y. Amenomiya, V.I. Birss, M. Goledzinowski, J. Galuszka and A.R. Sanger, Catal. Rev. Sci. Eng., 22 (1990) 163. [5] T.R. Baldwin, R. Burch, G.D. Squire and S.C. Tsang, Appl. Catal., (1991) 137. [6] T.R. Baldwin, R. Burch, G.D. Squire and S.C. Tsang, Appl. Catal., 75 (1991) 153. [7] G. Mortiers, M. Cassir, C. Piolet and J. Devynck, Electrochim. Acta, 36 (1991)1063. [8] A.G. Appleby and S. Micholson, J. Electroanal. Chem., 112 (1980) 71. [9] P. Tilman, J.P. Wiaux, C. Dauby, J. Glibert and P. Claes, J. Electroanal. Chem., 167 (1984) 117. [lo] C. Moneuse, M. Cassir, G. Martin and J. Devynck, Appl. Catal., 85 (1992) 147. Ell] I.M. Dahl, K. Grande, K-J. Jens, E. Rytter and A. Slagten, Appl. Catal., 63 (1991)163. [12] D.B. Fox and E.H. Lee, Chemtech., (1973) 186. [13] C. Moneuse, M. Cassir, C. Piolet and J. Devynck, Appl. Catal., 63 (1990) 67. [14] S.J. Conway, J. Szanyi and J.H. Lunsford, Appl. Catal., 56 (1989) 149. [ 151 Y. Otsuka, M. Kuwabara and A. Tomita, Appl. Catal., 47 (1989) 307. [16] J.A.S.P. Carrero and M. Baerns, J. Catal., 117 (1989) 258. [17] H. Mazurek, U.S. Patent 4,655,2621. [18] R. Burch, G.D. Squire and S.C. Tsang, Catal. Today, 6 (1990) 503. [19] T.R. Baldwin, R. Burch, E.M. Crabb, G.D. Squire and S.C. Tsang, Appl. Catal., s ( 1 9 8 9 ) 219. [20] R. Burch, G.D. Squire and S.C. Tsang, Appl. Catal. , 46 (1989) 69. [21] Kh. M. Minachev. N. Ya. Usachev, V.N. Udut and Yu. S. Khodakov, Russ. Chem. Rev., 57 (1988) 22. [22] K. Otsuka, M. Hatano and T. Komatzu, in D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion, Elsevier, Amsterdam, 1988, p.383. [23] S.J. Conway and J.H. Lunsford, J. Catal., 131(1991) 513. [241 T. Dupuis and M. Viltange, Mikkrochim. Acta, 2 (1963) 233. [25] I. Lukas and S. Strusievici, Z. Anorg. Allgem. Chem., 315 (1962) 324. [26] D.L. Baulch, R.A. Cox, P.J. Crutzen, R.F. Hampson Jr., J.A. Kerr, J. Troe and R.T. Watson, J. Phys. Chem. Ref Data, 1(1982) 329. [27] G.C. Fettis, J.H. Knox and A.F.T. Dickenson, J. Chem. SOC.,(1960) 4177.

a

335

C. Schild (Bayer AG, Leverkunsen, Germany): Do you have an idea about how one could arrange to characterize these systems under reaction conditions?

R Lago (University of Oxford, Oxford, U.K): Despite the practical difficulties, many high temperature (up to 1500°C) spectroscopic techniques such as EPR, NMR, IR,Raman and UV and electrochemical techniques have been applied to the characterization of molten salt systems (several excellent reviews are available in ref. (1)). In our case, for the CoC12/NaV03 system, vibrational spectroscopy (IR and Raman) and EPR could provide valuable information about the cobalt-vanadium-oxygen species present under the reaction conditions. The experimental arrangement would be similar to those described in the work of Karydis &I. (2) for an EPR reactor cell and in the IR studies described in ref (1). 1. R.J. Gale and D.G. Lovering (eds), Molten Salt Techniques, Plenum Press, 1991, Vol. 1-4. 2. D.A. Karydis, K.M. Eriksen, R. Fehrmann, G.N. Papatheodorou and N.J. Bjerrum, Materials Sci. Forum, 73-75 (1 99 1) 115.

J. Santamaria (University of Zaragoza, Zaragoza, Spain): If steady-state operation is to be achieved in systems operating in the redox mode (including molten salt system), the oxygenstoring species must be transferred to and from a regenerator unit. In any such system, the amount of oxygen that can be supplied to the reaction is an important operating parameter. Have you measured the oxygen storage capacity (on a weight bases) of your system? If so, how does it compare to other systems capable to operate in the redox mode, such as the reducible oxides used in the ARC0 process? R Lago: We agree that the total amount of oxygen stored in the molten mixture is a very important parameter if the steady-state operation is to be achieved. Experiments are being carried out in our laboratory to measure the amount of oxygen stored in the catalyst, therefore no data is available at this moment. However, in the literature it has been shown that some molten mixture can take up a large amount of oxygen (more than 75% wlw) and will release them in a reducing environment (ref. (3)). It would certainly be interesting to compare molten salts with solid oxides under the same conditions. The other important parameter we would like to point out is the high oxygen mobility in molten mixtures which may be suitable for a membrane reactor applications. Work is in progress to obtain data on oxygen mobility over our catalysts. 3. W. Sundermeyer, Angew. ChemJnternat., Edit., 4 (1965) 225.

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V. CortCs Corbcriin and S. Vic Bcllon (Edrmrs), New Developments in Selecrive Oxidation I1

0 1994 Elsevier Scicncc B.V. All rights rcservcd.

331

THE ACTIVE OXYGEN SPECIES IN OXIDATIVE COUPLING OF METHANE OVER Li/CaO AND Na/CaO USING N,O AND O2 AS OXIDANTS A.G. Anshits', V.G. Roguleva and E.V. Kondratenko Department of Chemistry, Krasnoyarsk State Technical University, Kirenskii Str. 26, Krasnoyarsk, 660074, Russia Institute of Chemistry of Natural Organic Materials, K. Marx Str. 42, Krasnoyarsk, 660049, Russia

Abstract The reactivity of N 2 0 and 0, in the methane conversion has been studied over Li/CaO and NdCaO catalysts in the field of solid solution (with a low amount of alkali promoter <2.5 at. %). It was found that the replacement of 0, by N,O causes the increase in formation rate of C,H, and sharp decrease of the rate of CH, conversion to CO. The concentration of Fe3+impurity ions in doped CaO was shown to influence on activity of used catalysts in the presence of different oxidants (N,O or 0,).The conception of oxidant (02or N,O) activation over different centers of studied catalysts was proposed. 1. INTRODUCTION

The problem concerning catalyst active centers and oxygen active forms in the oxidative coupling of methane is unsolved because of the diversity of the catalyst used in this process. However, there is evidence that certain types of oxygen species are involved in the methane conversion. Following the earlier studies of Lunsford and co-workers [11, Li+Ocenters were aproposed for the activation of CH, over Li/MgO catalysts with a high amount of alkali promoter. Sinev et al. [2] have shown that BaO, was capable of converting CH, to CzH6at relatively low temperature. Further [3], Lunsford et al. have observed peroxides by XPS on BaPbO, catalyst. On the other hand [4] the formation of two oxygen species over M/CaO (M=Li or Na) catalysts in the range of solid solution has been proposed when Q2disproportionated on the structural defects of catalysts. The many investigators studied oxidative coupling of methane, when N,O was used as oxidant, and it was shown that only one oxygen species (0 or 0.) was formed by N,O activation [5-71. Nitrous oxide is also known to be a more selective but less active oxidant than molecular oxygen for methane conversion over Li/MgO catalyst doped with a high concentration of alkali promoter (5 wt. %) [6] and rare earth metal oxides [8]. However, the better catalytic activity was achieved for Li/CaO (Li< 1.5 at. %) catalysts using N,O in 1

to whom all correspondence should be addressed; the present address: Institute of Chemistry of Natural Organic Materials, K. Marx Str. 42, Krasnoyarsk, 660049. Russia

338

comparison with 0, as oxidant [9]. It may be caused by the formation of different oxygen species generated from 0, and N20or various activation procedures of the involved oxidants. Thus, it is interesting to make a detailed comparative study of the differences between N,O and O2 activation on the point defects of the catalyst structure using Li/CaO and Na/CaO catalysts doped with a low concentration of alkali promoter (< 2.5 at. %). The present contribution deals with studying oxidative ability of two oxidants (N20 and 0,) in the reaction of oxidative coupling of methane over lithium or sodium promoted CaO in the range of solid solution.

2. EXPERIMENTAL Catalysts of Li/CaO and Na/CaO were prepared from high purity grade CaO (Fe<0.01; Zn<0.005; Mn, Cu Fe3') to compensate for the excess negative charge. It is also necessary to note that the special doping by iron was not used. ~

3. RESULTS AND DISCUSSION

To elucidate the role of point defects in the catalyst by the oxidant (both 0, and N,O) activation and the nature of active oxygen species taking part in methane conversion, the CaO catalytic system promoted with a low amount of alkali promoter (Na or Li) was used. Experimental data comparing the catalytic properties of Li/CaO and Na/CaO systems in oxidative coupling of methane using 0, and N20 as oxidants are given in Figures 1 and 2.

339

180

90

0

200

100 CONCENTRATION O F Fe3+*

pcig

Fig. 1. Formation rates of C,H, (circle) and CO (triangle) for CH,-0, mixture versus Fe3+ concentration; open symbols for Li/CaO; solid symbols for NaiCaO. As seen from Fig. 1, the rates of CO and C,H6 formation is similar. It may be testified that the active oxygen species taking part in the formation of mentioned products are formed simultaneously. The found increase of the formation rates of C2H, and CO versus Fe3+ ions concentration is caused by increasing the concentration of active catalyst centers. As mentioned in the experimental section, the incorporation of Li' or Na+ ions into the CaO lattice leads to stabilization of high valence stage of the impurity iron ions (F2' - > Fe3+). The appearance of Fe3' ions seems to be test for formation of solid solution of M i c a 0 (M=Li or Na) and reflect the common defectivity of catalysts. Also it is necessary to note that the similar correlation between the concentration of the alkali promoter and the formation rates of C& and CO was not found. This fact confirms the importance of solid solutions for t h e formation of active catalytic centers. The replacement of molecular oxygen by N,O causes an increase in the rate of formation of C,H, and sharp decrease in CO production rate (Fig. 2 ) . It is necessary to note that the ratio of COIH, products in changed from 0.6-0.7 for CH4-02mixture to 0.3-0.4 for CH,-N,O for all studied catalysts (Table 1). Decreasing rate of CO formation and changing ratio of CO/H, products using N,O as oxidant may be caused by modification of the route of the conversion of CH, to CO as compared with the use of 0, as oxidant. The weak dependence of the formation rate of CO on studied catalysts (Fig. 2) when N,O is used as oxidant, may be caused by gas phase formation of carbon monoxide. Earlier Aika and co-workers [lo] have shown that the presence of 02cuq was necessary to form carbon monoxide from CH,-0, mixture (equation 1).

340

180

90

CONCENTRATION OF Fe3+ *

pc/g

Fig. 2. Formation rates of C2H, (1) and CO (2) for CH,-N,O mixture versus Fe3+ concentration; solid symbols for NdCaO.

Thus, a low rate of formation of CO using CH,-N,O mixture over studied catalysts may testify to a small amount of @-, species. Based on experimental and literature [ 1 11 data the gas phase route of CO formation, independently of the catalyst, may be proposed when N,O is used as oxidant (equation 2): CHs3 CH,

+ 02-cu8 * CH,O- + e-

+ H,O

+

CO

+

CO

+ 3/2H, + 2e-

+ 3H, (gas phase reaction)

C0/H2=0.66 (CH,-OJ

(1)

CO/H, =0.33 (CHd-N 2 0 )

(2)

However, the use of different oxidants (0,or N,O) does not result in a change in the apparent activation energy (Ed for ethane formation (Table 1). The experimental data presented in Fig. 1 and 2 and similar values of E, for both CH4-02and CH,-N20 mixtures would suggest that the CH, molecule is activated by the same active atomic oxygen species.

34 1

Table 1 Catalytic properties of Li/CaO and Na/CaO in the oxidative coupling of CH, Catalyst

[Fe3+],

(at. %)

S, m2/g

Oxidant

pc/g 100

CO

CO,

kJ/mol

5.3

7.5

8.1

328

0.6

14

2.2

6.5

327

0.4

0,

0.8

1.2

1.0

298

0.6

N@

5.2

0.8

2.5

280

0.4

N,O 15

Ea(C2H6) CO/H,

0.9

(0.4) Na/CaO (0.1)

molec. CH, m2 * s

C7H, 0 2

Li/CaO

w*lO'*

1.5

Thus, the N20activation results in generation of only one oxygen'species which reacts with methane to form CH, radicals, the nature of which being similar for 0, and N,O. While for CH4-02 mixture, based on the routes of CO (equation 1) and ethane (as for CI-&-N,O mixture) formation we propose participation of two oxygen species (@-cus and 0).This fact disproportionated into two active oxygen forms confirms earlier [3] proposed idea of b-, taking part in the formation of CO and C2H,. However, in spite of the similar function of formation rates of C,-hydrocarbons with Fe3+content for 0, and N 2 0 (Fig. 1, Fig. 2), the differences of formation rates of ethane may be caused by oxidant activation of different active centers of the catalysts. The incorporation of Li' or Na+ into the CaO lattice has already been shown to result in the stabilization of Fe3' impurity ions in calcium oxide. These ions may be active centers of catalysts as well as to take part in the steps of electron or hole transfer. On the other, the Fe3' impurity ions may be a test for the presence of oxygen defects in the oxide lattice (potential active centers). To recognize the nature of active centers in the catalysts, which are necessary for oxidant activation, a study of N 2 0 decomposition was carried out under conditions of the oxidative methane coupling. The generally accepted mechanism of N20 decomposition over numerous oxides [12] consists of two steps:

(0)3 ( )

+

1/2 0,

(4)

( ) - anion vacancy

Equation (4) is the reversible adsorption of molecular oxygen yielding the same surface form as that resulting from decomposition of N,O molecules, this stage being the limiting step. The consequence of this mechanism is an inhibition effect of 0, as ascertained by investigations of numerous oxides of rare earth elements [13].

342

To confirm this mechanism over used catalysts, the N 2 0 decomposition was studied under the conditions of methane oxidation using the following mixtures: CH,:N20:02=30:10:X, where X=O-15 vol. %. The dependence of partial pressure of oxygen on the decomposition rate of nitrous oxide is given in the Fig. 3.

*

w

3

I 0

4

8

12

16

CONCENTRATION OF O,, vol. % Fig. 3. Rates of N2 and C,H, formation versus oxygen concentration; open symbols for Li/CaO (0.1 at. %), solid symbols for NdCaO (2.5 at. %).

The increasing oxygen concentration is not seen to influence the decomposition rate of N20 but the activity of the formation of ethane is increased. The rate of GH, formation for CH4:N20:0,=30: 10:15 vol. % mixture is equal to the sum of rates of ethane formation for CH4:N,0=30:10 and cH4:02=30:1S vol. % mixtures. Thus, the addition of another oxidant (9)results in the increasing concentration of active species which may be caused by activation of dioxygen and nitrous oxide over different active centers. The independence of N,O decomposition rate from oxygen concentration confirms that the former mechanism is not valid for the Li/CaO and NdCaO catalysts used here. N 20 decomposition over ZSM-5 zeolite promoted by iron also does not depend on oxygen pressure [14]. Other scheme of N 2 0 decomposition has been proposed [14] in which N,O takes part both in oxidation and reduction of catalyst surface:

( )-active center, part of which is iron

As seen from Fig. 1 and Fig. 2, the rate of GH, formation is higher for the CH,-N,O mixture than for CH,-O2 mixture. However, the increasing Fe3+ concentration results in

343

equal rates of C,H, formation for both mixtures. The reason for the decrease in rates of N 2 0 decomposition and ethane formation in CH,-N20 mixture, that is observed for Li/CaO ([Fe3+]=180*10'6spin/g) (Fig. l), may be caused by the limited solubility of Li+ in CaO. In this case the lithium ions promote the surface segregation of transition metal ions. It leads to the creation of strong adsorbed oxygen species and decrease of the number of active centers which decompose N,O. Similar decrease of the rate of N,O.decomposition was observed by Pepe [I51 over Li/Co/MgO catalysts. The effective N20 decomposition occurs on the point defects of MgO isolated from each other when doped by transition metal ions [16]. In our opinion, the single impurity ions of Fe3+ may be active centers for NzO decomposition over the studied catalysts. It may be proposed that molecular oxygen is not activated on isolated (single) defects because the N,O decomposition rate does not depend on the presence of 0,. Probably, in order to activate 0, the pair centers, part of which is either Fe3+or structural oxygen defect, are necessary. 4. CONCLUSIONS

1. The replacement of 0, by N,O in the oxidative coupling of methane over solid solutions of Li/CaO and Na/CaO catalysts doped with a low amount of alkali promoter leads to an increase in the rate of ethane formation and decrease in the conversion rate of CH, to CO. A correlation of rate formation of the two reaction products (GH, and CO) for CH,-O, and one product (C,H,) for CH,-N,O with the concentration of Fe3' point defects is observed. In the case of 0, two oxygen species (0and b-) are formed while only the atomic oxygen form can be generated at the N,O decomposition. 2. The rate of N20 decomposition over Li/CaO and Na/CaO in the range of solid solution is not found to depend on the presence of molecular oxygen. The nature of sites for oxygen and nitrous oxide activation is not similar. The presence of a single isolated defects (Fe3+ type) is necessary to activate the N20, while the O2activation may occur on the pair centers of Fe3+or structural oxygen defects. Acknowledgment. This work was supported by the Krasnoyarsk regional Science Foundation under Grant N. 2F0062. REFERENCES

1. D.J. Driscoll, W. Martir, J.-X. Wang and J.H. Lunsford, J. Amer. Chem. SOC.,107 (1985) 58. 2. M. Yu. Sinev, V.N. Korchak and O.V. Krylov, Kinet. Katal., 27 (1986) 1274. 3. K.C.C. Kharas and J.H. Lunsford, J. Amer. Chem. SOC.,111 (1989) 2336. 4. A.G. Anshits, N.P. Kink, V.G. Roguleva, A.N. Shigapov and G.E. Selyutin, Catal. Today 4 (1989) 399. 5. K.-i. Aika, M. Isobe, K. Kido, T. Moriyama and T. Onishi, J. Chem. SOC.,Farad. Trans. I , 83 (1987) 3139. 6. G.J. Hutchings, J.R. Woodhouse and M.S. Scurrell, J. Chem. SOC.,Farad. Trans. I, 85 (1989) 2507. 7. V.G. Roguleva, E.V. Kondratenko, N.G. Maksimov, G.E. Selyutin and A.G. Anshits,

344

Catal. Lett. 16 (1992) 165. 8. K. Otsuka and T. Nakajima, Inorganica Chimica Acta., 120 (1986) L27. 9. V.G. Roguleva, M.A. Nikiphorova, N.G. Maksimov and A.G. Anshits, Catal. Today 13 (1992) 219. 10. K. Aika and J.H. Lunsford, J. Phys. Chem. 81 (1977) 1393. 11. T. Tomita, K. Kokuchi, T. Sakamoto, K. Ishida, A. Morija, Proc. 1l t h World Petrol. Cong., London, (1984), v. 4, p. 407. 12. E.R.S. Winter, J. Catal. 34 (1974) 431. 13. E.R.S. Winter, J. Catal. 15 (1969) 144. 14. G.I. Panov, V.I. Sobolev and Kharitonov, J. Molec. Catal. 61 (1990) 85. 15. F. Pepe, Gazzetta Chimica Italiana 103 (1973) 1201. 16. A. Cimino, La Chimica e L’Industria 56 N1 (1974) 27.

DISCUSSION CONTRIBUTION M. SINEV (I. of Chemical Physics, Moscow, Russia): It is well-known that the products of oxidative coupling can be produced in the process of reduction of Li/MgO mixed oxide with methane with the same (or similar) rates as in the co-feeded (C&+O,) process. How can it be understood from the point of view of your mechanism including atomic oxygen formation?. A.G. ANSHITS (I. of Chemistry of Natural Organic Materials, Krasnoyarsk, Russia): The similar rates of the methane oxidation both in the process of reduction and in the co-feeded process were achieved over Li/MgO catalysts with a high concentration of alkali promoter. The surface of these catalysts consists of a phase of carbonate of alkali dopant. But it is wellknown that methane reacts with carbonates with formation of methyl radicals. However, the catalysts, which we use, are the solid solutions and no phase of alkali promoter is formed on the catalyst surface. That is the process of the oxidant activation with the formation of active oxygen species which plays an important role in the methane oxidation over our catalysts.

V. CortCs Corberin and S. Vic Bcllon (Editors), New Ueveioprnenls in Seleciive Oxidation I1 0 1994 Elscvicr Science B.V. All rights reserved.

345

Spectroscopic Characterization of Surface Oxygen Species on Barium-Containing Methane Coupling Catalysts Michael P. Rosynek, Dhammike Dissanayake, and Jack H. Lunsford Department of Chemistry, Texas A & M University, College Station, Texas 77843, U.S.A. 1. ABSTRACT

Promotion of MgO, CaO, and ZnO with 2 mol% Ba imparts high activity and C, selectivity to these oxide supports for the oxidative coupling of CH, at 800-850°C. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) of C02 have been employed to characterize the nature of surface species on both the fresh and used catalysts. These techniques have established that a direct correlation exists between the oxidative cou ling activity of these promoted oxides and their ability to form surface peroxide ions (0;) in the presence of gaseous O,, but that activity is not directly related to support basicity. Selective poisoning of CH,/CD, isotopic exchange reactions indicate that at least two different types of sites that are capable of activating C-H bonds in CH, exist on these catalysts. 2. INTRODUCTION

The oxidative coupling of methane to C, hydrocarbons has been extensively studied during the past several years [l-31. Although most of the catalysts, such as alkaline earth and lanthanide oxides, that are both active for methane conversion and selective for the formation of C2 products [4,5] are intrinsically basic in nature, disagreement still exists regarding the chemical/structural nature of the surface sites that are responsible for activating CH, on these materials. Unambiguous characterization of the catalytically significant sites on such basic oxides is complicated by their marked, but differing, susceptibilities to poisoning by the CO, byproduct of the coupling reaction. Most investigators agree, however, that a partially reduced surface oxygen species, e . g . , 0-,C+ or O:-, plays an important role in the methane activation process. In the case of Li-promoted MgO, for example, considerable evidence indicates the involvement of Li+Osites in the formation of the CH,. radicals that are believed to initiate the reaction [6]. On various perovskite catalyst systems, superoxide (02 and peroxide (O$) sites have been suggested [7,8]. We recently examined a series of Ba/MgO catalysts [9] and employed x-ray photoelectron spectroscopy to demonstrate that 0;- species, existing in a Ba02 phase, are responsible for their oxidative coupling activity. In order to ascertain further the effect of support acid-base properties on oxidative coupling behavior, we have extended our

346

previous study to include Ba supported on several additional oxides. We have determined that on supports of sufficient, although differing, basicity, such as MgO and ZnO, surface peroxide sites are invariably involved in methane activation. The use of non-basic oxide supports, such as A1203, however, results in inferior C, selectivity during methane oxidation. 3. EXPERIMENTAL

All catalysts were prepared by adding a quantity of the desired oxide support to an aqueous solution containing an appropriate concentration of dissolved Ba(N03). Each resulting slurry was stirred for 16 h, evaporated to dryness, crushed into 20-45 mesh granules, and then calcined in air for one hour at 800°C prior to use. The compositions and BET-N, surface areas of the pure supports and synthesized catalysts are summarized in Table 1. Methane (99.99%), oxygen (99.9%), helium (99.995%) , and carbon dioxide (99.9%) were used without additional purification. CD4 was obtained from Isotec, Inc. and had an isotopic purity of 99.4 atom% D.

Table 1 Surface Areas of Catalysts and Oxide Supports (m2/g) CatalystBupport

Fresh*

used**

MgO BdMgO (2 mol% Ba) CaO BaKaO (2 mol% Ba) ZnO BdZnO (2 mol% Ba)

68 9.3 17 9.7 3.3 1.7 294 81

36 7.7 9.6 7.4 1.o 1.o 105 77

A1203

BdA1203 (4 mol% Ba)

*After initial treatment in 0, at 800°C for 10 h. **Following exposure to a CH4:0,:He = 5:1:6 reaction mixture for 12 h at 850°C. Catalytic reaction and CO, poisoning experiments were performed using 50 mg of catalyst in a 2.85 mm I.D. downflow tubular quartz reactor, of a design described previously [8,9]. All reactions were run at 850°C and 1 atm total pressure, using a feed mixture of CH4:O, = 50:5 cm3/min, and with 2 to 20 cm3/min of CO, added, when necessary, to achieve a desired CO, partial pressure. The total flow rate was maintained at 120 cm3/min by appropriate adjustment of the diluent He flow. Methane oxidation reaction mixtures were analyzed by gas chromatography, and product mixtures from CH4/CD, isotopic exchange reactions were analyzed using a Hewlett-Packard Model 5971A mass sensitive GC detector.

347

Temperature-programmed desorptions of adsorbed CO, were performed using a stainless steel TPD system consisting of a thermal conductivity detector and a quartz reactor containing 100 mg of catalyst. Previously calcined samples were heated to 970°C in flowing He, cooled in pure flowing CO, to 120"C, and then flushed with He for 3 h. The sample temperature was increased at 16"C/min to 970"C, and maintained isothermally, if necessary, at the latter temperature until desorption was completed. XPS spectra were acquired on a Perkin-Elmer Model 5500 spectrometer, using a Mg anode at 300 Watts, a pass energy of 58.7 eV, and a step increment of 0.125 eV. All measured binding energies were adjusted with respect to the Ba 34,, peak at 779.7 eV. The latter was calibrated in selected samples by referencing it to the Au 4f7,, peak at 83.8 eV from a small Au spot deposited on the sample. Near-surface compositions were calculated from peak areas in each spectral region, using appropriate sensitivity factors for each line. Catalyst samples for XPS analysis were treated in an external quartz reactor system and then transferred in situ into a detachable 0-ring-sealed stainless steel transport vessel that was subsequently fitted directly onto an evacuable inlet port on the spectrometer, thus allowing completely anaerobic transfer of treated samples into the instrument. Catalyst samples were prepared in the form of pressed wafers and, prior to spectral analysis, were given thermal and chemical treatments that duplicated those employed in the catalytic reaction experiments. Spectra were obtained both after initial sample treatment in 0, at 800°C (fresh) and after subsequent exposure to a CH4:02:He reaction mixture (used) at the same temperature. 4. RESULTS

4.1. Methane Oxidation Behaviors and Effect of CO, Poisoning Addition of 2 or 4 mol% of Ba increased both the overall methane oxidation activity and the C2+ selectivity of B 30 Torr of Added C 0 2 each of the four oxide supports, as shown 0 60 Torr of Added C 0 2 by the solid bars in Figs. 1 and 2 [9]. The I al enhancement of CH, conversion was most > apparent for ZnO and MgO; activity of the 6 10 latter under these reaction conditions 2 increased almost three-fold when 2 mol% 0 of Ba was added. The promotional effect n 5 was least pronounced in the case of CaO, which is itself a relatively active and n selective catalyst for methane coupling, even in the absence of added Ba. Although the addition of Ba to y-Al2O3 markedly improved both its activity and C2+ selectivity, this non-basic material remained Figure 1 Effect of added CO, on CH, an inferior selective oxidation catalyst. conversion at 850°C. (CH40, = lO:l, The addition of 30 or 60 Torr of balance = He to 1 atm.) CO, to the reactant stream had virtually no

348

effect on the activity of any of the four pure oxide catalysts, as shown by the hatched bars in Fig. 1. Similarly, among the Bapromoted catalysts, CH, conversions over both BdCaO and BdAl,O, were largely unaffected by the presence of added CO, in the feed. However, the activities of BdMgO and BdZnO decreased markedly with increasing CO, partial pressure in the feed. In no case was the C2+ selectivity of any of the catalysts affected appreciably by the presence of added CO,. For BdMgO and BdCaO, in fact, the selectivity increased slightly at higher CO, partial pressures (Fig. 2).

,100

0"

B 30 Torr of Added COP 0 60 Torr of Added C 0 2

9 80 6o

0 4o v,

20 0

Figure2 Effect of added CO, on C2+ 4.2. XPS Characterization of Catalysts selectivity at 850°C. (CH,:O, = 10:1, The identities of surface species and balance = He to 1 atm.) the nature of catalytic sites on these oxides were investigated using x-ray photoelectron spectroscopy. Figure 3 presents typical XPS spectra in the 0 1s region for each of the Ba-containing catalysts, obtained following initial treatment in 0, at 800°C and after subsequent exposure to a 10: 1 CH,:02 mixture for 12 h at the same temperature. Prior to XPS analyses, the used samples were cooled to ambient temperature in an 02:He flow in order to ensure the complete removal of surface hydroxyl species that may have been formed by the water byproduct of the oxidation reactions. The presence of such surface OH species precludes unambiguous determination of peroxides (Og-),since the two species have nearly identical 0 1s binding energies [8]. The 0 1s spectra in Fig. 3 reveal the presence of at least two different types of near-surface oxygen species on each of the catalysts, except for BdAl,O,. This feature is most apparent in the case of BdCaO, for which two distinct peaks are observed, but is also present in BdMgO and BdZnO, as indicated by the asymmetry of the 0 1s peaks for these materials. The prominent peak observed at -529 eV for each of the latter three catalysts is due to 0,- species in the oxide support and in BaO (if present), while the smaller peak at -531 eV (assuming OH groups are absent) may result from either a Ba2+0g- and/or a Ba2+C0g- entity. As described previously [9], the individual contributions from these two oxygen species may be separately ascertained by using the accompanying C 1s spectrum for each material (Fig. 4) to determine the amount of CO$ from the peak at 288.5 eV. The intensity of the latter is then subtracted from the deconvoluted 0 1s peak intensity at 531 eV to quantitatively obtain its 0;- component. The near-surface compositions of each fresh and used catalyst, obtained using this technique, are summarized in Table 2. After cooling in O,, a surface peroxide (O$) species was Observed for each of the Ba-containing catalysts, except BdA1,O3, but not for any of the four ure oxide supports, and is thus ascribed to a BaO, phase. Similarly, the carbonate (CO, ) species observed on these same catalysts is due to BaCO,, since none of the four oxide supports forms a stable carbonate at the treatment temperature used. It should be noted that although the

!-

349

0 1s binding energy for 0,- in Al,O, (531.5 eV) is virtually coincident with that in Ba2+O$-, thus precluding separate determination of the various oxygen species in BdAI,O,, the absence of a C Is peak at 288.5 eV indicates that a C0:- species is not present on this material.

Ba/CaO

v)

a

Ba/MgO

0

4.3. TPD Characterization of Catalysts Temperature-programmed desorption of adsorbed CO, was employed to further elucidate the nature of surface species on the various catalysts. Among the pure oxide supports, only CaO exhibited a CO, desorption peak, due to decomposition of a CaC03 phase, which occurred at 720°C (Fig. 5). This phase could not exist, however, in the 800-850°C temperature range employed for methane oxidation. The addition of Ba promoter produced additional peaks in BdCaO at 770°C and 920°C. The former peak may be due to decomposition of a Ba-modified CaC0, phase, since this temperature is too low for BaC0, decomposition, which is ascribed to the peak at 920°C. Ba promotion also led to the formation of a BaC03 phase on both BdMgO and BdZnO, as indicated by CO, desorption peaks at 930°C and 850”C, respectively.

fresh

G ”

.-

v)

c

al ” c c

F

4-

0

al

iil

L 540

,

.

, I

I .

530

520

Binding Energy, eV

Figure 3 XPS spectra in 0 1s region, following 0, treatment at 800°C (fresh), and after subsequent exposure to CH4:02 reaction mixture at 800°C (used).

5. DISCUSSION

5.1. Nature of Active Sites for Methane Coupling In a previous study of a series of BdMgO catalysts [9], we obtained convincing spectroscopic evidence that the activity of these systems for methane coupling is primarily due to the existence of a BaO, phase on their surfaces. Indeed, Ba0, has been reported to react stoichiometrically (i.e., non-catalytically) with CH4 to produce C, products [lo]. The specific activity of our BdMgO catalysts increased with increasing Ba loading, up to a level of ca. 4 mol% Ba, above which it declined due to an increasing fraction of the Ba occurring as BaC0,. In the present study, XPS spectra of both the fresh and used catalysts (Fig. 3) indicate that peroxide formation can also occur on both BdCaO and BdZnO. Hence, we conclude that, although there appears to be no direct correlation between the basicity of the host oxide and the activity/selectivity of the resulting supported Ba catalyst for the selective oxidation of methane, a surface BaO, phase is responsible for promoting

350

the coupling reaction over BdMgO and BdZnO. In the case of CaO, the most basic of the four oxide supports studied, it must be noted that this oxide, unlike MgO and ZnO, is itself an active and reasonably selective catalyst for methane oxidation, even without added Ba. As a result, promotion with 2 mol% Ba causes relatively little improvement in the activity of CaO (Fig. 1). In fact, additional XPS results (not included in Table 2) indicate that, under typical reaction conditions, most of the Ba in BdCaO exists as BaC03 and makes only a secondary contribution to the activity of this catalyst.

290

280

Binding Energy, eV

/i_

WZnO

Ba/AI20,

Figure 4 XPS spectra in C 1s region, following 0, treatment at 800°C (fresh), and after subsequent exposure to CH4:02 reaction mixture at 800°C (used).

Although catalytic activity for methane coupling on BdMgO, BdCaO, and BdZnO may be ascribed to surface peroxide entities, Figure 5 Temperature-programmed which promote homolytic cleavage of C-H bonds in CH, to form CH,. radicals, desorption of CO, (heating rate = 16"C/min to 970°C). additional types of sites that catalyze heterolytic C-H bond cleavage, may also exist on these materials. Figure 6, for example, compares the effect on a 0.5 mol% BdMgO catalyst at 850°C of adding very small amounts of CO, to the methane coupling reaction (under non-O,-limiting conditions) and to another reaction, viz. , CH,/CD, exchange, that also requires C-H bond activation. Unlike the behavior observed for much higher partial pressures of added CO, (Fig. I), catalytic activity for methane oxidation was virtually unaffected by these low levels of CO, addition, while that for the isotopic exchange reaction decreased sharply, even at CO, partial pressures as low as 0.1 Torr. Hence, it is apparent that at least two different types of sites that are capable of activating C-H bonds in CH, exist on BdMgO. The coupling reaction requires the 02- centers of BaO,, which are sustained at these high temperatures only by the continuing presence of molecular O,, 200

400

600

000 970 Temperature, 'C

Table 2 Surface Compositions of Catalysts (mol%) Catalyst

02-

0’5-

cof-

C

Ca/Mg/Zn/Al

Ba

47.4 44.7 42.6 38.6 40.0 34.4 28.0

8.9 4.3 7.0 7.0

39.4 35.0 29.2 29.0

4.2 6.7 10.7 7.0

Fresh CaO MgO ZnO BdCaO BdMgO BdZnO BdA1203 Used BdCaO BdMgO BdZnO Ba/AI,O3

52.6 55.4 57.4 38.8 44.8 51.9

9.5 7.4 6.7

2.1 1.8

2.1 1.8

8.8 10.2 9.6

2.6 1.7 0.6

2.6

65.0* 42.6 44.6 49.4

1.7

0.6

64.0*

*Includes 02-from both A1203 and BaO, and 0’5-(if any) from Ba0,. while the isotopic exchange reaction occurs independently on another type of site, probably M2+-02-pair sites in MgO and/or BaO, which do not require the presence of gaseous O2 for their existence.

5.2. Susceptibility to Poisoning by CO, A characteristic feature of many of the basic oxide catalysts used for methane oxidation is their susceptibility to poisoning by the carbon dioxide byproduct of the coupling reaction. XPS spectra of used catalysts reveal that this deactivation is typically due to the formation of surface carbonate species, which block the catalytic sites used for CH, activation. Since none of the four pure oxide catalysts investigated in the present study forms a thermally stable carbonate at the 850°C reaction temperature employed for these experiments, the addition of CO, to the reactant feed stream had virtually no effect on their CH, conversion activities, as shown in Fig. 1. Among the four Ba-promoted catalysts, however, the activities of both BdMgO and BdZnO decreased markedly with increasing partial pressure of added CO,, while those of BdCaO and Ba/A1203 were largely unaffected by added C02. The contrasting behaviors of these two groups of catalysts are the result of two factors. As noted above, XPS spectra indicate that virtually all of the Ba in Ba/CaO exists as BaC03 under typical reaction conditions, and that CaO itself accounts for most of the observed methane conversion activity of this catalyst. Hence, since CaCO, is not stable at the 850°C reaction temperature employed, CaO (and BdCaO) is relatively insensitive to CO, poisoning. For the other three catalysts, methane conversion activities are due almost solely to a surface BaO, phase, and their comparative

352

susceptibilities to poisoning by CO, are 10 governed by the relative stability of the BaCO, phase, whose formation from the active peroxide sites destroys them and is 0 responsible for catalyst deactivation. The n 0 TPD results in Fig. 5 show that both 6 3 BdMgO and BdZnO exhibit a CO, 0 8 desorption peak (due to BaC03 4 ; decomposition) at 1800"C, while g 3 BdAl,O, does not form a surface 2 carbonate, presumably due to the acidic nature of this support. Thus, although the catalytic activity of Ba-containing catalysts 0 for methane coupling is not directly related CO, Partial Pressure, Torr to support basicity among the host oxides examined in this study, basicity does play a role in determining the susceptibility of the catalysts to CO, poisoning. With Figure 6 Effect of CO, partial pressure on increasing basicity of the support, i.e., CH4/O, (CH4:0,:He = 5:1:200) and MgO > ZnO > A1,03, the deactivating CH4/CD4 reactions (CH4:CD4:He = BaC0, surface phase becomes increasingly 1:1:20) over 0.5 mol% BdMgO at 850°C. less thermally stable, and the catalyst's sensitivity to poisoning by CO, correspondingly decreases.

6. ACKNOWLEDGEMENT The authors gratefully acknowledge financial support of this research by the United States National Science Foundation.

REFERENCES 1. Lee, J.S. and Oyama, S.T., Cutal. Rev.-Sci. Eng. 30, 161 (1988). 2 . Amenomiya, Y.,Birss, V.I., Goledzinowski, M., Galuszka, J., and Sanger, A.R., Cutal. Rev.-Sci. Eng. 32, 163 (1990). 3. Mackie, J.C., Catal. Rev.-Sci. Eng. 33, 169 (1991). 4. Driscoll, D.J., Martir, W., Wang, J.X., and Lunsford, J.H., J . Am. Chem. Soc. 107, 58 (1985). 5. DuBois, J.L. and Cameron, C.J., Appl. Cutul. 67, 49 (1990). 6. Wang, J.X. and Lunsford, J.H., J. Phys. Chem. 90, 5883 (1986). 7. Kharas, K.C.C. and Lunsford, J.H., J. Am. Chem. SOC. 111, 2336 (1989). 8. Dissanayake, D., Kharas, K.C.C., Lunsford, J.H., and Rosynek, M.P., J. Cutal. 139, 652 (1993). 9. Dissanayake, D., Lunsford, J.H., and Rosynek, M.P., J. Cutul. 143, 286 (1993). 10. Moneuse, C., Cassir, M., Piolet, C., and Devynck, J., Appl. Cutul. 63, 67 (1990).

353

J.M. Bregeault (UniversitC Pierre et Marie Curie, Catalyse et Chimie des Surfaces, Paris, France): How can we discriminate between 0:- and 0; ? What about the relative stabilities of these two species at your reaction conditions? M.P. Rosynek (Texas A&M University, Department of Chemistry, College Station, and superoxide (0; ) species can, in principal, be Texas, U.S.A.): Peroxide distinguished by XPS, since their 0 1s binding energies differ by ca. 1 eV. However, we observed no evidence for an additional peak in the deconvoluted 0 1s envelopes of any of the catalysts studied that could be attributed to superoxide. Similarly, our Raman spectroscopy studies, which could also distinguish these two species, indicated that 0; was not present on Ba/MgO under coupling conditions. In any case, I would expect 0; to be considerably less stable than 0;- under typical oxidative coupling reaction conditions.

(G-)

J.A. Navio (Universidad de Sevilla, Instituto de Ciencia de Materiales, Sevilla, Spain): The identification of surface peroxide species 0;- by only XPS spectra in the 0 1s region seems to be more or less complicated because surface hydroxyl groups must be removed and CO$ must be taken into account. Did you try to identify such species by other complementary techniques such as IR spectroscopy, chemical analytical tests, or TPD experiments of both the fresh and used catalysts?

M.P. Rosynek: As described on page 4 of our paper, all samples for XPS analysis were cooled from reaction temperature to ambient conditions in flowing 02/He in order to prevent the formation of surface hydroxyl species, whose 0 1s binding energy is virtually identical to that of peroxide. We verified the absence of surface OH species on these cooled catalysts in separate experiments by allowing them to equilibrate with gaseous D2 at 500-600°C for 2 h in a sealed vessel and observing no detectable formation of H2 or HD. The technique that we used to deconvolute the 0 1s XPS envelope into its O;-, C63, and 02'components is described in the paper and in more detail in Reference 9. As discussed during the oral presentation of the paper, we have also recently begun to employ Raman spectroscopy to allow in situ studies of these catalysts under reaction conditions. These recent studies have confirmed the existence of peroxide species in the presence of CH,/02 reaction mixtures at 850°C.

E.A. Mamedov (Institute of Inorganic and Physical Chemistry, Baku, Azerbaijan): Can you expect the formation of oxygen peroxide ions on the surfaces of other kinds of catalysts, such as lead, manganese or bismuth oxides, whose chemical natures essentially differ from that of alkaline earth oxides? M.P. Rosynek: We have not characterized the surface properties or catalytic behaviors of the metal oxides that you cite for methane coupling. However, I would not expect extensive formation of surface peroxides on non-basic or transition metal oxides. We have recently completed a detailed characterization of a Mn/Na,WO,/SiO, catalyst using XPS, IR, and TPD techniques, and observed no evidence for a surface peroxide species, despite the fact that this material is very active and highly selective for oxidative coupling of CH,.

354

G.L. Schrader(Iowa State University, Department of Chemical Engineering, Ames, Iowa, U.S.A.): The in situ Raman spectroscopy studies are a powerful method, but is it possible that other species are present on the surface which are representative of peroxide decomposition, especially since the intensity of the "peroxide" band decreases significantly at the high temperatures? M.P. Rosynek: The Raman spectra to which you refer (which were shown during the oral presentation, but which were not included in the written version of the paper) are reproduced at right. We believe that the attenuation caused by increasing sample cof temperature of the Raman band due to the 1 0-0 stretching vibration in Ba2+0$-, which occurs at 842 cm-' for a sample at 25"C, is 100°C simply due to a decrease in steady-state 3QO'C concentration of the peroxide species. With increasing sample temperature, a given 500'C partial pressure of gaseous 0, is able to sustain a progressively smaller concentration of BaO,. If 0, is removed from the reactant gas stream at 8OO-85O0C, 750 aoo s o 1050 1100 the Raman band due to 0;- immediately Wavenumbers, cm-' disappears, even in the absence of CH4. The observed broadening of the 0-0 In situ Raman spectra of 0.5 mol% Ba/MgO stretching band and its shift to lower exposed to 10: 1 CH4:02 reaction mixture at frequency with increasing temperature are the indicated temperatures. typical for these catalysts, and are similar to those reported previously for Raman spectra of L~,O single crystals. *

-

* Ishii, Y. et al., J . Am. Ceram. SOC.74, 2324 (1991). M. Baerns (Ruhr-University Bochum, Bochum, Germany): In your contribution, you have emphasized the possibility of homolytic splitting of CH, as the initial step in OCM, refemng to CH4/CD4 exchange experiments. From transient experiments with respect to the CH4/CD, exchange reaction and the OCM reaction, we could derive that CH, radical formation and H/D exchange occur on different sites. In the latter case, OH groups are involved, while for CH, formation, surface oxygen species, including basic 02-, might be involved. Thus, from the H/D exchange in the methane molecules no final conclusion should be drawn with respect to homolytic vs. heterolytic splitting of methane to form CH, radicals.

M.P. Rosynek: It should be emphasized that the CH,/O2 oxidation and CH4/CD4 exchange reaction results shown in Fig. 6 were obtained in separate experiments, i.e., the

355

HID exchange depicted was studied in the absence of 0,(and on catalysts that, following pretreatment in 0,at 80O0C, contained no surface OH groups, as verified by D, exchange measurements). The data in this Figure are simply intended to illustrate that two differing reactions, both of which require activation and cleavage of the C-H bond in CH,, must occur on different types of surface sites, as shown by their contrasting responses to added CO,. When 0, was added to the CH4/CD4 reaction mixture over BdMgO at 850°C, on the other hand, virtually no H/D exchange was observed because, as shown in Fig. 6, the small amount of CO, produced even at very low conversions of CH, is sufficient to almost completely poison the CH4/CD4 exchange reaction. Our conclusion that the C-H bond breaking which occurs during CH, radical formation is homolytic is based on the assumption that, as asserted by several previous authors, HID exchange in CH,/CD, mixtures involves heterolytic C-H bond cleavage, and the CO, poisoning results in Fig. 6 indicate that surface sites capable of such heterolytic C-H bond breaking are completely deactivated by CO, under typical oxidative coupling conditions.

M.Sinev (Academy of Science of Russia, Institute of Chemical Physics, Moscow, Russia): Your evidence for homolytic C-H bond cleavage seems to be very clear. But you included the contribution of a heterolytic mechanism too. Do you have evidence for this?

M.P. Rosynek: As discussed in my response to Professor Baerns' comment above, we believe that the C-H bond cleavage involved in CH, radical formation occurs exclusively by a homolytic mechanism on these catalysts. Surface sites that might otherwise be capable of heterolytically breaking the C-H bond in CH, are completely poisoned by the CO, byproduct of the methane coupling reaction. In the absence of O,, which is required for CO, formation, the strongly basic surface sites that are capable of heterolytically cleaving the C-H bond in CH4 are available to catalyze reactions such as CH,/CD, exchange. R.K. Grasselli (Mobil Central Research Laboratories, Princeton, New Jersey, U.S.A.): My question is a technological question. Where do we go from here? The maximum C,, product yields obtainable from CH, oxidative coupling appear to lie at about 20 % , which appears to be a commonly reached barrier. On the one hand, we are beginning to fall off the Periodic Table on the lower left-hand side, while on the other, the halide-mediated CH, activation is environmentally unacceptable. Would you be willing to comment on this?

M.P. Rosynek: There does appear to be an apparent upper limit of - 20 to 25 % to the C2+ yield, as observed empirically by numerous workers. (Indeed, modelling calculations have suggested that the C2+ yield may be mechanistically limited because of unavoidable homogeneous oxidation of C,H, and C2H4 to CO,.*) Moreover, from a commercial standpoint, the C2+ yield must be achieved at as high a C, selectivity as possible, in order to minimize 0, consumption, heat generation, and loss of carbon to CO,. Although we may becoming "Periodic Table-limited," it is not necessarily on the lower left-hand side. In fact, the best overall performance reported thus far for the oxidative coupling of methane in the co-feed mode has been obtained with a Mn/Na2W04/Si0, catalyst developed recently by a group of Chinese investigators, who obtained 65% selectivity to

356

C2+ at a CH, conversion of 37% at 800"C.** (Using a somewhat longer contact time and higher CH4/02 ratio, we have obtained 8 1% C2+ selectivity at 20%CH, conversion over this catalyst at 800°C.) Although further refinements in catalysts and promoters may lead to small incremental improvements in C, yield, I do not expect that significant advances in performance, e . g . , to 50% C2+ yield at 80% C, selectivity, will result solely from refinements in catalyst composition. Major advances are more likely to occur as a result of improvements in reactor design, such as developing a technique for introducing fresh 0, reactant continuously along the catalyst bed in a perfectly mixed mode to maximize C, selectivity. *

** Shi, C. et al., Catal. Today 13, 191 (1992). Fang, X. et al., J . Molec. Catal. (China) 6 , 255 (1992).

V. CortCs Corberan and S. Vic Bcllon (Editors), New Developmenls i n Seleclive Oxidalion I/ 1994 Elscvier Scicncc B.V.

351

SELECTIVITY CONTROL BY OXYGEN PRESSURE IN METHANE OXIDATION OVER PHOSPHATE CATALYSTS M. Yu. SineP, S. Setiadiband K. Otsukab %w.titute of Chemical Physics, Academy of Sciences of Russia, Moscow, Russia. Tokyo Institute of Technology, Tokyo, Japan. Oxidative transformations of methane are studied over a series of phosphate catalysts. Over Zr-P and Zn-P catalysts the shifts from formaldehyde to C,-hydrocarbons were observed at oxygen concentration and temperature variations indicating their formation via common intermediate, likely CH,-radicds. Different products selectivities over different catalysts in the same reaction conditions demonstrate the participation of the surface in products formation.

1. INTRODUCTION Selective partial oxidation of methane remains as one of the topics of the day in heterogeneous catalysis. During the recent decade since the work of Fang and Yeh [l] extensive studies of oxidative coupling of methane (OCM) led to creation of the efficient catalysts for this process [2,3] and to improvement of the understanding of reaction mechanism [4,5]. The progress in studies of methane partial oxidation to oxygenates (methanol and formaldehyde) is not so rapid. High yields of the products were obtained and kinetic and mechanistic studies were carried out over Mo- and V-containing catalysts in the presence of nitrous oxide as oxidant [6-81. The good results reported for the process in the presence of molecular oxygen are not reproducible. Information on the mechanism of the process is still lacking. The main purpose of this study was to establish the pathways of products formation and some features of methane oxidation reaction mechanism over various phosphate catalysts. 2. EXPERIMENTAL

Metal phosphates were used as the catalysts for methane oxidation. The samples were prepared by precipitation from aqueous solutions of corresponding nitrate and phosphoric acid of the following compositions: Fe:P = 1:2.2 (denoted as Fe-P); Zn:P = 1:2 (Zn-P); Zr:P = 1:0.6 (Zr-P (0.6)) and 1:l (Zr-P (1)); Zr:Ce:P = 0.95:0.05:0.6 (Zr/Ce-P); Zr:La:P = 1:0.3:0.6 (Zr/La-P). The precipitates were dried at 120 "C, calcined in air at optimal temperatures which allowed to achieve the highest selectivity with respect to partial oxidation products (see Table 1) and stabilized by treatment for 2 h. in reaction conditions. Methane and molecular oxygen were used as reactants and helium as diluent. A fixed-bed

358

quartz reactor (inner diameter - 7 mm) was used. Free volumes before and after the catalyst bed were filled with crushed quartz to minimize the contribution of gas-phase oxidation. On-line GC-analysis was used to measure the concentrations of reactants and products (hydrogen, carbon oxides, C,-hydrocxbons, methanol and formaldehyde).

3. RESULTS The main products of methane oxidation were formaldehyde, ethane, ethene, carbon oxides, water and hydrogen. Methanol concentration in the products was almost negligible. S(C,.$ and the ratio HCHO/C2.sboth decrease upon The total selectivity S, = S(HCH0) increase of the reaction temperature (see Table 1).

+

Table 1 Catalytic properties of phosphates in methane oxidation (0.5 g of catalyst, 50 ml/min. of the mixture CH4:0,He=1:1:3) CH, Conv.,

CO

co2

H,: CO

5.6 5.9 9.0

52.3 58.7 67.2

0 0

0

0

0 0

36.1 26.0

26.5 32.3

37.4 41.7

0 0

-0 0.1

2.10

16.3

9.7

68.9

5.1

0.63

675 700 725

0.40 2.00 4.00

33.6 32.1 13.8

11.2 12.6 20.8

55.2 47.5 61.7

0 7.8

3.7

0.23 0.25 0.50

Zr/Ce-P (700)

675 700

0.32 1.82

35.5 17.3

8.3 3.6

43.7 56.8

12.5 12.3

0.20 0.38

Zr/La-P (700)

700

61.6

0

10.6

17.1

72.3

0.85

Catalyst

t, "C

%

HCHO

Fe-P (600)'

675 700 725

0.40 0.70 1.50

42.2 35.4 23.8

Zn-P (600)

700 725

0.40 0.70

Zr-P (1) (700)

700

Zr-P (0.6)

(600)

*

Selectivity, % C,,,

- calcination temperature, "C.

The decrease of total flow rate W (see Fig. 1) leads to the linear rise of conversion accompanied by the partial loss of HCHO and by more complex change of selectivity to C2,$. Over Zr-P samples at 700 "C the minimal selectivity to G,% was observed at W = 50-200 ml/min.*g.

359

In the presence of the Fe-P catalyst H, and CO, are absent in the products. Increase of oxygen partial pressure leads to the rise of methane conversion and decrease of selectivity to HCHO (see Fig. 2-4). Over Zn-P and Zr-P catalysts significant amounts of H, are produced. Total selectivity S, over Zr-P and Zn-P catalysts is slightly dependent on oxygen concentration. However, the distribution of the products is very sensitive to oxygen concentration (see Fig.3). Addition of Ce and La to zirconium phosphates leads to the change of their catalytic performance. Over ZdCe-P the increase of P(0J from 5 to 20 kPa leads to an increase of H,/CO ratio from 0.14 to 0.38. Simultaneously a decrease of S, was observed, but the rise of HCHO/C, ratio is not so evident as over Zr-P and Zn-P. Zr/La-P is the most active one among the phosphates tested in this study. At 700°C it does not produce any formaldehyde and gives the highest yield of hydrogen at the ratio H,/CO ( - 1.1) almost independent on P ( 0 J . The amounts of ethene and ethane are comparable over Zr/La-P. The change of P ( 0 J from 5 to 20 kPa leads to the rise of C,H,/C,H, ratio from 0.5 to 1.8. 4. DISCUSSION

Different factors such as reaction temperature, flow rate and reactants concentrations are important for the yield of the products. But their effect on reaction pathways changes from one catalyst to another. The results obtained upon variation of experimental conditions demonstrate that products formation pathways and some features of methane oxidation mechanism are very sensitive to the catalysts composition. In case of Fe-P the absence of hydrogen in the reaction mixture indicates that the main pathway of consecutive transformation of formaldehyde over this catalyst is the oxidation to CO and water. In contrast with Fe-P, high concentrations of hydrogen were observed over Zr-P catalysts. H,/CO ratios rising up to - 1 at low flow rates point to the decomposition of formaldehyde into H, and CO as the way of its consecutive transformation [9]. The main distinctions in catalytic performance of different phosphates were observed upon oxygen pressure variations. In all cases methane conversion was increased proportionally upon increase of oxygen pressure. But the trends in selectivities are completely different reflecting the differences in the mechanism. Over Fe-P at P ( 0 J variation from 5 to 20 kPa the selectivity to formaldehyde decreases but S(CJ increases. Probably the rise of oxygen pressure leads to the acceleration of methane transformation both into formaldehyde and ethane, but HCHO becomes less stable at higher P ( 0 J over Fe-P due to its consecutive oxidation to CO and water. Over Zn-P and non-modified Zr-P we observed an opposite change of selectivities to HCHO and C,-hydrocarbons: S(HCH0) increased and S(CJ decreased upon increase of P ( 0 J . Evidently this group of catalysts possesses a low activity in formaldehyde oxidation. Addition of small amounts of easily reducible cation (Ce) to zirconium phosphate enhances the redox character of the catalyst and shifts its performance to that of Fe-P: upon increase of P ( 0 J the total selectivity decreases and the increase of the fraction of HCHO in S, becomes not so pronounced as over Zr-P.

360 c H 4 Camr.,u

CH4 Camr.,l

I

- 60

: /

6 -

- 40

*u 4

- 20

a-

‘ 0

10

6

0

16

20

P(O& ItP.

+

x x

-2n-P

0 - Zr-P(1) 0 - Zr/le-P

(rroh, Y-&xrSl

x

- Zr-P (0.6) - Zr/Ce-P -Fe-P

Flg.7 Methane converslon and roducts selectlvltles vs. reversed flow rate over Zr-P ( 0 3 ) boo%, Pop= 20 kPa) Flg.2 Methane converslon vs. Po2 (700%, 100 ml/mln,’g)

60 -

20

~

0

-

+ X.

- Zn-P

- Zr/Ce-P

X

0

0.8 -

w

0.4

0

- Zr-P (0.6) - 2riLe-P

0 - Zr-P(1)

x

Flg.3 Total selectivity S , vs. Po2(7OO0C, 100 ml/mln.‘g)

Flg.4 F

- S(HCHO)/St

vs. P~2(7000C, 100 ml/mln:g)

-Fe-P

Modification of Zr-P with lanthanum makes the catalyst very active. Only carbon oxides and C,-hydrocarbons were detected as carbon-containing products. The ratio H,/CO close to 1 indicate that HCHO decomposes rapidly to H, and CO over strong basic sites. Thus, the addition of cations imparting the foreign functions (redox or strongly basic) to zirconium phosphate leads to disappearance of the effect of HCHO/C, ratio regulation by P ( 0 J variations. Recently the effect of reversible selectivity switch from CO and ethane to formaldehyde in methane oxidation over MgO was observed [lo]. Unfortunately, it was impossible to separate the effects caused by changes in residence time and in oxygen concentration and to make a definite conclusion about the origin of this phenomenon since the only varied parameter was the total flow rate of the reactants. In our experiments we observed a parallel decrease of selectivities to HCHO and CzrS at high flow rates. Therefore we can suppose that the residence time has no effect on the reaction route. The high kinetic order of the recombination reaction with respect to CH,-radicals noted in [lo] may be the reason for the increase of S(CJ at low flow rates. The redistribution of the products at nearly constant total selectivity S, depending on P ( 0 J points to the existence of common intermediates for OCM and oxygenates formation. A number of data obtained since the paper of Lunsford et al. [111proved that OCM proceeds via methyl radicals formation. Obviously CH, radicals and the products of their transformations are the common intermediates for ethane and HCHO formation over phosphate catalysts. In fact, CH, radicals once formed in the reaction of a methane molecule with an active site /O/s on the surface /O/s

+ CH,

+

/OH/s

+ CH,

(1)

can give coupling products via recombination

2 CH,

+

C2H,

(2)

or oxygen containing compounds (oxygenates) in homogeneous and/or surface assisted reactions. In heterogeneous-homogeneouscatalytic oxidation of methane the equilibrium state in the reaction CH,

+ O2 < = > CH,Oz

(3)

is the key-factor for the products distribution [12,13]. The selectivity to C,-hydrocarbons should be a function of temperature and oxygen partial pressure but should slightly depend on the rate of radicals formation, i.e. on activity of the catalyst. Reaction (3) is followed by the subsequent homogeneous steps including oxygenates formation via reactions: i) at low temperatures

+ CH,OH + 2 CH,O + 0,

2 CH302 + HCHO 2 CH,O,

+

0 2

(4) (5)

362

+ CH,

CH302 CH30

+ CH,

+.

+.

2 CH,O

CH30H

(6)

+ CH,

(7)

ii) at high temperatures CH,

+ O2

+.

HCHO

+ OH

(8)

The role of the catalyst surface in the CH,-radicals transformations is not clear. Both the gas-phase and the heterogenous steps are considered to participate in the competition between coupling and oxidation [10,14]. If the surface of the catalysts plays a significant role in radicals transformations and products formation, the heterogeneous reactions such as

+ CH,O / /s + CH30 /O/s

+.

-+=

/OH/s +CH,O

/ O / s +CH,

(9) (10)

will be another factor of selectivities control. The rate constants of the steps (9) and (10) and products selectivities in the same reaction conditions should vary depending on thermochemical properties of the catalyst [ 151. High H-atom affinity of active surface oxygen species (high 0-H binding energy) and increase of surface concentration of active oxygen species /O/s will accelerate the formation of HCHO in reaction (9). High values of oxygen binding energies or decrease of /O/s concentration leads to regeneration of methyl radicals via step (10) and to the prevalence of the coupling reaction. The variation of oxygen pressure shifts the selectivity by changing the state of the surface. Thus, the regulation of HCHO/C,., ratio by P ( 0 J and temperature indicates that the common intermediate for oxygenates and coupling products exists and the equilibrium state in reaction (3) is a key-factor for products distribution in partial oxidation of methane over a series of catalysts. The variations of the relative amounts of formaldehyde and ethane from one catalyst to another at the same temperatures and oxygen partial pressures is an experimental evidence for surface-assisted products formation and the significant role of radical-surface interactions [ 15,161 in methane oxidation. REFERENCES

1. T. Fang and C. Yeh, J. Catal. 69 (1981) 227. 2. K. Otsuka, J. Japan. Petrol. Inst. 30 (1987) 385; J. S. Lee and S. T. Oyama, Catal. Rev.-Sci. Eng., 30 (1988) 249; J. R. Anderson, Appl. Catal. 47 (1989) 177; G . Hutchings, M.S. Scurrell and J. R. Woodhouse, Chem. SOC.Rev., 18 (1989) 251. 3 . E.E. Wolf, (ed.), Methane Conversion by Oxidative Processes, Van Nostrand Reinhold, New York, 1992. 4. J. H. Lunsford, Catal. Today, 6 (1990) 235. 5 . M. Yu. Sinev, V. N. Korchak and 0. V. Krylov, Uspekhi Khimii, 58 (1989) 35 (Rus. Chem. Rev., 58 (1989) 22). 6. M. J. Brown and N. D. Parkyns, Catal. Today, 8 (1991) 305. 7. H.-F. Liu, R.-S. Liu, K. Y. Liew, R. E. Johnson and J. H. Lunsford, J. Am. Chem.

363

SOC., 106 (1984) 4117. 8. K. J. Zhen, M. M. Khan, OC. H. Mak, K. B. Lewis and G. A. Somorjai, J. Catal. 94 (1985) 501. 9. M. Yu. Sinev, G. A. Vorob'eva and V. N. Korchak, Kinetika i Kataliz (Rus. Kinetics and Catalysis), 27 (1986) 1164. 10. J. S . J. Hargreaves, G. J. Hutchings and R. V. Joyner, Nature (London), 348 (1990) 428. 11. D. J. Driscoll, W. Martir, J.-X. Wang, J. H. Lunsford. J. Am. Chem. SOC., 107 (1985) 58. 12. M. Yu. Sinev, V. N. Korchak and 0. V. Krylov, Kinetika i Kataliz, 28 (1987) 1376. 13. M. Yu. Sinev, V. N. Korchak and 0. V. Krylov, Proc. 9th Int. Congress on Catalysis, Calgary 1988. Ed. M. J. Phillips and M. Ternan, v.2, p. 968-973. 14. H. Zanthoff and M. Baerns, Ind. Engng Chem. Res., 29 (1990) 2. 15. M. Yu. Sinev, Catal. Today, 13 (1992) 561. 16. Y. Tong, M. P. Rosynek and J. H. Lunsford, J. Phys. Chem., 93 (1989) 2896.

364

R.K. GRASSELLI (Mobil Research & Development Corp., Princeton NJ, USA): In your Table ? you show that Zr/La-P catalyst gives a CH, conversion of 61.6% at 700 "C and no formalcehyde while Zr/Ce-P gives a conversion of 1.82% with 17.3% HCHO selectivity, and Zr-P gives 2.1 % conversion with 16.3% HCHO selectivity. The first result seems out of place, and perhaps is a typographical error. If an error, then I have no question, if not, then I feel that if we assume that O2 influences the Zr/Ce-P system because of the Ce(3 +)/Ce(4 +) multivalent redox possibility, while La(3 +) is valence invariant in Zr/La-P, then while these two cations could give significantly different results, I would then expect the Zr/La-P to behave similar to Zr-P, and this is not the case, is it possible that you lost temperature control in the Zr/La-P system? M. Yu. SINEV (Inst. of Chemical Physics, Moscow, Russia): The data presented in Table 1 is not a misprint. In fact, La-doped zirconium phosphate is the most active in this series giving complete conversion of oxygen at 700 "C. I cannot exclude overheating in the catalyst layer, but though the catalytic behavior of this system is totally different from others. We assume that high La loading (La:Zr = 0.2:l)leads to the formation of separated highly active La-containing phase determining high activity of Zr/La-P sample. In case of Zr/Ce-P sample the loading is low (Ce:Zr = 0.05:0.95) and solid solution formation is probable due to similar ionic radii of Zr(4+) and Ce(4+) (0.082 and 0.088 nm respectively). Low formaldehyde selectivity of Zr/La-P catalyst (even at low temperatures when conversion is much less and heat evolution is not enough for loss of temperature control) is caused by rapid decomposition of HCHO to CO and hydrogen over strong basic oxygen bonded to La(3+) cations. So, there are several reasons for Zr/La-P sample to display different catalytic performance compared to undoped and Ce-doped zirconium phosphate.

J. HABER (Inst. of Catalysis and Surface Chemistry, Krakow, Poland): You are assuming that the intermediate is CH302.Is this intermediate formed in the gas phase or at the surface? In the latter case it could be possible to separate the centres generating the methyl radicals and surround them by centres adsorbing oxygen, what should strongly increase the selectivity to oxygenates. M. Yu. SINEV: Reaction CH, + 0, (+ M) <=> CH,02 (+M) in gas phase is very rapid, and the time of achievement of the equilibrium at atmospheric pressure is < lo5s. Therefore the contribution of the surface in CH,02 radicals formation can be substantial only in case of catalysts with tight pores. If the radical-surface collisions frequency is more than gas phase impact factor, the heterogenous reactions such as O(ads) + CH, + CH,O, can be significant for CH,02 radicals formation. However, the increase of diffusion path usually leads to the loss of oxygenates in secondary heterogenous decomposition and total oxidation reactions. So, it is difficult to increase the selectivity to oxygenates in this way. B.K. HODNETT (University of Limerick, Limerick, Ireland): With reference to the step CH,02 I wish to draw you two recent studies. The in your mechanism: CH, + O2 <=> first by Baiiares et al. (1) who demonstrated that when CH, + I8O2is passed over Mo0,/Si02 catalysts the product HCHO featured exclusively I6O. In our laboratory we have studied this reaction over V205/Si02catalysts in the TAP reactor (2).In the first series of experiments we showed that CH, is not actuated in the absence of gas phase oxygen. Another result was that HCH160 formed when pulses of CH, I8O2were fed to the reactor. We proposed therefore that the reaction mechanism is:

+

365

1. M. A. Baiiares, I. Rodriguez-Ramos, A. Guerrero-Ruiz, J. L. G. Fierro; Studies in Surface Science and Catalysis, L. Guczi et al. (ed.), p. 1131, 1993. 2. Karthenser et al., Catal. Lett., in press. M. Yu SINEV: The fate of free methyl radical once formed in CH4 molecule interaction with the surface active site is strongly dependant on the competition between homogeneous processes and reactions on the surface. If the catalyst pore diameter is less (or comparable) than mean free path in gas and probability of radical capture by surface site is close to 1, the role of reaction in adsorbed layer (or in coordination sphere of surface ions) in products formation will be predominant. This situation can take place in case of catalysts containing multi-valent cations such as V, Mo and W on supports with high surface area. The surface area of Mo03/Si0, catalyst you have mentioned was relatively high (69 m2g-I):similar values can be assumed for V,O,/SiO, samples. In this case the contribution of lattice oxygen in product formation will be principal and your scheme is completely correct. The phosphate catalysts we used are of relatively low surface area (0.5-10 m’g-’) and do not contain the ions which can cause the efficient capture of CH3 radicals. This can lead to another proportion in contribution of gas and lattice oxygen species in products formation.

J.J. LEROU (Du Pont & Co., Wilmington DE, USA): Could you comment on the effect of water vapor on selectivity? Could cofeeding water constitute another means to control the selectivity? Did you carry out H,O cofeed experiments?

M. Yu. SINEV: We did not cany out H,O cofeed experiments, but this influence can exist because water is able to change the relative concentrations of the surface sites in different states, for example, by shift of equilibrium between hydroxyls and anion vacancies: 2 [OHIS <=> [IS + [O]S + H,O. L. Ya. MARGOLIS (Inst. of Chemical Physics, Moscow, Russia): The reaction scheme for methane oxidation to formaldehyde has been repeatedly published. Which steps in your mechanism are new and what kind of experimental data they are based on? M. Yu. SINEV: The reaction schemes discussed in literature described formaldehyde formation at methane oxidation (i) in homogeneous gas phase reaction or (ii) in reaction localized on the surface of the catalyst. More realistic scheme should account both homogeneous and heterogeneous steps including radical-surface interactions. This approach is realized in the scheme discussed here. It is based on the data concerning the formation of free radicals during catalytic oxidation of methane obtained by several authors since J. Lunsford et al. (1) and on analysis proceeding from the experimentally established Polanyitype correlations in heterogenous chemistry of free radicals (2). (1) D. J. Driscoll, W. Martir, J.-X. Wang, J. H. Lunsford: J . Am. Chem. SOC.,107 (1985) 58. (2) M. Yu. Sinev; Catal. Today. 13 (1992) 561.

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V. CortCs Corbcran and S. Vic Bcllon (Editors), New Developments in Seleciivc Oxidaiion II

0 1994 Elsevier Science B.V. All rights reserved.

367

Isotopic Labeling Studies on Oxidative Coupling of Methane over Alkali Promoted Molybdate Catalysts S. A. Driscoll and U. S. Ozkan Department of Chemical Engineering The Ohio State University Columbus, Ohio, 43210-USA The addition of the alkali promoters Li, Na, and K to MnMo04 has been shown to increase the catalytic activity and selectivity for the oxidative coupling of methane at 700OC (1). This promoter effect has been further investigated through transient isotopic labeling technique using oxygen and methane isotopes under steady-state reaction conditions and by the addition of C02 to the feed stream. 1. INTRODUCTION

There has been a large volume of literature published about the catalytic conversion of methane (2-13). Although extensive studies have been performed, especially on the oxidative coupling of methane, questions still remain about the active form of oxygen participating in these reactions. In this study, MnMo04 catalysts doped with alkali promoters were synthesized and characterized using Xray diffraction, X-ray photoelectron spectroscopy, laser Raman spectroscopy, and thermal analysis techniques. Kinetic measurements were performed under steadystate conditions. Isotopic labeling technique was also used at steady-state by switching between 1 6 0 2 and 1 8 0 2 and between 1*CH4 and 13CH4 in the feed stream. The effect of C02 in the gas phase was also examined by the addition of C02 to the feed stream. Thermal analysis techniques were also used to probe the interaction of various species with the catalyst surfaces.

2. EXPERIMENTAL The manganese molybdate catalyst was prepared through a precipitation reaction between ammonium heptamolybdate and manganese chloride. Alkali promoted catalysts containing either Li, Na, or K were prepared through wet impregnation of the calcined molybdate with the alkali carbonate followed by drying in an oven overnight to drive off the water. The pure manganese molybdate and the alkali promoted catalysts were calcined in oxygen for 4 hours at 800 OC. These catalysts were characterized through a number of techniques including BET surface area using krypton (Micromeritics 21OOE Accusorb), X-ray diffraction (Scintag PAD V diffractometer with Cu K, radiation), X-ray photoelectron spectroscopy (Physical Electronics/Perkin Elmer, Model 550), laser Raman

368

spectroscopy (Spex 1403), and temperature programmed reduction (TPR) with hydrogen, and temperature programmed desorption (TPD). A quartz fixed-bed reactor with 9 mm O.D. and 5 mm I.D. was used for the catalytic reaction experiments. The diameter was reduced to 2 mm after the catalyst bed to allow rapid exiting of the gas stream. The isothermal portion of the quartz tube was determined to be 20 mm long. The catalyst bed length for the isotopic labeling studies ranged from 8 to 11 mm, with a quartz wool plug inserted to hold the bed in place. For the equal residence time experiments to determine the effect of the addition of C02 to the feed, quartz powder was mixed with the catalyst to maintain the bed length at 15 mm. The total surface area used for each experiment was 0.1 m2. Blank studies using an empty reactor, or a reactor filled with quartz chips revealed minor conversion under reaction conditions, mainly to C2H6 and HCHO. The feed gas composition was maintained using mass flow controllers (Tylan), and the reaction gas composition was continuously monitored during the isotopic labeling experiments by a quadruple mass spectrometer (HP 5989A MS engine). The steady-state experimental setup has been described previously (1). The experiments to examine the effect of C02 addition to the feed were performed in two steps. Reactant and product concentrations were first obtained with no added CO2 using a methane/oxygen/nitrogen mixture (2:1:2) at a flow rate of 10.3 cms(STP)/min. The feed composition was then changed keeping the overall flow constant by replacing one half of the nitrogen with C02. This resulted in a feed gas mixture of methane/oxygen/nitrogen/carbondioxide at 2:l :l :l. The same setup was used for the isotopic labeling experiments. Switching the feed gas from unlabeled source to a labeled gas involved a 4-port Valco valve in the gas flow stream. The feed gas source was switched from I602/He (Matheson) to 1802/He (Icon, 99 atom % pure l 8 0 ), or from 12CH4 (Matheson) to 13CH4 (Isotec). The oxygen isotopic labeling experiments were performed both in the presence and absence of methane in the feed gas, while all of the methane isotopic labeling experiments were performed under steady-state reaction conditions. The overall flow rate remained at 9.3 cms(STP)/min for all isotopic labeling experiments. Helium was substituted for methane during the rnethanefree oxygen isotopic labeling experiments. The isotopic labeling studies also included experiments to measure the gas phase hold-up time in the system by switching from an inert gas (argon) to oxygen/heliurn at the steady-state concentrations and following the argon decay. The argon concentration decay curve was also used to determine the gas phase hold-up correction factor in calculating the integrated amounts for the reaction products. Blank reactor experiments were also performed to examine the contribution of reactor wall activity or homogeneous reactions to the results. 3. RESULTS AND DISCUSSION

3.1 Characterization As has been previously reported ( l ) , the addition of the promoter ion to manganese molybdate resulted in a decrease in the catalyst surface area. No difference was found between the binding energies of Mn and Mo for fresh and spent catalysts through X-ray photoelectron spectroscopy. Na and K were present in detectable quantities on both fresh and spent catalysts, but the low sensitivity of

369

the technique to Li prevented its detection. Laser Raman spectra revealed no major changes in the molybdate structure of the promoted catalysts compared to pure MnMo04. The effect of the promoter on the X-ray diffraction patterns compared to the pure MnMo04 revealed mainly changes in the relative intensities of the pattern, with no new phases observed. The X-ray diffraction patterns of spent catalyst samples remained unchanged.

3.2 Transients for oxygen exchange The blank isotopic switch experiment over quartz wool revealed no formation of cross-labeled oxygen, indicating that there was no interaction between oxygen and the quartz wool and that the homogeneous scrambling did not occur. The transient oxygen isotope concentrations obtained in the absence of methane showed that almost no cross-labeled oxygen ( l 8 0 l 6 O ) was formed over pure MnMo04 catalyst, and only small quantities were formed over Li- and Na-promoted catalysts. However, the 1602 signal for these catalysts remained greater than the impurity concentration which was obtained from the blank run. Over the Kpromoted catalyst, however, the formation of the cross-labeled oxygen was very pronounced, and the decay of the 1 6 0 2 signal was considerably slower. The "surfacekubsurface" oxygen readily available for exchange from the catalyst was obtained from the isotope concentration curves by integration of the total l60 content, after corrections for gas phase hold-up and for bulk diffusion, and multiplying by the oxygen flow rate. The bulk contribution is defined as the continued offset in the l60content of the exiting stream after "pseudo steady-state" is reached and it refers to the continued replenishment of surface/subsurface oxygen by diffusion from the catalyst lattice. The K-promoted catalyst was able to incorporate the greatest amount of l60into the gas phase oxygen, both overall and in terms of surface/subsurface oxygen, with the ability to exchange oxygen decreasing as MnMo04
Transients of reaction products for isotopic labeling studies Under the conditions used for the transient isotope studies, the % conversion of methane over the catalysts was 3.6, 4.2,7.9 and 9.6 for pure MnMo04 and the K-, Li- and Na-promoted MnMo04 catalysts, respectively. The quantified products included COX, C2H& C2H4, and HCHO, with the calculated production of H20 based on the relative yields of the other products.

3.3

3.3.1 1 6 0 2 / 1 8 0 2 isotopic labeling The oxygen transients obtained under reaction conditions showed no increase in the formation of l 6 0 l 8 O over any of the catalysts, in fact the small amount of cross-labeled oxygen formed over the Na- and Li-promoted catalysts actually decreased under reaction conditions. Also decreasing was the concentration of l 6 0 2 in the total oxygen for the Na- and Li-promoted catalysts, as well as the unpromoted MnMo04. However, the K-promoted catalyst was seen to exhibit the same behavior as was seen under methane-free conditions, with cross-labeled oxygen constituting a significant portion of the total oxygen at concentrations that were not significantly different from those obtained in the absence of methane.

Details of the results of the transient isotopic labeling experiments under reaction conditions have been discussed previously (14). In summary, the lattice oxygen was found to be incorporated into the oxygenated reaction products in greater amounts than into gas-phase oxygen for the Li- and Na-promoted MnMo04 and the unpromoted MnMo04 catalysts. During the 15 minutes each of the isotope transients were followed, essentially all of the oxygen incorporated into HCHO was found to be 160. For the other products, the l60incorporation changed for the different catalysts. The K-promoted catalyst was found to incorporate less I6Ointo all of the products but, as previously mentioned, was still the most active catalyst for the formation of 160i80through oxygen exchange. H20 transients showed significant differences among the four catalysts in that both the Li- and Napromoted catalysts were found to use l60from the lattice in water formation, with no indication of site regeneration through the labeled 1 8 0 2 from the gas phase. Both the MnMo04 and K- promoted MnMo04 catalysts were found to use I8O in H20 formation with the K-promoted catalyst incorporating greater amounts of the labeled oxygen throughout the experiment. The sum total of I6O incorporated into the reaction products together with gasphase oxygen was calculated and is presented in Table 1, along with the amount incorporated into reaction products only. The total incorporation of l60into the product stream was found to increase as MnMo04 c Kc Li c Na. The order changes however when considering only the reaction products, which increases as K c MnMo04 c Li c Na.

Table 1. Comparison of total l60incorporation into the gas phase and into the reaction products for unpromoted and alkali promoted manganese molybdate catalysts. Catalyst

MnMoO4 Li/MnMo04 Na/MnMoo4 K/MnMo04

Total 6O incorporation over 14 min (atoms/m2)

lo2' 24.4 x lo2' 26.1 x lo2' 11.5 x

15.0 x 1020

6O incorporated into reaction products over 14 min (atoms/m2) 9.9 x 1020 23.3 x lo2' 24.2 x 1020 4.3 x 1020

3.3.2 12CH4/13CH4 isotopic labeling The 1% transient results from the isotopic labeling studies are plotted as normalized isotope concentrations versus time out to one minute after the switch in Figure 1. The transients for the unpromoted MnMo04 catalyst show the rapid fall in the formation of unlabeled carbon dioxide that was slightly faster than that of carbon monoxide, but slightly slower than the methane decay. The transients for

371

K/MnMo04 catalyst show a rapid decay of carbon dioxide, with a slower carbon monoxide decay transient. The transients for both the Li- and Na-promoted MnMo04 catalysts show a rapid decay in both 12CH4 and 1*CO with the decay of CO so close to that of methane that the two sets of data points are almost indistinguishable. The carbon dioxide decay for each of these catalysts is slightly slower than those of the methane and carbon monoxide. 1 .oo

1 .oo

0.75

0.75

C(t) 0.50

c ( t ) 0.50

0.25

0.25 0.00

0.00

0.0 0.2 0.4 0.6 0.8 1.0

0.0 0.2

Time (min)

0.4

0.6 0.8 1.0

Time (min)

1 .oo

1 .oo

0.75

0.75

C(t) 0.50

c ( t ) 0.50

0.25

0.25 0.00

0.00 0 . 0 0.2

0.4

0.6 0.8 1.0

0.0 0.2 0.4 0.6 0.8 1.0

Time (min)

Time (min)

Figure 1. Transient curves for 12CH4 isotope labeling experiments: a)MnMo04 , b)WMnMo04, c)Li/MnMo04, and d)NdMnMo04 . 3.4 Effect of C 0 2 addition to the feed

The homogeneous reaction proved to be generally insignificant in our reaction studies. The conversion obtained for an empty reactor, or for a reactor filled with quartz chips was found to be minimal under most of the conditions used in this study. The major products of the homogeneous reaction were C2H6 and formaldehyde, with only minor amounts of COX formed. For a blank study using carbon dioxide enriched feed there was no observable difference in product formation. The addition of C02 to the feed for the methane coupling reaction did not appear to have a significant effect on the product distribution, or the catalytic

372

activity of any of the catalysts examined. The CH4 conversion remained close to the runs with no added CO2 in the feed, and no change in the formation of CO, catalytically formed carbon dioxide, or C2 hydrocarbons was observed. Different investigations into the effect of excess carbon dioxide in the feed have produced different results, quantifying changes in the catalytic activity or selectivity to C2 hydrocarbons (7-10). For our study, the fact that there is no effect is supported by our isotopic labeling studies using methane in which the unlabeled carbon dioxide was found to have a very small residence time. This type of result has also been found for the Sm2O3 system by Peil et al. (7), where it was suggested that the active sites may not serve as significant readsorption sites for the product carbon dioxide. This explanation would appear to be the case for our studies.

3.5. TPR/TPD Studies As previously reported ( l ) , the resulting profiles from the TPR studies using hydrogen as the reducing gas were similar for the alkali promoted and the unpromoted catalysts in that they consisted of two reduction steps. The TPR patterns showed a shift in the temperature required for first step, the reduction of the manganese molybdate to Mn2M0308 and MnO, to higher temperatures with MnMo04 < K< Na < Li. The final reduction products were MnO and Mo. The intermediate and final forms were identified by X-ray diffraction, and found to be the same for all catalysts. The temperature programmed desorption characteristics of the catalysts using oxygen, methane, carbon monoxide, and carbon dioxide are also investigated. Preliminary studies have revealed significant differences in the desorption profiles for each of the catalysts. One of the important differences was in the TPD profiles obtained after degassing the catalyst surfaces. These profiles showed a strong oxygen desorption peak at temperatures lower than 600°C over K-MnMo04, while oxygen desorption was seen to take place at much higher temperatures over the other catalysts. This result seems to suggest that the high coupling activity for the potassium-promoted catalyst may be explained by the absence of a highly active surface oxygen species over this catalyst at reaction temperatures. 4.

SUMMARY

Differences were observed between unpromoted MnMo04 and alkalipromoted MnMo04 catalysts in the ability to form cross-labeled oxygen during oxygen isotopic labeling studies, and in the incorporation of 1 6 0 into the reaction products for the methane coupling reaction. l*CH4/13CH4 studies revealed differences in the relative times that carbon monoxide and carbon dioxide spend on the surface for the different catalysts. No changes were observed when excess carbon dioxide was added to the feed. We have found that the catalytic selectivity to C2 hydrocarbons for the oxidative coupling of methane over the alkali-promoted and unpromoted MnMo04 catalysts can be related to the interaction of the catalyst surface with gas-phase oxygen. The isotopic labeling studies when combined with the TPD experiments suggest that the differences observed in the product distribution may be related to the effect of the promoter on the interaction of the surface with

373

the methane molecule, which in turn changes the ease with which the methyl radical is released into the gas phase before it is oxidized into COX . 5.

ACKNOWLEDGMENT

The financial support provided by the National Science Foundation through Grant CTS-8912247 is gratefully acknowledged. REFERENCES 1.

2. 3. 4. 5.

6.

S.A. Driscoll, L. Zhang, and U.S. Ozkan, in Catalytic Selective Oxidation, Oyama, S.T., and Hightower, J.W., Ed.; ACS symposium series 523, American Chemical Society: Washington, D.C., (1993) 340. G.E. Keller, and M.M. Bhasin, J. Catal., 73 (1982) 9. T. Ito and J.H. Lunsford, Nature, 314 (1985) 721. S. K. Agawal, R. A. Migone, and G. Marcelin, Appl. Catal. , 53 (1989) 71. K.P. Peil, J.G. Goodwin, Jr., and G.J. Marcelin, J. Catal. , 131 (1991) 143. K.P. Peil, J.G. Goodwin, Jr., and G.J. Marcelin, J. Physical Chem. , 3 (1989) 5977.

7

K.P. Peil, J.G. Goodwin, Jr., and G.J. Marcelin, in Natural Gas Conversion,

8.

A. Holmen et al. (Eds), Elsevier: Amsterdam (1991) 73. T. Suzuki, K. Wada, and Y. Watanabe, Appl. Catal., 59 (1990) 213.

9.

G. Hutchings, M.S. Scurrell, and J.R. Woodhouse, Catal. Today, 6, (1990) 399.

11. 12.

K. Aika and T. Nishiyama, in Proceeding of the 9th International Congress on Catalysis; Calgary, Phillips, M.J., and Ternan, M., Ed., Ottowa: Chemical Institute of Canada, 2, (1988) 907. Aika, K., and Nishiyama, T., J. Chem. SOC.,Chem. Commun., (1988) 70 C. Mirodatos, A. Holmen, R. Mariscal , and G.A. Martin, Catal. Today, 6

13.

(1990) 601. E.E. Wolf (ed.), Methane Conversion by Oxidative Processes, Van

10.

Nostrand Reinhold Catalysis Series, Van Nostrand Reinhold,New York, 1992. 14.

S.A. Driscoll and U.S. Ozkan, J. Phys. Chem., 97 (1993) 43.

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L. YA MARGOLIS (Inst. Chem. Phys., Moscow, Russia) : Your catalysts are expected to accelerate the methane oxidation to formaldehyde and methanol. Did you observe those compounds among the reaction products and what was the isotopic composition? U.S. OZKAN (The Ohio State University, Columbus, Ohio, USA) : We observed substantial quantities of formaldehyde in the product stream and it was found to contain unlabeled oxygen almost exclusively. There was no methanol in the product stream. However, we can not rule out the possibility of methanol formation over these catalysts. In a separate study, we found methanol to be very reactive under the reaction conditions used. So even if methanol was to form over these catalysts, it would be expected to further react to COX and water quite readily.

X. VERYKIOS (Inst. Chem. Eng., High Temp. Proc., Patras, Greece) : In your steady-state tracing experiments in which labeled C was used, you showed CO originating from carbonaceous species accumulated on the surface, in contrast to C02 where signal decreased fast. So you differentiate the CO and C02 formation routes. Can you elaborate on that? Also, if you have quantified the results of labeled oxygen, have you noticed more than one monolayer of oxygen participating in the reaction, and, if yes, what is the effect of the promoter on that?

U.S. OZKAN : The most important finding of the experiments where carbon was labeled was that over potassium-promotedcatalyst there appears to be a carbon species which stays on the surface longer, whereas over the other catalysts, the surface residence times of carbon species are all very short and the interaction of methane with the surface leads to complete oxidation very readily. We are suspecting that this may be an indication of a longer "surface life" of methyl radicals on the K-MnMo04 catalysts and this may be related to the higher C2 selectivity observed over these catalysts. We are in the process of repeating some experiments which will allow us to compare the surface residence time of C2 species. To answer your second question, the amount of unlabeled oxygen participating in the reaction was higher than a monolayer by several folds. It appears that there is a rapid diffusion of oxygen from the catalyst bulk to the surface and this diffusion is considerably faster in the Li- and Na-promoted catalysts than it is in K-promoted or unpromoted catalysts. One possible explanation for this phenomenon could be the formation of some defect structures by the incorporation of Li and Na ions into the molybdate lattice. These defect structures, in turn, could facilitate a faster diffusion path for oxygen.

G.L. SCHRADER (Iowa State University, Ames, Iowa, USA) : The quantification of the ' 6 0 2 and 1802 exchange is very convincing, but under methane oxidation conditions (where a substantial amount of C02 is formed) would scrambling of the oxygen labeled in C02 in the gas phase (and on the surface) lead to an uncertainty in the knowledge of whether lattice oxygen is incorporated in HCHO?

375

U.S. OZKAN : Secondary exchange of oxygen through a scrambling process is certainly a possibility and needs to be looked at. For that reason, one needs to be cautious in trying to quantify the absolute percentages of unlabeled oxygen in the reaction products. However, the fact that the formaldehyde in our product stream consisted almost exclusively of the unlabeled isotope leads us to conclude that the oxygen incorporated in HCHO is lattice oxygen. J. HABER (Polish Academy of Sciences, Krakow, Poland) : A question may be raised as to the mechanism of the incorporation of alkali metals into the manganese molybdate : they may form a separate phase of alkali molybdates or become incorporated substitutionally into the manganese molybdate lattice. In the latter case, one would expect the formation of lattice defects, whose charge would have to be compensated by the formation of higher valence manganese ions. Do you see any traces of such ions in your XPS spectra?

U.S.OZKAN : There was a difference of about 0.3-0.4 eV between the Mn 2p3p binding energy of K-MnMo04 and those of Li-MnMo04 and Na-MnMo04. However, we are not sure if this difference is significant enough to indicate a higher valency for Mn ions. J.C. VOLTA (I.R.C., Villeurbanne, France) : Should the signal observed around 965 cm-1 on your Raman spectrum for the Li/MnMo04 be connected with some MOO, species strongly attached to the MnMo04 matrix, and should this be connected with the peculiar feature observed on the TPD experiment on the same material?

U.S.OZKAN : All promoted catalysts used in this study are prepared by impregnation of manganese molybdate obtained in the same batch. The fact that this feature was observed only on one of the promoted catalysts and not on the other promoted catalysts or the unpromoted MnMo04 catalyst leads us to think that it is not due to an MOO, species, but more likely, due to the interaction of Li with the molybdate lattice.

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V. CortBs Corberan and S. Vic Bcll6n (Editors), New Developmenrs in Selective Oxidarion II 1994 Elsevier Science B.V.

371

METHANE COUPLING OVER Sm-AI MIXED OXIDES P. Malet, M.J. Capitan. M.A. Centeno, J.J. Benitez, I. Carrizosa and J.A. Odriozola Departamento de Quimica lnorganica e lnstituto de Ciencia de Materiales de Sevilla. Centro mixto Universidad de Sevilla-CSIC. P.O. Box 874, E-41080 Sevilla, Spain

The oxidative coupling ot methane have been carried out over a series of aluminium and samarium mixed oxides prepared by a coprecipitation method XRD and EXAFS analysis have been performed for determining the structure of the catalysts From these data a coordination polyhedra of samarium that gives rise to a maximum in the activity and selectivity to C, is described

1. IMTRODUCTlON

The oxidative uimerization of methane is commonly accepted to proceed preferentially over basic oxides. Early, rare earth oxides have been shown as very effective in producing C, nydrocarbons (ethane and ethylene) [I]. The modification of the crystalline structure of Sm,O, from cubic to monoclinic has been reported to decrease the activity and selectivity to C, [2]. The addition of promoters to Sm,O, favours the formation of the monoclinic phase resulting in poorer C, yields [2][3]. In a previous study we have shown that there is a relationship between the structure of the supported phase of Sm,O,/AI,O, catalysts prepared by an impregnation method and the reactivity in the oxidative coupling of methane. So. oxide-like phases are more active than perovskite-like phases. However, the higher activity per site is encountered in a 5% Sm,O, supported catalyst, in which the structure of the active phase is different of that of C-Sm,O, or SmAIO, [4]. Kato et al.[5] propose the formation of a P-AI,O, phase when coprecipitating aluminium and lanthanum ions. To correlate the structure of the catalytic system and the activity to C, well defined samarium and aluminum mixed oxides have been prepared by a coprecipitation method and the reactivity in the oxidative coupling of methane measured. These data, discussed in conjunction with the previously reported for Sm,O, supported on y-Al,O, [4], let us propose a model for the most active site in samarium catalysts. 2. EXPERIMENTAL

The catalysts were prepared by a coprecipitation method previously described [ 5 ] . Samarium oxide (Sigma Chem. Co., 99.99% pure) in adequate amounts for preparing 5 and 30% molar Sm,O, solids was dissolved in HNO, 4M (Merck, a.r. grade) in such a way that the final pH of the solution was 0.6-0.7. Water and the adequate amount

378

of AI(NO,),.SH,O (Panreac, 99% pure) were added to the previous solution. By adding IOOml of NH,OH solution (Merck, a.r. grade) the precipitation of a mixed samarium and aluminium phase was achieved. The catalysts were obtained by crushing the solid in an agate mortar and calcining in air for two hours at 1073, 1273 or 1473K. In the following the catalysts will be named XSmAI-T, where X stands for the molar percentage of Sm,O, and T for the calcination temperature. DRIFTS experiments have been carried out in a Nicolet 510P spectrometer in which a diffuse reflectance cell (Spectra-Tech) was fitted. A reasonable signal-to-noise ratio was obtained by coadding 200 interferograms. The structure of the obtained phases has been analyzed by XRD and EXAFS. These EXAFS data were obtained at DCI EXAFS-3 station (LURE) and were analized by a procedure previously described [4]. The activity test was carried out in a quartz U-shaped fixed bed flow reactor at 988K, using a 1 5 1 CH,:O, mixture. The total flow of the reactants was 60 ml/min. The methane conversion was kept in every case below 5%. 3. RESULTS AND DISCUSSION

Two different XRD patterns are observed for the prepared catalysts. Perovskite phases were identified for the samples calcined at 1473K and for 30SmAI-1273; in addition to this phase, diffraction lines corresponding to cr-Al2O3can be observed for the 5SmAI-1473 catalyst. The remaining catalysts present an XRD pattern that can be assigned either to y-AI,O, or to amorphous phases. In any case, the presence of p-A1203phases, as reported by Kato et al. [ 5 ] , is observed. The DRIFTS spectra of y-A120, IS domlnated by a band at ca. 1050 cm-' which is associated to AI-0 stretching modes. The addition of samarium shifts its position to 1018 cm-' for the 5SmAI catalysts and to 961 cm-' for the 30SmAI. The position of the band remains unchanged on rising the calcinationtemperature, indicatingthat the shift cannot be associated to a structural change. A detailed interpretation of this shift is given elsewhere [6] and it has to be associated to the combination of the intrinsic frequencies of Sm-0 and AI-0 bonds having the same symmetry. The higher mass of the samarium ion shifting the observed frequency to lower wavenumbers. This result clearly states the presenceof a chemical interactionbetween samarium and aluminium oxides which may modify the catalytic properties of the mixed oxides. The specific surface areas,, , S , of these catalysts are shown in table 1, for the sake of comparison the values obtained for a series of alumina samples prepared by the same method are included. The addition of samarium results in a decrease of the,,S, Table 1. Specific surface areas, m2 Q-',of the studied catalysts. Calcination temperature (K)

Sm203

(% M)

1073

1273

1473

0

167

127

22

5

131

51

5

30

30

8

12

379

whatever the calcination temperature. On increasing the calcination temperature the surface area is dramatically reduced which is in accordance with the formation of a-AI2O3and/or perovskite phases. In order to obtain the local structure of the prepared phases for the catalysts with no defined XRD structure, EXAFS measurements were carried out. Since X-ray diffraction data reported above indicate that SmAIO, are formed in several catalysts, the EXAFS spectrum of this solid and of that of the initial samarium oxide have been measured and analyzed. The parameters obtained in the analysis of the EXAFS spectra of these compounds are used as input parameters in EXAFS-data analysis for the catalysts. Figure 1, shows the oscillatory EXAFS function X(k) at the samarium L,,, edge for Sm,O, and SmAIO,. The imaginary part and the absolute value of the associated k3weighted Fourier Transforms (F.r> performed on a k interval 3.0-1 1SA-’ are shown in figure 2. The radial distribution functions around samarium in both crystalline compounds have been calculated from crystallographic data given by XRD [7].The procedure followed in this work to fit the EXAFS spectra of these compounds implies the use of the minimum number of shells which reproduce the experimental XRD distributions. Best fit techniques in the k and R spaces, shell isolation by means of inverse FTs and subtraction of the calculed Sm-0, Sm-Sm and Sm-AI contributions from the primary EXAFS function have been employed. This allows us to reproduce the EXAFS spectra (dotted lines) using five shells whose fit parameters are shown in table 2 for Sm,O, and SmAIO,. Taking into account the complexity of the structures,

!

---T-

SmAlG3

r3

Y

*

x

A

4

6

8

k (A-’)

10

4

6

8

k (A-’)

10

4

6

8

10

k (A-’)

Figure 1. k3-weighted EXAFS functions (solid lines) for A) crystalline Sm,O, and SmAIO, compounds; B) 5SmAI catalysts and C) 30SmAI catalysts. The best fit functions are included in the figure as dashed lines.

I

Figure 2

I

I

I

I

I

FT functions corresponding :o the oscillatory functions shown in figure 3

t17e coordination numbers and shell distances fit reasonably with those deduced tron-I

XRD. P. detailed description of the data analysis procedure for lanthanide oxides is given elsewhere [ 8 ] .

The oscillatory EXAFS functions X(k) at the samarium L,,, edge of the catalysts as a function of the samarium content and the calcination temperature are shown in figure 1 (solid lines). On increasing the calcination temperature and the samarium content the oscillatory functions become more structurated. This fact can be associated with the structural change observed in the XRD profiles of these catalysts on rising the temperature. The k3-weightedITSof the catalysts (3.0 < k < 115k')are shown in figure 2 (solid line). It can be observed the presence of a maximum in the FT at ca. 3 . d corresponding to Sm-AI scatterers for the catalysts calcined at 1473K and for the 30SmAI-1273 catalyst in good accordance to the XRD results. As a first guess in the fitting of the EXAFS spectra of the catalysts the parameters obtained for the crystalline compounds were used. Initially a minimum number of shells was set for the fitting, the number of shells was increased when the difference spectrum between the experimental and the fitted one implies the presence of a new shell at R values close to those found in crystalline solids. The parameters obtained for A o2 and R are considered adequate when they are similar to the ones obtained for the crystalline compounds. Table 3 shows the best fit parameters obtained for the catalysts following this procedure.The calculated spectra have been included in figures 1 and 2 as dotted lines. It can be seen that the average coordination number for the oxygen shells increases on increasing the calcination temperature reaching a maximum for the crystalline perovskite structure. In all cases the average coordination

38 1

Table 2. EXAFS Parameter Values for Crystalline Sm,03 and SmAIO,. Compound

Sm203

SmAIOz

(A2)

Backscat.

N

R(A)

0

3.0

2.38

0.008

0

3.6

2.64

0.037

Sm

1.1

3.31

0.008

Sm

6.0

3.67

0.010

Sm

4.5

4.18

0.01 1

0

2.6

2.32

0.016

0

3.1

2.64

0.019

0

6.0

2.90

0.014

Ai

8.8

3.25

0.028

Sm

5.7

3.78

0.010

Ao2

numbers obtained for 5SmAI catalysts are iower than the corresponding to 30SmAI catalysts. A detailed observation of the data let us remark the presence of perovskite phases in all the catalysts when they are calcined at 1473K, this being independent on the samarium content. However, the temperature at which the perovskite phase appears increases on decreasing the samarium content. The catalysts whose structure cannot be determined by XRD present coordination polyhedra around samarium consisting of 6-7 oxygens and numbers of samarium and aluminium atoms near to one. We can explain this fact by the presence of isolated samarium clusters embedded in a y-Al,03 matrix having a first coordination sphere close to the one presented by C-Sm,O, like phases. The average coordination number of aluminium, close to one, implies that the coordination polyhedra of samarium is dispersed in the y-Al,O, matrix which presents a disordered spinel structure [9]; so, the aluminium ions are randomly distributed around samarium thus justifying the low coordination number. In the second oxygen shell of the 5SmAI-1473 catalyst shows a contraction of the Sm-0 distance of 0.19A that is clearly above the accuracy of the technique. This fact and the average coordination number of aluminium and samarium ions let us propose that samarium ions are present as small clusters immersed in the a-A1,03 matrix. The structure of the matrix set this contraction in the Sm-0 distance. The catalytic activities were measured at 988K for the series of catalysts studied. Methane conversion was kept in every case around 5%. The detected products were CO, CO, CH , , and CH ,, and in some cases traces of C, hydrocarbons. The methane conversion and the activity towards the main reaction products are shown in table 4. Two different behaviours can be observed as a function of the samarium content. The catalysts containing 30% molar samarium oxide show a decrease in the activity to C, on increasing the calcination temperature, while the 5SmAI catalyst presents a maximum in the activity when calcined at 1273K.

382

Table 3. EXAFS Parameter Values for the Coprecipitated catalysts. In parenthesis appears the crystalline phases shown by XRD. Catalyst

(A')

Backscat.

N

R(A)

0

3.0

2.43

0.012

0

2.4

2.69

0.028

0

3.3

2.43

0.015

0

2.5

2.70

0.029

Al

0.9

3.28

0.01 1

Sm

0.6

3.81

0.003

0

2.2

2.28

0.008

0

2.1

2.45

0.005

0

5.2

2.96

0.015

A1

8.5

3.23

0.016

Sm

2.4

3.81

0.004

0

3.6

2.40

0.023

0

4.1

2.69

0.026

30SmAI-1073

Al

0.8

3.24

0.013

(-1

Sm

0.7

3.74

0.006

0

1.7

2.37

0.010

0

3.3

2.61

0.019

0

5.3

2.89

0.019

Al

8.3

3.23

0.015

Sm

4.9

3.80

0.009

0

2.3

2.37

0.01 I

0

3.0

2.65

0.01 7

0

5.4

2.95

0.010

Al

8.7

3.23

0.015

Sm

5.3

3.79

0.009

5SmAI-1473 (a-A1,03 + SmAlO,)

30SmAI-I273 (SmAIO,)

30SmAI-1473

(SmAIO,)

Ao'

383

Table 4. Methane conversion and activity in the oxidative coupling of methane at 988K. The activity is expressed as molecules g-' 1017 CATALYST

CH, conv.

cox

c2

C2H4

C2H6

5SmAI-1073

11.60

10.01

1.47

0.77

0.70

5SmAI-1273

12.19

10.17

1.74

1.79

0.95

5SmAI-1473

9.45

8.24

1.21

0.32

0.89

30SmAI-1073

11.97

10.05

1.92

0.77

1.15

30SmAI-1273

8.70

7.51

1.19

0.39

0.80

30SmAI-1473

10.06

8.84

1.22

0.39

0.87

Whatever the calcination temperature or the samarium content the catalysts having perovskite-like structures present similar activities to C., Besides this, these catalysts show lower activities than the ones with oxide-like structure, in agreement with the structural effects observed for y-Al,O,-supported Sm20, catalysts [4], and to the increase in the activity and selectivity reported by lmai et al. [ l o ] for amorphous LaAIO, phases. The activity of the 5SmAI-1273 sample fits within the values corresponding to Sm,OJAI,O, catalysts prepared by impregnation techniques having similar samarium loading [4]. In an attempt to identify the nature of r' . - -?' the most active site, the activity to C2 was i :3 plotted against the average coordination 4, numbers of the first and second I coordination spheres. Figure 3 shows the result obtained for the first coordination * i (I) 1.6 sphere. It can be concluded, from inspection of the figure, that an activity 3 i: maximum is attained for a coordination a, 1.4 i polyhedron of samarium containing 6-7 0 i E 1 oxygen atoms in the first coordination 1.2 4 sphere and a second one, not shown in L I I Ii the figure, with a low coordination 0 4 6 8 10 12 number of samarium and without O x y g e n coo r d i n a t i o n n urn ber aluminiums. In conclusion, the samarium Figure 3' Of the coordination sphere which results in an selectivity to C, as a function optimum activity and selectivity in the of the average coordination oxidative coupling of methane can be number of oxygen for the described as SmO, polyhedra isolated in studied catalysts. the y-alumina matrix. This result clearly , ?

<-

1

--

1

4i

\'

4

'b ~

384

matches the one obtained for the supported phases [4]. So, a dependence of the activity and selectivity of the catalyst on the structural phase in which the active Sm-0 center is immersed can be postulated. ACKNOWLEDGEMENTS

Financial support has been obtained from Comision lnterministerial de Ciencia y Tecnologia (PB88-0257 and PB89-0642). REFERENCES. 1. Y. Amenomiya, V.I. Birss, M. Goledzinowski, J. Galuszka and A.R. Sanger, Catal. Rev., 32 (1990) 163.

2.

S.J. Korf, J.A. Roos, J.M. Diphoorn, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Catal. Today, 4 (1989) 279.

3.

A. Kiennemann, R. Kieffer, A. Kaddouri, P. Poix and J.L. Rehspringer, Appl. Catal., 6 (1990) 409.

4.

M.J. Capitan, P. Malet, M.A. Centeno, A. Munoz-Paez, I. Carrizosa and J.A. Odriozola, J. Phys. Chem., 97 (1993) 9233.

5.

A. Kato, H. Yamashita, H. Kawagoshi and S. Matsuda, J. Am. Ceram. SOC.,70 (1987) 157.

6.

L.J. Alvarez, J. Fernandez-Sanz,M.J. Capitan and J.A. Odriozola, THEOCHEM., in press.

7.

a) D.T. Cromer, J. Phys. Chem., 61 (1957) 753; b) M. Mareuo, P.D. Demier and J.P. Remska, J. Solid State Chem., 4 (1972) 11.

8.

M.J. Capitan, P. Malet, 1. Carrizosa and J.A. Odriozola, J. Chem. SOC.Faraday I, submitted.

9.

L.J. Alvarez, J. Fernandez-Sanz, M.J. Capitan and J.A. Odriozola, Chem. Phys. Lett., 192 (1992) 463.

10.

H. Imai, T. Tagawa and Kamide, J. Catal., 73 (1987) 394

M. SINEV (I.Chemical Physics, Moscow, Russia): In your calculations of the activation of CH, in absence of oxidant, you assumed the heterolytic mechanism of H f (or H-) abstraction. The values you obtained for the activation barrier are of more than 40 Kcal.mol-’, which is higher than AH of the process for more than 30 kcal.mol”. The values of E, obtained from redox experiments are always much less (from 10 to 35 Kcal.mol-l). For homolytic H-atom abstraction from CH, the activation energy is only of about 10 Kcal.mol-’ higher than AH. Why do you exclude the possibility of one-step homolytic abstraction of H-atom over the oxidative O-containing active site. J.A. ODRIOZOLA (Dpt. Inorganic Chemistry, Sevilla, Spain): A detailed description of our calculations will be given elsewhere [I]. The activation barrier for the heterolytic

385

dissociation of the AI(OH), and La(OH), clusters in vacuo are of 46 and 42 Kcal.mol-’, respectively. At the MP2 level these values slightly decrease to 39 and 38 Kcal.mol”. These values are in the upper limit of those found in the redox experiments you mention. The homolytic mechanism has been already considered by Borve and Pettersson and the barriers found were 7-10 Kcal-mol.’ [2], our results agree with the values given by Mirodatos and coworkers [3] for: CH, CD, CH,D + CHD, in which they assume an heterolytic rupture of the C-H bond. More recently, using TAP experiments the same group found an heterolytic and reversible adsorption of CH, over La,O, catalysts [4]. In addition to this, there is not evidence for the presence of radicalary oxygen species on the surface of rare earth oxides in the conditions of the OCM reactions and only transient species has been proposed. From that point of view seems to be more likely the heterolytic rupture than the homolytic one.

+

-

1.

M.J. Capitan, J.A. Odriozola, A. Marquez and J. Fernandez Sanz; J. Phys. Chern., submitted.

2.

K.J. Borve and L.G.M. Pettersson, J. Phys. Chem., 95 (1991) 3214; 95 (1991) 7401.

3.

C. Mirodatos, A. Holmen, R. Mariscal and G.A. Martin; Catal. Today, 6 (1990) 601.

4.

S. Lacombe and C. Mirodatos; preprints of EUROPACAT-1, Montpellier, September 12-17, 1993, vol. 1, p. 171.

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V . Corks Corbcrin and S. Vic Bcll6n (Edilors), New Developneni.s in Seleclive Oxidaiion I / 0 1994 Elscvicr Scicncc B.V. All rights rcscrvcd.

387

CATALYTIC REACTOR ENGINEERING FOR THE OXIDATIVE COUPLING OF METHANE. USE OF A FLUIDIZED BED AND OF A CERAMIC MEMBRANE REACTOR A. Santos, C. Finol, J. Coronas, D. Lafarga, M. Menendez and J. Santamaria. Departamento de lngenieria Quimica. Universidad de Zaragoza. 50009 Zaragoza. Spain. 1. ABSTRACT A study has been carried out of fluidized bed reactors and of membrane reactors as contactors for methane oxidative coupling. Both reactors are capable of providing a low oxygen concentration in the gas phase in contact with the catalyst, which can be expected to increase the selectivity to hydrocarbons in the system studied. Other operational characteristics such as temperature gradients within the reactor are also discussed. 2. INTRODUCTION

Since the potential of methane oxidative coupling as a source of ethane and ethylene was established, most of the research carried out in this area has been concerned with developing better catalysts for the process. In spite of this, the yields obtained to date are still too low to justify the use of the process at a commercial scale, except perhaps in a few specific applications. From the chemical reaction engineering point of view, the methane oxidative coupling process is an oxidation reaction with valuable intermediate products (ethane and ethylene), whose further reaction (in this case deep oxidation into carbon oxides), must be prevented. In any such system, the mode in which the reactants are contacted has a strong influence in determining the selectivity towards the desired product. In addition, an examination of the kinetics of methane oxidative coupling reported in the literature shows that for most catalysts, the apparent reaction order for oxygen in the deep oxidation reaction is higher than in the coupling reaction. This means that modes of contact which maintain a low oxygen concentration throughout the reactor should favour the desired versus the undesired reaction. The requirement of a low oxygen concentration in the gas phase may interfere with the need of obtaining acceptable conversion levels. One way to solve this problem would be to use a redox-type catalyst (a reducible metal oxide), which can supply lattice oxygen for the reaction. These solids would be continuously transferred to and from a regenerator unit where they would be reoxidated with gas phase oxygen in the absence of hydrocarbons. This is the base of the ARC0 process ( l ) , which uses two fluidized beds in a similar fashion to that of the fluid catalytic cracking process (FCC).

388

Apart from the circulating unit, there are other contacting devices which may satisfy the requirement of a lower oxygen concentration better than the conventional fixed bed reactor. Among these, the fixed bed reactor with a distributed oxygen feed has been studied both theoretically (2,3) and experimentally ( 4 3 . Although different interpretations have been proposed to explain the results obtained, these clearly indicate that a distributed feed reactor is potentially advantageous to improve the selectivity obtained at a given conversion level. Membrane reactors can also be used to distribute the oxygen feed, and have been employed for the oxidative coupling process (6,7). However, because of the type of membranes employed in previous works, very low permeation fluxes were obtained, and the yields reported to date are still low. Finally, another device which may provide a low oxygen concentration in contact with the catalyst is the fluidized bed reactor, which has also been used for methane oxidative coupling (e.g. 8-1 1). In this case, the bubbles in the bed would ideally act as an oxygen reservoir, from which oxygen would be transferred to the emulsion phase in the bed as the reaction takes place, thus keeping a low oxygen concentration in contact with the catalyst. This work reports the ongoing research at the University of Zaragoza regarding more suitable contacting devices to improve the yield obtained in the oxidative coupling process.

3. EXPERIMENTAL Although in principle most oxidative coupling catalysts could benefit from reaction environments with a low oxygen concentration, all the experiments in this work have been carried out using a Li/MgO catalyst since it is probably the best known among the methane oxidative coupling catalysts. The catalyst was prepared by impregnating fused MgO with an aqueous solution of Li2CO3 to give a Li concentration of approximately 3% by weight. After evaporating under constant stirring the resulting paste was further dried at 140 "C for 12 hours and calcined at 800 "C for 16 hours, then ground and sieved to the appropriate size (usually between 100 and 250 microns). BET area measurements yielded 0.2 m2/g after calcination and impregnation with Li2C03. The different reactor concepts under study in our laboratory are illustrated in Figure 1. The selectivity obtained in the conventional fixed bed reactor (1.A), can be improved by distributing the oxygen feed using several discrete oxygen injections along the bed (1.B). Alternatively, a membrane tube can be used to effect the oxygen distribution (1 .D to 1.F).In this case, a modified porous alumina tube was used, instead of the usual nonporous type, which allowed much higher permeation fluxes. The active phase can be in a catalytic bed packed inside the tube (l.D), deposited in the membrane (1.E), or both (1.F), which can be effective as a means to increase the contact time of the gas with the catalyst. In this work, the results obtained with a fluidized bed reactor (FBR, Figure l.C) and with a nonpermselective non-catalytic ceramic membrane reactor (NCFR, Figure 1.D ) are reported. The fluidized bed reactor was a 30 mm. i.d. quartz tube which used a porous quartz plate as gas distributor. The reaction was carried out in the bubbling fluidization regime by feeding undiluted CH4/02 mixtures. A vibratory device helped

3x9

1.A Single Feed, Fixed Bed reactor (SFR).

4 Products

1.B Distributed Feed, Fixed Bed Reactor (DFR).

CH4

Products

1.C Fluidized Bed Reactor (FBR).

1.D Noncatalytic Ceramic Wall, Fixed Bed Reactor (NCFR).

4 Products I/////

1.E Catalytic Wall Reactor (CWR).

1.F Catalytic Wall, Fixed Bed Reactor (CWFR).

CH4

Products

Figure 1 . Different reactor concepts under test in our laboratory.

390

to avoid particle agglomeration. Reaction temperatures were varied between 750 and 850 "C. Temperature readings were taken at different heights during the experiment to measure any gradients caused by the exothermic reaction. The membrane reactor was prepared by modification of a commercial porous A1203 microfiltration membrane. Sol-gel techniques were employed to deposit silica or alumina in the membrane structure in order to obtain adequate values of the oxygen flux. The undesired catalytic properties of the membrane were also modified by impregnation with suitable alkali promoters (usually Li2CO3). Details of the preparation and properties of the modified alumina tubes will be given in a separate paper (12). The modified ceramic tube was enclosed within a stainless steel outer shell. Oxygen flows in the shell side at controlled pressures of up to 600 kPa. Methane was fed to the inside of the ceramic tube, which was packed with the Li/MgO catalyst. The reaction takes place as the oxygen permeates through the membrane, at a rate governed by the applied pressure differential. The contact of the methane and the reaction products with the hot stainless steel enclosure was prevented by using graphite seals at both ends of the ceramic tube.

The flow of reactants to the FBR and to both sides of the NCFR was controlled by means of mass flow controllers, and a supplementary flow measurement was provided at the reactor exit to measure post-reaction flowrates. Catalyst loadings were about 3-4 and 30-40 g for the NCFR and the FBR respectively. In both cases the reaction products were analyzed by gas chromatography. Carbon mass balance closures were in all cases better than 5%, and usually better than 3%.

4. RESULTS. 4.1. Fluidized bed reactor. Fluidized bed reactor operation was essentially isothermal, with a maximum temperature difference in the reactor of about 15 "C, even when operating at CH4 conversions well above 20%. The fluidized bed reactor also showed a good performance in terms of fluidization behaviour. Velocities as low as 1.2 times the minimum fluidization velocity were employed without loss of fluidization due to particle agglomeration. The loss of catalyst fines by elutriation was usually negligible. Figures 2 and 3 show respectively the conversion of CH4 an 0 2 and the selectivities to hydrocarbons at 800 "C, as a function of the CH4/02 ratio, using undiluted feeds, with a total flowrate of 1500 Ncm3/min, and a catalyst loading of 40 g. The results shown in Figures 2 and 3 are preliminary, non-optimized reaction data. This means that it is very likely that under different reaction conditions and/or fluidization regimes still better yields are obtained. However the few experimental data gathered so far show that yields in excess of 16% can readily be obtained by choosing the appropriate reaction conditions, in this case by working at the lowest gas flowrate compatible with the bubbling fluidization regime.

39 1 90

35

85

h

8

w

cu

0

30

80

S

.-0

z

Q 5 C

25

15

0

0

I0 21 1

2.511

31 1 C H41O2

3.511

411

Figure 2. Methane and oxygen conversions in a fluidized bed reactor, as a function of the CH4/02 ratio. (Temp= 800 "C,W/F= 0.027 g min/Ncm3).

21 1

2.511

311 C H4/ 0,

3.511

1

Figure 3. Selectivity to hydrocarbons in a fluidized bed reactor, as a function of the CH4/02 ratio. (Temp= 800 "C, W/F= 0.027 g min/Ncm3). 4.2. Porous ceramic membrane reactor. As mentioned above, the use of a porous, non-permselective modified ceramic membrane allows a smooth distribution of one of the reactants (in this case oxygen), along the bed. This was achieved by applying the required pressure of oxygen at the shell side to obtain the desired CH4/02 ratio. A supplementary condition to be fulfilled consisted in attaining a high enough flow of oxygen through

392

the porous membrane so as to prevent back-permeation of methane to the shell side. To this end, preliminary experiments were carried out at different oxygen fluxes for a given methane flow on the tube side. After stabilization, gas samples were taken on the shell side and analyzed for carbon oxides. This allowed the selection of reaction conditions under which the permeation of methane to the shell side was negligible. The results obtained using the ceramic membrane reactor are shown in Figures 4 and 5 as selectivity versus conversion at a given CH4/02 overall ratio. For comparison purposes, the figures also show the results obtained in a quartz reactor in which methane and oxygen were cofed at the reactor entrance. It can be seen that the use of the membrane wall as an oxygen distributor is significantly advantageous with respect to the cofeeding of methane and oxygen at the reactor entrance. Selectivity improvements higher than 10 percentage points can be obtained by working with the membrane reactor, as shown in figure 5 at a conversion of around 20 %.

5

Figure 4. Selectivity versus CH4 conversion for a ceramic membrane reactor and for a quartz reactor with cofeeding of CH4 and 0 2 . Nominal temperature: 750 " C , CH4/02=2:1. As for the temperature control, both the membrane reactor and the quartz reactor with cofeeding of methane and oxygen showed significant temperature gradients along the bed, with steady-state temperature maxima more than 60 "C higher than the nominal temperature. A comparison of the temperature profiles inside the catalytic bed in the membrane reactor and in the fluidized bed reactor is shown in Figure 6. It can be seen that the temperature in the fluidized bed reactor is much more homogeneous than in the membrane reactor. However, the permeation of oxygen across the membrane was found to be advantageous in terms of uniformity of temperature, when compared to the conventional fixed bed reactor. The

393

temperature readings along the bed, obtained with the membrane reactor showed a much smoother temperature profile, with a temperature peak which was about 15 "C lower than that obtained under cofeeding of both reactants. 75

NCFR

O O

0 55

-

50

I

15

20

"

"

I

'

"

'

1

"

'

'

2s 30 Conversion CH4 (7'0)

35

Figure 5.-Selectivity versus CH4 conversion for a ceramic membrane reactor and for a quartz reactor with cofeeding of CH4 and 0 2 . Nominal temperature: 750 "C, CH4/02=311. 850

0

0

825

NCFR

0

800

0

L

S

o-

% 0

I Q

Q

s

0 115

% 0

I-

750

725

0

0.2

0.4

0.6

0.8

1

Normalized Reactor Length

Figure 6. Temperature versus reactor length for the fluidized bed reactor and for the membrane reactor. Reactor length was 0.14 m for the membrane reactor and 0.12 for the fluid bed reactor. The conditions for the fluidized bed reactor were as follows: Total flow rate: 1500 Ncm3/min, CH4/02=9:1, methane conversion, 16.2%.

394

For the membrane reactor: Total flow rate: 350 Ncm3/min, CH4/02=3.6:1, methane conversion, 19.7% An additional advantage of the ceramic membrane reactor follows from the increase in safety obtained by distributing the oxygen feed along the bed. In this way it is possible to operate safely at CH4/02 feed ratios well inside the explosion limits, since the reactants are not mixed at the entrance of the reactor, and a low oxygen partial pressure is maintained throughout the catalyst bed.

5. CONCLUSIONS. The results presented in this work show the potential of fluidized bed reactors and of non-permselective non-catalytic ceramic membrane reactors to increase the yields obtained in the oxidative coupling of methane. The study of these and of other contacting devices, such as those depicted in Figure 1, or those involving circulation of solids, may help to bridge the gap between the best laboratory results reported to date and those necessary to bring the process to full scale commercial application.

ACKNOWLEDGEMENTS. Financial support from DGA (Project P-IT-4/91) and of DGICYT (Project PB900920) is gratefully acknowledged.

REFERENCES. 1. C.A. Jones, J.J. Leonard and J.A. Sofranko, Methane Conversion, US Patent No. 4665260 (1987). 2. J. Santamaria, E. Miro and E.E. Wolf, Ind. Eng. Chem. Res., 30 (1991) 1157. 3. J. Santamaria, M. Menendez, J.A. PeAa and J.I. Barahona, Catal. Today 13 (1982) 353. 4. V.R. Choudhary, S.T. Chaudhary, A.M. Rajput and V.H. Rane, J. Chem. SOC. Chem. Commun., 20 (1989) 1526. 5. K.J. Smith, T.M. Painter and J. Galuszka, Catal. Lett., 11 (1991) 301. 6. K. Omata, S. Hashimoto, H. Tominaga and K. Fujimoto, Appl. Catal., 51 (1989) L1. 7. A.G. Anshits, A.N. Shigapov, S.N. Vereshchagin and V. Shevnin, Catal. Today, 6 (1990) 593. 8. J.H. Edwards, R.J. Tyler and S.D. White, Energy and Fuels, 4 (1990) 85. 9. J.H. Edwards, U.T. Do and R.J. Tyler, Catal Today, 6 (1995) 435. 10. G. Follmer, L. Lehmann and M. Baerns, ACS Prep.Div. Pet. Chem., 33 (1988) 452. 11. R. Andorf and M. Baerns, Catal. Today, 6 (1990) 445. 12. J.Coronas, M. Menendez and J.Santamaria, sent to Chem. Eng. Sci. (1993).

V. CortCs Corbcran and S. Vic Bcllon (Editors), New Developmenis in Selecrive Oxrdation If 0 1994 Elsevicr Science B.V. All rights reservcd.

395

CATALYSTS FOR OXIDATIVE COUPLING OF METHANE : RELATIONSHIP BETWEEN STRUCTURAL FEATURES AND CATALYTIC BEHAVIOURS Bi203-M203

E.A.Mamedova, N.T.Shamilova, S.Badrinarayanb

V.P.Vislovskiia,

P.N. Joshib and

aInstitute of Inorganic and Physical Chemistry, 2 9 Azizbekov Avenue, 370143 Baku, Azerbaijan bNational Chemical Laboratory, 411008 Pune, India

Abstract

Yttrium and ytterbium solid solutions in Bi,O as well as pure chemical compounds , like BiYO, or Bi,Mn,O,,, are low selective for oxidative coupling of methane.#uch higher activity and selectivity are characteristic of the polyphasic catalysts containing Bi,O, in combination with other bismuth-containing compounds that seems due to the cooperation of these phases. 1. INTRODUCTION

The oxidative coupling of methane to ethane and ethene ( C r compounds) is of great industrial and scientific importance. In the last decade, considerable progress has been made in this field. Various aspects of methane coupling reactions, including the catalysts developed, have been repeatedly reviewed [l-31. Oxides of elements from Group I1 of the periodic table and oxides of rare earth metals both promoted with alkali metal salts are most intensively studied as catalysts. Many of them are monophasic solids acting, according to Lunsford’s concept [ 3 ] , through active sites of the type [M+O-]. Much less attention has been paid to bismuth-containing systems, although this metal belongs to elements whose oxides are expected to catalyze the reactions of hydrocarbon oxidative dehydrodimerization [4]. The idea of investigating these systems is further justified by the experimental data reviewed in [5], according to which some binary oxides based on Bi,O, show appreciably high catalytic activity in the oxidative coupling of propene and toluene when consisting of two and more phases. As for the oxidative coupling of methane, only two papers mentioned that the activity of a two-phase catalyst could be favourably compared to those of similar one-phase catalysts. One of them deals with the phases issued from the decomposition of BiSn pyrochlore oxides [ 61 The other one, basing on o u r data, r e p o r t s [ 7 ] a cooperation between Bi,O, and Bi,Mn,O,,.

.

396

The present study deals the reactivity towards methane coupling by evaluating catalytic performance of a series of Bi,O,-M,O, catalysts which include monophasic as well as polyphasic solids. Attempts have been carried out to investigate the relationships between the catalyst structure and the activity. 2.

EXPERIMENTAL

Bismuth-aluminium and bismuth-manganese oxide catalysts were prepared by mixing a solution of bismuth nitrate in aqueous nitric acid with a solution of aluminium or manganese nitrate in water, coprecipitating the metal hydroxides with ammonia, drying at 110 C and calcining at 800 C for 8 h. A mixture of separately prepared Bi203 and Bi2Mn,0,, (1:l by weight) was prepared by suspending the powders in n-pentane, agitating and evaporating under agitation. Bismuth-yttrium and bismuth-ytterbium catalysts were prepared by wet mixing the required amounts of high purity grade Bi203 and Y203 or Yb20,, followed by grinding, drying at 110 C and calcining in two steps at 650 C for 5 h and at 1000 C for 10 h. The samples will be designated by the atomic proportion of bismuth: for example 0.20BiY corresponds to Bi/(Bi+Y)=0.20. Catalyst surface and phase compositions were studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) The surface areas were determined by argon thermal desorption using a standard chromatographic apparatus. Methane oxidative conversion was carried out in a continuous flow, fixed bed reactor. All experiments were performed using 8 ccm of catalyst (particle diameter 0.5-1.0 mm) and the composition of the reactant gases: methane/oxygen/ nitrogen=30/20/50 with a total gas flow of 50 ml/min. The temperature of reaction was 730 C for BiAl and BiMn catalysts, and 750 C for BiYb and BiY samples. Reaction products were analyzed by gas chromatography.

.

3.

RESULTS

3.1.

Bi203-Yb,03

system

Data on catalytic properties of the bismuth-ytterbium oxide system are given in Figure la. The main products of methane oxidative conversion over these and other catalysts described beneath were ethane, ethene and carbon dioxide. No production of carbon monoxide nor other oxygen-containing compounds was observed. Pure bismuth and ytterbium oxides catalyze the oxidehydrodimerization reaction with similar C2 selectivities equaled to ca.30% at 750 C. Binary samples are less selective. The X-ray patterns of the 0.80BiYbI 0.67BiYb and 0.6OBiYb catalysts presented in Figure 2 reveal only the presence of the Bi20, phase. The Yb203 phase can clearly be observed along with Bi203 in the samples with more than 40 at.% Yb, indicating this concentration to correspond to the solid solution limit of

397

ytterbium in Bi203. No indication for the formation of chemical compound was found.

Figure 1. Extent of methane conversion ( 0 ) and C2 selectivity ( 4 ) as a function of the Bi,O,-M,O, catalyst composition

I

I

Figure 2. XRD spectra of the BiYb oxide catalysts: oBi,O,, Yb-0,. L 3 Figure 3. XRD spectra of the BiY oxide catalysts: O B i 2 0 3 / BiYO,, A Y,O,.

X X

The XPS Bi4f doublet was used for characterizing the state of bismuth at the surface. For both 0.80BiYb and 0.50BiYb samples the binding energy (BE) of the Bi4€,,, peak was equal to 158.8 eV. The difference between the BE of the Bi4€,,, and Bi4€, peaks amounted to 5.3k0.1 eV. Those of Bi203 were found to be Jhe same that is consistent with literature data [8] As for the X P S Yb4p spectrum, its parameters were close to those of Yb,?,. This seems to confirm that the Bi and Yb species in the studied BiYb oxide catalysts were very similar t o those in BiZ03 and Yb20,,

.

398

respectively. The surface atomic ratios Bi/ (Bi+Yb) for the freshly prepared as well as for the used catalysts, calculated from the XPS intensities, were always close to those of the bulk. System Binary BIY oxides display, as is seen from Figure lb, higher total activity and lower selectivity with respect to the C, hydrocarbons formation in comparison with the individual oxides. Both activity and selectivity did not change significantly upon approaching the steady state of reaction. According to the XRD data presented in Figure 3 , the 0.85BiY and 0.75BiY catalysts were yttrium solid solutions in Bi?O,. The 0.50BiY was composed exclusively of the BiYO, phase. This phase predominated in the 0.33BiY, the rest of which was constituted of 3.2. Bi,0,-YK03

'Z03.

Parameters of the XPS Bi4f and Y3p lines for both freshly prepared and used BiY catalysts were found to be close to those of Bi,O, and Y,O,. Their surface compositions, calculated from the XPS intensities, were comparable to bulk compositions. Bi,O,-Al,O, system Though pure alumina catalyzes only methane deep oxidation, its combinations with Bi,O, show appreciable selectivity regarding the oxidative coupling reaction (Figure lc). BiAl oxides demonstrate higher total activity as well. Under the conditions studied, the highest overall yield of ethane and ethylene was found for the 0.60BiA1 catalyst and was equal to 12% at 730 C . The phase composition of the coprecipitated BiAl oxides was rather complicated. As is seen from Table 1, the most selective O.GOBiA1 catalyst contained, besides BiZ03 and A1203, a series of BiA10, and Bi2,A1203,. After chemical compounds, such as Bi,A1,0,, catalyst operation for several hours, the XRD diagram revealed no changes in phase composition. 3.3.

Table 1 Surface area and phase composition of BiAl oxide catalysts Bi/ (Bi+Al) ratio 0.00 0.20

S

(m2/9) 60.5

0.50

41.2 19.8 12.1

0.60

8.5

0.80

8.8

1.00

6.3

0.40

Phases (XRD)

y-A1203 y-Al,O, + BiZA1,O, + BiA10, Bi,A1,0, + BiA10, + a-Bi203 BiA10, + Bi2A1,0, + Bi2,A1,0,, + cr-Bi,O, BiAIOS + Bi,,A1,0,, + Bi,A1,0, + CY-B~,~, BiA10, + cu-Bi,O, + Bi,,Al,O,, Q - Bi-0,

399 Bi2O3-Mn2O3 system From the data on catalytic properties of this system presented in Figure Id, it follows that the highest activity and selectivity are characteristic of the 0.33BiMn sample which provides a 14% yield of the C, hydrocarbons at 730 C. After rising the temperature to 780 C, yield increases to 18%. The detailed characterization of BiMn oxide system has been done elsewhere [7]. Here, we would like to mention that the binary coprecipitated catalysts were biphasic solids the surface of which was enriched in bismuth. They consisted of Bi2Mn4010 and Bi,O, or MnZ03, excepting the 0.33BiMn sample which was composed almost exclusively of the Bi2Mn4010 phase, with only traces of Bi,O,. Under the reaction conditions, the BizMn4010 compound decomposed partly (about 2 5 - 3 0 % ) to Bi,O, and Mn,O,. Along with this, additional enrichment of the surface in bismuth took place during the catalytic reaction (Table 2). 3.4.

Table 2 Characterization of fresh and used BiMn oxide catalysts 0.3 3BiMn

fresh Surf.area (m2/g) 3.8 Phases (XRD) Bi,Mn,Olo Bi/ (Bi+Mn) (XPS) C, yield ( % )

a

4.

- after

0.624 l.oa

10 min of work;

0.33BiMn

used

fresh

3.6

4.9

Bi2Mn40,, Bi,O,, Mn203 0.785 14.4b

Bi,Mn,O,o Bi203 0.728 4.ga

- after

+ Bi,O, used 5.0

I

Bi,Mn,O,,, Bi,O,, Mn,O, 0.768 15.3b

150 min of work at 730 C

DISCUSSION

Although the Bi,O,-M,O, catalysts studied are only moderately selective for the oxidative coupling of methane to C, compounds, the results we report give clear evidence that the overall performance of similar catalysts strongly depends on their phase composition. According to the results of catalytic activity measurements, two groups of catalysts can be distinguished: one of them includes BiY and BiYb oxides which are less selective in comparison with the corresponding individual oxides, and the second one consists of BiAl and BiMn oxides showing much higher activity and selectivity than pure oxides. The surface areas of the BiYb and BiY catalysts have been found to vary from 0 . 5 to 0.8 m2/g and from 0.9 to 1.1 m2/g respectively, showing no correlation to Bi/(Bi+M) ratio as well as to activity and selectivity. On the other hand, the Bi/(Bi+M) ratio greatly influences their phase composition. Taking into account the XRD data, lower selectivity of BiYb and BiY oxides

400

can be attributed to the formation of solid solutions on the basis of Bi203 which are known [9] to possess higher mobility of lattice oxygen. This idea is consistent with the data [lo], according to which at concentrations of up to 4 0 at.%, ytterbium and yttrium readily dissolve in the Bi,O, forming monophasic bicomponent oxide. When the concentration exceeds this limit, a heterophasic mixture of oxides and/or new compounds, like BiYO,, are formed. Poor selectivity of BiYO, (the 0.50BiY sample) seems to be due to its perovskite structure which is characteristic of many catalysts for total oxidation of hydrocarbons Ell]. A pure Bi,Mn,O,, compound (the freshly prepared 0.3 3BiMn sample) is also non-selective in oxidative coupling of methane. This catalyst, as one can see from Table 2, has a very poor activity, yielding mainly carbon dioxide, during the first half hour of work, but the yield of C, hydrocarbons increases as the reaction proceeds, to reach its steady-state value. As XRD shows, the system is able to catalyze the oxidative coupling of methane when it contains several phases. An emerging explanation for this kind of situation is the occurence of a Itremote control" [12] when certain oxide donates spillover oxygen to the phase on which the catalytic reaction takes place. Using a formal similitude with the data [ 121 , one could speculate that Bi2Mn,010 would be the donor phase, and Bi203would be the acceptor phase. In this case, a mechanical mixture of Bi,O, and Bi2Mn,010 should exhibit an appreciable activity and selectivity from the start, as the cooperating phases are already present. It was found (Table 2), however, that both activity and selectivity of the mechanical mixture (0.33BiMn + Bi205) continued to increase as a function of time-on-stream, indicating that other phenomena contributed to the increase of these characteristics of catalyst. We believe that an active and selective BiMn catalyst of a composition more complicated than at the start, is formed under the influence of the reaction mixture. It is possible that Bi203 is advantageously supported on the BizMntOlo-likephase, as our X P S data might suggest. The challenge is now to prepare BiMn catalysts that do not contain initially, nor decompose to, the Mn203 and Mn30, phases which are responsible for the total oxidation of methane. Different picture is observed for the BiAl oxides which do not change both catalytic properties and phase composition as the reaction proceeds. So! the formation of a new catalytic composition under the influence of reaction media, as it was proposed for BiMn oxides, can be excluded. Essentially, three explanations could be given for higher activity and selectivity of BiAl oxides in comparison with Bi,O, and A1,0,: - formation of a new oxide phase associating Bi and Al, which would be more active and selective than the pure oxides; these could be BiAlO,, BiZAl,Op or Biz4A12039: - surface contamination of Bi203 by aluminium, supposing that the contaminating species, when associated with the surface, has higher catalytic performance: - cooperation between the Bi,O, and one (or two) of the BiAlO,, Bi2A1409, Bi,,Al,O,, phases , according to a remote control effect [I21*

40 1

The results presented above are not enough for finding out which phenomenon mainly contributes to a synergy between Bi,O, and phases containing aluminium, and further studies are needed. 5. CONCLUSION

Pure chemical compounds, like BiY03 and BiZMnCO,O1as well as Bi20,-based solid solutions show poor selectivity in the oxidative coupling of methane. Much higher activity and selectivity are characteristic of catalysts composed of Bi,O, and Bi-containing compound such as Bi,Mn,O,, and possibly oxide phases associating Bi and Al. The cooperation between these phases can be tentatively explained by the formation of a more complicated catalytic composition under the influence of the reaction mixture, or by the occurence of a remote control process. ACKNOWLEDGMENT

We gratefully acknowledge the financial help of Indian National Science Academy for the stay of one of us (E.A.M.) in Pune. REFERENCES 1 Kh.M.Minachev, N.Ya.Usachev, V.N.Udut and V.S.Khodakov, Russ.Chem.Rev. , 57 (1988) 385. 2 Y.Amenomiya,V.I.Birss, M.Goledzinowski, J-Galuszka and A.R.Sanger, Catal.Rev.- Sci.Eng., 32 (1990) 163. 3 J.H.Lunsford, Catal.Today, 6 (1990) 235. 4 E.A.Mamedov Russ.Theor.Exper.Chem., 24 (1988) 366. 5 V.D.Sokolovskii and E.A.Mamedov, Catal.Today, 14 (1992) 331. 6 J.L.Dubois and C.J.Cameron, Chem.Lett., (1991) 1089. 7 I.Baidikova, H.Matralis, J.Naud, Ch.Papadopoulou, E.A.Mamedov and B.Delmon, Appl-Catal. A, 89 (1992) 169. 8 G.E.Muilenberg (ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden-Prairie, MN, 1979, p.162. 9 P.G.Gellings and H.J.M.Bouwmeester, Catal.Today, 12 (1992) 1. 10 P.D.Battle, G.Hu, L.M.Moroney and D.C.Munro, J.Solid State Chem., 69 (1987) 30. 11 L.Ya.Margolis and O.V.Krylov, in 0.V.Krylov and M.Shibanova (eds.), Catalytic Total Oxidation of Hydrocarbons (Russ.Kinet.Catal.Problems, V01.18)~ Nauka, MOSCOW, 1981, p.120. 12 L.T.Weng and B.Delmon, Appl.Catal., 81 (1992) 141.

This Page Intentionally Left Blank

V. CoriCs Corberin and S. Vic Bcll6n (Editors), New Developrrienls i n Scl~ciiveOxidarion I / 0 1994 Elscvier Scicnce B.V. Ail rights rcscrvcd.

403

OXIDATIVE CONVERSION OF METHANE OVER MgO/ZSM-5 CATALYSTS

P. Kovachevaa, N. Davidovaa and A . H . Weissb aInstitute of Kinetics and Catalysis, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria bDepartment of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609, USA Increased selectivity to C2 hydrocarbons in the oxidative conversion of methane is obtained as a result of the addition of 1-10 wt.% MgO to ZSM-5. Sodium promotion of MgO/ZSM-5 improves the C2 selectivity to l o o % , but leads to lower stability of the catalyst in the reaction conditions examined. It is established, that the availability of zeolitic basic sites is of great significance for the interpretation of the catalytic activity and selectivity of MgO/ZSM-5 catalysts. 1.INTRODUCTION

Oxidative dimerization of methane is one of the possible routes for its conversion to more valuable hydrocarbons. Basicity is an important characteristic of the catalysts which display activity and selectivity in this reaction. The good performance of the lead containing zeolites in the oxidative conversion of methane and the assumed role of the zeolitic basic sites in this reaction prompted our interest towards studying zeolite basicity [1,2]. Although scarce, there are studies in the literature which examine the oxidative conversion of methane over zeolites without sufficiently considering however the basic properties of the zeolites [3,4]. Well known are the basic properties of MgO used in the same reaction. With a view to developing catalysts with improved basic and catalytic properties, in the present work zeolite ZSM-5 was modified with various amounts of MgO and was additionally promoted with sodium. The catalytic performance of the samples was tested towards the oxidative conversion of methane. The surface basicity was measured by temperature programmed desorption of C02.

2.EXPERIMENTAL

The catalysts were prepared on zeolite ZSM-5 (Si/A1 = 22) by impregnation with aqueous solutions of Mg(N03)2.6H20 at 809O'C. Following impregnation the samples were dried at 120'C and calcinated at 850°C for 2 h. In this way 4 samples with MgO content of 1, 5, 10 and 20 wt.% were obtained. The sample containing 10 wt.% MgO was additionally promoted with 5 and 15 wt.% Na by impregnation with aqueous solutions of NaCl as described above. The composition of the catalyst samples obtained is presented in Table 1. Table 1 Composition of the catalyst samples

Sample

MgO, wt.%

1MgO/ZSM-5 5Mg0/ZSM-5 10MgO/ZSM-5 20MgO/ZSM-5 5Na,lOMgO/ZSM-5 15Na,10MgO/ZSM-5

1 5

10 20 10 10

Na, wt.%

5

15

The catalytic experiments were conducted in a fixed-bed, flow-type reactor, at atmospheric pressure. The reactor was a quartz tube (i.d. 8 mm) packed with 1 g of the catalyst, held in place with 2 quartz wool plugs. The samples were activated in flowing air at the reaction temperature for 2 h before use. Then a feed consisting of methane and air at a total flow rate of 60 ml/min and CH4 to O2 ratio of 4.5 : 1 was admitted to the reactor at temperatures of 800 and 850'C. The compositions of the reaction products were analyzed with an on-line gas chromatograph. Two columns, operated in parallel were used for separation - Porapak QS and Molecular sieve 5A. The detailed calculations of methane conversion and selectivities to C02, C2H4 and C H are given in [2]. 2 ,6 Temperature programmed desorption (TPD) of C02 was carried out in a quartz reactor as follows: the catalyst (1 g ) , held in place with 2 quartz wool plugs was preheated in a stream of dry argon for 2 h up to 800'C for removing humidity and other adsorbates. Then C02 was fed with a rate of 20 ml/min for 30 min at 50'C. Physically adsorbed C02 was desorbed with Ar for

405 30 min. Chemically sorbed C02 was desorbed in the temperature range 50-8OO'C at a heating rate of lO"/min and Ar rate of 20 ml/min. The desorbed C02 was analyzed using a thermal conductivity detector.

3.RESULTS AND DISCUSSION

Methane conversion and selectivity over the catalysts examined are shown in Figure 1. MgO displays certain activity for methane conversion at 800'C. The introduction of 1 wt.% MgO on ZSM-5 decreased the activity but enhanced the selectivity to C2 hydrocarbons. Upon rising the MgO loading of

MgO,% Na ,%

100 -

1 -

C2H4

-

-

C2H6

-

5

15

co2

Figure 1. Conversion(open bar) and selectivities(hatched bar) as a function of MgO and Na loadings on ZSM-5 a) 800'C; b) 850°C.

406

the catalyst to 10 wt.% the conversion of methane kept practically constant whereas the C2 selectivity further increased. The very low catalytic activity of the sample 20MgO/ZSM-5 was accompanied by the highest selectivity to C2 hydrocarbons observed by us 79% (82% at 850'C) The changes in activity and selectivity of the MgO/ZSM-5 samples at 850'C were close to these observed at 800'C. Most strongly affected by the temperature increase from 800 to 850°C was the C2H4/C2H6 ratio: the value obtained at 850°C considerably exceeded that at 800'C. This was attributed to both increased dehydrogenation capacity of the catalysts at the higher temperature and accelerated homogeneous gas-phase reactions between the reaction products. Promoting of 10MgO/ZSM-5 with 5-15 wt.% Na lead to a surprising effect: disappearance of C02 from the reaction products. The Na promoted catalysts displayed 100% selectivity to C 2 hydrocarbons, the amount of C2H4 in the C2 products being the highest one registered by us at these temperatures(70%). A similar effect on C2H4 selectivity was reported by Otsuka et al. for LiCl promoted Sm203 [5]. Evidently, Na promoting of 10MgO/ZSM-5 depressed the level of full oxidation of methane. Even 5% Na loading on the catalyst were sufficient to achieve the depressing effect. The latter was attributed to: reduction of the specific surface area (reduced porosity) of the samples [6]; carbonate formation; changes in the number of active sites [ 7 ] . The role of basicity of the active sites is also to be considered. The main task of the basic sites is the dehydrogenation of methane and the formation of the reactive CH3 radicals [8]. Moreover, these sites which constitute the greater part of a basic surface, react much weaker with the olefines (C2H4) obtained as reaction products, compared to the neutral or acidic surfaces, thus indirectly depressing the full oxidation of olefines. The activity of the Na loaded samples dropped with time, which was not typical for the samples containing no Na. This effect was related to the sensitivity of the reaction to the concentration of the promoter which decreased by evaporation at the high reaction temperatures [9], The reliable interpretation of the catalytic performance of the samples should involve the basicity data obtained by the TPD of C02. The TPD plots of C02 adsorbed on the catalysts examined, are shown in Figure 2. The different performance of the various samples reflects in the characteristics of the TPD plots which are: temperature of maximum desorbed C02 - Tmax; amount of desorbed C02; position and shape of the TPD peaks. The maximum C02 desorption under the experimental conditions was observed in the following temperature ranges: 100-200, 280380, 420-450, 450-600 and 670-800°C. According to the

-

407

characteristics of the TPD plots, the examined samples were divided in 3 groups.: MgO - curve 1; MgO/ZSM-5 - curves 2,3 and 4; Na,MgO/ZSM-5 - curves 5 and 6. The TPD plot of MgO shows the desorption of small amounts of C02 in a broad temperature range: 120-42O'C. Small amounts of residual C02 desorbed above 650°C. The TPD plot of ZSM-5 loaded with 1-10 wt.% MgO shows 3 expressed temperature ranges of C02 desorption. Most intense is the temperature range 450-600°C characteristic for the sample 10MgO/ZSM-5. The increase in MgO loading from 1 to 10 wt.% resulted in a slight shift of TmqX to the higher temperatures in the range 450-600'C, accompanled by a change in the peak shape. This observation gave grounds to assume a correlation between the catalytic selectivity (Figure 1) and the basicity of the ZSM-5 samples loaded with 1-10 wt% MgO. The sample 20MgO/ZSM-5 manifested a TPD plot of much lower intensity and hence a lower basicity. The latter does not correlate with the

Y 0

x .-

4-

cn C a, -I--

C H

I

l

l

I

I

I

I

J

I

400

200

I

I

600

T,OC

I

I

,

800

Figure 2. TPD profiles of the catalyst samples: 1 - MgO; 2 - 1MgO/ZSM-5; 3 10MgO/ZSM-5; 4 - 20MgO/ZSM-5; 5 - 5Nar10MgO/ZSM-5; 6 - 15Na,lOMgO/ZSM-5.

-

408

high selectivity of this sample but corresponds to its low catalytic activity. It follows then that the optimal catalytic activity, selectivity and basicity is manifested by ZSM-5 samples containing not more than 10 wt.% MgO. How does the additional Na promoting of the sample 10MgO/ZSM5 reflect on its basic active sites? The introduction of 5 wt.% Na resulted in a surprising disappearance of the basic sites able to adsorb C 0 2 under the experimental conditions. It was unlikely to assume that basic sites disappear from the sample surface upon introducing an alkaline ion. More probably, highly unstable basic sites of a different type were formed on the sample surface under the effect of Na. The instability, i.e. the decrease in the number of these basic sites during the reaction was undoubtedly due to Na evaporation during the preliminary calcination and during the reaction itself. The changes observed in the basicity, catalytic activity and selectivity were assigned to a complex interaction between Na, the active phase and the support. As a manifestation of this interaction the depressed level of full oxidation of methane over Na loaded 10MgO/ZSM-5, resulting in 100% selectivity to C2 hydrocarbons was considered. The availability of basic sites of a different type reflected also on the performance of the sample promoted with 15 wt.% Na. A new intense asymmetric peak characteristic for C 0 2 desorption at 2 8 0 - 3 8 0 ° C appeared on the TPD plot of this sample which was not registered for the other samples examined. It was therefore concluded that basic sites of sufficient stability were formed at a higher Na content, which could be registered on the TPD plot. The amount of desorbed C 0 2 suggested that the basic active sites of the sample additionally promoted with 15 wt.% Na were responsible for its higher catalytic activity compared to the sample promoted with 5 wt.% Na. The relatively low temperature of maximum C 0 2 desorption - TmaX was attributed to the unstable catalytic activity of the sample. Summarizing, the TPD of C 0 2 proved to be a very sensitive method for the characterization of the basic sites of the catalyst samples. The availability of basic sites (together with acidic sites) in the MgO modified ZSM-5 samples is of great significance for the interpretation of their catalytic activity and selectivity in the reaction of oxidative conversion of methane to C2 hydrocarbons taking place as a rule in the presence of basic catalysts. The authors w i s h to thank the National Fund "Scientific Research" of the Bulgarian Ministry of Science and Education for the financial support.

409

REFERENCES

1. N . D a v i d o v a , P . K o v a c h e v a and A.H. Weiss, C a t a l y s i s Today, 13 (1992) 625. 2. P. K o v a c h e v a , N. D a v i d o v a and A . H . Weiss, C o l l e c t . C z e c h . C h e m . C o m m u n . , 57 (1992) 2548. 3. J. D o n g , Y. L i u and Q. X u , J . C a t a l . ( C u i h u a X u e b a o ) , 13 (1992) 387. 4. H. T s u i , F. Y a g i and H. H a t t o r i , C h e m i s t r y L e t t e r s , N o . 11 (1991) 1881. 5. K. O t s u k a , Q. L i u and A . M o r i k a w a , J . C h e m . S O C . , C h e m . C o m m u n . , N o . 8 (1986) 586. 6. E. Iwamatsu, T. Moriyama, N. T a k a s a k i and K. A i k a , J . C a t a l . , 113 (1988) 23. 7. A. Machocki, A. D e n i s , T . B o r o w i e c k i and J . B a r c i c k i , Appl. C a t a l . , 72 (1991) 283 8. C . Jones, J. L e o n a r d and J . Sofranko, J . C a t a l . , 103 (1987) 311. 9. E. Miro, M . Santamaria and E . Wolf, J . C a t a l . , 124 (1990) 465.

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V . Cortcs Corberin and S . Vic Bell6n (Editors), New Deveioprnenrs i n Seleciive O x i d d o n I / 1994 Elscvier Sciencc B.V.

41 1

Surface characterization and catalytic behaviour of Li/MgO in oxidative coupling of methane C.L. Padro, W.E. Grosso, G.T. Baronetti, A.A. Castro and O.A. Scelza Instituto de Investigaciones en CatAisis y Petroquimica, Facultad de Ingenieria Quimica (Universidad Nacional del Litoral) - CONICET, Santiago del Estero 2654, (3000) Santa Fe, Argentina - Fax: 54 42 553727. ABSTRACT

In this paper a study by XRD, XPS and SEM of the Li/MgO catalyst with different Li content is presented. The catalytic behaviour and the surface composition strongly depends on the Li content. Besides, from the experimental results it can be inferred that di-oxygen species (like 0;) would be present in the catalyst surface and a complex equilibrium between these species and 0-centers could take place. 1. INTRODUCTION

One of the most active and selective catalysts used in the oxidative coupling of methane is based on lithium-doped magnesium oxide. This catalyst type has been widely studied since the work reported by Ito et al. [l]. The nature of active centers in the Li-MgO catalyst is an important topic which has been focused by several authors. Thus, Wang and Lunsford [2] by using EPR found that [Li'O-] species would be the centers for methane activation. However, the [M+O-]centers type were not detected in other catalysts such as NdMgO or K/CaO [3]. Otsuka et al. [4] proposed that di-oxygen species would be responsible for methane activation, based on the finding that Na, Ba and Sr peroxides are very active compounds for the oxidative coupling of methane. In this line, Ross et al. [5] proposed (through a kinetic study of the methane dimerization on Li/MgO) that the rate determining step involved the reaction of methane with di-atomic oxygen species. Another point of view on the behaviour of the surface of the Li/MgO catalyst was given by Martin and Mirodatos [6]. These authors found that a sinterized core of MgO is covered by a Li,CO, film and the high selectivity is developed as a consequence of the interaction between the two components. In this paper, we report a study of the Li/MgO catalysts prepared by different procedures. The catalyst samples were characterized by XRD, SEM and XPS and tested in the oxidative coupling of methane (OCM) in order to elucidate the different crystallographic phases and surface species.

412

2. EXPERIMENTAL MgO and Li/MgO catalysts were prepared by two different procedures: sluny (S) and precipitation (P). Thus, the MgO-S catalyst was obtained by putting MgO (from Merck) in contact with deionized water at room temperature. Then, the remaining water was evaporated under stirring by heating the slurry until a thick paste was obtained. The MgO-P catalyst was prepared by precipitation of Mg(OH), from MgC12.6 H,O solution with a NK40H solution. The ammonia aqueous solution was slowly added until reaching pH =9. Then, the precipitate was washed several times until the absence of C1' was achieved in the washing water, and then dried at 393 K during 12 h. Li/MgO catalysts (S or P) were prepared by adding Li2C03to a portion of a Mg(OH), obtained by either slurry or precipitation. The amount of Li2C03used was such as to obtain the desired Li content in the samples. The Mg(OH), - Li2C03- H,O mixture was kept at room temperature under stirring, and then the remaining water was evaporated. The samples were dried overnight at 393 K. The dried MgO and Li/MgO samples prepared by different procedures were calcined in an air stream in two steps. The first at 738 K during 1 h and the second one at 1023 K during 3 h. The following notation was adopted : xLi/MgO-S (or P), where x denotes the weight percent of lithium in the sample. The different catalysts were tested in OCM at 1023 K and 1 atm of total pressure. The reaction was carried out in a quartz fixed bed reactor by using a CH,/air mixture as a feed (CH,/02 molar ratio = 2). The residence time was 3.38g s/cm3 CH, (STP). Before reaction, calcined catalysts were heated from room temperature up to reaction temperature in flowing N,. Calcined samples of catalysts were characterized in an XRD diffractometer Rich Seifert Iso-Debyeflex 2002. Specific surface area was measured in a sortometer Acussorb 2100-Micromeritics by using Kr as adsorbate. Also, some samples were characterized by using a JEOL 35-C scanning electron microscope. X-Ray photoelectron spectra (XPS) were obtained with an ESCA 750, Shimadzu instrument, using Mg K a radiation. Samples were pretreated in a chamber directly connected to the analyzing system. Surface charging was observed for all the samples. Binding energies (BE) for MgO and Li/ MgO catalysts are reported as referenced to the Mg 2p level at 50.8eV with an accuracy of k0.2eV. For pure Li2C03sample, the peaks were aligned to Li 1s level (56.3 eV) established by reference to Mg 2p level in Li/ MgO samples. When a multiple peak was observed, a curve fitting was performed by the Van Citter method using Gaussian and Lorentzian components. &i/Mg), and (OK), atomic ratios were calculated by using the area values under the corresponding peaks, the Scofield photoionization cross sections, the mean free paths of the electrons and the instrumental function given by the ESCA manufacturer. 3. RESULTS AND DISCUSSION

Table 1 summarized the results of surface areas (S,) of the fresh samples (previously calcined at 1023 K in air), chemical composition, and catalytic parameters in OCM reaction: methane conversion (XCH4) and selectivity to C, hydrocarbons (So). It must be indicated that the lithium contents in used catalysts, determined by chemical analysis were very close to those of the fresh one, the difference being no more than 10%.

413

Table 1 Characteristics and catalytic parameters in OCM for the different catalysts Catalyst

Li (wt %)

sg (m2/g>

XCH,

sa

(%I

(%)

Li/MgO-S

0 0.5 1.0 6.1 13.7

59.6 7.4 2.9 1.3 0.9

26.8 32.1 32.9 30.9 24.8

7.8 30.8 29.9 23.2 33.7

Li/MgO-P

0 4.7 10.0

14.7 0.8 0.7

20.6 25.7 22.5

27.2 31.9 30.2

Table 1 shows that the surface area sharply decreases as the lithium amount increases. Similar effects were also observed by several authors [1,7].It must be noted that MgO-P displays a higher selectivity to C2 compared with that corresponding to MgO-S. This difference can be attributed to the difference between the surface areas for both catalysts. In fact, high concentration of active sites for the non selective oxidation of methane can be expected for the MgO catalyst with a high surface area [8]. However, other factors such as small quantities of impurities occluded during the precipitation step in the Mg(OH), could contribute to the high selectivity to C, hydrocarbons. Moreover, it can be observed that the selectivity to C2 is enhanced by the addition of Li to MgO. The effect of Li addition on methane conversion is also positive, although in a minor proportion than that observed for the selectivity to C,. Likewise, it was observed that both methane conversion and selectivity increase for low content of the alkali-metal, but they decrease for high Li content. These effects were also detected by other authors [ 1,9]. XRD patterns of the fresh catalyst can be summarized as follows. For undoped catalyst, the characteristic lines of MgO and Mg(OH), were found. For Li-doped MgO, additional lines attributed to Li,CO, and Li,O, were also found. XPS spectra of the Li/ MgO samples with different Li loading were taken after an oxygen treatment "in situ" at 773 K. These spectra showed a complex 0 1s profile (Figure 1) probably due to the fact that the catalytic surfaces were strongly hydroxylated and/or carbonated. Three peaks ( 533.8 - 533.3, 531.4 and 529.9 - 528.8 eV ) can be distinguished in the 0 1s profile. The C 1s profile of Li/MgO samples was also complex, showing the peak related to the adventitious carbon and two additional peaks at 291.4 and 287.6 - 286.4 eV, respectively. In order to assign the 0 1s and C 1s peaks for Li/MgO, additional XPS spectra of highly pure MgO and Li,C03, without previous treatment, were also taken. XPS of pure MgO showed two 0 1s peaks at 531.4and 528.8eV, respectively. The first one can be assigned to oxygen from OH- and CO f species, while the second one is due to the lattice O= [9].The C 1s profile of MgO displays two peaks: one at 284.6eV due to the adventitious C, and the other at 286.4 eV assigned to MgC03. XPS spectra of Li,CO, showed only one 0 1s peak at 533.5 eV and one C 1s peak at 291.4 eV, besides the peak from the adventitious carbon. Returning now to the spectra of Li/MgO, the peak at 529.9 528.8eV can be assigned to the O= lattice of MgO, the peak at 5 3 1 . 4 ~ be due to the

4 14

540 530 BINDING ENERGY,eV

5;

Figure 1. XPS 0 1s spectra after 0, treatment "in situ" at 773 K for Li/MgO-S and P.

presence of Mg(OH), and MgC03. The 0 Is peak at higher BE in the Li/MgO spectra can be associated with the oxygen of Li2C03.In the same way the C 1s peaks at 291.4 and 287.6-286.4 eV for Li/MgO-S can be assigned to Li2C03and MgCO,, respectively. However, the surface ratio between the 0 1s peak (at 533.8 - 533.3 eV) and the one corresponding to C 1s at 291.4eV (Li,C03) for the different Li/MgO samples was always higher than the stoichiometric ratio corresponding to Li,CO,. Thus, another oxygen species besides Li2C03would contribute to 0 1s peak at 533.8 - 533.3 eV. In order to investigate the nature of these species, additional XPS experiments on Li/MgO samples with Li loading up to 6.1 wt% were performed. In this case, XPS spectra were taken after treatment "in situ" at 973 K in a C0,-free oxygen atmosphere. Under these conditions, which are similar to those used in the calcination of catalysts, a lower surface carbonation and hydroxylation can be expected. The XPS spectra of Li/MgO treated under these conditions show an 0 1s peak corresponding to O= at 530.4 eV and other peak at 532.4 eV. A strong decarbonation of the surface can also be observed (Figure 2). In fact, the C 1s peak at 291.4 eV strongly decreased after the treatment at 973 K. To elucidate the nature of the species responsible for the peak at 532.4 eV the following evidence must be taken into account. According to Anderson [lo], the Li,CO, treated in 0, at 896 K ( temperature close to its melting point) is in equilibrium with the corresponding oxide and peroxide. Besides, the following equilibrium between 0, and CO, in molten carbonate salts has also been proposed [I 11. 'LO,

+ co;

CO,

+ 0,

Furthermore, we found by XRD measurements that Li,C03, LiOH and Li,O, were the main phases in a sample of pure Li2C03treated in oxygen at 973 K. These evidence indicate that lithium peroxide could be present in LilMgO catalysts after the thermal treatment at high temperature. Besides, another evidence can be obtained from the XPS measurements. In fact, the difference in the BE between O= and other oxygen species is often a better

415

540

530 520 BINDING ENERGY, eV

290 280 BINDING ENERGY, eV

Figure 2. 0 Is and C Is XPS spectra of 1 and 6.1Li/MgO-S catalysts after O2 treatment "in situ" at 973 K.

indication of the kind of oxygen species rather than the absolute value of the BE [ 121.The RE of the lattice 0- in pure lithium oxide was reported by Hoenigman and Keil [13] as positioned at 530.8 0.14eV. It must be noted that the difference in the BE between the unidentified 0 1s peak (532.4 eV) and that of the lattice O= of lithium oxide in our XPS of Li/MgO was about 2 eV. A similar trend was found by Kharas and Lunsford [14], who reported that the difference between the 0 Is binding energy of peroxide anions in Na20, and BaO, and the lattice oxygen in decarbonated surfaces, are 2.4 and 1.9 eV, respectively. In consequence, the above evidence can indicate that the peak at 532.4eV in Li/MgO catalysts could be attributed to di-oxygen species like peroxide. From our catalytic results, an enhancement of the selectivity to C, is produced when Li is added to MgO. However, the Li addition not only caused an inhibition of the deep oxidation centers, but also produced new active sites for methane activation. In fact, taking into account the methane conversion (Xa4), the specific surface area (S,) and the residence time (W/F), the average specific rate (r) was calculated by the equation: r = [X,,,]/~/F][S, ].Figure 3 shows that r drastically increased with the addition of small quantities of lithium, and became almost constant for the higher Li loading. Besides, the lithium addition to MgO produces great surface changes. Figure 4 shows that the surface Li enrichment depends not only on the Li content, but also on the temperature of the thermal treatment of the samples. At one given temperature the (Li/Mg), ratio strongly increases when the (Li/Mg),,, increases up to 0.06, then it is kept constant until 0.36(Li/Mg),,, is reached. For (Li/Mg)buk > 0.36, the (Li/Mg), again increases. The step in the (Li/Mg), ratio would indicate the formation of surface layers with about the same composition. For (Li/Mg),,, > 0.36,the lithium, probably as Li,CO, forms a surface layer

416

"0

2

4 6 8 10 12 LI CONTENT. w t '10

Figure 3. Average specific rate as a function of the lithium content for Li/MgO-S ( 0 ) and P (m)

4

0

02

04 06 (Li/Mg)BULK

08

Figure 4. (Li/Mg), vs. (Li/Mg),,, for Li/MgO-S after different treatment temperatures: ( 0 ) 773 K, ( m ) 973 K.

14

over MgO. It was also observed that the (Li/ Mg), ratio diminishes when the treatment temperature increases from 773 to 973 K for a given bulk composition. This could indicate a lithium loss or a lithium migration to the deep layers of the MgO structure. Morphological changes in catalytic particles were observed by SEM when increasing amounts of lithium are added to MgO. Undoped MgO particles exhibit a "flakes" structure (Figure 5a). After Li addition up to 6.1 wt% (for P and S catalysts), the flakes structure is preserved, but agglomerates of these flakes surrounded by a diffuse cloud can be observed. However, the picture is quite different for higher Li contents. Thus, Figure 5d shows a very heterogeneous surface with the appearance of a molten phase, where a minor proportion of the surface kept the flakes structure. For Li content higher than 6 wt% there is not a great modification of the average specific rate, and the surface is very rich in the alkali-metal. The morphology of the particles (SEM) is very different from that of the catalysts with lower Li content. Moreover, XPS of the 13.7 Li/MgO-S revealed that the 0 ls/C 1s ratio is slightly higher than that of the stoichiometric one for Li,C03, which is the main surface species. Thus, it can be inferred that the Li,CO, would be heterogeneously spread on the surface in the 13.7 Li/MgO-S catalyst, and the active centers would be attributed mainly to the Li,C03 surface layer. From the evidence shown in this paper, we can conclude that surface di-oxygen species, like peroxide, would be present in the lithium-doped MgO catalysts. It is claimed that the active species for methane activation are [M'O-] centers produced by Li substitution in the lattice of MgO [2] and this process can be accompanied by an increasing concentration of point defects [15]. However, for high Li content, our results revealed that the outer surface layers are mainly composed by Li,C03. In consequence, it can be assumed

417

(a) MgO-S

(b) 6.1Li/MgO-S

(c) 4.7Li/Mg0-P

(d) 13.7Li/MgO-S

Figure 5. SEM microphotographs

that the surface di-oxygen species can be the precursors of active sites for methane activation.The contribution of peroxide in the OCM was first reported by Sinev et al [16]. A complex equilibrium between di-oxygen species (like 0,)and 0-centers could occur [13,17].In fact, according to Kirik et al. [18] the dimerization of methane on this type of catalysts is due to the presence of surface peroxide oxygen species. At the temperatures used in this reaction, these species disproportionate to produce an oxygen lattice ion and an oxygen center, which is apparently the active sites for the H abstraction from CH,. With respect to the effect of the different procedures used in the preparation of Li/MgO catalyst on the surface characteristics, Figure 1 shows that there is no difference in the nature of the surface species. A similar morphological characteristic was also observed for both catalyst types, such as shown in Figure 5. The higher specific average rate for the catalysts prepared by precipitation is probably due to the presence of small amounts of impurities trapped in the Mg(OH), lattice during the precipitation procedure.

418 CONCLUSSIONS - The surface composition strongly depends on the lithium content. For low lithium content, a lithium enrichment was observed, but at high alkali-metal content the MgO is almost covered by a Li,C03 layer. - The evidence obtained by XRD measurements would indicate the presence of peroxide species in Li/MgO catalysts. Furthermore, di-oxygen species (like peroxide) would be present in the catalytic surface after the lithium addition, and an equilibrium between these species and 0-centers could take place during the methane activation.

ACKNOWLEDGEMENTS The authors thank Ariel Calcaterra for the experimental assistance, and to Japan International Cooperation Agency for the donation of the ESCA Spectrometer. This work was supported by CONICET and Universidad Nacional del Litoral.

REFERENCES

1. 2. 3. 4. 5.

T. Ito, J. Wang, C. Lin and J. H. Lunsford, J. Am. Chem. SOC.107 (1985) 5062. J. Wang and J.H. Lunsford, J. Phys. Chem. 90 (1986) 5883. C. Lin, T. Ito, J. Wang and J.H. Lunsford, J. Chem. SOC.109 (1987) 4808. K. Otsuka, A. A. Said, K. Jinno and T. Komatzu, Chem. Lett. (1987) 77. J. A. Roos, S. J. Korf, R . H. J. Veehof, J. G . Van Ommen and J. R . H. Ross, App. Catal. 52 (1989) 131. 6. C. Mirodatos, G . A. Martin, J. C. Bertolini and J. Saint-Just, Catal. Today 4 (1989) 301. 7. J.S.J.Hargreaves, G.J. Hutchings, R.W. Joyner and C.J.Kiely,Catal. Today,. 10 (1991) 259 8. E. Iwamatsu, T. Moriyama, N. Takasaki and K. Aika, J. Catal 113 (1988) 25. 9. X. D. Peng, D. A. Richards and P. C. Stais, J. C a d . 121 (1990) 99. 10. B. K. Anderson, Docthoral Thesis, Techn. Univ. Denmark Lingbi, 1975. 11. A. J. Appleby and S. Nicholson, J. Electroanal. Chem. 38 (1972) app 13, ibid, 112 (1980) 71. 12.3. L. Dubois, M. Bisianx, H. Mimoum and C. J. Cameron, Chem. Lett. (1990) 967. 13. J. R. Hoenigman and R. G. Keil, App. Surf. Scien. 18 (1984) 207. 14. K. C. Kharas and J. H. Lunsford, J. Am. Chem. SOC.111 (1989) 2336. 15. J.S.J.Hargreaves, G.J. Hutchings, R.W. Joyner and C.J.Kiely,J. Catal. 135 (1992) 576. 16. M.Y. Sinev, V.N. Korchak, O.V. Krylov, Kinet. i Katal., 27 (1986) 1274. 17. M. M. Freund, F. Freund and F. Batllo, Phys. Rev. Lett. 63 (1989) 2096. 18. N. P. Kirik, V. G. Roguleva. G. E. Selyntin, E. A. Proshina, A. G . Anshits, Kinet. i Kataliz 30 (1987) 6.

V . CortCs Corbcrlin and S. Vic Bc116n (Editors), New Developmenls 0 1994 Elscvicr Scicncc B.V. All rights rcscrvcd.

111 Seleclive

Oxidulion //

4 19

SOLID - GAS PHASE INTERFACE ANALYSIS O N Z r 0 2 : CORRELATION BETWEEN CH4 OXIDATION ACTIVITY AND WORK FUNCTION MEASUREMENTS D. BOUQUENIAUX, L. JALOWIECKI-DUHAMEL, and Y. BARBAUX

Laboratoire de Catalyse Heterogkne et Homogene, U.R.A. C.N.R.S. D04020, BAt C3, Universitt des Sciences et Technologies de Lille. 59655 Villeneuve d'Ascq Cedex, France. SUMMARY

Good correlations are observed, on Zr02(CaO), between CH4 oxidation activity and work function measurements. In particular, a similar activation energy of about 25 Kcal/mol. is obtained. For temperatures lower than 720"C, the order in reactivities C2 > > CH4 is maintained which confirms a heterogeneous C2 yield limitation. The ratio C2Hq/C2& becomes higher than 1at temperatures higher than 610°C and then it increases linearly with temperature, therefore it is possible to inverse the reactivity between C2H6 and C2H4. Moreover, different types of sites are concerned in the catalytic reaction, CH4 is found to react with 02- species whereas the C2 hydrocarbons (C2H6, C2H4) interact with 0-species. 1. INTRODUCTION

The challenge of the oxidative coupling of methane is to obtain acceptable conversions and selectivities. Labinger anticipated the experimental limit of 25-30% by proposing a theoretical upper limit of 30% (1).Regardless of the catalyst nature and reaction conditions, a25% limit has beenconfirmed recentlyby McCarty (2). As acontribution to this discussion, a study has been carried out, on Zr02(CaO), parallely on the catalytic oxidation of CH4 and work function measurements in similar conditions. A major advantage of stabilized zirconia catalyst, as compared to most other oxides, is that it is highly stable at temperatures higher than 1000°C and difficult to reduce. This ceramic does not change morphology nor specific surface area easily and has a much longer catalytic life than more selective C2 catalysts. Moreover, promoting Z r 0 2 with Na+C1- via a sol-gel process, an effective catalytic system can be obtained (3). Doping to produce stable oxide catalysts, has only been investigated recently, since most catalysts such as Li/MgO deactivate easily. It has been suggested that oxygen vacancies in yttria doped Z r 0 2 may be important for synthesis gas reactions (4). The oxygen vacancy in stabilized Z r 0 2 is a 0 2 - vacancy and is reportedly adjacent to Zr4+ sites rather than Y3+ (5). But, the nature of the active oxygen species (02-, 0-,02-) in the conversion of methane is still under controversy (6). It is mainly

420

admitted that 0 2 - species is associated with total oxidation and 0-species is attributed to partial oxidation. Whereas, Bielanski and Haber postulated that 0 2 - should preferentially lead to partial oxidation (7); on some oxides such as PbO/Al2O3 it has been suggested that 0 2 - ions with reduced coordination could be responsible for methane coupling reaction (8) and on Bi2O3, a-Bi203-Mo03 it has been proposed that a hydrogen atom is abstracted by lattice 0 2 - ions in sites of low coordination (9). Dynamic work function measurements is a suitable technique for the identification of the nature of adsorbed species ( 0 2 - ,0-, 02-) (lo), and it has been already applied to a large variety of materials, it is used in the present study to investigate ZrO2 in the conversion of methane. 2. EXPERIMENTAL

2.1. Catalytic Activity The catalytic oxidation of CH4 (CH4/O2 = 2) has been performed under atmospheric pressure by co-feeding the nitrogen diluted reaction gases (CH4/O2 = 2) into an alumina tube reactor (15 mm I.D., 48 cm long) on Z r 0 2 (CaO) (Degussa, 0.3 m2 g-l, lg). The total flow rate was 25 ml min-1 (down flow) and the reaction temperature was in the range 500-750 "C. The reaction products were sampled on-line using an automatic sampling valve (Valco) connected directly to the reaction flow and analyzed by gas chromatography (Varian).

-

2.2. Work Function Measurements The work function of the samples has been measured by the vibrating capacitor (Kelvin) method with a graphite reference electrode and the potential measuring cell has been connected to a gas flow system which allowed controlled streams of oxygen and/or hydrocarbons (Cl-C3). Details of the measurements have been already published (1 l), the work function values are relative to the graphite electrode potential : V = Vgra bite - Vsample and an increase of the value of the potential difference indicates that tge surface becomes more negative. 3. RESULTS AND DISCUSSION 3.1 Catalytic Activity A classical behaviour has been obtained for the catalytic oxidation of CH4 as presented in Figure 1. The conversion of CH4 is increasing as a function of temperature up

to a value of 33% for T = 740"C, the activation energy obtained is of 25 Kcal/mol. The C2 yield reaches a value of 4 % at T = 660°C but the C2 selectivity -calculated as in (12) - is maximum at T = 640°C (Figure 2). An evolution of the products distribution as a function of the temperature is obtained, C2H6 is not always the major product among the C2 obtained, as often observed in the literature on various catalysts (12). C2H6 presents a maximum for T = 590°C and then it decreases whereas C2H4 is maximum for T = 640°C. In Figure 3 it is shown that for T<610"C the C2Hq/C2Hg ratio is lower than 1 and only for higher temperatures it increases linearlywith the temperature. So, at about T = 610"C, an inversion in reactivity exists between C2H6 and C2H4.

42 1

100

h

x

V

z

3

3

zy

z

0 L

80

I -

-

-

1

I h

w

1

.

60-

v

h

I

10 -

4-

.3

1 I

20 1 I

' I1

0)

0 0

0

0 ' '

0 q . F

0 0

0'

. , .

I

,

, .

I

TEMPERATURE ("C) cg2 C2F4

c 2 p $0

Figure 2 : Methane conversion products distribution as a function of temperature on Z r 0 2 (corresponding to experiment shown in figure 1).

TEMPERATURE ("C)

Figure 3 : C2Hq/C2Hg ratio as a function of temperature on Z r 0 2 (corresponding to experiment shown in figure 1).

TEhlPERATlJRE ("C)

Figure 4 : Work function evolution under 0 2 as a function of temperature on Zr02.

422

3.2. Work Function Measurements 3.2.1. Oxygen atmosphere Thework function measurements under oxygen-argon mixture ( P = 0.05 atm) have been performed in the temperature range 50-500°C and are reported in Figure 4. Different regions of temperatures are observed where the potential values are quite constant or presenting small variations : 150-225"C, 230-300°C and > 375°C. Between these regions, the work function value increases rapidly, the surface becoming more negative with the temperature.

,

On the surface, an equilibrium exists between the adsorbed oxygen species and gaseous oxygen (10): (1) 0,+ n e - H O ; y a d s l The mass action law is :

where k g is Boltzman's constant, T is the absolute temperature, e the electron charge and n the number of electrons transferred from the solid to the adsorbed species during the sorption process. From this equation, a relation between the work function value and the partial pressure of 0 2 can be obtained :

I/

=

k 7-

-In I I (?

Po,

+

constant

(3)

The value of n depends on the equilibrium between the gaseous molecule and the adsorbed oxygen species. The possibilities are : n = l n = 2 n=4 The work function measured on Z r 0 2 under 0 2 has been reported as a function of temperature (Figure 4), then for one temperature the study can be performed as a function of the 0 2 pressure, which permits to evidence the regions of existence of the different oxygen species (o~-,o-, 02-1. The slope of the straight lines obtained by plotting Vvs In P o ,at a given temperature leads to the value of n and therefore to the nature of the adsorbed oxygen species. On Z r 0 2 (CaO), 0-appears to be the dominant species within the 230-300°C temperature range, the 02- species is found to exist for lower temperatures, whereas for temperatures higher than 3'75°C the 02- species are present on the surface (Table 1).

423

Table 1 : Temperature

oxygen Species

300 375 500

0200202-

4.0

4.3

The 02- species is found to be in equilibrium with the 0 2 gas phase for temperatures higher than 375°C. This equilibrium is affected by the presence of hydrocarbons and depending on the hydrocarbon present (CH4, C2H4, CzHg), various oxygen species are involved. 3.2.2. Interaction between Hydrocarbons and Oxygen Species. On Zr02, in absence of gaseous oxygen, under hydrocarbon atmosphere, the work function varies as a function of time; when the surface is saturated with adsorbed oxygen, the hydrocarbons react rapidly, depending on the temperature. As an example in Figure 5 is presented the work function evolution measured on Z r 0 2 under C2H4 atmosphere at 400°C. As the work function values depend on the oxygen species concentration, the kinetic parameters of the reaction between any hydrocarbon and these active species can be determined as follow : k HC

H C + O ~ ~ ~ , , ,+ , ‘products’

(7)

and the reaction rate is :

Therefore,

By integrating these equations, the following relationship can be obtained : 111 ( I/’ -

1- )

= -h

P HT t

+

cor2stwrrt

(10)

where 1’ is the work function value in the absence of adsorbed oxygen species on the surface. ~

Therefore, the kHC values can be determined from these equations and it appears that even if the reaction rates depend on the temperature, the C2 compounds react faster than methane whatever the temperature applied, in the range of temperatures studied. Ethane reacts about three times faster than methane and ethene reacts about 10 times faster than methane at 500°C.

424

The work function measurements are technically limitedup to atemperature of %NIT. For higher temperatures, the measurements are difficult to perform. As the speed rates depend on the temperature, the work function evolution as a function of time after introducing the hydrocarbon at the surface allows to determine an inversion point in reactivity. By reporting the logarithm of the reaction speed for each hydrocarbon as a function of 1/T, the inversion point corresponds to the intersection of the straight lines obtained (Figure 6). The hydrocarbon reactivity corresponds in fact to the reducing power of each hydrocarbon and depends on the temperature, the order in reactivity CH4 < C2H6 < C2H4 becomes CH4 < C2H4 < C2Hg for temperatures higher than 620°C and lower than 720°C. This inversion in the selectivity C2H4 - C2H6 observed, is also well correlated with the catalytic tests for which a temperature of 610°C is evidenced as the inversion point in the reactivity of C2H4 and C2H6 on Zr02. For temperatures lower than 720"C, the C2 reactivity remains much higher than that of CH4 which confirms the heterogeneous C2 limitation. For very high temperatures ( > 720°C) the order in reactivity seems to become C2H4 < CH4 < C2H6. 80C 12

\

10

-2 7oc

z rl

- 8

i I

>

\

600

6

4

500 0

60

120

1

180

1.1

1.2 1.3 1.4 1/T . 1000 K-

1.5

6

TIME (seconds)

Figure 5 : Work function variations measured on Z r 0 2 under hydrocarbon atmosphere (C2H4 0.05 atm) at 400°C.

Figure 6 : In v as a function of 1/T for the various hydrocarbons on Zr02.

Moreover, by plotting In(dV/dt) as a function of 1/T, the activation energy value is determined. For the overall CH4 conversion a value of 26 Kcal/mol is obtained which is in good agreement with the value obtained previously by catalytic test (25 Kcal/mol). For C2H6 and C2H4 the activation energies obtained are respectively 25 Kcal/mol and 8 Kcal/mol. So a much lower activation energy is obtained for ethene whereas quite similar values are obtained for methane and ethane. 3.2.3. Interaction Zr02 - Hydrocarbon - Oxygen Mixtures

Under methane + 0 2 mixture, thework functionvalue decreases first and then reaches a constant value in few hours. This steady-state work function value depends on the initial

425

/P ratio and is related to the oxygen species which participate to the reaction process. The initial decrease of the work function value corresponds to a redox mechanism of the superficial sites

'i

H C + So,-f ' p i o d u c t s '

+

S,,,+ n e -

O2+ S R e d

where S o x and SRed are the superficial sites respectively, oxidized and reduced. At the steady-state the rates of eqn. (11) and eqn. (12) are equal, and the observed variations are in accordance with the following law :

The active oxygen species involved in the reaction mechanism can be deduced from the slope of the curve VVSIn Preactants (Figures 7 and 8). As presented previously (Table l), on Z r 0 2 the 02- species IS found to be in equilibrium with the 0 2 gas phase for temperatures higher than 375°C. This equilibrium is affected by the presence of hydrocarbons and depending on the hydrocarbon present (CH4, C H4, C2H(j), various oxygen species whereas C2Hg and species are involved (Table 2). CH4 is found to react with C2H4 involve 0-species at 500°C.

02

i"

-2 i

-2

-1.5

-1

-05

111 PO2

rg4qti

qif

Figure 7 : Work function variations measured on Z r 0 2 at 500°C under hydrocarbon+ 0 2 atmosphere as a function of the logarithm of oxygen pressure.

-201

1

I

-2.5

1

-2

1

-1.5

-1

-0.5

In PCII

C g 4 c z j 6 C$4

Figure 8 : Work function variations measured on Z r 0 2 at 500°C under hydrocarbon+ 0 2 atmosphere as a function of the logarithm of hydrocarbon pressure.

426

Table 2 : Oxygen - Hydrocarbon Mixture

PHC atm

PO2 atm

n

Oxygen Species

CH4 CH4

0.1 0.1 - 0.4 0.1 - 0.3 0.1

4.3 3.9

0202-

C2H6 C2H6

0.1 0.1 - 0.4 0.1 - 0.3 0.1

2.2 1.9

00-

C2H4 C2H4

0.1 0.1 - 0.4 0.1 - 0.3 0.1

2.1 2.5

00-

Even if for higher temperatures than 500°C one cannot eliminate the possibility of 0 2 - species reacting with the C2 hydrocarbons, different oxygen species are found to be involved in the catalytic reaction, so different types of sites are certainly concerned. Methane reacting with 02- species, is in agreement with an heterolytic rupture of CH4 (with abstraction of a hydrogen atom) involving an 0 2 - species in a low coordination site (8,9). And the active site for methane transformation could be constituted of a Zr4+-02pair with an anionicvacancy. Bielanski and Haber postulated that 02- should preferentially lead to partial oxidation (7), confirmed on V205/Ti02 (13) as well as on Bi2Mo3012 systems (11) but usually it is assumed that 0 2 - species is responsible for total oxidation (6). Anyway, considering the high reactivity of C2 products with the oxygen species, a C2 yield limitation should be observed with increase of CH4 conversion. In case of a heterogeneous limitation, each C2 hydrocarbon should compete for the active site and react with the The higher reactivity ofthe C2products leads to alimit of their amount oxygen species (0-). and to a steady-state (14). REFERENCES A. Labinger and K. C. Ott. J. Phys. Chem., 91, (1987) 2682. (1) J. G. McCarty, A. B. McEwen and M. A. Quinlan, Stud. Surf. Sci. Catal., 55, (1990) (2) 417. A. Z. Khan and E. Ruckenstein, Appl. Catal., 90, (1992) 199. N. B. Jackson and J. G. Ekerdt, J. Catal. 126, (1990) 31. C. R. Catlow, A. V. Chadwick, G. N. Greaves and L. M. Moroney, J. Am. Ceram. Soc. 69, (1986) 272. Y. Amenomiya, V. I. Birss, M. Goledzinowski, J. Galuszka, and A. R. Sanger, Catal. Rev. -Sci. Eng., 32(3), (1990) 163. A. Bielanski, and J. Haber, Catal. Rev. Sci. Eng., (1979) 19. M. Yu. Sinev, G. A. Vorobieva, and V. N. Korchak, Kinet. Katal. 27, (1986) 1164. D. J. Driscoll and J. H. Lunsford, J. Phys. Chem., 89, (1985) 4415. Y. Barbaux, A. Elamrani and J. P. Bonnelle, Catal. Today., 1, (1987) 147. J. M. Libre, Y. Barbaux, B. Grzybowska and J. P. Bonnelle, React. Kinet. Catal. Lett., 30, (1982) 249. J. A. Ross, A. G. Bakker, H. Bosch, J. G. van Ommen and J. R. H. Ross, Catal. Today, 1, 133 (1987). B. Grzybowska, Y. Barbaux and J. P. Bonnelle, J. Chem. Res. (M), (1981) 650. A. Cherrak, R. Hubaut and Y. Barbaw, J. Chem. Soc. Faraday Trans., (1992) 88.

V. CortCs Corberiin and S. Vie Bcll6n (Editors), New Deweiuprnenls in Seieclive Oxldaiion II 0 1994 Elscvicr Scicnce B.V. All rights reserved,

421

The R61e of Structural Defects and Oxygen Migration in La203 for the Oxidative Coupling of Methane M.S. Islam and D.J.Ilett Department of Chemistry, University of Surrey, Guildford, Surrey GU2 5XH, UK.

Atomistic computer simulation techniques have been applied to La203 in order to investigate key solid state properties that are relevant to its catalytic behaviour. We have examined the rBle of point defects and oxygen ion migration, the energetics of dopant substitution, as well as the formation of 0- hole species which are believed to act as H atom abstraction sites.

1. INTRODUCTION

Rare earth sesquioxides have attracted considerable attention as effective catalysts for the oxidative coupling of methanel-5. Several studies have found that Sr promoted La203 exhibits the highest activity with good stability at reaction conditions. The investigations on the La203 material have largely focused on sclectivity/activity properties and gas phase chemistry. It is clear, however, that the precise relationship between the solid state defect structure and the activity of pure and doped La203 is not well established, and is crucial to the proper understanding of its mode of operation. In an attempt to clarify these issues we apply computer simulation techniques to La203, which are well suited to exploring the solid state at the atomic level6, and have been successfully applied to a range of materials including zeolites’ and oxide superconductors8. 2. SIMULATION METHODS

The simulations were carried out using energy minimisation techniques (embodied in the widely used CASCADE code’). The interatomic potentials are based on the Born model of the solid, which includes a long range coulombic interaction, and a short range term to model the repulsions and Van der Wads attractions between the electron charge clouds; the shell model is

428

used to describe the electronic polarisability of the component ions. Defect calculations employ the established Mott-Littleton methodology involving a two-region approach6. La203 crystallises into a hexagonal structure10 with the unusual seven co-ordination of the cation (Fig. 1). The unit cell consists of one independent La ion and two independent 0 atoms. The La and O( 1) form double hexagonal layers of alternating La and 0 and these layers are held together by O(2). The potential parameters for La203 are derived from electron-gas methods (listed in the Appendix), which were found to accurately reproduce the complex structure and were recently applied to pressure simulations of La2Cu041’. These are non-empirical potentials and hence were not derived by fitting to experimental data.

O! I

/

0

Fig. 1 Hexagonal structure of La203 . Arrow indicates most favourable pathway for oxygen migration. Integral ionic charges are presumed, i.e. 3- for La and 2- for 0, which enables a simple definition of hole states (as 0-)and the useful conccpt of isovalent or aliovalent dopant substitution. Details of all the potential and shell model parameters are given elsewhere’2.

429

3. RESULTS AND DISCUSSION 3.1 Intrinsic Disorder Calculations were first performed on the energies of isolated point defects (vacancies and interstitials) and these were combined to give the Frenkel and Schottky energies shown in Table 1 . In all cases, the lattice ions surrounding the defect were allowed to relax in the energy minimisation procedure. Table 1 Calculated formation energies of Frenkel and Schottkv disorder Type

Defect equilibrium

Frenkel La3+

LaL:

02-

0,x

Schottky

2La,,X + 30,"

+ Lai"'

7.24

+ 0;"

2.57

= V,,"'

= V,"

E (eV per defect)

= 2VL,"'

+ 3v," + La203

3.34

From examination of the calculated energies in Table 1 it is clear that the simulations predict that the predominant mode of intrinsic disorder is that of the oxygen Frenkel type. This result accords well with work of Anshits et all3 and Kofstad14 who postulated that oxygen Frenkel defects would dominate in pure La203. Hence, ionic conduction will be undoubtedly controlled by the diffusion of oxygen defects, although the magnitude of the Frenkel energy suggests that the defect concentration will be very low in the pure material. It is interesting to note that the hexagonal A-type structure of La203 is closely related to the C-type of other rare earth sesquioxides which, in turn, are derived from the fluorite structureIO. Therefore, oxygen vacancies and interstitials might reasonably be expected in La203 since anion Frenkel disorder is commonly observed in fluorite-structured oxidesl4. In addition, we have considered redox processes that may lead to a degree of nonstoichiometry. Highly positive values (>9eV) for the oxidation and reduction reactions suggest that deviation from ideal stoichiometry is not significant in this materialI2, in agreement with the known properties of the materiall3,l4. 3.2 Oxygen migration It is well established that solid state diffusion is of central importance to the mode of

operation of this oxidation catalyst. However, only a few studies

4,15

have focused on oxygen

430

diffusion in the La203 system: early conductivity measurements of Etsell and Flengasls obtained activation energies of 17.1 kcalmol-1 (0.74 eV) for CaO doped La2O3, which they attribute to the motion of 0 2 - ions through either a vacancy or interstitial mechanism. Milne et a116 have reported conductivity measurements and obtained an activation energy of 0.6-0.7 eV for the undoped limit. More recently, from isotope exchange studies, Kalenik and Wolf4 have determined oxygen self-diffusion coefficients (Do) for the pure and promoted catalyst, with the 1% Sr/La203 system exhibiting the highest value. The precise atomistic mechanism controlling ion transport is, however, uncertain. After extensive examination of vacancy, interstitial and interstitialcy mechanisms, our calculations identify interlayer vacancy migration between adjacent O( 1) sites as the lowest energy path with

an activation of 0.63 eV (shown in Fig.1). This result is consistent with isotope exchange experiments4 which show fast oxygen diffusion. Moreover, the calculated activation energy is in good agreement with the observed values from ionic conductivity measurementsl5. All interstitial mechanisms considered had high energy barriers, which implies that they would have negligible contribution to oxygen transport. 3.3 Dopant Substitution A number of experimental studies have shown that the addition of alkali and alkaline earth dopants (especially Sr2+) to La203 is effective in promoting the catalytic properties. A wide range of activities and selectivities arc cxliibited which have yet to be optimised. Our simulation approach is based on assessing the energetics of dissolution of such aliovalent ions and the nature of the charge compensating defects. The most straightforward mode of dopant incorporation into the host matrix is as a substitutional ion at a La3+ site with compensating oxygen vacancies. The resulting energies of solution for a series of alkali and alkaline earth ions are presented in Table 2 and are also plotted versus ion radius in Fig 2. Two points emerge from these results. First. the most favourable solution energy and hence the highest solubility is predicted for Sr. This is clearly illustrated in Fig. 2 which reveals a degree of correlation between the calculated solution energy and the size of the dopant ion, with a minimum at Sr2+ (close to the La3+ radius of 1.06 A). It is significant that our results accord well with experimental studies which have demonstrated how the addition of Sr lcads to the highest activity of a range of dopants2.4.I7. Second, the lower solution energies for alkaline earth ions suggests a higher solubility range than the alkali metals. The calculations therefore predict that alkaline earth doped La203 would show the greater activity, which is consistent with observation. We recognise, however, the difficulty in assessing the relative activity/selectivity properties of the promoted catalyst since a diverse range of reaction conditions have been employed.

43 1 6 -

w+

Ba

0

+

K

!

Solution Energy (eV/do pant)

31 1 '

0 4

0.6

0.7

0.8

0.9

1

1.2

1.1

Ion Radius

1.3

1 4

1.5

(A)

Fig. 2 Calculated energetics of solution as a function of ion radius for alkali and alkaline earth dopants (Note that the La3+ radius is 1.06 A). Table 2 Energies of solution for alkali and alkaline earth dopants with oxygen vacancy compensation. M

Defect Equilibrium 'h M 2 0 + La,

ax

M,,"

4.78 3.82 4.47 5.09

LA*

Na+

K+ Kb'

MO + LaL: Mg2' Ca2+

Sr2

+

Ba2'

Esol (eV/dopant)

+ V," + % Laz03

=

h4,-a'+ % V,"

+ 'h La20, 3.94 2.30 1.71 4.93

In view of these high solution energies it is possible that the catalytic properties are associated with exsolved Li20 and Na20, but modified by interaction with the La203 host. Indeed, it is

432

believed that the rBle of Li and Na is to poison the catalyst surface for the total oxidation reactions'8. It is noteworthy that alternative compensation mechanisms involving cation interstitials or vacancies were also considered, but resulted in solution energies that were less favourable by at least 3 eV. This confirms the view that the majority defects created by incorporating these dopant ions will be oxygen vacancies (particularly at low oxygen pressures). Indeed, even for small additions of SrO the dopant-controlled vacancy concentration will far exceed that arising from thermal disorder and thereby enhance the oxygen mobility. We therefore conclude that the net flux of oxygen through the doped solid (and to the surface) will be high.

3.4 Hole centres in the doped oxide Various spectroscopic studies17J9 have demonstrated that the active sites in the Li/MgO catalyst are 0- hole species, which are stabilised by the formation of (Li+O-) centres, and facilitate hydrogen atom abstraction from methane. These studies also report that the majority of the 0- centres are located in the bulk of the material. However, the issue of the active site for rare earth sesquioxides is not as clear, although the participation of 0- and 0 2 2 - peroxide species has been proposed1J0J. To render alkaline earth doped La203 catalytically active, it is necessary to treat the material with gaseous oxygen co-fed with methane. In terms of defect chemistry this leads to the oxidation (or 'filling') of oxygen vacancies by molecular oxygen with consequent formation of hole states. This oxidation reaction may be expressed as: V,"

+

% 0 2 (g) =

0,"

+ 211'

where h' is an electron hole modelled as a substitutional 0-. The calculated energy for the oxidation reaction is reported in Table 3 for the two independent oxygen sites. Table 3 Energies of the Oxidation reaction (oxygen vacancy to hole) in doped La203 Oxygen site

E,, (eV)

The first point to emerge from the results is that the O(2) position is the most favourable 0- site. Moreover, the relatively small encrgy for Eox indicates that the equilibrium could be displaced by changes in oxygen partial pressure. At low pressures (or low oxygen activity), oxygen vacancies are likely to predominate. As the oxygen partial pressure increases, the annihilation of

433 0 2 - vacancies by gas-phase oxygen results in the creation of 0-centres. In other words, except at low oxygen partial pressure, we would expect 0- holes to predominate over oxygen vacancies and oxidation to enhance the catalytic activity. This oxidation reaction would also explain the

observed inactivity of the catalyst in the absence of gas phase oxygen. 4. CONCLUSIONS

It has been shown that computer simulation methods provide a useful way of investigating key solid state properties of the La203 material that are relevant to catalytic oxidation. Four main results emerge from the study: 1) Anion Frenkel disorder is calculated to be the predominant intrinsic defect (albeit at very low concentrations), with negligible deviation from ideal stoichiometry. This accords with the known properties of the oxide and suggests that the defect population will be largely controlled

by dopants. 2) The majority compensating defects, created by addition of alkaline (or alkali) dopants, will be 0 2 - vacancies, and hence the net flux of oxygen through the host solid will be high. The highest solubility is calculated for Sr which is consistent with experimental studies which find the highest activity for Sr promoted La203. 3 ) The following oxidation reaction is energetically favourable:

Vg..

+

?40 2 (g)

=

0,x

+ 211'

and suggests that the 'filling' or annihilation of oxygen vacancies by gaseous oxygen (present as one of the reactants) will create 0- hole centres (h'), which are believed to facilitate hydrogen abstraction. 4) Solid state diffusion will be associated with an interlayer oxygen vacancy mechanism. Moreover, the low activation energy (- 0.6 eV) suggests that the oxygen vacancies consumed in the oxidation reaction will be readily replenished by facile diffision through the bulk and to the catalyst surface. Studies currently underway include detailed simulations of surface structures and segregation of dopants, and will be extended to the investigation of protons in La203. Acknowledgements

DJI is supported by a SERC/CASE studentship with BP, Sunbury. We wish to thank M.Leslie, J.McNally, S.Ramdas and S.Parker for helpful discussions.

434

APPENDIX : Interatomic potentials for La203 a) Short-range V(r) = Aexp(-r/p)-C/r6 Interaction

NeV

PlA

CIeV A-6

La3+...La3+

8579 1.74 5700.52 576.94

0.22030

6.863 38.9365 0.0

~ a 3.... +020 2 - . ... 0 2 -

0.29885 0.33536

b) Shell model Species

YIe

WeVA-2

La3+

-6.00 -2.50

460.0 27.0

02-

REFERENCES 1. K.D.Campbel1 et al., J. Phys. Chem. 92, (1988) 750. 2. J.M.DeBoy and R.F.Hicks, J.Chem.Soc.Chem.Commun. (1988) 982. 3. G.J.Hutchings et al., Chem.Soc.Rev. 18, (1989) 25 1 . 4. Z.Kalenik and E.E.Wolf, CatdLett. 9 (1991) 441; Catal.Today 13 (1992), 255 5. T.LeVanet al., Catal.Lett.14 (1992), 321. 6. C.R.A.Catlow and W.C.Macluodt (Eds), Computer Simulations of Solids, Lecture Notes in Physics, 166 (1982) Springer Berlin. 7. J.O.Titloye et al., J.Phys.Chem. 95 (1991), 4038. 8. M.S.Islam and CAnanthamohan, Phys.Rev.B 44 (1991), 9492; JSolidState Chem 100 (1992), 371. 9. M.Leslie, SERC Daresbury Laboratory Report, No. DLISCIITM3IT (1982) unpublished. 10. H.R.Hoekstra, Znorg. Chem. 5 (1 966) 754; Wells, Structural Inorganic Chemistry, Oxford University Press, Oxford (1984). 11. X.Zhang et al.,J.Phys.Chem.Solids 53 (1992) 761. 12. D.J.Ilett and M.S.Islam, J.Chem.Soc., Faraday Trans (in press). 13. A.G.Anshits, E.N.Voskresenskaya and LLKurteeva, Catal.Lett. 6 (1990) 67. 14. P.Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides,Wiley, New York (1972). 15. T.H.Etsel1 and S.N.Flengas, JEElectrochemSoc. 116 (1969) 771. 16. S.J.Milne, R.J.Brook and Y.S.Zhen, British Ceramic Proc. 41 (1989) 243. 17. G.J.Hutchings et al, J.Chem.Soc., Ftrrmky Trans.1 85 (1989) 2507. 18. R.Burch, G.D.Squire ans S.C.Tsang, Applied Catalysis, 43 (1988) 105. 19. J.X.Wang and J.H.Lunsford, J.Phys.Chem. 90 (1986) 5883. 20. K.Otsuka, K.Jinno and A.Morikawa, JCataZ. 100 (1986) 353. 21. C.H.Lin, K.D.Campbel1, J.X.Wang and J.H.Lunsford, J.Phys.Chem. 9 (1986) 534.

V. C o r k Corberin and S. Vic Belldn (Editors), New Developmenls in Selecrive Oxrdation II 1994 Elsevier Science B.V.

Kinetic Simulation of Oxidative Coupling of Methane i n t h e Gas P h a s e

V.I.Vedeneev, O.V.Krylov, V.S.Arutyunov, V.Ya.Basevich, M.Ya.Goldenberg and M.A.Teitel'boim. N.N.Semenov Institute of Chemical Physics, Russian Academy of Science, ul.Kosygina 4, Moscow. 117334 A kinetic model of methane oxidative coupling in the gas phase is worked out on the basis of previously proposed kinetic scheme of methane oxidation with addition of a number of elementary ethane oxidation reactions. The model includes 1 8 8 elementary reactions and well fits experimental data on methane oxidative coupling. The inclusion o f additional source of CH3 radicals in the model to simulate catalytic methane activation shows a pronounced increase in methane oonversion and C2-selectivity. INTRODUCTION

Methane oxidative coupling is one of the most rapidly developing and industrially perspective methods of natural gas transformation into valuable products [I I . In an overwhelming majority of the investigations the process is conducted at atmospheric pressure and T=900-1200 K with the use of catalysts. But under some conditions, e.g. at higher pressures the process can also be carried out as a homogeneous gas phase reaction [21. Moreover it was shown that f o r most catalysts the heterogeneous-homogeneous mechanism of this process takes place [ 3 ] . Therefore a study of the mechanism of homogeneous methane oxidation coupling can reveal more details relating to the correlation between homogeneous and heterogeneous steps of the prooess and promote the establishment of the optimal conditions of the process with higher conversion and C2-selectivity. A number of investigations p o i n t s out to a limited yield of C2-hydrocarbons, its maximal value being 26% 141, although there exist data indioating higher yields up to 30% [II . According to the proposed scheme of heterogeneoushomogeneous mechanism 131 the first step of the process is the generation of methyl radicals by the oxide (MO) surface MO + CH4 MOH + CH3 But the CH3 radicals generation is also the first step of

-

.

435

436

homogeneous methane oxidation [51. As a first approximation it may be proposed that the role of the oatalyst consists in additional generation of CH3 radicals. That is why it is interesting to study the influence of additional source of CH3 radicals on conversion and selectivity of methane oxidative coupling. The purpose of this work is to check whether or not kinetic model of gas phase methane oxidation not specially adjusted f o r particular experiments on methane oxidative coupling can give a satisfactory description of available experimental data and reveal the role of the catalyst in the process. MODEL CALCULATION AND DISCUSSION

The previously developed kinetic model of methane oxidation to methanol and formaldehyde (69 elementary reactions) [51 which was thoroughly tested and confirmed [61, was taken as the basis f o r the kinetic model of methane oxidative coupling. It is worthwhile to note that in order to obtain a detailed kinetic model of methane oxidation with the formation of oonsiderable amount of ethane and ethylene it is necessary to have a sufficiently detailed kinetic model of ethane oxidation. Such a model including 94 reversible elementary reactions (altogether 188 reactions ) has been developed and used in the following calculations. The complete model will be published elsewhere. Let us discuss some specific features of the developed kinetic model and consider some key elementary reactions. Recombination of methyl radicals C2Hg (1) CH3 t C H 3 was taken as the main reaction of ethane formation. The rate constants of the reactions (I) and ( - I ) depend both on the temperature and on the pressure. Their values for pressures 1 , 5 and 10 atm were taken from 171. At higher pressures the limiting value k00 can be used. The reactions of all active particles H, 0, OH, H02, CH3, CH30 and CH300 with ethane were taken into consideration. Rate constants of these reactions were taken from analysis of the published data with the exception of the ethane reactions with H02 and CH300. Rate constants of the last two reactions are somewhat increased as compared with the similar methane reactions at the expense of the activation energy decrease by 2 kcal/mole (in accordance with Polanyi-Semenov rule). Ethylene formation is described by reactions C2H5 (+M) C2H4 t H (M) (a)

-

-

C2H5

+

O2

-3

C2H4

+

H02

(I)*

437

Reaction

(a)

-

is probably an effeotive reaction. Reaotions

C2H5 + 0 2 -

-

C2H502 C2H502 C2H4 + H02 (V1 are also included into the model. Values of the rate oonstants of the reactions (I-V) were aooepted close to those cited in paper 171. Chain branching reactions were also considered C H 0 + CH4 (C2H6) C2H500H C2H50 + OH 252 but these reactions apparently do not play any essential role at low enough pressures and high temperatures 183. The developed model of partial gas phase methane oxidation has been tested with the m e of published experimental data on methane oxidative coupling into ethane and ethylene obtained in the absenoe of catalysts. The results of papers [9,10] where experimental data were presented in detail a r e ohosen f o r simulation. The authors [91 specially noted that the conditions f o r optimal ethane and ethylene formation with minimal yield of carbon oxides and C3-hydrooarbons were selected.

-

-

5*

/---

2

1 / 8

0 20 40:

&cla;

E)!

0 -J

w

(fl 20

*

0-

0

I

1

Experimental [91 (points) and calculated (solid lines) selectivities of products against oxygen conversion.

F i g u r e 1.

438

Besides that the experimental conditions of this work = (temperature 1044 K and reagents ratio CH4:02:N2 0.8:0.08:0.12) are also typical for methane oxidative coupling. A somewhat excessive pressure (4.1 atm) is necessary f o r guaranteeing homogeneous performance of the reaction. A comparison of experimental data from fig.8 in 191 with calculations according to our model is presented in fig.1. A s in [91 oalculations were performed f o r i6Othermal conditions and a good agreement with experiments is observed. A correspondenoe of calculated and experimental reaotion times within a factor of 2 was also obtained. From our point of view are very important the results obtained on simulation of experimental data presented in [lo]. This work is the only one where the temperature profile of the reaction was obtained. This allows us to the process in perform an adequate simulation of non-isothermal conditions. On the base of the published 1103 temperature profile in the reactor in the absence of oxygen the value of the heat transmission coefficient was estimated and then it was used for simulation of methane oxidation process. A comparison of thus obtained temperature profile with experimental one is shown in fig.2. 1 200

1 1

1100

1 a00 Y I-=

900

1

“ “1 ’

700 0.h

2

0.55

0.40

0.h

1.60

TIME, s Figure 2. Temperature profiles in the reactor. Dash lines experimental results [ l o ] , solid lines - calculations. I reagent flow with oxygen admixture; 2 - reagent flow without oxygen admixture.

439

In Table 1 calculated and experimental data on selectivity of the base products and times of the process are listed for the maximal temperature 1018 K whioh is the most favorable one for the C2-hydrocarbons formation. Besides the general observation of a good acoordance it is necessary to pay speoial attention to the practical coincidence of such an important parameter as the process time. Table 1. C2H6

Selectivity, % C2H4 co

26

exp. cal.

22 27.8

20.2

'6,

6

c02

38 47.5

5

1 .og

1.3

1.01

It is also very interesting t o compare the results of the simulation with those obtained in experimental work [I1 I carried out at high pressure (62 atm.), which is typioal for methane oxidation into methanol but at a much higher temperature (823-873 K) whioh is rather characteristic for methane oxidative coupling into C2-hydrocarbons. Taking into account the high pressure in the reactor and constant (r2%) temperature the calculation was performed for isothermal conditions and without consideration of the reactions of heterogeneous radicals decay. The main simulation results of three experiments from the work [I11 are shown in Table 2. Table 2. ~~~

__

-~ ~

~

~

Conversion,% t,s CH4 O2 CO

~~

C02

Seleotivity, % CH30H CH20 C2t

. . cal .

exp. &3 T=738 K, [CH41:[021=0.962:0.038; 0.7 2.9 80 58.2 6.9 34.9 0.9 2.8 80 39.6 3.0 39.0 17.1 2.6 4.0 100 48.5 3.8 35.4 5.8

exp.

.

exp. T=823 K, [CH41:~021=0.962:0.038: (0.6 3.4 100 54.8 10.4 24.3 0.09 4.0 100 44.9 2.0 25.8 16.9

10.6 10.0

exp. cal

.

exp. #5 T=823 K, [CH41: [02]=0.848:0.152; (0.6 12.6 100 56.7 9.6 1.5 0.2 14.8 100 60.3 10.5 17.4 4.7

12.2 6.0

cal.

&5 T=953 K, [CH4]: [02]=0.848:0.152; 0.01 15.4 100 59.0 6.1 10.1 9.6

exp cal

cal

0.02

5.6

12.7

440

The temperature in the exp.J&3 (738 K) is typical for methane oxidation to methanol, and the exp.@4 and l&5 were performed at higher temperature (823 K) when the selectivity of methane formation decreased but the C2-hydrocarbons selectivity increased. Taking into account the high concentration of oxygen and short reaction time it may be proposed that in exp.J65 the real reaction temperature is somewhat higher. Increase of reaction temperature during simulation leads to a better fit with experimental results on C2-selectivity. Thus, the worked out gas phase model satisfactorily describes the prooess of homogeneous gaseous methane oxidation to ethane and ethylene in a wide range of conditions. It was interesting to carry out a comparison between calculated and experimental data for typical conditions of heterogeneous methane oxidative ooupling and to clear the question whether addition of external initiation in suoh conditions can give an increase of conversion and C2-hydrocarbons selectivity. Typical conditions: T=973-1079 K, P=l atm, CH4:02:N2= 10:1:8 were chosen for this examination. The simulation showed a good acoordanoe with experimental results on the selectivity of ethane, ethylene and carbon dioxide formation in the absence of the catalyst [121. A noticeable divergence was observed only for the selectivity of CO formation, but the calculated total selectivity for CO and formaldehyde formation practically coincided with experimental CO selectivity ( CHZO selectivity was not indicated in 12I 1. This fact by itself proves that the main source of CO formation under these conditions is formaldehyde. To simulate the heterogeneous catalyst action an external source of initiation was introduced into the scheme. This source generated CH3 radicals with a rate exceeding 10 times the rate of their thermal generation. This value is still less than the2Tate of radical generation on the most active catalysts (210 CH3 radicals from 1 g of Sm203 per second at 1023 K [131). Such an artificial introduction of CH3 radicals at equal reaction times leads to a roughly double increase of methane conversion and some increase of C2-hydrocarbons selectivity. Thus, the results of the simulation shows that in the typical conditions of catalytic methane oxidative coupling, an additional initiation even in the absence of the catalyst leads to values of methane conversion and product selectivity which are comparable with those obtained in catalytic experiments. One of the most important results obtained in this simulation is also the discovery of a negative temperature coefficient of methane oxidation in the temperature range 700-900 K and the observation of cold flame phenomena. These

44 1

results will be discussed elsewhere. CONCLUSION

The kinetic model of methane oxidation into methanol and formaldehyde [ 5 1 supplemented by an additional set of ethane oxidation reactions (altogether 1 8 8 elementary steps) allows to describe a very broad range of experimental conditions including the conditions of methanol formation (20-100 atm, 650-750 K) and those of methane oxidative coupling into ethane and ethylene (1-5 atm, 900-1200 K). The caloulated methane conversions and product selectivity oorrespond well to experimental data. This model can be useful also for describing heterogeneous-homogeneous methane oxidation. As the first step we examine in this work the influence of an additional source of CH3 radicals which can simulate the action of the catalysts. REFERENCE5

1. O.V.Krylov, Kinetika i Kataliz. 34 (1993) 18.

T.R.Baldwin, R.Burch, G.D.Squire, S.C.Tsang, Appl.Catal., 74 (1991) 137. 3 . M.Yu.Sinev, V.N.Korchak, O.V.Krylov, Proc. 9th Intern. Congress on Catalysis (Calgary), Chem. Inst. of Canada, (eds.) M.J.Phillips, M.Terner 2 (1988)899. 4. J.G.McCarty, A.B.McEwen, M.A.Quinlan, in New Developments in Selective Oxidation, Centi.G. and Trifir0.F. (eds.), Elsevier, Amsterdam, 1990. 5. V.I.Vedeneev, M.Ya.Goldenberg, N.I.Gorban' and M.A.Teitel'boim, Kinetika i Kataliz. 29 (1988)7. 6. W.Rytz, A.Baiker, 1nd.Ehg.Chem.Res. 30 (1991) 2287. 7. P.Dagaut, M.Cathonnet, J.-C.Boettner, Int.J.Chem.Kinet., 23 (1991) 437. 8. J.C.Mackie, Catal.Rev.-Sci.Eng. 33 (1991)169. 9. H-Zanthoff,M.Baerns, 1nd.Eng.Chem.Res. 29 (1990)2. 10. O.T.Onzager, R.Lodeng, P.Soraker, A.Anundskaas, B.Helleborg, Catal.Today.4 (1989)355. 1 1 . D.E.Walsh, D.J.Martenak, S.Han, R.E.Palermo, 1nd.Eng.Chem. Res.31 (1992) 1259. 12. J.W.M.H.Geerts, Q.Chen, J.M.N.van Kasteren, K.van der Wiele, Catal.Today. 6 (1990) 519. 13. K.Otsuka, M.Hatano, Q.Lin, A.Morikawa, J. Catalysis 100 (1985) 353. 2.

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V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation II

0 1994 Elsevier Science B.V. All rights reserved.

443

Oxidative Coupling of Methane over Ti-La-Na catalysts S.T. Brandaoa, L. Li e t t i a P.L. Villaa A. Santuccib, R. Millinic, 0. Forlanil; and D. Sanfilippol;.

S.

Rossinib,

a Dipartimento d i Chimica Industriale ed Ingegneria Chimica "G. Natta" del Politecnico, P.zza L. d a Vinci 32, 20133 Milano, Italy.

Snamprogetti, Via Maritano, 20097 S. Donato Milanese, Italy. C

Eniricerche, Via Maritano, 20097 S. Donato Milanese, Italy.

ABSTRACT In t h i s paper we r e p o r t on t h e preparation, characterization and catalytic a c t i v i t y i n t h e Oxidative Coupling of Methane (OCM) of TiLaNa oxide catalysts. XRD analysis performed on t h e calcined samples indicate t h e presence of La(OH)3, La203, La0.66Ti02.993. In t h e samples with higher sodium content t h e formation of La2NazTi3010 h a s also been detected. The preparation and characterization of t h i s phase has also been reported. A l l t h e c a t a l y s t s show appreciable activity and selectivity in t h e OCM reactions, t h e b e s t performance being displayed by T i 1LalNa2 sample. XPS analysis performed on t h i s catalyst indicates t h a t an oxidic environment surrounds lanthanum during reaction whereas sodium segregates a s carbonate. The acidlbase properties have also been addressed. Results indicate t h a t a l l t h e c a t a l y s t s show significant surface basicity, and a correlation e x i s t s between t h e base properties and t h e selectivity t o C2+ i n t h e OCM. 1. INTRODUCTION

Among a l a r g e number of c a t a l y s t s proposed f o r t h e oxidative coupling of methane (OCM), rare-earth oxides and alkali-promoted rare-earth oxides show promising catalytic properties. In particular, it has been reported t h a t lanthanum oxide shows significant activity in t h e production of methyl radicals from methane [l]. Recently, t e r n a r y c a t a l y s t s composed of a tetravalent element (of t h e IV A and IV B groups), a t r i v a l e n t element (of t h e I11 B group) and a basic component h a s been claimed t o be active i n t h e OCM reaction [2]. In t h i s paper we r e p o r t t h e main r e s u l t s obtained i n t h e physico-chemical and catalytic characterization of a group of t h i s family, i.e. catalysts containing Ti, La and Na. The s t r u c t u r a l characterization of La2Na2Ti3010, performed by full-profile f i t t i n g method of t h e X-ray powder diffraction p a t t e r n (Rietveld analysis [3]), is also reported.

444 2. EXPERIMENTAL

Catalysts preparation - Ti-La-Na c a t a l y s t s with Ti/La/Na atomic r a t i o s l / i / X (X=O, 0.5, 1, 2, 3, 4) were prepared by mixing solid T i 0 2 i n t o a water solution of lanthanum and sodium n i t r a t e s (99% purity Fluka products). The s l u r r i e s obtained were concentrated by evaporation under s t i r r i n g a t about 353 K and finally dried a t 383 K. The dried samples were calcined i n a i r a t 1073 K f o r 4 h o u r s before catalytic activity runs. The LazNa2Ti3010 sample w a s prepared according t o t h e c i t r i c acid complexation method. A c i t r i c acid solution of Ti-isopropoxide, lanthanum a c e t a t e and sodium a c e t a t e was prepared and evaporated under vacuum. The solid precursor obtained w a s calcined i n a i r a t 1073 K f o r 4 hours. Physico-chemical characterization - XRD spectra were collected on a Philips PW1050/30 v e r t i c a l diffractometer equipped with a pulse-height analyzer; CuKa radiation ( A = 1.54178 A) was used. Data were collected stepwise i n t h e 5 5 29 5 70", with 0.05" 29 s t e p s i z e and 5 s accumulation time. XRD d a t a f o r LazNazTi3010 were collected in t h e 4 5 29 5 loo", with 0.03" 28 s t e p s i z e and 15 s accumulation t i m e . Refinement was performed using t h e software package WYRIET [4]. The Pearson VII peak profile function was used, while t h e background intensity was described by a s i x t h o r d e r polynomial with refinable coefficients. The contribution of Kal and Ka2 radiation (2:l intensity ratio) t o t h e reflection profile was considered. Variation of FWHM with t h e scattering angle was described by t h e Caglioti, Paoletti and Ricci equation [5]. XPS s p e c t r a were collected on a VG ESCALAB 200-C instrument equipped with a hemispheric analyser operating in Constant Analyser Energy (CAE) mode. The decomposition of isopropyl alcohol (IPA) w a s used t o characterize t h e acid-base properties of t h e catalysts. A i r s a t u r a t e d with isopropyl alcohol was fed t o a fixed-bed microreactor (inside diameter 7 mm and heated length 10 cm) kept a t 573 K. The products were analysed by conventional on-line gas-chromatography by using three columns in parallel arrangement: a 5 A molecular sieve and a porapak QS columns with thermal conductivity detectors f o r t h e analysis of COX and water, and a poraplot Q capillary column with a flame ionization detector f o r t h e analysis of hydrocarbons and oxygenates. Catalytic a c t i v i t y runs - The catalysts (1000 mg, 20-40 mesh) were tested in t h e OCM reaction (T=1023 K measured a t t h e bottom of t h e catalyst bed, P = l atm, GHSV=2500 h - l ) by cofeeding methane and a i r in t h e same apparatus used f o r t h e IPA decomposition experiments. 3. RESULTS AND DISCUSSION 3.1. Bulk phase composition Phase composition of Ti-La-Na samples calcined i n a i r a t two different temperatures (873 and 1073 K) and a f t e r catalytic activity runs a r e reported i n Table 1. A complex phase composition is apparent f o r a l l t h e samples having atomic r a t i o s Ti/La/Na = 1 / 1 / x . A t 873 K La(OH)3, La203 and La0.66Ti02.993 were observed in a l l t h e samples. I t is worthy noting t h a t no crystalline phases involving t h e a l k a l i cation were detected, also in t h e samples with a higher sodium content. On increasing t h e calcination temperature up t o 1073 K, formation of La2Na2Ti3010 occurs, a t l e a s t in t h e samples with higher sodium content, together with a small amount of an unknown phase

445 Table 1. Major XRD lines of the Ti-La-Na catalysts

Calcined at 873 K

Discharged catalysts

Calcined at 1073 K La203.Ti02 Ti02 66Ti02. 993

La(OH)3 La203 66Ti02. 993

La203.Ti02

ti02

66Ti02. 993

i

1a203

~~

Na2La2Ti3010 unknown ohase La La?. OH 67Tib2.993

La(OH13 La203 66Ti02. 993

Na La2Ti3010 unznown Dhase

La203 Na La2Ti3010 unznown phase -(OH13 La203 Na La2Ti3010 ungnown phase La(OH) 3

unknown phase

1a203 La2C05

which is presently under investigation. On the samples discharged from t h e reactor, the formation of La2C05 was observed. In order t o b e t t e r understand t h e role of the LazNa2Ti3010 phase in the OCM reaction, a structural study was undertaken. The prepared sample La0.5Na0.5TiO3 which was considered contained small amounts of unimportant f o r an accurate structural characterization of the main phase and t h e main reflections were subtracted from t h e XRD pattern. A starting structural model was adopted on the basis of the data reported f o r SrqTi3010 [6,7,8]. The pattern was entirely indexed on t h e basis of the tetragonal I4/mmm space group. Refinement converged t o Rp=8.84%, Rwp=11.31%, S=2.16, with a good agreement between experimental and calculated XRD patterns (Figure 1). Atomic parameters and main geometrical data a r e reported in Table 2. The structure consists of three [ T i 0 6 1 octahedra thick perovskite slabs, stacked a t a distance of (a+b)/Z

446 t o each other. Lanthanum ions a r e located i n t h e cube-octahedral cavities within t h e s e s l a b s , with a l k a l i m e t a l ions occupying t h e semicubeoctahedral sites i n t h e i n t e r l a y e r region. [Ti061 octahedra appear t o be s l i g h t l y elongated i n t h e c-direction s o t h a t t h e La and N a coordination polyhedra display a s m a l l deviation from t h e ideal symmetry. 3.2. Catalytic a c t i v i t y runs Table 3 summarizes t h e r e s u l t s of t h e OCM reactivity t e s t s performed over t h e Ti-La and Ti-La-Na catalysts. Methane conversion ( X C H ~ )and carbon s e l e c t i v i t i e s (S) t o t h e v a r i o u s products have been reported. The binary c a t a l y s t shows a high selectivity t o COX, while sodium significantly enhances t h e C2+ selectivity. The b e s t catalytic performance was obtained with T i / L a / N a atomic r a t i o 1/1/2, which exhibits a yield of C2+ products close t o 12%. Upon f u r t h e r increasing of t h e alkali content, a decrease i n t h e c a t a l y t i c performance w a s observed. The La2Na2Ti3010 c a t a l y s t is active b u t not very selective in t h e OCM reaction (Table 3, sample 3/2/2).

h

6 000

m

5

z Z

3 0 0

4000

- 2000 I

I

! 0

20

40

60

2-THETA

80

100

(DEGREES)

Figure 1. Experimental (I), calculated (---) and difference profiles f o r L a ~ N a 2 T i 3 0 1 0 . Vertical b a r s indicate t h e position of Bragg reflections. Indeed t h e s e l e c t i v i t y t o C2+ hydrocarbons (28 %) is lower than f o r all t h e o t h e r Ti-La-Na investigated catalysts. However, a significant improvement of c a t a l y t i c performance w a s achieved by increasing t h e GHSV t o 4600 h - l and by decreasing t h e C H 4 / 0 2 r a t i o t o 3: a three-fold increase i n t h e yield t o Cz+ products h a s been observed i n t h i s case.

447

Table 2. Atomic parameters and main geometrical d a t a parentheses) f o r LazNa2Ti3010 (a).

0

0

0

0

0 0 0

1/2

0

0 0 0

0 0

0

1/2

0 0.1361 0

0.0685(9) 0.1356(6) 0.2052(5) 0.4322(1) 0.2912(4)

(A,

O,

e.s.d.’s i n

1.0 1.0 2.5 2.5 2.5 2.5 1.0 2.0

~~

Ti(1)-O(1) Ti(l)-0(2) Ti(2)-0(2) Ti(2)-0(3) Ti(2)-0(4)

4 x 1.92(1) 2 x 1.96(2) 1.94(3) 4 x 1.92(1) 1.98(2)

La-O(l) La-0(2) La-O(3) Na-O(3) Na-O(4)

4 4 4 4 4

x 2.729 x 2.710

x 2.73 x 2.84 x 2.712

O(l)-Ti(1)-0(2) 90.0(4) 0(3)-Ti(2)-0(4) 90.4(4) 0(2)-Ti(2)-0(4) 180.0(5) 0(2)-Ti(2)-0(3) 89.6(5) Ti(l)-O(2)-Ti(2) 180.0(9) (a) Tetra onal,

space group I4/mmm 139 Int. Tables of Cryst.), AiNr’ formula weight 627.48, 2=2, a=3.8333(1)% c=28.660(1)A V=421.07(2) DCalc=4.949 Mg/m3. The most active c a t a l y s t (i.e. T i / L a / N a = 11112) and t h e monophasic La2NazTi3010 sample were a l s o t e s t e d under long term operations (100 h). Both c a t a l y s t s showed high s t a b i l i t y under reaction conditions with no significant changes i n methane conversion and selectivity t o C2+ products. The La2Na2Ti3010 sample displayed a l s o a high s t r u c t u r a l s t a b i l i t y since n e i t h e r v a r i a t i o n i n t h e XRD p a t t e r n nor major loss of sodium (6.2 %! w/w i n t h e discharged c a t a l y s t vs. 7.1 % w/w i n t h e f r e s h catalyst) have been observed. On t h e o t h e r hand, both s t r u c t u r a l evolution and g r e a t e r loss of sodium ( p a r t i c u l a r l y during calcination) were detected i n t h e T i l L a l N a 2 c a t a l y s t in s p i t e of t h e s t a b i l i t y of catalytic performance. 3.3. XPS measurements XPS analysis w a s performed on t h e sample with b e s t catalytic activity. Figures 2 a and 2b show t h e 01s region f o r f r e s h and used Ti/La/Na 1 / 1 / 2 c a t a l y s t s , respectively. I t is worthy t o note a massive presence of carbonate r e l a t e d oxygen peak (531.4 eV) together with a less intense oxide r e l a t e d oxygen peak (529.2 eV). Pretreatment of t h e sample with oxygen a t high temperature r e s u l t e d i n a decrease of intensity of carbonate peak (Fig. 2c) and a higher separation of carbonate and oxide peaks, indicating a higher oxidic n a t u r e of t h e surface. Comparing t h e La3d region of f r e s h and pretreated catalyst, an increase i n t h e shake-up s p l i t t i n g value (3.5 t o 4.1 eV) was observed. The same shake-up value w a s observed f o r t h e pretreated sample (4.1 eV) and f o r t h e used sample (4.0 eV), suggesting t h a t during reaction a more oxidic atomic environment s u r r o u n d s lanthanum atoms i n lanthanum oxide of oxycarbonate.

448

Table 3. Results of t h e c a t a l y t i c a c t i v i t y tests performed over Ti-La-Na catalysts. Operating conditions: feed CH4 (50% v/v), 0 2 (loo), N2 (40%); P= 1 atm; T= 1023 K, GHSV=2500 h-l.

1/1/0

17.6

16.0

54.0

12.8

16.0

5.1

i/i/o.5

18.6

9.5

46.7

23.0

20.7

8.1

1/1/1

17.4

0.0

47.9

22.8

25.2

9.0

1/1/2

20.0

0.0

41.7

29.9

23.2

11.6

1/1/3

18.8

3.4

45.8

22.6

25.1

9.5

1/1/4

19.0

1.9

47.1

21.5

26.3

9.7

31212

13.7

6.1

66.1

7.2

20.8

3.8

3/2/2a

25.9

5.0

58.0

17.4

18.8

9.6

a T=1073 K ; GHSV=4600 h-';

CHq/02=3

Moreover, OKLL Auger spectrum can be explained as superimposition of sodium carbonate and Ti-La-Na oxide phases features, indicating t h a t sodium segregates as carbonate on t h e c a t a l y s t surface. Summarizing, XPS analysis suggested t h a t lanthanum displays an oxidic c h a r a c t e r while sodium shows a carbonate one. Transient experiments performed over t h e same Ti-La-Na catalyst were i n agreement with t h e formation of carbonates during catalytic activity t e s t s ~91. 3.4. Acid-basic properties of Ti-La-Na catalysts It is generally accepted t h a t t h e basic character of t h e catalyst surface is e s s e n t i a l f o r obtaining high C2+ selectivities i n t h e OCM reaction [lo]. The decomposition of isopropyl alcohol (IPA) w a s suitably used f o r characterising t h e acid-basic properties of t h e catalyst surface [ 111. Figure 3 shows t h e r e s u l t s obtained i n t h e IPA decomposition experiments at 573 K over d i f f e r e n t Ti-La-Na c a t a l y s t s vs. t h e a l k a l i nominal bulk content. For comparison, t h e selectivity t o C2+ products i n t h e OCM reaction is a l s o reported. The main reaction products were acetone and C02, with minor amounts of propylene (not reported i n t h e figure). High selectivity t o acetone and t h e corresponding very low formation of propylene clearly indicate t h a t f o r all t h e investigated c a t a l y s t s t h e s u r f a c e is basic i n nature. C 0 2 may derive from t h e successive oxidation of acetone, indicating t h e oxidative p r o p e r t i e s of t h e catalysts.

449

G 81 N O I N G ENERGY

(eV)

Figure 2. XPS s p e c t r a of t h e T i l L a l N a 2 sample i n t h e 01s region. a) fresh; b) used; c) f r e s h 02-treated at 873 K Figure 3 also shows t h a t t h e maximum acetone selectivity corresponds t o t h e sample with Ti/La/Na atomic r a t i o = l/l/Z ( N a z 20% w/w), which gives t h e highest C2+ yield i n t h e OCM reaction. This f a c t confirms t h a t surface basicity play a very significant r o l e in obtaining high C2+ selectivity i n t h e OCM reaction i n good agreement with o t h e r l i t e r a t u r e d a t a [1,10,12].

CONVERSION /SEL ECTl V I T Y 100 r

1

90

c

I

60

c

I

13

-

1

5

10

15 20 (XNa b u l k )

25

30

35

Figure 3. Isopropyl alcohol decomposition over various Ti-La-Na catalysts. Feed: IPA (1%v / v ) i n a i r , flow r a t e 60 Ncc/min; catalyst weight 0.8 g ; T=573 K ; P = l a t m . A IPA conversion; selectivity to: 0 C3H6O; 0 C02; *C2+ i n t h e OCM.

450 4. CONCLUSIONS

The following conclusions can be derived from o u r study: 1. Ti-La-Na c a t a l y s t s prepared via t h e s l u r r y method show appreciable activity and selectivity i n t h e OCM reaction, t h e b e s t performance being displayed by t h e sample TilLalNa2; 2. XRD performed on t h e calcined c a t a l y s t s indicates f o r a l l t h e samples a complex phase composition. The presence of La(OH)3, La203 and La0.66Ti02.993 h a s been observed togheter with t h a t of La2Na2Ti3010 i n t h e Na-rich samples. Modifications in t h e XRD s p e c t r a a r e also evident upon reaction under OCM conditions; 3. Pure La2Na2Ti3010 (with only t r a c e amounts of La0.5Na0.5Ti03) could be synthesized by t h e c i t r i c acid complexation method; t h i s sample shows a limited s t r u c t u r a l and composition changes during t h e OCM reaction; 4. In t h e case of t h e TilLalNa2 sample XPS, analysis indicates t h a t an oxidic environment s u r r o u n d s lanthanum during reaction while sodium segregates as carbonate: 5. The r e s u l t s of IPA decomposition experiments shows t h a t t h e Ti-La-Na c a t a l y s t s u r f a c e is basic i n nature, and t h a t a correlation e x i s t s between t h e base p r o p e r t i e s (formation of acetone from IPA) and t h e selectivity t o C2+ in t h e OCM.

REFERENCES 1. V.R. Choudary and V.H. Rane, J. Catal., 130 (1991) 411. 2. 0. Forlani, S. Rossini, D. Sanfilippo; U S Patent Application No. 9/650968 (1988). 3. H.M. Rietveld, J. Appl. Cryst., 2 (1969) 65. 4. J. Schneider, Proceedings "Int. Workshop on t h e Rietveld method", Petten Ed., p. 71 (1989): A. Sakthievel, and R.A. Young, ibid., p. 48 (1989). 5. G. Caglioti, A. Paoletti and F.P. Ricci, Nucl. Instrum., 3 (1958) 233. 6. S.N. Ruddlesden, P. Popper, Acta Cryst., 11 (1958) 54. 7. Vallino M., A t t i Acc. Sci. Torino, C1. Sci. Fis. Mat. Nat., 117 (1983) 85. 8. J. Gopalakrishnan, and V. Bhat, Inorg. Chem., 26(1987)4299. 9. S. Rossini, S.T. Brandao, 0. Forlani, L. L i e t t i A. Santucci, D. Sanfilippo and P.L. Villa,Proceedings "10th International Congress on Catalysis", L. Guczi, F. Solymosi and P.Tbt6nyi Eds, p.2237 (1993). 10.S. Becker and M. Baerns, J. Catal., 128 (1991) 512. l l . M . A i , J. Catal., 40 (1975) 327. 12.A.Z. Khan, E. Ruckenstein, J. Catal., 139 (1993) 304.

V . CortCs Corbcran and S. Vic Bcllon (Editors), New Developments i n Selective Oxidation I /

0 1994 Elsevier Science B.V. All rights reserved.

45 1

Gas Phase Oxidation of Benzene to Phenol using Pd/Cu Salt Catalysts. - Effect of Counter Anion in Copper SaltsKazuo Sasaki”, Tomoyuki Kitano”, Toshihiro Nakai”, Miki Mona, Sotaro Ito”, Masahiro Nittab, and Katsuomi Takehira“ ”Department of Applied Chemistry, Hiroshima University, Kagami Yama, Higashi-Hiroshima, 724 Japan. bKure Research Laboratory, Babcock Hitachi K.K., Takara, Kure, 737, Japan. ‘Department of Surface Chemistry, National Institute of Materials and Chemical Research, 1-1 Higashi Tsukuba, Ibaraki, 305 Japan. The catalyst, Pd, Cu,(PO,),/SiO,, impregnated by H,PO, exhibits a powerful activity for the title reaction. The rate of phenol production obtained is O.Smmol(h.gcat)-’ that is about ten times higher than that obtained with the Pd, CuSO,/SiO, catalyst studied previously. The improvement of the catalyst activity may be ascribed to the presence of a liquid film of phosphoric acid over the silica surface.

1. Introduction Direct conversion of benzene to phenol has been one of the central interests in industrial chemistry. Many trials have been put in practice in the past. Even if we limit the survey to the recent publications, we readily find several interesting works. Fujiwara reported that the coordinated complex of palladium with o-phenanthroline is an efficient catalyst 111. Moro-oka uses p-0x0-binuclear iron complex 121, while Kimura works with microcyclic polyamines [3]. In some system, nitrous oxide is used as the oxidant 141. In comparison to these rather sophisticated catalysts and reagents used by other workers, our reaction system is composed of very simple traditional catalyst and reagents. The catalyst is basically Cu(1) ion , which is either fixed on some suitable support or in some cases dissolved in solution. Cu(1) ion activates dioxygen by electron transfer to produce OH radical in cooperation with proton. Pd serves as the auxiliary catalyst, which regenerates Cu(1) from Cu(I1) ions with the aid of suitable reducing agents such as hydrogen. The oxidant is ordinary dioxygen. The role of the copper redox couple can be compared with that of enzyme monooxygenases in biological systems. The usefulness of our system has already been demonstrated in some liquid phase oxidation reactions [5-91. Application to the reaction in gas phase, which seems to be more practical for industrial applications, has also been reported [lo-ll]. This paper deals with further developments, laying stress on the effect of counter ions in copper salt on the catalysts.

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2. Experimental section 2.1 Preparation of catalyst Catalysts used in this paper are divided basically into two groups; one is prepared from cupric sulfate, which has been used customarily in our previous works, and the other from cupric phosphate which are newly tested in this paper. The former group of catalysts were prepared by depositing both PdC1, and CuSO, simultaneously from an aqueous solution on a given amount of silica gel (Merck Kiesel Gel 60). For further detail of this catalyst preparation, ref. 6 should be referred. The latter group, phosphate based catalysts, was prepared in a dual process: Pd was first loaded on the silica surface with ion exchange and then cupric phosphate was deposited on it from an aqueous solution. Since the solubility of cupric phosphate is rather low, it was necessary to acidify the depositing solution and phosphoric acid was mainly used for this purpose. For preparing 1 gram of 500 Cu catalyst, it was necessary to use 3 mmol of the acid. It should be mentioned that, because of its non-volatility, whole of phosphoric acid does not detach from the silica surface during the dry-up process and remains in the catalyst. Catalysts are denoted in this paper as xCu-yPd, where x and y stand for number of micromoles of metal species per gram of the silica support. In one batch, 10 g of catalyst was prepared and portioned for several measurements. At a given reaction condition, the catalyst was sustainable for more than 30 hours of reaction without any loss of the activity. When, however, the reaction was interrupted after a given time, 3 to 5 hours, a remarkable loss of activity appeared in the successive measurements by some unknown reasons. Accordingly, each individual measurement was done with fresh catalyst. In some experiments, catalysts prepared by ion exchange technique were used to which readers should refer our previous paper [7]. 2.2 Reactor and product analysis The reactor used is a Pyrex glass tube having diameter 18mm and length 38cm. The catalyst, normally 2 grams, was placed over the sintered glass plate located half way the vertically hold tube reactor, which was surrounded by an electric furnace. For the purpose of preheating the reactant gas mixture before entering the catalyst zone, the catalyst bed was covered by a thick layer (ca. 4 cm in depth) of silica gel. The temperature was monitored inside the catalyst bed and the reading was fed back to a regulator to control the variation within f. 10°C. Benzene (and water by occasion) was supplied by means of a micro feeder, and both hydrogen and oxygen as well as nitrogen were supplied through three independent flow regulators. To the lower end of the tube, a small glass tube containing ethanol was placed to trap soluble products, of which aromatics were analyzed by means of HF'LC. The trapping liquid used was ethanol which enabled us to detect and analyze water, another important reaction product, by means of gas chromatography. In the ethanol trap, an appreciable amount of benzene is captured so that the liquid volume increases with increasing reaction time, although some of them may be vaporized. In

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order to minimize these effects, a given amount of acetone as the internal standard for determining the amount of water was added to each sample before GC analysis. Trap ethanol was renewed at every sampling. The exhausting gas was passed through an aqueous solution of Ba(OH), to capture CO, before releasing it into the atmosphere.

3. Results 3.1. Effect of temperature and moisture Using 2g of a catalyst (500 CuSO4-50 Pd), we first studied the effect of temperature over the range from 140 "C to 300 "C. The phenol yield reached a maximum at ca. 200 "C and then decreased with increasing temperature. Instead, production of carbon dioxide became predominant at temperatures exceeding 250°C. It should be noted that an appreciable effect of moisture appears in the present system. For instance, a two times high catalyst activity is obtained when the catalyst is preliminarily wetted. Similarly, simultaneous feeding of steam in the reactant gas mixture also doubles the yield of phenol (see Fig. 2) [ll].

3.2. Effect of catalyst composition and preparation. Using PdCI, and CuSO, the effect of catalyst composition per gram silica on the yield of phenol has been studied from several aspects. When palladium content y was fixed at 5 kmol per gram silica (ca. 0.05% in weight ) and the copper content x was varied, the yield of phenol first increased with increasing copper content upto 300 pmol per gram silica and becomes constant (Fig. 1A). This means that one Pd atom can activate ca. 60 Cu ions ( x l y = 30015 ) on average. A remarkable fact is that the r (

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Fig. 1 Effect of the catalyst composition. Catalyst: A; x CuS0,- 5Pd / SO,, B;500CuS04-y Pd / SiO, , (2g) Partial presuure: P(O,):P(H,):P(Bz):P(N,) =0.05:0.05:0.4:0.5 Total flow rate: 90 ml/min. 200°C,

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single component Pd catalyst exhibits no catalytic activity indicating that the low valent copper species is the key reactant. This contrasts to some reactions occurring in acetic acid, where a high catalytic activity was found with some Pd catalysts free from copper species. The latter point is quite sensitive to the presence of C1- ion and was discussed in refs. 6 and 7. When, on the other hand, palladium content is varied and that of copper is fixed (500pmol Cu per gram silica), Fig. 1B is obtained. Although the phenol yield increases with increasing amount of Pd, the increase is not linear but logarithmic with regard to the amount of Pd showing that a too much use of Pd is insignificant and not recommended. The turnover number per hour with respect to Pd decreases with increasing amount of Pd: the value being 8 at y=l. The reason why the catalytic activity does not increase proportionally to the Pd content may be worth noting. The main reason should be ascribed to the fact that the active sites, on which oxygen is activated by receiving electron, are not of Pd but of

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Time 1 h Fig. 2 Performance of differently prepared catalysts. Catalyst: 500CuS04-50Pd/Si0, (2g), Inert gas was either N,(full) or H,O(empty). Square symbols (A) correspond to catalysts prepared with impregnation and circles (B,C) with ion exchange. Other conditions were the same as Fig. 1. Fig. 3 Effect of partial presuure of reactant gases. P(NJ=balance; (A):variable P(H2) at fixed P(02)=0.05 and P(Bz)=0.4; (B):variable P(Bz) at fixed both P(H,) and P(0,) at 0.05. (C):variable P(0,) at fixed P(H,)=0.05 and P(Bz)=0.4 Other conditions were the same as Fig.2A

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Cuo). In the present reaction system, Pd is merely an auxiliary catalyst which mediates electron transfer from hydrogen atoms to Cu(I1) ions and , the electron transfer to Cu(I1) ion is performed quite efficiently : one Pd atom can drive 60 Cu ions on average. Accordingly, when the Cu content is fixed at a constant value (x = 500 in the case of Fig. lA), the increase in Pd content does not result in a linear increase in phenol production. All catalysts hitherto used were prepared by impregnating both the Pd and Cu salts simultaneously from one solution. When the fixation of Pd is done with ion exchange technique and the impregnation of Cu salt is followed to it, a definite improvement of catalyst activity appears (Fig. 2). In this figure, curves A and B represent the accumulation of phenol during 5 hr reaction observed with impregnated and ion exchanged catalysts, respectively. The reactivity of catalysts prepared with ion exchange is roughly twice as high as the impregnated catalysts. This may be related to the difference in either the degree of dispersion of Pd or relative distribution of Pd to Cu on the silica surface. Curve C indicates the enhancing effect of moisture. The curve was obtained by replacing nitrogen, which was normally fed in the reactant gas for the safety purpose, with steam.

33. Composition of reactant gas. By keeping the total flow rate constant, effect of the reactant concentration was studied for each of the components, 4,0,, and benzene, respectively. When hydrogen was tested, for instance, flow rates of the other components were fixed and the variation in hydrogen rate was compensated for with nitrogen. Results obtained are illustrated in Fig. 3, where the scale of abscissa is expressed in terms of partial pressure. All curves in Fig. 3 rise almost linearly at lower pressures indicating that the reaction is first order with respect to each reactant and reach to a flat plateau. The phenol yield at the plateau is the largest in curve A for hydrogen than those for oxygen ( curve B) and benzene (curve C), with which the plateau values are more or less the same. It is interesting that the curve A saturates at a point where P(H,)/P(OJ equals to 2. 3.4. Reactivity of catalysts prepared from copper phosphate. It is interesting to show that when the catalyst was prepared from cupric phosphate in place of sulfate used above, a remarkable increase in the catalyst activity was observed. Since the phosphate salt does not dissolve readily in water, an addition of certain acid is effective to enhance the dissolution of it. When phosphoric acid is used, whole the acid added remains on the catalyst surface because of its non-volatility. Accordingly, we have to make clear whether the enhancing effect comes from cupric phosphate or from phosphoric acid. Fig. 4 shows results of a series of experiments conducted for verifying this point. As is indicated by curve A in Fig. 4, the catalyst prepared from cupric phosphate with the aid of phosphoric acid exhibits the largest reactivity. Other curves show the performance of catalysts of different nature. Obviously the catalyst made of cupric sulfate with the aid of phosphoric acid exhibits

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better performance (curve B) than that made of cupric phosphate with the aid of sulfuric acid (curve C), indicating that the nature of impregnating acid plays an essential role. Although the reason why is not clear, we may point out two possibilities. The first one is related to the relative volatility of the two acids. Although sulfuric acid is not so volatile by itself, but it readily escapes in the form of mist into the moist atmosphere. On the contrary, phosphoric acid is thought to remain stably over the silica surface by forming a liquid film. If this is the case, copper ions will get mobility in the liquid film to enhance the chance of contact with immobilized Pd sites. Another possibility is that phosphate group blocks the surface sites of palladium so as to reduce number of sites available for hydrogen adsorption. The decrease in surface concentration of hydrogen will retard the production of water by consuming useful intermediates, %O,, HO,, and OH, and thus the relative yield of phenol will increase.

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Fig. 4 Reactivity of the catalyst prepared from copper phosphate. catalyst: 500Cu-50Pd/SiO, (2g) with acid, Phosphoric acid; 3mmol/g, Sulfuric acid; 4.5mmol/g, A;cupric phosphate with phosphoric acid, B;cupric sulfate with phosphoric acid, C;cupric phosphate with sulfuric acid, D;cupric sulfate with sulfuric acid, Other conditions were the same as Fig. 1. Fig. 5 Effect of catalyst nature on the yield of carbon dioxide. All conditions were the same as Fig. 4.

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Regarding the latter possibility, we also determined yields of water and carbon dioxide besides the phenol yield shown in Fig. 4. Interestingly, the yield of water was not affected by the nature of catalysts and the rate of its production was roughly 5 mmol/h,g-cat. for all the catalysts studied. In contrast, a large effect of catalyst nature appeared in the yield of carbon dioxide as is shown in Fig.5. It is quite interesting to see that the catalyst efficient for phenol production is inefficient for carbon dioxide and vice versa. These results will indicate that the water forming reaction should be distinguished from reactions forming phenol and CO,, since the former reaction was not affected by the process of catalyst preparation. Probably the former reaction occurs exclusively on the palladium sites and the latter two on the copper sites, A mechanistic analysis indicated that the rate of phenol producing reaction can be well explained on this view. Detailed report will follow successively [12]. Fig. 6 shows the change in catalytic activity of the phosphate based catalysts as a function of Pd content, y. The scale of ordinate is expressed in terms of the production rate. Note that the scale for water is expressed in terms of mmol, while others are in pmol. In contrast to the similar study with the sulfate based catalyst (Fig.2 B), the curve for phenol rises steeply at the region of very low value of y and becomes flat at y>5. The corresponding rate is 350pmol (h,g-cat)-l. which is almost one order of magnitude higher than the rate with the sulfate based catalyst. The rate of water production increases, on the other hand, rather slowly and the increase still continues 500 -6

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Fig. 6 Typical performance of the phosphate based catalyst. Catalyst: 500Cu-yPdlSi0, (2g), cupric phosphate with phosphate acid, Other conditions were the same as Fig. 1,

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gradually at the region of higher Pd content. As a result, the percentage conversion of hydrogen to water increases from 50% at y =5 to 87% at y =SO. The use of the 5 Pd catalyst is by all means advantageous, e . g . , cost saving due to low Pd content, high rate of phenol production, decreasing the wasteful consumption of hydrogen, high turnover value, and SO on. The production of carbon dioxide is very small at the whole range of y: conversion of benzene to CO, being less than 0.01 % . The amount of phosphoric acid impregnated in the catalyst was found to have an appreciable effect on the phenol yield. Although 3 mmol of phosphoric acid per gram silica was impregnated in the above experiments, the phenol yield increased definitely by reducing the amount of acid to 2 mmol, which is the lowest limit for completing the dissolution of the phosphate salt. The rate is 470 pmol (h.g-cat)-' and the turnover frequency per hour regarding Pd is more than 90. Corresponding conversion of benzene to phenol is 1.1 %.

Acknowledgement This work was financially supported by Japan Polyurethane Industry Co. Ltd., to whom we are indebted. References T. Jintoku, H. Taniguchi and Y. Fujiwara, Chem. Lett. (1987) 1865. 1. N. Kitajima, M. Ito, H. Fikui, Y. Moro-oka, Extend. Abstr. 21 Symposium on Oxid. 2. Reactn., Nagoya 1988, 92. E. Kimura and R. Machida, J. Chem. SOC.,Chem. Commun. (1984) 499. 3. 4. M. Iwamoto, J. Hirata, K. Matsukami and S. Kagawa, J. Phys. Che., 87 (1983) 903. 5. T. Kitano, Y. Kuroda, A. Itoh, L-F. Jiang, A. Kunai and K. Sasaki, J. Chem. SOC., Perkin Trans. 2, (1990) 1991. Y . Kuroda, M. Mori, A. Itoh, F. Yamaguchi, T. Kitano, K. Sasaki and M. Nitta, J. 6. Mol. Catal., 73 (1992) 237. 7. A. Itoh, Y . Kuroda, T. Kitano, Z-H. Guo, K. Sasaki, J. Mol. Catal., 69 (1991) 215. 8. Y. Kuroda, A. Kunai, M. Hamada, T. Kitano, S. It0 and K. Sasaki, Bull. Chem. SOC. Jpn, 64 (1991) 3089. 9. A . Kunai, T. Kitano, Y. Kuroda and K. Sasaki, Catal. Lett., 4 (1990) 139. 10. T. Kitano, T. Wani, T. Ohnishi, Jiang L-F., Y. Kuroda, A. Kunai and K. Sasaki, Catal. Lett., 11 (1991) 11. 11. T. Kitano, Y . Kuroda, M. Mori, S. Ito, K. Sasaki and M. Nitta, J. Chem. SOC., Perkin Trans. 2, in press. 12. T. Kitano, M. Nitta, K. Sasaki, to be published.

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DISCUSSION CONTRIBUTION J.-M. BREGEAULT (Univ. Pierre et Marie Curie, Paris, France): Did you try to analyze the Pdo and Pd2+ contents of the catalyst prepared from cupric phosphate with the aid of phosphoric acid (which exhibits the largest activity) and can you compare the results with the other catalysts? T. KITANO (Hiroshima Univ., Higashi-Hiroshima, Japan): As far as the Pd content i n catalyst is sufficiently high (e.g. 100 pmol/g-SiO,), the existence of metallic Pd i s detectable as we have done using XRD, ESCA and CO adsorption measurements (1). The detection with the present catalyst was failed, however, because of its too low Pd content ( < 10 pmol Pd/g-SiO,). (1) Y . Kuroda et ul., J. Mol. Catal., 73 (1992) 237.

T. MALLAT (Swiss Federal Inst. Technol., Zurich, Switzerland): You suggested that Pd" activates hydrogen and Cu' activates oxygen, and the formed hydrogen peroxide reacts with benzene. On the contrary, I suggest that both oxygen and hydrogen are activated by (metallic) palladium and the Cu+"+ system catalyses the oxidation of benzene to phenol with H202(or OH) and suppresses the inefficient decomposition of H,O, to H,O and 0,. What is your opinion? T. KITANO (Hiroshima Univ., Higashi-Hiroshima, Japan): We do not deny your suggestion as a possibility. But, we rather assume the mechanism proposed here, because the kinetic analysis based on the assumed mechanism explains the experimental results quite well (2).

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V. CortCs Corberan and S. Vic Bellttn (Editors), New Developments in Selective O x i d d o n II 0 1994 Elsevier Science B.V. All rights reserved.

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Low Temperature Gas-Phase Selective Oxidation of 1-Buteneto 2- Butanone on Supported P W z O 5 Catalysts Gabriele Centi, Matteo Malaguti and Giuseppina Stella

Dept. of Ind. Chem. & Materials, V.leRisorgimento 4,40136 Bologna (Italy), Fax: +39-51-644-3680. The influence of the reaction conditions and some aspects of the reaction network and changes in surface reactivity during the catalytic reaction in the selective oxidation of 1-buteneto 2-butanone (methyl ethyl ketone, MEK) over supported Pd/V205 based catalysts at reaction temperatures of around 120°C are reported. In partid a r , the change in the selectivity to MEK as a function of time-on-stream, the effect of a higher temperature treatment, the influence of the 1-butene, 0 2 and H20 concentrations and the reaction temperature, the nature of the support and the surface reactivity of MEK are shown. It is suggested that the catalytic behavior of Pd/V205 on alumina catalysts, characterized by the presence of a maximum in the yield and selectivity to MEK, derives &om the presence of a rate controlling effect of the desorption of MEK 1. INTRODUCTION

The development of catalytic systems active at low temperature in heterogeneous gas-phase reactions of selective oxidation is an interesting and promising new field of research, because it offers the opportunity to develop a new type of selective oxidation reaction and a new type of applications (for example, the synthesis of fine chemicals which, at higher reaction temperatures, may decompose). On the other hand, heterogeneous gas-phase processes offer distinct advantages with respect to homogeneous reactions as regards separation costs and therefore it is interesting to explore new synthetic possibilities for heterogeneous gas-phase selective oxidation processes. However, limited examples exist in the literature for low temperature selective oxidation catalysts and the Pd/V205 system is one of the few selective catalysts. In this system, the noble metal is the active component, but the transition metal oxide is required for reoxidation of the noble metal reduced by the catalytic oxidation. This catalytic system may be viewed as an heterogeneous counterpart of the homogeneous Wacker catalysts based on Pd and a second element (CuClz or more recently V- heteropolyacids [l]),the function of which is to facilitate the reoxidation of Pdo to Pd2+by gaseous oxygen. Supported Pd/V205 catalysts have been studied mainly for the

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selective oxidation of ethylene to acetaldehyde 12-61, but recently the synthesis of butanone (methyl ethyl ketone, MEK) from 1-butenehas also been reported [71. MEK is a valuable industrial product with a world production of over one million tons per year, but currently is mainly synthetized using a two-step process involving the hydration of 1-butene to form 2-butanol followed by the gas-phase catalyzed dehydrogenation to MEK.The direct one-step catalytic oxidation of 1-butene to MEK through a Wacker-type mechanism thus appears to be an interesting industrial alternative. On the other hand, there are various other reasons for interest in this reaction. In fact, the synthesis of MEK from 1-butene through a Wacker-type mechanism involves a type of mechanism of selective heterogeneous oxidation different from known examples, such as the allylic- type mechanism (acrolein from propylene) where after the abstraction of an allylic H and formation of a n-bonded ally1 complex there is the selective insertion of a structural 0 atom from the transition metal oxide. In the heterogeneous Wacker-type mechanism there is an addition of a hydroxyl group to a coordinated olefinic molecule (hydroxypalladation)followed by rapid rearrangement and H-transfer. The oxygen inserted in the hydrocarbon derives thus from the water molecule and not from gaseous oxygen or from the transition metal oxide. Therefore, it is interesting to analyze the synthetic potentialities of this different type of mechanism of selective heterogeneous oxidation, but the limited information available in the literature about this kind of reaction indicates the necessity for extensive investigation on the key factors governing the surface reactivity of the Pd/V205 catalysts in the low temperature oxidation of 1-butene. This reaction, in fact, is both interesting for its potential industrial application and as a model reaction for the study of the reactivity of noble metals on transition-metal-oxideselective oxidation catalysts. 2. EXPERIMENTAL

Supported Pd/V205 were pre ared by an incipient wet impregnation method using an aqueous solution of VO'-oxalate (obtained by reduction of NH4V03 with H2C202) and microspherical yA1203 (Rhone-Poulenc 535), Ti02 or Ti02-Al203 (20% wt as alumina) pellets as the supports. Titania-based supports were prepared by a gelsupported precipitation method as reported elsewhere [81. After drying and calcination at 400°C (5 h), the supported vanadium samples were further impregnated with an aqueous solution containing PdC12 and NaCl (molar ratio 1:8) and then dried at 120°C. The final molar composition of the samples was 0.98% PdC12, 7.63% V205, 7.84%NaCl, (% in moles), and the remainder being the support ( d 2 0 3 , Ti02 or TiO2d203).

Catalytic tests were carried out in a glass continuous-flow fixed-bed microreactor working at atmospheric pressure. An axial thermocouple allowed the control of isothermicity of the catalytic bed. The composition of the feed was regulated using a series of flow controllers to mix already calibrated gas mixtures stored in cylinder. Water was dosed to the feed using a high-precision infusion micropump. Reagents

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Fig. 1 Dependence of the reactivity at 120°C of PdNzO5 on alumina catalyst on the time-on-stream in 1-butene oxidation. Reaction conditions as reported in the experimental part. and reaction products were analyzed using an on-line gas chromatograph equipped with a Porapak QS column and a flame-ionization detector. Any eventual formation of carbon oxides and the conversion of 0 2 was monitored using a separate gas chromatograph operating with a Carbosieve-II column and a thermoconducibility detector. If not otherwise indicated, the usual reaction conditions were 0.8% 1-butene, 20% 0 2 and 20% H20 in helium. The total flow rate (at room temperature) was 3.6 L/h (at STP and including water) using 2.5 g of catalyst.

3.RESULTS AND DISCUSSION 3.1 Transient Change in Surface Reactivity Reported in Fig. 1 is a typical change in the surface reactivity of Pd/V205 on alumina catalyst in the selective oxidation of 1-butene at 120°C as a function of the time-on-stream. The conversion is initially high, but progressively decreases to a nearly constant value after approximately 4-6 hours of time-on-stream. The yield to MEK, on the contrary, is initially very low and then increases up to a maximum value (in the 30-40% range) after around one hour. For longer times, the yield of MEK reaches a nearly constant value of around 20-25%. Consequently, the selectivity to MEK also passes through a maximum value (about 70-75%). The selectivity to the other products [mainly acetaldehyde (Acet) and acetic acid (HAc)] is generally low at this reaction temperature (lower than 5%).Carbon oxides and acetone are also detected with selectivities lower than about 1-2%.The carbon balance is always lower than loo%, especially for the shorter contact times indicating the probable formation of

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Fig. 2 Comparison of the infrared spectra (KBr disc technique) of PcW2O5 on alumina before (a) and after 6 hours of time-on-stream in 1-butene oxidation (b). The difference spectrum is also reported (b-a).

sigruficant amounts of products which do not desorb from the catalyst surface at 120°C. In fact, the deactivation observed is not irreversible and thermal treatment at a higher temperature (300°C in air, but also in N2) can restore the initial catalytic behavior. In particular, the yields of MEK and the dependence on time-on-stream observed in Fig. 1 can be repeated several times in subsequent cycles of reaction with an intermediate higher temperature treatment. During this high temperature treatment, the desorption of various reaction products (mainly MEK, acetaldehyde and acetic acid, but also 1-butene) is observed indicating their strong adsorption on the catalyst surface during the catalytic reaction. In fact, the comparison of the infrared spectra of the fresh Pd/V205 on alumina catalyst with the same sample after 6 hours of time-on-stream evidences the presence of additional bands in the 1350-1800 an-'region (Fig. 2) attributed to vc=oin MEK and acetic acid or acetaldehyde (1710 and 1720 an-', respectively) and to carboxylate and carbonate species (1570 an-' and 1470 and 1410 an-'), confirming above indications. The gas chromatographic analysis of the products of thermal desorption from the catalyst after the catalytic tests confirm the presence of large amounts of MEK and acetic acid on the surface of the catalyst. It may thus be concluded that significant amounts of reaction products, in addition to water, are adsorbed on the deactivated Pd/V205 on alumina catalyst and that the high temperature treatment induces the desorption of these species.

3.2 Influence of the Reaction Conditions Summarized in Table 1 is the dependence of the catalytic activity of Pd/V205 on alumina catalyst on reaction temperature and concentrations of 1-butene, 0 2 and H20 in the feed. Reported as indexes of the catalytic behavior are the yield and selectivity

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Table 1 Effect of the reaction temperature and of the l-butene, 0 2 and H20 Concentration on the selectivity and yield to MEK afber 1or 6 hours of time-on-stream in the oxidation of l-butene over PcW2O5 on alumina catalyst.

to MEK after 1 hour of time-on-stream [transient (7') activity, corresponding approximately to the maximum yield of MEK - see Fig. 11 and after 6 hours [corresponding to the nearly steady-state (NSS) behavior - see Fig. 11. The increase in the reaction temperature in the 120-180T range decreases both the T and NSS yield and selectivity due to a parallel increase in the selectivity to by-products (acetaldehyde, acetic acid, acetone and carbon oxides). At 180°C low selectivities to MEK are found and acetaldehyde becomes the main reaction product, even though the conversion of 1-butene at the NSS conditions increases to about 55%.Reaction temperatures lower than 120°C make it possible to obtain higher selectivities to MEK (up to over 90%), but with low yields due to low l-butene conversion. The increase in concentration of l-butene from 0.8 to 3.0% leads to a decrease in both T and N S S formation of MEK, but not a corresponding increase in the selectivity to by-products. Only the formation of adsorbed products increases and thus more rapid deactivation is observed. The effect of the 0 2 and H20 concentrations is also shown in Table 1. In both cases, the maximum in the selectivity and yield is observed for an intermediate value (around 20%). In the case of 0 2 , the selectivity after 1 hour increases decreasing the 0 2 concentration. This is probably related to a double role of 0 2 . Oxygen not only is involved, together with vanadium oxide, in the reoxidation of palladium, but also is involved in the parasite reactions of l-butene (or products) transformation. The concentration of around 20% thus is probably the compromise between these two opposite effects, but for the shorter times-on-stream (where the vanadium oxide buffer capacity is not sensibly altered by the absence of oxygen) the negative effect of the parasite reactions prevails. Therefore, the maximum selectivity to MEK after 6 hours is observed for a concentration of 0 2 of around 20%, but after 1

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Table 2 Comparison of the catalytic behavior in 1-butene oxidation and surface areas of PdN2O5 supported on different oxides. Reaction conditions as reported in the experimental part.

hour a higher selectivity (even though low yield) is shown in the absence of 0 2 . In the case of H20, on the contrary, both the yield and selectivity to MEK passes through a maximum for a concentration of around 20% after 1 or 6 hours of time-on-stream. This is related to the role of H20 in the reaction mechanism, but also to a probable saturation of the active sites at the higher H20 concentrations. Indeed using 40% H20, a decrease in the conversion of I-butene is observed together with a decrease in the selectivity to MEK. It is also interesting to note that in the absence of H20 in the feed, an initial selective formation of MEK is observed, but the activity rapidly declines in around 1 hour of time-on-stream. The initial selectivity (after 5 min) is over 80 % with a yield of MEK of over 45% and thus higher than in the presence of H20. This apparently may contradict the role of water in the reaction mechanism A preliminary dehydration of the catalyst at 300°C before the catalytic tests, however, shows that MEK does not form in the absence of water vapour when the water adsorbed on the catalyst is also removed. This confirms the role of water in the synthesis of MEK on this catalyst, but also evidences that probably absorbed water is more reactive for the reaction and that coordination of water inhibits the surface reactivity. 3.3 Influence of the Nature of the Support

The effect of the nature of the support for Pd-doped vanadium-oxide is summarized in Table 2 where the B.E.T. surface area (before and after the catalytic tests) and the catalytic behavior of the samples supported on alumina, titania and a titaniaalumina (20% wt of A12031 mixed oxide are reported. Contrary to that found by in our case titania-based samples show worse performances in Scholten et al. [7,9], comparison with Pd/V205 on alumina, especially in terms of selectivity to MEK. However, it should be noted that the catalytic behavior roughly correlates with the surface area of the samples, in agreement with the indication that the catalytic behavior of these samples is influenced negatively by the presence of a strong adsorption of reagents/produds of reaction which reasonably is less relevant in the higher

467

100

-

A A1203 t V205-A1203 PdN205-AI203

X Sum ylelds prod.

0

50

100

150

Time, min

Fig. 3 Conversion of 2-butanone (MEW at 120°C on alumina, v205-&03 and Pdon alumina samples as a function of time-on-stream. Reaction conditions as in the exp. part, but feeding MEK instead of 1-butene. The sum of the yields of acetone and acetaldehyde (see also text) in the case of PdN2O5 on alumina is also reported.

V2O5

surface area samples. 3.4 Surface Reactivity of MEK

Figure 3 summarizes the results of tests done at 120°C feeding MEK instead of 1butene together with 0 2 and H20. On pure alumina, MEK is initially strongly adsorbed on the sample without any formation of reaction products. When vanadium-oxide is supported on alumina a strong adsorption of MEK is also noted, but in this case some reaction products are also formed (mainly acetaldehyde, but also acetone) which explain, however, only about 30% of the carbon balance (the remaining being due to adsorbed species). On the catalyst (Pd/V205 on alumina) the initial adsorption of MEK is also high with the formation of acetone and acetaldehyde (in a ratio of about 2:l) the amount of which progressively increases with time-on-stream. It should be noted that in the absence of gaseous 0 2 a stronger adsorption of MEK is observed with limited formation of only acetone. The formation of acetone and acetaldehyde thus mainly derives from the surface reaction of MEK in the presence of gaseous 0 2 , even though acetone may also form from MEK in anaerobic conditions. 1-Butene, on the contrary, gives rise to a limited adsorption on the samples without Pd. Reaction products are not formed in the absence of 0 2 , whereas they are formed in the presence of 02/H20 acetic acid and acetaldehyde, especially at higher reaction temperatures, in agreement with Takita et al. [lo]. On the other hand, it is reasonable to assume that the formation of significant

468

amounts of adsorbed products during the catalytic reaction (of the order of lo4 moles per gram of catalyst after 1 hour of time-on-stream) inhibits the surface reactivity of the catalyst leading to a decrease in the conversion and selectivity to MEK. The maximum in the yield and selectivity to MEK observed as a function of the time-on-stream (Fig. 1) may thus be interpreted as deriving from a rate controlling effect of MEK desorption: initially the selectivity and yield increases due to the increased amount of adsorbed MEK which, on the other hand, leads to a deactivation of the surface reactivity. There is thus a progressive further decrease in the yield and selectivity until nearly-steady-state conditions are reached (see Fig. 1) characterized, however, by lower yields and selectivities to MEK as compared to the maximum values observed. 4. CONCLUSIONS

The catalytic behavior of Pd/V205 on alumina catalysts is characterized by the presence of a maximum in the yield and selectivity to 2-butanone (MEK) which derives from the presence of a rate controlling effect of the desorption of MEK. The analysis of the influence of the reaction conditions on the transient and nearly-steady-state activity of the catalyst and the effect of the nature of the support are in agreement with this indication showing one of the limits to be overcome in the design of low temperature catalysts for selective oxidation reactions. 6. ACKNOWLEDGEMENTS

Financial support from Koninklijke/Shell-Lab., Amsterdam (The Netherlands) for this research is gratefully acknowledged.

REFERENCES [ l l J. Vasilevskis, J.C. De Deken, R.J. Saxton, P.R. Wentrcek, J.D. Fellmann, L.S. Kipmis, PCT Int. Appl. WO 8701,615 (1987) assigned to Catalytica. [21 k B . Evnin, J.A. Rabo,P.H. Kasai,J. Catal.,30 (1973) 109. [31 L.Forni, G. Terzoni, I d . Eng. Chem. Proc. Des. Dev., 16 (1972) 288. [41 L.Forni, G. Gilardi, J. Catal., 41 (1976) 338. E51 E.van der Heide, M. Zwinkels, A. Gerritsen, J.J.F. Scholten, Appl. CutuZ.A: Gkmrul,86 (1992) 181. [61 E. van der Heide, M. de Wind, A.W. Gerritsen, J.J.F. Scholten, in Proc. 9th Int. Congress on catalysis, M.J. Philips and M. Ternan Eds., The Chem. Inst. of Canada Pub.: Ottawa 1988, Vol. 4,pag. 1648. [71 E.van der Heide, J A M . Ammerlaan, A.W. Gerritsen, J.J.F. Scholten, J. Molec. Catal., 55 (1989) 320. [81 G. Brambilla, G. Centi, S. Perathoner, A. Riva, in Proc. 2* Europ. Conf on Advanced Materials, T.W. Clyne and P.J. Withers Eds., The Institute of Materials Pub: London 1992, Vol. 3, p. 287. [91 A.W. Stobbe-Kreemers, M. Soede, J.W. Veenman, J.J.F. Scholten, in Proc. 10th Int. Congress on Catalysis,Budapest 1992. [lo1 Y. Takita, K.Nita, T. Maehara, N. Yamazoe, T. Seiyama, J. CutuZ., 50 (1977) 364.

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J.-M. BREGEAULT (Catalyse et Chimie des Surfaces, Univ. P. et M. Curie, Paris, France): Are your results in favour of a heterogeneous Wacker-type mechanism? Why can you consider nucleophilic attack by a hydroxyl group (see your introduction)?What is the origin of this hydroxyl group? (It is worth noting that the mechanism in Homogeneous Catalysis involves nucleophilic attack by a water molecule). G. CENTI (Dip. Chimica Ind. e dei Materiali, Bologna, Italy): We have not studied in detail the mechanism of reaction using labelled molecules and thus our interpretation of the reaction mechanism is only based on the effect of the reaction variables and of the reactivity of the possible intermediates. These results show a Wacker-type mechanism with only some difference in the mechanism of reoxidation of reduced vanadium-oxide. The hydroxyl group responsible of the addition t o the olefin coordinated in the allylic form derives &om the activation of water molecule by Pd. Our results suggest that adsorbed water be apparently more active in this reaction than gaseous water. On the other hand, we noted that Pd-V205 on alumina is more active in the oxidative dehydrogenation of see-buthyl alcohol t o MEK suggesting that a hydration-dehydrogenation mechanism is also possible. Present data, however, cannot further discriminate between these two hypotheses of reaction mechanism.

H. MIMOUN (Firmenich, Geneve, Switzerland): People have been trying for about 20 years to make stable supported Wacker catalysts. You reoxidize your catalyst by 0 2 why other reduce the catalyst. What should be done according t o your opinion? G. CENT1 (Dip. Chimica Ind. e dei Materiali, Bologna, Italy): Our results clearly show that two phenomena contribute in the deactivation of heterogeneous Wacker-type catalysts: the progressive reduction of vanadium-oxide (the rate of reoxidation is lower than the rate of reduction) and the formation of significant amounts of adsorbed reagentfproducts (reasonably these adsorbed products also limit the reoxidation of reduced vanadium-oxide). The treatment at higher temperature in 0 2 eliminates both negative effects and thus allows the regeneration of the e@yst. In fact, for the synthesis of MEK from 1-butene is necessary to have Pd ions. Reduced Pd is not active for this reaction. Some authors have prereduced similar kind of catalyst, but these samples were used for other type of reactions. These authors reduced the catalyst to make an alloy with a second element such as Sb. This alloy is then converted in the presence of gaseous oxygen, but the catalysts possess different properties than before the reduction. However, these catalysts are not active for the synthesis of MEK. 0. KRYLOV (Inst. of Chem. Physics, Russian Acad. of Sciences, Moscow, Russia): In our laboratory in the Institute of Chemical Physics (V. Bychov et al.)interesting phenomena of self-activation of V2O5 in the presence of small amounts of Pt were observed. During methane oxidation at low temperature (150-200°C) it was possible to evolve up to 20% oxygen without destruction of lattice. Perhaps, your effect of initial activation of PdN2O5 is due to the same reason, i.e. the formation of active vacancies. Did you not measure XRD in situ? G. CENTI (Dip. Chimica Ind. e dei Materiali, Bologna, Italy): Unfortunately, va-

410

mdium-oxide on alumina is amorphous and therefore any information can be derived from XRD. About the possibility that the induction time is connected to the formation of vacancies in vanadium-oxide, I do not believe that this can be applied for the synthesis of MEK. In fact, we observe an initial decrease of the conversion of 1-butene and a strong increase in the selectivity to MEK with a contemporaneous lack of carbon balance. It is diflicult to explain these phenomena with the hypothesis of formation of oxygen vacancies in vanadium-oxide. We also know both from IR data (Fig. 2) and from thermodesorption data that significant amounts of MEK remain adsorbed on the catalyst. We also know that MEK is adsorbed in significantly high amounts on the catalyst. All these indications thus suggest that MEK be selectively formed during the initial activation time, but do not desorb. It is corrected, however, that further studies are necessary to better investigate this initial stage.

E. BORDES (Univ. Technol. de Compiegne, France): Since you workoat very low temperature, the mobility of 0-V is very low, so the reoxidation of Pd is difficult. I noticed that when temperature increases selectivity in MEK decreases on PdV205/A2203. Did you observe the same trend with Pd-V205pI’i02,the oxygen mobility of which should be higher?

G. CENTI (Dip. Chimica Ind. e dei Materiali, Bologna, Italy): Increasing the reaction temperature, the decrease in the selectivity is due to the increase in the rate of the parallel reaction of oxidative cleavage of 1-butene with formation of carbon oxides and products like acetic acid. Vanadium-oxide is mainly responsible of these side reactions at high temperatures. On the other hand, we do not observe significant difference regards the effect of the reaction temperature in the case of Pd on vanadium-oxide supported on T i 0 2 or Al2O3. It should be noted that in o u r samples a loading in vanadium-oxide on alumina up to around 10% wt does not lead to the appearence of any detectable crystalline V205. This shows the good dispersion of vanadium-oxide also in the case of alumina support. Therefore, probably, we do not observe significant differences from samples supported on titania. Other authors in the case of the oxidation of ethylene to acetaldehyde, however, found superior performances for the samples supported on titania.

E.A. MAMEDOV (Inst. Inorganic Physical Chemistry, Baku, Azerbaijan): On of the most effective catalysts for propylene oxidation to acetone is tin-molybdenum and tin-titanium oxides that show very stable activity and do not need the regeneration. Did you test these systems in the oxidation of 1-butene? If you did, please compare them to your catalyst.

G. CENTI (Dip. Chimica Ind. e dei Materiali, Bologna, Italy): Propylene oxidation to acetone is a much easier reaction than 1-butene oxidation to MEK. Pd-VZ05 on alumina samples also shows a high activity and stability in acetone synthesis differently from the synthesis of MEK. On the other hand, tin-molybdenum catalysts show a low selectivity in the oxidation of 1-butene to MEK. In addition, the interest of these heterogeneous Wacker-type catalysts is for their low temperature activity and for the possibility of application to other similar type of selective oxidation syntheses involving a Wacker-type mechanism for which tin-molybdenum samples are not effective catalysts.

V. Cort6s Corberan and S . Vic Bell6n (Editors), New Developments in Selecrive Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

47 I

The adsorption of oxygen on Ag and Ag-Au alloys: Mechanistic implications in ethylene epoxidation catalysis Dimitris I. Kondarides and Xenophon E. Verykios Institute of Chemical Engineering and High Temperature Processes. Department of Chemical Engineering, University of Patras. P.O. Box 1414, GR 26500 Patras, Greece. The adsorption of oxygen on Ag and Ag-Au alloy catalysts supported on aX-AL,O, is investigated employing microgravimetric, TPD and TPSR techniques. It is shown that oxygen exists on Ag surfaces in three modes: molecular, atomic and subsurface. Alloying with Au results in enhancement of the molecularly adsorbed species, weakening of the atomic oxygen-Ag bond and gradual reduction of the subsurface oxygen species. Turnover frequencies of epoxidation and combustion drop to zero over alloy surfaces containing 40% Au or more. 1. INTRODUCTION The mechanism of ethylene epoxidation on Ag surfaces has attracted considerable interest

due to the industrial as well as fundamental importance of this catalytic process [1,2]. Significant controversy exists concerning mechanistic details, particularly the mode of oxygen adsorption and its participation in surface transformations. In general, three kinds of oxygen species are believed to exist in the adsorbed mode under reaction conditions, atomic oxygen, mono-and/or multi-coordinated, molecular, and subsurface oxygen. The role of each of these species in ethylene epoxidation and combustion routes is still not well-understood. In the present study, the interaction of oxygen with pure Ag surfaces as well as with surfaces modified by alloying with Au is investigated employing various techniques. Ethylene epoxidation and combustion kinetics are determined over a wide range of ethylene and oxygen partial pressures, in the temperature range of 200-260°C and mechanistic information is derived by coupling results of different techniques. 2. EXPERIMENTAL 2.1 Catalyst preparation and characterization Supported Ag catalysts were prepared by impregnation of low surface area (- lm2/g) a A1203 with AgNO,, following a procedure described elsewhere [ 3 ] . Supported Ag-Au alloy catalysts were prepared by simultaneous impregnation of the support with known amounts of mixed silver cyanide and gold cyanide dissolved in an aqueous solution of ethylenediamine, at 7OOC. The resulting solid was dried at 110 "C, calcined at 400 "C for 10 h and reduced under H2 flow for 8 h, at 350 "C. Total metal loading of all catalysts was 15 wt%. Alloying was examined

by X-Ray Diffraction (XRD). Spectra were obtained in the range of 30 to 80", and the

472

diffraction peaks due to the (1 1 I), (200), (220) and (3 11) lattice planes were observed. The lattice parameter, a, and the mean crystallite size were calculated for each catalyst from the position and peak width at half-maximum intensity of the diffraction peaks, respectively. Surface composition of bimetallic Ag-Au catalysts was determined by XPS. The X-ray source was a Mg and A1 double anode while the exciting radiation used was the Mg-K, line (1256.3 eV). Carbon Clsi/, was used to calibrate binding energies. Surface composition of bimetallic particles was calculated from the Ag 3d5,2 and Au 4f,,, peak intensity ratios. XRD and XPS experiments were conducted on samples "as prepared" , i.e. without any prior pretreatment. In certain cases, similar analyses were conducted on catalyst samples which had been exposed to reaction conditions.

2 . 2 Oxygen adsorption studies Kinetic studies of oxygen adsorption were conducted with a vacuum ultramicrobalance in the temperature range of 40 to 32OoC, at oxygen pressures between 15 and 200 Torr, as described elsewhere [3]. The Ag-Aula-Al203 samples, in powder form, were placed in the microbalance sample pan and exposed to a number of oxidation-reduction cycles at 32OoC, to clean the surface and obtain reproducible results. Exactly the same pretreatment procedure was followed for all catalysts investigated. The sample was then reduced under H2 for 3 h, cooled to the adsorption temperature and exposed to a known pressure of oxygen for 15 h. Temperature programmed desorption (TPD) and surface reaction (TPSR) experiments were conducted using an apparatus which has been described elsewhere [3]. The catalyst samples, in powder form, were placed in a quartz microreactor and were exposed to exactly the same pretreatment procedure as for the gravimetric experiments. Oxygen adsorption occurred at temperatures between 25 and 4OO0C under 0 2 flow for 30 min. The sample was then cooled to room temperature and the system was purged with He to remove gas phase oxygen. It was subsequently heated, with a heating rate of 20°C/min, under 40 cc/min He flow for the TPD experiments or under 40 cc/min of 3.5% C2H4/He flow for the TPSR experiments. Oxygen and/or COz, C2H4 and C2H4O at the effluent of the microreactor were detected using a quadrupole mass spectrometer. For the measurement of C 0 2 in the mixtures C02/C2H40. the peaks at mass numbers and 29 and 44 were used. Fragmentation patterns of C2H40 (ratio of 44/29), as well as sensitivities of C 0 2 and C2H4O were obtained based on known calibration mixtures of the pure components in He. For the measurement of C2H40/C2H4, the peaks at 44 and 28 were used.

2 . 3 Kinetic experiments Kinetic experiments were conducted in the temperature range of 200 to 26OoC, at atmospheric pressure, employing a tubular stainless steel reactor encased in a furnace. Feed flow rates were measured and controlled with thermal mass flow meters and control valves, while the partial pressure of the reactants were varied over a wide range. Reactant and product gas compositions were analyzed chromatographically, employing a T.C. detector. Kinetic

473

experiments were conducted under conditions where intraparticle and interparticle heat and mass transport resistances did not influence measurable kinetic parameters.

3 . RESULTS AND DISCUSSION 3 . 1 Catalyst Characterization T h e bulk composition of Ag-Au alloy catalysts was examined using XRD. Diffraction peaks due to the (1 lo), (200), (220) and (3 11) lattice planes were observed in all cases. T h e appearance of a single peak from each plane indicates the existence of monophasic Ag-Au alloys, as expected, since Ag and Au are known to be completely miscible and to form solid solutions at any composition. The lattice parameter, a,, was estimated from the position of the diffraction peaks and the resulting values are shown in Fig.1 (curve a) as a function of bulk alloy composition. It is observed that a. decreases with increasing Au content and goes through a minimum at 60 at.% Au, in agreement with results of composition. AI1 The values of the other studies [4,5]. lattice parameter shown in Fig. 1 comFig.1: Lattice parameter (a) and surface pare favorably with values of bulk Agcomposition (b) of Ag-Au alloy catalysts Au alloys of similar composition as a function of bulk composition. The dashed reported in the literature [6].Small difline (c) is the surface composition of alloys ferences may be attributed to the fact without surface enrichment. that in the present study relatively small alloy particles are employed as opposed to massive alloys. XRD profiles were also used to estimate the mean diameter of the metal particles, which was found to be 96+4 nm over the entire series of alloy compositions. The variation of the mean particle size, within the stated range, was random with respect to alloy composition, indicating that, although the total metal atom content is different (due to the differences in atomic weights of Ag and Au), this does not influence measurably the mean particle size. The large size of the metal crystallites is primarily due to the low surface area of the carrier and the large metal content of the catalysts. Surface composition of the Ag-Au alloy particles was determined by XPS. Integrated line intensities obtained from the areas under the peaks in the scans were corrected for differences in photoionization cross section, sensitivity, and electron escape depth, to approximate relative

BUG

474

atom abundance and were normalized to the C1,1/2 photoionization line, which was used as a reference. Surface composition is shown in Fig. 1 (curve b) as a function of bulk composition, along with the 45O line (c) which indicates the surface composition expected if surface enrichment did not take place. It is apparent that an enrichment of the surface with Ag is observed. This is due to the lower melting point of Ag as compared to Au and to the higher affinity of Ag for oxygen, both of which factors favor enrichment of the surface with Ag. It must also be pointed out that the surface composition shown by curve b of Fig. 1 represents the average composition of the top 5-10 atomic layers. If the composition of the outermost layer could be determined, higher enrichment of the surface with Ag would have been observed.

3 . 2 Oxygen adsorption The rate of oxygen adsorption on the Ag-AuIa-Al203 catalysts was determined gravimetrically in the temperature range of 40 to 32OoC. It was found that the experimental oxygen uptake curves can best be described using the Elovich equation whose integrated form is the following:

1

where p and y are temperature-dependent parameters and to is defined by: to = (p y P). , where P is the oxygen pressure. The adsorption curves obtained with the 95% Ag-5% Au catalyst are plotted in the coordinates of the Elovich equation and are shown in Fig. 2(A). Similar results were obtained with all other catalyst compositions. It is observed that each curve consists of two intersecting linear segments indicating the existence of two kinetically distinguishable adsorption processes. The first process is dominant at low coverages, between 0.1 and 0.5, depending on adsorption temperature, while the second at higher coverages. In the analysis which follows it is assumed that the low-time adsorption process does not contribute significantly to the overall adsorption process which takes place at high-time values. Thus, in essence, the rapid adsorption process is assumed to be completed upon initiation of the slower adsorption process. Values of the parameters p and yare estimated from the slopes and intersects of the linear segments of Fig. 2(A). Arrhenius type plots of the parameter p, as shown in Fig. 2(B) can be used to estimate the activation energy of adsorption at zero surface coverage. Fig. 2(B) shows that the linear segments of Fig. 2(A) of low time values reveal the existence of two distinct adsorption processes with widely different activation energies. Thus, three oxygen adsorption processes on Ag-Au alloy catalysts are distinguished. These are attributed to almost non-activated dissociative adsorption which takes place at low surface coverages (low-slope part of line a), molecular adsorption (line b), and subsurface diffusion of adsorbed oxygen (high-slope part of line a). Alloying Ag with Au results in significant alterations in all three uptake modes. The influence of Au content of the catalysts on the activation energies of adsorption and subsurface diffusion is illustrated in Fig. 3. It is observed that the activation energy of dissociative oxygen adsorption (a) increases while that of molecular adsorption (b) decreases with increasing Au content. The activation energy of subsurface diffusion (c) increases drastically with increasing Au content, indicating that alloying of Ag with

475

Au strongly hinders this process. The microgravimetric experiments also reveal that the activation energy of dissociative oxygen adsorption on Ag is strongly dependent on surface coverage, E=E,+21.6 0, 0<0<0.5, while the dependence of the activation energy of the non-dissociative adsorption is

1 .o e,

M (d

k

0.8

Q)

0.6

u 4 0.4 k

7 0.2 0.0

Fig.2: (A) Adsorption curves of oxygen at various temperatures (PO =50 Torr), plotted in the coordinates of the integrated Elovich equation. (B) Arrhenius pfot of the Elovich parameter p calculated from the intersects of the first line segments of Fig.:! (A). Line segments (a) correspond to low time-values, while line (b) corresponds to high time-values. significantly weaker, E = E, + 9.3 0.This observation can be explained assuming that oxygen adatoms occupy multiatom sites, in which case, the preadsorbed oxygen species induce stereochemical restrictions to the process. The high values of E at non-zero coverage then describe the activation energy of formation of the multiplet sites by surface rearrangements. Molecular oxygen has been suggested to adsorb on mono-coordinated sites. The variation of E with surface composition (Fig. 3) can then be explained as follows: Gold acts as a diluent on the catalyst surface, destroying multiplet Ag sites required for dissociative adsorption, which is reflected in enhanced activation energy for adsorption with increasing Au content of the surface. On the other hand, the destruction of the multiplet sites favors molecular adsorption on monocoordinated sites, and thus reduced E. The increased activation energy of subsurface oxygen diffusion with increasing Au content is probably related to the reduced lattice parameter of the alloys (Fig. l), the larger size of the Au atoms relative to Ag, and the weakened Ag-0 interaction in the presence of Au (see below). The existence or not of an adsorbed diatomic oxygen species on the surface of Ag at elevated temperatures has been a subject of controvercy. In the present study, the presence of adsorbed diatomic oxygen was verified by Surface Enhanced Raman Spectroscopy (SERS) [7]. A Raman band at 815 cm-l was observed at temperatures as high as 35OoC which did not scramble oxygen isotopes. On the basis of theoretical calculations this band was assigned to the v (0-0)vibration of a molecularly adsorbed oxygen, probably lying with its axis perpendicular to the Ag surface. In contrast, other investigators [8,9] have reported that molecularly adsorbed oxygen dissociates andor desorbs at temperatures as low as -lOO°C.

476

0' '

0 10 20 Surface comp., atom

' '0 30 "/,,Au

Fig.3: Variation of activation energies of dissociative (a) and molecular (b) oxygen adsorption at zero surface coverage, and of subsurface oxygen diffusion (c), with surface composition of Ag-Au alloy catalysts.

Further evidence for the existence of three oxygen adspecies on Ag and Ag-Au alloy surfaces is provided by TPD experiments. Typical TPD spectra obtained over the 95 at% Ag-5% Au catalyst are shown in Fig.4 (A) for adsorption temperatures between 100 and400°C. The species desorbing at low temperatures (T<2000C) is assigned to weakly adsorbed molecular oxygen. The fact that the mass spectrometer signal at low temperatures is due to oxygen originating from the catalyst surface (desorbing) and not from other sources was verified experimentally with blank experiments. The peak at approximately 285OC is assigned to atomically adsorbed oxygen, and the hightemperature peak to subsurface oxygen, as suggested by results of isotope exchange experiments followed by TPD [3]. The subsurface oxygen species appears only when adsorption takes place at high temperatures and its quantity increases with

2001 , Tad ("C) 150 -

a: 100 b: 200 c: 300

Surface

Tad :400"C

Au

300 a:

0

250

0"

200

a

150

El00 -

a

-

50 -

100 50

a

OO

200

T

400

("c)

600

0

200

T

400

600

I

("c)

Fig.4: (A) TPD spectra obtained after oxygen adsorption at 100, 200, 300 and 4OO0C for 30 min. Surface Ag-Au composition: 95-5 atom%. (B) TPD spectra obtained after oxygen adsorption at 4OO0C for 30 min on catalysts with 0, 5 , 15 and 24 atom% Au (surface).

477

increasing adsorption temperature. The mode of oxygen adsorption is significantly affected by the presence of Au on the surface, as illustrated in Fig. 4(B) in which T P D spectra obtained over aloy catalysts of different surface composition, following adsorption at 4OO0C, are shown. It is apparent that the peak which corresponds to subsurface oxygen decreases in magnitude with increasing Au content of the surface and is essentially eliminated over surfaces which contain 24 at.%Au or more. Furthermore, the peak which corresponds to dissociative oxygen shifts towards lower temperatures with increasing Au content of the surface which implies that the Ag-0 bond is weakened in the presence of Au. From similar TPD experiments of the Ag-Au alloy catalysts of variable surface composition the following observations have been made:

Po. :0.0990 atm P=:0.0225

atm

30 .

25-

C2H40

Po. :0.0990 atm PGH,:0.0225 atrn

10 30, 20 Surface comp., at. /o

40

Au

Fig.5: Turnover frequencies of ethylene epoxidation and combustion at various temperatures, as a function of surface Ag-Au composition. i) The population of molecularly adsorbed oxygen is enhanced relative to that of atomic oxygen with increasing Au content of the surface. ii) The atomic oxygen TPD peak shifts to lower temperatures with increasing Au content indicating that the presence of Au atoms results in weakening of the A g - 0 bond. iii) Subsurface oxygen diffusion is strongly hindered by AU and is essentially non-existent in catalysts containing 24 at.% Au on the surface, or more. iv) The enthalpy of adsorption of atomic oxygen, estimated from shifts of peak temperature with heating rate in the T P D experiments, decreases linearly with increasing Au content of the surface from approximately 40 kcaVmo1 over pure Ag to - 17 kcal/mol over a surface containing 24% Au. These observations indicate that in addition to geometric influences induced by the presence of Au atoms on the catalyst surface, electronic type interactions might also be operable. A net transfer of charge from Ag to Au is taking place upon alloying due to the different electronegativity of the two metals. The induced electron defficiency of Ag atoms affects the mode of oxygen adsorption, leading to weakening of the A g - 0 bond, since the adsorption of oxygen is an electron acceptor process. The weakening of the A g - 0 bond is proportional to the Au content of the surface, as would be expected from near-neighbor electronic interaction considerations.

478

3 . 3 Kinetic measurements Turnover frequencies of ethylene epoxidation and combustion over Ag-Au/a-A1203 catalysts were determined in the temperature range of 200 to 26OoC, varying the partial pressures of both reactants over a wide range. TOFs obtained at various temperatures are shown in Fig. 5 as a function of surface composition of the catalysts. Both TOFs, of epoxidation and combustion reaction, increase with increasing Au content of the surface, pass through a maximum at 5-15 at.% Au, depending on reaction temperature, and drop to zero when the Au content of the surface reaches 40 at.%. The variation of TOF is correlated with the variation of the enthalpy of adsorption of atomic oxygen, which is a measure of the Ag-0 bond strength. Lowering of AH results in decreased surface coverage but higher reaction rates due to weakened adsorbate-metal bond. When AH becomes very low (at high Au contents) the oxygen surface coverage is very low and the reaction rates decrease. This reasoning explains the "volcano-type'' dependence of TOFs on the Au content of the surface. The shift of the maximum toward higher Au content with increasing temperature is related to subsurface diffusion of oxygen which seems to be essential for the reactions to take place. Subsurface oxygen diffusion is favored at higher temperatures and hindered by high Au contents. The fact that TOFs drop to zero when the surface contains 40% Au or more is due to very weakly adsorbed atomic oxygen and lack of subsurface oxygen on these catalysts. Finally, the nearly identical variation of TOFs of both reactions with surface composition implies that both reaction routes proceed via identical surface elementary steps and that both products originate from the same intermediate species, the formation of which is probably the rate controlling step of the process.

REFERENCES 1. 2. 3. 4. 5. 6.

X.E. Verykios, F.P. Stein and R.W. Coughlin, Catal. Rev. Sci. Eng., 22 (1980) 197. R.A. Van Santen and H.P.C.E. Kuipers, Adv. Catal., 35 (1987) 265. D.I. Kondarides and X.E. Verykios, J. Catal., 143, (1993) 481. N. Toreis and X.E. Verykios, J. Catal., 108 (1987) 161. R.N. Herrera, E.N. Martinez and A. Varma, AIChE Ann. Meeting, Chicago L, 1985. W.B. Pearson, Handbook of Lattice Spacings and Structure of Metals, Pergamon, London, 1967. 7. D.I. Kondarides, G.N. Papatheodorou, C.G. Vayenas and X.E. Verykios, Ber. Bunsenges. Phys. Chem., 97 (1993) 709. 8. B.A. Sexton and R.J. Madix, Chem. Phys. Lett., 76 (1980) 294. 9. C. Backx, C.P.M. De Groot and P. Biloen, Surf. Sci., 104 (1981) 300.

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B. DELMON (UniversitC Catholique d e Louvain, Louvain-la-Neuve, Belgium): Your investigations are very careful, and your results on the existence of the various oxygen species very stimulating. It is disappointing that your system does not give any clue to selectivity improvements. You rightly invoke possible geometric factors. Recently, Ertl and coworkers investigated A g surfaces in methanol oxidation to formaldehyde and found a very special surface structure of silver, stable only at the temperature of reaction (substantially higher than in your case). It seems, thus, white relevant to focus on surface structure. You made a very careful characterization of your catalyst, including surface, when fresh. Did you check whether the surface remains unchanged after catalytic work?

X. VERYKIOS (University of Patras, Greece): This is indeed a very interesting question which could be asked for essentially any catalytic system. It is of particular importance for Agbased catalysts, under ethylene epoxidation, since many observations tend to support the notion that the structure of the catalyst might be altered under reaction conditions. W e have observed, for example, that this system requires an unusually long period of time to reach steady-state conditions, and that kinetic parameters are strongly influenced by pretreatment conditions of the catalyst. In order to obtain good and reproducible SERS spectra, a lengthy pretreatment procedure was required. It has also been found that kinetic parameters are sensitive to the details of surface structure, particle size and particle morphology. All these observations support your statement. To answer your question, we did examine used catalysts by XRD and XPS and did not notice measurable alterations. This does not necessarily mean that no structural changes take place. Such changes, if thcy occur, could be of the type not detected by these techniques or they could be reversible. In situ measurements are necessary in order to provide a complete answer to your question. L. MARGOLIS (Institute of Chemical Physics, Moscow, Russia): What is the role of oxygen dissolved in subsurface layers of Ag-Au in ethylene epoxidation. X. VERYKIOS: The role of dissolved or subsurface oxygen in the Ag-Au catalysts could be a direct one or an indirect one. Our results showed that in alloys which are rich in Au there is no dissolved oxygen and, simultaneously there is no epoxidation or combustion activity whatsoever. Thus, it could be argued that subsurface oxygen is absolutely necessary for catalytic action. However, it was also shown that the heat of adsorption of atomic oxygen decreases with increasing Au content, or with decreasing quantity of dissolved oxygen. From this perspective it could be argued that dissolved oxygen is necessary in order to facilitate dissociative oxygen adsorption which, in turn, leads to catalytic action. This is an indirect role of subsurface oxygen. W e hope to he able to elucidate the exact role of dissolved oxygen when the analysis of the kinetic study over these catalysts is completed, as well as the ISSTK study, which is currently in progress.

480

M.F. PORTELA (Technical University of Lisbon, Lisbon, Portugal): Have you observed formation of acrolein with silver-gold alloys? What is in your opinion the role of subsurface oxygen in the epoxidation of ethylene over silver?

X. VERYKIOS: Formation of acrolein was not observed either on pure Ag or on Ag-Au catalysts. As I discussed earlier, subsurface oxygen seems to be playing a very important role in this catalytic system. I would like to add that subsurface oxygen seems to be affecting both reactions (epoxidation and combustion) in the same direction. For this reason, no substantial variations in selectivity are observed. On the other hand, this observation may reveal important mechanistic aspects for the two reaction routes. J. LEROU (Du Pont CR & D, Wilmington, USA): Did you observe any de-alloying during your experiments? Did you observe changes of the surface composition of the alloys under reaction conditions?

X. VERYKIOS: Spent catalysts were examined by XRD to determine whether any dealloying had taken place. No evidence for de-alloying was found. In fact, Ag-Au alloys are thermodynamically very stable and form solid solutions over the entire composition range. Surface composition, on the other hand, can be influenced by the atmosphere under which the alloy particles exist. It is conceivable, therefore, that the surface structure is altered under reaction conditions. To examine this, in situ techniques need to be applied. The surface composition of used catalysts, examined ex situ by XPS, was found not to be measurably different than that of fresh catalysts.

V. CortCs Corberin and S. Vic Bellon (Editors), New Developrnenrs i n Seleclive Oxidolion 11 0 1994 Elscvier Science B.V. All rights reserved.

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Detailed Modeling of Transport-Kinetics Interactions of Ethylene Epoxidation at High Vacuum and Atmospheric Pressures G.D. Svobodaa. J.T. Gleavesa and P.L. Mill& aDepartment of Chemical Engineering, Washington University, One Brookings Drive, Box 1198, St. Louis, MO 63 130-4899 USA bDuPont Company, Experimental Station, P.O. Box 80262, Wilmington, Delaware 19880-0262 USA The activation of silver powder for ethylene epoxidation is investigated at vacuum and atmospheric pressure using transient response techniques based on a modified version of the TAP (Temporal Analysis of Products) reactor system. Comparisons between transient responses based on temperature programmed surface reaction and step response experiments at atmospheric pressure to those obtained at high vacuum using alternating pulses of oxygen and ethylene-d4 suggest that the activation process for silver powder is qualitatively the same in both pressure regimes. A detailed model that describes the incorporation of oxygen into the surface and subsurface sites is developed to interpret the TAP reactor data under high vacuum. Kinetic rate and transport parameters are obtained by matching the experimental and model predicted oxygen breakthrough transient responses. The results indicate that the activation process involves a concurrent filling of surface and subsurface sites, but that the latter sites are filled at a much slower rate. 1. INTRODUCTION

The heterogeneous selective oxidation of ethylene to ethylene oxide over silver based catalysts has become a significant world wide industry since its development in the 1950's (13). One important aspect that is not well understood and has received relatively little attention in the literature is the mechanism of the activation process that leads to a catalytic surface. A clearer understanding of the activation process is also important for producing more selective catalysts. From a fundamental view point, silver's unique character makes it an extremely important catalytic system and developing a greater understanding of its operation would add to our overall knowledge of oxidation processes. In silver catalyzed epoxidation, the selective oxygen species preferentially adds across the C=C double bond. However, oxygen covered silver surfaces can also give products similar to those found in metal oxide catalyzed reactions. For example, Madix and coworkers (4-6), recently found that butene can be oxidized to butadiene, dihydrofuran, furan and maleic anhydride by oxygen atoms chemisorbed on silver single crystals. The initial activation of butene involves the abstraction of an acidic allylic hydrogen similar to the activation of propene by bismuth molybdate or the activation of butene by vanadyl pyrophosphate (7,8). In these reactions, oxygen reacts as a Bronsted base and a nucleophile. Atomically adsorbed oxygen on Ag (1 10) surfaces can also add across the C=C double bond of olefins such as norbornene, styrene or 2,3-dimethylbutene (6,9).In these cases, the C-H bonds are much more weakly acidic. Thus, chemisorbed oxygen atoms can display decidedly different chemical characteristics depending on the nature of the reacting hydrocarbon. The reactive nature of an oxygen adspecies may also be influenced by the electronic characteristics and structure of the silver surface. Experimental evidence and theoretical

482

calculations indicate that more than one form of atomically adsorbed oxygen exists on a silver surface (10.11).It has been suggested that subsurface oxygen plays a key role in forming the selective surface for epoxidation ( I Q 1 2 ) . Van Santen and coworkers (13)have proposed that activation of a silver surface initially involves the filling of subsurface sites and that ethylene oxide production does not occur until after the subsurface is fully populated. Recently, Bukhtiyarov and coworkers (14) investigated ethylene epoxidation on polycrystalline silver foils and determined that two forms of atomically adsorbed oxygen are present on surfaces active for ethylene epoxidation. They also observed a strong promoting effect of carbon during the formation of subsurface oxygen (15) and suggested that carbon stabilizes surface defects (14) that are necessary for ethylene oxide production. These findings are consistent with studies by Grant and Lambert (16)which show that treatment of a Ag (1 11) surface with only oxygen is not sufficient for ethylene epoxidation, but that exposure to a reaction medium is necessary. Most of the previous reaction reaction studies have primarily focused on the reactivity of equilibrated silver catalysts having known compositions. The primary objective of this work is to study the mechanism of silver activation, and the role of subsurface oxygen in forming a selective catalyst using transient response techniques. The particular studies conducted here are performed at both high vacuum and atmospheric pressure using a modified TAP reactor system (17)to examine the effect of pressure on the mechanism of silver activation. A fundamental model is described that accounts for the various transport-kinetics interactions that occur during oxygen incorporation into the surface and subsurface sites. It is shown that excellent agreement is obtained between the expenmental and model-predicted oxygen breakthrough transient responses from which numerical values for oxygen transport and kinetic parameters are obtained.

2. EXPERIMENTAL Key aspects of the modified TAP reactor system used to perform the transient response experiments and particulars on the experimental protocals are summarized below.

2.1. Modified TAP Reactor System The reactor system used in this study is a modified version of the TAP reactor system (1719) developed by Gleaves and Ebner. A schematic of the system is shown in Figure 1. The principal modification is a movable high pressure sealing assembly that permits operation from to 2500 torr. When the sealing assembly is engaged, the reactor effluent is split between a vacuum bleed and an external vent that is connected to a back pressure regulator. The back pressure regulator is used to control the reactor pressure in the range of 100 to 2500 torr. Effluent from the vacuum bleed is detected by a quadrupole mass spectrometer (QMS) and flow from the back pressure regulator is monitored by a gas chromatograph. When the high pressure assembly is disengaged, the reactor is continuously evacuated and all of the reactor effluent is sent to the QMS. Switching between the high pressure (1OO-25Ootorr) and vacuum (
483

oxygen and ethylene-d4 feed reservoirs for a known number of pulses. Temperature programmed experiments (TPD and TPSR) were performed by heating the microreactor in a linear fashion at a ramp rate of 10 or 20" C per minute. Continuour F l o ~ Val\e (2) igli Pressure Assembly

Turbomolecular

Acquisition System

Figure 1. Schematic of modified TAP reactor system with movable high pressure assembly disengaged for vacuum operation. Reactant gases were obtained from Matheson, MDM Scientific, and Isotec and were used as received without further purification. Reactant gases included Matheson ultra high purity helium (99.999%),Matheson high purity ethylene (99.7%),Matheson high purity argon (99.7%),Matheson ultra high purity oxygen-16 (99.8%),MDM Scientific & Chemical ethylene-d4 (99%),and Isotec oxygen-18 (99%). T o eliminate support effects, polycrystalline silver powder was used as the catalytic substrate. Its activation was studied at vacuum and atmospheric conditions using both steady flow and transient experiments. A typical reactor charge contained 0.45-0.50 grams of polycrystalline Ag powder. Powder samples were obtained from Aesar Johnson-Matthey (99.995%purity) and sieved to give 300-350 p m particles.

484

2.3. Catalyst Pretreatment and Activation Pnor to each activation experiment, the silver samples were initially preconditioned for eight hours by alternately oxidizing and reducing the surface following the procedure described by Czanderna, et a1 ( 1 7). The pretreatment procedure was performed with the high pressure assembly engaged. A cycle consisted of first oxidizing with a 25 cc/min oxygen flow at 800 torr and 525 K for 30 minutes and then reducing with a 25 cc/min hydrogen flow at 800 torr and 625 K for 30 minutes. The sequence was repeated a total of eight times. The surface area was determined before and after pretreatment by the BET method using krypton adsorption and was typically 0.11 - 0.12 m*/g. In some experimental sequences, the same sample was used a number of times. After each reaction, these samples were restored to their pretreatment state by exposure to a 25 cc/min oxygen flow at 800 torr and 525 K for 30 minutes followed by a 25 cc/min hydrogen flow at 800 torr and 625 K for 30 minutes. In addition to experiments performed with pretreated silver, experiments were also conducted with preoxidized silver samples. The preoxidized samples were prepared by exposing a clean silver sample to 650 Torr of oxygen at 473 K for 1 hour. Oxygen adsorption was determined by measuring the reactor pressure drop over the 1 hour exposure period. For a typical sample, the pressure drop corresponded to an oxygen uptake of 1x1019 0 atomdgram of silver. SJ

3. RESULTS

Experimental results on activation of silver powder during transient experiments at atmospheric pressure and high vacuum pressure are described. These form the basis for the detailed model that describes oxygen incorporation in a later section.

3.1. Temperature Programmed Results. Pretreated and preoxidized silver exhibited no reactivity toward an ethyleneiargon mixture at reaction temperatures (443 - 543 K) and atmospheric pressures (750-800 torr). Similarly, the desorption spectrum of a pretreated sample showed no evidence of oxygen desorption when the sample was heated in vucuo to 673 K. These results are consistent with those of other workers (13,20-28)using powders and single crystals. Pretreated silver became active when ethylene/argon and oxygedargon mixtures were simultaneously fed over the silver at 800 torr and at temperatures above 423 K. Both ethylene oxide and carbon dioxide were produced. Figure 2 shows a typical three dimensional temperature-intensity-mass number plot collected during a TPSR experiment. In this example, the oxygen-16 to ethylene feed ratio was 3: 1 and the total flow rate was 25 cc/min. Similar spectra were obtained using other oxygen to ethylene feed ratios. The TPSR spectrum has reactant peaks at mle = 32, 16,28, 27,26 and 40 corresponding to the parent and fragment peaks of oxygen, ethylene and argon respectively. A product peak at m/e=44 begins to appear at temperatures above 423 K and corresponds to the parent ion of both ethylene oxide and carbon dioxide. Changing the feed to ethylene-d4 gives rise to new product peaks at m/e=48 and m/e=46 as well as the peak at m/e=44. The former two peaks are indicative of ethylene-d4 oxide. 3.2. Step-up Transient Results Activation of pretreated silver was performed under isothermal conditions at 493, 523 and 543 K and 800 torr using a step transient format. A typical spectrum collected at 493 K, obtained by simultaneously pulsing ethylene-d4 and oxygen- 18 from separate pulse valves into a continuous helium flow are plotted in Figure 3. In this example, the oxygen to ethylene ratio was 2: 1. As observed in the steady flow TPSR experiments, the pretreated sample is readily activated while the preoxidized samples remain inactive. Integration of the transient responses for ox gen, which includes the oxygen incorporation into products, corresponds to about 7.2 x lo1 molecules. The ethylene conversion is nearly constant at about 1% after 20 seconds.

2

485

% .- I

1"

Atomic Mass Units

Figure 2. TPSR spectra of the activation process over pretreated silver powder at 800 tom with a 3: 1 oxygedethylene feed ratio.

d e = 48

Figure 3 . Transient response spectra collected at 800 torr when step inputs of oxygen-18 and ethylene-d4 are fed over pretreated silver powder.

486

The transient responses of C D4180 and C1802 are delayed relative to the ethylene response. The rise time in the C1%0 2 response is more rapid then the C2D4180 response indicating that during the initial stages of oxygen incorporation, carbon dioxide production is favored. The C1802 response mirrors that of the ethylene input and reaches a maximum at the point that oxygen breakthrough begins. This indicates that carbon dioxide production is not affected by oxygen that is incorporated during the rise portion of the oxygen transient response. In contrast, the C2D4180 response resembles the oxygen response and begins to plateau when the oxygen uptake is approximately 90% complete. The lag in the C2D4180 response indicates that the sites that produce ethylene oxide form more slowly than the nonselective C@ producing sites.

3.3. Pump-Probe Results Alternating pulse sequences of oxygen followed by ethylene-d4 using a pump-probe format were performed at high vacuum to determine the effect of total pressure on silver activation. The transient responses for ethylene-d4, oxygen- 16, carbon dioxide, and ethylene-d4 oxide obtained at m/e = 32,44, and 48, respectively, (not shown) demonstrate that oxygen is readily chemisorbed by a pretreated silver surface. The oxygen uptake, corrected for oxygen incorporated in reaction products (is., ethylene oxide, carbon dioxide and water), is plotted in Figure 4 along with the carbon dioxide production. Oxygen is reversibly adsorbed at reaction temperatures and some desorption will occur between oxygen pulses. This is not accounted for in the oxygen uptake calculations. However, at temperatures below 493 K, the desorption rate is relatively slow compared to the pulsing rate and oxygen loss due to desorption is negligible. Since gas phase ethylened4 is not present during the oxygen pulse (the pulse separation between the ethylene-d4 pulse and the next oxygen pulse is 2.0 seconds), carbon dioxide production must result from the reaction of oxygen with a carbon containing adspecies. Carbon dioxide production during the oxygen pulse reaches a maximum within 65 oxygen pulses and remains constant, indicating that the surface carbon reaches a steady state. The pulse width of the CO2 pulse formed during the oxygen input is essentially constant, while the pulse width formed during the ethylene-d4 input varies with oxygen coverage. In the latter case, the C 0 2 pulse width decreases with increasing coverage. The shape difference between the two sets of C@ pulses indicates that the formation mechanisms are not equivalent. The C@ pulse formed during the oxygen input is the broader indicating the formation mechanism is slower. As indicated in Figure 4, the C 0 2 production initially increases with oxygen uptake, goes through a maximum and then rapidly decreases. Ethylene-d4 oxide production also increases and then decreases with oxygen uptake but more slowly than C02. As a result, the reaction selectivity to ethylene-d4 oxide increases with increasing oxygen uptake.

.

ow 0

,

loo

.

, 200

.

, 300

.

, 400

.

0 500

Pulse Number

Figure 4. Carbon dioxide production and oxygen uptake as a function of pulse number obtained by integrating area of pump-probe pulses.

487

3.4. TAP Reactor Model

The results given in the previous section provide the basis for a mathematical model that describes the incorporation of oxygen into the surface and subsurface sites of the silver powder. The model given here applies to the high vacuum mode of operation. Interpretation of the atmospheric pressure data follows similar reasoning and is omitted due to brevity. The basic concept requires accounting for the temporal and spatial distribution of oxygen in the gasphase and on the catalyst surface and subsurface. This is performed by development of microscopic oxygen mass balances when transport-kinetic interactions are described in terms of fundamental gas transport and kinetic-rate constants. During the initial stages of an oxygen incorporation experiment, oxygen is unevenly distributed over the length of the catalyst bed. The surface and subsurface sites at the front of the catalyst bed are filled first and those at the reactor exit more slowly. The oxygen incorporation process can be described by a mathematical model similar to that reported by Svoboda et al. (29) with modifications to include subsurface sites and nonlinear adsorption kinetics. Assuming oxygen chemisorption is a dissociative process (30),and oxygen migration into the subsurface is first order (31), equations (1) - ( 3 ) below can be used to model oxygen incorporation. The key model assumptions are: 1) Knudsen diffusion is the predominate mode of gas transport, 2 ) oxygen desorption is negligible, 3) diffusion from the subsurface to the surface is negligible, 4)the catalyst bed is isothermal, 5) the silver powder is nonporous, 6) changes in the number of surface sites are negligible, and 7) reaction is negligible. With these assumptions, the mass balances for oxygen in the gas-phase, the catalyst surface and the catalyst subsurface are:

In equations (1) - ( 3 ) , C is the gas phase oxygen concentration, 8 is the fractional surface coverage, Os is the fractional subsurface concentration normalized to the surface coverage, Oss is the subsurface saturation concentration normalized to the surface coverage, De is the Knudsen diffusion coefficient, ka is the adsorption rate constant, k, is the subsurface incorporation rate constant, ps is the surface site density, and E b is the reactor bed void fraction. With appropriate boundary conditions (29), equations ( I ) - ( 3 ) describe the time dependent concentration profile of oxygen in the reactor voids, on the silver surface, and in the silver subsurface over the reactors axial coordinate. The oxygen output response obtained at the QMS, Y(t), is determined by evaluating the flux at the reactor exit using equation (4).

Equations ( I ) - ( 3 ) are solved numerically for an assumed set of parameters, P = [Eb De ps Bss ka kS]T using methods that are described in standard texts on the subject (32). The vector of unknown parameters P is determined by minimizing the mean square relative error between the model-predicted and experimental breakthrough curves. Minimization of the mean-square relative errors was performed using Marquardt's method (33). Figure 5 shows a typical comparison between an experimental and model-predicted oxygen breakthrough curve. The experimental data points were taken from pump-probe data at 240" C. The model parameters are: ps= 1 x 10-4mole cm-3, ka = 2 x 108 cm3mole-lsec-~,k, = 8 x 102 sec-1,and Oss = 2. The ratio of subsurface to surface oxygen at saturation (Oss) indicates that = 33% of the oxygen present at saturation occupies surface sites. This is reasonably close to

488

the 50% value reported by van Santen for oxygen incorporation in polycrystalline silver powders at 200 "C (22).

30-

t , , ,

0 0 -

0

500

SOD

lYl0

2000

I

2300

'

I

3000

'

I

3100

'

I

4W0

'

I

45DO

'

SOD0

Pulse Number

Figure 5. Oxygen breakthrough curve (open circles) and model fit (solid line) using transportkinetics model. The value obtained for k,can be used to calculate the sticlung coefficient (so) for oxygen on clean silver powder at 240 "C using the relationship (34):

where (5 = (density of Ag)(measured surface area)/p,. Substituting the above value for ka and 0 = 3 x lo8 cm2/mole into Equation 5, gives This value lies within the range of = 5x literature values for the initial dissociative sticking coefficient on the (110) face = 5 - 30 x at 200 "C (30) and the (1 11) face 1 x 106) at 217" C (35) of silver single crystals. The modeling results yield values for the sticking coefficient, and the ratio of surface to subsurface oxygen that are consistent with those of other workers. The results indicate that the activation process involves a concurrent filling of surface and subsurface sites, but that the latter sites are filled much more slowly.

-

(so

(so

4. DISCUSSION

The results of the high and low pressure transient activation studies indicate that the activation process follows a similar pattern in both pressure regimes since oxidation products are formed during the initial incorporation of oxygen. The low pressure pump-probe experiments indicate that C 0 2 and C2D4O production begins after the first oxygen pulse when total oxygen adsorption is below .05 monolayers. Also, the surface is less selective at the two pressure extremes during the initial stages of the activation process, and selectivity increases with oxygen uptake. At high vacuum pressures and normal reaction temperatures, the predominant oxygen adspecies is atomic oxygen (28,36). Molecularly adsorbed oxygen has a significantly shorter surface lifetime and will either desorb or chemisorb dissociatively ( 2 5 ) . In pump-probe experiments, the concentration of molecularly adsorbed oxygen is negligible during an ethylene pulse since the oxygen and ethylene inputs are separated in time. As a result, molecularly

489

adsorbed oxygen is not expected to play a significant role in ethylene conversion. Previous TAP reactor studies using oxygen isotopes (25),as well as several other studies (I0,26),have indicated that atomically adsorbed oxygen and not molecularly adsorbed oxygen is the active species in ethylene epoxidation. Results of the pump-probe experiments presented in this paper are consistent with those findings. In addition to reacting with ethylene, atomically adsorbed oxygen can migrate to the subsurface where it is blocked from directly participating in an oxidation reaction but can modify the electronic properties of the surface. Specifically, reaction of subsurface oxygen with silver surface atoms will withdraw electron density from the surface. This will increase the electrophilic character of silver surface atoms and oxygen adspecies (10).Recent theoretical calculations by van den Hoek, et. al. (37) on Ag (1 10) clusters indicate that subsurface oxygen may also reduce the bond energy between silver and adsorbed oxygen and convert the repulsive interaction between adsorbed oxygen and ethylene into an attractive interaction. As a result, the barrier for epoxide formation disappears. Results from the oxygen incorporation modeling prove that the reaction selectivity is strongly influenced by the incorporation of subsurface oxygen. Comparison of the C2D4O selectivity curve and the oxygen incorporation curves shows that the rise in C2D4O selectivity parallels the increase in subsurface oxygen, but not the increase in surface oxygen. In contrast, the maximum C@ production occurs when the surface coverage begins to plateau, and C a production rapidly decreases with increasing subsurface concentration. These results provide evidence that incorporation of subsurface oxygen is crucial to the formation of a selective surface. They are also consistent with the idea that subsurface oxygen modifies the reactive characteristics of the oxygen adspecies. The evidence presented in this study suggests that the selective catalytic site is comprised of both surface and subsurface oxygen species, and a partially oxidized silver atom. In addition, the site may include one or more carbon adspecies. As well as stabilizing the defect site, the carbon adspecies may promote site isolation and prevent over oxidation of adsorbed ethylene molecules. Deactivation of the silver surface occurs when the surface is completely oxidized, the surface carbon is depleted, and no silver sites are available for ethylene adsorption. In this regard, it is important to note that the amount of oxygen incorporated during both the high and low pressure activation processes, and in the oxygen pretreatment experiments was = 1019 0atomslgram of silver. Since the active catalyst surface contains approximately the same quantity of oxygen as an inactive surface, it follows that the former is not simply a reduced state of the latter. Rather, the active surface must contain unique sites that are absent in the inactive surface. This result provides additional support for the supposition that carbon plays a role in creating and stabilizing ethylene epoxidation sites. 5. SUMMARY AND CONCLUSIONS

A novel modification of the TAP reactor system has been described that permits transient

response experiments to be performed at both vacuum and atmospheric pressures. This system was used to investigate the activation of silver powder for ethylene epoxidation. The experimental results indicate that activation at high and low pressures is qualitatively the same. In both regimes, C02 production proceeds more rapidly than ethylene oxide production. Low pressure pump-probe experiments indicate that significant quantities of C 0 2 are produced during both the oxygen and ethylene inputs but that ethylene oxide is formed only during the ethylene input. The C02 product pulses generated during the oxygen input have a different transient response from those generated during the ethylene input. The intensity and shape of the C02 and ethylene oxide transient responses change with increasing oxygen incorporation. Oxygen incorporation at low pressures was described using a model that accounts for transport, surface, and subsurface reaction steps. The results indicate that the activation process involves a concurrent filling of surface and subsurface sites, but that the latter sites are filled much more slowly. The model-predicted surface coverage and subsurface concentration was compared with the ethylene oxide selectivity. The results were consistent with current models

490

of ethylene epoxidation that indicate that subsurface oxygen is necessary for adsorbed oxygen to react to epoxide. A multistep mechanism that explains the activation process through carbon adspecies which play a role in the formation and stabilization of catalytic sites could be proposed using this data as the basis. REFERENCES 1. Voge, H.H.; Adams, C.R. Adv. Catal.1967, 17, 151. 2. Zomerdijk, J.; Hall, M. Catal. Rev. Sci. Eng. 1981, 23, 163. 3. Detwiler, H. R.; Barker, A.; Richarz, W. Helv. Chim. Acta. 1979,62, 1689. 4. Roberts, J.T.; Capote, A. J.; Madix, R.J. Surf. Sci. 1991, 253, 13. 5. Roberts, J.T.; Capote, A. J.; Madix, R.J. J. Am. Chem. Soc. 1991,113, 9848. 6. Roberts, J.T.; Madix, R.J. J . Am. Chem. SOC.1988, 110, 8540. 7. Grasselli, R.K.; Burrington, J.D. Adv. Catal. 1981, 30, 133. 8. Centi, G; Trifiro, F; Ebner, J.R.; Franchetti, V. Chem. Rev. 1989, 28, 400. 9. Hawker, S.; Mukoid, C.; Badyal, J.P.S.; Lambert, R.M. Suv. Sci. 1989,219, L615. 10. van Santen, R. A.; Kuipers, H. P. C. E. Adv. Catal. 1987,35, 265. 11. Carter, E. A.; Goddard, W. A. I11 J. Catal. 1988, 1 1 2 , 80. 12. Grant, R. B.; Lambert, R. M. J. Chem. Soc., Chem. Commun. 1983, 662. 13. van Santen, R. A.; DeGroot, C. P. M. J. Catal. 1986, 98, 530. 14. Bukhtiyarov, V. I.; Boronin, A. I.; Savchenko, V. I. Surf. Sci. Lett. 1990,232, L205. 15. Boronin, A. I.; Bukhtiyarov, V. I.; Vishnevskii, A. L.; Boreskov, G. K.; Savchenko, V. I. Surf. Sci. 1988, 201, 195. 16. Grant, R. B.; Lambert, R.M. J . Catal. 1985, 92, 364. 17. Cleaves, J. T.; Ebner, J. R.; Kuechler, T. C. Cat. Rev. Sci. 1988,30, 49. 18. Ebner, J. R.; Cleaves, J. T., U.S. Patent 4626412, Dec. 1986. 19. Cleaves, J. T.; Harkins, P. T., U. S. Patent 5,039,489 1991. 20. Czanderna, A. W. J. Phys. Chem. 1966, 70, 2120. 21. Barteau, M. A.; Bowker, M.; Madix, R. J. J . Catal. 1981, 67, 118. 22. Backx, C.; Moolhuysen, J.; Geenen, P.; Van Santen, R. A. J. Catal. 1981, 72, 364. 23. Campbell, C. T.; Paffett, M. T. Surf. Sci. 1984, 143, 517. 24. &gas, N. C.; Cleaves, J. T.; Mills, P. L. New Developments in Selective Oxidation, Elsevier (Amsterdam) 1990; pp. 707 (1990) 25. Cleaves, J. T.; Sault, A. G.; Madix, R. J.; Ebner, J. R. J. Catal. 1990, 121, 202. 26. Grant, R.B.; Lambert, R. M. J. Chem. SOC., Chem. Commun. 1983, 58. 27. Force, E. L.; Bell, A. T. J. Catal. 1975,40,356. 28. Bowker, M.; Barteau, M.A.; Madix, R. J. Surf. Sci. 1980,92, 528. 29. Svoboda, G. D.; Cleaves, J. T.; Mills, P.L. Znd. Eng. Chem. Res. 1992, 31, 19. 30. Engelhardt A.; Menzel D. Surf. Sci. 1976,57, 591. 31. Borg, R. J.; Dienes G . J.The Physical Chemistry of Solids,Academic Press, New York, N.Y .,1992. 32. Jenson, V. G.; Jeffreys, G. V. Mathematical Methods in Chemical Engineering, Academic Press, New York, N.Y., 1977. 33. Seinfeld, J. H.; Lapidus, L. Mathematical Methods in Chemical Engineering, Vol. 3, Process Modeling, Estimation, and Identification, Prentice-Hall, Englewood Cliffs, NJ, 1974.

34. Rieck, J. S.; Bell, A. T. J. Catal. 1984, 85, 143. 35. Hall, P. G.; King, D. A. Surf. Sci. 1973, 38, 129. 36. Barteau, M. A.; Madix, R. J. Surf. Sci. 1980, 97, 101. 37. van den Hoek, P.J.; Baerends, E.J.; van Santen R. A. J. Plzys. Chem. 1989, 93, 6469

49 1

J. F. Brazdil (BP CHEMICALS, Cleveland, Ohio, USA) : Have you looked at the effect of chloride on oxygen adsorption and subsurface oxygen, since chloride is important in commercial silver catalysts? J. T. Gleaves (Washington University, St. Louis, Missouri, USA) : We have not looked a t the effect of chloride on oxygen adsorption and subsurface oxygen. That is a topic for future investigation. B. Delmon (Universite Catholique de Louvain, Belgium) : I am stricken by the S-shaped curve corresponding to the incorporation of subsurface oxygen. Your model explains this kind of curve. But S-shaped curves are also found when nucleus-forming processes are rate-determining. Investigations with clean surfaces, at the Fritz-Haber Institute in Berlin and other places, show that the formation of surface "phases" constituted of adsorbed species (e.g., CO on Pt) are new surface structures induced by subsurface oxygen (several noble metals, perhaps Ag) involve a rate-limiting nucleation step. This suggests that the hypotheses of a rate-limiting nucleation should indeed be considered. This might have consequences with respect to the real role of promoters in Ag catalysts. Their efficiency might be related t o easier nucleation of the subsurface-oxygen-containing surface phases. One may speculate that these dopants might form nucleates, by creating more phases from which phases would develop. This is a n effect of dopants often observed on clean surface experiments. If this is true, the curves corresponding t o subsurface oxygen should lose or attenuate their S-shape when dopants are present. Did you investigate doped Ag catalysts with your techniqes?

J. T. Gleaves : Our model indicates that subsurface oxygen plays a critical role in the formation of the activehelective surface but does not rule out that site formation may involve surface rearrangement. It is important to note that our results suggest that both oxygen and carbon (from ethylene) are necessary for the formation of an active, selective catalytic site. This site may involve a unique surface structure containing a cluster of silver atoms, subsurface oxygen species, and surface oxygen species. Carbon may play a role in stabilizing this site. G. Emig (Universite of Erlangen, Nurnberg, Germany) : Two questions: 1) You have a large number of parameters in your TAP reactor model. It is always a problem to derive reliable estimates for these parameters on the basis of experimental data. Could you determine some of these parameters by independent methods and confirm in this way the validity of your model?

J. T. Gleaves : The parameters that appear in the TAP reactor model given here include (a) the effective Knudsen diffusion coefficient of species z and the adsorption and kinetic rate constants. We have shown in a previous publication (G. D. Svoboda, J. T. Gleaves, and P. L. Mills, I&EC Research, 1992, 31, 19-29) that the effective Knudsen diffusion coefficient for species z can be independently determined by time-domain analysis of inert gas-pulse data and using the

492

theoretically-based expression for the effective Knudsen diffusion coefficient. This leaves the adsorption and kinetic rate constants as the unknown model parameters since the remaining parameters, such as the bed porosity and packed-bed length, are independently measured. I didn't have time in my presentation to explain the statistical techniques that we used to assign confidence limits and other statistically-based measures to our final parameter estimates. These are described in the doctoral thesis of one of my co-authors, G . D. Svoboda. I would like t o mention that we have compared our TAP reactor-based values for the initial sticking coefficient and the ratio of surface-to-subsurface oxygen t o literature values where the latter were obtained by different experimental methodologies. Our values determined from interpretation of the TAP reactor are in good agreement, which provides independent verification that our model and parameter estimates are valid. 2) Was it possible to apply your model also under atmospheric pressure conditions? I assume that some changes in the governing mechanisms of the system will occur.

J. T. Gleaves : The transport-kinetics model used to interpret the TAP reactor data under high vacuum obviously cannot be directly applied to the transientresponse data obtained near atmospheric pressure. This is the case, since these two operating modes correspond t o the Knudsen and continuum regimes, respectively, so that the models used to describe the gas pulse transport are clearly different. We have modeled the transport of inert gas pulses in the TAP reactor at atmospheric pressure quite successfully using a classical twoparameter axial-dispersion model. Application of this model to the interpretation of transient-response data for ethylene epoxidation under atmospheric pressure is a topic for future study. I want t o mention that the behavior of atmospheric pressure step-response experiments appear t o be in qualitative agreement, at least, with the high-vacuum, pulsed experiments.

U.S . Ozkan (Ohio State University, Columbus, Ohio, USA) : In your results, the ethylene oxide selectivity seems to follow the subsurface oxygen concentration very nicely, however the C02 selectivity doesn't follow the surface oxygen concentration. C02 selectivity levels off where surface oxygen concentration decreases. Is there an explanation for this? J. T. Gleaves : C02 production decreases due to decreasing ethylene conversion and increasing selectivity to ethylene oxide. Conversion decreases as the surface becomes fully oxidized. A fully oxidized surface is inactive. J. C. Vedrine (IRCENRS, Villeurbanne, France) : Your TAP technique looks attractive to try to characterize intermediate species. A question arises to me: when after a pulse you are analyzing two species as CO2 and C2H40, the former appearing first, you conclude that C02 is formed first. In fact if both products are "adsorbed" with different energies, their departure will depend on adsorption

493

strength and not on formation time. Can you discuss on this point on the technique?

J. T. Gleaves : TAP-transient response curve shapes are determined by gas pulse transport rates, the rates of adsorption and desorption, and the rate of surface reaction. The latter two can be distinguished experimentally by feeding a product and observing the curve shape. In the case of the CO2 pulse shape, the width is primarily due to CO2 adsorption, indicating that C 0 2 forms a strongly bound species on a partially-oxidized silver surface. However, our conclusion that C 0 2 is formed first as the surface is being activated is not related to the pulse shapes but t o the fact that no ethylene oxide is formed when the surface is first exposed to oxygen.

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V . CortCs Corbcran and S. V i c Belldrn (Editors), N e w Deveiopmenis i n Seiecrive O x i d d o n l/ 0 1994 Elscvier Science B.V. All rights rescrvcd.

495

Doping effects in ethylene epoxidation over potassium promoted silver catalysts V. Lazarescu"', M . Stanciub and M . Vass" "Institute of Physical Chemistry, Spl. lndependentei 202, 77208 Bucharest, Romania blnstitute of Research, Technological Engineering and Design for Oil Distilleries, Bd. Republicii 291A, 2000 Ploiesti, Romania. The promotion action of the K + ions on the silver-catalyzed selective oxidation of ethylene t o ethylene oxide is correlated with their effects on the oxygen adsorption and bulk diffusion and explained as consequences of the changes induced into the metalkarrier electronic interaction.

1. INTRODUCTION It is well-known from the patent literature that the alkali metals have a significant 1 promoter action in the silver-catalyzed ethylene epoxidation. Grant and Lambert [I assigned this effect t o a negative influence of the alkali metal on the further reaction of the ethylene oxide, its isomerization to CH,CHO, that undergoes a rapid combustion, Van Santen [21 considers, on the other hand, that alkalis act only in cooperation with chlorine, stabilizing the formation of the silver oxychloride anion, supposed t o be the ethylene epoxidation agent. Both mechanisms refer, however, t o Cs, found t o rest only on the catalyst surface [ I ] . As K, unlike Cs, exhibits a pronounced diffusion into the metal bulk, strongly accelerated by the oxygen presence [3], the mechanism of its action is expected t o be different. It has been recently suggested [41that the alkali neutralization of the acidic sites of the alumina support could be responsible for the increased ethylene oxide yield observed in such cases. It is the aim of this paper t o propose an alternative explanation for this effect.

2. EXPERIMENTAL The experiments were performed on potassium-promoted catalysts containing 15.5% Ag supported on a-alumina doped beforehand with Me"0. The samples were prepared by a conventional impregnation technique with silver lactate coupled with a calcination step in flowing nitrogen. The promoter was added as KOH alcoholic solution in the impregnation step. The catalysts prepared thus had a potassium

Present adress: Universitat Ulrn, Abt. Elektrochernie, Albert-Einstein-Allee 1 1, 7900 Ulrn, Germany

496

content ranging from 1.4x 1 0 . ~t o I .4x 10.' g%. The catalytic tests were carried out in a laboratory reactor of integral type with recirculation and gas chromatographic analysis. The ratio between the ethylene and air flow rates was 1:lO and the temperatures, between 2 3 4 and 260 OC.

3. RESULTS AND DISCUSSION The addition of the potassium ions t o the alumina-supported silver catalyst was found t o have a positive influence on the selectivity t o ethylene oxide only in case of the lowest concentration, the higher ones yielding opposite effects (see Figure I).

$5L \

t

t 2

*I5

52

W

cn 50

'

0

3.5 7.0 10.5 K+ CONCENTRATION / mg yo

140

Figure 1. The variation of the selectivity to ethylene oxide with the potassium content of the alumina-supported silver catalysts This behaviour resembles closely with that showed by the same catalysts in their interaction with oxygen (see Figure 2). As reported earlier 151, the surface potential exhibiting a maximum value for the sample with the lowest potassium content, decreases rapidly when the potassium concentration increases. Even the amount of the dissolved oxygen, estimated by an electrochemical technique (6.71, greatly enhanced when K + ions are present, shows a tendency of decreasing for the higher K + concentrations. Both effects have been previously explained as consequences of the metal/support electronic interactions enhanced by the K + ions. According t o Schwab [81, a-alumina doped beforehand with Me"0 forms a p-type semiconductor. The addition of the K + ions during the catalyst preparation is expected to bring about an enhancement of its p-character and thus, of the electron transfers from the metal t o the carrier, noticed t o take place in such cases [91. This assumption, which is consistent with the observation that the state of the metal dispersion increases with the K + content [51 is in a good agreement with a recent report of Lambert et al. [I01 too. They found a significant population of potassium in the alumina support and proved that the alkali spillover from the silver t o alumina is extremely facile. The redistribution of the electrons between the metal and the support, yielding a slight electron deficit into the silver lattice, explains well the results obtained. The

497

Q7l

0

I290

35 7.0 10.5 K + CONCENTRATION I m g O h

u.0

Figure 2. The variation of the surface potential on oxygen adsorption ( 0 ) and subsurface oxide layer formed electro-chemically ( o ) with the potassium content of the alumina-supported silver catalysts. increase in the electrochemically formed subsurface oxide layer can be a consequence of an easier access of 02ions into the metal bulk allowed of the contracted size of the slightly positivated silver sites as long as the diminution of the metal crystallites does not become an hindering factor. The gradual inhibition of the oxygen adsorption, known to be an electron-demanding process, is, on the other hand, the expected effect of the progressively diminution of the electron density around the surface silver atoms. The small increase in the surface potential observed for the lowest K+ concentration could be due t o a higher dipole moment value associated with a more weakly adsorbed species, in the first step. As both the surface and the subsurface oxygen were found [11,12]to be necessary for the ethylene epoxidation, it is simple t o understand why only the smallest K + concentration promoted it: the higher ones, although can promote the oxygen submergence under the surface, inhibit the adsorption process. If the reaction takes place by the electrophilic attack of the 0 ,, on the olefinic n-bond, as it has been proposed [12,131,the epoxide yield must be favoured by a lower electron charge on the OdS, whatever the ethylene is adsorbed [I21 or in the gas phase [ 131.The role of the electron transfers inducedlenhanced b y the K + ions is t o create the silver sites slightly deficient in electrons on which the negative charge on Oadsis reduced. This process could be only the result of the overall charge transfers between the metal and the carrier or, most probably, the effect of the increased amount of the subsurface oxygen. The role of the subsurface oxygen was shown [I 31 t o be complex: it reduces not only the bond energy between the silver and the adsorbed oxygen but may convert also the repulsive interaction between the adsorbed oxygen and the gas phase ethylene into an attractive one, favoring thus

498 the epoxidation reaction. The close parallelism between the variation of the selectivity and the surface potential on oxygen adsorption with the K + content suggests that the reaction proceeds through a Eley-Rideal mechanism, as van Santen [131has proposed. In case of an adsorbed ethylene reaction, the selectivity should decrease more rapidly as the oxygen adsorption when the K + content increases, since the higher the electron deficit of the surface silver sites is, the greater the IIelectron transfer from the adsorbed ethylene towards the metal and thus, the lower the probability of an electrophilic attack of the ,O , are.

4. CONCLUSIONS The behaviour of the potassium promoted silver catalysts t o the oxygen adsorption/absorption as well as in the ethylene epoxidation might be a consequence of the doping effects of the potassium on the electronic interactions taking place between the silver and the carrier. The enhancement of the charge transfers from the metal t o the carrier brought about by the I<+ ions is supposed t o allow an easier transport of the 0’- from the surface to the bulk and to weaken the oxygen-silver bond strength, both of them of potential importance for the ethylene oxide formation. The selectivity of the catalytic process increases, however, only for the lowest K + concentration, because the oxygen interaction with the silver sites more and more deficient in electrons is gradually inhibited.

ACKNOWLEDGEMENTS One of us (V.L) gratefully acknowledges a fellowship from Alexander von Humboldt Stiftung allowing of the documentation stage of this work.

REFERENCES R.B. Grant and R.M. Lambert, Langmuir 1 (1985)29. R.A. van Santen, in Proc. 9th Internat. Congress Catal., Calgary, 1988, J. Phillips and M. Ternan (eds.), The Chemical Institute of Canada, Ottawa, 1988, Vol. 3, p. 1152. 3. M. Kitson and R. M. Lambert, Surf. Sci. 109 (1981)60. 4. J. K . Lee, X. E. Verykios and R. Pitchai, Appl. Catal. 44 (1988)223. 5. V. Lazarescu, M. Vass, A. Popa, M. Stanciu and M. Tolan, in Proc. Vlth Internat. Symp. Heterogeneous Catalysis, Sofia, 1987, Part 1, 265. 6. V. Lazarescu, 0. Radovici and M. Vass, Electrochim. Acta 30 (1985)1407. 7. V. Lazarescu, M. Vass, 0. Radovici, C. Dasoveanu and M. Stanciu, Rev. Roumaine Chim. 30 (1985)933. 8. G. M. Schwab, Advan. Catal. 27 (1978)I. 9. X. E. Verykios, F. P. Stein and R. W. Coughlin, J. Catal. 66 (1980)147. 10. R. M. Ormerod, K. L. Peat, W. J. Wytenburg and R. M. Lambert, Surf. Sci. 1.

2.

269/270(1 992)506. 11.

C. Backx, J.

Moolhuysen, P. Geenen and R.A. van Santen, J. Catal. 72

(1981)364. 12. 13.

R. B. Grant and R. M. Lambert, J. Catal. 92 (1985)364. P.J. Van der Hoek, E.J. Baerends and R.A. van Santen, J.Phys. Chem. 93

( 1 989)6469.

V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation If 0 1YY4 Elsevier Science B.V. All rights reserved.

499

A temperature programmed surface reaction study of the catalytic epoxidation and total oxidation of ethylene on silver C. Henriquesa,M.F. Portelaaand C. Mazzocchiab aGrecat - Grupo de Estudos de Catalise Heterogenea, Chemical Engineering Department, I.S.T., Technical University of Lisbon, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal bDipartimento di Chimica lndustriale ed lngegneria Chimica Giulio Natta, Politecnico di Milano, P u a . Leonard0 da Vinci, 20133 Milano, Italy The influence of the interaction temperature between oxygen and the catalyst, on the ethylene reactions over silver was studied. Catalytic effect of the different oxygen species was detected.

1. INTRODUCTION Ethylene epoxidation by silver catalysts is an important process of the petrochemical industry. A large number of publications, which deal with the interaction between oxygen and silver, in the presence and in the absence of the olefin, are quite contradictory, chiefly about the nature of the surface oxygen species (molecular andlor atomic) and about the way they are active for epoxidation andlor total oxidation. Most of the more recent work, seems to evidence that a third type of oxygen appears at the silver surface under reaction conditions and that its presence is determinant for the epoxidation reaction [ I -91. 2. EXPERIMENTAL

The unsupported silver catalyst was prepared by reduction of diamino-silver species, Ag(NH3)2+, with formaldehyde, followed by drying at 393K for 12h. For the temperature programmed experiments, an apparatus consisting on a thermal conductivity cell connected to a quadrupole mass spectrometer was used. A system of two 6-ports valves provided the connection to an "U" shape quartz cell, placed in a temperature programmed furnace. Helium (3.6 I h-1) was used as carrier, after oxygen trapping and drying (molecular sieves 13X). Several runs were performed, under similar conditions, in order to obtain data from about 15 mle different signals, which correspond to the main fragments of the species reported as being involved in the reaction mechanisms of ethylene and oxygen over silver (oxygen, carbon dioxide, methane, formaldehyde, formic acid, ethylene, ethylene oxide, acetaldehyde, acetic acid and oxalic acid) [6, 7, 10-121. About 0.2 mg of catalyst was used in each test.

500

X-ray diffraction analysis evidences that the catalyst is pure and well crystallised. The BET surface area is about 4.5 m*g-1. For all the tests, ethylene and oxygen (L'Air Liquide) were used after drying (molecular sieves 4A and 13X); hydrogen (L'Air Liquide) was used after oxygen trapping and drying (molecular sieves 13X). Prior to each experiment, the catalyst was reduced with hydrogen (5th-1gcat-1) at 623K during 2h, followed by cooling to room temperature and purge with He. The preoxidation of the samples was done by exposing a previously reduced sample to a flow of oxygen (51h-1gcat-l)at a given temperature, for 2h, followed by cooling to room temperature and purge with He. The interaction of ethylene with a catalyst sample (preoxidized or not) was done by exposing it to a flow of olefin (51h-lgcat-l), at a given temperature for 2h, followed by purge with He.

3. RESULTS AND DISCUSSION 3.1. Temperature programmeddesorption (TPD) In addition to TPSR experiments, pure reactants (ethylene and oxygen) were put in contact with the catalyst, and temperature programmed desorption (TPD) runs were carried out

3.1. I . TPD of ethylene on the reduced catalyst Pure ethylene was made to interact at 298K and 473K with the catalyst, previously reduced with hydrogen. In both cases, no signals attributable to ethylene or to any other products were detected. These results are consistent with reported studies [I31 that evidenced that ethylene does not adsorb on the complete reduced silver surface. On the other hand, it was confirmed that the used reduction conditions, provide complete elimination of adsorbed oxygen. 3.1.2. TPD of oxygen over the reduced catalyst Several runs were performed, in which oxygen was made to interact at 298K and 473K, with the catalyst previously reduced with hydrogen. The results are shown in Figure 1, where the intensities of the rnle = 32 a.m.u. and 16 a.m.u. (oxygen fragments) signals are presented as a function of the temperature. No other signals were detected. The spectra exhibit two desorption processes, with maxima at 535K (a peak) and 580K (0 peak). The ratio between mle = 32 a.m.u. and mle = 16 a.m.u. signal intensities, are constant and similar to those obtained with gaseous oxygen. On the other hand, the spectra recorded were similar, when the oxygen was adsorbed either at 298K or at 473K. This fact is in contradiction with several authors that present results in which the adsorption at higher temperature gives rise to a desorption process with a maximum at a very high temperature (700 to 1000K) [2-4, 141. Our results are understandable taking into account that 1OOOK (above which the temperature rise is no longer linear) is probably not high enough to detect subsurface species.

501

Literature [2-4] reports a desorption process with a maximum in the range 570/620K, assigned to the recombination and desorption of adsorbed atomic oxygen, in good agreement, with our 0 peak.

Figure 1 - TPD Spectra of oxygen:

- m/e = 32 a.m.u.;

+ - m/e = 16 a.rn.u

The existence of adsorbed molecular oxygen, has been reported by Campbell et. al. [I51 and Grant et. al. [I61 and other authors, that assigned it to desorption processes with maxima in the range 380/460K. It seems difficult to assign our CY peak to such a species, due not only to the great difference of the desorption maxima, but also to the fact that recent studies indicate that molecular oxygen cannot exist adsorbed over silver above 300K [3, 171. Our CY and 0 peaks would represent the desorption of the same kind of oxygen (probably atomic species), adsorbed over sites with different adsorption energies, evidencing the silver surface heterogeneity, probably due to the existence of different crystalline planes. 3.2. Temperature programmed surface reaction (TPSR)

Interacting the pure olefin with the catalyst, preoxidized in different conditions, at temperatures between 400 and 473K, no signals were detected when performing the TPSR runs. Only when this interaction occurred at the ambient temperature, it has been possible to detect signals from the analytic devices. 3.2.1. interaction of ethylene at 298K with the catalyst preoxidized at 298K

The results of the experiments are presented in Figure 2 . Only the signals corresponding to C02 (mle = 44 a.m.u.) and 0 2 (m/e = 32 and 16 a.m.u.) were detected. Carbon dioxide exhibits a maximum at 425K. The spectra of oxygen desorption evidences a partial reduction of the catalyst, due to its interaction with ethylene.

502

With these experimental conditions, no signals were detected ascribed either to ethylene oxide and its isomerisation products or to products involving the C=C bond rupture.

0,lO

0.08

-s

E' 0,os

2 0,04 0,02

0,oo

Figure 2 - TPSR spectra of ethylene adsorbed at 298K, on the catalyst, preoxidized at 298K. 3.2.2. Interactionof ethylene at 298K with the catalyst preoxidized at 473K The results of the experiments are presented in Figure 3. Only the signals corresponding to m/e = 44, 29 and 15 a.m.u. were detected. The signals corresponding to mle = 29 and 15 a.m.u. shows a single process, with a maximum at 395K. The signal corresponding to m/e = 44 a.m.u. presents three distinct desorption processes: two at low temperature, with maxima at 395K and 430K, and one, at high temperature, with the maximum at 805K. These results indicate that ethylene oxide (m/e = 44, 29 and 15 a.m.u.) is a product of reaction, and desorbs by a process with a maximum at 395K, while carbon dioxide (m/e = 44 a.m.u.) is formed and desorbs by two processes, one at low temperature, (maximum at 430K) and other at high temperature, (maximum at 805K). No oxygen was detected, indicating that it has been totally consumed. In these experimental conditions, no signals were detected ascribable either to secondary products of ethylene oxide (isomerisation products) or to products resulting from the C=C bond rupture. 3.2.3. Interaction of ethylene at 298K with the catalyst preoxidized at 473K and conditioned at 555K under helium These runs were performed in order to study the effect of the elimination of the low-temperature oxygen species (apeak), in the behaviour of the catalyst, in TPSR conditions.

503

In Figure 4 are shown the spectra of oxygen termodesorption (mle = 32 and 16 a.m.u.), after the pre-treatment with helium. They present a single desorption process, with a maximum at 595K, corresponding to the 0 peak, previously detected. The results of the TPSR experiments in the above mentioned conditions are presented in Figure 5. Only the signals corresponding to mle = 44, 29 and 15 a.m.u. were detected. The signals corresponding to mle = 29 and 15 a.m.u. show a single process, with a maximum at 395K. The signal corresponding to mle = 44 a.m.u. presents two distinct desorption processes: one at low temperature, with the maximum at 395K and the other at high temperature, with the maximum at 805K.

0,lO 0,08

g

B

I

I.

We=Ma.rnu.)

i T = 805K

0,06

I

u)

I0.04 0,02

0,oo

I 0,lO

0,08

273

375 ' 4 7 3 573 673 773 873 973 1073 Temperature (K)

f

273 373 473 573 673 773 873 973 1073

273 375 473 573 673 773 873 973 1073

Temperature (K)

Temperature (K)

Figure 3 - TPSR spectra of ethylene, adsorbed at 298K on the catalyst preoxidized at 473K. These results indicate that ethylene oxide (m/e = 44, 29 and 15 a.m.u.) is a product of reaction, and desorbs by a process with a maximum at 395K, while carbon dioxide (m/e = 44 a.m.u.) is formed and desorbs, after this pre-treatment, following a single process at high temperature, with a maximum at 805K.

504

No oxygen was detected, indicating that has been totally consumed. No signals were detected ascribable either to secondary products of ethylene oxide reactions (isomerisation products) or to products involving the C=C bond rupture.

Figure 4 - TPD spectra of the catalyst preoxidized at 473K and conditioned at 555K, under helium: - m/e = 32 a.m.u.; 0 - m/e = 16 a.m.u.

+

3.2.4. Interaction of ethylene at 298K with the catalyst preoxidized at 473K and conditioned at 750K under helium These runs were performed in order to study the effect of the elimination of both oxygen species ( a and 6 peaks, detected in TPD runs), in the behaviour of the catalyst in TPSR conditions. No signals in the mass spectrometer were detected in these conditions.

4. CONCLUSIONS The following main conclusions may be drawn from the results of the experiments carried out with this unsupported silver catalyst: - the pre-oxidation at 298K, brings about (i) C 0 2 formation by a low-temperature process and (ii) no ethylene oxide formation; - the pre-oxidation at 473K, brings about (i) ethylene oxide formation and (ii) C 0 2 formation by a high-temperature process, remaining active the low-temperature process; - the pre-oxidation of the catalyst at 473K, followed by its conditioning at 555K in helium, in such a way that the oxygen of the low-temperature desorption process ( a peak), is removed, brings about (i)the elimination of the low-temperature process of C02 formation, keeping the high-temperature process and (ii)the preservation of the ethylene oxide formation; - the pre-oxidation at 473K, followed by conditioning at 750K in helium, in such a way that the oxygen, corresponding to the desorbed species of the TPD experiments

505

(aand 0 peaks), is removed, causes the elimination of all the reactivity of the silver catalyst. A summary of the evolution of the different compounds detected in TPSR experiments, is presented in Table 1

0,lO 0,08

Z’0.06

-?

g 0,04 0,02 0,oo 273

3 7 i 473

573

673 773 873 973 4073

Tenpwdwe (K)

03’0 0.08

T

\

-

0,06

c

1.JpI.Y-u., , T = 395K

0,02

0,oo 273

373 473 573 673 773 873 973 1073 Tenperatwe (K)

g 0,04 0.02 0.00 273

373 473 573 673 773 873 973 1073 Tenperawe (K)

Figure 5 - TPSR spectra of ethylene, adsorbed at 298K over the catalyst, preoxidized at 473K and conditioned at 555K,under helium These results evidence that the pre-oxidation of the catalyst at 473K, induces the formation of a type of oxygen (not detected in TPD experiments) that is necessary not only to the ethylene oxide formation but also to the high temperature carbon dioxide formation. On the other hand, it is clear that the oxygen related to the (Y peak is only responsible for the low-temperature process of CO2 formation, while the 0 peak is responsible either for the ethylene oxide formation or for the high-temperature process of COz formation, provided the pre-oxidation is carried out at 473K. This seems to indicate that the same oxygen species is active for both epoxidation and total oxidation reactions. It also shows that the presence of an

506

oxygen species, induced by the pre-oxidation at 473K (probably a subsurface species, according to the literature) is involved in these last processes.

Compounds

Preoxidation Temperature 473K 298K Condit. Temp. Condit. Temp.

Ethylene Oxide C02 (low-temp.) C02 (high-temp.) Oxygen

+

-

+

5551750K

-

+ +

+

-

555K

750K

-

+

-

+

-

-

-

These results are in good agreement with those presented by different authors [5-9, 151 who have found that epoxidation only occurs with the simultaneous presence of both adsorbed and subsurface oxygen species. It is noteworthy that it was possible, at least partially, to assign a specific catalytic activity to the oxygen species detected at the silver catalyst.

REFERENCES 1 R.W. Joyner and M.W. Roberts, Chem. Phys. Letts., 60 (1979) 459. 2 M. Bowker, P. Pudney and G. Roberts, J. Chem. SOC.,Faraday Trans. I, 85 (1989) 2635. 3 C. Rheren, G. Isacc, R. Schlog and G. Ertl, Catal. Letters, 11 (1991) 253. 4 G. Rovida, F. Pratesi, M. Maglietta and E. Ferroni, J. Vac. Sci. Technol., 9 (1972) 796. 5 C. Back, J. Moolhuysen, P. Geenen and R.A. van Santen, J. Catalysis, 72 (1981) 434. 6 R.B. Grant and R.M. Lambert, J. Chem. SOC.,Chem. Commun. (1983) 662. 7 R.B. Grant and R.M. Lambert, J. Catal., 92 (1985) 364. 8 R.A. van Santen and C.P.M. De Groot, J. Catal., 98 (1986) 530. 9 R.A. van Santen, Proc. gth Int. Cong. Catal., Calgary, The Chem. Inst. Canada, vol. 3, 1988, p. 1152. 10 A. Orzechowsky and K.E. MacCormac, Can. J. Chem., 32 (1954) 443. 11 E.L. Force and A.T. Bell, J. catal., 40 (1975) 356. 12 N.W. Cant and W.K. Hall, J. Catal., 52 (1978) 81. 13 G. Rovida, J. Phys. Chem., 80 (1976) 150. 14 M. Dean and M. Bowker, Appl. Surf. Sci., 35 (1988) 27. 15 C.T. Campbell and M.F. Paffett, Surf. Sci., 143 (1984) 517. 16 R.B. Grant and R.M. Lambert, Surf. Sci., 146 (1984) 256. 17 R.A. Van Santen and H.P.C.E. Kuipers, Adv. Catal., 35 (1987) 265.

V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation II 1994 Elsevier Science B.V.

507

HRTEM and TPO Study of the Behaviour under Oxidizing Conditions of some Rh/CeO, Catalysts S. Bernal, G. Blanco, J.J. Calvino, G.A. Cifredo, J.A. PCrez Omil, J.M. Pintado and A. Varo Departamento de Quimica Inorganica. Facultad de Ciencias Quimicas. Universidad de Cadiz. Apartado 40. Puerto Real. 11510. Cadiz. Spain. Fax: 34-56-834924. A series of Rh/CeO, catalysts reduced at temperatures ranging from 523 K to 1173 K has been investigated by 0, Pulses-TPO and HRTEM. Rhodium crystals can be fully oxidized to Rh,O, by heating them at 773 K in flowing 0,(5%)/He. The ceria oxidation state could also be determined in every case. Upon reduction at 1173 K, metal decoration and encapsulation phenomena do occur. This prevents the reoxidation of part of the rhodium crystals.

1. INTRODUCTION Ceria can be considered as a major component of the so-called Three Way Catalysts (1,2). Because the oscillating lean/rich conditions under the TWC's work (3,4), the oxygen storage capacity of ceria (5,6) constitutes a key property allowing to widen the AirFuel operation window of the TCW's (7). For the past few years a good deal of work has been done an MIGO, catalysts (8-12). Very often, they were investigated as model systems of the TWC's. It would be noted, however, that in a few cases, the studies have dealt with the oxidiLed forms of the catalysts (6,13,14). This work reports on the behaviour under oxidizing conditions of a series of Rh/CeO, catalysts reduced at temperatures ranging from 523 K to 1173 K. Oxygen Pulses, Temperature Programmed Oxidation (TPO) and High Resolution Transmission Electron Microscopy (HRTEM) were used as experimental techniques.

2. EXPERIMENTAL

The Rh/CeO, catalysts studied in this work were prepared by the incipient wetness impregnation technique from an aqueous solution of Rh(NO,),. The support, a 99.9% pure ceria, with a surface area of 4 rn'g', was supplied by Alpha Ventron. After the impregnation treatment, the sample was dried in air, at 383 K, for 10 h, and further stored in a desiccator until its reduction "in situ". The metal loading was 2.4 %. The Oxygen Pulses and Temperature Programmed Oxidation (TPO) studies were carried out in an experimental setup similar to that described in ref. (15). This device was equipped with a thermal conductivity detector. The gascous mixture used for both pulses and TPO experirncnts was: 0,(5%)/He. The flow rate of either pure He or the mixture 0,(5%)/He was always: 60 cm3.min-'. The heating rate in the TPO experiments was: 10 Kmin-'. The sample weight used in these experiments was 200 mg. Before use, the gases, N-50 type (99.9990 %), from SEO, were further purified by passing them through either a series of deoxo and zeolite

traps or a zeolite trap only (O,/He mixture). The High Resolution Transmission Electron Microscopy (HRTEM) images were obtained with a JEOL JEM-2000-EX instrument, equipped wit! a top entry specimen holder and an ion pump. The structural resolution was better than 2.1 A. The samples to be investigated were prepared as reported elsewhere (16).

3. RESULTS

3.1. Oxygen Pulses and TPO Experiments The series of Rh(2.4%)/Ce02 catalysts investigated here were prepared by reducing for 1 h the Rh(N03)JCe02 precursor system with flowing H2 (60 crn3.min-') at 523 K, 773 K, 873 K, 973 K or 1173 K. Earlier studies from our Laboratory have shown that the reduction to Rh(0) of the ceria supported rhodium nitrate above occurs at around 423 K (17). Accordingly, the treatments applied here lead to fully reduced metal catalysts in every case. After reduction and evacuation for 1 h at the reduction temperature (To ensure the complete elimination of the hydrogen chemisorbed on ceria (18), the catalyst reduced at 523 K was further evacuated at 773 K), the samples were treated with 0,(5%)/He pulses, at 295 K, till no further oxygen consumption could be observed, Figure 1; then the gas flow was switched from pure He to 0,(5%)/He and the TPO experiment was run, Figure 2. In addition to the traces recorded for the series of catalysts mentioned above, Figure 2 includes, for comparison, the TPO diagram for bare ceria reduced at 973 K, evacuated at 973 K, and further treated with 0,(5%)/He pulses, at 295 K, in the same way as the Rh/CeO, catalysts. The TPO diagrams in Fig. 2 show a relatively narrow peak at 363 K (LTP),also observed for the bare ceria sample, the intensity of which increases as the reduction temperature does. Likewise, the traces for the Rh/CeO, catalysts show a second asymmetric much broader feature peaking at around 773 K (HTP). Upon separate integration of the two TPO peaks above, and taking into account the oxygen uptake at 295 K determined from the pulse experiments, we have been able to estimate the total amount of oxygen consumed by the series of Rh/CeO, catalysts investigated here. Table 1 accounts for these results. Data corresponding to bare ceria are reported elsewhere (18). Table 1 Amounts of Oxygen (Expressed as apparent O/Rh ratio) uptaken by the Rh/CeO, Catalysts TPO Redn./Evac. Temp. (K)

5237773 7731773 873/873 9731973 1173/1173

0, Pulses

LTP

HTP

0.9 0.8 3.4 5.1 6.4

0.0 0.1 0.2 0.5 1.0

1.6

1.7 1.7 1.7 1.0

Ceria Redn. Degree (% of Ce,O,)

8 8 30 46 55

509

Fig. 1. Amount of oxygen chemisorbed by the RhiCeO, catalyst reduced at 1173 K as determined from an O,(S%)/He pulse cxpcrimcnt at 295 K.

15 30 45 60 75 90 105

N u m b e r of P u l s e s

I

I

I

I

Fig. 2. TPO Diagrams corresponding to Rh/CcO, catalysls reduced at 523 K (A), 773 K (B), 873 K (C), 973 K (D) and 1173 K (E). The samples were evacuated at the rcduction temperature (except the one rcduccd at 523 K, which was further cviicualcd at 773 K), and treated w i t h O1(S%,)/He pulses at 295 K before running the TPO experiment. The trace for bare CeO, (F) rcduced at 973 K and pretreated as ~nclicstcdabove has also been included for corn par ison. ~

400

800 T e m p e r a t u r e (K)

1200

510

3.2. High Resolution Transmission Electron Microscopy Study Figure 3 reports on the HRTEM images recorded for the Rh/CeO, catalyst reduced at 773 K and further treated with oxygen, for 1 h, either at 373 K (Fig. 3A) or 773 K (Fig. 3C). The micrograph in Fig. 3A, which is similar to those reported in ref. (10) for the the same Rh/CeO, system just reduced at 773 K, shows clean, well faceted rhodium microcrystals seating on the ceria surface. This would suggest that the low temperature (373 K) oxygen treatment at 373 K does not strongly modify the metal particles. Also worth of noting, as already discussed in refs. (10,19), the images reported in Fig. 3A show the occurrence of a well defined epitaxial relationship between rhodium and ceria consisting of (111) planes of Rh growing parallel to (111) planes of the support. Figure 3C accounts for the HRTEM study of thc Rh/CeO, catalyst reduced at 773 K and further reoxidized at 773 K. In contrast with that reported for the catalysts treated with oxygen at 373 K, the microstructural characterization of the sample reoxidized at 773 K shows the occurrence of Rh,O,. Thus, the fringe structure of the particle marked with an arrow in Fig. 3C can be interpreted as due to (012) planes of the hexagonal phase of Rh,O,. Likewise, the corresponding selected area electron diffraction pattern, Fig. 3D, shows the existence of hexagonal rhodium sesquioxide. Earlier studies dealing with the oxidation of metallic rhodium with dry'air (20) are also in agreement with our observations. In effect, it is reported in ref. (20) that for treatment temperatures below 973 K, as is our case, the hexagonal sesquioxide phase is formed whereas for higher temperatures orthorhombic Rh,O, is found. We have also carried out a statistic analysis of the particle size of the rhodium particles in both, the reduced and oxidized phases. In the case of the low temperature oxidation treatment, no modification of either the mean size or the distribution curve can be noted; on the contrary the mean size of the particles in catalysts oxidized at 773 K is significantly larger than that of the corresponding reduced sample. Thus, the mean rhodium particle size for Rh/CeO, reduced at 973 K is: 3.7 nm, whereas for the oxidized catalyst the value is: 6.4 nm. This would be consistent with the lattice expansion inherent to thc formation of an oxidized phase. Furthermore, the occurrence of some sintering of Rh,O, during the oxygen treatment should also be considered.

4. DISCUSSION The results reported here allow one to draw a number of conclusions worth of being outlined. First of all, the feature peaking at 363 K in the TPO diagrams of Fig. 2 can be assigned to ceria oxidation, a process which in accordance with the pulse experiments takes place to a large extent at 295 K. This interpretation would be in agreement with occurrence of the same peak on pure ceria, Fig. 2 F, the absence of significant modifications on the rhodium microcrystals deduced from the HRTEM study in Figure 3, as well as with some earlier studies from the literature (18,21). Since bare ceria does not show any further oxygen uptake, Fig. 2F, the second much broader peak observed in the TPO diagrams would reasonably be assigned to the rhodium oxidation. The separate integration of the two peaks above provides some further irsights into the processes occurring throughout the whole TPO experiment. In effect, it can be deduced from Table 1 that for reduction temperatures ranging from 523 K to 973 K the oxygen consumption associated to the high temperature peak remains constant. Also interesting, the O/Rh w e have determined upon integration of this peak is in good agreement

511

Fig.3.- H R E M imagc (A) and Diffraction pattcrn (H) o f thc Rh/CeO, sample reduced at 773K and further oxidized at 373K. Noticc thc (1 I I ) planes oT rhodium lying parallel to the planes with the same indices in the support. The small arrows in the S A E D pattern point to (111) reflections of metallic rhodium; big arrows to ccria (1 1 I ) planes. (C) HREM image and (D) Difraction pattcrn corresponding t o thc samplc rcducccl a t 773K and reoxidized at 773K. T h e arrows in (D) indicate thc positions ol (102) rcllcctions in the RhZO, phase, also present in the particle shown in the bright field imagc (C).

512

with that required to fully oxidize the metallic rhodium to sesquioxide (O/Rh: 1.5). This would indicate that even in a fairly diluted oxygen atmosphere the =ria supported rhodium can be completely oxidized at the ordinary operation temperature of TWC's. Moreover, the broadness of the TF'O feature associated to rhodium oxidation suggests, as the results reported by US in ref. (14) for a Rh/CeO, catalyst with very high metal dispersion, that the smaller the rhodium microcrystals the easier their oxidation. In other words, a significant fraction of the supported rhodium can result fully oxidized at temperatures well below 773 K. These findings are in agreement with the results reported in the literature for the oxidation of rhodium dispersed on alumina and titania supports (22,23). For the catalyst reduced at 1173 K, the oxygen consumption associated to the high temperature peak, O/Rh: 1.0, is significantly smaller than expected. The HRTEM study of this catalyst, Figure 4, has allowed us to suggest an interpretation for this.

Fig. 4. HRTEM images corresponding to the Rh/CeO, catalyst reduced at 1173 K. In effect, the analysis of the HRTEM images corresponding to the catalyst reduced at such a high temperature shows the occurrence of both rhodium decoration and encapsulation phenomena. Support migration onto the metal, already observed on RhRiO, catalysts (24,25), as well as support sintering would be the major reponsible for these effects. To our knowledge, no HRTEM evidence of decoration phenomena like those depicted in Fig. 4 has earlier been reported for M/CeO, catalysts. In accordance with this observation, the TPO diagram can be interpreted as follows: consequently to the reduction treatment, about one third of the total rhodium particles present in the catalyst would become encapsulated, the support acting as protective cover which prevents the metal reoxidation. This work shows that metal and support contributions to the total oxygen consumption can be estimated separately. Accordingly, the results obtained from our P u l s e s m O experiments can be used to determine the actual oxidation state of ceria after each of the reduction

513

can be used to determine the actual oxidation state of ceria after each of the reduction treatments applied. As is discussed in refs. (14,18,26), when considered the ceria redox state, it would be distinguished the so-called reversible reduction state, the one associated to chemisorbed hydrogen or any other reducing probe molecule, which can be reverted by simple evacuation; from the irreversible state, the one associated to oxygen vacancies. In the present case, because the reduction treatment included a further evacuation aimed at eliminating the chemisorbed hydrogen, we are determining the ceria irreversible reduction degree. Table 1 accounts for this estimate expressed as percentage of CeO, reduced to Ce,O,. To summarize, we have investigated the behaviour under oxidizing conditions of a series of Rh/CeO, catalysts reduced at temperatures ranging from 523 K to 1173 K. It has been shown that the combination of 0, Pulses at 295 K and TPO techniques can fruitfully be used to estimate the contributions of both metal and support to the total oxygen uptake. Our results show that rhodium can be completely oxidized to Rh,O, under conditions similar to those under which the three way catalysts operate. Our study also shows that the reduction treatments applied here lead to supports presenting a wide range of reduction degrees. For the highest reduction temperatures, the HRTEM images show the occurrence of metal decoration/encapsulationeffects, which can protect the rhodium microcrystals against oxidation.

ACKNOWLEDGEMENTS This work has received financial support from the CICYT, Project with Reference PB-92/0483. The HRTEM images reported here were obtained at UCA Electron Microscopy Facilities. We also thank Mr. M.J. Romero for his help in the statistic analysis of the HRTEM images.

REFERENCES 1. 2. 3. 4. 5. 6.

7. 8. 9.

10. 11.

12. 13. 14.

B. Engler, E. Koberstein and P. Schubert, Appl. Catal., 48 (1989) 71. J.Z. Shyu and K. Otto, J. Catal., 115 (1989) 16. S.H. Oh and C.C. Eickel, J. Catal., 112 (1988) 543. P. Lijof, B. Kasemo and K.E. Keck, J. Catal., 118 (1989) 339. H.C. Yao and Y.F. Yu Yao, J. Catal., 86 (1984) 254. G.S. Zafiris and R.J. Gorte, J. Catal. 139 (1993) 561. T. Miki, T. Ogawa, M. Haneda, N. Kakuta, A. Ueno, S. Tateishi, S. Matsuura and M. Saio, J. Phys. Chem. 94 (1990) 6464. G.S. Zafiris and R.J. Gorte, Surface Sciencc, 276 (1992) 86 G. Munuera, A. Fernandez, and A.R. Gonzalez-Elipe, In Catalysis and Automotive Pollution Control II; A. Crucq, Ed.; Elsevier: Amsterdam, 1991, p. 207. S. Bernal, F.J. Botana, J.J. Calvino, M.A. Cauqui, G.A. Cifredo, A. Jobacho, J.M. Pintado and J.M. Rodriguez-Izquierdo, J. Phys. Chern. 97 (1993) 4118. A. Trovarelli, G. Dolcetti, C. Leitenburg, J. Kaspar, P. Finetti and A. Santoni, J. Chem. SOC.Faraday Trans., 88 (1992) 131 1 . P.J. Levy and M. Primet, Appl. Catal. 70 (1991) 263. T. Jin, Y. Zhou, G.J. Mains and J.M. White, J. Phys. Chem., 91 (1987) 5931 S. Bcrnal, J.J. Calvino, G.A. Cifredo, J.M. Rodriguez-Izquierdo, V. Perrichon and A.

5 14

15. 16. 17. 18. 19. 20. 21.

22. 23. 24. 25. 26.

Laachir, J. Catal., 137 (1992) 1. G. Blanchard, H. Charcosset, M. Forissier, F. Matray and L. Tournayan, J. Chromatogr. Sci., 20 (1982), 369. S. Bernal, F.J. Botana, R. Garcia, Z. Kang, M.L. Mpez, M. Pan, F. Ramirez, J.M. Rodriguez-Izquierdo, Catal. Today 2 (1988), 6.53. S. Bernal, F.J. Botana, R. Garcia, F. Ramirez and J.M. Rodriguez-Izquierdo, Mater. Chem. Phys., 18 (1987), 119. S. Bernal, J.J. Calvino, G.A. Cifredo, J.M. Gatica and J.M. Pintado, J. Chem. SOC. Faraday Trans. 89 (1993) 3499. M. Pan, J.M. Cowley and R. Garcia, Micron Microscopica Acta, 18 (1987) 16.5. L.A. Carol and G.S. Mann, Oxidation Met., 34 (1990) 1. A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J.C. Lavalley, J. El Fallah, L. Hilaire, F. Le Normand, E. QuCmerC, G.N. Sauvion and 0. Touret, J. Chem. SOC. Faraday Trans., 1991, 87, 1601. J.C. Vis, H.F.J. van’t Blik, T. Huizinga, J. van Grondelle and R. Prins, J. Catal., 95 (198.5) 333. A.D. Logan, E.J. Braunschweig, A.K. Datye and D.J. Smith, Ultramicroscopy, 31 (1989) 132. S. Bernal, J.J. Calvino, M.A. Cauqui, G.A. Cifredo, A. Jobacho and J.M. RodriguezIzquierdo, Appl. Catal. 99 (1993) 1. A.D. Logan, E.J. Braunschweig, A.K. Datye and D.J. Smith, Langmuir, 4 (1988) 827. S. Bernal, J.J. Calvino, G.A. Cifredo, J.M. Rodriguez-Izquierdo, V. Perrichon and A. Laachir, J. Chem. SOC.Chem. Comm. (1992) 460.

V. CoriCs Corberin and S. Vic Bellon (Editors), New Developmenls in Selective Oxidation II

0 1994 Elsevicr Science B.V. All rights reserved.

515

Redox molecular sieves as heterogeneous catalysts for liquid phase oxidations R.A. Sheldon*, J.D. Chen, J. Dakka and E. Neeleman Delft University of Technology Laboratory for Organic Chemistry and Catalysis Julianalaan 136, 2628 BL Delft The Netherlands SUMMARY

Redox molecular sieves have been synthesized by isomorphous substitution of chromium in the framework of silicalite-1 and AIPO-5. CrS-1 was shown to be an effective catalyst for the oxidative cleavage of styrene to benzaldehyde at 70 "C. CrAPO-5 catalyzes the selective oxidation of secondary alcohols to the corresponding ketones, alkylbenzenes to acetophenones and cyclohexane to cyclohexanone, using TBHP or 0, as the terminal oxidant. Both CrS-1 and CrAPO-5 could be recycled without loss of activity and no leaching of chromium from the catalyst occurred. In the case of CrAPO-5 evidence is provided for the reaction taking place inside the cavity of the molecular sieve. 1. INTRODUCTION

Catalytic oxidation is the single most important technology for the conversion of hydrocarbon feedstocks to industrial chemicals [ 13. Moreover, increasing environmental pressure has focussed attention on fine chemicals manufacture where stoichiometric oxidations with inorganic reagents such as dichromate and permanganate, are still widely used [2]. Consequently, there is a growing incentive to replace such antiquated technologies with cleaner, catalytic alternatives which do not produce excessive amounts of inorganic salt-containing effluent. Due to the low volatility and limited thermal stability of many fine chemicals, processing is often restricted to the liquid phase at moderate temperatures. Traditionally, catalytic oxidations in the liquid phase generally employ soluble metal salts as catalysts. However, solid catalysts offer several potential advantages over their homogeneous couterparts, e.g. ease of recovery and recycling and enhanced stability. Moreover, site-isolation of redox metals in inorganic matrices can provide oxidation catalysts with unique activities and selectivities. Thus, two major problems associated with soluble oxometal complexes are (a) the propensity of many oxometal species (e.g. titanyl, Ti'" = 0) to undergo agglomeration to inactive fi-oxo oligomers and (b) oxidative destruction of organic ligands. In principle, both of these problems can be circumvented by site isolation of discreet, monomeric oxometal

516

species in an inorganic matrix, whereby the latter functions as a thermodynamically stable ligand. One approach to designing stable solid catalysts with unique activities is to incorporate redox metals, by isomorphous substitution, into the lattice framework of a molecular sieve, such as silicalites, zeolites and aluminophosphates (alpos). We have coined the generic name redox molecular sieves [2-41 to describe such materials which can be regarded as ‘mineral enzymes’. Redox molecular sieves possess several advantageous properties compared to conventional metal-supported catalysts. Unlike amorphous materials, such as alumina and silica, molecular sieves have a regular microenvironment with homogeneous internal structures consisting of uniform, welldefined cavities and channels. They also provide the possibility of observing shape selective catalysis and, by ’fine tuning’ the size and hydrophobic/hydrophilic character of the redox cavity, designing ’tailor made’ oxidation catalysts. One can also expect more pronounced solvent effects since the molecular sieve can be regarded as a second solvent that extracts the substrate out of the bulk solvent. Last, but not least, we anticipate that incorporation of the redox metal into the stable lattice of a molecular sieve will afford a catalyst with enhanced stability towards leaching, a problem often encountered with metal-supported oxidation catalysts.

PHOSPHATE

ALUMINA

SILICALITES

V -VAPO -CrAPO

V Ti-ZSM-5 Ti- 5

-CoAPO,etc.

TS- 1 TS- 2

vs- 1 CIS- 1 Ti-ZSM-48

Figure 1. Types of redox molecular sieves. Various types of molecular sieves are, in principle, amenable to isomorphous substitution by transition metal ions (Figure 1). High-silica molecular sieves, e.g. silicalite, possess hydrophobic cavities and, hence, are suitable for oxidations of

517

organic substrates with aqueous hydrogen peroxide. Aluminophosphates, in contrast, have hydrophilic cavities that render them unsuitable for reactions with aqueous hydrogen peroxide. They can be successfully employed, however, with alkyl hydroperoxides or molecular oxygen. The pore dimensions of various molecular sieves are collected in Table 1. Table S Pore dimensions of molecular sieves Structure type

Isotopic framework structure

Ring number

Pore size

MFI MEL ZSM-48 FAU MOR BEA MCM-41

ZSM-5 ZSM-11 ZSM-48 y,x Mordenite Beta MCM-41

10 10

5.6 * 5.4 5.1 * 5.5 5.4 * 4.1 1.4 * 1.4 6.1 * 1.0 1.6 * 6.4 40-100

AEL AFI

AIPO4-11 AlPO4-5

so

12 12 12 unknown 10 12

6.3 * 3.9 1.3

Dimension

Incorporation of metals Ti, Zr, V Ti, V Ti Ti, Fe Ti, Fe Ti, Fe Fe

1 1

Co, Mn, Cr, V, Ti Co, Mn, Cr, V, Ti

A landmark in the development of redox molecular sieves was the titanium(1V) silicalite (TS-1) catalyst developed by Enichem workers [5-71. This truly remarkable catalyst mediates a variety of industrially useful oxidations with 30% aqueous hydrogen peroxide under very mild conditions. Examples include olefin epoxidation, phenol hydroxylation, cyclohexanone ammoximation and alcohol oxidations. Some of these processes have been or are being commercialized. More recently titanium(1V) has been incorporated into other molecular sieves, e.g. silicalite-2 [8], ZSM-48 [9] and zeolite beta [lo]. As part of an ongoing research program on redox molecular sieves we have synthesized and characterized a range of redox alpos, silicalites and zeolites. They were found to exhibit interesting catalytic behaviour in a variety of liquid phase oxidations using 02,H202 and R02H as the primary oxidant. 2. EXPERIMENTAL 2.1. Catalyst preparation Chromium silicalite (CrS-1) was hydrothermally synthesized by three different methods. For CrS-1 preparation with added H 2 S 0 4 we essentially followed the literature procedure [ll]. Synthesis in the presence of ammonia [12] or fluoride [13] was analogous to corresponding preparations of MFI type zeolites reported in the literature. In order to obtain pure CrS-1 (free of quartz) the autoclave was rotated at 320 rpm during the crystallization step. CrAPO-5 was hydrothermally synthesized in a 300 ml Teflon-lined autoclave by essentially following the reported procedure [ 141, using the following molar ratios: 0.05 Cr2O3:0.9 AI2O3:P2O5:Pr3N:50 H20. Crystallization was performed at 175 "C for

518

24 h. The crystalline material was subsequently calcined by heating the sample from room temperature to 500 "C at a rate of 60 "C/h and maintaining at 500 "C for 10 h. 2.2. Characterization of CrS-1 and CrAPO-5

X-ray diffraction (XRD) powder pattterns were recorded on a Philips PW 1877 automated powder diffractometer using CuKa radiation. Scanning electron microscopy (SEM) spectra were obtained using a Jeol JSM-35 scanning microscope. The samples were coated with an Au evaporated film. Diffuse reflectance electronic absorption spectroscopy (DREAS) spectra were measured by a Hitachi 150-20 UV-VIS spectrophotometer equipped with a diffuse reflectance unit, then recorded from wavelength 190 to 900 nm. Element analysis of calcined samples were performed by using an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin-Elmer Plasma-11). 3. RESULTS AND DISCUSSION 3.1. CrS-1 catalyzed oxidative cleavage of olefins with H,O?

Although TS-1 catalyzes a wide variety of selective oxidations there are certain synthetically useful reactions, e.g. oxidative cleavage of olefins (reaction l), for which titanium is not an effective catalyst. By analogy with known chemistry of chromium(V1) we envisaged that chromium(V1) silicalite would be a good catalyst for this reaction. Our assumptions were confirmed in a recent patent [ l l ] which reported the oxidative cleavage of methyl acrylate and methyl methacrylate, to the methyl esters of glyoxylic and pyruvic acids, respectively, using CrS-1 with aq. H,O, in acetonitrile at 40 "C. RCH=CHR'

+ 2H,O,

Cat.

> RCHO

+ R'CHO + 2H,O

(1)

The synthesis and characterization of chromium modified silicalite-1 has been described [ 15, 161. However, after calcination the chromium was easily leached from the catalyst by water [15] and it was considered unlikely that chromium had been incorporated into framework positions. A major problem associated with the incorporation of chromium in the silicalite framework is the propensity for chromium(II1) to undergo dimerization, via hydroxo bridge formation (reaction 2) at the high pH typical of hydrothermal synthesis conditions:

In order to circumvent this problem the hydrothermal synthesis conditions have to be modified. We have used three different approaches to suppress dimerization: by adding H,SO, to reduce the pH to 9-9.5 as reported [ l l ] or by the addition of NH, or NH,F. Ammonia stabilizes monomeric chromium(II1) species via the formation of

519

amine complexes and fluoride effects dissolution of silica at neutral pH [14]. The as-synthesized catalysts were green and contained chromium(II1). After calcination at 500 “C they were yellow and contained chromium in the hexavalent state. The catalysts were shown by ICP-AES analysis to contain ca. 1% chromium. The MFI topology of the CrS-1 was confirmed by comparing the XRD spectra of samples with that of silicalite-1. Interestingly, we found that a pure catalyst was obtained only when the autoclave was rotated rapidly during the crystallization, otherwise substantial amounts of quartz crystallized simultaneously with the CrS-1 (see Figure 2).

!

Figure 2. XRD spectra of chromium silicalite samples: (a) without rotation of the autoclave; (b) 150 rpm; (c) 320 rpm.

520

The catalytic activities of CrS- 1 samples, synthesized by the various procedures, were evaluated in the oxidative cleavage of styrene with 35% H202 in dichloroethane at 70 "C (Table 2). The major products were benzaldehyde and l-phenyl-1,2-ethanediol, together with smaller amounts of styrene oxide and phenylacetaldehyde. The latter results from rearrangement of styrene oxide and is the major product of TS-1 catalyzed oxidation of styrene with H202 IS]. The highest conversions were observed with CrS-1 prepared by the ammonia method. CrS-1 prepared by the fluoride method gave a slightly lower conversion and roughly the same selectivities. The catalyst prepared by the H,SO, method gave substantially lower styrene conversions and selectivity on H,02 consumed (although the selectivity to benzaldehyde on styrene converted was high). Table 2 CrS-1 catalyzed cleavage of styrene with 35% H202 at 70 "C. The effect of catalyst preparationa 2 e q H,O, I

I

>

PhCRO +

'

PhCH-CH

l e q H,O,

I

OH

H.CO

OH

'

(11)

Synthesis method

a

'

+ PhCH-CH

I

0

(111)

Conversion (YO)

+ PbCH,CHO

(I?')

Selectivity (%)

H202

Styrene

I

11

111

IV

on H,o,~

F'

90

26

57

31

6

6

90

NH3

98

34

52

35

4

9

100

H2S04

99

9

85

0

1

1C

31

Conditions: 100 mmol styrene, 50 mmol 35% H202, 0.1 g catalyst (containing ca. 1% Cr; 0.02 mmol) and 20 g of 1,2-dichloroethane, 4 hrs at 70 "C. Selectivity to I-IV on H202 consumed. Some polymeric material was also detected.

We also made a cursory examination of the effect of temperature and solvent on the reaction using the NH3-based catalyst (Table 3). Dichloroethane proved to be the best of the three solvents tested and reactions at 70 "C gave better results than those

52 1

at 55 "C or 40 "C. Table 3 The effect of solvent and temperature on CrS-1 catalyzed cleavage of styrenea Solvent

Toluene

a

Temp. "C

MTBE~

70 70

DCEC DCE DCE

70 55 40

Conversion (%)

Selectivity (%)

H202

Styrene

I

I1

III+IV

48 84 98 67 24

9 4 34 17 3

77 78 52 73 68

0 12 35 17

8 10 13 14 24

0

on H,O,

61 10 100 89 23

For conditions see Table 2. The catalyst was prepared by the ammonia method. MTBE = methyl tert-butyl ether. DCE = dichloroethane.

Although the conditions have not yet been optimalized for the maximum yield of cleavage products this method appears to have synthetic potential for the cleavage of olefins to aldehydes. Moreover, in one experiment with the NH3-based CrS-1 we demonstrated that the catalyst could be recycled 3 times without any loss of activity. Analysis of the mother liquors by ICP-AES showed that no chromium had been leached into the solution during the reaction. 3.2. CrAPO-5 catalyzed oxidations of secondary alcohols The oxidation of primary and secondary alcohols to the corresponding carbonyl compounds is an important synthetic transformation. Traditionally it was performed with stoichiometric amounts of chromium(VI) reagents [ 17, 181. However, because of the serious environmental problems associated with chromium-containing effluent attention has been focussed on the use of catalytic amounts of chromium in conjunction with, e.g. tert-butylhydroperoxide (TBHP), as the stoichiometric oxidant [18]. Obviously the use of a recyclable, solid catalyst would offer additional advantages in this respect. Since the discovery of the aluminophosphate (AlPO,) molecular sieves in 1982 [19] much attention has been directed towards the incorporation of various elements into the framework of these molecular sieves [20]. However, little is known about the potential of redox metal substituted aluminophosphates as catalysts for liquid phase oxidations. We envisaged that chromium-substituted AlP04s would probably be effective recyclable catalysts in conjunction with TBHP. To this end we have synthesized CrAPO-5 according to a literature procedure [ 141. As-synthesized CrAPO-5 was green and contained chromium in the trivalent state. After calcination at 500 "C the catalyst was yellow and DREAS showed that most of the chromium was present as Cr(V1). ICP-AES analysis showed that a Cr content of up to 1.5% could be achieved. The as-synthesized and calcined CrAPO-5 were characterized by XRD which gave well-defined, reproducible patterns. SEM showed that the particle sizes (from 5 to 70 pm) and morphologies of CrAPO-5 were

522

dependent on the precise synthesis conditions. The results of CrAPO-5 catalyzed oxidations of some secondary alcohols with TBHP at 85 "C are shown in Table 4. Good to excellent selectivities to the corresponding carbonyl compounds were observed with respect to both substrate and TBHP in most cases. Carveol (V) underwent chemoselective oxidation of its alcohol group, to give carvone (VI), without any attack at its double bonds. The diol (MI) was selectively oxidized at the secondary alcohol group to give (VIII). Table 4 CrAPO-5 catalyzed oxidations of secondary alcohols with TBHP at 85 'Ca Substrate

a-Ethylbenzyl alchol a-Methylbenzyl alchol Cyclohexanol Carveol 1-Phenyl-1,2ethanediol a

Time (h)

Product

Conversion

Selectivity (%)

(Wb Substrate

TBHP

7

propiophenone

77

100

91

16 12 16

acetophenone cyclohexanone carvone a-hydroxyacetouhenone

77 72 62

96 85 94

89 73 66

54

73

40

16

Conditions: substrate, 10 mmol; TBHP, 5 mmol; CrAPO-5 (0.14 mmol, chlorobenzene (solvent), 10 ml; stirred at 85 "C for 16 h under N,. Conversion of substrate based on the amount of TBHP charged.

Interestingly when the oxidation of a-methylbenzyl alcohol with TBHP was carried out in air instead of N, a yield of acetophenone on TBHP of 216% was obtained. This suggested that 0, could also act as the terminal oxidant. Subsequent

523

experiments confirmed this as shown in Table 5. The best results were obtained using a small amount (10 mol %) of TBHP to initiate the reaction. Table 5 CrAPO-5 catalyzed oxidations of secondary alcohols with OZa

a

Conversion (%)

Selectivity (%)

cyclohexanone

30

97

acetophenone

31

96

propiophenone a-tetralone 1-indanone

38 26 78

90 73 72

Substrate

Product

Cyclohexanol a-Methylbenzyl alcohol a-Ethylbenzyl alcoholb a-Tetralolb 1-Indanolb

Conditions: substrate, 250 mmol; 0, pressure 5 atm or 20 atm air; TBHP 25 mmol; CrAF'O-5 3.65 mmol Cr; chlorobenzene (solvent), 65 ml; 3 A molecular sieve (drying agent), 6 g; 110 "C, 5 h. Conditions: substrate, 50 mmol; O,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 mmol; chlorobenzene, 5 ml, 110 "C, stirring 1000 rpm, 19 h.

In one experiment with a-methylbenzyl alcohol and TBHP at 85 "C the CrAPO-5 catalyst was filtered, washed 3 times with chlorobenzene and recalcined before reuse. The catalyst was recycled 4 times without any noticeable loss of activity or selectivity. DREAS spectra showed that most of the chromium remained in the hexavalent state within the AlPO, framework after recycling. Hence, we conclude that CrAPO-5 is a stable, recyclable catalyst for the selective liquid phase oxidation of secondary alcohols to the corresponding ketones, using TBHP as the terminal oxidant. We tentatively assume that the reaction involves oxidation of the alcohol by an oxochromium(V1) species followed by reoxidation of the reduced chromium(1V) by hydroperoxide. When 0, is used the chromium(1V) is reoxidized by the a-hydroxy hydroperoxide derived from autoxidation of the substrate. 3.3. CrAPO-5 catalyzed oxidations of hydrocarbons By analogy with the chemistry of soluble chromium complexes [18] we reasoned that CrAPO-5 should also be an effective catalyst for benzylic oxidations with TBHP. Indeed, CrAPO-5 catalyzed the selective oxidation of ethylbenzene and tetralin with

TBHP. As in the case of the alcohol oxidations (see earlier), when the tetralin oxidation was carried out in air a selectivity of the product higher than 100% on TBHP consumed was observed. Subsequent experiments showed that CrAPO-5 was an effective catalyst for the autoxidation of benzylic hydrocarbons to the corresponding ketones. A small amount (10 mol %) of TBHP was added to initiate the reaction. For example, tetralin was oxidized to a mixture of a-tetralone (64%), a-tetralol (7%) and a-tetralin hydroperoxide (20%). Furthermore, it was shown that the CrAPO-5

524

could be recycled 5 times without loss of activity (Table 6). Recalcination of the catalyst between cycles was not necessary.

Table 6 Recycling of CrAPO-5 in the autoxidation of tetralin at 100 "Ca Cycle nr.

1 2 3 4 5c a

Selectivity (%)

Conversion (%> 44 58 57 53 57

a-tetralone

a-tetra101

64 65 60 61 65

7 6

5 5 6

THP~ 20 24 30 32 24

Conditions: tetralin, 50 rnrnol; 0,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 rnmol Cr; 100 "C, stirring, 1000 rpm, 10 h. THP = a-tetralin hydroperoxide. The CrAPO-5 was regenerated by calcination at 500 "C for 5 h.

Similarly, cyclohexane was oxidized at 115 "C to give a mixture of cyclohexanone (64%), cyclohexanol (10%) and cyclohexyl hydroperoxide (9%), together with (di)carboxylic acids (13%), at 3% cyclohexane conversion. 3.4. CrAPO-5 catalyzed decomposition of alkyl hydroperoxides CrAPO-5 is also an effective, recyclable catalyst for the decomposition of alkyl hydroperoxides (see Table 7).

525

Table 7 CrAPO-5 catalyzed decomposition of alkyl hydroperoxidesa

RO,H

Cyclohexyl tert-Butylb Cumene Triphenylmethyl a

Conversion (%)

Solvent

87 49 24 1

C6H12

C,H,Cl C,H,Cl 1,2-C2H,C1,

Selectivity (%) Ketone

Alcohol

86

13 93 86

5 2

Conditions: R02H, 2.9 mmol; CrAPO-5, 0.029 rnmol Cr; 12 ml of solvent; 70 "C, stirring loo0 rpm, 5 h. 50 "C.

Secondary hydroperoxides, such as cyclohexyl hydroperoxide, afford a high yield of the corresponding ketone according to the stoichiometry: R

/K

\,

'

1-I

I=

[CrL,P+51

= ;

2

C1J-I

0

4

H;CI

(7)

li

Evidence for the reaction taking place inside the cavity of the molecular sieve was provided by the observation that triphenylmethylhydroperoxide, which cannot be accommodated in the cavity of CrAPO-5, was not decomposed. In contrast, homogeneous chromium acetylacetonate and the supported Cr02C12/silica-alumina were effective catalysts for the decomposition of this hydroperoxide (Table 8). Table 8 Accessibility of Cr-containing catalysts in the decomposition of Ph3C02Ha Catalyst Cr(acac)3 Cr02C12/Si02-A1203 CrAPO-5 a

Conversion (%) 75 72 1

Conditions: Ph,C02H, 2.9 mmol; catalyst, 0.029 mmol Cr; 1,2-dichloroethane, 12 ml; 70 OC; stirring 1000 rpm; 2 h.

Furthermore, recycling experiments showed that the CrAPO-5 retained its activity and that no leaching of chromium from the catalyst occurred during the reactions.

526

4. CONCLUDING REMARKS

We have shown that redox molecular sieves containing chromium isomorphously substituted in a silicalite-1 or AlPO-5 framework are stable recyclable catalysts for liquid phase oxidations. CrS-1 is an effective catalyst for oxidative cleavage of olefins with aq. H,O, and CrAPO-5 is an effective catalyst for the oxidation of hydrocarbons and secondary alcohols with TBHP or 0,. The scope and mechanism of these and related liquid phase oxidations mediated by redox molecular sieves are being further investigated. 5. REFERENCES

1. R.A. Sheldon and J.K. Kochi, ‘Metal-Catalyzed Oxidations of Organic Compounds’, Academic Press, New York, 1981. 2. R.A. Sheldon, CHEMTECH (1991) 566. 3. R.A. Sheldon, in Topics Curr. Chem., 164 (1993) 21. 4. R.A. Sheldon and J. Dakka, in Tagungsbericht 9204, Proceedings of the DGMK Conference ‘Selective Oxidations in Petrochemistry’, Goslar, Germany, Sept. 1618, 1992, pp. 215-225. 5. U. Romano, A. Esposito, F. Maspero, C. Neri and M. Clerici, Chim. Ind. (Milan), 72 (1990) 610. 6. M.G. Clerici and P. Ingallina, J. Catal., 140 (1993) 71. 7. M.G. Clerici, Appl. Catal., 68 (1991) 249. 8. J.S. Reddy and S. Sivasanker, Catal. Lett., 11 (1991) 241; J.S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 58 (1990) L1. 9. D.P. Serrano, H.X. Li and M.E. Davis, J. Chem. SOC.,Chem. Commun., (1992) 745. 10. M.A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem. SOC., Chem. Commun., (1992) 589. 11. M. Kawai and T. Kyora, Japanese Patents, JP 0358,954 and JP 0356,439 (1991) to Mitsui Toatsu Chemicals; CA 115 (1991) 48863d and 48864e. 12. M. Ghamami and L.B. Sand, Zeolites, 3 (1983) 155. 13. J.L. Guth, H. Kessler, J.M. Higel, J.M. Lamblin, J. Patarin, A. Seive, J.M. Chezeau and R. Wey, ACS Symp. Ser., 398 (1989) 176. 14. E.M. Flanigen, B.M.T. Lok, R.L. Patton and S.T. Wilson, US Patent, 4,759,919 (1988) to Union Carbide Corp. 15. J.S.T. Mambrin, E.J.S. Vichi, H.O. Pastore, C.U. Davanzo, H. Vargas, E. Silva and 0. Nakamura, J. Chem. SOC.,Chem. Commun., (1991) 922. 16. U. Cornaro, P. Jim, Z. Tvaruzkova and K. Habersberger, in ‘Zeolite Chemistry and Catalysis’, P.A. Jacobs et al., Eds., Elsevier, Amsterdam, 1991, pp. 165-172. 17. G. Cainelli and G. Cardillo, ‘Chromium Oxidations in Organic Chemistry’, Springer-Verlag, Berlin, 1984. 18. J. Muzart, Chem. Rev., 92 (1992) 113. 19. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. SOC.,104 (1982) 1146. 20. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannon and E.M. Flanigen, J. Am. Chem. SOC.,106 (1984) 6092; S.T. Wilson and E.M. Flanigen, Eur. Pat. Appl., 132,708 (1985).

521

F. TRIFIRO (U. Bologna): In liquid phase homogeneous oxidation three types of oxidation are known: radical chain, oxygen transfer and redox. I think it is not correct to call the selective oxidation with titanium silicalite a redox type catalysis. The high selectivity is due to the fact that it is an oxygen transfer type catalysis, the key property of titanium silicalite being that it does not exhibit redox type catalysis and, hence, does not decompose H202. R.A. SHELDON (TU Delft): According to my definition, which not everybody will agree with, a redox catalyst is something which catalyzes an oxidation (or reduction) reaction. The name does not imply whether or not the catalytic cycle involves a valence change of the metal.

B. GRZYBOWSKA (I. Catalysis, Krakow, Poland): You have pointed out the importance of 'site isolation' as one of the advantages of redox molecular sieves and have drawn attention to links between various branches of selective oxidation. I would like to stress that in high temperature (gas phase) oxidations site isolation is advantageous only when the oxide support provides for rapid diffusion of 02ions. For example, when isolated vanadyl or molybdenyl species are supported on TiO, (a non-reducible oxide under the reaction conditions) the selectivity in partial oxidations, e.g. o-xylene to phthalic anhydride or butene-1 to maleic anhydride, is lower than with oligomeric oxometal species in which M-0-M-0 chains are present. R.A. SHELDON (TU Delft): The point I am making is that isolated oxometal species are more reactive than p-oxobridged oligomers, both in the gas phase and the liquid phase. In the liquid phase this is an advantage as the extra reactivity is necessary. In the gas phase at high temperatures, in contrast, site isolation can give catalysts that are too reactive, resulting in complete combustion of the substrate to carbon oxides and water. R.K. GRASSELLI (Mobil, Princeton): Just to clarify your statement that Cr(V1) is located within the cavities of the molecular sieves, I suppose that you mean that the Cr(V1) is attached to the internal surface but does not necessarily occupy a framework position. R.A. SHELDON (TU Delft): Our experimental evidence is consistent with the Cr(V1) being attached to the internal surface, probably as you suggest, at a defect site.

R.K. GRASSELLI (Mobil, Princeton): As far as I know Cr(V1) containing

528

materials are environmentally undesirable (unlike Cr(II1) materials) and might be difficult to commercialize. Could you comment on this? R.A. SHELDON (TU Delft): We have not observed any leaching of chromium from the catalysts but I am not sufficiently acquainted with the regulatory constraints to judge what the barrier to commercialization would be.

J.M. BREGEAULT (U. P&M Curie, Pans): Did you analyze the liquid phase for chromium, e.g. by atomic absorption spectrometry? Is there any leaching of chromium?

RA. SHELDON (TU Delft): We have indeed shown by atomic absorption spectrometric anaysis that no chromium is present in the liquid phase after the reaction in oxidations catalyzed by CrAPO-5. B. SULIKOWSKI (I. Catalysis, Krakow): What is the siting of Cr(1II) in the as-synthesized Cr-silicalite and of Cr(VI) in the calcined material? This is of interest in view of the recent discussion of the siting of Ti in Ti-silicalite. According to a recent paper [l] the Ti is not located in a regular T site but occupies a framework defect position. [l] D. Trong On, A. Bittar, A. Sayari, S. Kaliaguine and L. Bonneviot, Catal. Lett. 16,85 (1992). R.A. SHELDON (TU Delft): More characterization studies are necessary in order to clarify the exact structure of the chromium site. In the case of the Cr(V1) in the calcined catalyst it is likely that the Cr occupies a framework defect position. L. SIMANDI (Centr. Res. Inst. Chem., Budapest): Do coordinating polar solvents and products compete with the substrate for the metal site within the cavity? R.A. SHELDON (TU Delft): The extent to which polar products and solvents

compete with the substrate is very much dependent on the hydrophilicity of the molecular sieve. Silicalite has a hydrophobic cavity and preferentially adsorbs non-polar substrates. Hence, oxidations with H,O, are favorable with silicalitebased catalysts since there is no competitive adsorption of the water coproduct. Aluminophosphates, in contrast, have hydrophilic cavities and are not suitable for H202 oxidations. Moreover, polar solvents and products can compete with the substrate for adsorption into the cavity. Hence, oxidations catalyzed by metalsubstituted aluminophosphates should be carried out with R0,H or 0, in

529

non-coordinating solvents with simultaneous removal of any water that is formed.

J. LEROU (DuPont, Wilmington): The rates of your reactions are rather slow. Will you be able to optimize your catalysts to make them competitive with current commercial processes, e.g. for the oxidation of cyclohexane. RA. SHELDON (TU Delft): Up till now we have been primarily investigating the scope of these systems and have paid little attention to optimization of rates. Of course in practice such catalysts will probably be used in fixed-bed operation and that is what we intend to study when we address the question of optimization.

H. MIMOUN (Firmenich, Geneva): Styrene is known to cleave relatively easily with radical-type oxidation catalysts. Did you try any olefins having allylic hydrogen atoms? R.A. SHELDON (TU Delft): Up till now we have not tried olefins having allylic C-H bonds. As you imply, one can expect allylic oxidation to compete with

oxidative cleavage in such substrates. We intend to investigate this.

This Page Intentionally Left Blank

V . CortCs Corbcrin and S. Vic Bell611 (Editors), N e w Developmtenis in Sekciive Oxidurion I/ 0 1994 Elscvicr Science B.V. All rights reservcd.

531

Influence of the Synthesis Procedure and Chemical Composition on the Activity of Titanium in Ti-Beta Catalysts M.A. Camblor, A. Corma, A. Martinez, J. Perez-Pariente and S . Valencia lnstituto de Tecnologia Quimica, UPV-CSIC, Camino de Vera s/n, Universidad Politecnica de Valencia, 46071 Valencia, Spain.

Ti-Beta samples with different Ti and Al content and synthesized by different procedures have been characterized by XRD, i.r., SEM, Diffuse Reflectance Spectroscopy (DRS) in the UV-Vis region, and XPS. The catalytic activity of the samples was measured using the oxidation of 1-hexene as a test reaction. The Ti and Al content and the distribution of both elements along the crystallites of Ti-Beta was seen to affect the intrinsic activity of the Ti. The simultaneous presence of Al and Ti in the same crystal region reduces the intrinsic activity of titanium for the oxidation reaction. Moreover, it is found that the intrinsic activity of the Ti atom in Ti-Beta is lower than in TS-1. 1. INTRODUCTION

The field of application of zeolites was greatly enhanced when the ENl's group synthesized a Ti-containing zeolite having the MFI structure. This material, which was named as TS-1, was able to carry out the oxidation of a variety of organic molecules under relatively mild conditions and using HO , , as oxidant (1,2). After this success, a Ti-silicalite having MEL structure was also synthesized and proved to have similar properties than their MFI analogous for the selective oxidation of hydrocarbons (3). More recently, the synthesis of a Ti-ZSM-48 material has also been claimed (4). However, only relatively small molecules can be effectively oxidized on these medium pore zeolite derivatives due to the restrictions imposed to bulkier molecules to reach the active sites in the zeolite cavities (2). Recently, we have synthesized for the first time a large pore Ti-containing zeolite isomorphous to zeolite Beta (5),denoted as Ti-Beta. This material was shown to give a higher activity than TS-1 during the oxidation of cycloalkanes and cydoalkenes (5,6), but the activity per Ti site was higher on TS-1 in the case of molecules with less steric restrictions, such as 1-hexene and 1-dodecene (6). In contrast to what occurs for TS-1, Ti-Beta samples can not be prepared, up to now, in the absence of aluminium. Moreover, it has been shown (6) that the presence of Al in Ti-Beta affects its catalytic activity. On the other hand, the activity of TS-1 has been reported to change somewhat with the synthesis conditions (7).

532

The aim of this work has been to evaluate the influence of the Al content and distribution, and synthesis procedure on the specific activity of the Ti centers present in the framework of zeolite Ti-Beta. Then, samples with different Ti and Al content and prepared by two synthesis methods have been characterized by a variety of techniques (XRD, i.r., SEM, DRS, and XPS) and catalytically tested in the l-hexene oxidation reaction. The catalytic results have been compared with those obtained on a standard TS-1 sample (Euro TS-1) under the same reaction conditions. 2. EXPERIMENTAL 2.1. Synthesis procedures

Method I&Tetraethylammoniumhydroxide (TEAOH) (40% aqueous solution, K < 1 ppm, Na < 3 ppm, Alfa) was diluted with the required amount of water, and to this solution tetraethylorthotitanate(Alfa) and amorphous silica (Aerosil200, Degussa)were added and hydrolyzed at room temperature with stirring. Theg, a solution of aluminium nitrate (Merck), and in one case, Na' and K', were added to the reaction mixture. Method R; Tetraethylorthosilicate(TEOS, Merck) was hydrolyzed in TEAOH and water at room temperature with stirring. After the hydrolysis of the alcoxide, a clear solution was formed. After that, TEOTi and water were added. Finally, a solution of aluminium nitrate in water and TEAOH was also added, and the resulting mixture maintained under stirring until complete evaporation of the ethanol formed upon hydrolysis of the alcoxide. According to this method, a Ti-Beta sample was also prepared in such a way that the titanium was incorporated into the crystals after the incorporation of aluminium (sample 9 in Table 1). In all cases, the resulting gels were crystallized in 60 ml PTFE-lined stainlesssteel autoclaves at 408 K under rotation at 60 rpm for selected periods of time. After cooling the autoclaves the samples were centrifuged at 10.000 rpm and the recovered solids were washed until pH = 9, dried at 353 K, and finally calcined at 853 K. The TS-1 sample used in this work as reference was the Euro TS-1 catalyst, with a Ti content of 1.7 Wh TiO, measured by XRF. 2.2. Characterization

The crystallinityof Ti-Beta samples was determined by X-ray powder diffraction (XRD) on a Phillips PW-1830 spectrometer using the CuK, radiation, after dehydration of the samples at 383K for 1 h and further rehydration over a CaCI, saturated solution (35% relative humidity). The appearance of the 960 cm-' i.r. band was followed by i.r. spectroscopy in a Nicolet FTlR 710 using the KBr pellet technique. The total amount of Ti in the samples was measured by X-ray fluorescence (XRF) in an Outokumpu XMET 840. Diffuse Reflectance (DR) spectra were done on a Shimadzu UV-2101 PC spectrometer equipped with a diffuse reflectance attachment using BaSO, as reference.

533

The surface composition of Ti-Beta samples was determined by X-ray photoelectron spectroscopy (XPS) on an ESCAlAB-200R spectrometer. Binding energies were corrected for charge effects by reference to the C,,peak at 284.9 eV. Scanning Electron Microscopy (SEM) has been used to determine the crystal size of the samples. The average diameter of the crystallites was seen to be below 0.2 pm for all the Ti-Beta samples. 2.3. Catalytic experiments The 1-hexene oxidation reaction was carried out in a glass flask with reflux and magnetic agitation. In a typical experiment, 33 mmol of 1-hexene, 23.57 g of methanol as solvent, and 0.264 g of H,O, (35 wt%) were mixed and stirred at 298K. Then, 200 mg of catalyst were added to the mixture. Aliquotes were taken at different reaction times and analyzed by GC-MS on a capillary column (5% methylphenylsilicone, 25 m). The unreacted H,O, was determined by standard iodometric titration. 3. RESULTS AND DISCUSSION

Table 1 shows the chemical composition of Ti-Beta synthesized by the procedures described above. High crystalline samples are obtained by both methods, except when the synthesis is carried out in the presence of alkali cations (sample 6). Moreover, an increase of the interplanar d-spacing (XRD) and the intensity of the 960 cm-' i.r. band when increasing the Ti content was found for the samples prepared from alkali-free reaction mixtures, which can be taken, in principle, as an evidence of Ti incorporation into the zeolite framework (8).

-Table 1: ompositi

1

of Ti-Beta sampl

-

b

Sample

Method of synthesis

A

*

; svnthesized

In gel T'"OJAl,O,

bv different DrOCedUreS. In zeolite

Ti/(Si+Ti)

96 Beta XRD

Ti/(SitTi)

T'"OJAI,O,

0.01 6

108

TiO,

(wt%) 2.1

400

0.008

98

A

400

0.048

91

0.032

104

4.0

A

800

0.016

86

0.021

119

2.7

A

800

0.016

82

0.027

246

3.5

4

400

0.048

85

0.040

210

5.2

A'

400

0.016

63

0.044

116

5.7

0

400

0.016

84

0.024

105

3.1

0

400

0.016

83

0.029

207

3.8

0

--

0.048

80

0.032

205

4.2

Sample synthesized in the presence of alkali cations: (Na+K)/T'"O, = 0.12

5 34

The DR spectra in the UV-Visible region of calcined samples prepared from Aerosil and TEOS are shown in Figure 1. No peak at about 330 nm corresponding to anatase is detected, even in those samples with the highest Ti content. Sample 6, synthesized in the presence of alkali cations, shows an intense very broad band at 270 nm, attributed to hexacoordinated Ti species belonging to an amorphous titanosilicate phase having probably Ti-0-Ti bonds. This band is only hardly detected in the Ti-Beta samples crystallized without alkali cations. These samples show other bands at 205,215 and 225 nm. Dehydrated TS-1 shows a band at - 205 nm, which has been assigned to isolated Ti atoms in tetrahedral coordination (9). This band shift to - 230 nm by hydration, the Ti coordination increasing from 4 to 6-fold. If the same assignment applies for Ti-Beta, it can be seen in the Figure that an increase of the Ti content seems to increase the proportion of framework Ti atoms with low coordination numbers.

-

190

345

WAVELENGTH (nm)

WAVELENGTH (nm)

-

FIGURE 1. Diffuse Reflectance UV-Visible spectra of calcined Ti-Beta samples prepared by methods A ( a ) and B (b). Numbers in Figure correspond to sample numbers in Table 1.

535

However, the XPS measurements shown in Table 2 revealed the presence of minor amounts of an anatase-like phase on the surface of calcined Ti-Beta samples prepared according to method A, as can be seen from the values of Ti(2p3,J binding energies close to that of anatase (458.4 ev). The fact that TiO, was not observed in the DR-UV spectra of these samples suggest that the anatase phase is present as finely dispersed particles on the crystal surface. Table 2 Binding Enerl Ti-Beta Sam

(ev) of the Ti (2p,)

peak and bulk and surface composition of calcined

?S.

AIISI

TiISi

*

Sample

BE (eV)

Bulk'

Surface

Bulk'

Surface

1

458.0 459.8

0.016

0.010

0.019

0.016

2

458.0 459.8

0.033

0.022

0.020

0.013

5

458.2 459.6

0.042

0.094

0.010

0.000

6

458.7 459.4

0.046

0.036

0.018

0.021

7

459.8

0.025

0.027

0.020

0.000

8

459.8

0.030

0.035

0.010

0.000

9

459.8

0.033

0.063

0.010

0.000

From atomic absorption analysis.

The Ti and Al content of the zeolite surface is also shown in Table 2. There it can be seen that all the samples prepared from TEOS (samples 7 to 9) show an aluminium-free surface, while this situation only occurs in those samples synthesized from Aerosil with a high Si/AI bulk ratio (sample 5). The presence of Al in the framework was seen to affect the catalytic activity of Ti-Beta samples in a wide range of Ti and Al compositions and prepared according to method A (6). Thus, the l-hexene conversion decreased when increasing the Al content, and an almost linear relationship between the initial reaction rate and the Ti minus Al content of Ti-Beta samples was found. This results could suggest that the activity of Ti-Beta is mainly determined by the Ti atoms in the parts of the framework with Si and Ti atoms only (named as "effective" Ti in Figure 2). According to the XPS results above presented, different Al gradients can exist in Ti-Beta samples depending on the synthesis procedure (Aerosil or TEOS), so, in principle, one should expect differences in catalytic activity. However, the results presented in Figure 2 show that the same correlation found for samples prepared from Aerosil between the initial reaction rate and the Ti and A1 content applies to samples prepared from TEOS, eventhough the latter probably have a higher proportion of Ti atoms in Al-free regions of the crystals.

536

10

8

6

4

2

Olf 0

I

I

I

I

I

0.5

1

1.5

2

2.5

3

(Ti-A1)lC.U.

FIGURE 2. Initial reaction rate in the oxidation of 1-hexene as a function of the "effective" Ti content of Ti-Beta samples prepared by methods A (A)and B ( 0 ) .

The negative effect of Al on the activity of Ti-Beta catalysts can be related to changes in the electronegativity of the zeolite lattice induced by the presence of aluminium. According to this, one should also expect different turnover (activity per Ti atom) for samples with different Ti and Al composition. Results of Table 3 show that, in general, the same trend is observed for the initial reaction rate and the intrinsic activity of Ti centers, that is, both turnovers increase when decreasing the Al content at constant Ti content. It can be seen that this occurs irrespective of the method of synthesis used to prepare the Ti-Beta samples, except if the synthesis is carried out in the presence of alkali cations. In this latter case (sample 6, not included in Table 3), and owing to the formation of an amorphous titanosilicate phase (see Fig. la), a very low oxidation activity was obtained.

531

Table 3: Intrinsic activitv of Ti in TS-1 and Ti-Beta for the oxidation of l-hexene. Sample

A1lu.c.

42.8

-

1

1.oo

1.16

0.6

2 3 4 5 7

2.01 1.32

1.20 1.06 0.52 0.60 1.20 0.61 0.62

1.5 2.1

0.2 1.6

Euro TS-I

8

9

a

1.71

2.53 1.51 1.83 2.03

Turnover" fmol oxidmol Ti)

v x106 (moVrnol.s)b

Ti1u.c.

4.9

2.8 1 .o 3.7 7.9

1.o

3.0 2.9 0.6 3.0 6.8

(Activity per Ti atom) 1 (%XRD Crystallinity ) x 100. Calculated at 1 h reaction time V, = initial reaction rate.

At this point, it has to be said that sample 8 was obtained from the same gel that sample 7, but with a higher crystallization time. Then, the crystals of sample 8 can be considered to be formed by two different parts, one central part having the same chemical composition, and theoretically the same activity per Ti atom, than sample 7, and an outer shell in where all the Ti was incorporated in absence of aluminium (no Al was detected in the surface of sample 7). Then, the higher turnover obtained for sample 8 with respect to sample 7 must be adscribed to a much higher intrinsic activity of the Ti centers in Al-free regions of the crystals. The above results confirm the hypothesis that the maximum activity of Ti-Beta catalysts should occur in Al-free samples. A very close situation could be expected in sample 9, which gives the higher turnover. Nevertheless, even in this case, the activity per Ti atom in Ti-Beta is about 6-7 times lower than in TS-1 (Table 3). The lower intrinsic activity of the Ti sites in Ti-Beta as compared to TS-1 could be explained, in principle, taking into account the different Ti species found in the DR spectra of Ti-Beta. These spectra showed, besides the band of isolated tetrahedral Ti species (- 205 nm) which are the only appearing in dehydrated TS-1, other zeolitic Ti species with higher coordination numbers (bands at - 215 and - 225 nm). These latter species are thought to have a much lower catalytic activity than the tetrahedral ones, then decreasing the "average" turnover of the catalyst, which considers all the Ti species. Nevertheless, one can not reject the idea that the Ti centers in Ti-Beta could be, intrinsically, less active than those of TS-1 owing to the different zeolite structure. In this way, the electronic density around the Ti atoms, which is related to their red-ox potential, can be different due to the different T-O-T angles existing in the two structures, in a similar way to what occurs with the acidity of the protons associated with the framework aluminium in different zeolites.

538

ACKNOWLEDGEMENTS Financial support by the Comision lnterministerial de Ciencia y Tecnologia of Spain (MAT 91 -1 152) is gratefully acknowledged. We also thank to Professor P.A. Jacobs for providing the Euro TS-1 sample. REFERENCES

1.

2. 3. 4.

U. Romano, A. Esposito, F. Maspero, C. Neri and M.G. Clerici, "New Developments in Selective Oxidation", G. Centi and F. Trifiro (eds.), Elsevier, Amsterdam (1990)33. D.R.C. Huybrechts, L. De Bruycker and P.A. Jacobs, Nature, 345 (1990)240. J.S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 58 (1990)L1. D.P. Serrano, H. Li and M.E. Davis, J. Chem. SOC.,Chem. Commun., (1992) 745.

5. 6. 7.

M.A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem. SOC., Chem. Commun., (1992)589. M.A. Camblor, A. Corma, A. Martinez, J. Perez-Pariente and J. Primo, Stud. Surf. Sci. and Catal, 78 (1993)393. M. Guisnet et al. Eds., Elsevier, 1993. D.R.C. Huybrechts, P.L. Buskens and P.A. Jacobs, J. Mol. Catal., 71 (1992)

129. 8. 9.

M.A. Camblor, A. Corma and J. Perez-Pariente, Zeolites, 12 (1993)82. F. Geobaldo, S.Ekwdiga, A. Zecchina, E. Gianello, G. Leofanti and G. Petrini, Catal. Lett., 16 (1992)109.

539

DISCUSSION CONTRIBUTION H. MIMOUN (Firmenich SA, Geneva, Switzerland): (i) Why do you work in the

presence of large excess of olefin?; (ii) Do you have consecutive cleavage reactions? A. MARTINEZ (I.Tecnologia Quimica, Valencia, Spain): (i) From an industrial point of view, it is more interesting to work with an excess of olefin, while consuming all the hydrogen peroxide during the reaction. In this way, separation of unreacted olefin from the products and recycle to the reactor can be easily accomplished; (ii) We have studied the epoxidation reaction using different olefin/H,O, ratios. No products from the oxidative cleavage of the double bond were detected even at lower olefin/H,O, ratios than that used in this work. J.A. NAVIO (I. Ciencia Materiales, Sevilla, Spain): My question is related with your preparation procedure. Often, the use of organic metal precursors in the synthesis of catalysts by the sol-gel chemistry leads to the presence of a relatively significant amount of residual carbon, which can affect the catalytic activity. (i) Have you any comment about this possible situation on your catalysts?; (ii) Did you try to use inorganic precursors, such as TiCI,, for the preparation of your catalysts?. A. MARTINEZ: (i) This situation commonly occurs during the preparation of amorphous materials, such as amorphous silica-titania catalysts. This is not the case of crystalline Ti-zeolite catalysts, for which the hydrolysis of the alcoxide during the synthesis is complete. In this case, the Ti atoms are incorporated into the zeolite framework occupying tetrahedral positions (1) and no Ti-alcoxide bonds are observed by 13C NMR after the synthesis of the Ti-Beta catalysts. (ii) Although theoretically possible, we didn’t try the synthesis using inorganic precursors, such as TiCI,, due to the difficult manipulation of the reagent. 1. T. Blasco, M.A. Camblor, A. Corma, J. Perez-Pariente, J.A.C.S., in press. M. CROCKER (Shell-Laboratorium, Amsterdam, The Netherlands): Presumably the Ti-Beta catalyst contains acidic sites. During the epoxidation of 1-hexene, do you observe secondary reactions occurring on these sites?

A. MARTINEZ: We observe the selective opening of the epoxide ring on the acid sites of Ti-Beta catalysts to give the corresponding glycols and glycolethers. As these are consecutive reactions, their extent increases when increasing olefin conversion. Moreover, the rate of epoxide solvolysis has been seen to depend on the type of olefin used (1). 1. A. Corma, M.A. Camblor, P. Esteve, A. Martinez, and J. Perez-Pariente, J. Catal, in press.

540

F. TRlFlRO (Dep. Chim. Ind. e Mater., Bologna, Italy): In epoxidation with Ti-silicalite and H,O, the nature of the solvent is important. Further advantages or differences in the use of Ti-Beta zeolite may be that the optimal solvent is different from that used with Ti-silicalite. My question is: drid you try different solvent?

A. MARTINEZ: Indeed, we observed different reactivities depending on the type of solvent used. For the oxidation of olefins, alcohols were more effective than ketones. Among the different alcohols tried (methanol, ethanol, and t-butyl alcohol), methanol and ethanol were the most effective. 1-hexene and cyclohexene give almost the same activity on both alcohols (l), in contrast with the results reported for l-pentene on TS1, for which methanol gave the best results (2). This was explained in terms of increasing electrophilicity and steric constraints. The results on the large pore Ti-Beta show that this catalyst imposes less steric hindrance than TS-1.

1. A. Corma, M.A. Camblor, P. Esteve, A. Martinez, and J. Perez-Pariente, J. Catal., in press. 2. MG. Clerici, and P. Ingallina, J. Catal., M, 71, (1993).

V. Cortis Corberin and S. Vic Bell6n (Editors), New Deveiopnienls in Seleclive O x i d d o n If 0 1994 Elsevier Science B.V. All rights reserved.

SELECTIVE O X I D A T I O N OF AMMONIA TO HYDROXYLAMINE W I T H PEROXI DE ON T I TAN1 UM BASED CATALYSTS M . A . M A N T E G A Z Z A ~ , G. LEO FAN TI^, Z E C C H I N A ~ , s. B O R D I G A ~

G.

P E T R I N I ~ , M.

'ENICHEM ANIC, Centro Ricerche d i Bollate, 20021 B o l l a t e ( M I ) , I T A L Y

541

HYDROGEN

PA DO VAN^,

V i a S.

Pietro

A.

50,

2 D i p a r t i m e n t o d i Chimica I n o r q a n i c a , Chimica F i s i c a e Chimica 10125 d e i M a t e r i a l i d e l l ' U n i v e r s i t i i d i T o r i n o , V. P. G i u r i a 7 , T o r i no, I T A L Y

ABSTRACT

.

The s y n t h e s i s of h y d r o x y l a m i n e by o x i d a t i o n of ammonia w i t h h y d r o g e n p e r o x i d e on t i t a n i u m b a s e d c a t a l y s t s i s r e p o r t e d , Titanium s i l i c a l i t e i s t h e b e s t c a t a l y s t f o r t h e r e a c t i o n . The i n f l u e n c e of some r e a c t i o n p a r a m e t e r s on t h e main and s i d e r e a c t i o n s i s d i s c u s s e d and t h e r e a c t i o n n e t w o r k i s p r o p o s e d . The r o l e o f t i t a n i u m i s p o i n t e d o u t by r e s u l t s of s p e c t r o s c o p i c studies.

1. I N T R O D U C T I O N Hydroxylamine i s of g r e a t i n d u s t r i a l i m p o r t a n c e a s i n t e r m e d i a t e . More t h a n 9 5 % of hydroxylamine p r o d u c t i o n i s used t o p r o d u c e c y c l o h e x a n o n e oxime i n t h e caprolactam process. I n d u s t r i a l p r o d u c t i o n of h y d r o x y l a m i n e i s c a r r i e d o u t by t h e r e d u c t i o n of nitrogen oxides with s u l f u r dioxide o r by c a t a l y t i c hydrogenation ( r e f . 1 ) . I n a l l cases t h e product i s a n aqueous s o l u t i o n of a s a l t , r a t h e r t h a n f r e e hydroxylamine. Titanium s i l i c a l i t e (TiS) ( r e f . 2,3) i s a very s e l e c t i v e catalyst i n o x i d a t i o n r e a c t i o n s w i t h hydrogen peroxide, p a r t i c u l a r l y i n t h e l i q u i d p h a s e ammoximation o f cyclohexanone t o c y c l o h e x a n o n e oxime ( r e f . 4 ) . I n a p r e v i o u s work w e d e m o n s t r a t e d t h a t t h e ammoximation r e a c t i o n proceeds v i a t h e hydroxylamine i n t e r m e d i a t e ( r e f . 5 , 6 ) . I n t h e f i r s t s t e p , c a t a l y z e d by T i S , ammonia a n d h y d r o g e n p e r o x i d e r e a c t t o g i v e hydroxylamine which t h e n r e a c t s w i t h c y c l o h e x a n o n e t o g i v e t h e oxime. In this communication w e r e p o r t t h e r e s u l t s of further

542

i n v e s t i g a t i o n s on t h e c a t a l y t i c o x i d a t i o n of ammonia to hydroxylamine. T h i s r e a c t i o n i s i m p o r t a n t from a n i n d u s t r i a l p o i n t of view, because f r e e hydroxylamine can be o b t a i n e d d i r e c t l y from ammonia w h i t h o u t f o r m a t i o n of ammonium s a l t s .

2. E X P E R I MENTAL C a t a l y s t s samples w e r e s y n t h e s i z e d a c c o r d i n g t o r e f . 3. A l l samples were c h a r a c t e r i z e d by e l e m e n t a l a n a l y s i s , XRD,

N2 a d s o r p t i o n a t 7 7 K and s p e c t r o s c o p i c t e c h n i q u e s . The main f e a t u r e s of T i S samples were h i g h c r y s t a l l i n i t y and absence of e x t r a - framework T i . The ammonia o x i d a t i o n was c a r r i e d o u t under He atmosphere i n a j a c k e t e d g l a s s r e a c t o r equipped w i t h a mechanical s t i r r e r and a condenser. Aqueous hydrogen p e r o x i d e (30 w t % ) was f e d t o t h e s l u r r y o b t a i n e d by d i s p e r s i n g t h e c a t a l y s t i n an aqueous (1: 1 v / v ) h e a t e d a t t h e ammonia ( 1 5 w t % ) - s o l v e n t mixture A t the end, a f t e r c o o l i n g , t h e gaseous d e s i r e d temperature. r e a c t i o n p r o d u c t s were a n a l y z e d by gas chromatography. The c a t a l y s t w a s f i l t e r e d o f f and t h e hydroxylamine, a f t e r r e a c t i o n w i t h cyclohexanone, was determined as cyclohexanone oxime by g a s chromatography. N i t r i t e s and n i t r a t e s were d e t e r m i n e d by HPLC a n a l y s i s . The hydroxylamine o x i d a t i o n was c a r r i e d o u t i n t h e same way; ( 5 0 w t % ) and H202 were f e d s e p a r aqueous s o l u t i o n s of NH2OH a t e l y t o t h e aqueous ammonia/catalyst s y s t e m .

3. RESULTS AND DISCUSSION 3. 1 C a t a l y s t e v a l u a t i o n The r e s u l t s of ammonia o x i d a t i o n on d i f f e r e n t c a t a l y s t s , t - b u t a n o l ( T B A ) , a r e shown i n Table 1. A l l t h e y i e l d d a t a based on hydrogen p e r o x i d e .

in are

Table 1 O x i d a t i o n of ammonia o n d i f f e r e n t c a t a l y s t s Catalyst

Catalyst (wt %)

Silicalite Ti02 Ti02/Si02 TiS ~~

1. 0 1. 8 4. 8 21. 3 63. 7

-

8 3

60

6

8 2 ~

NH20H y i e l d (mol%)

Ti

(wt%)

2 ~~

~~

Reaction conditions: s o l v e n t TBA; t e m p e r a t u r e 80' C; molar r a t i o 30; r e a c t i o n t i m e 0 . 5 hour.

NH3/H202

543 I n t h e absence of a c a t a l y s t t h e r e a c t i o n does n o t t a k e p l a c e . The h y d r o x y l a m i n e y i e l d i s n e g l i g i b l e a n d t h e main p r o d u c t i s oxygen r e s u l t i n g from h y d r o g e n p e r o x i d e d e c o m p o s i t i o n . With s i l i c a l i t e a l m o s t t h e same r e s u l t s a r e o b t a i n e d . Only T i - c o n t a i n i n g c a t a l y s t s s u c c e e d i n t h e o x i d a t i o n of ami s more e f monia t o h y d r o x y l a m i n e . Amorphous s i l i c a - t i t a n i a f i c i e n t t h a n t i t a n i u m o x i d e , b u t t h e y i e l d i s s t i l l low. With b o t h c a t a l y s t s t h e main r e a c t i o n p r o d u c t i s n i t r o g e n . T i S i s t h e most s e l e c t i v e c a t a l y s t a n d h y d r o x y l a m i n e i s t h e main r e a c t i o n product. The d a t a show t h a t t h e h y d r o x y l a m i n e y i e l d i s r e l a t e d t o t h e n a t u r e o f T i s p e c i e s r a t h e r t h a n t o i t s amount i n t h e c a t a l y s t . isolated T h i s i s a n o t h e r example o f t h e p e c u l i a r i t y of framework t i t a n i u m atoms i n TiS. 3. 2 R e a c t i o n parameters The i n f l u e n c e of s e v e r a l r e a c t i o n p a r a m e t e r s o n T i S c a t a l y z e d o x i d a t i o n o f ammonia i s d i s c u s s e d below. Solvent e f f e c t The r e s u l t s o f ammonia oxidation i n different s o l v e n t s , m i s c i b l e and non-miscible w i t h water, a r e shown i n T a b l e 2. I t has been found t h a t

Table 2 S o l v e n t e f f e c t on ammonia o x i d a t i o n Solvent

NH3/H202

molar r a t i o

NH20H y i e l d

(mol%)

32 H20 t h e r e a c t i o n c a n be p e r formed i n many s o l v e n t s : n-BuOH 16 t -BuOH 12 w a t e r , a l c o h o l s , amides, aromatic hydrocarbons. MeOH 16 The h y d r o x y l a m i n e y i e l d Toluene 11 r a n g e s from a b o u t 30% t o CH3CONH2 13 5 0 % , based on hydrogen peroxide. Good y i e l d s R e a c t i o n c o n d i t i o n s : temp. have been obtained a l s o 1. I w t % ; r e a c t i o n t i m e 0. 5 i n s o l v e n t s non-miscible w i t h w a t e r , l i k e t o l u e n e and n-butanol.

53. 5 39. 2 50. 1 46. 1 51. 0 32. 1

80' C;

TiS

hour.

E f f e c t of NH3/H307 molar r a t i o The r e s u l t s o f ammonia o x i d a t i o n i n TEA a t d i f f e r e n t N H 3 / H 2 0 2 m o l a r r a t i o s a r e r e p o r t e d i n T a b l e 3. The i n c r e a s e o f t h e N H 3 / H 2 0 2 r a t i o r e s u l t s i n a n i n c r e a s e of the hydroxylamine yield. At ratios higher than 100, h y d r o x y l a m i n e c a n be o b t a i n e d w h i t h y i e l d s a s h i g h a s 7 0 % . R e a c t i o n b y p r o d u c t s a r e n i t r o g e n , n i t r o u s o x i d e , n i t r i t e s and n i t r a t e s . N i t r o g e n i s t h e main b y p r o d u c t .

544 Table 3 E f f e c t of NH,/H202 Reaction

t i m e (h)

molar r a t i o on ammonia o x i d a t i o n Yield (mol%)

NH 3/H202 molar r a t i o

0. 5

7

0.3 0. 6 0.5 0.3

12 17 31 120

Reaction conditions: solvent c o n c e n t r a t i o n 1. 7 w t % .

TBA;

NH20H

N2

39. 0 50. 1 57. 1 63. 7 70. 3

49. 2 36. 5 32. 4 16. 4 11. 8

temperature

80' C;

TiS

E f f e c t of c a t a l y s t c o n c e n t r a t i o n The r e s u l t s of ammonia o x i d a t i o n a t i n c r e a s i n g c a t a l y s t c o n c e n t r a t i o n a r e shown i n Table 4. The hydrogen p e r o x i d e c o n v e r s i o n i s always complete e x c e p t f o r t h e t e s t w i t h o u t t h e c a t a l y s t (55 m o l % ) . By i n c r e a s i n g t h e c a t a l y s t c o n c e n t r a t i o n i ) t h e hydroxylamine ii) the y i e l d promptly i n c r e a s e s and r e a c h e s a p l a t e a u , n i t r o g e n and n i t r o u s o x i d e y i e l d q u i c k l y r e a c h e s a maximum and t h e n d e c r e a s e s s l i g h t l y , i i i ) t h e n i t r i t e s and n i t r a t e s t r e n d i s s i m i l a r t o t h e n i t r o g e n one b u t t h e i r d e c r e a s e i s more pronounced, I V ) t h e oxygen, t h e main p r o d u c t i n t h e a b s e n c e of TiS, becomes negligible even a t the lowest catalyst concentration. Table 4 E f f e c t of T i S c o n c e n t r a t i o n on ammonia o x i d a t i o n Yield ( m o l % )

T iS

(wt%) 0.

a

1. 1 1. 7 3. 3

NH2OH 1. 0 29. a 59. 2 57. 9 62. 9

N2 2. 3 34. 2 28. 1 30. 0 28. 5

N20 7. 4 5. 5

5. 4 3. 6

NO; 1. 4 19. 5 3. 5 2. 9 3.0

NO;

02

0. 7

30. 0

9. 6 2. 1 1. a 1. 9

2. 2 1. 4 1. 0 0.7

R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; t e m p e r a t u r e 80' C; NH3/H202 molar r a t i o 17; r e a c t i o n t i m e 0.5 hour.

545 E f f e c t of temperature The r e s u l t s of ammonia o x i d a t i o n a t d i f f e r e n t t e m p e r a t u r e s a r e shown i n T a b l e 5. A t t e m p e r a t u r e s from 6 0 ' t o 8 O ' C t h e hydroxylamine y i e l d is a l m o s t c o n s t a n t a n d t h e h y d r o g e n p e r o x i d e c o n v e r s i o n i s comp l e t e . A t 5 0 ' C t h e c a t a l y s t a c t i v i t y and t h e hydroxylamine y i e l d decrease. The b e h a v i o u r of b y p r o d u c t s i s q u i t e d i f f e r e n t . By d e c r e a s i n g t h e t e m p e r a t u r e n i t r o g e n and n i t r o u s o x i d e d e c r e a s e w h i l e n i t r i t e s , n i t r a t e s a n d oxygen i n c r e a s e . Table 5 E f f e c t o f t e m p e r a t u r e on ammonia o x i d a t i o n Temperature ( "C)

H202 conv. (mol%) ~

80 70 60 50

NH20H ~~

99 98 91

~

39. 0 41. 8 39. 1 21. 7

46

Yield (mol%) N2 N20 NOS

NO;

O2

0.4 0.4 1. 4 4. 3

0. 5 0. 3 0.2 1. 3

~

49. 2 42. 2 40. 6 a. 6

5. 4 5. 4 4.

2. 1 3. 7 5. 2

7

a.

1. I

1

R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; NH3/H202 m o l a r r a t i o 7; c o n c e n t r a t i o n 1. 7 w t % ; r e a c t i o n t i m e 0. 5 h o u r .

TiS

E f f e c t of hydrogen p e r o x i d e f e e d r a t e The r e s u l t s o f ammonia o x i d a t i o n a t d i f f e r e n t h y d r o g e n p e r o x i d e f e e d r a t e a r e r e p o r t e d i n T a b l e 6. N o i n f l u e n c e on t h e hydroxylamine y i e l d i s o b s e r v e d by v a r y i n g t h e f e e d r a t e from 190 m l / h t o 4 ml/h. The h y d r o g e n p e r o x i d e f e e d r a t e a f f e c t s m a i n l y t h e n i t r i t e s , n i t r a t e s a n d n i t r o g e n y i e l d s . N i t r i t e s and n i t r a t e s d e c r e a s e while n i t r o g e n i n c r e a s e s almost p r o p o r t i o n a l l y . Table 6 E f f e c t of H202 f e e d r a t e on ammonia o x i d a t i o n H202 feed

rate (ml/h)

time (min)

190 9 4

1 21 51

~~

NH20H 53. 8 52. 1 53. 0

Yield ( m o l % ) N2 N20 27. 4 36. 5 38. 7

3. 4 3. 6 3. 7

NOS

NO3

O2

a.

2. 9 0.7 0.7

0.6

4 3. 1 2. 1

0.

a

0.9

~

R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; t e m p e r a t u r e 80' C; m o l a r r a t i o 1 2 ; T i S 1. 7 w t % ; r e a c t i o n t i m e 1 h o u r .

NH3/H202

546

3 . 3 Hydroxylamine o x i d a t i o n w i t h hydrogen p e r o x i d e The r e a c t i v i t y of hydroxylamine towards hydrogen p e r o x i d e , w i t h and w i t h o u t TiS, has been i n v e s t i g a t e d i n o r d e r t o v e r i f y t h e s t a b i l i t y of hydroxylamine i n t h e r e a c t i o n medium. The r e s u l t s a r e r e p o r t e d i n Table 7 . Hydroxylamine i s o x i d i z e d i n b o t h n o n - c a t a l y z e d (homogeneous) and c a t a l y z e d ( h e t e r o g e n e o u s ) r e a c t i o n s . I n t h e a b s e n c e of t h e c a t a l y s t t h e r e a c t i o n i s n o n - s e l e c t i v e . N i t r o g e n , n i t r o u s o x i d e and n i t r i t e s a r e formed i n a l m o s t t h e same y i e l d . The f o r m a t i o n of a r e l e v a n t amount of oxygen p o i n t s o u t t h a t a l s o t h e hydrogen p e r o x i d e decomposition t a k e s p l a c e . With TiS t h e r e a c t i o n i s v e r y s e l e c t i v e and n i t r o g e n i s t h e main p r o d u c t .

Table 7 Hydroxylamine o x i d a t i o n w i t h hydrogen p e r o x i d e Yield ( m o l % )

TiS

(wt%)

3

N2

N20

NO:

NO

-

21

1. 7

86

25 9

25 1

2 1

02

15 1

Reaction c o n d i t i o n s : N H 4 0 H ( 7 . 4 w t % ) 100 m l ; temperature molar r a t i o 0. 9; r e a c t i o n t i m e 2 . 5 hours.

8O'C;

NH20H/H202

3. 4 R e a c t i o n network The r e s u l t s p r e s e n t e d p o i n t o u t t h a t t h e ammonia o x i d a t i o n t o hydroxylamine i s c a t a l y z e d by TiS. The r e a c t i o n i s v e r y f a s t . The hydroxylamine produced can f u r t h e r r e a c t w i t h hydrogen nitrogen, nitrous p e r o x i d e to g i v e more o x i d i z e d p r o d u c t s : o x i d e , n i t r i t e s and n i t r a t e s . A s e v i d e n c e d i n Table 7 , n i t r o g e n and n i t r o u s o x i d e d e r i v e b o t h from n o n - c a t a l y z e d r e a c t i o n s i n t h e a b s e n c e of TiS and from c a t a l y z e d r e a c t i o n s i n t h e p r e s e n c e of T i S . A s r e p o r t e d i n Table 3, a l a r g e e x c e s s of ammonia i n c r e a s e s t h e hydroxylamine y i e l d and c u t s down t h e n i t r o g e n r e s u l t i n g from t h e c a t a l y z e d c o n s e c u t i v e hydroxylamine o x i d a t i o n , wich i s c o m p e t i t i v e w i t h t h e ammonia o x i d a t i o n . N i t r i t e s and n i t r a t e s a r e produced by n o n - c a t a l y z e d oxidation of t h e hydroxylamine, which i s c o m p e t i t i v e w i t h c a t a l y z e d r e a c t i o n s when t h e c a t a l y s t a c t i v i t y i s low, f o r i n s t a n c e a t low c a t a l y s t c o n c e n t r a t i o n ( s e e Table 4) and low t e m p e r a t u r e ( s e e Table 5 ) . The o v e r a l l r e a c t i o n network can be r e p r e s e n t e d by t h e f o l l o w i n g equations:

547

NH3

+

H202

T iS

TiS,

c

H202

NH2OH

N2

+

N20

H202

1 H20

+

O2

3 . 5 S p e c t r o s c o p i c c h a r a c t e r i z a t i o n of t h e a c t i v e T i c e n t r e s T i S h a s b e e n i n v e s t i g a t e d by s e v e r a l s p e c t r o s c o p i c t e c h n i q u e s t o e l u c i d a t e t h e i n t i m a t e mechanism o f t h e c a t a l y t i c r e a c t i o n b e t w e e n ammonia a n d h y d r o g e n p e r o x i d e , t h e r e a s o n why T i i n T i S i s s o a c t i v e a n d how i t c a r r i e s on i t s c a t a l y t i c a c t i v i t y . Under vacuum c o n d i t i o n s t h e most i m p o r t a n t s p e c t r o s c o p i c f e a t u r e s a s s o c i a t e d w i t h T i c a n be summarized a s f o l l o w s : 1 ) I R band a t 9 6 0 cm-' a s s o c i a t e d w i t h a S i - 0 s t r e t c h i n g mode i n [ S i 0 4 ] u n i t s p e r t u r b e d by a d j a c e n t [ T i 0 4 ] u n i t s or w i t h a s t r e t c h i n g mode o f [ T i 0 4 ] u n i t s ( r e f . 3 , 7 ) ; 2 ) o p t i c a l t r a n s i t i o n a t 48000 cm-l w i t h l i g a n d t o m e t a l c h a r g e t r a n s f e r ( C T ) c h a r a c t e r i n t e t r a c o o r d i n a t e d and i s o l a t e d T i ( I V ) ( r e f . 3, 8 ) ; 3 ) X-ray a b s o r p t i o n i n T i K p r e - e d g e r e g i o n , w i t h p e a k p o s i t i o n , f u l l w i d t h h a l f maximum (FWHM) and i n t e n s i t y i n d i c a t i n g t h a t T i i s t e t r a c o o r d i n a t e d and i n a symmetry v e r y c l o s e t o a perfect tetrahedron (ref. 9). The w h o l e r e s u l t s p o i n t o u t t h a t i n a w e l l s y n t h e s i z e d T i S a n i s o m o r p h o u s s u b s t i t u t i o n o f framework S i atoms by T i o n e s t a k e s p l a c e a n d t h e n a l l T i atoms a r e i s o l a t e d and i n t e t r a h e d r a l coordination. A f t e r t h e a d s o r p t i o n of molecules ( f o r instance water o r ammonia) t h e f o l l o w i n g c h a n g e s h a v e b e e n o b s e r v e d : 1 ) a s h i f t o f T i ( 1 V ) CT band t o l o w e r f r e q u e n c y d u e t o T i ( 1 V ) h e x a c o o r d i n a t e d complexes formed by l i g a n d a d d i t i o n ( r e f . 5 ) 2 ) a d i s a p p e a r a n c e of t h e p r e - e d g e p e a k r e p l a c e d by a new w e a k e r a b s o r p t i o n c h a r a c t e r i z e d by v e r y l a r g e FWHM, c l e a r l y i n d i c a t i n g t h e f o r m a t i o n of d i s t o r t e d o c t a h e d r a l s p e c i e s (ref. 9). These r e s u l t s , t o g e t h e r w i t h t h o s e o b t a i n e d by v o l u m e t r i c a d s o r p t i o n m e a s u r e m e n t s , d e m o n s t r a t e t h a t framework T i atoms i n T i S a r e a b l e t o a d s o r b up t o two l i g a n d s t o r e a c h t h e i r t y p i c a l hexacoordinated s t a t u s . The main s p e c t r o s c o p i c e v i d e n c e o f t h e i n t e r a c t i o n o f T i S w i t h h y d r o g e n p e r o x i d e i s t h e a p p e a r a n c e of i ) a band a t 43000 cm-' d u e t o t h e i n s e r t i o n of two w a t e r m o l e c u l e s i n t h e T i ( 1 V ) c o o r d i n a t i o n s p h e r e and i i ) a s t r o n g band a t 2 6 0 0 0 cm-' a s s o c i a t e d w i t h a CT from a h y d r o p e r o x o - t y p e s p e c i e s ( r e f . 5 ) .

548 When ammonia i s dosed on t h e preformed p e r o x o - s p e c i e s the band a t 26000 cm-l s h i f t s t o 2 7 5 0 0 cm-' and t h e n d e c l i n e s givaqueous i n g t h e same s p e c t r u m o b t a i n e d by a d i r e c t d o s a g e of hydroxylamine s o l u t i o n on TiS. S i m i l a r l y t h e ammonia d o s a g e on p r e a d s o r b e d hydrogen p e r o x i d e a band a t 1590 cm-I i n d i c a f o l l o w e d by I R s p e c t r o s c o p y g i v e s t i n g t h e hydroxylamine f o r m a t i o n ( r e f . 5 ) . I n c o n c l u s i o n t h e c a t a l y t i c s t e p of f o r m a t i o n of hydroxylamine c a n be r e p r e s e n t e d as f o l l o w s :

03Ti-OH

H20

0 3 T i OH) ( H 2 0 I 2

1

NH3

H2°2

03Ti ( OH) ( H 2 0 ) 2

NH 3

-

03Ti(OH) (NH3) ( H 2 0 )

1

H2°2

03Ti(00H)(NH3)(H20)

-c 03Ti(OH)(NH20H)(H20)

REFERENCES

J. N. A r m o r i n " C a t a l y s i s of Organic R e a c t i o n s " , J . R. Kosak ( e d . 1 , Dekker, New York, 1984, 4 0 9 M. Taramasso, G. Perego, B. N o t a r i , U.S. P a t . 4. 410. 5 0 1 (1983) A. Zecchina, G. Spoto, S. Bordiga, A. F e r r e r o , G. Petrini, G. L e o f a n t i , M. Padovan i n Z e o l i t e Chemistry and C a t a l y s i s , P . A . J a c o b s e t a l . ( e d s . ) , E l s e v i e r , Amsterdam, 1991, 251 Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. 4 ) P. Padovan, G. Petrini, S. Tonti, P. Gervasutti i n "New Developments i n S e l e c t i v e O x i d a t i o n " , G. Centi et al. ( e d s . ) , E l s e v i e r , Amsterdam, 1990, 4 3 Petrini, 5 ) A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. G. Leofanti, M. Padovan, M.A. Mantegazza, P. Roffia, L. P r o c e e d i n g s of t h e 1 0 t h I n t . Congr. on C a t a l y s i s - P a r t A, Guzci e t a l . ( e d s ) , Akademiai Kiadb, Budapest, 1993, 719 M . A . Mantegazza, M . Padovan, G. P e t r i n i , P. R o f f i a , I t . P a t . Appl. M I 91A001915 ( 1 9 9 1 ) A. Zecchina, G. Spoto, S. Bordiga, M. Padovan, G. Leofanti, G. G. P e t r i n i i n " C a t a l y s i s and A d s o r p t i o n by Z e o l i t e s " , Ohlman e t a l . ( e d s . ) , E l s e v i e r , Amsterdam, 1992, 671 F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. L e o f a n t i , G. P e t r i n i , C a t a l y s i s L e t t e r s 1 6 ( 1 9 9 2 ) 109 S. Bordiga, F. Boscherini, S. Coluccia, F. Genoni, G. L e o f a n t i , L. Marchese, G. P e t r i n i , G. V l a i c , A. Zecchina, i n press

549

G. CENT1 (Dip. C h i m . I n d . e M a t e r i a l i , Bologna, I t a l y ) : I n t h e k i n e t i c network f o r hydroxylamine s y n t h e s i s and t r a n s f o r m a t i o n you proposed t h a t N 2 0 i s formed t o g h e t e r w i t h N2 by o x i d a t i o n of NH20H. However, i n s e v e r a l c a s e s you r e p o r t e d t h a t t h e N2 y i e l d i s a b o u t one o r d e r of magnitude h i g h e r t h a n t h a t of N20. My q u e s t i o n i s t h e r e f o r e whether t h e N 2 0 f o r m a t i o n may d e r i v e from a d i f f e r e n t s i d e r e a c t i o n , l i k e t h e r e a c t i o n of n i t r a t e s p e c i e s w i t h ammonia, o r N20 i s an i n t e r m e d i a t e i n t h e a b e t t e r u n d e r s t a n d i n g of o x i d a t i o n of NH20H t o N2. I n f a c t , t h e mechanism of N 2 0 f o r m a t i o n may a l l o w t o l i m i t t h i s byproduct which f o r m a t i o n may be p r o b l e m a t i c i n commercial ammoximation r e a c t i o n s . (ENICHEM, C e n t r o Ricerche B o l l a t e , Bollate For what concerns t h e N 2 0 f o r m a t i o n from a s i d e r e a c t i o n between n i t r a t e s and ammonia w e have v e r i f i e d t h a t b o t h ammonium n i t r a t e and n i t r i t e a r e s t a b l e i n ammonia s o l u t i o n i n t h e r e a c t i o n c o n d i t i o n s , even i n t h e p r e s e n c e of the catalyst. I n o u r r e s u l t s t h e r e i s no e v i d e n c e of a r e l a t i o n between N2O and N2 l i k e betweeen a n i n t e r m e d i a t e and a f i n a l p r o d u c t , as you s u g g e s t e d . The b e h a v i o u r of t h e s e p r o d u c t s i s s i m i l a r , i. e. when n i t r o g e n i n c r e a s e s also n i t r o u s o x i d e i n c r e a s e s , s o w e b e l i e v e t h a t N 2 0 i s a f i n a l p r o d u c t and n o t a n i n t e r m e d i a t e i n t h e NH20H oxidation. W e a g r e e t h a t a b e t t e r u n d e r s t a n d i n g of t h e mechanism of N20 f o r m a t i o n c o u l d be h e l p f u l a l s o f o r t h e ammoximation r e a c t i o n . Anyway i n t h e ammoximation of cyclohexanone t h e p r o d u c t i o n of N 2 0 i s negligible.

M. A. MANTEGAZZA ITALY):

F. T R I F I R O ' (Dip. Chim. Ind. e M a t e r i a l i , Bologna, I t a l y ) : O n t h e b a s i s of t h e work you have done on ammonia o x i d a t i o n you a r r i v e t o p r o p o s e t h a t t h e mechanism of ammoximation r e a c t i o n i s t h r o u g h NH2OH i n t e r m e d i a t e . However s c i e n t i s t s ( J i r u and a l . 1 claimed t h a t t h e main i n t e r m e d i a t e c a n be cyclohexanone imine. My q u e s t i o n i s : do you have a l s o k i n e t i c e v i d e n c e t h a t NH20H i s t h e t r u e i n t e r m e d i a t e i n ammoximation r e a c t i o n ? M. A. MANTEGAZZA: We h a v e n ' t done any k i n e t i c s t u d y on t h e ammoximation r e a c t i o n . Anyway t h e e v i d e n c e t h a t k e t o n e s w i t h l a r g e m o l e c u l a r s i z e , unable t o d i f f u s e i n t o t h e c a t a l y s t , g i v e good y i e l d s i n ammoximation (ref. 5 ) rules out t h e hypothesis of the c y c l ohexanone i m i ne i n t e r m e d i a t e . The ammoximation mechanism v i a NH20H i n t e r m e d i a t e i s based o n t h e following evidencies ( r e f . 5 ) : i ) there i s a r e l a t i o n

550

between t h e k e t o n e r e a c t i v i t y i n t h e ammoximation and i n the o x i m a t i o n r e a c t i o n : i n t h e c a s e of k e t o n e s w i t h low r e a c t i v i t y , t h e o x i m a t i o n i s t h e r a t e d e t e r m i n i n g s t e p i n t h e ammoximation r e a c t i o n i i ) t h e i n t e r m e d i a t e NH20H has been i s o l a t e d i n y i e l d c l o s e t o t h e ammoximation y i e l d .

V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation II 0 1994 Elsevicr Science B.V. All rights reserved.

55 1

PALLADIUM CATALYZED OXIDATION OF BENZENE TO PHENOL USING MOLECULAR OXYGEN

,-

Alexandre T. Cruz, Luis C. Passoni and Carol H. Collins

Instituto de Quimica, Universidade Estadual de Campinas Caixa Postal 6154,13081-970 Campinas, SP (Brazil) Summary: Benzene can be effectively oxidized to phenol and phenyl acetate in acetic acid/acetic anhydride at 135OC with 45 bar of oxygen in the presence of palladium acetate. Only catalytic amounts of potassium dichromate as a co-oxidant are necessary but a nucleophile such lithium acetate should be present in sufficient concentration to reduce the formation of biphenyl. Under these reaction conditions a turnover number of 40 was obtained producing a 0.5 mol L-1 solution of phenol plus phenyl acetate. A reaction mechanism is proposed which takes these findings into account. 1. INTRODUCI'ION

The palladium-catalyzed direct hydroxylation or acetoxylation of benzene has been studied for more than 25 years [1,2]. Early work in acetic acid, in the presence of a suitable nucleophile, produced phenyl acetate and biphenyl with low ( < 5 ) turnover numbers [2], while metallic palladium precipitated at the end of the reaction [l].In more recent publications, phenol was obtained with a turnover number of 12 when 1,lO-phenanthroline was used as a stabilizing ligand, in the presence of carbon monoxide to avoid the formation of biphenyl [3,4]. If the palladium acetate concentration was reduced to l ~ l O mol%, - ~ turnover numbers as high as 500 were reported [ 5 ] ,but the total amount of phenol produced was only 0.55 mmol. Heteropolyacids of vanadium can be used as strong oxidizing agents in the palladium-catalyzed oxidation of benzene. In the absence of a nucleophile, biphenyl is the principal product with a total turnover number of 23, while in the presence of sodium acetate, phenol plus phenyl acetate are the principal products with the much lower turnover number of 4.2 [6]. In the presence of air, hydrogen as a reducing gas and supported palladium and copper salts as catalysts, benzene can be oxidized at room temperature [7,8]. On the other hand, the yields obtained under these conditions are normally below 1 mmol. We report here that molecular oxygen in the presence of palladium acetate in acetic acidlacetic anhydride, with potassium dichromate as a co-oxidant and lithium acetate as a nucleophile, oxidizes benzene to phenol, phenyl acetate and biphenyl with good yields and a turnover number of 40 at 135OC. At 45 bar of molecular oxygen, 30 mmol of phenol plus phenyl acetate per mmol of palladium are obtained, which corresponds to an approximately

552

0.5 mol L" solution under reaction conditions. The importance of the co-oxidant and the nucleophile is discussed in terms of a possible mechanism for the formation of the oxidation products. ZFXPERIMENTAL

All chemicals were reagent grade. Benzene was dried by refluxing over sodium wire/benzophenone and then distilled. Acetic acid was also distilled. All other reagents were used without further purification. In the preliminary experiments, 23 mL of acetic acid, 50 mmol(5.1 g) of acetic anhydride, 100 mmol (7.8 g) of benzene, 0.5 mmol (0.11 g) of palladium(I1) acetate, 10 mmol of the co-oxidant and 12 mmol of the nucleophile were stirred in a 50 ml round-bottom flask at room temperature until total dissolution of the solids (approximately 10 min). The flask was then equipped with a reflux condenser, a thermometer and an oxygen inlet tube and the oxygen flow regulated at 0.2 mL s-'. The flask was immersed in a preheated (110°C) oil bath and the reaction mixture refluxed for 24 h. The reaction temperature was approximately 9OoC. Solids formed during the reaction were removed by filtration and the homogeneous solution was analyzed by gas chromatography. In the experiments under oxygen pressure, the solution of the reagents in acetic acidacetic anhydride was introduced into a 100mL stainless-steel autoclave, equipped with a glass vessel to avoid contact of the reaction mixture with the autoclave walls. The autoclave was closed and pressurized with oxygen to the indicated pressure. The autoclave was then immersed in an oil bath, preheated to the desired temperature, and the reaction mixture magnetically stirred for 24 h. After the reactions, the unreacted oxygen was vented and the reaction mixture filtered for chromatographic analysis. Toluene (0.10 mL) was added as an internal standard and the reaction mixtures analyzed with a CG 37 gas chromatograph, equipped with a 3 m x 1 . 8 packed column (OV-101 on Chromosorb W-HP) coupled to a flame ionization detector. After 5 min at 2OoC, the oven temperature was programmed at 10°C min-l to 17OoC which was maintained for 10 min. The observed retention times were: toluene (13.1 min), phenol (20.5 min), phenyl acetate (22.0 min), biphenyl (28.0 min). The products were quantified by comparison of the peak areas with calibration curves of the pure compounds. 3. RESULTS AND DISCUSSIONS 3.1 Preliminary Experiments

Initial experiments, without a nucleophile and a co-oxidant, gave no oxidation products under reflux conditions (9OoC, 1 bar of 0 2 ) . Since the literature [2] indicates the need of both a co-oxidant and a nucleophile these compounds were evaluated. As water is considered to be disadvantageous for the oxidation reaction [9], 50 mmol of acetic anhydride was added to all reactions. In a first series of experiments the nucleophile was varied. As can be seen in Table 1, alkali acetates strongly promote the formation of phenyl acetate (PhOAc); biphenyl (Ph-Ph) and a very small amount of phenol (PhOH) were obtained as by-products. Lithium acetate gave the highest yield of phenyl acetate and the best turnover number (7.4), which

553

is in agreement with results reported in a patent [lo]. Thus lithium acetate was used in subsequent experiments. Table 1. Effect of different nucleophiles on benzene oxidation (0.5 mmol of Pd(OAc)2, 100 mmol of benzene, 10 mmol of KzCr~07,12 mmol of nucleophile, 23 mL of HOAc, 50 mmol of AcOAc, 1bar of 02, 9OoC, 24 h). NucleoDhile LiOAc NaOAc KOAc Ha(0Ach

PhOAc (mmol) 2.9 2.5 2.2 0.5

PhOH (mmol) <0.1 <0.1 <0.1 <0.1

Ph-Ph (mmol) 0.4 0.1 0.7 <0.1

TN 7.4 5.4 7.2 1.1

In a second series of experiments, the co-oxidants were tested. As already reported by Henry [2], a strong co-oxidant is necessary for the formation of PhOAc. Copper acetate is not strong enough and gave poor results (Table 2). Potassium dichromate gave both the best yield of PhOAc and the highest turnover number and was, therefore, used in the experiments under pressure. Table 2. Effect of different co-oxidants on benzene oxidation (0.5 mmol of Pd(OAc)2,100 mmol of benzene, 10mmol of co-oxidant, 12mmol of LiOAc, 23 mL of HOAc, 50 mmol of AcOAc, 1bar of 0 2 , 90°C, 24 h). Co-oxidant K2Cr207 Wn04

CrOs Cu(OAc)2

PhOAc (mmol) 2.9 1.4 0.6 <0.1

PhOH (mmol) <0.1 <0.1 <0.1 <0.1

Ph-Ph (mmol) 0.4 0.1 0.3 co.1

TN 7.4 3.2 2.4 0.1

3.2 Reactions under oxygen pressure Using the amounts of reagents and solvents previously established, the oxygen pressure was fixed at 15 bar (25OC)and the temperature varied between 100°C and 175OC. At 100°C the results were similar to those found under reflux conditions. At 115OC a significant oxygen consumption was observed (the oxygen pressure fell from 15 to 6 bar) and PhOH was the main product (Figure 1).By increasing the temperature up to 135OC, the quantity of PhOH maximized, while the quantity of PhOAc was further reduced and that of Ph-Ph stayed approximately constant.

554

8 -

90

X PhOH

110

130

150

170

t e m p e r a t u r e (OC) Figure 1. Oxidation products as a function of the reaction temperature (0.5 mmol of Pd(OAc)z, 100 mmol benzene, 10 mmol K2Cr207,12 mmol of LiOAc, 23 mL of HOAc, 50 mmol of AcOAc, 15 bar of 02,24 h).

At 135OC nearly all oxygen was consumed (the final oxygen pressure was 2 bar) and a turnover number of 20 was obtained. The reaction time under these conditions was approximately 8 h, as the oxygen pressure did not change after this time. At reaction temperatures above 135OC the total quantity of oxidation products was reduced. At 175OC the turnover number fell to 9 and oxygen consumption was lower (final pressure of 5 bar). This is not in agreement with the literature [ 10,111,where temperatures between 150°C and 2OO0C are considered optimal for benzene acetoxylation. On the other hand, PhOAc and Ph-Ph may suffer further acetoxylation at these reaction temperatures [6]. Since 135OC was the best temperature for this reaction (Figure 1)this temperature was chosen to study the amount of the nucleophile needed. The quantity of co-oxidant was reduced to 1 mmol. As can be seen in Figure 2, the quantity of PhOH increased almost linearly with the increase in lithium acetate concentration. On the other hand, the quantities of PhOAc and Ph-Ph reached a maximum at approximately 5 mmol of lithium acetate and were then reduced. This is somewhat surprising as one would expect an increase in the quantity of PhOAc, and not that of PhOH, with increased ammounts of lithium acetate. As expected, a high concentration of lithium acetate markedly reduced the quantity of Ph-Ph, but it is not clear why, under these conditions, the phenyl group is not preferentially attacked by the acetate ion (for details see section 3.3).

555

--

8 -

0

E E

X PhOH PhOAc A PhPh

0

6 -

Y

u)

c 0 1 Q 0 L

n

0

I

I

0

3 lithium

6 9 acetate (mmol 1

12

Figure 2. Oxidation products as a function of the concentration of the nucleophile (0.5 mmol of Pd(OAc)z, 100 mmol benzene, 1 mmol K2Cr207.23 mL of HOAc, 50 mmol of AcOAc, 15 bar of 0 2 , 135OC, 24 h).

The highest turnover number (34) was observed with 5 mmol of lithium acetate. Thus the effect of increasing the oxygen pressure was studied under these conditions, in an attempt to reduce the quantity of the undesired Ph-Ph and to produce more PhOH and PhOAc. As can be seen in Figure 3, this was successful: at 45 bar of oxygen pressure, 10 mmol of PhOH and 5 mmol of PhOAc were obtained (total turnover number of 4O), which corresponds to approximately 0.5 mol L-' solution of these products in acetic acid. On the other hand, the system still deactivated, forming a green-brown precipitate, which showed that potassium dichromate was reduced under these reaction conditions. Palladium acetylacetonate can be used instead of palladium acetate without loss of activity or selectivity for the products. Palladium chloride and methallylpalladium chloride gave the same product distribution with only a slight loss of activity. On the other hand, bis(benzonitri1e)palladium chloride was less active and formed PhOAc and Ph-Ph as the main products, showing that stronger ligands influence the catalytic system.

556

1 0 -- X PhOH

--

0 PhOAc 8 .

A PhPh

0

E

-E

6.

v)

c =I

V 0 L

4 '

n

12

17

22

27 32 37 o x y g e n pressure (bar)

42

47

Figure 3. Oxidation products as a function of the oxygen pressure (0.5 mmol of Pd(OAc)2, 100 mmol of benzene, 1 mmol of K2Cr207,5 mmol of LiOAc, 23 mL of HOAc, 50 mmol AcOAc, 135OC, 24 h).

3.3 Considerations about the reaction mechanism There seems to be no doubt that the reaction is initiated by an eletrophilic attack of the palladium acetate on the aromatic ring to give a phenylpalladium(I1) intermediate (eq. 1) ~~4~61.

Ph-H+Pd(OAc)2

LiOAc

-HOAc -PhPdOAc-

Li2PhPd(OAc)3

(1 1

In the presence of a sufficiently high concentration of alkali acetate, a phenyl(acet0)palladate anion is formed which, in the presence of a strong oxidizing agent, reacts to give PhOAc and palladium acetate (eq. 2) [2,6].

Li2PhPd(OAcI3

+ 2HOAc

'I

0

PhOAc+Pd(OAcI2+

2 L i O A c + H20 ( 2 )

557

Oxygen molecules alone can not effect this oxidative cleavage; the reaction stops at phenylpalladium acetate and no oxidation products are observed. Acetic anhydride promotes this reaction as it removes the water. If the concentration of lithium acetate is reduced, less phenyl(acet0)palladate anion is present. This could mean that dinuclear palladium complexes are formed, which may produce Ph-Ph by oxidative cleavage (eq.3).

OAc, 2 PhPd OAc SP hPd/ 'OAc/

PdPh-

"0" Ph-Ph+ HOAc

2Pd(OAc),+H20(31

On the other hand, Lyons [6] believes that Ph-Ph is formed from diphenylpalladium(I1) by reductive elimination (eq.4).

- +. Ph . -U I.

PhPdOAC

I ,

Ph2Pd

Ph-Ph

-HOAC

+Pd(0)

(41

This would explain the formation of palladium(0) at low oxygen pressure and/or in the absence of a strong oxidant. On the other hand, a high concentration of the nucleophile is more important for the prodution of PhOAc than is the concentration of the strong oxidant. This favors a dinuclear mechanism for the formation of Ph-Ph (eq.3). However, it is not clear why a high oxygen pressure and a high concentration of the nucleophile favor the formation of PhOH and not of PhOAc. The active oxygen, transferred from the dichromate to the complex, could oxidize palladium(I1) to palladium(1V) (eq.5) or insert into the phenyl-palladium bond (eq.6).

Li2PhPd(OAcI3

"0"

HOAc

2LiOAc

"0" Li2PhPd(OAcI3 -Li,PhOPd

+ PhPd(OAcI3

-

PhOAc+ Pd(OAcI2 ( 5 1

(0A~)~-Li~Pd(OAc14+

PhOH

(6)

Certainly the reaction shown in eq. 6 explains the formation of PhOH more easily. However, most authors [6,8,12] favor the formation of a palladium(1V) intermediate complex, as shown in eq. 5. The system deactivates, even at high initial oxygen pressure, due to slow reduction of the strong oxidant. Green chromium (hydr)oxides are formed during the reaction, which then stops as the phenylpalladium acetate is no longer oxidatively cleaved. Furthermore, metallic palladium is then formed by reductive elimination of oxidation products. This deactivation could perhaps be avoided, if the reactions were run in an open system which

558

would allow maintaining the initial oxygen pressure throughout the reaction. A co-oxidant which reacts more efficiently with molecular oxygen would also avoid deactivation. These factors are currently under study. We believe that a revised reaction system should allow the direct oxidation of benzene to phenol with high turnover numbers and high final concentration of the products under the described reaction conditions. 4. CONCLUSIONS

The reaction system described herein effectively oxidizes benzene, giving good yields of phenol plus phenyl acetate with a turnover number of 40. However, potassium dichromate, used as a co-oxidant, is slowly reduced, causing deactivation of the reaction system, and should be substituted by a more effective co-oxidant system. Furthermore, the oxygen pressure used to maximize the turnover number is quite high, which might cause explosions. We are currently working to modify the conditions to use lower oxygen pressures, as well as to obtain similar turnover numbers to those observed in the Wacker process as well as to explain the formation of phenol as the principal reaction product.

Acknowledgements: This work was financed by the Fundaqao de Amparo a Pesquisa do Estado de Silo Paulo (FAPESP). Fellowships from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) are acknowledged. REFERENCES

1. H.-J. Arpe and L. Hornig, Erdoel, Kohle, Erdgas, Petrochem. Brennst.-Chem., 23 (1970) 79. 2. P.M. Henry, J. Org. Chem., 36 (1971) 1886. 3. T. Jintoku, H. Taniguchi and Y. Fujiwara, Chem. Lett. (1987) 1865. 4. T. Jintoku, K. Takaki, Y. Fujiwara, Y. Fuchita, and K. Huaki, Bull. Chem. SOC.Jpn., 63 (1990) 438. 5. T. Jintoku, K. Nishimura, K. Takaki and Y. Fujiwara, Chem. Lett. (1990) 1687 and (1991) 193. 6. J.E. Lyons in "Oxygen Complexes and Oxygen Activation by Transition Metals", A. E. Martell and D.J. Sawyer (eds), Plenum Press, New York, 1988, p. 233. 7. A. Kunai, K. Ishihate, S. Ito and K. Sasaki, Chem. Lett. (1988) 1967. 8. A. Kunai, J. Kitano, Y. Kwoda, J. Li-Fen and K. Sasaki, Catal. Lett., 4 (1990) 139. 9. V.M. Vlasenko, V. Ya. Vol'fson, S.A. Solov'ev and G.F. Kalapusha, Ukr. Khim. Zh. (Russ. Ed), 44 (1978) 932; C.A. 90: 91596. 10. T. Kamitoku (to Idemitsu Petrochemical Co.), Jpn. Kokai Tokkyo Koho, JP 01,211,540 (1989); C.A. 112: 55227. 11. S. Venkateshwar and M.B. Rao, Indian Chem. Manuf., 14 (1976) 18; C.A. 86: 55108. 12. L.M. Stock, K.-T. Tsen, L.J.Vorvick and S.A. Walstrum, J. Org. Chem., 46 (1981) 1757.

559 J. Barrault (Lab. Catalyse, Poitiers, France): We have shown in our laboratory (see

paper P.2: liquid phase catalytic oxidation of toluene with H202-Fe11 and 02-FeII) that the stability and the selectivity of a catalytic system could be significantly increased if the reaction was done in an electrochemical cell, Have you carried out similar experiments and what are the results?

U. Schuchardt (I. de Quimica, UNICAMP, Campinas, B r a d ) : We have not carried out any reactions in an electrochemical cell.

J. A. Navio (I de Ciencia de Materiales, Univ. Sevilla, Spain): Concerning the

reaction mechanism described in your paper, do you have any experimental evidence, other than literature data, to support your considerations about the mechanism? (for instance identification of dinuclear palladium complexes or diphenylpalladium(I1) species) U. Schuchardt : We have not isolated any reaction intermediates. The considerations about the reaction mechanism are based on literature data and on the product distribution obtained in the reactions. The reaction mechanism will be studied in more detail during the continuation of this work.

L. .J. Simandi (Central Research 1. for Chemistry, Budapest, Hungary): How do you explain the regeneration of the reduced co-oxidant Cr(-)? 0 2 is not shown in your scheme. U. Schuchardt The reduced co-oxidant is reoxidized by molecular oxygen to chromium(V1). as indicated by the oxygen consumption during the reaction course On the other hand, the reoxidation is not complete and some chromium(II1) is formed which is totally inert under the reaction conditions described in this paper We. therefore, believe that the co-oxidant can only be reoxidized if it maintains an intermediate oxidation number With a more efficient stirring system we are able to avoid the formation of chromium(II1) and can keep the co-oxidant in an active form On the other hand, severe over-oxidation is now observed and benzophenone and fluorenone are obtained as side-products We are presently trying to find reaction conditions which would avoid this over-oxidation

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V. CortCs Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidation I I 0 1994 Elscvier Science B.V. All rights reserved.

561

Partial oxidation of cinnamyl alcohol on bimetallic catalysts of improved resistance to self-poisoning T. Mallat, Z . Bodnar, M. Maciejewski and A. Baker

Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Ziirich, Switzerland The oxidation of cinnamyl alcohol to cinnamaldehyde by air in an aqueous alkaline solution has been studied in a three phase slurry reactor. The structure of the Pdalumina catalysts promoted with submonolayer quantities of Bi was characterized by TEM-EDX and XPS. The studies show that partial coverage of the active Pto sites by Bi adatoms improves the catalysts by suppressing deactivation and increasing conversion and selectivity. Electrochemical, FTIR and TPO investigations indicate that catalyst deactivation is caused by selfpoisoning. Irreversibly adsorbed by-products are formed during the catalyst pre-reduction with the alcohol substrate under a nitrogen atmosphere and during the oxidation reaction. The successive oxidation of active sites during the oxidation reaction is a consequence of selfpoisoning. 1. INTRODUCTION

The noble metal catalyzed oxidation of primary alcohols to aldehydes may be carried out (i) in neutral or moderately alkaline aqueous solutions and (ii) in organic solvents, such as heptane or ethyl acetate [l]. The latter has minor practical importance due to safety reasons. In aqueous medium the presence of a base is advantageous to avoid catalyst deactivation. In neutral or slightly acidic medium the oxidation of primary OH groups may be totally suppressed [2]. Good aldehyde yields in aqueous alkaline medium have been reported for those instances in which both the substrate and the product are at least partly soluble in water and the carbonyl group is located next to an aromatic ring or a C=C double bond [3-51. Contradictory explanations exist on the nature of catalyst deactivation, which makes high catalysthubstrate ratio necessary and hinders the industrial application of the method. In the literature deactivation has been attributed mainly to the over-oxidation ("oxygen poisoning") of the noble metal [6,7]. According to the generally accepted dehydrogenation mechanism of alcohol oxidation, only the metallic surface sites are active and their oxidation should results in a loss of activity. Deactivation of polycrystalline Pt during the electrocatalytic oxidation of alcohols has been explained by the formation and strong adsorption of linear- or bridge-bonded CO [8]. However, it was proved by in-situ FTIR spectroscopy that CO poisoning of Pt is characteristic only for the destructive adsorption of short chain primary alcohols [9]. We proposed recently that the primary reason of deactivation during the oxidation of higher molecular weight secondary alcohols to ketones is the formation of organic byproducts [lo-131. Their irreversible adsorption on the active sites results in low reaction rates and a successive oxidation of the platinum metal which can falsely be interpreted as "oxygen poisoning".

562

The best catalysts are Pt or Pd promoted with Bi, Pb or other heavy metals [14-161. Two main roles of promoters have been proposed. By decreasing the size of active site ensembles, they can suppress the initial destructive adsorption of the alcohol substrate leading to irreversibly adsorbed byproducts [lo-121. When the catalyst surface is in a partially oxidized state during reaction, the promoter adatoms may behave as new active centers which adsorb OH better than the platinum metal [17] or act as new type of adsorption sites by complexation with the substrate [18]. In this paper we will discuss the nature of catalyst deactivation and the role of promoter using as a model reaction the liquid phase oxidation of cinnamyl alcohol to cinnamaldehyde. Pt/alumina promoted by Bi has been used as catalyst and a wateddetergent system as solvent. The advantage of this system has been shown in the selective oxidation of waterinsoluble secondary alcohols to ketones [12, 191. 2. EXPERIMENTAL

The Bi-promoted catalysts were prepared by consecutive reduction of a submonolayer of Bi onto a commercial 5 wt% Pdalumina (Engelhard E 7004, Pt dispersion 0.30 determined by TEM). An unsupported B e t catalyst (Bi/Pt = 0.06) for cyclic voltammetry was prepared similarly [ 171. The oxidation reactions were performed in a glass batch reactor, equipped with magnetic stirrer, reflux condenser and thermometer. The potential of the catalyst slurry during reaction was measured with a Pt rod collector electrode against a Ag/AgCmCl(sat) reference electrode [19]. Before reaction the catalyst was pre-reduced in situ under a nitrogen atmosphere (20 min) with the alcohol substrate (3.5 g) in 30 cm3 aqueous solution containing 0.14 g Li,C03 and 0.14 g dodecylbenzenesulfonic acid sodium salt detergent. The reactions were performed at 40 "C for 290 min; the oxidant was air. The reactor worked in a mass transfer limited regime, controlled by the air flow rate (10 cm3min-') and the mixing rate (1500 min-'). Conversion and selectivity were determined by GC analysis [lo]. The main products were cinnamaldehyde and cinnamic acid. High resolution transmission electron microscopy (HRTEM) was carried out using a JEOL 200 CX microscope. For the microanalytical investigations scanning transmission electron microscopy (STEM) was applied ( E O L 2000 FX 11), in combination with an energy dispersive X-ray spectrometer @DX) LINK AN 1085s with a Si/Li detector system. Temperature programmed oxidation (TPO)experiments were performed on a Netzsch STA 409 thermoanalyzer. Evolving gases were monitored by a Balzers QMG 424 mass spectrometer, connected to the thermoanalyzer by a heated capillary. 45-50 mg sample was heated with 10 "C.min-' rate in an air flow of 30 cm3min-'. FTIR spectra were recorded by a Perkin-Elmer FT-IR 200 instrument in the reflectance mode. 5 wt% catalyst in KBr was studied in air. Typically, 50 scans at a resolution of 8 cm-' have been accumulated. Difference spectra corrected with the background (catalyst without treatment) are shown. The electrochemical cell and polarization method used for cyclic voltammetry have been described previously [20]. 2 mg catalyst powder on a carbon paste electrode was polarized in a 0.1 M aqueous Li,C03 solution with 0.5 mV.s" scan rate. All potentials are referred to a hydrogen electrode in the same solution.

563

3. RESULTS AND DISCUSSION 3.1. Structure of Bi/Pt/alumina catalysts

On the basis of literature data and preliminary experiments it was supposed that the unambiguous interpretation of the role of promoters in the liquid phase oxidation of alcohols necessitates the preparation of a series of bimetallic catalysts with similar structure (dispersion, particle size distribution, etc.), but different surface composition. A special technique has been developed in our laboratory to deposit submonolayers of Bi adatoms onto the surface of supported Pt particles. By optimizing the method it has been found [12,17] that the slow deposition of Bi by hydrogen onto unsupported Pt from extremely dilute, moderately acidic aqueous solutions results in the formation of Bi submonolayers up to 0 = 0.5. At higher coverage bulk Bi crystallites are also formed. The Bi deposition onto a 5 wt% Pt on alumina catalyst was studied by electron microscopy. EDX analysis revealed that the 3-4 nm Pt particles were partially covered by Bi. The average Bi/Pt ratio was close to the predicted value. Some bulk Bi crystallites also developed by covering and interconnecting the Pt particles, forming 10-30 nm agglomerates. An extreme example, a well-crystallized "rod" containing more than 90 % Bi, is shown in Fig. 1. The higher the overall Bi/Pt ratio in the catalyst, the higher was the amount of Bi deposited onto the alumina support. Note that Bi3+does not react with molecular hydrogen; its reduction necessitates hydrogen adatoms adsorbed on surface Pt atoms (Pt,) or spilt over to the support.

Figure 1. Electron micrograph of a Bi (Pt) crystal (ca. 30 nm) in a Bi/Pt/alumina catalyst (Bi/P& = 0.50).

5 64

X P S investigation supported the above results. Two types of Bi species were found, Bio and Bi-. The surface Bi/Pt ratio increased with increasing overall Bi content in the catalyst. 3.2. Influence of Bi-promotion on the partial oxidation of cinnamyl alcohol The reaction rate and the final conversion were very low with the Pdalumina catalyst. A partial coverage of Pt particles by Bi adatoms suppressed the deactivation and increased the final conversion and selectivity towards cinnamaldehyde (Fig. 2). There is an optimum in the B e t , ratio above which both conversion and selectivity decrease.

Conversion, Selectivity, %

loo -

Selectivity

-* - -:.

_ _ _ _ - - _- -- - -

E. mV

.._

_-.-.-.:.*---x .

80 _ : '

Conversion 600

60-

t

OH!,

.....

I

:-

Had.

400

y/i;P _.

20

0' -

0 0

I

0 0.2

0.4

,

I

100

O

,

200

I

300

0.6

Bi/Pt, ratio

Figure 2. The catalytic performance of Bi/Pt/alumina catalysts.

Time, min

Figure 3. Catalyst potential during reaction as a function of B e t , atomic ratio.

The oxidation state of the bimetallic catalysts during pre-reduction and alcohol oxidation was followed by measuring the potential of the catalyst slurry under open-circuit conditions (Fig. 3). For the interpretation of the potential values, the cyclic voltammograms of similarly prepared Pt and B i P t catalysts were also measured in the same aqueous alkaline solution, in the absence of organic compounds. The voltammograms indicate that hydrogen sorption is characteristic for potentials below 0.5 V. Above this value both the uncovered surface Pt atoms and the Bi adatoms are successively oxidized by OH adsorption [17]. The approximate potential corresponding to the transition from reduced to oxidized catalyst surface is marked in Fig. 3.

565

A comparison of Figs. 2 an 3 indicates that the deactivation of the unpromoted Pt/alumina catalyst is accompanied by a fast oxidation of its surface. The catalyst potential during pre-reduction and alcohol oxidation is decreased by Bi-promotion. The lower the catalyst potential, the higher is the final conversion and the selectivity towards cinnamaldehyde. 3.3 Nature of catalyst deactivation

We propose that the primary reason for deactivation is the self-poisoning of Pt. The initial, destructive adsorption of the alcohol substrate leads to the formation of various irreversibly adsorbed products, which cover a fraction of the surface Pt atoms. The contaminated surface cannot be reduced efficiently by the substrate, which is indicated by the relative positive potential of the Pt/alumina catalyst after the 20 min pre-reduction step under a nitrogen atmosphere (0.4 V, Fig. 3). A partial coverage of Pt by Bi adatoms decreases the size of active site ensembles and suppresses deactivation. The low impurity content of the metal surface is indicated by the lower catalyst potential after the pre-reduction step (0.1 - 0.15 V, Fig. 3). We suggest that the by-product formation needs larger active site ensembles than the adsorption and oxidation of alcohol to aldehyde. Between 5 and 90 % conversion the catalyst potential increases by almost 0.4 V, even in the best case (Bi/Pt, = 0.5). This value is much higher than the one expected from the change in the alcoholketone ratio (Nemst equation). The unexpectedly high potential is attributed to by-product formation during the oxidation reaction. The increasing blocking of the active sites lowers the rate of alcohol oxidation. At constant oxygen feed the catalyst potential increases, according to the mixed potential theory [21]. The increasing catalyst potential is an indication of by-product formation and adsorption on the active sites during reaction. The most likely side reactions are the aldol dimenzation of cinnamaldehyde and the further oxidation of the dimer or polymer [13]. Note that the catalyst potential increased by less than 0.1 V between 5 and 90 % conversion of diphenyl carbinol or 1-phenylethanol to the corresponding ketones, as the formation of irreversibly adsorbed by-products was negligible in those cases [ 12,221. Bi promotion has only minor influence on the increase of catalyst potential during the oxidation of cinnamyl alcohol suggesting that the by-product formation during reaction is insensitive to the size of active site ensembles. The cyclic voltammetric, 'IT0 and FTIR measurements supplied evidence for the selfpoisoning of Pt. The hydrogen sorption region of the cyclic voltammogram of a Pt powder catalyst is shown in Fig. 4,curve a. Curve b represents the voltammogram after the treatment of Pt according to the pre-reduction step in the general oxidation procedure and after a further, careful washing with deionized, distilled warm water (60"C) for removing the weakly adsorbed organic species. The change in hydrogen sorption on Pt (the area under the curves of the positive sweeps) indicates that more than half of the active sites are covered by irreversibly adsorbed products after the initial contact of Pt with cinnamyl alcohol. Unfortunately, the change in the hydrogen sorption characteristics of Bi/Pt catalysts could not be interpreted unambiguously due to the low amount of hydrogen adsorbed on these catalysts. The irreversibly adsorbed products of cinnamyl alcohol adsorption on alumina-supported catalysts during the pre-reduction step was studied by TPO. Curve u and b in Figure 5 represent the CO, evolution originating from the oxidation of organic species on the freshly

566

prepared sample or on a sample exposed to air for two months, respectively. The oxidation of the similarly treated Bi/Pt/alumina sample resulted in a curve comparable to curve Q. The oxidation of organic impurities on the untreated commercial Pdalumina is shown by curve c. The amount of organic species (expressed as carbon) on Walumina after pre-reduction, calculated from the difference of curves (I and c. is about 1 wt%.

Intensity. rn 44/A*lO-'*

1. rnA

a

0

200

400

600

800

1.ooo

E, rnV

Figure 4. Cyclic voltammograms of a Pt powder before (a) and after (b) cinnamyl alcohol adsorption.

0

100

200

300

400

500

600

Temperature, 'C

Figure 5. CO, evolution during TPO of pre-treated Pt/alumina (a,b), untreated Pt/alumina (c) and alumina (d).

Alumina support was treated with a mixture of cinnamyl alcohol (substrate), cinnamaldehyde and cinnamic acid (main products) and washed carefully with warm water. (During the pre-reduction step the surface oxides of Pt are reduced to metal and the cinnamyl alcohol "reducing agent" is partially oxidized to aldehyde and acid.) The corresponding curve d is considered as a reference. One may conclude from the curves shown in Fig. 5 that the maxima between 100 and 250 "C cannot originate from the oxidation (burning) of the substrate or the products. The source is likely some by-product formation during the destructive adsorption of cinnamyl alcohol on Pt. Bi promotion did not influence the nature of this by-product. The reactivity of by-products towards oxygen - expressed by the low-temperature peak of CO, evolution decreases after two months storing in air (shift of curve h to higher temperatures between 100 and 250 "C). Note that the possibly instable products of alcohol adsorption cannot be detected by this way due to the inecitable exposure of the sample to air before TPO.

561

FTIR studies of the above samples gave further evidence for the destructive adsorption of cinnamyl alcohol on Pt and Bflt particles (Fig. 6). On alumina (trace c) the characteristic IR peaks of the strongly adsorbed cinnamyl alcohol, cinnamaldehyde and cinnamic acid are seen. The interaction of cinnamyl alcohol with the metal surface results in a large amount of strongly adsorbed, unidentifiable products. Their presence obscures the observation of the peaks characteristic of the alumina support after pre-treatment.

1800

1700

1600

1500

1400

1300

1200

wavenumber, cm-1 Figure 6. FTIR spectra of pretreated BiPdalumina (a), Pt/alumina (b) and alumina (c).

4. C O N C L U S I O N S

The Bi/Pt/alumina catalysts developed for the selective oxidation of alcohols to carbonyl compounds have basically the same structure as the unpromoted Pt/alumina except that a fraction of the surface Pt atoms is covered by Bi adatoms. A fraction of Bi is deposited as bulk metal crystallites, closely connected with the Bi/Pt particles. The bulk Bi deposition has presumably no influence on the catalytic performance. Bi promotion suppresses deactivation and increases eleven times the cinnamaldehyde yield. Due to the special properties of the prepared catalysts (only surface composition is changed), the influence of promotion could be unambiguously related to a decrease in the size of the active Pto ensembles, which lowered the formation and irreversible adsorption of byproducts. The self-poisoning of Pt and Pdalumina catalysts could be proved by electrochemical, TPO and FTIR measurements. The role of Bi in this process is indicated by the considerably lower catalyst potential during the pre-reduction of the catalyst and during the oxidation reaction.

568

ACKNOWLEDGEMENTS

Financial support by the "Kommission zur Forderung der wissenschaftlichen Forschung" and Hoffmann - La Roche AG, Switzerland is kindly acknowledged. Thanks are also due to P. Hug and A. Reller for their help in catalyst characterization. REFERENCES

1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

16. 17. 18. 19. 20. 21.

22.

K. Heyns and L. Blazejewicz, Tetrahedron, 9 (1960) 67. H. Kimura, K. Tsuto, T. Wakisaka, Y. Kazumi and Y. Inaya, Appl. Catal. A, 96 (1993) 217. P. Vinke, W. van der Poel and H. van Bekkum, in M. Guisnet et al. (eds.), Heterogeneous Catalysis and Fine Chemicals TI., in Stud. Surf. Sci. Catal., Vol. 59, Elsevier, Amsterdam, 1991, p. 385. J. Le Ludec, US Patent No. 4 026 950 (1977). K. Wedemeyer and H. Fiege, Ger. Offen., No. 2 943 805 (1981). J. M. H. D i r k and H. S. van der Baan, J. Catal., 67 (1981) 14. P. Vinke, D. de Wit, A. T. J. W. de Goede and H. van Bekkum, in P. Ruiz and B. Delmon (eds.), "New Developments in Selective Oxidation by Heterogeneous Catalysis", in Stud.Surf. Sci. Catal., Vol. 72, Elsevier, Amsterdam, 1992, p. 1. R. Parsons and T. VanderNoot, J. Electroanal. Chem., 257 (1988) 9. L. W. H. h u n g and M. J. Weaver, Langmuir, 6 (1990) 323. T. Mallat and A. Baiker, Appl. Catal. A, 79 (1991) 41. T. Mallat, A. Baiker and J. Patscheider, Appl. Catal. A, 79 (1991) 59. T. Mallat, Z. Bodnar and A. Baiker, in S. T. Oyama and J. W. Hightower (eds.), Catalytic Selective Oxidation, ACS Symp. Ser., Vol. 523, 1993, p. 308. T. Mallat, A. Baiker and L. Botz, Appl. Catal. A, 86 (1992) 147. H. Fiege and K. Wedemeyer, Angew. Chem., 93 (1981) 812. B. M. Despeyroux, K. Deller and E. Peldszus, in G. Centi and F. Trifio (eds.), "New Developments in Selective Oxidation", in Stud. Surf. Sci. Catal., Vol. 55, Elsevier, Amsterdam, 1990, p. 159. H. van Bekkum, in F. W. Lichtenthaler (ed.), "Carbohydrates as Organic Raw Materials", VCH, Weinheim, 1990, p. 289. T. Mallat, Z. Bodnar, A. Baiker, 0. Greis, H. Striibig and A. Reller, J. Catal., 142 (1993) 237. P. C. C. Smith, B. F. M. Kuster, K. van der Wiele and H. S . van der Baan, Appl. Catal., 33 (1987) 83. T. Mallat, Z. Bodnar and A. Baiker, in M. Guisnet et al. (eds.), "Heterogeneous Catalysis and Fine Chemicals 111", in Stud. Surf. Sci. Catal., Vol. 78, Elsevier, Amsterdam, 1993, p. 377. T. Mallat, T. Allmendinger and A. Baiker, Appl. Surf. Sci., 52 (1991) 189. J. Koryta and J. Dvorak,Principles of Electrochemistry, Wiley, Chichester, 1987. T. Mallat, Z. Bodnar and A. Baiker, Proceedings of the DGMK Conference "Selective Oxidations in Petrochemistry", Goslar, Germany, 1992, p. 237.

569

J. VEDRINE WC-CNRS, Villeurbanne, France): You have favoured the geomemcal effect

with respect to electronic effect to explain the role of Bi to limit deactivation of Pt particles. To check it you could have changed either Bi content or Pt particle size as far as you can show clearly that Bi is on top of Pt particles not on the side of these particles. X P S data experiments are not sufficient for me because the electron mean free path is of the order of Pt particle (3-5 nm). Change in X P S data for Bi/Pt ratio may then be more sensitive to dispersion of both Bi,O, and Pt on the alumina support. Can you comment more precisely? T. MALLAT (ETH, Ziirich, Switzerland): We mentioned in the paper that the electron

microscopic study combined with EDX analysis proved that the Pt crystallites were partially covered by Bi and only a small fraction of the promoter was located on the alumina support, closely connected with the Pt particles. A more detailed electron microscopic analysis of these type of catalysts has been published elsewhere (see ref. [17] in the paper). Knowing that at the ambient temperature the reduction of Biw necessitates activated hydrogen (H,), preferential deposition of Bi onto Pt is not astonishing. In case of carbon support which possesses good conductivity the situation is different, as the discharge of the promoter and the ionization of the pre-adsorbed hydrogen may be locally separated.

J. BARRAULT (Lab. Catalysis, Poitiers, France): You proposed in your paper that the role of bismuth was related unambiguously to a decrease in the size of active Pto ensembles. (i) Have you results on the effect of the Pt dispersion on the stability of the catalyst? (ii) Is there no participation of bismuth to the activation of cinnamyl alcohol? (iii) What are the variations of hydrogen chemisorption observed with promoted and unpromoted Pt/Al,O, catalysts? T. MALLAT (ETH, Ziirich, Switzerland): (i) These type of experiments are presently going on in our laboratory. (ii) The adsorption and oxidation of alcohols take place on surface Pt atoms; Bi adatoms alone are inactive in the reaction. It is possible that OH adsorbed on Bi, is involved in the oxidation of the substrate adsorbed on Pt. This mechanism has been suggested for the oxidation of 1-methoxy-2-propanolto methoxy-acetone (see ref. [ 171 in the paper). However, during the oxidation of cinnamyl alcohol, at conversions below 15-20 %, Bi, is in a reduced state (Fig. 3) and the initial rate of the oxidation reaction is still more than ten times faster on promoted catalysts compared to unpromoted Pt. At higher conversions the contribution of Bi, cannot be excluded, but for this explanation we have to assume that the reaction mechanism is a function of conversion. (iii) One cannot determine the hydrogen sorption by a conventional chemisorption technique, as the necessary pre-reduction at 200-300 "C may cause surface restructuring of the bimetallic system. An electrochemical study of similarly prepared but unsupported Bi/Pt catalysts showed that at ambient temperature Bi does not adsorb hydrogen and the higher the Bicoverage of Pt, the lower is the amount of hydrogen adsorbed on the bimetallic system (see also ref. [I21 in the paper). X . VERYKIOS (Univ. Patras, Patras, Greece): If the influence of Bi is exclusively a

geometric one, you should see similar effects by variation of the dispersion of the metal,

570

especially at high dispersions. How can you exclude the possibility of an electronic interaction effecting the strength of adsorption bonds of the poisons on the catalyst surface? T. MALLAT (ETH,Ziirich, Switzerland): We are aware of the fact that the geometric and

electronic effects of a promoter cannot be really separated. However, in our case we could not see any sign of electronic interaction between Pt and Bi. The X P S analysis showed that the electronic state of Pt was independent of the presence of Bi promoter. An electrochemical study of hydrogen sorption on similarly prepared but unsupported Pt and B e t catalysts proved that only the amount of adsorbed hydrogen decreased with increasing Bi/Pt ratio, the H-Pt bond strength distribution remained unaltered (see ref. [ 121 in the paper). On the other hand, the dispersion of Pt is the same in all used catalysts and we have unambiguous evidences that a considerable fraction of Pt is covered by Bi. On this basis we proposed that the size of active site ensembles should play a decisive role in suppressing deactivation. We do not think that a study of the influence of R dispersion could reveal the nature of deactivation, as at high dispersions not only the size of the active site ensembles are smaller, but also the proportion of coordinatively unsaturated Pt atoms is higher. H . MIMOUN (Firmenich S. A., Geneva, Switzerland): Pd-Bi and Pt-Bi couples are known to be homogeneous catalysts for oxidation of alcohols. How can you be sure that you have no leaching during reaction? T. MALLAT (ETH, Ziirich, Switzerland): There were neither Bi nor Pt detectable by ICP-

A E S analysis in the liquid phase after reaction and, according to X P S analysis, the surface composition of the fresh and used catalysts were identical. In other reactions, such as the oxidation of L-sorbose to 2-keto-L-gulonic acid the leaching of both metallic components could be detected by X P S and ICP-AES analysis (1). Note that in the latter case the leaching of the metallic components had a negative influence on the reaction rate and selectivity. 1.

C. Bronnimann, P. Hug, T. Mallat and A. Baiker, J. Catal., (submitted).

V . Corlcs Corbcrin and S. Vic Bcllon (Editors), N e w Dcwioprricnls in SeleclLve Oxidnfion // 0 1994 Elscvicr Scicncc B.V. All right5 reserved.

57 1

Novel tungsten catalysts grafted onto polymeric materials: a comparison with phase transfer catalysis J.-M. Bregeaulta, R. Thouvenotb, S. Zoughebia, L. Sallesa, A. Atlamania, E. Dupreya, C. Aubrya,F. Robert b and G. Chottardb aDepartement de Chirnie, Universite Pierre et Marie Curie, Catalyse et Chimie des Surfaces, URA 1428 du CNRS, Case 196, 4, place Jussieu, 75252 Paris Cedex 05, France bDepartement de Chimie, Universite Pierre et Marie Curie, Chimie des Metaux de Transition, URA 419 du CNRS, Case 42, 4, place Jussieu, 75252 Paris Cedex 05, France

S UMM A R Y The reaction of hydrogen peroxide with tungstic acid, " H 2 W O 4 " , or H3[PWi 20401.aq and orthophosphoric acid has been studied, and the formation of novel oxoperoxotungstophosphate species, [ P W X O Y ] ~has - been demonstrated. Some of them have been isolated and can be grafted onto polymeric supports. They are active oxygen-to-olefin transfer agents, in association with H202; these novel catalysts are compared with phase transfer systems. 1. INTRODUCTION

As part of a study on oxidation reactions in a biphase medium and/or by phase transfer catalysis (PTC), we became interested in the comparison of anionic peroxo complexes formed from hydrogen peroxide and heteropolyacids,' Hn[XM12040].yH20 (denoted HPA; X=P, Si; M=Mo, W) with those obtained from H202 and tungstic acid, "H2WO4", in the presence of orthophosphoric acid.2 It has been shown that the Venturello complex, Q3[P04{W0(02)2}4]. 1,where Q+ is an onium group, can be prepared by employing Ishii's method involving HPA and H2O2.3 More recently, tetrakis(oxodiperoxo~olybdo)phosphate(3-)complexes, Q3[PMo4024], were isolated and analysed by various physicochemical methods (Xray crystallography, vibrational spectroscopy, 31 P NMR, etc.). We showed that the [PMo4024]3- anion is identical to the species described by a Russian group in the " H ~ M o O ~ / H ~ O ~ / H ~ P O ~ / 2 - a m i n o p y rsystem.4 i d i n e " Other anionic species were isolated; some of them are being studied by other groups. We examine in this paper novel dinuclear species, particularly [HP04{W0(02)2}2]2- (denoted PW2), which can

572

be considered as good potential candidates for PTC, for grafting onto inorganic supports or for introduction into ion-exchange resins to generate organic resinsupported catalysts. 2. EXPERIMENTAL SECTION. W~O~O}I, Preparation of [ N ( C ~ H ~ ) ~ ~ ~ [ H P O ~ { 2. Tungstic acid (2.5 g, 10 mmol) was added to?3O0h H202 (7 ml, 69 mmol). After 40 min stirring at 60°C,followed by centrifugation (15 min at 2000 rpm), 6M H3P04 (0.85 ml, 5.1 mmol) was added to the supernatant liquid. To the clear solution (solution A), tetrabutylammonium chloride (5.45 g, 20 mmol) dissolved in 10 ml water was added. After stirring (5-15 min), a white precipitate was filtered off, washed with 10 ml of distilled water, 10 ml of diethyl ether and air dried. The analysis (C, H, N, P, W) is in good agreement with the chemical formula. Catalytic tests (PTC). Hydrogen peroxide (30%; 1 ml; 9.8 mmol) was added to the "H2WO4" sample (0.13 mmol) along with 2 ml of water. After 15 min stirring (necessary for the precursor to react completely), after addition of 0.01 7 mmol H3P04, the solution was transferred to a Schlenk tube containing a CHC13 solution of Arquad 2HT@, Aliquat or cetylpyridinium chloride (5 ml, 0.1 n(W)sn(Q+)<1.5n(W)). After the two phases had been stirred for 2 min, l-octene (1 ml, 6.5 mmol) was added. The mixture was then stirred under reflux at 60°C for 4h. The organic phase was analysed by GC on a Girdel apparatus equipped with a 3 m 10% OV 105 on WHP Chromosorb column. 1,2-Epoxyoctane was the major product; traces of 1,2-octanediol and heptanal were detected by GC-MS coupling. Preparation of organic resin-supported catalyst and catalytic tests. Beads of Amberlyst A26@ (0.5 g) in distilled water (5 ml) were stirred slowly for 30 min, then filtered off and dried under vacuum. They were added to a solution of 2 (= 2.1 meq/g of resin) in CH2C12 or toluene and stirred for 12-24 h at room temperature to exchange; the mixture was then filtered, the beads rinsed with CH2C12 (or toluene), then dried. The dried beads were pale yellow. Polymer-supported PW2 (0.5 g) was placed in the organic solvent (2.5 ml); 0.5 ml H202 (30%) and then (R)-(+)-limonene, 2 (0.2 ml, 1.2 mmol) were added and the triphase mixture stirred magnetically for-5 h at 20°C. The organic layers were analysed on a Delsi 30 gas chromatograph equipped with a 0.25 rnrn x 50 m OV 1701 capillary column and a flame ionisation detector linked to a Delsi Enica 10 electronic integrator (4a and 4 b give well resolved peaks). Preparation of silica-supported PW2 species. Catalytic tests. Dehydrated porous silica (0.5 g, 263 m2 g - l ; 1.52 cm3 g-') was added to a solution of 2 (0.5 g) in 1 ml CH2C12 (C2H4C12 or toluene). The system was stirred for 1-12 h at room temperature. The complex adsorbed on silica was then obtained by filtering; it was washed with the organic solvent (4 x 3 ml) and dried on a porous plate. The catalytic tests were similar to those involving PW2/Amberlyst A26@.

573

Preparation of silica-supported PWn species (n = 1-3). Catalytic tests. Solution A (see preparation of 2), porous silica (2 g) and methanol (2 ml) were mixed and the system stirred for 1 h 2 room temperature. The solid was then filtered off, rinsed with MeOH (4 x 1 ml) and then air dried. All catalysts were pretreated in air at 330-500°C for 2-5 h after temperature programming (15°C h-l). Catalytic tests. Oxidation solution: terf-Butyl alcohol (150 ml) was kept at 30°C and mixed with 45 ml of 30% H202. The solution was stirred with anhydrous MgS04 (40 g) for 3 h and filtered. The PW2/SiO2 (0.5 g) was dispersed in 3 ml of the oxidation solution. (R)-(+)Limonene (1 ml, 6 mmol) was added and the mixture then stirred at 20°C. The organic phase was analysed directly by GC. 3. RESULTS AND DISCUSSION. 3.1. Spectroscopic characterization of oxoperoxo anions formed in the

"H3[PW12040].aq/H20~" system. The 3 i P NMR spectra of this system were first studied with [H202]o/[W] = 1, C w = l M and pHs1. They exhibit several peaks and the concentrations of the corresponding species change with time so dramatically that it seems difficult to assign all the 3 l P NMR signals (Figure 1) to known phosphato-oxoperoxotungstates or phospho-oxotungstates. It appears, however, that addition of H202 to the [PW12040]3- solution leads immediately to the partial degradation of the Keggin anion, as shown by the appearance of the lines (a) around 0 ppm, characteristic of low-condensed peroxo species (low W/P ratio, vide M a ) . The concentration of these species reaches a maximum after about 3-4 min. The growth of other species occurs after about 10 min, at the expense of the starting [PW12040]3- moiety (line *) but also of these transitory species. Lines b and c can be tentatively assigned to monovacant [PW11039]7- and other plurivacant 0x0 species, whereas the shielded lines around 20 ppm could be due to relatively highly condensed oxoperoxo species.

a

"H3[PW12040].aq/H202" system: [H20210NV = 1 ;

5 74

For excess H 2 0 2 ( [ H 2 0 2 I o / [ W ] = 7), the 3 1 P spectra of aqueous solutions at equilibrium present prominent peaks in the +1 to -4 ppm region, some with well resolved tungsten satellites (Figure 2). The relative intensities of the satellites with respect to their central line allowed us to assign these lines to phosphatooxoperoxotungstates with the following W/P ratio: 1 , 2, 3 and 4 for +0.3, -0.3, -1.5 and -3.5 ppm, respectively. The measured coupling constants corresponding to the four "PWxOy" species are significantly different (Table I ) , which allows us to correlate the 3 1 P signals with the 1 8 3 W NMR doublets between -660 and -680 ppm. Moreover, the intensity variations of the 3 1 P NMR lines, as the H 3 P 0 4 concentration increases, appear to be correlated with the intensities of the l 8 3 W counterparts but uncorrelated with each other. These observations and the relative intensities of the tungsten satellites are fully consistent with the assignment of these signals to "PWxOy" species (x = 1-4; y depending on the nature of the phosphato-oxoperoxo species). The results (Figures 1-2) show that partial degradation of the [ P W 1 2 0 4 0 ] 3 - unit occurs, even at low [ H 2 0 2 I o / [ W ] ratios, and generates low-condensed phosphato-oxoperoxotungstate species and other tungsten-based species. [PW,O,]'

I

[Pw,O,]~-

Figure 2. 162 MHz 3 1 P NMR spectra of the [PW,O Iwanions; "H 3 [ P W1 2 0 4 0 ] . a q / H 2 0 2 " system: [ H 2 0 2 ] o / [ h ] = 7:1, 300 scans Table 1 3 1 P and

1 8 3 W NMR ( [ H 2 0 2 ] / [ W ) = 7).

6a -3.5 -1.5 -0.3 +0.3

data for the aqueous phosphato-oxoperoxotungstates

31P 2 ~ ~ - p b , C Ire1 W) 25.0 (4W) 15 24.4 (3W) 22.9 (2W) e

30 38 17

ga,d -667 (d) -668 (d) -676 (d) -675 (d)

'83W 2J~-pb 25.1 24.1 22.8

Ire1 P o ) 9 13 11

assignment [PW40mla[pw3Onl~[PW20plY[PWO&

a 6 in pprn relative to H 3 P 0 4 (31P) and W042- (183W), respectively; negative values correspond to shielding with respect to the reference; J in Hz k 0.2 Hz; number of W in parentheses, determined from relative intensity of the tungsten satellites; resonance, satellites not resolved.

multiplicity in parentheses; (d) = doublet;

broad

515

The "PW,O," complexes can also be generated in separate experiments from the "H2W04/H202/H3P04" systems;5,6 the NMR results demonstrate the existence of several equilibria which are tentatively summarized by the scheme:

n [Wj-0-0

n [WfO-O

r

- 1

[W-yO-0

Scheme 1

Let us mention that the same kind of chemistry can be developed with other assembling ligands (e.g. As043-; SiO&; etc.). We have chosen to study more closely species, i.e. the "PW20 " unit which can be isolated as a one of these "PW,O," tetrabutylammonium salt: ( ~ - B u ~ N ) ~ [ H P W ~-O I ~ ] ,

f

3.2. Crystal and Molecular Structure of 2. The asymmetric unit consists of a nearly tetrahedral assembling anion, HPO42- and a neutral moiety, [W202(p-02)2(02)2] which is a rather unique building block for the two anions [HPW2014]2- (ref.7) and [PW4024]3- (ref.2). In the neutral moiety, the tungsten atoms are seven-coordinated by oxygen atoms in a pentagonal bipyramidal arrangement ( PBP Y - 7 ) . Two anionic units [HPW2014]*- are held together by two equivalent rather strong hydrogen bonds (dO.0 = 2.51 A).

Figure 3. Dimeric anionic species in 2 showing the hydrogen bonds between two units The hydrogen atom of this H-bridge is characterized by a signal at 5.4 ppm7 (MAS + echo method NMR spectrum of a polycrystalline sample of 21. This neutral moiety has two distinct pairs of peroxo ligands which have not been observed for peroxo

576

tungsten complexes with O X O , ~oxalate,g sulfatelo or carbonatel as assembling ligands. Infrared and Raman spectra suggest that the structure of the anion in 2 is maintained in organic solvents (Figure 4). The 31P and 183W NMR spectra o t a n acetonitrile solution of 2 - are also in agreement with the persistence in solution of the [HP04{W(O)(p-O2)(02)}2]2- anion with the same overall structure as in the solid state, although solvent effects on the 6 and J values are observed. Solubilization in organic solvents probably requires ion-pairing through specific interactions between the anion and the onium cation, which could induce slight geometrical and/or electronic modifications in the oxoperoxoanions and hence explain the observed NMR variations. The protonation of the peroxidic ligands could also account for such a difference in aqueous H202 solutions.

4.000 1 .GOO

3 200 1 200

0 800

0.400

Figure 4. Raman spectra of 2: (a) polycrystalline sample; (b) in acetonitrile.

3.3. Catalytic Tests (Phase Transfer Catalysis PTC). Epoxidation of alkenes (mainly 1-octene and terpenes) by Q3[X04{M0(02)2}4] precursors or by dinuclear complexes shows that in many cases the order of increasing activity of phase transfer systems is: [ P 0 4 { M O O (0 2 ) 2 } 4 ] 3 < [ A s 0 4 { M o O ( 0 2 ) M 3 ' 5 [ w 2 0 3 ( 0 2 ) 4 ( H 2 0 ) 212- < [ p o 4 { w o (O2)2I4l3- = [HPO4{WO(O2)2}2]2- I [AsO4{WO(O2)2}4]3-. With these systems (PTC) the molybdenum or tungsten complexes are dissolved in the reaction mixtures (aqueous and organic phases, Figure 5). The assembling ligand " X ' causes no reaction but a peroxo group can transfer "active oxygen" to the olefinic substrate. Several parameters must be adjusted to optimize conversion and selectivity; Figure 6 gives an illustration of the [Q+]/[W] effect on the overall yield of 1,2-epoxyoctane.

511

Q = NR'R2R3R4; ...

M = W, Mo;...

Figure 5. Principle of phase transfer catalysis.

Q'clQ-(cH,)~,

-

n

0

05

a'iw

1

1J

CH,

a

=

"CPC"

-

"Aliquat"

Figure 6. Optimization of [Q+]/[W] with the "H2W04/H202/H3P04/Q+X-/CHC13" systems. Catalytic test: epoxidation of 1-octene (6.5 mmol); H202 10% (9.8 rnmol); 60°C; 4h; n(W)/n(l-octene)o = 2%; n(P043-)= 0.017 mmol. These curves (Figure 6) show a dramatic effect of the nature of Q+; Arquad 2HT@could be selected as one of the best phase transfer agents. The position of the maximum has to be related to the existence of several speciess-7 in the aqueous phase (see scheme l ) , the equilibria depending upon the nature and the concentration of Q+CI- in the two phases. Moreover, we have to consider that the transfers of HOO-Q+ and of inactive solvates, [Q+CI-....H202],12compete with that of peroxidic oxygen by anionic activated species, [(X),W(0),(02)z]an- (Figure 5).

578

3.4. Why graft dimeric units on inorganic surfaces? The recycling or re-use of the tungsten precursors or catalyst is not, in our hands, a simple operation. Moreover, it is difficult to rationalize the design of a heterogeneous system with a solid catalyst involving the basic elements of this new class of epoxidizing agents. Several attempts have been devoted to capillary impregnation or "dry impregnation" of inorganic supports (or adsorption) with solutions of Keggin units, e.g. H3[PW12040].aq, H3+n[PM012-nVn040].aq,etc. and to subsequent thermal treatments to prepare heterogeneous catalysts. Some of these studies give evidence for a redistribution of the metal(s) between several surface species during the thermal treatment.13 Moreover, aqueous solutions of heteropolyacids (HPA) with the Keggin structure, H3[PM12040] (M=Mo, W) are also degraded in the presence of excess H202 to form peroxo species: [PO4{MO(O2)2}4]3[HP04{W0(02)2}2]2- (vide supra) and [M203(02)4(H20)2]2-, etc.; it appears that these anions are responsible for the catalytic activity of phase transfer systems involving H3[PM12040]. These experimental facts and a comparison of the structures of all these anionic species led us to consider the [HP04{MO(02)2}2]2- anion (or their analogues, i.e. units less condensed than [PM4024]3-) as privileged precursors for the preparation of heterogeneous catalysts. Even if the peroxo groups are unstable and decompose during thermal treatments to give 0x0 groups, they will be potential sites which can be easily regenerated as surface complexes according to a well known process:

Scheme 2

It should be noted that monomeric tungstate species, WO42-, are predominant at pH>8 and low concentrations, while oligomeric non-peroxidic species, [Hx~.W,~.Oz~.]w'are predominant at pH<8 and high concentrations,l4 while the phosphato-oxoperoxotungstate species are generated in the pH 1-2 range. Then, in the latter case, the protonated surface hydroxyls of the inorganic support, Z--OH2+ (scheme 3) can be involved for the creation of adsorption sites. Among the amphoteric oxides (Si02, Zr02, Ti02, Cr2O3, Al2O3, MgO, etc.) silica is certainly worth trying with acidic aqueous solutions of HPA (non-peroxidic or peroxidic) which are initially at pH<2. Under these conditions, silica is an anionic exchanger according to the scheme:

Z-O-+

H+-

Z-OH

H'

c--Z-OH,

+ Scheme 3

In this paper we compare mainly PTC (vide supra) and heterogeneous catalysis. Catalysts suitable for conducting epoxidation reactions can be made I) by depositing (n-BuqN)2[HPW2014] on silica, which gives an efficient catalyst and alleviates the problems of discharge of W into the environment; h) by impregnating silica with a solution of phosphato-oxoperoxotungstate species containing

[HPO4{WO(O2)2}2]2- and other low-condensed species. Subsequent thermal treatment of the loaded support gives an active catalyst ; iil) by ion exchange resins with alkylammonium groups which are commercially available. We shall consider this organic support first and then the preliminary results with silica-supported complexes.

3.5 Organic resin-supported catalyst from organic media. The main advantage of using polymer-supported catalysts is their easy separation from the reaction mixture and usually high catalytic activity along with greater stability. This part deals with the synthesis of polymer-supported "PW2" catalysts and their evaluation in a catalytic test. Amberlyst A26@ was one of the macroreticular ion exchange resins used. It has high porosity ( 7%) and medium N(CH3)3+CI-, surface area (30 m2.g-') which exposes most of the ionic groups, in a non-aqueous system. We find that polymer-supported "PW2" beads are active catalysts for epoxidation of alkenes and, in this work, of (R)-(+)-limonene. In a typical catalytic experiment a triphase mixture was stirred vigorously at RT for 3-5 h. Table 2 shows high conversions and high selectivities in favour of the monoepoxide, 4 , even after long reaction times.

j--

3

4a cis-isomer

42

5

6

trans-isomer

main products

by-products

It was observed that H202 decomposition remains negligible up to pH 4. To compare the activity of resin-supported complex with that of unsupported complex, two sets of experiments were run simultaneously keeping the main parameters constant. It was observed that the reaction involving unsupported catalyst (PTC) is nearly 3 times faster than the reaction using resin-supported catalyst. 3.6. Silica-supported PW2 species from organic solvent. A solution of ( ~ - B u ~ N ) ~ [ H P W ~2, O in I ~ CH2C12, ], C2H4Cl2 or toluene was used to support 2 on dehydrated porous silica. The tungstenkarrier ratios were about 0.015:l by weight. Comparison of Figure 7 with the spectra (Figure 4), shows that effective sorption of the salt has been achieved. Other spectrometric studies are in agreement with this observation. Catalytic tests (see experimental section) were carried out. Table 2 shows that high conversions and high selectivities can be attained, even after 3 or 4 batches, but leaching was observed.

580 1

,.

, ,.

800.0

400.0

1200.0

1600.0'

Figure 7. Raman spectra of 2 supported on silica.

3.7. Silica-supported PWn species (n = 1-3)from aqueous media. With the "H2W04/H202/H3P04" system, the formation of "H3[PW2014]" can be favoured and used to prepare silica-supported "PW2" species firstly according to scheme 3 (the HPW20142- anion being trapped by the Z--OH2+ species). Catalysts corresponding in theory to coverage of one monolayer by the acid component were prepared by adsorption equilibria with the necessary quantity of aqueous (or MeOH/H20) solutions of the acids, assuming that the PW2 species occupies 25 A2. All catalysts were thermally treated in air at 330-500°C for 2-5 h after temperature programming (1 5"C.h-1). Catalyst characterization will be presented elsewhere. Preliminary tests have been conducted in a two-phase system with t-BuOH-Hz02 mixtures. The crude catalysts (see Table 2) are highly active and give initially - to form 1,2-diols, - is also catalysed by the monoepoxides, 4. Ring-opening of 2 thermally treatedsupported PWn species. Table 2. Epoxidation of limonene by H202. Run Catalyst Time (h)

Substrate conversion

la Q N " 2 0 1 4 1 1 2b PW2/Amberlyst A26@ 3-5

90 70-95

3b

100 71

4

Q'2[HPW2014]/Si02 PWnO,/SiO2

3-4 4-5

($+~JI)

4a/4b

(YO) yield (yo) 72 68-94 95 I 1

Diole

yieId-( Yo) 0.95 0.85 1.25

14 traces 0 67

aGeneral procedure for epoxidation with PTC: CHC13 (5 r n l ) / l O % H202 (3 rnl, 9.8 rnrnol); Teflon-coated bar driven externally by a magnetic stirrer; lirnonene (6.2 rnrnol); [W]/lirnonene = 2%; phase transfer agent (QCI) = Arquad 2HT8, bsee text, Q' = (mBu4N).

581

In spite of the low pH of the catalyst solutions (pH = 1-2), the activity of these supported "PWn" materials can be traced not only to Bronsted acidity but also to a metal effect. Controlled addition of amines can suppress the epoxide ring-opening. The oxidizing power of these catalysts is sometimes high enough for oxidative olefin cleavage. Work is in progress to control these systems better and to design tailormade catalysts. At present, it is possible to epoxidize the substrate with low release of W into the liquid phase of the tri- or biphase mixtures. REFERENCES 1. Y. Ishii, K. Yamawaki, T. Yoshida, T. Ura and M. Ogawa, J. Org. Chem., 52 (1987) 1868; S. Sakaue, Y. Sakata, Y. Nishiyama and Y. Ishii, Chem. Lett., (1992) 289. 2. C. Venturello, R. D'Aloisio, J. C. J. Bart and M. Ricci, J. Mol. Cat., 32 (1985) 107; C. Venturello and M. Gambaro, J. Org. Chem., 56 (1991) 5924. 3. J.-M. Bregeault, C. Aubry, G. Chottard, N. Platzer, F. Chauveau, C. Huet and H. Ledon, In "Dioxygen Activation and Homogeneous Catalytic Oxidation", L. I. Simandi (ed.), Elsevier, Amsterdam (1991) 521; C. Aubry, G. Chottard, N. Platzer, J.-M. Bregeault, R. Thouvenot, F. Chauveau, C. Huet and H. Ledon, Inorg. Chem., 30 (1991) 4409. 4. L. Salles, C. Aubry, F. Robert, G. Chottard, R. Thouvenot, H. Ledon and J.-M. Bregeault, New J. Chem., 17 (1993) 367 and references therein. 5. C. Aubry, Thesis, Universite P. et M. Curie (1991). 6. A. C. Dengel, W. P. Griffith and B. C. Parkin, J. Chem. SOC.,Dalton Trans., (1993) 2683. 7. L. Salles and J.-M. Bregeault, to be published. 8. F. W. B. Einstein and B. R. Penfold, Acta Crystallogr., 17 (1964) 1127. 9. M. Hashimoto, T. Ozeki, H. Ichida, Y. Sasaki, K. Matsumoto and T. Kudo, Chem. Lett., (1987) 1873. 10. M. Hashimoto, T. Iwamoto, H. lchida and Y.Sasaki, Polyhedron, 10 (1991) 649. 11. R. Stromberg, Acta Chem. Scand., Ser. A, 39 (1985) 507. 12. E. V. Dehmlow and S. S. Dehmlow, "Phase transfer Catalysis", 3 rd. Edn., VCH Verlagsgesellschaft mbH, Weinheim, 1993. 13. C. Rocchiccioli-Deltcheff, M. Amirouche, G. Hewe, M. Fournier, M. Che and J. M. Tatibouet, J. Catal. , 126 (1990) 591 ; K. Bruckman, J. M. Tatibouet, M. Che, E. Serwicka and J. Haber, J. Catal., 139 (1993) 455. 14. L. Karakonstantis, Ch. Kordulis and A. Lycourghiotis, Langmuir, 8 (1992) 1318.

DISCUSSION H. Mimoun (Firmenich Geneva, Geneva, Switzerland): The fact that, in contrast to neutral Moo5 or W05 complexes, only one of the peroxidic oxygen atoms transfers to olefins seems to be due to the bridging structure favoring hydroperoxidic species. Do you have any idea concerning the mechanism of the oxygen transfer reaction?

J.-M. Bregeault (Universite P. et M. Curie, Paris, France): It seems that in the case of W0(02)2HMPT the two peroxo groups react with the substrates at different rates. The transfer of the first peroxide oxygen been faster than the second one. At that time the mechanism of the epoxidation reaction is not clear and further experiments must be done to compare the Sharpless proposal and yours.

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V. CortBs Corbcrin and S. Vic Bell6n (Editors), New Developments in Selective Oxidation I1

0 1994 Elsevier Science B.V. All rights reserved.

583

Selective oxidation of cyclopentene and cyclohexene by hydrogen peroxide catalyzed by heteropolyacids Kwan-Young Lee, Koshi Itoh, Masato Hashimoto, Noritaka Mizunoll, and Makoto Misono" Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyoku, Tokyo 113, Japan Oxidation of cycloolefins by hydrogen peroxide catalyzed by various Keggin-type heteropolyacids, H ~ P M O ~ ~ - x=O~ W12, ~ were O ~ studied ~ , in homogeneous tri-n-butyl phosphate solution, placing stress on the effects of the constituent elements of heteropoly anion. Remarkable synergistic effect of mixed addenda atoms, Mo and W, was observed and high yields of dialdehyde were obtained. A reaction scheme was proposed based on the time course and stoichiometry of reaction, and the reactivity of products.

1. INTRODUCTION By utilizing the acidic and redox properties of heteropolyacids (HPA) [l], several large scale synthetic processes have been already commercialized [2J. The hydrations of propene, isobutene, and n-butene utilize HPA as acid catalysts and oxidation of methacrolein is an example using it as an oxidation catalyst. The acidity and oxidizing ability can be controlled by the change of the constituent elements (counter cation, central and addenda atoms of the anion cluster) in a systematic way, which makes the catalyst design possible. The effect of the cation has been examined widely for HPA in solid-state. Cs2.5Ho.gPW12040 shows very high activity i n acid catalysis 131, and Mo, V mixed-coordinated HPA are active for several oxidations (e.g., pentane, methacrolein, and isobutyric acid) [ 11. Recently, epoxidation and hydroxylation of olefins and aromatics using hydrogen peroxide, which is a clean oxidant, with titanosilicate [4,5] or HPA are worthy of note. Venturello's PO4W 0 4 or ( P 0 4 [ W ( 0 ) ( 0 2 ) 2 ] 4 ) 3 -161, Ishii's P M 0 1 2 0 4 0 ~ -or PW120403- [ 7 ] , Schwegler's lacunary PW110397- [8], Finke's SiWg(Nb02)30377- 191, and Hill [lo] and Lyon's 1111 transition metal-substituted HPA are the catalysts recently developed based on HPA. Furukawa et al. recently reported a remarkable synergistic effect for the formation of dialdehyde from olefin with Mo, W mixed-coordinated HPA catalysts [ 121. Dialdehydes are important intermediates for synthesis of various chemicals. However, the mechanism of the synergistic effect was not clarified in the study. In this study, it was attempted to synthesize dialdehydes by selective oxidation using hydrogen peroxide and HPA catalysts, and to elucidate the reaction mechanism, placing stress on the effects of constituent elements of heteropoly anion.

* To whom correspondence should be addressed. ll Present address: Catalysis Research Center, Hokkaido University, Sapporo 060, Japan.

5 84

2. EXPERIMENTAL 2.1. Catalysts H ~ P M O ~ ~ (which - ~ Wwill ~ be O abbreviated ~ ~ as Mol2-xWx; x=O-12) was prepared from Na2HP04.12H20, N a 2 W 0 4 * 2 H 2 0 , and N a 2 M o 0 4 * 2 H 2 0 solutions. After the aqueous solutions were mixed at 80°C for l h , hydrochloric acid was added at room temperature. Then, heteropolyacid was extracted by diethyl ether and crystallized at room temperature. As described below, the mixed-coordinated HPA are nearly statistical mixtures of Mol2-xWx, x = 0-12. H3PW12040, H3PMo1204(), HqSiMo12040, and HqSiW12040 were purchased from Nippon Inorganic Color and Chemical Co., Ltd. and used after extraction by diethyl ether and recrystallization. H5BW 12040 was prepared according to the literature [ 131. H4CW 1 2 0 4 0 [14] was kindly donated by Prof. T. Kudo of the University of Tokyo. 2.2. Reaction Reaction was c a m e d out in a 200 ml flask to which stirrer, condenser, thermometer, and sampler were attached. Catalyst (6 hydrate, 0.18 mmol), olefin (0.17 mol), and 10 wt% H 2 0 m B P (tri-n-butyl phosphate) (0.1 mol H202) were introduced into the reactor in the order at 0°C to prevent an abrupt increase of temperature during the mixing of the reactants, and then the temperature was increased to 30 "C in 5 min. During the periods, it was confirmed by gas chromatography (GC) that no reaction occurred. The solution of reaction was homogeneous. Reactants and products were analyzed by GC (FID, PEG 20M) and the concentration of hydrogen peroxide was determined by iodometry. 10 wt% H 2 0 m B P solution was prepared by the distillation of 35 % hydrogen peroxide and TBP solution at 20 torr and 45OC. 2.3. Characterization The catalysts prepared in this study were characterized by lPNMR (JNMGX-~OOMHZ, JEOL CO.) and 1R. Samples for NMR measurements were prepared by dissolving them in TBP. CD3Cl was used for spin lock, which was placed in the outer part of a dual cylinder. As an external standard, 85 % hydrophosphoric acid was used. IR was measured using JEOL 10 with an MCT detector. An IR cell with KRS-5 windows was used for liquid samples. Solidstate samples were dispersed on the surface of a silicon wafer and measured. 3. RESULTS AND DISCUSSION 3.1. 3lPNMR of Mo, W mixed-coordinated HPA The 31PNMR spectra of Mo6W6 in TBP solution ( 5 molL7o) are shown in Fig. la, in which 13 peaks were observed from -3.2 ppm of Mo12 to -14.3 ppm of W12 with a central peak at -9.0 ppm. The integrated peak intensities observed approximately agreed with the theoretical intensities calculated according to the binomial distribution, eq. ( l ) , as shown in Fig. 2 indicating that Mo6W6 was composed of 13 kinds of mixed-coordinated HPA in a statistical distribution of Mol2-xWx (x=6.3, which was calculated from the integrated intensity). Solidstate 31PNMR showed a similar pattern as that of solution even though peaks were broader in the solid-state, which indicated that this catalyst was already a mixture as it was prepared. Schwegler et al. reported that Mo12 and W12 easily exchanged addenda atoms in aqueous solution [lS]. A TBP solution of Mo12 and W12 mixture, however, showed only Mo12 and W12 peaks, so that n o mixing of addenda atoms occurred i n non-aqueous TBP solution. Other mixed-coordinated HPA ( M o I l W l , MogW3, Mo3W9, and M o l W l 1 ) were also mixtures of those with different ratios of Mo and W.

585

When H 2 0 2 was introduced into the TBP solution of Mo6W6 at room temperature, new peaks appeared at 6.2 and 4.8 ppm as shown i n Fig. lb. Since the peroxide of cetylpyridinium salt of Mo12 showed a peak at 7.2 ppm in 3lPNMR 1161, these peaks may be assigned to peroxides that originated from polyanions. It is noted for the peaks of unreacted polyanions that the species with high Mo content decreased to a larger extent than those with high W content as shown in Fig. 2 indicating that the formers are more easily peroxidized.

I

n

v -

0 1 2 3 4 5 6 7 8 9 1 0 1 1 2

1

1

1

1

8 6 4 2

1

1

1

1

1

1

1

1

1

0 -2 -4 -6 -8 - 1 0 - 1 2 - 1 4 - 1 6

PPm

Fig. 1. 3lPNMR spectra of Mo6W6 i n (a) TBP and (b) H 2 0 y T B P solutions.

x in Mol2-xWx Fig. 2. Distribution of Mol2-xWx (x=6.3) -, statistical (binomial) distribution; - , Mol2-xWx in TBP; , Mol2-xWx i n H202 / TBP.

3.2. Time course of reaction and reaction mechanism Time courses of the oxidation of cyclopentene with Mo12, Mo6W6, and W12 are shown in Fig. 3. Cyclopentene (1) decreased with time, while cyclopentene oxide glutaric aldehyde ( g ) ,and cyclopentanediol (2)increased. Induction period was observed at the beginning of reaction for the catalysts with high W content. All of these products were formed simultaneously from the beginning and the composition of the products was nearly constant at the early stage of the reaction i n the three cases. In the later stage of Mo6W6, no change was observed for the concentrations of 1 and 4. However, 5 increased gradually and 2 decreased instead. It was confirmed by a separate experiment that in the reaction of 2 under the same conditions, 4 and 5 were produced very rapidly. The induction period observed with high W content was considered to be caused by the slow rate of formation of peroxide which was differentiated in Fig. 2. Time course of the oxidation ofcyclohexene with Mo12 is shown in Fig. 4 [18]. At the early period of reaction, the time course was essentially the same as that of cyclopentene. Therefore, similar reaction schemes are suggested for both olefins (Fig. 5 ) . According to the scheme for cyclohexene (Fig. Sa), 2 mol of H 2 0 2 are consumed for the formation of 1 mol of 4_ (adipoaldehyde), and 1 mol of H 2 0 2 for 2 and 3. For the oxidation of cyclohexene with Mo12, H 2 0 2 was calculated to run OLII in SO min according to the stoichiometry. At that moment, as shown in Fig. 4, the reaction was accelerated, and 9 and 1decreased with a ratio of 1 to 1 while 2 and 5 increased. This is different from the case of cyclopentene shown in Fig. 3b. In the case of cyclopentene with Mo6W6, although H 2 0 2 was consumed after 80 min, the anomaly was not observed.

(z),

586

e0.2

-

1

I

a

. tb c

x i , 4

0.3

0

2

U 0

; 0.2 -0 C (21

EO.1 (21

c

0

(21

$

0

60

120 180

240

300

Time / min

0

0

60

120

180 240

Time / min

300

E0.2

. v)

c

0 3

U

2

a ,0.1 (5:

m

c

C

m 0 m

Fig. 3. Time courses of the oxidation of cyclopentene by H202 with (a) Mo12, (b) Mo6W6, and (c) W12.

c

2

0 0

60

120 180

240

300

Time / min -0.02

E" . v,

c

0 3 -0

0

60.01 -0 C

Fig. 4. Time course of the oxidation of cyclohexene by H202 with Mo12 [18]. Mo12: 0.026 mmol, Cyclohexene: 0.017 mol, Reaction temp.: 60°C, 7 wt% H202/TBP (0.00SS mol H202).

(21

c

c c (21

0

m

a,

K

O 0

50

100

Time / min

150

587

The dialdehyde, 4, was considered to form during the GC analysis from hydroxy hydroperoxide, X _ [12,181. This was confirmed by using IR and iodometry as follows. Cyclopentene oxide (2)was chosen as a starting reactant to simplify the reaction system and oxidized by H202. After the composition became constant, a portion of the solution was taken from the reactor and analyzed by IR and GC. For reference, IR of TBP solution of glutaric aldehyde was measured, of which the concentration was controlled to be the same as in the reaction system. The absorption band ofv(C=O) of the aldehyde was observed at 1724 cm-1 for the reference sample, while it did not exist in the reaction system. The amount of active oxygen measured by iodometry was consistent with the amount of aldehyde produced. However, both solution showed the aldehyde by GC analysis. These results indicate that phydroxy hydroperoxide with active oxygen, X_, existed in the reaction system as a precursor of aldehyde, and then the precursor was transformed into aldehyde during heating in GC. The reactivity of the precursor (P-hydroxy hydroperoxide, X) was thought to induce the differences in the time course between the systems of cyclopentene and cyclohexene after the consumption of H 2 0 2 (Figs. 3 and 4). So, the precursors, X_, were prepared by the oxidation of epoxide, 2, and their reactions with olefins, 1,were investigated. Initial reaction rates obtained are shown i n Table 1. The reaction rate of cyclohexene with its hydroxy hydroperoxide was 7 times faster than that for cyclopentene, which suggests that the differences between the two systems after the consumption of H 2 0 2 were primarily due to the difference in the reactivity of the precursors, X. Table 1. Initial reaction rates of phydroxy hydroperoxides with olefins. -d&ldt

(mol dm” h-’)

X

aE

1.4

10-2

10-2

Fig. 5. Reaction schemes for the oxidation of (a) cyclohexene and (b) cyclopentene by H 2 0 2 with HPA.

3.3. Synergistic effect due t o the mixed coordination of M o and W Furukawa et al. have reported :I synergistic effect of the mixed coordination for the oxidation of cyclopentene by H 2 0 2 [ 121. They compared only the yield of glutaric aldehyde obtained after 3 ti at 3 ° C . However, since the yield shows an induction period and moreover tends to be saturated with time (Figs. 3 and 4), a more quantitative comparison is desirable. Therefore, time courses were examined at 30°C i n the present study, in order to minimize the effect of the temperature rise caused by the exothermic reaction. Figure 6 shows the yield of glutaric aldehyde at 1. 2 , and 3 h for the mixed-coordinated HPA. The yield in Fig. 6 is defined by eq. (2).

588

Yield of glutaric aldehyde (GA) (mol%) =

GA produced (mol) H 2 0 2 fed (mol)

x 100

The differences were more remarkable among the HPA for the initial activities. Mo6W6 showed the highest activity; the yield reached 70 % at 2 h. On the other hand, when a mechanical mixture of Mo12 and W12 was used as catalyst, the synergistic effect was not observed. The yield was about the average of Mo12 and W12 as shown in Fig. 6, and in 3lPNMR, only two peaks of Mo12 and W12 were observed after the reaction for 3h. It means that the mixing of the addenda atoms did not occur during the reaction. Therefore, it can be coqGluded that the synergistic effect appeared only by the mixed coordination in the same anion unit. Burdett et al. have made a quantum-chemical calculation of all possible structures of P M 0 1 2 - ~ W ~ 0 4 0 3using EHMO [ 171. They suggested that the synergistic effect was brought about because the stability of highly reduced states increased upon asymmetric substitution from either W-0-W or Mo-0-Mo to Mo-0-W (namely it becomes a stronger oxidant). However, as indicated by 3lPNMR, there is a possibility that those mixed-coordinated HPA tends to produce peroxo species more easily. In case of the oxidation of cyclohexene by Mo6W6, it was difficult to follow the time course of the reaction accurately at 30°C because the temperature increased significantly due to the exothermic reaction. So, its reactivity was compared to that of Mo12 at 20°C. Figure 7 shows the first order plots of the initial states for the two catalysts. Synergistic effect was also observed for cyclohexene as in the cyclopentene system. The reaction rates of cyclohexene with Mo12 and Mo6W6 were two times higher than those of cyclopentene at 2OoC, respectively. 75

1

. .-

0

3

6

9

x in Mol2-xWx

1; ?

Fig. 6, Synergistic effect by Mo, w mixedcoordinated HPA. A mechanical mixture of Mo12 and W12.

0

60

120

180

240

Time / min Fig. 7. First order plot for the oxidation of olefins by H,O, at 20°C. (a) C5, Mo12; (b) C6, Mo12; (c) C5, Mo6W6; (d) C6, Mo6W6 (C5: cyclopentene, C6: cyclohexene).

3.4. Selectivity effected by the ratio of W to Mo The selectivities of the oxidation of cyclopentene with mixed-coordinated catalysts, which are defined by eq. (3), are shown in Fig. 8. Each selectivity was obtained before the expiration of H202, because the selectivities were almost constant in this period. As shown in Fig. 8, the production of the diol, which was formed by hydration of oxide, increased with the W content in the Keggin anion. This fact indicates that the acidity of HPA is reflected in the increase of the selectivity to diol, since the acidity increases with the W content.

5 89

Selectivity of glutaric aldehyde (mol%)

=

GA (mol) (GA + Diol + Oxide) (mol)

x 100

(3)

3.5. Effect of water The content of water had significant influences on the rate and selectivity. In the cyclohexene system, the contents of water could be changed by the addition of CHC13 or MgS04. The relationship between the reaction rate and .Lelectivity, and the concentration of water was investigated and the results are plotted in Fig. 9 [IS]. As the water concentration decreased, the rate of aldehyde formation was significantly improved, suggesting that the coordination of olefin to anion was suppressed by water. The decrease in the selectivity to aldehyde in the presence of water may be caused by the hydration of oxide to diol.

0.4

.....

I MolW11 y

..... ..... ..... \ ~.

u

~

I

w12 I

I

I

I

1

. -I

20 4 0 60 80 100 Selectivity / o/o Fig. 8. Selectivity for the oxidation of cyclopentene by H20, with Mo, W mixed-coordinated HPA.

0.8

2

.

0

Concentration of water I rn~l-drn-~ Fig. 9. Effect of water on (a) the selectivity and (b) the rate. 0 :CHCI,; 0:MgSO, (see text).

3.6. Effect of central atoms Redox potential of HPA is known to be controlled by the valence of the central atom. The catalytic activities of Mo12 and W12 catalysts with different kinds of central atoms for the oxidation of cyclohexene are plotted in Fig. 10 [18]. The catalytic activity approximately increased with the increase in the valence of the central atom; B3+ < Si4+ < C4+ 5 Ps+. These trends, including the result for polyatom; Mo > W, indicate that the catalytic activity increases with the increase in the redox potential of the polyanion. 3.7. Kinetics Dependency of the reaction rate on the concentrations of cyclopentene, catalyst, and H 2 0 2 were examined. Since Mo12 did not show induction period and its rate was more controllable, Mo12 was used as the catalyst. The reaction rate was first order to the initial concentrations of cyclopentene and catalyst, respectively, and zeroth order to H202. The rate dependencies for Thus, the rates are expressed for both cyclohexene were the same as those for cyclopentene. . . reactions by eq. (4). rate = k [olefin] [catalyst] [H2021° (4)

590

1

1

1

5 Valency of central atom

3

4

I

Fig. 10. Dependency of the activity for the oxidation of cyclohexene with hydrogen peroxide on the central atom of heteropoly anion. Reaction conditions are the same as in Fig. 4.

6

4. ACKNOWLEDGMENT This study was supported in part by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.

REFERENCES I . M. Misono, i n L. Guczi, F. Solymosi, and P. Tetenyi (eds.), Proc. 10th Int. Congr. on Catalysis, Akademiai, Budapest, 1992, p. 69. 2. M. Misono and N. Nojiri, Appl. Catal., 64 (1990) 1. 3. T. Okuhara, T. Nishimura, H. Watanabe, and M. Misono, J. Mol. Catal., 74 (1992) 247. 4. B. Notari, Stud. Surf. Sci. Catal., 37 (1988) 413. 5. T. Tatsumi, M. Nakamura, S. Negishi, and H. Tominaga, J. C. S. Chem. Commun., 1990, 476. 6. a) C. Venturello, E. Alneri, M. Ricci, J. Org. Chem., 98 (1983) 3831; b) C. Venturello, R. D. 'Aloisio, J . Org. Chem., 53 (1988) 1553. 7. a) Y. Matoba, H. lnoue, J. Akagi, T. Okabayashi, Y. Ishii, M. Ogawa, Synth. Commun., 14 (1984) 865; b) Y . Ishii. T. Yoshida, K. Yamawaki, M. Ogawa, J. Org. Chem., 53 (1988) 5549. 8. M. A. Schwegler, M.Floor, H. van Bekkum, Tetrahedron Lett., 29 (1988) 823. 9. N. Mizuno, D. K. Lyon, and R. G. Finke, J. Catal., 128 (1991) 84. 10. a) C. L. Hill and R. B. Brown Jr., J . Am. Chem. Soc., 108 (1986) 536; b) M. Faraj and C. L. Hill, J. Chem. Soc., Chem. Commun., 1987, 1487. 11. J. E. Lyons, P. E. Ellis, and V. A. Durante, Stud. Surf. Sci. Catal., 68 (1991) 99. 12. H. Furukawa, T. Nakamura, H. Inagaki, E. Nishikawa, C. Imai, and M. Misono, Chem. Lett., 1988, 877. 13. The Chemical Society of Japan (ed.). "Shin Jikken Kagaku Koza," Vol. 8, Maruzen, Tokyo, 1977, p. 1415. 14. T. Kudo, Nature, 312 (1984) 537. IS. M. A. Schwegler, J . A . Peters, and H. v a n Bekkum, J. Mol. Catal., 63 (1990) 343. 16. T. Ishihara, Master's thesis, the U n i v . o f Tokyo, 1989. 17. J. K. Burdett and C. K . Nguyen, J. Am. Chem. Soc., 112 (1990) 5366. 18. N. Mizuno, S. Yokota, I. Miyazaki, and M. Misono, Nippon Kagaku Kaishi, 1991, 1066.

59 I

E. BORDES (Universite de Technologie de Compiegne, Compikgne Cedex, France): About the bad performance you obtained with mechanical mixtures (Fig. 6 ) , my comment is that, for this kind of "activation", synergetic effects are found where solidsolid contracts are made. When you put these solids into solution, these contacts should exist no more, and so no synergetic effect would appear.

N. MIZUNOY (The University of Tokyo, Tokyo, Japan): Thank you for your comment. The remarkable synergetic effect in the present homogeneous system caused by the coexistence of Mo and W in the same anion can not be derived by the solid-solid contacts.

H. MIMOUN (Finnenich S A , Geneve, Switzerland): Epoxides are known to be consecutively cleaved by Mo peroxo species. How can you be sure that the HPA frame remains during the reaction and is not cleaved to single Mo peroxo species. N. MIZUNO: New peaks appeared in 3 l P - N M R of the H 2 0 m B P solution (Fig. l b ) are thought to be the active species for this reaction. The structure of the species has not been clearly assigned yet, even though we speculated as peroxide of heteropoly anion. Its partially decomposed forms such as Venturello type (1) or other Mo peroxo species can be considered as possible ones, as well. Further studies using 3'P-NMR are in progress.

7 Present address: Hokkaido University, Sapporo, Japan.

1. C. Venturello, R. D'Aloisio, J. C. J. Bart, M. Ricci; J. Mol. Catal., 32 (198.5) 107.

This Page Intentionally Left Blank

V. CortCs Corberin and S. Vic Bellon (Editors), New Deveioptnents in Selective Oxidation I / 0 1994 Elsevier Science B.V. All righls reserved.

593

Highly selective epoxidation of olefins on mono-transition-metalsubstituted Keggin-type heteropolytungstates by molecular oxygen in the presence of aldehyde N. Mizuno,* T. Hirose, and M. Iwamoto Catalysis Research Center, Hokkaido University, Sapporo 060, J a p a n Mono-transition-metal-substituted Keggin-type heteropolytungstates (PW11-M (M = Co, Cu, Fe, Ni, Mn)) have been found to catalyze the epoxidation of cyclohexene by molecular oxygen in the presence of aldehyde a t 303 K. The catalytic activities were i n t h e order of PW11-Co >> -Mn 2 -Fe > -Cu 2 -Ni. The present oxidation system was the most effective for the epoxidation of cyclohexene in acetonitrile among 0 2 + isobutyraldehyde, iodosylbenzene, hydrogen peroxide, and 0 2 alone. Isobutyraldehyde and pivalaldehyde having secondary or tertiary carbon adjacent to the carbonyl carbon were effective additives, while aldehydes such as butyraldehyde and acetaldehyde were less effective. PWii-Co also catalyzed the selective epoxidation of 1-decene and styrene. 1. INTRODUCTION

The strong acid or oxidizing properties of heteropolyanions a n d sometimes t h e i r unique basicity induce a lot of s t u d i e s on t h e heterogeneous and homogeneous catalysis [ 11. An additional attractive a n d important aspect of the heteropolyanions is t h e inherent stability toward the oxygen donors or molecular oxygen itself [2-51. Therefore, for example, heteropoly compounds are useful catalysts for the liquid-phase oxidations of alcohols [61, ally1 alcohols 171, olefins [8,91, alkynes [lOl, 0-unsaturated acids [111, uic-diols [12], phenol [13], and amines [141 with hydrogen peroxide, epoxidation of olefins and the oxygenation of paraffins with iodosylbenzene or t-butyl hydroperoxide [2-5,15], a n d allylic oxygenation of cyclohexene by molecular oxygen [16]. However, there appears no report on the epoxidation of olefins with molecular oxygen on heteropoly compounds. The epoxidation of olefins has become important both in industrial process and organic synthesis because epoxide is one of the most useful synthetic intermediates. Many c a t a l y s t s s u c h as r u t h e n i u m , molybdenum, and titanium complexes have been reported to be active for the reaction with peracids or peroxides since t h e pioneering work of H a w k i n s on v a n a d i u m pentaoxide [17]. Although t h e catalytic epoxidations with molecular oxygen under mild conditions are rewarding goals, only a little is known of the reaction [lS,191. In this study we have

594

tried to use heteropolyanion a s a oxidatively r e s i s t a n t ligand a n d wish to report the new catalytic system for the h i g h yield epoxidation of olefins on mono-transitionm e t a l - s u b s t i t u t e d Keggintype heteropolytungstates shown in Figure 1.

2. EXPERIMENTAL 2.1. Catalysts and reagents The tetra-n-butylammonium s a l t s of the transition-metalsubstituted Keggin-type heteropolytungstate complexes, {PWll(M"+)039)(7-n)(M = C O ~ + , Figure 1. Polyhedral representation Cu2+, Fe3+,Niz+, Mn2+; denoted of mono-substituted Keggin-type by PW11-M) were prepared by heteropolytungstate. The internal the slight modification of the black t e t r a h e d r o n represents the method reported i n ref. [20]. PO43- core and the white octahedra T h e f o r m a t i o n of K e g g i n M r e p r e s e n t WO6 fragments. structure a n d the composition represents Co, Cu, Fe, Ni, or Mn ion. were confirmed by IR and/or 31P NMR and the elemental analysis, respectively. Bis[1,3-bis(p-methoxyohenyl)-1,3-propanedionato] nickel and iron were prepared according to t h e ref. [19] and the formation was confirmed by the elemental analysis and IR spectra. Co(0Ac)z and CoCl2 were commercially obtained. Cobalt(II1) acetate (denoted by Co3O(OAc)6) was prepared according to the ref. [21]. All organic substrates were purchased in t h e i r highest commercial purity and purified before use. 2.2. Reaction

The catalytic oxidation were carried out according to ref. 1161. A typical reaction was performed a s follows: The catalyst (0.25 pmol) was introduced into a sealable glass vial (40 cm3) containing a magnetic stir bar, 3 cm3 of t h e solvent, 250 pmol of the substrate, and 1000 pmol of aldehyde and t h e vial was sealed. Then the glass vial was attached to a vacuum line, cooled to 77 K, and degassed by three freeze-pump-thaw cycles. The vial was allowed to warm to ca. 293 K and 1 atm of 0 2 gas was introduced to the system. The reaction vessel was t h e n placed at 303+1 K and vigorously stirred. The reaction vessel was removed from the bath every 10 - 20 min to be refilled the tube with 1 a t m 0 2 . The reaction solution was periodically sampled by syringe and analyzed by 1H NMR and gas chromatography on Unisole F-200 and FFAP columns.

595

Carbon balance for each reaction was more than 95% 3. RESULTS AND DISCUSSION 3.1. Epoxidation of cyclohexene The time course of cyclohexene oxidation by molecular oxygen on PW11-Co i n the presence of isobutyraldehyde is shown in Figure 2. The major product w a s cyclohexene oxide a n d s m a l l a m o u n t s of 2 -cyclohexen- 1-01 a n d 2-cyclohexen- 1-one. Isobutyraldehyde was oxidized to isobutyric acid. The a m o u n t s of cyclohexene oxide, 2-cyclohexen-l-one, and 2-cyclohexen-1-01 after 1 h were 82 ymol, 13 pmol, and 7 pmol, respectively, and the ratio showed little change with time. F u r t h e r addition of isobutyraldehyde a n d cyclohexene gave identical catalytic activities and no structural change i n PW11-Co was observed by IR. The results prove t h a t PW11-Co was stable under t h e conditions employed.

1000

- 500

E1

-

100

0

0

20

40

60

Time / min Figure 2. The time course of cyclohexene oxidation by molecular oxygen on PW11-Co in the presence of isobutyraldehyde i n acetonitrile at 303 K. 0 ,cyclohexene; 0 , cyclohexene oxide; m , 2-cyclohexen-1-one + 2-cyclohexen-1-01; A, isobutyraldehyde; A , isobutyric acid. PWI1-Co, 0.25 ymol; cyclohexene, 250 ymol; isobutyraldehyde, 1000 pmol; Po,, 1 atm; acetonitrile, 3 cm3

596

Table 1 Oxidation of cyclohexene on mono-transition-metal-substituted heteropolytungstate catalysts a t 303 K Product 1 pmol Entry

Catalyst COOH

1

PW,,-co

82 (338

20

355

2

PW,,-Mn

28 (11)

9

119

3

PW,,-Fe

25 (10)

8

104

4

PW,,-cu

11 (4)

5

49

5

PW,,-Ni

10 (4)

1

42

6

pw12

12 (5)

6

59

7

none

9 (4)

4

37

Catalyst, 0.25 pmol; cyclohexene, 250 pmol; isobutyraldehyde, 1000 pmol; solvent, acetonitrile (3 cm3); reaction time, 1 h. a) Numbers in parentheses are the % conversions to cyclohexene oxide.

The catalytic oxygenations of cyclohexene in the presence of various catalysts in acetonitrile a r e summarized in Table 1. Each reaction system was homogeneous. A similar product distribution to t h a t on PW11-Co yvas observed for t h e other PW11-M (M = Cu, Fe, Ni, Mn). Without any PW11-M catalysts the conversion was less t h a n 4%. The conversion level on [(n-C4H&N]3PW12040 was almost the same as t h a t of t h e blank experiment, suggesting that t h e transition m e t a l s introduced a r e active centers. PW11-Co was t h e most active for the reaction among the mono-transition-metal-substituted polyanions a n d the order of the activities were PW11-Co >> -Mn 2 -Fe 2 -Cu > -Ni. I n addition, PW11-Co was t h e most active among Co-containing catalysts a n d t h e order of t h e catalytic activities was PW11-Co > Co3O(OAc)6 > Co(0Ac)s > CoCl2 with the relative ratio of 1.0 : 0.76 : 0.68 : 0.62, respectively, suggesting t h a t the presence of PW1 loss7- lacunary heteropolytungstate enhances t h e activity. The enhancement of t h e catalytic activity of Ir+ upon the support on polyanion was also observed 1161.

591

Table 2 Oxidation of cyclohexene by molecular oxygen in the presence of various aldehvdes a t 303 K ~

Products f clmol Entry

Aldehyde 0

1

pCHO

0

0"; 0'

Acid

45

12

229

2

ACHO

45

16

220

3

&HO

82

20

355

30

7

135

22

6

102

103

18

268

4

5

6

7

-CHO

A C H O +CHO

O C H O

No reaction

PW,,-Co, 0.25 pmol; cyclohexene, 250 pmol; aldehyde, 1000 pmol; solvent, acetonitrile, 3 cm3 ; reaction time, 1 h. 3.2. Effects of oxidant, aldehyde, and solvent

Hill and Brown reported that the PW11-Co and -Mn were active among various transition metal-based catalysts for the epoxidation of olefins with iodosylbenzene i n acetonitrile [3]. We have also confirmed i n separate experiments that t h e yields of cyclohexene oxide by using iodosylbenzene and hydrogen peroxide a s oxidants (cyclohexene (250 pmol) + N2 (1 a t m ) + oxidant (1000 pmol) + PW11-Co (0.25 pmol) i n acetonitrile) were 8% and 0%, respectively. No cyclohexene oxide was formed in the 0 2 alone system. However, it should be noted in Table 1 t h a t t h e 0 2 + aldehyde system gave the 33% yield of cyclohexene oxide. The result shows the effectiveness of the present oxidation system. The catalytic activity was dependent on the kinds of aldehydes as shown in Table 2. The order of t h e effectiveness was pivalaldehyde > isobutyraldehyde >> butyraldehyde = propionaldehyde > valeraldehyde

598

Table 3 Epoxidation of olefins on PW,,-Co, Ni(dmp),, and Fe(dmp), at 303 K TONa/ h-* Olefin

0

PW,,-Co

Ni(dmp1,

328 (8d

204 (65)

116 (> 95)

72 (68)

48 (64)

8 (37)

Fe(dmp1, 76 ( 8 4

4 (44)

Catalyst, 0.25 pmol; olefin, 250 pmol; isobutyraldehyde, 1000 pmol; solvent, acetonitrile, 3 3 cm ; reaction time, 1 h. a) Turnover numbers, mol epoxide formed / mol catalyst used. b) Numbers in parentheses a r e the selectivities to the epoxides. c) Solvent, 1,2-dichloroethane. > isovaleraldehyde > benzaldehyde. It follows t h a t the aldehydes having secondary or tertiary carbon next to t h e carbonyl carbon such as isobutyraldehyde a n d pivalaldehyde a r e effective additives for t h e epoxidation. Further studies on the effect of the acid formed should be needed. The catalytic activity of PW11-Co was also solvent dependent. I n all solvents, t h e primary product was cyclohexene oxide. The activity decreased as follows: Dichloromethane > acetonitrile > N , N dimethylformamide > dimethyl sulfoxide (PW11-Co, 0.25 p m o l ; isobutyraldehyde, 1000 pmol; 02, 1 atm; solvent, 3 ems), with the relative epoxidation activities of 10 : 7 : 1 : 0, respectively. I t is noted t h a t the yield of cyclohexene oxide reached 75% a f t e r 1 h by u s i n g dichloromethane as a solvent.

3.3. Epoxidation of 1-decene and styrene The results of the oxidation of cyclohexene, 1-decene and styrene on P W i i - C o , N i ( d m p ) ~ ,a n d Fe(dmp)3 are shown in Table 3. By using PW11-Co, not only cyclohexene but also 1-decene a n d styrene were converted to t h e corresponding epoxides: 1-Decene was selectively oxidized to 1,2-epoxy-decane and the yield reached 61% after 69 h. I n the oxidation of styrene, styrene oxide and benzaldehyde were obtained i n 64% and 36% selectivity, respectively. The selectivities t o cyclohexene oxide, 1,2-epoxy-decane, and styrene oxide were comparable to or higher

5 99

than those of Ni(dmp)a and Fe(dmp)3, which have recently been reported to be very active for the epoxidation in the presence of aldehydes [19]. I n addition, it should be noted here t h a t the catalytic activities of PW11-Co for the epoxidation of cyclohexene, 1-decene, a n d styrene were greater than those of Ni(dmp)2 and Fe(dmp13. 4. CONCLUSION

To summarize, the above results demonstrate: (1)The first example of aerobically induced catalytic epoxidation of olefins by molecular oxygen on a mono-transition-metal-substituted heteropolytungstate species; (2) that the preferred catalyst is t h e cobalt complex, PWliCoO3g5-; (3) t h a t the presence of the oxidation-resistant basic oxide ligand, Pw1103g7-, enhances the catalytic rate; and (4) t h a t the conversion and selectivity to t h e epoxide in t h e preferred PWllCoO3$- + 0 2 + aldehyde system were greater than a n d comparable to or higher than those of very active Ni(dmp)e and Fe(dmp)3 for the epoxidation, respectively. 5. ACKNOWLEDGMENT

This work was supported in p a r t by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.

REFERENCES 1. Reviews on catalysis by polyoxometalates, for example, see: (a)M. T.

2. 3.

4. 5. 6.

7. 8.

Pope, Heteropoly and Isopoly Oxometalates; Springer-Verlag, Berlin, 1983. (b)M. Misono, Catal. Rev. Sci. Eng. 29 (1987) 269; 30 (1988) 339. (c)M. T. Pope and A. Muller, Angew. Chem. Int. Ed. Engel., 30 (1991) 34. (d)Y. Ono, Perspectives i n Catalysis, J. M. Thomas and K. I. Zamaraev (eds.), Blachwell Sci. Publ., London, 1992, pp 431. (e)N. Mizuno and M. Misono, J. Mol. Catal., in press. Chem. Commun., (1987) 1487. M. Faraj and C. L. Hill, J. Chem. SOC., C. L. Hill and R. B. Brown, J. Am. Chem. SOC.,108 (1986) 536. C. L. Hill (ed.), Activation and Functionalization of Alkanes, John Wiley & Sons: New York, 1989. D. Mansuy, J-F. Bartoli, P. Battioni, D. K. Lyon, and R. G. Finke, J. Am. Chem. SOC.,113 (1991) 7222. K. Yamawaki, T. Yoshida, H. Nishihara, Y. Ishii, a n d M. Ogawa, Synth. Commun., 16 (1986) 53; M. Daumas, Y. Vo-Quang, and L. VoQuang, Synthesis, (1989) 64. Y. Matoba, H. Inoue, J. Akagi, T. Okabayashi, Y. Ishii, and M. Ogawa, Synth. Commun., 14 (1984) 865. C. Venturello, E . Alneri, and M. Ricci, J. Org. Chem., 48 (1983) 3831; C. Venturello, R. D'Aloiso, J. C. J. Bart, and M. Ricci, J. Mol. Catal., 32 (1985) 107; C. Venturello and R. D'Aloisi, J. Org. Chem., 53 (1988) 1553.

600

9. Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida, and M. Ogawa, J. Org. Chem., 53 (1988) 3587; M. Schwegler, M. Floor, and H. van Bekkum, Tetrahedron Lett., (1988) 29. 10. F. P. Balistreri, S. Failla, E. Spina, and G. A. Tomaselli, J. Org. Chem., 54 (1989) 947. 11. T. Oguchi, Y. Sakata, N. Takeuchi, K. Kaneda, Y. Ishii, and M. Ogawa, Chem. Lett., (1989) 2053. 12. Y. Sakata and Y. Ishii, J. Org. Chem., 56 (1991) 6233. 13. M. Shimizu, H. Orita, T. Hayakawa, Y. Watanabe, and K. Takehira, Bull. Chem. SOC.Jpn., 64 (1991) 2583. 14. S. Sakae, Y. Sakata, Y. Nishiyama, and Y. Ishii, Chem. Lett., (1992) 289. 15. R. Neumann and C. A-Gnim, J. Chem. SOC., Chem. Commun., (1989) 1324. 16. N. Mizuno, D. K. Lyon, and R. G. Finke, J . Catal., 128 (1991) 84. 17. R. A. Sheldon and J. K. Kochi (eds.), Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981; J . P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke (eds.), Principles and Applications of Organotransition Metal Chemistry, University Science Books, Mill Valley, 1987; E. G. E. Hawkins, J . Chem. SOC., (1950) 2169; T. Katsuki and K. B. Sharpless, J. Am. Chem. SOC., 102 (1980) 5974. 18. J. P.Collman, M. Kubota, and J. W. Hosking, J. Am. Chem. SOC.89 (1967) 4809; J . E. Lyons, Tetrahedron Lett., (1974) 2737; I. Tabushi, and A. Yazaki, J. Am. Chem. SOC.,103 (1981) 7371; S. Itoh, K. Inoue, and M. Matsumoto, J. Am. Chem. SOC., 104 (1982) 6450; Y. Matsuda, H. Koshima, K. Nakamura, and Y. Murakami, Chem. Lett., (1988) 625; J.-C. Marchon and R. Ramasseul, Synthesis, (1989) 389. 19. T. Yamada, T. Takai, 0. Rhode, and T. Mukaiyama, Bull. Chem. SOC. Jpn. , 64 (1991) 2109; T. Takai, E. H a t a , T. Yamada, and T. Mukaiyama, Bull. Chem. SOC.Jpn., 64 (1991) 2513. 20. D. K. Lyon, W. K. Miller, T. Novet, P. J. Domaille, E. Evitt, D. C. Johnson, and R. G. Finke, J. Am. Chem. SOC.113 (1991) 7209. 21. J. J. Zioltkowski, F. Pruchnik, and T. Szymanska-Buzar, Inorg. Chim. Acta 7 (1973) 473.

60 1

R. Neumann (Hebrew Univ., Jerusalem, Israel): I n your mechanistic suggestion you claim that t h e formation of peracid is not catalyzed by PW11-Co whereas t h e olefin epoxidation is catalyzed by PW11-Co. Should not the reverse situation be true? N. Mizuno (Hokkaido Univ., Sapporo, Japan): We consider t h a t both steps a r e catalyzed by PW11-Co. F u r t h e r mechanistic study is in progress. R. A. Sheldon (Delft Univ. of Tech., Delft, The Netherlands): In your mechanism you consider a n oxocobalt (Con+=O) species to be the active oxidant i n the epoxidation step. Did you also consider the possibility that a peroxo tungsten species (Ws+-OOR or W6.Z: ) could be the active oxidant by analogy with other known systems? N. Mizuno (Hokkaido Univ., Sapporo, Japan): Yes, we did. T. Mlodnicka (I. of Catalysis, Krakow, Poland): (1)Have you checked the formation of carbon dioxide which generally results from t h e reaction of cobalt complexes with peroxyacids? (2) Can you tell more about t h e character of the cobalt-oxo species which you propose in your reaction scheme?

N. Mizuno (Hokkaido Univ., Sapporo, Japan): (1)Yes, we observed the formation of carbon dioxide. (2) No, we can't tell at present. We also consi$r t h e possibility that a peroxo tungsten species (Ws+-OOR or W6+:6 ) could be the active species. Further detailed mechanistic study should be necessary.

J.-M. Bregeault (Univ. of P. e t M. Curie, Paris, France): A lot of compounds, mainly transition metal complexes, a r e known a s active catalysts for catalytic epoxidation of olefins by dioxygen w i t h a coreducer (Mnz+; Fe3+; NiZ+; Pr3+; Mo6+ and more recently Ru, Rh, Pd, Ti, Sm, ... complexes). The differences you observed between "PW11-Co" a n d cobalt complexes do not seem significant if the oxidation of cyclohexene is considered a s a test reaction. Do you find the same order of reactivity with terminal olefins? N. Mizuno (Hokkaido Univ., Sapporo, Japan): We did not carry out the comparison experiments by using terminal olefins a s reactants. We would like to do such experiments.

This Page Intentionally Left Blank

V . CoriCs Corbcrrin and S. Vic Bcll6n (Editors), New Developrrienrs in Selecrive Onidarion I / 0 1994 Elsevicr Science B.V. All rights reserved.

603

SELECTIVE OXIDATION OF CYCLOHEXENE WITH MOLECULAR OXYGEN CATALYZED BY TRANSITION METAL SUBSTITUTED POLYOXOMETALATES Dujie Qin, Guojia Wang and Yue Wu Department of Chemistry, Jilin University Changchun, 130023, China

Abstract The transition metal substituted heteropolyoxometalates [PMi103y(Z"+'L)] (M = Mo, W ; Z = Mn2+, Fe3+,Co2', Ni2+,Cu2+)have been found to be effective catalysts for oxidation of cyclohexene with molecular oxygen under mild reaction conditions. IR and XPS results show that the introduced transition metal ions seem to be the active centers for the activation of molecular oxygen, though Mo6+ or W6+ in polyoxometalates can also participate in the reaction.

1. INTRODUCTION The homogeneous catalytic oxidation of saturated and unsaturated hydrocarbons has been actively researched in the last 10 years. Numerous metalloporphyrin-basedsystems have been found to be effective catalysts for the homogeneous oxidation of hydrocabons by various oxygen atom donors. The investigation of these systems has furthered substantially our understanding of the mechanism of homogeneous oxidation and the study of new catalyst systems. A major obstacle in the development of metalloporphyrin-catalyzed oxidation reactions into practical processes is the instability of porphyrin catalysts. As early as 1973, Baker [l] noted that substituted monolacunary polyoxotungstates, such as Keggin anion [PWllO,, (Zn+H20)]'"~y), ligate the heterometal (Z"+) in a pseudo-porphyrin environment. In addition, this kind of "inorganic-porphyrin" should be highly oxidation resistant. Hill [2], Lyon [3] and their co-workers reported respectively the epoxidation of olefins with iodosylarenes catalyzed by monosubstituted Keggin [(n-C,H,),N],HPW, ,O,, or Dawson 012-[(n-C4H9)4~11nP2W,7061 polyoxotungstateanions and found that their catalytic activity was similar to that of metalloporphyrin in the mentioned reaction. Recently, Mizuno et al. [4] demonstrated the catalytic oxidation of cyclohexene with molecular oxygen by the catalyst precursors [(n-C,H,),NS1,Na,[(l,5-COD)Ir.P,Wl,Nb,O6,], etc. We report here that the selective oxidation of cyclohexene with molecular oxygen catalyzed by transition metal monosubstituted heteropolyoxometalates [(nC4N,),N]sPM,l(Z"CL)(M = Mo, W; Z = Mn2+, Fe3+,Co2+,Ni2+, Cu2+,L = unknown) at mild reaction conditions. We compare these results with those obtained from substituted heteropolyoxotungstatesand metalloporphyrins. It has been found that the main active center is the introduced transition metal although Mo6+or W6+in monosubstituted polyoxometalates could participate in the reaction.

2. EXPERIMENTAL 2.1. Preparation of catalysts ] .=5 H ~ 0 In a 250 ml beaker, about 10 g of aqueous-soluble Nq[PZ(H,O)- - ~ ~ ~ 0 ~ ~(Z Mn”, Fe3+,Co2+,Ni”, Cu2+),prepared by modification method [5], was dissolved in 50 ml of H,O respectively. Then a solution of [(n-C,H,),N]Br was added in molar ratio 1:6 and precipitate, was formed. The organic solvent the raw product, [(~-C,H,),N],PZ(B~)MO~~O~~ soluble products can be obtained by means of isolating by extraction with CH2C12 and evacuation of the solvent and drying [ 6 ] . 2.2. Catalytic measurements The reaction equipment was composed of a reaction bottle equipped with magnetic stirrer and a pipe supplying oxygen. The reaction temperature was controlled by cycling constant temperature water. In all experiments, 50 ml cyclohexene and 0.05 mmol catalyst was added into the reaction bottle containing 10 ml of 1,2-dichloroethane as organic solvent. 0, used as oxidant was bubbled at the rate 10 ml/min and the reaction time was recorded. The reaction products were identified by GUMS and quantitatively determined by GC. IR spectra were recorded with Nicolet 5DX FT-IR instrument. XPS measurement was carried out on VG ESCALAB MK2 photoelectron spectrometer. All binding energies were calibrated at the position of the C 1s. 3. RESULTS AND DISCUSSION

3.1. Catalytic oxidation of cyclohexene with molecular oxygen at 70°C. [(n-C,H,),Nl’ salts of [PM11039(Z’+L)](M = Mo, W; Z = Mn2+, Fe3+, Co”, Ni2+, Cu2+) were applied as catalysts in the oxidation of cyclohexene. Table 1 shows that the substituted polyoxomolybdates exhibit a remarkable activity in oxidation of cyclohexene. Table 1 Oxidation of cyclohexene by O2 catalyzed by various transition metal monosubstituted polyoxomolybdate complexes Yield (%) Catalyst

Epoxide

Allylic alcohol

Allylic ketone

PMnMo PFeMo PCuMo PNiMo PCoMo

4.0 4.0 2.7 4.7 4.5

31.9 28.3 30.1 25.5 22.3

29.6 28.1 25.4 24.4 22.5

Catalyst PZM, 0.05 mmol; Cyclohexene, 50 mmol; 1,2-dichloroethane, 10 ml; Flowing rate of O,, 10 ml/min; Reaction temperature, 70 “C.

605

Comparing these results with that of oxidation of cyclohexene by PhIO catalyzed by substituted polyoxotungstates [2,3],it can be seen that main products are different. In our experiments, main products are cyclohexene-2-01- 1 and cyclohexene-2;one-I. The product distribution shows that the reaction may proceed by a radical mechanism. The order of the catalytic activity of the monosubstituted polyoxomolybdates in the described reaction is as follows: PMnMo > PFeMo > PCuMo > PNiMo > PCoMo This order may be related to the change in the binding energy of substituting ions from ZSO, to PZMo (Table 2). Comparing of activities of oxidation catalyzed by PZMo, PZW, and ZTPP (Fig. 1), it Table 2 Comparison of binding energies of Z in ZSO, and PZMo ZSO, Z Mn co Ni

cu Zn

50

PZMo

z PSI2

z P112

P312

(ev)

(eV>

(eV>

ZP,,, (eV)

642.7 782.6 857.8 935.5 1023.1

654.5 798.8 875.8 955.4 1046.3

641.3 781.4 856.7 934.1 1022.2

653.1 797.4 874.7 954.1 1045.3

El PZMO PZW ZTPP

40

-

OO \

u

30

-I -4

20

>1

10

0

Figure 1. The product yields of cyclohexene oxidation with molecular oxygen by PZMo, PZW and ZTPP (Z = Mn2+, Fe3', Cu2+)catalysts. A: Epoxide; B: Allylic alcohol; C: Allylic ketone. Reaction conditions as in Table 1.

4 z P312) (ev)

1.4 1.2 1.1

1.4 0.9

606

can be seen that PZW gives results similar to those found for PZMo, but different from the results produced by ZTPP. In spite of the difference in the activity, the yield of epoxide is higher for the reaction catalyzed by the substituted polyoxometalates than that obtained in the reaction catalyzed by metalloporphyrins. This results indicates that Mo" or W6+ ion exert some effect on the reaction course.

3.2 IR spectra In order to investigate the reaction intermediate, the IR spectra of fresh PMnMo, PMnMo+02 and PMnMo after reaction were studied (Fig. 2).

$00

1000

700.0

4-J.0

Figure 2. The change of IR spectra for PMnMo system. a: PMnMo; b: PMnMo+O,; c: after reaction

1 10

1000

700.0

4 10.0

WAVENUMBERS (CM-') Figure 3. The IR spectra of PMnMo and PMnW after 0, was added. a:PMnMo 0,; b: PMnW 0,

+

+

Comparing the three IR spectra, we can see that new peaks appear at 949,591 and 5 19 cm I , and splitting of peak u(0-P) become obvious after O2 addition and these changes vanished after reaction. These IR changes demonstrate that oxygen interacts with PMnMo anion to form an activating intermediate which seem to be Mn peroxo-complex [ 7 ] . This means that the catalytic active center in PMnMo is substituted transition metal ion Mn2+ . Comparing the IR spectra of PMnMo with that of PMnW after addition of O2 (Fig. 3), it is clear that in the IR spectra of PMnW three new peaks also appear at 95 1, 594 and 5 19 cm'. The position of these peaks is very close to that produced by PMnMo after O2 exposure. This result shows that the catalytic active center should be Mn2+in both PMnMo and PMnW systems.

607

3.3. Reaction mechanism Comparing the kinetic curves of the investigated reactions catalyzed by PMnMo and PMnW (Fig. 4 and Fig. 5), it can be found that the kinetic curves are of "S"shape and the reaction activities for the two studied systems are close.

80

1

Time (h)

Time (h)

Figure 4. The effect of H202on kinetic curve for PMnMo system. -:PMnMo; X: PMnMo+H,O,.

Figure 5. The effect of H202on kinetic curve for PMnW system. *:PMnW; X: PMnW +H20,.

This result suggests the catalytic characters of active centers and the reaction mechanisms are similar for the mentioned reactions catalyzed by PMnMo and PMnW. The reaction needs an induction period. A small amount of H202added into the reaction mixture produced some changes in the first part of the kinetic curve. The induction time was also reduced. So it is suggested that the reaction seem to be a radical mechanism from this kinetic result and reaction products distribution. On the basis of above experimental facts and published results [%lo], the reaction mechanism of cyclohexene oxidation catalyzed by PZM (Z = Mn2+,Fe3+,Co2+,Ni2+, Cu"; M = Mo, W) could be expressed as follows:

( PZM,, ) +OO-

+

+

PZM,,

+

H+

(2)

608

Due to the presence of Mo or W, small amount of cyclohexene oxide could be obtained in the following pathway:

4. SUMMARY The transition metal substituted heteropoly compounds are effective catalysts for the cyclohexene selective oxidation with molecular oxygen under mild reaction conditions. From kinetic behavior and product distribution, we can assume that the reaction proceeds by a radical mechanism and the substituting transition metal ions in polyoxometaltates are the active centers for the activation of molecular oxygen, though Mo6+ or W6+ in polyoxometalates can also participate in the reaction. The IR and XPS studies carried out for polyoxometalates justify these conclusions. REFERENCES L.C.W. Baker, Plenary Lecture 15 Int. Conf. on Coord. Chem., Proceedings, Moscow. 1973. 2. C.L. Hill and R.B. Brown, J. Am. Chem. SOC.,108 (1986) 536. 3. D. Mansuy, J-F. Bartoli, P. Battioni, D.K. Lyon and R.G.Finke, J. Am. Chern. SOC., 113 (1991) 7222. 4. N. Mizuno, D.K. Lyon and R.G. Finke, J. Catal., 128 (1991) 84. 5. D. Qin, G . Wang, M. Li, Y. Wu, Wu Ji Hua Xie Xie Bao (Chinese J. Inorg. Chem.), 8 (1991) 124. 6. D.K. Lyon, W.K. Miller, T. Novet, P.I. Domaille, E. Evitt, D.C. Johnson and R.G. Finke., J. Am. Chern. SOC.,113 (1991) 7209. 7. A.E. Martell and D.T. Sawyer Edited "Oxygen Complexes and Oxygen Activation by Transition Metals" Plenum press, New York, 1988, p. 17. 8. T. Lyons, " Aspects of Homogeneous Catalysis " Vol. 3D, Reidel Publishing Company (1977), P1. 9. N. Emanuel, Kinet. Katal., 15 (1974) 891. 10. H. Arzournanian, J. Organometal. Chem., 82 (1974) 161. 1

V. CortCs Corberan and S . Vic Bellon (Editors), New Developments in Selective Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

609

NOVEL CATALYSTS FOR OLEFIN CLEAVAGE USING HYDROGEN PEROXIDE A. Johnstone, P.J. Middleton, W.R. Sanderson, M. Service, P.R. Harrison

Solvay lnterox Research and Development P.O. Box 51, Moorfield Road, Widnes, Cheshire, WA8 OFE, England

ABSTRACT

Transition metal oxides are well-known reagents for cleaving olefinic doublebonds. Due t o toxicity and expense, catalytic amounts of the metal species, along with a co-oxidant are favoured. Whilst hydrogen peroxide and ruthenium give good results for many olefins, poor results are obtained for, amongst others, terminal double bonds. This paper reports the use of a second metal t o extend the range of cleavage reactions using hydrogen peroxide as oxidant, with good efficiency of peroxide usage.

INTRODUCTION

The oxidative cleavage of olefins can be carried out via a number of routes; the products resulting depend on the starting alkene and the oxidant used. The reaction can be summarized as follows:

If the R-groups are all alkyl, then the resulting ketone is usually not oxidized further. In the case of one of the R-groups being hydrogen however, then further oxidation of the resulting aldehyde, can occur to the acid. On an industrial scale this is often carried out using probably the oldest method, ozonolysis [ I I. This is used for example in the cleavage of oleic acid, and in the production of glyoxylic acid. Disadvantages of ozonolysis are its being a capital intensive process, and the high electricity consumption. Usually it is carried out for only very large-scale production.

610

Traditional methods for smaller-scale processes have involved the use of stoichiometric oxidants such as osmium or ruthenium tetroxide, or potassium permanganate [21. These reagents, whilst being efficient have the obvious drawbacks of being toxic (particularly osmium and ruthenium) and expensive. Developments have therefore concentrated on using catalytic amounts of various metals. So, for example, the Milas Reagent OsO,/H,O, is used for hydroxylation [3], and cleavage t o aromatic aldehydes [41. Other examples are the use of osmium or ruthenium tetroxide with sodium metaperiodate t5,61. Sodium hypochlorite has also been used as oxidant by Sharpless [71 but was apparently less effective. More recently, work has been undertaken using peracetic acid or hydrogen peroxide as secondary oxidant, and also a number of other transition metals have been examined for their catalytic activity towards alkene cleavage. Such systems would have the advantage, especially in the case of hydrogen peroxide, of the byproduct of the co-oxidant being easy to handle. So, for example, Warwel et al. [81 describe the use of an Re,O, system with hydrogen peroxide, as does U.S. Patent 3,646,130, the latter giving only low yields for cyclododecene cleavage. Barak et al. [91 use ruthenium trichloride with hydrogen peroxide in the presence of a phase-transfer catalyst (PTC) such as didecyldimethylammonium bromide (DDAB), in the oxidation of alcohols. Extending the above mentioned Barak system t o olefin-cleavage is quite successful, but not in the case, however, of terminal double bonds in compounds such as I-octene. Another drawback, is the use of chlorinated solvents, such as ethylene dichloride, which would be undesirable on an industrial scale. Investigations showed [I01 that t-butanol functions well as a solvent in cleavage reactions. The present paper reports the use of a t w o metal catalyst system, t o extend the range of cleavage reactions. Molybdenum and ruthenium are used in conjunction with a phase-transfer catalyst and a carboxylic acid - in catalytic amounts - t o cleave olefin functions in a variety of positions.

EXPERIMENTAL

The experimental procedure used in all reactions is outlined below using 1octene as sample substrate. Reagents used are commercially available and were used without further purification. To tert-butylalcohol (50 ml) was added molybdenum trioxide (0.2 g), ruthenium (Ill)chloride hydrate (0.03 g), didecyldimethyl-ammonium bromide (0.4 g, 80% w / w solution in ethanol), I-octene (5.0 g) and heptanoic acid (0.1 g). This mixture was heated t o reflux and the hydrogen peroxide ( 3 5 % w / w solution) added dropwise over a period of 1 hour. The reaction was then allowed t o continue for a further 3 hours, after which time the mixture was cooled and analysis carried out. H.p.1.c. analysis was carried out on a Perkin-Elmer LC250 instrument, with detection at 250 nm. G.C. analysis was carried out using a Perkin Elmer 8000 series capillary chromatograph.

61 1

RESULTS AND DISCUSSION Investigations have shown that, whilst ruthenium can be a good catalyst for the cleavage of olefins with hydrogen peroxide as oxidant (for example, styrene) [10,111, it is relatively inactive towards terminal olefinic compounds such as 1octene. (This is perhaps an indication that the Ru (VIII) state is not reached in these catalytic systems as this can perform this reaction). However, ruthenium had been observed to be very effective at cleaving diols, and so a number of other metals were studied as co-catalysts converting the olefin t o the diol. This diol can then be cleaved using a ruthenium/hydrogen peroxide system. Table 1 shows the results obtained:

TABLE 1 Comparison of Metals in the Hydroxylation of 1-0ctene"

Metal Compounds

H2WO4 Re207 MOO, ~~~~~~~~

Conversion

Diol

Acid

(%I

94.5 99.2 95.4

Selectivity

Yield

Selectivity

Yield

50.2 21.3 49.1

47.4 21 .I 46.9

Trace

Trace

2.5 5.9

2.4 5.6

~~

aSolvent: Acetic Acid (50 ml); I-octene (5.0 g 89.3 mmol); Catalyst (0.2 g); H,02 (as 35%, 147 mmol) 7OoC/4 hrs

Previous work in house [lo], has shown that t-butanol is the best solvent for the ruthenium system. However, none of the above metals gives any reaction a t all in this solvent, with hydrogen peroxide as oxidant. Furthermore the use of ruthenium in acetic acid gave rise to unwanted side-reactions such as polymerization and the formation of esters. As a result, a mixed solvent system was tried which overcame both of these problems. Comparison of tungsten and molybdenum (Table 2) indicates that rnolybdenum/ruthenium is a more effective system than tungsten/ruthenium. Additionally, it was found that only catalytic amounts of acid are necessary for the reaction t o proceed smoothly. It has also been shown that acetic acid can, in fact, be replaced by the acid resulting from the cleavage of the olefin. This provides superior results, and with fewer overall components, gives easier separation.

612

TABLE 2 Molybdenum or Tungsten, with Ruthenium in a Mixed Solvent Systemb Metal Compounds

Solvent t-BUOH:ACOH

Conversion

Heptanoic Acid

(% w/w) MoO,/RuCI, H,WO,/RuCI, MoO,/RuCI, MoO,/RuCI, MoO,/RuCI,

1 :I 1:l 9: 1 50: 1 200: 1

Selectivitv

Yield

32.3 17.9 42.6 57.2 59.2

27.3 11.o 40.4 55.6 57.9

84.7 61.4 94.8 97.2 97.0

bMo0,/H2W0, (0.2 g); RuCI, (0.03 g); Total Solvent ( 5 0 ml); I-octene ( 5 . 0 g) H20, ( 3 5 % w/w, 309 mmols) Reaction run at 8OoC/4 hrs A further important factor in this system is the phase-transfer catalyst. As reported by Barak [91, didecyldimethylammonium bromide (DDAB) functions well in this rBle, and this is demonstrated in the following comparison with tetraethylammonium bromide (T.E.A.B), and Aliquat 336 (Tricaprylmethylammonium chloride). The results comparing these are shown below in Table 3.

TABLE 3 Comparison of the Phase-Transfer Catalysts in 1-octene Cleavage" Phase Transfer Catalyst (PTC)

Aldehyde Yield

Acid Yield

(%I

(%I

Bu0H:AcOH (w/w)

Aliquat 3 3 6

25.9

41.8

200: 1

T.E.A.B

7.3

42.4

200: 1

D.D.A.B.

Trace

60.9

200: 1

D.D.A.B/T.E.A.B. ( 1 : l )

22.0

35.7

200: 1 ~

~~

"1-octene (5.0 9); MOO, (0.2 9); RuCI, (0.03 9); t-BuOH (50 g) Reaction Temperature: 8OoC/4 hrs; PTC (0.4 g) H,02 (as 3 5 % w/w, 309 mmols) The reason for the superiority of D.D.A.B. is unclear. Previous work on ruthenium [91 reported that the size of the quaternary ammonium cation was unimportant. In [ I 21 however, it is shown that increasing the lipophilicity of the cation, increases its capability in H,O, extraction into the organic phase. This latter effect is perhaps playing a rBle here. The above molybdenum/ruthenium system has been extended to other alkenes, and has shown itself t o be capable of cleaving carbon-carbon double

613

bonds occurring in a variety of positions. Conversions are usually over 90%, with the acid t o aldehyde ratio produced varying from substrate t o substrate. Table 4 shows examples of the effect on different substrates.

TABLE 4 Cleavage of Double Bonds; Effect of Substrated

(YO)

Substrate

Conversion (YO)

Oleic Acid

100

(Azelaic) 100 (Nonanoic) 43

Styrene

100

42

2-nonene

100

62

Stilbene

100

4-CI-Styrene

100

Acid Yield

(YO)

48.5

63.4

30

32.1

(Heptanoic) 42

Castor Oil ~

Aldehyde Yield

~

~

~

~~

~

~~

~

~

~

~

~

~

~

dSolvent: t-BuOH (50 ml); MOO, (0.2 g); RuCI, (0.03 g); D.D.A.B. (0.4 g) Expected Acid ( - 0.1 g); Temperature: 8OoC/4 hrs The side products were not identified, although polymerization products cannot be ruled out. Consideration of the peroxide consumption shows, that as four moles of hydrogen peroxide are theoretically required to convert one mole of alkene into the respective acids, approximately 50% of it is being consumed in side reactions - a typical H20,:substrate used is 7-8:l. Given that Ruthenium is wellknown for the active catalysis of the reaction:

-

and that in the octene and styrene systems the possibility also exists of oxidation of formic acid produced t o carbon dioxide, then these results are quite good.

CONCLUSIONS Using hydrogen peroxide as an oxidant, the molybdenumlruthenium catalyst system described above, has shown itself t o be effective in cleaving carbon-carbon double bonds. High conversions are obtained, and the two-metal system has a much wider range than ruthenium on its own. The use of the carboxylic acid resulting from the cleavage of the olefin, in catalytic quantities is also a useful feature. It reduces the number of components in the system, which is useful both for work-up and for any possible recycling of materials which may be required.

614

REFERENCES

1. 2. 3. 4.

5. 6. 7.

8.

9. 10. 11. 12.

U S Patent 2813113 to Emery Industries, 1957. J.L. Courtenay in "Organic Synthesis by Oxidation with Metal Compounds" edited by W.J. Mijs, C.R.H.I. de Jonge. N.A. Milas, S. Suismann: J.A.C.S., 59 (1937) 2345. U.S. Patents 2414385 and 2437648 to Research Corporation, New York. R. Pappo, D.S. Allen Jr., R.U. Lemieux, W.S. Johnson: J. Org. Chem., 21 ( 1956) 478. S. Sarel, Y. Yanuka; J. Org. Che., 24, 2018, (1956). P.H. J. Carlsen, T. Katsuki, V.S. Martin, K.B. Sharpless: J. Org. Chem., 4 6 (1981) 393. S. Warwel, M. Rusch gen Klaas, M . Sojka; Tagungsbericht 9 2 0 4 Proc. DGMK Conference, "Selective Oxidation in Petrochemistry", (M. Baerns and I. Weitkamp, eds.), Goslar 1992, p.161. G. Barak, J. Dakka, Y. Sasson: J. Org. Chem., 53, (1988) 3553. M . Service, Unpublished Work. G. Barak, J. Dakka, Y. Sasson: J . Chem. SOC., Chem. Commun., (1987) 1266. E.V. Dehmolow, S.S. Dehmlow, "Phase Transfer Catalysis", 2nd Edition, Verlag Chemie.

DISCUSSION CONTRIBUTION

H. MIMOUN (Firmenich SA, Geneva, Switzerland): Do you think that cleavage results from generation of RuO, species by reaction of RuCI, with peroxo Mo complexes or that epoxides are intermediately formed and cleaved by Ru? M. SERVICE (Solvay lnterox R&D, Widnes, UK): We believe that the molybdenum and ruthenium species act separately on the olefin substrate with initial hydroxylation by Mo-peroxy species followed by cleavage by, probably, Ru (VI). J-M. BREGEAULT (Univ. Pierre et Marie Curie, Paris, France): Can you give us any

information concerning the decomposition of hydrogen peroxide by your catalytic system in the reaction conditions? M. SERVICE (Solvay lnterox R&D, Widnes, UK): We have only looked at the optimisation of peroxide consumption in the cleavage of 1-octene and here there is still work t o do. Currently we operate with 6 moles H20,t o 1 mole substrate. No peroxide remains at the end of reaction.

V. CortCs Corberan and S. Vie Bellon (Editors), New Developments in Seleciive Oxidation II 0 I994 Elsevier Science B.V. All rights reserved.

615

Novel One-pot Synthesis of Indigo from Indole and Organic Hydroperoxide Yoshihisa Inouea, Yoshihiro Yamamotoa, Hiroharu Suzukib and Usaji Takakia "Department of Catalyst, Central Research Laboratow, Mitsui Toatsu Chemicals, Inc., 1190, Kasama-cho, Sakae-ku, Yokohama, Japan bDepartment of Chemical Engineering, Faculty of Engineering, Tokyo Institute of Technology, 2- 12-1, 0-okayama, Meguro-ku, Tokyo, Japan

1. SUMMARY

Indigo was prepared in an excellent yield by the reaction of indole with organic hydroperoxide catalyzed by molybdenum complex in a liquid medium under mild conditions. This is the first example to prepare indigo effectively by the selective oxidation of indole[ 11.

2. INTRODUCTlON

Indigo is a very important compound as a dyestuff, and a large number of synthesis routes have been studied. In industry, indigo is produced by a traditional method in which aniline and chloroacetic acid or aniline, HCN and formaldehyde are used as starting materials. In this process, it requires malti-step organic reactions and rather severe conditions. On the other hand, it is very attractive target for chemists to prepare indigo directly by selective oxidation and dimerization of indole which has a similar framework structure to that of indigo. Indole, which used to be obtained from the fraction of coal tar, is now produced

on an industrial scale by the reaction between aniline and ethylene glycol catalyzed by heterogeneous silver catalyst and is easily available as a raw material[2]. Some studies on the oxidation of indole have been reported, including photo oxidation[3], anodic oxidation[4], ozonization[5] and oxidations with manganese dioxide[6], persulphates[7], hydrogen peroxide[8] or peracids[9]. However, the object of these reports was to study the reactivity of indole and not to produce indigo. Even when indigo was detected, it was nothing but one

616

of side-products formed in very small amounts. Accordingly, they are not satisfactory processes for the preparation of indigo. On the other hand, there have been no reports up to now on the oxidation of indole utilizing an organic hydroperoxide as an oxidant. This paper describes novel one-pot process for the preparation of indigo by the reaction of indole with an organic hydroperoxide.

3. EXPERIMENTAL PROCEDURE The experimental procedure is explained below with a typical example. All reagents are commercially available and can be used without further purification. To a tert-butyl alcohol (15Og) solution consisting of indole (lO.Og, 85.4mmol). acetic acid (OSlg, 0.85mmol) and molybdenum hexacarbonyl (22.5mg, 0.085mmol), was added a 82wt.% solution of 1methyl- I-phenyl-ethylhydroperoxide(i.e., cumene hydroperoxide) in cumene (34.9g, 188.0mmol as cumene hydroperoxide).

This mixture was then allowed to

warm up to 86°C (refluxing), and stirred for 7 hours. A deep-blue solid was gradually precipitated from the solution. After the reaction, the reaction mixture was filtered. The resulting solid was washed with methanol and then dried under reduced pressure, whereby

9. log of a deep-blue solid was obtained. Elemental analysis, solid state 13C-NMR and IR spectra revealed that the solid is indigo. The yield of indigo based on charged indole was 81.3%.

4. RESULTS and DISCUSSION We have now found that the reaction of indole with an organic hydroperoxide takes place very smoothly in the presence of molybdenum catalyst and afford indigo in an excellent yield (Scheme 1).

Scheme 1

+ H

2

R-0-0-H

617

In this reaction one equivalent of indole requires 2 equivalents of the organic hydroperoxide to form one half equivalent of indigo; one equivalent of the organic hydroperoxide is consumed for the oxidation of the 3-position of indole and the other one for the oxidative coupling of two indole frameworks forming double bond between the each 2positions.

Consequently, the organic hydroperoxide is turned to 2 equivalents of the

corresponding alcohol and one equivalent of water is generated. A number of metallic complexes have been examined for the catalytic activity. Some of

the results are shown in Table 1.

When the reaction was carried out in the absence of

catalyst, indigo was obtained only in very poor yield (entry 1).

While the yield was

considerably enhanced (entries 2-8) in the presence of a molybdenum complex. A titanium complex also catalyzed the reaction, but the activity was quite low (entries 9-10). Metallic complexes of the other metals of the groups IVa, Va and VIa of the periodic table also showed low activity.

Thus the molybdenum complexes are considered to be the most

Table 1 Reactions i n the presence of Catalysts”) entry

Catalyst

1ndole:CHPb):Catal.

SolventC) Temp.

Time

Yield

“C

hr

74)

Toluene Toluene

80 80

10 5

4.3 49.7,

molar ratio

I 2

1:5:0 1 : 5 : 0.01

3

None [M@acac)2]2e) Mo(C0)6

1 : 3 : 0.001

Cumene

100

5

59.0

4

N~P-MO~)

1 : 3 : 0.01

5 6

[Mo(OAc)2@) [Mo(OOCPh)2]2 MO(C0)6 MoO?_(a~ac)2~)

80 80

5 5

53.4 37.6

80

5

49.6

100 100

5

1 : 2.2 :0.001

Cumene Cumene Cumene TAA~) TAA~)

5

70.8 66.5

1 : 3 : 0.01 1 : 3 : 0.01

Toluene Toluene

80

5

9.8

80

5

6.9

7

8

9 10

Ti(0-1Pr)4 TiO(acac)ze)

1 : 3 : 0.01 1 : 3 : 0.001

1 : 2.2 :0.001

a)lO.Og (85.4mmol) of indole. b)CHP=cumene hydroperoxide. c1300g (entries 1-6, 9-10) or

15Og (entries 7-8). d, based on indole. e)acac=,cetylacetonate. naphthenate. &OAc=acetate. h)TAA=tert-amyl alcohol.

f)Nap-Mo=rnol ybdenum

618

The oxidation state of molybdenum does not

favorable catalyst for the present reaction.

influence the catalytic activity to a large extent. It seems that any molybdenum complexes of lower valence are oxidized to higher valence in situ and the resulting high valent molybdenum complexes behave as the true active species. Reactivity of some secondary or tertiary organic hydroperoxides has been examined. The results are summarized in Table 2.

There are not much differences in the yield of

indigo. All organic hydroperoxides listed in Table 2 are equally applicable to the reaction. The organic groups of the hydroperoxides might not affect the reactivity to a large extent and, therefore, cumene hydroperoxide was chosen for the further experiments because it is the raw material of phenol production and is easily available on an industrial scale. Table 2 Reactions with various Organic Hvdromroxides a) entry

Organic Hydroperoxide

Solventb'

Yieldc) %

~

<

Cumene

57.8

2

Cumene

50.2

3

Toluene

42.0

4

To1uene

50.3

5

Toluene

49.2

6

Toluene

47.2

1

a) lOg(85.4mmol) of indole, indole : organic hydroperoxide : catalyst=l : 3 : 0.01 (molar

ratio), 80°C , 5hr. bh0Og. C)basedon indole.

619

Table 3 Preparation of Indigo from Indole and Cumene Hydroperoxide under Various Conditionsa) entry

solventb)

additive A

additive B NLc):Cd).catal."):add.A:add.B molar ratio

Temp. Time

Yield0

'C

hr

%,

1

1 : 2.2:

0001 :

0:

0

86

6

725

2

1: 3:

0.001:

0:

0

100

5

716

3

1: 3 :

0.001 :

0:

0

100

5

695

4

1: 3:

0.001:

0:

0

100

5

552

5

1: 3 :

0.001 :

0:

0

100

5

590

6

1: 3 :

0001:

0:

0

100

5

-184

7 9

1: 3 :

0.001:

0.1:

0

100

5

632

PlqSiOH

1: 3 :

0.001:

0:

0.1

100

5

657

Ph3SiOH

1: 3 :

0.001 :

0.1:

0.1

100

5

699

PhCOOH

n PhCOOH

10

AcOM

1: 3 :

0.001:

0.1:

0

100

5

780

II

PhCOOH

1: 3:

0.001

0.1:

0

100

5

78 1

1: 3 :

0.001:

0:

0.1

100

5

717

1 : 2.2:

0.001 :

0.1 :

0

86

7

813

12 13

Ph3SiOH

AcOI-I

'

a)lOg (85.4mmol) of Indole. b)15Og (entnes 1 and 13) or 300g (entnes 2-12). c)NL=indole. d)CHP=cumene hydroperoxide. e)catal.=Mo( C0)6.

abased on indole. g)TBA=tert-butyl

alcohol. h)TAA=tert-amyl alcohol. l)CM=cumene. J)TOL;.toluene.

In order to find out more favorable reaction system, the reaction was carried out under various conditions. I t was found that choice of the solvent is very important and that some organic compounds when added to the reaction system improve quite eif'ectively the yield of indigo. Results are summarized in Table 3 . As shown in entrics 1 and 2, among all of the solvents examined the most favorable solvent was tertiary alcohol,

When tert-butyl alcohol or tert-amyl alcohol was used, the

yield of indigo exceeded more than 70% which was 10-20%higher than that when aromatic hydrocarbon u'as used (compare entries 1 and 2 with entries 5 and 6). Secondary alcohol also impro\.ed the yield (entry 3), but somewhat higher conversion of hydroperoxide was observed. This observation must be due to the fact that secondary alcohol tends to react with hydropcroxide t o afford ketone compound, and consequently the hydroperoxide is additionall). consumed.

620

The improvement in the yield was also observed when the reaction was carried out in the presence of a small amount of carboxylic acid, such as benzoic acid and acetic acid. AS shown in Table 3, the yield increased by several percents (compare entries 7, 10 and 11, with entries 5, 2 and 2 respectively). The mechanistic explanation for the effect of carboxylic acid is not clear yet. Since this effect appears even when the reaction is carried out in the absence of catalyst, the carboxylic acid may activate either hydroperoxide or indole. In hydrocarbon solvent system, addition of a silanol compound was also effective to improve the yield (compare entry 8 with entry 5). Furthermore, when the silanol was used together with carboxylic acid further increment in the yield was observed (see entries 7-9). This fact indicates that the function of the carboxylic acid and of the silanol are independent

of each other. On the other hand, when the silanol compound was employed in the tert-amyl alcohol system it did not change the yield (compare entry 2 with entry 12). It seems that the function of the alcohol and of the silanol are essentially identical and that the function is the stabilization of molybdenum catalyst by forming an alkoxy or silanoxy complex. Applying those finding and further optimizing the reaction conditions, we now obtain indigo in more than 80%yield as shown in entry 13. From the mechanistic point of view, this reaction involves at least two basic steps, namely the oxidation of the 3-position of indole and the dimerization of indole framework. In order to find out an intermediate to make clear the reaction mechanism, the HLC-Mass spectrum measurement under inert gas environment was carefully made for the reaction solution at early reaction stage within approximately 15 minutes.

One HLC peak with

considerably high intensity showed the parent ion peak at 133 on mass-spectrum.

This

number is the molecular weight of indoxyl, which is the 3-position oxidized indole (see Scheme 2 ) . The mass-spectrum coincided with that of authentic sample of indoxyl prepared as indicated in ref.[10]. The existence of indoxyl in the early reaction stage was thus proved. It was also found that indoxyl is easily oxidized even by air to form indigo almost quantitatively. It is well known that indoxyl is the intermediate in the conventional industrial process, although it is the salt of alkaline metal. Now, we believe that indoxyl is an intermediate of the reaction. The proposed mechanism is shown in Scheme 2. The first step is oxidation of the 3-position of indole to indoxyl. Molybdenum complex catalyzes this step, because it was found that in addition to the molybdenum complex used from the beginning of the reaction, the equal amount of the complex added t o the reaction mixture when the reaction proceeded more than 2 hours does not make any effect on the rate of reaction and the yield of indigo. A molybdenum-peroxy complex (2, should be an active

62 1

Scheme 2

OH

-1

d

indigo

H

indoxyl

species as catalyst and it may react with indole to form five-membered metallacycle complex (3). Through the dissociation of 3 indoxyl is then formed. This mechanism is quite similar to that of the catalytic epoxidation of olefins by organic hydroperoxides proposed by H. Mimoun et al.[ll]. The high selectivity of the 3-position oxidation is due to electronic reason. It is well known that n-electrons of indole are localized on the 3-position. One example of the experimental evidence of this fact is the Friedel-Crafts reaction of indole in which an electrophilic substitution occurs mainly at the 3-position of indole. In the present reaction, the electron rich 3-position of indole must be attacked selectively by the electrophilic oxygen atom bound to molybdenum atom of the molybdenum-peroxy complex[ 111. It has been reported that indoxyl is oxidized to form indigo with radical mechanism in basic solution[l2]. In the present reaction, oxidative coupling of indoxyl to form indigo may also be radical reaction, though it is in neutral solution. Attempts to more thoroughly elucidate the mechanism of this latter step are currently under way in our laboratories. In conclusion we have shown a novel and simple method to prepare indigo by the reaction of indole with an organic hydroperoxide catalyzed by molybdenum complex. This selective oxidation is noteworthy in pure chemistry and applied chemistry as well.

REFERENCES 1. (a) U. Takaki, H. Suzuki, Y. Yamamoto, S. Aoki and I. Hara (Mitsui Toatsu Chem., Inc), US Patent No. 4 992 556(1991). (b) Y . Yamamoto, U. Takaki, S. Aoki and I. Hara (Mitsui Toatsu Chem.,Inc.), US Patent No. 4 973 706(1%U)

622

2. T. Honda, F. Matsuda, T. Kiyoura, K. Terada (Mitsui Toatsu Chem., Inc.), EP Patent No. 69 242 (1983). 3. A. Yoshimura and T. Ohno, Photochem. Photobiol., 48 (1988) 561. 4. C.J. Nielsen, R. Stotz, G.T. Cheek and R.F. Nelson, J. Electroanal. Chem., 90 (1978) 127. 5. B. Witkop, Justus Liebigs Ann. Chem., 556 (1944) 103. 6. B. Hughes and H. Suschitzky, J. Chem. SOC.,(1965) 875. 7. C.E. Dalgliesh and W. Kelly, J. Chem. SOC., (1958) 3726. 8. (a) A.K. Sheinkman, H.A. Klyuev, L.A. Rbibenko and E.X. Dank, Khim. Geterotsikl. Soedin., 11 (1978) 1490; (b) T. Kaneko, M. Matsuo and Y. Iitaka, Heterocycles 12 (1978) 471. 9. (a) S. Sakamura and Y. Obata, Bull. Agr. Chem. SOC.Japan, 20 (1956) 80; (b) B. Witkop and H. Fiedler, Justes Liebigs Ann. Chem., 558 (1947) 91. 10. B. Capon and F. Kwok, J. Am. Chem. SOC., 111 (1989) 5346. 11. (a) H. Mimoun, Angew. Chem. Int. Ed. Engl., 21 (1982) 734; (b) P. Chaumette, H. Mimoun and L. Saussine, J. Organomet. Chem., 250 (1983) 291. 12. G.A. Russell and G. Kaupp, J. Am. Chem. SOC.,91 (1969) 3851.

DISCUSSION CONTRIBUTION

J.F. BRAZDIL (BP Chemicals, Cleveland, U.S.A.): At the present yields you have shown, is your process economically advantaged over the existing multi-step process for indigo synthesis? Y. INOUE (Mitsui Toatsu Chemicals, Yokohama, Japan): Indole is now produced on an industrial scale from aniline and ethylene glycol and it can be inexpensive raw staff. All other raw materials and a catalyst are also easily available in industry and not expensive. Furthermore, new process is very simple and indigo is obtained through the one-pot reaction. Therefore, our process is economically advantaged over the conventional complicate process at the present yields.

V. CortCs Corberh and S. Vic Bell6n (Editors), New Developments in Selective Oxidation II 0 1994 Elscvier Science B.V. All rights reserved.

623

Selective Oxidations with Short-lived Manganese(V) Eva Zahonyi-Bud6 and Laszlo I. Simandi Central Research Institute for Chemistry of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary 1. INTRODUCTION

Manganese(V) intermediates have long been assumed to be involved in the oxidation of organic and inorganic compounds by permanganate ion [l-51, but little is known about the nature of their reactions. Hypomanganate ion has a fair degree of stability only in concentrated alkali [6] and could be detected in moderately alkaline solutions by stopped-flow rapid scan spectrophotometry as a short-lived intermediate during the oxidation of sulfite ion [7,8]. In the neutral or acidic solutions in which oxidations by permanganate ion are usually carried out Mn(V) escapes detection by both analytical and kinetic methods. A possible way of detecting short-lived manganese(V) in a given reaction would be the addition of a suitable reactant to the starting solution which would react with this intermediate, thus competing with the main reaction. In other words, the induced oxidation of an added substrate by Mn(V) should be accomplished. The term "induced oxidation" refers to a situation where a compound, which cannot be oxidized by the oxidant used, undergoes oxidation when another reducing agent (the inductor) is added to the system. Induced oxidations involving chromate ion have been widely studied [9], but only a few examples are available in which permanganate ion is the oxidant [ 101. In an attempt to examine the reactions of the putative h4n(V) intermediate, we have designed systems in which it is generated via the very fast reduction of permanganate ion with arsenite(II1) in the presence of various reducing substrates, In these systems arsenite(II1) plays the role of an inductor, leading to the oxidation of otherwise unreactive or only moderately reactive substrates.

2. EXPERIMENTAL Measurements were carried out by recording the successive UV-Vis spectra of the reacting solution on a Hewlett-Packard 8452 Diode Array Spectrophotometer combined with a HiTech Scientific stopped-flow type accessory. Reactions were run in buffered solutions, in the presence of pyrophosphate in a 20-fold excess over permanganate to avoid disproportionation of the Mn(II1) formed. The ionic strength was kept constant. Permanganate ion was added in excess over A@), ensuring that the latter should be fUy consumed in all cases. Substrates were added to the As(II1) solution before mixing with permanganate. Owing to the fast rate of

624

the Mn(VI1) - As(II1) reaction, hnetic measurements could not be made as the reactions to be discussed are complete within the mixing time of the stopped-flow instrument available to US. Measurements are based on analyzing the spectra of the product solutions. Residual permanganate and the stable manganese products, Mn(II1) and soluble Mn(IV), were measured by comparison with the known spectra of the individual species. Absorption coefficients of Mn(IV) were calculated from the absorbance of Mn(II1) and Mn(IV) formed in a known concentration ratio in the reaction of Mn(VI1) with As(II1) in the absence of other substrates. 3. RESULTS AND DISCUSSION We have found that the oxidation of phosphorous acid (H3PO3) as well as various organic compounds (alcohols, glycols, hydroxyacids, carboxylic acids, etc.) can be induced in the permanganate - arsenous acid system. This is demonstrated by the experimental finding that permanganate is consumed in excess of the amount used up by As(II4 alone, with no other reactant present. According to kinetic measurement in the absence of As(III), direct oxidation of each of these compounds by Mn(VI1) is negligible during the time intervals involved in our studies. Induced reactions can be characterized by the induction factor IF, which is the ratio of oxidation equivalents consumed by the acceptor (added substrate) and the inductor, As(II1) . It depends on the ratio of reactant concentrations. At sufficiently high acceptor concentrations IF reaches a limiting value, which may provide usehl information on the valence changes occumng in the redox process involved. In our system the limiting value of IF was found to be unity. The typical dependence of IF on the acceptor concentration is illustrated in Figure 1 in the case of ethanol as acceptor.

c

1.0 IF

I

I

1

I

625

In the absence of other reductants arsenic(II1) reduces permanganate to a mixture of Mn(II1) and Mn(1V). With an efficient acceptor present, mainly Mn(II1) is formed (Figure 2) which cannot be reduced hrther in the presence of pyrophosphate. We have observed

’t Figure 2. Permanganate consumption and stable manganese products as a function of acceptor concentration in the Mn(VI1) - As(II1) - Acceptor system (Acceptor: ethanol; conditions the same as in Figure 1) that not more than one half of the oxidizing capacity of permanganate is used up in the oxidation of the acceptor, which confirms that the intermediate formed in the reaction of Mn(VI1) with As (111) is indeed manganese(V): Mn(VI1) + As(II1)

d

Mn(V) + As(V)

(1)

The manganese(V) is hrther reduced to Mn(JI1) either by As(II1) or the acceptor molecule S, but may also undergo disproportionation:

Mn(V)

+ As(II1)

Mn(V) + S 3Mn(V)

A

d

+

Mn(II1)

+

h(V)

Mn(II1) + P

2Mn(IV) + Mn(VI1)

(2) (3) (4)

With increasing concentration of the acceptor (S), reaction (3) gradually becomes predominant. In the limiting case the overall reaction can be described by equations (1) and (3). It is important to emphasize that direct reduction of Mn(1V) by the acceptor and the involvement of Mn(II1) in the induced oxidation can be ruled out. Control experiments show

626

that if the acceptor is added to the solution after complete oxidation of As(III), no changes in the spectra can be detected. The data obtained in the induced oxidation of various acceptors are shown in Table 1. Table 1 Induction factors (IF) in the Mn(VI1) - As(II1) - Acceptor system [Mn(VII)],= 4 . 0 ~ 1 0-4 M; [As(III)IO= 4 . 0 ~ 1 0 -M; ~ [pyrophosphate],= 8 . 0 ~ 1 0 M -~ IF = oxidation equivs consumed by S/ oxidation equivs consumed by As(II1)

IF Acceptor (S)

S[MI

PKa pHZ1.04

None H3P03 HPO( OEt)( OH) HPO(OEt)2 Ethanol

0.0

0.02 0.02 0.02

1.0; 6.0 0.8

0.02 0.10

0.34 1 .O

n-pro pano i-Propanol Ethylene glycol

0.10 0.10 0.02 0.10

pHz3.8

0.0

0.82 (pH=O) 0.73 0.74 (pH=O) 0.0 0.0 (pH=O) 0.0

pH=6.8

0.0

0.15

0.40 0.75 0.91

0.16 0.40 0.66 0.93

0.73 0.58 0.46 0.79

0.35 0.23 0.30 0.63

0.20

0.52 0.62

0.76 0.64 0.68 0.76

0.32

Glycolic acid Tartaric acid Formic acid

0.02 0.02

3.60 3.0; 4.3

0.02

3.7

0.48

Oxalic acid

0.10 0.02

1.2; 4.2

0.88 0.39

Formaldehyde Acetaldehyde

0.02 0.02

0.36 0.33

0.30

0.34 0.14

In general, the oxidation reactions of the acceptors used are typically not very rapid. In the oxidation of alcohols and aldehydes the rate-limiting step is usually hydride ion or H-atom transfer from a C-H bond to the oxidant, which requires a considerable activation energy. Especially surprising is the high reactivity of phosphorous acid, because its oxidation requires

621

the cleavage of the strong P-H bond, which is a slow process in all known cases (see e.g. [ 11131). We have concluded that the fast oxidation of phosphorous acid by Mn(V) can be interpreted by 2-electron transfer followed by fast deprotonation of the product, rather than by hydride or H-atom transfer. We proposed a mechanism involving the formation of a mixed anhydride of hypomanganous and phosphorous acids, in which the bridging oxygen atom offers a convenient path for inner-sphere electron transfer [ 141, equation ( 5 ) .

OH I

[O3Mn-OW2- + HO-P=O I

H

OH slow

--+

[03Mn-O-P=Ol2-

+

Mn(II1) + P(V) ( 5 )

-H20

H

For anhydride formation to take place, at least one OH-group should be available on P(II1). Deprotonation indeed suppresses the induced oxidation and at higher pH, where only HPO32- is present, it cannot be observed at all. We have also found that diethyl phosphonate and the anion of monoethyl phosphonate have no effect on the oxidation of As(II1). In contrast to this, induced oxidation of the protonated monoester does readily take place [ 131. These results are in accordance with the proposed mechanism: if the acidic protons of phosphorous acid are removed or replaced by ethyl groups, anhydride formation is not possible and oxidation by Mn(V) is inhibited. The reactivity of the organic compounds listed in Table 1 may also be attributed to their ability to form 0-bridged compounds with hypomanganous acid due to the presence of OH groups (alcohols, glycols, hydroxy acids). These species can be regarded as mixed anhydrides or as manganate(V) esters, depending on the nature of the reacting OH groups. Aldehydes may react in their hydrated form when OH groups are available. The oxidation of formic and oxalic acid may involve the hydroxy groups but an outer-sphere reaction is also conceivable. The reaction of simple alcohols with Mn(V) is also influenced by the pH, although in the pH range studied there are no protonation equilibria involving either the alcohols or As(II1). Protonation of hypomanganous acid may be responsible €or this effect , which seems to favour ester formation with alcohols over the competing reactions. In many respects the behaviour of manganese(V) resembles that of chromic acid. Fast formation of Cr(V1) esters in the oxidation of alcohols and aliphatic aldehydes, as well as that of a mixed anhydride in the oxidation of phosphorous acid is well established [ 1,3,9,12]. The overall oxidation rate is determined by the cleavage of the C-H or P-H bond, respectively, in contrast to similar reactions with Mn(V), where the rate-limiting step is the formation of an 0bridged intermediate or electron transfer through the bridge. 4. CONCLUSIONS

The results of this work have shown that short-lived manganese(V) shows a reactivity pattern toward some organic substrates which is distinctly different from that of permanganate

62 8

ion. In that sense it can be regarded as a selwtive oxidant and further studies may lead to evidence that it certainly is a short-lived intermediate in oxidations by permanganate ion. Acknowledgments. This work was supported by the Hungarian Research Fund (Grant No. 4074). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

R, Stewart, in Oxidation in Organic Chemistry, Part A, K.B. Wiberg (ed.), Academic Press, New York, 1965, Chapter 1. F. Freeman, Rev. React. Species Chem. React., 1 (1976) 179. D.G. Lee, in Oxidation of Organic Compounds by Permanganate Ion and Hexavalent Chromium, Open Court, La Salle, IL, 1980. D.G. Lee, in Oxidation in Organic Chemistry, Part D, W.S. Trahanovsky (ed.), Academic Press, New York, 1982, p. 147 L.I. Simindi, in The Chemistry of the Functional Groups, Supplement C, S. Patai, Z. Rappoport (eds), Wiley, Chichester, New York, Brisbane, Toronto, Singapore, 1983, Chapter 13. J.S. Pode, W.A. Waters, J. Chem. Soc. (1956) 717. L.I. Simindi, M. J&y, Z.A. Schelly, J. Am. Chem. Soc., 106 (1984) 6866. L.I. Simindi, M. J&y, C.R. Savage, Z.A. Schelly, J. Am. Chem. Soc., 107 (1985) 4220. L.J. Csinyi, Induced Reactions, in Comprehensive Chemical Kinetics, C.H. Bamford, C.F.H. Tipper (eds), Vol. 7, Elsevier, Amsterdam, London, New York, 1972, Chapter 5. J.H. Merz, G. Stafford, W.A. Waters, J. Chem. Soc. (1951) 638. A. Viste, D.A. Holm, P.L. Wang, G.D. Veith, Inorg. Chem., 10 (1971) 631. G.P. Haight, Jr., M. Rose, J. Preer, J. Am. Chem. Soc., 90 (1968) 4809. 8. Z5honyi-Bud6, L.I. Simindi, Inorg. Chim. Acta, 205 (1993) 207. 8. Zihonyi-Bud6, L.I. Simindi, Inorg. Chim. Acta, 191 (1992) 1.

DISCUSSION CONTRIBUTION

J.-M. BRkGEAULT (Universitk P. et M. Curie, Paris, France): You are considering on your slides (schemes) several redox processes. As the E" values depend on the pH, ligands, etc., are the redox potentials in manganese chemistry directly relevant to your system? L.I. SIMANDI (Central Res. Inst. Chem., Budapest, Hungary): Yes, they certainly are or would be if they were available. The reactions discussed take place in acidic solutions, where both manganese (V) and manganese (VI) are thermodynamically unstable and very reactive. It might be possible to calculate standard redox potentials for these species but at the moment no experimental technique for this purpose seems to be accessible.

V. CortCs Corbcrin and S. Vic Bcllon (Editors), New Devebpmenrs in Selecrive Oxiduiion I / 0 1994 Elsevier Scicnce B.V. All rights rcservcd.

CATALYTIC OXIDATION OF POLYOLS: N E W NONRADICAL MECHANISM OF OXYGENATION

EXAMPLE

629

OF

A.M.Sakharov, 1.P.Skibida Institute of Chemical Physics, Russian Academy of Sciences. 117977 Moscow, Kosygin street, 4 (Russia) The low temperature polyols oxidation by 0 2 in alkaline aqueous solutions was studied in the presence of different catalysts, the Cu2+ salts being the most active among them. The strong dependence of the oxidizability of polyols on their structure, nature of base and partial pressure of dioxygen was established. Mechanism have suggested, accordin to that the oxidation of polyols occurs via the formation of a ternary complex [C$+...RO-m...02] (where ROwm is anionic form of polyol). The labile Q -0xyhydroperoxide was proposed to be a reactive intermediate. In the case of open-chain polyols (sorbitol, mannitol) oxidation the deep destruction and the formation of low molecular acids takes place. Oxygenation of non-reducin carbohydrates (methyl-D-glucoside, sucrose) yields ring-opened oxymonocar oxylates as a primary stable products. Monocarboxylates was obtained with the selectivity of 80% at the conversion range of non-reducing carbohydrates up to 40%.

i?

Introduction The autoxidation of organic compounds in the presence of molecular oxygen occurs, as a rule, by the radical chain (one-electron) mechanism [l]. The presence of highly reactive free radicals results in a large number of side reactions and, thus, in a low selectivity of the formation of desired products. The oxygenation of aliphatic ketones and alcohols in alkaline media in the presence of copper complexes at 30-50°C seems to present one of the not numerous examples of one-stage two-electron reduction of dioxygen with formation a oxy- or oxohydroperoxide as a primary labile compound [2,3]. It was found that, like for some enzymatic systems, the transfer of substrate to the anionic form is of the great importance. The catalyst particlpation in the reaction increases the reactivity of anionic form of substrate towards 02. The interaction of [Cu+”..A-] adducts and 0 2 occurs by the two-electron mechanism. The catalyst takes part in the charge transfer thus eliminating the violation of the spin conservation rule in the two-electron reduction of oxygen [2,3]. ‘The different substituted ketones, alcohols and other compounds with acidic 0t i or C-ti bounds includin those rather inert towards dioxygen can be oxidized using such a system with extreme y high rate and selectivity at the moderate temperatures. The present investigation is devoted to the study the effectiveness of catalytic system in question in the oxidation of polyols that are known to be essentially inert to O2 and can be easily deprotonated in base media.

f

Experimental The oxidation of polyols were carried out in a glass reactor supplied with highspeed mixer at 30 - 8QOC. The catalyst was formed in situ by dissolving CuCI,, CuS04 or Cu(Ac) in water solutions of polyols. l h e concentration of polyols was varied from 0.1 to M. The polyols oxygenation was carried out using NaOH, KO€& Ca(OH)2 or

3

630

I

I3a(Otl 2 as bases. l h c reaction begins irnrricdiatcly after adding a basc. ?'he reaction was f o lowed by 0 base and polyols consumption and products accumulation (measured using G L t and HPLC techniques). Oxygenation of open-ring p o l y o l s . I t was found that in contrast with earlier investigated oxidation of mono-atomic alcohols [ 3 ] the polyols oxidation can take place in the aqueous soiutioiis with the essentially high rates. Figure 1 presents the kinetic curves o f oxygen uptake in the oxidation of 20% aqueous solution of various polyok i i i the presence of CuC$ and Ca(0H)z at W C . Oxygen consumption stops after the base is ncutralized by acids formed in the course of reaction (Fig.1, curves 1,2). It was found that the rate o f oxidation of polyols is extraordinary sensitive to riature of polyol. Sorbitol (I) and niannitol (11) are oxidized with very high rates (Fig.1, curves 1,2). The rates of oxidation of this polyols at the optimum conditions are coinmensurable with the rates of oxidation of aldo- arid ketohexoses (glucose, mannose, fructose, sorbose) known to be very reactive towards dioxygen. The rate of 0 2 uptake in dulcitol (111) oxidation is i n several times lower than that of sorbitol in spite of the structures of both carbohydrates are very similar (Fig.1, curve 3 ) . aO*, M

I

I

0.3

0.2

0.1

// i

0

4

5 11

Figure 1. Oxygen uptake in polyols oxid;ltion (20'% in water): I-sorbitol, 2-mannito1, 3dulcitol, 4-iriosito1, 5-gliccroi, (i-diaccion-L-sortlosc. I C U C I ~=I 5.10 - 3 M, I C ~ ( O H =) ~0.3 ~ M,

50"C, p 0 2

=

1 atm.

Figurc 2. Oxygen consumptioii in sorbiiol o x i d a h n in rhc [trcscnce of 0.3 M Ca(Oll)2 (1) arid diflcrcnl quatitics of NaOlI: 0.5 N - (Z), 0.15 N - ( 3 ) a i d 0.05 N - (4). Sorbilol 1.0 M, [ C U C I ~=] 5.10-3 M, 5OoC, pO2 = 1 aim.

Unexpectedly low rates of oxidation were observed in the inositol, glycerol and diaceton-L-sorbose oxidation (Fig.l,curves 4-6).

F

Y QH PH HOCH2-$! - 7 - F - F-CHZOH OH OH H H ( 1 )

OH51 H HOCHz-C - 6 9 - e-CH20H OH H OH OH

-

(11)

63 1 OH

HocH2-R

H

-

6 -

H

-

bH 8H (1111

OH C-CH20H

k

The kinetics of catalytic oxidation of polyols drastically depends also on the nature of bases employed. The highest rate of mannitol and sorbitol oxidation at 305OoC can be achieved in the presence of Ca(OH)2. Figure 2 presents the kinetic curves of 0 uptake for the oxidation of 20% aqueous sorbitol solutions in the presence of 8uCl2 and Ca(OH)2 (curve 1)and various amounts of NaOH (curves 24). In fact at any concentration of NaOH or KOH one can not achieve so high rates of oxygenation as in [Cu2+ - C a ( 0 H ) ] system. The initial rate of sorbitol anzmannitol oxidation does not depend on the amount of Ca(0H)Z in the system in the interval of 5 to 200 g.1-1. This is due to the low Ca(0H)Z solubility in water solution of polyols and the inertness of the solid part of Ca(OH)2 in the reaction. The use of C a ( 0 H ) instead of NaOH leads not only to increase of the rate of oxidation but, apparent$, changes also the reaction pathway. First of all the distributions of the products formed during of polyols oxidation are essentially different for [Cu2+ - NaOH] and [Cu2+ - Ca(0H) 1. In both cases acids are the main products. The yields of acids forming during s o r h o 1 and mannitol oxygenation in the presence of NaOH and Ca(OH)2 are given in the Table 1.As can be seen from Table 1, formic and glycolic acids are practically the solely products of oxy enation of these polylols when NaOH are used as a base. In the presence of C!a(OH) oxycarboxylates C3 - C5 are formed in a large uantity. In both cases the amounts 0% non-acidic products (glycerol, erythritol, pentitoll) are negligibly low. Table 1. Distribution of the products of oxygenation of 20% water solutions of sorbitol and ma~itol.([CuCl2]= 5.10-3 M, 50OC). Base Substrate

M

Products, M

02

M HCOOH HOCH2COOH

C3-C5 acids

Mannitol

NaOH, 0.5

0.34

0.27

0.21

traces

Mannitol

Ca(OH)2 0.3

0.30

0.23

0.14

0.25

Sorbitol

NaOH, 0.5

0.33

0.25

0.22

traces

Sorbitol

Ca(OH)2 0.3

0.33

0.19

0.17

0.30

The catalytic oxidation of polyols in basic sqlutions proceeds via vicinal diol cleavage. This is confirmed by the very low rates of diaceton-L-sorbose and glycerol oxidation which has not the vicinal diol groups. It is possible nevertheless that durin sorbitol and mannitol oxygenation not only the vicinal diols cleavage but also the oxi ation of primary alcohol groups takes place. Cq -. carboxylates are very reactive in the reaction conditions and could be easily oxidized to produce low-molecular oxycarboxylates. In fact the rate of oxidation of D-gluconic acid is more than fives time higher than that of sorbitol. Moreover, i n the

ti

632

mixture of sorbitol and D-gluconic acid the latter are oxidized preferably. Only after all D-gluconate is completely consumed sorbitol starts to oxidize. The rate of oxidation of polyols in alkaline media depends on the concentration and structure of [ C U ~...A-m] ' complexes. Alkoxydes do not reduce Cu2' to Cu' at anaerobic conditions with measurable rates. However durin the oxidation of polyols some reducing agents are formed and the low soluble (%(I) complexes begin to accumulate in the solution after the 0 2 supply breaking off. The renewal of oxygen blowing through the solution results in rapid transfer Cu(1) to Cu(I1). In the course of polyols oxygenation under 0 2 pressure near 1 atm. all the catalyst is transfered in Cu(I1) form. Up to the catalyst concentration of about 2.10-2 M the dependence of the oxidation rate on concentration of copper salts is close to linear. As mentioned above oxidizability of polyols strongly depends on the structure of Cu(I1) - alkoxides. Anionic forms of glycerol, inositol, diaceton-L-sorbose do not form active cupric alkoxides. Activity of cupric alkoxydes may be strongly depressed by adding of complexing agents. Figure 3 presents the kinetic curves of oxygen uptake in the oxygenation of 10% aqueous sorbitol solution in the presence of CuC12 (curve 1)and the complexes of Cu(1I) with o-phenanthroline (curves 2,3)and ethylenediamine (curves 43). Even at low concentration of complexing agents the rate of oxygen uptake strongly decreases. Copper-o-phenanthroline complexes are known 141 to be a very pronounced oxidant, but the addition of o-phenanthroline decreases the rate of oxidation in the same extent that ethylenediamine. The latter unlike o-phenanthroline decreases the oxidation-reduction potential of copper ions. Oxidation of carbohydrates containing glycosidic bonds.

It was found that in the presence of bases and cupric ions non-reducing su ars such as methyl-D-glycoside and saccharose can be oxidized with sufficiently igh rates. The rate of oxygenation of this non-reducing carbohydrates is lower than that of sorbitol and mannitol oxidation. Thus it was more convenient to study kinetics re ularities of oxidation of these polyols oxidation at more higher temperature: 60 80% c .I It was found that the main regularities of open-ring polyols and non-reducin sugars are quite different. In the latter case the high oxygenation rate can be achieve only if NaOH or KOH but not Ca(OH)2 are used as bases in contrast to open-ring polyols oxidation. Fi ure 4 presents the dependences of the rates of catalytic oxidation of sucrose versus tie partial pressure of 0 in the presence of NaOH (curve 1) and Ca(OH)2 (curve 2). As it can be calculate3 from the dependence of w 0 on pO2, the reaction order on oxygen in the presence of NaOH is more that 1 wgereas no2 = 1/2 is caracteristic of sucrose oxidation in the presence of Ca OH)2. This fact demonstrates clearly the essential differences in the mechanisms o oxygen activation under these conditions. The deviation of the reaction order 1102from unity in the presence of NaOH is caused by deactivation of some part of the catalyst at low oxygen pressure. Table 2 represents percentage yields and distribution of the products formed in the methyl-D- lucoside oxidation using [Cu(II) - NaOH - 0 ] system. The main oxidation pro ucts are acids; the reaction stops after the compfete neutralization of the alkali by acids formed. The conversion degree of methyl-D-glucoside increases with the initial concentration of NaOH in solution. The ring-opened oxymonocarbonic acid (IV) appears to be the primar oxidation product. The subsequent oxidation of ( IV ) leads to the cleavage of C-

1

9

f

d

E

633

bond and the formation of low-molecular acids and C02.

OH l,i IIOOC-c

FH20H

- c

AH

h

-

0

-

II

I1

I

I

CH2011 I

C-CIi20EI

IV

0

V

2

4

h

0

0.5

1.0

PO*,

Figure 3. Oxygcn uptake i n sortiitol oxidation in I tic prcscncc of dillcrcnt Cu(1I) complcxcs. CuCl2 without ligand, 2 -Io-phcnI-S.10-3 M, 3 - lo-phcnl = .5.10-2 M , 4 - (EDA] = 5.10-j M, 5 - IEDAj = 4 . 1 V 2 M. [Cu(Il)] = S.10-3 M, .5O"C, [Ca(011)2l = 0.2 M, [Sorbitol] = 0.5 M , 1

-

pO2-1 a m .

Figure 4. The dcpcndenccs of sucrose (20% in watcr) oxidation ratc versus oxygcn partial prcssurc 1 - NaOH- 0.4 N, 2 - [Ca(OH)2] = 0.2 M . [CuC12] = %lor3 M, 70°C.

Ilowever even at the initial stage of oxygenation (low quantity of NaOfl corisuiried) the perceptible amounts of formic acid are detected. It seems that formic acid in this process is formed not only due to the oxidation of (IV) but also i n some concurrent pathwa . In Figure 5 the percentage yield of(IV) (curve 1)and of the sum of the formic, lycolic and oxalic acids (curve 3 ) are plotted versus the NaOil consumed. As follows froni this Figure the high selectivity of (IV) formation (near 80%) is observed at the range of NaOl-I consumed from 0.05 to 0.2 M ( 3 . 0 - 40% of substrate conversion). 'I'he selectivity of ( IV ) formation at 80% of substrate conversion is not higher than 20%. The rates of oxidation of saccharose in (Cu(1r) - NaOH - 0 J system exceed several time the rate of methy-D-glucoside oxidation. Neverthefess the kinetics behavior of these two non-reducing carbohydrate to acids are quite similar. The first

at

634

stage of sucrose oxygenation is vicinal glycol cleavage with formation of the ringopened monocarboxylates ( V ). The dependence of percenta e yield of ( V ) on NaOl-l consumed is presented on Fig.5 (curve 2). One can see roni this Figure that I ICOOI I is formed as a main low molecular acid (Fig5 curve 4 . At the early stagc of reaction the concentration of sucrose consumed is approximate y equal to the sum of ( V ) and formic acid formed.

/

Table 2. Percentage yield of [he products or methyl-D-glucoside (10% in water) oxygenation. L o w molecular acids

NaOn. M

IV

02.

ncmn

M

N

(coos)2

HOCH~COOH

%

N

%

N

8

X %

N

8

80.0

no

10.0

2.3

no

12.3 0.10

80.0

0.004

2.0

traces

11.5

0.15

75.0

0.040 10.0

0.02

5.0 0.03

22.0

0.16

40.0

0.38

0.066

10.2

0.02

3.1

0.10 15.4

28.7 0.18

27.3

0.58

0.145

14.5

0.03

3.0

0.23 23.0

40.5

20:O

0.03

0.020

0.003 10.0

0.13

0.086

0.013

10.0

0.003

0.20

0.11

0.019

9.5

0.40

0.25

0.65 1.0

[CuCl-J

=

traces

7.0

0.024

0.20

1.10-2 M, 75OC

%I

0.

0.4

0.8

NaOH, M

Figurc 5. The dcpcndcnccs of pcrccntagc yicld of acids in mclhyl-D-glucosidc (0.5 M) (curve 1,3) and sucrose (0.6 M) (curve 2.4) oxygcnalion on NaOH consumed. 1 - acid (IV), 2 - acid (V), 3 - sum of formic, glycolic and oxalic acids, 4 - formic acid. (CuCIzj = 5.10-3M, 75"C, pO2 = 1 atm.

635

The kinetic regularities of saccharose oxidation in [Cu(II) - Ca(OH)? - 02] system differ strongly from those, obtained using NaOH as a base. First o all the rate of oxidation in the first case is several times lower. Also the distribution of the products differs from that obtained for Cu(1I) - NaOH system: the yield of ringopened monocarboxylate ( V ) at low conversion degree of sucrose decreases from 80% (for NaOH) to 20-25% (for Ca(OH)2) and C j - C5 oxycarboxylates are formed as main products (the concentration of such acids is more than 50% of the total). It is important that the distribution of the acids formed in the course of oxygenation of sucrose in the system [Cu(lJ) - Ba(OH)2 - 0 J resemble to be quite close to that obtained in the presence of NaOH but not Ca(Ok),. In the presence of Ca(OH)2 apparently the cleavage of 1,Cglucosidic bonds takes place, that leads to the formation of aldohexose or aldopentose as reactive intermediates. The oxygenation of latter results in low molecular acids formation. Discussion Some kinetic peculiarities of polyols oxidation in the system [Cu(II) - base - 021 permit to consider this reaction as an example of new type of dioxygen activation: -

unusually strong dependence of the polyols oxidizability on their structure;

- the drastically influence of the base nature on the distribution of the oxidation products ; - the dependence of the reaction rate on oxygen partial pressure that is nontipical for the autoxidation ; - extremely high rates of oxygenation of non-reducing carbohydrates at moderate temperatures.

The mechanism of non-reducing carbohydrates oxygenation

As shown above polyols in alkaline media do not reduce Cu(I1) ions with measurable rate. Thus the reaction (2)

which is caracteristic of the mechanism of reducing carbohydrates oxygenation (aldo-, ketohexose, reducing disaccharides) is of no importance in the overall mechanism of catalytic oxidation of non-reducing carbohydrates. This conclusion is confirmed by the date concerned the influence of ophenanthroline on the rate of polyols oxidation. Cu(I1) -0-phenanthroline complexes are more strong oxidizing reagents at pH > 7 than Cu(I1) ions but addition of ophenanthroline diminishes the rate of 0 2 uptake (Fig.3). The rate of reducing carbohydrates oxidation is known to be determined mainly by the concentration of anionic form of substrate, not remarkable depend on the substrate structure and the nature of base, and very slightly depend on 0 2 pressure. The high rates of reducing sugars oxidation are observed also without catalyst. It is obviously, that the main features of catalytic oxidation of polyols by 0 2 are quite different from those of reducing sugars oxidation, that occurs preferably via one-electron mechanism. In contrast the kinetic regularities of polyols oxidation are quite similar to those

636

of aliphatic ketones. Two-electron mechanism of oxygen reduction in copper catalyzed ketones oxidation in the presence of bases was proposed [2]. We believe that similar mechanism is valid also for polyols and other non-reducing carbohydrates oxidation. The first stage of polyols oxidation in the presence of Cu(I1) salts and bases is the deprotonation of substrate and formation of cupric alkoxide (reaction 3) which is very stable in basic solution in the absence of oxygen: OH

-

+

Cu2'

n

7 -

- F

k2

OH-

---

Cu2+

7( 3 ) ...R1 - C - h - R2 k

OH

OH

o=o

k

bH

VI

--*

VI

+

\on

CU2'

+

HO2-

02

on cu2+.c?c

-09

\

i

(5)

OH-

H

-

..

I 2 4

--+

*o

C3

-

+

ncoon +

c 5 monocarboxylates Hooc-cn20n

(7)

ca2+

The low electron-donating ability of akoxide anions in water solution permits to exclude the possibility of realization in o u r system of one-electron reaction (8) Cu2+.-.A-

-..

O=O

--* CU2+...A*

+

02-

(8)

The electron-transfer from aions to 0 2 is known to occur only in the strong alkali aprotonic solutions [5]. We propose that the participation of Cu(1I) ions in electron transfer from the coordiwdted alkoxide to 0 (reaction 4) permits the one-step two-electron oxygen reduction to oxyperoxide &I). The electrons transfer from anion to 02 can proceed with no changes in multiplicity of the system owing to catalyst participation in this process [ 3 ] . It is obvious that thermodynamically favorable two-electron oxygen reduction does not require the participation of anions with so high donating ability as in one-electron reduction [5] and could occur in the mild conditions.

637

On the other hand the transferring of two electrons from coordinated alkoxide to dioxygen with simultaneous formation -C-00- bond (reaction 4) makes high demands to the structure and physical-chemical properties of ternary complexes [Cu(II) ...A- ...021. That is why oxidizability of polyols can varied in so large scale. For instance the rate of D-gluconate oxidation is more then 10 times higher than that of tartrate. We believe that in methyl-D-glucoside and sucrose oxidation the labile Q oxyhydroperoxide (VI) decomposes with the formation of HO2- and ring-opened acids ( IV ) or ( V ) (reaction 5), that is well known [lo]. The rate of H02disproportionation was found to be very hi at the conditions under study. As mentioned above the oxidation o polyols with ring-opened chain (sorbitol, mannitol) in the system [Cu(II) - NaOH - 021 leads to formic and glycolic acids formation. The formation of ( VI ) in ring-opened polyols oxygenation is followed by the deep destruction of substrates (reaction 6). It is known that Ca2+ - ions are the effective cross-linking agents [ 6 1. Thus the observed differences in the products distribution in the open-ring polyols oxidation when using Ca(OH)2 and NaOH as bases could be the effect of a very strict structure of ternary complexes [Cu2+..A-..Ca2+], that prevents at least partially the deep destruction of polyols (Scheme 1reaction 7). Polyoxymonocarboxylates ( 1V ) and ( V ) are oxidized with moderate rates to dicarboxylates. Percentage yield of oxydicarboxylates at the high conversion degree of non-reducing carbohydrates oxidation reaches 30-35% from total acids. Catalytic system [Cu(II)...base ...021 may be used also for polysaccharides oxidation. Thus it was found that kinetic regularities of starch, amilopectin, dextrane oxidation are quite similar to those of sucrose oxidation. The water soluble ringopened polymeric acids (carboxylates) are the primary stable products of polysaccharides oxygenation in the presence of Cu(I1) - salts and NaOH. These are formed via vicinal diol cleavage. At low degree of polysaccharides oxidation ([glucose unit] [NaOH] = 5 - 10) reaction occurs without the considerable 1.4-glucosidic bon s cleavage.

P

d

References 1. N.M.Emanue1, E.T.Denisov, Z.K.Maizus. Liquid-Phase Oxidation of Hydrocarbons. (Plenum Press. New York), 1967. 2. A.M.Sakharov, I.P.Skibida, Izvestia AN SSSR, ser.khim, No 3, 523-528

(1980)

3. A.M.Sakharov, 1.P.Skibida. J.Molec.cat.,48 N o 2-3, 157-174 (1988).

4. B.R.James, RJ.P.Williames. J.Chem.Soc., 2007-2019 (1961). 5. G.A.Russel1, A.G.Bemis, E.J.Ceels, E.G.Jansen, A.J.Moye. Adv.Chem.Ser., 174-202 (1968).

6. R.J.P.Williams. Cell Calcium. 13, 355-362 (1992).

75,

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V. CortCs Corberin and S. Vic Bellon (Editors), New Developments in Selective Oxidation I1 0 1994 Elsevier Science B.V. All rights reserved.

639

Copper-catalyzed oxidative decarboxylation of aliphatic carboxylic acids F.P.W. Agterberga, W.L. Driessena, J. Reedijka, H. Oeveringb and W. Buijsb a

Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands DSM Research, P.O. Box 18, 6160 MD, Geleen, The Netherlands

ABSTRACT

The copper(l1)-catalyzedoxidative decarboxylation of aliphatic carboxylic acids proceeds through an initial inner sphere one- or two-electron transfer, yielding a carboxyl radical or cation respectively. The carboxyl radical resulting from one-electrontransfer rapidly decarboxylates,after which the aliphatic radical may react with dioxygen -if present- to a ketone, alcohol or aldehyde. This occurs by Cu(l,ll) or H+-catalyzeddecompositionof the intermediate(hydro) peroxide. The aliphatic radical can also be oxidized by mononuclearCu(ll) to a carbocation, which yields an ester by reaction with a carboxylic acid (anion), or an alkene by P-H elimination. The carboxylate cation, formed by initial two-electron transfer, can perform an electrophilic attack on an a-C of another carboxylic acid, yielding a new carboxylic acid with an ester group. It can also undergo 0-H elimination, yielding an alkene carboxylic acid. 1. INTRODUCTION

Saturated aliphatic carboxylic acids generally are difficult to oxidize [la]. The oxidation of carboxylic acids has been widely studied, but a-hydroxy, a-carbonyl, and dicarboxylic acids have received far more attention than monocarboxylicacids. This reaction can be accomplished by very active free atoms or radicals, which are also able to oxidize unactivated hydrocarbons, i.e. CI', OH', CH,'. These reactions are comparable to the homolytic decomposition of diacylperoxides. RCO,H

+ X'

RCO,'

+ HX

4 R'

+ CO,(g) + HX (R= alkyl)

(1)

Carboxylic acids can also be oxidized by thermal decomposition of some of its transition metal salts. The oxidative decarboxylation of several (aliphatic) metal carboxylates [2, 31, like those of Mn(lll) [3a, 41, Co(lll) [3b,5], Ce(lV) [3c,6] and Pb(lV) [3d,7], has been investigated. Pb(IV) has without doubt been studied most extensively. The oxidative decarboxylation of aliphatic metal carboxylates generally is a concerted process, involving an inner sphere one-electron transfer from the carboxylate anion to the metal ion, thereby yielding a carboxyl radical. This carboxyl radical then rapidly decarboxylates to yield an aliphatic radical [3]. Except for those carboxylic acids, which form stable radicals, the reaction is relatively slow. (RCO,)M"+

+ R' + co, + M("-')+

(2)

It is well known that both aliphatic and aromatic Cu" carboxylates may also undergo oxidative decarboxylation [a], but the reactions of the latter are more selective than those of

640 the former, according to Nigh [9]. We have found that this selectivity depends very much on the structure of the aliphatic carboxylic acid. Carboxylic acids have also been oxidized by Cu(lll) [lo], but this trivalent metal ion acts as an outer sphere oxidant and will therefore not be regarded further in this study. Despite the large amount of work performed in this area, there still is ample discussion about the mechanism of the reaction. This fact, along with our general interest in homogeneous copper catalysis, urged us to reinvestigate the mechanism. Substituted aliphatic carboxylic acids were chosen to investigate the effect of the substituents on the activity and the selectivity of the oxidative decarboxylation reactions. 2. RESULTS AND DISCUSSION

2.1. Oxidative decarboxylation of some aliphatic carboxylic acids The reactions of various carboxylic acids in the presence of Cu(ll), added as CuO to the pure carboxylic acid, were investigated. In the presence of dioxygen, the reaction is catalytic, because the Cu(l) formed is reoxidized to Cu(ll). The main products are listed in TABLE I.

TABLE I. Results with various aliphatic carboxylic acids. Substrate R1R2CHC0,H

[CUI gas mmol (mol%) (Uhr)

temp.time Productsa ("C) (hr)

R1,R2=Ph

1405 (1.48) air (20)

184 4.5 Ph,CO Ph,CHCO,CHPh, 180 2.5 Ph,CO Ph,CHCO,CHPh, 180 2.4 Ph,CHCO,CHPh, 180 1.2 PhCHO PhCH2C0,CH,PhC PhCH,CO,CHPhCO,H 180 4.2 PhCOCH,CH, C,H,CHPhCO,CHPhC,H, C,H,CHPhCO,CH(CH,)CH,Ph

118

(4.91) air (10)

712 147

(5.00) N, (10) (5.00) air (10)

R1=Ph R2=C,H5

122

(4.99) air (10)

Rl=CH,Ph R2=H

133

(5.00) air (10)

Rl=Ph R2=H

yield mmol I mol% 405 I 84 76 116 57 I 65 28 I 32 18 I 23 I 39 24 I 4 1 101 17 4.3 I 11 7.8 I 20 8.2 I 2 1 -d I 60 -dl12

180 3.7 PhCHO PhCH,CH,CO,CH,Ph PhCH=CH, PhCH,OH PhCOCH, R1,R2=-(CH2),-= 2270 (2.72) 10% 0, (20) 200 4.0 C H C0,H' C:HYog C6H,00h 1860 (5.23) 10% 0, (20) 215 4.0 CH,(CH,),COCH,CH, CH,CH=C(C4Hg)C02H C,H,CH=C(C,H,)CO,H CH,(CH,),CH=CH,'

R1=C,H, R2=n-C4Hg

-d

+

I5

-d/8 -d I 5 380 181 63 113 25 15.3 26 18.1 193 I 60 103 I 3 2

besides CO, and H,O, if air is present; plus some Ph,C=O due to traces of 0, present; plus ester formed by addition to o or p phenyl carbon;d not determined; cyclohexane carboxylic acid; Cyclohexenecarboxylic acid; cyclohexene; cyclohexanone; plus isomers.

a

'

64 1 The products can be divided into three groups: (i) oxygenatedproducts: ketones, aldehydes and alcohols; (ii) dehydrogenated products: alkenes and alkene carboxylic acids; (iii) substituted products: esters. It is likely that the oxygenated products are formed by 0, attack on an intermediate radical, and that the oxidative substitutions and eliminations require the intermediacy of carbocations. The latter may be formed directly by a two-electron transfer from the substrate to two Cu(ll) ions, or by the oxidation of intermediate free radicals by Cu(ll) [ll]. The alkenes and esters may thus be formed due to a "lack" of dioxygen, in cases where all oxygen is needed for Cu(l) reoxidation. The rate of reaction is then limited by 0, diffusion [12]. To learn more about the mechanism of the reaction, diphenyl acetic acid, Ph,CHCO,H, has been investigated in detail. 2.2. Structure of the catalyst

When Cu,O or CuO is dissolved in carboxylic acids at elevated temperatures, the Cu' or Cu" carboxylate is formed according to equation (3) or (4) respectively. In this way our catalysts have been prepared in situ. In the presence of O, the Cu' carboxylate is rapidly oxidized to the Cu(ll) salt. Cu',O

+ 2 RCO,H

Cu"0

+ 2 RCO,H + Cu"(O,CR),

2 Cu'0,CR

+ HO ,

(3)

+ HO ,

(4)

The copper(l1) carboxylatesformed usually have the dimeric Cu" acetate-like structure [13 \I with carboxylic acids as axial ligands. The structures and magnetic properties of dimeric Cu carboxylates have been reviewed several times, most recently by Kato and Muto [14]. The structure of copper(l1) diphenylacetate,the catalyst in the case that Ph,CHCO,H is the substrate, has been determined by X-ray analysis of a single crystal obtained by recrystallization of Cu" diphenylacetatefrom acetonitrile. The presence of dinuclear units has been confirmed by EPR investigation of the reaction mixture. The coordination of the substrate carboxylate anion to two Cu(ll) ions provides both the possibility of initial one-electron or two-electron transfer.

Figure 1.

Projection of the molecular structure of [Cu,(Ph?CHC0,)4 The lattice acetonitrile has been omitted for clarity.

(CH,CN),]

.2CH,CN.

642 2.3. Reaction of diphenylacetic acid

Products The main products of oxidative decarboxylationof Ph,CHCO,H are benzophenoneand the ester diphenylmethyldiphenyl acetate (vide supra), resulting from oxygenation and oxidative substitution, respectively. The stoichiometric equations are (5) and (6).In figure 2 the concentrations of reactants and products for a typical run are shown.

+ 0, + Ph,C=O + CO, + HO ,

Ph,CHCO,H

+ 1/2 0, + Ph,CHCO,CHPh,

2 Ph,CHCO,H

+ CO, + HO , I air flow 10 Uhr II air flow 14 Uhr

Ill air flow 20 Uhr

50 E

20

Time (minutes)

Figure 2.

Relative concentrations of Ph,CHCO,H reaction with 1.48 mol% Cu(ll).

reacted, and products formed in the

It is seen in the figure that near the end of the reaction the dioxygen consumption quantitatively equals the substrate conversion, indicating that some of the 0, must have been consumed in side reactions. Kinetics

Because the reaction rate is limited by dioxygen diffusion, the kinetics of the initial reaction of the Cu" carboxylate could not be measured in the catalytic system, but could be determined separately in the absence of 0,. The order in Cu concentrationwas determinedby the fractional life-periodmethod. The halflife period was found to be equal to 2522 minutes, for several half-life periods, at different initial concentrations of Cu(ll). The rate of CO, evolution decreased to almost zero after one equivalent per Cu(ll) dimer was formed. Also, the plot of In ([CO,]/[Cu,]) versus gave the best straight line. In fact slightly more than 1 equivalent of CO, per Cu dimer is formed, due to thermal decompositionof the resulting Cu' carboxylate[i51.Indeeddiphenylmethane(Ph,CH,) is present in small amounts next to the main product Ph,CHC0,CHPh2. These results indicate the rate-determiningstep to be an intramolecular inner sphere oneor two-electron transfer of the dinuclear Cu" carboxylate.

643

--

A -2-

4

t

. ... '0

0

'

-3-

Figure 3. First-order kinetics of thermal decomposition of Cu"(Ph,CHCO,),. Selectivity The selectivity of the reaction of Ph,CHCO,H was found to depend both on the Cu(ll) concentration and the temperature. In figure 4 the relative rate of benzophenone and ester formation is shown, at different catalyst concentrations. Figure 5 shows the temperature dependence hereof.

1

1

11

>

PhZCHC02CHPh2

445

0

Figure 4.

2

4

6 8 1 [Cu] (mol%)

0

Relative rates of product formation, at different Cu concentrations.

1

2

455

465

47

Temperature (K)

Figure 5.

Relative rates of product formation, at different temperatures.

At higher Cu concentrations, more ester is found at the expense of benzophenone.This can be explained by realizing that (i) with more Cu, more 0, is needed for the reoxidation of the resulting Cu(l), as a result less 0, is available for oxygenation, and (ii) more Cu(ll) is available for the oxidation of an intermediate radical. These results thus support an initial one-electron transfer, yielding a carboxyl radical which -after rapid decarboxylation [2]-yields an alkyl radical. The presence of diphenylmethyl radicals, Ph,CH', was confirmed by the observation of traces of Ph,CHCHPh,, formed by the dimerization of two Ph,CH' radicals. Reaction with 0, yields benzophenone, whereas reaction with Cu(ll) yields the ester via the carbocation.

644

At higher temperatures the ester is favoured over the ketone. This is due to an increase of the rate of initial one-electron transfer, whereafter even more dioxygen is needed for Cu(l) reoxidation, leaving less 0, for ketone formation. The oxidation of radicals by Cu(ll) requires a mononuclear Cu(ll) species [ l l ] , which is present in solution due to the equilibrium in equation (7) [16]. The presence of some mononuclear Cu(ll) has also been confirmed by EPR analysis of the reaction solution.

Cu",(O,CR),

c

2 CU"(O~CR)~

(7)

The main route of decomposition of the (hydro)peroxidedoes not occur through the acidcatalyzed rearrangementof the hydroperoxide, because this would mainly yield benzaldehyde and phenol [ l b,17], which are only found as by-products, making this route a side reaction. Also the main route cannot proceed through the Haber-Weiss mechanism [la], since this would yield benzophenoneand diphenylmethanolin approximately1:1 ratio. Diphenylmethanol was not observed, but if formed, it would immediately yield the ester by acid-catalyzed condensation with Ph,CHCO,H, as was confirmed by addition of diphenylmethanolduring the reaction. In literature the alcohol was reported even to be the main product in the Cu" 2ethylhexanoate-catalyzed decomposition of tert-butylhydroperoxide[19]. As can be seen in figure 4, when the catalyst concentration approaches zero, the yield of benzophenone approaches 100%. It is thus unlikely that hydroperoxidedoes play an important role. Therefore another intermediate peroxo complex, perhaps a Cu-peroxo complex, must be important. The oxidative decarboxylation of Cull (Ph,CHCO,), thus involves an initial one-electron transfer only. The question is whether this is the case in general for all aliphatic carboxylates. 2.4. Other aliphatic carboxylic acids In all cases, except for cyclohexane carboxylic acid and 2-ethylhexanoic acid, traces of alkanes (R-R), formed by alkyl radical dimerization, were found. This indicates that in most cases initial one-electron transfer can occur. Alkene carboxylic acids and their esters however, formed in the cases of cyclohexane carboxylic and 2-ethylhexanoic acid, cannot be formed from alkyl radicals or cations. In this case an intermediate carboxylate cation is required. This must be due to initial two-electron transfer in the dimeric catalyst complex. The alkene carboxylic acid is then formed by p-H elimination from the carboxylate cation, followed by a H-shift.

[RR'CH-CHR"-CO,]+

+ RR'C=CR"CO,H + H+

(8)

Decarboxylation of the carboxylate cation cannot be excluded at this point; additional experiments to verify this are in preparation. It appears that the carboxylate cation itself can also perform electrophilic attack. With phenylaceticacid, the ester PhCH,CO,CHPhCO,H was found, which must be formed by attack of [PhCH,CO,]+ on the a-C of PhCH,CO,H. Surprisingly alkene esters were also found with 2-phenylbutanoicacid; these must be formed by reaction of an alkyl cation with an alkene carboxylic acid. The character of the peroxo species, resulting from 0, attack on an aliphatic radical, depends on the structure of this radical. With 3-phenylpropionoic acid, benzaldehyde is the main product. It is very likely that in this case the hydroperoxide is the dominating intermediate, its acid-catalyzed decomposition yielding benzaldehyde and methanol (not detected). This is different from the Ph,CH' radical, which was found mainly not to react via the hydroperoxide. Rearrangement of the intermediate radicals and cations is also possible. Acetophenone is observed in minor amounts with 3-phenylpropionoic acid, whereas the expected aldehyde, PhCH,CHO, is found only in trace amounts, along with traces of the corresponding alcohol, PhCH,CH,OH. This must be due to rearrangementof the PhCH,' radical.

645

Rearrangementof the carbocation is also possible. With phenylacetic acid also esters with the ester group attached on the ortho and para positions of the phenyl group. In general both initial inner sphere one- and two-electron transfers of dimeric Cu" carboxylates are possible, depending on the stability of the intermediate carboxyl radical or cation. 3. MECHANISM OF THE OXIDATIVE DECARBOXYLATION REACTION

A schematic proposal for the general mechanism of Cu(ll)-catalyzed oxidative decarboxylation of aliphatic carboxylic acids is depicted in figure 6.

Figure 6. Proposedmechanism of the Cu(ll)-catalyzeddecarboxylation of aliphatic carboxylic acids, X=RCO;. The mechanism suggested by Toussaint et al. [8b] for the oxidation of diarylacetic acids is not in a reement with our findings. They propose an oxygenating cupryl (Cu"l=O) species, from Cu carboxylate with O,,yielding benzophenone via the a-hydroxycarboxylate. This reaction, performed at 75 "C in acetonitrile, also was not catalytic.

9

Ongoing experiments deal with a detailed study of the electronic effects of phenyl substituted phenylacetic acids on the reaction kinetics, and with determination of the thermodynamic activation parameters.

646

REFERENCES 1. W.A. Waters, Mechanisms of oxidation of organic compounds, Wiley, New York 1964, (a) p.99; (b) p.45. 2. R.A. Sheldon and J.K. Kochi, Metal catalyzed oxidations of organic compounds, Academic Press, New York, 1981, p.142. 3. Organic syntheses by oxidation with metal compounds, W.J. Mijs and C.R.H.I. De Jonge (Eds.), Plenum, New York 1986; (a) W.J. De Klein, p.293; (b) F. Freeman, p.334; (c) T.-L. Ho, p.582; (d) M. Lj. Mihailovic, Z. Cecovic and Lj. Lorenc, p.787. 4. R. van Helden, A.F. Bickel and E.C. Kooyman, Red. Trav. Chim. Pays-Bas, 80 (1961), 1257 and refs. therein. 5. (a) D. Mishra and J.K. Sthapak, J. Ind. Chem. Soc., 47 (1970), 822; (b) A.A. Clifford and W.A. Waters, J. Chem. Soc., (1965), 2796. 6. H. Firouzabadi and N. Iranpoor, Syn. Comm., 14 (1984), 875. 7. (a) R.A. Sheldon and J.K. Kochi, Org. React, 19 (1972), 279; (b) J.D. Bacha and J.K. Kochi, Tetrahedron, 24 (1968), 2215. 8. (a) S.C. Goyal, L.K. Saxena, J. Ind. Chem. Soc., 62 (1985), 443; (b) 0. Toussaint, P. Capdevielle and M. Maumy, Tetrahedron Lett., 25 (1984), 3819; (c) 0. Toussaint, P. Capdevielle and M. Maumy, Tetrahedron, 40 (1984), 3229; (d) M.P. Sharma and J.N. Chatterjea, J. Chem. Tech. Biotechnol., 33A (1983), 328; (e) W.W. Kaeding, H.O. Kerlinger and G.R. Collins, J. Org. Chem., 30 (1965), 3754; (f) W.G. Toland, J. Am. Chem. Soc., 83 (1961), 2507 and refs. therein. 9. W.G. Nigh, in Oxidation in organic chemistry, 5b, W.S. Trahanowski (Ed.) Academic Press, Orlando 1973, p.1. 10 C.P. Murthv, 6.Sethuram and T. N. Rao, Oxid. Comm., 2 (19811, 13. 11. C.L. Jenkins and J.K. Kochi, J. Am. Chem. SOC.,94 (1972)', 843'and refs. therein. 12. (a) G. Astarita, Mass transfer with chemical reaction, Elsevier, Amsterdam 1967; (b) P.V. Danckwerts, Gas-liquid reactions, McGraw-Hill, New York 1970. 13. J.N. van Niekerk and F.R.L. Schoening, Acta Cryst, 6 (1953), 227. 14. M. Kato and Y . Muto, Coord. Chem. Rev., 92 (1988), 45. 15. (a) H.L. Aalten, G. van Koten, J. Tromp, C.H. Stam, K. Goubitz, A.S. Mak, and A. van der Kerk- van Hoof, Red. Trav. Chim. Pays-Bas, 108 (1989), 295; (b) H. Malenberg, M. Nilsson and R.A. Schambach, Chem. Scr., 19 (1982), 190; (c) T. Cohen, R.W. Berninger and J.T. Wood, J. Org. Chem., 43 (1978), 837; (e) A. Cairncross, J.R. Roland, R.M. Henderson and W.A. Sheppard, J. Am. Chem. Soc., 92 (1970), 3187. 16. (a) I. Uruska, J. Zielkiewicz and M. Sparakowska, J. Chem. SOC.,Dalton Trans., 1990, 733; (b) R.G. Ehirud and T.S. Srivastava, Inorg. Chim. Acta, 173 (1990) and refs. therein. 17. J. March, Advanced organic chemistry, 3rd ed., Wiley, New York 1985, p. 991. 18. F. Haber and J. Weiss, Proc. Roy. SOC.,Ser. A147 (1934), 332. 19. W.H. Richardson, J. Am. Chem. Soc., 88 (1966), 975. Acknowledgements

We are grateful to Dr. C. Versluis (Utrecht University) for the GCMS analyses, and to Dr. A. Spek (Utrecht University) for solving the X-ray crystal structure.

V. CortCs Corberin and S . Vic Bell6n (Editors), New Developments i n Selective Oxidarion If

0 1994 Elsevier Science B.V. All rights rcserved.

647

CYCLOHEXANEOXIDATION BY THE GOAGG"' SYSTEM: FORMATION OF IRON (HYDR)OXIDE PARTICLES AND REACTIVATION

U. Schuchardt, C.E.Z. Krahembuhl and W.A. Carvalho Instituto de Quimica, Universidade Estadual de Campinas Caka Postal 6154,13081-970Campinas, SP (Brazil) Summary: Cyclohexane oxidation with hydrogen peroxide by the GoAgg"' system forms iron (hydr)oxide particles which are not active in the oxidation reaction. Addition of hydrochloric acid together with hydrogen peroxide reduces the hydrolysis of the iron complexes, allowing accumulation of 0.26 M of oxidized products with 100%selectivityand 44% efficiency in only 60 min of total reaction time. Perchloric acid shows a similar efficiency for avoiding hydrolysis. On the other hand, the chlorine ion is an essential ligand for the catalyst; in its absence efficiency and selectivity are significantly reduced. INTRODUCIlON

The efficiency of cyclohexane oxidation by the classical process is unsactisfatory as only 4% of cyclohexane is converted and the selectivity for cyclohexanone plus cyclohexanol (one 01) is only 80% [l].The process can be improved by first oxidizing cyclohexane to cyclohexylhydroperoxide which, in a second step, is selectively decomposed to one + 01. Using a passivated reactor and 10% (v/v) fert-butanol as a stabilizer, we were able to accumulate 7% of cyclohexylhydroperoxideand 2% of one + 01 after 100 min of reaction time at 155'C [2]. The hydroperoxide can be rapidly decomposed in the temperature range of 80-100°C in the presence of soluble transition metal compounds, giving one + 01 with a selectivity of at least 95% [3]. The utilization of cyclohexylhydroperoxidefor epoxidation of propylene in the presence of a soluble molybdenum catalyst in the same temperature range allows the production of propylene oxide with a selectivity of 70% or more [4]. On the other hand, this makes the process very complex as the stabilizing agent has to be recycled and the selectivities are very sensitive to impurities [2]. During the last five years we have investigated the possibility of using the Gif system for industrial cyclohexane oxidation [5].We found that cyclohexane can be very effectively oxidized by hydrogen peroxide at room temperature in the presence of iron(II1) chloride in pyridine-acetic acid (GoAggI1 system), giving mainly cyclohexanone, with an efficiency with respect to hydrogen peroxide of 91% [6]. The reaction needs 10 h to complete. The addition of picolinic acid (GoAgg"' system) strongly accelerates the reaction, reducing the reaction time to 15 min [7] but also reducing the efficiency to 53% [8]. When we used this system to accumulate oxidation products, we found that a new portion of iron(II1) chloride

+

648

had to be added after each reaction in order to maintain the catalytic activity of the system [8].We were then able to accumulate six reactions, obtaining 8.7 mmol of oxidized products per 33 mL of solvent mixture (0.26 M solution) with 100% selectivity for one + 01 after 90 min of total reaction time (Table 1). Upon further addition of hydrogen peroxide and iron(II1) chloride, the efficiency of the system was reduced and the mass balance (mb) no longer closed, showing that side products were being formed [8]. After twelve accumulations we obtained 12.1mmol of oxidized products (0.37 M solution)with only 19% efficiency. Approximately 24% of the cyclohexane had reacted to give other products. Table 1 Time dependence of efficiency and concentration of one + 01 in the accumulation reactions (100 mmol of cyclohexane, 3 mmol of picolinic acid, 28 mL of pyridine, 5 mL of acetic acid, 2OoC; every 15 min: 1 mmol of FeCl3.6H20,lO mmol of H202). t(min) 15 30 45 60 75 90 105 120 135 150 165 180

one(mmo1) ol(mmo1) 2.66 0.23 0.35 4.19 5.53 0.41 6.65 0.43 7.10 0.34 8.16 0.52 8.83 0.64 0.68 9.26 9.89 0.62 10.47 0.79 10.95 0.74 11.33 0.76

one/ol 11.6 13.7 13.5 15.5 20.9 15.7 13.8 13.6 16.0 13.3 14.9 14.9

effic.(%) 56 50 38 35 29 28 26 24 23 22 21 19

mb(%) 100 100 100 100 100 100 89 88 84 79 80 76

conc.(M) 0.09 0.16 0.18 0.22 0.23 0.26 0.29 0.30 0.32 0.34 0.36 0.37

We wish to report here our results on the reactivation of the catalytic system and on a more efficient accumulation of the oxidation products. Furthermore we suggest that the chloride ion is an essential ligand for the catalytic system. 2. EXPERIMENTAL

All reagents and solvents were analytical grade. Cyclohexane was purified by washing with conc. sulfuric acid, water, 5% sodium hydroxide solution and water and then distilled. The oxidation reactions were performed in a closed 125mL Erlenmeyer under an argon atmosphere, using 28 mL of pyridine, 5 mL of acetic acid, 1.68 g (20 mmol) of cyclohexane and 270 mg (1 mmol) of FeCb.6H20. The reaction flask was placed in a thermostated water bath at 2OoC, the reaction mixture was magnetically stirred at 500 rpm and the reaction was initiated by the addition of 1.0 mL (10 mmol) of 30% H202. In the

649

accumulation reactions 1.0 mL (10 mmol) of 30% H202 and 1.0 mL of 1 M HC1 in acetic acid were added every 15 min. The reaction mixture was analyzed with a CG-37 gas chromatograph equipped with a packed column (5% Carbowax 20M on Chromosorb W-HP) coupled to a flame ionization detector. After 5 min at 80°C, the temperature was programmed at 8OC min-' to 170OC. Cyclooctane was added as an internal standard and the observed retention times were: cyclohexane (1.8 min), cyclooctane (3.8 min), cyclohexanone (11.1 min) and cyclohexanol (14.8 min). The efficiency of the reaction with respect to hydrogen peroxide was calculated taking into account that 1 mol of hydrogen peroxide is needed to produce 0.5 mol of cyclohexanone or 1 mol of cyclohexanol, respectively. The turbidity measurements were performed using a Micronal turbidimeter model B250. The solutions were diluted with pyridine to 5% (v/v) and the measurements made with visible light at a 90' angle. 3. RESULTS AND DISCUSSION

In order to understand why it was necessary to add iron(JI1) chloride after each reaction in the accumulation tests, we tried to explain the color change from light yellow to dark brown during the reaction course. Turbidity measurements showed the formation of colloidal particles of iron (hydr)oxide. By comparison with standard dispersions, it was This found that the iron (hydr)oxide particles were in the range of 1 to 5 nm (50-60 nU). is expected as the pH of the reaction mixture (5.6) is high enough to deprotonate the hexaaquoiron(II1) cation forming p-(hydr)oxodiiron(III) complexes which are considered to be the active species for the oxidation of cyclohexanewith hydrogen peroxide [9]. Under reaction conditions the p-(hydr)oxodiiron complexes further hydrolyse to polynuclear complexes and finally to iron(hydr)oxide particles (Scheme 1) which are not active in the oxidation of cyclohexane but simply decompose the hydrogen peroxide.

+ py l-PYH+ [FeOx ( OHly

I,

Scheme 1. Formation of di- and polynuclear iron (hydr)oxide complexes and iron (hydr)oxide particles under GoAgg"' reaction conditions. We tried to avoid the hydrolysis of the p-(hydr)oxodiiron complexes by reducing the pH of the reaction mixture. At pH values below 5 the system became less effective, which

650

we believe is due to the more difficult formation of the p-(hydr)oxodiiron complexes. A pH value of 5.2-5.3 was found to be the most favorable as the system remained very active and the formation of iron (hydr)oxide particles was slower. By addition of 1mmol of HC1 in acetic acid together with 10 mmol of hydrogen peroxide after each reaction, we were able to obtain 9.35 mmol of oxidized products per 36 mL of solvent mixture (0.26 M solution) after 60 min of reaction time. The overall efficiency,with respect to hydrogen peroxide, was 44% and the mass balance (mb) closed perfectly, showing that no side products were formed (Table 2). Table 2. Effect of the addition of HCl in acetic acid on the accumulation reactions (100 mmol of cyclohexane, 1mmol of FeCl3.6H20,3 mmol of picolinic acid, 28 mL of pyridine, 5 mL of acetic acid, 2OoC; every 15 min: 10 mmol of H202, 1mL of HCl in HOAc) t(min> 15 30 45 60 75 90

one(mmo1) ol(mmo1) 2.75 0.44 5.32 0.95 7.12 1.21 8.24 1.11 8.38 1.25 8.45 1.23

one/ol 6.3 5.6 5.9 7.4 6.7 6.8

effic.(%) 59 58 52 44 36 30

mb(%) 101 101 102 98 99 100

conc.(M) 0.10 0.18 0.24 0.26 0.26 0.26

The acidification of the reaction mixture can also be performed with 1M perchloric acid in acetic acid. The results were slightly inferior, giving 8.9 mmol of oxidized products per 36 mL of solvent mixture (0.25 M solution) after 60 min of reaction time with 42% of efficiency (Table 3). Table 3. Effect of the adition of HC104 in acetic acid on the accumulation reactions (100 mmol of cyclohexane, 1mmol of FeCl3.6H20,3 mmol of picolinic acid, 28 mL of pyridine, 5 mL of acetic acid, 2OoC; every 15 min: 10 mmol of H202, 1 mL of HC104 in HOAc). t(min) 15 30 45 60 75

one(mmo1) ol(mmo1) 2.70 0.34 5.08 0.47 6.47 0.74 7.87 1.05 8.00 1.18

one/ol 8.0 10.8 8.7 7.5 6.8

effic.(%) 57 53 46 42 34

mb(%) 100 101 100 100 102

conc.(M) 0.09 0.16 0.21 0.25 0.25

65 1

On the other hand, substitution of iron(II1) chloride by iron(II1) perchlorate reduces the efficiency of the system in the accumulation reactions. As shown in the Table 4, only 4.67 mmol of oxidized products (0.14 M solution) were formed in 6 accumulations compared to 8.68 mmol of oxidized products observed with iron(I1I) chloride (Table 1).The efficiency was reduced from 26% to 14% and the one/ol ratio dropped from 15.7 to 4.2, showing a significant reduction of the selectivity of the system. This shows clearly that the chlorine ion is essential to maintain the system active and selective for the production of cyclohexanone and corresponds to the results obtained by Nappa and Tolman [lo], who found that tetraphenylporphyrin iron(II1) looses its activity for cyclohexane oxidation if the axial chlorine ligand is substituted b perchlorate. We, therefore, believe that chlorine is an essential ligand for the GoAggIX system. Table 4. Accumulation reactions in a chlorine-free system (20 mmol of cyclohexane, 1 mmol of Fe(C104)3,3 mmol of picolinic acid, 28 mL of pyridine, 5 mL of acetic acid, 2OoC; every 15 min: 10 rnmol of H202, 1 rnmol of Fe(C104)3. t(min) 15 30 45 60 75 90

one(mmo1) ol(mmo1) 1.93 0.22 2.27 0.30 0.58 3.02 0.71 3.41 0.83 3.63 0.90 3.77

one/ol 8.8 7.6 5.2 4.8 4.4 4.2

effic.(%) 41 24 22 19 16 14

mb(%) 100 100 100 98 97 97

conc.(M) 0.07 0.08 0.11 0.12 0.13 0.14

4.CONCLUSIONS The GoAgg"' system deactivates due to hydrolysis of the active y-(hydr)oxodiiron(III) complex to form iron (hydr)oxide particles in the nm range. The hydrolysis rate can be reduced by the addition of hydrochloric acid in acetic acid, which allows obtaining one + 01 with a concentration of 0.26 M and 100% selectivity in 60 min of reaction time. This value is comparable to a 0.32 M solution of one + 01 obtained with 80% selectivity in 40 min in the classical oxidation process. Perchloric acid may also be used for acidification of the reaction medium but the chlorine ion is an essential ligand for the catalytic system. We are presently looking for a pyridine-free solvent system which avoids the hydrolysis of the catalyst while maintaining the high selectivity and the efficiency of the process. Acknowledgements:The authors thank Sir Derek H. R. Barton, Texas A&M University, for his interest in our work and for sending us his manuscripts prior to publication. This work was financed by the Fundacgio de Amparo a Pesquisa do Estado de SFio Paulo (FAPESP).

652

Fellowships from the Conselho Nacional de Desenvolvimento Cientifico e Tecnol6gico (CNPq) and from FAF'ESP are acknowledged.

REFEmNCEs

1. K.U. Ingold, Aidrichimica Acta, 22 (1989) 69. 2. U. Schuchardt, W. A. Carvalho and E. V. SpinacC, Synlett, in press. 3. W.A. Franco Jr. and U. Schuchardt, in "Proceedings of the XI Simp6sio Iberoamericano de Catidisis", Instituto Mexican0 del Petr6leo y Universidad Autonoma Metropolitana, MCxico, 1988, p. 1503. 4. M.N. Sheng, J.G. Zajacek and T.N. Baker I11 (to Atlantic Richfield Co, New York, N.Y.), U S Patent 3,862,961 (1976); C.A. 84:135449. 5. U. Schuchardt, W.A. Carvalho, R. Pereira and E.V. SpinacC, in "Proceedings of the 5th International Symposium on the Activation of Dioxygen and Homogeneous Catalytic Oxidation",A.E. Martell and D.T. Sawyer (eds.), Plenum Press, New York, in press. 6. U. Schuchardt, C.E.Z. Krahembiihl and W.A. Carvalho, New J. Chem., 15 (1991) 955. 7. G. Balavoine, D.H.R. Barton, J. Boivin and A. Gref, Tetrahedron Lett., 31 (1990) 659. 8. U. Schuchardt, C.E.Z. Krahembiihl and W.A. Carvalho, Abstracts of gth Intern. Symp. Homogeneous Catal., Amsterdam, 1992,p. 261. 9. D.H.R. Barton and D. Doller, Pure Appl. Chem., 63 (1991) 1567. 10. M.J. Nappa and C.A. Tolman, Inorg. Chem., 24 (1985) 4711.

V. CortCs Corberan and S . Vic Bcllon (Editors), New Developmenis i n Seleciive Oxiduiion /I 0 1994 Elsevier Science B.V. All rights reserved.

653

Oxidation of Cyclohexane Catalyzed by Polyhalogenated and Perhalogenated Manganese Porphyrins P. Battioni", R. Iwanejkob,D. Mansuy' and T. Mlodnickab* aLaboratoirede Chimie et Biochimie Pharmacologiques et Toxicologiques, Universite Rene Descartes, Paris, France. bInstituteof Catalysis and Surface Chemistry Polish Academy of Sciences, Krakow, Poland. 1.ABSTRACT

The system composed of manganese halogenated porphyrins bearing halogen substituents on the meso-phenyl rings or perhalogenated porphyrins bearing halogen substituents on both the meso-phenyl and pyrrole rings and hexa-aquomagnesium (II) bis (2-carboxylatomonoperoxybenzoic acid) appeared an efficient catalytic model system for hydroxylation of cyclohexane. The activity of the system is enhanced by the presence of nitrogenous-bases added in 25 fold excess over the catalyst concentration. The presence of halogen substituents at the B-pyrrole positions increases the stability of the porphyrin complexes in the oxidizing medium. A comparisonwith the correspondingsystem based on iron porphyrins is also given. 2. INTRODUCTION

Model systems composed of metalloporphyrins and oxidizing agents have often been used for liquid phase oxidation of hydrocarbons (for recent reviews see 11-31). The course of the reaction, the yields of products and the selectivity of the system appeared to be dependent on the character of the metal centre, the structure of the porphyrin ligands as well as on the oxidant used. It is also a well known fact, that the presence of substituents on the meso-phenyl rings and especially those situated at ortho positions exerts a significanteffect on the reactivity of metalloporphyrins. Similarly, strongly coordinating molecules which are potential axial ligands are important factors capable of modifying the catalytic behaviour of the metalloporphyrin complexes.

*To whom the correspondence should be addressed.

654

Recently, a new group of metalloporphyrin complexes has been synthesized and investigated [4-91. These porphyrins bear halogen substituents not only on the meso-phenyl rings but also on &carbons of the pyrrole rings. Manganese and iron complexes of these perhalogenated porphyrins appeared much more resistant against oxidative degradation and often better catalysts for oxidation of hydrocarbons than the corresponding porphyrins with only halogenated phenyl rings [6,9]. 3. EXPERIMENTAL

Fig. 1 show? the formula of the investigated porphyrins. The investigated manganese tetraarylporphyrins were synthesized according to the procedure described in [4,5,7]. The purity of the porphyrin complexes was checked by taking UV-VIS and NMR spectra which did not reveal traces of any putative impurities. In a standard experiment the solution of the catalyst, 4-t-butylpyridine and cyclohexane in CH,CI, was introduced to a thermostated glass reactor of 10 mL volume equipped with a magnetic stirrer. Then aqueous solution containing magnesium monoperoxyphthalate (MMPP) and tetrabutylammonium chloride which played role of the phase-transfer agent was added. The ',otal volume of the reagent solution was 3 mL. The experiments were carried out for 45 minutes under aerobic conditions at room temperature. The amounts of products were determined by G.C. analysis using Chrom 5 apparatus equipped with l m column filled with Carbowax. MMPP used was purchased from Aldrich (tech. 80%)and the active oxygen was determined by iodometric titration. ByNCl (Fluka) was of purity grade. Commercial CH2C1, was redistilled from CaH,. Cyclohexane (analytical grade) purchased from POCH Gliwice was passed through a short alumina column before use. The amounts of the substrates and products given in Tables correspond to the volume of the reagent solution.

LIGAND

X

Y

Z

TDCPP TPFPP TDCP-R-Br,P TDCP-R-CIaP TPFP-R-Br,P

C1 H

H

F C1 C1 F

F H

H Br C1 Br

H

F

Figure 1. Structures of the investigated porphyrins.

655

4. RESULTS A M ) DISCUSSION

Catalytic oxidation of cyclohexane with magnesium monoperoxyphthalate (MMPP) in dichloromethane-water solution containing Bu4NCI as the phase transfer agent has been investigated. Manganese tetraarylporphyrins with halogenated phenyl rings as well as the porphyrins bearing halogen substituents on both the meso phenyl and pyrrole rings have been used as catalysts. The obtained results are summarized in Table 1. Table 1 Oxidation of cyclohexane in the presence of manganese halogenated and perhalogenated porphyrins. Complex

Yield (mmol)

Turnover*

alcohol ketone alcohol ketone Mn(TPFPP)Cl 0.117 +4-t-BuPy 0.040 Mn(TPFP-O-Br8P)CI 0.060 4-t-BuQ 0.102 Mn(TDCPP)CI 0.082 +4-t-BuPy 0.103 Mn(TDCP-B-C18P)CI 0.085 +4-t-BuQ 0.152 Mn(TMP)Cl 0.018 +4-t-BuPy 0.066

+

0.198 0.325 0.008 0.016 0.029 0.140 0.024 0.385 0.007

0.025

0.6 0.1

7.5 6.4 2.8 0.7 3.5 0.4 2.6 2.6

126 146 27.2 47 44 97.2 43.6 215 10 36

TPFPP - 5,10,15,20-tetrakis@entafluorophenyl)porphyrin, TMP - 5,10,15,20-tetrakismesitylporphyrin, TDCPP - 5,10,15,20tetrakis(orth0-dichlorophenyl)porphyrin, TPFP-8-Br8P - 2,3,7,8,12,13,17,18-octabromo-5,10,15,20tetrakis@entafluorophenyl)porphyrin, TDCP-O-Cl,P - 2,3,7,8,12,13,17,18-octachloro-tetrakis(ortho-dichlorophenyl)poqhyrin. cyclohexane - 0.7mmo1, catalyst - 0.0025mmo1, MMPP:catalyst = 560, Bu,NCl:catalyst = 4, reaction time 45min., room temperature, aerobic conditions. 'turnover = [alcohol] &etone]/[catalyst].

+

The investigated manganese-porphyrin complexes appeared suitable catalysts for oxidation of cyclohexane under mild conditions. Their activities vary, however, to a large extent as a function of the degree of halogenation of the porphyrin ligand as well as on the presence of N-base. The data of Table 1 show that the introduction of the halogen substituents to the pyrrole rings of the manganese porphyrins bearing already such substituents on the phenyl rings brings about a decrease in the catalytic activity of manganese tetrapentafluorophenylporphyrin and does not affect tetra(orth0-dichloropheny1)porphyrin.

656

The decrease in the activity is generally followed by an increase in the selectivity to alcohol. The addition of the N-base such as 4-t-butylpyridine enhances the activity of all investigated systems. This effect is most evident in the case of the perhalogenated manganese porphyrin: Mn (TDCP-&Cl,P)Cl for which the highest yield of the products corresponding to 77% conversion of cyclohexane has been found. For the sake of comparison the results obtained for manganese tetramesitylporphyrin Mn(TMP)Cl are also given. The activity of this porphyrin in the investigated system is very low even in the presence of 4-t-butylpyridine. The influence of other N-bases such as n-hexylimidazole and imidazole was also examined. It appeared that the effect of n-hexylimidazole was comparable to that of 4-tertbutyl-pyridine while imidazole yielded rather poor results. The observed rise in the activity of the investigated systems was also dependent on the N-base : catalyst ratio. The best results were obtained for a ratio of 25. However, for the values within 0 - 1, at lower conversion of cyclohexane, higher selectivity to alcohol was observed. As it has already been mentioned the porphyrins bearing halogen substituents on both the phenyl and pyrrole rings are expected to exhibit higher stability in the oxidizing medium. In fact, spectrophotometric measurements performed at the beginning and at the end of the reaction, showed that, in contrast to tetramesitylporphyrin, manganese halogenated and perhalogenated porphyrins were still present after the reaction had been completed. However, the degree of the decomposition varied with the porphyrin applied. It has also been noticed that the presence of N-bases inhibited the process of the oxidative degradation during the reaction course. The order of the stability of the investigated porphyrins in the presence of the N-base is the following: Mn(TDCP-&Cl,P)Cl > Mn(TPFP-P-Br,P)Cl > Mn(TPFPP)Cl = (83 %) (67%) (35%) = Mn(TDCPP)Cl

> Mn(TMP)Cl (0%)

The numbers in the parentheses show the fraction of the initial porphyrin concentration found at the end of the reaction. In the absence of N-base the degradation was more important. As seen from Table 1 in the absence of the N-base manganese tetrapentafluorophenylporphyrin was the most active catalyst and the presence of 4-tbutylpyridine did not improve appreciably its activity. It should be noted, however, that the increase in activity on addition of N-base is simultaneous with the loss of selectivity to alcohol. In a separate experiment cyclohexanol was used as the substrate and the system appeared to carry out its oxidation to ketone. It may be thus concluded that in the presence of N-base the generated 0x0 species are more reactive. Surprisingly, the corresponding halogenated and perhalogenated iron porphyrins showed much lower catalytic activity than the manganese porphyrin. They appeared also less stable in the reaction medium except for Fe(TDCP-@-Br,P)Clwhich showed a stability similar to its manganese analogue. It should be noted that iron tetramesitylporphyrin was totally decomposed at the end of the reaction. For the sake of comparison some results obtained for iron porphyrins are given in Table 2. Some similarities in the behaviour of the iron and manganese porphyrins are observed. Thus iron tetrapentafluorophenyl-porphyrinis the most active catalyst in the absence of Nbase though, the yield of products is much lower than that found for its manganese analogue.

657

Table 2 Oxidation of cyclohexane in the presence of iron halogenated and perhalogenated porphyrins. Complex

Yield (mmol) alcohol ketone

0.090 0.017 Fe(TPFPP)CI Fe(TPFP-8-Br,P)Cl 0.0'75 0.015 Fe(TDCPP)CI 0.008 Fe(TDCP-8-Br,P)Cl 0.010 Fe(Th4P)Cl

alcohol ketone

5.3 5.0 00

00

Turnover

42.8 36.0 3.2 4.0

Conditions the same as in Table 1

The presence of the halogen substituents in the pyrrole rings results in lowering the catalytic activity in the case of iron tetrapentafluorophenylporphyrinand does not affect much that of iron tetra(orth0-dichloropheny1)porphyrin. In contrast to what was observed for the manganese porhpyrins, the presence of N-base in the system containing iron porphyrins brings about rather negligible and hazardous effects. This may be explained in terms of different coordinating abilities of manganese and iron porphyrins. These latter attain six coordinate geometry much more easily than five-coordinate manganese porphyrins and thus are more accessible for all potential axial ligands present in the reagent solution. The iron porphyrins seem to be, however, more selective catalysts to alcohol which in some cases is the only product of the reaction. It has been reported that increasing substitution of halogens into the porphyrin ring results in enhanced general oxidation activity of metalloporphyrins and has been rationalized in terms of increased electrophilic reactivity of the 0x0 intermediate towards the substrate and higher stability of the complex under the oxidizing conditions [6,9]. However, the investigated systems employed such oxidants as iodosylbenzene [6] or molecular oxygen [9] and the reactions were carried out in organic phase. Thus iron porphyrins containing perhalogenated pyrroles have been found to be much better catalyst for the hydroxylation of poorly reactive alkanes such as pentane or heptane by PhIO than the corresponding iron porphyrins without halogens on the pyrrole rings and yields as high as 80% were obtained [6].The relatively lower yields of the oxidation products obtained in our system with the perhalogenated porphyrins can be related to high affinity of these prophyrins for the potential axial ligands present in the investigated biphasic system and competing with the bulky molecules of the oxidant for the metal coordination sites. Especially, the presence of water and the hydroxyl groups, which are prone to form stable complexes with the prophyrin molecules, may exert an inhibiting effect on the catalytic activity of the perhalogenated metalloporphyrins. Our investigations on epoxidation of propene in the same systems and under the same conditions prove that the initial reaction rate is higher in the case of the perhalogenated porphyrins than that found for the correspondig halogenated porphyrins.

658

However, the perhalogenated porphyrins loose much faster their catalytic activity then their halogenated analogues, without loosing its porphyrinic structure as indicated by the presence of the Soret band [lo]. This finding supports the given above explanation for the observed lower total activity of the perhalogenated porphyrins. Further investigations on the interactions of the perhalogenated porphyrins with various oxidizing agents, which are currently carried out, may bring better understanding of the observed phenomena.

REFERENCES 1. 2. 3. 4. 5. 6.

7. 8. 9. 10.

D. Mansuy, Pure Appl. Chem., 59 (1987) 759.

B. Meunier, Chem. Rev., 92 (1992) 1411. D. Ostovic and T. C. Bruice, Acc. Chem. Res., 25 (1992) 314. T. G. Traylor and S. Tsuchija, Inorg. Chem., 26 (1987) 1338. T. G. Traylor and S. Tsuchija, Inorg. Chem., 27 (1988) 4520. J. F. Bartoli, 0. Brigaud, P. BattioniandD. Mansuy, J. Chem. SOC.,Chem. Commun., (1991) 440. D. Mandon, P. Ochsenbein, J. Fischer, R. Weiss, K. Jayaraj, R. N. Austin, A. Gold, P. S. White, 0. Brigaud, P. Battioni and D. Mansuy, Inorg. Chem., 31 (1992) 2044. 0. Brigaud, P. Battioni, D. Mansuy, New. J. Chem., 16 (1992) 1031. P. E. Ellis Jr. and J. E. Lyons, Coord. Chem. Rev., 105 (1990) 181. P. Battioni, R. Iwanejko, D. Mansuy and T. Mlodnicka, Proc. Europacat - 1 Montpellier, September 1993, vol. 1, p. 330.

V. Cortes Corberan and S. Vic Bellon (Editors), New Developmena in Selecuve Oxidation II

0 1994 Elsevier Science B.V. All rights reserved.

659

Polymer supported iron catalysts for the oxidation of cyclohexane Ki-Won Jun, Eun-Kyung Shim, Seong-Bo Kim and Kyu-Wan Lee' Catalysis Research Division, Korea Research Institute of Chemical Technology,

P.O. Box 9, Daedeog-Danji, Taejon 305-606, Korea Catalytic oxidation of cyclohexane was studied under the mild condition by using hydrogen peroxide formed from molecular hydrogen and oxygen in the original place. Iron chlorides were immobilized on poly (4-vinylpyridine) cross-linked with divinylbenzene and could be used successfully as the catalysts.

1. INTRODUCTION The development of efficient catalytic systems which are able to do the hydroxylation of alkanes under mild conditions has recently become of considerable interest 0 -81. Many monooxygenase-mimetic catalysts utilize transition metal complexes such as metallo-porphyrins [5-81. Homogeneous catalysts are often more active and more selective; however, they are often less durable and they are not readily separated from the reactants and products. Recently, several heterogeneous catalysts that are active for the hydroxylation under mild conditions have been reported [9-161. One of them is the bi-catalytic system which combines the ability of palladium metal to convert molecular hydrogen and oxygen into hydrogen peroxide with the ability of iron ions to use hydrogen peroxide to hydroxylate hydrocarbons [IS]. We have been applying the Fe-Pd bi-catalytic system to the oxidation of cyclohexane. The mixture of silica supported iron oxide and palladium was previously shown to be an efficient catalyst for the oxidation of cyclohexane using acetone solvent E17,181. Here we describe the results of om observation on the oxidation of cyclohexane with in situ produced hydrogen peroxide catalyzed by iron salts supported on 4-vinylpyridine-divinylbenzenecopolymer (PWDB).

*

To whom correspondence should be addressed.

660

2. EXPERIMENTAL 2.1. Catalyst preparation As a polymer matrix, a copolymer of 4-vinylpyridine and divinylbenzene (Koei Chemiacal Co., divinylbenzene content 6%) was used. Immobilization was carried out by stirring PWDB with a methanol solution of the required metal salts such as FeC12'4H20, FeCkj.6Hz0, Fe(N03)3,9Hz0, or FeS04.7Hz0 at reflux temperature for 15 h. The polymer, after filteration, was adequately washed with methanol and acetone and then dried in UCICLIO.The degree of anchoring was determined by atomic absorption analysis of the iron ions remained in the solution. The iron content of the samples was varied from 0.5 to 16 wt.-%. Two PdO/silica catalysts containing 0.6 and 1.3 wt.-% palladium were prepared by the impregnation of PdClz onto silica gel (Kiesel gel 60, surface area = 426 m2/g). The catalysts were subsequently dried at 15O'C for 2 h and calcined at 4OOC for 3 h in an air stream. 2.2. Catalyst charaterization X-ray photoelectron spectroscopic (XPS) measurement was carried out on a VG ESCALAB MK LT photoelectron spectrometer, which was equipped with Al anode operated at 15 KV and 20 mA. All binding energies (BE) were referenced to 284.6 eV of C 1s in the polymer support.

2.3. Cyclohexane oxidation The reaction was conducted in a round-bottomed flask equipped with gas bubbler and refrigerated condenser (maintained at -2O'c) as follows: a polymer supported catalyst and a PdO/silica were added to a solution of cyclohexane in acetone and the gases of hydrogen and oxygen were bubbled through the stirred reaction mixture at 30'C under atmospheric pressure. The products were analyzed by gas chromatograph over a 5% OV-17 on chromosorb column with a flame ionization detector. 3. RESULTS

AND DISCUSSION

3.1. Catalyst testing of polymer supported metal chlorides Preliminarily, iron(II) chloride, manganese(II1 chloride, cobalt(II) chloride, copper(II) chloride and ruthenium(III) chloride were supported on PVPDB and their activities were examined. Their testing results are given in Table 1. Amongst them, the catalyst FeCldPWDB exhibits the highest catalytic activity. The activity of other PWDB supported metal chlorides was found to be negligible or absent.

66 1

Table 1 Oxidation of cyclohexane with metal chlorideA'VPDB catalysts" Yieldb (mole%) Catalyst

~~

Cyclohexanol Cyclohexanone Total

FeCIflVPDB

2.43

0.51

2.94

MnCIdPVPDB

0.25

0

0.25

CoCIdPVPDB

0.21

0

0.21

CuCWPVPDB

0

0

0

RuCWVPDB

0

0

0

~

"Condition: metal chloridflVPDB, 1 g; PdO/silica (Pd 1.3%), 1 g; cyclohexane, 5 g; acetone, 20 ml; Hz, 20 d m i n ; 0 2 , 20 d r n i n ; temperature, 30'C; reaction time, 3 h. 'Yields are based on cyclohexane.

3.2. XPS results X-ray photoelectron spectra were acquired in order to investigate possible variations in the BE of nitrogen or iron electrons (Table 2). The Fe 2pyz binding energies of polymer supported iron salts do not show significant variations, whereas the N Is binding energies show differences depending on the catalyst samples. Table 2 BE of Fe 2pm and N 1s electrons measured by XPS Sample

BE (eV) Fe 2Pm

N 1s

~~~~~

711.7

-

-

398.7

FeClflVPDB (Fe 12%)

711.1

398.7

FeCWVPDB (Fe 9.6%)

711.1

399.2

Fe(N03)dPVPDB (Fe 9.5%)

711.2

400.7, 406.3

FeSO4/pvpDB (Fe 16%)

711.3

401.3

Fec12'4HzO

PVPDB

Torresponding to No3'

662 Fig. 1. shows the N 1s photoelectron spectra of the catalysts corresponding to pyridyl-group nitrogen. For the pure PVPDB, a simple band at BE of 398.7 eV is observed. As the Fe content of FeCWVPDB is increased, the complicated bands are appeared indicating that two or more types of nitrogen exist. The bands at higher binding energies are interpreted to be attributed to the coordination of pyridyl-group nitrogen towards iron ion. The observed BE-shift of N 1s electrons increases in the order: FeCldPVPDB < FeCWVPDB < Fe(N03)dPVPDB < FeSOdPVPDB. This sequence should be in accordance with the increasing strength of pyridyl group ligation to iron salt which seems to be varied by the electron-withdrawing power of counter anions C1-, NO3- and SO:-. The surface Fe/C and N/C ratios of FeCldF'VPDB samples evaluated by XPS are plotted against iron content in Fig. 2. It is readily seen that the surface Fe/C is almost proportional to the iron content. The surface N/C ratio decreases as the iron content is increased up to 4.6% which corresponds to the pyridyVFe ratio of 9, thus indicating that the pyridyl nitrogens is covered selectively with the iron chlorides. However, the surface N/C ratio is increased finally as the iron content is further increased. This suggests that all the iron chloride cannot exist immediately on the pyridyl nitrogen because of the steric hindrance of cross-linked polymer chain. It is likely that there are two or more types of coordination, i.e. strong and week coordinations. This is also supported by the complicated bands of N 1s photoelectron spectra as shown in Fig. 1.

3.3. Catalytic activity of polymer supported iron compounds Table 3 shows the performances of the PVPDB supported iron salt catalysts for the oxidation of cyclohexane. The different catalysts show different activities depending on the kind of iron salt. The catalytic behaviors of PVPDB supported iron nitrate and sulfate are not consistent with the previous observation about their pure iron salts [181: iron(III> nitrate itself is inactive for this reaction while iron(II> sulfate itself is more active than iron(II) chloride. It is most likely that the catalytic behaviors of PVPDB supported iron salts are affected by pyridyl ligand. The order of activity is FeCldPVPDB > FeCUVPDB > Fe(N03)dPVPDB > FeSOdPVF'DB: the sequence being in accordance with the increasing BE-shift of N 1s electrons observed in XPS. This implies that the strong pyridyl ligation to iron salt lowered catalytic activity.

663

405

403

401

399

397

395

393

Binding energy (eV) Fig. 1. Nitrogen 1s photoelectron spectra of (a) PWDB, (b) FeCldPWDB (Fe 0.5%), (c) FeCWVPDB (Fe 2.5%), (d) FeCWVPDB (Fe 4.6%), ( e ) FeCld PVPDB (Fe 12%), (f) FeCld"VPDB (Fe 9.6%), (g) Fe(N03)d'VPDB (Fe 9.5%), (h) FeSOdPWDB (Fe 16%).

664

- 0.02 1

0

.

2

I

.

4

I

.

I

6

.

8

I

.

10

0.00 12

Fe content (wt %)

Fig. 2. The Fe/C and N/C ratios of FeCldPVPDB obtained from XPS.

Table 3 Oxidation of cyclohexane with iron salt/PVPDB catalysts“ Fe Catalyst

Yieldb (mole%) Cyclohexanol

Cyclohexanone Total

FeCldPVPDB (Fe 12%)

2.25

0.25

2.50

F e C W W D B (Fe 9.6%)

1.49

0.07

1.56

Fe(N0MPWDB (Fe 9.5%)

0.51

0.10

0.61

FeSOdPWDB (Fe 16%)

trace

“Condition: iron salVPWDB, 1 g; PdO/silica (Pd 0.6%), 1 g ; cyclohexane, 5 g; acetone, 20 ml; HZ, 20 ml/min; 02, 20 d m i n ; temperature. 30°C; reaction time 3 h. bYields are based on cyclohexane.

665

The total yield of cyclohexanol and cyclohexanone, and the catalyst turnover are presented in Fig. 3 as a function of the Fe content in FeCM PWDB. The yield increases upto a Fe content of 4.6% and then remains almost unchanged. The catalyst turnover reaches a maximum for the Fe content of 2.5%. The fact that the Fe content = 0.5% catalyst shows relatively low catalyst turnover can be interpreted by that excessive coordinating pyridyl nitrogen due to the high pyridyne ratio inhibits the oxidation with iron ions. When the iron content is hqh, however, attached iron chlorides seem too many to be exposed mostly on the surface.

h

CH

0

PaJ

a r t

h

I

0

2

.

4

I

6

.

,

8

.

,

10

.

~0 12

Fe content (wt %)

Fig. 3. Cyclohexane oxidation to cyclohexanol and cyclohexanone with polymer supported iron(II> chloride. Condition: FeCM'VPDB, 1 g; PdO/silica (Pd 1.3%), 1 g; cyclohexane, 5 g; acetone, 20 ml; €32, 20 d m i n ; 02, 20 rnl/min; temperature. 30°C; reaction time 3 h.

666

ACKNOWLEDGMENT We wish to thank Prof. Y. Kurusu for genernously providing copolymer samples of 4-vinylpyridine and divinylbenzene.

REFERENCES 1. R.H. Crabtxee, Chem. Rev., 85 (1985) 245,and references therein. 2. D.H.R. Barton, M.J. Gastiger and W.B. Motherwell, J. Chem. Soc., Chem. Commun., (1983) 731. 3. D.H.R. Barton, J. Boivin, M. Gastiger, J. Morzycki, R.S. Hay-Mothemell, W.B. Motherwell, N. Ozbalik and K.M. Schwartzentmber, J. Chem. Soc. Perkin Trans. I, (1986) 947. 4. A.M. Khenkin, V.S. Belova and A.E. Shilov, Catal. Lett., 5 (1990) 211. 5. D. Mansuy and P. Battioni in C.L. Hill (Editor), Activation and Functionalization of Alkanes, John Wiley, New York, 1989, p. 1%. 6. D. Mansuy, J.-F. Bartoli, J.-C. Chottard and M. Lange, Angew. Chem. Int. Ed. Engl., 19 (1980) 909. 7. S. Banfi. A. Maiocchi, A Moggi, F. Montanari and S. Quici, J. Chem. Soc., Chem. Commun., (1990) 1794. 8. M.J. Nappa and C.A. Tolman, Inorg. Chem., 24 (1985) 4711. 9. N. Herron, G.D. Stucky and C.A. Tolman, J. Chem. Soc., Chem. Commun., (1986) 1521. 10. T. Tatsumi, M. Nakamura and H. Tominaga, Catalysis Today, 6 (1989) 163. 11. V.S. Belova, A.M. Khenkin, V.N. Postnov, V.E. F’rusakov, A.E. Shilov and M.L. Stepanova, Mendeleev Commun., (1992) 7. 12. Y. K u s u and D.C. Neckers, J. Org. Chem., 56 (1991) 1981. 13. L. Barloy, P. Battioni and D. Mansuy, J. Chem. Soc., Chem. Commun., (1990) 1365. 14. C. Bowers and P.K. Dutta, J. Catal., 122 (1990) 271. 15. M.G. Clerici, Appl. Catal., 68 (1991) 249. 16. N. Herron and C.A. Tolman, J. Am. Chem. Soc., 109 (1987) 2837. 17. K.-W. Jun and K.-W. Lee, Chemie Ingenieur Technik, 64 (1992) 637. 18. K.-W. Jun, K.-W. Lee, E.-K. Shim and N.-S. Cho, Appl. Catal. A, 96 (1993) 269.

V. Cortks Corberan and S. Vic Bell6n (Editors), New Developments in Selective Oxidation II 1994 Elsevier Science B.V.

667

SELECTIVE OXIDATION OF 2-MERCAPTOBENZOTHIAZOLE Milan Hronec, Magda Stolcova and Tibor Liptay Department of Organic Technology, Slovak Technical University 812 37 Bratislava, Slovak Republic

Abstract Preparation of 2-benzothiazolesulfonic acid and sulfenamides derived from cyclohexylamine, morpholine and tert-butylamine by metal catalyzed oxidation with molecular oxygen or hydrogen peroxide is described. At moderate reaction conditions these compounds are produced in 80-95 % yield. Continuous oxidation of MBT with H 2 0 2 is very sensitive to pH of the reaction medium and molar ratio of H202/MBT. In catalyzed oxidation the type of metal catalyst, amine and solvent as well as the partial pressure of oxygen have an important impact on the rate and selectivity. We suggest that the rate determining step is reoxidation of the metal catalyst by oxygen.

Introduction 2-Mercaptobenzothiazole (MBT) is a starting material for the preparation of e.g. 2,2’-dithiobis(benzothiazole), various sulfenamides, N-alkylbis(benzothiazolylsu1fen) amides, o-aminothiophenol, benzothiazolesulfonic and sulfinic acids. These materials are used as accelerators for rubber vulcanization, as intermediates for the production of antioxidants, regulators of plant growth, etc. Depending on the type of oxidizing agent, composition of reaction medium and operating conditions MBT is oxidized to a variety of products (Scheme 1). Thus, in aqueous, alcoholic or hydrocarbon solutions, MBT or its salts are oxidized, e.g. with permanganates, hypochlorides or hydrogen peroxide, to benzothiazolesulfonic acid [1,2], 2,2’-dithiobis(benzothiazole),[3] or in the excess of primary or secondary amines to corresponding sulfenamides [4]. Commercial processes use as an oxidizing agent preferrably an aqueous solution of NaOC1. Although these technologies produce the products with very high yields serious ecological problem is the formation of a great amount of salty water. This can be solved by using clean oxidizing agents, e.g. hydrogen peroxide or molecular oxygen. In the former case, the limiting factor is the price of H202. The most attractive way is the oxidation with molecular oxygen in the presence of metal catalysts. In alcoholic or aqueous media in the presence of ammonia or lower trialkylamines (e.g. tri- linebreak methylamine), the product is 2,2’-dithiobis(benzothiazo1e)[5]. In the excess of primary or secondary amines selectively are formed sulfenamides IS].

668

Scheme 1

+ H2N-R

In the present paper is described the selective oxidation of MBT with HzOz and molecular oxygen in the presence of metal catalysts to benzothiazolesulfonic acid and sulfenamides, respectively.

Experiment a1 Chemicals 2-Mercaptobenzothiazole was a commercial product with a purity of 98.7 %. Amines ( 2 9 9 %), cyclohexylamine, morpholine, t-butylamine, diisopropylamine, 2-methylmorpholine were purified by distillation and stored under nitrogen. Potassium salt of 2-benzothiazolesulfonic acid (BTS03K) and sulfenamides derived from the tested amines were purified by crystallization. Hydrogen peroxide (32 %), solvents and other reagents were used without purification, Catalysts. Ni(I1) acethylacetonate [Ni(AcAc)z]was synthetized according to [7].Metal phthalocyanines (MPc) and cobalt phthalocyaninetetrasulfonate (CoTSPc) were prepared by the method described by Weber and Bush [8]. Other metal catalysts were of analytical grade purity. A p p a r a t u s a n d procedure. Oxidation of MBT with H 2 0 2 was carried out continuously in a 50 ml glass reactor equipped with a stirrer (2 500 rpm) and electrodes for pH measurement. Reactants, aqueous solution (ca 15 %) of potassium salt of MBT in excess of KOH and aqueous solution of hydrogen peroxide (32 %) were separately pumped into the reactor with peristaltic pumps using teflon tubings. The reaction mixture overflowed from the reactor into the vessel, which was periodically exchanged (in 1-3 hr intervals), and its contents weighted and analyzed for products and peroxides. The reactor and all tubings were heated in a water bath. Catalytic oxidations of MBT with oxygen or air were carried out in a 250 ml stainless steel reactor fitted with a magnetic impeller system operating up to 3 000 rpm and having the air inlet at the bottom and outlet through a condenser, pressure and

669 temperature regulator, electric heating mantle and outlet for products probe. The teniperature of reactants charged in the reactor was increased gradually (8-10 min) under applied air or oxygen pressure. The rate of oxygen consumption was measured in a 50 ml stainless steal reactor connected with a flexible metal capillary to the apparatus for measuring the oxygen consumption at constant pressure. The rate was measured over the range of speed of reactor agitation where transport fenomena did not limit the reaction rate. Analyszs. The main product of MBT oxidation in aqueous KOH solution with HzOz is potassium salt of 2-benzothiazolesulfonic acid. This salt was isolated from the reaction mixture by crystallisation at 10 "C overnight. The white crystals after filtering off and washing with cooled water were dried at 50 "C, weighted and analyzed by HPLC (interiial standard 4-methoxybenzylalcohol) and NMR. The ammount of BTS03K soluble in inother liquor (ca. 5-8 %) was determined by HPLC and included for calculation of the yield. Concentlation of peroxides (calculated as H z 0 2 ) in the reaction mixture was determined iodometricaly in samples withdrawn from the reaction mixture and quickly inserted into iodine solution. Sulfenamides obtained by oxidation of MBT and amines were isolated from the reaction mixture by precipitation with water to ca. 20 % concentration of amine in the solution. Both the procedure and the method of analysis of products were the same as described previously [9]. Identification of corresponding sulfenamides was carried out by NMR spectroscopy.

Results and discussion Prelimiriary results of MBT oxidation with HzOz showed that the reaction performed discontinuously and continuously provides different results. Since experiments from continuous reactor provide higher reproducibility and direct rate data this type of reactor was applied for further studies. In Fig.1 is shown the influence of the molar ratio of H?O;?/MBT on the yield of BTSOSK at two ratios of KOHIMBT. As can be seen the same yield of BTSO3K is obtained at lower H 2 0 z / M B T ratio, ca. 3.5 when the KOH/MBT ratio is lower. The stationary concentration of peroxides in the reaction mixture is independent of KOH concentration and it is almost zero also at high Hz02/MBT ratio . However, when KOH/MBT ratio is 5 1.35 the yield of BTSOSK significantly decreases and the product is dark and sticky. Obviously at pH 5 7.5, oxidation of MBT proceeds with low selectivity. Very high rate and selectivity of MBT oxidation with HzOz are evident from Fig.'. As it is seen the space-time yield of the continuous stirred reactor in mol of formed BTS03K per volume of liquid in the reactor per hour is very high and conversion of MBT is practically complete. In the absence of strong alkali hydroxides and preferrably in alcoholic media, oxidation of -SH group in MBT with HzOz proceeds gently. In this case the main reaction product is 2,2'-dithiobis(benzothiazo1e). However, if the reaction mixture contains also primary or secondary amines the final product is the corresponding sulfenamide. The mentioned products of MBT oxidation can be obtained also by catalytic oxi-

670

dation of MBT with molecular oxygen. It necessitates the presence of selective metal catalyst which catalyzes the oxidation of the thiol group only to desired oxidation step. 100

'

0

°

Y

2

4 Y1

rnolw Y2

Wtw

50

2

0

Fig.1 The effect of molar ratio of H202 / MBT on the yield (yl) of BTSO3K and concentration of peroxides in the reaction mixture at various KOH /MBT ratios. Conditions : 55 "C, volume of liquid in the reactor 15 ml, flow of aqueous mixture KOH MBT (15.3 mol %) 33.8 g h-l ,KOH / MBT (0)1.5 : 1, (.)2: 1

Fig2 Space-time yield of continuous reactor in mol BTS03K dmP3h-' Conditions : 55 "C, molar ratio H 2 0 2 /MBT = 4.5 : 1, KOH : MBT =1.5 : 1

+

Table 1 Effect of the type of metal catalyst on the yield of sulfenamides and its purity Catalyst

Amine"

M B T : amine (moll

T "C

Cu-acetate Cy clohexylamine 1: 7 75 Co-acetate Ni( AcAc)2 MnPc CoC12 Morpholine 1 : 4.2 65 COPC 1 : 4.2 60 1 : 6.5 65 CoTSPc t-Butylamine a catalyst 0.15 wt %, 2.1 % water, oxygen pressure 3 atm

Yield mol%

Assay

70.6 36.7 21.1 71.9 86.4 89.7 93.1

91.6 99.1 92.2 95.2 96.9 97.2 98.1

%

In Table 1 is shown the effect of the catalyst type on the rate of oxidative condensation

67 1

of MBT with amines, yield and purity of products. Homogeneous catalyst on the base of cuprous salts are very active. However, at temperature above 50-60 "C due to overoxidation of formed sulfenamides the selectivity of oxidation is low. High activity and selectivity possess some metal phthalocyanines. The pressence of sulfa groups in the peripheral site of the phthalocyanine ring has highly positive effect. With these catalysts the yield of sulfenamides is above 90 mol 9% and the purity of products after precipitation and simple washing with water about 97-98.5 %. Differences between selectivities of metal phthalocyanines and metal salts are more significant at higher reaction temperatures (Fig.3). Metal phthalocyanines are sufficiently selective also at temperature 75 "C. The influence of catalyst concentration on the product yield is similar for all catalysts (Fig.4) and passes through a maximum.

Fig.3 Influence of temperature on the yield of N-cyclohexyl-2-benzothiazole sulfenamide catalyzed by copper acetate ( 0 ) and CoPc ( 0 ) . Conditions : MBT / cyclohexylamine 1 : 8.5 ; catalyst 0.15 wt % ; HzO 2.1 %. po2 = 3 atm

Fig.4 The effect of catalyst concentration. Conditions, see Fig.3, 65 "C, (0)Cu acetate (a)CoPc; (e)CoTSPc

The amine used for the preparation of sulfenamide has two functions. It serves as a reagent and also as a solvent for MBT and products. Fig.5 reveals the dependence of the rate of oxidation on the type of amine. The sequence of the reaction rate: morpholine > cyclohexylamine > 2-methylmorpholine > diisopropylamine does not correlate with the basicity of the amines. As conductivity measurements show, there is some dependence between the rate and the concentration of thiol anions formed from MBT in the corresponding amine. In order to investigate the role of amines as a solvent, the effect of polar and nonpolar solvents on the oxidation process was investigated, at constant molar ratio of MBT:morpholine . From the results it is obvious (Fig.G), that in the presence of toluene

672

or dimethylformamide the rate of oxygen consumption is higher than in pure morpholine

(Fig.5). In alcoholic solvents the reaction proceeds very slowly. However, higher rates of oxidation in the presence of some solvents are accompanied by the formation of higher ammount of by products. For example, in comparison with experiments in pure morpholine, the selectivity of sulfenamide formation drops by about 8 % in the presence of toluene as a solvent and by more than 30-40 ’% in acetone. This sharp decrease of selectivity was observed also with other amines. 400

600 02

02

cm3

cm

400 200 200

I L

0

50

Fig.5 Oxidation of MBT in different amines catalyzed by CoTSPc. Conditions see Fig.3, 1 - morpholine 2 - cyclohexylamine, 3 - 2-methylmorpholine, 4 - diisopropylamine

t,

min

100

Fig.6 Oxidative condensation of MBT and morpholine (1:l.l mol) in 1 - toluene, 2 - dimethylsulfoxide, 3 - methylethylketone, 4 n-butanol, 5 - only morpholine. Conditions, see, Fig.3, catalyst CoPc, 65°C -

Kinetic measurements confirmed that the rate of oxygen consumption increased with partial pressure of oxygen in the range 1-6 atm. The catalyst and its type plays an important role in oxidation process of MBT with amines but does not decompose during the reaction. After isolation from the reaction medium it can be used several times without any loss of activity. On the bases of the results and the literature data about thiols oxidation, the mechanism of catalytic oxidative condensation of MBT with amines can be depicted by the following reaction scheme

ArSH

ArS-

+ HzNR + PcM +

A r S - ...P c M ...0

ArS-

+

2

----f

ArS-

P c M...0:

-

R

ArS- ...PcM ...0

0 2 -----t

+ H3N+

ArS-

-+

+

ArS-

(1) 2

(2)

P c M ...0;

(3)

+

(4)

PcM ...0;-

673

Pc M... 0;-

+

BATS.

ilrSSAr

+

ArSSAr

+

2H3N'-R

HzNR

PcM

+

H20

+

11202

+

2RNH2

(5)

(6) ----f

ArSNH-R

+

HSAr

(7)

It is known that metallomacrocycle complexes easy coordinate in axial position strong electron donors [lo] e.g. thiolate anions or amines and thus favour the activation of molecular oxygen. We suggest that in the mechanism of MBT oxidation both formation and decomposition of the ternary complex between metal catalyst, thiolate anion and molecular oxygen (steps 2 and 3) play a dominant role. The rate determining step is reoxidation of catalyst by molecular oxygen. The observed effect of the type of amine and solvent on the rate of oxidation is probably connected with their influence on the concentration of thiolate anions in solution and the formation of the ternary complex. The first stable product of MBT oxidation is 2,2'-dithiobis(benzothiazo1e). However, in excess of amine it is converted to corresponding sulfenamide without participation of catalyst. Amines used for the preparation of sulfenamides determine besides the selectivity and th? rate of oxidation the stability of formed sulfenamide against subsequent oxidation which proceeds on sulfur atom to undesirable producti.. Similar influence has also soin? solvents.

References

1. US Pat. S o . 2. 179 897 (1937). 2. T. Kimijima , J . Soc. Chem. Ind. Japan, 45 (1942) 957. 3. M.J. Camille. Eur. Pat. No. 08.548 (1982). 4. R.F.A. Poomans, P.G.J. Delbrassinne and A.S. Cobb, Eur. Pat. No. 314 663 (1988). 5 . H. Zongel, hI.Bergfeld and L. Eisenbuth, DE Pat.No. 2.944.225 (1981). 6. S.A. Cobb and J.D. Williams, Eur. Pat. No. 029.718 (1981). 7 . R.G. Charles and M.A. Pawlikowski, J.Phys.Chem., 62 (1958) 440. 8. J.H. Weber and D.H. Busch, Inorg. Chem. 4 (1965) 469. 9. M.Hronec and L.Malik, J. Mol. Catal., 35 (1986) 169. 10. E.C. Niederhoffer, J.H. Timmons and A.E. Martell, Chem. Rev., 84 (1984) 137.

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V. CortCs Corberan and S. Vic Bellon (Editors), New Developmenu in Selective Oxidulion II 0 1994 Elsevier Science B.V. All rights reserved.

SELECTIVE OXIDATION

675

OF GASEOUS HYDROCARBONS BY MICROBIAL CELLS

G.A. Kovalenkoa, V.K. Sokolovskiib "Institute of Catalysis RAN, Novosibirsk, 630090, Russia. bUniversity of Witwatersrand, Chemistry Department, 2050 WITS, Johannesburg, RSA.

Selective oxidation of methane and propene by methanotrophic bacteria, possessing the monooxygenase activity has been investigated. It was shown, the formation of the partial oxidation products (methanol, propylene oxide) occurred with 100% selectivity in the mild reaction conditions. Immobilization of microbial cells on inorganic supports was carried out.

1.

INTRODUCTION

The direct partial oxidation of saturated and unsaturated hydrocarbons to corresponding oxy-product: alcohols, aldehydes, epoxides performed by some microorganisms'strains can be of a great interest for the chemical industry of organic synthesis because of the high selectivity of biocatalysis and the biotechnological manufacture is energy-saving (owing t o mild reaction conditions) and ecologically secure for environment. The advantage of biocatalytical processes may be illustrated by the comparison of the reaction parameters o f the direct methane oxidation t o methanol via the chemical and biochemical methods (Table 1). Table 1. Comparison of the reaction parameters of the methane partial oxidation to methanol via the chemical and biocatalvtical Drocesses. Reaction parameters

Chemical process [ I I

Biocatalytical process [2,31

Temperature, OC Pressure, atm Methane conversion, YO Methanol selectivity, YO

450 - 550 25 - 65 8 - 10

2 0 - 40 1 27 - 61 100

38 - 83

The main disadvantage of biotechnological processes seemed t o be a low concentration of product formed in reaction medium, for example, methanol concentration did not exceed 1 YO[41.

616

The direct oxidation of methane t o methanol is catalysed by methane monooxygenase enzymatic complex of methanotrophic microorganisms [41. In the case of olefin oxidation b y the microorganism strain, epoxide formation occurs. As the heterogeneous catalytic process is more available than homogeneouos one the immobilization of microorganism strains on the solid carrier is the important step for their practical application. The economic validity of the heterogeneous biotechnological process based on immobilized bacterial cells depends strongly on the cost of biocatalysts developed. Methods of adsorption of microorganisms on inorganic supports which are cheap, mechanically durable and biologically inert seems as very promising for this aim [51. In the present study, the biocatalytic activities of some strains of methaneutilizing bacteria and reaction conditions were investigated for partial oxidation of methane to methanol and propene to propylene oxide. Methods of immbolization of these microorganisms on inorganic supports were developed.

2.

EXPERIMENTAL

2.1

Microorganisms Bacterial cells of methanotrophs Methylococcus capsulatus IMV 3021 , Methylosinus trichosporium IMV 301 1, Methylomonas sp. GY-J-3 were grown in a medium composed of mineral salts: potassium nitrate, 1 g/l; potassium dihyrophosphate, 0.4 g/l; dipotassium hydrophosphate, 0.4 g/l; sodium chloride, 0 . 3 g/l; magnesium sulphate, 0.3 g/l; calcium chloride, 0.02 g/l; ferric chloride, 0.001 g/l. The medium with 10 YO cells inoculum was saturated b y gaseous mixture containing 1 0 - 20 YO vol. of growth substrate (methane, propane, ethylene) in air via bubbling through the medium for 10 -1 5 min. The the 5 0 0 ml Erlenmeyer flasks were shaken on a rotary platform at 3 0 OC and 1 5 9 rpm for several days with the cell suspension being resaturated b y gaseous growth substrate mixture every 2 days of cultivation. Cultures were harvested b y centrifugation at 10,000 g for 1 5 min and resuspended in 20 n M phosphate buffer PH 7.0 - 8.0 for performing oxidative biotransformation of methane and propylene. 2.2

Immobilization

Immobilization of microorganisms was performed b y adsorption of bacterial cells on solid mineral-based supports. Carbon-mineral supports were obtained by successive carbonization of mineral matrix during unsaturated hydrocarbons pyrolysis 161. Bacterial cells were adsorbed under the following conditions: ambient temperature, occasional stirring for 1 8 - 20 h, with a ratio of the adsorbent weight t o the cells suspension volume of 1 : 10. The optical density loss during the adsorption process was taken as a measure of the extent of adsorption. Cells adhesion o n the glass vessel walls during assay did not exceed 5 % of initial cells amount. The approach of double immobilization via "embedding" of enzymes in the pores of inorganic carriers b y silica hydrogel was developed in the previous w o r k [71.

2.3

Determination of Biocatalytical Activity The activities of resting cells in suspension was determined at 32OC in the

671

hermetic stirred-tank, 5 0 ml bioreactor, which contained 1 0 m l of cells suspension ( 1 -8 m g of cells per ml). Sodium formate (20 m M ) as a co-factor (electrons donor) was added t o the reaction medium. All hydrocarbons studied and oxy-products produced were analyzed b y gas chromatograph w i t h the flame ionization detection. The 2m-long column was packed w i t h Porapak Q (or N). The temperature was set at 150OC. The activity of immobilized bacterial cells was determined at 32OC b y a circulation system that consisted of a series-connected peristaltic pump, 5 0 ml jacketed magnetically stirred glass mixing vessel that contained 10 m l of 20 m M phosphate-buffer p H 6.0 - 8.0, and packed-bed glass column bioreactor filled w i t h the heterogeneous biocatalysts (5 -6 9 ). Gaseous mixture (hydrocarbon and oxygen) was passed through the mixing vessel t o saturate buffer solution. Ten-microliter samples of the outlet stream of the bioreactor were injected t o the chromatograph column to determine concentration of products.

3.

RESULTS AND DISCUSSION

3.1

The native microorganisms' activities Biocatalytic activities of resting cells of methanotrophs were tested at 32OC in the reaction media: 0.02 M phosphate buffer, p H 7.0; 20 mM sodium formate; 0.2 m M dissolved oxygen. For the reaction of propylene epoxidation the concentration of propylene in the solution was 3.0 mM, for the reaction of methane partial oxidation the concentration of methane dissolved was 5.5 p M . The results obtained are presented in Table 2.

Table 2 Biocatalytic activity of some strains of methanotrophs in the reactions of selective oxidation of methane and Dropylene (reaction conditions are reported in the text). Rate of product formation, nmol/min, m g cells (dry weight) Microorganisms CH,

.+

CH30H

C3He + CHS-CH---CH, 0

Meth ylococcus capsulatus IMV 3021

25.9

31.6

Meth ylosinus trichosporium IMV 3 0 1 1

18.3

14.4

Meth ylomonas sp. GY -J - 3

19.5

20.7

In the case of methane oxidation the inhibitor of the methanol dehydrogenase activity of methanotrophs was added. A number of inhibitors were tested: the high

67 8

concentration of phosphate-ion (100 mM), sodium chloride (100 mM), iodacetate ( 3 m M ) and ethylenediamine tetracetic acid (EDTA, 1 - 40 mM). The better results were obtained with ethylenediamine tetraacetic acid. This inhibitor was added in reaction media at concentration of 10 mM. The tested microorganisms perform the oxidative biotransformations of methane to methanol and propylene t o propylene oxide w i t h 100 % selectivity, n o other oxyproducts were detected in reaction media under corresponding conditions studied. One can see f r om Table 2, the rates of the reactions of C-H bond oxidation and double C = C bond epoxidation have similar values. Taking into account the essential difference of hydrocarbon concentration in the reaction medium (3.0 mM for propene and 5.5 v, M for methane) th e possibility of the limitation of the reaction by a substrate diffusion can be rejected. It can be supposed that the rate-limiting step of both reactions is the formation of oxidizing active species in the process of oxygen activation with a co-factor participation. The general scheme of the reactions can be presented as following:

1.

DH2

2.

EO

*

+ 02 + + R --

*

lim. E

----+

-+

RO

+

EO

+

D

E

where DH2, D - reductive, and oxidative forms o f co-factor;

*

E, EO - free, and bonded t o an active oxygen species monooxygenase; R, RO - hydrocarbon, and selective oxidation product (alcohol or epoxide). The activity observed depended on the microorganisms’ grow th phase at the point of harvesting and o n the cell concentration in the reaction medium. The maximum enzymatic activity was found t o be observed in the log-phase of the intensive gr owt h of bacteria studied and at the suspended cells concentration in the reactor below 1 mg of cells (dry weight) per ml. The decrease of the rate of the reaction w i t h time occurred during methane to methanol oxidation in the stirred-tank bioreactor under the conditions studied (Fig. 1) because of the depletion of substrate and co-factor during reaction. Addition of these components t o the reaction medium caused the increase of the rate observed. The rate of propylene oxide formation was also decrease with time (Fig.2). In this case the reason can be the well k n o w n toxic effect of product o n monooxygenese [8].Another reason was (as for the reaction of methane oxidation) a consumption of co-factor (NADH) during propene epoxidation. The addition of external NADH regenerating cosubstrate-sodium formate in the reaction medium restored the monooxygenase activity up to 90-100 % (Fig.2)

3.2

Immobilization of bacterial cells The development of an economically feasible biotechnological process includes a design of heterogeneous biocatalyst b y the bacterial cells immobilization. As

679

c

2 8 E

1

1 :

1

5 6

Reaction Time, hours Fig.1. Kinetics of methanol formation b y Methylosinus trichosporiurn IMV 301 1. Reaction conditions as in Table 2.

ZI

E

Reaction time, hours Fig. 2 Kinetics of propylene oxide formation b y Methylococcus capsulatus IMV 3021. Reaction conditions as in Table 2.

A - addition of sodium formate ( 10 mM) t o reaction medium; A - sodium formate w a s not added preliminary. Arrows indicate addition of sodium formate.

680

mentioned methods of adsorption on inorganic supports possess a high commercial potential [5]. The adsorption ability of the cells of the methanotrophic bacteria on three groups of supports based o n silica, alumina, mineral, carbon has been studied in this work. The first group of carrier was formed b y polar supports based on silica and alumina that were distinguished by specific surface area and pore size. The second group consisted of carbonized mineral supports biphilic b y nature w i t h hydrophobic centres on the surface, hydrophobic centres being formed b y carbon particles deposited on the walls of large pores during carbonization. The third group included only carbon supports. Microorganisms adsorption occurred predominantly on the geometrical surface of the support particles. Actually, the eightfold decrease of the average diameter of the support particles from 0.55 m m t o 0.071 m m by grinding caused more than the four-times increase of Methylosinus trichosporium IMV 3011 adsorption value o n carbonized alumina. The adsorption value of microorganisms studied on the mentioned three groups of supports are presented in Table 3.

Group

Specific area 2 m/g

Amount of cells adsorbed,

YO

Polar supports SiO, 110 10-1 8 0 - A1203 7 0 Table 3.e - ~ 1 2 0 3 55-84 0 Adsorption of Methylosinus trichosporium IMV 2301 y - A1203 0 0 1 on inorganic supports. 1(Ambient 3 temperature, 20 m M phosphate buffer pH 7.0, duration of cells adsorption 18-20 h, Biphilic supports 1 .O-I.2 mg/ml). initial cells concentration 0 . 3 % C on 8 , y - A1203 0 73-1 7 3 7 - 1 4 % C on 8 , y - A1203 84-1 8 0 22-30 20 % C on 8 - A1203 100 26 5-10 % C o n aluminasilica mineral 115 28-32 Carbon support Activated carbon P-29

384

20-36

68 1 were better adsorbents than a- and @-alumina, probably because of the enhanced acidity of the surface (compared with high-temperature a, @-modificationsof alumina) that provided electrostatic cells-supports interactions for the cells binding on the polar surfaces. The adsorption of microorganisms on carbonized supports was rather tight and irreversible. Amount of desorbed cells did n ot exceed 7-10 % of the adsorption value w h e n phosphate buffer p H 7.0-8.0 or solutions of sodium dodecylsulfate, sodium chloride, ethanol and distilled water were used as a medium for desorbing cells. A f e w techniques for cells immobilization on inorganic supports can be used to obtain heterogeneous biocatalyst: i) adsorption in sterile conditions of biomass grown and harvested previously; ii) cell cultivation in the presence of optimal adsorbent; iii) preliminary cells adsorption, then subsequent cultivation of already adsorbed cells; and iv) double immobilization of cells. As it was s h o w n earlier the biocatalyst obtained by double immobilization via "embedding" enzymes inside pores of inorganic supports b y silica hydrogel combine effectively the advantages of inorganic support (mechanical strength, good hydrodynamics parameters) w i t h a high biocatalytic activity and stability [71. This approach was performed for immobilization of bacterial cells in the present study. Biocatalytic properties of microorganisms w i t h respect t o propylene epoxidation are presented in Table 4.

Table 4. Biocatalytic properties of bacterial cells immobilized on carbonized silica-alumina. Reaction conditions: 32OC; 2 0 m M phosphate buffer pH 7.0, saturated b y mixture of 2 0 YOpropene, 2 0

Methods of immobilization

Meth yiosinus trichosporium IMV 301 1 A imm,*

1

Meth yiococcus capsuiatus IMV 3021

A rel,% **

I

A imm,*

A rel,

YO

Adsorption

15

Cultivation in the presence of adsorbent

32

Preadsorption w i t h following cultivation

45

Double immobilization

103

+* *A i m m - activity of immobilized cells, nmol/min/mg of dry cells; A re1 = A imm/A susp, A susp

-

activity of cells in suspension.

**

682

One can see f r om Table 4, the biocatalytic properties of immobilized microorganisms depended largely o n the method of immobilization. The method of double immobilization seemed t o be a more available and universal technique, providing an active biocatalysts that retained up t o 1 0 0 % of initial resting cells activity. These biocatalysts also possessed higher operation stability: after the 4 h operation double immobilized Methylosinus trichosporium I M V 3 0 1 1 retained 100% of initial activity against 50% for cultivation in the presence of adsorbent and 3 8 % for preadsorption, followed b y cultivation of adsorbed cells.

4.

CONCLUSIONS

The biocatalytic properties of methane-utilizing microorganism strains were investigated with respect t o methane oxidation t o methanol and propene epoxidation to propylene oxide. The microorganisms studied: Methylococcus capsulatus IMV 3021, Methylosinus trichosporium I M V 3 0 1 1 and Methylomonas sp. GY-J-3 performed these reactions with 1 0 0 % selectivity t o corresponding oxy-product. The immobilization of the bacterial cells o n inorganic supports w as carried out. Carbonized alumina-silica mineral was found t o be the optimal adsorbent for bacterial cells studied. Double immobilization of active bacterial cells was available and universal methods for all strains yielding a biocatalysts with a high activity and stability.

REFERENCES 1. 2. 3. 4. 5. 6.

7. 8. 9.

P.S.Yarladda, L.F.Morton, N.R.Hunter, H.D.Gesser, Ind. Eng. Chem. Res., 2 7 ( 1988) 262. D.O.Mountfort, Y.Pybus, R.Wilsson, Enzymes Microb. Technol., 12 (1990) 343. P.K.Mehta, S.Mishra, T.K.Ghose, Biotechnol. Bioeng., 3 7 ( 19 9 1 ) 551. Yu.R.Malashenko, V.A.Romanovskaya, Yu.A.Trotchenko, Methane-oxidizing microorganisms, Nauka, Moscow, 1 9 7 8 . G.A.Kovalenko, V.D.Sokolovskii, Biotechnol. Bioeng., 39 ( 1992) 522. L.N.Rachkovskaya, E.M.Moroz, V.F.Anufrienko, E.A.Levitskii, T.M.Kriksina, Izv. Sib. Otdel. Akad. Nauk.SSSR, Ser.Khim., 5 (1982) 34. V.D.Sokolovskii, G.A.Kovalenko, React. Kinet. Catal. Lett., 2 2 ( 1983) 125. L.S.E.Brink, J.Tramper, Enzyme Microbiol. Technol., 9 (1 9 8 7 ) 6 1 2 . G.M.Stephens, H.Dalton, Trend. Biotechol., 5 (1 987).

683 R. NEUMANN (Hebrew Univ., Jerusalem, Israel): The oxidation of methane or propylene includes the use of dioxygen as oxygen source and sodium formate as electron source (cofactor). What is the yield of the oxidation in terms of sodium formate, i.e. what is the efficiency of the reducing agent?

G.A. KOVALENKO ( I . of Catalysis, Novosibirk, Russia): Unfortunately, w e did not measure the rate of the sodium formate consumption. But the lower limit of selectivity can be evaluated from the balance. For the experiments with the concentration of sodium formate of 20 m M usually the concentration of methanol about 10 m M was reached. Taking into account that the essential amount of methanol was evaporating with the gaseous stream, the selectivity of reaction with respect t o sodium formate can be evaluated to be higher than 5 0 YO. B. DELMON (Univ. Catholique, Louvain-la-Neuve, Belgium): Biocatalysis is fascinating and it is very interesting t o compare, during this Congress, biocatalysis with other types of catalysis. Biocatalytic process have usually 2 drawbacks: i) a dilution of reagents and products, a large reactor volume, a low productivity and a high separation cost; ii) a high cost of nutrient. Could you tell us the concentration of products in the effluent in your reactions? In addition t o sodium formate, what are the other nutrient you use? What is the price of nutrient, per Kg of, e.g. propylene oxide? G.A. KOVALENKO: Definitely the low concentration of products is the one of the major problem of the selective oxidation by microbial cells, because these products (like methanol and propylene oxide) are poisons for bacteria. In our experiments the products concentration was below 1%. But if the product is rather volatile (e.g. propylene oxide) it could be easily separated from the gaseous stream. About nutrient. We used only sodium formate, it is rather cheap. Moreover, for propylene epoxidation the reaction can proceed without any external donors of electrons (like sodium formate). The substrate (propylene) can serve as a cofactor itself. J. HABER (I. of Catalysis and Surface Science, Krakow, Poland): We studied the some bacteria and found that they contain t w o types of enzymatic systems, which are able t o catalyze different reactions. Did you try t o separate the soluble enzyme from that contained in the membrane and compare their reactivity?

G.A. KOVALENKO: Our strain also contain t w o types of enzymatic systems, possessing activity with respect t o oxidative transformation of substrate. For methane oxidation these are the methanemonooxigenase which provide methane oxidation t o methanol and alcoholdehydrogenase for consecutive transformation of methanol. And it was mentioned we tried t o use some inhibitor of second enzymatic system in order to suppress the consecutive transformation of product. About activity of soluble part of enzymes, we did not measure this activity. But with respect t o monooxygenase activity this enzymatic complex contain the chain of enzymes, which are working in cooperation on the membrane and it could be supposed, a separate enzyme in solution is not working. But second enzymatic system alcoholdehydrogenase can work in solution too.

This Page Intentionally Left Blank

V. Cortes Corberin and S. Vic Bell6n (Editors), New Deveiopnienls in Selecrive Oxidution II 0 1994 Elsevier Science B.V. All rights reserved.

685

Selective Enzymatic Oxidations by using Oxygen as oxidizing agent: Immobilizationand Stabilization of FNR, a NADP+ regenerating enzyme

T. Bes", R. Fernandez-Lafuenteb, C. M. Rosellb, C. Gomez-Moreno" and J.M. Guisanb. a

Departamento de Bioquimica y Biologia Molecular. Universidad de Zaragoza. Spain. lnstituto de Catalisis. C.S.I.C. Madrid. Spain.

We have developed a new strategy t o prepare very active and very stable derivatives of the enzyme ferredoxin-NADP+-reductase (FNR). This strategy is based on a very controlled process of multipoint covalent attachment between the enzyme, through its amino groups, and very dense monolayers of linear aldehyde groups on support surfaces. Because of the excellent properties of this enzyme and the ones of its derivatives, w e have been able t o regenerate a complex and interesting enzymatic cofactor, NADP', by using molecular oxygen as oxidizing agent and under a very wide range of experimental conditions. In this way, by coupling our efficient oxidative system t o a number of NADPdepending redox enzymes, w e should be able t o develop very specific and very selective oxidative processes (dehydrogenation, deaminations, hydroxylations.. .) under very mild oxidizing conditions (e.g. at room temperature and by using molecular oxygen as unique oxidizing agent 1.

1 .- INTRODUCTION.

Enzymatic oxidations are a very promising tool t o develop new synthetic routes for selective modifications of fine-chemicals, pharmaceuticals and so on. As compared t o conventional catalyst, enzymatic routes appear t o be clearly advantageous: enzymes are able t o catalyze very selective and specific oxidative processes under mild experimental conditions, that is, involving chiral, complex and labile compounds. However, in addition t o their exceptional properties, enzymes are also complex and labile catalyst and hence, the preparation of industrial enzyme derivatives and the design of the reaction conditions are really a complex and difficult goal. Some redox enzymes, e.g., oxidases, are able t o directly use oxygen as oxidizing substrate. However, the most of redoxenzymes (hydroxylases, dehydrogenases, etc) utilize complex and expensive cofactors (e.g., NAD+, NADP') as oxidizing substrates. Hence, the industrial implantation of such enzymatic processes present some additional difficulties.

686 The continuous and complete performance of simple oxidative processes requires the simultaneous use of a second enzyme able t o regenerate the reduced cofactor by using simple and inexpensive oxidizing substrates. In this way, the finding of new interesting second-enzymes becomes a critical and key point t o develop such interesting processes at an industrial scale. Ferredoxin-NADP+-reductase, FNR, is a very interesting enzyme able t o oxidized NADPH t o NADP+ by using oxidized methylviologen (MV) as substrate. Oxidized MV

+ NADPH

<---FNR --->

Reduced MV

+ NADP'

Since methylviologen can be directly oxidized by molecular oxygen, this enzymatic process, if carefully designed, can be used t o reoxidize NADPH from oxygen, the simplest, more inexpensive and the less harmful oxidizing agent.

In this way, FNR derivatives could be coupled t o a number of redox enzymes (enzyme-11, the ones requiring NADP' as cofactor, in order t o perform direct oxidations by using molecular oxygen as substrate.

0,

+ substrate

--- FNR,

Enzyme-I, NADP', MV

--->

oxidized product

Because these excellent prospects of FNR as industrial enzyme, w e have tried t o develop new immobilization techniques which allow us t o prepare very active and very stable immobilized derivatives. Hence, in this communication w e present an study on the immobilization 81 stabilization of FNR by multipoint covalent attachment t o aldehydic supports (see Figure 1 ) . By using different combinations of the different variables that control the intensity of these enzyme-support multiinteraction process w e have been able t o prepare a number of FNR-agarose derivatives having very different activity / stability parameters. In addition w e have tested the behavior of these derivatives under very different experimental conditions in order t o test their ability t o be coupled as secondenzyme t o any type of selective enzymatic oxidation (by using different first-enzymes, substrates with different solubility and very different reaction conditions).

2.- MATERIALS AND METHODS. 2.1. Materials. Ferredoxin-NADP' reductase was purified from Anabaena PCC 71 1 9 as previously described ( 1 ). Crosslinked 10% glyoxyl-agarose gels were prepared as previously described (2) and they are now commercially available from Hispanagar S.A. ( Burgos, Spain, Facsimile No. 34 - 47 - 20 03 2 8 ) .

687

lMMOBlLiZATlON BY MULTiPOlNT COVALENT ATTACHMENT

MACROSCOPIC LEVEL

MOLECULAR LEVEL

Figure 1.

2.2. Preparation of one-point attached FNR (amino) - agarose (aldehyde) derivatives. They were prepared by using glyoxyl agarose w i t h a very low degree of activation (5 /./Equivalents of aldehyde per mL. of support). 10 mL of aqueous suspension of glyoxylagarose were mixed w i t h 10 mL of 0.1 M bicarbonate-carbonate buffer and w i t h 4 mL of a FNR solution in 50 mM TRIS/HCI buffer, pH 8.0 and containing 36 pgrs of protein per mL. The pH was adjusted t o pH 10.05 and the suspension was very gently stirred a t 25 "C for one hour. Then 25 mgrs of solid NaBH, were added and the suspension stirred again for 30 minute. After reduction, the derivatives were washed with 0.1 M phosphate pH 7.0 and finally with distilled water. In recent papers (31,w e have demonstrated that the enzymatic derivatives, obtained in these experimental, have one unique covalent attachment. Some logical and experimental evidences are: i. these derivatives are prepared by using very low activated supports on wich, by steric reason, each enzyme molecule cannot form more than one covalent attachment. ii. the derivatives of trypsin (41,penicillin G acylase (31,lipase from Clostridium cylindracea (5),and 13-galactosidase from Acinetobacter oryzae, prepared by this method, were exactly as stable as the corresponding soluble enzyme. From here, w e name at these derivatives like one-point attached derivatives.

2.3.Preparation of multipoint attached FNR (amino) - agarose (aldehyde) derivatives. They were prepared as described above but now using very highly activated glyoxylagarose gels ( containing 200pEquivalents of aldehyde per mL of support). In addition, the reaction time was now 5 hours and the temperature was also 25°C. This set of experimental conditions (buffer, reaction time, pH and temperature) were found t o be the best t o obtain very accurate activity/stabilization parameters from previous experiments where w e have prepared very different multipoint attached FNR-agarose derivatives (6).

688

By the other hand, we know the covalent multipoint attachment because has also been evidenced with other enzymes. A number of results confirm their existence: i. these derivatives have prepared by using our most activated gel. Thus, for example with penicillin acylase, each enzyme molecule can theoretically form more than 30 covalent linkages w i t h the support. ii. the derivatives prepared with other enzymes (3-5) are more stable than the soluble enzyme. Hence, from a logical point of view and from the results previously obtained w i t h trypsin (4), the highly increased stability can only be explained in terms of an increase in the intensity of the enzyme-support multipoint attachment. iii. furthermore, the amino acid analysis of trypsin derivatives confirm the formation of very intense multipoint covalent attachment, approximately 7 residues for each trypsin molecule have reacted w i t h the activated support. 2.4. Enzymatic assays. a.- Diaphorase activity. It was assayed by following the spectral changes that occur at 620 nm as a consequence of the bleaching of 2,6 dichlorophenol-indophenol (DCPIP) mediated by the enzyme in the presence of NADPH (7). b.- Photoreduction of NADP'. Steady-state reduction of NADP' by FNR in the presence of photochemically reduced methyl viologen was carried out in a 3 mL bull-necked anaerobic spectrophotometric cell provide with magnetic stirring device as previously described (8). The decrease in absorbance at 602 nm due t o oxidation of methyl viologen was followed spectrophotometrically . 2.5. Stability assays. Soluble enzyme and immobilized derivatives were incubated under very different experimental conditions. A t different times aliquots of these suspensions were withdrawn and the remaining activity was measured by using the diaphorase assay previously described. We have performed very different stability tests in order t o know the ability of our derivatives t o be coupled t o selective oxidations catalyzed by very different enzymes on different substrates and under very different conditions (pH, temperature, presence of organic solvents...). We have performed two sets of comparisons: a.- One-point attached derivatives versus soluble enzyme: i.- inactivation in 0.1 M phosphate, pH 7.0, 60 "C without stirring. ii.- inactivation in 0.1 M acetate pH 5.0, 45 "C without stirring. iii.- inactivation in 0.1 M phosphate pH 7.0, 24 "C under vigorous stirring (1,000 rpm).

iv.- inactivation in biphasic systems ( 1 :1 ethyl acetate / 0.1 M phosphate pH 7.0) at 25 "C and under vigorous stirring (1,000 rpm)

689

b.- Multipoint attached derivative versus one-point attached one. i.- inactivation in 0.1 M phosphate, pH 7.0, 68 "C. ii.- inactivation in 0.1 M acetate pH 5.0, 45 "C. iii.- inactivation in 0.1 M bicarbonate pH 10.0, 37 "C. iv.- inactivation in 70 O h ethanol in 0.1 M phosphate pH 7.0, 25 "C. v.- inactivation in 0.1 M phosphate pH 7.0 saturated with ethyl acetate,

25°C.

3.-RESULTS AND DISCUSSION 3.1. Activity of FNR-agarose derivatives. One-point attached derivative preserves fully active after immobilization in both catalytic assays (oxidation and reduction of NADP+). In fact these derivatives preserves exactly 100% of the catalytic activity of the soluble enzyme that has been immobilized. On the other hand, multipoint attached derivatives also preserve a very important percentage of activity (approx. 60 %) in spite of being attached t o the support through a number of covalent linkages (9). These results are similar t o many others obtained in our laboratory for the preparation of derivatives of a number of industrial enzymes (9).In fact, the poorly distorting effect of our strategy for immobilization-stabilization of enzymes is due to t w o main features of our immobilization technique: i.- each single one-point attachment between the enzyme and the activated support only involves the transformation of one primary amino group on the protein surface into a secondary amino one having a very similar pK value, ii.- multipoint attachment may be developed under mild conditions and it is not associated t o important distortions on enzyme structure because of: the absence of steric hindrance for the amine-aldehyde linkages, the reversibility of each single attachment and so on.

3.2 Stability of one-point attached FNR-agarose derivatives. Table 1 clearly exemplifies the different stabilizations promoted by simple one-point covalent immobilization. A t neutral pH the time-courses of inactivation of non-stirred solutions of soluble enzyme and non-stirred suspensions of immobilized derivative are exactly identical (stabilization = 1). This datum, in addition to the full recovery of catalytic activity, strongly supports the idea that this one-point attached derivatives have exactly the same conformation corresponding to the soluble native enzyme. As commented above, this immobilization method only promotes the transformation of only one primary amino group into a secondary one having very similar pK value. Hence this immobilization method may be considered almost the simplest and the least distorting one ("pure immobilization").

690

STABIUZATION BY 'PURE IMMOBIUZATIOW MECHAN I!

STABIUZATION

EXPERIMENTAL CONDITIONS

1

pH 7.0, no dlrrlng

10,ooo

PH 7.0, wlth 8th&lg

A

2,m

pH 5.0, wlth rtlrrlng

B

1,m

PH 7 6 , l : l watu:Ethyl Ac0lat.p wnh .tlrrh@

C

Table 1. Inactivation of One-point immobilized derivatives as compared to soluble enzyme under different experimental conditionsb (see Methods)." Stabilization is defined as the ratio between half-life of derivatives and the one corresponding t o soluble enzyme. Mechamisms of stabilization: A. Prevention of interaction with air interfaces, B. Prevention of aggregation at the isoelectric point, C. Prevention of interaction with solvent interfaces.

On the other hand, when inactivations are carried under more extreme conditions (with vigorous stirring in aqueous or biphasic systems or at the isoelectric point) we observe very important stabilization factors associated with this "pure immobilization". In fact these non-distorted immobilized molecules are now included inside the porous structure of the support which protects them from interactions with interfaces. In addition, the full dispersion of these covalently immobilized enzyme molecules also prevents them from any intermolecular process which may occur with enzyme in a soluble fashion (e.g. aggregation at the isoelectric point). From a more practical point of view these activity/stability results strong suggest the convenience of using immobilized enzyme derivatives to develop selective oxidations catalyzed by enzymes. In this case, at first glance immobilization of enzymes does not appear as strictly necessary because of the use of membrane reactors to "immobilize" the recycling cofactor, e.g. polyethyleneglycol-NADP+. However the results here presented clearly exemplifies the additional advantages of the use of immobilized enzyme derivatives even in this type of reactors. In this way we could: i.- try t o use oxygen as oxidizing agent (e,g, by bubbling pure oxygen or air inside the reactor), ii.- use biphasic systems or organic cosolvents to oxidize poorly soluble substrates, iii.- control of pH by titration of the reaction mixture with concentrated solutions (acidic /basic) under vigorous stirring and so on.

69 I 3.3. Stability of multipoint attached FNR-agarose derivatives. In addition t o the stabilizing effects reported above w e have also found very important additional stabilizations promoted by the multipoint covalent attachment of the enzyme on the activated support (figure 3).

STABIUZATION BY MULTIPOINT COVALENT ATTACHMENT STABILIZATION

WERIMENTAL

CONDITIONS~

600

pH 7.0

200

pH 10.0

200

pH 5.0

2,000

pH 7.0, 70% Ethanol

500

pH 7.0, Ethyl Acetate

Table 2. Inactivation of Multipoint attached derivatives as compared t o inactivation of onepoint attached one under different experimental conditionsb (see Methods). a Stabilization is defined as the ratio between half-life times of both derivatives.

Now, opposite t o that found for one-point attached derivatives, w e have found very interesting stabilizing effects under every experimental conditions. These results suggest that "rigidification of the 3D structure of the enzyme" should be the main mechanism of stabilization. This "rigidification" as well as the exact extension of the multipoint attachment has already been demonstrated in our laboratory for a number of industrial enzymes. A more detailed description of the strategy and mechanism of stabilization of FNR by multipoint attachment t o agarose-aldehyde gels will be the matter of a forthcoming paper.

4.- CONCLUDING REMARKS

Very interesting w e have t o remark the additive effect of both stabilizations. Hence multipoint attached FNR-agarose derivatives have real stabilizations corresponding t o the stabilizing effect of "pure immobilization" plus the stabilizing effect promoted by "rigid; f ication " .

692 In this way, these derivatives, preserving 60% of activity corresponding t o the soluble enzyme, are, in some cases, up t o six orders of magnitude more stable than the soluble enzyme. Therefore, the excellent properties of FNR as well as the dramatic improvement of its stability properties by multipoint covalent immobilization make this FNR agarose derivatives very adequate t o act as second enzyme in many very interesting selective oxidations catalyzed by enzymes. At first glance, the performance of these complex processes require cofactor regeneration by using soluble and labile enzymes and "sacrificial" oxidizing substrates. Now we may be able t o perform these processes by using oxygen as oxidizing agent as well as by using very active and extremely stabilized FNR-agarose derivatives.

REFERENCES 1. J.J. Pueyo and C. Gomez-Moreno, Prep. Biochem. 21(4) (1991), 191. 2. J.M. Guisan, Enzyme Microb. Technol 10 (1 987), 375. 3. Guisan, J.M., Alvaro, G., Fernhndez-Lafuente, R., Rosell, C.M., Garcia, J.L., Tagliani, A. Biotechnology Bioengineering. 42 (1 993), 455. 4. Blanco, R.M., Calvete, J.J., Guisan, J.M. Enzyme Microb. Technol. 1 1 (1989), 353. 163. 5. Otero, C., Ballesteros, A., Guisan, J.M. Appl.Biochem.Biotechno1. 19 (1 988), 6. T. Bes, R. Fernandez-Lafuente, C.M. Rosell, C. Gomez-Moreno and J.M. Guisan, summitted for publication to Biotech. Bioeng. 7. J. Sancho, M.L. Peleato, C. Gomez-Moreno and D.E. Edmonson, Arch. Biochem. Biophys. 260 ( 1 988), 200. 8. J.J. Pueyo and C. Gomez-Moreno, Enzyme Microb. Technol., 15 (4)(1992), 8. 9. R.M. Blanco and J.M. Guisdn, Enzyme Microb. Technol, 11 (19881, 227.

V. Cortes Corberan and S . Vic Bellon (Editors), New Developments in Sekcrive Uxidarion fI 0 1994 Elsevier Science B.V. All rights reserved.

693

ESR study of photo-oxidation of phenol at low temperature on polycrystalline titanium dioxide M.J. L6pez-Muiioza,J. Soria", J.C. Conesa" and V. Augugliarob %stituto de Catklisis y Petroleoquimica, C. S .I.C., Campus Universidad Autonoma, Cantoblanco, 28049 Madrid, Spain bDipartimento di Ingegneria Chimica dei Processi e dei Materiali, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy The formation of oxygen radicals on hydrated Ti02 (anatase) samples irradiated at 77 K in the near-UV region and their interaction with adsorbed phenol molecules have been studied by electron spin resonance. The results indicate that the irradiation determines the generation of 0 2 H - and 0- radicals which react with adsorbed phenol molecules. Phenol-derived radicals are generated after warming the sample at room temperature. A reaction mechanism is proposed on the basis of the observed species. 1.

INTRODUCTION

Oxidation of organic substances at ambient temperature, in presence of semiconducting oxides stimulated photochemically, is receiving much attention, at present, as an effective means of removing several kinds of trace pollutants in water [l]. The use of Ti@ for this type of process is widely accepted because of its activity and stability under reaction conditions [2] and much work has been devoted to determine the photocatalytic properties of this oxide [3, 41. In these studies some of the intermediate species in photooxidation reactions have been identified [5]. However, there are still some disagreements on the nature of the active sites originating the photoreactions [6, 71. A widely accepted view considers that the active sites for this type of reaction are OH- radicals, generated through capture of photogenerated holes by OH- groups at the titania surface. In this case, however, if OHgroups were to be used for the reaction and a constant catalytic activity were maintained, the catalysts should exhibit, together with their photoreactivity and at the same photoreaction conditions, an easy mechanism to recover lost hydroxyl groups. This is not the case of dehydroxylated Ti02 treated with wet air at mild temperature IS], but it is not known how easy is the hydroxylation of Ti02 in contact with liquid water. In the present work, in order to obtain evidences on the nature of the active sites involved in the photooxidation reaction, the radicals formed during oxygen photoadsorption on hydroxylated anatase surfaces and the types of paramagnetic species formed by reaction of those radicals with phenol molecules adsorbed from the gas phase, have been studied by electron spin resonance.

694

Two types of TiO, samples, with different anatasehtile ratios, have been used to verify how the presence of rutile in different concentrations affected the type of radicals generated by oxygen photoadsorption and how the phenol oxidation was affected by this modification. 2.

EXPERIMENTAL

TiO, samples from Aldrich, sample I (S,,, = 12 m2/g, with anatasehtile ratio of 86/14), and from Degussa, sample I1 (P25-type , SBET= 50 m2/g, with anatasehtile ratio of 60/40), have been used as photocatalysts. The powdered samples were placed in a vacuum quartz cell assembled with greaseless stopcocks capable of maintaining a dynamic vacuum better than 3 ~ 1 0 N/m2. .~ The samples were then outgassed at 298 or 373 K and irradiated with or without introducing oxygen (SEO, high purity grade). For irradiating the samples a 125 W medium pressure Hg lamp (Sylvania GTE) was used; the cell-lamp system was set in a cylindrical chamber with walls covered with aluminum foil, so that the tube with the Ti02 sample (which was kept inmersed in a liquid N, bath in all irradiations) was almost uniformely irradiated. The distance between the sample and the lamp was 10 cm; a Pyrex sheet, placed between the lamp and the cell, was used for cutting off all radiation with wavelength shorter than ca. 300 nm. ESR spectra were obtained at 77 K (without intermediate warming of the sample to room temperature, unless otherwise stated) using a Bruker ER2OOD spectrometer, working in the X-band. For calibration a DPPH standard (g = 2.0036) was used. In those cases where oxygen had been previously adsorbed, the sample was outgassed at 77 K before the ESR measurement, to eliminate any 0, excess that could broaden the signals of surface species. The procedure for low temperature phenol adsorption was as follows. Phenol (RhBnePoulenc, Rectapur Class) was placed in a side cell connected to the sample cell. Then, the system was outgassed while keeping both cells at 77 K. After that, the phenol cell was warmed at 298 K and, when steady conditions were reached, the connection between the two cells was opened for 2 minutes. Only the lowest extreme of the sample cell, containing the TiO, powder, was kept immersed in the liquid N2 bath, in order to allow the phenol vapour to reach the titania powder instead of being completely condensed in the sample walls. 3.

RESULTS

Figure 1 shows the ESR spectra obtained for sample I after consecutive cycles of irradiations under vacuum and in contact with 0,. The spectrum of sample I outgassed at 298 K and irradiated for 30 minutes under vacuum is presented in Figure la. It is formed mainly by signal A with g, = 2.018, g, = 2.008 and g, = 2.002 and signal B with gL = 1.991 and g II = 1.960. These signals are not observed after leaving the sample a short time at room temperature. A subsequent oxygen adsorption at 298 K followed by irradiation during 30 minutes originated spectrum lb, which is formed by signal A, with smaller intensity than in spectrum la, signal B and a new signal C, overlapping signal A, that can be distinguished by a broad peak at g = 2.034. After outgassing at 298 K and a new irradiation spectrum lc was obtained; it is formed by signal B and a small signal C ; signal A, if present, is clearly smaller than in Fig. la. Finally, when the sample was again contacted with 0, at 298 K and irradiated, spectrum Id was produced. It is formed mainly by signal B and signal C stronger

695

than in the previous cases. To study the influence of irradiation time on the intensity of the signals, a fresh sample I was irradiated several times for 30 minutes with and without 02. Signal A showed a maximum of intensity between 30 and 60 minutes while the intensity of signal C increased steadily with irradiation time.

3

b

C

Figure I. ESR spectra of sample I obtained after consecutive irradiation cycles: Irradiated after outgassing at 298 K (a) and (c); irradiated after oxygen adsorption at 298 K (b) and (d).

Figure 2. ESR spectra of sample I1 obtained after consecutive irradiation cycles: Irradiated after outgassing at 298 K (a) and (d) and 373 K (b); irradiated after oxygen adsorption at 298 K (c) and (e).

The spectra obtained for sample I1 after being subjected to different treatments are reported in Figure 2. After outgassing at 298 K and irradiation, sample I1 presented spectrum 2a, formed by signals A, B, C and two new signals: D with g, = 2.026, and E with g, = 1.975 (the other components of these signals are not resolved). When a new portion of sample 11was evacuated at 373 K and irradiated, spectrum 2b was obtained. It is formed by the same signals of the previous case but with a larger contribution of signal C. A subsequent contact with 0, at 298 K and irradiation produced spectrum 2c, mainly formed by signal C. A new evacuation at 298 K and irradiation led to spectrum 2d formed by signal E with g , = 1.975 and g = 1.950. A final contact with O2at 298 K and irradiation yielded spectrum 2e, formed mainly by signal C and small signals B and E. The effect produced on sample I by phenol adsorption at 77 K is presented in Figure 3. Spectrum 3a, corresponding to sample I outgassed at 298 K and irradiated in vacuum, is mainly formed by signal A and signal B. Subsequent adsorption of phenol at 77 K (without intermediate warming of the sample to room temperature) gave spectrum 3b which is formed by signal F with g, = 2.018, g, = 2.009 and g, = 2.003 and signal B. Further irradiation produced spectrum 3c, formed by a larger signal F, signal B and a small contribution of signal C.

,

696

Figure 4 shows the effect of phenol adsorption at 77 K on sample 11. After outgassing at 298 K, oxygen adsorption at 298 K and irradiation, signals C and E were mainly obtained (spectrum 4a). Phenol adsorption led to the disappearence of both signals (spectrum 4b).

25 C

C

Figure 3. ESR spectra of sample I irradiated after evacuation at 298 K (a) followed by phenol adsorption at 77 K (b) and a new irradiation (c).

Figure 4. ESR spectra of sample I1 irradiated after evacuation at 298 K and oxygen adsorption at 298 K (a) followed by phenol adsorption at 77 K (b).

Figure 5. ESR spectra of sample II after irradiation followed by phenol adsorption and several minutes at 298 K, without oxygen (a) with oxygen (b).

691

When irradiation was carried out on a sample 11, on which phenol had been adsorbed at 298 K, the spectra reported in Figure 5 were obtained. Depending on the absence or presence of oxygen during the irradiation, two different signals appeared respectively: signal G, with g, = 2.012, g, = 2.007 and g, = 1.999 (spectrum 5a) and signal H (spectrum 5b) which presents hyperfine structure and can be simulated assuming coupling to a single magnetic nucleus with I = 1/2, g-tensor principal values g, = 2.013, g, = 2.005 and g, = 1.997, and hyperfine coupling constants 1 A, 1 = 5 . 3 5 ~ 1 0and ~ I A, I = 5 . 6 ~ 1 0cm-' ~ (A, is not resolved, but an upper limit for its magnitude of 3x104 cm-' can be estimated by simulation). The signal intensity of this radical increases by further irradiation in presence of oxygen and decreases in absence of oxygen. The g values of all the different signals, together with their assignments and references to similar signals found by other authors, are collected in Table 1.

Table 1 g values and assignments of ESR signals

g,

Signals A B C D E F G H

+

gl

2.018

g2

1.960 2.002 (2.002)

2.008 (2.008)

2.034 2.026

g3 2.002

2.008 1.991 1.975

2.018 2.012 2.013

g I1

1.950 2.009 2.007 2.005

2.003 1.999 1.997

Assignments

Ref.

0- stabilized by OH-

4 Ti3+in anatase 11 0,H . 3 0- stabilized 3 Ti3+in rutile 11 0--adsorbed phenol Phenol derivative radical Phenol derivative radical 13

+ +

This work

4. ASSIGNMENT OF SIGNALS

Signals A and D are observed after UV-irradiation of fully hydroxylated titania samples and tend to disappear by succesive irradiations or outgassing treatment at 373 K. These results suggest that the radicals are in some way related to species that can be removed by irradiation or outgassing treatments. In a hydroxylated surface, such removable species can be ascribed only to some specific type of labile hydroxyl groups and/or to adsorbed water molecules. As indicated by Munuera et al. [9], acidic hydroxyl groups are most likely to be removed at lower outgassing temperature, while basic hydroxyls are more stable and can capture photogenerated holes more easily [lo]. Signal A presents g-values close to those corresponding to a signal observed by Howe et al. [4] after an anatase sample, prepared by a different method and containing different impurities, was UV-irradiated at 77 K . This signal was assigned to 0- radicals stabilized by hydroxyl groups at the anatase surface. This assignment is probably also valid for signal A. Signal D, observed more clearly for the

698

sample with a higher rutile content, has been previously assigned also to stabilized 0- species [3]. By considering arguments similar to those applied to signal A, signal D can be assigned to 0- radicals stabilized by acidic hydroxyl groups in rutile. The basic hydroxyl groups can also trap the holes to produce OH. radicals, but these react further (see below) and cannot be observed by ESR in our working conditions. Signals B and E are observed after irradiating TiO, in vacuum and present g < gl < g, as expected for Ti3+ ions. Signal B is observed more strongly for sample I, which has a very high anatase content, while signal E is observed when the sample contains a larger proportion of rutile. These signals are assigned to fully coordinated Ti3+ions in anatase and rutile, respectively, in agreement with previous assignments by different authors [ 11,121. Signal F, which presents g values very similar to those of signal A, is formed after phenol adsorption on sample I which contains the radicals originating signal A. This similarity in g values suggests that it may belong also to an 0- radical, but the larger linewidth indicates that it experiences a noticeable interaction with the phenol molecule (perhaps due to enhancement of the unpaired electron spin relaxation through interaction with the aromatic ring). Some sort of weak 0--phenol complex would thus seem to be involved in this species. Signals G and H, formed after phenol adsorption, present g values different from those normally observed for oxygen radicals or Ti3+ions and should be assigned to two different phenol derivative radicals. Signal H has been assigned in a previous work [ 131 to a diphenoltype radical formed after the attack of phenol by 0 2 H radicals, in agreement with the observed dependence on the presence or absence of 02. 5. DISCUSSION

Irradiation at 77 K under vacuum of fully hydrated anatase samples with different rutile contents produces mainly signals due to Ti3+ions and 0- species which can be stabilized in fully hydrated surfaces:

+ TiO, + Ti4+ h + + 0,-

Ti02 (e-

hv

+

e-

+ Ti3+ +

+ h+)

0-

The complete hydroxylatiodhydration would help the stabilization of photogenerated electron-hole pairs at certain sites in Ti02, without the possibility of reaction with adsorbed molecules. 0- species are normally difficult to detect in Ti02 (at least under conditions of substantial surface dehydration, which is the case in most literature studies), the reason being that they have great tendency to transfer the hole to hydroxyl groups producing OH. radicals which then dimerize to H,O,; the fully hydrated state, however, seemingly makes it possible to keep a certain amount of them stable at low temperature. Signal A appears with lower intensity in sample 11, suggesting that the stability of this 0- species is lower in rutile; this effect may be related to a distribution of adsorbed water or hydroxyl types different than in anatase. In parallel with process (3), the photogenerated holes can be also trapped directly by the most basic OH- groups, forming OH- radicals [lo] which may further react with other species, even at 77 K:

699

+

h+ OH2 OH.

+ +

OH. H202

Process (5) consumes some OH- groups, that in the absence of water are not replaced. This would lead gradually to a partially dehydroxylated surface, facilitating the access and the adsorption of 0, molecules onto the exposed Ti ions on the sample surface, whereas the fully covered conditions that seemingly are required to keep some 0- radicals stable disappear. If the sample is warmed at 298 K this reaction may occur also using the holes trapped as 0-, as said above. These effects can explain the intensity maximum of signal A observed at an intermediate irradiation time and the increase of signal C after consecutive irradiations in contact with oxygen. Species 0 2 H -can be formed by two different processes, having origin respectively in the capture of holes or electrons:

Hole process: Following (3, H202 OH*+ H,O

+

+ 02H*

Electron process: O2 e020,-+ Hf O,H*

+

-+

+

where the 0,- (and subsequently 02H)species would be stabilized transiently by coordination to accessible surface Ti ions. The formation of 0,H- by the hole process will contribute also to the consumption of OH- groups, On the other hand, the electron process requires: i) the elimination of some hydroxyl groups or water to facilitate the access of O2 molecules to the surface Ti ions; and ii) adsorbed O2 molecules. According to the results obtained, the experimental conditions needed to form significant amount of 02H. species agree with requirements of a previous elimination of OH- groups (by irradiation or outgassing) and of the presence of adsorbed oxygen. Therefore, although it can not be excluded that some 0 2 H . radicals can be formed by the hole process, most of the species detected by ESR are probably formed through the electron process. The protonation of the 02-radical could take place also through the reaction: 02-(ads) + H,O

-+

OH2-

+ 0,

(9)

as indicated by Stone [14, 151, but the hydroxylation of Ti02 by UV irradiation in presence of water, although very interesting, is not a very easy process [8]. The reactivities of the two radical species, 0- and OzH-, toward adsorbed phenol molecules seem to be different. At 77 K phenol adsorption produces only a broadening of signal A, while the 02H. radicals react producing some intermediate (and unidentified) diamagnetic species. By considering that other authors report that the oxidation of phenol with low power lamps requires a certain activation time [16], it may be proposed that the W species, which are the first ones to be observed by ESR, are not very reactive toward phenol, and that the 0 2 H *radicals, formed particularly when the TiOz surface is accessible to O2 molecules, are the actual initiators of the phenol (photo)oxidation at low temperature. After this initial phenol activation with O2H., occumng even at 77 K with the formation of

diamagnetic species, different stable radicals may be detected depending on the presence or absence of oxygen; at least some of these (the radical represented by signal H, which seems to be the most stable) may be ascribed to diphenol species, in agreement with results in the literature [5] about the nature of the intermediate products formed in the photooxidation reaction.

ACKNOWLEDGMENTS Thanks are given for the financial help obtained from the EC (Program STEP, project nr. CT91-0133). M.J.L.M. thanks also the P.F.P.I. program of the Ministerio de Educacion y Ciencia for a Ph D grant, under which this research was undertaken.

REFFCRENCES

1. M. Schiavello (Ed.), Photocatalysis and Environment. Trends and Applications, Kluwer Academic Pu. Dordrecht (The Netherlands), 1988. 2. N. Serpone and E. Pelizzetti (Fds), Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. 3. A.R. GonzAlez-Elipe, G. Munuera and J. Soria, J. Chem. SOC.Faraday I75 (1979) 748. 4. R.F. Howe and M. Gratzel, J. Phys. Chem. 91 (1987) 3096. 5. K. Okamoto. Y. Yamamoto, H. Tanaka and A. Itaya., Bull. Chem. SOC. Japan. 58 (1985) 2015. 6. C.S. Turchi and D.F. Ollis, J. Catal. 122 (1990) 178. 7. D. Lawless, N. Serpone and D. Meisel, J. Phys. Chem. 95 (1991) 5166. 8. J. Soria, J.C. Conesa, V. Augugliaro, L. Palmisano, M. Schiavello and A. Sclafani, J. Phys. Chem. 95 (1991) 274. 9. G. Munuera, V. Rives-Amau and A. Saucedo, J. Chem. Soc. Faraday I 7 5 (1979) 736. 10. A.R. Gonz&lez-Elipe, J. Soria, G. Munuera and J. Sanz, J. Chem. SOC.Faraday I 76 (1980) 1535. 11. J.C. Conesa and J. Soria, J. Phys. Chem. 86 (1982) 1392. 12. M.T. Blasco, J.C. Conesa, J. Soria, A.R. Gonzaez-Elipe, G. Munuera, J.M Rojo and J. Sanz, J. Phys. Chem. 92 (1988) 4685. 13. J. Soria, M.J. Lhpez-Muiioz, V. Augugliaro and J.C. Conesa, Colloids Surf. (in press). 14. R.I. Bickley and F.S. Stone, J. Catal. 31 (1973) 389. 15. R.I. Bickley, G. Munuera and F.S. Stone, J. Catal. 31 (1973) 398. 16. V. Augugliaro, L. Palmisano, J. Soria and M.J. Upez-Muiioz (to be published).

70 1

G. BUSCA (I. of Chemistry, Faculty of Engineering, Genova, Italy): You used Ti02 preparations constituted by mixtures of anatase and rutile. For these preparations one question arises whether the surface is constituted by rutile or by anatase or both. As for example, in a recent publication based on TEM data it is reported that Degussa P25 is constituted by bulk anatase with surface rutile. IR data instead support that anatase is present and predominates on P25 surface. My questions are: Why you have not used pure anatase? M.J. L O P E Z - m O Z , J. SORIA, J. C O M A and V. AUGUGLIARO (I. Catiilisis, Madrid, Spain): We have used Aldrich anatase (anatasehtile ratio 86/14) because it was the purer anatase sample available to us. G. BUSCA: Have you information on the location of the two phases on the surface? M.J. LOPEZ-MI&OZ, J. SORIA, J. CONESA and V. AUGUGLIARO: The UV radiation penetrates only a short distance from the surface, therefore the type of Ti+3ions generated by UV-irradiation can give an idea of the surface composition. For Aldrich anatase almost all observed Ti3+ ions corresponded to the signal with gL= 1.991. This signal indicates Ti3+ion in anatase. For P25 most of the Ti3' ions were in rutile probably indicating that most of the surface was formed by rutile.

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V. CortCs Corberin and S. Vic Bell6n (Editors), New Developments in Selective Oxidation If 0 1994 Elsevier Science B.V. All rights reserved.

703

Partial oxidation of benzene over the carbon whisker cathode added with iron oxide and palladium black during 02-H2 fuel cell reactions Kiyoshi Otsuka, Mitsuhiro Kunieda and Ichiro Yamanaka Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan

One-step synthesis of phenol and hydroquinone from benzene was carried out at ambient temperature by applying an 02-H2 fuel cell system under short-circuit conditions. The cathode of carbon whisker added with Fez03 and Pd black showed high electrocatalytic activity in the formation of oxygenates. The additives showed a marked synergism. The reaction mechanism via hydroxyhexadienyl radical was proposed on the basis of kinetic results. The phenol/hydroquinone ratio was insensitive to the reaction conditions, suggesting that both products are formed in parallel from a common reaction intermediate. The synergism of Fez03 and Pd black can be ascribed to the increase in the concentration of hydroxyl radical generated on the cathode. Addition of a load in the outer circuit improved the efficiency for the formation of oxygenates. Thus, the reaction system may better be operated by cogenerating the oxygenates and electricity.

1. INTRODUCTION We have proposed a new method for the direct synthesis of phenol by applying 0 2 - H ~fuel cell reactions at ambient temperature [l-31. During the 02-H2 fuel cell reaction, dioxygen is activated on the cathode as follows: OZ-(cathode)+ 2H+ + 2e- 4 0*-(cathode)+ H20 (1). The active oxygen species ( O * ) initiates the hydroxylation of benzene at room temperature [ l - 3 1 . Recently, we have found that the cathode of carbon whisker added with iron oxide and Pd black has the highest electrocatalytic activity for hydroxylation of benzene under short circuit conditions. The purpose of this work is to describe the electrocatalytic function of the cathode f o r the synthesis of phenol and hydroquinone. The formation of hydroquinone, which has not been paid attention previously [l-31, is followed carefully in this work. The reaction mechanism and the optimum reaction conditions are discussed on the basis of kinetic and electrochemical studies.

704

2. EXPERIMENTAL The fuel cell reactor used in this work is shown in Figure 1. The cathode was prepared from a mixture of carbon whisker, metal oxide o r metal blacks and Teflon powder (5mg). The mixture was pressed and shaped into a wafer (22mm diameter, 0.1mm thickness) The iron salts-added on a hot plate. cathode was prepared from the carbon whisker impregnated with the iron salts by wet impregnation method. The total weight of each cathode was 70mg. The anode and cathode thus prepared were attached to each side of the silica wool disk ( 2 . 0 ~ thickness, ~ 26mm diameter) The which contained H3PO4 (lM, lml) .

02

Vent

H2 f-

H3Po4 aq Figure 1. The fuel-cell reactor for oxidation of benzene.

anode was prepared from a mixture of graphite, Pt black and Teflon powder by the same hot-press method. Oxidation of benzene was carried out under the following experimental conditions unless otherwise stated. (Cathode): Oxygen (10lkPa) was bubbled into benzene (40ml) by a flow rate of 5ml(STP)*min-l. (Anode): Hydrogen (49kPa) and water vapor (IkPa, to keep the electrolyte always wetted) were passed with argon carrier (48kPa). The reaction was started by shorting the circuit at 303K. A l l the products dissolved in benzene, in the cathode and in the electrolyte were analyzed by gas-chromatography after the reaction for 3h. The electrolyte and the cathode were renewed before each run of experiment. A constant voltage was applied between the anode and the cathode using a variable power supplier. The potentials at the anode and the cathode during benzene oxidation were measured with reference to a KC1-saturated Ag/AgCl electrode (denoted as Ag/AgCl hereafter). The efficiency of oxygenate formation (O.E.) during 02-H2 reaction was defined as follows, amount of the sum of phenol and hydroquinone ( O . E . ) = 100% X (2) amount of water estimated from the charge passed where, the charge passed was assumed to be due to the formation of water.

3. RESULTS AND DISCUSSION 3.1. Active electrocatalysts for benzene partial oxidation We have already reported that among the carbon materials tested the carbon whisker pretreated with aqueous HNO3 solution is the most active

705

cathode for oxidation of benzene [3]. On the basis of this carbon whisker (denoted as CW), we improve the catalytic activity of the cathode by adding various metal oxides o r metal blacks. The results are summarized in Table 1. For all the cathodes in Table 1, the main products were phenol and hydroquinone with small amount of biphenyl, cyclohexanol and cyclohexanone less than lpmol. Therefore, the discussion in this work will be confined to the formations of phenol and hydroquinone hereafter. Thus, the selectivity to hydroquinone in Table 1 was calculated by excluding the formations of biphenyl, cyclohexanol and cyclohexanone. The results in Table 1 indicate that Table 1 MnO2, Fe2O3, CuO and Effects of various additives in the CW cathode on Pd black clearly inoxidation of benzene. crease the rate of C.P.b) Products/pmol Select. O.E. formations of oxygenCathodea) ates compared to /mF PhOH HQC) to HQ/% /% the CW cathode without 14.2 14.4 41.8 7.4 0.69 cw additives. Pd black 13.4 12.6 0.77 45.0 6.7 v205/cw 15.1 16.3 8.4 0.65 40.8 Cr203/CW shows the highest en13.9 8.6 14.5 0.82 48.2 Mn02/CW hancing effect on the 15.9 16.1 51.7 10.5 0.78 Fe203/CW formation of oxygen13.1 15.9 7.9 0.73 40.0 coo/cw 12.2 12.7 ates. Another char1.00 55.5 8.0 cuo/cw 17.7 12.7 8.4 0.71 36.9 Sn02/CW acteristic of this 7.7 15.7 65.9 13.1 2.05 Pd-black/CW additive is to enhance 1.9 17.7 5.4 2.87 20.8 Pt-black/CW the charge passed (C.P.) considerably. a) Content of additives: metal oxides (5mg), metal Thus, the cathode with blacks (20mg). b) Charge passed in 3h. Pd black decreases the c) Hydroquinone. oxidation efficiency (O.E.). In contrast t o the effect of Pd black, Fez03 improved the oxidation efficiency 3.2. Coaddition of Pd black and iron compounds We have suggested that coaddition of Pd black and FeC13 exerts the synergism on the formation of phenol[31. We describe here the synergistic effects of Pd black and iron compounds on the formations of phenol and hydroquinone in more detail. Table 2 shows the effects of various iron compounds added to the cathode of Pd-black(20mg)/CW. A l l the iron compounds added to this cathode enhanced the formations of phenol and hydroquinone remarkably as well as the oxidation efficiency. However, the selectivity to hydroquinone was not affected appreciably. Among the iron compounds tested, Fe3O4 showed the highest enhancing effect on the formation of the oxygenates. However, the Fe203-added cathode showed the highest stability and reproducibility in the

706

oxidation of benzene Table 2 to oxygenates. Effects of iron compounds added to the Pd(20mg)/CW Therefore, the cathode on the results of benzene oxidation. cathode coadded with Fe C.P. Products/pmo 1 Select. O.E. Fez03 and Pd black compoundsa) was chosen as one of PhOH HQ /mF to HQ/X /% the representatives none 2.05 65.9 13.1 16.6 7.1 of the cathodes in Fe203 2.16 99.7 20.7 17.2 11.2 2.50 107.8 19.8 15.5 10.2 Table 2 for further FeS04 Fe (NO3)3 2.00 90.1 18.1 16.7 10.8 studies. FeC13 2.12 98.1 20.4 17.2 11.2 The effect of Fe-powder 2.55 110.1 19.1 14.8 10.1 Fez03 on the reaction Fe304 2.65 120.9 23.3 16.2 10.9 with different contents (1.0-10.0mg) a) Content of Fe compounds: Fe2O3, Fe-powder, Fe304 (Zmg), iron salts (0.5mol%). in the Pd-black (ZOmg)/CW cathode has been examined under the standard reaction conditions. The rates of phenol and hydroquinone formations and O.E. increased sharply with a rise in the The rates and O.E. reached plateaus at the content of Fe2O3 at < lmg. quantity of Fez03 above l.Omg. I 115 These observations suggest that only a part of the added Fe2O3 is effective for the partial oxidation of benzene. The selectivity to hydroquinone and the charge passed did not change appreciably with the content of Fe2O3. Appreciable enhancing effect on the oxygenate formation was observed for all the iron compounds tested irrespective of the iron metal, iron oxides o r iron salts (Table 2). This is surprising because the degree of dispersion of the additives in the cathode must be 0 1 2 3 4 5 6 quite different among the Reaction t i m e / h additives. We speculate that Figure 2. Oxidation of benzene as iron cations (Fe2+ as described functions of reaction time. later) dissolved in the (0) , phenol ; , hydroquinone ; electrolyte (H3PO4) are adsorbed , hydroquinone selectivity; on the surface of CW. A part , charge passed; ( 0 ), 0.E . of these surface cations might

(0) (v)

(a)

707

be responsible f o r the enhancement in oxidation of benzene. The iron compounds used in Table 2 must be sufficient to supply an excess number of iron cations than those effective for the reaction. The rates of oxygenate formation (phenol plus hydroquinone) for the cathodes of CW, Pd(BOmg)/CW, Fe203(2mg)/CW and of (Pd(ZOmg),Fe203(2mg))/CW were 49.2, 79.0, 59.7 and 120.4pmo1, respectively. The result for the (Pd, Fe203)/CW clearly indicates the synergism of Pd black and Fez03 for the formation of oxygenates. Similar synergism was also observed between Pd black and other iron compounds (FeS04, Fe(N03)3 and FeC13). 3.3. Kinetic curves of the reaction The kinetic curves f o r the formations of oxygenates were measured for the Pd black(20mg) and Fe203(2mg)-coadded carbon whisker. The amount of products, the charge passed and O.E. are plotted as functions of the The results were measured by batch way. The reaction time in Figure 2 . results at each reaction time were obtained independently using a fresh substrate, fresh electrolyte and a renewed cathode. The kinetic curves in indicate that the reaction proceeds steadily in good Figure 2 reproducibility. At the early stage of the reaction (5 1 h), the amount of phenol and hydroquinone increased proportionally to reaction time, suggesting that both oxygenates are produced in parallel. Therefore, the selectivity to hydroquinone did not change appreciably during the b 0 / reaction. 40

3

5 . C

0 3.4. Effects of 02 and H2 partial '3 pressures E The effects of 02 and Hg 8 "0 20 results of pressures on the the reaction were studied for of (Pd-black(BOmg), cathode This Fe203(2mg )/CW(48mg). 0 50 100 was denoted as cathode Pressure of 0 2 1 kPa The (Pd,FezOg /CW hereafter. partial pressure of oxygen in the Figure 3 . Effect of partial cathode compartment affected pressure of oxygen. (Pd,FezOg)/CW strongly the charge passed and the cathode. Reaction time 1 h. rate of oxygenate formation as can See Figure 2 for the symbols. be seen in Figure 3. The increase in these values are accelerated with a rise in the partial pressure

708

of oxygen. However, the selectivity to hydroquinone was not affected by changing the partial pressure of oxygen. In the absence of oxygen at the cathode (P(02)= 0), a constant current (4mA) flowed under the short-circuit conditions due to the electrochemical hydrogen permeation from the anode to the cathode driven by the difference in hydrogen concentration. In contrast to the strong influence of oxygen described above, neither the charge passed nor the rates of formations of phenol and hydroquinone depended on the partial pressure of hydrogen (25-100kPa) in the anode compartment. These observations indicate that the anode reaction (HZ 2H+ + 2e-) over the Pt black electrode is not the rate-determining step. The cathode reaction must control the oxidation of benzene.

+

3.5. The reaction under externally applied voltage The reaction at a constant applied voltage has been observed L 15 E for the (Pd,Fe203)/CW cathode under the standard reaction conditions. 10 The results are shown in Figure 4 as functions of the applied 10 5 voltage. The negative voltage on the abscissa was controlled by a I 0 variable resistor in the outer circuit. Zero applied voltage means the short-circuit conditions. It is surprising that although a positive applied voltage increases the charge passed exponentially, the rate of formation decreases for both phenol The and hydroquinone (Figure 4). increase in the charge passed under positive applied voltage can be -0.6 -0.4 -0.2 0 0.2 0.4 0.6ascribed to the pumping of hydrogen Applied voltage I V from the anode to the cathode, evolving hydrogen into the cathode Figure 4. Oxidation of benzene as compartment. The hydrogen pumped functions of the applied voltage. onto the cathode might react with (Pd, Fe203)/CW cathode. Reaction the active oxygen transforming into time 1 h. See Figure 2 for the water. This must decrease the symbols. steady state concentration of the active oxygen, and consequently reduce the formations of oxygenates under positive applied voltage. The application of a negative voltage o r a load in the outer circuit decreases, of course, the charge passed as can be seen in Figure 4. The

:I

.

w

? 6

709

charge passed at the applied voltage of -0.2V decreased by about 40% compared to that at zero applied voltage. However, the rate of oxygenate formations (phenol plus hydroquinone) decreased by only 13%. Therefore, the oxidation efficiency of the oxygenate production shows maximum ( 1 6 . 4 % ) at an applied voltage of -0.2V. Under these reaction conditions we can cogenerate electricity and useful oxygenates with high O.E. The anode and the cathode potentials (vs Ag/AgCl) were measured as functions of the applied voltage at the same reaction conditions as those of Figure 4 . The open circuit potentials for the anode and the cathode were -0.24 and +0.55V, respectively. The cathode potential under shortcircuit conditions was -0.21V. The selectivity to hydroquinone in Figure 4 does not depend on the applied voltage in the range of - 0 . 2 to + 0 . 4 V , which corresponds to the The standard redox cathode potential of -0.01 to -0.53V (vs Ag/AgCl). potentials for Ee3'/Fe2+ and Ee2+/Fe are 0.57 and -0.64V (VS Ag/AgCl), respectively. Therefore, it is to be noted that most of the working iron cations must be in Fez+ state under the reaction conditions in this work. 3 . 6 . Reaction mechanism

We have suggested that OH radical could be the active oxygen species responsible for the oxidation of benzene over the cathodes of CW, FeC13/CW, and (Pd,FeC13)/CW [ 3 ] . The OH radical may be generated on the cathode as an intermediate o r a by-product during 02-H2 fuel cell reactions under short-circuit conditions. (At the cathode) Hf 02 (HOz., H202) H20 (3) H+, e-\i.:iiy H+,e-

+

We have assumed that OH radical attacks benzene, forming hydroxycyclohexadienyl radical as reaction intermediate. Further oxidation of this intermediate by dioxygen produces phenol [ 3 ] . For the oxidation of benzene on (Pd,Fe2Og)/CW cathode in this work, we believe that the reaction proceeds through the same reaction mechanism as However, our previous paper has not that proposed previously [ 3 ] . described about the formation of hydroquinone. We would propose the modified mechanism including the reaction path for the formation of hydroquinone as demonstrated in Figure 5. This mechanism hypothesizes that phenol is formed either via the electrochemical oxidation of hexadienyl radical (A) in step 11' o r via step 11, I11 and IV. As pointed out earlier, the selectivity to hydroquinone is insensitive to the partial pressures of oxygen (Figure 3 ) as well as to the cathode These observations strongly suggest that the potential below -0.01V. contribution of step 11' for the formation of phenol can be neglected under the standard reaction conditions. The parallel kinetic curves for the

710

formations of phenol and hydroquinone in Figure 2 suggest the parallel paths (IV and V) in Figure 5. The strong oxygen pressure dependence on the formations of oxygenates (Figure 3) can be ascribed both to the increase in the rate of step i in eq.(3) and to that in step I1 in Figure 5.

Figure 5. Reaction mechanism for oxidation of benzene.

The synergism of Pd black and Fe2O3 f o r the synthesis of phenol and hydroquinone may be explained in terms of the cooperative actions of the two additives. (i) Addition of Pd black increases the rate of reduction of dioxygen (step i in eq.(3)) o r the charge passed (Table 1). (ii) The iron compounds improve the oxidation efficiency (Table 2 ) , which can be ascribed to the increase in the concentration of .OH radicals (step iii in eq.(3)) probably through reaction ( 4 ) , H202 + H+ + Fe2+ d .OH + H20 + Fe3+ (4) as has been suggested in Fenton's reagent [ 4 - 6 ] . Thus, coaddition of Pd black and Fez03 remarkably improves the yield of oxygenates.

REFERENCES 1. K. 2. I. 3. K. 4 . F. 5. N. 6. C.

Otsuka, I. Yamanaka and K. Hosokawa, Nature, 345(1990)697. Yamanaka and K. Otsuka, J . Electrochem. SOC., 138(1991)1033. Otsuka, M. Kunieda and H. Yamagata, J. Elecrochem. SOC., 139(1992)2381. Haber and J . J . Weiss, Proc. Roy. SOC. London, Ser.A, 147(1934)332. Uri, Chem. Rev., 50(1952)375. Walling and R.A. Johnson, J. Am. Chem. Soc., 97(1975)363.

71 1

DISCUSSION CONTRIBUTION T. MALLAT (E.T.H., Zurich, Switzerland): You proposed a new method in your lecture for the direct transformation of benzene to phenol. But according to your data, under the best conditions in 1 hour you produced less than 5 mg phenol with 20 mg Pd metal. This method seems to be rather expensive for phenol production. The selectivity values were measured at conversion below 0.1 %. Do you have data at higher conversion? K. OTSUKA (Tokyo Inst. Tech., Tokyo, Japan): We are improving the electrocatalytic activity of the cathode. At this moment, as you commented, our method is quite expensive for phenol production because of low yield of phenol: 2% is the highest yield of phenol we obtained. However, this method is generally applicable for the production of oxygenates which are much more expensive than phenol. We believe the yield can be improved one order of magnitude by modifying the cathode.

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V. CortCs Corberin and S. Vic Bell6n (Editors), New Developmenis in Selective Oxiduiion II 0 1994 Elsevier Science B.V. All rights reservcd.

713

Influence of operational variables on the photodegradation kinetics of Monuron in aqueous titanium dioxide dispersions V. Augugliaro", L. Cavallerob,G. Marc?", L. Palmisano" and E. Pramaurob "Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita & Palermo, Vide delle Scienze, 90 128 Palermo, Italy bDipartimentodi Chimica Analitica, Universita di Torino, Via P. Giuria 5, 10125 Torino, Italy The degradation of Monuron [ N - (Cchlorophenyl) - N, N dimethyl urea] in aqueous TiO, dispersions irradiated in the near-UV region has been investigated using a Pyrex batch photoreactor. The influence on the degradation kinetics of pH, initial Monuron concentration and catalyst concentration has been studied. The mineralization of the pollutant was also investigated. Measurements of photon absorbed flows allowed to determine the quantum yield values; they were found to increase by increasing the initial pH of the dispersion. 1. INTRODUCTION

The pollution of the aquatic environment by harmful organic chemicals, as insecticides and herbicides, is a great concern in last years all over the world. Among the new promising methods for water purification, the photocatalytic one has gained increasing attention in recent years owing t o its applicative potentialities [l,21. Besides the establishment of the fundamental aspects of the method, assessments of the process economics [3, 41 in comparison with other methods have been also reported. The herbicide Monuron is a compound largely used in agriculture due t o its inhibitoring effect on the photosynthesis, but unfortunately it is a pollutant very persistent in the environment. In the present paper the results of photocatalytic degradation of Monuron in aqueous dispersions of TiO,, mainly anatase, using a Pyrex batch photoreactor are reported. The photoprocess was studied at different pH of dispersion and the influence of other operational parameters such as initial substrate concentration and catalyst concentration was investigated. 2. EXPERIMENTAL

High purity Monuron [ N - (4-chlorophenyl)- N, N dimethyl urea] was purchased from Laboratory Dr. Ehrenstorfer (Germany). H,SO, (Carlo Erba, RPE),

714

NaOH (Merck) and brucine (10,ll Dimethoxystrychnine, Carlo Erba, RPE) were reagent grade. TiO, P25 (Degussa, about 80 % anatase and 20 % rutile) [51 was used as photocatalyst. The determination of TiO, surface area (ca. 44 m2/g)was performed by means of the dynamic BET method using a Micromeritics Flowsorb 2300 apparatus with molecular nitrogen as the adsorbate. A Pyrex batch photoreactor of cylindrical shape (o.d., 8 cm, height, 16 cm) containing 0.5 1 of aqueous dispersion was used. The concentrations of TiO, and of the pollutant varied in the ranges 0.05-1.0 g/l and 0.02-0.1 g/l, respectively. The initial pH of the dispersion were: 1, 3,5.8, 7, 9,and 11 and were adjusted by adding H,SO, or NaOH, except for the case of pH=5.8. The photoreactor was provided with some ports in its upper section for the passage of gases, for sampling and for pH and temperature measurements. A 125 W medium pressure Hg lamp (Helios Italquartz, Italy) was immersed within the photoreactor. A radiometer UVP "UVX Digital", leaned against the external wall of the photoreactor at a fxed height, was used to measure the photon flow transmitted out of the photoreactor in different experimental conditions: without catalyst and with different concentrations of catalyst in the aqueous solution at the various pH investigated. The photoreactivity experiments were performed at 300 K. The dispersion was saturated with oxygen before starting the irradiation by bubbling pure 0, at atmospheric pressure; during the run also pure oxygen was continuously bubbled into the dispersion. The system was magnetically stirred, and samples for analyses were withdrawn at fmed intervals of time. The runs performed with the aim of studying the kinetics of Monuron photodegradation lasted about 40 minutes while those performed in order t o obtain a complete mineralization lasted about 8 hours. The quantitative determination of Monuron was performed measuring the absorption of the pollutant at 244 nm using an UV-Vis spectrophotometer (Varian DMS 901, after separation of the catalyst. The mineralization of the pollutant was monitored by a TOC analyzer (Carlo Erba TCM 480).For a selected run (initial pH=5.8, initial Monuron concentration = 0.08 g/l, TiO, concentration = 1.0 g/l) the experimental procedure was implemented in the following way: the gas outlet from the photoreactor was bubbled through a saturated Ba(OH), solution to trap CO,. The total amount of CO, produced during the photodegradation run was determined as BaCO,. Nitrates, product of complete oxidation of Monuron, were determined at the end of the runs by the brucine colorimetric method [6]. 3. RESULTS

No degradation of Monuron was observed in the absence of light andor of catalyst andor of oxygen. Table 1 reports the values of pH versus the time for runs with starting pH values of 3.0, 5.8, and 9.0. As it may be noted, the pH values decrease by increasing the time; the experiments performed with initial pH of 5.8 and 9.0 reach almost similar pH values (4.0 and 4.3)at the end of the run. In Figure 1 the TOC concentration and the Monuron concentration, expressed as carbon concentration, are reported as a function of the reaction time for pH values of 1and 11. The percentage of mineralization after 8 hours of irradiation was about 88% and 75% in the case of initial pH equal to 1 and 11 respectively,

715

while for pH values of 3, 5.8, 7.0, and 9.0 the mineralization was virtually complete, i.e. about 96-98%. The experiment in which the determination of CO, was carried out as barium carbonate confirmed the previous value of mineralization. The data obtained from the transmitted light measurements, performed at different catalyst concentrations and pH values of 3, 5.8, and 9.0, well fitted the following relationship:

Po= Pi exp[-ECcaJ.

(1)

Pi is the photon flow incident on the hspersion; it was measured at the external surface of the photoreactor in the absence of the catalyst and its value was of 3.97.10-6 einsteids. Po is the photon flow transmitted out of the photoreactor, E the extinction coefficient of dispersion and Cc, the catalyst concentration. Table 1 pH values of dlspersion at dlfferent reaction time. time[s]

0

240

480

720

960

1200 1440 1680 1920 2160 2400

3.00 3.00 2.99 2.98 2.98 2.98 2.98 2.97 2.97 2.97 2.96 5.80 5.70 5.00 4.70 4.40 4.30 4.20 4.15 4.10 4.05 4.00 9.00 6.70 5.95 5.15 4.90 4.60 4.50 4.45 4.40 4.35 4.30

20

-0.0

.

carbon [mgill 10

-

0 -

20

carbon [mgAl

0

.

O.

0

10

0

.

.I

0 0 0 I.

I

0

. .. . 0

0

-

0

.I

0

0

1 1

0

I

.

c

Figure 1. Total organic carbon concentration (0)and Monuron concentration ( H I , expressed as carbon, vs. run time. Catalyst concentration, 1.00 g/l. Initial pH of dispersion: a) 1.0; b) 11.0.

716

By applying a least square best fitting procedure on the data, the values of E were determined; they are reported in Table 2 as a function of dispersion pH. Equation 1 indicates that, as expected, the transmitted light decreases by increasing the concentration of the catalyst; from the E values it may be noted that negligible values of Po are reached for catalyst concentration of about 0.4-0.6 gA (POPi< 0.005). Concentrations of catalyst higher than the previous values are useless as the light absorption does not increase. Table 2 Values of the extinction coefficient, E, of dispersion as a function of pH. PH E [Ugl

3.0 9.42

5.8 13.35

9.0 8.74

4. DISCUSSION

The chemical kinetic model, which is capable of explaining the experimental results, is the following one [7]. The rate determining step is the reaction between OH radicals and Monuron molecules upon the Ti0 surface. Two types of sites are considered to exist: the first ones can adsorb Idonuron in competition with its oxidation compounds while the second ones can adsorb only oxygen. On this ground the reaction rate for the second order surface decomposition of Monuron may be written in terms of Langmuir-Hinshelwood kinetics as: r = k" 0,, O,On

in whch k" is the surface second order rate constant, 0 the fractional coverage of the sites by hydroxyl radicals and the fractiond coverage of the sites by Monuron molecules. By considering that all the runs have been performed at a constant concentration of oxygen in the dispersion, the ,8 is a constant in the experimental conditions used. By substituting the Langmuir expression for and by defining kc = k'.B,,, the rate equation becomes: r = kc KMOu [MonY(l + KMon[Monl + Ci Ki [I$

(3)

in which [Monl is the Monuron concentration, E(Mon and Ki are equilibrium adsorption constants and Ii refers t o the various intermediate products of Monuron degradation. By assuming that the adsorption coefficients for the organic molecules present in the reacting mixture are equal, i.e. that the following relationship holds:

KMOn EMonl+ Xi Ki [IJ = KMOu [Monl,

(4)

where [Monl, is the initial Monuron concentration, equation 3 becomes: r = k [Mon]

(5)

717

d~erk e = kc + KMon[Monl,) is the pseudo first order lunetic constant of Monuron photo ?gradation reaction. For all the runs the rate of degradation process exhibits a first order kinetics with respect to the Monuron concentration. Figure 2 reports the values of k vs. [Monl, obtained by applying a least-square best fitting procedure to the photoreactivity results. It can be noticed that the reaction rate increases by increasing the pH, even if the increase is very slight from pH=5.8 to pH=9.0. The data reported in Figure 2 show an inverse dependence of k on the initial Monuron concentration, as also reported in other investigations [7-111 performed on the photocatalytic degradation of different organic molecules. Figure 3 reports the values of k vs. the catalyst concentration for runs carried out at equal reaction conditions. It can be noticed that the values of k increase by increasing the catalyst concentration up to about 0.4-0.6 gA; for higher concentrations k is almost constant. This feature suggests that the photoreactivity level results from a balance of two opposite effects occurring at increasing catalyst concentrations: the beneficial one due t o the catalyst presence and the detrimental one due to the shielding phenomena. The transmitted light measurements indicate that the light is almost completely absorbed for catalyst concentration ranging between 0.4 and 0.6 gA. In the same range, also a constant value of k is obtained, thus indicating a correspondence of the highest photoreactivity with the highest photon absorption. Table 3 reports the values of quantum yield at different pH for runs carried out with initial concentration of Monuron of 50 mgA and with catalyst concentrations of 0.4 and 0.6 gA. At the previous values of catalyst concentration the transmitted photon flows are negligible so that it can be assumed that all the

5

k. lo4

k. lo4

[s-ll

[s-ll

5

0

Figure 2. Rate constant, k, vs. initial Monuron concentration, [Mon],. Catalyst concentration, 0.40 gA. Symbols: 0 , pH=3.0; U , pH=5.8; A , pH=9.0.

Figure 3. Rate constant, k, vs. catalyst concentration, Ccat. Initial Monuron concentration, 60 mgA. Symbols as in Figure 2.

718

photon flow emitted by the lamp is absorbed by the dwpersion. The figures of quantum yield reported in Table 3 have been calculated for the very first moments of the irradiation run.It may be confidently assumed that at the start of the degradation runs only Monuron molecules are present in the reacting mixture so that the aliquot of absorbed photons useful for the photoreaction is utilized only for Monuron degradation. The quantum yield values increase by increasing the dispersion pH thus indicating a positive role of OH- groups on the process. By applying a least square best fitting procedure on the k vs. [Monl, data correlated by the following equation: l/k = l/(kc.KMon)+ (l/kc)[Monl,,

(6)

the values of k, and KMonwere obtained; Table 4 reports these values for the used catalyst concentrations. A satisfactory fitting of the experimental data to eqn. 6 was obtained; this fact indicates that the model is adequate for describing the process, although it has been recently suggested that other mechanisms involving reactions in homogeneous media can also give rise to similar rate forms [121. The KMonvalues, reported in Table 4, seems t o indicate an independence of the adsorption equilibrium constant with respect t o the catalyst concentration (i.e. with respect to the light intensity inside the dispersion), but a dependence on Table 3 Quantum yield values for different pH of dispersion and catalyst concentration; initial Monuron concentration, 50 mgA. TiO, concentration

Quantum yield

WI

pH = 3.0

pH = 5.8

pH = 9.0

0.40 0.60

9.80.10-3 1.00-10-~

1.39.lo-' 1.61.lo-'

1.65.10-' 1.75.lo-'

Table 4 Second order rate constant, kc [mgl-l s-l], and adsorption equilibrium constant of Monuron, KMon[Vmgl, at different initial pH of dispersion and catalyst concentration. TiO, concentration [gAl 0.05 0.10 0.40 0.60 1.00

pH = 3.0

pH = 5.8

pH = 9.0

lo3

kc.103

KMon

kc.103

KMon

k;

9.6 12.7 16.6 19.6 22.4

0.15 0.14 0.12 0.12 0.14

19.2 27.9 32.6 34.1 31.9

0.03 0.06 0.03 0.10 0.09

20.0 33.5 31.1 32.8 33.1

KMon

0.06 0.03 0.09 0.10 0.07

719

the initial pH. The different values of KMo obtained by varying the pH suggest that the mechanism of adsorption depenas on the different forms in which the substrate is present in acid and basic medium (the higher pH values affect the solvation of Monuron molecules by water and therefore the adsorption equilibrium) and by the acid-base properties of the catalyst surface which is strongly affected by the pH of the dispersion [131. The k, values reported in Table 4 show an evident dependence on the catalyst concentration: kc increases by increasing the catalyst concentration even if almost constant values are obtained for pH equal to 5.8 and 9.0 and catalyst concentrations higher than 0.1 g/l. From the data reported in Table 4 it may be noted that the dispersion of KMonvalues is greater than that of kc ones. This fact can be justified by considering that the KMonvalues are calculated as the ratio between the slope and the intercept of the straight line expressed by eqn. 6 and, therefore, they are very sensitive to data scattering. The enhancement of the reaction rate as well as of the quantum yield by increasing the pH (at least up to pH=9), is probably due to increased concentration of physisorbed OH- groups at higher pH values. The primary step of a photocatalytic process is the generation of electron-hole pairs in the semiconductor [1,21. The pairs may recombine in the bulk or migrate to the surface; recombination process can be avoided if the pairs are separated and subsequently trapped by suitable sites. Hole trapping is carried out by surface hydroxyl groups (OH-s);this process is assisted by physisorbed hydroxyl groups (OH-J. Surface hydroxyl groups interact with positive holes so that hydroxyl radicals (OH,) form 112, 141:

OH-s

+

h'

+

(7)

OH.s.

Surface hydroxyl ralcals thus interact with physisorbed hydroxyl groups to produce surface hydroxyl groups and physisorbed hydroxyl radicals: OHs

+

OH-p + OH-s

+

OH-p.

(8)

Physisorbed OH- ions, acting as charge carrier species, ultimately participate in a process of charge transfer between the semiconductor and the electrolyte. It is evident, therefore, that OH- ions participate in the hole trapping process thus allowing the charge separation step to improve. The non-significant difference in photoreactivity between the runs at initial pH of 5.8 and 9.0 can be attributed t o the decrease of pH during the experiments (see Table 1). The complete mineralization of Monuron, in fact, determines the production of hydrogen ions. The global stoichiometry of the mineralization can be represented by the following equation: 2C1-C,H,-NH-CO-N(CHJ2 + 270,

+

18C0,

+ 4N0,- + 2C1- + 6H' + 8H,O.

(9)

An exhaustive investigation on intermediate compounds is reported in Ref. 7; in that study, carried out by using HPLC and GC-MS t e c h q u e s , together with the expected formation of several hydroxyaromatic derivatives, the unexpected hydrophobic compound 4-chlorophenyl isocyanate was recognized as a major reaction intermediate. It is worth noting that the mineralization in the case of pH 1and 11was incom-

720

plete, after 8 hours of illumination, while for the other pH it was virtually complete. From the observation of E'igure 1 it may be noticed that during the degradation run the TOC concentration is always higher than the Monuron concentration, expressed as carbon, especially in the first 2-3 hours of irradiation. This finding obviously indicates that the Monuron mineralization proceeds by intermediate steps in which stable species are formed. In the case of the run at initial pH of 1, the total carbon concentration remains constant during approximately the first two hours of illumination, although the fast disappearance of Monuron occurs. Similar behaviour can be observed for the run at initial pH of 11;for these runs the TOC concentrations do not reach negligible values for long reaction times. This feature indicates that the intermediate compounds which form at pH=l and 11 are different and more stable than those formed in the 3-9 pH range. The pH seems to be able t o change the degradation paths; this feature is not very useful in decontamination field but it would be of great interest in preparative photocatalytic processes.

ACKNOWLEDGEMENTS Financial supports from MURST (Rome) and CNR (Rome)are acknowledged.

REFERENCES 1. M. Schavello (ed.), Photocatalysis and Environment. Trends and Applications, Kluwer, Dordrecht, 1988. 2. E. Pelizzetti and N. Serpone (eds.), Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. 3. D.F. Ollis, in M. Schiavello (ed.), Photocatalysis and Environment. Trends and Applications, Kluwer, Dordrecht, 1988, p. 663. 4. N. Serpone, in J.R. Norris Jr. and D. Meisel (eds.), Photochemical Energy Conversion, Elsevier, New York, 1989, p. 297. 5. R.I. Bickley, T. Gonz6lez-Carreii0,J.S. Lees, L. Palmisano and R.J.D. Tilley, J. Solid State Chem., 92 (1991) 178. 6. H.J. Taras, A.E. Greenberg, R.D. Hoak and M.C. Rand (eds.), Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington D.C., 1971, p. 461. 7. E. Pramauro, M. Vincenti, V. Augugliaro and L. P a h s a n o , Environ. Sci. Technol., 27 (1993) 1790. 8. K. Okamoto, Y. Yamamoto, H. Tanaka and A. Itaya, Bull. Chem. SOC. Jpn., 58 (1985) 2015. 9. V. Augugliaro, L. Palmisano, A. Sclafani, C. Minero and E. Pelizzetti, Toxicol. Environ. Chem., 16 (1988) 89. 10. V Augugliaro, L. Palmisano, M. Schiavello,A. Sclafani, L. Marchese, G. Martra and F. Miano, Appl. Catal., 69 (1991) 323. 11. R. Matthews and S. McEvoy, J . Photochem. Photobiol. A, 64 (1992) 231. 12. C.S. Turchi and D.F. Ollis, J. Catal., 122 (1990) 178. 13. M. Schiavello, Electrochimica Acta, 38 (1993) 11. 14. P. Salvador, J. Electrochem. SOC.,128 (1981) 1895.

V. CortCs Corberan and S. Vic Bell611(Editors), New Deveiopmenis in Selaciivc, Oxlduiion I1 0 1994 Elsevier Science B.V. All rights rescrved.

72 1

HETEROGENEOUS PHOTOCATALYTIC OXIDATION OF LIQUID ISOPROPANOL BY Ti02, Z r O 2 AND ZrTiOg POWDERS J.A.Navio

*

and G.Col6n

Instituto de Ciencia de Materiales de Sevilla. Centro Mixto CSIC-Universidad de Sevilla and Dpto. de Quimica Inorgihica. Facultad de Quimica. 41012-Sevilla, Spain.

The photocatalytic oxidation of liquid isopropanol to acetone, in the presence of air, has been used as a test reaction to differentiate between the photoactivities of a processed zirconium titanate, ZrTiO4, and its two mther oxides, Ti02 and ZrO2. The experimental results, mich include values of rate of acetone production per surface unit and quantum yield, indicate that ZrTiO4 is much less photoactive than Ti02 (Degussa), although it exhi bits a similar photoactivity to that of Zr02 (Degussa).

Excitation of a semiconductor particle with a photon of eneg gy greater than the band-gap induces charge separation by crea ting an electron-hole pair. The capture of the photo-generated hole by interfacial electron transfer from an adsorbed acceptor allows for efficient oxidation and reduction, respectively, on a common surface[l] . The photocatalysed oxidation of gaseous isopropanol upon Ti02 (rutile) surface has been investigated by Stone et al. [2-31;this photo-reaction has been shown to be quite complex. The initial product is acetone, but the subsequent photo-oxidation of the latter produces formic acid and acetaldehyde. Ultimately the pro ducts of prolonged photo-oxidation are carbon dioxide and water. On the basis of the observed product distributions, a chain me chanism has been proposed[41. Mechanistic studies of the photo catalytically-induced oxidation of liquid isopropanol have been reported by Cundall et a1.[5-61 and by Egerton and King[7] .Their reported results do not support a chain mechanism for acetone formation.

*

Author to whom all correspondence should be addressed The financial support of the Ministerio de Educaci6n y Ciencia (Acci6n Integrada Hispano-Francesa, Grant HF-048, 1992) is gra tefully acknowledged.

722 On the other hand, Irick [81 has shown that the photocataly tic oxidation of liquid isopropanol to acetone in the presence of air is a simple reaction capable of differentiating between the photoactivities of several semiconductor materials. An homogeneous zirconium titanate, practically pure, has been prepared by using a sol-gel method. Processing and characterize tion has been previously reported [9]. In this paper, results related with the photocatalysed oxidation of liquid isopropanol on UV-illuminated suspensions of either Ti02, Zr02 and ZrTiO4 are presented. Discussion will be established by comparing the differences in the diffuse reflectance spectra and in the rate of acetone produced during the photo-reaction , between ZrTiOq and its two mother oxides. EXPERIMENTAL

2. 2.1.

Materials

Powdered zirconium titanate, practically pure, was processed following the sol-gel method previously described by us [9] .The amorphous solid was precipitated by hydrolysis in an alcoholic solution containing equimolar amounts of Tic14 (Merck, 99.99%) and ZrOC12 (Fluka AG, 43-44%, Zr02) in the presence of an excess of hydrogen peroxide. The precipitate was washed,dried and calci ned at 7000C for 2 h; the solid obtained had the structure of crystalline ZrTiOq. A zirconia gel powder was also prepared via hydrolysis of ZrOC12 using aqueous solution of ammonium hydroxi de at pH=ll. After washing and drying, this amorphous zirconia powder was calcined at l O O O Q C for 2 h; the solid obtained had the structure of crystalline Zr02 in the monoclinic phase and will be named hereafter Zr02 (hp). Commercial titanium dioxide, Ti02 and zirconium dioxide, Zr02, were supplied by Degussa, and before use were calcined in air at 5 0 0 Q C for 2 h. X-ray diffrac tion pattern showed that Zr02 (Degussa) is constituted by a mix ture of the monoclinic and mainly the tetragonal phases. Thermal treatments of the above samples were chosen on the basis of their previously reported characterization [9-111. Isopropanol and other reagents were analytical grade wherever possible. Gases used in the ambient atmosphere were of the hi ghest purity ( ) 99.99%) supplied by SEO. 2.2.

Techniques

The BET surface areas for the photocatalysts were measured by N2 adsorption at 78K. Diffuse reflectance spectra (UV-V DR) were obtained with a Perkin-Elmer Lambda 9 spectrophotometer using Bas04 as a referen ce. The Kubelka-Munk function was used to express the experimeg tal data. Photochemical reactor, light source and methods The photocatalytic oxidation of isopropanol was carried out

2.3.

723

in an Applied Photophysics Ltd. photochemical reactor equipped with a 400W medium pressure mercury-arc lamp, radiating predo minantl at 365-366 nm. This lamp produces more than 5x1019 phg tons s- within the reaction flask. It was contained in a dog bled-glass immersion well, through which water was passed for cooling. A borosilicate glass sleeve was used to remove short wavelength radiations (less than 300 nm). A gas inlet reaction flask (400 mL) was used: a double surface condenser was fitted to the reaction flask to prevent "creep" and loss of vapor.

Y "

The photocatalysts (1.5 g of each) were independently suspen ded in pure dried isopropanol (300 mL). Air or pure oxygen was bubbled through the suspension and a positive pressure of the gas was maintained during the period of the illumination (6 h). The photocatalyst was separated by centrifugation, to analyze the liquid phase. Samples of the photoreaction mixture were ang lyzed by a Hewlett-Packard gas chromatograph (model 5890).A 2.1 m column of 10% polyethylene glycol on Chomosorb W at 343K, with N2 carrier gas flowing at 1.6 mL s-1, showed a good separation of diethyl ether, acetone and isopropanol. Diethyl ether was ; sed as an internal standard, and equal volumes of the centrif; ged photoreaction mixture and of a standard solution of diethyl ether in isopropanol were thoroughly mixed before injection of a 5 PL sample into the chromatograph. The chromatograph was previously calibrated using varying concentrations of acetone and a fixed concentration of diethyl ether in isopropanol. Thus the concentration of acetone in the photoreaction mixture was obtained from the ratio of peak heights given by acetone and diethyl ether. 3.

RESULTS AND DISCUSSION

Diffuse reflectance spectra of ZrTiOq and of its two mother oxides Zr02 and Ti02 are shown in Fig.1. All the samples have spectra with bands in the same position, but the intensities are different from one sample to the other. As previously established [121, isoelectronic substitution, such as Zr4+ for Ti4+ in Ti02, does not change the concentration of electrons at T=OK in the conduction-band edge. According to the UV-V DR spectra in Fig.1, the equimolar substitution of Zr4+ for Ti4+ does not significan tly improve the spectral response of ZrTiOq if it is compared with those obtained for Ti02 (Degussa) and Zr02 (Degussa). In fact the experimental values for the band-gap, deduced from the UV-V spectra, reveal only small rise of ca. 0.04 eV in the absorption edge of ZrTiOq compared with Ti02. Regarding the conditions necessary for acetone production from isopropanol, the results presented in Fig.2 show that ill; mination, photocatalystand a source of oxygen are all necessary for measurable reaction rate to occur. It should be noted that the photoactivity with oxygen exceeded that with air, indicating that acetone formation is a function of the oxygen pressure.

724

Figure 1 W-Visible Diffuse Reflectance Spectra for samples: (a) Ti02; ( b ) zr~i04;(c) Zr02 (hp); and (d) Zr02 (Degussa). W

Y

Figure 2

Conditions necessary for aceto ne production from isopropanol: A ,02,W; 0 , photocatalysts, 02; photocatalyst~, N2 W; 0, Ti02, air, W; 0, Ti02, 02,

w.

0

1

2

3

4

timeihows

5

6

725

Fig. 3 shows acetone evolution during the photocatalytic oxi dation in air of isopropanol over the indicated photocatalysts. No reaction product other than acetone could be detected by GC; acetone formation follows a zero order kinetics. Rates of aceto ne production per gram of catalyst are compared in Table 1. It is note-worthy that the order of photocatalytic activity per surface unit follows the sequence,

0

I

I

I

1

1

2

3

4

I

I

5

6

time/ hours Figure 3

Acetone production durinq the photocatalytic oxidation of isopropanol: 0, Ti02; 0, Zr02(kgussa); b , Zr02(hp) ; I? , ~r~iO4.

726

Table 1 Acetone production by several catalysts during the photocatalysed oxidation (at 310K) of isopropanol Photocatalyst ~

Reaction rate (initial) mlh (g-cat) -1

Surface area Quantum ield In2 9-1 x 10

Y

~~

Ti02(Degussa)

zro2 ( Desussa1

*

Zr02(hane prepared)

10.01

46.5

5.05

1.09

35.4

0.55

0.94

23.4

0.47

0.78

39.5

0.39

L

ZrTi04

*

See tex? for the preparakion procedure

It should be considered that when a reaction shows a zero order kinetics, it indicates that the rate determining step is a phenawnon which is not an operative variable at the used experimental conditions. For exanp1es:a) the photon absorption rate by the powders could be too low so that the reaction rate is governed by the photon absorption; or b) a step, such as adsorpth of reactants or desorption of products from the w d e r could be the rate dc termining step: etc. On the other hand it should be taken into account that for the determination of an activity order among different photocatalysts, it is necessary that: i) the experimental system in which the powders are tested is always at equal reaction conditions: and that ii) it is sensitive only to the powder nature. All of these considerations could rise the queg tion is if a zero order reaction can be used as a "test" reaction. Although the photocatalytic oxidation reaction of liquid isopropanol to acetone merits further attention, hawever Cundall et al. [ 51 have investiga ted a number of parameters controlling the photoactivity of several semicog ductor powders, which have been taken in account in the present work. It may be worthy noting the following facts: A) Photoassisted oxygen isotope exchange (OIE) over Ti02 (Degussa,P-25)and other nonporous specimens occurs at much higher rate than over Zr02 [131 and over the present zr~i04[14]. B) The rate of photoassisted oxygen isotope exchange over a nonporous zirco nium dioxide was greater than Over ZrTiO4 [15]. C) Similar conclusions can be drawn from the photoconductance measurements [13-161 .

Therefore, although it will be, in principle, improper to establish an as tivity hierarchy among the powders considered in this work, only by the test reaction proposed by Irick [ 81 , however the photocatalytic activity predis ted for ZrTi04 in canparison with Ti02 and Zr02 [161, follows the same his rarchy that we have already observed in the present work. Thus we can proE se tentatively that the sequence observed f.or the rate of acetone prcductb per surface unit sems to be correlated with the intrinsic photoactivity of the stuhed semiconductor powders.

127

Quantum yields, q.y.r defined as the ratio between the mles of produced 5 cetone and the moles of absorbed photonsr have been estimated and are repor_ ted in Table 1. In calculating these values of q-y., it was assumed that a l l ra&ation entering the reaction vessel was absorbed by the photocatalyst.In principle, the values of q.y. could be used for comparing the photoactivity of the different semiconductor powders, because there are similar sequence between the q.y. values and the rate of acetone production per surface unit for the photocatalyts studied here. However, the use of q.y. values,as cai culated here, can be accepted only if there is the experinental evidence t h a t the acetone production pathway does not change by changing the photocg talyst, what up the present still as hypothesis. The order given above is valid only for samples studied here, because the photoactivity could be hfferent for the same solid according to the mrphg logical and textural properties of samples [141. In fact, differences in tocatalytic activity and diffuse reflectance can be found by comparing t m for Zr02(Degussa) and Zr02 (hp). With respect to the mechanism, we infer that the mechanism involved se transformations is parallel to that thought to occur in the photocataly tic oxidation of monoalcohols [ 4 1. An electron-hole pair is created as a re sult of optical excitation oE the semiconductor ( S C ) : (SC) + hv C h+-

e- (exciton) --I 'h + e-

Following Stone et al. [ 2-31 , the photogenerated holes are trapped by surface OH; , followed by trapping of the photoelectrons by molecular O gen :

-

(11 q

Reactions (2) and (3) are followed by:

OH,

+ Me2CHOH

Me2eOH

+ H20

(4)

The H20 produced in reaction (4) regeneratFs the surface hydroxyl groups, Finally, formation of acetone frcm Me2COH could occur in the follmq ways : CH,.

Me2kOH + HO; Me2COH + O2

-

2Me2kOH 2 HO;

Me2C0

H202

Me2C0 + "0; Me2C0

____)

+

H2°2

+ Me2CHOH +

O2

728 The H202 formed, by either reaction (6) or (9), has been cog sidered by Cundall et al. [5] to take no further part in the reaction unless it is first decomposed by additional photoeles trons. 4 . CONCLUSIONS

Zirconium titanate is constituted by the same elements as ti tania and zirconia and shows very similar optical absorption properties. However, comparison of their rate constants in the photocatalytic oxidation of liquid isopropanol to acetone cleag ly indicates that ZrTi04 seems to be much less photo-active thin Ti02, although it shows a similar photoactivity per surface writ than that for Zr02. REFERENCES

M. Schiavello, "Basic Concepts in Photocatalysis", in: Photocataly sis and Environment, M.Schiavello (ed.), NATO-XI, Series C, Vo1.237, by Kluwer Academic Publishers, Dordrecht, (1978)p.351. 2. R. I. Bickley and F.S.Stone, J.Catalysis, 31 (1973) 389. 3. R.I.Bickley, G.Munuera and F.S.Stone, J-Catalysis,31 (1973) 398. 4. R.I.Bickley, in "Catalysis" Vol. 5, Specialist Periodical Reports, Ro yal S o c . Chem. (1982) 325. 5. R.B.Cundal1, R.Rudham and M.S.Salim, J.Chem. SOC. Faraday Trans. I, 72 (1976) 1642. 6. R. B.Cundal1, B.Hullme, R.Rudham and M.S.Salim, J.0il Col. Chem.Assoc., (1978) 351. 7. T. Engerton and C.J.King, J.011 Col. Chem.Asscc., 62 (1979) 386. 8. G.Irick, J.App1. Polymer Sci., (1972) 2387. 9. J.A.Navio, F.J.Marchena, M.Macias, P.J.Sanchez-Soto and P-Pichat,J . E ter. Sci., 27 (1992) 2463. 10. J.A.Navio, F.J.Marchena, M.Macias and P.J.Sanchez-Soto, in: Ceramic To day Tomorrow's Ceramics, P. Vincenzini (ed.), Materials Science PbnG graphs, Vol. 66B, Elsevier, Amsterdam, (1991) p.889. 11. J.A.Navio, M.Macias, M.Gonzalez-Catal&n and A.Justo, J.Mater.Sci., 27 (1992)3036. 12. G. Bin-Daar, M.P. Dare-Edwards, J.B.Gcodenough and A.Hamnett, J.Chem. Soc., Faraday Trans.1, 79 (1983) 1199. 13. J.M.Herrmann, J-Disdierand P-Pichat,J.Chem.Soc., Faraday Trans.1, 77 (1981) 2815. 14. H.Courbon, M-Formentiand P.Pichat, J.Phys. Chem. 81 (1977) 550. 15. H.Courbon and P.Pichat, Ccmpt. Rend. Acad. Sci. (Paris)C285 (1977) 171. 16. H.Courbon, J.Disdier, J.M.Hernnann, P.Pichat and J.A. Navio, Catalysis Letters, 20 (1993) 251. 1.

V. CortCs Corbcran and S. Vic Bcll6n (Editors), New Developments in Selective Oxidmion II 0 1994 Elsevier Science B.V. All rights reserved.

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Effect of the state of vanadium on the properties of titanium phosphatebased catalysts for oxidation of toluene J. Soriaa, J.C. Conesa”, V. Villalbab, A. Aguilar Elguezabal” and V. Cortes Corberan” ”Instituto de Catalisis y Petroleoquimica, C.S.I.C, Campus U.A.M. Cantoblanco, 28049 Madrid, Spain Centro de lnvestigaciones Quimicas, Cerro, La Habana (Cuba)

SUMMARY

The effect of the addition of different amounts of vanadium during the preparation of titanium phosphate samples has been studied by XRD and ESR, and the catalytic properties of the resulting materials for the partial oxidation of toluene have been tested. The results indicate that the type of titanium phosphate formed depends on the vanadium concentration; the stability of these phosphates and the vanadyl ions located in them under reducing conditions determine the catalytic properties of the thus obtained catalysts in selective redox processes.

1. INTRODUCTION Vanadia-titania mixed materials are catalysts of choice for some selective oxidation reactions [I], but lack mechanical resistance. Addition of phosphoric acid improves their mechanical performance but, at the same time, modifies the phase composition of the catalysts by forming phosphates [2], thus influencing their catalytic properties. In order to study in detail how the catalytic properties of the vanadiahitania system are modified by the use of phosphoric acid as a binder, we have examinated the effect of adding a cation originating active centers for selective oxidation (vanadium) into a nonselective phase such as titanium phosphate. There are few studies on the properties of the ternary oxide system V-Ti-P, mainly focused on the effect of the amount of P,O,. In this sense, it has been reported that addition of small amounts of P,O, to a V-Ti-0 catalyst increase both activity and selectivity for the partial oxidation of butene [3]. In this work we have studied the changes produced in the structure and reactivity of Ti phosphate materials by the introduction of different amounts of vanadium ions during their preparation. These results are then related with the observed influence of the amount of added V on the partial oxidation of toluene over these catalysts.

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2. EXPERIMENTAL 2.1 Catalysts preparation The catalysts were prepared by impregnation of anatase powder (Carlo Erba) with aqueous solutions of ammonium metavanadate (BHD) having three different vanadium concentrations, followed by drying the samples in air at 473 K. The materials thus obtained were impregnated with phosphoric acid aqueous solutions and calcined at 823 K. The resulting phosphorus content was 15 YOw/w of P,O, in all the catalysts, and the obtained vanadium concentrations were 0.7, 2 and 3 % w/w of V,O, in the samples denoted hereafter as V-0.7, V-2, and V-3 respectively. An additional V-free sample (sample V-0),prepared by impregnation of the anatase material with the same amount of phosphoric acid and calcined at the same temperature, was used as reference. In all samples the surface area was 10 m2 g-’, as measured by the BET method. The samples were studied without any further previous treatment. 2.2 Catalysts characterization X-ray diffraction (XRD) measurements were carried out on a Philips PW1010 diffractometer using nickel-filtered copper K a radiation. Electron spin resonance (ESR) spectra, except where otherwise indicated, were obtained at 77 K with a Bruker ER 200D spectrometer operating in the X band, and using a DPPH standard (g=2.0036) to calibrate the magnetic field/frequency ratio. The samples were placed in specialpurpouse ESR quartz cells, provided with greaseless stopcocks, where they could be outgassed or treated under controlled gas atmosphere at different temperatures (vacuum manifold base pressure = Pa) and subsequently transfered to the ESR spectrometer without contact with air. Computer simulation of the ESR spectra using second order perturbation theory was performed where necessary for more accurate evaluation of the spin-Hamiltonian parameters. 2.3 Catalytic testing Catalytic activity of the samples for oxidation of toluene was measured in a tubular, plug-flow isothermal glass reactor at atmospheric pressure in the temperature range 673-748 K, using a reacting mixture to1uene:oxygen:helium in molar ratio 1: 13:38, total flow gas 140 mllmin. Toluene, introduced by a micropump, was evaporated in a preheating zone. The catalyst bed contained 0.20 g of catalyst, prepared by pelletization and fragmentation into particles of 0.42-0.50 mm particle size, and diluted with SIC tips of the same size to avoid hot spots. Reactants and products were analyzed on-line by GC, using two packed columns: 5A molecular sieve for 0, and CO, and Porapak Q for the rest. All connections between reactor and GC sampling valve were kept at 490 K to avoid condensation of high boiling point products. The possible relevance of homogeneous reaction contribution was determined by performing test in the absence of catalyst; it was found that below 800 K this reaction pathway can be ignored. Toluene conversion and yields of products were expressed as mol % on a carbon atom basis. In all experiments carbon balances were within 100*5%.

73 1

3. RESULTS

3.1 X-ray diffraction (XRD) Figure 1 shows the XRD patterns of the four samples examined. As it can be seen, the main crystalline component in all samples was anatase, which presented a similar contribution to the diffractograms in all cases, except for sample V-3 in which anatase lines were noticeably smaller. Apart from the TiO, contribution, all patterns presented lines at spacings d = 3.92 and 3.21 A, which together with a line at d = 3.50 A, unobservable due to overlapping with the main anatase reflection, are the three strongest lines of TiP,O, [4]. This pyrophosphate phase was better crystallized in sample V-3 (figure I d ) , while in samples V-0.7 and V-2 (figures l b , and I c ) it presented lower crystallinity than in sample V-0 (figure l a ) . Sample V-3 presented also lines at d = 3.31, 3.23 and 3.16 A, which correspond to the three main reflections of Ti,0(P0,)2 [5]. All of the significant diffraction lines observed can be ascribed to either one of the three phases mentioned.

28

Figure 1. XRD pattern of catalysts. A) 3.2 Electron Spin Resonance V-0; b) V-0.7; C) V-2; d) V-3. The ESR spectra of the three Vcontaining samples recorded before any pretreatment (figure 2), revealed the presence of only one paramagnetic species in each of them showing, in the three cases, axial symmetry and resolved hyperfine structure of eight lines. The observed signal (signal A) was the same for samples V0.7, and V-2, where it had ESR parameters g,, = 1.919, g, = 1.981 (resulting in = 1.960), A,,= 1 8 . 7 ~ 1 0cm-’ -~ and A_ = 7 . 5 ~ 1 0 . ~cm” (figure 2a), while for sample V-3 the signal presented narrower linewidth and different parameters (signal B): g,, = 1.925, gi = 1.969 (giving
132 2.0036

al

I $I

200 G

I \

Figure 2. ESR patterns of: sample V0.7 as prepared (a), and after reaction and exposure to air (b); V-3 as prepared (c).

w

Figure 3. ESR spectra of samples afler reduction at 673 K:V-0.7 (a) and V-3 (b) recorded at 77 K; and V-0.7 recorded at 295 K (c).

(1=7/2) in the V(IV) state; these must be forming vanadyl (V=O)2' groups instead of being incorporated as V4' ions having only single V-0 bonds, since these ions would present the hsc A,,< 15.2~10"cm-' [6]. These (V=O)" groups are most probably placed in phosphate compounds, because when they are bound to anatase their ESR signals present A,, values noticeably lower than those found here for signals A or B [7]. The narrow linewidth of both signals, particularly in the case of signal B, indicates that these vanadyl groups are magnetically isolated and occupy very homogeneous positions in two different diamagnetic (titanium) phosphates. By outgassing treatments at different temperatures, T, between 295 and 773 K, the intensity of the vanadyl signal changed, reaching a maximum for T, = 673 K and decreasing markedly at T, = 773 K for samples V-0.7 and V-2, while for sample V-3 the signal decreased only slightly after reaching the maximum. Upon heating the samples at different temperatures, T, under H, in static conditions (closed ESR cell, PHZ= 10 torr), the vanadyl signals decreased with increasing T, for T, >573 K, indicating that the corresponding vanadium ions were reduced to V3'. For T, = 673 K the vanadyl signal was very small for all the samples. But, while for samples V-0.7 and V-2 a new signal C appeared with g, = 1.925, g2 = 1.898 and g3 = 1.857 ( = 1.893) showing a substantial intensity (figure 3a), for sample V-3 the observed similar signal C', having g1 = 1.957, g2 = 1.909 and g3 = 1.873 ( = 1.910), showed an intensity ca. ten times lower (figure 3b). Signals C and C' were not observed when the spectra were taken at 295 K: in these conditions the spectra showed only very small vanadyl signals overlapping a broad signal D with g 4 . 9 6 (figure 3c). Signals C and C', with no hyperfine structure present values lower than the value of the vanadium signals A and B, and close to the value observed for Ti3+ ions in reduced titanium phosphates [a]; they can therefore be assigned also to Ti"

733

ions, present probably in two different titanium phosphate compounds. The disappearance of both signals when the spectra were obtained at 295 K supports this assignment. Due to the short spin relaxation time of the Ti” ions, their signal is usually not observed at room temperature, as it is well known in the case of Ti3+ in anatase. The signals of Ti” ions in anatase or rutile present different parameters with larger g values, than those observed for signals C and C‘. These evidences indicate that the Ti” ions detected are not placed in the anatase phase, but are due to the reduction of titanium phosphates. Signal D with cg> value similar to the vanadium signals and observed at 295 K must be assigned to V(IV) ions. The absence of hyperfine splitting structure must be due to line broadening produced by dipolar interactions between vanadium ions, which would indicate that these ions have agglomerated.

3.3 Toluene catalytic oxidation In the conditions used, conversion of toluene on these samples started at 673 K, being carbon oxides (CO,) and benzaldehyde (BzA) the main products, accounting for more than 95% of conversion. At high conversion levels, maleic anhydride and condensated aromatic ring products also appeared in minor quantities. Figure 4 shows the influence of reaction temperature in catalytic activity. Although the vanadia content of sample V-2 is three times higher than that of sample V-0.7, no significant difference was found in the activity of both samples, very close to that of the sample without vanadium (V-0).

40T----4 l i

t

2

I 10 20 O l

I

798

648

Figure 4. Influence of temperature on catalytic activity for toluene oxidation: A V-0; 0 V-0.7; 0 V-2; + V-3.

-

0

0

10

20

30

40

Figure 5. Evolution of selectivity to BzA with total conversion of toluene. Symbols as in Fig. 4.

134

However, with a further increase of only a 50% of the vanadium content of sample V-2, a significative improvement was observed for sample V-3, which activity was roughly double that of sample V-0.7. This improvement cannot be accounted for by a modification of the surface area, because it was similar for all the samples, nor by the number of vanadium atoms in the samples, as shown above. Consequently, it should be related to the change in the phase composition of the sample, and specifically to the appearance of Ti,O(PO,),. More differences were observed in the selectivity to BzA. As shown in figure 5, the initial selectivity to BzA increased with the increase of the vanadium content. As conversion increased, the selectivity decreased for every sample, but V-containing samples remained more selective than that without vanadium. Evenmore, the difference in selectivity of samples V-0.7 and V-2 decreased with conversion, disappearing above 20 mole%, while selectivity of V-3 remained higher in the whole range studied. These results indicate that vanadium insertion in the titanium phosphate framework modified the reducibility properties of the active centers, thus decreasing the rate of the secondary transformation of benzaldehyde.

4. DISCUSSION

XRD results indicate that all samples contained anatase and the titanium pyrophosphate TiP,O,; besides sample V-3 contained also Ti,O(PO,),. The ESR results indicate that Ti3’ ions in titanium phosphates were formed by reduction under H, for T, > 673 K. Ti3+ ions are also formed when it is exposed to H, at those temperatures in anatase, but presents a different signal. The absence in our samples of this anatase Ti3+ signal indicates that the anatase phase is not affected by the reduction process. As the described reduction treatment should affect mainly the sample surface, it can be inferred that the anatase content indicated by XRD is not forming part of the sample surface. This conclusion could be expected considering the method used for the preparation of the samples; the reaction of phosphoric acid with anatase can form external titanium phosphate layers, leaving anatase nuclei inside the catalyst particles. The observation by XRD of a smaller amount of anatase for sample V-3 than for the other samples, although the same amount of phosphoric acid was used to prepare all of them, is probably related to the observed formation of Ti,0(P0,)2, a phosphate with a ratio Ti/P=l. The formation of a titanium phosphate with this Ti/P ratio implies obviously the reaction with phosphoric acid of a larger amount of anatase than when the resulting titanium phosphate presents a ratio Ti/P=0.5. XRD results also show that the vanadium content has an influence on the type of titanium phosphate formed, but this technique cannot provide information about the state of the vanadium ions, due to their low concentration in these samples. The ESR results indicate that vanadium ions, in the form of isolated (V=O)” vanadyl groups, occupy relatively homogeneous positions which are located in the same specific titanium phosphate phase for samples V-0.7 and V-2. Considering that a compound that offers these conditions for location of impurities should be crystalline and that the only phosphate detected in these samples by XRD is TiP,O,, we assign signal A to

735

(V=O)" centres located in this pyrophosphate compound. The XRD results also indicate, on the other hand, that the presence of low vanadium concentration diminishes the crystallinity of TiP,O,. This suggests that these vanadyl cations were incorporated into this compound before its crystallization, possibly by ion exchange into the layers of Ti(HPO,),.nH,O, which is the known precursor of the pyrophosphate phase [9], and remained in certain specific positions in the resulting phosphate structure formed during the calcination treatment. For sample V-3 the parameters of signal B indicate a different location of the vanadyl groups; according to the small linewidth they should occupy also magnetically isolated positions, but now in a different crystalline titanium phosphate phase. XRD indicates the formation of Ti,O(PO,), in this sample; therefore we assign signal B to (V=O),' groups located in this compound with a crystalline structure more suited to acommodate impurities than the pyrophosphate [lo]). The same signal has been observed in another study in which by a different preparation method, the same Ti phosphate was obtained using of vanadium ions as promoters [II]. The intensity of signal B, which is found to be similar to that of signal A in sample V-0.7 eventhough the latter has a vanadium content four times lower, indicates that most of the vanadium ions in sample V-3 were not observed by ESR, probably because they are present as diamagnetic ions (V5+ or ( ~ 0 ) ~ ' ) . The ESR study of the samples after reduction treatments indicates that the redox stability of the vanadium ions is in someway affected by their location; their reducibility seems to be lower in Ti,O(PO,),, since the decrease caused in the (V=O),' signal by outgassing at 773 K is less important when the vanadium is located in Ti,O(PO,), (sample V-3) than when it is present in TiP,O, (samples V-0.7 and V-2). A more marked difference is observed in the reducibility of the Ti compounds upon treatments under H,; for Ti,O(PO,), and vanadium-free TiP,O,, Ti is far less reduced at T, = 673 K than in vanadium-containing TiP,O,. The fact that vanadium ions are reduced at lower temperature than titanium ions in these phosphates indicates that some (V=O)" groups are placed at the phosphate surface, where they can be more easily affected by external conditions modifying their oxidation state without the need of a significant reduction of the resting compound. The different titanium reducibility in samples V-2 and V-3 indicates that vanadium-free TiP,O, is reduced at higher temperature than vanadium-containing TiP,O,, probably because there is an important interaction between the vanadium ions and the titanium phosphate which favours the reduction of the pyrophosphate once the vanadium is reduced. In the case of vanadium containing Ti,O(PO,), this interaction must be smaller and Ti3' ions from this phosphate (or from TiP,O,) are observed for higher T,. The catalytic results also show differences in the behaviour of sample V-3, as compared with the other vanadium-containing catalysts, that cannot be related exclusively to the higher vanadium content. The higher selectivity to BzA of samples containing vanadium in Ti,O(PO,), should be related to (V=O)" characteristics in this compound, because a cooperative effect of the titanium phosphate can be discarded considering its higher stability to reduction under H,. Considering that the vanadium ions in the pyrophosphate were introduced by ion exchange during the formation of its layered precursor compound, where they would occupy sites between the layers and most of them should remain buried in the TiP,O, structure. While in Ti,O(PO,), the vanadium ions will occupy specific positions in its structure, a larger number of

736

vanadium ions are probably located at the catalyst surface for the latter case. The small intensity decrease of the ESR signal by outgassing at T, = 773 K indicate a relatively higher stability of the (V=O)" under mild reduction conditions, which can have an important effect when these groups are active in partial oxidation. An EXAFS study of vanadium in sample containing Ti,O(PO,), has shown that the V environment is characterized by comparatively high average number of V=O bonds [12]. Many authors agree that selective oxidation is due to vanadyl groups at surface [13,14], which agree with the results of this study, in which the increase of activity and selectivity to BzA of sample V-3 can be explained as a consequence of the increased number of active sites for selective oxidation available at catalyst surface. REFERENCES [I]V. Nikolov, D. Klissurski and A. Anastasov, Catal. Rev.-Sci.Eng., 33 (1991) 319. [2] J. Soria, J.C. Conesa, V.M. Villalba and M. Castro, Actas Xlll Simp. Iberoam. Catal. 2 (1990) 183. [3] M. Ai, Bull. Chem. SOC.Japan, 50 (1977) 355. [4] H.F.Mc Murdie, M.C. Morris, E.H. Evans, B. Paretzkin, W. Wong-Ng and Y. Zhang, Power Difraction J. 2 (1987) 52. [5] N.G. Chernornkov, I.A. Koroshunov and M.I. Zhuk, Rus. J. Inorg. Chem. 27 (1982) 1728. [6] H. Narayana and L.Kevan, J. Phys. C, 16 (1983) L863. [7] E. Serwicka and R.N. Schindler, Z. Phys. Chem., NF, 127 (1981) 79. [8] M. Makamura, K. Kawai and Y. Fujiwara, J. Catal., 34 (1974) 345. [9] J. Soria, J.E. lglesias and J. Sanz, J. Chem. SOC.Faraday Trans., (in press). [ l o ] Von Walter Gebert and Ekkehart Tillmans, Acta Cryst., 831 (1975) 1768. [ I 1] J. Soria, J.C. Conesa M. Lopez-Granados, J.L.G. Fierro, J.F. Garcia de la Banda and H. Heinemann, (to be published). [I21 M. Lopez-Granados, J.C. Conesa, P. Esteban, H. Dexpertand D. Bazin; Proc. 2"d Eur. Conf. Progr. X-ray Synchrotron Res., (Rome 1989) A. Baleron, E. Besview, S . Molulio, Eds., Editrice Compositori, Bologna (1990) 551. [I31 Israel E. Wachs, R.Y. Saleh, S.S.Chan and C.C. Chersich, Appl. Catal., 15 (1985) 339. [14] M. Gasior, T. Machej, J. Catal., 83 (1983) 472.

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J. Haber (I. of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland): Do you have any data indicating whether there is segregation of the vanadium containing phase at the surface?. Relatively small influence of vanadium on catalytic activity seems to indicate that vanadium is in the bulk. J. Soria (I. de Catalisis y Petroleoquimica, Madrid, Spain): The ESR spectra indicate that

the vanadyl groups are very well dispersed in the sample. The temperature needed to reduce the vanadyl groups suggests that most of them are located in the bulk.

B. Delmon (Universite Catholique de Louvain, Louvain-la Neuve, Belgium): This is a contribution to the discussion initiated by Prof. Trifiro and Haber. I wonder whether there would not be some formal analogy between VO,/TiO(PO,) and VOJanatase, where a mutual structural effect occurs. The major effect is the stabilization of an intermediary oxide of V, namely of a stable mixed valency oxides (T.Machej, P. Ruiz, B. Delmon). Do you believe one could also assume, in your case, that a mixed valency, stable, V species would be present (e.g. as rafts, or monolayers on TiO(PO,)?.

J. Soria: We cannot exclude the possibility that part of the vanadium ions are present as V5', particulary when the vanadium concentration is below 3%; however, the formation of a segregated mixed-valence phase is very unlike because it should have been detected by ESR.

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V . Corks Corherin and S. Vic Bcll6n (Editors), N e w Deveiopmmls in Selecrive Oxdarion I1 0 1994 Elscvier Scicncc B.V. All rights rcscrved.

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Quantum - chemical description of the oxidation of alkylaromatic molecules on vanadium oxide catalysts J.Habef , R.Tokarz" , M.Witkob 'Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul Niezapminajek, 30-239 Cracow, Poland bFritz-Haber Institut der MPG, Faradayweg 4-6, D-1000 Berlin 33, Germany Aleksander von Humboldt Fellow, on leave from the Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Cracow, Poland

SUMMARY SINDO method was used to calculate the interactions developing on approach of benzene or toluene to a cluster of six edge- and comer-linked vanadium-oxygen square pyramids which represent an element of the (010) plane of V,05 catalyst. Most exothermic is adsorption with ring plane parallel to the plane of the cluster, resulting in strong interactions of carbon atoms with surface vanadium and oxygen atoms and formation of carbon deposit or total oxidation products. Perpendicular end-on adsorption of toluene at the comer-bridging oxygen leads to abstraction of two hydrogens from the methyl group and formation of strong carbon-oxygen bond to give the precursor of benzaldehyde. Concomitantly the V-0-V bonds are dramatically weakened facilitating desorption of the product. In the case of benzene perpendiculary adsorbed species are weakly bonded and may serve as intermediates in electrophilic oxidation to maleic anhydride by coadsorbed oxygen molecules.

1. INTRODUCTION Oxidation of benzene and toluene on vanadium oxide monolayer catalysts has been a subject of many studies [1,2 and references therein]. Many attempts to identify the reaction intermediates in oxidation and ammoxidation of toluene by IR spectroscopy and to elucidate the mechanism of the reaction have been undertaken [3-131. These studies lead to the conclusion that in the case of toluene the reaction starts with the formation of the benzyl intermediate, which interacts with surface lattice oxygen to form, consecutively, adsorbed benzaldehyde and benzoic acid precursors. These may either desorb as products of selective nucleophilic oxidation or may be further oxidized to carbon oxides, or undergo degradation of the aromatic ring with the formation of maleic anhydride and carbon oxides. Much less information is available as to the initial step of the reaction of benzene which then becomes

740

oxidized to maleic anhydride or undergoes total oxidation. Many results indicate that it is an electrophilic oxidation by adsorbed oxygen species 114-161. In order to find the mechanism of the initial activation of the molecule and specify the factors determining the choice of the pathway by the reacting system quantum chemical calculations were carried out of the interactions developing on approach of benzene or toluene molecule to a cluster composed of two or six vanadium-oxygen square pyramids assumed to be a model of supported vanadium oxide monolayer catalyst. 2. MODELS

AND METHOD OF CALCULATIONS

The semiempirical INDO calculations [17-191 were carried out of the interactions developing on approaching benzene or toluene molecule to the vanadium-oxygen cluster taken as the model of a vanadium oxide catalyst. V20, and V,02, clusters built of two and six edgeand corner-sharing V-0 square pyramids respectively were chosen [20] (Fig. 1).

&

MODEL I

i I

S m (A)

Figure 1. Models. Model 1 is the smallest structural element which can mimic the existence of two different oxygen atoms: vanadyl oxygen, coordinated to one vanadium atom, site (A), and bridging oxygen, coordinated to two vanadium atoms, site (B2). Model 2 contains all symmetry elements of the V,O, structure and moreover, illustrates also the presence of bridging oxygen atom (B3) coordinated to three vanadium atoms. Calculations were carried out for the toluene or benzene molecule approaching the cluster along a reaction pathway perpendicular to the plane of the cluster from above three different adsorption sites, the vanadyl oxygen, site (A), the bridging oxygen between the two vanadium atoms, site (B2) and the exposed vanadium atom, site (C), (see Fig.1). The plane of the aromatic ring of the approaching molecule was oriented either perpendiculary to the axis of approach (the side-on adsorption with the plane of the molecule parallel to the plane of the cluster) or along this axis (end-on adsorption with the molecule attached perpendicularly to the plane of the cluster through the methyl group (toluene) or the C-C bond (benzene). The calculations were carried out for the experimental geometry of the free toluene and benzene molecule 121,221, and for the distances R [V- O(apical)J = 1.58 A and R [VO(pIanar)] = 1.87 A [23-251.

74 1

The total energy of the system as a function of the distance was examined. The distance was measured between the given adsorption site and: a) the center of the ring in the case when the plane of the ring is parallel to the surface b) the C atom of the methyl group when toluene is adsorbed end-on c) the C atom or C-C bond of benzene ring depending on the way of adsorption. In all cases the plot of the total energy as a function of the reaction coordinate showed a minimum corresponding to the formation of an adsorbed complex.

3. RESULTS OF CALCULATIONS 3.1 Total energy of adsorption. Fig. 2 (a) and (b) illustrates the total energy of the system composed of the V209 cluster and a benzene or toluene molecule respectively, plotted as a function of the reaction coordinate.

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IT v-0-v

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REACTION COORDINATE [A]

REACllON COORDINATE [A]

Figure 2. Changes of the total energy on approaching benzene (a) or toluene @) to different adsorption sites of the V,O, cluster as a function of the reaction coordinate. In both cases the parallel (side-on) adsorption is characterized by the greatest exothermic effect, the exposed vanadium (site C) being the most preferred adsorption site. The end-on adsorption of toluene can take place on all three adsorption sites (A), (B2) and (C), the bond strength of its adsorption on site (332) and (C) being comparable. The evidently preferred site for perpendicular adsorption of benzene is the exposed vanadium atom (site C).

142

3.2 Adsorption with the plane of aromatic ring parallel to the surface. Fig. 3 illustrates the changes of the total energy observed when a benzene molecule oriented parallely to the plane of the V,09 cluster is moved at the distance 0.8 A above this plane along its axis. It may be seen that the most stable adsorption complex is formed when the centre of benzene ring is located above the vanadium atom (site C). The second less stable adsorption complex appears when the centre of benzene ring is between the vanadium atom and bridging oxygen atom. The lower insert illustrates the values of the changes (A in %) of the diatomic energy contributions in the transition complex at the equilibrium distance in respect to the isolated V,O, cluster and the benzene molecule. A positive value of the A function suggests that the appropriate bond becomes weaker, whereas the negative one indicates the strengthening of the bond. In the case of bonds formed between adsorbate and cluster atoms the absolute values of the diatomic energy contributions are shown. Strong interactions of carbon atoms with surface oxygen and vanadium atoms result in the destruction of the adsorbate complex and the formation of the total oxidation products. E"l

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3.6

REACTION COORDINATE [A]

Figure 3. Changes of the total energy of the system V,O, cluster+benzene when benzene with its plane parallel to the plane of the cluster is moved along the axis of the cluster at the distance 0.8 A above its plane, the centre of the ring moving from the point above the external oxygen to that above bridging oxygen as indicated in the upper insert. The lower insert shows the A % values for the adsorption complex at the point of minimum energy.

743

Fig. 4 shows the changes (A in W ) of the diatomic energy contributions in the transition complexes in respect to the isolated V209cluster and the toluene molecule. There is a strong tendency to the formation of bonds between carbon and oxygen atoms. At the same time all bonds between carbon atoms of the toluene molecule are considerable weakened. Also bonds within the cluster are strongly affected. Similary as in the parallel adsorption of benzene these processes are equivalent to the destruction of the adsorbate complex and the formation of the total oxidation products.

I

t

I

Figure 4. Changes (in 970) of the diatomic contributions in side-on adsorption complex in respect to the isolated V209cluster and toluene. 3.3 Adsorption with the plane of aromatic ring perpendicular to the surface. Calculations were carried out for the perpendicular adsorption of toluene molecule on the V,O, and VaOzo clusters. Adsorption of the nonactivated and the activated toluene molecule on the three different adsorption sites: vanadyl oxygen, site (A), bridging oxygen, site (B2) and vanadium atom, site (C), (see Fig. 1) was studied. Activation of toluene was assumed to take place by abstraction of one or two hydrogen atoms from the methyl group followed by the simultaneous formation of one or two OH groups with the surface oxygen atoms of the catalyst. Results of the calculations lead to following conclusions: i) toluene molecule becomes bonded to all studied adsorption sites leading to the formation of stable adsorbate complexes; ii) adsorption on the vanadium atom is the most stable but a precursor of the oxygenated product is not formed. Adsorption at the vanadium atom site is thus equivalent to blocking of the surface; iii) adsorption of toluene molecule at oxygen sites leads to the formation of a precursor of the oxygenated product (benzaldehyde);

744

iv) comparison of the adsorption of toluene at different oxygen sites (A and B2) leads to the conclusion that the bridging oxygen site is the most preferential one for the adsorption of toluene; v) formation of the transition complex with activated toluene molecule as a ligand is energetically more favourable than the formation of the complex with a non-activated molecule. Adsorption of toluene species activated via abstraction of two hydrogen atoms turns out to be the most stable. The process of abstraction of the third hydrogen atom from the methyl group is energetically expensive. The formation of complexes of the toluene molecule adsorbed at sites (A), (B2) and (C) affects the bonds in the benzene ring to a small degree only but leads to a significant weakening of the C-H bonds in the methyl group and of the V - 0 bonds in the V,O, cluster. The fact that the C-H bonds in the methyl group become weaker suggests the abstraction of two hydrogen atoms resulting in the formation of adsorbed complex of the C6H5-CHspecies as a ligand characterized by lower energy than complexes of GH,-CH, or C,H,-CH, species. Fig. 5 summarizes the changes (A %) of the diatomic energy contributions and the energies of bond formation in the adsorbate complexes of toluene at the bridging oxygen site before and after abstraction of two hydrogen atoms.

9.2%

P

Figure 5. Changes (in X) of the diatomic energy contributions in adsorbate complex (activated and non-activated toluene) at bridging oxygen site in respect to the isolated cluster and toluene. The formation of transition complex with the C,H,-CH species leads to the dramatic destabilization of the bonds of oxygen playing the role of adsorption site with its nearest neighbours. This may be taken as an indication that the transition complex formed in the case of adsorption at the bridging oxygen site is a precursor of the benzaldehyde. A precursor of the oxygenated product may be formed also on adsorption at the vanadyl oxygen site however

745

the destabilization of the V - 0 bond is in this case much smaller and the desorption of a product molecule much less probable.

CONCLUSIONS Calculations of the diatomic contributions of the C-H bonds in the methyl group of the toluene molecule approaching the cluster in end-on orientation, as a function of the distance from the bridging oxygen indicate that these bonds become weakened already at fairly long distance, their strength decreasing when the molecule approaches nearer and nearer to the adsorption site. Thus, adsorption should be considered as a dynamic process representing a reactive chemisorption, in which the simultaneous transfer of two hydrogen atoms from the methyl group to surface oxygen atoms takes place in the course of adsorption. Simultaneously the carbon atom becomes linked to the bridging oxygen atom, whereas the bonds of the latter with its neighbours in the cluster are significantly weakened, the decrease of the V-0-V bond strength amounting to more than 80%. Oxygen can be thus easily extracted from the cluster in form of benzaldehyde leaving behind a vacancy. This series of consecutive elementary events, representing the nucleophilic oxidation of toluene to benzaldehyde on vanadium oxide catalysts is shown in Fig. 6. EIevl

+,

- 10.3 10

*\A

- 10330 - 10340

-10350

t -

A

J y I :\

I

-10360 - 10370

-

-10380

REACTION COORDINATE [A]

Figure 6. The sequence of elementary steps in oxidation of toluene. However, it should be borne in mind that parallel adsorption of toluene, resulting in the destruction of the molecule and formation of carbon deposit or total oxidation products is energetically more favourable. This may explain the difficulties in attaining high selectivity in the gas phase oxidation of toluene over vanadia catalyst because it would require tailoring of a catalyst in which side-on adsorption would be hindered in comparison with the end-on adsorption. Similar arguments seem to be valid for the oxidation of benzene. It should be remembered that an adsorption complex of benzene linked perpendiculary to a vanadium atom

146

was assumed as an intermediate in the electrophilic oxidation of benzene to maleic anhydride by adsorbed oxygen molecule [14-161.

ACKNOWLEDGEMENT Financial support through the grant (2 0720 91 01) from the State Committee for Scientific Research of Poland is kindly acknowledged. The calculations were partly performed at Fritz-Haber Institut der MPG, Abteilung Theorie, Faradayweg 4-6, D-1000 Berlin. One of the authors (R.T.) thanks for providing computer facilities and computer time during her stay ther.

REFERENCES

1. A. Bielanski, J. Haber, “Oxygen in Catalysis”, Marcel Dekker, Inc., New York 1990. 2. P.J. Gellings, in Special Periodical Reports, Catalysis Vo1.7, The Royal Society of Chemistry, London 1985 p.105. 3. J. Haber and M. Wojciechowska, Catal. Lett. 10 (1991) 271. 4. M. Niwa, H. Ando and Y. Murakami, J.Catal49 (1977) 92; 70 (1981) 1. 5. A.J. van Hengstum, J.G. van Ommen, H. Bosch and P.J. Gellings, Appl. Catal., 8 (1983) 369. 6. P. Cavalli, F. Cavani, I. Manenti and F. Trifiro, Catal. Today 1 (1987) 245. 7. B. Grzybowska, M. Czenvenka and J. Sloczynski, Catal. Today, 1 (1987) 157. 8. M. Santi and A. Anderson, Ind. Eng. Chern. Res., 30 (1991) 312. 9. S.L.T. Anderson J.Catal., 98 (1986) 138. 10. J. Zhu, B. Rebenstorff and S.L. Anderson, J. Chem. Soc. Farad. Trans. I, 85 (1989) 3629; 3642. 11. J. Zhu and S.L. Anderson, Appl. Catal., 53 (1989) 251. 12. A.J. van Hengstum, J. Pranger, S.M. van Hengstum-Nijhuis, J.G. van Ommen, and P.J. Gellings, J. Catal., 101 (1986) 323. 13. G. Busca, F. Cavani and F. Trifiro, J. Catal., 106 (1987) 471. 14. E. Broclawik, J. Haber and M. Witko, J. Mol. Catal., 26 (1984) 249. 15. M. Witko, E. Broclawik and J. Haber, J. Mol. Catal., 35 (1986) 179. 16. R.W. Petts, K.C. Waugh, J.Chem. Soc., Faraday Trans 1, 78 (1982) 803. 17. A. Golebiewski, R.F. Nalewajski, M. Witko, Acta Phys. Pol., A51 (1977) 617. 18. A. Golebiewski, M. Witko, Acta Phys. Pol., A51 (1977) 629. 19. A. Golebiewski, M. Witko, Acta Phys. Pol., A57 (1977) 585. 20. M. Witko, R. Tokarz, J. Haber, J. Mol. Catal., 66 (1991) 205. 21. F.A. Keidel, S.H. Bauet, J. Chern. Phys., 25 N6 (1956). 22. “Table of interatomic distances and configuration in molecules and ions” The Chemical Society, Burlington House, W. 1 1958 London, 23. A. Bystrom, K.A. Wilhelmi, 0. Brotzen, Acta Chem. Sand., 4 (1950) 1119. 24. H.G. Bachman F.R. Ahmed, W.H. Barnes, Z. Krist., 115 (1961) 115. 25. D.J. Cole, C.F. Cullis, D.J. Hucknall, J. Chern. Soc. Faraday Trans 1, 72 (1976) 2185.

147

G. BUSCA (I. di Chemica, Genova, Italy): Can you perform the same calculations on different faces of V,05, and considering V ions lacking of in-plane oxide ligands?

J. HABER (I. of Catalysis and Surface Chemistry, Cracow, Poland): Quantum-chemical calculations using cluster approach can be performed for cluster of any size and geometry. The real size of the chosen cluster depends, however, on the computational method and computational facilities. Generally, the semiemiprical treatments allow to consider much bigger cluster than the ab initio approaches. G. BUSCA: The formation of methyl-diphenyl-mete as a by-product of toluene oxidation on V,05 planes proceeds through abstraction of a single H atom. What happens in you calculations if you suppose breaking one C-H bond only?

J. HABER: Quantum-chemical calculations can be carried out by keeping the geometry(ies) of reacting molecule(s) frozen or by allowing the changes of geometries of reactants. In the approach without geometry optimization one can assume a priori a cleavage of bond(s) and what follows to study the reaction with substrate(s) already activated. Such studies concerning the activation of toluene via abstraction of 1,2 and 3 C-H bonds in the methyl group are described by Witko, Haber and Tokarz in [l]. When the geometry optimization procedure is applied the breaking of bond(s) is defined by the interaction between the reactants and cannot be assumed as input parameter. 1. M. Witko, J. Haber, R. Tokarz; J. Mol. Catal., 80 (1992) 457

R.K. GRASSELLI (Mobil Centrum LAB, Princeton, N.Y., USA): A few years back, I have developed with my group, non-vanadium containing catalysts, which are multicompenent molybdenum-oxide based catalysts, and which are extremaly selective (and active) to convert toluene to benzaldehyde. My question is: have you done or are you planing to do quantumchemical calculations on these systems? I believe it would be a very interesting undertaking.

J. HABER: The question of the dependence of the changes in diatomic energy contributions i.e. modifications of chemical bonds on the type of metal-ion and its atomic characteristrics is certainly one of the central issue in catalysis. We plane to attack this problem by taking for calculations a series of transition metal cations. B. GRZYBOWSKA-SWIERKOSZ (I. of Catalysis and Surface Chemistry, Cracow, Poland): Following the comment of dr Grasselli, I‘d like to recall another system much more selective in oxidation of toluene and its paraderivatives than vanadia: in 1986 at the I”‘ Workshop on Oxidation in Louvain-la-Neuve we presented manganese telluromolybdate as a promising catalyst for the oxidation of these hydrocarbons [l]. It would be interesting to make calculations on this compound.

148

1. J. Catal. Today, l(1987).

B. GRZYBOWSKA-SWIERKOSZ: It is know that toluene oxidation on vanadia catalys produces besides benzaldehyd and COXalso considerable amounts of maleic anhydride [11. How can you explain this reaction in terms of your model? J. HABER: The product of catalytic reaction depends on several factors ammong them the mutual orientation and activation of the reacting molecules. The influence of these factors on the products of oxidation reaction were studied for benzene and toluene molecules and are discusses in papers [2-41. 1. Czerwenka, Grzybowska, Gasior, Bull. Acad. Polon. Sci., 1987 2. E.Broclawik, J.Haber, M.Witko, J. Mol. Catal., 26 (1984) 249. 3. M.Witko, E.Broclawik, J.Haber, J. Mol. Catal., 35 (1986) 179. 4. M.Witko, E.Broclawik, J.Haber, J. Mol. Catal., 45 (1988) 183.

V. CortCs Corberin and S. Vic Bellon (Ediiors), New Deveioprnents i n Selecrive Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

749

CHARACTERISATION OF V205-Fe203-Cs2S04CATALYSTS FOR THE GAS-PHASE OXIDATION OF FLUORENE TO 9-FLUORENONE F. Majunke, S . Trautmann, M. Baerns

Ruhr-University Bochum Chair of Industrial Chemistry, D-44780 Bochum, Germany

Abstract Unsupported V205-Fq03-catalysts partly doped with cesium were characterized by their catalytic performance for the title reaction and their physico-chemical properties. Catalytic properties were correlated with surface and bulk composition, acidity and reducibility of the surface and oxygen chemisorption. By adding cesium surface acidity decreased while selectivity to 9-fluorenone was markedly increased. The addition of cesium sulfate did not significantly influence the bulk structure as derived from XRD and IR-solid spectroscopy. For catalysts with a high iron content the results on oxygen uptake derived from oxygen pulsing led to the conclusion that the V-Obond was weakened by the addition of cesium, which was enriched on the surface. The decrease of Lewis-acidity determined by pyridine adsorption was considered to be responsible for a lower degree of adsorption of the basic aromatic ring system whereby its propensity to total oxidation was reduced; this rational is in agreement with reaction rate laws previously published which showed that fluorene and 9-fluorenone adsorption became negligible when cesium was added to the V205-Fq03-catalyst.

1. INTRODUCTION Catalytic results obtained for the title reaction as well as for the oxidation of anthracene and phenanthrene to their quinones and dicarboxylic anhydrides have shown that the addition of iron oxide to vanadia led to an increase in selectivity of products formed by innerring oxidation; further selectivity enhancement was obtained by alkali doping 11-31; concomittently the selectivity of products from ring destruction was markedly reduced. It is the objective of the present contribution to relate the catalytic performance to the surface and bulk properties of catalysts consisting of various proportions of V205, Fq03 and C S ~ S O ~ ; the properties considered included surface acidity and composition, oxygen adsorption capacity, reducibility, and finally the phase composition of the catalyst bulk. 2. EXPERIMENTAL

2.1 Catalyst preparation Most of the catalysts used were prepared by coprecipitation and evaporation of water of an acidic solution of ammoniumvanadat, ferrous oxide and cesium sulfate to which ammo-

nia was added and subsequent evaporation of water (method a, for details see /4-6/). Some catalysts were obtained b:/ .synthesizing FeV04 (method b, see in /7, 8/); after calcination of this precursor at 623 G it was doped with cesium sulfate by wet impregnation with a cesium sulfate solution. Aiter additional drying at 393 K the final catalyst was calcined at 623 K for 12 hours before being used in catalytic oxidation of fluorene. The composition and BET-surface areas of the various catalysts are listed in Table 1. Table 1 Bulk composition (atomic ratios) and BET surface areas of catidysts used for catalytic oxidation of fluorene No.

1 2 3 4

5 6 7 8 9

Catalyst composition V : Fe : cs 1 1 1 1 1 1 1 1 1

: 1.4

: 1.4 :1 :1 : 0.77 : 0.13 : 0.13 : 0.74

:0.74

: 0.06 : 0 : 0.06

: o

: 0.06

: : : :

0.06 0

0.06 O1

1 3 1 3 2 4 6 20 25

prepared by wet chemical synthesis of FeV04 and subsequent impregnation (all catalysts calcined at 623 K, except no. 7 calcined at 723 K)

2.2 Catalyst characterisation Specific surface area was determined by the 1-point BET method by low-temperature (77 K) adsorption of N2 after calcination of the catalyst sample in air. Surface acidity was measured by pyridine adsorption at 296 K using the DRIFTspectroscopy (Perkin-Elmer 1710; DRIFT cell Spectra Tech, model 0030-103). Spectra were recorded with 50 scans at a resolution of 4 cm-1. Powdered KBr was used as a reference for diffuse reflectance spectra. The recorded spectra were transformed to KubelkaMunk units. Before adsorption the catalyst samples were outgassed in flowing N2 (10 ml/min) at 573 K and then cooled down to 296 K. Surface composition and valence states of the key cations (V, Fe, Cs) were determined by XPS (Leybold-Heraeus, LHS 10 spectrometer; A1 cathode). Catalysts were investigated freshly calcined and after time on stream for ca. 96 h. After non-linear background substraction the energy spectra were fitted by deconvolution based on Gauss and Lorentz functions. The sensitivity factors of Wagner et al. /9/ were applied. Bulk composition of powdered catalysts samples was obtained by XRD (Cu-K,-radiation). Additionally, IR spectra (equipment see above) were recorded for catalyst samples (ca. 5 wt. % sample) diluted by KBr at room temperature. Oxygen adsorption was measured applying the GC-pulse method. Prior to oxygen adsorption at 623 K the catalyst samples were reduced in flowing hydrogen (1 ml/s) at 623 K for 1 and 3 h, respectively. TPR experiments (SETARAM DSC 111) for determining catalyst reducibility were carried out after cleaning of the surface in flowing helium (50 ml/min) for 30 min at 723 K. The TPR response were measured between 300 and lo00 K (heating rate: 20 Wmin). The off-gas stream was analyzed using a mass spectrometer (analysed products: H2, H20). A gas mixture of He and H2 (50 ml/min He, 5 ml/min Hz) was passed over the catalyst.

75 I

2.3 Equipment for Catalytic Experiments Fluorene was oxidized with air in an previously described electrically heated fixed-bed reactor (d = 0.8 cm, ltotal = 30 cm, lcat I: 5 cm) /lo/. The condensable products were separated off-line by GC analysis on a OV-1 capillary column (Sichromat 2, Siemens, FID) and by HPLC-analysis on a reversed-phase column /11/. Carbon oxides were analysed online by GC using a TCD (Delsi 11 series, TCD) and applying two columns packed with Porapack Q and molecularsieve 5-A. 3. RESULTS AND DISCUSSION First, catalytic results including also those published previously /2/ are presented. Then, the physical and physico-chemical data are reported. Finally, an attempt is made to correlate the effects of catalyst composition to its bulk and surface properties as well as to its catalytic performance.

3.1 Catalytic Performance 9-Fluorenone is an intermediate product formed directly from fluorene; it is consecutively oxidized to phthalic anhydride, which then further reacts to the carbon oxides. The complete reaction scheme was given elsewhere /1, 21. The effect of catalyst composition on maximum 9-fluorenone selectivity for the catalysts prepared by method (a) is illustrated in Figure 1. Addition of a minor amount of iron to V205 does not significantly change 9-fluorenone selectivity, whereas by higher amounts of iron (V : Fe 2 1 : 1) the selectivity is markedly increase. However, these effects are small compared to doping with Cs2S04 which results in a strong selectivity increase up to 98 %.

Figure 1 Dependence of maximum 9-fluorenone selectivity on catalyst composition F = 0.18 ~ mol m-jSTp, ~ ~c02 =~8.8 mol ~ mJSTp, ~ TR& ~ 600 to (V:Fe:Cs atomic ratio), C 670 K (see also /2/) For comparison, 9-fluorenone selectivity of catalysts prepared by method (b) is plotted versus degree of conversion at 600 K for doped and undoped FeV04 (see Figure 2). Impregnation of FeV04 with cesium sulfate led to an increase in 9-fluorenone selectivity similar as for catalysts synthesized by method (a). These results indicate that the FeV04phase alone is not responsible for the enhanced selectivity of 9-fluorenone. Catalysts prepared by method (a) with nearly the same composition (V : Fe : Cs = 1 : 0.77 : 0.06) led to a higher 9-fluorenone selectivity over the whole range of conversion (see also Fig. 1).

152

3.2 Catalyst properties Surface area (see Table 1). The solids prepared by coprecipitation (method a) have surface areas less than 10 m2g-1. Increasing the iron content as well as doping with cesium sulfate results in a surface decrease. In contrast to these solids the vanadia-iron-catalysts prepared by method @) had a significantly higher surface area. Nevertheless, doping with cesium led to a decrease, too. SNON lmol OO/ I

-

'O0I 80

-7

2ou 0 0

2 0 L o 6 0 8 0 1 0 0 X/mol% Cabdyst odoped *undoped

Figure 2 Dependence of 9-fluorenone selectivity on degree of conversion for Cs-doped and undoped FeV04 catalysts ( c F = ~ ~ ~c02 ~= 8.8 ~ mol m-3S-p, T = 600 0.18~mol~m-3,yp, K) Surface aciditv. As was suggested earlier Jl-41,surface acidity of vanadia-based catalysts is reduced by doping with cesium sulfate and by increasing the iron content. The relative amount of Lewis acidic sites as obtained from area below the absorption band at 1450 cm-I ascribed to pyridine coordinatively bound, and of Bronstedt acidic sites, obtained from the absorption band at ca. 1540 cm-l for a pyridinium ion, are given in Table 2 together with the reflectance (R,) at 1650 cm-I. Table 2 Lewis and Bronsted acidic sites of the metal oxide catalysts as derived from pyridine adsorption and their IR reflectance R, Catalyst atom ratio V : Fe : Cs : 1.4 : 0.06 : 1.4 : 0 :1 : 0.06

1 1 1 1 1 1

1 1 1 1

:1 :o : 0.77 : 0.06 : 0.13 : 0.06 : 0.13 : 0 : 0.74 : 0.06 : 0.74 : 0

:o

0 :1

:o

:0

below limit of evaluability

R,

(1650cm-1) %

62 49 71 57 64 35 31 59 58 39 91

integrated absorbance Lewis sites Bronsted sites (1450cm-1) (1540cm-1)

0.2

0.3

-1

-1

-1 -1

-1

< 0.1 -1

0.8 1.1 1.8 0.4 1.6

-1

0.5 -1 -1

0.1 0.5 -1

0.2

753

For catalysts with low surface area (see Table I) adsorption of pyridine was generally very weak and calculation of the absorption band areas did not lead to significant results in most cases. Since the reflectance properties (R,) of the solids were found to vary markedly with catalyst composition, comparison of IR results obtained for different samples is not easy. However, it appears obvious that the amount of acidic centers decreased with the addition of cesium sulfate and with the amount of iron added. The reducing effect of iron and alkali added to vanadia based catalysts on surface acidity was also observed by other researchers /12, 13/. It was assumed in our prior published work /1-41 that the acidic strength and the number of acidic sites is responsible for destructive oxidation of the hydrocarbon ring systems. This was explained by the strong adsorption of the electron-rich polycyclic aromatic hydrocarbons on acidic surfaces. If the acidic sites are blocked or their strength is diminished by basic compounds desorption of the aromatic compounds is facilitated and hence, total oxidation therefore prevented. Furthermore, the presence of basic surface sites enhances the abstraction of acidic hydrogen in the 9-position of fluorene resulting in a further increase of the selective reaction path towards 9-fluorenone production (compare also Table 3).

Surface composition. Surface composition quantified by the relative proportions of the various cations is shown in Table 3. All catalyst samples i.e., fresh and after exposure to fluorene oxidation show only Fe and V cations in their highest valence state: V5+ BE(2p3l2) = 517eV, Fe3+ BE(2p3/2) = 711eV Cs+ BE(4d5/2) = 75.leV ; the binding energies were independent of catalyst composition. The data clearly demonstrate that cesium is strongly enriched on the surface, whereas the iron content in the surface is smaller than in the volume of the catalyst compared to that of vanadium. The cesium enrichment on the surface goes along with the decrease of surface acidity as discussed before. For comparison, also the maximum fluorenone selectivity is included in Table 3. Table 3 Surface composition for catalysts before and after use in the catalytic oxidation of fluorene as obtained by XPS and maximum fluorenone selectivity achieved IIMX

Catalyst composition atomic ratio V : Fe : Cs

1 : 1.4 1 : 1.4 1 :1 1 :1 1 : 0.77 1 : 0.13 1 : 0.13 1 : 0.74 1 :0.74

: 0.06 :0 : 0.06

:o

: 0.06

: 0.06 :0 : 0.06 : 01

1

before reaction

after reaction

V : Fe

V : Fe

: Cs

1 : 0.6 : 0.36 1 : 0.76 : 0 1 : 0.89 : 0.69 1 : 0.42 : 0 1 : 0.43 : 0.28 1 : 0.05 : 0.06 1 : 0.15 : 0 1 : 0.7 : 0.19 1 : 0.68 : 0

: Cs

1 : 0.84 : 0.3 : 0.89 : 0 : 0.77 : 0.26

1 1 1 1

:0 : 0.28 : 0.15 1 : 0.08 : 0.06 1 : 0.26 : 0 1 : 0.63 : 0.18 1 : 0.7 : 0 : 0.5

SNON %

99 70 97 62 95 85 64 95 80

prepared by wet chemical synthesis of FeV04; reproduction of experiments give deviation of 15 %

Phase comDosition. The different observable crystalline phases of the various catalysts after calcination are given in Table 4; (it has to be taken into account that amorphous substances possibly present in the catalysts could not be quantified). FeV04 and FqV4013 were the main phases observed for catalysts with a V-to-Fe ratio < 1.3; the latter phase can be con-

154

sidered as a mixture of FeV04 and V2O5 1141. Catalysts with low iron content show only signals for crystalline V2O5.

Table 4 Bulk phase composition determined by XRD Catalyst' V : Fe

: Cs

FeV04

1 : 1.4 1 : 1.4 1 :1

: 0.06 :0 : 0.06

>80 80 25 100 75

1 1 1 1 1 1

:1 : 0.77 : 0.13 : 0.13 : 0.74 : 0.74

:o

: 0.06 : 0.06

Cristalline phases (amount in %) Fq03 FeV204 FeV308

10 5

10 10 25

5

V2O5

50 25

5

:0

: 0.062 : 02

F~V4013

100 95

95 100

5

* atomic ratio; * wet chemical synthesis of FeV04 XRD results were supplemented by IR absorption bands for the different catalysts (see Table 5). Bands at 1020 and 825 cm-l are characteristic for the O=V- and V-O-Vstretching vibration in V2O5 1151. The band between approximately 940 and 905 cm-* 1161 is significant for the V-0-bond in FeV04, where vanadia is tetrahedrically surrounded by 4 0-atoms. The main phases observed by XRD are also dominant in the IR spectra that is to say V2O5 for V : Fe = 1 : 0.13 otherwise it is FeV04.

Table 5 IR absorption bands of the different catalysts and literature data of V2O5 and FeV04 Catalyst' V : Fe v2°5

V2O5 I151 Fe203 FeV04 I161 1 : 1.4 : 0.06 1 : 1.4 : 0 : 0.06 1 :1 Y

1 1 1 1 1

:1 : 0.77 : 0.13 : 0.13 : 0.74 : 0.74

IR-band Y 1 cm-1

: Cs

:o

: 0.06 : 0.06 :0 : 0.06 :0

*

1021s 1020s

82Ovs 615-475s,br 825s 600-450s,br 6 15-540s,br,47&s 985sh,940w,905vs 830s 705sh,660s,500vs 98h,95osh,912vs,900sh 83Ovs 730sh,7O0vw,665vs,510~~ 971m,915m,9oclsh 830m 535s 99ovw,93Ow,910s 830s 73Ow,71Ow,67Ovs,62Ovs, 57Ow,53ow 99bw,950sh,912s 840s 730sh,700w,670vs 99bw,950vw,915vs 840s 73ovW,702w,673vs,510s 1024s 95osh,91osh 84Ovs 650-510s,br 1024s 95osh,9lash 82Ovs 620-480s,br 1020sh 99Ovw,95h,915vs 840s 73Ovw,7O0vw,672vs,510s 1020sh 990vw,950vw,915vs 840s 73Ovw,7O0vw,673vs,510s

atomic ratio; vs = very strong; s = strong, m = medium, w = weak, vw = very weak, sh = shoulder, br = broad

155

As has been reported by Fikis et al. /15/ doping of V2O5 with a cesium compound (V/Cs led to a band shift of 10 to 20 cm-l corresponding to a lower binding energy. The weakening of the V - 0 bonds by alkali doping have been also observed by other groups /15, 17-201 investigating oxygen exchange between catalyst and gas-phase. The effect can be explained when assuming the alkali compound is acting as an electron donor and hereby weakening the vanadium-oxygen bond. This effect was, however, not observed in present work. = 110.1)

Oxvgen uDtake. The uptake of oxygen of various catalysts depends on reduction time (see Table 6). It is clearly shown that the reducibility of the catalysts expressed by oxygen uptake was enhanced by alkali doping for catalysts with V : Fe > 1 : 0.13 prepared by coprecipitation. In contrast to this finding for catalysts as main V2O5 having phase (V : Fe = 1 : 0.13) oxygen adsorption is diminished by alkaline doping. Also the catalysts prepared by method (b) show a slight decrease in oxygen uptake with cesium doping. Table 6 Oxygen uptake for the catalysts determined by GC-pulse method (Trd = Tads = 623 K; 60 ml/min H2) after different times of reduction and the initial reaction rate rFI (cF1 = 0.4 Vol. %, calculated for T,, = 623 K, data from /2/) Catalyst atomic ratio V : Fe : Cs

1 1 1 1 1 1 1 1 I

: 1.4 : 0.06 : 1.4 : 0 :1 : 0.06

:1 :o : 0.77 : 0.06 : 0.13 : 0.06 : 0.13 : 0 : 0.74 : 0.062 10.74 : 02

no kinetic constants available;

prno1(O2) m-2 tR4 =

144 84 59 42 88 25 31 10 16

1h

+13 f 6 f 2 f 6 f 8 f 2 f 4 f 1 f 2

‘F1

tRd = 3 h

531 190 198 134 127 44 169 27 34

f30 f15 f18 f15 fll f 5 f28 f 3 f 3

pmoI m-2 s-1

0.180 -1

0.364 0.053 -1 -1

0.179 -1

-1

wet chemical synthesis of FeV04

TPR. The dependence of the relative rate of H2 consumption on reduction temperature differs for doped and undoped catalysts and the vanadia-to-iron ratios (1 : 1.4 and 1 : 0.13) in the bulk. The reduction-rate maximum at 920 K is moved towards lower temperatures by alkali doping for a catalyst having a V-to-Fe ratio of 1 : 0.13. For the undoped catalyst having a higher iron content a broad reduction peak appeared between 800-900 K, whereas two sharp signals (910 and 980 K) occured for the doped solid. The observed results are in agreement with previous works of Bosch et al. /21/ who investigated the reduction of unsupported V205 catalysts applying comparable experimental conditions (20 K.min-1, 10 cm3.min-1 Ar with 9 % H2). The first reduction peak at 920 K was related to the reduction of V205 to V 0,. The different results for catalysts with V : Fe = 1 : 1.4 can presently not be explained2 The results obtained by the GC-pulse method and TPR led to the conclusion that doping of catatalyst of higher iron content (V : Fe 2 1 : 1) enhance the reducibility of the catalyst sample.

756

r(H2)lumollsla (Katl

Figure 3 TPR-profiles of doped and undoped catalysts with a atomic ratio of V : Fe = 1 : 1.4 (a) and 1 : 0.13 (b), respectively (heating rate 20 Wmin, V(He) = 50 ml/min, V(H2) = 5 ml/min) CONCLUSION Activity and selectivity in fluorene oxidation to fluorenone was increased by doping of binary vanadia-iron oxide catalysts with cesium sulfate. The beneficial effect of such alkali doping is similar to observations for other hydrocarbon oxidation reactions such as oxidation of ethylene to ethylene oxide and napthalene to phthalic anhydride 122-24/. The increase in fluorenone selectivity could be correlated with the surface enrichment of cesium and the decrease in surface acidity (compare Table 3 and 2). The reducing effect of adding iron and alkali to vanadia-based catalysts on surface acidity was also observed by other researchers /12, 131. Furthermore, diminishing of the acidic strength led to a decrease of the adsorption constants and heats of adsorption of fluorene and 9-fluorenone as was shown previously 121. No significant correlation could be derived between oxygen uptake, phase composition and catalytic performance. The increase of oxygen uptake of the catalysts with higher iron content could be explained when assuming the alkali compound acting as an electron donor; hereby the vanadium-oxygen bond is weakened facilitating oxygen exchange as discussed before. However, weakening of the V-0 bond was not observed in the IR spectroscopic studies as outlined above. On the other side the undoped catalyst with V : Fe = 1 : 0.13 has a higher oxygen uptake as the doped one, but the cesium containing catalyst show also a remarkable increase of 9-fluorenone selectivity. Therefore, further work for elucidating this discrepancy in IR results, oxygen adsorption and catalytic performance is necessary. Acknowledgement. Support of this work by Deutsche Forschungsgemeinschaftis gratefully recognized.

REFERENCES

111 M.Baems, H. Borchert, R. Kalthoff, P. KiiBner, F. Majunke, S. Trautmann, A. &in, (eds. B. Delmon and J.T. Yaks), "New Developments in Selective Oxidation by Hekrogenous Catalysis", Stud. Surf. Sci. Catal., 12(1992). Elsevier Science Publisher B.V., 57 121 F. Majunke, M.Baems, H. Borchert, Preceed. lo* Internat. Congr. on Catalysis, Budapest, Hungary, 103 (1992) 131 B. Odening, P. m n e r and M. Baems, Proceed. DGMK-Conference "SelectiveOxidations in Petrochemistry", Goslar, Germany 1992,347

757

/4/ N.T. Do, R. Kalthoff, J. Laacks, S. Trautmann, M. Baems, ( 4 s . G. Centi and F. Trifiro), "New Developments in Selective Oxidation by Heterogenous Catalysis", Stud. Surf. Sci. Catal., 55, Elsevier Science Publisher B.V., 247 151 M. Baems, R. Kalthoff, P. K a n e r , A. Zein, Erdol-Erdgas-Kohle, 106 (1990), 166 /6/ FIAT Final Rep. No. 1313, Vol.1, 332 /7/ M. Touboul, A. Popot, J. Therm. Anal., x ( 1 9 8 6 ) , 117 181 M. Touboul, D. Ingrain, J. Less Comm. Met., (1980), 55 /9/ Handbook of X-Ray Photoelectron Spectroscopy, C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (eds.), publ. by Perkin-Elmer Corporation, Eden Praire, Minnesota 55344, 1978 /lo/ M. Baems, R. Kalthoff, P. W n e r , A. Zein, ( 4 s H. Kral and D. Behrens), Dechema Monographie 118 Katalyse, Verlag Chemie, Weinheim 1989, 231 /11/ T Z e i n , M. Baems, J. Chromatographic Sci., 22 (1989), 249 /12/ M. Ai, J. Catal., 2 (1978), 16 /13/ D.V. Fikis, W.J. Murphy, R.A. Ross, Can. J. Chem., s ( 1 9 7 8 ) , 2530 /14/ J. Walczak, J. Ziolkowski, M. Kurzawa, J. Osten-Sacken, M. Lysio; Pol. J Chem.; 3 (1985); 255 /15/ D.V. Fikis, K.W. Heckley, W.J. Murphy and R.A. Ross, Can. J. Chem., 56 (1978), 3078 /16/ E.J. Baran, I.L.Botto, Monatsh. f. Chem., m ( 1 9 7 7 ) , 311 /17/ T. Tamaka, R. Tsuchita~ni,M.Ooe, T. Funabiki and S. Yoshida, J. Phys. Chem., 90 (1986), 4905 /18/ K. Hirota and Y. Kera, J. Phys. Chem., 22 (1968), 3133 /19/ K. Hirota and Y. Kera, J. Phys. Chem., 22 (1969), 3973 /20/ D.B. Dadybujar, S.S. Jewner, E. Ruckenstein, Catal. Rev. Sci. Eng., 19 (1979), 336 /21/ H. Bosch, B.J. Kip, J.G. van Ommen, P.J. Gellings, J. Chem. SOC., Faraday Trans. 1, (1984), 2479 /22/ W.-D Mross, Catal. Rev. Sci. Eng., 25 (1983), 591 /23/ N.R. Foster, M.S. Wainwright, D.W.B. Westerman, Aust. J. Chem., %(1981), 1325 /24/ D.W.B. Westerman, N.R. Foster, M.S. Wainwright, Appl. Catal., 2 (1982), 151

a

758

J. Haber (Institute of Catalysis and Surface Chemistry, Crakow, Poland): Data shown in Table 6 seem to indicate that there is no correlation between selectivity and reducibility SO that probably the influence of cesium on basicity of the surface is the main factor ? F. Majunke (Chair of Industrial Chemistry, Ruhr-University Bochum, Bochum, Germany): The enhanced reducibility goes along with an increased formation of a new, x-ray amorphous F%VyO,-phase determined by ESR- and Mossbauer-spectroscopy (Results obtained from Dr. Briickner, Institute of Applied Chemistry, Berlin-Adlershof). Although this phase is not fully defined containing an axially disturbed iron-ion included, to which the enhanced selectivity may be ascribed. We certainly go along with you that cesium decreases surface acidity by which selectivity would be reduced. Reducibility may, however, related to catalyst activity. R.K. Grasselli (Mobil Central Research Lab., Princeton, USA): (a) The way you choose to write the formula of what you suspect to be the active phase of your catalysts, namely FeV03.5; might imply that the iron is three valent (Fe3+), and the vanadium is four valent (V4+). My question is which evidence do you have for these (or other distribution of the respective valencies) oxidation states under reaction conditions, and/or as starting materials or after reaction? (b) Is the excess Fe in iron rich composition found to be as a - F q 0 3 ?

F. Majunke (Chair of Industrial Chemistry, Ruhr-University Bochum, Bochum, Germany): (a) Our results obtained by XPS-spectroscopy before and after reaction as well as the ESRand Mossbauer-spectra gave no evidence for the existence of other oxidation states than Fe3+ and Vs+. Nevertheless, from a formal point of view your assumption on the phase composition might be correct, there is just more work to do to characterize the new ironvanadate phase more exactly. (b) Results obtained by Mossbauer-spectroscopy show significant bands characteristic for aFe203. G. Emig (Institute of Technical Chemistry, University of Erlangen, Erlangen, Germany): You didn't tell us the differences in the behaviour of the two catalyst preparations. (a) What was the reason for this marked difference in surface area ? (b) Did both preparations show the same activity/selectivity behaviour ? F. Majunke (Chair of Industrial Chemistry, Ruhr-University Bochum, Bochum, Germany): (a) Solids with high surface area, prepared by method b (compare 2.1), were filtered from an acidic solution; whereas the other solids were synthesized by evaporation of the basic solution (method a). It may be ascribed to these totally different ways of preparation that there is such a remarkable difference in the specific surface areas. (b) Both kinds of catalysts show the same activity/selectivity behaviour (compare also Figure 1 and 2); i.e., 9-fluorenone selectivity is increased by cesium addition. However, when using solids prepared by method b only a maximum selectivity of 92 - 95 mol% was achieved instead of 95 - 99 mol% for catalysts synthesized by method a.

V. Cortes Corberan and S. VIC Bellon (Editors), New Devekymenis zn Selecrive Oxidation If 0 1994 Elsevier Science B.V. All rights rcservcd.

759

Gas-phase catalytic oxydehydrogenation of ethylbenzene on

AlPO, catalysts

F. M. Bautista, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas andR. A. Quiros Organic Chemistry Department, Faculty of Sciences, University of Cordoba, Avda. S. Albert0 Magno s/n, E-14004 Cordoba, Spain*.

Abstract The oxidative dehydrogenation of ethylbenzene to styrene has been carried out o n several natural and synthetic AlPO, catalysts as well as o n several systems constituted by AlPO, and some oxides such as SiO,, ZnO o r A1,0,. Results obtained with these catalysts and another used as a reference indicate that catalytic activity is closely associated with the surface density of acid sites, and especially with those more accessible ones exhibiting medium-high strength. Best results were obtained with different synthetic AlPO, catalysts.

1. INTRODUCTION The industrial dehydrogenation of ethyl benzene t o styrene is carried out in vapor phase at 800-900 K on several iron oxide catalysts in the presence of superheated steam which is used t o provide the heat its endothermic character requires [l].In this respect,, considerable work has been done in recent years [2-41 to produce catalytic systems where oxygen may function directly as a hydrogen acceptor yielding water as a by-product and providing the thermodynamic driving force t o obtain a lower reaction temperature. Among the catalysts described in the literature for this reaction, metal phosphates showed high activity and selectivity [5-81. Furthermore, surface acid-base properties were closely related t o their catalytic behaviour either directly [6] o r through the formation of a catalytically active coke [7,8]. Following previous research on catalytic dehydrogenation of alkylbenzenes [9-121, we can now report results obtained in the oxidative dehydrogenation of ethylbenzene over several aluminum phosphates and aluminum phosphate-metal oxide systems exhibiting very different numbers of surface acid and basic sites and which have proved t o act as catalysts in several reactions of interest in fine chemistry [13,14] and petrochemical processes [15-221.

*The authors acknowledge the subsidy received from the DGICYT (Project PB89/0340),Ministerio de Educacion y Ciencia, and from the Consejeria de Educacion y Ciencia (Junta de Andalucia).

760

2. EXPERIMENTAL

2.1. Catalysts Twenty one different catalysts have been used: three amorphous aluminum phosphate/alumina systems obtained by calcination of natural phosphorous-bearing bauxites from Brazil: Pirocaua, Trauira and Sapucaia; three pure AlPO, (AP)and three A1P04-A1,0, (75:25 wtO/o) (APA1) systems, all obtained by precipitation, from aluminum chloride and H,PO, aqueous solutions, with aqueous ammonia (A), ethylene oxide (E) or propylene oxide (P); two AlFQ,-ZnO (APZn-A) systems of varying composition (3-1 and 1-3weight ratio); an A1P04-Si0,-E (APSi-E) system (20:80 wt%); a chemically pure Al,O,-A and ZnO-A obtained by precipitation from aluminum nitrate or zinc nitrate solutions by aqueous ammonia. A commercial Al,O,-C and a commercial SiO, from Merck were also used. Besides, four nickel oxide systems supported on a natural sepiolite (NiO-Sep) with nickel loading ranging between 7-21 wtoh were prepared by impregnation of nickel nitrate followed by calcination. Natural sepiolite supplied by Tolsa S.A. was also used as a catalyst. All these systems were used as catalysts after calcination a t 923 K for 3 h. Details on the characterization of some catalysts have been previously described [16,191. The surface area S ,,, and acid-base properties are collected in Table 1. A spectrophotometric method described elsewhere [15,21,22] was used t o measure the surface acidity by titration with pyridine (PY, pKa = 5.3) and 2,6-di-t-butyl-4methylpyridine (DTBMPY, pKa = 7.5) and the surface basicity by titration with benzoic acid (BA, pKa = 4.2).

2.2. Catalytic measurements Oxidative dehydrogenation reactions were carried out in a conventional fixed-bed type reactor previously described [9-121. I t was made of quartz with a continuousflow system a t atmospheric pressure and 733 K. By means of a microfeeder, a fixed stream of ethylbenzene (EB) with a feed rate F = 6 mumin and oxygen was administered after dilution with dried nitrogen at different 0,:EB molar ratios and different catalyst weight (W between 0.1 and 0.8 g) t o obtain different residence time values, W F in gCsth/gEB. Times on stream of 2 hours were developed with all catalysts studied. The reaction liquid products collected by traps cooled with dry ice were analyzed by GC with FID by using a column (2m x 0.3 mm) packed with 5% polyphenylether on Chromosorb W 80/lOo at 373 K. In addition t o styrene (ST), always obtained with high selectivity, reaction products of the dehydrogenation process on the different catalysts were found t o be benzene (B) and, in minor amounts, toluene (T), P-methylstyrene (MST) and benzaldehyde (BZ). Furthermore, thermal reaction was negligible.

3. RESULTS AND DISCUSSION According t o the results obtained with all the catalysts studied, the absence of external diffusion effects in the present experimental conditions are obtained for residence time, W/F, values down t o 0.077 h. In this interval, a first-order rate equation is found t o fit the data a t different residence times where it is possible t o apply the "differential reactor" conditions f o r the treatment of the rate data.

76 1

Table 1 Surface area, S,, Catalyst

AP-P AP-E

AP-A APA1-P APAl-E

APAl-A APSi-E

APZn-A(1:3) APZn-A(3:1) A120,-A Al,O,-C SiO, ZnO Sep NiO-Sep-7 NiO-Sep-12 NiO-Sep-17 NiO-Sep-21 Pirocaua Trauira Sapucaia

(myg) and acid-base (pmoug) properties of different catalysts Acidity

SBET

228 239 156 319 242 244 327 9 8 151 72 366 4

127 103 102 102 102 12 3 19

Basicity

PY

DTBMPY

BA

227 267 190 179 165 92 380 1 2 77 23 206 1 31 29 31 32 32 5 0 6

78 90 53 79 52 32 8 0 0 0 0 0 0 9 8 10 11 8 0 0 0

166 266 200 774 577 535 70 29 20 450 191 164 2 174 134 130 142 124 0 0 0

Besides, the average particle size of catalysts used (c0.149 mm) determines that the reactions were not influenced by internal diffusional limitations [9-121. The influence of 0, : EB ratio on conversion and ST selectivity is shown in Fig. 1, where it can be seen that best results are obtained for an equimolecular 0, :EB ratio. It is also interesting t o note that while conversion exhibits a maximum, selectivity monotonously decreases o n increasing the oxygen proportion. Respect t o the effect of time on stream results obtained with all catalysts studied is close similar t o that shown in Fig. 2 for APA1-P. Conversion was always increased up gradually t o a stationary value after about 20-40 minutes on stream. According t o this, the standard working conditions t o carry out the reaction test for every catalyst studied were W/F = 0.077 h; W = 0.4 g; and 0, : EB = 1at 733 K. Results obtained for all catalysts a t a time on stream of 1 hour are compiled in Table 2. These results show the high selectivity obtained with every catalyst studied, up to 95°/o, with the exceptions of SiO, and the APSi-E system, also containing silica. With respect t o catalytic activity, the highest values of EB conversion and ST yield were obtained by the three different AlPO, catalysts. The results obtained were very similar t o those described by Vrieland [7], with a wide variety of metal pyrophosphates where the best results were obtained with aluminum as metal cation. APAl catalysts exhibited an intermediate behaviour between alumina and

762

l

o

0

s

2

1

0

0 , : EB m o l a r ratio

Figure 1. Influence of the 0, : EB ratio on EB conversion ( 0 ) and ST selectivity (0) on APA1-P catalyst at standard reaction conditions.

20

40

60

80

100

t i m e o n stream (mi.)

Figure 2. Influence of time on stream on EB conversion ( 0 ) and ST selectivity (v) o n APA1-P catalyst at Wfl = 0.077 h; W = 0.4 g; 0, : EB = 1 and T = 733 K.

AlPO, systems. Thus, in those catalysts constituted by AlPO,, Al,O, or mixed systems the conversion range was between 47 and 25%. Moreover, although the EB conversion and ST yield depend o n the precipitation medium, the most important influence is that of catalyst composition. Sepiolite and different Sepiolite supported NiO catalysts were also catalytically active exhibiting conversion levels between 10 and 27%. The only natural phosphate/alumina system showing

763

Table 2 Catalytic performance of different systems at standard reaction conditions -

Catalysts

AP-P AP-E AP-A

APA1-P APA1-E

APA1-A APSi-E

APZn-A(1:3) APZn-A(3:l)

A120,-A A120,-C Si0, ZnO SeP NiO-Sep-7 NiO-Sep-12 NiO-Sep-17 NiO-Sep-21 P'lrocaua Trauira Sapucaia

EB Conversion ("/.>

48.0 44.6 47.5 42.1 37.6 34.7 3.1 1.5 2.6 28.2 25.2 4.8 2.4 17.1 27.7 22.9 17.4 10.6 4.3 2.9 21.5

Product yields(%) B

T

MST

1.o 0.9 0.9 1.o 0.9 0.8 0.1 0.1 0.1 0.9 1.o 0.1 0.1 0.3 0.5 0.3 0.4 0.1

0.4 0.4 0.3 0.6 0.5 0.4

-

BZ

-

0.2 0.3 0.1

0.1

1.1

-

0.2 0.1

0.1 0.5 0.2 0.2 0.1

0.1

0.2

0.3

0.1 0.3 0.1

-

0.1 0.1

0.2

Selectivity ST

46.6 43.4 46.2 40.5 36.3 33.6 2.9 1.4 2.5 27.0 24.1 3.5 2.3 16.8 26.4 22.3 16.6 10.4 4.2 2.9 20.7

97.1 97.1 97.4 96.4 96.4 96.8 92.9 95.0 96.8 95.9 95.6 71.9 94.5 97.1 95.5 97.7 95.9 97.5 98.5 98.1 96.4

oxydehydrogenating activity was Sapucaia. All the other catalysts were rather inactive, down 5%. The presence of a dark black carbonaceous material deposited o n the surface of the initially white solids was a general behaviour in all catalysts studied. Besides, additional experiments carried out with AP-P showed that a deactivated catalyst can be reactivated by removing the coke formed by reoxidation of the used catalyst by air for 15 minutes under standard conditions. The reoxidized catalyst showed the same results as obtained previously with the fresh catalyst. Because in many previous works the role of acid sites was emphasized either for the oxydehydrogenation itself [4] o r for the formation of active coke on the catalyst surface where the oxydehydrogenationprocess develops properly [5-81,a correlation matrix using all the data in Tables 1 and 2 was built in order t o determine the influence of textural and acid-basic properties of the systems o n their catalytic behaviour. Results obtained in the regression analysis of the well correlated parameter pairs are shown in Table 3. The results in Table 3 show that a relationship exists not only between catalytic activity and surface acidity and basicity but also between the former and BET surface area. However, according t o the results, correlations obtained between catalytic properties and basicity could not be significant being obtained as a

764

Table 3 General expression of the correlation y = ax + b obtained between surface and acid-base properties of catalysts in Table 1 and the corresponding catalytic properties in Table 2 Y

EB conversion EB conversion EB conversion EB conversion EB conversion EB conversion ST Selectivity ST Selectivity ST yield ST yield ST yield ST yield ST yield ST yield BA

a

X ~~

~

PY DTBMPY BA SBET

PY/SBET DTBMPY/S BET SBET

PY -DTBMPY PY DTBMPY BA SBET PY/SBET DTBMPY/SBET DTBMPY

~

____

0.06 0.45 0.05 0.06 26.70 110.18 -0.02 -0.02 5.95 43.91 4.88 6.03 2593.37 10754.78 4.16

b

Significance

(%o)

~

15.5 11.9 11.3 12.4 8.6 10.2 5.0 97.0 1491.1 1134.3 1086.5 1206.9 814.9 969.9 110.7

93.6 100.0 99.9 95.9 99.5 100.0 97.2 93.4 93.3 100.0 99.9 95.3 99.5 100.0 99.5

consequence of the double correlation between catalytic activity and BA and between the latter and DTBMPY. In this connection, while the corresponding values of specific basicity (BNSBET) did n o t show any influence on catalytic activity, the best correlations were obtained taking into account specific acidity values obtained through the quotients PY/SBET and DTBMPY/SBET,respectively. These values represent the number of acid sites per unit of support BET surface area, which is the surface density of acid sites or specific acidity of catalysts. On the other hand, taking into account the lower pKa of PY with respect t o DTBMPY as well as the higher steric effects of tert-butyl groups in the latter, we have to conclude that only a fraction of the most accessible surface acid sites of medium-high strength are catalytically active in this reaction. This is obtained not only from the lower significance values of PY/SBETwith respect t o DTBMPY/SBET, but also from the absence of any correlation between catalytic activity and PYDTBMPY/SBET or PY-DTBMPY, which represent all acid sites of medium-low strength. This type of acid site, as well as BET surface area, seems t o be, however, responsible for a decrease in the selectivity of the process if we consider the negative values of slopes in the corresponding correlations shown in Table 3. These correlations obtained do n o t produce definitive conclusions with respect t o the role of acid-basic sites in the oxidative dehydrogenation mechanism. Thus, since the partipation of basic sites may not be definitively excluded, we can n o t exclude a mechanism where a peculiar distribution of acid-base pairs o n catalyst surface could be considered advantageous for coke formation, and hence for subsequent acceleration of the reactions [23]. The best agreement can be obtained with respect t o the participation of surface acid sites of medium-high strength in the reaction through the formation of a

765

catalytically active coke [8]. However, a t the present time we can also consider the existence of another concerted mechanism carried out directly o n the Lewis acid sites of the catalyst [lo] which according t o Brozyna and Dziewcki [6] could produce ethylbenzene dehydrogenation a t the same time that active coke does. This concerted mechanism [lo] considers the direct transfer of two hydrogen atoms t o a singlet oxygen molecule. The oxygen adsorption on a Lewis acid site overcomes the spin barrier between the stationary triplet state and the activated singlet state, which lets it participate in a concerted hydrogen transfer process through a six-membered cyclic transition state just as the cycloaddition t o endoperoxides does [24]. In this connection, a hydrogen peroxide molecule is postulated as an intermediate reaction product obtained in a first step beside to a styrene molecule. The 1:l stoichiometry of this slowest step could explain the maximun obtained in Fig. 2 for an equimolecular 0, : EB ratio. In the fastest non-catalyzed second step, the hydrogen transfer of ethylbenzene t o H,O, gives rise t o two H,O and a new styrene molecule. The presence of coke, as well as the secondary products obtained, may be very adequately explained in the context of the present mechanism by the action of a nonspecific oxidant agent like hydrogen peroxide over styrene. Thus, the presence of benzaldehyde as a secondary reaction product, may be easily explained through a n epoxystyrene intermediate, obtained by the action of hydrogen peroxide over the olefinic double bond of styrene.

4. CONCLUSIONS On the basis of these results, we may conclude that the density of surface acid sites plays a n important role in the catalytic behaviour of catalysts studied. Thus, on increasing the surface density of acid sites, especially in those more accessible and exhibiting medium-high strength, we might, in general, obtain an increase in EB conversion as well as in ST yield. In this respect, the most interesting results were obtained with AlF’O, catalysts. Furthermore, the catalytic activity of AP can be modified by the incorporation of different oxides such as SiO,, ZnO o r A1,0, and thus, a variable decrease in catalytic activity is always found. Consequently, AlPO, can be a catalyst, o r a t least a component of a tailored AP-metal oxide catalyst, t o obtain the most appropriate activity and selectivity in oxydehydrogenation of ethylbenzene.

REFERENCES 1 H. Kung, Ind. Eng. Chem. Prod. Res. Dev., 25 (1986) 171. 2 Z. Dziewiecki and P. Hydzik, React. Kinet. Catal. Lett., 46 (1992) 159. 3 J.J. Kim and S.W. Weller, Appl. Catal., 33 (1987) 15. 4 T. Tagaw, T. Hattori and Y. Murakami, J. Catal., 75 (1982) 56. 5 A. Schraut, G. Emigh and H. Hofmann, J. Catal., 112 (1988) 221. 6 K. Brozyna and Z. Dziewiecki, Appl. Catal., 35 (1987) 211. 7 G.E. Vrieland, J. Catal. 111 (1988) 1. 8 G. Bagnasco, P. Ciambelli, M. Turco, A. La Ginestra and P. Patrono,Appl. Catal., 68 (1991) 69.

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9 10 11

12 13 14 15

16 17 18 19 20 21 22 23 24

F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, and J.M. Marinas, J . Catal., 107 (1987) 181. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, and J.M. Marinas, J . Catal., 116 (1989) 338. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, and J.M. Marinas, Bull. Chem. SOC.Jpn., 62 (1989) 3670. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, and J.M. Marinas, React. Kinet. Catal. Lett., 41 (1990) 295. J.A. Cabello, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, J. Org. Chem., 49 (1984) 5195. J.M. Campelo, A. Garcia, F. Lafont, D. Luna, and J. M. Marinas, Syn. Commun., 22 (1992) 2335. J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas andM.1. Martinez, Mater. Chem. Phys., 21 (1989) 409. F.M. Bautista, A. Blanco, K.E. Besller, J.M. Campelo, A. Garcia, D. Luna, J . M. Marinas and A.A. Moreno, R o c . 12th Iberoamerican Symp. Catal., Rio, Brazil, 1990, p. 440. J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M.S. Moreno, J. Chem. SOC.Faraday Trans. I, 85 (1989) 2535. A. Blanco, J.M. Campelo, A. Garcia, D. Luna and J . M. Marinas, Appl. Catal., 53 (1989) 135. J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M. Martinez-Cunquero, R o c . 1l t h Iberoamerican Symp. Catal., Guanajuato, Mexico, 1988, p.799. A. Blanco, J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and A.A. Moreno, J. Catal., 137 (1992) 51. J.M. Campelo, A. Garcia, J.M. Gutierrez, D. Luna and J.M. Marinas, Canad. J. Chem., 61 (1983) 2567. J.M. Campelo, A. Garcia, D. Luna, and J.M. Marinas, Canad. J. Chem., 62 (1984) 638. Z. Dziewiecki and A. Makowski, React. Kinet. Catal. Lett., 31 (1986) 9. D.H.R. Barton, R.K. Haynes, G. Leclerc, P.D. Magnus and I.D. Menzies, J. Chem. SOC.Perkin Trans. I., (1975) 2055.

761

DISCUSSION CONTRIBUTIONS J. C. VEDRINE (Inst. Rech. Catal., CNRS, Villeurbanne, France): I am surpresed by your correlation o n acidity and EB conversion because such reaction does not involve acid sites. Don't you think that carbon deposit (occuring a t the beginning) is doing the job?. If not, you have t o give a mechanism for the reaction and t o describe the acid sites (Lewis, Bronsted ...).

D. LUNA (Org. Chem. Dept., Cbrdoba, Spain): Our results don't exclude the participation of surface active carbon and respect t o the participation of acid sites

we have proposed a concerted mechanism in [lo] which considers the direct transfer of two hydrogen atoms of ethylbenzene t o a singlet oxygen molecule adsorpted on a Lewis acid site.

J. R. EBNER (Monsanto Corp. Res. Lab., St. Louis, Missouri, USA): Wath is the explanation for the decrease in conversion with increase in 0, / EB ratio?. Does this represent a case of loss of surface carbon which is the "active oxydehydrogenation surface"?.

D. LUNA: Really this could be in fact an effect of a decrease in the surface active carbon but also it could be a consecuence of the concerted mechanism [lo] where the direct transfer of two hydrogen atoms to a singlet oxygen molecule yielding styrene and hydrogen peroxide is the slowest step which exhibits a stechiometric relation between oxygen and EB. In this conection, in Fig. 1 we did n o t obtain a decrease but a maximun for 0, / EB = 1 ratio.

F. TRIFIRO (Ind.& Mat. Dept., Bologna, Italy): It has been found by Drago (1) that carbon molecular sieve are very active in oxydehydrogenation of ethylbenzene. Fron this paper we can deduce that also with phosphates based catalysts carbonaceous adsorbed phase are the active species in oxidation. Do you still believe that acid sites play a role in the mechanism of oxidation?. 1.Grunewald and Drago, J. Mol. Catal., 58 (1990) 227. D. LUNA: Yes, of course because data in Table 3 indicates a clear correlation between the density of surface acid sites and the catalytic behaviour. However, respect t o the role of acid sites we think that discussion is still open. Thus, a possibility could be the participation of acid sites in the production of active coke.

G. EMIG (Inst. Tech. Chem., Erlangen, Deutschland): First a question. We found some years ago, that zirconium phosphate was the best catalyst for the oxydehydrogenation of ethylbenzene. I can n o t see what the adventages are in using aluminium phospates instead. Then a comment. You should have o n your AlPO, a very similar mechanism which we found for Zr-Phosphate. Some active coke o n the surface must be the intrinsic catalyst here acting in a type of organic redox couple. We proved that by separating the redox process into a reduction and a reoxidation phase.

768

D. LUNA: To know the catalytic behaviour of some well characterized compounds is always interesting in Catalysis. Besides, in the present case, the high resistence t o deactivation, the easy and complete reactivation by reoxidation as well as the good results obtained in activity and selectivity indicates that amorphous AlPO, can be a n excelent catalyst. On the other hand, price and enviroment impact are lower in aluminium phosphate than in zirconium phosphate. Respect t o the reaction mechanism, we first of all focus our attention in understand what kind of surface sites are able to promote the reaction. The possibility that active coke take part in the reaction as a true catalyst is not excluded by o u r results. In such case we consider that the density of surface acid sites play an important role in produce the surface active coke.

J. HABER (Catal. and Surf. Chem. Inst., Cracow, Poland): The problem of correlation between catalytic activity in oxidative dehydrogenation of ethylbenzene and the acidity of the catalyst has been addressed in many studies in early 1980s. I t has been established that the reaction is catalyzed by the carbon deposit which plays the role of the active phase, but the formation of the appropriate carbon deposit depends o n the presence of the acid sites a t the surface of the solid, hence the observed correlation between catalytic activity and catalyst acidity. The formation of quinone-type active sites at the surface of carbon deposit was postulated t o be responsible for the oxidative dehydrogenation.

D. LUNA: Basically I agree with you. However, at the present time we could not exclude the direct participation of acid sites in the reaction. Probably, the total activity come from both contributions and in every case the participation of every one depend o n the nature of the compund used as catalyst.

V. Corlks Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidmion II 0 1994 Elsevier Science B.V. All rights reserved.

769

Selective gas-phase dehydrogenation of cyclohexanol with magnesium orthophosphates M. A Aramendia, J. Barrios, V. Borau, C. Jimenez, J. M. Marinas, F. J. Romero, J. R. Ruiz and F. J. Urbano.* Department of Organic Chemistry, Faculty of Sciences, University of Cordoba, Avda. San Albert0 Magno s/n, E-14004 Cordoba, Spain. Abstract

The results obtained in the gas-phase dehydration-dehydrogenation of cyclohexanol (CHOL) over variously synthesized magnesium phosphates are reported. The activity and selectivity of the catalysts towards production of cyclohexanone (CHONE) and cyclohexene (CHE) was found to be related to the synthesis variables. FMNal solid consisted of a-NaMgPO, at 773 K, where it selectively converted CHOL into CHONE. In hydrated form, it produced CHE selectively at the same temperature.

1. INTRODUCTION The dehydration of alcohols to olefins and their dehydrogenation to aldehydes and ketones have both been thoroughly studied in relation to homogeneous and heterogeneous catalysis (1,2). As regards heterogeneous catalysis, alcohol dehydration has been investigated on a variety of substances including aluminas (3), modified aluminas (4), SiOJAI,O, systems ( 5 ) , sulphates (6,7), boron phosphates (8), cadmium (9), zinc (lo), nickel (11) and aluminium ( 1 2 ~ 3 )While . some basic catalysis reactions have been identified in this context, most authors relate the formation of the olefins with the population of Brdnsted or Lewis surface acid sites, which in turn vanes with the thermal pretreatment to which the catalyst is subjected during the synthetic process (14,15). The oxidation of alcohols has been assayed both on some of the above-mentioned catalysts and, particularly, on copper@) oxides (16), silver(1) oxides [in the presence or absence of oxygen (17)], gold (lS), tin oxide (19), chromium(LU) oxides for selective oxidation to aldehydes (20) or ketones (21), and supported metal systems ( e g . Pd/SiO,), with which the dehydrogenation of cyclohexanol to cyclohexanone may proceed to the phenol form (22,23). On the other hand, in the presence of various oxide mixtures [e.g. Cu(U)/Zn(II) (24), Cu(II)/Co(n) ( 2 5 ) ] , or carbon-supported Ni (26) the reaction stops selectively at cyclohexanone.

*The authors gratefully acknowledge financial support from DGICyT (PB92-0816) and Consejeria de Educacion y Ciencia de la Junta de Andalucia.

770 The mechanism for alcohol dehydrogenation remains poorly known and controversial; it seems clear, though, that the alcohol must be adsorbed at electron-deficient surface sites via the electron pair of the oxygen atom (27). According to Matsumura et al. (28), the active sites in the dehydrogenation of ethanol over sicalite-1 are bridging oxygen atoms arising from dehydration of neighbouring -OH groups at high temperatures. However, adsorption of the alcohol at two types of acid and basic sites has also been claimed in which water would be eliminated via a concerted mechanism. Such sites have been found to be pairs of cations and HPOi- ions, or OH- or PO:- groups, in the dehydrogenation of alcohols over hydroxyapatites. The type of site at which alcohol dehydration and dehydrogenation take place seems to be present in the thermal pretreatment of the catalyst. Magnesium orthophosphates are stable solids up to 800 C whose synthesis was described elsewhere (30,3 1). These solids have scarcely been used in organic processes notwithstanding their excellent performance in the gas-phase dehydration-dehydrogenation of cyclohexanol, as shown in this work.

2. EXPERIMENTAL 2.1. Catalyst synthesis To an aqueous solution containing 232 g of MgC1,6H,O and 115 g of Na,HPO, was added 3 N NaOH dropwise up to pH 9. The precipitate thus formed was allowed to stand, after which it was filtered and air-dried, which yielded solid FM. A portion of this solid was suspended at 343 K and added saturated Na,CO, dropwise. The solid obtained after 24 hour's standing, FMNal, was filtered and air-dried. Subsequently, each of the solids was calcined stepwise according to the following temperature programme: 1 h at 473 K, 1 h at 573 K, 1 h at 673 K and 1 h at 773 K. After calcination at 773 K, FMNal solid was washed with water several times until no chloride was detected in the washings (AgNO, reaction), and was thus made ready for reaction (FMNa1-773-W solid). A portion of the washed catalyst was calcined stepwise up to 773 K, which yielded FMNa1-773-W-773 solid). 2.2. Chemical and textural properties of the catalysts The specific surface area of the synthesized solids was determined by the BET method on a Quantasorb Surface Area Analyser from Micromeritics ASAP 2000. Acid, basic and oxidizing sites were determined from the retention isotherms of various titrants dissolved in cyclohexane, viz cyclohexylamine for acid sites, phenol for basic sites and phenothiazine for oxidizing sites. Application of the Langmuir equation provided the amount of titrant adsorbed in monolayer form, X,, as a measure of acid, basic and oxidizing sites (32). 2.3. X-ray diffraction analyses X-ray diffraction patterns were recorded on a Siemens D 500 diffractometer using CuK, radiation. Scans were performed between 20 = 5 and 20 = 70. 2.4. Reactor Reactions were carried out in a glass tubular reactor of 20 mm i.d. that was fed at the top with cyclohexanol by means of a SAGE 35 propulsion pump whose flow-rate was controlled by means of a nitrogen flow-meter. The reactor was loaded with 4 g of catalyst, over which 5 g of glass beads acting as vaporizing layer was placed. The temperature was controlled by

77 1 means of an externally wrapped heating wire that covered the height of both the catalytic bed and the vaporizer and was connected to a temperature regulator. The reactor outlet gases were passed through a condenser and onto a collector that allowed liquids to be withdrawn at different times. No diffusion control was detected, nor was any of the reactor elements found to contribute to the catalytic action under the reactor working conditions (feeding at 0.15-60 ml/min; temperatures between 473 and 823 K; nitrogen stream at 100 ml/min; 2-5 g of catalyst) in blank assays.

2.5. Product analysis The collected samples were analysed by gas chromatography on a 2 m 1/8" i.d. column packed with Carbowax over Chromosorb P-10% CW 20 M, using a linear temperature programme (from 333 to 423 K at 30 K/min). The products obtained were identified by comparison with standards and their structure confirmed by mass spectrometry. 3. RESULTS AND DISCUSSION Table 1 summarizes the textural properties and the acid, basic and oxidizing site which sodium carbonate was added concentrations of the catalysts. Both FM and FMNa 1 -to in the synthetic processhave a small surface area relative to those of similarly synthesized conventional catalytic solids such as SiO,, A1,0, and AlPO, (12,13,32). The area of FM catalysts decreases with increasing calcination temperature and time. However, the addition of Na,CO, (FMNal catalyst) seemingly had a stabilizing effect as the catalyst surface area did not follow this trend. Table 1 Chemical and textural properties of the catalysts Catalyst TCdC (K)

sspcc (m2/g)

FM 573 24

FMNa 1

673

773

17

15

Acidity (10' mol/g)

-

60.1

60.6

Basicity (10" mol/g)

-

13.0

20.3

Oxid. sites (10' mol/g)

-

10.5

11.0

573 9 ~

~

673

773

11

10

11.4

12.0

9.4

5. I

9.9

11.0

The table only shows the acidity and basicity of the solids that were found to have some catalytic activity. No reaction with CHOL was detected at a temperature below 573 K. The acidity did not vary significantly with the calcination temperature at which the solids were active; on the other hand, the basicity increased (FM catalysts) or decreased (FMNal catalysts) with increasing calcination temperature, depending on the particular solid. The addition of sodium carbonate during the synthetic process did not seemingly affect the population of oxidizing sites. Table 2 shows the variations of such properties on subjecting FMNa1 catalyst to washing and subsequent recalcination. A comparison with Table 1 reveals that washing resulted in substantially increased surface area, acidity and basicity. This suggests that washing with abundant water gives rise to major structural changes. This hypothesis is also supported by

172 the x-ray diffraction results. FMNal catalyst is a crystalline solid including such species as Na,Mg(CO,)CI, NaCl and Mg,(P04),*8H,0 up to 773 K. As the calcination temperature is increased, the bands for chlorocarbonate species disappear (by 773 K, the sole bands observed correspond to NaCl and a-NaMgPO,, which remain stable up to 873 K). If the crystalline solid (FMNal) is washed with abundant water, it decreases its crystallinity and only NaCl is detected in its composition. These changes are concomitant with those observed in its specific surface, acidity and basicity. Prior to calcination, FM solid consists of a crystallhe mixture of NaCl and Mg3(P0,).22H,0; however, its crystal structure is destroyed by the time the calcination temperature reaches 673 K. Table 2 Chemical and textural properties of the washed catalysts Catalyst

Sspec

(m2/s)

Acidity (lo6mol/g)

(lo6mol/g)

Basicity

Oxid. sites (lo8 mol/g)

Fh4Na1-773-W

95

48.6

19.5

5.0

FMNa 1-773-W-773

21

49.0

44.5

18.4

FMNal-773-W-Used*

15

70.3

34.7

17.8

't

After reaction

3.1. Influence of the reaction temperature After the optimal working conditions for the reactor were established (viz N, flow-rate = 100 ml/min; feed rate = 0.47 ml/min; amount of catalyst = 4.0 g; W/F = 0.15 h), conditions under which no diffusion phenomena were observed, the most suitable reaction temperature as regards conversion and selectivity was determined. Table 3 shows the results obtained in the conversion to CHE and CHONE at different temperatures by using FM catalyst calcined at 773 K.

Table 3 Variation of the conversion with the reaction temperature for FM catalyst

r,,,,

=

T,,,, (K) 673

49.5

47.0

773

84.6

40.8

XT

'CHE

20 min

t,,,

=

80 min

XT

XCHE

XCHONE

2.1

27.8

22.0

5.5

42.9

73.2

27.7

44.4

'CHONf?

At a reaction temperature of 673 K, only CHE was produced selectively. As the reaction time increased, conversion to CHE decreased considerably, though. On the other hand, at 773 K, the overall conversion increased substantially with time. Such an increasing trend is seemingly consistent with the appearance of CHONE since the amount of CHE produced was virtually the same as under the previous conditions. At longer reaction times (80 min), production of CHE again decreased markedly while that of CHONE remained constant. The results obtained in these experiments suggest that CHE and CHONE are yielded via different mechanisms at different active sites that come into play at different reaction temperatures. Both processes are deactivated also differently. Production of olefins by dehydration of

773

alcohols is known to be strongly deactivated by carbonization, which does not seem to be related to the dehydrogenation reaction by which CHONE is produced. Figures 1 and 2 show the CHE and CHONE selectivity results obtained with FM catalysts, which were used at the same reaction temperature as they were calcined. All behaved similarly. Thus, CHE was produced selectively at low temperatures; such a selectivity was preserved throughout the temperature range studied. CHONE started to appear at appreciable concentrations at 723 K. This resulted in an increase in the overall conversion seemingly independent of CHE production. At higher temperatures, the catalyst lost some activity, which affected its selectivity towards CHE, but not that towards CHONE. These results also seemingly confirm that the dehydration and dehydrogenation reactions take place at active sites of a different nature.

-67SK

" ( ' " ' ' ' ' ' I 0

102030406060708090

t

(-1

Figure 1. CHE selectivity VB. time at various temperatures w i t h FM catalyst.

0

102090406060708090

t Figure 2. CHONE selectivity vs. time at various temperatures with Fld catalyst.

3.2. Influence of the nature of the catalyst Table 4 compares the conversion at 80 min and the selectivity towards CHE and CHONE of FMNa1 catalysts calcined at various temperatures. The reaction temperature used was the same as the catalyst calcination temperature in all instances. Table 4 Selectivity to cyclohexene and cyclohexanone for FMNal catalysts T,,,, (K)

x, (%I

673

2.5

0.06

0.93

723

19.5

0.01

0.98

773

66.2

0.01

0.99

S,

~CH0i.E

774 In view of these results, FMNa1 catalyst, to which sodium carbonate was added in the synthetic procedure, behaves differently from catalyst FM. The former exhibited a high selectivity towards CHONE and yielded a virtually insignificant amount of CHE throughout the temperature range assayed. At a given reaction temperature, the overall conversion obtained with FMNal catalysts was lower than that provided by FM, however, they were virtually inactive in the conversion of CHOL to CHE. Table 5 compares the activity and selectivity results for two catalysts that were subjected to this treatment. FMNa1-773-W catalyst was obtained by washing FMNa1-773 catalyst with abundant water and recalcining it subsequently. Table 5 Effect of washing on the selectivity of FMNa1-773 catalyst Parameter Overall conversion (%)

FMNa1-773 64.0

FMNa 1-773-W 98.9

Selectivity towards CHE

0.01

0.85

Selectivity towards CHONE

0.99

0.13

As can be seen, washing of this catalyst resulted in substantially increased specific surface, acidity and basicity. The activity and selectivity were also markedly altered as a result. The selectivity of FMNa1-773 catalyst, which initially produced CHONE alone, changed dramatically -towards CHEon washing. The washed catalyst, FMNa1-773-W, featured a higher conversion than the starting catalyst and produced minor amounts of CHONE. Such a marked change in the selectivity must be related to dramatic structural and surface changes probably arising from surface rehydration processes. The FMNal-773 solid, initially crystalline, became amorphous on washing and calcination at 773 K, which also altered its chemical and textural properties dramatically.

3.3. Comparison with other magnesium oxides In order to compare the activity results obtained with those provided by our catalysts, we subjected a commercially available magnesium oxide [Mg(OH),, Probus ref. 3225 ] and another synthesized by us to the same reaction conditions. The synthesized oxide (M) was prepared similarly to FMNa1, but no Na,HPO, was added to the reaction medium. Precipitation in 3 N NaOH yielded a solid that, once dry, was subjected to the same calcination procedure as FMNal solid. The x-ray diffractograms showed that both the commercially available and the laboratory prepared oxide consisted of a mixture of brucite and periclase. Table 6 summarizes the chemical and structural properties of these magnesium oxides calcined at different temperatures. Commercially available Mg(OH), has a larger specific surface and a higher concentration of acid, basic and oxidizing sites than our magnesium orthophosphates. On the other hand, the magnesium oxide labelled M has a lower acidity than the magnesium orthophosphates at the same calcination temperature, even though its population of oxidizing sites is substantially larger.

775

Table 6 Chemical and textural properties of magnesium oxides M Tcdc

(K)

673

Sspec (m2/g)

Mg(OH), com. 773

773

18.5

24.5

Acidity (lo6 mol/g)

4.1

32.9

Basicity ( lo6 mol/g)

38.7

50.6

147.0

Oxidizing sites (10' mol/g)

318

105

306

2115

Even though both magnesium oxides feature an excellent overall conversion, only that synthesized by the authors possesses a selectivity as high as that of the magnesium orthophosphate FMNal . Commercially available magnesium hydroxide produces not only CHE and CHONE, but also large amounts of compounds of high molecular weight and condensation derivatives, the proportion of which is scarcely significant at low temperatures, but rises considerably by 773 K. Table 7 Activity of magnesium oxides in the conversion of cyclohexanol M

Mg(OH), corn.

T,,,, (K)

673

723

773

653

743

773

Overall conversion (%)

7.4

24.4

74.4

16.4

79.5

99.0

Select. CHE

0.01

0.01

0.01

0.03

0.08

0.08

Select. CHONE

0.99

0.99

0.98

0.70

0.61

0.53

REFERENCES 1. J. M. Winterbottom, "Catalysis" Vol. 4, Royal SOC.of Chemistry, London 1981. 2. M. Hudlicky, "Oxidation in Organic Chemistry", ACS, Monograph 186, Washington, DC, 1990. 3. H. Pines and J. Manassen, Adv. Catal. 16 (1966) 49. 4. J. M. Parera and N. S. Figoli, J. Catal. 14 (1969) 303. 5. F. F. Roca, L. Mourgues and Y. Trambouze, J. Catal. 14 (1969) 107. 6. T. Takeshita, 0. Ohnishu and K. Tanabe, Catal. Rev. 8 (1974) 29. 7. T. Yamaguchi and K. Tanabe, Bull. Chem. SOC.Jpn. 47 (2) (1974) 424. 8. J. B. Moffat and J. F. Neeleman, J. Catal. 34 (1974) 376. 9. F. Nozaki and H. Ohta, Bull. Chem. SOC.Jpn. 47 (6) (1974) 1307. 10. A. Tada, H. Itoh, Y. Kawasaki and I. Nara, Chem. Lett. (1975) 517. 11. S. Mahowski and J. M. Tyblewski, J. Colloid Interface Sci. 71 (3) (1979) 560. 12. J. M. Campelo, J. M. Marinas and R. Perez-Ossorio, An. Quirn. 74 (1978) 86. 13. J. M. Campelo and J. M. Marinas, Afinidad 38 (1981) 333. 14. J. M. Parera, Ind. Eng. Chem. Prod. Res. Dev. 15 (4) (1976) 234. 15. J. B. Pen, J. Phys. Chem. 69 (1965) 211, 230.

116 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

Y-M. Lin, I. Wang and Ch-T. Yeh, Applied Catal. 41 (1988) 53. N. Kh. Valitov and S. M. Lakiza, Zhur. Fiz. Khim. 49 (1975) 3139. G. C. Bond, Gold Bull. 5 (1972) 11. M. Hino and K. Arata, Chemistry Lett. (1990) 1737. J. C. Collins and W. W. Hess, Org. Synth. Collect., Vol. 6 (1988) 644. M. Richter and G. Ohlmann, React. Kinet. Catal. Lett. 29 (1985) 211. J. A. Gonzalez-Marcos, J. J. Alvarez-Uriarte, J. I. Gutierrez-Ortiz, A. T. Aguayo and J. R. Gonzalez-Velasco, Appl. Catal. 60 (1990) 1. J. Wanninger, Z. Paal and P. Tetenyi, Acta Chim. Acad. Sci. Hung. 97 (1978) 439. Y.-M. Lin, I. Wang and T. C. Yek, Appl. Catal. 41 (1988) 53. Mitsubishi, Chem. Ind. Ltd. Jpn. Patent 136241 (1980). Y. Uemichi, T. Sakai and T. Kanazuka, Chem. Lett. 777 (1989). J. Sedlacek and M. Krauss, React. Kinet. Catal. Lett. 2 (1975) 57. Y. Matsumura, K. Hashimoto and S. Yoshida, J. Catal. 122 (1990) 352. J. A. S. Bett and W. K. Hall, J. Catal. 10 (1968) 105. T. Kanazawa, T. Umegaki and H. Kawazoe, I International Congress on Phosphorus Compounds, p. 107, IMPHOS, Rabat, 1977. A. Y. Bakaev, U. A. Dzis'ko, L. G. Karakchiev, E. M. Moroz, G. N. Kustova and L. T. Tsikova, Kinetika i Kataliz, Vol. 15, no. 5 (1974) 1275. M. A. Aramendia, V. Borau, C. Jimenez, J. M. Marinas and F. Rodero, Colloids and Surfaces 12 (1984) 227.

DISCUSSION CONTRIBUTION J. A. NAVIO (Instituto de Ciencia de Materiales, Sevilla, Espana): Taking into account yours catalyst's preparation and concerning with their characterization, did you found any experimental evidence for the possible presence of remained surface or bulk chloride ions on your starting catalysts?

F. J. ROMERO (Dpto. Quimica Organica, Univ. Cordoba, Espana): No, the catalysts were washed until no chloride was detected in the washings. We do not know whether any residual chloride ions remained in the inside or on the surface of the crystallites. We are currently conducting surface experiments by using X P S and EDAXS in order to clarify this points.

B.DELMON (Univ. Catholique de Louvain, Belgium): This deals with the variability of your catalysts. A proposal would be that basic sites, associated with a surface excess of Mg, would be the dehidrogenation sites, and acid-base sites, namely a well balanced proportion of Mg and P, the dehydration sites. Did you make X P S measurements to evaluate surface Mg/P ratios?

F. J. ROMERO: In fact, the surface population of acid and basic sites might be related to the proportions of Mg and P. We will find out as soon as the X P S results are available. This paper only deals with the catalyst syntheses and the surprising activity and selectivity results obtained in the dehydration-dehydrogenation of cyclohexanol. Our aim is to go deeper into the subject in order to acquire a better knowledge of the structure-activity relationship for this type of solid.

V. Cortts Corberan and S. Vic Bell6n (Editors), New Developments in Seleciive Oxidation I1 0 1994 Elsevier Science B.V. All rights reserved.

777

Effect of K-doping on 2-propanol adsorption, desorption and catalytic oxidation over vanadia-titania G. Buscaa, V. Sanchez Escnbanob, P. Forzatti”, L. Lietti” and G. Ramisa Istituto di Chimica, Facold di Lngegneria, Universith, P.le J.F. Kennedy, 1-16129 Genova, Italy. Depastamento de Quimica Inorganica, Universidad, P”. de la Merced, E-37008 Salamanca, Spain. Dipartimento di Chimica Industsiale e Ingegneria Chimica ”G. Natta”, Politecnico di Milano, P. L. daVinci 32, 1-20133 Milano, Italy.

a

The adsorption, desorption and catalytic oxidation of 2-propanol has been investigated over a vanadia-titania catalyst without and with potassium doping. 70 % yields in acetone can be obtained at 443 K over vanadia-titania, with propylene and carbon oxides as the main by-products. Potassium doping causes a slight reduction in activity but a significant increase of acetone selectivity. Acetone yields exceeding 85 % are obtained at 573 K. FTIR and TPD data show that the key adsorbed intermediates are 2-propoxide groups that give propylene by elimination of an hydroxy- group, and acetone by oxidative dehydrogenation. K-doping is found to decrease strongly the surface acidity. Consequently, the elimination reactions giving propylene is inhibited, while acetone adsorption is also weakened, making easier its desorption as such. Both these effects favour higher acetone selectivities. 1. INTRODUCTION

Vanadia-titania catalysts are largely used in the industry for the selective oxidation of oxylene to phthalic anhydride (1,2). The mechanism of this reaction is very complex (3-6) and the catalyst performances are related to their properties both according to their red-ox and their acid-base character. Accordingly, in spite of the presence of alkali sulphates in industrial catalysts as stabilizers, alkali metals are reported to significantly decrease thc catalytic activity of vanadia-titania for alkylaromatic oxidation (7,s). Vanadia-titania catalysts have been found to be efficient also for other selective oxidation reactions, and, in particular, in alcohol oxidations, like methanol oxy-esterification to methylformate (9-1 1) and ethanol oxidation to acetaldehyde or acetic acid (12). This paper summarizes the results of a study of the behaviour of pure and K-doped vanadia-titania in the adsorption, desorption and catalytic oxidation of 2-propanol. The aim of this work is to improve the understanding of the behaviour of vanadia-titania catalysts and of doping of them. However, it can also be regarded as an explorative work for an oxide-catalyzed oxidative dehydrogenation process to acetone. In fact, acetone is mainly

778

produced as a by-product of the phenol synthesis via cumenyl hydroperoxide (Hock process) or by direct oxidation of propylene in liquid phase (Wacker-Hoechst process). Synthesis of acetone by 2-propanol oxidation was performed industrially in the past over metal catalysts (Ag or Cu) at relatively high-temperatures (673-873 K) and with selectivities near 85-90 % (13). This reaction has received attention in recent years in the surface science and catalysis literature (14). 2. EXPERIMENTAL The Vanadium-titanium oxide catalyst was prepared using an anatase-type TiO, obtained by precipitation from TiC1, and ammonia followed by calcination (surface area 60 mZ/ g). Dry impregnation was performed with a boiling water solution of ammonium metavanadate, followed by calcination in air at 720 K for 3 h. The loaded amount was 3 3 % as V,O, by weight. The resulting surface area of the catalyst was 48 m2/g. K-doping was performed with a second impregnation of KOH (1 % K w/w). All reagents were from Carlo Erba (Milano, Italy). Temperature Programmed Desorption (TPD) and flow reaction experiments were performed in a quartz tubular fixed bed microreactor (I.D. 7 mm). 160 mg of fresh catalyst (60-100 mesh) were used in each run. In a typical TPD experiments after saturation at 313 K with 2-propanol, the catalyst was heated up to 773 K in He (60 Ncc/min) at 15 Wmin. The gases exiting the reactor were analyzed by both a quadrupole mass detector (QMD) and on line G C (Flame Ionization Detector, FID). In the case of flow reaction studies (total flow rate = 60 Ncclmin), the reactor feed consisted of He (90.6 % v/v), 0, (7.8 %), N, (0.5 %) and 2-propanol(l.1 %). The 2-propanol conversion and selectivities were measured by on-line G C analysis. For details on the experimental set-up, see ref. 15. For the FT-IR adsorption and oxidation experiments the pure catalyst powders were pressed into self-supporting disks and activated by outgassing in the IR cell at 720 K for 2 h. The IR spectra were recorded with a Nicolet MX1 Fourier transform instrument, using IR cells to perform measurements in controlled atmospheres.

3. RESULTS 3.1. Catalytic behavior in flow reactor. The catalytic behavior of vanadia-titania in 2-propanol conversion is described in Fig. 1 (upper curves). 2-propanol conversion starts at 373 K and grows rapidly up to 450 K where it is complete. Acetone, propylene, acetic acid and carbon oxides are the predominant products. Selectivity to acetone is very high at low temperatures but decreases progressively in favour of selectivity to propylene, carbon oxides and acetic acid in the Trange 373-523 K. Selectivity to acetic acid is always small in these conditions. The maximum yield in acetone is near 70 % at 453 K. The catalytic activity of vanadia-titania is essentially due to the V,O, component. In fact 2-propanol conversion over pure TiO, is still negligible at 473 K (< 3 %) and it is complete only at 550 K, where the main product is propylene. Potassium significantly weakens the catalytic activity of vanadia-titania, as shown in Fig. 1 (lower curves). In fact, 2-propanol conversion is strongly decreased at the same

119 I*/*)

CONVERSION / S E L E C T I V I T Y

I

/i

LO

p r o p y leo e

--

11 350

.L

LOO

L50

500

TEMPERATURE

550

60

iK)

Figure 1. 2-propanol conversion (open triangles), - and selectivities to acetone (oDen ~* squares), propylene (full cyrcles), COX in flow reactor as a function of reaction temperature. Upper curves: vanadia-titania catalyst. Lower curves: K-doped vanadiatitania catalyst.

I JUU

c-nn I Y V

TEMPERATURE

7 I nn YY

(K)

Figure 2. 2-propanol TPD traces over vanadia-titania (full lines) and K-doped vanadia-titania (dotted lines).

reaction temperature with respect to the K-free catalyst. The selectivity to acetone is always very high, also at high temperature where 2-propanol conversion approaches 100 %. Accordingly, selectivities to both propylene and COXare very low up to near 773 K. This results in an acetone yield near 85 % at 573 K.

3.2 Temperature Programmed Desorption of 2-propanol. The desorption profiles of the main species produced upon 2-propanol adsorption at 3 13 K over the V,O,-TiO, catalyst are reported in Fig. 2 (full lines). The overall FID signal presents a main maximum centered near 420 K with shoulders at both sides. The QMD traces show that the peak with maximum at 420 K is due to the evolution of acetone, whereas the shoulders at low and high temperatures originate from the desorption of intact 2-propanol (TM= 370 K) and from the evolution of propylene (T, = 464 K), respectively. Thc evolution of water (a broad peak with T, = 547 K) and CO, (730 K) is also evident. The oxy-dehydrogenation activity producing acetone is related to the vanadium oxide component, since in the same conditions only traces of acetone were observed over TiO, (16). Also the dehydrating activity is favored by vanadium oxide since the propylene

780

desorption peak is observed at lower temperatures than on TiO,. K-doping influences strongly the behaviour of V,05-Ti0,, as shown in Fig. 2 (dotted lines). Only 2-propano1, acetone and water are detected. The peak relative to 2-propanol desorption is relatively much stronger and shifted to higher temperatures (T, = 393 vs. 370 K), while that relative to acetone evolution is weakened but also shifted upwards (T, = 480 vs. 420 K). The formation of propylene and CO, were no longer observed.

3.3 FT-IR study of 2-propanol adsorption and oxidation. The spectra of the adsorbed species arising from isopropanol adsorption over the pure support TiO, are reported in Fig. 3 (a,b). The sharp band at 1468 cm-' and the shoulder at 1455 cm-I are due to the asymmetric deformation modes of the two methyl groups while the split band at 1387,1366 cm-' is due to the corresponding methyl symmetric deformation modes.The band at 1332 cm-' is due to the deformation of the CH mode and characterizes secondary alcohols and their derivatives. As discussed previously (17,18) the relatively broad band at 1290 cm-I is due to the C-0-H deformation mode of coordinatively adsorbed 2-propanol. This mode is observed at 1252 cm-' in monomeric 2-propanol in CC1, solution. The very intense mode centered near 1130 cm-' is instead due to the C - 0 stretching of isopropoxy-groups, probably coupled with C-C stretchings, and characterizes the dissociated adsorbed form of isopropanol. This mode is found at 949 cm-I in monomeric isopropano1 and shifts strongly upwards by dissociation, up to superimpose the C-C stretchings and methyl rockings found, in CCl, solution, at 1161, 1141 cm-1(strong doublet), 1105 (weak) and 1075 cm-' (medium). These bands can also be found, nearly at the same position, in the adsorbed forms. So, IR spectra show that 2-propanol adsorption on TiO, produces two adsorbed species: i) coordinatively bonded undissociated 2-propanol; and ii) 2-propoxygroups produced by dissociative adsorption. Upon progressive heating under outgassing, the band at 1290 cm-' progressively decreases in intensity up to disappear at 473 K, together with a sharp component near 1168 cm". Simultaneously, all bands due to CH deformations only partially decrease in intensity while the component at 1387 cm-I disappears leaving another one at 1382 cm-I. These modifications clearly point to the progressive and complete desorption of undissociated 2-propanol below 473 K, when isopropoxy groups are still definitely stable. At higher temperatures (473-523 K) the bands of 2propoxides decrease and completely disappear. These IR data perfectly agree with the TPD pattern, showing that 2-propanol desorbs below 473 K while propylene is produced in the temperature range 473-523 K by an elimination reaction of isopropoxy groups, leaving an adsorbed OH group (as it is indeed found). Lewis acid-Br+nsted base sites, where dissociative isopropanol adsorption occurs (17), is responsible for alcohol dehydration on TiO,. No other species (i.e. acetone) are found after isopropanol adsorption and desorption on titania, according to IR and TPD. The FT-IR spectra of the adsorbed species of 2-propanol on vanadia-titania at r.t. (Fig. 3, c-e) are similar qualitatively to those observed on the pure support, with few small differences. The CH deformation modes are observed almost at the same frequencies: 1468 and 1455 cm-' (asymmetric CH, deformation), 1385,1370 cm-' (symmetric CH, deformation), 1330 cm-I (CH bending). However, the COH deformation mode expected in the region 1300-1250 cm-' is only present, if any, extremely weak, showing that undissociated adsorbed 2-propanol is present in very small amounts. Accordingly, also the sharp band

78 1

e

'' I '

c P L D

1800

1600

1400

1

1200 wavenumber

cm-1

Figure 3. FT-IR spectra of the adsorbed species arising from 2-propanol adsorption over TiO, (a and b) and vanadia-titania (cg) and following evacuation at room temperature (a,c), 373 K (d), 420 K (e) and 473 K (b,f) and 523 K (8).

,

I

1600

1400

1200 wavenumber

1' I0 cm-1

Figure 4. FT-IR spectra of the adsorbed species arising from 2-propanol adsorption over K-doped vanadia-titania and following evacuation at room temperature (a), 373 K (b), 423 K (c) and 473K (d).

near 1165 cm-' is very weak. On the contrary, the C - 0 stretching of 2-propoxy groups is very strong at 1140 cm-I. However, while the bands due to isopropoxy-groups decrease progressively by heating under outgassing in the range 300-473 K, other bands first grow and later decrease down to disappear. In particular, sharp bands at 1680 and 1250 cm-' grow very strong at 423 K but are completely disappeared at 523 K. They are due to C=O stretching and C-C-C asymmetric stretching of adsorbed acetone (19). O n the other hand, strong bands at 1565-1540 and at 1440 cm-I, with a weaker one at 1355 cm-I, are very strong at 523 K and later decrease up to disappear. These bands are due to acetate species, as confirmed by adsorption of acetic acid. Again IR data agree well with TPD data (Fig. 2) that show evolution of small amounts of 2-propanol, of acetone and later of small amounts of propylene and COXfollowing 2propanol adsorption on vanadia-titania. From these data it seems straighforward to deduce that acetone is produced by oxidative dehydrogenation of isopropoxy-groups bonded to vanadium cations. The small amount of propylene produced is likely due to elimination of an OH group from isopropoxy-groups bonded to vanadium too. In fact, the temperature of propylene evolution on vanadia titania is distinctly lower than that measured on pure TiO,, while the C - 0 stretching of 2-propoxy-groups is observed at higher frequency on vanadiatitania than on pure titania. It is possible to assume that the oxidative dehydrogenation and the dehydration of 2-propanol (giving rise to acetone and propylene, respectively) are competitive reactions occurring at the same site, on vanadia-titania, while the non-dissociative

782

adsorption observed mainly on TiO, anatase occurs over different sites. The detection of acetate ions by FT-IR agrees with the production of acetic acid in the flow reactor. ET-IR spectra indicate that carbon oxides observed both in TPD experiments and in the flow reactor are likely produced by acetone overoxidation, with further oxidation of the intermediate acetate species. The FT-IR spectra of the adsorbed species arising from 2-propanol adsorption on Kdoped vanadia-titania are shown in Fig. 4. Again the spectra recorded at room temperature can be interpreted as due to a mixture of undissociated 2-propanol and to 2-propoxygroups. However, the detection of two rather broad components that can be assigned to a C - 0 - H mode (at 1285 cm-' but also at 1390 cm") point to the existence both of coordinated undissociated species (whose OH group is nearly free from H-bonding, as shown by the 60H band at 1285 cm-') and of H-bonded species (whose &OH mode is shifted up to 1390 c m ~ ' ) .The last species is indicative of the presence of basic sites formed by K doping, to which the alcohol molecule can bond with its own hydrogen atom. Moreover, the relative intensities of these 6 0 H modes with respect to the C - 0 stretching mode of 2-propoxygroups points to the relative amount of undissociated adsorbed alcohol with respect to the dissociated one that is by far stronger on K-doped samples than on "pure" vanadia-titania. Finally, the C - 0 stretching mode at 11 15 cm-' is evidence of a strongly lowered C - 0 bond order of 2-propoxy- groups by K doping (1140 cm-' on K-free samples). By heating under outgassing, these species progressively disappear while small bands at 1708 and 1235 cm ' appear and raise their maximum intensity at 423 K. These bands are due to C=O stretching and C-C-C asymmetric stretching of acetone, adsorbed in a much weaker form than on K-free vanadia-titania, where these bands are found at 1680 and 1250 cm-'. However, at 473 K the K-doped surface is already completely clean from any adsorbed species. Again IR data agree with TPD data, that show 2-propanol desorption in the 300473 K range, and acetone evolution near 490 K, as well as no COXevolution. 4. DISCUSSION

The FT-IR and TPD data reported above allow us to propose several details of the mechanism of isopropanol conversion over vanadia-titania and on the effect of K-doping on it. The data discussed here can be explained assuming the following reaction scheme:

112 0, (gas)

\

2 e- + H'

+ (CH,),C=O

(ad)

CH,-CH=CH, (gas)

+ OH- (ad)

I

(CH,),C=O (gas)

overoxidation products (ad) ----->COX

783

where the oxidative dehydrogenation of the alcohol to the ketone and its dehydration to propylene are competitive reactions occurring at the same site. The active site is certainly constituted by vanadium ions, that are reduced by the electrons produced by the oxidative dehydrogenation reaction and are reoxidized by gaseous oxygen. In fact, our FT-IR and TPD data performed in the absence of dioxygen (either gas-phase or adsorbed) show that vanadium centers are able to dehydrogenate 2-propanol to acetone without direct involvement of 0,. The reaction mechanism is of the Mars-Van Krevelen type (20), as expected. The acetone yield over vanadia-titania catalysts is lowered by the competitive reaction giving propylene (that is reversible, as shown by propylene adsorption (19)) but also by the successive overoxidation of acetone to acetic acid and carbon oxides with the intermediacy of adsorbed acetate species, as observed by FT-IR spectra. K-doping has a significant adverse effect on catalytic activity. However, it increases acetone selectivity and yield. This is the result of a strong inhibition of both the competitive dehydration reaction and the successive overoxidation reaction, as it is evident from TPD and FT-IR experiments. The IR spectra of isopropoxy-groups on the catalysts differ significantly for the position of the C - 0 stretching. In particular, this mode is found at distinctly lower wavenumbers on the K-doped sample than on pure vanadia-titania. This indicates that potassium causes a lowering of the Lewis acidity of the vanadium sites where 2propoxy- groups are bonded. The inhibition of both acetone and propylene formation by K can be attributed to the perturbation of vanadium sites, where dissociative adsorption occurs, by potassium. However, the perturbation effect on the oxidative dehydrogenation activity is weak, while that on dehydration (a truly acid-catalyzed step) is strong. K-doping results in the disappearance of the acetate species observed by FT-IR after acetone overoxidation over vanadia-titania, as well as in a lower perturbation of adsorbed acetone species. This can be again related to the lowering effect on the Lewis acidity, with a consequent lower electronwithdrawal effect on the carbonyl group of adsorbed acetone and, consequently, its faster desorption and its lower tendency to undergo nucleophilic attack by surface anions. According to IR data, some conclusions can also be proposed on the kinetics of the elementary steps. Dissociative adsorption is certainly a fast phenomenon occurring easily and quickly already at room temperature, while both oxidative dehydrogenation and dehydration oC alkoxy- groups are certainly slower, being only observed at higher temperatures. The dehydration route is certainly slower than the oxidative one on vanadia-titania in contrast with pure titania where only dehydration is observed. This is related to the activity of vanadium oxide species in undergoing redox cycles, in contrast to Ti4+.The dehydration way is further slowed down relative to the oxidation one by, K-doping, while acetone desorption is sped up. Mainly for this reason, the successive overoxidation of acetone to COXappears to be inhibited by K-doping. The effect of K-doping of vanadia-titania with respect to the catalytic oxy-dehydrogenation of 2-propanol parallels that reported with respect to the selective oxidation of oxylene. Also in this case the activity is lowered but selectivity to the desired product phthalic anhydride is enhanced by potassium. According to previous studies (1-6) a key intermediate in o-xylene oxidation is o-tolualdehyde, that can in part react further to otoluic acid, whose decarboxylation is followed by total combustion. This parallel nonselective way, being an overoxidation of a carbonyl compound, is favoured by its strong

784

adsorption on the acidic catalyst. However, like in the present case, this way can be inhibited by K-doping. This explains the increased selectivity in phthalic anhydride by potassium doping. These data also suggest that acetone can be produced by 2-propanol oxy-dehydrogenation over vanadia-titania-based catalysts, at lower temperatures and with higher yields as compared to the metal catalyzed processes. ACKNOWLEDGEMENT This work has been supported in part by CNR, Progetto Finalizzato Chimica Fine. REFERENCES 1 . M.S. Wainwright and N.R. Foster, Catal. Rev. Sci. Technol. 29 (1975) 211. 2. H.G. Franck and J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer Verlag, Berlin, 1988. 3. L.H.S. Andersson, J. Catal. 98 (1986) 138. 4. R.Y. Saleh and I.E. Wachs, Appl. Catal. 31 (1987) 87. 5. G.C. Bond, J. Catal., 116 (1989) 531. 6. G. Busca, in "Catalytic Selective Oxidation", J. Hightower and T. Oyama eds., ACS, Symp. Ser. 523, Amer. Chem. Soc., Washington, 1993, p. 168. 7. M. Kotter, D.X. Li and L. Riekert, in "New Developments in Selective Oxidation", G. Centi and F. Trifirb eds., Elsevier, 1990, p. 267. 8. J. Zhu and S.L.T. Andersson, J. Chem. Soc. Faraday Trans. I, 85 (1989) 3629. 9. A.J. Van Hengstum, J.G. vanommen, H. Bosch and P.J. Gellings, Proc. 8th Int. Congr. Catalysis, Berlin, 1984, Vol. 4, p. 297. 10. E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and P. Tittarelli, Ind. Eng. Chem. Res. 26 (1987) 1269. 11. A.S. Elmi, G. Busca, C. Cristiani, P: Forzatti and E. Tronconi, in "New Developments in Selective Oxidation", G. Centi and F. Trifirb eds., Elsevier, 1990, p. 305. 12. N.E. Quaranta, V. Cortes Corberan and J.L.G. Fierro, in "New Developments in Selective Oxidation by Heterogeneous Catalysis", P. Ruiz and B. Delmon eds., Elsevier, 1992, p. 147. 13. J.M. Tedder, A. Nechvatal and A.H. Jubb, Basic Industrial Chemistry, Wiley, New York, 1975, Vol. 5 , p. 181. 14. P.D.A. Pudney, S.A. Francis, R.W. Joyner and M. Bowker, J. Catal. 131 (1991) 104. 15. L. Lietti, E. Tronconi and P. Forzatti, J. Catal. 135 (1992) 400. 16. G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatti and F. Bregani, Langmuir, 8 (1992) 1744. 17. P.F. Rossi, G. Busca, V. Lorenzelli, 0. Saur and J.C. Lavalley, Langmuir, 3 (1987) 52. 18. G. Ramis, G. Busca and V. Lorenzelli, J. Chem. Soc. Faraday Trans. 83 (1987) 1591. 19. V. Sanchez Escribano, G. Busca and V. Lorenzelli, J. Phys. Chem. 94 (1990) 8939. 20. R.D. Srivastava, Heterogeneous Catalytic Science, CRC Press, Boca Raton, USA, 1988, p. 45.

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B. GRZYBOWSKA (I. of Catalysis, Krakow, Poland): 1) Comment. W e have observed similar effect of K on selectivity to partial oxidation products in oxidative dehydrogenation of propane on V,O,/TiO, catalysts, and similar effect on selectivity to acetone in anaerobic decomposition of isopropanol (Poster 35 at this congress). In addition to modification of acid-basic properties we have found also that doping with potassium of V,0s/Ti02 catalysts affects also the oxygen sorption changing the type of oxygen species from 0-to 02-. 2) Question. Rough calculation from the data you've given shows that the coverage of TiO, with vanadia is well below monolayer, the K content being relatively high at the same time. Where, in your opinion, is located than potassium; on titania surface and or on VOx species? G. RAMIS (I. di Chimica, Genova, Italy): When vanadia-titania catalysts are doped with potassium (I), we observe in the FF-IR spectra a shift at lower frequencies of the bands due to fundamental and first overtone of V=O stretching of isolated vanadyles that are predominant on this catalyst. It is known that strong basic ligands, mostly if bonded in equatorial position, decrease V=O stretching frequency. So w e think that vanadyl cations are perturbed by K giving rise to O=V-0-K bridges. 1. L. Lietti, P. Forzatti, G. Ramis, G. Busca, F. Bregani; Appl. Catal. B: Environ., in press.

U.S.OZKAN (Dept. of Chemical Engineering, Columbus OH, USA): I have a comment in regard to Dr. Grasselli remark. W e have been working with alkali promoters in a different catalytic system, namely simple molybdates in oxidative coupling of methane. Our expeiimental observations agree with this prediction in that the selectivity increases in the order of Li
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V. CortCs Corberan and S. Vic Bellon (Edilors), New Developmenis in Seleciive Oxidation I f 0 1994 Elsevier Science B.V. All rights rescrved.

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Selective Oxidation of Methanol on Iron-Chromium-Molybdenum Oxide Catalysts D. Klissurskia, V. Rivesb, Y. Peshevaa, I. MitovC and R. Stoyanovaa aInstitute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria bDpto. de Quimica Inorganica, Universidad de Salamanca, Salamanca, Spain CInstitute of Kinetics and Catalysis, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria ABSTRACT Parallel studies of FeMoO and FeCrMoO catalysts for methanol oxidation to formaldehyde have shown that they possess commensurable activities and selectivities, the FeCrMoO catalyst being more stable. Mossbauer, ESR and XRD studies revealed that upon reduction with methanol, both catalysts have different behaviour. The presence of CX-~(MOO,)~ in the FeCrMoO catalyst retards its reduction. Under the experimental conditions used, the main intermediate reduction product of the FeCrMoO catalyst is a-FeMo04, while under identical conditions reduction of FeMoO leads to a-FeMoO, and P-FeMo04 phases.

1. INTRODUCTION

A Fe2(Mo04)3-Mo03-basedsystem is the only catalyst which has found industrial application in the selective oxidation of methanol to formaldehyde from the early sixties u p to now [l].Any improvement of its selectivity and thermal or mechanical stability and the duration of its industrial application could be of the utmost interest [2,

31. Previous studies [4-71 have shown that iron-chromium molybdenum oxide catalysts display catalytic activity and selectivity similar to those of industrial catalysts,

788

but with a higher stability. Then, it is worthwhile to perform comparative studies on the reactivity of both types of catalysts with respect to methanol oxidation. The present paper reports on results of the catalytic activity of Fe-Mo-0 and Fe-CrMo-0 catalysts and the nature of the products formed upon their reduction with methanol and further reoxidation. The aim of the study is to analyze the role of both metals (Fe and Cr) in the mixed catalyst, as well as the increased stability due to the presence of Cr.

2. EXPERIMENTAL

A mixed FeCrMoO catalyst with a Mo/(Fe+Cr) atomic ratio of 2.5/(0.5+0.5) was obtained by coprecipitation from continuously stirred aqueous solutions of iron and chromium nitrates, and ammonium paramolybdate. NH,OH was used to maintain pH=3.8. The samples were dried in air and heated at 54OOC in an air flow for 4 h. FeMOO and CrMoO catalysts (Mo:Fe=Mo:Cr=2.5:1)were prepared as described elsewhere [ 8 ] .Moo3 was obtained by decomposition of ammonium heptamolybdate and its surface was stabilized at 450°C. Catalytic activity and selectivity measurements towards methanol (MeOH) oxidation to HCHO were performed in a gradientless flow-circulation apparatus; the experimental details have been described elsewhere [4]. Reduction of the catalyst was carried out as follows: (i) the catalyst was heated up to a given temperature in an argon flow (120 ml/min), the temperature being maintained for 2 h; (ii) gaseous MeOH (4.5% vol in Ar) was flowed over the catalyst; (iii) the catalyst was cooled down in Ar. Reoxidation was eventually carried out in air at 400 or 540OC. Samples are described as Fe(Cr)MoO/T/ t (T=reduction temperature, "C; t=reduction time, min). The Mossbauer spectra were recorded in an electromechanical spectrometer working in the constant acceleration mode. The spectrum of a-Fe was used as a reference to measure the isomer shifts. The spectra were processed by the least square method assuming Lorentz-shaped lines. The amounts of FeMoO and FeCrMoO catalysts used corresponded to 15 mg/cm2 iron. The ESR measurements were recorded in the 100380 K range as a first derivatives of absorption lines in the X-band with an ESR220/Q spectrometer (ex-GDR) using 100 kHz modulation. The X-ray diffraction (XRD) patterns were recorded with a DRON-1 apparatus, using Cu K a radiation.

789

3. RESULTS

Catalytic performance. Results obtained from a comparative study of FeMoO and FeCrMoO catalysts are summarized in Fig. 1. The catalytic activity and selectivity for

4

I

the FeCrMoO catalyst are similar to those of a FeMoO catalyst under identical experimental conditions. In addition, when the process is carried out under extreme conditions (4O0-42O0C), the activity of the FeCrMoO catalyst FeMoO(I) FeCrMoO catalyst remains practically unchanFig. I.-Experimental results on catalytic activity for me- ged even after 100 h, while thanol oxidation on an industrial catalyst, FeMoO(I), that for the industrial catalyst and on catalyst FeCrMoO. (sample I) decreases by about 20%. This is indicative for a favourable catalyst stability upon incorporation of Cr. Long run experiments in a pilot plant are in agreement with these results [9]. Previous Mossbauer spectra of the FeCrMoO catalyst have shown that during the catalytic process a partial reduction takes place, and the steady state composition of the catalyst is different from the initial one. In order to understand these results and to analyze the stabilizing role played by Cr, the interaction of FeMoO and FeCrMoO with MeOH vapour has been studied. Mossbauer spectra. The spectra of the initial samples (Fig. 2A and 3A) are poorly resolved quadrupole doublets with identical parameters, close to those previously reported [8,10]. The isomer shift ( 3 ) and quadrupole splitting (AEQ) values are typical of high-spin, octahedrally coordinated Fe(II1) ions in a slightly distorted octahedral field of Fe,(MoO,), (monoclinic P1,J. After treatment in MeOH/Ar, the products formed from both (FeMoO and FeCrMoO) catalysts had different phase compositions, i.e., they showed remarkably different behaviour while they are being reduced. The spectrum for sample FeMo0/320/40 (Fig. 2D) corresponds to overlapping of four doublets: one due to initial Fe2(Mo0& (3=0.44,AEQ=0.19 mm/s), two (3=1.15,AEQ=0.93 and 3=1.13, AEQ=2.54) due to P-FeMoO,, and one (d=l.02, AEQ=1.75) due to aFeMoO,. After reduction at higher temperatures (sample FeMo0/350/60, Fig. 2E), only P-FeMo0, (the thermally stable phase) is detected. During reduction of catalyst FeCrMoO (Fig. 3) formation of an intermediate, distorted phase of Fe(II1) molybdate having anionic vacances, is observed, with final for100

Y

4%

4%

190

mation of a-FeMoO,; phase P-FeMo04 was not detected in any case. The relative percentages of every phase have been plotted in Fig. 4 vs. the reduc-

li

91

..,....!...,... , ! .

-3-2--1

0 f

V E LO C ITY

~

1

(

s

.

4

[ rnrn/ s

1

s I

Fig. 2.-Mossbauer spectra of FeMoO catalysts (A) fresh (B) FeMo0/320/20 (C) FeMo0/320/30 (D) FeMo0/320/40 (E) FeM 00/350/60.

Fig. 3.-Mossbauer spectra of FeCrMoO catalysts (A) fresh and FeCrMo0/320/20 (B) FeCrMo0/320/30 (C) FeCrMo0/320/40 (D) FeCrMo0/350/60 (E) FeCrMo0/350/150.

tion time. ESR spectra. In all cases, a single Lorentz-shaped line was recorded, due to antiferromagnetic interactions between Fe(II1) and Cr(II1)ions in the corresponding molybdates [ll].The g-factor values of the samples were 2.019 (FeMoO), 1.983 (CrMoO)

791

and 2.001 (FeCrMoO), the peak-to-peak linewidths (AHpp) 3 increasing as the temperature was decreased (Fig. 5). This [ broadening is also observed for MeOH-reduced FeMoO 40 ._ c and specially for FeCrMoO, 20 but the linewidth-temperature L dependence curves show the same shapes. However, no 0 40 80 120 160 reduction tirnehin change is observed for the MeFig. 4.-Reduction product as a function of reduction tiOH-reduced CrMoO sample. me for FeMoO (filled symbols) and FeCrMoO (empty This Same temperature-depensymbols). dence is also observed for sample FeCrMo0/350/60 reoxidized at 400 and 540°C, Fig. 5, indicating that the reduction and reoxidation processes are reversible. In addition, the decrease in the intensity of the ESR signal is more pronounced for Fe1100 MOO; taking a value of 1.0 for the in900 tensity of the signal a, for the unreduced samples, relative in- Q 700 I tensities were 0,1, U 500 0.8 and 0.4 for samples FeMoO, 300 CrMoO and FeCr100 200 300 400 MOO reduced at TIK 32OOC for 40 min, Fig. 5.-Change in the peak-to-peak linewidth in the ESR spectra of and 0, 0.3 and 0.05 the samples with the recording temperature: (a) CrMoO and CrMo0/320/40; (b) FeMoO and FeCrMoO; (c) FeMo0/320/40; when reduced 60 (d) FeCrMo0/320/40; (e) FeCrMo0/350/60; (f) FeCrMo0/350/60 min at 350°C. reoxidized 540/120; (g) FeCrMo0/350/60reoxidized 400/120 X-rav diffraction. The XRD patterns for fresh and reduced sample CrMoO are coincident, with maxima corresponding to Cr2(Mo0& and Moo3 [12,13]. Peaks due to Fe2(MoO4I3and Moo3 are identified for fresh FeMoO catalyst. For catalyst FeMo0/320/40 the presence of a strongly distorted octahedral structure of 100

I

I

2

792

Fe2(Mo04),together with a-FeMoO, and P-FeMoO,, can be observed, this last one being the only phase in catalyst FeMo0/350/60. The presence of a separate phase of Moo3 can be hardly concluded, because of overlapping of its diffraction maxima with those of Fe,(MoO,),. Identification of separate phases of Fe2(Mo04)3and Cr2(Mo04)3in fresh catalyst FeCrMoO was not possible. Only the existence of an ortho-rhombic phase with the Fe2(Mo04), structure, in addition to Moo3, can be stated. Reduction of sample FeCrMo0/350/60 leads only to a small distortion of the Fe2(Mo04)3structure, together with formation of a-FeMoO,. Reduction under milder conditions (FeCrMo0/320/40) leads also to formation of a-FeMoO,, but distortion in Fe2(MoO4I3is stronger; in the sensitivity limit, P-FeMo0, was not identified in any case.

4. DISCUSSION

From the data obtained, the conclusion emerges that explicit determination of the phase composition in the fresh FeCrMoO catalyst is unattainable; our results cannot distinguish between a mixture of Fe(II1) and Cr(II1) molybdates, or Fe2-xCrx(Mo04)3 mixed (solid) solutions. However, the study carried out under reducing conditions provides useful information on the role of Cr and the stability of partially reduced FeMOO,CrMoO and FeCrMoO catalysts. Stabilitv of the catalysts under reducing conditions. The three series of catalysts show different stabilities towards reduction with MeOH. Data indicate that the most stable catalyst is CrMoO. FeMoO and FeCrMoO are, however, reduced, but with different rates:. the reduction rate is higher for FeMoO than for FeCrMoO, specially at the first stages of reduction (Fig. 4); decomposition of the initial Fe,(MoO& proceeds much more slowly in FeCrMoO than in FeMoO, as shown by Mossbauer data (Fig. 2 and 3 ) . The first conclusion that can be drawn is that the presence of Cr2(Mo04)3in catalyst FeCrMoO retards its reduction. Nature of the reduction products. Reduction of FeMoO leads to formation of intermediate a- and P-FeMoO,, the final product being P-FeMoO,, while under the same conditions FeCrMoO is reduced only to a-FeMoO,. Some authors [14, 151 have also indicated that Fe2(Mo04)3is reduced to P-FeMoO,, formation of a-FeMoO, being also possible depending on the experimental conditions. So, the presence of Cr(II1) in the mixed catalyst favours the reduction of Fe2(Mo04), to a-FeMo0, under conditions that lead mainly to P-FeMoO, upon reduction of FeMoO. The preferential accumulation of an a-FeMo0, phase, whose structure would

793

considerably differ from the initial one, means that the structure of Fe2(Mo04), in reduced FeCrMoO is significantly deformed. Angelov [111 has shown that changes in the stoichiometry of Fe2(Mo04), leads to an alteration in the relative positions of the [Fe06] and [Moo4]polyhedra, i.e., changes in the Fe-0-Mo angle and hence, in the exchange interactions between the nearest paramagnetic Fe(II1) ions, thus accounting for the signal broadening. The strong broadening of the ESR signal for samples FeMoO and FeCrMoO indicates the change in stoichiometry of Fe2(MoO4),during reduction with MeOH. As expected, the nonstoichiometry of Fe2M03012-xincreases as the reaction time and temperature does, but, before its complete transformation into Fe(I1) molybdate, such stoichiometry changes are reversible (reoxidation of Fe2Mo3OI2-,is achieved at 54OoC,Fig. 5). These changes in stoichiometry in reduced FeMoO and FeCrMoO catalysts would account for the increase of the isomer shift of the Fe2(Mo0& doublet in both catalysts. Finally, it should be noted that the reduction of FeCrMoO leads to accumulation of a phase (a-FeMo04) which structure considerably differs from that of the initial one.

5. CONCLUSIONS

1. Incorporation of Cr into FeMoO catalysts favours an enhanced stability, the acti-

vity and selectivity towards methanol oxidation remaining practically unchanged. 2. The presence of Cr2(Mo04), in the FeCrMoO catalyst retards its reduction by methanol vapour. 3. Under the experimental conditions used, the main intermediate reduction product of the FeCrMoO catalyst is a-FeMoO,, while under identical conditions reduction of FeMoO leads to a-FeMoO, and P-FeMo0, phases. 4. Reduction and reoxidation of FeCrMoO are reversible processes. Strong changes in the stoichiometry of the initial Fe2(Mo04), phase is followed by its reduction to FeMoO, in FeCrMoO, favouring preferential formation of a-FeMo04 as intermediate reduction product. 6. ACKNOWLEDGEMENTS

DK acknowledges a sabbatical grant from MEC, Spain (SAB92-0302).

794

REFERENCES 1. Catalyst Handbook, 2nd Edition, Ed. by M. V. Twigg, Wolfe Pub. Ltd., London, 1989, p. 501.

2. M. Carbucicchio and F. Trifiro, J. Catal., 62 (1980) 13. 3. P. Courty, H. Ajot and B. Delmon, C. R. Acad. Sci. Paris (C), (1973) 1147. 4. G. M. Bliznakov, D. Klissurski, K. Cheshkova and V. Savova, Chim. Ind. (Sofia), 54 (1979) 157. 5. D. Klissurski, V. Rives, Y. Pesheva, I. Mitov and N. Abadzhieva, Catal. Lett., 18 (1993) 265. M. del Arco, C. Martin, V. Rives, A. M. Estevez, M. C. Marquez and A. F. 6. Tena, J. Materials Sci., 24 (1989) 3750. 7. M. del Arco, C. Martin, V. Rives, A. M. Estevez, M. C. Marquez and A. F. Tena, Materials Chem. Phys., 23 (1989) 517. 8. D. Klissurski, I. Mitov, K. Cheshkova, S. Angelov and K. Petrov, Kinet. Katal., 30 (1989) 403. 9. Bulgarian Patent no. 45524. 10. R. R. Zakhirov, G. M. Bartenev and A. D. Tsyganov, Zhur. Struc. Kihm., 16 (1975) 610. 11. S. Angelov, EMARDIS, Pravatz-Bulgaria, July 7-10, 1989, Ed. N. Yordanov, World Scientific, London (1989), p. 359. 12. ASTM, file 5-0508. 13. L. M. Plyasova and L. M. Keffeli, Neorg. Mater. (Russ.), 3 (1967) 906. 14. M. Carbucicchio and F. Trifiro, J. Catal., 45 (1976) 77. 15. N. Burriesci, A. Genaro and M . Petrera, React. Kinet. Catal. Lett., 15 (1980) 171.

V. CortCs Corberrin and S. Vic Bellon (Editors), New Developments in Selective Oxidation I1 0 1994 Elsevier Science B.V. All rights reserved.

795

CREATION OF NEW SELECTIVE SITES BY SPILL-OVER OXYGEN VIA a-Sb204 IN THE OXIDATION OF ETHANOL R. Castillo1, P.A. Awasarkar2, Ch. Papadopoulou3, D. Acosta4 and P. Ruiz Unit6 de Catalyse et Chimie des Mat6riaux Divisks, Place Croix du Sud 2/17, 1348 Louvain-laNeuve (Belgium), 1 Monomeros Colombo Venezolanos s.a.. Barranquilla, Colombia, 2 National Chemical Laboratory, Pune, India 3 Chemical Department, University of Patras, Greece Instituto de Fisica, UNAM,Mexico. Two system were studied. The oxidative dehydrogenation of ethanol to acetaldehyde over Fe2(Mo04)3 + a-SbO4 catalysts and the oxidation of ethanol to acetic acid over SnO;! + Moo3 + a-Sb204 catalysts. Catalysts were formed by mechanical mixtures. In catalysts containing C X - S ~anOimportant ~ increase in the selectivity was observed. To explain the synergy it is proposed that a-SkO4 produces spill-over oxygen which creates (or maintains) selective sites on Fe2(Mo04)3 and on Sn02 + M0O.j. Two mechanisms could explain the creation of selective sites by spill-over oxygen : i) maintaining the surface of Fe2(MoO4)3 in a high oxidation state, avoiding the segregation of reduced phases and ii) promoting the cristallizationof Sn@ + M a mixtures. avoiding reduction and degradation of M a

1. INTRODUCTION. Some oxides, called donors, are able to dissociate molecular oxygen to form mobile oxygen species that migrate to the surface of other oxides, called acceptors, where they react to create and/or regenerate the selective catalytic sites, thus explaining the synergetic effect observed when two separate phases are mixed together. This is the remote control mechanism (1).

During the oxidation of isobutene. spill-over oxygen produced by a-SbO4 controls the selective catalytic sites on Sn02, Moo3 and Fe2(Mo04)3, principally inhibiting their transformation to reduced non selective sites, and/or burning out either the precursors of coke or the coke itself (2,3,4). In this work, we present two examples in which spill-over oxygen coming from aSb204 creates (and/or regenerates) new catalytic sites on two different oxide catalysts, improving significantly the selectivity of the desired product. Examples concern two other reactions: (i) the oxidative dehydrogenation of ethanol to acetaldehyde (ACET) and (i) the oxidation of ethanol to acetic acid (HAC). Catalysts were mechanical mixtures of the oxides prepared separately. This method allows to minimize the mutual contamination of the components forming the oxides and facilitates the interpretation of the results. Two different catalysts were used: (i) for ACET, pure Fe2(Mo04)3 and a-SkO4 and a biphasic catalyst formed by a mechanical mixture of both oxides, and (ii) for HAC a mechanical mixture (Sn+Mo) composed of Sn02 and M o a , and a mixture of Sn+Mo with a-SkO4. Catalysts with high and low SnO;! and Moo3 surface area were used.

796

2. EXPERIMENTAL METHODS

2.1 Catalyst preparation. Pure grade reagents were used in all cases. Pure oxides i) Fe2(Mo04)3: 45.9 g of Fe(N03).9H20 and 40 g of citric acid, C&lgO7.H2O, were dissolved in 400 ml distilled water, then 30 g of (NH4)6Mo7024.4H20 were added to the solution. After evaporation of the solvent in a rotating vessel at about 50°C under reduced pressure, the solid obtained was dried at 100°C overnight in a vacuum oven, and then calcined at 500°C for 24 h. ii) Pure a-SbO4 was prepared by calcination of SbO3 in air at 500°C for 20h. iii) SnO2 (low surface area): 22.5 g of stannous chloride were dissolved in 50 ml of water. A few drops of diluted HC1 were added to the solution, then the solution was neutralized with diluted ammonia (1: 1) and filtered. The solid was dried at 110°C for 16h and calcined at 6000C for 8h. iv) SnO;! (high surface area): a 0.1 molar tin (IV)chloride solution was prepared using distilled water. Hydrochloric acid was added to get about a 1M concentration of that compound in the solution. The solution was filtered. Urea (NH2CONH2) was added in order to get a 0.15 M solution. The solution was stirred and transfered into plastic bottles tightly closed. These bottles were put in water bath at 80°C for 6 days, filtered and washed with distilled water, then dried at 1lOOC for 8h and calcined at 500°C for 8 h. v) Moo3 (low surface area): was obtained by thermal decomposition of (NH4)&0-&4.4H20 in air at 500°C for 20h. vi) Moo3 (high surface area): was obtained by dissolving in distilled water 12g of (NH&jM07024.4H20 and 22.6 g of oxalic acid, under stimng at 40°C. The solution was evaporated under vacuum, then dried at 80°C for 20h and calcined at 300°C for 20h and at 500°C for 16h. Mechanical mixtures. The mechanical mixtures were prepared by dispersing two oxide powders in different proportions in n-pentane under agitation and ultrasound for 10 minutes. This operation was followed by evaporation of the solvent at room temperature under agitation and reduced pressure and finally drymg overnight at 80°C. The composition of mechanical mixtures was defined by their mass ratio: Rm= weight of oxide A/(weight of oxide A+ weight of oxide B). Three types of mechanical mixtures were prepared i) Sb+Fe, that is, a-SkO4 (B) and Fe2(Mo04)3 (A) with Rm = 0.0,0.25,0.5, 0,75 and 1.0 ii) Sn+Mo, that is, Sn@ (B) and Moo3 (A) with Rm = 0.4 iii) Sn+Mo+Sb, that is, a mixture of Sn+Mo (B) and a-Sb204 (A) with Rm = 0.2 2.2. Catalytic activity measurements.

The oxidative dehydration of ethanol to acetaldehyde (ACET) and the oxidation of ethanol to acetic acid (HAC) were canied out in a conventional futed bed microreactor at atmospheric pressure. The reactor was a Pyrex tube with an internal diameter of 8 mm. A smaller tube of 4 mm diameter introduced inside the reactor contained a thermocouple in order to measure the temperature of the catalytic bed. The catalyst was screened and the fraction between 500-800 micrometers was used. The feed consisted of a mixture of N i 0 2 saturated with ethanol at 25°C. The catalytic reaction conditions were: i) ACET: atmospheric total pressure; partial pressure of ethanol, 66 mm Hg; oxygen/ethanol molar ratio: 0.25. 0.5. 1.0. and 1.5; total feed 50 ml/min. The amount of catalyst used was calculated so that the amount of Fe2(MoO4)3 either when used pure or in the

797

mixtures be always 200 mg. In the control reaction, 200 mg of a-Sb2O4 were used. The temperature of reaction was 350°C and the duration of a catalytic experiment 6h. ii) HAC : atmospheric total pressure; partial pressure of ethanoI, 47 mm of Hg; oxygen/ethanol molar ratio, 3.0; total feed 3ml/min. The overall weight of catalyst was always 3g for the Sn+Mo mixture, and 3.6 g for the Sn+Mo+Sb mixture. The temperature of reaction was between 200 and 300°C. The furnace was heated slowly up to 170°C in about one h. At this temperature a 100% conversion of ethanol was observed. The principal products of the reaction were ethylene and acetaldehyde, and no acetic acid was observed. After reaching this temperature, the reactor was further heated at a rate of SoC./min. Ethylene and acetaldehyde production decreased and acetic acid production increased. At 200°C and afterwards for every 10°C rise of temperature till u)o”C, products were analysed by gas chromatography. The yields (Y) are expressed as the number of moles of acetaldehyde or acetic acid produced per moles of ethanol in the feed, and the conversion (C) as the total fraction of ethanol transformed. The selectivity ( S ) of the reaction was calculated as Y/C. 2.3. Physico-chemical characterization The BET surfaces measurements were canied out in a Micromeritics Asap 2000 instrument at 77K, using krypton as adsorption gas. X-ray diffraction analyses were made with a high-resolution X-ray diffractometer Siemens D-500 or on Rigaku D MAX/III VC diffractometer, both using a Ni-filtered CuKa radiation (1= 1S404 A). X-ray photoelectron spectroscopy analysis was obtained with a Vacuum Generator ESCA-3 MK I1 spectrometer operated at 14 KV and 20 mA, using MgKa radiation (1253,6 eV). The residual pressure inside the analysis chamber was below 10-8 torr. All binding energies were calculated using the Cls line of carbon as reference (284,8 eV). The samples containing iron molybdate and antimony oxide were studied with a JEOL Temscan lOOCX microscope, equipped with a Kevex energy dispersive spectrometer for X-ray microandysis. The samples were dispersed in water using an ultra-sonic vibrator and deposited on carbon films supported on copper grids. They were examined by means of E M . SEM and electron microanalysis (AEM). Tin, molybdenum and antimony samples were studied in a JEOL l00CX electron microscope equipped with a Kevex apparatus (CTEM and AEM). SEM images were obtained in a JEOL SEM 5200 electron microscope.

3. RESULTS 3.1. Catalytic tests 3.1.1 ACET: Results as function of the mass ratio are presented in Table 1. u-SkO4 is inert in this reaction. Fe2(Mo04) is very active. Conversion is practically unchanged when aSk O4 is admixed. Selectivities of mechanical mixtures are higher than those observed in pure Fe2(Mo04)3. A selectivity maximum is observed for a mass ratio Rm of 0.5. A synergetic effect on the yield is also observed. Yields are also high for mechanical mixtures. A maximum is observed also at Rm = 0.5. Figure 1 shows similar effects on selectivity for different oxygen/ethanol molar ratios. Similar curves were obtained for the yield The highest selectivity is observed for the higher surface area sample at temperatures of about 240°C.

798

Table 1. Oxidation of ethanol to acetaldehyde. Conversion, yield and selectivity of Fe2(Moo4)3 and their mechanical mixtures with antimony oxide. Temperature: 350°C; oxygedethanol molar ratio = 0.5. MeChanical Conversion (%) Yield (%) Selectivity (%) tIliXtlUeS

a-SbO4 Rm = 0.25 Rm = 0.50 Rm = 0.75 Fe2(Mo04)3

inert 82.3 87.0 83.0 87.0

inm 70 80 68 68

inert 85 92 82 78

3.1.2. HAC: The conversion is 100%.The results concerning selectivity are presented in Table 2. The selectivity in acetic acid is higher with the mechanical mixture of Sn@+MoO.j having the higher surface area. When the mixtures are mixed with a-St~O4.the selectivity increases significantly.

z

80

k

->*

60

c'

40

U

0

W J -

W v)

20 0

0.25

0.75

1.25

OXYGEN (molar rotio) ETHANOL

Fig. 1. Oxidation of ethanol to acetaldehyde. Selectivity as function of oxygedethanol molar ratio. Temperature: 350°C.m: R m = 1.0; D: F b = 0.75; 1: Rm = 0.5; n: Rm = 0.25.

799

TABLE 2. Oxidation of ethanol to acetic acid. Selectivity to acetic acid as a function of temperature for S n a + MOO3 and S n a + M o Q + a-Sb2O.4 mixtures. Surface area of fresh and used (in parentheses) samples are also indicated.

280

118.7

133.7

123.9

128.5

300

115.9

17.4

111.0

113.9

3.2.- Characterization of samples. 32.1 Iron mlybdate and a-Sb204 system The specific surface area of pure Fe2(Mo04)3 and their mechanical mixtures with aSbO4 (about l a g ) increases significantly during catalytic test for all samples (between 3 to 7 times). XRD analyses show that in pure iron molybdate and in mechanical mixtures, iron was reduced to Fe+2 forming FeMoO4 during the test. X P S analyses for pure Fe2(Mo04)3 and the mechanical mixture with Rmd.5 show that for fresh samples binding energies of F e 2 p 3 ~and M o 3 d g ~are 71 1.9 and 232.8 eV, which correspond to Fe+3 and Mof6 in Fe2(Mo04)3. After test the binding energy of Fe2p312 decreases in all samples (to 711.0 eV), the binding energy of Mo-jdgn remaining unchanged. In all mixtures the Sb3d3n binding energy remains unchanged. For pure iron moIybdate and its mechanical mixtures the X P S Mo/(Mo+Fe) atomic ratio decreases slightly after tests (from about 0.8 to about 0.7). In used mechanical mixture, the X P S Sb/(Mo+Fe) atomic ratio decreases significantly (for b 4 . 5 , from 0.83 to 0.3) Electron Microscopy analyses indicate that after the test with pure Fe2(Mo04)3. small particles are observed as they disintegrated from Fe2(Mo04)3. In the presence of sb@4, there are less of these small particles. No indication of contamination between the particles was observed by AEM. 3.2.2 Tin-Molybdenum-Antimony system. The BET surface area measurements (Table 2) show that the surface area decreases slightly after catalytic test The XRD analysis of fresh and used mechanical mixtures show the spectra characteristics of the pure oxides. No indication of formation of SnSb04 or any other new phase was observed. The relative intensity of the peaks are the same as for pure oxides. No shift in the peak position was observed. For the pure high surface area SR+MOsamples, the

800

intensity of the peaks of Sn@ and Mo03 increases after test, the increase being more important for the latter. When the sample is mixed with a-Sb204, the increase in the peak intensity is significantly higher. Electron microscopy analyses show that, when SnO2+MoOg mixtures work alone, some crystallites of Moo3 become thin, as can be seen in micrographs 1 obtained in CTEM mode. The loss in the thickness of Mo crystallites become evident from changes in the contrast with respect to pure samples. SEM analysis, show for the fresh Sn+Mo mixture, the plaquette ofM00-j supporting homogeneously the small particles of Sn@. After the test the observation of plaquettes of Mo03 is difficult. They are less numerous. The small particles of S n a seem to form aggregates. In used mixtures with u-Sb204, the loss in the thickness of molybdenum crystallites is lower and the observation of plaquettes of Moo3 is less difficult. They are more numerous compared to those observed in used pure Sn+Mo mixtures.

Micrograph 1. CTEM micrographs of used SnOz + Moo3 mechanical mixtures (high surface area). B: M a ; B1: thin MoO-j; A: SnO2 (supported). 2 cm: 1356 A.

Analytical microscopy shows that the Sn+Mo mixture is enriched in molybdenum after test. The atomic ratio, (Mol Sb+Mo), increases from 33.3% for the fresh sample to 50.1%, and the tin atomic ratio decreases from 66.5 to 49.2%. In mixtures with u-Sb204, the same phenomenon occurs, the molybdenum atomic ratio, (Mo/Sn+Mo+S b), increases from 30.8 to 46.2. Simultaneously the tin atomic ratio decreases from 56.4 to 47.9% and the antimony atomic ratio from 12.8 to

5.9%.

No new phase was observed by selected area electro diffraction. 4. DISCUSSION

4.1.-Evolution of the catalysts during catalytic test. 4 .I .I.-Iron-molybdate-antimony system. Previous results are necessary to understand the evolution of the Fe2(Mo04)3 and their mixtures with a-SkO4 during the catalytic reaction. Both samples were submitted to a long catalytic test (30h) under the same experimental conditions (4). Characterization results of these samples, by XRD, XPS, CTEM, High resolution electron microscopy and AEM, indicated that, when iron molybdate works alone, FeMo04 and MOO:! were formed. In mechanical

80 1

mixtures the reduction is lower. No MoO2 was observed. No indication of contamination between phases was observed and in particular, antimony oxide is not modified during the. reaction. The increase in the surface area and the enrichement in molybdenum of the samples was explained by the segregation of these new reduced phases. These results and those obtained during the characterization of samples used in the present work (SBET, XRD,X P S , AEM), allow as to suggest that in the present work, in which less reacted samples are concerned, the same phenomena occur during the test and that the antimony oxide in mechanical mixtures acts as LI separate phase. 4.1 2 Tin-Molybderuun-Antimonysystem. In presence of antimony oxide, a decrease in the loss of thickness (degradation) of M0o-j particles and an increase in the cristallizationphenomena were observed. In spite of an enrichement of molybdenum concentration in the sample no formation of new phases was detected. Previous studies (2,3,4,5),in which very sensitive surface techniques were used, show in a conclusive way that when SnO2 or Moo3 is mixed mechanically with a-Sb204 or impregnated with Sb ions (or vice-versa), there is no indication of formation of a new phase and that the impregnation ions detache from the surface of support during catalytic reaction. No mutual surface contamination takes place. However, we cannot exclude that between Sn@ and MoO3, some type of contamination could arise. These results allow us to suggest that the most probable picture of Sn+Mo+Sb mechanical mixtures used in the present study can be considered as composed of S n a , Moo3 and probably an additional contaminated MoISn02 phase in close contact with non contaminated a-SbO4. Our suggestion is that, in this system, a-SbO4 is probably also present as a separate phase.

4.2.4nterpretation of the catalytic results. The addition of a-Sb204 to Fe2(MoOq) and SnO2 + Moo3 mechanical mixtures modifies the catalytic selecrivity of samples. In all cases the conversion of the mixtures remains nearly unchanged. Although it cannot be completely excluded that other phenomena could play a role in the explanation of these results, we suggest as a plausible explanation a cooperation between the phases forming the catalyst and, more precisely, it is proposed that the increase in the selectivity is explained by an increase in the number of selective sites on Fe(MoO43 and Sn@+Mo@ thanks to spill-over formed on a-SbO4. The structut of iron molybdate is very unstable (namely reduction to FeMooli + MoOx when subjected to strong reaction conditions such as a low oxygen/ ethanol molar ratio for a long time) (4). However, in the presence of antimony oxide, only iron is partially reduced. These results show that pure iron molybdate is not able to reoxidise itself during the catalytic test. During catalysis the reduced sites must be oxidized immediately, otherwise a reduced surface would be formed and selectivity would decrease. Lattice oxygen coming from iron molybdate cannot compensate for a loss of oxygen on the surface. Our suggestion is that the increase in the acetaldehyde yield is due to the presence of spillover oxygen emitted by a-ShO4. which helps prevent the reduction of Fe2(Mo04)3 and consequently maintains the selectivity at a high value. In other words, a - S b 0 4 creates or maintain selective sites on iron molybdate during catalysis. This mechanism explains satifactory the results presented in Figures 1. When the oxygen/ethanol molar ratio increases, more selective sites are created in the mixtures and selectivity increases. The effect is higher with a higher concentration of a-SbO4. The decrease in the selectivity when the oxygen/ethanol molar ratio is too high, that is when the partial pressure of oxygen is too high, is explained by a higher formation of electrophylic species on iron molybdate (probably as 0). This mechanism can also explain the influence of the concentration of a-SbO4 on the yield and selectivity at constant partial pressure of oxygen presented in Table 1. The maximum

802

yield is observed at the composition at which a good balance between the number of active sites present in Fez(M004)3 and the flux of oxygen spill-over, which is able to irrigate the actives sites, takes place. This is observed at Rm4.5. When the concentration of a-SkO4 decreases, the flux of oxygen spill-over decreases and consequentlyyield and selectivity decrease. When a-Sb204 increases, the number of active sites to be irrigated decrease and consequently the yield decreases. On the contrary, selectivity increases when the amount of a-SkO4 in the mixture increases because more spillover is able to create selective sites. The slight decrease in selectivity observed when the concentration of a-SkO4 is too high could be explained by an overoxidation of the surface of iron molybdate due to the high formation of oxygen spill-over, as in the case of high partial pressure of oxygen. In conclusion it is proposed that a-SbO4 creates new active sites on Fez(Mo04)3. These new selective sites are formed (or maintained) thanks to the reaction of spill-over with their surface, maintaining the catalysts in a high oxidation state. The reasons for the synergy between SnOz and Moo3 will not be discussed in this work. We will rather concentrate our discussion on the role played by a-SbO4. The addition of a-Sb2O4 to Sn+Mo mixtures increases the selectivity to acetic acid. Since the conversion is the same in both cases, this indicates that the role of a-SbO4 is essentially to transform the non selective sites on Sn@+MoOj mixtures into selective ones, without changing the total number of catalytic sites. As in the case of Fez(Mo04)3 mixtures, results support the proposition that the creation of selective sites is due principally to oxygen spill-over. The number of selective sites increases when the surface area of SnOz and M o Q used in the mixtures increases. This is explained because in this case the number of actives sites is higher and simultaneouslythe number of contacts between a-sk04 and the oxides increase, thus the number of active sites which can be irrigated by the oxygen spill-over is higher, increasing the number of selective sites and the selectivity in acetic acid. The selectivity reaches a maximum when the temperature increases. An increase in temperature favours the total oxidation, but simultaneouslyenhances the rate of migration of oxygen spill-over. The selectivity depends on both factors. A good balance has been observed at about 250°C for lower surface area and about 2MoC for higher surface area samples. The mechanism by which selective sites are created on Sn@+MoOg mixtures remains speculative.It is proposed that it is similar to the one which operates on iron molybdate. During catalytic reaction, Moo3 and Sn@ (and probably a new contaminated MoISnOz phase) cristallize. Oxides are reduced continuously. Reduced sites should be reoxidised continuously. These oxides alone are not able to reoxidise the reduced sites (2.3). The reduction modifies the structure of the oxides, giving deeper reduced structuresdegrading or destroying the cristallites and avoiding cristallization. In presence of a-SkO4, reduction is inhibited, no degradation is observed, active sites are reoxidised, crystallization is improved and selectivity is higher. REFERENCES

1.-L.T. Weng, P. Ruiz and B. Delmon, Studies in Surface Science and Catalysis "New Developments in Selective Oxidation by Heterogeneous Catalysis" (P. Ruiz and B. Delmon, eds.). Elsevier, vol. 72, pp. 399-413,1992 2.-L.T. Weng, N. Spitaels, B. Yasse, J. Ladrikre, P. Ruiz and B. Delmon., J. Cata1.,132, 319 -1991 3.-B: Zhou, E. Sham, T. Machej, P. Bertrand, P. Ruiz and B. Delmon, J. Catal, 132.157.199 1 4.-Y.L. Xiong. R. Castillo, L. Daza, P. Ruiz and B. De1mon.Vth International Symposium on Catalyst Deactivation, June 24-26,1991, Evanston (IL), U.S.A. Studies in Surface Science and Cata1ysis"CatalystDeactivation 1991". (C.H. Bartolomew and J.B. Butt, eds.), 68, 425, 1991. 5.- L.T. Weng, B. Yasse, J. Ladriere, P. Ruiz, B. Delmon, J.Cata1, 132, 343, 1991 ,~

~

V. CortCs Corberin and S. Vic Bclldn (Editors), New Deveiopments in Selecrive Oxidation II 0 1994 Elscvicr Science B.V. All rights reserved.

803

Effect of titania on the properties of alumina supported molybdena catalysts F. Requejo', N. Quarantab, J.M. Coronado", J. Soria' and H. Thomasd "Dpto. Fisica, Fac. Ciencias Exactas, UNLP. Argentina bUniversidad Tecnol6gica Nacional, Regional San Nicolas, San Nicolas. Argentina' 'Institute de Camisis y Petroleoquimica, Cantoblanco, Madrid, Espaiia. "CINDECA, UNLP, Argentina

Abstract Different characterization techniques and a catalytic reaction, oxidation of ethanol, have been used to determine the properties of molybdenum oxide supported on alumina containing titanium oxide. The characterization techniques indicate that titanium ions are very well dispersed and strongly interacting with alumina. On this support, the reduction of molybdenum oxide starts at lower temperature than when supported on pure alumina and, at low reduction temperature, a certain stabilization of Mo" ions against deeper reduction is favored. This higher stabilization of Mo5+ could explain the higher selectivity to acetaldehyde observed when titanium is present in the sample. 1. INTRODUCTION The modification of the catalytic properties of metal oxides by supporting them on different oxides is a well documented effect (1,2). In a previous study on the modification of the molybdenum oxide properties as a result of the interactions with different supports, we observed significant differences in the effects produced on MOO, by A1,0, and Ti0,(3). In both cases, molybdenum dispersion was high, but it showed different reducibility due to different type of interactions with the supports. In order to know if the different interactions were going to determine the location of molybdenum when A1,0, and TiO, were present in the same catalyst, in the present work we have studied the properties of MOO, on a support of alumina containing titanium ions. To determine the characteristics and distribution of titanium and molybdenum in the different samples, we have used x-ray diffraction, temperature programmed reduction, infrared spectroscopy, scanning electron microscopy, x-ray photoelectron spectroscopy and electron spin resonance. Selective ethanol oxidation has been used to determine how the modifications introduced in the molybdenum oxide by the presence of titanium affected its catalytic properties.

I

Investigador CICPBA.

804

2. EXPERIMENTAL The starting material Al,O, (Condea, S,,, = 214 m2.g-') was added to a solution of titanium isopropoxide in hexane at 295 K (6 ml of solution per g of alumina). Acetylacetone, in a molar ratio 1:l to Ti(i-OPr)4, was also added to the solution. After stiring for l h under N2, the mixture was heated up to 673 K in flowing N2. Its subsequent calcination at 873 K in air for 3h produced a sample I with a titanium content of 12% Ti02 w/w. Molybdena-supported catalysts were prepared using the excess water impregnation method from ammonium heptamolybdate (AHM) solutions. The solutions contained the required amounts of molybdate to yield catalysts with 8% MOO, w/w (equivalent to about half a monolayer) on each support, Al,O, (sample 11) and Ti/A120, (sample 111),and 16% MOO, w/w on Ti/AI,O, (sample IV). The AHM solutions were maintained at pH=6 by addition of NHJOH). The excess water was removed using a rotary evaporator, while heating at 373 K. The moist materials were left overnight (15h) at ambient temperature, and then dried at 383 K for 8h. Finally, the samples were air-calcined in two steps: 523 K for 2h and, then, 773 K for 4h. The specific surface areas were measured with an Accusorb Micromeritics equipment. The XRD patterns were recorded using a Philips PW 1390 diffractometer using nickel-filtered copper K a radiation. A FT-IR Bruker IF5 66 spectrometer was used to obtain the IR spectra of the samples diluted with KBr. For the SEM study a Philips 505 electron microscope was used, equipped with an EDAX 9100 accesory for microanalysis studies. XP spectra were acquired with a Shimadzu ESCA 750 spectrometer with a magnesium anode (MgKz = 1253.6 eV) operating at 8kV and 30 mA. All binding energies (BE) were referenced to the C 1s line at 284.6 eV and A1 2p line at 74.3 eV. The ESR spectra were obtained at 77 and 295 K with a Bruker ER200D spectrometer operating in the X-band. A DPPH (g=2.0036) was used for calibration. The powdered samples were placed within a vacuum quartz cell assembled with greaseless stopcocks capable of maintaining a dynamic vacuum better than 3 x 10-2Nm-2. After outgassing at 295 K, H, (50 Torr) was introduced in the cell and then heated at different temperatures in the 373-773 K range. For the TPR study a mixture of 10% H, in N2 was used, with a flow of 25 cm3/min at 760 Torr. Table 1. Characteristics of the samples Sample

A1203

I I1 I11 IV

Composition

SET (m2.g-')

214 12%TiO,/Al,O, 192 8 % MoO,/Al,O, 200 8%Mo0,/ 12%TiO,/AI,O, 178 16%Mo03/12%TiO,/Al,O, 164 2 %MoO,/TiO,

ESR parameters M O+~

'41203

= g~ =

1.945 gil = 1.866 1.952 g l , = 1.910

g~ = g~ =

1.978 g l l = 1.917 1.956 g l l = 1.900 1.946 g l l = 1.875

g~ =

The samples (50 mg) were first outgassed at room temperature and 373 K. The heating rate up to 1273 K was 5K/min. The catalytic activity tests were carried out in a fixed bed, flow reactor, at normal pressure and at temperatures in the 433-553 K range. A mixture of 5 % mole ethanol (Merck 99.8 ~ 0 1 %and ) chromatographic air (99.99 ~ 0 1 %was ) used as reactant. The spacial velocity was F/W = 1.04 pmol of ethanol/g,,.s Catalyst samples (500 mg) were diluted with l m m 0 diameter quartz particles to obtain a bed volume of 5 ml. The reactants and products were analyzed on line by gas chromatography, with a Shimadzu GC-94. Two columms, molecular sieve 5A for CO and 0, analysis and Porapak Q for the other compounds, were used. Mass balance calculated on C atom basis, were between 100 f 5 % . The conversion and selectivity to products was expressed as the % mole ratio of ethanol transformed to ethanol introduced, and of ethanol transformed to each product to total transformed ethanol, respectively.

3. RESULTS AND DISCUSSION Surface area and XRD The values obtained for specific surface area, S,,,,

of supports and catalysts are presented

in Table 1. They indicate that impregnation of Al,O, with titanium oxide produces a decrease of 10% in its surface area. The incorporation of 8% MOO, to this binary support decreases the SBET value by 7%, while a decrease of 14% is obtained for the sample with 16% MOO,. The XRD patterns showed only broad lines that were assigned to y-alumina; none XRD line could be unequivocally assigned to Ti02or mobybdenum oxides. Considering that crystals with sizes higher than 4nm are detected by this technique, it can be concluded that titanium and molybdenum oxides are well dispersed in these samples or are forming amorphous oxides. TPR Temperature programmed reduction profiles of alumina and sample I were very similar, without any defined reduction peak up to 1273 K, figure la and Ib. This result suggests that TiO, aggregates have not been formed on the alumina surface, after the calcination treatment of sample I, or they are well dispersed and stabilized by interactions with alumina, which is in agreement with the absence of any TiQ line in the XRD pattern of this sample. For sample I1 the TPR profile shows two peaks at 810 and 1138 K, figure lc. Unsupported MOO, also presents two reduction peaks, at 1002 and 1139 K, attributed to reduction of MOO, to MOO, and MOO, to metallic molybdenum, respectively (4). The detection of the two peaks, at temperatures not too different from those of MOO,, for sample I1 evidences the formation of MOO, aggregates, but the shift to lower reduction temperature for the transformation of MOO, to Moo2 indicates that the interaction with the support favours MOO, reduction, in agreement with previous results (3). This result points to a good dispersion of the molybdenum on alumina. However, the MOO, formed after this first reduction step seems to be unaffected by the support and its reduction temperature is not modified. The profiles corresponding to samples I11 and IV also show two peaks, (722 and 1098 K) and (715 and 1110 K), evidencing that molybdenum oxide aggregates have been formed, figure Id and le. The displacement to lower temperature of the reduction MOO, -+ MOO,

806

Figure 1.TPR profiles of samples: Al,O,(a) I@) II(c) III(d) and IV(e).

Figure 2. IR spectra of samples: A1203(a)II(b) I(c) III(d) and IV(e).

indicates that these aggregates are interacting with the support, formed by alumina modified by the presence of titanium, even more strongly than when they were on pure alumina, favouring their dispersion and the reduction under hydrogen. In a lower degree, the MOO, reduction is also affected by the support interactions. The present results, showing a higher reducibility of MOO, when titanium ions are incorporated, are in line with those of Tanaka et al. ( 5 ) , which indicated that MOO, loaded on titania, with 10wt%or less, could be reduced to nearly zero-valent molybdenum with hydrogen at 773 K, in contrast to MOO, on alumina, which can be reduced to below Mo4' only under more severe conditions. Considering that the titanium-molybdenum oxide interaction favours molybdenum reduction more than aluminamolybdena interaction, the displacement toward lower temperature of the MOO, MOO, transition and, particularly, the MOO, reduction, in the TPR profiles corresponding to samples I11 and IV, indicates that MOO, is directly interacting with titanium ions. The effect of the higher molybdenum concentration in sample IV is evidenced by the larger intensity of the peak corresponding to the MOO, reduction, but not by a variation of the reduction temperature, which, considering the higher reducibility of MoO,/TiO,, should depend on the titanium content.

-

807

Table 2. B.E. values of molybdenum and titanium and surface composition ratios obkuned by XPS and chemical analysis. Sample ~

Mo 3d5l2 (eV)

Ti 2~312 (ev)

Ti/A1

Mo/AI

XPS

Chem

459.0

0.074

0.076

459.1 459.2

0.068 0.074

0.076 0.076

XPS

Chem

0.035 0.032 0.083

0.031 0.031 0.064

~~

I I1 I11 IV

232.6 232.1 232.1

Infrared spectra FT-IR spectra, in the 1000-400 cm-' range, of the different samples are presented in figure 2. A1,0, shows a very large absorption from 520 to 870 cm-' (6), figure 2a. When this support is impregnated with molybdenum the spectrum is only modified by the observation of a not very prominent maximum at 520 cm-', probably because absorption above 600 cm-' has decreased a little. figure 2b. A similar effect, but far more pronounced, is observed for sample I, figure 2c. The increase of the A1,0, transmission is an effect of the interaction of the titanium ions with the alumina support, favoured by the good titanium dispersion. A maximum at 528 cm-' and shoulders at 740 and 419 cm-' are observed, but it is not easy to determine if they are due to A1,03 or TiO,. When Mo is added to Ti/AI,O,, sample 111, weak new shoulders are observed at 826 and 953 cm-', figure 2d, which are more prominent for sample IV containing 16% MOO,, figure 2e. Therefore, these shoulders should be associated with the formation of MOO,. Similar bands have been found by Ng et al. for molybdenum oxide supported on TiO, (7). This result supports the evidences obtained by TPR in the sens that in sample I11 and IV the I MOO, aggregates are directly 50 G interacting with titanium. After using these catalysts for ethanol oxidation, the IR spectra did not show any modification, due to the treatment, in the wavenumber range studied, which indicates that the MOO, is not significatively modified during the reaction. SEM A scanning electron ' microscopy study showed that Figure 3. ESR spectra, at 77 K, of sample I11 treated samples containing titanium under static H2 at 373 K(a) 573 K(b) 673 K(c) and 773 presented particles with better K(d). defined edges and slightly larger size than the other samples,

808

however no TiO, particles were observed. EDAX microanalysis showed that the titanium concentration values along the sample surface were in the 9-16% TiO, w/w range, close to the value expected if the titanium ions were randomly distributed in the whole alumina support. These results apport further evidence to to conclude that the titanium cations on alumina are well dispersed.

XPS The binding energy values of Mo 3d5l2 and Ti 2~312of the different samples are presented in Table 2. The B.E. value corresponding to titanium (459.1 eV) indicates that, in all the samples, these cations are present as Ti4' and its oxidation state is not affected by the presence of molybdenum. However, the Mo level with a B.E. of 232.6 eV for sample 11, corresponding to Mo6+ (3), is shifted 0.5 eV toward lower energy when titanium ions have been previously incorporated to Al,03. This small shift, checked by repetitive measurements in different samples, indicates that the presence of titanium is, in some way, affecting the molybdenum characteristics. In the case of samples I11 and IV, as was also observed for MOO, on TiO, (3), the titanium-molybdenum oxide interaction seems to induce a higher electron density in the Mo 3d orbitals. The existence of this interaction is a proof of the presence of titanium in the coordination sphere of, at least, a part of the molybdenum ions. The values of the Ti/Al, Mo/Al and Ti/Mo atomic ratios calculated from XPS spectra and chemical analysis are presented in Table 2. For the three samples containing titanium (Ti/Alhm < (Ti/Al)am is observed. Considering that XPS is a surface spectroscopy and the chemical analysis indicates distribution in the whole of the sample, that result can be interpreted in the sense that the titanium cations are not only located on the Al,03 surface, but,

0

20

40

60

80

100

Conversion [m . ethanol%]

Figure 4. Ethanol conversion versus temperature for sample I1 (0)and I11 (V).

Figure 5 . Selectivity vs. conversion for sample II(-) and III( ...). Acetaldehyde (0), acetic acid (V) and CO,(O).

probably, also forming a solid solution with A1203or an amorphous aluminate. Most of these titanium ions, incorporated to the alumina bulk, will not be detected by XPS and (Ti/Alhps will be smaller than (Ti/Alhm. In the case of molybdenum ions on alumina, sample 11, (Mo/Alh,, is a little higher than (Mo/Al),,, indicating that these cations are not so well dispersed as titanium. The incorporation of low concentration of titanium ions to the support, sample 111, decreases the molybdenum content at the sample surface, as indicated by the lower value of (Mo/A1hm in relation to sample 11, perhaps through interactions with titanium ions in the inner parts of the particles. For higher Mo concentration, sample IV, the higher value of (Mo/Alk, indicates that the stabilization of molybdenum ions at the surface increases.

Considering that the TPR and XPS results indicate some molybdena-titania interactions in samples I11 and IV, a method to obtain additional evidence on the modifications introduced by these interactions is to study by ESR the reducibility of those oxides. ESR spectra of sample 111, after thermal treatments under static H,, at different temperatures T, in the 373-773 K range, are presented in figure 3. For T, = 373 K, the spectrum is mainly formed by a signal with g~ = 1.952 g,, = 1.910, figure 3a. The same spectrum, although with different intensity, is obtained at 77 and 295 K. This ESR signal broadens and increases its intensity with increasing T,, reaching an intensity maximum for T, = 573 K, figure 3b. This increase indicates the reduction of molybdenum ions at the sample surface, which is produced at lower Tr than the needed for the bulk reduction that originates the TPR peak in figure 1. For T, > 573 K the intensity of the ESR signal decreases with increasing T,, figures 3c and 3d, due to a deeper reduction of the molybdenum ions. None of the spectra show evidence of the possible formation of Ti3+ions. The ESR spectra of molybdenum on TiO,, reduced under H2, present several narrower MoS+ signals with g-values close to those observed for sample 111, Table 1, and showing an intensity maximum for Tr = 473 K, with lower intensity than in the case of sample I11 (3). Sample I1 reduced under H2 shows a Mo5+ signal broader and with lower g-values, Table 1; the maximum intensity of this signal is found for Tr = 773 K, with lower intensity than for sample I11 or Mo03/Ti02. To give an interpretation of the differences in the variation of the Mo’’ signal intensity with T,, when MOO, is supported on different oxides, we must consider that the intensity value of the MO” signal is more related to the stabilization of this particular oxidation state than to the reduction degree of the sample, because a direct reduction Mo6+ + Mo4+ is not detected by ESR. However, if the reduction takes place at a low temperature, the observation of Mo” signals will be more likely than when the reduction conditions favours the formation of Mo4+ ions. Therefore, the observation of the maximum at lower temperature and with higher intensity than in the case of Mo/A120, (3) indicates a certain stabilizacih of molybdenum as Mo5+ ions under these experimental conditions, and that the reducibility of MOO, is favoured by the addition of titanium. In the case of Mo/TiQ the maximum was observed even at lower temperature. These results indicate, again, that titanium and aluminium are affecting the reduction properties of the molybdenum ions, but in an intermediate way to that observed for molybdenum on alumina or titania, probably because the titanium ions interacting with MOO, are affected by the alumina, modifying their properties.

810

Catalytic properties Ethanol oxidation reaction on samples I1 and 111 showed similar conversion versus temperature curves for both catalysts, figure 4; however, the products distribution was different, figure 5. For sample 111, the products were mainly acetaldehyde and C02, while for sample 11, acetic acid was also obtained, decreasing the selectivity to acetaldehyde and maintaining the selectivity to C02. Other products, present at concentations lower than 5%, were ethyl acetate and ethylene; for conversions higher than 80% CO was also produced. The similar values of the conversion levels measured for both samples indicate that the number of active sites was not modified by the addition of titanium, confirming the idea that the active sites were molybdenum ions; their surface concentration in both samples is not too different as indicated by XPS. The differences in selectivity are probably related to the higher ability to stabilize MoS+ions of the catalyst containing titanium, sample 111. The stabilization of this oxidation state should difficult the oxygen incorporation to the adsorbed molecule, favouring acetaldehyde formation. The generation of Mo4' ion would favour the production of acetic acid at low conversion and C 0 2 and CO above 80% conversion. Matsuoka et al. (9) have observed a similar effect when studying methanol oxidation on Mo/Ti02 and Mo/AI,O, catalysts. They found that the selectivity to the corresponding aldehyde was higher for Mo/TiO,.

Acknowledgments The authors thank Ing. L. Cornaglia (INCAPE) for the XPS measurements and G . Volle (CINDECA) for the FTIR results. REFERENCES 1. S. Okazaki, M. Kumasaka, J. YoshidaandK. Kosaka, Ind. Eng. Chem. Prod. Res. Dev. 20, 301 (1981). 2. W.W. Swanson, B. J. Streusand and G.A. Tsigdinos, "Proc. Chemistry and Use of Molybdenum". Clymax Molybdenum Company. Ann Arbor, Michigan 1982. 3. C. Ciceres, J.L.G. Fierro, J. Lkzaro, A.L. Agudo and J. Soria, J. Catal. 94, 1477 (1990). 4. P. Amoldy, J.C.M. de Jonge and J.A. Moulijn, J. Phys. Chem. 89, 4517 (1985). 5. K. Tanaka, K. Miyahara and K. Tanaka, Bull. Chem. SOC.Jpn. 54, 3106 (1981). 6. N. Giordano, Chim. Indus. 61, 283 (1979). 7. K.Y .S. Ng and E. Gulary, J. Catal. 92, 340 (1985). 8. C. Louis and M. Che, J. Phys. Chem. 91, 2875 (1987). 9. Y. Matsuoka, M. Niwa and Y. Murakami, J. Phys. Chem. 94, 1477 (1990).

V . CortCs Cotbcrin and S. Vic Bcllon (Editors), New DeveiopriIenr.5 i n Selectlvc Oxidallon I /

1994 Elsevier Science B.V.

811

Selective Dehydrogenation of Ethanol Over Vanadium Oxide Catalyst N.E.Quaranta*, R.Martino, L.Gambaro, and H.Thomas**. Centro de Investigacidn y Desarrollo en Procesos Catalfticos (CINDECA). Calk 47 No. 257. 1900. La Plata. Argentina.

ABSTRACT: Oxidative dehydrogenation of ethanol to acetaldehyde over V,O, -aAl,O, catalyst, was studied by using a flow reactor at temperatures ranging from 433-573K. The effect of the space velocity on conversion and selectivity to partial oxidation products was studied by modifying the flow rate of the ethanol-air mixture for a given catalyst mass. Acetaldehyde was the mayor product, with acetic acid, ethyl acetate, methanol, ethyl ether and carbon oxides as minor products. After the reaction, a reduced vanadium oxide (V,O,) was identified in both catalyst by FTIR Spectroscopy and X Ray Diffraction. 1. INTRODUCTION: A number of oxides are active for selective oxidation of ethanol, and the main product depends on the specific system involved. Unsupported V,O, and supported V,05 on different carriers (TiO,, SiO,) have been studied for this reaction. V,O, (1) and V,O,-SiO, (2,3) are very active and selective to acetaldehyde. In the case of TiO, support ( 3 ) , a high selectivity to ethanal is observed at low conversion (<20%), while a considerable formation of acetic acid and carbon oxides take place at higher conversions. The objective of the present research was to prepare a catalyst consisting of V,O, supported on high density a-alumina, that while maintaining the unsupported V,O, properties, can improve vanadium oxide performance for the selective oxidation of ethanol with respect to unsupported V,O,, due to a higher heat dissipation and catalytic bed isothermy.

2. EXPERLMENTAL: 2.1. Catalysts Preparation Unsupported V,O, was prepared by precipitation in a hydrochloric solution of ammonium vanadate neutralized with NH,OH, experimental details were given in a previous work (4). The supported catalyst was prepared by supporting V,O, on a-alumina from a suspension of the oxide in pure ethanol. The suspension was evaporated in a rotary device at 343K in oxygen atmosphere. The support was a-alumina of high density. Small particles 3 mm diameter were used. Then, the samples were burnt in a Pt crucible (to avoid the formation of vanadium bronzes) at temperature 823K during 24h.

*

.*

On leave from CICPBA. La Plata. Argentina. To whom correspondence should be adressed.

812

2.2. Catalysts characterization The samples of unsupported and supported catalysts were characterized by X-Ray diffraction (XRD), IR spectroscopy (FTIR), scanning electronic microscopy (SEM) and temperature programmed reduction (TPR). A Philips PW 1390 Diffractometer using CuK radiation and Ni-filter was used for crystallographic analysis. The X-ray diffractograms were performed in the interval 2 0 between 10" and 60°, at 2"/min. The X-ray tube was operated at 40 KV and 20 mA. The IR spectra were recorded by an Infrared equipment of transformed Fourier IFS66 Bruker in the region 1500 to 400 cm-'. The samples were measured in the form of BrK tablets. The spectra thus obtained showed an average of 32 determinations. Supported V,O, samples for characterization were obtained by scraping off the surface of V,O,-a alumina particles. The microstructural measurements were carried out by means of a Philips 505 scanning electronic microscope with an X-ray detector, and secondary and retrodiffused electrons detector. The tested samples were small broken particles from the supported catalyst. The preparation of the sample surface (coated with gold) was made with a Balzers SCD 030 equipment, whose working conditions were fixed in 30 mA current and 30 sec. exposure time. The TPR profiles were obtained with a flow cell connected to a gas mixing line. The samples (c.a. 40 mg.) were first degassed at ambient temperature and then at 373K to remove the adsorbed water. The mixture of gases used for reduction was 10% H, in N, with a flow of 25 cm3/min, under a pressure of 760 torr. The heating up to 1000 "C was conducted out at 5 "C/min.

2.3. Catalytic Activity Catalytic activity tests were made in a stainless steel tubular fixed bed reactor (2.5 cm diameter and 1 meter length) at atmospheric pressure in the temperature range 423-573K. The reactor was electrically heated with a melted salt chamber, thus allowing a greater isothermal bed length. The mass of V,O, in catalyst samples under study was of 250 mg. The supported catalyst filled a volume of 5 ml. The V,O, powder was diluted up to that volume with inert quartz particles (particle diameter 2 mm). Ethanol (Merck p.a. 99.8 ~ 0 1 %and ) chromatographic air (99.99%)were used as reactives. Different compositions of the reaction mixture were obtained through a glass saturator. The V,O,-a alumina sample was tested at different space velocities (0.4, 1.0 and 2.0 mol et./(g.cat.sec)), obtained by varying the feed flow rate of the ethanol-air mixture at constant catalyst mass. Catalytic activity measurements on the unsupported catalyst were carried out at space velocity of 1.O mol et./(g.cat.sec.),this flow rate value was selected because a grater selectivity to acetaldehyde was previously observed at this reaction conditions. The reactants and products were analysed on-line by gas chromatography with a SHIMADZU GC-9A equipment attached to the reactor. Two columns were used, molecular sieve 5A for the CO and 0, analysis, and Porapak Q for the rest of the compounds. Mass balances of 100 5 were obtained. Conversion and selectivity to products were calculated on the carbon atom basis, expressed as molar percentages of ethanol transformed to ethanol fed, and of ethanol transformed to each product to total ethanol transformed, respectively.

3. RESULTS AND DISCUSSION 3.1. Catalysts characterization X-ray diffraction spectra of fresh sample of both unsupported V,O, and scraped vanadium pentoxide of V,O,-a alumina only showed the diffraction lines corresponding to the orthorombic structure of crystalline V,O,, See figure la.

813

I

Figure 1. X-Ray diffraction spectra. a. fresh samples of unsupported V,O, and V,O,-a alumina. b. Used unsupported V,O,. c. Used V,O,-a alumina.

After several days of reaction (15-30 days), the diagrams of the used samples showed decrease in some of the lines corresponding to vanadium pentoxide, the disappearance of others and the presence of diffraction lines which were identified as belonging to V,O, reduced vanadium oxide. Figures l b and l c allow to see that both samples present almost identical diagrams, as a first evidence that the active phase coincides in both cases. Figure 2 shows the IR spectra of the fresh and used catalysts obtained in the 1200-400 an-' region. The supported and unsupported fresh samples showed the same spectra, with the characteristic bands of bulk V,O,. Used samples of both catalysts also showed coincident spectra, which corresponded to V,09 phase of the vanadium oxides. This is in agreement with the results obtained by X-ray Diffraction. Other authors have already found the V,O, crystalline phase when exposing vanadium pentoxide to organic vapours (5). a

814

Figure 2. IR spectra.

......... fresh samples

-a. Used unsupported V,O,.

b. Used V,O,- aAl,O,. Figure 3 shows a photograph of a small broken particle of the V,O,-a A1,0, catalyst, obtained with a magnification of 1000x. The figure depict a compact "shell" of typical V,O, small crystallites (6) formed on the support. Shell thickness was measured on several samples and the values ranged from 5 to 30 microns. The same grain morphology exhibited by this sample was observed in the used catalyst. Results were in accordance with those of Maciejewski et a1 (6) who found through electronic microscopy, that the original grain morphology of V,O, samples is not modified during reduction. The TPR profiles for both catalysts turned out to be identical, showing reduction peaks corresponding to the lower oxides V,O, and V,O, at 475 and 645 K temperatures respectively. This means that the tested V,O, samples possess the same reduction capacity. 3.2. Catalytic activity Results from the catalytic activity tests are shown in Figures 4 and 5 , where total conversion and selectivity to acetaldehyde are presented according to temperature. Under low conversions (<15%), no influence of different space velocities was observed for the supported sample. Under greater conversions, the decrease of the space velocity leads to an increase of the conversion at a given temperature, but at the same time to a decrease in the acetaldehyde selectivity. This is due to the fact that the greater the residence time for the same active phase surface, the greater the probability of reactive adsorption (> conversion), while the possibility of product readsorption and the production of more oxidated compounds is also greater (< ethanal selectivity).

815

Figure 3 . Scanning electron micrograph of a small broken particles of V,O,. Magnification 1ooox. The minor products obtained were methanol, ethyl acetate, ethylic ether and acetic acid, and their concentrations were below 7% in all cases. CO, and small quantities of CO were found as total oxidation products. It was observed that CO, formation at low space velocities occurs at the expense of reduced selectivity to acetaldehyde. In these figures we can also notice the behaviour of the powdered V,O, sample, tested at 1.O mol et./(g.cat.sec) space velocity. The unsupported catalyst present greater conversions at constant temperature than the V,O,-a alumina catalyst measured under the same working conditions. On the other hand, the selectivity to acetaldehyde is remarkably lower, with formation of acetic acid as another mayor product, were than 40% in some cases. Taking into account that the surface of the active phase and the bed volume are similar in both cases. This different behaviour may be due to the different distribution of both sample within the vertical reactor, The former differences can in form cause noticeable changes in the dissipation of heat produced by the reaction which in turn affect the catalytic bed isothermy. This is based on the observation of the temperature at which the formation of acetic acid starts. In the experiments carried out with V,05- a A1,0, catalysts, this temperature is independent of space velocities and is equal to 503K. For the unsupported vanadium pentoxide, the production of acetic acid takes place at 483K, probably due to the existence of higher temperature zones within the catalytic bed, caused by the poor homogeneity achieved when the packing powdered catalyst and the inert diluent was made.

816

Finally, the V,O, sample prepared under the conditions described in the present research maintains the bulk V,O, structural properties the oxidation-reduction capacity, and the same active phase in the ethanol oxidation reaction if compared with the unsupported V,O, catalyst, while improves perceptibly the selectivity to acetaldehyde. The preparation method described allows to obtain a vanadium pentoxide catalyst with the same fundamental properties than the bulk catalyst without interaction with the support, and leads to more uniform distribution of the catalyst into the bed.

Ethanol oxidat ion

'0

Temperature

(K)

Figure 4. Ethanol oxidation on supported V,O,-a alumina catalyst as a function of temperature at different space velocities and unsupported V,O, catalyst as a function of temperature at space velocity of 1.O mol et./(g.cat.sec).

817

Ethanol oxidat ion

(mol.tt/q.cat .sec.)

440

460

480

500 520 540 Temperature ( K )

560

5 10

Figure 5. Selectivity to acetaldehyde as a function of temperature.

4. CONCLUSIONS The preparation method used in this research, whose objective was to obtain a more homogeneous distribution of V,O, catalyst in the bed was a successful, since it did not modify the structural properties nor the vanadium oxide oxidation-reduction capacity. At the same time performance as catalyst was improved for the selective oxidation reaction of ethanol to acetaldehyde, probably due to a greater heat dissipation and catalytic bed isothemy. ACKNOWLEDGMENTS Thanks are due to Graciela Valle for the help in X-ray and IR essays, and to Nestor Bemava for the assistance in chromatography.

818

REFERENCES 1. L.Wang, K.Eguchi, H.Arai and T.Seiyama, Chern.Lett. (1986) 1173. 2. S.Oyama, K.Lewis, A.Carr and G.Somorjai, Proc. 9th Int. Congr. Catal. Calgary (Ed. M.J.Phillips and M.Teman), Vo1.3 (1988) 1489. 3. N.Quaranta, V.C.Corberin and J.G.Fierro, "New Developments in Selective Oxidation by Heterogeneous Catalysis" (P.Ruiz and B.Delmon, Eds.) Studies in Surf. Sci. Catal., Elsevier, Vo1.72 (1992) 147. 4. N.Quaranta, L.Garnbaro and H.Thomas, J.Cata1. 107 (1987) 503. 5. Francois ThCobald, Rev. Roumaine Chim. Vo1.23 6 (1978) 887. 6. M.Maciejewski, A.Reller and A.Baiker, Thermochimica Acta, 96 (1985) 81.

V. Cortes Corbcran and S. Vic Bcllon (Editors), New Developrnenls i n Selectwe O x r d d o n 0 1994 Elscvier Science B.V. All rights reserved.

819

Cesium promotion of iron phosphate catalyst and influence of steam on the oxidative dehydrogenation of isobutyric acid to methacrylic acid. J. Belkoucha, 8. Taouka, L. Monceauxa, E. Bordesa, P. Courtinea and G. Hecquetb aDepartement de Genie Chimique, Universite de Technologie de CompiPgne, B.P. 649,60206 CompiPgne Cedex, France, bElf-Atochem, Tour Aurore, 92080 Paris La Defense 2 Cedex, France. When cesium is added to iron phosphates in the oxidative dehydrogenation of isobutyric acid to methacrylic acid, both catalytic and non catalytic reactivities of the catalyst are modified, with or without steam. It is shown that both cesium and steam are responsible for regulation of the redox mechanism, and for stabilization of the catalysts by decreasing coke formation.

1- INTRODUCTION

As shown extensively by Millet et al., several iron phosphates are catalysts for the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) [l-31. They found a new phase called Fe3(P207)2, that they claim to be the active and selective one, responsible for the good performance of P/Fe = 1/1 catalysts. FegPg031, which is obtained by oxidation of pure Fe2P207, would be mainly composed of this new phase Fe3+2Fe2+(P207)2and of a-Fe2Og in a 2/1 molar ratio. The latter has been detected only by Mossbauer spectroscopy and must be amorphous because no other experimental evidence exists to suggest that it is crystalline. Dekiouk et al., studied the kinetics and investigated the composition of bulk and surface of catalysts containing an excess of phosphorus and silica. They emphasized the role of acid phosphates which should be present on the surface in presence of steam [4]. However the best catalytic properties are claimed for catalysts containing cesium (or other alkaline metal) and an excess of phosphorus (P/Fe > 1/1) [5].To understand the roles of Cs, P and steam, we have, first, examined their effects during the preparation of P/Fe = 1/1 catalyst, and second, studied the reactivity of samples with and without steam, in oxidizing and reducing conditions. Finally, the behavior of pure FeP04 and Fe2P207 has been compared with Cs-Fe-P-0, in catalytic and non catalytic conditions.

820

2- EXPERIMENTAL 2-1. Preparation of catalysts and characterization. FeP04 (quartz) was synthesized from an aqueous solution containing a stoichiometric mixture of NH4H2P04 and Fe(N03)3.9H20 heated to dryness, and the residue was dried at 393 K for 24 hrs. After grinding, calcination was performed at 573 K for 12 hrs and then at 723 K for two days. By the same method CsxFeP~+yO, catalysts were prepared by adding to the preceding mixture x CsN03 and y NH4H2P04 before evaporation and calcination. Typically x = 0.15 and y = 0.24 were used [5,6]. CsFeP207 was prepared as above with stoichiometric amounts. The dried solid was calcined first at 593 K and then at 823 K after grinding. Fe2P207 was prepared by reduction of FeP04(Q) in wet H2/N2 = 10/90 (psteam= 24 mmHg) at 723 K for two days. Its reoxidation in air at the same temperature yielded a black brown compound identified as FegPg031 based on weight gain in thermal analysis (TGA). X-Ray diffraction (XRD), electron microscopies (SEM, TEM) and infrared spectroscopy were used extensively to characterize the compounds at different stages of preparation, and before and after catalytic tests. 2-2. Catalytic and non catalytic reactivities The catalytic reaction was studied at 688 K in an integral stainless steel microreactor (30 cm3) fed with 5 mol.% of IBA in N2, 0 2 and steam (02/IBA = 0.75, H20/IBA = 12), at contact time z = 0.7 sec. The analysis of effluents was made with two on-line gas chromatographs [6]. Samples examined were pure phases Fe2P207, FegPgO31, CsFeP207, the catalyst CsxFeP~+yOz, and two mixtures of CsFeP207/Fe2P207 (MM0.15 and MM0.35 for 0.15/1 and 0.35/1 molar ratios respectively) obtained by mechanical grinding. The behavior of these solids in oxidizing and reducing conditions was examined by use of thermal analysers (TGA/DTA, heating rate 5 K/min) in dry or wet atmosphere of 0 2 / N 2 = 20/80 and H2/N2 = 10/90 (feed rate 12 l/hr) respectively. 3- RESULTS

3-1. Characterization of CsxFePl+yOZ(x = 0.15, y = 0.24).

Although the conditions of preparation and calcination at 723 K of FeP04 and of CsxFePl+yO, are the same, pure FeP04 is well crystallized (quartz-type structure), unlike Cs,FeP~+yOzwhich exhibits only two lines of FeP04 tridymitetype [6]. The state of crystallization is improved after calcination at 823 K, and peaks of FeP04(T) are unambiguously identified. After 7 hrs at 923 K, FeP04(Q) (high temperature) and CsFeP207 are formed. By TGA/DTA of CsxFePl+yOzin air a small loss of weight (0.4 %) occurs near 863 K and an exothermic signal at 983 K (no weight change), which is reversible with decreasing temperature, is assigned to the transformation of FeP04 quartz from low to high temperature forms. The small loss is due to water as evidenced

82 1

by following the peak 18 on the mass spectrometer coupled to TPD experiments in vacuum at 853 K. It could be related to the presence of iron pyrophosphate acid, HFeP207, which releases water near 873 K and is transformed into Fe(P03)3 and Feq(P207)3 [8]. The actual presence of HFeP207 could not be easily checked because of its low amount (y-x = 0.09) which hinders its detection by XRD, but the 0.4 YO loss is consistent with this hypothesis. The formula of CsxFeP~+yO, in air and below 873 K could be therefore written as (CsFeP207)x(HFeP207)y-x(FePO~)~-y. Because it crystallizes as FeP04(T), the tridymite structure of which is very loose and open, it could also be considered as a solid solution of ortho and pyrophosphate of iron, hydrogen and cesium: CsxHy-xFe(P207)y(P04)~-y, or CsxHy-x(FeP~07)y(FeP04)~-y [6]. It must be recalled indeed that FeP04(T) is stable only when dopants like A1 (replacing partly Fe) or alkaline cations, or even phosphorus are present. In the case of CsXFeP1+,0,, the tridymite structure would be stabilized not only by protons, which are necessarily present to balance the negative charges, but also by cesium cations occupying the large cavities. 3-2. Catalytic reactivity

The conversion CIBAand selectivity SMAAin MAA at 488 K and 2 = 0.7 sec are plotted against time in Fig. 1 for Fe2P207, Fe#@31, and for mechanical mixtures MM0.15 and MM0.35. The main by-products are carbon oxides, acetone, propylene and a little acetic acid. CsFeP207 was found nearly inactive. At the end of the catalytic experiments the phases inside the catalysts were identified by XRD (Table 1). Table 1 Catalytic results and identification of phases by XRD after 24 hrs. Catalysts

a: XRD after 13 days,

Phases after catalytic test (24 hrs)

IBA conv. SAMA mol.% mol.%

Yield mol.%

performance at 24 hrs.

After a transitory state which lasts from 3 to 7 hrs according to the sample, a plateau is reached which corresponds to the steady state. Fe~P207and FesPsOsl behave differently at first: CIBAand SMAAincrease for Fe2P207 while they decrease for FesP8031. After stabilization they decrease slightly with time (Fig. 1).Used catalysts in each case showed both FesP8031 and Fe2P207. The mechanical mixtures

822

OI\\*

.-. .

t

. .-. t

4 7

5

0

I5

10

25

20

30

.

TIME [ h n )

TIME (hm) 3

* *

c

10

20

30

Figure 1. Conversion of IBA (a) and selectivity in MAA (b) vs time at 688 K for Fe2Pz07, FegP8031, MM0.15 and MM0.35 (CsFeP207/Fe2P207 = 0.15 and 0.35 ).

A

2 :II

CONVERSIOS

+-+--+-+-+ f

2

4

6

8

10

D

12 TlME(days

Figure 2. Conversion of IBA and selectivity in MAA and by-products vs time for Cs,FeP~+yO,(x = 0.15, y = 0.24) (688 K, z = 0.7 sec).

823

MM are more stable and their catalytic properties are better than those of Fe2P207, at first and at the steady state (Fig. 1, Table 1).FegPg031 is also detected, together with CsFeP207 and Fe2P207 which are initially present in the fresh catalyst. The catalytic properties of Cs,FePl+yO, were examined during two weeks (Fig. 2). After 17 hrs of running, conversion and SMAA were 83 and 76 mol.% respectively. After 6 days, during which the steam feed stopped unexpectedly for a while, conversion and selectivity were stabilized ca. 76 mol.%. As already mentionned, the fresh sample of Cs,FePi+yO,is poorly crystallized in the FeP04(T) solid solution, but after 13 days of working the crystallinity is improved, and the three phases already identified in MM samples are also observed (Table 1). Moreover the catalyst is extensively coked as seen each time the catalyst works without steam in the feed. The comparison between these catalysts shows that the best performance is obtained with Cs,FeP~+yO,which is only slightly better than MM0.35 of similar stoichometry. At the steady state oxidized FegP~O31and reduced Fe2P207 forms of Fe-P-0 (P/Fe = 1/1) are present in all catalysts, including pure phases, whatever the initial compound is. These phases can be therefore considered as the redox partners, to which is added CsFeP207 when Cs and an excess of P are present. Although the latter is not active by itself, a synergetic effect is observed since catalytic properties of Fe-P-0 are enhanced. 3-3. Non catalytic reactivity

TGA/DTA of the oxidation of Fe2P2O7 in 02/N2 and of the reduction in H2/N2 of the products obtained were performed in dry or wet atmosphere. In dry air, pure Fe2P207 is oxidized in two steps, at 723 K in FegPgOgl which is stable up to 893 K, and further in FeP04(Q) up to 1073 K (Fig. 3). The reduction of FeP04(Q) (or of tridymite) yields directly Fe2P207, and runs faster in wet than in dry H2/N2 [6]. TGA were also conducted in isothermal conditions (723 K). In dry air the oxidation of Fe2P207 stops at FegPg031 and the reduction of the latter in dry H2/N2 gives back Fe2P207. Several redox cycles of this kind can be performed. Here, the rate of each reaction decreases in wet atmosphere and therefore steam hinders these reactions (Fig. 4). In the same conditions Cs,FePI 0, behaves somewhat differently. Initially the +y. sample is the FeP04(T) solid solution containing Cs. First, its reduction in dry H2/N2 yields Fe2P207 (Fig. 4B), which is reoxidized (in dry 02/N2) directly in FeP04(T) while CsFeP207 is present. Let us recall that starting with pure Fe2P207 would lead to FegP8031 (Fig. 4A). This step of reduction is also by far faster than when starting with pure FeP04(Q). Several redox cycles can be reproducibly made between FeP04(T) and Fe2P207 (in presence of Cs). Second, in wet atmosphere, the oxidation of FezP207 stops at Fe8PgO31, which gives back Fe2P207 by reduction. Similarly, several redox cycles can be performed between FegP8031 and Fe2P207 (in presence of Cs and H20). Oxidation and reduction run slower than in dry atmosphere. To summarize, and by comparison with pure phases, the presence of CsFeP207 can be correlated with the changes observed after the first reduction, which are (i), reoxidation up to FeP04(T) instead of FegP8031 in dry atmosphere, and, (ii), reoxi-

824

Figure 3. Thermal analyses (DTA/TGA) of the oxidation of Fe2P207 in air.

B

J.

0

15

30

45

60

timeimin)

Figure 4. Reduction (B) and reoxidation (A) of Fe-P-0 (P/Fe = 1/11 and Cs-Fe-P-0 (P/Fe = 1.24) in dry or wet atmospheres. Cs, wet; (4 without Cs, dry; (.el without Cs, wet. Legend: (4Cs, dry; (4

825

dation up to FegPg031 in wet atmosphere only, with higher rates than for pure phases but lower rates than in dry atmosphere. Another point is that the solids obtained in wet atmosphere are easily identified because of their better crystallinity. CsFePzO7 is therefore detected beside FegPgO3i or Fe2P207 according to the oxidizing or the reducing atmosphere used. 4- DISCUSSION

It has been already shown that Fe-P-0 catalysts work by means of a redox mechanism involving lattice oxygens of the solid [ 3 ] . All the experiments performed in this study show that Fe2P207 and FegPg031, the latter being obtained by reoxidation in the operating catalytic or non catalytic conditions provided steam is present, are the two stable partners of the redox system. Let us notice that the remarkable reproducibility of redox cycles is not consistent with the eventuality of amorphous iron oxide beside Fe3(P207)2. The presence of Cs as CsFeP207 stabilizes Fe3+, and increases the rates of both oxidation of Fe2P207 and reduction (by H2) of FegP8031. In turn, the activity is increased compared with non Cs-containing catalysts. The reason why an excess of phosphorus is used in the most efficient catalysts would be therefore related to the necessary formation of CsFeP207 inside the initial tridyrnite structure of fresh catalyst, while keeping the right stoichiometry P/Fe = 1/1 for FeP04 and redox partners. The role of steam is multiple. The catalytic phases obtained either by reduction or by oxidation in the presence of steam are in better state of crystallization and become less reactive as shown for the oxidation of Fe2P207 (Fig. 3 ) . Consequently the first action of steam is to modify the size and the texture of the crystallites, and to moderate the reactivity of CsxFeP~+yO, by limiting the oxidation of Fe2P207 present to FegPg031 (instead of FeP04). The second action of steam is to moderate both reactions, as shown for pure phases as well as for CsxFeP~+yO,,resulting in making the rates of oxidation and reduction closer to each other. Therefore the combined action of Cs and of steam can be understood as a regulation of the redox Fe2P207/Fe$@31 involved in the catalytic reaction, and of the amount of Fe3+ active sites. A comparison with the oxidative dehydrogenation of butene to butadiene which is made on iron oxides is fruitful because several points are common: the large amount of steam (H20/HC = 10-12), the addition of alkaline cations which enhance activity and selectivity, and the coking of catalysts especially in low amount of steam. In the case of iron oxides the role of steam has long been presented as a thermal diluent, an oxidant or a coke remover. Recently a study of the surface in operating conditions has evidenced another role which would be to convert the catalytic surface to hydroxyl forms such as Fe-OH-Fe (hydroxylation of an oxygen linked to two Fe) and 0 - F e O H - 0 (hydroxylation of a surface Fe), more adapted because more basic. These specie would be able to initiate the dehydrogenation by the ahydrogen abstraction as a proton from butene [9,10]. Steam would also limit the adsorption and reaction of butene owing to competitive adsorption of molecular water. However IBA (MAA) is not butene (butadiene) and the last proposition is

826

no more valid since, on the contrary, steam is useful in helping MAA to desorb, as with heteropolyoxometallates active and selective for IBA-MAA [ l l ] . Surface acid ortho and/or pyrophosphates would be the sites responsible for this function, as proposed formerly [2]. Another difference is that, although alkaline cations (which increase 0 2 - or OH- basicity) are useful to maintain the active Fe3+ state, potassium ferrite KFe02 has been claimed to be the active phase [9,10], whereas CsFeP207 itself is not active and yields mainly acetone from IBA [2,6]. In a similar way, we propose that neighboring iron cations wearing surface hydroxyls could act as active sites allowing dehydrogenation of IBA, the selectivity in MAA being controlled by the action of steam on acid phosphates. REFERENCES J.M.M. Millet, J.C. Vedrine and G. Hecquet, in G. Centi and F. Trifiro Eds., New Developments in Selective Oxidation, Stud. Surf. Sci. Catal., 55 (1990) 833. 2. J.M.M. Millet and J.C. Vedrine, Appl. Catal., 76 (1991) 209-219. 3. C. Virely, M. Forissier, J.M.M. Millet and J.C. Vedrine, J. Molec. Catal., 71 1992) 199-213. 4. M. Dekiouk, N. Boisdron, S. Pietrzyk, Y. Barbaux and J. Grimblot, Appl. Catal., A, 90 (1992) 51-60 ;ibid., 61-72. 5. D. Chelliah, Ashland Oil Inc. (USA), FR. 2 498 475 (1982). 6. J. Belkouch, ThPse, CompiPgne (1991). 7. P. R6my and A. Boull6, C. R. Acad. Sci. Paris, 253 (23) (1961) 2699. 8. F. D'Yvoire, Bull. SOC.Chim. Fr., 6 (1962) 1224-1246. 9. B.J. Liaw, D.S. Cheng and B.L. Yang, J. Catal., 118 (1989) 312-326. 10. B.L. Yang, D.S. Cheng and S.B. Lee, Appl. Catal., 70 (1991) 161-173. 11. M.J. Bartoli, L. Monceaux, E. Bordes, G. Hecquet and P. Courtine, in P. Ruiz and B. Delmon Eds., New Developments in Selective Oxidation, Stud. Surf. Sci. Catal., 72 (1992) 84-92. 1.

827

J. Vedrine (IRC, Villeurbanne, France): Your results complement those of Millet et al., concerning the role of Cs in the stabilization of tridymite FeP04 by adding steam and studying chemical mixtures. Could it be possible that mechanical mixtures between Fe2P207 and Cs20 give the same result, or at least affect acidic features. You have also made a correlation between oxidative dehydrogenations of butene on iron oxide catalyst and of of isobutyric acid to methacrylic acid, stating that the first step involves H abstraction on basic sites (OH- type). However butene is a basic compound while IBA is obviously acidic. How can you imagine the first step on the same basic OH- site. E. Bordes (UTC, CompiPgne, France): We are not presently able to assign the increase of performance obtained by addition of Cs either to CsFeP207 as a phase, or to Cs itself as a cation. In the latter case Cs would indeed affect acidity since it reinforces OH- or 0 2 - basicity, and so similar results could be obtained with a mixture Fe2P207/Cs20. To answer the second question, the abstraction of H(-C-C=O) (IBA) or H(-C-C=C) (butene) can be done by similar OH-, whatever the acidity or basicity of the molecule as a whole which affects mainly further steps. Hydroxyls such as OH(-Fe-0-Fe) displayed on e.g., Fe2O3, are indeed slightly different from OH(-Fe-0-P) on e.g. FeP04.

G . Emig (Institut fur Technische Chemie, Erlangen, Germany): When we used heteopolyacid catalysts for your reaction we saw also a very marked effect by the addition of water. We observed an optimal IBA/H20 ratio of 12. If one applies higher ratios a negative effect on IBA conversion is observed, because of competitional adsorption on the catalyst surface. Why do you use H2O/IBA = 12 and what happens if you go to lower values? Such a high amount of water could certainly not be beneficial for a commercialisation of your process! E. Bordes (UTC, CompiPgne, France): The H20/IBA ratio of 12 is indeed an economic handicap for the process. However a decrease of the amount of steam leads to a strong decrease in catalytic properties due partly to extensive coking of the catalyst. Stabilization of the best performance is observed for H2O/IBA = 12-15.

J. M. Millet (IRC, Villeurbanne, France): We have proposed some years ago that cesium plays a role in the stabilisation of a tridymite phase with P/Fe ratio greater than one, and that CsFeP207 itself has no catalytic role. You propose in this work that it does have a role. Since CsFeP207 has been shown to be inactive and since it is present in the catalyst in too a small amount to postulate a support effect, how do you explain the synergy effect that you observed with mechanical mixture of CsFeP207 and Fe~P207? Furthermore, your results (MM0.15 and MM0.35) seem to show that this synergy effect is quite independant of the amount of CsFeP207. E. Bordes (UTC, CompiPgne, France): CsFeP207 is indeed poorly active but we do observe a promoting effect when it is present beside active phases. Its amount is not low because Cs,FeP~+yO, (x = 0.15, y = 0.24) corresponds

828

approximately to CsFeP207/Fe2P207 = 0.35/1 (molar ratio). Catalytic results are higher with mechanical mixtures MM than with pure phases (Fe2P207 or FegP8031) which means that a synergetic effect exists, even though the contacts are not optimized. Moreover MM0.35 behaves similarly to C~0.15FeP1.240~. Other ratios should be prepared to be sure that the optimum ratio lays between 0.15 and 0.35, or closer to 0.35, as we suppose.

M. Sinev (I. of Chemical Physics, Moscow, Russia): You explain the influence of water by competitive adsorption between water and hydrocarbon. But your data show (Fig. 4) that the influence of water is stronger on reoxidation than on reduction. So, 1)What is the origin of the influence of water on reoxidation of catalysts? 2) Which of the two processes (reduction or reoxidation) is responsible for the influence of water on steady-state catalytic process?

E. Bordes (UTC, Compiegne, France): All data are not presented here. A kinetic study performed on pure phases has shown that (6), comparing oxidation and reduction without and with steam, the activation energy of oxidation Eox is slightly modified (20 and 23.5 kcal.mo1-1 respectively), whereas for reduction ERed (9 kcal.mo1-1) is multiplied by 2 or 3 according to the temperature range (similar values of preexponential factors). The adsorption of H20 on oxidized sites can be proposed as the moderating factor of reduction step. When Cs is present the main effect of steam is to limit the reoxidation of Fe2P207 to FegPg031.

V. Cort6.s Corberan and S. Vic Bellon (Editors), New Developments in Selective Oxidaiion II 0 1994 Elsevier Science B.V. All rights reserved.

829

Iron Hydroxysilicates: New Selective and Active Isobutyric Acid Oxidative Dehydrogenation Catalysts P. BONNET1, J.M.M. MILLE rl, J.C. VEDRINE' AND G . HECQUET2.

Institut de Recherches sur la Catalyse, CNRS, 2 avenue A. Einstein, 69626 Villeurbanne Cedex (France). Departement Recherche-Developpement-Innovation, ATOCHEM, 4 cours Michelet, Cedex42, 92091 Paris La Defense (France). Ilvaite, CaFe3+Fe2+2Si2@O(OH),a natural iron hydroxysilicate sample has been shown to be active and rather selective for oxidative dehydrogenation of isobutyric acid to methacrylic acid (MAA). However the catalytic properties were lower than those obtained iron oxy- and hydroxyphosphates. The selectivity in MAA was observed to decrease with the fast electron transfer increase as evidenced by Mossbauer spectroscopy (2.5 average oxidation state of iron rather than separated 2 and 3 ones). The rate of this electron fast transfer, which corresponds to an electron hopping over two iron cations, decreases when vaccancies are present along the Fe cations chains, resulting in higher selectivity in MAA. It was also shown that the presence of water in the gas feed was much less crucial for ilvaite than for oxy-and hydroxyphosphates. This was related to the higher thermal stability of ilvaite for dehydration,majoritarely in the 873-1073 K domain against 623-823 K for hydroxyphosphates in the catalytic reaction domain (633-723 K).

INTRODUCTION In recent years increasing attention has been focused o n new processes of production of methyl methacrylate. One of them includes a step which corresponds to the oxidative dehydrogenation of isobutyric acid to methacrylic acid (1). Up to now two types of catalysts are

known to be active and selective for this reaction, heteropolyanions and iron phosphates. The lack of long scale stability of the heteropolyanions let the iron phosphates based catalysts be a good candidate. Several studies have recently been devoted i n our laboratory to the study of iron phosphates for the oxidative dehydrogenation of isobutyric acid (1-5). We have been able to show that several phases with different compositions could be active and selective. All these phosphates were presenting common features, namely they appear to contain both femc and ferrous cations and hydroxyl groups. From these results it is proposed that the catalytic site could be described as a local arrangement of Fe octahedra. In order to confirm these results and to find new phases, we have undertaken the study of another iron hydroxylated compound, namely the ilvaite, an iron hydroxysilicate with the formula CaFe3+Fe2+,Si2O@(0H). This phase contains both femc and ferrous cations and hydroxyl groups. A comparison of the results obtained for hydroxyphosphates and the ilvaite is presented.

830

EXPERIMENTAL The ilvaite sample was kindly supplied by the Museum National d'Histoire Naturelle (Paris -France). The rock sample was crushed before charactenzadon. Mossbauer measurements were performed at room temperature (3). Standard least square fit procedures using Lorentzian lines were used to derive isomer shifts ( 6 ) (relative to aFe), quadrupolar splittings (A), line widths (W) and relative absorption areas. The accuracy for these parameter calculations was 0.02 mm.s-1. X-ray diffraction patterns were recorded using a Siemens D500 dffractometer and CuKa radiation. The chemical composition of the solids were determined by atomic absorption and their surface areas measured by nitrogen adsorption, using the BET method. The catalytic activities of the different samples were tested at atmospheric pressure using a flow microreactor. The reactor was charged with about 50 mg of catalyst and rapidly heated to the reaction temperature under a gas mixture containing isobutyric acid (IBA), 0 2 and H 2 0 diluted i n nitrogen. The conditions previously used to compare phosphates and hydroxyphosphates based catalysts were used; they were as follows: reactor temperature : 658 K, total flow rate : 1 cm3.s-1 and partial pressures for IBA, H20, O2 and N2 : 5.86, 72.0, 4.26 and 19.2 kPa respectively. With these conditions the conversion was around 10% for ilvaite and 15% for the hydroxyphosphates. The effect of water partial pressure was studied by keeping constant the IBA and 0 2 partial pressures as given above and varying the water partial pressure from 0 to 72 kPa. Product analysis was performed by gas chromatography as previously described (1).

RESULTS AND DISCUSSION The S e e and CaJFe ratios determined by chemical analysis were equals to 0.70 and 0.37 which showed that the samples had a cornposition close to that of pure ilvaite. The Fe3+/(Fe3++Fe2+)ratio calculated from Mossbauer spectroscopy results (0.36) was very close to the theoretical value (0.33); i n the same manner the OH/(OH+O) ratio determinated from thermogravimetry (0.116) was i n good agreement with that of the pure mineral (0.11 1 ) . The specific surface area measured using the BET method was equal to 1.12 m2.g-I.The X-ray diffraction pattern of the sample corresponded to that given in the literature (6).

FIGURE 1 : Schematic representation of the Fe-octahedra ribbons in the structure of ilvaite showing the 8d (A) and 4c (B) octahedral hire occupied by iron cations.

83 1

Ilvaite cnstallizes in the monoclinic system, with a space group P21/a. The cell parameters of the sample were calculated and found to be comparable to those cited in the literature (6). The structure is a framework of infinite edge sharing double chains of Fe(8d)OsOH octahedra connected by ( S i 2 0 ~ )groups, ~running all in the c direction, as schematized on figure 1. The six coordinated interstices of the framework are occupied by Fez+ (4c ) and the seven-coordinated interstices by Ca2+.The 8d site (A) is randomly occupied by Fe3+ and Fe2+ with a ratio 1:l. The Fe(4C)06 octahedron (B) shares four edges with Fe(8d)OsOH octahedron. Ilvaite shows a cristallographic phase transition at 346 K (7,8). In the high temperature form the atomic connectivity is the same as in the low temperature form. However the mirror plane normal to the c axis is lost and the 8d sites split into two 4e sites with an ordering of the Fe3+ and Fe2+ over these two sites. Moreover between 400 and 520 K a n electronic delocalisation occurs between the 4e sites.

r

1

3.30 2.5

3.45

2.4

3.40

1.2 , -2

r 0

t

tl

a 2

b

I

4

velocity (rnrnjs)

velocity ( r n r n i s )

FIGURE 2 : Mossbauer spectra of the ilvaite sample recorded at 295 K (a) before catalysis and (b) after catalysis (48 hours on stream). Solid lines are derived from least-square fits. TABLE 1 : Mossbauer parameters computed from the spectra of the ilvaite sample, recorded at 295 K before and after catalysis (48 hours on stream). compound

site

6

W

A

relative intensity

(mm.s-i)

(%I

Fe3+(8d) before catalysis Fe2+(8d) Fe2+(4c) Fe2.5+(8d)

0.53 0.28 1.30 0.97 0.28 2.18

31 27

1.16 0.26 2.21 0.78 0.57 1.53

30 11

Ilvaite after catalysis

0.59 0.28 0.9 1 0.36

1.18 1.96

23 21

1.03 0.25

2.38

31

0.71 0.36

1.57

25

I Iv aite

Fe3+(8d) Fe2+(8d) Fe2+(4c) Fe2.5+(8d)

832

The Mossbauer spectrum of ilvaite at 295 K consists of four doublets (Figure 2 and Table 1). Two ferrous doublets correspond to the cations occupying the 8d and 4c positions. A femc doublet corresponds to the cations sharing the 8d position. The last doublet characterized by 6 = 0.78 mm.s-1, A = 1.53 mm.s-1 and larger half width was associated with mixed valence cations. Such electron delocalisations at room temperature in ilvaite with the monoclinic structure has already been reported (8,9). The observation of these mixed valence cations by Mossbauer spectroscopy indicates that the frequency of the electron hopping process between two adjacents Fe3+ and Fez+ is about the mean life time of the Fe excited nucleus (lo-*s-*).These mixed valence cations are characterized by 6 and A values intermediate with respect to usual Fe3+ and Fez+ cations Mossbauer parameters. The ilvaite sample was studied in catalysis at different temperatures in the same temperature range as that used for the hydroxyphosphates.The results are gathered in table 2. First it can be observed that the same reaction products were obtained for CaFe3+Fe2+$i2O7O(OH)as for the hydroxyphosphates. The selectivity in MAA increased with increasing temperature whereas in the same time those in acetone and C02 decreased, the activation energy in the formation of MAA (136 kJ.mol-1) being much higher than that corresponding to the formation of acetone and C02 (78 and 72 kJ.mol-l). Similar phenomenom was observed on the phosphates and hydroxyphosphates. The results obtained at 658K were compared to those obtained for the hydroxyphosphates presenting the highest and the lowest yields in MAA, namely the barbosalite Fe3+2Fe2+(PO4)2(OH)2and Fe3+4(Po&(OH)3 which were presenting common structural feature (4) (Table 3).

TABLE 2 : Catalytic data of ilvaite at different temperatures ; PRO : propene, ACE acetone, MAA : methacrylic acid. temperature

selectivity (%)

(K)

C02 PRO ACE MAA

658 676

9 6

703 723

6 4

11

10 12 12

38 31 26 21

rate of formation of MAA (10-8 mol-1m-2)

42 53 56 63

89 216 492 866

TABLE 3 : Catalytic data of the oxy- and hydroxyphosphates and ilvaite at 658 K; PRO : propene, ACE : acetone, MAA : methacrylic acid. B : barbosalite.

phases Fe3+dP04)dOH>3 Fe3+2Fe2+(P0&(0H)2 B CaFe3+Fe2+,Si2q0(OH)

selectivity (%) CO;! PRO ACE MAA

7 6

6 7

9

11

41 21 38

46 66 42

rate of formation of MAA (10-8mo1.s-1.m-2)

55 223 89

The results show that the catalytic properties of ilvaite are good but not the best, i.e. intermediate between those of these hydroxyphosphates. However it can be observed that at

833

higher temperature (723 K) ilvaite became a very performent catalyst. The Mars and van Krevelen type mechanism proposed for the phosphates and hydroxyphosphates catalysts can very well be applied to the hydroxysilicate catalyst. In the presence of lattice oxygen ions of basic character in the sense of Pearson (labil02.) the first step of the IBA oxidation could be envisaged as an acidbase reaction of protons transfer and the second step as a redox process of electrons transfer from the ally1 species to the metal cations of the catalysts lattice. The two steps may proceed as one concerted reaction of mixed acid-base redox character (1,3). Like hydroxyphosphates, the ilvaite catalyst owes its catalytic activity to its possible oxido-reduction by an reduction-hydroxylation (or oxidation dehydroxylation) process (3). The effect of water partial pressure was studied as described in the experimental section. The results obtained are presented in figure 3. Contrarily to what was observed with the hydroxyphosphates, water partial pressure has onl) a small influence on both the activity and the selectivity of the ilvaite. The selectivities of this catalyst was pratically not affected even at low water partial pressure. With hydroxyphosphates and phosphates in the absence of water addition the reaction rates decreased until no product could any longer be detected (4.10). The results obtained with ilvaite are different since without added water the solid remained active. The large dependence of the hydroxyphosphates catalysts upon water has been explained by the fact that they were undergoing a total dehydration in the temperature range of the catalytic reaction as shown in figure 4. Water was proposed to be involved in displacing the corresponding hydration equilibrium in order to avoid a complete dehydration of the catalysts (4).

loo

F

3

0

0

100

20

50

L

99

2 0

-al a

U

0

20

40

60

80

FIGURE 3 : Variations of the selectivity (%) and of the rate of MAA formation mol.s-1.m-*)at 658 K as a function of the added water partial pressure. The results of the thermogravimetric analysis of ilvaite, showed that at the temperature of catalysis (623-723 K) the loss of water was low and that most of the water loss occured between 873 and 1073 K (Fig. 4). In the case of ilvaite, large quantities of water were therefore unnecessary, to displace the hydration equilibrium. However in such conditions, the mobility of the protons in the ilvaite was not enhanced by the vicinity of the bulk dehydration temperature, as it is usually admitted, which leads to a lower reoxidation rate than those observed on hydroxyphosphates and consequently to a less active catalyst.

834

FIGURE 4 : Direct thermogravimetric curves of Fe3( P 0 4 ) 2 ( O H ) 2 barbosalite (a), Fe3(P04)2(0H)2 lipscombite (b) and ilvaite (c). After catalytic reaction for 48 hours, the sample was characterized by X-ray diffraction and by Mossbauer spectroscopy. The powder pattern showed that the monoclinic form of ilvaite was always present after catalysis and that no observable phase transition had taken place. The Mossbauer spectrum was comparable to that recorded before catalysis and was fitted with the same four doublets (Table 1). The intensity of the doublet corresponding to the mixed valence cations has substantially increased. This allowed to clearly confirm that the cations involved in the electronic delocalisation were occupying 8d sites. In the conditions of catalysis (658 K) the electron delocalization involves all the cations in the 8d positions. The same type of fast electron transfer was observed in several phosphates and has been correlated with a decrease in h4AA formation and to an increase in acetone and C02 formation (5) (Table 4). Fast relaxation electron exchange was only observed for phosphates with long F e - 0 clusters. The observation of such a phenomenon for ilvaite which presents fully occupied chains of F e - 0 octahedra is thus understandable. From previous results it was suggested that the active site for MAA formation were small clusters of Fe-0 octahedra like in barbosalite after catalysis or in Fe3(P030H)4, the active phase of iron phosphates based industrial catalysts ( 3 5 ) .

TABLE 4 : Catalytic data of the hydroxyphosphates ( 5 ) and ilvaite at 658 K; PRO : propene, propene, ACE : acetone, MAA : methacrylic acid. B : barbosalite, L : lipscombite.

835

These results tend to show that the catalytic properties of ilvaite were more related to an electron delocalisation taking place in the compound at the temperature of catalysis rather than to the substitution i n the solids of phosphorus by silicium. This leds us to suggest that other ferrous and femc hydroxysilicates presenting not such electron delocalisation and consequently only small cluster of face or edge sharing Fe-0 octahedra, could be very good candidates as catalysts for the oxidative dehydrogenation of IBA.

CONCLUSION The results presented in this paper show that an iron hydroxysilicate, CaFe3+Fe2+2Si2070(0H)namely the ilvaite, could be successfully used as catalyst in an oxidative dehydrogenation reaction. The activity of this phase in the transformation of isobutyric acid into methacrylic acid is suggested to be related like in the hydroxyphosphates which have already been demonstrated as performant catalysts, to the presence in the same phase of both femc and ferrous cations and of labile oxygen and hydroxyl groups. The catalytic properties of ilvaite at 658 K, are lower than those of most of the phosphates cataly:its, but at higher temperature these properties become interesting Another characteristic which makes this type of solid attractive as catalysts for this reaction is the fact that it is less dependent upon large amount of water added to the gas feed. This can be explained by the fact that contrarily to the hydroxyphosphates the ilvaite looses only a small amount of constitutional water in the temperature range of the catalytic reaction and that consequently only small amount of water is necessary to displace the hydration equilibrium in the right direction. The presence in the structure of ilvaite of continuous chains of iron octahedra, which gives rise to a fast electron transfer between adjacent iron cations appears, as in the case of hydroxyphosphates, to be detrimental to the formation of MAA. This observation confirms as it has earlier been proposed, that the best catalytic sites for the reaction are small clusters of 3 or 5 Fe-0 octahedra (3,5) and tends to suggest that hydroxysilicates containing such small clusters would be valuable candidates as catalysts.

ACKNOWLEDGEMENT Dr. Skroy, from Museum National d'Histoire Naturelle (Paris-France), is gratefully acknowledged for supplying the ilvaite sample and ELF-ATOCHEM for financial support.

REFERENCES C. Virely, M . Forrissier, J.M.M. Millet and J.C. Vedrine, J. Mol. Catal. 71 (1992) 199. J.M.M. Millet, J.C. Vedrine and G. Hecquet, in "New Developments in Selective Oxidation" Stud. in Surf. Sci. and Catal.,G. Centi and F. Trifiro(Ed) Elsevier Amsterdam, 55 (1990) 833. 3 J.M.M. Millet, Ph D Thesis Lyon (1990) no 259-90. 4 D. Rouzies, J.M.M. Millet, D. Siew Hew Sam, J.C. Vedrine, submitted to J. Catal. (1993). 5 J.M.M. Millet, D. Rouzies and J.C. Vedrine, submitted to J. Catal. (1993). 6 S. Ghose, P.K. Sen Gupta and E.O. Schlemper, Amer. Mineral.. 70 (1985) 1248. 7 Y. Takeuchi and N. Haga, Zeit. fur Krist., 163 (1983) 267. 8 K. Xuemin, S. Ghose and B.D. Dunlap, Phys. Chem. Minerals, 16 (1988) 55. 9 G. Amthauer and G.R. Rossman, Phys. Chem. Minerals, 11 (1984) 37. 10 M. Dekiouk, N . Boisdron, S. Pietrzyk,Y. Barbaux,J. Grimblot, Appl. Catal., 90 (1992) 62. 1 1 B. Ech-Chahec, F. Jeannot, B. Malaman and C. Gleitzer, J. Solid State Chem 74, 47 (1988). 1

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V. CortCs Corberin and S. Vic Bcllon (Editors), New Developrnenls in Selective Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

837

THERMOLYSIS OF HETEROPOLYACID H3PMoiz040 AND CATALYTIC PROPERTIES OF THE THERMAL DECOMPOSITION PRODUCTS IN OXIDATION OF ACROLEIN TO ACRYLIC ACID T.V.Andrushkevich, V.M.Bondareva, L.M.Plyasova, G.S.Litvak, A.V.Ziborov

R.I.Maksimovskaya,

G.Ya.Popova,

Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia

Conversion of H3PMoiz040.13H20 heteropolyacid (HPA) has been studied within the 150-70O0C temperature interval. The products of thermal decomposition were identified and their catalytic properties were studied in acrolein oxidation.

INTRODUCTION 12-molybdophosphoric heteropolyacid (Mo-P HPA) and its salts appear to be efficient catalysts for numerous heterogeneous oxidation reactions [ 1-31 . They serve as a basis for the catalysts of methacrylic acid production via methacrolein oxidation [ 1-51 and oxidative dehydrogenation of isobutyric acid [6,7] . HPA also catalyzes oxidation of acrolein into acrylic acid [8,91. A peculiarity of Mo-P HPA significant for catalysis is its thermal instability [ l o ] . At the temperatures below 2OO0C the crystalline water is removed and the secondary HPA structure is rearranged. The existence of anhydrous HPA was registered in practically all studies but the temperature regions of its stability were found to differ considerably: 164-200°C [ I l l , 116-412'C [121, 270-395OC 1131, 320-340OC [ l o ] . At these temperatures the primary Keggin structure is known to be retained. At higher temperatures acid protons of the HPA are removed together with a part of oxygen atoms. This provides the primary structure distortion and the irreversible HPA decomposition. The total destruction of the Keggin anion yielding a mixture of phosphorus and molybdenum oxides occurs at 430-450 OC. Some intermediate products of the anhydrous HPA decomposition were referred to in [12-151. Thus, in [I21 this "anhydrous product" was observed within 477-757'C, in [ 131 an "anhydride" was registered but within a rather narrow temperature range (380-410 O C ) . In [14] some nonidentified products including X-ray amorphous ones were mentioned to form besides MOO,. The state and activity of HPAs under various conditions were often discussed in the reference literature in the relationship with their thermal instability [ 161. Thus, it was found in [8,9,16,17], that deactivation of catalysts occur upon decomposition of the Keggin structure. This deactivation is revealed in the loss of activity [17] or, sometimes, selectivity [8,9]. On the contrary, in [18,19] the HPA catalytic properties in the methanol oxidation to formaldehyde and the 2-propanol oxidation to acetone were found to be improved upon its thermal decomposition.

838

Here, we have studied thermolysis of the bulk molybdophosphoric acid H,PM0,204~.nH20 and described the catalytic properties of the thermolysis products in the acrolein oxidation. EXPERIMENTAL

Mo-P HPA was prepared from a commercial reagent via ether extraction followed by crystallization, filtering and drying in air. According to X-ray phase and chemical analysis, NMR and IR spectroscopies, the obtained sample was recognized to be the 12-Mo-P heteropolyacid with the Keggin structure and H3PMo12040.13H20 composition. The high temperature X-ray studies were performed in the X-ray chamber (HTXC) installed at the "D 500" (Siemens) diffractometer using K, - monochromatic emission from Cu within 25-50OoC. Silicon was used as internal reference. Besides, X-ray analysis of the samples heated at various temperatures in a derivatograph was performed. The registration rate was varied from 0.5 to 1 grad 20/min. MOO, lattice parameters were determined from the diffraction patterns using reflexes (122), (171) and (081) not overlapping with those from other phases. Thermal analysis of the samples was performed with the Hungarian derivatograph Q-1500 D; the conic platinum crucibles with covers were used, the heating rate was 10 grad/min, the sample weights were 100 and 1000 mg.

31P NMR spectra (121.47 MHz) were recorded with Bruker CXP-300, spectral width was 10 and 50 kHz, pulse duration was 10 p s , pulse delay was 30 and 90 s , number of transients was from 10 to 600. For narrowing the peaks of solid samples the MAS technique was used. The sample rotation rate was around 3-4 IfHz. After the thermal treatment the samples were tested for solubility in water and P NMR of the water extracts were recorded. The catalytic experiments on the acrolein oxidation were performed in a flow reactor (di,=10 mm) with the coaxial thermocouple pocket (d,,,=2 mm) under atmospheric pressure. The 0.25-0.50 rnrn catalyst fraction was used. The reaction mixture composition (v01.x) was: 0.4-0.5 C3H40 + 0.8-1.0 O,, balance helium. The composition of the starting reagent and the reaction products were analyzed on-line by chromatography with thermal conductivity and flame ionization detectors on the columns filled with Porapak-Q (C3H40, C3H402, C2H4O2, C02) and molecular sieves NaX (02, N2 and CO). RESULTS A N D DISCUSSION

Fig. 1 shows the X-ray patterns of the samples obtained in HTXC via heating in air and after a 15 min exposure at each temperature required. The X-ray pattern recorded at room temperature corresponds to H3PMo12040.13H20 [8]. The patterns obtained at 20O-35O0C are similar though differ significantly from the previous one. According to the thermal analysis data all crystalline water is removed at the temperatures lower than 2OO0C and, thus, the patterns observed within 20O-35O0C can be assigned to the anhydrous HPA: H3PMo12040.

839

Beginning from 38OoC new diffraction lines appear with d/n= 7.10, 3.89, 3.81, 3.56, 3.42, 3.28 and 2.62 A, while the intensity of the lines related to H3PMo12040 becomes weaker. Reflexes at 7.10, 3.81, 3.56 and 3.28 A belong to Moo3 [20], while those at 3.89, 3.56, 3.42 and 2.62 can be assigned to a new phase. The new phase line intensities increase with the calcination time and temperature increase up to 43OoC. The temperature increase to 45OoC provides a complete disappearance of the H3PMo,2040 lines and the decrease of those from the new phase, meanwhile the intensity of the Moo3 lines continues to grow. The picture observed in HTXC is of a qualitative character and does not allow to determine the temperature of the phase transitions precisely due to inhomogeneity of the temperature field. Visually the color of the samples is uneven. Therefore a detailed thermal investigation with the stepwise X-ray scanning after calcination at each temperature required was accomplished.

12.00 8.00

0

d/n A

b

5

I

3.00

4.00

1

I

10

2.5

2.50 1

3.5

'

29P)

Fig. I . X-raj patterns of the samples obtained in situ at 25 C ( l ) , 200°C (2), 35OoC (3), 38OoC (4), 400°C ( 3 ,42OoC ( 6 ) , 450°C (7), 500°C (8).

DTA and TG curves for the 100 mg sample (Fig. 2a) show endothermal dehydration reactions with the considerable weight loss corresponding to the complete crystalline water removal at the temperatures below 2OO0C as well as another dehydration process and a small additional weight loss at 40OoC. After varying the sample weight, heating regime and conditions of thermal exposures (duration, temperature) we have found that the conversion observed at 4OO0C is really a superposition of the endothermal and exothermal processes (Fig. 2b, 2c). The endothermal peak should be ascribed to the 13 mg weight loss corresponding to the removal of 1.5 moles of H20. This Fig. 2 Thermal curves (TG an d DTA) of HPA samples: a) sample weight 100 mg, dynamic regime; b) sample weight 1000 mg, dynamic regime; c) sample weight 1000 mg, dynamic regime with isothermal exposure at 380 C for 200 min.

840

water can be removed almost completely during a long (200 min) thermal exposure at 385OC till the constant weight. In this case at further heating the DTA curve exhibits no endo-peak and only exo-peaks are observed (Fig. 2c). The residual water weighs ca. 0.5 mg. This corresponds to the HxPMo12038.5+x/2 composition, where ~ ~ 0 . The 1 . X-ray pattern of this sample is rather legible and differs form that of the anhydrous HPA by.some shift of the reflexes and distribution of their intensities. The residual water removal (Fig. 2c) yields a new phase transition and provides an exothermal peak at 445OC. According to the X-ray phase analysis the sample contains Moo3 and the new phase observed earlier in the HTXC. The phase with a similar X-ray pattern was also qbserved in 1211 but was not identified. The further temperature increase results in the decomposition of the new phase yielding an exothermal peak at 520OC. Note, that the temperature of the latter depends on the degree of the preliminary dehydration of the sample (Fig. 2b, 2c). The less is the water content in the sample the higher is the temperature of the new phase decomposition. The X-ray patterns of the samples calcined at 500-7OO0C are practically identical and correspond to Moo3. The DTA curve in this temperature range exhibits two small endothermal peaks at 585 and 63OoC. Their nature remains unclear yet. The endoeffect observed at 77OoC is reversible and can be ascribed to Moo3 melting. However, this temperature is less than the reference temperature of the pure Moo3 melting, that is 801OC 1221, this can relate to the phosphorus dissolving in molybdenum oxide. Indeed, a precise X-ray study has shown a deviation of the lattice parameters of MOO, obtained (a=3.96205 A, b=13.8925 A, ~ = 3 . 6 9 3 6A) ~ from those of the pure MOO, (a=3.962g6 A, b=13.85s3 A, ~ = 3 . 6 9 6 4A). ~

-

7

4 6

3

2

. 1 L 0 ..

20

-20

. d

ppm

@. 3. 3 1 p NMR MAS spectra of initial HPA (1) and samples, calcined at 200 OC (2), 350 O C (3), 380-400 o c (4), 420 oc (s), 450 oc ( 6) and 500-700 O C (7)

Fig. 3 presents 31P NMR (MAS) spectra of the samples after various thermal treatments in the derivatograph. The samples calcined at 4OO0C and higher temperatures were preliminarily exposed at 385OC for 200 min and then heated to the mentioned temperatures in a dynamic manner. Initial HPA (H3PMo,2040. 13H20) gives a single line with 6=-3.6 ppm and ca.30 Hz line width, which is in a good agreement with the reference data [23]. After treatment at 20O-35O0C yielding anhydrous HPA H3PMoI2O4, the line is broadened and shifted to the lower field (6= -1.7 to 2.0 ppm). These changes are reversible and the initial spectrum is regenerated after the sample rehydration in air. The samples retain a complete solubility typical for HPA crystalline hydrates. The line from the sample calcined at 385OC (anhydride H , P M o ~ ~ O ~ ~ . ~is+narrowed ,/~) again and shifted to the higher field (6=-2.8 ppm). F,posure in air at the 100% humidity regenerates p NMR spectrum of the hydrated HPA, but the process occurs much slower, than with the samples heated at T <35OoC. Anhydride appears

84 1

to dissolve slower than anhydrous HPA and gives 31P NMR spectrum usual for H ~ P M O , , O solutions. ~~ The 31P NMR MAS spectrum of the sample calcined at 42OoC (fig. 3) differs significantly from the previous ones: a broad line (around 1000 Hz) at -8.3 ppm appears and a weak HPA line at -3.8 ppm is hardly recognizable. The further temperature increase does not change the spectrum significantly, only the line shift to -11 ppm is registered. The water leaching tests combined with the 31P NMR determinations of the Mo/P ratios in the several successive extracts (each washing was carried out during 1-3 minutes) show that the phase compositions of the samples calcined at 420-45OoC and above 5OO0C are different. It has been found that besides almost insoluble Moo3 the samples calcined at 500-70!j°C contain a water-leachable phase with Mo/P=l / 1 ratio. P205 is not observed by 'P NMR MAS in these samples. Thus, after Mo-P HPA degradation phosphorus remains in the composition of the MOP phase which in not observed by X-ray phase analysis. We assumed that this phase is molybdenyl pyrophosphate, (Mo02)2P207 described in [24], where it was shown to be X-ray amorphous at temperature below 65OoC. Indeed, the X-ray pattern of the sample calcined at 1000°C exhibits lines typical for molybdenyl pyrophosphate [24]. The phase with M o / P = l / l ratio is already present in the samples calcined at 420 - 45OoC along with Moo3 and another water-leachable phase with Mo/P=12/1 ratio, the former being washed out faster. According to the 31P NMR and DTA data the water-leachable phase with Mo/P=12/1 ratio has a P M 0 ~ 2 0 3 composition ~.~ and is identified with "the new phase" observed by X-ray analysis in the same samples. It is still able to regenerate into Mo-P HPA in the presence of water vapour, but this process is extremely slow and it is only evident in several months of the sample storage at high humidity ( > 80x4. At the - ~ ( ~ -3.7 ] same time on dissolving the phase in water heteropolyanion [ P M O ~ ~ O m) arises immediately and together with (-1:l) another PMo complex observed by "P NMR s' nal at -24.3 ppm. In solutions this complex transforms directly into IPMo~~O~,]! during several weeks at room temperature. We identified it as [PMo12038] , that is the first detected heteropolycation". "

(PMo,2038,5)n phase is likely to be a structural analog of VMoll+x. The latter was indicated upon decomposition of a 12 series silicon-vanadium-molybdenum HPA [ 251 . Thus, considering all the data obtained we can suggest the following pathways of molybdophosphoric HPA thermal decomposition:

Mo%(O.OlP)

+ (MOO~)~P~O,

842

Number of sample

of calcination

Phase composition

I 2 3

350 385 420

H3PMoiz040 PM0120385 (=0 05H20) PMo,,O, ,+MOO? + (Mo0,)2P70,

21

21 7 20 7 14 3

4

530

MOO: + (M002)2P207

1.5

6.4

5

530 530

Moo3 Moo:

2.3 4.4

1.4 0.75

6

Temperature

Surface (m2/g) 30 25

Activity, 1 o9 (mol/m2c) ( ~ - 1 5 ~Tr350°) ~.

w

- Moo3 promoted with P

Fig. 4 shows the change of HPA catalytic properties at 18o0C as a function of the operation time. The initial sample of H ~ P M O , ~1 3~ H, 2~0 . composition was dried in air, then it was heated in the air flow at 180°C and stored at this temperature for 10 min. Then, the reaction mixture was supplied. T h e catalytic properties appear to change significantly with operation time (see Fig. 4): the selectivity towards CO and C 0 2 decreases, the selectivity towards acrylic acid increases, the increase of activity is registered as well. Note, that a pronounced disbalance observed during 1 2 3 time ( h ) 5 the first operation hours gives evidence for the strong acrolein adsorption. Such Fig 4. Selectivities ( S ) towards: acrylic acid an evolution of the catalytic properties (curve 1 -4,CO (curve 2-9, C02 (curve 3-*), probably relates with the degree of acetic acid (curve 4 - 3 ; acrolein conversion (curve HPA hydration. According to X-ray 5) as functions of operation time. to= 180°C, phase and thermal analysis the sample catalyst weight 8.9 g, contact time 9.5 s. H ~ P M O ~13H ~ O0~ ~crystalline . is b .it transforms hydrate before catalytic performance. After a 10 hours operation at 180' C to H ~ P M O ~ ~ O ~ O . ~ . S H ~ O . The samples calcined at 350-38S°C differ considerably with respect to their catalytic properties, though were shown to retain their Keggin primary structure (see the Table and Fig. 5 ) . Th e selectivity towards acrylic acid decreases with the increase in acrolein conversion, while that towards carbon oxides (CO in particular) rises sharply and is the higher the lower is the temperature of the sample thermal treatment. This phenomenon also can relate to HPA hydration degree. The samples calcined at 180-385°C can be easily rehydrated in air and it is difficult to reproduce their initial activity and selectivity during repeated tests. Stationary activities and selectivities are attained after several hours of operation.

843

The samples calcined at temperatures exceeding 4OO0C are rather stable and t h e i r catalytic properties a r e reproducable. They are characterized by high selectivity towards acrylic acid.

l0OC

s (%)

We have successfully separated a phase of molybdenum oxide promoted with phosphorus. For this purpose the sample calcined at 530 O C was washed with water. The effluents and solid sedimen were accurately controlled NMR. (MoO~)~P,O, with "P compound appears to be spread as a film over the surface of the promoted L MOO,, since during the stepwise 6 washing the sample color changed 2 from light-green to grey. The green 20 40 60 Ro 100 solution provided the spectrum corresponding to the Mo:P=l :1 ratio. x(%) The dried solid sediment was Moo3 Fig. 5 . Selectivities (S) towards: acrylic acid (a), according to X-ray phase analysis but CO (0) and C02 (e) as the functions of acrolein possessed the melting point lower than conversion (X) at 350 'C. the reference one (770 against 801'C). Besides, it was even less active and selective than MOO, obtained from ammonia Daramolvbdate: the selectivitv towards acrylic acid was 9'x at the 16% conversion of acrblein. Thus, the cat&tic properties of the samples calcined at 420 and 53OoC are determined by PMo12038.5 and ( M o 0 2 ) 2 P 2 0 7 , respectively. Since according to X-ray analysis the sample calcined at 420°C contains ca. 70 mass.% of P M 0 ~ ~ 0 3 8phase , ~ and its activity is calculated with respect to the overall surface, the activity of this phase appears to be comparable with that of HPA with a high dehydration degree (samples 1,2). The comparison of catalytic properties of HPA and its thermal treatment products shows that the presence of Keggin structure is not responsible for the best catalytic performance of HPA in acrolein oxidation to acrylic acid. The (PMo12038.5)n phase and (Moo2)2 P 2 O 7 compound possess higher selectivity towards icrylic acid than HPA itself. REFERENCES

5.

Misono M., Sakata K., Yoneda Y., Lee W.Y.// Proc. Int. Congr. Catal., 71h., p.1 (1981). Ai M.// J. Catal., V.71, p.88 (1981). Otake M., Onoda T.// Shokubai (Catalyst), V.18, p.169 (1976). Mizuno N.,Watanabe T., Misono. / / Bull Chem. Soc. Jap., V.64, N o l , p.243 (1991). Vazhnova T.G., Korchak V.N., Timoshenko V.I. et al.// Kinet. Katal., V.29, No2, p.392

6. 7.

( 1 988). Ai M. Bull. Chcm. SOC. Jap., V.61, Nos, p.2949 (1988). Kuzzinger K., Emig. G., Hoffman H.// 8th Int. Congr. Catal. Weinheirn, V.5, p.499 (1984).

8.

Black J.B., Scott J.D., Sericka E.M., Goodenough J.B.// J. Catal., V.106, N o l , p.16 (1987).

1. 2.

3. 4.

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9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

Bruckman K., Haber J., Lalik E., Senvicka E.M.// Catal. Lett., V.l, p.35 (1988). Spitsin V.I., Chuvaev V.F., Bakhtchisaraitsev S.A.// Dokl. Akad. Nauk SSSR, V.160, Nos, p.658 (1965). Nikitina E.A., Buns E.V.// Zh. Neorg. Khim., V.2, No3, p.510 (1957). Duval C.// Anal. Chim. Acta, V.20, Nol, p.20 (1959). Jerchkewitz H.G., Alsdorf E., Ficher H. et aL// Z. Anorg. Allg. Chem., B.526, s.73 (1985). Black J.B., Clayden N.J., Gai P.L. et a l J / J. Catal., V.106, Nol, p.1 (1987) Lunk H.I., Chuvaev V.F., Spitsin V.I.// Dokl. Akad. Nauk SSSR, V.247, Nol, p.121 (1978). Misono M.// Catal. Rev.-Sci. Eng., V.29, No2-3 p.269 (1987). Haeberie T.. Emig G.// Chem. and Eng. Technol.. V.11, No6, u.392 (1988). Hideo Orita; Takishi Hayakawa, Masao- Shimizu, Katsuomi Takehira// Appl. Catal., V.77, Nol, p.133 (1990). Rocchiccioli-Deltcheff C.. Amirouche M.. Herve G.. et aL// J. Catal. V.126, No3.. u.591 . ( 1 990). ASTM 5-508. Yurieva T.M., Shokhireva T.H., Goncharova O.I., Boreskov G.K.// Kinet. Katal., V.26, No6, p.1439 (1985). Rabinovitch V.A., Khavin V.Ya.// Reference Book, L.: Khimia", p.85 (1991). Massart K., Contant K., Fruchart J.M., et al.// Inorg. Chem., V.16, p.2916 (1977). Kierkegaard P.// Arkiv Kemi.. B.19, Nol. s.1 (1962). Andruihkevich T.V., Plyasova L.M.,' Kuznetsova T.G., et al.// React. Kinet. Catal. Lett., V.12, No4, p.463 (1979).

V. CortCs Corberan and S. Vic Bell6n (Editors), New Developments in Selecrive Oxrdation II 0 1994 Elsevier Science B.V. All rights reserved.

845

SELECTIVE OXIDATION OF ALDEHYDES OVER V-Mo-OX/SiO2 CATALYSTS J.Machek,J.&achula,J.Tichy-Dept.of Physical Chemistry, University of Chemical Technology Pardubice, Czech Republic L.J.Alemany,F.Delgado,J.M.Blasco-Dept.of Chemical Engineering, University of Malaga , Spain

ABSTRACT Oxidation of acrolein has been studied ove;4+V-Mo-Ox catalysts, differing by the relative amount of which depends on temperature,time and atmosphere of annealing. The most selective catalyst was also used for oxidation of propanal and isobutanal.

INTRODUCTION The Vanadium-Molybdenum oxide system has found a lot of applications in numerous partial oxidation processes [l], including the selective oxidation of acrolein to acrylic acid [2,3,15]. The catalysts with high vanadium content form solid solutions,(M0,V~-~)2O5, as well as some intermediate compounds such as V2MoO8 or V9Mo6040. Enriched molybdenum catalysts are the most active and selective in acrolein oxidation, the active component being VM03O11 [2,4]. The phase composition of V-Mo-Ox catalysts strongly depends on V/Mo ratio, temperature of annealing and presence of reducing or oxidizing atmosphere in the final annealing [ 4 , 5 ] . The activity and selectiv'ty of V-Mo-Ox catalysts are related to the presence of the Vt+ and V5+ sites. The selective oxidation of acrolein require o 1 optimum acidity [6],but also a certain ratio of V sites which seem to be associated with the inlet and VS+' outlet sites of oxygen. The molybdenum specie romote the lattice oxygen, keeping the ratio of V4+ to V'+'sites at a certain value [ 7 ] . The aim of our study was to pre are the Mo-V-Ox/Si02 catalysts with different content of,'4V to analyze their catalytic activity in acrolein oxidation and to study the mechanism of the oxidation of acrolein, propanal and isobutanal.

846

EXPERIMENTAL Catap+st preparation. The catalyst for the study of the effect of v content on acrolein oxidation was prepared by mixing two solutions - the first one containing (NH4)6Mo7024 and the second one NH4V03 and ethylenediamine. Silica (Aerosil) was added to the solution formed and the pH was adjusted to final value 7,5 by adding ammonia under vigorous stirring. The suspension obtained was evaporated in a turbine drier A / S NA (Denmark) at 360 K. The powder catalyst was further annealed in air at 450 K for 6 hours. The chemical composition of the catalyst prepared was as follows : 17,25 %wt. Mo, 1,92 %wt. V, 64%wt. Si02. The catalytic activity in acrolein oxidation was studied on catalyst samples differing by the temperature and the atmosphere of annealing during the final treatment (6 hours), which resulted in diffyent V4+ contents. Determination of relative V amount. The catalyst samples were dissolved in diluted (1:l) sulfuric acid and the relative amount of tetravalent vanadium in the catalyst sample was analysed by potentiometric titration with 0,Ol M Ce(S04)~and colorimetric method at 770 nm wawelenght using a FEK-60 N spectrophotometer . XRD analysis was performed on a Joel JDX-85 diffractometer with LiF monochromator and CuK, radiation. W-VIS DR spectra were recorded by a Jasco double beam spectrometer equipped with an integrating Bas04 as reference. FT-IR spectra were obtained by means of a Perkin Elmer 1720 spectrometer using KBr pellets. Laser Raman spectra were recorded with a Dilor multichannel spectrometer using an Spectra Physic Ar+ Laser (514,6 nm) The catalyst activity tests. The measurements of the catalysts activity in acrolein, propanal and isobutanal oxidation were carried out in a flow apparatus with an integral reactor and over a catalyst with 0,4-0,6 mm grain size. The feed composition and the reaction products were analysed by means of a Chrom 42 gas chromatograph .

.

RESULTS a)Characterization of V-Mo-0, catalysts. In Table 1 the temperature and atmosphere o l + annealing are shown together with the relative amount and the catalytic activity in acrolein oxidation. The VaQ zontent decreases with increasing temperature and time of annealing. The XRD patterns recorded after calcination at different temperatures (6h, 573,603, 623, 673 and 773K) showed the presence of the Moog and vM03O11 phases (characteristic diffraction lines d= 0,400 nm anf+0,356 with nm) and a strong decrease of the relative amount of V increasing temperature of annealing in air. The relative intensities of characteristic lines of VM03O11 were lowered because of the reoxidation of the catalyst bulk. However, the direct catalyst treatment in the reaction mixture for acrolein oxidation (4% C3H40, 7% 02, 17% H20, N2) - catalyst preactivation in the microreactor, resulted in a very high

847

relative amount of V4+ in catalysts (between 90-96 rel.%) not changing with the varying annealing temperature. The continous absorption in the visible region clearly points to the presence of free electrons and consequently to the presence of partially reduced cations sites which is in line with the chemical analysis of the catalyst samples. The higher intensity of this feature than in the Moo3 and V205 samples points to an electronic inte action of Mo and V sites with stabilization of reduced V sites and Mo6+ ones. The intensities of the electronic absorptions at 500 nm and t e wing extending to the lower frequencies increase both with V content [13]. The previous FTIR and Laser Raman Spectroscopy [13] showed that the main absorption does not correspond to the superposition of Moog and V2O5, and this eventually indicates that the oxygen arragement of Moo3 is modified by the presence of vanadium. This is in line with a stabilization of a newly phase structure formed .

'+

:atalyst T(K)

1

TABLE 1

Atmosphere V4+content

573 603 623 673 773 573 603 623 673

8+

air air air air air react.mixt react.mixt react.mixt react-mixt

(%I 57

96

44

84 80 22 7 96 97

28 11 8

95 94 92 90

96

90

b) Acrolein ox'dation. The catalysts preactivated in situ in the reactor (V >90 rel.%) showed high activity in acrolein

i+

oxidation Xacr=90-96% and selectivity to acrylic acid Saa>90% The oxidation of V4+ to V5+ and thus the l o s s of activity was strongly dependent on the temperature and atmosphere during the direct annealing process in the r actor. Under actual reaction conditio s the reduction of V" partially occurs, increasing the V6+ content in the catalyst used (this time previously annealed in air) However at high annealing temperatures (T>623 K) the Vs+ reduction is very difficult because the mobility of V4' ions is so high that the main reaction becomes reoxidati n of catalyst. The catalysts with high relative content of V " and the VMo3011 active component [2] present high activity and selectivity in acrolein oxidation (see Table 1). To compare catalytic activity of V-Mo-Ox/Si02 catalysts for acrolein oxidation [3,5,8] and for the oxidation of the other

848

aldehydes the experimental study of propanal and isobutanal oxidation was carried out u in the catalyst sample L (1.e. the one having the highest V" :ontent). c) The catalytic tests. The measurements of the catalytic activity and the effects of the reactants on the aldehyde oxidation with V-Mo-Ox/Si02 (sample L) were performed in a flow apparatus under reaction conditions, when both the influence of external and internal diffusions were negligible and the oxidation in the gas phase did not occur. The oxidations of acrolein, propanal and isobutanal were found to be independent of their concentration in the range 3-10 % vol., but were strongly dependent on oxygen concentration, and, in acrolein and propanal oxidation, also on the concentration of water (Fig.l,resp.Fig.2).

I Conversion X ( X )

60

40 ,

0 ACROLE I N

'r 0

Fig.1

0 PROPANAL

20 7

0 ISOBUTANAL

4

8 wl.% 0

12

2

0 ISOBUTANAL 0 Fig.2

10 m1.%

20

30

H 0 2

Fiq.1 : Effect of oxyqen concentration on conversions of -ac;olein x c,3propanal xPr y d isobutanal XI), (acrolein :W/F = 0.6 g h?dm ,F = 5 dm /h,4% mol.C3H4OI02 vari ble 17% moJ.Hz0,N2.T = 250°C ;provanal :W/F = 2.0 g h/dm',F 5 dm /h,5% thol.~3~60, 02 varia le,ZO% mo+.H2OlN2;T = 270°C ;isobutanal : W/F = 2.4 g h/dm ,F - 5 dm /h,5% mOl.C4H@ ,02 variable,lO% mol.H20, N2;T = 250°C) Fig.2 : Effect of water concentration on conversions of acro ein,propanal and isobutanal (acrolein : W/F = 0.6.g h/dm4! , F = 5 dm3/h, 4% C3H49, 7% 02, %O variable,N2; T = roDanal :W/F = 2.0 g h/dm ,F = 5 dm /h,5% mol.C3H 0,l % Zol.02.H 0 variable,Nz;T= 270"C;isobutanal : W/F=2.4 g hy dm', F = 5 dm3 /h,5% mol.C4HgO ,8% mol.02,HZO variable,NZ;T= 250°C)

I

9

-

849

The oxidation of acrolein takes place with high selectivity to acrylic acid and only small amounts of carbon dioxide are formed, while in propanal oxidation the propanoic and acetic acid are formed [9]. In opposite during isobutanal oxidation acetone and methacrolein are mainly formed but only small amounts of acids - methacrylic and isobutyric acids [lo]

Fig.3 : Dependence of conversion X and yie d Y on contact time W/F a,acrolein oxidation (F = 5 dm /h,4% mol.C3H4Of7% moj.O2,17% mol.H20fN2.T = 250"C);b,propanal oxidation (F = 5 dm /h,5% mol.CgH6O,lb% mo1.02,2 % mo1.H2OfN2;T = 2 5 0 ° C ) c,isobutanal oxidation ( F = 5 dm /h,5% mol.CqHg0 ,8% mol.02, 10% mol.H20, N2;T = 250°C) Symbols : Acrolein - Acr,acrylic acid - Aa,propanal - Pr, propanoic acid - Pafacetic acid - Aca,isobutanal - Ib,acetone - Ac,methacrolein - Ma.

4

DISCUSSION The V-Mo-Ox catalysts, which are the most active an selective +'V contents in acrolein oxidation, are characterized by high and by the presence of active component VM03O11. This compound can be considered like the final component in a series where Me-0 have uniform distances in an octahedron structure. Two main features characterize this compound :l,the vanadium is completely reduced to four valent oxidation state and 2,it has a loose layer structure that ensures high mobility of oxygen.

850

The catalytic tests in aldehyde oxidation carried out on the catalyst sample L , which is the mixture of active component VM0301~ and Moo3 supported on Si02, can reflect catalytic properties of this active component. The characteristic feature of aldehyde oxidation is strong dependence of acrolein, propanal and isobutanal conversion on oxygen concentration and their independence of aldehyde concentration. This could be explained by supposing redox mechanism of oxidations and that the slowest reaction step in aldehyde oxidation is reoxidation of catalyst by gaseous oxygen [11,8]. The acrolein and propanal conversion is strongly influenced by water present in the reaction mixture. It was found [12] that the effect of water on acrolein oxidation might be explained by the formation of a surface acrylate complex from which the gaseous acrylic acid desorbs easily in the presence of water. Surface propionate complex, which is probably also formed during propanal oxidation, is decomposed by the water in the reaction mixture following desorption of propanoic acid from this complex. It is worth noticing that in acrolein and propanal oxidations the main products are acids - acrylic, propanoic and acetic acid, respectively. Opposite to isobutanal oxidation where only small amounts of acids (isobutyric and methacrylic acid) are formed, while acetone and methacrolein are dominant in the reaction products and water concentration has negligible effect on isobutanal conversion. The nature of aldehyde will determine whether hydride or hydrogen abstraction prevails [14]. In acrolein and propanal oxidations an abstraction of the aldehydic hydrogen atom results in the formation of surface R-COO-Cat complexes. Hydrogen abstraction is easy for acrolein and propanal while hydride abstraction take places in isobutanal oxidation (see scheme I). In acrolein oxidation the surface acrylate complex is stabilised by the formation of a TI - bond which delocalizes along the whole intermediate specie. Propionate surface complex is not able to create such kind of TI - bond and can be attacked by gaseous oxygen in a side reaction forming acetic acid and carbon dioxide. Interaction of isobutanal with the catalyst surface cannot take place producing an isobutyric acid surface complex because of preferable hydride abstraction from secondary carbon atom of molecule. After hydride abstraction the isobutanal surface complex easily reacts with oxygen forming mainly acetone and partially methacrolein.

CONCLUSIONS

1,the V4+ content and the preactivation of V-Mo-Ox catalysts have a strong effect on acrolein conversion 2,the slowest step in acrolein, propanal and isobutanal oxidation is the catalyst reoxidation 3 , water vapor facilitates the transformation of the surface

85 1

complexes to acids and their subsequent desorption in acrolein and propanal oxidations 4,in acrolein and propanal oxidation abstraction of the aldehydic hydrogen from the surface complex takes place opposite to isobutanal oxidation where probably an abstraction of an hydride atom occurs 5,the catalyst surface must contain ions in a high oxidation state able to form complexes with the reactants through ubonds and also ele ents having amphoteric or weak basic properties (such as +'V sites). Thus acid and redox properties are determining' in the catalytic system. In the present catalyst those properties are primarily associated with the molybdenum and vanadium active components. LITERATURE

1.Hucknall D.J. : Selective Oxidation Acad.Press ,London - New York 1974

of

Hydrocarbons,

2.T.V.Andrushkevich,L.M.Plyasova,G.G.KuznetsovafV.M.Bondareva, T.P.Gorshkova,I.P.Olenkova,N.I.Lebedeva : React.Kinet.Cata1. Lett. 12, 463 - 467 (1979) 3.J.Tichy,J.Svachula,J.MachekfN.Ch.Allachverdova :React.Kinet.

Cata1,Lett. 31, 159 (1986) 4.G.G.Kuznetsova,G.K.Boreskov,T.V.Andrushkevich,L.M.Plyasovaf N.G.Maksimov,I.P.Olenkova : React.Kinet.Catal.Lett. 12, 463 467 (1979) 5.T.V.Andrushkev~ch,V.M.Bondareva,G.Y.PopovafL.M.Plyasova New Developments in Selective Oxidation by Heterogenous Catalysis,Studies in Surface Science and Catalysis, V01.72~pp.91-100. 1992 Elsevier Science Publishers 6.V.M.Bondareva,T.V.AndrushkevichIE.A.Paukshtis : React.Kinet. Catal.Lett. 32, 72 (1986) 7.Zhen Xiang Liu,Yu Qing,Shang Xie Qi,Kan Xie,Nai Juan Wu,Qi Xun Bao : Appl.Cata1. 56, 207 (1989) 8.J.Tichy,J.MachekIJ.Svachula : React.Kinet.Catal.Lett.25, 231 (1984) 9.J.Svachula,J.TichyIJ.Machek :Catal.Lett. 3, 257 (1989) lO.J.Machek,J.Tichy,J.Svachula

:React.Kinet.Catal.Lett.

49,

209 (1993) : J.Cata1. 66, 347 (1980) 12.J.Tichy,A.A.Davydov : Col1ect.Czech.Chem.Commun. 48, 834 (19761 i3 . L J Alemany ,M C .Jimenez,E Pardo ,J .Machek ,J .Svachula

ll.J.F.Brazdil,D.D.Suresh,R.K.Graselli

. .

.

.

React.Kinet.Catal.Lett.(accepted) 14.G.C.GrunewaldIR.S.Drago : J.Am.Chem.Soc. 113, 1636 (1991)

15.T.V.Andrushkevich

:

Cata1.Rew.S~.33, 213(1993)

852

SCHEME I ACROLEIN

PROPANAL

I

CHi"CH'C,%

,:. 0 0

Reoxidation of catalyst

ISOBUTANAL

V . Cortcs Corbedn and S. Vic Bell6n (Edilors), New Deveioprnenls in Seieclive Oxidntion I / 0 1994 Elsevier Science B.V. All rights reserved.

853

Diacetyl synthesis by the direct partial oxidation of methyl ethyl ketone over vanadium oxide catalysts. E.McCullaghl, N.C. Rigas*, J.T. Gleaves2 and B.K. Hodnettl* 'Dept of Chemical and Life Sciences, University of Limerick, Limerick, Ireland. 2Dept of Chemical Engineering, Washington University, St. Louis , MO 63 130-4899, U.S.A. ABSTRACT The selective oxidation of butan-2-one to diacetyl has been studied in the temperature range 200-380°C over vanadium oxide and vanadium-phosphorus oxide catalysts. In addition to diacetyl, the principal reaction products detected were acetic acid, acetaldehyde, methyl vinyl ketone, propionaldehyde and carbon dioxide. Detailed steady state and transient kinetic analysis indicate that there are three distinct reaction pathways which lead to the observed product distribution. In the first of these diacetyl and methyl vinyl ketone are formed via a common intermediate, namely acetoin: CH3COCH2CH3 ---->CH3CO(CHOH)CH3 ------>CH3COCOCH3 + CH3COCHzCH2

Evidence for this reaction route include the fact that acetoin was detected as a reaction intermediate in Temporal Analysis of Products (TAP) and when acetoin was fed to the reactor it was converted into diacetyl and methyl vinyl ketone. The second reaction pathway observed involved the oxidation of the enol form of methyl ethyl ketone with the formation of acetic acid and acetaldehyde This reaction predominated at high oxygen partial pressures and represented a significant route away from diacetyl formation in these conditions. The third reaction pathway observed was the decomposition of methyl ethyl ketone to two molecules of acetaldehyde via a diol intermediate: CH3COCH2CH3 ------>CH3(CHOH)(CHOH)CH3 ----> 2 CH3CHO

Evidence that a second pathway was involved in the formation of acetaldehyde emerged from the fact that the molar ratio of acetaldehyde to acetic acid was always greater than unity in spite of the fact that acetic acid was more stable than acetaldehyde in our reaction conditions. The second pathway was confirmed when propiophenone was fed to the reactor. Cleavage of the enol form of propiophenone should lead only to the formation of benzoic acid and never benzaldehyde. In our reaction conditions benzaldehyde was in fact observed in the reaction products, confirming that a molecule bearing the aldehyde functional group could form on the carbonyl side of the substrate. INTRODUCTION

A limited number of studies of the selective oxidation of butan-2-one (methyl ethyl ketone, MEK) to diacetyl (DA) have now appeared in the open literature [l-61. This selective oxidation is of interest from a fundamental point of view because it represents the

854

functionalization of a hydrocarbon in a position alpha to an existing functionality and its practical interest lies in the fact that DA is a food additive with a significant annual production, presently supplied by the reaction of MEK with ethyl nitrite in hydrochloric acid, followed by hydrolysis of the resulting oxime 171. Most of the studies of MEK selective oxidation which have appeared to date emphasise that C-C bond scission products, such as acetaldehyde (AcH) and acetic acid (AcOH), represents the major competing route to diacetyl formation. In addition varying amounts of methyl vinyl ketone (MVK) and propionaldehyde (PrH) have also been reported[ 1-61. A reasonably wide range of oxides have been tested for this reaction and Takita et a1 [2,3] found that DA formation was accelerated on basic or amphoteric oxides such as Co304, NiO, ZnO, and CuO, while the scission products were more important on acidic oxides such as MoO3, V2O5, W03 and Cr2O3. Ai [ 13 has suggested that a peroxy intermediate, namely CH3-CO-HCOOH-CH3 is central to an understanding of this system. He has proposed that breakdown of this intermediate by scission of the 0-0bond and the adjacent C-H bond leads to DA, whereas scission of the 00 bond and the adjacent C-C bond releases equimolar quantities of AcH and AcOH. Other workers[4] have proposed a similar intermediate since they envisage that @-(ads) reacts with MEK to form the peroxy intermediate which subsequently breaks down to either DA on the one hand or AcH and AcOH on the other. However, this postulate cannot explain the full range of products normally observed from this reaction. In particular it not easy to see how this intermediate can lead to the formation of MVK or PrH. In addition the proposed intermediate would predict an AcH : AcOH ration of 1 : 1, whereas many of the studies cited above report different AcH : AcOH ratios [ 1-41. We have also studied this reaction over vanadium oxide catalysts and have postulated that a different series of intermediates is involved in this reaction [5,6]. Evidence for the occurance of these intermediates has been presented already [S-81, but the principal pieces of experimental evidence from that work will be reviewed here and new data will be introduced also.

EXPERIMENTAL VPO and V2O5 catalysts were used in this study. The latter was used as received and two preparation methods were used for the former, namely one based on an organic solvent and another based on oxalic acid as reducing agent. Full details have been presented elsewhere [5]. Below a shorthand notation will be used to refer to the VPO catalysts so that 1.2PV(ox) and 1.2PV(dir) will refer to a VPO catalysts with a P/V atomic ratios 1.2 : 1, prepared by the oxalic acid method and isobutanol methods respectively. The testing apparatus was a continuous flow system operated at ambient pressure. Unless otherwise stated the MEK partial pressure was 12.2 torr and the oxygen partial pressure was 98 torr. Testing was normally carried out in the temperature range 200-350°C. Products were analysed by on-line gas chromatography [5]. Propiophenone and reaction products were insufficiently volatile to be vaporized and as such were delivered through a septum into a heated zone just before the reactor using a Graseby Medical MS26 syringe driver. The propiophenones were dissolved in a sufficient quantity of the inert solvent, n-pentane, to allow 1.64 x lo-' mol min-' of the propiophenone to be admitted in 5 ml of solution per 24 hour cycle. The resultant molar ratios of propiophenone:02:N2 of 1:8:54 entering the reactor, corresponded to partial pressures of 12.2 torr and 95.8 torr for propiophenone and oxygen respectively, in an overall gas flow of 25 ml min-'. The corresponding W/F was 1.2 g s ml-l. Reactor effluent samples were collected for 2 hours in 1 ml of DMF, and subsequently analysed off-line. Product identification was by GCMS. The apparatus consisted of a Hewlett Packard 5890 GC configured with a direct capillary interface to a 5970 Mass Selective Detector, 5970C Chemstation and 7673A

855

Autosampler. Separation was on a 50 m x 0.25 mm i.d. Carbowax 20M column . Spectral identification was aided by reference to the Wiley Library. Full details of the experimental conditions employed for the TAP work have been presented elsewhere [6]. Briefly V2O5 was the only catalyst studied in this way . The catalyst was pretreated in situ by allowing l602 to pass for 0.5 hours at 300°C . Pulse sizes were usually ca. 1017 molecules pulse-'. RESULTS and DISCUSSION Selectivity to DA from MEK over 1.2PV(ox) is presented for two operating temperatures in figure 1. Clearly the process does not appear to be highly selective in the experimental conditions presented here. However, high partial pressures of oxygen in the feed gas favour the C-C bond scission products, and much higher selectivities to DA can be achieved by operating the system in anaerobic conditions [8]. This fact combined with the results of a kinetic study [ 5 ] points to a Mars and van Krevelen type mechanism [9] for DA formation from MEK. This was further verified by the TAP results [6] shown in figure 2. When MEK and l*@ were passed over V2O5 in the TAP reactor the product DA features only 160, obviously emanating from the lattice of the catalyst, in conditions where there was no appreciable exchange between 1 8 a and the lattice [6].

40

0 0

I

I

I

1

20

40

60

80

100

Percentage Conversion of MEK

Figure 1. Selectivity to DA as a function of conversion of MEK over l.ZPV(ox) at 300 (0) and 350°C ( 0 ) .p M E K = 12.2 torr; p AIR = 98 torr; p N Z = 650 torr; W/F = 0.12 1.2 g s ml-1.

-

Figure 3 shows that for each catalysts studied DA formation passed through a maximum as the reaction temperature was increased. Generally speaking this maximum occured at lower temperatures as the reducibility of these catalysts was increased. Hence, the maximum yield occured at the lowest temperature for V2O5 and this compound is much more readily reduced

856

then the VPO catalysts [lo]. There was also a general tendency for DA to decompose more readily as the reaction temperature was increased. When DA was fed into the reactor AcOH was the major decomposition product, particularly above 30OOC. A further feature to emerge from this work was that there was always a striking similarity between the appearance of DA and MVK [ 5 ] . Acetoin (CH3COCHOHCH3) is a hydroxylketone, dehydration of which can lead to an unsaturated ketone,(MVK), and dehydrogenation of which can lead to a diketone (DA). Indeed when this compound was fed to the reactor in typical reaction condition these two products were observed [5], and its involvement in the reaction was confirmed when this intermediate was directly observed in the TAP apparatus [6].

.70E-4

I N T E

m/e = 86 (CH3C160C160C

N

s I T Y

.44N

I

I 0 I I)

m/e = 88 (CH3C160ClsOCH3)

5

' 0

124

249

374

499

SECONDS

Figure 2. Multipulse experiments showing formation of DA in the presence of oxygen-18 over vanadium pentoxide at 300°C[6] Figure 4 illustrates the influence of oxygen partial pressure in the feed on the selectivity to AcH and AcOH. The formation of these products can be readily explained in terms of the oxidative cleavage of the enol form of MEK, in an analogous manner to that observed in conventional organic chemistry. However, this reaction route would predict an AcH : AcOH ratio of 1 : 1, whereas the observed dependence, was always greater than 1 : 1, and dependent of the oxygen partial pressure. These data suggest that oxidative cleavage of the enol form of MEK cannot be the only route to AcH formation. The detection of propionaldehyde in small amounts in the reaction products is further evidence that the enol route alone cannot be the only one, because this cleavage product could never be produced even from the thermodynamically less stable CH2=CHOHCH2CH3 form of the enol. In reality, the ratios of AcH : AcOH shown in figure 4 are overestimates of the true values, because AcH tended to decompose when fed separately to the reactor in typical reaction conditions whereas AcOH was stable.

857

10

a

n c

0

~

9

0

F

8

190

220

250

280

31 0

340

Reaction Temperature / "C

% Yield of DA over (0) l.ZPV(ox), (0) l.ZPV(dir) and ( W ) V 2 0 5 Figure 3: at the temperatures indicated. 40

I

I

I

I

30

s 10

-

-L

0 0

20

40

I

--

60

I

80

100

Oxygen Partial Pressure / torr

Figure 4. Influence of oxygen partial pressure on the selectivity to AcH (0), A c O H ( 0 ) and PrH(W) over 1.2PV(dir) at 300°C. Similar results have been observed during other investigations of MEK oxidation, but explanations have not been put forward. Hence, Takita et a1 [2,3] found that on a series of C0304-SeO2 catalyst AcH appearing at higher selectivities than AcOH. In an oxidizing

858

environment, such as that prevailing in their system, it is difficult to account for this behaviour and they suggest another pathway for AcH formation via: MEK + 0.502 -> 2CH3CHO but no further explanation was offered. Ai[ 11 also discovered this imbalance in favour of AcH but again no explanation was given. Indeed Ai observed that the ratio of AcH to AcOH increased with an increasing conversion. A complicating factor with MEK is that each of the major cleavage products is a C-2 fragment so making mechanistic considerations difficult. To try to overcome these difficulties a series of experiments were designed which replaced MEK with Propiophenone, namely C6H5COCH2CH3. Oxidative cleavage of the enol form of this compound can yield benzoic acid and AcH as the only possible products. Table 1 lists the products observed when this substrate was passed over V2O5 in the presence of gas phase oxygen. Essentially the same range of products was observed when the para position on the aromatic ring of propiophenone was substituted with CH3- or CH3O-.

Table 1 Product identification from Propiophenone oxidation over V 2 0 5 by GCMS analysis.

C6H,COC2H5 C,H,COCH=CH, C6H,COCOCH3 A significant feature of these results is that all the corresponding major products observed during MEK partial oxidation were observed also when propiophenone was used as substrate, indicating that the presence of an aromatic ring on the substrate did not significantly effect the nature of the interaction with the catalysts surface. Another important feature was that benzaldehyde was observed in the reaction products, which cannot be explained on the basis of an enol mechanism only. Instead it is proposed here that a diol intermediate is involved in the reaction network, as shown in scheme 1. The formation of the diol intermediate is envisaged to follow acid catalysed hydration and cleaves oxidatively to form two aldehyde molecules [ 111. In support of this proposed network sufficient Bronsted and Lewis acidity have been associated with the surface of VPO catalysts[l2-141; indeed in the presence of water vapour some Lewis acidic sites convert into Bronsted sites. Finally, diol oxidation is a well known reaction in conventional organic chemistry[151, and when butane-2,3-diol was passed over our catalysts in typical reaction conditions large amounts of AcH formed at low temperatures.

859

I

Scheme 1

This mechanism overcomes the problem associated with the work of Ai [ I J and Yamazoe et al. [4] who reported AcH : AcOH ratios greater than unity, whereas AcOH is more resistant to oxidation than AcH. This postulate now represents a coherent explanation for the imbalance. The presence of PrH and formaldehyde observed in our system, and PrH detected by others[l,4], is also readily explained by this mechanism, namely PrH is formed through hydration of the less favoured enol of MEK,i.e., CH2=CHOHCH2CH3. The general reaction network for the oxidation of MEK may now be written in a complete format and this is shown in scheme 2. This scheme also illustrated why the selectivity to DA is relatively low in this system and provides a useful comparison with several other selective oxidation processes. In any selective oxidation process the ratio of k l : k2 : k3 is critical in determining the overall selectivity. For MEK oxidation the ratio k l : k2 is always low and this value is determined primarily by the ease of formation and subsequent oxidation of the enol. Even at very low levels of MEK conversion the selectivity to DA (or indeed DA + MVK) usually does not exceed 30 mole% so that even in these conditions the competing route of C-C bond scission via an enol route is important, and perhaps is even the primary factor which determines the eventual selectivity. Maintaining the oxygen partial pressure at a minimum value and even working in anaerobic conditions does appear to increase the ratio of k l : k2 because oxidative cleavage of the enol to form AcOH does involve a Langmuir-Hinshelwood type mechanism involving adsorbed molecular oxygen [ 5 ] . DA and MVK formation on the other hand operate through Mars and van Krevelen type mechanisms [ 5 ] . However, the k2 route cannot be completely eliminated because the AcH formation via the diol appears to continue even at very low oxygen partial pressures and even in anaerobic conditions this route still appears to operate. The ratio of k l : k3 is determined primarily by the stability of the reaction product in the reaction conditions. Generally DA does not decompose to an appreciable extent below 250°, so that catalysts which are active at low temperatures lead to low relative values for k3.

860

kl

co;

CH3COCHOHCH3

/-

-H20

CH3COCH2CH3

1

CHCOCH=CHz

(and CHZ=C(OH)CH2CH3)

+H20

CH3CHOHCHOHCH3

I

(and CHZOHCHOHCH2CH3)

-

2 CH3CHO CH3CH2CHO + HCHO

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

M. Ai, J. Catal., 89 (1984) 413. Y. Takita, K. Inokuchi, 0. Kobayashi, F. Hori, N. Yamazoe and T. Seiyama, J. Catal., 90 (1984) 232. Y. Takita, F. Hori, N. Yamazoe and T. Seiyama, Bull. Chem. SOC.Jpn., 60 (1987) 2757. N. Yamazoe, S. Hidaka, H. Arai and T. Seyiama, Oxidation Communications, 4, NOS.1-4 (1983) 287. E McCullagh, J B McMonagle and B K Hodnett, Appl. Catal A: General 93 (1993) 203 E McCullagh, N C Rigas, J T Gleaves and B K Hodnett, Appl. Catal A: General 95 (1993) 183 E McCullagh, Thesis, University of Limerick, 1991 E McCullagh and B K Hodnett, Appl. Catal A: General, 97 (1993) 39 P. Mars and D.W. van Krevelen, Chem. Eng. Sci., (Special Suppl.), 3 (1954) 41 G Poli, I Resta, 0 Ruggeri and F Trifiro, Appl. Catal., 1 (1981) 395 E. McCullagh, J.B. McMonagle and B.K. Hodnett, in 'Heterogeneous Catalysis and Fine Chemicals II', (M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier, Eds.), Elsevier, Amsterdam, 1991. S.J. Puttock and C.H. Rochester, J. Chem. SOC.Faraday Trans. 1, 82 (1986) 2773. S.J. Puttock and C.H. Rochester, J. Chem. SOC.Faraday Trans. 1, 82 (1986) 3013. S.J. Puttock and C.H. Rochester, J. Chem. SOC.Faraday Trans. 1, 82 (1986) 3033. A. Streitwieser and C.H. Heathcock, 'Introduction to Organic Chemistry', McMillan, London, 1976.

V. CortCs Corberan and S . Vic Bellon (Editors), New Developments in Seleclive Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

861

SELECTIVE OXIDATION OF HYDROGEN SULFIDE ON A SODIUM PROMOTED IRON OXIDE ON SILICA CATALYST R.J.A.M. Terorde, M.C. de Jong, M.J.D. Crombag, P.J. van den Brink,

A.J. van Dillen and J.W. Geus. Department of inorganic chemistry, Debye institute, University of Utrecht, P.O. Box 80083, 3508 TB Utrecht, The Netherlands. Abstract In the SUPERCLAUS process H2S is converted to sulfur by selective catalytic oxidation with 0 2 . It is shown that in the part of the silica supported iron oxide catalyst bed where no H2S is present, sulfur can be oxidized to sulfur dioxide, causing a sharp decrease in selectivity. This oxidation is shown to proceed already on the pure silica surface, which shows that no redox couple of the catalyst is needed to oxidize the sulfur to S 0 2 . Addition of sodium to the silica support, turns out to be a possibility to inhibit this reaction, thus resulting in a catalyst exhibiting high selectivities over a much wider range of temperatures.

1.

INTRODUCTION

Hydrogen sulfide, released by desulfurization processes of oil refineries and natural gas plants, is often converted into elemental sulfur by the Claus process. However, due to thermodynamic limitations, 3 to 5% of the H2S feed is not converted to sulfur and has to be dealt with by an alternative process. Catalytic selective oxidation of the remaining hydrogen sulfide to elemental sulfur according to the SUPERCLAUS process has proven to be an attractive procedure to treat Claus tail gas [l-31: H2S + 1/2 0 2

+I/,

S,

+ HzO

(n=6-8)

(1)

In order to provide a high selectivity to sulfur, reactions leading to SO2 have to be suppressed. SO2 production by oxidation can occur by either a consecutive (2) or a parallel (3) route: I/,

s, + 0 2 * so2

862

In addition to these reactions, the hydrolysis of sulfur, i.e. the reverse Claus reaction, can also produce SO5 31,

S,

+ 2 H20 & 2 H2S + SO2

(4)

Because Claus tail gas contains large concentrations of water vapor (up to 30%), establishment of the equilibrium (4)can result in appreciable concentrations of s02.

Iron oxide on silica has proven to be an appropriate catalyst in the catalytic selective oxidation of hydrogen sulfide [3-71. It provides a sulfur yield of about 95 % (figure 1).However, this optimum performance is only established within a narrow range of temperatures. The typical temperature of maximum yield for this catalyst is 240°C. Up to this temperature the H2S conversion is not yet complete, beyond this temperature the sulfur yield drops due to a decrease of the selectivity, i.e., due to the formation of SO2 (figure 1). Because reaction (1) is a very exothermal reaction, it has to be expected that in a large commercial reactor a significant adiabatic rise of temperature will take place. Because of this effect it is difficult to keep the temperature in the lower parts of the catalyst bed from exceeding the temperature of maximum yield. Research on the reason for the mentioned decrease in selectivity is thus of very much interest. In the range of temperatures where not all of the H2S is converted, independent of this conversion the selectivity remains constant. In former publications [4,7] we showed that in this situation, where at least a trace of H# is still present through the whole of the catalyst bed, the formation of SO2 is probably due to the deep oxidation reaction of H2S to SO2 (3), according to the parallel route. We also showed that the exhibited selectivity is affected by the combination of active phase and support material [7], and by the preparation procedure of the catalysts, i.e. the dispersion of the active component [51. However, we never found a catalyst sample exhibiting a strongly decreasing selectivity before the conversion of H2S was complete. This suggests that the strong decrease in selectivity that occurs beyond the temperature of 100% conversion, is determined by another mechanism of SO2 formation. This could be the sequential oxidation of sulfur to SO2 (2), andor the hydrolysis of sulfur (41, because a t these temperatures elemental sulfur is the most available sulfurspecies. 2.

EXPERIMENTAL

Preparation of the catalyst samples The catalyst samples were prepared by impregnation of preshaped bodies of silica. After drying and calcining at 500°C the catalyst contains highly dispersed iron oxide particles; no X-ray diffraction maxima of iron oxide could be

2.1

863

detected. Electron microscopy showed that the catalyst contained iron oxide particles of 2-5 nm that homogeneously covered the surface of the support. A detailed description of the preparation method has already been published El. The unpromoted silica support consisted of extrudates of Aerosil OX50 (Degussa) having a specific surface area, determined by N2 adsorption, of 45 m2/g. The average pore radius was 35 nm, and the pore volume 0.8 cm3lg. The sodium promoted silica extrudates contained 3 wt.% sodium. The texture was comparable to that of the OX50 extrudates. Measurement of catalytic performance The activity and selectivity were measured in a continuous microflow apparatus a t atmospheric pressure. Of the catalyst bodies a sieve fraction (0.420.63 mm) was made. 0.400 g (1ml) of the sieve fraction was placed into a quartz reactor (I.D. 10 mm). These catalyst bed dimensions ensured plug flow conditions. In some of the experiments 0.400g of a sample of pure or sodium promoted silica was placed underneath the catalyst bed in the downflow reactor. For these samples the same sieve fraction as mentioned above is used. On top of the catalyst bed 2 cm of glass spheres were placed to preheat the gasmixture, and to create turbulent gas flow conditions. The feed composition consisted of 1vol.% H2S, 5 vol.% 02, and 30 vol.% H20, and balance He. The oxygen concentration was chosen higher than the stoechiometry given by (1) to prevent the formation of iron sulfide [8].The gas mixture was passed through the catalyst bed at a feed rate of 200 d(stp)/min. The 0 2 , H2S and SO2 content of the effluent was analysed with a gas chromatograph (Carlo Erba 6000) containing a 25 m poraplot Q and 7 m Poraplot U column. The temperature in the reactor was vaned stepwise (10 "C every i8 min) from 180 to 320°C and down again to 180°C. The cycle was performed three times, taking together 27 hours per experiment. This procedure was necessary to establish stable performance of the catalyst [3]. For all of the experiments the presented performance results describe the third downward part of the temperature cycle. 2.2

3.

RESULTS A N D DISCUSSION

3.1 The sodium promoted iron oxide on silica catalyst. Figure 1shows the typical performance of an iron oxide on silica catalyst; the conversion of H2S and the selectivity to sulfur. The resulting sulfur yield, the product of conversion and selectivity, can be read from this figure. While the low yield a t low temperatures is determined by the activity of the catalyst, the decreasing selectivity causes the yield to drop a t higher temperatures.

864

"Y 10

190

m

220

240

260

zm

3m

TeJq3ab.m("0

Figure 1: The performance of the 5 wt.% Fe,O, on silica (0x50)catalyst. -0-H,S conversion, selectivity to

a-

sulfur

320

im

m

220

240

260

zm

303

320

Tempab.m("O

Figure 2: The performance of sodium promoted 5 wt.% Fe203on silica catalyst. -0H2S conversion, -0-selectivity to sulfur, compared to the standard catalyst: -0-H,S conversion, Iselectivity to sulfur

This drop of selectivity observed for this sample is due to the oxidation of sulfur to SOZ. This reaction apparently only takes place when there is no HzS available, i.e. when the conversion has become 100%. When the temperature of the catalyst is raised, the part of the catalyst bed that becomes devoided of H2S broadens. This means that an ever growing part of the catalyst meets with the conditions in which the formed sulfur can be converted to S02. Application of sodium promoted silica instead of pure silica as a support for the iron oxide shows a different behaviour. Figure 2 shows the performance of such a catalyst. Also the catalyst from figure 1 is plotted in this figure. The selectivity curve of the sodium promoted sample shows a different behaviour. The selectivity remains high over a wider range of temperatures; also at temperatures at which H2S is already completely converted the selectivity remains high. This indicates that the oxidation of sulfur t o SO2,is suppressed by the presence of the sodium additive. Also another effect is observed in figure 2. The apparent activation energy of the oxidation of HzS has changed. For the sodium promoted catalyst an apparent activation energy of 58 kJImole can be calculated, while for the unpromoted catalyst this is 65 kJImole. This lower apparent activation energy for the promoted catalyst is already indicated in figure 2 by the smaller ramp of the conversion curve. The activation energy a t this part of the temperature range has been investigated thoroughly [4]. The reoxidation of the catalyst is pointed out as the rate determining step, and the measured activation energy is ascribed to this reaction. A change of the activation energy in this step, thus results in a change in reactivity of the iron species, responsible for the redox reaction. A reaction between the sodium- and the iron species could possibly be causing this effect.

865

From these experiments is it not clear what causes the positive effect that sodium addition has on the selectivity behaviour of the catalyst. In some way it keeps the sulfur from being oxidized to SO2 when the catalyst bed becomes devoided of HzS. Possibly this effect is related to the change in apparent activation energy t h a t is observed at lower temperatures. An acid-base effect of the sodium on the support surface can also be involved.

The influence of the support on the formation of SOz. When a sample, which performance is shown in figure 1, is placed on top of a second sample, the composition of the flow entering this second sample can be found in figure 1. At temperatures higher that 240°C no H,S will be present anymore and only sulfur, a small concentration of SO, a n d water vapor will be fed to the second sample. First we will discuss the results obtained with the bare support, silica, a s the second sample. These results are contained in figure 3a, for reason of comparison together with the results already shown in figure 1. 3.2

Bare silica (OX 50)

I0

0

LYI

LW

220

240

260

Tempwature1%)

-.-

EC

300

320

Figure 3a: The performance of a 5 wt.% Fe203 conversion, selectivity. on silica catalyst, -0the same catalyst with a bed of pure silica placed underneath; -Gconversion, and selectivity

-*-

Figure 3b: Experimental setup of the cspcriments represented by the -0a n d -4-symbols in figure 3a.

The most striking difference between both experiments is t h e selectivity at temperatures beyond 240°C. When silica is p u t underneath the catalyst bed, a much sharper decrease in selectivity is observed. Obviously conversion of sulfur to SO2 takes place in the silica bed, resulting in a lower selectivity. This reaction does not occur at temperatures below 240°C where not yet all of the HBS is converted in the upper catalyst bed; the selectivity curves of both of the experiments coincide, showing t h a t no extra SO, is formed. This is remarkable because a t these temperatures there are already appreciable amounts of sulfur

866

available. The conversion curves show no differences a t all, indicating that oxidation of H2S does not take place in the silica bed.

I

Standard iron oxide on silica catalyst. Sodium promoted silica

Downflow gasstream

i 1m

m

m

240

260

Temperabse(Q

-.-

280

300

321)

Figure 4a: The performance of a 5 wt.% Fe203 on silica catalyst, -0conversion, selectivity. the same catalyst with a bed of sodium promoted silica placed underneath; -Aconversion, and -A-selectivity

Figure 4b: Experimental setup of the experiments represented by the -Aand -Asymbols in figure 4a.

Figure 4a shows the experiment where instead of pure silica, a sodium promoted silica support sample is underneath the catalyst bed. The performance of this combination does not show any difference to the performance of the separate catalyst sample. This shows that on sodium promoted silica, in contrast to pure silica, no sulfur is converted to S 0 2 . An explanation for the formation of SO2 from sulfur on pure silica could be the proceeding of the reverse Claus reaction (4).However in that case also H2S would be formed. As we mentioned earlier, the oxidation of H2S doesn't occur on pure silica. This means that, if the Claus reaction would be responsible for the extra SO2 formation, H2S would actually have to be found, thus resulting in a conversion lower than 100%. Between 260 and 300°C this effect is actually seen in all of the experiments. The re-produced H2S results in a conversion slightly lower than 100%. In figure 3a it can be seen that this effect is slightly bigger for the experiment where bare silica is underneath the catalyst bed, than for the experiment with only the catalyst. This means that reverse Claw is indeed involved. However, it can only explain a small part of the SO2 formation. Another explanation for the formation of SO2 from sulfur could be the oxidation of sulfur. However, because no redox couple is available on pure silica, molecular oxygen should be the oxidizing reactant. Apparently silica is capable of activating sulfur to some extend. Highly reactive sulfur radicals could very well be the reactive sulfur species formed. It is known in literature that elemental sulfur, because of the pi-bonding between the sulfur atoms, behaves as a Lewis-

867

base [9]. An electrofilic attack of a Lewis-acid could cause the cleavage of a sulfur-sulfur bond, and the formation of sulfur radicals [lo]. It is perhaps peculiar that a relatively inert support like silica, can exhibit strong acidic sites. However, the fact that addition of sodium to silica causes the observed effect, strongly suggests an acid-base effect. Sodium is often added to a catalyst to introduce basic sites or to annihilate acidic sites. The presence of sodium at the silica surface possibly suppresses the formation of highly acidic sites, because of the formation of sodium sulfate, that does not show strong acidity. In this way the formation of sulfur radicals could be suppressed on sodium promoted silica. Unfortunately we were not able to identify these acidic surface sites on silica yet. The reaction of sulfur radicals with oxygen is not unknown in literature. Steyns [Ill studied the reaction of oxygen with polymeric sulfur. He proposed reaction scheme ( 5 ) for the reaction of oxygen with sulfur radicals. It shows that only when there is insufficient H2S, SO2 will be formed. In this scheme it is assumed that H2S will be present as S,H, which is very probable under our reaction conditions [4,12,13].

Although Steyns studied the reaction at temperatures beneath 180°C, reaction ( 5 ) could very well be valid our system. Van den Brink [41 proposed that the -S,-S-0. intermediate from (5), might react with water to give SO2 and H2S; an oxygen assisted hydrolysis of sulfur:

-S,-S-0. -SP2

+ HzO

+-S,H2 + SO2 -S,-1+ + H2S

-j

This would explane the connection between the oxidation of sulfur to SO,, and the occurence of the reverse Claus reaction, as we observed in our experiments.

4.

CONCLUSIONS

It is shown that the addition of sodium to the iron oxide on silica catalyst has a possitive effect on the catalyst performance. The formation of SO2 from sulfur, viz. the sequential oxidation reaction, on an unpromoted catalyst takes place as soon as all of the H2S converted. By using sodium as a promotor this reaction is shifted to higher temperatures.

868

It appears that the positive effect of the sodium is not related to its effect on the active phase, iron sulfate, but to its effect on the silica. It is shown that in the absence of H2S the oxidation of sulfur can readily take place on bare silica. Because on silica no redox couple is available, it is very probable that in the mechanism of this reaction sulfur radicals that can react with molecular oxygen are involved. The formation of these radicals can probably be subscribed to the occurence of highly acidic sites on the silica surface under reaction conditions. Addition of basic sodium, annihilates these sites and thus suppresses the formation of sulfur radicals. ACKNOWLEDGEMENTS GASTEC N.V. Apeldoorn is greatly acknowledged for their financial support. 5. 1. 2. 3

4 5.

6. 7. 8.

9. 10. 11. 12 13

REFERENCES P.H. Berben, A. Scholten, M.K. Titulaer, N. Brahma, W.J.J. van der Wal and J.W. Geus, in Surf. Surf. Sci. Catal. (Catalyst Deactivation), 34, 303-316, (1987). B.G. Goar, J.A. Lagas, J . Borsboom and G. Heijkoop, Sulfur, 220, 44-47, 1992. P.J.van den Brink, AScholten, A.J. van Dillen, J.W. Geus, E. Boellaard, and A.M. van der Kraan, in Stud. Surf. Sci. Catal. (Catalyst Deactivation), C. Bartholomew (Eds.), Elsevier, Amsterdam, 68, 515-522, (1991). P.J. van den Brink, Ph.D. thesis, University of Utrecht, The Netherlands, (1992). P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen and J.W. Geus., in Stud. Surf. Sci. Catal. (Preparation of Catalysts V), B. Delmon, P.Grange, P.A. Jacobs, and G. Poncelet (Eds.), Elsevier, Amsterdam, 63, 527-536, (1990). P.J. van den Brink, R.J.A.M. Terorde, J.H. Moors, A.J. van Dillen and J.W. Geus, in Stud. Surf. Sci. Catal. (Selective Oxidation), G. Poncelet and B. Delmon (Eds.), Elsevier, Amsterdam, 72, 123-132, (1991). R.J.A.M. Terorde, P.J. van den Brink, L.M. Visser, A.J. van Dillen and J.W. Geus, Catal. Today 17,217-224, (1993). P.H. Berben, Ph.D. thesis, University of Utrecht, The Netherlands, (1992). Schmidt, in elemental sulfur chemistry and physics, B. Meyer (ed.), Interscience Publishers, New York, (1965). Meyer, Chem Rev. 76, 367, (1976). M. Steyns, F. Derks, A. Verloop, P. Mars, J . Catal., 42, 96, (1976) J.B. Hyne, E. Muller, and T.K. Wiewiorowski, J. Phys. Chem. 70(11),3733, (1966) F. Feher and G. Winkhaus. Z. Anow. Allg.. Chem 292.210. (1957).

V. Cortes Corberan and S. Vic Bellon (Editors), New Developments in Selecrive Oxidation 11 0 1994 Elsevier Science B.V. All rights reserved.

869

E f f e c t of morphology o f hone comb SCR c a t a l y s t s on t h e r e d u c t i o n o f NOX and t h e oxi a t i o n o f SO2

5

A. Beretta*, E. Tronconi*, L.J. Alemany**, J. Svachula***, P. Forzatti*

* Dipartimento d i Chimica I n d u s t r i a l e e Ingegneria Chimica del Politecnico, P.zza L. d a Vinci 32, 20133 Milano (Italy). **

O n leave from Department of Chemical Engineering, Campus Teatino, 29071 University of Malaga (Spain).

*** On leave from Department of Physical Chemistry, University of Chemical Technology, 53210 Pardubice (Czechoslovakia). W e present a systematic s t u d y of t h e influence of morphological p r o p e r t i e s of monolithic SCR c a t a l y s t s on NOx reduction and SO2 oxidation, including optimization of t h e pore s t r u c t u r e with respect t o t h e i n d u s t r i a l c o n s t r a i n t s on both NH3 s l i p and SO3 formation. 1. INTRODUCTION

Denitrification processes based on Selective Catalytic Reduction (SCR) of NOx with NH3 a r e gaining widespread application i n many countries all over t h e world. Commercial c a t a l y s t s for t h e SCR processes a r e based on V205-W03/Ti02 systems. They are employed i n monolithic s t r u c t u r e s with honeycomb matrices o r i n plate-type form. These c a t a l y s t s a r e required t o e x h i b i t high DeNOx activity, high s t a b i l i t y and low SO2 oxidation activity. In f a c t , SO3 produced by oxidation of SO2 present i n t h e f l u e gases r e a c t s with NH3 and H 2 0 t o form ammonium sulfates, which may deposit i n t h e cold equipments downstream of t h e SCR reactor. This problem is s o important t h a t i n d u s t r i a l specifications f o r SCR processes include upper l i m i t s on t h e o u t l e t concentration of SO3, corresponding t o SO2 conversions as low as 1 - Z%, and on t h e NH3 s l i p typically less t h a n a few ppm. Installation of SCR DeNOx u n i t s involves huge capital investments, and t h e r e is a s t r o n g economic motivation t o optimize t h e performances of t h e commercial catalysts. Though its origins d a t e back t o t h e ‘ ~ O S , only i n recent years papers aimed at rationalizing t h e SCR process have appeared i n t h e scientific l i t e r a t u r e . Attention h a s been focused on t h e analysis of t h e monolithic SCR r e a c t o r , which o p e r a t e s under essentially isothermal conditions because of t h e v e r y low concentrations of reactants. Buzanowski and Yang [ 11 have presented a simple analytical one-dimensional model which however is n o t appropriate f o r i n d u s t r i a l SCR plants operating with substoichiometric NH3/NOx r a t i o s and with large monolith channel openings. Beekman and Hegedus [2] have published excellent work on modelling of SCR monolith reactors: once validated, t h e i r model has been applied t o s e a r c h t h e optimal morphological properties of t h e catalyst, and t h e computed r e s u l t s have provided guidance f o r t h e development of a new c a t a l y s t exhibiting t h e expected activity improvement. However, t h e

870

a u t h o r s assumed very simple f i r s t - o r d e r kinetics f o r t h e SO2 oxidation reaction. A recent systematic study of SO2 oxidation over monolithic SCR c a t a l y s t s i n o u r laboratory [3] has pointed out t h a t t h e kinetics a r e actually more complex, involving both inhibiting (NH3) and promoting (NO) effects due t o interactions with t h e species involved in t h e DeNOx reaction. Thus, t h e SO2 oxidation kinetics may be indirectly affected by t h e pore s t r u c t u r e , too, since it influences t h e intraporous concentration gradients of NO and NH3. In previous papers w e have reported t h e development of a comprehensive mathematical model of the SCR reactor. First, multidimensional modelling of t h e monolith channels [4] has been applied t o investigate t h e role of external (gas-solid) diffusion in order t o clarify t h e influence of channel geometry and of t h e reaction kinetics on t h e interphase mass t r a n s f e r process. I t has been shown t h a t a simple lumped parameter model based on t h e analogy with t h e Graetz problem with constant wall temperature yields adequate predictions of NO conversions f o r r e a l i s t i c SCR plant conditions, and reproduces successfully published d a t a on t h e influence of flow r a t e and channel size on NO reduction. In a subsequent paper [5], account of intraporous resistances has been introduced by an approximate analytical solution suitable f o r specific Rideal SCR kinetics, and t h e lumped reactor model validated against experimental d a t a on a predictive basis, a f t e r determining independently t h e i n t r i n s i c SCR kinetics over a catalyst ground t o very f i n e particles t o prevent diffusional intrusions. The r e s u l t s have confirmed t h e adequacy of t h e model i n predicting t h e experimental effects of monolith length, area velocity (AV), reaction temperature and NH3/NO feed r a t i o (a) on NO reduction efficiency [ 6 ] . In t h i s work t h e SCR reactor model is extended t o include oxidation of SO2 t o SO3 on t h e basis of realistic kinetics. Then, a systematic analysis is performed on t h e influence of t h e catalyst morphological properties on both NOx reduction and SO2 oxidation. Finally, optimization of t h e catalyst pore s t r u c t u r e is discussed in t h e light of t h e i n d u s t r i a l c o n s t r a i n t s on NH3 s l i p and on SO2 conversion. 2. SCR REACTOR MODEL

The mathematical description of t h e SCR reactor used in t h e following r e s t s on some simplifying assumptions: a ) conditions a r e identical in a l l t h e monolith channels ( t h e case of non-homogeneous distribution of t h e reagents h a s been already t r e a t e d in [ S ] ) ; b) t h e r e a c t o r is isothermal due t o t h e high dilution of t h e reagents: about 500 pprn NO, 400 ppm NH3, 1000 ppm S 0 2 ; c) axial mass diffusion phenomena a r e negligible with respect t o t h e convective contribution; d) developing laminar flow is assumed in the i n l e t section of t h e monolith channels. For t h e present purposes, our treatment of diffusion and reaction inside t h e monolithic reactor has been extended t o include both t h e DeNO, reaction and SO2 oxidation. The r e l a t e d kinetic and diffusional aspects are briefly summarized below. 2.1 NO,

reduction

In l i n e with t h e hypothesis of an Ealy-Rideal mechanism (reaction between adsorbed ammonia and gas-phase NO), we have adopted t h e r a t e expression: RDeNOx = kDeNOx CNOx

*

KNH3 CNH3 /

* KNH3 CNH3)

(1)

87 1

where kDeNOx is t h e i n t r i n s i c kinetic constant and K N H ~is t h e adsorption equilibrium constant of ammonia. Equation (1) accounts f o r t h e experimentally confirmed f i r s t o r d e r kinetics i n NO and zeroth order in NH3 (if K N H ~C N ~ 3 > > lb)u, t is able t o reproduce t h e limiting dependence on ammonia in t h e case of substoichiometric feed r a t i o s ( a = CNH3/C0N0<1),a s used i n i n d u s t r i a l practice t o prevent formation of ammonium bisulfate. Due t o t h e high i n t r i n s i c r a t e of reaction, t h e DeNOx process s u f f e r s from strong diffusional limitations both in t h e gas and within t h e solid phase, s o t h a t only a t h i n superficial layer of t h e channel wall is active a s denoxing catalyst [5]. The Wakao-Smith "random pore model" of intraporous diffusion in solids with bimodal pore s t r u c t u r e has been adopted, a s it well s u i t s t h e morphology of present commercial SCR catalysts. Given t h e morphological properties of t h e catalyst (micro and macropore radius, micro and macropore volumetric fractions), such a model provides estimates of t h e effective diffusivities of NO and NH3. The knowledge of both t h e effective diffusivities and t h e kinetic constants contribute t o evaluate t h e efficiency f a c t o r of t h e reaction according t o an analytical expression presented i n [ 51. Concerning diffusion of t h e reagents from t h e gas phase t o t h e catalytic surface, a specific s t u d y [ 4 ] h a s confirmed t h e adequacy of a "lumped" description which simplifies t h e gas-phase radial concentration profiles of t h e reagents a s s t e p functions and yields t h e radial flow a s t h e product of a m a s s t r a n s f e r coefficient (local Sherwood number, Sh) and of t h e difference between bulk and w a l l values of t h e concentration. Invoking t h e analogy with t h e corresponding heat t r a n s f e r problem, a proper estimate of t h e axial profile of Sh has been found in t h e available solutions of t h e well-known Graetz-Nusselt problem with constant wall temperature and developing laminar flow. 2.2 SO2 oxidation Based on a systematic s t u d y of SO2 oxidation over SCR catalysts, t h e following kinetic expression w a s derived [3]: RS02->S03 = k l cS02

+

Lk2 cS02 cS03(1

+

k3 CNO)I/DEN

(2)

In Equation (2), t h e first t e r m accounts f o r a low residual activity unaffected by s a t u r a t i o n o r inhibition of t h e catalytic s i t e s with f i r s t o r d e r dependence on SO2 concentration. The second, discussed in detail in [ 3 ] , r e l i e s on t h e hypothesis of a redox reaction mechanism: t h e catalytic sites involved a r e supposed t o be dimeric vanadium oxide sulfates. Such a mechanism can explain both t h e long conditioning t i m e s required by t h e catalyst to reach a steady-state a c t i v i t y and t h e variable order i n SO2 feed content experimentally found f o r t h e reaction r a t e : it increases with SO2 concentration a t low SO2 feed contents due t o formation of additional active s i t e s , b u t decreases f o r SO2 contents exceeding 200 ppm due t o surface s a t u r a t i o n effects. Equation (2) is also able t o reproduce all t h e o t h e r observed effects of feed composition: kinetic r u n s have shown in f a c t t h a t SO2 conversion is asymptotically independent of oxygen, depressed by water, strongly inhibited by ammonia, and slightly enhanced by NO. Diagnostic calculations have pointed t h a t t h e overall oxidation process is not limited by e i t h e r i n t e r o r intraphase diffusional

812

resistances, as opposite t o t h e DeNOx reaction. Dedicated experiments have confirmed i n f a c t t h a t SO2 oxidation occurs i n t h e whole catalytic volume. 2.3 M a s s balances The SCR r e a c t o r model consists of: i) t h e differential gas-phase balances of NO and NH3 along t h e axial coordinate (combining axial convective and t r a n s v e r s e diffusional fluxes): ii) t h e algebraic continuity equations f o r NO and NH3 across t h e gas-solid interface; iii) t h e axial d i f f e r e n t i a l m a s s balance of SO2 i n t h e gas phase; iv) t h e global m a s s balance of SO3 along t h e channel axis. Notably, SO2 and SO3 concentrations are assumed t o be constant over t h e channel cross section due t o t h e absence of i n t e r p h a s e gradients. 0 2 and H 2 0 concentrations a r e taken as constant throughout t h e reactor, due t o t h e i r large excess. The mathematical description refers to a single monolith channel r e p r e s e n t a t i v e of t h e e n t i r e r e a c t o r , according t o assumption a) above. Solutions of t h e 6 equations yields axial profiles of both NO conversion (i.e. NOx reduction efficiency) and SO2 conversion once t h e following input parameters a r e provided: a) t h e geometry of t h e monolith channel; b) feed composition, flow r a t e , T and P; c) t h e kinetic constants included i n t h e r a t e expressions (1) and (2): d) t h e morphological properties of t h e catalyst. 3. INFLUENCE OF THE CATALYST MORPHOLOGICAL PROPERTIES ON NOX AND

SO2 CONVERSION.

The s t u d y h a s assumed a "high d u s t " SCR honeycomb catalyst with s q u a r e channel openings 6 mm wide and a 1.4 mm w a l l thickness. For s t a n d a r d operating conditions (T=380"C, where estimates of t h e kinetic c o n s t a n t s and of t h e effective diffusivities were available [5], P = l atm, AV=33 Nm/h, AV standing f o r t h e r a t i o flow-rate/monolith geometric area, a=0.8), we have investigated t h e dependence of both NO conversion and SO2 conversion on t h e pore s i z e d i s t r i b u t i o n of t h e catalyst. In line with l i t e r a t u r e indications, o u r parametric s t u d y w a s so constrained: t h e c a t a l y s t morphology w a s t a k e n as bimodal with micro and macro pore r a d i i ranging between 40.f-13011 and 700&5000.f, respectively. Such intervals include values typical of commercial catalysts, as w e l l as more extreme values f o r investigation purposes; t h e t o t a l volumetric porosity w a s limited t o 0.7 v/v. Fig. 1 shows t h e calculated NO conversion as a function of t h e volume f r a c t i o n of micropores Emi, with t h e micropore r a d i u s as a parameter. Each c u r v e e x h i b i t s a maximum located n e a r Emi=0.35, resulting from t h e superposition of two opposite contributions: t h e increment of t h e surface area with growing E m i and of t h e diffusivity coefficients with growing ems, respectively. Concerning t h e effect of t h e pore size (Figures 1 and 2 ) f o r a fixed value of Ems, t h e NO conversion increases with growing rma (which f a v o u r s diffusion) and decreases with increasing rmi (which reduces t h e s u r f a c e area). Consequentely, t h e maximum conversion is achieved a t Emi=0.35, rmi=40.f, rma=5000W. Our r e s u l t s a r e i n agreement with those r e p o r t e d i n a s i m i l a r SCR c a t a l y s t design work [ 2 ] . On t h e o t h e r hand, SO2 conversion t u r n s o u t t o be uniquely controlled by s u r f a c e area. In f a c t , as i l l u s t r a t e d i n Fig. 3, catalyst morphologies with d i f f e r e n t pore size d i s t r i b u t i o n s b u t comparable surface areas yield t h e same values of SO2 conversion. Figure 3 shows t h a t such a dependence is manifest i n t h e case of both excess (a>l)and defect ( a c l ) of

873

NH3; we can a l s o appreciate t h e inhibiting effect of ammonia upon SO2 conversion which decreases heavily when unreacted NH3 becomes available inside t h e c a t a l y t i c w a l l (a>l). I t is s o confirmed t h a t , even though t h e r a t e of SOz oxidation is a function of t h e i n t e r n a l profiles of NO and NH3 concentrations (commonly subjected t o diffusional resistances), SO2 conversion does n o t s u f f e r from physical limitations. This is hardly surprising, as it must be noted t h a t under steady-state conditions, except f o r a t h i n s u p e r f i c i a l c a t a l y s t layer, t h e i n t e r n a l NO and NH3 profiles a r e f l a t a c r o s s most of t h e w a l l thickness, and a r e governed by stoichiometric c o n s t r a i n t s (with CNH3 and CNO = 0 i n t h e case of a < l and a > l , respectively), independent of t h e c a t a l y s t morphology. Since SO2 and SO3 a r e n o t affected by i n t r a p o r o u s limitations, too, t h e global effect is t h a t t h e r a t e of SO2 conversion is v i r t u a l l y independent of t h e pore size d i s t r i b u t i o n f o r fixed values of s u r f a c e area. Quite a d i f f e r e n t behaviour is revealed when simulating t h e case of equimolar feeds of NO and NH3 (a=l). Figure 4 shows i n f a c t t h a t catalyst morphologies with equal surface a r e a s r e s u l t i n SO2 conversions increasing with growing Ems. Such d a t a indicate t h a t t h e r a t e s of i n t e r n a l diffusion of NO and NH3 acquire h e r e an important r o l e i n determining t h e SO2->SO3 activity, which is controlled by t h e extension of NH3 i n t e r n a l gradients. The trend i n Figure 4 can be rationalized considering t h a t t h e effective diffusivity of NO is favoured t o a g r e a t e r e x t e n t than t h a t of NH3 by incrementing t h e f r a c t i o n of macropores i n t h e catalyst. Accordingly, t h e average i n t r a p o r o u s NH3 concentration, a s well as its inhibiting effect on SO2 oxidation, decreases upon incrementing gma. For a = l , then, high SO2 conversions r e s u l t from a compromise between high surface a r e a s and l a r g e f r a c t i o n s of macropores; consequentely, i n t h i s special case, t h e highest SO2 conversions a r e no longer associated with t h e highest surface a r e a s ( E m i = l ) b u t with Emi=0.8, rmi=4OA, rma=5000A. On t h e contrary t h e desired lowest conversions still correspond t o t h e lowest surface areas. 4. OPTIMAL DESIGN OF SCR CATALYSTS

We have t h e n addressed t h e constrained optimization of t h e SCR c a t a l y s t morphology, adopting t h e area velocity AV as t h e objective function t o be maximized, with c o n s t r a i n t s on NH3 s l i p and SO2 conversion of 2 ppm and Z%, respectively. Notably, f o r assigned gas flow and geometric section of t h e monolith channels, t h e search of t h e maximum AV s t a n d s f o r t h e minimization of t h e c a t a l y s t volume. The following operating conditions have been assumed as r e p r e s e n t a t i v e of t h e i n d u s t r i a l practice: T=380 "C, P = l atm, a=0.8. The last condition, combined with t h e ammonia s l i p c o n s t r a i n t , makes t h e required NO conversion equal t o 79.6%. We have also fixed t h e macropore r a d i u s at 5000& having t h e parametric study indicated t h a t tmi and rmi a r e responsible f o r t h e strongest effects upon NO and SO 2 conversions. Figure 5 shows AV level curves; each one identifies on t h e E r n i / r m i plane t h e s e t of morphological configurations t h a t ensure NO conversion =0.796 f o r t h e same value of c a t a l y s t volume. It is apparent t h a t f o r a fixed c a t a l y s t load t h e desired NO conversion can be achieved taking advantage of e i t h e r good d i f f u s i v i t i e s (high macropore void fraction, l e f t p a r t of t h e curves) o r l a r g e s u r f a c e a r e a s (high micropore void fraction, r i g h t p a r t of t h e curves). A s expected, t h e maximum AV (=11.7 Nm/h) corresponds t o t h e optimal c a t a l y s t morphology defined i n t h e previous section, which allows savings of about 17 % on t h e catalyst volume with

874

respect t o a reference morphology with monomodal pore size distribution and rmi=130 A. The optimal DeNO, morphology does s a t i s f y t h e constraint SO2 conversion 5 2%. On t h e contrary, f o r AV values lower t h a n 11.7 Nm/h t h e morphologies (increasing i n number with decreasing AV) t h a t s a t i s f y t h e NH3 s l i p c o n s t r a i n t a r e associated t o wide ranges of possible SO2 conversions. On t h e AV level curves i n Figure 5, points corresponding t o 1%,1.5% and 2% conversion levels of SO2 a r e displayed. I t is apparent t h a t SO 2 conversion decreases along each curve proceeding from t h e r i g h t t o t h e l e f t , since c a t a l y s t s u r f a c e a r e a s decrease along t h e same direction, too. The dependence of SO2 conversion on t h e pore s t r u c t u r e a t constant AV is represented more comprehensively by Figure 6, where t h e curve f o r NO conversion =0.796 and t h e s e t of curves f o r constant S O 2 conversion have been drawn.

5. CONCLUSIONS The DeNOx efficiency depends on both i n t e r n a l diffusional resistances and s u r f a c e area of t h e catalyst; f o r a t o t a l porosity of 0.7 v/v, t h e parameters of t h e optimal bimodal pore s t r u c t u r e are: Emi=0.35, rmi=40A, rma=5000A, which o f f e r t h e b e s t compromise between t h e values of t h e effective d i f f u s i v i t i e s of t h e r e a c t a n t s and of t h e catalyst surface area, and r e s u l t i n a =: 17% reduction of t h e catalyst volume with respect t o a monomodal catalyst. However, t h e optimum is apparently not very sharp: o u r calculations suggest t h a t nearly optimal conditions can be achieved by a v a r i e t y of d i f f e r e n t pore s t r u c t u r e s , associated, on t h e o t h e r hand, with remarkably d i f f e r e n t values of SO2 conversions. In f a c t , SO2 conversion is always essentially proportional t o t h e c a t a l y s t s u r f a c e area, s o t h a t t h e influence of c a t a l y s t morphology is s t r o n g e r t h a n on NOx reduction, SO2 conversions spanning over an order of magnitude f o r t h e conditions of o u r study. When t h e feed r a t i o a is d i f f e r e n t from 1, t h e SO2 conversion is unaffected by diffusional limitations and does n o t change with t h e c a t a l y s t morphological properties f o r a fixed s u r f a c e area, even though SO2 conversion is much lower i n t h e case a>l t h a n f o r a
875

5. E. Tronconi, P. Forzatti, J.P. Gomez Martin, S. Malloggi, Chem. Eng. Sci., 47 (1992) 2401. 6. J. Svachula, N. Ferlazzo, P. Forzatti, E. Tronconi, F. Bregani,

1nd.Eng.Chem.Res. 32 (1993) 1053. % NOX C o n v e r s i o n

Figure 1.: Calculated percent NO conversion v e r s u s micropore void fraction, for d i f f e r e n t values of micropore r a d i u s

70.001 57 50 6 5 . 00

62.50 60.00 57 . 5 0

I

%

I

I

55.00 0.00

1

J

I

0.20 0.40 0.60 0.80 Void F r a c t i o n of Micropores

l.( 10

NOx Conversion

66.50

".'%OO

Figure 2. : Effect of macropore r a d i u s on NO conversion f o r fixed micropore r a d i u s (rmi = 40 A) and void fraction (Emi=0.215).

' 1500

% SO,

2500 3500 4500 Macropore Radius [ A )

5500

Conversion

Figure 3. : Dependence of SO2 conversion on catalyst surface area when a is different from 1; AV = 11 Nm/h.

3 50 3 00

2 50 2 00

1 50 1 .oo

0 50 0 00

0

50

100

150

S u r f a c e Area

200 (m2/cm3)

250

300

876 % SO,

Conversion

Figure 4.: Dependence of SO2 conversion upon both surface area and pore volume distribution when a=l. AV 11 Nm/h

3.50 3.00 2.50

€mi

€mi

€mi

2.00

--

0.428 0.714 1.

1.50

1 .oo

0.50 0.00

0

50

100

150-

Surface Area M i c r o p o r e Radlus

200 (m2/cm3)

250

300

(A)

Figure 5. : Level curves of t h e objective function AV = F(rmi,Emi,NOconv.=79.6%). Upon each curve symbols identify t h e combinations of (rmi, corresponding t o selected values of SO2

0

Void F r a c t l o n o f Micropores Micropore Radius 130

120 110

100 90 80 70 60

50 4#

(8)

Figure 6. : The lines represent combinations of (rmi, €mi) corresponding t o specified values of SO2 conversion at AV=10.6 Nm/h. For t h e same AV, t h e curve, representing combinations of (r,i, €mi) t h a t provide NO conversion = 79.6 %, is also displayed.

877

Author Index Abon. M........... . . . . . . . . . . . . . . 67 41. 795 Acosta. D .......... . . . . . . . . . . Agterberg. F.P.W. . . . . . . . . . . . . . . . . 639 Aguilar ElguezAbal. A . . . . . . . . . . . . . . 729 Alemany. L.J. . . . . . . . . . . . . . . . 845. 869 293 Anderson. A . . . . . . . . . . . . . . . . . . . . Andrushkevich. T.V. . . . . . . . . . . . . . . 837 337 Anshits. A.G. .................... 769 Aramendia. A .................... Arutyunov. V.S. . . . . . . . . . . . . . . . . . . 435 Augugliaro. V . . . . . . . . . . . . . . . . 693. 713 143 Auroux. A . . . . . . . . . . . . . . . . . . . . . . Awasarkar. P.A. . . . . . . . . . . . . . . . . . 795 Bacherikova. I.V. . . . . . . . . Badrinarayan. S. . . . . . . . .

. . . . . . . . . . . . . 749

. . . . . . . . . . . . . 167 561 Baiker. A . . . . . . . . . . . Barbaux. Y . . . . . . . . . . . . . . . . . . 151. 419 Baronetti. G.T. . . . . . . . . . . . . . . . . . . 411 Barrault. J . . . . . . . . . .

Bouqueniaw. D . . . . . . . . . . . . . . . . . . 419 Brandao. S.T. . . . . . Brkgeault. J.M. . . . . Buijs. W. . . . . . . . . . Busca. G . . . . . . . . . . . . . . . . . . . . 253. 777

. . . . . . . . . . . . . 759 Capitan. M.J. . . . . . . . . Carvalho. W.A. . . . . . . . . . . . . . . . . . . 647 Castellani. F. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795 . . . . . . . . . . . . . . . 411 . . . . . . . . . . . . . . . 713 . . . . . . . . . . . . . . . 93 Centeno. M.A. . . . . . . . . . . . . . . . . . . . 377 . . . 281. 461 Centi. G . . . . . . 515 Chen. J.D. . . . . . . . . . . . . . . . . . . . . . .

Basevich. V.Y. . Bautista. F.M. . . . . . . . . . . . . . . . . . . 759 819 Belkouch. J . . . . . . . . . . . . . . . . . . . . . . 377 Benitez. J.J. . . . . . . . . . . . . . . . . . . . . . 869 Beretta.A . . . . . . . . . . . . . . . . . . . . . . Bergna. H.E. . . . . . . . . . . . . . . . . . . . 233 507 Bernal. S. . . . . . . . . . . . . . . . . . . . . . . 685 Bes. T. . . . . . . . . . . . . . . . . . . . . . . . . . 507 B1anco.G . . . . . . . . . . . . . . . . . . . . . . . 845 Blasco. J.M. . . . . . . . . . . . . . . . . . . . . .

Contractor. R.M. . . . . . . . . . . . . . . . . . 233 113. 537 Corma. A . Coronado. J.M. . . . . . . . . . . . . . . . . . . 803 Coronas. J . . . . . . . . . . . . . CortCs CorberBn. V . . . . . .

. . . . . . . . 551 Cucinieri Colorio. G . . . . . .

. . . . . . . . . . . . . . 829

Bosma. E.J. . . . . . . . . . . . . . . . . . . . . .

183

Dakka. J . . . . . . . . . . . . . . . . . . . . . . . . 515 Davidova. N . . . . . . . . . . . . . . . . . . . . . 403 55 Daza. L. . . . . . . . . . . . . . . . . . . . . . . . . Dejoz. A . . . . . . . . . . . . . . . . . . . . . . . . 113 845 Delgado. F. . . . . . . . . . . . . . . . . . . . . . 67 Delichbe. P. . . . . . . . . . . . . . . . . . . . . . 41. 55 Delmon. B. . . . . . . . . . . . . . . . . . . .

878 Dissanayake. D . . . . . . . . . . . . . . . . . . .345 Driessen. W.L. . . . . . . . . . . . . . . . . . .639 367 Driscoll. S.A. .................... 571 Duprey. E. . . . . . . . . . . . . . . . . . . . . . . Ebner. J.R. . . . . . . . . . . . . . . . . . 167. 203 243 Emig.C . . . . . . . . . . . . . . . . . . . . . . . . Eon. J.-G. .................... Fernkndez-Lafuente. R . . . . . . . . . . . . . 685 Fierro. J.L.G. . . . . . . . . . . . . . . . . . . . 103 387 Finol. C. . . . . . . . . . . . . . . . . . . . . . . . Foresti. E. . . . . . . . Forget. D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 Forlani. 0. . . . . . . . . Forzatti. P . . . . . . . . Fumagalli. C. . . . . . . . . Gama Freire. F. . . . . . . . . . . . . . . . . . . 31 . . . . . . . 814 Gambaro. L.A. . . . . .

Hirose. T. . . . . . . . . . . . . . . . . . . . . . . Hodnett. B.K. . . . . . . . . . . . . . . . . . . . ... Horowitz. h.S. . . . . . . . . . . Hronec. M . . . . . . . . . . . . . . . . . . . . . . Hutchings. G.J. . . . . . . . . . . . . . . . . . .

593 853 233 667 213

Uett. D.J. . . . . . . . . . . . . . . . . . . . . . . . Inoue. Y. . . . . . . . . . . . . . . . Islam. M.S. . . . . . . . . . . . . . . . . . . . . . Itoh. K. . . . . . . . . . . . . . . . . . . . Ito. s. . . . . . . . . . . . . . . . . . Iwamoto. M . . . . . . . . . . . . .

427

Jalowiecki-Duhamel. L. . . . . . . . . . . . . JimCnez Mpez. A . . . . . . . . . . . . . . . . . .... JimBnez. C. . . . . Johnstone. A . . . . . . . . . . . . . . . . . . . . Jong de. M.C. . . . . . . . . . . . . . . . . . . . ...... Joshi. P.N. . . . . Jun. Ki-Won . . . . . . . . . . . . . . . . . . . .

419 103 769 609 861 395 659

427

Garcia. A . . . . . . . . . . . .

. Gengembre. L. Geus. J.W. . . . . . . . . . .

. . . . . . . . . . 233

. . . . . . . . . . 151

Gleaves. J.T. . . . . . . . . . . . . 203. 481. 853 Goldenberg. M.Ya. . . . . . . . . . . . . . . . 435 Golinelli. G . . . . . . . . Gbmez-Moreno. C. . . . . . . . . . . . . . . . 685 . . . . . . . . . . 41 Gotor. F.J. . . . . . . . .

Grzybowska. B. . . . . . Guamieri. F. . . . . . . . . . . . . . . . . 281

Kimura. Naomasa . . . . . . . . . . . . . . . . 271 Kim. Seong-Bo . . . . . . . . . . . . . . . . . . 659 .. . . . . 451 Kitano. T. . . . . . . . . . . . . . . . . . 787 Klissurski. D . . . . . Komashko. G.A. . . . . . . . . . . . . . . . . . 265 Kondaridez. D.I. . . . . . . . . . . . . . . . . . 471 . . . . . . 337 Kondratenko. E.V. . . . 403 Kovacheva. P . . . . . . . . . . . . . . . . . . . . Kovalenko. G.A. . . . . . . . . . . . . . . . . . 675 Krahembiihl. C.E.Z. . . . . . . . . . . . . . . 647

Kuldarni. M.P. . . Kunieda. M . . . . . . . . . . . . . . . . . . . . .

703

GuisBn. J.M. . . . Haber. J . . . . . . . . . . . . . . . . . . . . 265. 739 243 Hacker. C.-J. . . . . . . . . . . . . . . . . . . . . 293 Hansen. S. . . . . . . . . . . . . . . . . . . . . . Harrison. P.R. . . . . . . . . . . . . . . . . . . . 609 Hashimoto. M . . . . . . . . . . . . . . . . . . . 583 Hecquet. G . . . . . . . . . . . . . . . . . . 819. 829 499 Henriques. C. . . . . . . . . . . . . . . . . . . . Hermann. J.M. . . . . . . . . . . . . . . . . . . . 31 75 Hermann. K. . . . . . . . . . . . . . . . . . . . . . 167 Hess,N.J. . . . . . . . . . . . . . . . . . . . . . .

Lazarescu. V . . . .

. . . . . . . . . . . . . . . 667 837 Litvak. G.S. . . . . . . . . . . . . . . . . . . . . .

879

Lbpez Nieto. J.M. . . . . . . . . . . . . . . . . 113 L6pez.Muiioz. M.J. . . . . . . . . . . . . . . . 693 Lorenzelli. V . . . . . . . . . . . . . . . . . . . . 253 Luna.D . . . . . . . . . . . . . . . . . . . . . . . . 759 . . . . . . . . . . . . . . . . 345 Machek. J . . . . . . . . . . . . . . . . . . . . . . . 845 Maciejewski. M . . . . . . . . . . . . . . . . . . 561 Magaud. L. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .749 Maksimovskaya. R.I. . . . . . . . Malaguti. M . . . . . . . . . . . . . . . . . 461 Malet. P. . . . . . . . . . . . . . . . . . . . . 377 Mallat. T. . . . . . . . . . . . . . . . . Mamedov. A.Kh. . . . . . . . . . . Mamedov. E.A. . . . . . . . . . . . . . . 125. 395 Mantegazza. M.A. . . . . . . . . . . . . . . . . 541 Martinez. A . . . . . . . . . . . . . . . . . . . . . 537 Martino. R . . . . . . . . . . . . . . . . Massardier. J . . . . . . . . . . . . . . . . . 67 Matsuura. Ikuya . . . . . . . . . . . Maza-Rodriguez. J . . . . . . . . . . . . . . . . 103 Mazzocchia. C. . . . . . . . . . . . . . . . . . 499

Odriozola. J.A. .

........

Oevering. H . . . . . . . . . . . . . . Olier. R . . . . . . . . . . . . . . . . . . . . 213 0livera.Pastor. P. . . . . . . . . . . . . . . . . 103 Oliveri. G . . . . . . . . . . . . . . . . . . 253 Otsuka. K. . . . . . . . . . . . . 57. 703 . . . . . . . . . . . . . 183 . . . . . . . . . . . . . 367

. . . . . . . . . . . . . 541

PCrez Om& J.A. ............ PCrez.Pariente. J . . . . . . . . . . . . . . . . . Pesheva. Y . . . . . . . . . . . . . . . . . . . . . . Petrini.G . . . . . . . . . . . . . . . . . . . . . . . Pintado. J.M. . . .

507 537 787 541

Pramauro. E . . . . . . .

Quaranta. N . . . . . . . . . . . . . . . . . 803. 814 Quin. Dujie . . . . . . . . . . . . . . . . . . . . . 603 Quirbs. R.A. . . . . . . . . . . . . . . . . . . . . 759

MenCndez. M . . Middleton. P.J. . . Millet. J.M.M. . . . . . . . . . . . . . Millini. R . . . . . . . . . . . . . . . . .

Mlodnicka. T. . . . . . . . . . . . . . . . . . . . Monceaux. 1. . . . . . . . . . . . . . . . . . . . . Mori. M . . . . . . .

Ramis. G . . . . . . . . . . . . . . . . . . . 253. Reedijk. J . . . . . . . . . . . . . . . . . . . . . . . Requejo. F . . . . . . . . . . . . . . . . . . . . . . Rigas. N.C. . . . . . . . . . . . . . . . . . . . . .

653 819

Nakai. T. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41. 55 Naud. J . . Navio. J.A ............ Neeleman. E . . . . . . . . . . . . . . . Nilsson. R . . . . . . . . . . . . . . . .

777 639 803 853

. . . . . . . . . . . . . . 571

. . . . . . . . . . . . . . 345 Roullet, M. . . . . . . . . . . . . . . . . . . . . . . 67 Ruiz, J.R. . . . . . . . . . . . . . . . . . . . . . . 769 Ruiz, P . . . . . . . . . . . . . . . . . . . 41, 55, 795

880 Sakharov. A.M. . . . . . . . . . . . . . . . . . .629 571 Salles.L. . . . . . . . . . . . . . . . . . . . . . . . SananCs. M.T. . . . . . . . . . . . . . . . . . . . 213 Shnchez Escribano. V . . . . . . . . . . . . . 777 Sanderson. W.R. . . . . . . . . . . . . . . . . .609 443 Sanfilippo. D . . . . . . . . . . . . . . . . . . . . 387 Santarnaria. J . . . . . . . . . . . . . . . . . . . . 387 Santos. A . . . . . . . . . . . . . . . . . . . . . . . 443 Santucci. A . . . . . . . . . . . . . . . . . . . . . . Sasaki. K . . . . . . . . . . . . Scelza. O.A. . . . . . . . . . . Schrader. Glenn L. . . . . . . . . . . . . . . . .19 . . . . . . 551. 647 Schuchardt. U . . . . . .

845 Tichy. J . . . . . . . . . . . . . . . . . . . . . . . . . 739 T0karz.R . . . . . . . . . . . . . . . . . . . . . . 305 Tournow. M . . . . . . . . . . . . . . . . . . . . 749 Trautrnann. S. . . . . . . . . . . . . . . . . . . . 93. 221 Trifirb. F. . . . . . . . . . . . . . . . . . . . 869 Tronconi. E . . . . . . . . . . . . . . . . . . . . . Tsang. S.C. . . . . . . . . . . . . . . . . . 315. 327 Uihlein. K. . . . . . . . . . . . . . . . . . . . . . Urbano F.J. . . . . . . . . . . . . . . . . . . . .

.

243 769

Valencia. S. . . . . . . . . . . . . .

. . . . . . . . . . . . . . 861 Seifullayeva. J.M.

. . . . . . . 495

Shamilov. N.T. . . . . Versluijs.He1der. M . . . . Sinev. M.Yu. . . . . Sisler. G.M. . . . . Skibida. I.P. . . .

. . . . . . . . . . . 357

. . . . . . . . 787 Suzuki. H . . . . . . . . . . . . . . . . . . . . . . . Svachula. J . . . . . . . Svoboda. G.D. . . . .

615

Takaki.U . . . . . . . . . . . . . . . . . . . . . . . 615 451 Takehira. K . . . . . . . . . . . . . . . . . . . . . Talyshinskii. R.M. . . . . . . . . . . . . . . . 125 819 Ta0uk.B . . . . . . . . . . . . . . . . . . . . . . . Teitel’boirn. M.A. . . . . . . . . . . . . . . . . 435 Terorde. R.J.A.M. . . . . . . . . . . . . . . . . 861 Thomas. H.J. . . . . . . . . . . . . . . . . 803. 814 Thompson. M.R. . . . . . . . . . . . . . . . . . 167 571 Thouvenot. R . . . . . . . . . . . . . . . . . . . .

. . . . . . . 183

Villalba. V. . . .

Wang. Goujia . . . . . . . . . . . . . . . . . . . 603 Warringa. P.A. . . . . . . . . . . . . . . . . . . 183 151 Wcislo. K. . . . . . . . . . . . . . . . . . . . . . . 403 Weiss. A.H. . . . . . . . . . . . . . . . . . . . . . Witko. M . . . . . . . . . . . . . . . . . . . . 75. 739 195 Wolf. G.-U. . . . . . . . . . . . . . . . . . . . . . 603 Wu. Yue . . . . . . . . . . . . . . . . . . . . . . . Xiong. Y.L. . . . . . . . . . . . . . . . . . . . . . .

41

Yamarnoto. Y. . . . . . . . . . . . . . . . . . . . 615 703 Yarnanaka. I . . . . . . . . . . . . . . . . . . . . . York. A.P.E. . . . . . . . . . . . . . . . . 315. 327 ZBhonyi.Bud6. E. . . . . . . . . . . . . . . . . Zazhigalov. V.A. . . . . . . . . . . . . . . . . . Zecchina. A . . . . . . . . . . . . . . . . . . . . . Ziborov. A.V. . . . . . . . . . . . . . . . . . . . Zoughebi. S. . . . . . . . . . . . . . . . . . . . . Zou . Joe Y . . . . . . . . . . . . . . . . . . . . . . .

623 265 541 837 571 19

881

Subject Index ab initio calculations . . . . . . . . . . . . . . 167 ab initio cluster model . . . . . . . . . . . . . . 75 Acrolein oxidation . . . . . . . . . . . . 837. 845 Acrolein. from propane . . . . . . . . . . . . 305 Acrylonitrde. from propane . . . . . . . . . 281 Aldehydes oxidation . . . . . . . . . . . . . . 845 Aliphatic carboxylic acids . . . . . . . . . . 639 Auryl aromatic oxidation . . . . . . . . . . . 739 ALPO, catalysts . . . . . . . . . . . . . . . . . . 759 Alumina-boria . . . . . . . . . . . . . . . . . . 143 Aluminium niobate. as support . . . . . . . 83 Ammonia oxidation . . . . . . . . . . . . . . . 541 Antimony-tellurium oxides . . . . . . . . . . 55 Aromatics formation. from methane . . 315 Atomistic computer simulations . . . . . 427 Attrition resistance . . . . . . . . . . . . . . . 233 Aurivillius phase . . . . . . . . . . . . . . . . . 305 Bacteria immobilization . . . . . . . . . . . . 675 Barium carbonate . . . . . . . . . . . . . . . . 315 Benzene oxidation . . . . . 221. 451. 551. 703 Bimetallic catalysts . . . . . . . . . . . . . . . 561 Binary bismuth oxides . . . . . . . . . . . . . 395 675 Biocatalysis . . . . . . . . . . . . . . . . . . . . . Biphasic epoxidation of 2-alkenes . . . . 571 Bismuth-molybdenum on TiO, . . . . . . 305 Butane oxidehydrogenation . . . . . . . . . 113 n-Butane oxidation . . . . . . . 167. 183. 195. . . . . . . . . . . . . . . . . . . 213.233. 243. 265 1-butene oxidation . . . . . . . . . . . . . 19. 461 C, hydrocarbons adsorption . . . . . . . . . 253 C.C. paraffins oxidehydrogenation . . . 125 C..C. hydrocarbons oxidation . . . . . . . 221 Carbon whisker cathode . . . . . . . . . . . 703 Catalyst activation . . . . . . . . . . . . . . . . 203 Catalyst regeneration . . . . . . . . . . . . . 243 Cesium sulfate . . . . . . . . . . . . . . . . . . . 749 Cinnamyl alcohol oxidation . . . . . . . . . 561 Circulating fluidized bed reactor . . . 1. 233 CO. as oxidant . . . . . . . . . . . . . . . . . . 159 Copper complexes . . . . . . . . . . . . . . . . 629 Copper phosphate . . . . . . . . . . . . . . . . 451 Cyclohexane oxidation . . . . . . . . . 647. 659 Cyclohexanol dehydrogenation . . . . . . 769 Cyclohexene oxidation . . . . . . 583. 593. 603 583 Cyclopentene . . . . . . . . . . . . . . . . . . . . CH./CD. isotopic exchange . . . . . . . . . 345

Chromia pillared clay . . . . . . . . . . . . . 103 Chromia-silica . . . . . . . . . . . . . . . . . . . 315 Chromium molecular sieves . . . . . . . . . 515 Deactivation of bimetallic catalysts . . . 561 Dehydrogenation of cyclohexanol . . . . 769 Diacetyl synthesis . . . . . . . . . . . . . . . . 853 Dioxygen as selective oxidant . . . . . . . 685 Doping effects. . . . . . . . . . . see Promotion Effect of dopants in lanthana . . . . . . . 427 Effect of water vapor . . . . . . . . . . . . . . 845 Electrocatalytic oxidation of benzene . . 703 Enzymatic processes . . . . . . . . . . . . . . 685 Epoxidation of ethylene . . . . . . . . . 471. 481. 495. 499 of other olefins . . . . . . . . . . . . 571. 593 ESR of V (VI) in doped NaVO. . . . . . 327 ESR of Mo-Ti-Al-0 . . . . . . . . . . . . . . 803 ESR of V-P-Ti-0 . . . . . . . . . . . . . . . . 729 ESR of oxygen radicals in TiO. . . . . . . 693 Ethane oxidehydrogenation . . . . . . . . . . 93 Ethane partial oxidation . . . . . . . . . . . 143 Ethanol oxidation . . . . . . . . . . . . 795. 803 Ethylbenzene oxidehydrogenation . . . . 759 Ethylene epoxidation . . . 471. 481. 495. 499 EXAFS of Sm-Al oxides . . . . . . . . . . . 377

Ferredoxin-NAPD-reductase . . . . . . . . 685 Fluidized bed reactor . . . . . . . . . . . . . 387 Fluorene oxidation . . . . . . . . . . . . . . . 749 Formaldehyde. from methane . . . . . . . 357 FTIR of pyridine adsorption . . . . . . . . 103 FTIR of C, hydrocarbons adsorption . . 253 FTIR of 2-propano1 adsorption . . . . . . 777 Fuel cell . . . . . . . . . . . . . . . . . . . . . . . 703 Furan. from 1-butene . . . . . . . . . . . . . . 19 Gallium oxide-faujasite . . . . . . . . . . . . 133 Gallium oxide reduction . . . . . . . . . . . 133 GoAgg"' system . . . . . . . . . . . . . . . . . . 647 Heterogeneous catalysts for liquid phase oxidation . . . . . . . . . . 515, 537. 541. 561 Heterogeneous photacatalysts . . . . . . . 721 Heteropoly compounds . . . . . 571. 593. 603 Heteropoly tungstates . . . . . . . . . 571. 593 Heteropolyacids . . . . . . . . . . . . . . . . . 583

882

Heteropolyacids thermolysis . . . . . . . . 837 Heteropolycation . . . . . . . . . . . . . . . . . 837 1-hexene oxidation . . . . . . . . . . . . . . . 537 Homogeneous copper catalysts . . . . . . 639 Homogeneous metal catalysts . . . . . . . 667 Honeycomb catalysts . . . . . . . . . . . . . 869 HRTEM studies . . . . . . . . . . . . . . . . . 507 Hydrogen peroxide as oxidant 541.571. 583 . . . . . . . . . . . . . . . . . . 609. 647. 659. 667 Hydrogen sulphide oxidation . . . . . . . . 861 Hydroquinone . . . . . . . . . . . . . . . . . . . 703 Hydroxylamine. from ammonia . . . . . . 541 Hydroxylation mechanisn . . . . . . . . . . 551 829 Ilvaite . . . . . . . . . . . . . . . . . . . . . . . . . Immobilization of enzymes . . . . . . . . . 685 In situ XRD of V-P-0 . . . . . . . . . . . . . 183 In situ XRD of heteropolyacids . . . . . . 837 In situ SERS (surface enhanced Raman spectroscopy) . . . . . . . . . . . . 471 Indigo synthesis . . . . . . . . . . . . . . . . . . 615 Indole selective oxidation . . . . . . . . . . 615 Influence of support pretreatments . . . 125 Iron (111) chloride . . . . . . . . . . . . . . . . 647 Iron-chromium-molybdenum oxides . . . 787 Iron hydroxysilicates . . . . . . . . . . . . . . 829 Iron molybdate . . . . . . . . . . . . . . . . . .795 Iron-molybdenum-antimony oxides 4 1. 795 Iron oxide-silica . . . . . . . . . . . . . . . . . . 86 1 Iron-palladium bi-catalytic system . . . . 659 Iron phosphate . . . . . . . . . . . . . . 357. 819 Isobutene oxidation . . . . . . . . . . . . . 41. 55 Isobutyric acid . . . . . . . . . . . . . . . 819. 829 Isolated V5+ions . . . . . . . . . . . . . . . . . 113 Isopropanol oxidation . . . . . . . . . 721. 777 Isotopic labelling . . . . . . . . . . . . . . . . . 367 Kinetic simulation . . . . . . . . . . . . . . . . 436 Lanthana . . . . . . . . . . . . . . . . . . . . . . . Li/CaO . . . . . . . . . . . . . . . . . . . . . . . Li/MgO . . . . . . . . . . . . . . . . . . . . . . . Liquid phase epoxidation . . . . . . . . . . Lithium acetate . . . . . . . . . . . . . . . . . .

427 337 4 11 593 551

Magnesium ortophosphate . . . . . . . . . 769 Maleic anhydride . . . . . . . . . . 19. 195. 203. . . . . . . . . . . . . . . . . . . . . . . 213.221. 243 commercial scale process . . . . . . . . . 233 Manganese oxide . . . . . . . . . . . . . . . . . 315

Manganese molybdate . . . . . . . . . . . . . 367 . . . . . . . . . . . . 387 Membrane reactor 2-mercaptobenzotiazole (MBT) . . . . . . 667 Methacrylic acid . . . . . . . . . . . . . 819. 829 Methane . . . . see Oxidative Coupling of Methane Methane activation by CO. . . . . . . . . . 159 Methane oligomeration . . . . . . . . Methane partial oxidation . . . . . . 357. 675 Methanol oxidation . . . . . . . . Methyl ethyl ketone. synthesis Methyl ethyl ketone (MEK) ox MgO/ZSM-S . . . . . . . . . . . . . . . . . . . 403 Mixed valence compounds

Molybdena-titania . . . . . . . . . . . Molybdena-titania-alumina . . . . . . . . . 803 Molybdenum oxide . . . . . . . . . . . . . . . . 67 Molybdenum complexes . . . . . . . . . . . 615 Molybdenum-ruthenium . . . . . . . 609 Monocarboxylates synthesis . . . . . . . . . 629 Morphology of SCR catalysts . . . . . . . 869 Mossbauer spectra . . . . . . . . . . . . 787. 829 Multicomponent molybdates . . . . . . . . . 19 N. 0 as oxidant . . . . . . . . . . . . . . Na/CaO . . . . . . . . . . . . . . . . . . . . . . . Ni/Al.O. catalysts . . Nickel-Vanadium-Ant

337

. . . . . . . . . . . . . . . . . . . 125 . . . . . . . . . . . . 19

NMR MAS spectra

..

Oxidative coupling of methane . . . . . 327. 337. 345. 357. 367. 377. 387. 395. . . . . . . . 403.411.419. 427. 436. 436. 443 . . . . . . . . . . . 609 Olefin cleavage . . . . . . . . . . . 615 Organic hydrope Oxidative dehydrogenation of ethane . . . . . . . . . . . . . . 103. 143. 159 of ethanol . ...... 814 of ethylbenzene . . . . . . . . . . . . . . . . 759 of isobutyric acid . . . . . . . . . . . 819. 829 of paraffins . . . . . . . . . . . . . . . 113. 159 . . . . . . . . . 83. 151. 159 sm . . . . . . . . . . . . . . 67 869 Oxidation of SO, . . . . . . . . . . . . .

883 Oxidative decarboxylation . . . . . . . . . . 639 Oxidative condensation . . . . . . . . . . . . 667 Oxygen adsorption . . . . . . . . . . . . . . . . 471 Oxygen defects . . . . . . . . . . . . . . . . . . 337 Oxygen migration . . . . . . . . . . . . . . . . 427 Oxygen radicals . . . . . . . . . . . . . . . . . . 693 Oxygen spillover . . . . . . . . . . . . . . . . . 795 Oxygen storage . . . . . . . . . . . . . . . . . . 203 Oxygen species . . . . . . . . . . . . . . 419. 499 Oxygen-18 labelling . . . . . . . . . . . . . . . . 67 Oxygenation of carbohydrates . . . . . . . 629 Palladium acetate . . . . . . . . . . . . . . . . 551 Palladium-copper salts . . . . . . . . . . . . 45 1 Palladium-vanadia on alumina . . . . . . . 461 345 Peroxide ions . . . . . . . . . . . . . . . . . . . . Phase transfer catalysis . . . . . . . . 571. 609 Phase contamination . . . . . . . . . . . . 41. 55 Phase cooperation effect . . . . . . . . . . . 293 Phenol. from benzene . . . . . . 451. 551. 703 Phenol photooxidation . . . . . . . . . . . . . 693 Phosphate catalysts . . . . . . . . . . . . . . . 357 Phosphates. Tetragonal type . . . . . . . . 271 Phosphato-oxoperoxotungsten complexes . . . . . . . . . . . . . . . . . . . . 571 Photodegradation . . . . . . . . . . . . . . . . 713 Photooxidation . . . . . . . . . . . . . . . . . . 721 Photooxidation mechanism . . . . . . . . . 693 Phthalic anhydride . . . . . . . . . . . . . . . . 221 . 659 Polymer-supported catalysts Polyols oxidation . . . . . . . . . . . . . . . . . 629 Process simulation . .

Promotion by cerium . . . . . . . . . . . . . . 357 Promotion by cesium . . . . . . . . . . 749. 819 Promotion by lanthanum 265 777 Promotion by potassium Promotion by sodium . . . . . . . . . 403. 861 . 729 Promotion by vanadium 1. 293 Propane ammoxidation . . . . . Propane oxidation . . . . . . . . . . . . 271. 305 Propane oxidehydrogenation . . . . 113. 133 ......................... 271. 305 2-propanol oxidation . . . . . . . . . . . . . . 777 Propylene allylic oxidation . . . . . . 1. 67. 75 Propylene epoxidation . . . . . . . . . . . . . 675 Pu1se.studies. butane oxidation . . .

.

Quantum chemical description . . . . . . 739 Raman spectra of gallium-faujasite . . . 133 Raman spectra of supported molybdates 19 Reaction modelling . . . . . . . . . . . . . 1. 869 Reactor simulation . . . . . . . . . . . . . . . . . 1 Recyclable catatalysts for liquid phase 515 oxidation . . . . . . . . . . . . . . . . . . . . . Redox molecular sieves . . . . . . . . . . . . 515 Reduction of VOP0,.2H2O . . . . . . . . . 213 Remote control . . . . . . . . . . . . . . . . 41. 55 Rhenium oxide . . . . . . . . . . . . . . . . . . 609 Rhodium on ceria . . . . . . . . . . . . . . . 507 Ruthenium chloride . . . . . . . . . . . . . . . 609 Samarium-aluminium oxides . . . . . . . . 377 SCR reactor model . . . . . . . . . . . Selectivity control . . . . . . . . . . . . . . . . 357 Sepiolite . . . . . . . . . . . . . . . . . . . 113. 759 305 Sillen phase . . . . . . . . . . . . . . . . . . . . .

Sodium metavanadate . . . . . . . . . . . . . 327 Solid solutions Stabilization of enzymes . . . . . . . . . . . 685 Structural deffects . . . . . . . . . . . . . . . . 427 Sulfenamides synthesis SUPERCLAUS process Supported homogeneous catalysts . . . . 571 Supported metal catalysts . . 451. 461. 471. . . . . . . . . . . . . . . . . . . 481. 495. 499. 507 Supported tungsten complexes . . . . . . . 571 Supported vanadium oxides . . . . . . . . . . 83 Surface characterization . . . . . . . . . . . 411 TAP reactor . . . . . . . . . . . . . 203. 481. 853 Tetrahydrofurane (THF) . . . . . . . . . . . 265 Ti-Beta synthesis . . . . . . . . . . . . . . . . . 537 541 Ti-silicalite . . . . . . . . . . . . . . . . . . . . . Tin-molybdenum-antimony oxides . . . . 795 Titanium dioxide . . . . . . . . . . . . . 693. 721 Titanium phosphates . . . . . . . . . . . . . . 729 Titanium-lanthanum-sodium oxides . . 443 Toluene oxidation . . . . . . . . . . . . 729. 739 TPD of CO. . . . . . . . . . . . . . . . . 345. 403 TPSR technique . . . . . . . . . . 481. 499. 615 Tungsten oxide on silica . . . . . . . . . . . 315 Tungstenic acid . . . . . . . . . . . . . . . . . . 609

884

V4' ions in SnO. rutile matrix . . . . . . . . 93 V4' ions in V-Mo-0 . . . . . . . . . . . . . . 845 Vanadia-alumina . . . . . . . . . . . . . 83. 814 Vanadia-silica . . . . . . . . . . . . . . . . . . . 3 15 Vanadia-titania . . . . . . . 151. 221. 253. 777 Vanadium. oxidation states . . . . . . 75. 281 Vanadium oxide . . . . . . . 75.739. 814. 853 Vanadium-antimony oxides . . . . . 281. 293 Vanadium-iron oxides . . . . . . . . . . . . . 749 Vanadium-magnesium oxide . . . . 113. 253 Vanadium-molybdenum oxides . . 221. 845 Vanadium-phosphorus oxides . . . 213. 221. ...................... 233.253. 853 activation and pretreatment 183. 195. 203 polytypsim . . . . . . . . . . . . . . . . . . . . 167 structure . . . . . . . . . . . . . . . . . . . . . 183

Vanadium-phosphorus-lanthanum oxide 265 Vanadium-sepiolite . . . . . . . . . . . . . . . 113 Vanadium-tin oxides . . . . . . . . . . . . . . . 93 Vanadyl pyrophosphate . . . . . 195. 203. 243 structural studies . . . . . . . . . . . . . . . 167 o-Xylene oxidation . . . . . . . . . . . . . . . 221 Work function . . . . . . . . . . . . . . . . . . . 419 Zinc phosphate . . . . . . . . . . . . . . . . . . 357 Zirconia . . . . . . . . . . . . . . . . . . . 419. 721 Zirconium phosphate . . . . . . . . . 243. 357 Zirconium titanate . . . . . . . . . . . . . . . 721

STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, Universityof Pittsburgh, Pittsburgh, PA, U.S.A.

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Preparation of Catalysts LScientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet The Control of the Reactivityof Solids. A Critical Survey of the Factors that Influence the Reactivityof Solids, with Special Emphasison the Control of the Chemical Processes i n Relation t o Practical Applications by V.V. Boldyrev, M. Bulens and B. 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 Propertiesof Metal Clusters. Applications to 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 b y Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September9-11,1980 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, June30-July4,1980. Parts A a n d 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, Bechyiie, September 29-October 3,1980 edited by M. Laznirka Adsorption a t the G a s S o l i d and LiquidSolid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud. 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. Ji& and G. Schulz-Ekloff Adsorption on Metal Surfaces. A n 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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Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Preparation of Catalysts 111. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September &9,1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-1 6,1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jik, V.B. Kazansky and G. Schulz-Ekloff Catalysison 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 Acidsand Bases. Proceedingsof an International Symposium, Villeurbanne (Lyon), September 25-27,1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis o n Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June28-29,1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors byYu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, 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 International Conference, Bowness-on-Windermere, September 15-19,1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerveny New Developments in ZeoliteScience and Technology. Proceedings of the 7th International Zeolite Conference,Tokyo, August 17-22,1986 edited by Y. Murakami, 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 I-4,1986 edited by B. Delrnon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chernisorption edited by P. Wissmann Synthesis of High-silica Alurninosilicate Zeolites edited 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 B. 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 Chemicalsfrorn Natural Gas,Auckland, April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S.Vurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings of the 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22,1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I),Bad Soden a.Ts.,April26-29,1987 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 HeterogeneousCatalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17,1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Paal Catalytic Processes under Unsteady-State Conditions by Vu. 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 ofthe 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 4-8,1988 edited by H.G. Kargeand J. Weitkamp Photochemistry o n 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 Confererce,Amsterdam, July 10-14,1989, PartsAand 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,

November27-December2.1988 Volume 51 Volume 52

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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. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedingsof 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 in Catalytic and Separation Technology edited by M. Misono, V. 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 International Symposium on Recent Developments in Olefin Polymerization Catalysts, 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 578 Spectroscopic Analysis of Heterogeneous Catalysts. Part B: 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 HeterogeneousCatalysis and Finechemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 60 Chemistry of MicroporousCrystals. Proceedings of the International Symposium on Chemistryof Microporous CrystaIs,Tokyo, June 26-29,1990 edited by T. Inui, S. Namba andT. Tatsumi Volume 61 Natural Gasconversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of PorousSolids 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 CatalystsV. Proceedings of the Fifth International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, September3-6,1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon Volume 64 New Trends in CO Activation edited by L. Guczi Volume 65 Catalysis and Adsorption b y 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 Heterogeneous Catalysis. 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. Wichterlova

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Poisoning and Promotion in Catalysis based o n Surface Science Concepts and Experiments by M. Kiskinova Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13,1990 edited by A. Crucq N e w Developments in Selective Oxidation b y Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by P. Ruiz and B. Delmon ProgressinCatalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff,Alberta,Canada, May25-28,1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P. Tetenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of theThird International Conference on Spillover, Kyoto, Japan, August 17-20,1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals 111. Proceedings of the 3rd International Symposium, Poiters, April 5 - 8,1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D.Duprez, G. Perot and C. Montassier Catalysis: An Integrated Approach t o Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22,1992 edited by Motoyuki Suzuki Natural GasConversion II. Proceedings of theThird Natural Gas Conversion Symposium, Sydney, July 4-9,1993 edited by H.E. Curry-Hyde and R.F. Howe New Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, September 20-24,1993 edited bv V. Cortes Corberan and S.Vic Bellon

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