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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

...

Vlll

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

- 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 H,O + CO

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

+ 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 transacross 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

176

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 transacross 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

177

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.

178

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.


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

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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.

191

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 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

193

(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.

UniversiY

B


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

195

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.

198

80 T = 380°C

60 8

60

I

40

40 U

20 0

10

80 60 8

I

20

-

I

I

I

30

40

0

30

60

90

.

+

40

a time [min]

80

conversion [%] T = 460'C

2 60 2

60

r_

8

c

0

I

40 200

f

10

20

30

=

40

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.

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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.

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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

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130

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I

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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

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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

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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.

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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

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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

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1000

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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.

229

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.

230

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).


V. CortCs Corberin and S. Vic Bcllon (Editors), New Deveioprnenls i n Seleclrve Oxidation //

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

233

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;

234

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.

235

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

237

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

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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.

238

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.

239

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

240

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].

242

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.

243

-

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

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'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. 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.


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.

293

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.

299

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.

30 1

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?

302

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


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.

305

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 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,

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.

323

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.

324

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?

325

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 ( 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.


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 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 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.


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


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 /


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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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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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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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).

(2) ready for publication.


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

466

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.

469

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 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.


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) 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.

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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.


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) %

~

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.


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.


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

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.


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.


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.

129

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 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.

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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.


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.

-5150

-5200

-5350

-5400

-

11

A-

IT v-0-v

-5250 -

-5450

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

-5230 -5250

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-

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-

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-

-5350 -5370

I

I

0

0.6

I

I2

I

I

I

I

1.8

2.4

3

3.6


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


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).

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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.

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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

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.

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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/C0N0S03 = 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

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