Heterogeneous Catalysis and Fine Chemicals III: Proceedings of the 3rd International Symposium, Poitiers, April 5-8, 1993 (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 78

HETEROGENEOUS CATALYSIS AND FINE CHEMICALS 111

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates VOI. 7a

HETEROGENEOUS CATALYSIS AND FINE CHEMICALS 111 Proceedingsof the 3rd International Symposium, Poitiers, April 5-8,1993

Editors

M.Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier Laboratoire de Catalyse en Chimie Organique (URA CNRS 3501,Faculte des Sciences, Universite de Poitiers, 40Avenue du Recteur Pineau, 86022 Poitiers, France

ELSEVIER

Amsterdam -London

-New York -Tokyo

1993

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box211.1000AE Amsterdam,The Netherlands

Library o f C o n g r e s s Cataloging-in-Publication D a t a

Heterogeneous c a t a l y s i s and f i n e c h e m i c a l s I11 proceedings of the 3 r d i n t e r n a t i o n a l s y m p o s i u m . P O l t i e r S . A p r i l 5-8. 1 9 9 3 / e d i t o r s . M. Guisnet Let a l . 1 ( L a b o r a t o i r e de c a t a l y s e en c h i n l e o r g a n i q u e . F a c u l t e des s c i e n c e s . U n i V e r S i t e de P o i t i e r s ) . p. c n . -- ( S t u d i e s i n s u r f a c e s c i e n c e a n d c a t a l y s i s , 7 8 ) Includes Inoexes. ISBN 0 - 4 4 4 - 8 9 0 6 3 - 7 (acid-free1 1. Heterogeneous catalysis--Congresses. I . G u i s n e t . M. I!. U n i v e r s i t e d e P o i t i e r s . L a b o r a t o i r e de c a t a l y s e e n c h l m l e ::I. S e r i e s organique. '993 Oi505.H4632 541,3'95--dc23 93-26769

...

CIP

ISBN 0-444-89063-7

0 1993 Elsevier Science Publishers 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 Publishers B.V., Copyright & Permissions Department, P.O. Box 521,1000 A M 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 tothe copyright owner, Elsevier Science Publishers 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

V

CONTENTS xiii xv xvi xv i xvii

Foreword Preface Scientific Committee Organizing Committee Financial Support PLENARY LECTURES

Trends and Opportunities with modern hydrogenation catalysts I. Dodgson

1

Catalytic oxidations with hydrogen peroxide : new and selective catalysts M.G. Clerici

21

Basic Catalysts and Fine Chemicals H. Hattori

35

Solvent effects in Heterogeneous Catalysis : Application to the synthesis of Fine Chemicals L. Gilbert and C. Mercier

51

RESEARCH PAPERS 1. HYDROGENATION AND RELATED REACTIONS

Phenyl vs. carbomethoxy group effect on selectivity during hydrogenation and exchange of a,b-unsaturated esters over modified and unmodified deuterated Raney nickel G.V. Smith, R. Song, J.M. Delich and M. Bartok

67

The role of the support in selective hydrogenations promoted by Cu/AI,O, N. Ravasio, M. Antenori and M. Gargano

75

Liquid-phase hydrogenation of acrolein to ally1 alcohol on supported cobalt catalysts Y. Nitta. T. Kato and T. lmanaka

83

Selective hydrogenation of aromatic aldehydes using precious metal catalysts on new high surface-area TiO, supports M. Bankmann, R. Brand, A. Freund and T. Tacke

91

vi

Kinetic comparison of enantioselective hydrogenations A. Tungler, T. Tarnai, A. Deak, S. Kemhy, A. Gybry, T. Math6 and J. Petr6

99

Enantioselective metal complex catalysts immobilized on inorganic supports via carbamate links B. Pugin and M. Muller

107

Stereoselective thymol hydrogenation : comparative study of charcoal-supported, platinum, rhodium and iridium catalysts M. Besson, L. Bullivant, N. Nicolaus-Dechamp and P. Gallezot

115

The selective hydrogenation of 6-chloro-2(1H)-hydroxyquinoxaline-4-oxides to 6-chloro-2(1H)-quinoxalinone R.E. Malz Jr., M.P. Reynolds and C.J. Fagouri

123

Study of the hydrogenation of methyl benzoate to benzaldehyde on various metal oxides A. Aboulayt, A. Chambellan, M. Marzin, J. Saussey, F. Mauge, J.C. Lavalley, C. Mercier and R. Jacquot

131

Enantioselective hydrogenation of a-ketoacids using platinum catalysts modified with cinchona alkaloids H.U. Blaser and H.P. Jalett

139

Surface organometallic chemistry on metals ; selective hydrogenation of citral on silica supported rhodium modified by tetra-n-butyl germanium, tin and lead B. Didillon, J.P. Candy, F. Le Peletier, O.A. Ferretti and J.M. Basset

147

Hydrogenation of citral in the liquid phase over new bimetallic Ni-M catalysts supported on graphite J. Court, J. Jablonski and S. Hamar-Thibault

155

Hydrogenation of cinnamaldehyde and citral over Ru supported catalysts S. Galvagno, C. Milone, G. Neri, A. Donato, R. Pietropaolo

163

Selective hydrogenation of cawone on Pt and Pt-Au catalysts G. Del Angel, R. Melendrez, V. Bertin, J.M. Dominguez, P. Marecot and J. Barbier

171

Selective catalytic hydrogenation of bifunctional compounds over amorphous nickel alloys B. Tbrbk, A. Molnar, K. Borszeky, E. Toth-KBdar and I. Bakonyi

179

vii

Stereoselective hydrogenation of D-fructose to D-rnannitol on skeletal and supported copper containing catalysts M. Hegedus, S. Gobolos and J.L. Margitfalvi

187

Furfural - Hydrogen reactions, manipulation of activity and selectivity of the catalyst T.B.L.W. Marinelli, V. Ponec, C.G. Raab and J.A. Lercher

195

Selective hydrogenation of a, b unsaturated compounds in the presence of cobalt catalysts J. Barrault, M. Blanchard, A. Derouault, M. Ksibi and M.I. Zaki

203

Selective hydrogenation of crotonaldehyde over Pt derived catalysts C.G. Raab, M. Englisch, T.B.L.W. Marinelli and J.A. Lercher

21 1

Crotonaldehyde hydrogenation over Pt-TiO, catalysts : variations of activity and selectivity with the partial pressure of crotonaldehyde R. Makouangou-Mandilou, R. Touroude and A. Dauscher

219

Selective hydrogenation of fatty acid ethyl esters on sepiolite-supported Ni and Ni-Cu catalysts F.M. Bautista, J.M. Campelo, A. Garcia, R. Guardeiio, D. Luna, J.M. Marinas and M.C. Ordoiiez

227

Selective hydrotreatment of rapeseed oil on nickel-cerium mixed oxides modified by Al additive A. Alouche, R. Hubaut, J.P. Bonnelle, Ph. Davies and D. Lamberi

235

Kinetics of the liquid-phase stereoselective hydrogenation of 44ertbutylphenol over rhodium catalyst D.Yu. Murzin. A.I. Allachverdiev and N.V. Kul’kova

243

Liquid phase catalytic hydrogenation of benzophenone : Role of metal support interaction, bimetallic catalysts, solvents and additives P.S. Kumbhar and the late R.A. Rajadhyaksha

251

Solvent effects in selective hydrogenation : Catalytic hydrogenation of Acetamido-4 hydroxy-2 butyrophenone F. Grass, J.M. Grosselin and C. Mercier

259

Factors influencing activity and selectivity of palladium catalysts J. Petro, T. Mallat, A. Tungler, T. Mathe and E. Polyanszky

267

Selective hydrogenation of maleic anhydride by modified copper chromite catalysts G.L. Castiglioni, M. Gazzano, G. Stefani and A. Vaccari

275

viii

Selective hydrogenation of dinitriles to arninotriles on Raney catalysts S.B. Ziernecki

283

Hydrogenation of dinitriles into diarnines. Influence of the nature of dinitrile on activity and selectivity of the reaction P. Marion, M. Joucla, C. Taisne and J. Jenck

29 1

Selective hydrogenation of 3-butenonitrile and 2-butenonitrile on palladium exchanged titanium pillared montrnorillonite A. Lamesch, H. del Castillo, P. Vandetwegen, L. Daza, G. Jannes and P. Grange

299

One step synthesis of dissyrnetrical arnines R,NR' from nitriles in the presence of copper catalysts J. Barrault, S. Brunet, N. Essayern, A. Piccirelli, C. Guirnon and J.P. Garnet

305

A process for coproduction of mono- and diarninoalkylated glycols K.S. Hayes and T.A. Johnson

313

New process for isophoronediarnine synthesis J.P. Gillet, J. Kervennal and M. Pralus

32 1

Catalytic synthesis of 2-rnethylpyrazine over Zn-Cr-O/Pd. A simplified kinetic scheme L. Forni and R. Miglio

329

Properties of sol-gel derived Ru/Cu/SiO, catalysts and role of water in the selective hydrogenation of benzene to cyclohexene with the catalysts F. Mizukami, S . 4 Niwa, S. Ohkawa and A. Katayarna

337

The partial hydrogenation of benzene and of toluene over ruthenium catalysts - The effect of salt addition on the selectivity to (methyl)-cyclohexenes M. Soede, E.J.A.X. van de Sandt, M. Makkee and J.J.F. Scholten

345

Shape-selectivity of Pt on carbon fibers catalysts S. Kogan, M.V. Landau, M. Herskowitz and J.E. Koresh

353

On the XPS-surface characterization of activated carbons resp. Pd/C catalysts and a correlation to the catalytic activity R. Burrneister, B. Despeyroux, K. Deller, K. Seibold and P. Albers

361

Thiophene synthesis by dehydrogenation of?etrahydrothiophene on chromium catalysts A. Cornrnarieu, E. Arretz, D. Duprez and C. Guirnon

369

ix II. OXIDATION

Partial oxidation of water-insoluble alcohols over Bi-promoted Pt on alumina. Electrochemical characterization of the catalyst in its working state T. Mallat, 2. Bodnar and A. Baiker

377

Selective oxidation reactions over vanadium silicate molecular sieves P.R. Hari Prasad Rao, K. Ramesh Reddy, A.V. Ramaswamy and P. Ratnasamy

385

Selective oxidation of organic compounds over the large pore Beta-Ti zeolite M.A. Camblor, A. Corma, A. Martinez, J. Perez-Pariente and J. Primo

393

Selective photocatalytic oxidation of hydrocarbon compounds over zeolites 0. Beaune, A. Finiels, P. Geneste, P. Graffin, A. Guida, J.L. Olive and A. Saeedan

40 1

Photocatalytic oxygenation of hydrocarbons on TiO,/iron-porphyrin-hybrid catalysts E. Polo, R. Amadelli, V. Carassiti and A. Maldotti

409

Selective oxidation of alkenes on a zeolite supported iron phthalocyanine catalyst A. Zsigmond, F. Notheisz, M. Bartok and J.E. Backvall

41 7

Some physical correlations with the catalytic activity of Mo(VI)-grafted carboxylated resins used as epoxidation catalysts E. Tempesti, E. Ranucci, C.L. Bianchi, V. Ragaini, L. Giuffre, G. Airoldi and C. Mazzocchia

425

Photocatalyzed oxidation of 1,4-pentanedioI on UV-illuminated suspensions of ZrTiO, powders J.A. Navio, M. Garcia Gomez, M.A. Pradera-Adrian and J. Fuentes-Mota

43 1

Selective electrocatalytic oxidation of sucrose on smooth and upd-lead modified platinum electrodes in alkaline medium P. Parpot, K.B. Kokoh, B. Beden, E.M. Belgsir, J.-M. Leger and C. Lamy

439

Selective oxidation of substituted aromatics using different peroxides C. Marchal, A. Tuel and Y. Ben Taarit

447

Catalytic hydroxylation of phenol by hydrogen peroxide. Kinetic study and comparison between solid acids and titanosilicates M. Allian, A. Germain, T. Cseri and F. Figueras

455

X

Reductive coupling of cyclic ketones on reduced TiO, ( 0 0 1) single crystal surfaces H. ldriss and M.A. Barteau

463

Oxidative dehydrogenation of isobutyric acid to methacrylic acid over heteropolysalts of composition K,(NH,),PMo,O, : Effect of catalyst pretreatment and composition on the activity and selectivity S. Albonetti, F. Cavani, M. Koutyrev and F. Trifirb

471

Heterogeneously catalyzed ammoximation of cyclohexanone with molecular oxygen in vapor phase D. Collina, E. Pieri, D. Pinelli , F. Trifiro, G. Petrini and G. Paparatto

479

The Mars and van Krevelen mechanism for oxidation reactions used for a selective reduction reaction - Influence of surface OH-groups on the selectivity. E.J. Grootendorst and V. Ponec

487

111. ACID-BASE CATALYSIS

K10-Montmorillonites as catalysts in Diels-Alder reactions : influence of the exchanged cation C. Cativiela, F. Figueras, J.M. Fraile, J.I. Garcia, M. Gil, J.A. Mayoral, L.C. de Menorval, E. Pires

495

Capsule membrane phase transfer catalysis : selective alkaline hydrolysis and oxidation of benzyl chloride to benzyl alcohol and benzaldehyde G.D. Yadav, P.H. Mehta and B.V. Haldavanekar

503

Selective acylation of sugar derivatives catalyzed by immobilized lipase A.T.J.W. de Goede, M. van Oosterom, M.P.J. van Deurzen, R.A. Sheldon, H. van Bekkum, F. van Rantwijk

513

Modified zeolites as active catalysts in Friedel-Crafts acylation D.E. Akporiaye, K. Daasvatn, J. Solberg, M. Stocker

52 1

Acid-catalyzed ketonization of mixtures of low carbon number carboxylic acids on zeolite H-T J.A. Martens, M. Wydoodt, P. Espeel and P.A. Jacobs

527

Reactions of ketoximes and aldoximes over solid acid catalysts T. Curtin and B.K. Hodnett

535

Zeolite catalysed rearrangement of aromatic amines T. Stamm, H.W. Kouwenhoven and R. Prins

543

xi

A selective process for the synthesis of para-nitrophenol C. Maliverney, M.H. Gubelmann and J. Susini

551

Catalytic behaviour of Lewis acid-base sites on alkali-exchanged zeolites M. Huang and S. Kaliaguine

559

Aromatic hydroxyalkylation using (Silico) aluminophosphate - molecular sieves M.H.W. Burgers and H. van Bekkum

567

Comparative study of isopropylation and cyclohexylation of naphthalene over zeolites : shape selective synthesis of a 2,6-dialkylnaphthalene P. Moreau, A. Finiels, P. Geneste, F. Moreau and J. Solofo

575

Stereoselectivity of the deisopropylation of methyl dehydroabietate C. Pereira, F. Alvarez, M.J.M. Curto, B. Gigante, F.R. Ribeiro and M. Guisnet

581

An in situ I3C-NMRstudy of the mechanism of cumene - n-propylbenzene isomerization over H-ZSM-11 1.1. Ivanova, D. Brunel, J.B.? Nagy, G. Daelen and E.G. Derouane

587

Kinetic study of the acylation of thiophene with acyl chlorides in liquid phase over HY zeolites A. Finiels, A. Calmettes, P. Geneste and P. Moreau

595

Zeolite catalyzed acylation of heterocyclic aromatic compounds. I - Acylation of benzofuran F. Richard, J. Drouillard, H. Carreyre, J.L. Lemberton and G. Perot

601

Catalytic vapour-phase nitration of benzene on modified Y-zeolites : Influence of catalyst treatment L.E. Bertea, H.W. Kouwenhoven and R. Prins

607

AIP0,-TiO, catalysts. V. Vapor-phase Beckmann rearrangement of cyclohexanone oxime F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and M.S. Moreno

615

Post-synthetic improvement of the basic character of caesium exchanged X and Y zeolites by occluded caesium oxides. Applications in condensation reactions 1. Rodriguez, H. Cambon, D. Brunel, M. Lasperas and P. Geneste

623

Catalytic transfer reduction of ketones over oxide catalysts J. Kijenski, M. Glinski, J. Czarnecki, R. Derlacka and V. Jarzyna

63 1

xii

Selective nng-opening of an epoxide on silica supports M. Do Ceu Costa, R. Tavares, W. Motherwell and M.J.M. Curto

639

Diels-Alder cycloaddition reaction between dihydropyran and acrolein over various H-form zeolites R. Durand, P. Geneste, J. Joffre and C. Moreau

647

Rearrangement of acetals of 2-bromopropiophenone as a test reaction to characterize the Lewis sites in large pore zeolites F. Algarra, A. Corma, V. Fornes, H. Garcia, A. Martinez and J. Primo

653

Shape selectivity in the zeolite-catalyzed Fischer indole synthesis M.S. Rigutto, H.J.A. de Vries, S.R. Magill, A.J. Hoefnagel and H. van Bekkum

66 1

Contribution to the study of isobutene condensation with formaldehyde catalyzed by zeolites E. Dumitriu, D. Gongescu and V. Hulea

669

lsomerisation of a-acetylenic alcohols into a,b-ethylenic carbonyl derivatives in vapor phase C. Mercier and P. Chabardes

677

lsomerization of a-pinene over TiO, : kinetics and catalyst optimization A. Severino, J. Vital and L.S. Lob0

685

Vapour phase hydrolysis : a new access to 2,2,2-trifluoroethanol P.J. Tirel, C. Doussain, L. Gilbert, M. Gubelmann, H. Pernot and J.M. Popa

693

Conversion of acetone into methylisobutylketone on Pt HZSM5 catalysts. Influence of the hydrogenating activity on the rate and on the selectivity L. Melo, E. Rombi, J.M. Dominguez, P. Magnoux and M. Guisnet

70 1

Author index

707

Subject Index

71 1

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

715

xiii

FOREWORD Heterogeneous Catalysis, widely used in petroleum refining and petrochemistry, plays an ever increasingly important role in Organic Synthesis (Speciality and Fine Chemicals). This development is quite natural as processes employing solid catalysts are definitely more advantageous from an environmental or functional point of view. than non-catalytic processes or those using soluble catalysts. However, because the interaction between functional compounds and surface sites is necessarily complex, the effect which the catalyst surface has on the reaction mechanisms is still not very well understood. This is the reason why the development of selective catalysts is particularly difficult. The aim of the Third International Symposium on Heterogeneous Catalysis and Fine Chemicals was to make possible a wide discussion of all these basic and practical aspects among industrial and academic researchers, manufacturers and users of solid catalysts. The 3rd Symposium was, like the preceding ones (1988 and 1990) organized by the "Laboratoire de Catalyse en Chimie Organique" of the University of Poitiers (France) within the framework of International Symposia of the National Center of Scientific Research (CNRS). The program was prepared by the European Pilot Committee which was created at the 1990 Symposium and included four invited lectures, 83 oral and poster communications and a panel discussion. Three out of the four lectures and 12 of the 29 oral communications came from Industry, sometimes in collaboration with Universities. The interest shown in the Symposium by industrial researchers was also demonstrated by their significant participation (roughly a third of the 270 participants). One of the outstanding features of the Symposium was the panel discussion on the topic of Clean Processes for Fine Chemicals Synthesis led by the following industrial and academical specialists : Dr. P.C. Gravelle (PIRSEM-Paris, coordinator), Dr. R. Bader (Ciba Geigy, Bide), Prof. W. Hoelderich (University of Aachen), Dr. J. Kervennal (Atochem, Pierre-Benite), Dr. C. Mercier (Rhbne-Poulenc, Saint-Fons) and Prof. H. van Bekkum (University of Delft). An exhibition of equipment, chemical products and catalysts took place in parallel to the Symposium and was presented by a score of firms on April 5, 6 and 7. The present volume contains the text of the lectures followed by those of the communications, the latter being classified under three main topics: Hydrogenation, Oxidation and Acid Catalysis. All the communications were read by two referees, who obliged authors in many cases to present revised versions. In most of the papers the emphasis is placed on the selectivity of the reactions, taking into consideration all its aspects: chemo-, regio- and stereoselectivity (including enantioselectivity). If the comprehension of the parameters (physicochemical characteristics of the catalysts, operating conditions, etc.) which determine this selectivity sometimes imposes the choice of simple reactions, various syntheses of complex products of industrial interest are also reported. The Organizing Committee is particularly grateful to the members of the European Pilot Committee for their help in the choice of communications and in reviewing the papers. We also thank the technicians, the secretaries of the "Laboratoire de Catalyse en Chimie Organique" and the young researchers of ATLAS who participated enthusiastically and efficiently in the organization of the Symposium.

This Page Intentionally Left Blank

xv

PREFACE La Catalyse Heterogene, largement utilisee en Raffinage du Petrole et Petrochimie joue un rdle de plus en plus important en Synthese Organique (Chimie des Specialites et Chimie Fine). Cette evolution est tout a fait naturelle, les procedes utilisant des catalyseurs solides presentant du point de vue de I'environnement. de la mise en oeuvre... des avantages certains sur les procedes non catalytiques ou sur ceux utilisant des catalyseurs solubles. Toutefois les interactions entre composes fonctionnels et sites superficiels etant necessairement complexes, I'effet que la surface du catalyseur a sur les mecanismes de reaction reste encore ma1 compris. C'est la raison pour laquelle la mise au point de catalyseurs selectifs est particulierement difficile. L'objectif de ce troisieme Colloque International sur le theme Catalyse Heterogene et Chimie Fine etait de permettre un large debat sur tous ces aspects fondamentaux et pratiques entre chercheurs de I'lndustrie et de I'lJniversite, fabricants et utilisateurs de catalyseurs solides. Ce troisieme Colloque etait, comme les precedents (1988 et 1990), organise par le Laboratoire de Catalyse en Chimie Organique de I'Universite de Poitiers dans le cadre des Colloques lnternationaux du Centre National de la Recherche Scientifique (CNRS). Le programme etabli par le Comite Europeen de Pilotage Cree lors du precedent Colloque comprenait 4 conferences invitees, 83 communications orales et affichees et une table ronde. 3 des 4 conferences et 12 des 29 communications orales venaient de I'lndustrie ou de collaborations entre chercheurs de I'lndustrie et de I'Universite. L'interet des chercheurs Industriels pour ce Colloque etait egalement confirme par leur participation importante (environ 1/3 des 270 participants). La table ronde portant sur les Procedes Propres en Chimie Fine a ete animee par des specialistes industriels et universitaires : Dr. P.C. Gravelle (PIRSEM-Paris, coordonnateur), Dr. R. Bader (Ciba Geigy, BBle), Pr. W. Hoelderich (Universite d'Aix la Chapelle), Dr. J. Kervennal (Atochem, PierreBenite), Dr. C. Mercier (Rhdne-Poulenc, Saint-Fons) et Pr. H. van Bekkum (Universite de Delft). Une exposition de materiel s'est tenue en parallele avec le Colloque les 5, 6 et 7 avril, avec presentation de materiel, produits chimiques et catalyseurs par une vingtaine de Societes. Ce volume contient les textes des conferences suivis de ceux des communications classees en trois grands themes : Hydrogenation, Oxydation et Catalyse Acide. Toutes les communications ont ete examinees par deux rapporteurs ce qui a conduit dans de nombreux cas les auteurs a presenter une version revisee. Dans la plupart des papiers I'accent est mis sur la selectivite des reactions, tous les aspects : chimio, regio, stereoselectivite (y compris enantioselectivite) etant consideres. Si la comprehension des parametres determinant cette selectivite impose parfois le choix de reactions simples, de nombreuses syntheses de produits complexes d'interet industriel sont egalement reportees. Le Comite d'organisation remercie particulierement les membres du Comite Europeen de Pilotage pour leur aide dans le choix des communications et la revision des papiers. Nos remerciements vont aussi au personnel technique et de secretariat du Laboratoire de Catalyse en Chimie Organique et aux jeunes chercheurs d'ATLAS qui ont participe avec enthousiasme et efficacite a I'organisation de ce Symposium.

xvi

SCI ENTlFIC COM MITTE E

A. ANDERSSON, University of Lund, Sweden M. BARTOK, University of Szeged, Hungary H.U. BLASER, Ciba-Geigy, Switzerland A. CORMA, University of Valencia, Spain B. DELMON, University of Louvain-LaNeuve, Belgium G. DESCOTES, University of Lyon, France F. FIGUERAS, CNRS, Montpelier, France G. FLECHE, Roquette, Lestrem, France L. FORNI, University of Milan, Italy P. GALLEZOT, lnstitut de Recherches sur la Catalyse, Villeurbanne H. VAN BEKKUM, University of Delft, The Netherlands

P.C. GRAVELLE, PIRSEM, Paris, France H.HOELDERICH, University of Aachen, Germany J. KERVENNAL, Centre d e Recherches, Atochem, Pierre-Benite, France G. MATTIODA, Hoechst, Stains, France R. MAUREL, Jaunay-Clan, France CI. MERCIER, Rh6ne-Poulenc, Saint Fons, France D. OLIVIER, Departernent Chimie, CNRS, Paris, France B. SILLION, lnstitut Francais du Petrole, Solaize, France K. SMITH, University of Swansea, United Kingdom

ORGAN I2ING COM M ITTEE

M. GUISNET J. BARRAULT and D. DUPREZ J. BARBIER, C. BOUCHOULE, C. MONTASSIER and G. PEROT ATLAS 86 (Student Association of the Catalysis Group of Poitiers)

Chairman Secretaries Members

xvii

FINANCIAL SUPPORT

The Organizers are grateful to their generous sponsors :

- CENTRE NATIONAL DE LA RECHERCHE SClENTlFlQUE (CNRS) - CONSEIL GENERAL DE LA VIENNE - CONSEIL REGIONAL POITOU-CHARENTES

- PROGRAMME INTERDISCIPLINAIREDE RECHERCHES SUR LES SCIENCES POUR L'ENERGIE ET LES MATIERES PREMIERES (PIRSEM)

- UNIVERSITE DE POITIERS AND FACULTE DES SCIENCES - VILLE DE POITIERS - CIBA-GEIGY - DERIVES RESlNlQUES ET TERPENIQUES - INSTITUT FRANCAIS DU PETROLE

- JANSSEN CHIMICA - RHONE-POULENC

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous CataIysis and Fine Chemicals 111 0 1993 Elsevier Sciencc Publishers B.V. All rights reserved.

1

Trends and Opportunities with Modern Hydrogenation Catalysts

Ivor Dodgson Johnson Matthey, Materials Technology Division, Orchard Road, Royston, Hertfordshire. SG8 5HE, United Kingdom

Abstract The paper reviews trends and opportunities with mainly precious metal hydrogenation catalysts. Environmental opportunities are described with the development of catalysts to manufacture HFC 134a to replace the ozone depleting CFC 12 and also the potential replacement of nickel catalysts by ruthenium in sugar hydrogenations to eliminate nickel contaminated waste waters. Recent advances in our understanding of the reaction mechanism for cinchona alkaloid modified Pt catalysts for enantioselective hydrogenations are discussed. Examples are given of bimetallic catalysts to improve selectivity, activity and precious metal inventory costs.

1.

Introduction

Around 90 per cent of all chemicals involve a catalyst at some stage of their manufacture. Hence, catalysis is critical to the chemical industry. The Worldwide market for catalysts in 1989 was estimated to be US$ 5,100 million [l]. The market for catalysts for organic synthesis as op osed to petroleum 100 million. Of refining, polymerisation, anti-pollution catalysts etc. was this, it was estimated that the Worldwide market of supported precious metal catalysts for the fine chemicals industry was $45 million.

f

Supported precious metal catalysts are used mainly for hydrogenation and dehydrogenation reactions. The catal st operating cost contribution in a batch process is typically less than .€O.lSdg reaction product and often less than 10%of the added value of the catalytic step for a fine chemical process.

2

There are many challenges facing the Fine Chemical industry in which catalysis and in particular precious metal catalysis has a key role. These include:a)

Environmental Pollution

Industry is seeking high yield, zero waste processes. Companies are evaluating the replacement of some base metal catalysts by more selective precious metal catalysts to eliminate troublesome by-product formation and/or contaminated waste waters. In the case of Chlorofluorocarbons (CFC's) industry is trying to develop cost effective processes to manufacture non-toxic Hydrofluorocarbons (HFC's) of zero ozone depleting potential to replace existing CFC's. b)

Asymmetric Hydrogenations

In the pharmaceutical industry, chirality issues are causing particular problems in the development of new chemical entities but also opportunities for the s o called "racemic switch" when an established racemate product approaches patent expiry. Bimetallic Catalysts

Improved analytical techniques to study individual crystallites on commercial catalysts assist the development of improved supported bimetallic catalysts. These catalysts can give improved activity and/or selectivity over the monometallic catalyst and reduced precious metal inventories. This paper will discuss each of these areas of opportunity and illustrate them with specific examples. As a result of the author's background with Johnson Matthey, many of the examples will relate to supported precious metal catalysts.

2.

Environmental Pollution

2.1

Hvdrofluorocarbons as CFC Substitutes

a

Information from the World Meteorological Organisation [2] and other sources confirm depletion of the ozone layer and provide evidence that the production and use of CFC's should be phased out. The Montreal Protocol, an international agreement regarding CFC production and use, was strengthened in 1990 to require a phase out of production in developed countries by 2000 and b 2010 in developing countries. A Du ont study [3] estimated that about 60% ofYprojected CFC demand in the year 20 0 will either be eliminated by improved conservation practices or will be satisfied by non fluorocarbon alternatives. The commercial development of CFC alternatives is a multifaceted problem involving toxicity testing, safety, environmental acceptability, materials compatibility and energy efficiency requirements as well as the manufacturing process.

B

One of the most promising CFC substitutes is HFC 134a ( C F ~ C H Z F )as an alternative to CFC 12 (CF2C12) for use as a refrigerant.

3

Unlike the simple fully halogenated CFC's which can only be made in a single step, there are many potentially viable routes to the alternatives, several of which involve a catalytic hydrogenation step [4].

CF2CICF2CI

7

/ \ 11'

CF3CFCI,

CF,CICChF 113

\

1140

CF,CHFCI

\

CF,CH,CI

/ 124

Figure 1 Potential Routes to HFC 134a

The higher price of HFC 134a compared to that of CFC 12 is due to the lack of a single step synthesis and the estimated Worldwide market of only 100,000 MT p.a. moves HFC 134a towards a Fine Chemical. Two routes have received the most attention in the literature; those based on HCFC 133a (CF3CH2CI) and CFC 114a (CF3CC12F)

4

One Step Process from TCE CHCl= CC12 + H F TCE

---+

CF CH2Cl+ C F CH F Q33a 3i34i

Attempts at a single step high yield route to 134a have not been very successful. Dow [ 5 ] obtained only 3% 134a and 78% 133a using an "oxygenated chromium fluoride" catalyst at 300-400°C. Increasing the temperature to 500°C over a modified Cr (VI)/Al2O3 gave only 20% 134a and 50% 133a [6]. Two Step Process via HCFC 133a Conversion of a -CH Cl roup to -CH2F is a very difficult transformation often requiring expenzve Wuorinating a ents. The reaction of H F with 133a requires a large excess of H F to drive t e reaction to a reasonable conversion 6-10 moles HF/mole 133a to obtain a 30% single pass yield of 134a at sf0-400"C [7].

R

Extensive recycling of organic and H F is required and catalyst lifetime is a problem. However, subsequent work by Dupont [8] over a variety of metals on an aluminium fluoride support gave extended catalyst lifetimes. Three Step Process Via CFC 114a The other route attracting considerable attention involves 114a as an intermediate. CCI - CC12 + C12 + HF d +CE

CF CFCl2 + CF2CI.CF C1 j14a I ?4

d

(1)

CF3CFC12 + H2 4 CF3CH F + HCl 114a h a The hydrogenation reaction can be achieved over a palladium catalyst in the va our phase at 200-450°C as described in [9]. It is claimed by Montedison [I! that the symmetrical isomer 114 is only partially hydrogenated to 124 (C 2Cl.CHF2). There are several steps in the hydrogenation of 114a.

7' HCl 1. HCl 'F H F CF3CFC12 d CF3CHCIF d CF3CH2F d CF3CH3 114a

124

13423

143a

Mosikawa of Asahi Glass [ l l ] has studied the selectivity of four platinum group metals in this hydrogenation reaction.

5 Table 1 - The Selectivity of the Platinum Group Metals in the Hydrogenation of 114a.

Catalyst Pd Pt Rh

Ru

114a Conversion @)

124 6 45 10

100 100 100 24

Reaction Conditions :

Selectivity (%I 134a 143a

10

Temperature Contact Time H2/114a mole ratio

15 25 48 6

19 30 42 84

250°C 7 sec 4

The result for the ruthenium catalyst appears strange. Although the catalyst has the lowest activity it caused the highest level of overhydrogenation to 143a. Results with a Johnson Matthey Ru catalyst in this reaction gave low conversions but predominantly underhydrogenation to 124. Nevertheless our results were in broad agreement with Pd being the most selective catalyst. Mosikawa and others have identified several potential problems for the reaction affecting the catalyst lifetime.

(9

The conversion of 114a to 134a is very exothermic with a H of approximately 40 kcal/mole. Hence performing the reaction at high temperatures will cause thermal sintering and catalyst degradation.

(ii)

114a feedstock prepared by reaction (1) will always contain some symmetrical isomer 114 (CFzCI.CF2CI). Symmetrical isomers can undergo partial hydrogenation and subsequent hydrogen halide elimination to form olefins such as CF2=CF2 [4,12]. These can polymerise on the catalyst, particularly with alumina supported catalysts, to reduce the catalyst life.

(iii)

Mosikawa claims that palladium catalysts have insufficient acid resistance and describes the development by Asahi Glass of a palladium alloy catalyst with improved durability. Johnson Matthey has also developed a Pd/C catalyst with successful stable performance in a pilot plant durability test.

Hence it can be seen that the successful desi n of a durable catalyst can give a cost effective process for the production o 134a. This should lead to the replacement of the ozone depleting CFC 12 and the improved environmental consequences.

P

6

2.2

Elimination of Nickel From Waste Waters

Other aspects of environmental pollution are:the generation of unwanted by-products the generation of contaminated waste waters the disposal of spent catalyst All three materials require safe and responsible disposal which can be expensive. These environmental costs can change the economic viability of one process route versus another for a particular compound. In general, precious metal catalysts offer advantages over base metal catalysts because of their higher activity. This allows their use at lower operating temperature and pressure, often giving im roved selectivity to the desired product and less by-product formation. n addition, spent catalyst is taken back by the catalyst supplier to recover the precious metals. This is not always the case with base metal catalysts. The noble nature of the precious metal usually prevents leaching into the solvent and subsequent contamination of waste waters. This can be illustrated by the potential replacement of nickel catalysts by ruthenium in the hydrogenation of sugars. The most important sugar alcohols produced industrially today are sorbitol, mannitol and xylitol by the hydrogenation of glucose, invert sugar and xylose respectively

P

CHO

CH20H I H-C-OH

H-&-OH HO-&-H

L

H-LOH +&-OH

AH,OH

HO-A-H H-L-OH H-L-OH

AH2OH

Glucose

Sorbitol

Raney nickel and supported nickel catalysts are currently used for almost all sugar hydrogenation processes. Typical glucose hydrogenation reaction conditions for batch, slurry and continuous, fixed bed catalytic processes are ~31. Parameters

Batch, slurry

Temperature "C Pressure (bar) pH value Catalyst conc. with respect to glucose (96) H?/glucose molar ratio

120 - 150 30 - 70 5.0 - 6.0

140 170 ca 5.0

3-6

2.5 - 3.0 8-1O:l

-----

Continuous, fixed bed

7

Both processes use 45-50% aqueous glucose solution as feedstock. Nickel is leached into the reaction medium resulting in nickel losses, end product purification and the inevitable disposal of nickel containing waste materials. Wisniak [14,15] has compared precious metal catalysts with Raney nickel in the hydrogenation of glucose, fructose and xylose. It was concluded that the activity of the catalytic metals decreased in the order Ru > Ni > Rh > Pd B.J. Arena of UOP [16] has studied recently the use of Ru/AI 0 catalysts in the fixed bed hydrogenation of glucose. Arena has identiied physical deterioration of the alumina support and poisoning by gluconic acid, sulphur and iron as possible causes of catalyst deactivation. When some of these factors were minimised, substantial improvements in catalyst stability were observed. Wisniak has studied the variation of reaction feedstock and commercial catalysts over a range of reaction parameters which allows the direct comparison of ruthenium catalysts against a Raney nickel catalyst in a batch slurry process as shown in Table 2. Table 2 -

Comparison of Ruthenium and Nickel Catalyst Activity in the Hydrogenation of Glucose __.

Catalyst

Catalyst Loading (%)

Pressure (psig)

Temperature (“C)

5% Ru/C Ra-Ni

3 3

600 600

130 130

37.5 8.1

5% Ru/C Ra-Ni

3 3

600 600

100 100

14.5 4.1

5% Ru/C Ra-Ni

3 3

400 400

130 130

29.0 6.4

-

Reaction Rate Co tantk (109;23min-1) 0

The ruthenium catalyst is more active under each of the sets of reaction conditions above. The ability of ruthenium to operate at lower temperatures offers the opportunity for less caramelisation and other by-product formation. Ruthenium catalysts cannot compete with nickel catalysts for sugar hydrogenation on a direct comparison of catalyst cost contribution because of the difference in metal value. However, environmental legislation and the increasing cost of waste disposal and waste water treatment is changing the situation and presenting opportunities for ruthenium catalysts for the future.

3.

Asymmetric Hydrogenation

The issue of chirality in the harmaceutical industry has grown in importance over the last few years. t is now widely recognised that the separate enantiomers of a drug can behave very differently in all of their interactions with the human system e.g. thalidomide. Many of the target receptors for new drugs are themselves asymmetric and increased specificity of interaction with these receptors is more likely to arise from an asymmetric molecule. Under the recent FDA uidelines, pharmaceutical companies will have to furnish rigorous justification k r FDA approval of racemates 1171. Hence most new drug development involves the preparation of the specific enantiomer.

f

For major pharmaceutical companies, the period during which a product is still on patent is the critical time for establishing high sales and high mar ins to recoup R & D expenditure and generate profits. However, product I FPatent Expiry" heralds the probability of generic entry. For an existing racemate that is being marketed, a roduct line extension may be achieved b switching to a single stereoisomer orm, the so-called "racemate switch". his switch may assist brand extension in several ways [18].

P

* * * *

::

by roviding either improved efficiency or a reduced side-products prohe. by enabling advanced drug delivery formulations to be more successfully applied. by providing, in a small number of cases, an improved cost versus activity ratio for bulk products. and, where all the activity is in one isomer, allowing preparations that appear twice as active as equivalent racemate preparations.

There is a wide range of chemistry available for chiral synthesis

*

* * * *

chiral catalysis using synthetic chemical catalysts to produce single isomer compounds. chiral resolution using chemical resolving agents and crystallisation to separate isomers from a mixture. asymmetric synthesis using reagents to control the stereochemistry through complete synthesis of a pure isomer. advanced chemical synthesis from chiral pool raw materials such as amino acids, sugars and products from fermentation. enzyme and cell biocatalysts for resolution of racemic compounds and for asymmetric synthesis of pure isomers.

This section will review some latest developments in the area of asymmetric hydrogenation using sup Jorted metal catalysts. It will not cover homogeneous or biocatalysis. The fie d has been extensively reviewed by Blaser and Miiller [19] and 1 will only describe new work published since that 1991 review.

r

In the Review, Blaser describes his own Group's work with cinchona alkaloid modified platinum catalysts in the enantioselective hydrogenation of d -keto esters.

9

Peter Wells and his Group at Hull University have studied this system further to seek to gain a fundamental understanding of the reaction mechanism to allow an extension to other metals and other reaction substrates. Following on from the Orito work [20], Wells and co-workers studied the hydrogenation of methyl p t e (MeCo.C€)OMe> to methyl lactate (MeCH(0H)COOMe) over supported platinum catalysts modified with cinchona alkaloids.

R = -CH=CH2 Cinchonidine R = -CHz.CH Dihydrocinchonidine B = See table

3

The platinum catalyst, the well characterised 6.3% Pt/SiO powder EUROPT-1 in the following examples, was pretreated by stirring wit2 the modifier solution at ambient temperature prior to the introduction of methyl pyruvate and performing the hydrogenation at 10 bar pressure [21]. Table 3 - The Effect of Modifier Structure on Activity and Optical Yield Modifier Structure

R

B --

C2H3 -C2H5 PhCH2C2H3 o modifier

Rate (m mol h-lg-l) 1290 1095 102 50

Optical Yield (%)

60 70 2 0

The results show that there is no optical yield in the absence of modifier. The presence of modifier not only generates an optical yield, but also increases the reaction rate 20 fold. Modification with benzylcinchonidinium chloride (B=PhCH2) almost eliminates both optical yield and the rate enhancement. Hence the reaction model has to explain both chirality and an enhanced reaction rate.

10

Cinchonidine in its lowest energy state is L-shaped and it can approach a Pt (100) or Pt (111) surface in a configuration that would permit adsorption by the quinoline moiety without conformational disturbance [22]. The model proposed by Wells involves the flat adsorption on the platinum surface in a non-close packed array, thus leaving exposed shaped ensembles of platinum atoms. 511.

n

Y

511.

Figure2-

B

A representation of (a) a non-closepacked array of L-shaped cinchonidine molecules onto a Pt surface and (b) sites A, B and C

The asymmetric hydrogenation site A is accessible to methyl yruvate provided +)-lactate on adsorption is in the configuration required to give hydrogenation. The spatial relationship of adsorbed methyl pyruvate to molecule x of cinchonidine is such as to permit the hydrogen bonding interaction that is proposed to occur when the reactant is converted to its half-hydrogenated state.

A-(

Cinchonidine molecule Y directly prevents methyl yruvate adsorbing in a form that would ive S-(-)- lactate a s product and mo ecules Z1 and 2 2 form the remainder o the boundary of the asymmetric hydrogenation site. Such boundary molecules are obviously not required at the edges of the Pt crystallites (site B). Site C represents an area of unmodified surface where slow raceniic hydrogenation may proceed [23].

P

P

The rate enhancement has been attributed to a stabilisation, by H-bonding, of one of the two half hydrogenated states formed during pyruvate conversion to lactate. Since the rate determining step is expected to be the conversion of the half hydrogenated states to product, any process that stabilises a half hydrogenated state (ie increases its concentration or lifetime) increases the observed reaction rate.

11

- * Figure 3 -

*

H-bonding between the quinuclidine-N of the absorbed modifier cinchonidine and one of the half-hydrogenated states for methyl pyruvate conversion to R-methyl lactate.

Returning to Table 3, quaternisation of the alicyclic-N at B by a benzyl group almost eliminates both the optical yield and the rate enhancement. This is to be expected on the basis of the model; since the modifier is no longer L-shaped, enantioselectivity is lost and the stabilisation of the reaction intermediate by H-bonding cannot be achieved, so rate enhancement is also lost. This system has been extended to evaluate other metals. Blaser has reported a moderate optical yield with rhodium catalysts, but no success with Pd, Ru and Ni [24]. Wells and co-workers [25] have achieved some success with iridium. Table 4 -

Enantioselective Hydrogenation of Methyl Pyruvate Catalysed by Cinchonidine Modified Ir/silica catalyst in Ethanol Solution at 293 K and 10 bar pressure.

Catalyst 20%Ir/Si02b 2O%Ir/SiO2 20%Ir/Si02 20%Ir/Si02 7% Ir/Si02 7% Ir/SiO?

Calcination Temp (K) 773 3 73 573 773 573 773

Reduction Temp (K)

Initial R T (mmol h' g-')

523 523 523

523 523 523

50 1560 1400 370 630 70

Optical Yielda

(%I 0 31 33 13 14 3

a = at high conversions b = not modified by cinchonidine (racemic reaction) The racemic rate was low and the enantioselective rate very fast (like Pt). There is, however, a curious particle size effect. If the Ir particles are made bigger by increasing the Ir loading but keeping the calcination temperature constant, then the optical yield and the reaction rate both increase.

12

However, if the Ir particle size is increased by increasing the calcination temperature but keeping the reduction temperature constant then the optical ‘eld and reaction rate fall. The particle size effects have been confirmed by ?EM. The best optical yields over Ir/SiO 66.5% R, 33.5% S) are nearly as good as the best yields over Ir/CaC03 (69% k,\1%S). The Pt/SiO catalyst EUROPT-l/cinchonidine modifier system has also been evaluated ?or substrates similar but different to W ’ -ketoesters [26]. 2,3-Butanedione is only a small step away from methyl pyruvate. Hydrogenation of one group would yield (R) or (S) -3-hydroxy-butane-2-one. If the other carbonyl function is hydrogenated as well, the resulting molecule is d,l or meso - 2,3 - butanediol. Unless the reaction proceeds very selectively, five hydrogenation products are possible thus making analysis difficult. Table 5 - Enantioselective Hydrogenation of 2,3-Butanedione Catalysed by Cinchonidine modified EUROPT-1 at 10.2 bar Pressure Solvent during modification

Solvent during reaction

CHzC12 CH2C12 CH C12 Et& CH2Cl2 CH2C12

CH2C12 CH2C12 CH C12 Et& EtOH EtOH

Temp (K)

273 293 303 293 273 293

Rat? (mmnl h’ g- )

775 1650 900 1650 1550 5200

Optical yield (%)

38 21 18 8 23 12

a = optical yield of CH3 CH(0H)CO CH3 Optical yields are lower than the corresponding reaction for methyl pyruvate. As with methyl pyruvate, a lower reaction temperature results in a higher optical yield albeit at a lower reaction rate. The influence of solvent is much more ronounced than with methyl pyruvate showing that modification in a solvent (C82CI ) which does not interact with the catalyst is of benefit. The reaction ra?e of 5,200 is very fast, but gives a poor optical yield which is at odds with the methyl p ruvate reaction model. Chiral hydrogenation has also been achieved with Ph.Cd.CO.Ph [26]. At present there are very few reaction types catalysed by chiral modified supported catalysts. However, our understanding is increasing and the scope of the systems is expanding. The opportunities are increasing and there is little doubt that modified supported catalysts are preferable to homogeneous catalysts from a handling and separation viewpoint provided that the catalyst performance is satisfactory.

-

13 4.

Bimetallic Catalysts

4.1

Imtxoved Selectivitv

Increasing attention is being focused onto bimetallic catalysts to improve the catalyst selectivity and/or activity and also in some cases to reduce the cost of the metal inventory. Examples will be given in each category. Gamma-Butyrolactone (GBL,) is an intermediate for the manufacture of Pyrrolidones which have a wide range of uses such as speciality solvents, functional monomers and pharmaceutical intermediates. GBL can be made by the vapour phase dehydrogenation of 1,Cbutanediol over a copper/ umice catalyst at 200°C. It is also available as a by-product with THF in the avy McKee two step butane diol process starting from maleic anhydride (MAN) via diethylmaleate [27].

b

BD

MAN

MF

GBL

Recent patent literature describes the development of MAN hydrogenation processes to selectively produce GBL in high yield.

0

0 SAN

MAN

GBL

t

Phillips Petroleum 281 describe a two step trickle column reactor process via succinic anhydride S A N ) . The preferred catalyst for the first step is a 0.6% Pd, 25% cobalt-oxide on silica catalyst operating at 120°C and 900-1500 psig. The more demanding second step uses a 4% nickel, 2.5% nickel as nickel oxide on silica catalyst operating at 220°C and 900-1500 p i g . The reaction is very exothermic and the best result quoted in the patent is a 93.2% conversion of S A N with a 91% selectivity to GBL. Mitsubishi [29] describe a continuous liquid phase process using a ruthenium-organic phos hine stabilised homogeneous catalyst. The best result quoted was a 99.2% {AN convers' n with 93% selectivity to GBL and 7% highboilers at 205°C and 40 Kg cm-!fG hydrogen partial pressure. The process uses a high boiling point polyether solvent to facilitate the separation of the GBL from the catalyst. The UCB patent [30] describes a liquid phase process using a bimetallic catalyst whereby both steps are performed sequentially in the same autoclave with the second stage at higher pressure and temperature.

14

The best result quoted was a 96% conversion of S A N to 97% GBL at 235°C and 95 bar (second stage) with a 2% Pd, 23% Ni/Si02 catalyst using THF solvent. Table 6 -

Bimetallic Catalyst Performance at 50 bar (first stage) and 95 bar, 235°C (second stage) in THF solvent with a constant catalyst weight and reaction time

Catalyst

Conversion (mole %)

16% Ni, 2% Pd/Si02 16% Ni, 2% Pt/SiO 16% Ni, 1.7% M o d 0 2 51% Ni, 5% Mo/Si02

92.0 52.7 61.3 62.0

Selectivity to GBL (mole %) 94.7 93.8 94.6 95.2

_ I

-__

The results demonstrate inferior activity with N i P t and Ni/Mo catalysts but good selectivity ir, all cases. Table 7 - The Effect of Metal Loading under the Same Reaction Conditions as Table 6

Catalyst 22.5% Ni, I .O% Pd/SiO2 22.5% Ni, 1.9% Pd/Si02 22.5% Ni, 2.3% Pd/Si02 8.9% Ni, 2.3% Pd/Si02

Reaction Time (min) 300 180 180 330

Conversion (mule %) 87.4 92.4 95.4 89.6

Selectivity to GBL (mole 9’0) 96.4 95.9 96.0 96.0

The results show that a 25% metal loading and an approximate 101 NiPd ratio gives the best activity and high selectivity to GBL. The above literature demonstrates that bimetallic catalysts are allowing the development of highly selective MAN hydrogenation processes to make GBL. 4.2

Imuroved Activity

The synergistic effect between certain metals can allow the development of higher activity catalysts. These can lead to

*

shorter reaction times and lower by-products where by-product formation is time dependent.

15

*

improved selectivity by the ability to operate under milder reaction conditions

*

lower catalyst loadings and improved filtration times.

An example of the synergistic effect between palladium and platinum can be found in the catalytic selective reduction of nitrate ions to hydroxylamine ~311.

NO-3

+ 2H' + 3H2 ----->NI-I+30H + 2H20

This is one step in the Stamicarbon caprolactam process whereby the reaction is performed i n a phosphate buffer system using a catalyst activator. Overhydrogenation will form ammonium salts and underhydrogenation nitrogen and nitrous oxide. Although not a Fine Chemical example it clearly demonstrates the synergistic effect of platinum on palladium. Table 8 -

Activity and Selectivity to Hydroxylamine as a Function of Palladium/Total Metal Ratio in a 5% (Pd + Pt)/C Catalyst Catalyst Composition (% total metal)

---

Pd

Pt

Activity

Selectivity to Hydroxylamine

g (mol meta1.h

(%)

0.97 3.60 4.34 4.20 3.03 0.27

87 72 67 71 61 37

--

. 1 1 4 1 4 1 1 1 1 -

100 91 79 71 51 0

0 9 21 29 49 100

-

The results show that palladium on its own is more active and selective than platinum which favours the formation of ammonia. However, the addition of platinum to form a bimetallic catalyst increases the activity of the palladium catalyst four fold with a 1520% loss of selectivity in the range of 10-30% Pt content. 4.3

Precious Metal Inventory Cost Reduction

Rhodium is the most active metal for the catalytic hydrogenation of aromatic ring systems. It is also very expensive (7 Dec 1992 price ca f44,OOOMg) because of its demand in autocatalysts. In comparison, at the same time the price of palladium was only 6% that of rhodium. Hence a bimetallic catalyst which can allow thrifting of the rhodium can offer a substantial precious metal inventory cost reduction.

16

One such example involves the hydrogenation of a substituted pyridine.

H

Table 9 -

Substituted Pyridine Hydrogenation with Various Catalysts Under the Same Reaction Conditions ~

~~~

Catalyst

Selectivity

(%I

iE E!Y!%/N203 4% Pt, 1%Rh/N 0

4.5% Pd, 0.5% Rf?/d203

94 96 - 98 96 - 98 96 - 98

Reaction Time (hr) 9 5 5 5 ~~

The results show that the rhodium can be thrifted down to 0.5% in combination with palladium (cheaper than platinum) with improved activity and selectivity. This would represent a 85% metal cost reduction which could dramatically affect the economic viability of a process.

4.4

Analvtical Techniaues

Analytical techniques have advanced to give high lateral spatial resolution. It is now possible to measure the chemical composition of l n m individual crystallites of a supported bimetallic catalyst. This will allow the development of catalyst preparation methods to make bimetallic catalysts of more homogeneous composition. This could be particularly important where a monometallic crystallite generates an unwanted by-product. The technique involves a scanning transmission electron high brightness electron source, a field emission gun sutticient current in a 1-2nm diameter probe to excite a useful X-ray intensity. More information about the FEG-STEM is given in [32]. The theoretical sensitivity can be as low as a few atoms of one minor element within a particle largely composed of a different element but this does vary with the element combinations.

5.

Conclusions

Catalysis is a fascinating and exciting field and offers a great deal towards meeting the Fine Chemical process targets of improved efficiency and environmental compatibility.

17 Industry is demanding high yield, zero waste processes. Improved hydrogenation catalysts have been described arising from modification with another metal or in the case of enantioselective hydrogenation by means of modification with a chiral compound. Increased fundamental understanding of the latter is allowing the expansion of the technology to other catalytic metals and other types of reaction substrate.

+

Environmental le islation is resulting in the development of new markets. The phase-out of CF 's o ens up new opportunities for hydro enation catalysts in the manufacture Eof lH ! C 134a and other CFC substitutes. he cost of treatment of nickel contaminated waste water from sugar hydrogenation processes could encourage the change to ruthenium catalysts. Acknowledgements

I would like to thank my colleagues at Johnson Matthey and in particular Ken Griffin, Brian Harrison and Dave Grove for their ideas and critical review of the paper. In addition, Peter Wells (University of Hull) and John Titchmarsh (AEA Technology) for their contributions and Alison Neilson for preparing the manuscript.

References 1.

J. A. Cusumano in JM Thomas and K. I. Zamaraev (Editors), "Perspectives in Catalysis", Blackwell Scientific Publications, Oxford UK (1992) 1-33

2.

World Meteorological Organisation (1988) Report of the Ozone Trends Panel, Report No. 18

3.

Dupont Alternatives Circular # H-07421, August 1989

4.

L. E. Manzer, Catalysis Today, 13 (1992) 13-22

5.

R. P. Ruh, R. A. Darrs, M. R. Broadworth, U.S. Patent 2,885,427 (1959)

6.

I. Sobolev, U. S. Patent 4,782,643 (1988)

7.

S. L. Bell, US. Patent 4,129,603 (1978)

8.

L. E. Manzer, U. S. Patent 4,922,037 (1990)

9.

J. I. Darragh, G.B. Patent 1,578,933 (1980)

10.

C. Gervasutti, L. Marangoni, W. Marra, J. Fluorine Chem 19 (1982) 1-20

11.

S. Mosikawa, ACS Conference, Washington USA, Aug 1990

12.

K. Sakata, Japanese Patent Sho 42-66690 (1967)

18

13.

R. Albert, A.Stratz and G. Vollheim in W. Moser (Editor), "Catalysis of Organic Reactions", Marcel Dekker, New York, 1981,421-439

14.

J. Wisniak and R. Simon, Ind. Eng. Chem. Prod. Res. Dev 18-1 (1979) 50-57

15.

J. Wisniak, M. Hershkowitz and S. Stein, Ind. Eng. Chem. Prod. Res. Dev, 13-4 (1974) 232-236

16.

B. J. Arena, Applied Catalysis 87 (1992) 219-229

17.

FDA's Poli Statement €or the Development of New Stereoisomeric Drugs, Chirality 48992) 338

18.

A. Richards, Chimicaoggi Oct (1992) 15-18

19.

H. U. Blaser and M. Muller in M. Guisnet (Editor), "Heterogeneous Catalysis and Fine Chemicals II", Elsevier Science Publishers, Amsterdam, 1991, 73-92

20.

Y. Orito, S. Imai and S. Niwa, Nippon Kagaku Kaishi 1979, 1118; 1980, 670; 1982,137

21.

G . Webb and P. Wells, Catalysis Today 12 (1992) 319-337

22.

I. M. Sutherland, A. Ibbotson, R. B. Moyes and P. B. Wells, J. Catal 125 (1990) 77

23.

K. E. Simons, A. Ibbotson and P. B. Wells in "Catalysis and Surface Characterisation" (Editors T. J. Dines, C. H. Rochester and J. Thomson) Royal Society of Chemistry, Cambridge (1992) 174-183

24.

H. U. Blaser, H. P. Jalett, D. M. Monti, J. F. Reber and J. T. Wehrli in "Studies in Surface Science and Catalysis 41" (Hetero eneous Catalysis and Fine Chemicals), M. Guisnet et a1 (Editor) l!lsevier, Amsterdam 1988 p.153-163.

25.

H. Plum, P. Johnston and P. B. Wells unpublished work

26.

W. Vermeer and P. B. Wells unpublished work

27.

A. M. Brownstein, "Chemtech", August (1992) 506-510

28.

G. Bjornson and J. J. Stark, U.S. Patent 5,086,030 (1992)

29.

C. Mi azawa, K. Takahashi, H. Kameo, S. Isogal and M. Otake, EP 420062 (19913

30.

J-L. Dallons, P. Jacobs, J. Martens, P. Tastenhoye, I. Vanden Eynde and A. Van Gysel, EP 339012 (1989)

19

31.

C. G . M. van de Moesdijk, 'The Catalytic Reduction of Nitrate and Nitric Oxlde to Hydroxylamine : Kinetics and Mechanism", PhD Thesis (1979)

32.

J. M. Titchmarsh, "Microscopyand Analysis",September (1992) 9-11

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 @ 1993 Elsevier Science Publishers B.V. All rights reserved.

21

Catalytic oxidations with hydrogen peroxide: new and selective catalysts Mario G. Clerici Eniricerche SPA, Via Maritano 26, 20097 S. Donato Milanese, Italy

Abstract Recent results in the field of catalytic oxidations with hydrogen peroxide are reviewed. Most effective catalysts fall into three categories: metallorganic compounds, phase-transfer catalysts, redox zeolites. Metalloporphyrins and Ptphosphine complexes are representative of first category. Mo and W polyoxometalates and related systems, in association with phase transfer agents, belong to the second one. Titanium silicalite (TS-I) is the most studied redox zeolite. The oxidation of nitrogen and sulphur compounds and Fenton-like reactions are not reviewed. 1. INTRODUCTION

The role of hydrogen peroxide in organic synthesis has grown steadily over the years, as reflected by scientific and patent literature. There are a number of reason for this, which are worth mentioning. The “active oxygen” content of H202, 47% of its weight, is much higher than that of other oxidants, such as NaCIO, KHSO,, ROOH. Water is the only byproduct. As a result, less reagent is needed on a wt/wt basis and no inorganic salts are produced in the reaction, not even NaCl or KHSO,. Also, aqueous hydrogen peroxide is a stable reagent, provided it is handled and stored in the correct manner. Most common precautions are those against metallic impurities and organic contamination. Useful information on safety and hazards are found in the literature or can be obtained by the manufacturers [ I , 21. The catalytic hydroxylation of olefins with H202 was first reported by Milas in 1936 [3, 41 and studied in more detail by Mugdan and Young a few years later [ 5 ] . Nevertheless, the use of H,O, in catalysis has been far less successful than that of organic peroxides, such as TBHP and peracids [2, 6-81. The reasons for this are worth mentioning here. Most important organic substrates and aqueous H202 are mutually insoluble. Water and polar solvents, which are most suited to H20,,compete with the latter for active sites, poisoning the catalyst. Water-sensitwe products, such as the epoxides, can be decomposed, at the reaction conditions, by the aqueous medium IS].

22

These drawbacks have been in part overcome by the discovery, in recent years, of new and effective catalysts which will be the subject of this review. Fentonlike reactions and the oxidation of sulphur and nitrogen compounds will not be considered. Emphasis will be put on selective catalysts, discovered over the last decade, and especially on phase transfer [9] and redox zeolite catalysts [lo-I I]. 2. HOMOGENEOUS CATALYSTS

2.1 Early results In the early literature, the oxidation of unactivated double bonds, catalysed by various Group IV-VI metal oxides, results into vicinal rruns-dihydroxylation. Epoxides were not produced even by the most active catalyst, tungstic acid [4-51. Conversely, a$-unsaturated acids [ 12-131 and ally1 alcohols [ 14-1 51 were efficiently epoxidized, under controlled reaction conditions. Epoxides were eventually obtained from unactivated olefins, by modifying the catalysts or by using anhydrous hydrogen peroxide. The use of Group IVB metal-alkyl compounds R,MOH (M = Sn, Pb; R = Alkyl) with W(CO), and Mo(CO),, increased the yields up to 70-80% [2]. Near anhydrous conditions were proposed for the epoxidation of propylene and isobutylene [2, 161. Water added with aqueous H,O, and produced in the reaction was continuously removed by azeotropic distillation, with 85% yields to propylene oxide. However, the use of organometallic compounds and the azeotropic distillation of water are both unsatisfactory. The instability of \MOH, in an oxidising medium, is a severe limitahon to the catalyst life. Anhydrous organic solutions of hydrogen peroxides are potentially dangerous. Double bond cleavage to form aldehydes or carboxylic acids has been reported. The reaction occurs in non aqueous solvents, at relatively high temperature. p-Carotene, isoeugenol and anethole yield Vitamin A aldehyde [17], vanillin and anisaldehyde [18], respectively, in the presence of OsO, and V,O,. At near room temperature, the dihydroxylation is favoured. Dicarboxylic acids are produced from cycloolefins, with Re,O, or OsO, as the catalysts [19-201. In either cases, yields do not exceed 50%. With Pd catalysts, ketones are produced [21-221. It is apparent that epoxides, which are valuable intermediates in organic synthesis, could not be obtained by a general, selective and facile reaction of olefins with H,O,. The epoxidation of unactivated olefins occurred with low yields, or with catalysts characterised by insufficient working life, or in anhydrous media. Further, alkane and aromatic hydroxylation was only known for Fenton type catalysts. With the latter, yields and selectivity are generally unsatisfactory 171.

2.2 Recent developments Efficient epoxidation catalysts, have been discovered over the past decade. Re [23], Mo [24], W [24-251, Mn [26-321, Fe [26-281 porphyrins have been reported. Mn and Fe porphyrins, in the presence of imidazole or other donor axial ligands, catalyse epoxidation with high yields and stereoselectivity (retention of

23

configuration) [27]. Reaction rates are further accelerated by the addition of catalyk amounts of carboxylic acids and lipophylic donor ligands [30]. Epoxides yields of about 90% are produced, even by weakly reactive olefins, such as a-olefins. Catalyst deactivation is reduced, by introducing either bulky or electron withdrawing substituents in the rneso-phenyl rings. Different reaction mechanisms have been proposed for Mo [24], and for Mn and Fe [27, 301 porphyrins.

B&(&

HO

OOH

Iron complexes of cyclam (1,4,8,11 -tetraazocyclotetradecane), and related ligands, are epoxidation catalysts for cyclohexene and other olefins, using aqueous 30% H20? in acetonitrile or methanol [33]. The reaction is characterised by stereospecificity and low allylic oxidation. It is interesting to note that even anhydrous ferric chloride FeCI,, in dry acetonitrile, activates hydrogen peroxide to epoxidize alkenes, though yields do not exceed 60% [34]. Under analogous catalyses the stereoselective reaction conditions, tris(acetylacetonato)iron(IlI) P-epoxidation of cholesterol and its analogues [35]. This catalyst is unique in that it yields only rruns-epoxides from either truns and cis-olefins [36]. However, a large excess of H,O, is needed, as a result of competitive decomposition.

Worth mentioning is the asymmetric epoxidation of cis p-methylstyrene to an optical active epoxide, using Fe(II1)-bleomycin and related complexes [37]. Enantiomeric excess is 45%, H,O, yields are very low. Various L,Pt(CF,)(OH) complexes are selective catalysts for the epoxidation of terminal olefins, in monophasic (THF or EtOH) and biphasic media (CH,Cl,/H,O) [38-401. Attempts to epoxidize cyclohexene and cis 2-hexene failed. Diphosphine complexes, with rigid five-membered chelate rings, proved to be the best catalysts. Chiral diphosphine ligands give optically active epoxides, with enantiomeric excess up to 41% [39]. Molybdic and phosphomolybdic acids bound to 60 nm colloidal anion exchange polymer catalyse the epoxidation of cyclooctene with 90% selectivity [41-421. Only 12% epoxide is formed in the absence of colloidal particles. 1-Octene does not react. The higher activity of the peroxomolybdate bound to latex is attributed to the higher local concentration of alkene and catalyst in the polymer. The vicinal dihydroxylation of double bonds, with Re,O, [43], or MeReO, [MI, has been further investigated. Based on the different addition mechanism of OsO,

24

and MeReO, on the double bond of various substituted (R)-2-hydroxy-3-butenoic acids, all four possible 2,3-dihydroxy-y-butyrolactones can be obtained [45]. Depending on the derivatives and reagents used, diastereomeric excesses achieved are in the range of 840%. L,Pt(CF,)X complexes (with L,= tetraaryldiphosphines and X = CH,CI,, OH, OPh) are hydroxylation catalysts of phenol and phenol ethers, with 70% H,O, [46]. In all cases, high ortho-selectivity (up to 95%) is observed. Phenylalanine is hydroxylated by a water-soluble iron porphyrin complex to tyrosine and dihydroxy phenylalanine (DOPA) in good yields [47]. Other hydroxylation catalysts of various aromatic substrates are halogenated porphyrin complexes of Fe and Mn [48-491. Mn- and Fe-tetraarylporphyrins catal yse the direct hydroxylation of saturated C-H bonds with 30% H,O, [27, 50-521. Because of further oxidation, a mixture of alcohols and ketones IS produced. With cycloalkanes, the hydroxylation is fast and selective. In n-alkanes and alkylbenzenes, the reaction stops after a few turnovers. Baeyer-Villiger oxidation of akyl- and aryl-substituted C,-C, cycloalkanones, steroid ketones and branched chain aliphatic ketones is catalysed by arsonated polystyrene resins [53]. Larger size cycloalkanones and linear ketones react much slower. Water miscible and immiscible solvents can be used. With the latter, the resin behaves as an effective catalyst and a phase-transfer agent (triphase catalysis). The same compounds are also epoxidation catalysts. More recently, a method for the preparation of phenols by the oxidation of aromatic aldehydes and ketones has been reported. The most convenient catalysts are nitro-substituted arylseleninic acids and corresponding diselenides [54]. A major drawback with organometallic catalysts is the chemically instability of organic ligands under oxidative conditions. Competitive degradation of porphyrin and imidazole ligands has been observed [32, 521. This is minimised by introducing halogenated phenyl substituents at the meso-position of the porphyrin, but in the long term degradation cannot be totally eliminated.

3. PHASE-TRANSFER CATALYSTS As pointed out earlier, the lack of a common solvent, for aqueous H,O, and certain organic substrates, may result in a slow reaction rate and poor selectivity. This serious limitation has been circumvented with the aid of phase transfer catalysis, a well known technique in organic synthesis [55]. It consists in the transfer of a water soluble oxidant species into the immiscible organic phase, as a quaternary ammonium or phosphonium salt. Two main results are achieved by this technique. The reaction rate is increased, due to higher concentration of the oxidant species in the organic phase. Acid catalyzed side reactions are decreased, by keeping the products in the organic phase. The fust example reported was the efficient epoxidation of water insoluble olefins with H,O,, under acidic conditions [9]. Subsequently, phase transfer

25

catalysis was extended to oxidation of acetylenes, oxidative cleavage of double bonds and vicinal diols, oxidation of alcohols, and hydroxylation of aromatics. Most studied catalysts are constituted by molybdate or tungstate salts, associated with phosphoric or arsenic acids. Heteropolycompounds are also used. Ammonium or phosphonium lipophilic salts constitute the phase transfer agent. Sometimes phosphate and arsenate components are omitted, or a lipophilic ligand is used instead of the quaternary ammonium cation. Their use as homogeneous catalysts, in alcohol solvents, is also known. M04-2/ X04-3/ R4N+ (M=Mo,W

H3PM12040 / R4N+

X=P,As)

The epoxidation of terminal and internal olefins is carried out with dilute H,O, (<35%), below 70°C [9, 56-67]. For most olefins, the optimum pH is close to 2 [56]. The yields are generally hgh (sometimes higher than 90%) and decrease for lower olefins, whose epoxides are quite soluble in the aqueous phase. The reaction is characterised by electronic effects prevailing over steric effects of the substituents, and by regio and stereoselectivity. The epoxidation of cis and trans olefins occurs with retention of configuration. 4-Vinylcyclohexene and pregna4(Z), 17(2O)-dien-3-one yield 1,2-epoxy-4-vinylcyclohexane and (Z)17a,20-epoxypregn-4-en-3-one respectively [57]. The epoxidation of 5-vinylbicyclo[2.2.1IheptIheptane [59], selectively. Dicy2-ene affords exo-2,3-epoxy-5-vinylbicyclo[2.2.1 clopentadiene derivatives are regio and stereoselectively epoxidized in good yields.

&

--/

exo

Different orientation effects are exerted by hydroxy and acetoxy substituents, at the same position [64]. Allylic alcohols are smoothly epoxidized at ambient temperature to give the corresponding epoxy alcohols in good yields. With geraniol, only the allylic double bond is epoxidized (98% selectivity). 4-Methyl3-penten-2-01 yields threo-3,4-epoxy-4-methyl-2-pentanol.Other olefins, such as 1-hexen-3-01, give a mixture of threo and erythro isomers [59]. The oxidation of alcohol group, to yield the corresponding unsaturated ketones, preferentially occurs by decreasing the concentration of catalyst and hydrogen peroxide

26

and by increasing the pH [68-691. Vinyl acrylates are only epoxidized at the vinyl positions [65].

Under different reaction conditions, vicinal diol production [70] or C=C double bond oxidative cleavage to carboxylic acids occurs [59, 711. Dialdehydes are produced from cycloolefins, by tungstic acid as catalyst in t-butanol [72]. Secondary alcohols yield ketones, while primary alcohols produce aldehydes or carboxylic acids [59, 68-69, 73-74]. Different products are obtained from glycols, under different reaction conditions. 1,2-Diols are cleaved to ketocarboxylic acids and dicarboxylic acids [58, 751, or oxidised to a-hydroxy ketones [76]. The latter can be obtained directly from the olefins, with lower selectivity [77]. Lactones are formed by 1,4-diols and other a,o-diols [78]. Internal alkynes predominantly yield a$-epoxyketones [79], or 1,2-diketones and carboxylic acids if Hg(AcO), is added as the cocatalyst [80]. Terminal alkynes yield a-ketoaldehydes and carboxylic acids. Few studies refer to the oxidation of aromatics. The hydroxylation of benzene to phenol [81] and the oxidation of alkylaromatics to arylcarboxylic acids [82] have been claimed. The oxyfunctionalization of saturated C-H bonds has not been reported. A q u a t e r n a r y a m m o n i u m tetrakis(diperoxotungsto)phosphate, [(C,H,,)4N+],[P0,[WO(02)2]4]~-, has been isolated and characterised by Venturello et ul. [56]. This and an analogous Mo species, produced in the reaction medium by heteropolycompounds or M0,-2/H,P04 (M = Mo, W) and hydrogen peroxide, are possible intermediates in the epoxidation of olefins [56, 59, 831. 4. REDOX ZEOLITES

The synthesis of titanium silicalites TS-1 [lo] and TS-2 [84-851, with MFI and MEL structure respectively, opened new opportunities in the oxidations with H,O,. TS-I and TS-2, the former being the most studied, show similar properties in catalysis. Catalytic sites are isolated titanium atoms, incorporated into the zeolitic framework, in a channel system of about 0.55 nm average diameter [86-881. Different Ti-peroxo species, at lattice position, result from complex equilibria between TS-I, H,O,, and protic molecules [89-911.

27

4.1 Titanium silicalite (TS-1) TS-1 is an efficient and selective catalyst for the oxidation of various organic molecules with H,O,: sulphur and nitrogen compounds, alcohols, olefins, aromatic and aliphatic C-H bonds [ 11, 92-93]. Selectivity is the result of the electrophilic properties of active oxidant species and of the shape selectivity. The latter arises from diffusion of reagents and products and from steric constraints in the transition state (restricted transition state shape selectivity). Molecules having a cross section larger than about 0.6 nm cannot diffuse to TS-1 active sites and are not oxidised. This restricts TS-I catalysis to almost linear molecules and mononuclear aromatic compounds, bearing small or no substituents. On the other hand, small molecules can be selectively oxidised in the presence of bulkier ones. Primary, secondary, and tertiary amines yield N-substituted oximes, hydroxylamines, and N-oxides, respectively [93-941. Secondary alcohols are selectively oxidised to corresponding ketones. From primary alcohols, aldehydes or carboxylic acids are produced, under low or high conversion conditions, respectively. It is noteworthy that the oxidation of methanol is sufficiently slow, to allow its use as the solvent of choice for most reactions. The rate of oxidation of primary and secondary alcohols decreases with increasing chain length and number of substituents. The OH group position is important as well. 2-Pentanol reacts 13 times faster than 3-isomer [93].

The epoxidation of olefins is best carried out in methanol at near room temperature [95-961. The reaction is fast and selective with unhindered olefins. Yields are almost quantitative. Turnover rates as high as 2 s-' have been observed for propylene [95]. Proposed active species is a five membered ring peroxo complex, formed by a Ti-OOH species and a protic molecule ROH, at lattice positions [96]. This shows electrophilic properties. However, the epoxidation of allyl chloride and allyl alcohol, is still fast at 40'C. Epichlorohydrin and glycidol are produced with 75% and 85% selectivity, respectively. Styrene yields P-phenyl acetaldehyde [97]. Recently reported C=C double bond cleavage [98] is likely the result of impure TS-I, as shown by Millini et al. [87]. Ally1 methacrylates are epoxidized at allylic position 1931. Dienes and diallylcarbonates are monoepoxidized [93].

H

O

G

*

HO

H O e O H

+

HO&

The hydroxylation of phenol to cathecol and hydroquinone, has already been commercialised by Enichem [93]. Anisole and other substituted benzenes are similarly hydroxylated by TS-l/H,O,. Shape selectivity plays a major role in these reactions. As a result, p-hydroxylation occurs to a higher extent than

28

with homogeneous catalysts. Production of tarry by-products, by further oxidation, is minimised [93, 99-1001.

H202

\

C , H2

\ 0 CHOH

+

>C=O

Aliphatic C-H bonds, at secondary and tertiary positions, are oxidised under mild conditions to corresponding alcohols and ketones [89, 101-106]. Primary C-H bonds do not react. Methanol is rather more stable towards oxidation than paraffins and therefore can be used as solvent for the latter [89]. The reaction can be performed as well with aqueous H,O,, in a two liquid phase system. As a result of shape selectivity, linear alkanes react faster than branched and cyclic ones. CH, groups nearest to terminal positions are preferentially oxidised: P-CH, > y-CH,. Linear alkylbenzenes are oxidised preferentially at the side chain. Electron withdrawing substituents, such as chlorine, carbonyl, hydroxyl and ester groups, exert a retarding effect on vicinal C-H bond reactivity enhancing regioselectivity [89]. Thus, the oxidation of 1-chlorohexane and methylheptanoate occurs at 0-1 and 0-2 positions. Only mono-oxyfunctionalization is observed in lower alkanes.

>C=O

+ NH3

+

H202

-

0 ‘C=NOH

+ 2H20

Ammonia is easily oxidised to hydroxylamine and subsequently to nitrogen oxides. In the presence of ketones, ketooximes are selectively produced. A new process, based on this route, for the production of cyclohexanonoxime is now under study [ 103-1041. As a general rule, the epoxidation of olefins is the faster reaction of those catalysed by TS-I. The oxidation of alcohols and the oxyfimctionalization of alkanes are somewhat slower. The hydroxylation of aromatics is slowest. With sterically hindered molecules, the reactivity order might be totally different. Over the past few years several reports on reactions attributed to TS-1 have appeared in the literature. In a number of cases, their value is questionable because the composition of the catalyst is unknown [87]. Actually, impure TS-I samples are easily obtained with low purity reagents. These impure samples are admixtures of TS-I with various amounts of other titanium compounds, such as amorphous and crystalline TiO,, and Ti-silicates, which have different catalytic properties. Thus, reported yields are often the result of various competitive oxidation pathways.

4.2 Other redox zeolites The remarkable properties of TS-I, have stimulated research on other redox

29

zeolites. Al-, Ga-, Fe-TS-I have been prepared [105]. The presence of strong acid sites lead to the production of glycol derivatives in the oxidation of olefins. Fe-TS-I showed lower selectivity on H,O,. The synthesis of other medium pore zeolites, V-silicalites VS-I [ 106-1071 and VS-2 [ 1081, and Ti-ZSM-48 [109], has been reported. Almost no data their catalytic properties are available so far. The synthesis of large pore Ti-containing zeolites, Ti-Y [I101 and Ti,Al-P [ I 1 I], has recently been claimed. Preliminary catalytic studies show poor results.

5. CONCLUSIONS Important results have been acheved, in the past few years, in the design of efficient catalysts for oxidations with hydrogen peroxide. Some of them are organometallic complexes, which may suffer from oxidative degradation in oxidising media. Phase transfer and redox zeolite catalysts, which have complementary properties, are more promising. With the former catalysts, water insoluble and larger molecules are oxidised by hydrogen peroxide. Smaller molecules, soluble in aqueous and alcohol media, are preferentially oxidised with TS-1. Although titanium silicalite was discovered over a decade ago, studies on other redox zeolites are just beginning. This is a field in which the most promising results are anticipated. The synthesis of large pore zeolites, containing Ti or other redox metal atoms, can open entirely new routes to catalytic oxidations and new products. 6. REFERENCES 1

E.S. Shanley, in Organic Peroxides (D. Swern, Ed.), Vol. 111, p. 341. Wiley, New York, 1972. 2 J.P. Schirmann and S.Y. Delavarenne, Hydrogen Peroxide in Organic Chemistry, Edition et Documentation lndustrielle, Paris, 1979. N.A. Milas and S. Sussman, J. Am. Chem. SOC., 58 (1936) 1302. N.A. Milas, J. Am. Chem. SOC., 59 (1937) 2342. M. Mugdan and D.P. Young, J. Chem. SOC., (1949) 2988. D. Swern in Organic Peroxides (D. Swern, Ed.), Vol. 11, Wiley, p. 345. New York, 1972. 7 R.A. Sheldon and J. Kochi, Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981. 8 R. Landau, G.A. Sullivan and D. Brown, Chemtech, 9 (1979) 602. 9 C. Venturello, E. Alneri and M. Ricci, J. Org. Chem., 48 (1983) 3831. 10 M. Tamarasso, G. Perego and B. Notari, U.S. Patent 4 410 501 (1983). 1 1 U. Romano, A. Esposito, F. Maspero, C. Neri and M.G. Clerici, in New Developments in Selective Oxidation ( G . Centi and F. Trifiro, Eds.), Studies in Surface Science and Catalysis, Vol. 55, p. 33. Elsevier, Amsterdam, 1990. 12 G.B. Payne and P.H. Williams, J. Org. Chem., 24 (1959) 54.

30

K.S. Kirshenbaum and K.B. Sharpless, J. Org. Chem., 50 (1985) 979. Z. Raciszewski, J. Am. Chem. SOC., 82 (1960) 1267. H.C. Stevens and A.J. Kaman, J. Am. Chem. SOC., 87 (1965) 734 A.M. Mattucci, E. Perrotti and A. Santambrogio, J. Chem. SOC , Chem. Commun., (1970) 1198. 17 N.L. Wendler, C.’ Rosenblum and M. Tishler, J. Am. Chem. SOC., 72 (1950) 13 14 15 16

234. 18 19 20 21 22 23

N.A. Milas, U.S. Patent 2 414 385 (1947). G.W. Parshall, US. Patent 3 646 130 (1972). N.A. Milas, U.S. Patent 2 395 638 (1946). M. Roussel and H. Mimoun, J. Org Chem., 45 (1980) 5387. J. Tsuji, H. Nagoshima and K. Hori, Chem. Lett., 1980) 257. J. Buchler, S. Kruppa, M. Schmidt and G. Prescher, Eur. Patent 308 790 and

308 791 (1989). 24 G. Lagemaat, W. Drenth, M. Schmidt, G. Prescher and G. Goor, J. Mol. Catal., 62 (1990) 119. 25 G. Goor, G . Prescher and M. Schmidt, in New Developments in Selective Oxidation (G. Centi and F. Trifirb, Eds), Studies in Surface Science and Catalysis, vol. 55, p. 71. Elsevier, Amsterdam, 1990. 26 J.P. Renaud, P. Rattioni, J.F. Bartoli and D. Mansuy, J. Chem. SOC., Chem. Commun., (1985) 888. 27 P. Battioni, J.P. Renaud, J.F. Bartoli, M. Reina-Artiles, M. Fort and D. Mansuy, J. Am. Chem. Soc., I10 (1988) 8462. 28 A. Robert, B. Loock, M. Momentau and B. Meunier, Inorg. Chem., 30 (1991) 706. 29 P.L. Anelli, S. Banfi, F. Montanari and S. Quici, J. Chem. SOC., Chem. Commun., (1989) 779. 30 S. Banfi, F. Legramandi, F. Montanari, G.L. Pozii and S. Quici, J. Chem. SOC.,Chem. Cormnun., (1991) 1285. 31 S. Banfi, A. Maiocchi, F. Montanari and S. Quici, Gazz. Chim. Ital., 120 (1990) 123. 32 A.M.A. Rocha Gonsalves, R.A.W. Johnstone, M.M. Pereira and J. Shaw, J. Chem. SOC., Perkin Trans. 1, (1991) 645. 33 N. Wonwoo, R. Ho and J.S. Valentine, J. Am. Chem. SOC., 113 (1991) 7052. 34 H. Sugimoto and D.T. Sawyer, J. Org. Chem., 50 (1985) 1784. 35 M. Tohma, T. Tomita and M. Kimura, Tetrahedron Lett., 44 (1973) 4359. 36 T. Yatnamoto and M. Kimura, J. Chem. SOC., Chem. Commun., (1977) 949. 37 Y. Koku, M. Otsuka and M. Ono, Chem. Lett., 4 (1989) 611. 38 R.A. Michelin and G. Strukul, J. Am. Chem. SOC., 107 (1985) 7563. 39 A. Zanardo, F. Pinna, R.A. Michelin and G. Strukul, Inorg. Chem., 27 (1988) 1966. 40 R. Sinigaglia, R.A. Michelin, F. Pinna and G. Strukul, Organometallics, 6 (1987) 728. 41 S. Srinivasan and W.T. Warren, Polym. Mater. Sci. Eng., 64 (1991) 355. 42 S. Srinivasan and W.T. Warren, New J. Chem., 15 (1991) 693.

31

43 S. Warwel, M.K. Ruesch and M. Sojka, J. Chem. SOC., Chem. C o m u n . , (1991) 1578. 44 W.H. Hemnann, J. Organomet. Chem., 382 (1990) 1. 45 H. Yu and H. Simon, Tetrahedron, 47 (1991) 9035. 46 A. Marsella, S. Agapakis, F. Pinna and G. Strukul, Organometallics, 11 (1992) 3578. 47 T. Shimidzu, T. Iyoda and N. Kanda, J. Chem. SOC., Chem. Commun., (1981)1206. 48 S. Tsuchiya and M. Seno, Chem. Lett., (1989) 263. 49 M.N.Carrier, C. Scheer, P. Gouvine, J.F. Bartoli, P. Battioni and D. Mansuy, Tetrahedron Lett., 31 (1990) 6645. 50 P. Battioni, J.P. Renaud, J.F. Bartoli, and D. Mansuy, J. Chem. SOC., Chem. Commun., (1986) 341. 51 S. Banfi, A. Maiocchi, A. Moggi, F. Montanari and S. Quici, J. Chem. SOC., Chem. Commun., (1990) 1794. 52 A.M.A. Rocha Gonsalves, R.A.W. Johnstone, M.M. Pereira, J. Shaw, A.J.F. Sobral, Tetrahedron Lett., 32 (1991) 1355. 53 S.E. Jacobson, F. Mares and P.M. Zambri, J. Am. Chem. SOC., 101 (1979) 6938. 54 L. Syper, Synthesis, (1989) 167. 55 C.M. Starks. J. Am. Chem. SOC., 93 (1971) 195. 56 C. Venturello, R. D’Alosio, J.J. Bart and M. Ricci, J. Mol. Catal., 32 (1985) 107. 57 C. Venturello and R. D’Alosio, J. Org. Chem., 53 (1988) 1553. 58 Y. Matoba, H. Inoue, J. Akagi, T. Okabayashi, Y. Ishii and M. Ogawa, Synth. Commun., 14 (1984) 865. 59 Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida and M. Ogawa, J. Org. Chem., 53 (1988) 3587. 60 Y. Ishii, K. Yamawaki, T. Yoshida, T. Ura and M. Ogawa, J. Org. Chem., 52 (1987) 1868. 61 M. Schwegler, M. Floor and H. Van Bekkum, Tetrahedron Lett., 29 (1988) 823. 62 M.W. Droege and R.C. Finke, J. Mol. Catal., 69 (1991) 323. 63 L.J. C s h y and K. Jaky, J. Catal., 127 (1991) 42. 64 Y. Ishii and M. Ogawa, Sekiyu Gakkaishi, 29 (1986) 419, in Chem. Abstr. 106, 176070. 65 Y. Fort, A. Olszewski-Ortar and P. Caubere, Tetrahedron, 48 (1992) 5099. 66 0. Bortolini, F. Di Furia, G. Modena and R. Seraglia, J. Org. Chem., 50 (1985) 2688. 67 J. Prandi, H.B. Kagan and H. Mimoun, Tetrahedron Lett., 27 (1986) 2617. 68 C. Venturello and M. Gambaro, J. Org. Chem., 56 (1991) 5924. 69 B.M. Trost and Y. Masuyama, Tetrahedron Lett., 25 (1984) 173. 70 C. Venturello and M. Gambaro, Synthesis, (1989) 295. 71 C. Venturello and M. Ricci, Eur. Patent 122 804 (1984). 72 D. Jin#a, X. Xinhua, C. Haiying and J. Anren, Tetrahedron, 48 (1992) 3503.

32

73 0. Bortolini, V. Conte, F. Di Furia, G. Modena, J. Org. Chem., 51 (1986) 266 1. 74 K. Yamawaki, H. Nishihara, T. Yoshida, T. Ura, H. Yamada, Y. Yishii and M. Ogawa, Synth. Commun., 18 (1988) 869. 75 C. Venturello and M. Ricci, J. Org. Chem., 51 (1986) 1599. 76 Y. Sakata and Y. Ishii, J. Org. Chem., 56 (1991) 6223. 77 Y. Sakata, Y. Katayama and Y. Ishii, Chem. Lett. (1992) 671. 78 Y. Ishii, T. Yohiada, K. Yamawaki and M. Ogawa, J. Org. Chem., 53 (1988) 5549. 79 Y. Ishii and Y. Sakata, J. Org. Chem., 55 (1990) 5545. 80 F.P. Ballistreri, S. Failla, E. Spini and G.A. Tomaselli, J. Org. Chem., 54 (1989) 947. 81 S.W. Brown, A. Hackett, A. Johnstone, A.M. King and W.R. Sanderston, WO 9214691. 82 Y. Saito, S. Araki, Y. Sugita and N. Kurata, Eur. Patent 193 368 (1986). 83 C. Aubry, G. Chottard, N. Platzer, J. M. Bregeault, R. Thouvenot, F. Chauveau, C. Huet and H. Ledon, Inorg. Chem., 30 (1991) 4409. 84 G. Bellussi, A. Carati, M.G. Clerici, A. Esposito, R. Millini and F. Buonomo, Belg. Patent 1 001 038 (1989) 85 J.S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 58 (1990) L1. 86 G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, in Proceedings 7th Internutionul Zeolite Conference (A. Murakami, A. lijima and J.W. Ward, Eds), p. 129. Kodansha, Tokyo, 1986. 87 R. Millini, E. Previde Massara, G. Perego, G . Bellussi, J. Catal., 137 (1992) 497. 88 M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrini, in Structure und Reuctiwity of S u ~ u c e s (C. Morterra, A. Zecchina and G. Costa, Eds), Studies in Surface Science and Catalysis, Vol. 48, p. 133. Elsevier, Amster dam, 1989. 89 M.G. Clerici, Appl. Catal., 68 (1991) 249. 90 G. Bellussi, A. Carati, M.G. Clerici, G. Maddinelli and R. Millini, J. Catal., 133 (1992) 220. 91 M.G. Clerici, P. lngallina and R. Millini, in Proceedings of 9th Internutionul Zeolite Conference (R. von Ballmoos, J.B. Higgins and M.M.J. Treacy, Eds). Butterworth-Heinemann, Montreal, 1992. 92 B. Notari, in Innovution in Zeolite Muteriuls (P.J. Crobet, W.J. Mortier, E.P. Vansant and G. Schulz-Ekloff, Eds), Studies in Surface Science and Catalysis, Vol. 37, p. 413. Elsevier, Amsterdam, 1988. 93 U. Romano, A. Esposito, F. Maspero, C. Neri and M.G. Clerici, Chim. Ind. (Milan), 72 (1990) 610. 94 S. Tonti, P. Roffia, A. Cesana, M.A. Mantegazza and M. Padovan, Eur. Patent 314 147 (1989). 95 M.G. Clerici, G . Bellussi and U. Romano, J. Catal., 129 (1991) 159. 96 M.G. Clerici and P. Ingallina, J. Catal., 140 (1993). 97 C. Neri and F. Buonomo, Eur. Patent 102 097 (1986).

33

98 J. S. Reddy, U.R. Khire, P. Ratnasamy and R.B. Mitra, J. Chem. Soc., Chem. Commun., 1234 (1992). 99 B. Kraushaar-Czametzki and J.H.C. Van Hooff, Catal. Lett., 2 (1988) 43. 100 A. Tuel, S. Moussa-Khouzami, Y. Ben Taarit and C. Naccache, J. Mol. Catal., 68 (1991) 45. 101 T. Tatsumi, M. Nakamura, S . Negishi and H. Tominaga, J. Chem. SOC.,Chem. Commun., (1990) 476. 102 D.R.C. Huybrects, L. De Bruycker and P.A. Jacobs, Nature, 345 (1990) 240. 103 P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti and P. Gervasutti, in New Developments in Selective Oxidations (G. Centi and F. Trifiro, Eds), Studies in Surface Science and Catalysis, Vol. 55, p. 43. Elsevier, Amsterdam, 1990. 104 A. Zecchna, G. Spoto, S. Bordiga, F. Geobaldo, G. Petrini, G. Leofanti, M. Padovan, M. Mantegazza and P. Rofia, in Preprints of 10th Internutionul Congress on Cutuly.sis, p. 105. Budapest, 1992. 105 G. Bellussi, A. Carati, M.G. Clerici and A. Esposito, in Preprution of Cutulysts Y (G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, Eds), Studies in Surface Science and Catalysis, Vo1.63, p. 421. Elsevier, Amsterdam, 1991. 106 M.S. Rigutto and H. Van Bekkum, Appl. Catal. 68 (1991) L1. 107 Ci. Bellussi, G. Maddinelli, A. Carati, A. Gervasini and R. Millini, in Proceedings of 9th Internutionul Zeolite Conference (R. von Ballmoos, J.B. Higgins and M.M.J. Treacy, Eds). Buttenvorth-Heineman, Montreal, 1992. 108 P.R. Hari Prasad Rao and A.V. Rmaswamy, J. Chem. Chem., Chem. Comrnun., ( 1992) 1245. 109 D.P. Serrano, H.X. Li and M.E. Davis, J. Chem. Soc., Chem. Commun., (1992) 745. 110 C. Ferrini and H.W. Kouwenhoven, in New Developments in Selective Oxidution (G. Centi and F. Trifirb, Eds), Studies in Surface Science and Catalysis, vol. 55, p. 53. Elsevier, Amsterdam, 1990. 1 1 1 M.A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1992) 589.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicnls III 0 1993 Elsevier Suence Publishers B.V. All rights reserved.

35

Basic catalysts and fine chemicals Hideshi Hattori Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan Abstract Catalytic behaviors of solid base catalysts for fine chemicals synthesis as well as the fundamental reactions are described. The reactions included are double bond isomerization of olefins, addition of hydrogen and amines to conjugated dienes, dehydration, dehydrogenation, reduction, alkylation, aldol addition and condensation, Wittig-Horner and Knoevenagel reactions, dehydrocyclodimerization, and ring transformation. The characteristic features of different types of solid base catalysts, zeolites, metal oxides, solid superhases and non metal-oxides, are summarized. 1. Introduction

In sharp contrast to solid acid catalysts which are used in wide varieties of reactions in petroleum refining, petrochemical processes and fine chemicals production, solid base catalysts have been used to a small extent. Although Ba(OH)2 is well known to catalyze aldol addition of acetone and described in every textbook of organic chemistry, the other types of solid base catalyst have been studied in recent years. In 1955, Pines reported that alumina supporting metallic Na showed a high activity for double bond isomerization of olefins[l], which was practically the initiation of the studies of solid base catalysts. Thereafter, not only the catalysts supporting basic compounds like Na, but also single component metal oxides were found to show activities for various base-catalyzed reactions. In addition to the single component metal oxides, zeolites containing different metal ions to control the acid-base properties and non metal-oxides are increasingly studied. Advantages of the use of solid base catalysts in fine chemicals syntheses are pointed out to be as follows. 1 Separation of the products from the catalyst is easy. 2 Catalysts are repeatedly used without disposal of the used catalysts. 3 The reaction temperature can be raised because a solvent is not necessarily used. 4 Continuous processes can be designed by use of a solid catalyst.

Besides these points, reactions specific to the surface reactions are expected, and

, because of the fixed state of the active sites on the surfaces, co-existence of acid sites and basic sites on the same surface are possible. In the present paper, selected reactions including fundamental reactions in which solid base catalysts are used are introduced in the first part, and the characteristic features of different types of catalysts are described in the second part.

36 2. CATALYTIC BEHAVIORS OF SOLID BASE CATALYSTS IN SEVERAL

REACTIONS 2.1. Double bond isomerization Although 1-butene isomerization to 2-butene is not an attractive reaction from the view point of organic synthesis, the reaction has been extensively studied over many solid base catalysts because the reaction is a good test reaction to probe the surface properties and surface reaction mechanisms. Over solid base catalysts, 1-butene isomerizationproceeds by the scheme as shown below[2]. At first, an allylic H in 1-butene is abstracted in the form of an H t by basic site to form allyl anions as intermediates. The allyl anions are stabilized on the matal cations on the surfaces. In the form of allyl anion, cis form is more stable than trans form. Therefore, cis-2-butene is predominantly formed at the initial stage of the reaction. The reaction involves intramolecular H transfer, which was evidenced by co-isomerization of 1-butene do/de[3,4]. The products consist mostly of do and de 2butenes. The reaction takes place at room temperature or below over most of the solid base catalysts. Over MgO, for an example, the reaction occurs even at 223K if the catalyst is properly activated.

tH'

CHyCH /...-. \

CH~=CIi-CH-CH~

'

-2 1 CH2

a

tHt

,CH:Cti CH;

@

3

tHt

\

aiz cis-2-butene

,a13

I-bulenc

/CH=Gi

c -

t Hi

__t

tH'

/CH=CH

/a2

mz rrans-2-butene

Based on the results of 1 -butene isomerization, solid base catalysts were applied to the double bond isomerization of olefins having more complexed structure such as pinene, illudadiene[5], 5-vinylbicyclo(221)heptene[6]etc., as shown below. Over alkaline earth oxides and Na/NaOH/A1203, the double bond isomerization selectively takes place. The reactants contain three membered and four membered rings in the molecules. If solid acid catalysts were used, the ring opening reactions would occur, and the selectivity for double bond isomerization should markedly decrease. One characteristic feature of solid base catalyst is a lack of C-C bond cleavage ability. Therefore, the double bond isomerization selectively occurs without C-C bond cleavages. As mentioned above, the solid base catalysts are highly active for double bond isomerization, it is possible to carry out the reaction at a low temperature. This is advantageous especially for the olefins like 5-~inylbicyclo(221)heptene which is unstable at high temperature. Because of this advantage, Na/NaOH/A1203 is used in an industrial process for the selective double bond isomerization of 5vinylbicyclo(221)heptene.

37

For the double bond isomerization of unsaturated compounds containing hetero atoms such as N and 0, solid base catalysts act as an efficient catalyst. Solid acid catalysts would be poisoned by hetero atoms and show no activity. In contrast, the active sites of solid base catalysts interact little with hetero atoms and, therefore, solid base catalysts act as efficient catalysts. The reaction mechanisms for double bond isomerization of unsaturated compounds containing N and 0 are essentially the same as those for 1-butene isomerization. The basic sites abstract an H+from the reactant to form allyl anion as an intermediate[7]. The reaction scheme for allylamines is shown below. In this scheme too, the intermediate of allyl anion is more stable for cis form than for trans form, and the products are mostly in the form of cis. I

I

I

tH'

CH.7."

//& (-342

\

N-

I

/" =" \ N-

"3

I

lsomerization of safrol to isosafrol proceeds at 300 K over Na/NaOH/AI203[6].

safrole

isosafrole

38

Over solid base catalysts, double bond isomerization of ethers proceeds as shown below. 3-Methoxycyclohexene is unreactive probably because allylic H is difficult to be abstracted for the structural reasons. The activities of alkaline earth oxides are in the order of CaO z SrO > La2Op MgO >> Tho2 , ZrO2[8] c=c-c-0-c-c

-

c-c=c-a-c-c

0"C

0" C

c - 0 0

-

c - 0 0

No reaction

2.2.Addition of hydrogen(hydrogenati0n) and amines(amination), and reduction Conjugated dienes like 1,3-butadiene and isoprene undergo hydrogenation over certain types of solid base catalysts such as alkaline earth oxides[9,10], rare earth oxides[l0-12], Zr02[13], and Th02[14]. Alkali metal oxides are not so active as compared to the catalysts mentioned above. In the hydrogenation of conjugated dienes, the products consist exclusively of mono-olefins; no further hydrogenation to alkanes scarcely take place. The monoolefins are difficult to be hydrogenated over the solid base catalyst. A large difference in the reactivity between conjugated dienes and mono-olefins is one of the characteristic features which distinguish the solid base catalysts from metallic hydrogenation catalysts. For an example, 1,3-butadiene undergoes hydrogenation at 273 K on MgO, while 2-butene hydrogenation needs a reaction temperature above 473 K. The large difference in the reactivity arises from the stability of the intermediates. An ally1 anion which is the intermediate of conjugated diene hydrogenation is much more stable than an alkyl anion which is the intermediate of hydrogenation of mono-olefin. The reaction scheme of 1,3-butadiene hydrogenation is shown below where H is replaced by D for clarity. The products contain two D atoms at the terminal C atoms if D2 is used in stead of H2.

39

Amines also undergo addition reaction with conjugated dienes over the solid base catalysts which exhibit activity for hydrogenation[l5]. Primary and secondary amines add to conjugated dienes to produce unsaturated secondary and tertiary amines, respectively. The reaction mechanisms are essentially the same as those for hydrogenation in the sense that heterolytic dissociation of hydrogen molecule ( H2 H+ + H- )and amine (RNH2 -+ H+ + HNH- )are involved in the reaction. The sequence that anion and Ht successively add to 1,4 position of conjugated dienes is common to hydrogenation and amination. The reaction mechanisms for addition of dimethylamine to 1,3-butadine, for example, are shown below. The reaction takes place at 273 K over CaO.

-

CH3,

NH + ci12+0?CH3

/

-

H3C

“6 &2t

a

3

Hi 02-

Direct hydrogenation of aromatic carboxylic acids to the corresponding aldehydes has been industrialized by use of modified Zr@ catalyst by Mitsubishi Chemical Co.[l6]. Although the reaction mechanisms are not clear at present, the hydrogenation and dehydration abilities of Zr02, which are associated with basic properties, seem to be important for promoting the reaction. By modification of ZrO2 with metal ions such as Cr and Mn, the activity is increased, crystallization is suppressed and the coke formation is avoided. ArCOOH

+ H2

--

ArCHO t HzO

Zr@ promotes transfer hydrogenation. 1,3-Butadiene undergoes transfer hydrogenation with 1,3-~yclohexadieneto form l-butene over ZrO2[17]. The selective formation of l-butene is in contrast to the hydrogenation with H2 where trans-2-butene is selectively formed. Meerwein-Pondorf-Verley reduction is the hydrogenation in which alcohols are used as a source of hydrogen, and one of the hydrogen transfer reactions. M-P-V reduction of aldehydes, ketones and esters are efficiently catalyzed by hydrous Zr02 catalyst[l8]. In these reactions, 2-propanol is the best for hydrogen source. X zeolites ion-exchanged with Cs+ and Rb+ were used for M-P-V reduction of aldehydes[l9]. The mechanisms proposed for the zeolites are shown below. The reaction is initiated by abstraction of an H+from alcohol by basic sites of the catalyst.

2.3. Dehydration and dehydrogenation In general, alcohols undergo dehydration to olefins and ethers over solid acid catalysts, and dehydrogenation to aldehydes and ketones over solid base catalysts. However, certain solid base catalysts promote dehydration in which the mechanisms and product distribution differ from those for acid-catalyzeddehydration. Characteristic features of base-catalyzed dehydration are typically observed for 2butanol dehydration. The products consist mainly of l-butene over rare earth oxides[20], Th02[21], and Zr02[22]. This is in contrast to the preferential formation of 2-butene over acidic catalysts. The initial step in the base-catalyzed dehydration is the abstraction of an H+at C-1 of 2-butanol to form anion. Base-catalyzeddehydration of alcohol has been industrialized[24]. Dehydration of 1-cyclohexylethanol to produce vinylcyclohexane was developed by Sumitomo Chemical Ltd. Zr02 was used as a catalyst. In the dehydration of 2-alcohol to corresponding 1-olefin over ZrO2, the selectivity for 1-olefin strongly depends on the amounts of Si contained in ZrQ as an2 impurity, calcination temperature, and treatment with bases. By treatment of ZrO2 with base such as NaOH, the selectivity for 1-olefin is increased, and the by-product 2-olefins were markedly reduced.

The conversion of alkylamines to nitrile proceeds efficiently over Zr02[23]. The conversion of secondary amines and tertiary amines to nitriles require both acid and base sites. A high activity of ZrO2 is caused by acid-base bifunctional properties of Zra.

41

y acid

Et3N

CH3CN

t

H2

C2Hq

t

NHJ

acid

EtZNH

EtNH2<

'ZH4

CZH4

Intramoleculardehydration of monoethanol amine to ethyleneimine has also been industrialized by Nippon Shokubai Ltd. The catalyst used in the process is Si-alkali metal-P-0 mixed oxide. The metal oxide catalyst possesses both weakly acidic and basic sites. Because monoethanolamine has two strong functional groups, weak sites are sufficient to interact sufficiently with the reactant. If either acid sites or basic sites are strong, the reactant interacts too strongly and forms much of undesirable by-products. It is proposed that the acid and basic sites act cooperatively as shown below[25]. The composition of the catalyst is adjusted to control the surface acid and base properties. A selectivity of 78.8 % for ethyleneimine was obtained for the catalyst composed of Si/Cs/P/O in the atomic ratio 1/0.1/0.08/2.25. .i-7 N H O W N " 2

Y-o-'+-f

-n20

tl,

M : acid site (Si or P)

2.4. Alkylation 0 : base site In general, alkylation of aromatics occurs at ring position over acid catalyst, while side chain alkylation takes place over basic catalyst[26]. Toluene undergoes side chain alkylation with methanol to produce ethylbenzene and styrene over Cs ion exchanged X zeolite. The first step in this reaction is dehydrogenation of methanol to formaldehyde which undergoes aldol condensation with toluene to form styrene. Ethylbenzene is formed by hydrogenation of styrene. The basic sites in the zeolite catalyst are considered to be active sites and participate in both alcohol dehydrogenation and aldol condensation.

WKOH/A1203 is efficient catalyst for alkylation of isopropylbenzene with olefins such as ethylene and propene. In this case too, selective alkylation at side chain occurs due to anionic mechanisms as proposed by Suzukamo et al.[6]. The reaction occurs at 300 K. The basic sites of WKOH/A1203 are sufficiently strong to abstract H+from isopropylbenzene at a low temperature.

42

2.5. Aldol addition and condensation Aldol addition of acetone to diacetonealcohol is well known to be catalyzed by Ba(0H)n. Alkaline earth oxides are also active for the reaction. Considering that the activity of MgO increases on addition of a small amount of water as well as the results of the tracer experiments, the active sites are suggested to be surface OH1271. By use of the catalyst possessing both acid and basic sites, the product diacetonealcohol undergoes dehydration to mesityl oxide. If hydrogenation ability is further added to the catalyst, mesityl oxide is hydrogenated to methylisobutylketone(MIBK) .

43

Butyraldehyde undergoes aldol addition to form the dimer over solid base catalysts[28]. The dimer undergoes either dehydration to form 2-ethylhexenal (2EHA) or Tischtschenko reaction with butyraldehyde to form trimeric glycol ester. Over alkaline earth oxides, the selectivity for the trimer is higher, whereas over alkali added A1203, the dimer is preferentially formed[28]. For Tischtschenko reaction, acidic sites are thought to be involved in the reaction. The metal cations of alkaline earth oxides act as an acid site to promote Tischtschenko reaction, but the cations of alkali metal oxides are too weak to act as the acid site toward butyraldehyde. Addition of alkalis enhances the basic properties and suppresses the acidic properties, and therefore suppresses the route to Tischtschenko reaction to form the trimer. H

+

C3H7-CHO

CzHs

I

-

GHs-CH-CHO

I

C3H7-CH-CH-CHO

I

-

GHS

I

.H20

C3H7-CH=C-CHO

OH

2EHA

dirner

C2HS

C2HS

I

C,H,-CH-CH-CHO

I

+

I

C3H7-CH0

+

C H CH-CH-CH2-O-C-C3H7

II

7 - ~

OH

OH

0

trimeric glycol ester

diincr

Benzaldehyde undergoes Tischtschenko type reaction to produce benzylbezoate over alkaline earth oxides[30]. The catalytic activity correlates well with the amount of basic sites, and the basic sites are believed to be active sites. The reaction proceeds as follows. The slow step of the reaction is H- transfer from (I) to (11). In this reaction, not only basic sites (0 ions) but also acidic sites (metal cations) are participating as shown above. $H5

0zC-H

+ -G.o-

-

76H5

'0-C-H -G.A(1)

7 6 H 5 F

Q2-C-H c6HS

-(&-

+

@

7sHS $-H

?

-&.O-

-

7sHS H-C-H

76H5

a+

-Q'

.0-

+

0

-Ca.0-

44 The formation of unsaturated compounds by the reaction of methanol with ketones, esters and nitriles involve aldol condensation. The reactions generally expressed as shown below proceed efficiently over transition metal modified MgO at about 673 K[31]. The reaction is initiated by dehydrogenation of methanol followed by aldol addition and dehydration successively. By addition of transition metals such as C$+, Fe3+and Mn2+,the basicity of MgO is increased and the dehydrogenation ability of MgO is enhanced. R C b Z t CH30H

CH2=CR-Z t H2 t H20 M"+ - MgO

Z = -CN, -COR, -COOR, -6H5 R = -H, -CH3 M"+= C$+, Fe3+, Mn*+etc. 2.6. Wiltig-Horner reaction and Knoevenagel reaction Aldehydes react with nitriles over solid base catalysts such as MgO and ZnO to proceed W-H and K reactions as shown below(32j. For both reactions, abstraction of H+from the nitriles by basic sites to form anions is the initial step.

0-

CHO t NC-CH,

-

2.7. Dehydrocyclodimerizationof conjugated dienes Conjugated dienes such as 1,3-butadiene and isoprene react over Zr@ and MgO to produce aromatics at about 650 K[33]. Two mechanisms are possible, one involves Diels-Alder reaction followed by isomerization and dehydrogenation, and the other involves anionic intermediates. Over MgO, 1,3-butadiene primarily produces o and pxylenes, which will not be formed via Diels-Alder reaction. Over Zr02, the main product from 1,3-butadiene is ethylbenzene. Dehydrocyclodimerization proceeds mainly via Diels-Alder reaction for ZrO2 and via anionic mechanisms for MgO.

2.8. Ring transformation Oxygen atom in a ring position can be transformed to N or S by use of zeolite catalysts whose acid and base properties are adjusted by ion exchange[34]. Hoelderich summarizes the relation between acid-base properties and the selectivity[35]. Increasing the basic properties enhances the activity and selectivity for ring transformation of 0 into S with H2S. For the reaction of (2), the activity order is CsY > RbY z KY > NaY > LiY, which coincides with the strength of basicity. The reaction is retarded by addition of HCI but not retarded by pyridine, which also

45 suggests basic sites are operating in the reaction, though the mechanisms are not clear.

c(r>

tH2X

+

0

tH20

X

I -

Dehydrocyclodimerization

Diels-Alder mechanisms

--o c=c

H

L Anionic mechanisms

D-A reaction

G

n

Q

Isomerization + @-c Dehydrogenation

~~~~~~~n~

46

3. CHARACTERISTIC FEATURES OF SOLID BASE CATALYSTS OF DIFFERENT

TYPES 3.1 Zeolites The characteristic features of zeolites result from their ion exchange ability. By selecting the ions to be exchanged as well as Si/AI ratio[36], the acid and basic properties can be controlled. The basic properties are enhanced by exchange with alkali ions. Among alkali ions, Cs+ ion is most effective for enhancing the basicity. However, the basic sites are not clarified yet as to the location relative to the exchanged cations. The ab es to abstract an H+for ion exchanged zeolites are not strong as compared to other solid base catalysts like MgO. The basicity is markedly enhanced by addition of alkali ions in excess of ion exchange capacity. Hathaway and Davis reported that decomposition of the impregnated cesium acetate in the zeolite cavities resulted in the generation of catalyticallyactive basic sites which were more active than those present in ion-exchangedzeolites[37]. We also reported that the addition of alkali jons resulted in generation of basic sites by use of TPD of adsorbed COP[38]. The strong basic sites for ion added zeolites are suggested to be caused by alkali metal oxides formed in the zeolite cavities[39]. Fine particles of MgO were introduced in the zeolite cavities by use of magnesium methoxide in the hope that the shape selectivity is added to the basic properties of Mg0[38]. The fine particles of MgO exhibit the catalytic activities for l-butene isomerization and 1,3-butadiene hydrogenation, but their basic properties are weak as compared to MgO bulk. The charge separation between Mg and 0 is not sufficient to generate strong base sites for the fine particles. The shape selectivity, however, was observed in allylbenzene double bond isomerization. The MgO bulk showed activity, but the fine particles in the zeolite cavities did not show appreciable activity in a liquid phase reaction. Shape selectivities in base-catalyzed reactions were reported by Corma et al. for alkaline-substituted sepiolites and alkali ion exchanged zeolites in the reaction of benzaldehyde with esters[40]. 3.2. Single component metal oxides

Alkaline earth oxides are most extensively studied and representative solid base catalysts. The ability to abstract an H+from reactants is very high. For instance, allylic H of olefins can be abstracted as a Hi- even at 223 K, which is evidenced by showing the activity for double bond isomerization of butene at this temperature. Among rare earth oxides, sesquioxides show high activity for base-catalyzed reactions. The activities are comparable to those of alkaline earth oxides. The rare earth oxides which are stable at higher oxidation states of metal cations such as Ce02, Tb407, and PrsO11 show weakly basic properties. ZrQ and Tho2 show similar catalytic behaviors. Although their basic properties are not so strong, they possess weakly acidic properties, and ,therefore, show the acid-base bifunctional catalysis. Zr02 and Tho2 as well as rare earth oxides showed dehydration activity for alcohols. One of the features commonly observed for single component oxides is strong dependency of the activity and selectivity on the catalyst pretreatment temperature. Once the surfaces are exposed to air, the surfaces are immediately covered with water and C02, and lose the catalytic activities. To reveal the basic properties on the surfaces, the oxides need to be pretreated at a high temperature to remove strongly

47

adsorbed species. The strong basic sites appear on removal of water, C02 and 0 2 . It is especially required to pretreat properly when the reactant is weakly adsorbed on the surfaces. In the case of the reactants which can interact strongly with the surfaces, the reaction may proceed by replacing water, C02 and 0 2 by the reactant. 3.3. Solid superbases Several trials were undertaken to prepare the catalysts possessing very strong basic sites. Suzukamo et al. succeeded in preparing solid base catalysts possessing basic sites stronger than H. 35 by treating A1203 with Na20 followed by addition of metallic Na[6]. K can be used in stead of Na. These catalysts are employed in the commercial process as described earlier. We added NaN3 to MgO and heated above 573 K to decompose NaN3 to metallic Na so that MgO surfaces react with metallic Na[41]. The resulting catalyst exhibited strong basic properties and showed a high activity for decomposition of methylformate to CO and methanol. 3.4. Non metal-oxide catalysts

Alumina supported KF has been introduced by Ando et a1.[42] and Clark et a1.[43]

as a basic catalyst for many types of the reactions such as emethylation of phenol, crown ether synthesis, Michael addition of various nitroalkanes to unsaturated carbonyl compounds etc. For the organic syntheses mentioned above, the KF/A1203 was used without thermal pretreatment. We found that KF/A1203 becomes more active as it is pretreated at proper temperatures. The activity of KF/A1203 for 1-butene isomerization showed a maximun at the pretreatment temperature of 623 K[44] as shown in Fig. 1.

, 200

300

400

m-

I

500

Pretreatment ternpepature / " C

Fig. 1 Variation of the activity of Fluka KF/A1203(F 5.5mmol/g) for 1-butene isomerization at 273 K as a function of pretreatment temperature

48

The basic sites of KF/A1203 have not yet been identified. F ions were proposed to interact with an H of the reactant[42,43]. On the other hand, the relevant species is surface KOH resulting from the following reaction[44]. 12KF

+

A1203

+

3 H20-

3 K3AIF6

+

6KOH

Baba et al. prepared low-valent lanthanide species introduced into zeolite by impregnation from Y and Eu metals dissolved in liquid ammonia followed by evacuation at 773 K[46]. The resulting compounds catalyzed 1-butene isomerization by anionic mechanisms. TPD results with the catalysts suggest that metal irnides such as EuNH act as basic sites. 4. CONCLUSIONS

Among the great number of organic reactions, solid base catalysts are employed only in a limited number of the reactions. One reason for the limited use of solid base catalysts arises from a quick deactivation while handling under atmosphere; the catalysts should be pretreated at high temperatures and handled in the absence of air prior to the reaction. If this care is taken, solid base catalysts should promote a great number of reactions. Recently, methods of preparing catalyst and characterizing the surface properties have been developed. Deep insight into the surface reaction mechanisms and the functions required to promote reactions will enable to design the solid base catalysts active for desired reactions. 5. REFERENCES

1 2 3 4 5

6 7 8 9 10 11 12 13 14 15 16 17

H. Pines, J. A. Veseley, and V. N. Ipatieff,J. Am. Chem. SOC.,77 (1955) 6314 H. Hattori, "Adsorption and Catalysis on Oxide Surfaces" ed. M. Che and G. C. Bond, Elsevier, 1985. p.319. J. W. Hightower and W. K. Hall, J. Am. Chern. SOC.,89 (1967) 7778. A. Satoh and H. Hattori, J. Catal., 45 (1976) 36. H. Hattori, K. Tanabe, K. Hayano, H. Shirahama, and T. Matsurnoto, Chern. Lett., (1979) 133. G. Suzukarno, M. Fukao, T. Hibi, and K. Chikaishi, "Acid-Base Catalysis" ed. K. Tanabe, H. Hattori, T. Yamaguchi, and T. Tanaka, Kodansha-VCH, 1988, p.45. A. Hattori, H. Hattori, and K. Tanabe, J. Catal., 65 (1980) 246. H. Matsuhashi and H. Hattori, J. Catal., 85 (1984) 457. H. Hattori, Y. Tanaka, and K. Tanabe, J. Am. Chern. SOC.,98 (1976) 4652. Y. Tanaka, Y. Imizu, H. Hattori, and K. Tanabe, Proc. 7th Intern. Congr. Catal., Tokyo, 1980, p.1254 H. Hattori, H. Kurnai, K. Tanaka, G. Zhang, and K. Tanabe, Proc. 8th Nation. Symp. Catal. India, Sindri, 1987, p. 243. Y. Irnizu, K. Sato, and H. Hattori, J. Catal.,76 (1982)65. Y. Nakano, T. Yarnaguchi, and K. Tanabe, J. Catal., 80 (1983) 307. Y. Imizu, H. Hattori, and K. Tanabe, J. Catal., 57 (1979) 35. Y. Kakuno and H. Hattori, J. Catal., 85 (1984) 509. T. Maki, T. Yokoyama, and K. Fujii, Shokubai(Catalyst),35 (1993) 2. T. Yamaguchi and J. W. Hightower, J. Am. Chern. SOC.,99 (1977) 4201.

49

18 H. Matsushita, S. Ishiguro, H. Ichinose, A. Izurni, and S.Mizusaki, Chern. Lett., (1985) 731. M. Shibagaki, K. Takahashi, and H. Matsushita, Bull. Chern. SOC.Jpn., 61 (1988) 3283. 19 J. Shabtai, R. Lazer, and E. Biron, L. Molec. Catal., 27 (1984) 35. 20 A. J. Lundeen and R. van Hoozen, J. Org. Che., 32 (1967) 3386. 21 T. Tornatsu, T. Yoneda, and H. Ohtsuka, Yukagaku, 236(1968) 236.(in Japanese) 22 T. Yarnaguchi, K. Sasaki, and K. Tanabe, Chern. Lett., (1973) 1017. 23 6.-Q. Xu. T. Yarnaguchi, and K. Tanabe, Appl. Catal., 64 (1990) 41. 6.-Q. Xu, T. Yarnaguchi, and K. Tanabe, Appl. Catal., 75 (1991) 75. 24 K. Takahashi, T. Hibi, Y. Higashio, and M. Araki, Shokubai(Catalyst), 35 (1993) 12. 25 M. Ueshirna, H. Yano, and H. Hattori, Sekiyu Gakkaishi, 35 (1992) 362. 26 T. Yashima, K. Sato, T. Hayasaka, and N. Hara, J. Catal., 26 (1972) 303. 27 G. Zhang, H. Hattori, and K. Tanabe, Appl. Catal., 36 (1988) 189. G. Zhang and H. Hattori, Appl. Catal., 40, (1988) 183. 28 G. Zhang, H. Hattori, and K. Tanabe, Bull. Chern. SOC.Jpn., 62 (1989) 2070. 29 H. Tsuji and H. Hattori, to be published. 30 K. Tanabe and K. Saito, J. Catal., 35 (1974) 247. 31 H. Kurokawa, W. Ueda, Y. Morikawa, and Y. Moro-oka, "Acid-Base Catalysis" Kodansha-VCH, 1989, p.93. 32 J. V. Sinisterra, Z. Mouloungui, and M. Marinas, J. Col. Inter. Sci., 115 (1987) 520. 33 H. Suzuka and H. Hattori, J. Mol. Catal., 63 (1990) 371. 34 Y. Ono, Heterocycle, 16, (1981) 1755. 35 W. F. Hoelderich, "Acid-Base Catalysis" Kodansha-VCH, 1989, p.1. 36 D. Barthorneuf, Stud. Surf. Sci. Catal., 65 (1991) 157. 37 P. E. Hathaway and M. E. Davis, J. Catal., 263 (1989) 263. 38 H. Tsuji, F. Yagi, and H. Hattori, Chern. Lett., (1991) 1881. 39 H. Tsuji, F. Yagi, H. Hattori, and H. Kita, Proc. 10th Intern. Congr. Catal., Budapest 1992, p. 40 A. Corrnaand R. M. Martin-Aranda,J. Catal., 130 (1991) 130. A. Corrna, V. Fornes, R. M. Martin-Aranda, H. Garcia, and J. Primo, App. Catal., 59 (1990) 237 41 T. Ushikubo, H. Hattori, and K. Tanabe, Chern. Lett., (1984) 649. 42 J. Yarnawaki and T. Ando, Chem. Lett., (1979) 755 and (1980) 533. 43 J. H. Clark, D. G. Cork, and M. S.Robertson, Chern. Lett., (1983) 1145. J. H. Clark, Chern. Rev., 80 (1980) 429. 44 H. Tsuji and H. Hattori, to be published. 45 L. M. Weinstock, J. M. Stevenson, S. A. Tornellini, S.-H. Pan, T. Ytne, R. 6. Jobson, and D. F. Reinhold, Tetrahedron Lett., 27 (1986) 3845. 46 T. Baba, G. J. Kim, and Y. Ono, J. Chern. SOC.Faraday Trans., 88 (1992) 891.

This Page Intentionally Left Blank

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicnls 111 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

51

Solvent effects in heterogeneous catalysis : Application to the synthesis of fine chemicals L. GILBERT and C. MERCER Rh6ne Poulenc Recherches. Centre de Recherches des Carrieres - B.P. 62 - 85, Avenue des Freres Perret - 69 192 St-Fons Cedex FRANCE. I

ABSTRACT Solvent effects in heterogeneous catalysis are examined in terms of physical or chemical modifications to control the chemo-, regio- and stereoselectivity of a reaction. The main factors affecting selectivity are reactant solubility, polarity, reactivity or acido-basicity of solvents and competitive chemisorption of products and solvents. In the special case of molecular sieves, selectivity control of a reaction by competitive adsorption, diffusion or shape selectivity and confinement catalysis are also examined.

INTRODUCTION The use of solid catalysts for the production of fine chemicals has found a broad range of applications in the field of organic chemistry. In the liquid phase, where solvents are often used to solubilize reactants, facilitate heat transfer and avoid coke formation on the catalyst, the choice of the solvent is crucial, influencing both the activity of the catalyst and the selectivity of the transformation. There are numerous studies concerning solvent effects on selectivity in heterogeneous hydrogenations (1). Similar effects are observed in oxidation or acid catalysed reactions. In addition, molecular sieves can be viewed as solvents for the species that they contain (2) and therefore, even in vapor phase reactions, modifications of activity or selectivity can be attributed to solvent effects. In a first part, the solvent variation influencing, the reactant solubility, polarity, reactivity and chemisorption of products as a way to control the chemo, regio and stereoselectivity will be emphasized. In the second part, the special case of molecular sieves and the three main routes to modify the reaction selectivity will be discussed. Such transformations include : competitive adsorption or diffusion of reactants, shape selectivity and confinement catalysis.

52 1. SOLVENT EFFECTS IN HETEROGENEOUS HYDROGENATION

1.1 Elementary steps o f an hydrogenation In a triphasic system, (gas, liquid, solid) hydrogenation is a global process which can be divided in three steps : - chemisorption of reactants (Hz, substrates); - reaction on the surface of the catalyst; - desorption of products. Assuming that all reactions presented are under chemical control and not limited by diffusional phenomena (3), solvents can influence the reactions selectivity at each step.

1.2 Physical effects of solvents 1.2.1 Effect on solubility

Hydrogen The solubility of hydrogen is not only dependant on the pressure in the gas phase but also on the solvent nature. Unfortunately, varying the nature of the solvent, only influence the transfer from the gas phase to the liquid phase which is by far not enough : .

Organic substrates During the hydrogenation of 1 to lysinamide byproduct : .

2 (4), pipecolinamide 2 is obtained as a

02N-(C

The selectivity of the reaction can be altered by controling the concentration of substrat as illustrated by Table 1 Table 1 : Influence of the substrat concentration on the selectivit Substrat concentration Yo wlw 65 1 3 5 76 120 85 I 1 5 The hydrogenation of the nitro to the amino function furnishes two molar equivalents of water. A kinetics study demonstrates that this reaction is zero order in nitro. Water can then hydrolyse oxime in ketone 5 which can be transformed into 3 by reductive amination :

53

The rate determining step is the hydrolysis of the oxime which is first order in water. Therefore, lowering the concentration of substrate decreases the amount of water while the selectivity increases. 1.2.2 Polurity :Dielectric constant and haptophilicity

The stereoselectivity of the hydrogenation on a cyclic molecule containing a polar group can be controlled by the hydrophilic - hydrophobic interactions between solvent, substrate and catalyst support. Selective hydrogenation of 6 to trans 7 or cis 8 isomers is greatly influenced by the solvent (5). I

I

n

I

The polar group (CH20H) can interact strongly with polar solvents such as DMF or EtOH. In this case, the hydrogenation takes place on the opposite face leading to the trans isomer 7. With a non polar hydrophobic solvent (Hexane), the polar group tends to chemisorb on the catalyst which is then the more polar. By addition on the same face, cis isomer 8 is obtained preferentially. The use of a more polar support for the catalyst (SO2 or A l 2 0 3 ) may have favored the formation of the cis isomer. Table 2 : Influence of the solvent polarity on enantioselectivity Solvent Polarity Er 718 37 94 I 6 DMF EtOH 25 94 16 Diglyme 81 119 Hexane I79 39/61

1

I

I

1

However, the relation between stereoselectivity of hydrogenation and solvents polarity is not always so clear as shown by the hydrogenation of 0-octalone 9 to trans decalone-3 and cis-decalone-3 1(6).

54

a, -h0.@J Solvent

H

H

0

The selectivity as a function of polarity is opposite for protic and aprotic solvents. This is a consequence of compared stability of intermediate conformers.

A Polarity

NMP

&r

Er

Polartty

__ 3 4 M e O H

30-

Acetone 21

-

-

I

__

25 ElOH

--

18 nBuOH

--

I I ‘BUOH

+

u*o 4,4 -.

Due to solvent effect, the hydrogenation of P-hydroxyketone 12 (7) furnishes selectively the cis diastereoisomer 13 rather than the anti 14.However, in the case of this aliphatic, polfinctionalised substrate the solvent effect is decreased.

OH

OH

0

OH

OH

H2 - 20 bar

12 --

25°C

I

Table 3 : Influence of the solvent polarity on the diastereoselectivity in the hydrogenation of 12 Solvent Polarity E, 13 1 1 4 MeOH 34 85 115 CH,CI, 9 76 / 24 PhCH3 68 / 32 Hexane 64/38

I

I

I

0

55 1.3 Chemical effects of solvents

1.3.1 Reactive solvent While, palladium catalyses the hydrogenation of phenol in cyclohexanone and subsequently the formation of its acetals which is then hydrogenate;

Ruthenium, on the contrary, tend to avoid this sequence, as illustrated by the two following exemples (8) :

H, - 3 3 bar

Yields : Ru (5 %)/C Pd (5 %)/C

100 Yo 16 Yo

0% 84 Yo

In the production of n-butyl-4 hydroxy-3 acetanilide 16 from l5, we want to avoid the formation of and so examine the influence of the solvent nature on the selectivity (9)

Compound 12 is the result of a Friedel-Crafts reaction of the carbocation intermediate. Changing the solvent from ethylene glycol dimethylether to alcohol modifies the reactive intermediate from Ltja to u b or B c . The hydrogenolysis of JfJb (or 18c) is more difficult than 18a which leads to a lower stationary concentration in carbocation, therefore minimizing the formation of 12.

56

AcNH

-or

AcNH

18s ( R

=

Et)

J&=

In this example, the concentration in substrate is critical as the formation of 16 is first order and the formation of 11is second order in fi. In addition one has to make sure that the chemical controle is obtained for hydrogen in order to limit the formation of 11. in

1.3.2 Solvent acido-basicity

a)Competition between hvdrogenation and hydrogenolvsis When competition between hydrogenation and hydrogenolysis exists it is possible to favor hydrogenolysis by increasing the solvent’s acidity while increasing basicity limits the hydrogenolysis (10) as illustrated by Grey’s example (1 1)

p) Selective hvdrogenation of aromatic compounds The selective hydrogenation of heterocyclic aromatic compounds is possible using strong acids. In a neutral medium pyridine is hydrogenated faster than benzene as its chemisorption is easier. In an acidic medium, the chemisorption of pyridine is greatly disfavored by protonation (1 2).

57 These observations are clearly demonstrated by the selective hydrogenation of isoquinoleine

19 (13)

:

W N 19 --

- WNI-I+m Pt

-

H,

1 atm.

Selectivity Solvent MeOH MeOH + HCI (1N) MeOH + HCI (4N) HCI

87 30 13 I

13 70 87 97

y) Polarity inversion bv protonation

gives I different products as a function Hydrodechlonnation of tetrachloro-2,3,4,5 aniline & of the pH (14). In a neutral medium aniline is obtained under very smooth reaction conditions. While dichloro-3,s aniline can be obtained very selectively in an acidic medium. y

CI

Pd’C 100°C H, - I atm

CI I

CI

6 N“2

2

@(‘I

20

+

selec~ivity

4HCI 90 %

EtOH

The selectivity of the hydrodechlorination is then a function of acidity as shown by the following table giving the amount of chlorine substituted at each position and the initial speed of the reaction :

Acid

Concentration

% substitution

Vinitial

2

4

3

5

25

0

0,25

40

0

20

40

7N

0,15

15

0

80

5

8N

0,20

7

0

91

-

H2S04

1N

H2S04

7N

H2S04 + LiCl (3,s M) HCI

100

0 -

75

2

~~~

To interpret the influence of acidity, one can assume that the hydrodechlorination proceeds via a nucleophilic substitution by addition of an hydride. Therefore, in a neutral medium (or a

58

GCI

not acidic enough medium to make sure that the protonation of the aniline is completed) steric and electronic effects will favour the substitution at the five position. In a very acidic medium, protonation of aniline will reverse the polarity on the nucleus desactivating it for the SNAr. The reaction will be slower and the substitution will start at the four position. d

QCI

6

CI

5 \4 &-

s6+ CI

5

p + s\4

CI

1.3.3 Solvents can modqy chemisorptions The fine choice of a solvent can greatly controle parallel or consecutive reactions by modifying the adsorption equilibrium. a)Selective hvdrogenation of nitroaromatics to hvdroxylamines In the hydrogenation of a nitroaromatic it is possible to limit the reaction at the hydroxylamine stage using pyridine or piperidine as a solvent (16).

The reaction kinetics follow the Langmuir-Hinshelwood model. Pyridine forces the desorption of hydroxylamine, then it can not be hydrogenated into aniline

i

r

ArN02

ArNHOH

This selective reduction can be used to produce intermediates for biologically active molecules (1 7) :

59

hNHO;.E C02H

H21 -5 °1Cbar

Me0 OCH2Ph

b

PtIC T H F - DMSO - NH,OH

.N H

Me0 OCH2Ph

Me0

A

Ho 0

yield 8 3 %

CH3

The latter exemple is a very selective hydrogenation without hydrodehalogenation or hydrogenolysis of the benzylic group.

0) Selective hvdrotzenation of acetylenic to ethvlenic unsaturation Hydrogenation of acetylenic to ethylenic is a classical example of selectivity control using an additive which can realise a competitive chemisorption An exemple studied at RP (1 8) is the hydrogenation of dehydrolinalyl acetate 2 to linalyl acetate 23 avoiding the formation of dihydrolinalyl acetate 2 Lindlard catalyst (Pd - CaC03 + Pb - quinoleine) enables this selective reaction but this Tatalyst is not very stable and recycling is difficult With Pd/C the amount of 24 is 5 % 'urification of 23 is very difficult and the application (perhmes) requires a product with less han 3 % of 3 This can be achieved by adding 2 % of pyridine to the reaction medium

-22

Pd 1 c

no solvent conversion 99,8 % pyridine 2 % conversion 100 %

0Ac

__ 24

23 --

yields yields

94,s % 98,5 '3'0

0Ac

5%

__

The same explanation as for selective hydrogenation of nitroaromatics can be given assuming that the reaction follows a Langmuir Hinshelwood kinetics Another exemple of application in the field of agrochemicals can be found in the synthesis of ethylenic diol 3 (19)

60 OH

H, - 2 bai

I

OH I

40°C

x

Pd (5 % ) I

c

X

-25

26 At

without

O

N

1,s h.

conversion (%) selectivity125 98

46

2. HETEROGENEOUS CATALYSIS BY MOLECULAR SIEVES 2.1 Competitive adsorption and diffusion Molecular sieves can also be viewed as polar solvents. Polarity of the zeolite is linked to the global charge of the framework. Therefore a decrease in SiIAI ratio enhances the zeolite polarity : a silicalite is hydrophobic when HZSM-5 is hydrophilic. Polarisability of a product contained in a zeolite can be modified by varying its hardness or softness. A variation of the global charge of the framework has a low influence on polarisability. However, hardness or softness can be greatly modify by substitution of the tetragonal component. Surface curvature (defined as the ratio of the Van der Waals critical radius of the sorbed or reactant molecule to that of the pore or cage in which it is sorbed) will influence the chemisorption of substrates and the chemical evolution of the sorbed species because this species must be considered as solvated by surrounding zeolite framework. Those effects will influence diffusion and adsorption of substrates.

2.1.1 Vaporphase hydroxylation of aromatics The vapor phase hydroxylation of aromatics by nitrogen oxide is catalysed by HZSM-5 zeolites (20). Studies of the influence of the Si02 I A1203 ratio shows an optimum of activity. However, in a HZSM-5zeolite the strength of acid sites is undependant of the Si02 I A1203 ratio; by increasing the Si02 I A203 ratio we only decrease the number of sites. The maximum activity observed can only be interpreted in term of solvent effect. By increasing the Si02 I AI2O3 ratio, we increase the hydrophobicity of the zeolite (21) therefore the solvatation of the aromatic while the solvatation of nitrogen oxide is disfavored. Therefore we observed a maximum of activity when concentrations of aromatic and nitrogen oxide are as close as possible. This is illustrated by the hydroxylation of fluorobenzene :

61 Solvability

Conversion of PhF (%)

of reactants

1

IS

--

10

--

s

-I

I

50

I 100

1 1%

Molar ratio

SiOj40,

I 200

1

2.1.2 Asymmetric epoxidntion

Molecular sieves, acting as a cosolvent, have recently been shown to greatly improve the stereoselectivity in some titanium - catalyzed reactions (22). Usually the zeolite selectively traps small molecules like water or alcohol to avoid the destruction of the catalyst or to form the chiral catalyst more efficiently. The epoxidation as initially described employs a stoechiometric amount of catalyst. The major improvement of the original procedure is the use of molecular sieves to allow the asymmetric epoxidation to be carried out with just 5 - 10 % catalyst (22a) Ti(OiPr)4 ( 5 W )

yield 65 W O h o H enantiomeric e x c e s s 90 I

&OH

(+)-

DIPT (6 0 % )

' B ~ O O H CH,CI, 0 ° C / 5 h. molecular s i e v e s 4 A

(+)-

DIPT Ho

HO"

Oi Pr Oi Pr

The presence of molecular sieves significantly increased both enantioselectivity of the epoxidation as illustrated by the following curves. 100

80

n

T

60

F a x)

0 0

20

40

60

Time (min)

80

100

conversion

and

62

The hydrophilic molecular sieves serve as a trap for water protecting the titanium catalybt Another way to look at zeolites is to look at them as microscopic catalytic reactors. Selectivity control can be achieved either by shape selectivity (23) or by a confinement effect (24). In the first case the selectivity comes from repulsive effects : - shape selectivity for reactants : they can not have access to the active site - shape selectivity for products : they can not leave the active site - shape selectivity for transitions states : steric constraints disfavor the formation of transitions states leading to some by products. In the case of confinement effect, activity and selectivity of the catalyst come from attractive effects : - molecular recognition - preorganisation - preferential interactions. 2.2 Shape selectivity 2.2.1 Selective oxidation with H202 catalyserl by titanosilicrtte

The titanosilicate having a pentad structure has been found to catalyse the oxidation of a variety of substrates with an aqueous solution of H 2 0 2 (25). Tatsumi el ul. (26) have also shown that this titanosilicate catalysed the oxidation of unreactive alkanes with reactant shape selectivity. The variation of activity in oxidation of hexane and cyclohexane is explain in terms of limited diffusion of cyclohexane. This was proved by elution chromatography

Substrate

Turnover

Selectivity

(mol/mol-Ti)

2-01

3-01

Hexane

7.00

17

39

Cyclohexane

0.37

----------

01 50

Heptane

4.50

16

47

17

Octane

0.50

28

34

22

4-01

Zone

3-one

34

10

4-one

one 50

__________

10

8.0

2.0

9.0

4.0

3.0

---------- ----------

The oxidation rate decreased in the order hexane > heptane > octane in relation with the decrease of diffusion into zeolites. The titanosilicate catalysed the hydroxylation of phenol by E l 2 0 2 to afford a mixture of hydroquinone and catechol. The following curves due to C. Naccache el cil. (27) show a great solvent effect on selectivity when methanol vs acetone is used.

63

Product dlrtrlbutlon

Producl dlrlribullon 1.0

. ---. -.

-

I + *,.'

0.6 o.4

0.0

@)

. 0

Time ( m i d

30

60

90

120

o.2 0.0

@)

-.--.-.-*.//-.-.----

i

0

30

60

Tima (rnin)

90

120

Product distribution on TS-1 during the oxidation of phenol . (M Hydroquinone , Catechol) I ) In methanol

2) In acetone

When the reaction is carried out in methanol the reaction gives initialy equal amounts of hydroquinone and catechol but the selectivity increases with reaction time This was explained assuming that catechol is produced on the external surface of the zeolite, whereas hydroquinone formation prevails at the internal catalytic sites by shape selectivity After a short period, the activity of the external surface sites was killed by cocking and hydroquinone is obtained selectively On the contrary, acetone prevents the formation of coke on the external surface by dissolving it. The selectivity of the reaction will then be the sum of external and internal catalysis. 2.3 Selectivity and solvent effects in the chlorination of aromatics The selective preparation of polychloroaromatics is an important goal in industry of fine intermediates. L type zeolite enables selective liquid phase chlorination. Some recent studies show solvent effects on this chlorination. A. Botta el al. (28) show that K-zeolite L enables the selective dichlorination of biphenyl 22 to give the 4,4' isomer 2.In addition, Lewis acid catalysed chlorination of biphenyl gives mixtures consisting mainly of 29. 30 - 22.

64

In the reaction run neat, it is not possible to explain the selectivity using the concept of shape selectivity because zeolites of similar pore diameter give different results. A. Botta et al. assume that the ionic radius, charge and spatial arrangement of the cations and their force fields are responsible for the activation of C12. Table 6 Product distribution (YOwt)

Catalyst

27729

30

32

15.4

6.4

13.2

23.7

9.1

K-zeolite s).

16.5

1.2

3.2

25.5

47.0

K-zeolite L

5.0

2.7

2.2

16.8

66.5

H-zeolite L

7.5

2.9

6.5

25.4

45.8

K-zeolite L t CICHzCOzH

2.1

2.7

0.4

8.1

83.7

FeC13

1.0

28

At lower temperature, the use of solvent gives an important solvent effect, CH,C12 being the best : yield o f 3 : 89.0 %. The solvent seems to favor diffusion and transport processes in the zeolite channels, the activation of the reactants is due to confinement catalysis A similar effect is observed by K. Shinoda el af.(29) in the chlorination of benzene, the best solvent for activity and selectivity being dichloro-l,2 ethane (EDC) 2.4 Confinement catalysis A typical exemple is due to A. Corma et af. (30). Organometallic complexes 35 linked to a dealuminated Y zeolite with larges micropores (> 20 A), are used to hydrogenate N-acyl dehydrophenylalanine with high activity and enantioselectivity L- Pro I I n e

N H

I

N

Rh+

d

U NH

NHR

35-a R = CMe3 -_35-b R = (CH,),Si(OEt),

H 'Rh f

U 26 S u p p

=

_3 _7

=

Supp

Silica Zeolite

65

NHCOCH, \I/

conversion 100 %

H, - 5 bar 65°C

Catalyst

-

[

H

I

35-a 36

36

enantiomeric excess : 54,4 YO I, 58,O Yo I, 94.2 Yo

A. C o m a et al. assume that the positive effect of the support on the activity can be attributed to the increase concentration of hydrogen due to the zeolitic support. A preferential interaction between the substrate and the zeolitic surface increases the enantioselectivity. Another explanation can be given assuming that the zeolite acts as a supramolecular ligand. The activity and selectivity of the catalyst are then a consequence of the solvatation by the zeolite framework (surface curvature) (24). CONCLUSlON From litterature examples and internal studies we have shown that : - the solvent is an essential part of the catalytic system and very often the right choice will allow a good control of selectivity. - molecular sieves often act as a supramolecular solvent leading to particular control of reactivity and selectivity. REFERENCES 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)

12) 13) 14) 15)

a) P.N. Rylander, Catalysis in Organic Synthesis, 155, (1978) b) L. Cerveny and V. Ruzicka, Advances in catalysis, 3 0 , 335, (1981) Zeolites for the production of fine and specialty chemicals Catalytica study No 4 1892 (1 990) J.F. Jenck in Heterogeneous Catalysis and Fine Chemicals 11', eds M. Guisnet et al., Elsevier Science Publishers B.V., Amsterdam, 1991 R. Fuhrmann, J. Pisanchyn and F. KoK Ann. N.Y. Acad. Ser. 214, 243 (1973) H. W. Thompson, E. Mc Pherson and B.L. Lences, J. Org. Chem. 41, 2903, (1976) R.L. Augustine, Advan. Catalysis 25, 63, (1 976) G. Allmang. and C. Mercier, Rhdne-Poulenc, unpublished results S. Nishimura, T. Itaya and M. Shiota, Chem. Commun. 422, (1967) J.M. Grosselin, F. Grass, Rhdne-Poulenc, unpublished results PN Rylander and L. Hasbrouck, Engelhard Ind. Tech. Bull., 3, 148 (1968) M. Grey 'Catalysis of organic reactions, 307, (1988) F.W. Vierhapper, J. Org. Chem., 40, 2729 (1975) J.Z. Ginos, J. Org. Chem., 40, 1191 (1975) G. Cordier and Y. Colleuille in 'Catalysis of organic reactions' eds J.R. Kosac, p. 197, Marcel Dekker Inc. NY (1986) a) L. Cerveny, V. Ruzicka, Catal. Rev., 24, 503, (1982)

66

16) 17) 18) 19) 20) 2I)

22)

23)

24) 25) 26) 27) 28) 29) 30)

b) Selective catalytic hydrogenation, Catalytica study No 41 88CH (1989) J. Leludec FR 72- 19130 to Rh6ne-Poulenc Chimie (1 972) M. Von Pierre, Helv. Chim. Acta, 72, 1554, (1989) C. Mercier, Rh6ne-Poulenc, Unpublished results R.M. Baillard, Y. Aranda, J.M. Mas, R. Jacquot EP 259 234 to Rh6ne-Poulenc Agro (1 988) a) M. Gubelmann and P.J. Tire1 EP 341 165 - 271041 to Rhdne Poulenc Chimie (1989) b) Y. Ono, E. Suzuki and K. Nakashiro Chem. Lett., 953, (1988) a) Zeolites 1990 : Synthesis, Characterisation and Applications Catalytica study N" 4 1902 (1 990) b) L. Leherte, D.P. Vercauteren, E.G. Derouane, J.M. Andre Stud. Surf Sci. Catal., 37, 293, (1988) c) E.G. Derouane, Chem. Phys. Lett., 142,200, (1987) a) Y. Gao, R.M. Hanson, J.M. Klunder, S.Y. KO, H. Masamune and K.B. Sharpless J. Am. Chem. SOC.109, 5765 (1987) &l 3949 (1990) b) K. Mikami, M. Terada and T. Nakai, J. Am. Chem. SOC. c) K. Narasaka, H. Tanaka and F. Kanai, Bull. Chem. SOC.Jpn., 64, 387 (1991) a) E.G. Derouane, in "Intercalation Chemistry", eds M.S. Whittingham and M.S Jacobson, Academic Press, New York, 1982, Chp 4 b) S.M. Csicsery, Rev. Appl. Chem., 58, 841 (1986) a) E.G. Derouane, J.M. Andre, A. A. Lucas, J. Catal., 110,58, (1988) b) E.G. Derouane, Catalytica Highlights, 18,4, (1992) a) G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, Stud. Surf Sci. Catal , 28, 129 (1986) b) B. Notari, Stud. Surf. Sci. Catal., 37, 413 (1988) T. Tatsumi, M. Nakamura, S. Negishi and H . Tominaga, J. Chem. SOC.,Chem. Commun., 476, (1990) A. Tuel, S. Moussa-Khouzami, Y . Ben-Taarit and C. Naccache, J . of Molecular Catalysis, 68, 45, (1991) A Botta, H J Buysch and L Puppe, Angew Chem Int Ed Engl , 30, 12, 1689, (1991) T Nakamura, K Shinoda and K Yasuda, Chem Lett 1881, (1992) a) F Sanchez, M Iglesias, A Corma and C del Pino, J Mol Cat , 7 0 , 369, (1991) b) A Corma, M. Iglesias, C del Pino and F Sanchez , J C S Chem Commun, 1253, (1991)

Authors would like to thank Prof, E.G DEROUANE for fruithll discussions.

M.Guisnet et al. (Editors), Heterogeneous Cnfalysis nnd Fine Chemicals !I! 0 1993 Elsevier Science Publishers B.V. All rights reserved.

67

Phenyl vs. carbomethoxy group effect on selectivity during hydrogenation and exchange of a$-unsaturated esters over modified and unmodified deuterated h e y nickel Gerard V. Smith"ib,Ruozhi Songb,Joseph M. Delichb, and Mihaly Bartok" "Molecular Science Program and bDepartment of Chemistry and Biochemistry Southern Illinois University, Carbondale, Illinois 62901, U.S.A. CDepartmentof Organic Chemistry, Jozsef Attila University, Szeged, Hungary

Abstract A series of variously substituted a$-unsaturated methyl esters were hydrogenated with deuterium (deuteriumated) over deuterated Raney nickel, modified deuterated Raney nickel, and PdC. The deuterated Raney nickel catalysts were prepared by leaching Ni-A1 alloy with NaOD in D 0. Each gram of deuterated Raney nickel can furnish approximately 45 to 50 m% of D, (STP), which is readily exchanged even from traces of atmospheric water. Deuterium distributions are similar for deuteriumations using either D or N, gas overlaying reaction mixtures of deuterated Raney nickeI and subs&ate in dioxane. However, N, produced less deuterium in the products because of atmospheric water contamination. L-glutamic acid modification of deuterated Raney nickel also produces less deuterium in the products, but deuterium distributions remain unchanged. Raney nickel does not catalyze simple 1,2-cis (syn) addition of deuterium across C=C double bonds but distributes deuterium among carbons contiguous to the double bonds. The distributions are influenced by structure and substituents; however, the results are adequately explained by the half-hydrogenated state mechanism. 1. INTRODUCTION Raney nickel is widely used in industry for hydrogenations of fine chemicals. This unique catalyst has two sources of hydrogen available for catalytic hydrogenation, internal and external. Considerable internal hydrogen is formed during catalyst preparation from Ni-A1 alloys [l],and subsequent reaction of adsorbed water produces internal hydrogen, which is available for hydrogenation. In practice, external hydrogen gas is added to accomplish hydrogenation. A well known relationship exists between the activity of Raney nickel and its hydrogen content. Removing internal hydrogen by heating lowers catalytic activity for ethene hydrogenation. Activity decreases in proportion to the quantity of hydrogen evolved 111. This relationship also exists for the hydrogenation of nitrobenzene [21, and acetone [31. Diluting nickel with cobalt in the alloy also leads to decreased activity for cyclohexene hydrogenation because less hydrogen is chemisorbed on the catalyst [41. However, not all internal hydrogen is catalytically active for hydrogenation. Temperature program desorption measurements on Raney nickel revealed that surface hydrogen associated with the low-temperature desorption peak is involved primarily in hydrogenation [51; and neutron inelastic spectroscopy

68

measurements show only loosely bound hydrogen active in the hydrogenation of benzene [61. To improve understanding of mechanistic aspects of Raney nickel's selectivity and activity, a series of seven variously substituted a,P-unsaturated methyl esters were hydrogenated either under deuterium (deuteriumated) or under nitrogen over deuterated Raney nickel. For comparison, some of the molecules were deuteriumated over Pd/C, and in some instances the Raney nickel was modified with L-glutamic acid. 2. EXPERIMENTAL

2.1. Materials

Substrates for hydrogenation were methyl cinnamate (I), methyl-a-methyl cinnamate (111, methyl-P-methyl cinnamate (III), methyl-a-phenyl cinnamate (IV), dimethyl itaconate (V), dimethyl mesaconate (VI), and dimethyl citraconate (VII). Their structures and t h e names of t h e corresponding hydrogenation products are given in Table 1. Table 1 Structure of the Substrates

Substrate

bc

X

a

I

-CO,CH,

-H

-4,

-H

I1

-CO,CH,

-CH,

-4,

-H

I11

-CO,CH,

-H

-4,

-CH,

N

-CO,CH,

-4,

-4,

-H

V"

-CO,CH,

=CH, -CO,CH,

VI

-CO,CH,

-CH,

VII

-CO,CH,

-CH,

-CO,CH, -H

-H,-H -H -CO,CH,

Product 1. Methyl 3-phenyl propionate 2. Methyl 2-methyl-3phenyl propionate 3.Methyl 3-methyl-3phenyl propionate 4. Methyl 2,a-diphenyl propionate 5. Dimethyl methylsuccinate 5. Dimethyl methylsuccinate 5. Dimethyl methylsuccinate

"No double bond between a and p carbon atoms Methyl cinnamate (I) from Eastman Organic Chemicals was recrystallized from acetone-water solution and then sublimed. Dimethyl itaconate (V) was from Chas. Pfizer Company and purified by sublimation and preparative gas

69

chromatography. Esters 11, 111, an d IV were prepared from t h e corresponding acids by t he "Diazald" 171 method of esterification. The resulting esters were purified by column chromatography followed by sublimation. E s t e r VI w a s prepared from t h e corresponding acid by the Fisher Methanol Method [ 8 ] a n d ester VII was made from the corresponding anhydride according to the method of Hope [9]. T h e resu ltin g esters were distilled a n d t h e n f u r t h e r purified by preparative gas chromatography. The purities of all th e esters were verified by G.C. a nd NMR. Dioxane, used as t h e washing agent for catalyst preparation a n d also as a reaction media, was spectrograde quality (Matheson, Coleman & Bell). In initial experiments no precautions were tak en to exclude wa te r from dioxane but this led to w a t e r contamination. I n later experiments dioxane w a s stored over a molecular sieve, Linde Type 3A (Matheson, Coleman & Bell). Deuterium g a s was obtained from Liquid Carbonic Corporation a n d h a d an isotope purity of 99.7%. The 99.8% heavy water (D,O), used for preparations of deuterated Raney nickel a nd as a washing agent, was from Isotopes and Stotler Isotopes Company. For each experiment a fresh catalyst was prepared using one of the following methods: RNiD(A). Deuterated Raney nickel catalyst (RNiD) was prepared by leaching Ni-AI alloy with sodium deuteroxide (NaOD) following a procedure suggested for light hydrogen Raney nickel 1101. Sodium deuteroxide solution was prepared by adding freshly cu t sodium metal to heavy w a t e r i n a d r y b a g filled with d r y nitrogen. The alloy, 1.5 g, was added to 25 mL of 20% NaOD solution during a ten minutes period. Th e resulting mixture was heated at 80°C for 45 min a n d then washed t e n times with 3 mL portions of D,O a n d finally washed five times with 5 mL portions of dry dioxane. RNiD(B). This catalyst was prepared with th e s a m e procedure as RNiD(A) except NaOD was prepared in air and no precaution was taken to exclude water from the dioxane. RNiD(C). RNiD(A) was stored under 10 mL of dry dioxane after preparation. Each day t he dioxane was changed an d 10 mL of fresh dry dioxane was added. This was done for five days. MRNiD(A). L-glutamic acid modified RNiD was prepared by first leaching 1.5 g of Ni-Al alloy with 20% NaOD for 45min a t 8OoC,washing with D 0, placing in 225 mL of 2% L-glutamic acid-D,O solution at a pH of 5.1, a n d h o ld n g at 0°C for 90min. MRNiD(B). Light Raney nickel was first prepared a n d modified w i t h Lglutamic acid using light water (H,O) and sodium hydroxide as described above. The resulting light catalyst was deuterated as follows: first, 1.5 mL dioxane a n d 10 mL D,O were added to the catalyst and the mixture shaken under a deuterium atmosphere for 24 hours. Then, the liquid was decanted a n d 15 mL dioxane and 5 mL D,O were added an d th e mixture shaken under deuterium for eight hours. The gaseous atmosphere was replaced with deuterium every two hours. 2.2. A p p a r a t u s and Procedures Deuteriumations were performed at approximately 750 mm Hg in a glass app a r a t us similar to one previously described [ll].Following preparation, the catalyst was pippetted directly into t h e reaction vessel and mixed with weighed substrate (either pure o r dissolved in dioxane). The reaction vessel was attached to the reaction system and the contents frozen with liquid nitrogen. The system w a s t hr i c e evacuated a n d filled with deuterium. T h e reaction mix tu r e w a s brought to a constant temperature (2527°C) by circulating water through th e jacket of t he reaction vessel, an d the reaction was started by agitating the reaction vessel with a vortex mixer. After completion, determined by deuterium

70

uptake, t h e reaction mixture was analyzed by g a s chromatogra h y (GC). T h e GC column was also used to separate the individual components or NMR analysis.

P

2.3. Nuclear Magnetic Resonance Spectroscopy (NMR) Proton NMR spectroscopy revealed deuterium incorporation i n the substrates and products. Using th e methoxy singlets as internal standards of 3.00 protons, t h e a m o u n t s of d eu teriu m i n each position of t h e recovered s u b s t r a t e s a n d products were determined from integration. In each case the number of deuterium atoms were calculated from an average of three to five consecutive integrations.

3. RESULTS AND DISCUSSIONS Availability of internal deuterium i n the various deuterated Raney nickels was evaluated by performing deuteriumations of s u b s tr a te s I1 a n d I11 u n d e r nitrogen atmospheres in dioxane at 25°C. The volumes of deuterium was calculated from percent conversions of t h e known amounts of s u b s tra te s . Typical results are summarized in Table 2. Table 2 Determination of Internal Deuterium Expt.

Catalyst

# ~~

Weight (g)

Substrate Reaction (g) Time (hr)

~

E14A E14B" El9 E20 E13Bb E13A E7 E8 E9 E 10 E 12c

Percent Vol. of D, Reduction (mL) ~

RNiD(A) RNiD(A) RNiD(C) RNiD(C) RNiD(A) RNiD(A) RNiD(B) RNiD(B) MRNiD(A1 RNiD(B) RNiD(B)

.86 .86 .84 .84 .84 .90 $83 .86 .83 .83 .83

I1 (.4000) I1 (.3990) I1 (.3078) I1 (.3004) I1 (.4054) I11 (.4144) I11 (.4021) I11 (.4043) I11 (.4031) I11 (.4060) I11 (.2094)

2.0 2.0 2.0 2.0 2.0 2.0 6.0 6.0 6.0 2.0 2.0

76 15 82 57 10 76 80 82 56 84 74

~~

45.1 8.9 38.2 21.8 6.2 44.4 49.5 49.3 34.6 52.3 23.7

"The catalyst used in E14A was washed three times with dry dioxane and then reused as catalyst for E14B. bThe catalyst used in E13A was washed three times with dry dioxane a n d then reused as catalyst for E13B. T h e catalyst used in E l 0 was washed three times with dry dioxane and then shaken for 48 hours under deuterium. The catalyst took up 28mL deuterium. The duplicate experiments in Table 2 (E7, E8, E l 0 or E13A, E14A) show th a t approximately 45 to 50 m L of D, per g ram of catalyst are available from t h e deuterated Raney nickel under the conditions of the investigation. The amount

71

decreases a s the catalyst is stored in dioxane for a long period of time (E19,E20) or modified with L-glutamic acid (E9). Deuterium distributions for reaction products and recovered substrates a r e shown in Tables 3 an d 4 respectively. Reproducibility is demonstrated by duplicate r u n s in Table 3 (E31B, E4A, E4B under D, a n d E7, E8, E l 0 under N,). Certain results from P d C are also included in Table 3 for comparison. Table 3 Deuterium Distributions in Saturated Products Expt

Catalyst

Substrate Gas

a

P .94

a

bt

bc

Total D

E3B

RNiD(B)

I

.75

D19D E 1A E13B" E14A El9 E20

Pd/C RNiD(B) RNiD(A) RNiD(A1 RNiD(C) RNiD(C)

I1 I1 I1 I1 I1 I1

.83 .43

D20A D28C D31B D31Bc E4A E4B E7 E8 El0 E12d E9 E 13A

PdC MRNiD(A) RNiD(B) RNiD(B) RNiD(B ) RNiD(B) RNiD(B) RNiD(B) RNiD(B1 RNiD(B) MRNiD(A) RNiD(A)

111 I11 I11 I11 111 111 I11 111 I11 111 111 I11

.92 .55 .47 .45 .43 .60 .40 .43 .41 .12 .39 .40

D17 D23A

PdC MRNiD(B)

rv

D28B D32B

MRNiD(A) RNiD(B)

V V

.22 .34

.21 .17

1.21 1.58

1.64 2.09

D34A D32A

RNiD(B) RNi D(B )

VI

.54 .86

.43 .87

1.01 .55

1.98 2.28

l Y

VII

.91 1.41 .37b .46 .32 .23b .12b

1.75 .04 .40 .40 .48 .17 .18

.93 .40 .52 .52 .51 .41 .21 .27 .32

.10 .34 .23 .26 .24 .04

.32 .35

.17 .26

.06

1.12 .85 1.8gb

1.78 2.10 1.13 1.73 .55 .20

.13 .36 .47 .15 .OO

.12 .67 1.14 1.05 1.03 1.14 .77 .86 .92 .42 .94 .88

1.97 1.62 2.19 2.12 2.31 2.38 1.64 1.80 1.69 .54 1.82 1.89 1.97 1.88

'See footnote b in Table 2 baand p positions could not be resolved for individual analyses. 'NMR spectrum was recorded on a 100 MHz NMR spectrometer and th e rests were recorded on a 60 MHz spectrometer. dSeefootnote c of Table 2

72

Table 4 Deuterium Distributions in Recovered Substrates Expt

Catalyst

Substrate Gas

E13B" RNiD(A) E14A RNiD(A) E8 RNiD(B) E9 MRNiD(A) E13A RNiD(A)

I1 I1

N2 N2

I11 I11 I11

N2 N2 N2

a

P .09 .03

.03 .07 .10

a

b .09

.13

,

be

.19 .13 .02 .19 .17

Total D .37 .29

.40 .27 .32

.45 .53 .17

"See footnote b of Table 2 Internal deuterium contained in deuterated Raney nickel readily exchanges, even with traces of atmospheric water. This is revealed by the low deuterium contents of reaction roducts in experiments E12, E13B, E19, and E20 (see Table 3). In particular, t e catalyst in E l 9 and E20 was prepared in a dry bag and stored in dry dioxane, but the dioxane was replaced fresh daily for five days. During the short time of these replacements fresh dioxane absorbed from the atmosphere light water, which exchanged with the internal deuterium of the catalyst. Subsequent GC analysis confirmed that dry dioxane readily absorbs water. However mass spectral analysis disclosed no deuterium exchange in dioxane stored over RNiD(A) for five days. Deuterium distributions are similar for products from reactions using overlayers of either D or N2 gas (see Table 3), deuterated Raney nickel, and substrate in dioxane (k2produced less deuterium in products because of atmospheric contamination from light water). Therefore, the mechanism for incorporation of deuterium is independent of the source of deuterium. Modification of deuterated Raney nickel with L-glutamic acid also results in lower deuterium contents in the products but does not drastically alter deuterium distributions (experiments D28B and D28C). In contrast to other work on C=C double bonds [12], little asymmetric induction was obtained. For example, the deuteriumation of 111 over MRNiD(A) and IV over MRNiD(B) produced products with optical purities of 0.4 and 1.1%respectively. Low enantioselectivities from these catalysts might result from processes associated with extensive exchange. Raney nickel does not catalyze simple 1,2-cis ( s y n ) addition of deuterium across C=C double bonds [131. This is confirmed in the products by unsymmetrical deuterium distributions between a and P positions and by deuterium distributions among the various positions on contiguous carbons. The half-hydrogenated state mechanism adequately explains most of the results. The different deuterium concentrations in cx and P positions can be rationalized by the different stability of the half-hydrogenated states (a- and P-monoadsorbed species a s shown below), and the distribution among the contiguous carbons can be explained as a result of repeated second point adsorption.

K

73

a-monoadsorbed species

P-monoadsorbed species

The more stable the a-monoadsorbed species, the more extensive are the D-H exchanges on the P-carbon, and vice versa. The relative stabilities of these two monoadsorbed species depends on the kind of substituents on the a and P carbons. The slightly greater stabilizing effect of carbomethoxy over phenyl is revealed by the slightly larger deuterium content at the P-carbon compared to the a-carbon (experiment E3B). This stability might be due to the formation of a pi-ally1 species involving the carbonyl double bond and the a-carbon. A methyl group introduced a t the a-carbon, as in 11, leads to a more stable a-monoadsorbed species; a s a result, much more deuterium i s found at t h e P-carbon (experiment E l A ) . However, when t h e phenyl group is replaced by a carbomethoxy group as in VI and VII, the effect of the methyl group diminishes and deuterium is more evenly distributed between a and P-positions (experiments D32A and D34A). A terminal double bond, as in V, favors large deuterium incorporation at the terminal carbon (methyl group in the product), whereas the same structure with a n internal double bond, as in VI, incorporates less deuterium in t h e methyl group. Changing the E structure of VI to the less stable Z, VII, produces yet a third deuterium distribution, in which more deuterium is incorporated into a and ppositions of Z than of E. This may result from adsorbed VII (adsorbed at its si face) adding one deuterium from the surface to the (3-carbon (making a half hydrogenated VII shown below), isomerizing by rotation about its newly formed C"-CP bond to the less sterically hindered transoid conformation, and, thereby, storing deuterium in a P-position relatively inaccessible to the surface.

a =

CH,,

x = b,

= CO,CH,, b t = H

74

4. CONCLUSIONS 1. Deuterium originating from either adsorbed D,O or D, gas is identical. 2. Deuterium distributions are not altered by asymmetric modification of Raney nickel. 3. Raney nickel does not add deuterium simply 1,2-cis instead extensive scrambling occurs between existing hydrogens and deuteriums with those positions adjacent to phenyl groups exhibiting greater exchange. 4. The half-hydrogenated state mechanism adequately explains the results with some evidence for pi-ally1 surface species.

5. REFERENCES

1 R. J . Kokes, and P. H. Emmett, J . Amer. Chem. SOC., 82, (1960) 4497 2 N. I. Popov, D. V. Sokolskii, I. S. Svets, L. A. Kolomytsev, and S. L. Kan, Zh. Fiz. Khim., 47, (1973) 1725 3 I. Nakabayashi, T. Hisano, and T. Terazawa, J . Catal., 58, (1979) 74 4 J . P. Orchard, A. D. Tomsett, M. S. Wainwright, and D. J . Young, J . Catal., 94, (1983) 189 5 J. Heiszman, J. Petro, A. Tungler, T. Mathe, and Z. Csuros, Acta Chim. Acad. Sci. Hung., 86, (1975) 117 6 A. J. Renouprez, G. Clubnet, and H. Jobic, J . Catal., 74, (1982) 296 7 Th. J . De Boer, and J. H. Bocker, Organic Synthesis, Coll. Vol. IV,p. 250, N. Rogjohn, (ed.), John Wiley and Sons, Inc., New Youk, (1963) 8 L. F. Feiser, Experiments i n Organic Chemistry , D. C. Heath and Co., Boston (1957) 9 R. J . Hope, Chem. SOC.,(1912) 900 10 Y. Izumi, H. Fukawa, S. Komatsu, and S. Akabori, Bull. Chem. SOC.Japan, 35, (1962) 1703 11 G. V. Smith, and M. C. Menon, Ann. N.Y: Acad.Sci.,158, (1969) 501 12 M. Bartok, Gy. Wittmann, Gy. Gondos, and G. V. Smith, J . Org. Chem., 52, (1987) 1139 13 K. Hirota, R. Touroude, M. Miyasaka, and J. Matsumara, Res. Inst. Catal., Hokkaiko Univ., 25, 37 (1977)

M. Guisnet et al. (Editors), Hrtcrogmeous Cutalysis und Fine Chemicals III 0 1993 Elsevier Sdence Publishers B.V. All rights reserved.

75

The Role of the Support in Selective Hydrogenations promoted by CU/A203 N.Ravasio*. M.Antenori, M.Gargano Centro C.N.R. MISO, Dipartimcnto di Chinlicit del1'Univsrsit~t.via Amendola, 173 1-70126 B x ~ ,Italy

Abstract The hydrogenation of a-ionone has been carried out in the presence of different copper catalysts supported on inorganic oxides. Results show that A1203 is responsible Tor the a to fi isomerization reaction, while all the catalysts showed the same chenioselectivity. Comparison of different catalysts in the hydrogenation of the steroidal ene-dione 1 evidenced a significant difference in regioselectivity when using Cu/SiOz. Introduction We recently reported that prereduced Cu/Al2O3 is an effective catalyst for the selective hydrogenation of the C-C double bond of or$-unsaturated carbonyl compounds when an isolated olefinic bond is also present in the molecule, showing unique chenioselectivity for a heterogeneous catalyst 111. However, the hydrogenation of a-ionone in hydrocarbon solvents gave mixtures of products owing to partial isonierization of the unconjugated C-C double bond:

+

H2

a

a - I-I

&& e

+

P-H

76

The isomerization reaction can be suppressed by using ethers as solvents, t h u s suggestin participation of weakly electrophilic Cu(1) surface sites or of hard Al$+ acidic sites. An isomerization step is also involved in the hydrogen transfer reaction of steroidal homoallylic alcohols which takes place in the presence of Cu/Al2O3 121. This process recently allowed us to obtain 95% 5p isomer in the hydrogenation of ergosterol under inert atmosphere while 98% 5a isomer was obtained under H2 pressure with the same catalyst [31. We therefore thought it of interest to investigate in more detail the role played by the support in selective hydrogenations promoted by copper catalysts. It is well known, e.g., that the support can strongly influence copper reduction through its ligand properties 141 or through electronic effects (5). Here we report results obtained with different catalysts in the hydrogenation of a-ionone, showing that the isomerization reaction is carried out by Al2O3. During this study some differences among the catalysts used were found and this prompted u s to investigate the influence of the support on chemo- and stereo-selectivity during the hydrogenation of steroidal enones.

EXPERIMENTAL

GC analysis w a s performed on a Hcwlett-Packard 5880 instrument, FI detector, equipped with a 35% diphenyl/65% dimethylpolysiloxane capillary column. (SPB-35. 30m) or a methylsilicone (SPB-1, 30111) Reaction products were identified by comparing their retention times and IR, 1H NMR and M S spectra with those of commercial products or those reported in the literature. Cu/SiO2. Cu/Cr203. Cii/Mn02 and CujTiO2 (Degussa P25) were prepared by chemisorption on the support from a [Cu(NH3)4]++solution following the procedure already described for Cu/Al2O3 Ill and have a copper content of 7-10%. Cu/ZnO (8.4%) was prepared by coprecipitation 161. Girdler G 13 is a commercial Mn-free copper chromite catalyst (40% Cu). Cu/A12O3, Cu/SiOz, Cu/CrzOs and Cu/ZnO were dehydrated and prereduced before use at 27OOC in H2, whereas Cu/MnOz, Cu/TiO2 and G13 were dehydrated at 270°C and reduced with Ha increasing the temperature from 100 to 200OC. Hydrogenation procedure. The substrate (0.5 mmoles) was dissolved in toluene (6 ml) and the solution heated to 90°C. then transferred, under H2,into the reaction vessel where the catalyst (200 mg) had been previously pretreated. The final charge of H2 was adjusted to 1 atm with a mercury leveling bulb, stirring was begun and H2 uptake measured through a mercury sealed gas burette.

77

The Lewis acidity of Al3+ ions can account for the isomerization reaction. Thus, the enone moiety would cohordinate through its more negative end, the oxygen atom. and rearrangement of the species formed would give the more stable conjugated isomer (Scheme 1).

II'

Scheme I Although active sites in transitional aluminas, particularly those involved in alcohols dehydration (7.81, are still poorly characterized, isomerization of l-butene to cis- and trans-butene-2 on y-Al2O3 has been extensively studied 191 and a general agreement has been reached on

78

Lewis acid sites, consisting of coordinatively unsaturated aluminium ions, being the primary seat of this type of activity in pure alumina. In our case, no isomerization took place when a-Al2O3 was used as the support, t h u s showing that defect sites and perhaps tetrahedrally coordinated Al ions [lo]. present only in the y form, can play a role in this kind of activity. Only in the presence of Cu/ZnO small aiiiounts of the p-isomers were formed (8-10%) when the reaction was carried out in toluene according to the Lewis acidity of Zn2+ ions. Some differences were observed among the catalysts used. Although these reactions are carried out without a rigorous check of stirring speed (magnetic stirring). some differences i n activity are apparent. Cu/TiO2 and Cu/SiO2 appear to be the most active ones, while if Cu/MnOn. Cu/TiO2. or, to a lesser extent, Cu/ZnO are used a poor mono/dihydrogenation selectivity is found. the alcohol being formed before the starting material is complctcly reduced. In order to get some inlorniations on the effect of the support on hydrogenation stereoselectivity we carried out the hydrogenation of 4androsten-3.17-dione 1 in toluene at 60°C and 1 atm of H2: results are collected in Table 2 and compared with those already reported for CU/A203 [ 101.

2 3

4

6

1

All the catalysts reported in 'I'abk 2 showed the same chemoselectivity already reported for Cu/Al2O3. Thus. the conjugated enone is selectively hydrogenated while the saluratcd keLo group at C 17 remains unchanged.

1

H

2

H 3

Stereoselectivity is almost independent on the catalyst used, but a small increase was observed in the presence of Cu/Cr203. Very similar results were obtained when a commercial copper chromite with high copper content was used. As these two catalysts, activated in different ways, are very different as far as the surface copper is concerned, this enhanced stereoselectivity appears to be d u e to different absorption of the substrate on the Cr2O3 surface. However, copper/chromium catalysts show very low mono/dihydrogenation selectivity and do not allow to separate the saturated ketone even by limiting the H2 consumption.This difference in

79

To monitor the product distribution versus H2 uptake , 20 pL samples were withdrawn from the reacting solution through a viton septum and analyzed by GC. The equivalents of hydrogen were calculated on the basis of the molar amount of H2 consumed per mole of substrate. Compound 4 was separated from the reaction mixture by medium pressure chromatography on silica, using EtzO/light ether (1:3)as eluent, and identified through its IR (vco=1740 cm-I), UV [EtOH kmax242, 275(sh) nm]. M S (m/z 270, m+l=21%,m+2=2%). l H NMR [(CDC13) 5.93(dm, lH), 5.61(m. 1H). 5.40 (m,lH) ppm] and 1% NMR [(CDCls), 221.1, 141.6. 128.8, 125.3, 122.1, 52.0. 48.5, 47.7, 35.8, 35.3. 33.7. 31.5, 31.4, 30.6, 23.0, 21.8, 20.3. 18.8, 13.7 ppm] spectra.

RESULTS AND DISCUSSION Results obtained in the hydrogenation of a-ionone with different supported copper catalysts are reported in Table 1. Among the catalysts used, only that based on Cu dispersed on y-Al203 produced significant amounts of the p-isomers. thus showing that this support is responsible for the a to p isomerization reaction. We can also exclude cohoperative effects between the metal and the support as this reaction took place with the same rate in the presence of both Cu/y-N203 and y-AlzO3 alone (Fig.l).

40 6o

1 0

0

b-ion/A1203

M

a-jon/A1203

0 a-ion/Cu-At

1

2 3 tirne(h)

4

5

Fig.l Isomerization of a-ionone in the presence of Cu/Al2O3 and of y-Al2O3 at 60°C.

80

The hydrogenation of 1 in the presence of Cu/SiO2 show some interesting features. Formation of a secondary product takes place from the very beginning of the reaction IFig.3).

0

E0

6o

F

40

.-

A 0

t

W

P

4-ene-dione 1 3,5-dlene 4 Bb-dione2 5a-dione2 5b-01~3

20

0 1,o

0,o

eq H2

Fig.3 Products distribution versus H2 uptake in the hydrogenation of 1 in the presence of Cu/SiOz Formation of this product. identified a s 3,5-androstadien-17-one 4 is worth noting for two reasons. First of all it can form through acidic dehydration of the allylic alcohol [ 121. thus representing an indirect evidence of 1.2 hydrogen addition to the enone nioiety. Therefore, in the presence of Cu/SiO2 hydrogenation of the steroidal enone is not regioselective as both 1,4 and 1,2 hydrogen addition products are formed a t the same time. Moreover, the conjugated diene accumulates in the reaction conditions reaching about 33 Yo of the reaction mixture while the conjugated ketone and also the saturated ketone are reduced, thus showing a dramatic difference in the reaction rates of the two functions, that is, a remarkable chemoselectivity. Work is in progress now to elucidate the contribution of Cu(1) on the reduced Cu/SiO2 surface 141 and of the weak Broensted acidity of the silanol groups to this reaction.

81

behaviour with respect to a-ionone hydrogenation (see Table 1) may well be due to different absorption of the cyclic enone on the catalyst surface. On the contrary, in the presence of Cu/Al2O3 the two hydrogenation steps are always well separated unless an alkylic substituent is present a to the carbonyl [l]. Table 2a Comparison between different copper catalysts in the hydrogenation of 1 after 1 eq H2 consumed

100

-

60 80

B

.-0

p

40

-

20

-

?

P

4-ene-dione 1 5b-3-0ne 2 Y

5b-3-01s 3

0 0,o

5a-3-0170.2

I

1 .o eq. H2

Fig.2 Products distribution versus H2 uptake in the hydrogenation of 1 in the presence of commercial copper chromite (C 13)

82

1) N.Ravasio. M.Antenori, M.Gargano, M.Rossi , J , Molec.CataZysts, 74 (1992) 267-274 2) N.Ravasio, M.Gargano. V.I-’.Quatrat-o, M.Rossi,ll 1nt.Syrnp.Het. Cat. Fine Chern., St. Surface Sciericc urtd Cutcihysis 5 9 (1991) 161 3) N.Ravasio, M.Gargano, M.Rossi. J.01-g.Chern., 58 (1993) 1259 4)M.A.Kohler, H.E.Curry-Hide, A.E.Hughes. B.A.Sexton, N.W.Cant, J.Catalysis 108 (1987) 323 5) H-W Chen. J.M.White. J.G.Ekerdt, J.Catalysis 99 (1986) 293 6) V.DiCastro, M.Gargano. N.Ravasio, M.Rossi, in G.Poncelet, P.A.Jacobs. P.Grange and B.Delmon(Eds.), Preparation of Catalysts V, Elsevier. Amsterdam 199 1, p.95 7) C .R.Narayanan. S.Srinivasan, A.K. Ilatyc. R.Gorte. A.Biaglow. J.Catalysfs 138 (1992) 659 8) E.C.DeCanio. V.P.Nero, J.W.I3runo. J.Catalysis 135 (1992) 444 9) H.A.Benesi, B.H.C.Winquist in: Advances i n Catakysis, vo1.27, Academic Press, 1978, pp. 123- 125 10) C.S.John, N.C.M.Alma, G.Hays. AppL. Cutahpis 6 (1983) 341 11) N.Ravasio. M.Rossi. ,J.Org.C/iern. 56 ( 1 99 1) 4329 12) Eck, Van I’cL1rscrn. ilollingsworth. J.Ar?i.Cliern.Soc. 6 1 (1939) 171

M. Guisnet et al. (Editors), Heterogeneous Cntdysis and Fine Chemirals Ill Q 1993 Elsevier Science Publishers B.V. All rights reserved.

83

Liquid-Phase Hydrogenation of Acrolein to Ally1 Alcohol on Supported Cobalt Catalysts Yuriko Nitta, Takashi Kato, and Toshinobu Imanaka Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osakil 560, Japan

Abstract In the selective hydrogenation of acrolein to allyl alcohol with supported Co catalysts, the use of cobalt chloride as the starting salt in catalyst preparation was not effective but the influence of support, solvent, and added Fe cations was significant in contrast with the hydrogenation of crotonaldehyde. A higher reduction temperature or Co loading of Co/silica-alumina catalysts resulted in a larger size of Co crystallites and a higher selectivity to allyl alcohol. The causes of the structure sensitivity in the selective hydrogenation of a$-unsaturated aldehydes have been discussed based on these findings. A relatively high selectivity to allyl alcohol up to 33% a t 50% conversion was obtained in 2-propanol with a Co/silica-alumina catalyst reduced at 500°C for lh. 1. INTRODUCTION

Catalytic hydrogenation of a$-unsaturated aldehydes to give unsaturated alcohols is a m u c h more difficult problem than the hydrogenation producing saturated aldehydes or saturated alcohols. Various platinum metals have been used for dccades, but reports on nickel or cobalt catalysts are limited [I]. Nickel catalysts are known to catalyze the hydrogenation to saturated aldehydes, whereas cobalt catalysts are suitable for selective hydrogenation to unsaturatcd alcohols [2, 31. We have rcported that a Co/SiO, catalyst prepared by a precipitation method from cobalt chloride exhibits high selectivity and relatively high activity for the hydrogenation of cinnamaldehyde and crotonaldehyde to corresponding unsaturated alcohols, and that the selectivity increases with increasing mean size of cobalt crystallites (Dc) in the catalysts [4, 51. The enhanced selectivity of the catalyst prepared from cobalt chloride has been explained by the effects of residual chlorine both in the H2-reduction stage and i n hydrogenation stage; the former leads to a n increased Dc and the latter depresses the hydrogenation of C=C double bond. Richard eta]. [6] also reported a similar structure sensitivity in the hydrogenation of cinnamaldehyde on platinum catalysts; selectivity into cinnamyl alcohol is much higher on large facetted metal particles than on small particles. They attributed this to a steric effect of the phenyl group which hampers the molecule to adsorb parallel to the flat metal surface thus favouring the adsorption and hydrogenation of the carbonyl

84

group with respect to the C=C double bond [7]. Although cobalt catalysts cannot be prepared with a dispersion as high a s in platinum catalysts, supported cobalt catalysts usually have broad size distributions of cobalt crystallites. Taking into account the facts that the crystallite sizes obtained by the X-ray line broadening reflect the information on relatively large crystallites and that smaller particles have relatively large contribution to the overall hydrogenation, the structure sensitivity with cobalt catalysts could be explained in a similar way as with platinum catalysts. Therefore, the present work is intended to make clear the causes of the structure sensitivity and the factors governing the selectivity of cobalt catalysts by employing acrolein as the reactant without any bulky substituent around the C=C double bond, and also to optimize the preparation and reaction conditions with cobalt catalysts.

2. EXPERIMENTAL Catalyst precursors (Co loading = 50wt%, unless otherwise noted) were prepared by a deposition-precipitation method with sodium carbonate and cobalt chloride or nitrate as described previously [4, 51. Supports used for the catalyst preparation are silica (Silica Gel No. 1, Nacalai Tesque, Inc., 600 m2g-'), y-alumina (AKP-G, Sumitomo Chemical Co., 110-170 m2g-l), titania (JRC-TIO-4, 150 m2g-'), silicaalumina (JRC-SAL-2, 560 m2g-'), silica-magnesia (JRC-SM-I, 642 m2g-'), carbon (Activated Charcoal, Wako Pure Chemical Industries, 1300 m2g-'), zirconia (NS-OY, Nippon Shokubai Kagaku Kogyo Co., 27 m2g-'), zeolite (JRC-Z-HM20, 299 m2g-'), and magnesia prepared from Mg(OH), by calcination at 600°C for 4 h (4 m2g1). The supports with JRC in the name are the Reference Catalysts of the Catalysis Society of Japan. One gram of a dried precursor was reduced by heating in an H, stream for 12 h at a temperature necessary to give a reduction degree higher than 90%. The hydrogenation of acrolein (0.4M), or crotonaldehyde (0.3M) for comparison, was carried out usually in ethanol at 30°C under 1.0 MPa of hydrogen by using a pressure-resistant glass autoclave equipped with a magnetic stirring system (Co/substrate = 0.6 in molar ratio). The initial hydrogenation rate (ro) was obtained from the conversion of the reactant measured by gas-liquid chromatography. The selectivities at different conversions were determined as the molar percentages of the unsaturated alcohol in all the products. Thermogravimetric analyses (TGA) of catalyst precursors were carried out with a Shimazu DT-30 thermal analyzer by heating in a stream of hydrogen to 800°C at a rate of 10°C min-'. The mean crystallite size of Co (Dc) in the reduced catalyst was calculated from the half-maximum breadth of the (1 11) peak of f.c.c. Co metal in the powder X-ray diffraction (XRD) pattern after correction for instrumental broadening

PI. 3. RESULTS AND DISCUSSION As shown in Table 1, catalysts prepared by using cobalt chloride as the starting salt had low activities and relatively low selectivities in the hydrogenation of acrolein irrespective of the support employed, although they were highly selective in the

85

Table 1 Effects of starting cobalt salt and support on the properties of resultant cobalt catalysts Acrolein No. Starting salt 1 2 3 4

Chloride Chloride Chloride Chloride

5 Nitrate 6 Nitrate 7 Nitrate 8 Nitrate 9 Nitrate 10 Nitrate 11 Nitrate 12 Nitrate 13 Nitrate 14 Raney-Co a b c d

support SiO A1 Tib, SO,-Al,O,

6,

2'6,

Tib, Si0,-Al,O, C ZrO Mg6 SiO -MgO Zeohte

Crotonaldehyde

Reduction conditions

Dca

400°C,2h 350"C, l h 350"C, l h 400°C l h

9 14 31 15

l.OxlO-' 7.4 3.2x10-, 10.7 4.1x10-, 5.0 4 . 8 ~ 1 0 - ~9.6

2.7~10-' 7.4x10-, 6 . 4 1~O-, 3.7xlO-,

80.0 65.6 80.0 73.4

400°C,2h 350"C, l h 350"C, l h 400"C, l h 300"C, lh 350"C, lh 400"C, l h 350"C, l h 350"C, l h

7 12 17 9 25 16 6 17 20 4

7.9~10-' 6.6 1.2~10-' 4.0 3.0~10-' 2.2 52x10-' 13.3 2.6x10-, 6.1 1.3x10-, 0.0 3.6~10-L 0.0 5.0x10-' 4.6 2.3~10-' 6.2 1.l x 10-1 5.3

4.4~10-' 1.7~10-' 2 . 7 lo-' ~ 4 . 2 lo-' ~ 1.ox 10-1 3.1x lo-, 1.1

32.0 29.0 25.0 33.2 24.0 28.4 12.1

4.7~10-'

4.1

b r0

s50"

r0

s,,.

Mean crystallite size (nm) of cobalt determined by XRD line broadening. Initial reaction rate (mmol min-lgco-'). Selectivity (%) to unsaturated alcohol at 50% conversion. Prepared from 50% Raney-alloy by leaching at 50°C for lh.

hydrogenations of crotonaldehyde and cinnamaldehyde [4].On the other hand, the catalysts prepared from cobalt nitrate had high activities, and the selectivity depended on the support employed, strongly in the hydrogenation of acrolein but not so much in the hydrogenation of crotonaldehyde. Acidic support, such as silica-alumina, seemed to be favourable, while the use of basic support, e.g., magnesia, resulted in a n immediate formation of hemiacetal with no or very low selectivity to ally1 alcohol. These findings show that the residual chlorine in the catalysts prepared from cobalt chloride is not effective on the hydrogenation of acrolein, suggesting this reaction is rather insensitive to the size of cobalt crystallites. Chlorine atoms remaining on the surface of metal crystallites are supposcd to be preferentially located at the sites of lower coordination numbers, such as edges and corners. When the substituent on f3-carbon of the substrate is bulky enough, the chlorine atom hinders the substrate from adsorbing parallel to the metal surface thus favouring the adsorption and the hydrogenation of the carbonyl group just as in the case on large facetted metal particles, even if the metal crystallites are relatively small (scheme 1). On the other hand, the selectivity in the hydrogenation of acrolein will not be affected by the residual chlorine atoms. In other words, this means that the structure sensitivity in the hydrogenation of a$-unsaturated aldehydes with bulky substituent on (3-carbon

86

Scheme 1 is attributable to the steric effect as dcscribed by Giroir-Fcndler ct al.171. Figure 1 shows the effect of the reduction temperature of a Co/silica-alumina catalyst, prcpared from cobalt nitrate, on the selectivity to allyl alcohol. Thc higher reduction temperature rcsultcd in a catalyst with a larger crystallitc size of cobalt and a higher selectivity. The presence of residual Co2+in the catalysts reduced at lower temperatures could have some electronic effect on the hydrogenation. Howcvcr, cobalt salts added into the reaction mixturc scarcely affected the sclcctivity as will bc shown below. Thc results for the catalysts with diffcrcnt cobalt loadings also

40

I

8

-

-

0 L

8 m

--x ;;i

~

I

J

I

I

I

"

A 450'c,1 h red u ced , 4OO'C.l h red u ced ,

30 - 0 350'C,lh red u ced ,

40-

.

o 500'C,lh red u ced , Dc=15nrn

\

1

-

D c = l l nrn Dc= 9nrn Dc= 8nrn

30: -

20-

0

x

u

u

0

-e,e, v)

10:

-o--+-+ I

I

I

I

J

I

I

I

I

I

I

I

I

I

--

u

'5 .- 10

I

1 1 -

-

20-

1

0 0 0 ,

CO loading 50.0wt0/o Dc=l5nm Co loading 33.3wt0/o Dc= 9nrn Co loading 20.0wt0/0 Dc= ,

,

I

,

,

!

Figure 1. Selectivities to allyl alcohol on Co/silica-alumina catalysts. a) Co: 50wt%, solvent: cthanol. b) Reduced at 500°C, solvcnt: 2-propanol.

,

,

I

87

supported the size effect in the hydrogenation of acrolein; a higher cobalt loading led to a larger Dc and a higher selectivity to allyl alcohol (Figure 1b). Therefore, it can be said that the selectivity of a series of catalysts, prepared by using a given salt and support, depends on the size of Co crystallites even in the hydrogenation of acrolein. In other words, the structure sensitivity in these reactions cannot be explained only in terms of the steric effect of bulky substitucnts on P-carbon of the substrate. As electronic effects of additives have been claimed for the selective hydrogenation of a,b-unsaturated aldehydes [9-141, the influence of metal salts was examined in the hydrogenation of acrolein with Co/silica-alumina catalysts. As shown in Table 2, a small amount of Fe (I1 or 111) chloride added into the hydrogenation mixture exerted a favourable effect on the selectivity to allyl alcohol. The other metal chlorides examined were less effective on the catalyst performance. Therefore, not chloride anion but Fe cation seems to have positive effects on the hydrogenation of carbonyl group as illustrated by Richard et al.[ 121. A series of Co-Fdsilica-alumina catalysts, prepared by a coprecipitation of cobalt and iron species on silica-alumina, had decreasing selectivity (S o) and Dc with increasing amount of Fe addition when they were reduced at 500°C tor lh, while the selectivity of the catalysts reduced at 400°C for l h was increased by Fe addition of 5-10% in spite of thc decrease in Dc of these catalysts (Figure 2). In the hydrogenation of crotonaldehyde, however, the selectivity was always decreased by coprccipitation of Fe. Differential thermogravimetric (DTG) profiles of these coprecipitated catalysts (Figure 3), clearly indicate t h a t there still remains considerable amount of unreducetl Fe species after reduction a t 400°C. Therefore, the electronic effect of incompletely-reduccd iron species seems to be larger than the effect of Co size (ensemble effect) on the hydrogenation of acrolein. Table 2 Effects of metal salt addition on the hydrogenation of acrolein with a Co/silica-alumina catalyst reduced at 400°C for 1 h Metal salt a None SnCl, 2H,O FeCl, 4H20 FeCl, FeCl; 6H,O CSCl MnCI, 4H,O CuCl CuCl, 2H20 CoCl, 6H,O Fe(NO,), 9H,O a b c

b r0

5.2x 10-1 5.Ox 10-l 1 ~5x10-' 1.8x10-' 1 . 6lo-' ~ 1.9~10-l 2 . 3 10-' ~ 1.9x10-' 7.2x10-, 2.3~10-' 2.1x 10-1

C

C

C

s20

s50

S80

__

11.3 11.6 18.3 16.5 17.8 9.8 12.8 12.7 5 .0 12.7 21.1

__

_ _ _ I

13.3 14.1 17.2 17.9 17.8 8.4 11.9 12.1 4.9 12.7 11.5

11.1 13.6 14.0 17.6 16.2 7.0 11.1 11.5 3 .0 10.9 4.7

M/Co=0.05 (molar ratio). Initial reaction rate (mmol min-'gCo-l). Selectivity (%) to allyl alcohol at 20, 50, or 80%conversion.

88 40

I

I

I

I

60

I

A - 40

Ec

-

0”

- 20

I

Figure 2. Effects of Fe addition on the selectivity to ally1 alcohol(0 0 ) or crotyl alcohol ( A , A ) and on the mean crystallite size of Co of Co-Fe/silica-alumina catalysts reduced at 400°C ( 0,A,O) or 500°C (0,A,+).

(0,b)

20% 2_

I

I

1

I

I

I

100

200

300

400

500

600

Ternperature/”C

Figure 3. DTG-in-H, profiles of the Co-Fe/silica-alumina precursors prepared by coprecipitation of Co and Fe(II1) nitrates.

89 From these findings, it can be concluded that the structure sensitivity in the selective hydrogenation of a,b-unsaiurated aldehydes such as crotonaldehyde and cinnamaldehyde largely depends on the steric effect of the bulky substituent on @-carbon of the substrate, but that another interpretation, e.g., an electronic effect, should be proposed for the size effect in the hydrogenation of acrolein. In order to improve the selectivity to allyl alcohol, we examined the effect of various solvents for the hydrogenation with a Co/silica-alumina catalyst. As shown in Table 3, the use of 2-propanol much improved the selectivity to allyl alcohol, while it only slightly improved the selectivity to crotyl alcohol in the hydrogenation of crotonaldehyde. The hydrogenation of acrolein in 2-propanol was accompanied with the formation of only a negligible amount of hemiacetal. Additional experiments will be needed to explain the solvent effect in the hydrogenation of acrolein. A higher selectivity, i.e., 33% allyl alcohol at 50% conversion, was obtained on the catalyst reduced at 500°C for l h with a higher concentration (1.OM) of the reactant i n 2propanol. The concentration effect may be interpreted by the stronger adsorption of acrolein than that of ally1 alcohol on Co catalysts. The higher the concentration of acrolein, the more easily allyl alcohol is replaced by acrolein molecule before being hydrogenated to saturated alcohol. Similar effects were observed in the hydrogenation of crotonaldehyde and cinnamaldchyde [5]. Table 3 Effect of solvent on the hydrogenation of acrolein and crotonaldehyde with a Co/silica-alumina catalyst reduced at 400°C for l h -

a

No. Solvent 1 Methanol 2 Ethanol 3 2-Propanol 4 t-Butanol 5 Ethyl acetate 6 n-Hexane 7' ?-Propano1 8' 2-Propanol 9 Ethanol 10 2-Propanol 11 n-Hexane

Substrate Acrolein Acrolein Acrolein Acrolein Acrolein Acrolein Acroleind Acrolein Crotonaldehyde Crotonaldehyde Crotonaldehyde

r0

4.3~10-1 5 . 2 lo-' ~ 4.3~10-I 3 . 0 10-I ~ 12x10-' 2.3~10-' 2.8x 10-1 2.4~10-l 4.2~10-' 2 . 81~0-I 2 . 4 lo-' ~

b

b

S*O

s*o

8.0 11.3 27.6 11.5 32.4 8.2. 33.9 36.5 37.0 40.6 49.5

7.9 13.3 26.9 11.3 24.1 8.0 30.1 32.9 33.2 37.7 42.0

6.3 11.1 23.6 8.3 15.9 7.4 24.8 28.3 28.2 33.2 31.8

Initial reaction rate (mmol min-'gC0-l). Selectivity (%) to allyl alcohol at 20, 50, or 80% conversion. c The catalyst was reduced at 500°C for l h instead of 400°C for l h . d Substrate concentration was 1.OM instead of 0.4M.

a b

b

S20

90

4. CONCLUSIONS

1. In the selective hydrogenation of acrolein to allyl alcohol with supported cobalt catalysts, the use of cobalt chloride as the starting salt in the catalyst preparation was not effective, but the influence of support, solvent, and added iron cations was significant in contrast with the hydrogenation of crotonaldehyde. These findings suggest that the hydrogenation of acrolein is not sensitive to the size of cobalt crystallites, but sensitive to the electronic state of cobalt surface. 2. The selectivity of a series of catalysts, prepared by using a given salt and support, increased with increasing size of cobalt crystallites even in the hydrogenation of acrolein. 3. The structurc sensitivity in the selective hydrogenation of a,p-unsaturated aldehydes largely depends on the stcric effect of the bulky substitucnt on 6-carbon of the substrate, while another interpretation should be proposed for the size effect in the hydrogenation of acrolcin. 4. A relatively high selectivity to allyl alcohol up to 33% at 50% conversion was obtained in 2-propanol with a Co/silica-alumina catalyst reduced at 500°C for 1 h. 5. REFERENCES

I P. N. Rylandcr, Catalytic Hydrogenation in Organic Syntheses, Academic Press, New Yorkbndon, 1979. 2 K. Hotta and T. Kubornatsu, Bull. Chcrn. Soc. Jpn., 42 (1969) 1447; 44 (1971) 1348; 45 (1972) 3118; 46 (1973) 3566. 3 Y. Nitta, T. lrnanaka and S. Tcranishi, Bull. Chcrn. SOC.Japan, 53 (1980) 3154. 4 Y. Nitta, Y. Hiramatsu and T. Imanaka, J . Catal., 126 (1990) 235. 5 Y. Nitta, K. Ueno and T. Imanaka, Appl. Catal., 56 (1989) 9. 6 D. Richard, P. Fouilloux and P. Gallezot, in "Proc. 9th lnt. Congr. Catal.", eds. M. J. Phillips and M. Tcrnan, Vol. 3 (The Chemical lnstitutc of Canada, Ottawa, 1988) p.1074. 7 A. Giroir-Fendlcr, D. Richard and P. Gallezot, Catal. Lett., 5 (1990) 175. 8 B. E. Warren, J. Appl. Phys., 12 (1941) B75. 9 Z. Poltarzewski, S . Galvagno, R. Pietropaolo and P. Staiti, J. Catal., 102 (1986) 190. 10 D. Goupil, P. Fouilloux and P. Gallezot, React. Kinet. Catal. Lett., 35 (1987) 185. 11 C. S. Narasirnhan, V. M. Deshpande and K. Rarnnarayan, J. Chcrn. SOC.,Cheni. Cornrnun. (1988) 99. 12 D. Richard, J. Ockelford, A. Giroir-Fendler and P. Gallezot, Catal. Lett., 3 (1989) 53. 13 D. G. Blackmond, R. Oukaci, B. Blanc and P. Gallezot, J. Catal., 131 (1991) 401. 14 T.B.L.W. Marinelli, J.H. Vleeming and V. Ponec, Proc. 10th lnt. Congr. Catal., Budapest, 1992.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

91

Selective Hydrogenation of Aromatic Aldehydes Using Precious Metal Catalysts on New High Surface-Area TiO, Supports

M. Bankmann, R. Brand, A. Freund and T. Tacke Degussa AG, Inorganic Chemical Products Division, Research and Development, P.O.Box 1345, D-6450 Hanau 1, Germany

Abstract New high surface-area TiO, extrudates, based on pyrogenic and precipitated TiO,, were used as supports for precious metal (Pd, Pt, Rh) catalysts. The precious metal catalysts were employed in the selective hydrogenation of aromatic aldehydes which have different substituents 1 (R = COOH, CH,, CI). Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation reaction can be directed either toward a benzyl alcohol derivative 2 or toward a methyl-substituted compound 3. The benzyl alcohol is the reaction product obtained using catalysts which have weak or no acidic properties. If acidic catalysts are used, hydrogenolysis to the methyl-compound 3 dominates. Selectivity changes in the order from l a to l c . If an alcohol is used as solvent, side reactions, e.g., ether formation or aromatic ring saturation, can be observed. In the case of catalysts on precipitated TiO,, the benzyl ether is obtained at high selectivity in almost quantitative yield. The rate of hydrogenation is determined by the precious metal profile as well as by the electronic properties of the substituents which are effective in the following order: COOH > CI > CH,.

1. INTRODUCTION

Selectivity is one of the most important goals of the use of heterogeneous catalysis in the production of fine chemicals. Recently, we have reported on new high surface-area supports based on Degussa’s pyrogenic 1. These catalyst supports, impregnated with precious metals, have TiO, [l

92

been further investigated to demonstrate their unique physicochemical characteristics in catalytic reactions. In this study, aromatic aldehydes having different substituents were subjected to selective hydrogenation in liquid phase. The 4-carboxy(l a), 4-methyl-(l b), and 4-chlorobenzaldehyde (1c) were chosen as representative examples because of their significance and of the different electronic properties of their aromatic rings [2]. Based on the substrate and the reaction conditions, as many as five products can be expected (Fig.1).

8 X

X

+ Hz/Solvent Cat.

+

Q

CHO

X = COOH X eCH3 IXIXmCI

+

+

CHzOR

[2a-c~=~ R

E

OCHzCH3

0 X

X

CHI

others and

CH3

[3e-cl X

=H

Figure 1. Selective hydrogenation of aromatic aldehydes

2. EXPERIMENTAL

Precious metal catalysts on TiO, extrudates having anatasdrutile ratio, which range from 75/25 to 0/100, were prepared by the pore-volume impregnation method. After being dried and calcined in air, they were reduced in a hydrogednitrogen atmosphere. In the same manner, a reference catalyst on formed precipitated TiO,, which consisted of pure anatase, was prepared. Varying acidic properties were obtained by changing the impregnation procedure and the phase composition of the titania support. The classification of catalysts in strong or weak acidity was done by pH

93

measurement of an aqueous 4 wt.-% catalyst suspension [3]. Catalysts classified as strong had a pH value of below 3.5, whereas the pH value of weakly acidic catalysts was in the range of 3.5 to 6. The same correlation was found for formed supports before the impregnation step.

As reported by Nakabayashi et.al. decreasing acidic properties of pure anatase were linked with decreasing BET-surface area due to increasing cristallite size of TiO, [4]. A similar correlation between acidity and BET surface area could also be observed in our experiments. However, it has to be emphasized that in our case the ratio of anatase to rutile is a subject to change. In general, weakly acidic catalysts were obtained by choosing supports, which have a high rutile content, and by using a minimal concentration of anions, such as halides or sulfate, during catalyst preparation [5]. In contrast, the use of precipitated titania which contained substantial amounts of sulfate introduced during its manufacture, gave rise to strong acidity in the final catalyst [6]. Typical characteristics of prepared catalysts are given in Table 1.

Table 1 Characteristic properties of PM/TiO, catalysts PMniO,

PM [wt Yo]

Phase Ratio AnataselRuth

Acidic pH [3] Properties

Precious Metal Profile

CO adsorption [ml CO/g Cat.]

Pd 1 Pd 1C Pd 2 Pd 4 Pd 5 Pd 6 Pd 7 Pt 1 Rh 2

0.5 0.5a) 1 .o 0.5 0.5 0.5 0.5 1.o 1 .o

75/25 10010 75/25 01100 75/25 75/25 01100 75/25 75/25

strong strong strong weak strong weak weak weak strong

uniform shell uniform uniform shell shell shell

0.43 0.09 0.50 0.33 0.32 0.31 0.22 0.38 1.71

2.9 3.1 2.8 3.8 3.1 4.4 5.2 3.5 2.9

shell uniform

a) PrecipitatedTiO, is used.

All catalyst tests were performed at 150 "C and 10 bar of hydrogen partial pressure in a one-liter stainless steel autoclave which was equipped with a catalyst basket. In each run, 500 ml of a 1 wt. YO ethanol or aqueous solution of the substituted benzaldehydes la-c were hydrogenated in the presence of 2 g of catalyst. Sampling was done after 10, 30, 60, 120 and 240 minutes. In the case of la, the reaction mixture was analyzed by HPLC. GC was used to analyse the products of 1b and 1 c.

94

3. RESULTS AND DISCUSSION 3.1. Hydrogenation of 1a The benzyl alcohol 2a was obtained at a selectivity greater than 90 % as the main product of the hydrogenation in aqueous phase when each of the catalysts, with the exception of Pd 5, was used. The most favourable results were obtained using either catalyst Pt 1 or catalysts Pd 4 and Pd 6 which have the lowest acidity of all of the catalysts investigated. This can be attributed to the preparation of these catalysts without the introduction of interfering anions and/or to the pure rutile support.

The highest selectivity to the hydrogenolysis product 3a was achieved using catalyst Pd 5 which is, not unexpectedly, one of the most acidic catalysts in Table 2. In this case, the use of a catalyst support based mainly on anatase and of palladium chloride as the PM precursor is particularly advantageous.

Table 2 Test results using PMTTiO, catalysts Catalyst

Variable Parameter

Selectivity

Pd 1 Pd 2 Pd 5 Pd 6 Pt 1 Pd 4

Reaction time PM loading PM profile Reaction ti

’

at Conversiona) 2a

3a

[“/.I

[“/.I

[“/.I

98.5 99.4’) 97.1d) 84.ge) 99.gC) 98.2 99.3 97.3c)

96.9 93.8 94.7 88.1 44.7 96.8 97.6 98.0

1.7 4.8 5.0 11.8 55.1 2.6 2.0 1.4

Lovctivity PM 100 Yo rutile

Others [> 1 71 . 1.4 1.4

a) after 2 h except where otherwise indicated b) and PM profile c ) after 4 h d) after 1 h e) after 0.5 h

As far as the rate of hydrogenation is concerned, a high surface concentration and dispersion of the precious metal is preferred because of existing mass-transport limitations. This can be accomplished by two different means: deposition of the precious metal in an external shell of the support

95

or less efficiently by increasing the PM-loading of a uniformly impregnated catalyst. Catalyst samples Pd 1 and Pd 5 illustrate this correlation in the formation of methyl compound 3a. Similar observations were made regarding the intermediate product 2a.

3.2. Hydrogenation of 1b Unlike the case with l a , water had to be replaced by ethanol as solvent in order to form a homogeneous liquid phase which contains 4-methylbenzaldehyde 1b. As a result, the diethyl acetal is temporarily formed. Because of the reversibility of the reaction, it has not been taken into further account.

Table 3 Hydrogenation of 4-methylbenzaldehyde 1b Catalyst

Variable Parameter

Selectivity at Conversion after 4 h PA]

Pd 5 Pd 6 Pd 7 Rh

Low acidity 100 % rutilea) PM

78.9 95.3 62.9 78.8

2b

3b

[“I/.

Pol

16.2 64.4

76.8 74.2 33.2 76.1

Others [> 1 Yo] 4b 21.9 4b 7.7 4b 2.4 4b 15.5 5b 6.3 6C

Pd 1 C

Strong acidity

74.1

2.1

4b 99.0

a) and low acidity

Despite the change of solvent, the correlations between product selectivity and catalyst properties were the same as those observed before. In general, the conversion of l b after 4 hours is lower than that of l a and it is more difficult to achieve high selectivity in regard to benzyl alcohol 2b. The hydrogenolysis reaction predominates. Typical product compositions contain approximately 75 % of 3b and 25 YO of other products. One of the latter is ethyl ether 4b which arises from the acid-base catalyzed reaction between benzyl alcohol 2b and ethanol. The amount of the ether, which is formed, depends on the acidity of the catalyst. It is well worth noting the remarkably high selectivity of catalyst Pd 1C. This catalyst was prepared on precipitated titania which is considered to be far more acidic than fumed TiO, P25 and is also having different kinds of acid sites generated

96 by sulphate [6]. Other PM/TiO, catalysts, such as Rh 2 gave rise to additional reactions, e.g., aromatic ring saturation or decarbonylation to toluene 6c.

3.3. Hydrogenation of 1c In addition to the reaction pathways of the previously discussed substrates, 4-chlorobenzaldehyde 1c can be hydrodehalogenated. Indeed, the main product obtained using the Pd catalysts on fumed titania was toluene 6c which is the hydrogenolysis product of 4-chlorotoluene 3c. No benzyl alcohol 2c was formed. Apart from about 50-60 % of toluene, 4-chlorotoluene 3c, ether 4b and saturated aromatic ring product 5c are present in the range of between 5 and 30 YO. In this case, the different acidic properties of the Pd catalysts on fumed titania are of minor importance. The release of hydrogen chloride from the hydrodechlorination reaction compensates for the initial differences of the catalyst acidity.

Table 4 Hydrogenation of 4-chlorobenzaldehyde 1c Catalyst

Variable Parameter

Selectivity at Conversion after 4 h [“I./

Pd 5

2b

3b

[“A]

Pol

Others [> 1 Yo]

78.2

18.7

4c 15.0 5c 3.1 6C 62.2

30.1

4c 10.2 5c 4.8 6C 54.9

Pd 7

100 o/o rutile and low acidity

91.2

Pd 1C

strong acidity

84.7

4c 100

As could have been seen from an analysis of the product during the course of the reaction, only the hydrodechlorination of 3c took place. No other possible dehalogenated products, e.g., benzyl alcohol or benzyl ethyl ether, were detected. This might explain why, particularly in the case of catalyst Pd l C , the chlorosubstituted benzyl ether (4b) was the only product formed in high yield. Because of the kind and strength of acidity,

97 the intermediate benzyl alcohol (2c) was intercepted by ether formation thus preventing further reaction. Taking these results and the previous ones into account, a general reaction scheme for the selective hydrogenation of aromatic aldehydes is outlined in Figure 2.

X

mb

Y

A

+ 2 EtOH I

- ca

+Hz0 I

- HzO

- 2 EtOH

*

CHO

OEt

EtO

X

- Ha0 + H z 0 I - EtOH

EtOH 1

I

CH2 OH

T

I

I

CH3

Figure 2. Reaction scheme aldehydes

CH20Et

CH3

for

the

CH3

selective

hydrogenation

of

aromatic

4. CONCLUSION

PM catalysts on new high surface-area TiO, supports were employed in the selective hydrogenation of aromatic aldehydes. Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation can be

98

directed toward a given product as follows: Benzyl Alcohols 2

Benzyl Ethers 4

Low Acidity

0

- Preparation method - Support based mainly on rutile 0

Low PM Loading

0

Pd as Precious Metal (occasionally Pt) Strong Acidity

0

- Preparation method

- Support based on pure anatase (precipitated)

Hydrogenolysis 3

0

Strong Acidity

- Support based mainly on anatase (fumed) - Reaction time

The rate of hydrogenation is determined by the precious metal profile and the electronic properties of the substituent in the 4-position of the aromatic aldehyde. Unlike the electron-donating methyl group, electronattracting groups, such as chlorine or carboxyl, increase the reaction comparing rate. This is in agreement with similar observations made 4-carboxybenzaldehyde with the unsubstituted derivative [7]. Based on the present and unpublished experimental results, the order is: COOH > CI > CH,.

5. REFERENCES

(11 M. Bankmann, R. Brand, B. H. Engler and J. Ohmer, Catalysis Today, 14 (1992) 225. [2] a) Amoco Corp., USP 4,812,594 (1 989) and USP 4,721,808 (1988). b) Teijin Ltd., JP 53 002 441 (1978). [3] Measurements were performed analogous to the ASTM method D 3830-80. [4] H. Nakabayashi, N. Kakuta and A. Veno, Bull. Chem. SOC.Jpn., 64 (1991) 2428. [5] J.A.R. van Veen, Z. f. Phys. Chem., 162 (1989) 215. 16) C. Morterra, J. Chem. SOC.Faraday Trans. I , 84 (1988) 1617. [7] L. Kh. Freidlin E. F. Litvin, R.N. Gurskii, G. K. Oparrina and R. V. Istratova, Izv. Akad. Nauk SSR. Ser. Khim., 1972, 1738.

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I11 Q

1993 Elsevier Science Publishers B.V. All rights reserved.

99

Kinetic comparison of enantioselective hydrogenations A. Tungle9, T. Tamaia, A. Deakb, S. KemCnyb, A. Gyoric, T. Mathea and J. Petr$ aResearch Group for Organic Chemical Technology of the Hungarian Academy of Sciences, H-l 1 1 I Budapest , Muegyetem rkp 3. bDepartment of Chemical Engineering, Technical University of Budapest, H-I52 1 Budapest, P.O. B o x 9 1. cDepartment of Organic Chemical Technology, Technical University of Budapest, H- 152 1 Budapest, P.O. Box 91.

Abstract Kinetic investigation of the Pdcarbon-(S)-proline system in reduction of isophorone and acetophenone and comparison with known enantioselective hydrogenations were camed out. An addition reaction of (S)-proline with the substrates, and the selective hydrogenation of this adduct results in the optically active products, the kinetic analysis verified this mode of action: these are rather diastereoselective reactions.

1. INTRODUCTION

Besides several diastereoselective heterogeneous catalytic hydrogenations [ 1-31 only two enantioselective hydrogenation reactions are known: the reduction of 0-keto-esters with Raney-nickel modified by tartaric acid and of pyruvic acid esters with Pt modified by cinchona alkaloids. Garland and Blaser [4] described the reduction of pyruvic acid ester as a "ligand-accelerated" reaction: with the adsorption of the modifier new active sites are generated on the catalyst surface. On these new centers the selective reaction is faster and the increased reaction rate is accompanied by greater enantioselectivities. This proposed reaction kinetics describes two mechanisms, exhibiting no formal difference: one of them is a competitive reaction on modified and unmodified sites, the other is a competitive reaction between the free reactant and a possible compound of reactant and modifier on similar sites.

The reaction was also described with a template model too [5-71which presumes a nonclose packed ordered array of adsorbed cinchona alkaloid molecules. Wells supposed three types of active sites. Two of them are producing (R) or (S) product depending on the type of cinchona alkaloid and the third one that is an active site which is not templated produces the racemic product. This model concurs with that of proposed by Blaser. Klabunovskii [8] suggested a kinetic model with two types of active sites on the Raneynickel modified by tartaric acid producing on modified sites optically active, on unmodified sites racemic hydroxy-butyric acid ester from the p-keto-ester. The reaction was found to be first order regarding the substrate and the reaction rate in reversed ratio with the optical yield. The differences between the two systems are (i) the reaction on modified sites is faster then on unmodified ones (Pt-cinchonidine) (ii) the hydrogenation of pyruvic acid ester is enantioselective even if the cinchonidine is only solved in the reaction mixture without premodification of the catalyst. Recently we published about enantioselective hydrogenations with palladium on carbon catalysts where @)-proline 1. was the chiral auxiliary. C=C double bond (isophorone:3,3,5trimethyl- 1-oxo-2-cyclohexene) and carbonyl-group (acetophenone) were reduced with eels (enantiomeric excess (ee) [%I= ([RI-[S])/([R]+[S])xlOO) up to 80% [9-121. An addition and/or condensation between (S)-proline 1. and the unsaturated reactants 2 was proved, the chemo and diastereoselective hydrogenation of this adduct 3, 4 results in the optically active products (Scheme 1 .).

ac; I

H

1

2

3

I

Scheme 1

In order to verify the existence of a chiral intermediate with kinetic methods too, we studied the effect of concentration of the chiral auxiliary on rate and enantioselectivity in hydrogenation of isophorone and acetophenone with PdC-(S)-proline.

2. EXPERIMENTAL

Isophorone, acetophenone and (S)-proline were (Fluka) p.a. grade, the solvents (Reanal) p.a. grade used as received. The catalyst was 10% Pd on carbon, trade name Selcat A [ 131, its BET surface 1 100 m2/g, Pd dispersity D>O.7 measured by Hz and CO adsorption. The hydrogenations were camed out in an autoclave equipped with a magnetically driven turbine stirrer (> 1500 rpm) under 5 bar pressure and at room temperature. After a given hydrogen consumption (measured with Biichi BCP 6010) reaction was stopped, a sample w a ~

101

taken, worked up and the rest was hydrogenated further. The products were separated from (S)-proline (and its derivatives) with acidic extraction and then distilled. For the detailed workup procedure see ref. I0 and 1 1. The conversion and the product composition were determined by GC. The optical rotation was measured by Perkin-Elmer 241 automatic polarimeter, the optical rotation of the R-(+)-Iphenylethanol is [aI2'D=+42' (neat), of (S)-(+)-dihydroisophorone is [aI2'D=+29' (neat) [ 101.

3. RESULTS AND DISCUSSIONS During hydrogenation of reaction mixtures containing isophorone 5 and (S)-proline 1 the following reactions and intermediates are possible [9-121: (S)-proline

+

1

racemic dihydroisophorone 7 1+

isophorone 5

+ t

carbinolamine 6 -H20& ?

LL H2

(E)- and (Z)-iminium salt + enamine 8-9 10 H2 active dihydroisophorone + semi-hydrogenated products 7a 411-!3 +H20 H2 -1 N-alkylated proline 14

Scheme 2

Detailed kinetic study of such a complicated system could not be carried out, because the could not be concentration of intermediates (E, Z iminium salts 8, 9 and enamine determined, not even estimated. The results of rate measurements are shown on Figure 1. and 2. It is obvious that with increasing ratio of the chiral auxiliary and with conversion reaction rate decreases, enantioselectivity increases. It means that the reaction order respecting isophorone is not zero, and the enantioselectivity can not be attributed to competitive reactions taking place on two types of active sites.

a)

102

- 50o’-=?\o

x?; ;

- 40

mmol -30 hour

3’

-20 -10 X‘ 0,Ol 0,l

0,3

1 S-proline ;sop horone

Figure 1. Reaction rates (0)and enantiomer excesses (x) as a function of (S)-proline/ isophorone molar ratioa.

”’,’

”0’~’

/ 1

1

,

2

50

0

1

1

-,

10

1

~

mol H2

Figure 2. Rate of hydrogen consumption eels (x) observed at various conversions expressed in consumed Hp? a2.0 g 10% Pd/C (Selcat A), 25oC, 5 bar, 0.4 mol isophorone, 800 cm3 methanol. b0.4mol proline. (0) and

A more detailed, but still a partial kinetic study was carried out in the hydrogenation of acetophenone-(S)-prolinemixtures according to the following reaction scheme (1 I):

Scheme 3

Definitions: concentration of acetophenone 15, c1 capo initial concentration of acetophenone, c,, concentration of proline 1,

103

c p initial ~ concentration of proline, concentration of adduct 16, fraction of (+)-adduct, concentration of (+)-phenyl-ethanol 18, 18a, concentration of (-)-phenyl-ethanol 18, I 8a, concentration of alkylated (S)-proline l7, kl-4 reaction rate constants (k2 only for one enantiomer). Cad

nu c2 c3 c4

According to Scheme 3 the hydrogenolysis of the C-N bond of the adduct 16 leads to optically active 18a. If the hydrogenolysis proceeds with retention of the configuration, as we suppose, the 1 OO[nu-( I-nu)] [%] gives the diastereoselectivity of adduct 16 formation. Since the adduct 16 can not be isolated because of its instability, neither the diastereomeric purity of 16 nor the stereochemical course of the hydrogenolysis can be determined. Rate measurements have shown that the hydrogenation is of first order with respect to acetophenone. The differential and balance equations describing the system are the followings: Capo = cad

+ c 1 f c2 + c3 + c4

cprO = Cad + Cpr + c4

dcl/dt = kincad - 2 k 2 ~ -1klpC1(cpfi - capo + ~1 + ~2 + ~ 3 ) dc2/dt = k2cI t k3nu(cap0 - cl - c2 - c3 - c4) dc3/dt = k2c1 t k3( I - nu)(capO- c1 - c2 - c3 - c4) dcq/dt = k4(cap0- c 1 - ~2 - ~3 - ~

4 )

Scheme 4.

On Figure 3. the reaction rates are depicted as a function of conversion, on Figure 4. enantiomer excesses, on Figure 5 . the variation of initial rates and ee's with the ratio of the chiral auxiliary to acetophenone and on Figure 6. the observed eels at various reaction rates are shown. On Figure 6 the rectangle represents the trend. The most important observations from these are the followings: (i) the hydrogenation of acetophenone is a first order reaction in itself and with triethylamine or with (S)-proline, but is zero order if both triethylamine and (S)-proline are present together, (ii) ee increases slightly with conversion (Figure 4.) and with increasing (S)-proline ratio (Figure 5.) but decreases linearly with increasing reaction rates (Figure 6.), (iii) quantity of (S)-proline recovered from reaction mixture shows that with increasing initial proline concentration the alkylation reaction becomes more significant.

104

;:

rate

15

1 1 0

-

mol % min g cat

x\

10

x\

0,s

5

.-y/---

I

conv.% Figure 3. Reaction rates as a function of conversion in the hydrogenation of acetophenone [Pd/C-(S)-proline]. (1.~)(2: )(3.0)(4.A )(5.11 )(6: aThe numbering correlates with Table 1

1 S-pro APh Figure 5 . Effect of (S)-proline/acetophenone molar ratio on the initial reaction rate and ee’s. [in the presence of (S)-proline 0 , x] [in the presence of (S)-proline and triethylamine 0 ,A ]

/

0-O

I

10

y o

o

I

I

I

I

I

I

50

I

I

100 conv. %

Figure 4. Enantiomer excesses as a in the function of conversion hydrogenation of acetophenone [PcUC-(S)-proline]. (3.0 )(4.A )(6: )(lo.+ )a

0,l

0,2

0,3

r mol% min g c a t

Figure 6. Enantiomeric excesses versus reaction rates.

105

In Table 1. and in Table 2. the reaction parameters, the measured conversion and ee values as a function of time, the quantity of the recovered (S)-proline and the computed nu and k values are given. The latters which were computed with the given equations (Scheme 4.) and with HookeJeeves minimalization algorythm show that this model (Scheme 3. and 4.) describes the experimental results with satisfactory correctnes [deviations between the computed and the measured values: 4% (conversion) and 10% (enantioselectivity)]. Table 1 Hydrogenation of acetophenone PrIAph time (min) No. molar conversion ratio ee’s (Yo) 14 135 38 95 0.539 0. I42 0.663 0.289 132 0.099

355 0.234

recovered (S)-proline (”/)

545 0.23 1

160 375 78 0.535 0.975 6.2 7.1 249 800 98 0.3 0.52 0.99 10.4 10.9 275 830 73 0.5 1 .o 0.48 10.25 10.9 850 392 71 1 .o 1.0 0.55 10.7 10.7 194 93 59 0.5 0.57 1 .o 11.4 9.4 300 144 0.3 61 0.39 0.9 14 13.7 790 40 1 51 0.4 0.5 1.0 12.2 12.6 600 100 297 66 0.38 1.o 0.15 0.8 1 13 16 18 10% PdIC catalyst (Selcat A) [ 131, 25 OC, 5 bar hydrogen pressure, 0.2 mol acetophenone, 200 cm3 methanol. a160 mg catalyst, b4 g triethylamine, C320 mg catalyst. 0. I

25 0.096 5.0 41 0.12 8.2 54 0.132 8.2 48 0.15 7.3 21 0.19 7.4 50 0.08 13.9 10.5 0.08 12.4 58 0.1

61 0.226 5.8 89 0.25 8.8 115 0.25 9.4 116 0.3 8.6 42 0.29 8.6 84 0.16 12.4 197 0.24 11.1

106

Table 2 Computed values for the hydrogenation of acetophenone No. computed values nu kl, kl" k2 k3 k4 3a 0.66 0.140 0.120 1.10-4 3.1.10-5 9.10-5 4a 0.66 0.076 1.5.10-3 I . I 0-4 0.076 8. I 0-4 5a 0.66 0.075 0.065 1.10-4 1.10-6 1.4.10-5 6a 0.66 0.064 l.10-4 0.072 1.12.10-4 1.1.10-4 1.10-4 7c 0.66 0.060 0.070 5.10-5 8.10-5 10% P d C catalyst (Selcat A) [ 131, 25 O C , 5 bar hydrogen pressure, 0.2 mol acetophenone, 200 cm3 methanol. a 160 mg catalyst, C320 mg catalyst.

In order to comment the large variation of kin, the improvement of the computation method and more measurements are needed, both are in progress. 4. CONCLUSIONS The kinetic approach confirmed our presumption that the optically active 1-phenylethanol arised from the hydrogenolytic cleavage of the C-N bond of the adduct of (S)-proline and acetophenone. The reasons for the relatively low enantioselectivities are: (i) the low diastereoselectivity ( 100[nu-( 1 -nu)]=32% with the presumption of retention in the hydrogenolysis} of the adduct forming reaction and (ii) the ratio of competitive reactions namely the hydrogenation of the free acetophenone and that of the adduct. The hydrogenations of isophorone and acetophenone in the presence of (S)-proline show similarities: the effect of PdC-(S)-proline system is based on the addition reaction of the reactants and (S)-proline in solution and on the chemoselectivity of Pd. Both hydrogenations should be termed diastereoselective rather than enantioselective, since the asymmetric induction takes place in the adduct molecules. 5 . REFERENCES

1 2 3 4

J.D. Morrison, H.S. Mosher, Asym. Organic Reactions, Prentice Hall, New York 1971. E.L. Eliel, J. Chem. Ed. 57. (1980) 52. A. Tungler, M. Acs, T. Mathe, E. Fogassy, Z. Bende, J. Petro, Applied Cat. 17 (1985) 127. M. Garland, H.U. Blaser, J. Am. Chem. SOC.1 12 (1990) 7048. 5 G. Webb, P.B. Wells, Catalysis Today 12 ( 1 992) 3 19. 6 J. Thomas, Angew. Chem. Adv. Mater. 101 (1989) 1105. 7 P.B. Wells, Faraday Discuss. Chem. SOC.87 ( 1 989) 1 . 8 E.1. Klabunovskii, A.A. Vedenyapin: Asym. Kataliz, Gidrog. na Met.Nauka,Moscow, 1980. 9 A. Tungler, M. Kajtar, T. Mathe, G. Toth, E. Fogassy, J. Petro, Catal. Today, 5 (1989) 159. I0 A. Tungler, T. Mathe, J. Petro, T. Tamai, J. Mol. Cat. 61 (1990) 259. 1 I A. Tungler, T. Tarnai, T. Mathe, J. Petro, J. Mol. Cat. 67 (I991 ) 277. 12 A. Tungler, T. Tamai, T. Mathe, J. Petro, J. Mol. Cat. 70 (1991) L5. 13 T. Mathe, A. Tungler, J. Petro, Hung. Pat. 177 860 (l98l), U.S. Pat. 4 361 500 (1982).

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

ENANTIOSELECTWE METAL COMPLEX CATALYSTS INORGANIC SUPPORTS VIA CARSAMATE LINKS

107

IMMOBILIZED ON

B. Pugin* and M. Muller Central Research Services, CIBA-GEIGY,R-1055.629, CH-4002 Basle, Switzerland

Abstract Using O=C=N-(CH2)3-Si(OEt)3 as a linker, chiral diphosphine ligands containing an amine function were bound in a new and very efficient way to silicagel supports with different textures. The corresponding cationic rhodium complexes are very active catalysts for the enantioselective hydrogenation of methyl-acetamidecinnamate(ee's up to 94.5%).They can easily be separated and successfully reused. Good catalysts with "molecular weights" as low as 5 kD can be obtained, since a high loading does not affect the catalytic performance, as long as the pores of the support are large.

1. INTRODUCTION

Chiral diphosphine complexes of Rh, Ru and Ir are very active catalysts for the enantioselective hydrogenation of a large variety of substrates containing C=C, C=O and C=N double bonds. So far however, only few of these systems are applied in an industrial scale. This reflects that the scale up of this technology still contains problems. Among others, (e.g. air sensitivity, long term activity and selectivity) ways have to be found to separate and recover the soluble catalysts. Where simple methods like destillation or recristallisation cannot be applied or fail, new methods are required. One possible approach is to anchor homogeneous catalysts to organic polymers or to inorganic supports. Many efforts have been made in this field (see overview in 111 and reviews 12-61). To be of practical use, immobilized enantioselective catalysts should meet the following requirements: - General and efficient preparation meth,d: a general method is important, since it is not yet possible to predict which ligand and support will be optimal for a given substrate and process. - Reasonable "molecular iueigh.t":for practical reasons, the "molecular weight" of a catalyst immobilized to a support should not exceed 10 kD per mole of metal complex.

108

G o d catalytic properties: the performance (selectivity, activity, productivity) of immobilized catalysts should be comparable to the performance of corresponding free catalysts. - Sim.ple and com.plete separation: separation should be achieved by a simple filtration and at least 95% of the catalyst should be recovered. - &use: reuse is not a requirement but from an economic point of view would be a great advantage. The immobilized catalysts known so f a r do not meet all these requirements. At present, the following system is one of the best [71:

-

It has a good productivity and is extremely selective (ee's up to 100% are claimed for the hydrogenation of methyl-acetamidecinnamate),it can be reused and its linker potentially could be applied to all phosphine ligands that are functionalized with an NH or OH group. Disadvantages are its circumstantial preparation and its prohibitively high "molecular weight" (50-200 kD). We have recently developed a general, simple and efficient method where we used isocyanato-alkyl-triethoxysilanelinkers to immobilize chiral diphosphine ligands bearing an NH or OH group to inorganic supports 18,91. In this work we describe their preparation and their performance in rhodium catalyzed enantioselective hydrogenations, using methyl-acetamidecinnamate as a test substrate. To obtain immobilized catalysts with reasonable "molecular weights", supports with high specific surface areas have to be used and a high loading of ligand must be achieved. In this context we also present the results of our study on the influence of pore size and loading on the performance of immobilized catalysts.

2. RESULTS AND DISCUSSION 2.1. Preparation of the Immobilized Ligands The NH group of the functionalized diphosphine ligands la-3a reacted quantitatively with the isocyanate group of the commercially available 3-isocyanatopropyltriethoxysilane linker 4, yielding the trialkoxysilane-diphosphine ligands lb-Sb (Figure 1).The method can be extended to diphosphine ligands bearing an OH-group. Due to their lower reactivity the reaction is carried out in presence of a catalyst like triethylamine or dibutyltin dilaurate.

109

Y=N,O

la

k-rn,&,&

Figure 1. Synthesis of the immobilized ligands

Table 1. Supports and reaction conditions for immobilization igand

Support Immob. Conditions Part. SBETD m l ) pmol Lig. Temp. time Size OHti) added [pm] [m2/gI [nm] perm2 pmoUm2] [OC] [h] Grace 332 35-70 325 19 11.6 0.46 70 5.5 Merck loo2) 200-500 308 14 10.5 0.44 80 7 Merck l0V' 200-500 310 14 10.2 0.46 70 5.5 0.46 70"' 5.5 Merck l 0 d ) 200-500 310 14 10.2 9 Merck loo4' 200-500 322 14 0.46 70 5.5 Merck l 0 d ' 200-500 310 14 10.2 0.46 90 24 Merck loo4) 200-500 322 14 9 1.2 90 16 0.46 70 5.5 Merck60, 60-200 369 10 puriss. 0.46 70 5.5 Merck40 60-200 592 4.4 0.47 80 22.5 Merck40 60-200 592 4.4 9 Merck loo4' 200-500 322 14 0.45 70 5 0.43 70 5.5 Merck 1004' 200-500 322 14 9 Name

lc Id le If lg lh li lk 11 lm 2c

3c

Lnalytic Result! Loading "hIW imm. Lig. [pmoUm2] BD] 0.24 12.9 0.18 17.5 0.25 12.5 0.31 10 0.35 9 0.38 8.2 0.63 4.9 0.22 9.5 0.13 0.31 0.28 0.27

11.3 4.7 11.3 11.6

I ) : D w = mean pore diameter calculated according to Wheeler [201, Batch A, pretreated by washing with 2n HCl, 3, Batch B, untreated, 4, Batch C, pretreated by washing with 2n HC1, 5 ) : addition of cat. amount p-toluenesulfonic acid (3 mg / g support), 6 ) :surface OH-groups

110

Isocyanato-alkyl-triethoxysilanes are commercially available or their preparation is described I l 0 , l l l and good enantioselective catalysts with diphosphine ligands that contain carbamate or carbamide groups are known [12,131. The price, one has to pay, to use these linkers, is the introduction of an NH or OH handle onto the ligand. However, since this allows the immobilization to various different supports [4,71 or e.g. to render ligands water soluble r141, such a functionalization should always be taken into consideration when designing new ligands. The ligands lb-3b were immobilized on several different silicagels by gently stirring a slurry of the support and ligand in toluene for 5-24 h a t 70-90 OC (see Table 1). The silicagel-supported ligands were then washed with MeOH to remove unreacted ligands and dried under vacuum. Table 1 illustrates that under mild immobilization conditions (70 O C ) the loading (number of immobilized ligands per area unit) strongly depends on the type or even the batch of the support used. This cannot be rationalized with differences in texture or with the number of surface OH groups r151 that are present in a large excess. However, when the immobilization is carried out above 80 O C or in presence of catalyhc amounts of an acid or a base and with longer reaction times, yields of immobilization up to > 80% can be obtained. Steric Considerations

It is interesting to envisage the situations that can occur with a high and with a low loading (Figure 2). Assuming, that i. the ligand Ib can comfortably be placed into a box of a size of 1.6 nm x 1.4 nm x 1.1 nm, ii. the ligands are immobilized in a monolayer, and iii. the surface is flat, then the maximum possible loading can be calculated to be in the range of 0.7 - 1 pmol/m2. Due to the bending of the surface in the pores, the real maximum possible loading will be even lower. This means that with a typical silicagel support (specific surface area = 300 m2/g) catalysts with a minimum "molecular weight" in the range of 3300 4800 D can be obtained. It seems unlikely that a significantly denser loading can be achieved, since there is no obvious force that could cause steric crowding. This is supported by ligand h: even with a large excess of ligand and under severe immobilization conditions, the loading did not exceed 0.63 pmoVm2.

0.1

Figure 2. Pictorial view of ligand loading

07

1:O

Loading [pmol/m2]

111

We can also estimate, how low the loading must be, to avoid interactions (e.g. dimerization) between immobilized metal complexes . With our linker, the distance between the anchoring points of the ligands should be at least 4 nm. This corresponds to a loading of 0.1 pmoVm2, that in reality will be lower, because an equal distribution of the ligands is unlikely. Therefore, interactions will occur even with ligand Id that has the lowest loading (0.18 pmoVm2).

2.2. Catalytic Hydrogenations with Immobilized Ligands The catalysts were prepared in situ from the immobilized ligands and a solution of TRh(COD)21BF4. The hydrogenation of methyl-acetamidecinnamate was used as a test reaction (Table 2). Table 2. Hydrogenation of methyl-acetamidecinnamate[MAC] with free and immobilized cationic rhodium catalysts Support Support D m l ) Loading Name pmol lig. [nm] perm2 Bppm free h g m d (ref.) Grace332 19 0.24 lc Merck loo2) 14 0.18 Id If Merck 1 0 d l 14 0.31 Merck loo4’ 14 0.35 lg lh Merck l 0 d ’ 14 0.38 li 0.63 Merck loo4) 14 lk 0.22 Merck 60, 10 puriss. 11 Merck40 4.4 0.13 lm Merck40 4.4 0.31 ’yrphos free ligand (ref.)

Results. re-use :onv Time ee

Ligand

2c Diop 3c

Merck 1004) 14 free ligand (ref.) Merck

loo4’

13

0.28

0.27

[%] [min] [%I 100

14

94.5

100 100 94 100 100 100 99

16 20 29 32 30 28 26

92.4 93 92 93.5 91.7 91.9 92.5

100 100

16 23

91.3 92.1

100

18

94.5

100

14

93.1

200 1 200 1 200O6I 59

98 30 99 90 100 56

89.8 89.3 92

33

114

86.8

1000fil 59

200 200 200 20071 200 20071

200

1 1 1 1 1 1 1

88 71.9

78

87.3

1

100 40 100 19

100

2007) 2007’

1

100

69.9

100

10

69.4

15

1)s 2 ) , s l v4) me table 1. Reaction conditions: Catalyst prepared in situ in methanol from 1eq. [Rh(COD),]BF, and 1.2 eq. of ligand. [MAC]=O.lm; “:0.3m. AU reactions in methanol, except 7 , in methanol /tetrahydrofurane = 8/2.

With all types of ligands catalytic performances comparable with those of the free analogues can be obtained. The immobilized catalysts can easily be separated by decantation or filtration and be reused, sometimes even with better activity and selectivity. Some ion selective mass spectroscopy measurements

112

revealed that in first reactions the leaching of rhodium is typically 4%6% and that it can go down to less than 1% when the catalyst is reused. This may be an indication that the supports still contain some not immobilized ligands and that they should be washed more extensively before use.

Influence of Loading and Pore Size The ligands lc-lm in Table 2 are listed in the order of decreasing pore sizes. Large pores: supports with large pores ( D m : 10-19nm) give catalysts with good activities and selectivities (ee: 91.7-94.5%).Using silicagel Merck 100 the loading was varied in the range of 0.18-0.63pmoVrn2. From a geometric point of view (Figure 21, intermolecular interactions are already possible at 0.18pmol/m2 and will be strongly favoured with higher loadings. Since all the catalysts are similarly active and selective we conclude that with cationic rhodium complexes no interactions occur that affect their catalytic properties. This is in contrast to neutral rhodium catalysts, where deactivation due to the formation of dimeric complexes has been described [16,171. Sm.all pores: measurements of pore size distributions revealed that from all supports studied, only Merck 40 has a considerable portion of small pores labout 50% of the surface is in pores with a size < 4nmj. Possibly, the lower selectivity (ee: 86.8-89.8%) obtained with this support may be a consequence of tight caging of immobilized catalyst molecules. The lower activity observed with high loading (ligand h) indicates that a dense coating of small pores with ligand molecules can reduce or even block mass transport. From all results we can conclude that immobilized catalysts with "molecular weights" as low as 5 kD with good performance can be prepared as long as supports with large pores are used.

3. EXPERIMENTAL PART All reactions involving phosphines and the manipulations in the hydrogenation experiments were carried out under argon. The toluene used in the immobilization reactions was dried with sodium and distilled under argon. The silicagel supports were obtained from Merck and Grace. Some were treated with 2n HCl (3h, 2 5 OCj, then washed 15 times with a large excess of distilled water and finally dried under vacuum, first a t 80 O C (2h) and then at 350 O C (12 h) (Table 1). The BET surface areas were determined from adsorption isotherms [ 181 and the pore size distribution [191 by standard methods from the desorption isotherm of N2 at 77K (apparatus: Omicron, OmnisorpTM 1001.The mean pore size D m was calculated according to r201. The number of surface OH-groups was measured by methyllithium titration r151. Before titration the samples were dried overnight at 180 O C under a high vacuum of 10-6mbar. The ligands la, (2S,4S~-N-(tert.-butyloxycarbonyl)-4-(diphenylphosphino)-2(diphenylphosphinomethy1)-pyrrolidine (BPPM)and (4S,SSj-2,2-dimethyl-4,5bis(diphenylphosphinomethyl)-1,3-dioxolane (DIOP)were purchased from Fluka, 3-isocyanatopropyltriethoxysilane 4 from Petrarch Systems Inc.. Ligand 2a and (3R,4Rj-l-benzyl-3,4-(bisdiphenylphosphine)pyrrolidine (PYRPHOS)

113 were synthesized according to 1211, Ligand according to [91 and [Rh(COD)2BF4 using the method described in [221. Generd method for the preparation of the immobilizable ligands -lb-3b 1.1 equivalents of 3-isocyanatopropyltriethoxysilane 4 were slowly added to a solution of a ligand (la, or S i in CH2C12 or toluene (10 ml / mmol ligand) and the solution stirred for two hours at room temperature. After evaporation of the solvent under reduced pressure, the crude product is obtained as an oil. The product was worked up by the following methods: a) the oil was stirred in alcohol for 20 h to convert the excess of isocyanate to the corresponding carbamate and then used without further purification, b) the oil was stirred in hexane until it solidified, washed with hexane and dried with high vacuum, yielding the pure product as a white solid, c) purification by chromatography (Merck 60, diethyl ether). Ligand lb: Preparation in toluene, method b). Yield: 95%. 31P-NMR (CDC13): 6 -8.79 (s), -22.90 (s); MS: m/z 700 (M+'). Ligand 2b: Preparation in CH2C12, method b). Yield: 95%. 31P-NMR (CDC13) : 6 -11.7 (s); MS: m/z 686 (M+'). Ligand 3 b Preparation in toluene, method a). Yield: 96%).31P-NMR (CDC13) : 6 -23.12 (s), -23.44 (6); MS: m/z 788 (M+'). Immobilization: The support (3-15g) was first dried at 130 OC for 5 h under high vacuum (< 5 0 1 0 -torr). ~ Then it was set under argon, cooled down and a solution of the immobilizable ligand in toluene added. The standard concentration of the ligand was 0.025m, except for ligand li (0.08m) and ligand (0.05m). The slurries were then gently stirred for the times and at the temperatures given in Table 1. After cooling, the support was washed 5 times with degassed methanol (6-10 d g support) to remove unreacted ligand. The loadings obtained are given in Table 1. Hydrogenation Reactions (Table 2): 1.2 equivalents of supported ligand were gently stirred in a schlenk flask with 1 equivalent 1Rh(COD)2lBF4 dissolved in 3 ml methanol, until the support was orange and the solution colourless. This process was fast for all ligands (< 5min.) except for (about 1 h). After addition of the substrate solution, the atmosphere of argon was changed to hydrogen (1 bar), and the magnetic stirrer switched on (1000 rpm). Gas chromatography was used to monitor the reaction (column: SE 54, 15m) and to determine the ee (column: Chirasil-L-Val, 50m). With PYRPHOS and ligand &, the reaction mixture was pressed with a steel capillary into a degassed 50 ml steel autoclave and the hydrogenations carried out with a Hz pressure of 59 bars.

a

4. CONCLUSIONS

Diphosphine-rhodium catalysts bound to inorganic supports via isocyanato-alkyl-trialkoxysilanelinkers meet all requirements to be of practical use: i. the preparation method is probably the most simple, general and efficient known so far, ii. immobilized cationic rhodium complexes with "molecular weights" as low as 5 kD exhibit a comparable performance in enantioselective catalytic

114

hydrogenation like their free counterparts and iii. they can easily be separated and successfully reused.

5.ACKNOWLEDGEMENTS We thank Heidi Landert and Andrea Schwendemann for the experimental work. We are also grateful to Hans-Ulrich Blaser, Rolf Bader and Martin Studer for the valuable discussions and for the critical reading of the manuscript.

6.REFERENCES 1 R. Selke, K. Hauptke and H.W. Krause, eJ. Mol. Catal.,56 (1989) 315, and literature cited therein. 2 M. Capka, Collect. Czech. Chem. Commun., 55 (1990) 3 F.R. Hartley, The Chemistry of the Metal-Carbon Bond, 4 (1987) 1163. 4 ,J. Hetfleis, Studies in Surface Science and Catalysis, 27 (1986) 497. 5 J.K. Stille, J. Macromol. Sci.-Chem., A21 (1984) 1689. 6 C.U. Pittman, in P. Hodge and D.C. Shenington (eds.), Polymer-supported Reactions in Organic Synthesis, ,John Wiley & Sons Ltd., (1980) 249. 7 U. Nagel and E. Kinzel, tJ. Chem. SOC.,Chem. Commun., (1986) 1098. 8 B. Pugin, F. Spindler and M. Muller, EP 496699-A1 (25. ,Jan. 1991) 9 B. Pugin, F. Spindler and M. Muller, EP 496700-A1 (25. Jan. 1991) 10 Patent, Dow Corning Corporation, FR 1.371.405 (4. Sept. 1964). 11 Patent, Shin-Etsu Chemical Industry Co., Ltd., ,JP 01275587 A2 (6. Nov. 1989) 12 K. Achiwa, ,J. Am. Chem. SOC., 98(25) (1976) 8265. 13 I. Ojima and N. Yoda, Tetrahedron Lett., 21 (1980) 1051. 14 U. Nagel and E. Kinzel, Chem. Ber. 119 (1986) 1731. 15 K. Unger and E. Gallei, Kolloid. Z.-Z. Polym., 237(2) (1979) 358. 16 M. Czakova and M. Capka, J . Mol. Catal., 11(1981) 313. 17 Z.M. Michalska, M. Capka and J . Stoch, .J. Mol. Catal., 11 (1981) 323. 18 S. Brunauer, P.H. Emett and E. Teller, tJ. h e r . Chem. SOC.,60 (1938) 309. 19 C. Pierce, J . Phys. Chem., 57 (1953) 149. 20 A. Wheeler, in P.H. Emmet (ed.), Catalysis, Vol. 2, Reinhold, New York ( 1955) 116. 21 U. Nagel, E. Kinzel, ,J. Andrade and G. Prescher, Chem. Ber. 119 (1986) 3326. 22 R.R. Schrock and J.A. Osborn, eJ. Am. Chem. SOC.,93(12) (1971) 3089.

M.Guisnet ct al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights rcserved.

115

Stereoselective thymol hydrogenation: comparative study of charcoalsupported, platinum, rhodium and iridium catalysts M. Besson, L. Bullivant, N . Nicolaus-Dechamp and P . Gallezot Institut de Recherches sur la Catalyse (CNRS) 2 avenue Albert Einstein - 69626 VILLEURBANNE CEDEX- FRANCE

Abstract The catalytic properties of Pt, Rh and Ir catalysts in the reduction of thymol have been compared, with respect to their activities and stereoselectivities in the formation of menthones and menthols. The reduction over Pt and Rh proceeds essentially via the ketone intermediates, whereas the direct route is predominant on Ir. The formation of the cis isomers is always highly favored. The stereoselectivity of thymol reduction is controlled by the relative proportions of these two routes and by their stereoselectivities.

1. INTRODUCTION

The catalytic hydrogenation of thymol is of importance for the preparation of (k) menthol [ 11, out it may also be used as a model reaction for the reduction of substituted aromatics. The factors controlling the cishrans ratios in the alcohols obtained on hydrogenating substituted phenols have been studied in detail in the case of cresols [2], but data are lacking for other alkylphenols. Thymol is reduced to the corresponding cyclohexanols (menthol, neomenthol, isomenthol and neoisomenthol), directly or via the cyclohexanones (menthone, isomenthone). Some hydrogenolysis (p-menthanes) may also occur. We have investigated the changes in the product distribution and in the stereoisomeric composition of the menthol isomers, using well characterized Pt, Pd and Ir catalysts with the same high dispersion (1-2 nm large particles) and on the same active charcoal support. Moreover the hydrogenation of pure menthone was studied on the same catalysts and under the reaction conditions used for thymol hydrogenation.

116

> N I+ -(

Ho NIML

HO

x3 C-M

IML

H6

Ho ML

NML

N 3 t-M

TH = thymol; IMN = isomenthone; MN = menthone; NIML = neoisomenthol; IML = isomenthol; NML = neomenthol; ML = menthol; c-M = cis-menthane; t-M = trans-menthane

2. EXPERIMENTAL Catalvsts: The catalysts Pt/C (4.18 wt% Pt), Ir/ C (2.24 wt% Ir) and Rh/C (4.0 wt% Rh) were prepared by ion - exchange with Pt (NH3)42+, Ir(NH3)53+ or € U I ( N H ~ ) ~ ~ + cations of a charcoal support (CECA SOS, 1400 m2.g-') previously functionalized by NaClO oxidation as described elsewhere [3]. After reduction by flowing hydrogen at 573K, the particles are in the size range 1-2 nm and homogeneously distributed throughout the grains. Hvdrogenation experiments: The reduction of thymol or menthone was conducted in a well-stirred autoclave as described in 141, The catalyst (0.15-0.50 g) suspended in cyclohexane was activated by stirring the mixture for two hours at the temperature of the reaction under 3MPa H2 pressure. Then 0.05 mol of thymol or menthone was introduced under H2 pressure and the reaction was started by stirring under 3 MPa H2. Gas chromatographic analyses of samples of the reaction medium, withdrawn at regular time intervals were performed on a Durabond Wax (J&W) capillary column.

117

3. RESULTS AND DISCUSSION 3.1. Hydrogenation of thymol over platinum In a previous paper [4], the product distribution as a function of the hydrogen consumption, given in figures 1 a, b for two different temperatures, were obtained on a 4.18% Pt/C catalyst (100% H2 consumed correspond to three moles of H2 consumed per thymol). The hydrogenation via the ketone intermediates was the preferred route (75%) and the formation of cis isomers was highly favored. Thus the hydrogenation via the ketones gave 71 % of isomenthone which was subsequently hydrogenated with a 96% selectivity to neoisomenthol. On increasing the temperature, the amount of ketonic intermediates which accumulate in the reaction medium increased; thus at 373K (figure Ib) large amounts of menthones remained as all the thymol was converted. timelmin)490

2955

30 4 1 0 I

100

(b)T=373K

(a)T=3131

80

8

C

.-0 60 c .-

c 60 .-0

.-v)

v)

4-

0

0

Q

5

40

40

0

0 0

20

20

0

0 0

20 4 0 60 80 % H2 consumed

100

0

20 4 0 60 80 % H2 consumed

100

Figure 1. Product distribution vs. hydrogen consumption during thymol hydrogenation over 4.18% Pt/C. The kinetical study of the different reaction pathways showed that the rate constants corresponding to the reaction from the menthones increased much less with temperature than those from thymol. This statement is fbrther supported by hydrogenation experiments on pure menthone. The apparent activation energy in the same conditions is found to be lower (18 kJ.mo1-l) than that for the reduction of thymol (63.5 kJ.mol-l). Thus a temperature increase accelerates the disappearance of thymol, but has less effect on the rate of reduction of the menthones.

118

In the course of the thymol reaction, the distribution of menthols isomers changed. Figure 2 shows that the ML/(ML+NML) ratios decreased: the lower the temperature of reaction, the larger the decrease. To check these results, the hydrogenation of' pure menthone was conducted under the same reaction conditions. Figure 3 shows that the ML/(ML+ NML) ratios are constant during menthone conversion; they are ranging from 0.24 to 0.32 at 313 and 373 K respectively. This indicates that higher temperatures favor the formation of menthol which is the thermodynamically more stable isomer.

0,40 A

=0,35 2

z

f 0,30

50,25

-

0

I

A 0

T=333K T=353K

20 O/O

t

4 0 6 0 80 100 H2 consumed

Figure 2. Relative concentration of ML compared to NML as a function of H2 consumption during thymol hydrogenation over 4.18% Pt/C.

-

0,20 0

20

010

4 0 6 0 80 1 0 0 H2 consumed

Figure 3. Effect of temperature on the stereoselectivity of menthone hydrogenation over 4.18 % PtlC

The stereoselectivity ofthe direct hydrogenation of thymol to ML or NML is given by extrapolating at 0% conversion the curves giving the ML/(ML+NML) ratio (figure 2). This stereoselectivity (ca.0.4) is different from that obtained from the reduction of pure menthone (0.24 to 0.32, figure 3). As the thymol reaction proceeds, the amounts of ML and NML produced directly from thymol decrease at the expense of those issued from menthone; accordingly the stereoselectivity becomes closer to that of menthone reduction.

3.2. Hydrogenation of thymol over rhodium The reduction of thymol over the 3.27% Rh/C catalyst was performed between 313 and 373K. The product distributions obtained at 313K and 373K as a function o f the hydrogen consumption is shown in figures 4 a, b.

119

time(min)

600 5750

100

100

80

80

s .-5 60 .-

time(min) 200 1500

s

5 60

.c .-

c v)

0

v)

0

40

40 0 0

0

20

20

0

0

0

20 4 0 6 0 80 100 YOH2 consumed

0

6 0 80 1 0 0 H2 consumed

20 4 0 %O

Figure 4. Product distribution vs. hydrogen consumption during thymol hydrogenation over 3.27% Rh/C. The initial selectivities to IMN and MN are ca. 60% and 15% respectively, whatever the temperature (compared to 50% and 20% on platinum), but because of the slow rate of hydrogenation of these ketones under the present reaction conditions, they accumulate in the reaction medium up to a maximum value of 75% at nearly complete conversion of' thymol. The isomenthone and menthone are slowly reduced to the alcohols, only when the thymol has practically disappeared from the reaction medium. The rate of reduction is so slow that complete reduction of the menthones could not be achieved i n a reasonable time under the present conditions. The weak activity of rhodium for the reduction of menthones is in agreement with the fact that rhodium is less active than platinum for the reduction of C = O bonds [ 5 ] . Moreover the presence of an isopropyl group in ortho position could even decrease further the rate of C =O reduction. For instance, Cherkaev et al. [6] showed that during hydrogenation of alkylphenols on Rh catalysts, ketone formation is favored by the steric hindrance effect of a bulky ortho substituent. Similarly. the hydrogenation of 2-methylcyclohexanone on Rh proved to be very difficult 171 and 2-substituted cyclohexanone was not hydrogenated at all [S], whereas 3- and 4-methylcyclohexanones were reduced readily. The ratio (MN +ML+NML+t-M) / (IMN+IML+NIML+c-M) (products issued from MN over those issued from IMN) has been plotted at different temperatures as a function of the conversion (figure 5). It increases sharply at 75% hydrogen consumption i.e. when thymol has been completely converted. This can be attributed to an epimerization of IMN to MN, which occurs as thymol is no longer present on the catalyst surface.

120 0,15 -

017

373K 353K

BK

h

25

zf $o15 +

30.10~

zi

f

:2L 2 &0,3

zs 011

Figure 5 . Ratio of "menthone type" products to "isomenthone type" products vs. hydrogen consumption over 3.27% Rh/C

z

2 0,05-

t

100% conversion of thymol

0,oo-

'

I

'

1

'

1

.

Figure 6. ML/(ML+NML) ratio vs. hydrogen consumption during reduction of thymol over 3.27% Rh/C

The ratio ML/(ML+NML) (figure 6) remains constant (ca. 0.06) as long as thymol is present; since the menthones are not hydrogenated at this stage, this ratio reflects the stereoselectivity to cis and trans alcohols produced directly from thymol Then, this ratio increases sharply after 75% hydrogen consumption. This may be attributed either to the isomerization of NML into ML, or to a marked change in the stereoselectivity of MN hydrogenation. The former hypothesis can be discarded on the basis of experiments conducted on a mixture of ML and NML in cyclohexane close to the composition ML/(ML+NML) ca. 0.06; indeed the composition of this mixture does not change when treated at 373 K over Rh/C. By reducing at 353 K pure menthone on this catalyst, under the same conditions as for thymol hydrogenation, we checked that the reduction was slow as expected, and gave a high selectivity to ML since the ML/(ML+NML) ratio was as high as 0.4 compared to 0.06, value found initially for thymol hydrogenation. Thus the higher selectivity to menthol can be clearly attributed to the differences in the stereoselectivity of MN - > (ML, NML) compared to that of direct route TH - > (ML, NML). The amount of p-menthanes produced by hydrogenolysis is low and never exceeds 2 % . As in the case of the platinum catalyst, the amount of hydrogenolysis products increases linearly as long as thymol is present, indicating that they are produced directly from the thymol rather than from the menthones or the menthols. Accordingly, the reduction of menthone never yields p-menthanes.

121

3.3. Hydrogenation of thymol over iridium Thymol was hydrogenated within the temperature range 333-373 K on the 2.24% Ir/C catalyst. The hydrogenation of thymol on iridium proceeds as on platinum (figure 7), but much lower amounts of IMN and MN are detected in the reaction medium: 5 and 1%respectively.

time(min) 200

1500

ML

0

20

40 60 80 % H2 consumed

100

Figure 7. Product distribution vs. hydrogen consumption during thymol hydrogenation over 2.24% Ir/C (p-menthanes not represented) These low yields may be attributed either to a specially high rate of hydrogenation of the ketones on iridium or to the fact that the direct route is by far the major one. The first possibility can be discarded, because the iridium catalyst has a relatively low activity in the reduction of pure menthone between 333 and 373 K; it is lower for instance than that of platinum catalyst. Therefore the direct hydrogenation of thymol to the menthols is the most favored route on iridium catalyst. Little isomerization of the menthones is observed in the course of the reaction since the ratio (MN +ML+NML+t-M) / (IMN+IML+NIML+c-M) remains constant during a whole run; it slightly increases from 0.35 at 333 K to 0.47 at 373 K, higher temperatures favoring the formation of the more stable products. The selectivity to ML increases with temperature (333 to 373 K), the ML/(ML+NML) ratio varying between 0.18 and 0.22 for thymol hydrogenation and between 0.23 and 0.31 in the case of pure menthone hydrogenation. However, the selectivity to NIML is always highly favored (ca. 60%).

122

The Ir/C catalyst produces large amounts of hydrogenolysis products; thus up to 11% of p-menthanes are formed at 333 K, and p-isopropyl toluene is detected in increasing amounts with temperature up to a 0 . 7 % yield at 373 K. The formation of aliphatic hydrocarbons may also occur. The presence of these hydrogenolysis products may explain the deactivation of the catalyst which occurs mainly at higher temperatures.

4. CONCLUSIONS

These experiments show that on all catalysts, the formation of the menthols occurs via two pathways: (i) via intermediate cis and trans cyclohexanone intermediates and (ii) directly from thymol. The proportion of the two routes depends on the nature of the metal: very small amounts of the ketones are initially formed over Ir, while the initial selectivities to the menthones are 75% over Pt and Rh. The maximum yields in menthones are high over Rh, because the rate of hydrogenation of these ketones is much lower than that of thymol, due to the bulky ortho isopropyl group. The configuration of the ketones and alcohols is controlled by the adsorption of the molecules on the metal surfaces. Thus IMN, where the methyl and isopropyl groups are in cis position, is predominantly formed. NIML and NML are the most abundant alcohols as might be expected from a cis approach of the adsorbed hydrogen. The product stereoselectivity depends also on the relative occurrence of the direct and ketone routes, as these two routes may give different selectivities. Hydrogenolysis occurs extensively over Ir. The p-menthanes are formed directly from thymol.

5. REFERENCES 1 R. Emberger and R. Hopp, Spec. Chim., 7 (1987) 193. 2 Y . Takagi, Sci. Pap. Inst. Phys. Chem. Res. (Jpn), 64 (1970) 39. 3 D. Richard and P. Gallezot, "Preparation of Catalysts IV", Studies in Surface Science and Catalysis 31, Editors B. Delmonet al., Elsevier, 1987, p. 71. 4 M. Besson, L. Bullivant, N. Nicolaus and P. Gallezot, J. Catal., 140 (1993) 30. 5 P.N. Rylander, "Catalytic Hydrogenation in Organic Syntheses" Academic Press, 1979, p. 82. 6 V.G. Cherkaev and L.A. Shutikova, Internat. Congr. Essent. Oils, 7th, 1977, 7 (1979) 513. 7 P.N. Rylander and D.R. Steele, Engelhard Ind. Tech. Bull, 3 (1963) 125. 8 S. Mitsui, H. Saito, Y. Yamashita, M. Kaminaga and Y . Senda, Tetrahedron, 29 (1973) 1531.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers

--

THE SELECTIVE EYDROGENATION OF Russell E. Fagour i

- u

Malz

123

B.V. All rights reserved.

Jr.,

Michael

6-CHLORO-2 l la)-HYDROXYOUINOXA-

P.

0

Reynolds,

. &

Christopher

J.

Uniroyal Chemical Company, 280 Elm Street, Naugatuck, CT, 0 6 7 7 0 U.S.A.

I.

ABSTRACT

We selectively hydrogenated 6-chloro-2(1H)-hydroxyquinoxaline-4oxides to 6-chloro-2(1H)-quinoxalinone, using sulfided and nonsulfided catalysts. The catalyst of choice is platinum sulfide. Our catalyst studies included sulfided and non-sulfided platinum, palladium, rhodium, ruthenium, sulfided nickel, Raney nickel, and cobalt. 11.

INTRODUCTION

A wide variety of pharmaceutical and agricultural chemicals are

based off 6-halo-2(1H)-quinoxalinone, (I), seen in Figure 1.

Figure 1 Certain piperazinylquinoxalines and other based 2-quinoxalinol derivatives are found to have Central (Nervous System) Serotoninmimetic activity'. Other substituted quinoxalines have shown fungicidal activity2. 6-halo-2(1H)-hydroxyqu~noxal~ne-4oxides are known intermediates for herbicides such as, CibaGeigy'S Agile3, Nissan Chemical's AssureI3 and Uniroyal Chemical's Pantera. These useful compounds are produced by the selective hydrogenation of substituted N-oxides, specifically 6chloro-2 (1H)-quinoxaline-4-oxide, (11), seen in Figure 2.

124 0-

H

(11) Figure 2 One can produce 6-chloro-2(1H)-quinoxalinone, via the N-oxide by, a multi-step method by condensing 4-chloro-2-nitroaniline, (111), and diketene,(IV), to produce 4-chloro-2~-nitroacetoacetanili.de, (V) Patents and literature*, reveal the reaction is carried out in the presence of a nucleophillic amine (Figure 3 ) .

’

.

O Q N 4

0

I

II

(111)

(IV)

(V) Figure 3

Earlier work revealed that refluxing (111) and (IV) at high temperature produced poor quality material in low yield.4 Additional work using glacial acetic acid and mercuric acetate in place of the nucleophillic amine was successful.4 However, this route makes isolation of the product difficult. The most promising route uses non-nucleophillic amines as a catalyst such as triethylamine in an aromatic hydrocarbon s01vent.~ The 4-chloro-2~-nitroacetoacetanilide (V) subsequently undergoes intramolecular cyclization in the presence of an alkaline base such as KOH, or NaOH to produce 6-chloro-2(1H)-hydroxyquinoxaline-4oxide (11). The cyclization reaction is influenced by the type of base, molar ratios of base, solvents, reaction time and temperature.4*5 Several by products can be formed during the intramolecular cyclization. Intermolecular condensation of the N-oxide (11) with a carbanion of (V) gives 6-chloro-3-methyl-2(lH)-qu~noxal~none(VI), while intramolecular rearrangement of (11) leads to 6-chloro-3-hydroxy-2 ( 1H)-quinoxalinone (VII).’ Figure 4 shows these cyclization impurities.

125

(VI)

Figure 4

(VII)

The N-oxide is hydrogenated t o the final product, 2(1H)-quinoxalinone (I), and can be seen in Figure 5.

6-chloro-

0-

I

I

H

H

(11)

(1) Figure 5

Figure 6 shows the potential side products formed during the hydrogenation of the N-oxide (11). All of these products, except (IX), are yield diminishing impurities. Compound (IX) can be reoxidized t o the desired intermediate (11).

(VIII)

Literature6 reports the catalytic hydrogenolysis of N-oxides

126

occurs easily unless the access to nitrogen-oxygen bond is severely hindered. Rates for the hydrogenation of pyridine N-oxides indicate that 5% Rh/C is the most active when compared to 5% Ru/C, Pt/C, and Pd/C. However Rh is unselective reducing the ring as well as the N-oxide.6*7 Earlier studies‘ on substituted pyridine-N-oxides over 5% Pd/C reveal that carbon-carbon double bonds and halogens are reduced before the N-oxide. However, hydrogenation of specific functional groups (i.e., nitro, carbon-carbon double bonds, halogens) depends on several variables. One can readily reduce a nitro substituted pyridine-N-oxide leaving the N-oxide untouched.‘-’ The final selectivity in the hydrogenation of N-oxides has been shown to depends on the functional groups present and their position, the pH of the reaction media, and the catalyst system The reduction of 6-chloro-2(1H)-hydroxyqu~noxaline-4-oxides can be completed by chemical reduction or catalytic hydrog e n a t i o n . 4 ~ 5 b 9 B ’ 0The successful chemical reduction has been reported using triphenylphosphine alone or in conjunction with iron, zinc, tin, sodium arsenite, ammonium sulfide, or sodium dithionite under alkaline conditions. This route is fairly expensive and gives low yields, large aqueous wastes and product isolation difficulties. A similarly expensive process employs hydrazine in the presence of Raney nickel catalyst in alkaline conditions.9 The reported yields range from 88 to 96%. Additional work’ with Raney nickel uses very low pressures of hydrogen in place of hydrazine. The yields are comparable. We investigated” the use of platinum metal sulfides, specifically platinum, rhodium, ruthenium, palladium, as well as sulfided nickel, Raney nickel, and cobalt.

JII. EXPERIMENTAL We completed the hydrogenations of 6-chloro-2(1H)-hydroxyquinoxaline-4-oxides to 6-chloro-2(1H)-quinoxalinone using either a 1 liter stainless steel or 300 milliliter stainless steel magnetically stirred autoclave. Each reaction used a concentrated aqueous potassium hydroxide solution with an indicated amount of 6-chloro-2(1H)-hydroxyquinoxaline-4-oxide and catalyst. All the catalysts were obtained from one of the following commercial vendors Engelhard Industries, Degussa Corporation, or Johnson Matthey Incorporated. The formulation of these catalysts are propriatary. In each experiment, we sealed the vessel, purged twice with nitrogen, once with hydrogen, and pressurized with hydrogen to the indicated pressure. We heated the reaction mixture with agitation to the specified temperatures and held for the indicated time. At the end of the reaction, we removed the product with the aid of distilled water and filtered off the catalyst at 6 O o C using Celite. The filtrate is neutralized at 65-70°C to a pH of 6.5 to 7.0 After neutralization the product is isolated by filtration. We analyzed with a Varian LC equipped with a Supelcosil LC-DP 25cm x 4.6mm, (5 micron Diphenyl) column.

.

127

Table 1 reveals our results with metal catalysts which had not been tested for the hydrogenolysis of (11) to (I). The results indicate that PtS, is a superior catalyst with a higher assay and yield of product compared to the unsulfided 5%Pt, 5%Rh, 5%Ru catalysts. At higher catalyst levels 5%Rh gave a lower yield. 5%Ru, which failed at a low catalyst level, gave some reaction at a 10 fold increase in catalyst. Raney Ni proved inferior to PtS, while Co/kielseguhr did not catalyze the reaction.

g

Table 1

U

NOg

PREV IOUBLY UNTESTED METAL CATALYBTB' Catalyst 5%Rh 5%Rh 5%Ru 5%Ruc 67%Coc Ni(R)d 3%PtS, a. b. c. d.

Isolated Wt.

0.40 0.08

3.0 1.5

45

52

70

87

0.10

5.7 5.2 4.7 0.3 5.2

1.00 2.50

6.20 1.20

c. d. e. f.

75

--

-42 --

-31 --

76

74

56

90

92

83

.

REDUCTION OF (11) WITH Pd and PdB, CATALYBTB'

Catalyst TVDe U..drv Pd 1.25' Pd 1.25' Pd 1.00d PdS, 2.00e PdS, 1.8af

b.

--

23 61

325 ml. (0.10 moles) of (11) at 6 O o C and 100-200 psig H 2 . All catalysts are supported on carbon except Co and Ni. Assay is by HPLC (RA%) Yield is based on Isolated Wt.% x RA% assay. Run at 12OOC Catalyst weight is for wet Raney Nickel.

Table 2

a.

Product (I)

RXN Time

RXN Time At Temp. (Hr

.)

1 : 00

Product (I) Assav(RA%) Yield(%)

--

--

0.75 0.08 2.50 5.75

.

All catalysts are supported 125 mls of (I1 (0.055 moles on carbon. Assay is by HPLC (RA%) Yield is based on Isolated Wt.% x RA% assay. 6OoC, 100-200 psig. 3OoC, 450-700 psig. 6OoC, 600-700 psig. 2ooc, 100-200 psig.

.

128

We examined Pd and PdS, catalysts for the hydrogenolysis of (11). Pd failed to yield any substantial product at 30-60°C as reported in the literature. PdS, at lower temperatures and pressures produced product in low yield. The results can be seen in Table 2. We examined various sulfided metal catalysts for the hydrogenolysis of (11). Our results in Table 3 indicate that PtS, is far superior to other metal sulfides or Pd for the hydrogenation of 6-chloro-2(1H)-hydroxyquinoxaline-4oxides to 6-chloro-2 (1H)-quinoxalinone. Based on the yield , RhS , appears t o be the next most selective catalyst followed by PdS,, RuS, and NiS,

.

.

Table 3

j

Catalyst TvDe q * * d r y P t S X C 0.13 Rhs, 2.00= RuS, 1.90'

NiS,

3.80'

PtS,C

0.12d

a. b. c.

d. e. f.

L

-F

I

m

a

Product (I)

RXN Time

2.00 5.90

92 91 75

5.00 4.00

91 92

5.00

72 56

18 18

72

.

A _ - catalys s are supported 125 mls of (I1 (0.055 moles. on carbon with ,he exception of Ni. Assay is by HPLC (RA%) Yield is based on Isolated Wt.% x RA% assay. Experiments run 17 days apart showing stability of starting material, using PtS, sample 1. 6OoC, 100-200 psig. 2OoC, 600-700 psig. 6OoC, 600-700 psig.

.

We found various PtS, samples had substantial difference in quality and yield of the product. One PtS, sample demonstrated that lowering the catalyst level by a factor of 3 increases the cycle time by a factor of 6, but does not diminish the quality or yield of the final product. These results can be seen in Table 4.

129

-Table 4

PtS, Catalyst

-XAAU&EUU

RXN Time

Isolated Wt.

Product (I)

Yield(%)

Assav(RA%l Yield(%)

1 1 2

0.125 3.0 80 90 73 0.042 17.2 77 92 71 38.6b 2.3 50 55 28 3 0.125 5.0 78 71 55 a. 140 m1.(0.064 moles) of (IV), 6OoC, 100-200 psig. Assay is by HPLC (RA%). Yield is based on Isolated Wt.% x RA% assay. b. This run was done in a 5 gallon autoclave with 13,110 ml. (6.372 moles) of (IV)

.

Table 5

RXN Time At TemD,.(Hr.1 5.75b 4.50b 3.00b 2. O O b 3.10' 1.30' 2.25' 5.20d 7.70d 8.70e 11.90f 10.10d 2.40d 4.55d 3. l o d a. b. c. d. e. f.

PROCESS OPTIMISATION

etSX&UW& 1

1

1 1 1 1 1 1 1 1 1 1 1 4 5 2

Fssav (RA%1 82 93 87 79 58 91 93 91 96 88 59 92 96 92 92

Product (I . 1 . Yieldf%) Yield(%)BOC' 58 69 67 71 64 72 70 82 46 47 65 65 50 51 79 74 74 73 59 55 55 53 95 94 76 78 80 80 60 61

.

Isolated Wt. % Yield (I) x {Assay (I)+Assay (11)+Assay (IX)} This assumes (11) can be recycled and (IX) can be oxidized back to (11). 0.75 9. dry PtS,, 325 m1.(0.15 moles), 6OoC, 100-200 psig. 0.75 9. dry PtS,, 325 m1.(0.15 moles), 6OoC, 600-700 psig. 1.20 9. dry PtS,, 650 m1.(0.30 moles), 60°C, 100-200 psig. 0.40 g. dry PtS,, 650 m1.(0.30 moles), 6OoC, 750 psig. 0.40 g. dry PtS,, 650 m1.(0.30 moles), 100°C, 100-200 psig.

130

Using the PtS. catalyst, we completed a process optimization of the hydrogenation. Operating at higher temperatures and pressures clearly leads to unselective hydrogenation lowering the yield. The yield loss is most likely due to dechlorination leading to water soluble by-products which are lost in work up of the product mixture. The results are in Table 5. V. 1. 2. 3.

VI.

CONCLUBION6

Sulfided catalysts are unique for the reduction of 6-chloro2(1H)-hydroxy-quinoxaline-4-oxides to 6-chloro-2(1H)-quinoxalinone. PtS, is the catalyst of choice. Non-sulfided catalysts produce inferior product in yield and purity. Operating at higher temperatures and pressures with PtS, leads to a lack of selectivity. RLEFWNCEB

R.D. Hartman, E . L . Engelhardt, W.C. Lumma, Jr., and W.S. Saari, J. Med. Chem, 1981, 24, 93-101. 2. G.A. Carter, T. Clark, C . S . James, and R.S.T. Loeffler, Pestic, Sci., 1983, 14, 135-144. 3. Pesticied Manual, British Crop Council, 8th Edition, 1987. Davis, Process for Preparing 6-Halo-2-Chloroquin4. R.F. oxaline, U . S . Patent 1987; 4,636,562. 5. K. Makino, K. Morimoto, and G. Sakata, Heterocycles, 1985, 23, No.1. 6. P.N. Rylander, Catalytic Hydrogenation over Platinum Metals, 1967. 7. R . L . Augustine, Catalytic Hydrogenation, 1965. a. M. Freifelder, Practical Catalytic Hydrogenation, 1971 9. T. Ishikura, Process for Producing 2-Quinoxalinols, 1986; U . S . Patent 4,620,003. 10. R.E. Malz, Jr., J. W. Sargeant, and J.A. Feiccabrino, Process for the Selective Reduction of 2-Hydroxyquinoxaline4-OxidesI 1989; U . S . Patent 4,814,444. 1.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals I11 63 1993 Elsevier Science Publishers B.V. All rights reserved.

131

Study of the hydrogenation of methyl benzoate to benzaldehyde on various metal oxides. A. Aboulayta, A. Chambellana, M. Marzina, J. Sausseya, F. Maugea, J.C. Lavalleya*, C. Merciefi and R. Jacquotb aLaboratoire Catalyse et Spectrochimie URA CNRS 41 4,ISMRA-UNIVERSITE, 6 Boulevard du Marechal Juin, 14050 CAEN Cedex, France b e n t r e de Recherches des Carrieres RhBne-Poulenc, 85 Avenue des FreresPerret, BP 62,69192 Saint-Fons Cedex, France.

ABSTRACT

The hydrogenation of methyl benzoate to benzaldehyde was studied on various metal oxides under atmospheric pressure at 300 and 350°C. ZnO, Zr02 and CeO2 presented high activity and selectivity whereas Ti02 (anatase) was found to be less active. MgO and A1203 were completely inactive. The infrared study of species formed under working conditions on ZrO2 confirmed the presence of benzoate species on basic sites as shown by methyl benzoate adsorption on Zr02 pre-exchanged by H2180. Most of the benzoate species were inactive towards H2 under the conditions used. It was concluded that acidic or basic sites are not essential for the studied reaction. By contrast, the redox properties of the metal oxides seem to be an important factor. 1.

INTRODUCTION

Aromatic aldehydes (benzaldehyde, vanillin, heliotropin. ..) are of economic importance in the perfume and flavor industries. Progress in their manufacturing technology has recently been reviewed [l]. Among the different production processes used, such as oxidation, halogenation or formylation, the direct hydrogenation of carboxylic acids (or their esters) to the corresponding aldehydes seems very attractive due to its simplicity and low cost. Several metal oxides, such as cerium, praseodynium, thorium or uranium oxide, or, even better, yttrium or zirconium oxide, generally supported on a-AlzO3, have been patented, being active and selective for this process [2].However, no fundamental work has been published on the reduction of aromatic carboxylic acids and esters on metal oxides

132

except for an in situ IR study on Y2O3 [3] and a recent work by Maki et al. [4] on zirconia. The former shows that surface benzoate ions are intermediate species and the authors suggest that hydrogen is transferred to the final products via surface hydroxyl groups [3]. The latter tends to relate the activity to surface acidbase properties of Zr02 : non acidic zirconia having Ho in the range of +6.6 to +7.2 show high activit [4]. The aim o the present study is to compare the activity and selectivity of various metal oxides towards methyl benzoate hydrogenation into benzaldehyde and to relate the results to surface properties of the catalysts.

Y

2.

EXPERIMENTAL

The hydrogenation reaction of methyl benzoate was carried out at atmospheric pressure in a flow reactor at 300 and 35OOC. The catalysts (0.1 g) were first activated at 35OOC under pure H2 for 2 h. Then methyl benzoate (P = 2.7 Torr) was carried from a thermostated regulator in the H2 flow (flow rate : 40 ml.min-1). The reaction products were analysed by an on line gas chromatograph equipped with a capillary column and a flame ionization detector. ZrO2 (70 m2 9-1) and Ti02 (70 m2 g-1) were prepared by hydrolysis of the corresponding metal alkoxides [5]. MgO (250 m2 g-1) was obtained by thermal decomposition at 55OOC of commercial magnesium hydroxide (Merck). Other catalysts were commercial products : ZnO (Kadox-15, 12 m2 g-l), Ce02 (RhdnePoulenc, 120 m2 g-I), A1203 (Degussa, 100 m2 9-1). FT-IR measurements were performed on a Nicolet MX-1 spectrometer. The samples were pressed into self-supporting wafers (diameter : 16 mm, weight m.20 mg) and activated in situ. Two types of IR cell were used. A reactor-cell [6] working as a continuous flow reactor, coupled to an on line gas chromatograph, allowed recording of the IR spectra of adsorbed species during the reaction (flow rate : 15 ml min-1 ; catalyst activation temperature : 4OOOC). Static measurements were performed in a conventional cell, the catalysts generally being activated at 550OC. 3.

RESULTS and DISCUSSION

Catalytic activity The stationary state of the reaction was found to be reached in two hours. Under such conditions the activity and selectivity of the oxides studied are given in Table 1. Alumina and magnesium oxide were inactive under the conditions used. These activity measurements can be compared to those reported by Yokoyama et al. [4] for the hydrogenation of benzoic acid at higher temperatures (400 to 440°C). They confirm that ZrO2, ZnO and CeO2 are active. Ti02 is found to be less active but more selective. Conversely, MgO and A1203 are inactive, indicating that basicity or acidity alone is not an essential factor for catalysis.

3.1.

133 Table 1 Hydrogenation of methyl benzoate at 300°C Conversion Selectivitv Catalyst of methyl (W benzoate % CEH~CHOCEHF;CH~OHCEHF;CH~ Zr02 Ce02 ZnO Ti02

14 15 16 3

86 80 68 100

Other products A* B*

2 2

14 12 30

6

CsHsCH3

CfiH6

Heavy products

*A : C&jCOOCH2C6H5 6* : C6H&H = CHC& HvdroQenationof methyl benzoate at 350°C Conversion Selectivity Catalyst of methyl (W benzoate yo C6HsCHO CsHsCH20H Zr02 Ce02 ZnO Ti02

74 66 43 23

63 59 70 83

5 3 2

1 9 12 2

1

31 28 16 15

The reduction of benzaldehyde was also studied at 350°C. The activity of the various metal oxides, particularly that of Ce02 and Ti02, was lower than that found for methyl benzoate. Therefore, benzaldehyde is less easily reduced than methyl benzoate explaining the high selectivity of these metal oxides for the hydrogenation of the latter (Table 1).

Mechanistic considerations It has been reported [3] that surface benzoates are intermediate species for the reduction of benzoate esters into benzaldehyde on Y2O3. Therefore, we have studied the formation mechanism of such species and their reactivity towards hydrogen. We report hereafter the results obtained on ZrO2. 3.2.

- Adsorption of methyl benzoate Figure 1A shows the IR spectra (1 700-900 cm-1 range) of species formed from introduction of successive doses of C6H5COOCH3 at 300°C on Zr02 activated at 550°C. The first dose leads to the appearance of main bands at 1536 and 1435 cm-1. They characterize va(C02) and vs(C02) vibrations of benzoate species, respectively [7]. Their wavenumbers and the va(C02) - vs(C02) splitting are in favour of a bidentate benzoate species, hereafter called A. Other bands at 1594, 1495, 1453 and 1309 cm-1 are due to ring stretching vibrations. The origin of the high intensity of that at 1594 cm-l is due to a coupling with the V a (C02) stretching frequency [a]. In the v(CH) frequency range (not shown), the band at 3060 cm-1 is assigned to ring v(CH) modes, whereas others at 2950, 2925 and 2815 cm-1

134

B

I

d d-C C

c-b

b

b-a

8

00

WAVENUMBER

1foo

boo

1500 1500 li00 WRVENUNBER

Figure 1. A) IR spectra of species formed from introduction of successive doses of methyl benzoate, at 3OO0C, on ZrO;! activated at 550°C. 6)Subtraction of two successive spectra. characterize the presence of methoxy species [9]. The v(C0) bands at 1154 and 1041 cm-1, corresponding to such species, are assigned to type I and type II methoxy groups, respectively. This assignment is based on results which showed that ZrO2 presents two types of hydroxyl group and two types of anionic vacancy (D) according to whether they are bonded to one or two surface Z++ ions : X

I Z++ Type I

Type II

Further addition of C6H5COOCH3 generally increases the intensity of the bands, except for those characterizing the methoxy groups, which tend to disappear. Subtracted spectra (Figure 16) show that methoxy I species (v(C0) band at 1154 cm-1) are first affected. They also indicate that the spectrum of the second benzoate species formed, hereafter called B, is different from that of s ecies A : the vs(C02) band intensity is much weaker whereas that of the 1453 cm- band is enhanced.

P

- Formation mechanism of

benzoate species In order to specify the adsorption mechanism of methyl benzoate, the same experiment was performed on ZrO2 previously exchanged by H2180 at 550°C. Adsorption of C6H5COOCH3 does not lead to any new band near 1120 and 1020

135 cm-1 indicating that formation of 18OCH3 species does not occur [9]. By contrast, 160 1 8 0 surface exchange since, for instance, the va(C02) frequency is situated at 1516 cm-I (species A) or 1528 cm-1 (species B) instead of 1536 cm-1 in the unexchanged sample. We deduce that the dissociative adsorption of methyl benzoate arises from the

v(C02) bands due to benzoate species are sensitive to the

t

breaking of the C-0bond of C6H5 - OCH3, the C ~ H S C O group being chemisorbed on basic 02-sites, the methoxy group on coordinatively unsaturated (cus) Zr4+ ion. The co-existence of Lewis acid and basic sites is therefore necessary for methyl benzoate adsorption.

- Reactivity of

benzoate species towards hydrogen This study was performed either under static or dynamic conditions. In static conditions, H:! (200 Torr) was introduced on Zr02 on which C&5COOCH3 were chemisorbed. Heating for 2 hours at 300°C does not modify the spectrum of benzoate species much. Subtracted spectra show that only benzoate species B disappear (Figure 2).

?

N

L? d

w

0

z

a

m?

3m m

a

L? 0

1

1$00

1500

1300

WAVENUMBER

Figure 2. Infrared spectra on species formed from : a) Methylbenzoate chemisorption at 300°C on Zr02 activated at 550°C b) After heating for 2 hours at 300°C under H2 c) Subtracted spectrum (b-a). For dynamic studies, the IR cell reactor was used. After zirconia activation in hydrogen at 4OO0C, the temperature was reduced to 200°C and methyl benzoate (2.7 Torr) was fed with the hydrogen flow (15 ml. min-1). At this temperature, the catalyst activity was low (conversion = rn 3 "/.). The first species formed on the surface were methoxy groups, partly formed at the expense of free zirconia OH groups at 3763 (type I) and 3654 cm-1 (type 11). Then benzoate species appeared

136

with time (Figure 3), at first mainly at the expense of type I methoxy species. The relative intensities of the va(C02), vs(C02) and 1453 cm-1 bands confirmed that formation of benzoate species A occurs first. Subsequently, as shown by subtracted spectra, benzoate species B appear, mainly at the expense of type I I methoxy species. When the stationary state was reached, the methyl benzoate flow was switched off. Under H2 flow, only benzoate species B disappear which agrees with

?

N

a

?

50

m,

3650

3250

MAVENUMBER

2650

2650

1850

1850

lhS0

1650

1450

1250

1650

650

1650 1250 UAVENUMBER

1650

b50

UAVENUMBER

B 29- 19 19- 15 15-10

?

10-1

cn m a

7-5

?

5-4

MAVENUMBER

Figure 3. A) Variation with time (in min.) of the IR spectra of adsorbed species formed when reducing methyl benzoate by H2 on Zr02 at 2OOOC (spectra recorded under flow conditions). B) Subtraction of two successive spectra.

137 the result obtained under static conditions (Figure 2). This study shows that the first benzoate species formed are not active towards H2 and play the role of spectator. The others (species B) disappear under H2 flow and lead to benzaldehyde. The existence of two types of benzoate species can be related to the presence of two types of adsorption site on Zr02, as already evidenced by methanol adsorption [9].The unreactive benzuate species seem to

0 be formed on basic sites in the vinicity of type I Zr4+ ions, whereas the reactive

cl

species occur close to type II Zr4+ Zr4+ sites.

- H2 activation

Under flow conditions, on Y2O3, King and Strojny [3]observed the formation of new surface OH groups at 3640 cm-1, attributed to OH groups in very weak interaction with the benzoate species. The authors concluded that hydrogen was transferred to the final products via surface hydroxyl groups. In the experiment reported in figure 3, no new OH groups appear when adsorbing C6H5COOCH3 on ZrO2, suggesting that H2 activation occurs differently. H2 adsorption has been studied on different oxides, mainly by IR spectroscopy [lo].On ZnO, at room temperature, two main processes have been identified, yielding two species, either Zn-H and 0-H (type I adsorption, rapid and reversible) H H\ / / \ Zn and 0 or bridged Zn 0 species (type II adsorption, slow and irreversible). The corresponding heats of adsorption are 40 kJ.mol-1 and 14 kJ.mol-1. The latter very low value has been considered as evidence for diffusion of H2 into the bulk [ll]. On other oxides, such as MgO [12]or ZrO2 [13]activated at temperatures higher than 600°C,Mg-H or Zr-H species have also been observed. Nevertheless, Kondo et al. reported that dissociatively adsorbed hydrogen species formed on ZrO2 were not active towards ethene hydrogenation [14].A similar result has been reported on ZnO [15]for propene hydrogenation. Nevertheless, nothing proves that such results can be extended to methyl benzoate hydrogenation. Heterolytic dissociative adsorption of hydrogen has generally been related to the basicity of the surface oxide ions [16].However, Busca pointed out that chromia and chromites, very often used as hydrogenation catalysts, did not seem to be particularly basic [17].He suggested that other properties may be involved such as a slight reducibility. Indeed, in contrast to alumina and magnesia which are not reducible by H2, at least at 350°C,it is well known that ZnO heated under H2 becomes non stoichiometric [18].H2 is incorporated into CeO2 at temperatures lower than 230°C and forms a compound with the composition CeO2Ho.17 [19,201. At higher temperatures, ceria reduction occurs as shown for instance by magnetic susceptibility measurements [21].Ti02 is also reducible : the temperature programmed reduction spectrum exhibits a peak at about 380°C whereas, simultaneously, the sample color becomes slighly blue, indicating the appearance of Ti3+ ions [22].In the case of Zr02, it has been reported that H2 treatment leads to the formation of water, which should create oxygen vacancies with a partial reduction of the surface (231.Indeed, a recent paper reports the formation of Z$+ ions at the surface of a monoclinic ZrO2 sample evacuated at 400°C [24]. It is difficult to determine how H2 is activated on reduced metal oxides. Further experiments are in progress on this point.

138

4.

CONCLUSIONS

ZrO2, ZnO and Ce02 present high activity towards hydrogenation of methyl benzoate to benzaldehyde. Ti02 is found to be less active but more selective. The inactivity of A1203 and MgO for the hydrogenation of methyl benzoate tends to show that acidic or basic properties of metal oxides are not directly connected to their hydrogenating power. In contrast, redox properties seem to play a more important role. 5.

REFERENCES

T. Maki and T. Yokoyama, Jap. Synth. Org. Chem., 49 (1991) 195. E.J. Strojny, Dow US Patent No. 4 328 373 (1982). S.T. King and E.J. Strojny, J. Catal., 76 (1982) 274. T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima, T. Maki, Appl. Catal., 88 (1992) 149. 5 C. Lahousse, A. Aboulayt, F. Mauge, J. Bachelier and J.C. Lavalley, submitted to J. Mol. Catal. 6 J.C. Lavalley, M. Maache and J. Saussey, Proceed. SPlE Conf., San Diego, Infrared Technology XVI, 1341 (1990) 244. 7 R.P. Groff, J. Catal., 79 (1983) 259. 8 A.S. Wexler, Spectrochim. Acta, 23A (1967) 1319. 9 M. Bensitel, V. Moravek, J. Lamotte, 0. Saur and J.C. Lavalley, Spectrochim. Acta, 43A (1987) 1487. 10 F. Boccuzzi, E. Borello, A. Zecchina, A. Rossi and M. Camia, J. Catal., 51 (1978) 150. 11 B. Fubini, E. Giamello, G. Della Getta and G. Venturello, J. Chem. SOC.,Farad. Trans. I, 78 (1982) 153. 12 S. Collucia, F. Boccuzzi, G. Ghiotti and C. Morterra, J. Chem. SOC.,Farad. Trans. I, 78 (1982) 2111. 13 J. Kondo, H. Abe, Y. Sakata, K. Maruya, K. Domen and T. Onishi, J. Chem. SOC., Farad. Trans. I, 84 (1988) 51 1. 14 J. Kondo, K. Domen, K. Maruya, and T. Onishi, J. Chem. SOC.,Farad. Trans. I, 88 (1992) 2095. 15 T. Okuhara, T. Kondo and K. Tanaka, J. Phys. Chem., 81 (1977) 808. 16 K. Tanabe, In "Catalysis by Acids and Bases" (B. lmelik et al., Eds), p. 1 Elsevier, Amsterdam, 1985. 17 G. Busca, J. Catal., 120 (1989) 303. 18 F. Boccuzzi, C. Morterra, R. Scala and A. Zecchina, J. Chem. SOC.,Farad. Trans., 2, 77 (1981) 2059. 19 J. Fierro, J. Soria, J. Sanz and J. Rojo, J. Solid. State Chem., 66 (1987) 154. 20 J. Rojo, J. Sanz, J. Soria and J. Fierro, Z. Physik. Chem., 152 (1987) 149. 21 A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J.C. Lavalley, J. El Fallah, L. Hilaire, F. Le Normand, E. Quernet-6, G.N. Sauvion and 0. Touret, J. Chem. SOC.,Farad. Trans, 87 (1991) 1601. 22 K. Hadjiivanov, J. Saint-Just, J.M. Tatibouet, M. Che, J. Lamotte and J.C. Lavalley, to be published. 23 N.B. Jackson and J.G. Ekerdt, J. Catal., 101 (1986) 90. 24 C. Morterra, E. Giamello, L. Orio and M. Volante, J. Phys. Chem., 94 (1990) 31 11. 1 2 3 4

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III (B 1993 Elsevier Science Publishers B.V. All rights reserved.

139

ENANTIOSELECTIVE HYDROGENATION OF a-KETOACIDS USING PLATINUM CATALYSTS MODIFIED WITH CINCHONA ALKALOIDS.

H.U.Blasefl and H.P. Jalett Central Research Services, CIBA-GEIGY AG, R 1055.6, CH-4002 Basel Abstract The enantioselective hydrogenation of a-ketoacids using modified heterogeneous catalysts can be carried out with moderate to good (>80%)optical yields. The influence of catalyst type, modifier and solvent on reaction time and optical yield was investigated for the hydrogenation of 4-phenyl-2-oxobutyric acid. R catalysts with several cinchona derivatives as modifiers in (aqueous) alcohols give the best results. This is in contrast to the well studied hydrogenation of a-ketoesters where the highest optical yields are obtained in solvents with low polarity. Temperatures above 25 OC lead to a decrease of the optical yields. The effect of the modifier concentration on reaction time and optical yield suggests a "ligand-accelerated" type of reaction. While several types of a-ketoacids can be hydrogenated with good optical yields, the K-or Na-salts are not suitable. INTRODUCTION The selective preparation of chiral compounds is a topic of current interest. A very elegant way of achieving this goal is the application of enantioselective catalysts. From a technical point of view, heterogeneous enantioselective catalysts are often preferable to homogeneous ones because of their handling and separation properties [11. Unfortunately, there are only very few useful heterogeneous systems and they are usually quite substrate specific i.e. only certain substrate types are transformed with high selectivity. Tartrate modified nickel catalysts and cinchona modified platinum catalysts are highly enantioselective hydrogenation systems that are able to hydrogenate P-functionalized ketones and a-ketoesters. respectively, with optical yields of > 90% [l]. The use of cinchona modified Pd catalysts for the enantioselective monodehalogenation of dichlorobenzazepinone has also been described [2]. (R)-4-phenyl-2-hydroxybutyric acid ethyl ester is an important intermediate for the synthesis of the angiotensin-converting enzyme inhibitor benuzepd [3] (see SCHEME 1). Its preparation via hydrogenation of the a-ketoester has been developed and scaled-up into a production process (10-200 kg scale, chemical yield >98%,ee 7942%).One drawback of this process is the instability of the a-ketoester during distillation and storage. The hydrogenation of the a-ketoacid followed by esterification is an alternative route to the desired hydroxyester. 4-Phenyl-2-oxobutyric acid is readily accessible via condensation of pyruvic acid with benzaldehyde followed by hydrogenation in presence of a Pd/C catalyst [4].

140

The present publication describes the application of cinchona modified platinum catalysts for the enantioselective reduction of a-ketoacids (SCHEME2). In particular, we report on the enantioselective hydrogenation of 4-phenyl-2+x~butyric acid which was investigated and optimized by a systematicvariation of catalyst, solvent, modifier and reaction conditions.

SCHEME 1

0

R +OH+

H2

0

- OH+ catalyst

clnchona modHler

0

R enantiomer

: G o H

0

s enantiomer

SCHEME 2 EXPERIMENTAL Materials: Catalysts (5% metal content, pre-reduced in H, at 300-400 OC before use) and solvents were of commercial origin. Substrates and modifiers were either commercially available or prepared according to literature procedures. Benzylidenepyruvicacid was obtained by condensation of benzaldehyde and pyruvic acid in presence of KOH,the acid hydrate was is* lated by pouring a warm, saturated solution of the K-salt into excess 2N aqueous HC1 [4a]. 4-Phenyl-2-oxobutyric acid was obtained either by selective hydrogenation of benzylidenep p v i c acid (5% PdC, EtOH, RT, 1 bar H,) [4b] or by condensation of ethyl 3-phenylpropionate with oxalic acid diethyl ester followed by hydrolysis and decarboxylation. In both cases,

141

the crude a-ketoacid was purified by precipitation from hot toluene by addition of water. The dried a-ketoacid hydrate, mp 36-38 OC,was used for the hydrogenation studies. HvdroEenation studies: All reactions were carried out in a three-phase sluny reactor with magnetic stirring (ca. lo00 rpm), at 2S-3OoC and 70-100 bar. The reactions were run to completion and the conversion was checked by gas chromatography (column: OV 101,2m, SOOC) after esterification of the h ydroxyacids (EtOH, HCl, RT). For technical reasons, the time (minutes) required for the uptake of 100% hydrogen is given as a qualitative measure of the catalyst activity. Optical yields were determined by gas chromatography on a chiral capillary column (Chirasil-&)-Val, 50 m, ISOOC) after derivatization of the hydroxyesters with isopropyl-isocyanate. ee [%I = 100 x I[Rl- [Sll/ ([Rl + [Sl). RESULTS AND DISCUSSION For the reasons described above, 4-phenyl-2-oxobutyric acid was used as model substrate in order to elucidate the qualitative and quantitative influence of different reaction parameters. If not otherwise indicated, (R)-4-phenyl-2-hydroxybutyricacid is formed in excess in all cases.

Effect of catalyst type, modifier structure and solvent a. One-dimensional Screening. First preliminary experiments were camed out in toluene in presence of 10,ll-dihydrocinchonidine( H a ) in toluene with a Pt/A1203catalyst, in our experience one of the best systems for the enantioselective hydrogenation of a-ketoesters [ l]. Optical yields between 40 and 50% were obtained. This was encouraging because it was by no means clear that the basic modifier would still be effective in presence of a large excess of a carboxylic acid [ 5 ] . In order to improve the enantioselectivity (ee), the effect of several types of catalysts, modifiers and solvents was investigated by changing one parameter at a time.

Clnchonldlne (Cd) derlvatives (2 = H) Cd HCd

OAcHCd OMeHCd deoxoHCd norcinchol

R

Y

vinyl

OH

ethyl "

"

Clnchonlne R = vinyl; Z = H

OAc OMe

"

H

CH,OH

OH

Qulnlne

Qulnldlne

R =vinyl; Z = OMe

R = vinyl; Z = OMe

SCHEME 3

142

Table 1: Optical yield (ee, %) and hydrogenation time (t,,,%, min) for the hydrogenation of 4-phenyl-2-oxobutyric acid. Influence of catalyst type, modifier structure and solvent. If not indicated otherwise, the R enantiomer was formed in excess. (Abbreviations see SCHEME 3)

Pt/A120$ Pt/AI,O,

64 68

Pt/C

51

Pd/AI,O, Rh/AI,O, Ru/AI,O,

8 34 8

120 60 240 120 900

quinine cinchonidine HCd OMeHCd OAcHCd deoxoHCd norci nchol N-Me-Cd+ClN-benzyCCd+CI cinchonine cinchonidine6) quinidine6) cinchonine6)

150 75 60 75 150 120 240 600 60 420 40 60 60

1) modifier HCd; solvent i-PrOH 2) catalyst 5R94; solvent i-PrOH 3) modifier HCd; catalyst 5R94 4) E 4759

(Engelhard) 5) JMC 5R94 (Johnson Matthey) 6) substrate ethyl pyruvate; solvent EtOH/10%H20;5R94

MeOH MeOH/1O%H,O EtOH EtOH/5%H20 EtOH/1OYoH20 EtOH/20%H20 i-PrOH i-PrOH/lO%H,O n-BuOH i-BuOH i-BuOH/lO%H,O 1-BUOH AcOEt AcOi-Pr i-Pr,O dioxane n-hexane toluene

43

60 56 60 68

56 68 69 61 68

72 56 68 50

25 58 55

52

CHZCI,

40

AcOH

58

120 80 160 60 60 60 60 75 180 60 60 100 160

120 360 420 120 300 140 140

Reaction conditions: 2 g substrate, 20 ml solvent;0.1 g Catalyst; 10 mg modifier; 100 bar H,; 20-30 ' C The results summarized in Table 1 allow several conclusions:

Catalysts: Pt catalysts are clearly best suited, Rh gives moderate ee-values, while Pd and Ru catalysts show both low activity and enantioselectivity. The same pattern was also observed for a-ketoesters [6]. Modifier: The effect of the modifier structure is also quite similar to that found for a-ketoesters I71. Cinchonidine derivatives and quinine lead to an excess of the (R)-hydroxyacid while the pseudo-enantiomeric cinchona alkaloids (cinchonine and quinidine) give preferentially (S)-product but with much lower enantioselectivity. Changing the substituent Y at C, has only an effect on the degree of asymmetric induction but not its direction. OMe and OH are more effective than OAc or H. An interesting exception are the N, alkylated Cd derivatives which are completely ineffective in the case of the ester. Here, N-methyl-Cd+Cl- gives a small excess of the R-enantiomer while N-benzyl-Cd+CI- leads an 33% excess of (S)-4-phenyl-2hydroxybutyric acid! Solvent: The effect of the solvent on the enantioselectivity is markedly different from the one reported for the hydrogenation of a-keto esters [la, 81. This is illustrated in Figure 1 where optical yields are plotted in function of the dielectric constant. While solvents with low DE are

143

preferred for the hydrogenation of a-ketoesters (best ee between DE 4 and 8). a-ketoacids are best hydrogenated in aqueous alcohols @E between 20 and 30). At the moment we have no satisfactory explanation for these differences in behavior. It is possible that the carboxylic acid which at least in principle can dissociate in some solvents interacts differently with modifier that is thought to be adsorbed at the active site.

ee (%I

20

.

20(:)

20 40 60 80 100

20

40 60 80 100

HCd/Pt/A1203

20

40

60

80

100 DE

Figure 1: Influence of the dielectric constant of the solvent on the enantioselectivity for the hydrogenation of a) ethyl pyruvate [la], b) 4-phenyl-2-oxobutyric acid ethylester [la], c) 4-phenyl-2-oxobutyric acid (conditions see Table 1). b) 23-1-factorial design experiment. Combinations of the best catalysts (Pt/A1203 E 4759 and Jh4R 5R94); modifiers (HCd and OMeHCd); and solvents (i-BuOH and EtOH/lO%H,O) were tested in two abbreviated factorial design experiments. The results are summarized in Table 2. The combination of catalyst JMC 5R94 and modifier 0-methyl-dihydrocinchonidine in EtOH/10%H20 led to the best results until now: 85% ee, tlW%=60 min for 4-phenyl-2-0x0-

Table 2: Effect of modifier, catalyst and solvent on optical yield of @)-4-phenyl-2-hydroxybutyric acid arid (R)-lactic acid madlfler

catalyst

solvent

eel)

OMeHCd OMeHCd OMeHCd OMeHCd

E 4759 5R94 E 4759 5R94 E 4759 5R94 5R94

2-BuOH 2-BuOH EtOHll O%H2O EtOH/lO%H,O EtOHll O%H2O EtOHll O%H,O 2-BuOH

76 78

HCd HCd HCd

1) substrate 4-phenyl-2-oxobutyric acid; 2) substrate pytuvic acid Reaction conditions see Table 1

85 58 72 74

ee2)

63 79

50 64

144

butyric acid. In each case, replacing one of these optimal components with the less effective one leads to a decrease in enantiosclcctivity.

Effectof temperature, and catalyst, modifier and substrate concentration Several factorial design experiments wen carried out in arder to elucidate these effects using OMcHCd modified N C 5R94 in EtOH/lO% HzO.Catalyst and substrate concentration as well as the ratio of catalyst to modider have negligible effects on the optical yield If the modifier concentration is too low or the temperature >25 OC then a decrease in enantioselectivity is observed. The effect of the modifier concentrations on ee and average rate was further investigated and is depicted in Figure 2. There is a smooth increase of rate and optical yield with increasing OMeHCd concentrations in agreement with a "ligand-accelerated"type of reaction as proposed for the hydrogenation of a-ketoesten [9].Indeed. the experimental points can be modeled with a satisfactory fit by assuming an equilibrium between unmodified and modified Pt surface sites (Keq = 3500 Vmol); rate constants of 0.1 5-l and 0.7 s-l, respectively and an intrinsic enantioselectivityof 78% for the chirally modified sites. While the general picture is the same for both a-ketoesters and acids, in the latter case much higher modifier concentrations are necessary to obtain a fully modified system. This might be due to the more polar solvent used for the hydrogenation of a-ketoacids, where the cinchona modifier is better soluble and therefore less strongly adsorbed on the R surface.

88 I

rate

Figure 2. Effect of modifier

80

60 40 . ' I -

20 .!x,*.

I ,

I

'*"

-X

I -1 - 1 -

0

-

x

I

0 0.00

2.00

4.00 6.00 [OMeHCd] (mmoWI)

145

Hydrogenation of various a-ketoacids In order to test the generality of this hydrogenation reaction, several a-ketoacids were hydrogenated In most cases, no a m p t was made to hther optimize the reaction conditions. The results summarized in Table 3 $huw that it is indeed possible to hydrogenate different a-ketoacids with cinchona modified WA12qcatalysts with moderate to good optical yields. The activity of the catalysts is satisfactory but rather high pressrues and modifier wncentrations are necessary in order to obtain these good results. "he direct hydrogenation of thc benzylideneppvic acid to (R)4phenyl-2-hydroxybutyric acid deserves a special comment. Since. under the same reaction conditions the hydrogenation of the saturated a-ketoester gives exactly the same ee, it is likely that the C=C bond is hydrogenated first, followed by the reduction of the C=Obond. Table 3. Best optical yields obtained for different a-ketoacids solvent

best 88

modifier

catalyst

MeOHCd HCd

5R94 E4759

EtOW1O%H20 EtOH

HCd

E4759

EtOH

50

MeOHCd

5R94

EtOHll096H20

72

MeOHCd

5R94

EtOH

69

MeOHCd

E 4759

toluene

51

norcinchol

E4759

H20

2

norcinchol

E4759

H20

2

0

P h ~ c c a i 0

P h d C O O H

82 5oI56

0

Am" 0 d C O O H 0 PhXCOOH

0

Reaction condition see Table 1

CONCLUSIONS We have demonstrated that the enantioselective hydrogenation of a-ketoacids can be carried out with moderate to good optical yields using cinchona modified Pt catalysts in alwholic solvents. For the synthesis of (R)-4-phenyl-2-hydroxybutyricacid we describe results which form a good basis for the development of a production prccess, since the starting materials are readily available and not very expensive. Fmm our experience with these chirally

146

modified heterogeneous catalysts, the chances arc good that the optical yields can be improved further by a careful optimization of the reaction parameters. While the detailed mode of action of the modified catalyst is not understood, our results are consistent with a "ligand accelerated" catalysis. In this model, the modifier is reversibly admrbed on surface Pt atoms, thereby leading to a chiral active site that on the one hand is mom active than the unmodified one and on the other hand is able to control the stexeochemistry of the hydrogen addition. ACKNOWLEDGMENTS We would like to thank Dr. G. Sedelmeier for several substrate samples and for helpful discussions and Dr.M. Studer for a critical review of the manuscript.

REFERENCES For recent xeviews see a) H.U. Blaser and M. MUller, Stud. Surf. Sci. Catal., 59 (1991) 73. b) H.U. Blaser, Tetrahedron: Asymmetry, 2 (1991) 843. H.U Blaser, S.K.Boyer and U. Pittelkow, Tetrahedron: Asymmetry, 2 (1991) 721. S.K.Boyer, R.A. Pfund, R.E. Portmann, G.H. Sedelmeier and Hj. Wetter, Helv. Chim. Acta, 71 (1988) 337. G.H. Sedelmeier, H.U. Blaser and H.P. Jalett, EP 206993 (1986). a) E.D. Stecher and H.F. Ryder, J. Am. Chem. Soc., 74 (1952) 4392. b) P. Cordier, C. R. Acad. Sci., (1955), 564. The effect of acids as solvents and additives has been described in H.U. Blaser, H.P Jalett and J. Wiehl, J. Mol. Catal., 68 (1991) 215. H.U.Blaser, H.P. Jalett, D.M. Monti, J.F. Reber and J.T. Wehrli, Stud. Surf. Sci. Catal., 41 (1988). 153.

H.U. Blaser, H.P. Jalett, D.M. Monti, A. Baiker and J.T. Wehrli, Stud. Surf.Sci. Catal., 67 (1991) 147.

J.T. Wehrli, A. Baiker, D.M. Monti, H.U. Blaser and H.P. Jalett, 3. Mol. Catal.. 57 (1989) 245.

M. Garland and H.U. Blaser, J. Am. Chem. Soc., 112 (1990) 7048.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals Ill 0 1993 Elsevier Sdence Publishers B.V. All rights reserved.

147

Surface Organometallic Chemistry on Metals; Selective hydrogenation of citral on silica supported Rhodium modified by tetra-n-butyl Germanium, Tin and Lead. *Didillon, B., *Candy, J.P., **LePeletier, F., *Ferretti, O.A. and *Basset, J.M. *I.R.C.-CNRS, 2 Av. A. Einstein, 69626 Villeurbanne Chdex, and E X I L 43 Bd du 11 Nov. 1918,69100 Villeurbanne. (France) **I.F.P., 1&4 Av. de Bois-Preau 92506, Rueil Malmaison Cedex. (France).

Abstract Bi-metallic Rh[M(n-C4Hg)xly/Si02 (M=Ge, Sn, Pb) catalysts can be obtained by surface organometallic chemistry on metals. With Ge and Sn based complexes, stable organometallic fragment can be obtained for which x = 2 and y 2 0.3. These catalysts exhibit increasing activities and selectivities for citral (cis and trans) conversion into geraniol (and nerol) when"y" vanes from 0.3 to ca. 1.With the Rh-Pb system, no butyl groups remain on the catalyst surface when "y" increases. These catalysts exhibits a decay of activity when "y" increases. 1. INTRODUCTION

Surface organometallic chemistry on metals deals with the reactivity of organometallic complexes with metallic surfaces 11-61. By controlled reaction between tetra-n-butyl tin and silica supported rhodium catalyst, a new catalytic material is obtained for which the chemoselectivity can be governed by the coverage of the metallic surface with the organometallic compound. On this material, a-p unsaturated aldehydes, (for example citral cis and trans) may be hydrogenated either into the corresponding unsaturated alcohols (for example geraniol and nerol) or into the saturated aldehydes (for example citronellal). For this kind of reaction, which is a good test of chemoselectivity, pure rhodium does not exhibit any selectivity and leads, besides unselective hydrogenation, to reactions of carbon-carbon bond breakage. Addition of tetra-n-butyl tin so that the coverage of the rhodium metallic surface is equal to 0.2, increases drastically the chemoselectivity for the hydrogenation of the conjugated C=C double bond. If the coverage of the metallic surface by the tin complex reaches a value close to unity, there is a total inversion of the chemoselectivity and the catalyst becomes fully selective for the hydrogenation of the C=O double bond [51. It was therefore interesting to see whether or not the addition of other group IV metal alkyls (namely Ge and Pb) would also produce

148

such drastic effects. If this were the case, one should have a better basis for a general interpretation of the phenomenon. 2 EXPERIMENTAL PART Monometallic startinn catalvst: Rhodium is impregnated on silica (Degussa Aerosil, 200 m2/g) by cationic exchange of the hydroxyl groups of the silica surface by the chloropentammine-rhodiumcomplex lRhCl(NH3)5C12]++in aqueous solution, with NHq+ ions (pH 10) as competitors. After filtration and washing with water, the solid is calcinated in dry air at 573 K and treated under hydrogen at 573 K. The metallic particle size of the catalyst is in the range of 1.0 to 1.5nm as determined by electron microscopy. The dispersion of the catalyst (D=Rhs/Rh, where Rhs are the surface rhodium atoms) measured by hydrogen adsorption [7] is 0.8, a value which is in close agreement with that deduced from the metallic particle size observed by .electron microscopy 181. Bimetallic catalvst DreDaration: After reduction under hydrogen at 573 K, 250 mg of the monometallic starting material (2.5.10'5 mol of Rh) are placed under argon in the reaction vessel (stainless steel autoclave, well stirred by a magnet), with 10 ml of nheptane and a given amount of M'(n-CqH& (so that O<M'/Rhs<2; M=Ge, Sn, Pb). The hydrogen pressure in the autoclave is then raised to 5 MPa and the tempertature is regulated at 373 K. Depending on the expected value of M/Rhs, the reaction time could vary from 30 to 1140 minutes. Blank experiment shows that in this condition, the amount of M'(n-CqH9)4fixed on the support alone is negligeable. Catalytic reaction: The reduction of citral is performed in situ, in the same autoclave. After cooling down the reactor to room temperature and reducing the hydrogen pressure, a solution of 0.9 ml of citral (citral/Rh = 200) and 0.4 ml of tetradecane (internal standard) in 10 ml of n-heptane is introduced in the autoclave containing the catalyst. The hydrogen pressure and the temperature are then raised to respectively 7.6 MPa and 340 K. The kinetics of the reaction is followed with time by analysis of samples of the liquid phase. In these conditions, the catalytic activity could be expressed by the first order rate constant k (h-l.g-11, following equation (1) [51, where m is the mass of catalyst expressed in gram, [Citrallo and [Citrallt are the citral concentration expressed in mol-1 at the begining of the reaction and after the time t (h). Ln I[Citrallt/[Citrall0}= - k*md

(1)

Citral is an a-/3 unsaturated aldehyde which offers three kinds of unsaturations. For the first hydrogenation step, depending on the selectivity, three different products could be obtained geraniol and nerol (ge+ne),citronellal (cal) or 3,7-dimethyl-2-octenal. In our reaction conditions, 3,7-dimethyl-2-octenal is never observed. In a second step, geraniol and citronellal give citronellol which can be reduced into 3,7-dimethyloctanol.

149

cimnellol

For a given reaction time (t), a) the citral conversion (Conv.)is expressed as : (Conv.)t = 1OO.[Citrallt/[Citrall~ b) the yield for geraniol and nerol formation (Yge+ne)tis expressed as: (Yge+ne)t= 100.[Geraniol+Nerollt/[Citrall~ c) the yield for citronella1formation (Ycal)t is expressed as: (Yca1)t = lOO.[Geraniol+Nerol]t/[Citra1]0 Estimation of the number of butvl groups remaining on the bimetallic catalvsts: After reduction under hydrogen at 573 K, 500 mg of the monometallic starting material (1.104 Mole of Rh) are placed under hydrogen (atm pressure) in a "Schlenk" tube with 10 ml of n-heptane and the desired amount of M'(n-CqHg)4 (so that O<M'/Rhscl; M'=Ge, Sn, Pb). The suspension is stired by a magnet. The tempertature of the reactor is raised to 373 K and regulated at 373 K for the desired time. Then the temperature of the "Schlenk' tube is quickly raised to ca. 210 K in a cool bath for one hour. The quantity of M(n-C4H9)4 remaining in n-heptane solution and the quantity of butane evolved are followed by analysis of samples of the liquid phase. In a blank experiment, given amounts of n-butane was introduced (at 298 K) in the "Schlenk" tube with 10 ml of n-heptane and the temperature of the "Schlenk' tube was quickly raised to ca. 210 K in a cool bath for one hour. By analysis of known amounts of the liquid phase, we have checked that the n-butane introduced (at 298 K)into the "Schlenk" tube is fully (>95%)dissolved in the n-heptane. 3. RESULTS

Bimetallic catalvsts ureparation The bimetallic catalysts are obtained by reaction of M(n-CqHg)4 (M=Ge, Sn or Pb) with the reduced Rh/SiO2 catalyst in the liquid phase (n-heptane) under hydrogen pressure (5 MPa) at 373 K. The amount of M' fixed on the Rh/Si02 catalyst

150

depends on the quantity of M'(n-CqHg)4 introduced and on the reaction time (Table 1) Table 1: Amount of M (Ge, Sn or Pb) fixed on the Rh/Si02 catalyst (b), as a function of the reaction time (in braket) and of the amount of M'(n-C4Hg)4 introduced M / ~ (a) s Ge/Rhs (b) 0.1 0.5 1.o 2.0 2.0

0.1 (20) 0.25 (20') 0.30 (20') 0.60 (20') 0.92 (1140')

0.1 (20) 0.42 (20') 0.72 (20') 0.92 (20')

0.1 (20') 0.49 (20') 0.96 (20')

For Ge, Sn and Pb, the maximum amount of M fixed corresponds to a M'/Rhs ratio of ca. 1. This value is obtained after 20 minutes of reaction for M'=Pb and Sn, but 1140 minuts are needed for M'=Ge. These results are not surprising since the MC bond strengh decreases according the following sequence: Ge>Sn>Pb [9]. During the hydrogenolysis reaction of the organometallic, some of the alkyl groups are evolved as butane. The corresponding catalysts could be describedby the average formula: R ~ s [ M ( ~ C & I(M'=Ge, ~ ) ~ ] Sn ~ and Pb; O<x<4;OGe>Pb. Regarding the hydrogenation of the conjugated C=O double bond, the selectivity progressively increases with the M/Rhs ratio (y) to reach values as high as 98% for M'=Sn. The selectivities which are the highest for M'/Rhs =1 vary according to the sequence Sn>Ge>>Pb. Regarding the first order kinetics, if the Rh-Pb systems, for which a continuous decay is observed, are excluded, the Rh-Ge and Rh-Sn systems exhibit similar behaviour: First a strong decrease of activity followed by a continuous increase of activity with y. The minimum of activity occurs for a "y" value close to 0.3. Obviously, the observed trend can be explained by a succession of chemical events occuring at the surface of the particle during the hydrogenolysis of the organometallic fragment: Selective poisonning at low coverage of some surface

151

rhodium atoms which are not very selective for hydrogenation and formation of a new catalytic material at high coverages. The chemical modification of the rhodium surface by the organometallic fragment can be followed by the estimation of the number of butyl groups remaining on the particle as a function of the coverage of the surface rhodium atoms by the organometallic fragment. n c

'9,

c

i

X

W

2

+

4

2

* 0

-

n

1

0

0.8 Ge/Rh,

0.4

'9,

c

k W 2 4

2

0

..

/

100

c

'9, .k W

n

s?

W

2 4

9 Q)

2

F

.\

x

CU

I Y I

0 0

n

0

0.4 0.8 Pb/Rhs Figure: Rate constant (k) of citral hydrogenation and maximum yields toward geraniol and nerol or citronella1 obtained for the three systems Rh~[M(n-C@g)~]y (M'=Ge,Sn and Pb) and for y varying from 0 to 1.

152

Estimation of the number of butvl eroutw remaininv on the bimetallic catalvsts: During the hydrogenolysis of M(n-C4H9)4 on Rhs/Si02, (Phydrogen'l atm, T473 K), n-butane is evolved. A surface organometallic (MI)complex grafted on rhodium is formed which can be formulated by the average formula Rhs[M(n"@@39)xly(O<x<4; O
M'/Rhsa (introduced)

Pb

0.50 1.10 0.35 2.0 0.66 2.0

I,

Sn 11

Ge I,

M'/Rhsa =

t (min)

C4/M

M/RhS

(4

(Y)

30 30 30 30 30 1140

0.0 0.0 0.0 1.3 0.7

0.50 0.73 0.35 0.93 0.25 0.92

1.8

21

For M=Pb, the butyl groups are fully hydrogenolysed whatever the amount of tetra-n-butyl lead introduced. For this metal, the value of x is always egal to zero (for O
Our results must be discussed on the basis of a possible relationship between the structure of the metallic surface modified by the organometallic fragments and the corresponding activities and selectivities. Regarding the structure of the rhodium surface modified by the organometallic compound, it appears that at low coverage there is a complete (Pb and Sn) or almost complete (Ge) hydrogenolysis reaction leading consequently to "naked Pb, Sn, or Ge atoms on the rhodium surface. Due to the necessary low coverage of the rhodium particle to achieve this complete hydrogenolysis, it seems reasonable to assume that this reaction will occur selectively on low coordination sites which are present at the corners or edges of these particles. The fact that a

153

progressive coverage of the rhodium particle (up to ca. 8=0.3) by the "naked germanium, tin or lead atoms result both in a decrease of activity and an increase of selectivity in the reaction of C=C bond hydrogenation can be easily explained by a selective poisonning effect of these low coordination rhodium sites by these Ge, Sn or Pb adatoms. However the effect is not identical from one metal to the other: Tin and germanium have similar effects, whereas lead is giving much less pronounced effects. This could be explained by the fact that lead is more mobile on the particle and can be located at various sites of the particle. In contrast tin and g e r m a n i h would be located in very specific sites of the particle. A completely different situation occurs at higher coverage of the particle by the organometallic fragments. First the supported group IV metal is no longer naked and organometallic fragments are stable on the surface: For Sn and Ge respectively 1.3 and 1.8 butyl groupdgrafted M atom remain on the particle at coverage close to unity. Considering that at low coverage the tin and germanium atoms are naked or almost fully "naked", one can estimate the average formula of the grafted organometallics as RhsSn(nC&)2.3 and RhsGe(n-QH9)2.4.It is this new catalytic material which exhibits the high activity and selectivity for the selective hydrogenation of the conjugated C=O double bond. Interestingly the two systems which have some remaining butyl groups are the most selective ones. One is therefore tempted to ascribe the high selectivity to the presence of the organometallic fragment. There are several possible explanations for such behaviour. In previous works [51 we have ascribed this selectivity to an electronic effect of the grafted organometallic which is formally oxidized. The adsorption and reaction of the citral molecule would occur by its carbonyl group which could coordinate to the tin or germanium atom via the lone pair of electrons on the oxygen atom [10,11]. Interestingly, the electron work function attributed to Ge, Sn and Pb decreases in the following order: Ge (5.0 eVb Sn (4.42 eV) > Pb (4.25eV>[121. There is also a possible steric effect of the organometallic fragments which could prevent the q-2 adsorption of the C=C double bond on the surface. In contrast the q-1 adsorption of the carbonyl function by its oxygen atom is less sterically demanding and can still lead to the hydrogenation to the alcohol. 5. REFERENCES

1 2

3 4

Margitfalvi,J., Hegedus, M., Gobtilos, S., Talas, E. K., Szedlacsek, P., Szabo, S., Nagy, F., "Proc. 8th Int. Cong. Catal., Berlin 1984 vol IV",Dechema, Frankfurtan-Main, 1984; p. 903 Travers, C., Bournonville, J.P., Martino, G., "Proc. 8th Congr. Catal., Berlin 1984 vol. IV. Dechema, Frankfurt-an-Main, 1984; p. 891 El Mansour, A., Candy, J.P., Bournonville, J.P., Ferretti, O.A. and Basset, J.M. Angew. Chem. Int. Ed. 1989, 347. Agnelli, M., Louessard, P.,El Mansour, A., Candy, J.P., Bournonville, J.P. and Basset, J.M., Catalysis Today 1989,h 63

a

5

6 7 8 9 10

Didillon, B., El Mansour, A., Candy, J.P., Bournonville, J.P. and Basset, J.M., Studies in Surface Science and Catalysis, "HeterogeneousCatalysis and Fine chemicals 11". Guinet et al. (Editors), Elsevier (Pub.) 1991, 137 . Didillon, B., Candy, J.P., El Mansour, A., Houtman, C. and Basset, J.M., J. Mol. Catal. 1992,24,43. Candy, J.P., Ferretti, O.A., Mabilon, G., Bournonville, J.P., El Mansour, A., Basset, J.M. and Martino, G., J. Catal. 1988,112.201. Van Hardeveld, R., Hartog, F., Surf.Sc. 1969,a 189. Riviere, P., Riviere-Baudet, M., Sat& J.,Comprehensive Organometallic Chemistry, Wilkinson, G., Stone, F.G.A., Abel, E.W., (Eds.), Oxford, V01.2, 1982, p. 411 Poltarzewski, Z.,Galvagno, S., Pietropaolo, R., Staiti, P., J. Catal., 1986,102, 190

11 12

Galvagno, S., Poltarzewski, Z., Donato, A., Neri, G., Pietropaolo, R., J. Mol. Catal., 1986, 365 Handbook of Chemistry and Physics, 67th Edition, Robert C. Weast, Melvin J. Astle. and William H. Beyer (Editors), CRC Press, Inc., Florida.

M. Guisnet et al. (Editors),Heterogeneous CataJysis and Fine Chemicals 111 0 1993 Elsevier science Publishers B.V. All rights reserved.

155

Hydrogenation of citral in the liquid phase over new bimetallic Ni-M catalysts supported on graphite. J. Courta, J. Jablonskia'l S. Hamar-Thibaultb a Universite Joseph Fourier - LEDSS1, (CNRS, URA332),

BP 53X, 38041 Grenoble Cedex, FRANCE.

-

b INPGrenoble ENSEEG, L.T.P.C.M., (CNRS, URA29),

BP 75, 38402 Saint Martin dHeres Cedex, FRANCE.

Abstract

We prepared Ni-M (M = Al, Cr, Cu, Co and Mo) catalysts supported on graphite, at low temperature, by coreduction of metal salt mixtures (NiX2, MX2) deposited on this support with sodium naphthalene as reducting agent. Quantitative microanalyses performed by STEM/EDX showed that the two metals were evenly distributed over graphite leaflets. The activity and the selectivity of these catalysts in the hydrogenation of citral to citronellal and citronellol have been compared with that of unsupported bimetallic catalysts, with the same atomic composition and prepared by the same procedure. It appeared that the nickel surface area of the supported catalysts was notably higher than that of the unsupported ones, but the support had almost no effect on the catalytic properties. 1. INTRODUCTION

On nickel catalysts the selectivity of the hydrogenation of citral to citronellal and then to citronellol is low but it may be substantially raised by modifications of the catalytic system with chromium or by the use of carriers [l]. We reported previously [2] that over un-supported Nil Oo-xMox catalysts, prepared by coreduction of mixtures of salts with naphthalene sodium as reducting agent, high yields in citronellol are observed in 2-propanol. We have taken advantage of the high reduction potential of the reducing agent to prepare Ni-M (M = Al, Cr, Cu, Co and Mo) catalysts supported on graphite at low temperature. The physicochemical characteristics of these catalysts were determined and the behaviour of these catalysts was investigated in the hydrogenation of citral in terms of rate of hydrogenation and selectivity.

on leave from the lnstitut of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw (Pologne).

156

2. EXPERIMENTAL 2.1. Catalysts preparation

The starting material was P high surface area graphite (LONZA, HSAG 300rn2g-1). It was washed twice with hot 2M hydrochloric acid to eliminate iron and other impurities, then with hot distilled water and dried at 458K for 4 days. An additional thermal treatment under a nitrogen stream was performed at 700K for 8 hrs. For bimetallic Ni-M catalysts, the support was co-impregnated with an aqueous solution of appropriate composition of the two metal salts (Nix2 and MXn) by using the incipient wetness technique. It was then dried at 333K under vacuum, in the presence of molecular sieves. In order to obtain the desired total metal loading the procedure was repeated two or three times. The salts which were used are given in Table 1. All the catalysts were prepared by the procedure described for Ni. Nickel acetate (17mmol) deposited on 9.589 of support was added in portions to a solution of sodium naphthalene (68mmol) in 200ml of dry THF under an argon atmosphere. At the end of the addition, the mixture was heated at its reflux temperature for 4h. The catalyst was then washed three times with THF, seven times with 99.5% ethanol and stored in oxygen free ethanol. 2.2. Reactlon conditions

Hydrogenations in the liquid phase were carried out at 353K in a 250ml static reactor, under constant hydrogen pressure (1.01MPa). The citral concentration in 2propanol was 0.1 95mo1.1-1. The reaction kinetics were followed by gas-liquid chromatographic (GLC) analysis of samples withdrawn from the reaction mixture. For the GLC, a Supelcowax 10 wide-bore capillary column ( 30mx75mm ID, l m m thickness ) was used with helium as the carrier gas at a flow rate of 5mVmm. The column temparature was isothermal at 413K.

2.3. Catalyst characterlzations The total surface areas (SBET)were determined by means of nitrogen adsorption at 77K and the metallic surface area (SNi) by using reactive adsorption of 3methylthiophen in the liquid phase according to a protocol already published [3]. It has been shown previously that the hydroctssulfurization of thiophene on chromium [4] and molybdenum [5] promoted Raney nickel occurs only on the nickel sites.The total metal load expressed in percentage weight and the catalyst compositions expressed as an atomic ratio percentage were determined by chemical analysis and are given in Table 1. Fine scale microstructure and metal dispersion were investigated by scanning transmission electron microscopy (STEM -VG501) equipped with energy dispersive X-ray analysis (EDX) both globally and at point level with a lateral resolution of 2 nm.

157

Some experiments were performed by Auger spectroscopy (Riber) in order to analyse interactions between graphite and deposited metals. Due to the small amounts of Ni and M (= 9% and l?A0),numerous scans were recorded in the energy ranges of interest. 3. RESULTS AND DISCUSSION

3.1. Characterization of the catalysts

The BET surface area values obtained for the catalysts were smaller than those observed for the support. The loss in specific surface area fluctuated between 4% (Ni) and 33% (NiMo12). Both chromium and molybdenum at high concentrations increased the metallic surface area, but the highest promoting effect was observed with aluminium. In the presence of Cu and Co, the metallic Ni surface area was much lower. Table 1 Physicochemical characteristics of Ni-M supported catalysts. Catalysts Salts

Ni NiAc2 Nil2

NiAIg.5 AIL3

NiColO CoAcn

NiCrl2 Cr12

NiCulO CuAc2

NiMog

NiMo6 NiMoip Mo02Br2

Metal load (Wh) 8.7 10.4 9.2 7.5 9.1 10.4 8.8 Composition WNi at%(a) 3.6 10.9 11.2 11.0 2.8 5.7 3 10 14 18 STEMlEDX Auaer 20 8 20 Surface rn2g-1 288 260 285 249 265 240 212 BET(b) 39 104 32 72 26 47 41 metallic (c) (a) obtained by chemical analysis. (b) per gram of catalyst (c) per gram of metallic element L stands for CH3CHOHC02-

9.7 12.9 15 15 207 68

The catalysts are named according to the metal salt composition of the aqueous solution used to impregnate the support. Thus, for exemple, NiColo was prepared from a mixture containing 90% nickel acetate and 10% cobalt acetate. The atomic percentage which was determined by chemical analysis (Table 1) of the catalyst : 10.9% is in good agreement with the initial salt ratio : 11.1%. The same observation stands for NiCulo. But with metals of higher oxidisability, in particular with aluminium, the atomic ratio percentage is much lower that the initial salt ratio. One could assume that aluminium is leached out by the alcoholate ion which results from the large excess of sodium naphthalene i.e; the alcoholate ion would act in a way similar to that of the hydroxide ion in the preparation of Raney nickel. The very high value measured for the nickel surface area might support this assumption. From transmission electron microscopy pictures, it appeared that the graphite leaflets were identical in the starting material and in the supported catalysts and that

158

the samples did not contain any other particles. The electron diffraction patterns were not modified by the metallic elements, i.e. we only observed the graphite structure.

Figure 1. Transmission Electron Image and STEMIEDX analysis of NiColo

159

Quantitative microanalyses performed by STEM/EDX with a resolution of 2 nm showed that the metals were deposited on the graphite leaflets. In the NiCrl2 catalyst, the Cr/Ni atomic percentage varied between 12% and 16%, therefore the two metals were evenly distributed over graphite. The same observation stands for NiM0.12 catalyst, but in the NiColo and NiCulo catalysts, the distribution was less homogeneous and higher fluctuations of the atomic percentage were observed (18 f 6 at% in NiCulo). In the Auger spectra of the supported catalysts, the carbon peak at 272 eV, with the characteristic shape of graphite did not present any fine structure at lower energy, which would indicate a strong interaction between nickel and graphite as shown previously during deposition of nickel on graphite by CVD [6]. The Ni peaks in the high energy range ( LMM : 714, 773 and 848 eV), as well as in low energy range (MNN : 60 eV) were well in evidence in each catalyst and the Ni/C ratios were in good agreement with those measured by chemical analysis. In spite of the low M/Ni atomic percentage, the Auger peaks of M were well in evidence at 925 eV for Cu, 770 eV for Co and 181 eV for Mo. However, in the NiCrl2 catalyst, a small amount of oxygen gave a peak at 512 eV which overlapped with the chromium peaks at 489 and 527 eV. One can see that the cleaning of the catalysts was very efficient since only a small amount of Na was detected at 997 eV.

3.2. Catalytic properties Reaction Scheme 3,7-dimethyl-2,6 octadienal (citral, A) has three sites of hydrogenation: the conjugated double bond, the carbonyl group and the isolated double bond. The hydrogenation of the conjugated double bond gives 3,7-dimethyl-6 octenal (citronellal, B) and the hydrogenation of the carbonyl group gives cis and trans 3,7 dimethyl 2,6 octadien-1-01(E). Compounds B and E undergo further hydrogenation to 3,7 dimethyl 6 octen-1-01 (C), the totally hydrogenated product (D) resulting from the hydrogenation of either C or 3,7 dimethyloctanal (F). However, for the series of supported catalysts the major portion of the hydrogenated product results from the sequence:

nerd and germid (E) \

/

citral (A)

citronellal (B) citronellol (C) \ 3,7 dimethyl,ocfan-a/ (F)

3.7-dimethyl octan 1-01 (D)

--+.

Activity of the catalysts From the initial hydrogenation rates (10) expressed in mmo1.s-1 per gram of metal (first row in Table 2), it appears that: cobalt and copper decrease the activity whereas chromium, aluminium and especially molybdenum have a large promoting

160

effect. The low activity of NiCulo alloy, could be related to the large atomic ratio Cu/Ni observed by Auger spectroscopy. Also Included in Table 2, are the initial *hydrogenation rates ,( in mmo1.s-1 per square metre of nickel surface area ) together with the results abtained with unsupported bimetallic Ni-M catalysts which have been prepared with the same procedure [7]. It appears that whereas the nickel surface areas of the supported catalysts are two times (Ni), three times (NiA19.5) and seven times (NiCrl2) higher than that of the unsupported catalysts, the hydrogenation rat8s are the same for both series of catalysts. But Ni-Mo catalyts have a different bahaviour, in the supported ones, molybdenum has a large promoting eYfect, which is not observed in the unsupported ones. Molybdenum and chromium are the onty ones among the metals we added to nickel which increase its acliwlty. Selectivity The hydrogenation of citral has been reviewed recently [l]. Under our experimental conditions hydrogenation of citral to citronellol proceeded stepwise with citronellal as an intermediate as illustrated in Fig.3. The maximum yield in citronellal was obtained when 1 rnol of hydrogen was consumed .per mol of citral, i.e: 33 percent, that in citronellol when two moles of hydrogen were consumed.

-_

I

citral citronella1

-

0

- d.octanal

+citronellol -

0

20

40

60

80

. A- . . d.octano1

100

Figure 3. Hydrogenation of citral on NiCr12: product composition as a function of the percent of hydrogen consumed.

161

The citronellal and citronellol selectivities are not modified by the support, with one exception, the yield in citronellol in the presence of molybdenum catalysts. This yield increases fram 85% to 95% over the supported catalysts, whereas it was constant and equal' tb 96% over the three unsupported catalysts with the same compositions [2]. Over the Ni catalyst the citronellal yield (Table 2) was lower than that reported (99.5%) over Ni/A1203 [a], but the citronellol yield was higher than that observed (78.5%) over Ni-CraOs [9] in the same solvent without additive. The addition of sodium carbonate, increased the citronellol yield [lo]. Table 2: Catalytic properties of the different catalysts.

M

WiAlg.5

NiColO

NiCrl2

NiCulO

NiMog

NiMog

NiMol2

1.1

2.6 25 (271

0.8 26

3.6

0.6 24

4.3 92 (25)

3.6 88 (28)

5.6

50 (43)

93.0 94.2 Citronellol W.5 81.0 ( ) non supportedcatalysts.

93.0 82.6

94.0 82.2

90.3 87.0

Catalysts Activity (ro) mrno1.s-lg-1 mmo1.s-1m-2 x 103

27 (25)

Selectivity yield'/'

Citronella1

89.0

86.0

89.2 90.5

83 (37)

89.0 95.0

The AVNi ratio percentage in NiA19.5 was choosen to allow comparison of the supported nickel duminium catalyst with Raney nickel. The actual AVNi ratio is smaller than that expected, however the citronellal yield is higher than that we observed with Raney nickel prepared from a commercial alloy (84%), but the citronellol yield is smaller: 81Yo instead of 86%. A process has been patented [i 11 for the production of citronellol substantially free from 3,7-dimethyl-octan-l-o1 by hydrogenation of geraniol in a mixture containing citronellol in the presence of Raney colbalt. We did not observe such an effect, over NiColo, the hydrogenation of the double bond in the 6 position of citronellol proceeds more readily than over Ni since the citronellol yield is equal to 82.6%. In the presence of chromium promoted Raney nickel, citral can be hydrogenated to citronellol in yields in excess of 95% [12]. NiCrl2 is amongst the most selective catalysts we prepared in the hydrogenation of citral to citronellal but the citronellol selectivity is low. However, chromium promoted Raney nickel has to be used in methanol in the presence of KOH in order to be selective [13], we did not carry out the reaction under these conditlons. Over NiMox, the citronellol selectivity increased with the molybdenum atomic composition, a 95K citronellol yield is observed with NiM012 which is close to that reported in the presence of unsupported Niloo-xMox catalysts prepared with the same process [l]. We made no attempt to optimize this result and higher yields could probably be obtained.

162

4. CONCLUSION

Metal salt mixtures (NiX2, MXn) deposited on graphite can be reduced with naphthalene sodium into bimetallic supported catalysts. The process allows one to prepare catalysts with metals as highly reducible as aluminium at low temperature. It is important to notice that the nickel surface area of supported Ni-M catalysts is always larger ( up to seven times ) than that of the unsupported ones prepared by the same procedure. Quantitative microanalysis performed by STEM/EDX showed that the two metals are evenly distributed over graphite leaflets. Graphite as support does not modify the selectivities of the catalysts we prepared; nickel chromium catalysts have to be chosen in order to obtain citronella1 whereas nickel molybdenum catalysts have to be preferred for the production of citronellol.

Acknowledgements

We are indebted to LONZA LTD Inorganic Chemicals C.H. 5643 Sins for a gift of the support. 5. REFERENCES 1 L. Cerveny and V. Ruzicka, Seifen, ole, Fette, Wachse, 114 (1988) 605-609. 2 J. Court, F. Janati-ldrissi and S. Vidal,

"Heterogeneous Catalysts and Fine Chemicals II" M. Guisnet et al (edit), Elsevier Science, Amsterdam, 1991 pp 193-200. 3 S. Sane, J.M. Bonnier, J.P. Damon and J. Masson, Appl. Cat., 9 (1984) 69-83. 4 J.M. Bonnier, J.P. Damon, B. Delmon, D. Doumain and J. Masson, J. Chim. Phys. 84 (1987) 889-895. 5 S. Hamar-Thibault, J. Masson, J. Chimie Physique 88 (1991) 219-232. 6 J.P. Coad and J.P. Riviere, surf. Sci., 25 (1971) 609-619. 7 F. Janati-ldrissi, Thesis University Joseph Fourier, Grenoble - 1992. 8 D.V. Sokol'skii, A.M. Pak, M.A. Ginsburg and V.A. Zavorin, Kinet. Katal., 20 (1979) 645-650. 9 D.V. Sokol'skii, A.M. Pak and S.R.Konuspaev, Kinet. Katal., 20 (1979) 884-890. 10 A.M. Pak, S.R. Konuspaev, G.D. Zakumbaeva and D.V. Sokol'skii, React. Kinet. Catal. Lett., 16 (1981) 339-342. 11 E. Goldstein, U.S. Pat. 3275696 (sept. 27, 1966). 12 R.S.De Simone and P.S. Gradeff, US. Pat. 4029709 (sept. 14, 1977). 13 P.S. Gradeff and G. Formica, Tetrahedron Letters, 51 (1971) 4681-4684.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals IIZ 0 1993 Elsevier Science Publishers B.V. All rights reserved.

163

Hydro enation of cinnamaldehyde and citral over Ru suppo ed catalysts

A

S. Galvagnw, C. Milonea, G. Nerib, A. Donatob, R. Pietropaolob a Universita di Messina, Dipartimento di Chimica Industriale, Sant'Agata di Messina, 98166 Messina, Italy

buniversita di Reggio Calabria, Facolta di Ingegneria, via E. Cuzzocrea 48, 89100 Reggio Calabria, Italy

Abstract Citral and cinnamaldehyde have been hydrogenated over a series of Ru catalysts supported on carbon and alumina. In the hydrogenation of citral selectivity to unsaturated alcohols remains constant regardless of the Ru particle size. In the hydrogenation of cinnamaldehyde, cinnamyl alcohol is instead preferentially formed on the larger Ru particles. Addition of tin to ruthenium decreases the catalytic activity but increases (up to 90%) the selectivity to unsaturated alcohols. It is suggested that the preferential hydrogenation of the C=O group occurs on sites associated with tin ions. The influence of the nature of the support and salt precursors is also discussed.

1. INTRODUCTION The selective hydrogenation of unsaturated aldehydes to unsaturated alcohols is an important reaction for the production of f i e chemicals. Even though the selective formation of unsaturated alcohols has been viewed as a difficult problem, recent investigations have shown that the selectivity of a metal catalyst can be very much improved by: a) varying the metal particle size [1,21, b) adding promoters [3,41, c) changing the catalyst support 151. Most of the work on the selective hydrogenation of unsaturated aldehydes has been carried out on cinnamaldehyde and few other studies are available on aliphatic aldehydes which can be of potential industrial interest. Within the class of aliphatic unsaturated aldehydes, hydrogenation of citral is important for its interest in the perfumery industry and from a scientific point of view since it offers three unsaturations: an aldehyde group, a conjugated double bond and an isolated double bond. In this communication we report the results obtained on a series of Ru samples having a different Ru dispersion and promoted by addition of SnCl2, in order to verify: i) the possibility of reducing selectively the C=O group without

164

hydrogenating the C=C double bonds; ii) the effect (under the same experimental conditions) of support, preparation methods, metal particle size and presence of promoters on the hydrogenation of two different unsaturated aldehydes, namely cinnamaldehyde and citral.

2.EXPERIMENTAL Monometallic Ru samples were prepared by incipient wetness impregnation of activated carbon (CHEMVIRON SCXII 80-100 mesh, surface area 900-1100 mVg) and alumina (Rhone-Poulenc GFC 200C, surface area 200 m2/g) with aqueous solutions of RuCl3 having the appropriate concentration of metals. After impregnation, the catalysts were dried a t 120 "C and reduced at 350 "C under flowing hydrogen. A R d N 2 0 3 sample was also prepared by using ruthenium acetylacetonate (Ru(acac)~)in toluene solution as precursor. Ru(acac)3 was decomposed at 250 "C under nitrogen flow and reduced to metallic ruthenium at 350 "C in flowing H2. Monometallic Ru samples were prepared with a Ru content ranging from 0.5 to 10 wt.% (tab. 1). Table 1 Composition and characterization of monometallic Ru catalysts catalysts

A B C D E

F

G

Ru (wt.%)

support

precursor

comu

0.5 1 2 5 10 1 4

carbon

RuC13

0.34 0.45 0.26 0.19 0.13 0.20 0.26

I1

11

a,

$8

It

!I

(9

11

A1203 9,

II

Ru(acac)~

Bimetallic Ru-SdC catalysts were prepared by co-impregnation of carbon with aqueous solutions of RuCl3 and SnClz. The amount of Ru was kept constant at 2 wt.% whereas Sn content was varied between 0 and 2 wt.%. Chemisorption of CO was measured in a conventional pulse system at room temperature. Details on the characterization results are reported elsewhere [2,6,71. Hydrogenations of citral (Z and E) and cinnamaldehyde were carried out in a 100 ml four-necked flask fitted with a reflux condenser, dropping funnel, thermocouple and a stirrer head. The catalyst was added to the required amount of solvent (95% ethanol) then treated at 70 "C for 1 h under H2 flow. After cooling at reaction temperature (60"C), the organic substrate (0.5ml) was injected through one arm of the flask. The progress of the reaction was followed

165

analyzing by GC a sufficient number of microsamples withdrawn from the reaction mixture. Preliminary experiments carried out with different amount of catalyst, stirring rate and catalyst grain size showed the absence of diffusional limitations.

3. RESULTS AND DISCUSSION

3.1. Monometallic Ru catalysts In the hydrogenation of aJ-unsaturated aldehydes over monometallic R d C catalysts several products were formed and the selectivity to unsaturated alcohols was generally low. In the hydrogenation of citral (mixture of Z and E) the isomers geraniol and nerol were obtained with a selectivity of about 35%. The other reaction products were citronellal and citronellol (scheme 1). Isopulegol, a cyclic isomer of citronellal, was also obtained. Products formed by hydrogenation of the isolated double bond were never detected. Selectivity to geraniol + nerol was found constant on all samples regardless of the Ru loading.

po

H2

Scheme 1. In the hydrogenation of cinnamaldehyde over the same R d C catalysts, selectivity to cinnamyl alcohol changed with Ru content. The influence of Ru loading on the selectivity to cinnamyl alcohol is reported in Fig. 1. The highest selectivity to cinnamyl alcohol was observed on the 10 wt.% Ru sample. On the R d C samples an increase of the Ru loading results in the formation of larger Ru metal particles. This is supported by the chemisorption data, which show a decrease of the CO/Ru ratio (table l), and by a TEM analysis [21 which has shown that the average Ru particle size increases from 3 nm on the 0.5 wt.%

166

Ru/C up to 17 nm on the 10 wt.% RdC. A higher selectivity to cinnamyl alcohol on the larger metal particles has been previously reported on Pt catalysts [ll. Selectivity values reported in fig. 1were determined at a constant conversion of about 40%. It should be however noted that within a large range of conversion (10430%) selectivity to cinnamyl alcohol did not change significantly. This is in agreement with a reaction mechanism similar to that reported in scheme 1 in which unsaturated alcohols are produced through a parallel reaction. The same effect of conversion on the selectivity to geraniol t nerol was observed in the hydrogenation of citral. It should be also noted that, on both reactions investigated, increasing the metal content the overall catalytic activity (per gram of Ru) decreased. A linear relationship between catalytic activity and metal surface area was found indicating that the specific rate of hydrogenation (per atom of Ru on the surface) is independent of the Ru particle size. The different influence of Ru particle size on the products selectivity in the hydrogenation of citral and cinnamaldehyde can be related to a steric effect. To explain the results obtained in the hydrogenation of cinnamaldehyde it has been, in fact, suggested [l]that on the larger metal particles the aromatic ring, which is not bonded to the surface, lies to a distance exceeding 0.3 nm [81. Under these conditions the unsaturated aldehyde is tilted and the adsorption of the carbonyl group is favorite facilitating the hydrogenation of the C=O bond. In the case of citral, where no aromatic ring is present, a steric effect can play only a minor role. No difference in the products distribution is in fact observed on the samples with different metal particle size.

sel,to cinnamyl alcohol (96)

701

40

t

10 2o 01

0

2

4

6

8

10

11

Ru (wt%) Fig. 1. Selectivity to cinnamyl alcohol as a function of ruthenium loading.

167

Hydrogenation of citral over RdAl2O3 showed a selectivity to unsaturated alcohols lower than that obtained on RdC. Moreover the products distribution was dependent on the Ru precursor used. On the Ru/&o3 sample prepared from RuC13 (sample 'F)selectivity to unsaturated alcohols was about 15% (Conv. 5 50%) and the main reaction product was the acetal of citronellal (S= 50%)obtained by reaction of the aldehyde with ethanol used as solvent. When RdAl2O3 prepared from acetylacetonate (sample ' G )is used (Fig.21, selectivity to unsaturated alcohols remains low (15%) whereas citronellal become the main reaction product and the acetal is obtained in a negligible amount (< 1%).At higher conversions, 3,7-dimethyloctanol is formed by hydrogenation of citronellol. The preferential formation of citronellal on sample 'G and of acetal on sample 'F' is likely to be related to the higher acidity of the catalyst prepared from RuC13. It is in fact known that synthesis of acetals is catalyzed by an acidic medium. On the basis of the results obtained on RdC, the lower selectivity to geraniol t nerol observed on samples ' F and 'G, with respect to the carbon supported catalysts, cannot be ascribed to a different metal particle size. Therefore a support effect which modifies the catalytic properties of Ru and/or the presence of impurities are likely to be responsible for the observed behaviour. Within this contest, it should be noted that the carbon used as support contains about 1000 ppm of Fe whereas the amount of Fe on the alumina support is < 200 ppm. The beneficial effect of adding Fe to promote C=O bond hydrogenation is well known [9,10].

10

o

50

im

150 200 250

so

9 ~ )

time (min) Fig. 2. Hydrogenation of citral over Ru/d203 prepared from acetylacetonate (sample 'GI. * , citral; D , citronellal; v , geraniol+nerol; o ,citronellol; , 3,7dimethylodanol.

168

3.2.Bimetallic Ru-Sn catalysta Addition of tin has been found to modify strongly the catalytic performance of RdC. Fig. 3 shows the influence of the Sn/(Sn+Ru) atomic percent (Sn%) on the catalytic activity. Increasing the Sn% the initial rate of hydrogenation of citral and cinnamaldehyde decrease. Samples with a Sn% higher than 40 were found inactive. On all samples hydrogenation of citral was slightly faster than the reduction of cinnamaldehyde.

Fig. 3. Hydrogenation of cinnamaldehyde ( o and citral content on the initial rates.

(A

1: influence of the Sn

Characterization of the Ru-Sn/C catalysts by CO chemisorption (Table 2) has shown that addition of tin decreases the amount of Ru atoms on the surface. The CO/Ru ratio which was about 0.26 on the monometallic RdC, decreased down to a negligible value on the samples having a Sn% 240. A detailed characterization of the bimetallic samples by TEM [71 has ruled out the possibility that the lower CO/Ru ratio and the lower catalytic activity observed on the Ru-Sn samples is due to the formation of larger Ru particles. The electron mi~oscopyanalysis has in fact shown that, at low tin loadings, metal particles are even smaller than those measured on the monometallic R d C sample. Only at the highest Sn% large aggregates made of clusters of small crystallites are observed. These results suggest that one effect of tin is that of poisoning the Ru active sites thus decreasing CO chemisorption and catalytic activity. From the chemisorption results and assuming a chemisorption stoichiometry CO/Ru = 1, the initial turnover rates (per atom of Ru on the surface), towards the hydrogenation of citral (Ncitr) and cinnamaldehyde (Nchn) have been calculated and reported in Table 2. It can be noted that on both

169

substrates the turnover rates increase with increasing the Sn content. A larger increase was observed in the hydrogenation of citral. Table 2 Bimetallic Ru-Sn/C samples. Characterization and initial turnover rates towards citral (Ncitr) and cinnamaldehyde (Ncim) hydrogenation. Ru= 2 wt.%. Sn4Voa 0 5 10 20 30 40

comu

Ncitrb 2.84 2.48 2.76 5.09 22.55

0.261 0.220 0.115 0.096 0.003

NCinllb 2.03 1.63 2.18 2.48 12.12

-

C

-

a lOo*Sn/(Sn+Ru)atomic ratios b [molecules Ru(s)-1 8-11 * l@ C not detectable

Addition of tin modifies also the products distribution. Increasing the Sn%, selectivity to unsaturated alcohols increases up to 90% (fig. 41, with a corresponding decrease in the selectivity to products obtained by hydrogenation of the conjugated C=C double bond. The changes observed in the turnover rates and selectivity indicates a promoting effect of Sn in the hydrogenation of the carbonyl bond.

sel. to unwhuki alcohols (96)

1001

2ot 0' 0

10

20

90

40

I

I 50

100 sn/(sn+Ru) Fig. 4.Influence of the Sn/Ru ratio on the selectivity to cinnamyl alcohol ( a ) and geranioltnerol (A)

170

This is in agreement with previous investigations on the hydrogenation of monofunctional molecules containing C=C and C=O bonds over Ru-Sn catalyst which have shown a positive effect of tin in the reduction of saturated aldehydes and no variations in the turnover rate in the hydrogenation of olefins 161. The reported results show that tin plays different roles in modifying the performance of the Ru/C catalysts. On increasing the Sn content the number of Ru surface atom decreases. However the increase in the specific activity of hydrogenation of the C=O group indicates that new and more active sites are formed. We have previously suggested that preferential hydrogenation of C=O occurs on sites associated with ionic tin. On these sites the carbonyl group is polarized, facilitating the hydrogen transfer from an adjacent Ru-H site [2,61. This is also in agreement with a microstructural characterization [71 which has shown that the Ru-SdC catalysts are made mainly of metallic Ru particles and ionic tin. No Sn" particles have been found. The absence of Sn" has been confirmed by a Mossbauer analysis carried out on samples which have not been exposed to air. The parallel trend observed on both citral and cinnamaldehyde in the variations of activity and selectivity with addition of tin support the conclusion that the same mechanism operates for both substrates. 4.ACKNOWLEDGMENT This work has been carried out with the financial support of CNR (Progetto Finalizzato Chimica Fine e Secondaria 11).

5. REFERENCES 1 D. Richard, P. Fouilloux and P. Gallezot, in M. J. Phillips and M. Ternan (eds.), Proc. 9th Int. Cong. Catalysis, Calgary, 1988, Chem. Institute of Canada, Ottawa, 1988, p. 1074. 2 S. Galvagno, G. Capannelli, G. Neri, A. Donato and R. Pietropaolo, J. Mol. Catal., 64 (1991) 237. 3 Z. Poltarzewski, S. Galvagno, R. Pietropaolo and P. Staiti, J. Catal., 102 (1986) 190. 4 D. G. Blackmond, R. Oukaci, B. Blanc and P. Gallezot, J. Catal., 131 (1991) 401. 5 B.Sen and M.A. Vannice, J. Catal., 115 (1989) 65. 6 S. Galvagno, A. Donato, G. Neri and R. Pietropaolo, Catal. Lett., 8 (1991) 9. 7 G. Neri, J. Schwank, S. Galvagno and R. Pietropaolo, J. Catal. (submitted). 8 E. L. Garfunkel, C. Minot, A. Gavezzotti and M. Simonetta, Surf. Sci., 102 (1986) 190. 9 S. Galvagno, A. Donato, G. Neri, R. Pietropaolo and D.Pietropaolo, J. Mol. Catal., 49 (1989) 223. 10 D. Goupil, P. Fouilloux and R. Maurel, React. Kinet. Catal. Lett., 35 (1987) 223.

M.Guisnet et al. (Editors),Heterogeneous oltulysis and Fine Chemicals IU 18 1993 Elsevier Science Publishers B.V. All rights reserved.

171

SELECTIVE HYDROGENATION OF CARVONE ON Pt AND Pt-Au CATALYSTS G. DEL ANGEL., R. MELENDREZ. P. W E C O T E I and J. BARBIER=

a) b) c)

V. BERTIN.

I

J.M.

DOMINGUEZb

Universidad Autonoma Metropolitana Iztapalapa, Departamento de Quimica Apdo. Postal 55-534, Mexico 09340, D.F., Mexico Instituto Mexican0 del Petroleo Eje Central Lazaro Cardenas 152, 07730 Mexico 14, D.F., Mexico Laboratoire de Catalyse en Chimie Organique, URA CNRS 350, Universit6 de Poitiers, 40, Avenue du Recteur Pineau 86022 - POITIERS Cedex France.

Abstract

The initial selectivity of platinum catalysts in carvone hydrogenation depends on the platinum dispersion. For example carvotanacetone (2-methyl-5-isopropylcyclo 2ene-1-one) is the major product on catalysts with large platinum particles, whereas carvomenthone (2-methyl-5isopropylcyclohexanone) is the major product on highly dispersed platinum catalysts. The previous platinum catalysts were modified by gold addition using a surface Redox reaction : Au3+ ions were reduced by hydrogen preadsorbed on the Pt surface area. The addition of gold to platinum on the highly dispersed catalyst modifies the initial selectivity toward that obtained with large platinum particles. In agreement with comparable electronic affinities of Pt and Au and comparable selectivities in 0-xylene hydrogenation on Pt and Pt-Au catalysts, such change in selectivity in carvone hydrogenation cannot be explained by a ligand effect. On the other hand, results can be explained by assuming that gold adds selectively on the corners or the edges of the platinum particles. X-ray emission analysis of platinum-gold catalysts bears out that gold decorates the platinum particles rims pointing out that low coordination platinum atoms which are selectively deactivated by gold deposition, are responsible for the direct transformation of carvone into carvomenthone.

172

INTRODUCTION

One of the biggest challenges in the development of the fine chemistry, is the selective reduction of polyfunctional organic molecules. Heterogeneous catalysis is playing an important role in this kind of reactions (2). The use of solid catalysts presents the advantage of handling and separation properties. Applications of supported metallic catalysts in selective hydrogenation have been made and the results are motivating (3-6). Therefore, the selectivity pattern can be modified by several means : size of the metallic particles, interaction between metal particles or precursors, support and alloying procedures ( 7 - 8 ) . So, optimization of the solid catalysts requires detailed knowledge of the reaction modifier. The carvone molecule is known in organic chemistry ( 9 1 , the interest to study the selective hydrogenation of this molecule is due to three possible sites in which the reduction can occur. In a previous study on Rh catalysts supported on SiO,, TiO, and MgO, an effect of the precursor and the support was observed on the activity and selectivity (10). desorbed

Carvone

Carvotanacetone

Ca rvoment hone

Ca rvoment hol

The aim of this work is to study the effect of Pt dispersion and the addition of a second metal (Au) in the selective carvone hydrogenation. EXPERIMENTAL The precursor catalyst (All was the European Catalyst Pt/Si02 6 %, whose preparation was described by Bond et Wells (11). This catalyst was preparared by exchange of Pt (NH,), C1, and Pt (NH,), (OH), with the silica support (sorbosil grade AQ U30 Silica Gel, 185 m2/g). The catalyst was dried at 105OC and reduced at 4 0 0 ° C in a flow of hydrogen for 0.5h, after this, the catalyst was calcined in air flow at 300°C for 2h and then reduced by H2 at 500°C during 2h. Part of that parent catalyst has been sintered under O2 diluted with N2 (1% O,/N,) at 575°C for 10h or 800°C f o r 3h giving catalysts B1 and C1 after reduction at 500°C.

173

Sold h t a ~deposited by the refilling method (12,131, which consists in a Redox surface reaction between chemisorbed hydrogen on platinum and the cation of the second metal (Au) according to the following scheme : Pt-H

+

AUC3>- .

Pt Pt

Some quantity of solution HAuC1, (twice the required for this reaction) was added to the pattern catalyst. After reaction, the catalyst thoroughly water-washing was dried in an oven at 120°C all night. The bimetallic catalyst was activated whether by direct reduction under hydrogen at 500°C for 2h (catalysts Alb, Blb, Clb) or calcined under air at 300°C for 2h, and reduced at 500°C (catalysts Ala, Bla, Cla). The metal accesibiAAty of the catalysts was determined by gas chemisorption (hydrogen, oxygen) at room temperature in a conventional volumetric equipment. The widely admitted adsorption stoichiometries are H/Pt = O/Pt = 1. No chemisorption of hydrogen and oxygen on gold was observed at this conditions (14). The carvone hydrogenation reaction, in n.hexane solutfon (lOOml), was carried out in a high pressure Parr reactor at 100°C (10). The total hydrogen pressure was 20 atm. The reaction products were analysed by gas chromatography. The detected products were carvotanacetone, carvomenthone and carvomenthol. Hydrogenation of orxylene was carried out in a conventional flow reactor at conversions less than 10 %. The reactant was o-xylene from Phillips (99.99 % ) . Hydrogen (756.3 Torr) was saturated with vapors of o-xylene at 14°C (3.7 Torr) and passed through the reactor. The products of the reaction were analyzed in a gas chromatograph coupled to the reactant system, the only products detected were 1,2 cis and trans dimethylcyclohexane. Activity per site (TOF) and selectivity were calculated for t=O. RESULTS

The dispersion and particle size values of catalysts are reported in Table 1. The different treatments carried out on Pt/SiO, allowed to obtain catalysts with variable dispersions (B1 and Cl). For the bimetallic Pt-Au catalysts the amount of gold deposited is near to the theoretical amount, taking into account the accessibility of the monometallic Pt catalyst (the number of Au atoms deposited is 1/3 of the number of accessible Pt atoms).

174

Table 1

:

Characteristics of the Pt/SiO= and Pt-AujSiO, catalysts.

AccesCatalyst Metal(wt%) Pretreatment("C1 Theoreticalsibility Au calcination Reduction H/T(%) Pt A1

B1 C1 Ala Bla Cla Alb Blb Clb

-

6 6 6 6 6

6 6 6

6

-

300 575 800

0.80 0.55 0.21 0.86

300 300 300

0.50 0.21

-

500 500 500 500 500 500 500 500 500

-

0.97 0.58 0.14 0.97 0.58 0.14

48 29 7 32 24 5 35 24 5

Hydrogenation of carvone

Activities per site for the carvone hydrogenation on the various monometallic catalysts are shown in Table 2 . It can be observed a particle size effect : large particles ( C 1 ) are more active than smaller ones (All. This particle size effect is also observed in the selectivity pattern. Partial hydrogenation (ex0 double bond) is the main reaction on large particles, the main product being carvotanacetone. On the other hand, small particles are more selective towards carvomenthone, the two double bonds (exo, endo) are hydrogenated. Table 2

Catalyst

:

Activity (TOF) and Selectivity on Pt/SiO, catalysts for carvone Hydrogenation TOF min-'

Selectivity

Carvotanacetone Carvomenthone Carvomenthol A1 B1 c1

97

31

42

52 48

208

69 48 52

-

-

Activities and selectivities of bimetallic catalysts for hydrogenation of carvone are reported in Table 3. The results show a decrease of the specific activity of bimetallic catalysts compared to that of monometallic ones. Gold addition also modifies the selectivity patterns. The partial hydrogenation of the exo double bond is increased on both large and small particles : on these catalysts carvotanacetone is the main product. The activation mode (calcined-reduced or direct reduction) does not modify the catalytic properties (activity and selectivity are comparable) of the bimetallic catalysts.

175

Table 3 : Activity and Selectivity of Pt-Au/Si02 catalysts for carvone hydrogenation Selectivity

Catalyst TOF min-l

Carvotanacetone Carvomenthone Carvornenthol Ala Alb Bla Blb Cla Clb

-

35 15 25 20

-

6

5a 53 53

42 46 46

49

50

is

44

64

35

Hydrogenation of 0-xylene

-

:

It was previously shown (15) that the selectivity for xylene hydrogenation (cis and trans dimethylcyclohexane) sensitive to electronic density modification. In order estimate electronic transfer on Pt catalysts induced by addition, Table 4 shows that it was not observed any change selectivity for the 0-xylene reaction on Pt-Au catalysts. Table 4

:

Selectivity cis

-

is to Au in

Selectivity for o-xylene hydrogenation on Pt/Sio, and Pt-AujSiO,

Catalyst

A1 Ala Alb B1 Bla Blb c1 Cla Clb

0-

Trans

11

23

72

28 22 23 23 22 24 23 22

ia 77

I7 18 76

71 ia

Compositional analysis of Pt-Au particles :

All the microanalysis work was done in a Jeol-100 CX electron microscope fitted with a STEM unit, a X-ray detector and a Tracor-Northern 5500 console.The compositional microanalysis was carried out by energy dispersive spectrometry on bulky Adams Pt particles modified by Au deposition. Table 5 gives the mean composition of Pt particles. It appears that Au is preferentially deposited on particle rims (edges and corners) than on flat planes.

176

Table 5 : Local E . D . S . analysis of Pt and Au on Adams Pt particles modified by A u addition.

FACES

Pt

Au

99.8

0.2

% Atom. ~~

RIMS 'k A t o m

98.5

1.5

DISCUSSION

According to the reduction reaction, one atom of Au'~ is deposited on 3 accessible Pt atoms, therefore a diminution of l/3 in the platinum active surface must occur. The results of elementary analysis and hydrogen chemisorption of bimetallic catalysts are in agreement with this hypothesis. Whatever the dispersion of the catalyst, the number of gold atoms deposited is nearly 1/3 the number of Pt surface atoms. Gold is selectively deposited over platinum because the HAuC1, solution does not exchanged with the silica support and no miscibility is expected in the region of the metal contents. These results suggest that the reduction reaction was almost total and without any notable formation of isolated Au particles. The higher activity observed in large Pt particles goes along with previous results reported in the literature in the hydrogenation of unsaturated hydrocarbons. According to this observation the active sites in small particles, composed of atoms with low coordination number, are irreversibly deactivated by a strong adsorption of hydrocarbon molecules (16-19). Large particles show higher selectivity towards the carvotanacetone product. This result suggests that this molecule desorbs easily from dense planes. On the contrary an small particles, it seems that the carvotanacetone-platinum bond is stronger leading to the reduction of the two double bonds. For Pt-Au bimetallic catalysts the selectivity pattern shows that the addition of gold to the small particles (All leads to a large formation of carvotanacetone. This means that their behaviour is similar to the one of the large particles. Such change in selectivity could be explained by assuming that gold, by a ligand effect, could change the electronic properties of platinum. The selectivity of o-xylene hydrogenation reaction (cis and trans dimethylcyclohexane) has shown to be sensitive to electronic density modifications (15): it has been proposed that transformation requires a long

177

residence time of the olefin precursor on the surface. An olefin will be more strongly bonded on an electron deficient site (15). If the addition of Au produces an electronic structure modification a change in stereoselectivity must be observed. It has not been observed any change in selectivity for the oxylene reaction on Pt-Au (Table 41, cis products en bimetallic catalysts are comparable to the monometallic catalysts. This result is in good agreement with the electronic affinities of Pt and Au which are comparable (2.1 eV and 2.3 eV respectively). Therefore, it is difficult to expect a ligand effect for the Pt-Au system. On the other hand the change in selectivity for carvone hydrogenation could be explained by assuming that the atoms of Au are deposited preferantially on edges, corners, leaving free large planes, on which a rapid desorption of carvotanecetone can occur. For low dispersion catalysts, the addition of A u also induces an increase in the carvotanacetone formation. However the variation in selectivity is lower to the one obtained on a catalyst with higher dispersion. The magnitude of this effect is justified by the small number of low coordination atoms that exist in large particles ( 140 A) in comparison with the large number of low coordination atoms that exist on the small 20 A). The previous results can be explained if particles ( we consider that the hydrogenation of this molecule follows a consecutive mechanism (scheme 1) without desorption where K',>>K, when the molecule is preferentially adsorbed on the planes (higher carvotanacetone formation). On the other hand K f 2 < < K 2when the molecule is adsorbed on the low coordination atoms (carvomenthone is favored). In accordance with this mechanism the carvomenthol formation is expected after a certain induction period. In conclusion, the change in selectivity in carvone hydrogenation on Pt-Au/SiO, catalysts prepared by Redox reaction can be explained in terms of a preferential deposition of Au on low coordination sites as it was pointed out by local microanalysis. ACIW(rWLEDGE"TS

We are indebted to CONACYT and CNRS for the financial support. REFERWCES

1.

P.N. Rylander, Catalytic hydrogenation in Organic Synthesis, Academic Press, New-York, pag. 55, 1979.

178

2.

Heteregeous Catalysis and fine chemicals 11, Proc. Int. Simp., Poitiers, 1 9 9 0 , M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Editors, Elsevier, Amsterdam, 1 9 9 0 ,

3.

J.A. Cabello, J.M. Campelo, A. Garcia, A. Luna and J.M. Marinas, J. Org. chem., 5 ( 1 9 8 6 ) 1 7 8 6 .

4.

J.M. Campelo, A . Garcia, D. Luna, J.M. Marinas, React. Kinet Catal. Lett., Vol. 1 8 , No 3-4, ( 1 9 8 1 ) 3 2 5 .

5.

M. Bonet, P. Geneste and M. Rodriguez, J. Org. Chem., 45 ( 1 9 8 0 ) 40.

6

P. Le Maux, G. Simonneaus, J. of Mol. Catal., 26 ( 1 9 8 4 ) 195.

7.

G. Del Angel, B. Coq, R. Dutartre and F. Figueras, J. Catal., 87 ( 1 9 8 4 ) 27.

a.

G. Del Angel, B. Coq, and F. Figueras, J. Catal., 95 ( 1 9 8 5 ) 167.

9.

B.M. Tcost, Science 2 1 9 ( 1 9 8 3 ) 2 4 5 ; b) J.A. Osborn, F.H. Jardine, J.F. Young, G. Wilkinson, J. Chem. SOC. (A) ( 1 9 6 6 )

10.

R. Gomez, J. Arredondo, N. Rosas and G. Del Angel, Studies in surface science, 59 ( 1 9 9 1 ) 1 8 5 .

11.

G.C. Bond and P.B. Wells, Appl. Catal., 1 8 ( 1 9 8 5 ) 2 2 5 .

12 *

J.C. Menezo, F.M. Denanot, S. Peyrovi and J. Barbier, Appl. Catal., 1 5 ( 1 9 8 5 ) 353.

13.

J.C. Menezo, L.C. Hoang, C. Montassier and J. Barbier, React. Kinet. Catal. Lett., 46 ( 1 9 9 2 ) 1.

14.

J.R. Anderson, K. Foguer and R.J. Breakspere, J. Catal., 57 ( 1 9 7 9 ) 4 5 8 .

15.

M. Viniegra, G. Cordoba and R. Gomez, J. of Mol. Catal., 58 (1990) 107.

16.

J.P. Boitiaux, J. Cosyns and E. Robert, Appl. Catal., 32 ( 1 9 8 7 ) 1 4 5 .

17. J.P. Boitiaux, J. Cosyns and E. Robert, Appl. Catal., 32 ( 1 9 8 7 ) 1 6 9 .

l a . M. Boudart and H.S. Hwang, 19.

J. Catal., 39 ( 1 9 7 5 ) 44.

R. Pitchai, S.S. Wong, N. Takahashi, J.B. Butt, R.L. Burwell Jr. and J.B. Cohen, J. Catal., 9 4 ( 1 9 8 5 ) 4 7 8 .

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III CO 1993 Elsevier Science Publishers B.V. All rights reserved.

179

Selective catalytic hydrogenation of compounds over amorphous nickel alloys Btla Toroka, &id

bifunctional

MolnBra, KBroly BorszCkya, Enik6 T6th-KBdBrb and Imre Bakonyib

aDepartment of Or anic Chemitry, J6zsef Attila University, and Centre for Catalysis, Surface and Materia Sciences, D6m tCr 8, Szeged, H-6720, Hungary

P

bResearch Institute for Solid State Physics of the Hungarian Academy of Sciences, H1525 Budapest, P.O. Box 49, Hungary

Abstract The hydrogenation of bifunctional carbonyl compounds 2 2,4,4-tetramethyl-l,3cyclobutanedione (l), 5-hexen-2-one ( 2 ) and (-)-verbenone was studied over amorphous Ni-P and Ni-B alloys. The amorphous Ni alloys were Ni-P and Ni-B powders prepared by chemical reduction, and Ni-P foils pre ared by electrolytic reduction or rapid quenching. A commercial Raney Ni and 3% NiPSi02 served as reference catalysts. The amorphous alloys exhibited lower catalytic activities, but much higher selectivities as compared to the polycrystalline nickel catalysts. Selective monohydrogenation of 1,3diketone (1) to the corresponding hydroxy ketone took place on all amorphous catalysts, except on Ni-P foil prepared by electrolytic reduction. Here, exclusive ring-opening attributed to acidic centres occurred. The hydrogenation of unsaturated carbonyl compounds (2 and 3) took place in two consecutive steps. At 373 K the selective saturation of carbon-carbon double bond occurred, while at 398 K the carbonyl group too was reduced to a hydroxy group, yielding saturated alcohols. In contrast, the conventional pol crystalline catalysts yielded a mixture of the corresponding hydroxy ketone and diol rom 1 , and saturated ketones and alcohols from 2 and 3, even at low temperature.

[i))

Y

1. INTRODUCTION

Rapidly cooled amorphous metal alloys have recently generated much interest in catalysis research. Progress in this field IS discussed in two review articles [1,2]. Amorphous alloys prepared by different methods are of only limited use in synthetic orgamc chemistry, mainly in hydrogenation and dehydrogenation reactions. Their applications include the selective dehydrogenation of alcohols to carbonyl compounds on Cu-Zr [3], the transformation of vicinal dinitro compounds to olefins [4] and the selective hydrogenation of unsaturated aldehydes on Ni-Zr [5] and on Co-B [6]. Relevant reactions, reductions with NaBH4 assisted by Ni and other metals (Cu, Pd, Fe, Mn, Rh etc.), were studied recently [7]. Mechanistic studies revealed that reduction of the carbon-carbon double bond in the presence of CoC12 took place via hydride ion transport over the surface of Co-B alloy precipitated under the reaction conditions [8]. Since nickel is frequently used in a wide variety of organic transformations, we decided to study various nickel catalysts in the reduction of bifunctional organic compounds. Amorphous alloys with different nickel contents were prepared and tested in

180

the hydrogenation of dicarbonyl and unsaturated carbonyl compounds (Formulae). The activities and selectivities of these catalysts were compared with those of traditional olycvstalline nickel catalysts. Our major goal was to learn more about the catalytic gehavlour of amorphous Iuckel alloys and to explore the possibility of their use in the liquid-phase hydrogenation of dicarbonyl and unsaturated carbonyl compounds.

Formulae 2. EXPERIMENTAL

p r p h o u s alloys were pre ared by rapid- uenchin (Ni-P(rq)) [9] (cooling rate 10 Ks- ), chemical reduction of 8iCY with NaB8, (Ni-Bf [lo], chemical reduction of NiCl2 with NaH2PO (Ni-P(ch)) [l and electrolytic r d ction (Ni-P(e1)) [12]. A commercial Raney Ni ?Carlo Erba pr 85.9 m g-1) and 3% Ni/Si02 (Cab0-Sil, BDH product, ABE 241.6 m as reference catalysts. The compositions oTimo hous alloys were determined with a JOBIN YVON 24 Inductively Cou led Plasma ( I C g Sequential spectrometer on the basis of the following lines: B - 249.6 8 nm, Ni - 221.647 nm, P - 213.618 nm. The alloys were dissolve$ !O.l g+O.Ol mg) in a 1:l mix re of 2 M hydrochloric acid and 2 M nitric acid (20 cm-) and were diluted to 500.0 cm . The concentration of the calibration solutions were 0, 20 and 200 200 ppm. ppm. The arno hous character of the alloys before reaction was investigated with PERKIN ELME DSC-2 differential scanning calorimeter (heating rate: 20 Kmin- , nitrogen atmosphere). The surface (BET area) of the alloys before reaction was determined in the conventional adsorption apparatus. The h drogenation reactions were carried out in the liquid phase (373 and 398 K) in a B E R G J O F BAR 845 hydrogenation autoclave under constant hydrogen pressure (70 atm) in ethanol as solvent. In eve case 50 mg of catalyst and 100 m of substrate were used. 2,2,4,4-Tetramethyl-1,3-cyc obutanedione, 5-hexen-2-one and ?-)-verbenone (2,6,6-trimethylbicyclo[3.l.l]hept-2-en-4-one) were purchased from Aldrich Chemical co. Products were anal sed by GC (Chrom 4 a aratus, flame ionization detector, 3.2 m Carbowax 20M column! and by GC-MS (an I?€?€ 5890A gas chromatograph equipped with an HP 5970B MSD, 50 m HP-1 capillary column).

f

7

9

a

%

7

3. RESULTS AND DISCUSSION

The compositions and the surface area of the nickel alloys are to be seen in Table 1.

181

Table 1 Compositions (atomic %) and the surface (m2g-l) of investigated amorphous Ni alloys

O0

Alloys

Ni

Ni-B Ni-P(e1) Ni-P(rq) Ni-P(ch)

64 80 82 83

P

20 18 17

1

B

BETarea

36

3.66 0.01 0.012 1.25

The data reveal that the alloys used contain various amounts of nickel. The crystallization behaviour of the catalysts was studied by DSC. Typical DSC curves are shown in Fig 1. Note that the temperature range of crystallization and the shape of the curve did not depend significantly on the of sample mode preparation. The observed crystallization peaks prove the amorphous character of these Ni-P and Ni-B alloys.

I

I

373

473

I 573

I

I

673

773

TEMPERATURE (K)

Figure 1. DSC curves of nickel alloys used

3.1. Hydrogenation of 2,Z2,4,4-tetramethyl12-cyclobutanedione

Results on the catalytic hydrogenation of 2,2,4,4-tetramethyl-1,3-cyclobutanedione (1) are shown in Table 2.

182

Table 2 Hydrogenation of 2,2,4,4-tetgamethyl-1,3-cyclobutanedione(1) over amorphous and polycrystalline nicke 1 catalysts

Catalyst Ni-B

Selectivity ketol (%) 100

diol (%)

ring-opened product (%)

stereoselectivity (cis/trans)

100

Ni-P(e1) Ni-P( rq)

100

Ni-P( ch)

98

1

Ni/Si0241

95

5

1 41/59

Raney Ni24h 100 38/62 *Reactions were carried out at 398 K. The conversion was 100% in all cases except NiP(rq) (35%). In general, the reaction time was 50 h. 1 is a unique molecule since the reduction takes place by the known keto mechanism [ 131, and Wagner-Meerwein rearrangements cannot occur. It can be seen that the activities of the amorphous alloys are lower than those of the polycrystalline catalysts. Formation of the corresponding diol was not observed on the amorphous catalysts, while the crystalline catalysts either produced the diol selectively, or a mixture of the diol and the hydroxy ketone was formed. The fundamental reason for the lower activity and higher selectivity of the amorphous alloys is their rather small surface area. Of the amorphous alloys studied, Ni-B and Ni-P alloy owders prepared by chemical reduction exhibited higher activities than those of N i - e alloys prepared by electrolytic reduction or rapid uenchin . This difference in activity can be attributed to an oxide layer covering the sur?ace rof $i-P foils [l]. It is necessary to point out, however, that the comparison of activities is based on unit catalyst weight. Obviously, this comparison does not take into account the real surface area of the nickel samples, nor active site densities. An unexpected transformation, the exclusive formation of ethyl 2,2,4-trimethyl-3oxovalerate, talung place via ring scission with the participation of solvent ethanol, was observed on Ni-P foil pre ared by electrolytic reduction. After preparation this foil was treated with sulfuric aci to dissolve the Cu plate used as substrate to deposit the amorphous Ni-P foil. We attribute this unusual transformation to acidic centres of the catalyst formed during the latter treatment. In an independent experiment, 1 was reacted without any catalyst in 1 M ethanolic hydrochloric acid at 398 K. The ring-opened ketoester was the only product formed, indicating that the transformation is an acidcatalysed process. The mechanism proposed to account for the selective ring-opening is to be seen in Fig. 2.

B

183

T

Figure 2. Formation of ethyl 2,2,4-trimethyl-3-oxovaleratevia ring scission of 1 The formation of the same ring-opened product was interpreted in terms of a different mechanism earlier [14].This unique property of 1 offers a simple, convenient method for testing the acidic character of supported catalysts.

3.2. Hydrogenation of 5-hexen-2-one The transformation of unsaturated compounds in general takes place at much higher rates. The results of the hydrogenation of 5-hexen-2-one (2) are shown in Fig. 3. In a similar investigation, Baiker and co-workers studied the hydrogenation of rranr-2hexen-l-a1 over amorphous Ni-Zr alloy and found that the hydrogenation occurred in two consecutive steps [5].The same phenomenon was observed over Ni-B and Ni-P alloys. However, the further hydrogenation of 2-hexanone formed after saturation of the carbon-carbon double bond did not occur at 373 K. After elevation of the temperature to 398 K, the carbonyl group also underwent hydrogenation, but at a much lower rate. This is in marked contrast to Raney Ni, which did not show any selectivity, producing a mixture of 2-hexanone and 2-hexanol even at 373 K.

184

5

10

20

15

25

35

30

40

TIME (h)

a. Change in concentration of 2-hexanone vs time curve 1 .o

0.8

0.6

0.4

0.2

0.0 5

10

15

20

25

30

35

40

TIME (h)

b. Formation of 2-hexanol vs time curve

Fieure 3. Hvdroaenation of 5-hexen-2-one over amomhous and Dolvcrvstalline nickel caralysts ( b-Ni-Li>(ch),0-Ni-B,A-Ni-P(el), h-Ni-P(rq): 0-Raney hi,-n13%Ni/Si02). Temperature: 373 K-( 1-398 K.

185 From a synthetic point of view it is important to point out that over amorphous Ni catalysts only the first hydrogenation step takes place at 373 K (C=C reduction) and higher temperature is necessary for the second step (C=O reduction). Thus, an appropriate selection of reaction temperature guarantees the corresponding product with excellent selectivity.

3.3. Hydrogenation of (-)-verbenone The results of the hydrogenation of (-)-verbenone (2,6,6trimethylbicyclo[3.l.l]hept-2-en-4-one)(3) are to be found in Table 3. This compound is a practically important molecule used as the starting material in the synthesis of a sex pheromone [15]. It is a conjugated unsaturated molecule with a hindered rigid bicyclic structure and as such offers a possibility for comparison with the behaviour of the openchain nonconjugated unsaturated ketone 2. Table 3. Hydrogenation of (-)-verbenone over amorphous and polycrystalline nickel catalysts* Catalyst

time h

conversion %

verbanone %

Ni-B

4 34

100 100

100

Ni-P(e1)

6 36

100 100

100 80

Ni-P(rq)

7 38

100 100

100 95

Ni-P(ch)

10 44

100 100

100 100

Ni/Si02

11

100

100

verbanol % 95

5

5

Raney Ni 8 100 2 98 *Reaction temperature 373 K (first line), then 398 K (second line), for every catalysts Earlier studies with complex metal hydrides (LiAlH [16], and NaBH4 [17]) indicated the formation of a mixture of cis- and trans-verbenof. In our experiments over amorphous Ni alloys, the hydrogenation of verbenone resulted in the formation of verbanone and verbanol. This means that the same consecutive two-step reduction occurred as observed in the case of 2. Under a propriate reaction conditions (373 K), selective saturation of the carbon-carbon doub e bonds yielded verbanone. At higher temperature (398 K), selective formation of the saturated alcohol (verbanol) was observed in most cases.

P

186

4.CONCLUSION Ni-P and Ni-B amorphous alloys pre ared by different methods exhibit interesting catalytic properties. The applicability o these alloys as catalysts in liquid-phase hydrogenations was demonstrated by studying the selective partial or total hydrogenation of 1,3-dicarbonyl and unsaturated carbonyl compounds. The hydrogenation of dicarbonyl compound over amorphous Ni alloys gave hydroxy carbonyl product with high selectivity. An unexpected transformation, the exclusive rinp-opening reaction of 2,2,4,4tetramethyl-l,3- clobutanedione, was observed on NI-P foil (prepared by electrolytic reduction). The ecisive role of acidic centres in the ring-opening reaction was proved. The hydrogenation of 5-hexen-2-one yielded 2-hexanone at 373 K with excellent selectivity, while at 398 K 2-hexanol could be prepared. Appropriate selection of the reaction temperature allows the selective preparation of either compound. On the basis of these results the selective hydrogenation of (-)-verbenone, a principal building block in pheromone synthesis, was carried out to produce either verbanone or verbanol with high selectivity.

P

7

5. REFERENCES 1 2 3 4 5

6 7 8 9 10 11 12 13 14 15 16 17

A. Molnar, G.V. Smith and M. Bartok, Adv. Catal, 36 (1989) 329 A. Baiker, Faraday Discuss. Chem. SOC.,87 (1989) 239 A. Molnar, T. Katona, M.Bart6k and K. Varga, J. Mol. Catal., 64 (1991), 41 A.A. Madjdabadi, R. Beugelmans and A. Lechevallier, Synth. Commun. 1 (1989) 1631 A. Baiker, J. De Pietro, M. Maciejewski, B. Walz, in Structure-Activity and Selectivity Relationship in Heterogeneous Catalysis (R.K. Grasseli and A.W. Slight, Eds.), Elsevier, Amsterdam, 1991, 169 Y.Z. Chen and K.J. Wu, Applied Catal., 78, (1991) 185 R.C. Wade, J. Mol. Catal., 18 (1983) 273 J.O. Osb , S.W. Heinzman and B. Ganem, J. Am. Chem. Soc. 108 (1986) 67 E.W. Co lings, C.E. Mobley and R.E. Maringer, AIChE Journal, 74 (1978) 102 H.C. Brown and C.A. Brown, J. Am. Chem. SOC.,85 (1963) 1003 S. Yoshida. H. Yamashita, T. Funabaki, T. Yonezawa, J. Chem. SOC.Chem. Commun., (1982) 964 E. Tcjth-KAdar, I. Bakon i, A. Solyom, J. Hering, G. Konczos and F. Pavlyak, Surf. Coat. Techn., 31, (1987) P. Geneste, M. Bounet and M. Rodriguez, J. Catal., 57 (1979) 147 R.H. Hasek, E.U. Ulam, J.C. Martin and R.G. Nations, J. Org. Chem., 26 (1961) 700 R.M. Silverstein, J.O. Rodin and D.L. Wood, Science, 154 (1966) 509 C.A. Reece, J.O. Rodin, R.G. Brownlee, W.G. Duncan and R.M.Silverstein, Tetrahedron, 24 (1968) 4249 M.A. Cooper, J.R. Salmon, D. Whittaker and U. Scheidegger, J. Chem. SOC.B. 1967/II 1259

I

11

ACKNOWLEDGEMENT We acknowledge the support provided for this research by the Hungarian National Science Foundation through grants OTKA T 4311.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 Q

1993 Elsevier Science Publishers B.V. All rights reserved.

187

STEREOSELECTIVE HYDROGENATION OF D-FRUCTOSE TO D-MANNITOL ON SKELETAL AND SUPPORTED COPPER-CONTAINING CATALYSTS M. HegedOs, S. Gobolos, J.L. Margitfalvi* Central Research Institute for Chemistry of the Hungarian Academy of Sciences, 1525 Budapest, POB 17, Hungary Abstract The hydrogenation of D-fructose over different copper based skeletal and supported catalysts has been investigated. The influence of (i) the conditions of catalyst preparation, (ii) the addition of second metal, (iii) the type and amount of modifiers used and (iv) the reaction parameters on the rate of hydrogenation and the selectivity of mannitol was studied. The introduction of Co or Fe to the copper alloy increased the specific copper surface area of the Raney-copper catalysts, but decreased the selectivity of mannitol. The addition of boron and iron to the alloy resulted in a slight increase in the selectivity of mannitol. In the presence of sodium halides and sodium borate the mannitol selectivity increased over all types of skeletal catalysts. The highest selectivity of mannitol, around 88 %, was obtained on supported Cu/CPG catalysts (CPG=Controlled Pore Glass). The high selectivity of mannitol of the above catalyst was attributed to the presence of boron containing surface moieties, such as B-(OH)2, in the support.

1. INTRODUCTION

D-mannitol is widely used as sweetening agent and finds also different application in the food industry and related areas [I]. D-mannitol can be directly prepared from mannose or by stereoselective hydrogenation of D-fructose. However, the hydrogenation of D-fructose in aqueous solution over different heterogeneous catalysts leads to the formation of two isomers, i.8. D-mannitol and D-sorbitol, near to a ratio of one to one [l]. It was observed earlier that upon hydrogenation of D-fructose high selectivities of D-mannitol could be obtained only over supported copper catalysts [2]. Further selectivity improvements were observed by addition of sodium borate to the reaction mixture [2].

* To whom correspondence should be addressed

188

It has been suggested that in the presence of borate ions the improved selectivity of mannitol is due to the formation of an adduct between the carbohydrate and borate ions [2]. It was evidenced by NMR that in the about adduct the beta-furanoseform of the carbohydrate prevails [2,3]. The above form of the carbohydrate is more favourable for the formation of D-mannitol than for the formation of D-sorbitol [2].

In this work different supported and skeletal copper catalysts were investigated in the hydrogenation of D-fructose to D-mannitol. The aim of this work was to find the modes and ways to increase the selectivity of D-mannitol. 2. EXPERIMENTAL PART 2.1. Catalyst preparation. Preparation of Ranev-copper catalvsts. Raney-copper catalysts (particle size: 0.09-0.31 mm) were prepared by dissolving aluminium from a binary (35-50 wt % Cu) or ternary (35-45 wt % Cu, 0.1-10 wt % Co, Fe, Cr, Zn, or B) copper-aluminium alloy with 10 or 20 wt % NaOH solution at 20 or 50 OC. Designation of the catalyst was done in the following way: the number before the metal indicates the metal content in the parent alloy, the number after the dash is the serial number of the catalyst prepared from the given alloy. Preparation of s u ~ prted o copper catalvsts. Two different supports were used to prepare supported catalysts. Silica gel with specific surface area of 200 m2/g, pore volume of 1.5 cm3/g, and particle size of 0.02-0.20 mm was used without any further treatment. The controlled pore glass (CPG) support (particle size: 0.045-0.10 mm) was prepared as described elsewhere [4,5]. CPG support with surface area of 33 m2/g, pore size 75 nm, pore volume 0.59 cm3/g was used. The catalysts were prepared by incipient wetness impregnation of the support with Cu(ll) nitrate in the presence of citric acid followed by drying in air at 230 OC. The dried samples were calcined at 500 OC for 5 hours followed by reduction in a hydrogen atmosphere at 300-400 OC.

The pore volume of the catalysts were determined by mercury porosimetry. The metallic surface area of both Raney-type and supported catalysts was measured by titration with N20 [6]. Characteristic properties of Raney copper and supported copper catalysts are given in Table 1.

2.2. Hydrogenation of fructose. The hydrogenation of 20 wt% fructose in water was carried out in a 250 ml SS stirred autoclave under 40-70 bar hydrogen pressure at 50-75 OC. The reaction

189 products were analysed by liquid chromatography using a 8 x 250 mm column filled with Ca modified ion exchanged resin (Katex KH-08). Eluent: water; column temperature: 85-90 OC. Two approaches were used for product analysis: (i) determination of the conversion and selectivity after a certain reaction time; (ii) determination of the time dependence both of the conversion and selectivity. In most of the cases instead of the selectivity values (SM), the product ratio, i.e. the ratio of D-mannitol to Dsorbitol, (M/S) was calculated.

Table 1. Characteristic properties of Raney-copper and supported copper catalysts. No

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Catalysts

35Cu-10 35Cu-11 35Cu-13 45Cu-07 45Cu-08 45Cu-15 50Cu-03 50Cu-04 5OCu-06 5ocu-10 5ocu-12 5oCu-13 1B53Cu-1 7Zn40Cu-2 5co45cu-1 5co45cu-2 5co45cu-4 3Cr50Cu-1 3Fe5OCu-2 Cu/SiO,-1 Cu/SiO,-2 Cu/SiO,-3 Cu/SiO,-4 CU/CPG-l Cu/CPG-4

Composition, % (a) Cu

Al

40.0 55.0 85.0 70.0 77.0 n.a. 63.0 67.0 82.0 n.a. 74.0 75.1 82.0 83.5 65.0 65.0 68.0 n.a. 75.0 10.0 20.0 25.0 30.0 20.0 20.0

35.0 35.0 5.0 10.0 4.6 n.a. 24.0 16.0 6.0 n.a. 6.8 5.4 7.0 2.8 8.0 10.0 3.5 n.a. 11.0

-

-

sCu

Others m 2 h

0.1 1.5 5.9 5.5 4.8 7.9 4.2

1.53 3.81 9.17 9.21 10.92 14.56 2.48 3.94 7.43 11.83 8.28 8.70 4.48 9.28 14.34 16.96 19.19 7.90 16.07 2.46 4.30 4.61 5.20 2.24 2.71

V,

rO

lo4 (b) M/S

(C)

cm3/q mot x m-* x min-’ n.a. 0.18 n.a. n.a. 0.20 0.20 0.29 n.a. 0.15 0.53 0.19 0.15 n.a. 0.20 0.13 0.10 0.09 n.a. 0.16 n.a. n.a. n.a. n.a. n.a.

n.a.

2.72 1.64 0.14 1.66

1.97 1.96 1.94 1.90

2.14 2.00 1.53 1.28

1.83 1.87 1.83 1.88

1.96 2.75 1.05

1.92 1.75 1.75

1.39 1.98 1.25 2.38 2.57 2.62 2.57 2.16 1.92

1.81 1.77 1.99 1.89 1.79 1.82 1.77 4.57 4.53

(a) composition of catalysts (the difference from 100 % is attributed to the formation of different metal oxides and hydroxides; (b) reaction condition: 50-60 OC, 50-60 bar; (c) measured at around 20 % of conversion.

190

3. RESULTS AND DISCUSSION 3.1.Preparation of Raney-copper catalysts.

Variable experimental parameters in the preparation of Raney-copper catalysts were as follows: (i) temperature of leaching, (ii) concentration of NaOH solution used, (iii) absence or presence of excess NaOH during the leaching process, (iv) absence or presence of different additives during the leaching process. The specific copper surface area (ScU) of Raney-copper catalysts was always high if the leaching process was carried out at 50 O C , instead of 20 OC (see entries 1 and 3 in Table 1.). Similar effect was observed when excess NaOH was used in the leaching process. The high extent of leaching of the aluminium from the alloy resulted in a catalyst with low activity. In some cases the formed AI(OH)3 was responsible for the decrease of the catalytic activity. In the presence of additives such as carbohydrates the extent of removal of AI(OH)3 formed could be improved. Raneycopper catalysts containing high residual aluminium content had low ScU value. Details on the preparation of Raney-copper catalysts will be given elsewhere [7].

3.2. Hydrogenation of D-fructose over Raney-copper catalysts. The hydrogenation of D-fructose was carried out in aqueous solution at pH 6-8, 60-70 OC, and 50-60 bar. Initial rates of hydrogenation (ro) and M/S ratios measured at 20 % of conversion are given in Table 1. The activity of catalysts was strongly influenced by the copper content of the parent alloy, the specific copper surface area, and addition of a second metal. Contrary to that only minor changes were observed in the M/S ratio. The highest M/S values were obtained over boron and iron containing catalysts. Catalysts with high specific copper surface area showed lower activity than catalysts with lower surface area (see entries No 1-3 and 7-9 in Table 1.). The addition of iron and cobalt resulted in a significant increase in the Scu values. The only catalysts having high activity and high specific copper surface area was 7Zn40Cu-2, however, this catalyst had low selectivity of mannitol.

191 40

80

2.0

z

30

C

a?

f 40

.%

dc 20

L

>

I

s 20

10

temperature, OC

0

" 1 '

40

50

Hydrogen pro.

0

70

80

80

20

0

Figure 1.

40

ure, b

60

80

Figure 2.

Figure 1. Dependence of conversion and D-mannitol/D-sorbitol ratio (M/S) of the reaction temperature. Experimental conditions: catalyst - 45Cu-08, 50 bar, substrate/catalyst = 12.9 Figure 2. Dependence of the conversion and D-mannitol/D-sorbitol ratio (M/S) of the hydrogen pressure. Experimental conditions: catalyst - 45Cu-08, 50 OC, substrate/catalyst = 10.6 40

1-p

50

2.1

-

4 40

0

r

f! 1.8

2

> 0

r c c

10

-1

1.6

0

1.3

3

5

7

Figure 3.

0

1

ae c'

-f 30 0

e

8

20

10 0

1

2

3

4

Figure 4.

Figure 3. Dependence of conversion and D-mannitol/D-sorbitol ratio (M/S) of the pH value. Experimental conditions: catalyst - 45Cu-13: , 50 OC, 50 bar, substrate/catalyst = 10.6 Figure 4. Dependence of conversion and D-mannitol/D-sorbitol ratio (M/S) of the concentration of sodium borate. Experimental conditions: catalyst - 45Cu-08, 50 OC, 50 bar, pH = 5, substrate/catalyst = 8.7

192

The dependence of the conversion and M/S ratio of the reaction temperature, hydrogen pressure and pH is shown in Figure 1-3, respectively. As emerges from these figures the conversion values measured after 160 minutes of hydrogenation showed a strong dependence of the reaction temperature and hydrogen pressure. A very sharp decrease of the conversion was observed at pH below 4 (see Figure 3). Contrary to the above findings the M/S ratios measured after 160 minutes of hydrogenation were only slightly changed upon increasing the reaction temperature, hydrogen pressure and pH. 3.3. Hydrogenatlon of D-fructose over supported copper catalysts.

Results obtained over silica and CPG supported catalysts are also included into Table 1. Silica supported catalysts showed moderate activity. Over these catalysts the M/S ratio was below 2, i.e. the enantioselectivity of silica supported copper catalysts was not better than that of the Raney-copper catalysts. The activity of CPG supported catalysts was slightly lower than that of the silica supported catalysts, however, the D-mannitol selectivity showed the highest value. Over these catalysts in the absence of additives M / S ratios above 4, or D-mannitol selectivity values around 80 % could be obtained. 3.4. Hydrogenation of D-fructose in the presence of different modifiers.

Sodium halides and sodium borates can be used as modifiers for hydrogenation of D-fructose to improve the selectivity of D-mannitol. The effect of sodium borate on the rate of hydrogenation of D-fructose and the M/S ratio is shown in Figure 4. This figure shows that upon increasing the concentration of sodium borate significant decrease in the conversion value takes place. However, similarly to earlier findings [2],in the presence of sodium borate substantial increase in the M/S ratio can be achieved. Similar effect, a strong increase in the M/S ratio, was observed by adding different sodium halides to the reaction mixture as shown in Table 2. As emerges from these results both sodium halides and sodium borates appeared to be effective modifiers both for Raney-copper and supported copper catalysts. It is suggested that the role of the above modifiers is selective site blocking. In addition to the site blocking effect, in the presence of borate ions, the modifier-subtrate interaction can also be involved in the improved selectivity . Makkee has suggested earlier [2] that the improved selectivity of D-mannitol observed in the presence of

193

borate ions can be attributed to the formation of a cyclic adduct between the borate ion and the D-fructose.

Table 2. Hydrogenation of D-fructose in the presence of sodium halides. No Catalysts 1 2 3 4

5 6 7 8 9 10 11 12 13 14 15

Sub/Cat Concentrationa moi x 10 3

35Cu-12 35Cu-12 35Cu-12 35Cu-12 35Cu-12 7Zn40Cu-2 7Zn40Cu-2 7Zn40Cu-2 7Zn40Cu-2 7Zn40Cu-2 45Cu-15 45Cu-15 45Cu-15 4%~-15 45Cu-15

9.6 9.6 9.6 9.6 9.6 12.3 12.3 12.3 12.3 12.3 18.2 18.2 18.2 18.2 18.2

0.0 a.7b 13.1b 26.2b 174.9b

0.0 16.gC 35.4c E~2.3~ 101.6c 0.0 3.0d 4.2d 6.0d 12.0d

Reaction rate, ro Conversion mol/l x m2 x min % 42 27 14 18 18 53 36 32 38 36 64 32 17 15 2

2.67 1.77 0.26 0.68 0.47 2.75 1.24 0.61 0.52 0.63 2.14 1.16 0.62 0.45 0.24

M/S 1.59 3.52 3.76 4.63 4.90 1.75 2.07 2.02 2.31 2.26 1.83 2.37 2.34 2.46 1.96

a) of the modifier, b) NaBr, c) NaCl d) Nal; t = 60 OC, P = 50 bar.

Table 3. Hydrogenation of D-fructose in the presence of sodium bromide and sodium borate Catalysts Sub/Cat

Modifer Concentration Reaction Conv. Sel. (SM) mol x 10 -3 time, hours % %

35Cu-12 10.0 35Cu-12 10.0 50Cu-05 3.7 5ocu-12 9.1 50Cu-11 8.7 5C045C~-01 8.0 5co45cu-04 11.1 3Fe5OCu-01 8.3 15Zn40Cu-9 8.3 CU/CPG-1 11.1 CU/CPG-l 6.7 Cu/CPG-4 11.1

NaBr NaBr NaBr NaBr Naborate Naborate Naborate Naborate Naborate

1.70 2.61 5.63 18.00 30.26 30.26 30.26 30.26 21.28

Naborate Naborate

21.28 21.28

t = 60-65 OC, P = 50 bar.

29 29 23 24 23 32 24 24 23 24 29 48

54 68 86 86 97 93 89 73 97 78 97 97

78.1 74.2 72.6 76.5 79.6 82.7 79.2 78.0 80.0 80.4 87.4 88.2

M/S

3.56 2.87 2.64 3.25 3.89 4.77 3.79 3.55 4.00 4.13 6.90 7.47

194

By using sodium halides or sodium borate the hydrogenation of D-fructose could be carried out almost to the completion. These results are shown in Table 3. In these experiments upon using Raney-copper catalysts the highest selectivity of Dmannitol, 82.7 %, was observed over a cobalt containing catalysts in the presence of sodium borate. Even higher D-mannitol selectivity, i.e. 88.2, was observed over CPG supported copper catalyst in the presence of sodium borate. 4. CONCLUSIONS

Raney-copper catalysts showed relatively high activity in the hydrogenation of Dfructose to D-mannitol. The selectivity of D-mannitol over these catalyst was around 60-65 %. The highest D-mannitol selectivity, i.8. values around 85-88 %, was obtained over Cu/CPG catalyst in the presence of sodium borate. This high selectivity could be maintained up to 95-97 % conversion. The high D-mannitol selectivity obtained over CPG supported copper catalysts needs further discussion. It is known that the surface of CPG is enriched with B-OH and B-(OH)2 moieties in the SiO, matrix [8]. We suggest that the above surface moieties are involved in the overall selectivity control. If in the borate-substrate adduct the beta furanose form of the carbohydrate determines the enantiodifferentiation step in the formation of D-mannitol [2], then due to the presence of surface B-(OH)2 moieties the concentration of the beta-furanose form of the carbohydrate should also increase resulting in the high selectivity for the formation of D-mannitol.

5. REFERENCES

1 J.F. Ruddesledem, A. Stewart, D.J.Thompson, R.Whelan, J. Chem. SOC. Faraday Disc., 72 (1981) 207. 2 M.Makkee, A.P.G.Kieboom and H.Bekkum, Carbohydr. Res., 128 (1985) 225. 3 H.Pelmore and M.C.Symons, Carbohydr. Res., 155 (1986) 206. 4 W.Haller, Chem. Anal.(N.Y.) 66 (1983) 535. 5 S.Gobolos, E.TAlas, K.Hanko, M.HegedCs and J.Margitfalvi, Epitoanyag, 44 (1992) 27. 6 J..W. Ewans, MSWainwright, A.J. Bridgewate and D.Y. Yung, Appl. Catal., 7 (1983) 75. 7 J.Margitfalvi and M. Hegedus, to be published. 8 H.L.Krauss and D.Naumann, Z.Anorg. Allg. Chem., 430 (1977) 23.

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

FURFURAL - HYDROGEN REACTIONS, MANIPULATION ACTMTY AND SELECTMTY OF THE CATALYST

195

OF

T.B.L.W. Marinelli", V. Ponec', C.G. Raabb and J.A. Lercherb 'Gorlaeus Laboratories, Leiden University, P.O. Box 9502,2300 RA Leiden, The Netherlands bInstitut fur Physikalische Chemie, TU Wien, Getreidemarkt 9,A-1060 Vienna, Austria

Abstract The geometric and promoter effects on the selectivity to furhryl alcohol were studied in the hydrogenation of furfural over various series of platinum catalysts. The results obtained with the PtCu catalysts indicate that the size of the active site does not affect the selectivity. Promotion of the platinum catalyst can lead to a considerable rise in the selectivity to furfuryl alcohol. The hydrogenation over the series of PtSn catalysts showed the influence of the reaction conditions. The experiments under isothermic conditions resulted in up to 80% selectivity for hrfuryl alcohol, while the selectivity dropped to approximately 40% if non-isothermic conditions were applied. This change in selectivity is attributed to the self-poisoning of the catalyst at high temperatures. INTRODUCTION The existence of numerous patents [l-31 indicates commercial interest in the hydrogenation of hrfural to furfuryl alcohol. By a more fundamental study we expect to gain more information on the adsorption of furfural on the catalyst surface, and on the influence of geometric and promoter effects on the selectivity to furfuryl alcohol. This information might also be useful for a better understanding of the behaviour of hydroxymethylfurfural [4], a molecule with still increasing industrial interest. Figure 1 shows the pathways along which furfural, according to literature [5], can react with hydrogen. The reaction pathway in which we are most interested is the hydrogenation of furfural 1 to hrfuryl alcohol 3. In a previous study on the selective hydrogenation of aliphatic a,R unsaturated aldehydes (R,R,C=C%-CR,=O) to the unsaturated alcohols [61 we found that the main problem is how to suppress adsorption on the C=C bond and simultaneously enhance that on C=O bond. A literature search and the results of our earlier study revealed that very little can be won by varying the metal, but that the use of a proper promoter can enhance the selectivity considerably.

196 I

8

no-OH

\

Fig.1: Reaction scheme of the hydrogenation of furfural 1. Furfural (FF) 2. 2-Methyl-furan (MF) 3. Furfuryl Alcohol (FA) 4. Tetrahydro-furfuryl Alcohol (THFA) 6. Tetrahydro-furan (THF) 6. Furan (F) 7. 1-Pentanol (1-P) 8. 1,5-Pentanediol (1,5-P)

The C=C bond in furfural is made much less reactive (compared with that in the above mentioned aldehydes) by the structure of the molecule. The reactions to be suppressed here are the hydrogenolyses leading to either 2 or 6. The routes 1 to 2 and 1 to 6 are the most important side-reactions which were observed in the hydrogenation of furfhral over platinum catalysts. The reaction of 1 to 6 is a CO elimination and reaction 1 to 2 is most likely a common hydrogenolytic fission of a C-0 bond. To study the role of promoters we have chosen platinum a s active metal in the bimetallic catalytic system. Platinum belongs to the most active metals for hydrogenation, while it shows a low activity for various hydrogenolytic fissions.

EXPERIMENTAL Materials: The support material used in this study was a very pure and inert silica (Aerosil 200 from Degussa (A200), surface area 200 m2/g). The precursors for the preparation of the Pt-Cu and the (un)promoted platinum catalysts were Cu(N03);3H,0 (J.T. Baker), H,Pt(OH), (converted into H,PtCl, by HC1) (Johnson Matthey Chemicals), and SnC1;2H,O (>95%), FeCl,, NH.,VO, (99.5%), GeC1, (99.99%), Fe(N03);9H,0 (99%) (Janssen Chimica), Ti(OC,H,), (Johnson Matthey Chemicals), GaCl,, NaCl (>99.5%) (Merck). Hydrogen (Hoekloos 3.0, >99.90% purity) used was further purified by passage through a BTS column and a column containing molecular sieve. Furfural (Merck, >99%) was distilled under nitrogen for use. Catalyst preparation: All the catalysts were prepared on Aerosil 200 using an incipient wetness technique. Different series of platinum catalysts were synthesized: Pt-Cu catalysts with a varying Cu/Pt atomic ratio and a total metal loading of 5.0wt%, a series of promoted 5wt% PtX catalysts (X=promoter

197 and Pt:X=4:1 at%) and finally a series of Pt-Sn catalysts with a constant Pt content and variable Sn loading. Apparatus and procedure: The vapour-phase hydrogenation experiments were performed in a flow system. Hydrogen was passed through a saturator, filled with (liquid) furfural (at about 70”C), and subsequently through the reactor, containing the catalyst on a glass grid. The gas flow rate was regulated by a calibrated mass flowmeter. Samples of furfural and its products from hydrogenation were taken every 30 minutes with the help of an automatic sampling valve and analyzed by a gas chromatograph equipped with a FID detector and connected t o an integrator. Product separation was achieved using a 5-mxU8” stainless steel column packed with 15% Carbowax 20M +2% KOH Chromosorb WAW, 80/100 mesh. The separation of furan and 2-methyl-furan was not quantitative, their presence was checked by GLC-MS analysis. Each catalyst (3 to 5 rng) was reduced in situ a t 573 K by a 15 mVmin hydrogen flow during 4 hours. Subsequently, the reactor was cooled down t o the reaction temperature (170°C) under hydrogen. Furfural was then added to the stream by passing a 15 d m i n hydrogen flow through the saturator. Partial pressures of hydrogen and furfural were 978 and 33 mbar, respectively. The hydrogenation reaction was monitored either as a function of time, with a constant reaction temperature of 170°C (isothermic), or as a function of temperature (non-isothernic).In the latter experiments a temperature program from 170°C to 210°C and back was used. The temperature was raised (and subsequently lowered) each hour by steps of 5°C. At each temperature two samples of the reaction mixture were analyzed by gas chromatography.

RESULTS A short study revealed that the selectivity to furfuryl alcohol cannot be increased by making alloys of platinum with copper (see Table 1). This is a situation which reminds us of the results obtained with acrolein [6]. In Table 1 the selectivity and yield for furfuryl alcohol are shown. The other products formed are furan and 2-methyl-furan; no tetrahydro-furan was observed. With the series of promoted platinum catalysts it was seen that those promoted by compounds containing non-transition elements (Sn and Ge) appeared to be promising, the selectivities to furfuryl alcohol were 40.0% and 16.8%, respectively. Promoters like V and Ti compounds did not enhance the selectivity above that of pure platinum and the sodium promoter even caused a lowering of the selectivity to furfuryl alcohol (Figure 2 and Table 2). The values plotted in Figure 2 for the promoted platinum catalysts were obtained in the non-isothermic way. In the non-isothermic experiments the temperature was stepwisely varied in the range of 170” to 210°C.

198

Table 1 Hydrogenation of furfural over PtCdAerosi1200 catalysts, under isothermic and 'non-isothermic' conditions. ISOTHERMIC (170°C)

NON-ISOTHERMIC

SF,

YP;

(%)

( 10-2)

Time' (hours)

SF, (%)

( 10.2)

("0

1 4 8 16

7.3 14.2 12.4 16.2

362.3 446.3 335.1

210 200 190 180 170

2.9 4.2 5.7 6.1 7.9

245 292 258 235 212

PtCu 4:l

1 4 8 16

2.0 3.0 3.3 3.2

151.4 155.0 142.4 113.7

PtCu 1:l

1 4 8 16

4.2 9.6 12.1 11.0

165.0 304.8 336.4 244.6

210 200 190 180 170

4.9 5.8 6.4 7.5 9.7

207 198 180 166 154

PtCu 1:4

1 4

2.3 9.1 10.0 7.6

59.8 36.0 26.2 17.2

Catalyst Pt

8 16

YFAl

408.4

Tlapeiion

1. Time on stream 2. in arbitrary units, per gram catalyst

Fig.2: Selectivity in furfuryl alcohol formation as a function o f the position of the promoting compound cation in the periodic table. The pure platinum catalyst has a s, of 7.9%.

199

Figure 3 shows the selectivities to furfuryl alcohol obtained in the hydrogenation of furfural over the series of PtSn catalysts. The main products formed from furfural were furd2-methyl-furan (in approximately 1:4 ratio) and furfuryl alcohol. An interesting shift in the selectivities was observed when the temperature of the catalyst was changed. In both types of experiments the shown selectivities were measured a t 170°C after 16 hours on stream. The conditions of the reaction, however, were either isothermic or non-isothermic before the selectivity was determined (Figure 3). It can be seen in Figure 3 that the selectivities for furfuryl alcohol in the hydrogenation reaction under isothermic conditions are much higher than under non-isothennic conditions.

isothermic non-isothermic

0 0

0

0

cn

20 0 1 I

I

I

I

I

I

Fig.3: Selectivity to Furfuryl alcohol obtained in the hydrogenation of Furfural over 5wt% PtSn /A200catalyst under isothermic (170°C)and hon-isothermic’ conditions.

DISCUSSION

It has been observed in agreement with Gon Seo and coworkers [51 that in the gas-phase hydrogenation of furfural the products formed are 2-methyl-furan, furfuryl alcohol and (at higher temperatures) traces of tetrahydro-furfuryl alcohol. Analysis with GLC-MS showed that in the hydrogenation over the (udpromoted platinum catalysts used in this study besides 2-methyl-furan also furan was formed. Resinous material and ring-decomposed products, like 1pentanol and 1,5-pentadiol, can also be found in liquid-phase hydrogenation reaction [31. The study of Gon Seo et al. concerned the role of copper in various copper-chromium oxides and in Pd-CuY and Ni-CuY zeolite catalysts. The

200

authors concluded that the Cu(I1) ion was responsible for the high selectivity to furfuryl alcohol and the absence of tetrahydro-furfuryl alcohol formation. The authors further speculated that Cu(I1) interacts with the furan ring of f u f i r a l and prevents thus the furan ring to be hydrogenated. Already in the thirties, copper-chromite promoted by addition of an alkaline earth oxide, was a favourite commercial catalyst for various hydrogenations. In the gas-phase hydrogenation of furfural copper catalysts have been used mainly to avoid hydrogen addition on the furan ring. However, an undesired further reduction of the furfuryl alcohol to 2-methyl-furan can sometimes occur. Bremner et al. [9]studied in details the reaction of furfural over a number of copper catalysts. These studies showed that in the hydrogenation of furfural, high temperatures (> 300°C) or the addition of chromite to the copper catalyst favoured the formation of 2-methyl-furan1whereas low temperatures (c 200°C) or the addition of alkali-metal containing compounds favoured the formation of furfuryl alcohol. Raney nickel has been also used in continuous processes for the conversion of furhral to tetrahydro-furfuryl alcohol [lo]. Kaufmann et al. [113 reported that pure platinum catalysts are not useful for the hydrogenation of furfural t o tetrahydro-furfuryl alcohol because of the occurence of extensive hydrogenolysis. On the other hand, a metal not selective in its pure state can offer suflicient space for a study of promotion effects. Table 2 Hydrogenation of furfural over promoted Pt/Aerosi1200 catalysts under ’nonisothermic’ conditions. The selectivity and activity’ values are taken at Treeetion = 170°C. FURFURYL ALCOHOL

Catalyst

(%)

S

Y (10.2)

7.9

212

PtSn

40.0

258

PtGe

16.8

596

PtV

7.6

245

PtTi

6.7

218

PtNa

2.1

29

Pt

~~

1. in arbitrary units, per gram catalyst

201

The 5wt% platinum catalyst supported on silica used in this study gave a selectivity for furfuryl alcohol of 7.9% (Table 2) in the gas-phase hydrogenation of furfural. As can be seen in Figure 2 the selectivity to the most desired product can be, indeed, enhanced by promotion. The values in Table 2 show that the yield of furfuryl alcohol can be increased while the selectivity for this compound has been also enhanced. This points to a selective activation of the C=O group by the promoter. We have to admit that it is still an open question in which form e.g. Sn exerts its promotion effect, A recent discussion [8,9] revealed that both oxidic tin species as well as metallic tin alloyed with Pt can coexist in PtSn catalysts. We suggest that an oxidic form of tin is the promoter, acting in a similar way as the promoting oxides do in syngas reactions [121. As already mentioned above the alloying of Pt with a much less active metal like Cu does not increase the selectivity to furfuryl alcohol. We expect that the inactive Sno atoms in Pt do not act differently. The fact that the non-transition elements form sometimes better promoters than the transition elements is still surprising but, actually, not new. Earlier papers (see e.g. [6,13]) report the same conclusion. A study, the results of which will be published soon [14], revealed that the selectivity in the hydrogenation of the unsaturated aldehydes is determined by the relative probability of adsorption of the C=O and C=C groups. The oxides of transition elements are binding the C=C group better than the oxides of non-transition elements and that aspect seems to be decisive for the resulting selectivity. When the addition of hydrogen to the C=C group is sterically hindered (like in furfural) the transition-metal oxides (as e.g. Fe) are also good promoters [ E l . The large changes in the selectivity to furfuryl alcohol in the hydrogenation of furfural over the PtSn catalysts (Figure 3) show the influence of the reaction conditions applied. The selectivity measured a t 170°C in the 'non-isothermic' regime is lower than in the isothermic experiment. Obviously a t higher temperature the catalyst is self-poisoned by carbonaceous deposits or polymers made from furfural or its fragments. Since this deposition of firmly adsorbed species decreases the selectivity we concluded that it takes place in the nearest neighbourhood of the promoter species. Actually also during the isothermic experiments the activity and selectivity decrease with time (although much less than after a high temperature treatment) and the values shown in Figure 3 are collected after 16 hours on stream. It is evident that each technology based on this or similar types of catalysts should minimize the formation of deposits for example by using low temperatures or high hydrogen pressures.

202

CONCLUSIONS T h e selectivity t o f u r h r y l alcohol is not influenced by the size of t h e active site. Promotion of the platinum catalyst b y compounds containing non-transition elements enhances the selective activation of t h e carbonyl group considerably. In the hydrogenation over the PtSn catalysts it is shown that the selectivity to f u r h r y l alcohol is dependent on t h e reaction temperature. A t higher temperatures the catalyst is probably self-poisoned by carbonaceous deposits or polymers made from hrfural or its fragments and t h e selectivity decreases.

ACKNOWLEDGMENTS The investigations were supported by The Netherlands Foundation for Chemical Research (SON) with financial aid from The Netherlands Organization for Scientific Research (NWO). Thanks are also due to the Johnson Mathey Technology Centre (Reading, U.K.) for the loan of the platinum salts.

REFERENCES 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15.

K. Kiimmerle, Ger.Offen. DE 2837022 A1 (1980). A.A. Kozinski, U.S. Patent 4,185,022 (1980) J. McEvoy and H. Shalit, US. Patent 3,374,184 (1968); E. Haidegger, Ger.Offen. DE 2740057 A1 (1979); E.A. Preobrazhenskaya et al., U.S. Patent 4,261,903 (1981); L.J. Frainier and H. Fineberg, Ger.Offen. DE 3007139 C2 (1982); H. Hinnekens, Ger.Offen DE 3425758 A1 (1985). Ph.D. Thesis P. Vinke, Delft 1991. Gon Seo and Hakze Chon, J.Catal., 67 (1981) 424. T.B.L.W. Marinelli, J.H. Vleeming and V. Ponec, Proc. 10th Intern. Congress on Catalysis (Budapest 1992). B.H. Davis, Proc. 10th Intern. Congress on Catalysis (Budapest 1992). J. Schwank, K. Balakrishnan and A. Sachdev, Proc. 10th Intern. Congress on Catalysis (Budapest 1992). J.G.M. Bremner, and R.K.F. Keeys, J.Chem.Soc. (1947) 1068; (1949) 1663. F. Starkey, and J.G.M. Bremner,-British Pat. 608,540 (1948). W.E. Kaufmann and R. Adams, J.Am.Chem.Soc.,45 (1923) 3029. V. Ponec, Catal. Today, 12 (1992) 227. S. Galvagno, Z. Poltarzewski, A. Donato, G. Neri, and R. Pictropaolo, J.Molec.Catal., 35 (1986) 365; Z. Poltarzewski, S. Galvagno, R. Pietropaolo, and P. Staiti, J.Catal., 102 (1986) 190; S. Galvagno, A. Donato, G. Neri, R. Pietropaolo, and D. Pietropaolo, J.Molec.Catal., 49 (1989) 223; D. Goupil, F. Fouilloux, and R. Maurel, React.Kinet.Cat.Lett., 35 (1987) 185; D. Richard, P. Fouilloux, and P. Gallezot, Proc. 9th Intern. Congress on Catalysis, 3, 1074 (Calgary 1988). T.B.L.W. Marinelli, and S. Nabuurs, unpublished results. P. Beccat, J.C. Rertolini, Y. Gauthier, J. Massardier, and P. Ruiz, J.Catal., 126 (1990) 451.

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

203

Selective hydrogenation of a$? unsaturated compounds in the presence of cobalt catalysts J. Barrault, M. Blanchard, A. Derouault, M. Ksibi and M.I. Zaki(*) Laboratoire de catalyse ,URA CNRS 350 ,ESP , 4 0 av du Recteur Pineau , 86022 Poitiers cedex ,FRANCE. (*) On leave fiom Minia University, El Mnia 615 19, EGYPT

Abstract The reduction of cobalt acetylacetonate with triakylaluminium leads to species composed of zerovalent cobalt and u ~ e d u c e dcobalt species. The exact composition depends upon the AVCo ratio and the activation process. When we used a catalyst corresponding to a N C o = 1, activaded with hydrogen at 180°C, we obtained an increase of the selectivity of the hydrogenation of 2-pentyl-2-nonenal into 2-pentyl-2-nonenol. 1. INTRODUCTION

The preparation of unsaturated alcohols from the corresponding I Y , ~unsaturated aldehydes is the object of extensive investigations. Since Adkins' copper chromite type of catalysts (1,2), several propositions concerning the effect of a support, various promotors and a second metal (3-7) have been forwarded and yet, in many cases, we have not been able to explain the modifications obtained. More recently research concerning the bimetallics Pd-Fe, Ru-Fe, Pt-Fe (Co,Ru) have been carried out showing that the selectivities for unsaturated alcohols can be favoured (8,9). Moreover it has been shown for Pt-Fe catalysts that the increase in selectivity is accompanied by an electron transfer from iron to platinum (10). The aldehydic double bond is then more easily adsorbed on these induced sites (1 1 ) . Furthermore the deposit of group VIlI metals on graphite has led also to an appreciable selectivity for unsaturated alcohols ( 1 2- 15). GALLEZOT and al have defined the different factors that must be taken into account to explain the catalytic properties (13). This implies the electronic properties, the geometrical effects as well as the aldehyde configuration. A quite similar description of this has been given to report the results obtained with catalytic systems modified by tin (16-18). In many of these investigations cinnamaldehyde hydrogenation has been chosen as a model reaction . Taking into account the importance of the electronic and the geometrical e!€ects certain results of selectivity obtained with the above molecule cannot be rationalized to other substrates. As for us and for basic practical reasons we have chosen to study the selective hydrogenation of 2-pentyl-2-nonenal and, in certain cases, compare the selectivity of this reaction with the one of cinnamaldehyde reaction. Furthermore we have used as catalysts, system whose preparation involved steps similar to those involved in the preparation of ZIEGLER type catalysts which, though initially developed for the polymerization of olefins, have been used since for various reactions. In particular in our laboratory, we have used them for the selective hydrocondensation of carbon monoxide, or methanol, into light olefins ( 19,20)

204 and, more recently, for the selective hydrogenation of nitriles (21) and (Y,P unsaturated carbonyl derivatives (22). In this paper we report the results obtained with such cobalt catalysts. 2. EXPERIMENTAL The catalysts were prepared by reducing the cobalt acetylacetonate dissolved in benzene, under a inert atmosphere of argon, usiig a known quantity of triethylaluminium (as a function of the desired AYCo ratio). The solution immediatly became black and the metallic particles formed were able to be stabiied by butadiene at 0°C. The solvent employed during the hydrogenation reaction was dodecane or propylene carbonate. The benzene was then evaporated under a controlled atmosphere and the degradation products were then removed and analyzed, the temperature being increased up to 200°C. The catalyst thus obtained was used "in situ". AU the catalytic hydrogenations (2-pentyl-2-nonenal or cinnamaldehyde) were carried out in a 200-ml static reactor under atmospheric pressure with a continuous flow of hydrogen and at temperatures ranging from 50°C to 120°C (23). Activity and selectivity values were obtained by gas phase chromatography analysis of the liquid mixture on a Cp SIL5 capillary column. Characterization of the catalysts employed in liquid phase was difficult. However we devised an experimental procedure to observe the preparation of these catalysts "in situ" by FTIR spectroscopy. We investigated the reaction between precursors, then the development of the compounds formed as function of temperature and reactional medium. 3. CATALYTIC RESULTS 3.1. Hydrogenation of 2-pentyl-2-nonenal

We examined the influence of each parameter connected with the preparation of the catalyst : influence of the reductant, the preparation and activation atmosphere, and

temperature on the catalytic properties. Here we have chosen a few of the most demonstrative examples.

* Influence of the activation atmosphere Table 1 Hydrogenation of 2-pentyl-2-nonenal. Influence of activation conditions on the properties of cobalt. T = 80°C ;P = 0,l MPa ;solvent = dodecane. Activation

Relative activity

I-&, 120°C

&, 180°C

5 5

(CO&), 180°C

100

Selectivity (%) 2p-nod 2p-2-nonenol isownversion = 50 YO 10 42 5.5 55.5 43

10

2p-nonanol 48 39 41

conversion = 100 % H2, 120°C 0 90 10 64 H,,180°C 2 32 (CO,k), 180°C 0 0 I00 As these catalysts had been used first in the (C0,HJ reaction they were submitted to a pretreatment by (C0,H.J at 180°C before being employed. This activation is not "a priori"

205

necessary in hydrogenation reactions of carbonyl derivatives. We investigated effects of various pretreatments and results obtained are given in Table 1. We can see therein that a slight modification in the activation process of the catalyst had affected considerably its activity and selectivity. The reduction of the solid by hydrogen produces a catalyst about 20 times less active than one resulting from a syngas activation. On the other hand the development of the selectivity in the unsaturated alcohol is quite the contrary. Indeed a more sigmficant selectivity after reduction of the catalyst by hydrogen at 180°C than after any other treatment can be observed. As the comparison was made at reactant isoconversion (SO?!) a conversion effect cannot be evoked as far as the same result was obtained at total conversion (Table 1).

* Intuence of fhe reductant content ( A K o ratio) The results of the experiments performed, and given in Table 2, show that decrease of the reductant leads to a definite increase of the selectivity for unsaturated alcohol and a decrease of the activity. The presence of non-metallic cobalt is therefore necessary for the activation of the carbonyl bond and for its hydrogenation. Table 2 Hydrogenation of 2-pentyl-2-nonenal. Muence of the reductant content (AVCo) on the catalytic properties of cobalt. T = 80°C ; P = 0.1 MPa ; solvent = dodecane. Activation at 180°C with hydrogen. Results obtained at isoconversion of 50 %. Avco Relative Selectivity (%) activity 2p-nonanal 2p-2-nonenol 2p-nonanol 0.2 inactive 1 5.0 35.0 60 0.5 1.o 5 5.5 55.5 39 1.5 11 37.0 16.0 47 3.2. Cinnamaldehyde hydrogenatioo

As pointed out in the introduction, this molecule has often been chosen as a model in the study of the reactivity of hydrogen with a double carboncarbon bond and a carbonyl hnction. We have therefore examined the hydrogenation of this aldehyde in the presence of a few of our catalysts (Table 3).

Table 3a Hydrogenation of cinnamaldehyde in the presence of a cobalt catalyst (AvCo = 1). T = 50°C ; P = 0.1 MPa ; solvent = propylene carbonate. Relative Selectivity (%) Conversion (%) activity HCAL COL HCOL 50 1 11.5 84 4.5 90 6.5 73 20.5 HCAL : hydrocin. Ald. ;COL : Cin. Alcoh. ;HCOL : hydrocin. Alcoh. Table 3b Hydrogenation of 2-pentyl-2-nonenal in the same solvent and under the same conditions. Relative Selectivity (%) Conversion (%) activity 2p-nonanal 2p-2-nonenol 2p-nonanol 50 0.3 17.0 23.0 60 90 7.5 17.5 75

206 Under the same experimental conditions great differences in the activity and selectivity could be observed. Indeed the catalyst chosen was about 3 times more active in cinnamaldehyde hydrogenation than in that of 2-pentyl-2-nonenal. Furthermore the selectivity for unsaturated alcohol (COL) was also very high even at almost total conversion, while the selectivity for 2-pentyl-2-nonenol was about 20% under the reactional conditions that were chosen (the solvent was propylene carbonate instead of dodecane). Owing to the presence of an aromatic nucleus the olefinic bond was harder to hydrogenate and this molecule can not be taken as representative of the selective hydrogenation of linear unsaturated carbonyl derivatives. 4. INFRARED CHARACTERlZATION OF COBALT CATALYSTS

The preparation of the catalysts was studied "in situ" by Infra-Red spectroscopy and we first examined the catalysts corresponding to an AVCo = 1 4.1. Co(acac),

+ AI(Et), (AvCo = 1)

After adding AI(Et), to Co(acac), dissolved in benzene there was a ligand exchange reaction leading to the formation of Al(acac), andor EtAl(acac), (appearance of a band at 1289 cm-1 at the expense of a band at 1260 cm-1 due to Co(acac), ) (Figure la). Other consistent changes could be observed over the following Frequency regions 1650-1500, 14801325, 1250-1175, 1000-900, 835-760, 600-540 and 510-450 cm-1. Some of these changes were due to Cocontaining species thus produced (Figure 1b) ;namely, the emergence of weak absorptions at 1250 and 826 cm-1 due respectively to Cr (CH,) and out of plane CH deformation of olefine groups bound to metal atoms (Co) (24). The change of solvent (benzene to dodecane) did not change the IR-spectrum. * The sample was then heated at 180°C under hydrogen and cooled down to room temperature. The strong absorptions in the (1600-1500 cm-I) region corresponding to u (C-C) and v (C-0) vibration of M(acac) disappeared. This result was consistent with a marked drop of the absorption at 1289 cm-1. On the other hand there was a strong composite absorption between 1175 and 1125 cm-I which could be attributed to u (C-C) / v (C-0) of metal-alkoxide species (25). * After (C0,H.J at room temperature The formation of two metalcarbony1 absorptions could be observed; an initial absorption around 1950 cm-l (at I5 min) and a subsequent absorption at 1991 cml (at 65 min) (Figure 2a). These two carbonyl absorptions could be associated with bridging(MJ0) and terminal (M-CO) species, respectively. When outgassing CO under hydrogen at 80°C, the CO, in the gas phase and the terminal (MCO) species were suppressed. But a new band at 1790 cm-I appeared (Figure 2b) which could be attributed t o multicentered adsorption of CO. * When (CO-) reaction was carried out at 80"C, Figure 3 shows the changes in the spectral behaviour with timeen-stream. There were two carbonyl absorptions around 1990 (M-CO) and 1850 cm-l (bridged species) which increased markedly with time. After outgasing under hydrogen at 80°C the 1850 and especially 2000 cml bands predominated. The increasing of the ( C 0 , b ) reaction temperature rendered the CO adsorption much stronger and irreversible and proved the presence of coordinatively unsaturated Co metal centers.

207

1300

I250

I200

600

570

540

510

480

(a) WAVENUMBER ( c m - l ) (b) WAVENUMBER ( cm-' ) Figure 1 LR spectra ofthe solution obtained after addition of Al (Et) to Co(acac),, AVCo = 1, at RT (-) Co(acac), , ( ) mixture [

,

/

; I

I I

~

\

I

'\

',

,-. -,.

\

'

'\,-,/

,\

, 2150

'\,2OqO I

0)

1850

(

, 8

I ,

I

I

1

I

I

1

1

I I

1

1700

1

,

I

,

I

0.013 a.u.

;

I

1

'

I

<

$1

11

- (d)l

I(C)

II

v

I

0.023 a.u.

1 -.-rc

-

2000

T I

1900

(d)

(c) (b) (a)

1800

WAVENUMBER ( cm-' )

Figure2 IR spectra of the solution obtained after (C0,H.J reaction at room temperature for 1Smin (a), 65min (b), 140min (c), 270min (d) and after outgassing in hydrogen for 20h (e)

0.04 a . u . '

1

1

I 1 I

1 1 \

I

I

I , I ,

2100

1

(

I

'I

I (

I

I

\

\

AL-i

,(d)

--__-

/(C)

-Jb) --(a)

L

2150

2000

1850

1

1700

WAVENUMBEH ( cm-' )

Figure3 IR spectra of the solution obtained after (C0,H.J reaction at 80°C for 15min (a), 60min (b), 180min (c) and after outgassing in hydrogen (d)

208 4.2. Influence of the AVCo ratio

When the amount of reductant used was changed during the preparation of the catalyst the variations observed in the IR spectra were the following : For AVCo of 0.5 the Co(acac), species are always present in the medium. However during the treatment by hydrogen at high temperature they transform preferably into alkoxide species. The species of the catalyst are then composed of reduced and unreduced cobalt in equal quantities. If the amount of reductant is increased (AVCo = 1.5) the cobalt acetylacetonate is almost entirely reduced and the catalyst species are composed essentially of metallic cobalt. 5. ANALYSIS OF THE GASEOUS PRODUCTS FORMED DURING TEE REDUCTION OF Co(acac), WITH AIEt,

For an AyCo ratio of 1 and at room temperature, ethane was the main product (93%) when the reduction was carried out under hydrogen #en the temperature was increased to 180°C there was a hrther decomposition of intermediate species into ethane which remained the main product. From the mass balance of these analysis one can propose the following reactions Co(acac), + AI(Et),

-

Co + Co(acac), + AI(Et),acac + Al(acac), 4 + hydrocarbons (C2&) metal Co (alkoxide)

Aluminium would be in Al(Et),acac (66%) and in Al(acac), (33%) About 80% of the cobalt would be zerovalent while the remainder (not reduced at room temperature) would be transformed into alkoxide species at 180°C with hydrogen (see above) For an AVCo ratio of about 1 5 , ethane was also the main decomposition product at room temperature But the amount of methane increased significantly with the temperature and represented 50% of the hydrocarbons formed at 180°C in the presence of hydrogen This result could indicate a change of the catalyst composition even if we cannot explain clearly the origin of the methane ethane hydrogenolysis, AlEQacac decomposition, 7 6. CONCLUSION

In this work we have shown that Ziegler type cobalt catalysts are rather selective for the hydrogenation of unsaturated carbony1 compounds into unsaturated alcohols. Nevertheless the selectivity was more important with cinnamaldehyde than with other aldehydes, which showed the effect of the aromatic cycle on the stabilization and the adsorption of the conjugated double bond The reaction of triethylaluminium with cobalt acetylacetonate (depending on the AVCo ratio) can lead to composite catalytic species containing zerovalent cobalt and unreduced cobalt. But the final catalytic properties also depend on the activation process which seems to m o w specifically the state of unreduced cobalt. We propose that, after a hydrogen treatment at 18O"C, a fraction of cobalt is transformed into alkoxide species and that the corresponding composite sites are involved in the selective hydrogenation.

209

REFERENCES 1091.

1

H. ADIUNS and R CONNORS, J. Am. Chem. SOC.,1931,

2

H. ADKINS, "Reactions of hydrogen with organic compounds over copperchromium oxide and nickel catalysts", University of Wisconsin Press, 1938.

3

M.A. VANNICE and B. SEN, J. Catal., 1989,115.65.

4

G. PAJONK and S.J. TEICHNER, Tatalytic hydrogenation", L. Cerveny Ed., Studies in 277. Surface Science and Catalysis, 1986,

5

S. GALVAGNO, Z. POLTARZEWSKI, A. DONATO, PIETROPAOLO, J. Chem. SOC.Chem. Comm., 1986,1729.

6

L. GUCZI and Z. SCHAY, "Catalytic hydrogenation", L. Cerveny Ed.,Studies in Su&ce Science and Catalysis, 1986,27- 3 13.

7

D. RICHARD, J. OLKELFORD, A. GIROIR-FENDLER and P. GALLEZOT, Catal. Lett., 1989, 53.

8

D. GOUPIL, Thesis, Lyon (France), 1986.

9

P. FOUILLOUX, "Heterogeneous Catalysis and Fine Chemicals", M. GUISNET et al Ed., Studies in Surface Science and Catalysis, 1 9 8 8 , a 123.

10

B. MORAWECK, P. BONDOT, D. GOUPIL, P.FOUILLOUX and A.J. RENOUPREZ, J. Physique C8,1987, 297.

11

D. GOUPIL, P. FOUILLOUX and R. MAUREL, React. Kinet. Catal. Lett., 1987, 5 3

G.NERI and

R.

185. 12

A. GIROIR-FENDLER, D. RICHARD and P. GALLEZOT, a) in "Heterogeneous Catalysis and Fine Chemicals", M. GUISNET et al Ed., Studies in Surface Science and 171. b) Catal. Lett.,1990, 175. Catalysis, 1988,

a

13

P. GALLEZOT, A GIROIR-FENDLER and D. RICHARD, in "Catalysis of Organic Reactions" W.E. PASCOE Ed., 1992, 1.

14

S. GALVAGNO, G. CAPANNELLI, G. NERI, A. DONATO and R. PIETROPAOLO, J. Mol. Catal., 1 9 9 1 , s 237.

15

C.S. NARASIMHAM, V.M. DESHPANDE and K. RAMMARAYAN, J. Chem. SOC., Chem. Comm., 1988,99.

16

B. DIDILLON, A EL MANSOUR, J.P. CANDY, J.P. BOURNONVILLE and J. BASSET, in "Heterogeneous Catalysis and Fine Chemicals", M. GUISNET et al Ed., 1988,

137.

17

O.A. FERRETTI, J.P. BOURNONVILLE, G. MABILLON, G. MARTINO, J.P. CANDY and J.M. BASSET, J. Mol. Catal., 1991,g 283.

18

V.M DESHPANDE,K. RAMNARAYAN and C.S. NARASHIMAN, J. Catal., 1990, 174.

210

19 20 21

M. BLANCHARD, D. VANHOVE, F. PETIT and A MORTREUX, J. Chem. Soc. Chem. C o r n , 1979,604. M. BLANCHARD, D. VANHOW, RM.LAINE, F. PETIT and A MORTREUX, J. Chem. SOC. Chem. Corn., 1982,570. M. BLANCHARD, J. BARRAULT and A. DEROUAULT, in "Preparationof Catalysts 687. V", G. PONCELET et Al.Ed., Studies in Surface Science and Catatysis, 1991,

22

M. KSIBI, Thesis, Poitiers (France), 1993.

23

C. BECHADERGUEi-LABICHE, S. MAILLE,P. CANESSON, M. BLANCHARD and D. VANHOVE,in "Preparationof Catalysts IV",B. DELMON Ed.,Studies in Surface Science and Catalysis, 1988, & 725.

24

K. NAKAMOTO, in "IR and Raman Spectra of Inorganic and Coordination Compounds",J. WILEY Ed., New-York, 1978,383.

25

J.LYNCH, Anal. Chem., 1964, & 2332.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

21 1

Selective Hydrogenation of Crotonaldehyde over Pt Derived Catalysts C.G. Raab", M. Englisch", T.B.L.W. Marinellib and J.A. Lercher' "Institut fur Physikalische Chemie und Christian Doppler Laboratorium fur heterogene Katalyse, TU Wien, Getreidemarkt 9, A-1060 Vienna, Austria bGorleus Laboratories, Leiden University, P.O.Box 9502, 2300 RA Leiden, The Netherlands

Abstract The hydrogenation of crotonaldehyde was studied over five catalysts with platinum as the major catalytically active component. The catalytic activity for hydrogenation of the C=O bond decreased in the order PtGa/SiO, > Pt/TiO, > P t S m O , > PtNi/SiO,. Pt/SiO, was one order of magnitude less active than the other catalysts investigated and catalyzed only the hydrogenation of the C=C bond. Two different types of sites were concluded to be active for C=O bond hydrogenation, i.e., (i) the metal-oxide-interface (i.e. Pt-TiO, and Pt-GaOJ and (ii) bimetallic moieties as in Pt-Sn and Pt-Ni with a small positive charge on the less noble constitr.ent. Hydrogenation of the C=C bond was concluded to occur primarily on the pure metal surfaces. 1. INTRODUCTION Group VIII metals supported on silica are known to hydrogenate preferentially the C=C double bond of a,Sunsaturated aldehydes (as e.g. crotonaldehyde). Only for a few metals, like 0 s and Co, a significant activity for the hydrogenation of the C=O group was reported [l]. Improved selectivity for carbonyl group hydrogenation was described for platinum catalysts modified by Sn [2,3,41, Fe [5,6,7] and Ni [8]. In these catalysts the less noble metal was suggested to be in a non zero valent state under reaction conditions. In addition to the promoting effect of non noble metals on Pt, the use of partially reducible supports were also reported to improve the activity for the carbonyl group hydrogenation [9-131. In this study we compare five Pt derived catalysts with respect to their catalytic activities for hydrogenation of crotonaldehyde (CH,-CH=CH-CHO, CrHO).

212

In the primary reaction step either butyraldehyde (CH,-CH,-CH,-CHO, BuHO) or crotylalcohol (CH,-CH=CH-CH,OH, CrOH) were formed. In the secondary reactions butan-1-01 (CH,-CH,-CH,-CH,OH, BuOH) and butane were found to be reaction products. Three non noble metal additives (Ni, Sn and Ga) and the partially reducible support TiO, were used to modify the catalytic properties of Pt in comparison to Pt/SiO, . In two of these cases (Ni, Sn) the non noble metal may form a n alloy with Pt, while in the other cases Pt is suggested to be decorated with understoichiometric metal oxide particles (PtiTiO,, PtiGaO,) [lo]. 2. EXPERIMENTAL

The P t derived catalysts were prepared by co-impregnation of the support (Aerosil200 with 200m2/g,TiO,P25 with 25m2/g,both from Degussa) with aqueous solutions of the metal chlorides (NiCl,.6H,O, GaCl,, SnC1,.2H,O). The concentration of the non noble metal additives were 30 mol% for Ni, 20 mol% for Ga and 5 mol% for Sn. The catalysts, their composition and the results of hydrogen chemisorption are summarized in Table 1. Hydrogen chemisorption was measured in an all glass system. 0.5 - 1.5 g of the precursor were reduced in flowing H, (200 ml.min"), with a heating rate of 10 Wmin up to 673K. The reduction was completed by holding at 673K for two to four hours. Adsorption was carried out at 298K and initial pressures between 3x104 and 5x104Pa. The isotherms were obtained by subsequently gradually decreasing the pressure. The total amount of adsorbed hydrogen extrapolated to zero pressure was used to calculate the fraction of accessible metal atoms. Hydrogen with a purity of 99.999 vol% was used without further purification. Table 1 Catalyst composition and hydrogen chemisorption results a t 293 K (reduced a t 673K) catalyst

metal loading

Pt

H adsorbed [molec./g,,*10'9]

H,$metal

[%I

[Wt%l

content [mol%l

Pt/SiO,

7.2

100

6.01

26

PtNi/SiO,

5.8

70

6.60

28

PtGa/SiO,

5.0

80

0.89

5

PtSdSiO,

5.0

95

0.94

6

Pt/TiO,

7.2

100

1.61

7

The gas phase hydrogenation of crotonaldehyde (2-butene-l-a1from Aldrich, purity > 99.5%, used without further purification) was carried out in a quartz glass tubular reactor under atmospheric pressure. Typically, 1 - 10 mg of the chloride precursor were loaded into the reactor. In order to obtain a suitable catalyst bed

213

and to avoid temperature gradients, the catalysts were diluted with quartz beads (i.e., to 2-4 wt% catalyst). The reactor temperature was measured outside the catalyst bed at the reactor wall. Crotonaldehyde was introduced into the H, stream by means of a saturator. Catalytic measurements were performed between 353K and 413K at partial pressures of 60 and 953 mbar of the aldehyde and hydrogen, respectively. Total reactant flow rates were 2 . 5 ~ 1 mol/s. 0 ~ The conversion was kept below 10%.The reactor emuent was analyzed by means of a HP5890 gaschromatograph, equipped with a 30m J&W DB-WAX capillary column and a flame ionization detector. Catalysts were reduced in situ under hydrogen by increasing the temperature from 353K to 673K at 10 Wmin. After holding the catalyst at 673 K for 1 hour, the reactor was cooled to reaction temperature under hydrogen before the reaction was started.

3. RESULTS The turnover frequencies (TOF) for the overall reaction, the selectivities as well as the apparent energies of activation are compiled in Table 2. PtNUSiO,, PtGdSiO,, PtSdSiO, and PtiTiO, showed similar TOFs while Pt/SiO, was approximately one order of magnitude less active than the other catalysts. It should be noted that the catalytic activity decreased significantly for all catalysts with time on stream. During the first 60 minutes on stream a decrease of approximately 30%of the overall activity was observed. It is noteworthy that the reactions leading to the formation of butyraldehyde and crotylalcohol showed dif€erent rates of deactivation as will be outlined below. The TOF values presented in Tab. 2 correspond to the activities after 60 minutes time on stream. The apparent energies of activation for the overall reaction were very similar for Pt/SiO,, PtNUSiO, and Pt/TiO,, i.e. 42,43 and 46 kJ/mol. Those for PtGdSiO, and PtSdSiO, were 33 and 27 kJ/mol, respectively. Table 2 Hydrogenation of crotonaldehyde: TOFs, apparent energies of activation for the overall reaction and selectivity of Pt derived catalysts at 353K (conversion ~ 1 0 % ) . selectivity [mol %I

TOF

E&PP)

catalyst

[moVHd,.sl

ckJ/moll

HC

BuHO

BuOH

CrOH

Pt/SiO,

0.03

42

0.2

98.3

1.5

0

PtNUSiO,

0.45

42

0.3

92.0

5.7

2 .o

PtGdSiO,

0.43

33

0.5

37.1

6

56.4

PtSdSiO,

0.36

27

0.5

63.1

5.5

30.9

pmo,

0.29

46

0.9

46.1

6.7

46.3

Except for PtGdSiO,, butyraldehyde was the main reaction product at 353K.

214

Formation of butanol as well as small amounts of hydrocarbons (HC) were observed over all samples. Over Pt/SiO, crotylalcohol was not formed in detectable concentrations. The addition of Ni to Pt led to a selectivity of 2%. However, the combination of Pt with Ga and Sn or the use of a partially reducible support like TiO, led to a significant increase in the rate of crotylalcohol formation (i.e., the selective hydrogenation of the carbonyl bond of crotonaldehyde). The highest selectivity to crotylalcohol was observed for PtGdSiO, (56%), while it was lower for Pt/TiO, (46%) and PtSdSiO, (31%).

0

600

1000

1500

2000

TON [molecules converted / accessible metal atom] Figure 1

Rates of hydrogenation of crotonaldehyde to butyraldehyde (B) and crotylalcohol (A) as a function of time on stream at 353 K over PtNYSiO, ( - ) and PtSdSiO, (----I

1.00

1

0.90

5;r: ~ 0 . 8 0 O

:E

!2 0.70

.d

c,

j

0.60 0.60 0

600

1000

1600

2000

TON [molecules converted / accessible metal atom] Figure 2

Rates of hydrogenation of crotonaldehyde to butyraldehyde (B) and crotylalcohol (A) as function of time on stream at 353 K over PtGa/SiO,( - ) and Pt/TiO, (---->

215

Over the freshly reduced catalysts the rate of crotyldcohol formation decreased always faster than that of butyraldehyde with time on stream. Thus, the selectivity to butyraldehyde increased slightly, while that to crotylalcohol decreased. When the relative activities (i,e., the TOFs normalized to their "initial" values after 20 minutes time on stream) were plotted against the turnover number per exposed metal atom (TON), a significantly difference in the decrease of the rates of reaction to CrOH and BuHO was observed. Over PtNi/SiO, and PtSdSiO, the decrease of the rate of crotylalcohol formation was significantly higher than that of butyraldehyde formation (Fig. 1). Over Pt/TiO, and PtGa/SiO, both reactions decreased with approximately the same rate, the difference after a TON of 500 being less than 10% (Fig. 2). 4. DISCUSSION

The results (as compiled in Table 2) show that the catalytic properties of a platinum catalyst with respect to hydrogenation of the double bonds of crotonaldehyde can be subtly modified by (i) the addition of non noble metal atoms or (ii) by the proper choice of the oxide support. Both options not only increased the overall activity in comparison to a standard Pt/SiO, catalyst, but also increased the selectivity to the hydrogenation of the carbonyl group. In agreement with other authors [2,6,17], we showed previously that the enhancement of the selectivity to hydrogenate the carbonyl group was affiliated with the presence of polarity a t the metal surface. This polarity may originate from two effects: (a) Addition of other metals to Pt may cause a change in the oxidation state induced by subtle changes in the electronic environment around the individual metal atoms. Charge transfer might, e.g., occur when the electronegativities of the two metals in the alloy differ substantially from each other. (b) Metal ions or metal oxide particles may decorate the surface of the Pt crystallites and act as relatively strong electron pair acceptor sites [18] for the free electron pairs a t the oxygen of the carbonyl group. For PVNi bimetallic catalysts, we found that Ni formed a stoichiometric PtNi compound 1201 and it was concluded from XANES measurements that Ni has a slightly positive charge in that alloy [15]. It could be demonstrated that the rate of hydrogenation of crotonaldehyde to crotylalcohol increased with increasing concentration of the PtNi phase. Therefore, we speculated that the positive charge on the Ni atoms might increase the interaction of the carbonyl group with the catalyst surface. The increased interaction in turn might intensify the polarization of the carbonyl bond and enhance the rate of hydrogenation of this functional group 18,153.The positive effects of the presence of the bimetallic PtNi phase are, however, partially compensated by the increase of the rate of hydrogenation of CrHO t o BuHO. Note that we have demonstrated recently by in situ Pt-XANES that the interaction between the free electron pairs at the oxygen of various oxygen containing unsaturated molecules and the unoccupied electronic states above the Fermi level of Pt is very small [19]. This may indeed explain the need of polarity

216

at the surface, which will act as electron pair acceptor with a u type interaction between the adsorption surface site and the oxygen of the C=O group. We showed in that study that the strong interaction between the n electrons of the C=O group of crotonaldehyde and the Pt surface did not lead to a marked enhancement of the rate of the selective hydrogenation of that group. By analogy, we conclude that a situation similar to that of PtNi/SiO, also exists with PtSdSiO,. Addition of Sn to a PVNylon catalyst was found to be a successful means to increase the selectivity for hydrogenation of acrolein t o allylalcohol [21. These results were confirmed recently by Marinelli et al. [41. This higher selectivity (also obtained for the hydrogenation of cinnamaldehyde t o cinnamylalcohol) was due to a significant increase of the rate of hydrogenation of the C=O group. The rate constants for the overall reaction, however, passed a maximum value between 20 and 40 % Sn. Although not explicitly concluded, the reported increase in the lattice constants of Pt after addition of Sn, might indicate the formation of a n intermetallic compound [2]. EXAFS measurements will be performed with the present catalyst to clarify that point. The significantly higher rate of the C=O group hydrogenation over PtSdSiO, in comparison to that over PtNi/SiO, suggests that the bimetallic sites in PtSdSiO, have a higher activity than in PtNi/SiO, (see Fig. 3). Because we can exclude a higher concentration of bimetallic sites at the surface of the metal particles, we speculate that Sn has a higher positive charge than Ni. Further XANES measurements under reaction conditions are necessary to clarify the importance of charge transfer in this case. It is also interesting to note that the deactivation as function of time on stream is very similar for PtNi/SiO, and PtSdSiO,. With both catalysts, the rate of hydrogenation to CrOH decreases significantly faster with the number of catalytic cycles (number of turnovers, TON) than the rate of hydrogenation to butyraldehyde. For Pt/TiO,, accessible Ti" cations present in substoichiometric TiO, particles decorating the Pt metal crystallites were concluded to be the electron pair acceptor sites necessary for enhancing the activity for C=O group hydrogenation. It was shown previously by us that the catalytic activity of Pt derived catalysts supported on TiO, can be explained quantitatively by sites consisting of pairs of Pt and Ti"' [13]. Because the electron pair acceptor strength of these cations is certainly higher than that of Ni or Sn in an PtNi or PtSn alloy a stronger interaction with the C=O group is anticipated. This explains well the higher selectivity found with PtJI'iO, in comparison to PtNi/SiO, and PtSn/SiO,. Because gallium is known to be very difficult to reduce, we speculate that at least part of it might be present in a non reduced form resulting in positively charged gallium oxide species similar to the titanium oxides on the platinum crystallites. The present data do not allow to speculate if the higher selectivity to crotylalcohol is due to a higher concentration or a higher strength of the catalytically active ensemble. It is striking, however, that with these two catalysts we observed the rate of crotylalcohol and butyraldehyde formation to decrease in parallel with time on stream. Note that this is distinctively different to the situation observed with PtSn/SiO, and PtNi/SiO,. For these catalysts the rate of hydrogenation to crotylalcohol decreased faster than that to butyraldehyde. The reasons for the

217

difference in the deactivation, however, are unclear at present. At this point it should be emphasized that we cannot completely rule out the possibility that clusters of NiO and SnO on the surface of platinum particles may be active sites formed under reaction conditions. In such a scenario one might conclude that it is the lower charge on the Ni and Sn cations in the oxide clusters that cause the lower selectivity towards C=O group hydrogenation in comparison to the Ga or Ti promoted catalysts. We showed previously [8]that partially or fully oxidized Pt catalysts did not enhance the rate of C=O group hydrogenation. Thus, the presence of fully reduced platinum and of a positively charged promoter was concluded to be indispensable for the catalytic hydrogenation to crotylalcohol. The differences in the activity decay suggest, however, that the surface structures of PtNi/SiO, and PtSdSiO, must differ substantially from those of PtGdSiO, and PtrriO, 6. CONCLUSIONS

I t was shown that the promotion of Pt by non-noble metals like Ni, Sn, Ga and Ti increased significantly the rate of hydrogenation of the carbonyl group of crotonaldehyde. The presence of a combination of platinum and of a metal with a fractional positive charge or a metal cation acting as electron pair acceptor site was concluded to be indispensable for hydrogenation of the C=O group. Based on the decay of the catalytic activity, we concluded that two types of such a platinum promoter combination existed in the catalysts studied: (i) the bimetallic phases of Pt-Ni and Pt-Sn and (ii) the interface between Pt and understoichiometric titanium- and gallium oxide particles. The enhancement of the rate of the catalytic C=O group hydrogenation is concluded to be caused by electron pair donor acceptor interactions between the positively charged sites and the carbonyl oxygen.

ACKNOWLEDGEMENTS We gratefully acknowledge the support of this work by the "Fonds zur Forderung der Wissenschaftlichen Forschung" under project P6912.

6. REFERENCES

1 2

P. Rylander, in "Catalytic Hydrogenation over Platinum Metals", Academic Press New York and London, 1967. S. Galvagno, Z. Poltarzewski, A. Donato, G. Neri and R. Pietropaolo, J.Molec.Cata1. 35, (1986) 365.

218

3

4

5 6

7

8 9 10 11 12

13 14 15 16 17 18

19 20

Z. Poltarzewski, S. Galvagno, R. Pietropaolo and P. Staiti, J.Cnta1. 102, 190 (1986). T.B.L.W. Marinelli, J.H. Fleeming and V. Ponec, in "Proc. 10"' ICC", Budapest 1992. D. Goupil, P. Fouilloux, R. Maurel, React.Kinet.Cata1.Lett. 35, Nos 1-2, (1987) 185. D. Richard, J. Ockelford, A. Giroir-Fendler and P. Gallezot, Catal.Lett. 3,(1989) 53. P. Beccat, J.C. Bertolini, Y. Gauthier, J. Massardier and P. Ruiz, J.Cata1. 126, (1990) 451. C.G. Raab, J.A. Lercher, J.Molec.Cata1. 75, (1992) 71. M.A. Vannice and B. Sen, J.Catal. 115, (1989) 65. M.A. Vannice, J.Molec.Cata1. 59, (1990) 177. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, React.Kinet.Catal.Lett. 29. No 2. (1985) 311. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, Appl.Catal. 25, (1986) 181. C.G. Raab, J.A. Lercher, Catal. Lett., submitted 1992. C.G. Raab, J.A. Lercher, J.G. Goodwin Jr. and J.Z. Shyu, J.Catal. 122, (1990) 406. A. Jentys, B. McHugh, G.L. Haller and J.A. Lercher, J.Phys.Chem. 96, (19921, 1324. G.L. Haller and D. R. Resasco, Adv.Cata1. 36, (1989) 173. D. Goupil, P. Fouilloux and R. Maurel, React. Kinet. Catal. Lett., Vol. 35, NOS 1-2 (1987), 185 V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York 1978 A. Jentys, M. Englisch, G.L. Haller and J.A. Lercher (to be published) A. Jentys, G.L. Haller and J.A. Lercher, J. Phys. Chem. (to be published Jan. 1993) 2

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals ZZI Q 1993 Elsevier Science Publishers B.V. All rights reserved.

219

catalysts: variations of Cmtonaldehyde hydrogenation over activityandselectivitywiththepartialp~~ofcrotonaldehyde. R. Makouangou-Mandilou a, R. Touroude a and A. Dauscher b a Laboratoire #Etudes de la RRactivit6 Catalytique, des Surfaces et Interfaces, U.A. 1498 CNRS-ULP-EHICS, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France. b Laboratoire Mixte CNRS-Saint Gobain, U.M.R. 37, CRPAM, BP 109, F-54704 Pont-A-Mousson Cedex, France.

Abstract Hydrogenation reactions of crotonaldehyde were studied at 60°C in the gas phase and in a pulse mode on a 4.7 % PtJ"iO2 catalyst reduced a t 200°C, as well as the hydrogenation reactions of the mono-dihydrogenated products: butyraldehyde and crotyl alcohol. By varying the partial vapor pressure of crotonaldehyde from 8 t o 25 Torr, we found that the total reaction rate is zero order with respect t o crotonaldehyde pressure. Nevertheless, the crotyl alcohol yield is enhanced when the P~2/Pcrotonaldehyderatio is increased. Additional evidence is found providing the existence of two types of sites, leading either to C=C or C=O hydrogenation. INTRODUCTION Selective hydrogenation of a,P unsaturated aldehydes into allylic alcohols remains a difficult task to achieve, especially in the gas phase, when working with heterogeneous catalysts. It becomes, however, more and more studied. Monometallic catalysts supported on A1203 or Si02 lead mostly, like in the liquid phase, to the formation of the saturated aldehyde [1,2,31. Touroude [ l ] found hydrogenation of the C=O bond in acrolein t o be effective only on monometallic Pt and Os/Al2O3 catalysts. To improve the selectivity towards unsaturated alcohols, the use of additives, bimetallic catalysts or easily reducible supports has been proposed but the results can be different, depending on the nature of the metal itself. They would act on one hand on the polarization of the C=O bond so a s to favour its reactivity and/or on the reactivity of the C=C bond in order to diminish it. Addition of potassium to Ru/SiOa catalysts leads to a n enhancement of the selective hydrogenation of 3-methyl-2-butenal [31 while it has only little effect on Pt/SiOa catalysts during acrolein hydrogenation 141. In the same way, sulphur has a benefit effect on CdAl2O3 [21 but not on Pt/SiOz catalysts [41. Iron

220

enhances by a factor 5 the global hydrogenation rate in Pt-Fe bulk catalysts a s compared to Pt bulk catalysts, but not specifically the selectivity towards C=O hydrogenation [51. In fact, Marinelli et al. [4], studying a lot of additives, observed a n enhancement of selectivity for PtlSi02 catalysts associated with a n oxide of non transition metal (Sn > Ge > Ga). The presence of the two metals in Ni-CdAl203 catalysts is necessary to enhance the selectivity, each of them having a specific role r6.71, like in Cr-Cu catalysts [8]. Time dependence on stream has been put forward for the selectivities 171. Recently, it has been shown that Pt/TiO2 catalysts present a rate increase for the carbonyl group which is markedly enhanced on high temperature (500°C) reduced catalysts where metal-support interactions become efficient with TiOx species decorating the platinum particles [9,10]. Yoshitake and Iwasawa [ l l l observed changes in the reaction pathway with Pt/NbaO5 catalysts for the critical reduction temperature of 350°C. In this work, we present results obtained during hydrogenation of crotonaldehyde over Pt/Ti02 catalysts reduced at low temperature (200°C) for which we already found a good selectivity [12]. Attention was focussed on the influence of the PHn/Pcrotonaldehyde partial pressure ratios. Additional studies with the mono-dihydrogenated intermediate reaction products (crotyl alcohol and butyraldehyde) were also performed.

EXPERIMENTAL The 4.7 wt % PtfI'i02 catalyst used in this study was prepared by adding dropwise, followed by thorough mixing, a n aqueous solution of hexachloroplatinic acid to a Ti02 support (2 cm3 of H2PtCl6,6H20 solution per g of support) from Tioxide (specific area: 46 m2 g-1, porous volume: 0.5 cm3 g-1). The small excess of water was slowly evaporated on a hot plate. The catalyst was then dried overnight at 120°C and stored under air until use. Each sample was dried again under air a t 120°C just before use. Reduction of the catalyst was performed during 16 h a t 200"C, reached a t a temperature of 3°C min-1, "in situ" in the reactor of the catalytic apparatus already described [13], under a hydrogen flow of 50 cm3 min-1 a t atmospheric pressure. The reactor was then slowly cooled down to the reaction temperature (60°C) under hydrogen. Pulses of 25 or 50 p1 of crotonaldehyde-CROALD (Fluka, puriss) were passed over the catalyst a t partial vapor pressures of 8f1,13f2 and 25f2 Torr (PHJPCROALD = 95, 60 and 30 respectively). The hydrogen flow was varied between 20 and 80 cm3 min-1 so as to modify the overall conversion. Duration of the pulses were therefore comprised between 40 and 10 min. In the same way, pulses of crotyl alcohol-CROALC (Sigma, purum) were performed with partial vapor pressures of 1, 7 f l and 12f2 Torr (PH$PCROALC = 760, 110 and 60, respectively), while pulses of butyraldehyde-BUTNAL (Fluka, puriss) were studied a t a partial vapor pressure of 7k1 Torr (PH~PBUTNAL = 110). Changes of activity and selectivity were monitored as a function of time during each pulse. Samplings were taken every 2 f 1 min by the way of syringes that were stored until analysis. It was checked that there is no loss or specific adsorption of the products in the syringes. The samplings are always performed when the pressures, recorded by catharometers placed before and

221

after the reactor, respectively reactant and reactant + product pressures, are the same. The reaction products were analyzed by gas chromatography on a 1540 carbowax packed stainless steel column of 4.5 m length and 118 inch diameter. Processing of the data were done using a Delsi-Niermag device. No noticeable activity was detected neither in the catalytic apparatus alone nor on the Ti02 support itself.

REsuLls During the hydrogenation reaction of CROALD, several products were detected: CROALC and BUTNAL that result from the hydrogenation of the C=O and C=C bonds, respectively, butanol (BUTNOL) and different hydrocarbons (HC) of which the nature was not determined on the chromatographic column used. Changes in activity and selectivities were observed during the course of the pulses that are reported in figure 1 for the 3 CROALD partial pressures (PCROALD) studied and for a total gas flow of 30 cm3 min-1. Similar features were observed with other gas flows. Crotonaldehyde hydrogenation over a 4.7 % PVTiO2 catalyst reduced at 200°C: activities and selectivities obtained after stabilization (reaction temperature = 6OoC) gas flow PCROALD conv. Torr (a) %

cc min-1

18 31 67 ~

8

8 8

act. (b)

33 23 1 8 2 0 10 20

time (C)

mjn

17 9 5

SCROALCSBUTNALSBUTNOL SHC %

%

%

%

14 25

54(d) 42 53

23(d) 25 13

9 8 6

28(d)

~~

m

13 15 13 12 19 63 11 7 34 13 9 19 7 22 Q 10 6 4 20 3.5 78 13 21 66 8 5 a0 25 11 17 12 19 63 12 6 30 25 6 2 0 7 19 619 7 5 48 25 4 2 0 5 13 76 7 4 (a) overall conversion (b) activity expressed in pmole of CROALD having reacted per second and per g of Pt calculated by aF/w (a: conversion, F: flow rate, w: Pt weight) (c) duration necessary to reach the steady state (d)value obtained at the end of the pulse (steady state not reached) During time on stream, the overall conversions decrease (transient state: TS) t o finally reach a stable value (steady state: SS), depending on PCROALD (fig. la). Nevertheless, when considering the calculated activities, expressed in pmole of CROALD having reacted per second and per gram of platinum, no is observed (fig. lb).The final rates, when the S S is influence of PCROALD reached during the pulse, are similar and are equal t o 19f4 pmole gpt-l s-1.

222

activity

conversion

80

rn ._

I""

75 25

tg

1

50 u 0

30 t

60 40

20

0 10 20 time on stream (rnin)

0

SCROALC

10 20 time on stream (rnin)

.

SBUTNAL

80 I

20 10 n "

-?u

40

0

10 20 time on stream (rnin)

0

SBUTNOL

20 .

30

15

-

20

10

-

10

5-

U

0

10 20 time on stream (rnin)

10 20 time on stream (rnin)

0

I

I

Eigure 1 Variation of conversion (%) [la], activity (pmole s-1 gpt-1) [lb] and selectivities (%I in CROALC [lcl, BUTNAL [Id], BUTNOL [le] and HC [1Q during time on stream of CROALD on a 4.7 Pt/TiOz catalyst at 60°C for P H ~ ~ C R Oratios A L Dof 95 (o), 60 (A) and 30 ( 0 ) (gas flow = 30 cm3 min-1).

223

selectivity

selectivity

100

75

50 25

n 0

100 time on stream ( m i d

200

0

10 20 time on stream ( m i d

100

_El 50

ip

75

30

El

-

50’

-

257==25

I

I

ure 2 Hydrogenation of CROALC a t 80°C on a 4.7 Pt/TiO2 catalyst. Variation of selectivities ( 0 HC, o BUTNAL, A BUTNOL) and conversion (+) (in =1 figs 2a and 2b, conversion = 100%) during time on stream. [2al: PCROALC Torr, gas flow = 37 cm3 min-1; [2bl: PCROALC = 7 Torr, gas flow = 35 cm3 min-1; [2c, 2dl: PCROALC = 12 Torr, gas flow = 67 and 38 cm3 min-1, respectively. The same value is obtained whatever the gas flow (table 11, showing that no diffusion phenomena occur. The duration after which the SS is observed is (table 1). inversely proportional to the gas flow but is independent on PCROALD The selectivity in CROALC (SCROALC) sharply increases fiom 0 to 22+4 % during time on stream (fig. lc). At the SS, SCROALC is slightly enhanced when PCROALD is low (table 1)except for the lowest gas flow. BUTNOL (SBUTNOL) and HC The selectivities in BUTNAL (SBUTNAL), (SHC) show a clear tendency t o decrease during time on stream (figs Id, e and f, is enhanced from 42 t o 68 % when PCROALD respectively). At the SS, SBUTNAL is increased (fig. lc). The greatest differences are observed between 8 < PCROALD < 13 Torr (fig. lc, table 1). At the opposite, SBUTNOL and SHCare the highest for low PCROALD(figs l e and 10. It is more pronounced for BUTNOL (SBUTNOL decreases from 25 to 7 % when PCROALD increases from 8 to 25 Torr). Similar features are obtained with other gas flows (table 1). Stable

224

selectivities in HC and BUTNAL are generally more rapidly obtained than that

of BUTNOL and CROALC. Hydrogenation reactions of CROALC, performed a t 80°C, lead to overall equal to 1and 7 conversions of 100 % all along time on stream for both PCROALC Torr. Nevertheless, changes in selectivity were observed during the pulses, the (figs 2a and 2b). shape varying with PCROALC When PCROALC = 1 Torr (fig.2a1, only HC are formed initially. Then, SHC decreases t o roughly 10 % in about 40 min with a concomitant enhancement of the selectivity in BUTNOL. Few isomerisation reactions in BUTNAL are obtained. When PCROALC = 7 Torr (fig.2b1, BUTNOL is the main product formed (SBUTNOL = 78 %). A slow decrease (increase) is observed during time on The SS is reached after about 20 min. stream for SHC(SBUTNOL). = 12 Torr (figs 2c and 2d), the overall conversion i s then When PCROALC lower than 100% and decreases during time on stream. The selectivities are quasi stable all along the pulses (SBUTNOL = 54+4, SHC= 25k2 %). Here, the = 17+2 %). isomerisation reaction becomes important (SBUTNAL The hydrogenation reactions of BUTNAL (PBUTNAL= 7 Torr), performed a t 8OoC, lead to overall conversions of 80 %, with quasi no variation of the distribution in reaction products (SBUTNOL = 94 %, SEIC= 6%)during time on stream. Similar results were observed with higher PBUTNAL.

DISCUSSION In a previous study [12], the pulse injection method allowed us to detect on a 1.9 wt % Pt /Ti02 catalyst, the presence of strongly and weakly poisoned sites, effective during the first pulse of CROALD and during all the following ones (reproducible slate), respectively. The variations of activity and selectivity during time on stream are different in the initial and reproducible states. The experiments performed in this study occur on a catalyst in its reproducible state. So, the observed decrease of activity is due to the presence of these weakly poisoned sites, activity that is recovered a t the beginning of each new pulse. From the present study it is clear that, either in the transient state or in the steady state, the total activity of the catalyst is independent on the CROALD pressure. It suggests that in our experimental conditions the catalytic surface is fully covered by CROALD in both cases. At first sight, large variations in selectivities are observed. This point will be discussed further but first and foremost it is worthwhile to note that, for a same conversion, the selectivities in the transient period differ from those observed in the steady state. Therefore the two regimes will be distinguished: transient state and steady state.

225

Transient state The duration of this period depends on the gas flow so as the total quantity of gas (Hz+CROALD), having passed over the catalyst before the reaching of the SS, is constant (250 ? 50 cm3; table 1: gas flow x time) whatever the partial pressure of CROALD. This period corresponds to the accomodation of the catalytic surface to the gas mixture. It is a reversible process because it is observed at the beginning of each new pulse. During this period, the variations of the different product yields show that the hydrogenation of CROALD into BUTNAL is largely poisoned as well as the consecutive hydrogenation of CROALC into BUTNOL and HC. On the contrary the hydrogenation of CROALD into CROALC is not affected. In other words, the transient period corresponds t o a selective poisoning of part of the C=C hydrogenation sites while C=O hydrogenation sites are not poisoned.

Steady state Using various PHB/PCROALD ratios and flow rates, different conversion ranges have been obtained. Therefore the selectivities have t o be cautiously examined. Actually, if one considers the mole fraction of each formed products as a function of the conversion, BUTNAL varies linearly with the conversion (up to 38 %) independently on the PH2/PCROALJ) ratios while CROALC increases with this ratio increasing. Moreover, it appears that BUTNOL and HC are secondary products, issuing mainly from consecutive CROALC reaction steps. These results agree with the higher reactivity of CROALC as compared to BUTNAL and CROALD, leading mainly to BUTNOL and HC.

CONCLUDING REMARKS

A discussion about the nature of the catalytic sites is out of the scope of this paper. Nevertheless, these results provide additional evidence that two different sites are involved in respectively C=C and C=O hydrogenation. The C=C hydrogenation reaction could occur on the metallic sites that would be largely poisoned in the first minutes of the reaction, while the C=O one occurs a t the interfacial sites between metal and support, as already postulated for catalysts supported on Ti02 [9, 11, 141. Nevertheless, in this study, the catalysts are only reduced a t 200°C, meaning that they do not present the metal-support interaction state as originally defined by Tauster et al. [151 and further assigned to migration of TiO, species onto the metallic particles. Actually, we have to postulate the existence of other interfacial sites, particularly reactive for C=O hydrogenation. As we have observed that these interfacial sites are sensitive t o hydrogen pressure since the CROALC yield is found enhanced with the increasing of the P H ~ / P C R O A Lratio, D they could be constituted by "hydrogen rich Pt-TiO2" ensembles from which hydrogen could be released in an homolytic way such as t o favorably polarize the C=O bond. Munuera and Co [16] showed that hydrogen is incorporated in Rh/TiO2 catalysts under an hydride form leading to HTiO, species of which mobility onto metallic particles is enhanced, giving interfacial Rh-HTiO, species.

226

The results presented here tend to show that H-Pt-Ti02 sites exist even after reduction a t low temperature and that they are able to play an important role in selective hydrogenation of a$ unsaturated aldehydes.

1 R. Touroude, J. Catal., 65 (1980) 110 2 M.D. Padley, C.H. Rochester, G.J. Hutchings, I.P. Okoye and F. King, 10th Inter. Congress on Catalysis, Budapest (19921, Preprints p. 329 3 A. Waghray, J. Wang, R. Okaci and D.G. Blackmond, J. Phys. Chem., 96 (1992)5954 4 T.B. Marinelli, J.H. Vleeming and V. Ponec, 10th Inter. Congress on Catalysis, Budapest (19921, Preprints p. 199 5 P. Beccat, J.C. Bertolini, Y. Gauthier, J. Massardier and P. Ruiz, J . Catal., 126 (1990)451 6 H. Noller and W.M. Lin, J . Catal., 85 (1984) 25 7 S.S. Lawrence and J.A. Schreifels, J. Catal., 119 (1989) 272 8 R. Hubaut, M. Daage and J.P. Bonnelle, Applied Catal., 22 (1986) 231 9 M.A. Vannice and B.H. Sen, J . Catal., 115 (1989) 65 10 A. Jentys, C.G. Raab and J.A. Lercher, 10th Inter. Congress on Catalysis, Budapest (19921, Preprints p. 314 11 H. Yoshitake and Y. Iwasawa, J. Catal., 125 (1990) 227 12 R. Makouangou, A. Dauscher and R. Touroude, 10th Inter. Congress on Catalysis, Budapest (19921, Preprints p. 327 13 F. Garin, G. Maire and F.G. Gault, Nouv. J. Chim., 5 (1981) 553 14 M.A. Vannice, Catal. Today, 12 (1992) 255 15 S.J. Tauster, S.C. Fung and R.L. Garten, J . Amer. Chem. SOC., 100 (1978) 170 16 G. Munuera, A.R. Gonzalez-Elipe, J.P. Espinos, J.C. Conesa, J. Soria and J. Sanz, J. Phys. Chem., 91 (1987) 6625

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

227

SELECTIVE HYDROGENATION OF FATTY ACID ETHYL ESTERS ON SEPIOLITE-SUPPORTED Ni and Ni-Cu CATALYSTS.

F.M. BAUTISTA, J.M. CAMPELO, A. GARCIA, R. GUARDERO, D. LUNA, J.M. MARINAS and M.C. ORDOREZ. Department of Organic Chemistry, University of Cordoba, San Albert0 Magno Av., E-14004 Cordoba, Spain. ABSTRACT Liquid-phase selective hydrogenation of ethyl 1 inolate to ethyl oleate has been carried out on nickel catalysts supported on sepiolite as well as on several different supports. The influence of metal loading and Ni-Cu alloying has been studied as well. The results indicate that catalytic activity and selectivity correlate closely with some textural and/or acidbase properties of the support and selectivity increases with metal loading. Furthermore, as a general rule, Ni-Cu alloying improves in selectivity. INTRODUCTION The usual, widely used catalyst in edible oil and fat hydrogenation is still the standard combination of nickel with kieselguhr. However, various promotors such as A1203, Ti02, Zr02 or MgO are actually added in the catalyst formulation [1,21. Kieselguhr, a natural silica, is used as a support not only due to its relatively low cost but also because it acts as a filter aid, promoting selectivity, and, of course, providing a high surface area. In this respect, clay mineral sepiolite, a hydrous magnesium silicate: SiI2Mg8O32.nH20 [ 31, used currerltly as an industrial adsorbent especially in processes involving refining of mineral or fatty oils, has also proven to be an excellent support for nickel catalysts [ 4 , 5 1 in selective hydrogenation reactions. This report evaluates the catalytic behaviour of sepiolite-supported metal catalysts in the hydrogenation of fatty acid esters in order to verify the potential use of such clay supports in production-scale hydrogenation of oils and fats. EXPER MENTAL The syntheses of Ni and Ni-Cu supported catalysts were carried out by impregnation o f the supports to incipient wetness with aqueous solutions of nickel or nickel and copper as previously described 15-11 1. Bulk nickel was

228

obtained by reduction of nickel oxide (Merck, p.a.). An as received natural sepiolite (Sep) from Vallecas (Madrid) supplied by Tolsa S.A. was used as support with nominal chemical composition of Si02 62.0, MgO 23.0, A1203 1.7, Fez03 0.5, CaO 0.5, K20 0.6, Na20 0.3, weight loss from 293 to 1273 K 10.5%. In order to evaluate the effect of the support, we used several supported nickel catalysts (20 wt% Ni) previously studied in the liquidphase selective hydrogenation of 1,4-butynediol [ 51 as a reference. The support components of these catalysts are not only habituals such as silica (SO2, Merck); alumina (A1203, Merck); or active carbon (C, Panreac), but they also include three different AlP04 prepared according to Kearby [121 by precipitation from aluminum chloride and phosphoric acid using ammonium hydroxide solution (A1PO4-F), ethylene oxide (A1P04-E) and propylene oxide (AlP04-P); and three A1P04-A1203 (75-25 wt.%) systems similarly obtained (A1P04-A1203-F, E and P, respectively). The detailed syntheses procedures and textural properties have been published elsewhere [ 5 .1 They are sumnarized in Table 1, where the surface acidity and basicity of supports are also shown. These values were determined by a spectrophotometric method [I31 that allows titration o f the irreversibly adsorbed benzoic acid (BA, pKa= 4.19) or pyridine (PY, pKa= 5.251, employed as titrant agents o f basic and acid sites, respectively. Three nickel catalysts (Harshaw Chemie B.V. were also used as a reference: a Ni-5333 T (20 w t % Nil, a Ni-5132 P (64 w t % Ni) and a Ni-3210 T (35 w t X Nil. The metal loadings of the catalysts were determined by Atomic Absortion Spectrometry (AAS) The corresponding metal surface areas were calculated from the average crystal1 ite diameter, obtained by X-Ray Diffraction (XRD) measurements as reported in previous papers [5-111. The values obtained are summarized in Tables 2 and 3. Some catalysts were also studied by Transmission Electron Microscopy (TEM) on a Philips EM 300 type equipment. An IBAS I 1 KONTRON digital image analyser [I4 1 was used for the automatic counting of metal particles by directly using negative images. Fig. 1 shows a positive image of the Ni/Sep catalyst obtained by a Canon color laser copier 200 after negativefpositive image digital treatment. The values of crystallite diameters obtained by this method are exhibited in Fig. 2, where the values obtained from XRD and final metal loadings from AAS are also shown. Both procedures give the same results. Hydrogenation reactions [ 5-11 I were carried out in a conventional low pressure hydrogenator at controlled temperature conditions. Ethyl 1 inoleate

.

229

TABLE 1 Textural and acid-base properties of different supports. V d Acidity Basicity SUPPORT SBET (m2 9-11 (ml 9-11 (nm) ( mol 9-11 ( mol g-1) 31 174 Sepiolite 203 0.54 5.3 A1 O3 72 0.24 2.7 23 191 366 0.68 3.5 206 164 Sd2 743 0.55 1.5 124 132 C 228 0.94 2.5 227 166 A1 PO4-P 242 0.52 4.3 267 266 A1 PO4-E 156 0.68 3.6 190 200 A1 PO4-F 31 9 0.68 4.2 326 774 A1 P04-A1203-P 242 0.54 4.5 208 577 A1 P04-A1203-E 244 0.37 3.1 187 535 A1 P04-A1203-F Ni-bulk 16

---

---

---

---

TABLE 2 Support influence on catalytic activity ( p m o l s-l g'l) and selectivity, S ( X ) , in the hydrogenation o f ethyl linolate, rL, and ethyl oleate, rO, on 20 w t X supported nickel catalysts. Nickel surface, SNi, in m2 gNi-l. SUPPORT 'N i rL PO RL,O KL,O 5 Sepiolite

40a 46.0 45 11.5 27 2 8.9 t12$3 26 5.9 AlP04-P 32 47.8 A1 PO4-E 66 58.4 AlP04-F 56 13.9 A1 P04-A1203-P 42 2.5 A1 POq-A1203-E 32 3.6 AlP04-A1203-F 103 6.3 Ni-bul k 13 3.9 Ni-5333 T 1 30b 2.7 Ni-5132 P 1 93b 6.4 Ni-3210 T 1 25b 8.7 a metal loading 28.3 wtj. b BET Surface area in m g-1.

sio

0.91 2.70 3.45 0.98 1.92 1.02 0.70 0.31 0.23 0.20 0.01 1.35 3.15 4.39

3.6 I .4 0.3 1.3 15.8 4.7 2.6 4.4 9.3 14.6 27.2 16.3 30.3 39.8

0.1 0.3 0.1 0.2 0.6 0.1 0.1 0.6 0.6 0.5 0.1 8.3 15.1 20.1

88.4 74.4 50.0 73.2 97.1 90.8 84.5 90.3 95.2 96.9 98.3 97.2 98.5 98.8

and ethyl oleate (Fluka p.a. 1, hydrogen (99.999 X , S.E.O.) and methanol, ethanol, I-propanol, 2-propanol, 1 -butanol , cyclohexanol and THF (99X, Panreac) and DMF (Merck)were used as solvents without further purification. Most hydrogenation reactions were carried out in 25 ml o f 0.5 M solutions of substrate in methanol, at 323 K, under initial hydrogen pressure of 0.41 MPa with 0.3 g of catalyst (particle size <0.149 mm). One set of reactions was carried out with Ni(39.4 wt%)/Sep in the pressure range of 0.3-0.7 MPa,

230

Fig. 1. TEM micrograph of 28.3 wtX Ni/Sep catalyst, with 120.000~. substrate concentration of 0.5-3 M, catalyst weight range of 0.05-1 g, in order to test the influence of these parameters on the catalytic activity and selectivity and to prove the lack of mass transfer limitations in the range of the studied operation variables. The influence of different solvents was also investigated (Fig. 3). Since we obtained zero-order kinetics with respect to the olefin concentration as well as to the initial pressure o f hydrogen, the initial reaction rates can be considered to be the corresponding rate constant values. The reaction mixtures were analyzed by GC with FID and a column packed with 10 X ethylene glycol succinate in 80/100 Chromosorb GAW-DMCS at appropriate intervals of hydrogen uptake. RESULTS AND DISCUSSION From the results of the AAS experiments, we can conclude that most sepiolite structure is destroyed during the catalyst synthesis due to the loss of constitutional water. Thus, we may assume that the support in Ni/Sep catalysts is basically made up mostly of a 3Si02.2Mg0 amorphous mixture. However, X-Ray and TEM experiments indicate that a smaller portion of sepiolite structure remains undamaged. Thus, in Fig. 1 we can see metal particles supported on the typical neddles constituting the sepiolite structure. In the consecutive process : H2

CH2)7-COOCH3 CH3 ( CH~)~-CH=CH-CH~-CHPCH-( CH3( C H ~ ) ~ - C H P C H - ( C H ~ ) ~ - C O O C H ~

c

H2

-

CH3( CH2)j 64OOCH3

231

Fig. 2. Metal loading influence on catalytic activity and selectivity in the hydrogenation of ethyl linolate and ethyl oleate on sepiolite supported nickel catalysts. Simbols and units as those described in Table 2. the relative reactivities of linoleic and oleic ethyl esters, R L , ~ , as well as the corresponding relative adsorption constants, KL,o, were successfully obtained by introducing a modification into the classical kinetic equation of competitive hydrogenations [ 10 .1 The selectivity values, S, indicating the relative concentration of hydrogenated compounds [ 151 were also obtained. TABLE 3 Influence of copper loading on catalytic activity and selectivity, in the hydrogenation of ethyl linoleate and ethyl oleate on Ni-Cu bimetallic catalysts. Symbols and units as those described in Table 2. SUPPORT XNi XCU S N ~ rL Po RL,O KL,O S Sepiolite Sepiolite Sepiolite Sepiolite C C C C AlP04-P A1 PO4-P A1 PO4-P A1 PO4-P

22.6 20.3 18.5 16.3 20 20 20 20 20 20 20 20

7.9 0.6 0.3 0.1 7.0 0.6 0.3 0.1 7.0

0.6 0.3 0.1

72 70 77 61 59 52 57 50 84 82 65 85

0.43 0.51 0.71 0.60 1.23 1.27 1.79 1.44 0.21 0.64 0.89 0.66

0.45 0.43 0.46 0.13 0.44 0.16 0.40 0.17 0.38 0.14 0.33 1.38

18.6 26.0 37.8 24.2 11.5 25.0 33.9 1.5 60.6 21.8 129.4 15.5

19.7 21.9 24.3 5.1 4.1 3.5 7.5 0.2 107.8 4.6 48.2 32.5

97.5 98.2 98.8 98.1 96.1 98.2 98.6 76.7 99.2 97.9 99.6 97.1

232

Fig. 3. Influence of the dielectric constant of solvents (E, in Debyes) on catalytic activity in the selective hydrogenation of substrates on 39.4 w t %

Ni/Sep catalyst. Simbols and units as those described in Table 2. According to the results in Fig. 3, in all solvents studied, rL > ro and, on increasing dielectric constant values of alcohol solvents, rL and ro increase, as previously obtained in several olefinic compounds [ 1 6 1, as well as S, RL,o and KL,-,, according to the results obtained in the liquid-phase selective hydrogenation of 1,4-butynedioI [ 5 .1 Thus, most experiments were carried out using methanol, the best solvent. With respect to the support influences (Table 21, they promote important changes in catalytic activity and selectivity. Thus, in order to determine the influence of textural and acid-basic properties of the supports on the catalytic properties of supported nickel catalysts, a correlation matrix using all the data in Table 1 and Table 2 was built. Results of the regression analysis of the well correlated parameter pairs are shown in Table 4. According to these, it is seen that the catalytic activity decreases on those catalysts whose supports exhibits a higher number of basic sites. Furthermore, selectivity increases with higher BET surface area and/or high pore volume and/or high surface acidity, as measured with PY. Accordingly, the best results for rL are obtained with Ni/Si02 and NiISep and for ro with Harshaw catalysts and Ni/A1203, so that rL / ro values differ strongly for different supports, ranging between 2, in Harshaw catalysts, and 50, in Ni/Sep. However, selectivity and R L t O follow a different sequence due to the double dependence, RL,o = (rL / ro) KL,o. Thus, the high selectivity in Harshaw

233

TABLE 4 General expression of the correlation y = ax + b obtained between some surface properties o f the supports in Table 1 and the corresponding catalytic properties of supported nickel catalysts in Table 2. Y X a b Significance(%) In rL BA -0.0036 3.745 97.9 In ro BA -0.0035 0.942 99.8 -185.231 1 -0.634 94.8 1n KL,O $;ET -0.6374 -2.839 90.4 In KL,0 1I P Y -53.0550 -0.934 99.6

;;

In s$0 In s '/RL,O 1/RL,0

1/RL,O

1 1$$ET 1$$ET 1/PY

1/PY

-41 .3829 -0.1642 -1 0.0634 206.3400 0.7949 46.6001

4.618 4.740 4.531 -0.434 -0.992 0.044

97.5 97.9 97.7 99.6 99.5 99.0

catalysts is explained by its higher KL,0 value while in Ni/Sep it can be associated with the highest value in rL / ro. All these facts, as well as the influence of metal loading on catalytic activity and selectivity, may be associated with metal-support interaction effects. An increase in metal loading in Ni/Sep catalysts (and in the metal/support ratio) leads in general (Fig. 2 ) to an increase of activity as well as to a higher selectivity, due to the increase in both rL / ro and KL,O values. Finally, the influence of Cu as a second metal in the selectivity is closely related to its influence in the K L , ~parameter. Thus, regardless of which support is used, the highest selectivity 0 9 8 % ) is closely related to the significant increase in K L , ~ , especially when Ni-Cu are in the proportion 20-0.3. Besides, catalytic activity is lower in bimetallic catalysts. Thus, taking into account that changes in the relative adsorption constant values of competitively hydrogenate substrate pairs have been used to probe changes in the electronic structure of platinum and other Group VIII metals [ 1 7 1 as well as in Ni-Cu alloys 112 I, the electronic influence of Cu promoting such an important change in R L , ~ and KL,0 ought to be considered as being the first responsible for the selectivity improvement promoted by the addition of this element to supported nickel catalysts. CONCLUSIONS We can conclude that Sepiolite could be an adequate support component to enable tailored Ni-Cu catalysts in oil and fat hydrogenation by taking into account how similar the results are when sepiolite is used as the support

234

instead o f A1P04 (which is the best o f all studied supports) and its comparatively lower cost as well a s the fact that, the Ni-Cu alloy (especially with a 20/0.3 proportion) exhibits substantially higher selectivity than the monometallic supported nickel catalyst, including comnercial catalysts we studied. ACNOWLEDGEMENTS The authors gratefully acknowledge the subsidy received from the Comisidn de Investigacidn Cientf fica y TBcnica (DGICYT, Project P889-0340), Ministerio de Educacidn y Ciencia as well as the financial aid from the Consejerfa de Educacidn y Ciencia d e la Junta de Andalucfa. The authors would also like to thank Harshaw Chemie B. V. for providing some catalyst samples and wish to acknowledge the grammatical revision of the manuscript carried out by Prof. M. Sullivan and the valuable help provided by Prof. F. Gracia of the Cell Biology Department in the use o f IBAS I 1 KONTRON. REFERENCES 1 R. J. Grau, A. E. Cassano and M. A. Baltanas, Catal. Rev. Sci. Eng., 30 (1988) 1. 2 J. W. E. Coenen, Ind. Eng. Chem. Fundam., 25 (1986) 43. 3 Y. Grillet, J.M. Cases, M. Francois, J. Rouquerol, and J.E. Poirier, Clay and Clay Minerals, 36 (1988) 233. 4 Ger. Offen.2., 108 (1971) 276. 5 F. M. Bautista, J. M. Campelo, A. Garcfa, R. Guardeiio, D. Luna and J. M. Marinas, in "Heterogeneous Catalysis and Fine Chemicals II", Studies in Surface Science and Catalysis Vol. 59, M. Guisnet et al., Eds., Elsevier, Amsterdam, 1991, p.269. 6 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, Appl. Catal., 3 (1982) 315. 7 J. M. Campelo, A. Garcia, J. M. Gutierrez, D. Luna and J. M. Marinas, Appl. Catal., 7 (1983) 307. 8 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, J. Chem. SOC., Faraday Trans. I, 80 (1982) 223. 9 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, J.Catal., 97 (1986) 108. 10 F. M. Bautista, J. M. Campelo, A. Garcfa, R. Guardeiio, D. Luna and J. M. Marinas, J. Catal., 125 (1990) 171. 1 1 F. M. Bautista, J. M. Campelo, A. Garcfa, R. Guardeiio, D. Luna and J. M. Marinas, J. Mol. Catal., 67 (1991) 91. 12 K. Kearby, in: Proc. 2nd. Inter. Congr. Catal., (Technip. Ed.), Paris, 1961, p. 2567. 13 J. M. Campelo, A. Garcia, J. M. Gutierrez, D. Luna and J. M. Marinas, Can. J. Chem., 61 (1983) 2567. 14 W. Kllditz, Praktische Metallographie, 18 (1981) 105. 15 L. Cerveny, J. Vopatova and V. Ruzicka, React. Kinet. Catal. Lett., 19 (1982) 223. 16' F. h. Bautista, J. M. Campelo, A. Garcia, R. GuardeRo, D. Luna, and J. M. Marinas, J. Chem. SOC. Perkin Trans. 11, (1989)493. 17 T. T. Phuong, J. Massardier and P. Gallezot, J. Catal., 102 (1986)456.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals !I 0 1993 Elsevier Science Publishers B.V. All rights reserved.

235

Selective hydrotreatment of rapeseed oil on NickelCerium mixed oxides modified by Al additive A. Aloucheqb, R.Hubauta, J.P. Bonnellea, Ph. Daviesb and D. L a m b e d

auboratoire de Catalyse HCtfiro h e et Homogene URA CNRS 402, UniversitC des Saences et Technologies de Ldle,99655 Villeneuve d'Ascq CCdex (France).

bSociCtC de la Raffinerie B.P. et ELF de Dunkerque, Route de I'Ouvrage Ouest, B.P. 4519,59381 DUNKERQUE CEDEX 1. Abstract Some new rare-earth based oxide catalysts are used to partially hydrogenated the rapeseed oil. The binary oxide Ce-Ni-0 presents a good selectivity in the partial hydrogenation but a large extent of Z/E isomerization. The ratio of the iodine value (IV) variation over the pourpoint (PP) variation is lower than one. The introduction of aluminium in the catalyst formulae leads to a significant decrease in the pourpoint variation due to the quasi-elimination of the Z/E isomerization. The oils obtained are more resistant to oxidation. 1. INTRODUCI'ION

Fats and oils are required for each are soybean and rapeseed use of this oil is the preparation of oil is also used in many has to remain liquid at low to avoid olymerisation. fatty acid ound to glycerol (Table 1). As the o gen absorption for linolenic acid, linoleic acid and oleic acid is respectively 800:100:1, g e commercial oil must contain a low percentage of trienic and dienic comounds. In order to fulfil these conditions, the initial rapeseed oil must be semidroeenated to decrease the iodine value. Moreover, with artial hydrogenation of the it IS also important to avoid increase the pourpoint. tor a hydrogenated oil, the pourpoint depends on both the saturated compound content and the extent of the Z/E lsomerization which occurs during the experiment. The E isomer has always a meltmg oint higher than the Z isomeric counte art (1). Unfortunately, the conjugated double gonds systems are more reactive than met ylene interrupted double bonds in competitive hydro enation, and the catalytic hydrogenation is often followed by a Z/E isomerization&

E

1:

236

Table 1 Initial constitution of the commercial rapeseed oil. (%)

Fatty Acids

palmitic acid 5.1 C16 : Oa 0.2 C16 : l(Cis) gdidic acid 1.5 C18 :0 stearic acid 60.1 C18 : l(Cis) oleic acid 21.9 C18 : 2(Cis, Cis) linole'ic acid linolenic acid 11.2 C18 :3(Cis, Cis, Cis) a :the index are indicative of the number of double bonds So far, only some homogeneous catalysts are able to partiall hydrogenate theve etable oils without changes of the cis configuration (3,4). However,tie association of nic el and ceria is really interesting because nickel is very active for hydrogenation and ceria can store hydrogen in its bulk (5).So, the aim of this work being to obtain a maximum of monoethyleniccompound,by a selective hydrogenation, with a minimum of Z E isomerization,some cerium and nickel mixed solids,prepared by a new method in our aboratory (6), have been tried.

f

(

2. EXPERIMENTAL

2.1. Catalysts The catalysts were prepared from nitrates of cerium, nickel and aluminium, if any, by copreci itation of the corresponding hydroxides using potassium hydroxide, followed by ra id fi tration to avoid the growing of the cristallites. The solids were then washed with lot water until neutral pH was reached and dried under air during 18h at 120°C. The solids were reduced under hydrogen flow at 300°C for 16 hours before test.

P

2.2. Catalytic tests The catalytic reactions were carried out in a catalytic flow microreactor at atmosheric pressure and various temperatures. The catal ic bed (lg) was covered by silica. The reaction conditions were the fo lowing: the oil (?O(wt%) in cyclohexane) was introduc d with a flow of 0.12 mlxmin-1 simultaneoulsy with hydrogen (flow = 20 mlxmin- 1. After evaporation of the solvant, the products were successively treated by sodium methoxide, methanol and sulfuric acid to obtain the free-esters before analysis. The final products were analysed by gas chromatography with a flame ionization detector and AT-FILAR (Altech) capillary column (30m, 1.d = 0.32 pm, film thickness = 0.25 pm) at 140°C. The extent of the h drogenolysis reaction C1g ---- C16 is always lower than 0.5 % and the total amount of tie C1g compounds is equal to 100 in all the tables of results. The extent of Z/E isomerizationis calculated as the all-Z isomericcompound missing for each corn onent after the experiment.The iodine value (IV),which is representative of the num er of double bonds present in the molecule, is measured by the Wis' method and the pourpoint (PP),related with the trouble of the solution, by the ASTM tl 97 method. The activities are expressed as the percentage of reactants and products present in the effluents.

f

!

3. RESULTS AND DISCUSSION 3.1. Binary oxide Ce-Ni-0 (Ni(Ce = 2) Activities and selectivities of the binaxy Ce-Ni oxide (Ni/Ce = 2) are dependent

237

on the e rimental conditions. The decrease in the initial iodine value (Iv)is a linear r e c i p r a b c t i o n of the temperature (figure l), and is directly proportional to the hydrogenation activity.

Figure 1

Rapeseed oil hydrotreatment on Ce-Ni-0 (Ni/Ce = 2). Iodine value versus the reaction temperature.

The product distribution is strongly dependent u on the temperature (figure 2), but it is mainly the amount of monoeth lenic com oun which increases whatever the ternerature from 60.1% in the initidoil up to !t 8.9% and 77.6% at respectively 190°C and 50°C). Lore than 60% of the total hydrogenation comes from the transformation of diethylenic into monoethylenic compounds.

B

Figure 2

r

Rapeseed oil h drotreatment on Ce-Ni-0 (Ni/Ce = 2). Product distribution as a function o the reaction temperature.

238

y

But,even if the bin oxide (Ni(Ce = 2) appears as an active and selective catalyst, the urpoint chan es w ich occur in the same temperature range remain too significant : -25°C fore!t initial oil u to + 15°C after hydrotreatment at 250°C (the main reason for this drastic increase is the igh Z/Eisomerization (figure 3)) and,in despite the good activi and the fair selectivity poly-monoethylenic compounds, this binary oxide is not suitabe for the purpose. As previously shown, the introduction of aluminium in a r-chromium oxlde can induce a change of the hydro enation/isomerization ratio the Same procedure has been applied to the Ni-Ce- catalyst by adding Al.

8.0,

K

'r

?fK,

Figure 3

%

r

Rapeseed oil h drotreatment on Ce-Ni-0 (Ni/Ce = 2). Z/E isomerization as a function o the reaction temperature.

3.2. Ternary oxide Ce-Ni-AI-0 (Ce/AI = 1, Ni Ce+AI = 2) Table 2 summarizesthe results obtaine with this catalyst in the temperature range 190"C-300"C. We notice an important decrease of the hydrogenation activity, the iodine value is only 5% lower than the initial value after hydrotreatment at 300°Cwhile it is more than 25% of the decrease at 250°C on the Ce-Ni-0 catalyst. Conversely, the percentage of change for the pourpoint are 8% and 160% respective1 Clearly, the presence of aluminium entails a significant change in the behaviour o the catalyst; however, the selectivity of the latter catalyst remains ood enough (as an example, there is no C18:O compound made up). But, the most strifing effect of the presence of aluminium is the very low extent of isomerizati0n.h order to maintain this low isomerization but improve the hydrogenation activity, a catalyst containing more nickel has been prepared.

d

fy

.

33. Ternary oxide Ce-Ni-AI-0 (Ce/AI = 1, Ni/Ce+AI = 5) As normall e ected, this catalyst is more active than the previous one (figure 4) and the iodine vdeTecrease is about 7% at 250°C.

239

Table 2 Rapeseed oil hydrotreatmenton Ce-Ni-Al-0 ;( Ce/Al = 1 ;Ni/Ce + Al = 2). Iodine value, pourpoint, product distribution (%) and Z/E Isomerization (%) as a function of the reaction temperature. Reaction temperature (“c) Initial oil Iodine value Pourpoint

117

c18:o %

1.7

c18:1 %

63.4

190 116

210 116

230 117

-25

Z/EIsom c18:2 % Z/E Isom

23.1

c18:3 % Z/E Isom

11.8

1.7 64.7 0.7 22.2 0.0 11.4

1.7 64.8 0.5 22.6 0.0 10.9 4.4

4.4

1.7 64 0.6 22.7 0.0 11.6 5

300 110

-23 1.7 68.6 0.9 20.6 3 9.1 5.1

Table 3 Rapeseed oil hydrotreatment on Ce-Ni-Al-O;(Ce/Al = 1 ; Ni/Ce+Al = 5) Z/E isomerization (%) as a function of the reaction temDerature. Reaction temperature (“C) Initial oil -25

Pourpoint c18:1 % c18:2 % C1~:3%

I

I

-18

-16

-14

-15

3

4

5.9

6.7

0.0

0.0 0.0

0.0 0.0

3 0.0

0.0

I

Table 4 Rapeseed oil hydrotreatment. Ratio of the iodine value variation A ( I V ) over the pourpoint variation A ( P P ) on rare-earth oxide based catalysts. Catalyst

A(IV)

A(PP)

7

2

3.5

8

10

0.8

30

40

0.75

A(IV>

A(PP)

Ce-Ni-Al-0 Ce/Al = 1 Ni/Ce+ Al =2 Ce-Ni-Al-0 Ce/Al =l;Ni/Ce+Al =5 Ce-Ni-0 Ni/Ce =2

240

Although the isomerization activity remains lower than that observed with the b' Ni-(3-0 system, it is interesting to notice that it is essentiallythe C18:1compound whic undergoes the Z/Echange table 3).Eventually, the product distribution obtained in this case is less interesting that t e one observed over the previous tern oxide because the unsaturated compounds are mainly replaced by a saturated produ7figure 5).

h

T

Figure 4

Ra eseed oil hydrotreatment on Ce-Ni-A-0 (Ce/Al = 1, Ni/Ce t Al = 5). Io&e value versus the reaction temperature.

Figure 5

Rapeseed oil hydrotreatment on Ce-Ni-Al-0 (Ce/Al = 1, Ni/Ce+Al = 5). Product distribution as a function of the reaction temperature.

The aim of this work bein to decrease the initial iodine value (IV)of the ra eseed oil without a lar e increase in tf;e pourpoint (PP), the table 4 shows clearly that t ie ternary oxide with aki/Ce+Al ratio equal to 2 is the best catalyst to do that. Indeed, only this catalystleads to aA ( I I/ ) / A ( P P )ratio larger than one. This ratio remains almost constant (-0.8) for CeNi2G and Ce0.5A0.5Nig9 whereas it is 4.5 times higher with

241

This is true whatever the temperature is (fi e 6). Moreover, the DSC Ce Al05Ni 0,. (D%renud !&anrungCalorimetry) experiments performe under oxygen atmosphere ( P o , = 35 bars, T = 130°C) show that the enthalpy of the oxidation reaction increases with the iodine value (fi re 7),but, the resistance to the oxidation is not a linear function of the iodine value un er the experimental conditions chosen (figure 8). Obviously, the introduction of aluminium gives rise to a product which becomes more resistant to the oxidation.

dg"

r

CeAIN12

Initial oil b

CeNQ

CeAINiS 8

+

A

Iodine value 120,

-25

-15

-20

-10

5

0

-5

10

15

20

Pourpoint'C

Figure 6 Rapeseed oil hydrotreatment. Iodine value Vs the pourpoint. Oxidation test by l3.S.C T'C=130 .oxygene pressure=35 bar W d r w n a l e d over CeNi2 a1 21o'C

Wdropenaied O W Ce0.3AI0.5N13 t l 21O'C

lnilid oil

A

Enthalpy (ioulelgl 16W

1

1100

90

95

1W

105

110

115

120

Iodine value

Figure 7

Oxidation enthalpy values (DSC experiments) versus the iodine value of partially hydrogenated rapeseed oil.

Some experiments performed on a solid havin a Ce/Al ratio of 1.5 show that the hydrogenationactivity of this third tern oxide,ric er in Ce, is close to that observed for the ternary systemwith a Ce/Al ratio of 1. owever, the Z/E isomerization becomes more hportanfeven though it remains lower than on the Ni/Ce+Al =5 counterpart. Conse-

3

fl

242

quentl it appears that the best compromise to obtain an intersting IV decrease and a small change is Ce0,5Al0,5Ni20,. Work is in progress to improve the formulae of the ternary oxide systems. Oxidation lest by D S C

h

T'C=i30 .Oxygene pressure=35 bar HydiOQensled over HydiOQenaled olel CeNc2 81 21O'C 0 0 54\10 5N15 .I 21O'C

m

lnlllll dl

A

8

90

95

I00

105

110

115

120

Iodine value

Figure 8

Time of resistance to oxidation versus the iodine value of partially hydrogenated rapeseed oil.

4. CONCLUSION

r

Because of the resence of high1 unsaturated fat acids (as indicated by a high iodine value IV = 117p the rapeseed oi shows poor stab&. It must be artially hydroenated to increase this stability.The rapeseed oil conversion and the pro uct distribution %oth depend on the Ni/Ce ratio of the mixed Ni-Ce-oxides and on the tem erature. These catal sts appear more selective than Ni alone but lead to an im ortant Z / isomerization whic Pves rise to a drastic increase in the ou oint value. e use of a ternary oxide (Ce-Ni-AI) allows a decrease in the extent o!the%/E isomerization. The results depend on the relative amount of each metal and the isomerization can be almost totally eliminated. Moreover, from some DSC experimentsunder an oxidative atmosphere,it appears that the resistance to oxidation can be improved, even at relatively high temperatures.

K

cf

.pn

E

5. REFERENCES

1 2 3 4 5 6 7

F.D. Gunstone and I.A. Ismail, Chem. Ph s. Lipids ., 1 (1967) 264. E. Ucciani, Stud. Surf. Sc. and Catal., 41 6988) 33. E.N. Frankel, J. Am. Oil. Chem. SOC.,47 (1970) 35. US Patent No.3543821-A (1970). L Tournayan, N. Romeu Marcillio and R. Frety, Ap 1. Catal., 78 (1991) 31. A. Alouche, R. Hubaut and G. Wrobel., to be publis ed. R.Hubaut and J.P. Bonnelle, React. Kinet. Catal. Lett., 47, 1 (1992) 73.

K

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals ZZI d 1993 Elsevier Science Publishers B.V. All rights reserved.

243

Kinetics of the liquid-phase stereoselective hydrogenation of 4-tertbutylphenol over rhodium catalyst D.Yu.Murzin a*, A.I.Allachverdiev and N.V.Kul'kovab aLERCS1,Instutut Le Bel,ULP,4 rue Blaise Pascal,Strasbourg,6707O,France bKarpov Institute of Physical Chemistry,Obukha 10,Moscow,103064,Russia

Abstract The kinetics of the liquid-phase hydrogenation of 4-tertbutylphenol in n-hexane was studied a t 0.4 - 4 MPa and 40-1OOOC .The kinetic model which describes experimental data with good accuracy is presented . 1.INTRODUCTION Problems of selective and stereoselective catalytic hydrogenation are of great importance in production of fine chemicalsSome aspects of selectivity in catalytic hydrogenation were discussed in [ 1,2] .The kinetics of liquid-phase hydrogenation of aromatic compounds,including phenol ,were investigated in [351 and a scheme of the reaction mechanism was presented .In this case complexes containing a molecule of the substance and one or two molecules of hydrogen are successively formed ,and the latter is isomerized into adsorbed cyclohexene or its derivative.1somerization of a complex containing one molecule of hydrogen does not take place because of thermodynamic reasons [41. In [5] the differential equations for the dependence of reactant concentrations as a function of time in the quasi-stationary reaction in a closed system were deduced and used for the description of experimental data in the case of hydrogenation of phenol over a platinum catalyst . Though liquid-phase hydrogenation of alkylphenols was intensively studied [6],kinetic m(Jdels, which represent the main features of the reaction were practically not reported with exception of 2-tertbutylphenol hydrogenation to cis and trans 2-tertbutylcyclohexanol over VIII Group metals[ 7,SI.Such reliable models are necessary not only for reactor design,but they can provide also information about mechanism of stereoselective hydrogenation,therefore being useful for catalyst optimisation.Frequently in kinetic studies actual reaction network is reduced t o single overall reactions and the rate equations are simplified to an extent when they stop reflect main features of the reaction.Quantification of the kinetics of such complex reactions as liquid-phase hydrogenation of alkylphenols therefore is of absolute necessity for investigation of the reaction mechanism. The hydrogenation of the phenyl ring in 4-tertbutylphenol is known t o yield 4tertbutylcyclohexanol (cis and trans isomers 1 ,that is used as an important intermediate for the fragrance industry [6,9] .Rhodium catalysts are characterized by high activity and stereoselectivity towards cis isomer the only *Permanent address:Karpov Institute of Physical Chemistry,Obukha 10,Moscow,103064,Russia

244

one which is of commercial interest 191, therefore we studied the kinetics of the liquid-phase hydrogenation of 4-tertbutylphenol over rhodium catalysts .

2.EXPERIMENTAL The kinetic experiments were performed in a shaked reactor with a n external heating -cooling system.The volume of the reactor was 75 ml.The frequency of double movements of the reactor was 150 min-1 and the amplitude was 15 cm.The pressure in the reactor was measured with a standard manometer and it was constant during experiment.The reactor temperature was kept within the range of 1%' of the fixed values.The hydrogenation reactions were carried out a t 40 to lOOOC and hydrogen pressure 0.4 - 4 MPa.N-hexane was chosen a s a diluent and the initial mole fraction of 4-tertbutylphenol was 0.14 and 0.3.Before an experiment the reactor ,containing 4-tertbutylphenol and catalyst was blown through with H 2 (for 1 h).During the course of the reaction samples of the solution were taken off and analysed by FID gas chromatography.The catalyst particles were less then 50 pm.A fresh portion of catalyst was taken for each experiment.We used catalyst of 3 wt.% Rh on activated carbon (BAU grade).The catalyst was prepared by adding activated carbon to water solution of RhC13 and reducing by HCOOH.The catalyst was washed from Cl-.The specific area of the carrier and the surface area of the rhodium catalyst was estimated by N2 and H2 adsorption respectively.The specific area of the support was equal t o 806 m2lg and the surface area of the rhodium was 5.7 m2Ig. The proportionality of the efficiency of the reactor t o the amount of catalyst,the calculation of the rate of diffusion of dissolved hydrogen to the outside of the catalyst particles and the efficiency factor show that the reaction took place in the kinetic region.

3.RESULTS AND DISCUSSION The following characteristic observations were made :the hydrogenation rate increases with increasing hydrogen pressure up to high partial pressures of hydrogen afterwards the zero order with respect t o H2 i s observed .The zero order with respect to the substrate ( 4-terbutylphenol) is observed in all experimental range.An investigation with different mole fractions of diluent show that hexane adsorption on catalyst is weak and there is no influence of it on reaction rate.The mole fractions of the intermediate (4-terbutylcyclohexanone) and the reaction products at fixed mole fraction of the substrate are almost independent on hydrogen pressure.When there is no substrate in the reaction mixture the mole fraction of 4-terbutycyclohexanone is not zero and this value increases with increasing hydrogen pressure.The cis I trans ratio decreases all along the experiments.A typical curve concentration -time is presented in Fig. 1.For the convenience of presenting results the initial mole fraction in Fig. 1 is equal to unity. Conversion of alkylphenols to alkylcyclohexanols and alkylcyclohexanones is always considered as consecutive reaction with the corresponding alkylcyclohexanone as an intermediate [8l,where, for the description of experimental data , a Langmuir-Hinshelwood model was proposed assuming noncompetitive adsorption of organic species and hydrogen.In several cases the pressure dependent "shunt-reaction" was added to the model ,assuming parallel formation of alkylcyclohexanol ommiting the ketone step. It was also proposed in [8] t h a t i n the case of 2-tertbutylphenol both cis- a n d t r a n s 2-

245

tertbutylcyclohexanols can be produced either by hydrogenation of alkylphenol or alkylcyclohexanone,and cis-trans isomerization occurs only through dehydrogenation via 2-alkylcyclohexanone.Thismodel does not take into account possible formation of alkyltetrahydropheno1,which was proposed a s a n intermediate as early as in 1928 [10].The model which was proposed in 181 can't also describe orders in respect to hydrogen more then unity.Such orders were reported in the case of the liquid-phase hydrogenation of several aromatic compounds and discussed in [lll.

Y

0

Time ( h )

2

Fig 1.Hydrogenation of 4-tertbutylphenol over Rh a t 4OoC and 3.0 MPa. The kinetic model of the liquid-phase hydrogenation of alkylphenols was proposed in [12].According t o this model the adsorbed enol form of alkylcyclohexanone is either hydrogenated into trans alkylcyclohexanol or it can undergo tautomeric transformations to form alkylcyclohexanone,which can be replaced on surface by substrate.In [121 the step of replacement was assumed to be an equilibrium step.In this case at the time of depletion of alkylphenol in the system only alkylcyclohexanol remains.When the step of adsorption is nonequilibria,the mole fraction of partly hydrogenated product is not zero when there is no substrut in the reaction mixture.Previously the nonequilibria desorption of 2-tertbutylcyclohexanone was proposed in [8] .As it was pointed out in [12] the cis- trans isomerization of alkylphenols exists not through dehydrogenation with formation of corresponding cyclohexanone.Such configuration isomerization ( definition from [13] ) is essentially influenced by hydrogen,which is astoichiometric component of the reaction .An activated complex which consists of molecules of hydrogen and hydrocarbon was proposed for such reactions [lZl.According to the kinetic data the reaction rate of isomerization is dependent on hydrogen pressure and the reaction order towards hydrogen decreases up to zero with hydrogen pressure increasing [ 13l.Therefore we can assume the intermediate formation on surface of an adduct of hydrogen molecule and the molecule of alkylcyclohexanol.Hence the basic reaction network of the liquid-phase hydrogenation of alkylphenols can be represented by the scheme in Fig.2. For the computation of rate laws ,the theory of complex reactions [14,15] was used.According to this theory elementary reactions are grouped into steps(stages).Chemical equations of steps contain reaction participants and

246

intermediates (surface species).Overall reaction equations can be obtained by summation of chemical equations of steps multiplied by stoichiometnc numbers (positive,zero or negative ).Such numbers must be chosen in a way that the overall equations contain no surface species.A set of stoichiometric numbers of steps was defined as a reaction route [ l l l .

OH

OH

Fig 2.Reaction network for the hydrogenation of alkylphenols.

In the case of 4-terbutylphenol, when overall reactions are irreversible , the reaction mechanism can be described by 4 reaction routes and written as follows N(2) N(3) N (4) 1.ZA +HZw ZAH2 1 1 0 1 2.ZAH2+H2 H ZAHq 1 1 1 0 3.ZAH4 + ZE 1 1 1 0 4.ZE e ZY 1 0 1 0 5.ZY+H2 oZYH2 1 0 0 0 6.ZYH2 -+ ZC 1 0 0 0 7.ZC+A Z ZA+C 1 0 0 -1 8.ZE + H2 e ZEH2 0 1 0 0 (1) 9.ZEH2 -+ ZT 0 1 0 0 1O.ZT + A S ZA +T 0 1 0 1 11.ZY +A e ZA +Y 0 0 1 0 12.ZC + H2 ZH2C 0 0 0 1 13.ZH2C H ZT +HZ 0 0 0 1 N( 1) : A+3H2 = C ;N (2) : A+ 3H2 = T ; N( 3) : A+ 2H2= Y ;N (4): C= T Here Z is a surface site ,A,Y,E,C,T, molecules of 4-tertbutylphenol,4tertbutylcyclohexanone,tetrahydrophenol and cis a n d t r a n s -4tertbutylcyclohexanol ,AH2 etc -intermediate complexes.Stages 7 and 10 are equilibria or quasiequilibria,which is shown by sign Z in mechanism (1).0n the right side of the reaction steps stoichiometric numbers for different routes are reported.

247

The Group VIII are known to adsorb considerable quantities of hydrogen rapidly.According to [4] the state of the adsorbed hydrogen resembles that of hydrogen dissolved in Pd,except that it is present within a few atomic diameters of the surface and not in the whole bulk of the metal.The adsorbed hydrogen consists of protons inserted in the near-surface layers,their electrons being cooperated together with the electrons of the metal.The hydrogen does not compete for sites with the molecules adsorbed on the surface.If the adsorbed hydrogen is in equilibrium with the hydrogen dissolved in the liquid phase the reaction rate will not depend on the extent of filling of the sites for hydrogen adsorption and the activated complexes will be in equilibrium with both the adsorbed and the dissolved hydrogen.As it was pointed out in [4] the reaction rate will be the same whether the activated complexes are formed from the adsorbed hydrogen or from the dissolved hydrogen,therefore it is convenient to express the reaction rate as a function of hydrogen pressure in the gas phase in equilibrium with the liquid phase rather than a s a function of hydrogen concentration in the liquid phase. Following [4] in the derivation of kinetic equations it was assumed that the surface of the catalyst is uniform or quasi-uniform and organic compounds form a n ideal liquid mixture ,hence their activities are equal to their mole fractions.The mole fraction of hydrogen in the liquid phase can be neglected,because its solubility is low. The nature of intermediate complexes on the surface of the catalyst can be considered using the "aromaticity principle" which was proposed by M.I.Temkin for explanation of the Balandin's multiplet theory of heterogeneous catalysis [ 16l.According to this principle ,in heterogeneous catalysis,the reaction occurs predominantly through intermediate compounds o r transition states in which the atoms of the reactants and the catalyst form rings having aromatic character . For example complex ZAH2 could be a doublet complex containing two atoms of catalyst and it resembles naphtalene derivative .Such complex is formed without loosing the aromatic benzene ring with some additional resonance energy. The kinetic equations were deduced from reaction mechanism (1)and compared with experimental data.Detailed kinetic analysis of the mechanism (1)will be presented elsewhere [17]. For the determination of the rate constants of the elementary reactions the sum of the squares of the relative deviations of the experimental and calculated values of mole fractions were minimized,using the system of automation of kinetic computations [18].0ur calculations show ,that stage 1) is irreversible, stage 4) is irreversible and fast ,stage 13)is irreversible and therefore it is possible t o describe the experimental data using the equations,presented in Appendix.Experimenta1 and calculated values of mole fraction of 4 tertbutylphenol as a function of parameter m*t/n,where m is mass of the catalyst$- time ,and n -initial amount of substrate are presented in Table 1 as an example ,at 8OOC. According t o derived kinetic equations,selectivity towards Y (4tertbutylcyclohexanone) is independent on hydrogen pressure P, up to high conversions and only a t N A =O the mole fraction of N y is a function of P.Differently the mole fractions of NC have practically no dependence in that case on hydrogen pressure in correspondence with experimental data.Experimenta1 and calculated values of Ny and NC as a function of conversion a t 60 O C are reported in Fig.3.It can be seen from Fig.3 that up to high conversions there is no dependence on hydrogen pressure of mole fractions of components Y and C as a function of NA.The difference in behaviour of Y and C a t different P is only when

248

NA is equal to zero. In Table 2 we present values of Ny and NC a t 6OOC and total conversion of NA at several hydrogen pressures. Such behaviour of Ny and NC is in good correspondence with mechanism (1).

Table 1 Hydrogenation of 4-tertbutylphenol a t 8OoC.Comparison of experimental and calculated values of m*t/n (g Wmole), a t different NA. 1* 2*

3*

P=0.5 MPa 0.65 2.5 2.1 0.3 5.0 4.1 0.1 6.6 5.4

* 1 - NA. ; 2 NY 0.5

2

1

1

3

P=1.0 MPa 0.78 0.67 0.75 0.44 2.0 1.9 0.28 2.7 2.5 0.1 3.3 3.1 m*t/n

;3

2

1

3

P=2.0 MPa 0.6 0.75 0.9 0.2 1.5 1.8 0.05 2.2 2.2

2

3

P=4.0 MPa 0.75 0.26 0.49 0.52 0.22 0.78 0.1 1.0 0.005 1.3

0.42 0.84 1.3 1.5 1.64

- (m*tJn)calc. NC

I

0,5

/"

a)

1

1

0 1 -N

1-NA

A

Fig.3.Mole f r a c t i o n s o f Y and C as a f u n c t i o n o f c o n v e r s i o n a t P:

0

- 0.5 MPa,

o -

1.0 M P a ,

Bll -2.5 MPa,

QP

-4.0 MPa

Table 2 Values of N y and NC a t NA = 0 and different P ( 60OC 1

P, MPa NY NC

0,42 0,09 0,50

0,52 0,11 0,48

1,02 0,12 0,49

4,02 0,15 0,50

The mean relative deviation of the calculated mole fractions from the experimental values is equal t o 18%, therefore kinetic model describes experimental data with good accuracy. As it was pointed out above the ratio of

249

cis and trans isomers in our case was not constant during a kinetic run.It can be obtained from mechanism (1) that,when stage 4 is equilibrium and if the rate of configurational isomerization is small the cis / trans ratio is constant during the reaction .Such dependences were observed, for examplejn the hydrogenation of 2-tertbutylphenol over Ni/SiOz, Co/SiOz, Pt/Al2O3, Rh/C, R d C [7,81. When the hydrogenation is of zero order with respect to the substrate and hydrogen the reaction rate is equal to the rate of isomerization of the adsorbed adduct of substrate and two molecules of Ha (kg).The dependence k3 with temperature is as follows k=1.42 108exp ( - 48000/RT)

(2)

where R is in J/mole K.As it was shown in [3] the preexponential factor in the Arrhenius equation in the case of isomerization reaction in the adsorbed state ,is close to 1012 s-1,i.e. t o the frequency of the vibration which is transformed into movement along the reaction coordinate,if the transmission coefficient is equal to unity.Assuming that the number of sites of adsorption is 1019 m-2,this factor in the Arrhenius equation for the turnover number in the zero region with respect t o the substrate and H2 corresponds to a transmission coefficient 10-4.Such nonadiabaticity (in the quantum mechanical sense) was previously observed in the case of hydrogenation of phenol and other aromatic compounds [31. The mechanistic considerations discussed above can be used also for the explanation of pH effects on stereoselectivity.It was formulated in [19l,that axial (i.e.cis)) alcohols are produced in acidic media and equatorial in basic.According to 1201 in acid media an adsorbed enol is proposed a s an intermediate and an adsorbed enolate anion in basic media with corresponding formation of axial and equatorial alcohols from these species.In contrast to it basing on mechanism (1) we suppose that acidic properties either of the media or of the catalyst itself can change the rate of keto-enol transformations,in the same way as it is established for acis-base homogeneous catalyzed transformation of ketone and enol[2 11.Such tautomeric transformation is a key step ,which can greatly influence stereoselectivity of the overall complex reaction of alkylphenol hydrogenation.In the case of Rh ,due t o the fact that the step 4 in mechanism (l),which corresponds to this transformation, is irreversible and fast a high selectivity in respect to cis isomer can be observed.In distinction from Rh, on Ni catalyst in phenol hydrogenation ,for example ,the rate of this step is small [22],therefore in the case of Ni mainly trans isomers are obtained.In order to obtain a high yield of cis isomers also the rate of alkylcyclohexanone hydrogenation must be high enough. Palladium catalyst containing dissolved hydrogen,for example, can act as an acid and therefore catalyzes enol-ketone transformation,but the rate of alkylcyclohexanone hydrogenation on Pd is rather slow in comparison with hydrogenation of alkylphenol [3l.In the case of Rh ketone hydrogenation is essentially faster ,than phenol hydrogenation .Therefore good stereoselectivity towards cis-isomers can be obtained ,using Rh as a catalyst.

4,CONCLUSION The kinetics of liquid-phase hydrogenation of 4-tertbutylphenol over RWC with formation of 4-tertbutylcyclohexanone ,cis and trans- 4-tertbutylcyclohexanol were investigated.The results were explained with the proposed reaction scheme.The dependence of selectivity and stereoselectivity as a function of experimental conditions was discussed.

250

SMPENDM The following kinetic equations were used for fitting experimental data. (m/n)t = (b+ c /P) (NAO-NA)

(3)

Ny= NA/( e - 1 1 - dP/e + ((NA+ dP)/(NAO+dP))c* (dP/e -(NAo+dP)/(e- 1))

(4)

Nc= eNAa /((e -1)* (1-a ) ) * ( ( N A O ) ~-- ~NA l-a) - (dP/e -(NAo+dP)/(e - 1))* * ( N A ~- N ~ ~ ( N ~ 0 l a* -e/e (NAO+ d€)e/( e-a

(5)

where

a = k12/klK7 * (1-k-12/( k-12+k13) ; b= l/k3 ; c=( k-2 + k3)/k2 k3 ;

d= k5k6/(k5+ktj)/kil ; e=k-11 k&j/(k-5+k6)/kll/kl ;

(6)

Here k l - constant of step l),etc.

6.REFERENCES S.L.Kiperman, Kinet.Katal.,No. 22 (1981) 30. D.Yu.Murzin,N.V.Kul'kovaand M.I.Temkin, Kinet.Kata1. ,No . 31 (1990) 983. V.G.Kotova,N.V.Kul'kova and M.I.Temkin, Kinet.Katal., No . 29 (1988) 148. M.I.Temkin,D.Yu.Murzin and N.V.Kul'kova, Kinet.Katal., No . 30 (1989) 637. V.G.Kotova,D.Yu.Murzin, A.G.Zyskin and N.V.Kul'kova, Kinet.Katal., No . 32 (1991)360. M.Bartok (ed.),Stereochemistry of Heterogeneous Metal Catalysis ,UK , Chichester, 1985. U.R.Datwyler , Ph.D. Thesis ,ETH,Zurich,l986. O.M.Kut,U.R.Datwyler and G.Gut, Ind.Eng.Chem.Res., No.27 (1988) 219. A.M.Pak.D.V.Sokolskii.R.C.Kuznetsova et al.. React.Kinet.Catal.Lett. .No. 33 (1987) 31. 10 M.V.Grignard,Bull.Soc.Chim.France., No.43 (1928) 473. 11 D.Yu.Murzin, Kinet.Katal., N o . 32 (1991) 1473. No . 33 (1992) 540. 12 D.Yu.Murzin and S.R.Konuspaev,Kinet.Katal., 13 0 .V.Bragin and A.L.Liberman ,Transformations of Hydrocarbons on Metal Catalysts ( in Russian ) , Moscow , 1981. 14 J.Horiuti and T.Nakamura ,Z.Phys.Chem .(Frankfurt am Main), No 11 (1957) 358. 15 M.I.Temkin , Advances in Catalysis , V 28 (1979) 173. 16 M.I.Temkin, Kinet.Katal. ,No. 27 (1986) 533. 17 D.Yu.Murzin, Kinet.Kata1. ,(in press). 18 G.M.Ostrovskii,A.G.Zyskin and Yu.S.Snagovskii,Comput.Chem. ,No. 11 (1987) 85. 19 D.H.R.Barton,J.Chem.Soc., (1953) 1027 . 20 R.L.Augustine,I3.C.Migliorini,R.E.Foscante,C.S.Sodano and M.J.Sisbarro, J.Org.Chem. ,No 34 (1969) 1075. 21 F.A.Carey and P.J.Sandberg ,Advanced Organic Chemistry,Part A,Plenum Press,New York,1977. 22 V.G.Kotova,D.Yu.Murzin and N.V.Kul'kova, Kinet.Katal., No .33 (1992) 452.

M. Guisnet et al. (Editas), Heterogeneous Cutulysis and Fine Chemicnls 111 1993 Elsevier Science Publishers B.V. All rights reserved.

Q

251

Liquid phase catalytic hydrogenation of benzophenone: Role of metal support interaction, bimetallic catalysts, solvents and additives P.S. Kumbhara and the late R.A. Rajadhyaksha Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India. *Present address: Laboratoire de Chimie Organique Physique e t CinCtique Chimique Appliqubes, URA 418 CNRS, Ecole Nationale Supbrieure de Chimie, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 1, France.

Abstract Liquid-phase hydrogenation of benzophenone was studied over well characterised supported monometalic Ni and bimetallic Ni-Cu and Ni-Fe (75:25, composition by mass) catalysts. Interesting changes in selectivity were obtained over the bimetallic catalysts. Different strategies such as use of Ti02 as a support, change in solvent and use of NaOH were tested to improve the activity and selectivity to benzhydrol. A Ni-Fe(75:25)/TiOZ catalyst combined with NaOH and methano1:water as solvent was found to be the most efficient system to selectively obtain benzhydrol. 1. INTRODUCTION

The activity and selectivity of metallic catalysts is known to be influenced by alloying and metal support interaction (MSI) (1).However, these concepts have been rarely applied in liquid phase hydrogenation, which are commonly encountered i n fine chemical synthesis. The present work stemmed from our results on hydrogenation of acetophenone (2) and was undertaken t o investigate the effect of alloying, MSI, solvents and additives on the hydrogenation of benzophenone over Ni based catalysts. Product of the reaction, benzhydrol, is a n important drug intermediate. The undesired product, diphenylmethane, is obtained by hydrogenolysis of benzhydrol. Conventionally, the reaction is carried out by using Zn/NaOH as the catalytic system. However, this process produces environmentally unwanted sludge and is expensive. Recently, the reaction has been carried out using Pd catalysts poisoned with Pb or modified by phosphine ligand (3.4).

252

2.1. Catalysts Hydrogenation of benzophenone was investigated using Ni, Ni-Cu(75:25) and Ni-Fe(75:25) (composition by mass) supported on S i0 2 (Aerosil 200) and T i 0 2 (P-25) of Degussa. The catalysts were prepared by impregnation of the respective nitrates i n a n appropriate proportion by the incipient wetness technique. The total metal loading was kept constant at 20% by weight. The catalysts were calcined i n air at 723 K for 4 h and were stored in dessicator until1 further use. Prior to use required amount of the catalyst to be tested was reduced i n Hz at 523 K for 2 h followed by another 2 h a t 723 K in a specially designed reducer (5) that prevented exposure of the catalyst to air. The catalysts were characterised by H2 chemisorption, degree of reduction measurements (D.R.), temperature programmed reduction(TPR), X-ray diffraction(XRD) of r e d u c e d - p a s s i v a t e d c a t a l y s t s , j n - s i t u e x te n d e d a b s o r p t i o n fine structure(EXAFS) (Ni-Fe) and Mossbauer spectroscopy (Ni-Fe). Details of these studies will be published elsewhere(6). 2 8 Reactionpdure Hydrogenation of benzophenone (0.065 mol) (Loba Chemie, A.R. grade) was carried out in a 100 ml Parr autoclave using appropriate solvents (35.5 ml, A.R. grade) and pre-reduced catalyst (0.24 g unless otherwise specified). The hydrogenation was carried out a t hydrogen partial pressure of 60.5 kg/cm2 and 408 K. The speed of agitation was 1550 rpm a n d was chosen after confirming the absence of diffusional limitations. Samples were periodically withdrawn and analysed on a gas chromatograph (Perkin Elmer) using a 2 m x 0.33 cm carbowax (KOH washed) on chromosorb W column with a flame ionisation detector. Authentic pure samples were Lsed for product identification and quantification. In some cases th e products were separated by column chromatography an d were characterised by infra-red (IR), H-NMR a n d mass spectral (MS) analysis.

3. RESULTS AND DISCUSSION

3.1. Characterisation of catalysts The XRD pattern indicated that Ni-CdSiOz was in a complctely FCC alloy phase. EXAFS data indicated t h a t major portion of Ni-Fe catalyst was in a FCC alloy phase with small am o u n t of unreduced FeO. Mossbauer spectroscopy showed t h a t th e Ni-Fe catalysts exist a s a mixture of supermagnetic an d ferromagnetic alloy with small amounts of Fe2+. TPR studies showed that both Cu and Fe improve the reducibility of NiO. Ni-CdSiOz was i n completely reduced state (100 % D.R.) as compared to partially reduced Ni/SiOa (76% D.R.). Compared to Ni/SiO2 both Ni-Cu a n d Ni-Fe catalysts showed decreased H2 chemisorption capacities which is characteristic of surface enrichment of Ni by Cu an d Fe respectively. Ni/"iO2 and Ni-Fe/TiOz

253

catalysts also showed suppression in H2 chemisorption, characteristic of the SMSI state (1). Details of these studies will be published elsewhere (6).

32. Hydrogenation of benzophenone in methanol Hydrogenation of benzophenone was first studied using methanol as solvent. A typical concentration profile for the reaction over Ni/SiO2 catalyst is shown i n Figure 1.

Q

* *

0

Benzophenone Benzhydrol

1-methoxy-1,l-diphenylmethane

100

200

300

Time, min

Figure 1. Hydrogenation of benzophenone over 20% Ni/SiOz catalyst. Reaction conditions: Benzophenone, 0.065 mol, Ni/SiOz, 0.24 g; Methanol, 35.5 ml; 308 K; H2 , 60.5 kg/cin2 I n addition to th e expected hydrogenation products (benzhydrol a n d diphenylmethane), th ere was formation of a by-product i n appreciable quantities. The by-product was separated by column chromatography (silica column) a nd was identified by IR, NMR an d MS as a n ether, 1-methoxy-1,ldiphenyl methane. The reaction was further studied by using SiO2 supported Ni-Cu and Ni-Fe an d Ti02 supported Ni an d Ni-Fe catalysts. The results a re summarised i n Table 1. The results are truly remarkable. There was a n increase in ether formation over Ni-Cu/Si02 an d Ni/Ti02, whereas it was negligible over Ni-Fe/SiOz. Ni/TiOe catalyst showed increased activity in agreement with th e results of

254

Vannice et al. (7) for carbonyl group hydrogenation. However, the catalyst showed increased ether formation. The most active a n d selective catalyst for formation of benzhydrol was Ni-Fe (75:25)/TiO2. The increased activity over NiA'iOz is probably due to the creation of highly active sites at the metal support interface (7). Further, increase i n activity over Ni-FeRiOz is due to synergetic metal support interaction a n d electronic effect of Fe as reported earlier for acetophenone hydrogenation (2). Table 1 Comparison of various catalysts for hydrogenation of benzophenone in presence of methanol ________---_______--____________________----------------

Catalyst

Time h

Conversion o/o

Selectivity to products (mol%l)a

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

I

I1

I11

________________________________________-------_----_---

Ni/SiOa 4.5 Ni/TiO2 2.5 Ni-Cu (75:25)/SiO$ 6.0 Ni-Fe (75:25)/Si02 4.5 Ni-Fe (75:25)/"302 2.0

89.3 92.6 83.8 85.9 86.7

68.3 52.0 46.4 84.5 84.5

19.7 37.5 43.7 4.6 1.7

12.0 10.5 9.9 10.9 13.9

________________________________________---_-----------I:benzhydrol;II:ether(l-methoxy-l,l-diphenylmethane~; 111: diphenylme t hane b Catalyst weight : 0.48 g Reaction conditions : same as in Fig. 1. a

To check the ether formation pathway, hydrogenation of benzhydrol was carried out in presence of methanol under similar conditions over selected catalysts. The selectivity to ether varied a s follows: Ni-Cu (75:25)/SiO2 (highest) >> Ni/SiOa >> Ni-Fe (75:25)/Si02 (lowest) From these results i t is clear th at the ether formation occurs due to the bimolecular reaction between benzhydrol and methanol in presence of catalyst and Ha. Based on this the overall reaction scheme can be depicted a s :

255

OH

i' e

0

*!-o

n

i

y

c

1+"3OH OCH3

Benzophenone

" i i

\ e

C

H

4

Diphenylmethane

Ether, 1-metohxy- 1,l -diphenylmethane

Mechanism of ether formation Formation of eth er (methoxycyclohexane) over Pd/C catalyst d u r in g hydrogenation of cyclohexanone in methanol was first reported by Nishimura e t al. (8). This scheme was used to prepare 4,4-dicyclohexyl methyl ether in 84% yield using Pd/C as the catalyst (9). Pines (10) in a n exhaustive study reported ether formation from various alcohols in presence of hydrogen over Ni based catalysts. He suggested th at ether formation was due to two types of sites present on Ni; one basic and the other having a n acidic character. These sites are created because of incomplete reduction of the metal. However, this reason was contradicted by Ponec et a1.(12) who showed th a t there was no co-relation between the ether formation and the reducibility of the metals, as Pt and Pd, which were completely reduced showed ether formation. I n the present case, contrary to t he arguments of Pines (lo), the competely reduced Ni-Cu/SiOz (D.R. loo%), showed increased eth er formation a s compared to partially reduced Ni/SiOz (D.R. 76% 1. Based on the present d a ta we propose the following mechanism for ether formation :

I0 : /\ 9" I 0 I CH3

Ph

r-

M I , M2 : Metal sites

MI

M2

We believe t hat ether formation is likely to occur as a result of nucleophilic attack of a methoxy species on the carbinol carbon as shown above. In Ni-Cu and Ni-Fe catalysts, the surface is known to be enriched by Cu and Fe, which was found to be also true in the present case from the hydrogen chemisorption dat a . T hus i n th e bimetallic catalysts, Ni atoms will be preferentially

256

surrounded by Cu or Fe atoms. Therefore, the above reaction is likely to involve a methoxy species bonded to Fe or Cu on the bimetallic catalysts. Evidence for this hypothesis is drawn from the d ata on heat of adsorption of methoxy species on these metals, which are as follows (12) : Fe: -87, Ni: -60, Cu: -8 kJ/mol. According to this d ata ease of formation of the methoxy species which is related to the yield of ether will be in the order Cu>Ni>Fe which indeed agrees with the results obtained i n th e present study. Similarly, increased e th e r formation on NiPTiO2 is also linked to the ease of formation of methoxy species over this catalyst (13). Another interesting point is the very small difference in the yield of the hydrogenolysis product diphenylmethane, over all the catalysts (Table 1). This is contrary to the results on C-C bond hydrogenolysis over bimetallic and Ti02 supported catalysts (1). This indicates that the sites for C - 0 hydrogenolysis are probably different from C-C bond hydrogenolysis.

35.Meet of solvents and additive Solvents a nd additives a r e known to influence activity a n d selectivity in hydrogenation reactions. In the present study we used four solvents having different dielectric constants (methanol, i-propanol, cyclohexane a n d 10% water in methanol) to check their influence on activity a n d selectivity over the most active Ni-FeA'iO2 catalyst. The results are summarised in Table 2. Table 2 Effect of solvents and additive (NaOH) on activity and selectivity to benzhydrol over 20%Ni-FeA'iOz catalyst. Solvent/ additive

Initial rate x 103 g mo lh g of cat.

Conversion of benzophenone o/o

Selectivity Dielectric to benzhydrol constant % of solvent

---_------------_---------------------------------------

Cyclohexane 2.63 84.0 58.8 i-Propanol 6.30 86.0 80.0 methanol 2.00 86.7 84.5 methanol+watera 4.11 90.1 91.0 methanol+NaOHb 0.61 86.2 94.3 (methanol+wate+ +NaOHb) 1.70 88.0 98.4 ________________________________________-------------_-a Water: 10% by volume of the solvent b NaOH: 0.1%by weight of benzophenone Reaction conditions : same a s in Fig. 1.

2.0 18.3 32.6 >>32.6

__

257

T he selectivity to benzhydrol is dependent on t h e solvent a n d can be correlated with the dielectric constant of the solvent; similar to earlier findings of Masson et al. (14) for acetophenone hydrogenation. Addition of NaOH results in an increase in the selectivity at the expense of drop in activity. However, this can be partly compensated by using methano1:water as a solvent. Using this combination i t was possible to achieve high selectivity to benzhydrol (98.4% selectivity at 88% conversion) at reasonable activity. 4. CONCLUSION

Interesting change in selectivity for ether ( 1-methoxy-1,l-diphenylmethane) formation during the hydrogenation of benzophenone in methanol is obtained over SiOa -supported bimetallic Ni-Cu and Ni-Fe catalysts. The formation of the ether is due to the bimolecular reaction between the product, benzhydrol a n d methanol in presence of catalyst and hydrogen. Ni-Cu (75:25)/&02 showed a n increase in ether formation whereas, Ni-Fe (75:25)/Si02 showed a suppression. The results suggest t h a t the methoxy species, h a s a possible role i n this reaction a nd i t s formation depends on the metal used. No suppresion in hydrogenolysis of the C - 0 bond was observed over bimetallic, as well a s Ti02 supported catalysts, indicating involvement of different type of sites than C-C bond hydrogenolysis. Ni-Fe (75:25)/Ti02 showed a maximum in activity a n d selectivity to benzhydrol due to the synergetic effect of MSI and the electronic effect of Fe. The nature of the solvent modifies activity and selectivity and depends on its dielectric constant. Addition of NaOH in controlled a mo u n ts improved selectivity. Benzhydrol can be obtained selectively ( > 98% ) a t high conversion of benzophenone (88%)over Ni-Fe (75:25)/Tio2 catalyst using methanol: 10% water as a solvent and NaOH(O.l%;by weight of benzophenone) as additive.

ACKNOWLEDGEMENTS Financial assistance from DST, Govt. of India is gratefuly acknowledged. PSK is thankful to UGC, Govt. of India, for senior research fellowship. PSK wishes to dedicate this paper to the memory of late Professor R.A. Rajadhyaksha.

1. 2.

3. 4. 5.

G.L. Haller and D.E. Resasco in Advances in Catalysis, 36 (1989) 173. P.S. Kumbhar, M.R. Kharkar, G.D. Yadav and R.A. Rajadhyaksha, J.Chem.Soc.Chem.Commun. (1992) 584. L.W. Gosser, US Patent No. 4302345 (1982). K. Kunerk, Ger.Offen. 2837022 (1980). P.S. Kumbhar, Ph.D. (Tech.) thesis, UDCT, Bombay (1992).

258

6.

7. 8. 9. 10.

11. 12. 13. 14.

P.S. Kumbhar, M.R. Kharkar, G.D. Yadav and R.A. Rajadhyaksha, to be forwarded for publication. M.A. Vannice and B. Sen, J.Catal., 113 (1989) 82. S. Nishimura and I. Takashi, J.Chem.Soc.Chem.Commun., (1967) 422. P.N. Rylander i n Catalytic Hydrogenation in Organic Synthesis, Academic Press New York, 1978. H. Pines in Advances in Catalysis, 35 (1987) 323 and references therein. V.Ponec, A.Vanderberg, J.Doombois and N.J. Ken, J.Catal., 54 (1978) 243. R.J. Madix, Catal.Rev.Sci.Engg., 26 (1984) 281. J . Falconer, B. Chen and L. Chang, J.Catal., 127 (1991) 732. J.Masson, P. Cividino, J. Bonnier and P. Fouilloux in Studies in Surface Science and Catalysis 59 (Heterogeneous Catalysis and Fine Chemicals 111, M. Guisnet et al. (eds.), Elsevier, Amsterdam, 1991, pp. 245-252.

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I11 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

259

Solvent effects in selective hydrogenation : Catalytic hydrogenation of Acetamido-4 hydroxy-2 butyrophenone. F, GRASS, J. M. GROSSELIN and C. MERCIER*

RhBne Poulenc Recherches. Centre de Recherches des Carribres - B.P. 62 85, Avenue des Frbres Perret - 69192 SAINT-FONS Cedex - FRANCE.

Abstract This example illustrates the influence of solvents on the selectivity and activity for hydrogenolysis in a triphasic gas / liquid / solid system. We use the concept of “reactive solvent”; the by-product 3 results froin a Friedel - Crafts reaction and can be controlled by changing the solvent and effect of dilution - 2 increases dramatically if hydrogenation is not under chemical control. This article attempts to suggest how the appropriate choice of solvent may improve selectivity.

-

INTRODUCTION

The presence of a liquid phase L in heterogeneous hydrogenation catalysis is useful for chemical reactivity control. Here we will discuss an example with a liquid medium in conjunction with G reactant and S catalyst. Two major reasons can be put forward for the presence of a liquid. First, a high temperature may not be suitable (thermosensitive fine molecules, low volatibility, ...). The second reason is that a liquid layer may be desired, to control the reaction path way, for instance by inhibiting or promoting secondary reactions in the liquid phase. From amongst the considerable number of cases, here are some examples of intentional addition of L component : - for acido-basic properties : Acidity for hydrodehalogenations [ 11, Basicity

for triazoles [2]. - for dielectric properties : hydrogenolysis (of C-0, C-X bonds) increases vs hydrogenation (cf double bonds) with higher & [3].

260

- for site modification : enantioselective a-ketoesters hydrogenation with alkaloid modified catalysts [4]. We want to illustrate these points with the example of the hydrogenolysis of product 1to 2, involved in the synthesis of METHYL BENZOQUAT, (anticoccidian product) (Scheme 1). The solvent plays a very important role in the activity and selectivity of this reaction.

Scheme 1 : Hydropenolysis step in MBO manufacture 0

OH multisteps

NHCOCH,

NHCOCH,

__ MBQ

Intermediates

By-product

4

3 CH3GHN 2 C

0

-

CO-Me L H (Methylbenzoquat)

O

C

H

3

8 b5

X=OH X=O OM E te NHCOCH,

EXPERIMENTAL

- Startinr materials : Solvents and catalyst (Engelhardt) were used as received. Compounds 1and 2 were prepared as described in the literature (RN CAS : 1: [028583-6221; 2 : [022932-84-91), - Analytical Drocedures : HPLC analyses were performed using a GILSON Model 302 instrument equipped with a UV detector TOUZARD-MATIGNON SPD-2A [Colum : Si 60 Lichrosorb (5 pm - 125 x 4 mm) - Eluent : Heptane / CHC13 / EtOH 95 : 80/11/9 v/v (1 ml.mn-1) - UV detection at 255 nm]. 1H and 13C NMR Spectra were obtained on a Brucker AC 200 spectrometer. IR and Mass spectra were used to confirm structures of intermediates (isolated by preparative HPLC or flash chromatography) and by-product 3.

261 vtlc reactiw : Catalytic reactions were performed in a 300 ml stainless-steel autoclave (SOTELEM). In a typical run, 20,8 g of substrate 1(96,4 % Purity - 90,4 x mole) and 0,65 g of 10 % Pd/C were weighed, then introduced into the autoclave H$O, (1 % molar / substrate) and Methanol (200 ml) were then added. The autoclave was purged and pressurized with 20 bar of hydrogen, and heated to 50°C. After the absorption ceased, the autoclave was cooled and hydrogen was vented. The reaction mixture was filtered in order to separate the catalyst and the liquid phase, after neutralisation with 3,6 ml NaOH, 0,5 N was analyzed by HPLC (after dilution).

-

RESULTS AND DISCUSSION We summarize in Table 1 the influence of solvents on activity and selectivity.

Table 1 : Influence of solvents and additives on activity and selectivity (Experimental) Conditions : P H = ~ 20 bar - T = 50°C - Catalyst : 10 % Pd/C (1.25 % w/w / substrate) I

Run

Additive: H2S04 (% w/w-substrate)

2

3

Intermediates

n

0

58

0

26 (4a)

n

1 %

I h

69

14

I

20h

Solvent

Yield

-

1

2 -

Me0 Me0

OMe

OMe

t

I

100

3

EtOH 100 %

0

4

EtOH 100 %

1 %

92

1.3

1.4 (4 b)

5

MeOH 100 %

1 %

96

2

0 (4 G)

100

On activity, comparison of runs 4 and 5 illustrates an increase by a factor of two on going from ethanol to methanol (polarity effect [3]). Addition of a catalytic quantity of sulfuric acid (run 4 compared to run 3) increases activity by a factor of ten [hydrogenolysis catalyst - [5]]. On selectivity, using alcohols in place of ether gives different intermediates (4b in EtOH, 4c in MeOH and 43 in dimethoxyethane) illustrating the concept of "reactive solvent" in the case of alcohols [6]. In order to further understand there results, we undertook a kinetic study.

262

*

The by-product (to avoid) was formed by a Friedel Crafts reaction on the intermediate EafbocW in the hydrogenolysis step. Changing the solvent - Modifies the intermediate, going from a benzylic alcohol to a benzylic ether (more difficult to hydrogenolyse), the consequence being limitation of the steady state concentration of carbocation [7b]. - Futhermore, the importance of the dilution on selectivity due to kinetic law on substrate concentration 1 should be noted : - order 1 for hydrogenation 1-+ 2. - order 2 for by product formation 3. In addition, normally a high dilution is desirable for good selectivity whereas the reverse is the case for productivity, thus a good compromise is 10 % w/w substrate. - It is also important to work under chemical control for hydrogenation and avoid mass transfer phenoma to increase the rate of hydrogenation versus the Friedel Crafts reaction (Scheme 2).

* We examined the product distribution during conversion (Figure 1) using alcohols : we did not detect any benzylic alcohol b as intermediate but instead found that the benzylic ethers & or & were the real intermediates. The formation of by-product 3 is very slow compared to hydrogenation. This point was confirmed by the synthesis of 2 starting from & and 2 in the absence of hydrogen (Scheme3).

Figure 1 : Product distribution during hydrogenation in methanol 20 b - T = 50°C - MeOH = 200 ml - 1: 20,8 g - 10 % Pd/C = 0,65 TT

10

50

100

150

t(min)

263

Scheme 2 : The possible mechanism and influence of solvent (

R=NH-ECH,

)

0

\ H,

R

d F

Intermediate

R'OH (via k e t a l )

OH

R

4s

&-

Intermediate

R

&

OR'

4b

or

4l

OH

CARBOCATION

/ "Pd-HS-"

FRIEDEL-CRAFTS

264

Scheme 2 : Synthesis of intermediates and by-product 2 to confirm origin : 4 h

&NaBH4,K, Me0

R

OH OH

OH

OR'

OMe

R

1

2 5 ° C - 30 mn

--

4a (yield

68 %)

R 4b (4c) I G e l d 90 % )

(R'=Et), EtOH 90 % +

2, 1

% H,SO,

50°C

-7 h

16 54

So from the kinetic study, we can propose the following steps (scheme 2) i) Reduction of ketone to alcohol (heterogeneous catalysis) ii) Etherification of alcohol (benzylic) 4a (homogeneous acidic catalysis) iii) Hydrogenolysis of benzylic ether & (heterogeneous catalysis) IV) Friedel-Crafts reaction between ether intermediate C and product of the reduction 2 (homogeneous acidic catalysis).

Step i) is very fast and step iii) is the rate determining step in hydrogenation [7]. We cannot exclude the formation of acetals from carbonyl 1 and lower alcohols and their subsequent hydrogenolysis to ether &, a well know reaction of Pd/C in methanol [6]. The rate of formation of by-product 3 is independant of palladium concentration. But on the other hand, the rate of formation of 2 is directly proportional to palladium concentration. This is very important in controlling the selectivity and you have to maintain the activity of the palladium catalyst (avoid poisoning...) in order to obtain good yields of 2.

265

In this paper, we present an example of selectivity in fine chemicals with a crucial role for the solvent in controlling the reaction : - By simple dilution effects (order 1 / order 2 kinetic law in substrate) - By influence of the dielectric constant on the rate (MeOH/ EtOH) - Solvents can also be reactive, changing the nature of the intermediates, promoting or inhibiting side reactions (Friedel-Crafts versus hydrogenation), This example illustrates the influence of solvents (and additives : H2S04) in a variety of ways which may not always be clearly distinguishable. The right combination of catalyst and solvent can help improve the performance of a reaction system [8].

-1. M. GUISNET and Coll. (Eds), "Heterogeneous Catalysis and Fine Chemicals", ELSEVIER, Amsterdam, 1988, p. 19 2. CIBA GEIGY, EP 343 640 (25-5-1988)

3. P.N. RYLANDER, Chemical Catalyst News, Engelhardt Corp. October 89 and ref. therein. 4. Ref.l p.153 5. P.N. RYLANDER, "Catalytic hydrogenation in organic syntheses" - Academic Press1979- Chap. 6 - p. 100 and ref. therein 6. Ref. 5 p.196

7. a) H. VAN BEKKUM and Coll., J. Catal., 2Q, 58 (1971) 2143 (1970) b) S. MITSUI and Coll., Bull. Chem. SOC. Jpn,

a,

8. C. MERCIER "Solvent effect on hydrogenation : use to achieve improved selectivity" to be published in Bulletin de la Soci6t6 Chimique de France.

This Page Intentionally Left Blank

M.Guisnet et al. (Editors), Heterogeneous Cntdysis and Fine Chemicals IIZ 0 1993 Elsevier Science Publishers B.V. All rights reserved.

267

Factors influencing activity and s e l e c t i v i t y of P a l l a d i u m Catalysts

J. Petr6, T. Mallat, A. Tungler T. Math6 and 6. Poly4nszky Department of Organic Chemical Technology, Technical University of Budapest, Miiegyetem rkp. 3 . Budapest, H-1111, Hungary

Abstract The most important factors effecting hydrogenating catalytic activity and selectivity are 1. The metal hydrogen system which could be varied by the way of catalyst preparation and reaction variables 2 . Dispersion of the metal and 3 . Applying promoters. Examples are given for the effects of factors mentioned above.

INTRODUCTION The aim of this paper is to draw attention of those who use Pd-catalysts to some variables influencing catalytic performance you often encounts in the praxis. The most important governing factors, are as follows 1. Metal-hydrogen system 2. Dispersion of metal(s) 3 . Effect of promoters The effect of these, in most cases, could not be separated from each other. In the following some remarks will be given to each item for illustration only.

EXPERIMENTAL All the catalytic hydrogenations were carried out in liquid phase room temperature and 1 bar hydrogen pressure. The rate of hydrogen uptake measured by volume. For characterization of metal-hydrogen system and that of promoted catalysts, electrochemical methods were used.(l) Details of experiments and of catalysts are given in the publications referred to at different topics.

268 RESULTS

AND DISCUSSION

Metal-hydrogen system-activity and selectivity Transition and noble metal catalysts are used most widely phase hydrogenation reactions in the fine chemical industry. On the basis of literature data and our research work of several decades it is known that the active sites of working catalysts consist of at least two components: metbl and hydrogen (to which, during the catalytic reaction, the organic compound may join). Thus, hydrogen affet'ts on the one hand the number and quality of active sites by "alloying" with the catalyst, and on the other hand it is one of the reaction partners. The metal-hydrogen system forming on the catalyst during its'preparation is extremely sensitive, and its quality depend on the partial pressure of hydrogen, temperature, alloying metal components, etc. The various forms of hydrogen sorbed on the surface as a function of time may also convert into one another. During catalytic hydrogenation this picture may be further complicated by a concurrent absorption of the reactant or the final product. As a consequence, hydrogen has an important role in the formation of active surface sites. It is generally accepted that hydrogen sorbed on the surface is different from molecular hydrogen in the gas phase: the former is in a more active state. The two simplest ways of changing the metal hydrogen system are: 1. desorption, followed by controlled readsorption of hydrogen on the given catalyst; 2. applying high surface area support. Point 1. is illustrated by the following example. During the catalytic reduction of ally1 alcohol,(2) the saturation of the double bond (and propanol formation) may also be accompanied by isomerization (with formation of propionaldehyde) and C - 0 or C-C bond cleavage (destruction) via hydrogenolysis, yielding gaseous products (propylene, propane, ethylene, acetylene). On surfaces covered with hydrogen, hydrogenation and isomerization start with the formation of a semi-hydrogenated intermediate: t Hads CHz=CHCH20H > CH~CHCHZOH for liquid

which, according to Sokolskii and Sokolskaia ( 3 ) , is the common intermediate of both the hydrogenated and isomerized products that is the aldehyde. This intermediate is attached to one site of the surface, and isomerization may also occur here. If, however, it can move to a doublet site, hydrogenation is possible.

On a hydrogen-free surface the mechanism of isomerization is different: first acrolein forms via dehydrogenation and then propionaldehyde is produced by hydrogen uptake. In addition,

269

gaseous hydrocarbons may also form by t h e h y d r o g e n o l y s i s of t h e bond. I t i s r e p o r t e d , (3-6) t h a t a-phase h y d r i d e w i t h a l o w e r hydrogen c o n t e n t d o e s n o t a t t a c k t h e d o u b l e bond a n d i s therefore t h e most s e l e c t i v e c a t a l y s t of t h e semi-hydrogenation of t r i p l e bond, whereas p-phase Pd-H i s a c t i v e i n t h e s a t u r a t i o n of t h e C=C double bond. I n o u r e x p e r i m e n t s a l l t h e c a t a l y s t s w e r e d r i e d i n a i r and s t o r e d a t r o o m temperature for a month, so t h e y does n o t c o n t a i n hydrogen. B e f o r e t h e e x p e r i m e n t s t h e c a t a l y s t s w e r e p r e hydrogenated. Figure 1 d e p i c t s t h e composition of t h e r e a c t i o n mixture as a f u n c t i o n of hydrogen uptake. Table 1 summarizes t h e results. C-0

Table 1

Hydrogen s o r p t i o n and s e l e c t i v i t y i n t h e hydrogenation of a l l y l a l c o h o l w i t h 20 mg of 10 % Pd on carbon c a t a l y s t s as a f u n c t i o n of t h e prehydrogenation ( P R ) i n 0.05 m o l d ~ n -s~o l u t i o n ( 2 )

Dry

80 80 80 80

-

PR time (dn) Electrolyte

-

60 120 60 80

-

E content

Conversion

(%I cm3g-l

3.84 4.00 4.74 2.57 3.00 0.24

weak strong

2.6 2.7 3.3 3.1 3.8 0

100 100 100 100 100 10

Selectivity

Isomerization reduction alcohol aldehyde (%)

21 28 33 30 48 0.5

71 69 60 68 52 9.5

3.4 2.5 1.8 2.3 1.1 19

Reaction medium : Na2SOq ( l i n e 5 ) ; H2SO4 ( l i n e s 1-4 and 6 ) The p r e f e r e n c e f o r i s o m e r i z a t i o n can be a t t r i b u t e d t o t h e moderate hydrogen s o r p t i o n c a p a c i t y d u e t o c o n t a m i n a n t s t o t h e acidic medium and t o t h e high d i s p e r s i t y of t h e c a t a l y s t . Under t h e s e c o n d i t i o n s , f3-phase palladium hydride, w h i c h can be a c t i v e i n C=C d o u b l e bond hydrogenation, may be a b s e n t . S a t u r a t i o n by hydrogen o f d i s p e r s e d p a l l a d i u m i n an acidic medium a t 1 bar r e s u l t s i n a - h y d r i d e p h a s e f o r m a t i o n o n l y (8). An a - p p h a s e t r a n s f o r m a t i o n i n 0.05 m o l dm-3 Na2S04 demands a h i g h e r p r e s s u r e t h a n p r e d i c t e d by t h e Pd-H s o l u b i l i t y d i a g r a m ( 4 ) or a c o n s i d e r a b l e i n c r e a s e i n t h e s a t u r a t i o n t i m e up t o 15-20 d a y s , e s p e c i a l l y i n t h e presence of contaminants. The l a c k of p-phase i n o u r d r y s a t u r a t e d c a t a l y s t c a n be presumed from t h e v a l u e of i t s electrode p o t e n t i a l ( 2 3 mv). These c o n d i t i o n s may be unfavourablo f o r t h e h y d r o g e n a t i o n of C=C d o u b l e bonds i n sulphuric acid. Beside t h e high c o n c e n t r a t i o n of s t r o n g l y a d s o r b e d a l l y l a l c o h o l c a n be f a v o u r a b l e for i s o m e r i z a t i o n i n acidic s o l u t i o n . Under these

270

c o n d i t i o n s t h e p r o p o r t i o n of i s o m e r i z a t i o n may be as h i g h as 7 0 % . I n c o n t r a s t , a n i n c r e a s e i n t h e prehydrogenation t i m e of c a t a l y s t s prior t o r e a c t i o n enhances t h e hydrogen c o n t e n t and also t h e proportion of weakly bonded hydrogen (p-phase). To t h e p o i n t 2 . I f t h e d i s p e r s i o n of a metal i s i n c r e a s e d by e i t h e r a special method of p r e p a r a t i o n or t h e u s e o f a s u p p o r t , n o t o n l y a q u a n t i t a t i v e change can be observed i n i t s p h y s i c a l properties ( s u r f a c e area, s o r p t i o n c a p a c i t y ) , b u t also q u a l i t a t i v e changes o c c u r . These are d u e t o changes i n geometry a n d e l e c t r o n i c s t r u c t u r e . With i n c r e a s i n g d i s p e r s i o n , t h e o r i e n t a t i o n of t h e m i c r o c r y s t a l s faces change, and t h e amount of c o o r d i n a t i n g u n s a t u r a t e d metal atoms i n c r e a s e s . The effect t h e way of p r e p a r a t i o n of metal powder ( 9 ) h a s t o t h e amount of o v e r a l l sorbed hydrogen (VH) and of metal s u r f a c e area (Ss) i s given i n T a b l e 2 ( 9 ) . Applying a c t i v a t e d carbon s u p p o r t ( 1 0 ) ( 5 w t % P d ) (Fig. 2 ) owing t o t h e h i g h e r d i s p e r s i o n t h e amount of absorbed hydrogen i s less t h a n t h a t of t h e adsorbed one. The r a t i o i s t h e reverse t h a t w a s found w i t h u n s u p p o r t e d Pd. W i t h i n c r e a s i n g metal c o n t e n t , as expected, t h e amount of sorbed hydrogen i n c r e a s e s , and so does t h e specific a c t i v i t y of t h e c a t a l y s t w i t h r e s p e c t t o u n i t mass, b u t it decreases related t o t h e u n i t mass of Pd i n t h e hydrogenation of carbonyl groups or double bonds (see T a b l e ..

t

I ImAl

Figure 1. Hydrogenation of a l l y 1 Figure 2 . Potentiodynamic curves of Pd-catalysts: c u r v e a l c o h o l w i t h 10% Pd/C c a t a l y s t ( 2 0 mg), i n sodium s u l p h a t e solu- 1 5 % Pd on activated carbon, curve 2 Pd powder ( 1 0 , 11). t i o n (see Table 1 ) . Reactant I 7.35 ml. 1, Ally1 alcohol ; 2 , propionaldehyde ; 3, propanol ( 2 ) .

271

Table 2 Relationships between reduction agent (R.A.), (VH) and surface area (SH) (9) Catalyst

R. A.

VH

cm3g-1 Pd Pd Pd Pd Pd Pd

HCOOH A2

treated at 470 K treated at 870 K NaBHq HCOH

78.9 89.4 70.2 66.1 60.6 64.0

sorbed hydrogen

SH

m2g-1 25 69 43 25 51 15

Table 3 Relationship between Pd-loading and catalytic activity in liquid phase hydrogenations (10) Catalyst

Pd content w t %

Pd/C Pd/C Pd/C Pd/C

Activity, cm3H~/min acetophenone cyclohexene 9-1 cat. mg-1Pd 9-1 cat. mg-1Pd

1 5 10 20

11 34 16 78

1.1 0.7 0.2 0.4

67 126 114 167

6.7 2.5 1.1 0.8

Effect of copper promoter Palladium-copper catalysts were investigated more thoroughly in our laboratory. With the addition of copper, the hydrogen peaks shift toward the lower potentials and peak heights decrease (12,13). Hydrogen contents determined from the areas below the peaks can be seen in Fig. 3. The amounts of loosely bonded and total hydrogen monotonously decrease with increasing copper content, that of strongly bonded hydrogen slowly decreases after reaching a maximum. On the sample containing 60 at% of copper the two types of hydrogen can already not be separated, and the catalyst with 80 at% copper content does not sorb hydrogen at all. From the oxygen region of voltammogram on the surface of Pd-Cu catalysts four phases could be distinguished : alloys rich in Pd and Cu, respectively, a PdCu3 compound phase and metallic Cu (14).

272

so LO

-3 0

20

-10

0

c -ham

0-lh

F i g u r e 3. Hydrogen c o n t e n t of Pd-Cu c a t a l y s t (measured by e l e c t r o c h e m i c a l method) ( 1 2 ) .

m kelophmn Nltob.nl.n

6

Figure 4. Relative a c t i v i t y o f Pd a n d i n Pd-Cu c a t a l y s t s by c o p r e c i p i t a t i o n

catalytic H2 c o n t e n t prepared (12).

T h e de pe nden ce of t h e a c t i v i t y of c a t a l y s t s on t h e i r c o p p e r c o n t e n t c a n be s e e n o n F i g . 4. ( 1 2 ) . For a l l t h e f o u r r e a c t a n t s t h e a c t i v i t y p a s s e s t h r o u g h a maximum t h e i n t e r p r e t a t i o n of which r e q u i r e s a t least t w o f a c t o r s of o p p o s i t e e f f e c t . The i n i t i a l i n c r e a s e i n a c t i v i t y i s caused presumably by t h e i n c r e a s i n g d i s p e r s i o n , although t h e change i n e l e c t r o n i c s t r u c t u r e ( l i g a n d e f f e c t ) may a l s o p l a y a role i n it. W i t h i n c r e a s i n g c o p p e r c o n t e n t t h e p r o p o r t i o n of c o p p e r - r i c h a n d p u r e c o p p e r p h a s e s i n c r e a s e s on t h e s u r f a c e , a n d t h e r e f o r e t h e a c t i v i t y r a p i d l y decreases from ca. 4 0 a t % of Cu a n d t h e 80 a t % Cu/Pd c a t a l y s t i s , i n a c c o r d a n c e w i t h t h e p o t e n t i o d y n a m i c investigations inactive. T h e a c t i v i t y e n h a n c e m e n t e f f e c t of c o p p e r i s much more s i g n i f i c a n t w i t h aceto p h en o n e and p h e n y l a c e t y l e n e t h a n w i t h t h e o t h e r t w o r e a c t a n t s . T h i s i s due presumably t o t h e f a c t t h a t t h e c h e m i s o r p t i o n of a c e t o p h e n o n e an d p h e n y l a c e t y l e n e , s t r o n g l y bonded t o t h e s u r f a c e , becomes more f a v o u r a b l e from t h e a s p e c t s of r e a c t i o n w i t h t h e i n c r e a s e of c o p p e r c o n t e n t . I n t h e Pd-Cu a l l o y t h e number of e l e c t r o n s i n t h e d-band o f P d i n c r e a s e s , which i n v o k e s a l i g a n d effect d e c r e a s i n g t h e h e a t o f chemisorption. The a c t i v i t y v a l u e s show a maximum for a l l r e a c t a n t s a t a b o u t 30 a t % Cu ( 8 0 w t % of P d ) . I t i s a l s o s e e n t h a t t h e t o t a l amount of H2 decreases and a t 30 a t % Cu i s a b o u t 5 0 % of t h a t of p u r e Pd. I n t h e same t i m e t h e r a t i o o f weakly t o s t r o n g l y bonded H2 also decreases. One of t h e most s t r i k i n g e f f e c t o f t h e c o p p e r f o r t h e s e l e c t i v i t y of P d - c a t a l y s t w a s found i n t h e so c a l l e d Rosemund r e a c t i o n s , by which a l d e h y d e s c o u l d be produced by r e d u c t i o n of acid-chlorides (15).

273

Using Pd alone there are two pathway of side reactions: 1.) the aldehyde formed reduces to alcohol following alkylation by the acid-chloride present,

its

2.) reduction of the acid-chloride group to methyl one along with water formation which resulted in emulsion and hydrolysis of yet not reacted acid-chloride.

Using Pd-Cu-catalyst (30 at% C u ) the selectivity and yield is improved especially for aromatic aldehydes production. For aliphatic aldehydes the Pd-Zn catalysts are suitable but both selectivity and yield are less than for most aromatic aldehydes. Pd-Se-catalysts show some stereoselectivity, too, claimed by a Hungarian Patent (16) at the hydrogenation of tetracycline derivatives. Good selectivity was achieved with Cu-promoted Pd-catalyets in the selective reduction of an oestradiole compound (17). A summary is given in the T a b l e 4. Table 4. Selectivities of promoted Pd-catalysts in liquid phase hydrogenations Reactant

I

Producl

Selecti“I t y %

Yield %

Catalyst

95

95

Pd - Cu

60

70

Pd - Zn

~100

E2

Pd-Cu Pt -cu

L!

REFERENCES 1. Hydrogen

in Catalysis, Theoretical and Practical Aspects Chemical Industries Series, Editors Z. Paal and P.G. Menon, Marcel Dekker Inc. New York (1986) 2. 6. Polyanszky and J. Petrb, Applied Catalysis: 62 (1990)

335-347 3. D.V. Sokolskii and

Gidrogenizacii,

A.M. Sokolskaia, Metalli-Katalizatori Izd. Akad. Nauk. Kaz. S.S.R., Alma-Ata,

1970 p.268 4. Hydrogen Effects 5. 6. 7. 8.

in Catalysis, Editors: W. Palczewska, Z. Pail and P.G. Menon, Marcel Dekker, New York, 1988 p.373 G.C. Bond and P.B. Wells, J.Catal., 5 (1965) 373 J. Zelinski and A. Borodzinski, Appl. Catal., 13 (1985) 305 G. Carturan, G. Faccin, G. Cocco, S . Enzo and G. Navazio, J. Catal., 76 (1982) 405 S. Schuldinger and J.P. Hoare, J. Electrochem. SOC., 111

(1964) 610 9. T. Mallat, E. PolyAnezky and J. Petr6, J. Catal., 44 (1976) 345-351 10. 6 Polyhszky, T. Mallit and J. Petr6, Akta Chim. Hung. 92 (1977) 147-156 11. T. Mallat and J. Petrb, Acta Chim. Hung. 112 (1983) 173-181 12. T. Mallat and J. PetrB, Acta Chim. Hung., 108 (1981) 381388 13. T. Mallat, J . Petrd and M. SchBffer, Acta Chim. Hung., 98 (1978) 175 14. T. Mallat, S. Szabo, J. Petr6, Applied Surface Sciences, 40 (1990) 309-313 15. Hung. Pat. 169 835 (1979) 16. Hung. Pat. 198 173 A (1987); US Pat. 4,960.913 (1990) 17. Hung. Pat. 168 073 (1975); US Pat. 4,021.374 (1976)

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

275

Selective hydrogenation of maleic anhydride by modified copper chromite catalysts G.L. Castiglionia, M. Gazzanob, G. StefaniCand A. Vaccaria 'Dip. Chim. Industr. e Materiali, Viale del Risorgimento 4,40136Bologna (Italy). bCFSM/CNR,Via Selmi 2,40127Bologna (Italy). 'Alusuisse Italia, Via Fermi 51,24020Scanzorosciate BG (Italy).

Abstract Cubic spinel-type phases containing an excess of copper ions may be obtained mainly by controlled oxidation of homogeneous copper-rich precipitates, via intermediate chromates. Part of the copper ions may be replaced by other bivalent cations, with corresponding changes in both thermal stability and catalytic properties. In particular the partial substitution of cadmium ions for copper ions gives rise to considerable deactivation in the hydrogenation of both y-butyrolactone (GBL) and maleic anhydride (MA), while substitution of zinc ions causes a slight decrease in the activity with GBL, but favours the hydrogenation of succinic anhydride (SA) to GBL. On the other hand, the partial substitution of magnesium ions for the copper ions gives rise to an increase in the productivity of GBL from MA, inhibiting the reactions of further hydrogenation and/or hydrogenolysis. 1. INTRODUCTION

The maleic anhydride (MA) business is curently undergoing a revival following some years of over capacity and low margins. Several plants in the USA, Far East and Europe are planned, being designed or under construction. Furthermore, considerable reduction in the price of MA will be attained by the construction of large fluid-bed maleic anhydride plants (about 50,000 Vy), according to AlusuisseLummus and BPAJCB technologies [l-31. Therefore, MA may be supplied easily and a t a reasonable price for the production of y-butyrolactone (GBL) and/or tetrahydrofuran (THF) [4-61.GBL and THF are excellent solvents: GBL is an important intermediate for the chemical industry, while THF is demanded mainly for spandex fibers and polyurethan elastomers [71. They are also examples of small-volume commodities, i.e. of those materials that lie between commodities, where price is the determining force, and specialities or functional chemicals, that require intimate knowledge of several end-use markets 131. Reduced copper chromite is already employed in the hydrogenation of vaporous maleic (MA) or succinic anhydride (SA) or their esters. However, it must be noted that the reaction proceeds consecutively, and GBL and THF are

276

MA

SA

G BL

THF

intermediates. The main by-products are alcohols (methanol, ethanol, propanol and butanol), acids (propionic and butyric), esters (combined with the above alcohols and acids) and others (acetone, methane, a.s.o.1. Therefore, the interest in developing new catalysts with the necessary requirements of activity, selectivity and life time is evident. Considering that GBL may be easily hydrogenated to THF, knowledge of the factors which influence this step is also important, in order to have the possibility to control the GBYTHF ratio. It has been shown that improved copper-containing catalysts for the hydrogenation of crude 0x0-aldehydes may be obtained by reduction of nonstoichiometric spinel-type (NSS) phases, in which part of the copper ions are substituted by another divalent element, with higher activity than the commercial copper chromite systems and, moreover, lower sensitivity towards poisoning [8,9]. The aim of the study reported here, was t o investigate the activity in the selective hydrogenation of vaporous MA of copper-containing catalysts prepared via NSS precursors, focusing attention on the role of the second divalent element on the nature and stability of the phases obtained by thermal decomposition of the precipitates, and on the catalytic performances. 2. EXPERIMENTAL

With the exception of Cat 1 (Cu/Cr= 33:67, as atomic ratio, i.e. with a composition corresponding t o the stoichiometric spinel), all samples contain 50% chromium. In samples 3, 4, and 5, 20% of the copper ions was replaced by zinc cadmium and magnesium ions, respectively. All catalysts were prepared by coprecipitation at pH 8.0 It 0.1 from a solution of nitrates of the elements with a slight excess of NaHC03, washed until the sodium concentration was lower than 0.1% (as NaaO), dried at 363K and calcined at 653K for 24h. The XRD powder patterns were recorded using a Philips PW 1050/81 difkactometer controlled by a PW 1710 unit and Ni-filtered CuK, radiation (I= 0.15418nm) (40kV, 40mA). The data were processed by an Olivetti M240 computer. IR spectra were recorded using the KBr disk technique and a Perkin-Elmer 1700 Fourier-transform spectrometer. Before the catalytic tests, the calcined samples were activated in-situ at atmospheric pressure in a flowing H2-N2 (5:95 v/v) stream, while the temperature was progressively increased from room temperature to 603K. The catalytic tests were carried out using 0.8-2.Ogof catalyst in a tubolar fixed-bed microreactor (i.d. 2mm, length 520mm), operating at atmospheric pressure in the 453-593K range. The reactor was fed with a stream of either GBL or MMGRL solution (60:40 w/w) in hydrogen (H2/C4 molar ratios in the range 150-180 and 50-80, respectively). The organic feedstock was introduced by an Infors Precidor 5003 infusion pump. The reaction products were analyzed in-line without condensation by using a C. Erba 4300 gas chromatograph equipped with FID and two columns (3.2 mm x

277 A0

2.0m) fitted with Poropack QS.

3.RESULTS AND DISCUSS1

r

With the exception of Cat 4, for which crystalline CdCO3 was detected, all the precipitates dried at 363K show XRD powder patterns typical of quasi-amorphous phases, identified as hydroxycarbonates on the basis of the IR spectra [lo]. However, the small amounts of carbonates detected in all samples by titrimetric analyses [ l l l suggest that chromium is mainly present in a highly hydrated polymeric form [121. By calcining precipitates 2-5 at increasing temperature, we observed at first (at about 573K) the formation of amorphous dichromate-type phases [ 131, which successively decomposed to CuO and cubic spinel-type phases, in analogy to that previously reported in the literature [14,151. It is worth noting that also for the 20 30 40 50 60 70 chromium-rich sample (Cu/Cr= 33:67) the 2 0 formation of tetragonal C U C ~ ~spinel O ~ XRD powder patterns Of (ICDD 34-424) took place via intermediate the calcined at 753K for CuCrO4, according to the following scheme: 24h. (a) CdCr; (b) CdZdCr; (c) Precipitate ------> CuCr04 + CrzO3 CdCdCr; (d) C r n d C r - ( A CuO; cuca4 ------> cucr204tetrug. + c u 0 ( 0) tetragonal spinel; (0) cubic CuO+ Cr2O3 -----> At higher calcination temperatures, spinel-type Phase. 753K ca., increased segregation of the oxide phases took place, together with a change from the cubic t o the tetragonal form for the spinel-type phase in Cat 2 (Cu/Cr= 5050) (Figure. 1). Therefore, cubic spinel-type phases may be obtained by controlled oxidation of precipitates with high copper contents, in agreement with the data of Bonnelle et al. [16,17]. Furthermore, the partial substitution of the copper ions increases the stability of the cubic form, with an effect related to the nature of the second bivalent element: CdZdCr > Cu/Mg/Cr > CdCcUCr > CdCr. However, for samples 2-5,the amounts of CuO detected by quantitative XRD analysis (18) approach a maximum of 50% of the value calculated on the basis of a phase composition CuO + stoichiometric spinels, showing the existence of a consistent fraction of copper ions which escape XRD detection, probably present inside the spinel-type phase or strongly interacting with it (19). Evidence for the presence of more elements inside the NSS phases, was found by investigating the CdCdCr sample heated at 753K in different conditions. This was possible because of the higher ionic radius of Cd2+ions in comparison with

278

Cu2+ ions (20). The compositions of the cubic 0.86 spinel-type phases determined on the basis of the lattice 0.855 parameter a, using Vegard's 0.85 law (211, with a linear interpolation between the 0.845 values reported in the ICDD data file, are in good agreement with those determined by quantitative 0.83 0 10 20 30 4 0 50 60 70 80 90 100 XRD and chemical analyses (Figure 2) and show that both Cd amount (atomic %) Cu2+ and Cd2+ ions are present in the spinel-type Figure 2. Composition of the spinel-type phases structure, with different ratios obtained after heating the Cu/Cd/Cr precipitate as a function of the heating at 753K in air (A) or in vacuum (B), determined conditions. Thus, it may be by linear interpolation on the basis of the a hypothesized that the values ( 0 ) [CuCr204 cubic ICDD 26-509 and formation of mixed NSS CdCr2Oa ICDD 2-10001 or by quantitative XRD analyses. phases, containing an excess (m) or chemical (0) of bivalent cations, is even more favoured for elements with ionic radii similar to that of Cu2+ions. The catalytic tests with GBL were carried out to take into account both that GBL is an intermediate in the hydrogenation of MA and that it is also the solvent employed to feed MA, considering that most of the usual solvents react preferentially with MA. For all the samples investigated, the tests carried out at T< 473K show mainly surface adsorption and/or condensation phenomena, while at T> 573K the reactions of further hydrogenation and cracking predominated. The CdCr catalysts (Cat 1 and 2) show similar catalytic behaviour, with a small increase in activity as a function of the copper content (Table 1).At the lowest temperatures investigated, the main products are THF and n-butanol, while at 548K the formation of significant amounts of ethanol were observed, evidence of considerable hydrogenolysis activity (5).In addition ethanol formation is favoured by increasing the copper content. The complement of the selectivity data of Table 1(as well as those of Table 2) has to be attributed both to the gas products (mainly hydrocarbons) and to the irreversible surface adsorption with tar formation. The partial substitution of the copper ions gives rise to a decrease in GBL conversion, with the following order: CdCd/Cr > Cu/Mg/Cr > CdZdCr. In particular, cadmium gives rise to considerable deactivation and the magnesium puts down almost completely the hydrogenolis activity, while smaller differences are observed with the zinc. In regard to the catalytic tests with the MNGBL solution (Table 2), it must be pointed out that a yield in GBL of 43.2%corresponds to the amount introduced as solvent, with theoretically complete absence of its conversion. The CdCr catalysts show similar behaviours, with again an increase in activity as a function of

3

Table 1 Catalytic activity in the hydrogenation of y-butyrolactone. P=O.1MPa; H2/GBL ratio= 150-180 (moVmo1)l.

:at. Composit. React. temp. (K) 1

Cu/Cra

2

Cu/Crb

3

Cu/ZdCr

4

5

CdCdICr Cu/Mg/Cr

485 518 548 485 518 548 485 518 548 485 518 548 485 518 548

GBL" conv. (%I

50.6 80.9 100.0 58.6 92.4 100.0 42.1 69.0 100.0 0.0 10.8 17.7 21.7 37.2 76.8

Selectivity (on carbon atom basis) (%) CH~O'

C2H60

Acetone

n-C,H,O

0.9 3.4 26.2 1.3 7.4 49.5

--

6.2 31.4

-----

2.1 8.0

Cu/Cr= 33:67; b) Cu/Cr= 5050 (as atomic ratio %). GBL= y-butyrolactone; CH40= methanol; C2H60= ethanol; n-C,H,O= i-C,H,,O= n-butanol

THF

n-C4H1,O

56.4 42.8 24.4 61.3 43.4 28.2 58.7 34.9 16.8

11.2 26.3 24.5 10.8 19.5 9.8 20.4 42.1 26.3

5.7

---

50.3 29.0 15.5

10.7 42.7 43.7

---

Butyric acid

--

t)

:)

n-propanol; THF= tetrahydrofuran

Table 2 Catalytic activity in the hydrogenation of a solution of maleic anhydride in y-butyroladone (60:40 w/w). IF=O.1MPa; HdC4 ratio= 150-180(mol/mol)l.

:at. Cornposit. React.

(K)

MAc GBLc conv. yield (8) (8) CH40c C@,O

485 518 548 485 518 548 485 518 548 485 518 548 485 518 548

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 37.0 67.8 81.7 100.0 100.0 100.0

temp.

1 . Cu/Cr" 2 3

cu/c,P Cu/Zn/Cr

4

Cu/CdCr

5

Cu/Mg/Cr

47.1 55.4 5.3 56.3 39.4 2.3 68.9 47.2 3.8 39.1 39.5 38.8 46.1 68.1 45.4

Selectivity (on carbon atom basis) (8) Acetone n-C,H80

--

__

0.9

1.4 0.4

__

--

__ 1.0 __ __ 2.8 __ ___ __ __ __

-_

1.9 0.4 0.6 0.8 0.8

THF n-C4HIoOButyric acid

1.8 31.5 24.8

__

39.1 27.8 0.9 39.6 24.7

__ __

-_

__ -__ __

0.7 1.7

1.8 15.1

--

33:67; b) Cu/Cr= 50:50(as atomic ratio 8). 1 MA= maleic anhydride; GBL= ybutyroladone; CH40= methanol; C$,O= ebahydrofuraq n-C4HloO= n-butanol;SA= SU&C anhydride.

__ 13.7 39.2 0.4 20.1 26.6

__

31.4 33.1

2.2 2.1

__ __

2.1 2.1 3.1

--

-_

4.8 2.2 3.2 2.9

0.5 41.8

0.7

-_

__

_-

__

--

SA 43.9 10.3 2.3 45.2 10.4 I

55.6

--

__

53.1 48.8 60.0 53.4

---

,) Cu/Cr=

ethanol; n-C,H,O= n-propanol; THF=

281

copper content. However, it is worth noting that SA is also present at the highest temperature investigated, together with an appreciable formation of ethanol. Therefore, for these catalysts the determining step seems to be the hydrogenation of SA to GBL, with further rapid reactions to other products. Also in this case, the partial substitution of Cd2+ions for Cu2+ ions gives rise to considerable deactivation and Cat 4 is the only catalyst studied for which the MA was not totally converted. On the other hand,the presence of the zinc ;Ann

;n-aenam

+h*

h.rd-nea-mGn-

activity towards SA, but does not B m o m significantly both the 6 0 M e r , hydrogenation and the 5 0 I ) hydrogenolysis activity. ,Y - 60 On the contrary, the partial 4 0 t substitution of magnesium for some 30 of the copper gives rise to the 2o .............................................. ..................................... ...........- 40 production of GBL at all the - 20 temperatures investigated, with 10 practically an absence of @O hydrogenolysis products and a 518 548 delay of the GBL hydrogenation Temperature (K) reaction. Therefore, the presence of the magnesium seems to decrease Figure 3. Catalytic activity of the Cu/Mg/Cr the hydrogenation activity towards catalyst in the hydrogenation of a W G B L GBL more than that towards MA. solution (60:40 w/w).IF= O.1MPa; HdC4 Furthermore, we observed that ratio= (A) 150-180; (€3) 50-80 (moVmo1). (0) decreasing the HdC4 ratio (Figure y-butyrolactone; ( 0 )tetrahydrofiwan; (m) 3) led not only to a shift in the succinic anhydride; (A ethanol; ( h ) temperature of the maximun of Acetone; (A1n-propanol; ( 0 n-butanol. GBL production, but also to the production of essentially only SA as a by-product, which can then be recycled or hydrogenated in a following step.

.// MY--

-- --,

_ / - -

4. CONCLUSIONS

CdCr or Cu/M/Cr (M= cadmium, zinc or magnesium ions) cubic nonstoichiometric spinel-type catalysts, containing an excess of bivalent cations,

282

may be obtained mainly by controlled oxidation of homogeneous coprecipitates. Direct evidence of the presence of Me r e n t ions inside the NSS structure was obtained by investigating the Cu/Cd/Cr system. It can be hypothesized that reduction of these spinel-type phases results in the formation of well dispersed metallic copper, the properties of which are different from those of metallic copper obtained by the reduction of CuO and are related to the nature of the second element present. However, this effect depends considerably on the nature of the organic substrate investigated, as shown by comparing the behaviours of the different samples in the hydrogenation of MA and crude oxo-aldehyde mixtures (8). In particular, in the case of MA hydrogenation, high productivity in GBL may be obtained by partial substitution of the copper ions with magnesium ions, which inhibits the reactions of further hydrogenation and hydrogenolysis much more that of MA hydrogenation. 6. REFERENCES

1 A. Wood, Chemical Week, (September, 1989)20. 2 N Harris and M.W. Tuck,Hydroc. Process. 69 (1990)79. 3 A.M. Brownstein, CHEMTECH, 21 (1991)606. 4 S.Minoda and M. Miyajima, Hydroc. Process., 49 (1970)176. 6 J. Kanetaka, T. Asano and S. Masamune, Ind. Eng. Chem. 62 (1970)24. 6 T. Aoki, Catalytic Science and Technology, 1 (1991)373 7 Mitsubishi Kase Co., CHEMTECH, 18 (1988)769. 8 G. Braca, A.M. Raspolli Galletti, F. TriGrb and A. Vaccari, It. Pat. Appl. 23,38lA ( 1989). 9 G. Braca, E. Foresti, A.M. Raspolli Galletti, M. Gazzano, F. Trifirb and A. Vaccari, 2nd Eur. Conf. on Catalysis "P,Sabatier", Dijon (F), September 17-24, 1991. 10 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley & Sons, New York, 1978. 11 Vogel's Textbook of Quantitative Inorganic Analysis, IV Ed., p. 309, Longmann, London, 1981. 12 N.N. Greenwood and A. Earnshaw, Chemistry of the Elements, p. 1196, Pergamon Press, Oxford, 1984. 13 J.A. Campbell, Spectrochim.Acta, 21 (1966)1333. 14 H. Charcosset, P. Turlier and Y. Trambouze, J. Chim. Phys., 61 (1964)1249. 16 H.Charcosset, P. Turlier and Y. Trambouze, J. Chim. Phys., 61 (1964)1267. 16 G. Wrobel, J. Arsene, M. Lenglet, A. d' Huysser and J.P. Bonnelle, Materials Chem. 6 (1981)19. 17 A. d' Huysser, G. Wrobel and J.P. Bonnelle, Nouv. J. Chem. 6 (1982)437. 18 H.P. Klug and L.E. Alexander, X.Ray DiEaction Procedures, p. 631,Wiley & Sons, New York, 1974. 19 J. Escard, I. Mantin and R. Sibut-Pinote, Bull. Soc. Chim. France, (1970)3403. 20 R.D. Shannon and C.T. Prewitt, Acta Crystallogr., B26 (1970)1076. 21 A.R. West, Solid State chemistry and its Applications, ch. 10,Wiley & Sons, New York, 1984.

M. Guisnet et al. (Editors),Heterogeneous Cntulysis und Fine Chemiarls IIZ 0 1993 Elsevier Science Publishers B.V. All rights reserved.

283

Selective hydrogenation of dinitriles to aminonitriles on Raney catalysts S.B. Ziemecki Central Research and Development, E. I. Du Pont and Co., PO Box 80262, Wilmington, DE 19880-0262,USA

Abstrctct High selectivity to aminonitriles can be achieved when liquid-phase hydrogenation of dinitriles over Raney catalysts is carried out in the presence of a n excess of methanol (containing an inorganic base) or ammonia. Results obtained by hydrogenation of dinitriles containing from 4 (succinonitrile) to 12 (dodecanedinitrile) carbon atoms are compared. Observed trends are tentatively explained by invoking an intramolecular transformation of the aminonitrile molecules in the preferred reaction media. Formation of ring structures (existence of which is supported by IR results) should hinder readsorption and hence further hydrogenation of aminonitriles to diamines.

1. INTRODUCTION Hydrogenation of aliphatic dinitriles is used in industry mainly to produce diamines. In this sequence of series-parallel reactions aminonitriles represent the half-hydrogenated intermediates of the process. Maximum observed yields of aminonitriles are between 20 and I45% at incomplete conversion of 50%) in all cases. Higher selectivity to aminonitriles has been dinirtiles (I achieved by using catalysts containing rhodium deposited on high-surface MgO, and carrying the reaction in the presence of a large excess of NH3 [l,21. The purpose of this work was to define conditions resulting in maximum yield of aminonitriles over commercial Raney catalysts. For the considered sequence of hydrogenation reactions

kl

A +2H, -B

net rate of formation of the intermediate B is d@/dt= kiCA - k 2 C ~ .Thus the maximum yield of the aminonitrile (B) depends on the value of the kllk2 ratio [31. Phenomenological rate constants hi and k2 may include adsorption

284

components. Properly selected reaction medium can influence adsorption equilibria of species of interest by interaction with either the catalyst surface, or the adsorbate. It is the intention of this paper to show that the latter may be the cause of observed enhancement of selectivity. 2.

EXPERIMENTAL

Liquid phase hydrogenation reactions were carried out in 300 mL autoclave equipped with a stirrer, provisions for heating and cooling, and a dip tube with filter. Samples of the reaction mixture were withdrawn periodically through the dip tube for gas chromatographic analysis, allowing for concentration profiles of components to be constructed. Reactions were carried out a t temperatures from 40 to 120 "C, and hydrogen pressures from 15 to 100 bars. Commercially available Raney Ni (including catalysts promoted by the presence of Fe Cr, Mo,and Co), and Raney Co catalysts were used in this work. Catalysts obtained from Degussa, W. R. Grace, and Activated Metals were tested. Typically 5 to 15 % (wt) of Raney catalyst was charged. Group VIII metals (Pd, Pt, Rh, and Ru) on various supports were tested, but found less satisfactory, except for Rh/MgO. IR spectra of methanolic solutions were taken in transmission mode using conventional equipment. Spectra of aminonitriles in liquid ammonia were obtained in internal reflection mode, using high-pressure attachment equipped with a sapphire rod.

+

3. RESULTSAND DISCUSSION. Hydrogenation of dinitriles containing from four (succinonitrile) to twelve (dodecanedinitrile) carbon atoms in the molecule was investigated. In the absence of solvents reaction leads to formation of diamines, and only low concentrations of aminonitriles are observed. For example, hydrogenation of neat adiponitrile (ADN) on Raney Ni catalysts gives the maximum yield of 6aminocapronitrile (ACN) I 50%, at ca. 70% ADN conversion. Thus interrupting the reaction a t lower conversions of the dinitrile is not an efficient way to produce aminonitrile, because extensive separations and recycle would be required. But in the presence of 10-fold (molar) excess of MeOH (containing 1.5% NaOH) the [ACNlmax > 75%, at ADN conversion ca. 85%. A strong effect of the reaction medium on selectivity to aminonitriles was found. Good solubility of H2 in the solvent is necessary, but not sufficient. The preferred solvents include simple aliphatic alcohols (with an inorganic base, for example 2 0.5% of NaOH or KOH, added), or liquid ammonia. The effect of solvents is not additive, ie. addition of liquid ammonia to methanol does not further enhance selectivity. In general selectivity to aminonitriles increases with dilution by above solvents. For example selectivity of conversion of ADN to aminocapronitrile increases with the amount of alcohol used, and reaches a plateau at ca. 10 mols MeOH per mol of ADN. Lesser excess of liquid ammonia solvent (5 fold molar, for ADN hydrogenation) is sufficient to achieve similar selectivity. It is interesting to note that the rate of dinitriles hydrogenation remains essentially insensitive to the amount of the solvent. Several other

285

solvents, as ammonium hydroxide, hydrocarbons such a s heptane and higher homologues, tetraglyme, several amines, and tetrahydrofuran (THF), were tested but found less satisfactory. Thus the enhanced selectivity to aminonitriles is not due to dilution alone. Presence of a n inorganic base, such as alkali metals hydroxides, methoxides or carbonates, is required when aliphatic alcohols are used as solvents. In the absence of a base in methanolic solutions of ADN the rate of hydrogenation decreases and byproducts (especially those formed by condensation) appear in excessive amounts. Yield of aminonitriles decreases with increasing temperature of the process. Looking from the practical point of view, selective hydrogenation of dinitriles to aminonitriles can be operated in the range from 50 to 80 C. At lower temperatures the rate of hydrogenation becomes too slow, and temperature control of this exothermic process becomes too involved to be practical. At higher temperatures rates of hydrogenation of both the dinitrile and the aminonitrile are greatly accelerated, and selectivity decreases. In addition formation of products of condensation (bis-hexamethylenetriaminein the case of ADN hydrogenation) increases, resulting in further decrease of selectivity. Several Raney Nickel catalysts were tested. The best for our purpose, and about equivalent in performance, were promoted with Cr + Fe, and with Mo.

c

C Q)

C

0 P

E 0 0

C

.-0

CI

0

z

c

0

100

time

200

(min)

Figure 1. Concentration profiles of adiponitrile (squares), aminocapronitrile (circles), and hexamethylenediamine (triangles) during ADN hydrogenation.

286

Concentration profiles of the main components of ADN hydrogenation are shown in Figure 1. At 70 C and 34 bars H2, in the presence of 5 mols of ammonia per mol of ADN, conversion of ADN is achieved within 180 minutes. Concentration of aminocapronitrile in the reaction mixture increases steeply, and reaches maximum of 72% at ca. 90% conversion of the dinitrile. At that time concentration of hexamethylenediamine is still below 10%. The same set of data is plotted vs. ADN conversion in Figure 2. The diagonal line indicates theoretical 100% selectivity of conversion of adiponitrile to aminocapronitrile. Data points corresponding to concentration of ACN in the reaction mixture show increasing deviation from this line with increasing conversion of ADN. A steep increase in hexamethylenediamine concentration is seen above ca. 90% conversion of ADN . Similar results were also obtained when 10-fold excess of methanol (containing NaOH) was used as solvent. The high selectivity to ACN is not limited to liquid ammonia medium of reaction. As seen in Figures 1 and 2, the maximum yield of aminocapronitrile observed in this experiment exceeds 70%. Kinetic analysis [31 indicates that in order to reach such yield of an intermediate in series-parallel reactions under consideration, the effective value of the kilk2 ratio (defined above) must be ca. 8. In the absence of solvent the kilk2 ratio, measured in independent hydrogenations of neat ADN and ACN containing only aqueous solution of NaOH, has

c C Q)

C

0

P

E 0 0

ADN conversion

Figure 2. Data of Figure 1plotted against conversion of adiponitrile. Diagonal line represents 100% selectivity of conversion to ACN. Symbols as in Figure 1.

287

value S 2 (compare Figure 3). The maximum yield of ACN found during hydrogenation of neat ADN is ca. 45%. Thus the observed enhancement of selectivity, associated with the presence of one of the preferred solvents, is accompanied by an apparent increase of the kllk2 ratio by a factor of ca. four.

1.5

c 0 .c

2

c

c a, u c 0 0

C -

0

1

2

3

4

time (h)

Figure 3. Comparison of rates of conversion of adiponitrile (squares) and aminocapronitrile (circles). Data from two independent experiments are shown. Effect of chain branching (ADN vs. methylglutaronitrile, MGN) was studied. The two CN groups in MGN are not equivalent due to presence of the side methyl group in molecule. The steric hindrance of the methyl group favors formation of 5-amino-2-methylvaleronitrile (AN 1)over 5-amino-4-methylvaleronitrile (AN 2).

288

(x

Yield of the sum of both of these isomeric forms AN) reaches 80% at ca. 90%of MGN conversion, while yield of the methylpentamethylenediamine (MPMD) remains below 5% up to ca. 95% MGN conversion. The ratio [AN 11/ [AN 21 remains constant at ca. 2.75 until all the MGN is converted. Then this ratio increases steeply because the unhindered C N group of AN 2 undergoes further hydrogenation to MPMD, while the CN group of AN1 remains protected by the steric effect of the adjacent methyl group. Thus the relative concentration of the two isomers is not a constant, but varies with time on stream (MGN conversion). This indicates that, similarly as ADN, MGN is preferentially adsorbed on active sites of the Raney catalyst, and AN 2 can undergo hydrogenation only after exhaustion of the available MGN. Hydrogenation of other members of the homologous series of dinitriles containing from four to twelve carbon atoms per molecule was undertaken, looking for optimization of aminonitriles formation. Selectivity to corresponding aminonitriles is lower for 7-and 8-carbon systems than that found for 6-carbons members, but higher than that for 12-carbon system. The behavior of sebaconitrile (Clo) during hydrogenation id similar to that of dodecanedinitrile (Cu). Figure 4 shows changes of selectivity to aminonitriles (at various dinitrile conversions) plotted versus the chain length of the starting dinitrile in the series. 00

80 60% conv.

60 7 0% 8 0%

40

-

L+-@ 9 0%

20

1

I

I

I

I

4

6

8

10

12

14

Number C atoms

Figure 4. Selectivity to aminonitriles vs. chain length, shown for various conversion levels of corresponding dinitriles.

289

Selectivity decreases when the chain length of dinitriles increases from six to ten carbon atoms per molecule, but this relationship levels off at each extreme of the range. We propose to explain this observation by invoking an intramolecular interaction leading to a heterocyclic (n+l) conformation, containing imino- form of the CN, in equilibrium with the linear form. Hence, for example, for the five carbon system:

NC(CH2)*NH2

*

p

==N ---solv.

This equilibrium is driven to the right by the presence of a base: addition of an inorganic base (as NaQH or KOH) to alcoholic solution, or by use of liquid ammonia aa the solvent. Above hypothesis is supported by IR data. For aminonitriles in methanol alone there are two prominent lines: a t 2248 cm-1 (CN group), and 1605 cm-1 (NH2 group). On addition of NaOH to this solution the intensity of the CN band decreases, with a concomitant appearance of a new band centered around 1660 cm-1, and assigned to vibration along the C=N bond. At room temperature ca. 25 to 30% of the adiponitrile undergoes such transformation. Analogous changes were observed in liquid ammonia solvent. Such cyclic conformation would impede readsorption of the aminonitrile, necessary for its further hydrogenation, on the catalyst surface. [On the other hand no changes in the IR spectra were detected in solvents which do not enhance the aminonitriles selectivity. This was shown to be true for THF and heptane, with or without addition of aqueous NaOH solution.1 While 6- and 7member nitrogen-containing heterocycles are common, increasingly more stress is involved in formation of higher-order rings. Thus the equilibrium will shift to linear form of aminonitrile (resulting in lower selectivity) with increasing number of carbon atoms in the chain. That is indeed the trend observed in this work. Maximum yield of the aminonitrile, and position of this maximum with respect to conversion of the starting dinitrile, depends on the ratio of rate constants kllk2 (where k l is the rate constant for conversion of the dinitrile to aminonitrile, and k2 is the rate constant of further hydrogenation of that aminonitrile to corresponding diamine). Change in the adsorptive properties of aminonitrile, associated with the existence of the cyclic form, results in lower effective value of the kg, thus increasing the effective value kllk2. Another trend uncovered is the increasing tendency towards formation of dimers of aminonitriles with decreasing carbon chain length. Such dimers are formed during the course of reaction by coupling diamine-diamine, diamine-aminonitrile, and aminonitrile-aminonitrile, with elimination of one molecule of ammonia per created bond. During hydrogenation of glutaronitrile (C5) up to 15% of aminonitrile is found in the form of dimers, and is counted as aminonitrile in the data presented in Figure 4. But for hydrogenation of succinonitrile (C4) more than 50% of the aminonitrile formed exists as dimers,

290

and oligomerizatiion continues even on standing at room temperature. This is the reason why data for C4 dinitrile hydrogenation is not included in Figure 4.

4. CONCLUSIONS. High selectivity to aminonitriles can be achieved when hydrogenation of dinitriles containing from 5 to 12 carbon atoms per molecule over Raney catalysts as Ni/CrFe, and Ni/Mo is carried out, at moderate conditions of temperature and pressure, in solvents such as MeOH + NaOH, or in liquid ammonia. Observed dependence of selectivity on the carbon chain length can be explained by an equilibrium between linear and heterocyclic forms of aminonitriles.

5. REFERENCES 1. US Patent No. 4,389,348(1983)and US Pat. 4,601,859(1986) 2. F. Mares et al., J. Catal. 11 (1988) 145 3. 0. Levenspiel, "Chemical Reaction Engineering", Prentice Hall, 2nd ed.

M. Guisnet et al. (Editors),Hderogenwus Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

291

HYDROGENATION OF DINITRILES INTO DIAMINES- INFLUENCE OF THE NATURE OF DINITRILE ON ACTIVITY AND SELECTIVITY OF THE REACTION MARION Philippe, JOUCLA Marc,TAISNE Chantal and JENCK Jean UMR 45 CNRSRP - Rhdne-Poulenc Industrialisation 24, avenue Jean Jau& F- 69153 - DECINES (FRANCE)

Abstract : Hydrogenation of dinitriles into primary diamines over Raney Nickel involves the formation of numerous by products. The nature of dinitrile and hydrogenation intermediates greatly modify the selectivity of the reaction and the catalytic activity The kinetic and thermodynamic aspects of the by products formation are discussed.

Introduction : Primary aliphatic diamines are very important intermediates in polymer industry. Tetramethylene diamine is a monomer for polyamide 4,6. Hexamethylene diamine is used in two main industrial productions. Copolymerisation with adipic acid gives rise to nylon 6,6. Reaction with phosgene leads to isocyanates which are monomers for painting industry. Methylpentamethylene diamine is a monomer for thermoplastics (DU PONTs DYTEK). The major route to produce those diamines is the hydrogenation of the corresponding dinitriles. The comprehension of the chemistry of these reactions is of first interest. Hydrogenation of aliphatic dinitriles over Raney nickel involves the formation of several unsaturated intermediates described many years ago by VON BRAUN (1)-scheme I-. The reactivities of dinitrile, diamine and above all, of the intermediates are responsible for the formation of numerous by-

HN=CH-(CHZ),-CH=NH

Scheme I : General scheme of hydrogenation of dinitriles We have classified these impurities according to the nature of the new created bond. Thus, there are three different kinds of by-products : - C-Cimpurities result from two reactions * The baso-catalyzed THORF'E-ZIEGLERs cyclization. * An unknown C-C coupling similar to pinacolic reaction.

292

- C-N by-products are more classical. They are secondary and tertiary amines and their Rrecursors secondary imines and enamines (2,3). - C-0 impurities result from hydrolysis of nitriles or imines. They are oRen undetected except for very slow hydrogenation reactions in aqueous medium. In this report, we will discuss about kinetic and thermodynamic aspects of the by-products formation. Then we will focus on the relationship between the selectivity of the reaction and the catalyst activity related to the productivity of the hydrogenation for an industrial point of view. 1. FORMATION OF 1,2-DIAMINOCYCLOALCANES (DCA) :

Several parameters affect the rate of formation of diaminocycloalcanes. At first, we have clearly shown that the formation of DCA occurred in a parallel pathway with the hydrogenation of dinitrile into aminonitrile. For example, no 1,2-diaminocyclohexane is formed in the course of the hydrogenation of aminocapronitrile (4). Diadsorption of the two nitrile functions is another major point : The DCA content increased with the ratio catalyst/dinitrile and, in semi-batch reaction, when the rate of addition of dinitrile was lowered (4). It is of importance to notice that the corresponding intermolecular byproduct has never been detected. Formation of DCA derivatives could occur through a reductive coupling of an intermediate diimino moiety geometrically fitted to allow C-C bond formation to overcome. In this way this reaction could be compared with the pinacolic coupling for carbonyl compounds (scheme 11) By mimetic reactions, 1,2-DiaminoCycloHexane was successfilly produced only when the hybridation states of carbon and nitrogen were sp2 (4). This result are in accordance with the litterature (6-6). Catalytic sites of formation of DCH seemed to be specific and rather reactive. They were easily deactivated or poisoned. The addition of sodium carboxylates (6% of nickel surface coverage) lowered the 1,P-DCH formation by 40% during the hydrogenation of adiponitrile without any loss of activity (semibatch test).

(G

C r N

C r N

a@NH

- [iccH""H ]+2H2

+H2

( In

CH= NH

H % ~ ~ , DCA

Scheme I1 :Proposed mechanism for the formation of DCA

293

The thermodynamic and kinetic aspects of cyclization leading to 1,2-diamino cycloalkanes will be discussed further.

2. THE BASOCATALYSED THORPE-ZIEGLER'S CYCLIZATION (AMCA) : The presence of two electron withdrawing groups on the same molecule gives rise in presence of a BRONSTED base to intramolecular reaction (7). The reaction scheme is described in scheme 111. The nucleophilic addition of the carbanion (ato the first electronwithdrawing group) to the second electrophilic center gives rise to C-C bond formation. For the hydrogenation of dinitriles, two unsatured intermediates and the reactant (dinitrile itself) could give rise to THORPE-ZIEGLERs adducts. We have shown that this reaction is competitive with the hydrogenation and it depends .essentially on catalyst and base concentrations. We have never detected the products resulting from an intermolecular reaction, while the intramolecular cyclization was fully characterized. C a N CN

8

CN

CN

Scheme I11 : THORPE-ZEGLERs reaction for dinitrile The product obtained by cyclization could exist under two tautomeric forms. NMR shows that only the enaminonitrile form is present in the hydrogenation medium. By further hydrogenation, this unsaturated compound gives rise to-2 Aminomethyl cycloalkylamine (AMCA). Kinetic and thermodynamic aspects of THORPE ZIEGLER cyclization during the course of dinitriles hydrogenation will be discussed with all the by products. of the

.. .

The formation of enaminonitriles which are the basocatalysed adducts lowered the rate of hydrogenation of the corresponding dinitrile (cf. figure n"1). In terms of industrial considerations, the productivity was decreased. We explain this poisoning effect by a competitive chemisorption of enaminonitrile and dinitrile and a rate of hydrogenation of the by-product very slow compared to the nitrile.

294 1

2.5

Figure 1 : HYDROGENATION OF ADIPONITRILE OVER RANEY NICKEL : Influence of THORPE-ZIEGLERs adduct on the catalyst activity.

3. C-NBY-PRODUCTS : The mechanism of formation of C-N impurities was first proposed by VON BRAUN (1). The nucleophilic addition of a primary amine to an unsaturated intermediate of hydrogenation which is a primary imine leads to an aminal. Loss of ammonia gives rise to a secondary imine, precursor of secondary amine by further hydrogenation. In the particular case of the hydrogenation of dinitriles, this reaction could occur in an intramolecular or intermolecular process depending on the double functionnality of molecules. Another nucleophile could be the secondary amine leading to enamines and tertiary amines. The reaction scheme for the formation of secondary amines is described in scheme IV. NraC-(CH2),, -C= N

1

+3H2

H2N-(CH2 ),+1-CHsNH

SchemeIV: Formation of secondary amines

+diamin;/

H2N(CH2)n+1-FH.NH.(CH2)n+2-NHZ NH2

1

+Ha

\

G

( a + l )r N H 2 I

1

+H2

(*) Those rings involving (n+l) carbon atoms other than mentioned are equivalent to (n+3)membered rings

295

In our conditions, we have only observed secondary imines and secondary amines. Moreover, we have shown that the C-N impurities are mainly produced during the hydrogenation of aminonitrile to diamine. They correspond to the reaction of primary diamine with aminoimine intermediate. In the case of the hydrogenation of adiponitrile we have shown that cyclic and linear secondary imines are in equilibrium through a transimination reaction (8). This reaction is charactarized with an equilibrium constant which was determined for adiponitrile hydrogenation.

nation: We have shown previously the importance of secondary imines on the rate of hydrogenation of the intermediate aminonitrile (9). The adsorption constant value of secondary imine was comprised between the adsorption constant values of dinitriles and aminonitriles. As the rate of hydrogenation of imine is slow comparatively to aminonitrile, the rate of the global reaction is controlled by the adsorption of secondary imines at the catalyst surface. So the ability of the intermediate aminoimine to generate secondary .imines . is very harmful for the catalytic activity : it leads to a chemical d e a c w n of

theROCEDUI1E;; Dinitriles were hydrogenated in a 150 ml autoclave with magnetic stirring under constant pressure and temperature. The Raney nickel was first washed, then weighed with a pycnometer and charged into the reactor with 50 ml of solvent containing ethanol, water and the corresponding diamine. The dinitrile (6mmoles) was charged in a dropping funnel. Then the autoclave was closed and purged repeatedly with nitrogen first then hydrogen. The reactor was heated under hydrogen pressure to the fixed reaction temperature. Dinitrile was introduced rapidly under stirring and this was considered as the zero time of the experiment. The rate of the reaction was measured by following the hydrogen consumption in the hydrogen supply. Samples were withdrawn regularly in the course of the hydrogenation and were analysed by gas chromatography. CUWONS 1 The catalytic hydrogenation of dinitriles to primary diamines involves the formation of by-products. We have proposed a classification for these impurities putting forehead the nature of the new created bond during their formation. Thus we distinguish the C-C by-products from the C-N ones. C - 0 impurities result &om hydrolysis of nitrile into amide and of imine into aldehyde.

296

C-C by-products formation occurs during the hydrogenation of dinitrile into aminonitrile, while C-N by-products are formed during the second stage of the hydrogenation ie from aminonitrile to diamine.

DINITRILE + 2 H2

-

-

DIAMINE AMINONITRILE +2 H2 C-N by-products

C-Cby-products

From a thermodynamic point of view, the hydrogenations of dinitriles are similar (AGr(8O0C)=-18,7kcallmol). The first step of hydrogenation gives the same kinetic rate whatever the dinitrile. Nethertheless, the ability of a dinitrile and its corresponding unsaturated intermediates to generate byproducts by cyclization reactions drastically modify the selectivity and the activity of the hydrogenation. Three kinds of cyclizations are observed :

- A reductive coupling leading to 1,2 diaminocycloalkanes (DCA). - A baso-catalyzed reaction giving rise by further hydrogenation to 2aminomethyl cycloalkylamines (AMCA).

- A nucleophilic addition of a primary amine to a primary imine leading by loss of ammonia to azacycloalkenes which are hydrogenated to azacycloalkanes (AZACA). Those different reactions involve the formation of three to eight membered rings during the hydrogenation of succinonitrile, glutaronitrile, adiponitrile and pimelonitrile respectively. All the cyclization products are not detected. The different impurities found are reported in table 1.

For each dinitrile and each kind of impurity, we get three informations.

- The nature of the cycle formed ie X membered ring

- AGr(T) for the reaction dinitrile + x H2 = impurity - The relative rate of formation of the impurity comparatively to the rate of formation of the corresponding diamine (vfidvfdiamine) at is0 conversion. For each impurity, we fixed arbitrarily this ratio equal to 1for the hydrogenation of adiponitrile. When this ratio is 0 this by-product has not been detected by gas chromatography that means its content is less than 100 ppm.

297

Table 1:Thermodynamic and kinetic aspects of by-products formation by-product dinitrile

DCA

SUCCINONITRlLE

+ 3,l kcaVmol

+ 5,O kcaVmol

- 0,3 kcdmol

0 5-MB RING - 16,5 kcaVmol 10 6-MB RING - 19,6 kcaVmol

0 4-MB RING + 3,7 kcaVmol 0 5-MB RING - 15,5 kcaVmo1

100 6-MB RING - 3,2 kcdmol 10 7-MB RING + 1,9 kcaVmo1

4-MB RING

GLUTARONITRILE

ADIPONITRlLE (*I

AMCA 3-MB RING

AZACA 5-MB RING

1

7-MB RING 6-MB RING 8-MB RING PIMELONITRILE - 12,3 kcaVmol - 18,6 kcaVmol + 3,5 kcaVmol 0 0 0 (*) All values are expressed comparatively to this reference for each kind of impurity. The catalytic test is well defined and the operating conditions are strictly identical. Thermodynamic calculations by BENSONs group contributions (10) show that cyclizations involving a six membered ring formation are favoured. Then, !%omthe easiest to the more difficult, come the 5-membered ring, the 8 , 4 and 3-membered ring. These calculations are in agreement with the well-known ring strains and eclipsed bonds. Hydrogenation results show that the cyclizations involving a five membered ring formation are greatly favoured relatively to 6 then 7-membered rings. Thus these reactions are under kinetic control. If we compare for one kind of cyclization, the rate of formation of different size of ring, we obtain results in agreement with those observed in classical organic chemistry (1112). Comparing the three reactions in our conditions, in terms of quantity of by-products, intramolecular nucleophilic addition is more important than reuctive coupling, itself more important than THORFE-ZIEGLERs cyclization. The ability of dinitrile or hydrogenation intermediates to be cyclized is very important. First, the selectivity decreases, then, the formation of by-products causes a chemical deactivation of the catalyst especially for AMCA and AZACA formations. In terms of industrial applications, catalyst consumptions are increased to maintain a constant productivity. Thus, it is easier to hydrogenate pimelonitrile than adiponitrile in our conditions, both in term of vltv sele-. Glutaronitrile hydrogenation is more difficult. Succinonitrile is the hardest substrate. In this case pyrrolidine is the major product. Considering the substrate hydrogenability, we have : Pimelonitrile 2 Adiponitrile > Glutaronitrile > Succinonitrile

298

CONCLUSION : Secondary reactions are of prime importance in the hydrogenation of dinitriles over Raney nickel. Though no difference was foreseeable, the ability of each dinitrile or the corresponding hydrogenation intermediates to be cyclized is drastically important in terms of selectivity and catalytic activity of the reaction. Formations of by-products by cyclization are under kinetic control for the different reactions observed.

Nomenclature : DN : DINITRILE PRIMARY DIAMINE DA : DIAMINOCYCLOALKANE DCA : AMCA : 2-AMINO METHYL-CYCLO ALKYLAMINE CI: CYCLIC IMINE AZACA AZACYCLOALKANE CSA : CYCLIC SECONDARY AMINE DIAMINEIMINE DAI : TA : TRIAMINE Succinonitrile : 1,a-dicyano ethane Glutaronitrile : 1,3-dicyano propane Adiponitrile : 1,4-dicyano butane Pimelonitrile : 1,5-dicyano pentane Aminocapronitrile : l-amino 5-cyano pentane REFERENCES : (1) J.VON BRAUN, G. BLESSING, F. ZOBEL, Chem,Ber.,l923, %, pp 1988-2001. (2) J. VOW, J. PASEK, Stud.Surf.Sc.Cat., Elsevier, 1986,22,pp 105-144. (3) P. T I N U P , Methoden der organischen chemie, HOUBEN WEYL, l Y&, pp 111-143. (4) P. MARION, PhD Thesis, University of LYON I, FRANCE, 1990. (5) S.F.PEDERSEN,E.J. ROSKAMP,J.Am.Chem.Soc.,1987, U.9, pp 3152-3154. (6) H.TANAKA, T.NAKAHARA, H.DHIMANE. S.TORII, Synthesis, 1989, pp51-52. (7) J. MARCH, Advanced Organic Chemistry, 3rd edition, WILEY, 1985. (8) P. MARION, P.GRENOUILLET,M. JOUCLA, J. JENCK Stud.Surf.Sc.Cat,Elsevier,B,pp 329-334. (9) M. JOUCLA, P.MARION, P. GRENOUILLET, J. JENCK, ORCS abstracts, Albuquerque, April 1992. (10) S.W.BENSON, F.R. CRUIKSHANK, D.M. GOLDEN, J.R. HAUGEN, H.E. O m , AS. RODGERS, R. SHAW, R WALSH, Chem.Rev., 1969@,3, pp 279-323. (11)AC. KNIPE, C. J.M. STIRLING, J. Chem. SOC. B, 1967, p.808 B, 1968, p l l l . (12) R.BIRD,C.J.M. STIRLING, J. Chem. SOC.

M. Guisnet et al. (Editors),HeterogeneousCatalysis and Fine Chemicals 111 63 1993 Elsevier Science Publishers B.V. All rights reserved.

299

SELECTIVE HYDROGENATION OF 3 BUTENONITRILE AND 2 BUTENONITRILE ON PALLADIUM EXCHANGED TITANIUM PILLARED MONTMORILLONITE Anne Lamesch, Hector del Castillo, Pascal Vanderwegen*, Loreto Daza**, Georges James*, Paul Grange

Unit6 de Catalyse et Chimie des Matdriaux DivisBs, Universitd Catholique de Louvain, Place Croix du Sud 2/17, 1348 Louvain-la-Neuve(Belgium) * CERIA-ISI, Brussels (Belgium) ** Instituto de Catdisis y Petroleoquimica, Madrid (Spain).

ABSTRACT Thermally stable titanium pillared montmorillonite have been synthesized and used as support for preparing palladium exchanged high microporous catalysts. Low Pd content does not modify the physico-chemical properties of the pillared support, while 3 wt% Pd deposited on the Ti montmorillonite slightly decreases the surface area and microporosity of the catalysts. Selective hydrogenation of 3 butenonitrile and 2 butenonitrile to butyronitrile is observed without isomerization. Progressive poisoning of the catalytic sites and possible diffusional limitations alter the activity of these catalysts. INTRODUCTION The last decade has seen a growing interest for the study of pillared clays and several papers on the preparation and characterization of these microporous materials appear in the literature. The main emphasis was oriented towards the preparation of new systems or catalysts presenting advanced structural-textural properties and thermal stability (1). Regarding the catalytic properties, several reactions have been proposed on clays as addition, dehydration, dehydrogenation, condensation, esterification or cracking or hydrocracking. Less attention has been given to the catalytic properties of pillared clays. Let us, however, mention some studies showing their potential applications in hydrotreating (2), acid catalyzed reactions (3-4) and some attempt in selective oxidations and epoxidations (5). In this last case, the pillared clay was used as a support on which the active phase is deposited. In the present study, we prepared high thermally stable Ti pillared clays and used the microporous solids as a support for Pd metal exchange and start preliminary catalytic experiments of this new hydrogenating catalyst in selective hydrogenation of 3 butenonitrile (B3N) and 2 butenonitrile (B2N).

EXPERIMENTAL Preparation of the Pd-Ti pillared montmorillonite The first step of the preparation consists in the synthesis of thermally stable, high microporous titanium pillared montmorillonite. Afterwards,

300

palladium is exchanged in order t o prepare two Pd-Ti PILC catalysts with two different palladium contents. The starting material is a sodium montmorillonite (Kunimine Industry Co. Ltd. - Kuni pure F).As the pH of a 4 g.1-1 clay suspension is 9.75 and the electric conductivity is 122 p.Scm-1, the Na excess is previously eliminated by several washings and centrifugation up to a final pH of 6 and an electrical conductivity of 5 p.Scm-1. The titanium pillaring solution is prepared from Tic14 and HC1 diluted in distilled water, the Tic14 and HC1 concentration being respectively 0.82M and 0.125M.Before pillaring, the solution is stirred for 3 h at 25 "C. The pillaring process consists in slowly adding the Na-montmorillonite (4 g.1-1) t o the hydrolyzed or partially hydrolyzed titanium solution (10 mmoles Ti/g clay). After 12 h under permanent stirring at 25 "C, the pillared clay is washed and centrifuged (5000 rpm for 5 mn). The pillared montmorillonite is then dried and calcined at 500 "C. The Pd is then exchanged [Pd(NH3)42+]in order t o prepare Pd-Ti PILC catalyst containing two different Pd loadings, namely 1 and 3 Pd wt% and labelled Pd-1 and Pd-3.

Characterization and evaluation of the catalysts Chemical analysis, X-ray diffraction, N2 adsorption and hydrogen chemisorption were done in order to evaluate the composition, structure, texture of the catalyst and the metal dispersion. The catalytic properties of the two Pd Ti PILC samples were evaluated by the study of the hydrogenation of 3 and 2 butenonitrile (B3N and B2N). All the hydrogenation experiments were done in a well stirred autoclave (Labor Zurich) under a constant pressure of 10 bar Druckriihrwerk Ingenieurburo S.F.S. at 20°C. The apparatus allows the introduction of the reactants under pressure and is equipped with sampling and flushing attachments and a thermocouple probe. Catalyst reduction is performed ex situ during 1 hour under a hydrogen flow of 20 mVmin in a temperature range between 520 and 570 K. The solvent used is a 95 % ethanolic solution. All the reagents are laboratory grade and freshly distilled before use. The products analysis was performed by gas chromatography. We used a Cp-SIL 5CB (50m x 0.53mm) chromatography column from Chrompack. RESULTS AND DISCUSSION The pillaring process allows to introduce a large amount of titanium oxide into the layers of the montmorillonite. The Ti02 content evaluated after calcination at 500 "C is 43 wt%. The prepared Ti PILC are thermally stable up to 600 "C without noticeable change of size (height) of the pillars that are around 13 8, a t 500 "C, as determined from XRD diffraction lines. This induces a high surface area microporous solid. The specific surface area and micropore volume, as determined from BET adsorption, are 360 m2g-1and 0.19 cmsg-1respectively at 500 "C. These values are very close t o those previously obtained (6-9) whatever the Ti precursor species, when using an appropriate pillaring process. After Pd exchange, the thermal stability of the catalyst is not modified as compared with the support. In addition, the height of the pillars is almost the

301

same as before Pd.exchange However, we note an important change in the textural properties. The surface area of the catalyst decreases by 7 and 30 % respectively when 1 or 3 Pd% is introduced into the layers of the Ti pillared montmorillonite (table 1).The introduction of the highest amount of Pd largely decreased the internal accessibility of the clay layers. In parallel, a decrease in the micropore volume is observed. Table 1. Sample Ti PILC Pd-1 Pd-3

Pd (wt%) 0 1.13 2.89

S BET (m2g1) Smicro (m2g1) Vmicro(cm3g1) 360 335 257

323 30 1 225

0.19 0.19 0.13

Hydrogen chemisorption brings crucial information concerning the Pd dispersion exchanged on the Ti PILC. Table 2 compares the metallic surface area and dispersion of the Pd particles evaluated from the H2 chemisorption. Table 2. Sample Pd-1 Pd-3

smet W g -9 720 450

D >1 1

From these results, it can be noticed that calculated dispersion value is very (too) high. In addition, the data obtained for the catalyst containing the lowest Pd content (1%) is higher than 1. These values are unrealistic and cannot give valuable informlation concerning the metallic surface areas and particle size. However, the extremely high hydrogen chemisorption content obtained could be explained in two different ways according to previously published results on conventional noble metal Ti02 catalysts (10-13). Partial reduction of Ti02 pillars or some hydrogen spillover from Pd to Ti02, enhanced by the small Pd size crystallites, could occur. It has been shown that the comparison of the hydrogenation kinetics of conjugated and non-conjugated unsaturated nitriles, and more specifically of 2butenonitrile (B2N) and 3-butenonitrile (B3N), could bring on some information on the role of dopes in metal catalysts (14). This has been generalized to the influence of other second order interactions (solvent effects, support or vicinal oxidic or acidic phase effects, ...1. Such interactions, that are not strong enough to influence the adsorption constants, decrease the rate constant of B3N hydrogenation, leaving B2N hydrogenation rate unchanged (16). This may be justified by a difference in reaction mechanisms: direct hydrogenation for B2N, cyclic transfer for B3N. This mechanistic difference is reflected in a difference in reaction rate: B3N is hydrogenated four to ten times faster, according to the solvent, than B2N.

302

-

5

0.25

- - 82N

Pd 1

0.125

0 Time Imnl

-

s

0.25

I

Pd-1- B3N

0 Time lmnl

-

5

0.25

Pd-3

- 83N

C

2

Pu 0

5

u 0.125

0

.A

I

0

150

300 Time - . lmnl -. ,

Fig. 1. Hydrogenation of B2N and B3N: nitrile ( 0 ) and butyronitrile (D) concentration.

303

In the case of pillared clays supported metal catalysts, the influence of the acidic sites on hydrogenation could be evidenced by this kinetic characterization. The hydrogenation of 3 butene nitrile B3N and 2 butene nitrile B2N is reported in figure 1 and table 3. Initial rates are calculated on the basis of product formation. When comparing the influence of Pd content on catalytic hydrogenation of B3N, an increase in the rate of reaction with the Pd content is observed. On the other hand, the rate of hydrogenation of B2N is lower than that of B3N on the same (Pd-1) catalyst. The preliminary results of the C4 unsaturated nitriles hydrogenation confirm several trends already observed on h e y nickel (14). Table 3 Initial rate * mol dm-3min-1gc,t-l mol dm-31nin-lg,~tal-~ 0.0938 Pd- 1 B2N 0.00106 2.52 B2N 0.126 PdC 5% 0.394 Pd-1 B3N 0.00445 Pd-3 B3N 0.011 0.380 Pd/C 5% B3N 0.337 6.74 * Initial rates are calculated on the basis of product formation Catalyst

Substrate

First, in the experimental conditions we have used, the hydrogenation of both B2N and B3N is 100 percent to the butyronitrile (BN). Secondly, the initial results collected clearly show that the apparent reaction orders, for both studied substrates, are different from zero and hence demonstrate that the product competes with the reactant for the adsorption of the catalytic site. Thirdly, in the hydrogen pressure range explored, the isomerization of B3N is never observed on any catalyst (Pd-1,Pd-3) or on the support free of metal. This is an evidence that no side isomerisation reaction occurs neither on the metal nor on the support. Finally, we may also notice the reactivity order of the substrates as previously observed on Raney nickel (14-15): B3N is more rapidly reduced than B2N. This fact precludes a hydrogenation mechanism of B3N through isomerisation to B2N. For the activity of the catalysts used, we find a good proportionality between the hydrogenation rates of B3N on Pd-1 and Pd-3. Taking into account the amount of catalyst and metal loading, this suggests that metal dispersion and metal distribution on the support are independent from the metal loading. However, the activity of these metal-loaded pillared clays is more than 10 times lower that for carbon-supported catalysts (15) for both unsaturated nitriles. This loss of activity could be explained in two ways: the first supposes a lower accessibility of the metallic surface in the case of the pillared clays catalyst as compared with carbon based catalysts. We may suppose some strong diffusional effect. This would indicate that, assuming a good repartition of the metal into the layers of the pillared clay, the most active accessible metallic sites would just be those near the outer edge of the clay. The second explanation could be a second order effect due to an interaction between the olefinic double bond and the support may induce a decrease of the hydrogenation rate of the B3N (16). Nevertheless, this last explanation cannot be

304

applied to the B2N hydrogenation rate decrease because the hydrogenation of this substrate is insensitive to second order interactions. At this stage of the study, we can confirm that this test reaction has a behaviour on this kind of catalyst similar to the one on Raney nickel. An extensive research of the hydrogen pressure, poisoning and solvent effects should be carried out to improve this comparison. The second point to be mentioned is that, in spite of the particular architecture of the catalytic site located inside the pillared layered structure, the hydrogenation activity and selectivity of B3N and B2N is not drastically modified as compared to conventional Raney nickel catalyst.

ACKNOWLEDGMENT We acknowledge the "Service d e Programmation de la Politique Scientifique - SPPS" Belgium for supporting this work.

REFERENCES 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Pillared Clays (R. Burch, ed.), Catal. Today, 2 (2-3- (19881, 185. M.L. Occelli, R.J. Renard, Catal. Today, 2 (2-3) (1988), 309. P.E.W. Vaughan, R.J. Lussier, Proc. 5th Int. Zeolite Conf. (L.V.C. Rees, ed.), Heyden Press, 1980,84. E. Kikuchi, T. Matsuda, Catal. Today, 2 (2-3) (19881, 297. B.M. Choudary, V.L.K. Valli, A. Durga Prasad, J. Chem. SOC.,Chem. Comm. (19901,721 and (19901,1186. A. Bernier, L.F. Admaiai, P. Grange, Appl. Catal., 77 (1991), 269. L.F. Admaiai, A. Bernier, P. Grange, 10th I.C.C., Budapest, 1992, in press. J . Sterte, Clays and Clay Minerals, 34 (6) (19861,658. S. Yamanaka, T. Nishihara, M. Hattori, Y. Suzuki, Mat. Chem. Phys., 17 (19871, 87. T. Tanaka, H. Kumagai, H. Hattori, M. Kudo, S. Hasegawa, J. Catal., 127 (19911, 221. J.C. Conesa, P. Malet, G. Munuera, J. Sanz, J. Soria, J. Phys. Chem., 88 (1984), 2984. H.R. Sadeghi, V.E. Henrich, J. Catal., 87 (1984), 279. Ch. Hong, Ch. Yeh, Mat. Chem. Phys., 20 (1988), 471. J.L. Dallons, G. Jannes, B. Delmon, i n Heterogeneous Catalysis and Fine Chemicals (M. Guisnet et al., eds.), Studies in Surface Science and Catalysis, 4,( 1988),115. P. Vanderwegen, G. Jannes, unpublished results. J.L. Dallons, G. Jannes, B. Delmon, Catal. Today, 5 (19891,257.

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals IIZ @ 1993 Elsevier Science Publishers B.V. All rights reserved.

305

One step synthesis of dissymetrical amines R2NR'from nitriles in the presence of copper catalysts J. Barraulta, S. Bruneta, N. Essayema, A. Piccirillia, C. Guimonb and J-P. GametC Laboratoire de Catalyse, URA CNRS 350, ESP, 40 av du Recteur Pineau, 86022 Poitiers Cedex, France. b) Laboratoire de Physicochimie mol6culaire, URA CNRS 474, av de l'Universit6, 64O00 Pau, France. C) CECA-ATOCHEM, C.A.L., Levallois, France. a)

Abstract Copper-chromite type catalysts supported by alumina or graphite and promoted with barium were used for the one step synthesis of tertiary fatty amines (R2NCH3 or RN(CH3)2) from nitrile, methanol and hydrogen. The surface composition of the catalysts was studied by XPS and by adsorption experiments. A correlation was found between the selectivity and the presence of a welldispersed CuCrO2 phase, stabilized with barium. Moreover the elements influencing the stability of the copper catalysts were also studied and we remarked the effect of the promoter or/and of the support on the variation of the copper surface area in the presence of water or ammonia. These modifications were examined in relation with the change of the catalytic properties with time-on-stream. 1 - INTRODUCTION

The main objective of this work consists in the direct catalytic synthesis of R2NCH3 or RN(CH3)z dissymetrical tertiary amines, which find many applications in the industry of tensio-active products. To this end acids, esters, alcohols, nitriles obtained from triglycerides of natural origin can be used as raw materials. Starting from nitriles the following are the reactions to be performed RCsN+H,+CH,OH > RC%N(CH,), + NH, + %O (1) or (RCH&NCH,

This type of reaction requires a complex, multifunctional catalyst with acid and hydrogenating properties. In a previous work we showed that these properties and hence the activity and the selectivity of the catalyst depended very much on the support and on the various promotors of copper which was chosen as a basic metallic element (1, 2). Furthermore the stability of the catalysts also depends largely on their composition and of certain products of the reaction such as water and ammonia (3). We have therefore studied the modifications of the bulk and surface compositions as well as those of the adsorption properties of supported (alumina or graphite) or promoted (barium) copperchromium

306

catalysts so as to account for the variations of the catalytic properties (3, 4). The main reactions presented in this work are the hydrogenation of lauronitrile (model molecule) and the N-methylation of the dodecylamine by methanol. + CH,OH

+CH,OH

2 - EXPEIUlMENTAL

The catalytic reactions were performed under pressure in a fixed bed continuous reactor and the experimental conditions appear in the tables of results. The surface composition of the catalysts was determined by XPS (spectrometer SSI, model 301) using an AlKa monochromatic radiation (10 kV, 12 mA). The catalysts were prepared by coprecipitation of copper and chromium hydroxides in the presence of graphite Lonza HSAG (300 m2/g) and alumina RPGFSC (200 m2/g) supports.

3 - RESULTS 3.1 - Catalyst characterization * The bulk composition and the main phases identified with X-ray diffraction are reported in table 1. Quite well crystallized phases on alumina supported catalysts were observed i.e. CuCr204 for unpromoted catalysts and BaCr04 for barium promoted catalysts. Less crystalline solids were obtained when graphite was used as a support. In this case different phases were observed i.e. CuCrOz and Cu20 which showed that the support had a significant effect on the bulk composition. Table 1 : Bulk cornposition of copper chromium catalysts. CuCr/A1203 Cu content (wt %)

5.4

CuCrBa/AlZO3 CuWgraphite CuCrBdgraphite 5.9 5.8 5.5 4.1 5.0 4.3 1.05 1.18 0.95 4.4

X-ray diffraction (identified Dhases)

BaCrO4

0 less crystalline

1.2 CuCrO, CU?O

* The X P S surface characteristics presented in table 2 also demonstrated that the surface composition of the catalysts depended very much on the support. Studying aluminasupported solids, a wpper surface enrichment not modified by the addition of barium (before reduction) was remarked. However after a reduction treatment with hydrogen at

307

623 K, we observed the dispersion of copper decreased in the absence of barium while the modification was less important in the presence of barium. Therefore in the presence of barium, the copper dispersion was higher and copper oxide was not totally reduced even after the reduction treatment. Table 2 : Surface atomic ratios of supported copper-chromium catalysts obtained from XPS measurements.

I

CuCr/Al203

I CuCrBdA1203 I

I

CuCdgraphite CuCrBdgraphite

I

(a) and (b) : non reduced and reduced (623 K with Hz)catalysts ; (c) : Cuo/Cu total (d) : C u k u total For graphite-supported catalysts, the presence of barium decreased the number of copper and of chromium atoms at the surface. From the relative intensity of the X P S signals it could be seen that the dispersion of copper varied with the barium content. In comparison with the alumina supported catalysts the Cur content was more significant whereas it was the contrary with Crw species which disappeared completely after addition of barium. These results are in agreement with XRD characteristics showing the presence of a CuCrO2 phase on the surface of the catalyst precursor.

3.2 - Catalytic properties Resch'viry of laumnitde with methanol and hydmgen in the prpseoce of chmmium cata@sts- Effrctof the s u p d

COWP

The catalytic results obtained with CuCr supported on alumina or graphite reduced at different temperatures are given in table 3. In the presence of Cu-Cr/alumina reduced at low temperature we noticed that didodecylamine was the major product while with unreduced Cu-Cdgraphite methylation reactions were preponderant ; Selectivity [RN(CH&

+ RNHCH, + R2NCH3] = 75%

After a reduction treatment of the Cu-Cr/Al203 catalyst at 623 K with hydrogen, the methylation reactions and the alkylation into tridodecylamine increase significantly. In the presence of other Cu-Cdgraphite catalyst reduced under the same conditions m a d y tridodecylamine could be observed (60%). It can be concluded that the methylation of dodecylamine or of didodecylamine is favoured when the reduction rate of the catalyst is low while it is the contrary for the formation of the tridodecylamine.

308

Table 3 : Conversion of Iauronit.de in the presence of copper chromium catalysts. Effect of the support and of a reduction treatment. T = 523 K, P = 5 MPa, RCN/CH,OHIH, = 1/10/50. Nitrile conversion

Catalyst

RNHCHl RN(CHl),

(%)

Selectivtty (%) R3NCH1 R2NH

RIN

others

95

5

15

30

22

20

8

cu5 4%

90

18

5

15

48

5

9

Cu5gCr4 ,/graphite ( 4 , (4

95

2

3

15

15

60

5

Cus &r4 ,/graphite

95

10

45

20

5

12

8

cu5 4%

dA201

(a), (4 hlA201 (b). (4

(c),

(4

On the other hand, for both catalysts under consideration a substantial decrease in selectivity in RN(CH3)2 can generally be observed with time on stream (more marked for the alumina supported catalyst than for the graphite supported). At the same time the selectivities in R2NH and in R2NCH3 increase which indicates that the condensation reaction of dodecylamine became faster than the hydrogenation and the methylation reactions.There can be various reasons for these changes in selectivity. However the results in table 3 show that the modifications of the degree of reduction of the catalysts both alumina and graphite supported lead to significant changes of selectivity which are similar to those observed during the reaction. Moreover a pretreatment of a Cu-Cr/Al203 catalyst under a hydrogen flow at 50 bar at 623 K during 12 hours confirmed this hypothesis (4). Hydrogen, therefore, plays a complex role in the conversion of nitrile. Moreover its excess is indispensable for observing methylation reactions. Indeed if a fraction of the hydrogen is replaced by nitrogen (table 4) the reactions of substitution by methanol are practically not observed, the main reaction being the formation of didodecylamine. Table 4 : Conversion of lauronitrile in the presence of a Cu-Cr/A1203 catalyst (reduced with hydrogen at 623 K for 10 h). Influence of hydrogen partial pressure. T = 523 K, P = 5 MPa, RCN/CH,OH/H, + N2 =1/10/10 + 40. Reaction time (h) 6 15 25

conri R

90 88

Y

35 42

:R ;

12 12

Selectivity (96) R2TCH3 R T T 3 R N ( r 2

20 15

12 10

0 t h

309

Effectof M u m The catalytic properties of copper-chromium catalysts can be strongly affected by the presence of additives such as barium (3, 4). The selectivity however depends a great deal on the content and on the localisation of this promoter (4). Indeed we observed that the modification of a Cu-Cdgraphite catalyst by barium impregnation led to a different selectivity from the one obtained with a CuCrBa catalyst coprecipitated in contact with graphite (table 5 ) . Table 5 : Nitrile conversion in presence of barium promoted Cu-Cdgraphite catalysts. Reaction conditions : see table 3. (Catalysts are used without reduction treatment). Conversion

Selectivity (%)

(%)

RNHCHt

95

3

RN(CHt)2 R2NCH3 R2NH 40

42

66

90

R3N

others

2

10

3

30

3

1

If in both cases a very substantial increase of the methylation reactions (after stabilization) can be observed, the coprecipitated catalyst is much more selective for dimethyldodecylamine (70%) than the former which gives equivalent amounts of this amine and of methyldidodecylamine. Furthermore the selectivity of the CuCrBa/graphite catalyst still changes at the beginning of the reaction. In the first hours of use the catalyst is indeed very selective for dimethyl dodecylamine ( 2 95%). This selectivity drops down to 70% after 30 hours reaction. At the same time the formation of methyldidodecylamine increases, which shows that the hydrogenating properties of the catalyst are changed in priority by the reaction. These variations of selectivity are much less significant than these observed by non-promoted catalysts.

Effectof mter and of snoaoOia on tlie stabi&y of catalp& When in the presence of methanol and hydrogen, lauronitrile is totally transformed into dimethyldodecylamine,water is the main coproduct : RC N + CH3OH + 2% --->

CHiOH

RC$NHCH,

+ HZO ->

RC%N(CH,), + 8

20

% As for formation of methyldidodecylamine it is accompanied by the formation of water and of ammonia. 2RC a N + 4% > [RC&]$JH + NH,

I 131

310

These latter products act like catalyst modifiers in particular of copper (5) and hence we were led to examine the modifications of the copper metallic surface area (N20 decomposition) following the addition of known amounts of water or of ammonia at 623 K. These experiments were performed "in situ" in the unit cell used for the metallic surface area measurement.

* CuCr(Ba)/graphite catalysts Figure 4 gives the variations of the accessible copper surface area as function of the addition of water and of ammonia [ S/So where So and S designate respectively the copper surface area before and after introduction of the modifier 1. The quantity of modifier is expressed relatively to the number of copper atoms initially accessible . It is easy to perceive that water has a significant inhibiting effect even if more subtle than that of ammonia on non-promoted catalysts. On the other hand after adding barium a substantial fraction of the initial metallic surface area is conserved even after adding significant amounts of water and of ammonia. Besides its considerable effect on the selectivity of nitrile transformation, barium by stabilizing the phases of the CuCrOz chromite type, has a very significant effect on the stability of the catalysts. * CuCr(Ba)/alumina catalysts Equivalent results were obtained with alumina deposited catalysts (figure 5). However in this case ammonia has a very marked inhibiting effect on non-promoted catalysts . It can be seen then that the addition of the promotor strongly increases the stability of the catalysts in the presence of ammonia and of water. However contrary to graphite supported catalysts the addition of a promotor does not allow to stabilize completely the catalyst in the presence of ammonia and of water. The comparison between these variations of accessible copper surface areas and the changes in selectivity and activity as function of time on stream makes a few analogies apparent : The modifications of the catalytic properties are more significant at the beginning of the reaction and it is also after the addition of the first modifier molecules to the catalyst that the variations of the surface area are the most significant. This is especially true after the addition of ammonia to the non-promoted CuCr/A1203 catalyst and of water to the CuWgraphite catalyst. After addition of barium to the latter catalyst the addition of small amounts of water and of ammonia decreases still more the accessible copper surface area but a stabilization can then be observed which can be connected to the change of catalytic properties.

31 1

0 75

050w

01s -

025

SO

H2O / Cu a

m

I00 H20 / Cu aOOeFS

120

Figure 4 : CuCr(Ba)/ Graphite Figure 5: CuCr(Ba)/A1203 Influence of water or of ammoniac on the copper surface area of CuCr(Ba) supported catalyst. (m) experiment carried out in presence of hydrogen, ( 0 ) CuCr, (*) CuCrBa

4 - CONCLUSION

All the results show that the methylations of primary and secondary amines are favoured by the presence on the surface of mixed phases of a CuCrO2 type stabilized by a promotor such as barium. Furthermore the increase in the total acidity of the catalyst by using an alumina support favours the production of the secondary amine (didodecylamine) resulting from the condensation of a primary amine with the corresponding imine. However the formation of tridodecylamine (not desirable) which can be favoured by the increase in acidity of the catalyst is also much increased, and rather unexpectedly, by the presence of metallic copper on the surface area of the catalyst, be it alumina or graphite supported. From a mechanistics point of view the reaction scheme proposed by VON BRAUN (6) and taken up by VOLF and PASEK (7) allows to account for the main results. However in the case of methylation reactions (Leuckart type reaction) in particular the methylation of the secondary amine, as well as for the formation of tridodecylamine it is apparently necessary to propose new reaction steps (8).

312

1) the primary amine methylation being faster than the one of didodecylamine the methylation step preceds the alkylation step. 2) a secondary amine transalkylation reaction into tridodecylamine and dodecylamine favoured by the relative increase of the metallic surface area of the catalyst. The examination of the evolution of the catalytic properties as function of the time on stream shows that these properties are largely modified at the beginning of the reaction when the Cu-Cr catalysts do not contain any promotor. Adding a small amount of barium in particular to the Cu-Cdgraphite catalyst strongly increases the selectivity in methylation (of the primary or of the secondary amine according to the mode of preparation of the catalyst) and the stabilization. We have shown that the modifications could be the result of the action of water andor ammonia on the catalyst. These modifications are less marked in the presence of a large excess of hydrogen (see figure 5) the reason why it is necessary to perform the synthesis of amines under these conditions. However wellchosen promoters such as barium can also allow to stabilize a copper chromite type of phase (CuCrO2) which appears more stable in relation to these poisons than metallic copper or other chromite phases. We shall pursue therefore this study in order to identify more precisely the phases present on the surface of the catalyst after various activation processes in the presence or not of amination reaction inhibitors.

Acknowledgements The authors of this paper wish to thank the firm CECA (ELF-ATOCHEM) for the profitable discussions and for their substantial financial aid.

REFERENCES 1

2

8

J. BARRAULT, M. SEFFEN, C. FORQUY and R. BROUARD, Stud. Surf. Sci. and Catal., Elsevier Ed., 1988, 4l,361. J. BARRAULT, G. DELAHAY, N. ESSAYEM, Z. GAIZI, C. FORQUY and R. BROUARD, Stud. Surf. Sci. and Catal., Elsevier Ed., 1991, 59, 343. Z. GAIZI, Thesis, Poitiers, France, 1990. N. SUPPO-ESSAYEM, Thesis, Poitiers, France, 1991. A. BAIKER and J. KUENSKI, Catal. Rev., Sci. Eng., 1985, 22, 653. J. VONBRAUN, G. BLESSINGandF. ZOBEL, Chem. Ber., 1923, 36, 1988. J. VOLF and J. PASEK, in Catalytic hydrogenation, L. Cerveny Ed., Stud. Surf. Sci. and Catal., Elsevier Ed., 1986, 22, 105. J. BARRAULT, S . BRUNET, N. ESSAYEM and C. GUIMON, J. Mol. Cat., In press.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemiculs I11 63 1993 Elsevier Science Publishers B.V. All rights reserved.

313

A Process for Coproduction of Mono- and DiaminoalkylatedGlycols Kathryn S. Hayes and Thomas A. Johnson Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195, USA

Abstract A process for coproduction of mono- and diarninoalkylated glycols has been developed which involves Michael addition of excess diethylene glycol to acrylonitrile followed by catalytic hydrogenation of the nitrile mixture over a chromium-promoted sponge cobalt catalyst.

1.

INTRODUCTION

Synthesis of diarnines 1 typically involves Michael addition of a glycol to a conjugated olefinic nitrile followed by catalytic hydrogenation of the dinitrile to give the primary diamine.

Base

H~CZCH-CEN + HO(CH,),O(CH2),0H

1

where m+n2l Use of excess nitrile is required to completely dicyanoalkylate the glycol. The olefinic nitrile then must be removed from the reaction product, and although distillation is the method generally employed, the olefinic nitrile tends to polymerize. This polymeric impurity may act as a poison for the hydrogenation

314

catalyst and therefore, must be removed by solvent extraction [l]. Another method for removal of the olefinic nitrile involves reaction with a primary or secondary amine. The nitrile product is hydrogenated along with the desired dinitrile and the diamine products are easily separated by distillation [2]. An alternative solution involves coproduction of the aminoalcohol and the diamine. In this case, the glycol is used in excess and consequently, removal of residual olefinic nitrile is unnecessary. Using this technique, we have developed a process to coproduce 3-aminopropoxyethoxyethanol (APEE, 2) and di(3-aminopropoxyethoxy) ether (DAPEE, 3). These products have found utility in coatings and epoxy curing agent applications, respectively.

2

3

Production of primary amines via hydrogenation of nitriles over sponge and supported metal catalysts is well-known and has recently been reviewed [3]. Hydrogenation of cyanoalkylated glycols poses a unique challenge as these materials are extremely unstable under basic conditions and readily decompose to the starting glycol and the unsaturated nitrile. Under hydrogenation reaction conditions, the nitrile is reduced to an amine which further reacts with imine intermediates of the cyanoalkylated glycol hydrogenation to produce undesired by-products. Farkas and Miller [4]demonstrated the production of N,N-di(3-aminopropyl)ethanolamine in 54% yield by batch hydrogenation of di(2-cyanoethy1)ethanolamine. The reaction was conducted in the presence of ammonia over sponge nickel catalyst at low temperature (4860°C)and high hydrogen pressure (2100 psig). Low reaction temperature was thought to decrease by-product formation. Kluger and Goineau [5] also used a sponge nickel catalyst to hydrogenate di(2cyanoethoxyethoxy) ether but found that the selectivity to the diamine product, DAPEE, could be improved by incrementally adding the dicyanoglycol to the hydrogenation reactor. The hydrogenation was carried out in the presence of ammonia and toluene or isopropanol as solvent. Reaction temperature typically was 135-145"C,and reaction pressure ranged from 1450 to 1700 psig. Seven cycles of the catalyst were demonstrated with 2 wt% of fresh catalyst added prior to each cycle after the second. More recently, chromium-promotedsponge cobalt has been found to be an active catalyst for nitrile hydrogenation. Examples of nitriles which have been successfully hydrogenated using this catalyst include 2-methylglutaronitrile [6], nitrilotriacetonitrile (71,and adiponitrile [a]. Hydrogenation of the latter nitriles typically is complicated by intramolecular cyclization reactions of the feedstocks. Use of the chromium-promoted sponge cobalt catalyst results in improved selectivities to linear products. Since our objective was to develop a batch process to coproduce equal volumes of APEE and DAPEE, use of a sponge metal catalyst was indicated based on the literature cited above. Therefore, our experimental program involved determining the appropriate acrylonitri1e:diethylene glycol ratio for the cyanoethylation

315

reaction, evaluating sponge metal catalysts for activity, and testing catalyst life and methods for regenerating catalyst activity.

2.

EXPERIMENTAL

The general procedures for the cyanoethylation and hydrogenation reactions are described below. Catalysts were obtained from Activated Metals, Inc. or W. R. Grace and Co. Crude reaction products were analyzed using a Hewlett-Packard 5890 GC. Products were isolated by distillation under reduced pressure in either a 1" x 12" or a 1" x 24" packed column. Cvanoethvlation. A 3-necked 500 ml round-bottomed flask equipped with overhead stirrer, thermometer, condenser, addition funnel, and N2 purge was charged with diethylene glycol (DEG, Aldrich) and LiOH (0.3 mole%, anhydrous), and the mixture was stirred under nitrogen until the LiOH dissolved in the DEG. The solution was heated to 55"C, then acrylonitrile (ACN, Aldrich) was added dropwise over 3.5 h while the temperature of the reaction mixture was maintained at 55-60°C. Upon completion of the addition, the solution was stirred for an additional 2 h at 55-60°C. The solution was cooled, and a sample was withdrawn for analysis by gas chromatography. Although the crude product was not neutralized prior to hydrogenation, the product must be neutralized prior to GC analysis. If base is present in the sample or if the GC insert is contaminated with base, retro-Michael reactions occur in the GC, and as a result, the level of ACN measured is much higher than the level actually present in the reaction product. The GC sample was neutralized with 34% H3P04 and then diluted with diglyme (internal standard) and methanol. The cyanoethylation reaction product was analyzed on a DB 1701 column (30 m x 0.32 mm, 1.O micron film, J&W Scientific). Calibration standards containing ACN, DEG, and diglyme as internal standard were prepared and used to calibrate the GC. The shelf life of these standards is short due to reaction of ACN. New standards should be prepared when GC analysis shows that degradation products have formed. A 1 liter stainless steel autoclave was charged with catalyst (13.1 g) and H20 (14.8 moles). The reactor was sealed and purged three times with nitrogen then purged three times with hydrogen and pressure tested with hydrogen at the desired operating pressure. Ammonia (3.3 moles) was added, the reactor was heated to 60°C, then the hydrogen pressure was increased to 8001300 psig. The nitrile product (310 g) was fed incrementally to the reactor via an HPLC pump at a rate determined by the rate of hydrogenation. Upon completion of the addition, the reactor was sampled, then reaction was continued either for an additional hour or until hydrogen uptake ceased. The contents of the reactor were pumped out through a dip tube which was fitted with a sintered metal frit. If the catalyst was reused, the reactor would be charged with H20 and NH3 then the procedure described above would be followed. The hydrogenated product was analyzed by GC on an HP-5 column (25 m x 0.32 mm, 1.O micron film, HewlettPackard). Four calibration standards which contained approximately 50% H 2 0 and different levels of DEG, APEE, and DAPEE were used to calibrate the GC. Diglyme was used as the internal standard.

w.

316

3.

RESULTS AND DISCUSSION

.Excess ACN was added incrementally to a solution of 0.3 mole% LiOH in DEG at 55-60°C. Samples were withdrawn during the addition to determine the ACN:DEG ratio required to give 1:1 mono:dicyanoethylated products. The results presented graphically in Fig. 1 show that approximately equal weights of mono- and dicyanoethylaled products are produced when 1.3 mole of ACN is added. The data therefore are in agreement with the theoretical value of 1.33. The ACN level of the crude product typically was <0.2 wt% when the solution was stirred an additional two hours at 5560°C after the ACN addition was complete.

U Mono:Di Ratio

..........

0.0 0.5 1.0

1.5

2.0

2.5

Ratio = 1

3.0

AC N:DEG Fig. 1. Weight ratio of mono:dicyanoethylated OEG as a function of ACN:DEG molar ratio.

.Reaction conditions for the catalyst screening studies were chosen based on previous work. The hydrogenation reactor was charged with catalyst and water, then purged, and ammonia was added. The reaction mixture was heated to the desired reaction temperature then the reactor was pressurized with hydrogen. The nitrile was fed incrementally at a rate consistent with the rate of hydrogenation. As compared to a strictly batch process, the incremental feed system gave higher yields of the desired products and minimized losses due to decomposition [5]. While there are several reports of using low levels of water in nitrile hydrogenations to maintain or improve catalyst activity [6,9],water has not

317

generally been used as a solvent for the reaction. No deleterious effects were observed by using a high concentration of water in the reaction mixture. Four sponge metal catalysts were screened for activity in hydrogenation of the crude mixture of cyanoethylated glycols. Of the catalysts tested, Cr-promoted sponge nickel and Cr-promoted sponge cobalt gave the highest yields of the desired amines (Table 1) and had the lowest concentrations of DEG in the products. When the rate of hydrogenation is slow, the retro-Michael decomposition of the cyanoethylated DEG feed competes effectively with hydrogenation. Propylamine is produced by hydrogenation of the acrylonitrile formed by the retro-Michael reaction. Propylamine reacts with the imine intermediates of the nitrile hydrogenation to give N-propylated by-products. Acrylonitrile reacts with the product amines to give aminopropylated by-products after hydrogenation. Table 1 Catalyst Screening Experiments Cr-promoted Cr-promoted Sponge Sponge Sponge Sponge Catalyst Nickel Cobalta Cobalt Nickel 1300 1300 Pressure, psig 1300 1000 36.90 7.64 Wt % DEG 15.06 8.75 2.91 36.28 Wt o/' APEE 32.10 39.94 Wt % ' DAPEE 29.44 47.47 0.20 50.46 aNo hydrogen uptake observed initially, temperature raised to 70°C,reaction terminated after 3.7h, 148 g of nitrile added. The Cr-promoted sponge nickel and the Cr-promoted sponge cobalt catalysts were chosen for further evaluation in life study experiments. As shown in Fig. 2, the activity of the Cr-promoted sponge nickel catalyst decreased significantly after the first use. In an attempt to increase activity, we followed the procedure described by Kluger and Goineau (51 and washed the catalyst with water after the second and third uses, and charged 0.05g of NaOH and 2 g of fresh catalyst after each wash. However, no improvement in activity was observed. The effect of washing the catalyst with 1% NaOH solution between uses was evaluated next. The results, which also are presented in Fig. 2,show that there always was a yield loss with each successive catalyst use. The loss, however, was not as severe when the catalyst was only washed with caustic. Similar results were obtained when the reaction pressure was increased from 1000 psig to 1300 psig. A possible explanation for the loss of catalyst activity upon reuse is poisoning of the metal surface by adsorption of reaction intermediates and/or products. Washing the Cr-promoted nickel catalyst with NaOH solution may either remove the bound species from the metal surface or remove additional aluminum from the structure to create fresh surface for reaction. In the life study experiments, loss of the DAPEE product was particularly evident, and therefore, to try to compensate, the ratio of ACN:DEG in the cyanoethylation step was increased to 1.5:l.The data from these experiments (Table 2)show that the desired level of DAPEE in the hydrogenated product could not be maintained,

31 8

100

w w

2n

80

+

W

W

--o- Test#1

60

a

a

a......

+..,..

Test #2

40

20

Catalyst Use # Fig. 2. Cr-promoted sponge nickel catalyst life test. Reactions conducted at 60°C, 1000 psig. Test #1: NaOH and fresh catalyst added after uses 2 and 3. Test #2: Catalyst washed with NaOH solution between uses. and the catalyst could not be successfully recycled. Since many of the byproducts observed are propylated derivatives of APEE and DAPEE, the ammonia concentration was increased to try to limit the reaction of intermediate imines with propylamine. However, the higher ammonia concentration results in increased decomposition of the nitriles relative to hydrogenation. Table 2 Effect of Increasing ACN:DEG Ratio and NH3 Level on Catalyst Life 1 2 1 2 Catalyst Use # NH3/Nit riIe 1.4 1.4 2.0 2.0 Wt Yo DEG 6.85 20.31 6.78 35.03 wt % APEE 34.06 26.98 33.87 14.13 Wt Yo DAPEE 47.58 12.05 47.1 2 2.28 6OoC, 1300 psig, Cr-promoted sponge nickel catalyst washed with NaOH solution between uses.

319

The life of the Cr-promoted sponge cobalt catalyst also was tested. As shown in Fig. 3, a small initial decrease in activity was observed between the first and second uses, then the activity remained essentially constant for the next six cycles. In contrast to the results obtained from the Cr-promoted sponge nickel catalyst, washing the catalyst with NaOH solution between uses appears to be unnecessary. This result may indicate that the nickel catalyst is more susceptible than the cobalt catalyst to poisoning by reaction intermediates and/or products.

-

W

90

a n +

80 -

W Q

W W

a

a

g

70

4 ....0....

-

-o-

Test#1

........+.....

Test #2

3

0

2

4

6

8

10

Catalyst Use # Fig. 3. Cr-promoted sponge cobalt catalyst life test. Reactions conducted at 6OoC, 1300 psig. Test #1: Catalyst washed with NaOH between uses. Test #2: No catalyst treatment between uses. Hydrogen pressure also was found to affect catalyst life. When the pressure was reduced from 1300 psig to 800 psig, the activity decreased significantly with each successive use of the catalyst. Loss of DAPEE yield again was evident. However, in contrast to the results obtained over the nickel catalyst, hydrogenation of a nitrile feed prepared by increasing the ACN:DEG ratio to 1.5:l resulted in the desired product ratio (Table 3). The activity of the catalyst also appeared to be relatively stable.

320

Table 3 Effect of Increasing ACN:DEG Ratio on Catalyst Life 1 2 3 Catalyst Use # Wt Yo DEC 4.48 5.50 10.15 Wt % APE€ 31.03 32.28 32.78 Wt Yo DAPEE 60.68 54.62 38.73 6OoC,1000 psig, 1.35:l (molar) NHg:nitrile, Cr-promoted sponge cobalt 4.

4 10.26 34.08 38.16 catalyst.

CONCLUSIONS

A process to coproduce equal volumes of APEE and DAPEE in 275% selectivity has been developed. Excess diethylene glycol is used in the cyanoethylation step which eliminates the necessity of recovering unreacted acrylonitrile. Use of Cr-promoted sponge cobalt as a catalyst for hydrogenation of polynitriles has been found to have advantages over other sponge metal catalysts. The catalyst exhibits high activity for nitrile hydrogenation as evidenced by the high product selectivities and the minimal by-product selectivities. Water can be used as a solvent for the hydrogenation reaction which offers cost and handling advantages over organic solvents. A major advantage is that the activity of the catalyst is maintained through several cycles without any treatments required between cycles. Both catalyst cost and processing time are reduced as a result.

ACKNOWLEDGEMENT

The assistance of Eugene G. Lutz and Carla M. Schadt in performing the experimental work is gratefully acknowledged.

5.

REFERENCES

1 F. Poppelsdorf, U.S. Patent No. 3 799 986 (1974). 2 R. V. C. Carr, T. A. Johnson, S. M. Galaton, and T. A. Albanese, U.S. Patent No. 5 075 507 (1991). 3 J. Volf and J. Pasek in L. Cerveny (ed.), Studies in Surface Science and Catalysis 27 - Catalytic Hydrogenation; Elsevier, Amsterdam, 1986, p. 105. 4 A. Farkas and F. Miller, U.S. Patent No. 3 377 383 (1968). 5 E. W. Kluger and A. M. Goineau, U.S.Patent No. 4 313 004 (1982). 6 F. E. Herkes, U.S. Patent No. 4 885 391 (1989). 7 M. B. Sherwin, S.-C. P. Wang, and S. R. Montgomery, U S . Patent No. 4 721 81 1 (1988). 8 Y.-J. Lin, S. R. Schmidt, and R. Abhari, U.S. Patent No. 5 105 015 (1992). 9 C. A. Drake, U.S. Patent No. 4 248 799 (1981).

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

321

New process for isophoronediamine synthesis

J.Ph.Gillet ; J.Kervennal ; M.Pralus

Elf Atochem - centre de recherche Rhbne-Alpes, rue Henri Moissan - B.P. 63, 69493 Pierre-Bbnite Cedex. France. ABSTRACT A new process for synthesis of 3-aminomethyl-3,5,5-trimethylcyclohexylamine (IPDA) has been investigated. The reaction was performed in two steps. In the first step bis (3-cyano-3,5,5 trimethylcyclohexylidene) azine (IPNA) was synthesized from 3-cyano-3,5,5 trimethyl- 1 0x0 cyclohexane (IPN) and hydrazine hydrate in solvent. The reaction yield was nearly quantitative. In the second step the azine (IPNA) was hydrogenated under mild conditions on a Raney nickel or cobalt catalyst in the presence of a small amount of ammonia. lsophorone diamine (IPDA) was obtained at high yields (90-95 %). But the main interest of a such process is to minimize the production of byproducts (aminoalcohol, azabicyclic compound, secondary amine) and to use less severe pressure conditions than those generally employed.

1. INTRODUCTION At the present time isophorone diamine (IPDA) is experiencing a good expansion in its traditional markets such as polyurethanes, paints and varnishes. For this reason a real interest has been shown in the synthesis of IPDA. Until now, this compound was synthesized by aminoreduction of isophoronenitrile (IPN).

Until the beginning of the eighties, the hydrogenation was carried out under severe temperatures (120"-150°C) and pressures (150-270bar) (ref.1,2). Over the last ten years, these conditions have been constantly improved. In 1987, Daicel patented a process operating at 120°C and 70bar to loobar, but the yield was only 89 % (ref.3). More recently, Union Carbide described a process using Raney cobalt, doped or not with chromium. The best yields (about 90 %) were obtained under a pressure of 80bar and at a temperature of 100"-120°C. But this process still generated several byproducts. (ref.4,5).

322

More recently, BASF patented a new two step process which minimized the byproduction of aminoalcohol (ref.6). The first step consisted in synthesizing the intermediate imine with acid catalyst and the second one in hydrogenating the latter, without intermediate isolation. The announced yields were 95 to 97 % and impurities decreased but the condiiions remained very severe (12O0C-250bar).To conclude, it did not seem possible to minimize the rate of impurities by operating under mild conditions and with yields better than 90 %. The main cause of byproduct generation is the hydrogenation of the ketone, giving aminoalcohol, separable with difficulty from isophoronediamine.

\

CN

L

'CN

1

cn2Nn2

So we looked for a reaction blocking the carbonyl group to avoid the side reactions and it was the reason why we investigated a new process passing through the azine formation. 2. RESULTS AND DISCUSSION

This new process involves two consecutive reactions. k i n e synthesis :

IPDA synthesis :

We tried a one pot reaction but it failed. The main reason being the unstability of hydrazine in the presence of catalysts such as cobalt or nickel. We noticed a large decomposition which prevents azine formation and subsequently the IPDA yield was very low (10-30 %). So, only a two step process was available.

323

2.1. k i n e synthesis The reaction between a carbonyl group and hydrazine is a classical reaction but in this case we obtained a bis(3-cyano-3,5,5-trimethylcyclohexylidene)azine which was a new product. The preferred hydrazine source was hydrazine hydrate since it is an industrial product easy to use. In order to obtain an industrially viable process different parameters of the reaction have been optimized. - catalyst influence : It is well known that the addition of acid greatly enhances the rate and yield in azine synthesis. In our case, we chose formic acid as catalyst due to its acidity and miscibility in the organic medium. Formic acid was used at a 2 % molar ratio vs. IPN. - solvent influence : The reaction can be conducted in miscellaneous solvents like xylene or alcohols. Nevertheless, polar solvents like alcohols are preferred. There were no great differences in selectivity between C1 to C4 alcohols but we preferred methanol for several reasons. The main one was the homogeneous medium and the good solubility of IPN at the reaction temperature. In addition, at room temperature, isophoronenitrile azine (IPNA) precipitated and the white crystals produced were easily recovered by filtration (isolated yield 96 YO vs.97,9 % for the reaction yield). By comparison, in xylene medium, aqueous hydrazine was not completely miscible and isophoronenitrile azine was partially soluble. Thereby the yield of the isolated product was only 76 % vs. 92,6 % for the reaction yield. - Reaction time : Good conversions and selectivities were obtained after two hours, with methanol as solvent and formic acid as catalyst. Below this time we observed traces of unconverted isophoronenitrile and especially hydrazone which precipitated with the isophoronenitrile azine. 2.2. IPDA synthesis : In the second step the intermediate was submitted to hydrogenation and isophoronediamine (IPDA) was obtained according to the global reaction below.

The main arameters of the reaction were optimized at the laboratc I with Raney nickel as catalyst (10 wt %). Firstly, we chose methanol as solvent because it is the most convenient solvent in azine synthesis while being efficient for hydrogen solubility. In addition there were no great differences in selectivity and yield compared with other solvents like upper alcohols and xylene. Moreover it can be easily recovered at the end of the reaction. Temperature and pressure were optimized and the best results were obtained with a temperature of 150°C and a pressure of 60bar. This temperature is a good compromise allowing a normal hydrogenation rate without loss of selectivity. Higher temperatures affect the selectivity. We observed formation of methylated IPDA (mono and di) and some other unknown products. If we operated below 100°C the reaction did not occur. The pressure was also chosen as low as possible to obtain a good selectivity in IPDA. Another point is the use of ammonia during this step. As it

324

is well known, ammonia is required to promote primary amine formation during nitrile or imine hydrogenation. We found that the lowest suitable NHdazine molar ratio was about 8. Below this value, selectivity in IPDA decreased and with lower ratio or without ammonia we observed secondary amine formation. This phenomenon was also noticed with Raney or supported cobalt catalysts. The main parameter was the choice of the catalyst. Generally catalysts such as Raney nickel or cobalt are convenient for hydrogenation of nitrite groups and are able to cleave the N-N bond of hydrazine by hydrogenolysis. We tested different catalysts under various forms.The main results are indexed in table I . Table 1 : influence of catalyst solvent : MeOH ; H, pressure : 60bar Catalyst

Temperature

Azine conversion

Selectivity

%

% 95 96 494 45

"C Raney Ni Raney Co PdlC RulC

150

100 100 96,6 87

I,

115 130

IPDA yield %

95 96 42 39

The best selectivity in IPDA was obtained with Raney cobalt or nickel but the supported forms were also suitable. Noble metals like palladium or ruthenium failed in this case. H, consumption is lower with Pd and we found unconverted azine (IPNA).

0

CO raney Pd/C

*- .

I

0

50

100

150

200

250

300

350

TlME (MN)

Figure 1. H, consumption in fonction of time and catalyst exp. conditions : azine : 123 mmol ; MeOH : 400 cm3 ; NH, : 984 mmol a) Ni or Co : 8 g (wet) ; 8 : 150°C ; P: 60 bar b) PdlC (5 %) : 2 g ; 8 : 115°C ; P:GObar

325

Finally, Raney nickel was preferred to cobalt due to its industrial availability and relatively low cost. But for a better optimisation of this process we studied the hydrogenation in detail.

2.3. Hydrogenation mechanism The results were obtained by continuous analysis of the hydrogenation medium. The reaction was performed in a 1 I autoclave equipped with a magnetically driven stirrer at a speed of about 1000 r/m. Samples were periodically withdrawn through a decanter tube from the bottom of the reactor. The main products were analysed by gas chromatography coupled with mass spectroscopy, NMR and infrared spectroscopy. At the beginning of the hydrogenation, isophoronenitrile azine (IPNA) is quickly converted to the azine with hydrogenation of the nitrile groups. This diamine exists in two isomeric forms (X3, X4) (ciso’ide or transoide). These two isomers are further hydrogenated and give IPDA. The intermediate hydrazine is not observed. The hydrogenation rates of X, and X, are different. Four or five hours were necessary to obtain the complete conversion of this intermediate diamine. All these obervations led to the following mechanism :

/D=N-N4 \C N

CN CN

CN

,

CH2NH2

r i

J pp

CHzNH2

ZH2

NH-NH

CH2NH2

Figure 2 : hydrogenation mechanism

C

‘

H 2N H 2

]

326

We can also compare the effect of Raney nickel and cobalt in this hydrogenation (fig. 3,4)

0

Figure 3.

50

100

RANEY Ni

l4

150

200

T

0

250

300

TIME (mn)

I

50

100

150

200

300

TIME (mn)

Figure 4.

RANEY Co

lsophoronenitrile azine is quickly converted to partially hydrogenated azines (X, and X,) during the first hour. The hydrogenation rate is faster with cobalt, that is confirmed by H, consumption. Moreover we observed only one isomeric form of the intermediate diamine with Raney cobalt. As we previously described, we can see a different behaviour between these 2 intermediates. For one of them (at the present time we are unable to distinguish between cisoi'de and transoi'de) hydrogenation rate is low. Another characteristic of this process relates to IPDA isomeric composition. Usually by reductive amination of isophoronenitrile, the isomeric ratio cisltrans is about 80120 but in our case the proportion of trans isomer increased considerably to reach a ratio of 50/50 to 44/56, depending on the nature of the catalyst (table 2).

327

cis

80 493 44

1

1

trans 20

50,5 56

1

I

process

1 commercial IPDA

azine way (cobalt) azine way (nickel)

Another interesting point is the low coproduction of impurities. Nevertheless we observed the formation of small amounts of low boiling points products (X,,X2) compared with IPDA. After analytical characterisation we assigned to the most important of them the azabicyclic (1,3,3-trimethyl-6-azabicyclo[3,2,1]octane) structure. In anhydrous conditions as in our case, byproduction of aminoalcohol is minimized. On the other hand, when water is present the proportion of aminoalcohol increases. These observations can be explained by the following mechanism.

+

C H ~ N .-.* HZ

CH2NH2

i CH2NH2

OH

Figure 5

&

CH2NH2

Conclusion : This reaction through formation of an azine opens a new route to primary amines from ketones and presents several real interests. In case of IPDA, the global yield is about 93 % and for the first time byproduction of impurities is lowered while using mild conditions. The most convenient catalyst for the hydrogenation step is a simple Raney nickel contrary to the classical process which often uses more sophisticated catalysts.

328 3. EXPERIMENTAL

Before use, isophoronenitrile (IPN) was recrystallized from diisopropylether MP:7O0C. a) typical procedure for bis(3-cyano-3,5,5-trimethyl cyclohexylidene) azine synthesis : The reaction was performed in a 500 cm3 glass reactor fitted with stirrer and condenser. 150 g of methanol and 82,5 g (0,5M) of isophorone nitrile (IPN) were placed into the reactor. While stirring the mixture, the catalyst was added : 0,53 g (1,15-10-2M) of formic acid dissolved in 8 g of water. Then 12,5 g (0,25M) of hydrazine hydrate were fed through the dropping funnel at room temperature under stirring, over a 1Omn period. The homogeneous mixture was heated at reflux for 2 hours. After cooling the suspension was filtered to remove azine. The crude product was washed with cold methanol and dried. The yield was 98 % (purity 99 %). IR :

MP : 191"-193'C 6 :(C=N) : 1641 cm-1 6 : (C=N) : 2234cm-1 13C and 1H NMR were in accordance with the structure. b) Hydrogenationof bis (3-cyano-3,5,5-trimethylcyclohexylidene) azine : The reaction was performed in a 1 I autoclave. 40 g (0,123M) isophorone nitrile azine were placed into the reactor with 400 cm3 methanol and 8 g Raney nickel (solvent wet weigh!). The apparatus was purged of air by nitrogen. Liquid ammonia was introduced 16,7 g (0,98M) at room temperature. The reactor was then heated to 150°C under gentle stirring. When the reaction temperature was reached, stirring was stopped and hydrogen was introduced until a total pressure of 60bar. Stirring was restarted and the reaction began immediatly. Hydrogen uptake under the above mentioned conditions ended after 4 or 5 hours. The mixture was filtered and solvent removed. The yield in IPDA was about 95 % (GC with internal standart).

REFERENCES 1 A.Sommer, R.Bruecker, Ger.Patent No.3 011 656 (1981) 2 J.Disteldorf, W.Huebel, L.Broschinski, Ger-PatentN0.3 021 955 (1981) 3 Y.Hirako, J.P.Patent No 62 123 154 (1987) 4 B.D. Dombeck, T.T.Wenze1, Eur.Patent No 394 968 (1990) 5 B.D.Dombeck, T.T.Wenzel, Eur.Patent No 394 967 (1990) 6 F.Merger, C.U.Priester, T.Witzel, G.Koppenhoeffer, W.Harder, Eur.Patent No 449 089 (1991)

M. Guisnet et al. (Editors), ~etnogenaous&tdysis a d Fine Chemicals III aD 1993 Elnevier Science Publishers B.V. All rights reserved.

329

CATALYTIC SYNTHESIS OF 2-METHYLPYRAZINE OVER ZN-CR-OPD A SIMPLIFIED KINETIC SCHEME

*

Lucio Forni and Roberta Miglio Dipartimento di Chimica Fisica ed Elettrochimica Universita' di Milano. Via C.Golgi, 19 20133 Milano. Italy ABSTRACT

The kinetics of cyclisation of ethylenediamine and propylene glycol to 2-methylpyrazine has been studied at 623 to 663 K and in excess of steam. under the optimised reaction conditions put in evidence by a previous extended research on this process. In spite of the complexity of the reaction, the present results allowed to define a very simple kinetic model, by means of which the behaviour of the system can be satisfactorily described through a set of only five pseudo-first order rate equations.

INTRODUCTION The most important usually accepted mechanisms of alcohol amination with ammonia or amines are: i) The so called dehydroamination, in which the dehydrogenation of the alcohol to a carbonyl derivative takes place first on a hydro-dehydrogenation catalyst, followed by reaction of the latter with the amino group to give an imino intermediate, further hydrogenated to the amine. An excellent review paper [l] has been published quite recently on this subject. ii) The so called dehydrative amination, taking place on a dehydration catalyst 12-41, in which a dehydration between the amino and the hydroxyl groups leads directly to the amine. A dehydrogenation with formation of a A1-dehydrogenated product may follow. quickly proceeding to a final heterocyclic compound, if the reaction is carried out on an a,@-aminoalcohol of the proper chain length 141. A n extended explorative study [5-101 carried out in our laboratory put in evidence the practical feasibility of a two-step catalytic route for the preparation of 2-amido-pyrazine. The first step of the process may be considered a double alcohol amination and consists in the cyclisation of ethylenediamine (ED) and propylene glycol (FG) to 2-methylpyrazine (MP), which can be carried out quite satisfactorily in vapour phase over a Zn-Cr-O/Pd catalyst. A further, detailed mechanistic study [11,121 showed that the main reaction can be described by a dehydration, followed by multiple quick dehydrogenation steps, perfectly in line with what suggested in literature [1,41.That study indicated also the role played by Pd in increasing selectivity. by favouring the dehydrogenation following the initial dehydration and suggested a possible Rideal-Eley step between adsorbed PG and ED coming from the gas phase, as rate-determining, at least at low ED partial pressure. However, the overall reaction proved to be quite complex, since up to ca.30 different species could be detected in the reactor effluent. Fortunately, most of them form only in trace, so that, by properly choosing the reaction conditions, a selectivity to MP exceeding 80% at total conversion can be attained. Hence, a simplified reaction scheme could be forecast, able to describe the general behaviour of the system by taking into account only the most important species. The present paper

reports on a kinetic study of the dehydration-dehydrogenation of ED+PG to MP, aiming at defining the principal kinetic equations of such a simplified scheme and at determining the values of the relative kinetic parameters, useful for the development of a more detailed pilot-plant study.

EXPERIMENTAL The Zn-Cr-O/Pd catalyst was prepared as described in previous work ill]. The principal characteristics y e : Z d C r atomic ratio33/1, Pd concentration 1 wt %, BET surface area 52.9 m /g, pore volume 0.4 cm /g, bulk density 1.12 g/cm 3 , particle size 0.15 to 0.18 mm. Details of the fixed-bed, continuous, microreactor assembly and of the GC analysis of the reactor effluent are given elsewhere [61. The identification of the various species was done by GC-MS. The overall mass balance around the reactor was ca. 100% (99f3. including the experimental error). The principal products, covering more than 98% of the organic matter, with respect to the reactants transformed, were MP, dihydro-2-methylpyrazine (DHMP), acetone ( A ) , pyrazine (PI and dimethylpyrazine (DMP). Due to the practical purpose of the present study, all of the analytical data take into account only these species, besides reactants, neglecting minor byproducts. RESULTS AND DISCUSSION

Kinetic data have been usually expressed in terms of conversion Ci (mol % of the reactant i transformed into products) and yield Yj (mol % of the j-th product/mol of converted ED), referred to the principal reactant ED. Differential reactor technique runs. Two series of runs have been carried out at 643 K and atmospheric pressure, by feeding 2 cm3/h of a 10 wt % aq. Fig.1 - Initial reaction rate ro vs. partial pressure of each reagent. ConFtant valueos 7.6 (p,, (Torr) of: (01 p,= in abscissa), (01 piG= 11.7 (piD in abscissa).

solution of ED+PG and charging 0.05 g of catalyst, diluted with pyrex beads. Some blank runs in the absence of catalyst, but in the presence of such beads, showed no conversion at any temperature. The range of partial

331 pressure explored for each of the two reagents was ca. one order of magnitude wide. Of course, the analytical samples for every run were collected only after the system had attained the steady state, f.e. ca. 4 hours after starting the reactants feeding. The results are shown in Fig.1.

The trend of the data for constant piD is not in contrast with a Rideal-Eley mechanism, controlled by the surface reaction between adsorbed PG and ED coming from the gaseous phase. A possible interpretation is given in Fig.2. In it the adsorption of PG takes place through the interaction of the couple of electrons present on the oxygen of the terminal hydroxyl with a Lewis acid site of the catalyst. The slow process is the attack of the carbon atom adjacent to the activated hydroxyl group by the amino group of the ED. The kinetic equation describing such a mechanism can be written as

100 -2

c

60 L

: I

I

1

2

0.1

0.3

3

4

Fig.4. Dependeace of reaction rate on p p,,= 11.7 Torr ED.

l/PK

Fig.3. Fitting of eq. l/r=m/pPc+q to exptl. data. k (mol of conv. ED/h x g cat.) being the kinetic constant, bi (Tom-', 1 Torr = 133 Pa) the adsorption equilibrium constant and pi (Torr) the partial pressure of the f-th species, respectively, the subscript w indicating water. In the present case p , is constant and, due to the excess of water, also bRw is practically constant, so that we have

l/r = m/pPc + q , where

332 pmbm and q = bpG/k pDbED. The fitting of eq. l/r = m/ppc+ q to our experimental data is shown in

m = (l+b#,,)/k

Fig.3. From the best straight line (least-squares method, corr. factor = 0.992) the values of m = 222.2 (h x g cat. x Torr/mol of conv. ED) and q = 12.94 (h x g cat./mol of conv. ED) were calculaoted. As for the results obtained at constant p (see Fig.1). the range of PG

experimental data should be divided into two parts. When <,p

12 Tori-, Eq.1

is still able to describe the behaviour of our system, since, for constant is also constant and r = k"p,. values of both ,p and pw, (l+b,,p,,+bpGp,) 1 for the best straight line (corr. factor = 0.999) Fig.4 shows a slope drawn through the experimental points for p < 12 Torr. However, beyond this ED

value, the data deviate markedly from linearity, indicating that the previous mechanism is accompanied by a competitive adsorption of ED. Therefore, the secondary reactions, especially the condensation of two molecules of ED to P, are no more negligible. Integral reactor technique runs. Our previous research 16,101 showed that the ED/PG molar ratio in the feeding mixture has to be controlled, in order to get a good yield to MP. Those results limit considerably the range of experimental conditions, under which integral reactor technique data, useful for practical purposes, can be collected. So it was decided to $ollect suFh data at the maximum value of the reactants partial pressure p and p, ED

40

r (hr.9 cat./rnol ED fed1

Fig.5. Integral reactor data. (A) D W , ( 0 ) MP. ( 0 ) P. (A) A, (01 DMP. Solid lines calc. by Model 3 eq.s with optim. parameters of Table 1. T = 623 K.

60

r 1hr.g cat./mol ED fed1

Fig.6. As for Fig.5. T = 643 K.

(7.64 and 11.67 Torr, respectively), still avoiding unacceptable wasting of runs were reactants to byproducts. Therefore, all the integral react:r carried out at atmospheric total pressure and by feeding 2 cm /h of an aq. solution containing 3.55 and 6.45 wt X of ED and PG, respectively, together

333 3

with the usual 16,101 small flow (3 Ncm /min) of N, as carrier gas. Catalyst weight ranged from 0.005 to 0.100 g and temperature from 623 to 663 K. Under these conditions. diffusive intrusions are negligible, as verified previously [lo]. The results, expressed in terms of Yi, are shown as experimental points in Fig.5-7. The trend of data agrees with the principal reaction sequence ED + PG + DHMP + MP, with DMP forming through a parallel reaction. Furthermore, as a first approximation, A has been assumed to form exclusively by dehydration of PG. As for P, several pathways can be imagined, e.g. by demethylation of MP, or by condensation of two molecules of ED, etc. Hence, the different models considered take into account one or more pathways to this species. A simplified kinetic scheme, taking into account all the mentioned hypotheses, takes the form ED+PG A D H M P 4 &4

&6

2

M

P &P

(2)

'L5

P

A DMP + P in which all pseudo-first order reactions with respect to the principal reactant can be assumed, except reaction 6, which is of second order with respect to ED and. of course, reaction 1, which is of overall second order, being first-order with respect to both ED and PG. I

0

20

40

60

80

1.50

r (hr 1 g cet./md ED fed)

Fig.7. As for Fig.5. T = 663 K.

-

'

I

1.55

Fig.8. Arrhenius plot, Table 3 data. (0) k ,. (A) k,, (A) k,, (01 k,, ( 0 ) k,.

The stoichiometric equations referring to this scheme are

-

ED + PG DHMP + 2 H,O + 2 H, D+PG - M P + 2 H , O + 3 H 2 P + H,O + H, + CH,OH ED + PG FG A + H,O ED + PG d 0 . 5 DMP + 0.5 P + 2 H,O 2ED - P + 2 N H 3 + 3 H ,

1.60

lo3/ T , K

+ 3 €I,

334 By referring to 100 mol of ED fed and by taking into account the feeding rates of the various substances (vide supra), the molar feeding ratios of the various species are ED:PG:H20:N2 = 100:142.9:8417.7:617.9. Hence =,p ( 1oO-nDm-nw-np3-2n -2n I/& i , =,p ( 142.9-nDm-nw-np -2n -nA)an i '5

p5

3

6'

( J = A,P.MP.DHMP,DMP), where n

and pj= nj/Zni

represents the mol of P

k'

formed through the k-th reaction. Furthermore, since nMeOH=n , n n = H

2nDHnp+3n +n +6n +3n UP

2

'3

5 '

6'

and

nW =

'6

8417.7+2nDm+2nUP+nA+np+4np ,

have Xn i = 9278.5+3nDHnp+4nw+nr+nDw+2np+ 7np +4np . 3

=2n ,

NH 3

p3

5

3

we

5

6

From our definition of yield Yi, we have ni = mol of ED fed to the reactor. S o , for no 400, ni ED summation including the "yield" of ED and PG, 1 . e . reactants, calculated by difference with respect system of rate equations takes the form

YiniD/lOO. n;, being the = Yi and Zni = ZYi, the the mol % of unconverted to the feed. Hence, the

dYDm/dt = (klYEDYPC-k2YDm-k5YDm )/I?f i dYw/d.c = (k2YDm-k3YUP)/ZY i dYDw/dt = k5YDm/ZY i dYA/dz = k4Y,/ZY i dYp/dr = (k3Yw+k5YDm)/ZY i

+

k6(YED/ZYi l2

(4)

The present scheme defines the molar ratios among the various species. However, it cannot give any information about the quantitative ratio between the amounts of P produced through different routes. This has been determined by formulating an hypothesis about these relative amounts and by verifying through a non-linear regression-optimisation procedure the agreement between calculated and experimental data. Three hypotheses have been formulated within the framework of the general scheme 2. The first one, referred to as Model 1, is based on the results of some auxiliary runs, carried out under properly chosen experimental conditions, so to isolate the particular reaction under study. For instance, by feeding an aqueous solution of pure ED, it was observed that the amount of P formed by condensation of 2 molecules of ED (reaction 6 in scheme 51 is about l(10 of the overall amount of P found under the same conditions, but feeding the standard solution. On the other hand, by feeding an aqueous solution of MP only, it has been noticed that the amount of P forming by dealkylation of MP is about 2/10 of the total obtained with the standard solution. As a consequence, in Model 1 the following assumption was done: n :n :n = 2:7:1. In this way, no explicit relation between P and DMP is '3

'5

6'

can be taken into account

needed, since the previous ratios among the n, 1

-implicitly through Eq.s 3 and the following expressions: p, (lOO-nDHnp-nUP-l. 8np)/Zni and p, = (142.9-nDm-nw-nA-l. 6np)/Zni. The second hypothesis (Model 2 ) gives more weight to the transalkylation (reaction 5 of scheme 2). by assuming that only the moles of P exceeding

33s DMP

those of

can form

through

( 100-nDHwp-nIIP-2/3nDIIP-4/3np )/Zn i

reactions and

3

=,p

or 6. Therefore, pm= ( 142.9-nDm-nw-nA-4/3nDw

-2/3np)/Xn i . It may be observed that with Model 1 the error affecting YDWp is less important than that affecting Yp. while the opposite occurs with Model 2. The third hypothesis (Model 3) assumes that the condensation of ED to P (reaction 6 of scheme 2) is negligible and that the excess of P, with respect to DMP, forms exclusively by dealkylation of MP. Therefore, and n = 0. n =n -n Pg P

DIIP’

nP = nDIIP

P

5

6

After modification of the system 4 of rate equations, according to the Table 1. Optimised kinetic parameters of Model 3 (Scheme 6) Parameter 623

k, k, k, k, k,

(mol/h x (mol/h x (mol/h x (mol/h x (mol/h x

g

g

g g g

cat. cat. cat. cat. cat.

x x x x x

2

atm 1 atm) atm) atm) atm)

Temperature, K 643 663

176. 330. 3.43 6.53 0.017 0.033 0.084 0.280 0.079 0.121

980. 7.40 0.046 0.790 0.136

Table 2. Arrhenius parameters for the reactions of Model 3 (Scheme 6).

Ea (kcal/mol)

1nA

ED+PG%DHMP

3526

33f5

D W % M P

16f6

14f5

M P 5 P

20f4

12f3

PG

4621

3521

1154

7f3

Reaction

A‘

D W ”-, DMP + P

various models, the kinetic parameters (kl to k6) have been evaluated by minimising the objective function N @ =

X 1=1

7 max

X (YE,l,JYc,l,,)2/D

J=O

indicating experimental and calculated data, respectively. The best fitting was obtained in any case with D = Models 1 and 2 gave practically identical results, with k6 values of the (mol/h x g cat. x ah2). This means that reaction 6 in scheme order of lo-' 2 is negligible and that Model 3, the scheme of which is ED+PG L D H M P L M J'r J.5 A DMP + P

P &P

in spite of the lower number of parameters. can represent satisfactorily our reacting system. The optimised parameters of Model 3 are shown in Table 1. The agreement between experimental data and the curves, calculated through Model 3 equations, together with the latter parameters, is shown in Fig.5-7. By means of the well-known equation In k = In A - Ea/RT and of Table 1 parameters, the Arrhenius plots of Fig.8 were drawn, from which the values of the apparent activation energy E, (kcal/mol) and of the preexponential factor A (mol/h

x g

cat.

x

atm") were evaluated (Table 2).

The principal conclusions one can draw from the present results are: i) The differential reactor technique runs showed that, within the explored range of reaction conditions, the overall reaction rate is practically of first order with respect to both reactants. ii) This allows to write an extremely simplified reaction model, in which only five reactions are considered. showing a satisfactory interpretation of the whole set of data, collected by the integral reactor technique. iii) No nore complex models are needed for practical purposes, since the reaction conditions cannot be very different from the present ones, in order to get good yield and satisfactory catalyst life, as reported [6,101. iv) The kinetic equations and parameters so obtained may constitute a safe basis for the design of a pilot plant for the further development of the process. REFERENCES

1) A.Baiker and J.Kijenski, Catal.Rev.-Sci.Eng., 27 (1985) 653. 2) Y.Takita, Y.Nishida and T.Seiyama, Bull.Soc.Chem.Jpn., 49 (1976) 3699. 3) W.W.Kaeding, US Pat. 4082805 (1978). 4) W.Hammerschrnidt, A.Baiker, A.Wokaun and W.Fluhr, Appl.Catal., 20 (1986) 305. 5) L.Forni, Appl.Catal., 20 (1986) 219. 6) L.Forni, G.Stern and M.Gatti, Appl.Catal., 29 (1987) 161. 7) L.Forni, C.Oliva and C.Rebuscini, J.Chem.Soc., Faraday I, 84 (1988) 2397. 8) L.Forni, J.Catal., 111 (1988) 199. 9) L.Forni, Appl.Catal., 37 (1988) 305. 10) L.Forni and S.Nestori, in M.Guisnet et al.(Eds.) Heterogeneous Catalysis and Fine Chemicals, Elsevier. Amsterdam 1988, p.291. 11) L.Forni and P.Pollese1, J.Catal., 130 (1991) 403. 12) L.Forni and R.Miglio, in M.Guisnet et al. (Eds.) Heterog.Cata1. and Fine Chemicals 11, Elsevier. Amsterdam, 1991, p.367.

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

337

Properties of Sol-Gel Derived Ru/Cu/Si02 Catalysts and Role of Water in the Selective Hydrogenation of Benzene to Cyclohexene with the Catalysts Fujio Mizukami", Shu-ichi Niwa", Shin Ohkawa" and Atsuhiko Katayamab "National Chemical Laboratory for Industry, 1-1, Higashi, Tsukuba, Ibaraki 305, Japan bNippon Steel Corporation, Advanced Materials and Technology Research Laboratories, 1618, Ida, Nakahara-ku, Kawasaki 211, Japan

Abstract Ru/Cu/SiO2 catalysts were prepared using sol-gel a n d impregnation methods, and activated with hydrogen before and after calcination. The four types of the catalysts obtained were characterized by TEM, XRD, XPS, and adsorption of hydrogen, benzene, cyclohexene and water. From the results, it was found that the catalysts which have fine metal particles and low adsorptive abilities for hydrogen and cyclohexene, for example, lwt%Ru/O. lwt%Cu/SiO2 prepared by sol-gel method and activated before Calcination, are suitable for the production of cyclohexene. It was also concluded that the roles of water in the partial hydrogenation of benzene with the R d C d S i O 2 catalyst are to expel and isolate cyclohexene from the catalyst surface. 1. INTRODUCTION

Catalytic hydrogenation of benzene to cyclohexene has been actively studied because of its great synthetic and industrial interest, and it has been found so far that, for the formation of cyclohexene, ruthenium and copper metals are the best catalyst and promoter, respectively, water is essential as a n additive in the reaction mixture or the solvent, the optimum reaction temperatures and pressures are 170-190 "C and 60-80 kg/cm2, respectively, ruthenium catalysts prepared by a traditional method such a s impregnation can effectively produce cyclohexene only in the presence of a large amount of sodium hydroxide, iron sulfate or cobalt sulfate in t h e reaction mixture, namely, t h e traditional ruthenium catalysts need to be poisoned by the corrosive additives for the cyclohexene formation, on the other hand, sol-gel derived ruthenium catalysts activated with hydrogen before calcination can hydrogenate benzene to cyclohexene in high yields in the absence of the corrosive additives, etc. [l-81. These results are very important and interesting, but most of them have been not rationalized fully. Here we will discuss the difference between the sol-gel and impregnation catalysts and the role of water from the view points of the surface properties of the catalysts and the solubilities of benzene, cyclohexene

338

and cyclohexane to water.

S.EXPE€UMENTAL Four types of catalysts were obtained by using two preparation methods of sol-gel and impregnation and two activation procedures before and after calcination. In the preparation of sol-gel derived Ru/Cu/SiOa catalysts 15, 61, at first, RuC13.nH20, CuC12.2H20 and Si(OCzH& were mixed in ethylene glycol, and then hydrolyzed with water. The resultant monolithic gel was dried under reduced pressure and powdered. The powder was activated at 400 "C for 8 h in a hydrogen stream before and after calcination (450 "C, 2h). In the preparation of impregnation catalysts, a t first, silica powder was obtained from Si(OC2H514, ethylene glycol, a small amount of HCl methanol solution and water in a similar manner to the above sol-gel method. Then, the powder was calcined at 450 "C for 2 h and impregnated with an ethanol solution of RuC13.nHaO and CuC12.2H20. This powder was activated in the same manner t o the above. The relative solubilities of benzene, cyclohexene and cyclohexane to water at 50-200 "C were obtained under vapor pressure of the four substances only and total 70 kg/cm2 of the four vapors and hydrogen as follows. An organic mixture (50-60 ml) of benzene, cyclohexene and cyclohexane (1:l:l) and 100 ml of water were stirred at the rate of 1500 r.p.m. for 10 min. in an autoclave with a withdrawal tube of solution, and allowed to stand for 2 h. Then, an aliquot of the lower aqueous layer was poured into an 1,4-dioxane solution in a sample tube through the withdrawal tube and analyzed by GC. The catalyst were characterized by inductively coupled plasma spectrometry (Jarrell-Ash ICAP-7571, TEM (Hitachi H-8001, EDS (Horiba EMAX-3000), XRD (MAC Science MXP 181, XPS (Shimadzu ESCA 850), and adsorption of hydrogen (Be1 Japan Belsorp 36), water, benzene and cyclohexene (Microscal Microcalorimeters Mark-3V).

3.RESULTS AND DISCUSSION

3.1. Electron microscope, X-ray diffraction and photoelectronspectra Figures 1 shows TEM images of the lwt%Ru/O.lwt%Cu/Si02 catalysts activated with hydrogen before and after calcination. Metal particles (2-10 nm and 2-50 nm) in the sol-gel catalysts activated before and after calcination are smaller than those (3-50 nm and 20-60 nm) in the corresponding impregnation ones, respectively. In the comparison of the catalysts activated before and after calcination, the size of metal particles is smaller in the catalysts without calcination than in the corresponding catalysts with calcination. These are consistent with the fact that the sol-gel catalysts activated before calcination show faster rates in the benzene conversion than the sol-gel catalysts activated after calcination and the impregnation catalysts [7]. As shown in Figs. 2 and 3, in accordance with the TEM observations, the catalysts without calcination give weaker and broader XRD peaks assigned to ruthenium metal than the corresponding catalysts with calcination, and the impregnation catalysts activated after calcination show much stronger and sharper peaks than the corresponding sol-gel ones. However, in the case of the catalysts without calcination, the sol-gel catalysts show stronger and sharper

339

catalyst. Fig. 1 TEM image of Iwt%Ru/O.lwt%,CdSiO~ impregnation : (a)activated before calcination, ( b ) activated after calcination : (c) activated before calcination, sol-gel ( d ) activated after calcination

340

?U 0

bLl 0

40 0

80 0

28

XRD pattern of impregnation catalyst.

Fig. 2

upper : 5wt%Ru/0.5wt%Cu/SiO~ lower : lwt%Ru/O. lwt%,Cu/SiOz right : activated after calcination left : activated before calcination

10 0

4ilk-

-

20

Fig. 3

GO 0

80 0

20 0

40 0

20

XRD pattern of sol-gel catalyst. upper : 5wt%~Ru/0.5wt'%~C~dSiO~ lower : lwt%Ru/O.lwt%Cu/SiO:! right : activated after calcination left : activated before calcination

GO 0

80 0

341

peaks than the corresponding impregnation ones. This is, a t first sight, contradictory to the TEM results, for big crystalline particles generally show stronger and sharper XRD peaks than small ones. But, this is reasonable if metal particles in the impregnation catalysts activated before calcination are amorphous in structure. Because it can be considered that, in the catalyst preparation by impregnation, Ru(C1)aCu bridges tend to be formed on the silica surface, the bridges remain just before the activation with hydrogen if the catalysts are not calcined, and induce the formation of structurally disordered metal particles. In the sol-gel procedure, such chlorine bridges are difficult to be formed, since metal ions are surrounded by the ethylene glycol ligands [91 which are used in large excess as the solvent. The above consideration could be supported by the facts that among the four types of the catalysts the impregnation catalysts activated before calcination have the highest chlorine contents and the impregnation catalysts calcined show bigger and sharper XRD peaks assigned to ruthenium oxides than the corresponding sol-gel ones. Four types of the catalysts (5wt%Ru/0.5wt%Cu/SiO2)were also examined by XPS. All the four X-ray photoelectron spectra were quite similar in shape to each other and showed peaks at about the same position (463.7 eV) which were attributable to zero valent ruthenium. But, from the etching times to obtain the spectra and the peak heights, it was estimated that ruthenium particles in the sol-gel catalysts were situated i n deeper position than those in the impregnation ones. As it has been known that the sol-gel catalysts have a lot of micro pores [6], it will be reasonable t o presume that ruthenium particles are in micro pores. Accordingly, one of the causes of the low hydrogenation activity of the sol-gel catalyst activated aRer calcination could be related to narrowing of the pore entrances with heating during the calcination.

3.2. Adsorption Table 1 shows the hydrogen adsorption of the catalysts which were activated with hydrogen at 400 "C for 8h and evacuated at 180 "C for about 2h. Hydrogen is more adsorbed on the impregnation catalysts activated before calcination than on the corresponding impregnation catalysts after calcination. This could be related to that metal particles in the impregnation catalysts activated before calcination are amorphous or disordered in structure and metal particles generally become bigger with calcination. Table 1 Amount of hydrogen adsorbed on RdCdSiO2 catalyst at 180 "C. Metal Amount(H2 moV(Ru+Cu) moll content Calcination Sol-gel Impregnation lwt%RdO.l%wtCu without 1.61X10-1 4.85X10-1 lwt%Ru/O.l%wtCu with 2.55X10-1 1.28XlO-1 5wt%Ru/O.5%wtCu without 1.60X10-1 530x10-1 5wt%Ru/0.5%wtCu with 1.57X10-1 2.16XlO-1 On the contrary, in the case of the sol-gel catalysts, hydrogen seems to be more adsorbed on the catalysts activated after calcination, nevertheless metal particles are slightly bigger with calcination as can be seen from the TEM results. It has been generally known that as metal particles in a supported

342

catalyst are highly dispersed or fine, the amount of hydrogen adsorbed on the catalyst increases [lo-121. According to this, the results of XRD and TEM are in conflict with those of hydrogen adsorption. But, these results would be quite reasonable, assuming t h a t metal particles in t h e sol-gel catalysts activated before calcination are strongly interacting with the support or silica because of their smallness in size and t h e interaction is considerably canceled with calcination 113-151,namely, if it is considered t h a t as t h e interaction weakens and t h e metal particle size increases, t h e adsorbed amount of hydrogen increases and decreases, respectively. Generally speaking, if the amount of hydrogen on the catalyst is not enough to hydrogenate benzene to cyclohexane, cyclohexene will be apt to be produced. From this point of view, the catalysts whose adsorptive abilities for hydrogen are low would be suitable for the production of cyclohexene. This could be one of the reasons why the sol-gel 1wt%Ru/0.lwt%~CdSiO~ without calcination is the best catalyst for t h e formation of cyclohexene (29.2% yield) among t h e four catalysts listed in Table 2 15-8, 161. Table 2 shows the heats of adsorption of benzene, cyclohexene and water on the catalysts a t room temperature by flow microcalorimetry. n-Hexane for the two organic compounds and methyl ethyl ketone for water were used as the liquid carriers in measuring the heats. The heats of adsorption of water a r e overwhelmingly bigger than those of benzene and cyclohexene, namely, a r e about twenty times for t h e sol-gel catalysts and about ten times for t h e impregnation catalysts. Heat of adsorption on lwt%Rd0. lwt%Cu/Si02 catalyst Adsorption h e a t (mJ/gc) Ad so r ba te ( g/d m 31* Calcination Sol-gel Impregnation Benzene 5.00 without 171 317 5.00 with 233 469 10.2 without 274 5 19 10.2 365 with 777 Cyclohexene 5.33 without 200 523 5 33 with 208 322 9.90 w 1 thou t 371 803 9.90 with 346 539 Water 4.99 without 4385 4938 4.99 with 4870 4350 10.6 without 6323 7334 10.6 with 6960 6163 *: g=Weight of adsorbate, dm3=Volume of carrier + adsorbate.

Table 2

There a r e only small differences between the heats of adsorption of water on the four catalysts, but considerable differences between the heats of adsorption of benzene and cyclohexene. Roughly speaking, the h e a t s of adsorption of cyclohexene and benzene on the impregnation catalysts are twice those on the sol-gel ones. These would indicate t h a t t h e catalysts tend to be covered with water (actually, t h e catalysts a r e very hygroscopic), benzene a n d cyclohexene adsorbed on the catalysts a r e easily substituted with water, cyclohexene on the

343

sol-gel catalysts is easier desorbed from the surface, and the sol-gel catalysts are relatively favorable for the production of cyclohexene.

33. Solubility Figure 4 shows the relative solubilities of benzene, cyclohexene a n d cyclohexane to water under vapor pressure and 70 kg/cm2. The solubilities of the three organic compounds to water are higher under 70 kg/cm2 than under vapor pressure a n d increase with a n increase in t h e t e m p e r a t u r e . Furthermore, it is found that benzene is much more soluble in water than cyclohexene and cyclohexane, and the differences between the solubilities of the three organic compounds to water become greater with a n increase in t h e temperature. These suggest that benzene can much easier approach to the active sites in t h e catalysts than cyclohexene a n d cyclohexane a s t h e temperature goes up, even if the catalysts are covered with plenty of water. 3.4. Formation mechanism of cyclohexene and role of water On the bases of the results and discussion stated above, we considered the role of water for the formation of cyclohexene (Fig.5). In the reaction mixture, the catalysts are in aqueous phase, and benzene and hydrogen have to pass the thick water layer surrounding the catalysts to react with each other on the active sites. When cyclohexene is produced from benzene, it is expelled by water and benzene newly coming from organic phase. High temperatures and pressures such as 170-190 "C and 60-80 kg/cm2 which a r e t h e optimum conditions for the production of cyclohexene are favorable for increasing the concentration of benzene in the water layer. If the desorption r a t e of cyclohexene from the surface is slow, cyclohexene is naturally converted to cyclohexane. But, if cyclohexene is expelled, it will be isolated from the active sites in the catalysts because of its low solubility to water. Accordingly, it could

r

1.4

r

1.4

x, 1.0

x, 1.0 0.8

,h 0.6

.-

.C

7

e

0.0

0.0

0

Temperature("C) Fig. 4

0

50

loo

150

200

Temperature("C)

Solubility of benzene ( 0 1, cyclohexene ( 0 and cyclohexane (01 to water under vaper pressure (broken line) and 70kg/cm2(solid line) (A) : One organic component and water (B) : Mixed three organic components and water

344

be concluded that the roles of water are roughly to expel and isolate cyclohexene from the catalyst surface.

Fig. 5

(1) Dissolution

(2) Adsorption

(4) Substitution

( 6 ) Isolation

(3)Reaction

Formation mechanism of cyclohexene

4.REFERENCES 1 M.M. Johnson and G.P. Nowack, J. Catal., 38 (1975) 518. 2 C.U.I. Odenbrand and S.T. Lundin, J. Chem. Tech. Biotechnol., 30 (1980) 677. 3 C.U.I. Odenbrand and S.T. Lundin, J. Chem. Tech. Biotechnol., 31 (1981) 660. 4 C.U.I. Odenbrand and S.L.T. Andersson, J. Chem. Tech. Biotechnol., 32 (1982)365. 5 S. Niwa, F. Mizukami, M. Kuno, K. Takeshita, H. Nakamura, T. Tsuchiya, K. Shimizu and J . Imamura, J . Mol. Catal., 34 (1986) 247. 6 S. Niwa, F. Mizukami, S. Isoyama, T. Tsuchiya, K. Shimizu, S. Imai and J. Imamura, J. Chem. Tech. Biotechnol., 36 (1986) 236. 7 S.Niwa, F. Mizukami, J. Imamura and K. Itabashi, Sekiyu Gakkaishi, 32 (1989) 145. 8 S.Niwa, F. Mizukami, J. Imamura and K. Itabashi, Sekiyu Gakkaishi, 32 (1989)299. 9 S.Niwa, E. Lopez, F. Mizukami and M. Toba, Yukagaku, 38 (1989) 938. 10 J.R. Anderson, "Structure of Metallic Catalysts", Academic Press ( 1975) 11 J.E. Benson and M. Boudart, J. Catal., 4 (1965) 704. 12 E. Kikuchi, K. Ito, T. Ino and Y. Morita, J . Catal., 46 (1977) 382. I3 S.T. Tauster, S.C. Fung and R.L. Carten, J. Am. Chem. SOC.,100 (1980) 170. 14 S.T. Tauster and S.C. Fung, J. Catal., 55 (1987) 29. 15 T. Lopez, A. Romero and R. Gomez, J. Non-Cryst. Solids, 127 (1991) 105. 16 S.Niwa and F. Mizukami, Yukagaku, 39 (1990) 105.

M. Guisnet et al. (Editors), Heterogeneous Caialysis and Fine Chemiurls 111 1993 Elsevier Science Publishers B.V. All rights reserved.

(b

345

The partial hydrogenation of benzene and of toluene over ruthenium catalysts - the effect of salt addition on the selectivity to (methyl-)cyclohexenes M. Soede, E.J.A.X. van de Sandt, M. Makkee and J.J.F. Scholten

Delft University of Technology, Department of Chemical Technology and Materials Science, Julianalaan 136, 2628 BL DELFT, The Netherlands

Abstract A study has been made of the batch-wise partial hydrogenation of benzene and of toluene to cyclohexene and the methyl-cyclohexenes, respectively, over ruthenium as a catalyst in the presence of an aqueous zinc sulphate solution. The reaction has been performed in a stirred autoclave at 423 K and at a total pressure of 5.0 MPa. Both the hydrogenation rates of benzene and of toluene are governed by diffusion processes, which can be shown by applying the Carbeny and Wheeler-Weisz criteria. The high selectivities and yields of cyclohexene and methylcyclohexenes are mainly explained by mass transfer limitation of hydrogen and cyclic alkenes towards the active catalyst surface. The lower toluene hydrogenation rate can be explained from the lower solubility of toluene in water as compared to benzene. By increasing the zinc salt concentration or by increasing the reaction temperature, the selectivities towards the various methylcyclohexenes change due to an increase of the ad- and desorption rates of the methylcyclohexenes at the surface of the catalyst. 1. INTRODUCTION

Cyclohexene can be produced by partial hydrogenation of benzene over a ruthenium catalyst at ambient temperature and pressure. In the presence of reaction modifiers, selectivities up to only 10%have been achieved [1,2]. Therefore, a study has been made of the batch-wise partial hydrogenation of benzene to cyclohexene over ruthenium as a catalyst in the presence of an aqueous zinc sulphate solution; in this case the reaction is performed in a stirred autoclave at 423 K and a total pressure of 5.0 MPa. From the patent literature [3-51 and from publications by Odenbrand and Lundin [6] and by Struijk et al. [7,8] it follows that under the conditions quoted above, selectivities to cyclohexene up to 80% and cyclohexene yields of 40 to 50% may be achieved. Cyclohexene is relatively weakly bound to the ruthenium surface, as compared with the other reaction intermediates benzene and cyclohexadiene. It, therefore, can desorb before further hydrogenation. A measure of the selectivity to cyclohexene is the relative reaction rate, r', given by the expression: (r&$ - r*) ri

-

'H

346

where r, is the rate of desorption of cyclohexene, r,,, the rate of readsorption of cyclohexene and rHthe rate of hydrogenation of cyclohexene. In the absence of water and of zinc sulphate in the reactor, the rate of readsorption of cyclohexene appeared to be so high that the selectivity to cyclohexene becomes negligible [7]. However, as soon as zinc sulphate and water are added, the surface of the ruthenium catalyst is partially covered by cations, due to the chemisorption of zinc sulphate. Owing to this the hydrophobicity of the ruthenium surface changes into hydrophilicity and the catalyst particles are surrounded by a stagnant water layer. As the production rate of cyclohexene is much higher than the rate of diffusion of cyclohexene through the stagnant water layer, it breaks through the water layer and dissolves in the organic phase. But now the rate of readsorption, Tad,, has become very low as this readsorption can take place only via diffusion through the stagnant water layer. Furthermore, rHis strongly lowered due to diffusional retardation of hydrogen and/or the organic substrate. Both these factors cause a strong increase of the selectivity to cyclohexene. A full analysis of the kinetics of benzene hydrogenation under these conditions shows that, due to the presence of the stagnant water layer, mass-transfer limitations of both hydrogen and benzene at the watedruthenium interface are introduced and that these transfers become rate-determining [7]. The rate of reaction and the cyclohexene selectivity is nearly totally determined by diffusion. Therefore, we might expect that replacement of benzene by toluene will result in a further slowing down of the rate of reaction, the solubility of toluene in water being lower than of benzene. In the present article this aspect will be verified. Also the effect of salt concentration and of the temperature on the selectivities towards the various methylcyclohexenes has been studied. 2. EXPERIMENTAL SECTION The ruthenium catalyst (mean particle size 25 pm, S,, = 75 m2.g-', mean pore size 60-70 nm) was prepared by reduction of ruthenium hydroxide, obtained by adding a sodium hydroxide solution to a ruthenium(1II)chloridesolution [6,7]. The construction of the experimental setup has been described earlier [7]. A standard quantity of ruthenium catalyst (0.20 g), 200 cm3of substrate, 75 cm3of doubly distilled water and 2.1 mmol of ZnS04.7H,0 were introduced into the stainless-steel autoclave, equipped with baffle bars. After purging with hydrogen, the pressure was elevated to 3.5 MPa. When the desired reaction temperature (423 K) was reached, the total pressure was adjusted to 5.0 MPa. The benzene or toluene hydrogenation was started by switching on the stirrer (stirring speed 1500 rpm). In the hydrogenation of toluene the reaction is carried out at temperatures, ranging from 403 to 443 K. Also different amounts of zinc sulphate are used in the hydrogenation reaction. The simultaneous hydrogenation of benzene and of toluene has been studied as well; in this case a mixture of 100 cm3 of benzene and of 100 cm' of toluene was introduced into the reactor.

347

3. RESULTS AND DISCUSSION 3.1 Hydrogenation of benzene and of toluene

Characteristic concentration profiles in the hydrogenation of benzene and toluene are shown in Figs. 1 and 2. The kinetics of the hydrogenation of benzene will depend on physical as well as on chemical parameters. Using toluene instead of benzene will change these physical and chemical parameters, but the reaction mechanism remains virtually the same. When no salt is added only chemical parameters influence the reaction rate. Experiments show that the initial reaction rate of the hydrogenation of benzene and of toluene, without adding salt, are approximately 8 mmol/s. At 423 K and 5.0 MPa the solubility of toluene in water is a factor of 4 lower than of benzene: 30.6 and 125 mol*m-’, respectively. The solubility of hydrogen in water is 36 mol/m3. The solubilities are calculated using the method of Tsonopoulos [9] and the solubility data of McAuliffe [lo]. On the bases of these solubilities the hydrogen mass transfer can be rate limiting in the benzene hydrogenation and the initial reaction rates of the benzene and toluene hydrogenation should be of the same order. The hydrogenation of toluene, however, appeared to run a factor 3-4 slower than the hydrogenation of benzene (See Table 1). Three explanations for this difference may be offered. First, the diffusion coefficient of hydrogen is larger than the average coefficient of the substrates, 20.10-9and 7.10-9m2/s respectively. Second, the concentration of hydrogen at the surface of the catalyst appeared may be higher than expected when normal Langmuir adsorption is assumed; the concentration is enlarged due to the enhancement effect, discussed by Vinke [l I]. And third, the maximum solubility

R

v

C

0 .c 0 c L

C

ar C U

0

0

0

40

80

120

160

200

Time (min)

Figure 1. Molar percentages of benzene (o), cyclohexene ( A ) and cyclohexane (0) in the hydrogenation of benzene over a ruthenium catalyst as a function of time.

0

40

80

120

160

200

Time (min)

Figure 2, Molar percentages of toluene (o),1-methylcyclohexene( A ) , 3+4-methylcyclohexene ( 0 ) and methyl-cyclohexane ( 0 ) in the hydrogenation of toluene over a ruthenium catalyst as a function of time.

348

of the substrates is less than calculated. The solubilities are calculated for solutions, in the absence of salt. In general the solubility of a compound in water decreases when the water contains salt. We assume that the decrease in the solubility is of the same order for both substrates. These explanations support the assumption that the hydrogen transport is not rate limiting. So, the above data confirm our earlier conclusion [7] that the transfer of the substrates through the stagnant water layer is rate-determining and that the initial reaction rates are linear with the solubility of the substrates in water. The extent of external and internal mass transfer limitation can be estimated by the methods introduced by Carberry and by Wheeler-Weisz [12]. A Carberry number (Ca) smaller than 0.05 means that diffusional retardation by external mass transport may be neglected. A Wheeler-Weisz group (WW) smaller than 0.1 means that pore diffusion limitation is negligible [ 13,141. From the data in Table 1 it may be concluded that, under the experimental conditions applied, both the hydrogenation of benzene and of toluene are externally diffusionally controlled and that at the same time we are dealing with internal diffusion of hydrogen and of the substrates, viz. in the pores of the catalyst.

Table 1 The observed reaction rates of benzene and of toluene compared with their solubilities in water at 423 K and 5.0 MPa. Carberry and Wheeler-Weisz numbers in the hydrogenation of benzene and of toluene over a ruthenium catalyst. (T =423 K; P = 5.0 MPa; 0.2 g ruthenium catalyst; 200 cm3 substrate; 75 cm3 water and 2.1 mmol ZnS0,.7H,O.) Hydrogenation of

Solubility in water (~OITTY?

~~~

~

Benzene Toluene

125 30.6

Observed initial reaction rate (mmol-s-') ~

1.52 0.39

~

For substrate Ca

For hydrogen

ww

Ca

WW

2.30 0.55

0.671 0.173

17.9 1.83

~~~

0.177 0.048

The difference in hydrogenation rate is also illustrated in the competitive hydrogenation of the mixture of benzene and toluene, see Fig. 3. The benzene is almost totally hydrogenated in the first stage and the toluene in the second stage of the reaction. The initial hydrogenation rate of benzene is now 15 times as high as of toluene. This difference in reaction rate can be explained by two factors. Firstly, the solubility of toluene in water is lower than the solubility of benzene and, in comparison with the hydrogenation of the pure substrates, the activity coefficient of benzene is increased with regard to the activity coefficient of toluene, just to the presence of toluene (see for instance [15]). Secondly, the adsorption coefficients of benzene and toluene will differ. The large difference in reaction rates points to the adsorption enthalpy of benzene to be higher than of toluene on ruthenium.

3.2 The influence of the concentration of zinc sulphate The influence of the amount of zinc salt added on the hydrogen consumption rate and on the initial selectivity to cycloalkenes in the hydrogenation of benzene is illtistrated by Struijk [8] and of toluene is shown in Fig. 4. By adding an amount of salt increasing from 0 to 12 mmol per 75 cm3 the reaction rate gradually decreases. Two factors may be put forward to

349 100

100 90 -

toluene

80 -

fl

20

10

40

30

60

50

70

80

90

100

Total conversion (7%)

Figure 3. Conversions and selectivities in the competitive hydrogenation of benzene and of toluene over a ruthenium catalyst in the presence of a zinc sulphate solution.

“k

-

v €

A

w

v

3

0

3

6

9

Amount of ZnS04

12

15

(mmol)

Figure 4. Influence of the amount of ZnS04.7H,0 on the hydrogen consumption rate and the initial selectivity in the hydrogenation of toluene.

explain this effect. Firstly, on adding more salt, the hydrophilicity of the catalyst particles increases by which the stagnant water layer becomes more dominant and hence the rate of diffusion of the reactants declines. But secondly, an increasing salt coverage will finally poison the catalyst to such an extent that the catalytic reaction becomes the slowest rate determining step instead of the diffusional transport [13].

350

Table 2 The initial selectivitiesto (methyl-)cyclohexenesin the hydrogenation of benzene and toluene. * = initial selectivitiesin the competitive hydrogenation as shown in Fig.3. (T = 423 K; p = 5.0 MPa; 0.2 g ruthenium catalyst; 200 cm3 substrate; 75 cm3 water and ZnS04.7H20.) Amount of salt added (mmol)

Initial selectivity to cyclohexene (96)

Initial selectivity to (1+3 +4)-methylcyclohexene(%)

0 2.1

3.5 43

9.2 54

2.1

*

44

*

65

*

A very important consequence of the addition of more zinc salt is the strongly increasing selectivity to the (methyl-)cyclohexenes. Due to the water layer the re-adsorption of the (methyl-)cyclohexenesis retarded very strongly. The solubility of cyclohexene in water is 6 times as small as the solubility of benzene, 21 (mol/m3), the solubility of the methylcyclohexenes is 4.7 (mol/m3)6.5 times as small as of toluene. Due to these lower solubilities the benzene and toluene will hydrogenate preferentially, before further hydrogenation of the (methyl-)cyclohexenes takes place, assuming that the substrate and the reaction intermediate have the same adsorption strength. From the data in table 2 it can be seen that the initial selectivity of toluene to the methylcyclohexenes is higher than the initial selectivityof benzene to cyclohexene; due to the steric hindrance of the methylgroup in the methylcyclohexenes they will desorb more easily than cyclohexene. It is interesting that the selectivity to 1-methylcyclohexene appeared to be higher than the selectivity to 3- and 4-methylcyclohexene.(From a thermodynamic point of view this ratio has to be larger than 20 at a 423 K and at 5.0 MPa.) Increasing the amount of salt added from 1.0 to 6.1 mmol the selectivity ratio 1methylcyclohexene/(3+4)-methylcyclohexene, S1/S3+4,decreases from 5.9 to 1.2 at 50% conversion (see Table 3). At 10% conversion this ratio decreases from 3.3 to 0.65 on increasing the salt concentration from 0.87 to 12 mmol. We conclude that the degree of coverage with cations influences the selectivity of the reaction. The higher the salt coverage, the higher is the selectivity to 3- and 4-methylcyclohexene. Increasing the temperature gives also a decrease in the ratio, S,/S3+4, (more 3- and 4methylcyclohexene are formed - see Table 4). More 3- and 4-methylcyclohexeneis formed at higher temperature. Due to the higher temperature the ad- and desorption rate of the product are enhanced. The residence time at the catalyst surface is reduced and the 3- and 4methylcyclohexenes will be converted in less extent to 1-methylcyclohexene. From this viewpoint the decrease in the ratio can be explained when salt is added. Due to the salt adsorbed at the surfaceof the catalyst the desorption rates of the methylcyclohexenes are enlarged. Zinc sulphatechanges not only the hydrophilicityof the ruthenium particle, but has also a catalytic effect. This might be due to an ensemble size effect; the sterical hindrance of smaller ruthenium ensembles. Besides this sterical argument the energetics of the reaction may play a role as well. Due to surface heterogeneity we are dealing with more weakly bonding ruthenium sites when the cation coverage is high.

351

Table 3 The influence of the amount of salt on the selectivity ratio l-methylcyclohexene/(3+4)methylcyclohexene, SI/S3+4, at three different conversions (10, 25 and 50%). (T = 423 K; p = 5.0 MPa; 0.2 g ruthenium catalyst; 200 cm3 substrate; 75 cm3water and ZnS04.7Hz0.) n.m.= not measured. Conversion

S1/S3+4

(96)

0

0.87

1.6

3.0

6.1

12

10

n.m.

25 50

7.1 10

3.3 4.4 5.9

1.7 1.7 1.8

1.1 1.o 1.7

0.86 0.74 1.21

0.65 2.8 n.m.

mmol

Table 4 The influence of temperature on the selectivity ratio l-rnethylcyclohexene/(3+4)methylcyclohexene, Sl/S3+4, at different conversions. (p = 5.0 MPa; 0.2 g ruthenium catalyst; 200 cm3 substrate; 75 cm3 water and 2.1 mmol ZnS0,.7Hz0.) Conversion (96)

s1/s3+4 403

413

423

433

443

10 25 50 62.5

2.6 3.6 4.4 4.8

75

5.5

2.8 3.1 4.0 4.2 5.0

1.7 1.7 1.8 2.0 2.3

1.6 1.8 2.2 2.5 3.0

1.3 1.3 1.6 1.7 2.2

K

4. CONCLUSIONS

In the presence of an aqueous zinc solution both rates of hydrogenation of benzene and of toluene are governed by mass transfer limitations. The high selectivities and yields of cyclohexene and of the methyl-substituted cyclohexenes are mainly due to the mass-transfer limitation of the re-adsorption of this intermediates.The Carberry and Wheeler-Weiszcriteria support these conclusions. The lower toluene hydrogenation rate can be explained from the lower solubility in water as compared with benzene. Upon zinc addition the selectivities towards cycloalkenes are increased, whereas the activity of the catalyst is diminished. In the hydrogenation of toluene the selectivitiestowards the various methylcyclohexenes is influenced by the salt coverage. The higher the amount of salt and the higher the reaction temperature, the higher is the selectivity to 3- and 4methylcyclohexene.

352

ACKNOWLEDGEMENT Thanks are due to DSM Research (Geleen, The Netherlands) for financial support.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

13 14 15

J. Struijk and J.J.F. Scholten, Appl. Catal., 62 (1990) 151. J. Struijk and J.J.F. Scholten, Appl. Catal., 82 (1992) 277. H. Nagahara and M. Konishi, Eur. Patent 0.220.525 (1986). K. Matsuoka, Jap. Patent 03,240,746 (1990). J. Struijk, Dutch Patent application No. 9101743 (1991). C.U.I. Odenbrand and S.T. Lundin, J. Chem. Tech. Biotechnol., 32 (1982) 365. J. Struijk, M. d'Angremond, W.J.M. Lucas-de Regt and J.J.F. Scholten, Appl. Catal. A: General, 83 (1992) 263. J. Struijk, R. Moene, T. van der Kamp and J.J.F. Scholten, Catal. A: General, 89 (1992) 77 C. Tsonopoulos and G.M. Wilson, AIChE. J., 29 (19.83) 1267. C. McAuliffe, J. Phys. Chem., (1966) 1276. H. Vinke, Ph.D.-thesis,"The effect of catalyst particle-to-bubble adhesion on the mass transfer in agitated slurry reactors", Amsterdam, 1992. M. Soede, E.J.A.X. van de Sandt, J. Struijk, M. Makkee and J.J.F. Scholten, "The partial hydrogenation of benzene and of toluene to (methyl-)cyclohexenesover modified ruthenium catalysts" in M.P.C. Weijnen and A.A.H. Drinkenburg (eds.), Precision Process Technology', Kluwer Acadamic, Dordrecht, 219-224. J.J. Carberry, "Physical-ChemicalAspects of Mass and Heat Transfer in Heterogeneous Catalysis" in J.R. Anderson and M. Boudart (eds.), Catalysis, Science and Technology, Vol 8, Springer Verlag, Berlin (1987) 131. G.W. Roberts, "The influence of mass and heat transfer on the performance of heterogeneous catalysts in gas/liquid/solid systems" in P.N. Rylander and H. Greenfield (eds.), Catalysis in Organic Synthesis, Academic Press, New York (1976) 1. G. Arich, I. Kikic and P. Alessi, Chem. Eng. Science, 30 (1975) 187.

M. Guisnet et al. (Editors),Heterogeneaus Cptulysisand Fine Chemicals III CD 1993 Elsevier Sdence Publishers B.V. All rights reserved.

353

SHAPE - SELECTIVITY OF Pt ON CARBON FIBERS CATALYSTS S. Kogana, M.V.Landaua, M. Herskowitza and J. E. Koreshb a Department of Chemical Engineering, Ben-Gurion University of the Negev b Chemistry Division, Nuclear Research Center Negev, Beer Sheva, Israel

Abstract Catalysts were prepared by impregnation of Pt inside the pore structure of carbon fibers. Care was taken to eliminate the active metal from the external surface of the support. A very high dispersion of Pt was measured. Four reactions were carried out in a fixed-bed reactor: competitive hydrogenation of cyclohexene and 1-hexene, cyclization of 1-hexene, n-heptane conversion and dehydrogenation of cyclohexanol. Three types of carbon fibers with a different pore size and Pt-adsorption capacity along with a Pt on activated carbon commercial catalyst were tested. The data indicate a significant effect of the pore size dimension on the selectivity in each system. The ability to tailor the pore structure to achieve results drastically different from those obtained with established supports is demonstrated with heptane conversion. Pt on open pore carbon fibers show higher activity with the same selectivity as compared with Pt on activated carbon catalysts.

INTRODUCTION

1.

Carbon fibers have been used as molecular sieve carbon (MSC) applied in gas separation processes. Previous studies (1,2) have shown that pore dimensions of ultramicroporous carbons could be gradually increased in the ran e of 3-681 and could serve as a MSC. Any two molecules which differ by 0.2 in their smallest dimension, in that range, could be completely separated by selective adsorption. This high resolution is not achieved by homogeneous enlargement of the pore dimensions since the activating process involves chemical adsorption of oxygen molecules and desorption of a 3A entity like CO or COa molecules. In essence it is a kinetic separation (1) with a difference of many orders of magnitude in the adsorbing rates between the two separated molecules. The pore structure model consists of constrictions in series and

x

354

wider pores in between. One of the main problems in a molecular sieve adsorbent which could exclude the use of it as a substrate for a catalyst is the slow diffusion inside the ultramicroporous regions (3). Minute enlargement of pore dimensions could drastically increase the diffusion rates in these regions for molecule which formerly hardly diffuse (4). The effective dimensions of an adsorbing or reacting molecule is dependent on the adsorbent-adsorbate interaction, in other words, the chemical nature of both the adsorbent and the adsorbate is playing a crucial role and should be taken into account while tailoring the pores structure. Only a few studies published in the literature have examined the possibility to utilize carbon fibers as support with special shape-selectivity properties to prepare catalysts (5-7). The purpose of this work is to test various carbon fibers as supports for Pt supported catalysts. Preliminary activity and selectivity data were obtained for hydrogenation, dehydrogenation and cyclization reactions. 2. EXPERIMENTAL

The raw material for the catalyst substrate was a fibrous carbon TCM 128 (Carbonne-Lorraine). The various supports were obtained by thermochemical treatment with air or C02 reaching up to 15% weight loss using the same procedures described previously (8). The preparation of the catalyst included several steps. Carbon fiber samples (0.1 - 0.5 g, thread diameter of about 1000 nm) were immersed in aqueous solutions of H2 Pt C1, (0.1% of Pt) for 6 hrs. The amount of Pt in solution was equivalent to 5% of the fiber weight. Spectrophotometric (wave length 262 nm) and colorimetric (using SnC1, at a wave length 403 nm) measurements were carried out to determine the Pt concentration in the liquid collected after removing the catalyst samples. Those samples were rinsed with water, and dried at 170-180°Cfor 5-6 hrs. Pt concentration in the various samples, determined by XR-fluorescence method, ranged from 0.5 to 2.5%, depending on the adsorption capacity of the carbon fibers. The samples were characterized by hydrogen chemisorption and XRD. A commercial catalyst, 0.5% Pt on activated carbon, manufactured by Engelhard, was tested to provide a reference for comparison. The gas phase reactions were carried out in a fixed-bed reactor. The equipment is described elsewhere (9).

355

RESULTS AND DISCUSSION

3.

Pt adsorption of the various samples measured as a function of time is plotted in Figure 1. Three groups were identified - strong, intermediate and weak adsorption. Pt concentration in the samples and the rate of adsorption are determined from this plot. All samples were reduced at 350°C with hydrogen at a space velocity of 1000 h-1 for 2 hrs. Their physical properties are listed in Table 1. According to XRD data, Pt in all the samples was amorphous, i.e., particle size was less than 4 nm. H to Pt ratio, measured by hydrogen chemisorption in samples based on CF-1 and CFi, was near unity. Table 1 Properties of catalysts support

CF-1 CF-s CF-i AC*

* **

%Pt

2.5 0.5 1.8 0.5

Effective constriction dimensions* *

Pt adsorption

large small intermediate

strong weak intermediate

0.5%Pt on activated carbon manufactured by Engelhard Constriction dimensions measured by molecular probe technique.

Four reaction systems were studied. All reactions were performed at atmospheric pressure. The range of operating conditions was set to allow a meaningful1 comparison of the activity and selectivity of each sample for a specific reaction. Only the initial performance was tested in experiments lasting for 3-8 hrs. Rates of reaction were calculated from low (5-10%) conversion data. The hydrogenation of 1-hexene and cyclohexene mixture (1: 1 molar ratio) was selective to the saturated products for all three catalysts. The rates of reaction at 100°C are listed in Table 2. Similar rates were measured for the two reactions in the case of CF-1 and AC while the rate of 1-hexene hydrogenation was almost three times as high as the cyclohexene hydrogenation in the case of CF-s. The specific rate on CF-1 was higher than the one on AC probably because of a better dispersion of Pt on the surface. The special effect identified during the experiments with CF-s is demonstrated in Figure 2 which shows the cyclohexene conversion as a function of 1-hexene conversion measured at the same operating conditions. While the conversion ofthe two reactants is almost identical for AC and CF-I, selective conversion ofl-hexene was measured for CF-s.

356

200

0

400

600

300

1000 1200 1400

Adsorption time.rnin

adsorption o n carbon fibers at 2OoC.

Fig. 1. H,PtCl, 0

CF-314

' 470

6o

V

CF-325

V

CF-315

1 t

0

10 20

30 40 52 oC '0 80 9cl Hexene- I conversion.%

I90

Fig. 2. Cyclohexene conversion as a function of Hexene-1 conversion at identical conditions.

0 O.S%Pt/AC

0 2.5%Pt/CF-314

V 0.5%Pt/CF-315

357

The cyclization selectivity of 1-hexene to aromatics and naphtenes was measured at low conversion levels of 2-15%. Again AC and CF-1 displayed a similar performance yielding about 70% selectivity and CF-s gave half this value, as illustrated in Figure 3. The dehydrogenation of heptane to olefins and aromatization to toluene was tested with several samples of catalysts, listed in Table 3. A gradual change in selectivity was measured from the high selectivity to toluene using AC or CF-1 catalysts to high selectivity to olefins obtained with CF-s. The cyclohexanol dehydrogenation was carried out at a level of 30-40% conversion. Cyclohexanone, phenol and hydrocarbons like cyclohexane, cyclohexene and benzene were analyzed. The results summarized in Table 4 show that the selectivity changes from 30% to 91% by using AC and CF-s, respectively. Table 2 Hydrogenation rates at 100°C Catalyst

Reaction rates, gmol/gPt.h

AC CF-1 CF-s WHSV = 25-50 h-l;

1-hexene

cyclohexene

46 86

40 79

22

8

H2 :RH = 10 (molar)

Table 3 Results of heptane conversion Catalyst

Temp, "C WHSV, h-l Conversion, % Selectivity,% toluene olefins other products

H2:RH = 10 (molar)

AC

CF-1

CF-i

CF-s

475

465

475

480

20

10

5

5

6

8

5

8.5

62 5

60 9

33

1

16

15

33

31

51

84

0

4

c A

W

0

0

Cyclization selectivity,%

359

Table 4

of cvclohexanol de-nat

Temp, "C Conversion, % Selectivity,% cyclohexanone phenol hydrocarbons

ion (WHSV = 25 h d a @ H = 3 molar) Catalyst

AC

CF-s

CF -i

300 37

350 39

400 31

30 64 6

91 0 9

64 29 7

The rates of reaction and conversions measured in the four reactions indicate a clear trend related to the diffusion and reaction inside the pores of the carbon fiber supports. Linear molecules like 1-hexene diffuse at a higher rate than larger molecules like cyclohexene in small-pore carbon fibers. Preliminary measurements of the critical pore size support those results. Furthermore side groups like OH in the cyclohexanol penetrate selectively into the small pores yielding cyclohexanone, while in large pores reactions involving the complete molecule take place leading to products like phenol along with cyclohexanone. The reactions of heptane conversion and 1-hexene cyclization indicate a selectivity pattern based on the size of the product molecules. Negligible conversion to toluene was measured in the small pores carbon fibers, while other carbon fibers gave results similar to those of the activated carbon. The preliminary results presented in this work demonstrate a significant molecular sieves effect of the support which can be utilized to tailor specific catalysts. 4. REFERENCES 1 2 3 4 5 6 7 8 9.

J.E.Koresh and A. Soffer, J. Chem.Soc. Faraday I, 76 (1980) 2457-71. J.E.Koresh and A. Soffer, J. Chem.Soc. Faraday I, 76 (1980) 2472-85. J.E.Koresh and A. Soffer, J. Chem.Soc. Faraday I, 77 (1981) 3005-18. J.E.Koresh and A. Soffer, J. Chem.Soc. Faraday I, 76 (1980) 2507-09. D.L. Trimm and B. J. Cooper, J. Catal., 31 (1973) 287. J.L. Schmitt and P.L. Walker, Carbon, 9 (1971) 791. H.C. Foley, ACS Symposium Ser., 368 (1988) 355. S.S. Barton and J.E. Koresh,J. Chem.Soc. Faraday I, 79 (1983) 1173. M.V. Landau, S.B. Kogan and M. Herskowitz, AIChE Annual Meeting, paper N 60V, Nov. 1-6, (1992).

This Page Intentionally Left Blank

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 aD 1993 Elsevier sdence Publishers B.V. All rights reserved.

361

On the XPS-Surface Characterieation of Activated Carbons reap. Pd/C Catalyets and a Correlation to the Catalytic Activity R. Burmeister, B. Despeyroux, K. Deller, K. Seibold, P. Albers Degussa AG, P.O. Box 1345, D-6450 Hanau 1

Abstract Powdered activated carbons, standardly used as supports for the manufacture of precious metal containing catalysts, have to be washed with strong mineral acids like HNO3 to reduce their ash content and to functionalize their surface. Even after the HNO3 washing step, lot-to-lot variations in morphology and surface chemistry occur. These variations, in particularly coming from the presence of N-containing groups after the HNO3 washing, can be attenuated by a further oxidative treatment with H202, leading to a drastic increase of.the catalyst activity. This phenomena is reported for the hydrogenation of cinnamic acid to dihydrocinnamic acid as a model reaction. 1. Introduction

The different steps in preparing a precious metal catalyst influence the overall physico-chemical characteristics and are. correlated to catalyst activity and selectivity in the desired chemical reaction. One other requirement which has to be met, is to have a reliable support material. This property is difficult to achieve for activated carbons as they are natural materials, even after having reduced their ash content by treatment with mineral acids. The XPS surface characterization.of activated carbons as support for Pd-catalysts resp. the Pd/C-catalyst itself can allow a better understanding of how to control the catalyst preparation and the final catalytic activity.

362

Changes in the spectrum of oxygen containing surface functional groups seriously affect the adsorption properties of activated carbons /1/ and the precious metal dispersion /2, 3, 4 / . The spectrum of surface functional groups can be modified by controlled acid treatment /3/, 2.

Experimental Part

2.1

Catalyst Preparation

99,8 g (dry base) of HN03-washedt powdered activated charcoal produced from beechtree-wood with a BET surface area of 900 - 1.000 m/g, a pore volume of 0,9 - 1 , O ml/g and a mean particle size distribution (50 % value) of 22 p n , which was already treated with HNO3 under standard conditions, were suspended in water, impregnated with a 20 % by weight aqueous solution of tetrachloropalladium acid and heated to 80 OC. Then precipitation of the precious metal by treatment with a 10 % by weight aqueous NaOH solution lead to the desired active phase. After filtration and washing a catalyst with 1 % palladium by weight on activated carbon was obtained and used without further treatment. 2.2

H202 treatment of the HNO3 washed activated carbon

130 g of the above described activated carbon (dry base) were suspended in 800 ml water. Then 50 ml of a 30 % by weight aqueous H202 solution were added and the suspension stirred for further 4 hours. After filtration the activated carbon was washed with 5 1 of distilled water. 2.3

Catalyet testing 200 mg of a catalyst prepared as above described were given

to a solution of 10 g cinnamic acid in 120 ml ethanol. This

suspension was transferred in a 250 ml agitation reactor equipped with gas agitator (stirrer type BRI from Buddeberg GmbH), thermometer and H2-supply. The hydrogen was distributed at normal temperatur of 25 O C at a stirring speed of 2.000 rpm, through the gas agitator into the solution. The reaction took place under a H2-pressure of 10 mbar above atmospheric pressure.

363

The catalyst performances were measured in the hydrogenation of cinnamic acid to dihydrocinnamic acid and the catalyst activity given in ml H2/g catalyst/min. The reaction time for the hydrogenation reaction was 5 min in all experiments, and measured between the 3rd and 8th minute starting from the beginning of the H2-supply (Ref. 5). 2 . 4 XPS-measurement8

The XPS-measurements were conducted using MgKa-radiation at a power of 200 W. The base pressure of the spectrometer system was 4x10- 10 mbar and during the measurements 2 - 8 ~ 1 0 -mbar. ~ The purity of the vacuum system was steadily monitored by residual gas mass spectrometry (Quadrex 200, Leybold) to exclude artefacts. A Leybold EA 200 energy analyzer with multichannel detection was operated in the AE=const. mode at pass energies of 150 eV (survey scans for the detection of all elements present on the surfaces investigated ) and 50 eV (detail scans for the binding energy analysis). The samples were measured as powders, supported on goldcoated stainless steel holders. In all cases the area analyzed was 1,5 cm-2 to provide reasonable sampling statistics. For spectrometer calibration, the Au 4f712-signal at 83,8 eV repeatedly has been controlled. Changes of the XPS-signals during the measurements were not observed. Therefore, surface conditions which are stable under ultrahigh vacuum conditions were recorded. The XPS-data were treated as follows: subtraction of the Xray satellites, smoothing by second order polynomial fit, Shirley-type background subtraction, peak integration and correction by relative sensitivity factors (Cls:0,2, Ols:O,61, Nls:0,36, Pd3d5/2:2.5). Line shape analysis was performed by standard GauR/Lorentz-procedures. The relative surface concentrations of Pd and N reported in Tab. 1 were calculated, considering all elements detected including traces of contaminants such as Fe, Na etc. The partially overlapping Olsand Pd3p3/2-signals were separated by numerical line shape analysis. It has been confirmed by flood gun experiments, that the signal positions and peak shapes measured were not influenced by electrostatic charging effects.

364

3.

Results

and Discussion

D i f f e r e n t l o t s o f commercially a v a i l a b l e , s t e a m - a c t i v a t e d and

HNO3 washed c a r b o n s from b e e c h t r e e wood were u s e d as c a t a l y s t s u p p o r t . Tremendous c a t a l y t i c a c t i v i t y v a r i a t i o n s w e r e s t i l l found a f t e r i mp reg n atio n w i t h a H2PdC16-solution u s i n g t h e l o w p r e s s u r e hydr o g en atio n o f cinnamic a c i d t o d ih y d ro c in n a mic acid as a model r e a c t i o n /5/. I n Tab. 1 t h e r e s u l t s of c a t a l y s t a c t i v i t y measurements are compared t o t h e s u r f a c e c o n c e n t r a t i o n s of Pd a n d N measured by XPS for t h r e e d i f f e r e n t l o t s of s u p p o r t s before and a f t e r p r e c i o u s metal im p reg n atio n . The l a s t decimal of t h e s u r f a c e -

c o n c e n t r a t i o n v a l u e s i s g i v e n o n l y f o r n u me r ic a l p u r p o s e s . Table 1

S u r f a c e c o n c e n t r a t i o n of n i t r o g e n compounds, d e t e r m i n e d by i n t e g r a t i o n of t h e N l s XPS s i g n a l group and c a t a l y s t a c t i v i t y ( A ) measured i n t h e h y d ro g en atio n of cinnamic acid Lot 1 N4) A5)

Lot 2 Pd6)

N4)

A5)

Lot 3 pd6) N4) A5)

pd6)

A c t i vat ed Carbon : original (or. 1 or./E202 3, or. /HN03/E2022)

Pd Catalyst

0.18 3) 1.01

0.65 3) 1.18

0.89 3) 1.49

:

or. /Pdl ) or./8202/Pd or. / B N O ~ / )P ~ ~ or./EN03/H202/Pd2)

3, 3,

0.91 1.65

100 124 45 46

0.65 0.64 0.46 0.96

0.73 3, 0.55 1.22

35 62 17 45

0.73 0.64 0.81 0.75

1.53 3,

0.34 1.99

82 108 36 18

0.82 0.64 0.46 1.00

1 ) l o w c o n c e n t r a t i o n HNO3 e x p o s i t i o n ; 2 ) e x c e s s i v e H N 0 3 e x p o s i t i o n ; 3) below t h e d e t e c t i o n l i m i t of XPS ( 0 . 1 ) ; 4 ) s u r f a c e c o n c e n t r a t i o n ( % ) of n i t r o g e n compounds o b t a i n e d by i n t e g r a t i o n of t h e N l s XPS s i g n a l group ; 5 ) c a t a l y s t a c t i v i t y i n m l (H2)/g ( c a t . ) x min/4/ ; 6 ) s u r f a c e c o n c e n t r a t i o n ( % ) of p a l l a d i u m , n.b. c o n s t a n t Pd concentration/dispersion a f t e r H 2 0 2 regeneration. The s u r f a c e - n i t r o g e n c o n t r i b u t i o n s measured on t h e c a rb o n a c e o u s s u p p o r t s i n t h e " o r i g i n a l " - s t a t e , i . e . a f t e r t h e HNO3treatment c a n b e e f f i c i e n t l y removed by s u b s e q u e n t H 2 0 2 t r e a t m e n t . T h i s i s shown i n f i g . 1 where t h e N l s - s i g n a l - r e g i o n measured before and a f t e r H202-treatment i s shown ( l o t 3 ) . The a c t i v i t y of t h e c o r r e s p o n d i n g P d - c a t a l y s t s p r e p a r e d o n t h e H z O 2 - t r e a t e d l o t s 1-3 w a s s i g n i f i c a n t l y enhanced, compared t o

365

t h e Pd-impregnated o r i g i n a l samples which p a r t l y showed a n a d d i t i o n a l increase o f t h e N-surface c o n c e n t r a t i o n a f t e r Pdi m p r e g n a t i o n (Tab. 1 ) . The l o w a c t i v i t y o f l o t 2 f u r t h e r m o r e i s due t o t h e p r e s e n c e of traces of o t h e r s u r f a c e i m p u r i t i e s l i k e Fe, A l a nd Mg which p a r t l y c o u l d be removed by t h e a d d i t i o n a l H202-elution. On a l l c a t a l y s t s samples o b t a i n e d o n t h e HzOz-washed s u p p o r t s , t h e same Pd-XPS-signal i n t e n s i t y was measured (Tab. 1 ) . An a d d i t i o n a l e x c e s s i v e HN03-treatment of t h e o r i g i n a l s a mp le s for removing i n o r g a n i c trace co n tamina n ts a n d e n h a n c in g t h e s u r f a c e c o n c e n t r a t i o n o f active C/O-functional g r o u p s i n t h e p r e s e n t cases o n l y l e a d t o a d e c r e a s e o f t h e h y d r o g e n a t i o n a c t i v i t y o f t h e f i n a l c a t a l y s t s (Tab. 1 ) . Under t h e s e circums t a n c e s a s u b s e q u e n t HzO2-application c o u l d n o t remove t h e s u r f a c e n i t r o g e n c o n t r i b u t i o n s . Furthermore, a p o s i t i v e i n f l u e n c e on t h e h y d ro g en atio n a c t i v i t y of t h e P d - c a t a l y s t s w i t h h i g h e r s u r f a c e - n i t r o g e n c o n c e n t r a t i o n s c o u l d n o t be observed (Tab. 1 ) . The c ha nge s i n t h e h y d ro g en atio n a c t i v i t y measured w e r e n o t correlated w i t h t h e p r e c i o u s metal surface- and b u lk -c o n c e n tra t i o n b u t w i t h t h e s u r f a c e - c o n t r i b u t i o n s of n i t r o g e n . F ur t he r m or e , no s y s t e m a t i c v a r i a t i o n s of t h e Pd-valency w e r e detected. T h i s i s shown i n f i g . 2 whe r e t h e P d - s ig n a l measured on t h e c a t a l y s t s , o b t a i n e d on l o t 1 -s u p p o r t w i t h d i f f e r e n t p r e t r e a t m e n t are compared.

'

3000

t I

"*

----

1

n---

~-

Pd

n.

I

I

\r-*yyr,

Lo

a

u

I

1

420

LOO

binding energy I eV

380

1

F i g . 1. XPS:Nls-signal r e g i o n , measured on activated carb o n ( b e e c h t r e e ) . I. O r i g i n a l s t a t e , i.e. steam-activated, HNO3treated ( l o t 3, Tab. l ) . I I . As I , a f t e r s u b s e q u e n t H202-treatment. Complete removal of n i t r o g e n containing surface-functionalgroups .

,

355

350

315

3LO

335

330

binding energy I eV 1

Fig . 2 . XPS:Pd3d5/2,3/2-signal group, measured on Pd/C-catal y s t s , o b t a i n e d on l o t 1 a c t i v a t e d c a rb o n s u p p o r t s (Tab.1). No systematic c o r r e l a t i o n s between XPS-peak shapes and p o s i t i o n s and t h e c a t a l y s t a c t i v i t y (Tab. 1 ) .

366

Generally, the surface contributions of traces of inorganic contaminants were lowered by H202- and enhanced in the case of excessive HN03-treatment. From fig. 1, trace I, qualitatively it can be derived, that different binding states of nitrogen were present on the carbonaceous supports. In Tab. 2 . , results of line shape analyses for roughly estimating the relative amounts of different nitrogen-containing surface groups present on the surface of lot 1 after different pretreatments are compared to the oxygen- and nitrogenconcentrations. Table 2 Results of Gauss-Lorentz-line shape analysis on the Nls-signal group measured on Lot 1 specimens. Binding energy and %contribution of the signals, forming the whole Nls-group measured. Surface concentration of oxygen and nitrogen (Os, Ns).

original % (or*1 eV

or. /H202

amine

amine/ ammonium

nitrite/ nitrate

25 398.6

26 401.1

-

-

NS

OS

49 401.1/406.9

0.65

4.63

-

-

6.50

or./H~O3 % (5 min) eV

22 398.8

33 400.8

45 405.8

1.37

7.74

or./HNO3 % (60 min) eV

traces 17 398.1 399.9

83 405.9

0.94

7.77

94 406.5

0.94

8.89

1.18

7.91

o r . / ~ ~ O 3% (180 min) eV or./AN03 % (180 min) eV (HZ021

-

6 400.1

-

100” 405.4/406.8

Reference Data Nls (eV) as given in : D. Briggs, M.P. Seah (ed.), Practical Surface Analysis Vol. 1 J. Wiley, Chichester, 1990, p. 599 ; C.D. Wagner, W.M. Riggs, L.E. Davis, J.F.Moulder, G.E. Muilenberg (ed.), Handbook of XRay Photoelectron Spectroscopy, Perkin Elmer, Eden Prairie MI, 1978, p. 40 SigNq 397.7 ; PhNH2 399.2 ; BN 398.1 ; NHqCl 401.7 ; NH3 398.8 ; NaNO2 403.8 ; B u N H ~398.7 ; NaN03 407.3.

367

In the original state, amine-, ammonium- and nitritehitratelike compounds were observed. With increasing the HNO3exposition-time, the nitrate-contributions increase and the amine/ammonium-contributions decrease. The surface-concentration of oxygen can be enhanced. The same trend can be seen for the surfaces of lot 2 and 3 samples in Figs. 3 and 4. Nls

Fig. 3. XPS:Nls-signal region, measured on lot 2 samples. Activated carbon, after 5 min (I) and 180 min (11) of subsequent HN03-treatment. ,‘f

I 620

L 00

380

binding energy I e V l

L20

410 500 binding energy eV I

390

Fig. 4. XPS t Nls-signal region, measured on lot 3 samples. Activated carbon, after 5 min. (I) and 180 d n . (11) of subsequent HNOj-treatment.

368

4. Conclurion The controlled HN03-treatment of raw activated carbons is a standard technique to improve the purity of the carbonaceous supports and, in modifying the surface functional groups, the interactions between the support and the precious metal phase.. A n excessive HN03-exposition on the other hand can lead to a serious worsening of the catalytic properties of the activated carbon due to the formation of different nitrogen containing surface functional groups. This effect can be compensated by an additional H202-treatment, as far as the HN03-exposition does not exceed a critical limit. These results can be helpful to minimize the influence of lot-to-lot variations of diffent activated carbon lots in the manufacture of precious metal containing catalysts and therefore to better tailor the final catalyst to the desired characteristics. 5. References /1/

F. Rodriguez-Reinoso, M. Molina-Sabio and M. A. Muneceas, J. Phys. Chem. 1992, 96, 2707-2713

/2/

C. Prado-Burguete, A. Linares-Solano, F. RodriguezReinoso, and C. Salinas-Martinez de Lecea J. Catal. 1989, 115, 98 106

-

/3/

P. Albers, K. Deller, B. Despeyroux, A. Schlifer and K. Seibold, J. Catal. 1992, 133, 467 478

/4/

D. Richard and P. Gallezot in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors). Preparation of 81 Catalyst IV, Elsevier, Amsterdam, 1987, pp. 71

-

-

/5/

Determination of the hydrogenation activity of powdered supported palladium catalysts, Degussa standard procedure Nr. A 101 including the test procedure Nr. 1.1.1.1, Qualitgtssicherungshandbuch for Chemical Catalysts, Labormanual 1

M.Guisnet et al. (Editors), Heterogenwus Catalysis and Fine Chemicals Ill CD 1993 Elsevier Science Publishers B.V. All rights reserved.

369

Thiophene synthesis by dehydrogenation of tetrahydrothiophene on chromium catalysts A. Commarieua, E. Arretza, D. Duprezb and C. Guimonc a Groupement de Recherches de Lacq, ELF, BP 34,64170 ARTlX France b Laboratoire de Catalyse, URA 350,86022 POITIERS Cedex France

C

Laboratoire de Physico-Chimie Moleculaire, URA 474,64000 PAU France

Abstract Dehydrogenation of tetrahydrothiophene (THT) was carried out at 380°C, 1 atm on supported chromium catalysts (20 wt.-% Cr2O3). Different supports were investigated : A12O3, Si02, TiO2, C and the results were compared to those obtained with an unsupported chromium catalyst. The following rank of activity was obtained (mmole h-lg-1) : Cr/C, 33 > Cr/A1203, 12 > Cr/SiO2, 2-12 == Cr/TiO2, 7. The selectivities to thiophene, relatively constant in the 40-100% range of conversion,were higher for C, Si02 and Ti02-supported catalysts (87-92%) than for Cr/A1203 (79%). The catalysts were characterized by XRD, XPS, TPR and oxygen uptake. The activity depends on the crystallite size of Cr2S3 (very small on Cr/C), on the degree of sulfidation (residual 02- ions being poison of the reaction) and on the surface reduction of chromium (CrII species).

1. INTRODUCTION Thiophene is utilized in Organic Chemistry as an intermediate in the synthesis of pharmaceuticals and of phytoproducts. Moreover since the go's, the polymerization of thiophene has been extensively studied, pol ythiophene being a conductor polymer with potential application in Electrochemistry.

370

Thiophene can be prepared (i) by sulfurization-cyclization of hydrocarbons (butane, butene or butadiene) or of alcohols (butanol, butanediol-1,4) (ii) by substitution of the oxygen by sulphur in the molecule of furan or (iii) by cyclization of linear sulfided compounds (diethylsulfide for instance). In the processes, the selectivity into thiophene is generally lower than 70% [l-41. An alternative way for thiophene synthesis is the dehydrogenation of tetrahydrothiophene (THT). This molecule is easily produced, with an excellent yield, by oxygedsulfur substitution in tetrahydrofuran. THT dehydrogenation was studied by different authors on sulfided Ni, Mo or Co-Mo catalysts. Recently Lacroix et al [5] performed an extensive study of the reaction on unsupported sulfides. In this paper we present a kinetic study of THT dehydrogenation on supported chromium catalysts which were, concurrently with ruthenium ones [ 5 ] , the most promising catalysts for this reaction. Two main reactions can occur : Dehydrogenation (DH) :

4 2 -H2 THT -> Dihydrothiophene -> Thiophene

Desulfurization (DS) :

- H2S THT > Butadiene

H2 ->

(1)

Butene, Butane (2)

The catalysts were also characterized by different techniques before and after their use in reaction.

2. EXPERIMENTAL

Six supports were used : GFS = RhBne Poulenc GFS alumina (200 m2g-1, impurities < 500 ppm). A = RhBne Poulenc A alumina (350 m2g-1, 0.4% Na), C = Picatal E 612 carbon (950 m2g-1,0.7% K), DBM = RhBne Poulenc DBM 250 silica (250 m2g-1, 0.2% Na), AERO = Degussa Aerosil silica (200 m2g-1, impurities < 500 ppm), P25 = Degussa P25 titania (50 m2g-1, anatase + rutile, impurities c 1000 ppm). The supports were peptized (AERO and P25), crushed and sieved to 0.1-0.2 mm. The catalysts were prepared by impregnation of these supports with aqueous solutions of chromium salts (generally, the nitrates). They are refered to as CrnX, where n is the percentage of chromium oxide (equivalent Cr2O3) and X the code of the support. They were calcined at 450°C for 4h and in situ sulfided in a stream of 10% H2S/H2 for 4h at 500°C and cooled to the ambient temperature in the sulfiding mixture. Without any contact with air, they were heated to the operating temperature under a 1.38 v o I . 4 H2S/N2 flow. The reaction was carried out at 380°C in a microreactor (THT, 5.2 torr; H2S, 10.4 torr; N2, 745 totr; weight hourly space

371

velocity : 0.3 to 1.2 h-1). The products were analyzed by G.C. using a PORAPLOT Q capillary column (25 m, i.d. 0.32 mm, 60-200°C, 6°C min-1). XRD characterizations were carried out in a Siemens D 200 diffractometer. JCPDS sheets 06-0504 of Cr2O3 and 10-0340 of Cr2S3 were used as references. XPS experiments were carried out in a SSI-301 spectrometer (source : AlKa at 1486.6 eV, 120 W). The spectra were standardized by using hexatriacontane as a reference (Cls at 284.6 eV). The major peaks of the supports were Cls at 284.2 eV for C, Si2p at 103.7 eV for AERO and at 104.0 eV for DBM, and A12p at 74.4 eV for A. Pure compounds (Cr2O3, Cr2S3, CrC13 and CrC12) were also used for measuring binding energies of Cr2p3/2 in different valence states and in different chemical environments. Oxygen chemisorptions were carried out on the calcined catalysts reduced in H2 (temperature-programmed reduction from 25 to 500°C at 4°C min-1) in order to evaluate the surface reducibility of chromium ions. The same technique was applied to sulfided catalysts in accordance with a method already used with NiMo [6] and CoMo [7] catalysts.

3. RESULTS AND DISCUSSION 3.1 Activity and selectivity of the bare supports The reaction was first carried out on the sulfided supports (no Cr added). The results are shown in Fig.1. The two silicas (AERO and DBM) and the carbon which were found totally inactive are not represented on the figure.

U

u)

w

Conversion

DHT

k

2 I-

el

0 w

Thiophene c4

-I

w

u)

A1203

C

A1203 A

Ti02 P 25

Fig. 1 : Activity and selectivity of the sulfided oxides (no Cr added). Catalyst weight : 120 mg. DHT = dihydrothiophene,C4 = butenes+butadiene

372

The two aluminas and the titanium oxide show a significant activity in THT conversion. Nevertheless the selectivity to thiophene is relatively poor particularly on aluminas (C4 formation). Active sites of the support (probably - SH groups) catalyze desulfurization at the expenses of dehydrogenation, thus confirming the high reactivity of THT (compared to thiophene) in desulfurization [8]. By-products (C1C3 hydrocarbons, alkylthiophenes and butylmercaptan) are produced in significant proportion (15-20%) on A1203 while Ti02 was the most active and selective support in dehydrogenation.

3.2 Activity and selectivity of the sulfided chromium catalysts Two series of experiments were carried out : at a constant space velocity (120 mg) and at a conversion close to 5096, obtained by changing the catalyst weight. The catalytic properties of an unsupported chromium oxide (29 m2g-1) were also determined. The results are given in Table 1. Table 1 Activity and selectivity of the sulfided chromium catalysts (1h-on-stream). Catalyst or precursor

Weight

Conversion %

0 2 0 3 (bulk) Cr20 A Cr20 GFS Cr20 DBM Cr20 AERO Cr20 c Cr20 P25

120 120 120 120 120 120 120

97.6 96.6 9x3 44.4 91.0 99.5 82.7

0 2 0 3 (bulk) Cr20 A Cr20 AERO Cr20 c

25 30 30 10

54.0 52.2

52.4 48.1

Activity (mrnol/g cat.h)

-__ __

Selectivities Thiophene DHT

C4 others

7.3

91.6 81.9 70.8 87.4 93.0 88.5 87.0

0.1 0 0 3.0 0.1 0 0.5

10.8

0.9 1.5 10.0 3.3 1.1 7.5 1.7

15.5 12.3 12.4 32.8

90.5 79.5 89.4 89.4

2.0 2.4 3.0 2.6

6.9 17.4 6.7 5.6

0.6 0.7 0.9 2.4

2.4

---

7.4 16.6 19.2 6.3 5.8

4

Chromium sulfide is the active component for the dehydrogenation : alumina which possesses desulfurization sites, confers a poor selectivity to the Cr/A1203 catalyst. We can note the very high activity of the Cr/C catalyst. The selectivity to dihydrothiophene (DHT) is always higher at a 50% conversion than at high conversion (80-100%). Results obtained at low conversion [9] confirm this tendency, the selectivity to DHT reaching 15-2096 at a 10-20% conversion, in accordance with the consecutive scheme (Eqn. 1) proposed for the dehydrogenation.

373

3.3 Stability of the sulfided chromium catalyst The changes in relative activity with time-on-stream are represented on Fig.2. The initial conversions are those, close to 50%, given in Table 1.

TIME (h) Fig.2 Deactivation of the supported chromium catalysts The following order of the catalyst stability is obtained : Cr20 DBM > Cr20 A > Cr20 C > Cr20 AERO. At this stage, no correlation can be found between the rates of deactivation and the nature of support (the two silicas DBM and AERO lead to catalysts with quite different stabilities). This stability probably is a complex function (i) of the morphology of catalyst, (ii) of its state of sulfidation and (iii) of the formation of coke precursors (particularly butenes and butadiene) during the reaction 191.

374

3.4 Catalyst characteristics The results obtained with the different methods of characterization are given in Table 2. The sulfidation is not total on Si02-supported catalysts while approaching 100% on Cr20 C. The extent of bulk sulfidation rs is an important parameter for the reaction : increasing rs by a prolonged duration of sulfidation increases catalyst activity (Table 2a). Even though there remains some chromium oxide in Cr/SiOz catalysts, the surface appears to be essentially sulfided (see XPS data in Table 2d : surface Cr2S3 and surface Cr2O3 can be distinguished by their differences in binding energies). TPR profiles (not represented here) show two main peaks : a lowtemperature peak in the 320-380°C temperature range and a high-temperature peak at 480-490°C. Hydrogen uptakes during TPR and oxygen uptakes after TPR (Table 2b) can be interpreted as follows : in the oxided catalyst, Cr is present in the tn valence state (with n 2 3, obtained probably by a mixture of t3 and t6 species). During TPR, chromium is essentially reduced in Cr2O3 plus a small amount of CrII species (x), so that : WCr = n - 3 t x

(1)

After TPR, these CrII are re-oxidized into CrIII by oxygen : O/Cr = XI2

(2)

Except in Cr20 A (45% CrVI), chromium is essentially (CrlSiO2) or exclusively (Cr/C) present as Cr2O3 (Table 2c). There exists a correlation between the reducibility of Cr (measured by the percentage of CrII after TPR) and the catalytic activity. A similar correlation is obtained with the O/Cr values measured after sulfidation. Owing to differences in the temperature programme (4h at 500°C in H2S/H2 versus l h at 500°C in HYAr after the temperature rampes) oxygen uptakes are higher after sulfidation than after TPR. However, after a 4 h-reduction in H2, the ratio O/Cr for Cr20 C is very close (0.18) to the value found after sulfidation (0.21). It seems thus that, both on reduced and on sulfided catalysts, oxygen titrates coordinatively unsaturated sites (c.u.s.) of CrII which would be the active sites of dehydrogenation. XRD and XPS results show that chromium is highly dispersed on carbon even though a slight sintering occurs upon reduction and sulfidation. In the calcined catalyst, all the chromium is in the t3 state and the support favors the formation of CrII c.u.s., which makes this catalyst particularly active in THT dehydrogenation. It was shown by XPS that the difference between the binding energies of Cr3+ and Cr2+ amounts to 1.9 eV (Cr2p3/2 at 577.4 eV for CrC13 and at 575.5 eV for CrC12. Nevertheless, owing to the high sensitivity of sulfided catalysts to air, CrII species could not be detected by this technique despite the use of a glove box coupled with the introduction chamber of the spectrometer.

375

Table 2 Catalyst characteristics (a) Extent of sulfdation rs (96 Cr2O3 ->

Cr20 DBM Cr20 AERO Cr20 c

29% 50% 1s = 100% IS= IS=

Cr2S3) versus activity A (mmol h - I g - 1 )

A = 2.4 A = 12.4 A = 32.8

IS 1s

= 65% = 73%

A = 3.8 A = 16.8

(b)Reducibility of chromium species (oxided catalysts) HICr VPR) 0.165 0.094 1.37 0.18

Cr20 DBM Cr20 AERO Cr20 A Cr20 c

(c)x

OICr (after TPR) 0.007 0.032 0.009 0.09

n

% CrII

3.15 3.03 4.35 3.00

1.4% 6.4% 1.8% 18%

OICr (after sulf.) 0.005 0.040 0.071 0.21

m:

Cr203 180-200 %, cristallites on the two silicas Cr2S3 evident on the two silicas, particularly on 0 2 0 AERO. Cr203 remains visible after sulfidation. No clear diffractogram of 0 2 0 3 and Cr2S3 on Cr20 C (well-dispersed Cr) Oxided : Sulfided :

-

(d) XPS :surface state of chromium State Cr20 DBM

calcined reduced (H2) sulfided Cr20 AERO calcined reduced sulfided Cr20 c calcined reduced sulfided Cr203 unsupp.,calc. 20 Cr203+80 Si02 mech. mixt. 20 C12O3+8OC mech. mixl. Cr2S3 unsupp.,re-sulf.

Cr2p.712 (eV) B.E. width (at Hl2) 576.3 3.3 575.8 3.1 574.0 3.0 3.0 576.4 576.4 2.9 574.6 3.9 576.3 2.6 576.2 2.7 575.0 2.4 2.9 576.4

CdSi (Cr20 Si02) or CdC ((2120 C) 2 20 4 1 2 2 20 11 6 (S/Cr=2.6)

__

19.5 4

574.5

2.1

--

376

4. CONCLUSIONS Chromium sulfide is a selective catalyst of THT dehydrogenation minimizing the C-S bond cleavage. The activity seems to be correlated to the reducibility of chromium into CrIl species. Carbon is an excellent support promoting both the dispersion of the active phase and favoring the creation of coordinatively unsaturated chromium species (probably CrII species). However further investigations are required to improve its stability.

5. REFERENCES

U.S. Patent 3 939 179 to Pennwalt (1974). U.S. Patent 3 822 2x9 to Tar Distillers (1973). U.S. Patent 4 143 052 to SNEA(P) (1979). Ger. Offen. 1 224 749 to BASF (1964). M. Lacroix, H. Marrakchi, C. Calais, M. Breysse and C. Forquy, in M. Guisnet el al., Ed., 2nd Int. Symp. Heter. Catal. Fine Chem., Poitiers, 1990, Stud. Surf. Sci. Catal. Vo1.59, Elsevier, Amsterdam, 1991, p.277. S. Brunet, S. Karmal, D. Duprez and G. Perot, Catal. Lett., 1 (19x8) 255. S. Karmal, Thesis, Poitiers (l98X) W.R. Moser, G.A. Rossetti Jr, J.T. Gleaves and J.R. Ebner, J . Catal., 127 (1991) 190. A. Commarieu, Thesis, Poitiers (1990).

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I11 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

377

Partial oxidation of water - insoluble alcohols over Bi promoted Pt on alumina. Electrochemical characterization of the catalyst in its working state T. Mallat, Z. Bodnar and A. Baiker Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092Ziirich, Switzerland

Abstract The oxidation of secondary alcohols to ketones by air has been studied in aqueous alkaline solutions, in the presence of dodecylbenzenesulfonic acid Na salt detergent. The self-poisoning of a 5 wt% Pdalumina catalyst could be suppressed by Bi deposition onto the Pt particles. The origin of catalyst deactivation (self-poisoning or over-oxidation) was investigated by measuring the potential of the catalyst slurry during the oxidation reaction. It is suggested that the over-oxidation of Pt is the result, not the reason of catalyst deactivation. The reliability of the potential measurement in the presence of a non-conductive support is discussed. The catalyst potential was found to provide useful information for controlling the rate of oxygen supply, which helps to avoid the over-oxidation of the active sites. After optimization of the economically and environmentally important variables, 97-99 % conversion and 95-100 % selectivity could be achieved in the oxidation of waterinsoluble alcohols. 1. INTRODUCTION

The liquid phase oxidation of alcohols on platinum metal catalysts can be carried out under mild conditions and with air as oxidant. Heptane, ethyl acetate or 2-butanone have been suggested as solvents for water-insoluble alcohols [ 11. However, only the aqueous phase oxidation is of practical importance, due to safety reasons [2]. The possibility of applying a "water-detergent'' system for water-insoluble substrates will be shown here using the example of the selective oxidation of secondary alcohols to ketones. A serious drawback of the process is the catalyst deactivation, which results in low reaction rates and the necessity of a relatively high catalyst/substrate ratio. There are numerous reports in the literature on catalyst deactivation, attributed variously to overoxidation of the catalyst [3-51,irreversible adsorption of by-products [6-81 or dissolution and re-deposition of Pt [5]. It has been suggested that the over-oxidation of active sites can be avoided by working at low and constant level of dissolved oxygen [9].

378 We propose here another possibility: controlling of the oxygen supply on the basis of potential measurement of the catalyst slurry during reaction. This technique provides direct information on the oxidation state of the active sites [ 10.1 11. The key question of the measurement is, whether the potential indicated by the collector electrode is the same as the mean potential of the catalyst particles. There are some basic rules which help in the selection of the proper electrode [12,13]. The electrochemical reaction at the surface of the electrode should be negligible compared to the reactions at the catalyst surface to avoid its influence on the potential response. The size (capacity) of the electrode, the frequency of collision and the resistance of the electrolyte and the electronic contact have also an influence on the measured potential [ 141. Most of the above knowledge on the "slurry electrodes" concerns unsupported or carbon-supported metal catalysts in the presence of hydrogen. The oxidation of secondary alcohols to ketones is more complex: the noble metal catalyst is usually covered by hydrogen and by organic substances, and there is dissolved oxygen and an alcohol/ketone mixture in the aqueous phase. The catalyst potential is a mixed potential [ 151 made up of contributions from the alcohol oxidation reaction, the oxygen reduction reaction and possibly also the side reactions, including the oxidation of the by-products. A further unknown factor is the role of non-conductive supports on the potential measurement. The aim of our work was to reveal the reliability and the limitations of the potential measurement and to show the usefulness of this information for interpreting the catalyst deactivation and optimizing the catalyst composition and reaction variables. 2. EXPERIMENTAL

The Bi-promoted catalyst (0.75 wt% Bi + 5 wt% Pdalumina; Bi/PtSwf = 0.50) was prepared by consecutive reduction of Bi onto a commercial 5 wt% Pt/alumina (Engelhard Escat 24, D = 0.30 determined by TEM). Preferential deposition of a Bi sub-monolayer onto Pt has been confirmed by TEM-EDX and electrochemical polarization methods [S]. An unsupported Bimt catalyst (Bi/Pt = 0.06) for cyclic voltammetry was prepared similarly. A commercial 5 wt% Bi + 1 wt% Pt + 4 wt% Pd/C catalyst was used as delivered (Degussa CEF 196 R A W ) . The oxidation reactions were performed in a glass batch reactor, equipped with magnetic stirrer, reflux condenser, thermometer and electrodes. The catalyst was prereduced in situ in a nitrogen atmosphere (30 min) with the alcohol substrate (3.0 g) in 30 mL aqueous alkaline solution of the detergent dodecylbenzenesulfonic acid sodium salt. The oxidant was air. The reactor worked in a mass transfer limited re ime, controlled by the air flow rate (2-20 mL.min-') and the mixing rate (1000-1500 mid ). Conversion and

7

selectivity were determined by GC analysis. More details can be found elsewhere [7,8]. The reaction conditions are listed in Table 1. if not otherwise stated. The electrochemical cell and polarization method used for cyclic voltammetry have been described previously [16]. 2 mg catalyst powder on a carbon paste electrode was polarized in a 0.1 M aqueous borax solution. The potential of the catalyst slurry was measured with a Pt md collector electrode against a Ag/AgCI/KCl,, reference electrode, unless otherwise stated. All the potentials in the paper are referred to a hydrogen electrode in the same solution.

379

3. RESULTS AND DISCUSSION 3.1. Measurement of the catalyst potential The possibility of measuring the real potential of a catalyst containing a nonconductive support was studied in preliminary experiments. In a hydrogen atmosphere 5 mg catalyst was enough to measure the real (theoretical) value with a Pt rod collector electrode (Fig. 1). The response time was short (<1 min), while an Au electrode had 10-30 min response time even with 10-20 mg catalyst. Note that a much higher amount of catalyst was used in the oxidation experiments. Ag and glassy carbon electrodes gave poor results. E.

E. V

0.1 l

0

20

40

60

80

Catalyst, mg

Figure 1. The influence of the amount of Pvalumina catalyst and the electrode material on the measured catalyst potential (74 O C , 0.1 g Li,CO,).

30

v

.

l

50

.

l

70

.

1

90

2L

110

130

Time. min

Figure 2. Potential fluctuations during the oxidation of diphenyl carbinol (0.1 g Bfldalumina).

In the presence of air and an alcohol/ketone mixture it was difficult to find a reference value to which the measured potential could be compared. A carbon paste electrode containing 5-20 wt% catalyst in graphite [ 161 could be used as a reference when the reaction rate and mixing speed were low. During the oxidation of 2-butanol to 2-butanone the potential of the Pt electrode laid within a f 10 mV potential range. However, higher deviations up to 50-100 mV were found when the electrode surface was contaminated with by-products. In general, the catalyst potential during alcohol oxidation was accepted only if the cleaning and renewal of the Pt electrode surface did not result in a different value. Au, which is also frequently used as a collector electrode, usually indicated a positive

380 deviation of 50-200 mV from the values measured by Pt. Note that for correct potential measurement it was recommended that the same material is used for the collector electrode and the catalyst [131. In some cases considerable fluctuations of the measured value were observed at 300-400 mV (Fig. 2). Note that this is the upper limit of hydrogen sorption on Pt (Fig. 3). At potentials higher than about 500 mV the oxidation of Pt becomes considerable and the fluctuation disappears. It is supposed that these fluctuations are due to adsorption and subsequent removal (oxidation) of organics.

3.2. Catalyst deactivation We suggest that there is a causal sequence between the three types of catalyst deactivation described in the literature [3-81: Chemical deactivation

+

over-oxidation

surface restructuring

The primary reason is usually the formation and irreversible adsorption of by-products ("chemical deactivation" or "self-poisoning"). Poisoning species may be formed during the initial adsorption of alcohol on Pt [17]. The formation of linearly or bridge bonded -CO during the adsorption of various alcohols has been proved by in situ spectroscopic methods. We found that the aldol dimerization and the oxidation with C-C bond cleavage of the ketone product are the origins of further contamination during the oxidation reaction [71. The over-oxidation of Pto active sites occurs if the rate of the surface chemical reaction is lower than the rate of oxygen supply [3,4]. We propose that in most instances the self-poisoning of the catalyst is the reason for the low rate of surface chemical reaction. Consequently, the over-oxidation of Pt is not the reason, but the result of the deactivation process. Note that most of the authors, who claimed that catalyst deactivation was due to over-oxidation of the active sites, had never measured the oxidation state of their catalyst [e.g. 3,4,181. We propose that there is a strong correlation between the role of promoter, the oxidation state of the catalyst and the catalyst deactivation, as it will be shown in the following section. The next stage of deactivation is the partial dissolution of the oxidized (M"') metal and a subsequent re-deposition onto bigger particles, resulting in an increase in particle size and irreversible deactivation 151.

3.3. The role of Bi promoter The influence of Bi promotion on the chemisorption properties of Pt is shown in Fig. 3. The electrochemical polarization curves in aqueous alkaline solution indicate that Bi adatoms suppress the hydrogen sorption on P&(0-0.45 V) and that the oxidation of the promoter by OH adsorption becomes considerable above 0.6 V [19]. The influence of Bi promotion on the oxidation of a-tetralol to a-tetralone is shown in Fig. 4. The unpromoted PValumina catalyst rapidly deactivates and only 34 % conversion is achieved in 5 h. The oxidation of the catalyst surface during reaction is clearly shown by the catalyst potential. There is no hydrogen on the surface from about 10 % conversion on (E > 0.45 V) and the OH coverage increases continuously up to complete deactivation. The by-product formation is suppressed but not eliminated by Bi promotion,

381

as indicated by the high catalysJalcoho1 ratio (10 wt%) necessary to achieve total conversion. There was some by-product desorption during reaction, which was detected by GC analysis. Nevertheless, the Bi/Pt/alumina catalyst is in a reduced state (E < 0.4 V) up to almost total conversion. It seems that the side reactions require larger active site ensembles than the alcohol oxidation reaction, which is in accordance with literature data [17]. This, together with the possible influence of the suppression of hydrogen sorption on Pt, results in a diminution of chemical deactivation. Rate, rnrnol rnin'

E. V

I, mA

0.15

Bi/P 0.:

h

0.1

0.05

0

1 0 .e

, ,

0.E

I

0.4

0.2 (

C 0

0.4

1.2

E, V

Figure 3. Positive sweeps of the voltammograms of Pt (----) and Bi/Pt (-) catalysts; (v = 0.5 mV.s").

20

40

60

a0

loo

Conversion, %

Figure 4. The role of Bi promotion on the rate of a-tetralol oxidation and on the catalyst potential.

The reaction shown in Fig. 4 was repeated using the same Bi/Pt/alumina catalyst after filtration and washing with water. In the second and third runs the reaction rate was lower, and the catalyst potential higher, than in the first one (Fig. 5). This is exp!ained by successive contamination of the catalyst surface resulting in a lowering of the conversion under otherwise identical conditions. Much higher reaction rates (lower catalyst loading necessary for total conversion) were achieved when the self-poisoning of the catalyst was negligible. An example is the partial oxidation of diphenyl carbinol. in which only a 1 wt% catalyst/substrate ratio was used (Fig. 6). As expected, the reaction rate and the catalyst potential increased with higher oxygen content of the gas phase. The surface chemical reaction became the rate limiting

step in 100 % oxygen and the catalyst deactivated above 80 % conversion. Unfortunately, the catalyst potential could not be measured reliably in the latter case. E. v

Rate. mmol inin'

E. v

Rate. mmol inin' 1 1 . 0

~0.1

-0.05

-0

0

20

40

60

80

100

Conversion, % Figure 5. Reaction rate and catalyst potential in the oxidation of a-tetralol, in the first, second and third runs.

0

20

40

60

80

100

Conversion. % Figure 6. The influence of oxygen partial pressure on the rate of diphenyl carbinol oxidation and on the catalyst potential (0.1 g Bi/Pt/alumina).

3.4. Optimization In the optimization process both the "one parameter at a time" method and a factorial experimental design were used. Our aim was to find an economic (low catalyst/alcohol ratio) and environmentally friendly ("low salt effluent") technology. Some interesting points will be considered in the following section. The measurement of the catalyst potential was found to be a useful guide for finding a suitable reaction temperature range. The minimum temperature is that at which the catalyst is rapidly (within a few minutes) reduced by the alcohol under a nitrogen atmosphere. The maximum temperature is usually 20-40 O C higher and characterized by increasing significance of the side reactions. The catalyst potential was found to be a reliable parameter in controlling the rate of oxygen supply from the gas phase to the catalyst surface. In general, the reactor worked in a transport-limited regime to avoid the over-oxidation of the active sites. The rate of the surface chemical reaction decreased with time due to the lower substrate concentration and to the deactivation processes. At a certain conversion the chemical reaction became the rate

383 limiting step and the active sites were successively oxidized and the catalyst deactivated. The oxidation of the catalyst is clearly indicated by a rapid increase in its potential. The transport limited regime could be restored simply by decreasing the rate of oxygen supply (air flow rate or mixing speed). Several type of detergents were tested concerning their oxidation stability, their influence on the reaction rate and the stability of the emulsion. Dodecylbenzenesulfonic acid sodium salt was chosen as a good compromise. The higher the pH of the aqueous solution, the higher was the reaction rate, but the correlation seems to be ambiguous. The nature of the anions and cations have also some influence on the reaction rate and selectivity, and the rate of aldol dimerization of the product increases with increasing pH. In the oxidation of several secondary alcohols an optimum in the catalyst composition Bi/Pt,, = 0.50 was found. The formation of a two-dimensional alloy on the surface efficiently sup, s e s the by-product formation and increases the conversion by a factor of 3 to 66. Some examples on the beneficial influence of Bi promotion are shown in Table 1. A three-component Degussa catalyst was used as a reference. This catalyst is, to our knowledge, the only commercially available alcohol oxidation catalyst, which has been developed for the selective oxidation of glucose to gluconic acid [20]. We suppose that the superior behavior of our bimetallic catalyst in the reactions studied is due to the more homogeneous distribution of Bi on Pt. Table 1 Oxidation of some secondaq alcohols to ketones over Bi-promoted Pt, unpromoted Pt and a reference catalyst (Degussa). Reaction conditions

a-tetralol

Diphenyl carbinol

I-pknylethanol

CatalysVROH (w/w) Li,COJROH (w/w) DetergenVROH (w/w) Temperature (OC) Reaction time (h)

0.10 0.0 1 0.05

0.01 0.01 0.003 75 3

0.02 0.01 0.01 60 4.5

85 5

Catalysts

Results

a-tetralol

Diphenyl carbinol

l-phenylethanol

Bi/Pt/alumina

Conversion (%) Selectivity (%)

99 95

99 100

97 99.5

Pdalumina (Engelhard)

Conversion (%) Selectivity (%)

34

Bi-Pt-Pd/C (Degussa)

Conversion (%) Selectivity (%)

66 89

1.5

7.5

99

54

79.5 99.5

384 4. CONCLUSIONS

The present studies indicate that the measurement of the "real" potential of oxidesupported bimetallic catalysts during the aqueous phase oxidation of alcohols is possible. The catalyst potential provides information concerning the oxidation state of the catalyst, which is essential determining the role of promoters and the nature of the deactivation processes. The in-situ electrochemical measurements were found to be helpful in the optimization of the process variables and in controlling the rate of oxygen supply.

5. REFERENCES 1. 2. 3. 4.

5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20.

K. Heyns and L. Blazejewitz, Tetrahedron, 9 (1960) 67. H. van Bekkum, in F. W. Lichtenthaler (ed.). Carbohydrates as Organic Raw Materials, VCH, Weinheim, 1990, p. 289. J. M. H. Dirkx and H. S. van der Baan, J. Catal., 67 (1981) 14. P. J. M. Dijkgraaf, M. J. M. Rijk, J. Meuldijk and K. van der Wiele, J. Catal., 112 (1988) 329. Y. Schuurman, B. F. M. Kuster, K. van der Wiele and G. B. Marin, Appl. Catal. A: Gen., 89 (1992) 47. P. C. C. Smith, B. F. M. Kuster, K. van der Wiele and H. S. van der Baan, Appl. Catal.. 33 (1987) 83. T. Mallat, A. Baiker and L. Botz. Appl. Catal. A: Gen., 86 (1992) 147. T. Mallat, Z. Bodnar. A. Baiker, 0. Greis, H. Sotibig and A. Reller, J. Catal., (in press) . P. Vinke, W. van der Poel and H. van Bekkum, in M. Guisnet et a1 (eds.), Heterogeneous Catalysis and Fine Chemicals 11, Studies in Surface Sience and Catalysis, Vol. 59, Elsevier, Amsterdam, 1991, p. 385. E. Miiller and K. Schwabe, Kolloid. Z., 52 (1930) 163. K. Heyns and H. Paulsen, Angew. Chem., 69 (1957) 600. J. Held and H. Gerischer, Ber. Bunsenges. Phys. Chem., 67 (1963) 921. J. F. van der Plas, E. Barendrecht and H. Zeilmaker, Electrochim. Acta, 25 (1980) 1471. B. Kastening. Ber. Bunsenges. Phys. Chem., 92 (1988) 1399. C. Wagner and W. Traud, Z. Electrochem., 44 (1938) 391. T. Mallat, T. Allmendinger and A. Baiker, Appl. Surf. Sci., 52 (1991) 189. R. Parsons and T. VanderNoot, J. Electroanal. Chem., 257 (1988) 9. H. Kimura. A. Kimura, I. Kokubo, T. Wakisaka and Y. Mitsuda, Appl. Catal. A: Gen., 95 (1993) 143. J. Clavilier, J. M. Feliu and A. Aldaz, J. Electroanal. Chem., 243 (1988) 419. B. M. Despevroux, K. Deller and E. Peldszus, in G. Centi and F. Triffiro (eds.), New Developments in Selective Oxidation, Studies in Surface Science and Catalysis, Vol. 55, Elsevier, Amsterdam, 1990, p. 159.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals Ill 0 1993 Elsevier Science Publishers B.V. All rights reserved.

385

SELECTIVE OXIDATION REACTIONS OVER VANADIUM SILICATE MOLECULAR SIEVES P.R. Hari Prasad Rao, K. Ramesh Reddy, A.V. Ramaswamy and P. Ratnasamy National Chemical Laboratory, Pune - 41 1 008, India.

Abstract The oxidation of various hydrocarbonssuch as n-octane,cyclohexane, toluene, xylenes and trimethyl benzenes over two vanadium silicate molecular sieves, one a medium pore VS-2 and the other, a novel, large pore V-NCL-1, in presence of aqueous H,O, has been studied. These reactions were carried out in batch reactors at 358-373 K using acetonitrile as the solvent. The activation of the primary carbon atoms in addition to the preferred secondary ones in n-octane oxidation and oxidation of the methyl substituents in addition to aromatic hydroxylation of alkyl aromatics distinguish vanadium silicates from titanium silicates. The vanadium silicates are also very active in the secondary oxidation of alcohols to the respective carbonyl compounds. V-NCL-1 is active in the oxidation of bulkier hydrocarbons wherein the medium pore VS-2 shows negligible activity. Thus, vanadium silicate molecular sieves offer the advantage of catalysing selective oxidation reactions in a shape selective manner. INTRODUCTION lsomorphoussubstitution of Al or Si by transition metal atoms in molecular sieves offers the possibility of shape selective redox catalysts. Titanium silicalites are the first such molecular sieves that have found commercial application in the hydroxylation of phenol to catechol and hydroquinone. Vanadium silicate molecular sieves with vanadium in the framework positions of both MFI [l-41 and MEL [5] structures have been reported recently. These vanadium silicates, like their titanium analogs, are active in the hydroxylation and oxidation of a number of organic substrates using H,O,. They catalyze the oxidationof alkanes under mild conditions with aqueous H,O, and, unlike the titanium silicates, are also able to oxyfunctionalize the primary carbon atoms of the alkanes to give corresponding primary alcohols and aldehydes [6]. Attempts at incorporating vanadium in a large pore molecular sieve led to the synthesis of vanadium-containing NCL-1 (V-NCL-1) NCL-1 is a novel, large pore, high silica zeolite (pore openings of r~ 0.7 nm), the synthesis of which has recently been reported [a]. Selective oxidation of bulkier hydrocarbons like trimethylbenzenes and naphthalenes catalysed by V-NCL-1 is an interesting application. This paper deals with the selective oxidation of n-alkanes and alkyl aromatic2 over two different vanadium silicates, one with MEL structure (medium pore), VS-2 and the other, the large pore molecular sieve, V-NCL-1,and demonstratesthepossibilityof carrying out shape selective oxidation reactions.

m.

386 EXPERIMENTAL

The hydrothermal crystallization of vanadium silicates was carried out using the following molar compositions: SiO, : 0.2 R,-OH or (R,N+ + NaOH) : x VO, : 30 H,O. Details of the synthesis and characterization of VS-2 and V-NCL-1are given in our earlier reports [5,7l. The samples were characterized by XRD, IR/FTIR, ESR, SEM, NMR and adsorption techniques. The catalytic oxidation of n-octane and cyclohexane with aqueous H202was carried out in a stirred autoclave (Parr Instruments, USA) of 300 ml capacity at 373 K under autogenous pressure. Typically, 100 mg of the catalyst, 2.53 g of 26 % (by wt.) aqueous HO ,, (alkane/&O, mole ratio of 3) and 5 g of alkane in 20 g of acetonitrile (solvent) were taken in the reactor. After completion of the reaction (8 h), 25 g of acetone was added to the products, which were then separated from the catalyst by filtration and analyzed. The oxidation of alkyl aromatics was carried out in batch reactors. 100 mg of catalyst was dispersed in a solution containing 1 g of reactant and 10 g of solvent (acetonitrile).The mixture was vigorously stirred and H,O,was added. After completion of the reaction (12 to 18 h), the products were separated from the catalyst and analyzed. All the products were analyzed by GC (HP 5880) using a capillary (cross-linked methylsilicone gum) column andflame ionization detector. The identity of some of the products was confirmed by GC/mass spectroscopy (Shimadzu, Japan, model GCMS-QP 200A). RESULTS AND DISCUSSION Characterization: All the vanadium silicate samples have been characterized to investigatetheir thermal stability, the location, co-ordination number and oxidation state of vanadium in different atmospheres by a variety of techniques. Some of the physical properties of representative vanadium silicates are given in Table 1. The oxidation reactions reported here are confined to calcined on air) samples of VS-2 and V-NCL-1 with SIN = 79 and 250, respectively. Absence of ESR signals due to paramagneticvanadium species indicatedthat all vanadium is in pentavalent oxidation state in these samples. For comparison, data on their respective silica polymorphs viz., silicalite-2 (S-2) and Si-NCL-1 are also included in Table 1. These samples as well as those with different SiN ratios, (Figs. 1 and 2) probably contain vanadium in the silicalite framework as may be inferred from the following observations:- 1) The unit cell parameters of V-samples increase linearly with vanadium content, as shown in Fig. 1 [5,-/1. On steaming at 923 K, a reduction in the unit cell parameter to the level of the vanadium-free silicalites indicates migration of vanadium (from the framework positions) in the steamed samples. Unit cell expansion has not been reported for VS-1 in previous studies [4] probabaly due to the low (less than 1%) incorporation of V. In all cases, the V4+is in non-tetrahedralpositions. Apparently, V ions influence the unit cell dimensions even when they are linked to the silicalite framework by grafting through surface silanol groups. 2) An absorption band around 965 cm-' (probably due to Si-0-V linkages) was observed in the IR spectra of all V-samples. The intensity of this band increases linearly with the vanadium content in the samples [5] (Fig. 2). This band is absent in the IR spectra of pure silicalite-2 and Si-NCL-1 samples as well as in silicalites impregnated with similar concentrations of

387 vanadium [5]. 3) The ESR spectra of the as-synthesized samples have well-resolved anisotropic 8-line hyperfine splitting indicating atomically dispersed vanadium. The integrated intensity of ESR signal increased with vanadium content in the samgles [5]. The 'g' values and hyperfine coupling constants (g = 1.932, g = 1.981; A,, = 185 G and AL = 72 G) are notably different from those observed for samples in which V02+are ion-exchanged into ZSM-5 [3]. It has been proposed that the vanadium ions are probably coordinated at defect sites where the concentration of Si-OHgroups is likely to be high (51. The sorption data given in Table 1 indicatethatthe pores of thevanadiumsilicates are free from any occluded materials (similar nitrogen and hydrocarbon sorption capacities of vanadium-incorporated and vanadium-free silicates) and that the pore dimensions of VS-2 and V-NCL-1 are different (from the sorption capacities for mesitylene).

,,

n

0

1 I 1

1

0.30 0

n

0-

1:;

7p

I

.

I

3

0

7

0

In

In c (

\

J

lrl

_1

(D

W

b, U

(0

- m c

'vz

I..

Lo

I

3

0.02

0.0 1

Figure 1. Unit cell volume vs. mole fraction of vanadium in VS-2 (a) and V-NCL-1 (b).

0.01

0.2

Figure 2. The ratio of intensitiesof 965/550 cm-' IR bands vs. mole fraction of vanadium in VS-2 (a) and V-NCL-1 (b)

Table 1 Physico-chemical properties of vanadium silicates

Catalyst

SIN

Surface

Micro pore

ratio

area, m2/g

volume,

Sorption capacity, wt.% Cyclohexane

Mesitylene

8.2 8.0 6.1 6.5

0.2 0.1 4.6 4.8

mllo

vs-2 Silicalite-2 V-NCL-1 Si-NCL-1

79 250

550 540 310 340

0.204 0.201 0.131 0.144

Unit cell

vo'Fet 5373 5354 2896 2864

388 Table 2 Oxidation of alkanes over vanadium silicate molecular sieves' Catalyst

VS-2 (SiN = 79)

Substrate

n-octane

V-NCL-1 (SiN = 250)

cvclohexane n-octane

cvclohexane

~~

Turnover numbe? &02selectivityc mole % Product distribution, wt.% 1-octanol 2-octanol 3-octanol 4-octanol 1-0ctanal 2-octanone 3-octanone 4-octanone cyclohexanol cyclohexanone othersd

2.3 43.4 4.6 5.9 4.6 3.8 3.2 21.7 18.0 13.8

_-

_244

2.0 33.0

4.6 39.5

__

2.1 8.8 4.8 5.2 3.6 22.5 21.1 15.0

33.3 60.7 6.0

-16.9

7.3 53.3

33.4 61.3 5.3

"Reaction conditions: catalyst = 0.1 g; alkane = 5 g; temperature = 373 K;alkane/H,O, (mole ratio) = 3; solvent (acetonitrile) = 10 g; reaction duration = 8 h. bMolesof reactant converted per mole of vanadium min-' H ' ,O, utilized for monofunctional product formation dMostlyoxygenates with more than one functional group and lactones Catalytic activity Oxidation of n-octane: The results on the oxidation of n-octane and cyclohexane, with aqueous H,O, over VS-2 and V-NCL-1 are presented in Table 2. The corresponding V-free silicalites and the silicalites impregnatedwith similar concentrations of vanadium have negligible activity for these reactions. It appears that only those vanadium ions which are incorporated through the hydrothermal synthesis of the vanadium silicates are active in these oxidation reactions. On both the vanadium silicates, the major products of oxidation of n-octane are the corresponding alcohols and their secondary oxidation products viz., the carbonyl compounds (aldehydes and ketones). An interesting observation is the formation of primary alcohols and aldehydes in addition to the secondary alcohols and their corresponding ketones. Thus, unlike titanium silicalites [9-111, vanadium silicates are able to activate the primary (terminal) carbon atoms of n-alkanes. This appears to be a characteristic feature of all vanadium silicates. The product distribution shows that the activation of carbon at the second position (among all the secondary C atoms) is preferred to others and follows the order, 2C > 3C > 4C > 1C. Among the two catalysts, the H,O, selectivity is marginally lower and the selectivity to mono-functional products is higher on V-NCL-1 than on VS-2. The secondary oxidation to aldehydes and ketones is very effective on both the catalysts. Interestingly,the ratio of total alcohols to (aldehydest ketones) in the product is 0.33 on both the catalysts. The formation of lactones and oxygenateswith more than one functional group was significantly lower over V-NCL-1.

389 Oxidation of cyclohexane: Compared to VS-2, V-NCL-1 is more active (TON = 7.3 vs. 2.0) in the oxidation of cydohexane to cydohexanol and cyclohexanone (Table 2). The YO, efficiency is also higher on V-NCL-1 (53.3 vs. 33.0). This superior performance may be partly due to the large-pore character and a lower concentration of vanadium in V-NCL-1. On both the catalysts, the mono-functional product selectivity is high (about 94-95%) and the cydohexanol to cydohexanone ratio is similar. The catalysts after separation from the products,washing and reactivation were found to be as active as the fresh, calcined samples.

Table 3 Oxidation of toluene over vanadium silicate molecular sieves' Turnover

HO ,,

Product distribution, wt. %

Catalyst

numbe?

selectivitf

benzyl alcohol

benzaldehyde

o-cresol p-cresold others"

vs-2 V-NCL-1

0.34 0.76

49.5 39.4

7.7 7.1

52.2 46.0

19.7 31.5

17.1 15.4

3.7

--

"Reaction conditions: catalyst = 0.1 g; toluene = 1 g; temperature = 358 K; toluene/H,O, (mole ratio) = 3; solvent (acetonitrile) = 10 g; reaction duration = 12 h. bhrlolesof reactant converted per mole of vanadium min" 'H,O, utilized (mole %) in the formation of benzyl alcohol, benzaldehyde and cresols. dContainsabout 1 wt.% of mcresol. "Mainly oxygenates with more than one functional group. Oxidation of Toluene: The oxidation of alkyl aromatics may lead to aromatic ring hydroxylation as well as the oxidation of side chain methyl substituents. With toluene, the cresols and benzyl alcohol are the primary oxidation products. Benzaldehyde is formed by the subsequent oxidation of benzyl alcohol (Table 3). Unlike titanium silicalites, the hydroxylation of the methyl substituent is the preferred reaction over both VS-2 and V-NCL-1. The selectivity to mono-functional products is almost 100 % on V-NCL-1 and 96.3 % on VS-2 (mainly o and p with small quantities of mcresol). However, the product distribution shows that between the two vanadium silicates, the formation of cresols, particularly o-cresol is more on V-NCL-1 than on VS-2 (the ortho position being more favourable for the aromatic electrophilicsubstitutionreaction). Being a large pore molecular sieve, there is no steric constraint for the formation of o-cresol on V-NCL-1. Indeed, the observed o/p ratio of about 2 is what is expected from purely electronic considerations in the absence of any product shape selectivity. VS-2, on the other hand, gives a ratio of about 1 for the two cresols, indicating the steric constraints in medium pore molecular sieves. The occurrence of benzylic oxidation products together with cresols suggests a Fenton type free radical mechanism. Another observation is the rapid oxidation of the initially formed benzyl alcohol to benzaldehyde on both the catalysts. However, further oxidation to benzoic acid was not observed under these conditions.

390 Table 4 Oxidation of xylene isomers over vanadium silicate molecular sieves' Catalyst Substrates

VS-2(SiN = 79)

V-NCL-1(SiN = 250)

exylene mxylene pxylene

0.1 Turnover number" 11.4 H202sel".mole % Product distribution, M.% 3-methylbenzyl alcohol -3-methylbenzaldehyde -2,6-dimethylphenol -2,4-dimethylphenol _-4-methylbenzyl alcohol 4-methylbenzaldehyde -2-methylbenzylalcohol 43.4 2-methylbenzaldehyde 43.8 12.8 otherd

0.06 14.5 34.7 49.5 15.8

-_ _-

__ ___ __

0.2 24.3

o-xylene mxylene 0.41 34.1

__

__

--

--

-_

__

32.1 49.6

__-

18.3

__

_-

__

-40.0 60.0

__

0.48 32.0 42.2 38.9 6.9 4.5

-_ ---

--

7.5

pxylene 0.44 36.1

__

__

--43.0 57.0

_--

__

"Reaction conditions: catalyst = 0.1 g; reactant = 1 g; solvent (acetonitrile) = 10 g; reactant/H202= 2 moles; temperature = 358 K; reaction duration = 18 h bMolesof reactants converted per mole of vanadium min". 'H,O, utilized for formation of methylbenzyl alcohols, corresponding benzaldehydes and dimethylphenols. dMainlyoxygenates with more than one functional group. Oxidation of xylenes: In the oxidation of xylenes one begins to see some differences betweenVS-2 and V-NCL-1 in termsof reactants shape selectivity. The resultsof the oxidation of o-,mand pxylenes on thetwo samples are summarised in Table 4. Firstly, the conversions on V-NCL-1 are higher than on VS-2 (TON of 0.41-0.48 vs. 0.05-0.2, respectively). On the medium pore VS-2, the H,O, selectivity decreases in the order, p > m- > o-xylene, indicating steric constraintsforthe three xylenes inthat order. Secondly,there are some vital differences in the product distribution. Irrespective of the difference in the pore dimensions of the two catalysts, and pxylenes do not undergo ring hydroxylation (no dimethylphenols were detected in the products). Apparently, none of the positions in the aromatic ring is activated for hydroxylation in these substrates. The oxidation of the methyl substituents in o-xylene leads to the formation of 2-methylbenzyl alcohol and 2-methyl- benzaldehyde. Similarly, oxidation of pxylene gives almost exclusively 4-methylbenzyl alochol and 4-methylbenzaldehyde on V-NCL-1. The selectivity to mono-functional products is 100 % for both these substrates on V-NCL-1. On the other hand, oxidation of mxylene leads also to the formation of dimethylphenols (by ring hydroxylation), in addition to giving predominantly 3-methylbenzyl alcohct and the corresponding benzaldehydes. These results indicate that the hydroxylation of aromatic ring (with H202as the oxidant) is probably an electrophilic substitution reaction while the benzylic oxidations arise probably through free-radical mechanism.

391

Table 5 Oxidation of 1,2,4- and 1,3,5-lrimethylbenzenesover V-NCL-1" Substrate

1,3,5-trirnethylbenzene

Turnover numbeP Y O 2 selectivity", mole % Product distribution, wt. % 2,4,6-trimethylphenoI 3,5-dimethylbenzyl alcohol 3,5-dimethylbenzaldehyde 2,4-, 2,5-, 3,4-dimethylbenzaldehydes 2,4-, 2,5-, 3,4-dimethylbenzyl alcohol othersd

1,2,44rimethylbenzene

0.4 33.0

0.3 28.9

12.7 27.2 46.2

---

---

13.9

-51.1 40.0 8.9

"Reaction conditions same as given in Table 4. bMolesof reactant converted per mole of vanadium m i d . H ' ,O, utilized in the formation of dimethylbenzyl alcohols, corresponding aldehydes and trimethylphenols. dMainly oxygenates with more than one functional group. Oxidationof trimethylbenzenes: In the oxidation of trimethylbenzenes, VS-2 showed negligibleactivity and H,O, selectivity. By contrast, due to its larger pore dimension, V-NCL-1 possessed significant activity in the oxidation of 1,3,5- and 1,2,4-trimethyIbenzenes (Table 5). With 1,3,5-trimethyIbenzene,the side chain oxidation of one of the methyl groups leads to the formation of 3,5-dimethylbenzyl alcohol and its aldehyde (27.2 and 46.2 W.%, respectively). In addition, hydroxylation of the aromatic nucleus also occurs, as seen from the formation of considerable amounts of 2,4,6-trimethylphenoI in the products (Table 5). With 1,2,44rimethylbenzene, the activation of the three methyl substituents leads to the formation of 2,4-, 2,5- and 3,4-dimethylbenzyl alcohols and the corresponding aldehydes. The hydroxylation of the aromatic ring is apparently not favourable with this substrate, as no phenolic products were detected.

CONCLUSiONS Two vanadium silicate molecular sieves, VS-2 and V-NCL-1 with medium and large pore dimensions, respectively, have been synthesised and their catalytic activity in oxidation reactions evaluated. Isolated vanadium ions, probably in framework positions, possesss unique catalytic activity and shape selectivity in oxidation reactions. So far, such behaviour has been found only in the case oftitanium silicalites. Our studies demonstratethat vanadium silicates also possess such features. The latter differ from the former in their ability to oxidise even the primary carbon atoms (in paraffins and side chain alkyl groups of aromatic hydrocarbons), and effect further secondary oxidation to a greater extent. V-NCL-1, with its large pore dimensions, enables the oxidation of bulky molecules like o and mxylenes and 1,3,5and 1,2,44rimethylbenzenes.

392

ACKNOWLEDGEMENT This work was partly funded by UNDP. Hari and Ramesh thank CSlR and UGC, respectively, for research fellowships. REFERENCES J. Kornatowski, M. Sychev, V. Goncharuk and W.H. Baur, Stud. Surf. Sci. Catal., 65 (1990) 581. 2. M.S. Rigutto and H. Van Bekkum, Appl. Catal., 68 (1991) L1. P. Fejes, I. Marsi, I. Kiricsi, J. Halasz, I. Hannus, A. Rockenbauer, Gy. Tasi, L. Korea 3. and Gy. Schobel, Stud. Surf. Sci. Catal., 69 (1991) 173. G. Centi, S. Perathoner, F. Trifiro, A. Aboukais, C.F. Aissi and M. Guelton, J. Phys. 4. Chem., 96 (1992) 2617. 5. P.R. Hari Prasad Rao, A.V. Ramaswamy and P. Ratnasamy, J. Catal., 137 (1992) 225. P.R. Hari Prasad Rao and A.V. Ramaswamy, J. Chem. SOC.,Chem. Commun., (1992) 6. 1245. K. Rarnesh Reddy, A.V. Ramaswamy and P. Ratnasamy, J. Chem. SOC., Chem. 7. Cummun., (1992) 1613. 8. R. Kumar, K.R. Reddy, A. Raj and P. Ratnasamy, 9th Intern. Zeolite Conf., Montreal, Canada, (1992) Paper A6; T. Tatsumi, M. NakamuraS. Negishi and H. Tominaga, J. Chem. SOC.Chem. Commun., 9. (1990) 476. 10. D.C. Huybrechts, L.D. Bruyeker and P.A. Jacobs, Nature, 345 (1990) 240. 11. J.S. Reddy, S. Sivasanker and P. Ratnasamy, J. Mol. Catal. 70 (1991) 335. 1.

M. Guisnet et al. (Editors),Heterogeneous Catdysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

393

Selective Oxidation of Organic Compounds Over the Large Pore Beta-Ti Zeolite M.A. Camblor, A. Corma, A. Martinez, J. Perez-Pariente, and J. Primo. lnstituto de Tecnologia Quimica, UPV-CSIC, Universidad Politecnica Valencia, Camino de Vera s/n, 46071 Valencia, Spain

Abstract The selective oxidation of linear and cyclic alkanes and alkenes of different sizes has been carried out using Ti-Beta zeolite in the presence of hydrogen peroxide. This zeolite shows a higher activity than TS-1 for the oxidation of cyclic olefins. Moreover, the influence of Ti and Al content on the activity of Ti-Beta samples is discussed. 1. INTRODUCTION

TS-1 and TS-2 titanium silicalites having MFI and MEL structures, respectively, have been shown to be effective catalysts for the selective oxidation of organic substrates using hydrogen peroxide under relatively mild conditions (1,2). Examples of reactions catalyzed by these Ti-containing zeolites are: alcohol oxidation (3),olefin epoxidation (4), ring hydroxylation of aromatic compounds (5), cyclohexanone ammoximation (6), and more recently, the oxidation of alkanes to a mixture of the corresponding alcohols and ketones (7,8). Although it is accepted that the catalytic activity of these materials has to be ascribed to the presence of Ti'" in the silicalite lattice, the exact nature of the active sites involved still remains unclear. However, a common feature is observed in all samples showing catalytic activity, that is the presence of an adsorption band at 960 cm-' in the infrared spectra (9),its intensity increasing when increasing the titanium content of the zeolite. This band has been assigned to Ti=O (10) or Si-0(Ti) groups (1 1) in the zeolite framework, which will form the actual oxidant sites after the addition of hydrogen peroxide. In the case of Ti-silicalites, despite their high activity and selectivity, the steric restrictions imposed by the pore size (- 5.5 A) limit their usefulness for the oxidation of relatively small organic molecules (8). In this sense, it has been shown (1) that the reactivity of TS-1 decreases by a factor of - 30 in the oxidation of cyclohexene with respect to 1-hexene, and that the epoxidation rate decreases when increasing

394

the chain length of linear olefins (1,12). Similarly, a substantial decrease in the reactivity of TS-1 when increasing the chain length and degree of branching in the oxidation of alkanes has also been reported (7,13). Then, it would be of great interest to obtain large-pore titanium zeolites which would allow the diffusion of bulkier organic molecules to the internal Ti sites, thus broadening the possibilities of these materials. Recently (14), we have reported the direct synthesis of the first 3-D large pore Ti-zeolite isomorphous to zeolite Beta, which was denoted as Ti-Beta. This zeolite was shown to be an active catalyst for the selective oxidation of cycloalkanes, i.e., cyclohexane and cyclododecane, achieving a higher conversion level than a reference TS-1 catalyst. The aim of this work is to show the possibilities of Ti-Beta as catalyst for the oxidation of linear and cyclic alkanes and alkenes with different sizes, and to compare its performance with a standard TS-1 sample.

2. EXPERIMENTAL 2.1. Synthesis

Ti-Beta samples with different Ti and Al contents were synthesized according to the following procedure: TEAOH (40% aqueous solution, K < 1 ppm, Na < 3 ppm, Alfa) was diluted with the required amount of water to which tetraethyl ortotitanate (Alfa) and amorphous silica (Aerosil200, Degussa) were added at room temperature with stirring. After that, a solution of aluminum nitrate (Merck) was also added. The resulting gels were poured into 60 ml poly(tetrafluoroethy1ene)-lined stainless-steel autoclaves and heated at 4 0 8 t l K in an oven under rotation at 60 r.p.m. for selected periods of time. Finally, after cooling the autoclaves the samples were centrifuged and the recovered solids were washed till pH = 9, dried at 353 K, and calcined at 853 K. 2.2. Characterization The incorporation of Ti into the zeolite framework was assessed by means of powder X-ray diffraction (XRD) in a Phillips PW 1830 spectrometer using the CuKa radiation, by measuring the expansion of the interplanar d-spacing corresponding to the most intense XRD peak in zeolite Beta (28 -- 22.4'), after dehydration at 383 K for 1 h and rehydration overnight over a CaCI, saturated solution (35% relative humidity). Moreover, mid-infrared spectra were recorded in a Nicolet FTIR 710 using the KBr pellet technique, in order to follow the appearance of the band at - 960 cm-', which is usually taken as an evidence that Ti is present in the zeolite lattice. The total amount of Ti in the samples was determined by X-ray fluorescence (XRF) in an Outokumpu XMET.

395 2.3. Catalytic Experiments

The oxidation experiments of n-hexane (Probus) and cyclohexane (Probus) were performed in a batch reactor at 373 K and autogeneous pressure. Typically, 56 mmol of alkane, 11.30 g of H,O, (35 Wh, Jansen), 17.35 g of acetone (Merck) as solvent, and 300 mg of catalyst were charged into the autoclave and heated under agitation (700 r.p.m.) until the reaction temperature was reached. After 4 hours, the reactor was cooled, the catalyst was separated by filtration and the reaction products were analyzed by G.C.-M.S. using a capillary column (5% mehylphenylsilicone, 25 m). The amount of H,O, in the products was determined by iodometric titration. The olefin epoxidation experiments were carried out in a glass flask with reflux and magnetic agitation. For 1-hexene (Aldrich) and cyclohexene (Fluka) oxidation, 33 mmol of olefin, 0.264 g of H,O, (35 Wh),23.57 g of methanol (Merck) as solvent, and 200 mg of catalyst were mixed and stirred at 298 K. In the case of 1-dodecene (Merck) and cyclododecene (Fluka), 33 mmol of olefin, 0.822 g of H,O, (35 WO), 23.57 g of ethanol (Merck), and 200 mg of catalyst were mixed and heated at 353 K under agitation. Samples were taken at different reaction times, and after catalyst separation, the H,O, and reaction products were analyzed as indicated above. An optimized TS-1 sample (EUROTS-1) has been tested in all the above reactions for comparison purposes. 3. RESULTS AND DISCUSSION

The chemical composition of Ti-Beta samples with different titanium and aluminum contents is given in Table 1. As a comparison, the titanium content of the reference TS-1 is also reported. It can be observed that the incorporation of Ti in the zeolite increases when increasing the concentration of Ti in the gel, and that the efficiency of Ti incorporation is higher at lower Ti concentrations, decreasing markedly when increasing the Ti content in the gel. Moreover, it can also be seen that for a given concentration of Ti in gel the incorporation increases when decreasing the Al content, as both elements compete for incorporation. However, at low Al and high Ti contents a sensible loss of crystallinity is observed. The increase in the interplanar d-spacing observed when the Ti content in the zeolite increases (Fig. 1) can be taken, in principle, as an evidence of Ti incorporation into the zeolite framework. In addition to this, all the Ti-Beta samples show the characteristic band at - 960 cm-' in the i.r. spectra, which intensity increases linearly with the Ti content of the zeolite, as it is shown in Figure 2. The intensity of this band decreases slightly for the sample with the higher Ti content. Similarly to what has been described for Ti-silicalite (1,9) this band disappears after impregnation of the Ti-Beta with H,O, with the formation of the corresponding yellow colored Ti peroxo complexes. The 960 cm-' band is restored after heating the sample at 353 K overnight.

396

-

Table 1 Composition of different Ti-Beta sarnDles In gel

T~~o,/AI,o,

Sample

Ti/(SitTi)

TiB-1

0.008

400

0.016

108

1.00

1.16

2.1

TiB-2

0.016

400

0.021

100

1.33

1.08

2.7

TiB-3

0.024

400

0.022

90

1.39

1.39

2.9

TiB-4

0.048

400

0.032

104

2.01

1.20

4.0

TiB-5

0.048

400

0.039

92

2.43

1.21

5.0

TiB-6

0.016

800

0.024

214

1.51

0.58

3.2

TiB-7

0.016

800

0.027

202

1.70

0.51

3.5

TiB-8

0.048

800

0.044

204

2.93

0.55

5.7

__

TS- 1 a

1.7

After calcination at 853 K.

3 961

-a 3 9 5 5 0

G

" m 2 I

U

395

L

m C m

b C -

(1

3945-

I

3 94

0

001

002

003

004

005

TI/(SI+TI) In zeoilw

Figure 1. lnterplanar d- spacing corresponding to the most intense XRD peak in zeolite Beta (20 = 22.4') as a function of the Ti content of the zeolite.

OCJl

0.02

003

004

005

Ti/(Si+Ti) in zeollte

Flgiire 2. Ratio of intensities of the 960 ana 795 cm-' i.r. bands as a function of the Ti content of the zeolite Ti-Beta

397

The behavior of Ti-Beta as a catalyst for the selective oxidation of different organic substrates is compared with that of TS-1, and the results are given in Table 2. Table 2 Activity and selectivity of Ti-Beta and TS-1 for the selective oxidation of different organic substrates. SelectwitV (rnol%) Substrate

Cahlysl

Reaction lime (h)

Turnover (rnolhnol-Ti)

Conversion

-0L

-ONE

Epoxide

TiB-6

4

27

5.7

21

79

-

TS-1

4

536

60.9

17

03

-

TiB-4

4

50

13.4

26

74

TS-1

4

111

12.6

57

43

-

1-Hexene

TiB-8

3

11

4.7

-

13

TS-1

3

47

6.0

Cyclohexene

TiB-8

3

10

4.3

TS-1

3

1

0.1

-

TiB-8

1

43

17.1

TS-1

1

73

10.6

-

TiB-8

1

10

3.8

TS-1

1

3

0.4

n-Hexane

Cyclohexane

1-Dodecene

Cyclododecene

Glycols'

(d%)

-

-

-

87

98

2

1

99

100

0

3

97

-

81

19

-

88

12

-

72

28

This value corresponds predominatly to the rnonoalkylglycolethers. In that Table it can be seen that TS-1 is much more active than Ti-Beta for the oxidation of n-hexane, the activity per Ti site being about 20 times higher in the former. However, this difference in activity is strongly reduced in the case of cyclohexane, provided this molecule has serious problems to diffuse inside the pores of TS-1, and hence, to reach the Ti active sites. It has also to be noticed that TiBeta shows a higher selectivity to cyclohexanone than TS-1 at a similar conversion level. TS-1 exhibits a slightly higher conversion, under the same reaction conditions, for the oxidation of 1-hexene, but its activity is lower for the oxidation of 1-dodecene than Ti-Beta zeolite. Nevertheless, the activity per Ti is still higher on TS-1 zeolite. However, in the case of cyclic olefins, for which TS-1 presents higher steric restrictions, this zeolite shows a very low activity, while the large pore Ti-Beta zeolite is still active to oxidize these bulkier molecules. No difference in turnover is found for l-hexene and cyclohexene for Ti-Beta, while this value decreases by a

398

factor of about 4 when going from 1-dodecene to cyclododecene. Indeed, cyclododecene is thougth to have serious steric limitations to penetrate the channels even in the large pore Ti-Beta zeolite. As can also be observed in Table 2 the product distribution is different for TS1 and Ti-Beta catalysts during the oxidation of olefins. While TS-1 gives almost selectively the epoxide, Ti-Beta produces the corresponding monoalkylglycolethers. This is explained on the basis of the acid sites associated to the presence of Al in Ti-Beta which catalyze the opening of the epoxide ring. Indeed, this also occurs in TS-1 when trivalent elements, such as Al, Ga, or Fe are introduced in the framework (15). The effect of Ti content on the catalytic activity of Ti-Beta was studied for the 1-hexene oxidation reaction. The activity was found to increase when increasing the Ti content of Ti-Beta samples with similar Al content. However, for a given Ti content the activity decreases when increasing the amount of Al in the zeolite framework. A linear correlation between the initial reaction rate and the number of (Ti-AI) per unit cell has been found, as can be seen in Figure 3. These results could be explained assuming that the change in electronegativity of the zeolite lattice due to the presence of Al produces a reduction in the red-ox ability of the Ti sites.

0

0.5

1

1.5

2

2.5

(Ti-AI)I U.C.

Figura 3 Influence of the number of (Ti-AI) ptx unit cell in Ti-Beta on the initial reaction rate for the oxldation of 1-hexen..

3

399 4. CONCLUSIONS

It has been shown that Ti-Beta zeolite is a useful catalyst for the selective This large pore Ti-zeolite takes oxidation of olefins in the presence of H,O,. advantage over the TS-1 when bulkier organic molecules are to be oxidized, as was shown for cyclododecane (14) and now for cyclic olefins. It has also been seen that the presence of Al in Ti-Beta affects negatively the red-ox properties of the catalyst by reducing the reaction rate. 5. ACKNOWLEDGMENTS

Financial support by the Cornision Asesora de lnvestigacion Cientifica y Tecnica of Spain (Project MAT 91-1 152) is gratefully acknowledged. We also thank Professor P.A. Jacobs for providing the EUROTS-1 sample. 6. REFERENCES 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15.

U. Romano, A. Esposito, F. Maspero, C. Neri and M.G. Clerici, in "New Developments in Selective Oxidation" (G. Centi and F. Trifiro, Eds.), Elsevier, 33 (1990). J.S. Reddy, R. Kurnar and P. Ratnasarny, Appl. Catal., 58, (1990) L1. A. Esposito, C. Neri, F. Buonorno, US Pat. 4,480,135 (1984). C. Neri, A. Esposito, 6. Anfossi and F. Buonorno, EP 100,119 (1984). A. Esposito, M. Tarnarasso, C. Neri and F. Buonorno, US Pat. 2,116,974 (1985). P. Roffia, M. Padovan, E. Moretti and G. De Alberti, EP 208,311 (1987). T. Tatsurni, M. Nakarnura, S. Negishi and H. Torninaga, J. Chern. SOC., Chern. Cornrnun., (1990) 476. D.R.C. Huybrechts, L. De Bruycker and P.A. Jacobs, Nature, 345, (1990) 240. D.R.C. Huybrechts, I. Vaesen, H.X. Li and P.A. Jacobs, Catal. Lett., 8 (1991) 237. G. Perego, G. Bellussi, C. Corno, M. Tarnarasso, F. Buonorno and A. Esposito, Stud. Ssurf. Sci. Catal., 28 (1986) 129. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. CAtal., 48 (1988) 133. T. Tatsurni, M. Nakarnura, K. Yuasa and H. Torninaga, Chern. Lett., (1990) 297. R.F. Parton, D.R.C. Huybrechts, Ph. Buskens and P.A. Jacobs, in "Catalysis and Adsorption by Zeolites" (G. Ohlrnann et al., Eds.), Elsevier, Amsterdam, (1991) 47. M.A. Carnblor, A. Corrna, A. Martinez, and J. Perez-Pariente, J.C.S., Chern. Cornrnun., (1992) 589. G. Bellussi, A. Carati, M.G. Clerici, A. Esposito, in "Preparation of Catalysis V", Elsevier, Amsterdam, (1991) 421.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

401

Selective photocatalytic oxidation of hydrocarbon compounds over zeolites.

0. Beaune, A. Finiels, P. Geneste, P.Graffin, A. Guida, J.L. Olive, A. Saeedan. Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees, URA 418 CNRS, Ecole Nationale Superieure de Chimie, 8 Rue de I'Ecole Normale, 34053 Montpellier Cedex 1, France.

Abstract The photocatalytic oxidation of various hydrocarbon compounds (alkyl and alkenylbenzenes) over irradiated semiconductor with wavelengths b 3 0 0 nm has been studied at room temperature first using TiO, and then using TiO, and zeolites. Alkylbenzenes led essentially to the corresponding a-ketones while for alkenylbenzenes there is a cleavage of the double bond with formation of the corresponding carbonyl compounds and oxygen addition leading to epoxide. Addition of acidic zeolite led to an increased or a decreased oxidation rate depending on the substrates but an increased selectivity was observed in all cases.The various results and the opposite effects observed with zeolite-added Ti02 are consistent with two different intermediates involved in the photoxidation mechanisms of alkylbenzenes and of alkenylbenzenes. 1. INTRODUCTION

Heterogeneous photocatalysis, applied to the oxidation at room temperature of organic substrates using optically excited semiconductor oxides and molecular oxygen is of intense interest and has received a great and increasing attention in the last few years [l-31.Thus various hydrocarbon compounds have been ironinvestigated over TiO, [4-81 and modified TiO, (Pt or Mo deposition [9-111, doped [12])as catalyst. On the other hand, the study of photochemistry of organic molecules adsorbed on solid surfaces (zeolites, microporous solids, ...) has attracted considerable attention and with zeolites, a shape selectivity and an However the use of zeolites intrazeolite photochemistry have been shown [13-151. has not been developed so far in heterogeneous photocatalytic oxidation using irradiated semiconductor materials. This paper is concerned with an attractive possibility involving both zeolite and semiconductor in photocatalytic oxidation. We study here the heterogeneous photocatalytic oxidation of various alkylbenzenes and unsaturated model compounds using TiO, and zeolite-added TiO, as catalyst. The influence of reaction parameters and the effect of zeolite addition are analysed.

2. EXPERIMENTAL SECTION

Experiments were carried out with a Degussa P-25 TiO, (ca- 70% anatase, 30% rutile, specific surface area 56 m2.g-1, non porous). Organic substrates were reagent grade quality and employed as received. The zeolites used were Y Faujasite type zeolites with different Si/AI ratios. HY2.5was supplied by Zeochem and dealuminated HY,o~,,~20 by Zeocat. The silanated zeolites were modified by Chemical Vapor Deposition Technique as reported previously [16,17].

Procedure: When zeolite was employed, it was calcined overnight at 500°C, in air, prior to use. The activated zeolite (0.3 g) was added to the organic compounds (10-2 M) in dichloromethane (3 ml) and was stirred at room temperature ; then the solvent was removed under reduced pressure. The substrate or the impregnated substrate was added to a magnetically stirred TiO, (0.3 g) acetonitrile (330 ml) suspension. The illumination through pyrex photoreactor was provided by a Philips HPK 125 W mercury lamp. The reaction was performed at room temperature in pure oxygen flow. Samples were taken and analysed, after centrifugation of the suspension, by flame ionization gas chromatography (OV,-capillary column: 25 m), GLC /mass spectrography and 13C-NMRspectroscopy. UV- abso rDtion method : The UV spectroscopy measurements were performed on a Varian Superscan series 3 spectrometer. The experimental procedure was as follows : a suspension of organic compounds (0.03 M), acetonitrile (330 ml) and catalyst, TiO, (0.3 g) or TiO, and zeolite (0.3 g of each), was stirred at room temperature. Samples were withdrawn periodically and the catalysts were separated by centrifugation. The amounts of organic sustrates in the supernatent were measured by UV absorption.

3. RESULTS AND DISCUSSION From the preliminary results it was confirmed that the photooxidation did not occur in the absence of semiconductor, oxygen or illumination respectively. Various alkylbenzenes and unsaturated model compounds have been investigated over TiO, and zeolite (HY,,) added Ti0, as catalyst (Tables 1, 3). of : -D In the case of the oxidation of isopropylbenzene selected as model compound, we have studied the effect of the amount of semiconductor and zeolite. As expected for a catalysed reaction, the conversion increases with the mass of the mg). This amount was chosen for the experiments catalyst until a maximum (~300 carried out afterwards. In presence of 300 mg of zeolite same maximum in the conversion was obtained.

403 plkvlbenzenes : The photocatalytic oxidation of alkylbenzenes leads essentially to the corresponding a-ketones. For example, as can be seen from Table 1 , linear alkylbenzenes give only one product (a-ketone) while ramified alkylbenzenes such as i-propylbenzene give two products with a selectivity equal to 84% in ketone and

Table 1 Photocatalytic oxidation of alkylbenzenes over TiO, and over zeolite-added Ti02 Substrate

Products (YO)

Conv.

Irr. time (h)

Catalyst

96 72

T102 Tt02 + HY20 T102+ HY 10 T102+ HY2.5

37

Ti02 T102+ HY20 T102+ H Y l o T102+ HY2 5

38

Ti02 Ti02 + HY20 Ti02 + HY 10

10

(“10)

Acetophenone Ethylbenzene

lsopropylbenzene

96 72

98 64

50

99 68

58

Acetophenone 84 98 =IOO

n-Propylbenzene

n-Butylbenzene

96 72

Ti02 Ti02 + HY20 Ti02 + HY 10

100

16

100

6

Phenylpropyl ketone 100

13

16

2 E

-....

100

39

34

2-Phenylpropan-2-01

%loo Proptophenone

96 72

____.

100 100 100 100

__.._.

100 100

1 Phenylbutyl ketone

n-Pentylbenzene

96 72

Ti02 Ti02 + HY20 Ti02 + H Y l o

2

100

20 6

100 100

.....

404

16% in tertiaryalcohol. Moreover, the conversion decreases as the hydrocarbon chain length is increased (37% conv. when R = Et, 2% when R = pentyl). This result seems to correlate with the steric factors.

Oxidation of alkylbenzenes over TiO, added zeolites leads to an increase in conversion (98% conv. with R = Et after 72 h irradiation against 38% after 96 h irradiation when TiO, alone was used). The same phenomenon is observed with isopropylbenzene (conversion changes from 84% to 98% ). This effect can be explained on the basis of the zeolites properties. The highest values of conversion are obtained for the most dealuminated zeolites (Table 1). It is well known that a dealuminated zeolite has strong acidic sites. Therefore, the zeolite acidity is enhanced by increasing the Si/AI ratio. These data seem to indicate that the zeolite acidity is an important feature in the photocatalysed oxidation of alkylbenzenes. Our overall results (Table 1) can be related with the zeolite acidity. As it is known, the zeolite sites are stronger for the HY, than for the HY,,. The conversion increases with the zeolite acidity (Table 1, when R = Et, the conversion changes from 50 to 98 with the increasing ratio Si/AI from 2.5 to 20). The question is to determine if the substrate is adsorbed on the external surface or inside the pores of the zeolite. A silanated catalyst for which the external sites are deactivated while internal sites are still active [17] must give some information. When same reactions are carried out with silanated zeolithe [17], a drastically decrease in conversion was observed (Table 2). The results are close to those obtained with TiO, alone.

Table 2 Photocatalytic oxidation of i-propylbenzene over silanated zeolites added Ti02 I 1 I I Produds (010)

Conv. ("1.) (72 h)

Catalyst

Ti02 + ( H Y z , ~ ) ~

Acelophenone

2-Phenylpropan-2-01

58

100

&

99

98

2

;

88

12

87

13

81

19

I

I

Ti02 + ( H Y ~ o ) ~

"

+(HY20)S

1

Ti02

n ; non silanated zeolite

s : silanated

zeolite

I I I

I

405 These results suggest that only external surface sites are active in the oxidation of isopropylbenzene. ated ComooundS : Under similar experimental conditions, the oxidation products of unsaturated compounds with an activated double bond by aromatic substitutent results essentially in cleavage of the double bond with formation of the corresponding carbonyl compounds. However, addition of oxygen leading to epoxide as secondary minor product is observed. In all cases, no oxidation of the aromatic ring is detected (Table 3).

When a zeolite is added to the medium, the reaction is slower (for example, it needs 25 hours for oxidation of 1,l-diphenylethylene against 7 hours when TiO, alone is used ; similar results were obtained with stilbene), but results in increasing selectivity. In the case of a-methylstyrene, the conversion is reduced when zeolite was added but the selectivity to carbonyl compounds is slightly increased (89O/, to 100%). Zeolites are well known as microporous solids and acid catalysts [la]. These two charateristic properties seem to be operating in the course of photooxidation (by adsorption of organic substrates or/and by the acidity influence on the reaction mechanism).

406

It was necessary to compare the adsorption of alkylbenzenes and studied olefines. In UV-absorption experiments undetectable adsorption for the substrates studied was generally observed over Ti02. When zeolite was added, adsorption was observed only for a-methylstyrene for which 40-45% of the initial amount was measured after 15 hours. These adsorption results and the opposite effect of zeolite in the oxidation of alkylbenzenes and the olefines studied lead us to investigate the reaction mechanisms which would be different for the two kinds of substrates. Results suggest that the involved intermediates are not be the same and the effect of zeolite should be taken into account. Based on these we propose following two mechanisms for the two different kinds of substrates : i) from the cation radical in the case of alkylbenzenes (schemel) where it can allow the transfer of proton; ii) from the dioxetane which can be stabilized by acidic zeolites in the case of olefinic compounds (scheme 2).

Scheme 1: TiO, 0 2

+

hu e~

@C H R R ' CHRR'

TiO*,

(e-+ hf

)

_____t

h+

+

+

0-.2

@- C H R R '

Zeo1-H' b-.

o-2

Zeol-

1\

@- CIHRR' + H'

e - R

+

R'OH

+

0 2

Q0

In the first case, the photo-excitation of semiconductor produces an electron and a positive hole (step 1). Oxygen serves as an electron acceptor for the conduction band electron (step 2) and the substrate (donor) gives an electron to the photogenerated hole at the surface of the Ti02particle forming an adsorbed cation radical (step 3). The zeolite (step 4) aids in the stabilization of radical-cation and proton transfer, so this step would be enhanced by the more acidic zeolites.

407 This hypothesis is confirmed particularly in the case of isopropylbenzene. In fact, two mechanisms are possible for oxidation of this compound:

When a zeolite is added, the rate of step (a) should be slowed down because a-methylstyrene is an intermediate product. On the contrary, an accelerating effect is observed, therefore, the step (b) appears to be the essential path. In the second mechanism proposed (scheme 2), there is a formation of dioxetane which is well known to be stabilized in strong acidic medium [19], so, the overall oxidation rate should be decreased as we observed.

Scheme 2: l +

\C=

C'

/

0 2

\

+

h+(BV)

->

+ e. (BC) > 0-0

I

I

'c-<-,\+,\

0

II

0

II

0

Investigations are being performed to gain further understanding of the photocatalysis of a-methylstyrene by Ti02 added zeolite for example in using other types of zeolites (with different structure and acidity).

408

4. CONCLUSION

In summary, the addition of zeolite to TiO, in the photocatalytic oxidation of alkylbenzenes and olefines studied shows two different behaviours. The use of both zeolite and semiconductor seems to be particularly interesting for increasing selectivity in the photooxidation. The various effects observed by comparing the results with and without zeolites are related to different mechanisms for the two types of substrates.

5. REFERENCES M.A. Fox, Photocatalysis - Fondamentals and applications, N. Serpone and E. Pelizzetti (eds.), J. Wiley, New-York, 1989, chap. 13, p. 421. 2 P. Pichat and M.A. Fox, Photoinduced Electron Transfer, M.A. Fox and M. Chanon (eds.), Elsevier, Amsterdam, 1988, part D. 3 M.A. Fox, Photocatalysis and Environment : Trends and Applications, M. Schiavello (ed.), Kluwer Academic Publishers, Dordrecht, The Netherlands, 1988, p. 445. M.N. Mozzanega, J.M. Hermann and P. Pichat, Tetrahedron Lett., 34 (1977) 4 2965. P. Pichat, J. Disdier and J.M. Hermann, Nouv. J. Chim., 10 (1986) 545. 5 M. Fujihira, Y. Satoh and T. Osa, J. Electroanal. Chem., 126 (1981) 277. 6 7 N. Djeghri and S.J. Teichner, J. Catal., 62 (1980) 99. 8 M.A. Fox and C.C. Chen, J. Photochem., 17 (1981) 119. 9 J.M. Hermann, W. Mu and P. Pichat, Stud. Surf. Sci. Catal., 59 (1991) 405. 10 M. Anpo and M. Tomonari, Denki Kagaku, 57 (1989) 1219. 11 K.R. Thampi, J. Kiwi and M. Gratzel, Catalysis Letters (1988) 109. 12 J.A. Navio, M. Garcia Gomez, M.A. Pradera Adrian and J. Fuentes Mota, Stud. Surf. Sci. Catal., 59 (1991) 445. 13 J.C. Scaiano, H.L. Casal and J.C. Netto-Ferreira, A.C.S. Symposium Series no 278, Organic Phototransformation in Nonheterogeneous Media, M.A. Fox (ed.), 1985, p. 21 1. 14 N.J. Turro, C.C. Cheng, X.G. Lei and E.M. Flanigen, J. Am. Chem. SOC.,107 (1985) 3740. 15 V. Ramamurthy, D.R. Corbin, N.J. Turro, Z. Zhang and M.A. Garcia-Garibay, J. Org. Chem., 56 (1991) 255. 16 M. Niwa, H.G. Karge and J. Weitkamp, J. Catal. Today, 3 (1 988) 11. 17 P. Moreau, A. Finiels, P. Geneste and J. Solofo, J. Catal., 136 (1992) 487. 18 a) A. Dyer, "An introduction to zeolite molecular sieves", Ed. J. Wiley, NewYork, 1988. b) Subhash Bhatia, "Zeolite catalysis: Principles and Applications", Ed. CRC Press, 1990. c) P. B. Venuto and P.S. Landis, "Advances in catalysis", Academic Press, New-York, 1968,t. 18, p. 259. 19 K. R. Kopecky, J.E. Filby, C. Mumford, P.A. Lockwood and J.Y. Ding, Can. J. Chem., 53, (1975), 1103. 1

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals IIJ Q 1993 Elsevier Science Publishers B.V. All rights reserved.

409

Photocatalytic Oxygenationof Hydrocarbons on TiOJlron-Porphyrin Hybrid Catalysts. E.Polo, R.Amadelli, V.Carassiti and A.Maldotti Centro di Studio su Fotoreattivitae Catalisi (C.N.R.), Dipartimentodi Chimica,Universita degli Studi di Ferrara, via L.Borsari,46 - 44100 Ferrara, Italy.

Abstract The heterogeneous photocatalytic mono-oxygenation of cyclohexane, methylcyclohexane and n-heptane has been carried out using TO, and iron-porphyrins surface modified TiO,. The modified catalysts show an interesting new reactivity with respect to both TiO, and the porphyrins used separately as photocatalysts in the heterogeneous and homogeneous phase, respectively. In particular, it is important to note that for the unmodified TO, the amount mono-oxygenated products obtained is higher when the modified oxide is used. This is clear when also the degree of complete oxidation to CO,, being much higher for the unmodified catalyst, is included in the calculation of the yields. 1. INTRODUCTION In the framework of the research on the oxidation of hydrocarbons under mild conditions, we reported recently on the mono-oxygenation of cyclohexane and cyclohexene photocatalyzed by iron-porphyrins in the homogeneous phase (1) and in the heterogeneous phase (2). In the latter case, the metal porphyrin is anchored to the surface of TO,. Absorption of light by the photoactive oxide results in an electron-hole chargeseparation:holes can oxidize the hydrocarbonto a radical species while electrons reduce the linked Fe(ll1)-porphyrinto Fe(1l)-porphyrin(2). The system is a "hybrid" one in that it consists of catalysts (the porphyrin and TO,) which are able to function separately in the photo-oxidationof hydrocarbons. The hybrid catal st showed a new reactivity with respect to both TO, and the porphyrin (2). d e are advancing in the study of the photocatalyticoxidation of hydrocarbons using the TiOJmetal porphyrin hybrid catalyst and, herein, we report on the importance of surface tayloring of the TiO, catalyst in order to improve the yields in cyclohexane oxidation. Results of the photo-oxidationof n-heptane and methylcyclohexane are also reported. In this case, the distribution of the photo-oxidation products provides some insight in how the regioselectivity is affected by the surface modification. 2. EXPERIMENTAL

2.1 Reagents and Catalyst Cyclohexane (Merck), methylcyclohexane (Aldrich) and n-heptane (Merck), spectrophotometric grade, were purified by elution through a dry-packed alumina-silica column and distilled before use. 3-aminopropyltriethoxysilane (Aldrich) was distilled each time before usage.

410

I ron(lI I) [5,10,15,2O-Tetrakis(4-suIphonatophenyl) porphyrin] [Fe(TPS PP)] was purchased from Porphyrin Products and its linkage to TO, has been carried out accordin to literature procedures (3). Pfe synthetized 5-(4-~arboxyrnethylphenyl)-lO, 15,20-tris-(2,6-dichlorophenyl)-

was covalently linked to the previously silanized TiO, surface. Titanium dioxide was commercially available (Degussa P-25, surface area 55 rn2/g). The surface-derivatized 30, powder was obtained by treatment with 3-aminopropyltriethoxysilane(3) followed by reaction with the proper porph rin. The surface coverage by the porphyrin was calculated to be about 30% of the tota surface.

Y

2.2 Analytics UV-Vis spectrawere recordedon a Perkin Elmer model Lambda 6 spectrophotometer equipped with an integrating sphere. Gas-chromatographic analyses were carried out with a Dani 8521 gas chromatograph equipped with a flame ionization detector and a Carbowax 20 M 5% on Chromosorb W-AW packed column. 2.3 Reaction Conditions In a typical experiment, the powder catalyst 4mg/ml) was suspended in the neat liquid hydrocarbon and irradiated at h > 360 nm. e sample was continuously stirred and kept under 0, atmosphere at 20'C. The light source was a Hanau Q 400 type medium pressure mercury lamp equipped with cut-off filters.

'rr,

3. RESULTS In order to avoid lengthy descriptions, we chose to present the data in the form of tables and figures whenever possible. 3.1 Photo-oxygenation of cyciohexane The new data reported in Table 1 and in Fig.1 are those for Ti0,-sil-Fe(TDCPP). The other data have been previously reported by us (2) and are shown here for comparison.

Table 1 Quantum yields for the oxidation of cyclohexane (@)

TiO, Ti0,-si I-NH, Ti0,-sil-Fe TPSPP) Ti0,-sil-FeITDCPP)

Cyclohexanone

Cyclohexanol

0.058 0.006 0.023 0.036

0.001 0.002 0.010 0.014

411 FeVDCPP) is a porphyrin where the easily attacked meso positions of the ring are protected by chlorine atoms against degradation. When it is used as a homogeneous photocatalyst in the oxidation of hydrocarbons, the turnover number increases significantly with respect to the non protected porphyrins (6,7).

0 TiO,

a) 6.0.10-'

TO,-sil-NH,

,.

4.0.10-'

@

TO,-sil-Fe(TPSPP)

a

TO,-sil-Fe(TDCPP)

,,

2.0.10'..

*

Cyclohexanone

Cyclohexanol

Figure 1. Products distribution in the photo-oxygenationof cyclohexane Compared to TO,-sil-Fe(TPSPP), the quantum yield for product formation globally increases when TO,-sil-Fe(TDCPP) is used as catalyst. Compared to unmodified TO,, the salient feature of the hybride catalyst is confirmed to be the production of cyclohexanol in greater amount. We observed only a slight increase in the turnover number for TO,-sil-Fe(TDCPP) compared with TO,-sil-Fe(TPSPP) at variance with the behaviour in the homogeneous phase. The higher turnover numbers for the former catalyst can then be ascribed to the higher quantum yield for products formation rather than to its greater stability (Table 1). In turn, the higher quantum yields observed may tentatively be attributed to an increase in the hydrophobicity of the surface when Fe(TDCPP) is used. 3.2 Photo-oxygenation of methylcyclohexane A cursory glance at the data reported in Figure 2 would reveal that the product distribution is only slightly modified by the derivatization. In reality, as for cyclohexane, the amount of alcohols among the products is higher when the derivatized TO, is used. From the data reported in Table 2, the calculated percent of alcohols increases from 22 for TO, to 34 for TO,-sil-Fe(TPSPP) and 46 for TO,-sil-Fe(TDCPP). For the formation of ketones, a close examination of the data of Figure 2 reveals non negligible differences in the regioselectivityfor the different catalysts. In particular, the ratio of the amount of 2-one with respect to 4-one is higher for the modified TO,. Since position 4 is less hindered, this behaviour may indicate that, for the formation of ketones, the interaction of methylcyclohexane with the surface is a more critical step with unmodified TO, than with derivatized TO,.

412

4

0

Ti02 TiO,-sil-NH,

1.5.10.'

0 TiO,-sil-Fe(TPSPP) Ti0,-sil-Fe(TDCPP) 1.0.1O'

5.0.1O 3

0.0

6 Figure 2. Products distribution in the photo-oxygenation of methylcyclohexane

Table 2 Quantum yields for the photo-oxygenationof methylcyclohexane (Qxl 07

TiO, Ti0,-sil-N H, Ti0,-sil-Fe TPSPP) Ti0,-sil-FelTDCPP)

Aldehyde

Ketones

Alcohols

1.5 0.3 0.5 0.7

32.7 3.8 11.2 19.7

9.3 2.1

5.0 9.1

413

3.3 Photoacygenationof n-heptane The data for the mono-oxygenation of n-heptane are shown in Table 3 and Figure 3. The formation of hydroxylated products, in this case, is not pronounced. However, the percent of alcohols with respect to ketones is almost double when the porphyrin-modified catalysts are used (Table 3), confirming the general tendency observed in this work. It is seen in Figure 3 that, for the different catalysts,thereis a different regioselectivity for the formation of ketones. In particular, the formation of the 2-one is more favoured with unmodified TO,. Since this is the less sterically hindered position, the behaviour is indicative (as for methylcyclohexane) of the fact that interaction of the hydrocarbon with the surface has a greater role in the case of unmodified TO,.

Table 3 Quantum yields for the photo-oxygenationof n-heptane(Qx107

TiO, Ti0,-sil-NH, Ti0,-sil-Fe(TPSPP) Ti0,-sil-Fe(TDCPP)

1.5.10'

I

Aldehyde

Ketones

Alcohols

0.4 0.1 0.3 0.5

34.1 4.7 12.1 15.9

2.1

I

1.6 1.9

4 3

7702

@

TO,-sil-FeFDCPP)

@

TO,-sil-Fe(TPSPP)

m 0.0

0.9

m

m ..

2-one 3-0n0 &one Figure 3. Regioselectivity in the formation of ketones in the photo-oxygenation of n-heptane.

414 4. DISCUSSION

From the analysis of the data reported above one could conclude that the catalytic activity of TO,/iron-porphyrin for the mono-oxygenation of the hydrocarbons is always lower than that of TO,. However, an important aspect must be taken into account, i.e., that complete oxidation of the hydrocarbons to CO, can occur in these systems.

1 0.6

0.4

0.2

0.0.

TO,

TO,-sil-Fe(TPSPP)

TO,-sil-Fe(TDCPP)

Figure 4. Quantum yields ratio of mono-oxygenation products to CO, for the oxidation of hydrocarbons on TO, and TO,/iron-porphyrin photocatalysts. When the production of CO, is accounted for, one obtains the interesting results reported in Figure 4, where the ratio of the quantum ield for the mono-oxygenation products formation to that of CO, is shown for the di erent catalysts employed. From an inspection of this figure we reach the important conclusion that, from the standpoint of fine chemicals production, the TO,/iron-porphyrin system is more efficient than unmodified TO,. Several factors are likely to concur in the behaviour of the "hybrid" catalysts toward photo-oxidation of hydrocarbons, and below we propose possible explanations also in the light of literature data. In illuminated dispersions of semiconductors, light induces an electron-hole charge separation at the surface of the particles and within their bulk. Redox reactions at the solid/solution interface can then occur upon capture of the electrons and holes by an oxidant and a reductant, respectively. Since a reduction and an oxidation reaction proceed simultaneously on the same surface (at the same rate, at steady-state conditions), the dispersed system can be compared to an ensemble of semiconductor microelectrodes at open circuit (8). In our case, and generally, where the reaction of interest is the oxidation of a substrate by holes, electrons are conveniently scavenged by molecular oxygen bubbled through the dispersion. Since TO, is a poor catalyst for the reduction of 0,, it is common to modi its surface by depositing metal atoms. Electron accumulation on the particles woul , otherwise, lead to recombination of the electron-hole pairs with a dramatic loss of efficiency for the reactions at the interface. The linkage of porphyrins or phtalocyanines to the surface provides an alternative way of creating electron acceptor surface states (2,3) that can mediate 0, reduction.

4

2

415

We stress that, when analyzing the data obtained with these systems, attention should be paid to the fact that the necessary intermediate step giving TO,-sil-NH, apparently causes a decrease in the activity of TO,. In particular, the silanized oxide becomes a poorer catalyst for the reduction of 0, than the unmodified one (2). On this ground, one reason for the increased activity observed when the iron porphyrins are subsequently anchored to the surface (through the NH, function), is the recovery of 0, reduction activity. This purely kineticcause does not explain the formation of hydroxylated species in the TO,-sil-porphyrin/hydrocarbon systems. Clearly, either intermediate species have a different reactivity or entirely different intermediates are formed. We envisage two possible reaction pathways: the reaction of hydrocarbon radical species with 0, or with the superoxide coordinated to the iron-porphyrin. The direct reaction of hydrocarbon radical intermediateswith 0, would yield peroxo species, ROO. or ROOH, which could eventually form alcohols and ketones. Although these species should form irrespective of whether the surface is modified or not, there could still be an influence of the surface on how they evolve. For instance, a possible route on TiO, is ROOH -+ (O;)sug

-

RO -+ OH- -+ 0,

(1)

while on porphyrin-modified oxide, the same species can yield alcohols via redox reactions involving the porphyrin (9). The second possible pathway involves the interaction of R. with [Fe(lll)-O;], formed from 0, reduction at the porphyrin centers on the surface: a species of the type Fe(ll1)-O-O-R is obtained. Solution phase studies indicate that this species produces hydroxylated compounds in the presence of traces of water or other added proton sources (10). Actually, this process, as well as atwo-electron reduction of Fe(lll) in the presence of 0, and protons, also yields the hypervalent intermediate Fe'=O. This species can be tracked down by assessing the ability of the system to give epoxidation of alkenes (1). In a comparison of TO, and TO,-sil-porphyrin, we failed, however, to observe an increase in the yield of epoxidation products both in systems containing cyclohexene and cyclooctene. The results reported here indicate that another factor is particularly important in determining the distribution of the reaction products, i.e., the degree of hydrophilicity of the surface. The oxidation of methylcyclohexane and n-heptane represents a useful probe to test the importance of the interaction of the substrates with the surface in determining the distribution of the products. The results obtained with underivatized TO, show that, in every case, functionalization in the less sterically hindered position is strongly preferred. This can be explained by reasonably assuming that the adsorption of the hydrocarbon precedes its oxidation. Since it is a hydrophilic surface, the interaction with the hydrocarbon will be the smallest possible. The derivatized surface is considerably more hydrophobic, and the approach of the hydrocarbonsto it is much less restricted. Significantly, in the TO,-sil-porphyrin system, the distribution of the oxidation products of methylcyclohexaneand n-heptane is less sensitive to sterical effects when the more hydrophobic FeFDCPP) is used instead of the tetrasulphonated FeFPSPP).

416

5. CONCLUSION

The heterogeneousphotocatalyticoxidation of cyclohexane,methylcyclohexaneand n-heptane by molecular oxy en (760 torr), at room temperature, has been carried out on TO, and TO, derivatize with iron porphyrins. The salient features of the modified TO, compared to the unmodified oxide are an increase in the formation of hydroxylated species among the reaction products, a different regioselectivity in the formation of ketones from methycyclohexane and n-heptane and a consistent decrease of CO, among the reaction products. This last point makes the system particularly attractive since it allows to improve the ratio of "useful" products to that of unwanted side chemicals. The data reported here show that, in the use of dispersed semiconductors for photosynthetic applications, the hydrophilicity of the surface is an important parameter that should be controlled, depending on the aims of the reasearch.

8

6 REFERENCES A. Maldotti, C. Bartocci, R. Amadelli, E. Polo, P. Battioni and D. Mansuy, J. Chem. SOC.Chem. Commun., (1991) 1487. 2 R. Amadelli, M. Bregola, E. Polo, V. Carassiti and A. Maldotti, J. Chem. SOC.Chem. Commun., (1992) 1355. 3 A.P. Hong, D.W. Bahneman and M. Hoffmann, J. Phys. Chem., 91 (1987) 21 12. 4 J.S. Lindsey, I.C. Schreiman, H.C. Hsu, P.C. Kearney and A.M. Marguerettaz, J. Org. Chem., 52 (1987) 827. 5 A.D. Adler, F.R. Longo, F. Kampas and J. Kim, J. Inorg. Nucl. Chem., 32 (1970) 2443. 6 C.K. Chang and F. Ebina, J. Chem. SOC.Chem. Commun., (1981) 778. 7 P.S. Traylor, D. Dolphin and T.G. Traylor, J. Chem. SOC.Chem. Commun., (1984) 279. 8 M. Neumann-Spallart and O.Enea, J. Electrochem. SOC.,131 (1984) 2767. 9 D. Mansuy, J-F. Bartoli and M. Momenteau, Tetrahedron Lett., 23 (1982) 2781 10 T.G. Traylor and Feng Xu, J. Am. Chem. SOC.,112 (1990) 178. 1

Acknowledgements This research was supported by the Italian National Council of Research (C.N.R.), Progetto Finaliuato Chimica Fine II and by M.U.R.S.T.

M. GuiBnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

417

Selective oxidation of alkenes on a zeolite supported iron phthalocyanine catalyst AZsigmonda, F. Notheisza, M. Bartoka and J.E.BackvalP aDepartment of Organic Chemistry, Jozsef Attila University, 6720 Szeged, Dom-ter 8, Hungary bDepartment of Organic Chemistry, University of Uppsala, Box 531, 751 21 Uppsala, Sweden

Abstract Iron phthalocyanine encapsulated in zeolites was used as oxygen activating catalysts in the triple catalytic aerobic oxidation of hydroquinone to benzoquinone, in the allylic oxidation of olefins and in the selective oxidation of terminal olefins to ketones. The catalyst proved active in the above reactions. It is stable towards self-oxidation and can be recovered and reused.

1. INTRODUCTION

The metal-catalyzedoxidation of organic compounds is of importance in industrial organic chemistry both in large-scale processes and in manufacturing of fine chemicals [l].An attractive oxidant in such processes is molecular oxygen because it is inexpensive and gives no environmentally harmful side products. Many processes based on molecular oxygen, however, require elevated temperatures and pressures and as a consequence there is a demand for mild aerobic catalytic processes. Macrocyclic metal complexes have recently attracted attention as possible oxygen activating catalysts in oxidation reactions [2-51. In one approach they are used as oxygen activating components in a triple catalytic system (Scheme 1) for oxidation of olefins [4,5] and alcohols [6]. This leads to very mild reactions reminiscent of aerobic processes occurring in living organisms. or f Fe(02lPc

cti

Scheme 1. Triple catalytic system for oxidation of terminal olefins.

418

r

This triple catal ic system allows the aerobic oxidation via a multistep electron transfer involving t ree redox systems Pdll)/Pd(O) - benzoquinone/hydroquinone MLoX/ML, where ML is an oxygen activating macrocyclic transition metal complex. Electron transfer occurs from the substrate to Pd(ll), giving Pd(O), followed by another electron transfer from Pd(0) to benzoquinone. The hydroquinone thus formed transfers electrons to the oxidized form of the metal macrocycle, which is reduced. The latter is reoxidized by electron transfer to molecular oxygen. A similar triple catalytic system was applied for allylic oxidation of cyclic olefins (eq. 1)

A number of metal macrocycles were tested as oxygen-activatingcom lexes and it was found that the cobalt macrocycle complexes efficiently catalyzed ast electron transfer from hydroquinone to dioxygen in the palladium-catalyzed aerobic oxidation of olefins. However, in the oxidation of terminal olefins to ketones with palladium acetate as the catalyst a small amount of strong acid was required to prevent precipitation of Pd(0). The presence of the strong acid limits the use of the cobalt macrocycles. However, good results were obtained with the iron phthalocyanine macrocycle. Iron phthalocyanine, Fe(Pc is a very stable, porphyrin analogue complex, which is very efficient in catalyzing t e electron transfer from h droquinone to oxygen. It is stable enough to be resistant toward degradation under t e reaction conditions employed.

P

k

K

However, the full potential of this catalytic system will be realized only if the macrocycle complexes can be made stable towards self-oxidation and can be recovered and reused. One possibility for the development of methods for the recovery and reuse of this catalyst is to li ate the macrocycle to the surface of a solid. Zeolites are well suited or the preparation of encapsulated complexes by virtue of the large supercages. Metallo-phthalocyanines encaged in zeolites have been proosed as enzyme mimics [7,8].Zeolite-encapsulated iron phthalocyanine catalysts Eave been used in h drocarbon oxidations; it was found that the resistance of the zeolite-enca ed compexes against oxidative destruction by far exceeded that of free iron phtha ocyanines [9,10]. In the present work, zeolite-encaged phthalocyanine catalysts were studied in the triple catalytic oxidation of olefins.

P

B

Y

419

2. EXPERIMENTAL Palladium diacetate, 1,2-dicyanobenzene (+ 98%) and ferrocene (+98%)were purchased from Aldrich and used as received. Other chemicals and solvents were obtained from commercial sources and were used as received. Iron phthalocyanine, Fe(Pc), was prepared from 1,2-dicyanobenzene according to the procedure reported by Byrne, Linstead and Lowe [ 111. 1,2-Dicyanobenzene (20 g) and iron owder (1 g) were combined with naphthalene (1.5 g) in a round-bottomed flask itted with an air condenser. The stirred mixture was heated at 523 K for 1 hour. After cooling, the solid material was broken up and washed with ether and hot acetone. Iron phthalocyanineencaged in zeolite, Fe(Pc)/Z, was prepared by adding 5 g of air-dried NaY to 50 ml of a solution of 84 mg of ferrocene in acetone, followed by air-drying at 343 K [lo]. The dried solid was mixed with 5 g of 1,2-dicyanobenzene and 15 ml of decalin, and was heated in an argon atmosphere. The solid material was soxhlet extracted with acetone, pyridine and again with acetone, until a colorless extract was obtained. Finally, the catalyst was dried at 343 K. The iron content of the catalyst samples was determined by chemical analysis after dissolution of the zeolite in concentrated sulfuric acid. The I.R. characterization was carried out by using the KBr technique. The amount of nitrogen in the catalyst was measured by the standard chemical method. The oxidation reaction was carried out at room temperature and atmospheric pressure. The solid materials were put into a glass reactor. The reaction vessel was purged with oxygen and put under 1 atm of oxygen. The liquid compounds were added from a syringe, and the stirring was started. The oxygen uptake was measured with a buret. The reaction was followed via the oxygen uptake and when the gas consumption had ceased the stirring was stopped, the reaction mixture was extracted and the organic layer was analyzed by gas chromatography. Oxidation of hydroquinone to benzoquinone. A solution of 68 mg (0.12 mmol, 5%) of Fe(Pc) and 250 mg (2.27 mmol) of hydroquinone in 5 mL of HOAc was stirred at room temperature under an 0, atmosphere. Use of Fe(Pc)/Z. In a similar experiment, 1 g zeolite-encaged iron phthalocyanine, Fe(Pc)/Z (0.02 mmol, 0.9%),was used (first run) instead of Fe(Pc), together with 250 mg (2.27 mmol) of hydroquinone in 5 mL of HOAc. When the oxygen consumption had ceased, another portion of hydroquinone (250 mg, 2.27 mmol) dissolved in 2 mL of HOAc was injected into the reactor (second run), the shaking was started and the oxygen uptake was measured again. When the gas consumption had ceased, the catalyst was filtered off, washed with methanol and acetone, and dried overnight at 333 K in an argon atmosphere. This catalyst was used to oxidize 250 mg of hydroquinone in a new experiment (first run after filtration). The catalyst was filtered off and the whole procedure was repeated three times. The yield was almost 100%. Oxidation of l-decene to 2-decanone. The reactor was charged with Pd(OAc), (17 mg, 0.076 mmol, 5%), hydroquinone (25 mg, 0.23 mmol, 20%), Fe(Pc) (43 mg, 0.076 mmol, 5%), and distilled N,N-dimethylformamide (1 mL). To this 0.12 mL of water and 8 NL of 60% aqueous HClO (0.076 mmol) were added. The reactor was purged with oxygen, and l-decene ((3.21 g, 1.5 mmol) was added. The reactor was stirred at room tem erature and the oxygen uptake was measured. Use of Fe(Pc&. in a similar experiment, 1.O g (0.2 mmol, 1.3%) Fe(Pc)/Z was used instead of Fe(Pc). When the oxygen consumption had ceased, another portion of l-decene (0.21 g, 1.5 mmol) was injected into the reactor, the shaking was started and the oxygen uptake was measured again.

P

420

When the gas consumption had ceased, the catalyst was filtered off, washed with methanol and acetone, and dried overnight at 333 K in an argon atmosphere. This catalyst was used for a new experiment. The liquid mixture was diluted with saturated NaCl solution and extracted with pentane and a pentane/ether mixture. The dried organic layer was analyzed by gas chromatography. 2-Decanone was the only oxidized product and GC analysis showed a 65% yield of ketone. Allyllc oxldatlon of cyclohexene. Pd(OAc), (22 mg, 0.1 mmol, 5%), hydroquinone (44 mg, 0.4 mmol, 20%), Fe(Pc) (5.7 mg, 0.01 mmol, 0.5%), and LiOAc .2H,O (102 mg, 1 mmol) were stirred in acetic acid (10 mL) for 20 minutes. The reaction was put under oxygen atmosphere and cyclohexene (0.20 mL, 2 mmol) was added from a syringe. The reactor was heated to 333 K with a water thermostat, and the oxygen uptake was measured. Use of Fe(Pc)/Z. In a similar experiment, 0.2 g (0.004 mmol, 0.2%) Fe(Pc)/Z was used instead of Fe(Pc). When the oxygen consumption had ceased, another portion of cyclohexene (0.20 mL, 2 mmol) was injected into the reactor, and the oxygen uptake was measured again. When the gas consumption had ceased, the catalyst was filtered off, washed with methanol and acetone, and dried overnight at 333 K under argon atmosphere. This catalyst was used for a new experiment. The reaction mixture was cooled to room temperature. Water (8 mL) was added and the mixture was extracted with pentane (4 x 15 mL). The combined organic layer was washed with water and sat. aqueous NaHCO,. The organic layer was dried and concentrated. The 2-cyclohexen-1-yl acetate was measured by gas chromatography. The yield was 80%.

3.RESULTS AND DISCUSSION The zeolite-encaged iron phthalocyanine catalyst was used to oxidize hydroquinone to benzoquinone, 1-decene to 2-decanone and cyclohexene to 1-acetoxy-2cyclohexene. The free iron phthalocyanine catalyst was also used for the same oxrdation reactions, and the results were compared with those obtained employing the supported catalyst.

3.1. Catalyst characterization

The extraction procedure removed all of the free iron from the zeolite-encagedcatalyst. Thus, all residual iron present is associated with encaged FePc. In accordance with Parton, Uytterhoeven and Jacobs [lo], the I.R. data showed that the characteristic lines of ferrocene and 1,2-dicyanobenzenewere absent. Analysis of the iron content allowed determination of the amount of iron phthalocyaninepresent in the zeolite. It was found that the metal macrocycle content of the zeolite was 0.02 mmol Fe(Pc) g catalyst. The I.R. and the nitrogen analysis data showed, that the amount of H pht alocyanine was much higher than the amount of Fe(Pc), just as it was publishegby Parton and coworkers [lo]. This is not surprising, because 1,2-dicyanobenzene was used in large excess over ferrocene.

b

421

3.2. Oxidation of hydroquinone to benzoquinone

The oxidation of hydroquinone to benzoquinone is regarded as a test reaction for evaluation of the catalytic activity of the metal macrocycle oxygen-activating complexes. The oxygen uptake for the oxidation of hydroquinone catalyzed by the free iron phthalocyaninecatalyst is depicted in Fig. 1. The catalytic activity of the free iron phthalocyaninewas similar to that observed by Backvall, Hopkins, Grennberg, Mader and Awasthi [5]. The zeolite-encapsulatediron phthalocyanine macrocycle catalyst was also used for the oxidation of hydroquinone (Fig. 2). When the oxygen uptake in the first run had ceased, 250 mg of hydroquinone was injected into the reactor (second run), and the oxygen uptake was measured again. When the gas consumption had ceased, the catalyst was filtered off and used in a new experiment to oxidize 250 mg of hydroquinone (1st run after filtration). The catalyst was filtered off again and used in new experiments (2nd, 3rd and 4th experiments after filtration). Only details on the 1st and 4th experiments are to be found in Fig. 2. The zeolite-encapsulatediron phthalocyaninecatalyst was also active in this reaction and the catalytic activity was similar to that of the free complex. After the injection of a new portion of hydroquinone, the catalyst showed almost the same activity as in the first run, indicating that there was no catalyst deactivation during the reaction. The supported catalyst can be filtered off and used in new experiments. The same catalyst was reused after filtration in four subsequent experiments without appreciable change in catalytic activity.

3.3. Oxidation of l-decene to 2-decanone.

The oxidation of l-decene in aqueous DMF with catalytic amounts of Pd(OAc),, hydroquinone and iron phthalocyanine was also studied (Fig. 3.). In the case of the free complex, after a relatively fast induction period (about 30 minutes), the reaction showed a linear uptake of oxygen, close to the consumption of the theoretical amount. The rate was similar to the literature value reported by Backvall and coworkers [5]. The zeolite-encapsulatediron phthalocyanine catalyst exhibited a similar behavior. When the oxygen uptake in the first run had ceased, l-decene was injected into the reactor (second run), and the oxygen uptake was measured again. A similar rate was measured in the second run as in the first one, i.8. the catalytic activity did not decrease during the oxidation reaction, in spite of the presence of the strong acid HCIO4 in the reaction mixture. When the oxygen consumption had ceased, the catalyst was filtered off, washed, dried and used in a new experiment (Fig. 3.). The catalyst reused in a new experiment showed a similar activity to that of the previous experiment.

422

25

0, uptake (cm’)

20 15 10 5 0

0

2

6

4

8

10 12 14 16 18 20 22 2 4 2 6

Figure 1. Oxygen uptake in the oxidation of hydroquinoneto benzoquinone catalyzed by iron phthalocyaninecatalyst.

-- I

-0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 t (h)

Figure 2. Oxygen uptake in the oxidation of hydroquinoneto benzoquinone catalyzed by zeolite-encapsulatediron phthalocyanine catalyst: (*) first run; (+) second run, after injection of a new portion of hydroquinone; (x) 1st experiment after filtration; (B) 4th experiment after filtration.

423

25 20 0, uptake (cm')

15 10 5

n -0

2

4

6

8

10 12 14 16 18 20 22 24 t (h)

Figure 3. Oxygen uptake in the oxidation of 1-decene to 2-decanone catalyzed by free and zeolite-encapsulatediron phthalocyanine catalysts: (*) Fe(Pc); (+) Fe(Pc)/Z, first second run, after injection of a new portion of 1-decene; filtered catalyst.

25

m

m X

+

20

0, uptake (cm3

15 10 5

0 0 2 4 6

8 10 12 14 16 18 2022 2426 282 t (h)

Figure 4. Oxygen uptake in the oxidation of cyclohexene to 1-acetoxy-2-cyclohene: (*) Fe(Pc); (+) Fe(Pc)/Z; (x) Fe(Pc)/Z, second run, after injection of a new portion of cyclohexene; (m) Fe(Pc)/Z, filtered catalyst.

424

3.4. Allyllc oxldatlon of cyclohexene.

The reaction of c clohexene in acetic acid in the presence amounts of Pd(OAc, hydroquinone and iron phthalocyanine sulted in a smooth oxidation (Fig. 4.). The iron phthalocyanine activity as that reported by BBckvell and coworkers [5]. The zeolite-encapsulated iron phthalocyanine catalyst was also active in this reaction, with an activity similar to that of the free complex. No decrease in catalytic activity was observed during the reaction, and after filtration the catalyst was reused in a new experiment without appreciable loss of catalytic activity (Fig. 4.).

4. CONCLUSIONS

Zeolite-encapsulated iron phthalocyanine proved to be an active and stable catalyst in the oxidation of hydroquinone and in the triple catalytic oxidation of l-decene and cyclohexene. Product distribution, selectivity and yield were similar to those obtained with free iron phthalocyanine. The oxidation of hydroquinone occurs in the zeolite domain, but the other reactions take place more probably in the bulk liquid. No decrease in catalytic activity was observed during the catalytic reaction. The zeolite-encapsulatedcomplex is easier to handle than the non-supported one, it can be removed from the reaction mixture by simple filtration and it can be reused in several subsequent catalytic runs with similar catalytic activity.

REFERENCES

1 R.A.Sheldon and J.K.Kochi, Metal-catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981. 2 B.Meunier, Bull. SOC.Chim. Fr., (1986) 578. 3 J.E.Backvall and R.B.Hopkins, J. Am. Chem. SOC., 109 (1987) 4750 4 J.E.Backvall, Stud. Surf. Sci. Catal., 41 (1988) 105. 5 J.E.Backvall, R.B.Hopkins, H.Grennberg, M.M.Mader and A.K.Awasthi, J. Am. Chem. SOC., 112 1990) 5160. 6 J.E.Backvall, R.L.Chowd ury and U.Karlsson, J. Chem. SOC., Chem. Commun.,

‘h

7 1;1991) .Meyer, 474D.W.Wohrle, M.Mohl and G.Schulz-Ekloff, Zeolites, 4 (1984) 30. N.Herron, J. Coord. Chem., 19 (1988) 25. 9 N.Herron, G.D.Stucky and C.A.Tolman, J. Chem. SOC.,Chem. Commun., (1986) 1521. 10 R.F.Parton, L.Uytterhoeven, and P.A.Jacobs, Stud. Surf. Sci. Catal., 59 (1991) 395. 11 G.T.Byrne, R.P.Linstead and A.R.Lowe, J. Chem. SOC.,(1934) 1017.

8

Acknowledgments Financial support (T 4182/92) from the Hungarian National Scientific Research Foundation (OTKA) and from the Eastern European Exchange Program of University of Uppsala is gratefully acknowledged.

M. Guisnet et al. (Editors), Heterogmeous Catalysis and Fine Chemicals 111 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

425

Some physical correlations with the catalytic activity of Mom)-grafted carboxylated resins used as epoxidation catalysts E.Tempesti.E.Ranucci Uipartimento di Ingqperia Meccanica-Universita di Brescia (Italy) C .L. Bianch,V.Ragaini Dipartimento di Chlmica Fisica-Univenith d~ Milano (Italy) L.Giuffie.G.Airo1di.C. hlazzoccha Dipartimento di Chimica Industriale-Politecnico di Milano (Italy)

Abstract

By using dfferently functionalized polymeric supports conventional &lo(\?) catalysts have been successfully hetempmized and tested in liquid phase epoxidation of cyclohexene. In order to assess if metal centers of lower oxidation states are involved in the reaction, some physical correlations njth the catalytic activity have been collected and rationalized in terms of different oxygen environments of the active catalytic site.

1. INTRODUCTION

Molybdenum contaimng catalysts are used rn many industnal processes. For example 110 naphtenates are active homogeneous cataljsts for the epoxldation of olefins (Halcon process) w t h organic hydroperoxidesl. Conventional Mo(VI) catal) sts have been successfully heterogenized by a multifaced approach starting fiom commercial1 available polymeric supports fmctionalized wth surface boromc293 or phosphonic 4y groups. 2. EXPERIMENTAL

With commercial resins it is not possible to control the number and distribution of acid groups. Ths results in a poor flexibility of structure and properties of the final catalyst.

426

To avoid this drawback we have synthesized two carboylated resins506

-

1

COOH

I

1

COOH

0

I

L

RESIN R1

0

II

I/ I

I

CROSSLINKED

COOH

2

2

In

CROSSLINKED RESIN R2

referred to as R1 and R2 by stepwise Michael polyaddition of di-nucleophiles such as a bis-aniine (X.Y-bis(2-carboxy ethyl) anline) and a bis-thiol (2,2’-ethane&thol) to an excess of an -unsaturated compound (1,4-bis-acryloyl-piperazineand 2,2’-bisacrjlarnido acetic acid respectively). On b0t.h R l m d F12 resins the Mo(V1) grafting procedure67 was performed in the presence of H2Q as follows. The resin (0.5 g, R1 or R2) is suspended in a ti;l(lldioxane solution (25m1/15ml: vlv) O f Nq$!q04 . H20 (1.5 g), the pH is fixed at 1.0 with HC1 1 M and H2Q (807r. v/v; 1 ml) is added. The slurry is stirred mildly at 40°C for 2 1. brought almost t.0 dryness under vacuum, washed with H20 (25.0 ml), dioxane (2(J.0 ml) and ether (20.0 ml) and finally dried under vacuum. (25-318 ?uIo,whv, as determined by atomic arlsorption).

3. RESULTS AND DISCUSSION h’e have found by EPR that the yields of epodde vary as a function of pentavalent Mo initially present8 on the grafted support. Indeed under the same activating conditions(1 h at 80’C in ethylbenzene with a fixed ROWMo ratio) the activity of the catalysts is definitely affected by the MooIMototratio which varies as a function of the diffei~ntH W M o ratios adopted in the graftlng procedure. TABLE 1 reports some typical results.

427

TABLE 1- Yield of Cyclohexene Oxide as a Function of Different Catalyst Activation Conditions

YIELD ' 3 3

RUM

H202/M0l

MoV/Mot& 5min. 15min. 30min.

a

b C

d

0.00 0.41

0.100 % 0.025 9" 0,010 To 0.010 %

0.83 1.66

0.0

0.0

0.0

3.2

7.1 20.6

1.4 27.2 53.8

31.9

56.6

72.4

60min. 3.6

44.1 74.4 83.7

I - Different H2OL/Jlo ratios used in the graftmg procedure. 2 - Evaluated by EPR. 3 - Reaction condltions: Mo 0.30 mmol; t-Butylhydmperoxide 9 mmol; Cyclohexene = 19.78 mmol; Tot-a1volume = 40 ml (made up with ethylbenzene);

-

-

Temperature = 80°C. All catalysts were further conditioned (tBHP/Mo 929.7;1 h reflux in ethylbenzene) before use. It thus appears that on increasing the quantity of H2Q used in the grafting procedure both a variation of the number of paramagnetic metal centers and a change in the ligand field s m n d i n g them can be related to the varied activity of the catalysts. To frrrther emphasize these findings and to better define the metal modifications whch occur under different activating conditions we have carried out an WS investigation within the same experimental condtions reported above R1 and R2 samples were analyzed under the following conditions: a) as grafted; b) as grafted but without HpQ in the grafting procedure; c) same as a) but activated as reported in TABLE 1, run d: d) same as c) but after the usual cyclohexene epoxidation. TABLE 2 reports also the observed bindmg enexyes relative to three samples chosen as standards, namely t,he inorgauc precursor used (Na2MoO4) and two known homogeneous catalysts Mo(0) CQ) [C5&N(CQ)2] * H20 and [Mo(O) (Q)2 (CsH&"Q)] indicated with (I) and (ID. The latter two are both characterized by 0-0 pcroxo bridges9 and do not require activation prior to use.

428

I

0

',

i

0

TABLE 2 - Binding Enerjpes (ev) from XPS of Molybdenum Containing Resins XqMoO4

M%d5/2 M%d3/2 231.8 234.9

FWMH 1.2

(A)

As grafted* R1 R2

R1 R2

(I) (11)

235.9

F"MH 1.5

235.9

1.8

$hid512 Mo3d3/2

232.7 ( L)

232.8 (bl)

A s graftedb

M e r activation

& h d 5 / 2 Mqd3/2

Mqd5/2 Mqd312

(D)

(D

233.5 233.6

236.3 236.7

M%d5/2 M%d3/2 232.8 235.9 (81 233.1 236.2

232.9 233.0

236.2

After use M%d5/2 M%d3/2 232.4 235.6

236.6

232.6

(F) 236.1

1.4

(0 abGrafting performed in the absence or in the presence of H2Q, The XPS (see. e.g.. Fig l-a) showed characteristic doublet peaks of hexavalent molybdenum ions. Referring specifically to R1 and R2 grafted in the absence of H2@, the full wdths of half maximum height of the Majd512 emission peaks were 1.5 eV and

429

__

..

-H

. . . .

,-', ',

I)

I ,

G

,

,

\

I

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

1

239.6

,

I

\

.....

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

242.6

1

\

I

236.6

I

1

233.6 B.E. (eV)

230.6

227.6

Fig. 1 - a Characteristic M O O doublets observed by X P S (For symbols refer to TABLE).

I)

t

I

G

C

-~

234 0

f

A

i 231'5

Fig. 1 - b Mo3d5/2 emission peak shifts as a function of different M o m electronic environments (For symbols refer to TABLE).

430

1.8 eV wluch are sensibly u e r than the Mw5/2peak of X"jqJloO4 (1.2 e\?. This broadening effectmay be due to interactions of the support with Mo ions. Finally, if reference is made to R1 and R2 grafted in the presence of H2Q and duely activated prior to use (see,e.g. Fig. 1-b) it may be assumed that the structure and environment of Mofi9 ion affects the electronic property (a degree of covalencylo) of the Xlo-0 bond by fairly advocating useful comparisons with (D and (11) in terms of 0-0peroxo bridges requirements onthe active catalytic site. 4. CONCLUSIONS

The results reported above are somewhat at variance with some recently published assumptions11concerning the formation of the active pol-mer-supported Mo(V1) catalytic center. Specifically in our view - reduced molybdenum('l')is not necessarily involved in the catalytic activity and - molybdenum activation does not necessarily involve oxidation of Mo(V to MdW.

5. REFERENCES

1 R. LandauG.A.Sul1ivan.D .BmwnChemtech (1979, 602.

2

3 4 5

6

7 8

9 10

E.Tempesti,L.Gi~,F.DiRenzo,C.Mazzocchia,G.Airoldi,Appl.Cat 46

(1986l.285. E.Tem~esti.L.Gi~.F.DiRenzo.C.MazzocchiaP,G~nchi~.Mol.Cat..45 (19881.255. E.Tempes ti.L.Giu~,F.DiRenzo.C.Mazzocchia,P.GronchiJ.Mol.Cat.dS (19891.371. P.Fenuti,E.Ranucci,E.Tempesti,L.Gi&,P.~l~ti~.Appl.Pol.Sc.,41. (199011923. P. Femrti, E.Ranucci, E,Tempesti, M.Casolaro, Makromol Chem., Macromol. Sjmp. 59(1W2)L)381. P.Fe~ti,E.Tempesti,L.Gi~,R.Ranucci,C.Maz~chi~S~d.Surf.Sc. and Cat.,59.431(1991). E.Tempes ti.F.Morazzoni.L.Gi&A.Manchi,G.Airoldd.Mazzocchia.Paper presented at the Seventh International Symposium on Relations between Homogeneousand Heterofineous Catalysis. Tokyo,l7-21/5/1992 S.E.Jacobson.R.Tang,F.Mares,lnorg.Chem..l7( 11) (1978). 3055. G.K.Boreskav,Proc.SthInt.Congr.Catalysis,2 (19721, 981. D.C.SherrinntonS.Simn. J.CataI..31.(1991).115.

M. Cuianet et al. (Editors),Ht!tnogmeowCatalysis and Fine Chemicals III (D 1993 Elsevier sdence Publishers B.V. All rights reserved.

431

PHOMCATALYSED OXIDATION OF lr4-PE"EDI0L ON W-IUUMINATED SUSPENSIONS OF ZrTi04 POWDERS J.A.Navioa, M.Garcia Ganezb, M1.A.Pradera Adrianb and J.Fuentes m ab aInstituto de Ciencia de Materiales,Universidad de Sevilla-CSIC,Apdo.l115, 41080,Sevilla,Spain, and Dpto. de W c a Inorg&ica,Facultad de -car 41012,Sevilla,Spain. bDpto. de Q u x c a Orghica,Facultad de Quhica,Universidad de Sevilla,41012, Sevilla,Spain.

Abstract The photocatalytic oxidation of lr4-pentanediolhas been investigated in W-illuminated acetonitrile suspensions of zirconium titanate, ZrTiOq, pcy der under oxygenated conditions. The temporal course of the photo-oxldation of this diol was monitored by GC-MS technique. A practically total regioche mica1 preference for oxidation of the primary hydroxyl group in 1,4-pentang diol was observed, with 4-hydroqpentanoic acid and its Y -1actone derivati ve being generated as the sole products. Mechanistic delineation for expla& ning the observed regioselectivity is proposed. 1. IMRammIm

Excitation of a semiconductor particle with a photon of energy greater than the band-gap induces charge separation by creating an electron-hole

pair. The capture of the photogenerated hole by an adsorbed donor and of the photogenerated electron by an adsorbed acceptor allows efficient o e dation and reduction respectively on a c m n surface. The surface, if che mically modified can influence not only the quantum efficiency of the orga nic redox reactions [ l ] but also the chgnical selectivity via specific acti vation of a potential-matched redox-active functional group. The surface also acts as a template for organizing the reactants, transition statesrand products after initiating the primary electron exchange. Relevant improVements have been reported in selective organic redox re% tions on illuminated semiconductor particles [ 2-3 1 Hawever, most investi gations have been concerned with functional-group activation, and little a$ tention has been devoted concerning the ability of an excited semiconductor to selectively initiate redox reactivity at one site in a multi-functional molecule. The photochemical oxidation of lr4-pentanediol (as a model canpound) on illuminated suspensions of Ti02 in oxygenated acetonitrile media has been previously investigated by Fox et al. [ 4 ] ; the photoreaction produces a canplex mixture of products, the canposition of which evolves with extended illumination period. At short reaction times, the photooxidative transfonng tion can be understood as involving initial oxidation of the alcoholic sites

.

432 to carbonyl canpounds, eqn.(l).

1

TiO,*

OH

CH$N

LCHO

02

Upon extrapolation to time

t

2

3

zero, a strong preference for oxidation of the . Zr02 sanple was much less active than

primary alcohol is attained [41

Ti02 as photocatalyst. The present study was undertaken to establish whether illuminated ZrTiO suspensians could induce preferential photooxidation at one site of a diof. The 1,4-pntanediol was chosen in order to cmpare our results with those previously reported by Fox et al. [4 1 when pure oxides, Ti02 or Zr02, were used.

Materials

l,4-pentanediol (Aldrich, 99% pure) was used as supplied without further treatmnt. The solvent used for the photocatalytic reaction was acetonitrile (Aldrich,HPLC grade Cataly8t prEparatj.Cn Pawdered zirconium titanate, practically pure, was prepared by same of us [ 51 using a sol-gel method. The precipitation of the solid was obtained by hydrolysis of an alcoholic solution of equimolar amounts of Tic14 (Merck, 99.99%) and of ZrCCl2 (Fluka ?!G, 43-44% ZrO2) in the presence of an excess of hydrogen peroxide. After washing, the precipitate was dried and calcined at 700X for 2 h; the solid obtained has the structure of ZrTiO4 and a s~ cific k e a of 39.5 m2g-1. The surface acidity (sum of Bronsted and Lewis si t e s ) was measured by a spectrophotmtric titration method [ 6 1 using pyri dine or benzoic acid as adsorbates. A surface acidity of ca. 0.06 p o l s m-2 and a surface basicity of ca. 0.13 p mls m-2 was found for this catalyst.

.

-q=3

W-Visible DRS (Diffuse Reflectance Spectranetry) Diffuse reflectance spectra were obtained with a Perkin-Elmer Lambda 9 spectrcphotaneter using Bas04 as a reference. The Kubelka-Munk function was used to express the experimental data. Photochemical reactor and light source The photocatalytic oxidation of 1,4-pentanediol was carried out in a Applied Photophysic Ltd. photochemical reactor equipped with a 400 W medium pressure mercury-arc 1 , radiating predaninantly at 365-366 nm. This lamp produces m r e than 5 x 1 7 photons s-l within the reaction flask. It was COG tained in a double-walled quartz-glass inmnersion well, through which water was passed for cooling. A borosilicate glass sleeve was used to remove short wavelength radiation (less than 300 nm). A gas inlet reaction flask (400 mL) was used: a double surface reflux condenser fitted to the reaction flask was

433 used in order to prevent "creep" and loss of vapour. kti-mds andanalysis

The photocatalyst, ZrTiO4 (250mg) was suspended in a solution of 1,4-pe! Oxygen was bubbled tanediol (624 mg, 6 m l s ) in acetonitrile (300 a). through the suspension and a positive pressure of the gas was mantained du ring the period of illumination (25 h). The photocatalyst was separated by centrifugation, to analyse the liquid phase. Identification of products was performed by GC-MS technique using a Kratos-MS 80 RE'A instrument fitted to a GC Carlo EYba. Separations were achieved on a CP-SIL 5 C.B.W.C.O.T. (25 m x 0.32 m) column whose tatperatwe was programned fran 30% (5 min.) up to 1502C (10 min.) at 15: min-l. In a separate experiment, the liquid phase of the reaction products was subjected, previous to GC-MS analysis, to a sily lation procedure according to the method described [ 7 ] In this case, an SPB-20 (30 m x 0.32 m) column was used and the temperature was raised fran SOX (1 min.) up to 2902C (26 min.) at a rate of 122 min-l.

.

3.

REjuLTs

DIs(IISsI0N

Diffuse Reflectance Spectroscopy Diffuse reflectance spectra of ZrTiO and its two mother oxides ZrO2 and Ti02 are shown in Fig.1. The three s q t e s have spectra with ban& in the s a m position, but the intensities are different fran one to the other. As has been previously established [ 81 isoelectronic substitution, such as Zr4+ for Ti4+ in Ti02, does not change the concentration of electrons at F =O K in the conduction and valence band but it does raise the conduction-band edge. According to the UV-V DR spectra in Fi .1, the equholar subst& tution of Zr4' for Ti4+ does not sign& a ficantly inprove the spectral response of ZrTiO4 if it is c v e d with those Samples Eg/eV obtained for Ti02 and ZrO2. In fact, the experimental values for the band-gap, deduced fran the UV spectra, re_ veal only a small rase of ca. 0.04 eV, in the absorption edge of zrTi04 if it is canpared with that for Ti02. Reaction performance Under the conditions of our experi mental photooxidation of 1,4-pentans diol, over a period of illumination of 25 h, using ZrTi04 as photocatalyst the GC-MS analyses shaved the appear? ce of only two adjoining product peaks 200 300 400 (major, m12.00 and minor,RT=12.05); bnrn their relative intensities increase with illumination times. Bath peaks Figure 1. W-V DR Spectra of shawed the smne mass-spectra. These samples (a) T102; (b) ZrTi04; ( c ) Zr02 results indicate, in principle, that under our experimental conditions two Caridation products are obtained which shaved the same mass-spectra.

434

Only a few of the several products generated (see Scheme I) under the photccatalysed oxidation of 1,4-pentanediol (1)using Ti02 or 21-02 [4 ]have been ruled out. Thus,the canparative differences in rqh values and intensities between our MS data and those included in our MS library for known spectra, leads to the rejection, fran our experimental results, of the fos mation of the following products: 4-oxo-l-~tanol(31, tetrahydro-2-n~tl-yl -2-furanol (51, 4-oxopentanoic acid ( 8 ) and 4,5-dihydro-2-mthylfuan (10). s(BEME1

Prcducts formed during the photccatalysed oxidation of 1,4-pentanediol (1) (ref.4)

6

3

8

Mixture of dimenc diastereomers 5

10

Although the canpunds 4-hydroxypentanal ( 2 ) , tetrahydro-5-methyl-2-furanol (4) and 4-oxopentanal ( 6 ) were not camercially available and their mass-spectra were not found in our library, their mlecular masses are incanpati ble with our MS data and therefore must also be rejected as one of our phg tcgenerated products. The sole products which seem to be canpatible with our MS results are the 4-hydroxypentanoic acid (7) and its Y -1actone der; vative named hereafter ( 9 ) . Although canpund ( 7 ) is not camrercially avails ble and its mass-spectrum is not described in the MS library, the MS data for the trhethylsilyl derivative of this acid is very well described in

435

.

the literature [7] Accordingly, the s i l y l a t i o n of the l i q u i d phase of t h e reaction products gave a GC* i n which the t r h e t h y l s i l y l derivative of the ccmpound (7) w a s identified. On the other hand, the Y -1actone derivative of carpound 7,named ( 9 ) , is ccmnercially available. The ccnparison between the mass-spectra of our photogenerated products and the corresponding t o the carpound ( 9 ) revealed t h a t i n our MS are included t h e spectrum of canpound ( 9 ) plus other peaks which could be catpatibles with canpound ( 7 ) . Based exclusively on the GC-MS and because of t h e close vecinity of t h e two GC peaks, it could be possible t h a t the acquisition of MS f o r both gave the same mass-spectra, which should be tentatively assigned t o t h e 4-hydr~ xypentanoic acid (7) and t o its Y-lactone derivative ( 9 ) , respectively. Although the chemical y i e l d of t h i s process has not been estimated, quali tative evidence by ccrrp3aring t h e relative i n t e n s i t y of the GC p e a k s f o r lr4-pentanediol (1)and 4-hydroxypentanoic acid ( 7 ) , showed t h a t t h i s values should be very low indeed, even a f t e r 25 h of i l l m i n a t i o n , i n s p i t e of t h e high s e l e c t i v i t y observed. Mechanistic approach In principle it can be infer t h a t the mechanism involved i n t h e ZrTiO4-photocatalyzed oxidation of lr4-pentanediol follows the same mechani srn previously described f o r these t r a n s f o m t i o n s over Ti02 or ZrO2 [ 4 1 which is p a r a l l e l t o the general proposed mechanism f o r t h e saniconductor photoca lysed oxidation of monoalcohols [91 Haever, the high r e g i o s e l e c t i v i t y *we f i n d f o r the oxidation of the primary alcohol functionality i n 1 , 4 -pentanediol (l),with 4-hydroxpentanoic acid (7) produced as p r a c t i c a l l y t h e only photogenerated product, c l e a r l y indicates t h a t c e r t a i n structural and surface differences between ZrTiO4 and Ti02 or ZrO2 must account f o r the regiochanical preference f o r oxidation of the primary alcoholhkn ZrTi04 is used.In f a c t , although Z f l i O 4 is constituted by the same elements a s t h a t of titania and zirconia, and sham very similar optical absorption p r o p e g ties, h a e v e r i t s s e l e c t i v i t y i n photoassisted oxygen isotape exchange (OIE) is quite different f r a n t h a t of Ti02 and ZrO2 [ 1 0 ] . On t h e other hand the photoadsorbed oxygen species on ZrTiO4 is exclusively 0- [lo] whereas 0on ZrO2 [ 111 and simultaneously 0- and 0; on Ti02[12f have been found. Taking i n t o account these data and t h e surface acid-base p r o p e r t i e s o f zrTi04, the mechanism s u n a r i z e d i n Schane I1 is proposed.

.

schene I1

Ti4' h+

+

===

h+vb

RCH20H

+

(0

e-cb

-

RlH20H

-H+

RkHOH

(ill (iii)

0,'

+

+ HO;

H+

R~HOH

HO;

-H2O

RCHOH 0-OH

RCO,H

An electron-hole pair is created, as a result of optical excitation of tfE semiconductor. The photcgenerated hole in zrTiO4 is capable of i n i t i a t w a single electron-axidation of the primary adsorbed alcohol. Deprotonation of this species will produce an 0-hydroxy radical. The photogenerated el% tron can be trapped by adsorbed oxygen to form superoxide adsorbed oxygen species. A sequential proton transfer to the superoxide adsorbed species would be expected to occur generating adsorbed hydroperoxy radi cals, OH; The interaction between the a-hydroxy and the hy droperoxy radicals will produce the 4-hydroxypentanoic acid ( 7 ) via dehydration of a hydroperoxy intermedia compound. This la ter dehydration process should be invoked as a concerted two-centre mechanism [13] on the acid (Zr') and basic (Ti') sites of ZrTiOq. On the other hand, an additional contribution for selective activation may lie in preferential adsorption effects. Thus a single-site adsorption has been observed in the electrochemical ox& dation of 1,2-propanediol [14] With a single adsorption site, that rz gion of the molecule more closely associated with the surface would be predicted to suffer the m r e rapid oxidation. In fact, Fox et a1.[4]have Shawn that primary alcohols are more effectively adsorbed than are seconds r y substrates.

.

.

4. OexWsIaJ

We can conclude, that under the experimental conditions repog ted here, for the photo-oxidation in oxygen of 1,4-pentanediol over illuminated ZrTiO4 particles, a practically total regiosg lectivity for oxidation of the primary hydroxyl group is obseg ved giving, based on the GC-MS study, the proposed 4-hydroxypen tanoic acid associated with its y-lactone derivative as pro duct. 5. ACKNOWLEDGEMENTS

This work was partially supported by "Junta de Andalucia" (Res. Sept.88) and by a Franco-Spanish Integrated Action (Spa nish Contract: HF-048,1992). One of us (M.G.G.) wishes to thank the "Ministerio de Educacidn y Ciencia" (Spain) for the award of a scholarship. Finally, we are grateful to Dr. Pierre Pichat (CNRS, Ecole Centrale de Lyon,France) for helpful discussions and continuing collaboration on this and related photocatalytic transformations. 6. REFERENCES

1 M.A.Fox, Adv. Photochem.,l3 (1986) 237. 2 M.A.Fox,"Photocatalytic Oxidation of Organic Substrates",in: "Photocatalysis and Environment, Trends and Applications", M.Schiavello (ed.), NATO-AS1,Series C, Vo1.237, by Kluwer Academic Publishers, Dordrecht, The Netherlands,l988, p.445. 3 P.Pichat and M.A.Fox,"Photocatalysis on Semiconductors"; in: "Photoinduced Electron Transfer", Part.D, by Elsevier Sci.

437

4 5 6

7 8 9

10 11 12 13 14

Publishers, The Netherlands, 1988, p.241. M.A.Fox, H.Ogawa and P.Pichat, J.Organic Chem.,54 (1989)3847. J.A.Navio,F.J.Marchena, M.Macias,P.J.Sanchez-Soto and P.Pichat J.Mater. Sci., 27 (1992) 2463. J.M.Campelo, A.Garcia, J.M.Gutierrez, D.Luna and J.M.Marinas, J.Colloid Interface Sci., 95 (1983) 544, and references cited therein. O.A.Mamer, J.C.Crawhal1 and S.S.Tjoa, Clin. Chim. Acta, 32 (1971) 171. G.Bin-Daar, M.P.Dare-Edwards, J.B.Goodenough and A.Hamnett, J.Chem. SOC., Faraday Trans. I, 79 (1983) 1199. R. I .Bickley, "Heterogeneous Photocatalysis", in: "Catalysis" Vo1.5, from Specialist Periodical Reports, The Royal SOC. of Chemistry, London, Great Britain, 1982, p.325 and references cited therein. H.Courbon, J.Disdier, J.M.Herrmann,P.Pichat and J.A.Navio, (submitted for publication). H.Courbon and P.Pichat, R.Acad. Sci. Ser., 285 (1977) 171. H.Courbon, M.Formenti and P.Pichat , J.Phys. Chem.,81 (1977) 550. K.Tanabe, in: "Solid Acids and Bases, Their Catalytic Activi ty", by Academic Press, New York (1970). H.Huser, J.M.Leger and C.Lamy, Electrochim.Acta, 33 (1988) 1359.

This Page Intentionally Left Blank

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

439

Selective electrocatalytic oxidation of sucrose on smooth and upd-lead modified platinum electrodes in alkaline medium. P. Parpot, K.B. Kokoh, B.Beden, E.M. Belgsir, J.-M. LBger and C. Lamy Laboratoire de Chimie 1, "Electrochimie et Interactions", URA au CNRS 350, Catalyse en Chimie Organique, UniversitB de Poitiers, 40, avenue du Recteur Pineau, F-86022 Poitiers cedex, France.

Abstract This work aims t o investigate the electrocatalytic oxidation of sucrose on smooth and upd-lead modified platinum electrodes i n order to find experimental conditions for selective electrosynthesis of high value added products. The preliminary results obtained show that the oxidation of sucrose on Pt-Pb electrodes leads mainly to C12 products, such as the 1'-monoacid and the 6-monoacid of sucrose.

1. INTRODUCTION

The electrocatalytic oxidation of sucrose has only been the subject of a few investigations. The chemical oxidation of sucrose was firstly mentioned in the works of Bresler (1)and Usch (2). Karabinos (3) analysed the oxidation products of fructose, glucose, glucono-y-lactone and sucrose in 0.5 M NaHC03. The author concluded that the main reaction products were C02 and H2O. Bockris et al. (4), investigated the electrochemical oxidation of different carbohydrates at platinum electrodes for their possible use in fuel cells. They noticed that the electroactivity was better in alkaline medium than in acidic medium, and that the reactivity of the molecule decreased with increasing molecular weights. Court (5) used cyclic voltammetry for comparing the activity of different catalytic metals towards the electrooxidation of sucrose. However, in spite of some attempts, the analytical techniques available a t that time did not allow the identification of the reaction products. Other studies (6, 71, although they were also carried out by cyclic voltammetry on noble metal electrodes (Pt and Au) and on nickel electrodes, aimed t o improve the amperometric detection of carbohydrates and were thus not directly related t o our purpose. The carboxylic acids derived from sucrose may find some use in pharmaceutical and agricultural chemistry. The uronic and 2-keto-aldonic acids obtained by hydrolysis of these compounds represent a great industrial

interest for the production of detergents, foods, emulsifiers and pharmaceuticals. Edye et aZ. (8)stated that with a catalyst consisting of platinum deposited on carbon, the chemical oxidation by oxygen, at 100°C and at a constant neutral pH, was highly specific for the production of carboxylic acids at the 6-and 6'-positions of sucrose. This work aims to improve the selectivity of the electrocatalytic oxidation of sucrose into its monocarboxylic acids without breaking the squeleton of the molecule.

Programmed potential electrolysis was the main electrochemical method used in this work. It consists in controlling the oxidation potential which is a key parameter of selectivity for an electrocatalytic reaction. The potential programme was composed of three potential plateaux (9,lO).The first one is set a t a low potential for adsorbing the molecule and/or the adatoms by underpotential deposition and the second one is the oxidation potential. The third one is used to desorb the poisoning species formed during the oxidation in order to regenerate the electrode. In fact, a recent study by in situ FTIR spectroscopy showed that the platinum electrode surface is covered, a t low potentials, by adsorbed carbon monoxide coming from the partial dissociation of carbohydrates (11). These adsorbed species are probably responsible for the electrode poisoning during the second potential plateau. The analysis of the reaction products was performed using High Performance Ionic Chromatography (HPIC, Dionex 4500i). It works with a ternary gradient of elution and includes an ion-exchange column (ASEiA, Dionex) and a conductimetric detector followed by a refiactive index detector. The 13C and 1H spectra of reaction products were obtained on a Brucker-WP 200SY NMR spectrometer (200 MHz) using CD30D solutions with tetramethylsilan or aqueous solutions ( D 2 0 ) with methanol as internal reference.

3. RESULTSAND DISCUSSION 3.1. Voltammetric study

EZectrocatalytic oxidation of sucrose on smooth platinum The electrocatalytic oxidation of sucrose on platinum and other metal electrodes is mentioned in a previous paper (12). As it is shown in figure 1, the maximum current density of the oxidation peak, observed a t 0.69 V/RHE during the positive potential sweep, is very low (imax=0.13mA cm-2). The modification of the so called "hydrogen region"

441

(O<E<0.45WRHE), shows that sucrose is more or less adsorbed on the surface. During the negative potential sweep, the oxidation process is very weak and no notable oxidation peak does appear. i / m cm ~ -2

0.1

-

0

-

-0.1 O . 1 I I

0

I

I

0.5

1.0

1 1.5

EN(RHE) Figure 1. Voltammograms of a platinum electrode, recorded at 50 mV.s-l, in 0.1 M NaOH , at room temperature, (-----I without sucrose, (-1 in presence of 0.01 M sucrose.

Electrocatalytic oxidation of sucrose on upd-lead modified platinum Lead adatoms have important effects on the electrocatalytic properties of platinum (13-15). U ~ cm A -2

I

0.5

0

-0.5

I-

I 0

I

0.5

I 1.0

I

1.5

EN(RHE) Figure 2. Voltammograms of an upd-lead modified platinum electrode, recorded at 50 mV.s-l, in 0.1 M NaOH, a t room tempkrature, in presence of 0.01M sucrose : M Pb2+ ;(----I 5 M Pb2+ , (--) 5.10-4M Pb2+, (-

In the case of the electrocatalytic oxidation of sucrose at a platinum electrode

442

in acidic medium, the presence of lead adatoms greatly enhances the oxidation current leading to a well-defined oxidation peak. This is particularly shown in figure 2. The under-potential deposition of lead adatoms occurs during the negative sweep of the potential. During the positive sweep, the first oxidation wave is shifted towards more negative potential and starts from 0.4VLRHE. The maximum current densities are obtained in presence of 10-5 M Pb2+ at 0.75 VLRHE. Otherwise, during the negative potential sweep an oxidation process is observed with a maximum current density at 0.62 VLRHE.

3 6 Electrolysesof SUQ'Ose Oxidation of sucrose on a smooth Pt electrode

Because of the very low current densities, the electrolysis was carried out using a cyclic variation of potential between 0.1 and 1.6 VRHE during 19 hours a t room temperature. The initial concentration of sucrose was 0.01 M. The electrocatalytic oxidation of sucrose leads mainly t o the 1'-monoacid of sucrose (l'-MAS), and to the 6-monoacid of sucrose (6-MAS) : CHzOH

0

Ho H

O

H

0

O H H l'-MAS

H O H

CHzOH O H H

6-MAS

Qualitative and quantitative analyses of the reaction products were also performed and the chemical yields are given in table 1 : Table 1. Oxidation products f sucrose obtained on a platinum electrode. Identified products % of oxidized sucrose Concentration (mM) I 6-MAS 0.13 I 25.3 l'-MAS 0.10 19.5 Glucuronic acid 0.05 4.9 1.9 Glucaric acid 0.02 Tartric acid 5.1 0.08 Mesoxalic acid 1.5 0.03 Glyceric acid 1.9 0.04 Glycolic acid 1.6 0.05 Glyoxylic acid 4.9 0.10 Formic acid

The conversion yield a t the end of the electrolysis was very low (5%). However, these first results allowed us to identify the oxidation products of sucrose on a smooth platinum electrode. Particularly, it was important t o confirm that it is possible to oxidize sucrose without breaking the acetalic bond.

443

Oxidation of sucrose on a Pt-Pb electrode Electrolysis of 10 mM sucrose was performed on a Pt-Pb electrode using a triple-pulse potential programme. The concentration of the precursor salt, Pb(C104)2, was 10-5M. The first potential pulse, fixed at 0.4 V/RHE during 1s, is used t o adsorb the organic molecule and to deposit in situ the metal adatoms onto the electrode surface. The potential of the oxidation plateau was set to 0.65V/RHE and held 15 s.The third potential pulse was fixed at 1.6V/RHEI during 0.5 s, allowing to reactivate the electrode by clearing out the poisoning species formed during the potential plateau. The oxidation products detected during this electrolysis and their chemical yield are represented in table 2 : Identified products 6-MAS l'-MAS Glucuronic acid Gluconic acid Tartric acid Mesoxalic acid Glyceric acid Glycolic acid Oxalic acid Formic acid

% of oxidized sucrose 11.1

Concentration (mM) 1.00 7.10 0.07 0.13 0.11 0.16 0.05

79.0 0.4

0.7 0.4

0.3 0.1 0.6

0.40

0.16

0.2

0.40

0.3

The main reaction product, 1'-MAS, was confirmed by NMR spectroscopy :

1'

c1

C2

C3

C4

C5

C6

93.7

72.7

74.5

70.8

73.40

61.7

C'1 183.3

C'2 106.1

C'3 78.6

C'4 77.7

C'5 81.5

C'6 62.9

Figure 3. 13C-NMR spectrum of the l'-MAS recorded in D 2 0 with methanol as internal reference.

The 13C-NMR spectrum of reaction products showed thirteen peaks. The intense peak quoted I, in the so called "carbonyl region", is attributed t o an

444 impurity coming from the solvent. The confirmation of the oxidation of the primary hydroxyl group of the fructose moiety into a carboxylic acid group, is based on the two peaks (at 61.6 and 62.9 ppm) in the CH2OH region and the peak (at 183.2 ppm) in the "carbonyl region" of the spectrum. Moreover, in comparison with the spectrum of sucrose, an important shift is observed for the resonance of the anomeric carbon atom of the fructose moiety due to the proximity of a carboxylic group.

Effect of the concentration of lead precursor salt The electrolyses were carried out with different concentrations of the Pb precursor salt ranging from 5 10-6 to 10-4M. Data on the electrolyses of sucrose for different concentrations of the precursor salt of lead adatoms are given in figure 4 :

S(w 100

80 60 40 20

I*+

0 1'-MAS

6-MAS

Figure 4. Electrocatalytic oxidation of sucrose a t an upd-lead modified platinum electrode. Selectivity (S)in 1'-MASand 6-MASfor different concentrations of the precursor salt of lead adatoms

The best selectivity was obtained with lO-sM af Pbz+.For this case, the ratio of l'-MAS/G-MASwas 7.1. 4. CONCLUSIONS

The chromatographic analysis performed during electrolyses on Pt-Pb electrodes showed that sucrose is mainly transformed in 1'-MAS (major product) and 6-MAS. A very few amount of degradation products were detected. However, it must be noted that two reaction products remained unidentified. The main role of lead adatoms seems to be a decrease of the formation of byproducts, and the appearance of new centers for the adsorption of hydroxyl species. required for the oxidation of an alcohol grcrup into a carboxylic group. Conversely, in the case of the oxidation of D-gluconic acid to 2-keto-D-gluconic acid, the change in selectivity in presence of lead adatoms was attributed by

445

Smits to the Pb2+ complexation with the substrate (16). In fact, many complexes of sugars and sugar derivatives with inorganic salts and bases were isolated in solid and often in crystalline forms. There was considerable evidence, based on various physical measurements, that sugar-cation complexes exist in solution (17). A suitable arrangement of some oxygen atoms from the two rings of the sucrose molecule may create the complex [sucrose]-Pb2+. The adsorption of this complex, during the first pulse of the programmed potential may lead to specific adsorption of the sucrose molecule involving a n interaction between the active sites and the primary hydroxyl group of sucrose. This may favor the formation of 1'-MAS in the case of fructose moiety o r of 6-MAS in the case of the glucose moiety.

ACKNOWLEDGEMENTS The authors are very gratefully to the GS "Sucrochimie" for having initiated this work, and supported it.

REEERENCES 1. M. A. Bresler, Bull. SOC. Chim. Fr., 8 (1866) 23. 2. K. Usch, Z. Elektrochem., 6 (1883) 539. 3. J. U. Karabinos, Revista Euclides, Vol. XIV, (1954) 211. 4. J. O'M. Bockris, K. J. Piersma and E. Gileadi, Electrochim. Acta, 15 (1964) 1329. 5. D. E. Court, Ph. D. Thesis, University of Southampton, England, (1984). 6. J. Hughes and D. C. Johnson, Anal. Chim. Acta, 132 (1987) 11. 7. L. M. Santos and R. D. Baldwin, Anal. Chim. Acta, 206 (1988) 85. 8. L. A. Edye, G. U. Msehan and G. N. Richards, Carbohydr. Res., 10 (1991) 11. 9. E. M. Belgsir, E. Bouhier, H. Essis Yei, K. B. Kokoh, B. Beden, H. Huser, J. M. LBger, and C. Lamy, Electrochim. Acta, 36 (1991) 1157. 10. K. B. Kokoh, J . M. LBger, B. Beden, H. Huser and C. Lamy, Electrochim. Acta, 37 (1992) 1909. 11. I. T. Bae, Xuekun Xing, C. C. Liu, and E. Yeager, J. Electroanal. Chem., 284 (1990) 335. 12. P. Parpot, K. B. Kokoh, B. Beden and C. Lamy, Electrochim. Acta, accepted. 13. R. R. Adzic, Israel J. Chem., 18(1979)166. 14. N. Furuya and S. Motoo, J. Electroanal. Chem., 72 1976) 165 ; 78 (1977) 243 ; 98 (1979) 189 ;99 (1979) 19. 15. G. Kokkinidis, J. Electroanal. Chem, 201(1986)217. 16. P. C. C. Smits, Ph. D. Thesis, University of Eindhoven, The Netherlands, (1984). 17. S.J.Angyal, Adv. Carbohydr. Chem. Biochem., 47(1989)1.

This Page Intentionally Left Blank

M.Guisnet et al. (Editm), Heterogeneous Cntulysis and Fine Chemiculs III (B 1993 Elsevier Science Publishers B.V. All rights reserved.

SELECTIVE OXIDATION

447

OF SUBSTITUTED AROMATICS

USING DIFFERENT PEROXIDES. C. Marchal, A. Tuel and Y. Ben Tadrit Institut de Recherches sur la Catalyse. C.N.R.S. 2, avenue A; Einstein. 69626 Villeurbanne CEDEX FRANCE. Abstract Substituted aromatic molecules can be oxidized under mild conditions using Ti or V substituted molecular sieves. The nature and the selectivities of the products formed strongly depend on the oxidant used in the catalytic reaction. H202 favours the hydrox lation of the aromatic ring whereas tert-butyl hydroperoxide is very selective in the si(Ye-chain oxidation. For V-substituted ole ular sieves, we have proposed a mechanism which involves the redox system Vsm,/V2 +.

1. INTRODUCTION. Direct oxidation or hydroxylation of aromatics and particularly alkylbenzenes, is a complex reaction consisting of a variety of parallel and consecutive reactions. Low temperature procedures involving hydrogen peroxide as the oxidant have been reported. They include hydroxylation over Ti-silicalites TS-1 gnd TS-2. Perego et al. [l]re orted that toluene and phenol could be oxidized at 80 C into a mixture of creso s and dihydroxybenzenes respectively. More recently, Hari Prasad Rao et al. observed that phenol [2] and toluene (31 could also be oxidized over VS-2, the vanadium substituted silicalite-2. In fact, the small pore opening of the MFI and MEL structures (= 5.5 A) makes peroxide capable of entering the zeolite Yet, tert-butyl hydroperoxide (TBHP) in the homogeneous epoxidation of olefins catalyzed by titanium alkoxides [4]. VAPOd is a vanadium substituted aluminophosphate with the MPO4-5 structure. This solid has the advantage to possess a structure with relatively large pore openings (7.3 A) thus allowin reactions with tert-butyl hydroperoxide. We have synthesize a series of transition metal substituted molecular sieves and investigated the oxidation of substituted aromatics over these catalysts using different oxidants. Phenol and toluene have been chosen because of their small size and also to determine the influence of a substituent on the nature and selectivities of the products formed.

P

B

448

2. EXPERIMENTAL. Ti-silicalite (TS-I) was synthesized following the procedure re orted by Taramasso et al. 151. Chemical analysis of the calcined sample gave Si/Ti = 5 and U.VVis spectroscopy confirmed the absence of extraframework oxides. V-Silicalite was synthesized with Si/V = 50, according to the method given in

op

[61.

VAPO-5 molecular sieve was prepared using Pseudoboehmite (Catapal B from Vista. 70 c/o Al2O3) as the aluminium source, hydrophos horic acid and tri ropylamine. Chemical analysis of the calcined sample gave a vana ium fraction V/( +AI+P) = 0.02. The catalytic reaction mixtures were refluxed at 353 K in a round bottom flask. They included 10 ml of solvent (methanol or water), 0.5 catalyst, 0.1 mole substrate and Oxidant/Substrate = 0.2. The monophasic ( heno + methanol) or biphasic (toluene + water) system was stirred vigourously at 1 00 rpm with a magnetic stirrer. The reaction products were periodically analyzed by Gas Chromatography using a Tenax GC column attached to a flame ionization detector. For the oxidation of toluene over VAPOS, Mass Spectroscopy was also used to identify the byproducts formed at the beginning of the reaction.

B

6

R f

3. RESULTS AND DISCUSSION. It has been widely reported during the last years that TS-1 was v e 7 active and selective in the oxidation of aromatics using H20 as the oxidant [6 In particular, it was successfully used for the hydroxylation of phenof into a mixture o catechol (the ortho isomer) and hydroquinone (the para isomer) and the hydroxylation of toluene into a mixture of cresols [ 11according to the scheme:

k

In the case of phenol, only the ortho and para isomers are formed. By contrast, all the isomers are observed in the case of toluene. Oxidation of the methyl group, i.e. formation of benzyl alcohol. benzaldehyde and/or benzoic acid has never been oberved. No products were detected when using TBHP as the oxidant : this was blamed on the size of TBHP which is robably too large to penetrate the zeolite channels. Similarly, VS-1 was ound to be very ytive in the hydroxylation of aromatics with H 02.Indeed, phenol was converted at 80 C into a mixture of dihydroxybenzenes, with seectivities comparable to those obtained over "rs-1. However, and as it was already reported by Hari Prasad Rao et al. (21 the H 02 efficiency was only about 50 %. Concerning the oxidation of toluene, VS- is not as selective as TS-1 since toluene is converted into a mixture of cresols and benzaldehyde, the latter being the major product formed.

F

T

T

449

These selectivities are very similar to those observed in the oxidation of toluene over TS-2 in acetonitrile [3]. Like in the case of phenol, the hydrogen peroxide efficiency did not exceed 55 c/o. Because of its large pore opening, VAPO-5 may catalyze the oxidation of aromatics using either H202 or TBHP. When using H202, results were similar to those obtained over VS-1, i.e. hydroxylation occured namely at the aromatic ring.

(42%) (58%) Nevertheless, side chain oxidation also takes place with toluene, su esting that the mechanism of oxidation is not influenced by the catalyst structure. Wit ten-butyl hydroperoxide, results are totally different. Phenol is not converted at all (even after several days) and only traces of catechol were detected. On the other hand, toluene is converted very selectively into a mixture of benzyl alcohol (AL), benzaldehyde (AD), benzoic acid (AC) and dibenzyle (DB). Furthermore, traces of cresols have also been detected by chromatography.

TI

Lg;g~~ (traces) OH

As the conversion increases, the amount of dibenzyle remains practically constant (Fig. 1). The selectivities of the different products chan e with the conversion, which is characteristic of consecutive reactions from toluene to enzoic acid. Dibenzyl and part of the alcohol are formed initially via a mechanism involvin radicals formation. Indeed, the same amounts of dibenzyl and ben 1 alcohol were o tained in the absence of catalyst. Benzoic acid is formed only after a ew hours and it’s selectivity reaches 113 5’% at the end of the reaction.

t

E

7

1

0.8 A

0.6

Ac

AD 0 AL .-E .-.> DB o

4-

OA

3 Q)

v)

0.2

0

10

20

30 Time (hrs)

40

so

Figure 1. Conversion of toluene over VAPO-5 and selectivities.

Moreover, while only 50 % of the hydrogen peroxide were consumed to form cresols and benzaldehyde, the TBHP efficiency is close to 100 c/o, indicating that the non catalytic autodecomposition of the peroxide is very slow. In the case of silicalites, crystals were in the form of very small hexagonal prisms of about 0.2 pm diameter whereas VAPOd crystals were ve large (10-20 pm). The size of the crystals as well as the use of large molecules like TB P are very likely responsible of the low reaction rate. When the original sample was sub’ected to mechanical grindin the crystal size was brought down to 5 pm and the cataiytic activity was increased. &at clearly shows that, under the working conditions, the reaction is limited by the diffusion of the substrate and/or products inside the catalyst channels.

i7

451

4. MECHANISTIC PROPOSITIONS. The experiments described above tend to suggest that the nature of the products formed depend on the oxidant used, but to a lesser extent, on the nature of the transition metal ion. Indeed, it has been shown that H 02 lead preferentially to the hydroxylation of the aromatic ring whereas TBHP oxidizes selectively the side-chain of the molecule. Several authors have investigated the nature of peroxovanadates ( V ) species formed in the presence of alkyl peroxides and their role in catalytic oxidation processes. Essentially, 3 main modes of interaction of hydrogen peroxide and alkyl peroxides with a suitable metal center can be envisaged, leading to species A, B and C respectively :

structure B in the case

distinction between H202 and Thus different mechanisms for the oxidation of substituted aromatics can be proposed, depending on the oxidant resulting in the formation of species B or C. Species C is well known to oxidize electron donor molecules such as olefines :

In the case of substituted aromatics two different epoxides are usually obtained :

+

0

+

R

-

O

9

0

the third isomer being generally absent because of the steric effect of R.

R

452

The next step of the reaction consists in the isomerization of the intermediate e oxide to the hydroxide driven by the preservation of the aromaticity of the hydroxo a kylbenzene :

P

HO

R Catalytic results obtained on the oxidation of toluene and phenol with H20 over V-substituted molecular sieves are in line with the formation of an active species t! as an intermediate during the reaction. Moreover, since the products obtained over TS- 1 are similar to those obtained over VS-1, it is conceivable that cyclic peroxo-titaium species are formed by the action of hydrogen eroxide on TS-1. The case of tBuOOH is more comp ex since partial oxidation of toluene also occurs in the absence of catalyst. This homogeneous oxidation which very likely proceeds via a homolytic process will not be discussed in the resent paper. Special attention will be given to the heterogeneous reaction in volving t e presence of VAPO5. It has been widely re rted that organic molecules. and especially methylsubstituted benzenes could easirreduce V5 ions incorporated in calcined VAPOd at temperatures below 100 C [l ] Therefore, it is conceivable that the mechanism ions by toluene molecules followed b the formation proceeds via the reduction of of tBuO' radicals on the Vy+ formed. Indeed, the chain complex leads to the formation of radicals, with the following scheme :

P

K

l4

+

fi

V 0 2 + + ROOH-

OVOOR+ + H+

+ RO'

OVOOR+-VO*+

Assumin that the possible structure for V5+ ions in a rigid lattice reported by Rigutto et al. [ l 1 in the case,of VS-1 can be applied for VAPOS molecular sieves, oxidation of toluene may be described by :

P

t

0

0 ' / 6 u

'L/ / \

-

v

O,

OH

1

/ \

+

t

BuO

453

tBuO' may react with toluene or oxygen to form radicals

Different terminations can be envisaged, leading to the formation of benzyl alcohol, dibenzyle and benzaldehyde. The transition metal ions may also play a role in consecutive reactions :

The autoxidation of aldehyde to form benzoic acid is also favoured by the presence of vanadium. The reaction is usually represented by :

Of course, this mechanism is rather simplistic and numerous secondary reactions have to be considered. Nevertheless, it has the advantage to explain the role played by the solid during the catalytic run and especially the importance of the V4+ /V5 + redox system.

5. CONCLUSION. Although substituted aromatic molecules can be oxidized under mild conditions over a variety of substituted molecular sieves and using different oxidants, the choice of the latter is very important to direct the nature of the products. In the presence of transition metal ions, hydrogen peroxide forms a cyclic metalloperoxide which is known to oxidize electron donor molecules. Therefore, H202 will preferentially lead to the hydroxylation of the aromatic ring. On the other hand, tBuOOH forms a chain complex with transition metal ions which severes to produce tBuO radicals. The mechanism proposed, although very incomplete and still under i vesti ation, allows to explain the side chain oxidation and involves the redox system Vp+/V4+ present in V-substituted molecular sieves.

454

6. REFERENCES. 1

2

G. Pere 0, G. Bellussi, C. Corno, M. Taramasso, F. Buono o and A. Esposito, in Y. durakami. A. Lijima and J.W. Ward (eds), Proc 7' Int Zeolite Conf., Tokyo, 1986, Elsevier, Tokyo ( 1986) p. 129.

R

P.R. Hari Prasad Rao, A.V. Ramaswamy and P. Ratnasamy, J. Catal.. 137 (1992) 225.

3

P.R. Hari Prasad Rao and A.V. Ramaswamy, J. Chem. SOC.Chem. Commun. (1992) 1245.

4

K.B. Sharpless, in "The Discovery of Titanium Catalyzed Asymmetric Epoxidation",Chemtech, November ( 1985) 692.

5

M. Taramasso, G. Perego and B. Notari, U S . Pat. No 4 4 L O 501 ( 1983).

6

G. Centi, S. Perathoner and F. Trifiro, J. Phys. Chem., 96 (1992) 2617.

7

U. Romano, A. Esposito, F. Mas ero, C. Neri and M.G. Clerici, in G. Centi and F. Trifiro (eds), "New Deve opments in Selective Oxidation", Elsevier, Amsterdam (1990) 33.

8

0. Bortolini, F. di Furia, G. Modena and P. Scrimin, J. Mol. Catal., 9 ( 1980) 323.

9

F. di Furia, G. Modena, R. Curci, S.J. Bachofer, J.O. Edwards and M. Pomerantz, J. Mol. Catal., 14 (1982) 219.

10

Sung Hwa Jhung, Young Sun Uh and H a h e Chon, Appl. Catal., 62 (1990) 61.

11

M.S. Rigutto and H. Van Bekkum, Appl. Catal., 68 (1991) LI.

P

M.Guisnet et al. (Editas), Hctnogenwus Catalysis and Fine Chemiurls ZIZ 0 1993 Elsevier Science Publishers B.V. All rights reserved.

455

Catalytic hydroxylation of phenol by hydrogen peroxide. Kinetic study and comparison between solid acids and titanosilicates. M. Allian, A. Germain, T. Cseri and F. Figueras Laboratoire de Chimie Organique et Cinetique Chimique Appliqdes, CNRS URA 418,ENSCM, 8 Rue Ecole Normale, 34053 Montpellier Cedex 1, France

Abstract Hydroxylation of phenol by hydrogen peroxide over solid acids exhibits an autocatalysis that has never been described in earlier works. The induction period is dependent on the acidity and is reduced by initial addition of dihydroxybenzenes or other electron-transfer agents. A new mechanism, initiated by the slow formation of dihydroxybenzenes in the induction period, should be considered. Comparison of various catalysts shows that the reaction is also dependent on the structure of the solid. Zeolites with too small a porosity are not active, according to a large space demand of the reaction. Catalysis by titanium silicalites does not show such behaviour : the reactivity is low but regular. Thus, our results show that valuable comparison between catalysts cannot be deduced from tests performed by stopping the reaction at a determined time, but that kinetic studies are essential.

INTRODUCTION The direct introduction of a new function in an organic substrate is certainly the best method to obtain "fine chemicals". However, the application of this concept is not always so obvious. For example, despite their industrial importance, phenols are essentially prepared, from aromatics, by multi-step processes. This is due t o the fact that the products are more reactive than the starting materials. Therefore, direct hydroxylation of aromatics is always a real problem.

0

Catalyst

+ H202

O

O

H

+

H,O

456

The heterogeneous catalysis of such a reaction, is a challenge which has shown a remarkable expansion since the discovery of titanium-containing zeolites and their catalytic properties in the activation of hydrogen peroxide [l]. The development of this catalysis has however taken place to the detriment of others. Thus, despite patents [2-51, the use of solid acids, such as zeolites or clays, has never been the subject of academic work. The absence of kinetic results is particularly prejudicial to a valuable comparison of the various types of catalysis. We present, here, a kinetic approach to the study of the hydroxylation of phenol by hydrogen peroxide over solid acids. This shows unexpected behaviour, which calls into question the mechanism of the acid catalysis. A more accurate comparison with the catalysis by titanosilicates will be deduced.

EXPERIMENTAL 1. Catalysts Number after the structure type code of zeolites denotes Si/Al ratio. Most of the catalysts were commercial : SA, a silica-alumina containing 13 wt% alumina, from Ketjen ; K 10, an acidified montmorillonite, from SiidChemie ; FAU 2.5, the Linde type Y molecular sieve SK-41, from Alfa-Products ; FAU 15, MOR 11 and MOR 49, dealuminated HY and Mordenites, were gifts from Zeocat ; MOR 6.5 was obtained from Norton ; MFI 25, a H-ZSM-5 zeolite, from Conteka. BEA 27, a beta zeolite, was synthesized according to Wadlinger et a1.[6]. Nafion-H, a perfluorinated sulfonic acid resin, was obtained by treatment of the potassium salt of the Nafion 501 resin (from DuPont) with 20% aqueous nitric acid. Heteropolyacids, H3PW12040 and H4SiW12040, were gifts from F. Lefebvre (IRC, Lyon). Sulfated Zirconia was synthesized according to Arata et al.[7]. Two titanium silicalites were used : TS-1 (Ti/(Ti+Si)= 0,019) with MFI structure, was supplied by Atochem ; TS-2 (Ti/(Ti+Si) = 0.0176) with MEL structure, was a gift from S. Kaliaguine (UniversiG LAVAL, QuBbec). Poisoning of silica-alumina was carried out by exchange for 20 h a t room temperature, in a 0.1 N solution of the selected sodium salt (1ml per g). (C1 = chloride ; Ac = acetate ;CO3 = carbonate ;PO4 = phosphate). Except for the heteropolyacids, all the catalysts were calcined before use : SA, FAU, MOR, MFI, TS-1 and TS-2 at 500°C, BEA at 450°C, sulfated zirconia at 350"C, K 10 at 300"C, and Nafion at 150°C. 2. Reactants Phenol, a Gen-Apex chemical, and hydrogen peroxide 30%, stabilized by sodium stannate (5 ppm), were purchased from Prolabo. N,N,N',N'-tetramethyl1,4-phenylenediamine hydrochloride (TMPDH), hydroquinone, catechol and resorcinol were obtained from Aldrich. 3. Procedure

The reactions were carried out, at 60°C under air, in a stirred batch reactor. Hydrogen peroxide was added at once to the solution of phenol. Generally, the

457

solvent used was water, but methanol or phenol itself could also be used. Products were analysed by HPLC (Shimadzu) using a C-18 column, U V detection and fluorophenol as a standard. In order to check the absence of external diffusion for our reaction conditions, the effect of the amount of catalyst on the initial rate was tested over FAU 15.

RESULTS 1. Characteristics of the kinetics of the acid catalysis A typical example of the evolution of dihydroxybenzenes as a function of time is presented in Figure 1. The conversion (%) is relative t o the initial hydrogen peroxide concentration. The predominant feature is the existence of an induction period, followed by an autocatalysis. Such behaviour had never been described before. Catechol (ortho-isomer) is the initial product observed, followed by hydroquinone (para-isomer). Resorcinol is not detected (< 0.5 %I. The para-selectivity increases with t h e time of reaction. The catechol concentration of catechol reaches a hydroquinone maximum, as observed in Figure 1.

This can be attributed t o a 0 €0 1 2 0 1 8 0 2 4 0 subsequent oxidation of the Time ( m i d dihydroxybenzene which is likely to Figure 1. Formation of dihydroxybe the rupture of the benzene ring. benzenes as a function of time. This occurs preferentially for the Phenol : 53 mmol ; H 2 0 2 (30%) : 17 mmol (2cm3) ; FAU 2.5 : 250 mg ; ortho derivative [8]. Further details methanol : 5 cm3 ; 60°C. will be given later 193. These secondary reactions, not only destroy some of the products, but also consume a large part of the hydrogen peroxide. Thus, they are responsible for the limitation of the conversion of hydrogen peroxide into dihydroxybenzenes. A good catalyst should therefore accelerate only the first hydroxylation step and not the secondary reactions.

458

The induction period is considerably reduced by initial addition of catechol (0.1 mol. per mol. of H202). Hydroquinone gives a similar effect. In this case, the transitory partial oxidation of this dihydroxybenzene into benzoquinone is unequivocally observed (Figure 2). In the same way, addition of the quinone also removes the induction period. These results imply : firstly, that the dihydroxybenzenes produced are responsible for the autocatalysis ; and secondly, t h a t a n oxidation-reduction mechanism is involved in this process. Suppression of the induction period is more efficiently obtained by the addition of a powerful electron-transfer agent, such as NNN”’-tetramethyl- 1,4-phenylenediamine hydrochloride (TMPDH). Effectively, less than 0.01 mol of additive per mole of hydrogen peroxide, is sufficient to eliminate the induction period (Figure 3). The increase i n t h e yield of dihydroxybenzenes observed, is essentially due to the formation of resorcinol in a ratio o l d p = 55/9/36. The appearance of this product can be considered as a sign of a change in the mechanism of the reaction.

dihydroxybenzenes

0

30

60

Time (min.)

Figure 2. Effect of initial addition of hydroquinone (1 mmol.). Phenol : 21 mmol ; H202 : 9.4 mmol ; FAU 2.5 : 50 mg ; water : 10 cm3 ; 6OoC

W h

-

with TMPDH

without

TMPDH

0

60

120

180

Time (min) Figure 3. Effects of N , N , N ’ , N ’ tetramethyl- 1,4-phenylenediamine hydrochloride (TMPDH) : 0.08 rnrnol. (dashed curves). Phenol : 21 mmol. ; H202 : 9,4 mmol. ; FAU 2.5 : 50 mg ; water 10 cm3 ; 60 “C.

459

The acid strength also acts upon the induction period. This is demonstrated by the study of the effect of the neutralisation of the acidity of silica-alumina (SA) by various sodium salts. An extension of the induction period is observed (Figure 41, which becomes more important when the base strength of the counter ion is increased. However, this effect is not accompanied by a significant alteration of the final yield, nor by a notable reduction of the rate at the inflexion point.

- 1

I

n

.

SA

-608

g

SAC1

SA-PO,

SA-CO3

-

.rl

ca-

B .

020-

u

.

00

5

1

0

1

5

2

0

2

5

Time (h) Figure 4. Effects of the neutralisation of silica-alumina (SA) by various sodium salts. Phenol : 21 mmol ; H202 : 9.4 mmol ; SA : 50 mg ; H 2 0 : 10 cm3 ; 60°C.

-

2 Effects of the nature of the catalyst. The nature of the catalyst also has a large influence upon the induction period, the rate and the yield of the hydroxylation. In addition to the results shown in Figure 5 , it should be noted that no reaction occurs, even after 24 hours, with nafion, sulphate-treated zirconia, mordenites or MFI.

1

1

SA TS-1 FAU15 X K10 A BEA 9 FAU2.5 +

60 n

8

w

g 40 .H rn

&

B

u0

20

0

0

1

2

3 Time (h)

4

5

6

Figure 5. Hydroxylation of phenol over various catalysts. Phenol : 21 mmol. ; H202 : 9.4 mmol. ; catalyst : 50 mg ;water : 10 cm3 ;60 "C.

460

With heteropoly acids, a selectivity of less than 10% to the dihydroxybenzenes is obtained, accompanied by trihydroxybenzenes. The absence of reactivity over nafion can be attributed to too low a n acidity of this resin. The same explanation can be given for the sulphated zirconia, because this solid loses its strong acidity in water. The lack of activity of MFI was unexpected because it is in contradiction to the patent from Chang and Hellring [3]. T h s result might be explained by too small a porosity of this zeolite, likewise for MOR. In this latter case, though the porosity is larger, its monodirectional structure is a n additional handicap. BEA, a three-directional channel system with larger opening, permits the reaction. The induction period is short, in accordance with a strong acidity, but the rate remains low. This latter point can always be attributed t o steric problems. The faujasites show a much better activity. Though the opening of their pores is not much larger than that of BEA, their three-directional structure has large cavities (supercages) that allow reactions with bulky transition states to take place. Comparison between these two faujasites gives more information. The dealumination (FAU 15) has two effects : on the one hand, the increase of the strength of acid sites eliminates the induction period; on the other hand, the formation of mesoporosities facilitates the diffusion of the 0 100 200 300 400 500 600 Catalyst (mg) primary product. Thus, the subsequent reactions are reduced Figure 6 . Effect of the mass of catalyst and the selectivity in on the yied of dihydroxybenzenes. Phenol : 21 mmol ; H 2 0 2 : 9,4 mmol ; dihydroxybenzenes is increased. water : 10 cm3 ; 60°C. The effect of the mass of catalyst to on the selectivity dihydroxybenzenes, shown in Figure 6 , illustrates the points discussed above. While the yield is independent of the amount of FAU 15, it decreases when the mass of FAU 2.5 increases. The longer the dihydroxybenzenes remain inside the catalyst, the more they are subject to consecutive reactions and the greater the consumption of hydrogen peroxide is.

461

The behaviours of the acidified montmorillonite (K 10) and of the silicaalumina (SA) are similar to the one of the dealuminated faujasite, except for the presence of induction. With SA, the best yield is in accordance with the best diffusion of the primary product through the amorphous structure of the catalyst. With all these acid catalysts, the final para-selectivity is low : between 41 and 47%. The variations are not in relation with the structure of the solid. The titanium silicalites, TS-1 and TS-2, have similar reactivities. Although they have a low acidity, they show no induction period. Their activity curves are regular, but the rates of reaction are low, however good yields are obtained after long reaction times. With these catalysts the para selectivity is higher than with acid catalysts : 68% with TS-1,57% with TS-2.

DISCUSSION The observation of an autocatalysis in the acid catalysed hydroxylation by hydrogen peroxide is a new fact which needs consideration. As verified, it signifies that the products - catechol and (or) hydroquinone - participate in the fast part of the reaction. During the induction period period, the acid reaction, presumably an electrophilic aromatic substitution, must occur alone. This would explain why the induction time is dependent on the acidity. When a sufficient amount of dihydroxybenzene has formed, a new faster mechanism occurs. This involves a molecule of dihydroxybenzene in an oxidation-reduction process, as shown by the detection of quinones. It is not very dependent on the acidity, though no reaction of phenol occurs in presence of dihydroxybenzenes without acid. This reaction, in which three species - the hydroxylating agent, one molecule of phenol and one molecule of dihydroxybenzene - participate, requires a large space. So, its catalysis is efficient only over solids having a large porosity. At the present time, it is not possible t o identify the exact role of the dihydroxybenzene in this reaction. Two possibilities can be considered : first, whether it acts like a reducing agent on the hydrogen peroxide to give the hydroxyl radical, in a similar way to the Fenton reaction [lo] ; or whether, after its oxidation by hydrogen peroxide into quinone, it causes a n electron transfer from the phenol to give the phenoxy radical, in a similar role to the nitrosonium cation in the nitrous acid catalysed nitration of phenols [ 111. In either case, this peculiar characteristic of the hydroxylation reaction explains the good behaviour

462

of the reduction-oxidation properties of the dihydroxybenzenes. Thus, i t is specific to the hydroxylation of phenol. Characteristics of the hydroxylation over titanium silicalites a r e different. No induction period is detected despite the absence of acidity, a n d the reaction occurs even though they have a low porosity, similar to t h a t of MFI. Nevertheless, the reaction must occur inside the channels of the catalyst since a shape selectivity is observed. Thus, the mechanism mu s t be different, though benzoquinone was also detected at the beginning of th e reaction [12]. The formation of surface titanium peroxocompounds h a s been postulated [ 131.

CONCLUSION The present study shows fundamental differences i n the hydroxylation kinetics of phenol using strong acid catalysts or titanium silicalites. With the latter, the reaction occurs slowly but regularly, while, with the solid acids, the reaction shows a n induction period followed by a very fast autocatalysis. These results cast doubt on the validity of the tests performed by stopping the reaction a t a determined time. They also call into question the mechanism of the acid catalysis, the homogeneous as well as the heterogeneous contribution. Finally, taking into account th at water is the best solvent for this reaction, solid acids should be considered as valuable catalysts for hydroxylation of phenol.

REFERENCES 1

2 3 4

5 6 7 8 9 10 11 12 13

G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo, A. Esposito, i n Y. Murakami, A. Iijima, J.W. Ward, (Eds.), Proc. Seventh Int. Conf. on Zeolites, Tokyo, (19861, Tonk Kodansha Amsterdam Elsevier, p. 129. H. S. Bloch, US Pat. 3 580 956 (1971). C. D. Chang, S. D. Hellring, US Pat. 4 578 521 (1986). M. Costantini, J.-M. Popa, Eur. Pat. 299 893 (1988). M. Costantini, J.-M. Popa, M. Gubelmann, Eur. Pat. 314 583 (1988). R. L. Wadlinger, G. T. Kerr, E. J . Rosinski, US Pat. 3 308 069 (1967). M. Hino, S. Kobayashi, K. Arata, J. h e r . Chem. Soc., 95 (1979) 6439. A. J. Pandell, J. Org. Chem., 4 1 (1976) 3992. A. Germain, M. Allian, F. Figueras, to be published. C. Walling, Acc. Chem. Res., 8 (1975) 125. M. Ali, J. H. Ridd, J. P. B. Sandall, S. Trevellick, J. Chem. SOC.,Chem. Commun., (1987) 1168. J . S. Reddy, R. Kumar, P. Ratnasamy, Appl. Catal. 58 (1990) L1. B. Notari, Stud. Surf. Sci. Catal., 37 (1987) 413.

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 C3 1993 Elsevier Science Publishers B.V. All rights reserved.

463

Reductive coupling of cyclic ketones on reduced Ti02(001) single crystal surfaces H. ldriss and M. A. Barteau Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark DE 19716

Abstract Reactions of cyclohexanone, cyclohexenone, and p-benzoquinone were investigated on Ti02(001) single crystal surfaces. On the reduced, sputtered surfaces, the following reactions were observed: partial reduction of the cycloketones to their corresponding alcohols and of p-benzoquinone to phenol; O C~HFJ=C~H from F J cyclohexanone reductive coupling to form C ~ H I O = C ~ H ~and and cyclohexenone respectively; and reductive couplingheduction of p-benzoquinone to benzene, biphenyl, and terphenyl. The reduced surface was oxidized upon adsorption of the ketones as evidenced by an increase of the intensity of the XPS Ti(2p312) peak at 459.1 eV corresponding to Ti+4 cations. The population of Ti+4 cations in the region sampled by XPS increased by 6.8%, 11.2%, and 16.8% upon adsorption at 300 K of cyclohexanone, cyclohexenone, and p-benzoquinone, respectively. The activity of the reduced surface for reductive coupling was highest for p-benzoquinone and lowest for cyclohexanone. The observation of reductive coupling on single crystal surfaces in ultrahigh vacuum suggests that gas-solid contact catalysis of this reaction may be feasible.

1. Introduction: Carbon-carbon bond formation is common to a number of processes in heterogeneous catalysis and includes reactions such as Diels-Alder [ 11, carbonylation [2], Fisher-Tropsch [3], and aldolization [4]. The low selectivity toward the desired products in many of these examples shows that a wide gap remains to be overcome in the understanding and control of catalyst behavior. One approach is to investigate model systems in order to construct correlations between the active sites, the adsorbed species, and the reaction products. Single crystal surfaces increasingly fill this role. The complicated nature of oxide catalysts (the presence of different thermodynamically stable phases and facets, of cations with multiple oxidation states, and of different oxygen species) is the driving force for the choice of oxide single crystals as models for in situ studies of catalytic reactivities. This work presents reactions of cyclic ketones and diketones on the surfaces of a Ti02(001) single crystal. To date three different types of C-C bond formation have been observed on different surfaces of this material depending on the prior pretreatments. Carboxylate coupling (of both saturated [5] and unsaturated [6]

464

intermediates) to ketones is a structure-sensitive reaction which has been observed only on the (1 14j-faceted TiOz(001) single crystal surface. The fully oxidized TiOZ(001) surface exhibits two stable faceted structures. These are identified as the {011)-faceted structure, obtained by annealing the sputtered surface at 750 K, and the (1 14j-faceted surface, obtained by annealing to 950 K. The latter surface contains Ti+4 cations 4-fold coordinated to oxygen; these can accommodate two adjacent carboxylates as required for coupling [5]. The second C-C bond forming reaction is the aldolization of acetaldehyde to crotonaldehyde [7]. This reaction was observed on both faceted structures, suggesting that it is relatively insensitive to surface structure. The third C-C bond forming reaction, reductive carbonyl coupling of aldehydes to form symmetric olefins [8, 91, occurs on the sputtered surfaces, and requires Ti cations in lower oxidation states than +4. In fact, carbonyl coupling of aldehydes and ketones to symmetric olefins was first observed as a stoichiometric liquid-solid reaction in slurry [lo]. It was proposed that the active site was Tio. The observation of this reaction on a Ti02(001) single crystal as a gas-solid process has permitted direct, in situ investigation of the required active sites. No Ti0 was observed by XPS [9,11] on surfaces active for this reaction. Moreover, no specific oxidation state of Ti+X cations (Ocxc4) could be considered as solely responsible for the reaction [9]. In the course of our attempts to understand this reaction and to carry it out catalytically, the reactions of cyclohexanone and cyclohexenone were studied by XPS and TPD on Ti02(001) surfaces. The reactivity of p-benzoquinone was also studied, since this diketone can couple at both ends, leading to polymer formation on the surface. The formation of polymeric monolayers can have wide applications including surface coating and doping of semiconductors in order to enhance their diode quality factors (121.

2. Experimental: The crystal preparation, mounting, and pretreatment procedures were described in detail elsewhere [5, 71. All experiments were done in a VG Scientific ESCALAB equipped with a twin-anode X-ray source, LEED optics, electron and ion guns and a UTI mass spectrometer (multiplexed with an IBM PC for TPD experiments [13]). Ketones (spectra grade, Aldrich) were contained in a glass sample tube attached to a dosing manifold; prior to use, they were purified by freeze-pump-thaw cycles. All TPD and XPS measurements were carried out at saturation coverages. TPD experiments were performed as follows. After sputtering the crystal [5,11], the surface was dosed and the chamber was pumped down until a stable background pressure was reached (ca. mbar); the crystal was then positioned in front of the mass spectrometer (1-2 mm from an orifice in the envelope enclosing the ionizer). Heating was initiated (1.2 Us)and the desorption flux was monitored with the mass spectrometer. As many as 100 masses were scanned simultaneously (at intervals of 4 s or less) during a single TPD experiment. XPS data were collected using the Al anode (operated at 600 W). For all spectra the C ( l s), O( 1s) and Ti(2p) regions were scanned. XPS spectra were collected before starting the TPD run, and at the end. TPD results are presented in terms of fractional yield, defined as the fraction of the total carbon appearing in each product. The TPD peak area of each product was multiplied by its mass spectrometer correction factor. The relevant correction factors determined using the procedure of KO et a / . [ 141 from the observed fragmentation patterns are as follows: cyclohexanone, m/e 98 (1 l ) , cyclohexenone,

465

m/e 68 (2.2), p-benzoquinone, m/e 108 (7.2), benzene, m/e 78 (2.5), biphenyl, m/e 154 (5.8), terphenyl, m/e 230 (12.3), cyclohexanol, m/e 82 (4), cyclohexenol, m/e 80 (2.4), and phenol, m/e 94 (2.3).

3. Results 3.1 XPS Ti(2p) region after adsorption of the ketones: The adsorption of cyclohexanone, cyclohexenone, or p-benzoquinone on reduced Ti02(001) surfaces at room temperature resulted in an increase in intensity of the lines corresponding to Ti+4 cations and a decrease of those of other Ti+X cations. Figure 1A presents the Ti(2p) region of a sputtered, reduced Ti02(001) single crystal. This spectrum was studied in detail elsewhere [9, 111 and consisted, by curve fitting [ l l ] , of lines at 459.1, 457.6, 456.1 and 454.9 eV attributed respectively to the Ti(2p312) binding energies of Ti+4, Ti+3, Ti+2, and Ti+'. Tio lines at 454 eV [ 1 1, 151 were not observed. Figure 1B presents the Ti(2p) region of the same crystal after adsorption of cyclohexenone at room temperature. An increase of the Ti+4 line at 459.1 eV was accompanied by a simultaneous decrease of the lines corresponding to the lowest oxidation state Ti+X cations (O<x<3). The relative populations of the different oxidation states of Ti cations before and (after) adsorption were as follows: Ti+4 27.5% (30.7'/0),Ti+3 26.7% (28.3%),Ti+2 24.4% (22.5%), and Ti+' 21.2% (18.4%). Thus, adsorption of cyclohexenone resulted in a decrease of Ti+' and Ti+2 cations and an increase of Ti+3 and Ti+4 cations. These results are in agreement with previous results for benzaldehyde [8, 91, which gave high yields of stilbene by reductive coupling, while oxidizing the surface. The differences between the influence of cyclohexanone, cyclohexenone and p-benzoquinone on the Ti(2p3/2) lines are compared in the course of the reaction studies below.

3.2.

Temperature programmed reaction of cyclohexanone, cyclohexenone and p-benzoquinone.

Figure 2A presents the desorption of products during cyclohexanone TPD. The products desorbed in three different temperature domains. The first, at 410 K, consisted of unreacted cyclohexanone. This peak was identified by its parent mass (m/e 98 (39)) as well as by its major fragments (m/e 55 (loo), m/e 42 (86), m/e 69 (37), and m/e 27 (60); the values in () are those of the relative intensities of the fragments. The second reaction domain was at 500 K where desorption of cyclohexanol (identified by its m/e 82 (45), 67 (23.5), 43 (12), 41 (21) and 27 (11)) fragments, and benzene (m/e 78, and m/e 77) occurred. Small amounts of cyclohexane (m/e 84), cyclohexene (m/e 82), and cyclohexadiene (m/e 80) also may have also desorbed, however the low signals for the common fragments of these molecules did not allow conclusive identification. The third regime was at slightly higher temperature, at ca. 520 K, where the reductive coupling product C6Hl o=C6H10 (m/e 164) desorbed. At the end of the TPD ramp carbon deposition was detected by the presence of a C( 1s) signal at a binding energy of 284.5 eV. The comparison between the C(1s) peak area at the end of the temperature ramp and that obtained after adsorption of cyclohexanone at room temperature indicated that only about 3% of the carbon initially contained in the adsorbed layer remained on the surface. No CO or CO2 were observed during the TPD run. Water desorbed with a peak maximum at about 500 K. This product

466

suggests that part of the cyclohexanone may be reduced to cyclohexene which might desorb, dehydrogenate to cyclohexadiene and benzene, or decompose completely. Part of the hydrogen evolved during this process participates in the reduction of a portion of the adsorbed cyclohexanone to cyclohexanol.

.....*. .. ... : .. . _i 2

.. . ..

......

. ..: ..,

.

'5. *'

.:<:. .. .. ..

.

I..

lenzeno

-.... .. ........ .

: 3.6

.

B

A

400 454

458 462 466 Binding Energy (eV)

Figure 1. XPS Ti(2p) region before (A) and after (B) adsorption of cyclohexenone, at room temperature.

600 400 600 Temperature (K)

Figure 2. TPD after adsorption of (A) cyclohexanone and (8) cyclohexenone.

The presence of reduced Ti cations [7] made possible the coupling of two molecules of cyclohexanone to form C6Hlo=C6Hlo. Investigation of the reactivity of cyclohexanone on the fully oxidized surface (obtained by annealing the surface to 750 K prior to adsorption of reactants at 300 K [ 5 ] ) indicated the absence of C6Hlo=C6Hlo and a decrease of the cyclohexanol yield. This latter result demonstrates the role of Ti suboxides for the reduction of ketones to alcohols as well as for the reductive coupling of two ketones to symmetric olefins. Figure 28 presents the desorption products of cyclohexenone-TPD. Results similar to those for cyclohexanone were obtained: first cyclohexenone desorbed at 440 K, followed by benzene and cyclohexenol at ca. 530 K and then by the coupling product, C6H8=C6H8, at ca. 550 K. As in the case of cyclohexanone TPD, no coupling product was observed when the experiment was performed on fully oxidized Ti02(001) surfaces. At the end of the TPD experiment the total XPS C(1s) area was about 15% of the initial value.

467

Table 1 presents the TPD products after adsorption of p-benzoquinone on the surface of the reduced Ti02(001) single crystal. Complete consumption of the reactant occurred, as indicated by the absence of its characteristic signals (e.g., m/e 108) during TPD. Phenol desorption was observed at 650 K resulting from partial hydrogenation of one side of adsorbed p-benzoquinone. In contrast to the results for the monoketones, the expected primary product of reductive coupling (O=C6H4= C6H4=O) was absent. However the desorption of benzene, biphenyl, and terphenyl occurred. These products are formed by reduction and reductive coupling. Benzene would result from the reduction of p-benzoquinone at both ends, biphenyl from the coupling of two molecules of p-benzoquinone from one side and the reduction from the other side, etc. It was not possible to detect the formation of tetraphenyl (m/e 306) and higher polymers since the maximum mass number accessible with the UTI 100 C mass spectrometer is 300. However, the yield of phenyl oligomers decreased with increasing product carbon number from 0.496 for benzene, to 0.23 for biphenyl and to 0.10 for terphenyl. If one assumes that these coupling products follow the Flory distribution, the estimated yield of products containing more than 3 phenyl rings would be 9.2% of the phenyl monomers and oligomers [16]. Table 1 Product desorption during O=CsH4=O TPD on the reduced Ti02(001) surface: Products water p-benzoquinone benzene phenol biphenyl terphenyl > 3 phenyl rings

Peak Temperature

Fractional Yield

500 - 800 K

not quantified 0 0.496 0.092 0.23 0.10 0.08 (calculated)

-

650 650 650 650

-

K K K K

XPS C(1s) after adsorption of the ketones: Figure 3 presents the XPS C(1s) spectra obtained after adsorption of cyclohexanone (A), cyclohexenone (6) and p-benzoquinone (C) on sputtered Ti02(001) surfaces at 300 K. Very close similarities are observed for the C(1s) data obtained after cyclohexanone and cyclohexenone adsorption (as was the case for their corresponding TPD (Fig. 2)). In both cases a peak centered at 284.9 eV with a FWHM of 2.4 eV appeared. Cyclohexenone adsorption gave a slightly higher C(1s) peak area than that of cyclohexanone (3.2 eV/ms compared to 2.9 eV/ms). Curve fits locating the principal (ring) peak below 285 eV, with shoulders at higher binding energy, were considered. The common shoulder at 286.4 eV in all three is in the region of C - 0 binding energy of alkoxides [17], and very probably of the expected pinacolate intermediates in reductive coupling [9]. A small peak at 288.0 eV was also observed and is most likely that of the unperturbed carbonyl of the ketone. Previous results for formaldehyde adsorption at 170 K on fully oxidized Ti0;!(001) surfaces gave a C(1s) peak at 288.0 [ l l ] . The curve fits of figure 3C (after p-benzoquinone adsorption) gave four peaks centered at 284.5, 286.1, 288.0 and ca. 290.5 eV. These peaks can be tentatively assigned

3.3

468

to the following species: -CH=CH-, C - 0 (of alkoxides and pinacolates) [9, 171, C=O (of adsorbed p-benzoquinone) [ 111, and traces of carboxylates [5], respectively. The comparison between these three spectra clearly shows that the C - 0 region (at 286.1-286.4 eV) is more pronounced in the case of p-benzoquinone (C), since that molecule contains twice the number of carbonyl groups of the other two.

282

286

290

294

Binding Energy

Figure 3. XPS C(1s) after adsorption of cyclohexanone (A), cyclohexenone (B), and p-benzoquinone (C) at 300 K.

4. Discussion: Reductive coupling of carbonyls, previously carried out as a stoichiometric liquid-solid reaction [lo], can be performed as a gas-solid reaction on reduced surfaces of Ti02, as shown by this work for ketones and that for aldehydes [8, 91. In situ investigation of the XPS Ti(2p312) lines indicated the absence of the Ti0 line even though the surface was active for coupling. These results clearly show that Ti0 is unnecessary, in contrast to previous proposals [lo]. In addition, no specific oxidation state appeared responsible for the coupling; Ti+l and Ti+2 intensities decreased upon adsorption of carbonyl compounds in this work, as in the case of benzaldehyde coupling to stilbene [9]; Ti+3 did not change appreciably. A summary of the observations from XPS and TPD investigations of the reactions of ketones and

469

diketones is presented in table 2. This table presents 1) the population increase of Ti+4 cations (calculated from the change of their XPS Ti(2p3l2) intensities), 2) the peak area of the XPS C(1s) at 286.4 eV (attributed the C-0species of alkoxides and pinacolates), and 3) the yield of coupling products in TPD after ketone adsorption. As shown, a correlation exists between the magnitude of the change in population of Ti+4 cations, the peak area of the C - 0 moieties and the coupling product yield. p-Benzoquinone produced the highest increase of Ti+4 cations, the highest peak area of adsorbed C - 0 species and the highest activity for reductive coupling. Table 2. Relation between the increase of the TiA cation population, the peak area of the C-0 species, and the coupling yield for adsorption/reaction of cyclic ketones on reduced Ti02

Reactants

% increase of Ti+4 cations

C(1s) area of the C - 0 species (eV/ms)

Coupling fractional yield

cyclohexanone cyclohexenone p-benzoquinone

6.8

0.66

0.085

11.2

0.88

16.8

1.80

0.13 0.36

The mechanism of the reductive coupling, following McMurry [lo],can be proposed as follows. Adsorption of the ketones in the presence of oxygen vacancies, Vo (and Ti+X) occurs at room temperature. Two molecules of adsorbed cycloketones react to yield adsorbed pinacolates. These pinacolates are deoxygenated to give the symmetric olefin with simultaneous oxidation of the titanium suboxide (oxygen deposition). p-Benzoquinone would react in the same way, with the only exception that it reacts at both ends. As an example, the formation of terphenyl from three molecules of p-benzoquinone requires 6 oxygen vacancies which it subsequently fills. In order to for this C-C bond formation to be catalytic, it is necessary to reduce these oxidized sites again. Co-feeding with a strong reducing agent such as CO can be an alternative. However, concurrent regeneration may not be a major problem, as several systems in catalysis require regeneration after each turnover. This regeneration might be achieved in a Catofinlike process (an adiabatic, fixed-bed, multiple reactor system, which operates in a cyclic manner to utilize/ regenerate the catalyst [19]), or in a moving bed type reactor where the catalysts flows, co-currently with the reactants, and is then removed from the reactor and regenerated in a separate area [20].It is worth mentioning that reductive coupling of aldehydes was recently carried out on reduced titania, ceria and iron oxide powders during TPD cycles [21]in this laboratory, indicating that translation of knowledge from single crystal to powder can be achieved, but not without difficulties.

5. Conclusions: The reduced, sputtered, Ti02(001) single crystal surface was active for the reductive Coupling Of C&ilo=O to C ~ H ~ O = C ~OfHC~HFJ=O ~ O , 10 C~HFJ=C~HFJ, and Of

470 O=CsH4=0 to benzene, biphenyl, terphenyl, etc. The coupling activity of the surface decreased in the the following order: p-benzoquinone > cyclohexenone > cyclohexanone. Fully oxidized Ti02(001) surfaces were inactive for reductive coupling. XPS Ti(2p3~)indicated that oxygen deposition occurred upon adsorptian of ketones on the reduced surfaces of TiO2, as evidenced by the increase of the Ti+4 signal (at 459.1 eV) and the decrease of those of other Ti+X cations; the magnitude of this increase was in the following order: p-benzoquinone > cyclohexenone > cyclohexanone. XPS C ( l s) upon adsorption of the ketones indicated the presence of a species at 286.4 eV attributed to alkoxide and pinacolate functions. Their concentration, upon adsorption of the three ketones, was correlated with the increased population of Ti+4. The observation of the reductive coupling of ketones (as well as of aldehydes [a, 91) on Ti02(001) single crystal surfaces via gas-solid reaction might open new routes for C-C bond formation in heterogeneous catalysis.

Acknowledgement We are grateful to the National Science Foundation (Grant CTS9100404) for support of this research.

6. References 1

2

3 4

5 6

7 8 9 10 11 12

13 14 15 16 17 18 19 20 21

R. Dessau, Mobil Oil Corporation, US Patent No 4,413, 115 (1983). H.M. Colquhoun, D.J. Thompson, M.V. Twigg, Carbonylation, Plenum, New York (1991) and references therein. A.B. Anderson, Catal. Rev. Sci. Eng., 21 (1980) 53 and references therein. K. Tanabe, M. Misono, Y. Ono, H. Hattori, New Solid Acids and Bases, Stud. Surf. Sci. Catal., Vol. 51, Kodansha, Tokyo and Elsevier, Amsterdam (1989). K.S. Kim, M.A. Barteau, J. Catal., 125 (1990) 353. H. Idriss, K.S. Kim, M.A. Barteau, in Structure Activity and Selectivity Relationships in Heterogeneous Catalysis, R. K. Grasselli and A. W. Sleight Eds., Stud. Surf. Sci. and Catal., Vol. 67, Elsevier, (1991) 327. H. Idriss, K.S. Kim, M.A. Barteau, J. Catal., 139 (1993) 119. H. Idriss, K. Pierce, M.A. Barteau, J. Am. Chem. SOC.113 (1991) 715. H. Idriss, M.A. Barteau, Appl. Surf. Sci., submitted. J.E. McMurry, Chem. Rev. 88 (1988) 733 and references therein. H. Idriss, K.S. Kim, M.A. Barteau, Surf. Sci., 262 (1992) 113. 0. Inganas, T. Skotheim, I. Lundstrom, J. Appl. Phys., 54 (1983) 3636. X.D. Peng, M.A. Barteau, Langmuir, 5 (1989) 1051. E.I. KO,J.B. Benziger, R.J. Madix, J. Catal. 62 (1980) 264. J. Lausmaa, B. Kasemo, H. Mattson, Appl. Surf. Sci., 44 (1990) 133. H. Idriss, M.A. Barteau, in preparation. K.S.Kim, M.A. Barteau, J. Mol. Catal., 63 (1990) 103. B.E. Khan, R.D. Rieke, Chem. Rev., 88 (1988) 733 and references therein. Chem. System., 8984 (1991). Kirk-Othmer Encyclopedia of Chemical Technology, third edition, Vol. 19, Reactor Technology (1983). H. Idriss, M. Libby, M.A. Barteau, Catal. Lett., 15 (1992) 13.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

471

Oxydative dehydrogenation of isobutyric acid to methacrylic acid over heteropolysalts of composition Kx(NH4)3..xPMoi2040: effect of catalyst pretreatment and composition on the activity and selectivity. S. Albonetti, F. Cavani, M. Koutyreva and F. Trif51-b Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4,40136 Bologna, Italy. a

On leave from the Institute of Chemical Physics, Academy of Sciences, ul. Kossygina 4, 117334 Moscow, Russia.

Abstract Heteropolysalts with composition Kx(NH4)3-xPMoi2040were prepared by precipitation; the compounds were tested as catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid. The heteropolysalts proved to be active and selective in the short-term, leading to methacrylic acid with selectivity close to the 70% at total reactant conversion. The calcination temperature of the samples, as well as their cationic composition, affected the performance; the best results were obtained with the (NH4)3PMoi2040compound, calcined at 380C. The effect of the main operative parameters (temperature, residence time and oxygen partial pressure) on the catalytic performance was studied. 1.INTRODUCTION In the petrochemical industry the introduction of unsaturations in hydrocarbons is mainly obtained by dehydrogenation. This kind of reaction is less suitable for the fimctionalization of fine chemicals, because the high temperature necessary for the endothermic reaction can lead to the decomposition of thermally unstable compounds. An alternative reaction consists in the oxidative dehydrogenation, that can be carried out at lower temperatures. A n example of this kind of reaction is constituted by the synthesis of methacrylic acid (MAA, intermediate of methylmethacrylate production) via the oxidative dehydrogenation of isobutyric acid (IBA), itself obtained from isobutyraldehyde (by-product of the 0x0 synthesis of nbutyraldehyde from propylene). This process constitutes one of the economically most interesting routes, alternative to the acetone-cyanohydrin process, which nowadays is the predominant process for the MAA production. Though the MAA can not be strictly regarded as a fine chemical (being

472

produced in very large amounts), its synthesis via the acetone-cyanohydrin process suffers fkom drawbacks that are typical of the fine chemicals production: formation of large amounts of low-value co-product (ammonium bisdfate, to be disposed as waste material, or pyrolyzed to sulfuric acid), utilization of stoichiometric amounts of toxic reactant (HCN), large number of synthetic steps, high energy consumption. Among the different catalysts that have been proposed for the oxydehydrogenation of IBA to MAA, heteropolycompounds with the Keggin structure lead to the highest selectivity (1-9);best reported results are for IBA conversions higher than the 90%, with selectivity to MAA close to the 75%. Optimal patented catalysts are heteropolyacids, with composition H3+xPMoi2-xVx040, eventually partially salified with alkali metals (10,ll). An important drawback of these systems, limiting their application as heterogeneous catalysts, consists in a long-term deactivation effect, related to the loss of molybdenum oxide (8); this occurs as a consequence of the primary structure decomposition. By supporting the heteropolycompound (51, or by co-feeding Moo3 in the reactor (8), improvements of the catalytic stability can be achieved. Moreover, the salification of the acid is known to lead to an increase in the stability of these compounds. All the heteropolysalts described in the literature have been prepared starting from the heteropolycompound in the acid form, successively exchanged by addition of the suitable alkali metal carbonate. The final materials are therefore characterized by the presence of some fiaction of heteropolycompound still in the acid form, not salified; this constitutes a factor of structural instability. In addition, the acid form is known to be detrimental for the selectivity in MAA formation, being responsible for the formation of the by-product propylene. Aim of the present work is to study the short-term catalytic performance in IBA oxydehydrogenation of mixed salts with composition K x ( N H ~ ) ~ - x P M o ~ ~ O ~ O , prepared with a method that allows to directly obtain salified compounds, without intermediate formation of the acid. The effect of calcination temperature and of composition on the catalytic behavior will be examined.

2.EXPERlMENTAL The Kx(NH4)3-xPMoi2040compounds (with x=O to 3, hereinafter designated Kx) were prepared by initial dissolution of (m4)3Mo7024, KNo3 and H3P04 in hot water, in the desired relative amounts; HN03 was then added to the solution, to lower the pH. At this pH the Keggin anion was formed, and in the presence of K' and (NH4)' the insoluble heteropolysalt immediately precipitated. The obtained compound was then dried at 120C overnight, and stepwise calcined up t o the desired final temperature. All the salts were characterized by the cubic crystalline structure; crystal size was similar for all samples, ranging from 5 to 10 m'cron . The s@m areas of samples p c i n e d at 320C were the following: Ko 4 mi? /g, Ki 212 m /g, Kz 6 m2/g K3 96 m /g. The Ki and K3 samples af'ter reaction were characterized by decreased value! of surface area. A sample with composition Ki(NHd3PMoiiVi040 (193 m /g at 320C) was prypared by addition of (NH4)vo3 to the solution. Reference H3PMoi2040 (4 m /g at 320C) was prepared by addition of concentrated HCl to a solution of Na2HP04 and

473

NamO04, followed by extraction of the acid with diethylether. Vapour-phase oxidation of IBA was carried out in a continuous-flow laboratory reactor, at atmospheric pressure. The standard feed composition was: IBA 2%,02 20%, H2O 4%. The total flow rate was 60 mL/min and the amount of catalyst used was 1 g (approximately 1 mL). The reaction products, kept at 200C to prevent condensation, were analysed by gas chromatography; a GP 10% SP-1200/1% H3P04 on Chromosorb WAW was used to separate IBA, propylene, acetone and U,with oven temperature programmed from 40 to lOOC (FID); a Carbosieve S column was utilized for analysis of CO and C02, with oven temperature programmed from 40C to 200C (TCD). All the catalysts were kept under reaction environment for approximately 50-70hours; along this period, after an initial unstable behavior, no deactivation phenomena were observed.

3.RESULTS AND DISCUSSION 3.1 Effect of temperature, residence time and oxygen content on the

catalytic performance All the salts prepared exhibited an initial unstable behavior within the first hours of operation, with a decrease in IBA conversion and increase in MAA selectivity; the stable catalytic performance was reached after 3-5 hours. The effect of the residence time on the IBA conversion and products selectivity at the temperature of 235C is drawn in Figure 1, for the Ko sample (ammoniacal salt) calcined at 320C. The proportionality between conversion and residence time allows to exclude diffision as the rate-determining step. Moreover, it is shown that the selectivities to the various products were substantially independent on the conversion. This is in favour of a reaction network constituted of parallel reactions (probably sharing a common reaction intermediate, obtained by IBA activation) for the formation of methacrylic acid, acetone plus C02, propylene plus CO, and carbon oxides from combustion (1,4,7). The dependence of catalytic performance on the temperature for the Ko sample calcined at 320C is shown in Figure 2. Also in this case the selectivities to MAA and acetone were substantially constant up to 280C, thus up to complete IBA conversion. The selectivity to propylene slightly decreased, with corresponding increase in COXformation. These trends suggest similar activation energies for the reactions of formation of the various products. It has been established in literature that the formation of the propylene occurs on the acid centers (1-7).On the contrary, the formation of acetone and MAA occurs via a redox mechanism, even if it was proposed that possibly different kinds of molybdenum sites can be involved in the two reactions (4,7).A common reaction intermediate has been proposed for MAA and acetone, but disagreement exists about the nature of this intermediate, either cationic (underlying the importance of surface acidity) (9,12) or radicalic (1). Figure 3 shows the catalytic behavior of the K3 sample. The dependence of conversion and selectivities on the temperature is similar to that exhibited by the Ko sample; the only difference consists in a decrease of the selectivity to MAA (and of acetone, too) approaching total IBA conversion, with an increase in combustion products. This effect is likely related to the higher surface area of the K3 catalyst, favouring consecutive reactions of MAA combustion.

474 IBA Conversion and selectivity. %

IBA conversion and seleclivity. %

-

W

l

t7-

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

60 ......................................

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

201.............................

Ire----.-------. I

I

I

I

2

4

6

8

10

220

I

I

I

I

I

I

230

240

250

260

270

280

rQSldenCelime, s

290

tomperature, C

2. IBA conversion and Figure 1. Il3A conv. (A) and sel. to MAA Fi ), acetone ( ), .propylene ( ) and se ectivities as fknctions of the COX ( L ) as fknchons of the contact tem erature of reaction. Residence time time. Temp. 230C. Catalyst: KO sample 1 s. ymbols and catalyst as in Fig. 1 (

*

p.. i

The network of reaction deduced from the data is summarized in the following scheme.

Figure 4 shows the effect of oxygen concentration in feed on the catalytic performance of the Ki catalyst, calcined at 320C. The IBA conversion was independent on the oxygen partial pressure in the examined range (thus, for oxygen-to-IBA ratios higher than the stoichiometric requirement), while selectivity to MAA decreased with oxygen, with a corresponding increase in C02 and propylene. This result is indeed surprising, because it was found ( 6 ) that, assuming a Mars-van Krevelen-type mechanism, the catalyst reoxidation is the rate-determining step under usual reaction conditions, and therefore the rate of IBA conversion is usually proportional to the oxygen concentration. The addition of transition metals (such as vanadium) favours catalyst reoxidation (6); under certain conditions (high oxygen-to-IBA ratio) catalyst reduction may therefore

475 IBA conversion and selectivity, %

-m i'

IBA conversion and selecliity, %

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

/ I

*Ot

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

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

60-

I ...........................................

t

-

40

20-

20

40

01 220

I

I

I

1

I

l

l

230

240

250

260

270

280

290

temperature, C

v

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

I

0

d

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

4

8

12

16

20

oxygen conc., %

3. IBA conversion and 4. IBA conversion and %%%ities as functions of the reaction 3Ziivities as functions of the oxygen temperature. Symbols as in Figure 1. in feed. Temp. 260C. Symbols as in Catalyst: K3 sample. Figure 1.Catalyst: Ki sample.

become rate-determining. In our case, instead, catalyst reoxidation is very quick, though the absence of vanadium in the Keggin unit; this feature must therefore be related either to the catalyst cationic composition or to the method utilized for the preparation, leading to an enhanced catalytic reoxidizability. The selectivity to MAA decreased on increasing the oxygen partial pressure, due to preferred formation of carbon oxides. The similar trends displayed by the various products with respect to the reaction parameters (temperature, residence time, oxygen concentration) would suggest that the same active site is responsible for the formation of all the products. This is in contrast with the literature indications, but the discordance can be related t o the composition of our salts: in the absence of protons and of structural acidity (responsible for the high amount of propylene formed on the heteropolycompounds in the acid form), the formation of low amounts of propylene may occur with a different mechanism, i.e. by decomposition of a common reaction intermediate. 3.2 Effect of the catalyst composition on the catalytic performance The substitution of protons with other cations, i.e. alkali or alkaline earth metals, is known to modify the catalyst reactivity. The role of cation has been correlated either to the metal electronegativity (affecting the reactivity of molybdenum atoms in the Keggin unit) (1,2),or to the modification in the compound acidity (affecting the interaction with the reactants) (9). More recently,

476

it has been demonstrated that the presence of K+ ions either in the secondary structure or in the support improves the stability of the heteropolycompound, favouring the formation of the cubic crystalline structure (5). Though it is actually difficult to distinguish between a role of catalyst redox properties and of surface acidity (a shell of heteropolyacid can not be excluded to form on the surface of salified compounds, above all in the presence of water in feed), the literature data point out that salification with alkali metals (1,9) or ammonium (9) leads to an enhancement of the catalytic activity in IBA oxydehydrogenation. Table 1 compares the steady-state conversion at 260C and the selectivity in MAA, acetone and propylene at 260C and in correspondence of total IBA conversion, for the compounds calcined at 320C. The Kx salts exhibited comparable catalybc behaviors, with some differences in the IBA conversion and in the maximum selectivity to MAA, the most active and selective catalysts being the ammonium-salified (Ko) and the Q;these were the samples characterized by the lowest values of surface area before reaction. Moreover, they were the only samples which did not exhibit a decrease in selectivity approaching total IBA conversion. This effect can be related to the absence of consecutive reactions of MAA combustion, which instead occurred in the high surface area compounds. The performance exhibited by these catalysts was surprisingly good (above all in terms of selectivity to MAA), remarkably better than that reported in literature for heteropolycompounds without vanadium (1,2,4,9).The reference acid showed a slightly lower activity, but above all a lower selectivity to MAA. The best selectivities obtained were similar to those reported for iron phosphate-based catalyst (13);this system, however, operates at higher temperatures (4000. The surface area of the compounds was not a determining factor for the catalytic activity; indeed, the best results (in terms of both activity and selectivity) were obtained on the Ko and K2 samples, that were characterized by the lowest values of specific surface area before reaction. This is in agreement with the indications given by Misono et al. (13), who reported that the IBA oxydehydrogenation is a bulk-type reaction, where reaction rates are independent on the surface area. Moreover, this points out the potential utilization of these heteropolysalts as versatile catalysts for oxidative dehydrogenation of bulky molecules, which could not otherwise diffise in the porous structure of highsurface-area compounds. The catalytic results obtained with the sample containing vanadium, Ki(NH4k1PMoiiVi040,are reported in Table 1, too. Table 1. Effect of ( mposition on the ca tlytic performance. Selectivitiesat total IBA conversion, % lesults at 260C

lCatalysts

BA conv.% MAA sel.,g 6AA

56 72 55 75 66 62

43 70 57 70 67 72

41

70 49

70 57 64

acetone propylene aceticacid COX

14 13 9 14 10 8

26 5 6 8 6 3

1 1 3 1

2 3

18 12 33

8 25 22

477

It is shown that the addition of vanadium led to a slight enhancement in the selectivity to MAA, both at 260C and at total IBA conversion, with respect to the Ki sample; the formation of acetone and propylene decreased, while that of COX increased. However, the improvement in performance obtained by vanadium addition was low, if compared to that reported in literature for vanadium-modified heteropolycompounds. 3.3 Effect of the thermal treatment on the catalytic performance The calcination treatment at temperatures higher than 320C led t o a decrease in the surface area for the catalysts characterized by initig high values. ARer c cination at 420C the Ki sample had a surface area of 3 m /g and the & of 40 m /g. All Kx samples, with the exception of the &, exhibited a partial structural decomposition after calcination at 420C. The K3 sample was instead intact, and began t o decompose only after thermal treatment at 500C. Figure 5a compares the conversion of IBA over three heteropolysalts (the Ko, Ki and K3 samples) and over the reference acid as functions of the calcination temperature; Figure 5b gives instead the selectivity to MAA in correspondence of the total IBA conversion. It is shown that all the Kxsalts maintained a relatively high activity, as well as almost constant selectivity to MAA, up to a calcination temperature of 420C; at this temperature the Ko and Ki salts were partly structurally decomposed. In the same range of temperature the reference acid showed a marked fall in activity.

%

IBA conversion. %

AMA selectivily, %

3 '

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

O

0

80- ...........................................

r

5

GO-.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 o L . . .........................................

350

400

450

temperature 01 calcination. C

500

jO0

350

400

450

500

temperature 01 calcination, C

Figure 5. IBA conversion at 260C (a) and selectivity to MAA at total IBA conversion (b) as functions of the calcination temperature on the Ko ( A 1, Ki ( m and & ( ) samples, and on the reference aad (1).

478

Only for calcination temperatures close to 440-45OC the activity of the Kx salts fell. The best performance was exhibited by the ammoniacal salt, which maintained the highest activity and selectivity. These tests indicate that the catalytic performance of the prepared salts remains relatively good even in conditions at which the structure partly decomposes; this makes particularly promising the utilization of these salts as catalysts also in the presence of particularly hard reaction conditions.

4.CONCLUSIONS Heteropolysalts with composition Kx(NH4)3-xPMoi2040, prepared by precipitation, showed a good catalytic activity and selectivity in IBA oxydehydrogenation over the short-term. Obtained results were comparable to the best ones reported in literature for heteropolyacids containing vanadium atoms. The catalytic performance was relatively stable even aRer tretaments that led to partial structural decompositions. The cationic composition affected the catalytic performance; the best results were obtained with the (NH4)3PMoi2040 sample, characterized by a low specific surface area. 6ACKNOWLEDGEMENTS

This work was sponsored by the Minister0 de1l'Universit.A e della Ricerca Scientifica (National Group on Structure and Reactivity of Surfaces).

6.REFERENCES 1 2 3

4 5 6 7 8 9 10 11 12 13 14

M. Akimoto, Y. Tsuchida, K. Sato and E. Echigoya, J. Catal., 72 (1981) 83 M. Akimoto, K. Shima, H. Ikeda and E. Echigoya, J. Catal., 86 (1984) 173. M.J. Bartoli, L. Monceaux, E. Bordes, G. Hecquet and P. Courtine, in "New Developments in Selective Oxidation, Studies in Surface Science and Catal., vol72", P. Ruiz and B. Delmon (Eds.), Elsevier Science, Amsterdam, 1992,Sl. V. Ernst, Y. Barbaux and P. Courtine, Catal. Today, 1(1987) 167. C. Desquilles, M.J. Bartoli, E. Bordes and P. Courtine, Proceed. DGMK Conference on "Selective Oxidation in Petrochemistry", Goslar, Germany,sept. 1992,69. Th. Haeberle and G. Emig, Chem. Eng. Technol., 11(1988) 392. 0. Watzenberger, G. Emig and D.T. Lynch, J. Catal., 124 (1990) 247. 0. Watzenberger and G. Emig, in "New Developments in Selective Oxidation by Heterogeneous Catalysis, Studies in Surface Science and Catalysis vol. 72" P. Ruiz and B. Delmon (Eds.), Elsevier Science, Amsterdam, 1992,71 G.B. McGarvey and J.B. Moffat, J. Catal., 132 (1991) 100. JP 60/150.834 A2, 1985, Nippon Shokubai. Eur. Patent 60.066 (1981) Standard Oil Co. M. Ai,J. Catal., 98 (1986) 401. J.M. Millet and J.C. Vedrine, Appl. Catal., 76 (1991) 209. M. Misono, N. Mizuno, H. Mori, K.Y. Lee, J. Jiao and T. Okuhara, in "Structure-Activity and Selectivity Relationship in Heterogeneous catalysis", R.K. Grasselli and A.W. Sleight (Eds.), Elsevier, Amsterdam,l991, p. 87.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

479

Heterogeneously catalyzed ammoximation of cyclohexanone with molecular oxygen in vapor phase D. Collina, E. Pieri, D. Pinelli, F. Trifirb* Dip. di Chimica Industriale e dei Materiali, V.le del Risorgimento 4,40138 Bologna, ITALY G. Petrini, G. Paparatto Enichem Anic, Bollate (MI), ITALY

Abstract The work reported here was carried out to investigate the reaction mechanism and to clarify the origin of the observed oxidation power of silica samples used in the reaction as the catalyst. These results will be obtained through a preliminary qualitative kinetic analysis of the reaction network; simple power-type equations were proposed to interpolate the catalytic data. A special designed catalytic test showed that the simple silica can not generate the oxime and that the oxidizing power is related to the presence of the tars. It was tentatively hypotyhesized that they should be involved in the activation of 0 2 necessary to the oxidation of the imine to oxime. Finally it was proposed that the real oxidizing species on the tars might consist of hydroperoxide groups on the tars surface. 1. INTRODUCTION Cyclohexanone oxime is an important intermediate for the production of caprolactam and nylon-6 (1). Recently, attention has been focused on a new reaction for its synthesis, called ammoximation whereby the cyclohexanone is transformed into the oxime by reaction with

4 NH,

Fig.1 Reaction Network

I '"

ammonia and an oxidizing agent. A first process, patented by Montedipe (2-5), is very near to industrial application. It is a liquid-phase process using a zeolite, titanium silicalite, as the catalyst and hydrogen peroxide as the oxidizing agent. A possible alternative to this process has been patented by Allied Chemical Corporation and studied by Armor et al. (6-12) and more recently by our research group (13-20). Our work involves gas-phase ammoximation with molecular oxygen as the agent and an amorphous silica as the catalyst. Figure 1 shows a complete reaction scheme drawn on the basis of previous catalytic tests (13,14) and in-siru FT-IR experiments (15). The data has been reported with the indication of gas phase contribution and of catalytic parallel and consecutive reaction. After an initial optimization of reaction conditions and reactor configuration was possible to work on the following more semplified reaction network where gas phase and consecutive reaction of oxime have been minimized (18): cyclohexanone oxime (CHO) cyclohexanone

------------ >

imine

____________ >

tars other volatile products (OW)

The second most important group of products consists of heavy products with molecular weight higher than that of a trimer of the cyclohexanone which can not desorb and remain on the catalyst as tars. Their amount is calculated by weighing the catalyst before and after the catalytic run. From elemental analysis, tars contain N with a molar C/N ratio near 6 and 0 also with C/0=12. Very poor information has been collected on their nature and origin; it was only hypothesized that they are formed by a radical polymerization process of the ketone, the corresponding imine and other partially dehydrogenated products that are present in the reaction atmosphere. Further characterization of the tars are in progress. The last class of products includes other volatile condensation compounds (OVP) whose yield is estimated roughly by subtraction of the yields of oxime and tars from the conversion. They are a mixture of many compounds, essentially dimers and trimers of cyclohexanone, produced by condensation. Some of them contain N and do not contain 0 such as deca-hydro-phenazine and N-cyclohexyl- cyclohexanone imine and, viceversa, others do not contain N but contain 0 such as the aldol condensation products of the ketone. Several catalytic tests were carried out during the research work with different catalyst (commercial silica samples and silicalites) and reported in the papers previously published under the standard conditions (14,17,19). In all the tests a complex evolution of the catalytic behaviour with time-on-stream was found. In particular, with the silica Akzo F-7, in the first 8 hours of time-on-stream the conversion, as well as the oxime yield, increases considerably indicating a process of activation of the catalyst. During the following 7 hours, the catalytic behaviour is approximately constant in a pseudo-stationary state. After about 15 h of time-on-stream, the activity decreases due deactivation phenomenon. The analysis of the time evolution of the catalytic behaviour shows that the rate of formation of tars is constant up to 20 hours of time-on-stream and, then, decreases to zer.3. The OVP yield decreases only slightly with time-on- stream up to 40 h when the OVP are the only products of the reaction. The oxime yield presents a completely different behaviour. Its rate of formation increases with time after approximately 10 hours and then decreases to zero more rapidly than the rate of formation of tars. This phenomenology was found with all the tested catalysts. At the present stage of the research, we focused attention on the explaination of the

481

two observed time dependent phenomena: i) an activation process whose consequence is an increase in the r-OVP rate of formation of the oxime alone, and ii) a following deactivation process which lowers the 0 2 4 6 8 rate of formation CH-conc. (mol%) of oxime and, then, of tars. The Fig.2 Influence of the concentrauon of ketone on the rates of formation of products. work reported here was carried out to investigate the reaction mechanism in order to clarify the origin of the observed oxidation power of silica samples used in the reaction as the catalyst.

Fl

2. EXPERIMENTAL The apparatus for the catalytic test consisted of a conventional glass micro-reactor with accumulation of the reaction products in a solvent, and gas-chromatographic analysis using an internal standard method. A complete description of the whole apparatus has been reported elsewhere (14). The following abbreviations will be used hereinafter: CH = cyclohexanone, CHN = cyclohexanone imine, CHO = cyclohexanone oxime, OVP = other volatile products. Standard conditions for the catalytic test were the following: reactant concentration in the reaction gas CH=2.8 mol%, NH3=35 mol%, 02=10 mol%, T=220°C, catalyst weight W=OS g, molar flow rate F=4.75E-5 mol/min, W/F=175 g.h/mol.cH, total flow rate V=40 d m i n (contact time 3.0 s). The formation rate rates x E+5 (mol/g.min) of tars was evaluated as follows, 2.5 assuming that they are a polymer of a compound with a molecular weight 1.5 equal to that of the r - CHO oxime: rtars=w/(l13*t*W) with w=tars 0.5 weight after a test of t-minutes, 113= 0 1 0 4 8 12 16 molecular weight Oxygen conc. (mol%) of the oxime, W=catalyst

I

Fig3 Influence of the concentration of 0 2 on the rates of formation of the products.

482 rates x E+5 (mol/g.min) 2.5

i

0

10

20

30

40

50

0

r-CHO

A

1-tars

0

1-OVP

60

ammonia conc. (mot%) Fig.4 Influence of the concentration of ammonia on the rates of formation of the products.

P

weight. The catalyst used in all the cata tic tests was a comm rcial amorphous silica by AKZO (AKZO F-7, surface area = 472 m /g, pore volume 2.0 cm5/g) which has been shown in previous work to be the best catalyst for the reaction (13,14). 3. RESULTS AND DISCUSSION

In order to optimize the rate of formation of oxime and at the same time obtain new information on the mechanism of formation of the several products, a first approach to the kinetic investigation was attempted under pseudo-stationary conditions when the best performance is achieved. Many catalytic runs were carried out with varying reaction temperature, contact time, and reactant concentrations using a large batch of homogeneous pre-activated catalyst. In this way, interference due to the change of catalytic behaviour with time- on-stream was minimized. The pre-activated catalyst was prepared by running a special catalytic test on a large scale until1 the maximum activity was reached. The data obtained under the standard conditions and various N H 3 , 0 2 and CH concentrations are reported in figures 2 , 3 and 4. The reaction rates were calculated supposing differential conditions. The conversions obtained in the tests were all less than 40% and were achieved by decreasing the amount of catalyst charged in the reactor. The data in figure 2 show the following apparent orders in regard to CH concentration of the reaction rates in a power- type equation: CHO=0.5, tars=0.8, OVP=1.0. The data in figure 3 show that the 0 2 concentration has a considerable influence on the reaction. In particular, the calculated apparent reaction orders are near 1 for CHO and tars, whereas there seems to be no influence on the condensation products (OVP). The data in figure 4 show that two different kinetic regimes might be present at different ammonia contents. For NH3 concentrations lower than 5 mol%, the rates of formation of CHO and tars depend approximately linearly on the P N H ~ .On the other hand, if the ammonia concentration is maintained higher than 5 mol%, an increase in the NH3 concentration results in a lower increase in the rate of formation of CHO, whereas the yield of tars seems no longer to depend on ammonia concentration. Finally, the rate of formation of the OVP seems not to depend on NH3 concentration over the entire range studied. To explain the variation in the rate of formation of CHO, it may be hypothesized that two different kinetic regimes are present for NH3 concentration lower and higher than 5 mol%. In particular, at low ammonia concentration the

483

rate limiting step should be the formation of the cyclohexanone imine which was demonstrated to be the primary product and the key intermediate for all the three parallel reaction shown in the simplified scheme reported in the Introduction section. At high ammonia concentration, that is under the conditions under which the best performance is obtained, the rate determining step should become the oxidation of the imine. In these conditions, infact, the reaction of formation of the imie from the ketone reaches the equiluibrium conditions and the rate of formation of the oxime depends on the ammonia concentration indirectly throw the equilibrium constant: cyclohexanone

+ NH3

------> imine

+

with

H20

.

CCHN c H 2 0

Keq= . . . . . . . . . . . . . . . . . . . . . . C C H . -3

In order to verify this hypothesis and to verify the predicted influence of water on the equilibrium concentration of the imine and, therefore, on the rate of formation of oxime, a new set of catalytic experiments was carried out vaaring the concentration at low ammonia concentration and in the presence of water added to the reaction atmosphere. Figure 5 shows the influence of the concentration of molecular oxygen on the reaction rates at low ammonia content (2.5 mol%). In these conditions no dependence of the product formation rates on P o 2 is observed. On the other hand, some catalytic tests carried out adding different amounts of water to the reaction atmosphere showed a negative effect on the conversion and on the imine concentration in the outlet gas phase. On the basis of the considerations made above and the approximation of pseudo-stationary state conditions, the following kinetic equations can be written to describe the catalytic behaviour under the two conditions considered:

mol%): Low ammonia content (4 WHO = kCHO PCH a PNH3 rtars =ktars PCH PNH3 P o 2

r o w = k o w PCH ites x E+6 (mol/g.min) /

9

/’ /

y’’

/’

-+ r-CHO

h ’

+ r-tars ~

0

5

10

15

20

25

conc.02 (mol%) Fig.5 Influence of the concentration of 02 for low content of NH3 (2.5 mol%).

High ammonia content (>S mol%): rCH0 = kCHo PCH a P N H ~ Po2 1 & I 2 0 r m = k m PCH Po2 r o w = k o w PCH

(4) (5)

(6)

With a
All the data collected on the three main class of products and presented in the previous sections allowed to formulate a first tentative superficial model which can account for the cata100

yield (mol%)

conv. (mol%)

1.2

0

40

- 0.3

20-

OWC' 0

10

'

'

'

'

'

'

'

20

30

40

50

80

70

80

'

0 90

time-on-stream (min) Fig.6 Time evolutions of the conversion and the yield in oxime during the fvst two hours of time-on-stream.

485 lytic behaviour observed. 4.1 Formation of cyclohexanone oxime

In the standard conditions, the rate limiting step is assumed to be the oxidation of the imine, in equilibqum on the silica surface with the ketone, to the oxime by some activated oxygen species 0 . These species are assumed to be generated by active sites whose concentration increases with the total tar content. Therefore, the following scheme may be suggested: +NH3 O* Cyclohexanone ---------- > imine ---------> oxime -H20

The corresponding rate equation for the rate of formation of oxime was deduced in the form of an empirical power-type equation and, as a first approximation, it can be used to describe correctly the experimental data. 4.2. Formation of tars

At this stage of understanding of the mechanism of formation of the tars, it can be proposed that they form by a polymerization reaction with a polyaddition mechanism on C=C and/or C=N bonds, which occurs at the silica surface and involves N containing compounds (i.e. the imine and/or condensation products) in equilibrium with the ketone. The polymerization must, in some way, depend on the presence of oxygen. It can be proposed that activated oxygen species may be involved in determining the number of sites responsible for initiation of the reaction in a radical polymerization mechanism. It can also be supposed that these activated oxygen species might consist in the radical superoxides generated by silica surface. The proposed mechanism would explain the high N content and the low 0 content of the tars as well as the constancy of the rate of formation of the tars (since oxygen activated species are not involved as reactant species, no dependence is hypothesized on the concentration of the sites responsible for 0 2 activation during the first 40 h). The decrease in tar formation after the 40th h can also be explained by the proposed mechanism since, in the end, the concentration of oxygen activated species decreases as a consequence of the destruction of most of the sites responsible for 0 2 activation. Further characterization experiments in progress should add important information in order to clarify all the above mentioned hypotheses. 4.3. Formation of the other volatile products (OVP)

Taking into account that the OVP are a mixture of different products, the apparent first order with respect to the ketone concentration and the fact that their rate of formation does not depend on ammonia concentration seems to indicate that i) their formation might occur at least partly in the gas phase by condensation reactions such as aldol condensation reactions catalyzed by ammonia, ii) they may be in equilibrium with the ketone. The first assertion was confirmed by experiments carried out without the catalyst which brought to paragonable yields of OVP. With regards to the second assertion, a certain decrease in the yield of the OVP as conversion increases is indeed observed. On the basis of this hypothesis, the simple kinetic

486

equation proposed that can efficiently describe the experimental data, can be proposed for the formation of the entire group of OVP. In spite of the great effort spent to understand the mechanism of the reactions involved in the ammoximation of cyclohexanone with molecular oxygen, there is still a great lack of knowledge on the reaction, in particular, with regards to the mechanism of the imine oxidation to oxime. The data reported in the present paper showed the fundamental role of the tars in the selective pathway. They are supposed to furnish the sites responsible for the oxidation of the imine in the gas phase or for the formation of activated oxygen species which can oxidize the imine adsorbed on the silica surface Bronsted sites. At this stage of the research, it can be hypothesized that the oxidizing species on the tars can consist in hydroperoxide groups on the tars surface. The future works will try to determine the real nature of the oxidative species present on the tars and involved in the mechanism of formation of the oxime. We will try to gain the required information investigating the oxidation power of the fresh and the aged catalysts in some model reactions of oxidation such as: methanol oxidation, cylohexylamine and cyclohexane oxidative dehydrogenation and cyclohenanol oxidation. The data which will be collected will be used to complete the surface model and to draw new kinetic equations to be verified in a fitting of the experimental data and to design new more selective catalysts.

Acknowledgments. The financial support from C.N.R. - "Progetto Finalizzato - CHIMICA FINE 2" (Rome) is gratefully acknowledged.

5. REFERENCES 1. K.Weissermcl and H.J. Arpe, Industrial Organic Chemistry, Springer Verlag, 1978,pp. 222-231. 2. P. Roffia et al., European Patent 208,31 1 (1987). 3. P. Roffia et al., European Patcnt 267,232(1988). 4. P. Roffia et al., US Patent 4,745,221(1988). 5. P. Roffia, G . Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti, P. Gervasutti, in G . Ccnti and F. Trifirb (eds.) New Developments in Selective Oxidation, Rimini, 1989. 6. J.N.Armor,U.S.Patent4,163.756(1979). 7. J. N. Armor, J.Amer.Chem.Soc., 102,1453 (1980). 8. I. N.Armor, JCatal., 70.72 (1981). 9. I. N. Armor, E. J. Carlson, S. Soled, W. D. Conner, A. Laverick, B. De Rites and W. Gates, J.Catal., 70,R4 (1981). 10. I. N. Armor, P. M. Zambri and R. Leming, J.Catal., 72,(1982)66. 11. J. N. Armor, P. M. Zambri, J.Cata1.. 73.57 (1982). 12. J.N. Armor, J. Catal., 83,487(1983). 13. D. P. Dreoni, D. Pinelli, F. Trifirb, in Proceedings, "12Simposio Ibero American0 de Catalise". Rio de Janeiro 1990,Inst. Brasileiro de Petroleo, (1990),Vo1.2, pp.305-312. 14. D. P. Dreoni, D. Pinclli, F. Trifirb, J. Mol. Catal., 69,171 (1992). 15. D. P.Dreoni, D. Pinelli, F. Trifirb, Busca, Lorenzelli, J. Mol. Catal., 71,111 (1992). 16. D. Pinelli, F.Trifirb, A. Vaccari, E. Giamello, G . F. Pedulli, Catal. Letters, 13.21 (1992). 17. D.P. Dreoni, D. Pinelli. F. Trifirb. H. Habersberger, Z. Tvaruskova. P. Jiru, Catal. Letters, 11,285(1992). 18. D.P. Dreoni, D. Pinelli. F. Trifib, in New Developrncnt in Selective Oxidation by Heterogeneous Catalysis 111, book series Studies in Surface Science and Catalysis. vo1.72,P.Ruiz and B. Delmon Eds., Elsevier Science pub., Amsterdam 1992,pp. 109-116. 19. D.P. Dreoni, D. Pinclli, F. Trifirb, H. Habersbcrger,Z. Tvaruskova, P. Jim, in Proceedings of the " 10th International Congress on Catalysis", Budapest 1992,in pub. 20. E. Pieri, D. Pinelli, F. Trifirb, Chem. Eng. Science, 47,2641 (1992).

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

487

The Mars and van Krevelen mechanism for oxidation reactions used for a selective reduction reaction - influence of surface OH-groups on the selectivity. E.J. Grootendorst and V. Ponec Gorlaeus Laboratories, Leiden University P.O. Box 9502, 2300 RA Leiden, The Netherlands

Abstract In this paper the Mars and van Krevelen mechanism for selective oxidation reactions is used to describe a selective reduction reaction. It will be shown that the reduction of nitrobenzene to nitrosobenzene via this mechanism is possible over various transition metal oxides. It will also be demonstrated that surface hydroxyl groups are the most important hydrogen source for the unwanted production of aniline. The selectivity of the reaction can be influenced by the number of surface hydroxyl groups.

INTRODUCTION Nitrobenzene can easily and without much investment be reduced batchwise to nitrosobenzene in the liquid phase, but this procedure creates serious environmental problems. By this reaction (1)considerable amounts of waste products (like Zn- and Cr-salts) are produced.

Therefore, the gas phase hydrogenation over appropriate oxidic catalysts has recently gained in importance [1,21.

4aa

Possible unwanted byproducts are: aniline, azobenzene and azoxybenzene. This reaction (2) is believed to follow the Mars and van Krevelen mechanism [31, as most of selective oxidation reactions do [41. In the oxidation mechanism a molecule is oxidized to a valuable product with oxygen from a metal oxide catalyst (e.g. oxidation of propene to acrolein), thereby creating an oxygen vacancy in the surface of the catalyst. This vacancy is then refilled with dioxygen. Favre et al. [51 have shown that this re-oxidation reaction can also be performed with a useful, so called selective reduction reaction. A n example of a selective reduction reaction is reaction (2). Reaction (2) requires oxygen vacancies in the oxide surface, which can be created by using a reductor like CO or CH,. In the experiments described here nitrobenzene is reduced selectively to nitrosobenzene and some molecules of nitrobenzene are oxidised to CO, CO,, H,O and so on. The latter reaction is required to create oxygen vacancies, needed for the reduction reaction (autoredox-reaction [2,61). The composition of the surface of the catalyst is not exactly known, but in any case hydroxyl groups are present on the surface. These hydroxyl groups can participate in oxidation as well as reduction reactions. One of the undesired side reactions upon reduction of nitrobenzene is hydrogenation t o aniline. Since a hydrogen source is needed for the undesired production of aniline, the question we attempted to answer is as follows: what is the role of surface OHgroups (or other hydrogen containing surface groups like CH,) in the reduction of nitrobenzene ?

EILPERIMENTAL The experimental technique used is simple. The oxide powder is placed in a reactor, which is a part of a high vacuum system (typical pressures: - 10.' mbar). The reactant (C,H,NO,, > 99,5% pure, Baker Chemicals BV or C,D,NO,, 99 atom% D, Janssen Chimica) is admitted batchwise at low pressures (0.2 mbar) to the catalyst and the course of the reaction is followed with a mass spectrometer (VG Instruments MM8-80s). After a certain reaction time (about 20 min.) the gas phase is removed by evacuation and a new batch of reactant is admitted to start the following experiment. Blank experiments have shown that the glass and stainless steel walls of the vacuum system have no influence on the formation of the products.

489

The oxidic catalysts used in this study are prepared as follows. a-Mn,O, is prepared by thermal decomposition of manganese(I1)hydroxide in air at T = 390 K. The hydroxide is precipitated from manganese(I1)nitrate (Mn(N0,),.4 H,O, Merck) solution with ammonia a t pH = 9. 'y-Fe,O, is prepared by oxidation of Fe30, at 350 K in oxygen (Air Products). a-Fe,O, is prepared by thermal decomposition of iron(I1)hydroxide in air a t T = 875 K. The hydroxide is precipitated from iron(II1)nitrate (Fe(NO,),.9 H,O, J.T. Baker Chemicals B.V.) in the same way as manganese(I1)hydroxide. The oxides are characterised by X-ray diffraction and surface area measurements. XRD measurements are performed on a Philips type PW1050 X-ray diffractometer using monochromated Cu-Ka radiation (k0.154178 nm). The specific surface areas of the oxides are determined by adsorption of molecular nitrogen, according to the single point B.E.T. method, with a Quantasorb Jr. flow apparatus supplied by Quantachrome. The surface areas of a-Mn,O,, aFe,O, and yFe,O, are 26.9, 28.6 and 86.0 m2/g respectively. Selectivities are calculated only for products containing nitrogen. For example the selectivity towards aniline is calculated as follows:

in which I is the intensity of the molecular ion peak (corrected for fiagmentation).

RESULTS Three different sources of "hydrogen" are available on the oxide surface: i) intrinsically present OH-groups; ii) OH-groups formed during the reaction and iii) various Cq-fragments. Due to oxidation of the phenyl ring (autoredox reaction) CO, CO,, H,O and CH, can be formed. In most experiments no water is observed in the gasphase. It is assumed that some water produced is used to create OH-groups on the surface (species ii). When using C,D,NO, as a reactant, species ii) and iii) contain deuterium. As already mentioned a hydrogen source is needed for the production of aniline and we want to know which one is the most active. Taking into account the rising coverage of the surface by deuterium, the fact to be established by this experiment is as follows: which aniline (C,D,NHD, C,D,ND, or C,D,NH,) prevails in the initial products. In all figures the calculated selectivities reflect the initial product distributions. These distributions are measured directly after admitting the reactant to the catalyst. Figure I shows that the main product obtained fi-om C$,NO, is C&NH,. Only after several runs (with C,D,NO, as reactant) on the same sample, deuterium

490

appears in the amino group (see batch number 4 and 5). It is known from continuous flow experiments that in a steady state the formation of aniline can drop to a trace level and formation of nitrosobenzene can gain a high steady state level (selectivity up to 90 %). This effect can not be seen here since the steady state is reached only after several hours of reaction in continuous flow.

Batch number Figure 1. Reaction of C,D,NO, on a-Mn,O, at T=573 K, Initial product distributions.

The hydrogenating power of surface OH-groups is further checked by another experiment. A fresh Mn,O, sample is treated with D, gas a t T=600 K, which leads to a partial conversion of OH-groups into OD-groups. After this exchange reaction the reaction of C,H,NO, on the catalyst a t T = 573 K is checked. Figure 2 shows the initial product distributions. It can be seen that all three possible isotopes of aniline are formed in the first runs. Due to incomplete exchange of the surface hydroxyl groups into deuteroxyl groups, the production of C,H,NH, is relatively high. The possible role of surface hydroxyl groups in the undesired formation of aniline is also studied with two more oxides. Figures 3 and 4 show the initial product distributions after admitting C,D,NO, to a-Fe,O, and y-Fe,O,, respectively. It is remarkable to see that on y-Fe,O, (figure 4) already in the first run C,D,NHD is formed, while on a-Fe,O, (fig.3) this happens only after batch 4. Also the good selectivity towards nitrosobenzene on y-Fe,O, is noteworthy.

DISCUSSION Some data on the reduction reactions of nitrobenzene are available in the literature. Shindo and Nishihara [7]detected nitrosobenzene as an intermediate in

491

Batch number Figure 2. Reaction of C,H,NO, on a-Mn,O, (D, exchanged) at T = 573 K initial product distributions.

Figure 3. Initial product distributions from C,D,NO, on a-Fe,O, a t T=573 K.

Figure 4. Initial product distributions from C,D,NO, on y-Fe,O, a t T=573 K.

the electrochemical reduction of nitrobenzene on a metal electrode (Ag on 1%). They propose the following reaction scheme: c

ti 6 5

NO,^ c H NOZ c H NHOH 5 C ~ H ~ N H , 2H. 6 5 6 5 2H'

A paper presented a t the previous symposium [8J shows that formation of aniline from nitrobenzene is also possible over oxidic solid catalysts. Kijcnski found that MgO is a good catalyst for formation of aniline, when hydrogen atoms are supplied from a hydrogen donor (like isopropanol). Formation o r

492

aniline is not accompanied here with any formation of nitrosobenzene (not even in traces). In our laboratory the catalytic activity of various non-transition metal oxides for the reduction of nitrobenzene was tested. All tested oxides (Al,O,, ZnAl,O,, MgO and ZnO) are practically inactive in reduction towards aniline, when no external hydrogen source is supplied to the surface together with nitrobenzene. However, many transition metal oxides, having cations able to switch between different valencies, appear to be all comparably active [91 in the reduction of nitrobenzene. With most of these oxides selectivities for nitrosobenzene as high as 90 % can be achieved. Formation of aniline also takes place. In the induction period of the reaction (1 to 4 hours) oxidation towards CO and CO, and formation of aniline are the most important reactions. After this the selectivity for aniline drops to 10 % or even lower. The above mentioned data (from the literature and our own) can be explained as follows. Two mechanism for the reaction of nitrobenzene are possible and can operate next to each other:

Mechanism 1 is a mechanism of the Mars and van Krevelen type (also called "redox mechanism"). This mechanism prevails on transition metal oxides. By this mechanism nitrosobenzene can be produced selectively. The second scheme (2) depicts a " hydrogenation - dissociation* " mechanism, which indiscriminatcly removes both oxygen atoms (* read: N - 0 bond dissociation). Obviously, the intermediates formed upon reaction by mechanism 2 are too reactive and only the final product aniline can leave the surface. Mechanism 2 will be the most likely one when a surplus of hydrogen is available (for instance with MgO) or on Cu- and Ag-electrodes (electrochemical reduction). The results presented in this paper indicate that reduction of nitrobenzene to nitrosobenzene proceeds via mechanism 1. The first argument is based on a simple comparison of oxides: only those oxides which tolerate oxygen vacancies (needed for Mars and van Krevelen mechanism) are active. Secondly, production of nitrosobenzene and aniline is accompanied always with formation of CO and CO, (not shown in the figures). When using O'*-labeled 'y-Fe,O, as a catalyst for the reduction of nitrobenzene, the isotope 0l8appears both in CO and CO,. This means that a t least a part of the oxidation of nitrobenzene is performed with lattice oxygen. In this way oxygen vacancies are created and selective reduction of nitrobenzene via the Mars and van Krevelen mechanism is possible.

493

The results presented clearly indicate that on all oxides tested surface hydroxyl groups are the most important hydrogen source for aniline production. Figures 1,3 and 4 demonstrate the product distributions after a reaction of C,D,NO, on a-Mn30,, a-Fe,O, and yFe,03, respectively. After a certain number of batches deuterium is incorporated in the amino group of aniline. These deuterium atoms originate from the phenyl ring. As already mentioned the ring is oxidized and, since no water is observed in the gas phase, most of the deuterium atoms stay on the surface. When a certain coverage of deuterium is achieved, deuterium atoms start to be incorporated in the amino group. It is not clear yet why on yFe,O, already in the first run C,D,NHD is formed. Watanabe and Seto [lo] report that y-Fe203becomes reduced under vacuum at temperatures higher than 523 K, releasing oxygen and turning dark coloured. Possibly, the different density of oxygen vacancies on the surface of a- and y-Fe,03 causes the observed difference in behaviour. With other oxides, reduced to a higher extent than the steady state average valence, the same differences are found. For instance with MnO (steady state form being Mn30,), we observed also a high production of aniline. All mentioned effects are currently being studied in more detail. The hydrogenating power of the surface hydroxyl groups is hrther confirmed by the results presented in figure 2. Although it is difficult to exchange more than about 50 % of all OH-groups into OD-groups (when using DJ, the conclusions are obvious. Reaction of C,H,NO, on a-Mn,O, (containing OD-groups) immediately results in formation of all aniline isotopes. Results of Maltha [9] support the results presented here. Continuous flow experiments have revealed that oxides prepared from hydroxides (i.e. having more intrinsically present OHgroups) produce more aniline than oxides prepared by dry reduction of a higher oxide. It is not clear by which mechanism aniline is formed. Reduction of nitrobenzene t o nitrosobenzene proceeds via mechanism 1. Nitrosobenzene itself is also a reactive molecule and can react in a consecutive reaction to aniline, but it is not clear whether this reaction takes place in our experiment. Direct reduction of nitrobenzene to aniline via mechanism 1 seems only possible on oxide surfaces having a high density of oxygen vacancies on the surface. Direct reduction via mechanism 2 is not expected, since a very high concentration of surface OHgroups is needed in that case.

CONCLUSIONS It can be concluded from the results presented above that reduction of nitrobenzene t o nitrosobenzene is occurring via the Mars and van Krevelen mechanism. Reduction to aniline always happens in the first stages of the reaction, but it is not sure by which mechanism this reduction proceeds. In any case, surface hydroxyl groups are the most important hydrogen source.

494

It is proven that the selectivity of the reduction of nitrobenzene is influenced by the way the catalyst is prepared. More specifically, the selectivity is dependent on the number of OH-groups present on the catalyst surface before the reaction is started.

REFERENCES 1. 2. 3. 4. 5.

6.

7. 8. 9. 10.

D.Dodman,K.W. Pearson and J.M. Woolley, Brit.App1. 1322531 (1973) H.G. Zengel and M. Bergfeld, Ger.Offen 2939692 (1981) P. Mars and D.W.van Krevelen, Chem.Eng.Sci. 3 (1954)41 G.I.Golodets, Stud.Surf.Sci.Catal.56 (1990)693 T.L.F. Favre, P.J. Seijsener, P.J. Kooyman, A. Maltha, A.P. Zuur and V. Ponec, Cat.Letters 1 (1988)457 T.L.F. Favre, Ph.D.thesis, Leiden University 1991 H. Shindo and C. Nishihara, J.Electroanal.Chem., 263 (1989)415 J. wellski, M.Glillski and J. Reinhercs, Stud.Surf.Sci.Catal. 41 (1988)231 A. Maltha, S.C. van Wermeskerken, T.L.F. Favre, P.A.J.M. Angevaare, E.J. Grootendorst, C.A. Koutstaal, A.P. Zuur and V. Ponec, Cat.Today 10 (1991) 387 H.Watanabe and J. Seto, Bull.Chem.Soc.Jpn., 61 (1988)3067

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Wence Publishers B.V. All rights reserved.

495

K10 Montmorillonites as catalysts in Diels-Alder reactions: influence of the exchanged cation C. Cativielaa, F. Figuerasb, J. M. Frailea, J. I. Garciaa, M. Gila, J. A. Mayorala*, L. C. de Menorvalb, E. Piresa aDepartamento de Quimica Organica. lnstituto de Ciencia de Materiales de Arag6n. Universidad de Zaragoza-C.S.I.C., 50009 Zaragoza. Spain. bLaboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees (URA 418 du C.N.R.S.), E.N.S.C.M., 8, rue de I'Ecole Normale, 34053 Montpellier Cedex. France.

Abstract K10 montmorillonites exchanged with different cations, dried at 120°C or calcined at 55OoC, are used as catalysts in Diels-Alder reactions of methyl and (-)menthyl acrylates with cyclopentadiene. In general, calcined clays give rise to better conversions and selectivities. Zr(lV) and specially Ti(IV) clays display the best catalytic activities. However, the best asymmetric induction is achieved with Cr(lll) and Ca(ll) calcined clays. Clays containing easily reducible cations behave differently due to the cyclopentadiene polymerization via radical cations. 1. INTRODUCTION The Diels-Alder reaction, which is a powerful tool for the total synthesis of a variety of natural products [ l ] , has long been known to be catalysed by Lewis acids. This methodology has its drawbacks; namely the use of large quantities of catalyst and the problem of disposing of environmentally hazardous residues. Therefore, the development of new catalytic systems able to overcome these problems is an interesting task. Clays [2] and zeolites [3] have been reported as good catalysts for DielsAlder synthesis. Recently, we have studied the Diels-Alder cycloaddition between methyl acrylate (1) and cyclopentadiene (2) (Figure 1) and have shown that the solvent [4], the calcination of the solid [5] and the exchanged cation [6] play a decisive role. We now report the results obtained from the reactions of cyclopentadiene with methyl and (-)-menthy1 acrylates, catalysed by K10 montmorillonites exchanged with different cations and dried at 120°C or calcined at 550°C.

496

1

COOCHB

2

3x

3n Figure 1

2. EXPERIMENTAL 2.1. Preparation and characterizatlon

of the catalysts.

K10 montmorillonite was purchased from Aldrich. Cation exchange was performed by gradually adding of the clay to a stirred solution of the cation (Table 1) at room temperature and stirring the suspension for 24 h. After exchange, suspensions were filtered and washed with deionised water. Resulting solids were dried on a thin bed at 120°C in an oven and ground in a mortar. The solids were equilibrated over saturated salt solutions in order to give reproducible water contents. Calcination was carried out in air (25-30 ml/min) with the following temperature program: 20°C 1O°C/min 120°C 1"C/min 550°C (10 h) 1"C/min 4OOC.

-

-

-

-

-

-

Table 1. Methods of cation exchange for 10 g of clay. cation

H(I) Fe(lll) Zn(ll) Cu(ll) V(IV)

ce(lll) Ce(lV) Ca(ll) Zr(lV) Cr(lll) [S] Ti(IV) [9]

m

salt

conc.

volume

clay

1M 1M 1M 1M 1M 0.25 M 0.25 M 0.25 Ma 1M 0.1 M 0.1 MC 0.8 Me

125 ml 125 ml 125 ml 125 ml 125 ml 240 ml 267 ml 167 ml 125 ml 250 ml 1.25 I 126 ml

K10 K10 K10 K10 K10 K10 K10 K10 Na(l)-K10 K10 in1 I HpOb K1Od K10 in 2.5 I HpOb

aln H2SO.q 1 M. bSolution of the cation was gradually added to the clay suspension. cNapCO3 (125 mmol) was radually added to the solution of Cr(lll) and the solution was refluxed for 36 h. Suspension was refluxed for 1.5 h and then filtered. @Tic14was added to HCI (4 ml, 6 M) under Ar atmosphere. The mixture was then diluted by slow addition of deionised water (122 ml).

8

497

Surface areas of some representative clays, namely Zn(ll), Fe(lll), Ti(lV), Zr(lV) and Ce(lll), were calculated from BET isotherms determined at 77 K. In all cases the values obtained fall within the range of 220-240 m*/g, with the exception Zr-clay whose values are around 190 m2/g. The number of acid sites was determined by stepwise thermal desorption of ammonia at 373 K, then swept by a flow of dry nitrogen while the temperature was raised by steps of 50 K. The amount of ammonia evolved from the solid was monitored by conductimetry. X-ray diffraction patterns were recorded on a Phillips computer-driven X-ray diffractometer using CuK,1 radiation. 2.2. Reaction procedures.

Methyl acrylate and acryloyl chloride were purchased from Merck. (-)Menthol was purchased from Aldrich. Reagents were used without further purification. a) Reaction between methyl acrylate (1) and cyclopentadiene (2). Preweighed cation-exchanged montmorillonite (1.25 g) was dried at 120°C overnight or calcined by the above-described method. The flask was charged with the catalyst and methylene chloride (15 ml) under Ar atmosphere at 20°C. Methyl acrylate (1) (0.645 g, 7.5 mmol) and freshly distilled cyclopentadiene (2) (1.485 g, 22.5 mmol) were added via syringe. The reaction flask was shaken for 24 h and the reaction monitored by gas chromatography (FID from Hewlett-Packard 5890 II, cross-linked methyl silicone column 25 m x 0.2 mm x 0.33 pm, helium as carrier gas 17 psi, injector temperature 23OoC, detector temperature 250°C, oven temperature program 50°C (3 rnin), 25"C/min, 100°C (9 rnin), retention times: methyl acrylate (1) 2.7 min, exo cycloadduct (3x) 12.7 min, endo cycloadduct (3n) 12.9 rnin). b) Reaction between (-)-menthy1 acrylate (4) and cyclopentadiene (2). (-)-Menthy1 acrylate was prepared according a procedure described in the literature [lo]. Preweighed cation-exchanged montmorillonite (1.5 g) was dried at 120°C overnight or calcined by the above-described method. The flask was charged with the catalyst and methylene chloride (15 ml) under Ar atmosphere at 20°C. (-)-Menthy1 acrylate ( 4 ) (0.630 g, 3 mmol) and freshly distilled cyclopentadiene (2) (amounts described in Table 2) were added via syringe. The reactions were monitored by gas chromatography (FID from Hewlett-Packard 5890 II, cross-linked methyl silicone column 25 m x 0.2 mm x 0.33 pm, helium as carrier gas 19 psi, injector temperature 230°C, oven temperature program 190°C (1 min), 2"C/min, 180°C (0 rnin), l"C/min, 170°C (5 rnin), retention times: (-)-menthy1 acrylate (4) 3.9 min, (6a+6b) 18.9 min, (5a) 19.6 min, (5b)20.0 min). Absolute configurations were assigned by comparison of gas chromatograms obtained in clay-catalysed reactions with those obtained in Lewis acid-catalysed reactions previously described by Oppolzer and coworkers [l I].

3. RESULTS AND DISCUSSION Table 2 gathers the results obtained from the reaction of methyl acrylate (1) with cyclopentadiene (2) (Figure 1). Except for Na(l) exchanged clay, whose structure collapses upon calcination, calcination improves both catalytic activity and endohxo selectivity.

Table 2. Results obtained from the Diels-Alder reaction between methyl acrylate (1) and cyclopentadiene (2), catalysed by cation exchanged K10 montmorillonites dried at 120°C or calcined at 550OC.

Catalyst

-------

Zn(ll)-l20 Fe(111)-120 Cu(II)-120 V(IV)-120

Cr(lll)-l20 Na(l)-120 Ca(ll)-l20 H(1)-120 Zr(lV)-120

Ti(lV)-120 Ce(ll1)-120 Ce(lV)-120 Zn(ll)-550 Fe(lll)-550 Cu(ll)-550 V( Iv)-550 Cr(l il)-550 Na(I)-550 Ca(ll)-550 H(l)-550 Zr(lV)-550 Ti(lV)-550 Ce(Ill)-550 Ce(lV)-550

30 min

2h

conversiona 31113x8

conversiona 31113xa

2 35 26 15 23 28 8 11 21 23 16 45 10 61 47 59 52 42 7 52 27 81 76 67 54

3.7 9.4 9.7 12.2 12.3 13.0 8.3 9.8 10.1 11.2 10.2 9.6 7.6 14.7 15.3 15.4 14.0 14.7 9.2 14.7 12.2 14.3 14.6 14.5 14.0

63 44 23 47 46 21 33 3OC 65 31 67 19 92 75 90 89 78 32 79 62 97d 95 87 82

24 h

9.4 10.0 12.5 12.1 12.2 8.3 10.0 10.6 11.4 8.5 9.6 6.5 14.7 15.0 15.6 14.5 14.7 9.1 14.6 12.3 14.4 14.6 14.4 13.4

conversiona 3n13xa 54 82 57 31b 78 83 44 87

3.7 9.1 9.8 12.2 12.3 9.9 7.8 8.3

99 77 91 74 99 97 99 99 99 65 98 92

11.3 6.5 9.4 5.2 14.6 14.8 15.9 14.5 14.5 7.9 14.0 11.7

99 99

14.1 12.8

aDetermined by gas chromatography. bAt this time an additional 3 eq. of diene were added and the reaction reaches, after another 24 h, 92 YO of conversion with endohxo = 9.3. CYo of conversion after 1.5 h. After this time further conversion was not observed. d% of conversion after 1.5 h. Among the non-calcined clays some cations, namely Cu(ll), H(I), Cr(lll), Ti(lV) and Ce(lV), deserve particular consideration. When Cu(ll) and H(I) exchanged clays are used as catalysts the reaction stops at low conversions,

499

which can be explained by a competitive polymerization of the cyclopentadiene which eliminates this reagent from the solution. In fact, when an additional amount of diene is added further progress of the reaction is observed, but endolexo selectivity decreases. The decrease in selectivity indicates that the polymers formed partially poison the catalyst, so that the percentage of the less selective non-catalysed reaction increases. The poisoning of the clay also accounts for the decrease in endo/exo selectivity with increasing conversions observed when Cr(lll), Ti(IV) and Ce(lV)-exchanged clays are used as catalysts. Except for Ce(lV)-K10 montmorillonite, this abnormal behaviour disappears when calcined clays are used as catalysts. Given that calcination eliminates internal water and, consequently, most of Brernsted acid sites [5], it can be concluded that B r ~ n s t e dacidity greatly favours the polymerization of the diene. However, in the case of Ce(lV) clay there must be an additional mechanism for diene polymerization. It has been reported [6] that the formation of radical cations accelerates this lateral reaction. In fact, EPR spectra of Ce(lV)-clays in the presence of cyclopentadiene show a narrow signal at g = 2.004 k 0.002 which could be characteristic of organic radicals. If conversions after short reaction times are considered indirect measurements of reaction rates, Ce(lll)-KlO is the most efficient of the clays dried at 12OoC, whereas Ti(lV) and Zr(lV) calcined montmorillonites are the best solids studied in this work. The number of acid sites in calcined clays was determined by adsorption of NH3 and desorption at several temperatures. Except for Zr(lV) clay, whose catalytic activity was less than predicted, there is a rough correlation between the conversion after 15 min and the number of acid sites determined by desorption at 100°C. However, the correlations are worse when desorption of NH3 at 200, 300 or 400°C are considered. Therefore, the reaction rate correlates better with the total number of acid sites than with the number of strong acid sites.

4 2

&COOR*& 6a

COOR' GbR

Figure 2 The heterogeneous catalysis of the benchmark asymmetric Diels-Alder reaction between (-)-menthy1acrylate (4) and cyclopentadiene (2) (Figure 2) with both dried [12] and calcined 151 Zn(ll)-clays has shown that selectivities close to

500 those obtained with homogeneous catalysts can be achieved. Furthermore, the calcined clay can be recovered without loss of activity [5]. The clays that led to good results in the above non-asymmetric reaction, were tested in this asymmetric Diels-Alder reaction.

Table 3. Results obtained from the Diels-Alder reaction between (-)-menthy1 acrylate (4) and cyclopentadiene ( 2 ) , catalysed by cation exchanged K10 montmorillonites calcined at 550°C. Exchanged cation

----Zn (II ) C Fe(llI) V(W Cr(ll1) Zr(lV) Ti(1V) Ce(II1)g Ce(lll) Ce(lV) Cu(l1) Ca( II)

2:4

3: 1 3:1 3:1 5:ld 6: 1 6: 1 3: 1 6:le 6: 1 3:1 6:le 6: 1 3: 1 3:1 6:lf 3: 1 3:1 3:1 3: 1 6:le 6: 1 3:1 6:le 6: 1 3: 1 6:le 6: 1 3: 1 6:le 6: 1 3:1 6:le 6: 1

time (h)

51 1 2 24 2 24 1 2.5 24 1 5.5 24 1 2 24 1 2 24 1 2 24 1 5.5 24 1 2.5 24 1 3 24 1 3 24

Oh

conversiona

60 46 67 99 35 76 25 48 77 30 73 89 35 48 85 52 63 86 13 33 69 30 64 78 5 12 43 11 28 46 37 65 86

5/6a

3.8 11.2 11.0 11.0 8.0 6.4 11.8 12.0 8.8 13.1 11.8 10.8 11.2 11.0 10.7 10.3 10.1 10.1 13.6 12.8 12.3 12.6 11.6 10.4 7.4 4.7 4.7 12.2 11.6 10.9 13.4 12.4 11.7

% d.e.a,b

6 41 41 41 39 33 44 44 39 52 52 49 36 36 35 38 38 38 47 48 47 45 45 43 36 18 16 41 44 43 50 50 48

aDetermined by gas chromatography. b5b is preferably obtained. Fief. 5. dAfter 2 h, 2 eq. of diene are added. eAfter 1.5 h, 3 eq. of diene are added. 'After 2 h, 3 eq. of diene are added. W a y dried at 120°C.

The results obtained from the reaction were determined by gas chromatography, (5a) and (5b) were assigned by a comparison of the gas chromatograms obtained in several clay-catalysed reactions with those obtained in several reactions of (-)-menthy1acrylate with cyclopentadiene using homogeneous catalysts, whose diastereoselectivities and absolute configurations have been described [ll]. As can be seen (Table 3) all these clays play a catalytic role increasing reaction rate, endohxo selectivity and diastereofacial selectivity. With this less reactive dienophile (4) the differences between clays are more important. With regard to reaction rate, Ti(1V) clay is the most efficient catalyst, leading to high conversion with a 3:1 diene:dienophile molar relationship. However, the best asymmetric inductions are achieved with Cr(lll) and Ca(ll) clays calcined at 550°C. Cu(ll), Fe(lll) and Ce(lV) calcined clays behave differently. When Fe(lll) or Ce(lV) clays are used as catalysts, both selectivities decrease with increasing conversions, which is particularly noticeable with Ce(lV) clay. With Cu(ll) clay the reaction stops at low conversions. This behaviour can again be attributed to the polymerization of cyclopentadiene. Given that calcination eliminates most of Bransted acid sites, a cation radical mechanism must be invoked for the extensive diene polymerization. Ce(lV), Fe(lll) and Cu(ll) are the most easily reducible cations of those used and their EPR spectra in the presence of cyclopentadiene show the above-mentioned signal of organic radicals. The structure of Ti(IV) clay deserves comment. All the clays show the same diffraction pattern and the same crystallinity, except Ti(lV)-KlO, which exhibits a relatively broad line at 0.454 nm after calcination at 55OoC, characteristic of titanium oxide. The appearance of this line simply reflects the hydrolysis of Ti-clay bonds by the moisture contained in the clay, and sintering of the Ti polycations in the course of this thermal treatment. Part of the titanium in this sample is therefore in the form of very small particles of Ti02 dispersed at the surface.

CONCLUSIONS Calcined clays are better catalysts than dried clays, which may be due to the elimination of Brarnsted acid sites upon calcination. When the exchanged cation is easily reducible low selectivities, due to the deactivation of the catalyst by cyclopentadiene polymers, are obtained. Ti(IV) calcined clay is the most efficient catalyst, but the best asymmetric inductions are obtained with Cr(1ll) and Ca(ll) calcined clays.

Acknowledgements. This work was made possible by the generous financial support of the Comisi6n Interministerial de Ciencia y Tecnologia (Project MATSO0778). We are also indebted to the Subdireccion General de Cooperacion lnternacional (Acci6n lntegrada Hispano-Francesa2438). We are indebted to Prof. Pablo J. Alonso for the carrying-out of the EPR spectra.

502

REFERENCES (a) E. J. Corey, N. M. Weinshenker, T. K. Schaaf, W. Huber, J. Am. Chem. SOC., 91 (1969) 5675. (b) E. J. Corey, H. E. Ensley, J. Am. Chem. SOC.,97 (1975) 6908. (c) R. V. Boeckman Jr., P. C. Naegely, S. D. Arthur, J. Org. Chem., 45 (1980) 754. (d) 0. Ceder, H. G. Nilsson, Acta Chem. Scand. B., 30 (1976) 908. (e) E. E. Smissman, J. T. Suh, M. Oxman, J. Daniels, J. Am. Chem. SOC.,84 (1962) 1040. (a) P. Laszlo, J. Luchetti, Tetrahedron Lett., 25 (1984) 1567. (b) P. Laszlo, J. Luchetti, Tetrahedron Lett., 25 (1984) 2147. (c) P. Laszlo, J. Luchetti, Tetrahedron Lett., 25 (1984) 4387. (d) P. Laszlo, H. Moison, Chem. Lett. (1989) 1031. (e) J. Cabral, P.Laszlo, Tetrahedron Lett., 30 (1989) 7237. (f) C. Collet, P. Laszlo, Tetrahedron Lett., 32 (1991) 2905. (a) J. Ipaktschi, Z. Naturforsch, 41b (1986) 496. (b) Y. V. S. Narayana Murthy, C. N. Pillai, Synthetic Commun., 21 (1991) 783. (a) C. Cativiela, J. M. Fraile, J. I. Garcia, J. A. Mayoral, F. Figueras, J. Mol. Catal., 68 (1991) L31. (b) C. Cativiela, J. M. Fraile, J. I. Garcia, J. A. Mayoral, E. Pires, F. Figueras,.J. Mol. Catal., 79 (1993) 305. C. Cativiela, J. M. Fraile, J. 1. Garcia, J. A. Mayoral, E. Pires, F. Figueras, L. C. de MBnorval, Tetrahedron, 48 (1992) 6467. C. Cativiela, J. M. Fraile, J. I. Garcia, J. A. Mayoral, F. Figueras, L. C. de MBnorval, P. J. Alonso, J. Catal., 137 (1992) 394. F. Figueras, A. Mattrod-Bashi,G. Fetter, A. Thrierr, J. V. Zanchetta, J. Catal., 119 (1989) 91.

M. S. Tzou, T. J. Pinnavaia, Catalysis Today, 2 (1988) 243. J. Sterte, Clays Clay Miner., 34 (1986) 658. 10 W. Oppolzer, M. Kurth, D. Reichlin, C. Chapuis, M. Monhaupt, F. Moffat, Helv. Chim. Acta, 64 (1981) 2802. 11 W. Oppolzer, M. Kurth, D. Reichlin, F. Moffat, Tetrahedron Lett., 22 (1981) 2545. 12 C. Cativiela, F. Figueras, J. M. Fraile, J. I. Garcia, J. A. Mayoral, Tetrahedron: Asymmetry, 2 (1991) 953.

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicnls 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

503

Capsule membrane phase transfer catalysis : selective alkaline hydrolysis and oxidation of benzyl chloride to benzyl alcohol and benzaldehyde G.D. YADAV*, P.H. MEHTA AND B.V. HALDAVANEKAR Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India.

Abstract Selectivity of multiphase reactions catalysed by phase transfer catalysts can be greatly improved by the use of the so called capsule membrane - PTC (CM-PTC) technique. We report here the theoretical and experimental analysis of the CM-PTC and InverseCM-PTC for exclusively selective formation of benzyl alcohol and benzaldehyde from the alkaline hydrolysis and oxidation of benzyl chloride, respectively. The theoretical analysis shows that it is possible to simultaneously measure rate constant and equilibrium constant under certain conditions. The effects of speed of agitation, catalyst concentration, substrate concentration, nature of catalyst cation, membrane structure, nucleophile concentration, surface area for mass transfer and temperature on the rate of reaction are discussed. 1, INTRODUCTION

Selectivity engineering is a new term that is coined with the engineering aspects of multiphase reactions that could be manipulated through the use of several techniques such as use of an additional immiscibleliquid phase, porous inert solids, particles smaller than diffusion film thickness, etc. in order not only to intensify the rates of reaction but also to improve greatly the selectivity of the desired product. We are particularly concerned in this paper with the selectivity engineering aspects of the capsule membrane phase transfer catalysis, which has interesting attributes, for the preparation of benzyl alcohol and benzaldehyde by selective alkaline hydrolysis and oxidation, respectively, of benzyl chloride. In the industrial process, benzyl alcohol is normally manufactured by refluxing benzyl chloride with an alkali over a very long period (-24 hrs). However, this process leads to significant formation of the byproduct benzyl ether [ 11, Several attempts have been reported in the literature to suppress the yields of the ether. Thus, it was thought

504

desirable to employ the technique of capsule membrane phase transfer catalysis (CMPTC) to enhance the rate of reaction and particularly the selectivity of benzyl alcohol [2]. Selective oxidation of benzyl chloride to benzaldehyde is another reaction of great commercial importance. There exist several methods to prepare benzaldehyde, both commercially and synthetically,but the oxidation of benzyl chloride directly to benzaldehyde in a single pot by using CM-PTC merits special attention. We have studied in our laboratory [3] the kinetics and mechanism of the liquid-liquid and solid-liquid phase transfer oxidation by using chromate and hypochlorite salts and it was thought desirable to study the selectivity engineering aspects of this reaction. Triphase catalysts, which are bound to a polymer matrix, were introduced in the late 1970s in order to overcome problems associated with liquid-liquid and solid-liquid phase transfer catalysts. However, binding a phase transfer ammonium, phosphonium or polyethylene glycol catalyst to the solid phase, leads to loss in activity and thus to an increase in reaction times. In 1985,Okahata and Ariga [2]introduced the concept of CMPTC, wherein the phase transfer catalyst is grafted onto the surface of a porous ultrathin nylon capsule membrane. The chief advantage of this technique is that the hydrophobic onium salt or polyethylene glycol grafted on the capsule membrane physically separates the organic and aqueous phase reactants because the inner phase contains the organic substrate in a suitable solvent and the outer phase is an aqueous phase containing the nucleophile. This technique changes the selectivity drastically because the phase transfer catalyst attached to long graft polymerchainscan move freely between the inner organic and the outer aqueousphase. The pore size distribution of the membrane also plays a very vital role in getting favourable selectivity.

1.1. Inverse capsule membrane phase transfer catalysis (ICM-PTC) There is a merit in having the aqueous phase nucleophile inside the capsule and the organic phase substrateas the bulk outside phase. This way, the capsulecan be reused several times and the process can be made economical. The aqueous phase byproduct salt could be washed easily with water, or digested with fresh aqueous solution of the substrate. We have named this process as inverse capsule membrane phase transfer catalysis (ICM-PTC) wherein the locale of the reaction is likely to be outer surface of the capsule. Some aspects of ICM-PTC are also reported in this paper. 2. EXPERIMENTAL METHOD Large, semipermeableultrathin nylon capsules were prepared from mines and acid chloride by interfacial polymerisation by using a drop technique. However, a small amount of crosslinking agent (trimesoyl chloride) was added to obtain a strong and hard capsule membrane [2]. Two ml volume of an aqueous solution containing0.8 M NaOH and a suitable amine (such as ethylenediamine /diethylenetriamine/ triethylenetetramine

= 0.38/0.6/0.75M)was added dropwise from a syringe to a mixture of chloroform (75 ml), cyclohexane (25 ml), terephthaloylchloride (1 mmole) and trimesoyl chloride(0.03

505

m o l e ) . Nylon capsules having ultrathin thickness (7.625 p)and small diameter (1.74mm), as measured by using high performance image analyser model (TN 8502, Tracor-Northen, USA) were obtained. Three different types of capsules, with different mines were prepared with ethylenediamine, diethylenetriamine and triethylenetetramine.

2.1. Supporting PTC on capsule The nylon capsules prepared with different mines were used to graft a suitable phase transfer catalyst (PTC). Initially capsules prepared with ethylenediamine were chosen to support the following PTCs for the alkaline hydrolysis. (i) Aliquat-336 (trioctylmethylammoniumbromide) (ii) Tetrabutylammonium bromide (TBAB) (iii) Cetyltrimethylammoniumchloride (CTMAC) A saturated solution of the desired catalyst was prepared in acetone, into which a known number and weight of capsules were added, stirred for 30 minutes and then allowed to soak for additional 3 hrs. Finally the capsules were filtered off and dried at 700 C for 2 hrs. An isopropanol wash was given to the capsule to remove any unbound PTC followed by drying. The difference in the weight of the capsule denoted the amount of catalyst bound onto the capsule. Thereafter the same capsules were digested with benzyl chloride for about 12 hrs to allow its diffusion inside the capsules. The time for digestion was based on a number of previous trials. After digestion, the capsules were filtered off and washed carefully from outside with isopropanol and dried gently. Once again they were weighed to determine the actual amount of the reactant benzyl chloride that had penetrated the capsules. In order to minimise errors associated with weighing, etc., a large number of capsules were employed for each experiment and utilised for the reaction. In the case of ICM-PTC, after the phase transfer catalyst was grafted on them, the capsules were digested with the aqueous phase containing the substrate. 2.2. Reaction procedure The reactions were studied in a 5 cm i.d. fully baffled mechanically agitated contactor, of 250 ml of total capacity and equipped with a 6-bladed pitched turbine impeller and reflux condenser. The aqueous phase containing the suitable reactant, namely NaOH or K,Cr,O, solution was added to the reactor and agitated at the reaction temperature, following which the requisite number of capsules containing benzyl chloride was added. In a typical experiment, 100 capsules of 1.74 mm diameter with a catalyst loading of 7.06E-06 gmol/sq.cm and [BnCl] of 2.45E-05gmolkapsule and 50ml of the outer aqueous phase of the desired concentration were used. The reaction was allowed to proceed to the desired time period and a few capsules were sampled out, washed, cooled and broken in chloroform to expose its organic content which was analysed by G.C. (Perkin Elmer, 8350). The concentration profiles were determined as a function of time for each case, from the chromatogramswhich were quantitatively

506

analysed with reference to synthetic samples. 5% OV-17on Chromosorb WHP (2m x 3.2mm) column was used for the analysis in both the cases.

I

Overall: RX(org.) + MY (aq.)

M+YS(Q+ xj

(Q+Y-)

R Y (org.) +MY(aq.)

PTC Q*Yt

Aq. phase

M+Y-

------- il - - - - - - tt - - - - - - R Y t (Q' X-) -(Q+Y')

t

+

M =Metal Cation ,RX=Substrate a ) Liauid-

RX

YGNucleophile ,X:Leaving

group

Liauid PTC -r-

Inner liquid

Inter face Organic phase

&,A

Outer phase volume, vo Capsule spongy layer

Ou te liquid f

bl

CM-PTC and

ICM-PTC

Figure 1. Capsule membrane phase transfer catalysis.

3. RESULTS AND DISCUSSIONS 3.1. Models of CM-PTC and ICM-PTC Okahata and Ariga [2] have presented a mechanistic model for CM-PTC wherein the organic phase reactant resides inside the capsule membrane and aqueous phase reactants outside. This model has been modified by Yadav and Mehta [4] and is shown in Figure 1. The interface between the inner and outer phases would lie in the spongy layer of the capsule, wherein the PTC polymer is grafted. The exact location of the interfaceinside the thin layer of the membrane would depend on several factors, including the

507

preferential wettability of the membrane, its pore size distribution,the external pressure exerted by the outside phase in the agitated mixture, etc. Since the reaction mechanism is of SN2 type, there should be an exchange of ions via the PTC on spacer chains or anchors which are danglingacross the aqueous and organic phases. The reaction is interfacial. Furthermore, the said hypothesis which discusses the microscopic phenomenon can only be tested by macroscopicobservations.Additionally the length of anchors, the thickness of the membrane and the thickness of the liquid films adjacent to the interface ought to be considered. Since the membranes are hydrophobic, there is a possibility of formation of micellar-like oil-water interface in the spongy layer but it can be discounted on the basis of other observationsthat are characteristics of true PTC, one of them being the linear dependence of rate on the concentration of the PTC. It appears that the reaction occurs either at the interface or at the immediate vicinity of the interface in the organic phase upto which the anchors are mobile. In view of the very small thickness of the membrane and small lengths of the anchors, for all practical purposes, the surfacearea of the capsule can be assumed to be the locale of the reaction,

I-

L-L inter face

' 1

Centre of capsule

'

zone Membrane

Figure 2. Capsule membrane PTC : concentration profiles. Figure 2 depicts the various steps. At steady state, the following specific rates based on unit surface area (gmoysq. cm min) hold. For complete analysis, see Yadav and Mehta [4].

Rate of transfer of [Y-,,I from the bulk exterior phase to the outer capsule (1) surface = r y = Rate of exchange of [Yms0]with [ +Q+X-,,] at the anchor dangling outside the surface (2) = Rate of transfer of [ +Q+Y-,,] from outer phase anchor to the inner phase (3) = Rate of transfer of substrate RX from inside bulk phase to the inside capsule surface (4) = Rate of reaction at interface, to generate RY (5) = Rate of transfer of product RY from the inner surface to the inside bulk phase of capsule (6) = Rate of transfer of +Q+X-,i generated at the anchor located inside, to the outer surface of capsule (7) = Rate of transfer of X-so generated at the anchor at the outer surface of capsule to the outside bulk phase (8) In the above equationsthe second subscriptrefers to the inner (i) or outer (0)phase. b refers to bulk and s to surface. k, is the rate constant. Vi and V, are the inner phase (capsule) and outer phase volume. Any of the above (8) steps could control or, for that matter, more than one step may control the overall rate. Resistance due to steps (1) and (8) can be eliminated by using high speeds of agitation. It can be argued that except step (5) all other steps are fast and therefore, the specific rate of reaction in the spongy layer can be calculated. The overall rate can be arrived at by eliminating the unknown concentrationterms and the total resistance would thus comprise of several resistance in series. For step ( 5 ) as the rate determining step the specific rate of reaction for n capsule was found out to be,

where [Q,] = gmoles of catalyst anchored on capsule/cm2 The above equation was integrated to get the values of K and k, under certain conditions [4].

3.2. Selectivity In both the systems, it was found that the capsule membrane technique was exclusively selective,in that the hydrolysisreaction led to the formation of only benzyl alcohol and the oxidation reaction to only benzaldehyde. Furthermore, the rates of reaction were substantially enhanced. For a liquid-liquid PTC hydrolysis, there is a substantial formation of the byproduct dibenzyl ether. The oxidation of benzyl chloride with chromate under L-L PTC also results into byproduct formation, including benzyl

alcohol, dibenzyl ether and benzoic acid etc. The polymer supported PTC is also selective to benzaldehyde but the rates were comparatively lower than the CM-PTC. In contrast, the CM-PTC and ICM-PTC reactions resulted into 100% selective formation of benzaldehydeat much higher rates. This exclusive selectivity was possible because of the occurrence of the reaction in the spongy layer of the capsule, which avoids the solvolytic reactions which normally occur in other L-L PTC cases [5]. The kinetic data were interpreted by employing thetheoretical model, under the various assumptions outlined before, because there were no complications arising out of formation of different products.

3.3. Effect of speed of agitation Figure 3 shows that beyond 750 rev/min for the hydrolysis (curve A) and beyond 500 revhin for oxidation the speed (curve B) had no effect on conversion and hence on the rates of reaction, thereby indicating absence of liquid-to-membrane surface mass transfer resistance both inside and outside the capsules. The reaction could be taken as kinetically controlled and governed by eq.(5) beyond the said speeds in each case. This was further confirmed by studying the effect of temperature and the values of activation energies, which will be discussed later. Since the capsules were well dispersed in the agitated outer phase the bulk concentration of benzyl chloride within a capsule would be uniform. Further experiments were conducted beyond these speeds which were safe to maintain the fidelity of the capsules. 3.4. Measurement of equilibrium constants and rate constants As delineated earlier, by careful manipulation of variables, it was possible to calculate the rate constant k, and the equilibrium constant K for both the systems. Initially, a rough estimate of K was obtained by assuming a pseudo-first order behaviour and then iterativeprocedure was followed to determine K and k, from two independent experimental data. The average k, (cm3/ gmol) and K values for the hydrolysis and oxidation reactions were found to be (700,0.5)and (446.95,4.96),respectively. It is interesting to know that the equilibrium constant values for the +Q+OH- are around 0.5 which are almost 10 times higher than that for the L-L PTC. This explains the enhancementin CM-PTC reactions. The +Q+HCr04-valuesare also high in comparison with those for L-L PTC.

3.5. Effect of catalyst concentration Both the initial rate of reaction and the left hand side of the integrated forms of the appropriate equations were plotted against the catalyst concentration under otherwise similar conditions as shown in Figure 4,which confirms that the both the reactions are true PTC, because the linear dependence of rate on catalyst concentration, unlike the micellar catalysis.

510

-

.-C

Q,

E E PJ m E V *

0

::r 20

a

'I X U

u OO 400 800 1200

a

0'

Speed, RPM

Figure-4. Effect of catalyst loading

Figure-3. E f f ect ot speed o f agitation

-5*1

10 20 30 40 50 6 0 7 0 Q t x106 gmole

35v 1 15

20 25 [BuCI] xlO$grnolo

Figure-5 .Eftect ot concentration of benzyl chloride

t, m i n

F i g u r e 4 Effect of d i f f e r e n t catalysts 25-

2

.-C

o E

20

5% I 5

25

0

Figure-70

0.1

0.2 [NaOH] ,gmole

0.3

Effect of concentration of nucleophile

I35

Figu re -7 b Effect of concentration of nucleophile

511

3.6. Effect of Digestion time or Concentration of Substrate Different amounts of concentrations of benzyl chloride were introduced into the capsules by varying the digestion period as well as by taking different number of capsules to study the effect of concentrationof benzyl chloride on the rate under otherwise similar conditions. Figure 5 shows the effect of concentration, where rates are directly proportional to the concentration of benzyl chloride under otherwise similar conditions. 3.7. Effect of structure of PTC cation

Three different phase transfer catalysts, namely, TBAB, Aliquat-336 and CTMAC were evaluatedunder otherwise similar conditions (Figure 6) in the case of the hydrolysis reaction only. The anchored TBAB was found to be superior to others in the following orders (k,, K). TBAB (700.0.4) > Aliquat-336 (269.2.17.05) > CTMAC (145.2.17.17). It appears that although K values for the more bulky catalyst are higher than TBAB, the less bulky TBAB anchor is more mobile across the interface, thereby resulting into higher rates of reaction because anion exchange is very high. The low K values for the TBAB are thus of no consequencebecause the anion exchangetaking place in the aqueous phase across the interface is much faster for TBAB than the other two catalysts.

3.8. Effect of structure of polymer membrane The capsules prepared with EDA, DETA and TETA which were anchored with TBAB as PTC were used to compare the effect of polymer structure on the rate of hydrolysis reaction. The k, are 700, 641.2 and 575.7, respectively. This could be attributed to better mobility of anchors in the porous network of EDA membranes. 3.9. Effect of concentration of nucleophile in aqueous phase Figures 7a and 7b show the effect of concentration of aqueous phase nucleophile Y-in the aqueous phase underotherwise similar conditions. The ionic strength of the solution were also calculated. It was found that for the ionic strength of aqueous phase in the alkaline hydrolysis case which was varied from 1.O x to 5 x giodlit for the hydrolysis and from 3.0 x to 24.48 x for the oxidation reactions, respectively.This was further confirmed by adding NaCl salt (log and 20g) to the aqueous phase in the beginning to vary the ionic strength as well as to study the effect of C1- on K. It is interesting to note that the conversions were unaffected thereby indicating that the K values were also not influenced, unlike the regular L-L PTC. There was no effect on the rate of reaction and hence rate constant. K is dependent on ionic strength among other factors, for L-L PTC but binding the PTC to the anchor results into enhanced rates.

3.10. Effect of surface area Here three different capsule sizes (1.1, 1.74 and 4.92 mm) were taken and the number of capsules were chosen in order to have same concentration of the substrate, nucleophile and the catalyst. The rate of reaction is directly proportional to surface area,

512

which supports the arguments put forward before that the reaction occurs in the spongy layer of the capsule surface. 3.11. Effect of temperature The Arrhenius type plots were made to study the effect of temperature on rate constant (k,). The activation energies were found to be 12S4 kcdgmol and 16.35 kcaV gmol for the hydrolysis and oxidation reactions, respectively. These values also suggest that there was no influence of mass transfer and the reactions occur at the capsule surface.

3.12. Inverse-CM-PTC The ICM-PTC reactions were conducted by taking the aqueous phase nucleophile inside and the substrate in organic phase outside, wherein the organic phase was diluted with toluene to maintain same volume of outer phase for 100 capsules. The nucleophile was stoichiometricallydeficient in both the cases, which were analysed by making use of eq. (22). The values of k, and K obtained were (220.15, 0.2618) and (80.9042, 15) respectively for the hydrolysis and oxidation reaction. These values were found to be less than the CM-PTC although the selectivity was 100%. The lower rates could be attributed to the change in polarity of the organic phase, change in K values due to deficiency of nucleophile, etc. Further work is in progress. 4. CONCLUSIONS A complete theoretical and experimental analysis of capsule membrane phase transfer catalysis was done for the alkaline hydrolysis and oxidation of benzyl chloride. It is possible to determine both rate constant and equilibrium constant for the same data. There is 100% selectivity to benzyl alcohol and benzaldehyde. There is tremendous scope for research on various aspects of the CM-PTC and ICM-PTC techniques to be exploited for the intensification of rates of variety of multiphase reactions and selectivity of desired products.

5. REFERENCES 1 2

3 4 5

R. E. Kirk and D. E Othmer, Encyclopedia of Chemical Technology, 3rd Ed., vol. 3,795, Wiley Interscience, New York, 1979. Y.Okahata and K. Ariga, J. Org. Chem., 51 (1986) 5064. G. D. Yadav and B. V.Haldavanekar, to J. Phys. Chem. (1993). G. D. Yadav and P. H. Mehta, to Catalysis Letters (1993). G. D. Yadav and B. V.Haldavanekar, to Langmuir (1993).

M. Guisnet et al. (Editors),HCrcrogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

513

Selective Acylation of Sugar Derivatives Catalyzed by Immobilized Lipase A.T.J.W. de Goede, M. van Oosterom, M.P.J. van Deurzen, R.A. Sheldon, H. van Bekkum and F. van Rantwijk Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Abstract Alkyl derivatives of glucose, galactose and fructose were acylated by lipase-catalyzed transesterification with alkanoic esters. The best results were obtained with immobilized lipases of the Candidu untarcticu type. Primary alcohol functions were acylated first; followed by secondary ones depending on the structure of the glycoside. The water activity in the reaction medium had a striking effect on both the rate and the selectivity of the process. The size and orientation of the alkyl substituent and the structure of the acyl acceptor were also found to exert a profound influence on the course of the reaction. 1. INTRODUCTION Interest in the chemistry and applications of renewable raw materials and of products derived from these is rapidly growing. In this respect alkylation and acylation of monoand disaccharides combine the essential features of two major renewable classes, viz. triglycerides and carbohydrates, while leading to bio-friendly surfactants and emulsifiers. Introduction of acyl groups by chemical means usually results in mixtures of compounds, although some exceptions are known'. Lipase-catalyzed transesterification, in which the sugar acts as an acyl acceptor, offers an elegant way for selective conversion of these multifuctional systems without resorting to protection-deprotection schemes2>? Under the usual conditions for enzyme-catalyzed (trans)esterification, the composition at equilibrium is often far from optimal. Complete conversion can be achieved by removal of alcohol and/or water by vacuum3' or chemical means, i.e. by the use of enolk or oxime4 esters. We endeavoured to fix the water activity at a low level by adding zeolite to the reaction mixture. Immobilized enzymes which are stable in these very dry media have recently become available5. In this paper we present the results obtained with lipase-catalyzed transesterification of alkyl derivatives of glucose, galactose and fructose.

514

2. EXPERIMENTAL

Rhizomucor miehei lipase immobilized on a macroporous anion exchange resin (lipozym IM 20) and Candida antarctica Ii ase SP 435 (immobilized on an unspecified carrier) were kindly donated b Novo NorJsk A/S. The a- and 8-methyl glucosides and galactosides were supplied by sigma. The allotted amounts of reagents were mixed and shaken for the time indicated in the text. Sam les were withdrawn at regular intervals and analyzed by reversed-phase HPLC, and by after trimeth Isilylation. Pure com ounds were oitamed by chromatography and identified by 'H and 13C NMR at 400 MEz. Full spectral data will be reported elsewhere.

&

3. RESULTS AND DISCUSSION 3.1. a-Alkyl glucosides

1-0-Alkylglucopyranosides (alkyl is C, to C,2) can be prepared by direct acetalization of glucose with the appropriate alcohol under proton catalysis. Thus, in the case of 1-0octyl glucoside the a-isomer can be easily and almost quantitatively obtained as its monohydrate by crystallization followed by recycling of the other product components6. Various immobilized 0 lipases were tested in the +3AO/C2H5 transesterification of I-0-octyl a-D-glucopyranoside ethyl acrylate, using the ( 1) latter with HO compound both as reactant \\*H\& HO and solvent. By far the best Ho O C a H l 7 Lipose HoOCaH, results were obtained with lipase preparations of the 1 C2H50H 2 Candida antarctica type7 (see Table 1).Acylation occurred mainly at the 6-0 position, in line with the usual preference of lipases for primary alcohol functions3. The resulting 6-0-acryl ester may serve as a starting material for specialty polymers. Acylation at the 2-0-position was the main sidereaction. The selectivity and rate of the C. antarctica lipase catalyzed reaction could be improved substantially by adding zeolite CaA which selectively adsorbs water and The role of water in lipase-catalyzed reactions has been studied extensively, but still remains enigmatic. A minute amount of water is essential for establishing the proper tertiary structure, although a low water concentration enhances the stability of the enzyme. Water also engages in unwanted side-reactions such as hydrolysis and it inhibits the desired reaction by acting as a competitor for the enzyme domain". At low water activity", the rate enhancing factors apparently prevail over the rate decreasing factors, as far as the lipases from C. antarctica are concerned (see Table 1). The amount of enzyme could even be reduced by 90 % (i.e. to 4 mg) when zeolite CaA was present, for the same conversion. Surprisingly, the parallel and consecutive esterification at the 2-position was slowed down in the presence of the zeolite. The opposite is true for RIz. miehei lipase: this enzyme is partially deactivated and becomes less selective at low water activity.

515

Table 1 Transesterificationa of l-O-octyl a-D-glucopyranoside (1)catalyzed by immobilized lipases. Zeolite

Without zeolite Time Conv. (h) (%I

Lipase

45 Rhizomucor Miehei Candida antarctica A t B 24 Candida antarctica B (SP 435) 4 a

With zeolite CaA

Select.

Time Conv. Select. (h) (%I (% 2)

(%'.I 73 89 99

43 94 95

100 6 4

12 99 99

66 99 99

Compound 1, 40 mg; ethyl acrylate, 4 mi; immobilized lipase, 40 mg; zeolite CaA, 0.4 g; 40".

This pattern changed when the transesterification of 1-O-methyl a-D-glucopyranoside (3) with ethyl butanoate in the presence of C. antarctica lipase SP 435 was investigated12. Without drying agent the 6-O-acylated compound 4 was the main product. In the presence of zeolite the reaction continued, although more sluggishly, until complete conversion to diesters was attained. The 2,6-diester 5 was mainly formed, together with a small amount of the 3,6-isomer (6). It would seem that the size of the axial substituent at the l-position profoundly influences the rate of reaction at the neighbouring equatorial 2-position. We tentatively ascribe this effect to steric interactions with the apolar residues of the aminoacids which surround the active site13. 0

HoOCH3

-

HO \

5

4

3

Table 2 Transesterificationa of l-O-methyl a-D-glucopyranoside (3) catalyzed by lipase SP 435 Drying agent

Cosolvent

4 (%)

98 Zeolite CaA Zeolite C ~ A ~ - B U O H ~ 95 a

5

6

(96)

(%)

95

5

Compound 3,40 mg; ethyl butanoate, 4 ml; lipase SP 435, 4 mg; zeolite CaA,0.4 g where appropriate; 5 days at 40'. Ethyl butanoate, 2 ml; t-butyl alcohol, 2 ml.

C3H7

C H 3 7

6

516 The solvent also has an effect on the selectivity, as became clear when 50 % t-butyl alcohol14 was added with zeolite present, and the reaction stopped at the mono-ester stage. It would seem that t-butyl alcohol competes with 4 for the active site of the catalyst. 33. 8-Alkyl glucosides

With 8-alkyl glucosides, the picture became more complex1*. Transesterification of 1-O-methyl- and -0ctylB-D-glucopyranoside(7a and 7b, respectively) with ethyl butanoate without drying gave mainly the 6-0-acylated products 8, as would be expected, but with zeolite present complete conversion to the diesters took place. The 3,6-diesters 9 were mainly formed, together with the 2,6-isomers (lo), depending on the size of the anorneric substituent. 0

HO

C3H7COOEt

Ho&Ok C

SP 435

HO

OH

7

H

0

OH

4

OH

8

R

=

b: R

=

a:

'

CH3 C8HI5

HO

Table 3 TransesterificatiotP of 1-0-alkyl 8-D-glucopyranosides (7) with lipase SP 435 8a

Drying agent

Cosolvent

Zeolite CaA Zeolite CaA

-

a

t-BuOHb

(%)

9a (%)

10a

8b

(%)

(%I

98

10b (%)

98 88

95

9b

60

12

40

95

Compound 7, 40 mg; ethyl butanoatc, 4 ml; lipase SP 435, 4 mg; zeolite CaA, 0.4 g; 5 days at 40" Ethyl butanoate, 2 ml;1-butyl alcohol, 2 ml.

3.3. Methyl galactosides

It has already been shown that the equatorial/axial configuration of the secondary alcohol groups in sugar derivatives profoundly influences the rate and selectivity of their

517

acylation, although the effect is rather dependent on which lipase is used". We found that, in the presence of lipase SP 435, the 1-0-methyl a-D-6-0-butanoylgalactopyranoside (12) was converted into the 2,6-diester considerably faster than 4. When the reaction was run in the presence of zeolite CaA, 14 was formed in a nearly quantitative yield, even in 50 % t-butyl alcohol solution (see Scheme 1). 0 C3H7COOEt H

HO

SP 435

50

Z t-BuOH

11 0

I-

50

7

99

Zeolite CoA

11 ,

Compound

36

I

40 mg; ethyl butonoote, 2 ml; lipose

sp 435, 4 0 mg; t-BuOH, 2 ml; zeolite CoA. 0 . 4 g; 50 h at 40'

Transesterification of I-@Methyl a-D-galactopyranoside (11).

Scheme 1.

'

e

O

C

H

Y

-.

C3H7COOEt

3

HO

50 % 1-BuOH

15

: (

:(

-

:( 27

Zeolite CaA

15 , 40

47

HH&OCH3

'&

52

mg; ethyl butonoote. 2 ml; lipose

SP 435. 40 mg; I-&OH.

Scheme 2.

1

-

16

Drying agent

Compound

C3H7

"&0CH3

lipase SP 435

HO

1

o\

2 rnl, zeolite CaA, 0 4 g, 50 h at 4 0'

Transesterification of 1-0-Methyl 8-D-galactopyranoside (15).

CH3

518

1-0-Methyl 8-D-galactopyranoside (IS)was rapidly acylated by ethyl butanoate in the presence of SP 435 to a mixture of the 6-mono-ester 16 and the 2,6- and 3,6- diesters 17 and 18, respectively (see Scheme 2). In the presence of zeolite CaA the di-esters were the sole products. 3.4. Alkyl fructosides

Alkyl fructosides have remained rather inaccessible and have, consequently, received only scant attention. We have found that the reaction of fructose and alkyl alcohols can be achieved by taking special precautions16. Initially, furanoid compounds are formed which are in the course of the reaction partly transformed into the B-fructopyranoside. Separation of the products by preparative HPLC allowed us to study the esterification of structurally homogeneous compounds16. 2-0-Dodecyl F D OC 2'25 CgH gCOOEt OC1ZH25 fructopyranoside (19) HOe 0 y c 9 H 19 was converted into the OH OH Lipase HQ 1-0-decanoyl ester 20 by 50 'Z a c e t o n e lipase catalyzed 19 20 transesterification with ethyl decanoate (Table 4). It should be noted that here Rhizotwcor miellei lipase is activated by zeolite, whereas the reverse was true in the case of 1 and ethyl acrylate; 20 is formed less selectively under these conditions, however. Without cosolvent, di-esters whose structure still has to be elucidated, were mainly formed.

*,

Table 4 Transesterificationa of 2-0-dodecyl fl-D-fructopyranosidecatalyzed by immobilized li pases. Zeolite Lipase Rhizomucor Miehei Candida antarctica SP 435 a

Without zeolite Time

Conv.

(h)

(%)

168 24

22 74

Select. (%20) 94 100

With zeolite CaA Time (h)

Conv.

168 24

60 96

(%)

Select. (%20) 82 100

Compound 19, 40 mg; ethyl decanoatc 2 ml; immobilized lipase, 40 mg; acetone, 2nd; zeolite CaA, 0.4 g where appropriate; 40".

Transesterification of 2-0-dodecyl cr-D-fructofuranoside (21) with ethyl decanoate and SP 435 lipase gave initially the 6-mono-ester 22 which was slowly converted into di-esters, mainly the 1.6-di-ester 23. Addition of zeolite CaA allowed selective formation of either 22 or 23, depending on the reaction time.

519

) g o ,

21

1””

Y

CSHISCOOEt

IY

040+p

6 C l2H25 C. ontarctico SP 435

23

22

170 h

Table 5 Transesterificationa of 2-0-dodecyl a-D-fructofuranoside (21) catalyzed by lipase SP 435

Drying agent

Zeolite CaA a

time (h)

mono-esters 22 div (%) (‘31)

25 168

75 59

4

25 75

93

0 10

di-esters 23 div (%) (%)

8

3 3

0 80

0 4

Compound 21,40 mg; ethyl decanoate 4 ml; lipase SP 435, 40 mg; zeolite CaA, 0.4 g where appropriate; 170 h at 40’.

4. Concluding remarks Summarizing, our results on the lipase-catalyzed transesterification of 1-0-alkyl glycosides first of all underline the potential of C. antarctica type lipases. Generally, the primary 6-hydroxyl function of glucosides and galactosides is esterified first. The rate and selectivity of the consecutive esterification of the secondary hydroxyl groups is strongly influenced by the water activity in the medium, the anomeric configuration of the glycoside and the configuration of the 4-hydroxyl group. The size of the anomeric substituent exerts considerable influence on the esterification of the secondary hydroxyl groups in the 6-0-acylated a-D-glucopyranosides 2 and 4; with an octyl group as the anomeric substituent (2), the reaction is quite sluggish whilst in the case of a methyl group (4) it proceeds readily. Regioselective acylation of the 2-hydroxyl group was found for 4 as well as for the 6-0-acyl-a-D-galactoside 12. The influence of the size of the anomeric substituent in the I-0-alkyl 6-0-acyl-8-Dglucopyranosides 8a and 8b was much less pronounced. A tendency towards acylation of the 3-position was observed for the glucosides 8 as well as for the galactoside 16. The alkyl fructosides 19 and 21 represent a novel class of reactants and the investigations are far from complete. It is already clear, however, that in this case also the primary hydroxyl functions are acylated first.

520

5. ACKNOWLEDGEMENT The authors wish to thank Novo-Nordisk A/& Denmark for supplying the sample of lipase SP 435. This work has benefited from numerous discussions with Dr. L. Maat, for which the authors wish to express their gratitude. Thanks are due to Miss W. Benckhuijsen, who performed part of the experimental work. Financial support by Unichema BV, Royal Gist-brocades BV, Suiker-Unie BV and the Innovation-oriented Research Program on Carbohydrates (10P-k) is gratefully acknowledged. 6. REFERENCES

10 11 12 13

14

15

16

K.S. Mufti, R.A. Khan, GB Appl. 80/22320 (1980) (Chem. Abstr., 96 (1982) 1631121). For a review see: D.G. Drueckhammcr, W.J. Hcnnen, R.L. Pedcrson, C.F. Barbas and C.M. Gautheron, T. Krach, C.-H. Wong, Synthesis, (1991) 499. a. M. Therisod and A.M. Klibanov, J. Am. Chem. Soc., 108 (1986) 5638; b. M. Therisod and A.M. Klibanov, J . Am. Chem. SOC.,109 (1087) 3977; c. S. Riva, J. Chopineau, A.P.G. Kieboom and A.M. Klibanov, J. Am. Chem. Soc., 110 (1988) 584; d. W.J. Hennen, H.M. Swcers, Y.-F. Wang and C.-H. Wang, J.J. Lalonde, M. Momongan, D.E. Bergbreiter and Wong, J. Org. Chem., 53 (1988) 4939; e. Y.-F. C.-H. Wong, J. Am. Chem. SOC.,110 (19%) 7200;f. 0. Kirk, F. Bjorkling and S.E. Godtfredsen, PCT WO 89/01480 (Chcm. Abstr., 114 (1991) 183876c);g. M.P. de Nijs, L. Maat and A.P.G. Kieboom, Rccl. Trav. Chim. Pays-Bas, 109 (1990) 429. V. Gotor and R. Pulido, J. Chem. Soc., Perkin Trans., 491 (1991). S. Pedersen and P. Eigtved (Novo-Nordisk A/S), PCT Int. Appl. WO 90/15868 (Chem Abstr., 114 (1991). 224573r). A.J.J. Straathof, H. van Bekkum and A.P.G. Kieboom, Starch, 40 (1988) 229. M. Ishii (Novo Industri A/S), PCT Int. Appl. WO 8802,775 (Chem. Abstr., 110 (1989) 20529t). C’ W.F. Holderich and H. van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 642. Full paper: A.T.J.W. de Goedc, W. Benckhuijsen, F. van Rantwijk, L. Maat and H. van Bekkum, submitted to Recl. Trav. Chim. Pays-Bas. M.J.S. Dewar, Enzymc, 36 (1986) X; M.J.S. Dewar and K.M. Dieter, Biochemistry, 27 (1988) 3302. The water content in water-saturated and CaA-dried cthyl acrylatc was determined at 1.68 and 0.0075 ‘K, respectively, by Karl-Fischcr titration, corresponding with a water activity of 0.004. Full paper: A.T.J.W. de Gocdc, F. van Rantwijk and H. van Bckkum, in prcparation. See: L. Brady, A.M. Brzozowski, Z.S. Dcrcwcnda, E. Dodson, G.Dodson, S. Tolley, J.P. Turkenburg, L. Christiansen, B. Huge-.lensen, L. Norskov, L. Thim, U. Menge, Nature, 343 (1990) 767; A.M. Brzozowski, U. Derewenda, Z.S. Derewenda, G.C. Dodson, D.M. Dodson, J.P. Turkenburg, F. Bjorkling, B. Huge-Jensen, S.A. Patkar, L. Thim, Nature, 351 (1991) 491. The combination of the slightly acidic CaA zcolite and 1-butyl alcohol would seem to raise the problem of Ca-catalyzed dehydratation of thc cosolvent. Experiments with methyl butanoate as acyl donor and zeolite N a A as drying agent gave results which were very similar to those obtained with the combination ethyl butanoate/CaA. The same was true when zeolite powder not containing a binding agent was used instead of pellets. a. P. Ciuffreda, F. Ronchetti and L. Toma, J. Carbohydr. Chem., 9 (1990) 125; b. P. Ciuffreda, D. Colombo, F. Ronchetti and L. Toma, J. Org. Chem., 55 (1990)4187; c. D. Colombo, F. Ronchetti and L. Toma, Tetrahedron, 47 (1991) 103; d. D. Colombo, F. Ronchetti, A. Scala and L. Toma, J. Carbohydr. Chem., 11 (1992) 89. Full paper: A.TJ.W. de Goede, M.PJ. van Deurzen, F. van Rantwijk and H. van Bekkum, in preparation.

M.Guisnet et al. (Editors), Heterogeneous Cofalysis and Fine Chemicals Ill Q 1993 Elsevier Science Publishers B.V. All rights reserved.

521

MODIFIED ZEOLITES AS ACTIVE CATALYSTS IN FRIEDEL-CRAFTS ACYLATION.

D.E.Akporiaye, K. Daasvatn, J. Solberg and M. SMcker. Senter for Industriforskning, P.O. Box 124 Blindern, N-0314, Oslo, Norway.

Abstract Studies of the acylation of toluene with La-exchanged zeolite Y has shown the dependence of the activity on the rare-earth cation content and the high selectivity to the para isomer. Activities increasing in the order benzoyl < acetyl < propionyl chloride were found for all catalyst modifications. 1. INTRODUCTION

The wide range of zeolite and aluminophosphate structures now known and the many avenues open for their chemical modification by isomorphous substitution of framework elements and/or ion-exchange has led to increasing interest in their application in organic and fine chemical synthesis. Friedel-Crafts acylation is one of such potential process in which an active and selective heterogeneous catalyst could have opportunities of replacing more traditional homogeneous systems [ll. The current use of conventional Lewis acid catalysts such as aluminium chloride can pose a number of problems related to the fact that greater than stoichiometric amounts of the catalyst is needed due to the formation of an intermediate complex [ll. Subsequent hydrolysis leads to the loss of the catalyst and the environmental consequences of its disposal. The advantages and commercial viability of a recoverable and regenerable heterogeneous catalyst systems have been recently illustrated [23 in the application of pillared clays to Friedel-Crafts acylation and alkylation. The earliest studies in the use of rare-earth modified zeolites [3,4]and pillared clays [5] in the acylation of toluene using aliphatic carboxylic acids of different chain lengths have already shown the high selectivities possible for the para substituted product, although the activities were found to be dependent on the chain length of the acid used. The acylation of the more reactive anisole using phenylacyl chlorides over HY and dealuminated HY [6] has also given very high para isomer selectivities and the importance of the zeolites Br~nstedmidity in catalysing the reaction was illustrated. This has been confirmed by similar studies on the acylation of anisole by phenylacetyl chloride and acetic anhydride over a number of different zeolite structure types 173. Alternative studies on the

522

acylation of 2-methoxynapthalene with acetic anhydride 181 over a variety of Hzeolites was also able to optimise the selectivities in favour of either of the two possible ketone isomer products. We have now investigated fbrther the importance of the modification of the zeolite on its activity in catalysing the acylation reaction. Of particular interest was the application of the rare-earth (RE) exchanged zeolites to acylation reactions using acid chlorides. Previous studies had either tested the RE modified zeolite systems with the carboxylic acid [3,41 or applied only the Hform of the zeolite to the acid chloride system 15-71. We were interested in the role of the rare-earth cation in enhancing the activity of the catalyst with the acid chloride. 2. EXPERIMENTAL

Samples with two different levels of La exchange and one NH,Y were prepared from a commercial Linde LZ-Y52 sample of NaY (SUM = 2.37, BET = 900 m2/g). The exchange procedure [91 involved the addition of a La(NO,), solution to 10 g of zeolite suspended in 2 1 of deionized water heated to 70"C, followed by stirring during 24 hr. A sample with 26%, La(26)Y and 70%, La(70)Y of the theoretically possible ion-exchange capacity were obtained as confirmed by elemental analysis. HY was obtained during the pre-calcination treatment of the same commercial zeolite material which had been converted to the ammonium form by exchange with ammonium nitrate solution several times. The acylation reactions were carried out under batch conditions at 110°C under atmospheric pressure using the substrate, toluene, as the solvent. For each test, the catalyst waR calcined for 7 hr in flowing air at 400°C before rapidly transferring 1 g while hot to the reaction vessel. The system was subsequently evacuated overnight while heating at 130°C. Each run commenced with the addition of the mixture of solvent (300 ml) + acyl chloride (18 or 12 mmol) + internal standards (decane and hexadecane) to the catalyst. Samples were taken out periodically during the period of 24 hours and analysed by GLC ((5%phenyl)methylpolysiloxane capillary column).

3. RESULTS The results of the studies of the effect of catalyst modification on the activity in the benzoylation of toluene are presented in Figure 1. Rather low yields were found for the reaction over HY zeolite, in contrast t o the much higher yields over the fully exchanged La(70)Y system. In all reactions carried out the major product was always the para substituted material with only minor amounts of the ortho and meta, as shown for the La(70)Y system in Figure 2b. In the

523

experiments carried out using the La exchanged catalysts, minor amounts of the acid were formed immediately after the addition of the reactants to the catalyst with very little subsequent change in the acid concentration during the reaction. This was attributed to residual amounts of water still present in the zeolite. A similar effect had also been found during the reaction of phenylacetyl chloride with anisole [6]. During the reaction of the HY catalyst with benzoyl chloride, an initial and rapid disappearance of the substrate is observed for the first analysis after 0.6 hr., despite the low yields of products. This can be attributed to the rapid uptake by the zeolite of the acid chloride from the solution. This process had been earlier confirmed in blank runs with the equivalent acid, during which approximately 2 mmol of the acid was completely taken up from the solution by 1 g of catalyst.

.

Ls(70)Y

0 Ls(26)Y

.

b(70)Y

HY

I

0

10

20

Ume / hour

30

-

70

T

0

P 0

0 Ls(2B)Y

10

HY

20

30

lime I hour

Figure 1 Reaction of benzoyl chloride (12 mmol) and toluene over lanthanum exchanged N a y a) %Yield of the para isomer, b) %Conversion of benzoyl chloride.

Even higher activities and para product selectivities over the La exchanged catalysts were observed for the reactions with the propionyl chloride system, as shown in Figure 2. In contrast to the large difference in activity between the La(26)Y and La(70)Y catalyst for the benzoyl chloride system, only a small difference in activity is observed between the two levels of La exchange for the

524

propionyl chloride system. Similar results were also observed for the reaction with acetyl chloride and toluene.

r

I -

0

5

10

15

llme I hour

20

25

ortho 0 mela

pare

M

A

lo 0

0

6

1 0 1 5 2 0 2 5

tlme I hour

a)

Figure 2 Reaction of propionyl chloride (18 mmol) and toluene over lanthanum exchanged NaY zeolites. a) %Yield of para isomer, b) Distribution of isomers over La(70)Y. 4. DISCUSSION

The relative activities of the different acid chlorides over the La(26)Y and La(70)Y system are presented in Figure 3. The observed increase in activity going from acetyl chloride to propionyl chloride follows the trends reported for the equivalent carboxylic acid system [31. However, the higher activity of both the aliphatic acid chlorides with respect to benzoyl chloride was an interesting result. This contrasted with the observed trend for an equivalent homogeneous system in which the benzoylation reaction gives higher yields than the acetylation reaction [l]. A number of factors could determine these differences one of which is the diffusional restraints and a higher tendency for deactivation of the zeolite in the case of the benzoyl chloride system.

525 ~~

Aortyl

0 Bonzoyl

Acolyl

Proplon.

*1

d

0

1

1

0

1

1

Umo I hour

2

0

2

0 Proplonyl

5

0

5

1

0

1

5

2

0

2

5

tlmo I hour

b)

Figure 3 Reaction of acetyl, propionyl and benzoyl chloride (18 mmol) over a) La(70)Y and b) La(26)Y. The role of the RE exchanged cations in the zeolite catalyst has been observed by Chiche et al. [3,41 for the acylation of toluene using aliphatic carboxylic acids of varying chain lengths. A similar increase in the catalyst activity with increasing La content is shown by our study for the acylation reaction of toluene using acid chlorides. The use of La-exchanged zeolites in combination with acid chlorides appears to give a promising system for the acylation of toluene, giving relatively high yields of the para acylated toluene. This is illustrated by comparison of the much higher yields of 44% of the para isomer (compared to 2% ortho and 1% meta) for the acylation reaction of toluene presented here using La(70)Y and acetyl chloride after 24 hr. at llO"C, to the negligible yields observed for the analogous reaction of acetic acid with toluene after 48 hr at 150°C over a CeNaY zeolite [3]. Similar results were also found by us in our preliminary studies. In our case the observed 36% yield of para product from the acylation of toluene with benzoyl chloride over La(70)Y at 110°C for 24 hr., compares with the negligible yields found using the same catalyst system and benzoic acid under the same experimental conditions. The importance of the Bransted acidity of the zeolite in catalysing the acylation reaction from the acid chloride has been shown by Corma et al. [6] by the decreasing catalyst activity with increasing replacement of the available Bransted sites by exchange with sodium cations. A reaction mechanism involving the initial adsorption of the acid chloride on the zeolite was proposed

526

on the basis of kinetic studies. For the RE exchanged catalysts, the increased activity found in hexane cracking [9]and the dehydration of cyclohexanol [6] with the level of rare-earth cation exchange in zeolite Y has been attributed to the formation of Bransted acid sites with enhanced acid strength compared to the equivalent HY system. The enhanced acid strength has been linked in particular to the presence of La(0HP species formed during the calcination pretreatment [lo] through a reaction of the type: La3+(H2O),(.O-Zeol),

-->

LaVH,O),JOH)

(.O-Zeol),

+

HO-Zeol

Our studies clearly indicate the significant modification of the catalyst activity due to the presence of the RE cation, which currently may be assumed to play the role of enhancing the catalytic properties of the generated Bransted sites. F'urther planned studies characterizing the acidity of these materials will be used to obtain some insight into the reasons for the different catalytic activities. 6. CONCLUSIONS

The low activity of HY zeolite catalysts for the acylation of toluene using benzoyl and propionyl chloride is in contrast to the much enhanced activities found for the lanthanum modified equivalents. The importance of the lanthanum component is confirmed by varying the level of exchange with high selectivities for the para-product being found for all catalyst modifications. "he Lamodifidacid chloride system is found to be a much more active system for the acylation reaction compared to the equivalent carboxylic acid. Higher activity for the aliphatic acylation compared to the benzoylation appears to indicate additional factors restricting the activity in the benzoylation reacton. 6. REFERENCES 1G.A.Olah, Friedel-Crafts and Related Reaction, Interscience, part I, (1964). 2 New Scientist 16th February (1992)28. 3 B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille and D. Pioch, J. Org. Chem., 61 (1986)2128. 4 C. Gauthier, B.Chiche, A. F'iniels and P. Geneste, J. Mol. Catal, 60 (1989)219. 6 B. Chiche, A. Finiels, C. Gauthier and P. Geneste, J. Mol. Catal, 42 (1987)229. 6 A. Coma, M.J.Climent, H. Garcia and J. Primo, Appl. Catal., 49 (1989)109. 7 G. Harvey,A. Vogt, H.W. Kouwenhoven, R. Prins, 9th International Zeolite Conference, Montreal (1992). 8 G. Harvey and G. Milder, "Zeolite Chemistry and Catalysis", Prague (1991). 9 R. Carvajal, P-J.Chu, J.H. Lunsford, J. Cat. 126 (1990)123. 10 B.Herreros, P.P. Man, J.-M. Manoli, J.C.S. Chem. Comm. (1992)464.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

527

Acid-catalyzed ketonization of mixtures of low carbon number carboxylic acids on zeolite H-T J.A. Martens, M. Wydoodt, P. Espeel and P.A. Jacobs

Centrum voor Oppervlaktechemie en Katalyse, KU Leuven, Kardinaal Mercierlaan 92, B-3001 HEVERLEE, BELGIUM

Abstract Binary mixtures of acetic, propionic and butyric acid are converted in the vapor phase over zeolite H-T. From hydroxyl stretching vibration spectra of zeolite H-T with adsorbed butyric acid, it is concluded that the carboxylic acids have access to the different proton locations in this zeolite. The acids undergo ketonization reactions inside the erionite cavities of the zeolite, and dehydration reactions into anhydrides on the outer surface of the zeolite crystal. The ketonization activity and selectivity is rationalized by transition-state shape-selectivity in erionite cages. Zeolite H-T is particularly suitable for converting an equirnolar mixture of propionic and butyric acid into 3-hexanone.

Introduction Acetic acid is known to undergo a vapor-phase ketonization reaction with formation of acetone on Bronsted acids in general, and an proton-zeolites in particular. On large-pore zeolites in their proton form, the ketonization reaction is followed by acid-catalysed self-condensation amounting to rnesitylene, rnesityl oxide and phorone as main products [I], the chemistry being essentially identical to that in mineral acids. In Hpentail zeolites with suitable acid site density, phorone isomerises to isophorone, which is cracked to yield 2,4-xylenol [l]. With propionic acid a similar chemistry occurs, but the formation of phenolics is severely suppressed by transition-state shape-selectivity effects [2]. With butyric acid, fast deactivation occurs, except for zeolites belonging to the erionite-offretite family, on which selective ketonization to 4-heptanone is possible [2]. This has been attributed to the presence of the so-called erionite cage and the occurrence of a cage effect, comparable to that obtained with alkanes [2]. Based on this knowledge, the selective formation of certain ketones has been attempted using mixtures of short chain carboxylic acids.

528

Experimental A sample of zeolite T with a Si/Al ratio of 3.6 was crystallized at 373 K from a gel with molar oxide composition corresponding to: (Na2O)5 2(K20)1 7(A1203)(Si02)17.5. The crystallization time was 166 h. The zeolite T crystais obtained showed an euhedral morphology with the largest dimension varying from 1 to 4 pm. As-synthesized zeolite T was ion exchanged with 1 N NH4Cl solution under reflux conditions. Infrared spectra were recorded with a double beam Perkin Elmer 580B spectrometer. A self-supporting wafer of compressed zeolite powder was suspended in the infrared beam in an home-made vacuum cell, permitting in-situ heating of the sample and adsorption of vapors. The catalytic experiments were performed in a continuous flow tubular microreactor. The catalyst bed consisted of 0.8 cm3 of 0.3-0.5 mm pellets, prepared by compressing of the NH4-T zeolite powder into flakes, crushing and sieving. The zeolite was activated in the reactor by deammoniation at 673 K.The vapors of the acids were diluted with helium. The reaction conditions are further specified in Table 1. The reaction products were analysed on-line with GC, using a capillary fused silica column, coated with CP Si15 (Chrompack) and F.I.D. detector. C02 and H20 were not analysed. Table 1. Reaction conditions for the conversion of carboxylic acidsa over zeolite H-T Exp.

PHe

PCICOOH

(MPa) (kPa) 1

0.1

2

0.1

1.3

PC2COOH PC3COOH WHSV &Pa)

(kPa)

1.3 1.3

1.3

m-9

T

(K)

0.32

737

0.3 1

743

a, CICOOH = acetic acid; C2COOH = propionic acid; C3COOH = butyric acid.

Results and discussion 1. Interaction of hydroxyl groups of zeolite H-Twith butyric acid Zeolite T is an erionite-offretite intergrowth [3]. Infrared spectra of the hydroxyl groups in zeolite H-T are shown in Fig.1. The deammoniated zeolite shows stretchin vibrations of h droxyl groups at wavenumbers of 3740 cm-', 3616 crn" and 3564 cm . The 3740 cm' vibration is ascribed to silanol groups terminating the framework. The

r

-H

band at 3616 cm-l is due to bridging Si-OH-AI groups in large void volumes, e.g. the erionite cages and the 12-membered ring pores of offretite. The vibration at 3564 cm-l is typical for erionite and is ascribed to S i - O H 4 groups in 6-rings [4]. Upon adsorption of butyric acid vapor, the intensity of the three OH bands is decreased (Fig,l), indicating that butyric acid has access to the different proton locations in the zeolite. Evacuation of

529

the adsorbed butyric acid in vacuo at 573 K restores the initial intensities, indicating that the chemisorption of butyric acid is reversible.

Fig.1. Hydroyl spectrum of zeolite H-

T at 373 K in vacuo (10 d a ) (A);

-

3900

3550

3200

after exposure to 0.4 kPa vapour of butyric acid at 373 K (B); after subsequent evacuation in vacuo (10mPa) at 573 K (C).

cm-'

2. Conversion of carboxylic acid mixtures An equimolar mixture of propionic acid and acetic acid was converted over zeolite

H-T. In Fig.2, the conversion of the acids is plotted against the weight of feedstock contacted with the catalyst. Over the whole experiment, the conversion of acetic acid is higher than that of propionic acid. For both acids, an induction period is observed. The conversion of the acids increases till ca. 1.6 g of acids is fed per g of catalyst. Subsequently, a slight deactivation is observed, followed by a period of stable conversions of ca. 20% for propionic acid and 55% for acetic acid. Much higher levels of conversion were reached when an equimolar mixture of butyric acid and propionic acid was fed to the zeolite H-T catalyst (Fig.3). An induction period was observed, during which the increase of the butyric acid conversion was more pronounced than for propionic acid. After a short period of activity decay, for both acids stable conversions of ca. 70% are reached. These experiments show that synergetic effects occur in mixtures of carboxylic acids. The reactivity of propionic acid is much higher in the presence of butyric acid compared to acetic acid. The molar composition of the reaction products obtained from the two acid mixtures is given in Figs.4 and 5. In the conversion of the acetic - propionic acid mixture, formation of acetone, 2-butanone, 3-pentanone and acetic anhydride is observed. The reaction products from the propionic - butyric acid mixture are 3-pentanone, 3hexanone, 4-heptanone, acid anhydrides and traces of hydrocarbons.

530

100 n

00

?R

,

/

/

-. .

60

aoetio aoid

\

/

Y

.-

-

\

\

/

-

\

/

\

/

*-5

/

/

t>

propionio aoid

A

-

- - - _-

I /

C

0 0

20

-

:

,

1.0

0.5

0.0

1.5

2.0

2.5

acids / catalyst

3.5

3.0

(g/g)

Fig.2. Conversion of acetic acid and propionic acid in an equimolar mixture of these compounds over H-T zeolite against time-on-stream (expressed as g of acids fed to the reactor per g of catalyst).

I

loo

;

90 n

?R

Y

E

t

'-

0 0

80

1

-

- - a

70

.*

- - - ----

I I

I I

60

I I I

50

propionio add

A

I I

butyrlo aold

I

6

40 0

I

I

I

1

1

1

I

I

2

4

6

8

10

12

14

16

acids / catalyst

18

(g/g)

Fig.3. Conversion of ropionic acid and butyric acid in an equimolar mixture of these compounds over H-+zeolite against time-on-stream (expressed as g of acids fed to the reactor per g of catalyst).

531 n

ap

Y

C

.-0

60

Y

3

.-n .-Ua0 L

50

Y

Y

40

0 3

30

i

20

U

-5

z

10 0 0.0

1

1.0

0.5

1.5

2.0

acids / catalyst

2.5

3.0

3.5

(gig)

Fig.4. Molar composition of reaction products from an equimolar mixture of acetic acid and propionic converted over H-T zeolite.

70 60 50

40 30 20 10 0 0

2

4

6

8

10

acids / catalyst

12

14

16

18

(g/g)

F i e 5 Molar composition of reaction roducts from an equimolar mixture of propionic acid and butyric acid converted over -Tzeolite.

R

532

The formation of ketones is explained by the occurrence of ketonization reaction of two carboxylic acid molecules. The ketone molecule contains one carbon atom less than the two acid molecules, and the position of the ketone function is determined by the size of the alkyl chains R1 and R2:

RlCOOH t R2COOH --> R1COR2 t C02 t H2 0

(1)

For a mixture of two carboxylic acids, two self-ketonization and one mixed ketonization are possible. Mechanistically, the ketonization reaction starts with the formation of acyl cations: t

RlCOOH

+ H-Z--> RlCO t Z- t H20

(2)

A second acid molecule is dissociated and transfers a proton to the basic site in the zeolite (Z') :

R2COOH

t

Z- -- > R2COO'

+ HZ

(3)

The acyl cation and the carboxylate anion react to form a ketone and C02:

+

R2COO t RlCO --> RZCOR1 + C02

(4)

or else, an anhydride: t

R2COO- t RlCO -- > R2COOCOR 1 The selectivity of the reaction towards anhydrides or ketones changes strongly with time on stream (Figs.4 and 5). Anhydrides are formed during the induction period and after the period of fast deactivation. Ketone formation is most pronounced during the period of maximum catalytic activity. In previous work [2] it was shown that the formation of anhydrides occurs predominantly on acid sites at the outer surface of the zeolite crystals. In the conversion of butyric acid, the formation of butyric anhydride was suppressed after a selective poisoning of the acid sites on the external surface with triphenylchlorosilane.The formation of 4-heptanone was not affected. The dissociation constants of acetic, propionic and butyric acid in aqueous solution are comparable (pK values of 4.75, 4.87 and 4.81, respectively). The stability of the acyl cations increases in the order: t

+

t

CH3CO < CH3CH2CO < CH3CH2CH2CO

(6)

These data do not offer an explanation for the synergism in the propionic-butyric acid mixture. Steric factors seem to govern this type of catalysis. The presence of erionite cages in the zeolite is a prerequisite for ketonization activity and stable catalytic performances [2]. Butyric acid is transformed into 4heptanone over zeolite T, erionite and ZSM-34 containing this structural element [2].

533

Other zeolite types including zeolite L, Y, Beta, mordenite and ZSM-5 catalyse the conversion of butyric acid far beyond the ketonization stage. The shape and dimensions of the erionite cage are indeed ideally suitable for the accommodation of two acid molecules only. The accommodation of a propionic and a butyric acid molecule in an erionite cage is illustrated in Fig.6. The erionite cage is a cylindrical cavity with dimensions of 1.3 x 0.63 nrn. Six 8-membered ring windows on the walls give access to neighboring cavities. The erionite cage is too small to accommodate a third acid molecule, explaining why in this system, condensation reactions can be terminated at the dimerisation stage.

Figure 6. Transition state for the ketonization of butyric and propionic acid in an erionite cavity.

During the induction period, acyl cations are probably generated inside the erionite cavities. The ketonization of butyric acid on H-T zeolite was found to obey first order reaction kinetics. This reaction order can be reconciled with a reaction mechanism in which the reaction of a molecule of butyric acid or its dissociated form with the butyric cation is rate determining. The activation energy for the ketonization of butyric acid amounts to 73 KJ/mole. The bimolecular ketonization of short-chain carhoxylic acids is known to be catalysed by oxides of cerium and thorium, supported on titanium oxides [ 5 ] . The present work shows that erionite-offretite zeolites are potential alternative catalysts.

534

Conclusions Zeolite H-T catalyzed the ketonization of short-chain carboxylic acids. The formation of anhydrides is a side reaction, occurring on the outer surface of the zeolite crystals. The propionic and butyric acid molecules seem to have the optimum size for a bimolecular ketonization reaction inside an erionite cavity. The ketonization of carboxylic acids is an example of zeolite specificity in catalysis, illustrating the necessity of strict adaptation of the transition state of the reaction to the intracrystalline porosity of the zeolite.

Acknowledgments JAM and PE acknowledge the National Fund for Scientific Research, Belgium for a research position and a fellowship, respectively.

References 1. Y. Servotte, J. Jacobs and P.A. Jacobs, Proceed. Int. Symp. Zeolite Catalysis, Siofok (Hungary), 1985, Acta Phys. Chem. Szeged. p.609. 2. M. Vervecken, Y. Servotte, M. Wydoodt, L. Jacobs, J.A. Martens, P.A. Jacobs, in 'Chemical Reactions in Organic and Inorganic Constrained Systems' R. Setton, ed., D. Reidel, 1986, p.95. 3. A. Cichocki, J. Chem. SOC.Faraday Trans. I 81 (1985) 1297. 4. P.A. Jacobs and W.J. Mortier, Zeolites 2 (1982) 220 5. R.F. Goldstein and A.L. Waddams, The Petroleum Chemical Industry, Spon, London, 1967, p.361.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

535

Reactions of ketoximes and aldoximes over solid acid catalysts. T.Curtin and B.K. Hodnett Dept of Chemical and Life Sciences, University of Limerick, Limerick, Ireland.

Abstract The Beckmann rearrangement has been studied over a range of solid acids, including zeolites and modified aluminas, in the temperature range 250-380OC. Generally ketoximes rearrange to the corresponding lactams, but acetoximes dehydrate to the nitriles. Coke formation was a major problem associated with all solid acids tested, giving rise initially to lowering in conversion and later to lowering in selectivity to the lactam product. There was a direct relationship between levels of coke formation and the decline in catalytic activity. Because coke formation was most prevalent over those catalysts with the largest numbers of surface basic sites, it is proposed that caprolactam polymerization and condensation reactions of byproducts, such as between aniline and cyclohexanone, involve surface basic sites. By contrast, there was a direct relationship between the surface concentration of acidic sites of intermediate strength and the rate of caprolactam formation from cyclohexanone oxime. Introduction The rearrangement of oximes over solid acids has been studied for some time although the total number of studies has remained low . Zeolite and alumina based solid acids have been mostly studied and the substrate most often used has been cyclohexanone oxime, with a view to its transformation into caprolactam [ 1-15]. A major problem associated with this reaction is coke formation and it appears with all catalyst types studied to date. The origin of coke formation have not been established, although there is a tendency to assume that it arises via a similar set of conditions to those which are responsible for coke formation in, for example, toluene disproportionation. This study looks at the rearrangement of a range of ketoximes and aldoximes over zeolite and alumina based solid acid catalysts with a view to elucidating the factors which determine the reactivity of the various oximes and the selectivity to the amide products. Another factor to be addressed is the mechanism of coke formation, in particular the role of acidic and basic surface sites in this process[ 161 .

Experimental The sodium, phosphorus, chloride, sulphate and boria modified catalysts were prepared by impregnating y-alumina, (surface area 95 m2g-l), with aqueous solutions of Na2C03, (NH&HP04 , N&CI , H2SO4 , or H3B03 , respectively. After stimng for 2 hours the excess water was slowly removed by boiling until a slurry formed, which was dried overnight

536

at 100OC. The zeolites HA, HY, H-mordenite and HZSM-5 (supplied by BDH) were obtained from the sodium forms, by repeated ion exchange with 1M ammonium hydroxide solution. The vapow phase Beckmann rearrangement reaction was carried out using a continuous flow system operated at ambient pressure. Helium was used to entrain the oximes into the vapour phase from a saturator (2.2 torr). The rearrangement of the oxime took place in the utube pyrex reactor loaded with 100 mg of catalyst. In normal operation the total gas flow rate was 30 ml min-I. On leaving the reactor the gas stream was cooled, the products were trapped and analysed by off-line gas chromatography. Temperature programmed desorption of ammonia and carbon dioxide and the techniques measurement of wt% coke formed on the catalysts have been fully presented elsewhere[ 14- 161.

Results and Discussion Conversion of cyclohexanone oxime and selectivity to caprolactam over a range of solid acids are presented in figures 1 (a) and (b). The corresponding data for cyclopenatanone conversion and selectivity to 6-valerolactam is presented in figures 2 (a) and (b). High conversions were achieved over B2OgAl203 and HZSM-5 and maintained for up to 10 hours on stream. However, these catalysts eventually lost their activities just as H-Mordenite and NaA1203 had over a shorter time frame. Deactivation was accompanied by coke formation, which was most pronounced on Na-AI203. Generally selectivities (figures 1 b and 2 b) remained high as long as the levels of conversion remained high, but selectivity over all catalysts declined when coke formation became excessive. Other products observed in this study included cyclohexanone, 5-cyanopent- 1-ene and aniline, corresponding to hydration, fragmentation and dehydrogenation products respectively. Generally selectivities to these products remained constant for the first 10 hours on stream in spite of declining conversions and the dehydrogenation product, aniline was most prevalent over the Na/A1203 catalyst. The relationship between selectivity to caprolactam and zeolite pore diameter is presented in Table 1 and clearly shows that oxime conversion and lactam selectivities were lowest over the zeolites with the smallest pore diameters.

Table I Relationship between pore size and selectivity to caprolactam Catalyst

Pore Diameter

Conversion a %

Selectivity a to Caprolacmn %

/A

HA H-ZSMS H-Mord H-Y

4.1

5.5 6-7 7.4

14 100 100 84

a Conversion and selectivity measured between 350 and 380°C.

4 48 55

62

537 100

80

60

z>

40

8

20

0

2

4

6

8

Tlmelh

0

2

4

6

8

Time/h

Figure 1. Conversion of cyclohexanone oxime (a) and selectivity to caprolactam (b) over B203/A12O3,30O0C I,[. Na/A1203,3OO0C [O], HZSM-5,35O0C 101and H-Mordenite, 350°C

[OI. 100 80

f

' z>

s 40

0

2

4

6

Tlmelh

8

Tlmelh

Figure 2. Conversion of cyclopentanone oxime (a) and selectivity to valerolactam (b) over Na/A1203,300°C [a],HZSM-5,35O0C [Q] and H-Mordenite, 350°C B203/A1203,300°C [HI,

[OI.

538 Acetaldoxime conversion in the temperature range 250-350°C over B2O3/Al2@ and HZSMS is presented in figures 3 (a) and (b). In each case the major product, corresponding to greater than 90% of the converted aldoxime. was acetonitrile, which arose from the dehydration of the oxime. Small amounts of the amide also formed.

260

300 TernperaturePC

340

150

200

300

400

TemperaturePC

Figure 3. Acetaldoxime conversion [W] and acetronitrile yield [Q] over (a) B203/A1203 and (b) HZSM-5.

The relationship between the total amount of cyclohexanone oxime passed through the catalyst bed and the decline in conversion is illustrated in figure 4(a) for the B203/A1203 catalyst, which was tested for up to 10 hours at 300°C during contact with cyclohexanone oxime in helium at various pressures. The rate of decline in conversion was dependent of oxime partial pressure, with the greatest decline observed at the highest oxime partial pressure. The corresponding data for selectivity to caprolactam are presented in figure 4 (b) and show that selectivity was greatest at the lowest oxime partial pressure in the feed. These data imply that coke formation is associated with a side reaction which proceeds along a parallel pathway to oxime conversion. One possible pathway is the polymerization of caprolactam, which is base catalysed [17], thus explaining the greater propensity for coke formation and deactivation over the more basic catalysts and the greater decay in catalytic activity at high oxime pressures. Another source of deactivation arises from Schiff base formation between aniline and cyclohexanone [18], both of which are byproducts during cyclohexanone oxime conversion to caprolactam. These products, in fact, were most abundent over the more basic catalysts used in this study. Interestingly the product of this condensation can condense once again with cyclohexanone via the Mannich reaction [ 181 leading eventually to polymer formation.

539 1001

-

.

-

I

-

-

'

1

-

Total oxime in contact with catalyst bed (g oxime/g catalyst)

Figure 4. Decline in conversion of cyclohexanone oxime (a) , at the pressures indicated, and selectivity to caprolactam (b), plotted as a function of the total amount of oxime passed through the catalyst bed. Characterization of these catalysts by TPD of C02 and NH3 is shown in figure 5. The B203/A1203 material featured a large concentration of acidic sites of intermediate strength, as indicated by the large desorption peak in the temperature range 2OO-35O0Cand the concentration of basic sites, measured by TPD of C02 was zero. By contrast, the Na/Al2O3 material did not possess any strong or intermediate strength acid sites, but it did possess a very large number of surface basic sites, as indicated by the TPD profile for C@ desorption. NH3 Desorption BIA1203

HZSM-5 H-Mord NalA1203 100

200

300

400

500

0

100

200

Temper8tureI"C

Figure 5. TPD of C02 and NH3 from the catalysts indicated.

300

400

TemperaturePC

500

600

540

There was a simple relationship between the rate of caprolactam formation over the range of modified aluminas studied and the surface concentration of intermediate strength acid sites, namely those from which ammonia desorbed in the temperature range 200-350OC. This relationship is shown in figure 6 and establishes a link between acidic sites of intermediate strength and caprolactam formation. Based on these data turnover frequencies were all in the range 0.8-1.8 x 10-3molecules of caprolactam formed per surface site of intermediate acidity per second.

0.5

I

I

-

0.4

0.3

-

0.2

-

0.1

-

1

9

PI

8 PI

7 4

8 3 2 1

0.0 ’--

I

I

1

0

2

4

6

8

Surface conc intermediate strength acid sites / umoi m-2

Figure 6. Relationship between the rate of caprolactam formation and the surface concentration of intermediate strength acid sites for the modified aluminas 1, Na/AI203; 2 , A1203; 3 , CI/A1203; 4, 2%B203/A1203; 5. 4%B203/A1203; 6, 7%B203/A1203; 7, S04/A1203; 8, 10%B203/A1203;9,20%B203/A1203.

A relationship was established between coke build-up on these catalysts over 8 or 27 hours on stream, as measured by thermogravimetric analysis, and the decline in conversion of the oxime observed over the same period. This relationship is presented in figure 7 and shows that coke formation is related to loss in catalytic activity. In fact, the greatest decline in conversion was observed over the most basic catalysts, namely Na/A1203, which featured close to 90% decline in conversion over 8 hours on stream. By contrasuhe most active and selective catalyst tested, namely 20 wt%B203/A1203 did not appear to possess basic surface sites (see fig 5 a>and its activity had declined by less than 20% in the first 8 hours on stream and by about 70% after 27 hours on stream.

541

6ot

6

1

m 4

3

2

4

6

8

10

Wt% coke after 8 or 27 hours

12

14

16

on stream

Figure 7: Influence of coke build-up on decline in conversion of the oxime over a range of modified aluminas at the temperatures indicated. (1) AI203; (2) 20wt%B203/A1203 (3) S04/A1203; (4) NdA1203; ( 5 ) Cl/A1203; (6) A1203; (7) 2wt%B203/A1203; (8) 4wt% B203/A1203; (9) 7wt% B203/A1203; (10) lOwt% B203/A1203; (1 1) 14wt% B203/A1203; (12) 20wt%B203/A1203 These findings may be rationalized in terms of the scheme outlined below: An active site of intermediate acidity will not completely remove the hydroxyl group from the oxime and after migration of the R' group the intermediate (I) is formed. If basic sites are present on the catalyst surface then a reaction with the partially charged carbon may occur and the rearrangement would not proceed. In the absence of basic sites rearrangement may proceed as follows:

Acid site A

R

\C = N Oxime

c.. /OH

.I

.OH.

. . ..* .. R-6

=N-R' (1)

542

The acid site, being of intermediate strength, is not strongly attached to the hydroxyl group, thus the reaction continues producing the enol form of the amide which subsequently tautomerises to the amide:

v

Y

A

i

.a

OH .... .... R

-c=

N-R'

6+

0

Y A

+

II R-C-NH-R'

J

Tautomerisation of enol form of amide

References D. England, US Patent, No. 2,634,269 (1953). 1. L. Werke, East Ger. Patent, No. 10,920 (1955) 2. BASF, Ger. Patent, No. 1,227,028 (1967) 3. F. W. Yates, R.O. Downs and J.C. Burleson, US Patent, No 3,639,391 (1972) 4. Y. Izumi, S.Sato and K. Urabe, Chem. Lett., (1983) 1649. 5. H. Sato, N. Ishii. K. Hirose and S. Nakamura, Studies in Surface Science and 6. Catalysis, 28 (1986) 755. A. Aucejo, M.C. Burguet, A. Coma and V. Fornes, Appl. Catal., 22 (1986) 7. 187. S. Sato, S. Hasebe, H. Sakurai, K. Urabe and Y. Izumi, Appl. Catal., 29 8. (1987) 107. H. Sakurai, S. Sato, K. Urabe and Y. Izumi, Chem Lett., (1985) 1783. 9. 10. S. Sato, K. Urabe and Y. Izumi, J. Catal., 102 (1986) 99. S. Sato, H. Sakurai, K. Urabe and Y. Izumi, Chem. Lett., (1985) 277. 11. E. Gutierrez, A.J. Aznar and E. Ruiz-Hitzky, Studies in Surface Science and 12. Catalysis, 59 (1991) 539. T. Curtin, J.B. McMonagle and B.K. Hodnett, Studies in Surface Science and 13. Catalysis, 59 (1991) 531. T. Curtin, J.B. McMonagle and B.K. Hodnett, Appl. Catal., 93 (1992) 75 14 T. Curtin, J.B. McMonagle and B.K. Hodnett, Appl. Catal., 93 (1992) 91 15. T. Curtin. Thesis. Universitv of Limerick. 1992. 16. A. Ravve; Organic Chemis6y of Macromolecules, Marcel Dekker, Inc., New York, 17. (1967). ,- A. Streitwieser and C.H. Heathcock, Introduction to Organic Chemistry, 3rd edition, 18. Macmillan Publ. Co., New York, 1989, p-722. I

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

543

Zeolite Catalysed Rearrangement of Aromatic Amines Th. Stamm, H.W. Kouwenhoven and R. Prins Laboratory for Technical Chemistry, Eidgenossische Technische Hochschule, Zurich, 8092 Zurich, Switzerland

Abetsact The influence of reaction conditions, catalyst parameters and reactants on the rearrangement of aromatic amines to substituted methylpyridines catalysed by zeolites has been examinated. Passivation of the outer surface has been used to increase the selectivity. Experiments have been carried out with different zeolite types, with substituted anilines and with naphthylamines in order to prove a new model for the mechanism of the reaction.

1. INTRODUCTION A process for the conversion of phenol to aniline using the H-form of ZSM5 at a n ammonia pressure of about 1000 Wa was described by Mobil Oil [1,2]. The formation of diphenylarnine and the rearrangement of aniline to 2-methylpyridine were observed as interesting side reactions [3]. The latter reaction was separately claimed in a patent and described to occur at 783 K and 28000 Wa [4]. Best results were obtained with a molar ratio of ammonia t o aniline of 1.5, be it a t low conversion. Typical side products of the rearrangement of aniline were acetonitrile and condensed aromatic compounds, like substituted (methylated) indoles and quinolines, and the catalysts showed a rapid deactivation by coke formation. Condensed heteroaromatic compounds are most probably formed on the outer surface of the catalyst, especially if it has a n MFI-structure 151. A similar reaction, the rearrangement of m-phenylenediamine was patented by Bayer (Figure 1)[6]. In this patent, a much higher conversion and a better selectivity than obtainable with aniline was reported. Two reaction mechanisms were proposed for the aniline rearrangement reaction [3,71. In this contribution, we will discuss this interesting reaction and report on some studies on process variables and on the reaction mechanism. As a model compound, we used m-phenylenediamine because of the higher conversion and relatively milder reaction conditions required for its conversion into 2amino-6-methylpyridine (a-aminod-picoline).

544

NH3 / ZSM-5

673K 19000kPa * N H 2 0 c H : o n v e r Selectivity: sion: 73.4 56.4 % NH2 ( NH3 Idiamine = 60 1 Figure 1. Rearrangement of m-phenylenediamine to a-amino-a'-picoline

2.EXPERDIENTAL Samples of ZSM-5, Beta and Y-sieve were obtained from CU Chemie Uetikon. The samples of ZSM-5, varying in crystallite size, and Nu-10 were laboratory synthesised according to published methods [8,9]. ZSM-5 and Ysieve were converted to H-ZSM-5 and USY by standard techniques and finally calcined a t 823 K.The template was removed from zeolite Beta by heating at 773 K, first during 3 h in a helium flow and subsequently in a n oxygen flow. The template free Beta was activated by a treatment with 0.1M hydrochloric acid at 298 K for removal of sodium followed by drying and calcination at 773 K. Some analytical data on the various materials are collected in Table 1. The optimal calcination method for zeolite beta was established by thermogravimetric analysis using a PL-Thermal Sciences STA 1500 apparatus. Chemical compositions of the zeolites were determined by atomic absorption spectroscopy on a Varian AAlO spectrometer after dissolution of the samples in hydrofluoric acid. The structure was confirmed by x-ray diffraction on a Siemens D-5000 diffractometer and with infrared spectroscopy on a Mattson Instruments Galaxy 2000 spectrometer. Total surface area, micropore area and micropore volume of the samples were determined by argon adsorption on a Micromeritics ASAP 200M volumetric analyzer using standard techniques. Crystal diameters were determined by scanning electron microscopy. Table 1. Analytical data 'the catalysts Catalyst CrystSize Si/Al ( p m) NU-10 1.3 39 USY 2.0 6 Beta 0.5 45 ZSM-5 0.2 a0 Silica alumina amorph. " 5"

AreaTotal ( m2/g) 136 506

624 398 433

AreaMjcrop. Microp.Vo1 ( rnVg1 (rnl/g) 101 0.05 448 0.21 616 0.18 259 0.12 7 0.00

545

The active sites on the outer surface were passivated by the reaction with a bulky silylchloride: 10 g of the acidic form of a zeolite was calcined one hour at 773 K and suspended in 100 ml of dry hexane. The slurry was mixed under nitrogen with a solution of 5 g of triphenylchlorosilane in 400 ml of dry hexane and stirred at 328 K during one hour. The hydrochloric acid gas formed was removed by flowing nitrogen and trapped in water. By titration, the number of active sites which reacted was calculated. The total number of active sites was assumed to be equal to that calculated from the silicon t o aluminium ratio. The silylated material was investigated with infrared spectroscopy to detect the aromatic C-H stretch vibration and the amount of organic material was determined by thermogravimetric analysis. Before the catalytic reaction, the catalyst was calcined at 823 K in air and the surface area was measured again. The catalytic reaction was performed as follows: 0.5 g of the activated catalyst and 2.5 g of the reactant were put in a 100 ml stainless steel autoclave, preheated at 398 K. The autoclave was filled two times with nitrogen of 20'000 kPa which was again released. After cooling, the autoclave was placed into an ice bath and filled with ammonia and was subsequently heated to reaction temperature with a heating rate of about 7 Kmin-1. Usual reaction temperature was between 550 and 680 K. Samples were taken during the reaction using a nitrogen blanketed two valve sampling system, dissolved in dry tetrahydrofurane and analysed using a HP 5600 gas chromatograph. Coke formation on the catalyst was determined by thermogravimetric analysis.

3. RESULTS AND DISCUSSION The optimal conditions for conversion and selectivity in the rearrangement of m-phenylenediamine were 593 K and 10000 kPa. Results obtained with various zeolites are given in Table 2. Table 2. Rearra SiIA1 Cryst. Conv. (%I Selectivities: Type: AmPic' CH,CN Aniline Cond." Others"' Size (pm) (%) Nu-10 39 1.3 8 B 1 12 46 17 Beta 45 0.5 I3 63 4 13 3 16 us Y 6 2 19 47 8 8 7 23 SiAl "5" amorph 19 64 3 8 6 19 ZSM-5 20 0.2 66 76 1 2 4 17 ZSM-121 32 4 12 122 5 10 7 56 -. Conditions:p =10000 kPa, T = 593 K, t=10 h, ammoniddiamine = 28 '2-amino-6-picoline, "condensed heteroaromatics, "'mainly methylated reactands, isomers and addition products

546

ZSM-5 is clearly the best catalyst. The large pore zeolites as Beta and USY have a lower activity and selectivity. Nu-10, a 10-ring zeolite with small pores [5] shows a low selectivity for aminopicoline. Formation of aminopicoline over Nu-10 is, however, an example of a reaction taking place at the outer surface. For comparison, a sample of silica-alumina was tested and the data in Table 2 show that i t is about equally active as USY at a higher selectivity. Several other zeolites such as ZSM-12 were investigated and by far the best results were obtained with the H-form of ZSM-5. But although the selectivity is quite high, it must be expected that also in the case of ZSM-5 a part of the reaction runs at the outside of the catalyst. And this led to some investigations of two catalyst parameters. Various ZSM-5 samples with approximately the same silicon to aluminium ratio but differing in crystal size were tested and it appeared that the yield increased with decreasing crystal size. Accordingly, the yield increased with increasing outer surface area (Figure 2). Another zeolite parameter which may have a strong effect on the conversion, is the silicon to aluminium ratio. Some ZSM-5 samples with approximately the same crystal size, but different silicon to aluminium ratios were tested. It appeared, that with increasing silicon to aluminium ratio, the yield of aminopicoline decreased probably because of the decreasing number of active sites (Figure 3). The best catalyst had a low silicon to aluminium ratio and a very small crystallite size. Yield (%I 60

Yield (%)

40

20

0

0

0

10

20 l / r (l/prn)

Figure 2. Influence of crystal size

0

200

400 Si/Al ratio

Figure 3. Influence of SUAl ratio

547

Reactions at the outer surface may influence the product compositions and it was therefore tried to selectively deactivate the sites on the outer surface, using triphenylsilylchloride, a bulky reactant which cannot enter the pores of ZSM-5. The results in Table 3 show that deactivation of the outer surface results in a somewhat better selectivity, albeit at lower conversion level.

Convers. Catalyst H-ZSM-5 ZSM-5 passiv.

(%)

66 24

Selectivities: Amino- AcetoAniline picoline nitrile 76 1 2 83 6 0

(%)

Cond. Arom. 4 0

Others 17 11

The deactivation reaction allows a rough estimate of the number of active sites at the outer surface of the zeolite, since the quantity of the hydrochloric acid gas produced is proportional to the number of hydroxyl groups that reacted with the silylchloride. When this quantity is compared with the number of active sites, calculated from the silicon t o aluminium ratio, one obtains the fraction of active sites accessible for the triphenylsilylchloride which depends on the crystal size. For ZSM-5 crystals of about 0.2 pm, we found that 13% of the active sites are accessible for the silylchloride. For a crystal diameter of 25 pm, obtained using the fluoride synthesis method, the percentage of accessible active sites is 1%.The IR spectra of the calcined zeolite and the treated and vacuum dried zeolite were taken and the differential IR spectra showed the vibration of the aromatic C-H bonding a t 2930 wavenumbers. There is a slight shift towards lower wavenumbers because of the distorted vibration of the surface bonded phenyl groups interacting with the surface. In order to learn more about the reaction mechanism and the scope of the aniline rearrangement, additional substrates were tested. Experiments with toluidines (methylpyridines) showed a substantial isomerisation and disproportionation of the substrate (Table 4) due to the methyl-shift reaction on the highly acidic catalyst. Selectivities are low due t o disproportionation reaction of the reactants and a large amount of di- and trimethylated reactants and several addition products were formed (which explains the missing 60% in the reaction of o-toluidine).

548

Table 4. n t of toluidines Selectivities (%) Lutidine Colli- AniToluidine Xyli2,3 2,4 2,5 2,6 dine line ortho meta para dine 2.7 0.5 0 1.6 0 14.2 12.7 1.4 6.7 0 12.8 0 6.5 25 3.0 6.9 29 6.6 0.3 1.1 1.2 1.5 1.7 18.5 43.1 18.2 10.0 Conditions:p -43000 kPa, T = 563 K, t=6 h, lutidine = di methylpyridine, collidine = 2,4,6-trimethylpyridine, xylidine = trimethylani.1ine Conv.

To prevent the methyl rearrangements we examined naphthylamines as reactants. Results in Table 5 demonstrate, that substantial conversions of both a - and P-naphthylamine over zeolite H-beta occurred a t reasonable selectivities. Besides isomerisation among the naphthylamines, a main product of a-naphthylamine is l-methylisoquinoline, while using pnaphthylamine 3-methylisoquinoline is a main product [lo]. These products cannot be explained by the ring enlargement mechanism proposed by Chang and Perkins [3]. Table 5. Conversions and selectivities for the rearrangement of a- and f3-naphthylamine (a-NA, P-NA) to methylisoquinoline (MeIQ) and methylquinoline

NA

a

a*

P P

Temp. ("C) 300 320 320 350

Conv.

(%I 28 9 19 50

Selectiv ities l-Me I& 3-Me IQ 2-Me Q n 2 71 3 15 2 7 1

(%I

P-NA

a-NA

25 7 45 45

*Others 46 22 37 45

The results of the naphthylamine rearrangements and also the rearrangements of aniline and m-phenylenediamine can be fully explained by a new mechanism as presented in Figure 4 [ l l l . We postulate that the reaction

549

starts with the addition of ammonia to the aromatic ring, probably preceded by a n enol-keto-type enamine imine isomerisation. After another enamine-imine isomerisation, the ring can open by means of a reverse aldol-type reaction. The ring closes again through addition of the amine group to the imine double bond. Elimination of ammonia from the resulting aminal gives the final methylpyridine product. This mechanism explains why a high ammonia pressure is required. Addition of ammonia to an aromatic ring is thermodynamically unfavourable. The final pyridine product, however, is more stable than the phenyl reactand and thus provides the thermodynamic driving force. Even an unfavourable ammonia addition preequilibrium is no obstacle for reaction if subsequent reactions are fast. The proposed reaction steps can be catalysed by acid-base type catalysts, explaining why zeolites and other solid acids in the presence of ammonia are observed to catalyse the reaction.

Figure 4. Reaction mechanism

4. CONCLUSIONS

The yield in the rearrangement of m-phenylenediamine is directly proportional to the number of active sites, which corresponds to the silicon to aluminium ratio of the zeolitic framework. The major part of the reaction, however, takes place on the outer surface of the zeolite. ARer passivating the active sites of the outer surface with a silylchloride, the conversion decreases, but the selectivity increases. The selectivity in the rearrangement of toluidines is lower due to the methyl-shift reaction ocurring at the highly acidic catalyst. The rearrangement reaction proceeds via an aldol-type mechanism as confirmed by the rearrangement of naphthylamines to methylquinoline and methylisoquinoline.

550

ACKNOWLEDGEMENT

The present work was supported by the Swiss KWF-project 1814.1 "Zeolites as Catalysts" as a joint project with CU Chemie Uetikon, Switzerland.

D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J.Phys.Chem. 85 (1981)2238 2 C.D. Chang and W.H. Lang, US Patent 4 380 669 (1980) 3 C.D. Chang and P.D. Perkins, Zeolites 3 (1983)298 4 C.D. Chang and P.D. Perkins, European Patent 0 082 613 (1982) 5 W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, 2nd Ed., Butterworths, London 1987 6 H. LeBlanc and L. Puppe, OffenlegungsschriftDE 3332687 A1 (1983) 7 H.C. van der Plas, Acc. Chem. Res. 11 (1078)462 8 J.L. Guth, H. Kessler, J.M. Higel, J.M. Lamblin, J. Patarin, A. Seive, J.M. Chezeau and R. Wey, Zeolite Synthesis, ACS Symp. (1989)176 9 P.J. Hogan, A. Stewart and T.V. Whittam, European Patent 0 065 400 (1982) 10 Th. Stamm,H.W.Kouwenhoven and R. Prins, Swiss Patent Appl. Nr.02 699/92-1 11 Th. Stamm, H.W. Kouwenhoven, D. Seebach and R. Prins, to be published 1

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemiurk 111 Q 1993 Elsevier Science Publishers B.V. All rights reserved.

551

A SEEECXWE PROCESS FOR THE SYNTHESIS OF PARA-NITROPHENOL C. Maliverney*, M.H. Gubelmann*" and J. Sushi*

* O

RHONE-POULENC RECHERCHES, Centre des CarriBres, BP 62 69192 Saint-Fons (France) author to whom correspondence should be sent. Present address: RHONE-POULENC RECHERCHES, Centre dAubervilliers, 52, rue de la Haie Coq, 93308 Aubervilliers (France)

Abstract The selective nitration of phenylcarbonate into the 4,4'-dinitro isomer has been described in the literature. In this work we show that basic solids, such as CsF and Cs2CO3 as well as basic organic resins, do catalyse the transcarbonation of the nitrated phenylcarbonate into paranitrophenol and recyclable phenylcarbonate, which is a considerable advantage compared with homogeneous stoichiometric techniques. The overall sequence is an attractive alternative for the synthesis of paranitrophenol.

INTRODUCTION Para-nitrophenol (PNP) is an essential starting material for existing technologies used in the production of analgesics such as N-acetylparaaminophenol (APAP) (paracetamol). It is usually obtained either by direct nitration of phenol (eq.1) or by the sequence involving the nitration of chlorobenzene and the subsequent hydrolysis of paranitrochlorobenzene (PNCB)(eq.2)[ 1].

CgHgC1

+ HNo3

1)NaOH; -NaCI

----> CIC6HgNO2 - - - - - - - - - - - - > HOCgHgNa -H20

2)H+

para

(2)

552

Both routes suffer from major drawbacks. In the direct nitration it is difficult to obtain a selectivity in PNP higher than 50 to 608, whereas in the hydrolysis of PNCB, corrosion of the reactors might occur and salty effluents are produced. Consequently, there is a strong need for selective and clean technologies in the production of PNP. Recent work in our company has dealt with the following reaction sequence: nitration of phenylcarbonate(PC)/transcarbonation of nitrated PC with phenol (Scheme 1).

i

1

I

Scheme 1. New route to para-nitrophenol.

The feasibility of the nitration of PC has already been shown and extensively studied [2-41. In this communication we report on the transcarbonation step catalysed by basic solids.

EXPERIMENTAL All organic reagents were either of commercial grade and used without further purification, or prepared according to literature work. Basic inorganic solids were of analytical grade. Basic organic resins were kindly provided by BAYER (Duolite) and DOW CHEMICALS (Dowex). A typical experimental procedure is as follows: bis(4-nitropheny1)carbonate (1.1g; 3.5mmol). phenol (0.7g; 7mmol). dichloromethane (20cm3) and the catalyst (0.5g; 1 to 2 millibasic function) were introduced into small one or three necked glass reactors, equipped with a condenser and a magnetic stirring. The mixture was refluxed with good stirring and analysed periodically by HPLC (column: CN Nucleosil 5um; 150x4.6mm; eluent: n-heptane/ methyltertbutyl ether/isopropanol/acetic acid = 98/1/1/0.2 (~01.96); UV detection at 260nm). All products observed were identified by standard spectroscopic techniques.

553

RESULTS AND DISCUSSION First of all, the thermal stability of 4,4'-dinitro was checked. As indicated in table 1, it appears that the transcarbonation does not occur in the absence of a catalyst, even at high temperature and in the absence of a solvent.

Table 1 Thermal stability of bis(4-nitropheny1)carbonate in the presence of phenol. Molar ratio Solvent temperature . . f°C) PhOH/4.4 dinitro 2 CH2CI2 20 2 CH2C12 100 4 PhOH 100 a) only traces (C lmol.%) of PNP detected by HPLC 1-

(4,4'-dinitro) duration (h) 4 4 24

Conv.4.4'-dinitro (%)

0 very low a) verv low a)

So far, the transcarbonation of the nitrated derivatives of phenylcarbonate (PC) by phenol has been described only in the presence of homogeneous catalysts such as 4-(dimethy1amino)pyridine [ 5 ] . The author has shown, that the 2,2' isomer is seven times more reactive than the 4,4'-dinitro. Indeed, the yield of PC after 4 hours is only 14% for the latter, compared with 100% for the former. Furthermore, the reactivity of 4,4'-dinitro can only be significantly enhanced in the presence of stoichiometric amounts of a base such as triethylamine [ 5 ] . Therefore, it was an interesting and important challenge to examine if the homogeneous base could be replaced by a heterogeneous basic catalyst, thus enabling a simpler and more economic process. The results of a screening of representative examples of basic solids are summarised in table 2.

554

Catalyst (0.5g)

- Conv.(%) 4,4'-dinitro

Yield(%) 4 - n i t r o PC

total

Conv.(%) phenol

Yield (%) PNP

none NEt3 b)

0 a) 99

6

88

94

98

C)

Na2C03

55 68 97 79 75 95

50 61 55 70 66 69

3 6 41

25

53 67 96 78 72 94

28 41 70 45 39 58

26 38 61 42 37 56

KF/AI203 58 NaOH/A1203 (10wt.46) 59 V0LCLAYd)e) 3 VOLCLAY d) 33

43

4

47

29

25

56 2.7 27

0 0 2

56 2.7 29

32 1.6 16

25 1.7 15

Table 2 Transcarbonation of bis(4-nitropheny1)carbonate with phenol catalysed by basic solids (molar ratio phenol/4,4'-dinitro=2; solvent: CH2CI2; temperature: reflux; duration: 5h).

K2c03 cszc03

crHco3 cs2HP04 CSF

8 6

DOWM s?@f)

17

16

0

16

9

8

DUOLITE A378 f)

77

59

18

77

47

48

a) see- table 1; b) 1 equivalent; c) PNP obtained as triethylammonium salt; d) natural clay of the montmorillonite type (water content is 12wt.%); e) dried at 100°C for one night under vacuum; 9 basic resin: STY-DVB-C6H4CH2NMe2 (STY=styrene; D V B = divinylbenzene).

It is shown,that the transcarbonation can indeed be catalysed by basic solids. Best performances are obtained with cesium salts such as CsHC03, C s 2 C O 3 , CsF and with organic resins bearing basic functions such as N,Ndimethylbenzylamine. A key factor for the use of this type of catalysts is the irreversible adsorption of PNP which has to be avoided. In fact, in the case of cesium carbonate, the formation of para-nitrophenate species on the catalyst has been shown by infrared spectroscopy. On the other hand, a comparative study indicates a decrease by a factor of 1.5 of the initial reaction rate of 4.4'-dinitro consumption, when Cs2C 0 3 was recycled in dichloromethane as a solvent.

555

Therefore, we tried to overcome this problem by changing either the nature of the solvent (---> increase of the solubility of the PNP and better desorption capability than CH2C12), either the catalyst itself. The solvent of choice was 2-nitropropane (table 3). Table 3 Effect of solvent in the transcarbonation of 4,4'-dinitro by phenol at 40°C (molar ratio PhOH/4,4'-dinitro=2; catalyst: K2C03(0.5g); solvent: 2Ocm3; duration: 5h). solvent CH2C12 i-PrN02

o n v . (%) 4,4'-dinitro PhOH

Yield (%) 4-nitro

PC

PNP

68 100

63 26

2 69

38 a ) 82 b )

41 83

a) 5 % of PNP is soluble; b) 66% of PNP is soluble

Results of table 3 clearly show that 2-nitropropane is an efficient solvent for the transcarbonation reaction. Nevertheless, one third of the PNP produced still remains on the catalyst. Basic organic resins are an interesting class of catalysts, because they enable a fine tuning of the density and of the strength of the catalytic sites as well as an easy access of the reactants to the catalytic site due to their swelling ability as a function of the solvent. Moreover, they are insoluble in organic media, even at high temperature, contrary to certain of their inorganic counterparts. The results obtained in this respect at low and high temperatures are shown in table 4 and in figure 1. Table 4 Use of basic organic resins in the transcarbonation of 4,4'-dinitro by phenol (molar ratio PhOH/4,4'-dinitro=2; catalyst: Duolite A378(0.5g), bearing N,Ndimethylbenzylamine groups; solvent: 2Ocm3; duration: 5h). solvent

tem per at u re Conv.(%) 4.4I-dinitro PhOH

Yield (%) 4 - n i t r o PC

40 40 100

58 70 30

("(3 CH2C12 i-PrNO2

77 94 100

47 59 83

18 24 69

PNP 45 57 83

As in the case of K2CO3, the use of i-PrN02 permits to obtain good performances, specially at high temperature in the case of resins.

556

When using this type of catalysts, desorption of residual traces of PNP is easily achieved by washing with slightly acidic or basic aqueous solutions.

. . M e ; -

-..-..--

..*...---

_..-C'

1

2

-- -- - - - - _ _ _ _

3

4

time (h) Figure 1 . Kinetic study of the transcarbonation of 4,4'-dinitro by phenol in the presence of a basic organic resin (molar ratio PhOH/4,4'-dinitro=2; solvent: i-PrN02; catalyst: Duolite A378(0.5g): a) 40°C; b) 100°C.

From a mechanistic point of view, our experimental results tend to indicate that the transcarbonation procedes in three elementary steps (eq. 3-5). The transcarbonation of the 4-nitro intermediate is slower than the transcarbonation of the starting 4,4'-dinitro, which is easily explained by the activating effect generated by each nitro group (kl > k2)(scheme 2).

I

4.4-dinim

4

4-Nu0

4

PC

--

Scheme 2. Elementary steps in the transcarbonation of 4,4'-dinitro with phenol.

In accordance with this study, the following catalytic cycle might be proposed:

P2H2 00co0No2

2 nlbo-4

/

dlnitro4,4'

558

CONCLUSIONS Work from the literature indicates that the nitration of phenylcarbonate is possible with very high regioselectivities in the 4,4'-dinitro isomer. In the present study, we have shown that basic solids do catalyse the transcarbonation of 4,4'-dinitro into PNP and recyclable PC. This is a remarkable advantage compared with homogeneous systems, where a stoichiometric amount of consumed base is required [ 5 ] . Best performances are obtained by using inorganic salts of cesium, such as fluorides and carbonates, or organic resins bearing weak basic functional groups, such as N,N-dimethylbenzylamine. The sequence "nitration-transcarbonation" is thus an attractive procedure for the selective synthesis of para-nitrophenol.

REFERENCES 1

2 3 4 5

H.G. Franck, J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer Verlag, 1988, pp. 180-181. German Pat. 2,557,614, 1975, to BAYER. German Pat. 2,549,036, 1975, to BAYER. M. Desbois, French Pat. 2,599,363, 1986, to RHONE-POULENC. D.I. Brunelle, Tetrahedron Lett., 23(1982)1739.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 83 1993 Elsevier Science Publishers B.V. All rights reserved.

559

Catalytic behaviour of Lewis acid-base sites on alkali-exchanged zeolites M. Huang and SKaliaguine' Departement de Genie Chimique et CERPIC, Universite Laval, Quebec (Canada) Correspondence to this author

Abstract Three different reactions, the reactions of methylbutynol, the condensation of acetone and the reactions of monoethanolamine were examined over alkali-exchanged zeolites. The results clearly showed the Lewis acid-base dependence of the activity and the product selectivity. The results suggest that Lewis basic sites or Lewis acid sites of zeolites should be considered as acid-base pairs. The negative charge on oxygen is an indication of zeolite Lewis base strength, while the charge on cation divided by the square of the atomic radius is a convenient approach to Lewis acid strength of the cations.

INTRODUCTION The Lewis acid-base properties of zeolites received a growing interest recently. A series of works('-6) suggested that the infrared and XPS spectra of adsorbed pyrrole can characterize both the Lewis basicity and the Lewis acidity of alkali-exchanged zeolites. The zeolite base strength decreases with an increase in SVAI ratio of the framework, and increases when the electropositivity of the counter alkali cation is raised. The Lewis acid strength of counter cations increases with Si/AI ratio and decreases with electropositivity. In other words, the alkali-exchangedzeolites have both Lewis acid and Lewis base centres and provide an easy way to tune up the relative strengths of these two sites. In the present .work, in order to complement the above knowledge, the reactions of 3-methylbutynol(MBOH), the condensation of acetone and the reactions of monoethanolamine(MEA) were examined over alkali-exchanged zeolites.

EXPERIMENTAL The bulk chemical composition of alkali-cation exchanged zeolites was established

560 by atomic absorption using a Perkin-Elmer (Model 11OOB) spectrometer. Table 1 lists the unit-cell compositions of all zeolite samples used in this work. Table 1

Unit-cell Composition of Zeolite Samples

LiX NaX

Kx RbX

csx

NaA NaY NaL NaM(mordenite) Na-ZSM-5

The MBOH and MEA reactions were carried out in a continuous flow reactor operated at atmospheric pressure. The MBOH vapour (-1 60 torr) and the MEA vapour (-1 00 torr) were introduced using Ar as the vector gas with a W/Fof 0.2 g h mol-' and 2.0 g h mol-' for MBOH and MEA reactions, respectively. The effluent gas was analyzed using a SIGMA 115 gas chromatographic system. The acetone condensation was investigated using the infrared spectroscopic method. Zeolite samples were pressed into self-supportedwafers of roughly 10 mg. Before infrared measurementthe samples were degassed at 400°C overnight (p = l o 5 Torr). Acetone vapour was introduced into the infrared cell at room temperature, then the temperature of the cell was increased to 180°C and maintained at this value for one hour. The samples were then evacuated at the same temperature and the spectra were recorded at room tem perature.

RESULTS AND DISCUSSION

MBOH reactions The reactions of 3-methylbutynolwere first recommended by Lauron-Pernotet a ~ ( ~ ) to characterize the acid-base properties of a series of metal oxides. The basic centres catalyze the cleavage reaction of MBOH, which produces acetone and acetylene, while the acid centres catalyze the dehydration of MBOH to 3-methyl-3-buten-1-yne (MBYNE) or the intermolecular rearrangement to produce 3-methyl-2-butenal (PRENAL). The productionof 3-methyl-3-butene-2-one(MIPK) and 3-hydroxy-3-methyl2-butanone (HMB) from MBOH was also reported in the presence of amphoteric sites. However, as pointed out by these authors, the kind of acid-base sites responsible for the different products remain unresolved. In the case of alkali-exchanged zeolites the framework oxygens adjacent to counter cations are the Lewis basic sites , while the counter cations should be regarded as Lewis acid sites. No Bronsted acid sites could

561 be detected by pyridine adsorption over these The MBOH reaction was then performed over a series of Na-exchanged zeolites. The products detected over the A, X and Y zeolites were only acetone and acetylene, but MBYNE appeared as another reaction product over L zeolite, and it became the dominant product over NaZSM-5 zeolite (96% selectivity). Small amounts of MlPK and PRENAL were also detected over L, mordenite and ZSM-5 samples. The completely different product selectivity revealed a completely different reaction route over Na-ZSM-5 from that over other alkali-exchanged zeolites. The charges on oxygen and on the cation can be calculated from Sanderson Electronegativity Equalization method(’*2) based on the atomic parameters recommended by Sanderson(’). The former is an indication of zeolite Lewis base strength(lS2),while we suggest that the latter divided by the square of the atomic radius can be an approach to Lewis acid strength of the cations. Figure 1 plots the change in both the conversion and the product yield (the selectivity time the conversion) with these charges. Clearly, the yield of acetone plus acetylene

L:

4 7

f

3

‘1140

s

‘0

E 8 5 0

0 0.5

A

. 0.6

’

0.7

0.0

0.0

Positive Charge on Cation/Radius2

0.2

0.3

0.4

”.,

Negative Charge on Oxygen

Figure 1. Conversion and product yield after 5 min of MBOH reaction at 180°C over Na-exchanged zeolites. 0 - 0 Conversion; 0 Yield of C,H2 + C,H,O; A - A Yield of Prenal t Mbyne. increased monotonously with the Lewis base strength and decreased with Lewis acid strength, while the yield of MBYNE plus PRENAL (mainly MBYNE) decreased monotonously with Lewis base strength and increased with Lewis acid strength. The Lewis base centres catalyze the cleavage reaction of MBOH, which produces acetone and acetylene, while the strong Lewis acid centres catalyze the dehydration of MBOH to 3-methyl-3-buten-1-yne(MBYNE).The strong basicity of alkali-exchanged A, X and Y zeolites results in almost 100% selectivity to. acetone and acetylene, whereas the strong Lewis acidity of alkali-exchanged ZSM-5 zeolites yields a high selectivity to MBYNE. Mechanisms for these two different reactions are drawn in Scheme 1 and 2 respectively:

562

O-H : ~

,at

~~

0

‘.

,

oa-

Mat

I

I

H

OH

SCHEWE 1

CH3

CH3

I

oa-

rn

/ / I ffffl1

CH,-C-C=CH

I

I

CH,-:-CZCH H

CH3

I

-)

CH,=C-C=rCH

(MBYNE)

- - - OH

+ H 2 0 + 0’-

d’

f1/1/,/

SCHEME 2

In these schemes it is emphasized that both Lewis acid and Lewis base sites take part in reactions. However, since the basicity of alkali-exchanged ZSM-5 is very weak, its role in scheme 2 may not be important. For example, one hydrogen atom of CH, group could be directly captured by the OH group, which is activated by the Lewis acid site. In any cases the different reaction products between scheme 1 and 2 are mainly caused by the different acid and base strengths. The O-H bond is stronger than the C-H bond, thus only the strong basic site can abstract a hydrogen atom from the OH group, finally resulting in the formation of acetone and acetylene. On the other hand, compared with scheme 1 the complete rupture of C - 0 bond in scheme 2 requires stronger attractive power of cations, i.e., strong Lewis acidity. That could be why the MBYNE cannot be formed over alkali-exchanged A, X and Y zeolites, even though the stronger basicity of these samples is in favour of the rupture of the C-H bond. ~

Condensation of acetone The aldol condensation of acetone to form diacetone alcohol is a well known reaction(g) catalyzed by basic catalysts. This is also regarded as one important reason for the deactivation of MBOH reaction over alkali-exchanged X and Y zeolites(”). Thus the pure acetone vapour was introduced into the infrared cell which contains fresh zeolite samples. It is found that the spectra after acetone adsorption are nearly completely the same as those obtained after MBOH reactions(”) except for the region around 3400 cm”, where the spectra after MBOH reaction at 180°C still showed some bands belonging to the residual adsorbed MBOH. Figure 2 shows the IR spectra of alkali exchanged X zeolites and other Na exchanged zeolites

563

NaA

I

M

L

0 - 1

Figure 2. IR spectra of acetone adsorbed on alkali-exchanged X zeolites and other Na-exchanged zeolites. After heating at 180°C for one hour and further evacuation.

after adsorption of acetone and further evacuation at 180°C. In the case of X zeolites a strong band located around 1700 (1708-1693)cm-’ appeared accompanied by bands at 3470(broad), 2995, 2920, 1675(1670-1675), 1423 and 1372 cm-I. The main band around 1700 cm-’ is assigned to the C=O stretching vibration (referred as v l ) of adsorbed acetone. However the 1423 cm-’ belongs to the CH,-CO bending frequency(”) and the 2920 cm-1’ belongs to the vaSof CH, group(”). The vs band of CH, group should be weaker(”) and may therefore correspond to a very weak band around 2850cm-’. Thus the presence of CH, groups may indicate the condensation of adsorbed acetone to form diacetone alcohol. Further evidence for the formation of alcohol comes from the broad band around 3470 cm-’, which usually suggests hydrogen bonded OH groups. The main C=O band intensity decreased in the order Na > Li > K > Rb > Cs. Another weak band around 1675 cm-’ is detected over Li, Na and K-exchangedX zeolites. Since the frequency is still in the C=O stretching region(‘ ’I, this means another form of adsorbed acetone with a weakened C=O bond (referred as v2 or second C=O band). The C=O band observed on Na-A zeolite possesses the lowest frequency (1666 cm-’) and weaker intensity. In contrast, the C=O frequencies observed on Y, L, mordenite (very weak) and ZSM-5 samples are higher (1713 cm-’) than that on X zeolites, meanwhile, the band around 1420 cm-’ became very weak for Y and L zeolites and absent for A,

564

and mordenite. That is, the acetone condensation does not occur over A, mordenite and became less important over Y and L zeolites. The 1420 cm-’ band was detected in the case of ZSM-5 zeolite, however, there is no 2920 and 3470 cm-’ bands but new bands located at 1675, 1630 and 1320 cm”. Thus we suggest that there is no condensation of acetone but some unidentified compound formed. The second C=O band (v2) was also detected for Y zeolite (1690 cm-’). Figure 3a and 3b plots the change in infrared band intensity of the C=O bond for X (vl), Y(v1) and other Na

,120

-C

.*

a,

c

C

M1

0 0.2

0.3

0.4

0.5

Negative Charge on Oxygen

0.4

0.6

0.8

Positive Charge on Cation/Radius*

> 1665

0.2

0.3

0.4

0.5

Negative Charge on Oxygen

Positive Charge on Cation/RadilJS2

Figure 3. Relationship between the C=O infrared band intensity and a. the negative charge on framework oxygen b. the positive charge on cation divided by square of the atom radius Relationship between the C=O infrared vibration frequencies ( v l and v2) and c. the negative charge on framework oxygen d. the positive charge on cation divided by square of the atom radius The open circle points represent samples where diacetone alcohol was formed. exchanged zeolites. The abscissas are respectively the charges on oxygen and on the cation (divided by the square of the atomic radius), as mentioned above, which are the indication of zeolite base strength and the Lewis acid strength of the cations respectively. Clearly a volcano curve is obtained between the band intensity and both the Lewis acid and Lewis base strength at least for the points represented as open circles. These points represent the samples where diacetone alcohol was detected from the presence of the 3470, 2920 and 1423 cm” IR bands. Thus not only the basic site but also the alkali cation which possesses high enough acid strength are necessary for the condensation of acetone. Figure 3c and 3d shows the correlations observed between the CO vibration frequencies and the base and

565 acid strength respectively. It is interesting to note that the data yielded two straight lines corresponding to the v l and v2 respectively. v2 was observed in LiX, NaX, KX, NaY samples and also denoted the CO bands of samples where diacetone alcohol was not present (ZSM-5, M and A) and was therefore ascribed to adsorbed acetone. MEA reactions Figure 4 shows the MEA reaction results over alkali-exchanged X zeolites, NaA and CsA zeolite. The main product over these basic zeolites is ethylenimine and a

*\

-

"U

n

lo(

Positive Charge o n Cation

,

Kadiusz

,

RbX

-0-0

'

csx

Negative Charge o n Oxygen

Figure 4. Conversion and product selectivity after one hour of MEA reaction at 400°C over alkali exchanged X and Na,Cs-A zeolites. 0 - 0 Conversion; 0 - 0 Selectivity to Ethylenimine; A - A Selectivity to Piperazine and pyrazine; A - A Selectivity to Acetaldehyde

volcano relationship was found between the selectivity and both Lewis base strength and Lewis acid strength. The piperazine and pyrazine were also detected and their selectivity seems to increase with the Lewis acidity but decrease with Lewis basicity. The selectivity to acetaldehyde showed another volcano relationship with both Lewis base strength and Lewis acid strength. Small amount of acetonitrile, ethylamine and ethylene were also found. These results are basically consistent with the observations of Ueshima et aI.(l2) over a series of metal oxides. The authors conclude that the acidic catalysts produced piperazine and pyrazine while the basic catalysts produced acetaldehyde, and only the presence of both weak acid and weak base sites may produce the ethylenimine. However, in the scope of zeolites, the alkali-exchanged X and A zeolites should be regarded as relatively strong basic zeolited2). A mechanism for the ethylenimine formation was suggested by Ueshima et aI.('*), where the first step is the breaking of the C - 0 bond. Since the N-H bond is stronger than the C - 0 bond, rather strong basic sites are required for breaking it. An alternative mechanism is therefore suggested as follows:

566

NH I

CH2

N H - -- CHz

H

OH

H - - - OH

06-

K6t

I

/ / / / f / l /

06-

NH

- CH2

+

M6t

i l l 1I / / /

CONCLUSIONS 1. Three different reactions are shown to be catalyzed by alkali-exchanged zeolites. The results revealed that Lewis acid-base sites on zeolites possess potential application in the production of final chemicals. 2. The product selectivity of these different reactions is in general both Lewis acidity and Lewis basicity dependent. In agreement with the proposal recently made by Kazansky(13),we suggest that the Lewis basic sites or Lewis acid sites of zeolites should be considered as acid-base pairs, in which both the framework basic oxygen and the neighbouring cation are important (see schemes 1, 2, 3 and figure 3). 3. The negative charge on oxygen is an indication of zeolite Lewis base strength, while we suggest that the charge on cation divided by the square of the atomic radius can be an approach to the cation Lewis acid strength.

REFERENCES 1 2

3 4 5 6 7 8

9 10 11 12

13

D.Barthomeuf, J.Phvs.Chem., 88, (1984) 42 M.Huang, SKaliaguine, J.C.S.Faradav Trans., 88(5), (1992) 751. M.Huang, A.Adnot, SKaliaguine, 1992, J.Catal.. 137, (1992) 322. M.Huang, S.Kaliaguine, in "Progress in Catalysis"(Ed. K.J.Smith and E.C.Sanford), Elsevier Sci. Publishers B.V., 1992, p291. M.Huang, S.Kaliaguine, J. Molecular Catal., in press. M.Huang, A.Adnot, S.Kaliaguine, J.Am.Chem.Soc., 114, (1992) 10005. H.Lauron-Pernot, F.Luck, J.M.Popa, Appl. Catal., 78, (1991) 213. R.T.Sanderson, "Chemical Bonds and Bond Energy", Academic Press, New York, 1976. G.Zhang, H.Hattori. K.Tanabe, A ~ p l Catal., . 40, (1988) 183. M.Huang, S.Kaliaguine, submitted to Catal. Letters. K.Nakanishi, "Infrared Absorption Spectroscopy", Nankodo Company Limited, Tokyo, 1962. M.Ueshima, Y.Shimasaki, Y.Hino, H.Tsuneki, in "Acid-Base Catalysis", (Ed. K.Tanabe, H.Hattori, T.Yamaguchi, T.Tanaka), Kodansha, Tokyo, VCH, Basel, 1988, p41 V.B.Kazansky, 1991, in "Catalysis and Adsorption by Zeolites" (Ed. G.Ohlmann et al.), Elsevier Scientific Publishing, Amsterdam, 1991, p.117.

M. Guisnet et al. (Editors),Heterogeneous Catalysis und Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

Aromatic Hydroxyalkylation using Molecular Sieves

567

(Si1ico)aluminophosphate

-

M.H.W. Burgers and H. van Bekkum

Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Abstract The hydroxyalkylation of aromatics with carbonyl compounds was investigated using the mildly acidic aluminophosphate molecular sieves as catalysts. Attention was focused on the condensationof phenol with formaldehyde (leading to diphenylmethane derivatives) and with isobutanal (aiming at monomeric compounds). ALPO-5 gave the best results of the catalysts tested, as the material suffered least from deactivation. Yields with this catalyst were, however, dependent on its synthesis method: structures synthesized in a fluoride-containing medium gave significantly lower conversions. The lower content of lattice defects in the latter materials might be responsible for this effect. For comparison some other zeolites were tested in the hydroxyalkylations. INTRODUCTION

Aluminophosphates (denoted as ALPO’s) are porous crystalline molecular sieves, consisting of AIO,/, and POd/,-tetrahedra. Since the first synthesis of these materials, reported in 1982 [1,2], a large number of different structures has been identified. The best known example is ALPO-5, a unidimensional structure with circular channels, surrounded by 12 T-atoms, thus giving an effective pore diameter of 7.3 A. It has been shown possible to perform isomorphous substitution [3]. Especially silicon is known to substitute into the framework; the materials thus obtained are known as silicoaluminophosphates or SAPO’s [4,5]. Silicon substitution in the framework can either take place by Si4+-O-Si4+-groups(replacing AI3+-O-P5+),or by isolated Si4+-atomsreplacing P5’ [3]. The first option does not change the charge of the lattice, while the second gives a negatively charged lattice, and thereby (in the protonic form, in which these materials are synthesized) acid activity. However, the acidity of the sites is lower than of that aluminosilicate molecular sieves. Although ALPO’s are neutral, and might be expected not to possess acid activity, several studies have indicated that this doesn’t have to be valid; Brernsted-acidity, both in the presence [6,7] and in the absence [8] of Lewis-acidity has been reported. The acidity was reported to be rather low [6,9], but cumene cracking was achieved to a limited extent [9].

568

The mild acidity of ALPO's and SAPO's makes these materials very interesting for acid catalysis in reactions where a low-acidity material is required, as coke formation may be decreased and catalyst life increased. Also, unwanted consecutive reactions may be prevented. The proton-catalyzed hydroxyalkylation of phenolic substances with carbonyl compounds is one of the reactions in which a weak acid catalyst is required, as strong acids will lead to oligomerization. The possible reaction routes of phenol reacting with the two aldehydes studied are shown in Figure 1. Especially with formaldehyde, monosubstituted products are difficult to obtain, because of the fast consecutive dimerization; when larger carbonyl compounds are applied (such as isobutanal), monomer formation might be enhanced because of steric reasons.

H

Figure 1. Reaction scheme of the acid-catalyzedtwo-step condensation of phenol with formaldehyde (a) and isobutanal (b), and dehydration and cyclization of the primary phenol/isobutanal product (c). Catalysis of molecular sieves in these reactions might lead to a higher selectivity towards mono-substituted products: application of zeolite H-Y in the phenol/formaldehyde condensation has been reported to improve monomer yields [lo]. Besides, it is also of interest whether the molecular sieves exhibit shape selectivity with respect to ortho- or para-substitution.

569

In the present study, (silico)aluminophosphates, esp. ALPO-5 samples, have been tested for their capacity in the hydroxyalkylationof phenol with formaldehyde and with isobutanal, a small as well as a more bulky aldehyde. The condensation of phenol has also been compared to other aromatics. EXPERIMENTAL

Synthesis of SAPO-5 was performed according to Lok et al. [4], example 12, using LUDOX LS (Du Pont de Nemours, 30% 90,) and aluminium isopropoxide as silicon and aluminium sources, respectively, and tripropylamine as template. ALPO-5 was synthesized similarly, excluding the silica, and coded ALPO-5-P. ALPO-5 was also prepared according to Lechert and Weyda [ l 11, using pseudo-boehmiteas aluminium source, for 16 h at 170'C; this batch was coded ALPO-5-B. A third batch of ALPO-5 was prepared according to Qiu et al. [12], sample 4 (using tripropylammoniumfluoride as template), and was designated ALPO-5-F. Materials were characterized by X-Ray Diffraction, Scanning Electron Microscopy and Temperature Programmed Ammonia Desorption (TPAD); Atomic Emission Spectroscopy was used to examine bulk compositions. TPAD was performed by activating the material up to 600°C in 10 ml/min. nitrogen, followed by ammonia saturation at 100"C and removal of non-adsorbedammonia. The measurement was carried out by heating with 5"C/min to 600"C, with the nitrogen-flow as carrier-gas. Catalytic reactions were performed in teflon-lined autoclaves. Besides additional phenol in some test experiments, no solvent was applied. When condensation was performed with formaldehyde, 1,3,5-trioxane was used as its source. In all experiments, the amount of catalyst was kept constant at 7.5 wt% with respect to phenol. In the comparison of different aromatics, all reactions were performed using 7.1 mg catalyst per mmol arene. When testing homogeneous aluminium catalysis, aluminium isopropoxide was used as source; other materials tested for their catalytic activity were y-alumina (obtained from AKZO-Ketjen) and ammonium-exchanged mordenite (Union Carbide, Si/AI = 10.3), US-Y (AKZO, Si/AI = 5), and ZSM-5 (home made, Si/AI = 25). All catalysts were activated in air by heating with 1 'C/min to 550% (or to 450°C, in case of US-Y and mordenite). Reaction products were identified by Mass Spectrometry; quantitative analysis was carried out by Gas Chromatography (GC). RESULTS

ALPO-5 samples contained a minor amount (< 5%) of cristobalite; however, a pure sample of this dense phase was found to be inactive in catalytic experiments. The phosphorus/aluminium ratio was 1 within experimental error (k 3 %). SAPO-5 had the composition A1:P:Si = 1:0.98:0.06.

570 The results of the TPAD-measurements are shown in Figure 2, and show the expected differences in acidity.

- ALPO-5

------ S A W - 5

---

HZSM-5

600

700

lo I

100

200

300

400

500

800

Temperature ('c)

Figure 2. Temperature ProgrammedAmmonia Desorption- plots of SAPO-5, ALPO-5-P and NH,+-ZSM-5. Nitrogen flow rate: 10 ml/min; heating rate 5"C/min. In some explortitory experiments test conditions were selected for the phenol/ formaldehyde condensation. It was found that catalytic test experiments could best be carried out at a relatively high temperature, 180°C, to increase conversions; generally a reactiontime of 4% h was found suitable to compare materials with higher and lower deactivation rates. Application of 1,4-dioxane as a solvent did not improve the selectivity much, and therefore the experiments were carried out solvent free. Selectivities could be improvedconsiderably when a higher phenol/formaldehyde-ratiowas applied (see for example Figure 3); but, again to compare different catalysts, a molar ratio of 2/1 was considered most suitable in the catalytic test experiments.

Using these standard conditions, a series of catalytic test experiments were performed. The results are given in Tables 1-3. When phenol/formaldehyde-experiments were carried out (Table I), no (hydroxymethy1)phenoIwas observed; the high selectivity to this mono-substituted product, found with H-Y [lo], could not be duplicated. Instead, dihydroxydiphenylmethane, the dimer shown in Figure la, was the main product. with oligomeric compounds as side products. The regioselectivitywas found to vary considerably.

571 100

-s

1

75 -

C

Figure 3. Formaldehyde conversion (v), dihydroxydiphenylmethane yield (A) and selectivity (0)upon condensation with phenol for 4% h at 180°C, vs. phenol/formaldehyde-molar ratio. Catalyst: SAPOd.

Figure 4. Formaldehyde conversion upon condensation with phenol for 4th h at 180°C, vs. reaction time. ALPO-5-P (0)and SAPO-5 (A) were used as catalysts. Phenol/formaldehyde molar ratio: 2/1.

Table 1. Formaldehyde conversion and dihydroxydiphenylmethane selectivities upon condensationwith phenol, for 4% h at 180°C (phenol/formaldehyde molar ratio 2/1). Catalyst

Conversion

ALPO-5-P ALPO-5-B ALPO-5-F SAPO-5

55 65

H-US-Y H-mordenite 7-alumina A?+ (homogeneous)

25

15 13 33 27 27

Dihydroxydiphenylmethane select. (%)

Regioselectivity

40 34 40 35 40 34 48 80

35 : 55 : 10 18 : 70 : 12 4 5 : 4 5 : 10 48 : 48 : 5 32 : 46 : 22 38 : 43 : 18 81 : 1 7 : 1 82: 17: <1

2,2':2,4':4,4'

To investigate the scope of aromatics to be converted, several monosubstituted benzene derivatives were tested in reactionwith formaldehyde, catalyzed by ALPO-5-P. The results are shown in Table 2.

In contrast to these experiments, phenol/isobutanal reactions (Table 3)gave mainly monoadducts. Only traces of hydroxyalkylphenols were observed; apparently dehydration is a fast consecutive reaction under these conditions.

572 Table 2. Comparisonof aromatic systems in the hydroxymethylationwith formaldehyde, for 4% h at 180'C. Catalyst: ALPO-5-P; arene/formaldehyde molar ratio: 2/1.Selectivity is given vs. formaldehyde consumed. Conversion (%) Phenol Anisole Toluene Chlorobenzene

56 26 3

Diphenylmethane selectivity (%) 40 80 > 50

<1

Table 3. Yield of monoalkylated products (in mole%, vs. isobutanal) after hydroxyalkylation of phenol with isobutanal for 4% h at 180°C (phenol/isobutanal molar ratio: 1/11.

Catalyst

ALPOB-B ALPOB-F SAPO-5 H-ZSM-5 7-alumina a

-

Conversion Monomer

23

60

- 90

4

12

35 65 60

7

3

Monomer composition

4 30 3 10

-

5

90 60 90 20 80

-

2 < 10 7

65 8

the side products found with alumina and ZSM-5 consisted of both diphenylalkanesand dialkenylphenols; with ALPO-5 and SAPOQ mainly the latter side products were formed.

DISCUSSION As shown in Figure 4, catalyst deactivation is an important factor in the hydroxyalkylation reactions.This makes quantitative comparison difficult, as each material has a balance between activity and deactivation. The main conclusion from the results is that materials having a low acidity, ALPO-5 (vide supra) and 7-alumina, give best results, because of their low rate of deactivation. Already when SAPO-5, having an acid strength between ALPO-5 and aluminosilicates, is used, a rapid deactivation was observed in the phenol/formaldehyde reaction. The relatively weak acidity of SAPO-5 is shown by the TPAD-results, as a chemisorption peak is found at 320'C comparable to literature data [I31 -, clearly lower than H -2SM -5 (450'C). The prolongedtailing in the TPAD-profile of ALPO-5-P is an indication for the acid activity of this material. The actual nature and geometry of the ALPO-5

-

573 sites are not clear; they might be related to lattice defects, giving P-OH and AI-OH groups [6,8,9], but octahedrdly coordinated aluminium can also play a role [8]. These effects might be related to the observed difference between Lewis- and Brnrnsted acidity; however, it must be stressed that the difference between synthesis methods can have substantial influence. It must be noted that the phenol/aldehyde reaction can be catalyzed by Brnrnsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). Inthe latter case one Lewis centre (e.g. A?+) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2’-dihydroxydiphenylmethane selectivity we obtained with homogeneous A13+catalysis and with y-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensationgives a low 2,2’-selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxylgroup plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Brnrnsted acid catalysts [17], can’t be ruled out. The above does not take into consideration steric factors which of course can govern regioselectivity inside a molecular sieve. Based on molecular models, the dimensions of 2,2’-dihydroxydiphenylmethaneare estimated at 7.3 x 5.5 A, of the 2,4’isomer at 7.2 x 5.6 A, and of the 4,4’-isomer at 6.2 x 5.6 A; therefore, formation of all isomers seems possible within the ALPO-5 channels.

For the phenol/isobutanal condensations, the dimeric diphenylalkane products are too large to be formed in the molecular sieve channels. Shape selectivity is assumed to be a major cause for the fact that the consecutive reaction towards diphenylalkane systems is of minor importance. The dimeric products found are likely to be formed at the external surface of the catalyst.. It may be noted that the primary product, hydroxyalkylphenol, seems to be subject to rapid dehydration underthe present conditions, as only traces of this compound are detected. The observed products are the alkenes, 1-(4- and 2-hydroxyphenyl)-2methylpropene, together with (in case of the ortho-isomer) the cyclized compound. This cyclization towards 2,2’-dimethyl-2,3-dihydrobenzofuran (see Figure 1c) is promoted by strong acid catalysts [18]. Our findings are in agreement with this observation: ZSM-5, the catalyst having the highest acid strength of the series tested, leads to a high selectivity of this isomerized product. When ALPO-5-P and ALPOB-B are compared to ALPO-5-F, the material synthesized in a fluoride-containingmedium, it is important to notice that the latter gives significantly lower conversions in both test reactions. As it is assumed that lattice defects are partially responsiblefor the catalytic activity of the ALPO-5-samples, this indicates that ALPO-5 synthesized in a fluoride-containing medium leads to samples having less structural defects. As a consequence, the ALPO-5 synthesis might be comparable to the MFI-synthesis, where fluoride-containingsynthesis methods are also reported to lead to materials essentially free from structural defects [19]. Additional characterization methods have to be applied, however, to confirm these findings.

574

CONCLUSION

(Si1ico)aluminophosphate-molecular sieves are interesting materials as catalysts for the condensationof phenol with carbonyl compounds, presumably because of the low acidity and the spacious constraints, decreasing catalyst deactivation. Especially ALPO5, having a very low acid strength, gives good yields in the reaction of phenol with formaldehyde towards dihydroxydiphenylmethanes. In case of the phenol/isobutanal reaction, mainly monosubstituted products are obtained; shape selectivity seems to retard a second condensation step. Ortho-substitutionprevails, possibly caused by a contribution of Lewis-acid catalysis. The yields are highly dependent on the synthesis method; ALPO-5 samples synthesized in a fluoride-containing medium give lower conversions. The lower content of lattice defects in the latter material is thought to be responsible for this effect. REFERENCES 1 2 3 4

5 6 7 8 9 10 11 12 13 14 15 16 17 18 I9

S.T. Wilson, B.M. Lok and E.M. Flanigen, U.S. Pat. 4.310.440 (1982). S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. SOC.104 (1982), 1146-1147. E.M. Flanigen, B.M. Lok, R.L. Lyle and S.T. Wilson, Stud. Surf. Sci. Catal. 28 (1986), 103-112. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannen and E.M. Flanigen, U.S. Pat. 4.440.871 (1984). B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, J. Am. Chem. SOC. 106 (1984), 6092-6093. G. Dworezkov, G. Rumplmayr, H. Mayer and J.A. Lercher, Stud. Surf. Sci. Catal. 21 (1985), 163-172. V.R. Choudhary and D.B. Akolekar, J. Catal. 103 (1987), 115-125. O.V. Kikhtyanin, E.A. Paukshtis, K.G. lone and V.M. Mastikhin, J. Catal. 126 (1990), 1-7. S.G.Hedge, P. Ratnasamy, L.M. Kustov and V.B. Kazansky, Zeolites 8 (1988), 137-141. T. Kiyora, Jap. Pat. 63.307.835 (1988). H. Lechert and H. Weyda, DECHEMA-Monogr. 118 (1989), 159-170. S. Qiu, W. Pang, H. Kessler and J.L. Guth, Zeolites 9 (1989), 440-444. C. Halik, J.A. Lercher and H. Mayer, J. Chem. SOC.,Faraday Trans.(l) 84 (1988), 4457-4469. Ya.B. Kozlikovskii and B.V. Chernyaev, J. Org. Chem. USSR 21 (1985), 2198-2201. A.J. Hoefnagel, J.A. Peters and H. van Bekkum, Recl. Trav. Chim. Pays-Bas 107 (1988), 242-247. M.J. Climent, A. Corma, H. Garcia and J. Primo, J. Catal. 130 (1991), 138-146. R.F. Parton, J.M. Jacobs, D.R. Huybrechts and P.A. Jacobs, Stud. Surf Sci. Catal. 46 (1989), 163-192, and references therein. A. Arduini, A. Pochini and R. Ungaro, Synthesis (1984), 950-953. S.A. Axon and J. Klinowski, Appl. Catal. A 81 (1992), 27-34.

M.Guisnet et al. (Editors), Hetmgmeous Catalysis and Fine Chemicals 111 0 1993 Elsevier sdence Publishers B.V. All rights reserved.

575

Comparative study of isopropylation and cyclohexylation of naphthalene over zeolites: shape selective synthesis of a 2,6dialkylnaphthalene. P. Moreau, A. Finiels, P. Geneste, F. Moreau and J. Solofo. Laboratoire de Chimie Organique Physique et Cinhtique Chimique Appliqubes,

URA 418 CNRS, Ecole Nationale Suphrieure de Chimie, 8 Rue de 1'Ecole Normale, 34053 Montpellier Cedex 1,France

Abstrad Liquid-phase isopropylation and cyclohexylation of naphthalene over a series of H mordenites and HY zeolites have been studied. These reactions can be carried out efficiently over HY zeolites. High conversions and efficient p, p selectivities are obtained after very short reaction times. The use of cyclohexyl derivatives, cyclohexyl bromide or cyclohexene, as alkylating agents leads to a n improvement of the p , p selectivity. Moreover, the 2,6dicyclohexylnaphthalene is easily separated from the reaction mixture by crystallization.

1. INTRODUCTION The interest of 2,6-dialkylnaphthalenes as starting materials in the production of polyester fibers and plastics with superior properties [l,21 and of thermotropic liquid crystal polymers 131 is clearly shown by the increasing number of recent patents relevant to their preparation and separationC3.51. However, the selective formation of 2,6-dialkylnaphthalenes is not obvious, not only with conventional Friedel-Crafts catalysts [6-81, but also over solid catalysts such as silica-alumina [9-111 or zeolites. The latter have been used in the gas phase methylation of naphthalene with methanol [12-141 . Large pore zeolites, such as H-mordenite or H-Y, led to a nonselective methylation of naphthalene, whereas a high p -selectivity was observed with the medium-pore H-ZSM-5. Liquid phase isopropylation of naphthalene with propene 13,151 or isopropylbromide [lSI over zeolites was more recently reported. It was found that isopropylation could be carried out efficiently over such catalysts with a good selectivity for the formation of 2isopropylnaphthalene and 2,6-/2,7-diisopropylnaphthalenes. We especially demonstrated that the use of zeolites modified by silanation of the external surface led to an improvement of the p -selectivity by suppressing the formation of triisopropyl derivatives [161. Nevertheless, the selective formation of the 2,6-isomer was not possible in any case, whatever the zeolites and alkylating agents were used. Such a

576

selectivity might be found with more hindered alkylating agents, such as cyclohexyl derivatives. The present paper is concerned with the comparative study of isopropylation and cyclohexylation reactions of naphthalene over various zeolites.

BEXPERIMENTAL Materials Analytical grade cyclohexane, naphthalene, isopropyl bromide, cyclohexyl bromide and cyclohexene (Aldrich Company) were used without further purification. Catalysts Dealuminated mordenite HM (Si/Al=9) was prepared fronn Zeolon 100-H (SUAb6.9 from Norton) by treatment in 1M HCl solution and 100°C for 3h [171. HY (WAk2.5) was an ultrastable zeolite from Chemische Fabrik Uetikon, Zurich (26-05-01). HY (WAk19.5) and HM (Si/A1=10.8) were provided by Zeocat, Montoir de Bretagne (ZF520 and ZM510 respectively). Procedure The alkylation reaction of naphthalene was carried out in a 0.11 stirred autoclave reactor. In a typical run, the autoclave was charged with l g of zeolite freshly calcined in air at 500"C, and a mixture of 5 mmoles of naphthalene and 10 mmoles of the alkylating agent (isopropyl bromide, cyclohexyl bromide) in 50 ml of cyclohexane, and heated t o 200°C. After cooling, the catalyst was filtered, and the reaction mixture analysed by g.1.c. (Altech OVI-capillary column, 10m or 25m, carrier gas Ha). The filtered catalyst, after drying and cduination, had the same activity as the fresh catalyst. When cyclohexene was used instead of cyclohexyl bromide, the procedure is the following: the autoclave was charged with naphthalene (5 mmol.), cyclohexane (50 ml) and the catalyst (1.0 g), and heating was started. At the same time, cyclohexene (10 mmol.) was added, drop by drop, by means of a stainless steel pressurized funnel, and the mixture was stirred in the same conditions as above. Isolation and purification of 2,6-dicyclohexylnaphthalene After cooling, the cyclohexane was evaporated; the crude product solidified at room temperature. The solid was then filtered and recrystallized from ethanol; after t w o recrystallizations, white crystals of 2,6dicyclohexylnaph thalene (m.p. = 152"C, lit. [ 18,191 m .p.= 151-152°C) were obtained. The structure was confirmed by GC-MS, 1H and 13C NMR spectroscopy, together with X-ray crystallography [20].

577

bRESUL'lSANDDISCUSSION Isopropylation and cyclohexylation reactions of naphthalene with isopropyl bromide and cyclohexyl bromide respectively were first studied over a series of mordenites and Y zeolites at 2OOOC in cyclohexane as the solvent. Whereas Y zeolites appear t o be very efficient catalysts in both reactions, Hmordenites present only a weak activity (12% for isopropylation and 6% for cyclohexylation). Moreover, mordenites lead preferentially t o a monosubstitution reaction; for example, as shown in a previous work [16], the major product of the isopropylation reaction over a H-mordenite Si/Al=9.0 is the 2-isopropyl naphthalene (p-isomer, 83%) Such results have already been interpreted in terms of molecular sieve effects, due to the constrained environment in the channels of the mordenite structure [15, 161. Table 1 gives, as an example of the efficiency of Y zeolites, the results obtained over an ultrastable HY zeolite (Si/A1=2.5),under the same conditions (200°C) for both reactions. Table I : Isopropylation and cyclohexylation of naphthalene over Y zeolites a

PP

Alkylating

jelectivity

Distribution of products 8

T i m e Naphth ( m i n ) Conv.

% 1,6+2,7DAN

%

Em

agent

iwProPY1 bromide

MAN^

DANC

TAN^

80

97

71

10

96

82

cyclo-

hexyl bromide

a Catalyst : ultrastable HY (SiIAl =2.5); T =200°C ;solvent: cyclohexane ; ratio: alkylating agent I naphthalene = 2 /1; b MAN : monoalkylnaphthalenes. C DAN :dialkylnaphthalenes. d TAN : trialkylnaphthalenes.

578

A very high conversion of naphthalene is obtained as soon as the temperature of 200°C is reached, and the reactions, which lead in both cases to dialkyl naphthalenes as major products, are nearly quantitative after only short reaction times. We have shown that, at lower temperatures (80°C), the main products are monosubstituted derivatives, which consist of l-alkyl and 2-alkyl naphthalenes. When the temperature is increased to 2OO0C, the amount of dialkylnaphthalenes increases drastically. Moreover, in both cases, 2,6 and 2,7-isomers are the main disubstituted compounds, as shown by the [ 2,6- +2,7dialkylnaphthalenesl / [ total dialkylnaphthalenesl ratio, which is greater than 70% (71% for isopropylation, 83% for cyclohexylation). These derivatives are formed by a consecutive alkylation of the monosubstituted 2-alkylnaphthalene (p-isomer), which is the thermodynamic product of the monoalkylation reaction. It is, in fact, well known [211 that, in the alkylation reactions of naphthalene, both a and p-isomer are formed as primary products, and that the a-isomer is easily rearranged into the thermodynamically more stable p-isomer (as shown, for example, by the thermodynamic effect in favour of the p-isomer for isopropyl derivatives a w p 1.5-98.5[8] 1. It can be seen, from Table I, that the use of cyclohexyl bromide as alkylating agent instead of isopropyl bromide yields an increasing amount of 2,6+2,7-dicyclohexylnaphthalenes (55% instead of 39% 1, together with a drastic decreasing of trialkyl derivatives. This is due to the steric hindrance of the cyclohexyl group compared with isopropyl, leading to a significant improvement of the p, p selectivity (82%versus 71% ). In both isopropylation and cyclohexylation reactions over Y zeoli tea, as much of 2,6-isomer is formed as the 2,7-isomer; the relative distribution of the two isomers is nearly the same (2,6-/2,7-ratio = 0.95 for isopropyl and 1.1for cyclohexyl). This is not surprising, since the two isomers have the same kinetic diameters (6.5 8, for isopropyl compounds, for example [161 ), and their production and subsequent diffusion in the pores or cavities of the Y zeolites certainly occur in the same way. The advantage of the cyclohexylation in comparison to the isopropylation is directly related to the physical properties of the 2,6-dicyclohexyl naphthalene. This compound can be effectively isolated from the reaction mixture by crystallization, as i t was reported in earlier studies of the cyclohexylation reaction over aluminum chloride [18,191. The 2,6-dicyclohexylnaphthalene, a crystalline compound (m.p.= 152OC) with a crystallographic center 1201 , is thus obtained in a 27% yield from the mixture of the cyclohexylation reaction of naphthalene with cyclohexyl bromide over the ultrastable HY zeolite.

579

Moreover, cyclohexene can be used as the alkylating agent instead of cyclohexyl bromide. Table I1 gives comparative results obtained over a dealuminated Y zeolite (Si/Al =19.5). Table I1 : Cyclohexylation of Naphthalene over Y zeolites a

PP

Alkylati ng

Time Naphth. ( m i n ) Conv.%

Distribution of products %

selectivity % 2,6+2,7DCN

m

agent DCNC

cyclohexyl bromidee cyclohexenee cyclohexenef

10

94

79

10

90

40

93

77

a Catalyst: dealuminated HY (Si /A1 =19.5); T=2OO0C;solvent: cyclohexane; ratio alkylating agent 1 naphthalene =2/ 1 . bMCN : monocyclohexy 1naphthalenes. CDNC :dicyclohexyl naphthalenes. dTCN : tricyclohexylnaphthalenes. enaphthalene and alkylating agent put together in the autoclave. fcyclohexene is added drop by drop into the autoclave charged with naphthalene and zeolite.

If the reaction is carried out under the same conditions (naphthalene and alkylating agent put together in the autoclave), a significant difference,due to dimerisation of cyclohexene, is observed mainly (monocyclohexylnaphthalenes as main products, low p, p selectivity). When cyclohexene is added drop by drop to the stirred mixture, identical results are obtained, both in conversion and selectivity.

580

4. CONCLUSION

The liquid phase alkq.ation reaction of naphthalene with hindered alkylating agents such as isopropyl and cyclohexyl derivatives can be carried out efficiently over HY zeolites. High conversions and efficient p, p selectivity are obtained after very short reaction times at 200°C.The use of cyclohexyl derivatives, cyclohexyl bromide or cyclohexene, as alkylating agents leads to a n improvement of the p, p selectivity, compared with isopropyl derivatives. In both cases, 2,6- and 2,7- disubstituted naphthalenes are predominantly formed, but, in the cyclohexylation reaction, the 2,6-dicyclohexylnaphthalene, a crystalline compound, can be isolated from the reaction mixture by crystallization. 5. REFERENCES

R.M.Gaydos, in Kirk Othmer Encyclopaedia of Chemical Technology, 3rd ed., R.E.Kirk and D.F.Othmer (eds.), Wiley, New York, 1981. 2 K.Kobayashi, 1.Dogane and Y.Nagao, Japan Kokai Tokkyo Koyo No 75 0076054(1975). 3 J.D.Fellmann, R.J.Saxton, P.R.Wantreck, E.G.Derouane and P.Massiani, PCT Int. Appl., WO No 90 0361 (1990). 4 K.Tate, YSasaki and T.Sasaki, Japan Kokai Tokkyo Koyo No 02 264733 (1990). 5 T.Fujita, H.Oono, K.Taniguchi and K.Takahata, Japan Kokai TokkyoKoyo No 03 167139 (1991). 6 F.Radt, in Elsevier's Encyclopaedia of Organic Chemistry, Naphthalene, F.Radt (ed.), Elsevier, New York, 1948. 98 (1976)1839. 7 G.A.Olah and J.A.Olah, J ; Am. Chem. SOC., 8 G.A.Olah, US Patent No 4 288 646 (1979). 9 W.M.Kutz and B.B.Corson, J. Am. Chem.Soc., 67 (1945)1312. 10 Teijin Co. LM., Japan Patent No 74 48949 (1974). 11 A.V.Topchiev, M.V.Kuraschev and I.F.Gavrilenko, Dokl. Akad. Nauk. SSRR, 139 (1961)124. I2 D.Fraenke1, MCherniavsky, B.Itah and M.Levy, J.Catal., 101 (1986)273. 13 K.Eichler and E.I.Leopold, Ger. Offen. No 3 334 084 (1985). 14 M.Neuber, H.G.Karge and J.Weitkamp, Catal. Today, 3 (1988)11. 15 A.Katayama, M.Toba, G.Takeuchi, F.Mizukami, S.Niwa and S.Mitamura, J.Chem.Soc., Chem.Comm., (1991)39. 16 P.Moreau, A.Finiels, P.Geneste and JSolofo, J.Catal., 136 (1992)487. 17 F.Fajula, R.lbarra, F.Figueras and C.Gueguen, J.Catal., 89 (1984)60. 18 E.Bodroux, Ann. Chim. Phys., 11 (1929)535. 19 E.S.Pokrovskaya and T.G.Stepantseva, J. Gen. Chem., 9 (1939)1953. a0 P.Moreau, J.Solofo, P.Geneste, A.Finiels,J.Rambaud and J.P.Declercq, Acta Cryst., C48 (1992)397. 1

M. Guisnet et al. (Editors),Hetmgencous Catalysis and Fine Chemicals 111 (0 1993 Elsevier Science Publishers B.V. All rights reserved.

581

Stereoselectivity of the deisopropylation of methyl dehydroabietate C. Pereiraa, F. Alvarezb, M. J. M. Curtoa, B. Gigantea, F. R. Ribeiroband M. Guisnetc a LNETI, DTQI, Est. Palmeiras, 2745 Queluz, Portugal.

b Grupo de Zeblitos, DEQ, IST, Av. Rovisco Pais,

C

1096 Lisboa, Portugal.

LACCO, Univ. Poitiers, 40 Av. Recteur Pineau, 86022 Poitiers, France.

Abstract The potential of zeolites as catalysts for the Friedel-Crafts deisopropylation of dehydroabietic acid methyl ester 1 was determined. HY zeolites were found as active catalysts converting 1 into methyl trans-podocarpa-8,11,13-trien-l5-oate, 3, through isopropyl transfer to toluene used as a solvent. -Thereis no formation of the cis isomer, contrarily to what is found with aluminium chloride, but other products resulting from the elimination of the methylcarboxylate group and cracking can be observed at 100 "C. A better selectivity of deisopropylation can be obtained by reducing the zeolite acidity through sodium exchange or through dealumination or/and by operating at a lower temperature.

1. INTRODUCTION

Alkylation of aromatic hydrocarbonswith zeolites is well established, as shown by numerous processes (e.g. Mobil-Badger process for ethylbenzene production, paraethyltoluene synthesis [l] ), patents and papers. Acid zeolites can also replace the highly corrosive acid catalysts (protonic: e. g. sulfuric acid or Lewis: e. g. aluminium chloride) currently used for the synthesis of more complex aromatic compounds 12-51.This is the case in particular for Friedel-Crafts and related reactions: alkylation, transalkylation, acylation, halogenation of aromatics 161. Zeolites offer numerous advantages: less or no corrosion, no waste or disposal problems, easy set-up of continuous processes, high thermostability. Moreover their acidity (nature, number and strength of the sites) can be easily adapted to the reaction considered by various treatments: thermal treatment, ion exchange, dealurnination... . Another of their advantages results from their well-defined pore

582

structure with molecular size pores and from the possibility of adjusting the size of the pores. Actually the selectivity of zeolites depends not only on the intrinsic properties of active sites but also on the pore structure (shape selectivity). In this work, acid zeolites are used for the Friedel-Crafts dealkylation of dehydroabietic acid methyl ester 1, a readily available hydrophenanthrene derivative and a useful starting material for the synthesis of industrial and/or physiologically important products. Results obtained in the deisopropylation of 1 using various zeolites are compared with those obtained with aluminium chloride.

1

2

3

2. EXPERIMENTAL 2.1 Materlals All the reagents were of analytical grade and dried and purified when necessary. Dehydroabietic acid methyl ester 1 was prepared by methylation with diazomethane [7] of the respective acid, obtained from commercially disproportionated rosin [a]. NaHY, NaY and HY zeolites (PQ Corporation and Conteka) and mordenite (Norton 900H) were commercial samples. HZSMS (Si/AI=40) was synthetised according to Mobil patents. Zeolites were calcinated at 520 "C under a dry air flow, for 12 hours. The characteristicsof the NaHY and HY samples are given in Table 1. 2.2 General procedure The zeolites were activated under a nitrogen dry flow for 12 hours at 500 "C, prior to use. All the reactions were carried out under N2 by stirring a solution of methyl dehydroabietate 1 (0.008 M) in 40 ml of dry toluene under N2 atmosphere with a zeoliteheagent ratio = 10. The samples were analysed, after evaporation of the solvent, by gas chromatography, using a CPSil5CB capillary column (50 m length , 0.25 mm interior diameter).

Table 1 Characteristics of HY zeolites : EFAl extraframework aluminium per unit cell; Si/AIframework ratio calculated from the unit cell parameter [9] Framework Crvstallinitv

-

583

3. RESULTS AND DISCUSSION With aluminium chloride at 30 "C the dehydroabietic acid methyl ester 1 is rapidly transformed into a mixture of 2, methyl cis - podocarpa-8,11,13-trien-15-oate(about 70%) and 3 , methyl trans - podocarpa-8,11,13-trien-15-oate (about 30Y0), i n agreement with the literature [lo, 111. The reactions with zeolites were carried out at a higher temperature (100 "C). With Nay, no transformation of 1 was observed, while HY4 was very active and converted 1 into various products, including the methyl trans - podocarpa-8,11,13trien-l5-oate, 3. The isomer cis, 2, was not observed. The other products were hydrocarbons (no 0 atom). Their formulas determined by GS/MS are given in Table 2, with their yield after 5 minutes' reaction. Table 2 Percentage of reaction products after 5 minutes' reaction over HY4 Product

Molar Weiaht

Formula

wt Yo

3

272

5P

9.3

COOMe

A

256

1.8

B

174

65.8

C

222

4.2

D

224

11.6

A (molar weight M=256) results from the simple elimination of the methylcarboxylate group, B (M=174) is probably isopropyl-l,2,3,4-tetrahydronaphtalene, C (M=222) and D (M=224) are isopropyl-hydrophenanthrenes. Cymenes were also observed, which shows that deisopropylation of 1 occurs through isopropyl transfer into toluene. Two types of acid mechanism have been proposed to explain alkyl transfer [12]. The first which is the more likely occurs through alkyl transfer from a protonated molecule of 1 to toluene UI

584

The second, which explains the disproportionation of xylenes, occurs through benzylic carbenium ions and diarylmethane intermediates. With these mechanisms the cis isomer 2 cannot be formed. The formation of this isomer when aluminium chloride is used as catalyst can be explained [ l l ] by simultaneous elimination of the isopropyl group and cleavage of ring 6 (followed by the reformation of ring 6).An alternative mechanism in which the opening of ring B precedes deisopropylation has been proposed [13].

The elimination of the methylcarboxylate group occurs probably through the following mechanism proposed for the decarbonylation of acids in concentrated sulfuric acid [14]: RCOOMe + H+

0 --b

R-f

0 R-C:

-+

RCO++MeOH

P;'

Me H

The carbenium ion formed through this reaction can be transformed into A through hydride transfer to an alkane molecule

It can also undergo successive cracking reactions leading finally to B, or cracking reactions and hydrogen transfer to olefinic cracking products with formation of C and D. Highly acid zeolites such as HZSM5 and HMOR are not active, probably due to the impossibility for 1 to diffuse in the pores of these zeolites. A better selectivity to 3 can be obtained by modifying the acidity of the HY catalyst. Figure 1 shows the formation of the products as function of time with a less acidic Y zeolite (NaHY4). 3 and B are the main products; both are directly formed from 1 with approximatively the same rate. Again no cis isomer 2 is formed showing that there is no cleavage of the 6 ring. At long reaction times 3 undergoes

585

secondary transformations leading mainly to C and D products. Unfortunately the activity of NaHY4 is much smaller than that of HY4 (100 times).

" i SO

2 t(h) Figure 1 - Distribution of reaction products (wt%) vs. time on NaHY4 With a dealuminated zeolite (HY20) the formation of 3 is faster than with HY4 and the selectivity to 3 is better, comparable to that found with NaHY4. This selectivity is considerably improved by operating at a lower temperature. At 70 "C only 3 and A are initially formed the 3/A formation rate ratio being around 10 (Figure 2).

t

40

Figure 2 - Distribution of reaction products (wt%) vs. time on HY20 4. CONCLUSIONS

HY zeolites are active catalysts for the deisopropylation of the dehydroabietic acid methyl ester. While with aluminium chloride a mixture of the cis and trans deisopropylated derivatives is obtained (which makes it difficult to obtain pure

586

compounds), only the trans isomer is formed with zeolites. This selective formation is probably related to the zeolite pore structure (shape selectivity effect).Unfortunately decarbonylation and cracking of the ring A accompany deisopropylation. Reducing the zeolite acidity by sodium exchange or b y dealumination has a positive effect on the deisopropylation selectivity. Trans isomer can be obtained with a high selectivity by operating at a lower temperature with dealuminated Y zeolites.

5. ACKNOWLEDGEMENTS

Junta Nacional de InvestigaCiio Cientifica e Tecnologica (JNICT) and the French Gouvernement are acknowledged for a grant to C. Pereira.

6. REFERENCES

1 2 3

4 5 6

7 8 9 10 11 12 13 14

N. Y. Chen, W. E. Garwood and F. G. Dwyer, Shape Selective Catalysis in Industrial Applications, Chemical Industries Series vol. 36, M. Dekker Inc., New York and Basel, 1989. H. G. Franck and J. W. Stadelhofen, Industrial Aromatic Chemistry, SpringerVerlag, Berlin and Heidelberg, 1987. W. F. Holderich and H. van Bekkum, in: H. van Bekkum et al (eds.), Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, Elsevier Science Publishers B.V., Amsterdam, vol. 58, 1991, p. 631. W. F. Holderich , in: P. A. Jacobs and R. A. van Santen (eds.), Zeolites: Facts, Figures, Future, Studies in Surface Science and Catalysis, Elsevier Science Publishers B.V., Amsterdam, vol. 49, 1989, p. 69. G. Perot and M. Guisnet, J. Mol. Catal., 61 (1990) 173. G. Perot and M. Guisnet, in: Weijnen and Drinkenburg (eds.), Precision Processs Technology, Perspective for pollution prevention, Kluwer Academic Publishers, 1993, in the press. D. F. Zinkel and J. S. Han, Naval Stores Rev., 96 (1986) 14. N. J. Halbrok and R. V. Lawrence, J. Org. Chem., 31 (1966) 4246. D. W. Breck and E. M. Flanigen, Molecular Sieves, Society of Chemical Industry, London, UK, 1968, p. 47. M. Ohta and L. Ohmori, Chem. Pharm. Bull., 5 (1957) 91. A. Tahara and A. Akita, Chem. Pharm. Bull., 23 (1975) 1976. M. Guisnet, in: B. lmelik et al. (eds.), Catalysis by Acid and Bases, Studies in Surface Science and Catalysis, Elsevier Science Publishers B.V.. Amsterdam, vol. 20, 1985, p. 283. E. Wenkert and B. G. Jackson, J. Amer. Chem. SOC.,80 (1958) 21 1. G. A. Ropp, A. J. Weinberg and 0. K. Neville, J. Amer. Chem..Soc., 73 (1951) 5573.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 @ 1993 Elsevier Science Publishers B.V. All rights reserved.

587

An in situ 13C-NMR study of the mechanism of cumene n-propylbenzene isomerization over H - Z S M - 1 1

-

1.1. lvanovaa, D. Brunelb, J. B.Nagya, G. Daelena and E.G. Derouanea-' Facultds Universitaires N. D.de la Pzix, Laboratoire de Catalyse, 61, Rue de Bruxelles, 8-5000 Namur, Belgium

a

Ecole Nationale Supdrieure de Chimie, Laboratoire de Chimie Organique Physique, 8, Rue Ecole Normale, F-34053 Montpellier, France

b

Abstract Cumene conversion under excess of benzene was studied over H-ZSM-11 in the adsorbed phase at 473 K by in situ 1% MASNMR. To follow the fate of different carbon atoms during the reaction, cumenes labelled with 13C-isotopes either on aor on p-positions of the alkyl chain or in the aromatic ring have been synthesized. The primary product of cumene conversion over H-ZSM-11 was found to be npropylbenzene. It is formed via intermolecular reaction of cumene and benzene. At long reaction times, the formation of n-propylbenzene is accompanied by complete scrambling of both cumene and n-propylbenzene alkyl chain carbon atoms and formation of toluene, ethylbenzene and butylbenzene. The rate of isomerization is higher than the rate of scrambling and fragmentation. 1. INTRODUCTION

Cumene is an important intermediate in the industrial production of phenol, acetone and a-methylstyrene. The large-scale production of cumene is based on the alkylation of benzene with propene over Friedel-Crafts [ l ] or phosphoric acid on silica catalysts [2]. Zeolites, namely ZSM-5 and ZSM-11, have also been shown to be potential catalysts for this process [3, 41. However, the formation of cumene (isopropylbenzene, IPB) on this catalysts is accompanied by its isomerization to npropylbenzene (NPB). The latter is considered as an undesired by-product with respect to further processing of cumene to phenol and acetone. Therefore, preventing the formation of NPB would enable the substitution of the current catalysts used in the industrial process by ZSM-5 or ZSM-11 type solid acids which have major advantages in terms of environmental protection, safety, and avoidance of corrosion. It is therefore of interest to look for possibilities to suppress the isomerization of cumene. For this purpose, understanding of the mechanism of isomerization might be of great help. However, no information on this mechanism is available at present. *To whom queries about this paper should be sent.

588

Cumene isomerization over H-ZSM-11 has been studied by 1%-MASNMR in the adsorbed phase, using 1% - labelled molecules. This technique allows one to observe different kinds of adsorbed entities, to distinguish unequivocally between mobile and adsorbed species, to monitor their fate during the course of the reaction, and to determine approximate reaction rates [5-81. As a result, reaction mechanisms or pathways can be proposed. To follow the fate of carbon atoms in the alkyl chain or aromatic ring during isomerization, cumenes labelled with 1% at either a-, or p-position of the alkyl chain, or on the aromatic ring were prepared. As over ZSM-5 or ZSM-11 catalysts, cumene is produced under excess of benzene in order to avoid propylene oligomerization and polymerization, the cumene - n-propylbenzene isomerization was studied under similar conditions. 2. EXPERIMENTAL

Labelled c o m . Propene 1-13C (99.9% 13C), isopropanol 2-13C (99.9% 13C), and benzene (5% 1%) were purchased from MSD isotopes. Propene 2-I3C (99.9% 1%) was prepared at 623 K by dehydration of isopropanol 2-lSC (99.9% 1%) on zeolite NaY with 100°/o conversion and selectivity. Syntheses of labelled cumenes were carried out by benzene alkylation with propene in the adsorbed phase over H-ZSM-11. To avoid propene oligomerization, benzene was always adsorbed first. To synthesize cumene labelled with 13C isotope in the a-position of the alkyl chain (cumene l ) , non-labelled benzena and propene 2-13C were used. Cumene 1% - labelled in the p-positions of the alkyl chain (cumene 2) was synthesized from non-labelled benzene and propene 1-1%. Cumene with 13C isotopes in p- positions of the alkyl chain and in the aromatic ring (cumene 3) was synthesized from all-ring labelled benzene (5% 13C) and a mixture of unlabelled propene and propene 1 - W (1 :l). All cumene preparations were performed in sealed 5 mm NMR tubes. Cumenes were obtained at room temperature with 100% conversion and selectivity. The formation and the purity was controlled by 1% MASNMR. a m d e Dr-. Zeolite H-ZSM-11 (Si/AI=25) was prepared by ammoniumexchange of Na-ZSM-11 and calcination at 823 K in air. The powdered samples (0.09;tO.Ol g) were packed into NMR tubes (Wilmad, 5 mm 0.d. or 5.6 mm 0.d. with constrictions). After 8 h heating at 573 K the catalysts were evacuated to a final pressure of 6.1 0-6 Torr and cooled down to room temperature before adsorption. The samples preparation conditions are shown in Table 1. In order to prevent reaction by local overheating, all the samples were kept in liquid nitrogen while sealing the capsules. Sealed samples were treated at a precisely controlled temperature (473 or 486 K) for various lengths of time. .rn sifu 13C-NMR measurements were carried out on CXP-200 and MSL-400 BRUKER spectrometers operating at 50.3 and 100.6 MHz, respectively. Quantitative conditions were achieved using high-power gated proton decoupling with supressed NOE effect (90' pulse, recycling delay 4s). The highly symmetrical 5.6 mm 0.d. sealed capsules were used in a standard Bruker MAS probe, while the 5.0 mm 0.d. sealed capsules were used directly in a home-made probe [9-10]. The spinning rates were up to 4kHz for the former and 2.5 kHz for the latter.

589

Table 1 Dle m Samples

o n s NMR tube

Label w

. l.

o

n d e d lmo1h.c.l e e CAa 5.6 j3- ( 9 9 Y 0 ~ ~ c ) 8 4 ring (5%13C) B 5.6 ring (5%l3C) 8 4 Cb 5.6 a- (99%13c) 8 1 p- (99%l3C) 8 1 Dlb 5.6 a b , c 5.0 B199Oh'3C) 8 1 a - To prepare this sample, cumene 3 was preliminarily synthesized in another sealed capsule. The latter was broken by a magnet in a thoroughly evacuated closed volume. Cumene was then released and trapped in a glass U-arm tube by freezing with liquid nitrogen, and finally loaded on the activated zeolite along with unlabelled benzene. b - Samples were prepared by adsorption of 9 molecules of benzene and 1 molecule of labelled propene per U.C. of a ZSM-11. This mixture was converted to cumene 1 or 2 at room temperature giving final molar ratio benzenekumene = 8:l. C - Sample D2 was sealed 1.5 cm higher than D1, resulting in about 3-5 times greater dead volume above the sample. 3. RESULTS AND DISCUSSION

The mechanisms proposed for the conversion of alkylaromatic compounds over acidic catalysts [12-14 ] suggest that, in principle, both inter- and intramolecular pathways are possible in the case of cumene - NPB isomerization. Some examples including carbocationic pathways are given on Scheme I. Scheme I

&-

INTRA

/

methyl shift ,--Wc Tphenyl s h E

&&

Intramolecular reaction (routes 1,2 and 3) may occur via a protonated phenylcyclopropane intermediate, as well as through a non cyclic intermediate. In the first case C1-C2 or C2-C3 cleavage is preferable, resulting in the formation of n-

590

propylbenzene with b-a-c or b-c-a ( 1 : l ) ordering in the alkyl chain. In the second case methyl or phenyl shifts are equally possible resulting in either b-a-c and b-c-a (1 :1) or a-b-c and c-b-a (1 :1) ordering in the alkyl chain, respectively. Beyer and Borbely [15], however, suggested that cumene isomerization proceeds more likely via intermolecular alkyl transfer. Intermolecular isomerization may occur [I61 1 ) through a bimolecular transition state in which the aromatic rings are bridged by an isopropyl group, with subsequent cleavage and transfer of the propyl group to the second aromatic ring (route 4); 2) by isopropyl group cleavage, followed by the formation of a cycloproponium ion with further alkylation of another benzene ring (route 5). In the first case the distributions of the carbon atoms in the alkyl chain might be a-b-c and c-b-a, whereas in the second one - a-b-c, c-b-a, b-ca, b-a-c, c-a-b, and a-c-b, are equally possible. Detection of intermediates seems to be rather difficult because of their high reactivity and short half-life. However, using cumene, labelled at carbon atoms in different positions, it is possible to trace the fate of each carbon atom in the course of the reaction and the distribution of the label in the final products. This gives insight in the possible mechanisms. The ability of 13C MASNMR to distinguish unambiguously between the different carbon atoms in the alkyl chain and the aromatic ring of cumene and NPB motivated our choice of this method to trace the label during the reaction. Table 2 lists the chemical shifts of the carbon atoms of cumene, NPB and some other byproducts which might be formed in the course of cumene conversion. Table 2 Chemical shifts ( 6 in ~ ppm) of reagents and main products Compound C-1 C-2 C-3 C-4 benzene 128.5 isopropylbenzene (IPB) 148.8 126.6 128.6 126.1 n-propylbenzene (NPB) 142.5 128.6 128.3 125.8 toluene (T) 137.8 129.2 128.4 125.5 ethylbenzene (EB) 144.3 128.1 128.6 125.9 sec-butylbenzene (BB) 148.4 127.9 129.3 126.8

C-a

C-p

C-y

34.4 38.3 21.3 29.7 42.3

24.1 25.0

13.9

15.8 31.7 22.2

12.2

Reprinted from [17]. In order to prove whether isomerization occurs through an intra- or an intermolecular pathway, experiments with either cumene or benzene labelled in the aromatic ring were carried out. In experiment A, cumene labelled in the ring as well as in the alkyl chain was used to follow the fate of the aromatic ring atoms and to simultaneously follow the cumene conversion to NPB and other products. Aromatic regions of the 1 % MASNMR spectra taken from sample A before and after treatments at 486 K for 60 and 540 min are shown on Figure la. The spectrum of initial unheated sample contains 3 lines: at 149 ppm, corresponding to 1%-l in cumene, at 128.5 ppm, ascribed to 13C-3,5 in cumene, and 1% in benzene and at 126.5 ppm - to 13C-2,4,6 in cumene. After heating for 60 min, the conversion of cumene to NPB estimated from the aliphatic part of the spectrum, was more than 50%; meanwhile the line at

591

Sample A

128+126

@

Q

n

Sample B

149

u 120

140

PPM

I

I

1

140

120

I

PPM

Figure 1 . 13C MASNMR spectra of (a) samples A and (b) sample B treated at 486 K for various lengths of time.

149 ppm, corresponding to 13C-l in cumene disappeared. However, the line at 142.5 ppm corresponding to l3C-l in NPB was not observed. The complete disappearance of the l3C-l resonance of cumene and the absence of NPB labelled in the aromatic ring demonstrates the intermolecular character of the mechanism of the cumene - NPB isomerization . Similarly, when using labelled benzene and unlabelled cumene (Sample B) a benzene - NPB aromatic ring label transfer was evidenced by the appearance of the resonance at 143 ppm (Figure l b ) . This confirms unambiguously the intermolecular character of cumene isomerization. The intermolecular mechanism of the isomerization reaction is further detailed by experiments with cumene labelled in the alkyl chain. The aliphatic regions of the l3C MASNMR spectra taken from samples C and D before and after treatment at 473 K for various lengths of time are shown in Fig. 2. Experiments C and D1, were run under similar conditions. The only difference is the position of the label in the starting cumenes. The initial spectra contain single resonances either at 34 (sample C) or 24 (sample D1) ppm, corresponding to cumene labelled either at a- or at p-position, respectively. Heating the samples for 15 min leads to conversion of a-cumene in p-NPB (25 ppm) and p-cumene to aNPB (38 ppm) and y-NPB (14 ppm), the latter 2 NPBs being obtained in equal amounts. This observation suggests that isomerization proceeds through bimolecular transition state as shown on scheme I, route 4. Further treatment of both samples for another 30 min results in the appearance of all the lines corresponding to cumene (34, 24 ppm) and NPB (38, 25, 14 ppm) alkyl chains. These lines appear because of the scrambling of the carbon atoms in the

592 Sample C

Sample C

630 min

134

15 rnin

25

45 min

630 min

x

Sample D2

@

15 min

24

I

I

I

40

20 PPM

I

l4

I

I

40

20

1

PPM

Figure 2. 13C-MASNMR spectra of samples C (a), D1 (c) and D2 (d) treated at 473 K for various lengths of time. Decomposed spectrum of sample C (b)

593

alkyl chain of the reagent and product. The rate of scrambling is lower than the isomerization rate as scrambling occurs after isomerization of cumene to NPB. Finally, after heating for 630 min, lines at 21 and 16 ppm, corresponding to the acarbon atom of toluene (T) and the p-carbon atom of ethylbenzene (EB), appear together with lines at 42, 22 and 12 ppm, which could be assigned to different atoms of sec-butyl benzene (BB), indicating thereby the fragmentation of NPB or cumene which accompanies the isomerization of cumene on H-ZSM-11. The final spectra can be decomposed in 11 lines as shown on Fig. 2b: lines 2,5, and 10 being ascribed to NPB alkyl chain atoms, 3 and 6 to IPB, 8 to T, 4 and 9 to EB, and 1, 4, 7 and 11 to BB. A similar distribution of line intensities is observed for samples C and D1, indicating that the system has reached an equilibrium. The lines corresponding to cumene are narrower compared to those of NPB. It indicates a faster motion of the isopropyl group and a stronger adsorption of the n-propyl group in comparison with the isopropyl chain. This might be due to a better accomodation of the straight alkyl chain of NPB along the wall of the zeolite channel. The narrowing of the lines corresponding to the alkyl chain of NPB from a- to ycarbon atoms indicates an increasing sequential mobility in that order. The conversion of cumene into NPB and fragmentation products T, EB and BB (calculated from integral intensities of the lines) is 90%, the selectivity towards isomerization is 50%. The next experiment (Figure 2d) was performed on sample D2 which is similar to D1 but differing from the latter in the pressure of the reagents (Table 1). Two differences emerge: 1. The rate of isomerization is at least two times lower than for D1. 2. The rate of fragmentation increases dramatically with decreasing pressure. Significant amounts of 1%-aEB and 1%-a T were observed within 45 min of heating. These results lead us to the conclusion that the rate of isomerization depends on the pressure of reagents. It provides another proof for an intermolecular isomerization pathway. Note that the fragmentation may occur also through an intramolecular mechanism. On the basis of above observations an intermolecular pathway involving diphenylethane as an intermediate could be suggested (Scheme 11).

Scheme I1

Despite the fact that the supposed intermediates look rather bulky, their cross sections are close to that of cumene, and therefore this mechanism could occur in the channels of H-ZSM-11.

594

4.

CONCLUSIONS

1. Cumenes labelled at various positions were prepared with 100% selectivity at room temperature by alkylation of benzene with propene over a H-ZSM-11 catalyst. 2. In the presence of an excess of benzene, the primary product of cumene conversion over H-ZSM-11 is n-propylbenzene. NPB is formed via intermolecular reaction of cumene with benzene. 3. At longer reaction times, the formation of n-propylbenzene is accompanied by complete scrambling of the carbon atoms in the alkyl chains of cumene and npropylbenzene, and by fragmentation towards TI EB and BB. The rate of isomerization is higher than the rate of scrambling and fragmentation. As the transition state in the cumene n-propylbenzene isomerization is rather bulky,the selectivity could be altered by modifying the channel or cavity size of the microporous catalysts.

-

ACKNOWLEDGMENTS

I . lvanova acknowledges financial support from the Belgian Program on Interuniversity Attraction Poles (PAI), initiated by the Belgian State Prime Minister Office, which has also funded this work. REFERENCES

1 2

3 4

5 6 7

8 9 10 11 12 13 14 15 16 17

H. Miki, US Patent No. 4 347 393 (1982). E.K. Jones and D.D. Dettner, US Patent No. 2 860 173 (1958). W.W. Kaeding and R.F. Holland, J. Catal., 109 (1988) 212. K.V. Topchieva, N.F. Meged, 1.1. lvanova and T.V. Limova, USSR US No.1234393 (1986). M.W. Anderson and J. Klinowski, Nature, 339 (1989) 200 J.F. Haw, B.R. Richardson, IS. Oshiro, N.D. Lazo and J.A. Speed, J. Amer. Chem. SOC.,111 (1989) 2052. K.P. Datema, A.K. Nowak, J. van Braam. Houckegeest and A.F.H. Wielers, Catalysis Letters, 11 (1991) 267. Y.S. Kye, S.X. Wu and T.M. Apple, J. Phys. Chem., 96 (1992) 2632. W. Buckerman. L.-C. de MOnorval, F. Figueras and F. Fajula, to be published. F. Rachdi, J. Reichenbach, L. Firlej, P. Bernier, M. Ribet, R. Aznar, G. Zimmer, M. Helmil and M. Mehring, Nature, submitted. G.A Olah, and J. Kaspi, Nouv. J. Chim., 2 (1978) 585. D.H. Olson and W.0. Haag, ACS Symposium Series, 248 (1984) 275. D.C. Santilli, J. Catal., 99 (1986) 327. G.A. Olah, J. Amer. Chem. SOC.,94 (1972) 808. H.K. Beyer and G. Borbely, New Developmentsin Zeolite Science and Technology, Proceedings of the 7th International Zeolite Conference, Eds. Y. Murakami, A. lijima and J.W. Ward, Elsevier, New York (1986) 867. P.A. Jacobs, Carboniogenic Activity of Zeolites, Elsevier, New York, 1977. E. Breitmaier and W. Voelter, Carbon-13 NMR Spectroscopy, VCH Verlag, Weinheim, 1987.

M. Guisnet et al. (Editors), Heterogenmus Catalysis and Fine Chemids III (B 1993 Elsevier sdence Publishers B.V. All rights reserved.

595

Kinetic study of the acylation of thiophene with acyl chlorides in liquid phase over HY zeolites A. Finiels, A. Calmettes, P. Geneste and P. Moreau Laboratoire de Chimie Organique Physique et Cinbtique Chimique Appliqubes, URA 418 CNRS, Ecole Nationale Supbrieure de Chimie, 8 Rue de 1'Ecole Normale, 34053 Montpellier Cedex 1, France.

Abstract The kinetic study of the acylation reaction of thiophene with acyl chlorides over dealuminated HY zeolites, in chlorobenzene as solvent, has been shown to follow a Langmuir-Hinshelwood type kinetic law. In liquid phase conditions, thiophene is slightly more adsorbed than the acyl chloride. Moreover, the study of the reaction with various substituted benzoyl chlorides X-C,H,-COCl shows that the nature of the substituent has very little influence on the initial rate.

1. INTRODUCTION

Thiophene and its derivatives are well known for their biological and pharmacological activities. In particular, acyl derivatives are precursors of compounds with anti-inflammatory, analgesic or anti-spasmodic properties [ l , 21; a representative example of such derivatives is the tiaprofenic acid ( 2(5'-benzoylthiophene)-propanoicacid) used for its analgesic activity [3]. Acyl derivatives themselves, such as the 2- (1'-piperidino) propanoyl thiophene, are used a s anti-spasmodic [41. The high reactivity of thiophene towards electrophilic substitution has been reported for a long time, and acylation reaction of thiophene has been widely studied. Lewis acids, mainly AlC1, and ZnClz [51 , and more recently SnC14 [6, 71 or FeC13 181, have been mostly used as catalysts. Acylation reactions of thiophene have been also investigated under heterogeneous conditions; a montmorilloni te clay and silica-alumina [9] and Nafion-H [lo] have been shown to be efficient catalysts of the acylation of thiophene with acetyl chloride.

596

Recently, it was reported that it was possible to perform this acylation in the gas phase on zeolite catalysts with a high selectivity [ll]. Our interest in the use of zeolites as shape selective catalysts in liquidphase acylation 1121 and alkylation [131 reactions of aromatic derivatives led us to consider their ability to catalyse the acylation of thiophene in relatively mild conditions, compared with the previous results 1111. The present paper deals with kinetic results obtained in this reaction, which leads t o the determination of the mechanism in liquid-phase under heterogeneous conditions.

2.EXPERIMENTAL The reactions were carried out in a standard glass apparatus. The experimental procedure was as follows. A solution of thiophene (10-2mol) and acid chloride (2. 10-2 mol) in 50 ml chlorobenzene, was heated to 100°C. The freshly calcinated zeolite (HY SUAl =15,150 mg, supplied by Zeocat, activation temperature 400°C during 6h, in air) was added to the stirred solution. Samples were withdrawn periodically and analysed by GLC using a OV1 capillary column (30 m ~ 0 . 2 2mm). Under these conditions, the reactions follow second order kinetics. The products isolated by column chromatography on silica with hexaneether as eluent were identified by 13C and 1H NMR and by GLC-mass spectroscopy. 3.RESULTS AND DISCUSSION

Preliminary results We have shown that HY zeolites are convenient catalysts for the highly selective preparation of 2-acylthiophenes in the reaction of thiophene with various aliphatic acyl chlorides (C, Hzn+lCOCl with n=1,3,5,7). For example, 2-butyroylthiophene is easily and quantitatively produced from the acylation of thiophene with butyroyl chloride at 1OO"C, over HY (Si/Al =15) zeolite, after 7 hours. Higher homologs lead to identical results (good selectivity and efficient conversion).

Kinetic study The acylation of thiophene (TI with butyroyl chloride (BC) at 100°C in presence of HY (Si/Al =15) in liquid phase, with chlorobenzene used as solvent (S), was chosen for the kinetic study. As usual for heterogeneous reactions, the reaction rate was found to increase linearly with the mass of catalyst (until 200 mg of zeolite) and then the rate approaches a constant value.

597

The initial rates of disappearance of thiophene were determined at different initial concentrations of each of the reactants (from 0.05 to 1mol.1-1) while the initial concentration of the other was kept constant (0.4 mol.1-1for butyroyl chloride and 0.2 mol.1-1 for thiophene). Figures 1 and 2 show the evolution of the initial reaction rate with the initial concentration of thiophene and butyroyl chloride respectively. The behaviour is similar for these two reactants. The initial acylation rate increases, passes through a maximum, and then decreases. We can note that the decrease is slightly more important with thiophene than with chloride.

c

0,m h

-

h

m Ce

u 0

M

2 .- 0.04-

g E

1

0,02

-

Y

0

>

0,oo

.

I

.

,

.

I

.

,

.

I

.

0,O 0 , 2 0 , 4 0,6 0,8 1 , 0 1 , 2

[Tlo (moV1)

0.00 . I . I . I . I . l . 0,O 0 , 2 0 , 4 0 , 6 0 , 8 1 , O 1 , 2

[BC]o (moV1)

Figure 1.Plot of vo against [thiophenelo Figure 2. Plot of vo against [chl~ride]~ [TI0 = 0.2 mol.1-1 [BC10 = 0.4 mol.1-1

These kinetic observations indicate a competitive activation of the two reactants, leading to an inhibiting effect a t high concentrations. The results can be interpreted in terms of a Langmuir - Hinshelwood mechanism for which the surface reaction takes place between thiophene and butyroyl chloride both adsorbed on identical active sites. The initial rate of the process, in liquid phase, is given by the equation (1).

taking into account that Zi hi [i] >> 1 as the reaction occurs in liquid phase and where h T,h BC and h s are adsorption equilibrium constants of thiophene, butyroyl chloride and solvent (chlorobenzene) respectively and k is the rate constant of acylation reaction.

598

Equation (1)can be written as follows:

As shown in Figures 3 and 4, the plots of against “0

CBCl,

-

(when

CSCI,

d T l:

and

[TI0

against

-

CBClo

@XIois constant )

(when [T 1, is constant )

[TI,

lie in a good straight line, which is characteristic of a Langmuir Hinshelwood mechanism with competitive adsorption over the same type of sites.

10

8

0

1 0

1

2

[Tlo/[BClo Figure 3. Linear transform of the curve shown in Figure 1.

3

0

1

2

3

4

5

6

[BC3 o/[Tlo Figure 4. Linear transform of the curve shown in Figure 2.

From these diagrams, the ratio of adsorption coefficients of thiophene and butyroyl chloride can be calculated: AT / h g c = 1.5 This values indicates that, thiophene is more adsorbed than butyroyl chloride.

599

Substituent effect. The acylation reaction of thiophene with various para substituted benzoylchlorides has been studied, over HY (Si/Al =15) zeolite, in the same conditions as described above. The initial rates observed in the reaction with p-methoxy (powerful electron donating group p =-0,27; p+ =-0,78)and p-NOz (powerful electron withdrawing group p = 0,78; p+ = 0,791benzoyl chloride are not very different: vo (OCH3) 2 vo (N02) The Hammett p-o relationship (or p-@) (Figure 5 ) shows that there is no significant effect of substituent X on the aromatic ring of benzoyl chlorides on the initial reaction rate. p

1

-2.6 I

-

-2.7

-

-2.8 0

>

-

M 0

-3,O-2.9

--3.1 3.1f -0.4

.

I

-0.2

.

.

II

0.0

.

.

II

0.2

.

.

NO2 I I

0.4

I

0.6

I 0.8

Figure 5. Plot of log vo against CF for HY catalysed acylation of thiophene with para-substituted benzoylchlorides. It is known that, in acylation reactions in homogeneous [14,15]o r heterogeneous [16]conditions, the rate determining step is the attack of the electrophilic species RCO+, previously formed in a fast step, on the aromatic ring. Electrophilicity of benzoyl cations X-Ar-CO+increases with the presence of electron-withdrawing substituent, while it decreases with electron-donating substituents on the aromatic ring. However, our results show that the nature of the substituent has only very little influence on the initial rate of the reaction. Such a result is assumed t o be directly related to the high reactivity of thiophene towards electrophiles. The rate of the attack of the benzoyl cation XArCO+ on the heterocyclic ring (slow step) is not affected by its electrophilicity, high or weak, once it is formed (fast step) . It can then be concluded that the mechanism of the acylation of thiophene over zeolites is identical to that described for aromatic derivatives.

600

4. CONCLUSION

Dealuminated H Y zeolites .*ave been shown to be efficient catalysts for the selective preparation of 2-acyl thiophenes in the reaction af thiophene with various acyl chlorides. The acylation reaction of thiophene with butyroyl chloride over HY (Si/Al =15) zeolite, in chlorobenzene as solvent, follows a Langmuir-Hinshelwood kinetic law, which involves the adsorption of the two reactants on identical sites of the catalyst surface . Moreover, the nature of the substituent has very little influence on the initial rate of the reaction of thiophene with various substituted benzoyl chlorides.

1 2 3 4

5 6 7 8 9 10 11

12

13 14

15 16

H.D. Harthough, in the Chemistry of Heterocyclic Compounds, J. Wiley (eds.), New-york, 1985, vo1.44. F. Clemence, 0. Le Martet, R. Fournex, G. Plassard and M. Degnaux, Eur. J. Med. Chem., Chim. Therap., 9 (1974) 390. E.M. Sorkin and R.N. Brogden, Drugs, 29 (1985) 208. J.J. Denton, R.J. Turner, W.B. Neir, V.A. Lawson and H.P. Sheld, J . Am. Chem. SOC.,71 (1949) 2048. H.D. Harthough, in the Chemistry of Heterocyclic Compounds; Thiophene and its Derivates, Monograph Inters. Publ., New-York, 1952. S.J. Rao, U.T. Bhalerao and B.D. Tilak, Indian J. Chem., 24B (1985) 1275. S.J. Rao, U.T. Bhalerao and B.D. Tilak, Indian J. Chem ,26B (1987) 208. A.M. El-Khawaga, M.F. El-Zhory and M.T. Ismail, Phosphorus and Sulfur, 33 (1987) 25. H.D. Harthough, A.I. Kosak and J.J. Sardella, J. Am. Chem. SOC., 69 (1947) 1014. H. Koniski, K. Suetsugu, T. Okano and J. Kiji, Bull. Chem. SOC. Jpn, 55 (1982)957. a) H. Lermer, W. Hoelderich and M. Schwarzmann, Ger. offen. DE No 3618964 (1987). b) W. Hoelderich, M. Hesse and F. Naumann, Angew. Chem., 27 (1988) 226. c) W. Hoelderich, Stud. Surf. Sci. Catal., 49 A (1989) 69. B. Chiche, A. Finiels, C. Gauthier and P. Geneste, J. Org. Chem., 51 (1986) 2128. P. Moreau, A. Finiels, P. Geneste and J. Solofo, J. Catal., 136 (1992) 487. G.W. Whealand, J. Am. Chem. SOC., 64 (1942) 900. G.A. Olah, Acc. Chem. Res., 4 (1971) 240. B. Chiche, A. Finiels, C. Gauthier and P. Geneste, Appl. Catal., 30 (1987) 365.

M.Guisnet et al. (Editors),Hetmgenmus Catalysis and Fine chmrifpls In (D 1993 Elsevier Science Publishers B.V.

All rights reserved.

601

Zeolite catalyzed acylation of heterocyclic aromatic compounds. I Acylation of benzofuran.

-

F. Richard, J. Drouillard, H. Carreyre, J.L. Lemberton and G. Wrot*. Laboratoire de Catalyse en Chimie Organique, URA CNRS 350. 40,avenue du Recteur Pineau, 86ooo POITIERS. ABSTRACT The acylation of benzofuran by acetic anhydride was canied out in the presence of Y zeolites in the liquid phase (6OoC, atmospheric pressure). It is shown that the reaction procedure has a significant influence on the activity of the catalyst. Deactivation takes place but the zeolite can be completely regenerated by reactivation in air. A reaction mechanism is proposed in which the acylium ion adsorbed on the zeolite reacts with non activated benzofuran. INTRODUCTION In aromatic acylation, present industrial practice involves stoichiometric amounts of metal halides as "catalysts"and of acylating agents. Aromatic heterocycles present exceptions. For example, catalytic amounts of SnC14 promote the reaction of benzofuran with acetic anhydride to give 40% 2-acetyl-benzofum [l]. More and more, solid catalysts like zeolites, clays or resins are used instead of traditional catalysts. Thus, zeolites are good catalysts for the acylation of non-heterocyclic aromatic compounds, both in the gas phase [2] and in the liquid phase [3]. The acylation of thiophene and of furan can also be carried out in the gas phase with ZSM-5 catalysts [4]. Laszlo and co-workers have shown that modified clays like montmorillonite doped with ZnCI2 can catalyse the reaction of arenes with substituted benzoyle chlorides in good yields [S] (70 to 100%). Delmas and co-workers have reported the acylation of furan by carboxylic acids with nafion-H [6] (sulfonic resin) and duolite [7l (ion exchanged phosphonic resin). One of the advantages of these catalysts is the safety of environment. Actually, the use of homogeneous catalysts causes problems of corrosion, waste and troublesome workups [8,9]. In this paper, we present the results obtained in the acylation of benzofuran in the presence of Y zeolites, in the liquid phase. EXPERIMENTAL Catalysts Y zeolites with different Si/AI atomic ratios were used (table 1). Their composition was determined by chemical analysis (CNRS, Service Central d'Analyse, Vernaison). The framework Si/Al atomic ratio was deduced from the unit cell parameter (determinedby X-Ray diffraction, ASTM method D 3942-80) using the equation given by Breck and Flanigen [ 101. SnC14 (99%)was purchased from Merck. Chemicals Benzofuran (99.5%) and the solvent chlorobenzene (99%) were purchased from Aldrich. Acetic anhydride (99%)was purchased from Janssen.

602 Procedure The reaction was carried out in a glass reactor equipped with a magnetic stirrer and heated in an oil bath. The experiments with SnC4 were conducted according to the procedure reported in the litterature [l]. The zeolites were activated just before use at 500°C under air flow in a fixed bed reactor for 12 hours. The catalyst was then transferred into the reaction vessel without exposure to ambient atmosphere. Table 1: Characteristicsof the zeolites (Aim: extra framework aluminum) Si/Alfmework atomic ratio zeolites unit cell formula

acid sites I g of zeolite

LzY82

NW.8H32.2A133Si1590384+4kF15.5

4.8

16.2 1020

CBV 760

N % . d 1.6A12.2Si189.80384,AIEiF9.8

86

0.8 1020

ZF 520

Nao.7HgsAliosi i820384Ak~5.7

18

4.8 1020

As shown in figure 1, the order of introduction of the reactants was critical. The conversion of benzofuran after 5 hours reaction was at least 10 times lower when benzofuran was introduced first. This suggests that there is a competition towards adsorption between the two reactants which is in favour of benzofuran. A standard procedure was defined in which the solvent (chlorobenzene) was added to the zeolite followed by the acylating agent (acetic anhydride). Stirring was turned on, and the mixture was heated to the reaction temperature (60°C) for 15 min before starting the addition of the substrate (benzofuran).Typical quantities of the ingredients were the following : zeolite, lg; chlorobenzene, 30 ml (295 mmol); acetic anhydride, 5.5 ml (58mmol); benzofuran, 0.5 to 5.4 ml(4.7 to 50 mmol). Samples of 0.3 ml were withdrawn and analyzed by gas chromatography using a DB1 capillary column (length : 30 m; diameter : 0.25 mm; film thickness : 0.25 pm). 2acetyl-benzofuran was the only product of mono acylation; 2,3-diacetyl-benzofuran was also formed when the reaction time exceeded 5 hours under standard conditions with ZF 520. High molecular weight non-identified by-products were also formed. 2-acetyl-benzofuran was identified by comparing its retention time to that of a commercial sample as well as by mass spectrometry and 1H NMR (both were in accordance respectively with Varian data and Aldrich NMR tables [111) after separation from the reaction mixture by flash chromatography. The stirring speed was varied between 250 and 500 revolutions per min without any change in conversion rate.

RESULTS AND DISCUSSION 1. Effect of reaction time

As shown in figure 1 for ZF 520, the activity of the catalysts seems to decrease significantly after about 5 hours of reaction. This can be due either to the deactivation of the catalyst by "coke" deposition andlor to the inhibition of the reaction by the products. 2. Comparison of zeolites with different SiIAI ratios Benzofuran acylation with acetic anhydride was carried out under the same experimental conditions in the presence of the three Y zeolites having different SiIAl atomic ratios (table 1).

603 As shown in figure 2, the best activity for 2-acetyl-benzofuran formation was obtained with ZF 520. If we compare this zeolite to LZY 82, we can assume that its higher activity is due to a higher strength of the acid sites because of dealumination [12] or to a lower level of deactivation by coke because of a lower density of acid sites [131. The intermediate behaviour of CBV 760 may due to the fact that it has a very low acidity due to the framework aluminum (it contains apparently a great quantity of extra framework of aluminum). conversion into 2-acetyl-benzofurao(md %)

conversion into 2 - a c e t y l - b f m (md %)

0 0

10

20

30

40

I

60

time (hours) *: benzofuran+ solvent. then acetic anhydride +: d c anhydride + solvent, then b f u r a n Figure 1: Acylation of b f u r a n with acetic anhydride at 60°C over Y zeolite (ZF520, ZEOCAT). Solvent: chlorobenzene.Effect of the p d m of introduction of the *taut.

time (hours)

+: HY Si/M = 86

*: HY SilAl = 18 o: HY SilAl = 4.8

Figure 2: Acylation of benzofuran with =tic anhydride at 60°C over Y zedites with d i f f m t SilAl atomic ratios.

If we compare the activities of the zeolites per acid sites deduced from the conversion obtained after 40 hours to that of SnC4 (table 2), we can see that ZF 520 has approximately the same activity as SnC4, while CBV 760 is about 4 times more active than SnC14 because of the very low amount of acid sites (high framework Si/AI atomic ratio). Table 2 : Comparison of the activities of the catalysts.

catalyst

SnC14

activity in moUmol mollmol H+

7.5

relative activity

1

zeolites LzY82

ZF 520

CBV 760

0.06

8.8

26.7

0.008

1.2

3.6

3. Effect of the quantity of catalyst

The initial activity in acylation (figure 3) is proportional to the amount of catalyst.

604 Ai (mmol2-a~etyl-beozofuren.h-~)

conversion into 2 - a c e t y l - b f m ( m d %)

0,

6 -

4 '

3'

2'

1 '

0

0.6

2

1.6

1

2.6

catalyst weight (g)

time (hours) *: 29 mmol 0: 13.3 mmol x: 4.7 mmol Figure 4 Acylation of benzofuran with acetic anhydride at 60°C over ZF 520. Effect of the substrate (benzofuran) Concentration. +: 50 mmol

Figure 3: Acylation of benmfuran with acetic anhydride at 60°C over ZF 520. Effect of the quantity of catalyst. A, : initial activity

4. Effect of reactant and product concentration

As can be seen in the figure 4, the mol percent conversion of benzofuran into 2-acetylbenzofuran decreases when the initial concentration of benzofuran increases. However, the specific rate of 2-acetyl-benzofuran formation increases and the kinetic order with respect to the substrate is of about 0.5. On the other hand, the concentration in acylating agent has practically no influence on the conversion of benzofuran (kinetic order close to zero). As shown in figure 5,2-acetyl-benzofuranhas a strong inhibition effect on the reaction. When the initial concentration of added 2-acetyl-benzofuranto the reaction mixture increases, the conversion of benzofuran decreases. The order of the reaction with respect to 2-acetylbenmfuran is of about -1 if we consider initial rates. conversion into 2-acetyl-benzofuran (mol %)

conversion into 2-acetyl-benzofuran(mol %)

." 00

-

301 :

+

+ +

I

./* I

210

0 0

6

I0

16

20

26

30

36

time (hours)

+: without 2-acetyl-benz. *: 2-acetyl-benz. 2.5 mmol 0 : 2-acetyl-benz. 5 mmol x: 2-acetyl-benz. 10 mmol

0

6

10

I6

20

26

30

time (hours)

+: activated catalyst, benzofurau 4.7 mmol *: regaerated catalyst. benzofuran4.7 mmol U: activated catalyst, benzofurau 29 mmol

Figure 5 Acylation of benzofurau with acetic anhydride at 60°C over ZF 520. Effect of the 2 - a c e t y l - b e m f m

x: regenerated catalyst, 29 mmol

conmuation.

Figure 6 Regeneration of the catalyst.

I

36

605 The acylation of benzofuran by acetic anhydride in the presence of zeolites involves two main steps : - the activation of acetic anhydride on the protonic sites of the zeolite (reaction 1) :

(CH,-C=O),O

+

ZOH

CH3 7H3@ I O=C--O--C=DH ,ZO(1)

- the transfer of the acylium ion from I to a benzofuran molecule from the liquid phase (reaction 2) :

Z0

+

CH,COOH

(2)

II

0

followed by the desorption of 2-acetyl-benzofuran. Assuming that reaction (2) is the rate-limiting step, the corresponding initial rate equation should be the following : r = k [I] [BF]

(3)

where [I] and [BF]stand for the concentrations in activated acetic anhydride and benzofuran respectively. In fact, [I] can be deduced from the Langmuir equation considering reaction (1) as a chemisorptionstep :

where h u and [AA] are the adsorption coefficient and the liquid phase concentration of acetic anhydride respectively.Then equation (3) becomes :

One may notice that the adsorption of the reaction product (2-acetyl-benzofuran) is neglected concerning the initial rate although, as we have seen, 2-acetyl-benzofuran is an inhibitor of the reaction. It is also considered that acetic acid is not adsorbed on the acid sites. According to this equation, the kinetic order with respect to benzofuran should be 1 unless the latter undergoes a strong physisorption on the zeolite leading to a kinetic order between 0 and 1 as is often the case with functional compounds [14,15]. The order with respect to acetic anhydride should also have a value between 0 and 1 depending on the

606 magnitude of h f i [AA] with respect to 1. Actually, it seems that acetic anhydride is strongly adsorbed on the acid sites since the kinetic order with respect to it is close to zero. 5. Regeneration of the catalyst The regeneration of the catalyst was examined with two different concentrations of substrate. After a first experiment, the zeolite was separated from the reaction mixture, washed with methylene chloride, dried for 2 hours at about 100°C and finally activated for 12 hours under dry air at 500°C. As can be seen in figure 6, the regenerated catalysts has approximately the same activity as the fresh one which indicates that "coke"if formed can be easily eliminated by the standard activation procedure. In both cases, the selectivity in 2-acetyl-benzofuran is between 90 and 100%. CONCLUSION

Dealuminated zeolites can be fairly active in the acylation of compounds like benzofumn although their performance depends very much on the reaction procedure. The reaction kinetics agree with a Rideal-type mechanism in which the adsorbed acylium ion reacts with a non activated benzofuran molecule. However, benzofuran seems to be strongly physisorbed on the zeolite as indicated by a cinetic order less than unity and a very slow reaction when i t is introduced first on the catalyst. A significant deactivation takes place after about 5 hours of reaction under the conditions used in this work, due to the inhibiting effect of the reaction product (or of byproducts). However, the catalyst can be completely regenerated after separation from the reaction mixture by using the standard activation procedure. REFERENCES 1. M.W. Farrar and R. Levine, J. Am. Chem. Soc., 72 (1950) 4433. 2. G. Friedhoven, 0. Immel and H.H. Schwartz, D.E. Pat. 2633458 (26.01.78); Chem. Abstr., 88 (1978) 137904J. 3. B. Chiche, A. Finiels, C. Gauthier and P. Geneste, J. Org. Chem., 51 (1986) 2128. 4. W.F. Hoelderich, H. Lermer and M. Scharzmann, Ger. offer. DE No 3618964 (1987). 5. A. Cornelis, A. Gerstmans, P. Laszlo, A. Mathy and I. Zieba, Catal. Lett., 6 (1990) 103; P. Laszlo and A. Mathy, Helv. Chim. Acta, 70 (1987) 577. 6. S. Fayed, M. Delmas and A. Gaset, Synth.Commun., 12 (1982) 1121. 7. M.E. Borredon, Z. Mouloungui, P. AudoyC, M. Delmas and A. Gaset, Revue Frangaise de Corps Gras, 31 (1984) 395. 8. R.A. Sheldon, International Conference on Precision Process Technology. Perspective for Pollution Prevention. Delft, the Netherlands, Kluwer Academic Publishers, 1992, in the press. 9. G. Pkrot and M. Guisnet, ibid. 10. D.W. Breck and E.M. Flaningen, Molecular Sieves, Society of Chemical Industry, London, 1%8, p.47. 11. C.J. Pouchert, The Aldrich Library of NMR Spectra, Edition 11, Vol. 2, p. 553A. 12. D. Barthomeuf, Mat. Chem. Phys., 17 (1987) 49. 13. M. Guisnet and P. Magnoux, Zeolites Microporous Solids : Synthesis, Structure and Reactivity, E.G. Derouane, F. Lemos, C. Naccache and F.R. Ribeiro, Kluwer Academic Publishers, 1992, p.457. 14. M. Marcziewski, J.P. Bodibo, F. Bouchet, P. Magnoux, G. Pkrot and M. Guisnet, Zeolites for Chemical Synthesis,AIChE Series, Spring Meeting, Houston, 1989. 15. G. Wrot and M. Guisnet, J. Mol. Catal., 61 (1990) 173.

M. Cuisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

607

Catalytic vapour-phase nitration of benzene over modified Y zeolites: influence of catalyst treatment Leopold E. Bertea, Herman W. Kouwenhoven, Roe1 Prins Laboratorium fur Technische Chemie, ETH Zurich, Universitatstrasse 6, 8092 Zurich, Switzerland

Abstract The catalytic vapour-phase nitration of benzene with aqueous nitric acid has been carried out on modified Y zeolites. The influence of various ultrastabilization and activation procedures on the catalytic activity and stability of the materials was investigated. The modified Y zeolites were characterized by physico-chemical methods, and correlations between catalyst properties and catalytic performance are discussed. It was found that modified Y zeolites are active catalysts for the vapourphase nitration of benzene, and that both nitrobenzene space time yield and catalytic stability are strongly dependent on the preparation procedure. Thus, stable catalysts for the vapour-phase nitration of benzene were obtained by acid treatment of low sodium ultrastabilizedY zeolites.

1. INTRODUCTION

The industrial nitration of aromatics, in particular of benzene and toluene, was developed in the two decades following the pioneering studies on benzene nitration [l] by E. Mitscherlich (1834).Traditionally the reaction is carried out in the liquid phase by reacting the aromatic compound with so called "mixed acid", a mixture of nitric acid, sulphuric acid and water [2].Concentrated sulphuric acid is used as catalyst, solvent and water binder. It is strong enough to protonate nitric acid, giving the nitronium ion, which is the electrophilic reactive species. The generally accepted mechanism for the nitration of aromatics with "mixed acid" was elucidated by lngold [2].Sulphuric acid acts as a solvent, increasing the aromatics concentration in the boundary acidic layer, where the reaction occurs. Both isothermal and adiabatic plants are used in industry for the continuous nitration of benzene and toluene. Despite conversions of about 98% or more in the case of benzene, the traditional "mixed-acid" process has some drawbacks. Firstly, concentrated sulphuric acid in a molar ratio toward the reacting aromatic close to unity (isothermal units) or larger (adiabatic units) is required by the process [3].After reaction, the spent sulfuric acid must be reconcentrated for recycling, or neutralized. The concentration step is energy consumptive and requires corrosion

608

resistant materials, whereas the neutralization produces large amounts of waste sulfates. Secondly, the nitrophenolic b -products of the "mixed acid" process are soluble in water, high1 toxic to marine li 8, and cannot be processed by standard biological treatments y3.41. since they have bactericidal properties. Traditionally they are extracted with benzene and incinerated, causing some safety problems owing to the thermal lability of such compounds, although a new solution involvin has temperature/pressure retreatment of the aqueous effluents (120 bar, 573 ) high recently been claime [4]. Thus, an acceptable decontamination of the aqueous effluents can be reached only at relatively high costs and energy consumption. Since the pioneering work of McKee and Wilhelm [5], who used silica gel as a solid catalyst for the heterogeneous vapour-phase nitration of benzene and toluene, meny additional catalytic systems have been proposed either as liquid phase or vapourphase processes, in order to avoid the problems related to the use of sulphuric acid. Nitric acid, NO / N 04,and acyl nitrates have been used as nitrating agents. Mercuric salts (k], sufphonated polyorganosiloxanes [7], acidic resins [8,9], mixed oxides [10,11], modified clays [11,12], zeolites [13-151, supported sulphuric acid [16] and supported sulfonic acids [17] have been reported as catalysts. A HN03 / anhydrous sulphate (as catalyst and desiccant) s stem for liquid phase nitrations has recently been described by Gubelmann et al. fia]. Since nitrobenzene is a base chemical, only HN03 and or NO, / N 0 can be considered as industrially useful nitrating agents [18]. As for the catalysf, t i e main requirements are a good nitrating activity, selectivity, time on stream stability, and re enerability. Zeolites may fulfil these requirements, and in contrast to sulphuric acid t ey are not corrosive materials. Some applications of commercial zeolites for the vapour-phase nitration of benzene were recently reviewed by Halderich and van Bekkum [ 191. In the present study, we investigated a series of systematically modified Y zeolites as catalysts for the vapour-phase nitration of benzene. The activity, selectivity and stability of the samples was measured in a microflow fixed bed integral reactor, at 443 K and atmospheric pressure. The influence of sodium and aluminum content, and of the textural properties on the catalytic performance was investigated. The effect of the different treatments on some physico-chemical properties of the catalysts was investigated by means of standard characterization techniques (SEM, AAS, XRD, low pressure Ar adsorption, *9Si MAS solid state NMR). In order to obtain viable vapour-phase nitration catalysts, it is important to study both the factors leading to a deactivation of the modified Y zeolites, and the conditions influencing the selectivity to the desired product. In the present study, we show that zeolites can afford nitrobenzene, free of nitrophenolic by-products, and containing only traces of dinitrobenzenes at high benzene and nitric acid selectivity.

Y

a

s

PI

2. EXPERIMENTAL

A set of systematically modified Y-zeolites was prepared using a commercial synthetic NaY (PY44-Na) obtained from CU Chemie Uetikon as the starting material. The NaY was subjected to a 3-fold NH4N0 (lM, 5 mVg, 363 K, 1 hr) exchange, and the resulting NH4Y was partially dried (393 K, 3 hr) before being subjected to a deep bed calcination under self steaming conditions at 773 K (3 hr), giving LTHY1, and at 1023 K (2 hr) iving HTHYl. The exchange/calcination procedure was repeated twice with LT!-I Y1 and HTHY1, resulting in LTHY2 resp. HTHY2, and LTHY3 resp. HTHY3. The HTHY1-3 samples were additionally leached with 2M

609

, 363 K, 4.5 hr), dried (393 K, 10 hr) and activated at 773 K (0.5 hr) leading(lo HN03 to HT m# Y1 D-3D. All the samples and treatments are listed in Table 1.

Table 1. Catalysts and treatments Sample Parent Treatment zeolite LTHY1 LTHY2 LTHY3 HTHY1 HTHY2 HTHY3 HTHY1D HTHY2D HTHY3D

NaY LTHY1 LTHY2 NaY HTHY 1 HTHY2 HTHY 1 HTHY2 HTHY3

3xlM NH4N03 (1 hr), 393 K (3 hr), 773 K (3 hr) 3xlM NH4N03 (1 hr), 393 K (3 hr), 773 K (3 hr) 3xlM NH4N03 (1 hr), 393 K (3 hr), 773 K (3 hr) 3xlM NH4N03 (1 hr), 393 K (3 hr), 1023 K (2 hr) 3xlM NH4N03 (1 hr), 393 K (3 hr), 1023 K (2 hr) 3xlM NH4N03 (1 hr), 393 K (3 hr), 1023 K (2 hr) 2M HNO3 (4.5 hr), 393 K (10 hr), 773 K (0.5 hr) 2M HNO3 (4.5 hr), 393 K (10 hr), 773 K (0.5 hr) 2M HNO3 (4.5 hr), 393 K (10 hr), 773 K (0.5 hr)

The total amount of Si, Al, and Na was determined by atomic absorption spectrometry on a Varian SpectrAA.10 spectrometer after dissolution of the samples in HF/H,SO,. A Siemens D-500 diffractometer (CuKa radiation) was used for the XRD investigations. The structure of the modified Y zeolites was checked in the 2-9 range 5-60, whereas the XRD crystallinity and the unit cell parameter of the previously equilibrated probes (laboratory air) was measured in the 2-9 range 25-61 (step size: O.0lo, step time: 10 s), using Si powder as the internal standard. The relative XRD crystallinity was calculated by summing up the intensities of 8 peaks between 2-9= 15.4 and 34 [20], and by arbitrarily taking the sum found for NaY as corresponding to 100% XRD crystallinity. 16 peaks between 2-8 = 10 and 33 were used for the calculation of the unit cell parameter The argon adsorption measurements were carried out on a Micromeritics SAP 2000M volumetric analyzer. Prior to analysis, the samples (100 mg) were degassed under vacuum Torr) at 723 K for at least 4 hours. The adsorption isotherms were taken at the tem erature of liquid nitrogen (77.3 K) in the relative pressure range P/P between 10- and 0.2 by adding subsequent argon doses of 3 ml (STP). The isotlerm data were computed according to the BET equation in the relative pressure range between 0.01 and 0.14 for the calculation of the BET total surface area. The t-plot method of Lippens and De Boer [21] was used to calculate both the micropore volume and the "external" (i. e. not microporous) surface area (ESA) of the samples. The method was adjusted by measuring the surface area of a non porous standard glass material [22] at the same conditions. Solid state 29Si MAS NMR measurements on a Bruker AMX 400 were used to investigate the degree of framework dealumination upon the treatments. The benzene nitration experiments were carried out at atmospheric pressure in a stainless steel fixed bed downflow microreactor (inner diameter 10 mm, length 80 mm) equipped with a central thermowell (1.5 mm cross section). The catalyst powder was pressed, crushed and sieved to pass 25 mesh and to be retained on a 45 mesh screen. 1.5 g (3.2 ml) of fresh catalyst were used for each run, and were packed between two plugs of glasswool, whereas 2 mm glass beads were put on top of the upper plug in order to complete mixing before the reaction. Benzene (0.03 ml/min)

3

B

610

-80

-100 -120 -80 -100 -120 -80 -100 -120

IPPMI

WMl

IPPMI

Fig. 1. 29Si MAS NMR spectra of the starting NaY and of the series of modified Y samples. The signals at -1 05, 100, -94, and -89 ppm (in the spectrum of Nay) correspond to Si (n Al) orderings in the structure, with n=0-3.

was fed with an HPLC pump, and HNO, 65 wt% with a syringe displacement pump. The WHSV of benzene was 1.0 h-l, the HNO, to benzene molar ratio was kept at 0.5. The reactor and separate vaporizers for benzene and nitric acid were placed in a fluidized sand bath at a constant temperature of 443 K. Nitrogen (10 ml/rnin) was used as the carrier as. The reactor effluent was recovered in acetone kept at 278 K and periodically ana yzed over a time interval of 1 hr. The first sample was taken after a lining out period of 1 hr. The runs were interrupted after about 20 hours on stream. HN03 was analyzed by acidimetric titration, whereas benzene and the products of nitration were analyzed by off line gas chromatography (cool on column injection), using o-nitrotoluene added to the collected probes as the internal standard.

a

3. RESULTS AND DISCUSSION

The measured ph sico-chemical roperties of the progressively dealuminated Yzeolites are listed in able 2. The NMR spectra of the samples are shown in Fig. 1. Four signals, at about resp. -105, -100, -94, and -89 ppm occur in the spectrum of the parent Na-Y, corresponding to 4 different Si (n Al) environments, with n = 0 - 3 [23]. The crystallites were identified by SEM as intergrowths of about 3 pm. The different treatments applied on the parent NaY yielded three distinct groups of modified Y zeolites. The samples treated hydrothermally at 773 K (LTHY 1-3) are highly crystalline materials, as reflected by the relative XRD crystallinity, the total BET surface area, and the micropore volume very close to the accepted value for the faujasite structure [24]. The increase of the ESA is due to the formation of mesopores, as expected upon Y hydrothermal stabilization [25].

f!

2g!4

61 1

Table 2. Physico-Chemicalproperties of the catalysts Sample Si/Ala) (bulk) NaY LTHY1 LTHY2 LTHY3 HTHY1 HTHY2 HTHY3 HTHYlD HTHY2D HTHY3D

2.2 2.3 2.5 2.7 2.3 2.9 4.2 31.2 32.0 22.9

Na20a) (wt%)

a,

rel.XRD cryst(%)b)

(A)

24.647(8) 100 84 24.63(1) 84 24.58(4) 24.537(5) 77 24.549(9) 74 81 24.38(1) 24.339(4) 70 XRD amorphous 24.20(1) 32 <0.006 24.28(1) 70 9.9 2.8 0.17 e0.006 2.8 0.18 <0.006 0.05 0.02

-

BET SA (m2/g)c)

ESA micro.Vol (m2/g)d) (cm3/g)d)

706 727 756 745 639 647 667 313 666 680

86 107 101 125 95 140 170 135 349 252

0.285 0.287 0.300 0.291 0.252 0.237 0.235 0.089 0.166 0.21 1

a) Measured by atomic absorption spectrometry. b, Reference: untreated Nay. c),d) Low pressure argon adsorption. dl t-Plot method of Lippens-De Boer, Harkins-

Jura equation. The removal of structural Al is evident from the decrease of the unit cell parameter ao, and the increase of the Si (0 Al) 29Si NMR signal, corresponding to a decrease of the Si (1 Al), resp. disappearance of the Si (n > 1 Al) signals, originally present in NaY. The samples which were treated at 1023 K (HTHY1-3) show nearly the same sodium content, but a higher structural dealumination level than LTHYl-3, as reflected by the lower unit cell parameters and the higher relative intensities of the Si (0 Al) as compared to the Si (1 Al) 29Si NMR signals. At lower sodium content, the larger Al removal from the structure due to the higher temperature induces a more developed mesoporous region, as indicated by the larger ESA in HTHY2-3, as compared to LTHY2-3. The total BET surface areas of the catalysts treated at 1023 K are smaller than those of the samples treated at 773 K, owing to a larger loss of crystallinity, also indicated by the smaller micropore volumes in HTHY1-3. Leaching of HTHYl with 2N HNO, resulted in an XRD amorphous material with 50% of the total BET surface area and 35% of the micropore volume, as compared with HTHYl. Acid treatment of HTHY2 leads to a 50% loss of XRD crystallinity, a decrease of the micropore volume and an unexpectedly high ESA, indicating a high degree of mesoporosity. On the other hand, acid treatment of HTHY3 results in a dealuminated Y zeolite with the same crystallinity, a high ESA and, at the same time, an only slightly lower micropore volume. The acid treatment removes the non structural aluminum to a larger extent, as indicated by the strong increase of the bulk Si/AI ratios. The decrease of the unit cell parameter upon acid leaching of the ultrastabilized HTHY2-3 indicates a further structural Al removal. Further changes of the Si (1 Al) and Si (0 Al) 29Si NMR signals comparing the ultrastabilized to the acid leached materials are outside experimental accuracy, due to the already very low Si (1 Al) signals in HTHY2-3. The poor structural stability of HTHY1 towards acid leaching, respectively the increasing stability of HTHY2-3 is due to the gradually decreasing structural aluminum content in the HT samples [26]. The results of the vapour-phase nitration experiments at both 4 and 20 hr on stream are reported in Table 3, including comparative results obtained with Zeolon 900H of Norton. The nitrobenzene yield is based on the molar input of HNO,, and the HNO, selectivity on the reacted HNO,. The benzene selectivity was always better

612

than 99.9%, the only condensed by-products being traces of dinitrobenzenes (m- and p-) with all catalysts except with HTHYlD and HTHY3D, where 100% benzene selectivit was observed. The nitrous gases arising from non-selectivethermal andor catalytic NO3 decomposition were not analyzed.

L

Table 3. Experimental results (at 4 and 20 hours on stream) Catalyst

Yield % NBz.~) 4 hr 20hr

Convers.% B z . ~ )Sel.% HNO3 4 hr 20 hr 4 hr 20 hr

LTHY1 LTHY2 LTHY3 HTHY1 HTHY2 HTHY3 HTHYlD HTHY2D HTHY3D Z900HC)

61.5 67.4 70.4 58.1 76.3 78.3 56.5 80.9 75.7 25.0

31 33 35 29 38 39 28 41 38 14

54.9 63.2 65.8 54.6 74.8 76.2 48.3 76.5 75.0 10.0

28 31 33 27 37 38 24 39 37 5

99.9 88 92 90 93 88 96 91 86 99.9

STY NBz (hT1) 4hr 20hr 0.48 0.53 0.55 0.45 0.60 0.61 0.44 0.63 0.59 0.22

99.9 88 90 93 92 86 97 92 91 99.9

0.43 0.49 0.51 0.43 0.59 0.59 0.38 0.59 0.59 0.03

Based on HN03 input. WHSV (benzene) = 1.0 h r l , HN03/benzene = 0.5-mol/mol. Benzene selectivit to nitrobenzene: 99.9%. Only traces of dinitrobenzenes were detected. Detection imit for the dinitrobenzenes: 0.02% based on benzene input. C) Comparable data obtained with Zeolon 900H (Norton). a) b)

The modified Y zeolites showed both higher activity and stability than the commercial catalyst Zeolon 900H (Norton) based on mordenite. The catalytic performance of the samples reflects the extent of dealumination, the sodium content, and the textural properties of the modified Y zeolites (Table 3). The dependence of the activity after 4 hr on the framework Al content, as calculated from the unit cell parameter with the Breck-Flanigen equation [27] is depicted in Fig. 2. At low sodium content, both nitrobenzene yield and stability (Table 3) increase with decreasing Al content. At relatively high sodium content (LTHY1, HTHYl), the beneficial effect of lower Al content is more than compensated by the deactivating presence of sodium. The highest combination of activit and stability is observed with the acid leached and previously ultrastabilized HTHY3 in spite of an even lower Al content in HTHY2D, owing to the higher crystallinity of HTHY3D (Table 2). A strong correlation is observed between the catalytic activity and the ESA of the modified Y zeolites, although a large ESA is not sufficient for high activity and stabilit , as shown by the poorer catalytic performance of HTHY 1 D as compared with HTH 2, both with similar ESA but different crystallinity (Tables 2,3). The increase of catalyst performance with increasin ESA, i.e. meso- and macroporous surface area, is due to the improved accessibi ity of the reactants to the active constricted zeolitic system, and an improved products desorption. In order to explain the observed increase of activity and stability with decreasing Al content, it is proposed that in the case of vapourphase nitration and other reactions involving strongly polar species, structural Al deactivates the zeolite due to its strong interaction with polar molecules. Accordingly, increasing their concentration in the micropores slows down the overall rate of nitration, thus more than compensating the enhancing effect due to Al acidity. In this case, no maximum activity is observed at an WAI ratio of about 7 (as for n-heptane

i

1

B

613

to the maximum acid strength SiAI limit predicted by structure, above which predominantly isolated structural Al atoms are present.

I

1

12

24

n Al /

36 U.C.

47

68

[-I

Fig. 2. Influence of the framework Al content on the nitrobenzene yield based on HN03 at 4 hr on stream. Although the benzene selectivity was very close to 100% in all experiments, virtually no dinitrobenzene (DNB) has been observed at both the lowest and highest HNO3 conversions that were obtained, i.e. on HTHYlD resp. HTHY3D. In the first case, the hi h selectivity is main1 due to the lower benzene conversion, whereas the absence of NB at the 75% H 0 conversion achieved over HTHY3D is probabl due to the combination of high crysfallinity, very low Al content (low acid site densityr and high mesoporosity (improved diffusion patterns) of HTHY3D.

Eb

NY

4. CONCLUSION

Modified Y zeolites are active catalysts for the vapour-phase nitration of benzene with 65 wt% HN03 (301. Their activity and stability depends strongly on the preparation procedure. The best catalysts were obtained by acid treatment of hydrothermally treated low sodium Y. A high crystallinity and extra-zeolitic surface area are required for active and stable nitration catalysts.

ACKNOWLEDGEMENT

a

The present work was supported by the Kommission zur Farderun der Wissenschaftlichen Forschung, project Nr. 1814/1, as a joint project with CU C emie Uetikon.

614

REFERENCES H.G. Franck and J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer Verlag, Berlin, 1988, p. 193 Ullmann's Encyclopedia of Industrial Chemistry, 5th edn. vol. A1 7, Verlag Chemie, Weinheim,l991, pp. 415-418 J.J. McKetta, Encyclopedia of Chemical Processing and Design, vol. 31, Marcel Dekker, New York, 1990, pp. 165-188 W. Larbig and G. Scharfe, in Produktionsintegrierter Umweltschutz in der chemischen Industrie, DECHEMA, 1990, pp. 63-64 R.H. McKee and R.H. Wilhelm, Ind. Eng. Chem., 28 (1936) 662 L.M. Stock and T.L. Wri ht, J. Or . Chem., 42 (1977) 2875 See also ibid., J. Org. Clem., 4471979) 3467 S. Suzuki, K. Tohmori and Y. Ono, Chem. Lett. (1986) 747 See also ibid., J. Mol. Cat., 43 (1987 41 G.A. Olah, V.V. Krishnamurthyand .C. Narang, J. Org. Chem., 47 (1982) 596 US Datent, 4 234 470, 1980, to Du Pont European patent, 184 569, 1985, to Monsanto US atent, 5 004 846, 1991, to Sumitomo A. Ckorn6lis, A. Gerstmans and P. Laszlo, Chem. Lett. (1988) 1839 J.M. Bakke, J. Liaskar and G.B. Lorentzen, J. PraM. Chem. 324 (1982) 488 See also US patent, 4 418 230, 1983, to AB Bofors US patent, 4 426 543, 1984, to Monsanto European patent, 53 031,1984, to Monsanto European patent, 402 207, 1990, to Sumitomo Y. Ono, K. Tohmori, S. Suzuki, K. Nakashiro and E. Suzuki, Stud. Surf. Sci. Catal. 41 (1988) 75 M.H. Gubelmann, C. Doussain, P.J. Tire1 and J.M. Popa, Stud. Surf. Sci. Catal. 59 (1991) 471 See also European patent application, 385 884, 1991, to Rhone-Poulenc W.F. H6lderich and H. van Bekkum, Stud. Surf. Sci. Catal., 58 (1991) 677 R. Szostak, Stud. Surf. Sci. Catal., 58 (1991) 162 S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd edn., Academic, London, 1982 Standard non porous glass (004/16818/00), LOT NO: 7-30-863-6 (Micromeritics) G. Engelhardt and D. Michel, High-ResolutionSolid-state NMR of Silicates and Zeolites, Wiley, Chichester, 1987 D.W. Breck and R.W. Grose, in Molecular Sieves, W.M. Meier and J.B. Uytterhoeven (Eds.), ACS Symposium Series 121, Washington D.C., 1973, p 319 H. Ajot, J.F. Joly, J. Lynch, F. Raatz and P. Caullet, Stud. Surf. Sci. Catal., 62 (1991) 583 J. Scherzer, in Catalytic Materials; Relationship between Structure and Reactivity, ACS Symposium Series 248, T.E. White Jr. et al. (Eds.), Washington D.C., 1984, p 157 D.W. Breck and E.M. Flanigen, Molecular Sieves, SOC.of Chem. Ind., London, 1968, p. 47 A. Corma, V. Fornes, A. Martinez, F.V. Melo, 0. Pallota, Stud. Surf. Sci. Catal., 37 (1988) 495 D. Barthomeuf, Mater. Chem. Phys., 17 (1987) 49 Swiss patent application, 1995 92-0, 1992

4

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

M.Guisnet et al. (Editors), Hete?vgmmusCatalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

AlP0,-TiO, catalysts. V. Vapor-phase rearrangement of cyclohexanone oxime

615

Beckmann

F. M. Bautista, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M. S. Moreno Organic Chemistry Department, Faculty of Sciences, University of Cordoba, Avda. S.Albert0 Magno EJn, E-14004Cordoba, Spain.

Abetract Vapor-phase Beckmann rearrangement of cyclohexanone oxime over AlPo, (AP) and AlF'04-Ti0, (AFTi, 26-76 wt%) catalysts was investigated. Apparent rate constants and activation parameters were calculated in terms of the kinetic model of Bassett and Habgood for first order reaction processes. In all cases the selectivity to E-caprolactam ( S a d increased with reaction temperature and, furthermore, at the same level of conversion, ApL'i catalysts exhibited better ,,S values than AP catalysts. Moreover, although the increase in TiO, content ,,S continuously increased. From the strongly decreased oxime conversion, the distribution of reaction products and their selectivity curves, which showed that amide and nitrile were obtained by parallel reactions, an outline of the catalyzed rearrangement was presented.

1.INTRODUCTION E-Caprolactam is an important starting material for the production of nylon-6. It is synthesized by the Beckmann rearrangement reaction of cyclohexanoneoxime catalyzed by a solid acid catalyst. Many solid acid catalysts, such as mixed boron oxide [l-31, Si0,-Al,O, [4,6], metal phosphates [6-81 and modified zeolites [3,9-121, are reported to catalyze the cyclohexanone oxime rearrangement. The acid function of the catalyst is essential to effect the rearrangement reaction. Moreover, AlPo~sare able to catalyse a variety of reactions of interest in petrochemical processes. Their texture, structure and acid-base character as well as their catalytic properties are dependent on a number of variables such as preparation method, P/Al ratio, or calcination temperature. Furthermore, these properties can be improved or modified by the incorporation of a metal oxide ( b O , , SiO,, TiO,, Zro, or LO). The catalytic behaviour of these materials depends on the metal oxide used, their composition and/or their modulated acid properties [13-191. In this research, we study the Beckmann rearrangement of cyclohexanoneoxime (OX)catalyzed by a series of ALPO,-TiO, catalysts, of varying composition (26-76wt% TiOJ, compared to the AlPo, catalysts similarly obtained.

616

2.1. Catalysts AW04 (AP)catalysts were obtained by precipitation, from aluminum chloride and H W 4 aqueous solutions, with aqueous ammonia (A), ethylene oxide (E) or

propylene oxide (P). AW04-Ti02(APT&)catalysts of varying composition (AlP04-Ti02:3-1, 1-1and 1-3weight ratio) were obtained by adding Ti02to a reaction medium where the precipitation of AlP04,from aluminum chloride and H J 0 4 aqueous solutions was initiated by the addition of aqueous ammonia (AFTi-A), ethylene (AFTi-E) or propylene oxide (AFTi-P).Furthermore,the total precipitation of AlPO, was carried out by the addition of aqueous ammonia. After filtration, washing and drying at 390 K, all the solids were calcined at 923 K for 3 h. Details on the characterization of all catalysts have been previously described [13,16,17]. The surface area,S and acid-baseproperties, measured by a spectrophotometricmethod, are collected in Table 1. Table 1 Surface area (mvg) and acid-base (ymovg) properties of AP and ApI'i catalysts Catalyst

,s

Acidity

PY

AN

DTBMW

Basicity

BA

227 AP-P 40 67 166 229 242 AP-E 267 102 90 266 109 AP-A 190 48 49 200 APR-P-31 224 66 238 43 268 167 m1-P-11 168 31 236 46 69 44 AH'i-P-13 16 126 16 86 264 m1-E-31 198 171 26 192 AFTi-A-31 416 43 146 60 PY pyridine; AN: aniline; D"BMPY 2,6-ditert-butyl-4-methylpyridine; B A benzoic acid. 2.2. Catalytic measuremente The reactions were carried out in a pulsed microcatalytic fixed bed reactor at temperatures in the range 473-673 K using dried nitrogen as the carrier gas. The catalyst in the reador was fist standardized under nitrogen flow (30mL m i d ) at a temperature of 473 K for 2 h. A methanovcyclohexanone oxime molar ratio of 6 and pulses of 1 yL were used. Reaction products were analyzed by GC with FID by using a column (2m x 0.3 mm) packed with 10% UCC on Chromosorb W 8W00 at 363 K (4 min)-463 K (16K/min). Reaction products of the rearrangement of cyclohexanoneoxime on the different catalysts were found to be e-caprolactam, cyclohexanone, 6-cyanopent-l-ene, 2-cyclohexen-l-one and, in minor amounts, aniline, 2-methyl-pyridine and other

617

unidentified produds. To further clarify the reaction pathway, we have carried out additional catalytic runs using e-caprolactam instead of cyclohexanone oxime. Thus, we have found that e-caprolactamdoes not undergo any reaction process at temperatures in the range 473-673 K and that neither e-caprolactam nor cyclohexanone oxime react with any of the other reaction products. Moreover, a small condensation reaction between cyclohexanone (and 2-cyclohexen-l-one)and aniline was found although the reaction products were insignificant. Furthermore, thermal reaction was negligible.

3. RESULTS AND DISCUSSION In the absence of a boundary layer and internal and external diffusional influences, the cyclohexanone oxime conversion follows the requirements of Bassett-Habgood kinetic treatment [%I]for first order reaction processes in which the rate determining step is the surface reaction:

In M1-&31=RT k~ (WP)

(1)

where GXis the total oxime conversion, k the rate constant of surface process, K the adsorption constant of the substrate on the catalyst, W the catalyst weight and F the flow rate of carrier gas. In A, Apparent rate constants (kK& at 673 K and activation parameters AH', A S and AG') for all catalysts are compiled in Table 2. A least-squares regression analysis shows correlation coefficients over 0.99. A t-test of significance, performed on the regression coefficients, show that these values are significant at levels over 1%.At least three measurements were used to calculate each

ma,

wp

Table 2 at 673 K and activation parameters ma,In A, AH', Apparent rate constants A S and A C ) for vapor-phase rearrangement of cyclohexanone oxime to e-caprolactam on A P and ApTi catalysts

mx)

E,

Catalyst

1nA'

AP-P

AP-A m - P - 31 m-P-11 ApTi-P-13 APTi-E-31 APTi-A-31

13.2 20.3 8.9 22.4 7.3 4.0 16.8 7.6

6.8 6.9 7.4 7.7 8.6 9.4 9.3 7.6

-A is expressed in movatm g s.

AS*

AG'

(Kcal/mol) (cab01 K) (Kcallh.lo1)

(mol/atm g s) (Kcalhol)

AP-E

AH'

-0.7 -1.0 -1.0 0.6 -0.1 0.4 1.7 -0.2

6.7 4.8 6.3 6.6 7.6 8.3 8.2 6.4

-172.2 -172.6 -172.8 -169.3 - 170.8 -169.9 -167.3 -171.0

104.3 103.7 106.3 103.7 106.4 106.6 104.1 104.4

618

The resulta in Table 2 show that a relationship exists between surface acidity and catalytic activity od AP-P > AP-A. Besides, the experimental data in Table 2 clearly show how the catalytic activity of AP can be modified by the incorporation of TiO,. The most striking feature of the activity studies is that among the tested AFTi catalysts, those obtained in propylene oxide and with lower TiO, content showed higher activity for cyclohexanone oxime conversion than did the other acid Catalysts. Thus, M i - P - 3 1 catalyst exhibits an increase in activity about twice higher than that for the starting AP-P catalyst. Moreover, although the rate constant depends on the precipitation medium, the most important influence is that of catalyst composition. Thus, we observe a strong decrease in catalytic activity as the TiO, content increases; however this decrease values (at 673 K) is higher at 60 wt% TiO, than at 76 wt?4 TiO, so that the KX on APTi-P-31, Apl'i-P-11and ApTi-P-13are, respectively, 22.4, 7.3 and 4.0. The same effect was found in cyclohexene skeletal isomerization [13,14] and in the alkylation of phenol with methanol [21,22]. So, the results can be well interpreted in terms of the number of more accessible strong acid sites (measured vs. AN and DTBMPY).

As far as the selectivity of the cyclohexanone oxime reaction is concerned, Table 3 compares the selectivities of E-caprolactam (SccApR),cyclohexanone (So&, 2-cyclohexen-l-one,&,S( 6-cyanopent-l-ene (S,) and aniline ( S , ) of APTi catalysts to that of AP catalysts under comparable extents of oxime conversion RX-60 mol%). Moreover, molar selectivities to other reaction products, including 2-methylpyridine, remained almost unchanged with the catalyst and strongly decreased with the increase in the reaction temperature. Table 3 Oxime conversion &J and product selectivities (S), a t &,*50 mol%, in the rearrangement of cyclohexanone oxime t o E-caprolactam over AP and AETi catalysts Catalyst

T (K)

Apl'i-P-31 APTi-P-11 APTi-P-13 APTi-E-31 Apl'i-A-31 AP-E AP-P AP-A APTi-A-31 APTi-A-31 APTi-A-31 AMY-A-31 APTi-A-31

523 673 673 648 673 473 623 673 473 623 673 623 673

&x

, ,s

so,

SENONE

(mol%) (molYo) (mol%) (mol%)

56.3 48.0 66.1 54.6 51.2 48.2 46.0 46.0 10.9 23.3 61.2 84.7 92.7

41.4 44.1 47.7 33.0 32.7 7.8 23.6 36.6 12.3 26.3 32.7 39.3 46.1

36.9 23.7 20.4 43.6 40.3 38.8 66.9 31.1 63.7 62.1 40.3 34.6 28.0

7.7 22.3 16.8 9.4 13.2 26.4 8.4 18.6 5.4 9.6 13.2 13.9 13.6

sCN

sAN

(mol%) (mol%) 4.3 4.7 5.8 6.3 5.8 6.0 6.0 6.0 3.0 3.2 6.8 6.5 6.2

3.1 3.0 3.2 2.4 1.6 4.6 3.6 3.6 1.8 2.0 1.6 3.3 3.0

619

As for the ,,S it should be noted that a decrease in selectivity was found upon increasing acidity. Thus, the more acidic catalyst (AP-E) showed the lowest ,,S value. Thus, independently of the conversion level, the values ,of, S were found to increase on going from AP-E to AP-P and to AP-A catalysts. In this sense, in the literature [l-3,7] a decrease in Sic, at increasing surface acidity is reported, as consequence of the accelerated decomposition of the cyclohexanone increases with reaction oxime into cyclohexanone. Besides, in all cases,-,S temperature and, at the same level of conversion, A F T 1 catalysts exhibited better ,,S values than AP catalysts. Furthermore, although the increase in TiO, content strongly decreases oxime conversion, the ,,S continuously increases. Moreover, the influence of reaction temperature on conversion and selectivity in the case of ATPI-A-31 catalyst is presented in Table 3. It can be seen how and ,,S values and increasing the reaction temperature increases both the hX drops, and , , ,S ,S and ,S increase with a rise in temperature. how, S Furthermore, for the most active catalysts, Seem increased with temperature although it began to decrease above 623 K due t o the rise in side reaction products. On the other hand, at all temperatures, catalysts deactivate with pulse number but catalyst decay, due to the formation of coke on the catalyst an@or to the formation during the reaction of basic products, such as aniline and 2-methylpyrib, is slight on AP and AFTi catalysts. values while Moreover, poisoning with pyridine (Table 4) slightly decreases GX ,,S remained almost unchanged. Furthermore, the addition of carbon disulfide or l-butanethiol produce a decrease in the oxime conversion as well as in the ,,S values (Table 4). In this sense, previous results [8] show that cyclohexanone oxime also rearranges to a great extent when a solution of oxime in neat pyridine is used as the reactant. Table 4 Poisoning experiments (at 573 K) in cyclohexanone oxime rearrangement over API'i-E-31 catalyst Poison

1 pL pyridine 1 pL s,c 1 pL butanethiol

&x

,s

SAN

SeCAPR

SONE

SFNONE

(mol%)

(mol%)

(mol%)

(mol%)

(mol%)

(mol%)

73.6 66.1 65.2 63.3

37.0 39.9 32.2 32.8

38.3 21.8 30.0 28.3

9.5 20.7 22.2 22.2

4.6 5.0 5.9 4.9

5.3 7.2 5.3 6.0

In order to obtain more knowledge about the reaction sequence, we have constructed the OPE curves by plotting fractional conversion to each reaction product against the cyclohexanone oxime total conversion&J, for different weight ratios of catalyst with respect to introduced oxime (Figure l), such as recently described by Best and Wojciechowski [23]. In the OPE curves, we have included experimental data corresponding to different temperatures and contact times on the same diagram. Using such a procedure, an insignificant scattering of the data

620

20

0

no

40

X,

80

loo

(moilbL)

5-cyanopent- 1-ene 4

2

I

0

20

40

60

80

Xm (mol%)

100

0

0

20

40

X,

60

80

KH)

(mollb)

Figure 1. OPE selectivity curves. is evident in the selectivity diagrams and clear tendencies can be observed from these curves. Thus, OPE selectivity curves indicated, in all cases, that E-caprolactam, cyclohexanone and 6-cyanopent-1-eneare primary products coming from cyclohexanone oxime by rearrangement, hydrolysis and fragmentation. However, cyclohexanone is an unstable product since a maximum o n its OPE curve is found. F'urthermore, 2-cyclohexen-l-one, aniline and 2-methylpyridine are secondary products. On the other hand, maprolactam seems t o be a primary plus secondary reaction product (Fig. 1). Thus, the reaction network of the cyclohexanone oxime conversion is complex due to the simultaneous intervention of a large number of reactions. An outline of catalyzed reactions was presented in Figure 2. The first step would imply the chemisorption of the oxime on the catalyst, and then either a rearrangement reaction could take place or a fragmentation process which would lead to the intermediate I (6-cyanopentyl carbenium ion) being unstable. This in its turn could evolve to a more stable structure such as 11. The intermediate I1 would provide an explanation for the appearance of 2-methylpyridine as well as the existence of an alternative pathway for the forma-

621

6 0 6 OCAT

N'

CATH

Rearr.

___)

1

___)

I

CATH H a

Fragm.

-

t H . O -0

cN %Hg Ritter

.

(1)

J

C=N

0J-r-I-

Figure 2. Reaction pathway. tion of e-caproladam according to its OPE curve. Finally, 6-cyanopent-1-ene and cyclohexanone could be obtained by proton loss from I and I1 and hydrolysis of the initial oxime, respectively. In this way, e-caprolactam and 6-cyanopent-1-ene formation follow parallel reaction paths (Fig. 1). 4. CONCLUSIONS

The results show that a relationship exista between surface acidity and catalytic activity. Besides, the catalytic activity of AP can be modified by the incorporation of TiO, and thus, m i - P catalysts with lower TiO, content showed higher activity for cyclohexanoneo x h e conversion than did the other acid catalysts. Also, a strong decrease in catalytic activity is founded as the TiO, mntent increases; however this % so that the values (at decrease is higher at 50 wtolb TiO, than at 76 ~ 6 TiO, 673 K)on AF'"i-P-31,AFE-P-11and AFE-P-13are, respectively, 22.4,7.3 and 4.0. Moreover, in all cases, ,,,S increases with reaction temperature and, at the ,,S values than AF' same level of conversion, Wi catalysts exhibited better catalysts. Furthermore, although the increase in TiO, content strongly decreases oxime conversion, the ,,S continuously increases.

wx

622

6. ACKNOWLEDGMENT8

The authors acknowledge the subsidy received from the DGICYT (Project PB8!3/Q340), Minisbrio de Educacion y Ciencia, and from the Consejeria de Educacion y Ciencia (Junta de Andalucia). They also acknowledge the linguistic revision of the manuscript carried out by Prof. M. Sullivan

6. REFERENCES

1 S. Sato, K.Urabe and Y. Izumi, J. Catal., 102 (1986)99. 2 S.Sato, S. Hasebe, H. Sakurai, K. Urabe and Y. Izumi, Appl. Catal., 29 (1987) 109. 3 T. Takahashi, K.Ueno and T. Kai, Can. J. Chem. Eng., 69 (1991) 1096. 4 0.Immuel, H.H. Schwarz, H. Starke and W. Swoden, Chem. Ing. Tech., 56 (1984)612. 6 Y.Murakami, Y.Saeki and K. Ito, Nippon Kagaku Kaishi, (1978)21. 6 J. Haber and V. Szybalska, Discuss. Faraday SOC.,72 (1981)263 7 A. Costa, P.M. Deya, J.V. Sinisterra and J.M. Marinas, Can. J. Chem., 58 (1980)1266. 8 A. Costa, P.M. Deya, J.V. Sinisterra and J.M. Marinas, Afinidad, 38 (1981) 226. 9 A. Aucejo, M. C. Burguet, A. Corma and V. Fornes, Appl. Catal., 22 (1986)187. 10 A. Thangaraj, S. Sivasanker and P. Ratnasamy, J. Catal., 137 (1992)262. 11 S. Sato, K.Takematau, T. Sodesawa and F. Nozaki, Bull. Chem. SOC. Jpn., 66 (1992)1486. 12 W.F. Hoelderich, Stud. Surf. Sci. Catal., 46 (1989)194. 13 J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M.I. Martinez, Mater. Chem. Phys., 21 (1989)409. 14 F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and A.A. Romero, Appl. Catal., in press. 16 P.M. Deya, A. Costa, J. V. Sinisterra and J.M. Marinas, Can. J. Chem., 60 (1982)36. 16 J.M. Campelo, A. Garcia, D. Luna, J, M. Marinas and M.S. Moreno, J. Colloid Interface Sci., 118 (1987)98. 17 J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M.S. Moreno, J. Chem. SOC.Faraday Trans. I, 86 (1989)2636. 18 A. Blanco, JlM. Campelo, A. G&ia, D. Luna and J. M. Marinas, Appl. Catal., 63 (1989)136. 19 J.M. C&pelo, A. Garcia, D. Luna, J. M. Marinas and M. Martinez-Cunquero, Proc. 11th Iberoamerican Symp. Catal., Guanajuato, Mexico, 1988,p.799. 20 D. Basset and H.W. Habgood, J. Phye. Chem., 64 (1960)769. 21 J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M.S. Moreno, Bull. SOC. Chim. Fr., (1988)283. 22 J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M.S. Moreno, Heterogeneous Catalysis and Fine Chemicals, Elsevier, Amsterdam, 1988,p. 249. 23 D.A. Best and B. W. Wojciechowski, J. Catal., 47 (1977)343.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals III 0 1993 Elsevier Science Publishers B.V. All rights reserved.

623

Post-synthetic improvement of the basic character of caesium exchanged X and Y zeolites by occluded caesium oxides. Applications in condensation reactions. I. Rodriguez, H. Cambon, D. Brunel*, M. Laspkras, P. Geneste. Laboratoire de Chimie Organique Physique et CinBtique Chimique Appliqukes,

URA 418 CNRS, Ecole Nationale SupBrieure de Chimie, 8 Rue de 1'Ecole Normale, 34053 Montpellier Cedex 1, France

Abstract Basic zeolites were prepared by in situ formation of caesium oxide by calcination of the parent acetate loaded in an increasing amount up to 26 caesium atoms per unit cell. X-ray diffraction and BET studies are consistent with good crystallinity and site accessibility retainings. C02 TPD results show homogeneous location of the basic species inside the pores with one caesium oxide per supercage. The results are fairly correlated with the initial rates of the Knoevenagel reaction of benzaldehyde and ethylcyanoacetate. These basic solids provide well-adapted selective microporous catalysts for condensation reaction. 1. INTRODUCTION

Increasing attention is now on solid catalysts possessing both basic and shape selective properties to perform selective base-catalyzed fine organic chemical reactions 11-93.Exchanged alkali zeolite demonstrated both these characters, however recent results show that these catalysts possess much less activity than sepiolites or hydrotalcites [10,111. In order to take benefit of the strong geometrical constraints of the zeolite voids, so as to improve the basic properties, basic moities insertion inside the The direct incorporation of pores was achieved by several authors [2-4,9,12,13]. alkali hydroxide during their synthesis by treatment of aqueous or methanolic solution [131can lead t o loss of crystallinity by A1-0 solvolysis. This phenomena can be avoided by using a mild method consisting of a thermal decomposition of neutral alkali salts previously occluded inside the zeolite void cavities [9]. In this respect, Martens et al. prepared supported sodium clusters on zeolites generated by impregnation of sodium azide followed by its thermal decomposition 12-41. In this work, we use the method previously perfected by Hathaway et al. based on the decomposition of caesium acetate inserted on CsNaX and CsNaY zeolites [9,141,which could lead to moderate basic sites useful for a large domain of base-catalyzed organic reactions using various solvents. The choice of ceasium exchanged X and Y zeolites as hosts is justified considering their

624

character more basic than the other cationic zeolites [5,6,151. Moreover, their large pore structure with three dimensional framework allowed access of rather hindered organic molecules to the intracrystalline basic sites. Different amounts of occluded caesium acetate were deposited on zeolites leading to a range of 0 to 26 caesium atoms per unit cell. The strength, the number and the location of the basic sites of the modified zeolites after thermal oxidative decomposition of the acetate were determined by CO2 stepwise TPD [161 in order to correlate them with their activities in the Knoevenagel reaction of benzaldehyde and ethylcyanoacetate. This condensation reaction has been used for studying the activities of alumina [171, aluminiumphosphatealumina [18], grafted silica [191, exchanged zeolites [71, sepiolites [lo] and dopped xonotlite [201. Experimental conditions were adjusted using caesium exchanged Y zeolite with 7 occluded caesium atoms per unit cell in view to allow comparison among all the prepared zeolites 1211.

2.1. Materials The starting materials for the preparation of the basic catalysts were NaX (13 X) zeolite from Aldrich-Chemie and NaY (Linde SK 40) zeolite from Union Carbide. Analytical grade C H ~ C O ~ CCH2(CN)C(O)OC2H5, S, CsH5C(O)H were from Aldrich-Chemie. Benzaldehyde was dried and then kept under nitrogen on basic alumina in order t o trap acid impurities, essentially benzoic acid.

2.2. Catalystpmparation The exchanged CsNaX and CsNaY zeolites were prepared according t o the general procedure [161. The chemical compositions of the unit cells were determined from elemental analyses. The calculated Cs exchanged NaX formula is Cs36Na5oA186Si1060384 corresponding to an exchange equal to 41.9 %. The calculated Cs exchanged NaY formula is Cs33Na23A156Si 1360384 corresponding to 57 % exchanged cations. The samples of modified X zeolites were prepared according to Hathaway [9,14], by impregating CsNaX (5 g) with aqueous solution of CH3C02Cs (12.8 ml) of appropriate concentration to obtain 4.5 (CsNaX 4 Cs), 9.1 (CsNaX 9 Cs), 11.4 (CsNaX 11 Cs), 15.9 (CsNaX 16 Cs) and 26.4 (CsNaX 26 Cs) per unit cell D61. The slurry was stirred a t room temperature until water was evaporated and then dried a t 80°C for 12 h. The occluded caesium oxide was formed during the calcination (550°C) by decomposition of the corresponding acetate (330°C) The Cs impregnated Y zeolite (CsNaY 7 Cs) corresponding to 7.4 Cs atoms per unit cell, was prepared similarly [211.

2.3. CharacterLpation ofmodified zeolites 2.3.1. X-ray powder diffraction

625

X-ray diffraction patterns of the samples after calcination were recorded on a Philips PW 1130 diffractometer in air using CuKa monochromated radiation (h = 1.54 A) 2.3.2. Volumetry Nitrogen adsorption-desorption isotherms at 77 K were determined with a volumetric devise equipped with an integral Barocel pressure transducer [221. Prior to the measurements, the samples (100 mg) were outgassed at 300°C under vacuum (up to 2.10-6 Torr) for 10 h. The specific surface areas were calculated by using the BET method and the micropore volumes according to both Dubinin and Singh methods. 2.3.3. Stepwise temperature-programmed desorption of C02 (TPD) The samples were pre-calcinated at 550°C (6 h) with a heating rate of 1"C.min-1 in a quartz cell in a flow (200 ml.min-1)of dry N2-02 mixture. After cooling to lOO"C, the zeolite ,was saturated during 6 h with a dry CO2 stream (12 ml.min-1) at atmospheric pressure. A flow of dry N2 (9.8 ml.min-1) filtered on a cartridge (% H20~0.4ppm) was then admitted. After temperature equilibration of the system, the zeolite sample was heated at a rate of 3°C /min, in steps of 50°C. A t each step temperature, the catalyst was maintained during 2 h. The desorbed CO2 was collected under a nitrogen flow in a NaOH solution thermostatized at 25°C. The amounts of evolved C02 were determined by titration continously performed by conductimetry using a conductivity cell (Schott LF 3100; Pt, Kd.01, connected to a Consort K 320 conductimeter. 2.4. Condensation reaction

General procedure: The zeolite sample (0.220 g) was calcinated in situ at 550°C (heating rate:l"C.rnin-l) under a stream of dried N2-02 mixture in a pyrex reactor equipped with a magnetic stirrer, a controlled heating and a sample tube with a frit to draw samples under controlled atmosphere. After temperature equilibration at 80"C, 50 ml of a solution of CH~(CN)C(O)OC~HS (1.86 g) in dried DMSO was introduced through a septum in the reactor. After (1.74g) was added. temperature adjustment, 5 ml solution of C&$(O)H Thus, the molarity of each reactant was 0.3 M. The progress of the reaction was monitored by periodically withdrawing samples and analysing them by G.C (Delsi 30) using 25 m OV-1 capillary column (oven programme: 8OoC, 3 min - 15"C/min - 200°C). Intermediate and final reaction mixtures were also analyzed by 1H and 13C NMR. The spectra were recorded in CDC13 solutions with 200 MHz Bruker and 360 Mz Bruker spectometers.

&RESULTS 3.1.Characterisation of the modified zeolites The comparison of the X-ray diffractograms of the various impregnated CsNaX n Cs before and after calcination with that of the parent exchanged

626

zeolite indicated that the structural framework is largely retained during the modification procedure [16]. The surface area and microporous volume of the modified zeolites are given in Table 1. Table 1. Surface areas and microporosities of the solids as a function of the Cs loading. Zeolite CsNaX CsNaX 4 Cs CsNaX 9 Cs CsNaX llCs CsNaX 16 Cs

BET surface area (m2 g-1) 432.0 398.7 395.9 373.8 314.8

Dubinin micropore volume (ml g-1) 0.21 0.17 0.17 0.16 0.13

Singh micropore volume (ml g-1) 0.18 0.16 0.16 0.14 0.12

The specific surface area as well as the pore volume decrease steadily with increasing Cs loading. The C02 TPD histograms of the modified and parent Cs Na zeolites are reported in Figure 1.

20

ro

2 K h

r:

bn

#

5 0

u

38 d

0 200

300

400

500

750

Temperature ("C) Figure 1. Evolution of the TPD spectra of the post-synthetically modified CsNaX 4 CS; zeolites as a function of the cesium loading: H CsNaX, CsNaX 9 Cs; E l CsNaX 11 Cs; H CsNaX 16 Cs; CsNaX 26 Cs. It has been shown that the experimental conditions allow titrating of the basic sites with good reproductibility [16]. Two main observations should be outlined: a) Up to 55OoC, the TPD spectra of the modified zeolites present gaussian type when the participation of the desorbed CO2 relative to the parent zeolite is substracted from each profile. They have maxima at 300°C and minima a t 500°C.

627

b) Above 55OoC, significant increasing amounts of desorbed CO2 appear for caesium loadings from 11to 26 atoms per unit cell. 82 Knoevenagel oondensation reaction tests.

The Knoevenagel condensation reaction of benzaldehyde with ethylcyanoacetate (Scheme 1)was first studied on CsNaY 7Cs in order to check the better conditions to control the different reaction parameters (solvent effect on the rate and on the selectivity of the uncatalyzed and catalyzed reactions, mass effect of the catalyst, concentration effect of both reactants) 1211.

a) C‘ H’

I

H’

‘CQEt

Scheme 1: Knoevenagel condensation

DMSO cancelled out the interference of the non-catalyzed reaction. Stoechiometric concentration of each reactant allowed to nearly suppress the successive Michael reaction (Scheme 2).

Scheme 2: Michael reaction.

-

Finally, 0.3 M solution of each reactant was chosen to allow a reasonable initial rate at 80”C, using all the modified prepared zeolites. The initial rates for all of them are listed on Table 2. Table 2. Initial rates of the condensation of benzaldehyde (0.3 M) with ethyl cyanoacetate (0.3 M) in DMSO solution(55 ml) on modified zeolites (0.220 g) at 80°C.

4. DISCUSSION

The impregnation of caesium acetate followed by calcination a t 550°C does not affect the host faujasite crystanillity as deduced from X-ray results. Moreover, the adsorption isotherms of the modified and the parent zeolites

628

belong to type I without hysteresis loop indicating absence of mesopores. The gradual trend of decreasing specific areas and microporous volumes with increasing loadings is consistent with the more and more large space occupied by occluded species. I t is noteworthy that specific area and microporous volume values remain sizeable even for the highest impregnation level, showing free accessibilities t o the internal sites. These results demonstrate that this loading method allows us preparation of modified zeolites without noticeable loss of microporosity. The C02 adsorption has been already used for measuring base strength distribution of [23,241, or for poisoning [15] the basic sites on solid catalysts. Nevertheless, on cationic zeolites, the contribution of the C02 adsorption on the cations could interfere with basic sites. However, CO2 TPD spectrum of the CsNaX shows small amounts of desorbed CO2 at low temperatures. This confirms the microcalorimetric results of Ginoux [251. Thus, this coverage can be taken away from the different histograms in the 100-200°C range. The similar pattern of the histograms in the range 200-550°C suggests that the basic species associated to the corresponding adsorbed C02, are unique whereas the sites desorbed at temperature higher than 550°C could be assigned to different species such as decomposition of pure caesium carbonate (decomposition temperature, 610°C)

10

0

20

30

Encapsulated Cs (mol unit cell" Figure 2. Evolution of the desorbed CO as a function of the cesium loa ing.

%

0

2

4

6

8

I

Desorbed C02 (mol unit Celt' Figure 3. v, as a function of basic species determined by CO, TPD.

Figure 2 illustrates the correlation between the total C02 evolved below 55OoC and the calculated amounts of caesium atom loading. The desorbed amounts are plotted on a straight line for loading lower than 16 caesium atoms per unit cell whereas, saturation is observed for higher number. This result suggests an homogeneous distribution of the basic species. Moreover, the slope of the straight line which is nearly equal t o 0.5 (0.44, r=0.984)

629

accounts for caesium carbonate species. Therefore it could be proposed that until 16 caesium acetate molecules impregnated per unit cell, the caesium species formed by calcination, are homogeneously located inside the cavities, with one ceasium oxide type structure per supercages. Above 16 caesium atoms per unit cell, more than two molecules of the parent caesium acetate cannot enter into the structure and remain on the external zeolite surface as caesium carbonate. As emphasized recently by Corma [261, condensation of benzaldehyde and ethylcyanoacetate, without solvent, is an interesting reaction to check the number and the strength of the basic sites. However, when this reaction is canied out in solution, solvent effect plays an important role on the kinetic of the reaction. For this reason, we have compared the activities of different zeolites calcinated at 55OoC,using similar experimental conditions (Table 2) Results reported in Table 2 illustrate that the initial rate increases with the caesium loading. In order to correlate the activities of the various zeolites with the true caesium oxide numbers included in the microporous voids, we have plotted in Figure 3 the initial rates versus mole number of desorbed CO2 up to 550°C. Up to 6 moles of CO2 corresponding to the number of caesim oxide molecules encaged, the plotted line is straight. This confirms that there is an homogeneous loading of caesium oxide inside the micropores up to 6 caesium oxides per unit cell. Nevertheless, the fact that the slope of the line increase when n is higher than 8, shows that the external caesium compound, probably carbonate (because the activation temperature used is 55OoC),catalyze also the condensation reaction. In this case, an homogeneous catalysis by these dissolved carbonate cannot be excluded t o explain this additional activity. The good selectivity obtained in the Knoevenagel product (95% at 90% conversion) versus the Michael ones could be explained either by a lower basicity of the encaged caesium oxide on zeolite compare to Mg-A1 hydrotalcites 1261 or by a shape selective properties of this modified zeolites. Complementary work is needed to determine the origin of this last result. 5. CONCLUSION

The mild method of loading caesium oxide inside the microporous voids of the X zeolite by the mean of calcination of the corresponding acetate previously inserted, allows to preserve the crystallinity of the solid supports. Up to eight caesium oxides per unit cell, these occluded basic species are homogeneously located essentially inside the micropores with one oxide per supercage. The zeolites thus modified, appear to be well-adapted catalysts to perform basecatalyzed reactions in fine organic chemistry. The fact that the catalytic reaction takes place inside the internal volume of these modified zeolites is promising for their further applications to organic reactions in order to improve the selectivities.

1

Y.Ono, in "Catalysis by Zeolites", B. Imelik, C. Naccache, Y. Ben Taarit, J.C. VBdrine, G. Coudurier and H. Praliaud (eds.), Stud. Surf. Sci.Catal., 5 (1980) 19

630

2

L.R. Martens, P.J. Grobet, W.J. Wermeiren and P.A. Jacobs, in "New Developments in Zeolite Science and Technology", Y. Murakami, A. Lijima and J.W. Ward (eds.), Stud. Surf. Sci. Catal., 28 (1986) 935 3 L.R. Martens, W.J. Wermeiren, P.J. Grobet and P.A. Jacobs, in "Preparation of catalysts IV",B. Delmon, P. Grange, P.A. Jacobs and G. Poncet (eds.), Stud. Surf. Sci. Catal., 31 (1987) 531 4 L.R. Martens, W.J. Wermeiren, D.R. Huybrechts, P.J. Grobet and P.A. Jacobsjn Proceeding 9th International Congress on Catalysis", Vol I, M.J. Phillips and M. Ternan (eds.), 1 (1988) 420. 5 D. Barthomeuf and A. Mallmann, i n "Innovation in Zeolite P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff (eds.), Stud. Surf. Sci. Catal., 37 (1988) 365. 6 D. Barthomeuf, G. Coudurier and J.C. Vedrine, Mater. Chem. Phys., 18 (1988) 553. 7 A. Corma, V. FornBs, R.M. Martin-Aranda, H. Garcia and J. Primo, Appl. Catal., 59 (1990) 237 9 P.E. Hathaway and M.E. Davis, J . Catal., 116 (1989) 263. 8 R.M. Dessau, U S . Pat. no 5,026,919 (1991). 10 A. Corma and R.M. Martin-Aranda, J . Catal., 130 (1991) 130. 11 a)H. Louthy, Thesis, Univesity Montpellier 11, 1993 b)D. Tichit, M. Lhouty, A. Guida, B. Chiche, A. Auroux, E. Garrone, F. Figueras, Submitted 12 D. Archier, Thesis, Claude Bernard University, Lyon I, 1989. I3 a) K.K. Dzkkumakaev, Y.I. Isakov, G.T. Fedolyak, K.M. Minachev, T.A. Isakov and A.D. Kagarlitskii, Kinet. Katal., 28 (1987) 856. b) P.T. Wierzchowski and L.W. Zatorski, Catal. Lett., 9 (1991) 411. 14 P.E. Hathaway and M.E. Davis, tJ. Catal., 116 (1989) 279. 15 E.J. Rode,P.E. Gee, L.N. Marquez, T. Uemura and M. Barzargani, Catal. Lett., 9 (1991) 103. 16 M. LaspBras, H. Cambon, D. Brunel, I. Rodriguez and P. Geneste, submitted 17 F. Texier-Boulet and A. Foucaud, Tetrahedron Lett., 23 (1982) 4927. la J.A. Cabello, J.M. Campelo, A. Garcia, D. Luna and J.M. Marinas, J . Org. Che1n.,49 (1984) 5195. 19 E. Angeletti, C. Canepa, G. Martinetti and P. Venturello, Tetrahedron Lett., 29 (1988) 2261. 20 S. Chalais, P. Laszlo and A. Mathy, Tetrahedron Lett., 26 (1985) 4453. 21 I. Rodriguez, D. Brunel, M. Lasperas, H. Cambon and P. Geneste, to be published F. Fajula, L. Moudafi, R. Dutartre and F. Figueras, Nouv. J . Chim., 8 (1984)207. 23 V.R. Choudhary and V.H. Rane, Catal. Lett., 4 (1990) 101. 24 X. Wang, G. Wang, D. Shen, C. Fu and M. Wei, Zeolites, 11 (1991) 254. 25 D. Amari, J.M. Lopez-Cuesta, N.P. Nguyen, R. Jerrentrup and J.L. Ginoux, J . Thermal Anal., 38 (1992) 1005 26 A. Corma, in "Synthesis/Characterization and Novel Applications of Molecular Sieve Materials", R.L. Bedard,T. Bein, M.E. Davis, J. Garces, Symp. Proc., 233 V.A. Maroni and G.D. Stucky (Eds.),Mater. Res. SOC. (1991) 17. 'I,

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I11 0 1993 Elsevier Science Publishers B.V. All rights reserved.

631

Catalytic Transfer Reduction of Ketones Over Oxide Catalysts J.KIJEfiSKI", M.GLINSKI, J.CZARNECKI, R.DERLACKA, and V.JARZYNA Laboratory of Catalytic Synthesis, Chemical Faculty, Warsaw Technical University (Politechnika), Koszykowa 75, 00-662 Warsaw, Poland

Summary The catalytic transfer reductions of ethyl methyl ketone, isopropyl methyl ketone, and 4-methylacetophenone were studied over the wide series of basic, acidic and semiconducting oxides supported on Si02. Most of them exhibited remarkable activity in the studied transformations. The existence of the strong oxide oxide interaction between deposited phases and Si02 was noted. The nature of catalytic active sites was identified using catalytic titration with Hammett indicators and tetracyanoethylene. An unforeseen modifying effect of n-propylamine, o-nitroaniline, and TCNE onto A1203 was observed which led to the enhancement of catalyst activity.

1. INTRODUCTION In our previous papers we demonstrated the practical potential of catalytic transfer reduction CTR) over MgO for the reduction of various types of unsaturated functional g r o ~ p s . l -One ~ of them was devoted to the reduction of ketones with isopropyl a l ~ o h o l . ~ ) Some other authors also have made the attempts to reduce ketones over oxide catalyst^.^-^) In the present work we have expended the series of the studied catalysts to include other main group and transition metals oxides: B2O3, Al2O3, LapO3, CuO, Moog, WOg, Fe2O3, PbO, V2O5, ZnO, ZrO2, Bi2O3, COO, Cr2O3, MnO2, and NiO. As model reactions the reductions of ethyl methyl ketone, isopropyl methyl ketone and 4methylacetophenone with isopropyl alcohol were chosen. Our previous efforts confirmed the participation of basic and/or one-electron donor sites of the MgO surface in the CTR process.' In the present work the catalytic titration using poisons suppressing various types of surface sites was applied for the identification of the centres responsible for the catalytic activity of studied 0xides.~19)

I

13)

2. EXPERIMENTAL 2.1. Catalysts MgO was prepared according to the procedure described elsewhere.lO) Before the reaction magnesia was calcined at 773K in the stream of air during 2 hrs. Si02 was the commercial Aerosil preparation of Degussa. Before the reaction silica was calcined at 673K in the stream of air during 2 hrs. The catalysts used in the supported form contained 5 or 20 wP/, of corresponding oxide on silica. They were obtained by the impregnation of

632 SiOp with the solutions of precursors using incipient wetness technique. The catalysts precursors were corresponding nitrates, ammonium salts, and H3BO3. The impregnated preparations were dried at 353K during 24 hrs and then calcined at 673K in the stream of the air during 2 hrs. X-ray diffraction and esr spectroscopy were used for oxides characterization.

2.2. Reactions The reactions were performed in a typical flow reactor with a fixed catalyst bed. Reactant solution (isopropyl alcohol:ketone, ratio 3) were dosed using a microdosing pump with HLSV 2 cm3 of solution per 1g of catalysthour. The activity measurements were performed after 1hr from the start of the reactants dosage. The product analysis was performed by gc using 4m column with 20% OV-101 on GasChrom Q.

2.3. Poisoning experiments The catalytic titration experiments were performed introducing various amounts of corresponding poison into the reactant feed. The amount of poison was of the range from 20 to 1500 pmoles per 1g of a catalyst.

3. RESULTS AND DISCUSSION

3.1. Reduction of ethyl methyl ketone From the investigated series of oxides only B2O3, CuO, Moog, WOg, and Fe2O3 were found to be completely inactive in ethyl methyl ketone reduction under adopted conditions. Other catalysts under study exhibited remarkable, however, deeply differentiated activity and selectivity in the catalytic transfer reduction (Table 1). The reaction proceeded according to the following equation: CH3COCH2CH3

+

(CH3)2CHOH -+ CH3CH(OH)CH2CH3

+ (CH&C=O

(1)

As side-products diminishing the reaction selectivity butenes were detected which were originated in the consecutive dehydration of 2-butanol. MgO, AI2O.3, ZnO, and Zr02 exhibited significant activity in the hydrogen transfer already at 423 K. Other supported oxide system started to catalyze the studied reaction at much higher temperatures. It is interesting to note that the load of an impregnated oxide phase also influenced strongly its activity/selectivity profile. The enlargement of the content of supported vanadia and chromia from 5 wP/, to 20 wP/, resulted in the appearance of the remarkable activity even at 423 K. 2-Butanol existed in the products mixture obtained over A1203 only at 423 K, at higher temperatures it was totally dehydrated to butenes on highly acidic centres of alumina.

633 Table 1 Yield of 2-butanol and reaction selectivity over various oxide catalysts Temp. 423 473 523 573 623 673 S Y S Y S Y S Y S Oxide M a (S)b Y (5 W/o) 49 0 0 77 61 76 100 77 100 79 100 80 MgO 0 0 0 0 0 0 33 100 50 100 49 100 PbO v2°5 0 0 26 100 52 88 45 80 13 43 0 0 V2O5C 20 80 31 76 ZnO 48 100 67 100 72 96 78 94 63 74 0 0 13 40 25 81 25 100 24 97 15 83 0 0 z*2 5 1 1 0 0 0 0 0 0 0 0 0 0 0 0 A1203 0 0 0 0 0 0 35 100 36 100 31 100 BipO3 coo 0 0 0 0 0 0 0 0 30 100 48 100 Cr203 0 0 0 0 0 0 22 100 41 100 49 100 Cr2O3C 60 63 70 79 9 2 7 8 100 0 0 0 0 7 1 0 0 0 0 La203 47 100 0 0 0 0 38 100 0 0 0 0 Mn02 NiO 0 0 0 0 0 0 0 0 0 0 2 2 1 0 0 a Y-yield, b S-selectivity, 20 wt%

3.2. Reduction of isopropyl methyl ketone. Nature of active sites lsopropyl methyl ketone was successfully reduced on bulk magnesia to 3-methyl-2butanol using isopropyl alcohol as hydrogen donor. Reaction proceeded according to the equation:

In the temperatures range of 423-498K 3-methyl-2-butanolwas the only product formed from the ketone and its yield reached 75% which is close to the thermodynamic value. Our previous results 1) indicated the basic and one-electron donor sites of MgO surface as responsible for the oxide activity in the CTR of carbonyl and other unsaturated groups. On the other hand, we did not ascertain the necessity of a co-action of acidic surface sites which is generally postulated when assume the existence of reaction intermediates analogous to the Meetwein-Pondorf-Verleyreaction. To identify more closely the sites active in hydrogen transfer the catalytic titration was performed using the poisons suppressing all types of surface centres existing on magnesia. For the selective poisoning of basic sites o-nitroaniline (pKa=19.0), 2,6-dimethylphenol (PKa-1 I ) , phenol (pKa=9.9), and benzoic acid (pKa=4.2) were used. The poisoning strength of this series increases, of course, with the decrease in pKa value. n-Propylamine ( ~ K B H + =3.2) was the poison of acidic centres. Tetracyanoethylene (E.A.=2.7eV) was used to eliminate one-electron donor sites, and m-dinitrobenzene (E.A.=I .7eV, pKa=l2.2) was used for the simultaneous elimination of basic and one-electron donor centres. The amounts of poisons used were extended from 10 to 1500 pmoles per I g of a catalyst to find the quantity causing the maximum effect. The poisons influence is depicted in Table 2.

634 Table 2 The yields of 3-methyl-2-butanolover MgO suppressed with various poisons. Poison Amount Reaction Yield of alcohol Temperature mol ?” pmo1e.g-1 K 423 73.5 473 75.0 o-nitroaniline 100 423 70.3 473 74.2 1500 423 0.0 473 2.3 100 423 27.8 2,6-dimethylphenol 473 67.2 500 423 3.4 473 46.2 phenol 20 423 19.4 473 56.1 50 423 0.0 473 17.5 benzoic acid 20 423 9.0 473 38.3 1000 423 73.6 n-propylamine 473 75.0 50 423 5.9 m-dinitrobenzene 473 20.3 10 423 9.0 TCNE 473 14.5 The data collected in Table 2 evidence the unquestionable responsibility of basic and/or one-electron donor centres for the activity exhibited by MgO in CTR. Simultaneously it is clearly shown that the elimination of all acidic surface sites by poisoning with n-propylamine did not result in any change of catalyst activity.

3.3. Reduction of 4-methylacetophenone

Reaction selectivity. The reduction of 4-methylacetophenone using isopropyl alcohol as a hydrogen donor was carried out over bulk magnesia and silica, and in the presence of the series of silica supported oxides, including MgO, Al2O3, ZnO, Cr2O3, and V2O5. The 4-methylacetophenone reduction is more complex process than the above discussed reactions of aliphatic ketones. 1-(4-Methylphenyl)ethanol produced by the hydrogenation of aromatic ketone can be easily dehydrated to 4-methylstyrene which would further undergo reduction to 4-methylethylbenzene under CTR conditions, according to the following equations: H3C-Ph-COCH3 + (CH&CHOH H$-Ph-CH(OH)CH3

+

+ H$-PhCH(OH)CH3 +

(CH3)2C=O

H3C-Ph-CH=CH2 + H20

H3C-Ph-CH=CH2 + (CH3)2CHOH

-+

H3C-Ph-CH2CH3 + (CH3)2C=O

(3) (4) (5)

635 Therefore, the total conversion of the starting 4-methylacetophenone to alcohol, alkenyl and alkyl derivatives would depict a catalyst activity in CTR process, while the total yield of 4-methylstyrene and 4-methylethylbenzene manifest the dehydrating activity of the studied oxides. Additionally, the appearance of alkylaromatic derivative in products mixture could be considered as a measure of the reducing power of an oxide system. As it was reported by us earlier, the reduction of carbon carbon double bond is the most difficult process among all realized by CTR using alcohols as hydrogen donors. Table 3 The distribution of 4-methylacetophenone transformation products. Catalyst Temp. C=Oa C-OHb K

c=cc

C-Cd

~~

Si02 bulk

523 673 523 Si02 bulk + TCNE 673 523 Si02 bulk + n-PrNH2 673 Si02 bulk + o - N O ~ C ~ H ~ N H ~523 673 523 MgO bulk 673 MgO bulk + TCNE 523 673 MgO bulk + n-PrNH2 523 673 MgO bulk + O - N O ~ C G H ~ N H ~ 523 673 523 MgO 673 523 MgO + TCNE 673 MgO + n-PrNH2 523 673 MgO + O - N O ~ C ~ H ~ N H Z 523 673 ZnO 523 673 ZnO + TCNE 523 673 ZnO + n-PrNH2 523 673 ZnO + O-NO2C6H4NH2 523 673

100.0 59.9 100.0 84.7 100.0 84.1 100.0 96.7 40.2 0.0 45.4 3.7 44.5 9.0 55.0 1.4 0 .o 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 35.6 3.1 37.0 5.5 46.8 9.4 42.5 10.6 0.0

0.0 0.0

0.0

0.0

0.0 0.0 27.3 2.6 53.6 46.7 75.8 21.4 66.3 25.7 87.3 66.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 40.1 0.0 15.3

0.0 0.0

0.0 0.0

0.0

0.0

15.9 0.0 3.3 24.2 92.8 17.6 85.5 8.7 78.5 2.5 84.6 100.0 84.2 100.0 85.9 100.0 85.9 72.7 90.4 46.4 50.3 24.2 78.6 33.7 71.6 12.7 32.4

0.0 0.0 0.0 0.0 4.1 0.0 5.3 0.0 3.1 0.0 3.4 0.0 15.8 0.0 14.1 0.0 14.1 0.0 7.0 0.0 3.0 0.0 0.0 0.0 2.8 0.0 1.o

636 Table 3 The distribution of 4-methylacetophenonetransformation products. Catalyst Temp. C=Oa C-OHb K 523 42.4 0.0 0.0 673 16.4 523 31.7 0.0 Cr2O3 + TCNE 0.0 673 13.5 Cr2O3 + n-PrNH2 523 27.2 0.0 0.0 673 18.4 Cr2O3 + o - N O ~ C ~ H ~ N H ~ 523 78.7 0.0 0.0 673 18.7 0.0 523 55.5 A1203 0 .o 673 25.1 523 0.0 0.0 A1203 + TCNE 0.0 673 15.5 523 0.0 0.0 A1203 + n-PrNH2 0.0 673 40.2 523 0.0 0.0 A1203 + O - N O ~ C G H ~ N H ~ 0 .o 673 7.5 0 .o 523 84.3 v205 673 95.4 0.0 523 87.3 0.0 V2O5 + TCNE 0.0 89.4 673 523 52.9 0.0 V2O5 + n-PrNH2 0.0 673 86.4 78.2 0.0 V2O5 + o - N O ~ C ~ H ~ N H ~ 523 0.0 673 74.2 a unreacted 4-meth lacetophenone;b 1-(4-methylphenyl)ethanol; C 4-methylstyrene;J4-methylethylbenzene cr203

c=cc

C-Cd

57.6 78.3 68.3 80.6 72.8 73.8 21.3 76.1 18.2 30.7 97.9 40.9 91.6 30.0 97.5 64.0 15.7 4.6 12.7 10.6 47.1 13.6 21.8 25.8

0.0 5.3 0.0 5.9 0.0 7.8 0.0 5.2 26.3 44.2 2.1 43.6 8.4 29.8 2.5 28.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

All studied catalysts exhibited remarkable activity in 4-methylacetophenonereaction with isopropyl alcohol (Table 3). Even over pure silica carrier significant amounts of 4-methylstyrenewere produced at the temperatures above 573K. It is worth noting that the activity and selectivity of bulk magnesia and magnesia supported on silica were substantially different. The second converted the whole amount of reactant into products mixture already at 523K. At the same conditions in the presence of bulk magnesia the conversion of 4-methylacetophenone did not exceed 60%. No traces of alcohol were detected over supported system within whole range of studied temperatures. For the bulk catalyst 1-(4-methylphenyl)ethanolwas the main reaction product at 523K, and even at 573K the products mixture obtained over this catalyst contained 22% of alcohol. This observation would suggest a considerable enhancement of an acidity of magnesium oxide when supported onto silica. Other investigated catalytic systems exhibited much lower activity in hydrogen transfer as compared with magnesia. Their activity would be ranged as follows: MgO >> CrpO3 > A1203 > ZnO > V2O5. Considering the proposed criteria all the supported catalyst are strongly acidic - no traces of the alcohol were detected in the products obtained in their presence.

637

Nature of active sites. o-Nitroaniline (pKa=l9.0), n-propylamine, and tetracyanoethylene were used as catalyst titrants in the amount of 100pmoles per 1g of a catalyst to identify the centres participating in the observed sequence of 4-methylacetophenone transformations.The conclusions from the performed poisoning were surprising. Only in the case of magnesia the obtained results were not far from the expected ones. For the both: bulk MgO and supported system o-nitroaniline was found to be the most effective poison. This is the next evidence for the importance of basic sites in CTR over MgO. Astonishing was, however, the practical lack of any effect of the introduction of TCNE into the reactants feed what was observed for the bulk, as well as for the supported magnesia catalyst. This observation would lead to the conclusion that one-electron donor sites do not take part in the studied transformations in opposite to their role in the earlier described CTR's over MgO.112) n-Propylamine adsorption suppressed the consecutive dehydration of 1-(4-methylphenyl)ethanolover the bulk MgO, and simultaneously it did not give any effect in the case of much more acidic supported catalyst. The activity of CrpO3 was significantly reduced by o-nitroaniline. The effect of the both remaining poisons was not spectacular. Similarly, in the case of ZnO o-nitroaniline was an effective poison for 4-methylacetophenone conversion. Nevertheless, also the presence of TCNE on the catalyst surface resulted in the remarkable diminishing of its activity. The vanadia catalyst was practically non-sensitiveto TCNE and o-nitroaniline. The unforeseen results were obtained during the catalytic titration of the alumina on silica system. All the used poisons caused the large enhancement of its activity in 4-methylacetophenoneconversion and its hydrogenating power, measured by the yield of 4-methylethylbenzene. The poisons independing on their chemical nature started to be positively modifying agents. These modifiers have to be considered as powerful when state that they act in low amounts - their concentration in the reactants feed equal to 1OOpmoles per l g of a catalyst. The similar surprising effect was observed during vanadia titration with n-propylamine.At the present state of our knowledge about CTR over oxides the observed phenomenon is difficult for the explanation. Recently, we began studies concerning the identification of new surface individuals formed during the co-adsorption of 4-methylacetophenone and the poisons used which lead to the enhancement of A1203 activity. 4. CONCLUSIONS 1. The wide series of metal oxides possessing differentiated chemical nature exhibits remarkable activity in the catalytic transfer reduction of aliphatic and aromatic ketones. 2. The supported oxide phases take part in the strong oxide oxide interaction with silica carrier which is reflected in the significant changes of MgO activity when deposited on Si02, and in Cr2O3 and V2O5 activity changes accompanying the increase in their load. 3. The centres responsible for the activity of various oxides in CTR using aliphatic alcohol as hydrogen donor are usually of basic character. Acidic centres are mainly responsible for the undesired consecutive dehydration. 4. Alumina supported on silica undergoes unforeseen modifying effect during catalytic titration with organic acid, base and electron acceptor. The observed phenomenon seems to be interesting from the practical point of view, however it would be an essential limitation for the catalytic titration method using the mentioned poisons.

638

5. REFERENCES 1 J.Kijerkki, MeGIinskiand JBeinhercs, Stud.Surf.Sci.Catal., Heterogeneous Catalysis and Fine Chemicals, ed. M.Guisnet, J.Barrault, C.Bouchoule, D.Duprez, C.Montassier and G.Perot, vo1.41 (1989) 231. 2 J.Kijeriski, M.Glinski, R.WiSniewski, and S.Murghani, Stud.Surf.Sci.Catal., Heterogeneous Catalysis and Fine Chemicals 11, ed. M.Guisnet, Elsevier, Amsterdam (1991) 169. 3 J.Kijehski, M.Glinski, J.Czarnecki, J.Chem.Soc.PerkinTrans.2, (1991) 1695-1698. 4 H.Niiyama and E.Echigoya, Bull.Chem.Soc.Jpn.,45 (1972) 939. 5 Y.Okamoto, T.lmanaka and S.Teranishi, Bull.Chem.Soc.Jpn., 45 (1972) 3207. 6 C.L.Kibby and W.K.Hal1, J.Catal., 31 (1973) 65. 7 K.Ganesan and C.N.Pillai, J.Catal., 118 (1985) 371. 8 J.Kijenski, B.Zielinski, S.Malinowski, J.Res.lnst.Catalysis, Hokkaido University, 27 (1979) 111. 9 J.Kijenski, A.Baiker, Catalysis Today, Acidic Sites on Catalytic Surfaces and Their Determination, 5 (1989) 1-120. 10 J.Kijenski, S.Malinowski, J.Chem.Soc., Faraday 1, 74 (1978) 250.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals III (Q 1993 Elsevier Science Publishers B.V. All rights reserved.

639

SELECTIVE RJNGOPENING OF AN EPOXIDE ON SILICA SUPPORTS

Maria do Cdu Costa, Regina Tavares, William Motherwell Marcel0 Curto

* and

Maria Jodo

Instituto Nacional de Engenharia e Tecnologia Industrial (INETI), Departamento de Tecnologia de Indlistrias Quimicas, 2745 Queluz, Portugal *Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY, United Kingdom

Absbract The intramolecular G,E-epoxycarbonyl rearrangement of 801,17epoxy-14,15-dinorlabdan-13-one I to give stereoselectively two diastereoisomeric ketals I1 or I11 was promoted in heterogeneous media using silicic porous solids as catalysts, making possible to choose a convenient selective system through an adequate combination of solvent, catalyst and temperature.

1. INTRODUCTION

Although in most preparations of fine chemicals the wet way is the dominant, the dry way has become usual for some academical studies19 and offers increased advantages which include faster reaction times, more economical transformations due to the absence of solvent and different selectivities in general.4 We report here results illustrating a silica gel, a silicalite and an aluminosilicate-promoted intramolecular cyclization with very high attendant stereoselectivity depending more on the presence or absence of solvent and the temperature of the reaction than on the type of support used.

640 2. RESULTSAND DISCUSSION

We have performed the stereoselective intramolecular 6,E-epoxy-

I (Scheme -carbony1 rearrangement of 8a,17-epoxy-14,1Bdinorlabdan-l3-one 1)for the high yield diastereoselective synthesis of the perfume ambergris5

ketals

(13S~8a,13;13,17-diepoxy-14,15-dinor1abdane I11

and

(13R)-

-8p,13;13,l'l-diepoxy-14,15-dinorlabdane I I. Previously reported methods 6-8 lead to mixtures of the ketals I1 and I11 in yields not higher than 30% and have the disadvantage of using toxic and/or expensive reagents, as well as the added disadvantage of producing mixtures of both isomers in which one of the components is odorless, thereby reducing the commercial value of the final product for application in the industry of p e r f ~ m e r y . ~ ~ , ~

I

I11

Scheme 1 Ketals I1 and I11 obtained through the intramolecular cyclization of 8a,17-epoxy-14,15-dinorlabdan-13-one I.

In organic media, high yields of isomer I11 were isolated when the cyclization of the a-epoxyketone I was carried out in the presence of acidic activated silica gel. Reaction of the a-epoxyketone I with neutral vermiculite produced quantitatively (90-100%)the ketal I11 in the adequate solvent and at mild temperatures.The same intramolecular cyclization of compound I carried out in dry media conditions, at 100°C, either over silica gel, vermiculite or silicalite produced the isomer I1 in quantitative yield (Table 1).

641

Table I

Cyclization of the 8a,17-epoxy-14,15-dinorlabdan-13-one I in the presence of porous solids.

REACTION CONDITIONS

FWODUCIS I11 (%) I1 (%)

Silica gelllOO°Cldry mediumL2 h

__--

100

SilicaliteI10OoC/dry m e d i u d 2 h

----

100

Vermiculi tell OO"C/dry medi u d 2 h

----

100

25

75

Silica gel activated with oxalic acid/ polar solventJAJ4 h

70

25

Silica gel activated with p -7bOWapolar solvent/l8"C/3 d

75

25

90-100

0-5

Silica 'gel activated with p -TsOWpolar solventfAM h

Vermiculite/18°C/apolar solventf3 h

From these results, and taking into account previous studies on the ketalization of epoxyketone I in homogeneous conditions or using other solid

catalyst^,^ a mechanistical proposal can be drawn based on the following points: i) the stereoeselectivity for compounds I1 or I11 is not dependent on the acidity; ii) the surface interaction seems to be a major factor i n the catalysis and iii) although carbocations are favoured intermediates in acidic medium, the participation of the active group SiO2 has been evidenced. lo,ll We suggest that selectivity depends on the way the molecule adsorbs on to the solid, through the epoxide or the carbonyl oxygen or both groups, and we picture these reactions in terms of the mechanisms outlined in Schemes 2 and 3.

642

I11

Weme 2

Proposed mechanism for the regio-selectivity of the transition state in the cyclization of a-epoxyketone I with vermiculite, in the presence of an organic apolar solvent.

The first elementary reaction step in wet conditions is believed to be the adsorption of the epoxycarbonylic compound I through the a-plane of the molecule on the solid support, as evidenced by the total absence of organic compounds in the organic reaction medium after 2 h (GLC).In the second step, the active centers participate in the opening of the epoxide ring and produce the isomer I1 or I11 depending essentially on the reaction conditions (temperature and solvent). The last step, consisting in desorption of the rearranged product by washing the solid with excess solvent, leads to a quantitative recovery of the product.

643

&

A __t

I

I

Scheme 3

I1

Proposed mechanism for the stereo-specific cyclization of a-epoxyketone I adsorbed through the oxygen of the oxirane ring, onto silica gel, silicalite or vermiculite, at 100-200°C, in dry media.

In both mechanisms proposed in Schemes 2 and 3, the stereoelectronic and thermodynamic control of the transition state through the formation of a six-membered ring is in agreement with the established requirements for intramolecular reactivity of precursor I in homogeneous

condition^.^

However, there is evidence of possible shape selectivity when the reaction is carried out in the presence of Vermiculite used, since although the dimensions of both isomers are similar (- 12 A x 7 A) and compare with the interlayer distance of the used vermiculite (14.50 A), the ratio of isomers is solvent and temperature dependent. This result points out to the importance of the solvent participation through physical parameters, namely a simple variable wetting capacity leading to a different interaction of the reactive molecules with the soli support, even when solvents are chemically inert.

644

3.CONCLUSIONS Using the same silicic supports, it is possible to alter the stereochemical course of the intramolecular epoxycarbonylic rearrangement t o produce stereoselectively two diastereomeric ketals essentially by working in wet or dry media or with activation of the support and at an adequate temperature. Since an overall interpretation of the precise role of silica gel, as well as zeolites and aluminosilicates in general, as surface catalysts has yet to be reached, further investigation is needed to understand the detailed reaction mechanisms taking place on the solid supports.

4. EXPERZMENI'ALPART

Gas-liquid chromatography (GLC) was used routinely to monitor reactions, in a Carlo Erba HRGC 5160 chromatograph, with a FID detector and a silicone DB-1 (bonded methyl silicone, J&W Scientific, Inc., Rancho Cordova, Calif6rnia column, 15 m x 0,25 mm x 0,lO pm. Helium N50 was used as carrier (2 mumin, 60 KPa, split ratio 1 O O : l ) and analyses were performed at 260°C (inj.); oven, 170°C (7 min); 2OOOC (2 min, 4"C/min); 285OC (15"C/min; 20 min). Melting points were determined on a Reichert Thermovar hot bench and are uncorrected. Infra-red spectra were recorded on a Perkin-Elmer 298, as liquid films in CHC13 solution. Proton and carbon-13 magnetic ressonance spectra were recorded on a General Electric QE-300 (300,65 MHz) in CDC13, with tetramethylsilane as the internal standard. Mass spectra were recorded on a KRATOS M S 25RF mass spectrometer operating at 70 el?

Silica gel MERCK 9385, 230-400 mesh has also been used either as a pure mineral or activated with organic acids. Analytical pure silicalite was a gift from Prof. Rees (Department of Chemistry, Imperial College, London). The vermiculite used in this work originates from South Africa and contains some pyrophyllite as contaminant. Its chemical composition is Si02 39.37%; A1203 12.08%; MgO 23.37%; Fez03 5.45%; CaO 1.46%; K20 2.46%; Na2O 0.80%; MnO 0.30%. The specific surface area is 7.53 m2g-l(BET, N2, after 2 h at 200"C, 0.06 mmHg) and the porosity is 2-50 nm.

645

4.1.1. General Fhcedures for the syntheses of (13S~~,1~13,17-diepxy-14,lS-dinorlabdane111 and (13R)-8~,13;13,17-diepxy-l4,l&dinorlabdane I1 4.1.With a solid activated support and in the presence of solvent A solution of 8a,l7-epoxy-14,15-dinorlabdan-l3-one I (10 mg) in dry toluene (5 ml) was added at 18-25OC to a flask containing recently activated

vermiculite. After transformation of the starting material, the solution was filtered and the residual vermiculite washed with toluene. The filtrate was dried and concentrated at reduced pressure to afford 113SMa.13:13.17 oxv-14.15-dinorlabdane 111 (go%), [alZoD=+ 18.9 (0.7 g, CHC131, Umax (CHC13): 3001,2950 (uCH), 2884 (uOCH~O),1461 (6CH2), 1390 (6CH3), 1270, 12OO,1112,1O40 (uCOC), 969,918,869 (GCOCcyclic) CXYI-'; 6lH (CDC13):0.86 (3H, s, l8-CH3), 0.89 (6H, s, 20-CH3 e 19-CH3), 1.41 (3H, s, 16-CH3), 3.35 (lH, d, 5 ~ 7 . 517-CHendo), , 4.30 (lH, d, J =7.5, 17-CHe,,) ppm; liUC (CDC13): 39.39 (C-1), 18.09 (C-2), 42.47 (C-3), 33.74 (C-4),53.98 (C-5),20.68 (C-6), 36.56 (C-7), 83.28 (C-8), 56.30 (C-9), 37.97 (C-lo), 18.94 (C-11),36.80 (C-12), 106.68 (C-131, 24.92 (C-16),74.11 (C-171, 22.40 (C-18),34.28 (C-19, 15.25 ((2-20) ppm; mlz 278(M, 5.0), 263 (8.8), 260 (4.21, 248 (5.3),245 (2.7), 236 (9.3),233 (5.8),218 (82.1), 215 (2.1),208 (3.7), 203 (27.4),190 (99.8),177 (12.9),175 (51.9),137 (44.4), 121 (42.6), 109 (65.9), 95 (39.71, 91 (29.81, 81 (41.51, 69 (41.51, 55 (3.0); (M+*) obtained, 278.2233; C ~ H 3 0 2requires: 278.2245 and f13RMB.13~13.174.4' (0.3 g, CHCl3), Umax die~oxv-14.15-dinorlabdane 11 (5%) [al2OD= +a

(CHC13): 2994,2960, 2949 (uCH), 2884, 2855 (uOCH~O),1464 (6CH2), 1390 (6CH3), 1263,1209,1116,1048(uCOC), 865 (GCOCcyclic) cm-l; GIH (CDC13): 0.86 (3H, S, l8-CH31, 0.88 (3H, s, 19-CH3), 1.10 (3H, S, 20-CH3), 1.43 (3H, s, 16-CH3), 3.32 (lH, d, J =6.6, 17-CHend0), 3.77 (lH, d, J =6.6, 17-CHeXo) ppm;

6%

( C & ) : 40.69 (C-l), 17.57 (C-2),42.64 (C-3),33.84 (C-4), 51.03 (C-51, 19.87 (C-6), 34.34 (C-7),82.36 (C-81, 55.46 (C-91, 39.19 (C-IO), 19.12 (C-W, 36-93(C-12), 109.08 (C-13), 25.67 (C-lS), 76.77 (C-171, 22.50 (C-18),34.51 (C-19), 17-11 (C.20) ppm; mlz 278 (M+-, 4.6), 263 (7.1),248 (6.9, 236 (18.2), 233 (7.1),218 (99.9),208 (2.6),203 (27.7), 190 (64.3),177 (13.8),175 (56.01, 137 (6 6 4 ,1 2 1 (44.8), 109 (53.5), 95 (41.7), 91 (31.51, 81 (44.4, 69 (42.2), 55 (42.6); (M+-), obtained: 278.2233; C ~ H 3 0 0 2 ,requires: 278.2245. Elemental analysis: 77.53% C; 11.07%; 11.38%0; C u H ~ 0 2requires: , 77.69% C; 10.79% H; 11.52% 0.

646

4.2.With a solid support in the absence of solvent The catalyst (silica gel 230-400 mesh, 0,4g; silicalite 0,5g; activated vermiculite, I (6,O 0,l g) was added to a solution of 8a,17-epoxy-14,15-dinorlabdan-13-one mg) in dry petroleum ether (3 ml), the solvent removed and the mixture heated to 150-200°Cfor 1-4 hours. Addition of solvent, filtration of the suspension and concentration of the filtrate produced the (13Rr80.13:.17--dinorlabdane I1 as an oil (6,O mg; loo%), identified as described in 4.1, by comparison with an authentic sample. 4.3. With acidic activated silica gel

To a suspension of silica gel (0,ll g) in dichloromethane (0,5ml) was added a 10% aqueous solution of oxalic acid (0,8ml) and a solution of 8cc,l7-epoxy-14,15-dinorlabdan-l3-one I (2,4 mg) in dichloromethane (1 ml). After reflux for 24 hours, the reaction mixture was treated with sodium bicarbonate, the organic phase separated, dried and concentrated to afford a mixture of (13R)-w3.17-*-14.15-dinorlabda I1 (25%),white needles m.p. 121-122°C (lit.12 m.p. 121-122°C)and 0 - 8 a . 1 3 : 3 . 1 7 - d i e ~ o x v - 1 4 . 1 5 - ~ I11 (70%),white rosettes, m.p. 112-115°C (lit.12 m.p. 115-116OC)also identified by comparison with authentic samples. &REFEzENCEs 1. Cohen, Z. and Mazur, Y., J. Org. Chem., 44 (1979)2318-2320. 2. Cohen, Z.,Varkony, H., Keinan, E. and Mazur, Y., Org. Synth., 8B (1980) 176-182. 3. Degl’Innocenti, A. and Walton, D. R. M., Tktrahedron Lett., 21 (1980)39273928. 4. Bram, G. and Loupy, A. Silica-supported reagents: reactions in dry media, in Preparative Chemistry using Supported Reagents, €? Laszlo (Ed.), Academic Press Inc., London, 1987, p. 387-400. 5. G. Ohloff, C. Vial, H. R. Wolf, K. Job, E. JBgou, J. Polonsky and E. Lederer, Helv. Chim. Acta, 63 (1980)1932. 6. H. R. Schenk, H. Gutmann, 0. Jeger and L. Ruzicka, Helv. Chim. Acta, 35 (1952)817. 7. L. Ruzicka et al., U.S. Pat. 3,144,465 (1964). 8. E. Demole, Experientia, 20 (1964)609. 9. a) M. C. Costa, phD Thesis, Faculdade de Farmdcia, Univ. Cldssica de Lisboa, 1992. b) Portuguese Patent N. 98 344, “Processo para a Produp3o de Espirocetais LabdPnicos do Tip0 &bar Cinzento“, 17th July 1991. 10. a) Laszlo, P., Homogeneous and heterogeneous reaction conditions, in: “Preparative Chemistry Using Supported Reagents”, Laszlo, P., (Ed.), San Diego, Academic Press, 1987, p. 3-12; b) Posner, G. H., Alumina and alumina supported reagents, ibid., p. 287-315. 11.Casal, B., Ruiz- Hitzky, E., J . Catalysis, !Z (1985)291-295. 12. Scheidegger, U., Schaffner, K., Jeger, O., Helv. Chim. Acta, 45 (1962)400.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemiculs ZZZ 0 1993 Elsevier Science Publishers B.V. All rights reserved.

647

Diels-Alder cycloaddition reaction between dihydropyran and acrolein over various H-form zeolites.

R. Durand, P. Geneste, J. Joffre and C. Moreau Laboratoire de Chimie Organique Physique et Cinbtique Chimique Appliquees, URA 418 CNRS, Ecole Nationale Supbrieure de Chimie, 8 Rue de 1'EcoleNormale, 34053 Montpellier Cedex 1,France

Abstract The Diels-Alder cycloaddition reaction of dihydropyran with acrolein was performed in the presence of various H-form zeolites such as H-Faujasites, H-p, H-Mordenites which differ both in their shape selective as well as their acidic properties. The activity of the different catalysts was determined and the reaction products were identified. High yields in cycloadduct were obtained over dealuminated HY (Si/Al=15) and Hp (Si/Al=25) compared to HM (Si/Al=lO).These results were accounted for in terms of acidity, shape selectivity and microporosity vs mesoporosity properties. The activity and the regioselectivity were then discussed in terms of frontier orbital interactions on the basis of MNDO calculations for thermal and catalyzed reactions by complexing the diene and the dienophile with Bronsted and Lewis acidic sites. From these calculations, Bronsted acidic sites appeared to be more efficient than Lewis acidic sites to achieve DielsAlder reactions.

INTRODUCTION The Diels-Alder cycloaddition reaction is of fundamental importance in organic synthesis, particularly in the field of perfumes, flavors, fragrances, fungicides, pesticides, pharmaceuticals ... This reaction is a double bond (dienophile) 1,baddition to a conjugated diene (4+2 cycloaddition), and the product is always a six-membered ring. Competing reactions are polymerization of the diene or dienophile or both and 1,2cycloaddition. However, yields are usually quite high. No catalyst is needed but this reaction was known for several years to proceed through catalysts with Lewis acidic properties (143,usually for reactions in which there is a C=O or C=N bond in one of the reactants.

648

More recently, catalysts such as silica (6),clays (7), zeolites (8-10) were also reported to catalyze the Diels-Alder reactions and two important concepts were invoked from these few reports : i- an acidic catalytic effect on the reaction rate and ii- a n entropic effect due to the presence of cavities in some zeolites. In order to get more information on these two effects we have performed the Diels-Alder reaction between clihydropyran and acrolein in the presence of various H-form zeolites such a s H-faiijasites, H+ and H-mordenites. This reaction was reported to proceed with difficulty under thermal conditions in the absence of catalyst (11). The catalysts tested in this work differ both in their shape selective as wcll a s their acidic propertics On the other hand, molecular orbital calculations (MNDO) have been performed to account for th e expcrimental results on the uncatalyzed and catalyzed reactions.

EXPERIMENTAL Experiments were carried out without solvent, a t O"C, by mixing in a round bottomed flask cquimolar amounts of dihydropyran and acrolein (0.12 mol) with 0.5 g of calcined H-form zeolite (6 hours a t 5OO0C, I W m i n ) . The reactions were followed by gas chromatography analysis on a OV 1 capillary column. Beta-zeolites were synthesized and characterized according to the procedure reported elsewhere (12). Dealuminated mordenite ZM5 10 (Si/AI=lO), fuujasites ZF510, ZF515 and ZF520 (Si/AL=lO, 15 and 20, respectively) were obtained from Zeocat and fau.jclsites CBV720 m d CBV740 (Si/AI=15and 20, respectively) from PQ Zeolites. For these dealuminated catalysts, total and framework SUAI ratios were given a s identical. The amorphous silica-alumina ( 13%J alumina) was obtained from Ketjen. Micropore and mesopore volumes were deduced from th e isotherms of' adsorption of nitrogen a t 77 K (?'-plot method).

RESULTS AND DISCUSSION Under thermal reaction conditions (150°C during 15 hours), a 5% overull yield of cis 1,8 dioxaoctahydronaphthalene 3 (Scheme 1) was obtained (11).

2,3-dihydropyran 1 -

acrolein

2 -

cis I ,8-diox3ochhydronaphthalerie 3-

Scheme 1. Major route to cis 1,8-dioxaoctahydronaphthalene t h r o u g h cycloaddition of di hydropyran with1 acrolein.

649

Some improvements for this reaction might be expected in the presence of acidic catalysts, particulary solid acidic catalysts such as silica-alumina and zeolites. Under these conditions, an increase in the rate as well as in the yield of the reaction was immediately observed. However, small amounts of by-products were detected. The possible reaction pathways outlined in Scheme 2 account for all the cycloadductsidentified.

-1 + 2

-

+

H-Zeol i te

0°C

I

a

-Y

4

f

I

+2t

f

Acrolein polymerisation

-5

6

Scheme 2. Reaction pathway for Diels-Alder cycloaddition of dihydropyran with acrolein over solid acidic catalysts. The Diels-Alder cycloaddition is known to proceed through a cis-addition in such a manner that cis 1,8-dioxaoctahydronaphthalene3 is the major product. Small amounts of its regioisomer, cis 1,5-dioxaoctahydronaphthalene4 and its 5 resulting from the acid trans isomer, trans-l,8-dioxa-octahydronaphthalene catalyzed rearrangement of 3 were also identified. Additionally, dimers and trimers resulting from self-addition of acrolein or addition of acrolein to the different dioxaoctahydronaphthaleneadducts (3.4.5J were also detected. Table 1 reports the initial reaction rates and the maximum yields for the conversion of dihydropyran and acrolein into cis 1,8-dioxa&ahydronaphthalene at 0°C. Table 1 Zeolite-catalyzed reaction of dihydropyran with acrolein at 0°C. Catalyst none Silica-alumina H-mordenite H-beta H-beta H-faqjasite H-faqjasite H-faqjasite H-faqjasite

Si/Al

v, 103 mol.min-lg1

5.7 10 13 25 2.5 10 15 20

5.2 11.8

Reaction time

(%)

15 h(150"C) 2h 2h 2h 50 min

lh 6.6 14.8 9.0

Maximum yield

45 min 10 min 30 min

3 5 1 1 66 65 5 39 62 52

4

5

3 4

1 2

2 5 4

1 2 1

650

From the results reported in this table, it can be pointed out that : i Acidic properties are most important to obtain high activity and yield as, for example, dealuminated H-Faujasite (Si/Al=15) and H-beta (Si/Al=25). Such a behaviour is in close agreement with the literature concerning the increase in the acidity of zeolites through partial dealumination, i.e. the numher of protonic species is decreased but their strength is reinforced. Characterization of Bronsted and Lewis acidity of zeolites is relatively well documented (13), particularly in the gas phase where it is possible to differentiate between the two types of acids from the analysis of products formed as, for example, in deamination of sec-butylamine (12). However, for liquid phase reactions, it is very difficult to correlate the results with Bronsted or Lewis acidity as the reaction conditions used are different from those used for characterization. When the Diels-Alder reaction is conducted in a solvent, it appears that the maximum for the activity of HY zeolites is obtained for a Si/Al ratio of 15. This maximum was also observed for esterification of carboxylic acids (141, methylthiolation of phenol with dimethyldisulfide (15), acylation of toluene with benzoic acids (15) or dehydration of fructose (151, and in solvents such as alcohols, water or hydrocarbons. If we assume that Lewis species are transformed to Bronsted ones in the presence of water as solvent, this would thus mean that the Diels-Alder reaction is preferentially catalyzed by Bronsted species ,the maximum observed at Si/Al=15 for HY zeolites being a good balance between the number and the strength of the protonic species. ii Zeolites with a tridimensional framework such as faqjasites and p zeolites are required to obtain high yields in the adduct, compared to zeolites with a bidimensional framework (H-mordenites).The presence of cavities in the former zeolites allows the adduct to be formed, whereas in the latter ones diffusional limitations may occu due to the presence of only one large-size channel (Table 2). iii The presence of these large cavities seems to corroborate the hypothesis concerning the concentration effect, compared to non microporous materials. This concentration effect would be equivalent to an increase in pressure (10). iv. Finally, the mesoporosity, which favors accessibility to acidic sites, is necessary for the reaction to take place, as illustrated in Table 2 for the HY series. The HY (2.5) zeolite, with a low mesoporous volume, appears to be inactive towards the Diels-Alder reaction, even if its acidity, in terms of meq H+/g,is higher than for other dealuminated faujasites.

Table 2 Initial reaction rates, accessibility, micropore and mesopore distribution for different catalyst families. Catalyst tSi/Al) HM (10) HY (2.5) HY (10) HY (15)

pore diameter microporosity mesoporosity (Angstr6ms) (Cclg) (Eclg) 7.0 x 6.5 2.6 x 5.7 7.4 I1 I1

HY (20)

II

Hg (13)

7.6 x 6.4 5.5 x 5.5

VO x 103 mol min-lgl

0.192

0.056

0

0.248 0.249 0.269 0.279 0.175

0.121 0.224 0.243 0.263 0.481

0 6.6 14.8 9.0 5.2

651

Quantum Chemical intermetation

A great number of theoretical works have been published on Diels-Alder reactions. Some of them deal with the interpretation of the increased reaction rate produced by Lewis acid catalysts (16-18). For the reaction reported in this paper, molecular orbital calculations were computed using the MNDO method (19) implemented in MOPAC (20) and performing full geometry optimization. The modelling of the catalytic activity was achieved by complexation of one reactant with either a proton (Bronsted acid) or with AlCl3 (Lewis acid). The calculated reaction enthalpies showed that, for a Lewis acid catalyst, acrolein ( - 2 5 . 2 kcal/mol) is more complexed th an dihydropyran (-8.2 kcal/mol). The same conclusion is obtained by comparison of protonation enthalpies of acrolein (- 196.4 kcal/mol) and dihydropyran (-177.5 kcallmol). In order to account for th e higher reaction rates, the well-known Frontier Molecular Orbital Theory was used. For th e thermal reaction, t h e smallest HOMO-LUMO energy difference for the couple diene-dienophile is calculated to be high (9.1 eV). In this case the magnitude of the frontier orbital coefficients agrees with the experimental regioselectivity (Scheme 3).

-8 -9.18 e"

I3

Scheme 3 : Orbital frontier interactions for thermal and catalyzed reaction For the catalytic reaction, it was assumed th a t adsorbed acrolein reacts on free dihydropyran, as suggested by the thermodynamical results. In fact, th e complexation of acrolein with a Lewis acid reduces the LUMO-HOMO gap to 6.9 eV and thus stabilizes th e transition state of t h e reaction. T h e acrolein protonation lowers again the orbital level of the diene and in this case the gap is lowered to 2.2 eV. In all three cases (uncatalyzed, Lewis and Bronsted acidcatalyzed) the major orbital interaction leading to the transition state occurs between the LUMO of acrolein and the HOMO of dihydropyran. Moreover the maximum overlap principle predicts the same major regioisomer, in agreement with experimental results.

652

From a qualitative point of view, both acid-catalyzed mechanisms may account for the observed reactivity, and Bronsted acid catalysts seem to be more efficient than Lewis ones. However, limitations of catalyst modelling prevent us from drawing further conclusions.

CONCLUSIONS The results confirm the high potentiality of large pore zeolites in the DielsAlder cycloaddition reactions which can be performed with high regio and stereoselectivity a t low temperature and pressure, and the validation of the quantum chemical approach to interpret both catalytic activity and selectivity for these reactions.

REFERENCES 1. E.C. Angell, F. Fringuelli, M. Guo, L. Minuti, A. Taticchi and E. Wenkert, J . Org. chem., 53 (1988) 4325 and references therein. 2. H. Hartmann, H. Abdel, F. Abdel, K. Surtor, J . Weetman and G. Helmchen, Angew. Chem., 99 (1987) 1188. 3. Z. Chen, S. Liaos and Q. Shen, Yingyong Huascue, 4 (1987) 72. 4. C. Collet and P. Laszlo, Tetrahedron Lett., 32 (1981) 2905. 5. P. Laszlo and H. Moison, Chem. Lett., (1989) 1031. 6. V. Veselouskii, A. Lozanova, A. Moiseenkov, A. Gybin and V. Smit, Izv. Akad. Nauk. SSSR, Ser. Khim., 4 (1987) 959. 7. C. Cativelia, J.M. Fraile, J.I. Garcia, J.A. Mayoral, F. Figueras, L.C. De Menorval and P.J. Alonso, J . Catal., 137 (1992) 394. 8. Narayana Murty, Y.V.S. and C.N. Pillai, Synth. Commun., 21 (1991) 783. 9. R.M. Dessau, J . Chem. Soc., Chem. Commun., (1986) 2167. 10. J. Ipaktschi, J . Naturforsch., 416 (1986)496. 11. R. Paul and S. Tchelitcheff, Bull. Soc. Chim. Fr., (1954) 672. 12. M. Lequitte, F. Fibweras, C. Moreau ans S. Hub, Appl. Catal., 84 (1992) 155. 13. H. Karge, "Catalysis and Adsorption by Zeolites", G. Ohlmann, H. Pfeifer and R. Fricke (Editors), Elsevier, Amsterdam, Studies in Surface Science and Catalysis, 65 (1991) 133. 14. A. Corma, H. Garcia, S. Ibarra and J. Primo, J. Catal., 120 (1989) 78. 15. Unpublished results. 16. I. Fleming, Frontier Orbitals and Organic Chemical Reactions, J . Wiley, New York, 1976. 17. O.F. Guner, R.M. Ottenbrite, D.D. Shillady and P.V. Alston, J. Org Chem., 52 (1987) 391. 18. V. Branchadell, A. Oliva and J . Bertran, J . Mol. Struct. (Theochem), 138 (1986) 117. 19. M.J.S. Dewar and W. Thiel, J. Am. Chem. Soc., 99 (1977) 4899. 20. J.J.P.Steward, J. Comp. Mol. Des., 4 (1990) 1.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicnls 111 8 1993 Elsevier Science Publishers B.V. All rights reserved.

653

Rearrangement of Acetals of 2-Bromopropiophenone as a Test Reaction to Characterize the Lewis Sites in Large Pore Zeolites F. Algarra", A. Corma, V. Fornes, H. Garcia, A. Martinez, and J. Primo. lnstituto de Tecnologia Quimica, UPV-CSIC, and 'Departamento de Quimica, Universidad Politecnica de Valencia, Camino de Vera s/n, 46071 Valencia (Spain).

Abstract Rearrangement of dimethyl and cyclic ethylene acetals of 2bromopropiophenone to 2-phenylpropanoates (3)can be catalyzed in the liquid phase at 130QCin the presence of metal ion exchanged large pore zeolites, including a novel crystalline structure. IR Spectroscopic characterization of the sites and comparison of the results achieved with the HY, LaNaY and the rest of the samples have shown that Bronsted acid sites and hard Lewis sites are responsible for the hydrolysis of the acetal, the major side reaction. On the other hand, an increase of the softness of the Lewis site tend to favour the competitive 1,2-alkoxy migration leading to dihydro-l,4-dioxin or 2-methoxypropiophenone versus the 1,2-phenyl shift giving 3.Finally, a small extent of debromination through a radical mechanism has also been observed. 1. INTRODUCTION

While it is well known how the acid strength of Bronsted sites depends on the physicochemical parameters of a zeolite, the situation in the case of Lewis centers is much less established. In particular the influence of the chemical composition and crystalline structure on the more tenuous concept of softness and hardness of the metal sites is a subject of recent interest [l]. In the present work, we have carried out the rearrangement of acetals of abromopropiophenone to 2-phenylpropanoates catalyzed by large pore zeolites containing Lewis metal ions. We have found that our results can be interpreted taking into account the Lewis nature of the active sites but also assuming that their softness-hardness is modified by the zeolite framework. 2-Arylpropanoic acids exhibit a potent analgesic and antiinflammatory activity [2], and some of them (Naproxen, Ibuprofen, Ketoprofen, Flurbiprofen, etc.) are currently among the best-selling non-steroidal antiinflammatory drugs. It is expected that their sales still increase in the forthcoming years [3].

654

One of the most general approaches to the synthesis of this type of compounds involves the Lewis acid promoted rearrangement of acetals of asubstituted propiophenones [4,5]. Besides silver salts, a large variety of soft and borderline Lewis acids have been found to be convenient catalysts for the 1,2-aryl shift, noticeably zinc halides in substoichiometric amounts [6].As final products need to be free of metal traces for human consume, the use of Lewis acids supported on microporous solids can be advantageous since a better recovering of the catalyst can be anticipated. Although it is known that the presence of electron-donor substituents on the aromatic ring greatly enhances the efficiency of the transposition leading to 2arylpropanoates, we have chosen for our study the unsubstituted phenyl group, which shows a low migratory aptitude. This will provide us a deeper understanding of the influence of the physicochemical parameters of the zeolite on the side reactions competitive to the 1,2-aryl shift.

2. EXPERIMENTAL 2.1. Materials The HY sample was prepared submitting a commercial NaY faujasite (Union Carbide UY-52) to three consecutive NH',. exchange and thermal decomposition (773 K) cycles following the protocol previously described [7]. Chemical analysis showed that the Na' content of the HY sample was less than 0.10 wt% of the zeolite. Preparation of the ZnHY catalyst was accomplished stirring at 353 K for 2 h the HY zeolite with a 1 M aqueous solution of ZnSO,, followed by drying and calcination at 773 K for 3 h. The ZnNaY sample was obtained by submitting the NaY zeolite to an initial exchange with a 0.05 M aqueous solution of Zn(NO,), at 353 K for 1 h, drying and calcination at 773 K for 3 h, followed by neutralization by stirring the solid with a 1 M aqueous solution of NaCl at room temperature for 24 h and a The final exchange-calcination process using a 1 M aqueous solution of Zn(NO,),. ZnX sample was prepared by submitting a NaX zeolite (Union Carbide, Si/AI = 1.2) to two consecutive exchange-calcination (773 K) steps with a 1 M aqueous solution of Zn(NO,), at 353 K for 1 h. Zn-ETAS and Zn-ETS samples were obtained by exchange-calcination of the starting ETAS and ETS materials, respectively, in the ammonium form with an aqueous solution of Zn(NO& under the same experimental conditions as above. Likewise, the AgNaY, HgNaY and LaNaY were prepared by submitting the NaY zeolite to the same ion exchange-calcination procedure but using aqueous solutions of AgNO, (1 M, pH 7), Hg(NO,), (0.1 M, pH 1) or La(NO,), (1 M, pH 3) respectively. A solid-to-liquid ratio of 10 was used in all the exchange steps. The amount of active metal ion per unit weight was determined by atomic absorption spectrophotometry after solving the solid with HF. Dimethylacetals of a-bromopropiophenone and a-bromoacetophenone were synthesized by stirring at 323 K for 2 h a solution of the corresponding phenyl ketone

655

(Aldrich) (20 mmol) in trimethyl orthoformate (2 ml) and methanol (20 ml) using rnethanesulfonic acid (0.2 ml) as catalyst. After this time, the mixture was partitioned between CH,CI, and a 0.1 M aqueous solution of NaHCO,, the organic layer dried over anhydrous NqSO,, the solvents removed and the residue distilled under vacuum. The cyclic ethylene acetal of a-bromopropiophenone was accomplished by heating at reflux temperature for 4 h a mixture of the propiophenone (20 mmol), ethyleneglicol (5 ml) and 4-toluenesulfonic acid (100 mg) in toluene (50 ml) using a Dean-Stark system for the azeotropic removal of the water formed. Then, the suspension was throughly washed with water, the organic phase dried over NqSO,, the solvent removed under vacuum and the residue submitted to flash column chromatography using mixtures of hexane-CH,CI, as eluent. Spectral data of starting acetals were in agreement with the reported values.

2.2. Reaction Procedure A solution of the corresponding acetal (100 mg) in chlorobenzene (50 ml) was poured onto the thermally activated (423 K, 1-3 Torr, 2 h) catalyst (350 mg) and the resulting suspension magnetically stirred at 403 K. The course of the reaction was periodically followed by GC (25 m capillary column of cross-linked 5% phenyl methyl silicone) analysis of the organic solution at times ranging from 0.25 to 20 h. At the end of the reaction the catalyst was filtered and submitted to continuous Soxhlet extraction with CH,CI,. The organic solutions were concentrated in vacuum, weighed and analyzed by GC-MS (HP 5988A spectrometer) and GC-FTIR (HP 5965A detector). Isolation of the reaction mixtures was carried out by flash column chromatography using mixtures of hexane-CH,CI, as eluent. 2.3. Spectral data of compounds 3a and 4a 2-Hydroxyethyl 2-phenylpropanoate (3a), IT-IR: 3650 (OH), 1755 (C=O ester); 'H-NMR: 7.3 (br s, 5H, C,H,), 4.2-3.7 (m, 4H, CH,CH,); MS: 194 (M+, 2). 2151 (5),150 (l), 133 (2), 105 (loo), 103 (16), 91 (6), 79 (13), 77 (23). 5-Methyl-6-phenyl-2,3-dihydro-l,4-dioxin (4a), FT-IR 3067 (w), 2976 (m), 2880 (m), 1678 (m), 1311 (m), 1228 (s), 1142 (m), 1054 (m), 914 (m), 760 (m), 696 (m); 'H-NMR: 7.8-6.9 (m, 5H, C,H,), 4.1-3.1 (m, 4H, CH,CH,), 2.2 (s, 3H, CH,); MS: 176 (M', 25), 131 (l), 115 (4), 105 (loo), 77 (31). 3. RESULTS AND DISCUSSION Treatment of the cyclic ethylene acetal l a in chlorobenzene at 403 K in the presence of the eight ion exchanged zeolites affords variable mixtures of 2bromopropiophenone (2), 2-hydroxyethyl 2-phenylpropanoate (3a)and 5-methyl-6phenyl-2,3-dihydro-l,4-dioxin (4a). Trace amounts of propiophenone, 2hydroxypropiophenone, 1-hydroxy-1 -phenylpropanone and 2-phenylpropanoic acid were also present in some reaction mixtures. Likewise, reaction of the dimethyl acetal l b gives rise to mixtures of 2bromopropiophenone (2), methyl 2-phenylpropanoate (3b) and 2-

656

methoxypropiophenone (4b),accompanied in some experiments with trace amounts of propiophenone, 1-methoxy-1 -phenylpropanone and, tentatively, both Z and E stereoisomers of the methyl enol ether of 2. The results attained are summarized in Tables 1 and 2. In order to establish a valid comparison, reaction of cyclic acetal l a was also carried out under the same experimental conditions using anhydrous ZnCI, and Hg,CI, as well as y-Al,O, as catalysts, The results are also included in the Table 1. Table 1 Results of the reaction of cyclic ethylene acetal la in chlorobencene at 403 K.

la

2

0 0 78 187 134

162 0 76 4 23 0 92 0 74 2 61

4a

1 4

100

91

0

0

98 86 89

66 23

X

7 19

4

1 1

2

0 14

15

5 t)

14

4

I

07 1 97 8 6

3 I 3

I I

0 15

0 09 I 4

58 50 80

6 048 8 5 19

75 9 28 1 (1 25 2 253 53

4

3X

7

/I 50

31 2

331 21

0 -33 2 5

10

2 6 1 2 1 3

0 05

0 4

0 12

3

0 12 0 21 0 12

.~

657

Table 2 Results of the reaction of dimethyl acetal 1b in chlorobencene at 403 K.

Catalyst

ZnNaY

m A s znE1s 2nx

LaNaY

Transition Conver metal content (%) (inmol g 1 )

1 87 092

100 100

0 74 2 61 0 94

93 995

100

Selectivities

khidr

(h 2

3b

4b

58 5

24

4 2

5 7

85 4 2 '1 842 98

8 5

4 5 7 5 5

19

10 6 8 5

I g I)

3b/4b

1 5 1 5

4 5 143

0 17 0 10 0 19 0 03

I

Almost all the hydrolysis product (90%) was a mixture of Z and E stereoisomers of methyl enol ether of 2.

a

3.1. Influence of the nature of the metal ion Taking into account that formation of the propiophenone 2 is the most important side reaction, we were interested in determining the exact nature of the active sites responsible for the hydrolysis of the acetal moiety. Aimed at this purpose, a characterization of the Bronsted and Lewis acidity of the centers was accomplished for the HY, ZnHY and ZnNaY samples by means of the pyridine adsorption method. Pyridine when adsorbed on solid acids, shows in the IR spectra specific bands assignable to pyridinium ion (1540 cm-') and Lewis adducts (1450 cm-I), which intensitiesare directly related to the population of both types of centers [8].The IR spectra of the 1750-1350 cm-' zone of pyridine adsorbed on these three ion exchanged zeolites are given in Figure 1. Although they show in all cases the presence of both types of centers, their relative intensities are in good agreement with the corresponding sample preparation procedure and a direct relationship between the ratio 2/(3a+4a), i.e. hydrolysis to rearrangement, and l,ss,J11,50,i.e. Bronsted to Lewis acid sites ratio, is obtained. Therefore, it can be concluded that each type of acid site catalyzes a different reaction, being the presence of adventitious Bronsted centers negative from the point of view of the catalyst selectivity.

8S9 Moreover, a comparison of the results achieved using the catalyst containing La3' with the other metal ion exchanged zeolites indicates that hard Lewis acids also do not catalyze the 1,2-aryl transposition. This finding fits well with the previous reported data for salts of these metal ions which shows that only soft and borderline Lewis acids can catalyze this rearrangement. On the other hand, looking for the origin of the water required for the hydrolysis step, control experiments using dried solvent and different substrate-tocatalyst ratio have shown that both water retained in the zeolite after the usual thermal activation and the solvent and ambient moisture are responsible for the hydrolysis. This undesirable side reaction has a much higher rate constant for the dimethyl than for the ethylene acetal as it is commonly observed for these types of acetals [9].

r 9

-I\

--

yvT-

Figure 1. i.r. spectra of piridine adsorbed

on HY (a); ZnHY (b); and ZnNaY (c)

Figure 2. Mechanism of the rearrangement reaction.

3.2. Influence of the zeolite framework on the softness-hardness

of the Lewis metal site. An important item in order to discuss the hardness and softness of the active sites is the relative selectivity of the products arising from phenyl (3a and 3b) or alkoxy migration (4a and 4b). In fact, as it can be seen in Table 1, the ratio between these two types of compounds W 4 a decreases along the softness of Lewis sites for the series ZnNaY, HgNaY and AgNaY. Furthermore, Tables 1 and 2 reveal that even for the series of Zn2’ containing catalysts, the ratio 3/4 varies significantly from the hard ZnCI, to the softer ZnNaX faujasite. Albeit these results could be taken for granted as it is well known that in general the surrounding sphere of the Lewis site strongly influences its softness and hardness [lo], the situation is much less understood when the cation is included within the voids of a zeolite. Our

659

experimental results indicate that the softness of the site increases as the framework Si/AI ratio decreases, which is in agreement with reported theoretical calculations [l]. On the other hand, the fact that the alkoxy shift to afford compounds 4 is favored by softer sites can be justified assuming that the rearrangement follows a SN2-like mechanism, in with the migrating group and the bromine leaving atom must be in a disposition anti periplanar (Figure 2). Therefore, the conformation leading to the 1,e-phenyl shift (I) must be different to that required for the alkoxy migration (I1 and 11') and an increase in the relative selectivity of compounds 4 would indicate a preferent stabilization of II, II' (or the transition states derived from them) over I. In fact, a recent X-ray study of the crystal structure of complexes of aryl a-bromoalkyl ketones with Lewis acids [ l l ] has established that the position where the acid is bound to the organic molecule depends on its softness, soft metal ions being coordinated to Br atom and the n-system of phenyl ring, lending support to our proposal of a greater contribution of conformations II and II' for these catalysts. 3.3. Influence of the framework composition and crystalline structure: ZnETAS and ZnETS catalysts Besides Y and X faujasites, we have also carried out the rearrangement using a novel large pore zeolite, whose structure is formed by three arrays of parallel cylindrical channels (75x75 A') crossing at right angles. Everywhere the three channels intersect a cubic supercavity is generated [12]. The difference between these two samples is the framework chemical composition, which contains titanium, aluminum, silicon and oxygen in the case of ETAS, while the ETS zeolite is composed by titanium, silicon and oxygen. As can be seen in the Tables, the behavior of these two new zeolites as catalysts in the rearrangement of acetals l a and 1b is similar to that found for the Y and X faujasites, although an appreciable diminution of the extent of acetal hydrolysis has to be remarked especially for the ZnETS sample. According to the 3/4 ratio, ZnETS is as soft as ZnNaX and appreciably softer than ZnETAS. When the analogous rearrangement of dimethyl acetal of abromoacetophenone to methyl phenylacetate was attempted under identical experimental conditions as acetals 1 in the presence of ZnETAS, acetophenone (4%) was the most characteristic product formed. In fact, as we have noted above, propiophenone is also formed in detectable amounts starting from acetals 1. Since it is unlike that hydride anions are involved under acidic conditions, the most reasonable mechanism for the debromination is the C-Br homolytic bond breaking followed by hydrogen abstraction from the medium of the resulting radical. Related precedents for a radical mechanism for the dehalogenation of alkyl halides are well documented in the chemical literature [13]. Aimed to explore the extent of these radical processes using our Lewis metal ion containing zeolites, we have treated the a-bromopropiophenone (2) in methanol/trimethyl orthoformate mixtures, which besides to be a better hydrogen donor medium than chlorobentene and hence more propiophenone should be now expected, could shift the equilibrium to the acetal 1b formation. The results obtained

660 show that radical processes leading to propiophenone occur cleanly on ZnETAS, and also on LaNaY but accompanied by acetalization and nucleophllic substitution. However, the ZnNaY sample shows that debromination is not a general pathway. 4. CONCLUSIONS

We proposgthat the reactionof acetals of 2-bromopropiophenone constitutes a test to distinguish between the different types of acid sites in a metal ion exchanged large pore zeolite. Thus, while Bronsted or hard Lewis sites produce hydrolysis of the acetal moiety, lI2-phenyl shift leading to 2-phenylpropanoate is catalyzed by soft and borderline Lewis sites. Moreover, the ratio between phenyl or alkoxy migration products appears to be controlled by the softness-hardness of the centers. A new large pore zeolite with two framework compositions (ETAS and ETS) have also been studied and it has been found that their chemical behavior is intermediate between Y and X faujasite. Finally, a small extent of debromination through a radical mechanism giving rise to propiophenone has been observed. 5. ACKNOWLEDGEMENTS

We thank to the Spanish DGICYT for financial support (Project PB90-0747). 6. REFERENCES

1. 2. 3. 4.

5.

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

A. Corma, G.Sastre, P. Viruelaand C. Zicovich-Wilson, J. Catal., 136 (1992) 521. S.H. Ferreira and J.R. Wayne, "Antiinflammatory Drugs", Springer-Verlag, New York, 1979, p. 321. S.C. Stinson, Chem. Eng. News, 1989, p.37. J.-P Rieu, A. Boucherle, M. Cousse, G. Mouzin, Tetrahedron, 42 (1986) 4095. M.A. Miranda, H. Garcia, in "Carboxylic Acids and Derivatives", The Chemistry of Functional Groups, Supplement 8, Chapter 26, S. Patai ed., Wlley, 1992. G. Castaldi, A. Belli, F. Uggeri, C. Giordano, J. Org. Chern., 48 (1983) 4658. M.J. Climent, A. Corma, H. Garcia and J. Primo, Appl. Catal., 51 (1989) 113. A. Corma, C. Rodellas, V. FornBs, J. Catal., 88 (1984) 374. J. March, "Advanced Organic Chemistry. Reactions, Mechanisms and Structure", J. Wiley, 4th Ed., New York, 1987. J.E. Huheey, "Inorganic Chemistry. Principles of Structure and Reactivity, Harper and Row, London, 1975, p. 229. T. Laube, A. Weidenhaupt, R. Hunziker, J. Am. Chem. Soc., 113 (1991) 2561. US Pat. 4,853,202 (1989), assigned to Engelhard Corporation. G.A. Molander, Chem. Rev., 92 (1992) 29.

M. Cuisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals IZZ 0 1993 Elsevier sdence Publishers B.V. All rights reserved.

661

Shape Selectivity in the Zeolite-Catalyzed Fischer lndole Synthesis M.S. Rigutto, H.J.A. de Vried, S.R. Magill, A.J. Hoefnagel and H. van Bekkum Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands. Fax (+ 31-15)-782655

Abstract It was found that zeolite beta is a highly shape-selective catalyst for the Fischer indole synthesis of 2-benzyl-3-methylindolefrom phenylhydrazine and 1-phenyl-2-butanone. A selectivity of 83 % for this isomer was obtained at full conversion. Combined results from catalytic experiments and sorption measurements indicated that the formation of the isomeric 2-ethyl-3-phenylindole is suppressed as a consequence of restricted transition state selectivity.

1. INTRODUCTION

Zeolites I r e known to catalyze the formation of various nitrogen-containing aromatic ring systems. Examples include the synthesis of pyridines by dehydrogenation / condensation / cyclization of C,-C, precursors [ l ] , the formation of methylpyridines by high-temperature isomerization of anilines [2],the amination of oxygen-containing heterocyclic compounds [3] and the Fischer indole synthesis [4,5]. The latter synthesis consists (see Scheme 1) of a condensation towards a phenylhydrazone followed by an acid-catalyzed cyclization with elimination of ammonia. The two reaction steps are usually combined in a one-pot procedure. As is shown in Scheme 1, the use of non-symmetrical ketones in the Fischer synthesis will result in two isomeric indoles, 4 and 5. A detailed reaction mechanism is described in refs. [6] and [7], but some important characteristics will be given here.

4 Present address: Rotterdam Polytechnic (HR&O), Chemistry Department, Kluyverweg 4, 2629 HT, Delft, The Netherlands.

662

The reaction is believed to involve a fast [3,3]-sigmatropic rearrangement of the enehydrazine tautomers 2 and 3, which in turn are formed from the hydrazone 1. The formation of 2 and 3 from 1, which is catalyzed by both Lewis and Brnrnsted acids, is R1

J/' 0

'R2

'\ Scheme 1. probably rate-determining [ 6 ] . Prochazka et al. [6] recognized that the spatially restricted reaction environment of zeolite pores can be a very effective catalytic tool for influencing the ratio of indole isomers formed. As the most striking example, the authors reported that the use of mordenite as a catalyst led to the selective formation of 2-benzyl-3-methylindole(5, R, = Ph, R, = CH,) from phenylhydrazineand 1-phenyl-2butanone in high yield, whereas according to the same report only 2-ethyl-3phenylindole(4) was formed when the reaction was performed in acetic acid. However, in attempts to reproduce those findings, we invariably obtained mixtures of both isomers. Since the use of molecular sieves in the shape-selectiveconversion of larger molecules in liquid-phase reactions has our keen interest, we felt prompted to make a more detailed study of the influence of the pore size and dimensionality of large-pore molecular sieves on the selectivity in the reaction of phenylhydrazine and 1-phenyl-2butanone. The results of that study are reported here.

2. EXPERIMENTAL

Materials 1-Phenyl-Bbutanone, phenylhydrazine and methanesulfonic acid were obtained from Aldrich and used as received. The following solid catalysts were used: silica-alumina Filtrol Grade 105 clay (Engelhard),Amberlyst 15 ionHA-HPV (Akzo, 25 wt. % AI,O,), exchange resin (Aldrich), NaY (Akzo, Si/AI = 2.4), zeolite beta (synthesized according to ref. [a], Si/AI= lo), mordenite (Union Carbide LZ-M-8, Si/AI = 10.4), mordenite (PQ-

663 zeolites, Si/AI = 56), mordenite (kindly supplied by H.W. Kouwenhoven, ETH Zurich, Si/AI =62), zeolite omega (synthesized according to ref. [Q], Si/AI=6.0), K-L (synthesized according to ref. [lo], Si/AI =3.0),AIP04-5 (synthesized according to ref. [ l l ] ) and SAPO-5 (synthesized according to ref. [ l l ] with the addition of Si(OEt),, Si/(Si + Al+ P) =0.05). Where necessary, the appropriate ion-exchange and/or calcination procedures were followed to obtain acidic, dehydrated materials. Modification of the outer crystal surface of zeolite beta - External surface silylation of zeolite beta A sample of beta was treated with a dilute solution of triphenylsilyl chloride in toluene at reflux. Comparison of diffuse reflectance and transmission FTlR spectra indicated selective silylation of the outer crystal surface. - Ekternal surface dealurnination of zeolite beta Dealumination of the outer surface of zeolite beta was performed by stirring an assynthesized sample overnight with a 0.025 M aqueous solution of EDTA, followed by thorough washing. The sample was then calcined for two hours at 550 "C (heating rate 1 'C/min). General procedure for the one-pot Fischer indole synthesis To allow comparison of the data, a procedure similar to that described in ref. [5] was used. The catalyst (3.00 g) was suspended into a solution of 1-phenyl-Zbutanone(1.48 g, 10.0 mmoles) in p-xylene (30 9). The mixture was then heated to reflux, and after 15 minutes phenylhydrazine (1.08 g, 10 mmoles) was added in one portion. Samples were taken at regular intervals and analyzed by GC using a CP Sil-5 CB capillary column. In several cases, the indole isomer mixture was isolated from the reaction mixture by vacuum distillation after removal of the catalyst and the solvent. 'H NMR shift values and coupling constants for 4 and 5 were identical to those reported in ref. [12]. The isomer ratios 4 5 were then determined by GC analysis and 60 MHz proton NMR spectroscopy, and agreed within two percent with the values obtained from GC analysis of the reaction mixture. The same amounts and conditions were used for the reactions carried out in refluxing acetic acid (30.0 g) and in xylene/methanesulfonic acid (30.0 g/1.15 9). Adsorption of 4 and h by molecular sieves The sorption experiments were carried out under nitrogen. A solution of 100 mg 4, 100 mg 5 and 200 mg 1,Bdiphenylbenzene in 5.00 g of 1,3,5-triisopropyIbenzenewas thermostatted at 140 "C. 500 mg of freshly calcined molecular sieve was added, and the uptake of both indole isomers was monitored by GC analysis. 1,2-diphenyIbenzene

664 was chosen as the internal standard after it had been verified that this compound could not be sorbed by zeolite Y at 140 'C.

3. RESULTS AND DISCUSSION

The results of our catalytic experiments are summarized in Table 1. An approximate 75:25 isomer ratio is observed when the reaction is catalyzed by organic acids, which agrees with previous observations [ 12,131. The result reported by Prochazka et al. in ref. [5] (see Table 1, entry 15) is probably erroneous and in fact contradicts their earlier work [13] (entry 18). The use of silica-alumina or acidic Filtrol clay results in only minor selectivity changes in favour of the 5 isomer. The use of (H,Na)-Y or H-mordenite further increases the selectivity for the 5 isomer, but the effect is small. In efforts to reproduce the results reported in ref. [5] (entry 17), we have tried several other mordenite samples, but all gave very similar results. In fact, it appeared that none of the molecular sieves having a one-dimensional channel system were effective in altering the reaction selectivity. (In this reaction, mordenite can be regarded as having a one-dimensional channel system, at least effectively, as the &ring pores are not expected to play any role in reactant accommodation or molecular transport.) Moreover, the catalytic activity of these systems was relatively low. A substantial improvement was, however, achieved when zeolite beta was used as the catalyst (entry 12). This catalyst was also found to be highly active, as was zeolite H-Y.

4- 11.9 x 9.0 x 4.3 A

5- 13.8 x 6.8 x 394 A

Figure 1. Molecular dimensions of the indole isomers 4 and 5.

665 Table 1. Fischer indde synthesis from 1-phenyl-2-butanoneand phenylhydrazineusing soluble and sdid acids. Entry

Catalyst

Conversion (%)"

Ratio of indole isomers 4 : 5 (%)

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

acetic acid2) rnethanesulfonic acid3) Amberlyst 15 resin Filtrol Grade 105 clay HA-HPV SiO,/AI,O, H-mordenite (UC)') H-omega (HN-L AIP04-5 H-SAPO-5 (H,Na)-Y H-beta H-beta. silylated') H-beta, surf. deal.')

100 100 100 100 100 95 56 40 35

this work

51 100 100 100 100

76 : 76 : 75 : 69 : 71 : 65 : 71 : 72 : 77 : 71 : 62 : 31 : 36 : 18 :

15 16 17 18

acetic acid acetic acid H-mordenite H-Y

100 100 100 100

4, 4,

4, 4, 4, 4,

100 78 7 83

: : : :

24 24 26 31 29 35 29 28 23 29 38 69 64 82 0 22 93 17

')Conversion of the hydrazone after 4 hours. 2)A~OHwas used as the solvent at reflux. 3)CH3S0,H:PhNHNH - 1 2.1 4)Thereaction was completed in less than one hour. 5)Sampleobtained from Union Carbide.6 External f - " ' surface silylated with Ph,SiCI (see text). ')External surface deluminated with EDTA (see text).

For the interpretation of the results of the catalytic experiments in terms of reactant accommodationor product diffusion, sorption experiments proved helpful.The data presented in Table 2 show the extent up to which the indole isomers 4 and 5 can be sorbed by some of the molecular sieves used in the catalytic experiments. The molecular dimensions of both isomers are shown in Figure 1. The materials with onedimensional channel systems are highly selective in sorbing the smaller indole isomer 5 from a mixture of 4 and 5, but they show only a very limited sorption capacity for this molecule. The three-dimensionallarge-porezeolites beta and Y on the other hand have a much higher uptake. Zeolite beta is nevertheless fully selective in sorbing the smaller 5 isomer, whereas zeolite Y also sorbs the larger 4 isomer and has only a slight preference for 5.

666 Table 2. Sorptlon of indole Isomers 4 and 5 onto large-pore molecular sieves Material

Channel system (sizes in A)')

Total indoles sorbed (wt. %)

H-Mordenite (HIK1-L

7.0 7.1 7.3 7.6 7.4

0.2 < 0.2 3.1 16.3 23.3

AIPO4-5

H-Beta (H,Na)-Y

x 6.5; 7 (1) x7.1; 13 (1) x 7.3; -- (1)2' x6.4; 11 (3) x 7.4; 13 (3)

Ratloof lndole Isomers sorbed (4 : 5) 0 : 100

7 : 9 3 0 : 100 42 : 58

')The geometries, taken from refs. [14] and [15], are described as AxB: C (D) where AxB denotes the size of the largest pore, C the estimated intersection or cavity size and @Jthe dimensionality of the channel system. In this case, mordenite Is regarded as having a one-dimensional channel system (see text). *)The structure of AIP04-5 does not possess any cavities.

For the zeolites with three-dimensional channel systems, the sorptionselectivities reflect the reaction selectivities rather well. The data suggests that zeolite beta should even be fully selective for the formation of 5 in the catalytic reaction. We assumed that in this case, the formation of isomer 4 is probably catalyzed by the outer surface of the crystallites. To test this assumption, efforts were made to minimize the outer surface acidity of zeolite beta. A silylation with triphenylsilyl chloride was performed, but the effect on the selectivity of the catalyst (Table 1, entry 13) was small. In view of the poor stability of the surface R3Si-0 bonds against ammonia or water (which are both liberated during the one-pot Fischer synthesis) this is not surprising. Dealumination of the outer surface proved to be a more effective treatment. As can be seen in entry 14 of Table 1, a catalytic experiment using the outer surface-deluminated material afforded isomer 5 with good selectivity. It is expected that the use of larger crystals of zeolite beta with well-developed faces, and hence a smalbr area of the external surface of the crystals, will give a further improvement.

The combined results from the sorption and catalytic experiments can be interpreted as follows: Catalysis of the indolization of 1 by soluble and solid, non-microporous Bransted acids gives a typical 4:5 isomer ratio of about 75:25.It was verified in separate experiments that no isomerisation of the indole products occurs under the conditions used in this study. At 140 "C,diffusion of the indole isomers into one-dimensional, non-intersecting

667

7.0-7.5 %, sized pores is very slow, which implies that counter-diffusion will not occur at all in these systems, and that intraporous reactions cannot occur at an appreciable rate. The use of molecular sieves with one-dimensional channel systems as catalysts therefore results in residual non-shape-selective catalytic activity caused by the outer surface of the crystallites. When H-Y and H-beta zeolites, which have three-dimensionalchannel systems, are used, intraporouscatalysis is dominant. The reaction selectivity is apparently closely related to the sorption selectivity. In the case of zeolite beta this means that the observed formation of the large indole isomer 4 can only be effected by acid sites located on the outer surface of the crystals. When intraporous formation of 4 would occur, it would lead to rapid deactivation of the catalyst because the molecule would neither desorb nor be converted (Although in zeolite beta the intraporous formation of 4 is largely suppressed, it is probably not completely inhibited; some deactivation was actually observed when 100 mg of zeolite beta was used in stead of 3 9). The fact that removal of outer-surface aluminium leads to a substantial selectivity improvement is consistent with this interpretation. As was mentioned before, isomerisation of the substituted indoles does not occur under the conditions used in this study. The selectivity of the catalytic reaction should therefore only depend on the relative rates of formation of the enehydrazines 2 and 3 in the conformationwhich allows their [3,3]-sigmatropic rearrangement to occur (which is the conformation drawn in Scheme 1). The sorption data and molecular geometries indicate that the formation of both enehydrazines 2 and 3 inside the channels of zeolite beta should be possible, but 2 is probably severely hindered in adopting the conformation required for indolization, given the fact that this conformation is even bulkier than the indole isomer 4 which is formed from it. The selective Fischer synthesis of 2-benzyl-3methylindole 5 catalyzed by zeolite beta is therefore a true example of transition state selectivity.

REFERENCES 1.

2.

W. Holderich and H. van Bekkum in "Introduction to Zeolite Science and Practice" (H. van Bekkum, E.M. Flanigen and J.C. Jansen eds.), Stud. Surf. Sci. Catal., 58 (1991) 698, and refs. cited therein. C.D. Chang and P.D. Perkins, Zeolites, 3 (1983) 298.

668

3. 4.

5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15.

H. Le Blanc, L. Puppe and K. Wedemeyer, Ger. Patent DE 3,332,687 (1975), to Bayer AG. P.B. Venuto and P.S. Landis in "Advances in Catalysis and Related Subjects" Vol. 18, (D.D. Eley, H. Pines and P.B. Weisz eds.), Academic Press, New York, London, 1968, p. 259. M.P. Prochazka, L. Eklund, R. Carlson, Acta Chem. Scand., 44 (1990) 610; M.P. Prochazka, R. Carlson, Acta Chem. Scand., 44 (1990) 614. A.W. Douglas, J. Am. Chem. SOC.,100 (1978) 6463. J. March, "AdvancedOrganic Chemistry", 3d ed., Wiley-Interscience, New York, 1985, p. 1032. R.L. Wadlinger, G.T. Kerr, E.J. Rosinski, U S . Patent Reissue 28,341 (1975), to Mobil Oil Corp. F. Fajula, S. Nicolas, F. Di Renzo, C. Guegien and F. Figueras in "Zeolite Synthesis", (M.L. Occelli and H.E. Robson eds.), ACS Symp. Ser., 398 (1989) 495. T.M. Wortel, Eur. Patent 96,479 (1983), to Exxon. S.Qiu, W. Pang, H. Kessler and J.-L. Guth, Zeolites, 9 (1989) 440. G. Baccolini, G. Bartoli, E. Marotta and P.E. Todesco, J. Chem. SOC., Perkin Trans. I, 1983, 2695. M.P. Prochazka and R. Carlson, Acta. Chem. Scand., 43 (1989) 651. W.M. Meier and D.H. Olson, "Atlas of Zeolite Structure Types", 3rd rev. ed., Butterworth-Heinemann, London, 1992. R.M. Barrer and H. Villiger, Z.Kristallogr., 128 (1969) 352.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals ZZZ Q 1993 Elsevier Science Publishers B.V. All rights reserved.

669

CC"RIBWI0N To THE S W Y OF ISOBWEBE CONDENSATION WITH FORMALDEHYDE CATALYZED BY ZEOLITES E. hnnitriu, D. Gongescu and V. Hulea Laboratory of Catalysis, Faculty of Chemical Engineering, 71 Splai Bahlui, Jassy-6600, Romania Abstract The behaviour of

the volcanic tuff, HZSM-5, HM and beta zeolites and in catalysing the condensation of isobutene with fonnaldehyde was canpared and the effect of the most important reaction parameters on the catalyst performances was systematically studied. Interesting selectivity to isoprene, coupled with resonable life, was obtained with beta zeolite, other catalysts giving satisfactory results. The correlation between acidic and catalytic properties of the catalyst in question has been revealed. SAPO-5

1. INrRoDuc!rI0N

The continuous increase of isoprene demand has determined the enhancement of researches focused on the elaboration of new and efficient routes for the industrial synthesis of this valuable diene. The acid-catalyzed condensation of fonnaldehyde and isobutene, known as Prins reaction, is already a recognized industrial route for the isoprene synthesis, being applied as two-stages process, condensation to form 4,4-dimethyl-l,3-dioxane in the presence of an aqueous sulphuric acid and its decomposition to isoprene using solid phosphoric acid catalysts. The direct production of isoprene, the vapor-phase one-stage process, is a very attractive route on account of its simplified procedure. The known catalysts proposed in the previous studies are based upon various oxides of Si, Al, P I Sb /l/, V /2/ etc, and the attempts to find catalysts having improved properties have led to the following important conclusions: (1) the possession of an acidic property is indispensable for a solid catalyst in the vapor-phase condensation (all catalysts proposed in the patent literature are acidic oxides); (ii) - the catalysts possessing strong acidic sites /2-4/ are not effective and there is a certain acid strength just proper to catalyze the reaction; (iii) the best results are obtained with the oxide mixtures which can generate both acidic and basic sites /2,5,6/. An elegant manner to answer to the point could be the use of molecular sieves if the following considerations are taken into account: there is a great variety of molecular sieves having different acidities and distribution of the acidic sites; the acid-base properties of molecular sieves can be easily rmdified by ion exchange, impregnation, dealumination and other methods; - due to the spatial constraints, imposed especially by the medim pores, the molecular sieves can inhibit the formation of bulkier products; - many molecular sieves possess a high catalytic stability and a good re-

-

-

-

670

sistance to coking. Besides the patent literature very few scientific reports are devoted to this subject /7,8/, and therefore a systematic investigation is needed. Our paper is intended to study the behaviour of some zeolites in Prins reaction and to emphasize the correlation between the zeolite acidity and its catalytic performances. 2. EXPERIMENTAL

1. Catalysts. HZSM-5 and beta catalysts were prepared fran cmnercially powder cake (CBV 8020 and CP 811 BL fran P.Q. Zeolites). SAPO-5 catalysts,

sample S-1 having the ccmposition (Sio~5&Alo.383Po,363)02and respectively (Si0.231Alo375P0.39&)02 for S-2, were prepared in our laboratory, following the well-known procedure /9/. HM (Si/A1 = 10) was prepared as previously reported /lo/. xDH showed that both the latter materials were fully crystalline, their diffraction patterns matching perfectly the corresponding ones reported in the literature. Pt-, NiO-, MgO- and P$5/SAKh5 were the same as those used in the work /ll/. The volcanic tuff (Mirsid deposit, Rcmania) used as row material canprised 60-62 % wt clin@dolite and its characteristics was already presented /12/. The pmder without binder was pressed into thick wafers, which were gently crushed and sieved, collecting the 40-60 mesh fraction. 2. Acidity measurements. The acidity of samples was studied by the TPD of amnnia in a nitrogen stream; the quantitative evaluation of acidity was performed by the acid titration of desorbed amnonia. 3. Reagents. Tertiary butyl alcohol (lBA), Fluka reagent, was used to generate in situ isobutene. Formaldehyde (FA), Merck reagent, was used as an aqueous solution containing 37 % wt HCHO and 12 % wt methanol as stabilizer. 4. Reaction procedure. Condensation m s were carried out in a pulse type microreactor. The microreactor consisted in a stainless steel tube (0. d. 6 mn; i.d. 3 n and 80 mn length) with the catalyst particles (20 mg; 5 mn high bed) packed between the quartz wool plugs. Before reaction, the catalyst was activated or regenerated in an air stream at 5OOOC for 2.5 h, followed by cooling to the reaction temperature in a nitrogen gas flow. Samples of 1.0 p1 reagent mixture (BA/FA = 1/1 mole) have k e n injected at constant temperature and the reaction products were analyzed on line by GC. 3. RESULTS AND DISCUSSION

In order to compare the behaviour of different zeolites in Prins reaction a series of catalysts having various acidities and distributions of the acidic sites has been chosen. Their acidic properties are shown in Figure 1 as NH3-TPD spectra. These zeolites have been tested in standard conditions: the same amount of catalyst, 30 ml/min flow rate of carrier gas, and 30OOC reaction temperature. The results are swmarized in Table 1. It must be mentioned, fran preliminary runs, that TBA is converted pranptly into isobutene and water, even at the low temperatures of 170' C, over every catalyst tested. The experiments of the methanol conversion perfonned with aqueous methanol solution having the same concentration as those of FA, have also indicated the formation of only a very small amount of light hydrocarbons. As it is knm, the main disadvantage of the pulse method is that it does not establish an equilibrium condition on the catalyst surface, but by injecting a series of pulses a steady-activity of the catalyst is obtained.

671

The results of condensation runs at steady-activity, the last column fran Table 1, demonstrate that the conversion is correlated to the catalyst acidity, it increases as the concentration of acid sites on the catalyst surface is increased, u1 * whereas the selectivity to isoprene ._ C can be better correlated to the dis3 tribution of the acid strength than the concentration of acid sites. HM ? catalyst shows an apart position, ? ._ though it has the greatest acidity, n the conversion at steady- activity 0 is rather low due to its fast deactivation by coking. At the same t h e , the results obtained after the first five pulses 2oo 300 500 600 reveal that only the volcanic tuff t emperature,OC and SAPO-5 catalysts have the highest selectivity which is kept conFigure 1. NH3-'IPD spectra of tested stant during all runs, whereas over zeolites; 18'C/min heating rate. the catalysts having strong acid sites the chemical transformations are more ccmplex, at initial stage various side reactions being implied. This fact is also pointed out by the chromatograms shown in Figure 2. As it 4-

L

Table 1 Main characteristics of catalysts and results of activity canparison runs in standard conditions

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

Catalyst

Surface After 5 pulses Steady-activity acidity, Conversion Yield Selectivity Conversion Yield Select. % mole % mole % mole % mole % mole % mole meq NH3/g

--------------------_____I______________---------------------------------

Tuff s-1 HZSM-5 Beta

m

0.155 0.433 0.453 0.702 0.992

4.21 5.96 21.89 11.42 8.43

4.21 5.96 8.61 8.08 5.35

100 100 39.33 70.75 63.46

3.38 4.80 18.63 27.86 7.85

3.38 4.80 12.79 26.15 7.49

100 100 68.65 93.84 95.33

can be Observed, at the beginning, isobutene undergoes reactions characteristic to the olefins over zeolites as disproportionation, oligcmerization, arcanatization and cracking. Further, our study was focused on the behaviour of SAPO-5 and beta zeolite, the first having the highest selectivity and the second being the most active catalyst. The acid properties of SAFC-5 samples can be modified either by the canposition of framework, or by ion-exchange or impregnation. These changes in the acid properties can have certain implications on the catalytic perfomces. This assertion is proved by the results shown in Figure 3, where it can be observed that the decrease of the phosphorus content and partially of the aluninium determines an increase of the activity, but the selectivity is kept constant. The difference between the framework cmpsitions deter-

672

CL

I

I - isoprene

M - methanol T - toluene X-xylene

Figure 2. Results of GC analysis of the organic phase after fifteen consecutive pulses of reaction mixture intrcduced on beta zeolite at 3OOOC (Carbowax 20M, programned temperature) mines a slight increase of the acidity from 0.433 meq NH3/9 catalyst for the sample S-1 up to 0.529 meq NH3/9 for the sample S-2.By modifying the SAPO-5 samples with Pt, NiO, MgO and P 2 05, according to the c m n 'JI I methods, the catalytic performances have been changed into an "\ unexpected manner. Thus, the best results have been obtained with Pt/S-1, the yield of isoprene is twice greater than that obtained over S-1, and the lowest yield is obtained using MgO/S-l. The loading degree was 0.4 % wt Pt and 4.5 % wt oxide. The selec0 -3 Y t *tivity was the same. From a practical pint of view, the best cunprdse seems 01 ' ' ' ' ' ' ' 1 1 2 3 L 5 6 7 0 9 1 0 to be reached with the beta zenumber of pulses olite which has the highest activity and a good selectivity (see Table 1). The possibilities in Figure 3. The yield of isoprene vs. the the use of this zeolite as catanumber of pulses over ( 0 ) S-1, 5-2, lyst in various processes have ( x ) Pt/S-l, ( b ) NiO/S-1, ( 0 ) P205/S-l, seldcm been described. It is able and (*I MgO/S-l. I

'

( 0 )

673 to alkylate benzene with dcdecene /13/, to form diphenylmthane fran benzene and trioxane /14/, to hydroxylate phenol /15/. Therefore, beta zeolite was selected for the following analysis aiming at the optimization of reaction conditions. In addition, this zeolite exhibits a high resistance to coking, as it can be seen fran Figure 4. The yield and the selectivity rise as the number of pulses increases attaining a maximum activity at about 15-18 pulses, then the yield decreases up to a steady-activity, whereas the selectivity progressively increases. LO

S 30-

>

-10

E 0, .-

0, 200

I

I

I

300

; U

0 LOO

t e m p e r a ture,OC

number of pulses

Figure 4. The yield of isoprene and the selectivity as function of the number of pulses; TBA/FA = 1/1; 30 ml/min flow rate of nitrogen; beta catalyst.

Figure 5. The conversion and the yield of isoprene vs. reaction temperature; TBA/FA=1/1; 30 ml/min flow rate of nitrogen; beta.

Further, the influence of certain reaction parmters on the catalytic perfomances of beta zeolite was investigated. Thus, the experimental results obtained by the pulse method indicate an optimum reaction temperature between 300-350°C (Figure 5), when the maximum yield of isoprene is achieved As it has been expected, the conversion increases as the temperature increases, but after 35OoC the yield of isoprene decreases due to the fact that the conversion of isobutene into the side reactions is intensified. The influence of the contact time on the catalytic performances was studLO Lo a~ ied by the change of the flow rate of a, O carrier gas, in the range 20-60 ml/min E (this range was selected in order to a' C void the troubles in the GC analysis of F0q a 0 reaction products). The results show Y3O30 that the yield to isoprene increases as .+ 0 the contact time is decreased reaching 2 o a maximum at about 40 ml/min, after aJ 0 which a slight decrease is observed / O , -20 (Figure 6). If the conversion curve is 20,

G

-

-*---T====*-

/

-

-!

--

flow o f carrier gas,ml/mln

Figure 6. The influence of the flaw rate of carrier gas on the conversion and the y i e l d of i s 9 prene a t 300OC; TBA/FA = 1/1.

~

---- -.-. -..-

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

could be explained by assuring that a t prolonged contact t i m e a certain amount of isoprene is converted i n t o secondary products, fran aranatics up to coke. Then an equilibrium betteen the condensation and aranatization reactions is

674

established, after which, at short contact t i m e , ithe yield of isoprene decreases, due to the fact that the aranatization reaction is probably faster than the condensation. As it was already mentioned, it is very interesting to study the correlation between the distribution of the acid strength and selectivity. On this point two kinds of experiments were performed: the saturing of the sites with increasing acid strength by pyridine irreversibly adsorbed at different constant temperatures (Figure 7a) and a consecutive poisoning by pyridine at 3OOOC (Figure 7b). In the first case before each condensation LO

LO. aJ 30

-

,--/a .

€200

s

'

aJ

-

-

30 -

03

' 1 ; sE" To *O-

10 -

10-

1

I

L

I

/o

I

I

I

1

I

Figure 7. The dependence of condematiFlg and aromatizing activity on the chemisorbed pyridine: (a) poisoning at saturation temperature; (b) poisoning by consecutive pulses; (1) conversion, ( 2 ) selectivity to isoprene. run the fresh activated catalyst was poisoned by five pulses of pyridine at a certain temperature, but in the second case puJ.ses of 0.5 p1 pyridine were alternated by pulses of reagent mixture; the reaction temperature was the same 3OOOC. Some interesting aspects can be emphasized these .experi. . .thefranisoprene ments: , (i) is . imnediately formed, without the first stage of cracking or disproportionation, (11) the conversion decreases as the concentration of acid sites decreases by poisoning and (iii) an unexpected change of the selectivity to isoprene is observed. Though the selectivity to isoprene increases as the stronger acid sites are poisoned, emphasizing the role of slight acid centers, however the poisoning favoures the side reactions rather than the condensation of isobutene with aldehyde. An explanation of the last aspect could consist of a 100 200 300 LOO 500 6( selective poisoning of the active t emperature,OC centers by the chemisorbed pyridine which is unfavourable to the condenFigure 8. TPD spectra of amonia on saion reaction. (1) fresh and ( 2 ) coked (1.2 % w t ) In order to clear up these recatalyst, and respectively of the isosults scrne NH3-TpD experiments on butene and formaldehyde; heating rate coked beta zeolite and separately 18OC/min; nitrogen as carrier gas.

675

isobutene and formaldehyde ~ u 1 fresh catalyst were performed. The results show that: (i) as it was expected the amnonia-TPD clearly reveals that the strong acid sites are bhcked by coking, (ii) the TPD spectrum of isobutene has the same alure as previously reported on other zeolites /16/,isobutene and its oliganerization or polymerization products are progressively desorbed, and (iii) the W D spectrum of formaldehyde shows that after a fast desorption on the weakeracid sites, a certain m u n t of formaldehyde or products of the subsequent conversion of formaldehyde is desorbed on the stronger acid sites. Analysing the above presented results same interesting aspects can be pointed out. As it is known, the Prins reaction is acid catalyzed and the following mechanism can be considered:

the route (1)being the most probable, it is specific to the two-stage process, but the route ( 2 ) can be taken into account for the one-stage vapurphase process. This has been demonstrateri by ccmputational mthcds ( W and MNW), the values of the heats of formation being presented on scheme. At low number of pulses the condensation is not so selective (Fiyure 2) due to the easily protonationof isobutene on Bronsted sites and thus a variety of side reactions such as oligomerization, cracking into smaller olefins, disproportionation and arcmatization appear. Some of these side reactions are catalyzed by the strong acid centers which are soon deactivated by the carbonaceous deposit ('Figure8). At this point the isoprene begins to appear (Figure 2) and the isobutene conversion to side reactions drops, only the arcmatization being significantly. It seem that after the achieving of the optimum distribution of acidic centers a definite ratio between condensation and arcmatization is established. This ratio is determined both by the reaction conditions ana by the zeolite structure, excepting the S A D 5 catalysts which are selective during all experiments. The influence of reaction conditions (temperature,contact time) may be connected with the kinetic: properties of the process and, in some extent, with the adsorption-desorption factors. On the other hand, in order to explain the effect of zeolite structure on the ocrnpetitive condensation and oligcmerization/aramatization reactions it is W r t a n t to take into account its acidity and porosity. By cunparison it can see from the results ob.tai,ned on various zeolites that the conversion regularly increases and the selectivity decreases with the increase of the surface acidity. This may be explained by considering that the condensation of isobutene with formaldehyde, as well as the undesired coking and polymerization reactions, are catalyzed by the acid centers, although of different strength. In any case, the weaker acid sites (see the volcanic tuff, SAPD-5, beta after coking) p m t e preferentially the condensation reaction, conferring a high selectivity to the catalyst. Up to this point the contribution of weaker sites is very clear, but if the results of runs carried out on the poisoned catalysts are taken into account the contribution of stronger acid sites is not so clear. when these

676

sites are blocked by coke the selectivity increases, but by poisoning with pyridine the ratio between condensation and arcmaatization is changed in the favour of the aranatization reaction. A question arises if these sites are involved into an activated f m of the formaldehyde molecule, as it is resulted fran the TPD of formaldehyde (though the TPD~offormaldehyde without the analysis of the aesorbed species can not be a rigourous base). However, if it is assumed that this contribution of the strong acid sites is true, it seems that for a very active and selective catalyst, before the acid-base co-operation, a co-operation between the acid centers of different strength is needed. 4. CONCLUSlON The results of the present study allow to conclude that: (i) Vapor-phase condensation of isobutene with formaldehyde can be performed over zeolites with resonable selectivity only when the acid sites of relative low strength are significantly presented on the catalyst surface. (ii) Beta catalyst showed the best coinpranise between activity, selectivity and durability. (iii) Reaction temperature should not exceed 35OoC and TBA/FA ratio should not be too high, to keep within resonable limits the formation of arcmatics and coke. (iv) The activity decay is essentially due to fouling by carbonaceous deposit. However the latter can be easily burned-off with air, so restoring ccrnpletely the behaviour of fresh catalyst. (v) A m r e detailed investigation on the role of catalytic sites is needed in order to establish the most favourable co-operation between sites. REFERENCES

1

Nicolescu, F. Avramescu and V Barbulescu, Rev. Roum. C h h , 35 (199) 193. 2 M. Ai, J. Catal., 1106 (1987) 280. 3 1. Yashm, T. Ohara, N. Yokoi and N. Hara, Nippon Kagaku Kaishi, (1974) 325. 4 M. Ai, J. Catal., 71 (1981) 88. 5 M. Ai, J. Catal., 100 (1986) 336. 6 M. Ai, J. Catal., 83 (1983) 141. 7 P.V. Venuto and P.S. Landis, Adv. Catal., 18 (1968) 259. 8 C.D. chang, W.H. Lang and W.K. Bell, Catalysis of Organic Reactions, W.R. Moser (ed.), Marcel Dekker, New York, (1981) 53. 9 E. Dumitriu, N. Bilba, M. Lupascu, A. Azzouz, V. Hulea, G. Czirje and D. Nibou, J. Catal., (in press). 10 B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. C a m and E.M. Flanigen, J. Am. Chm. Soc., 106 (1984) 6092. 11 S. Oprea, E. Dumitriu, V. Hulea, P. Onu, D. Ginju and K. Yousef, Rev. Chim., 36 (1985) 807. 12 E. Dumitriu, N. Bilba, D. Lutic and V. Hulea, Anal. Univ. Iasi,(in press) 13 L.B. Young, Eur. Pat. Appl 30, 084 (1981). 14 M.A. TDbias, U.S. Pat. 3,728,408 (1973). 15 C. Ferrini and H.W. Kouwehaven, Stud. Surf. Sci. Catal., 55 (1990) 53. 16 A. Baranski and S. Cockiewicz, Catalysis on Zeolites, A. Kallo and Kh.M. Minachev (eds.),Akadedai Kiado, Budapest, (1988)121. I.V.

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I I I Q 1993 Elscvier Science Publishers B.V. All rights reserved.

677

Isomerisation of a-acetylenic alcohols into a$-ethylenic carbonyl derivatives in vapor phase C. MERCIER* and P. CHABARDES RhBne Poulenc Recherches. Centre de Recherches des Carrieres - 69190 St-Fons - FRANCE. Abstract The catalytic isomerisation of dimethyl ethynyl carbinol 1 affording 0-methylcrotonaldehyde 2 was camed out in the presence of oxometallic derivates in heterogeneous vapor phase process. The Mo03-Si02 catalysts, prepared by impregnation are particulary efficient : a 75 YO selectivity could be obtained. The only by-product is methyl-2 butene- 1 yne-3. Mechanistic explanations are profinds and catalyst design are given in relation to the liquid phase homogeneous process.

-

INTRODUCTION The rearrangement of a-acetylenic alcohols into a-P ethylenic carbonyl derivates has been extensively studied. Different catalysts have been proposed : acid catalysts such as sulhric, hydrochloric or acetic acids which give rise to unselective rearrangements [1,2] and more recently, 0x0 derivatives of vanadium, molybdenum or tungsten in liquid phase [3]. Particulary in the case of the vanadates, good yields in isomerisation may be obtained with high dilution or with low transformation rate of the substrate thus giving low productivity The reaction has been extended to the corresponding esters, yielding the corresponding allenic esters when one uses group I B metal derivates [4] or Pd (11) compounds [ S ] . But three stages are required : esterification of the acetylenic alcohol, isomerisation, saponification to a, P-ethylenic carbonyl derivative. Recently P. CHABARDES has proposed new titanium / copper based catalysts.[6] which allow a simple, efficient and selective isomerisation of a-acetylenic alcohols in liquid phase. In heterogeneous catalytic conversions in the vapor phase good results have been obtained on Mo03-Si02 [7]. But we were unable to reproduce these results. This paper describes an investigation of the activity of MoO3-SiOz catalysts in vapor phase isomerisation of dimethyl ethynyl carbinol (DMEC), scope and limitation of that process.

-

EXPERIMENTAL The S O 2 supports used are pyrogenic silica from DEGUSSA, type Aerosil A 150, A 200 and A 300. (The number indicate the surface area in m2/g) with low porosity and low number of Si-OH (1,8 to 2 by nm2). Catalysts were prepared by impregnation of SiOz with aqueous solutions of (NH4)6Mo7024.4 HzO (Prolabo) of concentrations leading to a final MOO:, content from 2 to 25 wt % (Control by fluorescence X or atomic absorption). After drying at 80°C under 60 mm Hg overnight, catalysts were activated by calcination at 500°C in air, for 15 hr.

678

Acidimetric titrations were performed according to the Tanabe method (with Nbutylamine) [8]. Catalytic investigations were conducted in a quartz reactor with inner diameter 10 mm. We introduce DMEC by "seringe" under a regulated flow of nitrogen. Temperature is controled by captors. The reaction products collected between 0 to - 78°C (2 traps) were analyzed by vapor phase chromatography. [VARIAN Model 3700 with a SPECTRA-PHYSIC 4290 integrator Column : 2,s m x 1/4 ft : 20 YOcitroflex 4 on Chromosorb P 60/80 - Injector : 150°C Detector : 250°C - Oven : 60 + 120°C (6"C/mn) - Helium flow 30 mVmn - Internal Standard : Nonane]. These mentionned conditions make it possible to separate [the following :] Methyl-2 butene-l yne-3, Acetone, Isoprenylmethylketone, Isoprenal, Dimethylethynylcarbinol, Internal Standard, Mesityl oxide, Prenal (Increasing retention times). Authentic sample of Methyl-2 butene- 1 yne-3 was obtained by dehydration of DMEC [9]. This product present some safety problems : Eb760 33°C - AH"f = 53 kcal/mole ACD : 320 caVg at 200°C). Isoprenal was obtained using the litterature procedure [lo]. Other products are commercial from PROLABO, ALDRICH and RHONE-POULENC ANIMAL NUTRITION. Isopropenyl methylcetone was obtained by aldolisation - deshydration [ 1 I].

-

- RESULTS AND DISCUSSION The starting point of our study is a russian patent [7] claiming a vapor phase process on molybdenum. Si02 catalysts with a 95 % selectivity to isomerize DMEC 1 into Prenal 2 Scheme 1).

This rearrangement is well known in liquid phase and numerous mechanism studies of this [3,3] sigmatropic reaction on vanadates 0 = V(OR)3 appeared in the litterature [3,12]. But we found only four publications in heterogeneous vapor phase on that isomerisation. people from PUBLICKER [ 131 using phosphates catalysts obtained essentially methyl-2 butene-2 yne-3 : 3 (Scheme 2) like B E R G M A " [9]. Best results in Prenal2 are obtained with Zn3(P0&

+ H,O

A-

Zn,( P0,)2/Celite ______)

190 - 215°C

2

1

(75 %

/

25 %)

1

Selectivity

'

Scheme 2

36 %

44 %

2 %

679 After preliminary work on zeolite catalysts, A.V. MUSHEGYAN el al. [7] have developped a new, simple and efficient catalyst for selective isomerisation of DMEC 1 to Prenal2 and for these authors the important parameter is the "B" aciditv of the catalyst. By analogy to the liquid phase mechanism, and in particular V0LPI"s polysilylvanadates [12b] and works of Y. IWASAWA on tailored metal catalysts and Molybdenum oxides for ethanol oxidation [ 141 we proposed an hypothetical mechanism for isomerisation vapor phase with a tetrahedral Mo active center (Scheme 3). Scheme 3 : Mechanism Proposal

L

J

OH

H ' octahedral

c

>-( 4b Transesterification

A

b

O

H

=Lo

1

2

Three known reactions are known to proceed depending on acid base .properties of the . catalyst; high acidity or high basicity are prbhibited (Scheme

4).

Scheme 4 : Possibles reactions on DMEC Base" +

-

(Retrocondensation)

&LF& b& 24 0

1

2

"Acid'

3

4

0

680

-

MOLYBDENUM CONTENT We tried catalysts between 0.5 to 15 wt % (as MOO, content) by impregnation on Si02 A150. Table 1 summarizes the results : an optimum could be seen between near 7 % that can be correlated with a maximum of Bronsted type site [Polymolybdic acid] as observed by A.CASTELLAN [ 1 Sb]] (cf Fig. I). Table 1 : Effect of Molybdenum Content 8 = 300°C - SiO, A 150 %

MOO^ (wt)

Substrate Transformation %

Yield %

L

o

-

4

L

o

-

1

15 %

81

55

i2

1

2

7

97

77

14

5

3

4

98

70

II

2

4

1

95

70

12

2

5

0.5

85

63

15

1

Fip, I : Acidity = f(M003VXlO

meq I m2 cata]

2

1.5

1

0.5

0

0

5

5,5

10

15

100

- OXIDATION STATE OF MOLYBDENUM To verify our hypothesis of tetrahedral MoV* as active site, we prepared, using Y. I S A W h techniques [14b] different oxidation states of molybdenum (Scheme 5) : Scheme 5 : Molybdenum oxidation states

681

-

Mo (IV) >> Mo (II)]. These results lead to two comment : Identical results for M o v and MoIV could be explained by a transesterification followed by a [3,3] migration when only one "Mo = 0" bond is needed for the isomerisation to take place. Table 2 : Effect of the Oxydation State of Mo - 7 YOwt Moo3 - Si02 a1 50 - 300T As shown in table 2, (Mo (VI) ,

Oxydation state

Yield (YO)

Substrate Transformation ("h)

L

o

&

L

o

1

Moll

71

68

22

1

2

MoIV

95

75

16

2

3

Mo v1

98

77

14

4

Scheme 6 \

-90 /

\ IV Mo=O

Tran sesteri-fication

On the basis of our hypothetical mecanism no isomerisation is possible in the absence of "Mo = 0". The high yield in Prenal obtained with "MolI" is contradictow. Then the two possibilities are : - the postulate mechanism is wrong. - Molybdenum is reoxidized by the substrate during reaction...

682

-

ACTIVITY OF THE CATALYST AND REGENERATION We realise 3 cvcles (3 x 6 hr) with the same catalyst (7 YOMoO,/SiO, A150) (cf Fig.2). . We observe a rapid deactivation of the catalyst (substrate transformation falls down) while stay untouched the selectivity in prenal 2. No leak of molybdenum in the organic phase could be observed and deactivation is in relation with coking of methyl-2 butene-1 yne-3 on the catalyst. . The regeneration of the catalyst is easy : 500°C under air until the black powder becomes white. We found for each cycle the same initial activity and the same profile in deactivation. We have an easy regeneration but a problem of rapid desactivation by coking. Fig. 2 : Evolution transformation (TT) I Selectivity bv cycle (RT) (0 = 300°C - tc 0.8 s - 7 % Moo3 - Si02 A150)

-

t Transformation (TT) hr

- OTHERS OXIDES ON SIO? - A150 We prepared different oxometallic compounds by analogy with results obtained in liquid phase [3,12]. V205 is inactive in vapor phase and only W03 is as active as MOO:, while totally inactive in liquid phase [19]. Table 3 summarizes the results.

Catalyst

Substrate Transformation

Yields L

o

L o / &

1

7 OO/ Ta205 /Si02 A150

67 %

73

4

2

7 YOW03/Si02 A150

65 Yo

74

4.5

3

7 % Nb,05/Si02 A150

23 %

39

0.30

4

7 YOV20s/Si02 A50

28 %

55

2.5

5

7 % Re20$Si02 A150

0 Yo

I

I

683

-

EXTENSION TO OTHERS SUBSTRATES We try to generalize the reaction to different a-acethylenic alcohols and a-ethylenic alcohols which are known to give good results in liquid phase [3,19]. As shown in table 4, we obtain only dehydration product : Citral 5 and prenol 6 are not stable under the reaction conditions.

1 I

citral

3

L

O

p. cymhne

180°C

H

b

Prenol 6

+ HO ,

I so Drh n e

Substrate

8 ("C) reaction

Substrate Transformation

200°C

100 Yo

200°C

100 Yo

300°C

,,

' A 4 h A

A/

0

3 4 h

* Products

only

11

It

449 ,,

180°C

unique

,,

- CONCLUSION In this paper, we try selective isomerisation of DMEC 1into Prenal 2. It was impossible to reproduce' Assian results [7] and the best conditions for selectivity and productivity after 0 imisation are : .

7 % Moo3 /SO2 A150 (Impregnation)

. e=3oooc ,

tc

- 0.8 s

At

- 30 mn

RT-70-75%in

/b/*.

2

0 -

N2 .

1.6 1.h-1

RT

- 15 -25 YOin

Productivity > 1.2 kg.1-lh-I

3

684

The main advantages are the simple preparation and regeneration of the catalyst and the high productivity in heteregeneous vapor phase process. The main drawback is the formation of methyl-2 butene-1 yne-3, 3, leading to rapid deactivation of the catalyst by carbonisation. All attempts to reduce the formation of 3 failed and the reaction 1+ 2 is specific for prenal 2 and not generalizable to other substrates like dimethylvinylcarbinolor dehydrolinalol.

- ACKNOWLEDGMENTS I thank M. AUFRAND and G. ALLMANG for their collaboration in this study and RHONE-POULENC for permission to publish these results.

- REFERENCES I 2 3 J 5 6

7

N

Y 10

I1 12

13

I4 15

16 17

I8 19

s,

MEYER K.H. and SCHUSTER K., Ber, 8 19, (1922) RUPE H. and KAMBLI E., Helv. Chim. Acta., 9,672, (1926) CHABARDES P., KUNTZ E. and VARAGNAT J., Tetrahedron. 33, 1775 (1977) and references therein. SAUCY G . , MARBET R., LINDLAR H. and ISLER O., Helv. Chim. Acta, 112, 194s (1959) RAUTENSTRAUCH V., Tetrahedron Letters, 3,3845, (1984) CHABARDES P. (Rh3ne-Poulenc Sante) - FR Pat. 2.596.755 (3 apt. 1986) ICA 108,166 a) 680 Vl CHABARDES P., Tetrahedron Letters, 29, 6253 (1988) b) MUSHEGYAN A.V., DZHULAKYAN R. Kh. and TSAGIKYAN A.R., Kinetika i a) Kataliz, 25,77, (1984) MUSHEGYAN A.V., DZHULAKYAN R. Kh. and MARTIROSYAN Zh. G., S11.827 b) 477. (27 July 1979). TANABE K., J. Catal., 4 7 , 3 5 8 (1977) a) 827 (1955) JOHNSON B.O., J. Phys. Chem., 3, b) 82 (1986) ALSDORF E. and Coll., J. Catal., B, c) BERGMANN E.D., J . Amer. Chem. SOC.,22, 1218 (1951) CON FORTH J. and Coll.. J. Cheni. SOC.,827 (1958) COOK K.L. and WARING A.J., J.C.S. Perkin I, 536 (1973) and references therein a) PAULING H., ANDREWS D.A. 'and HINDLEY N.C., Hclv. Chim Acta. 5 9 , 1233 ( 1 976) b) ERMAN M.B., AUL'CHENKO 1.S , KHEIFITS L.A., DUVOLA V.G., NOVIKOV Y.M. and VOL'PIN M.E., J. Org. Chem USSR, l2, 931 (1976); Tetrahedron Letters, 2981 (1976) PUBLICKER, US Pat. 2.524 865 and 2.524 866 (1950) [CA : 4S, 1617 and 16181 a) IWASAWA Y. and G A r E S B.C., Chem. Tech., 173 (1989) and references therein b) IWASAWA Y. and Coll , J.C S. Faraday I, 2968 (1978) a) HOWE R H., J . Phys. Chem.. @, 1234 (1982) b) CASTELLAN A,, BART J.C., VAGHI A. and GIORDANO N., J . Catal., 42, 162 ( 1 976) c) OGATA A,, KAZUZAKA A,, YAMAZAKI A. and ENYO M , Chem. Letters, 15 (1989) d) MARCWKOWSKA K. and Coll., J. Catal., 97, 75 (1986) Kirk-Othmer Encyclopedia - Third Edts - "Silica" - V01.20 p.748 a) Kirk-Othmer Encyclopedia - Third Edts - "Molybdenum Compounds" - Vol. IS p.688 b) PERSDO'TTER I . and Coll., Acta. Chem. Seand., A4J, 83 (1986) a) BORESKOV G.K., Kinetic Y. Catalysis, 1240 (1985) [Engl.] b) ALSDORF E., J. Catal., %,82 (1986) FUJITA Y.,HOSOGAI T., NINAGAWA Y. and NlSHlDA T., Chem. Letters, 357 (1982)

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

685

lsomerization of a-pinene over Ti02: kinetics and catalyst optimization A. Severino, J. Vital, L. S. Lob0 Universidade Nova de Lisboa, FCT, Departamento de Quimica, 2825 Monte de Caparica, Portugal

Abstract a-Pinene gives a good yield of camphene over Ti02 catalysts. The optimization of selectivity and yield depends on the level of acidity, activation time, activation temperature and other factors. Modelling of the kinetic system is performed by lumping species and using Langmuir-Hinshelwood kinetics. XRD shows that Ti02 is present as anatase and IR shows that acidity is mostly of the Bronsted type. The specific surface of ca. 400 m2/g is mostly due to micropores. The average pore radius is 20 A. This may have an effect on selectivity.

1. INTRODUCTION

The acid catalysed isomerization of a-pinene proceeds via two types of reactions, one giving bi- and tricyclic products such as camphene, (J-pinene, tricyclene, and bornylene and the other giving rise to monocyclic compounds such as dipeiitene, terpinolene, tx-terpinene, y-terpinene and p-cymene [l]. Over solid catalysts such as clays, mineral oxides and inorganic salts,the main product is camphene [2], of particular interest as an intermediate in the synthesis of camphor. Camphor is of value due to its aroma and pharmaceutical properties. The production of camphene is usually carried out by isomerization of a-pinene over titanium oxide catalysts [3]. These are prepared by treating titanium oxide with an acid in order to obtain a layer of titanic acid on the surface of the oxide [4]. The reaction was reported as showing zero order [4] or a transition from first order at low conversion to zero order above ca. 30% conversion [5]. In view of optimizing the production of camphene it is of interest to have comprehensive kinetic data and models for the isomerization of u-pinene over titanium oxide. The catalyst is previously treated with sulfuric acid. It is of interest to know the effect on catalyst activity and selectivity of varying (i) the amount of sulfuric acid in relation to the amount of oxide; (ii) the amount of catalyst in relation to the amount of pinene; (iii) the time of catalyst activation; (iv) the temperature of catalyst activation.

686

2. EXPERIMENTAL

Instrumentation: Adsorption-desorption isotherms of nitrogen were determined with a CI Electronics microbalance. Infrared spectra were taken with a FTlR Mattson Galaxy instrument. XRD spectra were recorded on a Rigaku X-ray diffractometer. Gas chromatography analyses were carried out in a Konik HRGC 3000-C instrument equipped with a 30 m x 0.25 mm OV-1 column; splitter: 1/30; Injector temperature: 180" C; detector: FID; detector temperature: 250" C; oven:80" C isothermic by 5 min.; rate: 6" C/min.; carrier: H2. Catalyst preparation: The titanium dioxide catalysts were prepared by suspending 40 g of Ti02 (99% Merck) in a solution of the appropriate amount of sulfuric acid in 100 ml of super-dry methanol, during 4 hrs, with slow agitation. After this period of time the solvent was completely evaporated under vacuum. The resultant powder was then dried in air at 70" C during 22 hrs. Finally, the material was crushed down to a granulation of less than 1 mm, activated in air at convenient temperatures and times as described below, and stored under vacuum. lsomerization reactions: 40 ml of a-pinene (Terpex, 91%) were charged into a 250 ml glass reactor equipped with a reflux condenser, a stirrer and a temperature controller. The temperature of the vessel was kept constant with a thermostated bath. The reactions were started with the addition of the appropriate amount of catalyst. Samples were taken at regular time intervals. After addition of n-octane (Merck, 99%) as internal standard and appropriate dilution with n-hexane, the samples were analysed by gas-chromatography.

3. RESULTS AND DISCUSSION

Finding a model : Fig. 1 shows experimental data of a-pinene isomerization at 150" C catalysed by 4% of titanium oxide catalyst (conditions: 15 % H2SO4, activation temperature,135" C; activation time, 4 hrs). In this figure the concentrations of some species are lumped together in the way described below. For kinetic modelling purposes the three reaction schemes shown in Fig. 2 were used. In scheme I, P is pinene, C is camphene, B lumps the other bicyclic and tricyclic terpenes and M lumps the monocyclic terpenes. Fig. 4, to be referred below, shows the main lumped products and their structure. Scheme II includes a reversible reaction. Scheme Ill represents a lower degree of lumping showing terpinolene (T) and Dipentene (D) as separate species.

687

0

41T XIM

3'

Figure 2: Reaction schemes. P - a-pinene; C - camphene; D - dipentene; T - terpinolene; 6 - lumping of various bicyclic terpenes; M - lumping of monocyclic terpenes. Figure 1: Fitting of model 4 to data. - Pinene; 0 - Camphene; 0 - Dipentene I - Terpinolene; A - monocycloterpenes; bicyclic terpenes not shown.

0

Table 1 Variance analysis of the fitting of some models to the data. Models 1, 2 and 3 assume 1st order kinetics of all reactions. Model 4 assumes Langmuir-Hinshelwoodkinetics. MODEL

REACT SCHEME

1 2 3 4

I II 111 111

No LACKOFFIT PARA- Variance d f . METERS (~10-3) 80 4 15.36 5 10.50 79

7 10

9.85 2.55

119 116

PURE ERROR Variance d f ( x 0-3) ~

6.87 6.87 4.86 4.86

252 252 378 378

RATIO

2.24 1.53 2.03 0.53

F

TEST

u:l%

~

1.53 1.53 1.42 1.42

1.35 1.35 1.28 1.28

COMMENT

%a = l %

u=5%

Rejected Accepted Rejected Accepted

Rejected Rejected Rejected Accepted

5

Table 1 shows model discrimination by variance analysis based on the results of four replicated experiments. The kinetic models used were based on the reaction schemes of Figure 2. Models 1, 2 and 3 assume 1st order kinetics. Model 4 assumes Langmuir-Hinshelwoodkinetics according to the following equations: dCp/dt = -

(R1 t R 2

dCC/dt =

(R1

+ R 3 + R 4 + R5)W/V

- R6)W/V

dCe/dt = (Re +

(11

(2)

R2)W/V

(3)

- R7)WN

(4)

dCD/dt =

(R3

dCT/dt =

R4W/V

dCNJdt = (R5 + R7)W/V

(5) (6)

688

where C are the concentrations, W is the catalyst weight, V is the reaction volume, invariant, ki are kinetic constants, Ki are adsorption constants and Ri are the following Langmuir-Hinshelwood reaction rate expressions, where A is the adsorption term: R1 = k l Kp Cp/A R2 = k2 Kp Cp/A R3 = k3 Kp Cp/A R4 = k4 Kp Cp/A R5 = k5 Kp Cp/A R6 = k6 k Cc/A R7 = k7 KD CD/A

A = 1 +KPCP+KCCC+KDCD

(14)

The models were fitted to the data by nonlinear regression, using direct search as described elsewhere [6].No initial estimates were required, the search being self starting (all parameters equal to zero or one initially). The four models shown on table 1 are a sample of about 10 models tested, with different degrees of lumping and different kinetics (1st order or LangmuirHinshelwood). Model 4 was the only one fully accepted. Figure 1 shows the excellent fitting of the data to model 4. This model was thus used to fit the experimental data. The validation of the model is further supported by the good fitting of the rate and adsorption constants obtained from experiments at various temperatures to an Arrhenius type plot. Due to lack of space a full discussion is not presented here.

Optimizing the catalyst: In order to optimize the catalytic properties of titanium oxide impregnated with sulfuric acid, catalysts were prepared with different amounts of added sulfuric acid, with different activation times and activation temperatures. The catalyst was studied by X-ray diffraction, IR spectroscopy, and N2 adsorption (specific surface area and pore size distribution).

5 4

3

2 30

100

50

0

130

Figure 3. XRD spectra: 1- 0% HzSO4, activated 5 hrs at 450°C; 2-20% H2SO4, activated 2 hrs at 150°C; 3- 15% H2SO4, activated 5 hrs at 150°C; 4- 5% H2SO4, activated 2 hrs at 150°C; 5- 15% H2SO4, activated 2 hrs at 450°C.

689

Figure 4. a-Pinene isomerization. Lumping according to scheme I. 1, a-pinene; 2, camphene; 3, bornylene; 4, a-fenchene; 5, tricyclene; 6, dipentene; 7, terpinolene; 8, a-terpinene; 9, y-terpinene; 10, a-phellandrene. 100

3 rn

'9

80

0% 5% 20%

60

?

Figure 5. Pore size distributions of catalysts prepared with different amounts of H2SO4.

&

---c--. --t

4o

+ 3

20

0 0

20

40

60

r

80

'

10

(4

1.8 Figure 6. a-Pinene isomerization over 4% of Ti02 catalyst (act. temp.: 170" C; act. time: 2 hrs; H2SO4 : 15%). Global selectivity (S = CC/(CB+CD+CT+CM) ) versus fractional conversion (X).

S

1.0,

0.0

.

I

0.2

. , . 0.4 X

I

0.6

. 0.8

1 0.8-

2.0

- 1.8

P Figure 7. Effect of the amount of H2SO4 on initial reaction rate and global selectivity.

ci,

2 0

-1.6s

E

v

-1.4

0 L

- 1.2 0

10 %

y

20 30 so4 (w/w)

690

0.72-

.2.0

- 1.9

0.64* 3 0.560) 0.480 0.400 0.32 L 0.24. 0.16. 0

- 1.7S

- 1.6 - 1.5

-

.

,

.

15

10

5

.

I

Figure 8. Effect of activation time on initial reaction rate and global selectivity.

1.4 20

Activation Time (hrs)

0.800.72.

~1.8 -1.7

0.56.. 0.48

-1.6 -1.5s -1.4

-50.64-.

1

Figure 9. Effect of the activation temperature on initial rate of reaction and on global selectivity

v

- 8

0

0.40.. 0.32- . 120

I

.

.

140

1

.

I

.

160

I

-1.3 . 1.2 180

Activation Temperature ("C)

1

.

2

1 1.6

7

I.5

S

1.4

Figure 10. Effect of catalyst load on initial rate of reaction and on selectivity

1.3

1

2

3

4

5

6

7

% Catalyst ( wlw)

Table 2 Effect of temperature on rate constants (m0Vh.g). Numbering according model 4

160 156 150 145 140 135

1.725 1.503 1.167 0.524 0.357 0.265

0.268 0.247 0.191 0.077 0.047 0.037

0.430 0.329 0.299 0.126 0.089 0.064

0.178 0.180 0.12 0.047 0.029 0.021

0.267 0.233 0.187 0.071 0.047 0.035

0.012 0.007 0.003 0.001 0.001 0.000

0.057 0.080 0.040 0.009 0.004 0.002

691

The amount of sulfuric acid seems not to affect the crystallinity of titanium oxide as shown by X-ray diffraction (Fig. 3).The diffractograms are characteristic of anatase and show no apparent differences for different preparation modes. Specific surface areas were determined for samples of catalyst prepared with 0%, 5% and 20% of sulfuric acid by using the BET method were 350,380 and 420 rn'lg, respectively. These values are in agreement with the value of 350 m2/g reported in the literature [4]. The average pore radius r = 2VdSo, according to Wheeler's model was found to be 20 A in all cases (Vpis the volume of pores and So is the area by unit of mass). This value is consistent with the pore size distributions shown in Fig. 5, calculated from desorption isotherms using the Kelvin equation. The pore dimensions should be compared with the size of molecules, shown in Fig. 4, ranging from 5 to 10 A. This may have an effect on the selectivities observed, reported below. The infrared spectra of adsorbed pyridine shows the characteristic bands of protonated pyridine at 1480 cm-1 and 1540 cm-1 and a weak Lewis acidity at 1460 cm-1. The effect of various factors of catalyst preparation on selectivity was studied. Althougth the selectivity varies with conversion, as shown in Fig. 6 , it is constant above 30% conversion. We used the selectivity at 65% conversion, when comparing data. Figures 7 to 9 show how initial reaction rate and global selectivity are affected by the ratio sulfuric acid - titanium oxide, by the activation time and by the activation temperature. The effect of the ratio sulfuric acid - substrate on the reaction rate and selectivity is contradictory. In fact, selectivity decreases when the ratio H2S04-Ti02 increases (figure 7) and at the same time the initial reaction rate increases. However, the increase in catalyst activity is moderate when the amount of acid is higher than 5% and even decreases at high amounts. The selectivity can be understood by considering that the yield in monocyclic terpenes increases with the number of strong This observation reflects the role of sulfuric acid in catalyst preparation. acid sites In fact, sulfuric acid seems not to act as catalyst itself but only through the modification of the titanium dioxide surface. This surface has a limited extension and when the amount of sulfuric acid is increased the catalyst activity also increases but reaches a saturation value. Figures 8 and 9 show how catalyst activity and selectivity are affected by catalyst activation time and temperature, respectively. In both cases the catalyst activity decreases while selectivity increases when the activation time or the activation temperature are increased. An optimization problem arises. The increase in selectivity could be explained by the desorption of water from catalyst pores. The pores would become more accessible to the substrate molecules and shape selectivity would increase. The decrease of the acidity could be related with titanic acid dehydration. However, titanic acid dehydration is only reported to occur at higher temperatures (above 400" C) than those used in this work to activate the catalyst [8].A possible explanation for this loss in activity could be again the loss of adsorbed water. At low levels of activation the catalyst surface would retain some water adsorbed. Titanic acid would be dissociated and the catalyst would behave globally as a strong acid. The proton transfer to the a-pinene molecules would occur from adsorbed H3O+ ions and not from titanic acid OH groups. At higher activation temperatures or longer activation times the water content of the catalyst surface would decrease and the acid strength would also decrease.

m.

692

Operating conditions: Experiments were performed to select the best operating conditions, namely the temperature and the amount of catalyst to be used. Fig. 10 shows that 4% of catalyst by weight is required to get good reaction rates. Table 2 shows the kinetic parameters for temperatures in the range 135 to 160" C. Best selectivities are observed at low temperatures. At 160" C the conversion of pinene to camphene is 7 times faster and the selectivity in series is still good (kl/k6 140) but the selectivity in parallel kll(k2 t k3 + k4 t k5) is 1.5, lower than the value of 1.7 observed at 135" C.

-

5. CONCLUSIONS Successful kinetic modeling of this complex reaction system can be made wing Langmuir-Hinshelwoodkinetics and lumping the appropriate species. Ti02 catalyst is mostly microporous, has high surface area (ca. 400 m2/g), is in the form of anatase and shows mostly Bronsted acidity by IR. The effects of H2SO4 addition, activation time and activation temperature were established, so that optimized conditions can be selected. Operating conditions can also be optimised by selecting the temperature and using a catalyst load of about 4%. 6. AKNOWLEDGEMENTS

The authors thank JNICT for a grant to one of them (A.S.) and Dr. F. Bras Fernandes for the XnD spectra. 7. REFERENCES

J. Simonsen, The Terpenes, 2nd vol., J. Simonsen and L. N. Oeven (eds.), Cambridge University Press, 1957. V. P. Wystrach, L. H. Barnum and M. Garber, J. Am. Chem. SOC.79 (1957) 5786. Albert, R. M.,; Traynor, S. G.; Webb, R. L.; Fragrance and Flavor Chemicals, in Naval Stores - Production, Chemistry, Utilization, D. F. Zinkel and J. Russell (eds.), Pulp Chemicals Association, New York 1989. G. A. Rudakov, T. N. Pisareva and N. F. Ovsyukova, Gidroliz. Lesokhim. PROMST No. 6 (1981) 14. G. A. Rudakov, L. S. Ivanova, T. N. Pisareva, A. G. Borovskaya, Gidroliz. Lesokhim. PROM-ST No. 4 (1975) 7. L. S. Lob0 and M. S. Lobo, Computers and Chem. Eng. 15 (1991) 141. A. Stanislaus and L. M. Yeddanapalli, Can. J. Chem. 50 (1972) 61 . T. Tanaka, H. Kumagai, H. Hattori, M. Kudo and S. Hasegawa, J. Catal. 127 (1991) 221. M. Dul and M. Bukala, Chem. Stos. 15 (1971) 75.

M. Guisnet et al. (Editors),Heterogeneous Cntalysis and Fine Chemicals ZI! 0 1993 Elsevier Science Publishers B.V. All rights reserved.

693

Vapour phase hydrolysis : a new access to 2,2,2-trifluoroethanoI P.J. Tirel*, C. Doussain*, L. Gilbert*, M. Gubelmann**, H. Pernot**, J.M. Popa**, RhBne-Poulenc Recherches

* Centre de Recherches des Carrihres,

85, avenue des Freres Perret,

BP 62,69192 Saint-Fons

** Centre de Recherches d'Aubervilliers, 51, rue de la Haie Coq, 93308 Aubervilliers Abstract Hydrolyses of l-halo-2,2,2-trifluoroethanes (HCFC 133a, 133b, 134) in 2,2,2-trifluoroethanol (TFE) were studied in the presence of modified lanthanum phosphates. Thermodynamic equilibrium could be achieved when the reactions were carried out in the vapour phase. The order of reactivity and selectivity of the various catalysts seemed to be governed by impregnation of the lanthanum phosphate with the desired alkaline earth salt.

INTRODUCTION Trifluoroethanol (TFE) 1 is an industrially important organic intermediate in the manufacture of the trifluoroethoxylated anaesthetic (1)isoflurane 2 and desflurane 3.

Reported methods for the production of TFE involve hydrogenation of trifluoroacetic acid derivatives (2-9). Although highly selective, there are a number of significant shortcomings associated with these processes, including the price and access of raw materials and heat removal problems. The industrial process makes use of the nucleophilic substitution of l-chloro-2,2,2trifluoroethane 4 (HCFC 133a) with an alkaline earth acetate (eqn. 1). After acetolysis, TFE is obtained by saponification (eqn. 2) (10-14). CF3CH2C1

4

+

AcONa

-

CF3CH2OAc

d

+

NaCl

694

+

CF3CH20Ac

NaOH

-

CF3CHzOH

+

AcONa

(2)

Unlike the first step, the saponification is highly selective. The synthesis of the 2,2,2-trifluoroethyl acetate 5 requires high temperature (150-180°C) and pressure (15 bars). This reaction involves polar aprotic solvents such as Nh!lP, DMSO and sulfolane. The major drawbacks of this process are corrosion and waste water disposal. The most direct access to TFE is the direct hydrolysis of l-chloro-2,2,2trifluoroethane with water (eqn. 3). CF3CH2Cl

+

H20

CFQCH~OH+ HC1

4

(3)

1

To our knowledge this reaction is not described in the literature. From a chemical point of view the nucleophilic substitution of chlorine with water is not spontaneous due to the electron withdrawing effect of the trifluoromethyl group. On account of this, a preliminary thermodynamic study was first undertaken.

Thermodynamics Two chemical equilibria were studied :

- The direct hydrolysis of l-chloro-2,2,2-trifluoroethane (eqn.4) with the effect of an excess (x) of water (figure 1).

X H20 CF3CH2Cl

* d

CF3CH20H

+

HC1

(4)

- The dehydrohalogenation of l-chloro-2,2,2-trifluoroethane (eqn. 5) with the effect of a dilution (y) by nitrogen (figure 2). CF3CH2Cl

y N2

fl

CF2 = CHCl + HCI

This was the main side reaction anticipated. The thermodynamical data were obtained using the S.W.Benson (15-16) method.

695

Table 1 Calculated thermodynamic data for the hydrolysis (eqn. 4) T(K) A H R O (Kcdmole) ASR' (u.e) AGRO (KcaVmole) K

300

500

800

5,O 1.1 4,7 4 x 10-4

497 -0,8 591 6 x lom3

1000

499

498

-l,o

- l,o 28

597 10-3

55

53 10-3

Table 2 Calculated thermodynamic data for dehydrohalogenation (eqn. 5 ) T(K) (Kcallmole) ASRO (u.e) AGRO (KcaVmole) A H R O

K

300

500

800

1000

30,5 36,l 19,7

30,8 37,O 12,3

30,B 36,B 194

31,O 36,3 -5,3

0,4

14,2

6x

2x

As depicted in table 1 a n d 2, the reactions a r e not thermodynamically favoured because of the positive value of AG'R. Conversion (a)through hydrolysis of CF3CH2Cl was obtained as a function of temperature and the amount of water (XI. 01

(x,T) =

-(l+x)K+

4 (l+xI2K2 +4Kx(l-K) 2( 1-K)

I

T=lOOOK

where K = e

-A G$ RT

*

a2 (1-a)(x-a)

I

T=800 K = 0,02

I

X 1

10

5

20

i

5

10

Fig. 1. Conversion a for the hydrolysis versus temperature and the amount of water present. Fig. 2. Conversion

p

for the dehydrohalogenation versus temperature and dilution.

696

Conversion p of CF3CH2C1 in the dehydrohalogenation reaction was obtained as a h c t i o n of the temperature and the quantity of nitrogen (y) as diluent at atmospheric pressure. (y,T)=

- Ky + 4 K2y + 4K (1+ yXK + 1) 2(1-K)

-A G R O where K = e -RT

-

P2 (l-pXl+P+y)

This thermodynamic study showed that : - the hydrolysis reaction is possible (conversion eqn. 4 = 30 %, T = 500"C, x = 5 ) - an increase in temperature promotes the dehydrohalogenation reaction - an excess of water favours the hydrolysis but also the dehydrohalogenation (water as a diluent). This calculation helped selection of the experimental domain : gas flow reactor,

To= 400 - 500"C, Ratio CF3CH2C1 = 5 with the aim of finding a specific catalyst for the hydrolysis reaction.

H20

EXPERIMENTAL Experimental Procedure The reactions were performed in a vapour phase tubular quartz reactor packed with the catalyst (stationary bed) and heated in a shell oven under nitrogen at the test temperature. After thermal equilibria had been reached, nitrogen and HCFC were introduced via volumetric flow meters. Water was introduced with the aid of a syringe. The reaction products were collected after a n initial 30 minute period of conditioning a n d were analyaed by gas chromatography. The stationary phase used for analysis was a poraplot Q on silica (column length 10 m, diameter 0,32 mm). Catalyst Catalyst preparations have been previously discussed (17-18). Thus, the rare earth phosphates were prepared by conventional methods. They were then modified by introducing an impregnating compound (alkaline earth salt) either directly into the synthesis mixture, or after filtration and drying. After impregnation, this catalyst precursor was r.1lcined at 500°C for two hours. The product crystals were hexagonal in structu I c and the quantity of alkaline earth was measurable by elementary analysis.

RESULTS AND DISCUSSION The selected working conditions for a preliminary screening of catalysts were determined as discussed above "thermodynamics". Only modified rare earth phosphates (REP) were found to be effective for the h yd roly sis.

697

CF3CH2Cl

+

5H20

modified REP

CFQCH~OH+ HCl

In these first experiments the selectivity to TFE was moderate therefore a further investigation was carried out to increase the selectivity by modification of the REP.

Alkali Earth Effect - The catalyst was prepared as previously described (18)

These catalysts were then tested using a standard procedure (eqn. 6 , Table 3) CF3CH2Cl 1

+

H20 5

Vapour phase WHSV=lh-l LaP04, Alk 490°C

~

CF3CH20H

+

HCI

Table 3 Alkali earth effect LaP04, Alk

Conversion % CFQCH~C~

Yield % TFE

Selectivity % TFE

None Lap04 LaP04, Li LaP04, Na LaP04, K LaP04, Cs

0 4 6 18,6 26 26

0,4* 293 791 13,8 23,4

10 38 38 53 90

* GLC detection limit The presence of a n alkaline on the surface of lanthanum phosphate is necessary for catalytic activity. As depicted in table 3, a cation effect was observed and there was a direct correlation between the cation size and the catalytic effect.

698

Effect of Cesium Concentration

A series of catalysts was prepared by t h e same procedure b u t the neutralisation of the mixture was carried out with a 6 M solution of CsOH until the pH of the mother liquors reached 9. A portion of the product obtained by filtration was then washed either once o r twice with demineralized water to reduce the cesium concentration.

La203

H3P04 e L ~ P Ocs ~, 2)CsOH

washing b

LaP04, Cs

The cesium contents were measured in the dry samples. These catalysts were tested using the standard procedure (eqn.6, Table 4). Table 4 Influence of cesium concentration

cs % wlw

Conversion % CF3CH2C1 4,o 895 20,6 26.8

0 1 3 6

Selectivity % TFE

Selectivity % CF2 = CHC1

10,o 88,3 93,O 87,7

ND* 11,7 78 7,5

* ND : No determination Cesium concentration has a twofold effect. Firstly the conversion increases with the quantity of alkaline earth ; secondly, a selectivity optimum is obtained with 3 % wlw of cesium. With low weights of cesium dehydrohalogenation is the only side reaction giving a chemical balance of 100 %.

Effect of Cesium Salt (Cs) In these experiments a large quantity of lanthanum phosphate was prepared and after drying, impregnated with cesium salts. La2(CO3)3

H3PO4 -* impregnation Cesium salt

+

Lap04 LaPOq, Cs (3 % w/w)

These catalysts were tested using the standard procedure (eqn. 6,Table 5 )

699

Table 5 Influence of Cesium Source (Cs 3 % w/w) Cesium salt Cs2HP04 CsOH CsgLaClg CsF cs2so4 cs2co3 cs2c204 Cs Benzoate

Conversion % CF3CH2Cl 23,7 23 12 18,5 19,5 21 21,5 21

Selectivity TFE % 91 87 60 82 89 81 66 80

Selectivity % CF2 = CHCl 9 13 40

18 11 19 15 20

As depicted in table 5 the optimal activity was obtained with dicesium hydrogenophosphate impregnation (LaP04, Cs2HP04 3 % w/w). For some of these catalysts, the selectivity t o 2,2-difluorochloroethylenewas high ( = 40 %). Influence Of The Leaving Group Similarly experiments have been carried out using HCFC 134 and HCFC 133b. vapour phase WHSV = 1h-l CFBCH~X + 5H20 CF3CH20H + HX 49OOC LaP04, Alk The catalysts were those previously given in table 3. Table 6 Influence of the leaving group (X) LaP04, Alk

X

Conversion % CFQCH~X

LaP04, Na LaP04, K LaP04, Cs LaP04, Na LaP04, K LaP04, Cs LaP04, Na LaP04, K LaP04, Cs

c1 c1 c1

18,6 26,O 26,O

Br Br Br F F F

5,6 17,4 23,4 5

Selectivity % TFE _ _ _ _ _ ~ ~

83 11

38 53 90 84 95 96 > 95 > 95 > 95

700

Under these conditions, the hydrolysis of a l-halo-2,2,2-trifluoroethane with water was possible irrespective of the choice of halogen. The conversion followed the order Cl>Br>F and with cesium modified lanthanum phosphate gave in all cases, a selectivity greater than 90 %.

CONCLUSION

This paper reports, for the first time, the synthesis of trifluoroethanol (TFE) by hydrolysis of l-hal0-2,2,2-trifluoroethanes. A new catalytic phase has been defined and a synergy between support (REP) and modifyer (Cs) has been observed. The thermodynamic study has permitted selection of the experimental domain, and a good correlation with experimental results has been obtained (thermodynamic conversion = 30%, experimental conversion = 26 %). The simplicity of the approach, the availability of reactant, and the efficiency of the catalyst together give the reaction industrial potential. This study also provides insight into the catalytic behaviour of phosphates, and their potential for further application.

REFERENCES 1 W.G.M. JONES, Preparation an Industrial Applications of organofluorine compounds, R.E. Banks Edition, p 162 (1982) 2 Rhbne-Poulenc, EP 128059 (06.05.83) 3 Rhbne-Poulenc, EP 125951 (21.04.83) 4 Kali chemie A. - G. ; DE 3510883 (09.10.86) 5 Allied chemical Corp. ; NL 6512355 (31.03.66) 6 ABBOTT CAB. ; US 3970 710 (09.04.75) 7 ASAHI GLASS CO. ; JP 85 110196 (24.05.85) 8 SU 514803 (25.05.76); C.A. (17)123333 9 F. SWARTS ;CPTE REND. ACAD.SC1 p. 1261 (27.11.33) 10 Mitsubishi Metal, JP 60 120835 (02.12.83) 11 Mitsubishi Metal, J P 85 195987 (06.09.85) 12 Halocarbon Products, EP 101526 (20.08.82) 13 Boc Inc, EP 171248 (12.02.86) 14 Onoda Cement Co, US 4489211 (18.12.84) 15 S.W. Benson : “Thermochemicalkinetics” Wiley 2nd Ed. (1976) 16 S.W. Benson et Coll., Chem. Rev., 1969,279. 17 Rhbne-Poulenc, FR 9001264 (01.02.89) 18 Rhbne-Poulenc, US 5118651 (01.02.89)

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.

701

Conversion of acetone into methylisobutylketone on Pt-HZSM5 catalysts. Influence of the hydrogenating activity on the rate and on the selectivity. L. MELO1, E. ROMB12, J.M. DOMINGUEZ1, P. MAGNOUX1 and M. GUISNET1 1. URA CNRS 350,Laboratoire de Catalyse en Chimie Organique, UFR Sciences, 40, avenue du Recteur Pineau, 86022 Poitiers cedex, France

2. Universita di Cagliari, Dipartimento di Scienze Chimicue, via Ospedale 72, 0914 Cagliari, Italy

Summary The conversion of acetone was studied at 160°C under hydrogen flow on a series of PtHZSM5 catalysts with 0.03 to 0.55 wt % platinum. On HZSM5, acetone is transformed mainly into mesityloxide (MO) through successive aldolisation and dehydration on the acid sites. On RHZSM5, methylisobutylketone (MIBK) and propane (P) are the main products, small amounts of 2-methylpentane (2MP), of diisobutylketones (DIBK) an of mesityloxide (MO) are also observed. Only the hydrocarbons (P and 2MP) appear as primary products. A reaction scheme is proposed to explain the formation of the various products. MO is formed through acid catalysis, the other products through bifunctional catalysis. As can be expected from a bifunctional mechanism the rates of formation of P, 2MP, MIBK and DIBK first increase with nPt the number of accessible platinum atoms then remain constant. On the other hand the rate of MO formation decreases when nPt increases. Moreover like with other bifunctional reactions such as alkane hydrocraking, the greater nPt the greater the stability of the ca!alysts. INTRO D UCTlO N Methylisobutylketone (MIBK) which is used as solvent for inks and lacquers can be prepared from acetone through a catalytic three-step process : base-catalyzed production of diacetone alcohol (DA), acid dehydration of (DA) into mesityloxide (MO) then hydrogenation of (MO) on a noble metal :

With bifunctional catalysts (Pd-doped sulfonated resin (l), Pd-HZSM5 (2) and PtHZSM5 (3)...) operating under hydrogenating conditions the preparation can be carried out in one step.

702

Obviously the activity, the stability and the selectivity of bifunctional catalysts are determined by the characteristics of the hydrogenating and of the acid sites and particularly by the balance between the hydrogenating and acid functions. Thus for nheptane hydrocracking on PtHY catalysts (4) it was shown that a definite correlation existed between the catalytic properties of the PtHY samples and nPt/nA the ratio of the number of hydrogenating/acid sites. For low nPt/nA values, the activity per acid site was low, the stability weak and n-heptane led directly to all the isomerization and cracking products. For high values, the activity per acid site was maximum, the stability nearly perfect and n-heptane transformed successively into mono-branched isomers, bi-branched isomers and cracking products. It was concluded that the catalytic properties were determined by the number of reactions the intermediate olefins could undergo on the acid sites during their diffusion between two platinum sites (4). The object of this work is to specify the effect of nPt/nA on the activity, the stability of PtHZSM5 catalysts in the conversion of acetone into methylisobutylketone. A significant difference between this reaction and the hydrocracking of alkanes is that acetone conversion requires the consumption of hydrogen.

EXPERIMENTAL HZSM5 (Si/AI = 41) was prepared by the method proposed by Guth and Caullet (5). X-Ray diffraction and adsorption of nitrogen showed the good cristallinity of this zeolite sample. Eight Pt-HZSM5 samples (from 0.03to 0.54 wt o/a Pt) were prepared by the method previously described (6): exchange with Pt(NH3)$+ in competition with NH4+, calcination under dry air flow at 300°C for 6 hours, reduction under hydrogen flow at 500°C for 6 hours (6).For two samples (0.54 and 0.43 wt %) the dispersion determined by transmission electron microscopy was to be found equal to 50 and 60%. All the PtHZSM5 samples were characterized by their activity for toluene hydrogenation in a flow reactor at 110°C with PH2 = 0.9 bar and Ptoluene = 0.1 bar. The transformation of acetone was carried out in a flow reactor at 160°C with PH2 = 0.25 bar and Pacetone = 0.75 bar. Reaction products were analyzed on-line by gas chromatography with a CP Sil 5 CB capillary column.

RESULTS AND DISCUSSION 1. Characterization of the catalyst samples The unit cell formula of HZSM5 determined from the chemical analysis is H2.oNao.3A12.3Si93.7Olg2,which corresponds to a number of protonic sites of 2.1 1020 g-1. All the acid sites are strong : indeed the heat of ammonia adsorption is between 80 and 145 kJ mol-1 (7) ; the desorption of NH3 occurs only above 500 K. Table 1 gives the percentage of platinum of the various platinum samples and their activity for toluene hydrogenation per weight of catalyst and per weight of platinum. The hydrogenating activity of the samples is practically proportional to the platinum content, i.e. that the activities of platinum in all the samples are quasi identical. This means that the dispersion of platinum is nearly the same with all the catalysts (about 50 % as shown by TEM). One exception however, the 0.07 Pt-HZSM5 sample whose hydrogenating activity per weight of platinum and hence the platinum dispersion, are about twice lower. The number of accessible platinum atoms was estimated from the

703

hydrogenating activities. The values of nPtlnA, the ratio of the number of accessible platinum atoms to the number of acid sites are given in table 1. Table 1 Characteristics of the PtHZSM5 samples. AH hydrogenating activity per g of catalyst, A'H hydrogenating activity per g of platinum, nPt/nA ratio of the number of accessible platinum atoms to the number of acid sites. Pt AH A'H nPtInA Catalysts (M%.)

0.03 PtHZSM5 0.07PtHZSM5 0.16 RHZSM5 0.25 PtHZSM5 0.31 PtHZSM5 0.40 PtHZSM5 0.43 PtHZSM5 0.54 PtHZSM5

0.03

mole h-lg-l

mol.h-lg-l Pt

0.07

1.2 1.5

4.0 2.2

0.16 0.25 0.31 0.40 0.43 0.54

7.5

4.7

10.0 13.0 21.o 22.0 26.0

4.0 4.2 5.2 5.1

4.8

0.0020 0.0025 0.0110 0.0150 0.0200 0.0320 0.0340 0.0400

2. Trensformatlon of acetone Figure 1 shows the effect of nPt/nA (or of nPt since nA is here constant) on the initial activity of the PtHZSM5 samples in acetone transformation. Like in hydrocracking the

initial activity Ao first increases with nPt/nA and for a certain value of nPt/nA remains constant. This means that then the limiting step of acetone transformation does not occur on the platinum sites and is therefore catalyzed by the acid sites.

0.5

0 .lo1

0:03 nPt/nA

Figure 1. Initial activity of RHZSM5 samples as a function of nPt/nA.

Figure 2. Ratio of the final to the initial activity of RHZSM5 samples as a function of nPtInA.

704

The activity of the samples decreases with time-on-stream. However the greater nWnA the slower the deactivation. This is shown in figure 2 giving Af/Ao the ratio of the activities at 250 mn and at 25 mn. This increase of the stability with nPt/nA was also found in n-heptane hydrocracking with PtWHY catalysts (4). However with this latter reaction the AflAo ratio became close to 1 for high values of nPt/nA while here the maximum value is 0.7 and when increasing nPt/nA there is a slight decrease in the coking rate and in C/T the ratio of the rates of coking and of acetone transformation. Thus C/T passes from 0.03 for nPtlnA = 0.0025 (0.07 PtHZSM5) to 0.011 for nPt/nA = 0.040 (0.54 RHZSMS). The product distribution depends also on nPthA : with pure HZSM5 only mesityloxide (MO) and propene are formed. Both are primary products. Diacetone alcohol (DA) is not observed indicating that its dehydration is very rapid in comparison to its formation by aldolisation of acetone on the acid sites. Only traces of MlBK were found due probably to hydrogen transfer from coke precursors to mesityloxide (MO).

10

5

5

io

(xx)

5

10

(XX)

Figures 3. Transformation of acetone on 0.16 PtHZSM5. Yields (wt YO) in the products as function of the conversion (X %). With PtHZSM5 catalysts the main products are light hydrocarbons (essentially propane) and MlBK (Fig. 3a). Small amounts of 2-methylpentane (2MP), of diisobutylketone (DIBK) and of mesityloxide (MO)are also observed. Figure 3b shows that light hydrocarbons, 2-methylpentane and MO appear as primary products, MlBK and more particularly DIBK appearing as secondary products. The reactions involved in the product formation are indicated in scheme 1 :

705

w

H2

H+ PI

H2

I

All the products except MO are formed through bifunctional catalysis. MlBK results from a three-step scheme : aldolisation of acetone on acid sites, dehydration of DA then hydrogenation of MO. The formation of diisobutylketone (DIBK) involves the same series of reactions. Propane formation occurs through a three-step process with hydrogenation of acetone on the platinum sites, dehydration of isopropanol on the acid sites and hydrogenation of propene on the platinum sites. 2-Methylpentane being a primary product results probably from propene dimerisation on the acid sites followed by hydrogenation. As does the total activity the rates of formation of MIBK, propane, 2-methylpentane and DIBK first increase with nPt/nA then remain constant (Fig. 4). This means that above a certain value of nPt/nA the hydrogenation reactions are no longer the limiting steps of the bifunctional processes. On the other hand, the rate of MO formation decreases when nPt/nA increases then remains constant. For these high values of nPt/nA MO and MlBK are probably in their thermodynamic equilibrium ratio.

706

0

2 n

10

1 -

0.01

0.03

nPt/nA

0:Ol

0.b3

nPt/nA

Figures 4. Influence of nPt/nA on the rates of formation (Rf) of Methylisobutylketone (MIBK), Propane (P), 2-Methylpentane (2MP), Mesityloxide (MO), Diisobutylketone (DIBK). CONCLUSIONS

On PtHZSM5 catalysts, acetone is transformed mainly into methylisobutylketone, propane and 2-methylpentane. These reactions occurring through a bifunctional pathway their rate is mainly determined by the ratio between the number of hydrogenating and of acid sites. Like for the hydrocracking of alkanes, the rate first increases with this ratio then remains constant ; on the other hand the greater this ratio the greater the stability of the catalysts. ACKNOWLEDGMENTS

L.MELO acknowledges a fellowship from "Consejo de Desarrollo Cientifico y Humanistic0 de la Universidad Central de Venezuela". REFERENCES

1 "New Solvent Process", Ind. Research (1968), 25. 2 P.V. Chen, S.J. Chu, N.S. Chang, T.K. Chuang and L.Y. Chen, "Zeolites as Catalysis, Sorbents and Detergent Builders" 46 (1989) 231. 3 L. Melo, F. Chevalier, P. Magnoux and M. Guisnet, Xlll Simposio lberoamericano de Catalisis, 2 (1992) 775. 4 M. Guisnet, F. Alvarez, G. Giannetto and G. Pbrot, Catalysis Today, 1 (1987) 415. 5 J.L. Guth and Ph. Caullet, J. Chim. Phys., 83 (1986) 155. 6 G. Giannetto, G. P6rot and M. Guisnet, "Catalysis by Acids and Bases" 20 (1985) 265. 7 P. Magnoux, P. Cartraud, S. Mignard and M. Guisnet, J. Catal., 106 (1987) 242.

707

AUTHOR INDEX

C

A Aboulayt, A. Akporiaye, D.E. Albers, P. Albonnetti S. Algarra, F. Allachverdiev, A.I. Allian, M. Alouche, A. Alvarez, F. Amadelli, R. Antenori, M. Arretz. E.

131 521 361 471 653 243 455 235 581 409 75 369

B

Backvall, J.E. Baiker, A. Bakonyi, 1. Bankmann, M. Barbier, J. Barrault, J. Barteau, A. W o k , M. Basset, J.M. Bautista, F.M. Beaune, 0. Beden, B. Belgsir, E.M. Ben Taarit, Y. Bertea, L.E. W i n , J. Besson, M. Blanchard, M. Blaser, H.U. Bodnar, Z. Bonnelle, J.P. Borszeky, K. Brand, R. Brunet, D. Brunet, S. Bullivant, L. Burgers, M.H.W. Burmeister, R.

417 377 179 91 171 203,305 463 67,417 147 267,615 401 439 439 447 607 171 115 203 139 377 235 179 91 587, 623 305 115 567 361

Calmettes, A. Camblor, M.A. Cambon, H. Campelo, J.M. Candy, J.P. Carassiti, V. Carreyre, H. Castiglioni, G.L. Cativiela, C. Cavani, F. Chabardes, P. Chambellan, A. Clerici, M.G. Collina, D. Comrnarieu, A. Corma, A. Costa, M.C. Court, J. Cseri, T. Curtin, T. Curto, M.J.M. Czarnecki, J.

595 3!n 623 227,615 147 409 601 275 495 471 677 131 21 479 369 393, 653 639 155 455 535 581,639 631

D Daelen, G. Daasvatn, K. Dauscher, A. Davies, P. Dazal, L. De Goede, A.T.J.W. Del Angel, G. Del Castillo, H. Delich, J.M. Deller, K. De Menorval, L.C. Derlacka, R. Derouane, E.G. Derouault, A. Despeyroux, B. De Vries, H.J.A. Didillon, B. Dimitriv, E. Dodgson, I.

587 521 219 235 299 513 171 299 67 361 495 631 587 203 361 661 147 669 1

708

Dominguez, J.M. Donato, A. Doussain, G. Drouillard, J. Duprez, D. Durand, R.

171,701 163 693 601 369 647

E English, M. Espeel, P. Essayem, N.

211 527 305

F Fagouri, C.J. Ferretti, O.A. Figueras, F. Finiels, A. Fornes, V. Forni, L. Fraile, J.M. Freund, A. Fuentes Mota, J.

123 147 455,495 401, 575,595 653 329 495 91 431

G Gallezot, P. 115 Galvano, S. 163 227,615 Garcia, A. 653 Garcia, H. 495 Garcia, J.I. 431 Garcia Gomez, M. 75 Gargano, M. Gazzano, M. 275 Geneste, P. 401,575,595, 623,647 Germain, A. 455 Gigante, B. 581 Gil, M. 495 Gilbert, L. 51,693 Glinski, M. 631 Gtlboltls, S. 187 Gongescu, D. 669 Graffin, P. 401 Grange, P. 299 Grass, F. 259 Grootendorst, E.J. 487 Grosselin, J.M. 259 Guardeflo, R. 227 Gubelmann, M.H. 551,693 Guida, A. 401 Guillet, J.P. 321

Guimon, C. Guisnet, M. Gytlri, A.

305,369 581,701 99

H Haldavanekar, B.V. Hamar-Thibault, S. Hattori, H. Hayes, K.S. HegedUs, M. Herdkowitz, M. Hodnett, B.K. Hoefnagel, A.J. Huang, M. Hubaut, R.

503 155 35 313 187 353 535 661 559 235

I Idriss, H. Imanara, T. Ivanova, 1.1.

463 83 587

J Jabtonski, J. Jacquot, R. Jacobs, P.A. Jallet, H.P. Jannes, G. Jarzyna, V. Jenck, J. Joffro, J. Johnson, T.A. Joucla, M.

155 131 527 139 299 631 291 647 313 291

K Kaliaguine, S. Katayama, A. Kato, T. Kemeny, S. Kemnnal, 4. Kijenski, J. Kogan, S. Kokoh, K.B. Koresh, J.E. Koutyrev, M. Kouwenhoven, H.W. Ksibi, M. Kul'kova, N.V. Kumbhar, P.S.

559 337 83 99 321 631 353 439 353 471 543,607 203 243 251

709

N

L Lambed, D. Lamesch, A. Lamy, C. Landau, M.V. Lasperas, M. Lavalley, J.C. L&er, J.M. Lemberton, J.L. Le Peletier, F. Lercher, J.A. Lobo, L.S. Luna, D.

235 299 439 353 623 131 439 601 147 195,211 685 227,615

M Magill, S . R . 661 701 Magnoux, P. 345 Makee, M. 219 Makouangou, M.R. Maldotti, A. 409 551 Maliverney, C. 267,377 Mallat, T. 123 Maiz, R.E. 447 Marchal, C. 171 Marecot, P. 227,615 Marinas, J.M. 195,211 Marinelli, T.B.L.W. 291 Marion, P. 187 Margitfalvi, J.L. 527 Martens, J.A. 393,653 Martinez, A. 131 Marzin, M. 99,267 Mathe, T. 131 Maug6, F. 495 Mayoral, J.A. !XI3 Mehta, P.H. 171 Melendrez, R. 701 Melo, L. 51,131,259,677 Mercier, C. 329 Miglio, R. 163 Milone, C. 337 Mizukami, F. 179 Molnard, A. 647 Moreau, C. 575,595 Moreau, P. 615 Moreno, M.S. 639 Mortherwell, W. Murzin, D.Y. 243

Nagy, J.B. Navio, J.A. Neri, G. Nicolauz-Dechamp, N. Nitta, Y. Niwa, S. Notheisz, F.

587 431 163 115 03 337 417

0

Ohkawa, S. Olive, J.L. OrdoAez, M.C.

337 401 227

P Paparatto, G. Parpot, P. Pereira, C. Perez-Pariente, J. Pernot, H. Pbrot, G. Petrini, G. Petr6, J. Piccirelli, A,. Pieri, E. Pietropaolo, P. Pinelli, D. Pires, E. Polo, E. Polyansky, E. Ponec, V. Popa, J.M. Pradera, A.M.A. Pralus, M. Prasad Rao, P.R.H. Primo, J. Prins, R. Pugin, B.

479 439 581 393 693 601 479 99,267 305 479 163 479 495 409 267 195,487 693 431 321 385 393,653 543,607 107

R Raab, C.G. Rajadhyaksha, R.A. Ramaswamy, A.V. Ramesh Reddy, K. Ranucci, E. Ratnasamy, P. Ravasio, N. Reynolds, M.P.

195, 211 251 385 385 425 385 75 123

710

Ribeiro, F.R. Richard, F. Rigutto, M.S. Rodriguez, 1. Rombi, E.

581 601 661 623 701

S Saeedan, A. Saussey, J. Scholten, J.J.F. Seibold, K. Severino, A. Sheldon, R.A. Smith, G.V. Soede, M. Solberg, J. Solofo, J. Song, R. Stamm, T. Stefani, G. Stbcker, M. Susini, J.

401 131 345 361 685 513 67 345 521 575 67 543 275 521 551

T Tacke, T. Taisne, C. Tamai, T. Tavares, R. Tempesti, E. Tirel, P.J. Tbrdk, B. Touroude, R. Toth-Kadar, E. Trifiro, F. Tuel, A. Tungler, A.

91 291

99 639 425 693 179 219 179 471,479 447 99,267

V Vaccari, A. 275 Van Bekkum, H. 513,567,661 Van der Wegen, P. 299 Van de Sandt, E.J.A.X. 345 Van Deurzen, M.P.J. 513 Van Rantwijk, F. 513 Vial, J. 685

W Wydoodt, M.

527

v

Yadav, G.D.

Z Zaki, M.I. Ziemecki, S.B. Zsigmond, A.

203 283 417

711

SUBJECT INDEX

A Acid-base sites Acidity effect Acylation of -,benzofuran -,thiophene -,toluene Alkylation of -,naphtalene Al2O3, modified AIPO-4 AIPO-4 Ti&

559 91 601 595 521

AIPO-5

575 535 615 615 567

Ammoximation of -,cyclohexanone

479

‘3C MAS-NMR 507 Cu-chromite modified 279,305 Cyclic ketones, reductive coupling 463 Cyclization of -,ethylonediamine and propyleneglycol 329

D Deactivation Dealumination Dehydrogenation of -,cyclohexanol -,tetrahydrothiophene Diels-Alder

377,535 661 353 369 495,647

B

E

Base catalysts 551 Base oxides 35 Basic sites, characterization 623 Basic zeolites 623 Beckmann rearrangement 535615 Bifunctional catalysis 701 Bimetallic catalysts 1,147 -,Ni-Mlgraphite 155 -,Pt-catalysts 211 -,Pt-cu 195 -,R-Sn 195 -,Ni-Cu 251 -,Ni-Fe 25 1

Electrocatalyticoxidation of -,sucrose 439 Electrochemical characterization 377 Enantioselectivecatalysis 1,51,99,107,139,495 Epoxidation 21,425

C Confinement catalysis

51

H Halogenation Heteropolycompounds H202 oxidation of -,cyclic alkenes and alkanes -,alkanes and aromatics -,aromatics Hydrogen transfer Hydrogen pressure effect

51 21,471 21 393 385 447 631 219

712

Hydrogenation of -,4-acetamido 2-hydroxybutyrophenone 259 -,acetophenone 99 -,aromatic acids 35 -,aromatic aldehydes 91 -,aromatics to cycloalkenes 337,345 -,azine 321 -,benzophenone 25 1 -,butenonitriles 299 -,rbutyrolactone 279 -carvone 170 -,cinnamaldehyde 163 -,citral 147,155,163 -,crotonaldehyde 211,219 -, D-fructose 187 -,dinitriles 283,291 -,esters 67 -,fatty acid ethyl esters 227 -,furfural 195 -,isoquinoline 51 -,a-ketoacids 139 -,maleic anhydride 279 -,methylacetamide cinnamate 107 -, methylbenzoate 131 -,methyl pyruvate 1 -,nitrile 313 ,nitroar0mat ics 51 -,phenol 51 -,rapeseed oil 325 -,substituted N-oxides 123 -,sugars 1 -,4-tert-butylphenol 243 -,thymot 119 -a$-unsaturated aldehydes 83,203

-

-,a$-unsaturated carbonyl compounds -,verbenone Hydrolysis of -, benzylchloride -, halotrifluoroethane Hydroxylation of -,phenol Hydroxylalkylation

75 179 503 693 21,51 455 567

I Immobilized ligands 107 Iron phtalocyanine, zeolite supported 417 Ir spectroscopy 131,203 lsomerizationof 75 -,a-acetylenic alcohols 677 -,a-pinene into camphene 685 -,5-vinyl bicyclo(221) heptene 35

K Ketonisation of 527 -,acid mixtures Kinetic study 243,329,595,685 Knoevenagel condensation 623

L Lap04 Lewis sites Lipase immobilized

693 653 513

M Metal catalysts -,amorphous Ni alloys

179

713

-,a -,a sponge catalyst -,a, supported

203 313

Montmorillonite K10 MoO3-SiO2

495 677

83

-,Cr/C 369 -,cu,supported 83 -,IrK: 119 -,Mo complex, polymer supported 425 -,Ni 227 -,Ni-Cu 227 -,Pd catalysts 267 -,PdlC 67,361 -,Pd/Ti pillared montmorillonite 299 -, Pd/TiO2 91 -,FVA1203, Bi promoted 377 -,Pt, Au modified 170 -,m 119 -,Pt/C fibers 353 -,R,cinchona modified 139 -,R,Pb modified 439 -,Mi02 219 -,Rh/C 119,243 -,Rh, modified 147 -,Ru 345 -,Ru-Cu/Si02 337 -,Ru, Sn modified 163 -,sulf ided 123,369 Metallo-porphyrins 21 Metal oxides 131,631 487 Metal oxides = Fefl3-Mn304 Metal support interaction 25 1 Mo complexes, polymer supported 425 Model reactions 559,653 Molecular sieves, Ti or V substituted 447

N Ni-Ce mixed oxides Nitration of benzene

235 607

0 Oxidation of -, a1kenes 417 -,benzyl chloride 503 -,alcohols 21,377 Oxidative dehydrogenation of -,isobutyric acid 471

P Phase-transfer catalysis Phenol-aldehydecondensation Photocatalytic oxidation of -,alkanes -,alkyl and alkenyl benzenes -,1,4-pentanediol Poisoning Prins reaction Pt-HZSM5

21 567 409 401 43 1 631 669 701

R Raney catalysts 283-321 Raney Cu 187 Raney Nickel 291 -,deuterated 67 Reaction mechanism 587 Rearrangement of -,acetals 653 543 -,arylamines to methyl pyridine

714

-,cumene to n-propylbenzene -,epoxy carbonyl Redox zeolites Reductionof ,acylchlorides -,ketones -,nitrobenzene into nitrosobenzene Ring opening

-

587 589 21 267 631 487 179

S Shape selectivity 51 SiO2 479 Si02, activated 639 Solvent effect 51,259 Surface characterization 361 Synthesis of -,3-aminopropoxyethoxyethanol 313 -,rbutyrolactone 1 -,di (3-aminopropoxyethoxy)ether 313 -,2,6-dialkylnaphtalenes 575 -,cisl,8-dioxaoctahydronaphtalene 647 -,hydrofluorocarbones 1 -,hydroxylamine 1 -,indoles 661 -,isophorone diamine 321 -,pmethylcrotonaldehyde 677 -,methyl isobutylketone 701 -,2-methylpyrazine 329 -,methyl trans-podocarpa8,11,13 trien-15-oate 581 ,paranitrophenol 551 -,tertiary fatty amines 305 -,2,2,2-trifluoroethanol 693

-

Superbases

35

T Ti-Beta zeolites Ti-silicalite Ti02 -,porphyrins modified -,zeolites added Transalkylation Transcarbonation Tranesterification of -,sugar esters Transition state selectivity

393 21,455 463,685 409 401 581 551 513 661

v V molecular sieves

385

2 Zeolites 35,535,543,647,653 -,alkali exchanged 559 -,Beta 661,669 -,CaA 513 -,dealuminated HY 581,601,607 -,HT 527 -,HMOR 575 -,HY 575,595 -,HZSM11 587 -,Lay 521 ZrTiO4 43 1

715

STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T.Yates, Universityof Pittsburgh, Pittsburgh, PA, U S A . Volume 1

Volume 2

Volume 3

Volume 4

Volume 5

Volume 6

Volume 7

Volume 8 Volume 9

Volume 10

Volume 11

Volume 12

Volume 13 Volume 14

Preparation of Catalysts IScientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-1 7,1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and 6. Delmon Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28,1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September9-11,1980 edited by 6. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13- 15,1980 edited by B. Delrnon and G.F. Frornent New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June30-July4,1980. Parts Aand B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyiie, September 29-October 3,1980 edited by M. Laznirka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties -Applications. Proceedings of a Workshop, Bremen, September 22-24,1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jit6 and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. B h a r d Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4,1982 edited by C.R. Brundleand H. Morawitz

716 Volume 15 Volume 16

Volume 17

Volume 18

Volume 19

Volume 20

Volume 21

Volume 22 Volume 23 Volume 24

Volume 25

Volume 26

Volume 27 Volume 28

Volume 29 Volume 30

Volume 31

Volume 32 Volume 33 Volume 34

Heterogeneous Catalytic Reactions Involving Molecular Oxygen byG.1. Golodets Preparation of Catalysts 111. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July9-13.1984 edited by P.A. Jacobs, N.I. Jaeger, P. Ji& V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3,1984 edited by S. KaliaguineandA. Mahay Catalysis by Acidsand Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27,1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29,1984 edited by M. Che and G.C. Bond Unsteady Processesin Catalytic Reactors by Yu.Sh. Matros Physicsof Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoroi-Portorose, September 3-8,1984 edited by B. Driaj, S.HoEevar and S. Pejovnik CatalyticPolymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6,1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-19,1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cervenq New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22,1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Gucri and H. Knozinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1,1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases forthe Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4,1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P. Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1,1987 edited by B. Delmon and G.F. Froment

717 Volume 35 Volume 36

Volume 37

Volume 38

Volume 39

Volume 40

Volume 41

Volume 42 Volume 43 Volume 44

Volume 45 Volume 46

Volume 47 Volume 48

Volume 49

Volume 50

Volume 51 Volume 52

Volume 53

Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Methaneconversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S.Vurchak Innovation in Zeolite MaterialsScience. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by P.J.Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22,1987 edited by J.W. Ward Characterization of PorousSolids. Proceedings of the IUPAC Symposium (COPS I),Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1,1987 edited by J. Koukal HeterogeneousCatalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17,1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Pa61 Catalytic Processes under Unsteady-State Conditions by Vu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, September 44,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structureand Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16,1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference,Amsterdam, July 10-14,1989. PartsA and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November27-December 2,1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advancesin ZeoliteScience. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19,1989 edited by J. Klinowskyand P.J.Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5 4 , 1989 edited by D.L. Trimm, S.Akashah, M. Absi-Halabi and A. Bishara

718 Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura New Developments in Selective Oxidation. Proceedings of an International Volume 55 Symposium, Rimini, Italy, September 18-22,1989 edited by G. Centi and F. Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium Volume 56 on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25,1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A Methods of Surface Analysis edited by J.L.G. Fierro Volume 578 Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction t o Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 24,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium Volume 60 on Chemistry of Microporous Crystals, Tokyo, June 26-29,1990 edited by T. Inui, S. Namba and T. Tatsumi Natural Gasconversion. Proceedings of the Symposium on Natural Gas Volume 61 Conversion, Oslo, August 12-17,1990 edited by A. Holmen, K.-J. Jensand S. Kolboe Characterization of Porous Solids II. Proceedings of the IUPAC Symposium Volume 62 (COPS I I ) , Alicante, May 6-9,1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of CatalystsV. Proceedings of the Fifth International Symposium Volume 63 on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, September 34,1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon Volume 64 New Trends in CO Activation edited by L. Guczi Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, Volume 65 August 20-23,1990 edited by G. Ohlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonfured, September 10-14,1990 edited by L.I. Simandi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27,1990 edited by R.K. Grasselli and A.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26,1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13,1991 edited by FA. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova Volume 54

719 Volume 70 Volume 71

Volume 72

Volume 73

Volume 74 Volume 75

Volume 76 Volume 77

Volume 78

Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium ICAPoC 21, Brussels, Belgium, September 10-13,1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by P. Ruiz and B. Delrnon Progress in Catalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff,Alberta, Canada, May 25-28,1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F.Solymosi and P. Tetbnyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20,1993 edited by T. Inui, K. Fujimot0.T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals 111. Proceedings of the 3rd International Symposium, Poitiers, April 5 - 8,1993 edited by M. Guisnet, J. Barbier, J. Barrault,C. Bouchoule, D. Duprez, G. Perot and C. Montassier

This Page Intentionally Left Blank