Heterogeneous Catalysis and Fine Chemicals IV

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Studies in Surface Science and Catalysis 108 HETEROGENEOUS CATALYSIS AND FINE CHEMICALS IV

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol.108

HETEROGENEOUS CATALYSIS AND FINE CHEMICALS IV Proceedings of the 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12,1996

Editors H.U.BIaser Ciba-GeigyAG, Basel, Switzerland A. Baiker ETH Zurich, Zurich, Switzerland R. Prins ETH Zurich, Zurich, Switzerland

1997 ELSEVIER Amsterdam — Lausanne — New York — Oxford — Shannon — Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat25 P.O. Box 211,1000 AE Amsterdam, The Netherlands

ISBN 0-444-82390-5 © 1997 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, PO. Box 521,1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. 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 referredtothecopyrlghtowner,ElsevierScienceB.V.,unlessotherwisespecifled. 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

Contents Foreword Scientific Committee Organizing Committee Financial Support

XIII XIV XIV XV

Industrial and Engineering Problems 1

2 3

4

5

6

7

Homogeneous catalysis for fine chemicals synthesis - new trends and perspectives M. Beller Catalysis for agrochemicals: the case history of the DUAL herbicide R.R. Bader, H.U. Blaser Heterogeneous vs. homogeneous catalysis in manufacturing of terbinafine - a case study for route selection of an industrial process U. Beutler, C. Fleury, P.C. Fimfschilling, G. Penn, Th. Ryser, B. Schenkel Multiple use of palladium in a homogeneous and a consecutive heterogeneous catalytic reaction P. Baumeister, W. Meyer, K. Oertle, G. Seifert, U. Siegrist, H. Steiner The optimization of the catalytic hydrogenation of hydroxybenzamidines to benzamidines M.G. Scaros, P.K. Yonan, S.A. Laneman Catalytic hydrogenation reactors for the fine chemicals industries; their design and operation K.R. Westerterp, E.J. Molga, K.B. van Gelder Selective catalytic hydrogenation of 2-butyne-l,4-diol to c/y-2-butene1,4-diol in mass transfer efficient slurry reactors J.M. Winterbottom, H. Marwan, J. Viladevall, S. Sharma, S. Raymahasay

1 17

31

37

41

47

59

Alkylation andAcylation Reactions 8

9

10 11

Replacing liquid acids infinechemical synthesis by sulfonated polysiloxanes as solid acids and as acidic supports for precious metal catalysts St. Wieland, P. Panster Amine functions linked to MCM-41-type silicas as a new class of solid base catalysts for condensation reactions M. Lasperas, T. Llorett, L. Chaves, I. Rodriguez, A. Cauvel, D. Brunei Modified clay catalysts for acylation of crown compounds S. Bekassy, K. Biro, T. Cseri, B. Agai, F. Figueras Acylation of aromatics over a HBEA Zeolite; effect of solvent and of acylating agent F. Jayat, M.J. Sabater Picot, D. Rohan, M. Guisnet

67

75 83

91

12 13

14

15 16

17

18

Zeolite-catalysed acetylation of alkenes with acetic anhydride K. Smith, Zhao Zhenhua, L. Delaude, P.K.G. Hodgson Influence of the acidity and of the pore structure of zeolites on the alkylation of toluene by 1-heptene P. Magnoux, A. Mourran, S. Bernard, M. Guisnet Reductive O- and N-alkylations; alternative catalytic methods to nucleophilic substitution F. Fache, V. Bethmont, L. Jacquot, F. Valot, A. Milenkovic, M. Lemaire N-Methylation of aniline over AIPO4 and AlP04-metal oxide catalysts F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero Preparation of symmetrical and mixed secondary alkylamines over Raney nickel and supported copper catalysts S. Gobolos, M. Hegedus, E. Talas, J.L. Margitfalvi Synthesis of dimethylethylamine from ethylamine and methanol over copper catalysts Y. Pouilloux, V. Doidy, S. Hub, J. Kervennal, J. Barrault Catalyst acid/base properties regulation to control the selectivity in gasphase methylation of catechol L. Kiwi-Minsker, S. Porchet, R. Doepper, A. Renken

99

107

115 123

131

139

149

Enantio- and Diastereoselective Hydrogenation Reactions 19

20

21

22

23

24

25

Enantioselective hydrogenation of ethyl-pyruvate and isophorone over modified Pt and Pd catalysts A. Tungler, K. Fodor, T. Mathe, R.A. Sheldon Controlling the enantioselective hydrogenation of ethyl pyruvate using zeolites as catalyst support K. Morgenschweis, E. Polkiehn, W. Reschetilowski Kinetic modeling of the ligand accelerated catalysis in the enantioselective hydrogenation of ethyl pyruvate; influence of solvents, catalysts and additives H.U. Blaser, D. Imhof, M. Studer Modeling of kinetically coupled selective hydrogenation reactions: kinetic rationalization of pressure effects on enantioselectivity J. Wang, C. LeBlond, C.F. Orella, Y. Sun, J.S. Bradley, D.G. Blackmond Enantioselective hydrogenation of (£)-a-phenylcinnamic acid on cinchonidine-modified palladium catalysts: influence of support Y. Nitta, K. Kobiro, Y. Okamoto Enantio-differentiating hydrogenation of 3-alkanones with asymmetrically modified fine nickel powder T. Osawa, T. Harada, A. Tai, O. Takayasu, I. Matsuura Diastereoselective hydrogenation of a prostaglandin intermediate over Ru supported on different molecular sieves F. Cocu, S. Coman, C. Tanase, D. Macovei, V.I. Parvulescu

157

167

175

183

191

199

207

Vll

26

27

Diastereoselective hydrogenation of substituted aromatics on supported rhodium catalysts: influence of support and of thermal treatment M. Besson, P. Gallezot, C. Pinel, S. Neto Stereoselective reductions of aromatic compounds E. Auer, A. Freund, P. Panster, G. Stein, Th. Tacke

215 223

Chemoselective Hydrogenation Reactions 28 29

30

31

32 33 34

35

36

37

38

39

Selective reduction of nitro groups in aromatic azo compounds M. Lauwiner, R. Roth, P. Rys, J. Wissmann Selective catalytic hydrogenation of 2,4-dinitrotoluene to nitroarylhydroxylamines on supported metal catalysts M.G. Musolino, C. Milone, G. Neri, L. Bonaccorsi, R. Pietropaolo, S. Galvagno Kinetics and pathways of selective hydrogenation of l-(4-nitrobenzyl)1,2,4-triazole C. LeBlond, J. Wang, R.D. Larsen, C.J. Orella, A.L. Forman, F.P. Gortsema, T.R. Verhoeven, Y.-K. Sun Design of selective l-ethyl-2-nitromethylenepyrrolidine hydrogenation for pharmaceuticals production V.A. Semikolenov, I.L. Simakova, A.V. Golovin, O.A. Burova, N.M. Smimova Kinetic study of a nitroaliphatic compound hydrogenation V. Dubois, G. Jannes, P. Verhasselt Kinetics of the hydrogenation of citral over supported Ni catalyst P. Maki-Arvela, L.-P. Tiainen, R. Gil, T. Salmi Selective hydrogenation of a,p-unsaturated aldehydes to allylic alcohols over supported monometallic and bimetallic Ag catalysts P. Glaus, P. Kraak, R. Schodel Surface organometallic chemistry on metals; selective hydrogenation of acetophenone on modified rhodium catalyst F. Humblot, M.A. Cordonnier, C. Santini, B. Didillon, J.P. Candy, J.M. Basset Use of Ni containing anionic clay minerals as precursors of catalysts for the hydrogenation of nitriles D. Tichit, F. Medina, R. Durand, C. Mateo, B. Coq, J.E. Sueiras, P. Salagre The effect of co-adsorbates on activity/selectivity in the hydrogenation of aromatic alkynes S.D. Jackson, H. Hardy, G.J. Kelly, L.A. Shaw Simple preparation of bimetallic palladium-copper catalysts for selective liquid phase semihydrogenation of functionalized acetylenes and propargylic alcohols M.P.R. Spee, D.M. Grove, G. van Koten, J.W. Geus Catalytic hydrogenation by polymer stabilized rhodium G.W. Busser, J.G. van Ommen, J.A. Lercher

231

239

247

255 263 273

281

289

297

305

313 321

Oxidation Reactions 40 41

42

43

44

45 46

47

48

49

50

51

52

Epoxidation of cycloalkenones over amorphous titania-silica aerogels R. Hutter, T. Mallat, A. Baiker Selective aerobic epoxidation of olefins over NaY and NaZSM-5 zeolites containing transition metal ions O. Kholdeeva, A.V. Tkachev, V.N. Romannikov, I.V. Khavrutskii, K.I. Zamaraev Effect of preparation methods of titania/silicas on their catalytic activities in the oxidation of olefins M. Toba, S. Niwa, H. Shimada, F. Mizukami Heterogeneous catalysts from organometallic precursors: how to design isolated, stable and active sites; applications to zirconium catalyzed organic reactions A. Choplin, B. Coutant, C. Dubuisson, P. Leyrit, C. McGill, F. Quignard, R. Teissier Selective sulfoxidation of thioethers on Ti-containing zeolites under mild conditions V. Hulea, P. Moreau, F. Di Renzo AUylic oxidation of cyclohexene catalysed by metal exchanged zeolite Y O.B. Ryan, D.E. Akporiaye, K.H. Holm, M. Stocker Ammoxidation of methylaromatics over NH/-containing vanadium phosphate catalysts - new mechanistic insights A. Martin, A. Bruckner, Y. Zhang, B. Liicke Hydrogen peroxide oxidation of methyl a-D-glucopyranoside, sucrose and a,a-trehalose with Ti-MCM-41 E.J.M. Mombarg, S.J.M. Osnabrug, F. van Rantwijk, H. van Bekkum On the role of bismuth-based alloys in carbon-supported bimetallic Bi-Pd catalysts for the selective oxidation of glucose to gluconic acid M. Wenkin, C. Renard, P. Ruiz, B. Delmon, M. Devillers Selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxaldehyde in the presence of titania supported vanadia catalysts C. Moreau, R. Durand, C. Pourcheron, D. Tichit Dehydrogenation of methoxyisopropanol to methoxyacetone on supported bimetallic Cu-Zn catalysts M.V. Landau, S.B. Kogan, M. Herskowitz Butadione synthesis by dehydrogenation and oxidative dehydrogenation of 2,3-butanediol G.V. Isagulyants, LP. Belomestnykh Phase transition of crystalline a-Te2Mo07 to the vitreous P-form, surface composition, and activity in the vapor-phase selective oxidation of ethyl lactate to pyruvate over TeOj-MoOj catalysts H. Hayashi, S. Sugiyama, T. Moriga, N. Masaoka, A. Yamamoto

329

337

345

353

361 369

377

385

391

399

407

415

421

53

Selective oxidation with air of glyceric to hydroxypyruvic acid anu tartronic to mesoxalic acid on PtBi/C catalysts P. Fordham, M. Besson, P. Gallezot

429

Immobilized and Encapsulted Complex Catalysts 54 55

56 57

58

59

60

61

62

63

64

65

PDMS occluded Ti-MCM-41 as an improved olefin epoxidation catalyst I.F.J. Vankelecom, N.M.F. Moens, K.A.L. Vercmysse, R.F. Parton, P.A. Jacobs Epoxidation with manganese N,N*-bis(2-pyridinecarboxamide) complexes encapsulated in Zeolite Y P.P. Knops-Gerrits, M. L'abbe, P.A. Jacobs Selective oxidation of benzyl alcohol on a zeolite ship-in-a-bottle complex A. Zsigmond, F. Notheisz, Z. Prater, J.E. Backvall Oxidation of pinane using zeolite encapsulated metal phthalocyanine catalysts A.A. Valente, J. Vital Hydrogenation of carbonyl groups containing compounds over Pt(II)-salen complexes occluded in zeolites W. Kahlen, A. Janssen, W.F. Holderich Novel clay intercalated metal catalysts: a study of the hydrogenation of styrene and 1-octene on clay intercalated Pd catalysts A. Mastalir, F. Notheisz, Z. Kiraly, M. Bartok, I. Dekany Catalytic enantioselective addition of diethylzinc to benzaldehyde induced by immobilized ephedrine: comparison of silica and MCM-41 type mesoporous silicates as supports N. Bellocq, D. Brunei, M. Lasperas, P. Moreau The immobilization of sulfonated Ru-BINAP chloride by anion exchange on layered double hydroxides D. Tas, D. Jeanmart, R.F. Parton, P.A. Jacobs Regiospecific hydrosilylation of styrene by rhodium complexes heterogenised on modified USY-zeolites A. Corma, M.I. de Dies, M. Iglesias, F. Sanchez Polymer-supported Al and Ti species as catalysts for Diels-Alder reactions B. Altava, M.I. Burguete, J.M. Fraile, J.I. Garcia, S.V. Luis, J.A. Mayoral, A.J. Royo, R.V. Salvador Molecular imprinting; polymerised catalytic complexes in asymmetric catalysis F. Locatelli, P. Gamez, M. Lemaire Environmentally friendly catalysis of liquid phase organic reactions using chemically modified mesoporous materials A.J. Butterworth, J.H. Clark, A. Lambert, D.J. Macquarrie, S.J. Tavener

437

445 453

461

469

477

485

493

501

509

517

523

Zeolite and Clay Catalysts 66

67

68

69 70

71 72

73 74

75

76

77

Meerwein-Ponndorf-Verley and Oppenauer reactions catalysed by heterogeneous catalysts E.J. Creyghton, J. Huskens, J.C. van der Waal, H. van Bekkum Selective synthesis of monoglycerides from glycerol and oleic acid in the presence of solid catalysts S. Abro, Y. Pouilloux, J. Barrault Zeolite-catalysed hydrolysis of aromatic amides B. Gigante, C. Santos, M.J. Marcelo-Curto, C. Coutanceau, J.M. Silva, F. Alvarez, M. Guisnet, E. Selli, L. Fomi Hydration of a-pinene and camphene over USY zeolites H. Valente, J. Vital Dehydration of 2-(2-hydroxyethyl)-pyridine to 2-vinyl-pyridine over solid acid catalysts L. Fomi, D. Moscotti, E. Selli, I. Belegridi, M. Guisnet, D. Rohan, B. Gigante, C. Coutanceau, J.M. Silva, F. Alvarez The use of heterogeneous copper catalysts in cyclopropanation reactions J.M. Fraile, B. Garcia, J.I. Garcia, J.A. Mayoral, F. Figueras Reaction between haloaromatics over Cu-HZSM-5 zeolite; mechanistic study S. Vol, L. Vivier, G. Perot Selective isomerization of a-pinene oxide with heterogeneous catalysts A.T. Liebens, C. Mahaim, W.F. Holderich Reactions of 2,2-dimethyl-l,3-pi'opanediol with zeolites: correlation of selectivity with acidity H.U. Blaser, B. Casagrande, B. Siebenhaar Clay-catalyzed reactions of imidazole and benzimidazoles with propiolic esters M. Balogh, C. Gonczi, I. Hermecz Selective synthesis of cyclohexylcyclohexanone on bifunctional zeolite catalysts; influence of the metal and of the pore structure F. Alvarez, A.I. Silva, F. Ramoa Ribeiro, G. Giannetto, M. Guisnet Solid acid catalyzed disproportionation and alkylation of alkylsilanes T. Yamaguchi, T. Yamada, M. Shibata, T. Tsuneki, M. Ookawa

531

539

547 555

563 571

579 587

595

603

609 617

Miscellaneous Topics 78

Intramolecular ene reactions promoted by mixed cogels N. Ravasio, M. Antenori, F. Babudri, M. Gargano

625

XI

79

80

81

82

Rh^^ ions and Rh^^-diamine complexes intercalated in a- and y-zirconium hydrogen phosphate as stable and effective catalysts for the conversion of aniline or nitrobenzene to carbamates and/or N,N*-diphenylurea; Part 3 P. Giannoccaro, S. Doronzo, C. Ferragina 1,4-Butanediol conversion routes over bifunctional supported Co-Zn catalyst L. Leite, S. Kruc, Zh. Yuskovets, V. Stonkus, M. Fleisher, E. Lukevics, J. Stoch, M. Mikalayczyk Preparation of solid superbase catalyst and its application to the synthesis of fine chemicals G. Suzukamo, M. Fukao, T. Hibi, K. Tanaka, M. Minobe Synthesis of delicious peptide fragments catalyzed by immobilized proteases M.D. Romero, J. Aguado, M.J. Guerra, G. Alvaro, R. Navarro, E. Rubio

633

641

649 657

Author Index

665

Other volumes in the series

669


Foreword After three meetings in Poitiers, France, the 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals was held under the auspices of the New Swiss Chemical Society in Basel, Switzerland, from September 8 to 12,1996. 270 scientists attended the meeting, more than a third of them from in industry - reflecting the importance of catalysis not only as an academic but also as a practical science. The focus of the symposium remained unchanged: fundamental as well as applied contributions on the use of heterogeneous catalysis for the preparation of fine chemicals were presented and discussed. The program consisted of 4 plenary lectures, 28 oral contributions and around 90 posters covering a broad range of reactions and catalytic aspects. 82 of these contributions are collected in the present proceedings, grouped into the following 8 topical areas: -

Industrial and engineering problems (7 contributions) Alkylation and acylation reactions (11 contributions) Enantio- and diastereoselective hydrogenation reactions (9 contributions) Chemoselective hydrogenation reactions (12 contributions) Oxidation reactions (14 contributions) Immobilized and encapsulated complex catalysts (12 contributions) Zeolite and clay catalysts (12 contributions) Miscellaneous topics (5 contributions)

Compared to the first three symposia, there are two developments worth mentioning. First, the number of contributions describing stereoselective hydrogenation reactions has increased noticeably, pointing to the growing importance of stereochemically pure active compounds. Second, immobilized and encapsulated complexes are making a comeback. There obviously is still hope that such heterogeneous catalysts can be usefiil for solving special selectivity problems. The Organizing Committee would like to acknowledge the efforts of all members of the Scientific Committee who helped to select the oral and poster contributions and in addition reviewed most papers of the present proceedings. We would also like to thank the staff of AKM Congress Services, Basel (Switzerland) and all other persons who helped to organize the symposium.


Scientific Committee Chairmen Baiker, ETH Zurich, Switzerland H.U. Blaser, Ciba-Geigy AG, Switzerland R. Prins, ETH Zurich, Switzerland Members C. Andersson, University of Lund, Sweden Amtz, Degussa AG, Germany R. Bader, Ciba-Geigy AG, Switzerland M. Bartok, Jozsef Attila University, Hungary A. Corma, Universidad Politecnica de Valencia-CSIC, Spain B. Delmon, Universite Catholique de Louvain, Belgium I. Dodgson, Johnson Matthey Ltd., UK F. Figueras, IRC Villeurbanne, France L. Fomi, Universita di Milano, Italy P. Fiinfschilling, Sandoz AG, Switzerland P. Gallezot, IRC Villeurbanne, France E. Gehrer, BASF AG, Germany P. Gravelle, CNRS-PIRSEM, France J. Heveling, Lonza AG, Switzerland W. Holderich, Technische Hochschule Aachen, Germany J. Kervennal, Elf Atochem S.A., France K. Kiihlein, Hochst AG, Germany T. Mallat, ETH Zurich, Switzerland G. Perot, Universite de Poitiers, France F. Rossler, Hoffmann-La Roche AG, Switzerland F. Schmidt, Siid-Chemie AG, Germany R. Sheldon, Delft University of Technology, The Netherlands K. Smith, University of Wales, UK M. Spagnol, Rhone-Poulenc, France M. Studer, Ciba-Geigy AG, Switzerland H. van Bekkum, Delft University of Technology, The Netherlands E. Zimgiebl, Bayer AG, Germany

Organizing Committee R. Bader, Basel (Chairman) H.U. Blaser, Basel (Secretary) D. Fritz, Basel (Secretary's office) A. Baiker, Zurich; W. Graf, Visp; G. Perot, Poitiers; R. Prins, Ziirich (Members)

XVI

Financial Support The organizers gratefully acknowledge the financial support of the following sponsors: Canton and City of Basel Ciba-Geigy AG, Basel Degussa AG, Frankfurt a.M./D Engelhard Corporation, Iselin NJ/USA Hoechst AG, Frankfurt a.M./D Hoffmann-La Roche AG, Basel Johnson Matthey, Royston/UK Lonza AG, Basel New Swiss Chemical Society Novartis AG, Basel Sandoz AG, Basel Swiss Federal Institute of Technology (ETH), Ziirich

Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All rights reserved.

Homogeneous Catalysis for Fine Chemical Synthesis - New Trends and Perspectives Matthias Beller Anorganisch-chemisches Institut der TU Miinchen, Lichtenbergstr. 4, 85747 Garching, Germany

Abstract Homogeneous catalysis used for fine chemical synthesis is a success story of both organometalhc chemistry and organic synthesis. The wide scope for application of recently developed transition metal catalysts and ligands is illustrated by selected exanq)les. Emphasis is given on efficient catalytic CC-couphng reactions, atom economic processes, and new stereoselective methods. Apart fi-om the synthetic possibihties of homogeneous catalysts trends to overcome the basic problem of catalyst recychng are briefly reviewed. Here, two phase catalysis offers the most elegant solution. Recent achievements, e.g. the use of alternative two phase systems as well as the design of new hgands in this area are reported. Keywords: Homogeneous catalysis; Organometallic chemistry; Two phase catalysis; CC-coupling reactions; Asymmetric catalysis 1. Introduction In the area of classical fine chemicals significant changes are taking place worldwide because a number of estabHshed productions in the western world e.g. for chlorinated aromatics, Bnaphthol, resorcin, and others, which are characterized by large amounts of unwanted sideproducts and waste can not conête with productions ia industrially developing countries which operate under different conditions. As a consequence the development of profitable production of already established fine chemicals can only be achieved with innovative methods which have ecological and economical benefits. In this regard catalysts in general are of key in:q)ortance due to their abilities to open up new reaction pathways and to improvQ all kinds of selectivity (chemo-, regio-, and stereoselectivity) in a given reaction. Consequently it is possible to use cheaper feedstocks and to avoid unwanted side-products. Compared to heterogeneous catalytic systems homogeneous catalysts ofl:en show very attractive selectivities under remarkably mild conditions. Moreover, homogeneous catalysis is generally better understood on a molecular level which leads to a more rational driven design and variation of homogeneous catalysts. However, many published homogeneous systems show only poor activities (turnover fi-equencies) and hfetune (total turnover numbers). In addition, even with very stable catalysts recycling is often not possible. As a consequence catalyst costs are dominating for certain processes and prevent commercial application.

Various strategies have been pursued to overcome these problems. On the one hand the introduction of new hgands has proved to be extremely successfiil in gaining new reactivity and inq)roved activity and on the other hand the concept of two phase catalysis nowadays allows recychng of highly sophisticated catalysts and Ugands even in the synthesis of advanced organic building blocks. 2. New transition metal catalysts and ligands Traditionally the strength of homogeneous catalysis results from the concept of tuning catalyst properties by changing the electronic and steric environment of a given transition metal center. Here, the hgands play an extremely inq)ortant role. Thus, the introduction of new types of Hgands often parallels breakthroughs in catalytic appHcations. Selected examples from our own research as well as highhghts from the hterature in the 90's will be adressed and discussed: The palladium-catalyzed activation and subsequent fimctionalization of aryl haHdes has received increased attention in the last decade^l The enormous synthetic possibihties are demonstrated in Scheme 1 by the synthesis of aromatic intermediates such as cinnamic acid derivatives, styrenes, biaryls, benzoic acids, benzonitriles, or anilines. .CN

Scheme 1: Palladium-catalyzed activation of aryl haHdes Until now aromatic fine chemicals are often synthesized using classical stoichiometric organic reactions. In principle, catalytic methodologies based on the pioneering work of Heck^^ offer an interesting alternative for the generation of new carbon-carbon or carbonheteroatom bonds in a selective and economicaUy feasible manner. However, the reactions

generally suffer from serious limitations which have precluded widespread industrial apphcations so far^\ Typically, a large amount of catalyst (1-5 mol %) is needed for reasonable conversions and often catalyst recycling is hankered by early precipitation of palladium black. Even more in:q)ortant for industrial apphcations is that the attractive aryl chlorides are generally unreactive. In recent years there have been only few contributions towards catalyst improvement hke the use of highly basic and sterically hindered phosphines by Milstein'^^ Osbom^^, Alper^\ and Reetz^\ or the appUcation of high pressure conditions recently described by Reiser^l In 1991 at the Central Research Laboratories of Hoechst AG we became interested in the palladium-catalyzed olefination (Heck reaction) of aryl hahdes and aryl diazonium compounds^^ which is arguably one of the most powerfiil methods for the synthesis of substituted olefins. In collaboration with Herrmann and co-workers we have shown that active catalyst mixtures obtained by using in situ mixtures of Pd(II) salts and commercially available tri-o-tolylphosphine consist under the conditions of the Heck reaction primarily of cyclometallated palladacycles^^^

R

Pdcat

.^^^^^./'^^^^/^

base 135 °C

R

— - Qi R

^\ Pd cat. =

OAc

/< V

^< /^K

^

2

^

R^

X

4-CH3CO C02BU Br Br 4-CH3CO Ph 4-CH3CO Ph CI 4-CH3CO CO2BU CI

^-

Yield (%) >95 45 43 50

TON 1.000.000 450.000 43.000 50.000

R = o-tolyl Scheme 2: Heck reaction with palladacycles as catalysts The metallacycles themselves show outstanding catalyst activities for the reaction of aryl hahdes with e.g. alkyl acrylates, and styrenes (Scheme 2). In particular the C-C bond forming reactions using aryl chlorides and palladacycles as catalysts seem to be technically viable for the first time. Thus, we were able to reach turnover numbers in the range of 30.000 - 50.000, albeit at conversions of 30-50% for olefinations of aryl chlorides substituted by electron-withdrawing groups^^\ The importance of Heck reactions for the synthesis of pharmaceuticals is illustrated in the synthesis of 6-methoxy-2vinyhiaphthahn which is a possible intermediate for naproxen, one of the most knportant nonsteroidal anti-inflammatory drugs. The key step of process is the vinylation of 2-bromo-6methoxynaphthahn with ethylene (Scheme 3). Despite the possibihty of double arylation the Heck reaction proceeds highly selective in the presence of a palladacycle catalyst, thus yielding the mono-arylation product in 80% yield with turnover numbers of the catalyst above 10.000.

Pd cat.

^ CH30

+ HBr

base 135°C

CH3O

1. Hydrocyanation (DuPont) 2. Hydrolysis or 1. Hydroformylation(Takaya 2. Oxidation

Pd cat.

CO2H CH3O

Scheme 3: Synthesis of Naproxen via Heck technology Apart from the generation of C-C bonds extension of the Heck methodology towards C-N bond formation has been reported independently by Buchwald^^^ and Hartwig^^\ Aromatic amines can be prepared directly from amines and aryl bromides in the presence of a palladium catalyst and a stoichiometric amount of a strong base. Again a catalyst system containing the sterically hindered tri-o-tolylphosphine hgand gave the best results.

Br

|T

N(R')R"

Pdcat.

+ R'(R")NH

»

+ HBr

base toluene Buchwald: Pdcat. = [(o-tolyl)3P]2PdCl2orPd(dba)2/2P(o-tolyl)3 base = NaO/Bu Hartwig:

Pd cat. = [(o-tolyl)3P]2Pd or [(o-tolyl)3P]2PdX2; X = Br, C base = LiN(SiMe3)2, NaO/Bu, LiO/Bu

Scheme 4: Palladium-catalyzed amination of arylhaUdes Both electron-rich and electron-poor aryl bromides react in good yields, albeit with turnover numbers below 1000. Nevertheless, this new palladium-catalyzed amination protocol is a first step towards substitution of the traditional copper-mediated Ullmann condensation. In the area of polymerizations new Hgand systems have dramatically improved catalyst performance. The developed organometaUic complexes might also find widespread apphcation in organic synthesis. As an example metallocene catalysts offer new opportunities in asymmetric catalysis^"^^ after having proven to be industrially viable for olefin polymerization^^^

Very recently, palladium or nickel complexes in the presence of ligands with a 1,4diazabutadiene structural unit have shown superior activity for the coupling of fimctionalized olefins with non-fimctionahzed olefins^^. Although long known, this class of Hgands may also be able to effect transformations regarding fine chemicals not readily achieved witib the corresponding phosphine catalysts. Not only changing the hgand sphere but also the addition of a co-catalyst can dramatically increase catalyst-efficiency. In an interesting exanq)le is the cobalt-catalyzed amidocarbonylation. In a perfectly atom economic way N-acylamino acids can be produced fiôm simple amides, aldehydes and carbon monoxide. Although amidocarbonylation reactions which were originally developed by Wakamatsu in the early seventies^^^ cannot conq)ete commercially against cheap natural sources or fermentation, for non-natural amino acids this salt fi-ee process must be considered as a viable alternative to the conventional Strecker reaction.

9N

QO RR""

9II

I N - "

+

R-^-'^^H

H

+ CO

»-

«

R^'^N^'^C

i > 95% yield

Scheme 4: Atom economic synthesis of N-acylamino acids Recently, we discovered at the Central Research Laboratories of Hoechst AG that the addition of acids as co-catalyst dramatically improves the amidocarbonylation of Nalkylamides^^\ Under very mild conditions (50°C and 10-20 bar CO) extremely high conversions (> 99%) and selectivities (> 95%) can be achieved in the synthesis of sarcosinates. Most significantly a wide variety of non-natural N-acyl and N-acyl-N-alkyl amino acids such as arylglycines, arylalanines, and alkylglycines can be prepared in good to excellent yields^^\ Because of industrial appHcations of N-acyl amino acids as chelating reagents and detergents this method has aheady been up scaled to a 200 1 scale. Due to recent pharmaceutical interest in peptides containing non-natural amino acids an asymmetric amidocarbonylation would constitute a sophisticated tool for catalytic asymmetric synthesis. This leads over to new developments in the area of stereoselective catalysis. 2.1. Stereoselective catalysis It is undisputed that asymmetric synthesis has gained increasing importance both at the university level and in industry. The last ten years have seen enormous advances in asymmetric catalysis using transition metals and during the next decade fiirther progress will be made. The market for optically-pure pharmaceutical compounds grows faster than for classical fine chemicals. In 1994 bulk intermediates for pharmaceuticals reached a market volume of $9.2 biUion. At the beginning of the next century the potential market for synthetic chiral products in bulk form alone is estimated to be much more. Special interest in asymmetric catalysis has been paid towards the refinement of olefins because alkenes are the most versatile feedstock of the chemical industry and moreover for

organic synthesis. In future industrial realizations will be seen especially for oxidations and hydrogenations. Emerging technologies include the Sharpless dihydroxylation, the Jacobsen epoxidation, and hydrogenations with ferrocenyl phosphines and phospholanes. From the standpoint of general appHcabihty, and scope the osmium-catalyzed asymmetric dihydroxylation of alkenes (Sharpless dihydroxylation) has reached a level of effectiveness which is unique among asymmetric catalytic methods^^l In the presence of an optimized catalyst hgand system nearly every class of olefin can be dihydroxylated with high enantioselectivities.

' x ^ ^ p

K20s03(OH)2 K3Fe(CN)6 >> ligand t-BuOH, H2O

OH yields > 80% ee's > 90-99%

ligand:

êO,

quinidine derivatives

OMe

quinine derivatives

Scheme 5: Sharpless dihydroxylation The synthetic success of the Sharpless dihydroxylation (AD) is based on the hgand acceleration phenomenon^^^ and an incredible optimization program, which resulted in the preparation and testing of more than 500 hgands. Pseudoenantiomeric cinchona alkaloid derivatives support extremely efficient catalysis, as shown in the homogeneous dihydroxylation of 2-vmyhiaphthalene. Turnover fi-equencies of 3000 min'^ which mimic enzymatic catalysis have been achieved. Numerous synthetic apphcations of the asymmetric dihydroxylation have aheady appeared and some are of potential industrial interest including the synthesis of propranolol, diltiazem, 4-amino-3-hydroxybutyric acid, azole antifimgals, chloramphenicol, taxol side cham, and canq)tothecin intermediates^^^ (Scheme 6). An extension of the AD process, the asymmetric amidohydroxylation reaction has been reported by Sharpless et al. early this year^^l The resulting B-amido alcohols can be easily transferred to B-amino alcohols, which are an unportant structural element in pharmaceuticals. Although the enantioselectivities are generally lower conq)ared to the AD they can often be raised to enantiopurity by sinq)le crystallization due to the crystalline nature of the products. For the epoxidation of a variety of olefins chiral Mn(III)-salen complexes have been introduced by Jacobsen^^^ and subsequently Katsuki^'^^ These catalysts have emerged as the most enantioselective epoxidation catalysts uncovered to date. The system is particularly well suited for the epoxidation of c/5-disubstituted olefins and trisubstituted olefins. Trans-epo^dos are obtained with high enantioselectivities in the presence of an additional chiral quartemary ammonium salt^^l

OAc

OH H2N^

Propranolol

^COsH

GABOB

OH

Ar

NHBz

^^X/CH20H

^"

-OH >;^v^ O2N

OH Azole antifungals

NHCOCHCI2

Chloramphenicol

Scheme 6: Selected products from asymmetric dihydroxylations (AD) Epoxidations can be carried out at room temperature in a two phase system employing commercial bleach or a combination of oxygen and pivaldehyde. Due to the practical utihty of a number of epoxidations the salen catalyst derived from chiral 1,2-diaminocyclohexane has been prepared at the multihundred kilogramm level^^^ Nevertheless industrial apphcations seem to be difficult so far because of a Umited catalyst efficiency and stabiHty (TON < 1000). R'

Mn - Catalyst

R'

NaOCI

cis - olefins:

80 - 99% ee

trans - olefins: 20 - 40% ee trisubst. olefins: 50 - 70% ee terminal olefins: 35 - 70% ee

Scheme 7: Jacobsen epoxidations It will be interesting to see whether manganese salencon^lexes can be made more efficient or if more active epoxidation catalysts, e.g. methyltrioxorhenium^^^ in the presence of chiral hgands will lead to more practical solutions. Apart from oxidation reactions hydrogenations offer an easy access for the mtroduction of stereogenic centers in a given molecule. Again the preparation of new classes of chiral Ugands, especially phosphines led to significant progress. Although the enantioselective hydrogenation

of olefins has been extensively investigated, and relatively high enantioselectivities have been achieved with certain substrates synthetic procedures still need to be inq)roved. In this regard the so-called DuPHOS hgands 1 based on the 2,5-dialkylphospholane moitey proved to be well-suited for rhodium-catalyzed hydrogenation of enamides to give a wdde range of nonnatural amino acids^^^ Advantageously, both (E)- and (Z)-isomers of enamides can be hydrogenated in high enantiomeric excess to give products with the same absolute configuration. Even B,B-disubstituted acetamidoacrylates gave B-branched amino acids in up to 99% Qe^^\

R..„ /

1

R

/

1

PPh2

c; P-^.

'

o

CH3

Fe '

1: R = CH3,C2H5,i-C3H7

2

3

Scheme 8: New Ugands for asymmetric hydrogenations In the past the concept of C2 symmetric catalysts has played an inq)ortant role in the design and understanding of asymmetric catalysis. Even so, examples of chelating phosphines without C2 symmetry appear in the hterature with increasing fi-equency and open generally new opportunities. In this respect, chiral ferrocenyl hgands have at present a specific potential. An easy to use enantioselective ortho-hthiation of ferrocenylamidesfi*omthe Snieckus group^^^ offers availabihty of a number of new ferrocenyl hgands wdth high optical purity and extends the first mdustrial apphcations for ferrocenyldiphosphines^^l While Lonza Ltd. apphes the hgand 2 in a rhodium-catalyzed hydrogenation for a new biotin synthesis Ciba-Geigy performs an iridium-catalyzed hnine hydrogenation using 3 for the herbicide (S)-Metolaclilor^^\ New mrpetus for asymmetric hydroformylations came primarily fi-om Takayas phosphinephosphinite hgand (BINAPO) 4 which constitutes an enormous breakthrough^^\ In combination with rhodium the BINAPO hgand gave enantioselectivities up to 95% and i/n ratios > 86/14 in the hydroformylation of substituted styrene derivatives. Conversions are > 99% at substrate/catalyst ratios between 300 and 2000. Shortly afterwards, similar catalytic results were reported by Union Carbide with chiral diphosphinite hgands, e.g. 5^^\ After many years of stagnation these new catalysts now point the way towards fixture developments in asymmetric hydroformylation. Another impressive example for the importance of electronic asymmetry in the design of chelating chhal hgands was reported by RajanBabu and Casahuovo for the asymmetric hydrocyanation reaction^'^^ As chiral Hgands 3,4-phosphinites fi"om D-finctofiiranoside derivatives were synthesized. The imsymmetrical phosphinite with the more electron-deficient phosphorous at the C4-position of fiuctose gave superior enantioselectivities for the hydrocyanation of 6-methoxy-2-vinyhiaphthalene.

Ph^

^

CHO

RhI cat./ligand + CO + H2

^CHO Ph

OlVfe

Scheme 9: Chiral ligands for asymmetric hydroformylation Obviously, basic research is needed to provide a more comprehensive insight into the dependence of enantioselectivities on Ugand electronics which is only poorly understood compared to steric effects. Nevertheless the concept of electronic asymmetry to enhance enantioselectivity in other Hgand systems is very appealing and should be considered to a greater extent forfixtureligand optimization studies.

3. New applications of transition metal catalysts The search for new reactivity and new reactions is an mq)ortant target in homogeneous catalysis. A declared goal is the selective activation of C-H bonds under mild conditions. Although there are numerous exanq)les of stoichiometric C-H bond oxidative additions to transition metal centers, successfiil examples regarding catalytic fimctionalization of C-H bonds have been made only during the last five years. Notable advances have been achieved by Moore and coworkers who described in 1992 the or^/zoacylation of pyridine with olefins and carbon monoxide. The cluster compound triruthenium dodecacarbonyl has been used as catalyst (Scheme 10).

1.3mol% C4M9

N

9

Ri^(C0)i2 60 CO, 150°C C4H9

5%

Scheme 10: Catalytic acylation of pyridine

10 High regioselectivities and turnover frequencies of 300/h have been achieved. It is beheved that the cluster framework remains intact during the course of the acylation reaction and chelation of the nitrogen atom facihtates CH-activation^^l Until now no extension of this chemistry or ftirther results have appeared in the Hterature. Thus, the couphng reaction seems to be applicable only to pyridine derivatives. Similar chelation assistance has been used to affect the addition of ortho C-H bonds of aromatic ketones to olefins albeit in the presence of a totally different catalyst system. Here, Murai et al. use certain ruthenium conq)lexes, e.g. Ru(H)2(CO)(PPh3)3^^\ in refluxing toluene (Scheme 11). The chemical yields based on the aromatic ketones are often close to quantitative. The resulting products, ortho alkylsubstituted aromatic ketones, are not easily available by classical organic chemistry, demonstrating that new catalytic reactions can be new synthetic tools, too. Clearly, at present there are significant limitations to the range of olefins that are suitable for this reaction. Neither olefins having strong electron-withdrawing groups or electron-donating groups react yet. Probably modification of the catalyst may overcome these limitations in fixture.

RuH2(COXPPh3)3 < ^ R 110°C, 18h

Scheme 11: Alkylation of aromatic ketones (Murai reaction) Immediate extensions of the Murai alkylation are akeady underway, e.g. catalytic addition of alkynes to aromatic C-H bonds, and alkylation of 2-phenylpyridines with olefins^^\ Another exanq)le that the successfiil discovery of new reactions may effect fine chemical synthesis is the selective cross-metathesis of acrylonitrile with terminal olefins to give substituted acrylonitriles. This is the first time that an olefin fimctionalized directly at the double bond undergoes cross-metathesis^^l In the presence of Schrock's molybdenum catalyst Mo(CHCMe2Ph)(NAr)[OCMe(CF3)2] yields of 18-90% and total turnover numbers of 4-25 were achieved. As a matter of fact, the ubiquitious avaUabihty of terminal olefins combined with their low prices makes this methodology potentially usefiil for industrial appHcations.

4. New methodical developments In addition to the synthesis of specific Hgands and the resulting catalysts new ideas and concepts begin to combine with transition metal chemistry to open up new areas in homogeneous catalysis, hke catalysis under supercritical conditions^^\ colloidal catalysis'*^\ organometalHc electrocatalysis and multi-metallic catalysis"*^^ ("cooperative catalysis"). Beyond these methodical developments which will prove their utility in the next decade catalyst recycling is of crucial importance in homogeneous catalysis. In contrast to heterogeneous catalysis recychng of the expensive metal is usually difficult. To faciUtate catalyst/product separation often the attachment of a catalyst to an organic polymeric resin has been used'*^^ Although this concept workes nicely on a laboratory scale decreased activity and

11 the more serious leaching of catalyst under industrial conditions has prevented any apphcation so far. Todays best solution to surpass the recychng problem is Uquid/liquid two phase catalysis'^^l 4.1. Two phase catalysis The most hnportant methodical progress in homogeneous catalysis since 1980 has been the introduction of industrially feasable Hquid/Uquid two phase catalysis. This technique uses a homogeneous catalyst, dissolved in a hydrophiHc phase, advantageously water, while organic starting materials and products form a second phase. By sinaple phase separation the catalyst is separated from reactants and products. In relation to the reaction products the catalyst is immobihzed as well as heterogenized. Although industry created sophisticated processes already in the seventies (Shell higher olefin process) and early eighties (Ruhrchemie/Rhone Poulenc hydroformylation process)"*"*^ it is surprising that two phase catalysis has gained more academic interest only fakly recently. The following topics are currently investigated in the area of multi-phase catalysis: Synthesis of new hydrophihc Hgands, especially phosphines, has been pursued. The solubihty in hydrophiHc solvents (water) is achieved by introduction of polar substituents such as SO3H, -CO2H, or -NRs^ into the corresponding Hgand. Traditionally and by far the most examined class of hydrophiHc Hgands are sulfonated phosphines, e.g. trisulfonated triphenylphosphine (TPPTS). Often sulfonation of phosphines constitutes a problem because of concomitant oxidation of the phosphorous atom. Using a combination of boric acid and concentrated sulfiiric acid Herrmann and coworkers developed a very selective and efficient sulfonation technique'*^^ It is beheved that a super acidic medium is generated that protects the phosphorous atom by protonation. In order to use highly hydrophobic starting materials for two phase catalysis with water as hydrophiHc medium Hanson et al. synthesized surface-active phosphines. These form more active catalysts for the hydroformylation of 1-octene conq)ared to TPPTS"*^. Apart from ionic Hgands neutral hydrophiHc Hgands seem to be very interesting. Berbreiter et al. described polyaUcylene oxide substituted bis-(2-diphenylphosphinoethyl) amides'*^^ Because of an inverse tenq)erature-dependant solubiHty in water these Hgands solubilize a catalyst at room temperature in the hydrophiHc medium and at higher reaction temperatures in the organic phase ("smart Hgands"). By carefiil fine tuning the solubiHty change can be highly specific. Similar polyethyleneglycol modified phosphines (scheme 12) have also been described for 0x0 reactions'*^^ To enlarge the scope of "smart Hgands" we recently prepared a new class of sugar-containing phosphines'*^^ Based on a highly specific glycosidation reaction a large number of carbohydrate based triarylphosphines (scheme 12) are available for testing in new catalytic appHcations. In addition to aqueous biphasic systems a number of alternative two phase processes using non-aqueous Hquid/Hquid systems are emergmg. One exanq)le is Horvath's proposal of a fluorous biphase system (FBS)^^^ using the immiscibiHty of a fluorinated compound with organic solvents. Based on the increasing knowledge of two phase catalysis newfimechemical processes Hke butadiene telomerisation, aUylation with carbon nucleophiles, and the carbonylation of benzyl chlorides with water-soluble catalysts have aheady been commercialized or are Hkely to be industriaUy reaHzed in the near fiiture^'^^^

12 0(CH2CH20)nH

.-^^^^P—C^

SOsNa

^—0(CH2CH20)nH

H(OCH2CH2)nO

HO HO

X X = OH,NHAc

Scheme 12: Hydrophilic ligands for two phase catalysis Clearly, Hquid/Uquid two phase catalysis has not yet reached its culmination poiQt and will see fiirther apphcations in fiitm*e. 5. Conclusion and outlook On the verge of the 21. century we live in an interdependant global economy which has also effects on fine chemical synthesis. Following the general trend the chemicalfixturein Emope will be determined to a large extent by realization of new manufacturing processes for highly specialized products. In this area sophisticated structures for pharmaceuticals, agrochemicals, food additives, and other speciahties offer the best opportunities. For the purpose of fast and flexible apphcations homogeneous catalysis is ideally suited as an interdisciphnary science^ ^^ which apphes basic organometalHc chemistry in an efficient manner to organic synthesis. So far the focal point of organic synthesis was the development of new methods, often highly stereoselective reactions, but was not always usefiil for practical purposes. Traditional organometalHc chemistry was concentrated on the synthesis of new complexes but not so much on apphcations. This still creates a need for more research directed towards the inq)rovement of catalyst activities and productivities. Infixturethe combination of high level organometalHc chemistry with state of the art organic synthesis Hnked together by efficient catalysis wiH be crucial for the development of technicaUy usefiil processes for advanced organic building blocks. In this respect homogeneous catalysis wiU gain increasing inq)ortance in the industrial production of fine chemicals. Which trends wiH dominate homogeneous catalysis in academia the next years? The search for more active catalysts and easy catalyst recycling for aheady estabHshed methodologies wiU be of continuing interest. Atom economic processes using mdustriaUy viable starting materials wiH be considered to be of most importance. Concerning the inteHectuaUy appealing rational design of new catalysts one has to istinguish between catalyst development in general and catalyst optimization. Wlule mechanistic knowledge is and wiH be

13 the key to get a "catalyst lead structure" catalyst optimization will be in reality still very much empirical in nature despite all progress in computational chemistry. Fundamental research in the next decade will concentrate on more efficient catalysis for CHactivation, all kinds of catalytic additions of nucleophiles to double bonds as perfect atom economic procedures, and asymmetric catalysis. It is clear that Ugands will retain their inq)ortance for controlhng the metal catalyst. In order to improve hgand properties more and more comphcated, and thus more expensive Hgand systems will be designed. This seems especially true for asymmetric catalysis. Here, two phase catalysis will surely gain increasing importance due to the easy recycling not only of the metal but also of the appropriately designed Hgand. TraditionaUy carbonylation reactions are imderestimated in fine chemical business. Due to an abundance of starting materials and relatively inexpensive carbon monoxide or syn gas carbonylations wiU be enq)loyed more often to synthesize interesting building blocks: amino acids via amidocarbonylation, profenes by asymmetric hydroformylations or hydrocarboxylations, reductive and oxidative carbonylations towards urethanes and ureas, etc. In conclusion the development of homogeneous catalysis wiU have significant industrial intact and provide societal benefit in fixture. Besides this, it wiU be also much firn to participate actively in this scientificaUy interesting area.

Acknowledgements The author would like to thank the many coUeagues of the catalysis group at Hoechst AG for fiiendship and discussions. Special thanks go to Prof K. Kiihlein who initiated and supported always the catalysis research at the Central Research Laboratories of Hoechst AG. I particularly thank our coUaborators at the TU Miinchen Prof W. A. Herrmann and his coworkers and especiaUy my co-workers M. Eckert, J. Krauter, T. Riermeier, F. VoUmuUer, A. Zapf for then work and enthusiasm to join me in the areas of carbonylations. Heck reactions and two phase catalysis.

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16 [49] [50] [51]

M. Beller, J. G. E. Krauter, A. Zapf, Angew. Chem. Int. Ed. Engl. 36 (1997) in print. I. T. Horvath, J. Rabai, Science 266 (1994) 72. Recent monograph: B. Comils, W. A. Herrmann, Applied Homogeneous Catalysis, VCH, Weinheimi, 1996.


17

CATALYSIS FOR AGROCHEMICALS: THE CASE fflSTORY OF THE DUAL®HERBICn)E Rolf R. Bader and Hans-Ulrich Blaser*, Central Research Services, R-1055.628 CIBA-GEIGY AG, CH-4002 BASEL, Switzerland 1. SUMMARY The use of catalytic methods for the technical preparation of agrochemicals is illustrated by the case history of the herbicide metolachlor (trade name DUAL®), the most important herbicide for maize. The key step for the technical synthesis of the racemic compound is a reductive alkylation catalyzed by a Pt/C catalyst in presence of sulfuric acid. The commercial production of the biologically active S-enantiomers was made possible by the development of a new Iridium ferrocenyl-diphosphine catalyst system. Important aspects of the development of the two catalyst systems as well as in^)ortant prerequisites for the use of catalysts for the production of agrochemicals are discussed. 2. INTRODUCTION Metolachlor is at the present time the most important herbicide of Ciba-Geigy's Crop Protection Division. It is produced since 1978 in volumes of >10'000 tons per year and is sold under the trade name DUAL® . In the near future, an enantiomerically enriched form will replace the racemic mixture, leading to a reduction of the environmental load by ca. 40%. The case history that is presented here might not be prototypical for an agrochemical, but it serves several purposes. First, it is an impressive example demonstrating the importance of catalysis to the fine chemicals industry. Second, it illustrates how catalytic process technology evolves and, third, it describes how a technically feasible chiral homogeneous catalyst was developed for such a large volume product. The history of this project covers many years and several teams worked on many different aspects (Table 1). We will mainly discuss the problems related to the development of a technically feasible synthesis both for racemic and ^-metolachlor. Metolachlor was first described in 1972 [1]; it is an N-chloroacetylated, N-alkoxyalkylated ortho disubstituted aniline (Figure 1). The unusual fimctionalization pattern renders the amino function extremely sterically hindered. In addition, metolachlor has two chiral elements: a

18 chiral axis (atropisomerism, due to hindered rotation aroimd the C ^ - N axis) and a stereogenic center, leading to four stereoisomers. 1982 it was found that the two S-enantiomers provide most of the biological activity [2].

CIT^N'^CH''"

"'""uT^''"'

'^N-'^CH.CI

^N^CHJCI

ir ~^ ir aR.rS

metolachlor

^'"^N'^CH.CI

aS.rS

the active enantiomers

V^-'^N^CHJCI

w

aR.rR

aS.I'R

the inactive enantiomers

Figure 1. Structure and stereoisomers of metolachlor

Table 1. Milestones in the history of metolachlor 1970

Discovery of the biological activity of metolachlor (patent for product and synthesis)

1973

Decision to develop a production process

1974

First 100 kg of racemic metolachlor produced

1975

Pilot plant in operation (40001 reactor)

1978

Full-scale plant with a production capacity >10'OOO t/y in operation

1982

Synthesis and biological tests of the four stereoisomers of metolachlor

1983

First unsuccessful attempts to synthesize S-metolachlor via enantioselective catalysis

1985

Rhodium / cycphos catalyst gives 69% ee for the imine hydrogenation (UBC Vancouver)

1987

Discovery of new Iridium diphosphine catalysts that are more active and selective than Rh catalysts for MEA imine hydrogenation

1987

Ca-Sn-Pt catalyst for direct alkylation of MEA in the gas phase developed

1992

Patent for racemic reductive alkylation is granted

1993

Ir / ferrocenyl diphosphine catalysts and acid effect discovered

1993/4 Patents for rac-metolachlor expire 1995/6 Pilot results for S-metolachlor: ee 79%, ton I'OOO'OOO, tof >200'000/h, &st 300 t produced 1996

Full-scale plant for production of > lO'OOO t/y S-metolachlor starts operation

19 3, THE FIRST PRODUCTION PROCESS In August 1973 it was decided to develop and establish a technical process for the production of rac-metolachlor. Two synthetic approaches were proposed and tested in the laboratotry (Figure 2): The alkylation of 2-methyl-6-ethyl-aniline (MEA) with methoxyisopropanol (MOIP) and the reductive alkylation of MEA with methoxyacetone (MOA% followed by chloroacetylation. This was obviously a chance for heterogeneous catalysis.

Figure 2. Two synthetic routes to rac-metolachlor 3.1. Acid catalyzed alkylation It was weU known from the literature that anilines can be mono- or di-alkylated with alcohols in presence of acidic catalysts. However, preliminary experiments on several solid acidic catalysts in the vapor-phase gave complex mixtures of the desired product (NAA), as well as N-methyl-, N-dimethyl-, N-propyl- and N-isopropyl-Affi/4 as by-products. These can be e5q)lained by cleavage of the ether group of MOIP followed by reaction of the fragments with MEA, All attenq)ts to inprove the selectivity of this acid catalyzed alkylation failed. Therefore, the reductive alkylation route was chosen for fiuther investigations.

20

3.2. Reductive Alkylation Reductive alkylation of anilines is a very convenient and broadly used method to synthesize secondary and tertiary aryl amines. Many examples were described both in the primary literature and in reviews, nickel, palladium or platinum catalysts being recommended without any strong preference [3]. Surprisingly, we could not find any information on the reductive alkylation of highly sterically hindered anilines like MEA. The course of the reaction is well understood. In a first step, imines are formed by condensation of the aniline with the aldehyde or ketone, leading to an equilibrium where unreacted carbonyl compound and amine are present besides the imine. The resulting imines can either be isolated or hydrogenated directly out of the condensation equilibrium. For industrial applications the second method is preferred because it saves operational costs and costly isolation losses of raw materials are avoided. On the other hand, this in situ method rises a selectivity problem. The catalyst must be able to selectively hydrogenate the imine but leave the carbonyl con:q)ound untouched. This problem is aggravated when the rate of imine formation is slower than that of the hydrogenation step. This leads to even lower imine concentrations in the reaction mixture and as a result, an increase of the undesired carbonyl hydrogenation is likely. To circumvent this problem, the condensation reaction has to be catalyzed by the addition of small amounts of acids. For our process development we concentrated on the in situ method. i.e., the hydrogenation of the Schi£F-base formed in the equilibrium reaction of MEA and MOA to produce the Nalkylated aniline (with the working name NAA)(see Figure 2). Surprisingly, and in contrast to the literature, under moderate reaction conditions no hydrogenation took place with nickel and palladium catalysts. Hydrogen consumption started only at 80-90 °C with these catalysts but after consunq)tion of 1 mol of hydrogen, unreacted MEA was recovered in nearly quantitative yield. That means that MOA was hydrogenated preferentially to give MOIP whereas the imine was not reduced. Platinimi catalysts turned out to be more active and selective. With sulfided platinum on carbon catalysts, especially recommended for reductive alkylations [4], up to 75% ofNAA were formed at 50-60 °C in preliminary screening experiments. During later process development we found that unsulfided Pt/C catalysts were just as selective as the sulfided catalysts for the reductive alkylation, a significant advantagefi*omthe industrial point of view [5]. After intensive development work, first in the laboratory (con:q)rising >1500 experiments) and later in the pilot plant (also used for the production of the first commercial quantities), the following production process was established in early 1978 (see Figure 3): MOIP is dehydrogenated in the gas phase and MOA is isolated by azeotropic distillation with water. The MEA imine formed in situfi*omMOA and MEA is hydrogenated in a batch process at 5 bar

21 hydrogen pressure and 45-50 ^C using a 5% Pt/C catalyst. No additional solvent is used and the purified hydrogenfi*omthe MOA production can be employed. Under these conditions, the hydrogenation is very fest and small amounts of sulfuric acid must be added to catalyze the relatively slow condensation reaction ofMEA and MOA. NAA is worked up by continuous distillation in nearly quantitative yield and is then chloroacetylated to give rac-metolachlor. 0

Cu/ZnCrOx

OH

gas phase continuous process 300X

H2 used in hydrogenation

MOIP

MOA

CHjO^ NH^ Pt/C

jLôm^

liquid phase

^+ H2

batch process

H2O/H2SO4

&

Pt/C can be recycled

50'C, 5 bar MEA

NAA

CHjOv.^^^ ^ ^ ^ 4 H

1

Vi U NAA

+ P I P O ^ u 'î ^ . .2V..

fc

w

>

0

XX 1 YS^

u

metolachlor

Figure 3. The production process for rac-metolachlor 3.3. Some unusual phenomena of the reductive alkylation reaction Space-filling models of the imine gave us a first idea to understand the unusual preference for platinum as the catalytic metal. These models showed that the imine double bond is completely hidden by the two bulky ortho substituents, thereby preventing interactions with the metal surface. This led us to consider the possibility that not the imine but the isomeric enamine(s) shown in Figure 4 were hydrogenated. Enamine hydrogenation would also explain the strong preference for Pt, that in our experience is the catalyst of choice for this transformation. Since we were imable to detect any enamines by spectroscopic methods, we tested our hypothesis by carrying out deuteration experiments (Figure 5). Indeed, about 85 % of the products were bis-deuterated products with deuterium at the two a-positions of the

22 imine carbon atom. Based on this evidence we propose an equilibrium between the MEA-mme and the isomeric enamines present in very low equilibrium concentrations. Then, the observed high reaction rates require both a very &st isomerization as well as a very fast enamine hydrogenation by the platinum catalyst.

CHo

CH3

CH«

A/°^CH,

HN

N H3C

H3C.

CH^

Ql^

HN

and/or H3C

HC

^"3

CH^

Figure 4. Imine - enamine equilibrium CHjD

CH3

,CD

CH3 CD ^0.^ HN^ ^CHD CH3 1

.0.

1

fKÔU

^

HjCv,^

11

\ ; ^

J

*" Dj/Pt-Carbon

A^CH,

H3CV

U

,'

1J

\ ^ < = H ,

85% dg-Products

Figure 3. Deuteration experiment Another phenomenon was also not easy to understand. The catalyst seemingly showed almost no deactivation during its use in the reductive alkylation. On average, it is reused 70 times without much loss in activity or selectivity. The best catalyst lots lasted even up to 160 times! This behavior is quite unusual for a catalytic system, especially when some of the starting materials are recycled. We suspected some sort of an in situ regeneration during the catalyst cycle. While optimizing the operation procedure, we got a clue as to the nature of the regeneration. Before filtering off the catalyst, the hydrogen is replaced by technical nitrogen. In order to save hydrogen, we attempted to filter without first changing the gas atmosphere. To our surprise, the recycle rate of the catalyst dropped drastically and in some cases, the catalyst had to be replaced after only a few uses! We remembered that technical nitrogen as used in production plants contains up to one volume-percent of oxygen. Therefore, the working catalyst is exposed to traces of oxygen during the flushing procedure. We think that catalyst poisons like CO or higher molecular weight by-products are oxidized under these conditions.

23

thereby cleaning the catalyst siirface. In absence of oxygen such species accumulate and lead to a fast catalyst deactivation. 4. A 'DIRECT ALKYLATION PROCESS' Even though the production process described above was very eflScient, we went back to our first process idea: The synthesis of the NAA intermediate in one single process starting fi*om MEA and MOIP because this would allow to eliminate the dehydrogenation step and the distillation of the MOA, Obviously, acid catalysis did not work, therefore M. Rusek turned to multifunctional catalysts that are known to work by dehydrogenation - condensationhydrogenation mechanism. However, probably due to the steric properties of the MEA, it turned out to be very difficult to get the high activities and selectivities necessary for a commercial process. In the end, M. Rusek nevertheless succeeded to develop both the catalyst and find the reaction conditions to produce NAA directlyfi-omMOIP and MEA. At 200 °C in the gas phase NAA was produced with >98% selectivity at ca. 66% conversion over more than 1000 hours [7].

?*'

f

HN

0

H,C

MOIP

^^

1

if^""'

ii

NAA

A

yf

Dehydrogenation

Hydrogenation

t

CH,

Condensation

oX-V MOA

^CH,

1 MEA

^

H,C

< MEAImlne

Figure 6. Proposed steps for the direct alkylation of MEA with MOIP over the Pt-Sn/Si02(Ca) catalyst

24

Table 2. AUcylation of MEA with MOIP: Eflfect of catalysrt composition catalyst

conversion

selectivity

Pt - Si02

2%

29%

Pt - Si02 ( C a ^

3%

65%

Pt - Si02 Sn-doped

14%

93%

Pt - Si02 Sn-doped; Ca"^ 200°C, lbarH2

66%

97%

The key to the success was the development of a new bimetallic platinum tin catalyst on silica support that was treated with calcium salts. As shown in Table 2 all three components as well as the silica support are absolutely necessary to get a good catalysts performance, i.e. there is a remarkable synergy between the various elements. This promoted Pt catalyst catalyzes the alkylation of various sterically hindered anilines with both primary and secondary alcohols with high selectivities and acceptable conversions. [7]. It is remarkable that increasing steric demand either of the aniline or of the alcohol part, only marginally afifects conversion or selectivity. It seems quite reasonable to assume that enamines are involved also in this gas phase reaction. In the end, M. Rusek's very elegant process was not developed further. The two major reasons were the lack of a technical catalyst and the need for extensive (and therefore costly) changes in the production plant. 5. TOWARDS S'METOLACHLOR VIA ENANTIOSELECTIVE HYDROGENATION When it became clear that the two IS-enantiomers of metolachlor were responsible for most of the biological activity (see Fig. 1), there was the obvious challenge of finding a chemically and economically feasible production process for the active stereoisomers. Many methods allow the enantioselective synthesis of chiral molecules (that is the preferential formation of one enantiomer instead of the usual racemate). However, the selective preparation of ^-metolachlor was a formidable task, due to the very special structure and properties of this molecule and also because of the extremely eflBcient production process for the racemic product as described above. During the course of the development efforts, the following minimal requirements evolved for a technically viable catalytic system: ee >80%, substrate to catalyst ratio (s/c) >50'000 and turnoverfirequency(tof) >10'000 h'^.

25 5.1. Problem analysis and a first unsuccessful approach via enamide hydrogenation A careful aiialysis in 1982 lead to the conclusion that only an enantioselective catalytic process was feasible for an economical production of such large volumes . Further, the state of the art of enantioselective catalysis at that time indicated some chances of success for only two approaches: First, the hydrogenation of an enamide precursor of metolachlor (Figure 7) in analogy to the L-dopa process of Monsanto [8], even though nobody had ever tried to reduce such highly sterically hindered enamides using homogeneous chiral catalysts. Second, and more attractive from a practical point of view, the enantioselective hydrogenation of the imine intermediate that was produced in situ in the racemic reaction. However, enantioselective imine hydrogenation at that time was virtually unknown [9]. Accordingly, in 1982/3 we started to prepare all three isomers of the metolachlor-ermrmde shown in Figure 7. This was by no means easy and to our disappointment none of the catalysts described in the literature was active for the hydrogenation of either isomer and the feasibility study was terminated.

CHjOv,^^^

CH3O

[

CHjOv,^^^

CHjOx.

I

^

Figure 7. Enamide hydrogenation 5.2. The first success: Imine hydrogenation with Rh and Ir diphosphine complexes The next attempt at solving our problem was carried out in collaboration with a team at the University of British Columbia (UBC) who investigated the hydrogenation of both MEA and DMA imine with Rh diphosphine conq)lexes. They were indeed successful [10]: Under ambient conditions enantioselectivities in the range of 3 - 50% were obtained. The best optical yields of 69% ee were achieved using Rh(nbd)Cl]2/cycphos at -25°C (for Ugand structures see Figure 8). The best activities were observed in methanol/toluene but the maximum tof was only 15 hr^ at 65 bar, r.t., fer too low for an industrial application. Nevertheless, these results represented a remarkable progress for the enantioselective hydrogenation of N-aryl imines. Based on these results, we realized that the catalyst activity would be the critical issue. Therefore, F. Spindler, who was responsible for the project, was very much attracted by the results of Crabtree et al. who described an extraordinarily active Ir / tricyclohexylphosphine / pyridine catalyst that was able to hydrogenate even tetra substituted C=C bonds [11]. He decided to give iridium catalysts a try even though he was aware of their fest deactivation and

26 also of the very low activities of Ir diphosphine catalysts for the hydrogenation of enamides described by Brown [12]. The very first experiments with an in situ formed [Ir(cod)Cl]2/ diop catalyst were quite encouraging and an extensive screening of then available diphosphines, solvents, additives as well as an optimization of the reaction conditions was carried out. Here, we only summarize the best results [13], a more detailed report can be found in [14].

.CH,

R

0

^ R=CH3 (DMA-imine) R=C2l^5 (MEA-imine)

= H

xXi

PPhj PPH,

diop

bdpp

a

pph, cycphos ^PPh,

Figure 8. Imine hydrogenation: Structure of knines and of important ligands The highest optical yields were obtained with Ir-bdpp catalysts in presence of additional iodide (ee 84% at 0 °C) but the activity was disappointing. Better activities but with somewhat lower ee's were obtained for Ir-diop catalysts: Maximum turnover numbers of lO'OOO and higher, with an average tof of 250 h'^ could be achieved at 100 bar and 25 °C. When s/c ratios >10'000 were applied, the reaction did no longer go to completion. A major problem of these new Ir diphosphine catalysts was an irreversible catalyst deactivation. These results, especially the good enantioselectivities, were very promising and represented by fer the best catalyst performance for the enantioselective hydrogenation of imines at that time. Nevertheless, it was also clear that we could probably not reach our ambitious goals using Ir complexes with "classical" diphosphine ligands. Even though Ir/diop and Ir/bdpp catalysts showed much higher activities than the best Rh complexes for MEA imine, they were still far below the requirements: A new approach was clearly required. 6. A NEW LIGAND CLASS LEADS TO A PRODUCTION PROCESS Since we could not get stable catalysts with the known diphosphine ligands we started to test new types. Among others, we screened novel ferrocenyl-diphosphines (PPF) developed recently by Togni and Spindler [15]. Their mode of preparation (see Figure 9) allowed an eflScient fine tuning of the electronic and steric properties of the two phosphino groups, something

27 that is often very dfficult with other ligand classes. When they were tested in the hydrogenation of MEA imine there was a pleasant surprise: While the in situ catalyst derived from [Ir(cod)Cl]2 and the rather basic josiphos (R = Ph, R' = cyclohexyl) was not very active, the analogous catalysts with two diarylphosphino groups (R, R* = Ar) gave very promising results. Especially PPF-P(3,5-(CH3)2C6H3)2 (R = Ph, R = 3,5-xylyl) turned out to give an exceptionally active catalyst and, even more important, it did not deactivate!

Figure 9. Structure and preparation of ferrocenyl diphosphine ligands €250-1 o

rlOO 200*0010

1,200-

-80

^B

^^m

** 150-

-60

100-

-40 50*000

50-

-20

t best tof) e

200

15 1

1

1

Rh/ pAp

EZZDbesttof

-I

1

Ir/P'^P

1r/PPF

-r

-

A w

pilot results

Figure 10. Milestones of progress for the enantioselective hydrogenation of MEA imine (requirements: ee >80%, tof >10'000 h ^ s/c >50'000) In collaboration with H.P. Jalett and H.P. Buser, Spindler again carried out an extensive screening of diphosphines, solvents, additives as well as an optimization of the reaction conditions. Most remarkable was the effect observed when 30% of acetic acid were added to the reaction mixture resulting in a rate increase by a factor of 5 while the time for 100% conversion was more than 20 times shorter than without additives. Using optimized conditions, the isolated imine was hydrogenated at a hydrogen pressure of 80 bar and 50 °C using a substrate to catalyst ratio (s/c) of 750*000. Complete conversion was reached within 4 h. The

28

the initial tof exceeded I'SOO'OOO h"* and optical yields were >80%. I'OOO'OOO turnovers were achieved within 6 h. These results set a new standard for the enantioselective hydrogenation of imines (see Figure 10). One molecule of the Iridium catalyst can produce more than 500'000 molecules of S-NAA within two to three hours. The selectivity to the desired S-enantiomer is not extremely high but fulfills the requirement for the production of enantiometically enriched metolachlor. The technical handling of the organometallic catalyst precursor is rather easy, scale up presented no problems and at the moment, the production plant is being con:q)leted. 7. CONCLUSIONS The case history of this large volume herbicide demonstrates that * catalytic methods are very suitable for the synthesis of molecules of low to mediimi complexity in medium to large volumes as is often the case for agrochemicals. * imine hydrogenation is a very powerful synthetic method to produce sterically hindered Nalkyl anilines both chemo- and enantioselectively. The choice of the catalytic system is unusually important for getting the necessary high catalyst activities and selectivities. * catalysis by solid acid in the gas phase is unsuited for the alkylation of anilines with alcohols containing alkoxy groups. * a chiral switch fi'om the racemate to an enriched form is not only attractive for pharmaceuticals [16] but is also an inqx)rtant strategy for agrochemichals [17]. * the time for process development dependends very much on the state of the art of a given catalytic technology. It was quite short for the reductive alkylation, a well known method already in 1972, whereas it took more than 10 years to develop a suitable catalyst for the enantioselective imine hydrogenatioa * an empirical approach is the festest way to find or develop a catalytic system for a problem that has no close precedent. Mechanistic information is especially helpful in later stages of process development or for trouble shooting. 8. ACKNOWLEDGMENTS The results described in this case history are due to the eJBForts of several teams of very dedicated chemists, engineers and technicians and we would like to acknowledge their contributions. Reductive alkylation process: C. Gremmelmaier, P. Flatt, P. Radimerski, A. Balmer. Alkylation process: M. Rusek, B. Casagrande. Enantioselective hydrogenation: H.P. Buser, R. Hanreich, H.P. Jalett, U. Pittelkow, B. Pugin, F. Spindler, A. Wirth-Tijani, B. Eng, R. HSusel, S. Maurer, M. Parak, G. Thoma, N. Vostenka.

29 9. REFERENCES 1 2 3 4 5

6

7 8 9

10

11 12 13 14

15 16 17

C. Vogel, R. Aebi, DP 23 28 340 (Ciba-Geigy AG, 1972). H. Moser, G. Ryhs, H. Sauter, Z. Naturforsch. 37b (1982) 451. a) M. Freifelder, Practical Catalytic Hydrogenation, Wiley-Interscience, 1971; b) P.N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, 1967. US 3,336,386 (Uniroyal Inc., 1967): Sulfidation of catalyst leads to higher selectivity for imine hydrogenation. Sulfided catalyst are not stable for storage. Therefore, sulfidation has either to be carried out just before the start of the reaction or the inhibitor has to be added to the reaction solution. In addition, most sulfidation procedures are not easy to reproduce. Interestingly, this process was patented only much later when several patents describing less efficient syntheses of NAA had been published by other groups: R. Bader; P. Flatt, P. Radimerski, EP 605363-Al (Ciba-Geigy AG, 1992). M. Rusek, Stud. Surf Sci. Catal. 59 (1991) 359. D. Vineyard, W. Knowles, M. Sabacky, G. Bachmann, D. Weinkau£ J. Amer. Chem. Soc. 99 (1977) 5946. For recent overviews on enantioselective imine hydrogenations see H. U. Blaser and F. Spindler, ChimicaOggi, 1995, June, p. 11; H. U. Blaser and F. Spindler, Proceedings of Chiral Europe *94 Symposium, Spring Innovations, Stockport, UK, 1994, p. 69. W.R. Cullen, M.D. Fryzuk, B.R. James, G. Kang, J.P. Kutney, R. Spogliarich, I.S. Thorbum, US 4,996,361 (Ciba-Geigy AG, 1987); W. R. CuUen, M. D. Fryzuk, B. R. James, J. P. Kutney, G-J. Kang, G. Herb, I. S. Hiorbum and R. Spogliarich, J. Mol. Catal. 62 (1990) 243. R. Crabtree, H. Felkin, T. Fellebeen-Khan and G. Morris, J. Organometal. Chem. 168 (1979) 183. N. Alcock, J.M. Brown, A. Derome and A. Lucy, J. Chem Soc, Chem Commun. (1985) 575. F. Spindler and B. Pugin, EP Patent 0256982 (Ciba-Geigy AG, 1988); F. Spindler, B. Pugin and H. U. Blaser, Angew. Chem Int. Ed. Engl. 29 (1990) 558 . F. Spindler, B. Pugin, H.P. Jalett, H.P. Buser, U. Pittelkow and H.U. Blaser, in Catalysis of Organic Reactions, R.E. Malz Ed., Marcel Dekker Inc., New York, 1996, p.153. A. Togni, C. Breutel, A. Schnyder, F. Spkidler, H. Landert and A. Tijani, J. Am. Chem Soc. 116(1994)4061. S.C. Stinson, C&EN October 9 (1995) 45. G. M. Ramos Tombo and D. Bellus, Angew. Chem 103 (1991) 1219. H.P. Fischer, HP. Buser, P. Chemla, P. Huxley, W. Lutz, S. Mirza, G.M. Ramos Tombo, G. Van Lommen and V. Sipido, Bull. Soc. Chim Belg. 103 (1994) 565.



31

Heterogeneous vs. Homogeneous Catalysis in Manufacturing of Terbinafine A Case Study for Route Selection of an Industrial Process Ulrich Beutler, Christian Fleury, Peter C, Fiinfschilling, Gerhard Penn*, Thomas Ryser and Berthold Schenkel Sandoz Pharma Ltd., Technical Research & Development, Process Development Department CH-4002 Basle, Switzerland

Abstract The antimycotic agent terbinafine can be prepared from the vinylchloride 1 and tert.-butylacetylene 2 using different homogeneous or heterogeneous palladium-catalysts. Alternatively terbinafine was synthesized from the aUyl acetate 3 and N-methyl-N-(a-methylnaphtyl)-amine 4 with the aid of a polystyrene-supported Pd(PPh3)4-catalyst. AU processes gave terbinafine in high yield and stereoselectivity. For the selection of a new production process of terbinafine ecological, economical, and technical aspects are compared.

1. INTRODUCTION Terbinafine is an antimycotic agent, which is registered with the brand-name Lamisil®. Lamisn® is well tolerated and is used in topical and oral form for the treatment of fungal infections [1]. Terbinafine is the first manufactured drug substance containing an (£)-l,3-enyne substructure [2]. The former synthesis followed a 6-step sequence using the very toxic and nasty starting materials acrolein and phosphorus pentachloride [3]. In order to overcome the handling of these compounds and to reduce the number of process steps we have developed inter alia two synthesis for terbinafine using palladium catalyzed coupling-steps for the construction of the molecule. The advantages and disadvantages of the different couplingroutes are discussed. 2. EXPERIMENTAL 2.1 Catalysts As heterogeneous catalyst polystyrene-supported Pd(PPh3)4 [4] was used. The catalyst contained 0.7 weight-% of Pd after exchange reaction with Pd(PPh3)4. For experiments under homogeneous conditions Pd(PPh3)2Q2 or Pd(PPh3)4 were used as catalysts. Copper(I) iodide was used as co-catalyst.

32 2.2 Catalytic experiments (scheme 1) Careful exclusion of oxygen is necessary in all experiments (argon atmosphere). Heterogeneous experiments were performed in glass-reactors with conventional stirring or in loopreactors pumping the educt^roduct-solution over a bed of polymer-supported catalyst. Preparative homogeneous experiments were performed in glass-reactors following a standard protocol [5,6] whereas kinetic experiments with Pd(PPh3)4 or Pd(PPh3)2Q2 were performed in a copper autoclave. All analyses of product mixtures were done with the aid of GLC (internal standard method for quantitative measurements). Scheme 1

AcO

-=CH

'^Xi

2

homogendous or hetorogerraous Pckâtalyst / Cul

polymer-suppoiled Pcksatalyst

solverit butylamine / water

solvent methanol

Ha

Terbinafine

2.3 Synthesis of starting materials (scheme 2) Vinylchloride 1 can be prepared in a one-step reaction from N-methyl-N-(a-methylnaphtyl)-amine 4 and (E)-l,3-dichloro-propene in high yield [7]. The latter is commercially available or can be separated from an E/Z-mixture via rectification. Allyl acetate 3 was prepared by the following manner: tert.-butyl-acetylene was converted into the Grignard-compound with n-butyl-magnesium chloride followed by reaction with epichlorohydrin. Treatment of the resulting chlorohydrin 6 with NaOH in water gave the epoxide 7 in good overall yield. Isomerisation of 7 was achieved with butyllithium/ N J»J,N'^'tetramethyl-ethylenediamine in THF at -60 °C. Acetylation of the resulting allyHc alcohol with acetic anhydride furnished the required 6,6-dimethyl-hept-2-en-4-yne-l-yl acetate 3 as a 9:1

33

E/Z-mixture. The minor Z-isomer is the lower boiling component and pure E-isomer can be obtained via rectification. Scheme 2 I C I N X ^ / - CI NaOH 70-90'»C

-=CH

I.BuMgCI

:

^.

o

HO

2. ^—y

01

/20»G 01

6

NaOH,20*C

1.BuLI/TMEDA,-60*»C AcO 3

2. ACaO/NEtj

'' _^_ \7 o

E/Zca.9:1

3. RESULTS AND DISCUSSION 3.1 Reactions with homogeneous catalysis Coupling of tert.-butyl-acetylene 2 with vinylchloride 1 under homogeneous conditions in the presence of 0.05 mol-% of Pd(PPh3)2a2 or Pd(PPh3)4 and 5 mol-% of Cul in butylamine as solvent at 60 °C gave terbinafine-base 5 in > 95 % yield and excellent stereoselectivity. Kinetic experiments of this quite exothermic reaction have shown, that water has a slight influence on the rate of the reaction. First experiments were performed with Pd(PPh3)4 as catalyst. A model based on a kinetic first order in 1, 2, and the concentrations of the catalysts permits to describe the reaction rate in the whole of the concentration domain (equation 1). A slight reduction of the reaction rate constant after 80% conversion of 1 was observed. This can be explained by the the slight temperature increase observed at the beginning due to the fast exothermic reaction. A possible mechanism for the reaction imply the reversible formation of activated complexes between Cu"^ and 2 as well as Pd(PPh3)4 and 1. A quasi stationarity assumption (Bodenstein kinetics) for these activated complexes permitted to derive a kinetic model which was simplified in further steps.The reaction orders werefittedwith computer modeling for the

34

four species according to equation 1. The reaction orders were in all cases not significandy different from 1. -

^

= ^1 [ n [2 ] [Cul\ {Pd(PPh,),l

(1)

The system using Pd(PPh3)2Cl2 as a catalyst was studied in more details. An experimental plan with 3-4 levels (30 - 40 experiments) was designed. The concentrations of Cul, Pd(PPh3)2Q2, butylamine, water, 1, and 2, as well as the temperature and the reaction time were varied. The reaction should be first order in 1, 2, Cul, and Pd(PPh3)2Q2. However, the fit of the kinetic model with equation 1 was not as good as for the system using Pd(PPh3)4. It can be supposed, that the reduction of Pd(PPh3)2Q2 by 2 to a Pd^-species at the beginning of the reaction may produce different catalyst activities depending on the reaction conditions. Tert.butyl-acetylene can serve as reducing agent under formation of 2,2,7,7-tetramethyl-3,5octadiyne. The reaction rate is also dependent on the concentration of water and butylamine. A maximum reaction rate is obtained at a water concentration of about 1 mol/l. A linear model was adapted for determining the influence of the reaction conditions on the selectivity. The Cul-concentration does not affect the selectivity of the reaction, while an increase in the Pd(PPh3)2Q2-concentration reduces significandy the selectivity. The selectivity decreases slowly with time, since the product 5 reacts slowly to minor amounts of side products. The selectivity decreases also slighdy with increasing temperature and increasing excess of acetylene 2. 3.2 Reactions with heterogeneous catalysis Reaction of 1 with tert.-butyl-acetylene in presence of 0.36 mol-% of polystyrene-supported Pd(PPh3)4-catalyst gave terbinafine-base 5 with 90 % yield and excellent stereoselectivity. However, the turnover-rate of the re-used catalyst dropped dramatically in the second and third mn. A prolonged reaction time is necessary for complete conversion (table 1). Table 1 Influence of re-used polymer-supported Pd(PPh3)4-catalyst on the reaction time. Conditions: T = 40 ""C, 0.36 mol-% Pd-catalyst, 6 mol % Cul, butylamine as solvent mn

conversion

reaction time [h]

ration E/Z of 5

1 2 3

99.9% 97.6 % 98.7 %

17 40 68

99.6: 0.4 99.6:0.4 99.6: 0.4

Analysis of the crude product after thefirstmn showed a contamination of ca. 850 ppm of palladium. At the same time the polymer-supported catalyst contained up to 3 % of copper. Thus, a copper-palladium exchange led to the observed decrease of the catalyst activity. An altemative to the above discussed preparation of terbinafine is the palladium-catalyzed coupling of the allyl acetate 3 with N-methyl-N-(a-methyln2^htyl)-amine 4 [8]. This reaction

35

can be performed in a loop-reactor, where the reaction solution is pumped over a bed of polymer-supported Pd(PPh3)4-catalyst. The reaction proceeds very smoothly with 0.08 mol-% of Pd-catalyst at 50 °C in methanol as solvent. Starting from 99.5 % pure (E)-allyl acetate 3 the yield of 5 was 90 %. However, the E/Z-ratio of the product was only 97 : 3. The cycle can be repeated several times without loss of catalyst activity. 3.3 Route selection For the selection of the final production process ecological, economical, and technical aspects were compared. Criteria for the route-selection were: catalyst activity, stability and storage properties of the catalyst, number of reaction steps of the different processes, concepts of metal recovery and recycle, safety aspects of the coupling reactions, and other more. Table 2 Comparison of different routes to terbinafine route

reaction sequence

number of steps

catalyst

comment

A

4^1-^5

2

Pd(PPh3)4/CuI (homogeneous)

Pd-catalyst less stable

B

4-^l->5

2

Pd(PPh3)2a2/CuI (homogeneous)

catalyst stable and commercially available

C

4-^1-^5

2

Pd(PPh3)4 -polymer catalyst activity decreases very rapidly /Cul (heterogeneous)

D

2-^6-^7-^3-»5

4

Pd(PPh3)4-polymer (heterogeneous)

stable catalytic system but less selective coupling reaction and more reaction steps

Route B was chosen as the superior process since a stable, commercially available catalyst can be used at very low molar concentration (technically superior) [9]. The number of reaction steps in route B is lower than in route D and the E/Z-selectivity is significantly higher. Thus, route B is more economical. All metals can be recovered and recycled. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

G. Petranyi, N.S. Ryder, A. Stiitz, Science 1984,224,1239. A. Stutz, Angew. Chem. 1987,99, 323. A. Stiitz, G. Petrani, J. Med. Chem. 1984, 27,1539. B.M. Trost, E. Keinan, J. Amer. Chem. Soc. 1978,100,7779. Review: K. Sonogashira, Comprehensive Organic Synthesis, Vol. 3, p 521, Pergamon Press, 1990. V. Ratovelomana, G. Linstrumelle, Synth. Commun. 1991, i i , 917. D. Chemin, G. Linstrumelle, Tetrahedron 1994,50,5335. U. Beutler, J. Mazacek, G. Penn, B. Schenkel, D. Wasmuth, Chimia 1996,50,154. For a preliminary communication of this reaction see ref. 2. Route B is also superior to a recently published synthesis of terbinafine (5 mol-% of Pd(PhCN)2Q2 as catalyst). See: M. Alami, F. Ferri, Y. Gaslain, Tetrahedron Lett. 1996,37,57.



37

Multiple use of Palladium in a homogeneous and a consecutive heterogeneous catalytic reaction. P. Baumeister", W. Meyer\ K. Oertle", G. Seifert^ U. Siegrist" and H. Steiner^ 'Scientific Services CIBA-GEIGY AG, CH-4002 Basel, Switzerland ^Crop Protection Division CIBA-GEIGY AG, CH-4002 Basel, Switzerland ' Central Research CIBA-GEIGY AG, CH-4002 Basel, Switzerland

1. SUMMARY The Pd^^)-catalyzed reaction of aryl diazonium salts with monosubstituted alkenes [1] was found to be an interesting alternative to the weUknown Pd - catalyzed arylhalide alkene coupling (Heck t5ê reaction) or the copper mediated reaction of aryl diazonium salts with alkenes (Meerwein arylation) [2]. The reaction can be run without isolation of the diazonium salt in presence of only 0.5 to 1 mol% of the Palladium catalyst in a one pot procedure, in high jdeld and under mild conditions. The resulting styrene is reduced in a subsequent hydrogenation step with an in situ generated heterogeneous Pd-catalyst. The combination of three reaction steps without isolation of intermediates and the virtually complete recovery of the Pd-metal at the end of the reaction sequence makes this process [4] extremely efficient.

"

^

2

3

Figure 1. Reaction-Scheme

2. INTRODUCTION Benzenesulfonates carr3dng alkyl- substituents in ortho position to the suLfonato-group are intermediates in the synthesis of a new class [3] of potent herbicides. More conventional synthetic routes as, e.g. Friedel-Crafts alkylation fail to yield these products at economically acceptable costs.

38 Starting from readily available aniline-2-sulfonic acid we have developed a highly "atom efficient" production process for sodium-[2-(3',3',3'- trifluoropropl'-yl)]-benzenesulfonate based on the combination of three different reactions, among them a homogeneous and a heterogeneous Pd-catalyzed step.

3. RESULTS AND DISCUSSION The arylation of trifluoropropene with the diazonium salt of aniline-2sulfonic acid proceeds with extraordinary ease due to the outstanding reactivity of the olefinic compound and the relative stabiUty of the diazonium salt. The temperature must be set at a range where the relative rate of the arylation reaction versus decomposition of the diazonium salt is high. Experiments performed under elevated pressure (not cited here) showed the catalyst to develop its highest productivity under conditions with high olefin concentration. The low solubility of a gaseous olefin can be overcome, either by increasing the pressure, or by selecting a solvent with favorable solvating properties. In this work we found Pd(dba)2 to be the preferred catalystprecursor, since it is easily prepared according to the literature [5,6] by reacting an aqueous solution of H2PdCl4 with dibenzyhdeneaceton (dba). Contrary to the weU known classical Heck-type arylation with arylhalides, the so called Matsuda variation does not require the use of phosphine Ugands. Contacting the reaction mass with hydrogen gas after the completion of the arylation step, optionally by adding activated carbon as a carrier, generates a heterogeneous Pd on carbon catalyst with sufficient activity to hydrogenate the styrene formed. After the filtration of the catalyst, the product is isolated from the filtrate in very high yield. The Pd used to form the Pd(dba)2-precursor was reclaimed nearly without any losses (Pd-yield: 95%) after work up of the spent catalyst by incineration [7].

4. EXPERIMENTAL Preparation of diazosulfonate Aniline-2-sulfonic acid, is slurried in dry ethanoic acid at room temperature. Sulfuric acid is added to the reaction mixture. At between 18 and 20 °C aqueous sodium nitrite solution is added dropwise to the reaction mixture. After all nitrite is consumed, ethanoic anhydride is added and the resulting suspension is cooled to between 12 and 15 °C. Preparation of sodium-[2-(3',3\3'trifluoroprop-l'-enyl)]'benzenesulfonate At that temperature sodium acetate is added and stirring is continued at room temperature. Pd(dba)2 is added and 3,3,3-triQuoropropene is bubbled in. After the end of the mildly exothermic reaction no more nitrogen is evolved. The ethanoic acid is distilled off and the residue is dissolved in water.

39

Preparation of sodium'[2'(3',3',3'' trifluoroprop'l'-yl)]-benzenesulfonate The solution is transferred into a hydrogenation flask and activated carbon is added. Under the hydrogen atmosphere the heterogeneous catalyst is formed and the catalytic hydrogenation is carried out. The palladium containing catalyst is separated by filtration and washed with water. The aqueous solution of the product is concentrated in vacuo, the remaining ethanoic acid is neutralized and after cooUng to room temperature the precipitated product is filtered of and washed with NaCl brine. The wet cake is dried and the identity is confirmed by ^HNMR. The yield is higher than 90% for each individual step.

5. CONCLUSIONS The high potential of a skillful combination of different catalytic and noncatalytic steps to a high performing process is exemplified in the present work. The main achievements are: Accessibility of an intermediate in high 5rield and purity, which is not available economically and/or ecologically by more conventional synthetic methods. Low catalyst costs: (i) By using Pd(dba)2 as the catalyst-precursor for the Heck arylation step, prepared from readily available Palladium salts (aqueous H2PdCl4 solution). (ii) Use of the same Palladium for two different catalytic reactions by in situ generation of a heterogeneous Pd/C hydrogenation catalyst from the Heckreaction mass. (iii) Quantitative separation of the Palladium from the product stream by filtration of the spent hydrogenation catalyst (95% Pd reclaimed after refining).

REFERENCES 1 K. Kikukawa, K. Nagira, F. Wada and T.Matsuda, Tetrahedron, 37 (1981) 31. 2 C.S. Rondestvest, Organic Reactions, 11 (1960) 189; 24 (1977) 225. 3 W. Meyer and K. Oertle, EP 120'814 (to Ciba-Geigy AG), 1984. 4 P. Baumeister, G. Seifert and H. Steiner, EP 584^043 (to Ciba-Geigy AG), 1992. 5 M.F. Rettig and P.M. Maitlis, Inorg. Synth., 1990, 28, 110. 6 Y. Takahashi, T. Ito, S. Sakai and Y. Ishii, J. Chem. Soc, Chem. Comm., (1970) 17, 1065. 7 Data from workup of large amounts of spent catalyst by DEGUSSA



41

THE OPTIMIZATION OF THE CATALYTIC HYDROGENATION OF HYDROXYBENZAMIDINES TO BENZAMIDINES Mike G. Scarosa, Peter K. Yonana and Scott A. Lanemanb a G. D. SEARLE Skokie, IL 60077, USA bNSC Technologies, Mount Prospect, IL 60056, USA Abstract The hydrogenation of hydroxybenzamidine to benzamidine has been investigated with various catalysts and solvents. It was determined that palladium was the catalyst of choice, however, when employed with glacial acetic acid as the solvent, colloidal palladium was generated. When the solvent system was changed to water with 5 equivalents of acetic acid the colloidal problem was eliminated and the hydrogenation rate increased. The isolation of the benzamidine from water/acetone was instrumental in eliminating a key impurity. The various steps leading to a large scale manufacturing process are also discussed.

NH2

HO Ac

Results and Discussion Initial investigations successfully employed conditions using 4% Pd/C as catalyst at 60 °C and 60 psig hydrogen pressure in glacial acetic acid (Scheme I). The use of acetic acid served two purposes. Acetic acid was a good solubilizing solvent for the hydroxybenzamidine and it improved the reaction rate by either protonation of the hydroxyl group or by formation of an N-acetoxy intermediate by proton-catalyzed esterification. Both of these would produce a better leaving group during catalysis. Scheme I

5'"Y

If NH2

NR

4% Pd/C, HOAc

'NR H

60 °C, 60 psig H2

NH2 HOAc

42

The reduction of hydroxybenzamidine to benzamidine was studied with various acids, both organic and inorganic. A summary of reactions performed is Hsted in Table I. In general, the reaction proceeded very well when organic acids were employed. Organic acids, such as methanesulfonic acid, L-tartaric acid and acetic acid, produced benzamidine in good yields. Inorganic acids, such as HCl and HBr, resulted in poor reaction rates and incomplete reactions. As can be seen in Table I, the best conditions were found when acetic acid was used as a solvent. One possible reaction pathway of the reduction involved the formation of the N-acetoxy intermediate prior to hydrogenation, as shown in Scheme II. Proton-catalyzed esterification of hydroxybenzamidine would produce the "N-acetoxy derivative" with the respective organic acid which would then be carried onward to benzamidine by catalytic hydrogenolysis of the N-0 bond. This may explain why hydrogenation proceeded better with organic acids compared to inorganic acids. However, one should not overlook the possibility that halide ion can poison catalysts and inhibit certain hydrogenationsCil. Table I: 1 Catalyst

Conditions for initial experiments for the hydrogenation of hydroxybenzamidine to benzamidine. Catalyst Temp. Solvent Acid Isolated Yields H2 Load (%) Press. (%)

0

1

60^

30 91

1 1

60 psig

6or

81

HBr

60 psig

60°C

EtOH[a]

HCl

60 psig

1 1

18

EtOHW

MsOH

60 psig

88

1

12

EtOH/H20

6or 6or 6or

0 0 91[c]

1

1 5% Pt/C

43

EtOH[a]

HOAc

5 psig

23 °C

1 5% Pt/C

50

3A EtOH[b]

HOAc

60 psig

23r

1 4% Pd/C

20

HOAc

HOAc

60 psig

1 4% Pd/C

18

MeOH

L-Tartaric Acid

4% Pd/C

18

EtOH[a]

1 4% Pd/C

18

1 4% Pd/C 1 4% Pd/C a. b.

60 psig

Anhydrous ethanol 95% ethanol denatured with methanol

Hydrogenation of N-acetoxy derivative.

Scheme II ^

NRAC20^

H2N.

HO

^N

THE, 60 "C

rT^v

rrV^NR

%>' H

HaNÂ^

O H

AcO"

4% Pd/C

H2(60psig)

^

H2O, EtOH^ H g N V ^ 60 T

KT"^,,

fY NH

r^^ ^ C

N-Acetoxy derivative

Early studies probing a possible mechanism centered around the synthesis and reduction of the N-acetoxy intermediate. The N-acetoxy derivative was synthesized, as shown in Scheme II, by heating hydroxybenzamidine with AC2O in THF at 60 °C. The reaction mixture was a slurry from start to finish producing N-acetoxy in -88% yield. Hydrogenation of the N-acetoxy intermediate in EtOH with 4% Pd/C produced benzamidine in - 9 1 % yield. The N-acetoxy intermediate could be produced in situ during the hydrogenation by performing the reaction in the presence of 2 equivalents of AC2O in glacial acetic acid. This procedure had previously been

43

reported for the reduction of similar hydroxybenzamidines to benzamidines[2,3] The preferred catalyst was Pd/C (see Table I) with an organic acid (acetic acid), since platinum was more expensive and reduced the hydroxybenzamidine slowly. Colloidal Palladium It became very evident that during the course of the hydrogenation in acetic acid a small amount of colloidal palladium was being generated and that colloidal palladium was contaminating the isolated benzamidine. High RPMs and the use of a sparger to deliver the hydrogen at a high concentration to the surface of the catalyst (mass transfer) minimized colloidal palladium. "Palladium can be rendered insoluble if the metal (reduced metallic palladium or unreduced palladium oxide catalyst) can be maintained as a hydride during catalytic reduction. This can only occur if the rate of hydrogen arrival at the metal surface is greater than the rate of consumption"[4]. Potential solutions for improving mass transfer include the following: * Low catalyst loading

*

High hydrogen pressure

* Low temperature

*

HighRPM's

* Sparging of the hydrogen

*

Low substrate concentration

The only practical solutions were the sparging of the hydrogen and high RPM's. These were tried in the laboratory and incorporated in our pilot plant campaign. Unfortunately, while it was successful in lowering the colloidal content it was not sufficient to reduce the colloidal amount below the required 10 ppm. At about this time we became aware of a new product from Degussa, a macroporous organofunctional polysiloxane chemically bonded thiourea (DELOXAN) THP® that had a high affinity to precious and heavy metals. The manufacturer claimed that the use of the resin, either in suspension or in a fixed-bed, resulted in removal of metals from aqueous or organic solutions. When a small amount of the Deloxan resin was added to the filtered hydrogenation solution containing high amounts of colloidal palladium, stirred and filtered, the palladium content of the filtrate dropped well below the 10 ppm level (Table II). Table II:

Colloidal Palladium Levels Before/after Deloxan Resin Treatment^

Colloidal Palladium Level Before Resin Treatment

1

1 1 1 1 1 a b

(ppm)

^^^ ^^-^ ^^^ ^^-^ 15.8

Colloidal Palladium Level After Resin Treatment (ppm)b

95%).

0

20

40 60 t (min)

80

100

Figure 1: % product versus time using BA (1.75 g) EGA (1.87 g) and 0.22 g of catalyst in DMSO (55 ml) at G X A

0H-MTS1 NH2-MTS2a NH -MTS2b

GI-MTS3a

0.0

1.0 2.0 3.0 4.0 5.0 Grafted moieties x 10"* (mol)

Figure 2: Gomparison of the grafted primary and tertiary amine activities versus amino group content 0HMTS1 GI-MTS3a X NH2-MTS2a, 2b Reused NH^ - MTS A Gl- Pip MTS 4a I - Pip MTS 4b V I - Pip MTS 4b'

79 The catal5îc activities of immobilized primary groups (2a: initial rate ro = 0.5 10"^ m o l . m n ' l ; 2b: ro = 1.1 10"^ mol.mn"!) are demonstrated by comparison with the inactivity of anchored neutral function as chlorine (3a) and the very low activity observed with the parent mesoporous silica 1 (ro = 0.03 10'^ mol.mn"!), also reported in Figure 1. The initial rates of the condensation reaction are plotted versus molar grafted moiety numbers in Figure 2 for both primary and tertiary amino groups. The number of catalytic sites was varied using either different mass of the same solid in the case of 2a or using a constant mass of solids having different grafted amino group contents as 2a and 2b or 4a and 4b respectively. The activity of each of the two various catalyst series is linearly related to both mass of catalyst and amino site content. The interesting fact that the activity of each amino group is independent of both grafted chain density and nature of the surrounding groups is consistent with the absence of internal diffusional Hmitation effect. That emphasized the interesting properties of the mesoporous materials related to their large channels which can easily adsorb large molecules even though their surface are lined by organic chains leading to a decrease in the pore radiius. Another interesting result concerns the easy reuses of the NH2-MTS catalyst 2. Its activity is totally maintained during several batch experiments after reactivation which simply consisted in washing with methylene dichloridedi-ethyl ether mixture, water solution of sodium hydrogenocarbonate (2%) and pure water, successively then evacuation in vacuum at 150°C overnigth. That illustrated the good stabiUty of the grafted MTS solids as catalyst in organic solvent which is also demonstrated by the preservation of the textural characteristics in addition with the composition and the catalytic activity of the Pip-MTS material 4b* resulting from the washing of 4b with hot ethanol. Hence we conclude that the observed activation is effectively a true heterogeneous catalysis. On the other hand, it is noteworthy that the turnover of the two different amino groups determined from the straight Hne slopes is much higher for the primary amine than the tertiary one. That suggests two different activation mechanisms. The catalysis induced by tertiary amine groups would be a classical base activation of the methylene group of EGA followed by the nudeophilic attack of the carbonyl function. In the case of the primary amine, the condensation activity induced by the basic character of the amino group in the methylene activation would be enhanced when the carbonyl function of the other substrate was firstly activated by the linked primary amine via imine formation. This tj^Dical phenomenon, studied long time ago in homogeneous catalysis [14,18] could be represented in scheme 3. We propose a concerted mechanism as more probable than a multistage one because the imine intermediate possesses a greater basic character than the

80

amine function itself. Moreover a single amine site seems to be involved during each cycle due to the absence of an amine group density effect. Scheme 3

o

o NH2

>

o

N

/

C.

,c—o

Zn^'^-KlO > Cu^'^-KlO » KIO is compatible with the activities observed here, and suggests that the mechanism of formation of the carbocation could be similar, therefore occiu-s by oxidation.

87 4'Ac-B15C5 (%) UU -| ^,

80 60-

_.

^Zi

__ ^ — 5

4020 0 i (.

y

, 0.5

,

,

Fe3+-K10 -Cu2+-K10 - - 3 K - Zn2+-K10 "ik' O -

->Zi--

,

1.5 Reaction time (h)

Figure 1. Activity of the investigated catalysts. In order to evaluate the reactivity of Fe^^-KlO it is worth mentioning that KIO itself contains Fe in the octahedral layer (1.97%, Table 3), which has become at least partly accessible upon steam treatments or acid leaching during the manufacturing process. This Fe could be one reason of its activity. The exchanged Fe ^ represents a smaller amount (0.95%) but it is deposited at the surface, certainly more accessible then far more effective since the final conversion rises from 45 to 85%. The usual work-up procedure of the reaction mixture made with Fe^^-KlO under optimum conditions (Table 2) resulted in 4'Ac-B15C5 with 55% yield. This result gives a practical preparative method which is really competitive with the 65-70% yield of the P2O5 or polyphosphoric acid acylation methods and eliminates their drawbacks: difficult handling and mixing because of high viscosity, costly neutralization. 4. EXPERIMENTAL KIO clay is manufactured by high temperature acidic treatment from bavarian montmorillonite and was purchased from Siid Chemie as original sample. Cation exchange was performed by gradually adding 10 g KIO clay to 125 cm^, 1 mol/1 stirred solution of CuClj, FeClg or ZnCl2 at room temperature and stirring the suspension for 24 hours. After exchange, the suspensions were filtered and washed with deionised water. The resulting solids were dried on a thin bed at 100°C and ground. Specific surface areas were calculated fi'om BET nitrogen isotherms determined at -196°C on samples degassed at 250°C for 12h before the experiment. Chemical analyses were obtained by plasma emission spectroscopy. Acidity of the catalysts was measured by intensity of the IR bands of pyridine coordinated to Bronsted and Lewis sites respectively. For a quantitative characterization the area of the absorption bands was related to the area of a structural band of the clay in the same spectral region (values indicated as rel. int.) [10].

88

Table 3 Some characteristic data of the catalysts Catalyst

Specific surface Surface area of Fe content Metal retained Charge exchanged area micropores m^/g m^/g wt % wt% meq/g cat

KIO

229

2.5

1.97

Cu^'^-KlO 236

-

Cu''^=1.24

0.39

Zn^'^-KlO 213

-

Zn^'^=1.19

0.36

Fe^'^-KlO 239

10

Fe^"^ = 0.95

0.54

2.92

General reaction conditions [11]: Acetylation was made by acetyl chloride in 15 cm^ 1,2dichloroethane at 83°C (boiling point) using a batch reactor. The catalysts were normally heat treated at 250°C. After filtration the solvent was evaporated in vacuo, from the residue 3x15 cm^ dichloroethane was distilled to eliminate the traces of acetyl chloride. The dark, thick residual oil was repeatedly extracted with boiling n-heptane. After cooling the crystals were filtered. Yield: 55% isolated product (in the case ofFe' -KIO catalyst). The reactions were monitored by HPLC (CI8 reversed phase column, eluent methanolwater 50:50 v/v, UV-detection at 254 nm). 5. CONCLUSIONS Benzo-15-crown-5 has been acetylated efficiently using KIO clays ion-exchanged by Cu ^, Fe^^ or Zn^^. The best results were obtained with Fe^^-KlO. The catalytic properties are not related to the acidity of the solid and it can therefore be admitted that the C-Cl bond is activated by a redox mechanism, as proposed earlier for the alkylation of aromatic compounds by benzyl chloride. The results give a practical preparative method, competitive with the P2O5 or polyphosphoric acid acylation processes. The method can be generalized and was successfully applied in acetylation of other benzo-crown compounds.

REFERENCES 1. E. Lindner, K. Toth, J. Jeney, M. Horvath, E. Pungor, I. Bitter, B. Agai and L. Toke, Microchim. Acta I., (1990) 157. 2. J. Beger and M. Meerbote, J. prakt. Chem., 327 (1985) 2. 3. L. Toke, I. Bitter, B. Agai, E. Csongor, K. Toth, E. Lindner, M. Horvath, S. Harfouch and E. Pungor, .Justus Liebigs Annalen der Chemie, (1988) 349. 4. L. Toke, L Bitter, B. Agai, Z. Hell, E. Lindner, K. Toth, M. Horvath, S. Harfouch and E. Pungor, Justus Liebigs Annalen der Chemie, (1988) 549.

89 5. K. Toth, E. Lindner, M. Horvath, J. Jeney, I. Bitter, B. Agai, T. Meisel and L. Toke, Anal. Lett., 22 (1989) 1185. 6. K. Szabo, Diploma Thesis, Technical University of Budapest, 1990. 7. F. Wada and T. Matsuda, Bull. Chem. Soc. Jpn., 53 (1980) 421. 8. W.W. Paris, P.E. Stott, C.W. McCausland and J.S. Bradshaw, J. Org. Chem., 43 (1978) 4577. 9. S. Kano, T. Yokomatsu, H. Nemoto and S. Shibuya, Tetrahedron Lett., 26 (1985) 1531. lO.T. Cseri, S. Bekassy, F. Figueras and S. Rizner, J. Mol. Catal. A: Chemical, 98 (1995) 101. 1 l.T. Cseri, S. Bekassy, Z. Bodas, B. Agai and F. Figueras, Tetrahedr. Lett., 37 (1996) 1473.



Acylation of aromatics over a HBEA Zeolite. Effect of solvent and of acylating agent. F. Jayat\ M. J. Sabater Picot^ D. Rohan^ and M. Guisnet\ ^ URA CNRS 350, Catalyse en Chimie Organique, Universite de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France. Fax : 33 - 5 49 45 34 99 ^ Instituto de Tecnologia Quimica UPV-CSIC, Univ. Politecnica de Valencia, Avd. Los Naranjos S/n, 46022 Valencia, Spain. ^ Dept of Chemical and Life Sciences, University of Limerick, Limerick, Ireland.

Summary. A kinetic study of the acylation of phenol with phenyl acetate was carried out in liquid phase at 160°C over HBEA zeolite samples, sulfolane or dodecane being used as solvents. The initial rates of hydroxyacetophenone (HAP) production were similar in both solvents. However the catalyst deactivation was faster in dodecane, most likely because of the faster formation of heavy reaction products such as bisphenol A derivatives. Moreover, sulfolane had a very positive effect on p-HAP formation and a negative one on o-HAP formation. To explain these observations as well as the influence of phenol and phenyl acetate concentrations on the rates of o- and p-HAP formation it is proposed that sulfolane plays two independent roles in phenol acylation : solvation of acylium ions intermediates and competition with phenyl acetate and phenol for adsorption on the acid sites. Donor substituents of phenyl acetate have a positive effect on the rate of anisole acylation, provided however there are no difRzsion limitations in the zeolite pores.

Introduction. Fries rearrangement and acylation of aromatics are of great importance in many areas of the fine chemical and pharmaceutical industry. The manufacture of several major pharmaceuticals (e.g. Ibuprofen, (S)-Naproxen) involves an aromatic acylation step, whereas some synthetic fragrances of the musk type contain an acetyl group [1,2]. Considerable effort has been devoted to the development of solid acid catalysts, since the traditional catalyst, AICI3, causes many environmental problems. Tridirectional large pore zeolites (in particular HBEA and HFAU zeolites) are very promising catalysts for the synthesis in liquid phase of benzenic ketones [3-5]. Furthermore, Rhone-Poulenc has recently developed an industrial process using zeolite catalysts for the selective acylation in para position of arylethers [5]. The acylation of

92 aromatics is also very interesting from a fundamental point of view because of the general complexity of the reaction scheme. This paper is devoted to the acylation in liquid phase of phenol and anisole by aromatic acetates. Reaction mechanisms are presented and the effect of the solvent polarity which is known to play an important role on the rate and on the selectivity of zeolite catalysed reactions [6] and of the nature of the acylating agent are discussed.

Results and Discussion. 1. Influence of the solvent on the transformation of a phenyl acetate - phenol mixture. The transformation of an equimolar mixture of phenol (P) and of phenyl acetate (PA) (2.2M) was carried out in dodecane (2.3M) or in sulfolane (5.5M) solvents over 100 mg and 500 mg of H-BEA-10 and over 100 mg of H-BEA-70. 0-hydroxyacetophenone (o-HAP),phydroxyacetophenone (p-HAP), p-acetoxyacetophenone (p-AXAP) and phenol appear as primary products while bisphenol A and its mono and diacylated products as well as a low amount of hydroxy-benzoate of benzyle and traces of trimethylbenzene appear at high conversion only.

sulfolane

25 n 20 -

a dodecane

V""^""^^

y - ^

1^ 16
98%). On the other hand aromatic acetates were transformed through intramolecular and intermolecular acylation and through hydrolysis. Table 2 : Yield of p-methoxyacetophenone (mole %) obtained during the acylation of anisole with various acetates under the standard operating conditions (see experimental section) Acylating agents Phenyl p-tolyl 2-methoxyhydroquinone 2-methoxy2,4,6-trimethyl hydroquinone phenyl acetate acetate acetate phenyl acetate diacetate diacetate OAc OAc OAc QAc OAc OAc 3Me T OMe

or

p 9 p

Time (h)

0.25 24

CH3

3.0 12.5

4.0 15.2

^^^

3.7 14.0

1.4 1.5

sXy

¥ 1

OAc 2.5 10.3

0.05 0.05

The rate of anisole acylation depended on the acetate (Table 2). Initially it was about 1.5 times greater with p-tolyl acetate and with 2-methoxyphenyl acetate than with phenyl acetate, slightly lower with 2-methoxyhydroquinone diacetate, 2.5 times lower with the hydroquinone diacetate and very low with 2,4,6-trimethylphenyl acetate. The low reactivity of this latter acetate can be related to limitations in the rate of diffusion of this bulky compound in the BE A zeolite pores. Furthermore, a greater reactivity of this acetate was found with HFAU zeolites whose pore size is greater. Curiously, with hydroquinone diacetate (but not with the 2methoxyhydroquinone acetate), there was a quasi immediate deactivation. We are carrying out additional experiments so as to understand how the reactivity of aromatic acetates changes with their nature and the zeolite acidity and porosity.

Experimental. All the reactions were carried out at 160°C in a flask equipped with a cooler and a magnetic stirrer (600 rpm). Two HBEA zeolite samples were used as catalysts : HBEAIO (total and framework Si/Al ratios of 11 and 15.5 respectively, provided by PQ Zeolites : CP 811-DL-25) and HBEA70 resulting from the dealumination of HBEAIO by acid treatment [7] (total and framework Si/Al ratios of 72). The standard operating conditions were as follows : 100 mg catalyst (previously activated overnight in air at 500°C), 20 mmol of reagent (phenol or anisole) at a concentration of 2.2 M, 20 mmol of acylating agent (phenyl acetate, etc.) also

at a concentration of 2.2 M and the corresponding quantity of solvent (sulfolane, dodecane or sulfolane-dodecane mixtures). Small samples of the reaction mixtures (about 0.1 cm^) were taken at various reaction times, diluted with methylene chloride and analyzed by gas chromatography on a 25 m capillary column of CP Sil 8 CB.

Conclusion. Under mild conditions (liquid phase, 160°C) HBEA zeolites can catalyse the acylation of phenol with phenyl acetate. High selectivity to p-hydroxyacetophenone is obtained by using sulfolane as a solvent, which can be explained by a better dissociation of phenyl acetate into acylium ions due to a solvation effect. However a competition between sulfolane and phenyl acetate for adsorption on the active acid sites is also demonstrated. A preliminary investigation of the effect of the acylating agent shows that generally, donor groups in aromatic acetates have a positive effect on the rate of acylation provided they do not block the access of the acetate to the acid sites of the zeolite pores.

Acknowledgement. Financial support by the European Commission within the Human Capital and Mobility program (contract n° ERBCHRXCT940564) is gratefully acknowledged. F. Jayat gratefully acknowledges the 'Region Poitou-Charentes' for a scholarship.

References. 1. H. van Bekkum, A. J. Hoefhagel, M. A. van Koten, E.A. Gunnewegh, A.H.G. Vogt and H.W. Kouwenhoven, Stud. Surf. Sci. Catal. 83 (1994) 379. 2 H.G. Franck, J.W. Stadelhofer, in 'Industrial Aromatic Chemistry', Springer-Verlag Berlin Heidelberg, 1987. 3 A. Vogt, H.W. Kouwenhoven and R. Prins, Appl. Catal. A : General 123 (1995) 37. 4 F. Jayat, M.J. Sabater Picot and M. Guisnet, Catal. Lett.,41 (1996) 181. 5 M. Spagnol, L. Gilbert, R. Jacquot, H. Guillot, P.J. Tirel, A-M. Le Govic, Proc. 4**^ Int. Symp. on Heterogeneous Catalysis and Fine Chemicals, September 8-12, 1996, Basel, Switzerland. 6 L. Gilbert, C. Mercier, Stud. Surf. Sci. Catal., 78 (1993), 51. 7 C. Coutanceau, J.M. Da Silva, M.F. Alvarez, F.R. Ribeiro, M. Guisnet, J. Chim. Phys., to be published.

99

Zeolite-Catalysed Acetylation of Alkenes with Acetic Anhydride Keith Smith, *a Zhao Zhenhua,^ Lionel Delaude,^ and PhiHp K. G. Hodgson^ ^Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK ^BP Chemicals Ltd, Sunbury Research Laboratory, Chertsey Road, Sunbury-onThames, TW16 7LN, UK

Abstract Among various microporous adsorbents such as alumina, silica, clays, molecular sieves, etc., the HY zeolite was found to be best at promoting the acylation of 2,3-dimethyl-2-butene with acetic anhydride. The influence of numerous experimental p a r a m e t e r s on the course of the reaction was investigated. Variations in the silica/alumina ratio of the zeolite, or in the relative proportions of reagents and catalyst, markedly affected the yield of 3,3,4trimethyl-4-penten-2-one, whereas the reaction time and temperature were less influential. The procedure was extended to various other alkenes and it was possible to regenerate and to reuse the solid catalyst without significant loss of activity. 1.

INTRODUCTION

Friedel-Crafts acylation is one of the most important methods for the synthesis of ketones [1]. To achieve satisfactory reaction rates, "catalysts" such as aluminium chloride are usually needed in more than stoichiometric amounts because of complexation to starting materials and/or products. Work-up often involves hydrolysis, which leads to loss of the catalyst and causes problems with corrosion and disposal of potentially toxic wastes. Also, reactions are not always clean and may lead to mixtures of products. Recourse to recoverable and regenerable solid catalysts can overcome many problems of these types [2]. Therefore, the development of new heterogeneous catalytic procedures for the acylation of organic compounds has become a priority for the chemical industry. Significant advances resulting from the use of aluminosilicate solids were made during the last few years [3-6] and the first industrial application of zeolites in large scale Friedel-Crafts acylations was reported very recently [7]. However, most of the efforts devoted so far focused on the acylation of aromatic compounds. To the best of our knowledge, recourse to heterogeneous aluminosilicate catalysts for the acylation of alkenes has not yet been reported. Conventional methods for alkene acylation [8] involve the use of Br0nsted or Lewis acids such as sulfuric acid [9], boron trifluoride [10], zinc chloride [11], or

100

tin(IV) chloride [12]. In this pubhcation, we present the results obtained in the acetylation of various alkenes with acetic anhydride in the presence of zeolites. 2. EXPERIMENTAL 2.1 Materials Commercially available alkenes (Aldrich) were used as supplied. Acetic anhydride (Aldrich, 99%) was refluxed overnight over P2O5 and distilled under dry N2. Unless otherwise specified, zeolite HY refers to a sample supplied by Merck Ltd UK (product code B-157, Si02/Al203 = 29, specific surface area 700 m^/g). Hp, H-mordenite, H-ZSM5, and HY zeolites with other Si/Al ratios were gifts fi:om PQ Zeolites. All solid catalysts were calcined in air at 400 or 550°C for 2-5 h prior to use and cooled to room temperature in a desiccator over silica gel. 2.2 Acylation procedure Afi:*eshlycalcined zeolite catalyst was added to a mixture of 2,3-dimethyl-2butene (1), acetic anhydride, and chlorobenzene (internal standard). The suspension was stirred at room temperature or heated for a few hours (see Tables for details). The solid was filtered off with suction and rinsed with acetone. The filtrate was analysed by GC on a Pye Unicam Series 104 chromatographic system using a glass column packed with SE-30 stationary phase. A sample of pure 3,3,4-trimethyl-4-penten-2-one (2) was prepared and characterised according to literature indications [13] and used for calibration. Yields were determined using the internal standard method. 3. RESULTS AND DISCUSSION To start our investigations, we examined the conversion of 2,3-dimethyl-2butene (1) into 3,3,4-trimethyl-4-penten-2-one (2) as a model reaction (eq. 1). The choice of acetic anhydride as the acetylating agent was made in the light of related studies on the acylation of aryl ethers. Our work in this field had shown that acetic anhydride was the most efiective reagent for the Friedel-Crafts acylation of anisole in the presence of Hp zeolite. A lower degree of conversion was achieved with acetyl chloride, while hardly any reaction occurred with ethyl acetate or acetic acid [6].

H (1)

COCH3

ACoO

V

C

+

AcOH

(1)

(2)

The ability of numerous microporous adsorbents to catalyse the acylation of (1) was scrutinised. Amorphous materials such as alumina, silica-alumina, or zinc oxide afforded only traces of the product (2) or were totally devoid of catalytic activity. KIO montmorillonite clay and two types of aluminophosphate or silicoaluminophosphate molecular sieves were equally inefficient in promoting acylation, whereas acidic forms of zeolites were much better catalysts. As can be seenfi:-omTable 1, proton-exchanged aluminosilicates with the ZSM-5, p, or

101

faujasite Y structures led to significant amounts of the desired ketone within 2 h at 60°C. Only HX zeolite, which lacks strongly acidic sites, and H-mordenite, which has monodimensional pores, gave very low yields of (2). Table 1 Comparison of activity between various proton-exchanged zeolite catalysts Catalyst

Si02/Al203

GCYieldof2(%)

2.4 HX zeolite H-mordenite 35 H-ZSM5 80 Hp zeolite 25 HY zeolite 12 HY zeolite 40 All reactions were carried out using a l/Ac20/zeolite ratio mmol/0.05 g at 60°C for 2 h.

0.1 0.9 9 22 41 49 of 1 mmol/1.2

Since the most encouraging results were obtained with catalysts possessing the faujasite Y structure, various other ion-exchanged forms of this molecular sieve were prepared and their catalytic activity assessed. Replacement of the proton counter-ions with either sodium, magnesium, aluminium, iron(III), copper(II), lanthanum(III), or mixed rare earths reduced the yields of (2) to trace amounts. Conversely, impregnation of HY zeolite with ZnCl2 or FeCls led to highly active catalysts. Preliminary experiments with these composite materials revealed, however, t h a t they did not withstand h e a t t r e a t m e n t , t h u s compromising their chances of recycling and reuse. Therefore, research in this direction was abandoned and unmodified HY zeolite was used as a catalyst for all our subsequent studies. To complement the data in Table 1, we investigated further the influence of the Si02/Al203 ratio of zeolite HY on the outcome of the reaction (Fig. 1). A sharp increase in the yield of (2) was first observed when the Si02/Al203 ratio increased from 5.4 to 10.5. This threshold effect probably indicates that a specific high acidic strength must be reached in order for the catalyst to play its role. An optimum efficiency was attained with the sample having a Si02/Al203 ratio of 29, then the conversion rate slowly decreased as the number of acidic sites per unit cell of the crystalline aluminosilicate dropped. In order to investigate the influence of the amount of catalyst on the acylation rate, the proportion of zeolite HY (Si02/Al203 = 29) was varied between 0.02 and 0.4 g per mmol of (1) and the model reaction, carried out at 25°C, was monitored by GC. The results after 2 h are plotted in Fig. 2. The yield of (2) steadily increased with the proportion of catalyst. In addition, analyses of the reaction mixtures at various time intervals indicated that the acylation was almost complete within one hour or less. Extending the reaction time to 24 h did not result in any significant improvement. The 5delds increased by only a few per-cent or remained unchanged after 2 h at room temperature. As an alternative to altering the amount of catalyst, we examined the influence of the alkene/acetic anhydride molar ratio on the course of the reaction

102

g 60CM

D 0

> 0 0

40-

20-

/

0 - flC—

100 Si02/Al203 Figure 1. Effect of the Si02/Al203 ratio of zeolite HY on the 5deld of 2 (all reactions were carried out using a I/AC2O/HY ratio of 1 mmol/ 10 mmol/O.l g at 25°C for 2 h).

0

0.1

1 0.2

, 0.3

, 0.4

0.5

Amount of HY (g/mmol of 1) Figure 2. Effect of the amount of zeolite HY on the 5nLeld of 2 (all reactions were carried out using a I/AC2O molar ratio of 1/15 at 25°C for 2 h).

(Table 2). In a first set of experiments, a fixed amount of alkene (1) was acetylated using a 5-, 10-, 15-, or 21-fold excess of acetic anhydride. Taking into accoimt the experimental errors, identical 5delds of ketone (2) were obtained in the three latter cases. Thus, an optimum was reached for an AC2O/I molar ratio close to 10/1. Exceeding this limit only added to the cost of the process without any practical advantage. In a second set of reactions, the amount of anhydride was kept constant and the amount of alkene was progressively increased. This modification of the proportions of reagents also led to increased formation of the acylated product. The yield of (2) was maximum for a I/AC2O molar ratio of 8/1 and slightly decreased when larger excesses were used. The use of a ten-fold excess of alkene led to a higher 5deld than when acetic anhydride was used in the same excess under otherwise identical conditions. Excess amounts of the low boiling point alkene could also be separated from the reaction mixture by distillation and recycled more easily than acetic anhydride (b. p. 73°C and 138-140°C respectively). Therefore, recourse to an excess of alkene would be more efficient and more economic than the use of excess acetic anhydride in an industrial process. To continue our systematic study of the acylation of 2,3-dimethyl-2-butene, we examined the influence of the temperature on the reaction course (Table 3). Using various amounts of the HY catalyst, the acylation of (1) was carried out at 25 or 65°C. Surprisingly, increasing the temperature had only a minor effect on the yield of (2), which increased by just a few per-cent. This confirmed previous indications that the acylation proceeds very quickly and that the time allowed for the reaction (2 h) is sufficient to afford completion at room temperature. To confirm this h5T3othesis, we followed the time course of the model reaction at

103 Table 2 Influence of the alkene/anhydride ratio on the yield of ketone I/AC2O (mol/mol)

GCYieldof2(%)a

Conditions

A 38 1/5 A 45b 1/10 46 A 1/15 A 1/21 45 B 18 1/1 B 27 2/1 B 34 5/1 44 B 8/1 42 B 10/1 15/1 B 37 Conditions A: reactions were carried out using a 1/HY zeolite ratio of 1 mmol/0.14 g at 22°C for 4 h. Conditions B: reactions were carried out using a AC2O/HY zeolite ratio of 1 mmol/0.1 g at 25°C for 2 h. ^Yield based on the reagent in deficiency. ''The yield was 32% using conditions B. 23°C (Fig. 3). The results clearly demonstrate that conversion occured rapidly and that almost no more reaction took place after 1 h. Table 3 Influence of the temperature on the yield of ketone Amount of HY Zeohte (g/mmol of 1)

GC Yield of 2 (%) Reaction at 25°C

0.2 0.3 0.4 All reactions were carried out using a

Reaction at 65°C

55 56 62 59 64 61 I/AC2O molar ratio of 1/10 for 2 h.

Next, we tried to introduce a solvent in our system, instead of using only neat liquid reagents. Three experiments were carried out in ethyl acetate, dichloromethane, and chloroform respectively (Table 4). Dilution of the reaction mixtures in these polar organic solvents did not have any beneficial influence on the acylation, as the yields of (2) were consistently lower than that obtained in the absence of any solvent. Ethyl acetate had the most negative effect, probably because this Lewis base competes with acetic anhydride for coordination to the acidic sites of the zeolite catalyst. A series of reactions was also performed using different grades of acetic anhydride, viz., (i) not purified before use, (ii) refluxed over P2O5 and distilled

104

Table 4 Influence of solvents on the yield of ketone Solvent

GCYieldof2(%)

AcOEt 28 CH2CI2 41 CHCI3 50 none 58 All reactions were carried out using 1 (1 mmol), AC2O (15 mmol), HY zeolite (0.2 g) in 1.718 g of solvent at 25°C for 5 h.

0

2 4 6 Reaction Time (h)

24

Figure 3. Time course of the acetylation of 1 (the reaction was carried out using a I/AC2O/ HY ratio of 1 mmol/15 mmol/0.138 g at 23°C).

under N2 prior to use, (iii) contaminated with small known amounts of acetic acid (5 or 10 molar %). The results were unambiguous: while satisfactory yields are obtained if acetic anhydride is purified before use, only traces of the ketone (2) were obtained in the presence of even relatively small amounts of acetic acid. This impurity is inevitably foimd in acetic anhydride left in contact with humidity. Therefore, it is essential to remove the acid accompan5dng the anhydride as completely as possible before starting the acylation of (1) in the presence of zeolite HY as the catalyst. The generation of acetic acid during the reaction also explains why the catalyst is deactivated before conversion is complete. As an alternative to prior removal of acetic acid, we performed this operation in situ by adding phosphorus pentoxide to our reaction mixtures. The results showed that the association of P2O5 and unpurified acetic anhydride led to inferior 5delds compared against purified anhydride alone. Yet, adding the drying agent to already purified anhydride boosted the yield of ketone (2), but made the work-up more cimabersome. Having established the influence of the various experimental parameters on the acylation of 2,3-dimethyl-2-butene, we extended the procedure to a few other alkenes (Table 5). Unsubstituted cyclohexene gave a mixture of 1-acetyl-lcyclohexene and 3-acetyl-l-cyclohexene in almost equimolar amounts. The best overall yield was obtained by reacting a five-fold excess of acetic anhydride and 0.1 g of zeolite HY per mmol of alkene for 1 h at 25°C. Increasing the reaction time or the proportions of acylating agent and catalyst had a detrimental effect on the yields of the a,p- and p,Y-unsaturated ketones. Acid-catalysed side reactions and degradations probably account for these observations. The acylation of the trisubstituted double bond of 1-methylcyclohexene was easier to carry out and afforded a 60% yield of 6-acetyl-1-methylcyclohexene within 3 h. Similarly, ethylidenecyclohexane led to a satisfactory 66% 5deld of 3-(lcyclohexenyl)-2-butanone after 1 h. In the case of 2,4,4-trimethyl-1-pentene, three isomeric ketones were obtained, viz., 4-(2,2-dimethylpropyl)-4-penten-2-one and (E)- or (Z)-4,6,6-trimethyl-4-hepten-2-one in an overall 72% yield. The first isomer, with the terminal double bond, was the major product of the reaction, but

105

the GC conditions adopted for analysis did not allow us to fully separate the different compounds and to obtain quantitative determinations. Table 5 Acetylation of various alkenes Substrate

o o O-' XA

I/AC2O/HY (mmol/mmol/g) 1/5/0.1

Time GC Yield (h) (%) 1

23

Product(s) (See Text for Isomer Distributions) {

^—COCH3 /

1/10/0.2

3

V-COCH3

60 COCH3

1/10/0.25

1

66

0^

1/10/0.25

2

72

>O^C0CH3

^

^

COCH3 .COCH3

COCH3 All reactions were carried out at 25°C. To conclude this study, we examined the possibility of recycling and reusing the HY catalyst. Initially, the spent solid was simply recovered by filtration, washed with acetone, dried at 110°C, and reused. Under these conditions, the recycled aluminosilicate exhibited only poor catalytic activity, and the conversion of the alkene to ketone was limited to a few per-cent. When an additional calcination step was performed before reuse, on the other hand, the recycled material was almost as active as a fresh sample of zeolite HY. For instance, the yield of (2) dropped only firom 45 to 39% when catalyst regenerated by calcination in air at 400°C was used instead of the original fresh molecular sieve. Furthermore, it was possible to reemploy the same catalyst in a third, and even a fourth run, without any further decrease in yield, provided t h a t the solid was regenerated by calcination between each successive reaction.

106

4.

CONCLUSION

The above results clearly demonstrate that proton-exchanged Y zeolite is an efficient heterogeneous catalyst for the acylation of alkenes with acetic anhydride. The ease of separation and of regeneration of the spent solid is particularly attractive in view of possible industrial applications and contributes to the environmental friendliness of the process, together with the absence of any solvent. Nevertheless, a careful optimisation of the experimental parameters is required in order to achieve high 5delds of ketones. In the case of 2,3-dimethyl-2butene, we were able to obtain a 79% yield of 3,3,4-trimethyl-4-penten-2-one (based on acetic anhydride) by adopting the following conditions: 10/1 alkene/anhydride molar ratio, 0.2 g of HY zeolite per mmol Ac20, reaction temperature 25°C, reaction time 4 h. No alkene oligomerisation was observed. ACKNOWLEDGEMENT We wish to thank the British Government for an ORS Award to Z. Z. and PQ Zeolites for gifts of zeolite samples. Financial support from BP Chemicals and from the European Union within the Human Capital and Mobility Programme (Contract CHRX CT 940564) is gratefully acknowledged, as is the use of the EPSRC's Chemical Database Service at Daresbury [14] and the EPSRC Mass Spectrometry Service in Swansea. REFERENCES 1. G. A. Olah, Friedel-Crafbs Chemistry, Wiley-Interscience, New York, 1973. 2. K. Smith (ed). Solid Supports and Catalysts in Organic Synthesis, Ellis Horwood, Chichester, 1992. 3. H. van Bekkum, A. J. Hoefhagel, M. A. van Koten, E. A. Gunnewegh, A. H. G. Vogt and H. W. Kouwenhoven, Stud. Surf Sci. Catal., 83 (1994) 379 and references cited therein. 4. Q. L. Wang, Y. Ma, X. Ji, H. Yan and Q. Qiu, J. Chem. Soc, Chem. Commun., (1995) 2307. 5. F. Jayat, M. J. Sabater Picot and M. Guisnet, submitted. 6. K. Smith, Z. Zhenhua and P. K. G. Hodgson, manuscript in preparation. 7. M. Spagnol, L. Gilbert, R. Jacquot, H. Guillot, P. J. Tirol and A.-M. Le Govic, preprints 4th Int. Symp. on Heterogeneous Catalysis and Fine Chemicals, Basel, 1996, p. 92. 8. For reviews see C. D. Nenitzecsu and A. T. Balaban, in Friedel-Crafts and Related Reactions, Vol. Ill, G. A. Olah (ed), Wiley, New York, 1964, pp 10331052; J. K. Groves, Chem. Soc. Rev., 1 (1972) 73. 9. A. C. Byrns and T. F. Doumani, Ind. Eng. Chem., 35 (1943) 349. 10. K. Hideo, N. Yoshinori and H. Yasno, Jpn Pat. 05 163 189; Chem. Abstr. 119 (1993) 249581s. 11. P. Beak and K. R. Berger, J. Am. Chem. Soc, 102 (1980) 3848. 12. J. E. Dubois, I. Saumtally and C. Lion, Bull. Soc. Chim. Fr., II (1984) 133. 13. E. F. Kiefer and D. A. Carlson, Tetrahedron Lett., (1967) 1617. 14. D. A. Fletcher, R. F. McMeeking and D. Parkin, J. Chem. Inf. Comput. Sci., 36(1996)746.


107

Influence of the acidity and of the pore structure of zeolites on the alkylation of toluene by 1-heptene. P. Magnoux, A. Mourran, S. Bernard and M. Guisnet URA. CNRS 350, Catalyse en Chimie Organique, University de Poitiers, 40 avenue duRecteur Pineau - 86022 Poitiers cedex France. Summary The alkylation of toluene with 1-heptene was used as a model reaction for the synthesis of long-chain linear alkylbenzenes which are precursors of biodegradable surfactants. The effect of the pore structure and of the acidity of large pore zeolites : HFAU (framework Si/Al ratio from 4 to 100), HMOR (Si/Al from 10 to 80), HMAZ (Si/Al=10), HBEA (Si/Al=10) and of an average pore size zeoUte, HMFI (Si/Al=40) on their catalytic properties was determined in Kquid phase at OO^'C with a toluene/heptene molar ratio of 3. With the large pore zeolites the main reactions are alkene double bond shift and toluene alkylation which occur through a consecutive scheme. Some of the alkylation products, mainly triheptyltoluenes, remain trapped in the zeolite pores. With HMFI, alkylation products are only found in the zeolite pores, because of the impossibility of desorbing these bulky products from the narrow pores of this zeolite. The activity of large pore zeolites depends on their acidity but also on the ease of desorption of alkylates from their pores. In particular mesopores created during dealumination which facilitate the product desorption have a positive role on the zeolite activity. Thus, HFAU with a Si/Al ratio of 30 which has a relatively high acidity and which contains mesopores is the more active catalyst. Highly dealimiinated (hence mesoporous) HMOR samples are also active. Moreover, their selectivity to 2-phenylalkanes which are the most biodegradable isomers is much higher than that of the HFAU samples. By comparison of the compositions of the heptene mixture and of the monoheptyltoluene mixture in the liquid phase and in the zeolite pores, this shape selective preference can be attributed to transition state control. INTRODUCTION Linear mono C10-C13 alkylbenzenes (LAB) which are used in the production of biodegradable surfactants are produced industrially by benzene alkylation with linear alkenes, HF or AICI3 being used as catalysts (1,2). Because their degradabilityis very high, the 2-phenylalkanes are preferred to the other isomers (3). A great effort is now being made in order to substitute soUd adds (in particular zeolites) for these polluting, corrosive, industrial catalysts (4-9). That is why it is important to understand how the alkylating activity, stability and selectivity of zeolite catalysts change as a function of their physico-chemical characteristics (pore structure, acidity). The effect of these characteristics is investigated here on a model reaction, the alkylation of toluene with 1-heptene. The interest which this

108

reaction presents is due to the ease in the quantitative analysis of the complex reaction mixture : heptene isomers, all the monoheptyltoluenes, biheptyltoluenes, triheptyltoluenes, etc. This model reaction is carried out over large pore zeolites : HFAU (framework Si/Al ratio of 4,16, 30,100) which wiUbe called HFAU4, 16, 30 and 100, HMOR (Si/Al=10, 20, 40, 80), HMAZIO (Si/Al=10), HBEAIO (Si/Al=10) and over an average pore size zeolite: HMFI40 (Si/Al=40). RESULTS AND DISCUSSION 1. Physico-chemical characteristics of the zeolite samples The main characteristics of the zeolite samples are given in table 1. Their unit cell formula was drawn from their elemental analysis and from the nimiber of framework aluminium atoms per unit cell (NAI) estimated from the relationship between the wavenumber of IR structure bands and NAI- Among the zeolite samples, only HFAU4 and 100 and HBEAIO contained a large amount of extraframework aluminium species. Table 1 Characteristics of the zeoUte samples : unit cell formula, number of extraframework aluminium atoms ( N E F A L ) , volume (cm^^-l) of micropores (V^) and of mesopores (VM) and acidity : nimiber (lOÔg-l) of protonic acid sites estimated from the imit cell formula (NH"^) and of sites retaining ammonia adsorbed above lOO'^C (NiooX Zeolite HFAU4 HFAU16 HFAU30 HFAUIOO HMORIO HMOR40 HMOR80 HMAZIO HBEAIO HMFI40

Unit cell formula Na0.4Al39.2Sil52.8O384 Nao.3Alll.3Sil80.70384 Nao.l5Al6.2Sil85.80384 Nao.5All.9Sil90.l0384 Nao.05Al4.4Si43.6O96 Nao.05All.lSi46.9O96 Nao.OlAlo.6Si47.4O96 Nao.03Al3.3Si32.7O72 Nao.2Al3.9Si60.lOl28 Nao.oiAl2.lSi93.90i92

NEPAL

v»

VM

9.6 2.4 0.1 10.0 2.1 0 0 0 1.5 0

0.298 0.295 0.282 0.249 0.195 0.210 0.210 0.170 0.240 0.175

0.056 0.140 0.193 0.161 0.030 0.070 0.085 0.105 0.540 0.000

Nioo 20.3 10.2 3.9 5.7 3.2 1.8 0.85 2.7 9.3 6.3 2.2 2.0 1.2 0.8 9.2 6.0 8.4 7.1 2.4 2.5 NH^

Nitrogen adsorption shows that all the zeolites except HFAU4, HMORIO, HMAZIO and HMFI40 have in addition to micropores a significant mesopore volume. The very large mesopore volume of HBEAIO is due to intercrystalline voids resulting from the agglomeration of the very small crystallites of this zeolite (10). The number of protonic acid sites estimated from the unit cell formulas was compared to the nimiber of sites on which ammonia remained adsorbed above lOO^^C and SOO^'C. In the FAU and MOR series, the number of ammonia molecules

109 which remained adsorbed decreased with the theoretical number of protonic acid sites. However, with HFAU 100, the number of ammonia molecules detected was greater than expected from the unit cell formula, which coxild be attributed to the presence of a large amount of extraframework Al species. Furthermore the lower N A I the greater Sie proportion of strong acid sites (retaining ammonia adsorbed above 300°C). 2. Alkylationof toluene with 1-heptene With all the zeolites 1-heptene isomerizes into 2 and 3-heptenes, monoheptyltoluenes are formed by alkylation of toluene with heptenes, biheptyltoluenes by alkylation of monoheptyltoluenes. Non desorbed products are also found. Neither skeletal isomerization and dimerization of heptenes nor formation of C i 4 alkyl toluene are observed. 2-Heptyltoluene (Mi), 3-heptyltoluene (M2) and 4-heptyltoluene (M3) can be separated by GC : three peaks of M i and M2 and two peaks of M3 are observed, which correspond to ortho, meta and para isomers. Most likely the two major peaks correspond to the ortho and para isomers which can be expected with electrophiUc aromatic substitution. Double bond shift and toluene alkylation involve n-heptyl carbenium ions as intermediates. By considering the toluene alkylation mechanism it can be concluded that Ml results from toluene alkylation with 1- or 2-heptenes, M2 from alkylation of toluene with 2- or 3-heptenes and M3 only from alkylation with 3-heptenes (Figure 1).

c-c-c~c-c-c-c Ml

+ I-C7

ill \

^

c-c-c-c-c-c-c

O ^ -'

M2

^

c-c-c-c-c-c-c

+ 3-C7

M3

Figure 1 : Formation of monoheptyltoluenes Only a small amount (5 - data obtained at n = 5 and 10 are used.) Table 4 Alkylation of ethylamine with n-butanol on CuZn/Al catalyst (P=1.3 MPa, WHSV=0.7 h"!) NO

T

na

OC

1 2 3 4 5 6 7

175 178 190 195 205 191 193 201 208 190 201

1.3 1.3 1.3 1.3 1.3 5 5 5 5 10 10

Xb % 67.1 70.3 88.4 93.9 96.9 91.1 93.5 95.6 97.1 93.1 97.5

EtNHnBu 81.8 79.2 67.3 62.6 55.9 83.3 80.4 77.5 75.1 82.4 74.1

Selectivities, % nBu2NH nBuNH2 4.9 9.9 11.4 5.2 8.2 19.0 9.1 22.2 26.3 10.2 4.1 12.1 13.4 5.2 14.9 6.4 7.5 15.9 13.3 3.9 6.1 19.2

8 9 10 11 a) n=EtNH2/n-butanol molar ratio, b) X = conversion of n-butanol, c) yield of EtNHn-Bu

Yieldc % 54.9 55.7 59.5 58.8 54.2 75.9 75.2 74.1 72.9 76.7 72.2

137

Fig.4 gives further data on the effect of reaction temperature at diffferent EtNH2/n-BuOH molar ratios. As seen in Fig. 4 the selectivity of EtNHn-Bu is significantly higher when pure ethylaraine was used instead of 70 wt% EtNH2-H20 mixture. This can be explained by the fact that water is one of the reaction products in the alkylation of ammonia or an amine with an alcohol, therefore the addition of water to the ractants thermodynamically is unfavourable [1]. Neither the concentration of ethylamine (70 or 100 %) nor the amine/alcohol molar ratio had changed the reaction temperature (190 ^C) at which the highest yield of mixed amine was obtained (see Fig. 4).

90i EtNHz, n>5

210

220

Fig.4 Correlation between the yield of EtNHn-Bu and reaction temperature in the alkylation of ethylamine with n-butanol over CuO-ZnO-Al203 catalyst. (Data given in Table 3 and Table 4 are used; n>5 - data obtained at n = 5 and 10 are used.) Conclusions Raney nickel modified with Mg or V can be used for the highly selective preparation of symmetrical amines by the alkylation of ammonia with n-propanol or i-butanol. Upon modifying the Raney nickel catalyst with 0.5 wt % V or Mg, 4-5 % increase in the selectivity to secondary amines was observed and the selectivities reached 70-80 % at 90-95 % conversions. In the alkylation of ammonia with an alcohol symmetrical secondary amines can be obtained with 70 % yield over Mg or V modified Raney nickel catalyst at 220-240 ^C and ammonia/alcohol ratio of 1.5. In an industrial application 2-4 % increase of the selectivity results in an important finantial benefit. It was shown that pure (100 %) ethylamine and 70 % EtNH2 in water can be used for the preparation of N-ethyl-N-butylamine over a commercial CuO-ZnO-Al203 catalyst.

138 In the preparation of EtNHn-Bu from ethylamine and n-butanol over a commercial CuO-ZnOAI20 3 catalyts the highest yield, about 76 % was obtained at 190 °C and EtNH2/n-BuOH molar ratio 5 or above. Here we report the first time such a high yield in the preparation of a mixed aliphatic secondary amine over a copper-containing catalyst. References

1.

A. Baiker and J. Kijenski, Catal. Rev. Sci. Eng., 27 (1985) 653

2.

U.S. Patent 2,636,902 (1953), CA: 48, 3991 (1954)

3.

B. Cornilis, E. Wiebus, P. Ruprercht, W. Konkal (Ruhrchemie AG) Ger. Often. 2,624,63

(1977) 4.

L. T6th et al. (Nitroil, Hungary) Hung. Teljes HU 39,711 (1986)

5.

J. Margitfalvi, S. Grbrlrs, M. Hegediis and E. T~las, Stud. Surf. Sci. Catal., Vol. 41, Elsevier

Amsterdam, 1988 pp. 145-152 6.

M.A. Popov, Zh. Obsh. Khim., 18 (1948) 438

7.

N.S. Kozlov and N. I. Panova, Zh. Obsh. Khim., 26 (1956) 2602

8.

F.V. Belchev, N. I. Sujkin and S. S. Novikov, Izv. AN SSSR, OHN (1961) 649

9.

F.V. Belchev, Tr. Belorus Seljskohoz., Akad., 42 (1968) 232

10. A. Buzas, C. Fogarasan, N. Ticusan, S. Serban, I. Krezsek, L. Ilies, Ger. Often. 2,937,32 (1981) 11. L. W. Hoffman, R. M. Guertin (Pennsalt Chemical Co.) Fr. Demande 1,490,929 (1967) 12. D.Z. Zavelski, L. A. Lavrenteva, U.S.S.R. 170,517 (1965) 13. K. Klier, R. G. Herman, G. A. Vedage, EP P 127,874 A2 (1984), U. S. Patent 4,480,13 (1984) 14. S. Grbrlrs, E. T~las, M.Hegediis, J. L. Margitfalvi and J. Ryczkowski, Stud. Sure Sci. Catal., Vol. 59, Elsevier, Amsterdam, 1991, pp. 335-342 15. J. Antal et al. (Nitrogen Works P&, Hungary), Hung. Patent 206,667 (1987) 16. J. Margitfalvi, S. Grbrlrs, E. T~ilas and M. Hegediis, Stud. Sure Sci. Catal., Vol. 63, Elsevier Amsterdam, pp. 669-678 17. S. Grbrlrs, M. Hegediis, I. Kolosova and J. L. Margitfalvi, submitted to Appl. Catal. 18. J.W. Evans, M. S. Wainwright, A. J. Bridgewater and D. J. Young, Appl. Catal., 7 (1983) 75


139

SYNTHESIS OF DIMETHYLETHYLAMINE FROM ETHYLAMINE AND METHANOL OVER COPPER CATALYSTS. Y. POUILLOUX^ V. DOroY% S. HUB^ J. K E R V E N N A L ' ' and J. BARRAULT* ""Laboratoire de Catalyse, URA CNRS 350, ESIP, 40 avenue du Recteur Pineau , 86022 POITIERS CEDEX, FRANCE ^CRRA ELF - ATOCHEM, 69140 PIERRE BENITE, FRANCE

ABSTRACT : Over a copper chromite type catalyst, a DMEA (dimethylethylamine) yield of 70 % was obtained from monoethylamine (MEA) and methanol with diethylmethylamine (DEMA) as the main by-product. The formation of DMEA was increased to 85 % by just changing the basicity of the catalyst resulting from a change of the rate of the determining steps. The rate of the MEA condensation compared to that of the MEA methylation decreased. Moreover the mechanism of the second methylation step which could involve an intermediate amide ((MEFA) or aminoalkoxide) was different from that of the first methylation step. Keywords : Dimethylethylamine synthesis, monoethylamine reaction with methanol, copper or copper chromite catalysts, alkaline or alkaline-earth modifiers. 1. INTRODUCTION Light amines are important intermediates in chemistry and in the pharmaceutical industry. The substituted light amines are prepared from alcohol, ammonia and/or monosubstituted amine in the presence of a solid catalyst (1-4). The heterogeneous catalysis process requires the formulation of a multifunctional catalyst which at a first approximation presents (i) acidic properties (amine adsorption, dehydration,...) and (ii) a hydro-dehydrogenating function (methanol dehydrogenation, hydrogenation of imine and enamine intermediates). Previous works have shown that copper catalysts are selective in the dehydrogenation of esters (5-7), in the hydrolysis of nitrile (8), in the selective hydrogenation of nitrile or in alcohol amination (10). The catalyst systems such as copper chromite are often used for the preparation of substituted amines. These solids, however, are very sensitive to the presence of water and ammonia (formation of copper nitrides (12)). Moreover, the catalysts promoted by alkaline or alkaline-earth species are more stable than the unpromoted CuCr. For example, barium impregnated on copper chromite increases the stability of the active CuCr02 phase (13). Furthermore, the presence of barium or calcium on copper chromite catalysts influences strongly the selectivity to the methylation of amines : N-alkylation/N-methylation.

140 In our laboratory, we have shown that copper chromite doped with barium, calcium or manganese can lead selectively to dimethyldodecylamine from lauronitrile, ammonia, hydrogen and methanol but not to methyldidodecylamine (14). Among the light amines, the dimethylethylamine (DMEA) is quite an important product i.e. as a catalyst in polymerisation processes. DMEA can be prepared from the reaction of ethanol with dimethylamine or from the reaction of methanol with monoethylamine; H2 CH3CH2NH2 + 2CH3OH

CH3CH2N(CH3)2 + 2H2O

We report, in this paper on the properties of promoted copper for the main and the side-reactions and we propose a reactional scheme of monoethylamine transformation. 2. EXPERIMENTAL 2.1. Catalytic test The reaction was studied in a dynamic fixed bed reactor under hydrogen pressure (1.0 MPa) at 210°C. The molar ratio MeOH/MEA was 8.2, the ratio (MeOH + MEA)/H2 « 0.85 (where MEA : monoethylamine or ethylamine) and the catalyst weight « 5g (particle size 1.2-1.6 mm). The reaction products were analysed by a gas chromatograph equipped with a SGE BPl column (L : 25m ; ID : 0.3 mm ; thickness of film : 5 jiim). Each catalyst was characterised by its activity and selectivity under standard conditions. The activity was obtained from the reagent conversion, the selectivity being expressed as follows : - The first calculation refers only to monoethylamine (MEA) and to products resulting from the conversion of MEA: Si(%) =

— X 100 ZMEA-^Pi - The second refers to all the products formed during the reaction (example : TMA) ^Zr%)zz:—XIOO

2.2 Catalysts The catalyst used in this study was a copper chromite doped with barium (YPl). The other solids were prepared from that catalyst by impregnation with alkaline salts from Prolabo (LiNO^,, KOH, CSNO3). After impregnation, the catalysts were dried in a sandbath (120°C), and then calcinated at 350°C for 4 hours under a dry air stream. 3. RESULTS AND DISCUSSION 3.1. Monoethylamine methylation in the presence of modified copper chromites 3.1.1. Activity and selectivity of the reference catalyst (YPl) In the first part of our work, we examined the properties of a copper chromite catalyst for the selective synthesis of dimethylethylamine (DMEA) from monoethylamine (MEA) and methanol (MeOH). Under our experimental conditions at 230°C, this catalyst

141 was quite selective (70%) at total conversion of the reactant. However, at this temperature, there was a significant formation of trimethylamine (TMA). Table 1 N-Methylation of monoethylamine (M£A) in the presence of a YPl catalyst. Effect of the temperature Time

T(°C)

(h)

Conversion

TMA

Selectivity (except TMA) (%) ^

(%) MEA

EMA

DMEA

DEA

DEMA

(%)

32

210

89.0

13.0

61.0

2.4

23.8

6.3

37

230

100

0.1

68.0

-

31.7

23.6

44

190

36.0

64.3

11.8

20.1

3.7

-

Pj^ : 1.0 MPa, Catalyst weight: 5g, Contact time : 1.3 s. (a) Selectivity to product i with reference to transformed ethylamine: Si(%) = (nMEA)conv

-xlOO

Moreover, the study of the influence of residence time showed that methylethylamine was the primary product of the reaction, whereas dimethylethylamine (methylation of EMA) was a secondary compound. The other products, issued from condensation and methylation reactions, were DMEA ((C2H5)2NCH3), DEFA (H-CON(C2H5)2), TEA ((C2H5)3N) Moreover, we observed the formation of monomethylamine MMA (CH3NH2), dimethylamine DMA ((CH3)2NH) and trimethylamine TMA ((CH3)3N).. The overall reaction scheme of the transformation of MEA is ; MeOH

+ MeOH EMA

DMEA

H2O

H2O

MEA+ MEA >3

+ MeOH ^ -H2O

DEA

DEA + [CH3OH 3 MeOH + NH3

^

-H2 -H2O

DEMA

HCHO]

H2O -^

DEFA

>- TMA

In order to increase the DMEA selectivity, the YPl catalyst was modified. As the formation of DEA and DEMA was favoured by the presence of acid sites on the surface of the catalyst, we showed that the addition of alkaline or alkaline-earth elements decreased

142 the condensation reaction rate of MEA. Moreover the presence of an alkaline-earth agent like barium stabilised the active phase (CuCr02) and led to a more stable catalyst (12). 3.1.2. Effect of the addition of alkaline elements (15) The YPl catalyst was impregnated with lithium, potassium or cesium. Table 2 shows that the addition of an alkaline (with the exception of cesium) improves the selectivity to DMEA (up to 90%). The addition of a small amount of KOH (0.5 to 2%) decreases the quantity of DEMA formed without changing the activity. The impregnation of lithium leads to a similar effect. However, in the presence of Li catalyst, the TMA selectivity is much more significant. We think that lithium which has a smaller particle size than potassium, can be easily inserted in the copper chromite phase. It can thus modify the hydro-dehydrogenating properties of copper and change the rate determining steps of side-reactions (TMA formation). The TMA selectivity is reduced when the solid is impregnated with cesium. However, DEFA is formed and it seems that DEMA could be obtained from DEFA;

2MEA^;F=^

DEA

+ CH3OH -H2O ^ DEFA = ^ = ^ DEMA

Table 2 N-Methylation of ethylamine. Comparison of the catalytic properties of YPl catalysts modified with Li, K or Cs. Catalyst

time

Conv.

(h)

EMA

DMEA

DEA

DEMA

DEFA

(%)

0

90.1

0

9.9

0

32.5

TMA

Selectivity (:%) (except TMA)

Li* 0.2%

24

MEA (%) 100

K 3.5%

16.5

99

0

94.2

0

5.8

-

7.5

69

59

22.0

1.3

1.0

17.0

0

100

0.1

68.1

0

31.7

-

23.6

Cs YPl

37

* impregnated with nitrate salt. T : 230°C, PH2 time : 1.3 s.

10 MPa, Catalyst weight : 5g, Contact

It can be observed that the rate of the secondary methylation is much slower than the one observed over the unpromoted solid and it decreases when the size of the alkaline ion increases. Moreover, it seems that the mechanism of the second N-methylation step is different from the one involved in the first N-methylation step. The mechanism of the second N-methylation step for the formation of DMEA or DEMA requires an adsorption step of intermediates MEFA or DEFA via alkoxide species because there is no hydrogen linked to the carbon of the CO bond. The following concerted elimination-hydrogenation reaction can lead to DMEA ;

143 + H2 CH3CH2NHCH3 + (CHsOH =^ EMA

,CH2CH3 H2CN. I ^CH3 OH

H-C—H) =5= II O

-H2 ^CH2CH3 HCN^ II CH3 O MEFA DMEA and a similar mechanism can explain the formation of DEMA via the intermediate DEFA. CU3 CH3CH2N^ ^CH3

H2O

3.2. Reactivity of monoethylamine or other intermediates In order to corroborate the main steps of the synthesis of dimethylethylamine and of the main by-products, we studied the reactivity of some intermediates and products with or without reagents (MEA, MeOH) under the same experimental conditions ; i) In the absence of methanol (replaced by n-heptane), monoethylamine is transformed mainly into diethylamine (DEA), the deactivation of the catalyst being very fast due to an increase of the formation of ammonia (15). Baiker and Kijenski showed, for instance, that part of the copper was transformed into copper nitride during the amination of alcohols (12). In the presence of methanol, the monoethylamine surface coverage is lower and a decrease cf the DEA formation can be observed. Methanol acts as an inhibitor in the synthesis of DEA and as a promotor of the catalyst duration. Table 3 Reactivity of various intermediates with methanol over a YPl catalyst. Reagent

Selectivity (%)

Time

Conv.

(h)

MEA (%)

DMEA

EMA

8

100

DEA'

8

100

DMEA

10 45

TMA + EtOH

DMA

DEMA

(%)

(%)

95.1

4.9

5.0

4.9

4.7

95.3

2.5

2.2

6.8

0

100

5.4

33.1^

66.9

T : 210°C, PH2 10 MPa, Catalyst weight: 5g, Contact time : 1.3 s. (a) catalyst weight: Ig ; (b) ethanol

ii) The methylation rate of diethylamine with methanol is more significant (selectivity to DEMA : 95%) than that of methylethylamine EMA (Table 3). Indeed, we obtained a total DEA conversion with five times less catalyst weight than for the reaction with EMA. This

144

result was expected on account of the change of amine reactivity with the N-substitution. On the other hand, DMEA does not react with methanol or with DEMA and there is no formation of TMA or of TEA. Moreover, the study of DMEA reactivity shows that this compound is much less reactive and can be converted only into DMA and ethanol. The presence of ethanol is more difficult to explain. However, the water issued from the dehydration of methanol into dimethylether can react with DMEA especially in the presence of the fresh catalyst; H2O + DMEA 2 CH3OH

^

DMA + EtOH (CH3)20 + H2O

Figure 1 shows that in the absence of methanol, initially, monoethylamine (EMA) and methylethylamine (MEA) are transformed rapidly. Surprisingly, we observe that: I) the imine, CH3-CH2-N=CH-CH3 is the main product. This compound is the intermediate in the formation of DEA (reactions 1 and 2); CH3CH2NH2 ^

CHsCH^NH + H2

CH3CH = NH + CH3CH2NH2

-NH3 ^

(1) + H2

CH3CH2N - CHCH3 ^ imine DEA

(CH3CH2)2NH (2) DEA

and 2) the enamine, CH3-CH2-(CH3)-N-CH=CH2; compound formed from the reaction of MEA with ethylenimine (reaction 3) which is further hydrogenated into DMEA ; CH3^

CH3^ CH3 CH3^ NH + CH3CH = NH««—^ N - C H ; î—^ ^ N H - C H = CH2 (3) CH3CH2'^ CH3CH2'^ NH2 CH3CH2'^ EMA imine MEA enamine DMEA

The hydrogenation steps are the rate limiting steps over the fresh catalyst. After an experiment lasting two hours, we observed a dramatic decrease of the activity, specially of the MEA conversion and the disappearance of the intermediates. Furthermore, the main products, DEMA and DEA, were formed. These results show that the adsorption properties of the catalysts vary very much during the reaction since ethylamine was mainly adsorbed and led to DEA. We suppose that these significant modifications could be due to the polymerisation of reaction intermediates such as imine or enamine. The polymers could remain on the catalyst surface and modify the nature and the number of active sites. In previous works, we remarked that these secondary reactions could modify the catalyst surface (16,17).

145

100

> o o c o

2 > C o

o

Figure 1: Reaction between monoethylamine and methyiethylamine over the 2.5Li*YPl catalyst. Methyiethylamine EMA Conversion (M)^ Ethylamine MEA conversion (x). Selectivity to Diethylamine DEA (O), imine ofDEA (0), Diethylmethylamine DEMA (^), enamine ofDEh/lA (H"

0.03 -

y^^j^

^ 0.02-

0.01.^^^""'"^ '

0.00 H'

0.02

0.00

^

WW '

—1

_Ii-^

*

— 1—

'

0.04

1

0.06

0.08

Conversion, X, [-] Figure 1. Yields Yi of monomethylated products A, as a function of catechol conversion Xi. Similar behaviour was observed with other catalysts in the reaction:

pH

CH30H [Qj ^ (Ai) I -OCH3

CH30H \ ^

i ™^^"

AÔH (A4)

(A3)

Q T

+H2O

OH

C O T + H2O CH3

'CH3

Figure 2. Reaction scheme of catechol methylation at 260-300°C ( Xi < 0.05 ).

152 The experiments were carried out always under the conditions to form the products in parallel pathways. That allowed to evaluate the influence of catalyst acid/base properties on the reaction selectivity towards O- or C-methylation. 3.2. Comparison of catalysts activity and selectivity Catalytic activity and selectivity of pure and ion-modified aluminas are presented in Table 1. The activity and selectivity of aluminas used depend on the added ion and its concentration. It is seen from the data of Table 1 that activity increases with the decrease in surface basicity. This is in agreement with previous work regarding the reaction of phenol alkylation with methanol over oxides [5-10,16]. Table 1. Characteristics of the different catalysts. Ion content

Activity, -Ri

(at.%)

(mol-m-^-h-^)

S2

S3

S4

(|Limolb.a.-m'^)

Y-AI2O3

-

2.0910-^

0.71

0.26

0.03

2.59

P04^/Al203

5

3.2610-^

0.88

0.07

0.05

2.06

P04^/Al203

10

3.4010"^

0.89

0.06

0.05

BOs^/AhOs

5

8.7 MO-^

0.75

0.22

0.03

2.15

Li'-ZAhOa

5

2.93-10"^

0.60

0.39

0.01

3.00

Mg^^/AhOs

2.5

9.3510"^

0.55

0.41

0.04

2.65

Mg^^/AhOs

5

7.56-10-^

0.34

0.62

0.04

Mg^VAhOs

6

6.00-10-^

0.31

0.64

0.05

Mg^^/AhOs

7.5

5.1010"^

0.30

0.65

0.05

Mg^^/AhOs

10

4.64-10-^

0.45

0.51

0.04

Catalyst

Selectivity

Basicity

3.07

The modification of y-AI2O3 by boric acid increased the initial activity, but the catalyst was rapidly deactivated due to coke formation. Under steady-state conditions a much lower activity, compared to the case when pure Y-AI2O3 was used as a catalyst, was observed ( see Table 1 ). In comparison with boric acid, phosphoric acid supported on Y-AI2O3 did not exhibit important deactivation and showed 50% higher steady-state activity than the Y-AI2O3. Modification of Y-AI2O3 by Mg^"^ and Li"^ leads to the diffusion of these cations into the YAI2O3 lattice and results in non-stochiometric spinel formation: MgAl204 and LiAlsOg, as was confirmed by X-ray data [4]. The amount of incorporated cation seems to determine the surface basicity: the benzoic acid (b.a.) adsorption per surface unit of the catalyst increased with the Mg^"^ amount added ( Table 1). This way the surface basicity can be regulated without changing the structure of the catalyst surface or bulk, since Y-AI2O3 is known to have the same spinel structure with unit cell of cubic type and cell parameter a = 7.907 A.

153 To assess the surface acid/base site modification due to Mg^"^ and Li"^ incorporation into the lattice, FTIR spectroscopy of adsorbed methanol as a probe molecule was used. Methanol is known to interact with the acid-base pair (M""^, O^") of the oxide surface [19] and to form methoxy and hydroxyl groups by dissociation of its 0-H bond. The Vco (C-0 stretching) and VcH3 (C-H stretching of the methyl group) bands of the methoxy species are measured since they are known to be sensitive to Lewis acid site nature [20]. Fig. 3 shows spectra of methanol adsorbed on Y-AI2O3 and Mg^"^-modified Y-AI2O3. From the spectral observations it is seen that methanol forms methoxy species in which oxygen is co-ordinated to two cations as shown by the Vco band at 1090 cm'^ [21]. No new Vco band appears when Mg^"^ is incorporated into the lattice, but the maximum of the Vco band is shifted to higher values concomitantly the VCH3 bands are shifted to lower values with increasing Mg.2+.concentration

X)

10. Intermediate e.e.max are observed with EtOH, acetone and diisopropyl ether (DIPE). Both s and Kni/Km2 are significantly lower. It is interesting to note that similar e.e.-profiles are observed with EtOH and DIPE, even though the absolute rates are very different. A low e.e.max, is observed with CH3CN. This solvent shows a very low rate and a very low km/ku. This could be due to strong competitive adsorption of the solvent on modifiable sites. In AcOH, e.e.max and a first rate plateau is reached at very low modifier concentrations. In contrast to all other catalytic system, increasing the modifier concentration leads to a fiirther

179 increase in rate with no change in e.e.. This can either be interpreted that the doubly modified sites are also active or that further sites are modified (see below).

100

X 2.0E-04

D

ee obs AcOH

O

+ 1.5E-04

ee obs toluene —eecalc toluene

A

+ 1.0E-04 ^

ee obs DIPE ee calc DIPE r obs AcOH

g_ + 5.0E-05 j?

- r calc AcOH r obs toluene

robs DIPE

O.OE+00 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02

-r calc DIPE

HCd concentration (mol/l)

Figure 4. Measured and calculated ee. and rate curves for different solvents.

4.3. Additives Table 3 and Figure 5 show the results obtained in presence of quinoline, EtsN and thiourea. As seen in Figure 5, small amounts of additives have a strong influence on rate and ee.. Thiourea and the amines show a different behavior. With the amines, e.e.max is almost unchanged and the maximum rate is somewhat higher. However, more HCd is necessary to reach to maximum values. Tentative Explanation: Competitive, reversible adsorption of EtsN and quinoline and HCd on the modifiable sites. Using k^, K,n and s of the unmodified system, Kadd can be calculated (Scheme 2, results in Table 3). Table 3. Parameters obtained with different modifiers additive

k„

"^m

Km

s

Kni2

K-add

Xmax

e.e.max

-

1.8

53

32'000

0.94

1300

-

0.50

83%

quinoline

2.2

53

32'000

0.94

1300

3'600

0.50

82%

EtsN

2.2

53

32'000

0.94

1300

5'000

0.50

81%

thiourea

0.9

53

32'000

0.94

1300

-

0.05

63%

Xmax =fi-actionof modifiable sites. 50 mg JMC 94, [HCd] = 0-0.02 M, 20 ml toluene, 10 ml etpy, [additive]/[Ptsurf] = 1.7 (quinoline, EtsN) and 0.18 (thiourea), 20 ml toluene, 10 ml etpy, 100 bar H2. modifier

Pt„

->

base Pt,u

4

^

^*add

*^dd

Scheme 1: Competitive adsorption of bases and HCd on modifiable sites.

180 With thiourea, e.e.nax and ratCmax are significantly lower but they are observed at the same HCd concentration as without additive. This suggests, that thiourea poisons the modifiable sites irreversibly. This can be simulated by varying thefi*actionof modifiable sites while using km. Km and s of the unmodified system. The optimal fit is obtained at x max = 0.05, suggesting that 90% of the modifiable sites are poisoned by thiourea (one molecule of thiourea poisons 12 modifiable site). With these refined models, a good fit is obtained with all additives.

x1.E-04

100 T

o

ee obs toluene ee calc toluene

+ 8.E-05

75 +

+ 6.E-05 g^ o E + 4.E-05 B 2

50 +

D

eeobsEtSN

A

ee ot)S thiourea ee caic thiourea r obs toluene -r calc toluene robsEtSN

25 +

+ 2.E-05

' r caic ttoN r obs thiourea

I

"l^

\

h

O.E+00 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 HCd concentration (mol/l)

Figure 5. Measured and calculated curves with different additives

4.4. Kinetic Treatment The definitions and the kinetic treatment were described in detail by Garland and Blaser [4b]. Reversible adsorption of the modifier (adsorption constant Km) on unmodified surface platinum atoms, Ptu, creates modified Pt atoms, Ptm (see Figure 1) which catalyze the reaction with the rate constant km* and selectivity s^. In principle, two situations are possible: i) all Ru sites are modifiable ii) only part of all Ptu are modifiable. In [4b], both situations are discussed. With both approaches it is possible to fit the data presented in this paper. However, we think that for geometrical reasons, not all Ptm-atoms can be modifiable. Therefore, we arbitrarily assume for all our calculations that only 50% of all Ptatoms can be modified (xmax = 0.5), giving 50% Pt^ and 50% Pt^ in the fully modified system. As a consequence, the racemic reaction on unmodified sites contributes in variable degrees to the over-all reaction but is never negligible.

^ km and K are lump constants and not rate ccmstants. They (xmtzm the dependence of the rate on hydrogen pressure, ethyl pyruvate omcîtration and tenêrature. Since these parameters were kept constant during all experiments, the values of k are conq>arable. ^ s and S2 give the selectivity of the modified sites. s=0.9 means that 90% of all molecules of ethyl lactate produced on the corresponding sites have the R-configuration.

181

One HCd alkaloid occupies and / or blocks a certain number of modifiable platinum atoms. For geometrical reasons it was assumed that an asymmetric site consists of 15 Pt atoms and one adsorbed cinchona molecule. By calculating x^ (the fi*action of modified sites), the observed e.e. and rate can be expressed as a fimction of [HCdJtot (the total HCd concentration in solution) and [Ptsmf] (the concentration surface platinum atoms) [4b]. Adjustable parameters are Km, s, and km, and ku, is obtainedfi*oman experiment without modifier. At high modifier concentration ([HCd] >10"^M), this simple model is no longer valid and a refined model has to be designed [4]. This can be done by assuming that a second HCd molecule is adsorbed at high [HCd]. Again, two situations are possible: a) A second molecule of HCd adsorbs reversibly on a already modified site (Ptm) to form Ptm2 which can hydrogenate with km2, and S2. b) At second type of hitherto unmodified site v^th a much lower affinity of HCd is modified reversibly at high [HCd]. Again, in [4b] both situations are discussed. At least for AcOH, it is possible to fit the data presented in this paper with both approaches. Because a) can be used for all solvents, however, we decided to use only this approach here. Furthermore, to make the model as simple as possible, we made in contrast to [4] the assumption, that doubly modified sites are no longer active (km2 = 0) for all solvents except AcOH. Here, km2 > 0, and S2 = s. Since this second adsorption is only important at high modifier concentrations, the approximation HCdsoi « HCdtot used here is valid and Xm2 can be estimated easily form the definition of Km2. ^2— Km2 Xm [HCdtot]

E.e.obs and robs can again be expressed as a fimction of [HCd]tot with the adjustable parameters Km, Km2, s, km, and in AcOH with km2. With this somewhat simplified model, the data for all catalysts and solvents could be explained.

CONCLUSIONS Our refined models allow a good description of the concentration dependence of rate and e.e. with different Pt-catalysts and additives in various solvents. This is well in line v^th results reported for various modifiers [6a, 6c, 6d] and supports like Pt/Al203 [4], Pt/Si02 [6b], Pt-zeolite [6e] or Pt-coUoids [1]. With etpy as substrate, a qualitatively similar behavior is observed in all cases. Therefore, the concept of reversible adsorption of the modifier on the catalyst seems to be generally applicable. No simple correlation is found between e.e.max and the absolute values of ku, km. Km or Km2 Decisive for good results is a high acceleration factor (ratio of kjku) and a high intrinsic selectivity s. In some cases, also the ratio of Km / Km2 plays a role. A correlation between the model parameters and the properties of the solvents or the catalysts is impossible. Therefore, the models is only of limited value for the characterization of catalysts and solvents. On the other hand, the models allow to differentiate between reversible competition of the modifier and different bases, and the irreversible poisoning of thiourea. The optimal modifier concentrations for modifier screening is not very sensitive. Between 10"^ M and lO'^M, good results are obtained.

182 REFERENCES [1] H.U. Blaser, H.-P. Jalett, M. MuUer, and M. Studer, Catalysis Today, accepted for publication (1996). [2] H.U. Blaser, Stud. Surf. Sci. Catal. 59 (1991) 177. [3] J. Wang, Y. Sun, C. LeBlond, R.N. Landau and D.G. Blackmond, J. Catal. 161 (1996) 752. [4] a) H.U. Blaser, M. Gariand and H.P. Jalett, J. Catal. 144 (1993) 569. b) M. Garland and H.U. Blaser, J. Amer. Chem. Soc, 112 (1990) 7048. [5] H.U. Blaser, H.P. Jalett, D.M. Monti, A. Baiker, J.T. Wehrli, Stud. Surf Sci. Catal. 67 (1991) 147. [6] a) B. B. NCnder, T. Mallat, A. Baiker, G. Wang, T. Heinz and A. Pfaltz, J. Catal. 154 (1995) 371. b) K.E. Simons, P.A. Meheux, S.P. Griffiths, I.M. Sutherland, P. Johnston, P.B. Wells, A.F. Carley, M.K. Rajumon, M.W. Roberts and A. Ibbotson, Reel. Trav. Chim. Pays-Bas 113 (1994) 465. c) A. Tungler, T. Mathe, K. Fodor, R.A. Sheldon and P. Gallezot, accepted for publication in J. Mol. Catal. 1996. e) W. Reschetilowski, U. Bohmer and J. Wiehl, in G. Jannes and V. Dubois, Eds., "Chiral Reactions in Heterogeneous Catalysis'*, Plenum Press, New York, 1995, p. 111.


183

Modeling of Kinetically Coupled Selective Hydrogenation Reactions: Kinetic Rationalization of Pressure Effects on Enantioselectivity Jian Wang\ Carl LeBlond^ Charles F. Orella*^ and Yongkui Sun** John S. Bradley*' and Donna G. Blackmond''* ^Merck Research Laboratories, Merck & Co., Inc., Rahway, NJ 07065 U.S.A. ^'Max-PlancklnstitutfiirKohlenforschung, Mulheima.d. Ruhr D45470 Germany 1.

ABSTRACT

A two-site, two-step kinetic model is proposed to rationalize the observed effects of solution hydrogen concentration on enantioselectivity in the asymmetric hydrogenation of ethyl pyruvate using a dihydrocinchonidine-modified heterogeneous Ft catalyst. The model successfully predicted enantioselectivity at a hydrogen concentration outside the range used in the kinetic fit. This work demonstrates how the perturbation from equilibrium adsorption of the organic substrate on a heterogeneous catalyst may account for the observed effects of pressure on enantioselectivity. Both positive and negative hydrogen dependences on enantioselectivity may be rationalized using the same model. 2.

INTRODUCTION

Many asymmetric hydrogenation reactions have been shown to exhibit a marked dependence on enantioselectivity of kinetic variables such as pressure, although the hydrogenation of a-keto esters over Pt surfaces containing chiral modifiers (1-8) remains the only widely studied system in heterogeneous catalysis. In the homogeneous asymmetric catalytic hydrogenation of enamides, Halpem and coworkers (9) combined kinetic and spectroscopic measurements to develop a kinetic model to rationalize observed effect of hydrogen pressure on enantioselectivity that has become a textbook example (10). Recently, Boudart and Djega-Mariadassou (11) discussed these data as the only quantitative example of kinetic coupling of elementary steps in and between parallel catalytic cycles reported in the homogeneous, heterogeneous, or enzymatic catalysis literature. In the present study, this concept of kinetic coupling is shown to rationalize and predict the effect of pressure on enantioselectivity in the hydrogenation of ethyl pyruvate using dihydrocinchonidine-modified Pt/AljOg. The kinetic model is developed from a simple two-step mechanism in the parallel (R) and (S) branches of a catalytic cycle involving two different catalytic sites. In the pressure range investigated, the model is analogous to that developed in the homogeneous catalytic system discussed above and demonstrates the kinetic consequences of coupling in catalytic cycles for a heterogeneous catalytic system. 3.

EXPERIMENTAL

The organic substrate, ethyl pyruvate (Aldrich, > 99%) and the solvent 1-propanol (Aldrich, 99.5%) were used without further purification at a substrate concentration of 0.5-1 M. Dihydrocinchonidine, prepared by hydrogenation of cinchonidine (Aldrich) as described

184 previously (7), was used in a concentration of 100 mg/1. The catalyst employed in these studies was a 1 wt% Pt/AljOg (Precious Metals Corporation, prereduced, 4-11 g/1). Reactions were carried out at 303 K and at constant pressure ranging from 135-2500 kPa. A fiilly automated reaction calorimeter (Mettler RCl) was used for reactions up to 600 kPa. Higher pressure reactions were carried out in a stainless steel autoclave (PanInstruments). Since it has recently been shown that diffusion processes may play a role in determining enantioselectivity, care was taken in these experiments to insure that the reactions were kinetically controlled, as has been discussed previously (3,8). Enantioselectivity was constant after the initial induction period (GC, Chiraldex B-TA column). Calorimetric measurements provide a simple and rapid means of monitoring reaction rate on-line. The energy balance for an isothermal reacting system shows that the heat flow is proportional to the reaction rate: (1)

Qr=VrZ,MIr.n.i('-lt)

where qr is the heat released or consumed by the reaction, Vj is the volume of the reactor contents, (dCj/dt) is the reaction rate and AHrxn,i the heat of reaction of the ith reaction. When the heat flow of a reaction is calibrated as described previously (8a), qr gives a quantitative measure of the overall reaction rate. In terms of consumption of the reactant ethyl pyruvate ([EP]) this rate becomes: d[EP]_[EP\_^,-[EP\^^

Qrit)

(2)

Our previous studies of this system (8b) found strong agreement between conversions derived from analytical sampling and from fractional heat evolution, confirming that the concentrations of the R and S products may be determined from a mass balance and the definition of instantaneous enantioselectivity (%ee=100*(d[R]-d[S])/(d[R]+d[S]). The rate of R formation is given by Eq. (2):

^=y2h^}-{-^}H-)4«o{[£''i

-m.}^

(3)

The solution hydrogen concentration at any time t, [H2], may be found by integration of the hydrogen mass balance:

^ = .4«,]- -[«,])-(-«)

(4,

The term for the rate of consumption of ethyl pyruvate, d[EP]/dt, is given by the heat flow data. Gas-liquid mass transfer coefficients, k^â, and hydrogen solubilities, [H2]^^^ were measured as described previously (8a). Experimental rate and enantioselectivity data were acquired over a range of concentrations of both reactants, ethyl pyruvate and hydrogen. The experimental data were fit using the Excel Solver program (Microsoft) to Eq. (3) and Eq. (11) (see Results Section), which describes the relationship between enantioselectivity and hydrogen concentration predicted by the kinetic model. Since it has recently been shown (8c) that this catalytic system exhibits an unusual induction period in which increasing reaction rate and enantioselectivity are observed, experimental data obtained at conversions greater than 30% were used in kinetic modeling.

185 4.

RESULTS

4.1

Development of Proposed Mechanism and Kinetic Expressions Scheme 1 presents the mechanism used to develop a kinetic model for the asymmetric hydrogenation of ethyl pyruvate on dihydrocinchonidine-modified Pt/Al203 catalysts. Although a consensus concerning the origin of the effects of the modifier has not been reached, a metal site which is modified by an interaction with the cinchona alkaloid has been implicated in the enhancement of both rate and enantioselectivity. The model which we develop here consists of two separate catalytic sites, those which have been modified (Q^^d) ^Y ^ê presence of dihydrocinchonidine and those which have not (0^). We describe the enantiodifferentiation as occurring through the formation of two separate intermediate species upon the reversible adsorption of the substrate, ethyl pyruvate, on the modified sites. Unmodified sites may undergo reversible adsorption of the organic substrate and hydrogen. Reaction occurs through the irreversible addition of hydrogen, adsorbed on unmodified sites, to these intermediate species adsorbed on modified sites. From the more than ten-fold rate acceleration which we observe in the presence of the modifier, it may be inferred that the contribution to the observed catalytic behavior from the racemic pathway on unmodified sites is negligible, and it is not included in our model. However, the unmodified sites influence the enantioselective reaction by controlling the supply of adsorbed hydrogen atoms to the intermediate species formed on the modified sites.

organic substrate

modified catalyst site

(/?)-intermediate

(S)-intermediate

Scheme 1. Proposed reaction mechanism for the hydrogenation of ethyl pyruvate using a dihydrocinchonidine-modified Pt catalyst. The adsorption of the substrate on the modified site results in the formation of {R) and (5) intermediates which undergo hydrogenation with hydrogen adsorbed on unmodified sites.

0H

unmodified catalyst

unmodified catalyst (/?)-product

(5)-product

The rate expression for the asymmetric hydrogenation reaction may be written as the sum of the rate of reaction for the two intermediate species ©ÊP.mod ^^^ ®ÊP,mod ^^^h adsorbed hydrogen 0^.: r =

rS + Z = (h ®£P,mod + h®EP,mod )QH

(5)

As shown in Scheme 1, the rate constants k^ and k, in each pathway refer to the substrate adsorption and desorption steps on the modified sites, while the two k2's are constants for the irreversible hydrogenation steps for each pathway. Expressions for the intermediate species 0^ on unmodified sites and 0ÊP,mod ^ ^ ®ÊP,mod ^^ modified sites are given below. Adsorptiondesorption equilibrium on unmodified sites is assumed, with K^p and K^ referring to the

186 adsorption equilibrium constants for ethyl pyruvate and hydrogen, respectively, on the unmodified sites. The steady-state approximation is used to determine the concentration of the two intermediates on the modified sites.^This equation sets the change with time in 0\p„iod ^ ^ 0ÊP,inod ^ ^ ^ to zero and equal to production terms (adsorption step, rate constant ki)* minus consumption terms (dissociation and hydrogenation, rate constants k.j and kj).

(6)

{I + A:„'^[H,]^+A:^,[£/']}

9i^fiP.mod (7R)

k!\EP] ^£i»,mod

The rate expression for the R and 5 pathways which is found when Eqs. (6) and (7R,7S) are inserted into Eq. (5). This expression consists of three different parts: . .«v,. . 1 \ concentration driving forces] .«. r . « . . . rate{R) + rate(S) = [(R) kinetic term + (S) kinetic term] * ^ —^ ^ [adsorption term]

/ox (o)

Because adsorption-desorption equiUbrium is not assumed for ethyl pyruvate on the modified sites, the kinetic terms is a function of surface hydrogen concentration: (/?)-pathway kinetic term:

k^k2 k-i-^k^^H

(5)-pathway kinetic term:

(9R,9S)

k^^ AC_i "T K2^

fj

The concentration driving forces are given simply by the reactant concentrations [EP] and Q^. The adsorption term takes into account the presence of both the R and S surface intermediates formed from interaction of the substrate with the modified site and is given by: ^^

kÊP] k\^k^Qjj

^ k,'[EP] k^.x-^klBfj\

(10)

It is important to note that the concentration term and the adsorption terms above are identical in the R and S pathways, and hence any difference between (/?) and (5) rates (the origin of enantioselectivity) must result from differences in the kinetic terms for the two pathways. Positive reaction order in both ethyl pyruvate and hydrogen was always observed under the range of experimental conditions we studied, indicating that the uimiodified sites were not ^ The use of the steady-state approximation instead of assuming substrate adsorption-desorption equilibrium was a key feature of Halpem's studies (9) of pressure effects on enantioselectivity, and its implications for enantioselectivity in the current system will be discussed later in the text.

187 saturated. If we make an assumption that surface coverage on the unmodified sites is low (i.e., 1 » KH^^^[H2]^^^ + KEP[EP]), the rate expression for /^-production (with an analogous equation for ^-production) becomes: (11)

[EP][H,r k^,^k,'K-^[H,f

kÊP]

1 1

k'lEP]

1

L ^-^.

1

k'-.

Enantioselectivity is related to the ratio of (R) and (5) reaction rates: r«

l + ee _ k ^ \-ee klk',

ki,+klK^;^'[H,r k^+k^K'l^lH^]'"

(12)

Lumping the rate constants together illustrates the functional form of the hydrogen dependence: r" _ \ + ee _ a + b[H^\'^ 1 -ee

(13)

l + c[//,]"

It is important to note that a consequence of the use of the steady-state assumption for surface intermediates is that enantioselectivity is a function of hydrogen concentration. The model predicts that enantioselectivity ceases to be a function of hydrogen concentration at the limits of low pressure and high pressure: high pressure limit:

(14)

l + ee l-ee

-^high P

low pressure limit:

l + ee \-ee

k,'K/2

K^

k'Kj2

K'

(15)

While Eqs. (11-15) have been developed for the case of low surface coverage on the unmodified sites, the general model may also be used to consider other cases. When surface coverage of ethyl pyruvate on the unmodified sites becomes very high (i.e., KEP[EP] » 1 + K^^ [H2]'^^), the rate and enantioselectivity become complicated functions of hydrogen and ethyl pyruvate concentrations. This Hmiting case can give a reaction rate showing a negative order in ethyl pyruvate, which has been observed experimentally at very high ethyl pyruvate concentrations (12). ^H

ÊP

L

h

R

(16)

\H,

"^

KH}^[H2^ KEP[EP]

jcnm

kÊP]

1+

l^-l "^ l^l

KrplEP]

'^-l "^ '^2

KpplEP] KEP[EP]

KHy2[H2]y2

^r'

l + ee high OEP

/C_i

T" /C2

KEP[EP]

l-ee 2

KEP[EP]

(17)

4.2 Kinetic Model Applied to Experimental Results A comparison of experimental data to this kinetic model is given in Figures 1 and 2. The open symbols in Figure 1 show the relationship between enantioselectivity and hydrogen concentration over a ten-fold change in hydrogen concentration (8d). The solid curve in the figure is a fit of these discrete experimental data points to Eq. (12) using solution hydrogen concentrations up to 0.013 M (corresponding to the solubility of hydrogen in propanol at 600 kPa and 303 K). Experimental data for the rate of (R) production at 303 K two different pressures obtained from heat flow and analytical measurements (Eq. 3) are compared in Figure 2 with the fit of these data to Eq. (11). The more than four hundred discrete experimental data points collected in each experiment over the time interval shown in Figure 2 form an apparentiy continuous line in excellent agreement with the model. The predictive capability of the model was also tested by comparing the enantioselectivity obtained experimentally with that predicted from the model for a reaction carried out at higher pressures. This is shown in Figure 1, where the dashed line represents the model prediction from Eq. 12 and the filled symbols give the experimental result. Thus the model is shown to give an accurate prediction of the enantioselectivity obtained at hydrogen concentrations up to three times greater than that of the highest value used in the kinetic fit. 80-

O

Figure 1. Enantioselectivity as a function of solution hydrogen concentration. Solid line represents the fit of experimental data given by the open circles fit to Eq. (12). Dashed line represents the prediction of the model to higher pressures. Closed circles represent experimental data obtained outside the range used in the model fit.

Experimental Data Kinetic Model Kinetic Model Prediction

200,01

5.

0,04 0,02 0,03 Hydrogen Concentration (M)

0,05

DISCUSSION

The excellent agreement between the experimental data and the proposed model fit shown in Figures 1 and 2^ reveal the power of this simple two-step mechanism in rationalizing complex (and heretofore inexplicable) observed relationships between reaction variables. Further support for this model comes from its prediction of enantioselectivities at pressures outside the range used in its development, corroborated by independent experimental data. ^ Eqs. (11) and (12) yield six constants (where KH*'^ is combined with the constants kj'^ and k2^ to give a lumped constant in the irreversible hydrogenation step) in the kinetic fit. Data obtained over a range of ethyl pyruvate and hydrogen concentrations are required to obtain the solutions shown in Figure 2. Because of the similiarity in the form of the (R) and (S) contributions to the adsorption term for the modified sites (Eq. 10), the overall fit to the rate equation may produce a non-unique solution for the individual constants unless a wide pressure range is employed in obtaining the experimental data.

189

20

30

Time (min)

40

25

Time (min)

Figure 2. Comparison of experimental data for (R) production (solid lines) and kinetic model fit to Eq. (12) (dashed lines) for hydrogenation of 0.5 M ethyl pyruvate in n-propanol at 303 K and two hydrogen pressures. The experimental data curve is comprised of more than 400 data points for each reaction.

The proposed model demonstrates that a pressure dependence on enantioselectivity may be explained by the concept of kinetic coupling between two parallel branches of a reaction network. The model describes a reaction network in which the (R) and (S) pathways involve surface intermediate species occupying sites on a common catalytic surface and sharing a common reactant pool. Because the species in these two parallel pathways may communicate with each other through the reversible adsorption-desorption steps, the rate of formation of each product may be linked to the other through this kinetic coupling. Enantioselectivity is related to the ratio of rates of (R) and (S) production. Each rate has a dependence on hydrogen concentration which is a function of the rate constants for the elementary steps in its own branch of the reaction network. As a result, the individual (R) and (S) rates may have different sensitivities to changes in hydrogen concentration. The model predicts that changes in pressure may result in both increasing and decreasing enantioselectivity, with the trend being determined simply by the relative magnitudes of the rate constants in the network. This description of the reaction network does not require the postulation of different mechanistic pathways for the (R) and (S) branches, or even a different rate-Umiting step in the two branches of the same network. The critical feature which allows the pressure effect to be rationalized is simply the deviation from the Langmuir isotherm assumption requiring that adsorption-desorption equilibrium exists under all conditions of hydrogen pressure. This was elegantly demonstrated by Landis and Halpem (9), and more recently discussed by Boudart and Djega-Mariadassou (11), in the context of hydrogen pressure effects on enantioselectivity in a homogeneous asymmetric catalytic hydrogenation example. The model predicts that enantioselectivity ceases to be a function of hydrogen concentration at the limits of extremely high ([H2] becomes infinite) and low ([H2] approaches zero) pressure. Examination of the physical significance of these limits is informative. The low-pressure limit of Eq. (15) is in fact equivalent to the Langmuir assumption of adsorptiondesorption equilibrium for ethyl pyruvate on modified sites. The high pressure limit (Eq. (14)) gives the case where the rate-limiting step shifts from the irreversible hydrogenation step to the substrate adsorption step in each branch of the cycle. Here enantioselectivity is related simply to the ratio of the forward rate constants for the adsorption step. Another intriguing point in our model is that, while enantiodifferentiation occurs due to the interaction of the substrate with the modified sites, the unmodified sites play a significant role in determining both rate and enantioselectivity. Hydrogen availability is a driving force for the reaction, and competition between hydrogen and other adsorbates on the unmodified sites

190 may thus influence rate. The sensitive relationship between hydrogen availability and enantioselectivity shown in Eq. (12) reveals that the unmodified sites act as "gatekeepers" for hydrogen entry into the irreversible reaction step with the chiral surface intermediate. Thus it may be suggested that the characteristics of these achiral surface sites are as important in the ultimate determination of enantioselectivity as is the concept of the "chiral efficiency" of the modified catalyst sites. 6.

CONCLUSIONS

A mechanism involving reversible substrate adsorption and irreversible hydrogenation was proposed for both the (R) and (S) pathways in the asymmetric hydrogenation of ethyl pyruvate using a dihydrocinchonidine-modified heterogeneous Pt catalyst. This kinetic model gives a quantitative rationalization of the effects of hydrogen concentration on enantioselectivity in a heterogeneously catalyzed asymmetric hydrogenation reaction. The model provided an excellent fit to the experimental data and successfully predicted enantioselectivities attained at pressures greater than the highest value used to develop the model. The two branches of the network share the same pool of reactants and hence are coupled, resulting in this case in the perturbation from equilibrium adsorption of ethyl pyruvate under all conditions except the extreme of very low hydrogen pressure. Both reaction rate and enantioselectivity are sensitive functions of the ability of hydrogen to be delivered to the (R) and (S) surface intermediate species. The model may be applied to cases exhibiting positive, negative, or no pressure effect on enantioselectivity without invoking a change in mechanism or even in rate-limiting step to explain the observed enantioselectivity trends.

REFERENCES 1. 2. 3. 4. 5.

6. 7. 8.

9. 10. 11. 12.

Blaser, H.U., Garland, M., and Jallet, H.P., / CataL, 1993,144, 569. Wehrli, J.T.; Baiker, A.; Monti, D.M.; Blaser, H.U.; /. MoL CataL, 1990, 61, 207. Garland, M.; Jalett, H.P., Blaser, H.U.; in Heterogeneous Catalysis and Fine Chemicals II, Guisnet et al., eds., Elsevier, Amsterdam, 1991, p. 177. Schwalm, O., Weber, J., Minder, B., and Baiker, A., Int. J. Quantum Chem., 52, 191 (1994). a) Wang, G.; Heinz, T.; Pfaltz, A.; Minder, B.; Mallat, T.; and Baiker, A.; J.C.S. Chem. Comm., 1994, 2047; b) Minder, B.; Mallat, T.; Baiker, A.; Wang, G.; Heinz, T.; and Pfaltz, A.; /. Catalysis, 1995, 154, 371; c ) Minder, B.; Schurch, M.; Mallat, T.; Baiker, A.; Heinz, T.; and Pfaltz, A.; J. Catalysis, 1996,160, 261. a) Sutherland, I.M.; Ibbotson, A.; Moyes, R.B.; Wells, P.B.; /. Catal, 1990, 125, 11. b) Meheux, P.A.; Ibbotson, A.; Wells, P.B.; J. Catal, 1991,128, 387. Augustine, R.L.; Tanielyan, S.K.; Doyle, L.K.Tetr. Asymm.., 1993, 4, 1803. a) Singh, U.K.; Landau, R.N.; Sun, Y.; LeBlond,C.; Blackmond, D.G.; Tanielyan, S.K.; Augustine, R.L. /. Catalysis, 1995,154, 91; b) Sun, Y., LeBlond, C., Wang, J., Landau, R.N., and Blackmond, D.G., J. Catalysis, 1996, 161, 759; c) Wang, J., Sun, Y., LeBlond, C , Landau, R.N., and Blackmond, D.G., /. Catalysis, 1996, 161, 752; d) Sun, Y., Landau, R.N., LeBlond, C., Wang, J., and Blackmond, D.G., J. Amer. Chem. Soc, 1996,118, 1348. a) Landis, C.R.; Halpem, J. J. Am. Chem. Soc. 1987, 109, 1746; b) Halpem, J. in Asymmetric Synthesis, ed. by J.D. Morrison, (Academic Press, New York, 1985) p.41. Collman, J.P.; Hegedus, L.S.; Norton, J.R.; Finke, R. Principles and Applications of Organotransition Metal Chemistry, 2nd. Ed. (University Science Books, Mill Valley, 1987). pp. 538-541. Boudart, M.; Djega-Mariadassou, G. Catal. Lett. 1994, 29,1. It should be pointed out for clarity that the designations of (/?)- and (S)- in the Halpern work in Ref. (9a) are transposed in Ref (11). Blaser, H.-U., in Catalysis Today, Proceedings Pre-Congress Workshop on Organic Reactions, 11th ICC, Baltimore, MD, June 1996, in press.

^ This concept might be considered in other selective hydrogenation reactions, for example the case of unsaturated aldehydes where parallel pathways for C=C and C=0 bond hydrogenation exist.


191

Enantioselective hydrogenation of (£)-a-phenylcimiamic acid on cinchonidinemodified palladium catalysts : influence of support Y. Nitta^

K. Kobiro^*, and Y. Okamoto^

^ Niihama National College of Technology, Niihama, Ehime 792, Japan Department of Chemical Engineering, Osaka University, Toyonaka, Osaka 560, Japan

Significant support effects are observed in the enantioselective hydrogenation of (E)-aphenylcinnamic acid to (*S)-(+)-2,3-diphenylpropionic acid on cinchonidine-modified Pd catalysts, especially when the catalysts are pre-reduced at elevated temperatures. The amounts of the modifier and the substrate adsorbed on Pd are strongly influenced by the support employed, indicating the importance of the surface concentration of the modifier for obtaining a high enantioselectivity. A support having appreciable amounts of both acidic and basic sites with a moderate specific surface area is preferable; a Pd/Ti02 catalyst exhibited the best performance.

1. INTRODUCTION Enantioselective hydrogenation of C=C double bonds with heterogeneous catalysts has been a challenging subject [1, 2]. Perez and co-workers [3] showed that (£)-a-phenylcinnamic acid (1) was enantioselectively hydrogenated to (*S)-(+)-2,3-diphenylpropionic acid (2) with an optical yield of 30.5% using a cinchonidine-modified 5wt% Pd/C catalyst. The cinchonidinemodified platinum catalyst, a well-investigated catalytic system for the enantioselective hydrogenation of ketones (a-ketoesters) [1, 4], was found inactive for the hydrogenation of 1 [5]. On the other hand, Bartok and co-workers [6] applied the tartaric acid-modified RaneyNi catalyst, another well-studied system for the enantioselective hydrogenation of ketones (Pketoesters and alkanones) [2], to the hydrogenation of substituted cinnamic acids and their salts. They reported that the hydrogenation of 1 resulted in a very low optical yield of 0.21%, while the hydrogenation of the sodium salt of 1 gave an optical yield of 17%. Subsequent reports have shown that palladium is the superior metal for the enantioselective hydrogenation of alkenes [7-11]. In the course of the studies to improve the enantioselectivity of cinchonidine-modified Pd catalysts for the hydrogenation of 1, we found that the support materials used in the catalyst preparation, as well as the preparation method, had significant influence on the activity and selectivity of the resulting catalysts [12-14]. The highest optical yield (72%)ee) of 2 was obtained with a Pd/Ti02 catalyst under optimal reaction conditions [15]. In the present study. * Present address. ERATO, Japan Sci. Techn. Corp., 1-2-35-103 Onoharahigashi, Mino, Osaka 562, Japan

192 we have examined the effects of the nature of support materials on catalytic properties of the supported Pd catalysts, in order to obtain some insight into the surface state of the modified catalyst and the mechanism of this reaction.

COOH

c'nPd/Zr02>Pd/Ti02, while that of 1 is decreased in the reverse order; the tendency is eminent for the catalysts reduced at 473 K. These results indicate that the adsorption capacity

197 Table 4 Amounts of (jE)-a-phenylcinnamic acid and cinchonidine adsorbed on supported Pd catalysts Catalyst

Red. Temp.

Spd

m'/g

D^ Dc nm nm

c

_

e

d

DM

A.A.(mmol/g) PCA CDN

A.A.(^mol/m^Pd/ Molar ratio PCA CDN PCA/Pd CDN/Pd

298K 298K 298K 298K

4.1 14.6 11.6 12.7

5.1 1.4 1.8 1.6

;50 >

^

>-

30 20

^

^

40

10 30 20 40

0

60 80 100 120 Hydrogenation temperature / °C

Fig. 2 Effect of the hydrogenation temperature on OY(2-octanone) FNiP was treated with a hydrogen stream at 320°C. Modifying conditions:TA Ig and NaBr O.lg, pH 3.5, 100°C Reaction mixture: 2-octanone(32m mol), THF (10ml) and MCA (7.28g)

60 80 100 120 140 Hydrogenation temperature / °C Fig. 3 Effect of the hydrogenation temperature on OY(3-octanone) FNiP was treated with a hydrogen stream at 320°C. Modification conditions:TA Ig and NaBr O.lg, pH 3.5, 100°C. Reaction mixture: 3-octanone(32m mol), THF (10ml) and MCA (O) (7.28g) or acetic acid (D) (1.02g)

2.3. Hydrogenation of various 3-alkanones Enantio-differentiating hydrogenations of various 3-alkanones were carried out in the presence of MCA or PA (Table 6). (i) The addition of highly bulky MCA was more effective than that of PA for the differentiation of the ethyl and other alkyl groups, (ii) A difference of three carbon chains or more was needed for the effective differentiation in the presence of PA (heptyl, pentyl/ethyl; Entries 1 and 2), while that of two was enough in the presence of M C A (heptyl, pentyl, butyl/ethyl, Entries 1, 2, and 3). As a result, a bulkier acid, which may Table 6 Enantio-differentiating hydrogenation of various 3-alkanones OY/% Configuration Entry Substrate MCA PA of the product (R-COCH2CH3) 23 1 S 44 CH3(CH2)643 25 2 S CH3(CH2)417 41 S 3 CH3(CH2)325 2 S 4 CH3(CH2)232 15 S 5 (CH3)2CHCH2FNiP was treated with a hydrogen stream at 280°C. Modification conditions: pH 3.5, 1 l O T Hydrogenation temperature: lOOT

205 interact with the substrate over a wide area, is suitable for differentiating small differences in carbon chains. 3. EXPERIMENTAL All the chemicals except tetrahydrofuran (THF) were used as received. Commercial THF was dried over NaH overnight and then distilled. Raney nickel (RNi): Commercially available Ni-Al alloy (Kawaken Fine Chemicals Co., Ltd., Tokyo, Japan, Ni/Al=42/58, L9 g) was added to a 20% NaOH solution in small portions. The resulting suspension was allowed to stand at 100°C for Ih. After the supernatant was removed by decantation, the catalyst was washed with 25 ml of deionized water 20 times. Fine nickel powder (FNiP): One gram of commercially available fine nickel powder (Vacuum Metallurgical Co., Ltd., Chiba, Japan, mean particle diameter : 20 nm, specific surface area : 43.8 mVg, bulk density : 0.19 g/ml) was treated with a hydrogen stream for 0.5 h at the temperature described in the text. Reduced nickel (HNi): Nickel oxide (3.8 g) was reduced for Ih at 350°C in a hydrogen stream. Modification: l)RNi was modified with a 100-ml solution containing 1 g of (R,R)-tartaric acid ((R,R)-TA) and 6 g of NaBr (pH of this solution had been adjusted to 3.2 with 1 mol/dm^ NaOH) for 1 h at 100°C. After removal of the modification solution, the catalyst was successively washed with deionized water, methanol, and THF. 2) FNiP was modified with a 100-ml solution of 1 gof (R,R)-TA and the given amount of inorganic salt (pH of this solution had been adjusted with 1 mol/dm^ NaOH) for 1 h at the temperature described in the text. The modification over 100°C was caried out in an autoclave. The catalyst was washed in the same manner as RNi. 3) HNi was modified with a 300-ml solution of 3 g of (R,R)-TA and 0.3 g of NaBr (pH of this solution had been adjusted to 3.5 with 1 mol/dm^ NaOH) for 1 h at 100°C. The catalyst was washed in the same manner as RNi. Hydrogenation of 3-alkanones: The modified nickel catalyst thus obtained was used for the hydrogenation of the 3-alkanones (32 mmol) in the mixture of carboxylic acid (amount is described in the text) and THF (10 ml) under an initial hydrogen pressure of 9X10^ Pa. The hydrogenation temperature was also described in the text. Simple distillation gave a product of more than 98% (GLC analyses: 5% Thermon 1000 on Chromosorb W at 70-260T). Determination of OY: The OY of the reaction was evaluated using the optical purity of the product determined by polarimetry. OY (%) = ([aJi) of hydrogenation product / [a]^ of pure enantiomer) X 100 The specific optical rotations [a]^^ of the optically pure enantiomers are: (S)-3-hexanol, [af^ +7.13° (neat) [12]; (S)-3-heptanol, [af^ +8.13° (neat) [13]; (S)-3-octanol, [af^ +8.22° (neat)

206 [14]; (S)-3-decanol, [af^ +6.68° (neat) [13]; (S) 5-methyl-3-hexanol, [af^ +21.23° (neat) [15]. 4. CONCLUSIONS For the enantio-differentiating hydrogenation of 3-alkanones, a rather high OY (44%) was attained by the following method, (i) FNiP was used as the source of the catalyst, (ii) A highly bulky carboxylic acid such as 1-methyl-1-cyclohexanecarboxylic acid or 1adamantanecarboxylic acid was added to the reaction system, (iii) Hydrogenation was carried out at 100°C. Among the homogeneous catalysts and heterogeneous catalysts, this TA-NaBrMNi is a unique system giving good OY in the hydrogenation of 3-alkanones as well as 2alkanones. This study also suggests that FNiP is a promising material of the e.d. heterogeneous catalyst. REFERENCES 1. A. Tai and T. Harada, in Y. Iwasawa (Ed), Tailored Metal Catalysts, Reidel, Dordrecht, (1986) 265 and references therein. 2. T. Osawa, T. Harada, and A. Tai, J. Mol. Catal., 87 (1994) 333. T. Osawa, T. Harada, and A. Tai, Catalysis Today, to be published. 3. T. Ohkuma and R. Noyori, J. Synth. Org. Chem., 54 (1996) 553. 4. T. Kikukawa, Y. lizuka, T. Sugimura, T. Harada, and A. Tai, Chem. Lett., (1987) 1267. T. Osawa, T. Harada, and A. Tai, J. Catal., 121 (1990) 7. 5. T. Harada, A. Tai, M. Yamamoto, H. Ozaki, and Y. Izumi, Stud. Surf. Sci. Catal., 1 (1981) 364. 6. T. Harada, M. Yamamoto, S. Onaka, M. Imaida, H. Ozaki, A. Tai, and Y. Izumi, Bull Chem. Soc. Jpn., 54 (1981) 2323. 7. T. Osawa, A. Tai, Y. Imachi, and S. Takasaki, Chiral Reactions in Heterogeneous Catalysis, Edited by G. Jannes and V. Dubois, Plenum Press, (1995) 75. 8. T. Harada, Bull. Chem. Soc. Jpn., 48 (1975) 3236. 9. A. Bennett, S. Christie, M. A Keane, R. D. Peacock, and G. Webb, Catalysis Today, 10 (1991)363. 10. T. Osawa and T. Harada,5w//. Chem. Soc. Jpn., 57 (1984) 1518. 11. T. Harada, Y. Imachi, A. Tai, and Y. Izumi, Metal-support and metal-additive effects in catalysis, Lyon, Elsevier Publishing Company (1982) 377. 12. J. Kenyon and R. Poplet, J. Chem. Soc, (1945) 273. 13. R. H. Richard and J. Kenyon., J Chem. Soc, 103 (1913) 1923. 14. R H. Richard and J. Kenyon., J Chem. Soc, 101 (1912) 620. 15. P. A. Levene and R. E. Marker, J Biol. Chem., 90 (1931) 669.


207

DIASTEREOSELECTIVE HYDROGENATION OF A PROSTAGLANDIN INTERMEDIATE OVER Ru SUPPORTED ON DIFFERENT MOLECULAR SIEVES F.Cocu*, S.Coman^ C.Tanase*, D.Macovei" and V.I.Parvulescu" *- Chemical and Phannaceutical Research Institute, Vitan Avenue 112, Bucharest, Roanmia ^'-University ofBucharest,Department of Catalysis,B-dul RepubUcii 13,Bucharest 70346,Romania '- Institute of Physics and Materials Technology, Magurele-Bucharest, Romania SUMMARY Hydrogenation of a prostaglandin intermediate has been carried out on some Rumolecular sieves prepared via deposition of RuClj. Characterization of these catalysts using different tools like WAXS, XPS or EXAFS indicated differences due to both the different metal loading and the different structure and topology of the investigated molecular sieves (L, APO-34 and ZSM-5). The prostaglandin intermediate has two kinds of unsaturation, a double C=C bond and a prochiral C=0 bond. Therefore hydrogenation could imply two different selectivities. On simple Ru-molecular sieves, hydrogenation occurs with different reaction rates and selectivities as a function of the catalysts characteristics. Thus, d.e. of about 100% for a selectivity to the alcohol of 99% were obtained for Ru(12wt.%))-L. However, on these catalysts only the epi configuration was obtained. Modification of the catalysts using L(+) tartaric acid leads to the increase of the reaction rate and the decrease of the d.e. Adding of the pivahc acid to the reaction mixture determined a supplementary increase of the reaction rate. More important in this case is the change of the stereoselectivity, the main product becoming the natural configuration. INTRODUCTION Stereocontrolled synthesis of prostaglandin intermediates and of the numerous structural analogs found a large interest because of the therapeutic properties of these compounds. Some prostaglandin Ej and E2 analogues are known to show both antisecretory and cytoprotective activities [1,2], prostacyclin and isocarbacychn are useful therapeutic agents in cardio-vascular field because of their potent vasoactive properties [3] and F for the veterinar use [4]. Almost all these compounds retaining prostaglandin-like biological activity exhibit an 15-OH group located in an ally lie position with the same configuration as natural prostaglandins [5]: OH 0

^COOR R' OH

OH

PGF2a

6 OH

.COOR

OH

PGEj

The total synthesis of the typical molecular framework of these compounds is of a high complexity due to the presence of three or four chiral sites. Most of the total synthesis strategies start from cyclopentanic key intermediates which contain a proper configuration of the substituents [6]. Subsequent introduction of the two lateral chains could be performed through Wittig or Horaer-Emmons olefinations. Following this procedure some intermediates containing a, P-unsaturated ketone fragments of type 1, 2 or 3 are formed. Starting from these enones, a

208 diastereoselective reduction of ketone to the ally lie alcohol is necessary to obtain the natural configuration.

Few studies concerning catalytic hydrogenation are known in the literature. Up to now, mainly the hydrogenation of the double 5Z double bond present in (2) was investigated. Both homogeneous (in the presence of Wilkinson catalyst [7,8]) and heterogeneous catalysis (on 5%Pd/C [9,10] or SVoBh/Alfi,) have been used. The aim of this study was to investigate the diastereoselective hydrogenation of 1 on ruthenium supported on different molecular sieves. The reason for the use of the molecular sieves as support was to investigate if the different structure and topography of these materials could lead to a different behaviour of ruthenium in the 15-keto group hydrogenation. The influence of the ruthenium loading and of some modifiers like L(+) tartaric acid and enantiofacedifferentiating agents like pivalic acid was followed as well. EXPERIMENTAL Intermediates of type 1 (R' = -CH2-0-C6H4-Cl(m); n-CjH,,) as well as the standards of the possible reduction products were prepared by stereocontroUed synthesis starting from norbomadiene [6]. The products were characterized by ^H-NMR and '^C-NMR on a Varian 300 NMR-sp ectrometer. Ru supported catalysts have been obtained by deposition of Ru on molecular sieves (L, APO-34, ZSM-5) in the potassium form fi-om a solution containing 0.4 M RuClj (Fluka purity) [12]. After washing, the catalysts were dried in a vacuum stove at room temperature and then reduced in hydrogen (30 ml.min') at 500 ^C for 6 hours. The heating rate was of l^C.min ^ Modification of the catalysts with L(+) tartaric acid was carried out by stirring 2 hours at 70 ^C an 100 ml aqueous suspension containing 0.0IM acid. The correction of pH at 5.1 was made with NaOH solution. After 2 hours the catalysts were successively washed with water, methanol and anhydrous THF. Samples were characterised by elemental analysis, adsorption of N2 at 77 K, XPS, WAXS, EXAFS and FT-IR. Elemental analysis of Ru, Si, Al and P was performed by atomic emission spectroscopy with inductively coupled plasma atomization (ICP-AES). Adsorption and desorption curves of Nj at 77 K were obtained with a Micromeritics ASAP 2000 apparatus after degassing the samples at 200 °C under vacuum. XPS spectra were recorded using a SSI X probe FISONS spectrometer (SSX 100/206) with monochromatic Al Ka radiation. The spectrometer energy scale was cahbrated using the Au4f7/2 peak (of binding energy 93.98 eV). With the analyzer energy used (30 eV), the fiiU width at half maximum of 4f7/2 peak was 10 eV. Vacuum in the analysis chamber during analysis was 10'^-10"'° P. For calculation of the binding energies, the peaks of the C-(C,H) component coming from contamination carbon (284.8 eV) and of Sijp that exhibits very well resolved symmetric peak located at 103 eV were used as an internal standard. The composite peaks were decomposed by a fitting routine included in the ESCA 8,3 D manufacturer software. The surface composition of the investigated samples was determined using the same software. Bands assigned to Ru^ds/i^ ^hs^û ^^^ ^hp

209 respectively were followed. FTIR spectra were recorded with a Bruker IF 388 spectrometer. Spectra were recorded at room temperature between 4000 and 800 cm* with a resolution of 1 cm"'. The EXAFS spectra at the L edges of Ru were measured by the total photoyield current, for the catalysts as well as for RuOj and RUCI3, as standard compounds. The primary data were acquired with the use of the synchrotron radiation, in a range between about 100 eV below the Ru L3-edge and 450 eV above Ru Lj. Unfortunately the EXAFS at Ru L3 was not available beyond about 100 eV above the edge, due to the superposition with Ru Lj. The range above Ru Lj was Tree' until about 400 eV, where the K-edge of potassium of the support was superposed, but the EXAFS signal was too weak to provide a reliable information. Only the Ru Lj-edge was suitable for a further analysis of EXAFS, assuming a negligible influence from the high-energy part of Ru L3 EXAFS. However a limitation of the radial resolution is resulting from the narrow analysed range ('-ISO eV). The EXAFS spectra were averaged over 5 or 6 runs and Fourier transformed after a k^-weighting. Hydrogenation of the substrates was carried out in a stainless steel stirred autoclave under 2-10 atm. and temperatures ranging between 20-80 °C. Standard experiments used 40 mg substrate dissolved in 20 ml methanol or THE (in the case of the modified catalysts). In the case of the adding of the pivalic acid, the reaction product was neutralized with K2CO3, extracted in 100 ml anhydrous ethylic ether and dried on CaClj. Analysis of the reaction products was performed on a Varian 5000 HPLC using as column NUCLEOSIL 5-C-18 250x4 in the next conditions: eluent -water:acetonitrile:THF=70:30:2, flow rate-0.8ml.min"', detection UV 279nm. Optical selectivity was expressed in terms of diastereoisomeric excess (d.e.) which was defined as: de. % =

[R]-[S]

xlOO

[R] + [S]

RESULTS Catalysts characterization Chemical composition and textural properties of the investigated catalysts as well as the systems modified with tartaric acid are presented in Table 1. Table 1. Catalysts used in diastereoselective hydrogenation of intermediate 1 Support

Ru loading, wt.%

Ru-1

L

5.84

246

yes

yes

Ru-2

L

12.02

176

yes

yes

Ru-3

APO-34

3.39

78

yes

Ru-4

APO-34

6.21

62

yes

yes

Ru-5

ZSM- 5

3.08

278

yes

yes

Ru-6

ZSM-5

4.11

259

yes

Catalyst

Surface area, mlg-'

Untreated with tartaric acid

Treated with tartaric acid

Typical XRD lines of Ru were detected only in the case of the Ru(12.02wt.%)-L and Ru(6.21 wt.%)-APO-34 samples. However, deposition of the Ru in all these cases does not modify the crystallinity of the support. XPS results of the investigated samples are given in Table 2. XPS values are

210

Table 2. XPS results for ruthenium molecular sieves Catalyst

Untreated with tartaric acid eV

Treated with tartaric acid

0.S

Si2p

eV

eV

eV

280.89

281.32

532.18

103.1

Ru-2

280.42

281.06

532.22

103.1

Ru-3

280.23

Ru-4

280.11

280.56

532.27

Ru-5

281.71

282.33

532.15

103.1

Ru-6

281.29

532.12

103.1

Ru-1

1

532.25

consistent with the results of Cisneros and Lunsford on Ru-Y zeohtes [13]. These values indicate the existence of a greater localization of charge on L and APO-34 than on ZSM-5 although metal concentrations are high enough to observe such differences. However, differences of about 1.6 eV like that between Ru(6.2lwt.%)-APO-34 and Ru(3.08wt.%)-ZSM-5 suggest this is no an artefact. Subsequent treatment of these catalysts with tartaric acid seems to diminish the localization of charge. Some intrinsec metal characteristic features are also indicated by EXAFS analysis of these catalysts. By comparing the Fourier transforms of the tartaric acid treated catalysts with those of the standard compounds, a similarity to RuOj is quite evident in the first 4 A of the transforms. This claims for a quite oxidated state of the local Ru-environment in these catalysts. The shift of thefirstradial miaximum towards larger distances, characteristic of the Ru-Cl bond, suggests a certain CI contribution to the local environment of Ru, however still dominated by the oxygen of the support. Catalytic tests Hydrogenation of the substrate 1 could occur both to the carbonyhc and the double C==C bonds. Therefore the investigation of this reaction implies two selectivity aspects. One related to a chemoselectivity and an other to a stereoselectivity Figure 1 shows the evolution of the reaction rate and of the selectivity to the alcohol as well as to the configuration epi on the investigated catalysts. Concerning the influence of the support, the order of the activity is ZSM-5 > L > APO-34. The increase of the metal loading has as an effect a strong decrease of the reaction rate but an increase both in the selectivity to the alcohol and stereoselectivity to the epi-configuration. The best selectivities were obtained on Ru-2. Low metal loadings seems to Figure 1. Evolution of the reaction rate (-»-) ^f'^'T!^^ the hydrogenation of the and oftiieselectivity to alcohol ( . ) and to ^'"W^ ^=0 bond. However, we consider epi configuration (a). 80 "C. 2 aton.. 3 h. ^»**« behaviour of the Ru-5 catalyst is

211 noteworthy. It gives good diastereoselectivities (d.e. of about 98%) and high reaction rates for a mere 3.08 wt.% metal loading. In Figure 2 is shown the variation of the reaction rate and of the selectivity to alcohol and of the e.e. in the q)i form as a function of temperature on the Ru(12.02wt%)-L catalyst. As one can observe, the variation of the temperature has no influence on the reaction rate and on the stereoselectivity but a slow decrease of the selectivity to the alcohol has been evidenced. On the opposite, modification of the pressure in the range 2 - 1 0 atm. has as an effect a more evident change in the values of these parameters (Figure 3). Thus, except 2 atm. the stereoselectivity to the epi configuration is less than 100, at the same time the compound with the configuration of the natural analogous obtains. However, in the investigated conditions, the epi form is majoritar irrespective of the reaction parameters. The increase of the pressure in the above range also determines a decrease of the selectivity to the alcohol as well as of the reaction rate. The increase of the Ru/substrate ratio in the initial mixture of reaction exhibits a similar effect with that of the metal loading, i.e. determines an increase both in the stereoselectivity and in the selectivity to alcohol and a decrease of the reaction rate (Figure 4).

20

40

60

80

4

temperature, C

6

8

10

pressure, atm

Figure 2. Reaction rate , selectivity to Figure 3. Reaction rate , selectivity to alcohol ) and d.e. in the epi configuration (m) alcohol ) and d.e. in the epi configuration ) as a function of temperature on Ru-2. 2 atm., 3h. as a fixnction of pressure on Ru-2. 80°C, 3h. 100 n 90

'm

5: ,

^ T . l

^

^ ^

2.5

80

1

1

r 3

5P 70 > >

60. 50

1

40

8

30

1.5 § J ^

20

2 |

-0.5

10 . 0 \ Ru-2

Ru-3

Ru-5

Ru-2

Ru-3

Ru-5

Ru-2

Ru-3

Ru-5

lo

Ru/substratate molar ratio

Figure 4. Reaction rate (-#), selectivity to alcohol ) and d.e. in the epi configuration ( o ) as a fimction of Ru/substrate ratio. 80 ^C, 2 atm., 3h.

212 Tartaric acid modified Ru/molecular sieves catalysts were tested using tetrahydoforan as solvent. Under similar conditions, modified catalysts in the presence of tetrahydrofiiran exhibit reaction rates higher than unmodified catalysts in the presence of methanol (Figure 5). The effect of the solvent has been checked in the case of Ru-2 catalyst. Experiments performed at 10 atm and 80 ®C showed that in the presence of the tetrahydrofiiran the reaction rates are lower than in methanol. The catalysts modified with tartaric acid exhibit a lower selectivity to the q)i configuration than umnodified ones. The percent of the natural configuration is increased but the epi configuration is still dominant Moreover, under these conditions the selectivity to the alcohol is lower, because of the tendency to hydrogenate the double C=C bond. Introduction of the pivalic acid in the reaction mixture determines an important change as afiinctionof metal loading. Thus, in the case of high metal loadings (Ru(12.02wt%)L), both an imiportant change in the stereoselectivity and an increase in selectivity to the alcohol were induced. It is noteworthy that in this case an inversion in the stereoselectivity takes place, the main component being that with the natural configuration. Moreover, the de. in this cases reaches values of about 80 %. For low metal loadings, like Ru (5.84wt.%)-L one can suppose that the presence of the pivaUc acid poisons the catalyst surface. The effect of the ZSM-5 zeolite is again noticeable: selectivity to alcohol of about 48% and d.e. about 40% of the natural configuration were obtained even at low metal loadings (Ru-3.08wt.%)-ZSM-5.

Ru-1

Rii-2

Ru^

catalyst nature

Figure 5. Influence of the L(+) tartaric acid on the reaction rate ^, on the selectivity to alcohol ) and on the d.e. in the epi configuration (S). 80 ^C, 10 atm., 3h.

catalyst natura

Figure 6. Influence of the L(+) tartaric acid and of the pivalic acid on the reaction rate , on the selectivity to alcohol ) and on the d.e. in the natural configuration (o). 80 °C, 10atm.,3h.

DISCUSSION Catalyst characterization indicates that Ru deposition in a high loading on different molecular sieves mainly occurs on the external surface of these. However, as XPS and EXAFS measurements showed, some differences are induced by the features of the molecular sieves even at these high metal loadings. Chlorinefi*omthe RuCl, precursor has a certain contribution to these differences. Subsequent treatment with tartaric acid seems to induce a further delocalization of the charge on Ru irrespective of the molecular sieve support. Hydrogenation of the intermediate 1 on these catalysts occurs with different reaction rates as a function of the support nature and metal loading. In this case it is very difficult to assume a relation between the activity and the selectivity to alcohol or to the stereoselectivity to the epi configuration. However, some speculations could be envisaged. Ru-2 exhibits lower reaction rates but higher stereoselectivity to the epi configuration. The increase of the reaction

213 rate, like in the case of Ru-l, Ru-5 or Ru-6, seems to lead to the decrease of the d.e. to this configuration. Whether the selectivity is kinetically controlled and which is the best loading in order to achieve a high reaction rate coupled with a good selectivity remain two unsolved problems. The treatment with tartaric acid of these catalysts modifies both the reaction rate and the selectivity. We suppose that the increase of the reaction rate could be assigned to the intermediate presented in the Scheme 1. This is also consistent with the literature data published up to now [14-17]. Again, one can suppose the decrease in the d.e. of the epi configuration as a kinetically controlled process. The configuration of the intermediate 1 given in the Scheme 1 has been proved by us by 'H-NMR data of the vicinal coupling constants : Jio«iip= J, i2« *'iip,iop p 'Hz and Jgj, loj, == 3.2 Hz. O

" .O

R \ ,15

c

R=CH2-0-C^H5Cl

c O'

-

0

Scheme 1.

Ru

Introduction of the pivahc acid in the reaction mixture not only increases the reaction rate and decreases the selectivity to the epi configuration but changes the ratio between the two stereoconformers, the natural one becoming majoritar. Like in the case of the tartaric acid, we suppose that the effect of the pivahc acid is a kinetical one. Generation of a complex like the one in Scheme 2 generates a high reactivity which leads to the natural configuration as a main product (d.e. over 77%). These assumptions are also consistent with published data [18-19]. CH,

.H — O 0

0

1 3

^

c ,H' \ ^ 0 ^

R= CH2-0-C^H5CHm)

V R \ H

c

cHO^

X'-\ 0'

Ru

H

J^H

0

/ H H

Lô=-^ 1

^

Scheme 2.

214 CONCLUSIONS Hydrogenation of a prostaglandin intennediate like 1, on Ru supported on different molecular sieves like L, APO-34 or ZSM-5 occurs with different reaction rates as a Ainction of the support nature and the metal loading. In the investigated conditions, hydrogenation leads to the epi alcohol configuration as the main hydrogenated product. Modification of these catalysts with L(-H) tartaric acid has as effect on both the increase of the reaction rate and the decrease of the de. in the epi configuration, which is still the main alcohol product. Introduction of the pivalic acid determines a suplementary increase in the reaction rate and a decrease in the selectivity to the epi configuration. Moreover, in these conditions the natural configuration becomes the main alcohol hydrogenated product. The effect of both the tartaric and the pivalic acid seems to be due to the formation of active intermediate complexes among these and the reaction intermediate 1 (Scheme 1 and 2). The present woik is a step in order to obtain in mild conditions the prostaglandin intermediates with natural-like configuration that are of practical importance. REFERENCES l.T.Tanaka,K.Bannai,K.ManabeandS.Kurozumi,J.Label. Compounds Radiopharm.,29(1991)933. 2.SSugiura,TTanaka,K.Bannai andS.Kurozumi,J.LabeLCompounds Radiopharm.,29(1991)1042. 3.K.Manabe,T.Tanaka,S.Kuiozimii and Y.Kato,J.LabeLConûnds Radiopharm.,29(1991)l 108. 4. N.S.Crossley, Chem.Ind. (1976) 334. 5. F.E.Collins and S.W.Djuric, Chem.Rev., 93 (1993) 1533. 6. J.S.Bindra and R.Bindra, Prostaglandin Synthesis, Academic Press, New York, 1977. 7. E.Anggard and B.Samuelsson, J.Biol.Chem.,239 (1964) 4097. 8. G.K.Koch and J.W.Dalenberg, J.Label.Compounds, 6 (1970) 395. 9. E.J.Corey, R.Noyori and T.K.Scharf, J.Am.Chem.Soc.,92 (1970) 2586. 10. E.J.Corey and R.K.Varma, J.Am.Chem.Soc.,93 (1971) 7319. 11. F.H.Lincobi, W.P.Schneider and J.E.Pike, J.Org.Chem., 38 (1993) 1533. 12. V.LParvulescu, V.Parvulescu, S.Coman, C.Radu, D.Macovei, Em.Angelescu and R.Russu, Stud. Surf Sci.Catal., 91 (1995) 561. 13. M.D.Cisneros and J.H.Lunsford, J.Catal., 141 (1993) 191. 14. D.R.Richards, H.H.Kung and W.M.H.Sachtler, J.MoLCatal., 36(1986) 329. 15. H.Brunner, M.Muschiol, T.Wischert and J.Wiehl, Tetrahedron Asymmetry, 1(1990)159. 16. M.A.Keane and G.Webb, J.Chem.Soc.,Chem.Commun., (1991) 1619. 17. M.A.Keane and G.Webb, J.MoLCatal., 73(1992) 91. 18. T.Osawa, T.Harada and A.Tai, J.MolCatal, 87 (1994) 333. 19. T.Harada, T.Kawamura, S.Harikawa and T.Osawa, J.MoLCatal., 93(1994) 211.

Acknowledments The authors thank Dr.V.Marcu from lEC for helpful discussions and to Ministry of Education for the Grant No.407.


215

Diastereoselective hydrogenation of substituted aromatics on supported rhodium catalysts: influence of support and of thermal treatment M. Besson, P. Gallezot, C.Pinel and S. Neto Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, 69626 Yilleurbanne Cedex, France Abstract The diastereoselective hydrogenation of N-(2-methylbenzoyl)-f5j-proline methyl ester 1 into optically active 2-methylcyclohexane carboxylic acids was studied on rhodium catalysts supported on active carbon, graphite and alumina. The diastereoselectivity was highly dependent upon the nature of the support. Without modification of the catalysts by EDCA, a 40% d.e. was measured over ^hlPAÔ^,, in contrast to Rh/C or Rh^G catalysts which were unselective. The adsorption of the aromatic substrate via a specific face on Rh/ AI2O3 was interpreted in terms of electronic and/or steric factors on the basis of TEM and XPS studies. EDCA addition had comparatively little effect on the initial rates and d.e. of RhlKlÔ^, because the amine was preferentially adsorbed on the acidic sites of the alumina support. In contrast, the d.e. of carbon-supported catalysts increased from 0 to ca. 50% upon EDCA addition. The diastereoselectivity of Rh/AljOg catalyst was higher after thermal treatments under hydrogen prior to hydrogenation reaction. The optimization of rhodium-based catalysts (support and thermal pretreatment) and of reaction conditions (solvent, addition of amine) led to a final d. e. of 68%. 1. INTRODUCTION In the past few years, a considerable interest was focused on chiral catalysis, because of the need to develop new chiral compounds for pharmaceuticals and agrochemicals [1]. Optically active cyclohexyl compounds are often used as chiral building blocks [2]; they are also essential constituents of biologically active compounds [3]. However, their synthesis from the corresponding substituted aromatic molecules had never been achieved by homogeneous or heterogeneous catalytic hydrogenation. Different strategies can be developped to get enantioselectivity with solid catalysts, viz: (i) modification of a metal surface with a chiral compound-e.g. Pt-cinchona alkaloids and Ni-tartaric acid for hydrogenation of ketoesters; (ii) grafting of a chiral complex on a solid; (iii) use of a chiral support, e.g., chiral polymers [4]; (iv) covalent binding of the substrate with a chiral auxiliary before hydrogenation on a metal surface [5]. Recently, we have successfully used the last procedure for the diastereoselective hydrogenation of o-toluic acid, covalently bound to proline esters chiral auxiUaries [6-8]. Thus, hydrogenation of the A^-(2-methylbenzoyl)-f5'j-proline esters 1, performed on carbon supported rhodium catalysts, yielded essentially the cis diastereoisomers 2 and 3 (>97%) and transiently the cyclohexenic compound 6 (Figure 1). After cleavage of the proline auxiUary by hydrolysis in acidic medium, the optically active cis 2-methyl-l-cyclohexane carboxylic acids were obtained without racemization. It was shown that the diastereoselectivity of the reaction can be influenced with the addition of amines. Without additive, the hydrogenation was not selective as long as the aromatic substrate was present, because the steric hindrance was not sufficient to favor the adsorption of the aromatic ring on a specific face. However, during the consecutive hydrogenation of the cyclohexenic 6, a diastereoisomeric excess d.e. of ca. 17% was observed in favour of 2. The addition of various non chiral amines (e.g., //-ethyldicyclohexylamine EDCA) produced a configurational inversion in favour of 3 and an improvement in

216 diastereoselectivity attaining up to 50%. To interpret our experimental results on Rh/C catalysts, we have proposed, on the basis of NMR and molecular modelling studies, the existence of two conformers for the aromatic substrate in a 80/20 ratio, which adsorption on the rhodium surface was sterically oriented by the presence of amine surface ligands. Attempts of catalytic diastereoselective hydrogenation of vanillic acid derivatives resulted in poor enantioselectivities [9]. In the present work, the dependence of the catalytic activity and diastereoselectivity in the hydrogenation of the methylester 1, was studied on rhodium catalysts, as a function of the nature of the support (active carbon, graphite and alumina) and of catalyst pretreatments under hydrogen.

^^^^COCl

N

^COOH

I

COOMe

y C j sAcooH^H^L rfVô ^°^^^ (Qr'° A

2)HCI

^^^C^^^^

\^^Me

I catalyst I H2,5MPa

N^COOMe

N^COOMe ^ N - ^

(-"^"Y^^" ^Me

'Me cis

2 (1R,2S,2'S)

3 (1S,2R,2'S) cis

Figure 1. Synthesis of A^-(2-methylbenzoyl)-f5'j-proline methyl ester and main products obtained during diastereoselective hydrogenation. 2. EXPERIMENTAL The methyl ester 1 was prepared by coupling f^j-proline with 6>-toluic acid, via the corresponding acyl chloride, and derivation of the acid obtained to the methyl ester, as described in the previous report [7]. Catalysts 5%Rh/C and 3.7%Rh/Al203 (340 mlg'^) were obtained from Aldrich. Sample 4.2%Rh/G was prepared by ion-exchanging the functional groups of a graphite support (Lonza HSAG 300, 300 m .g'^) oxidized in NaOCl solutions, with Rh(NH3)5CP'' cations and reducing in flowing hydrogen at 3(X)°C [8]. The catalysts were characterized, using high resolution microscopy with JEOL lOOCX and 200EX microscopes. X-ray photoelectron spectroscopy measurements were performed on an Escalab 200R (Fisons Instr.) with a Mg K^^ source. The binding energy scale for Rh/Al203 was calibrated by setting the Al 2p peak at 74 eV. Thermal treatments of the catalysts under flowing hydrogen were conducted in an atmospheric pressure reaction cell connected with the XPS chamber. Catalytic hydrogenations were carried out in a 250mL, mechanicaly stirred autoclave (1250 rpm), under 5MPa hydrogen pressure at room temperature. The reactor was filled with 2.5 mmol of substrate 1 dissolved in 130 mL of alcoholic solvent, 0.07 mmol of rhodium, and, optionally, EDCA with a molar ratio EDCA/Rh of ca. 3.5, except otherwise stated. Samples of the reaction medium were analysed by gas chromatography (J&W DB1701 column). Initial rates were determined and the diastereoisomeric excess of the cis products 2 and 3 was calculated as: d.e. (%) = l(%2 - %3) / (%2 + %3)l x 100.

217 3. RESULTS AND DISCUSSION 3.1. Characterization of the catalysts Representative TEM micrographs of 5%Rh/C and 3.7%Rh/Al203 catalysts are shown in Figure 2 a and b. Rhodium particles in the size range 1-3 nm were observed whatever the support. In 5%Rh/C catalyst some larger particles up to 8 nm were also detected but at high resolution these particles appeared to be formed by the agglomeration of smaller ones. Studies on ultramicrotome sections of the catalyst grains revealed that these large particles were gathered near the external surface of the support. In 3.7% Rh/Al203, rhodium was under the form of 1-2 nm particles distributed all over the alumina platelets. Their contrast was comparatively lower than that of carbon-supported particles, suggesting that lens-shaped particles interacting with the support were present. In 4.2%Rh/G, the metal particles (mean particle size 2 nm) were selectively located along the edges and steps of graphite layers as already observed earlier [10].

Figure 2. Transmission electron microscopy: (a) 5%Rh/C, (b) 3.7% Rh/AiPg. Figure 3 gives the XPS Rh 3d spectra of 5%Rh/C and 3.7% Rh/Al203 catalysts, without pretreatment (a) and after exposure to hydrogen at room temperature (b) or 3(X)°C (c). A comparison of the spectra revealed significant differences, depending on the carrier. Deconvolution of the asymmetric Rh 3d5/2 peak of the non-treated catalysts resulted in three overlapping peaks at 309.5, 308 and 307. leV, attributed to Rh"\ Rh^ and Rh^ respectively. The spectrum of Rh/C consisted of an intense peak corresponding to Rh^ species and a small signal of oxidized rhodium which totally disappeared after treating under flowing hydrogen at room temperature. In contrast, in the alumina supported catalyst, rhodium was in a much higher oxidation state. Initially, rhodium was present mainly in the form of Rh^ and Rh"' species (ca.80%). After exposure to hydrogen at room temperature, a significant increase of the intensity of the Rh^ peak was observed, but residual Rh'" and Rh' species were still present (ca.50%). Heating at 300°C completed the reduction. The total intensity of the rhodium signal did not change after catalyst pretreatment, allowing to conclude that the dispersion and distribution of the metal particles were not modified by the reduction treatment.

218

Rh/ALOa

320

314 308 Binding energy (eV)

302

314 308 Binding energy (eV)

302

Figure 3. XPS Rh 3d spectra of 5%Rh/C and 3.7% Rh/AiPgi (a) without pretreatment, (b) treated by H2 at room temperature, (c) treated by H2 at 300°C. In conclusion, the TEM and XPS studies indicate that on the Rh/Al203 catalyst, the rhodium particles tend to spread on the alumina platelets and to be positively charged probably because they are interacting with the electron acceptor sites of the oxide support. 3.2. Effect of the nature of the support The methyl ester 1 was hydrogenated in ethanol, with or without EDCA, over Rh/C, Rh/G and Rh/Al203 catalysts. The initial reaction rates are given in Table 1 and the values of diastereoisomeric excess (de.) as a function of conversion of 1 and consecutive hydrogenation of 6 are represented in Figure 4 (a-f). Table 1. Initial reaction rates on catalysts (mol.h \molRt^') in ethanol.

without EDCA EDCA/Rh = 3.5 EDCA/Rh = 5

5% Rh/C

4.2 % Rh/G

3.7% Rh/Al203

15.5 3.5 0.7

4.1 0.3 not measured

14.5 8.6 6.4

Without amine, the initial reaction rate on Rh/G was 3 to 4 times lower than on Rh/C and Rh/Al203 which have almost the same activity. This can be attributed to steric constraints hampering the approach of the aromatic ring toward the rhodium particles located along graphite steps. With the addition of EDCA to the reaction medium, the rates of hydrogenation were found kower on all catalysts, but this effect was more pronounced on carbon supports, particularly on graphite, than on alumina. The lower deactivation of the alumina-supported catalyst may be attributed to the acidic sites on the surface of alumina which act as adsorption sites for the basic amine, thus decreasing the amount of amine covering the metal surface. In support of that hypothesis, the rate of hydrogenation decreased rapidly over Rh/C as EDCA was progressively added, whereas it remained almost constant over Rh/Al203, which suggests that the amine was adsorbed first on the support rather than on the metal. The high deactivation of Rh/G catalyst is probably due to the adsorption of EDCA on the metal which further hinders the metal particles already in not easily accessible positions along the graphite steps.

219 d.e. (%) 60 I

d.e. (%)

I 60 b.

-«%—^ 40 O Rh/C Rh/C,EDCA

8 20 c\l

-o—o—o-

0

O

-

40

Rh/C Rh/C,EDCA

20

|o

-oo-H

"^^^^—-————5

si

O-20 ^ 0

1 1 1 -20 40 60 80 100 conversion of 6 (%) 1

20 40 60 80 conversion of 1 (%)

100

1

0

d.e. (%)

20

1

1

1

de(%)

DU

c

Rh/G Rh/G,EDCA

CO

J

d

n

40

Rh/G

-

I 60

.

40

. - 20

§20 <M

0

c rN

1

on

1

I

|o

D

—n

Hi

1

1—1

1

1

20 40 60 80 conversion of 1 (%)

1

100 0

1

20

1 -20 40 60 80 100 conversion of 6 (%) 1

1

1

d.e. (%)

d.e. (%)

60

e J 40

CO

A

^

A

-A

Hi

0|

R-20

'^

A

A A 1 ..

A

A

A

A

-K-A—A

A

A-AT

Rh/AI203 Rh/AI203,EDCA 1

1

1, , i

1—

20

] 1

20 40 60 80 conversion of 1 (%)

A A

1

100

0

20

Rh/AI203 Rh/AI203,EDCA

0

-20 40 60 80 100 conversion of 6 (%)

Figure 4. Variation of the diastereoisomeric excess (%) with conversion of 1 and consecutive conversion of 6, using unmodified or modified (EDCA/Rh ca. 3.5) rhodium catalysts. As far as diastereoselectivity is concerned, active carbon and graphite catalysts showed the same trends (figure 4 a-d). Without EDCA, the methylester 1 was hydrogenated to isomers 2 and 3 with a negligible d.e. up to total conversion of 1. A substantial amount of 6 (20-25%) was formed which was hydrogenated preferentially to 2 when the conversion of 1 was completed. Thus, the d.e. increased to attain ca. 15-17% in favour of 2. The addition of EDCA lead to obvious changes, by inverting the selectivity in favour of 3 and the d.e. values were 47% and 8% for Rh/C and Rh/G catalysts, respectively. The behaviour of 3.7% Rh/Al203 was quite different (figure 4 e-f). Regardless of the presence of EDCA, the diastereoselectivity was in favour of 3 (38% and 30%, respectively) from the start of the reaction. Then, the d.e. value remained relatively constant during the course of the hydrogenation of 1, but it slightly decreased as the cyclohexenic compound 6, formed in

220

10% yield, was preferentially hydrogenated into isomer 2. The weak effect of the amine addition on the selectivity of Rh/Al203 could be explained by the low EDCA coverage of the metal surface since the amine is mainly adsorbed on alumina as discussed previously to account for the weak activity decrease (vide supra). To interpret the different diastereoselectivities obtained in the absence of EDCA on Rh/AljOg compared to Rh/C and Rh/G, three hypotheses can be put forward, viz: (i) The XPS study indicated that rhodium particles supported on alumina were slighdy oxidized even in the presence of hydrogen at room temperature, i.e. under reaction conditions. Electropositive rhodium could interact more strongly with substrate 1 either via the lone pair of electrons of the oxygen atoms or via the 7C-electrons of the aromatic ring. These stronger interactions might orientate the adsorption of the aromatic via a specific face thus exerting a diastereoselective control favouring the formation of isomer 3. (ii) A flat morphology of the rhodium particles on alumina may favour the adsorption of 1 via the less hindered face of the aromatic ring. There are some TEM indications that the morphology of rhodium particles are lens-shaped rather than spherical (vide supra). However, the Rh-particles in Rh/AljOj are quite small (1 to 3 nm), and therefore, not good models for a flat surface. Large facetted rhodium particles would be required to ascertain that metal surfaces can exert sterical constraints favouring the adsorption of the aromatic ring on a specific side, thus controlling the diastereoselectivity. (iii) The substrate may adsorb on the catalyst in such way that the aromatic ring interacts with surface rhodium atoms while the functional groups of the proline moiety interact with the oxygen anions of the alumina surface. This situation is not unlikely since the Rh-particles are small and lens-shaped. The preferential adsorption of the aromatic ring on a given face will then be mainly driven by the stereochemistry of the interaction (e.g., via hydrogen bonding) of the proline groups with the alumina surface. 3.3. Effect of pretreatments Attempts have been made to determine the optimum reduction conditions for obtaining the maximum diastereoselectivity. The ^h/A\20^ samples were heated to different pretreatment temperatures at l°C.min"' and kept for 3 h under flowing hydrogen, then cooled to room temperature under argon and transfered into the reactor. The hydrogenation reactions were then carried out in ethanol, with addition or not of EDCA. The initial rates of hydrogenation and the diastereoselectivities are given in Figure 5 a and b, respectively, as a function of hydrogen pretreatment temperatures. o -^^

b A

a A A

Rh/AI203 Rh/AI203,EDCA

E ^ 10 Ti02 (anatase/rutile 73/27) > Si02 > Ti02 the surface acidity decreases while the stereoselectivity increases. The conârison of runs 3 and 4 of Table 3 show that small changes of the reaction parameters can result in an improvement of the chemo- and/or stereoselectivity. A similar effect is observed in runs 8-10. Table 4 Influence of the kind of solvent on chemo- and stereoselectivity Run

Solvent

Reaction time

Conversion

[mini

[%1

Selectivity to amines

cis/trans ratio ammes

1

none

60

24.2

96.3

3.89

2

cyclohexane ^

330

99.4

82.4

2.80

3

isopropanol

300

99.8

69.2

2.27

Reaction parameters: alkylphenol R = 4-t-C4H9, 0.1 mol/mol phenol LiOH, 0.1 mol N-ethyl morpholine/mol phenol, 1% Pt + 4%Rh/C catalyst. ^ Residual conôsition: mainly alkylcyclohexanols, traces of alkylcyclohexane. ^ T = 423 K. Another parameter influencing the selectivity is the kind of solvent. If aprotic solvents like cyclohexane or no solvent were used under the same conditions, the activity was decreased, but both the chemo- and the stereoselectivity were increased considerably (see Table 4). The high chemoselectivity is in agreement with earlier results using phenol[l 1]. Table 5 Influence of the kind of alkyl substituent on chemo- and stereoselectivity Run

Substituent

Reaction time

Conversion

Selectivity to amines ^

[min]

r%i

r%i

cis/trans ratio amines

1

4-ethyl

120

> 99.9

75.8

n.d.

2

4-methoxy

140

95.0

42.9

1.18

3

4-isopropyl ^

100

> 99.9

76.0

2.63

4

2-tert. butyl ^

60

> 99.9

0

0

5

4-tert. butyl ^

120

99.7

74.4

2.48

6

4-tert. amyl

150

99.9

62.9

2.24

Reaction parameters: alkylphenol R = 4-t-C4H9, 0.1 mol LiOH/mol phenol, 1% Pt + 4% Rh/C catalyst. ^ Residual conq)osition: mainly alkylcyclohexanols, traces of alkjdcyclohexane. ^ 5% Rh/C catalyst.

228 Surprisingly, the bulkiness of the 4-substituent on the phenol ring did not have a significant influence on the cis/trans ratio of the formed alkylcyclohexylamines at the same reaction conditions, whereas a bulky substituent in 2-position made the reduction of the intermediate cyclohexanone more difficult. After one hour of reaction time, 2-tert. butylcyclohexanone was the only product formed. Prolonged reaction led to the slow formation of 2-alkylcyclohexanols. In the case of the methoxy-substituted phenol, hydrogenolysis of the ether group took place.

4. CONCLUSIONS Rhodium catalysts as well as bimetallic catalyst formulations with platinum have been found most suitable for the one-step hydrogenation of 4-alkylphenols to the corresponding cis-4alkylcyclohexylamines in the presence of ammonia (see Figure 2).

NH9 +NH3 -H9O

2H2

cat.

H2

H. cat.

H

NHo

cat. OH H

H

_^

H OH

Figure 2 Reaction scheme for the reductive amination of alkylphenols Particularly, the addition of bases - organic and inorganic - reduced the amount of alkylcyclohexanols as main by-products. Both the chemo- and the stereoselectivity of that reaction were in^roved by proper adjustment of these and other parameters such as solvent, support acidity and catalyst formulation. For optimum results a fine tuning of the parameter set will be necessary.

229 REFERENCES 1. M. Bartok et. al. Stereochemistry of Heterogeneous Metal Catalysis, John Wiley & Sons, Chichester, 1985,251-290. 2. R. Burmeister, A. Freund, P. Panster, T. Tacke and S. Wieland, Stud. Surf. Sci. Catal., 92 (1995), 343. 3. A. Tungler, T. Mathe, J. Petro and T. Tamai, Appl. Catal.,79 (1991), 161. 4. M. Freifelder, Practical Catalytic Hydrogenation, Wiley-Interscience, New York, 1971, 168 - 172. 5. P.N. Rylander, Hydrogenation Methods, Academic Press, London, 1985, 123 - 132. 6. M. Bartok et. al.. Stereochemistry of Heterogeneous Metal Catalysis, John Wiley & Sons, Chichester, 1985,419. 7. BASF AG, EP Patent No. 0 053 819 (1984). 8. Mitsui Toatsu Chemical, JP Patent No. 34677 (1974). 9. Air Products and Chemicals, Inc., EP application No. 0 392 435 (1990). lO.Firmenich SA, EP Patent No. 0427 965 (1990). 1 I.Abbott Laboratories, DE Patent No. 1 276 032 (1969).



231

SELECTIVE REDUCTION OF NiTRO GROUPS IN AROMATIC AZO COMPOUNDS M. Lauwiner, R. Roth, P. Rys and J. Wissmann Chemical Engineering & Industrial Chemistry Laboratory, Swiss Federal Institute of Technology (ETH), CH-8092 Zurich, Switzerland.

1. SUMMARY This work deals with the selective reduction of aromatic nitro compounds to the corresponding aromatic amines with hydrazine hydrate in the presence of catal3rtic amounts of a modified iron oxide hydroxide compound. The dependence of the rate of reduction on the nature and the position of additional substituents other than the nitro group was determined. The rate is enhanced by electron-withdrawing substituents and decreased by electron-donating groups. Moreover, our study on the range of application of this cheap iron oxide hydroxide modification as a H-transfer catalyst opened up a promising new route for the selective reduction of nitro groups in aromatic azo compounds. A series of monosubstituted 4-nitroazobenzenes were selectively reduced by hydrazine hydrate in the presence of the iron oxide hydroxide catalyst. The selectivity for the reduction of the nitro group vis-a-vis that of the azo bridge was increased with stronger electron-withdrawing properties of the substituent R. For 4-nitroazobenzenes with electron-donating substituents the rate constants of the reductive cleavage of the azo bridge and of the nitro group reduction are of the same order of magnitude. For the reduction of the nitro group (1) in the unsubstituted 4-nitroazobenzene and for the reductive cleavage of the azo function (3) in the corresponding 4aminoazobenzene (Scheme 1), the Arrhenius activation energies were determined to be Ea(l) = 81.5 kJ/mole and Ea(3) = 46.6 kJ/mole, respectively. Thus, the selectivity for the reduction of the nitro group vis-a-vis that of the azo function can be enhanced by higher reaction temperatures.

2. INTRODUCTION For the reduction of nitroarenes to aminoarenes by the catalytic hydrazine Htransfer reduction method, the classical hydrogenation catalysts Ni, Pd and Pt are most commonly used [1] [2]. In a more extended study [3] we were able to confirm previously reported observations [4] that these reductions can also be catalysed by modified iron oxides hydroxides. This method for the production of many aromatic amines offers several advantages compared to the conventional processes still employed in industry, such as the environmentally unfavourable Bechamp [5] and Zinin reductions [6]. It is an outstanding feature of the novel reduction method presented here that further reducible substituents in nitroazo compounds, such as

232

e,g, azo groups, are not reduced as long as the reaction conditions are carefully controlled. Aromatic azo compounds carrying free amino groups are widely used in the production of reactive dyes or as starting materials for further diazotizations in manufacturing polyazo dyestuffs. A possible way to synthesise these aminoazo compounds is to reduce selectively the corresponding nitroazo compounds. Nowadays, such a selective reduction is carried out at industrial scale using salts of sulphides, e.g. sodiima sulphide (Zmm-reduction) [6]. This process is cheap, highly selective, and the desired aminoazo compounds can be obtained in high yield. However, large quantities of waste products are disposed in an ecologically unfavourable way. Moreover, at low pH-values, the evolving of H2S gas might endanger the operating personnel. The aim of the work presented here was to examine the influence of additional substituents on the rate of the cataljîc reduction with hydrazine hydrate of nitrobenzenes and of a variety of nitro arenes carrying phenyl azo substituents. Particular interest was focused on the influence of substituents in nitroazo compounds on the selectivity for the reduction of the nitro group (1) vis-a-vis that of the azo bridge in reaction (2) or (3) (Scheme 1).

Scheme 1. Possible reactions of hydrazine hydrate with nitroazo compounds. 3. RESULTS AND DISCUSSION 3.1. Influence of substituents on the catalytic reduction of nitroarenes

2 0"^

+3 N2H4H2O

^ 2 K l + 3 N2 + 7 HoO FexOy(OH), ' '^"2

Scheme 2. Catal3rtic reduction of substituted nitrobenzenes.

233

The influence of substituents on the reduction rate of nitro arenes is most conveniently represented by the following Hammett ap-relationship, where ko = 1 mole/(l-min). log—= d p Thus, the initial pseudo 0*^-order rate constants of the catal5d;ic reduction of a variety of substituted nitrobenzenes were determined applying a tenfold excess of hydrazine hydrate.

,

-2.4-

P-Cl

-2.6

/ôm-CF3

-H y^ ^/Om-OCH3 yVm-CH3

-2.8

P-CH3

/

P-OCH3

-3 rm-OH / ^ -NH2 -3 9

!

1

1

-0.5

0 a

0.5

1

Figure 1. Hammett plot for the reduction of substituted nitrobenzenes by hydrazine hydrate in the presence of the iron oxide hydroxide catalyst. The a-values are taken from [7]. ko = 1 mole/(l-min). The corresponding Hammett plot (Figure 1) reveals that the rate of reaction is enhanced by electron-M;j^/i(irau;m^ substituents and decreased by electron-donating groups. The resulting slope of the op-relationship obtained by linear regression is p = 0.546 (correlation coefficient for 15 substituents: r^ = 0.994).

234

3.2. Influence of substituents on the selectivity and the reaction kinetics in the reduction of substituted 4-nitroazobenzenes As shown in Figure 1, the reduction of nitrobenzenes with the phenylazo substituent in para position, Le, of 4-nitroazobenzene» is approximately as fast as the reduction of 1,3-dinitrobenzene. It is interesting to note that the nitro group, and not the azo bridge, is selectively reduced. This fact is somewhat surprising considering the structiu'al similarity between the azo bridge, the hydrazine, and their respective reduction or oxidation products. In order to evaluate the influence of substituents on the selectivity behaviour of the described catalytic reduction of the nitro group vis-a-vis the reductive cleavage of the azo bridge, the Hammett plots for the competitive reductions (1), (2) and (3) of substituted 4-nitroazobenzenes were determined (Figure 2).

-3

-3.5

-4.5 +

-5 -0.5

0 a

0.5

Figure 2. Hammett plots for the competitive reductions (1), (2) and (3) of substituted 4-nitroazobenzenes by hydrazine hydrate in the presence of the iron oxide hydroxide catalyst. The a-values are taken from [7]. ki(0), k2(A), kaCo), ko = 1 mole/fl-min). For nitroazo compoimds with electron-withdrawing or weakly electron-do/iaî/i^ effects {e,g, R = -CI, -H, -CH3), the reduction of the nitro group is highly favoured.

235 Cleavage of the azo function in 4-nitroazobenzenes (2) could not be observed. After the corresponding aminoazobenzenes were formed, the subsequent reduction step (3) takes place, yet very slowly. For substituents with strong electron-dona^m^ properties (e,g. R = -NH2, -NHPh, -N(CH3)2 or -OH), the rates of the nitro group reduction (1) and of the cleavage of the azo function (2) in 4-nitroazobenzenes are of the same order of magnitude. The p-values of the Hammett plots for the different reactions (1), (2) and (3) are Usted in Table 1. Table 1 p-Values for the steps (1), (2) and (3) in the reductions of substituted 4-nitroazopenzenes by hydrazine hydrate in the presence of an iron oxide hydroxide catalyst. Reaction

Nimiber of substituted azobenzenes examined

p-Value

Correlation coefficient (r^)

9 4 10

1.25 0.61 0.29

0.989 0.999 0.923

Nitro reduction (1) Azo cleavage* (2) Azo cleavage** (3)

*azo cleavage in the substituted 4-Mi^roazobenzene **azo cleavage in the substituted 4-a7nmoazobenzene Table 2 Yields of the cataljrtic reduction of substituted 4-nitroazobenzenes with a stoichiometric amount of hydrazine hydrate.^>^ Substituent R m-Cl p-Cl -H m-CHs P-CH3 p-NHPh p-N(CH3)2 P-NH2 p-OH

a-Value [7] +0.373 +0.227 0 -0.069 -0.170 -0.450 -0.600 -0.660 -0.920

Yield/[%]

C(0R)2 + H2O

(1)

/ is an irreversible reaction on the alumina support. When the catalyst was activated in the

276

(a)

60

120

180

240

300

time/min

(b)

0

60

120

180

time/min

240

300

277

(c)

60

120

180

240

300

time/min

Fig. 2. The effect of citral concentration on the hydrogenation kinetics of citral at 70 "C. Symbols: (a) 0.05 M citral, (b) 0.1 M citral and (c) 0.2 M citral, ) citral (cis and ) citronellal ) citronellal diethylacetal and (A) citronellol. The continuous lines represent the fit of the model to the experimental data. current experiments at 350 "C there was not enough spillover hydrogen on the acidic catalyst surface and thus large amounts of acetal was formed. When the catalyst was activated at a higher temperature for a longer time the acetal formation was suppressed due to the more extensive formation of spillover hydrogen [1, 4]. With the same catalyst, which was reduced at 500 °C for 2 hrs and then at 350 °C for 2 hrs before the experiment the citronellal acetal formation could be lowered to 2 mol-% [1].

3.2. Modelling of the kinetics The following reaction scheme was used in the modelling of the reaction kinetics:

AT-

As A4

9/'

-*A8

(2)

278

where Aj and A2 denote the cis- and trans-citral, respectively, and A3 and A4 are the corresponding primary hydrogenation products, nerol and geraniol, respectively. A5 and A^ are citronellal and its acetal, A7 is citronellol and Ag is the final hydrogenation product, 3,7-dimethyloctanol. The cis-trans-isomerization (Ai'*A2) as well as the hydrogenation of nerol (A3->A7) and geraniol (A4->A7) were neglected in the quantitative treatment because these reactions were of very minor importance. The stoichiometry of the hydrogenation steps 1-5 and 7-9 can be written as follows A. + H2 -^ A-

(3)

and the stoichiometry of the acetalization step 6 is A5 + 2 ROH -> A^ + H2O

(4)

For each hydrogenation step the rate equation was assumed to have the simply first-order form with reagent (i): ^k= k^^""

cr KCi

(5)

where the product k^c^^J^ was lumped to a pseudo-constant, because the hydrogen pressure in the gas phase was constant in all experiments (PHO" ^ ^^^^ ^^^ ^ ^ solubility of hydrogen in the liquid phase was exclusively determined By its solubility in the solvent (the substrate concentrations were always very low compared to the solvent concentration). For the sake of simplicity, also the acetal formation reaction (reaction 6 in eq. 2) was assumed to follow first order kinetics with respect to citronellal (A5). The generation rates of the compounds were obtained from the stoichiometry (6) ^ = E (v.k^k) where v-j, denotes the stoichiometric coefficient of compound A- (i= 1, ...8) in the reaction k. The mass balance of hydrogen was ignored, since its concentration was assumed to be constant in the liquid phase. The generation rates of the compounds are related to their mass balances in the liquid phase. The mass balance for a compound can be written as follows: dq = PB^.

(7)

d^ where pg = m^JVi^, m^^^ and V^^ denote the mass of the catalyst and the liquid volume, respectively.

279

Based on this simple kinetic model, the rate constants (k^) were determined from the experimental data with nonlinear regression analysis. The objective function (Q), which was minimized in the regression, was defined as follows

2=EE

(8)

(cu-^f

where c- ^ is the experimentally recorded concentration of compound i at the reaction time t and q^ is the corresponding concentration predicted from the model (eq. 7). The reactor model equations (7) were solved numerically during the parameter estimation, using a semi-implicit Runge-Kutta method (Rosenbrock-Wanner method) suitable for stiff differential equations [5]. The objective function was minimized with Levenberg-Marquardt method [6]. The algorithms are available in the software package Reproche [7], which was used in all estimations. Some preliminary results from the estimation of the kinetic parameters are depicted in Figs 1 and 2, which represent the long time experiment as well as experiments with different substrate concentrations. The values of the kinetic parameters are listed in Table 1. As can be seen from the figure, the simple kinetic model is able to describe the main trends of the experimental data. Obviously some systematic deviation remained between the experimental and the fitted concentrations. This may be due to problems with the parameter estimation, which is going to be improved in the future.

Table 1. Kinetic parameters at 70 "C. parameter

Long time experiment

^0,citrar 0-05 M

Co, ,,^,^1= ^' ^ ^

CQ, îtrar ^'^

PBÎ

0.6210-^

0.10

0.6310-^

0.8310-^

PB^2

0.5710-^

0.9710-^

0.5710'^

0.8210-^

PB^3

0.6310-^

0.3910-^

0.5710-^

0.5110-^

PB^4

0.6910-'^

0.1610-'^

0.1410-^

0.3410-^

PB^5

0.3810-^

0.8010-^

O.lllO'^

0.8610-'^

PB^6

0.1510-^

0.3210-^

0.2310-^

0.3110-^

PB^7

o.ioio-^

0.3510-^

0.2110-^

0.1510-^

^

The lumped parameter k^ = p^ky., where PB= m^^JVi^; ^c^r ^-^^ § ^"^ ^L"^ 0.075 1. The unit of the k^ -values is min" .

280

4. CONCLUSIONS Based on the experimental information we can conclude that the main routes for the hydrogenation of citral over alumina supported nickel goes through citronellal to citronellol and finally to 3,7-dimethyloctanol, whereas the contribution of nerol and geraniol to the hydrogenation path is of very minor importance. The hydrogenation kinetics was described with first order rate laws with respect to the substrate molecules [2]. Acknowledgements The authors are grateful to Dr. R. Sjoholm and Mr. M. Reunanen (Abo Akademi) for the NMR and GC-MS analyses.

REFERENCES 1. Maki-Arvela, P., Tiainen, L.-P., Salmi, T., 8th International Symposium on Heterogeneous Catalysis, 5-9.10. 1996, Varna, Bulgaria. 2. Maki-Arvela, P., Tiainen, L.-P., Salmi, T., Vayrynen, J., 7th Nordic Symposium on Catalysis, 2-4.6. 1996, Turku, Finland, Book of abstracts, 017. 3. March, J., Advanced Organic Chemistry, J. Wiley&Sons, Third Ed., New York, 1985, 789. 4. Kramer, R„ Andre, M., J. CataL, 58, 1979, 287-295. 5. Kaps P., Wanner G., Num. Math. 38, 1981, 279. 6. Marquardt, D. W., An algorithm for least squares estimation on nonlinear parameters. SIAM J., 11, 1963,431-4. 7. Vajda, S. and Valko, P., 1985, Reproche - Regression Program for Chemical Engineers, Manual, European Committee for Computers in Chemical Engineering Education, Budapest, 1-36.


281

Selective hydrogenation of a,p-unsaturated aldehydes to allylic alcohols over supported monometallic and bimetallic Ag catalysts P. Claus^, P. Kraak'' and R. Schodel^ Înstitut fur Angewandte Chemie, Abteilimg Katalyse, Rudower Chaussee 5, D-12489 Berlin, Germany KataLeuna GmbH - a Member of the Tricat Group, D-06236 Leima, Germany

Summary Monometallic silver catalysts prepared by various techniques were found to control the intramolecular selectivity of the gas phase hydrogenation of crotonaldehyde by favoring the hydrogenation of the C=0 group compared to the C=C group which gave crotyl alcohol selectivities around 55 to 63 %. For sol-gel derived Ag/Si02 catalysts the influence of the hydrolysis conditions on the structural properties has been studied. They showed a narrower Ag particle size distribution at significantly lower crystallite sizes than catalysts prepared by impregnation or precipitation-deposition. Ag catalysts further modified by (i) alloying with a late transition metal and (ii) promoting with an early transition metal or a rare earth metal have been found to increase the selevctivity towards crotyl alcohol up to 85 %. 1. INTRODUCTION The selective hydrogenation of a,(3-unsaturated aldehydes to the corresponding allylic alcohols is an important reaction for the industrial production of fine chemicals as well as for fundamental research in catalysis. In the presence of conventional hydrogenation catalysts (e.g. supported Pt, Pd, Cu), a,P-unsaturated aldehydes are hydrogenated predominantly to the saturated aldehydes or even to saturated alcohols. Therefore, it is desirable to find catalysts that will control the intramolecular selectivity by hydrogenation preferably the C=0 group while keeping the olefmic bond intact. Several attempts have been made to develop a suitable catalytic system. Selectivity towards allylic alcohols was improved e.g. by metal ion additives in liquid phase hydrogenations [1,2], electron donor ligands [3] and SMSI effects [4], steric constraints on the metal surface or in the metal environment [5,6], and it depends on the nature and position of substituents on the C=C and C=0 group [7]. As shown recently durmg crotonaldehyde hydrogenation on a series of silica supported Rh-Sn alloy catalysts, the selectivity to trans- and cis-crotyl alcohol greatly increases with increasing tin content, reaching values between 65 and 74 % provided that the Sn/(Sn+Rh) atomic ratio is more than 40 % [8]. It is interesting to note that on silver surfaces a strong interaction of acrolein and allyl alcohol was observed by HREELS and TPD [9]. NEXAFS and TPSR studies indicated a bifunctional bonding on the surface resuhing in the formation of an allyloxy intermediate

282 [10,11]. Moreover, during the interaction of allyl alcohol with Ag surfaces hydrogenation of the C=C group did not occur [10]. Thus, consecutive hydrogenation of allylic alcohols to saturated alcohols would be expected to be prevented by Ag catalysts. The objective of the present study was to find out the potential of Ag catalysts for the selective hydrogenation of the conjugated C=0 group and, thereby, how varying the procedures for preparation of monometallic Ag catalysts and modifying Ag with a second metal effect the activity and intramolecular selectivity of these catalysts during the hydrogenation of a,Punsaturated aldehydes. Cadmium was selected in order to alloy because it forms solid solutions with Ag and has a lower electronegativity than Ag, which would be expected to influence the mode of adsorption of the a,P-unsaturated aldehyde by creating surface polarity. Ternary alloys (Ag-Cd-Zn) were found to hydrogenate acrolein to allyl alcohol [12]. Because Mn and La are known to be efficient promoters of the hydrogenation of CO [e.g. 13], they were additionally selected for preparing the catalysts of this study. 2. EXPERIMENTAL Ag based catalysts used in this study were prepared by three different methods. 1. Sol-gel derived catalysts: A uniform solution of Si(OC2H5)4 (tetraethoxy orthosilicate, TEOS) and AgNOs (bothfiromFluka) was obtained by dissolving them into a mixed solvent consisting of ethanol and water (molar ratio 1:2). Then, the hydrolysis and gelation of TEOS was carried out in acidic, neutral and basic media. The catalysts prepared by this method are denoted as Ag/Si02-SG1/A (16.1 mole% Ag), Ag/Si02-SG6/N (11.6 mole% Ag), and Ag/ Si02-SG7/B (11.2 mole% Ag), respectively. In the case of acid-hydrolyzed TEOS the pH was kept constant at 3.0 by adding several drops of IN HNO3 to the hydrolyzing solution, whereas in basic media the pH was adjusted to keep constant at 9.0 using ammonia. After refluxing the sol mixtures at 353 K white, grey and black gels were formed in the case of Ag/Si02-SG1/A, Ag/Si02-SG6/N and Ag/Si02-SG7/B, respectively, after a period of 30 to 60 min. The mixtures were continuously stirred for 1 h, and ethanol was removed at 313 K under reduced pressure using a rotary evaporator. Then, the sol-gel derived precusors were aged for a period of 48 to 72 h and subsequently dried at 373 K for 20 h. Finally, the samples were calcined in air at 673 K (4 h) and reduced in flowing hydrogen (5 1 h'^) at 623 K (3 h) to give the final catalysts. 2. Ag catalysts by impregnation: Catalysts prepared by conventional impregnation are denoted as Ag/Si02-IMP (14.5 mole% Ag), and Ag/Al203-IMP (4.6 mole% Ag), respectively, using silica (Aerosil 200, Degussa) or alumina (Aluperl 1540, Kalichemie). For the preparation of the former, an aqueous solution of AgNOa was used, and the catalyst was calcined at 448 K and reduced at 523 K. The latter was prepared using silver lactate. This catalyst was calcined at 463 K and reduced at 473 K. 3. Ag catalysts by precipitation-deposition: The catalysts denoted as Ag/Si02-P (71.7 mole% Ag), Ag-Cd/Si02-P (39.4 mole% Ag, 20.3 mole% Cd), Ag-Mn/Si02-P (62.5 mole% Ag, 21.1 mole% Mn) and Ag-La/Si02-P (40.5 mole% Ag, 15.9 mole% La) were preparedfi*omaqueous solutions of the corresponding metal nitrates and of sodium hydroxide at 353 K. Silica was dispersed in the suspension with vigorous sturing. After stirring for 6 h and standing overnight, the mixtures were filtered and washed with 1500 ml deionised water. The precursors were dried for 8 h at 373 K and reduced at 623 K.

283

The metal contents of the Ag catalysts were determined by atomic emission spectroscopy with inductive coupled plasma (AES-ICP). Nitrogen adsorption/desorption isotherms at 77 K and BET surface areas, respectively, were obtained after outgassing of the samples at 473 K for 2 h, using a Sorptomatic 1990 (Fisons). XRD patterns were created without exposure to air on a URD 6 diffractometer (Seifert FPM) using monochromatic Cu-Ka radiation. Mean crystallite sizes of monometallic catalysts were calculated from the Ag(lll) reflection, using the Scherrer equation. The metallic particle size was determined by transmission electron spectroscopy using a JEOL lOOC microscope. The gas phase hydrogenation of crotonaldehyde (Aldrich, destilled before use) was carried out in a fiilly computer controlled fixed-bed microreactor system at temperatures between 413 K and 533 K and a total pressure of 2 MPa which has been described in detail elsewhere [14]. The reactor effluent was analyzed on-line by means of an HP 5890 gas chromatograph equipped with a flame ionization detector. The separating column was a 50 m J&W DB-WAX capillary column, operated between 353 and 573 K. 3. RESULTS and DISCUSSION 3.1. Monometallic Ag catalysts Representative isotherms from three sol-gel derived Ag catalysts prepared under different conditions of pH are shown in Fig. 1. 120^-77

1 300^

1 600T

SBET = 259 Vp = 0.75 VM=0.07

,-^100

Figure 1. Nitrogen isothermes of sol-gel derived Ag/Si02 catalysts if hydrolysis was perfor2

1

1

med under acidic (la), neutral (lb) or basic (Ic) conditions (SBET in m g ;Vp, VMinmlg ). The isotherm of the Ag/Si02-SG1/A catalyst prepared via the acid-hydrolyzed route was of type I [15] as shown in Fig. la indicating the presence of a microporous system. The micropore volume (VM) represents about 88 % of the total pore volume (Vp). The distribution of the micropores determined by applying the method of Horvath and Kawazoe [16] was monomodal with a pore-size maximum of 0.5 nm. In the case of the catalysts prepared at higher pH (Fig. lb and Ic), the isothermes are of type IV and HI hysteresis according to lUPAC classification [15]. They show pronounced meso- to macroporosity with only little microporosity around 10 to 20 %. The average pore size of the catalysts Ag/Si02-SG6/N and Ag/ Si02-SG7/B produced with increasing pH of the hydrolyzing solution was 11 and 68 nm, respectively. Moreover, a decrease of the BET siuface areas and an increase of the pore volumes under basic sol-gel conditions was observed. The Ag catalysts prepared by impregnation and precipitation-deposition using commercial supports were mesoporous. The Ag particles of the reduced sol-gel derived catalyst Ag/Si02-SG1/A had a mean size of about 8 nm as determined from the line broadenmg of the Ag(l 11) reflection during the XRD

284 measurements which is significantly smaller than those of the mesoporous Ag catalysts prepared by precipitation-deposition and unpregnation (Âg= 30 nm). Furthermore, Fig. 2 shows that catalysts prepared by the sol-gel method exhibit a narrow Ag particle size contribution. A representative TEM micrograph of catalyst Ag/Si02-SG1/A is presented in Fig. 3 indicating the presence of smgly-twinned and multiply-twinned Ag particles on a nanometer scale. It is important to emphasize that these Ag catalysts were able to control the intramolecular selectivity of the hydrogenation of crotonaldehyde by favoring hydrogenation of the C = 0 group leading to crotyl alcohol (CyOH) compared to the C=C group (Tab. 1). The monometallic Ag catalysts gave selectivities up to 63 % CyOH. This is an unexpected result because monometallic catalysts (e.g. Pt [4,7,17], Rh [8], Ru [18]) with nonreducible supports would not be expected to produce CyOH as main product under gas phase conditions. Altough a direct comparision between the catalysts studied is not possible it seems that crystallite size effects on the CyOH selectivity do not exist with Ag catalysts [19]. Tablel Catalytic properties'^ (at p = 2 MPa, H2/CA = 20) of reduced monometallic Ag catalysts prepared by sol-gel technique, impregnation and precipitation-deposition catalyst Ag/Si02-SG1/A Ag/Si02-SG6/N Ag/Si02-SG7/B Ag/Si02-P Ag/Si02-IMP Ag/AbOa-IMP

T 413 413 413 453 453 453

XcA [%] 11.1 16.3 18.6 91.1 51.0 6.6

ScyOH

SBA

SBUGH

SEMK

[%1

[%]

[%1

[%]

56.2 62.8 57.0

26.8 35.2 40.2

10.0 1.0 1.0

4.8 0.1 0.2

58.5 60.4 47.6

29.3 37.8 45.5

11.5 1.3 0.8

-

SOP

tA

2.2 0.9 1.4 0.7 0.5 6.1

16 32 37 62 172 69

^ XcA= conversion of crotonaldehyde, S = selectivities of crotyl alcohol (CyOH), n-butyraldehyde (BA), n-butanol (BuOH), ethyl methyl ketone (EMK), other products (OP = hydrocarbons, allylcarbinol, 2-butanol, 2-ethylhexanal) rcA = catalyst activity [10 /mol g^g h~ ]

Considering the group of monometallic catalysts only those with partially reduced supports (e.g. Pt/Ti02 in the SMSI state [4]) showed remarkable selectivities toward crotyl alcohol as yet. Vannice [20] obtained a selectivity of 37 % with a high-temperature reduced Pt/Ti02 catalyst having an average crystallite size of 1.5 nm [20]. The catalyst pretreatment of Lercher et al. [21] led them to conclude that larger Pt particles are more active and more selective than Pt/Ti02 catalysts with small Pt particles. Interestingly, during acrolein hydrogenation over RU/AI2O3 at very low conversions (< 1 %) allyl alcohol selectivity did not change much with Ru particle size [18] which is similar to the results of the present study. Moreover, the sol-gel derived Ag catalysts prepared at higher pH (Ag/Si02-SG6/N and Ag/ Si02-SG7/B) exhibit a higher steady-state activity at 413 K than the microporous catalyst produced under acidic sol-gel conditions. Because the Ag catalysts were used as powders the higher activity suggests that the considerably wider pores (11 and 68 nm) of tiie meso- to macroporous Ag sol-gel catalysts give a higher effectiveness factor and thus minimize effects of pore diffusion control. Further experiments are necessary to clarify this point.

285

frequency

[%]

frequency

[%]

Ag/Si02-SG1/A Ag/Si02-P Ag/Si02-IMP Ag/Al203-IMP Figure 2. Ag particle size distribution determined by XRD from Ag(l 11) reflection in Ag catalysts prepared by sol-gel method (SG), precipitation-deposition (P) and impregnation (IMP)

Figure 3. High-resolution TEM micrograph of the Ag/SiOi-SGl/A catalyst prepared under acidic solgel conditions (STP: singly-twinned particles, MTP: multiply-twinned particles).

3.2. Ag catalysts modified by a second metal Powder X-ray pattern of the reduced catalysts Ag/Si02-P, Ag-Cd/SiOi-P, Ag-Mn/SiOi-P and Ag-La/Si02-P are represented in Fig. 4a. In the case of Ag-Cd/Si02-P they show clear evidence that the formation of two different Ag-Cd alloys had been achieved and that neither reflections due to discrete Ag or Cd crystallites nor due to oxide species were observed. Compared to the monometallic catalyst, the lattice constants obtained after deconvolution of the (220) reflection increased from 0.4081 nm to 0.4095 nm and to 0.414 nm for the reflecions at 20 = 64.279° and 20 = 63.480^ respectively.

286 The former indicates the formation of an a-AgCd phase [22] for which a Cd content of 21.8 at.% was estimated, and the latter corresponds to a cubic alloy structure which is richer in Cd [19]. Their mean crystallite sizes were 11.4 nm and 18.2 nm, respectively, which is significantly lower than that of Ag particles in Ag/Si02-P (27.6 nm). 10n

(^; 1)

(a)

(b)

XRD

TPR

*

8

6i

(220)

(0

c

I jl

»

/

V

[

^

.,.-.j2)

-J

®.

^ 30

2

it

^ ^ ,

®.'^l

Q

® 20

KD

E 1

(200)

,

^

40

, 20/^

^

50

,

^ 60

,

70

n-

T^'^V-.----

/ -^

\j:^=^

xoTT

300 350 400 450 500 550 600 650 700 T[K]

Figure 4. (a): X-ray diffraction patterns of modified Ag catalysts: ®Ag/Si02-P, ®Ag-Cd/ Si02-P, (DAg-Mn/Si02-P, ®Ag-La/Si02-P; reflections: (*) Ag; (#) and (T) Ag-Cd alloys at 20 = 63.480° and 20 = 64.279°, respectively; (b): TPR of these catalysts. The X-ray pattern for Ag-Mn/Si02-P and Ag-La/Si02-P showed low-intensity reflections of Ag, but no evidence of the formation of manganese oxides or La203. This implies that dispersed Mn and La oxides, if formed from the precipitates during reduction of these catalysts, are present as particles smaller than 4 nm. However, the results of TPR (Fig. 4b) suggest that reduction of the second metal occured not only in Cd, but also in Mn and La. Therefore, it might be assumed that partially reduced La20x and manganese oxide species are closely associated with the Ag phase. Alternatively, an X-ray amorphous composite could be formed during coprecipitation and drying exhibiting a perovskite-like phase with oxygen vacancies, making it easier to reduce. It should be noted that compared to Ag/Si02-P a decrease in the mean crystallite sizes of Ag, determinedfromthe (220) reflection, was observed (12.8 nm for Ag-Mn/Si02-P and 22.2 nm for Ag-La/Si02-P). Thus, it is obvious, that Mn and La additionally improve the Ag dispersion by preventing sintering of Ag particles. The product selectivities obtained under optimized reaction conditions using the selectivity to the desired product, crotyl alcohol, as the main parameter, and the corresponding conversions of crotonaldehyde are shown m Fig. 5. It can be seen that (i) alloying of Ag with a late transition metal (Cd) and (ii) modifying of Ag with an early transition metal (Mn) or a rare earth element (La) results in high selectivities of crotyl alcohol. In the latter case, by analogy to Pt/Ti02 catalysts in the SMSI state [4], interfacial sites may consist of accessible lower-valent metal cations or oxygen vacancies on the matrix adjacent to Ag sites, inducing an interaction with the free electron pair of the oxygen atom of the C=0 group and, thus, activating this functional group. In the former case the addition of Cd to Ag fills up the electron band of the atoms in the host lattice. Silver cadmium alloys belong to Hume-Rothery phases which are characterized by their metallic bonds and their valence-electron concentration [23]. As shown

287

for PtNi/Si02 and PtFe/C catalysts where electron transfer from the much more electropositive element (Ni, Fe) to Pt was evidenced by XANES measurements [24,25], a heteropolar bonding between Ag and Cd is to be expected [23] because they differ in the electronegativities. Therefore, in the case of the true alloy catalyst Ag-Cd/SiOi surface polarity of the bimetallic sites is suggested to be responsible for the adsorption of the C = 0 group leading to a high crotyl alcohol selectivity of 85 % at 65 % conversion. 100 90

^

533 K

453 K

70 wm crotyl alcohol (trans + cis) ^ n-butyraldehyde

M\

I 60 (0 C

453 K

50

^

40

g $

n-butanol

I I sum of trace products (ethyl methyl ketone, 2-butanol, allylcarbinol, 2-ethylhexanal) ^m conversion

30 20 10

aa

Ag-Cd/Si02-P (Ag/Cd=1.9)

Ag-Mn/Si02-P (Ag/Mn=3.0)

Ag-La/Si02-P (Ag/La =2.6)

Figure 5. Product selectivities and conversion for crotonaldehyde hydrogenation in the gas phase over modified Ag catalysts (p = 2 MPa, H2/CA = 20; W/FCA= 19 g h mol'^). Both types of catalysts have been successfully used for the hydrogenation of acrolein and acetophenone to improve the selectivity to allyl alcohol and phenylethanol, respectively [19]. Finally, it could be possible too, that the interaction of Ag with lanthanum or manganese oxides may result in a modification of the electronic environment of Ag because of the electron donation properties brought about theu* basicities. Therefore, in-situ XPS measurements are currently performed to assess the oxidation state of the metals in the working catalyts. CONCLUSIONS Activity and selectivity of monometallic Ag catalysts can be controlled by the preparation conditions leading to micro- and meso- to macroporous catalysts which are active and selective in the hydrogenation of crotonaldehyde. In Ag catalysts modified by a second metal, bimetallic sites exhibiting surface polarity and Ag^ particles in close contact with a partially reduced early transition metal or a rare earth element, or Ag species stabilized and incorporated in these oxides were concluded to be the active species in the working state of these catalysts. Simultaneous introduction of both metals during the sol-gel process under optimized hydrolyzing conditions could further increase the metal-promoter interaction and lead to well-tailored new hydrogenation catalysts.

288

Acknowledgements Partial financial support of this work by the Bimdesminister ftir Bildung, Wissenschaft, Forschung und Technik (BMBF) in project 03D0028A0 is greatly acknowledged. Assistance by Mrs. H. Miinzner and Mr. M. Lucas in experimental work has been greatly appreciated. The authors are grateful to Dr. Hofmeister (Halle) for creating TEM images. REFERENCES 1. S. Galvagno, A. Donato, G. Neri, R. Pietropaolo and D. Pietropaolo: J. Mol. Catal., 49 (1989) 223. 2. D. Richard, J. Ockelford, A. Giroir-Fendler and P. Gallezot, Catal. Lett., 3 (1989) 53. 3. A. Giroir-Fendler, D. Richard and P. Gallezot, Stud. Surf Sci. Catal, Vol. 41: Het. Catal. Fine Chem. (Eds. M. Guisnet et al.) Elsevier, Amsterdam, 1988,171. 4. M. A. Vannice and B. Sen, J. Catal., 115 (1989) 65. 5. P. Gallezot, A. Giroir-Fendler and D. Richard, Catal. Lett., 5 (1990) 169. 6. A. Giroir-Fendler, D. Richard and P. Gallezot, Catal. Lett., 5 (1990) 175. 7. T.B.L.W. Marinelli, S. Nabuus and V. Ponec, J. Catal, 151 (1995) 431. 8. P. Claus, Chem.-Ing.-Techn., 67 (1995) 1340. 9. R. N. Carter, A. B. Anton and G. Apai, Surf. Scl, 290 (1993) 319. 10. J. L. Solomon and R. J. Madix, J. Phys. Chem., 91 (1987) 6241. 11. J, L. Solomon, R. J. Madix and J. Stohr, J. Chem. Phys., 89 (1988) 5316. 12. A. Ueno, J. Kanai, K. Fujita, E. Nishikawa and K. Imai, Jpn. Pat. No. 01 127 041 (1989). 13. V. Ponec and G. C. Bond, Stud. Surf. Sci. Catal, Vol. 95: Catalysis by metals and alloys, Elsevier, Amsterdam, 1995. 14. M. Lucas and P. Claus, Chem.-Ing.-Techn., 67 (1995) 773. 15. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure & Appl Chem., 57 (1985) 603. 16. G. Horvath and K. Kawazoe, J. Chem. Eng. Jpn., 16 (1983) 470. 17. H. Bemdt, H. Mehner and P. Claus, Chem.-Ing.-Techn., 67 (1995) 1332. 18. B. Coq, F. Figueras, P. Geneste, C. Moreau, P. Moreau and M. Warawdekar, J. Mol. Catal, 78 (1993) 211. 19. P. Claus, P. Kraak and R. Sch6del, (to be published). 20. M. A. Vannice, J. Mol. Catal, 59 (1990) 165. 21. M. Englisch and J. A. Lercher, Proc. DGMK-Conf Sel Hydrog. Dehydrog. (Ed. M. Baems, J. Weitkamp), Nov. 11-12, Kassel, Tagungsbericht 9305,1993,255. 22. ASM Handbook, Vol. 3: Alloy Phase Diagrams (Eds. H. Baker et al), 10th Edition, ASM International, Materials Park, Ohio, 1992. 23. M. Elkier and B. Predel, Intermetallic Compounds, Vol. 1: Principles (Eds. J.H. Westbrook, R.L. Fleischer), Wiley, New York, Vol. 1,1994, Chapter 5. 24. B. Morawek, P. Bondot, D. Goupil, P. Fouilloux and A.J. Renouprez, J. Physique, 48 (1987)297. 25. A. Jentys, B.J. McHugh, G.L. Haller and J.A. Lercher, J. Phys. Chem., 96 (1992) 1324.


289

Surface Organometallic Chemistry on Metals; Selective Hydrogenation of Acetophenone on Modified Rhodium Catalyst. F. Humblot*, M.A. Cordonnier*, C. Santini*, B. Didillon**, J. P. Candy* and J.M. Basset* *COMS-CPE, 43 bd du 11 Novembre 1918,69626 Villeurbanne Cedex. (France) **I.F.P., 1&4 Av. de Bois-Preau 92506, Rueil Malmaison Cedex. (Frai\ce).

Abstract Bimetallic catalysts can be obtained by surface organometallic chemistry on metals. These catalysts are prepared by the controlled reaction under hydrogen between tetra n-butyl tin and silica supported rhodium particles. For a given amount of tin fixed, these solids exhibit increasing activities and selectivities for the conversion of acetophenone to phenylethanol. INTRODUCTION Surface organometallic chemistry deals with the reactivity of organometallic compounds with surfaces. The reaction of organometallics with metallic surfaces appears to be a very promising aspect of surface organometallic chemistry in the field of catalysis related to fine chemicals.^"' The reaction between silica supported rhodium and tetra n-butyl tin begins at room temperature. The hydrogenolysis of the butyl groups is not complete at temperatures below 373 K and a somewhat stable surface species of the general form Rhs[Sn(n-C4H9)x]y/Si02 is produced.° The value of y depends on the amount of Sn(n-C4H9)4 introduced (0Yield at 100% alkyne conversion determined b y G.C. peak area analysis. În all cases the byp r o d u c t w a s the fully saturated compound. ^ N o trans isomer w a s detected.

2.3. Catalyst characterization Transmission Electron Microscopy (TEM) m e a s u r e m e n t s o n t h e catalysts containing p a l l a d i u m a n d copper in a ratio of either 1:1 or 1:2 s h o w e d a good dispersion of the metal particles (2-5 n m ) over the support. Energy Dispersive Xray Analysis (EDAX) on the samples revealed the presence of both metals in each examined particle. By doubling the amount of copper a proportionate increase of copper content in the metal particles w a s detected as expected.

317 3. EXPERIMENTAL 3.1. General considerations Reactions were performed in an atmosphere of dinitrogen using Schlenk techniques. Toluene, benzene, diethyl ether and pentane were freshly distilled from sodium benzophenone-ketyl. All other solvents were used as received. The support material used was silica OX-50 (surface area 50 m^/g) which was purchased from Degussa. Before usage the silica was boiled in bi-distilled water and dried in vacuo at 200 °C for 3 d to increase the amount of silanol groups. Pd(OAc)2 (47.35 % Pd) was purchased from Degussa. The compounds 4-iodotoluene, n-butyllithium (1.6 M in hexane), 3-methyl-l-pentyn-3-ol, 2-butyne-l,4diol, phenylacetylene, diphenylacetylene, 1-ethynyl-l-cyclohexanol and 2-methyl3-butyn-2-ol were obtained from Acros. Other propargylic alcohols were prepared according to the literature [9,10] and purified by kugelrohr distillation and crystallization. The substrates were analysed by G.C., G.C.M.S. and ^H NMR and l^C NMR spectroscopy prior to use. Complex Cu2Li2(p-tolyl)4(Et20)2 was prepared according to literature procedures [3,4] and analysed by ^H NMR spectroscopy. The NMR spectra were recorded on Bruker AC200 (200 MHz) and AC300 (300 MHz) spectrometers at ambient temperature in NMR solvents obtained from ISOTEC Inc.. G.C. analysis were performed on Unicam PU4600 and PU610 apparatus with 30 m J&W Scientific DB-1, DB-17 and AT-SILAR capillary columns and flame ionization detectors. Product yields were determined by peak area analysis; response factors for selected substrates and products were found to be virtually identical. Internal standards were used in the initial stage of this study, but were found to influence the catalyst characteristics. G.C.M.S. was performed on a Unicam Automass apparatus combined with 610 series G.C. apparatus equipped with 30 m J&W Scientific DB-1 and DB-17 columns. TEMEDAX was performed on a Phillips CM 200 microscope equipped with a field emission gun. TEM-EDAX samples were prepared by application of a few droplets of a suspension of the catalyst in ethanol onto a holey carbon film which was supported by a nickel grid after which the ethanol was allowed to evaporate.

3.2. Preparation of the copper precursor [Cu2Li2(p-tolyl)4(Et20)2] Preparation of [Li(p-tolyl)]. To a solution of 8.76 g (40.2 mmol) 4-iodotoluene in ca. 30 mL toluene was added 1.05 equivalent of n-butyllithium at 0 "C. The resulting white suspension was stirred for 30 min., after which the slightly yellow solution was decanted. The white residue was washed with pentane (5 x 50 mL) and dried in vacuo. Prior to the next preparation step the white solid was dissolved in diethyl ether, centrifuged and decanted. After evaporation of the solvent a white solid was obtained. Yield 3.70 g (94%). Preparation of [Cu4(p-tolyl)4]. To a suspension of 2.09 g (14.6 mmol) CuBr in diethyl ether was slowly added a solution of 1.47 g (15.0 mmol) p-tolyllithium in

318 ca. 15 mL diethyl ether at -78 'C. After 1 h the suspension was allowed to warm to 0 "C, after which the intense yellow precipitate was isolated by decantation, washed with cold (0 'C) diethyl ether (4 x 50 mL) and dried in vacuo. Yield 1.34 g (62%). IH NMR (CeDe, 300 MHz): 6 8.02 (d, 8 H, 3/HH = 75 Hz, aryl), 6.86 (d, 8 H, 3/HH = 75 Hz, aryl), 1.96 (s, 12 H, CH3). Preparation of [Cu2Li2(p-tolyl)4(Et20)2]. To a suspension of 0.70 g (5.0 mmol) p-tolylcopper in diethyl ether (25 mL) was slowly added a solution of 0.53 g (5.4 mmol) p-tolyllithium in diethyl ether (20 mL) at 0 °C. The resulting greenish solution was stirred for 1 h during which time a white precipitate of the product formed. The solid was isolated by decantation, washed twice with pentane and recrystallized from diethyl ether. Yield 1.08 g (66%). ^H NMR (CeDe, 300 MHz): 8 8.37 (d, 8 H, 3/HH = 72 Hz, aryl), 7.07 (d, 8 H, 3/HH = 7.2 Hz, aryl), 2.78 (q, 3/HH = 72 Hz, OCH2CH3), 2.12 (s, 12 H, CH3), 0.74 (t, 3/HH = 72 Hz, OCH2CH3).

3.3. Preparation of the supported palladium-copper catalysts Silica supported catalysts with different metal loadings were prepared in several batches: Pd/Cu/Si02 (4.0 w% Pd, 2.1 w% Cu; Pd:Cu = 1:1) (I), P d / C u / S i 0 2 (4.0 w% Pd, 4.2 w% Cu; Pd:Cu = 1:2) (II). The catalysts were prepared in a reactor vessel of 250 mL. The reactor vessel was equipped with three baffles (120**) and mechanically stirred with a gascirculating stirrer (2000 rpm). A red ultrasonically pre-treated solution of palladium(II) acetate in ca. 35 mL toluene was added to an ultrasonically pretreated suspension of silica in ca. 150 mL toluene using a tube pump (Gilson Minipuls 2, equipped with a PVC tube, type GI 17942 internal diameter 1.52 mm and a Teflon injection tube, outer diameter 1.52 mm). The solution was injected at the height of the stirrer. The formed orange suspension was stirred for 24 h after which a yellow solution of the copper precursor in ca. 50 mL toluene was added by means of the tube pump. This resulted in a dark brown suspension. After stirring for 3 d, dihydrogen was introduced to the resulting black suspension during 3 h to be sure that the reduction process was completed. Stirring was stopped and the colourless solution was decanted from the settled material. The resulting black powder was washed twice with pentane and dried in vacuo at room temperature. Both the activity and selectivity of all catalysts batches were tested in the hydrogenation of 3-methyl-l-pentyn-3-ol before use.

3.4 Catalytic hydrogenations The hydrogenations were performed in a glass reactor vessel applied with a gas-circulating stirrer (2000 rpm) and three vertical glass baffles at atmospheric dihydrogen pressure. The reactor vessel was kept at 25 °C by circulating thermostated water through the wall of the vessel. In all hydrogenation reactions

319 the following procedure was executed. The reactor vessel was evacuated and filled with dinitrogen. The catalyst (ca. 35 mg) was added to the reactor followed by addition of 100 mL of ethanol. While stirring the (nitrogen) atmosphere was expelled out of the equipment by subsequent evacuation and flushing with dihydrogen (5x). The suspension was stirred for 1 h under dihydrogen. Next, without stirring, a solution of the substrate (5 mmol) in 1.5 mL ethanol was added with a hypodermic syringe. After the first sample had been taken the hydrogenation reaction was started by switching the stirring device on. Dihydrogen uptake was monitored using a gas burette system. G.C.(M.S.) samples were taken through a silicon septum with a hypodermic syringe. Substrates and products were analysed with G.C. and confirmed with G.C.M.S..

4. CONCLUSIONS The method described allows fast and consistent production of silica supported bimetallic palladium-copper catalysts in the liquid phase at room temperature, without the need for high temperature reduction. The catalysts show homogeneous dispersion of the mixed metal particles over the support surface and are ready to use immediately after preparation. The silica supported palladium-copper catalysts are selective in the hydrogenation of monosubstituted acetylenes giving high yields of either olefins or saturated hydrocarbons, depending on the reaction time. In addition, the catalytic system shows reasonable selectivity towards c/s-olefins in the hydrogenation of disubstituted acetylenes. REFERENCES [1]

Gutman, H.; Lindlar, H. Chemistry of acetylenes; Viehe, H.G., Ed., Marcel Dekker: New York, 1969. [2] Cussler, E.L. Diffusion; Mass transfer in fluid systems; Cambridge University Press: Cambridge, 1984. [3] Carturan, G.; Gottardi, V. /. Catal 1979,57, 516. [4] Travers, Ch.; Bournonville, J.P.; Martino, G. in Proceedings, 8th International Congress on Catalysis, Berlin, 1984; Dechema: Frankfurt-amMain, 1984; Vol. IV, 891. [5] Candy, J.P.; Didillon, B.; Smith, E.L.; Shay, T.B.; Basset, J.M. /. Mol Catal. 1994, 86,179. [6] Cohen, M.S.; Noltes, J.G.; van Koten, G. US Patent 4 222 898, 1980. [7] van Koten, G.; Jastrzebski, J.T.B.H.; Noltes, J.G. /. Organomet. Chem. 1977, 140, C23. [8] van Koten, G.; Jastrzebski, J.T.B.H.; Noltes, J.G. /. Organomet. Chem. 1978, 148,317. [9] Fleming, I.; Tonaki, K; Thomas, A. /. Chem. Soc, Perkin Trans. 11987, 2269. [10] Pittman, C ; Olah, G. /. Am. Chem. Soc. 1965, 87, 5632.



321

Catalytic Hydrogenation by Polymer Stabilized Rhodium G.W. Busser, J.G. van Ommen and J.A. Lercher University of Twente, Department of Chemical Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands. ABSTRACT The preparation, physico-chemical characterization and catalytic testing of polymer stabilized rhodium particles are described. Particles between 1 and 3.5 nm stabilized by polyvinyl-2-pyrrolidone and poly-2-ethyloxazoline were characterized by Transmission Electron Microscopy (TEM), X-ray absorption spectroscopy (XAFS) and liquid phase hydrogen/oxygen titration. Liquid phase hydrogenation of 4-f^rr-butylphenol was used as a test reaction. It was found that in contrast to a conventional carbon supported material, a polymer supported Rh did not lead to hydrogenolysis and isomerization. Larger catalyst particles and a higher concentration of polymer caused a higher selectivity to 4-f^rr-butylcyclohexanone. This has been attributed to the presence of well-reduced Rh. In contrast, the availability of electron deficient Rh is speculated to enhance the rate of hydrogenation to 4-^6rr-butylcyclohexanol. The preferential formation of the cis-isomtT of this alcohol was observed over all the catalysts.

1. INTRODUCTION Though not asfrequentlyused as oxide and carbon supports, polymers offer interesting possibilities to stabilize highly dispersed metal catalysts [1,2]. In polymer stabilized colloidal suspensions, the polymer acts primarily as a steric stabilizer via van der Waals interactions with the metal thus preventing agglomeration of the particles [3]. The adsorption of the polymer on the metal cluster/particle is considered irreversible in the sense that simultaneous desorption of all polymer segments is statistically unlikely [4]. When these materials are used as catalysts, it can be expected that the nature and strength of interaction between the polymer and the metal particles will determine the availability of the metal surface for reactants and products and, thus, activity and selectivity. In addition, the interacting polymer functional groups might induce changes in the electronic properties of the metal surface atoms [5]. As for all catalysts, well-characterized samples are necessary to be able to relate the catalytic performance to physico-chemical properties. Transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAFS) were used in this study to characterize the stabilized metal colloid. The necessity of such extensive characterization of particle size has been outlined by Harada et al [6,7] showing that the formation of aggregates may be overlooked and misinterpreted as large metal particles when using TEM alone. The actual availability of the polymer stabilized surface has been probed by hydrogen/oxygen titration adopted from the description of Bernard et al. [8]. Hydrogenation of 4-r^r^butylphenol was chosen as test reaction for two reasons. The first

322

is that it is a bulky molecule. Therefore, it was expected that there could be problems to reach the polymer-covered metal particles and getting indirect information on the degree of blocking and the flexibility of these chains might be possible. The second is that the conversion of the partially hydrogenated product, 4-r^rf-butylcyclohexanone, to the fully hydrogenated products, cis and trans 4-feAt-butylcyclohexanol, is reported to depend on the presence of charged metal atoms [9]. Therefore, we expect that a difference in the concentration of, e.g., Rh"" species in the catalysts will be reflected in different selectivities. In this paper we describe the preparation, characterization and the catalytic properties of polymer stabilized Rh particles for hydrogenation of 4-ferr-butylphenol. The combination of TEM, XAFS and a newly developed liquid phase hydrogen/oxygen titration technique is applied to characterize particle size and availability for the reactants.

2. EXPERIMENTAL 2.1. Preparation , high purity) and polymer (2.0 mmol Rhodium chloride (0.057 mmol, Aldrich, RhCl3 monomer units of polyvinyl-2-pyrrolidone or poly-2-ethyloxazoline, special grade, Aldrich) were dissolved in water (20 ml). This mixture was heated at 373 K for 2 hours. Then, it was rapidly mixed with 130 ml alcohol (methanol, ethanol, 1-propanol or 1-butanol, p.a., Merck) and heated to 348,353,373 and 393 K, respectively. Heating at that temperature was continued for 48 hours. Then, the colloidal solutions were cooled with liquid nitrogen, the solvent was evaporated under vacuum and the materials were redissolved in 1-butanol. To prepare a colloidreducedby hydrogen, a solution was prepared as described above, using methanol and heating at a lower temperature (340K). After removing the solvent and redissolving in butanol, the material was treated with hydrogen at 7 bar and 343K for Ihour. Average particle sizes were determined with TEM. For that purpose a drop of the colloidal solution was placed on a carbon covered copper grid (Balzers) and analyzed with a high resolution transmission electron microscope (model JEOL 200 CX). Particle size distributions were determined by optical inspection of the photographs. From this data, metal areas of the catalysts were estimated assuming spherical particle shape and a rhodium surface density of 1.66 10"^ mol Rh Ivcf [10]. As a reference material for characterization and testing, a commercial rhodium on carbon catalyst (5w% Rh, Aldrich) was used. 2.2. Physicochemical characterization X-Ray Absorption Fine Structure Spectroscopy XAFS measurements were done at the National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York, beamline X23A2 equipped with a Si (311) double crystal monochromator that did not require detuning. Concentrated solutions of catalyst (1.4 10"^ mmol Rh/g) were poured into a glass cell with 1 cm path length having poliimide windows (KAPTON). The samples were reduced in a gas stream of 5%hydrogen/95% helium at 343K for 30 min. XAFS spectra at the Rh-K edge (23220 eV) were recorded at liquid N2 temperature, the spectral resolution being 1 eV. The EXAFS analysis followed standard procedures as described in ref. [11]. The ^-weighted spectra were Fourier transformed within the limits k=4 to k=16. EXAFS of the first Rh-Rh shell was fitted using phase shift and amplitude functions obtained from a Rh foil under the assumption

323

of plane waves and single scattering. From the average coordination number an average particle size was determined using the correlation of Kip et al [12] assuming spherical particle shape. Hydrogen chemisorption In a typical experiment, a glass reactor (volume =110 ml) was filled with 100 ml of colloidal solution (0.23 mmol Rh and 8 mmol polymer in 1-butanol). Dissolved oxygen was removed by flushing with Ar after heating to 343 K. Subsequently, the catalyst was reduced with a mixture of 5% H2 in Ar. Hydrogen consumed was measured with a TCD. After reduction, the catalyst surface was oxidized by flushing with air for 20 min. Subsequently, the oxygen dissolved in the solvent was removed by flushing with Ar, the catalyst was rereduced following this procedure and hydrogen uptake was measured. Gas phase hydrogen chemisorption of the reference compound (5w% Rh/C) was done in a standard volumetric hydrogen chemisorption set up. The catalyst was reduced by pretreating it for one hour at 343K in a gas stream containing 5% H2 and 95% He (flow rate 50 ml/min). 2.3. Catalytic testing For catalytic hydrogenation of 4-tert-butylphenol, an enamel coated stainless steel reactor was filled with 50 ml of colloidal solution (4mg Rh, varying amounts of polymer) and 100 mg of 4-r^rr-butylphenol. The temperature was increased to 343K and dissolved oxygen was removed by flushing with N2 for 10 min. Subsequently, the hydrogen pressure was increased to 15 bar. Liquid samples were taken during reaction with a syringe and analyzed by GC/MS (column: CP-Sil-5CB-MS, 50m*0.25mm ID*0.12iim, temp.: 250 °C, apparatus: Varian Saturn 4D).

3. RESULTS 3.1. Physico-chemical characterization XAFS measurements Results of XAFS measurements are compiled in Table 1 with the results of the estimates of the TEM measurements (for details on TEM results see ref [13]). The catalysts are named according to the stabilizing polymer and particle size determined by TEM. Except for the PVP stabilized catalyst reduced with methanol, an excellent agreement exists between the size of the metal particles estimated from TEM and EXAFS. The methanol reduced sample shows the smallest particle size when estimated from EXAFS, while it appears to be much larger in TEM. At present we would like to speculate that this is due to the formation of agglomerates (diameter 3.5 nm) formed from very small particles (diameter 1.2 nm). Consequently, the catalyst is named Rh/PVP/3.5-a to indicate the difference with the sol containing primary particles of 3.5 nm in diameter. Determination of the metal sites accessible to hydrogen The percentages of the surface area accessible for hydrogen chemisorption of the samples investigated are compiled in Table 1. More specifically, the TEM measurements were used to derive the potentially accessible surface area of the colloid metal particles. These values were used to estimate the theoretical maximum uptake of hydrogen and oxygen using an adsorption stoichiometry of one hydrogen or oxygen atom per accessible metal atom. Hydrogen consumed during first hydrogen admittance was insufficient to reduce and cover all metal atoms. On the assumption that part of the physical surface of the metal particles might

324

be inaccessible due to sorbed molecules, the amount of chemisorbed hydrogen was related to the fraction of available metal atoms on the surface of the colloid particles as estimated by TEM. For the Rh/C catalyst, the metal surface area was determined by gas phase chemisorption. While comparing this value with the value obtained with Hquid phase hydrogen/oxygen titration, for the Rh/C catalyst an accessibility of 100% is determined. The larger polymer stabilized particles also seem to be able to accommodate hydrogen on a large fraction of its surface (88%). In contrast, small particles have only about 40% of their surface atoms available to interact with hydrogen.

Table I Catalyst Characteristics catalyst

reducing agent

average size (TEM) (nm)

average size * (EXAFS) (nm)

N

A a^

r

Perc. H2*' ^

(A^)

(nm)

(%) 42

Rh/PVP/1

H,

1

1.3

7.1

20

2.65

Rh/PVP/2

EtOH

2

1.9

8.4

8.3* 10-"

2.68

Rh/PVP/2.5

1-PrOH

2.5

1.9

8.5

12*10-^

2.67

Rh/PVP/3.5

1-BuOH

3.5

3.3

10.1

8.8*10-^

2.68

88

Rh/PVP/3.5-a

MeOH

3.5

1.2

6.4

1.5*10-^

2.66

30

Rh/POX/1

H,

1

Rh/C/3.5

44 104°

* determined using the method published by Kip et A/. [12] assuming spherical particle shape "^ percentage of surface atoms available for H2 chemisorption ° metal surface was determined with volumetric gas phase chemisorption

3.2. Catalytic testing Differences in the activity and selectivity for 4-r^r^butylphenol hydrogenation are displayed in Fig. 1. The variation in selectivity as a function of conversion shows that cis and trans-4-tertbutylcyclohexanol and 4-r^r^butylcyclohexanone are all primary products. It should be noted that 4-ferf-butylcyclohexanol is also produced as a secondary product via the hydrogenation of A-tertbutylcyclohexanone. With decreasing particle size and lower polymer concentration the tendency to hydrogenate the intermediately formed 4-f^rr-butylcyclohexanone to the alcohol increased. Always, the concentration of cw-4-r^rr-butylcyclohexanol was higher than the concentration of the ^ran^-isomer. Differences in stereo selectivity for different particle sizes stabilized by PVP were hardly significant. However, with increasing polymer concentration the cisitrans ratio was found to decrease. For the POX stabilized catalyst the selectivity to the ketone was low. Also, the cis/trans ratio of 4-rerr-butylcyclohexanol was lower compared with the PVP stabilized catalysts with a similar particle size.

325

In contrast to the polymer stabilized Rh particles, the Rh supported on carbon showed a significant concentration of products resulting from hydrogenolysis and isomerization. The relative selectivities to the hydrogenated products, however, were similar to those found with the PVP stabilized Rh of similar particle size.

Rh/PVP/1 ri53mMPVP

2000 4000 time (min) Rh/PVP/3.5 28mMPVP

0.3

6000 0

Rh/POX/1 r28mMPOX

2000 4000 time (min)

6000 0

2000 4000 time (min)

6000 0

20

6000

|Rh/PVP/3.5-a

lo.2 I 0.1 tf

-

H—^4—_^ 2000 4000 time (min)

6000 0

2000 4000 time (min)

4-rerf-butylcyclohexanone cis-4-tert-huty\cyc\ohcxsino\ trans-4-te rt-buty\cyclohex3ino\

O -

40 60 time (min)

80

hydrogenolysis isomerization

Figure 1. Products from hydrogenation of 4-f^rr-butylphenol; influence of polymer concentration and catalyst.

4. DISCUSSION 4.1. Preparation and physico-chemical properties The polymer stabilized Rh sols prepared by the present method show a strong influence of the reduction medium on the particle size. While a detailed description is given in ref. [13 ], we would only like to state here that the rate of reduction and the reduction potential seem to play a decisive role. As the rate of reduction increases (from butanol to hydrogen) the metal particle size in the stabilized sol decreases. This is unequivocally seen from TEM and EXAFS results, confirming that polymer stabilized sols with a well tailored and narrow particle size distribution

326 have been prepared. There seems to be a discrepancy for methanol reduced Rh particles. TEM suggests the existence of large particles, while the EXAFS results show very small particles. In this context it should be emphasized that the two techniques deliver complementary information. EXAFS provides primarily information on the arrays of metal atoms that are well ordered on an atomistic scale. TEM on the other hand gives information about the overall size of particles. Thus, we conclude that with methanol as a reducing agent "micro clusters", approximately of the size of one nanometer, are formed that are connected to form larger agglomerates while retaining their intrinsic short range order. A similar result has been reported by Harada et al [6,7] using also the two techniques to characterize Pt/Rh clusters suspected to form larger entities from very small crystallites. Hydrogen chemisorption shows that the fraction of surface atoms available for hydrogen chemisorption increases with increasing particle size (see Table 1). Note, however, that Rh/PVP/3.5-a was found to have a capacity to chemisorb hydrogen similar to the capacity of the small primary particles. This suggests that the local surface chemistry, possibly the higher abundance of edges and comers [14], rather than the physical size of a particle determines the interaction with the polymer and, hence, the availability of surface atoms for hydrogen. As the amount of hydrogen consumed was insufficient to account for complete reduction, we suggest that a fraction of the surface atoms remain ionic. We propose that these sites, i.e., Rh"^, are located preferentially at the edges and comers of the crystallites or completely isolated attached to the polymer. Since Rh/PVP/1, Rh/POX/1 and Rh/PVP/3.5-a contain a high concentration of edges and comers (small primary particles), these materials are expected to have the highest concentration of electron deficient sites. Indeed this is confirmed by results from CO adsorption experiments combined with IR spectroscopy [15]. High concentrations of gem-dicarbonyls, representative of the presence of Rh(I), are observed on these materials. In contrast, exclusively completely reduced atoms are found on large particles (Rh/PVP/3.5), exposing preferentially low index crystal planes. In analogy with the interactions between the polymer and the precursor [13], the interaction of the polymer with the metal particles is suggested to involve the carbonyl groups of the polymer. Since this is most likely a van der Waals (dipole-dipole) interaction, it can be expected that the presence of metal ions enhances the strenght, thus implying a stronger interaction with decreasing primary particle size. 4.2. Catalytic hydrogenation of 4-^^r^butylphenol Hydrogenation of 4-r^rr-butylphenol is an interesting test reaction to probe the stereo selective hydrogenation of complex aromatic molecules. It is generally accepted that the reaction proceeds at least to some extent via the formation of 4-r^rr-butylcyclohexanone [16], while a direct hydrogenation route to c/5/ifranj:-4-r^r^butylcyclohexanol is also possible. The present results fully confirm this model. Secondary conversion of 4-ferf-butylcyclohexanone to the alcohol was primarily observed with catalysts having a large concentration of electron deficient sites (see Fig. 1; Rh/PVP Inm and Rh/POX/lnm; 28mM polymer). Note that the most drastic secondary hydrogenation of 4-f^r^butylcyclohexanone occurs with Rh/POX/lnm, which is concluded to contain by far the highest concentration of electron deficient Rh. We attribute the higher catalytic activity to hydrogenate 4-rerr-butylcyclohexanone to a higher adsorption constant and/or to a stronger interaction of the carbonyl group with electron deficient sites. This is thought to be analogous to the promotion of the hydrogenation of C=0 bonds in unsaturated aldehydes by the presence of positively charged species (Lewis acid sites) [9,17]. For

327

these reactions it is thought that the electron pair donor - acceptor interaction polarizes the carbonyl bond thus promoting hydrogenation of the carbonyl group. In analogy, we propose that 4-tertbutylcyclohexanone is more strongly adsorbed on these cations than on the fully reduced metal. Increasing the polymer concentration leads to preferential blocking of the cationic sites and/or more constraints for adsorption on the metal particle, which results in a lower conversion of the ketone. Rh/PVP/3.5a shows an intermediate behavior between the catalytic activity of catalysts with very small and larger particles. The high concentration of electron deficient Rh in Rh/PVP/3.5-a should lead to hydrogenation of the ketone. However, the fraction of metal atoms available on the surface of the agglomerate, determined by hydrogen chemisorption, was about a third smaller than that of Rh/PVP/1, indicating a rather high concentration of polymer on the surface. This seems to markedly impede the secondary hydrogenation of the ketone to the alcohol. All catalysts are found to favor the formation of the cw-isomer of 4-r^rr-butylcyclohexanol, in agreement with some Rh catalysts described [18]. Note that for homogeneous Rh(I) catalysts, trans-4f^rf-butylcyclohexanol has been found to be the preferred product [19]. We suggest, that the specificity of the adsorption complex of the cyclohexanone intermediate controls the selectivity of the hydrogenation. Senda et al. [20] have shown this elegantly using the influence of particle size on the stereo selectivity for methylenecyclohexane hydrogenation. Small particles seem to allow both adsorption modes of methylenecyclohexane leading to the cis and trans isomers of methylcyclohexane, while large particles seem to prevent the transition state to the trans-isomer due to steric constraints. We propose that a similar influence prevails for hydrogenation of the 4-f^rf-butylcyclohexanone formed as an intermediate. For the polymer stabilized Rh sols the presence of electron deficient sites and the polymer on the surface are complicating factors. The limited information available from the present set of experiments suggests that the highest selectivity to cw-4-r^rr-butylcyclohexanol is found with the large particles of Rh/C (>80 mol%). Polymer stabilized catalysts showed a lower selectivity (approximately 60%) than Rh/C indicating that either some homogeneous Rh was in the solution or the interaction with the 4-f^r^butylcyclohexanone was less defined than on the carbon supported catalysts. Compared with a conventional carbon supported Rh, the absence of significant amounts of products from (structure sensitive) hydrogenolysis [21] and the (acid catalyzed) isomerization [22] is speculatively attributed to the presence of the polymer causing steric constraints and smaller ensembles of accessible metal atoms on the particles. The absence of any acidic function on the support prevents isomerization.

5. CONCLUSIONS Various sizes of rhodium particles stabilized in liquid phase by polyvinyl-2-pyrrolidone (PVP) and poly-2-ethyloxazoline (POX) have been successfully prepared by using different reductants. With a stronger reducing agent (such as H2), smaller particles were obtained than with a weaker reducing agent (such as butanol). The sample reduced with methanol forms agglomerates (3.5 nm in diameter) from small primary particles (approximately Inm). The concentration of partially reduced Rh, as concluded from the formation of dicarbonyl species from adsorbed CO (see ref [15]), increases with decreasing primary particle size. The availability of surface metal sites for chemisorbed hydrogen increases with increasing particle size. The selectivity in the hydrogenation of 4-r^r/-butylphenol varies with the particle size and

328

the presence of partially reduced Rh. As the carbonyl group of the ketone interacts more strongly with Rh"^ compared to metal atoms, it is preferentially converted to the alcohol over catalysts showing a higher concentration of dicarbonyl species after sorption of CO [15]. The concentration of Rh"^ was shown to decrease with increasing particle size. Thus it is expected and observed that catalysts with a larger particle size and a lower concentration of electron deficient sites will be more selective to the ketone than the alcohol. Higher concentrations of the polymer seem to reduce the concentration of electron deficient sites available for reaction and enhance the selectivity to the ketone. In contrast to results obtained with homogeneous catalysts[19], cw-4-r^r^butylcyclohexanol is the preferred product. This is attributed to the preferred addition of the hydrogen to the C=0 group bound parallel to the metal surface. In contrast to a conventional Rh/C catalyst, acid catalyzed skeletal isomerization of the 4-r^rr-butylcyclohexanol and metal catalyzed hydrogenolysis were not observed. ACKNOWLEDGMENTS Partial support of this work by the Onderzoeks Stimulerings Fonds of the University of Twente, is grateftiUy acknowledged. The XAFS measurements were carried out at the National Synchrotron Light Source (Beamline X23A2), Brookhaven National Laboratory, which is supported by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. REFERENCES 1. D.C. Sherrington, Pure&Appl. Chem. 1988, 60,401. 2. F. Ciardelli, C. Carlini, P. Pertici, G. Valentini, J. Macromol. Sci.-Chem.1989, A26, 327. 3. D.H. Napper, Polymeric Stabilization of Colloidal Dispersions, Academic Press, London, 1983. 4. M. Ohtdd, M. Komiyama, H. Hirai, N. Toshima, Macromolecules 1991, 24, 5567. 5. E. Tsuchida, Macromolecular Complexes - Dynamic Interactions and Electronic Processes, VCH Publishers, New York, (1991). 6. M. Harada, K. Asakura, N. Toshima, J.Phys. Chem. 1994, 98. 7. M. Harada, K. Asakura, Y. Ueki, N. Toshima, J. Phys. Chem. 1992, 96, 9730. 8. J.R. Bernard, C. Hoang-Van, S.J. Teichner, Journal de Chimie Physique, 1975, 72, 1217. 9. A.A. Wismeijer, A.P.G. Kieboom, H. Van Bekkum, React. Kinet. Catal. Lett. 1985, 29, 311. 10. J.J.F. Scholten, A.P. Pijpers and A.M.L. Hustings, Catal. Rev.-Sci. Eng., 1985, 27(1), 151. 11. T. Fukunaga, V. Ponec, J. Catal. 1995, 157, 550. 12. B.J. Kip, F.B.M. Duivenvoorden, D.C. Koningsberger, R. Prins, J.Catal. 1987, 105, 26. 13. G.W. Busser, J.G. van Ommen and J.A. Lercher, Advanced Techniques in Catalyst Synthesis, Ed. W.R. Moser, Academic Press, 1996, accepted for publication. 14. R. van Hardeveld, F. Hartog, Surf. Sci. 1969, 15, 189. 15. G.W. Busser, J.G. van Ommen and J.A. Lercher, in preparation. 16. S.R. Konuspaev, Kh.N. Zhanbekov, T.S. Imankulov, R.K. Nurbaeva, Kin. Katal. 1993, 34(1),82. 17. A. Jentys, M. EngHsch, G.L. Haller, J.A. Lercher, Catal. Lett. 1993, 21, 303. 18. D. Yu. Murzin, A. I. Allakhverdiev, N.V. Kul'kova, Kinetics and Catalysis 1993, 34,442. 19. M.J. Burk, T.G.P. Harper, J.R. Lee, C. Kalberg, Tetrahedron Letters, 35,4963. 20. Y. Senda, K. Kobayashi, S. Kamiyama, J. Ishiyama, S. Imaizumi, A. Ueno, Y. Sugi, Bull. Chem. Soc. Jpn. 1989, 62, 953. 21. M. Che, CO. Bennett, Adv. Catal. 1989, 36, 55. 22. J.A. Lercher, G. Mirth, M. Stockenhuber, T. Narbeshuber, A. Kogelbauer, Stud. Surf. Sci. Catal. 1994, 90, 147.


329

Epoxidation of cycloalkenones over amorphous titania-silica aerogels R. Hutter, T. Mallat and A. Baiker Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum CH-8092 Zurich, Switzerland

Epoxidation of a- and P-isophorone was studied using t-butylhydroperoxide and a titania-silica catalyst containing 20 wt% titania. The mixed oxide was prepared by the sol-gel aerogel technique. Good activities and high selectivities (up to 94 - 99 %) could be achieved. The activity of the aerogel in the epoxidation of aisophorone was compared to the performance of titania-silica xerogel, Mg-Alhydrotalcite and KF/alumina catalysts. Acid-catalyzed side reactions during the epoxidation of P-isophorone (isomerization to ot-isophorone and epoxide ring opening) could be suppressed by treatment of the aerogel with a weak base. The influence of solvent, temperature and peroxide structure on the performance of the titania-silica aerogel is discussed. 1. INTRODUCTION Various Ti- and Si-containing materials have been proposed as heterogeneous epoxidation catalysts, including titania-on-silica (Shell, [1, 2]), titania-silica mixed oxides [3-8], Ti-MCM-41 [9, 10], TS-1 [11] and Ti-P [12]. Only the first three types of catalysts are able to epoxidize bulky reactants with acceptable rate and good selectivity [13]. We have shown recently [4-6] that titania-silica, prepared by the sol-gel aerogel technique, possesses an amorphous mesoporous structure with high surface area and good dispersion of Ti in the silica matrix. These parameters were crucial for obtaining outstanding epoxidation activity and selectivity [13].

a -isophorone

p -isophorone

330

Most of the earlier studies described the oxidation of simple (electron-rich) cycloalkenes, such as cyclohexene and cyclododecene. Here we report the catalytic behaviour of titania-silica aerogels in the oxidation of cycloalkenones. The model reactions are the epoxidation of a- and P-isophorone, depicted in scheme 1. 2. EXPERIMENTAL 2.1 Catalyst preparation The sol-gel titania-silica mixed oxide contained 20 wt% titania. The catalyst was sjmthesized under acidic conditions [5]. The acidic hydrolysant was added to an isopropanolic solution of tetraisopropoxjrtitaniumCIV) modified by acetylacetone (molar ratio alkoxide : acetylacetone = 1:1) and tetramethoxysilicon(IV). The water : alkoxide : acid molar ratio was 5 : 1 : 0.09. The resulting gel was dried by semicontinuous extraction with supercritical COg at 40 °C and 240 bar, and stored in a closed vessel under Ar. After calcination in flowing air at 600 °C, the BET surface area was 648 m^g'^ and the specific pore volume 2.9 cm^g"^. Details on the synthesis and characterization of the sol-gel titania-silica by means of FTIR, Raman and UV-vis spectroscopies, Ng-physisorption, XRD, XPS, TEM and thermal analysis have been reported previously [5, 6, 14, 15]. For the treatment with bases, 1 g calcined sample was mixed in 25 ml of a 0.1 M aqueous solution of the base at 80 °C for 30 min, than filtered, washed with water, dried at 100 °C for 1 h and re-calcined at 600 °C for 3 h. 2.2 Epoxidation of isophorone The oxidation reactions were performed in a closed, mechanically stirred 100 ml glass batch reactor under Ar. For the epoxidation of a-isophorone, 0.2 g catalyst, 9 ml solvent, 7.2 mmol cumene (internal standard) and 77 mmol olefin were introduced into the reactor. The slurry was heated to the reaction temperature and the reaction started by adding 13.4 mmol t-butyl hydroperoxide (TBHP, ca. 3 M in isooctane)fi:-oma dropping funnel to the vigorously stirred slurry (n = 1000 min" ). For the epoxidation of P-isophorone, 20 ml ethylbenzene solvent, 61 mmol Pisophorone, 7.2 mmol cumene and 5.6 mmol TBHP or cumene hydroperoxide (CHP) were introduced into the reactor in this order. The solution was heated to 80 °C and 0.2 g catalyst was added. Conversion and selectivities were determined by GC analysis (cool on-column injection, HP-1 coliman). Products were identified by GC-MS and NMR spectroscopy. The initial rate (r^) is defined as the epoxide formation in the first 20 min. Hydroperoxide conversion was determined by iodometric titration. Selectivities are calculated as follows: ^peroxide ^^^ = ^^^ * [epoxide] / ([peroxide]0 - [peroxide]) Soiefm (^^) = 100 ^ [epoxide] / ([olefin]^ - [olefin])

331 3. RESULTS AND DISCUSSION 3.1 Epoxidation of a-isophorone Several solvents have been tested in the epoxidation of a- isophorone with tbutyl hydroperoxide (TBHP). The best performance of the aerogel was observed in low polarity solvents such as ethylbenzene or cumene (Table 1). In these solvents 99 % selectivity related to the olefin converted was obtained at 50 % peroxide conversion, independent of the temperature. Rasing temperature resulted in increasing initial rate and decreasing selectivity related to the peroxide. The low peroxide efiSciency is explained by the homolytic peroxide decomposition. Protic polar solvents were detrimental to the reaction due to their strong coordination to the active sites. There was no epoxide formation in water.

Table 1 Influence of solvents and reaction temperature on initial rates and selectivities in the epoxidation of a- isophorone; catalyst: 20 wt% titania - 80 wt% silica, oxidant: TBHP

Solvent

Temp.

\

°C

To

ôlefin

mmol/min, g

%

Conversion, % =

peroxide

%

50

70

50

70

Ethylbenzene Ethylbenzene Ethylbenzene Ethylbenzene

50 60 70 80

0.12 0.16 0.36 0.69

99 99 99 99

98 97 97 96

89 83 74 69

85 79 71 63

Isooctane Cumene 1,2-Dichlorobenzene l,l,2,2-C2H2Cl4 f-Butanol

60 60 60 60 60

0.10 0.16 0.15 0.15 0.01

95 99 93 90 78

_ ~

79 82 75 73 53

-

The reaction rate was rather low, compared to those observed in the epoxidation of cyclohexene or limonene (4-isopropenyl-l-methyl-1-cyclohexene) under otherwise identical conditions. The likely reason is the electron withdrawing effect of the carbonyl group in a position to the C=C double bond.

332

Very few examples can be found in the literature on the heterogeneously catalyzed epoxidation of a-isophorone. We compared the performance of the titaniasilica aerogel with that of a similarly prepared but conventionally dried xerogel. When using the xerogel in ethylbenzene at 60 °C, the initial rate was almost two orders of magnitude lower than that measured with the aerogel (for the latter see Table 1). Besides, the epoxide selectivities at 50 % peroxide conversion were only 85 and 2 %, related to the olefin or the peroxide, respectively. It seems that the xerogel catalyzed mainly the homol3rtic decomposition of TBHP. The strikingly different behaviour of aerogel and xerogel is attributed to their different pore structure. The aerogel was found to be mesoporous with an average pore diameter of ca. 10 nm, while the xerogel was microporous with a mean pore diameter of « 2 nm [4, 5]. The latter is definitely too small to accomodate the bulky reactant and the oxidant.

Table 2 Comparison of various heterogeneous catalysts in the epoxidation of a- isophorone

Catalyst

Oxidant

aerogel^

TBHP TBHP

KFI alumina^

TBHP

hydrotalcite^

H2O2

Temp. °C

Time h

Productivity g/g,h

5/1 5/1

80 50

2.8 18

1.0 0.47

1/4.5

20

24

0.021

20

72

0.094

Olefin/oxidant mol/mol

ca. 1 / 3

^ 20 wt% TiOg - 80 wt% SiOg ^ 5.5 mmol KF/g, data taken from [17] ^ [Mg]/[A1] = 2.8, data taken fi-om [17]

It is also interesting to compare various types of solid catalysts in the epoxidation of a-isophorone. Unfortunately, a real comparison is rather difficult, as the reaction conditions (temperature, oxidant, concentrations) are different for each catalyst. Due to the lack of information, the comparison shown in Table 2 is based only on the productivity, i.e. the amount of isophorone oxide produced in unit time using unit amount of catalyst. Two set of data were chosen for the 20 wt% titania - 80 wt% silica aerogel, and the best published data were chosen for the hydrotalcite [16, 17] and the alimiina-supported ICF [17, 18]. We assumed that the

333

conditions applied by the authors are at least advantageous for the latter two catalysts, though likely not the optimum. On the basis of these data it seems that the titania-silica aerogel is far the most active material for converting a-isophorone to the corresponding epoxide. Note that in case of strongly electron-deficient olefins usually the base catalyzed epoxidation with H2O2 provides the best results [16, 19].

3.2 Epoxidation of P-isophorone Preliminary experiments revealed that the selectivity of titania-silica aerogel in the epoxidation of P-isophorone was moderate. The selectivity related to the olefin converted was below 90 % at low temperature, and dropped rapidly at 80 °C or above. The most important side reactions were the formation of 3,5,5-trimethyl-2cyclohexene-4-hydroxy-l-one (2) by ring opening of the epoxide (1), and the isomerization of P- to a-isophorone (3), as shown in Scheme 2. Epoxidation of 2 and 3, and the oxidation at the OH group of 2 to a dicarbonyl compound were slow and the amounts of these by-products were usually around 1 % or less.

P-isophorone

The importance of the acid-catalyzed side reactions are illustrated in Table 3 by the product distribution obtained using either TBHP or cumene hydroperoxide (CHP) as oxidant. The epoxidation with TBHP is faster and considerably more selective. When using CHP, about 20 mol% of the coproduct 2-phenyl-2-propanol was dehydrated to a-methylst5n:*ene. It is likely that the simultaneously formed water increases the (Br0nsted) acidity of the aerogel and thus accelerates the ring opening and - to a smaller extent - the isomerization reactions. No oxidation products were formed in the absence of peroxide, as expected. Slow isomerization fi:'om P- to a-isophorone catalyzed by titania-silica was the only reaction observed. The data in Table 3 indicate that the simultaneous presence of peroxide and catalyst in the reaction mixture markedly accelerates the acid-catalyzed isomerization reaction.

334

Table 3 Influence of peroxide used as oxidant in the epoxidation of P-isophorone at 80 °C; catalyst: 20 wt% TiOg-SO wt% SiOg aerogel

Catalyst^ Oxidant^

r^,^

Conv.,^ %

Amount of

Molar product distr.^, %

3^, mmol + + + -

TBHP CHP TBHP

1.4 0.9 -

80 75 6

1

0.25 0.30 0.07 0

83 55 0 0

2

3 10 35 0 0

6 8 100 0

^ + indicates the presence, - the absence of catalyst or oxidant ^ mmol.min'V'^ ^ peroxide conversion after 6 h ^ determined after 6 h reaction time; the total amount of other products was 2 or less

Table 4 Effect of basic treatment of the 20 wt% TiO2-80 wt% Si02 aerogel on the epoxidation of p-isophorone with TBHP at 80 °C

Catalyst treatment

no treatment no treatment NaOAc NaOH

Conv.,^ % Spgj.Qxi^je, %

75 90 90 90

_ C

74 66 _ C

Molar product distribution,^ % 1

2

3

83 78 94 49

10 13 5 23

6 7 1 27

^ peroxide conversion ^ the total amount of other products was 2 % or less ^ not determined

335 Both the formation of 4-hydroxy-isophorone (2) from the epoxide (1) and that of a-isophorone (3) fromp-isophorone are catalyzed by the acidic sites of the aerogel. Neutralization of the Br0nsted acidic sites of titania-silica suppresses the isomerization reactions and favours the epoxide formation. The influence of the catalyst treatment with weak and strong bases is illustrated in Table 4. The best catalyst performance was achieved after treating the titania-silica aerogel with an aqueous NaOAc solution and recalcination in flowing air at 600 °C. 94 % epoxide selectivity related to the converted P-isophorone was obtained at 90 % peroxide conversion. Applying a strong base in similar concentration can have a detrimental effect on the epoxide selectivity. For example, titania-silica treated with NaOH provided more isomerization products than epoxide. This is an indication that bases are also excellent catalysts of the isomerization reactions leading to the formation of 2 and 3, in agreement with literature data [19]. The significant improvement of epoxide selectivity after ion-exchange with weak bases has already been reported for TS-1 [11, 20-22] and Ti-beta [23, 24].

4. CONCLUSIONS The epoxidation of two cycloalkenones, a- and P-isophorone, with alkyl hydroperoxides demonstrates that active and selective titania-silica aerogels can be prepared by the sol-gel method combined with extraction of the solvent with supercritical CO2 at low temperature. The key factors for obtaining high activity in the epoxidation of bulky cyclic olefins are the high Ti-distribution in the silica matrix, the mesoporous structure and high surface area. The electron deficiency of a-isophorone seems to aflect mainly the reaction rate, whereas the selectivity to epoxide is high (up to 99 %). A comparative study shows that the productivity (peroxide produced per unit time and unit amount of catalyst) of titania-silica is outstanding compared to other types of solid epoxidation catalysts. The epoxide selectivity is considerably lower in the other model reaction, the oxidation of P-isophorone. The acid-catalyzed side reactions could be suppressed by a treatment of the mixed oxide catalyst with a weakly basic salt prior to the reaction. The epoxide selectivity related to the olefin converted could be increased up to 94 % at 90 % peroxide conversion.

ACKNOWLEDGEMENT Financial support of this work by F. Hoffmann-La Roche AG, Switzerland, and the Kommission zur Forderung der wissenschaftlichen Forschung is gratefully acknowledged.

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

Brit. Pat. 1 249 079 (1971). R. A. Sheldon, J. A. Van Doom, C. W. A. Schwarm and A. J. De Jong, J. Catal., 31 (1973) 438. S. Imamura, T. Nakai, H. Kanai and T. Ito, Catal. Lett., 28 (1994) 277. R. Hutter, D. C. M. Dutoit, T. Mallat, M. Schneider and A. Baiker, J. Chem. Soc. Chem. Comm., 163 (1995). D. C. M. Dutoit, M. Schneider and A. Baiker, J. Catal., 153 (1995) 165. R. Hutter, T. Mallat and A. Baiker, J. Catal., 153 (1995) 177. Z. Liu, G. M. Crumbaugh and R. J. Davis, J. Catal., 159 (1996) 83. S. Klein, J. A. Martens, R. Parton, K. Vercruysse, P. A. Jacobs and W. F. Maier, Catal. Lett., 38 (1996) 209. A. Corma, M. T. Navarro and J. Perez-Pariente, J. Chem. Soc. Chem. Comm., (1994)147. T. Mashmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature, 378 (1995) 159. M. G. Clerici and P. Ingallina, J. Catal., 140 (1993) 71. A. Corma, P. Esteve, A. Martinez and S. Valencia, J. Catal., 152 (1995) 18. R. Hutter, T. Mallat, D. Dutoit and A. Baiker, Topics Catal., 3 (1996) 421. D. C. M. Dutoit, M. Schneider, R. Hutter and A. Baiker, J. Catal., 161 (1996) 651. D. C. M. Dutoit, U. Gobel, M. Schneider and A. Baiker, J. Catal., (in press). C. Cativiela, F. Figueras, J. M. Fraile, J. L Garcia and J. A. Mayoral, Terahedron Lett., 36 (1995) 4125. J. M. Fraile, J. L Garcia, J. A. Mayoral and F. Figueras, Tetrahedron Lett., 37(1996)5995. V. K. Vadav and K. K. Kapoor, Tetrahedron Lett., 35 (1994) 9481. M. Bartok and K. L. Lang, in A. Hassner (ed.). Chemistry of Heterocyclic Compounds, Vol. 42, Wiley, New York, 1985, p. 1. G. J. Hutchings and D. F. Lee, J. Chem. Soc. Chem. Comm., (1994) 1095. H. Gao, G. Lu, J. Suo and S. Li, Appl. Catal. A, 138 (1996) 27. G. J. Hutchings, D. F. Lee and A. R. Minihan, Catal. Lett., 39 (1996) 83. T. Sato, J. Dakka and R. A. Sheldon, J. Chem. Soc. Chem. Commun., (1994) 1887. T. Sato, J. Dakka and R. A. Sheldon, Stud. Surf. Sci. Catal., 84 (1994) 1853.


337

Selective Aerobic Epoxidation of Olefins over NaY and NaZSM-5 Zeolites Containing Transition Metal Ions O.A. Kholdeeva', A.V. Tkachev^ V.N. Romannikov*, I.V. Khavmtskii* and K.I.Zamaraev ' Boreskov Institute of Catalysis, 5 Lavrentjev Ave., Novosibirsk 630090, Russia ^ Novosibirsk Institute of Organic Chemistry, 9 Lavrentjev Ave., Novosibirsk 630090, Russia Aerobic epoxidation of different alkenes, including a number of natural terpenes, efficiently occurs under mild reaction conditions in the presence of isobutyraldehyde as a reductant and MNaY and MNaZSM-5 type zeolites (M=Co(II), Cu(II), Ni(II) and Fe(in)) as catalysts. Yields of the epoxidation products vary from 80 up to 99% depending on the olefin and catalyst. The reaction proceeds via chain radical mechanism, acylperoxy radicals being the main epoxidizing species. 1. INTRODUCTION Epoxidation of olefins has received much attention since epoxides are very important synthetic intermediates that can be regio- and stereoselective^ converted to a wide variety of oxygen-containing compounds [see for reviews ref 1-3]. Catalytic epoxidation with molecular oxygen under mild conditions is a process of challenge from both economic and ecological points of view. Very efficient catalytic systems for alkene epoxidation based on the combined use of dioxygen, branched aliphatic aldehydes and various transition metal compounds have been found recently [4-14]. Most of them describe the use of homogeneous catalysts and only few papers deal with applications of heterogeneous ones [10-12]. We report here on the catalytic properties of MNaY and MNaZSM-5 type zeolites (M=Co(n), Cu(n), Ni(n) and Fe(III)) in the alkene-aldehyde co-oxidation and discuss some mechanistic features of this reaction. 2. RESULTS AND DISCUSSION We have studied efficiency of MNaY and MNaZSM-5 type zeolites with M= Co(II), Cu(n), Ni(II) and Fe(III) in aerobic epoxidation using /raw^-stilbene as model substrate and isobutyraldehyde (IBA) as reductant. The resuhs are summarized in Table 1. Trans-stilbene epoxide was found to be the main oxidation product, isobutyric acid being the main product of transformation of IB A. Order of the catalytic activity of the metal ions introduced into NaY zeolites (Co > Cu » Ni, Fe, NaY) is similar to that obtained previously for M-substituted heteropolytungstates [13]. Pronounced catalytic activity of CoNaY and NiNaY zeolites was earlier observed for co-oxidation of linear alkenes with acetaldehyde at 70°C [15]. The extents of ion exchange that can be attained for NaZSM-5 catalysts are less than those for NaY

338 zeolites. In contrast to MNaY, content of M in MNaZSM-5 is less as compared to the total content of iron in zeolite (see Table 4 in Experimental). This is probably the reason that catalytic properties of MNaZSM-5 zeolites do not vary significantly for different M and are close to those of the parent NaZSM-5 catalyst. Selectivity of epoxidation is sufficiently high for all zeolites except for FeNaZSM-5 (Table 1). Low selectivity of the later zeolite arises probablyfiômsubstantial replacement of Na^ ions by H" during the ion exchange procedure (see Table 4) because of higher hydrolysis of Fe(III) ions as compared to the other M ions studied. It is known that H-forms of zeolites are able to catalyze epoxideringcleavage as well as some other transformations of alkenes and their epoxides [16]. Table 1 Aerobic epoxidation of/row^-stilbene in the presence of IB A and zeolite catalysts^*^ Zeolite (mg) CoNaY (35) CoNaY(1.8) CuNaY (35) FeNaY (35) FeNaY (78) NiNaY(35) NaY (35) without catalyst NaZSM-5(549) CoNaZSM-5 (549) CoNaZSM-5 (549) FeNaZSM-5 (549) CuNaZSM-5 (549)

Content of transition metal (xlO^mmol) 2.0 0.1 1.9 2.8^^> 6.2^^>

2.0 6.2^^^

0.1 0.1 6.1

1.9

Time (h)

Stilbene conversion (%)

Yield of transepoxide^^(%)

Ti

Too

3.5 2.5 18 6.0 9.0 9.5 7.0 5.0 3.5 7.0 6.0 5.0

100 100 100 34 96 69 35 100 42 100 100 100

98 89 98 80 80 98 65 55 84 90 87 65 83

^*^ Reaction conditions: stilbene 0.30 mmol, isobutyraldehyde 2.28 mmol, acetonitrile 3 ml, air 1 atm, 18°C. ^^ GLC yield based on alkene consumed. ^^"^ Total content o f iron. A s one can see fi'om Table 1, catalytic activity o f M N a Y and MNaZSM-5 zeolites is not the same despite the same content o f the transition metal. Activity o f C o N a Y is higher than that o f CoNaZSM-5 at the same content o f cobalt (respectively 100 and 4 2 % stilbene conversion for 3.5 h) and, on the contrary, the activity o f FeNaY is poorer as compared to FeNaZSM-5 ( 3 4 and 100% respectively for 6 h). These facts indicate that structure o f zeolite remarkably influences upon the oxidation process. Using the most active catalyst (CoNaY) w e have studied the oxidation o f olefins o f different structure and size o f molecules, including a number o f natural terpenes, namely, (+)-a-pinene (1), (+)-3-carene (2), (-)-caryophyllene (3) and dipentene (4). W e have found that even acidsensitive epoxides that are known to be prone to ring cleavage (caryophyllene epoxide, for example) can be obtained with high-to-excellent selectivity (Table 2). It is noteworthy that neither allylic oxidation nor overoxidation occurs in the systems studied. Diolefins give mono-

339 or diepoxides depending on the reaction time (see Table 2). 4,5-Monoepoxide of caryophyllene can be obtained with high regioselectivity (>99%) at 100% alkene conversion. Regioselectivity o f the epoxidation of dipentene is also high (96 and 4% of 1,2- and 8,9-epoxides respectively at 40% alkene conversion). These facts are in accordance with reaction ability of the carboncarbon double bonds to electrophylic reagents, including peroxy acids. Distribution of isomers (see Table 2) is similar to that observed earlier for epoxidation with peroxy acids [17]. Table 2 Aerobic epoxidation of terpenes in the presence of IB A and CoNaY catalyst^'^ Alkene

Solvent

^

1

JCX

2

C /-A

/

Alkene conversion (%)

Yield of epoxide^^ (%)

methylene chloride

5.0

100

95

methylene chloride

2.0

100

99

1,2-dichloroethane

2.0^^^^

100

99(d)

5.0

100

2.0

40

98(0

5.0

100

97(g)

3 1

Time (h)

methylene chloride

4^-8

4

rto(c)

99

^"^ Reaction conditions: alkene 0.30 mmol, isobutyraldehyde 2.28 mmol, CoNaY 35 mg, solvent 3 ml, air 1 atm, 24°C. ^^ Determined by ^H NMR. ^""^ For another sample of 3 the time of 100% alkene conversion was 9 h. ^^^ Only monoepoxide is formed; 4p,5a-/4a,5Pepoxide=85:15. ^^^ Mono-/diepoxide=2:l. ^^ l,2-/8,9-epoxide=24:l, â«5-/c/5-epoxide=47:26 {tranS'lcis- indicate the orientation of the epoxide oxygen relative to isopropenyl group in the 6-membered ring). ^^^ Mono-/diepoxide= 13:1. Caryophyllene 3, having strained /ran^-substituted carbon-carbon double bond in the ninemembered ring, is known to be one of the most reactive olefins in the reactions of electrophylic addition, including epoxidation by peroxy acids [17]. It was surprised that in our first series of experiments w e observed no significant difference in activity of 3 and 2 (Table 2). Moreover, different samples of caryopyllene (3) with the same N M R and GLC parameters, exhibit different activity when oxidation with 02/IBA/CoNaY system is performed. Kinetic curves for alkene consumption and epoxide accumulation have an induction period that differs from sample to sample. As a result, the period of time necessary for 100% caryophyllene conversion under the conditions specified in the Table 2 varied from 2 to 9 h. Additionally, w e have found that the reaction proceeds more slowly with increasing concentration of 3. When using 0.5 M of this alkene, the oxidation process does not proceed at all in a reasonable time period. We carried out our experiments with the samples o f caryophyllene isolated fi-om essential oil of

340 Eugenia caryopyllata that is known to be the natural source of eugenol - phenolic compound that might be an inhibitor of chain radical reactions. Different samples of caryophyllene could differ in the content of the phenolic microimpurity (eugenol) and, therefore, posses different activity. All these facts have lead us to the suggestion that the epoxidation process in 02/IBA/CoNaY system has chain radical nature, and negligible impurities can dramatically influence on the reaction rate. To the date, it is well established that two main types of species, namely, peroxy acids and acylperoxy radicals, which are produced during aldehyde autoxidation, can act as active epoxidizing species [7, 11-14]. The ratio of radical to non-radical pathways of epoxidation in alkene-aldehyde co-oxidation is dependent on both nature of reagents and reaction conditions [18]. Recently we have reported that stilbene epoxidation by O2/IBA in the presence of transition metal substituted heteropolytungstates proceeds via chain radical mechanism, acylperoxy radicals being the main epoxidizing species [13, 19]. We have extended our mechanistic study on 02/IBA/zeolite system and found that small additives of chain radical inhibitors, such as 1,4-hydroquinone and 2,6-di-tert-butyl-4-methylphenol (ionol), stop the reaction that restarts only after the complete consumption of an inhibitor (Fig. 1). We also have found that small additives of eugenol cause great increase of the induction period in both caryophyllene and stilbene epoxidation. These facts confirmed that epoxidation by O2/IBA over zeolite catalysts has chain radical nature. Some disadvantages of the described method of preparation of epoxides, namely, high sensitivity of the reaction rate to microimpurities of inhibitors and initiators and, as a consequence, poor reproducibility of the reaction time are due to the chain radical mechanism. However, the chain radical nature of the process turns out to be profitable for epoxidation of diolefins since monoepoxides can be readily obtained by addition of an inhibitor after the complete conversion of the starting compound to monoepoxide (to prevent the formation of diepoxides). Figure 1.

2.0 25 Time, h

Kinetic curves of alkene consumption (7-i) and epoxide accumulation (7' and 2*) for aerobic oxidation of trans-stilbene (0.1 M) in the presence of isobutyraldehyde (0.37 M) and CoNaY (11 mg) in MeCN at 24°C: 7, 7' - ionol (1x10'^ M) was added before the addition of IB A; 2, 2' and 5' - hydroquinone (1x10"^ M) was added after 56 min. and before the reaction, respectively.

To define the truthfiil order of the reactivity of the terpenes we have carried out competitive oxidation of 1, 2 and 3 in Oz/IBA/CoNaY system. As one can see from Fig.2, caryophyllene (3) is much more active than 1 and 2, the consumption of the two later olefins being started only after the quite complete consumption of 3. It should be mentioned that the results obtained in the competitive oxidation of terpenes 1-3 by O2/IBA in the presence of

341 homogeneous catalysts, for example Co(N03)2, are absolutely identical to those obtained with the zeolite catalysts. There is no considerable influence of steric restrictions on the epoxidation process in the later system. This indicates that interaction of epoxidizing species with alkenes takes place mainly at the outer surface of zeolite crystals. Otherwise, the oxidation of 3 should be much more slowly than that of 1 and 2. Nevertheless we can not exclude that generation of the active radicals as well as epoxidation of the alkenes with small kinetic diameter may, at least partially, proceed inside zeolite crystals. Figure 2. Competitive oxidation of a-pinene (0.033 M), 3-carene (0.033 M) and caryophyllene (0.033 M) by air oxygen (1 atm) in the presence of isobutyraldehyde (0.74 M) and CoNaY (34 mg) in MeCN at 24°C: 7 - caryophyllene, 2 - a-pinene and 3 - 3-carene.

The experiments with inhibitor, which was added when the reaction rate attained its maximum value (Fig. 1), showed that the role of Prilezhaev reaction in the epoxide formation is negligible and acylperoxy radicals are the main epoxidizing species. Perisobutiric acid (PEBAC) was detected by ^H NMR during both IB A autoxidation [7] and alkene-IBA co-oxidation [19]. To clarify its role in the epoxidation process we have studied stereochemistry of the epoxidation of cw-stilbene with PIBAC and O2/IBA both in the presence of CoNaY zeolite and without any catalyst (Table 3). It is well established that the alkene epoxidation with peroxy acids proceeds stereospecifically [3], whereas the radical epoxidation results in inversion of c/5-alkene configuration [18]. We have shown that c/W/raw^-epoxide ratio is greatly dependent on the presence of a catalyst and nature of the solvent. When PIBAC is used in 1,2dichloroethane in the absence of CoNaY catalyst, c/5-stilbene epoxide was presumably formed. Stereospecificity of the epoxidation of cw-stilbene in the O2/IBA/DCE system in the absence of catalyst was reported previously [20]. When using acetonitrile as a solvent, the cis-/transepoxide ratio is decreased as compared to 1,2-dichloroethane (Table 3). The use of CoNaY catalyst dramatically influences on the cw-stilbene epoxidation with PIBAC, /raw5-epoxide being preferably formed. The inversion of c/5-stilbene configuration was also observed for the 02/IBA/CoNaY system. Recently we have found that cobah-containing compounds, namely heteropolycomplexes, catalyze homolytic decomposition of PIBAC and therefore can increase the rate of degenerate branching in the chain radical process of the alkene-IBA co-oxidation. The results of this work demonstrate that the use of both acetonitrile and CoNaY catalyst greatly enhances the formation of âw5^-epoxidefi"omc/5-stilbene. These indicate that CoNaY zeolite, especially in an acetonitrile medium, mediates decomposition of PIBAC. In independent experiments without an alkene we have found that 54% of PIBAC are

342

decomposed in 2 h in the presence of CoNaY zeolite in an acetonitrile medium under the conditions described in Table 3. Table 3 Epoxidation of cw-stilbene by PIBAC and O2/IBA over CoNaY catalyst^*^ Solvent

Cis/trans-epo^de ratio^^

acetonitrile acetonitrile l,2.dichloroethane 1,2-dichloroethane acetonitrile acetonitrile

1:52 1:90 1:20 3.2:1 1.7:1 1^9^2

Oxidant 02/(CH3)2CHCHO^'^ 02/(CH3)2CHCHO 02/(CH3)2CHCHO (CH3)2CHCOOOrf'*^ (CH3)2CHCOOOrf*^^ (CH3)2CHCOOOH

^""^ Cw-stilbene 0.30 mmol, isobutyraldehyde 2.28 mmol or perisobutyric acid 0.30 mmol, solvent 3 ml, air 1 atm, 24°C. ^^ Determined by ^H NMR by comparison of lines with 5 4.15 (c/5-epoxide) and 5 3.66 (/raw^-epoxide) in CCU-CeDe mixture (9:1). ^""^ Isobutyraldehyde 1.14 mmol. ^^^ Without catalyst. The results obtained allow us to propose the reaction mechanism comprising the following elementary steps of the chain radical process leading to epoxide and isobutyric acid formation: RCO;+ = — ^

,^-' RCOg'^

(6)

RCHO + M""^*— RCO + M " ^ + H^

(1)

RCO+O2—»- RCO^'

(2)

RCO^' + RCHO— RCO3H+ RCO

(3)

RCO3H + M""— RCOJ+M"^ V OH"

(4)

RCO2 + RCHO — ^ RCO2H + RCO

(8)

R C 0 3 H + M"^^—» RC03+M"%H+

(5)

2RC0^ — ^ termination

(9)

RC03^'-^

Z^+RCOJ

(7)

3. EXPERIMENTAL Catalysts. Commercial sodium forms of Y (alumino-silicate lattice composition) and ZSM5 (iron-alumino-silicate lattice composition) zeolites containing not less than 95% of the corresponding main phase were used as starting material. Transition-metal-ion-exchanged forms, MNaY and MNaZSM-5, were prepared by well known ion exchange procedure at ambient temperature using 0.1 M aqueous solutions of the corresponding metal salts followed by filtration, washing with water, drying and calcination under air flow at 500-550°C for 2 h. The obtained catalysts were characterized by elemental analysis (Table 4). Materials. Trnw^-stilbene (Fluka AG) and (+)-a-pinene (Aldrich Chemical Company) were used as received. Cw-stilbene and perisobutyric acid (PIBAC) were prepared as described in [19]. (-)-Caryophyllene (>99%) and eugenol (95%) were isolated from the oil of Eugenia , 95%] caryopyllata by vacuum rectification. (+)-3-Carene (95%) and dipentene were prepared by rectification of the Pinus sylvestris turpentine.

343

Oxidation procedure. Alkene oxidation was carried out in a thermostated 20 ml Pyrexglass reactor equipped with a stirring bar and a reflux condenser. Isobutyraldehyde was added to a solution of alkene (0.1 M) in a solvent (3 ml) containing a catalyst, and the reaction mixture was vigorously stirred. Table 4. Elemental analysis of zeolite catalysts Content (%) Zeolite

NaY FeNaY CuNaY CoNaY NiNaY NaZSM-5 FeNaZSM-5 CuNaZSM-5 CoNaZSM-5

M 4.37^^^ 3.41 3.29 3.35

0.62^*^ 0.22 0.011

Na

Al

Fe

6.03 2.24 3.64 3.20 3.38 1.05 0.14 0.83 0.81

8.50 8.22 8.95 8.51 8.71 0.93 0.61 0.63 0.60

0.073 4.37^"^ 0.075 0.067 0.060 0.71 0.62^'^ 0.60 0.59

^""^ Total content of iron. Product analysis. The oxidation process was monitored by GLC ("Tsvet-500", 2mx3mm Carbowax 20M on Chromaton N-AW-HMDS for stilbenes and 15mx0.3mm SE-30 for other alkenes, Ar, FID). The reaction mixture was percolated through alumina (/=2 cm, 0 = 1 cm), concentrated at reduced pressure and the crude product was analyzed by ^H and ^^C NMR on a Bruker AM 400 instrument. Trans- and cw-stilbene epoxides, a-pinene epoxide, (+)-3carene epoxide, isomeric limonene and steroisomeric caryophyllene epoxides were identified by comparing NMR spectra of the products with the spectra of authentic samples prepared by traditional peracid oxidation of the corresponding hydrocarbons. The yields of /ra«5-stilbene epoxide and the degree of the alkenes conversion were measured by GLC using biphenyl as an internal standard. For other alkenes the crude product (50-70 mg) was separated by column chromatography on a silica gel column (/=10 cm, 0 = 1 cm, silica gel 0.040-0.100 mm, air-dried and activated at 130°C for 8 h,) using solutions of 0-10% diethyl ether in pentane as eluents. The following fi-actions were collected: Rf=0.85-0.90 (Et20-pentane 1:9, v/v) - starting hydrocarbons, RÔ.45-0.55 - monoepoxides, Rf=0.20-0.30 - diepoxies (on Silufol®).

4. CONCLUSIONS Epoxidation of alkenes can be performed efficiently by molecular oxygen (air) in the presence of isobutyraldehyde and NaY or NaZSM-5 zeolite catalysts containing transition metal ions. High selectivity of epoxidation is achieved (up to 99%) in spite of the chain radical nature of the reaction.

344

ACKNOWLEDGMENTS The research described in this publication was made possible in part by Grant No 96-0334215 from the Russian Foundation for Basic Research. REFERENCES 1. R.A. Sheldon and J.K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981. 2. K.A. Jorgensen, Chem. Rev., 89 (1989) 431. 3. D. Swem, in D. Swem (Ed.), Organic Peroxides, Vol. 2, Wiley-Interscience, New York, 1971, p. 355. 4. T. Yamada, T.Takai, and T. Mukaiyama, J. Syn. Org. Chem. Jpn., 51 (1993) 995. 5. T. Yamada, T. Takai, O. Rhode and T. Mukaiyama, Bull. Chem. Soc. Jpn., 64 (1991) 2109. 6. N. Mizuno, T. Hirose, M. Tateishi and M. Iwamoto, Chem. Lett. (1993) 1839, 1985. 7. M. Hamamoto, K. Nakayama, Y. Nishiyama and Y. Ishii, J. Org. Chem., 58 (1993) 6421. 8. S.-I. Mirahashi, Y Oda, T. Naota andN. Komiya, J. Chem. Soc. Chem. Commua, (1993) 139. 9. T. Nagata, K. Imagawa, T. Yamada and T. Mukaiyama, Bull. Chem Soc. Jpa, 68 (1995) 1455. 10. A. Atlamsani, E. Pedraza, C. Potvin, E. Duprey, O. Mohammedi and J.-M. Bregeault, C.R. Acad. Sci. Paris, ser. II, 317 (1993) 757. I I P . Laszlo and M. Levart, Tetr. Lett., 34 (1993) 1127. 12. E. Bouhlel, P. Laszlo, M. Levart, M.-T. Montaufier and GP. Singh, Tetr. Lett., 34 (1993) 1123. 13. O.A. Kholdeeva, V.A. Grigoriev, G.M. Maksimov and K.I. Zamaraev, Topics in Catalysis, (1996) accepted for publication. 14. P. Mastrorilli, C.F. Nobile, GP. SurannaandL. Lopez, Tetrahedron, 51 (1995) 7943. 15. S.A. Maslov, G Vagner and V.L. Rubailo, Neftechimia, 26 (1986) 540 (in Russian). 16. N.F. Salakhutdinov, E.A. Kobzar, D.V. Korchagina, L.E. Tatarova, KG. lone, V.ABarkhash, Zh. Org. Khim, 29 (1993) 316 (in Russian). 17. A.V.Tkachev, Khim. Prir. Soedin., (1987) 475 (in Russian). 18. A.D. Vreugdenhil and H. Reit, Reel. Trav. Chim. Pays-Bas, 91 (1972) 237. 19. O.A. Kholdeeva, V.A. Grigoriev, G.M. Maksimov, MA. Fedotov, A.V. Golovin and K.I. Zamaraev, J. Mol. Catal, accepted for publication. 20. K. Kaneda, S. Haruna, T. Imanaka, M. Hamamoto, Y. Nishiyama and Y. Ishii, Tetr. Lett., 33 (1992) 6827.


345

Effect of P r e p a r a t i o n M e t h o d s of T i t a n i a / s i l i c a s o n t h e i r C a t a l y t i c Activities in t h e O x i d a t i o n of Olefins M. Toba, S. Niwa, H. Shimada and F. Mizukami Department of Surface Chemistry, National Institute of Materials and Chemical Research, Tsukuba 305, Japan Titania/silica catalysts were prepared by a conventional procedure (precipitation) and a complexing-agent assisted sol-gel method. The effect of preparation methods of titania/silica catalysts on their properties and catalytic activities in the oxidation of olefins were examined. The sol-gel method gave the best dispersion of titania. In contrast, using the precipitation method, titania is deposited at the external surface of silica with formation of crystalline particles. The sol-gel catalysts are more effective for epoxidation of olefins because of the high dispersion of Ti in them.

1. INTRODUCTION It is known t h a t titania/silica can catalyze oxidation reactions [1-31. Especially, titanium-silicate-1 (TS-1) has been shown to be a very effective catalyst for oxidation reactions. In the TS-1 catalyst, most Ti atoms are isolated from each other by long chains of -0-Si-O-Si-O- and this structure gives high selectivity for the formation of epoxides from olefins [1]. We have reported t h a t properties of titania/silicas depend on their preparation methods and a complexing-agent assisted sol-gel method gives the most homogeneous titania/silicas [4]. In the sol-gel titania/silicas, Ti-O-Si bonds are more effectively formed and Si and Ti components are higher dispersed than those in conventional titania/silicas [41. Therefore, it is expected that the sol-gel titania/silicas are also effective catalysts for oxidation reactions. In this work, we prepared titania/silicas using a conventional procedure (precipitation) and a complexing-agent assisted sol-gel method, and examined

346 the effect of preparation methods of titania/sihca catalysts on their properties and catalytic activities in the oxidation of olefins. 2. EXPERIMENTAL 2.1 P r e p a r a t i o n of t i t a n i a / s i l i c a s Titania/silicas with different molar ratios of Ti to Si were prepared using precipitation and complexing-agent assisted sol-gel method [4]. complexing-agent assisted sol-gel method A series of the sol-gel titania/silicas were synthesized using diols, diketones and ketoesters as complexing agents. The typical procedure is as follows: tetraethyl orthosilicate and titanium iso-propoxide were mixed in a 2propanol-biacetyl (2.5 mol/ mol alkoxide) solution containing a catalytic amount of dimethyl sulfate, stirred and kept at 80°C for 3 h (step 1). When the solution appeared homogeneous, water (4mol/mol alkoxide) was added to the solution to hydrolyze the various diketone-metal complexes formed by ligand exchange reaction and evolution of monoalcohol at step 1 (step 2). The solution became viscous and finally coagulated into a transparent monolithic gel. The gel was dried at ca. 130~140°C under reduced pressure (step 3). Finally, the dry gel obtained was finely powdered and then calcined at 500°C for 4h. titania precipitation Silica supports were prepared by a complexing-agent assisted sol-gel method from tetraethyl orthosilicate and calcined at 500°C for 2h. Titanium isopropoxide was added to silica-2-propanol suspension and then water was added to the suspension. The resulting solid was dried at llO'^C and then calcined at 500°C for 2h. 2.2 C h a r a c t e r i z a t i o n Bulk Si/Ti ratios of titania/silicas were measured by X-ray fluorescence analyses carried out on a Seiko I n s t r u m e n t Co. SEA-2010. The X-ray photoelectron spectra were obtained with a Perkin Elmer ESCA5500 with a monochromatised Mg-Ka source. The Cls, 0 1 s , Si2p, and Ti2p lines were investigated and their binding energies were referenced to the C l s line at 285 eV. The X-ray powder diffraction patterns were obtained on a MAC Science

347

MXP-18 instrument using Cu-Ka radiation with a Ni filter. The X-ray powder diffraction patterns were obtained on a MAC Science MXP-18 instrument. 2.3 Epoxidation of olefins A typical reaction procedure was as follows. 10.0 mmol of olefin, 10.0 mmol of oxidant and 10 ml of solvent were charged into a 50 ml three-necked glass reactor equipped with a condenser and a magnetic stirrer. 50 mg of catalyst was added and the mixture was stirred at 60°C. Analysis was performed by gas chromatograph (column; OV-1 bonded 0.25mm X 50 m). 3 RESULTS A N D DISCUSSION 3.1 Bulk and surface composition The results of bulk Si/Ti compositions of titania/silicas characterized by Xray fluorescence analyses are shown in figure 2. Observed bulk Si/Ti ratios of sol-gel titania/silicas agree with those of precipitated titania/silicas and they are proportional to calculated ratios. The results of surface Si/Ti compositions of titania/silicas characterized by XPS are shown in figure 3. In order to calculate the surface atomic ratios, n(Si)/n(Ti), the following equation was used [5]: n(Si)/n(Ti) = {I(Si2p) / I(Ti2p3/2 ) }{a(Ti)/a(Si)} where I is the intensity and a is the sensitivity factor [6] (a(Si)=0.52, a(Ti)=1.2), respectively. The surface atomic ratios of n(Si)/n(Ti) of the sol-gel titania/silicas increase with increasing Si/Ti ratio of 1 to 5. However, those of the precipitated titania/silicas are almost constant and their values are low. These results mean that the distribution of titanium and silicon around the surface of catalyst depends on the preparation method. Therefore, there is a good correlation between the surface and bulk Si/Ti ratios in the sol-gel catalyst. This suggests that Si and Ti components are homogeneously dispersed at both the surface and inside of the sol-gel titania/silica. On the other hand, the content of titanium at the surface of precipitated titania/silica was much higher compared to the corresponding sol-gel titania/silica and did not depend on the bulk titanium content. These results mean t h a t the surface of precipitated titania/silica was covered with titania as is expected from the preparation procedure.

348

3 > 9 9 A

^1 0

_l

0

I

1 2

I

I

L

3

4

5

6

Si/Ti (calculation) F i g u r e 1 X - r a y f l u o r e s c e n c e a n a l y s e s of t i t a n i a / s i l i c a s sol-gel A precipitation

1 2 3 4 5 Si/Ti (calculation)

6

Figure 2 Surface atomic ratios of n(Si)/n(Ti) of titania/silicas calculated by XPS Spectra sol-gel A precipitation 3 ^ X-ray diffraction of titania/silicas The dispersion of components in the titania/silicas were characterized by X-ray powder diffraction. The results are shown in Figure 3. All precipitated titania/silicas showed patterns characteristic of anatase titania. However, solgel titania/silicas with Ti/Si ratios of less than 1 did not show clear diffraction peaks, indicative for their amorphous nature in spite of almost same bulk Si/Ti compositions. These results indicate that the crystallinity of the precipitated

349

titania/silicas is higher than that of the sol-gel ones, indicating the extent of aggregation of titania in the sol-gel titania/silicas is less t h a n t h a t in precipitated samples. Therefore, the dispersion of Ti in the sol-gel titania/silicas is higher than that of precipitated ones.

sol-gel SiA'i=l SiyTi=2

precipitation SiyTi=l iL

o o CD

o

1

10

100

Si/Ti Figure 4 Effect of preparation method on the reactivity of titania/silica catalysts in the epoxidation of cyclooctene with tert-butyl hydroperoxide in tert-butanol sol-gel O n precipitation Reaction conditions; 1.10 g (10.0 mmol) of cyclooctene; 1.13 g (10.0 mmol) of 80% tert-butyl hydroperoxide; 10 ml of tertbutanol, catalyst 50 mg; temperature, 60°C; time, 20h 3.4 Effect of solvent Table 1 shows the effect of solvent on the reactivity of the sol-gel titania/silica catalysts in the epoxidation of cyclooctene. The reactivities observed depend on the solvents. Acetonitrile is most suitable for the epoxidation. However, other nitriles such as propionitrile, isobutyronitrile and pivaronitrile do not give good results. In the case of tert-butanol, the conversion and selectivity for epoxide are relatively high. However, the conversion is low in the case of methanol. These results indicate that the reactivity of the catalysts in the epoxidation depends not only on the functional group of the solvent but also on the molecular shape of solvent. Clerici et al. proposed that the solvent coordinates to the catalytic center (Ti) of TS-1 in the epoxidation step and the order of the reactivity is in agreement with the order of electrophilicity and steric constraint of the solvent [7]. So, the facility of solvent coordination to

351 titanium must determine the reactivity of the sol-gel titania/silicas as well as titanosilicate. Table 1 Effect of solvents on the oxidation of cyclooctene with tert-butyl hydroperoxide COO Yield(%) Conversion(%) Solvent COO Select.(%) 8 7 85 Methanol 42 41 97 tert-butanol 36 98 36 1,4-dioxane 78 75 97 Acetonitrile 30 28 96 Propionitrile 30 26 88 Isobutyronitrile 29 23 79 Pivalonitrile 22 21 96 Benzonitrile 33 31 92 Cyclohexane 1 1 84 DMF Reaction conditions; 1.10 g (10.0 mmol) of cyclooctene; 1.13 g (10.0 mmol) of 80% tert-butyl hydroperoxide; 10 ml of solvent, catalyst (titania/silica Si/Ti=5, solgel) 50 mg; temperature, 60°C; time, 20h. COO = cyclooctene oxide

3.5 Effect of o x i d a n t Figure 5 shows effect of oxidant on the reactivity of the sol-gel titania/silica catalysts in the epoxidation of cyclooctene. The conversion in the epoxidation of cyclooctene with tert-butyl hydroperoxide sharply increases with reaction time. However, when the oxidant is hydrogen peroxide, the conversion is quite low

100 95^^ g

H85^ 5

10 15 Time (h)

'80 20

0

5

10 15 Time (h)

Figure 5 Effect of oxidant on the oxidation of cyclooctene in acetonitrile with sol-gel titania/silica (Si/Ti=5)

352 compared to that of tert-butyl hydroperoxide. Low cyclooctene conversion in the case of hydrogen peroxide is presumably caused by alteration of active sites of catalyst and nonproductive decomposition of hydrogen peroxide. Bellussi et al. demonstrated that hydrolysis of Ti-O-Si bond takes place with the formation of Ti-OH and Si-OH groups [8]. This means t h a t the structure of active site, that is, isolated Ti atoms by long chains of -0-Si-O-Si-O-, which gives high selectivity for the formation of epoxides is destroyed by water. The decomposition of active site must occur in the initial stage. Therefore, the conversion in presence of hydrogen peroxide slightly increase with reaction time. From the result of iodometric titration, the conversion of hydrogen peroxide is quite high. The nonproductive decomposition of hydrogen peroxide catalyzed by titania/silicas also gives low cyclooctene conversion. 4. CONCLUSIONS It was found that dispersion of Ti and Si components are accelerated by using the complexing agents. Titania/silicas prepared by the complexing agent-assisted sol-gel method are more homogeneous than the corresponding precipitated ones. The sol-gel catalysts are more effective for epoxidation of olefins because of the high dispersion of Ti component in them. Solvents and oxidants have great influence on the reactivity of the catalysts. REFERENCES 1. B. Notari, "Innovation in Zeolite Material Science" Elsevier, pp. 413-425 (1988). 2. D. C. M. Dutoit, M. Schneider and A. Baiker, J. Catal., 153, 165 (1995). 3. R. Hutter, T. Mallat and A. Baiker, J. Catal., 153, 177 (1995). 4. M. Toba, F. Mizukami, S. Niwa, T. Sano, K. Maeda, A. Annila and V. Komppa, J. Mol. Catal, 91, 277 (1994). 5. Z. Zsoldos, G. Vass, G. Lu and L. Guczi, Appl. Surf. Sci., 78 467 (1994). 6. C. K. J(|)rgensen and H. Berthou, Faraday Discus. Chem. Soc, 54, 269 (1972). 7. M. G. Clerici and P. Ingallina, J. Catal., 140, 71 (1993). 8. G. Bellussi, A. Carati, M. G. Clerici, G. Maddinelli and R. Mikkini, J. Catal., 133, 220 (1992).

Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) 1997 Elsevier Science B,V. All rights reserved.

353

Heterogeneous catalysts from organometallic precursors: how to design isolated, stable and active sites. Applications to zirconium catalyzed organic reactions. A. Choplin, a* B. Coutant, ^ C. Dubuisson, a p. Leyrit, ^ C. McGill, a F. Quignard ^ and R. Teissier.t> Înstitut de Recherches sur la Catalyse -CNRS, conventionne a TUniversite CI. Bernard, Lyon1, 2 avenue A. Einstein, 69 626 Villeurbanne Cedex, France Êlf-Atochem, Centre de Recherche Rhone-Alpes, 69 310 Pierre Benite, France SUMMARY The synthesis of silica supported mononuclear hydride, hydroxide and allkoxide zirconium complexes was achieved by reaction between an homoleptic alkyl complex and a silica showing only isolated surface hydroxyl groups, followed by mild post-anchoring treatments. They are heterogeneous molecular catalysts for the dehydrogenative coupling of silanes, the epoxidation of cyclohexene with H2O2 and the Meerwein-Ponndorf-Verley and Oppenauer reactions respectively. All could be recycled by simple washing, filtration without significant loss of activity and selectivity. The " solid" siloxy ligand induces steric and electronic effects, which intervene for the catalytic properties, but also for the stability of the supported complexes. 1. mXRODUCTION Although heterogeneous catalysts are often preferred for industrial processes, they are generally not as selective and efficient as the homogeneous ones. Supporting homogeneous catalysts either on inorganic or organic solids has so far lead to very few convincing results: the process of complex anchoring induces a modification of either the coordination sphere (electronic and steric effects) or the degree of oxidation of the metal, both being disastrous for the catalytic properties. Finally, the stability of the anchoring bond is very often weak under the catalytic experimental conditions: this is the origin of metal leaching [1]. When optimization of selectivity and activity is the concern, then it is important to have only one surface species. We will show here that it is possible to synthesize isolated well-defined zirconium sites on the surface of a silica and then to adapt the coordination sphere around the metal center to the target reaction. 2. RESULTS AND DISCUSSION Only one surface complex can form when an homoleptic complex is reacted with the surface of a silica pretreated so as to present only isolated surface silanols. Thus the reaction between ZrNp4 (Np = neopentyl) and a silica pretreated at 500°C under vacuum [2] leads to the formation of (=SiO)ZrNp3, A, as the only the surface complex [3]. Although this complex

354 is electronically unsaturated (formally an 8e" species), the bulkiness of the ligands (neopentyl and siloxy) prevents it being a suitable candidate for catalysis. The coordination sphere around the metal must then be transformed; its design is guided by comparison with related homogeneous catalysts. 2.1. Synthesis of the supported catalysts Complexes Cp2ZrR2 (R=H, alkyl) are known to be precursors of catalysts for the dehydrogenative coupling of primary and secondary silanes [4] and for the hydrogen transfer between ketones and alcohols (Meerwein-Ponndorf-Verley and Oppenauer reactions) [5]. These complexes and the presumed reactive catalytic intermediates are stabilized by electron rich cyclopentadienyl ligands. The "solid" siloxy ligands, involved in the silica bonded complexes A (covalent bond Zr-Og) are capable of stabilising efficiently highly reactive species, by preventing their dimerization; they enhance simultaneously the electronic unsaturation of the metal center, maintaining it as an 8e" species.

SiOZrNp3

H2,150°C^

(^SiO)3Zr.H

-

^SiOZr(OH)3

2

_ siOZr(OiPr)3

3

"20^ \

iPiOH, r t ^ ^

i

scheme 1

Surface zirconium hydrides 1 result from hydrogenolysis of the Zr-C bonds of A at 150°C. A monohydride zirconium supported complex is the major species formed, but some zirconium dihydrides are present along with surface silanes, [Sijg-H, the product of reduction of siloxane bridges by the very reducing zirconium hydrides [6]. These latter are catalytically inert. The surface zirconium hydrides are stable up to 200°C under vacuum or hydrogen. Complexes 1 were fully characterized by physical techniques (in situ IR, EXAFS) and their chemical reactivity determined (towards O2, H2O, R-X, ROH, olefins) [6]. Hydrolysis of A under very mild conditions (p(H20) = 22 torr, 25°C) leads to the formation of surface zirconium hydroxides, 2, with evolution of 3 mol neopentane/Zr. It is very difficult to prove unambiguously that the Zr sites remain isolated on the surface, but the absence of an absorption band in the UV spectrum and the catalytic activity of these solids (see below) strongly suggest that no large Zr02 particles have formed. Hydroxycomplexes of Zr (or Ti) are presumed intermediates in the mild oxidation reactions performed with H2^2Surface zirconium isoproxides, 3, are synthesized by alcoholysis of A with isopropanol. The reaction occurs with evolution of neopentane. Subsequent reaction with HCl in Et20 liberates 3.1 moles of isopropanol, confirming the stated stoichiometry. Based on a study by IR spectroscopy on the reactivity of complexes 1 with isopropanol (no intensity change of the band at 3747 cm"^ corresponding to isolated silanol groups), one can safely conclude to the stability of the (=SiO)-Zr bond under our conditions [6].

355

2.2 Activation of alkanes and silanes by surface zirconium hydrides, 1 We have previously shown that the mononuclear zirconium hydride complexes 1 activate, under very mild conditions, the C-H bond of alkanes, including methane [7]. The mechanism involves a four center intermediate, as proposed earlier for electrophilic activation of C-H bonds by group 3, 4 and lanthanides d° complexes [8]. Given the similarities of the energies of dissociation of C-H and Si-H bonds, it is not surprising at all that activation of Si-H bonds occurs with 1. Reactions of H/D exchange, followed by in situ IR spectroscopy, reveal that all types of silanes are activated, i.e. primary, secondary and even tertiary silanes [9]. D2, 50°C RxSiH4.x -^ RxSiD4-x + (4-x) HD

R = phenyl, alkyl

Whilst complexes 1 catalyze, at low temperatures (ca. 50°C), the C-C bond rupture in alkanes such as neopentane, isobutane, propane...,[10] with silanes, they catalyze the Si-Si bondformation . ^_

Figure 1: M a s s distribution of the polymers obtained with PhSiH3 as determined by G P C .

L

2000 3000 1500 2500 Figure 2: Gas phase I R spectrum of: (a) Et3SiH+ D2; (b): (a) after contact with 1 (3h, 60°C)

Preliminary tests have been performed so far with the primary silanes, PhSiH3 and nC6Hi3SiH3- With PhSiH3 for example, polymers are obtained with a mass centred at 1312 (calibration against polystyrene) and M^^Mj^ = 1 . 1 7 (Figure 1). Interestingly, for some silanes such as Et3SiH and Et2SiH2, t h e Si-H and C - H bonds are simultaneously activated, as evidenced by I R spectroscopy: v(Si-D): 1536 cm-^; v ( C - D ) : 2 2 1 8 and 2 1 8 7 cm-^ (Figure 2). D2, 50°C » (C2H4D)3Si-D (C2H5)3Si-H -*

356 This observation is further confirmed by GC-MS analysis.We are currently determining the conditions which favor the formation of either polysilanes or polycarbosilanes. 2.3 Epoxidation of cyclohexene and hydroxylation of phenol by hydrogen peroxide catalyzed by surface zirconium hydroxydes, 2. Two types of heterogeneous catalysts based on group 4 elements for the mild oxidation of olefins are currently used: the so-called supported and incorporated M (Ti, Zr) based solids; their most famous members are the Shell and ENICHEM catalysts respectively. The first is synthesized by reaction of Ti(0Et)4 or TiCl4 with a silica, followed by hydrolysis/condensation and calcination [11]; in the second, Ti is introduced as Ti(0R)4 as a reactant at the level of synthesis of silicalite [12]. Both have nevertheless severe drawbacks associated with poor stability in presence of H2O2 for the former and drastic steric limitations for the latter. The search for M based solids, as efficient as TS-1, but presenting no microporosity, is thus still challenging. Although the precise nature of the catalytic sites as well as the mechanism of oxidation with hydrogen peroxide are still a matter of debate, it is generally accepted that the active sites must be well separated from each other for the obtainment of a selective catalyst; this avoids, inter alia, the unproductive decomposition of H2O2. Supported complexes 2 should fullfiU this condition. Indeed, 2 catalyzes both the epoxidation of cyclohexene and the hydroxylation of phenol with H2O2 (table 1).

0^0

' O .Qr-'^Q^'-

For these two reactions, the activity of 2 is of the same order of magnitude as that of supported Ti based catalysts: this is rather unusual and was never reported so far [13].The selectivity for cyclohexene epoxide is high ([epoxide]/[diol] » 8) as is the selectivity for dihydroxybenzenes, when these results are compared with those obtained with Ti based amorphous solids. Finally, for both reactions, no Zr was detected in the catalytic solutions and the solids could be recycled after simple fihration, without significant loss of activity and selectivity. Table 1: Oxidations by H2O2 catalyzed by complexes 2 reactant

H2O2 conv.(%)c

product sel (%)d

Re

cyclohexene^

76

71

7.9

phenol^

98

31

0.6

^ mcata=2g; solvent: diglyme (30 ml); T= 353K, t(reaction): 3h, cyclohexene: 0.5mole; H2O2 (70% wt.): 25 mmole (added within Ih). ^ mâta = O.lOOg; solvent: phenol (lOg); T=333K; H2O2 (70%wt.): 10 mmoles; t(reaction): 24h. ^ by iodometric titration; ^ determined by VPC, (epoxide plus diol) or dihydroxybenzenzes; ^ R = [epoxide]/[diol] or [hydroquinone]/[catechol].

357

2.4. Reductions and oxidations by hydrogen transfer catalyzed by complexes 3. The Meerwein-Ponndorf-Verley and Oppenauer reactions are useful when highly selective reduction or oxidations are required, when hydrogenation with molecular H2 is not possible (presence of functional groups) or when suroxidation must be avoided. R''

MP

Me

RI

These reactions are currently performed using large amounts of A1(0R)3 a situation which makes the search for a catalyst potentially important [14]. Supported 3 may be considered as an analog of the recently reported molecular catalysts Cp2ZrR2/i-PrOH [5]. Table 3 shows the results obtained for the reduction of a number of ketones with 2-propanol. Table 3. (=SiO)Zr(Oi-Pr)3, 3, catalysed reduction of ketones with 2-propanol run 1

conversion (%) ^ run 2

run 3

66

67

67

28

n.p.

n.p.

35

38

37

^ after 20h; catalyst Zr (% wt.)=0.7; [substrate] / [Zr]=72.

Significant differences in reactivity are observed, which may be explained by either steric effects or by unfavorable rates of the reaction of substitution of the alkoxy group (from the ketone) by 2-propanol (an elementary step of the presumed mechanism) [14]. With diphenylketone, steric effects are certainly predominant, inhibiting its coordination to Zr. The unreactivity of 4-methyl-pentanone is not yet understood. Although a simple and direct comparison with literature data is difficult, it seems that the order of reactivity with 3 is very different from what was observed with the molecular analog [5] i.e. aromatic>alicyclic>aliphatic ketone. This strongly suggests that the "solid" siloxy ligand induces new properties.

358 Most interesting is the fact that catalyst 3 is easy to recycle by simple filtration without significant loss of activity. No Zr is detected in the solution. We have also checked that Zr(OiPr) 4 is not active under our experimental conditions: this is a strong argument in favor of true catalysis by supported complex 3. Complexes 3 also catalyze the reverse reaction, i.e. the oxidation of alcohols with a ketone (here benzaldehyde and acetophenone) (table 4) Table 4: (=SiO)Zr(OiPr)3 catalyzed oxidation of alcohols with carbonyl compounds. conversion (%)a run 1 run 2

substrate

oxidant

catalyst

0°"

PhCHO

1

40

32

PhCOMe

2

49

n.p.

PhCOMe

2

55 b

n.p.

PhCHO

1

69

n.p.

1

48 c

15

1

46

n.p.

pH

fT^V^

Qi

r-

PhCHO

^

(1) xatalyst Zr (% wt.) = 0.7, [substrate] / [Zr] = 72; (2): catalyst Zr (% wt.) = 0.8,[substrate] / [Zr] = 40. [oxidant]/[alcohol] = 5/1; solvent: toluene; T= 383K unless specified;^ after 6h; h solvent: octane; EtOH > MeCN > t-BuOH > Me2C0 > THF, whereas, over Ti-beta, the conversion of sulfide is very similar in all protic solvents, and much higher than that obtained in aprotic solvents. It has been shown that Ti-beta is more active than TS-1 in the oxidation of larger molecules. This order can be explained by both restricted transition state shape selectivity and diffiisivity effects of reagents and products.

REFERENCES 1. S. Oae, Organic Sulfiir Chemistry: Structure and Mechanism; CRC Press, Boca Raton, 1991, ch.6. 2. M. Madesclaire, Tetrahedron, 42 (1986) 5459. 3. C. Walling, Ace. Chem. Res., 8 (1975) 125. 4. Y. Watanabe, T. Numata and S. Oae, Synthesis (1981) 204. 5. O. Bortolini, F. Di Furia and G. Modena, J. Mol. Catal., 14 (1982) 53. 6. Y. Ogata and K. Tanaka, Can. J. Chem., 60 (1982) 848. 7. A. Arcoria, F.P. Ballistreri, G.A. Tomaselli, F.Di Furia and G. Modena, J. Mol. Catal., 24 (1984) 189. 8. S. Campestrini, V. Conte, F. Di Furia and G. Modena, J. Org. Chem., 53 (1988) 5721. 9. K.A Vassel and J.H.Espenson, Inorg. Chem., 33 (1994) 5491. 10. F. Di Furia, G Modena, R. Curci, S.J. Bachofer, J.O. Edwards and M. Pomerantz, J. Mol. Catal., 14 (1982) 219. 11. R.S. Reddy, J.S. Reddy, R Kumar and P. Kumar, J. Chem. Soc, Chem. Commun., (1992) 84. 12. A.V. Ramaswamy and S. Sivasanker, Catal. Lett., 22 (1993) 239. 13. A. Corma, M.A. Camblor, P. Esteve, A. Martinez and J. Perez-Pariente, J. Catal., 145 (1994) 151. 14. V. Hulea, P. Moreau and F. Di Renzo, J. Mol. Catal., 111 (1996) 325. 15. F. Di Renzo, S. Gomez, F. Fajula and R. Tessier, French Patent 9509436 (1995); PCT/FR 96/01209 (1996). 16. F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. Leofanti and G. Petrini, Catal. Lett., 16(1992)109.

Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) O 1997 Elsevier Science B.V. All rights reserved.

369

AUylic oxidation of cyclohexene catalysed by metal exchanged zeolite Y O.B. Ryan, D.E. Akporiaye, K.H. Holm, M. Stocker SINTEF Applied Chemistry, P.O.Box 124 Blindem, N-0314 Oslo, Norway 1. INTRODUCTION The selective oxidation of hydrocarbons, particularly using molecular oxygen, for the chemicals industry, is of immense interest as a possible route for the conversion of feedstocks to useful products. The use of solid catalysts to replace the more traditional homogeneous systems gives several potential advantages, including ease of product recovery, recycling of the catalyst and the potential to add unique selectivities as determined, for example, by the shape selectivity of the inorganic matrix. Previous studies on the allylic oxidation of olefms [1,2] have in general focused on the use of transition metal salts. Making use of molecular oxygen and an heterogenised active metal complex in this reaction provides an interesting alternative route to the activation of alkenes, compared to the current focus on Ti-zeoHte systems [3]. The oxidation of cyclohexene has been used as a model reaction by several investigators [4]. Different initiators, transition metal complexes or metal exchanged zeolites have been used to study this reaction in detail. The aim of many of these reports is to find catalysts that increase the selectivity towards one of the three expected oxygenated products 2-cyclohexen-l-one (1), 2-cyclohexen-l-ol (2) and cyclohexene oxide (3). In most cases an increased selectivity towards the epoxide product was desired. In our on-going studies on the applications of zeolites in organic synthesis, [5] we have investigated the potential for modified zeolites in this reaction in combination with molecular oxygen, as compared to the previous studies on homogeneous systems. The goal of this work is to find conditions which result in high selectivities to either of the products (1) or (2).

02 I

II

I

II

I

r.

370

2. EXPERIMENTAL l-Methyl-2-pyrrolidone (99 % pure) and cyclohexene (99 % pure) were obtained from Fluka Chemie AG. Cyclohexene was distilled and stored under argon prior to use. Chlorobenzene (99.5 % pure) was obtained from BDH Laboratory Supplies. Cyclohexene oxide (98 % pure), 2-cyclohexene-l-ol (96 %pure) and 2-cyclohexene-l-one (95 % pure) were obtained from Aldrich. Oxygen (99.95 % pure) was used as delivered. 2,6-di-r^'rf-butyl4-methylphenol was obtained from Fluka and used as radical scavenger. The zeolite catalysts were prepared by ion-exchange of NaY (Aldrich, Si/Al 2.7) with measured amounts of the relevent metal compounds (chromium acetate and cobalt nitrate). In a typical procedure 10 g of the zeolite was dispersed in 500 ml of dionized water. The metal salt solution was added dropwise to this vigorously stirred mixture, after which the mixture was stirred overnight, followed by filtering and drying of the material at lOO^'C. Chemical analysis of the samples (XRF) indicated the presence of 10 wt% Co in CoNaY-1 and 4 wt% Cr in CrY. The catalytic experiments were perfomed in a three-necked flask fitted with a twostage condenser system, a gas-inlet tube, and a teflon cock with septum for withdrawing samples. The condenser system consisted of a water-cooled lower part and a top cooled with crushed dry ice. In a typical experiment, chlorobenzene (3.0 ml, 29 mmol) as internal standard and l-methyl-2-pyrrolidone (1.7 ml, 18 mmol) were added to the reactor containing cyclohexene (25.0 ml, 245 mmol). The reaction mixture was bubbled with oxygen (approx. 20 ml/min) for ca. 10 minutes before adding 0.4 g of the catalyst. The temperature of the reactor was rapidly raised to the desired reaction temperature, which, unless otherwise noted, was 65 °C. The oxygen flow was maintained throughout the experiment and the slurry was stirred rapidly to ensure effective suspension of the catalyst. The course of the reaction was followed by gas chromatography.

3. RESULTS AND DISCUSSION The results from the catalytic oxidation of cyclohexene with different metalexchanged zeolite Y and l-methyl-2-pyrrolidone (NMP) are presented in Table 1. The cobalt exchanged zeolite with an Si/Al ratio of 2.7 (CoNaY-1) shows the highest conversion of cyclohexene (64 %). However, only one third of the converted cyclohexene results in the formation of the oxygenated products (1-3), this being the major product. The remaining is believed to end up as polymeric products [6]. Changing the Si/Al ratio to 5.0 (CoNaY-3) decreases both the conversion of cyclohexene and the yield of oxygenated products, without affecting the selectivity to the oxygenated products noteworthy. The chromium exchanged zeolite gives the same yield of oxygenated products as the Co and Na exchanged zeolite, but with a significantly lower conversion. However, the selectivity towards the cyclohexen-1-one product (1) is increased dramatically. Except for increased conversion, calcining the CrY catalyst at 400 °C does not have any important effect on the outcome of the reaction. However, the induction period is reduced from approximately 4 h to practically zero. To our knowledge, selectivities as high as 88 % towards 1 has not been reported for the catalytic, heterogeneous oxidation of cyclohexene. This indicates that the results obtained with the CrY/NMP system is quite novel. Exchanging the zeolite with cerium has no positive effect at

371 all. Only 1 % yield of oxygenated products is observed. In fact, CeY seems, to some extent, to inhibit even the polymerisation reaction. Our results with the CoNaY catalysts is quite comparable to the results of Lunsford and Dai [4]. They have reported the catalytic oxidation of cyclohexene with Co2.5NaY as catalyst and t-BuOOH as initator. They found a conversion of cyclohexene of 49.7 % with a product distribution of 1-3 of 53.0, 39.4 and 6.0, respectively.

Table 1 Catalytic Activity and Product Distribution of the Catalytic Oxidation of Cyclohexene by Metal-Exchanged Zeolite Y and l-Methyl-2-Pyrrolidone (NMP).^ Conversion*^ Yieldc Selectivity [%] Entry Catalyst l-one(l) l-ol (2) Oxide (3) [%] [%] CoNaY-Id 64 20 56 33 11 1 CoNaY-3e 44 53 10 31 2 16 88 18 40 7 CrY 5 3 20 86 9 47 5 4 Calcined CrY^ 1 19 50 CeY 33 5 17 a) reaction conditions: reaction temperature, 65 °C, reaction time, 24 h, catalyst, 0.4 g, cyclohexene, 25 ml, 245 mmol, NMP, 1.7 ml, 18 mmol,flowrate of O2, 20 ml/min, internal standard, chlorobenzene, 3.0 ml, 29 mmol. b) total conversion of cyclohexene in %. c) total yield of oxygenated products (1-3) in %. d) Si/Al ratio of 2.7. e) Si/Al ratio of 5.0. 0 catalyst calcined at 400 T for 7 h. The beneficial effect of NMP in the oxidation of cyclohexene by oxygen has been reported by Alper and Harustiak [1]. They studied the catalytic, homogenous oxidation of cyclohexene with several transition metal salts. The combined use of C0CI2 and NMP (1 atm O2) gave a total yield of 50 % of 1 and 2, with a selectivity towards the 1-one (1) of 84 %. This urged us to choose NMP as initiator in our reactions instead of ^Bu()OH, and thus, avoid the problems related to using water solutions of ^BuOOH. The role of NMP as an initiator can be understood on the basis of a study by Drago and Riley [7]. They found that NMP was oxidized by O2 at 75 °C to form 5-hydroperoxo-l-methyl-2-pyrrolidinone (4, eq. 1). Uncatalyzed, this reaction has a induction period of approximately 24 h. The reaction can be catalyzed by CoNaY-zeolite, while adding certain Co and Mn complexes increase the decomposition of the peroxide to A^-methylsuccinimide. The use of the CoNaY zeolite results in increased yield without any observed induction period. As will be discussed later, NMP appears to play a more important role than just as an initiator in the reaction with CrY.

dn

N O CH3

^ 75 T

"^

HOO

N (CH

O

(1)

4 In order to unveil some of the details concerning the mechanism of the catalytic oxidation of cyclohexene with CrY and NMP, some further experiments were performed.

372

The results of these experiments are summarized in Table 2. When heated at 65 ""C under oxygen atmosphere for 24 h, without the presence of a catalyst or an initiator, cyclohexene is converted to polymeric and oxygenated products (1-3) in approximately 33 % yield, of which the polymers count for 25 % (entry 1, Table 2). This indicates that there is a considerable conversion of cyclohexene in the bulk phase even without the presence of a catalyst and an initiator. This autooxidation of cyclohexene will be discussed later on (vide infra). Further, it can be concluded from the results in Table 1 that ion-exchanging the zeolite with chromium has a significant influence on the selectivity of the reaction. However, experiments with only the CrY catalyst present do not show the same kind of selectivity even though the conversion of cyclohexene and yield of the oxygenated products 1-3 are nearly the same as with the initiator present. The observed selectivity of the products 1-3 in this case are 66, 22 and 12%, respectively (entry 2 Table 2). Compared to the autooxidation of cyclohexene in the bulk phase, the conversion is increased with 10 % and the yield of oxygenated products is doubled. Entry 3 in Table 2 shows the result from an experiment with only the initiator, NMP, present. In this case, the conversion also increases to over 40 %, but the yield of oxygenated products only increases from 7 to 9 % compared to the autooxidation experiment (entry 1, Table 2). The selectivity towards the 1-one product (1) is slightly increased.

OOH

O

OH

02 |0 + polymers (2)

Table 2 Influence of Different Parameters on the Catalytic Activity and Product Distribution of the Catalytic Oxidation of Cyclohexene by Metal-Exchanged Zeolite Y and l-Methyl-2Pyrrolidone (NMP).^ Entry Catalyst Initiator Conversion^ Yield^ Selectivity [%] [%] [%] 1-one (1) l-ol(2) Oxide (3) 8 43 31 26 1 33 46 CrY 15 66 22 12 2 44 NMP 9 52 26 22 3 4d Cr-Y 9 NMP 0 nd nd nd 56 19 Cr-Y NMP 0 nd nd nd Cr-Y 20 6 95 4 1 NMP 6f 7g Cr-Y 2xNMP 49 25 86 8 6 a) reaction conditions unless otherwise noted: reaction temperature, 65 '*C, reaction time, 24 h, catalyst, 0.4 g, cyclohexene, 25 ml, 245 mmol, NMP, 1.7 ml, 18 mmol,flowingrate of O2, 20 ml/min, intenial standiu-d, chlorobenzene, 3.0 ml, 29 mmol. b) total conversion of cyclohexene in %. c) total yield of oxygenated products (1-3) in %, d) addition of radical scavenger (2,6-di-rerr-butyl-4-methylphenol) prior to addition of catalyst, e) addition of radical scavenger (2,6-di-rerr-butyl-4-methylphenol) after addition of catalyst, 0 reaction temperature 40 °C, g) NMP, 3.4 ml, 36 nunol, nd - not detected.

373

It is well recognized that the oxidation of cyclohexene with O2 in the presence of several transition metal catalysts is a free-radical chain reaction giving cyclohexenyl hydroperoxide as an intermediate (eq. 2). Some metal complexes are known to catalyze the formation of the hydroperoxide, while others catalyze the decomposition of the peroxide to the oxygenated products 1-3. The uncatalyzed bulk autooxidation of cyclohexene to polymeric and oxygenated products is also a radical reaction. This is clearly shown by the fact that no oxygenated products are observed when cyclohexene is heated at 65 °C under oxygen atmophere in the presence of a radical scavenger, such as 2,6-di-r6'rr-4-methylphenol. Only a small conversion of cyclohexene of approximately 4 % is observed, most likely to polymeric products. According to textbooks in organic chemistry [8], the mechanism of autooxidation of an oragnic compound can be depicted as shown in Scheme 1. A radical initator which removes a H-radical is needed to start the reaction. In the case of the bulk autooxidation of cyclohexene, oxygen is believed to act as initiator. Hence, no reaction take place when cyclohexene is heated at 65 °C under argon atmosphere. However, the reaction starts immediately when oxygen is added to the reaction solution. Performing the experiments under complete darkness has no unfluence on the outcome of the reaction. Scheme 1 RH

^ O2

R.

(H' is removed by initiator)

R.

+

ROO.

+ RH

ROOH

R.

+

RR or dispropotionation

R.

ROO-

0) (4)

+

R.

(5) (6)

It can, however, be questioned if the oxidation of cyclohexene in the presence of several transition metals is a free-radical chain mechanism. Arzoumanian et ai have reported that the catalytic decomposition of cyclohexene hydroperoxide by a rhodium complex in benzene is not a free-radical chain reaction. They found that the decomposition of each cyclohexene hydroperoxide gives rise to only one free radical [9]. In this case the decomposition products were found to be 18 % l-ol (2), 33 % 1-one (1) and 35 % polymers together with small amounts of water and oxygen. They were also able to identify two freeradical species in solution, namely the cyclohexenylperoxy radical (6) and the cyclohexenyloxy radical (7).

374

The 1-one (1) and l-ol (2) products can be formed by the known mechanism where two peroxy radicals dimerize to give an unstable tetroxyde intermediate which in turn decomposes with hydrogen transfer to give oxygen, 1 and 2 (eq 7).

( _ > H O >

It is evident from the experiments in this study that both the CrY catalyst and NMP must be present to achieve 20 % yield of oxygenated products and close to 90 % selectivity towards the 1-one product. However, it is also clear that the bulk autooxidation of cyclohexene gives a considerable contribution to the total conversion of cyclohexene. The experiments also suggest that this represents two different mechanisms, where the bulk autooxidation of cyclohexene proceeds as discussed above. This is in accordance with the fact that both CrY and NMP separately increase the conversion of cyclohexene, but without any increase in selectivity. Based on the considerations above the mechanism for the oxidation of cyclohexene with oxygen in the presence of CrY and NMP is suggested in Scheme 2. As the first step, NMP is oxidized to 5-hydroperoxo-l-methyl-2-pyrrolidinone (4). This reaction can be catalyzed by CrY inside a pore. However it most likely takes place in the solution outside the pores. This is supported by the fact that addition of the radical scavenger, 2,6-di-ten-4methylphenol, results in no formation of oxygenated products 1-3, at all. As can be seen from entry 5 and 6 in Table 2, it does not matter whether the radical scavenger is added prior to or after the addition of the CrY catalyst. The size of the radical scavenger ensures that it is too big to be able to enter the pores. When inside the pores, NMP hydroperoxide can decompose to the 5-oxy-l-methyl-2-pyrrolidone radical (8). This reaction can be catalytically assisted by the CrY zeohte. The radical 8 reacts with a cyclohexene molecule to give a cyclohexenyl radical, which can add oxygen to form the cyclohexenperoxy radical (6). As discussed above, this peroxy radical can give rise to all the three oxygenated products. However, in the presence of Cr, the special geometric cavity of a pore, or both, formation of only one product (1) results. The catalyzed oxidation is expected to be a fast reaction compared to the bulk autooxidation of cyclohexene. This is confirmed by the fact that even a higher selectivity is observed when the reaction is performed at a lower temperature (entiy 6, Table 2), and that less amounts of 2 and 3 is formed in the CrY and NMP catalyzed reaction than in the uncatalyzed bulk oxidation of cyclohexene. Doubling the amount of NMP does not have any effect on the product distribution, but does increase both the conversion and the yield of oxygenated products. This also indicates that the reaction giving the 2-one product takes place inside the pores of the zeohte, while the initiation of the reaction take place in the bulk phase.

375 Scheme 2

O2

HOO

N

U

C CH,

-OH

Crn-Y

Crn+l-Y

CHo

o

00.

O2

HO^N A

^

CH^ Cr-Y

+

"Cr-OH.

4. CONCLUSIONS Cr-exchanged zeolite NaY is found to convert cyclohexene selectively to cyclohexenl-one by oxidation with molecular oxygen, in contrast to an equivalent CoNaY system. The initiator, NMP, is found to play an important role in the transformation, both components being necessary to achieve the high selectivities observed. A reaction mechanism consistent with the experimental data is proposed.

376 REFERENCES 1 2 3

4

5 6 7 8 9

H. Alper and M. Harustiak, J. Mol. Catal. 84, 87 (1993). J.E. Lyons, Catal. Today. 3, 245 (1988). a) P.B. Venuto, Micropor. Mater. 2, 297 (1994) and references sited therein, b) A.Corma, P. Esteve and S. Valencia, J. Catal. 152, 18 (1995), c) A. Corma, M.A. Camblor, P. Esteve, A. Martinez and J. Perez-Pariente, ibid 145, 151 (1994), d) C.B. Khouw, C.B. Dartt, J.A. Labinger and M.E. Davis, ibid. 149, 195 (1994). a) P-H.E. Dai and J.H. Lunsford, J. Catal., 64, 184 (1980), b) T. Hosokawa, M. Takano, S-I. Murahashi, H. Ozaki, Y. Kitagawa, K. Sakaguchi and Y. Katsube, J. Chem. Soc, Chem. Commun., 1433 (1994), c) J. Guo, Q.Z. Jiao, J.P. Shen and D.Z. Jiang, Catal. Lett., 40, 43 (1996), d) A. Fusi, R.Ugo and G.M. Zanderighi, J. Catal., 34, 175 (1974). D.E. Akporiaye, K. Daasvatn, J. Solberg and M. Stocker, Studies in Surf. Sci, 59, (1993). A. Fusi, R. Ugo, F. Fox, A. Pasini and S. Cenini, J. Org.met. Chem. 26, 417 (1971). R.S. Drago and R. Riley J. Am. Chem. Soc. 112, 215 (1990). J.D. Roberts and M.C. Caserio, Basic principles of Organic Chemistry, W.A. Benjamin, Inc.: Menlo Park, CA, USA 1977, p 658. H. Arzoumanian, A.A. Blanc, J. Metzger and J.E. Vincent J. Org.met. Chem. 82, 261 (1974).


377

Ammoxidation of Methylaromatics over NH4*-contaiiiing Vanadium Phosphate Catalysts - New Mechanistic Insights Andreas Martin, Angelika Bruckner, Yue Zhang and Bernhard Liicke Institut fur Angewandte Chemie Berlin-Adlershof e.V., Abt. Katalyse Rudower Chaussee 5, D-12489 Berlin, Germany

SUMMARY NH4^-containing vanadium phosphate catalysts and (yO)2P2î for comparison were applied as catalysts for the ammoxidation of toluene. The reaction was also followed by in-situ ESR spectroscopy. TAP experiments were carried out to prove the effect of the vanadium valence state of the near-surface catalyst area on the catalytic properties as well as to reveal the role of NH4^-ions during the nitrile formation. Main results demonstrate that an active and selective ammoxidation on such catalysts requires i) adjacent, edge-sharing VOg units, running sufficiently as chains through the bulk, ii) an easy and fast change of the V valence state that should be +4 on the average and Hi) the presence of NH4^-ions as structural unit or generated by hydrolysis of V-O-P links.

1. INTRODUCTION The ammoxidation of substituted toluenes and methylheterocycles to the corresponding nitriles is an industrially important reaction [e.g. 1]. The synthesized nitriles are valuable intermediates in the organic synthesis of different dyestuffs, pharmaceuticals and pesticides. Recent studies have shown that the conversion rate of the substrate and the nitrile selectivity are strongly determined by the position, size and electronic effects of one or more substituents [2,3] (Eq. 1).

+ NH3 + 1.5 O2

Ri, R2 = -H, -CH3, -CI, -Br, -NO2, -O-CH3. -CN

+ 3H2O

(1)

378

Vanadium phosphates (VPO) of different structure are suitable precursors of very active and selective catalysts for the oxidation of C4-hydrocarbons to maleic anhydride [e.g. 4] as well as for the above mentioned reaction [5,6]. Normally, VOHPO4 Vi H2O is transformed into ^0)^'f^r^ applied as the Ai-butane oxidation catalyst. Otherwise, if VOHPO4 y^ H2O is heated in the presence of ammonia, air and water vapour a-(NH4)2(VO)3(P207)2 as XRD-detectable phase is formed [7], which is isostructural to a-K2(VO)3(P207)2. Caused by the stoichiometry of the transformation reaction (V/P = 1 => V/P = 0.75) (Eq. 2) and the determination of the vanadium oxidation state of the transformation product (« 4.11 [7]) a second, mixed-valent (V^/V^) vanadium-rich phase must be formed. VOHPO4

y2H20 + O2/NH3/H2O

^

a-(NH4)2(VO)3(P207)2 + % 0 ; '

(2)

Raman and IR spectroscopic experiments support these ideas [7]. Last but not least, in-situ XRD measurements showed reflections of NH4VO3 on equilibrated samples cooled down to r.t. after finishing the transformation process [8]. Otherwise, these reflections were absent if the transformed sample was flushed with a nitrogen flow before cooling down. Recently, in-situ Raman spectroscopy revealed the existance of NH4VO3 too [9]. Thus, it seems very likely that NH4VO3 could be formed from in-situ existing mixed-valent vanadium oxides and an excess of ammonium ions located on the surface. This paper summarizes catalytic data of the ammoxidation of toluene carried out on as-synthesized, pure a-(NH4)2(yO)3(P207)2, the transformed material a(NH4)2(VO)3(P207)2 + „VxOy" generated during the ammoxidation reaction from VOHPO4 V2 H2O, pure a-(NH4)2(VO)3(P207)2 samples differently treated with NH4VO3 and (VO)2P207 for comparison as well as results of in-situ ESR experiments, proceeding under comparable reaction conditions. Furthermore, the paper focuses on the significance of the vanadium valence state of the nearsurface area for catalytic activity and product selectivity and on the role of anmaonium ions during the nitrile formation mechanism. 2. EXPERIMENTAL 2.1. Catalysts VOHPO4 V2 H2O (VHP) has been prepared in aqueous solution as described in [10]. Pure a-(NH4)2(VO)3(P207)2 (NVPOgyJ has been synthesized as described in [7]. (VO)2P207 (VPP) was prepared by calcination of VHP under nitrogen up to 773 K and 3h whereas a-(NH4)2(VO)3(P207)2 + „VxOy" (NVPOô) was obtained after treatment of VHP with the ammoxidation gas mixture at 673 K and 6h. Furthermore, mixtures of NVPOgyn and NH4VO3 were prepared either by impregnation of NVPOgyn with an aqueous solution of NH4VO3 (NVPOj) or by mechanical mixing (NVPOn^) of the two compounds in a molar ratio of NVPOgyn ' NH4VO3 = 1 [11].

379

2.2. Catalytic runs The catalytic properties of the above mentioned samples were determined using a fixed bed U-tube quartz-glass reactor. The catalysts were applied as split (1-1.25 nmi, 1.5 ml each). The following reaction conditions were performed: molar ratio of toluene : air : anmionia : water vapour = 1 : 30 : 5 : 25, WIF = ca. 10 ghmol"^ and atmospheric pressure. The reactor outlet flow was analyzed by on-line GC. 2.3. Methods ESR measurements were performed with the cw spectrometer ERS 300 (ZWG, Berlin) equipped for in-situ investigations with a flow reactor and a gas as well as liquids (with vaporization) supplying system [12,13]. 0.4 g catalyst was applied each and the reaction conditions were comparable to catalytic runs as described above. The principle of the temporal-analysis-of-products (TAP) reactor unit has been described detailed elsewhere [14,15]. The equipment was used for the investigation of the effect of a prereductionZ-oxidation of a VPO catalyst nearsurface area on its catalytic properties [16] and for isotope experiments (^^NHs) [17] that should reveal the role of ammonium ions during the catalytic cycle. 3. RESULTS AND DISCUSSION Figure 1 depicts the toluene conversion on the described NH4^-containing VPO materials applied as catalysts in comparison to VPP which could be also regarded as NH4^-containing VPO system because such groups exist on the VPP surface under reaction conditions as well, e.g. by hydrolysis of V-O-P links [10]. 1C\

-,

o 60 -

y A

c 50 o 12 4 0 -

^

1 30o © 20 0)

1 10- J

^

^

A^.^'^'Ô^

^ A

tU H

600

-

—1 620

1 640

_l_. 660

R Z^LJ

—i 1

680

""'^

1— 700

T / K ^2

Figure 1. Toluene conversion during the ammoxidation reaction vs. reaction temperature on several VPO catalysts (+ - NVPOi, x - NVPO^,, A - NVPO^^, ° NVPO^^n, 0 - VPP).

380 However, the tests showed a very poor activity for the pure compound whereas NVPOao and VPP revealed a comparable high activity. The observed activity on the NH4VO3 treated samples is much higher. Thus, it seems likely that NVPOgyn acts as a less active matrix mainly, probably due to its structure that does not contain edge-sharing VOg octahedra units in contrast to VPP or other vanadium oxides (see Fig. 2). Therefore, NVPOgy^ can not expose appropriate neighbouring VOg sites on the surface assumed to be the active ones in the ammoxidation reaction. This is also supported by some structural calculations that have shown that a successful activation of a substrate molecule should be only possible if two adjacent, edge-sharing VOg units (distance of the vanadyl centres in trans position VPP approximately 3.4 A [18]) are present at the catalyst surface at least, taking a chemisorption step of the substrate molecule with the ji-system on a coordinatively unsaturated (Lewis site) vanadyl site into account (distance of the centre of the aromatic ji-system to the methyl group C-atom approx. 3 A).

^P

/A'

>L'

A/

3.4 A NVPO,,

VPP

"VÔ " domains or clusters located on the surface of NVPO,„, NVPO: and NVPO„

Figure 2. Schematic view of the VOg-octahedra units located on the (100) *basic plane' of NVPOg^n as well as VPP (idea of the chemisorption of a toluene molecule on a dioctahedra unit) and as domains or clusters on NVPOao, NVPOj and NVPO^.

Figure 3. Efficiency of the spin-spin exchange vs. reaction time (# A NHa/air, # B - NHg/air/ water vapour, # C NH3 /air/water vapour/ toluene, # D - NHg/air/ water vapour).

381

These considerations were confirmed by in-situ ESR spectroscopy. The VO^^ centres in the "VÔy domains assumed to be present in the more active catalysts NVPOi and NVPO^n as well as NVPOao are coupled by strong spin-spin exchange interactions. Accordingly, high values of the quotient of the 4th and the square of the 2nd moment (/^) of the ESR absorption signal are obtained (Figure 3) since this parameter is a measure for the exchange efficiency [12,13]. In the presence of toluene this value decreases slightly, indicating a perturbation of the spin-spin exchange interaction which is caused by the chemisorption of the aromatic ring system on a surface vanadyl site as discussed above. With respect to the catalytic results it appears that catalysts with effective spin-spin exchange interactions reveal higher catalytic activities. The reason therefore could be that the alterating electron density at a discrete surface VO^^ can be easily delocalized via the overlapping d-orbitals of the exchange coupled centres [13]. Additionally, a good catalytic performance imperatively requires effective exchange pathways for the electron transport through the catalyst bulli; isolated VO^^ sites as present in supported VPO catalysts with low vanadium loading are not involved in the catalytic reaction [13]. Figure 4 depicts the benzonitrile selectivity during the toluene ammoxidation runs in dependence on the reaction temperature. It is clearly shown that the selectivity data of NVPOgyn* NVPOao and VPP decrease slightly with increasing temperature. Otherwise, the nitrile selectivity of the two NH4VOQ treated samples drops drastically. This effect is caused by the existance of V of V2O5containing domains formed from NH4VO3, probably.

Figure 4. Benzonitrile selectivity during toluene ammoxidation runs vs. reaction temperature on several VPO catalysts (+ - ISTVPOi, x - NVPO^, A - NVPOao, NVPOgyÔ-VPP). Pulse catalytic experiments, using the TAP reactor equipment, toluene as feed and VPP as catalyst revealed a distinct dependence of the catalytic activity and

382

product selectivity on the oxidation state of the catalyst surface [16]. An ammonia-containing flow was applied for a reductive pretreatment of the catalyst near-surface area (V => V"^). The subsequently performed ammoxidation feed pulses showed that the toluene conversion decreased drastically. Otherwise, an oxygen-containing flow was used for a partial oxidation of the surface (V^^ => V^). The result was that the toluene conversion increased, but the nitrile selectivity droped and CO^ selectivity grew immediately (see Fig. 5). Thus, it seems very likely that also in the case of VPO catalysts used for the ammoxidation a growing part of surface V^" restricts the catalytic activity whereas an increasing part of V accelerates the catalytic process but the nitrile selectivity decreases by overoxidation towards total oxidation products.

Carbon dioxide/Q CO CD CO "cD

c

o

O) U) /

Benzonitrile

c o

Q. V)

a:

Toluene ^^^^

A\

0.3 0.1 -1

1

1

1

H

Ammonia pretreatments (near-surface reduction)

V'" "^—

A

A

.

^

A

Parent (V'^0)2P207

A ^—1

1

V^

1

1

1

Oxygen pretreat^

ments (near-surface oxidation)

Figure 5. Response signal area of pulse catalytic ammoxidation experiments on prereduced/-oxidized (VO)2P207 catalyst. Furthermore some TAP followed ammoxidation of toluene runs were carried out to investigate the role of anmionium ions during the nitrile formation mechanism. NVPOgyn was used as catalyst and an amimoxidation feed, containing ^^NHa [17]. The studies revealed that i) no gas phase ammonia reacts, but ii) the NH4^-ions of the catalyst participate in nitrile formation. Table 1 summarizes the calculated response signal areas of the generated benzonitriles (atomar mass unit (amu) 103 - ^"^N-benzonitrile, amu 104 - ^^N-benzonitrile) simultaneously measured by mass spectrometry. It seems, that NH4^-ions act as potential Ninsertion species in the ammoxidation cycle on NVPOgyn at least. The results show further, that not only NH/-surface ions react but also ^'*NH4^-ions of

383 deeper layers of the bulk move up. The remaining vacancies could be occupied again by ammonia molecules of the gas phase, generating new ammonium ions. Therefore, ^^NH4^ -ions could be incorporated into the catalyst structure on sites occupied before by the ^'^NH4^-ions. Recent temperature-progranmaed reaction spectroscopy (TPRS) as well as in-situ FTIR spectroscopy studies showed also that VPP could be considered to be an NH4^-containing system under reaction conditions at least [10]. Therefore, it seems very likely that the ammonium ions generated during the ammoxidation could be able to intervene in the ammoxidation mechanism as well. Table 1 Response signal area (a.u.) of ^^N-benzonitrile and ^^N-benzonitrile during pulse catalytic ammoxidation of toluene on pure a-(NH4)2(VO)3(P207)2 and ^^NHacontaining ammoxidation feed. Ammoxidation pulse series

1 2 3 4

^^N-benzonitrile 2.4005 1.7716 0.9410 0.6596

^^N-benzonitrile 0.0000 0.5805 0.7200 1.1024

In conclusion, the studies have shown that an active and selective ammoxidation on VPO catalysts requires adjacent, edge-sharing VOg units (dioctahedra units at least), running sufficiently as chains through the bulk. The catalyst structure should enable an easy and fast change of the vanadium valence state that should be +4 on the average, growing parts of V reduce the catalytic activity whereas increasing amounts of V^ promote total oxidation paths. Furthermore, NH4^-ions, existing as structural unit or generated by hydrolysis of V-O-P links seems to play the role of potential N-insertion sites. ACKNOWLEDGEMENTS The authors thank Dr. H.W. Zanthoff for his help in TAP experiments as well as helpful discussions. Financial support by the Bundesminister fiir Bildung, Wissenschaft und Technologie (grant-no. 423-4003-03D0001B0) is gratefully acknowledged. REFERENCES 1. R.G. Rizayev, E.A. Mamedov, V.P. Vislovskii and V.E. Sheinin, Appl. Catal. A: General, 83 (1992) 103. 2. A. Martin, B. Liicke, G.-U. Wolf and M. Meisel, Catal. Lett., 33 (1995) 349. 3. A. Martin and B. Liicke, Catal. Today, in press.

384

4. G. Centi (Ed.), Vanadyl pyrophosphate catalysts, Catal. Today, 16 (1993) 5. A. Martin, B. Liicke, H. Seeboth, G. Ladwig and E. Fischer, React. Kin. Catal. Lett., 38 (1989) 33. 6. A. Martin, B. Liicke, H. Seeboth and G. Ladwig, Appl. Catal., 49 (1989) 205. 7. Y. Zhang, A. Martin, G.-U. Wolf, S. Rabe, H. Worzala, B. Liicke, M. Meisel and K. Witke, Chem. Mater., 8 (1996) 1135. 8. L. Wilde, U. Steinike, A. Martin, G.-U. Wolf and B. Liicke; J. Solid State Chem., in preparation. 9. Y. Zhang, M. Meisel, A. Martin, B. Liicke, K. Witke and K.-W. Brzezinka, Chem. Mater., submitted. 10. H. Berndt, K. Biiker, A. Martin, A. Briickner and B. Liicke, J. Chem. Soc, Faraday Trans., 91 (1995) 725. 11. H. Berndt, K. Biiker, A. Martin, S. Rabe, Y. Zhang and M. Meisel, Catal. Today, in press. 12. A. Briickner, B. Kubias and B. Liicke, Catal. Today, in press. 13. A. Briickner, A. Martin, N. Steinfeldt, G.-U. Wolf and B. Liicke, J. Chem. Soc, Faraday Trans., in press. 14. J.T. Cleaves, J. Ebner and T.C. Kuechler, Catal. Rev. - Sci. Eng., 30 (1988) 49. 15. O.V. Buyevskaya, M. Rothaemel, H.W. ZanthofT and M. Baerns, J. Catal., 146 (1994)346. 16. A. Martin, Y. Zhang and M. Meisel, React. Kin. Catal. Lett., accepted. 17. A. Martin, Y. Zhang, H.W. Zanthoff, M. Meisel and M. Baerns, Appl. Catal. A: General, 139 (1996) L l l . 18. M.R. Thompson, A.C. Hess, J.B. Nicholas, J.C. White, J. Anchell and J.R. Ebner, Stud. Surf. Sci. Catal., 82 (1994) 167.


385

Hydrogen peroxide oxidation of methyl a-D-glucopyranoside, sucrose and a,a-trehalose with Ti-MCM-41 E.J.M. Mombarg, S.J.M. Osnabmg, F. van Rantwijk and H. van Bekkum Laboratory for Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands. 1. INTRODUCTION The present surplus of agriculturally produced carbohydrates acts as a powerftil driving force for the development of non-food applications for these compounds. Moreover, their lack of toxicity and ready biodegradability are very desirable properties in everyday applications. The oxidative cleavage of the C(2)- C(3) bond in starches and inulin would yield polycarboxylates with good calcium sequestering properties. Oxidation of the primary hydroxyl functions at C(6), on the other hand, would yield products which are structurally closely related to alginates and pectinates. A recently developed method for this type of oxidation is the TEMPO catalysed oxidation with hypochlorite as the primary oxidant . A combination of these two oxidation processes would yield interesting tricarboxylate type compounds whose properties have not yet been explored. Oxidation of long chain glucosides might give a new kind of detergent. Progress has been hampered, however, by the lack of a suitable oxidation catalyst. Titanium-based catalysts, would seem particularly attractive candidates, but the pore size of e.g. TS-1, is much too small to admit even a monosaccharide. Recently a number of synthetic approaches towards mesoporous titanium containing catalysts of the MCM-41 type have appeared in the literature ' . In the present paper we will describe the use of Ti-MCM-41 materials in the oxidation of the model mono- and disaccharides methyl a-D-glucopyranoside, sucrose and a,a-trehalose, and we will discuss the effect of the zeolite synthesis on the effectiveness in these reactions. Several preparative approaches of Ti-MCM-41 have been compared in the oxidation of these model carbohydrates. 2. RESULTS AND DISCUSSION A number of Ti-MCM-41 materials were synthesised according to published procedures ' and evaluated for activity in the oxidation of methyl glucoside in aqueous medium. Only the material which had been synthesised by Maschmeyer et al."^, which involves post-synthesis modification of all silica MCM-41 by bis(cyclopentadienyl)titanium dichloride, showed activity towards methyl glucoside. The conversion was low, however, because a vigorous decomposition of hydrogen peroxide predominated. We considered that this might be caused by crowding of the

386 titanium sites on the silica surface. The overall Si/Ti ratio amounted to 10, which is rather low if isolated titanium sites are desired, considering that all titanium sites are at the chaimel walls. Hence, we modified all-silica MCM-41 by the same method, but with an overall Si/Ti ration of 200. The resulting Ti-MCM-41 catalyst performed much better: the decomposition of the hydrogen peroxide abated after a few minutes and 60% conversion of methyl a-D-glucopyranoside in 20 h was achieved. 1-0Methyl glucuronic acid was the main oxidation product; tartronic acid accounted for the remainder. The formation of the latter product is ascribed to the hydrolysis of the tricarboxylate (see Scheme 1). The formation of glycolic acid and formic acid is ascribed to hydrolysis and/or oxidative break-down of the primary oxidation products. The rapid initial increase of 1-0-methyl glucuronic acid suggests that the primary (C(6)) hydroxyl group is selectively oxidised but is subsequently rapidly converted, most probably by glycol cleavage (left pathway in Scheme 1). If this is indeed the case 1-0-methyl glucuronic acid would be more sensitive towards glycol cleavage than methyl glucoside. OH

A"-

H O - ^COONa NaOOC^ OMe

OMe

OH

COONa

I

HO^X ^ OH OMe HO.Xx^

NaOOO ,^/OMe NaOOC

X

C032

COONa NaOOC^^/OMe NaOOC

Scheme 1. Possible pathways for the oxidation of methyl a-D-glucopyranoside to tartronic acid. The identified products (l-O-methyl glucuronic acid and tartronic acid) are within the frames. The formation of the products vs. time in the Ti-MCM-41 catalysed oxidation of methyl a-D-glucopyranoside is depicted in Figure 1.

387

Figure 1. The Ti-MCM-41(100 mg) catalysed oxidation of methyl a-D-glucopyranoside (5.0 g) with aqueous 35% hydrogen peroxide (25g). The concentration of the substrate and the oxidation products 1-0-methyl glucuronic (x), formic , tartronic ) and glycolic (^) acid v^-. time (h). The question arises whether the reaction is truly heterogeneous. The schematic representation of Maschmeyer suggests an easily accessible titanium site which might be susceptible to leaching. While this is probably irrelevant in the gas phase and in the organic media in which Ti-MCM-41 is commonly used, hydrolytic cleavage of titanium from the molecular sieve framework might easily take place in aqueous medium. The reaction system was checked for titanium leaching by removal of the solid catalyst from the reaction mixture after 1 h.

Figure 2. Sodium hydroxide consumption in a standard oxidation of methyl a-Dglucopyranoside over Ti-MCM-41 with hydrogen peroxide , removal of the heterogeneous catalyst after Ih and ftirther reaction of the solution ) and upon re-use of the solid catalyst (^).

388

In Figure 2 it is shown that the reaction continues unabated in the absence of the solid catalyst, whereas the recovered catalyst has lost the major part of its activity. The leaching of the titanium was further investigated by ICP-OES analysis. The silicium/titanium ratio of the Ti-MCM-41 as-synthesised is 230, while after the reaction this ratio was increased till 4720. We found that the native catalyst was hydrolytically stable under aqueous conditions, whereas in the presence of hydrogen peroxide rapid leaching was observed. Apparently the titanium hydroperoxide is more sensitive to hydrolysis than the native catalyst. The homogeneous titanium species is apparently an oxidation catalyst. A recent paper on Ti-MCM-41 also reports Tileaching in the liquid phase . Experiments with slow continuous hydrogen peroxide addition were also performed. After dissolving the substrate the hydrogen peroxide was added at a rate of 1.2 ml/h. The spectrum of products was not different from that obtained upon addition of the oxidant in one portion. Hence, the oxidation of 1-0-methyl glucuronic acid is also catalysed by homogeneous titanium hydroperoxide. Other substrates tested are the disaccharides a,a-trehalose and sucrose. These substrates were oxidised by adding the amount of hydrogen peroxide in one portion as well as by gradual addition of the hydrogen peroxide. The oxidation of the disaccharides is probably also catalysed by homogeneous titanium. Deep oxidation was observed leading to Ci - C4 mono- and dicarboxylic acids: formic acid, glycolic acid, tartronic acid and tartaric acid. Their formation vs. time is depicted in Figures 3 and 4 for a,a-trehalose and sucrose, respectively. A number of unknown products were also present in the product mixture; these products are more abundant when the reactions are performed using gradual addition of hydrogen peroxide. Because the unknown products elute shortly after the uncharged substrates, these are probably monocarboxylates. Further work on their isolation and identification is in progress.

Figure 3. The Ti-MCM-41 catalysed oxidation of a,a-trehalose with hydrogen peroxide. The concentration of the substrate (*) and the identified oxidation products formic acid , glycolic acid , tartaric acid (x) and tartronic acid (A) VS. time (h).

389

Figure 4. The Ti-MCM-41 catalysed oxidation of sucrose with hydrogen peroxide. The concentration of the substrate (*) and the oxidation products formic acid , glycoUc acid , tartaric acid ) and tartronic acid ) v^. time (h). In conclusion this system can be used for the formation of the uronic acids if a method could be developed for removal of the products before they are further oxidised. Homogeneous titanium is apparently a good oxidation catalyst, this is under further investigation. 3. EXPERIMENTAL All silica MCM-41 was synthesised by making a homogeneous mixture of Cab-0Sil (3.96 g, 66 mmol) and tetramethylammonium hydroxide solution 25 wt% Aldrich (12 g, 33 mmol). This was added to a mechanically stirred mixture of sodium silicate Aldrich (18.84 g, 154 mmol), silica Cab-0-Sil (13.8 g, 230 mmol) and water (84 g, 4.7 mol). After addition of a solution of cetyltrimethylammonium bromide Fluka (44.64 g, 122 mmol) in water (300 g, 16.67 mmol) the mixture was homogenised and put aside without stirring for 36 h. The resulting solid was washed with water and dried in vacuo at 60 °C. The product was calcined using the following temperature program: l°C/min to 70 °C, for 3h at 70 °C, l°C/min to 540°C, 10 h at 540 °C and stepwise to 20 °C. The material was analysed with XRD. 3.1. Synthesis of Ti-MCM-41 Bis-cyclopentadienyl titanium dichloride Aldrich (52 mg, 0.2 mmol) was dissolved in 40 ml chloroform and added to a suspension of MCM-41 (1 g, 16.6 mmol Si02). This mixture was stirred overnight at room temperature and triethylamine Janssen (0.25 |Lil, 0.8 mmol) was added. After stirring for another 4 h the mixture was filtered, washed with chloroform and dried. The resulting yellow solid was calcined using the same temperature program as described above. The resulting white solid was analysed using XRD. 3.2. Oxidation Reactions The oxidation of methyl a-D-glucopyranoside with Ti-MCM-41. Methyl a-Dglucopyranoside Janssen (5 g, 25.7 mmol) was added to a mixture hydrogen peroxide

390 solution 35wt% (25 g , 250 mmol) and water 25g at 70 °C. The Ti-MCM-41 (100 mg, 7.2 jLimol Ti) was added and the pH was adjusted to 4. Samples taken were analysed using HPLC (Organic acid and anion exchange) and ^^C NMR. 3.3. Oxidation reaction with gradually added hydrogen peroxide To a solution of methyl a-D-glucopyranoside (5 g, 25.7 mmol) in 25 ml water was added Ti-MCM-41 (100 mg, 7.2 îmol Ti) and 25 g, 35wt% hydrogen peroxide solution at a rate of 0.02 ml/min. The pH of both solutions were adjusted to 4. Samples taken were analysed on HPLC. The reaction of trehalose and sucrose are performed under the same conditions. 3.4 General Procedures All oxidation experiments were performed in a magnetically stirred, thermostatted reaction vessel of 100 mL. During the oxidation the pH was kept constant using a pH meter (Metrohm 654), a pH controller (Metrohm 614) and a motor burette (Metrohm 655) containing 2.00 M aqueous sodium hydroxide. Samples were analysed by HPLC on a system consisting of a Millipore 590 pump and a Perkin Elmer ISS-100 autosampler, a Shodex RI SE-51 RI detector, a Shimadzu SPD-6A UV detector at 215 nm, and a Spectra Physics SP4270 integrator. A Phenomenex organic acid column was used with aqueous 0.01 M trifluoroacetic acid as the mobile phase at 60 °C. A Benson BA-X8 anion exchange column was used at 85 °C to monitor the charged compound using aqueous buffer of 0.162 M ammonium sulphate and 0.038 M magnesium sulphate adjusted at pH 8 using ammonium hydroxide solution. 4. CONCLUSIONS In aqueous medium in the presence of hydrogen peroxide the titanium leaches easily out of Ti-MCM-41 synthesised by impregnation by bis-cyclopentadienyl titanium dichloride of an all silica MCM-41. The dissolved titanium catalyses the oxidation of methyl a-D-glucopyranoside to 1-0-methyl glucuronic acid. This product is sensitive to further oxidation to formic, glycolic and tartronic acid. In the oxidation of sucrose and trehalose monocarboxylate are probably formed beside Ci - C4 monoand dicarboxylates.

REFERENCES 1 2 3 4 5 6 7

A.C. Besemer, H. van Bekkum, Starch, 46 (1994) 101. A.E.J, de Nooy, A.C. Besemer and H. van Bekkum, Carbohydr. Res. 269 (1995) 89. A. Corma, M.T. Navarro, and J. Perez-Pariente, J. Chem. Soc. Chem. Comm., (1994) 147. T. Maschmeyer, F. Rey, G. Sanker, and J.M. Thomas, Nature, 378 (1995) 159 T. Blasco, A. Corma, M.T. Navarro and J. Perez Pariente, J. Catal., 156 (1995) 65. J. Kumar, G.D. Stucky, and B.F. Chmelka, Stud. Surf Sci. Catal, 84, (1994) 243. C. H. Rhee and J.S. Lee, Catal. Lett, 40, (1996) 261.


391

On the role of bismuth-based alloys in carbon-supported bimetallic Bi-Pd catalysts for the selective oxidation of glucose to gluconic acid M. Wenkin \ C. Renard ^ P. Ruiz ^ , B. Delmon ^ and M. Devillers a* Universite Catholique de Louvain, ^ Laboratoire de Chimie Inorganique et Analytique, place Louis Pasteur, 1 ^ Unit6 de Catalyse et de Chimie des Materiaux Divises, place Croix du Sud, 2 B-1348 Louvain-la-Neuve, Belgium The formation of various Pd-Bi alloys on the surface of carbon-supported PdBi(10%wt)/C catalysts and their role in the catalytic oxidation of glucose to gluconic acid by oxygen were investigated. Supported catalysts characterized by (Afferent Bi/Pd ratios were prepared from Pd acetate and Bi oxoacetate, and activated upon thermal heating at 773 K under nitrogen. The pure Bi2Pd, BiPd and BiPds alloys were prepared from the same precursors upon thermal heating at 873 K, 973 K and 1123 K, respectively. The catalytic performances of the supported and unsupported catalysts for the above reaction were measured by keeping the palladium weight constant. The catalysts were characterized by XRD, XPS and BET. Bismuth losses from the catalysts in the reaction medium were analyzed by atomic absorption spectrometry after 4 h running. For the supported catalysts, the highest performances are observed for Bi/Pd equal to 1, and for Bi/Pd = 0.5 when the gluconic acid yields are normalized with respect to the initial or residual Bi amount, or to the catalyst mass. The most active alloy is BiaPd which is also the one that loses Bi at the largest extent. 1. INTRODUCTION Bismuth is known for displaying very attractive properties as promoting element in heterogeneous catalysts for the selective oxidation of alcohols or aldehydes by molecular oxygen in aqueous solutions [1-9]. The conversion of glucose to gluconic acid, an intermediate in the food and pharmaceutical industries, is particularly well documented [10-12]. However, the actual origin of the promoting role of Bi and the question whether Bi-Pd alloys (and which of them) are present and do play a significant role in these catalysts remains under debate. In a previous work [13], we reported on the preparation of carbon-supported bimetallic Bi-Pd catalysts by the thermal degradation of Bi and Pd acetate-type precursors under nitrogen at 773 K and described their catalytic properties in glucose oxidation. The formation of various BixPdy alloys (BiPd, BiPda, Bi2Pd5) or, at least, associations on the surface of these catalysts during the activation step was heavily suspected. Alloy formation in supported bimetallic Pdbased catalysts has been mentioned several times in the literature in the presence of other promoting elements, like Pb or Te [14-16] and is sometimes assumed as responsible for the deactivation of the catalysts. Furthermore, bismuth was found systematically to dissolve in the reaction medium during the catalytic tests, the losses being significantly more extensive from the monometallic Bi/C than from the bimetallic PdBi/C catalysts. Glucose and gluconate in solution were shown

Corresponding author; Research Associate of the Belgian National Fund for Scientific Research.

392 to be both responsible for Bi dissolution. However, we demonstrated that the mere presence of Bi3+ in solution was not a sufficient condition to improve the catalytic activity of a monometallic Pd/C catalyst [13]. The present work reports on further experiments devoted to the role played by the various Bi-based alloys in the above reaction. Bimetallic PdBi/C catalysts with different Pd/Bi ratios, and the various BixPdy intermetallic compounds whose presence was suspected in the above catalysts were prepared and tested catalytically in the glucose oxidation reaction. Because preliminary experiments [17-18] indicated that an acetate route was a convenient way to generate Bi-based oxide-type catalysts, this procedure was selected to prepare all the catalytic materials examined wihin the frame of the present work. The catalysts were characterized by XRD and XPS . 2. EXPERIMENTAL 2.1. Starting materials An activated carbon supplied by NORIT was used as support. It corresponds to the trade name PKDA 10X30 (SBET = 550 m^.g-l) and is noted hereafter CQ. Its selected particle size is in the range 0.1-0.05 mm. Palladium(II) acetate (ACROS) and bismuth(III) oxoacetate, BiO(02CCH3) (obtained as described elsewhere [13]), were used as precursors for the incorporation of the active metal and the promoting element in the catalysts. 2.2. Preparation of the catalysts Carbon-supported bimetallic catalysts (Ac.lPd3Bi/Co - Ac.lPd2Bi/Co - Ac.lPdlBi/Co - Ac.5Pd2Bi/Co - Ac.3PdlBi/Co) were prepared by deposition from a suspension of carboxylate particles in n-heptane chosen as inert organic solvent. They are characterized by different Bi/Pd molar ratios (bold figures) and by a constant metal/catalyst weight percentage of 10. Among the selected Bi/Pd molar ratios are those corresponding to the stoichiometry of the pure Pd-Bi alloys whose presence was suspected in the course of previous experiments [13]. Other compositions were also considered to provide a broader investigation range. These catalysts were prepared according to the following procedure. The adequate amount of palladium acetate was dispersed in the presence of the activated carbon (2.7 g) in about 100 ml n-heptane under ultrasonic stirring for 30 min. After slow evaporation of the solvent at room temperature, the appropriate amount of bismuth oxoacetate was deposited on the obtained monometallic catalyst according to the same procedure. The bimetallic catalyst was then activated upon thermal heating under nitrogen at 773 K during 18h. 2.3. Preparation of the Bi-Pd alloys The pure Bi2Pd, BiPd and BiPda alloys were prepared from the same precursors according to the deposition procedure described above. The carboxylates were decomposed upon thermal heating under nitrogen. The degradation temperatures were determined from the binary Bi-Pd phase diagram [19] and fixed at 873 (24h), 973 (ISh) and 1173 K (18h) for Bi2Pd, BiPd and BiPds, respectively. 2.4. Catalytic measurements 2.4.1. Reaction conditions The selective oxidation of D-glucose into gluconic acid was selected as catalytic test reaction. The reactor vessel and the experimental conditions were described in detail elsewhere [13]. The pH of the reaction mixture was kept at a constant value in the range 9.25-9.45 by adcUng a 20 or 40 wt.% aqueous solution of sodium hydroxide with an automatic titrator (Stat Titrino 718) from METROHM. The base consumption was recorded in function of time. The glucose solution (72 g glucose in 400 ml) was heated in the reactor to 50°C. Once the temperature was stabilized, the catalyst was added to the solution and the oxidation reaction started by introducing oxygen (flow rate : 0.4 l.min-^) in the stirred (1000 rpm) slurry. Two

393 series of tests were carried out by keeping the palladium weight constant within each series, the first one with supported catalysts (mp(i=2.7 mg), the second one with the pure intermetallics (mp(i=30.1 mg). Depending on the Bi-Pd composition, the amounts used in the catalytic tests were in the range 45-148 mg. Measurements performed under these conditions with different stirring rates (in the range 1000-1800 rpm) confirmed the absence of diffusional limitations. After 4 hours reaction, the oxygen inlet was turned off and the catalyst was removed from the reaction mixture by filtration. The filtrate was then analyzed by HPLC, ^^C-NMR and atomic absorption spectrometry. The catalyst was washed with water, dried under vacuum at 30°C and analyzed by XPS and XRD. 2.4.2. Analysis of the reaction products The composition of the reaction mixture was determined by HPLC and ^^C-NMR spectroscopy. The bismuth and palladium losses from the catalysts in the reaction mixture during the catalytic tests were determined by analyzing the collected filtrates by atomic absorption spectrometry. Analytical conditions were described elsewhere [13]. 2.4.3. Expression of the catalytic results Because the ^^C-NMR analyses showed that gluconic acid was the only carboxylic acid generated in the reaction medium, the yields in gluconic acid (YGLU» %) were calculated directly from the NaOH consumption. The main side product is fructose due to isomerisation in the presence of oxygen and appears at an extent between 2.6 and 4.6 % yield when YGLU is larger than 10%. 2.5. Catalyst characterization techniques 2.5.1. X-ray diffractometry (XRD) Powder X-ray diffraction patterns were obtained with a SIEMENS D-5000 diffractometer using the Ka-radiation of a copper anode. The samples were analyzed after deposition on a quartz monocrystal sample-holder supplied by Siemens. The crystalline phases were identified by reference to the ASTM data files. 2.5.2. X-ray induced photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy was performed on a SSI-X-probe (SSX-100/206) spectrometer from FISONS, using the Al-Ka radiation (E = 1486.6 eV). The energy scale was calibrated by taking the Au 4f7/2 binding energy at 84 eV. The Cis binding energy of contamination carbon fixed at 284.8 eV was used as intemal standard value. The analysis of bismuth and palladium were based on the Bi 4f7/2 and Pd 3d5/2 photopeaks. The intensity ratios I(Bi4f7/2)/I(Bi4f5/2) and I(Pd3(i5/2)/I(Pd3(i3/2) were fixed at 1.33 and 1.5 respectively. 3. RESULTS 3.1. Characterization of the supported bimetallic catalysts XRD : Most bimetallic catalysts are characterized by poorly resolved XRD spectra, suggesting an amorphous or microcristalline structure. Metallic bismuth and an intermetallic compound (Bi2Pd) were however observed in the catalysts in which the Bi/Pd molar ratio is equal to 2. Figure 1 shows the X-ray diffraction pattem of the bimetallic supported catalyst Ac.lPd2Bi/Co. XPS: Representative XPS results are listed in table 1 for fresh and used bimetallic catalysts.

394

28r) Fig. 1 : XRD pattern of a bimetallic carbon-supponed AclPd2Bi/Co catalyst before use. Table 1 XPS data of fresh and used carbon-supported Pd-Bi/Co catalysts Catalyst

fresh BiyPd Pd/C Bi/C Bi/Pd Pd/C theor. theor. theor. exp. exp. xlOO xlOO xlOO

Ac.lPd3Bi/Co Ac.lPd2Bi/Co Ac.lPdlBi/Co Ac.2PdlBi/Co Ac.5Pd2Bi/Co Ac.3PdlBi/Co

3.00 2.00 1.00 0.50 0,40 0.33

0.18 0.25 0.42 0.62 0.70 0.76

0.54 0.51 0.42 0.32 0.28 0.25

11.16 12.65 4.07 1.56 0.95 0.73

0.25 0.32 0.93 2.09 2.50 2.70

used BVC Bi/Pd Pd/C Bi/C exp. exp. exp. exp. XlOO XlOO X 100 2.76 0.56 0.63 0.35 4.09 0.53 1.35 0.71 3.79 0.54 2.06 1.11 3.24 0.55 2.17 1.20 1.35 2.37 0.49 2.78 1.38 1.98 0.48 2.89

Bismuth and palladium appear in the metallic (Pd^, Bi^) and the oxidized form (Pd^^, Bi^+). The binding energy values associated with the Bi 4f7/2 photopeak lie in the range 157.2157.8 eV for Bi^ and 158,9-159.6 eV for Bi3+ and, for the Pd3d5/2 line, in the range 335.5336.1 for ?69 and 337.1-337.8 eV for Pd2+. The experimental Bi/Pd rados in the fresh catalysts are higher than the theoretical values calculated from the bulk composition of the catalysts, indicating a partial coverage of palladium by bismuth. This observation is in agreement with the sequential incorporation of Pd first, then Bi, during the preparation of these catalysts. The Bi/Pd molar ratio in the used catalysts always decreases to reach the value of 0.5 (between 0.48 and 0.56), suggesting that this particular composition might play an important role in the oxidation process. This decrease in the Bi/Pd but also in the Bi/C ratios and the increase in the Pd/C ratios after the catalytic tests is in line with the bismuth losses previously observed during the catalytic oxidation of D-giucose in D-gluconic acid.

395 3.2. Characterization of the pure intermetallics DRX : p-Bi2Pd which, according to the phase diagram [19], is metastable at low temperature, was found to transform into the stable a-BiPd phase and to lose 64% of its initial bismuth content during the catalytic operation. Small amounts of metallic palladium and aBi2Pd were also observed in the XRD spectra of this intermetallic compound after use, suggesting the following transformation : p-Bi2Pd -^ a a-BiPd + b a-Bi2Pd + c Bisol + d Pd a-BiPd3 and a-BiPd were found to be stable under the catalytic conditions. XPS : The XPS data collected on the pure intermetallics before use in the catalytic tests are are listed in table 2. As indicated by this table, palladium is again partially covered by bismuth (Pd/Bi exp- > Pd/Bi theor) and is mainly in the metallic form, while bismuth is present on the surface of tnese intermetalHc compounds in the oxidized form. Table 2 XPS data of intermetallic compounds intermetallic compound

Bi/Pd theor.

Bi/Pd exp.

Pd0/Pd2+

BiO/Bi3+

BiPd3 BiPd Bi2Pd

0.33 1.00 2.00

0.5 4.2 8.1

3.9 1.6 3.9

0.7 Bi3+ 0.1

3.3. Catalytic results 3.3.1. Supported bimetallic catalysts The catalytic results are Usted in table 3. Yields in gluconic acid (line 6) and the extent of bismuth dissolution in the reaction medium (lines 7-8) are given after four hours running. The bismuth losses are expressed in percents of initial bismuth loading on the catalyst. As shown by comparing lines 5 and 6 in table 3, there is an optimal value for the Bi/Pd molar ratio in the PdBi/Co catalysts : the catalytic performances are better in the Bi-rich region, but do not increase further above Bi/Pd equal to 1. However, when these data are normalized with respect to the initial (line 12) or residual (Une 14) Bi amount, or the catalyst mass (line 11), the highest values are found for tiie composition 2PdlBi. Bismuth was found systematically to dissolve in the reaction medium during the catalytic tests. In contrast with bismuth, palladium dissolution was never detected under the present experimental conditions. As indicated in line 7 of table 3, bismuth dissolution increases with the Bi/Pd molar ratio and, when the gluconic acid yields are normalized with respect to the amount of dissolved bismuth (line 13), the highest value is observed for the 5Pd2Bi/Co catalyst. Figure 2a illustrates the comparison between the evolution of either the bismuth losses or the gluconic acid yields, with respect to the Bi/Pd molar ratio. The gluconic acid yield increases with the Bi/Pd molar ratio till Bi/Pd is equal to 1 and then slightly decreases. On the other hand, bismuth dissolution increases further with the Bi/Pd ratio, showing that for Bi-rich compositions, the excess of Bi is leached from the catalyst without affecting significantly the catalytic activity.

396 Tables Catalytic performances of carbon-supported Bi-Pd/Q) catalysts (after 4 h reaction). Catalysts 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

lPd3Bi

lPd2Bi IPdlBi 2PdlBi 5Pd2Bi 3PdlBi

2:1 1:1 3:1 8.55-1.45 8.0-2.0 6.6-3.4 146.1 133.0 80.0 m cata (mg) 11.9 10.6 5.3 mBi(mg) 3.00 2.00 1.00 Bi/Pd (molar ratio) 37 31 38 YGLU(%) 66 58 Bi losses (%) 35 7.85 6.15 1.86 m Bi dissolved (mg) 3.44 4.05 4.45 mBiresidual(mg) 0.84 0.65 0.77 Biresidual/Pd (molar ratio) 0.21 0.28 0.48 YGLu/mcata(%-mg-^) 7.2 2.6 3.5 YoLu/mBi (%mg-l) 20.4 6.0 3.9 YGLU/mBidiss(%-mg-l) YGHj/mBires(%mg-l) 7.7 8.3 11.0 0.191 0.228 0.234 Rate (mol gluconic acid. (gPd)-l.(min)-') Bi:Pd(mol) Bi - Pd (wt. %)

1:2 2:5 5.0-5.0 4.4-5.6 48.2 54.0 2.7 2.12 0.50 0.40 15 30 11 31 0.84 0.23 1.89 1.86 0.36 0.35 0.31 0.56 11.1 7.1 65.2 35.7 7.9 16.1 0.093 0.185

1:3 4.0-6.0 44.7 1.77 0.33 10 13 0.23 1.54 0.29 0.22 5.6 43.5 6.5 0.062

3.3.2. Intermetallic compounds The catalytic results are listed in table 4 and illustrated in fig. 2b. As shown by these results, the three alloys exhibit a very different catalytic behaviour. The most active phase is pBi2Pd which is also the alloy that loses bismuth at the largest extent and transforms into aBi2Pd, a-BiPd and Pd during the test. BiPds is not leached during the catalytic reaction but is essentially inactive. Although we demonstrated previously [13] that the presence of bismuth in solution was not a sufficient condition to promote the catalytic activity, the partial dissolution of bismuth in the reaction medium may be necessary. When the gluconic acid yields are normalized with respect to the amount of residual bismuth (line 13), the highest values are observed for the Bi2Pd alloy. 3.3.3. Comparison alloys-supported catalysts Independently from variations in the specific surface areas and in the palladium masses engaged within each series of tests, the comparison between the performances of the intermetallics and those of the supported catalysts suggests that the alloys suspected on the surface of the bimetallic supported catalysts are not the only factor responsible for their catalytic behaviour. The multiphasic nature of the catalysts seems to be a key feature to account for their properties. For instance,when the gluconic acid yields are normalized with respect to rticata (Table 3, line 11), met engaged 0^^^ 12), or mfii residual (^^^ 14), the highest values are obtained for Ac.2PdlBi/C (Bi/Pd=0.5) whose composition does not correspond to a given intermetallic compound; in the same way, BiPds is inactive while the Ac.3PdlBi/C catalyst of the same composition is active; also, Bi2Pd is more active than BiPd but the corresponding supported catalysts display the same catalytic behaviour.

397

%

YGLU(%) Bi losses (%)

O T"

T"

T

T-

1 2 3 Bi/Pd molar ratio

Bi/Pd molar ratio

Fig. 2 : Evolution of the gluconic acid yields and the bismuth losses with respect to the Bi/Pd molar ratio in the supported catalysts (a) and the BixPdy inteimetallics (b) Table 4 Catalytic perfoimances of the various BixPdy intemietallics (t = 4h) Intermetallic compound 1

Bi2Pd

BiPd

BiPds

BirPd (mol)

2:1

1:1

1:3

89.3

49.8

3

m cata (mg) m Bi (mg)

148.4 118.3

59.2

19.7

4

Bi/Pd (molar ratio)

2.00

LOO

0.33

81

20

2

2

5

YGLU(%)

6

Bi losses (%)

64

5

0

m Bi dissolved (mg)

75.71

2,96

0

8

m Bi residual (mg)

42,59

56.24

19.70

9

Bi residuai/Pd (molar ratio)

1.41

1.87

0.65

cata (%.mg-l) YGLU/mBi(%.nig-l)

0.55

0.22

0.04

0.7

0.3

0.1

6.8

-

7

10 11 12

Y GLU/m Bi dissolved (%-nig-i)

1.1

13

Y GLU/m Bi residual (%.mg-l) Rate (mol gluconic acid. (gPd)-l.(min)-l)

1.9

0.4

0.1

0.045

0.011

0.001

14

398 4. CONCLUSIONS The catalytic performances of supported Pd-Bi/Co catalysts for the selective oxidation of glucose to gluconic acid and the Bi losses from the catalysts during operation were both found to be highly dependent upon the composition of the active phase. Bi losses were found to increase with the Bi content, without any relationship with the catalytic activity. The experiments performed with the pure intermetallics Bi2Pd, BiPd and BiPds showed that the intrinsic catalytic behaviour of these phases are very different; in addition, the most active phase, Bi2Pd, is the one that loses the largest amount of Bi, whereas BiPds, the most stable phase during operation, remains totally inactive. When the performances of the supported catalysts were compared with those of the pure intermetallics, the highest yields in gluconic acid were found for different compositions. This demonstrates that the behaviour of the Pd-Bi/Co catalysts is modulated by the presence of the support and the intrinsic activity of the various alloys. Furthermore, the multiphasic nature of the catalyst characterized by the Bi/Pd ratio of 0.5 might be responsible for the enhanced catalytic performances, given the fact that there is no pure intermetallics corresponding to that composition. Although there seems to be no simple relationship between the extent of Bi dissolution and the performances of these catalysts, partial leaching of the promoting element seems to be a key feature for their functioning. ACKNOWLEDGEMENTS The authors greatly acknowledgefinancialsupport from the Belgian National Fund for Scientific Research (F.N.R.S., Brussels) for the programme concerning the selective oxidation of glucose. The authors are grateful to NORIT for supplying the carbon support, to Dr. R. Touillaux and J.F. Statsijns for their assistance in the analytical part of this work, and to the F.R.I.A, Brussels and the Catholic University of Louvain for the fellowships allotted to M.W.

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399

Selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxaldehyde in the presence of titania supported vanadia catalysts C. Moreau*, R. Durand, C. Pouixheron and D. Tichit Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS/ENSCM, Ecole Nationale Superieure de Chimie, 8 Rue de I'Ecole Normale, 34053 Montpelher Cedex 1, France. Oxidation of 5-hydroxymethylfurfural to 2,5-furan-dicarboxaldehyde was performed in a batch reactor at 363 K in the presence of supported V205/ri02 catalysts with different vanadium loadings, and in toluene and methyl isobutyl ketone as the solvents. An air pressure of 1.6 MPa allowed the fast in situ regeneration of the catalyst and the complete transformation of the starting reactant. It appears that a multilayered V205/ri02 catalyst with a structure close to t h a t of bulk V2O5 is preferred since involving more V=0 species responsible for the oxidation of alcohols. In addition, a higher turnover frequency is obtained in the presence of methyl isobutyl ketone as the solvent. This is particularly suitable for a further development of the reaction on a pilot scale as far as 5-hydroxymethylfurfural is extracted with that solvent in the preceding chemical step and thus may be directly used in the oxidation step. 1. INTRODUCTION Since the last decade, there is a renewed interest to use carbohydrates as a source of chemicals, particularly for the preparation of non-petroleum derived polymeric materials from furanic compounds [11. One key compound, 5-hydroxymethylfiirfiiral, readily available with a high selectivity through dehydration of fructose and fructose precursors in the presence of H-form zeolites [2-4], is a suitable starting material for the preparation of further monomer units required for polymer applications, since containing two different functional groups at the positions 2 and 5. The oxidation of 5-hydroxymethylfurfural is a reaction of particular interest as far as complete oxidation yields 2,5-furan-dicarboxylic acid (FDA), a material which has properties and applications similar to those of both terephthalic and isophthalic acids [1,5]. Other partially oxidized compounds (Scheme 1) are all involved as intermediates for the preparation of surfactants, synthetic materials or resins [6]. Among them, another symmetric compound of interest is 2,5 furan-dicarboxaldehyde (FDC), used as such as an antifungal or as a precursor to symmetincal diamines and Schiffs bases.

400

Up to now, high yields of 2,5-furan-dicarboxaldehyde were only obtained in the presence of stoechiometric quantities of classical oxidants [7]. In the presence of noble metal catalysts, selective oxidation to 2,5-furan-dicarboxylic acid (FDA) and 5-formyl 2-furan-carboxylic acid (FFCA) were only reported [6,8,9]. HMF

FDC

HOHoC

OHC

HFCA

FFCA

HOH

OHC^ ^ 0 ^ ^COOH

FDA

'

Scheme 1 : Reaction scheme proposed for the oxidative dehydrogenation of 5-hydroxymethylfiirfural over noble metal catalysts [8,9]. In this paper, we wish to report on the selective oxidation of 5hydroxjnnethylfurfural to 2,5-furan-dicarboxaldehyde using vanadium oxide supported on titanium oxide with different vanadium loadings. If we take into account the large differences in the activation energies reported over V2O5 in the oxidation sequence benzyl alcohol - > benzaldehyde (Ea = 26 kJ/mol) and benzaldehyde - > benzoic acid (Ea = 55 kJ/mol) [10], those catalytic systems were then expected to stop at the aldehyde stage by working at low temperatuiê. 2. RESULTS AND DISCUSSION 2.1. Catalysts characterization The experimental data concerning centesimal analyses, BET surface areas calculated from nitrogen adsorption at 77 K and monolayers number calculated assuming a monolayer capacity of 4.0 wt % of V2O5 for P 25 of surface area 55 3 m^/g [11] are reported in Table 1.

401 Table 1 Centesimal analyses, BET surface areas and monolayers number Catalyst

Surface area (m^/g)

Support CPI

58 53 50 44 42

cpn

CPIII CPIV

V2O5 (wt %) ~ 2.40 3.07 9.34 15.03

V (wt%)

V2O5 (wt%/m2)

Monolayer number

-1.32 1.67 4.79 7.33

0.045 0.061 0.212 0.358

~ 0.60 0.77 2.33 3.76

X-Ray Diffraction analysis was performed on vanadia, titania support and supported V205/ri02 catalysts CP I-CP IV. For the vanadium loaded samples, vanadium oxide can be only identified for the catalysts having more than the single monolayer, CP HI and CP IV. From XRD experiments, it was also possible to calculate the mean value for the particule diameter, 400 to 600 A. From the chemical shifts measured by ^^V NMR spectroscopy for the catalysts CP I and CP II, it appears that the bands observed between - 640 and 710 ppm correspond to vanadium in a tetrahedral environment [12] with vanadium bound to the support [13]. For the catalyst CP III, an octahedral environment is observed. For the liighly vanadium loaded catalyst CP IV, the chemical shifts observed, - 300, - 650 and -1242 ppm correspond to those of bulk vanadium oxide, - 280, - 609 and - 1250 ppm [14]. This could correspond to the "disordered vanadium oxide" growing away from the surface as described by Bond et al., and indicative of the presence of V=0 V bonds [11]. In the literature, the presence of V=0 species located on [010] planes of V2O5 crystals would be considered as the active sites responsible for alcohol oxidation [15-17]. Confirmation of the presence of such V = 0 bonds was obtained by FT-IR spectroscopy with the presence of a band at 1016 cm"l for bulk V2O5 and for the highly vanadium loaded catalysts CP III and CP IV [18]. 2.2. Catalytic tests Experiments were performed according to the operating conditions reported in the experimental section, in the presence of the V205/Ti02 catalyst CP IV which was found to be more active. The activities of both V2O5 active phase and Ti02 support have been considered first in toluene as the solvent. After a reaction time of 4 hours, the conversions to 2,5-furan-dicarboxaldehyde (FDC) were 30 and 15 %, respectively. The corresponding selectivities to FDC were 80 and 95 %. In the presence of the V205/ri02 catalyst CP IV, a synergy effect was observed (Figure 1). The reaction is nearly complete after the same period of time, with a selectivity close to 90-95 %. This accelerating effect would result from the electron withdrawing character of the support which increases the positive charge on the V^+ ions [18], making easier the reduction of the supported vanadium oxide catalyst [19], and thus favoring the mechanism proposed by Subrahmanyam [17].

402 Conversion

[Products]

100 H / 50 H

If

l^^ 100

200

i^ 240

- r > time 300

Figure 1. HMF conversion vs time (min) for V2O5 active phase , Ti02 support (Q) and CP IV V205/Ti02 catalyst .

time

Figure 2. Products distribution (x 10^ mol in 50 ml of solvent vs time (min); HMF , FDC ) and by-products (o) over CP IV V205/n02 catalyst.

By plotting the concentrations in reactant and products as a function of time, first order kinetics are observed as illustrated in Figure 2. The initial reaction rates calculated fiôm these curves are then plotted as a function of catalyst weight (Figure 3) and initial concentration in 5-hydroxymethylfurfural (Figui-e 4).

Vo 10i

-T-> cata wt 1 Figure 3. Initial rates (x lO'^mol/s) vs catalyst weight (g) in toluene.

- > [HMF] 1 Figure 4. Initial rates (x 10'^ mol/s) vs HMF concentration (x 10^ mol in 50 ml toluene).

From Figures 3 and 4, it is then deduced that the reaction obeys a classical Langmuir-Hinshelwood mechanism with a maximum reaction rate constant of 9 IQ-'^ mol/s in toluene as the solvent. 2.3. Influence of vanadium loading Table 2 reports the initial oxidation rates expressed per m^ as a function of vanadium loading in toluene as the solvent. From this table, it can be seen that a plateau seems to be reached for a vanadiimi loading between 1.32 and 4.79 %, i.e. in the region corresponding to less than the single vanadium monolayer [20]. The

403 higher loaded catalyst is slightly more active, probably because of the positive effect of the dispersion of the active phase on the support and of its structure close to that of V2O5 with the presence of a larger number of V=0 species responsible for alcohols oxidation. Table 2 Influence of vanadium loading on the oxidation rates in toluene as the solvent vanadium %

0

monolayer 108Vo/m2

0.8

1.32

1.67

4.79

7.33

0.60

0.77

2.33

3.73

3.6

3.2

3.8

4.3

2.4. Solvent effect In methyl isobutyl ketone as the solvent, the plots of the initial rate constants as a function of the catalyst weight and the initial concentration in 5hydroxymethylfurfural are reported in Figures 5 and 6, respectively.

Vo

Vo 20

20 1

15H

10i

10H

5 0

/ r^ cata wt 1

Figure 5. Initial rates (x lO'^mol/s) vs catalyst weight (g) in methyl isobutyl ketone.

[HMF]

0 0

10

Figure 6. Initial rates (x lO^mol/s) vs HMF concentration (x 10^ mol in 50 ml methyl isobutyl ketone).

From both figures, it is clearly seen that the saturation phenomenon, of the substrate by the catalyst (Figure 5) or of the catalyst by the substrate (Figure 6), will occur for high catalyst weight and HMF concentration as compared to the corresponding behavior in toluene. No change in the reaction mechanism might be invoked to account for such a behavior; the apparent energies of activation are nearly the same, 64 kJ/mol in methyl isobutyl ketone and 77 kJ/mol in toluene. That means t h a t a higher turnover frequency is obtained in the presence of methyl isobutyl ketone as the solvent. However, it should be mentioned t h a t the selectivity to 2,5-furandicarboxaldehyde is high (^ 90 %) at moderate 5-hydroxymethylfuifural (HMF) concentration and catalyst weight. When the ratio substrate/catalyst is close to

404

0.5, a high selectivity can be maintained up to « 90 % whatever the solvent and at relatively high conversions as reported in Table 3. Table 3 Influence of the substrate/catalyst ratio on the selectivity to FDC : 0.4 g of CP IV catalyst, 0.2 g of HMF, 50 ml of solvent, 363 K, 1.6 MPa air. Solvent

Reaction time

HMF conversion

FDC selectivity

Toluene Toluene Toluene

Ih 1.5 h 4h

76% 81% 91%

87% 97% 93%

Methylisobutylketone Methylisobutylketone Methylisobutylketone

Ih 1.5 h 4h

26% 40% 66%

97% 89% 90%

Otherwise, the selectivity tends to drop probably because of the rapid saturation of the catalyst in the less polar solvent (toluene, see Figure 4) and the rapid decomposition of unreacted 5-hydro3^Tnethylfarfural on the acid catalyst, as illustrated in Table 4 for a substrate/catalyst ratio of 1.5 and in Table 5 for a substrate/catalyst ratio of 3. Table 4 Influence of the substrate/catalyst ratio on the selectivity to FDC : 0.4 g of CP IV catalyst, 0.6 g of HMF, 50 ml of solvent, 363 K, 1.6 MPa air. Solvent

Reaction time

HMF conversion

FDC selectivity


Ih 1.5 h 4h

33% 43% 57%

85% 88% 86%


Ih 1.5 h 4h

25% 29% 52%

70% 76% 77%

3. EXPERIMENTAL 3.1. Catalysts preparation The support used is Degussa P25 Ti02 (58 m^/g). The preparation technique consists of impregnation of the support with an aqueous solution of ammonium metavanadate [11]. The partially dehydroxylated support (10 g) is diluted with 250 ml of water, and, after addition of a given amount of ammonium

405 metavanadate, the mixture is acidified with concentrated hydrochloric acid up to pH = 2. After a stirring period of 24 hours, the solid is separated by centrifugation, washed several times with water, dried at 333 K and finally calcined at 773 K for 4 hours. Table 5 Influence of the substrate/catalyst ratio on the selectivity to FDC : 0.4 g of CP IV catalyst, 1.2 g of HMF, 50 ml of solvent, 363 K, 1.6 MPa air. Solvent

Reaction time

HMF conversion

FDC selectivity


Ih 1.5 h 4h

44% 61% 74%

32% 30% 22%


Ih 1.5 h 4h

28% 40% 84%

93% 98% 62%

3.2. Typical procedure Experiments were performed in a 0.1 1 reactor operating in the batch mode. The feed consists of 0.4 g of catalyst and 0.2 g of HMF in 50 ml of toluene or methylisobutylketone. The temperatui'e is 363 K, the air pressure 1.6 MPa, and the agitation speed 1000 rpm. Under those operating conditions, the reaction is not limited by external or internal diffusion. 3.3. Analyses Analyses were performed by HPLC using a Shimadzu LC-6A pump and UV Spectrophotometer SPD-6A detector. The columns used were Phenomenex Rezex monosaccharide-H-*- with trifluoroacetic acid (lO'^M) as the mobile phase for toluene as the solvent, and Spherisorb-CN with cyclohexane/methylene chloride/isopropanol (80/16/4 by volume) as the mobile phase for methyl isobutyl ketone as the solvent. 4. CONCLUSIONS The selective preparation of 2,5-furan-dicarboxaldehyde is easily achieved at low temperature in the presence of V205/ri02 P25 catalysts. A higher turnover frequency is obtained in the presence of methyl isobutyl ketone as the solvent. This is particularly suitable for a further development of the reaction on a pilot scale as far as 5-hydroxymethylfurfural is extracted with that solvent in the preceding chemical step and thus may be directly used in the oxidation step [22].

406 5. ACKNOWLEDGEMENTS Siidzucker A.G. is gratefully acknowledged for providing us with a sample of HMF, and Agrichimie for financial support.

REFERENCES 1. A. Gandini, "Comprehensive Polymer Science", First Supplement, S.L. Aggarwal and S. Russo Eds, Pergamon Press, Oxford, 1992, p. 527. 2. C. Moreau, R. Durand, P. Geneste, P. Faugeras, P. Rivalier, P. Ros and G. Avignon, french Pat. FR 2 670 209 (1992) , european Pat. EU 561 928 (1996), assigned to C.E.A. 3. C. Moreau, R. Durand, C. Pourcheron and S. Razigade, Industrial Crops and Products, 3 (1994) 85-90. 4. C. Moreau, R. Durand, S. Razigade, J. Duhamet, P. Faugeras, P. Rivalier, P. Ros and G. Avignon, Appl. Catal. A General, 1996, in press. 5. M. Kunz, "Inulin and Inulin-containing Crops", A. Fuchs Ed, Elsevier, Amsterdam, 1993, p. 149. 6. E.I. Leopold, M. Wiesner, M. Schlingmann and K Rapp, Geiman Pat. DE 3 826 073 (1990), assigned to Hoechst A G . 7. L. Cottier, G. Descotes, J. Lewkowski and R. Skowronski, Polish J. Chem., 68 (1994) 693, and references therein. 8. P. Vinke, H.H. van Dam and H. van Bekkum, in "New developments in Selective Oxidation", G. Centi andF. Trifiro Eds, Elsevier, Amsterdam, Studies in Surface Science and Catalysis, 55 (1990) 147. 9. P.Vinke, W.van der Poel and H.van Bekkum, in "Heterogeneous Catalysis and Fine Chemicals II", M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Eds, Elsevier, Amsterdam, Studies in Surface Science and Catalysis, 59 (1991) 385. 10. J. Zhu and S.L.T. Andersson, J. Catal., 126 (1990) 92. 11. M. GUnski and J. Kijenski, React. Kinet. Catal. Lett, 46 (1992) 387. 12. G.C. Bond, J. Perez Zurita, S. Flamerz, P.J. Ceilings, H. Bosch, J.G. van Ommen and B.J. Kip, Appl. Catal., 22 (1986) 361. 13. B. Jonson, B. Rebenstorf, R. Larsson and S.L.T. Andersson, J. Chem. Soc, Faraday Trans. I, 84 (1988) 3547. 14. S. Jansen, Y. Tu, M.J. Palmieri and M. Santi, J. Catal., 138 (1992) 79. 15. S.T. Oyama and G.A. Somorjai, Catal. Sci. and Technology, 1 (1991) 219. 16. H. Miyata, Y. Nagawa, T. Ono and Y. Kubokawa, J. Chem. Soc, Faraday Trans. I, 79 (1983) 2343. 17. H. Eckert and I.E. Wachs, J. Phys. Chem., 93 (1989) 6793. 18. M. Subrahmanyam and AR. Prasad, Appl. Catal., 65 (1990) L5. 19. J. Huuhtanen, M. Sanati, A Andersson and S.L.T. Andersson, Appl. Catal. A: General, 97 (1993) 197. 20. G. Deo and I E . Wachs, J. Catal., 145 (1994) 323. 21. A.J. van Hengstum, J.G. van Ommen, H. Bosch and P.J. Ceilings, Appl. Catal., 5 (1983) 207. 22. J. Duhamet, P. Rivalier, C. Moreau and R. Durand, Catal. Today, 24 (1995) 165.


407

DelQ^diogenation of M ethos^sopropanol to Methosyacetone on Suiqx>rted Bimetallic Cu-Zn Catalysts M.V.Landau, S.B.Kogan and M.Herskowitz The Blechner Center for Industrial Catalysis and Process Development, Ben-Gnrion University of the Negev, Beer-Sheva 84105, Israel Dehydrogenation of methoxyisopropanol (MOIP) on reduced Cu-Zn catalysts was studied in a fixed bed reactor at 200-300^C and atmospheric pressure. Alumina supported catalysts yielded a lower initial activity compared with the silica supported catalysts and displayed a lower deactivation rate. The main route for deactivation of Cu-Zn/Al203 was coking while that of Cu-Zn/Si02 was the crystallization of Cu^ phase. ZnO was essentially an inactive component, promoted the activity of supported Cu catalysts by modifjdng the structure and electronic state of Cu metallic phase and selectivity of Cu/Al203by modifjdng the surface of support. Oxidative regeneration of Cu-Zn/Al203 catalyst after 250 hours on stream recovered completely its initial activity. Kinetic experiments yielded a Langmuir-Hinshelwood type equation expressing the MOIP rate of dehydrogenation on Cu-Zn/Al203 1. INTRODUCTION Ortho substituted N-alkyl gmilines are intermediates for an important class of pesticides. They can be produced by reacting aniline with the proper alcohol in the presence of combined acid-based and metallic catalysts [1-3] or a polymetallic platinum-based catalyst that displayed a better performance [4-6]. Dehydrogenation of alcohol to the corresponding ketone [4,5] is one of the steps in the overall reaction. Significant deactivation of platinum-based polymetallic catedysts was reported in the study of the reaction of 2-Me-6-Et-aniline with MOIP [5,6], Then, acceptable yields of the corresponding N-alkylaniline could be achieved for more than 25 hours on streeun by purification of the feedstock and operating at optimal reaction conditions [6]. The reductive alkylation of aniline with the corresponding ketone was carried out in liquid phase in a stable mode [7-9]. Dehydrogenation of alkoj^rEdcohols to alkoxyketones could be performed with reasonable selectivity using bulk mixed oxide Cu-Cr or Cu-Zn catalysts [10]. The scope of this study was to develop an optimal supported bimetallic Cu-Zn catedyst for the selective production of methoxyacetone (JMA) from MOIP. 2.EXPERIMENTAL SECTION Catalysts preparation. Extrudated pellets of alumina (A-4191, Engelhard B.V.) with a surface area of 250 m^/g and pore volume of 1.06 cm^/g and silica (SiO2-1030, PQ Corp.) with a s\irface area of 290 m^/g and pore volume of 1.32 cm3/g were calcined at 550^C for 3 hours and impregnated with excess of 25% aqueous ammonia solution containing dissolved Zn(CH3COO)2 ( Merck 8800) and Cu(CH3COO)2( Merck 2710) for one hour. The pellets were separated from the solution, dried in air at 120^C for 3 hours and calcined at 300-450^0 for 5 hours. The Cu-Zn loading (from 5 to 15 wt.% of each oxide) and Cu/Zn atomic ratio were adjusted by changing the salts concentrations in the impregnation

408

solution. The Cu-Zn and Cu-Cr catalysts with high metal loading were prepared by mixing the same solution ( in case of Cu-Cr catalyst Zn acetate was replaced with (CH3C02)7Cr3(OH)2, Aldrich 31.810-8) with water-washed aluminimi hydroxide cake, precipitated from A1(N03)3 (Fluka 06275) aqueous solution by ammonia at pH = 8. Water was evaporated, the catalyst powder was dried at 120OC for 3 hours, calcined at 400^C for 5 hours and pelletized to pellets 1.5-2 mm in diameter. Platinum catalyst was prepared by impregnation of silica pellets with aqueous solution of H2PtCl6 (Aldrich 20.608), drying and calcining in air at 350oC. Prior to testing, all the catalysts samples were treated in N2 flow of 1000 cc/g-h at 140OC for 1 hour and then reduced in H2 flow of 700 cc/g-h at 140OC and I8OOC for 1 hour and 250OC for 2 hour. Catalysts characterization. Several characterization techniques were employed: Catalysts chemical composition, determined by ED AX, along with SEM microphotographs, was measured on a JEM-35CF (JEOL Co) instrument. BET surface area was determined according to the ASTM D-3663-84 procedure. Catalysts pore volume -was determined by water absorption. ESCA and XRD characterization methods were described in [6]. Catalyst 8.4 wt% Cu/Si02 reduced at 400^C in H2 for 2 hours was used as a standard for copper crystallinity calculations based on XRD data. TPR and TPO investigations were carried out with a device set up on a GC HP 5890 at a heating rate of 10°C/min. Coke in spent catalysts was determined by sample burning in air in a closed glass cell after extraction, washing and drying as descibed in [6] and by DTG (Mettler TG-50 instrument) in air at 6O-6OOOC ( heating rate of IS^C/min). Catalysts testing. The catalytic reaction sj^tem consisted of a fixed-bed SS reactor (21 mm ID.) heated electrically and controlled by Eurotherm controller. The temperature was measured by a chromel-alumel thermocouple in a central thermowell. Liquid MOIP (Aldrich, 98% purity) was pumped by an Eldex Lab. metering pump, evaporated and reacted on 5 g of catalyst diluted with 10 g of silica and loaded between two layers of 20 cc silica. Products were condensed at the reactor outlet, the hydrogen vented though a cooler and its rate was measured by Brooks 58601 mass flowmeter. The standard start of nm (SOR) test conditions were 200OC and LHSV = 2 h-1 for two hours, then raised to 2200C for two hours. Then the temperature and LHSV were adjusted according to required operating conditions. Calculations indicated no inter and intrapellet mass emd heat transfer limitations over the operating conditions in this study. The liquid products were anedyzed by (3C (HP 5890) equipped with Rtx (Restek) capillary column of 15 m length and 0.53 mm diameter. The only byproduct detected was acetone (A). Conversion of MOIP (X) and selectivity (S) to MA were determined as: % X = 100(1-CMOIP/CMOIPO) % S = 100[CMA / (CMA + CA)]

3. CATALYSTS PERFORMANCE The catalysts performance in MOIP dehydrogenation is given in Table 1. Alumina displayed significgmt activity with low selectivity to MA as a result of high demethoxylation activity. Introduction of copper increased the dehydrogenation activity and MA selectivity of alumina while introduction of zinc led to low activity and complete selectivity to MA. The catalyst containing both metals on alumina jdelded activity close to CU/AI2O3 but significantly

409

higher selectivity to MA. Silica had no activity while the activity of Cu/Si02 was close to Cii/Al203 with higher MA selectivity. Zn/Si02 displayed low activity with no A. The bimetallic silica catalyst displayed a significant activity promotion effect of Zn with no selectivity change. The high loading Cu-Zn and Cu-Cr catalysts displayed a much lower dehydrogenation activity and lower MA selectivities. The activity of Pt/Si02 was comparable with supported Cu-Zn catalysts with lower MA selectivity. Table 1 Catalj^ts support, active components and promoter affect its performance Cat^j^ts composition, wt.%

Testing conditions: Temperature, ^C LHSV,h-l

M t i S performance, % MOIP MA conversion selectivity

A1203 8% CUO/AI2O3 10% ZnO/Al203 8%CuO- 10%ZnO/Al2O3

270 270 270 270

7.0 7.0 7.0 7.0

25.9 45.3 2.0 46.2

54.2 93.3 100 96.3

Si02 10% CuO/Si02 13% ZnO/Si02 10%CuO-13%ZnO/SiO2

270 270 270 270

7.0 7.0 7.0 7.0

0 48.3 7.8 62.0

96.5 100 96.3

5%Pt/Si02 36%CuO-32wt.%Cr203/Al203 32%CuO-60%ZnO-Al2O3

300 250 300

7.0 1.0 1.0

51.4 70.1 60.1

88.6 71.3 90.2

The Cu-Zn catalysts supported on gdumina and silica showed different stability performance in long runs (Fig.l). The stability tests were started at 240OC and LHSV = 2.5 h"^. After a period of conversion decrease (about 40 and 100 hours for Cu-Zn/Al203 and Cu-Zn/Si02, respectively) a constant temperature increase of O.PC/h was maintained for both catalysts for additional 200-250 hours. Silica supported catalysts deactivated at a faster rate than the corresponding alimiina catalysts. After 300-320 hours a fast deactivation rate of both catalysts was recorded. Fig.2 demonstrates the effect of Cu/Zn atomic ratio on initied activity of CuZn/Al203 catalyst in MOIP dehydrogenation. The catalysts samples were csdcined at 350^0 before reduction with H2. The MOIP conversions were measured at two hours on stream at 240^0 and LHSV = 2 h-^. The loading of ZnO was kept constant at the level of 10 %wt., since its effect on the catalyst performance over the range 5-15 wt% was negligible. The dehydrogenation activity of Cu-Zn/Al203catalyst reached a maximum at Cu/Zn atomic ratios 0.60.75 close to equilibrium converion with essentially no change of MA selectivity. The Cu-Zn/Al203 (Cu/Zn = 0.7) catalysts calcination temperature before reduction affected significantly its activity and stability as shown in Fig.3. The initial MOIP conversion was measured at 260^C and 6.2 h^ and the deactivation rate (% conversion/hour) was calculated from the MOIP conversion decrease

410 100 90 80 70 B 60 50 u 40 a 30 20 U 10 0 0

Conditions: T SOR = 240oC: LHSV = 2.5 O

a

D

Cu-Zn/AI203: Cu-Zn/AI203: Cu-Zn/Si02: Cu-Zn/Si02:

h-1

MOIP conversion MA selectivity MOIP conversion MA selectivity

^3»Sl2-*l&S#S»:*^ 50

100

150

200

250

300

Run time, hour Figure 1. Effect of time on stream on performance of silica and alumina supported Cu-Zn catalysts ^ 50 B U

140' «20 . 5 0 . 6 0 , 7 0, 8 0 . 9 1.0

Cu/Zn Fig.2. Effect of Cu/Zn ratio on Cu-Zn/Al203 catalysts activity

Initial

MOIP

conversion,%

Catalysts deactivation %conv./h ' 10

rate,

300 350 400 450 Catalysts calcination temperature, oC Fig.3. Effect of Cu-Zn/Al203 catalysts calcination temperature on its performance

over a period of 50 hour runs. Increasing the catalysts calcination temperature from 300 to SSO^C yielded the best performance, increasing the activity by about 1.5 times and decreasing the deactivation rate by a factor of two. Further increase in calcination temperature increased the catalysts deactivation and at 450°C the initial activity decreased considerably. The catalyst 8 wt%CuO-10wt%ZnO/Al2O3 calcined at SSO^C with surface area 180 m^/g and poire volume 0.69 cm^/g was selected for further investigation. 4. EFFECT OF Zn ON ACTIVE Cu-METALUC PHASE The TPR spectra for Cu catalysts supported on silica and alimiina displayed two maxima at 220;260°C and 200;310^C (Table 2). The total hydrogen uptake in both cases corresponded to full reduction of Cu ( H2/Cu2+ « 1). The Cu cations supported on alumina were reduced mainly during the lower temperature

411

hydrogen uptake period. In the case of Cu/Si02 catalyst the reduction took place at the higher temperature while Zn supported on alumina was not reduced (Table 2) in agreement with the data presented in [11]. Two peaks in the TPR curves could be explained by intermediate formation of Cu+ cations [12] feasible in case of alumina support. One maximum at 260^C measured with CuZn/Al203 catalj^t also agrees with the data presented in [11]. An essential difference from the performance of monometallic Cu- and Zn-catalysts is that the ratio H2/Cu2+ exceeded one ( 1.42, Table 2). This result indicates the partial reduction of Zn in bimetallic catalyst in agreement with the data obtained with bulk Cu-Zn-oxide catalyst [13], probably due to hydrogen spillover [14]. Table 2. TPR data for supported Cu-Zn catalysts Metal content, Temperature of peak maximum, °C wt.% l-st 7.6 CU/AI2O3 7.5Cu/Si02 8.5 Zn/Al203 6.5 Cu-8.1Zn/ AI2O3

2-nd

Ratio of H2 uptaken to amount of ions Me2+ in samples , molecule/ion First peak Second peak

220 200 None

260 310 None

0.66 0.36 0

260

None

1.42 *) 0.64**)

0.38 0.64 0

Total 1.04 1.00 0 1.42 *) 0.64**)

*) H2/Cu2+ ; **) H2 / (Cu2++ Zn2+) The ESCA data (Table 3 ) confirmed the complete reduction of Cu in CU/AI2O3 and Cu/Si02 catalysts: B.E.=932.2-932.7 eV, and no reduction of Zn in Zn/Al203 catalyst: B.E. = 1021.2 eV corresponding to ZnO [15]. ESCA indicated the influence of Zn on the state of Cu metallic phase since the B.E. of Cu 2p3/2 electrons was decreased by 0.5 eV in reduced bimetallic catalyst compared with reduced CU/AI2O3. ESCA also confirmed the partial reduction of ZnO in bimetallic catalj^t since the B.E. of Zn 2p3/2 electrons was decreased by 0.3 eV compared with reduced Zn/Al203 sample (Table 3). An ejcpected result of ZnO partial reduction in bimetallic catalysts along with copper reduction is the enrichment of the surface with zinc as a low-melting metal. Zn/Cu atomic ratio in surface layers of metal particles according to ESCA is 2.3 times higher than it should be according to total analysis measured by EDAX. Comparing the M/Al ratios measured by the two methods indicates also enrichment of the alumina surface with Zn in bimetallic catalysts. XRD measurements indicated only Cu metallic phase in the freshly reduced Cu-Zn catalysts and no Zn metallic phase and ZnO in supported Zn catalysts. This is evident for the high dispersion of ZnO and metallic Zn. Those data did not indicate the formation of any Cu-Zn phase (the fixed 20 for Cu metallic phase). In the case of Si02 support, Zn acts as a typical structural promoter for CvP phase reducing its crystallinity by more than twice (Table 4) while no effect

412

Table 3 ESCA and ED AX data for Cu-Zn/Al203 catalysts Metal Bonding energy, content eV wt.%

Atomic ratio EDAX data

Cu2p3/2 Zn2p3/2 7.6 Cu 8.5 Zn 6.5 Cu-8.1 Zn

Atomic ratio ESCA data

Cu/Al Zn/Al Zn/Cu Cu/Al Zn/Al Zn/Cu

932.7 .

1021.2

0.07 -

0.07

932.2

1020.9

0.06

0.07

0.12 1.16

0.11

0.30

2.73

All the samples were reduced in H2 flow at 400OC , 1 h. Table 4. XRD data for supported Cu-Zn catalysts Metal content, S\q>port wt.% Cu Zn 8.4

--

8.4 8.4 6.0 6.0 9.6 7.3

— 10.5 10.5 9.5

State

Degree of Cu 2eofCu Domain size crystallimty,% metal phase, of Cu metal phase, rnn degrees

standard* ) assumed as 100 88 Si02 fresh**) 91 Si02 treated*** ) 37 Si02 fresh 81 Si02 treated AI2O3 fresh 56 102 AI2O3 fresh Si02

43.29

18

43.20 43.30 43.24 43.15 43.29 43.22

19 23 21 24 14 17

*) calcined at 6OOOC in air for 6 h and reduced in H2flowat 400OC for 2 h **) calcined at 350OC in air for 5 h and reduced at 140-250OC in H2 flow as described before ***) fresh catalyst treated in H2flowat 260OC for 100 h was recovered for AI2O3. This could be due to competition with intrinsic structure-forming function of this support. Moreover the crystallinity of copper in freshly reduced Cu-Zn/Al203 catalyst is higher than in CU/AI2O3. Treatment of Cu-Zn/Si02 catalj^t in conditions modeling the MOIP dehydrogenation for 100 hours led to significant crystallization of CvP phase (i.e. decrease of Cu® surface area) that could be a cause for deactivation. Taking into account the results described above, the promotion effect of Zn on the activity of Cu/Si02 catalyst could be eiqplained as a result of increasing the surface area of Cu^'phase. In the case of alimiina, Zn could also display some activity promotion effect. It should be considered that even thou^ the activities of CU/AI2O3 and Cu-Zn/Al203 catalysts shown in Table 1 were comparable, the MOIP conversion with CU/AI2O3 catalyst is not fully represented by Cu, as in

413

case of Cu/Si02, because of contribution of alumina support. Introduction of Zn in CU/AI2O3 does not reduce the crystallinity of CvP phase or its domain size (Table 4). Therefore this promotion effect could be attributed to the change in Cu^ electronic state as measured by ESCA. The main effect of Zn on CU/AI2O3 catalyst is the MA selectivity increase that could be a result of selective blocking the active sites on alumina surface responsible for MA demethoxylation. 5. CATALYSTS DEACTIVATION AND REGENEEIATION Monitoring the structure of CvP phase along with carbon content in silica and alumina supported Cu-Zn catalysts during MOIP dehydrogenation (Table 5) showed that alumina supported catalyst was deactivated only as a result of coking while the Cu-Zn/Si02 catalyst (especially during first 100 hours on stream) was deactivated mostly as a result of Cu^ phase crystallization. Coke deposition in alumina-supported catalyst was not accompanied by visible changes in catalysts texture (SEM) and surface area. Table 5 Changes in catedysts characteristics during MOIP dehydrogenation Time on stream, h*) MOIP conversion, %: Cu-Zn / AI2O3 Cu-Zn/Si02 Cu^ domain size, A: Cu-Zn / AI2O3 Cu-Zn/Si02 Cu® crystallinity, %: Cu-Zn / AI2O3 Cu-Zn/Si02 Carbon content, wt.%: Cu-Zn / AI2O3 Cu-Zn/Si02

0

2

10

20

77 100

220

240

--

64 64

62 60

54 53

50 43

48 41

43 35

38 34

142 210

160 215

152 213

174 225

182 175 235 240

~ 245

166 240

102**) _ 37 **) 42

103 ~

100 53

- 104 69 81

102 89

103 90

1.6 0.4

7.2 1.7

7.4 —

9.1 3.1

3.5

9.3 3.8

9.7 4.0

0 0

*) T = 260OC, LHSV = 6.2 h i

'^) Fresh as designated in Table 4

Oxidative regeneration of spent Cu-Zn/Al203 catalyst (Cu/Zn = 0.7) after operating250 hours on stream at 260^0 and LHSV of 6.2 h^ was carried out in flow of O2-N2 mixture (1 mol.% O2) of 500 NL/L/h, increasing the temperature from lOOOC to 280OC for 10 hours, maintaining at 280OC and 290OC for 14 and 3 hours, respectively. CO2 release started at 140^C with maximum combustion rate at 260-280^0. After 14 hours at 280^0, the CO2 concentration in the effluent gas was reduced to 0.1 mol%. Further temperature increase led to a CO2 concentration of 0.25 mol% and reduction to 0.08 mol. % at the end of regeneration. DTG indicated, in agreement with TPR data, a reduction of the weight of spent Cu-Zn/Al203 catalysts samples in temperature range 140-300^0. The performance of standard fresh Cu-Zn/Al203 catalyst was compared with the regenerated samples obtained after 250 hours of operation at 260^C and LHSV = 6.2 h-1. The results listed in Table 6 indicate that the first regeneration

414

slightly increased the initial activity and stability of the catalyst while the second regeneration did not change further the catalysts performance. Table 6 Effect of oxidative regeneration on performance of Cu-Zn/Al2Q3 catalyst Run#(250h)

1

Catalyst state Initial MOIP conversion*) Deactivation rate, % conv./h**)

Fresh 48.2 0.26

1 -st regeneration 2-nd regeneration 54.4 53.8 0.18 0.19

~~*)T=~240OC]uiSV^^ **) During first 50 h at 260^0 and LHSV = 6.2 h^. 6. KINETIC STUDY AND REACTOR MODELING A kinetic expression was derived based on data measured for the optimal alumina supported catalyst: r=v*^

\ '^MOIpyMOIP'^ "^ '^MAyMA*

— "^

'^HiOyHto")

where yj is the mole fraction of each component, Kp is the equilibrium constant and Kj is the adsorption constant of each component. Details of the kinetic study and modeling of a fixed bed pilot unit will be published elsewhere. Apriori simulation results based on the reactor and kinetic models were in good agreement with data measured over a range of operating conditions

REFERENCES 1. P.Rylander, Catalytic Hydrogenation in Organic Synthesis,Acad.Press, N.Y., 1979, Ch.lO,p. 165. 2. M.V.Klynev and M.L.Khidekel, Russ.Chem.Rev., 49 (1980)14. 3. J.Dlouky and J.Pasek, Coll.Chech.Chem.Commun., 51 (1989) 326. 4. M.Rusek, in M.J.Phillips and M.Teman (eds.), 9-th Intern. Congr.on Catalysis, Calgary, 1988,The Chem. Inst, of Canada, Ottawa, 1988, p.l 138. 5. M.Rusek, Stud.Surf.Sci.Catal., 59 (1991) 359. 6. M.V.Landau, S.B.Kogan and M.Herskowitz, Appl.Catal., 118 (1994) 139. 7. L.J.ROSS and S.D.Levy, US Pat. 4261926, 1981. 8. M.Kohler and W.Richarz, Chem.-Ing.-Tech., 57(4) (1985) 350 . 9. J.Volf and J.Pasek, Chem.Prum., 28(9) (1978) 464. 10. C.Gremmelmaier, Ger.Pat. DE 2801496,1978. 11. T.H.neish and R.L.Mieville, J.Catal., 90 (1984) 164. 12. R.G.Hermami, K.Klier, G.W.Simmons,B.P.Fmn, J.B.Bulko and T.RKobylinski J.Catal., 56 (1979) 407. 13. D.S.King and R.M.Nix, J.Catal. 160 (1996) 76 14. R.Burch, S.EGolunski and M.S.Spencer, Catal.Lett, 5 (1990) 55. 15.1.Grohman, B.Peplinski and W.Unger, Surface and Interface Analysis, 19 (1992)591.


415

Butadiene Synthesis by Dehydrogenation and Oxidative Dehydrogenation of 2,3-Butanediol G.V.Isagulyants and I.P.Belomestnykh. N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp., 47, 117913, Moscow, Russia

Abstract The results of a complex investigation of the process and catalysts for heterogeneous synthesis of butadione (diacetyl) are presented. A series of the dehydrogenating catalysts for this reaction was investigated. The attention was focused on the study of 2,3-butanediol dehydrogenation and oxidative dehydrogenation to butadione using vanadium-magnesium oxide catalysts and zinc-chromium oxide catalysts. Kinetic data and data on the reaction mechanism were obtained. Butadione conversion is believed to proceed via consecutive elimination of hydrogen molecules and intermediate formation of acetoin. The efficiency of zinc-chromium and vanadium-magnesium oxide catalysts in the reaction of butanediol dehydrogenation has been estabUshed. The optimum reaction conditions in butadione synthesis providing high yields and selectivity have been found. Experimental substantiation of principles for the purposeful synthesis of the catalytic systems mentioned above is considered. The catalysts were prepared based on these principles. 1. INTRODUCTION Highly reactive dicarbonyl compounds are widely used in organic synthesis. Some of them are present in the vegetable and animal products. Butadione, for example, gives butter its specific flavour. Synthetic butadione is often added to margarine. Butadione (diacetyl) is applied in the synthesis of drugs, dyes, photographic materials and other fine chemicals [1]. Among the methods of preparation of butadione the oxidation of 2-butanone is one of the most frequently discussed. Both catalytic end electrochemical methods were used. The conversion of 2-butanone varies from 10 to 84% and the yields of diacetyl do not exceed 40% [2-4]. The heterogeneous dehydrogenation of 2,3-butanediol is an alternative simple method of butadione synthesis [5]. We have investigated a series of the dehydrogenating catalysts for this reaction. Our attention was focused on two of them. Further study of 2,3-butanediol dehydrogenation and oxidative dehydrogenation to butadione was performed using zinc-chromium oxide catalysts and vanadium-magnesium oxide catalysts as well.

416

Previously we have studied such catalysts in hydrocarbon dehydrogenation and oxidative dehydrogenation reactions [6,7]. Instrumental methods such as XRD, X-ray, photoelectron spectroscopy, DTA, UV-spectroscopy, EM were used. It has been found that activity of the Zn-Cr catalysts is determined by the stoichiometric spinel ZnCr204 [8]. In the case of the vanadium-magnesium system the activity and selectivity depend upon the presence of ions V^"^ and Y^'^ grouped on the catalyst surface into clusters of 2-3 vanadium ions [9]. This was taken as a principal for the purposeful synthesis of the catalytic systems mentioned. In this work an attempt was made to spread the obtained experience on the dehydrogenation of alcohol groups. 2. RESULTS and DISCUSSION 2.1. Dehydrogenation on Zn-Cr oxide catalyst In the course of transformation of 2,3-butanediol the products of dehydrogenation of one and two alcohol groups were formed. Thus the reaction results not only in the formation of butadione but in formation of acetoin as well. The amounts of both products formed change markedly with the butanediol conversion degree. Dehydrogenation of butanediol on Zn-Cr-oxide catalyst in a wide temperature range allowed to obtain data on the content of acetoin and diacetyl at different conversions of 2,3-butanediol. Besides, the transformation of acetoin into butadione was studied (Figure 1.). One can see that at low temperature (310-340°C) when the conversion was less than 50% mainly acetoin was presented in the reaction products (diacetyl/acetoin molar ratio - 0,5). At 375°C the curve of acetoin content reaches 80 n

^

40 H

.a

-| ' r 450 350 400 Temperature, C Figure 1. Dehydrogenation of acetoin (to the left, pale dots) and of 2,3-butanediol (to the right) on zinc- chromium oxide catalyst, LHSV=1.6 h"^. 2*, acetoin (as initial material); 3*, butadione; 1, butanediol (as initial material); 2, acetoin (formed as intermediate from butanediol); 3,butadione. 300

417

maximum and at 385°C the diacetyl content becomes equal to that of acetoin. Further increase of the reaction temperature promotes both butanediol conversion and formation of diacetyl. At 420°C the ratio diacetyl/acetoin exceeds 1.8. The only byproduct was carbon dioxide. According to thermogravimetric data, the amount of products deposited on the catalyst surface during the run period did not exceed 2%. In the runs at 420°C and LHSV=1.6 h"^ the yield of the liquid products was as high as 92% and the content of butadione in the latter attained 60% mol. Unfortunately, further elevation of the temperature resulted in the increase of deoxygenating processes; formation of hydrocarbons becomes perceptible and the selectivity decreases. The results plotted at Figure 1 seem to be useful for elucidation of the reaction pathway. In the course of butadione formation two hydrogen molecules of 2,3butanediol must be ehminated. The elimination can proceed either simultaneously or step by step via consecutive elimination of hydrogen molecules and intermediate formation of acetoin. One can see that conversion of the latter into diacetyl proceeds faster and at lower temperature as compared to the conversion of butanediol. By increasing the temperature and conversion of butanediol the curve of acetoin formation passes maximum and the curve of diacetyl formation has an induction period. Thus, one can beheve that the conversion proceeds mainly via consecutive elimination of hydrogen molecules and intermediate formation of acetoin: CH3 CH(OH) CH(OH) CH3 ^ CH3 CH(OH) CO CH3 -> CH 3CO CO CH3. In general the results can be considered as promising for butadione synthesis; in particular because of high reactivity of acetoin, which can be easily converted into diacetyl. In order to increase the yields of the latter a two step process using repeated treatment of reaction products on the same catalyst can be proposed. 2.2. Oxidative dehydrogenation on V-Mg oxide catalysts Oxidative dehydrogenation gives us another opportunity to transform butanediol to diacetyl. The dehydrogenation processes are known to proceed with high conversions and at mild conditions when hydrogen acceptors are used. The method was widely used in our previous work in connection with synthesis of vinylaromatic and vinylheteroaromatic compounds. The V205/MgO catalytic system was investigated in detail and successfully used. The formation of the active structures and the efficiency of the catalysts was found to depend on the methods of MgO impregnation, on the nature of the initial V-containing material, on the V2O5 content and on the thermal treatment of the catalyst prepared. It enabled us to vary the content of octahedral coordinated V^"^ and V^"^ ions grouped on the catalyst surface into clusters of 2-3 vanadium ions [8,9] and, thus, obtain an efficient catalyst. Starting with testing of V205/MgO system in butanediol conversion some primary tests with ethanol dehydrogenation were carried out to simplify the catalyst selection. Table 1 presents the properties of vanadium-magnesium oxide catalysts subjected to the heat treatment. The temperature of the heat treatment determines both the textural and the catalytic properties of the catalyst. Similar to the dehydrogenation of ethylbenzene into styrene [10,11], the most active catalysts occurred to be those

418

subjected to heat treatment in air stream at 550°C. This activation mode results in the formation of a catalyst with a porous structure and a surface area that favor the process. Besides as was found previously [9], this treatment enriches the surface with V^"^ and V^"^ ions in octahedral and tetrahedral coordination. Table 1. Relationship between preheating temperature and catalytic activity in ethanol conversion at 400°C, LHSV-1.5 h"^, ethanol: oxygen = 1:1 (mol). Calcination temperature, °C

Surface area,

Pore volume,

m'/g

cm^/g

Alcohol conversion, wt%

120

40

0.3

20

550

100

0.7

63

750

60

0.35

35

850

40

0.2

20

The effect of vanadium content of the catalyst on the conversion of ethanol is shown on Figure 2. Again in agreement with the data obtained by oxidative dehydrogenation of ethylbenzene one can see the extreme dependence of the catalytic properties on the V2O5 content, with a maximum at 12% of the latter. This phenomenon was explained previously. At a low content of vanadium oxide (2-5%) only isolated vanadium ions in the matrix of the support occur on the surface. Associated clusters of 2-3 ions are formed when the content of vanadium oxide varies between 7-15 %. Such species were shown to be responsible for the oxidative dehydrogenation of the alkyl aromatics [8]. Further elevation of the V2O5 content leads to the formation of magnesium vanadates with regular structure. The latter display low reducibihty, adsorption capacity and, hence, low activity in oxidative dehydrogenation. Oxidative dehydrogenation of butanediol on the selected vanadium-magnesium catalysts allowed to reduce the reaction temperature of butadion synthesis by about 100°C. The reaction was studied in the temperature range of 160 - 350°C at LHSVequal to 1 h"i and butanediol : oxygen molar ratio equal to 1:1 (Table 2). Already at 250°C more than 85% of butanediol was converted. As in the dehydrogenation on Zn-Cr oxide catalyst, at low butanediol conversion acetoin is formed preferably. At 180°C diacetyl : acetoin molar ratio was equal 0.66. Maximum amount of acetoin in the reaction products was observed in the range of 180-220°C. With further increasing temperature the acetoin content declined in favor of diacetyl. This dependence of acetoin content on butanediol conversion allows it to extend the above conclusion of the intermediate formation of acetoin to oxidative dehydrogenation of butanediol on V205/MgO catalysts. The yield of diacetyl equal to 62.3% and the diacetyl : acetoin molar ratio equal to 2.3 were achieved at 350°C; the total selectivity of the formation of both products of

419

dehydrogenation was as high as for the reaction on Zn-Cr oxide catalyst and reached 98%. 80 n

I

40 H

1 1

O

1

I

I

1

I

10 20 30 Vanadium pentoxide content, wt %

Figure 2. The eflfect of V205/MgO catalyst content on the activity in ethanol dehydrogenation at 350, 375, 400 °C (1, 2, 3 respectively), LHSV=1 h'l, alc.:02=l:l mol. Table 2. Oxidative dehydrogenation of 2,3-butanediol on Mg-V oxide catalyst, LSHV 1.0 h"^ 2,3-butanediol : oxygen molar ratio=l. Temperature,

Content in the reaction products, mol.% Butadione Acetoin

Butanediol conversion, %

160 180

20 28

30 42

50 70

250

45.6

40

86

300 350

60

28 26

88 89

62.3

3. EXPERIMENTAL The reaction was performed in a quartz flow reactor with a fixed bed (2-40 ml) [6,8]. Parameters of the reaction were varied in a broad range: temperature 200420° C, LHSV 0.8-1.6 h"^ in the oxidative dehydrogenation butanediol : oxygen molar

420

ratio varied from 1:0.5 to 1:3.0. Samples of the liquid and gaseous products were taken every 15 min., the experiment duration was 2-5 hours. The reaction products were analyzed by GLC. The amount of products deposited on the catalyst surface was determined by the derivatographic technique. Zinc-chromium oxide catalysts were prepared by co-precipitation from aqueous solutions of corresponding nitrates with aqueous ammonia. The precipitated hydroxocompounds mixed with ZnO were dried at 120°C and the sUghtly wet product was then molded by squeezing out through orifices with diameter of 4 mm [6]. Vanadium oxide catalysts were prepared by impregnation of the MgO-support with aqueous solutions of ammonium vanadate. Samples were dehydroxilated at 120°C, then calcinated in air stream by gradual temperature elevation [8]. 4. CONCLUSIONS The efficiency of zinc-chromium and vanadium-magnesium oxide catalysts in the reaction of butanediol dehydrogenation has been established. To simpUfy the preparation of appropriate catalysts fundamental principles elaborated previously for catalytic systems under consideration has been used. The optimum reaction conditions in butadione synthesis providing yields of 60-62% and high selectivity have been found. Kinetic data and data on the reaction mechanism were obtained. The conversion of butanediol is believed to proceed via consecutive elimination of hydrogen molecules and intermediate formation of acetoin. REFERENCES 1. A.S. Sanina, S.I. Shergina, I.E. Sokolov, LA Kotljarevsky Bull. Acad. Sci. USSR, Div. Chem. 1981, 5, 1158. 2. T. Seiyama G.Takita Jap. Pat. 79, 132515 (1979). 3. G. Takita, K. Inokuchi, O.Kobajashi et al., J.Catal., 1984, 90(2), 232. 4. B. Mueller, H.Dietz, C. Stoekel Ger. (East) Pat. 238816, (1986). 5. T.Kritchevsky US Pat. 2462107, (1949). 6. LP. Belomestnykh, G.V. IsaguUants et al., Kinetika i Kataliz 1987, 28, 691. 7. G.V. Isaguliants, O.K. Bogdanova et al., Neftechimija 1970,10, 174. 8. A.V. Simakov, S.A. Veniaminov, LP. Belomestnykh, G.V. Isaguliants Kinetika i Katahz, 1989, 30, 68. 9. LP. Belomestnykh, G.V. IsaguUants et al.. Bull.Acad.Sci. USSR, Div.Chem.Sci. 1991, 40, 1751. 10. LP. Belomestnykh, G.V. IsaguUants et al., App. Surf. Sci., 1992, 72, 40. 11. O.B. Lapina, A.VSimakov, S.A. Veniaminov J. mol. cat. 1989, 50, 55.


421

Phase transition of crystalline a-Te2Mo07 to the vitreous p-form, surface composition, and activity in the vapor-phase selective oxidation of ethyl lactate to pyruvate over Te02-Mo03 catalysts H.Hayashi, S.Sugiyama, T.Moriga, N.Masaoka and A.Yamamoto Department of Chemical Science and Technology, Faculty of Engineering, University of Tokushima, Minamijosanjima, Tokushima 770, Japan Binary oxides, TeOi-MoOs, converted ethyl lactate selectively to ethyl pyruvate in the vaporphase fixed-bed flow system, showing a sharp maxmum in activity at a composition of MoOs* 2Te02. Phase transition of crystalline active phase, a-TeiMoOy, to the vitreous p-form and regeneration of a-form by recalcination are demonstrated with evidence of powder XRD. Metal-oxygen distances by EXAFS analysis are given for Te2Mo07 and the component oxides. XPS depth-analysis revealed enriched Te-content at surface, accounting for vitreous pTeiMoOy and regenerated a-form to be less active. On exposure of active crystalline a-phase to the surface by grinding the regenerated a-form, the activity level of fresh a-Te2Mo07 was reproduced. 1. INTRODUCTION Pyruvic acid is the simplest homologue of a-keto acids, which were extensively reviewed by Cooper et al.[l], covering various methods for their synthesis elegantly designed for laboratory procedure in organic synthesis, but the applications of catalytic processes are of more recent vintage [2-8]. Lead-modified palladium-on-carbon and related catalysts converted sodium lactate selectively to pyruvate in aqueous phase [3,6,7]. The advantage of running the reaction in liquid-phase over the gas-phase fixed-bed operation for the production of fine chemicals appears to be generally accepted in terms of inexpensive plant investment, stability and flexibility for operating conditions, and greater ease for renewal and making up of catalyst [8]. However, both ethyl lactate and ethyl pyruvate boil at the same temperature of 155°C and are similar in chemical nature, leading to unfavorable difficulties in separation[8]. Ethyl pyruvate was obtained in the liquid phase, but the conversion of lactate was usually 30-50% [3]. Thus, the separation of product pyruvate from unreacted lactate discourages the practical application of the liquid-phase oxidation of lactate.

422

Catalyst screening in the vapor-phase oxidation of ethyl lactate [4] showed a binary oxide, Te02-Mo03, to be an active catalyst to afford pyruvate with high selectivity, where the component oxide M0O3 showed a moderate activity, but the other component Te02 was less active only to give ethanol at a high temperature of 350-400°C. A synergy in activity was observed for the Te02-Mo03 catalysts calcined at SOCC in air, showing a sharp maximum at a composition of Mo03*2Te02. Crystalline a-Te2Mo07 was suggested as the active species with evidences of powder XRD, IR, DTA-TGA and SEM/EPMA [8]. Telluromolybdates are classified into three groups [9] of heteropoly [TeMo6024]^", substitutive [TeyMoi.y04]2- and additive [TeMoOe]^' telluromolybdate. The structure of hexamolybdotellurate is not the Keggin unit [PWi204o]^", well-known heteropoly anion, but the Anderson-type which consists of seven octahedra all lying in one plane [10], The six MoOe octahedra form a ring surrounding the central Te06 octahedron. Each M0O6 shares one edge with each of its two neighboring M0O6 octahedra. Each M0O6 also shai*es an edge with Te06 octahdron. Isomorphic substitution of molybdates at Mo^+ with Te^+ gives substitutive telluromolybdates [9] of wolframite structure such as Mn3TeMo20i2 and Co4TeMo30i6. Additive telluromolybdates composed of three oxides, e.g. ZnO-Te02'Mo03 (=ZnTeMo06), are prepared by solid phase reactions between Te02 and a molybdate. A binary oxide, Mo03'2Te02 (=Te2Mo07), is also an additive telluromolybdate. Tellurium is tetravalent in additive telluromolybdate, while hexavalent in heteropoly acid and substitutive telluromolybdate. Phase transition of crystalline a-Te2Mo07 to less active vitreous p-form and the regeneration of a-form by recalcination with evidence of powder X-ray diffraction(XRD), metal-oxygen distances analyzed by extended X-ray absorption fine structure (EXAFS) for Te2Mo07 and the component oxides, and enriched Te-content at surface by X-ray photoelectron spectroscopy (XPS), are given in the present paper. 2. EXPERIMENTAL The vapor-phase oxidation of ethyl lactate was carried out by a conventional fixed-bed flow apparatus at 300°C with a space veleocity of 3600 h-^ Ethyl lactate was supplied as a toluenic solution by a Microfeeder (Type JP-S, Furue Sci. Co., Tokyo) and diluted with O2/N2 to adjust the gas-phase composition of 5% ethyl lactate with 30% O2. The reactor effluent was scrubbed by ice-cooled 1-propanol and analyzed by gas chromatography with a Hitachi 163FID for organic species and a Yanaco G 2800-TCD for gases. The catalyst, Te02-Mo03, was prepared by kneading the component oxides with an appropriate small amount of water in an automatic porcelain mortal' for 2h. The resultant paste was spread over a glass plate, dried overnight at 80°C, crushed, and calcined at 500°C in air for 5h. Ethyl lactate was purchased from Wako Pure Chemicals, Osaka, and used as supplied. Ethyl pyruvate for the calibration

423

of GC analysis was obtained from Aldrich Chemical Co., Milwaukee. Te02 and M0O3 were obtained from Wako, Osaka, and Merk, Darmstadt, respectively. Powder X-ray diffraction of catalysts was measured by a MXP system of MAC Science Co., Tokyo. X-ray photoelectron spectra of the Te 3d5/2 and Mo 3d5/2 core electrons were measured by a Shimadzu ESCA-1000, irradiated with Mg Ka, and the observed binding energies were calibrated with 285.0 eV for C Is electron. The rate of argon-etching for the XPS depth-analysis is estimated as ca.2nm/mm for Si02 at 2 kV. X-ray absorption spectra near Mo K-edge and Te L3-edge were measured by the transmission method for boron nitride disk at Photon Factory (BL-7c) of National Laboratory for High Energy Physics, Tsukuba, Japan. 3. RESULTS

AND

DISCUSSION

3.1 Reaction network The major reaction is oxidative dehydrogenation at the secondary hydroxyl site of lactic acid, but the product pyruvic acid in its free-acid form is unstable to decompose. Thus the substrate was supplied as ethyl ester to protect the carboxyl moiety. Esterification is also of benefit to vapor-phase flow operation in making acids more volatile. Hydrolysis of ethyl lactate gives free pyruvic acid with further decarboxylation to actaldehyde. Ethanol, which is another fragment of ester hydrolysis, could be either oxidized to acetaldehyde or dehydrated to ethylene at higher temperature above 350°C. The reaction network is summerized in Scheme 1. 1/2 O2

CH3-CH-COOC2H5 OH

>CH3.C-COOC2H5 + H2O ^20/

0

CH3-C-COOH + C2H5OH

CH3CHO

C2H4

Scheme 1 3.2 Phase transition of crystalline a-TciMoOv to the vitreous p-Form Differential thermal analysis with thermogravimetry (DTA-TGA) has been made for a sample with a composition of Mo03-2Te02, which was kneaded with an appropriate amount of water and then dned overnight at 80°C prior to the measurement. A sharp exotherm was observed at 450°C without change in the weight, suggesting the solid phase reaction to give aTeiMoOv, followed by the two endotherms presumably phase transition and melting at 528°C and 542°C, respectively. Melting the Te2Mo07 at 600°C and then cooled to room temperature, the p-phase, an orange-yellow transparent glass, was obtained.

424

SEM/EPMA for TeOi-MoOa catalysts calcined at 400°C and 500°C were given in the previous paper [8], to compare the difference in morphology at temperatures below and above 450°C, at which the first sharp exotherm was observed in DTA. The solid-phase reactions between the two component oxides of TeOi and M0O3 have not yet occurred for Te02*2Mo03 calcined at 400°C, while those calcined at 500°C gave traces of the reaction. Domains composed of both elements Te and Mo, and of single Mo, were observed, but of single Te disappeared. A rapid regeneration of crystalline a-Te2Mo07 was observed, when calcined the vitreous p-form at 450°C in a porcelain crucible as shown in Fig.l. XRD pattern shows still amorphous phase after 5 min (a), but the orange-yellow transparent glass gradually turned greenish black in color and then opaque white powder of crystalline a-Te2Mo07 was obtained within 20 min (b).

(C)

(b)

JLJ

(a) 10

20

30

40

29 n Figure 1. Powder XRD evidence for rapid regeneration of a-TeiMoOv by calcination of the vitreous p-fonn. Calcined at 450"C for (a) 5 mm, (b) 20 min and (c) 5 h. 3.3 Structure of crystalline and vitreous Te2Mo07 a) M0O3 and Te02: The crystalline M0O3 is described as a layer structure [11,12] in which each layer is built up of distorted MOOG octahedra. Among six Mo-0 distances, each two distances aie similar together as shown in Table 1 (i) by the single crystal data [11] and thus thiee Mo-0 distances were observed in EXAFS analysis as given in Table l(ii) and (iii). The basic unit of the structure of Te02 is built up from four oxygen atoms coordinated to one tellurium atom to form a trigonal bipyramid with one of the equatorial position unoccupied [14]. A pair of the Te04 units are connected by edge sharing to [Te206] followed by the linkage at o

corners to chains of oxotellurium polyhedra [15]. The Te-0 distances [14] of 1.90A (equa0 tonal) and 2.08A (axial) are rather close together, and a single Te-0 was obtained in EXAFS (Table 1 (viii)). b) Crystal structure of a-Te2Mo07: The crystalline a-phase is monoclinic with a= 4.286, Z7=8.6i8, c=15.945 A, P=95.67", Z=4 and space group P 2i/c [16], where a pair of

425 Table I Metal-Oxygen Distances (A) for Te2Mo07 and the Component Oxides ^ i)

Sample

Method

M0O3

XRD

Mo-0(6) Mo-O(l) 1.73

1.67

Mo-0(2) Mo-0(7) 1.95

1.95

Mo-0(7) Mo-0(6) 2.25

1 2.23

Reference [11]

ii)

M0O3

EXAFS

1.727

1.948

2.264

This work

iii)

M0O3

EXAFS

1.70

1.95

2.29

[13]

iv) a-Te2Mo07

XRD

V) a-Te2Mo07 EXAFS vi)P-Te2Mo07 Sample

EXAFS Method

vii)

Te02

XRD

viii)

Te02

EXAFS

ix)a-Te2Mo07 XRD

1.745

1.699

1.935

1.947

2.138

2.589

[16]

1.677

1.937

2.238

This work

1.746

2.004

2.222

This work

Te-0(l)Te-0(2) Te-0(l)Te-0(4) Te-0(2)Te-0(3) 1 Te-0(2)Te-0(5)

[14]

2.082

1.903 1.882 1.886

This work 1.862

1.898

Reference

1.899

[16]

x)a-Te2Mo07 EXAFS

1.912

This work

xi)P-Te2Mo07 EXAFS

1.895

This work

*) For oxygen-numbering in parentheses, see ref.[16].

M0O6 octahedra are linked by edge sharing to [Mo20i()] unit and the double chains of distorted molybdenum octahedra connected at corners along the a-direction are linked by tetrahedral oxotellurium, Te^^, chains to buid up the three dimentional arrangement [16]. Each IVloO^ octahedron has a non-bridging Mo=0 (0.1745 nm; IR 906 cm'^) [17]. Three Mo-0 distances for crystalline a-Te2Mo07 obtained by EXAFS analysis were similar to those for M0O3 as in Table 1 (v). c) Short-range atomic order of P-Te2lMo07 glass: Melts of the binary Te02-]VIo03 system are easily fixed in a glassy state at normal cooling rates [18]. The glass-formation limit [19] in the system Te02-Mo03 are in the range of 12.5 to 58.5 mol%-Mo03 and the structure of Te02-Mo03 glasses were analyzed by the radial distribution function (RDF) of X-ray [15] and neutron diffraction [18] data. The short-range atomic order in glasses was suggested probably to be similai' to the arrangement in the crystalline state [18], and the vitreous p-form regenerates crystalline a-Te2Mo07 with great ease on heating at 450°C as shown in Fig.I. EXAFS data given in Table l(v)/(vi) and (x)/(xi) also provide evidence that phase transition of crystalline a-Te2lVlo07 to the vitreous p-form occurred without appreciable change in the metal-oxygen distances, but differences in three Mo-0 bond distances of p-form are a litde bit close together compared with those in the a-form. Te2Mo07 glass is composed of [Te04] and [M0O5] unit, and two of the latter linked together to [M02O8] complex[15]. Breaking of the [M0O6] octahedra chains along the longest M o - 0 bond in glass formation may add flexibility to the rigid stmcture with ci7stal symmetry.

426

0

1

2

3

ARGON ETCHING CYCLE

Figure 2. XPS evidence for enriched Te-content at catalyst surface. O : fresh a-Te2Mo07 O: p-TeiMoO? A regenerated a-TeiMoO?

0

0.2

0.4

0.6

T e / ( T e + Mo)

0.8

1.0

moi/moi

Figure 3. Effect of surface composition of Te2Mo07 on the activity for oxidation of ethyl lactate. 300°C, 5% EL, 30% O2, SV 3600 h-^ Black symbols are plotted based on surface composition, #: fresh a-, 4 : p - , A :regenerated a-phase. O : Te02-Mo03with various composition [8] for reference.

3.4 Surface composition of TeiMoOy X-ray photoelectron spectroscopic(XPS) analysis with argon-etching gives surface and depth profile of catalyst composition[20]. Figure 2 shows enriched Te-content at surface of Te/Mo= 3.8 and 2.9 for the vitreous p- and regenerated crystalline a-phase, respectively. The surface atomic ratio appears to be variable, and does not show any indications for a specific surface compound other than Te2Mo07. 3.5 Activity of a-, p- and regenerated a-Te2Mo07 Binary oxides, Te02-Mo03, showed a sharp maximum in activity at a composition of Mo03-2Te02 in the vapor-phase selective oxidation of ethyl lactate to pyruvate as given in Fig.3 (white circles), and a-Te2Mo07 was suggested as the active species[8]. The fresh aphase, prepared by kneading the component oxides in a molar ratio of Mo03:Te02=l/2 with an appropriate amount of water, dried and calcined at 500°C for 5h, showed a high conversion of ethyl lactate of 93% at a tentative standard condition for comparative reaction studies of 5% ethyl lactate, 30% O2 with SV 3600 h'^ at 300°C as in Fig.4(a). The vitreous p-phase, obtained by melting a-phase at 600°C followed by cooling to room temperature, was less active with 21% conversion of lactate as shown in Fig.4(b). Calcination of the p-phase at 450°C regenerates a-phase as evidenced by XRD in Fig. 1, but the regenerated a-phase did not repro-

427

100 53C 80 60 (a) 40 a 20 UJ 0( >o3 100

-o-

_ j

> Z

o o

100 80 60 \ ' 40 20 0

1 (c) »

a

0 0

- 4 -

-4^

100 80 60

HI) 60 40 20 0 TIME (h)

TIME (h)

Figure 4. Oxidation of ethyl lactate over various TeoMoOy. (a) fresh a-, (b) P- , (c) regenerated a-, as is (d) regenerated a-, ground and pelleted. O : lactate conv. # : pyruvate yield O: acetaldehyde yield. Conditions: same as in Fig.3.

duce the activity of fresh a-TeaMoOv as given in Fig.4(c). The unfavorable low activity of regenerated pure crystals of a-Te2Mo07 is as anticipated in reference to the activity pattern given in Fig. 3 (white circles), where drastic decrease in activity of TeOi-MoOs binary 3ystem with increasing Te-content has been shown in the region above Te/Mo=2. The obsei-ved activities of present catalysts: a-, p- and regenerated a-Te2Mo07, of which bulk compositions are the same, were found on the same activity pattern in Fig.3 (black symbols) when plotted against the surface composition. The activity level of fresh a-TeiMoOv was reproduced as shown in Fig.4(d), on exposure of active crystalline a-phase to the surface by grinding the regenerated a-form. 4. CONCLUSION Binary oxide, TeOi-MoGs, converted ethyl lactate selectively to pyruvate in a vapor-phase fixed-bed flow system. A synergy in activity suggested a-Te2Mo07 as the active species. Phase transition of crystalline active phase, a-Te2Mo07, to the vitreous p-form are demonst-

428 rated with XRD evidence. EXAFS analysis showed the phase transition occured without appreciable change in metal-oxygen distances. Depth-profile by XPS revealed enriched Te content at the catalyst surface, accounting for vitreous P-TeiMoOv and regenerated a-form to be less active. On exposure of active crystalline a-phase to the surface by grinding the regenerated a-form, the activity level of fresh a-Te2Mo07 was reproduced.

REFERENCES 1. A.J.L.Cooper, J.Z.Gions and A.Meister, Chem.Rev., 83 (1983) 321. 2. S.Sugiyama, S.Fukunaga, K.Ito, S.Ohigashi and H.Hayashi, J. Catal., 129 (1991) 12. 3. T.Tsujino, S.Ohigashi, S.Sugiyama, K.Kawashiro and H.Hayashi, J.Mol.Catal., 71 (1992)25. 4. S.Sugiyama, N.Shigemoto, N.Masaoka, S.Suetoh, H.Kawami, K.Miyaura and H.Hayashi, Bull.Chem.SocJpn., 66 (1993) 1542. 5. H.Hayashi, N.Shigemoto, S.Sugiyama, N.Masaoka and K.Saitoh, Catal.Lett., 19 (1993) 273. 6. H.Hayashi, S.Sugiyama, N.Shigemoto, K.Miyaura, S.Tsujino, K.Kawashiro and S.Uemura, Catal.Lett., 19 (1993) 369. 7. H.Hayashi, S.Sugiyama, Y.Katayama, K.Kawashiro and N.Shigemoto, J.Mol.Catal., 91(1994) 129. 8. H.Hayashi, S.Sugiyama, N.Masaoka and N.Shigemoto, Ind.Eng.Chem.Res., 34 (1995) 135. 9. J.Slocynsky and B.Sliwa, Z.anorg.allg.Chem., 438 (1978) 295. 10. G.A.Tsigdinos, Topics in Current Chemistry, Vol.76, 1978, p.36. 11. L.Kihlborg, Arkiv Kemi, 21 (1963) 357. 12. B.C.Gates, J.R.Katzer and G.C.A.Schuit, Chemistry of Catalytic Processes, McGrawHill, New York, 1979, pp.367-8. 13. M.Niwa, M.Sano, H.Yamada and Y.Murakami, J. Catal., 151 (1995) 285. 14. O.Lindqvist, Acta Chem.Scand., 23 (1968) 977. 15. Y.Dimitriev, J.C.J.Bart, V.Dimitrov and M.Amaudov, Z.anorg.allg.Chem., 479 (1981) 229 16. Y.Amaud, M.T.Averbuch-Pouchot, A.Durif and J.Guidot, Acta Cryst.,B32 (1976)1417. 17. E.J.Baran, I.L.Botto and L.L.Founier, Z.anorg.allg.Chem., 476 (1981), 214. 18. S.Neov, I.Gerasimova, B.Sidzhimov, V.Kozhukharov and P.Mikula, J.Mater.Sci., 23, (1988)347. 19. V.Kozukharov, M.Marinov and G.Gridorova, J.Non-Cryst.Solids, 28 (1978) 429. 20. D.Briggs and M.P.Seah (eds.). Practical Surface Analysis, 2nd Ed. Vol.1 - Auger and X-ray Photoelectron Spectroscopy, J.Wiley & Sons, Chichester, England, 1990.


429

Selective oxidation with air of glyceric to hydroxypyruvic acid and tartronic to mesoxalic acid on PtBi/C catalysts Peter Fordham, Michele Besson and Pierre Gallezot Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne, France Abstract Bimetallic platinum-bismuth catalysts, supported on active carbon, were employed to oxidise aqueous solutions of glyceric and tartronic acid with air, in a batch reactor. High selectivities for the corresponding keto-acids were obtained under acidic conditions. Glyceric acid was selectively oxidised to hydroxypyruvic acid and maximum yields were obtained at pH 5 (74% at 77% conversion). Tartronic acid was selectively oxidised to mesoxaUc acid with highest yields also being obtained at pH 5 (39% at 79% conversion). Analysis of the reaction mixture after 22 hours indicated that leaching of the platinum component of the catalyst was negligible but significant quantities of the bismuth promoter were present (10-17 mg/1). 1. INTRODUCTION The oxidation of alcohols with air on platinum group metals was discovered well before the turn of the century [1] but has attracted only occasional interest in intervening years. However, recent interest in this reaction has been stimulated by its potential application to the production of oxygenated substances for fine chemical use [2]. Appealing features include: its heterogeneous nature, enabling potentially expensive post-reaction separation processes to be avoided, and straight forward catalyst recycling; the absence of toxic or polluting effluents, which are frequently encountered in traditional stoichiometric oxidation processes employing mineral acids; and tiie ready availability and low cost of the solvent (water) and consumable reagents (air and, most often, an organic substance derived from a sustainable resource). For an in-depth and up-to-date account of the subject area, the reader is referred to the comprehensive review by Mallat and Baiker [3]. Much effort has focused on the use of this approach for the selective oxidation of carbohydrates [4-6], but interest has recently broadened to accommodate other biosustainable substances [7]. Thus glycerol, which may be oxidised to a range of useful molecules (see Figure 1), has come under scrutiny as a possible candidate for valorisation. The conversions of glycerol 1 to glyceric acid 2 [8,9] and dihydroxyacetone [9,10]; glycerol and glyceric acid to tartronic acid 3 [11]; glyceric acid to tartronic acid and hydroxypyruvic acid 4 [12]; and tartronic acid to mesoxalic acid 5 [13] have been studied. In general, oxidation of the primary function is favoured on platinum or palladium and the rate of reaction increases with pH. However, the reaction pathway may be altered to favour selective oxidation of the secondary function by adding a bismuth promoter and employing acidic conditions. The fundamental mechanism which drives oxidation is generally accepted to be oxidative dehydrogenation, which occurs on the surface of the metal, and the increase in reaction rate with pH has been interpreted as being due to either one of two mechanistic steps: deprotonation of the hydroxy 1 group, or desorption of the formed acid from the metal. The mechanism governing oxidation of the secondary alcohol function is not as yet fully understood, but a complexing mechanism between bismuth and the substrate has been proposed [14].

430

PtBi/C catalysts were reported earlier to enable selective oxidation of the secondary hydroxy function of glyceric and tartronic acid to hydroxypyruvic and mesoxalic acid, respectively [13]. In the work reported here, these two reactions were studied in more detail to determine the influence of the following parameters on selectivity and reaction progress: pH of the reaction medium, over-oxidation of targeted products, and leaching of catalyst components. OH HO^ J^ Ô OH

1 4

OH

o^-ô

->3

OH

OH

\

\

o

o OH

5

OH

OH

Figure 1: Carboxylic acids derived from glycerol. The objective was to improve understanding of the selective oxidation of the secondary alcohol function to the corresponding keto derivative and to determine the conditions which give maximum selectivity and yields. For catalytic reactions in triphasic reactors, an important aspect which needs to be addressed is the stability of the catalyst. Thus, the corrosion of both catalyst components under reaction conditions needed to be scrutinised but most particularly that of the promoter. 2. EXPERIMENTAL 2.1 Preparation of catalysts Platinum catalysts were prepared by an ion-exchange method [16,17]. Oxidised sites on the surface of an activated carbon support (CECA SOS) were created by pre-treatment with sodium hypochlorite (3%); the associated protons were subsequently exchanged with Pt(NH3)42'^ ions, in an aqueous ammonia solution, and reduction was carried out on the dry catalyst under a flow of hydrogen at 300°C. A surface redox reaction was subsequently employed to deposit the bismuth whereby the catalyst was suspended in a glucose solution, under an inert nitrogen atmosphere, and the required volume of a solution of BiONOs, dissolved in hydrochloric acid (IM), was added [18]. High resolution transmission electron microscopy (TEM) (Jeol lOOCX) was employed to determine the size of the metal particles on the surface of the catalyst support, and the composition of individual metal particles was ascertained (for thin sections cut with an ultramicrotome) using a field-emission scanning transmission electron microscope (STEM) (VG HE 501) (at 1.5 mm resolution) and an energy dispersive X-ray (EDX) analyser. The metal loading of catalysts was determined by ICP-AES (Spectro D), following dissolution in concentrated hydrochloric and sulphuric acids. Direct analysis of aqueous samples taken from the reaction medium, using the same analytical technique, allowed the corrosion of metallic components from the catalyst surface to be studied. 2.2 Reaction procedure Catalytic oxidations of glyceric and tartronic acids, in aqueous solution, were realised in a glass batch reactor housed in a thermostatically-controlled heating jacket. Fitted attachments

431 included: mechanical stirrer, air and nitrogen gas supply system, oxygen sensor (Ingold) and pH electrode (Radiometer) (see ref. [9] for detailed description of apparatus). An" aqueous solution of the reactant (300 ml; 0.1 mol 1^ plus the catalyst (0.2g, substrate/metal molar ratio = 500-600) was stirred vigorously (1200 rpm) with a steady stream of nitrogen bubbling through the suspension and heated to 50°C. At the required temperature, the supplied gas was switched to air (0.75 ml min'^) and, when necessary, the pH was maintained at a constant value by addition of aqueous sodium hydroxide, via a pump controlled by the pH meter. A Shimadzu LC-IOAS liquid chromatograph with UV (>.=210nm) and refractive index (RI) detectors mounted in series was employed to determine reactant conversion and distribution of oxidation products. An ion exchange column (Sarasep Car-H 300mm x 7.8mm i.d.) pumped at 0.4 mlmin-^ with a dilute solution of sulphuric acid (0.0004M or 0.025M) enabled separation and analysis of glyceric, tartronic, hydroxypyruvic, mesoxalic, oxalic and glycolic acids. Quantitative data were obtained from linear regressions derived from standard calibration curves covering the appropriate concentration ranges. 3. RESULTS AND DISCUSSION 3.1 Oxidation of glyceric acid Glyceric acid was oxidised on a Pt(4.3%)-Bi(3.9%)/C at pH 2, 4, 5 and 6. The initial reaction rates were determined as the initial slope of the conversion. Oxygen partial pressure measurements of the reaction medium showed an immediate rapid increase in dissolved oxygen, thus indicating that reactions took place in the kinetic regime (see Figure 2).

100

200

300

400

t (mins) Figure 2: Oxygen partial pressure, for the oxidation of glyceric acid, as a function of time at pH2 , pH5 (A) and pH6 . Figures 3(a-d) show the conversion and evolution of products as a function of time, and activity and selectivity data is presented in Table 1. Under acidic conditions, where the pH was determined by the acidity of the substrate and product acids (pH 2), an initial high rate of conversion was observed and very high selectivity in hydroxypyruvic acid was obtained. However, at about 50% conversion deactivation of the catalyst blocked reaction progress (maximum yield: 53% at 58% conversion). At pH 4, the initial rate was reduced slightly but catalyst deactivation did not occur and conversion advanced to 93% after six hours. However, as the reaction progressed the selectivity fell as hydroxypyruvic acid was over-oxidised to oxalic and glycolic acids. At pH 5, conversion was total after just four hours, and a maximum yield in hydroxypyruvic acid was obtained after 1.6 hours (74% at 77% conversion). Unfortunately, the rate of over-oxidation was also higher and the product was subsequently rapidly converted to oxalic and glycolic acids. At pH 6, the initial rate of reaction was at its highest but subsequently decreased, and oxalic and glycolic acids were evolved from the outset.

432

(a)

J^

4 time (h)

Figure 3: Product distribution for the oxidation of glyceric acid on Pt(4.3%)Bi(3.9%)/C at (a) pH 2, (b) pH 4, (c) pH 5 and (d) pH 6, as a function of time (A- glyceric acid, Ohydroxypyruvic acid, o- oxalic acid and - glycolic acid).

433

Table 1 Activity and selectivity data for the oxidation of glyceric acid to hydroxypyruvic acid. pH

Initial rate*

Maximum selectivity (%)

Maximum yield (%)

%Pt leached from catalyst

%Bi leached from catalyst

2

760

97

3 nm represents a sufficiently large distance between the clay lamellae for the reactants to enter the interlayer space and undergo hydrogenation there. Nevertheless, the reaction rate may also be influenced by solvation effects, i. e. the adsorption equilibrium of the liquid mixture (reactant, product and reaction medium) at the solid-liquid interface, which determines the concentration of each component in both the adsorption layer and the bulk phase.

4. CONCLUSIONS Novel type Pd-montmorillonite catalysts were prepared by controlled colloid synthesis. The samples exhibited a remarkable catalytic activity in the hydrogenations of 1-octene and styrene. The catalytic performance was found to be closely related to the particle size of Pd and the swelling properties of the clay host. In the solvents toluene, ethanol or tetrahydrofurane, active sites composed of interlamellar Pd particles were involved in hydrogenation. Since a considerable shape selectivity may also be achieved through swelling, Pd-HDAM samples may be regarded as promising consumer designed catalysts.

ACKNOWLEDGEMENT Financial support through Grant OTKA T016109 and T007530 is gratefully acknowledged.

484 REFERENCES 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

P. Laszlo, Science, 235 (1987) 1473. M. Balogh and P. Laszlo, "Organic Reactions using Clays", K. Hafner, C. W. Rees, B. M.Trost, J-M. Lehn, P. vonRague Schleyer, R. Zahradnik(Eds), Springer, Berlin, 1993. T. J. Pinnavaia, Science, 220 (1983) 365. P. Ravindranathan, P. B. Mall, S. Komameni and R. Roy, Catal. Lett., 6 (1990) 401. D. Fishman, J. T. Klug and A. Shani, Synthesis, 1981 (1981) 137. P. G. Gassman and D. A. Singleton, J. Am. Chem. Soc, 106 (1984) 7993. P. Laszlo and L. Lucchetti, Tetrahedron Lett., 25 (1984) 4387. R. Raythatha and T. J. Pinnavaia, J. Organomet. Chem., 218 (1981) 115. M. Choudary and P. Bharathi, J. Chem. Soc. Chem. Comm., (1987) 1505. Z. Kiraly, L Dekany, A. Mastalir and M. Bartok, J. Catal., 161 (1996) 401. E. P. Giannelis, E. G. Rightor and T. J. Pinnavaia, J. Am. Chem. Soc, 110 (1988) 3880. P. C. Aben, J. Catal., 10 (1968) 224. R. Burch, "Catalysis", G. C. Bond, G Webb (Eds), The Royal Society of Chemistry, London, 1985. I. Dekany, Pure and Appl. Chem., 64 (1992) 1499.


485

Catalytic enantioselective addition of diethylzinc to benzaldehyde induced by immobilized ephedrine : comparison of silica and MCM-41 type mesoporous silicates as supports. N. Bellocq, D. Brunei, M. Lasperas and P. Moreau Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique UMR 5618 - CNRS - Ecole Nationale Superieure de Chimie de Montpellier 8, rue de rEcole Normale - 34296 MONTPELLIER Cedex 5 - FRANCE.

The synthesis and characterization of a chiral amino-alcohol ((1R-2S)ephedrine) immobilized on MCM-41 type mesoporous silicas (MTS : Micelle Template Silicas) are described. The activity of these supported catalysts in the enantioselective addition of diethylzinc to benzaldehyde are reported, and compared with those obtained with the corresponding silica gel supported catalysts. The observed differences are discussed in terms of the nature of the grafting which depends on the support structure. 1. INTRODUCTION The enantioselective addition of organometallic reagents to aldehydes which affords optically active secondary alcohols is one of the most important and fundamental asymmetric reactions [1-3]. Among the various organometallic compounds, diorganozincs act as ideal alkyl donors for catalytic asymmetric alkylation. Monomeric dialkylzincs which have an sp-hybridized linear geometry are practically inert to aldehydes because the alkyl-metal bond is rather nonpolar. It has been especially shown that the addition of chiral auxiliaries, such as p-amino-alcohols, could greatly enhance the bond polarity and consequently the reactivity by creating a bent geometry where the Zn atom possesses a higher p character [3]. The use of optically active P-amino-alcohols as chiral auxiliaries in the enantioselective addition of dialkylzincs to aldehydes not only accelerates but also controls the stereochemical outcome of the reaction [4]. In the mechanism of alkyl transfer from the organometallic species to the carbonyl group in homogeneous medium, various bimetallic transition state models suggest that both amino and hydroxyl moieties are involved in the complexation of the zinc atom [3, 5].

486

Such a reaction can constitute an ideal model for the search of solid chiral catalysts. Heterogeneous catalysts have advantages over homogeneous catalysts in their easy separation and recovery from the reaction mixture. A few examples of chiral auxiliaries immobilized on some solid supports, such as polymers [6] and alumina or silica gel [7] have been recently reported for the above mentionned reaction [5]. In the case of alumina and silica as heterogeneous supports, the enantiomeric excesses obtained are moderate and the role of the solid is not clearly understood. The new generation of MCM-41 type mesoporous silicas (MTS : Micelle Template Silicas) which are characterized by a regular porosity, consisting of uniformly sized channels with pore diameters within a mesoporous range of 20100 A [8] have not been used in this type of enantioselective catalysis. Taking into account their characteristic structure, the insertion and grafting of functional molecules is possible as it has been shown recently in this laboratory [9]. The present work is concerned with the synthesis and the characterization of a chiral p-amino-alcohol ((lR-2S)-ephedrine) immobilized on MTS. We report also here the preliminary results obtained concerning the activity of these supported catalysts on the enantioselective addition of diethylzinc to benzaldehyde, compared with the corresponding silica supported catalysts. 2. RESULTS AND DISCUSSION 2.1. Immobilization of ephedrine over the support 2.1.1. Synthesis The immobilization of ephedrine over the support (mesoporous silica and amorphous silica gel respectively) was carried out via the reaction with 3chloropropyltrialkoxysilane (CPS). Two synthetic routes have been envisaged for the preparation of such functionalized solids [10]. In the first route (route a), CPS was modified by ephedrine according to a nucleophilic substitution in homogeneous conditions leading to the linear adduct (Figure 1). The functionalized solids could be obtained by the subsequent attachment of this linear adduct to the support. The second route (route b) involved the reverse procedure, i. e., grafting of the CPS on the carrier and the consecutive modifying of the resulting silanized support by ephedrine. In homogeneous conditions (route a), cyclic products resulting from the substitution of Si-OR groups of CPS by the OH group of ephedrine were obtained

487

together with the desired linear adduct (Figure 1) ; such products have been identified by means of GC/MS.

a

.hH

(RO)3Si

'N*^ CH,

Cychc products Figure 1

No separation of the various adducts was possible and, therefore, route b was prefered. Indeed, grafting of CPS on the silica surface prior to modification by ephedrine is supposed to protect the trialkoxysilyl groups and consequently, to limit such cyclization reactions. The immobilization of ephedrine over the support was thus carried out according to the second route. The substitution of the surface silanols with CPS was performed according to the classical procedure of silanization (Scheme 1):

1: MTS 2a, 2b 4 : Silica gel 5a, 5b 2 and 5 : respectively MTS and silica grafted by chloroalkylsiloxanes a : R = Me, b : R = Et Scheme 1

Elemental analysis (characterization Table 1) showed that residual alkoxy arms remained on the grafted silicon atom depending on the support. Therefore, when necessary, a basic treatment of the solids before modification, specially silica gel solids, has been performed in order to decrease the number of residual alkoxy arms according to scheme 2 : I 4-0^

NaHCOa 0.1 M ^^ Methanol

OR

yI

1-0^ 2b 5b

CI c : R = Et after basic treatment Scheme 2

488

These reactions were followed by the nucleophilic substitution of the chlorine by the basic amino group of ephedrine according to scheme 3 :

2a, 2b, 2c 3a, 3b, 3c 5a, 5b, 5c 6a, 6b, 6c (-)-ephedrine anchored MTS (3) and silica (6) Scheme 3

2.1.2. Characterization The solids thus obtained have been characterized by infrared spectroscopy, nitrogen volumetry, elemental analysis and thermogravimetry. The anchorage of the organic moieties on the MTS support was studied by infrared spectroscopy. The grafted chloroalkylsiloxane MTS spectra (solids 2a, b , c) exhibit a silanol band at 3741 cm-l with a lower intensity than the parent mesoporous and bands at about 2950 cm-l characteristics of -CH aliphatic stretching vibration. The grafted ephedrine mesoporous silica spectra (3a, b , c) show the same band and in addition, bands at 3033, 3068 and 3092 cm-^ characteristics of -CH aromatic stretching vibrations associated to the phenyl group of ephedrine showing that the modification occurred. The number of grafted chloroalkylsiloxanes (solids 2a, b ; 5a, b) has been calculated from elemental and thermogravimetric analyses. A good agreement between the two methods has been obtained and Table 1 shows the average number calculated.

Table 1 : Number of grafted species (m ol/g) Grafted choroalkylsiloxanes Number Residual mol/g xlO'* alkoxy arms 2a 14.3 0.8 2b 12.1 MTS 0.8 2c 12.9 0.5 5a 8.9 1.8 5.2 Silica 5b 1.6 5c 0.6 5.5

Grafted chiral ephedrine Number ModiHcation mol/g xlO'* rate (%) 58 8.3 3a 76 8.8 3b 58 8.5 3c 60 6a 5.3 84 4.9 6b 4.4 70 6c

489

The number of grafted chloro moieties is more important on MTS than on silica. Such a divergence could be due to a higher surface area of the MTS sample in the case of the modified solids b (R = Et), but could also be explained by the different number and nature of silanols for the solids a (R = Me). Elemental analysis gives also the C/Cl ratio which is often higher than the stoichiometric value. We explain this difference by the presence of residual alkoxy arms [11]. The average value of the number of residual alkoxy arms, which is deduced from this ratio, is shown in Table 1. It can be seen that this number is less important on MTS than on silica gel. These results can be explained by the number of silanols present near a grafted chlorosiloxane or by the hexagonal structure of MTS pores which can favour the formation of the Si-O-Si linkage [12]. Elemental analysis of the solids 2c and 5c shows that the basic treatment allows to eliminate alkoxy arms on silica gel, whereas this number remains practically constant on MTS. The number of grafted ephedrine species (solids 3a, b , c ; 6a, b , c) has been calculated by the same methods. All the grafted chloroalkylsiloxanes have not been modified by ephedrine and the modification rate (grafted ephedrine/all grafted species) is higher on silica than on MTS (Table 1). It is likely that the modification is disturbed by the steric hindrance in the pores of MTS. The nitrogen sorption isotherms of the functionalized MTS give informations on their texture and surface state [11]. All isotherms are of type IV [13], indicating the preservation of the mesoporous system during the grafting reactions. Data derived from the sorption isotherms of some samples are reported in Table 2. Table 2 : Textural parameters deduced from nitrogen sorption isotherms Solids Surface area Mesoporous Diameter (m^/g) volume (cc/g) (A) 0.74 1 962 31 0.42 2b 811 21 734 0.41 2c 21 674 3b 0.29 17 684 3c 0.28 17 Surface area, mesopore volume and pore diameter decrease uniformly with the organic coverage. Moreover, all isotherms indicate the preservation of the mesoporous system after the basic treatment.

490

2.2. Application : enantioselective addition of diethylzinc to benzaldehyde The two families of solids have been tested in the enantioselective addition of diethylzinc to benzaldehyde leading to the (R)-l-phenyl-propan-l-ol according to scheme 4 :

i^95%. The inorganic support was dried at 415 K under 0.01 torr before the anchoring process. 5.2. Heterogenisation of Rh-complexes. General Procedure A rhodium complex 4b or 5b, bearing a triethoxysilyl group (0.2 mmols) in dry dichloromethane was added to a suspension of zeolite (1 g, previously dried at 140X/0.01 mm Hg) in dry toluene and the mixture was stirred for 24 hours at room temperature. The solid was then filtered and Soxhlet-extracted with dichloromethane-diethyl ether (1:2) for 12 hours to remove the remaining non-bonded complex, and dried in vacuo. The analytical data for the supported complexes are shown in Table 3. Table 3. Analytical data of homogeneous and zeolite-heterogenised catalysts Catalyst

Elemental Analysis [Found(Calc)]

Atomic Absorption

Anchoring

%C

%H

%N

%Rh

(%)

4a

57.1(57.3)

5.4(5.1)

3.4 (3.0)

10.8 (10.9)

-

Zeol-4b

4.7 (4.71)

0.9(1.1)

0.5 (0.6)

0.8 (0.8)

81

5a

59.4(61.1)

5.5 (5.2)

2.1(2.4)

9.5 (9.0)

-

Zeol-5b

5.3 (5.32)

0.7(1.1)

0.6 (0.8)

0.7 (0.7)

70

506 3.3. Rhodium-catalysed hydrosilylation ofstyrene. In a typical run, benzene (1ml), styrene (208 mg, 2 mmol) and the catalyst (0.002 mmol, ratio catalyst-substrate, 1:1000) were put into a flask under argon, and silane (1.5 mmol) was added dropwise with stirring at selected temperature. The reaction was monitorised by g.l.c. using decane as internal reference^, the results are shown in Table 4. The crude products were isolated by distillation at reduced pressure in excellent yield The silane mixtures were analysed by g.l.c. and g.l.c.-mass spectrometry. The structures of the all products were confirmed by I.R. and ^H-n.m.r. spectroscopic data of isolated samples. Table 4. Data of turnover rates and selectivities for hydrosilylation ofstyrene catalysed by homogeneous and heterogenised cationic Rh-complexes at 38® C. MezPhSiH

PhzSiHi

EtsSiH

TOR* (Select.%)^

TOR(Select.%)

TOR(Select.%)

4a

7170 (>99)

5140 (>99)

19340 (97)

Zeol-4b

2140 (>99)

2960 (>99)

7280 (98)

5a

4100 (>99)

4900 (>99)

7200 (97)

Zeol-5b

790 (>99)

2140 (>99)

8280 (97)

Catalyst

* TOR: mol converted substrate»mol catalyst"'^h^ ^ % of lineal products 4. CONCLUSION The Rh-homogeneous and zeolite-anchored catalysts are active and unusual regioselective catalysts for hydrosilylation of styrene to 2-phenylethylsilanes. The supported catalysts present advantages over their homogeneous counterparts, such as, enhanced stability, increased selectivity to linear alkylsilanes, together the simplicity of recovering and recycling of the catalysts and the easier workup. It can be said that the activity of the heterogenised catalysts remains the same after five recycles, and a significant metal leaching have not been detected. Thus, these catalytic materials are a real alternative to classical homogeneous Pt- or Pd-catalyst for hydrosilylation of olefins to linear alkylsilanes, in the laboratory or in preindustrial environments. Acknowledgements The authors thank the Financial support from Direcci6n General de Investigacidn Cientffica y T6cnica (Project MAT-94-0359-C02-02). REFERENCES 1. S. Patai and Z. Rappoport, " The chemistry of organic silicon compounds", J. Wiley, Chichester, 1989; p. 1479; H. Brunner and cols., "Catalytic Asymmetric Synthesis" (Ed. I. Ojima), VCH Publisher Inc. New York, 1993, p. 303-322.

507 2. (a) F.R. Hartley, M. Eisen, T. Bernstein, J. Blum and H. Schuman, J. Mol Catal 43 (1981) 199; (b) F.R. Hartley, Supported metal complexes, D. Reidel, Dordrecht, 1985; (c) F.R. Hartley in F.R.Hartley (Ed.): ''The chemistry of the metal carbon bond", vol. 4, J. Wiley, New York, 1987, pg. 1163. 3. (a) U. Nagel and E. Kinzel, J. Chem.Soc.Chem.Comm., (1986) 1089; (b) A. Corma, C. del Pino, M. Iglesias and F. Sanchez, J. Chem.Soc.Chem.Comm., (1991) 1253. 4. A. Corma, C. del Pino, M. Iglesias and F. Sanchez, J. Organometal Chem., 431 (1992) 233. 5. A. Carmona, A. Corma, M. Iglesias A. San Jose and F. Sanchez, J. Organometal Chem., 492(1995)11. 6. A. Corma, M.I. de Dios, M. Iglesias and F. Sanchez, An. Quim., 91 (1995) 277. 7. The reaction were monitorised by analysis of samples at different times by g.l.c on a capilary cross-linked methylsilicone column (25m x 0.2mm, 0.3 jim) using nitrogen as carrier gas with decane as internal reference. The regioselectivity and quantification of side-product were measure by g.l.c. and g.l.c.-mass spectrometry of crude distillates. A typical analysis for PhMciSiH or Ph2SiH2 reactions was run using the following oven programe: 100°C(5 min), heating 20T/min to 200°C(6 min) and 200T(10 min); injector and detector at 280T.



509

Polymer-supported Al and Ti species as catalysts for Diels-Alder reactions* B. Altava,a M.I. Burguetc^ J.M. Fraile,^ J.I. Garcia,^ S.V. Luis,^ J.A. Mayoral ^ A.J. Royo^ and R.V. Salvador^ ^Departamento de Quimica Inorganica y Organica, E.S.T.C.E., Universidad Jaume I, E-12080 Castellon, Spain ^Departamento de Quimica Organica y Quimica Fisica, Institute de Ciencia de los Materiales de Aragon, Universidad de Zaragoza-C.S.LC, E-50009 Zaragoza, Spain

Several aminoalcohols and tartaric acid derivatives have been grafted to polystyrene resins. These polymers have been transformed into supported chiral Lewis acids by treatment with EtAlCli or, in the case of the diols derived from tartaric acid, with TiCl2(OPr*)2 and EtiAlCl. All these solids, as well as the non-chiral ones prepared from simple hydromethyl resins, are efficient catalysts in the Diels-Alder reaction of cyclopentadiene and dienophiles like methacrolein or acryloyl and crotonyl-l,3-oxazolidin-3-ones. In some cases the supported catalysts are much more active that the homogeneous analogues. Some of the catalysts can be recovered and reused without metal leaching, keeping an important catalytic activity and without changes in selectivity. The enantioselectivities obtained are minor than 20% ee, being lower than the ones attained with the homogeneous analogues, which shows that the polymer does not behave as a simple inert support.

1. INTRODUCTION Polymer-supported Lewis acids represent an important target as they can be applied for the catalysis of a number of different organic reactions of interest for the preparation of fine chemicals. From an applied point of view, those materials have several advantages associated with their easier separation and the potential of their use in continuous processes [1]. Additionally, the presence of the polymeric matrix can modify the activity and selectivity of the reaction under study. The Diels-Alder reaction is one of the most useful processes in organic chemistry. Study of Lewis acid catalysts that efficiently promote Diels-Alder reactions is receiving much attention in last years because of the great efficiency of this reaction in the preparation of complex structures. The role of the catalyst is important as it can greatly accelerate the process but also produce an increase in regio- and stereoselectivities. Several examples of highly * This work has been made possible by the generous financial support of the C.I.C. Y.T. (Projects MAT93-0224 andMAT96-1053).

510 enantioselective Diels-Alder reactions have been described by employing chiral Lewis acids as catalysts. The use of different heterogeneous catalysts, most of them non chiral, have been reported for this reaction. However, studies related to the preparation and use of polymersupported Lewis acid catalysts is scarce [2,3]. A limited number of reports on this subject have recently appeared, but most of them concern with the use of boron Lewis acids [4]. We have centered our work in the preparation of different resin-bound Ti and Al Lewis acids and their use as catalysts in Diels-Alder reactions.

2. RESULTS AND DISCUSSION Two different approaches can be used for the preparation of functionalized polymers: Polymerization of functionalized monomers and chemical modification of preformed resins. Initially we selected this second approach, using polystyrene-divinylbenzene resins, as this allows to work with materials of well known structural characteristics. Polymers containing hydroxyl groups were selected as starting materials for the preparation of Al and Ti supported catalysts, as it was considered that preparation of the catalysts would be carried out easily by ligand exchange on a MXn species (Scheme 1). (P)—OH

+

MXn

^

I (P>—OMXn-l

| Scheme 1

2.1. Non chiral catalysts Simple hydroxylic resins were obtained from chloromethylated resins (1), commercially available (gel-type resins) or prepared by chloromethylation of styrene units according to the method described by Itsuno (in particular for macroreticular polymers) [5]. Chloromethyl groups were reacted with acetic acid and EtsN to give polymeric esters 2 which were hydrolyzed with KOH-H20-methylglycol [6]. Quantitative transformation of chloromethyl into hydroxy methyl groups was observed in all cases.

® - ©

-^

(P)-^>-CH2Cl

-

©-^-CHjOCOCHj

-*

Scheme 2 Reaction of polymeric alcohols 3 with Ti(0Pr')2 CI2 in CH2CI2 at room temperature gave Ti catalysts 4. Ti content of the polymers was determined through plasma analysis and revealed that conversion of the -OH groups had been complete. An important difference between macroporous and microporous resins is the fact that for gel-type polymers both isopropoxy

511 groups seem to be substituted by polymer-bound oxybenzylic fragments. On the contrary, for highly crosslinked macroporous resins site isolation of hydroxybenzylic groups is achieved and only one isopropoxy group is substituted [7]. Loadings of the resulting polymers ranged from 0.4 to 1 mmol/g, Preparation of Al catalysts was initially accomplished by reaction of polymers 3 with Et AICI2 in CH2CI2 at room temperature. Functional conversions were very low (< 20%). Results could be increased by carrying out the reaction in refluxing CCI4, but conversions were never quantitative (< 60%), loadings of ca. 0.5-1 mmol/g being obtained. Catalytic activity of these polymer-supported Ti and Al species was assayed for the DielsAlder reaction between cyclopentadiene and methacrolein [6]. As polymerization of cyclopentadiene competes with the cycloaddition process, all reactions were carried out using an excess (1.5 times) of the diene. Catalysts to dienophile ratios of ca. 0.05-0.2 were used for the different experiments.

o^< 6

7

CHO

CHO Exo

R 9R

CHO Endo

Scheme 3 Table 1 Results obtained in the reaction of methacrolein with cyclopentadiene in the presence of catalysts 4-5 Catalyst^ Time(min) Id (%) Exo/endo ratio 7 5.3 1740 none 99 30 7.6 4a 83 30 6.7 4b 30 95 8.9 5a 30 36 8.0 5b a) 4a and 5a prepared from a microreticular resin, 1% crosslinked. 4b and 5b prepared from a macroreticular XAD-4 resin. As can be seen in Table 1, all the polymer-supported species tested catalyze very efficiently the process under consideration. The exact nature of the polymeric matrix affects to the catalytic activity, and thus, for instance, microreticular polymers (4a and 5a) gave better results than the macroreticular ones (4b and 5b), in particular for Al species. Even catalyst 5b was relatively efficient as the yield was 98% after 360 min. Exo/endo selectivities were much less affected, but, in general, the bulky nature of the polymeric backbone seems to be reflected in a small increase of the selectivity (to ca. 7 and 8 for Ti and Al catalysts respectively). 2.2. Catalysts prepared from polymer-supported chiral aminoalcohols Chiral aminoalcohols have been widely used for the preparation of polymer-supported catalysts and reagents [8, 9]. Polymer-bound aminoalcohols can be easily obtained from simple

512 compounds by reaction of the aminoalcohol or its hydrochloride with a Merrifield resin in the presence of a base, as is illustrated for prolinol in Scheme 4.

1

HO^

Scheme 4 Reaction of the anchored aminoalcohols with Ti(0Pr')2 CI2 or AlEtCl2 afforded the expected chiral catalysts 12-15 containing 0.6-0.75 mmol of metal/g of resin [10].

A^ 12

CbADV^

Ph

CfeAD^/ph

Activity of these supported Lewis acids was again assayed for the Diels-Alder reaction between methacrolein and cyclopentadiene and results were compared with those obtained for related homogeneous catalysts, such as 16 and 17, prepared from the corresponding Nbenzylated aminoalcohols. Results obtained for the catalysts derived from (»S)-prolinol are summarized in Table 2. Table 2 Results obtained in the reaction of methacrolein with cyclopentadiene in the presence of Al Lewis acids derived from (S)-prolinol Yield (%) Time (min) Exo/endo ratio " %t^ Catalyst 11.2 15 14 98 12 9.2 70 105 13 7 1 240 16 12ba 94 12.6 2640 0 a) Obtained from 12 by reaction with 1 mol of 4-r-butylphenol. b) Determined for the exo adduct. 3xR is the major adduct. As can be seen in Table 2, those polymers catalyzed very efficiently the process. As could be expected, catalytic efficiency decreases when the steric hindrance around the metal increases and when the number of Al-Cl bond decreases. Thus, for instance, catalyst 12b, where one AlCl bond had been replaced by one Al-O-Ar bond, showed to be about 200 times less efficient than the parent compound 12. As a matter of fact, supported catalysts were always far more

513 active than the corresponding homogeneous catalysts. This is particularly noticeable in the case of (iS')-prolinol derivatives (see Table 2) but was also observed for other systems. Thus, polymer-bound catalyst 14 derived from (7/?,25)-ephedrine was 5 times more efficient than homogeneous catalyst 17, and an even higher difference in activity (40 times) was observed when the pyrrolidinol derivative 15 was compared with its homogeneous analogue. These results can be explained by considering that formation of oligomers, which is known to be present in solution giving place to less active species, should be restricted in the supported catalysts because of site isolation [7]. When enantioselectivity is considered, the first point to be mentioned is the observation that, in general, enantiomeric excesses obtained for polymer-bound catalysts were lower than those for homogeneous analogues. In this respect, results obtained for polymeric catalyst 15 are significant. No asymmetric induction was observed for 15 but a 25% ee was observed for its homogeneous analogue. The very low activity of the homogeneous catalyst 16 precluded an accurate determination of the ee and the exo/endo ratio and a direct comparison with results obtained for related polymer-bound species. Data gathered in Table 2 are interesting as they show how an increase in the steric hindrance around the chiral ligand is not reflected, as should be expected, in an increase of the asymmetric induction but in a reduction of the ee values observed. The whole of these results indicate that the polymeric matrix does not act as an inert support but it has an important influence on the steric course of the reaction. This influence may be due to a differential shielding of the enantiotopic faces of the double bond and to a modification of the conformational preferences of the catalyst-dienophile complex. 2.3. Catalysts prepared from polymer-bound 1,2- and 1,4-diols derived from tartaric acid Study of catalysts prepared from polymer-supported ligands containing 1,2- or 1,4-diol functionalities is interesting as this structural factor would favor the formation of very stable chelate rings and provide, if using chiral auxiliaries, a well defined steric environment. Additionally, it has been shown that catalytic activity in solution of some of those catalysts is higher than that reported for aminoalcohol derivatives. Polymer-bound 1,2- and 1,4-diols could be prepared starting from (2/?,5/?)-tartaric acid [11]. Direct esterification with a Merrifield resin in basic media provided 1,2-diols (0.3-0.4 mmol/g) where both carboxyl groups were linked to the polymeric backbone. Treatment with AlEtCl2, AlEt2Cl2 and TiX4 species afforded catalysts 18-20. ®-CH202C^..O-AlCb (P)-CH202C

T (P)-CH202C^0H 18 Chart 2

o

(P)-CH202C>,^...0

T Aici (P>-CH202C

0» 19

T Tict @-Cll202C

^ 20

Et02C

.O-AlCfe

1 Et02C^0H 21

The catalytic activity of these Lewis acids was tested again for the reaction of methacrolein and cyclopentadiene. Some results are presented in Table 3.

514 Table 3 Results obtained in the reaction of methacrolein and cyclopentadiene in the presence of catalysts 18-21 Time (min) Temp. (°C) Yield (%) Catalyst Exo/endo %ee 18^ 1200 -78 21 80 24 13b 2880 74 -35 10.5 18 1440 43 25 8.1 19 0 1440 96 9.3 -35 20 3 a) 3xR is the major adduct. b) 3xS is the major adduct. In this case, the homogeneous catalyst 21 showed to be more active than polymeric catalyst 18. Diffusional limitations can be important in the polymers at the low temperatures required to obtain appreciable enantioselectivities. The possibility that chelate structures such as 19 should be present along with the desired functionality (18) has to be also considered. The decrease in the number of Al-Cl bonds in 19 is directly reflected in a lower catalytic activity. On the other hand, asymmetric induction observed for 18 is very similar to that found for the homogeneous catalyst 21, but the direction of the enantioselectivity is reversed. Thus, most likely, the increase in the crosslinking degree resulting from the double esterification of the tartrate moiety with the polymeric backbone should difficult the appropriate swelling of the resin and, accordingly, diffusion of the reagents to the active sites, but, at the same time, provides a rigid environment that favors asymmetric induction. The nature of the catalytic species is clearly different in solution and in the supported system. Polymer bound 1,4-diols (23) were obtained by reaction of alcoxylates derived from mono0-benzylated tartaric acid derivatives (22) and chloromethylated resins and then modification of the resulting polymers. Treatment with TiX4 species gave Ti catalysts 24 (Scheme 6).

(P^CH^C.^X ^^CH.oA-' BnC'V O 22

BnO''V R R 23

(PKH,0^4^^_^ BnC'V^ R R 24(a: R=H, b: R=Ph)

Scheme 5 Those polymeric Ti Lewis acids (24) catalyzed efficiently the reaction of cyclopentadiene and dienophiles such as crotonoyl- or acryloyl-l,3-oxazolidin-2-ones. Enantioselectivities observed were very low, but it has to be noted that no asymmetric induction was observed when an homogeneous analogue to 24 was used for the same reactions. 2.4. Recovering and reuse of the polymer-supported catalysts Potential recycling of the polymeric catalysts is a very important feature of supported systems. According to this, all polymer-bound catalysts prepared were recovered after the initial reaction, washed, dried and reused for the same reaction, under similar conditions. This procedure was repeated for several cycles. Results obtained showed that all resins partially lose

515 their activity with the use, in particular if initial rates are considered [6]. When the reaction times were slightly increased, good results were usually obtained. In general, Al catalysts seem to be more easily deactivated than Ti catalysts. Titanium chelate species such as 20 and 24 were the most efficiently recycled catalysts. Activity loss is not accompanied by metal leaching and, accordingly, changes in the structure of the catalytic sites or in the polymer, or reaction with residual moisture have to be considered to explain these results. Finally, when polymer-bound chiral ligands were involved, thorough washing of the used polymer allowed efficient recovering of the polymeric chiral auxiliary, and hence regeneration of the appropriate catalyst.

3. EXPERIMENTAL 3.1. Preparation of polymer-supported catalysts Under argon, 1 g of polymer was shaken with a slight molar excess of the Lewis acid (i.e., AlEtCl2 IM in hexanes or a 1:1 mixture of TiCl4 IM in CH2CI2 and Ti(OPr04) ^t -20°C for 20 min and then 1 h at room temperature in dry methylene chloride. The catalyst was filtered, thoroughly washed with CH2CI2 and kept in dry CH2CI2 under an argon atmosphere. 3.2. Diels-Alder reactions with supported catalysts Polymer-supported catalysts were suspended in dry CH2CI2 in a Schlenk tube under an argon atmosphere and the dienophile was added. The suspension was shaken and periodically monitored by GC. Then, the polymer was filtered, washed with dry CH2CI2 and kept under an argon atmosphere to be reused. The solvent and, in its case, the non reacted methacrolein were evaporated under reduced pressure and the cycloadducts were separated and purified by means of column chromatography on silica gel. After separation, the enantiomeric composition was analyzed by NMR in the major endo or exo cycloadduct using Eu(hfc)3.

4. CONCLUSIONS Polymer-supported Ti and Al Lewis acids, both chiral and non-chiral, can be easily prepared from polystyrene resins functionalized with fragments containing hydroxyl groups: simple alcohols, aminoalcohols or diols derived from tartaric acid. These polymer-supported Lewis acids are more stable than analogous in solution and can be used efficiently to catalyze the Diels-Alder reaction between cyclopentadiene and methacrolein, crotonoyl- or acryloyl-1,3oxazolidin-2-ones. The exact nature of the catalytic species can be very different when going from solution to heterogeneous systems, and this has to be taken into account to understand results obtained as well as in the design of supported catalysts. Minimization of oligomer formation is reflected in a higher catalytic activity of resin-bound species, but diffusional limitations of the polymeric matrix can act in the opposite way. For chiral systems obtained by anchoring of a chiral fragment on a preformed polymer, the role of the non-chiral polymeric backbone must be emphasized, as it seems to produce a decrease in asymmetric induction as compared with results in solution. Finally, these catalysts, in particular some Ti species, can be recovered and reused keeping a reasonable catalytic activity.

516 REFERENCES 1. 2. 3. 4. 5. 6. I. 8. 9. 10. II.

K. Smith, ed., Solid Supports and Catalysts in Organic Synthesis, Ellis Horwood, Chichester, 1992. H.U. Blaser, B.Pugin, in Chiral Reactions in Heterogeneous Catalysis, G. Jannes, J. Dubois, eds.. Plenum Press, New York, 1995. C. Cativiela, J.M. Fraile, J.I. Garcia, J.A. Mayoral, F. Figueras, L.C. Menorval, E. Pires,7.Cflf., 1992, 757,394. K. Kamahori, S. Tado, K. Ito, S. Itsuno, Tetrahedron: Asymmetry, 1995, 6, 2547. S. Itsuno, K. Uchikoshi, K. Ito, /. Am. Chem. Soc, 1990,112, 8187. S.V. Luis, M.I. Burguete, N. Ramirez, J.A. Mayoral, C. Cativiela, A.J. Royo, React. Polym., 1992, 50,1535. W.T. Ford, in Polymeric Reagents and Catalysts, W.T. Ford ed., ACS Symposyum Series, 308. ACS, Washington, 1986. S. Itsuno, J.M.J. Frechet, /. Org. Chem., 1987, 52, 4140. K. Soai, S. Niwa, M. Watanabe, /. Org. Chem., 1988, 53, 927. J.M. Fraile, J.A. Mayoral, A:J. Royo, R.V. Salvador, B. Altava, S.V. Luis, M.I. Burguete, Tetrahedron, 1996, 52, 9853. B. Altava, M.I. Burguete, S.V. Luis, J.A. Mayoral, Tetrahedron, 1994, 50, 7535.

517

MOLECULAR IMPRINTING POLYMERISED CATALYTIC COMPLEXES IN ASYMMETRIC CATALYSIS

F. Locatelli^, P. Gamez, M. Lemaire* Institut de Recherches sur la Catalyse, CNRS, Laboratoire de Catalyse et Synthese Organique, UCBL 1 /CPELyon, FRANCE 43 Boulevard du II Novembre 1918, 69622 Villeurbanne Cedex, Tel. : (33) 72 43 14 07 Fax : (33) 72 43 14 08

1. INTRODUCTION With the recent progresses in molecular imprinting, one can consider this as a new tool for the synthesis of enantioselective material applicable to both chromatography separation and catalyst preparation [1]. For example, using non covalent bonding, Mossbach et al. have prepared a stationary phase for HPLC in order to resolve l [2]. MIP's havefiinctionalgroups arranged in such a manner that they are complementary in shape and electronic features to the template. Therefore, Wulf et al. have selectively prepared L-Threonine with an enantiomeric excess of 36% by using a polymer which was imprinted with L-DOPA [3]. 1.1 Hydride transfer reduction The target reaction in this study is the reduction of prochiral ketones. In order to do this, we chose to use hydride transfer reduction to reduce phenyl alkyl ketones. This technique is attractive because high pressure and the use of H2 can be avoided. Reduction is carried out using a hydride donor solvent (mainly isopropanol). Under basic conditions, in presence of a rhodium catalyst, ketones are reduced in alcohol and isopropanol is oxidised in acetone. O

II

[OT

OH

OH

^ ^

[RhJIin*

1^

O

tBuOK/iPrOH

Figure I : hydride transfer reduction of phenyl alkyl ketone

The use of chiral ligands within the rhodium complex can render the catalyst enantioselective. For example Gladiali et al. have reduced acetophenone 2 to (S)-l-phenylethanol (ee 63%, yield 89%) using a catalyst having chiral alkyl phenantroline ligands [4].

Present address : Laboratoire de Chimie Organometallique de Surface, UMR CNRS-CPE 69616 Villeurbanne Cedex, FRANCE

518 2. RESULTS We have previously shown that N,N'-dimethyl-l,2-diphenylethanediamine 1 is a good ligand due to the fact that, in this case, the nitrogen atoms are stereogenic centres. It was first used in homogeneous catalysis [5] , then as a monomer to prepare chiral polyureas on which rhodium was deposited [6]. Finally, it was used to prepare a diamine-rhodium complex (similar to the homogeneous one) that was then polymerised. Ph

CH3HN

Ph

NHCH3

1 Figure II: N,N'-dimethyl-l,2-diphenylethanediamine Reducing acetophenone to phenyl-ethanol, polymerised (R,R) rhodium complex allows a lower selectivity of the same enantiomer to that observed in homogeneous phase (homogeneous reduction e.e. (S) 55%, heterogeneous reduction e.e. (S) 33%). 2.1 Molecular imprinting effect We wish to use the "molecular imprinting effect" to obtain highly enantioselective catalytic reaction. The new catalyst preparation allows us to polymerise the chiral rhodium complex in presence of optically pure l-(s)-phenylethanol as a template. A Typical procedure for imprinted polymerised rhodium complex synthesis is : in a round bottom flask under inert dry atmosphere of argon, 750mg (3.12mmol) of (1S,2S)-N,N'dimethyl-l,2-diphenylethane diamine 8 are dissolved in 4 ml of dichloromethane fi-eshly distilled from P2O5. 78 mg (0.32mmol) of catalytic precursor ([Rh(C8Hi2)Cl]2) are added and the solution stirred. Preparation of sodium 1-phenylethanolate : 9 mg (O.33mmol) of NaH 98% are introduced in a second round bottom flask. Then, 1.5 ml of dichloromethanefi-eshlydistilledfiômP205.and 36^1 (0.3mmol) of optically pure (R)-l-phenylethanol are added. This solution is stirred for one hour before being introduced into the flask containing the rhodium complex. After two hours stirring, a solution of diisocyanate (13a or 13b) and triisocyante ([-C6H3(NCO)CH2-]n n=3 Aldrich 11,130-9) in 1.5 ml of dichloromethane is added. The polyaddition is exothermic. The solution is stirred overnight at room temperature. The solvent is evaporated and the polymer is crushed and washed with 500ml of 2-propanol during 24 hours. Finally it is filtrated through a Millipore filter (w type, pore size O.lOjim). Elimination is monitored by GC on a chiral Cydex B SGE column, 25m x 0.25mm with the other enantiomer as internal reference, dried and sifted. Only particles with a size between 80 and 120jim were retained. Elemental analysis of the solvent shows no evidence for the presence of rhodium, therefore, it can be assumed that no leaching of the metal occurs.

519

Ph \

Ph

[Rh(cod)CI]2

/

H3C—N^

ONC-R-CNO

/Nf-CH3

'^V,

HH

H 3

CHjClj, r.t. overnight under argon

8

M >\ /U

,Nh—C—NH—R—O

N—C—NH—R-

Scheme I: Molecular Imprinted Polymer preparation Whereas the non templated polymer gives a selectivity of 33% e.e. (*S)-phenyl-ethanol, the imprinted-polymerised [bis-((R,R)-diamine l)-l-(5)-phenylethoxy-rhodium] complex allows an increase of 10% e.e. in imprinted alcohol. The utilisation of [bis-((R,R)-diamine l)-l-(/?)phenylethoxy-rhodium] complex shows a slight decrease of e.e.. These results may represent an imprinting effect. To be sure that the increase of enantioselectivity observed is not due to the leaching of residual template, we have performed reduction onto propiophenone ((C6H5)COC2H5) since we already knew that non-imprinted complexes are selective for this compound [7] . A similar increase of 19% e.e. between un-imprinted and imprinted polymer is observed. This cannot be due to any leaching as l is not present in the initial catalyst. Further trials were then carried out using (R)-l-phenylethanol as the template and propiophenone as the substrate to ensure that observed enantioselectivity was only due to the selectivity of the system. 2.2 Influence of cross-linker quantity Being a tri functional group molecule, triisocyanate is used as a cross-linking agent. The degree of cross-linking affects the stiffness of the MIP. It seems that the best compromise between activity and selectivity is obtained with a cross-linking ratio of 50/50 [8]. We then obtain an enantiomeric excess of 70%. 2.3 Influence of temperature and swelling of the polymer Increasing the temperature increases the selectivity of the reaction. This can be explained by the fact that the hot solvent (isopropanol) makes the polymer expand and then "core" sites can be reached by the substrate. There, cavities have a better defined shape making the system more selective. Therefore reaction should be performed at 60°C rather than at room temperature. The swelling-selectivity relationship is not absolute, the use of a very polar solvent such as N,N-dimethylacetamide, leads to such a swelling of the polymer that no more selectivity is observed. Thus, one can assume that this increase of selectivity is due to an

520 equilibrium between accessibility of the best defined sites of the system and stiffiiess of these sites. The activity-swelling relationship is corroborated by the fact that the polymer is more active if it is left for one hour in hot solvent before starting the reaction rather than if the reaction is started as soon as catalyst is in the reactor. The pre-expansion period is once again to the benefit of activity. 2.4 Mechanism We always obtain linear graphs (whatever the value of the parameters) for yield versus conversion graphs. Consequently, one can assume that the synthesis of the two enantiomers occurs via two parallel reactions on the same site without any interconversion. We have therefore proposed the following mechanism (Scheme II) : propiophenone approaches the hydride rhodium complex (described by Gladiali [4]) facing its Si or Re side. This leads to two different complexes and gives (R) or (S) phenyl propan-1-ol. The shape and electronic features of the cavity where the rhodium complex is, should influence the activation energy of these intermediates, thus promoting one of them to react much faster than the other(the one that is created by the Si side approach). (R)-l-phenylpropanol synthesis is favoured.

(R)-Ph6nyl propan-2al

(S)-Ph^yl propan-2oi

Scheme n : Proposed mechanism of reaction 2.5 Selectivity and Activity of MIP on different substrates We have then reduced substrates with structure similar of that of the acetophenone (Table I). Conditions were : 60°C temperature, one hour pre-expansion, cross-linking ratio 50/50, [tBuOK]/[Rh] = 6 (Table I). It can be seen that for 4'-trifluoromethyl acetophenone (entry 4), the initial rate is higher than for the other compounds. It seems that there is an activation of the ketone group by the three fluorine atoms through the aromatic ring. At the other end, no conversion was observed for isopropyl-formiate-benzoyle (entry 3). This can be attribute to the low activity of the a-keto-ester-group or because of the insolubility of the molecule in the polymer. Moving from propiophenone to butyrophenone (entry 2), activity goes down from 18.3 to 14.7 mmol.h"\g"^ of Rh. We can say that the longer the lateral chain the lower the reactivity. For entries (1) and (4) we have obtained an imprinted effect. The selectivity is different and depends of the structure of the substrate. An increase of 22 points was observed for propiophenone and of 8 points for 4'-trifluoromethyl acetophenone. The lower imprinting effect for 4'-trifluoromethyl acetophenone could be explained by the structure being less similar to that of acetophenone than with the propiophenone, and because of the high activity of the molecule. Not only are molecular imprinted polymers more selective, but they are also more active (5 to 10 times higher).

521 For propyl-phenyl ketone (entry 2) a decrease of 14% e.e. on the selectivity was observed. This can be explained by the structure of this compound which is too far from the acetophenone one. A mismatch effect is observed. Table I : Hydride transfer reduction of different substrates Entry

Substrates

imprinted Polymer e.e.(R) initial speed

non imprinted Polymer initial speed e.e.(R) mmol.h "^g"^ of Rh

mmol.h"^g"^ of Rh O

1

2

3

0 0

18.3

70

1.46

48

14.7

44

5.2

58

-

-

-

-

64

38

10

30

0

o

dlVr 0

1

3. Conclusion Polymerised preformed [(N,N'-dimethyl-l,2-diphenylethane diamine)2Rh] complex allows us to obtain enantioselective material. We have then shown that it is possible to imprint an optically pure template into the rhodium-organic matrix and to use the heterogeneous catalyst in asymmetric catalysis with an obvious template effect. The study of yield versus conversion graphs has shown that the mechanism occurs via two parallel reactions on the same site without any inter-conversion of the final products. Adjusting the cross-linker ratio at 50/50 allows us to find a compromise between activity and selectivity. Phenyl ethyl ketone (propiophenone) was reduced quantitatively in 2 days to (R)-l-phenyl propanol with 70% enantiomeric excess We have then shown that the imprinting effect is obvious for molecules related in structure to the template (propiophenone, 4'-trifluoromethyl acetophenone). It is not efficient if the structure of the substrate is too different to that of the template. Further experiments using these materials and on polymers prepared without chiral monomers are under investigation. 4. REFERENCES [1] G. Wulf, Arigew. Chem. Int. Ed Engl., (1995), 34, 1812-1832. [2]

L. Fisher, R. Muller, B. Ekberg and K. Mosbach, J. Am. Chem. Soc, (1991), 113, 9358-9360.

[3]

G. Wulf and J. VietmQiQr,Makromol. Chem., (1989), 190, 1727-1735.

522 [4]

a) S. Gladiali, L. Pinna, G. Delogu, S, De Martin, G. Zassinovich and G. Mestrioni, Tetrahedron : Asymmetry, (1990), 1, 635. b) G. Zassinovich, G. Mestrioni and S. Gladiali, Chem. Rev. , (1992), 92, 1051-1069.

[5]

P. Gamez, F. Fache, P Mangeney and M. Lemaire, Tetrahedron Lett, (1993), 34, 68976898.

[6]

P. Gamez, B, Dunjic, F. Fache and M. Lemaire,./ Chem. Soc, Chem. Commitn, (1994), 1417.1418.

17]

P. Gamez, B. Dunjic, C. Pinel and M. Lemaire, Tetrahedron Lett, (1995), 36, 87798782.

[8]

ratio calculated as below : cross-linking ratio = [sum of fimctional groups from triisocyanates] I [sum of functional groups from diisocyanates]


523

Environmentally friendly catalysis of liquid phase organic reactions using chemically modified mesoporous materials A. J. Butterworth, J. H. Clark*, A. Lambert, D. J. Macquarrie and S. J. Tavener Department of Chemistry, University of York, York YOl 5DD, England A range of novel solid catalysts based on chemically modified materials has been prepared and developed for use in important liquid phase reactions including selective oxidations, nucleophilic substitutions and Knoevenagel reactions. 1. INTRODUCTION The drive towards clean technology, with an increasing emphasis on the reduction of waste at source, will require a level of innovation and new technology that the chemical industry has not seen in many years 1. There is a particular need to reduce the environmental impact of many processes operated by the fine and speciality chemical industries. These are typically batch processes which are often unselective and inefficient, based on stoichiometric reagents and toxic solvents, require multiple separation stages and generate large volumes of toxic or corrosive waste. These problems can be largely overcome if genuinely catalytic, heterogeneous alternatives to environmentally unacceptable reagents can be developed. While heterogeneous catalysis is well established in large scale vapour phase reactions, it is rarely used in liquid phase systems. Microporous materials such as zeolites have limited potential in such systems because of slow diffusion and blockage by larger molecules, but many of the advantages of using porous solids should still be available through the use of mesoporous materials such as silica gels^'^. Recent developments in the design and application of supported reagent catalysts offer considerable potential for clean synthesis^. Synergistic effects between the support material and the reagent can lead to unexpectedly high activity. Thus "clayzic" is a potent solid acid catalyst for certain Friedel-Crafts reactions despite the low activity of the individual components^. This and other similar clay and silica-based solid acid catalysts have been shown to have many useful applications in liquid phase reactions and some of them have been commercialised as environmentally friendly replacements for conventional acidic reagents^. Effective methods of chemical surface modification of mesoporous materials, to create robust surface structures with high catalytic activities in liquid phase reactions, are essential for the future development of environmentally friendly heterogeneous processes. In this paper we demonstrate the value of this methodology in different areas of organic chemistry and catalysis. 2. RESULTS AND DISCUSSION We have prepared chemically modified mesoporous catalytic materials via three routes: (i) surface modification of preformed gels via chlorination in a fluidised bed reactor; (ii) surface modification of preformed gels via silylation; (iii) sol-gel techniques based on preformed silane monomers (Figure 1). Further chemistry on the surface of the materials is achieved by conventional solution methods or by further reaction in the vapour phase.

524 2.1 Catalytic oxidations The selective oxidation of organic substrates in the liquid phase provides routes to a wide range of important functionalised molecules including ketones, alcohols, aldehydes, carboxylic acids and epoxides. Current industrial processes often involve the use of stoichiometric quantities or large excesses of poisonous high oxidation state chromium, manganese and osmium reagents. Environmental and economic factors make the use of these reagents increasingly unacceptable^. Liquid phase oxidation processes based on lower oxidation state transition metals such as Co(II) are also known and form the basis of industrial processes for the oxidation of alkylaromatics. These may utilise catalytic quantities of metal and molecular oxygen as the consumable oxidant, but the conditions are often harsh, the reagent mixture corrosive (acid media is required and bromide is used as a promoter), and the chemistry rarely selective. The selective, catalytic oxidation of organic substrates in the liquid phase under moderate, environmentally acceptable conditions is an important target for many industrial sectors including the manufacture of pharmaceuticals and fine chemicals^.

silicaV-OH

(R0)3SiR sol-gel

CGI J A silicayci

silica ] - R

^

SI(0R)4 + RSi(0R)3

RMgBr

I Figure 1. Routes to chemically modified mesoporous materials A large number of supported reagents has been used for the partial oxidation of organic substrates in the liquid phase^-4. Support materials include silica gel, alumina, clays and molecular sieves. The importance of oxidation chemistry and the many advantages of using supported reagents has made this a well researched area but traditional materials such as KMn04-silica gel, Cr03-montmorillonite, and NaI04-alumina are stoichiometric in oxidant. This makes their use prohibitive in terms of reagent volume, cost and disposal difficulties. There are notable exceptions to this, including some catalytic molecular sieves containing vanadium, titanium and chromium centres!'^'% but there remains a need to develop new supported reagents that are resistant to metal leaching and can operate efficiently and selectively in the liquid phase under mild conditions. Reaction of a preformed cyanoalkylsilane with mesoporous silica gel followed by hydrolysis of the nitrile function leads to the efficient formation of a chemisorbed carboxylic acid which is capable of strongly binding transition metal ions including Cu(II), Mn(II), Ni(II), and Co(II)8 (Figure 2). Characterisation of the resulting cobalt-supported reagent reveals a loading of ca. 0.3 mmol g-1 of the metal ion. The metal is not removed by prolonged washing with dichloromethane, acetonitrile and acetone even though unsupported C0CI2 is soluble to some extent in all of these solvents. Thorough washing with water removes only ca. 50% of the cobalt. Interestingly, the water-washed material has almost identical activity to the unwashed catalyst in the catalytic oxidations described below. The material also enjoys excellent thermal stability with no weight loss, other than adsorbed solvent, below 300 ^C. Thereafter, evolved gas analysis shows the formation of carbon dioxide. The supported reagent is capable of catalysing oxidation reactions including the oxidation of alkylaromatics using air as the oxidant, and the selective oxidation of alkenes to epoxides (Table 1) in the presence of air and an aldehyde. Reactions with moderately active low-melting alkylaromatics do not require the use of a solvent and can be run in neat substrate. In the oxidation of diphenylmethane to benzophenone, only the substrate and catalyst are charged to the reaction vessel with an air sparge. The supported cobalt reagent (ca. 0.5% hr^ conversion at 110 ^C) is significantly more

525 active than unsupported cobalt(II) acetate ( 95%). Application of Lewis-acidic CaX gave acetalisation of the aldehydes as an important side-reaction. This could be prevented, however, by applying higher reaction temperatures. Unfortunately, the X-type zeolite/isopropanol system was not capable of reducing a,P-unsaturated aldehydes. Shapeselectivity was found in the selective conversion of citronellal under MPV conditions. In NaX there was enough space for the substrate to undergo an intramolecular ring closure to isopulegol wheras over CsX reduction to the linear citronellol was observed (Scheme 2). In the reduction of methylcyclohexanone isomers at 100°C it was observed that the 4-isomer reacted relatively fast and gave a thermodynamically determined product distribution (cis:trans = 24:76). The 2- and 3-methylcyclohexanone reacted more slowly and gave a kinetically determined product distribution (cis:trans = 62.5:37.5 and 23.5:76.5 for the 2- and 3-isomer, respectively). The mechanism was proposed to involve the formation of a surface isopropoxide group attached to a cationic site (basic mechanism). It could not be excluded, however, that incompletely coordinated Si- or Al-sites contributed to the catalytic activity (Lewis-acid mechanism).

NaX sel.86% conv. 87 %

isopulegol

CsX |1 JL

O

citronellal

sel.92% conv. 77 %

citronellol

Scheme 2. Shape-selective conversions of citronellal to isopulegol or citronellol under MPV conditions, after reference 14.

535 The reaction of cyclopentanol in the presence of cyclohexanone at 350°C over amorphous metal oxides and zeolites was studied by Berkani et al (scheme 3) [15]. MgO was found to be the most active catalyst for the hydrogen transfer reaction, followed by potassium impregnated gamma alumina (Y-AI2O3-K), Y-AI2O3 and CsNaX zeolites. For the zeolites, the MPVO activity decreased with decreasing cesium content. The reverse trend was observed for the acid catalysed dehydration activity. Addition of CO2 poisoned only the hydrogen transfer reaction while the amount of cyclopentene remained constant. It was therefore concluded that hydrogen transfer occurred only on the basic sites and dehydration only on the acid sites of the catalysts.

OH MPVO

+ H2O

H2O +

Scheme 3. Reaction of the cyclopentanol/cyclohexanone mixture at 350°C over various metal oxide catalysts, from reference 15. Recently, Creyghton et al reported the application of zeolite beta (BEA) in the stereoselective (> 95%) reduction of 4-/gr/-butylcyclohexanone to cz\s'-4-rer^butyl-cyclohexanol in the liquid phase [16,17]. This zeolite-based catalyst proved to be fully regenerable without loss in activity or stereoselectivity. This is of industrial relevance, as the c/^-isomer is a fragrance-chemical intermediate. Other active solid catalysts, including zeolites, invariably gave the thermodynamically more stable trans-isomQx. The activity of the BEA catalyst was found to increase upon increasing activation temperature. Furthermore, deep-bed calcination conditions gave a higher catalytic activity than a shallow-bed procedure, indicating a relation between the catalytic activity and the extent of framework dealumination since the former method results in a greater degree of auto-steaming. However, -^Âl-NMR spectra did not show any increase in octahedral aluminium. FT-IR results indicated a relation between the catalytic activity and the amount of aluminium which is only partially bonded to the framework (Lewisacid sites). The MPV mechanism was therefore proposed to involve a six-membered transition state which is formed upon chemisorption of a secondary alcohol on a Lewis-acid aluminium site and coordination of the ketone to the same site. A base mechanism was ruled out because of the low aluminium content (Si/Al=12), the absence of alkali or alkaline earth cations in the active H-BEA catalysts and the very similar activity of the Li-, Na-, K-, Rb- and Csexchanged catalysts. Furthermore, the catalyst could be poisoned by the base piperidine.

536 The transition states which lead to the cis- or trans-3[coho\ differ substantially in spatial requirements (Figure 1). That for the c/^'-isomer is more or less linear in form and aligned with the BE A channel while the formation of the trans-2i\co\v6[ requires an axially oriented (bulkier) transition state. Although the latter might still fit in the intersections of BEA it is questionable whether there is an active site available at the required position. More coordination possibilities are available for the c/5-transition state, which can easily be accommodated within the straight channels of BEA. The observed kinetically determined product distribution is thus satisfactorily explained by true transition state selectivity.

,H.

-Al-

r

.CH3 .CH3

,0

7-7~rr77^î^

r

.CH3 CHs

-Al

.0

zeolite

Figure 1. Transition states for the formation of c/5-4-r^r/-butylcyclohexanol (top) and trans-Aêrr-butylcyclohexanol (bottom). In addition to the stereoselective MPV reaction presented above, van der Waal et al. reported the catalytic activity of aluminium-free titanium beta ([Ti]-BEA) zeolite in the same MPV reaction [18]. Again, a very high selectivity of 98% to the cz^y-isomer was found which was also explained by a restricted transition state, here around a Lewis acid titanium site. The Lewis acid properties of tetrahedrally incorporated titanium in zeolite [Ti]-BEA had already become clear during catalytic studies on the epoxidation of olefins with hydrogen peroxide in alcoholic solvents. The oxophilic Lewis acidity of the titanium site was confirmed by UV-VIS which showed an increased coordination number for the originally 4-coordinated titanium atom upon adsorption of alcohols and water. Kinetically determined product distributions were also obtained in the MPV reduction of 2-, 3- and 4-methylcyclohexanone; the cis-, trans- and cisalcohol being the major products, respectively. The catalytic activity of [Ti]-BEA was found to be much lower than that of its aluminium analogue whereas its tolerance for water was observed to be much higher. The latter property, which is related to the hydrophobic character of the aluminium free zeolite, illustrates its catalytic potential in this type of reactions.

537 4. Conclusions Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BE A (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions.

Acknowledgement This work was financially supported by the Foundation for Chemical Research in the Netherlands (SON).

References [I] [2] [3] [4] [5] [6] [7] [8] [9] [10] [II] [12] [13] [14] [15] [16] [17] [18]

C.F. de Graauw, J.A. Peters, H. van Bekkum and J. Huskens, Synthesis, 10 (1994) 1007. L. Horner and U.B. Kaps, Ann. Chem., (1980) 192. G.H. Posner, A.W. Runquist and M.J. Chapdelaine, J. Org. Chem., 42 (1977) 1202. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, Appl. Catal., 34 (1987) 189. M. Gargano, V. D'Oranzio, N. Ravasio and M.J. Rossi, J. Mol. Catal., 58 (1990) L5. H. Kuno, M. Shibagaki, K. Takahashi and H. Matsushita, Bull. Chem. Soc. Jpn., 64 (1991) 312. J. Kaspar, A. Trovarelli, M. Lenarda and M. Graziani, Tetrahedron Lett., 30 (1989) 2705. J. Kaspar, A. Trovarelli, F. Zamoner, E. Farnetti and M. Graziani, Stud. Surf Sci. Catal., 59 (1991) 253. J. Kijenski, M. Glinski and J. Reinhercs, Stud. Surf Sci. Catal., 41 (1988) 231. J. Kijenski, M. Glinski, R. Wisniewski and S. Murghani, Stud. Surf Sci. Catal., 59 (1991) 169. J. Kijenski, M. Glinski, J. Czarnecki, R. Derlacka and V. Jarzyna, Stud. Surf. Sci. Catal., 78 (1993)631. N. Ravasio, M. Gargano, V.P. Quatraro and M. Rossi, Stud. Surf Sci. Catal., 59 (1991) 161. V.A. Ivanov, J. Bachelier, F. Audry and J.C. Lavalley, J. Mol. Catal., 91 (1994) 45. J. Shabtai, R. Lazar and E. Biron, J. Catal., 27 (1984) 35. M. Berkani, J.L. Lemberton, M. Marczewski and G. Perot, Catal. Lett., 31 (1995) 405. E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Chem. Soc, Chem. Commun., (1995) 1859. E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Mol. Catal., in press. J.C. van der Waal, K. Tan and H. van Bekkum, Catal. Lett., 41 (1996) 63.



539

SELECTIVE SYNTHESIS OF MONOGLYCERIDES FROM GLYCEROL AND OLEIC ACID IN THE PRESENCE OF SOLID CATALYSTS S. ABRO, Y. POUILLOUX and J. BARRAULT Laboratoire de Catalyse, URA CNRS 350, ESIP, 40 avenue du Recteur Pineau , 86022 POITIERS CEDEX, FRANCE ABSTRACT : The selective synthesis of glycerol monooleate can be performed in the presence of solid catalysts less corrosive and more easily reusable than homogeneous mineral acids. The study of various acid solids (zeolite, clay, ion-exchange resin) for the esterification of glycerol (coproduct of methyl ester synthesis) with oleic acid has shown that cationic exchange resins were the best catalysts for the selective preparation of monooleyl glyceride in mild experimental conditions. Indeed, a selectivity of about 90% for an oleic acid conversion greater than 50% is obtained. It seems that the activity and the selectivity is influenced by the resin structure ; depending on its crosslinking, the resin acts as a shape selective catalyst. L INTRODUCTION The main objective of this work consists in the synthesis of monoglycerides from glycerol and fatty acids issued from vegetable oils in the presence of solid catalysts. Indeed, the use of natural feedstocks presents several advantages : i) the diversity of the available products, ii) the renewable character of natural compounds. Moreover, in the chemical industry, the use of natural products opens an area of investigation of new processes and of new products some of which are quite different from those accessible by petrochemical paths. In our Laboratory, we are involved in a general programme on the selective transformation of fatty acids (or of methyl esters) and glycerol issued from sunflower oil (1,2). We have studied recently the preparation of esters of glycerol, specially monoglycerides, which are important intermediates for the manufacture of lubricants, emulsifiers, surfactants used in the industries of pharmaceuticals, cosmetics and food,... (3). CH2OH RCOOH + CHOH I CH2OH

O II CH2—O—C—R CHOH I CH2OH

+ H2O

R : 8 to 22 carbon atoms. Monoglycerides are obtained generally from glycerolysis or hydrolysis of triglycerides (4) or from the direct esterification of fatty acids by glycerol.

540 Alcohol esterification is usually catalysed by homogeneous catalysts such as sulfuric acid (5), para-toluenesulfonic acid (6) or bases such as sodium (potassium, ...) hydroxide (carbonate,...) (3,7). Unfortunately, it is well known that these bases favour the production of soaps. Moreover, homogeneous catalysts are corrosive, difficult to separate from the products and lead to excessive wastes (salts). As for the direct esterification of glycerol, previous works have shown that numerous solid and acid oxides could be used as catalysts (6, 8-11). In the presence of: i) tin or zinc chloride which are active at low pressure and at 200°C, glycerol is easily esterified, however dehydration and/or oxidation of glycerol can occur, ii) large pores zeolites, the patent of Aracil and Corma claims that monoglycerides can be obtained with a high selectivity (90%) in the same pressure range but at high temperatures (12). Hoelderich and Siegel also used different types of zeolites for this esterification reaction (13). iii) enzymes (supported on resins), it can be observed that there is either a selective formation of monoglycerides, or a mixture of mono, di and triglycrides (14-16). On the other hand, monoglycerides can be obtained selectively from the reaction of glycidol with fatty acids over an anionic ion-exchange resin (17). The aim of our work is to find a new type of solid catalysts in order to be able to control the selectivity. We present in this paper some results obtained with crosslinked porous polymers. Also, we compare the behaviour of different catalysts, in particular, ion-exchange resins, in the esterification of glycerol with oleic acid. The influence of the nature of the resin as well as its swelling properties are discussed. 2. EXPERIMENTAL 2.1. Catalytic tests The esterification was carried out at atmospheric pressure in a glass reactor equipped with a mechanical stirrer and heated v^th an oil bath. Moreover, we verified that the activity and the selectivity were independent of the stirring rate. The reaction was studied at 90°C during 24 hours. The molar ratio glycerol/oleic acid was 6.3, the weight ratio oleic acid/catalyst 4.5 and the catalyst weight 1 g. At the end of the reaction, the mixture was dissolved into ethanol and analysed with an HPLC equipped with a light scattering detector and an apolar column (Licrospher). The separation of different products was done by a gradient elution. The percentage of each compound was determined by using standardisation methods with methyl laurate as an internal standard. The conversion is expressed as follows ;

Conversion (%) = ^ . o / la

y'Oiki

. c +

iS oleic

Yi: stoichiometric coefficient of the i product ki: response factor of the i product The selectivity of each glycerol ester is the ratio : mono-, di- or triester/(oleic acid)transf-

541 2.2. Catalyst The catalysts used in this study were a zeolite (Zeocat HY 510, Si/Al = 10), a Montmorillonite clay pillared with Titanium species and cationic ion-exchange resins ; a macroporous lER Amberlyst 15 (Rohm & Haas) and resins with a gel structure (K1481 Bayer and Amberlyst 31 - Rohm & Haas). 3. RESULTS AND DISCUSSION 3.1. Comparison of the activity and the selectivity of acid catalysts In the first part of our study, the esterification of glycerol with oleic acid in the presence of different acid solids with a controlled porosity (zeolite, clay, ion-exchange resin) was studied (Table 1). Table 1 Esterification of glycerol with oleic acid. Comparison between various acid solid catalysts. Selectivity (%)

Conversion

Catalysts

(%)

Mono-ester^^^

Di-ester^^^

Tri-ester^^^

Without

1

-

-

-

Zeolite HY510

5

67

27

6

Montmorillonite-Ti

10

71

24

5

K1481 resin

49

78

21

1

ABS*

92

54

46

0

* ABS : benzenesulfonic acid, homogeneous reaction Reaction temperature : 90°C, reaction time : 24 h.

(a)

C17H23COOCH2

.. --

Monoester:

J-

C17H23COOCH2

CHOH Diester: C17H23COOCH CH2OH

C17H23COOCH2

I

I Triester iCizHssCOOCH

CH2OH

C^H23COoiH2

The esterification rate was very low when the reaction was performed without a catalyst. The reaction was much faster in the presence of benzenesulfonic acid. Over a solid catalyst, we observed the selective formation of monooleyl glyceride (mainly of the a form) in the presence of a catalyst since the monoglyceride selectivity was between 60 to 70%, the diester selectivity is of about 25% (mainly a,P €ster), the formation of triester being very small. Moreover, the oleic acid conversion varied significantly with the solid used as a catalyst. Thus, the activity of the HY 510 zeolite and of the titanium-pillared clay was lower than the one obtained over the K1481 cationic resin. The selectivity to monoglyceride in the presence of the K1481 resin was higher than the one observed using benzenesulfonic acid in a homogeneous reaction. These results seem to show that the activity of the catalyst depends mainly on the accessibility of the protonic centres to the oleic acid. Indeed the oleic acid could diffuse

542

slowly inside the bi or the tridimensional structure of the pillared clay or of the zeolite (pore size between 10 and 13 A). Moreover, the hydrophilic character of these materials could favour the glycerol adsorption while inhibiting the oleic acid adsorption and the esterification reaction. By contrast over gel resins, mainly the active centres located on the surface of the microspheres of the resin particles would be involved in the reaction. 3.2. Activity and selectivity of ion-exchange resins As the K 1481 resin for the esterification of glycerol with oleic acid was the most active solid catalyst of the series, we compared different cationic resins whose characteristics are presented in Table 2. Table 2 Characteristics of resins. Resin

K1481

Supplier

Type

Crosslinking

Acidity

Particle size

level^'^ (%)

(meq H'/g)

(mm)

Bayer

gel

8

4.8

powder < 0,05

Amberlyst 31

Rohm & Haas

gel

4

4.8

1.2 to 1.3

AmberlySt 119

Rohm & Haas

gel

8

4.8

1.2 to 1.6

12

5.0

1.2 to 1.6

20 "oporous Rohm & Haas macroporous ^^^ % Divinylbenzene (DVB) added to the polymer matrix.

4.8

1.2 to 1.6

s

Amberlyst 16 Amberlyst 15

The catalytic results (Table 3) show that the catalytic activity varies with the structure of the resin. Indeed, the gel resins are more active than the macroporous resins. The conversion of oleic acid is about 55% in the presence of the Amberlyst 31 catalyst whereas it is only of 35 % with Amberlyst 15 or 16 (macroporous type). Table 3 Esterification of glycerol with oleic acid in the presence of ion exchange resins. Influence of the resin structure. Catalysts

Conversion

Selectivity (%)

(%)

Mono-ester

Di-ester

Tri-ester

K1481

49

78

21

1

Amberlyst 31

54

90

7

3

Amberlyst 16

37

83

12

5

Amberlyst 15

36

75

22

3

Reaction temperature : 90°C, reaction time : 24 h. Formulation, see experimental.

543 Moreover, the selectivity to glycerol monoesters is higher than 90 % in the presence of Amberlyst 31 (4 % of Divinylbenzene (DVB)) and it seems that the crosslinking level influences the selectivity of gel organic polymers. With macroporous resins, the selectivity is much less influenced by the crosslinking density. In macroporous resins, two kinds of active sites can be distinguished : i) the sites on the surface of microparticles or in the macropores which are easily accessible by the reactants, ii) the sites located inside the polymer matrix with a greater acid strength but of limited accessibility. The (ii) sites availability or the diffusion of the reagents to these (ii) sites depends on the crosslinking degree. From the results, it seems that a DVB percentage of about 10 % is the value above which the reaction occurs only over (i) sites. On the other hand, gel resins have only a microporous network, a crosslinking density lower than 10 % and can swell in a polar solvent by solvation of sulfonic acid groups. The swelling of the resin is accompanied by a stretching of the crosslinked hydrocarbon matrix leading to the formation of pseudo-pores (20 to 40 A), the size of which depends on the DVB content. As a result these resins look like shape selective materials and that could explain the significant monoglyceride selectivity obtained with the lightly crosslinked (4 %) Amberlyst 31 sample. 3.3 Influence of reaction conditions on the catalytic properties of gel resins 3.3.1. Change of the activity and of the selectivity with reaction time Figure 1 shows that under standard conditions the oleic acid conversion increases linearly with reaction time and reaches 80 % after 48 hours. Thus, the initial activity is around 0.4mmol.h-i.g-i. 100 80

o

>

e o

60 40 20 4-

10

20

30

40

50

Time (h) Figure 1 : Esterification of glycerol with oleic acid in the presence of the K1481 resin. Conversion of oleic acid versus reaction time. Reaction temperature : 90°C As for as the monoester selectivity is concerned. Figure 2 shows that the formation of monoester decreases with increasing conversion, which is what can be expected from a kinetic point of view. At the same time, the formation of diester increases but the selectivity to glycerol trioleate is still very low.

544 100 80 ^ 60

Monoester

i

I 40 Diester C/2

20

^-4=-r-trir

Triester m

—=*t= 40 60 Conversion (%)

20

f

1

80

100

Figure 2 : Esterification of glycerol with oleic acid in the presence of the resin K1481. Selectivity to the esters of glycerol with the conversion of oleic acid. 3.3.2. Effect ofparticle size of the resin Two resins with the same physicochemical characteristics (crosslinking level : 8 % ; exchange capacity : 4.8 meq HVg) were studied. However, Amberlyst 119 has a larger particle size than the K1481 polymer (Table 4). The results show that the K1481 catalyst is more active by a factor of 2 than Amberlyst 119, which indicates that particle size influences the esterification rate. As the K1481 catalyst is a powder, it has a significant outer surface area and acid centres located on this surface are easily accessible. Table 4 Esterification of glycerol with oleic acid in the presence of gel resins. Influence of particle size. Resin

K1481 Amberlyst 119

Particle size

Conversion

Selectivity (%)

(mm)

(%)

Mono-ester

Di-ester

Tri-ester

powder

49

78

21

1

0.65

21

93

3

4

The decrease of the particle size induces a decrease of the osmotic pressure within pores and a faster solvation of the protonic sites. Under these conditions, the properties of the K1481 resin resemble those of a homogeneous catalyst. Thus, the oleic acid is a molecule having a significant lateral chain (CI8) and, when esterified with glycerol, a bulky terminal ester group. Therefore, it cannot or just very slowly diffuse in the micropores of K1481 which has a crosslinking of 8 %. The reaction occurs mainly on the surface of the microspheres. On the other hand, the comparison of the activity of K1481 with that of the Amberlyst 119 resin shows that the sites located on the surface are accessible only by oleic acid since the conversion is much lower with the resin in bead form.

545 The available outer surface on Amberlyst 119 is smaller than that of K1481. It seems that the oleic acid diffusion is inhibited by the Amberlyst 119 catalyst. 3.3,3. Effect of the amount of catalyst The results obtained with the K 1481 resin reported in Table 5 show that the acid conversion normally increases with the increase of the amount of catalyst in the medium. The variations of the activity and the selectivity observed in these experiments are similar to those presented in the previous paragraph concerning the influence of reaction time. It can thus be concluded that there is no significant modification caused by external diffusion phenomena. Table 5 Esterification of glycerol with oleic acid in the presence of the K1481 resin. Influence of the amount of catalyst. Selectivity (%)

meq H+/mmol

Conversion

OA

(%)

Mono-ester

Di-ester

Tri-ester

0.15

45

75

22

3

0.30

71

61

35

4

0.60

81

52

47

1

OA : Oleic acid ; Reaction temperature: 90°C ; reaction time : 24 h. 3.3.4. Thermodynamics and influence of the reaction temperature The glycerol esterification vyath oleic acid is exothermic (AH°R = -30 kJ.mol"^) and under our experimental conditions the equilibrium constant is above 270. The conversion decreases as expected with the temperature (see Table 6), leading to an apparent activation energy for the formation of the mono-ester (calculated for low conversion values) around 75 kJ.mor\ Table 6 Esterification of glycerol with oleic acid in the presence of the K1481 resin. Influence of the reaction temperature. Temperature

Conversion

(°C)

(%)

Monoester

Diester

Triester

90

5

97

3

0

110

23

97

3

0

140

72

43

57

1

Selectivity (%)

Reaction time : 2 h. The selectivity to monoglyceride is modified when the temperature increases since the diester is formed mainly at 140°C at higher conversion of the acid (Table 6). However, at 140°C, we observed the formation of the by-products resulting from the polycondensation of glycerol as well as from the esterification of polyglycerols. We believe that these new

546 reactions modify significantly the catalytic properties of the resin for the esterification of glycerol. 4. CONCLUSION The present study of the esterification of glycerol by oleic acid over different acid solids showed that the ion-exchange resins are the most active catalysts for the selective preparation of monooleyl glyceride. The zeolite and the clay used in this study are much less active than the sulfonic acid resins at low temperature. A comparison between macroporous resins and gel-type resins shows that the gel resins are the most active. The macroporous resins, which are strongly crosslinked, do not permit the swelling of the matrix and as a result the reactants cannot reach the internal active sites (the strongest acid centres); consequently the reaction occurs on the outer surface of the microspheres or in the macropores. Moreover, the selectivity to mono-oleyl glyceride is higher than 90 % at a conversion of 50 % over the gel resin Amberlyst 31. This selectivity is due to the structure of the gel resin which can easily swell in the glycerol favouring the diffusion of reactants to the internal sites. 5. ACKNOWLEDGEMENTS This study was carried out within the framework of a European Program concerning the valorisation of natural feedstoks (AAIR). The authors from the University of Poitiers are very grateful to the European Communities for their financial support and also to the ADEME and the "Region Poitou-Charentes" for their financial support for this research.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

A. Piccirilli, Y. Pouilloux, J. Barrault, J. Mol. Cat., accepted for publication. X. Caillault, Y. Pouilloux, J. Barrault, J. Mol. Cat. A: 103 (1995) 117-123. E. Jungermann, Cosm. Sci. Tech. Serv., 11 (1991) 97-112. K. Holmberg, E. Osterberg, EP Patent 237092. P. Marchal, Rev. Fran?. Corps gras, 32,11-12 (1985) 421-432. G. Devinat, J.L. Coustille, Rev. Frang. Corps gras, 30,11-12 (1983) 463-468. L. Rongsheng, Y. Hua, Z. Wuyang, W. Naixiang, Ind. J. Chem, 31 A (1992) 449 R. Schuch, R. Barrufaldi, L.A. Gioielli, Rev. Farm. Bioquim. Univ. S. Paulo, 20, 1 (1984)51-55. M. Martinez, E. Torrano, J. Aracil, Ind. Eng. Chem. Res., 27 (1988) 2179-2182. A. C. Bhattacharrya, D.K. Bhattacharrya, J. Am. Oil Chem. Soc, 64, (1987) 128-131. R. O. Feuge, E. A. Kraemer, A. E. Bailey; Oil and soap, 22 (1945) 202. J. Aracil, A. Corma, M. Martinez, WO Patent 94 /13617 ; 23 -06 - 94. W.F. Hoelderich, H. Siegel, BASF S. A, EP Patent 0312921; 26 -04 - 89. S. Mert, L. Dandik, Appl. Biochem. Biotech., 50, (1995) 333-342. A. Millquist, P. Aldercreutz, Enzyme Microb. Technol. ;16, 12 (1994) 1042. K. Kitano, Lion Corp ; EP Patent 0407959 ; 16 - 01 - 91. V. Rakotondrazafy, Thesis N° 985, INPT, Toulouse, France, 1994. C. L. Levesque, A. M. Craig, Ind. Eng. Chem., 40,1 (1948) 96-99.


547

ZEOLITE-CATALYSED HYDROLYSIS OF AROMATIC AMIDES B. Gigante. C. Santos, M. J. Marcelo-Curto, C. Coutanceau,^ J. M. Silva,^F. Alvarez,^ M. Guisnet,^ E. Selli ^ and L. Forni c INETI, IBQTA, DTIQ, Estrada do Pago do Lumiar, 1699 Lisboa Codex, Portugal ^ 1ST, Dep. Eng. Qufmica, Grupo de Zeolitos, Av. Rovisco Pais, 1096 Lisboa, Portugal b Univ. Poitiers, Fac. de Sciences, URA CNRS 350, 86022 Poitiers, France c Univ. di Milano, Dip. Chimica Fisica ed Elettroch., Via Golgi 19, 20133 Milano, Italy

ABSTRACT Amide hydrolysis is a key step in the widespread strategy of protection/deprotection of amino groups for synthetic purposes, usually carried out in homogeneous phase with mineral acids. It is shown here that under mild conditions (batch reactor, liquid phase, 75°C) large pore zeolites (HY, HBeta, HMOR) can catalyse the hydrolysis of various aromatic amides. The best results are obtained over HY zeolite samples with Si/Al ratios of 16 and 30: e.g. complete and selective hydrolysis of 2-nitroacetanilide after 2-4 hours reaction for a zeolite/substrate ratio of 0.5 g/mmol. For similar values of the Si/Al ratio HBeta and rather all HMOR samples are much less active than HY samples, which is probably related to diffusion limitations.

I. INTRODUCTION During the synthesis of polyfunctional compounds the amino group can be protected by acetylation, whereas, the regeneration of the amino group can be carried out by hydrolysis of the formed acetamides [1-3]. The conversion of amides to the parent carboxylic acids and amines is considered a routine procedure but in practice it is not always straightforward, since the amino group is not a good leaving group. Strong acid or base catalysts are needed.to catalyse hydrolysis, such as concentrated sulphuric or phosphoric acids, or strong alkali hydroxides, often causing side reactions or even decomposition of the products thus requiring special conditions [4-10]. On the other hand, due to the

548 importance of the amide unit in organic, as well as in biological systems, the mechanisms of acid and base promoted amide hydrolysis have become a subject of interest in recent years, particularly because they may provide models for the cleavage of peptide bonds in living systems [11-13]. As part of our research work to achieve new, simple, non corrosive and environmentally friendly processes for the acid hydrolysis of aromatic amides, we report herein a study of the hydrolysis of substituted anilides (Scheme 1) in liquid phase using zeolites with different pore structures and framework Si-to-Al ratios, as acid catalysts. NHC0R2

6 "^^

NH2

— —

1 + R2C00H

Scheme 1 2. RESULTS AND DISCUSSION In this study protonated large pore zeolites of different structures (HY, HBeta and HMordenite) and framework Si-to-Al ratios were used in liquid phase in a batch reactor. The zeolites were calcined at 500°C and the hydrolysis was conducted at 75°C. The procedure was optimised in terms of solvent, activation, type and amount of catalyst for the hydrolysis of nitroacetanilides, currently carried out with 10 % sulphuric acid [14], and then extended to other substituted amides. The reaction, followed by GC with nitrobenzene as internal standard, was clean and no by-products or degradation were detected. 2.1 Hydrolysis of nitroacetanilides The influence of the solvent was studied for the hydrolysis of 2-, 3- and 4-nitroacetanilide using an HY zeolite (Si/Al=30) as catalyst. From Table 1 it can be seen that the reaction rate was higher when a mixture of methanol-water (1:1) was used as solvent than with methanol or with water separately. The slower hydrolysis rate in water, when compared to methanol or to methanol-water, can be explained by the lower solubility of the aromatic amides. The hydrolysis in the presence of methanol could be due to the small amounts of water present in the commercial synthesis grade methanol used. While this is enough to accomplished the reaction, methanolysis cannot be ruled out.

549 Table 1 Hydrolysis of nitroacetanilides with HY zeolite (Si/Al=30) (2 g/mmol) in different solvents 1

NHCORi

Rl = Me R2=2-N02

Rl = Me R2 = 3-N02 Rl=Me R2=4-N02

Time (h)

Conversion^) (%)

H2O

5.5

100

MCOH1>)

3.5

97

MCOH/H2O

2.0

100

MCOH/H2O

2.0

H2O

7.0

100

MeOH/H20

99c)

4.5

98

H2O

6.0

98

MeOH/H20

4.5

99

a) Based on GLC analysis. Upon isolation of the amines, the molar yields were ca. 5-8 % lower than the conversion values.. b) MeOH for synthesis was used without being dried. c) Reaction with HY (Si/Al =30) previously used.

It can be emphasized that be the hydrolysis of 2-nitroacetanilide is slower with HY zeolite (100 %, 5.5 h) than with a 10 % H2SO4 solution (100 %, 45 min.) [14]. However the work-up with solid acid catalyst was much easier and after washing and calcination, the used HY catalyst exhibite little loss in activity when compared to the fresh catalyst, hence can be reused (Table 1).

c o U

Time (h)

Figure 1. Conversion-time plots for the hydrolysis of 2-nitroacetanilide in MeOH:H20 (1:1) with activated (o) and non activated (A) HY zeolite (Si/Al=30) (2 g/mmol).

550

Figure 1 states that the activation of HY (30) at 250 °C under nitrogen for 6 hours has no effect on the conversion of 2-nitroacetanilide. Consequently the activation step was omitted in the experiments which followed. As Figure 2 shows, after two hours reaction the conversion of 2-nitroacetanilide in MeOH:H20 (1:1) is complete when the amount of catalyst is equal or greater than o.5 g/mmol.

IOOT-

i

804-

/

S O

U

0.5

"+" 1

\— 1.5

HY (g/mmoI)

Figure 2. Hydrolysis of 2-nitroacetanilide with different amounts of HY zeolite (Si/Al=30) in a 2 h run. Figure 3 compares the conversion-time plots for the hydrolysis of 2-nitroacetanilide over zeolites with different pore structures (HY, HBeta and HMOR) and different framework Si/Al ratios. 100 T ^

80 60 +

c o

40 + 20 +

Figure 3. Conversion-time plots for the hydrolysis of 2-nitroacetanilide in MeOH:H20 (1:1) with zeolites (0.5 g/mmol) of different structure and Si/Al ratio: HY Si/Al=30 , HY Si/Al=16 (0), HY Si/Al>100 (x), HY Si/Al=40 (n), Hp Si/Al=10 (o), Hp Si/Al=20 (A), Hp Si/Al=65 (A), HMor Si/Al=60 (*), HMor Si/Al=80 (+), HY Si/Al= 4

.

551 For the same Si/Al ratio, hence for the same density of acid sites, HY zeolites are more active than HBeta zeolites and much more than HMOR zeolites. Since the acidity of HMOR zeolites is stronger than that of HY zeolites [15] their weaker activity indicates that the strenght is riot the determining factor for the hydrolysis activity. The activity of the HY zeolites depends significantly of their Si/Al ratio, a maximum activity being obtained for a Si/Al ratio of 30. The HY zeolite with a Si/Al ratio of 16 has practically the same activity but that with a framework Si/Al ratio of 4 is practically inactive. The low activity of this zeolite is most likely due to the presence of extraframework aluminium species, these species limiting the desorption of the products. It must be emphasized that all the HY zeolites, except that with a Si/Al ratio odf 4 present mesopores which can facilitate the diffusion of the reactants and of the products. From this comparison, it can therefore be concluded that both the acidity and the pore structure of the zeolites determine their hydrolysis activity. 2.2 Influence of substituents on the reactivity of aromatic amides Since, in homogeneous catalysis, the chemical nature of the amine moiety has a large influence on the hydrolysis rate [2,6,7], the previous conditions with zeolites were applied to several aromatic amides containing different substituents and to benzanilide. The results obtained (Table 2) show that the effect of substituents on the reaction rate is comparable to that observed in homogeneous catalysis [2,6,7]. The reaction rate gready increases in the presence of a strong electron-withdrawing group such as NO2, especially in thcortho position, due to the destabilization of the N-acyl bond by cumulative resonance and inductive effect. Although the mechanism may be contentious, some analogies between homogeneous [5-7, 11-13] and heterogeneous catalysis observed from the reaction under study, suggest that changes in the mechanism can occur all depending on the acidity of the medium used. The mechanism most likely (Scheme 2) involves protonation of the amide by the zeolite Br0nsted acid sites followed by the attack of the water, this attack being the limiting step of the hydrolysis.

552

Table 2. Hydrolysis of aromatic amides with an HY zeolite (Si/Al=30) as catalyst in H20/MeOH(l:l) Amines NHCOR 1

d>"Rl=Me,R2=H

|

Time

Conv. a)

Yield ^)

b.pytorr or m.p.

(h)

(%)

(%)

(Lit.) [16] C O

24

85

82

70/9 (71/9)

2

100

95

70-72 (72)

Rl=Me,R2=2-F

22

65

60

65/10(58/11)

Rl=Me,R2=4-F

24

23

20

84/19 (85/19)

Rl=Me,R2=2-Br

21

100

96

31-33 (32)

Rl=Me,R2=2-OH

29

1

-

Rl=Me,R2=4-OMe

20

30

28

56-58 (57)

3

100

98

138-140(139-140) 108-109(110)

Rl=Me,R2=2-N02

Rl=Me, R2=2-OMe, 4-N02 Rl=Me, R2=3-CH3,6-NO2

3

97

93

Rl=Ph,R2=H

12

2

-

Rl=Ph,R2=2-N02

20

55

50

70-72 (72)C)

a) Based on GLC analysis. b) Based on recovered product after purification The amines were characterized by their melting or boiling point and IR and MS spectra. c) Benzoic acid was also isolated (52%), m.p. 121-122X (122°C) [16].

slow

OH I R Î +0H2

Ar-NH2+

fast.

OH + i ArNH^C-R OH

ROH

Scheme 2

3. EXPERIMENTAL 3.1 Catalysts HY zeolites of different Si/Al ratio were supplied by PQ Industries, HBeta 10 was prepared from the commercial Na Beta (CP 806 from PQ) by threefold consecutive ion

553 carried out at 100°C with a 5 fold excess of a 10 N NH4NO3 solution, followed by calcination at 550°C. HBeta 20 and HBeta 65 were obtained by dealumination of HBeta, through acid leaching at 100°C for 4 h with a 10 fold excess of a 0.2 N or of a 1 N HCl solution, respectively. H Mordenites were supplied by Societe Chimique de la Grande Paroisse. All the catalysts were calcined under air at 500°C for 12 hours in a tubular oven. Activation was then carried out under nitrogen at 250 °C for 6 hours. 3.2 Chemicals and Equipment Reagents and standards were of analytical grade and were used without further purification. GLC analyses were performed using a J&W® DB-1 fused silica capillary column, 15 m, 0.25 |Li film thickness. 3.3 Reaction procedure A typical reaction procedure was as follows: a mixture of zeolite (5 g) and amide (10 mmol) in an appropriate solvent (25 ml) was vigorously stirred under reflux at 75°C (using a thermostated bath) under nitrogen atmosphere. The reaction was periodically monitored by GLC. At the end of the reaction, the mixture was filtered and the cake washed with methanol and diethyl ether. After drying with anhydrous sodium sulfate, the solvent was evaporated and the residue purified by distillation or crystallization. The known compounds were characterized by their GC/MS spectra, as well as by their melting points or boiling points, by comparison with standards.

4. CONCLUSION Preliminary results for the hydrolysis of aromatic amides indicate that HY zeolites are the most active catalysts, probably due to their tridirectional pore structure and large pore apertures, while other tridirectional HBeta zeolites and especially unidirectional mordenites were less performant. Although the influence of the crystalline structure of the zeolites has to be taken into account, in comparison with the homogeneous phase conditions achievable in liquid phase, it can be concluded that the HY zeolites with Si/Al between 15 and 30, which are the most active, can replace the strong mineral acids, thereby providing a new, simple, non corrosive and environmentally friendlier process. It is noteworthy that no secondary products were detected, even for the slower reactions. Further evaluation of this process is currently under progress.

554

ACKNOWLEDGMENTS Financial support by Fundaâo Calouste Gulbenkian and the EC within the Human Capital and Mobility Programme (Contract CHRXCT94-0564) is gratefully acknowledged. C. Santos also thanks JTI Programme for a grant.

REFERENCES 1. March, J. Advanced Organic Chemistry, Reactions, Mechanisms and Structure, 3rd Ed.,Wiley Interscience, NY, 1985, p. 338. 2. Barton, Sir D. and 011is,W. D. Comprehensive Organic Chemistry, The Synthesis and Reactions of Organic Compounds, Pergamon Press, London, 1979, Vol. 2, p. 1003. 3. Greene, T. W. and Wurts, P. G. M. Protective Groups in Organic Synthesis, 2nd Edition, Wiley, New York, 1991, p. 249. 4. Lothrop, W. C. J. Am. Chem Soc., 1942, 64, 1698-1700. 5. Duffy, J . A. and Leisten, J. A. J. Chem. Soc. 1960, 545-549 and 553-559. 6. Bamett, J. W. and O'Connor, C. J. J.C. S. Perkin II 1972, 2378-2381 and 1973, 220-222. 7. Giffeney, C. J. and O'Connor, C. J. J.C. S. Perkin II 1975,706-712 and, 1357-1360. 8. Vaugnh, H. L. and Robbins, M. D. J. Org. Chem., 1975,40,1187-1189. 9. Gassman, P. G.; Hodgson, P. K. G. and Balchunis, R. J. /. Am. Chem Soc. 1976, 98, ni 5-1216. 10. Flynn, D. L.; Zelle, R. E. and Grieco, P. A. /. Org. Chem. 1983,48, 2224-2226. 11. Bennet, A. J.; Slebocka-Tilk, H.; Brown, R. S.; Guthrie, J. P. and Jodhan, A. /. Am. Chem. Soc. 1990,112, 8497-8506; Slebocka-Tilk, H.; Bennet, A. J.; Brown, R. S.; Guthrie, J. P. and Jodhan, A. J. Am. Chem. Soc. 1990,112, 8507-8514. 12. Brown, R. S.; Bennet, A. J. and Slebocka-Tilk, H. Ace. Chem. Res. 1992,25, 481488. 13. Antonczak, S.; Ruiz-Lopez, M. F. and Rivail, J. L. /. Am. Chem. Soc. 1994,116, 3912-3921. 14. Vogel, A. I. A Text-Book of Practical Organic Chemistry, 3rd Edition, Longman Group Limited, London, p. 1076. 15. Rabo, J. A. and Gadja, G. J., Guidelines for Mastering the Properties of Molecular Sieves, D. Barthomeuf et al. Eds., NATO AST Series B: Physics, Vol. 21, Plenum Press, New York and London, 1990 p.273. 16. Dictionary of Organic Compounds, 5th Edition, Chapman and Hall, New York, 1982, Vol. I, p. 372 and 751; Vol.III, p. 2641.


555

Hydration of a-Pinene and Camphene over USY Zeolites H. Valente and J. Vital Departamento de Quimica, CQFB, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, 2825 Monte de Caparica, Portugal* The acid catalysed hydration reactions of a-pinene and camphene, respectively, using USY catalysts in aqueous acetone at 55 °C, are studied. The catalyst samples were prepared from zeolite Y by hydrothermal treatments at temperatures ranging from 450 to 850 °C. The so generated extra-framework aluminium species were kept in the samples. The main products of both hydration reactions are a-terpineol, in the case of a-pinene hydration and isobomeol, when camphene is used as the starting reagent. Although zeolite H-Y is not active as catalyst for these hydration reactions, USY catalysts show a reasonable activity and are very selective to the above mentioned terpenic alcohols. Selectivity increases with the relative concentration of Lewis sites. 1. INTRODUCTION a-Pinene is an important starting material in the manufacture of a variety of synthetic aroma chemicals [1]. Camphene is a well known intermediate in the synthesis of bomeol and camphor which are important fragrance materials [1]. The hydration of a-pinene (1) with aqueous mineral acids leads to a complex mixture of monoterpenes known as synthetic pine oil [2]. The main products are monocyclic terpenes, namely a-terpineol (9). The reaction mechanism has been extensively studied [3-7]. It is generally accepted that it proceeds through cation I (Scheme 1). Subsequent carbonium ion rearrangements leads to two parallel pathways. One yields bi- and tricyclic products such as camphene (2), borneol (3) and isoborneol (4). The other yields monocyclic products such as limonene (5), a - (6) and y-terpinenes (7) terpinolene (8), a-terpineol (9)and 1,8-terpine (10). Products from the cyclization of terpineol, like 1,8-cineol, can also be formed. By controlling the many reaction variables the process can be directed to produce a maximum of terpene alcohol's. The direct hydration of a-pinene and camphene to bomeol and/or isobomeol is of great interest since presently a two step procedure is used in industrial practice: acetolysis of camphene and subsequent hydrolysis of bomyl acetate [8].

Funding for this work from JNICT (Junta Nacional de Investigagao Cientifica e Tecnologica) through Grant PBIC/C/CEN/1061/92, is gratefully acknowledged.

556 The hydration of camphene leads mainly to isobomeol and bomeol [9], being the reaction product dominated by isobomeol (Scheme 2). The demands of environment proOH tection lead to an increasing search for 4 cleaner processes. The replacement of mineral acids by more selective catalysts or easily regenerable heterogeneous catalysts lies in this field Zeolites show unique catalytic properties due to their uniform pore size. Namura et al. [10] used various zeolites in the hydration of a-pinene to a-terpineol. Between the catalysts tested ferrierite made possible to achieve selectivities to terpineol as high as 69 %. In the presence of zeolite H-beta a-pinene undergoes rapid hydration in aqueous acetone, with high yield in a-terpineol (48%) [11]. When pure acetone is used as solvent, a new compound identified as a-terpinyl acetone, Scheme 1 - Acid catalysed isomerization and hyis formed. dration of a-pinene. 1: a-pinene; 2: camphene; On the other hand, the selective hy3: bomeol; 4: isobomeol; 5: limonene; 6: adration of a-pinene to bomeol, cataterpinene; 7: y-terpinene; 8: terpinolene; 9 alysed by an unspecified high silica zeoterpineol; 10: 1,8-terpine. lite, has been reported by Chen et al. [12]. Heteropoly acids [9] and natural mordenite [13] have been previously used in the hydration of camphene with high selectivities to isobomeol. The extra-fi-amework aluminium species (EFAL) generated by hydrothermal treatment of zeolite Y have a strong effect on catalytic activity and selectivity upon cracking, isomerization and alkylation reactions of hydrocarbons [14-17]. The effect of EFAL species on the liquid phase isomerization of a-pinene OH has also been studied [18]. In this work we study the effect of EFAL species in steam dealuminated USY zeolites OH with different framework compositions, on the hydration reactions of a-pinene and camphene, at 328 K. Scheme 2 Acid catalysed hydration of camphene.

557 2. RESULTS AND DISCUSSION 2.1 Characterisation of catalysts

Table 1 - Physicochemical characteristics of the catalyst samples. The number following USY refers to the temperature of the hydrothermal treatment; 3T means three consecutive hydrothermal treatments. Lattice Si/Al NEPAL Al-Na' Sample Si/Al' N N / NAI' ao CRX IR^ XRD'' (%) (A) 42.78 46.4 4.10 3.138 2.94 100 HY 2.55 3.62 24.671 39.33 40.3 6.61 3.766 3.59 USY450 2.76 0.95 24.614 94 USY500 35.75 1.20 24.581 36.7 10.22 4.228 4.71 2.93 82 USY550 32.0 4.97 1.60 24.552 33.6 18.60 2.67 4.70 78 4.845 6.54 USY600 2.89 1.80 24.545 32.8 17.65 82 31.05 USY650 28.9 21.51 5.623 7.51 100 27.19 2.90 1.80 24.509 USY700 7.803 8.54 19.81 2.95 2.00 24.441 21.8 28.69 61 USY750 8.607 15.00 16.59 2.82 3.40 24.425 19.9 30.51 80 USY800 2.86 11.11 20.84 15.8 34.64 7.50 24.386 20 8.36 USY850 2.90 12.70 0 USY3T 2.80 2.80 24.269 0.5 57.00 99 3.3 47.2 ^Calculated from Atomic Absorption data, ^ Calculated using NAI = 107.1(ao-24.238) [20]. "" Calculatedusing NAI = -1.401 V3+856.2 [19].

0 5 10 15 20 25 30 35 40 45 50 Figure 1 - Effect of dealumination on the relative concentrations of Lewis (L) and Bronsted (B) acid sites.

The physicochemical properties of the different USY catalysts are given on Table 1. The unit cell parameter, ao, and the crystallinity, CRX, obtained from XRD data were estimated by ASTM methods [24]. The number of aluminium atoms per unit cell, NAI, was calculated from ao [20]. The atomic framework Si/Al ratio was obtained from NAI [21]. The number of extraframework aluminium atoms per unit cell, NEPAL, was calculated by the difference between the total aluminium atoms, obtained by atomic absorption analysis (AA), and the

558 framework aluminium atoms NAI. Subtracting the number of sodium ions per unit cell, NNa+, determined by AA, from NAI, the number of tetrahedral aluminium atoms per unit cell not neutralised by sodium ions, a o Al-Na^, is obtained. U Crystallinity is not strongly affected by the steaming tem0 10 20 30 40 50 perature, except in the case of dealuminations carried out at Time (hrs.) 800 and 850°C. In the first case crystallinity decreases to 20% Figure 2 - Concentration profiles of a-pinene, w, and and in the second case the camphene, ZA. Hydration reaction over USY in aqueous crystalline structure has comacetone at 55 °C. The lines represent the fit to a first orpletely disappeared. When the der kinetics. dealumination temperature increases the lattice ratio Si/Al increases too. Simultaneously, NAI and Al-Na^ decrease. With three consecutive hydrothermal treatments it was possible to prepare a catalyst with a framework Si/Al ratio of 57 and still with a crystallinity as high as 99%. Figure 1 shows the relative concentrations of Lewis (L) and Bronsted (B) acid sites, calculated from IR spectra of adsorpted pyridine [22, 23], as a function of the number of framework aluminium atoms per unit cell, NAI. When NAI decreases and therefore NEFAL increases, [L]/[B] increases, meaning that the increase of extra-framework species corresponds to an increase of Lewis acidity.

I

2.2 Reaction studies The main product in the hydration of a-pinene for all the catalyst samples is a-terpineol. On the other hand, the hydration of camphene leads mainly to isobomeol. The study of catalytic activity towards the hydration of a-pinene or cam-

15 20 25 30 35 40

Figure 3 - Initial activity as a function of NAI and Al-Na^. 0 , 0 - a-pinene; A , A - camphene.

559

Figure 4 - Selectivity to a-terpineol (#) and isobomeol (A) as a function of NEPAL.

phene and selectivity towards aterpineol or isobomeol, respectively, was carried out in a batch reactor. In both cases a pseudo first order kinetics was observed, being a-pinene more reactive than camphene, due to the angle strain of the cyclobutane ring (Figure 2). This kinetic behaviour is in agreement with what was observed for the hydration of a-pinene over zeolite H-beta [11]. In the same way it suggests that a-pinene is not directly protonated on the zeolite's active site. Instead, a pinene is protonated by the wa-

ter molecules solvating the aluminium site [11]. For both substrates, a-pinene and camphene, initial activity (taken as the initial reaction rate, ro) shows a strong dependence on NAI (figure 3). In both hydration reactions, ro achieves a maximum value at values of NM around 22. However, in contrast to what was observed for the isomerization reaction of pure a-pinene over USY [18], the highest activity is now reached at a much lower value of NAI (22 instead of 36 for the isomerization of pure pinene [18]. Also in contrast to the isomerization of pure pinene is the abrupt loss in activity for high NM values. For NAI around 37 the catalyst samples are even inactive for the hydration of a-pinene. When the reaction is carried out over HY zeolites, no significant conversion is observed even after 170 h. This absence of cataljîc activity observed for the parent H-Y or for the low dealuminated catalyst samples, can be explained as follows: — For H-Y or for the low dealuminated catalyst samples the zeolite's inner 1.5 2 2.5 surface is very hydrophilic. The solvent inside [L]/[B] the zeolite pores is probably richer in water than in Figure 5 - Selectivity to a-terpineol (O)and isobomeol (A) the bulk solution. The and activity to a-pinene (#) and camphene (A) hydrations, layers of water molecules respectively, as a function of [L]/[B].

560 surrounding the acid centres form a barrier hindering the diffusion of the terpene molecules. — The increase of activity observed for increasing dealuminations is not only due to the increase of the acid strength of the Bronsted acid sites due to an inductive effect of EFAL species [14, 15], but also to the increase of the hydrophobicity of the zeolite's surface. On the other hand, for high degrees of dealumination the initial activity decreases with NAI, due to the decrease in the number of Bronsted sites per unit cell. The value of the residual activity when Al-Na+ is extrapolated to zero, is very small. This means that Lewis sites give a negligible contribution to the catalyst activity. Selectivity to a-terpineol in a-pinene hydration and to isobomeol in camphene hydration, increases with NEPAL (Figure 4). This is also contrasting with what is observed in the isomerization of pure a-pinene, where the curve selectivity vs. NEPAL shows a minimum [18]. Although Lewis sites exhibit a very low activity in the hydration reactions of a-pinene and camphene, they are very selective to a-terpineol and isoborneol respectively (Pigure 5). The highest selectivities — 70 % to a-terpineol, in the hydration of a-pinene and 90 % to isoborneol, in the hydration of camphene — are reached at the highest relative concentration of Lewis sites. This concentration, however, corresponds to the lowest catalytic activity. Selectivity to a-terpineol is particularly sensitive to the relative concentration of Lewis sites, changing about 20 % in the whole range of catalysts tested. 3. EXPERIMENTAL 3.1 Catalyst preparations USY zeolites with different Si/Al ratios were prepared by hydrothermal treatments from Y zeolites (Aldrich). The dealuminations were carried out at 450-850®C for 3h in 100% steam at. atmospheric pressure. Before and after each hydrothermal treatment the catalysts were ion exchanged twice with aqueous 2M NH4NO3 solution at 80°C, washed with destilled water and dried at 100°C. Pinally, all zeolite samples were calcined at 550°C for 3h. Por the catalyst sample USY3T three consecutive hydrothermal treatments were used: 730 °C, 3h; 760 ""C, 3h; 815 °C, 3h. 3.2 Characterisation The X-ray powder diffraction patterns were recorded on a Rigaku D/MAX III C diffractometer model with CuKa radiation (A,=1.5406 A). The unit cell parameters (ao) were calculated according to the ASTM procedure D 4938-85 [24]. The internal standard was siUcon (ao=5.43094 A). Infrared spectra were taken on a PTIR Gemini spectrometer. The samples were in the form of self-supporting pressed wafers prepared from a mixture of 1 mg of zeolite and 300 mg of KBr. Prior to analysis the samples were dehydrated under vacuum at 400°C for 3 h and then exposed to pyridine vapour at 120®C for 15 min. The bulk Si/Al ratio of the catalysts was determined by atomic absorption. 3.3 Catalytic experiments Catalytic reactions were carried out using 5.2 mmol terpene, 200 mg catalyst and 40% (v/v) aqueous acetone as solvent at 328 K, in 1 ml mini-vials magnetically stirred (1 vial per sample).

561 Samples were analysed by GC and GCMS using a Konic HRGC-3000C and a Fisons MD 800 instmment, respectively, both equipped with a 30m x 0.25mm DB-1 column. 4. CONCLUSIONS The main products of the hydration reactions of a-pinene and of camphene over USY zeolites are a-terpineol and isoborneol, respectively. In the hydration of a-pinene simultaneous isomerization takes place whereas with camphene, hydration is observed nearly exclusively. For both hydration reactions a pseudo first-order kinetics is observed, being a-pinene more reactive than camphene as a consequence of the angle strain of the cyclobutane ring. The activity of the catalyst samples for both hydration reactions depends strongly on NAI. For highfi-ameworkaluminium contents (fi-om 50, corresponding to the parent H-Y, to about 35) the catalyst samples are inactive. This is probably due to the hydrophilic properties of the zeolite's active surface. The catalytic activity increases with thefi-ameworkdealumination to reach a maximum value at NAI = 22. This increase is likely to be due not only to the increase in the acid strength of the Bronsted sites, but also to the changing in the hydrophilic/hydrophobic balance of the zeolite's active surface. Selectivities to a-terpineol, in the hydration of a-pinene and to isoborneol, in the hydration of camphene, can be as high as 70% and 90%, respectively. Selectivities to both terpene alcohols grow continuously with NEFAL. Apparently they are not affected by the changes in the acid strength. Although the Lewis sites give a very small contribution to the catalytic activity, they are very selective to the terpenic alcohols. a-Terpineol is particularly sensitive to changes in the relative concentration of Lewis sites. In the whole range of the relative concentrations of Lewis sites tested, selectivity to a-terpineol in the hydration of a-pinene changes of about 20%. REFERENCES 1. M. Albert, S. G. Traynor and R. L. Webb, in Naval Stores, D. F. Zinkel and J. Russel (eds) PULP Chemical Association, p. 479, New York 1989. 2. J. Kelly and A. E. Roll, in Naval Stores, D. F. Zinkel and J. Russel (eds) PULP Chemical Association, p. 560, New York 1989. 3. Valkanas, N. Iconomou, Helv. Chim. Acta 46, (1963) 1089-1096. 4. Indyk, D. Whittaker, J.C.SPerkin II, (1974), 313-317. 5. .M. Williams, D. Whittaker 7.C^. (B), 1971, 668-672. 6. M. Williams, D. Whittaker J. C5'. (B), 1971, 672-677. 7. Whittaker, in Chemistry of Terpenes and Terpenoids, A. A. Newman (ed.). Academic Press, p. 11, London, 1972. 8. E. Bean, Chem Br. 8 (1972), 386. 9. A. Schwegler, H. van Bekkum, Bull. Soc. Belg. 99, (1990), 113-120. 10. Nomura, Y. Fujihara, H. Tanaka, T. Hirokawa and A. Yamada, Nippon Kagaku Kaishi, 1 (1992) 63; CAl 16:129268m. l i e . van der Waal, H. van Bekkum, J. M. Vital, J. Molec. Catal. A, 105, (1996), 185-192. 12. Chen, X. Cai and S. Cao, Chin. Pat. 1.049.842, (1991). CA 115:P159483z.

562 13. Chen, Y. Li et al.. Faming Zhuanli Shenging Gongkai shuomingshu, CN 1052658A, (1989), CA 116:129323 a. 14. Garralon, A. Corma and V. Fomes, Zeolites,9 (1989) 84. 15. L. Wang, G. Giannetto and M. Guisnet, J. Catalysis, 130 (1991) 471. 16. V. Shertukde, W. K. Hall, J. -M. Dereppe, and G Marcelin, J. Catal., 139 (1993) 468. 17. Corma, A. Martinez and C. Martinez, Appl. Catal., 134 (1996) 169. 18. Severino, A. Esculcas, J. Rocha, J. Vital and L. S. Lobo, Appl. Cat., in press 19. Cairon, S. Khabtou, E. Balanzat, A. Janin, M. Marzin, A. Chambellan, J. C. Lavalley and T. Chevreau in Zeolites and Related Microporous Materials: State of the Art 1994 (Studies in Surface Science and Catalysis, Vol. 84), Elsevier, Amsterdam, 1994, p.997. 20. R. Sohn, S. J. DeCanio, J. H. Lunsford and O. J. O'Donnell, Zeolites, 6 (1986) 225. 21. H. C. van Hoof and J. W. Roelofsen in Introduction to Zeolite Science and Pratice (Studies in Surface Science and Catalysis, Vol. 58), Elsevier, Amsterdam, 1991, p.242. 22. W. Ward, 1 Catalysis, 9 (1967) 225. 23. W. Anderson and J. Klinowsky, Zeolites, 6 (1986) 455. 24. Anual Book of ASTM Standards, Vol. 503, D3942-85, 1987, p. 675.


563

Dehydration of 2-(2-hydroxyethyl)-pyridine to 2-vmyl-pyridine over solid acid catalysts L. Fomi*'*, D. Moscotti', E. Selli^ I. Belegridi^ M. Guisnet\ D. Rohan", B. Gigante^ C. Coutanceau^, J.M. Silva* and F. Alvarez" *Dipartimento di Chimica Fisica ed Elettrochimica, Universita di Milano, Via Golgi 19,1-20133 Milano, Italy ^HLfRA CNRS 350, Faculte de Sciences, Universite de Poitiers, 86022 Poitiers, France "CES Department, University of Limerick, Plassey Technological Park, Limerick, Ireland "^Departamento de Tecnologia de Industrias Quimicas, EBQTA, INETI, 2745 Queluz, Portugal TDepartamento de Engenharia Quimica, I.S.T., 1096 Lisboa, Portugal

2-(2-Hydroxyethyl)-pyridine was dehydrated to 2-vinyl--pyridine in liquid phase over solid acid catalysts, with very high selectivity and fairly good reaction rate at relatively low reaction temperature (160°C). The catalytic activity is well correlated with the presence on the catalyst surface of medium to weak Br0nsted acid sites. The analysis of "coke" left behind onto the catalyst and the effect of partial poisoning of catalytic activity by CO2 indicate that the reaction takes place through two mechanisms, involving either a Br0nsted acid site or a couple of acidbase sites.

1. INTRODUCTION 2-Vinyl-pyridine (VP) is gaining an increasing industrial importance. Indeed, besides the preparation of specialty polymers and copolymers, possessing particular technological properties [1], it is also increasingly and successfully employed as a versatile intermediate for the synthesis of several pharmaceuticals used for various diseases [2,3]. VP is currently manufactured by condensation of 2-picoline with aqueous formaldehyde to 2-(2-hydroxyethyl)-pyridine (HEP), in the absence of any catalyst, followed by dehydration of the alcohol, catalysed by either concentrated sulphuric acid or concentrated aqueous KOH in large excess [4,5]. However, this route is becoming economically less attractive, due to the coproduction of huge amounts of diluted exhaust basic or acid solutions to be recovered or disposed of Other processes were proposed, such as dehydrogenation or oxydehydrogenation

* To whom correspondence should be addressed. Fax No.: +39-2-70638129

564 of 2-ethyl-pyridine in vapour-phase over various catalysts [6], but none of them has yet gained commercial application. As for the presently employed process, a potential alternative to aqueous KOH or mineral acid is the use of a solid acid or base as catalyst for the dehydration of HEP, in order to make the process environmentally friendly [1,7]. When carried out by the usual homogeneous-phase route, dehydration reactions promoted by acidic catalysts take place rapidly through carbocation intermediates, but unfortunately with a low selectivity. On the other hand, basic catalysts, which promote the reaction through carbanion intermediates, are usually more selective. However, they easily deactivate when exposed to the atmosphere, owing to the formation of quite stable carbonates. Therefore, we undertook a preliminary mvestigation, aiming at comparing the behaviour of some solid catalysts, properly chosen through the usual systematic rules [8,9], with those suggested by literature [1,7]. Rather surprisingly, resuhs pointed to acidic solids as the most promismg catalysts, basic solids showing unacceptably low activity. Thus, basic catalysts were abandoned and the investigation continued on acidic solids only. The aim of the present work was then to look for a sufficiently active, selective and durable acid solid catalyst and to establish possible correlations between its physico-chemical characteristics and catalytic behaviour.

2. EXPERIMENTAL All chemicals were "pro-analysi" certified reagents and were generally used as supplied. Pyridine was distilled over KOH and collected and stored over either pre-dried zeolite 3A pellets or KOH beads. High purity (> 99.9995 vol %) cylinder gases were used as supplied. The catalysts screening was carried out on many different amorphous or crystalline silicoaluminates of various structure and different Si/Al ratio. The complete list of the catalysts tested is given in Table 1, together with their main characteristics. When available, commercial zeolites in protonated form or silica-alumina were selected. CJBV 500 Y-zeolite was in the anmionium-exchanged form and was decationated to the protonated form by heating in slowly flowing air (20-30 cm^/min) at 10°C/min up to 500°C and left at such a temperature for 6 h, followed by one night in slowly flowing nitrogen at the same temperature and cooling down to room temperature also in nitrogen flow. BETA M was synthesised by us, following a known patent [10]. HBETA 10 was obtained from the commercial NaBETA (CP 806 PQ) by 3 consecutive ion exchanges with a 5 times excess of a 10 N NH4NO3 solution at 100°C, followed by calcination at 550°C. HBETA 20 and HBETA 65 were obtained by dealumination of HBETA, through acid leaching at 100°C for 4 h with a 10 tunes excess of a 0.2 N or of a 1 N HCl solution, respectively. In standard dehydration runs, 2 g of catalyst powder, preactivated by heating overnight at 550°C in air, were slurried in 10 g of HEP. The reaction was carried out at 160°C in a small glass batch stirred tank reactor, equipped with thermostating jacket. Activity was expressed as mol % overall conversion of HEP (CHEP) or as mol % yield per gram of dry catalyst (YVP), obtained after 4 h of reaction. Reactant and products were analysed by gas chromatography (GC). The composition of the carbonaceous compounds ("coke") left behind on aged catdyst was determined as described elsewhere [11]. Surface acidity was determined by FT-IR analysis of catalysts after preadsorption of pyridine as probe molecule, according to a procedure already described [12].

565 Table 1 Characteristics of the employed catalysts and reaction yield under standard conditions Supplier

Code

Type

PQ PQ PQ Union Carbide UOP Toyo Soda Univ. Milano PQ I.S.T. Lisbon I.S.T. Lisbon UOP Akzo Engelhard

CBV 500 CBV 720 CBV 780 Y82 Y84 TSZ-330HUA BETAM HBETA 10 HBETA20 HBETA 65 M8 HA 100 5P F34

Y zeolite Y zeolite Y zeolite Y zeolite Y zeolite Y zeolite Beta zeolite Beta zeolite Beta zeolite Beta zeolite Mordenite Silica-alumina Montmorillonite

Si/Al ratio 2.75 16.5 41 2.4 2.95 2.95 12.5 12.8 24.2 66.5 9 2.5

YVP (mol%/g cat.) 41.0 12.4 5.8 17.8 37.2 28.5 11.5 16.9 9.0 0.8 1.5 2.4 4.4

Two sets of experiments were also carried out by means of a previously described temperature-programmed reaction (TPR) apparatus, equipped with a mass spectrometric (MS) detector [13] and operated isothermally (200°C) in the pulse mode. In the first set some 2 \x\ pulses of HEP were injected in the flowing carrier gas (ultrapure helium, > 99.9999 vol%) just before the catalyst bed (50 mg of Y84). The second set of experiments was carried out on anotherfi-eshbatch of the same catalyst under exactly the same conditions, but after poisoning the catalyst surface by some pulses of CO2.

3. RESULTS AND DISCUSSION 3.1. Catalytic activity In the absence of any catalyst, no conversion of HEP was ever observed, up to 8 h of reaction. The trend of CHEP obtained in preliminary standard runs in the presence of some of the most significant samples is shown in Fig. la, while the Yvp values obtained after 4 h of reaction are reported for all catalysts in Table 1. Protonated Y-zeolites appear as the most interestmg catalysts. The trend reported in Fig. la was observed also by slurrying the same amount (2 g) of catalyst in an initially larger amount (30 g) of HEP. This allowed to close the mass balance around the reactor very near to 100%, by reducing the influence of the hold-up of the various parts of the apparatus. Under these conditions mol % selectivity to VP (Svp) was always very close to 100%, the only detectable by-product being 2-methyl-pyridine in trace amounts. After reaction every catalyst appeared pale-straw coloured, due to the presence of some organic material ("coke"). The influence of catalyst mass and of reaction temperature was investigated with one of the best-performing catalysts (Y84). A sort of plateau was attained for catalyst mass >ca. 2.5 g, while, by loading 30 g of HEP and 2 g of catalyst, a steady increase of yield may be obtained

566 by increasing temperaturefrom150 to 160 °C. However, growing amounts of by-products are formed at higher temperature. The behaviour of Y84 was compared with that of the actually most widely employed homogeneous catalysts (aqueous KOH). Both these runs were carried out at 160°C, by loading 30 g of HEP. In one case the catalyst was Y84 (2 g), in the other it was a 20 wt % KOH solution (20 g). The results are shown in ¥ig.\b. As expected, the homogeneous catalyst was ca. three times as active as the heterogeneous one, allowing to attain 100% conversion of the reactant in 3 h, instead of 9 h. "Diiii£is'*(iii/z=:210) ii

TSZ330HUA

-—X--CBV500

O

^

F34 HA100SP BETAM

X

MB

Dla

(a)

Dlb

OwO--. D2 Q^rgaiated "diiiieis" (m/z =228)

fCH^<M

(b) -^2

-

(afcolul)

ca

GDI

30 20 F 10 0 0

OD2

"'IHineis"(iii^=315) O o % o ^ ^ ^

20 10 t(h) (C) Figure 1. (a) CHEP VJ. reaction time in standard conditions for some catalysts, (b) Comparison between Y84 (0) and 20 wt% aqueous KOH (D). (c) Y84 catalyst durability run. HEP feeding rate: 2 g/h; reacting solution withdrawal: 2 g/h.

.UO

'tô

. » ^ ^

-

^

T2

Figure 2. Main components of the "coke"

The catalyst life time and regenerability were also tested. A set of runs was carried out by a CSTR configuration of our batch reactor, which was modified, by adding a syringe pump, to feed 2 g/h of reactant and a proper device for withdrawing 2 g of reacting liquid mixture every hour on-stream. The results (Fig.lc) indicate a substantial constancy of conversion, at least up to 18 h on-stream. As for catalysts regenerability, some preliminary runs showed that all the solids tested, once taken out of the reactor and put in contact with the atmosphere, lose most of their activity. However, after heating overnight at 550°C in air, every catalyst reassumed the snow-white colour of thefi-eshsample and became active again. So a short series of reactionregeneration cycles was carried out under the usual batch reaction conditions (160°C, 2 g of catalyst, 30 g of HEP, reaction time 8 h), followed by overnight regeneration at 550°C in air. The overall CHEP values, measured at the end of the 8 h working time, indicate that the catalyst appears perfectly regenerable. Indeed, even an increase of activity was noticed after every regeneration.

567 3.2. Analysis of "coke" The analysis of "coke" was carried out on one sample of Y84 catalyst, representative of "standard" aged samples. Apart the reactant (HEP, ca. 65 wt% of the extracted matter), the product (VP, 10%) and the main by-product, (2-methyl-pyridine, 10%), the main components of "coke" revealed by the GC/MS analysis were species with m/z = 210, 228 and 315 (6%, 6% and 3%, respectively). The most stable carbocation intermediate, generated through acid catalysis, should be of benzylic nature. This intermediate, by reacting with a molecule of VP, can lead to a "dimer" with m/z = 210; by reacting with a molecule of HEP can lead to a symmetric dipyridic ether or to a dimeric alcohol, with m/z = 228; by reacting with the previously mentioned dimer(s) can form a "trimer", with m/z = 315. The most probable species so formed should be those shown in Fig.2. Though undetected in the reacting liquid mixture, the presence of these species in the "coke" confirms the usually accepted mechanism for the dehydration of alcohols by acid catalysis [14]. 3.3. Analysis of surface acidity The surface concentration of Br0nsted and Lewis sites was evaluated by integration of the absorption bands at 1545 and 1455 cm-^ due to adsorbed pyridinium ion and pyridine, with integrated molar extinction coefficients values 813 = 1.67 and 81^ = 2.22 cm jimol-^ [15]. Although absolute concentration of surface acid sites obtained by this method is affected by a 10 - 15% error [12], reliable and valuable information can usually be obtained on a relative scale. Moreover, in the present case pyridine is an ideal probe molecule. Indeed, being very similar in structure to the reactant involved in the present reaction, it is able to titrate exactly those acidic sites which can be reached also by HEP. Fig.3 shows that there is a fairly good linear correlation between the catalytic activity of investigated samples and the concentration of Br0nsted acid sites. An analogous correlation could not be found with Lewis acid sites. Thus Brensted sites confirmed to be responsible for catalytic activity in HEP dehydration.

Figure 3. Yw vs. concentration of Bronsted acid sites (measured at 150°C) of all the catalysts tested. 500

asilBs(Meq(g)

One can thus conclude that the greater is the concentration of surface Br0nsted acid sites, the higher should be the catalytic activity. However, when calculating the specific activity, i,e, the catalytic activity per Br0nsted acid site (Fig.4a), some specific features of each catalyst can be evidenced. For instance, every site of F34 appears very active for HEP dehydration, in spite of the low overall activity, which is due only to the low concentration of surface acid sites. Other catalysts, such as HBETA 20, show a low specific activity, but a rather good overall performance, possessing a relatively high number of Brensted acid sites. Of course, also the strength of the acid sites could play a role. In order to evaluate this parameter, for each catalyst we calculated the ratio between the concentration of Brensted acid sites measured after

568 pyridine adsorption, followed by evacuation at 150 or at 300°C, and the concentration of sites measured after adsorption, followed by evacuation at 100°C. The results are shown in FigAb. It may be noticed in this case that M8 and HBETA 20, though possessing the strongest acid sites, show the lowest specific activity, very likely because the high strength of the sites tends to adsorb the reactant quite tenaciously. On the other hand, F34 and HA 110 5P possess the weakest acid sites, though showing a fairly good specific activity (Fig.4a). It may thus be concluded that the activity of the present catalysts is very likely connected with the presence on their surface of a sufficiently high concentration of Br0nsted acid sites of medium-to-weak strength. 3.4. Analysis of catalytic behaviour Some experiments were also dedicated to the investigation of the evolution of reacting species and of the sorption-desorption behaviour of reactant and products, during the reaction. Before these experiments, the MS spectra of both HEP and VP were recorded by injecting 2 |j,l pulses of pure reactant or product at 200°C, in the absence of any catalyst. Manyfi-agmentsfor both components have common m/z value. However, the shape and intensity of the spectra of the two species are very different. The intensity of the HEP signals is very low with a lot of tailing (Fig. 5a), whereas the intensity of VP signals is much greater and the peaks observed are much sharper (Fig.Sb). This helps in differentiating between the species. Another important point worth noting is that H2O is produced during the dehydration of HEP to VP. Thus, by monitoring the amount of H2O produced, the extent of the reaction can be detected. During the first pulse of HEP on Y84 catalyst a considerable amount of H2O was produced (Fig. 5c), indicating that the dehydration of HEP took place on the surface of the catalyst. However no pyridic species were observed (Fig.5^. Presumably, they remained adsorbed on the catalyst surface. For the second pulse more H2O was produced and VP started to desorb fi-om the reaction sites of the catalyst (¥ig.5c,d). With the third pulse less water was produced andfiômthe shape of the peaks it appeared that both HEP and VP desorbfi-omthe surface of the catalyst (Fig.SeJ). At this stage the activity of the catalyst began to decrease and by the fourth pulse (not shown) only HEP desorbed from the catalyst, showing that the activity at this stage was very poor and all the active sites were blocked. Some experiments were carried out on the catalyst poisoned by CO2. In this case the first pulse of HEP produced less H2O than that injected on the unpoisoned catalyst (Fig.5g), while no pyridic species were again observed (Fig.5/?). For the second pulse some fiirther H2O was produced (JFig.Sg) and pyridic species started to be observed, but the shape of the peaks show that the latter have to be assigned almost exclusively to HEP (Fig.5/i). At this stage the activity of the catalyst was already greatly diminished, most of the active sites being blocked. The catalyst was then left for over 60 h in flowing carrier gas in an attempt to unblock the pores. However, its activity quickly decreased within thefirstfiirthercouple of HEP pulses. At the end of each of the two sets of pulse-mode reaction runs, the catalyst was subjected to a progressive heating (10°C/min) up to 550°C. Both catalyst samples released H2O, VP and HEP in the order, but the C02-poisoned sample in much lower amount. To confirm these findings some additional experiments by CSTR under standard conditions were carried out after treating two samples of activated Y84 catalyst with pure CO2, either at room temperature or at 160°C. After four hours of reaction the conversion of HEP dropped by 8.4 and 25.9% with respect to the unpoisoned catalyst, for the two samples, respectively. It is then evident that the activity of the catalyst is greatly reduced when its basic sites are poisoned by CO2.

569 1.0E-07 (b)

O.OE-tÔO

20

40

60

80

20

t(min)

40

60

80

t(min)

O.OE+OO

1.6E-07 (0 CO fiD

u. O

B

i

I o

5.0E-08

1.8E-09

)

1.0E-07

^

m 40 80 t(min)

0.0E-f00

120

I.OE^

1.00E-09

< I

5.00E-10

01 0.0E-t>00 I

'

'

^

1

0.00E-M)O 20

Figure 4. (a) Specific activity, i.e. per Bronsted acid site, (b) Fraction of Bronsted acid sites of high (D) and medium (0) strength (see text).

40 60 8C t(min)

Figure 5. Pulse-reaction experiments at 200°C. Mass spectra of (a) HEP and (b) VP in the absence of any catalyst. (c,d) First, second and (ej) third HEP pulse on Y84; (g,h)firsttwo pulses of HEP on poisoned Y84. (c,e,g) formation of water (m/z = 18); id/,h) formation of VP, desorption of HEP.

CO2 adsorbs on sites containing exposed cations and anions, forming acid-base pairs [16,17]. Indeed, the FT-IR spectra of Y84, after evacuation at 550°C, followed by treating with CO2 (250 torr) at increasing temperature, showed the appearance of several bands (2335, 1490 and 3265 cm"^), with intensities having a maximum just for C02-pretreating at 150°C. This clearly implies that the dehydration reaction can take place also through a pathway involving some basic as well as acidic sites, in a sort of concerted mechanism implying the removal of the OH group (on a Brensted acid site) and the simultaneous deprotonation at the P-position (on a basic site) [18,19]. The other, presumably most important reaction path, however, involves the formation of the carbocation intermediate, on Brensted sites only.

4. CONCLUSIONS VP can be obtained with very high selectivity and fairly good reaction rate by dehydration of HEP on solid catalysts possessing a high surface concentration of medium-to-weak Br0nsted acid sites and sufficiently large pores in a tridimensional array. At least two reaction

570 mechanisms seem to be active for the present alcohol dehydration. Thefirstone is the usually accepted reaction taking place on Br0nsted acid sites only, through the formation of a quite active carbocation intermediate. The second involves also the presence of basic sites, to promote the P-deprotonation of the side-chain, simultaneously with the removal of the OH group onto the Bronsted acid site. Thefinancialaid of EEC (Human Capital and Mobility Programme, Contract CHRX-CT940564) is gratefiilly acknowledged. Thanks are due to C. Canaflf for "coke" analysis.

REFERENCES 1. L.E. Tenenbaum, in E. Klingsberg (ed.). The Chemistry of Heterocyclic Compounds, Pyridme and its Derivatives, Part H, Interscience, New York, 1961, p.212. 2. B. Elvers, S. Hawkins, W. Russey and G. Schulz (eds.), UUmann's Encyclopedia of Industrial Chemistry (5th Ed.), VCH, Weinheim, A22, 1993, p.408. 3. (a) H. E. Reich, J. Am. Chem. Soc, 77 (1955) 5434. (b) RY. Mauvemay, German Patent No. 2 451 932 (1976). (c) R Petersen, German Patent No. 2 364 685 (1975). (d) M.E. Freed, US Patent No. 4 203 987 (1980). (e) G. Devaux, Bull. Soc. Pharm. Bordeaux), 114 (2) (1975) 70. (f) M.R. Bell, US Patent No. 4 307 102 (1981). (g) F.E. Janssen, Eur. Patent No. 151 826 (1985). (h) M. Abou-Garbia, US Patent No. 4 754 038 (1988). (i) C. Safak, J. Medic. Chem., 37 (7) (1982) 1276. (j) L. AeppU, Helv. Chun. Acta, 63 (1980) 630. (k) M. Abou-Garbia, Brit. Patent No. 2 180 535 (1987). (1) U.T. Bandurco, Brit. Patent No. 2 127 823 (1984). (m) J. Baldwin, Eur. Patent No. 431 945 (1991). (n) Y. Hasegawa, Japan Kokai No. 61 148 176 (1986). (o) A. Shiozawa, Chem. Pharm. Bull., 32 (1984) 553. 4. H.F. Kauffinanm, US Patent no. 2 556 845 (1951). 5. R Bachman and L. Minucci, J. Am. Chem. Soc, 70 (1948) 2381. 6. D. Moscotti and L. Fomi, Appl. Catal., A: General, 134 (1996) 263. 7. (a) L.F. Salisbury, c/o E. I. Du Pont de Nemours, Brit. Patent No. 632 661 (1949). (b) J. Mahan, US Patent No. 2 534 258 (1950). (c) F. Cislak and W. Wheeler, US Patent No. 2 786 846 (1957). (d) H. Thyret, German Patent No. 2 002 661 (1971). 8. D.L. Trinmi, Design of Industrial Catalysts, Elsevier, Amsterdam, 1980. 9. J.T. Richardson, Principles of Catalyst Development, Plenum Press, New York, 1989. 10. R.A. Innes, S.I. Zones and G.J. Nacamuli, US Patent No. 4 891 458 (1990). 11. M. Guisnet and P. Magnoux, Appl. Catal., 54 (1989) 1. 12. T. Barzetti, E. Selli, D. Moscotti and L. Fomi, J. C. S., Faraday Trans., 92 (1996) 1401. 13. L. Fomi, M. Toscano and P. Pollesel, J. Catal., 130 (1991) 392. 14. P.A. Jacobs, M. Tielen and J.B. Uytterhoeven, J. Catal., 50 (1977) 98. 15. C.A. Emeis, J. Catal., 141 (1993) 347. 16. C.L. Angell, J. Phys. Chem., 70 (1966) 2420. 17. D. Bathomeuf, in B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliand (eds.), Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 1980, Vol.5, p.55. 18. H. Pines and J. Manassen, Advan. in Catal., 16 (1966) 49. 19. F. Figueras, A. Nohl and Y. Trambouze, Trans. Faraday Soc, 67 (1971) 1155.

Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) C) 1997 Elsevier Science B.V. All rights reserved.

571

The use of heterogeneous copper catalysts in cyclopropanation reactions J.M. Fraile,^ B. Garcia,^ J.I. Garcia,^ J.A. Mayoral,^* F. Figueras^ a. Dpto Quimica Organica. Instituto de Ciencia de Materiales de Arag6n. C.S.I.C.Universidad de Zaragoza. Facultad de Ciencias. E-50009 Zaragoza (Espafia). b. Institut de Recherches sur la Catalyse. C.N.R.S. 2, Avenue Albert Einstein. 69626 Villeurbanne Cedex (France). Several copper-exchanged and CuCl2-supported solids, together with copper oxide, have been tested as catalysts in the benchmark cyclopropanation reaction of styrene with ethyl diazoacetate. The catalytic activity does not depend on the amount of copper but on the structure and pretreatment of the catalyst. The trans/cis selectivity also depends on the nature of the solid and with KlO-montmorillonite the cis-cyclopropane is predominantly obtained, so that the selectivity is reversed with regard to that observed with copper homogeneous catalysts. The use of several olefins confirms this tendency to reverse the selectivity obtained in solution and the electrophilic character of the reaction. The effect of the reaction conditions and the influence of the solvent are also analyzed.

1. INTRODUCTION Cyclopropanes are invaluable intermediates in organic synthesis [1] and they are present in several biologically active compounds [2]. Therefore, it is very interesting to develop efficient and selective methods to obtain these compounds. The direct transfer of carbene from diazocompounds to olefins catalyzed by transition metals is the most straightforward synthesis of cyclopropanes [3,4]. Reactions of diazoesters with olefins have been studied using complexes of several transition metals as catalysts. In most cases trans-isomers are preferably obtained, but the selectivity depends on the nature of the complex. In general the highest trans-selectivity is obtained with copper catalysts and it is reduced with palladium and rhodium complexes. Therefore, the rhodium mesotetraphenylporphyrin (RhTPPI) [5] and [(r|5-C5H5)Fe(CO)2(THF)]BF4 [6] are the only catalysts leading to a preference for the cis-isomer in the reaction of ethyl diazoacetate with styrene. With regard to the use of heterogeneous catalysts, copper bronze is a traditional catalyst in cyclopropanation reactions [7] and the use of zeolite CuNaX in the reactions of ethyl diazoacetate with several olefins has been described [8].

572 In view of this we have tested several heterogeneous copper catalysts in the benchmark reaction of ethyl diazoacetate (1) with styrene (2). Some of them have been also used in the reactions of ethyl diazoacetate (1) with cyclohexene (4), a-methylstyrene (6), trans-anQtholQ (8) and chalcone (10).

2. EXPERIMENTAL 2.1 Preparation and characterization of the catalysts. KIO montmorillonite and NaY zeolite were purchased from Aldrich, bentonite from Fluka and silica gel from Merck (silica gel 60), iron free synthetic laponite was generously provided by Chimie Labessor. 2.1.1 Cu(U)-exchanged clays. Cation exchange was performed by gradually adding the clay (10 g) to a stirred solution of 2.31 g of CUCI2.2H2O in 125 mL of water. The suspension was stirred at room temperature for 24 h, the solid was separated by centrifugation and washed with deionised water. Resulting clays were dried on a thin bed at 120°C, ground in a mortar and stored at constant humidity. Before use clays were dried at 120°C overnight or calcined in dry air with the following temperature program: 100°C-l°C/min-550°C (10 h)-l°amin-100°C. Copper contents were determined by plasma emission spectroscopy: Cu(II)-K10 montmorillonite (0.09 mmol g-1), Cu(n)-bentonite (0.70 mmol g-1) and Cu(II)-laponite (0.72 mmol g'l). 2.7.2 Cu(U)-exchanged NaY zeolite. The exchange was performed in the same way using 1 g of zeolite and 2.22 g of CUCI2.2H2O in 13 mL of water. The solid was separated by filtration, washed and activated before use as described above. The copper content of the zeolite was 0.57 mmol g"l. 2.1.3 Supported catalysts. 5 g of silica gel or KIO montmorillonite were gradually added to a solution of 1.71 g of CUCI2.2H2O in 20 mL of methanol, the solution was stirred at room temperature for 30 min and the solvent evaporated under reduced pressure. 2.1.4 Silylated Cu(n)-exchanged KIO montmorillonite. To a suspension of 1 g of Cu(II)exchanged KIO montmorillonite calcined at 550°C in 20 mL of dry toluene, Me3SiCl (2.5 mmol) was added and the mixture was heated at 60°C for 6 h under argon. The solid was washed with dry CH2CI2. Neither copper content nor the surface area and X-ray diffraction pattern were altered upon silylation. 2.2 Reaction procedures. Two methodologies were used. In the first one (method A) the catalysts were compared in the reaction between ethyl diazoacetate and styrene. In the second one (method B) several aUcenes were compared using conditions more suitable from a synthetic point of view.

573 2.2.7 Method A, Under argon, to a suspension of the corresponding catalyst (300 mg), styrene (1.04 g, 10 mmol) and n-decane (200 mg, internal standard) in dry CH2CI2 (10 mL) at room temperature, ethyl diazoacetate was added in fractions of 5 mmol (0.5 eq) in intervals of 20 min. Before each addition the reaction was monitored by gas chromatography. 2.2.2 Method B. Under argon, to a suspension of the corresponding catalyst (300 mg) in dry CH2CI2 (1 mL) ethyl diazoacetate (0.8 mmol) was added and the mixture was heated under reflux for 10 min. The suspension was cooled to room temperature and the correponding alkene (10 mmol) and n-decane (200 mg) in dry CH2CI2 (8 mL) were added. The mixture was stirred and a solution of ethyl diazoacetate (5 mmol) in CH2CI2 (1 mL) was added dropwise. The reaction was stirred for an additional 24 h, the catalyst separated by filtration and thoroughly washed with CH2CI2, diethyl ether and methanol. The results were determined by gas chromatography, the solvents evaporated under reduced presssure, the products separated by column chromatography on silica gel and their structures confirmed by NMR. 2.3 Chromatographic analysis. All the reactions were monitored by gas chromatography (FID from Hewlett-Packard 5890II, cross-linked methyl silicone column 25mx0.2mmx0.33|im, helium as carrier gas 20 psi, injector temperature 230°C, detector temperature 250°C) using n-decane as internal standard. 2.3.1 Reactions with styrene. Oven temperature program: 70°C (3 min)-15°C/min-200°C (5 min). Retention times: ethyl diazoacetate (1) 4.2 min, styrene (2) 4.9 min, n-decane 6.8 min, diethyl maleate 8.6 min, diethyl fumarate 9.0 min, (3cis) 11.7 min, (3trans) 12.2 min. 2.3.2 Reactions with cyclohexene. Oven temperature program: 50°C (3 min)-20°C/min-250°C (5 min). Retention times: cyclohexene (4) 3.2 min, ethyl diazoacetate (1) 5.6 min, n-decane 7.7 min, diethyl maleate 9.1 min, diethyl fumarate 9.3 min, (Sendo) 10.2 min, (5exo) 10.4 min. 2.3.3 Reactions with a-methylstyrene. Oven temperature program: 70°C (3 min)-15°C/min-200°C (5 min). Retention times: ethyl diazoacetate (1) 4.2 min, a-methylstyrene (6) 6.5 min, n-decane 6.8 min, diethyl maleate 8.6 min, diethyl fumarate 9.0 min, (7cis) 11.7 min, (Ttrans) 12.2 min. 2.3.4 Reactions with trans-anethole. Oven temperature program: 70°C (3 min)-15°C/min-200°C (5 min). Retention times: ethyl diazoacetate (1) 4.2 min, n-decane 6.8 min, diethyl maleate 8.6 min, diethyl fumarate 9.0 min, fran^-anethole (8) 10.3 min, (9cis) 15.0 min, (9trans) 16.4 min.

574 3. RESULTS AND DISCUSSION First of all we compared the behaviour of these catalysts in the benchmark reaction of ethyl diazoacetate (1) with styrene (2) (Scheme 1) using equimol amounts of both reagents or even a twofold excess of diazoacetate (Table 1). Under these conditions the selectivity with regard to diazoacetate was low and did not depend on the catalyst. This result was not unexpected because this reagent has a great tendency to dimerize and polymerize so a large excess of alkene is generally used.

r -

N2CHC00Et 1

2

+

Ph^

Ph..

COOEt 3cis

COOEt 3trans

Scheme 1 The selectivity in styrene depends on the acidity of the catalyst, given that acid catalysts are able to promote the cationic polymerization of this reagent. Therefore the lowest selectivity, with the catalysts activated at 120°C, is obtained when the most acidic KlO-montmorillonite is used to exchange Cu(n) or to support CuCl2. It is known that calcination eliminates water and reduces Br0nsted acidity, in fact selectivity is increased when the catalysts are calcined at 550°C or when acidity is even more reduced by silylation with MesSiCl [9]. Catalytic activity depends more on the nature and pre-treatment of the catalyst than on the content of copper. Thus, Cu(II)-exchanged KlO-montmorillonite displays an activity similar to the other clays and CuCl2-supported catalysts in spite of the very low amount of copper contained in that clay. Zeolite Y is less active than clays and does not promote the reaction at room temperature. Finally, calcination under dry air reduces the catalytic activity. A very interesting result is that the trans/cis selectivity depends on the nature of the catalyst. In most cases a normal trans preference, which reaches a value of 1.8 with the noncalcined Cu(n)-bentonite, is obtained. However, Cu(II)-exchanged KlO-montmorillonite is the only copper catalyst able to revert this selectivity leading to a slight cis preference. This result had only previously been obtained with RhTPPI [5] and [(r|5-C5H5)Fe(CO)2(THF)]BF4 [6]. It is not easy to give an explanation for this behaviour, but it is possible to discuss the influence or not of several factors. First of all it is important to note that the exchanged or supported copper is responsible for the catalytic activity, in fact the solids without copper did not catalyse the reaction. The influence of shape-selectivity must be discarded given that the normal trans preference is obtained with the microporous Y zeolite and the more sterically

575 c« 1-H

^c/5

rNiomT-Hooo>ncNcviaNr^rôooortr^râN

C2

O

OO

o o

-^ Ci

en ON

^

. CD

O

.9 ^

>\ ffi

o\ C/5

g

^ >^ ^

cd

fs

3

lO

;^ ^ i i U U

i

U

B

i

U

b

O

O

o

2 2 2 2 "in -^ i

U

i

U

i

U

i

U

cd

^

576 hindered cis-cyclopropane is preferably obtained with the mesoporous KlO-montmorillonite. It may be speculated that the changes in selectivity experimentally observed are due to the formation of small particles of copper oxide. In order to test this hypothesis we used this oxide as a catalyst in the same reaction. As expected it was less active than the catalysts used in this work, and it led to trans/cis=2.3, therefore the copper oxide is not responsible for the cis preference obtained with Cu(II)-exchanged KlO-montmorillonite. In view of this, it is difficult to offer an explanation for the changes in selectivity, although it may be speculated that they depend on the dimensionality of the solid and/or more probably on the isolation of the catalytic sites, in fact, the unexpected cis preference is observed with the catalyst containing the lower amount of copper. In order to improve the selectivity with regard to the diazoacetate we tested the reaction using a twofold excess of styrene. Furthermore, in order to have the maximum excess of alkene, the diazoacetate was added dropwise. In this way the selectivity in diazoacetate is increased from about 20% to about 50%, furthermore the selectivity with regard to the styrene is also noticeably improved. The KIO catalyst was recovered by filtration and washing. The recovered catalyst was slightly less active and the selectivities with regard to both reagents were also slightly lower, however the stereoselectivity was not modified. The solvent has also a great influence. In fact when the reaction was carried out without a solvent the conversion of styrene was very high, in spite of the fact this reagent was used in excess, but the yield of cyclopropanes was low. When acetonitrile was used as a solvent the reaction did not take place, which indicates that this solvent coordinates the active sites of the catalyst hindering the access of the reagents. Finally we tried to increase the abnormal cis preference by decreasing the temperature, but the stereoselectivity was not modified and the reactivity and selectivity were clearly diminished. The catalysts leading to the higher trans and cis preferences, namely Cu(II)-bentonite and Cu(II)-K10, were tested in the reactions of ethyl diazoacetate (1) with different alkenes (Scheme 2). The results obtained (Table 2) show that the yield and the selectivities with regard to both reagents increase with the electron-donor ability of the substituents of the double bond, which indicates the electrophiUc character of the reaction. Therefore the cyclopropanes coming from chalcone (10) could not be obtained. The use of Cu(II)-exchanged KlO-montmorillonite as a catalyst favours the formation of the sterically most hindered product, whereas the less hindered is favoured in homogeneous phase or with Cu(II)-exchanged bentonite.

577 Table 2. Results obtained in the reactions of ethyl diazoacetate (1) with several alkenes in CH2CI2 at 25°C using the reaction method B. % conv.b % yieldb trans/cisb endo/exob % sel. alkh % sel. 1^ alkene catalyst^ 53 79 84 97 81 100 ~

1.6 21 40 4 Cu-KlO 3.6 22 4 Cu-bentonite 28 26 0.7 31 6 Cu-KlO 1.2 30 31 6 Cu-bentonite 21 1.1 8 Cu-KlO 26 34 34 1.8 8 Cu-bentonite 0 10 Cu-KlO 0 — — 10 Cu-bentonite 0 0 a. Catalysts pretreated at 120°C. b. Determined by gas chromatography.

E t O O C ^ ^ ,H

^xv^

-L

H,^ C O O E t

A

' CP

B -

i

+

n

^

5exo

Sendo CH.

42 44 52 60 42 68 ~

Ph

,. Y ^

Ph.

CH3... Ph^

COOEt

COOEt CH3 7cis

Ttrans

CH3

OCHa

An

COOEt 9cis

'COOEt 9trans

COPh

COPh

O 1

+ PPhh- ^ >^ c^/^- ^ ^- pl h 10

Ph"

'COOEt llcis

Scheme 2

'COOEt lltrans

578 4. CONCLUSIONS Several copper-exchanged and CuCl2 supported solids promote the reaction of ethyl diazoacetate with alkenes to yield cyclopropanes. The activity depends on both the nature and the pre-treatment of these soHds. The solvent has a noticeable influence, so very bad results are obtained when the reaction is carried out in the absence of a solvent and the reaction does not work in the presence of co-ordinating solvents such as acetonitrile. The results obtained depend on the nature of the alkene and, in agreement with the electrophilic character of the reaction, they are better with alkenes possessing electron-donor substituents. The most interesting result is that the trans/cis stereoselectivity depends on the nature of the catalyst. In general the less sterically hindered cyclopropane (trans or exo) is the major product, as also happens with homogeneous copper catalysts. However, the use of Cu(II)-exchanged KIOmontmorillonite increases the amount of the most hindered (cis or endo) product which in some cases is the major one. Acknowledgements. This work was made possible by the generous financial support of the Comisi6n Interministerial de Ciencia y Tecnologia (Project MAT96-1053).

REFERENCES 1. See for instance: H.N.C. Wong, M.-Y. Hon, C.W. Tse, Y.-C. Yip, J. Tanko, T. Hudlicky, Chem. Rev., 89 (1989) 165. 2. See for instance: a) M. Elliot, A.W. Famhem, N.F. James, P.H. Needham, A. Pulman, J.H. Stevenson, Nature, 246 (1973) 169. b) D. Arlt, M. Jantelat, R. Lantzsh, Angew. Chem., Int. Ed. Engl., 8 (1981) 719. 3. a) J. Salaun, Chem. Rev., 89 (1989) 1247. b) P. Helquist in B.M. Trost ed.. Comprehensive Organic Synthesis. Pergamon Press, U.K. 1991. Vol.4, p 951. c) H.M.L. Davies, ibid. Vol.4, p 1031. 4. A. Demonceau, E. Abreu Dias, C.A. Lemoine, A.W. Stumpf, A.F. Noels, C. Pietraszuk, J. GuHnski, B. Marciniec, Tetrahedron Lett., 36 (1995) 3519 and references cited therein. 5. a) H.J. Callot, C. Piechocki, Tetrahedron Lett., 21 (1980) 3489. b) H.J. Callot, C. Piechocki, Tetrahedron, 38 (1982) 2365. 6. a) W.J. Seitz, A.K. Saha, D. Casper, M.M. Hossain, Tetrahedron Lett., 33 (1992) 7755. b) W.J. Seitz, M.M. Hossain, Tetrahedron Lett., 35 (1994) 7561. 7. R.C. Fuson, E.A. Cleveland, Organic Synthesis Collect.; Wiley, New York 1955. Vol. Ill, p 339. 8. J.C. Oudejans, J. Kaminska, A.C. Kock-van Dalen, H. van Bekkum, Reel. Trav. Chim. Pays-Bas, 105(1986)421. 9. J.M. Fraile, J.I. Garcia, J.A. Mayoral, T. Tamai, P.J. Alonso, J. Chem. Soc, Chem. Commun., (1996) 1981.


579

Reaction between haloaromatics over a CuHZSM-5 zeolite - Mechanistic Study. S. Vol, L. Vivier and G. Perot URA CNRS 350, Catalyse en Chimie Organique, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France. SUMMARY The reaction between haloaromatic compounds (R(|)X and (|)X : R = -F, -CI, -CH3, -CF3 ; X = -CI, -Br, -I) was studied in gas phase (400^C, atmospheric pressure) in a flow microreactor in the presence of a 2 wt% Cu-HZSM-5 catalyst. In most cases a highly selective ipso exchange of halogen atoms was obtained except with fluorine which was completely unreactive. The kinetic study of the reaction between bromobenzene and 3chlorofluorobenzene showed that the bromo compounds are more strongly adsorbed than chloro compounds and inhibit the reaction. The reactivity sequence of the halogen leavinggroup (-Br > -I > -CI » -F) as well as the positive value of the p reaction constant are in favour of a SRN mechanism although a mechanism through aryl copper complexes cannot be excluded. 1. INTRODUCTION We reported recently [1] that a reversible exchange of halogen atoms occurred between aromatic compounds in the presence of Cu-HZSM-5 (Equation 1):

(|)I (1.8) >(^C1 (1) > R|)C1 (0.45) »(\>¥, ¥2^ It should be mentioned that all these experiments were carried out independently (on different catalyst samples or on the same sample which was reactivated by treatment under air flow at 500°C overnight). Hence the relative reactivity values seem reliable since for instance the reactivity ratio between iodobenzene and chlorobenzene deduced from Experiments 2 and 5 is the same as the one obtained by multiplying the reactivity ratio between iodobenzene and bromobenzene (Experiments 3 and 6) by the reactivity ratio between bromobenzene and chlorobenzene (Experiments 1 and 4).

584 b) Influence of other substituents present on the aromatic ring A series of chlorobenzene derivatives with various substituents wê allowed to react in the presence of bromobenzene (Table 4). As in previous experiments contact time was varied in order to obtain low conversions and to compare conveniently the reactivities of the various compounds. With all the reactants shown in Table 4, except 4-chlorotrifluoromethylbenzene, the only products which were obtained were those resulting from the regioselective exchange of chlorine for bromine. A few other compounds were used as reactants but unfortunately led to by-products which made it difficult to compare their reactivities. Moreover in the case of 4-chlorotrifluoromethylbenzene, a reaction with potential pratical interest, the reaction led to a significant loss of copper from the catalyst as well as to the deterioration of the zeolite structure. Therefore, even if the reaction was quite selective in the case of 4-chlorotrifluoromethylbenzene its reactivity cannot really be compared to the reactivities of the other reactants. Table 4 Exchange reaction with bromobenzene of various aromatic compounds in equimolar mixture. Activity mmol. h"l. g-l m-Fc|)Cl m-Bn|)Cl m-Cl2 e s o r p t i o n t e m p e r a t u r e ("C)

90

1

80 70

T

RbY

2-methyl-propanal D 2-methyl-butanal NaY . a

Eeo

Fig. 4. Selectivity to 2methyl-propanal (I) and 2-

-

methyl-butanal

(II)

desorption temperature

.^50 § 40 T

I 30

° °' ° *a "

HY

vs. of

°

HY

ammoma. NaX

20 TZSM-5"

- ZSM-5

„

J

^ 250

m

-

10 1

1

S

350 450 550 D e s o r p t i o n t e m p e r a t u r e (°C)

The difference in size of the two probe molecule does not explain these results. Even though ammonia, as a small molecule, is able to penetrate through narrow windows of the zeolite structure which might be inaccessible for pyridine or for the diol, the acidity of these sites should not be very different from the sites in the larger cavities. The observed desorption temperatures probably mirror the complex interaction of different bases with the solid acids and could be a rather complex function of the acid strength (PKA) and maybe the hardness of an acid site according to Pearson [16]. It could well be that the nature of the interaction [17] of the "soft" pyridine with an acidic site is more like the one with 2,2-dimethyl-propanediol whereas ammonia is adsorbed preferentially on "hard" sites. However, this tentative interpretation does not fiiUy explain the large difference in adsorption temperatures, especially for the basic RbY zeolite, which showed a surprisingly high pyridine desorption temperature. It is possible that remaining acid sites - the zeolites were ion exchanged only once - are responsible for some of the inconsistencies. Indeed, the relative intensity (partial pressure) of the desorption peak of pyridine for RbY was rather low, compared to the TPD of H-type zeolites. To test the idea of the similarity of the adsorption properties of 2,2-dimethyl-propandiol and pyridine, some competition experiments were carried out, using Rb(Na)Y- zeolite as catalyst, which showed very high selectivity to 2-methyl-propanal (88%) and a high desorption

600 temperature (around 400 °C ). As shown in Fig. 6 a strong decrease in activity with increasing pyridine concentration was found as well as a gradual decrease of selectivity to (I). This behavior can interpreted as a blocking of the sites with higher desorption temperature than the reaction temperature of 300 "^C by pyridine. This would lead both to a lower activity as well as selectivity to (I) that is formed preferentially on these sites.

Fig. 6. Effect of pyridine addition to the feed on conversion and selectivity to 2-methyl-propanal (I). Catalyst Rb(Na)Y, 300 °C.

pyridine in the feed

Even though a complete interpretation of these observations is not possible it seems that strongly acidic centers with high desorption temperature of pyridine preferentially catalyze the formation of (I), whereas zeolites with weaker or moderate acidic centers (one important factor being the Si/Al ratio [14,15]) lead to rearrangement to (II). It is more difficult to propose a mechanism by which the two aldehydes are formed [18]. For the formation of 2methyl-butanal (II) the loss of a hydroxy group accompanied by a methyl shift is required. For the formation of the 2-methyl-propanal (I) the formal loss of a hydroxy and a methyl group is required. Or, in other words, we have to explain a preferential methyl shift versus a preferential methyl loss. Dehydration would require the formation of a primary carbenium ion in the first step so a concerted mechanism for OH abstraction accompanied by methyl shift or methyl loss is more likely. Acid attack with formal loss of one OH group of the 2,2-dimethyl-propanediol represents the first step which is followed by methyl and hydride shift leading to 2-methylbutanal (II) or followed by CHs^ abstraction and subsequent double bond shift and enolization leading to 2-methyl-propanal (I). The two routes differ in the migration of negatively or positively charged species, respectively. For the abstraction and migration of positively charged species a basic a basic material could be helpful, maybe explaining some of the inconsistencies found in our investigations. Conclusions The reaction of 2,2-dimethyl-propanediol over various solid acids does not lead to the desired 3,3-dimethyl-oxetane. The major products are 2-methyl-propanal (I) (cleavage) and 2-

601 methyl-butanal (II) (rearrangement). The ratio of the two products is controlled by the nature of the ion of the zeolite (varied by ion exchange) whereas the structure of the catalyst has little effect. Even though a correlation was found between the selectivity to (I) and (II) and the desorption temperature of pyridine, only a tentative explanation for the formation of the cleavage product (I) and rearrangement (II) could be put forward. References 1

H. Mueller, V. Dieter, BASF AG, DE 3308931 Al (1984)

2

W. Holderich, R. Fischer, W. Mesch, BASF AG, DE 3636430 Al (1987)

3

L.D. Brake, DU PONT, EP 0099676 Al (1983)

4

J. Topp-Jorgensen, HALDOR-TOPSOE, EP 0148626 A2 (1985)

5

M.H. Harandi, H. Owen, MOBBL OIL, US 5011506 (1991)

6

M.H. Harandi, H. Owen, MOBIL OIL, US 5015782 (1991)

7

S. Searles, R.G. Nickerson, W. Witsiepe, J. Org. Chem. 24 (1960) 1839

8

E.J. Vandenberg, J.C. MuUis, R.S. Juvet, T. Miller, R.A. Nieman, J. Polym. Science, Part A: Polymer Chem. 27 (1989) 3113.

9

Product Information: Ion Exchange and Metal-Loading Procedures, Linde & Union Carbide

10

G Lischke et al., J. Catal. 132 (1991) 229.

11

J.P. Joly, A. Perrad, Appl. Catal. A 96 (1993) 355.

12

N.Y. Toepsoe, K. Pedersen, E.G. Derouane, J. Catal. 70 (1981) 41.

13

M.J. vanNiekerk, J. Fletscher, CO. Connor, J. Catal. 138 (1992) 150.

14

U. Lohse, B. Parlitz, V. Patzelova, J. Phys. Chem. 93 (1989) 3677.

15

V.B. Kazansky, PA. Jacobs, Structure and Reactivity of Mod. Zeolites, Elsevier 1984, p. 61.

16

R.C. Pearson, J. Am. Chem. Soc. 85 (1964) 3533.

17

A. Corma, G. Sastre, R. Viruela, C. Ziovich-Wilson, J. Catal. 136 (1992) 521.

18

The authors gratefully acknowledge a referee for helpful suggestions concerning the reaction mechanism.


Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All riehts reserved.

603

CLAY-CATALYZED REACTIONS OF IMIDAZOLE AND BENZIMEDAZOLES WITH PROPIOLIC ESTERS Maria Balogh, Csaba Gonczi, Istvan Hermecz Chinoin Pharmaceutical and Chemical Works Ltd., Research Centre, H-1325 Budapest P.O.Box 110, Hungary

Michael addition of imidazole (1) and benzimidazole (5) to alkyl propiolate (2) can be promoted by KIO montmorillonite clay affording alkyl 3-(imidazol-l-yl)acrylate (3) and alkyl 3-(benzimidazol-l-yl)acrylates (6), respectively. Michael adduct of 2-mercaptobenzimidazole (8) undergoes cyclization in the presence of clay catalyst to 4-oxo-4H[l,3]thiazino[3,2-a]benzimidazole(9). Introduction Special attention has been called to environmentally friendly catalysis because of the increasing demand for cleaner industrial processes in recent years. The use of solid catalysts is likely to be especially important in thefriturein the manufacture of fine chemicals and intermediates ' . Clays are effective catalysts for a wide variety of organic reactions . Recently the prowess of KIO montmorillonite, as a strong Bronsted acidic catalyst, has been shown in cyclocondensation reactions . This presentation will focus on the assets and the usefulness of KIO montmorillonite for catalysis of the reactions of imidazole and benzimidazoles with propiolic esters. Results and Discussion Methyl 3-(imidazolyl)acrylate (3) prepared earlier in a two step procedure can be obtained in high yield from imidazole (1) and methyl propiolate (2) at ambient temperature. The presence of KIO montmorillonite speeds up the reaction but does not alter the E : Z isomer ratio determined by ^H-NMR spectroscopy. The distribution of the addition products (3 and 4) depends on the molar ratio of the reactants. Double Michael addition is favored by the use of excess imidazole leading to alkyl 3,3-bis(l-imidazolyl)propionate (4) (Table 1). Remarkable acceleration can be achieved in the reaction of benzimidazoles (5) (R = H, Ph) with propiolic esters (2) by the use of KIO montmorillonite: the reaction time can be decreased from 5-7 days to 1-7 hours (Table 2).

604

m in

'-H CN '^ OS o m en ^n m

o

in in to

r- t^ ^t* r f

OS r-H so ^ o en m vo so Tf vo o »o m

i

u O

OS

r-H m in m »n S so in

r-

00

in

(N

^

00

OH

£

S

CN

s

-^ X3

43 ^

X3

â ^ Z: Z: I^: Z: tr ^ »n ^

2 2 2 2 2 (N 0.2 wt% the cyclohexanone conversion is Umited by the acid steps hence the activity depends no more on the metal content. The stabilities of the catalysts were compared for identical values of the initial conversion. The nature and the percentage of the metal have no effect on the stability. Thus for an initial conversion of 30% the conversion after 3 hours' reaction is around 10% whatever the catalyst (residual activity equal to 0.35). The catalyst deactivation can be due to the retention inside the zeolite pores of heavy reaction products ("coke'' precursors) [8]. However a sintering of the metal particles can also occur owing to the presence of water resulting fi'om dehydration reactions. Indeed preUminary results indicate that, after removal of non desorbed products by oxidative treatment at 773 K, the acidity of the catalyst is completely recovered, which is not the case for the hydrogenating activity. Ao (mmol/h/g)

Selectivity to 4 (%)

50 -

> ? ^ 5 ^ = * — '\

40 -

r .

3020 i

10 -

/

0

80 -

A PtHFAU 0HFAU

60-

^^

^

^ 40 -

r

n U

PdHFAU

20 1

0.2

n U i

1

0.4

0.6

metal (wt,%)

Figure 2. Initial activity, Ao, of PdHFAU and PtHFAU catalysts vs. metal content.

0

I

0.2

1

0.4

0.

metal (wt,%)

Figure 3. Selectivity to cyclohexylcyclohexanone over PdHFAU and PtHFAU catalysts vs. metal content.

Figure 3 shows the change of the initial selectivity to cyclohexylcyclohexanone, calculated for an initial conversion of 30%, as a function of the metal content. For both series of catalysts a small quantity of metal (about 0.1 wt %) is enough to obtain the maximal selectivity value to the desired product. Nevertheless, contrary to what was found for the catalyst activity, the selectivity value strongly depends on the nature of the hydrogenating function (47% on PtHFAU against 75% on PdHFAU). The better selectivity of Pd catalysts was also observed with all the other bifunctional zeoUte catalysts. However, contrary to what was found

613

with HFAU catalysts, the initial activities of PtHMFI and PtHMOR were greater than those of the corresponding Pd catalysts. The lower selectivity of PtHFAU catalysts is due to the very rapid formation of Ce cycUc hydrocarbons (family 1). The same trend has been found in the case of acetone transformation [3]. This can be explained by the lower activity of the palladium relatively to the platinum to hydrogenate the C=0 bond. This lower activity which has been found in the case of cyclohexanone hydrogenation on platinum group metals was explained by a weaker adsorption of the ketone on Pd in comparison with Pt and Ru 19]. The lower activity of Pd for ketone hydrogenation is also responsible for the lower selectivity of PdHFAU catalysts for the compounds of family 2 whose formation involves cyclohexylcyclohexanone hydrogenation. The value of the ratio between cyclohexylcyclohexanone and cyclohexenylcyclohexanone (4/3) increases with the metal content (Pt or Pd) up to 0.2 wt%, then remains constant. Nevertheless this ratio is greater on PtHFAU than on PdHFAU catalysts (25 against 19 for a metal content of 0.2 wt%). This greater value found with PtHFAU catalysts is certainly due to their higher hydrogenating activity. 2.3. Influence of the zeolite pore structure Figure 4 compares the values of the initial activities of acid and bifunctional (0.2 wt% Pt or Pd) zeoUte catalysts. The most active acid catalyst is HBEA, HFAU is 1.3 times less active, HMOR and HMFI aluminosilicate about 3 times less active and HMFI gallosiUcate 30 times less active. The low activity of the gallosilicate was expected from the low strength of its acid sites [10]. However the difference in activity between the MFI gallo and aluminosUicate is more pronounced than in m-xylene isomerization. This suggests the existence of diffusion limitations during cyclohexanone transformation on the MFI zeoUte samples. These diffusion limitations are more pronounced with the gallosilicate sample for which the paralortho ratio found in m-xylene isomerization is greater than with the MFI aluminosiUcate sample [10]. Furthermore the greater activity of the HBEA sample could be due to the very small size of its crystallites hence to its large external surface area [11]. HMOR catalysts are generally less active than HFAU catalysts because of diffusion limitations in their monodirectional pores. The difference between these zeolites found in this work is very limited probably because mesopores created during the mordenite preparation by dealumination render the diffusion of organic molecules quasi tridirectional [12]. 0.2 wt% Pd exchanged catalysts are generally more active than the corresponding acid zeoUtes. An exception however: PdHBEA has the same activity as HBEA but the selectivities are totally different : as it could be expected the main reaction products observed on the acid zeolite are the cyclohexenylcyclohexanone isomers (selectivity equal to 74 % against 37% on PdHBEA, at 10% conversion) whereas only a selectivity value of 4% to cyclohexyl-cyclohexanone is observed. The most active bifunctional Pd catalyst is PdHFAU. This catalyst is about twice more active than PdHBEA, 3-4 times than PdHMOR and PdHMFI

614

aluminosilicate and 30 times than PdHMFI gallosilicate. These differences in activity cannot be explained by differences in acidity only. Most likely, other catalyst characteristics such as their porosity and their hydrogenating activity play also a significant role. Ao(mmol/h/g) PtHFAU PdliFAU 40 PtHMFI 30 PdHBEAl HBEA 20 PdHMFI 10 +P(iH-[Ga].MFI HMFI

PtHMOR HFAU IPdHMOR

HMOR

|H-[Ga]-MFI/ 0

Catalyst Figure 4. Transformation of cyclohexanone. Initial activities of acid and bifunctional (0.2 wt% Pt or Pd) zeolite catalysts. Table 1 shows that the product distribution on Pd catalysts depends on the zeoUte. PdHFAU and PdHMOR are the most selective to cyclohexylcyclohexanone. This is also the case when all the products which can be transformed into o-phenylphenol (3+4+5) are considered. However the hydrogenating activity of PdHMOR is weaker than that of PdHFAU. Indeed the cyclohexylcyclohexanone/cyclohexenylcyclohexanone ratio (4/3) is lower (Table 1). The difference in selectivity to 3+4+5 between PdHFAU, PdHMOR on the one hand and PdHMFI, PdHBEA on the other is partly due to the formation of Ce or Ci2 hydrocarbons (products 1 and 2) even if some other differences exist between the catalysts. In particular only 0.3% of Cs hydrocarbons are found in the products on PdHMOR against around 2% on the other catalysts; 10% of C12 hydrocarbons are found on PdHBEA against around 6% on the other catalysts. Furthermore there is a more significant production of the heavy products 6 with PdHMFI and PdHBEA than on PdHFAU and PdHMOR. This faster production of 6 is probably due to Umitations in the desorption of the reaction products 3 and 4 which can therefore undergo secondary transformations into heavy products. This faster production could also be due to the weaker hydrogenating activity of PdHMFI and PdHBEA (see in Table 1 the low value of the 4/3 ratio) if it is admitted that aldoHsation occurs more rapidly from the 3 alkylenic compounds than from the 4 compound because of a stronger adsorption on the acid sites.

615 Table 1 Transformation of cyclohexanone. Selectivities of bifunctional zeolite catalysts Selectivity to products (%) Catalyst

1

2

3

4

5

6

4/3 ratio

0.2PtHMFI 0.2PdHMFI 0.2PtHMOR 0.2PdHMOR 0.2PtHFAU 0.2PclHFAU 0.2PdHBEA

61.2 2.6 34.4 0.3 30.1 2.0 1.8

7.7 6.5 21.5 6.0 12.1 6.0 10.2

2.0 17.0 3.8 7.0 1.9 4.0 18.3

23.2 52.9 34.5 73.2 47.2 75.0 47.7

1.8 6.0 1.0 2.5 1.5 3.5 5.0

4.1 15.0 4.8 11.0 7.2 9.5 17.0

11.6 3.1 9.1 10.5 24.8 18.7 2.6

3. EXPERIMENTAL HZSM5, HFAU, HMOR and HBEA zeoUtes had framework and total Si/Al ratios of about 40. They were commercial Valflor zeolites supplied by PQ, or obtained from them by dealumination by acid treatment. The H-[Ga]-ZSM5 had a Si/Ga ratio of 35 and its synthesis has already been described [10]. The Pt and Pd zeolites catalysts were prepared by ion exchange with [Pt(NH3)4]Cl2 and [Pd(NH3)4]Cl2, respectively, followed by calcination under dry air flow at 573K and reduction under hydrogen at 773 K. The reaction was carried out in a flow reactor at 473 K, atmospheric pressure and PH2^Pcyclohexanone~ 3- Reaction products were identified by a GS/MS system and analyzed by gas chromatography using a CPSil 5 CB capillary column with 50 m of length and 0.25 mm of interior diameter [4]. In order to study the influence of contact time on the catalytic properties, different catalyst weights (0.07 - 0.6 g) and different flows of Liquid cyclohexanone were used (1.9 - 4.25 cm^/h).

4. CONCLUSIONS Bifunctional Pt or Pd zeolite catalysts (with large or average pore sizes) can catalyze in one pot the transformation of cyclohexanone into cyclohexylcyclohexanone which requires three successive steps catalyzed by acid sites : aldolisation and dehydration or by metal sites : hydrogenation. Pd catalysts are more selective than Pt catalysts, for palladium catalyzes preferentially the hydrogenation of C=C double bonds (compared to the C=0 bonds). PdHFAU zeolites because of their large pores and of their tridirectional pore system are the most active and selective catalysts. With these catalysts the formation of

616

cyclohexylcyclohexanone is not limited by the desorption of this bulky product from the zeolite pores, which is the case with Pd deposited in zeolites with narrower pores or with unidirectional pore systems.

REFERENCES 1. W.F. Holderich and H. van Bekkum, in "Introduction to Zeolite Science and Practice", (H. van Bekkum et al., Eds.), Studies in Surface Science and Catalysis, vol. 58, Elsevier, Amsterdam, 1991, p. 631. 2. P.V. Chen, S.J. Chu, N.S. Chang, T.K. Chuang and L.Y. Chen, in "Zeolites as Catalysts, Sorbents and Detergent Builders", (H.G.Karge and J. Weitkamp, Eds.), Studies in Surface Science and Catalysis, Vol. 46, Elsevier, Amsterdam, 1989, p. 231. 3. L. Melo, Ph.D.Thesis, Universite de Poitiers, 1994. 4. F. Alvarez, P. Magnoux , F. R. Ribeiro and M. Guisnet, J. Mol. Cat., 92 (1994) 67. 5. A. Mitschker, R. Wagner and P.M. Lange, in "Heterogeneous Catalysis and Fine Chemicals", (M. Guisnet et al., Eds.), Studies in Surface Science and Catalysis, Vol. 41, Elsevier, Amsterdam, 1988, p. 61. 6. P. Thomissen and J. Hubertu (Stamicarbon B.V.), Eur. Pat. Appl. EP 87187 (1983). 7. M. Guisnet and G. Perot, in "Zeolites Science and Technology", (F. R. Ribeiro et al., Eds.), NATO ASl Series E, Vol. 80, Martinus Nijhoff Publishers, The Hague, 1984, p. 397. 8. A.I. Silva, F. Alvarez, P. Magnoux and M. Guisnet, unpubUshed results. 9. C. Sungbom and K. Tanaka, BuU. Chem. Soc. Jpn., 55 (1982) 2275. 10. F. Jayat, I. Neves, M. Guisnet, M. Goldwasser, G. Giannetto and J. Papa, in "Proc. XIV Simposio Iberoamericano de Catalisis", Chile, 1994, p. 573. l l . C . Coutanceau, J.M. Silva, F. Alvarez, F.R. Ribeiro and M. Guisnet, J. Chim. Phys., in press. 12.N.S. Gnep, P.Roger, P. Cartraud, M. Guisnet, B. Juguin and C. Hamon, C.R. Acad. Sci. Paris, 309 QI) (1989) 1743.

ACKNOWLEDGEMENT Financial support by the EC within the International Scientific Cooperation EC-ALA/MED countries (Contract CIl*-CT94-0044) is gratefully acknowledged.


617

Solid Acid Catalyzed Disproportionation and Alkylation of Alkylsilanes T.Yamaguchl^, TYamada^, M.Shibata^, T.Tsunekl^, M.Ookawa^ ^Department of Applied Chemistry, Faculty of Engineering, Ehime University, Matsuyama 790-77, Japan '^Graduate School of Environmental Science, Hokkaido University, Sapporo 060, Japan Catalytic disproportionation of alkylsilanes such as diethylsllane (E2), triethylsilane (E3) and diethyldimethylsilane (E2M2) were examined at 373-623 K In a closed recirculation apparatus. Strongly acidic catalysts, SA, HY, MgY, CaY, S03/Zr02and alumina, exhibited high activities. Catalytic alkylation of alkylsilanes with oleflnlc and acetylenic compounds using solid catalysts was examined In a closed recirculation reactor at 373 - 473 K. Alkylation of dlethylsilane(E2) with these compounds took place smoothly on sllicaaiumlna (SA) and SOsfZrOz catalysts but not on alumina, which means protonic solid add catalyzed the reaction. n-Alkylated products were the main products and the /so-alkylated ones were the minor products regardless the type of olefins. The product distribution Indicates the reaction takes place via a nucleophlllc attack of olefins on a SI cation. 1. INTRODUCTION A hydrosllylatlon Is a well known method to create a Sl-C bond via the addition of Sl-H to a C-C multiple bond [1,2]. Chloroplatlnic acid Is used as a catalyst under a homogeneous liquid phase condition. The use of metal halldes for the disproportionation of some alkylsilanes Is found In the earlier work by Russell [3]. Although numerous works on the heterogeneous synthesis and conversion of hydrocarbons have been done, surprisingly only few research works have been reported on heterogeneous catalytic transformations of organosllanes, In spite of an increasing Importance of their application to various fields. Not only the conversion of homogeneous add catalyzed system to the heterogeneous one Is an Important challenge, but the development of fundamental chemistries of SI compounds in heterogeneous catalysis Is also quite attractive and Important. The catalytic disproportionation (or an alkyl exchange reaction) of alkylated aromatics such as toluene to yield xylenes Is a well-known add catalyzed reaction.

618

If the chemistry of carbon and SI Is similar, alkylsllanes may undergo disproportionatlon by using solid acid catalysts. This paper deals with the feasibility study of the catalytic disproportionatlon of alkylsllanes and the catalytic alkylation of diethylsilane with olefinic and acetylenic compounds by using solid acid and base catalysts. 2. EXPERIMENTAL The disproportionation (or alkyl exchange) and the alkylation reactions of alkylsilanes have been carried out in a closed recirculation reactor at 373 - 623 K and 373 - 473 K, respectively, by using 100 - 200 mg of catalysts. For the disproportionation reaction, 30 Torr of diethylsilane (E2), diethyldlmethylsilane (E2M2), and triethylsilane (E3) were used. For the alkylation reaction, 30 Torr of E2 and 30 Torr of alkylating reagents (propene, 1- and c/s-2-butene, 2-methyl-1butene, 1,3-butadlene, methylacetylene, ethylacetylene) were used. Cyclic olefins, nitriles, benzene and carbonyl compounds were also tested. Catalysts were clay minerals, sulfated Zr02(S03/Zr02), alumina, SiOa-AlaOa (SA) and various metal oxides. Prior to the reaction, the catalyst was evacuated at 773 K for 3 h. Product mixtures were analyzed by an on-line gas chromatograph equipped with an FID detector. Reaction products were separated by a gas chromatograph with a TCD detector and Identified by an NMR analysis. 3. RESULTS AND DISCUSSION 3.1. Disproportionatlon A reaction of alkylsilanes basically consists of the disproportionation 2R2SiH2

RSIHa + R s S i H

2 (R^)2 Si (R2)2

(R^)3Si R2 + R1 SI (R2)3

and the decomposition (cracking) reaction. R2S1H2

RS1H3 + R'

Figures 1, 2 and 3 summarize the results of the reactions of E2, E3 and E2M2 at 573 K, respectively Yield was estimated after 60 min reaction. SA, SOs/ZxOz, alumina and modified clay minerals were active for the disproportionation. Other oxide catalysts such as Ti02, Zr02, Na-Y, MgO and niobic acid were inactive for the reaction, instead only a decomposition reaction took place. Si02 was totally inactive. Acidic catalysts showed good catalytic activity, while ones with weak or non-acidic character were inactive. Solid bases were inactive for the disproportionation reaction. Though \NO3fT\O2 and niobic acid have an acidic character and are excellent catalysts for the olefin isomerlzation [4] and the olefin-

619

aldehyde condensation reaction to yield conjugated dienes [5], they were inactive too. Thus it is concluded that the disproportionation of alkylsilanes is catalyzed by strongly acidic catalysts such as SA and S03/Zr02. In a preceding paper [6-8], we reported the disproportionation of E2 and E2M2 on zeolite catalysts. Although the order of catalytic activity of zeolites were HY > MgY > Ca-Y > HM > HZSM-5 regardless the reactant, E2M2 is more sensitive for the kind of zeolites. The rate of disproportionation of E2M2 decreased more sharply than that of E2 in the order shown above. For the E2 reaction, the activity of Ca-Y was 30% lower than that of HY, however, for the E2M2 reaction, the activity of Ca-Y was only one-tenth of that of HY. HM showed very low activity for E2M2 reaction and HZSM-5 was totally inactive. This means small pore zeolites are unfavorable for the disproportionation reaction. The shape selective behavior is more pronounced in the E2M2 reaction. Figure 4 compares the relative reactivity of E2, E3 and E2M2 over three catalysts, S03/Zr02, SA and alumina. It is seen that the reactivity of these alkylsilanes is in the order of E2M2 > E3 > E2 on S03/Zr02 and SA. It seems that the more the central Si atom Is alkylated, the more the compound become reactive. The order, however, was reversed on alumina ; E2 was the most reactive. Thus the order on alumina catalyst is E2 > E3 = E2M2. By considering the fact that the disproportionation proceeds over acidic catalysts, a possible intermediate is a catlonic one. A siliconium ion and a silicenium ion are the possible intermediates. The former is produced by

AI203 AS/Zr-SAPO S03/Zr02 !>3>S>S>S*N!i>0S«Hi AI3+/SAPO Si02-A1203 AS/Zr-PILC Zr-PILC Na-Y W03m02 MgO Nb205-nH20 Active Carbon 1102 Zr02 Si02 1 1

JpsN

10

^1

D

C2 1 El 1

E3 1 15 yield / %

20

Fig. 1 Disproportionation of E2 at 573 K

Si02-AI203 S03/Zr02 AI203 W03m02

P 1i1 i ? ^ ^M ^ S ?M ?^ MT ^

Nb205-nH20 MgO Active Carbon

;

; 1;

i

"

™1

:

j

;

i

—^ 1

1 1

! i

1 1 11

:

5

\

i

i

i 15 yield / %

Zr02 TI02 SI02

n

;

T""

1

1 j

1

S E1 I H E2| S5 E4 [

i

Fig. 2 Disproportionation of E3 at 573 K

S03/Zr02 »SQQi9«Qi9CQ«KSiSQQ«9QQQQQiiQQi9^»M si02 AI203 H^^'^^^^'^^;^^^'^^^^s^^^-^^^ i i AI203 AS/Zr-PILC _ Zr-PILC ^ ^ W03/n02 MgO D CI Zr02 D C2 Nb205-nH20 H M4 Active Carbon B E1M3 Ti02 S E3M1 Na-Y H E4 Si02 30 yield / %

Fig. 3 Disproportionation of E2M2 at 573 K

620

the addition of a proton and the intermediate is five-coordinated. The latter is produced by the abstraction of a hydride Ion and three-coordinated. The change of reactivity order by two groups of catalysts may indicate the different reaction mechanisms are operative. Over S03yZr02 and SA, an alkyl-saturated reactant is the most reactive, while over alumina, less alkylated or more hydrogen-substituted one is more reactive. Thus we propose a protonation on alkylsilanes is operative on S03/Zr02 and SA, while a hydride abstraction initiated the reaction over alumina. It is known that alumina possesses only Lewis acidity and not protonic acidity The initiation steps may be as follows.

Et

Me

{

^Me

E2M2 Fig. 4 Reactivity of E2, E3 and E2M2 over SA, SO^/ZrO^ and alumina

+ H' Br0nsted acid (B)

Et , Me

>Y-H Et ""Me siliconlum Jon

3.2. Alkylation Si-H is believed to be equivalent to H-H and hence Si-H can add a C-C multiple bond to produce SiC-CH. The reaction is usually mediated by precious metal complexes such as chloroplatinic acid in a homogeneous liquid phase. This reaction may correspond to the alkylation of Si compounds with olefins and unsaturated compounds. A typical alkylation reaction is an acid catalyzed FriedelCrafts reaction, e.g. an alkylation of benzene with propene to yield

621 isopropylbenzene by using BFa or solid acids such as zeolite, or an alkylation of aliphatic hydrocarbons with olefins to yield branched, high octane number hydrocarbons by using sulfuric acid. In the benzene alkylation, the reaction consists basically of an electrophilic attack of carbocation produced from acid and olefins to an aromatic ring. So alkylated products are usually isoalkylated ones, since the formation of a secondary carbocation is more favorable. We aimed to throw a light to the followings. 1. Does a hydrosilylation or an alkylation between unsaturated compounds and alkylsilanes undergo over solid acids ? 2. If the reaction takes place, what is a favorable product, /so-alkylated or nalkylated one ?

well-known

Et Si Et H propene

and/or

r^ Et Si Et H

79

Since no information is available for the reaction using heterogeneous catalysts, catalyst screening test was first employed by using E2 and propene. Catalysts tested were metal oxides including solid acids such as S03/Zr02, SA, HY and solid base, MgO. Results are summarized in Fig. 5. Over S03/ZrC>2 and SA, the reaction took place S03/Zr02 K \ s \ ^ \ \ \ ^ ^ \ ^ \ \ \ \ ^ ^ ^ ^ I^^^^J^^îî^ smoothly at 373 K, though the SI02-AI203 Zr-SAPO r f _ disproportionation of E2 proceeded AS/Zr-SAPO H - Y ^ Z r 0 2 r^ higher temperature. Modified clay AI203h MgOB minerals also catalyzed the reaction at an W03/AI203 5 SAPOJ appreciable rate. Alumina and HY AI3+ySAPO J showed low activity and other catalysts W03/Zr02f W03/TI021 were almost inactive for the reaction. This 10 15 20 25 product yield / % clearly shows that strong solid acids can catalyze the alkylation reaction. The Fig. 5 Alkylation of E2 with propene at 373 K heterogeneous alkylation of E2 is an acid catalyzed reaction. It is interesting finding that the catalytic activity of alumina, which showed the highest activity for the disproportionation of E2, was very low for the alkylation. This may Indicate the reaction can not be catalyzed by Lewis acid. HY hardly catalyzes the reaction, though its activity for the disproportionation was high. This may come from the shape selective nature of microporous zeolite.

622 Propyldiethylsilane, the alkylated Table 1 Alkylatlon of E2 by Olefinlc product, was found to be an n-propyl form Hydrocarbons and not iso form which was confirmed by reactant yield / % product NMR analysis. This clearly shows that the 1-butene 81.7 «-form addition of propene to a Si atom >K^^ isobutene iso product could be produced. Hence 7.3 iso-form >%""" 1,3-butadiene Propyldiethylsilane (PrE2) is a sole disilyl 18.5 product ; no dipropylated product was /;-form 37.4 :>%^"" found. In the hydrosilylation reaction, it is methylacetylene known that the reactivity of E3 is higher iso-form 11.9 :>^'<S* than that of E2. It is still unclear why the reaction conditions : reaction temp. = 423 K, cat. wt.: 0.5g. dipropylated product was not found under E2 = 60 Torr, hydrocarbon = 120 Torr: yield after 3h reaction the present condition. Several possibilities may be pointed out. 1. Because of the lower concentration of PrE2, further alkylation was not pronounced. 2. The reactivity of trialkylated compounds is lower than that of dialkylated ones. 3. Because of the steric hindrance, trialkylated compounds hardly undergo further alkylation on the surface of catalysts. Preliminary investigation using E3 as a starting material suggested the reactivity of E3 was lower than that of E2. So first possibility could be neglected. Further investigations are necessary to clarify another two possibilities. Several alkylating reagents were tested, namely, 1-butene (1-B), c/s-2-butene (c/s-B), 2-methyl-1-butene (isobutene; /-B), 1,3-butadiene (1,3-BD), methylacetylene (MA), and ethylacetylene (EA). The reaction was carried out at 423 K using 0.5 g of SA as a catalyst. Initial pressures of E2 and hydrocarbons were 60 and 120 Torr, respectively. Yield were evaluated after 3 h reaction. Table 1 summarize the results. From this Table following characteristics can be pointed out. 1. n-Alkylated product was found to be a major product as in the case of propylation. Propene and 1-B gave n-form exclusively c/s-B also gave n-form as a major product. 2. Dialkylated species were not found. 3. Relative amounts of n-form and /-form varied depending on the structure of reactants. The result obtained from c/s-B is suggestive. If c/s-B directly alkylates E2, then the expected product should be /-form, since an E2 molecule can only attack the secondary carbon atom of c/s-B. The n-alkylated product from c/s-B must come from

623 a terminal olefin, I.e. 1-B. An analysis of butenes after the reaction showed almost equilibrated mixture of l-B, c/s-B and fra/?s-2-butene ; about 10% of 1-B and the rest 2-B. It is known that over solid acid catalysts isomerization of butene is usually rapid. So the relative reactivity of terminal carbon to an E2 molecule is calculated to be 65 times higher than that of secondary carbon. The preferential attack of E2 to a terminal carbon of terminal olefins strongly suggests that E2 attacks the carbon atom with the highest electron density. The relative amount of the formation of n-form and /-form may reflect the electron density of carbons in C-C multiple bonds. As seen in Table 1, n- / /-form ratio is high in olefinic compounds while that was low in acetylenic one, in which the charge distribution is more uniform than olefinic molecules. As stated above, Si-H is believed to be equivalent to H-H. A hydrogenation of 1,3-BD undergoes over metal oxide catalysts [9]. A characteristic feature of this type of hydrogenation is 1,4-addition of two H atoms, i.e. addition of two H at terminal position of 1,3-BD molecule. 1,2-Addition was found when the hydrogenation was carried out over Zr02 catalyst by using a hydrogen donor molecule, cyclohexadiene, instead of H2 [9]. Unlike to the hydrogenation by H2 over oxide catalysts, SI-H added to the 1,2-position of 1,3-BD to yield (JJ and no product from 1,4-addition was found. Product (1) has one olefinic double bond and this further reacted with E2 to yield product (2), a disilyl compound. >Si
Si
Si

Heterogeneous Catalysis and Fine Chemicals IV

Heterogeneous Catalysis Research Progress

Heterogeneous Catalysis Research Progress

Theoretical Heterogeneous Catalysis

Isotopes in Heterogeneous Catalysis

Isotopes In Heterogeneous Catalysis

Handbook of Asymmetric Heterogeneous Catalysis

Handbook of Asymmetric Heterogeneous Catalysis

Heterogeneous Catalysis in Organic Chemistry

Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis

Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis

Theoretical Aspects of Heterogeneous Catalysis

Heterogeneous Catalysis in Industrial Practice

Theoretical aspects of heterogeneous catalysis

Mass Transfer in Heterogeneous Catalysis

Heterogeneous Catalysis in Organic Chemistry

Principles and Practice of Heterogeneous Catalysis

Mechanical Catalysis: Methods of Enzymatic, Homogeneous, and Heterogeneous Catalysis

Fine Chemicals: The Industry and the Business

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Fine Chemicals Manufacture: Technology and Engineering

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Heterogeneous Catalysis and Fine Chemicals II: Proceedings of the 2nd International Symposium, Poitiers, October 2-5, 1990 (Studies in Surface Science and Catalysis)

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

Heterogeneous Catalysis for the Synthetic Chemist

Eco-Friendly Synthesis of Fine Chemicals

Selective Nanocatalysts and Nanoscience: Concepts for Heterogeneous and Homogeneous Catalysis

Modern Heterogeneous Oxidation Catalysis: Design, Reactions and Characterization

Heterogeneous Enantioselective Hydrogenation: Theory and Practice (Catalysis by Metal Complexes)

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Heterogeneous Catalysis Research Progress

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Theoretical Heterogeneous Catalysis

Isotopes in Heterogeneous Catalysis

Isotopes In Heterogeneous Catalysis

Handbook of Asymmetric Heterogeneous Catalysis

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Heterogeneous Catalysis in Organic Chemistry

Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis

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Theoretical Aspects of Heterogeneous Catalysis

Heterogeneous Catalysis in Industrial Practice

Theoretical aspects of heterogeneous catalysis

Mass Transfer in Heterogeneous Catalysis

Heterogeneous Catalysis in Organic Chemistry

Principles and Practice of Heterogeneous Catalysis

Mechanical Catalysis: Methods of Enzymatic, Homogeneous, and Heterogeneous Catalysis

Fine Chemicals: The Industry and the Business

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Fine Chemicals Manufacture: Technology and Engineering

Fine Chemicals: The Industry and the Business

Heterogeneous Catalysis and Fine Chemicals II: Proceedings of the 2nd International Symposium, Poitiers, October 2-5, 1990 (Studies in Surface Science and Catalysis)

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

Heterogeneous Catalysis for the Synthetic Chemist

Eco-Friendly Synthesis of Fine Chemicals

Selective Nanocatalysts and Nanoscience: Concepts for Heterogeneous and Homogeneous Catalysis

Modern Heterogeneous Oxidation Catalysis: Design, Reactions and Characterization

Heterogeneous Enantioselective Hydrogenation: Theory and Practice (Catalysis by Metal Complexes)

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